An intron of the eyeless (ey) gene contains an enhancer that regulates the eye specific expression of the gene in the eye disc primordia of embryos and in the eye imaginal discs of third instar larvae. The enhancer is located in the intron preceding exon 3 of ey. Moreover, a 212-bp enhancer element is necessary and sufficient for the enhancer function. It is partially conserved in Drosophila hydei and contains putative Pax-6 Paired domain binding sites. Several binding sites are required for the eye specific expression, and, therefore, it is proposed that a Pax-6-like molecule is a positive transactivator for the eye specific ey expression. This transactivator recently has been identified as twin of eyeless, the second Pax-6 gene in Drosophila (Hauck, 1999).

The Drosophila Pax-6 homologs eyeless and twin of eyeless are expressed in the eyes and in the central nervous system (CNS). In addition to the pivotal functions in eye development, previous studies have revealed that ey also plays important roles in axonal development of the mushroom bodies, centers for associative learning and memory. It has been reported that a second intron enhancer that contains several Pax-6 binding sites mainly controls the eye-specific expression, but the DNA sequences that control CNS expression are unknown. In this work, the transcriptional enhancer elements of the ey gene that are required for the CNS expression in various developmental stages have been dissected. CNS expression is independent of the eye-specific enhancer of the second intron. By systematic reporter studies, several discrete DNA elements in the 5' upstream region and in the second intron have been identified that cooperatively interact to generate most of the ey expression pattern in the CNS. DNA sequence comparison between the ey genes of distant Drosophila species has identified conserved modules that might be bound by the upstream regulatory factors of the ey gene in CNS development. Furthermore, by RNA interference and mutant studies, it has been shown that ey expression in the brain is independent of the activity of toy and ey itself whereas in the eye primordia it requires both, supporting the notion that ey and toy are regulated by parallel and independent regulatory cascades in brain development (Adachi, 2003).

The expression of Ey in the CNS is first detected in the ventral nerve cord (VNC) at stage 9 and in the early brain at stage 10. After germ band retraction, several clusters of Ey-expressing cells are observed in the embryonic brain. Ey is expressed in several clusters of cells in each of the embryonic brain neuromeres (b1, b2 and b3). In the first neuromere b1, a prominent cluster of Ey-expressing cells is observed at the most anterior region (in neuraxis), which gives rise to the MBs. Additional groups of cells are located dorsally (b1D), ventrally (b1V) and medio-laterally (brackets). In contrast, Ey is not expressed in the optic lobe primordia. In the ventral neuromeres Ey is expressed in a large segmental pair of cells located laterally as well as in a few pairs of cells located more medially. Later in the third instar larval stage Ey is expressed in MBs, in the medial part of the optic lobes and in segmental pairs of cells in the VNC. In the larval MBs, Ey is strongly expressed in all MB neurons. After metamorphosis, Ey expression is detected in many clusters of neural cells in the adult brain. Strong Ey expression continues in the Kenyon cells located in the posterior-dorsal region of the adult brain (Adachi, 2003).

A GAL4 enhancer trap, OK107, was initially isolated as an enhancer trap line that shows strong MB expression in the adult brain. Its P-element is inserted on chromosome 4, where ey is located. By PCR and sequencing, the exact location of the OK107 insertion was found to be 6.5 kb upstream of the first exon of ey. OK107 is confirmed to be expressed in the embryonic MB primordia as well as the larval MBs. Double immunostaining verified colocalization of OK107 expression and Ey in the brain, in that all GAL4-expressing neurons in OK107 indeed express Ey, although many Ey-expressing cells in other regions of the CNS do not express OK107 GAL4. In MBs, OK107 faithfully recapitulates the Ey expression throughout brain development. The coexpression of OK107 and EY in MBs was further confirmed in a complementary experiment, in which OK107 was used to drive a dominant-negative form of Ey. The phenotypes observed in this experiment reproduce the range of MB phenotypes observed in ey mutants (Adachi, 2003 and references therein).

Previous work led to the identification of the eye-specific enhancer at the 3' end of the second intron. Despite its robust transcriptional activity in the eye, minimal eye fragments such as 5D12 and D02 lack the potential to drive the LacZ reporter in the CNS, highlighting distinct requirements of cis-regulatory regions in the eye and CNS. In contrast, the E3.6 intronic fragment is able to drive CNS expression in reversed orientation, indicating a neural enhancer at the 5' end of the second intron, where conserved CNS motifs are confirmed (Adachi, 2003).

Apart from the intronic enhancer, a 5 kb upstream sequence drives strong LacZ expression in the CNS, including the MBs. This result is consistent with the robust MB activity of the GAL4 enhancer trap OK107, which is inserted 6.5 kb upstream of the first exon. Intriguingly, deletion analysis of the upstream region results in complex expression patterns that only partially overlap with the endogenous Ey pattern, with occasional ectopic activation of LacZ expression. Thus, these results argue for the existence of both positive and negative enhancer modules in the upstream and intronic sequences that cooperatively define most of the developmental expression pattern of ey in the CNS (Adachi, 2003).

However, the LacZ expression of ey12ER does not faithfully represent the endogenous ey patterns. This is particularly evident in the embryonic brain, in which expression of ey12ER is detected in virtually all the Ey positive neurons but its strength is not proportional to the endogenous Ey levels. Perdurance of LacZ protein might account for part of this discrepancy. Alternatively, it could be that additional modular elements are required to further refine the expression levels in different neurons. It is also noteworthy that none of the constructs drives LacZ expression in the pars intercerebralis, lateral horns and lateral neurons in the adult brain. These results suggest that enhancers for the expression of these adult neurons might be missing in the current series of reporter constructs. Indeed, it has been shown that regulatory elements located at the 3' end of the mouse Pax-6 gene control a part of expression in the developing pretectum, neural retina and olfactory region. Another set of reporter constructs and/or novel 3' enhancer trap lines are required to clarify this point (Adachi, 2003).

Identification of functional sine oculis motifs in the autoregulatory element of its own gene and in the eyeless enhancer

In Drosophila, the sine oculis (so) gene is important for the development of the entire visual system, including Bolwig's organ, compound eyes and ocelli. Together with twin of eyeless, eyeless, eyes absent and dachshund, so belongs to a network of genes that by complex interactions initiate eye development. Although much is known about the genetic interactions of the genes belonging to this retinal determination network, only a few such regulatory interactions have been analysed down to the level of DNA-protein interactions. An eye/ocellus specific enhancer of the sine oculis gene has been identified that is directly regulated by eyeless and twin of eyeless. This regulatory element has been further characterized and a minimal enhancer fragment of so has been identified that sets up an autoregulatory feedback loop crucial for proper ocelli development. By systematic analysis of the DNA-binding specificity of so the most important nucleotides for this interaction have been identified. Using the emerging consensus sequence for SO-DNA binding a genome-wide search was performed and eyeless has been identified as well as the signalling gene hedgehog as putative targets of so. These results strengthen the general assumption that feedback loops among the genes of the retinal determination network are crucial for proper development of eyes and ocelli (Pauli, 2005).

In-vitro data on the autoregulatory element with the known so target sequence of lz and the AREC3/Six4-binding site, the consensus sequence GTAANYNGANAYC/G was identified as necessary for SO binding to DNA. This consensus sequence was taken as a basis for scanning the Drosophila genome for similar sites. In total, 1632 putative so targets emerged from this survey. Out of the affected genes several candidates are already known to be involved in eye development (Pauli, 2005).

so gene activity is crucial for proper development of the entire visual system of Drosophila, including the larval visual system (Bolwig's organ), the optic lobe, the compound eye and the ocellus. An eye-specific enhancer of so, so10, has been identified that is regulated by ey and toy. When used as a driver for so, so10 is sufficient to rescue only eye development of so1 mutant flies but not ocellus development. A fragment of 27 bp, soAE, found downstream of so10, is sufficient to rescue the entire mutant phenotype of so1 mutant flies when combined with so10. The So protein itself binds to soAE and, in cooperation with Eya, forms an autoregulatory feedback loop that is essential for ocellus development (Pauli, 2005).

Since So binds to its own enhancer and autoregulation cannot initiate expression of a gene, the initiation of so expression in the ocellar region must be triggered by other means. The following model is proposed. Initiation of so expression in early third instar eye discs is mediated by ey and toy throughout the eye disc, including the ocellar precursors. Later, after this first induction, so cooperatively with eya can maintain its own expression in the ocellar region by a positive autoregulatory feedback. Thus, the initiation of so expression is mediated by so10, whereas for the maintenance of so, soAE is required. This is supported by the observation that so10, which is activated by ey and toy, mediates expression in early third instar larvae all over the eye disc and only later gets restricted to the compound eye part (Pauli, 2005).

In this model the specificity of so expression for ocellar precursor cells is provided by the expression pattern of eya; Eya protein can be found only in the ocellar region itself, where it specifically interacts with So, and no Eya is present in the proximity of these cells. The importance of eya is further strengthened by the fact that eya4 mutants show an eyeless and ocelliless phenotype. Therefore, to elucidate the mechanisms that control gene expression specifically in ocellar precursor cells, additional studies on eya are required (Pauli, 2005).

Positioned at the top of the hierarchy of the retinal determination network, ey is a potent inducer of ectopic eyes and is able to directly induce so and eya. Like ey, so and eya are able to induce ectopic eyes but only when co-expressed; so alone fails to do so (Pauli, 2005).

To accomplish this induction, eya and so need to feed back on ey, obviously by binding to the eye-specific enhancer of ey. In an ectopic situation, the feedback of so/eya on ey is strong enough to induce ey for ectopic eye formation (Pauli, 2005).

The function of this feedback loop in normal eye development remains to be elucidated. so and eya are both expressed posterior to the furrow and are important for neuronal development. Nevertheless, ey is tuned down posterior to the MF. The activity of the so-binding site in the ey gene might, therefore, be suppressed by other factors or by so itself during cellular differentiation posterior to the furrow. Since co-expression of ey, so and eya is elevated only in a few cells in front of the MF and within the MF, a possible role for this feedback loop might be to boost ey expression in front of and within the furrow, which leads to a strengthening of so and eya expression in just a few cell rows (Pauli, 2005).

For proper eye development, a well-balanced expression level of the genes belonging to the retinal determination network is crucial. Loss-of-function mutations, as well as overexpression of the eye specification genes ey, eya, so or dac during eye development, impede proper determination of the organ and result in a reduction in eye size. Therefore, it is hypothesized that a feedback loop of so on ey is also important for the fine-tuning of ey expression during normal eye development. Due to its previously proposed ability to activate as well as to repress the expression of genes, so is a potent regulator in this context (Pauli, 2005).

so belongs to the Six gene family. All Six proteins are characterized by a Six domain and a Six-type homeodomain, both of which are essential for specific DNA binding and protein-protein interaction. Based on the amino acid sequence of their homeodomain and Six domain, the Six genes were divided into three subgroups. Each of the three Drosophila homologues can be assigned to one of these subgroups: so is mostly related to Six1/2, optix to Six3/6 and DSix4 to Six4/5 (Pauli, 2005).

Promoter analyses of the mouse Six genes (Six1/2, Six4/5) revealed similar target sequence specificities for these mammalian counterparts of so. Six2, Six4/AREC3 and Six5 effectively bind to the same target sequence in a DNA fragment called ARE (Atpla1 regulatory element) that can be found in the Na,K-ATPase alpha1 subunit gene. Six1 and Six4 have been shown to bind to MEF3 sites in the myogenin and in the aldolase A muscle-specific (pM) promoters. Recently, mammalian Six4 has been shown to bind additionally to the transcriptional regulatory element X (TreX) within the muscle creatine kinase (MCK) enhancer (Pauli, 2005).

Comparison of all these sites confirms that the three nucleotides suggested to be the most important for So-DNA interaction are present and conserved within these motifs (nt. 1, 4 and 9 in the identifed binding site). In the case of the MEF3 site (which comprises seven nucleotides that include only two of the nucleotides important for So-DNA interaction), the original publications were examined to check if the third conserved nucleotide is also present, and in most of the cases its conservation has been verified. In fact, there is only one exception published in a study that describes two Six2 target sites (Pauli, 2005).

Nevertheless, by combining the vast majority of previous studies describing protein-DNA interaction of Six genes and this study of So-DNA interaction, it is inferred that So, Six1, Six2, Six4 and Six5 have very similar DNA-binding properties. In the case of so, it is proposed that the consensus sequence GTAANYNGANAY(C/G) marks a good starting point for the identification of additional targets of So, thereby helping to unravel the complex genetic interactions that orchestrate the development of the visual systems of Drosophila (Pauli, 2005).

A BEAF dependent chromatin domain boundary separates myoglianin and eyeless genes of Drosophila melanogaster

Precise transcriptional control is dependent on specific interactions of a number of regulatory elements such as promoters, enhancers and silencers. Several studies indicate that the genome in higher eukaryotes is divided into chromatin domains with functional autonomy. Chromatin domain boundaries are a class of regulatory elements that restrict enhancers to interact with appropriate promoters and prevent misregulation of genes. While several boundary elements have been identified, a rational approach to search for such elements is lacking. With a view to identifying new chromatin domain boundary elements genomic regions were examined between closely spaced but differentially expressed genes of Drosophila melanogaster. A new boundary element between myoglianin and eyeless, ME boundary, was identified that separates these two differentially expressed genes. ME boundary maps to a DNaseI hypersensitive site and acts as an enhancer blocker both in embryonic and adult stages in transgenic context. It is also reported that BEAF and GAF are the two major proteins responsible for the ME boundary function. These studies demonstrate a rational approach to search for potential boundaries in genomic regions that are well annotated (Sultana, 2011).

BEAF is the major player in the boundary function of ME boundary as evident from genetic data and the effect that BEAF has on the ME boundary is by direct binding to the ME region as is evident from the ImmunoFISH and ChIP data. It is already known that BEAF binds to the scsÂ’ boundary as a heterotrimer at the CGATA sites and ME boundary has similar arrangement of the CGATA sites. Some scattered CGATA motifs are also present in the ME boundary. ChIP data shows that BEAF binds to the core region of ME where two palindromic CGATA sites and one additional CGATA site are present. The importance of these BEAF binding sites is also evident from the fact that when these sites were mutated, boundary activity of ME is lost (Sultana, 2011).

The ME boundary also contains binding sites for GAF. The pattern of GAF binding sites in ME boundary is similar to that seen in the case of Fab-7 boundary present in the bithorax complex of D. melanogaster. This prompted an examination of whether GAF has any effect on the boundary activity of ME. The results show that GAF is also a positive regulator of the ME boundary function as loss of single copy of GAF results in partial loss of the boundary function of ME. This effect is by direct binding of GAF to the ME sequence as seen in the ImmunoFISH and ChIP experiments. In case of GAF, it was observed that the effect of loss of GAF was more dramatic in female flies, which was opposite to what was see in the case of BEAF. Since both these proteins, specially GAF, regulate a large number of loci and GAF has also been implicated in dosage compensation, it is likely that the sex specific effect seen here in the case of ME boundary may be a result of complex and indirect interaction of multiple factors (Sultana, 2011).

It is shown that both BEAF and GAF are needed for ME boundary activity. However, either BEAF or GAF (Trl) mutations alone were not sufficient for the complete loss of the boundary function. Since flies with BEAFAB-KO/BEAFAB-KO;P/TrlR85 genotype were lethal, it remains an open question whether BEAF and GAF can account for the complete boundary function of ME. Synthetic lethality in the double mutant BEAFAB-KO/BEAFAB-KO;P/TrlR85 does, however, suggest that these two proteins act in combination at the key loci and that this combination is essential for viability. There might be several such loci working as boundary elements and the double mutant combination, by abolishing or weakening a number of such boundaries, would cause misregulation of associated genes and lead to lethality (Sultana, 2011).

ME boundary function is by recruitment of BEAF and GAF along with, perhaps, several other proteins although BEAF appears to be the major player as mutation in BEAF binding sites abolishes boundary function. Relatively lower level of GAF enrichment at ME, as seen in ChIP experiments, may also indicate an indirect role of this protein at this locus. Minor but distinct effect of Polycomb and trithorax group mutations on ME boundary function was observed. The data, although suggestive and preliminary, indicate that ME boundary functions by recruiting multiple proteins, mutants of which lead to a partial loss of the boundary function. This mode of boundary function is similar to the other well studied gypsy boundary which depends on large number of factors including Su(Hw), Mod(mdg4, CP190 and dTopors that associate with lamina. Boundary function of gypsy was also shown to depend on Polycomb and trithorax group of proteins. While no prominent effect was seen of CTCF or CP190 on ME boundary activity, which is expected as ME region does not contain binding sites for these proteins, genome wide ChIP studies do detect association of these factors with ME. It is possible that ME may be part of nuclear structures where multiple boundaries cluster and number of factor participate even if not by direct binding to each boundary (Sultana, 2011).

In conclusion, a rationale to look for boundary elements in short intergenic regions that separate differentially expressed genes can be applied successfully. Although expression pattern of a number of genes has not been analyzed in many organisms, analysis in other model organisms and human can be used and by homology criteria, large part of a genome can be mapped for potential boundary elements. Once a boundary region has been identified, the precise mapping of the functional boundary element can be accomplished by DNaseI hypersensitivity and transgene based assays available in model systems. Such studies will help in understanding the genomic organization and regulatory environment of genes (Sultana, 2011).

Segregation of eye and antenna fates maintained by mutual antagonism in Drosophila

A general question in development is how do adjacent primordia adopt different developmental fates and stably maintain their distinct fates? In Drosophila, the adult eye and antenna originate from the embryonic eye-antenna primordium. These cells proliferate in the larval stage to form the eye-antenna disc. The eye or antenna differs at mid second instar with the restricted expression of Cut (Ct), a homeodomain transcriptional repressor, in the antenna disc and Eyeless (Ey), a Pax6 transcriptional activator, in the eye disc. This study shows that ey transcription in the antenna disc is repressed by two homeodomain proteins, Ct and Homothorax (Hth). Loss of Ct and Hth in the antenna disc resulted in ectopic eye development in the antenna. Conversely, the Ct and Hth expression in the eye disc was suppressed by the homeodomain transcription factor Sine oculis (So), a direct target of Ey. Loss of So in the eye disc caused ectopic antenna development in the eye. Therefore, the segregation of eye and antenna fates is stably maintained by mutual repression of the other pathway (Wang, 2012).

In l-L3 eye-antenna disc, although the expression domain of Ct/Hth and Ey/So are juxtaposed, so3 clones showed derepression of Ct and Hth only in the most posterior region (zone 4) but not in the more anterior regions behind MF (zones 2 and 3). Thus, there may be an additional mechanism to repress Ct and Hth expression. For Hth, the repression is by Dpp and Hh signaling in L3 eye disc. It has not been tested whether Ct is also repressed by Dpp and Hh (Wang, 2012).

For individual cells in the eye-antenna disc, the mutual repression provided a mechanism for a choice of bistable states, either eye or antenna fate. A bistable state can often be maintained by positive-feedback loop, in addition to mutual repression. Such a positive-feedback loop is known for the eye pathway, but has not been reported for the antenna pathway (Wang, 2012).

The mutual transcriptional repression mechanism is expected to work at the level of individual cells. Therefore, a salt-and-pepper mosaic pattern would be predicted unless there is additional patterning influence. The patterning gene dpp is expressed in the posterior margin of e-L2 eye disc, and is required for Eya expression at this stage. However, dpp is not required for the restricted expression of Ct and Ey. It is proposed that there is another patterning gene that biases the antenna disc to express Ct. Thus, the difference between eye and antenna primordia may be predetermined before the onset of Ct and Eya at e-L2 (Wang, 2012).

The subdivision of a developmental primordium into subprimordia with specific fates is a common requirement in development. For example, the mammalian ventral foregut endoderm differentiates into the adjacent liver and pancreas, and a bipotential population of foregut endoderm cells give rise to both liver and pancreas. The maintenance of such division by mutual antagonism has been reported before. For example, the division between the presumptive thalamus and prethalamus in Xenopus is due to the mutual repression by the Irx homeodomain proteins and the Fezf zinc-finger proteins. The boundary between optic cup and optic vesicle is maintained by mutual transcriptional repression between Pax6 and Pax2. The current findings provide a new example, with clear correlation, both temporal and causal, of gene expression changes and developmental fate specification (Wang, 2012).

The maxillary palp and ocelli are derived from specific regions in the eye-antenna disc. The maxillary palp fate does not become segregated from the rest of eye-antenna disc as late as late L3. The timing of ocelli fate decision is not clear. otd is required for ocelli development, and is the first marker for the ocellar region: it is ubiquitously expressed in the early L2 eye-antenna disc, and becomes restricted to the ocellar region in the eye disc in early L3. Thus, the palp and ocelli may be determined as subfields of the antenna disc and eye disc, respectively. This is consistent with the finding that hth>mi-ct+mi-hth (knocking down both ct and hth in their endogenous expression domain) resulted in the loss of palp, whereas so affected ocelli but not palp (Wang, 2012).

The results showed that Ct and Hth are repressed by So. Previous studies also found induction of Ct and Hth expression in so3 clones in a region far posterior to the MF in l-L3 eye disc. The fact that So represses Ct and Hth in two spatially and temporally distinct situations suggest that this is a conserved function of So (Wang, 2012).

Whether the repression of Ct and Hth by So is direct transcriptional repression is not clear. Ectopic So expression caused cell-autonomous repression of Ct and Hth, suggesting that the repression could be direct. Recently it was shown that So acts as a transcriptional repressor to repress ct transcription (Anderson, 2012). So may interact with a repressor and Groucho (Gro) is a likely candidate. So can bind to Gro and the So-Gro complex was postulated to repress Dac transcription in eye disc. The zebrafish So homologue Six3 interacts with Groucho and functions as a transcriptional repressor. The transcriptional co-repressor CtBP has been shown to functionally and physically interact with Ey, Dac and Dan. Whether the protein complex also involves So has not been determined. Overexpression of CtBP caused eye and antenna defect, but the phenotype was not affected by reducing so dose. Therefore, CtBP is probably not the co-repressor for So (Wang, 2012).

so3 clones caused non-autonomous induction of Ct in its surrounding wild-type cells. Similar non-autonomous induction of Dac has been reported in L3 disc. Elevated Delta was observed within the mutant clone and elevated activated N at the border of mutant clone, thus suggesting that the non-autonomous induction is due to N signaling to surrounding cells. Whether a similar mechanism operates in the L2 disc remains to be tested (Wang, 2012).

The finding that ey and toy do not repress Ct and Hth, in both gain-of-function and loss-of-function experiments, was initially perplexing. Clonal ey expression in the antenna disc did not repress Ct and Hth. In these clones, so-lacZ was induced, but not in all ey+ cells and at a level lower than the endogenous level in most cells in the eye disc. When ey was clonally induced at 29°C, Ct level was reduced. These results suggested that the ectopic ey and toy at 25°C induced so at a level not sufficient to repress Ct. The strength of Ey has been shown to be crucial for its ability to induce ectopic eye development. In the double knockdown of ey and toy in the eye disc, Ct and Hth were not induced. Judging from the eye disc phenotype and residual neuronal differentiation, the knockdown was not complete and may account for the failure to detect Ct and Hth derepression. Alternatively, additional factors, independent of ey and toy, may also repress Ct and Hth expression. This would be consistent with the weak effect of so3 clones in inducing Ct and Hth expression (Wang, 2012).

Hth expression is initially uniform in the eye-antenna disc but becomes restricted to the antenna disc in e-L2. In L3 eye disc, Hth expression is downregulated by Dpp and Hh, produced from the progressing MF and developing photoreceptors, respectively. However, Hth expression retracted from the posterior part of the eye disc in e-L2, even before the initiation of MF and photoreceptor differentiation. At e-L2, dpp and hh are expressed in the posterior region of the eye disc. It is possible that the early Hh and Dpp contributed to the repression of Hth from the eye disc, in addition to the repression by So (Wang, 2012).

The results showed that Ct and Hth represses ey transcription. The binding sites for both Hth and Ct in ey3.6 are required for its repression in the antenna disc, suggesting that both Hth and Ct bind to the ey3.6 enhancer directly. The ChIP assay results showed that both Hth and Ct can bind to the ChIP-1 fragment, which contains the binding site for Ct but not for Hth. This suggests that the Hth may bind through a Hth-Ct complex. However, as ectopic expression of either Hth or Ct is sufficient to repress ey transcription, the repression does not require the formation of the Hth-Ct complex (Wang, 2012).

In the RNAi experiments, knocking down Ct or Hth individually did not cause de-repression of the eye pathway genes. However, when the Ct- or Hth-binding site in ey3.6 was separately mutated, the repression of ey3.6 in the antenna disc was partially lost. One possible explanation for the discrepancy is that the RNAi knockdown was not complete. When the binding sites for both Ct and Hth were mutated, the de-repression of ey3.6 in the antenna disc was strongly enhanced. It is possible that both Ct and Hth contributed to the repression of ey transcription, and a threshold net amount of these repressors is required (Wang, 2012).

Hth physically interacts with Exd through the MH domain of Hth and the PBC-A domain of Exd to promote Exd nuclear localization. Hth generally acts as a transcriptional activator , but Hth and Exd can interact with En or UbxIa to repress transcription. Thus, Hth would need to interact with a repressor to repress ey. Ct can serve such a role. Ct can act as a transcriptional repressor by direct binding to a target gene. The human and mouse Ct homologues generally function as transcriptional repressor. However, as ectopic expression of Hth alone in the eye disc, in the absence of Ct, is sufficient to repress ey, Hth must be able to interact with an additional repressor (Wang, 2012).

This study found that Ct can also block the function of Ey when co-expressed with Ey. It is possible that the block resulted from the repression of toy transcription, which may reduce the strength of the feedback regulation of the retinal determination gene network (Wang, 2012).

Although Ct is expressed in L2 in the entire antenna disc, the phenotype caused by ct clones affected only restricted domains, perhaps owing to its later restricted expression. This study reports a novel function of Ct in antenna development. Ct and Hth function redundantly to repress the retinal determination pathway. Because of this functional redundancy, this Ct function was not revealed in ct clones (Wang, 2012).

hth or exd mutations caused antenna-to-leg transformation. Hth has a role in blocking eye development at the anterior margin of the eye disc, where Ct is not expressed. In the antenna disc, this function is masked because of the functional redundancy with Ct revealed in this study (Wang, 2012).

Even when both ct and hth were knocked down in their endogenous expression domain (hth>mi-hth+mi-ct), no significant transformation of the antenna to eye was seen in adult. One possible reason is that the hth>mi-hth+mi-ct caused lethality and the flies have to be raised at a lower temperature, thereby excluding a stronger phenotype. Another possibility is that the Dll expression in the antenna disc served to block eye development. Dll and hth are required in parallel for normal antenna development. Co-expression of Dll and hth can induce the formation of antenna structures in many ectopic sites. It may be the presence of Dll that blocked eye development and provided a leg identity to cause the distal antenna-to-leg transformation found in hth>mi-hth+mi-ct flies (Wang, 2012).

Dynamic rewiring of the Drosophila retinal determination network switches its function from selector to differentiation

Organ development is directed by selector gene networks. Eye development in Drosophila is driven by the highly conserved selector gene network referred to as the 'retinal determination (RD) gene network,' composed of approximately 20 factors, whose core comprises twin of eyeless (toy), eyeless (ey), sine oculis (so), dachshund (dac), and eyes absent (eya). These genes encode transcriptional regulators that are each necessary for normal eye development, and sufficient to direct ectopic eye development when misexpressed. While it is well documented that the downstream genes so, eya, and dac are necessary not only during early growth and determination stages but also during the differentiation phase of retinal development, it remains unknown how the retinal determination gene network terminates its functions in determination and begins to promote differentiation. This study identified a switch in the regulation of ey by the downstream retinal determination genes, which is essential for the transition from determination to differentiation. Central to the transition is a switch from positive regulation of ey transcription to negative regulation and that both types of regulation require so. These results suggest a model in which the retinal determination gene network is rewired to end the growth and determination stage of eye development and trigger terminal differentiation. It is concluded that changes in the regulatory relationships among members of the retinal determination gene network are a driving force for key transitions in retinal development (Atkins, 2013).

This work has found that a switch from high to low levels of Ey expression is required for normal differentiation during retinal development. A mechanism is presented of Ey regulation by the RD gene network members Eya, So, and Dac. Specifically, So switches from being an activator to a suppressor of ey expression, both depending on a So binding site within an ey eye-specific enhancer. It is additionally reported that the So cofactors Eya and Dac are required for ey repression posterior to the furrow but not for its maintenance ahead of the furrow, and are sufficient to cooperate with So to mediate Ey repression within the normal Ey expression domain (Atkins, 2013).

The results support a Gro-independent mechanism for the suppression of target gene expression by the transcription factor Sine oculis (So). An independent study has also shown that So can repress the selector gene cut in the antenna in a Gro-independent process though the mechanism was not determined (Anderson, 2012). It was observed that Ey is expressed at low levels posterior to the morphogenetic furrow. However, when so expression is lost in clones posterior to the furrow, Ey expression and ey-dGFP expression are strongly activated. This is not simply a default response of ey to So loss, as removing So from developmentally earlier anterior cells results in reduced ey expression. Knockdown of So specifically in differentiating cells using RNAi causes a similar phenotype, suggesting that an activator of Ey expression is expressed in differentiating photoreceptors. Mutation of a known So binding site in ey-dGFP results in activation of the reporter posterior to the furrow, supporting a model that binding of So to the enhancer prevents inappropriate activation of ey expression posterior to the furrow. Finally, in vitro it was observed that an excess of So is sufficient to prevent activation of the enhancer; in vivo overexpression of So can also suppress normal Ey expression. The observations are consistent with what in vitro studies have indicated about So function: when So binds DNA without Eya, it can only weakly activate transcription. However, the current work introduces a novel mechanism of regulation for So targets, in which So occupancy of an enhancer prevents other transcription factors from inducing high levels of target gene expression. The results also indicate that suppression of robust ey expression is an important developmental event. It is not yet clear if maintaining basal expression of ey, rather than completely repressing it, is developmentally important; however, it is possible that the ultimate outcome of a basal level of ey transcription may be necessary for the completion of retinal development (Atkins, 2013).

The results also show that eya is required for Ey suppression in vivo. However, consistent with its characterization as a transcriptional coactivator, in vitro analysis does not indicate a direct role for Eya in repression. Previous studies, and the current observations, indicate that Eya is required for the expression of So posterior to the furrow in the third instar. Additionally, reporter analysis shows that Eya regulation of ey requires the So binding site. It is proposed that the simplest model for Eya function in the suppression of ey is through its established function as a positive regulator of So expression, as it was observed that overexpression of So alone is sufficient to weakly repress Ey expression and to block reporter activation in vitro. This model could also account for the results reported regarding the inability of this UAS-so construct to induce ectopic eye formation. Briefly, the primary function of So in ectopic eye formation is to repress the non-eye program (Anderson, 2012). Overexpressing the So construct used in this study alone is not sufficient to induce this program, possibly because the transgene expression level is not sufficient; however, co-expression of the so positive regulator Eya is sufficient to induce robust ectopic eye formation. In light of the current findings, it is proposed that Eya co-expression is necessary to induce So expression to sufficient levels to block transcriptional activation of non-eye targets to permit the induction of the ectopic eye program; however other functions of Eya may play a role (Atkins, 2013).

It was further demonstrated that dac expression is required specifically near the furrow for Ey repression. In addition, this study showed that the So binding site is required for strong ey expression in dac clones near the furrow, suggesting that So activates ey in these clones. This suggests that repression by Dac occurs before the transition to repression by So, making Dac the first repressor of ey expression at the furrow, and identifying how the initiation of repression occurs before So levels increase. It was further shown that Eya and So are sufficient to repress ey expression in dac mutant clones anterior to the furrow, though not as completely as in cells that express Dac. This result indicates that Dac is not an obligate partner with Eya and So in ey repression, but is required for the full suppression of ey. One model would be that Dac and So can cooperate in a complex to modestly repress eyeless directly. This would be consistent with loss-of-function and reporter data as well as the observation that Dac and So misexpression can weakly cooperate to repress Ey anterior to the furrow. However, while a similar complex has been described in mammalian systems, previous studies have been unable to detect this physical interaction in Drosophila. An alternative model is that Dac suppresses ey expression indirectly and in parallel to Eya and So. A previous study has shown that dac expression is necessary and sufficient near the furrow to inhibit the expression of the zinc finger transcription factor Teashirt (Tsh). Tsh overlaps Ey expression anterior to the furrow, and can induce Ey expression when misexpressed. Furthermore, tsh repression is required for morphogenetic furrow progression and differentiation. In light of these previous findings, a simpler model is proposed based on current knowledge that Dac repression of tsh at the morphogenetic furrow reduces Ey expression indirectly. Future studies may distinguish between these mechanisms (Atkins, 2013).

In addition to the role of the RD gene network in ey modulation,signaling events within the morphogenetic furrow indirectly regulate the switch to low levels of ey expression. It has been shown that signaling pathways activated in the morphogenetic furrow increase levels of Eya, So and Dac; furthermore, it is proposed that this upregulation alters their targets, creating an embedded loop within the circuitry governing retinal development and allowing signaling events to indirectly regulate targets through the RD network. The identification of ey regulation by So posterior to the morphogenetic furrow represents a direct target consistent with this model (Atkins, 2013).

In conclusion, a model is presented that rewiring of the RD network activates different dominant sub-circuits to drive key transitions in development (see A model for dynamic RD gene network interactions during the third instar). To the interactions previously identified by others, this study adds that strong upregulation of So, dependent on Eya, results in minimal levels of ey transcription. It is proposed that the identification of this novel sub-circuit of the RD network provides a mechanism for terminating the self-perpetuating loop of determination associated with high levels of Ey, permitting the onset of differentiation and the completion of development. Together, these results give a new view into how temporal rewiring within the RD network directs distinct developmental events (Atkins, 2013).

Transcriptional Regulation

Mutations in four other Drosophila genes (eyes absent, sine oculis, eyes gone and eyelisch) have similar phenotypes but do not affect the expression pattern of eyeless, indicating that these genes are downstream or act in parallel with eyeless (Cheyelle, 1994).

Ectopic expression of sine oculis has little or no effect on antennal, wing, or leg disc development, while ectopic eyes absent expression often causes mild growth alterations resulting in extra folds in the epithelium and, rarely, formation of small ectopic ommatidial arrays in the antennal disc. Coexpression of so and eya leads to a dramatic increase in the development of ectopic eye tissue in antennal discs. These ommatidial arrays lead to adult eye structures. Ectopic so/eya induce eyeless expression in the antennal disc. When expressed in eyeless mutant discs, ectopic so/eya produces growth alterations, but ectopic eyes are not observed (Pignoni, 1997).

The Notch signaling pathway defines an evolutionarily conserved cell-cell interaction mechanism that throughout development controls the ability of precursor cells to respond to developmental signals. Notch signaling regulates the expression of the master control genes eyeless, vestigial, and Distal-less, which in combination with homeotic genes induce the formation of eyes, wings, antennae, and legs. Therefore, Notch is involved in a common regulatory pathway for the determination of the various Drosophila appendages (Kurata, 2000).

The intracellular domain of the truncated Notch receptor reflects a constitutively activated state (Notch activated, Nact) and the extracellular domain of the truncated receptor mimics loss-of-function phenotypes representing a dominant negative form (Notch dominant negative, Ndn). To examine the role of Notch signaling in early eye development, these truncated forms were expressed in the early eye imaginal disc, using the GAL4 system with the eye-specific enhancer of the ey gene. This eye-specific enhancer induces target gene expression in the eye primordia of the embryo and maintains expression throughout eye morphogenesis. In contrast to ey expression in the wild-type eye-antennal disc, the enhancer-driven reporter gene expression is not down-regulated in the differentiating cells posterior to the morphogenetic furrow but it extends all over the eye disc and into the area of the antennal disc where the rostral membrane is going to be formed. However, the expression in the antennal disc is quite variable from disc to disc. Consistent with previous loss of Notch function studies, crossing ey enhancer-GAL4 (ey-GAL4) flies to a stock carrying Ndn under an upstream-activating sequence for GAL4 (UAS-Ndn) results in a strongly reduced eye phenotype in all transheterozygous flies similar to that of the ey2 mutant. Inhibition of Notch signaling by misexpression of Hairless (H) and dominant negative forms of Delta (Dl) and Serrate (Ser) also leads to a reduction or complete absence of the eye (Kurata, 2000).

Activation of Notch signaling by crossing ey-GAL4 flies to a UAS-Nact line leads to significant pupal lethality, but all transheterozygotes that escape lethality show hyperplasia of the eyes with a significant increase in the number of facets. The disc overgrowth is found in all eye discs of ey-GAL4 UAS-Nact larvae, consistent with a role for Notch signaling in growth control of the eye imaginal disc. Furthermore, about 16% of the escapers form ectopic eyes on the rostral membrane of the head, which is derived from the antennal disc (Kurata, 2000).

Immunostaining of eye-antennal discs of ey-GAL4 UAS-lacZ UAS-Nact larvae using an ELAV antibody to identify the differentiating photoreceptor cells and a beta-galactosidase antibody to monitor the Nact protein shows that both the strong hyperplasia of the eye disc and ectopic eye formation in the antennal disc correlate with the expression of Nact. However, the time window for expression of the truncated receptors is critical. Transheterozygotes in which either Ndn or Nact were driven by the glass promoter GMR-GAL4, which drives expression in all cells posterior to the furrow only, show only a mild phenotypic effect. Ndn results in a roughening of the eye, whereas Nact produces a polished eye phenotype. Therefore, the timing of Notch signaling is of crucial importance (Kurata, 2000).

The reduced eye phenotype caused by expression of Ndn and the induction of ectopic eyes by the expression of Nact are similar not only to loss-and-gain mutants of ey but also resemble two other mutations acting downstream in the ey developmental pathway, eyes absent (eya) and dachshund (dac). Furthermore, a second Pax-6 gene of Drosophila, twin of eyeless (toy), was found to be an upstream regulator of ey capable of inducing ectopic eyes by inducing ey. To determine the epistatic relationship of Notch to those genes, the effect of Ndn on ectopic eye induction by ey and toy was studied (Kurata, 2000).

A dpp-enhancer GAL4 line (dpp-GAL4) was crossed to flies carrying both UAS-Ndn and UAS-ey or alternatively to UAS-Ndn and UAS-toy. Transheterozygotes from both crosses exhibit ectopic eyes on legs and wings in all flies. The size of the ectopic eyes is similar to those of the transheterozygous controls dpp-GAL4 UAS-ey and dpp-GAL4 UAS-toy, respectively, suggesting that Notch acts upstream of ey and toy. Double immunostaining of eye-antennal discs from transheterozygous ey-GAL4 UAS-Nact, UAS-lacZ using an anti-EY antibody to reveal EY protein and anti-beta-galactosidase antibody to indirectly reveal Nact demonstrates that ey expression is induced in all eye discs by the activation of Notch signaling. Moreover, strong ectopic expression of Ey protein has been observed. The ectopic expression pattern of Ey corresponds to that of lacZ reflecting the expression of Nact protein. Analysis of ey expression by in situ hybridization indicates that ey is induced at the transcriptional level. Similarly, ectopic expression of toy also is induced in the antennal discs of ey-GAL4 UAS-Nact larvae. Thus, activation of Notch signaling can induce toy and ey expression in antennal discs. Expression of Nact also correlates with the ectopic induction of photoreceptor cells as revealed by ELAV staining (Kurata, 2000).

Notch activation of ey and toy depends on the downstream effector of Notch, Suppressor of Hairless, because Su(H) mutant clones generated anterior to the morphogenetic furrow in the eye fail to produce adult structures, in agreement with a requirement for Notch signaling during eye morphogenesis (Kurata, 2000).

Consistent with the finding that ey acts downstream of Notch, the expression of Nact in an ey2 or eyR hypomorphic mutant background generates eyes of a reduced size. Approximately 72% of the ey-GAL4 UAS-Nact; ey2 flies that survived were found to have reduced eyes and about 15% of these flies had both a reduced original and a reduced ectopic eye. However, in addition to ectopic eyes Nact also induces ectopic antennae in 25% of these flies on the side of the head that is derived from the eye disc. Many of the induced ectopic antennae were complete with all three antennal segments and the arista (Kurata, 2000).

Based on these findings, a model is proposed to explain the difference between the eye and antennal pathway starting from a common signaling mechanism. Notch signaling induces the expression of both ey and Dll. However, in the eye primordia ey represses Dll and induces eye morphogenesis. By contrast, in the antennal disc ey is repressed by a repressor, resulting in Dll expression that confers antennal (ventral appendage) specificity. Two of the possible candidates for the repressor are the homeobox genes exd and hth, because both exd- and hth- mutant clones in the rostral membrane region of the antennal disc can result in ectopic eye development, which presumably is caused by derepression of ey. Both exd and hth also may function in conjunction with Dll, serving as corepressors. The present study extends the fundamental role of Notch by indicating that the implementation of entire developmental programs leading to appendage formation and organogenesis may be controlled by Notch activity (Kurata, 2000).

The posteriorly expressed signaling molecules Hedgehog and Decapentaplegic drive photoreceptor differentiation in the Drosophila eye disc, while at the anterior lateral margins Wingless expression blocks ectopic differentiation. Mutations in axin prevent photoreceptor differentiation and leads to tissue overgrowth; both these effects are due to ectopic activation of the Wingless pathway. In addition, ectopic Wingless signaling causes posterior cells to take on an anterior identity, reorienting the direction of morphogenetic furrow progression in neighboring wild-type cells. Signaling by Dpp and Hh normally blocks the posterior expression of anterior markers such as Eyeless. Wingless signaling is not required to maintain anterior Eyeless expression and in combination with Dpp signaling can promote Ey downregulation, suggesting that additional molecules contribute to anterior identity. Along the dorsoventral axis of the eye disc, Wingless signaling is sufficient to promote dorsal expression of the Iroquois gene mirror, even in the absence of the upstream factor pannier. However, Wingless signaling does not lead to ventral mirror expression, implying the existence of ventral repressors (Lee, 2001).

Two characteristics distinguish anterior from posterior behavior in the eye disc: growth occurs in the anterior, with the exception of the second mitotic wave, and differentiation occurs in the posterior. Wg signaling regulates both of these properties. Wg signaling promotes the growth of eye disc cells. Loss of axin causes dramatic overgrowth and outgrowth of cells in the eye disc, and this phenotype is due only to excessive Wg pathway activity, since it can be blocked by a dominant negative form of dTCF/Pangolin. The strength of the phenotype may reflect higher levels of Wg signaling than are induced by loss of sgg; perhaps Axin contributes to retaining Arm in the cytoplasm, in addition to promoting its phosphorylation. Vertebrate Axin has been shown to associate with mitogen-activated protein kinase kinase kinase 1 and activate the c-jun N-terminal kinase (JNK) pathway. However, JNK signaling does not appear to be essential for the growth or differentiation of cells in the Drosophila eye disc, and it does not contribute to the axin mutant phenotype in the eye. The ability of Wg signaling to promote overgrowth in the eye disc is consistent with the reduction in the size of the eye disc caused by loss of Wg signaling (Lee, 2001).

Although Wg signaling is sufficient to establish but not necessary to maintain anterior identity in the eye disc, Dpp signaling has complementary properties; it is sufficient to promote but not essential to maintain posterior identity. Ectopic Dpp can downregulate ey in the anterior, but loss of components of the Dpp pathway has a variable effect on ey expression and does not lead to hairless expression or induce reorientation of the furrow in neighboring cells. It is possible that this is due to redundancy with Hh signaling, which also has a weak and variable effect on ey expression. Clones that are mutant for cell-autonomous components of both the Hh and Dpp pathways do not differentiate, while loss of one or the other pathway only delays differentiation; however, even loss of both pathways does not reorient adjacent wild-type cells (Lee, 2001).

Downregulation of ey and photoreceptor differentiation appear to be independent events, since smo clones have a weaker effect on ey expression than Mad clones, despite their stronger effect on differentiation. Anterior Dpp can also downregulate ey over a much longer range than that over which it promotes photoreceptor differentiation, and loss of slimb downregulates ey without leading to ectopic differentiation (Lee, 2001).

The complementary effects of Wg and Dpp on AP polarity appear to be independent of one another. The effect of Dpp is not simply due to its repression of wg, and posterior Wg signaling can upregulate ey even when the Dpp pathway is also activated. However, activation of both pathways reveals the existence of another mechanism that distinguishes anterior from posterior, since the anterior of the disc is more sensitive to the effects of Dpp and the posterior is more sensitive to the effects of Wg. This leads to a striking repolarization of the eye disc when both pathways are activated, resulting in initiation of an ectopic morphogenetic furrow from the anterior margin as well as reduction or elimination of the normal posterior furrow. Since normal development requires anterior cells to be gradually converted to posterior by the action of Dpp and Hh, it is important for Dpp to overcome the effects of Wg in this region. This also underscores the importance of establishing the early expression patterns of wg and dpp (Lee, 2001).

Wg has three roles in early eye disc development: establishment of anterior identity, establishment of dorsal identity, and promotion of growth. Prior to furrow initiation, Pnr, expressed at the dorsal margin, activates wg expression in a broader domain; Wg then activates mirr and the other Iro-C genes throughout the dorsal compartment. Hh may contribute to the activation of these genes through Wg or act independently of Pnr. Upd, which is present at the optic stalk also contributes to the ventral repression of mirr. Mirr represses fng, forming a boundary of fng expression at the DV midline that leads to activation of N in this region. During furrow progression, Hh is expressed in the differentiating photoreceptors and Dpp in a stripe in the morphogenetic furrow. These two signals act to downregulate genes expressed in the anterior such as ey, and allow an anterior to posterior transition. Wg establishes the anterior state, probably at an earlier stage, while another factor (X) contributes to its maintenance. Other factors are necessary to modify the response to Wg in order to determine which cell fate should be specified; this is consistent with data suggesting that Wg signaling alters chromatin structure to allow access to transcription factors. A requirement for multiple signaling systems also ensures accuracy in cell fate determination (Lee, 2001).

The function of the Dpp and Hh signaling pathways in partitioning the dorsal head neurectoderm of the Drosophila embryo has been analyzed. This region, referred to as the anterior brain/eye anlage, gives rise to both the visual system and the protocerebrum. The anlage splits up into three main domains: the head midline ectoderm, protocerebral neurectoderm and visual primordium. Similar to their vertebrate counterparts, Hh and Dpp play an important role in the partitioning of the anterior brain/eye anlage. Hh and its receptor/inhibitor, Patched (Ptc), are expressed in a transverse stripe along the posterior boundary of the eye field. Hh triggers the expression of determinants for larval eye (atonal) and adult eye (eyeless) in those cells of the eye field that are close to the Hh source. Eya and So, which are induced by Dpp, are epistatic to the Hh signal. Loss of Ptc, as well as overexpression of Hh, results in the ectopic induction of larval eye tissue in the dorsal midline (cyclopia). The similarities between vertebrate systems and Drosophila are discussed with regard to the fate map of the anterior brain/eye anlage, and its partitioning by Dpp and Hh signaling (Chang, 2001).

Loss of hh results in a strong reduction of the head midline epidermis, a reduction in the size of the brain and optic lobe, and the total absence of the larval and adult eye primordium. Temperature-sensitive shift experiments of hhts2 embryos indicate that the phenocritical period for Hh function in Bolwig's organ development is between 4 and 7 hours. Aside from the larval eye, the primordium of the compound eye, which is marked from stage 12 onward by the expression of eyeless (ey), is also affected by the loss of hh. Heatshock induced overexpression of hh, as well as loss of ptc, causes an increase in larval eye neurons and optic lobe precursors. Interestingly, ectopic Hh activity is able to induce optic lobe and Bolwig's organ tissue in the head midline and thereby generate a cyclops phenotype similar to the condition described above for partial reduction of dpp. Applying heatshocks at different times of development indicates that the phenocritical period for the Hh induced cyclops is early, between 2.5 and 5 hours. Thus, heat pulses administered during this time cause fusion of the optic lobe and, at a lower frequency, of the larval eye without significantly increasing the number of optic lobe and larval eye cells. By contrast, later heat pulses (after 5 hours) lead to larval eye/optic lobe hyperplasia but no concomitant cyclops phenotype (Chang, 2001).

The transcriptional co-factor Chip acts with LIM-homeodomain proteins to repress eyeless and set the boundary of the eye field in Drosophila

Development involves the establishment of boundaries between fields specified to differentiate into distinct tissues. The Drosophila larval eye-antennal imaginal disc must be subdivided into regions that differentiate into the adult eye, antenna and head cuticle. The transcriptional co-factor Chip is required for cells at the ventral eye-antennal disc border to take on a head cuticle fate; clones of Chip mutant cells in this region instead form outgrowths that differentiate into ectopic eye tissue. Chip acts independently of the transcription factor Homothorax, which was previously shown to promote head cuticle development in the same region. Chip and its vertebrate CLIM homologues have been shown to form complexes with LIM-homeodomain transcription factors, and the domain of Chip that mediates these interactions is required for its ability to suppress the eye fate. Two LIM-homeodomain proteins, Arrowhead and Lim1, are shown to be expressed in the region of the eye-antennal disc affected in Chip mutants, and both require Chip for their ability to suppress photoreceptor differentiation when misexpressed in the eye field. Loss-of-function studies support the model that Arrowhead and Lim1 act redundantly, using Chip as a co-factor, to prevent retinal differentiation in regions of the eye disc destined to become ventral head tissue (Roignant, 2009).

Regionalization of the eye-antennal disc is a progressive process in which selector genes and signaling pathways specify the fates of different head structures. Clones of eye-antennal disc cells induced during the second larval instar can contribute to multiple organs, indicating that these cells retain developmental plasticity at this stage. The anteroposterior boundary of the wing disc is established much earlier; expression of the selector gene engrailed (en) specifically in the posterior cells during embryogenesis generates an affinity border that keeps the two compartments clonally separated. By contrast, the eye selector gene ey is uniformly expressed throughout the early eye-antennal disc, and only retracts to the eye field in the second instar. It was initially proposed that localized Notch signaling controls this retraction, as expression of dominant-negative forms of Notch in the eye disc abolishes ey expression and leads to antennal duplications. However, a later study demonstrated that loss of Notch function does not affect ey expression directly, but reduces cell proliferation in the retinal field, preventing the initiation of eya expression. This study shows that Chip and Lim1 are both necessary to repress ey expression in the anterior of the antennal disc. Additional factors probably help to restrict ey expression to the eye disc, because ey expression does not extend throughout the normal Lim1 expression domain in Lim1 or Chip mutant clones in the antennal disc (Roignant, 2009).

Since Lim1 mutant clones always misexpress Ey, but rarely misexpress Eya and never differentiate ectopic photoreceptors, additional proteins must interact with Chip to repress retinal differentiation. Awh is a good candidate because it is expressed at the ventral margin of the eye-antennal disc, its misexpression in the retina represses photoreceptor differentiation in a Chip-dependent manner, and loss of both Lim1 and Awh leads to ectopic photoreceptor differentiation in the ventral eye-antennal disc. Since ectopic photoreceptors differentiate only in the absence of both Lim1 and Awh, whereas Ey expansion is observed in Lim1 single mutants, Awh must control the expression of target genes other than ey. It may negatively regulate other genes involved in retinal determination, such as eya, or positively regulate genes important for head capsule development, such as Deformed and odd-paired (Roignant, 2009).

Like Chip, Hth is required to prevent retinal differentiation at the ventral eye-antennal disc boundary. Investigation of the relationship between Chip and Hth indicates that Chip is not required for Hth expression or activity. The ability of Hth to repress photoreceptor differentiation in Chip mutant clones rules out the possibility that Chip acts as a co-factor for Hth or an essential downstream mediator of its effects. The normal expression of Hth and its target gene wg in Chip mutant clones also make it unlikely that Chip controls the expression of Hth or its co-factor Exd. However, the possibility that Hth and Chip act in parallel poses the paradox that misexpressed Hth is sufficient to repress photoreceptor development in the eye field in the absence of Chip, but endogenous Hth is insufficient to do so in the head field. It is possible that Hth expression levels in the head field early in development are too low to repress the eye fate in the absence of Chip. Consistent with this hypothesis, it was found that overexpression of Hth in Chip mutant cells prevents ectopic photoreceptor differentiation. Similarly, overexpression of Awh or Lim1 prevents ectopic photoreceptor differentiation in hth mutant cells, suggesting that endogenous levels of these LIM-HD proteins are not sufficient to compensate for the absence of Hth. The two classes of transcription factors may normally act on different sets of target genes, but show some cross-regulatory ability when overexpressed (Roignant, 2009).

The boundary between the eye and the dorsal head appears to be established differently from the boundary in the ventral region. The LIM-HD gene tup is expressed at the dorsal eye-antennal disc boundary, in a pattern resembling the mirror image of the Awh pattern, and is capable of repressing photoreceptor development in a Chip-dependent manner. However, loss of Chip in this region does not lead to ectopic eye formation, although it can cause overgrowth and mispatterning of the head. In the absence of Chip, the GATA transcription factor Pannier (Pnr) and its target gene wg may be sufficient to maintain dorsal head fate. The ventral margin of the eye-antennal disc may be particularly susceptible to ectopic photoreceptor differentiation because of the high level of Dpp signaling there. A 5' enhancer element has been shown to direct dpp expression specifically in the ventral marginal peripodial epithelium of the eye-antennal disc. The ability of Dpp and Ey to synergize to drive retinal differentiation therefore makes it critical to repress Ey in this region, which is fated to form head capsule (Roignant, 2009).

In addition, this domain of Dpp overlaps with Wg present at the anterior lateral margin of the eye disc; the combination of these two growth factors induces proximodistal growth of the leg. One function of Chip and its partner proteins might thus be to repress the outgrowth that would otherwise be triggered by the combination of Dpp and Wg. Unlike growth of the wild-type eye disc, growth of Chip mutant regions appears to be Notch-independent, as they do not contain a fng expression boundary and do not show activation of the Notch target genes E(spl)mβ or eyg. Notch has been thought to trigger growth by inducing the expression of the JAK/STAT ligand Unpaired (Upd); however, a recent report describes an earlier function for Upd upstream of Notch, raising the possibility that upd expression is activated independently of Notch in Chip mutant clones. As hth mutant clones, or clones lacking the Odd skipped family member Bowl, frequently show ectopic ventral photoreceptor differentiation but rarely induce outgrowths like those seen in Chip mutants, the functions of Chip in growth and differentiation are likely to be separable (Roignant, 2009).

LIM-HD proteins also set developmental boundaries in other imaginal discs, acting in concert with other classes of transcription factors. In the wing disc, Tup specifies the notum in collaboration with homeodomain transcription factors of the Iroquois complex, and Ap specifies the dorsal compartment. Ap interacts with the homeodomain protein Bar and Lim1 with Aristaless to establish specific tarsal segments within the leg disc. LIM-HD proteins have also been implicated in vertebrate eye development, although those that have been studied appear to play positive roles. The Ap homologue Lhx2 is expressed within the mouse retinal field at the neural plate stage, and contributes to the expression of Pax6, Six3 and Rx. Lmx1b, the homologue of CG32105, is required for the development of anterior eye structures such as the cornea and iris, and is mutated in human patients with nail-patella syndrome, often characterized by glaucoma. Within the retina, loss of Lim1 results in mispositioning of horizontal cells within the amacrine cell laye. Drosophila Lim3 shows photoreceptor-specific expression, and might therefore have a positive function in eye development (Roignant, 2009).

In the central nervous system, LIM-HD proteins act combinatorially to specify different neuronal cell fates. In both Drosophila and vertebrates, combinations of Islet and Lhx3/4/Lim3 proteins regulate motoneuron specification and pathfinding. The ability of Chip to interact with LIM-HD proteins and other transcription factors as well as to dimerize enables it to form heteromeric transcription factor complexes. In the wing disc, the active complex is a tetramer containing two subunits each of Chip and Ap, whereas in motoneuron development the Chip homologue NLI can form either a tetramer with Lhx3 or a hexamer containing both Isl1 and Lhx3. The finding that Lim1 and Awh act redundantly to prevent eye development in the ventral head primordium, whereas Chip is absolutely required, seems most consistent with regulation of distinct subsets of target genes by independent Chip-Awh and Chip-Lim1 complexes; however, a contribution from a complex containing all three proteins, or even additional transcription factors, cannot be ruled out. The role of the Chip co-factor may be to coordinate multiple transcriptional regulatory complexes to restrict developmental fates within the eye-antennal imaginal disc, allowing it to give rise to the head cuticle as well as distinct external sensory structures (Roignant, 2009).

Targets of Activity

The dachshund (dac) gene encodes a novel nuclear protein that is required for normal eye development in Drosophila. In the absence of dachshund function, flies develop either without any eyes, or with severely reduced eyes. Targeted expression of dachshund is sufficient to direct ectopic retinal development in a variety of tissues, including the adult head, thorax and legs. This result is similar to that observed with the highly conserved Drosophila gene eyeless (ey), which can induce ectopic eye formation on all major appendages. dachshund and eyeless induce the expression of one another. dachshund is required for ectopic retinal development driven by eyeless misexpression. Dac protein is detected at the posterior margin of the eye disc prior to furrow initiation, and at positions both anterior and posterior to the furrow throughout furrow progression. Three independent results suggest that dac functions downstream of ey: (1) Misexpression of ey in the antennal, leg and wing imaginal discs is sufficient to induce ectopic dac expression in all discs. These results suggest that EY positively regulates dac expression. (2) Targeted expression of ey is unable to induce ectopic eye formation in a dac mutant backfround. (3) ey is expressed in a mutant dac background, indicating that dac is not required for ey expression. These results suggest that the control of eye development requires the complex interaction of multiple genes, even at the very highest regulatory levels (Shen, 1997).

The eyeless, dachshund, and eyes absent genes encode conserved, nuclear proteins that are essential for eye development in Drosophila. Misexpression of eyeless or dachshund is also sufficient to induce the formation of ectopic compound eyes. Like ey and dac, targeted expression of eya alone is sufficient to induce ectopic eye formation. However, in contrast to ey, the penetrance of the ectopic eye phenotype induced by either dac or eya alone is incomplete and, when induced, such eyes are small. When dac expression was strongly induced in all imaginal discs, ectopic eye development was observed only on the anterior surface of the fly head ventral to the antenna, in just 56% (61/109) of animals examined. In contrast to the low penetrance of ectopic eye formation induced by dac or eya expressed alone, coexpression of dac and eya induces substantial ectopic eyes on the head, legs, wings, and dorsal thorax of 100% of animals examined. On the head, the cuticle between the normal eye field and antennae is transformed into retinal cells such that the normal retinal field is expanded. Large patches of pigment are induced on the dorsal surface of the femur and tibia of all legs, which are severely truncated. Ectopic eya alone can induce small patches of glass expression in the pouch area of the wing disc with 25% penetrance. In no case has ectopic Glass staining been observed in leg discs with either dac or eya alone. However, when dac and eya are coexpressed, ectopic Glass staining is induced with 100% penetrance along the ventral margin of the eye-antennal disc, the dorsal half of the leg disc along the anterior-posterior compartment (A/P) boundary, and along the A/P boundary of the dorsal wing disc. In each case, the sites of ectopic glass expression in discs correspond to the positions of ectopic retinal development observed in adults. Taken together, these data demonstrate that dac and eya show strong genetic synergy to induce ectopic retinal development in Drosophila (Chen, 1997).

While eya expression in the eye disc does not depend on dac function, dac expression is greatly reduced in an eya2 mutant background, demonstrating that dac expression requires eya activity. Similarly, eyeless (ey) induction of ectopic dac expression is greatly reduced in an eya2 mutant background. These results suggest that dac may function downstream of eya. Consistent with this interpretation, eya is unable to induce ectopic eye formation in a dac mutant background. eyeless misexpression is sufficient to induce eya, suggesting that eya may be required for ey function. Indeed, ectopic retinal development driven by targeted ey expression fails to occur in an eya2 mutant background. Induction of eya expression by ey does not depend on dac activity, consistent with the idea that eya functions downstream of ey but upstream of dac. However, these genes do not act in a simple, linear pathway; targeted expression of dac and eya strongly induce the expression of one another, and eya is required for ectopic eye induction by dac. Misexpression of dac or eya is also sufficient to induce ectopic ey expression in the antennal disc. These results suggest that multiple positive-feedback loops exist among these genes during normal eye development and raises the possibility that ey may be required for ectopic retinal induction by eya and dac. Indeed, ectopic eye formation driven by coexpression of dac and eya is completely blocked in an eyeless2 mutant background, indicating that induction of ey is essential. It is proposed that a conserved regulatory network, rather than a linear hierarchy, controls retinal specification and involves multiple protein complexes that function during distinct steps of eye development (Chen, 1997).

The fly eyes absent (eya) gene, essential for compound eye development in Drosophila, has been shown to be functionally replaceable in eye development by a vertebrate Eya homolog. The relationship between eya and that of the eyeless gene, a Pax-6 homolog, critical for eye formation in both flies and man, has been defined: eya is found to be essential for eye formation by eyeless. Directing eyeless expression to discs that generate legs, wings and the antennal region of the head generates ectopic eyes in these regions. Eya is ectopically expressed in regions where eyeless directs ectopic eye formation. eya is essential for these ectopic eyes as ectopic eyes fail to form in eya mutants. eya can itself direct ectopic eye formation, indicating that eya has the capacity to function as a master control gene for eye formation. Directing ectopic eya expression to imaginal discs induces ectopic eyeless gene expression in the antennal region of the eye-antennal disc but fails to induce ectopic eyeless expression, even though ectopic ommatidia are formed and ectopic expression of Glass occurs. eya and eyeless together are more effective in eye formation than either gene alone: when expressed together, ectopic eyes are larger and form with higher penetrance than is the case with either eyeless or eya alone; expressed togther, eye formation occurs on the genitalia, a condition never observed in individuals with either gene alone. These data indicate conservation of the pathway of eya function between flies and vertebrates; they suggest a model whereby eya/Eya gene function is essential for eye formation by eyeless/Pax-6, and that eya/Eya can in turn mediate, via a regulatory loop, the activity of eyeless/Pax-6 in eye formation (Bonini, 1997).

The Pax-6 gene encodes a transcription factor with two DNA-binding domains, a paired domain and a homeodomain, and is expressed during eye morphogenesis and development of the nervous system. Pax-6 homologs have been isolated from a wide variety of organisms ranging from flatworms to humans. Since loss-of-function mutants in insects and mammals lead to an eyeless phenotype and Pax-6 orthologs from distantly related species are capable of inducing ectopic eyes in Drosophila, it has been proposed that Pax-6 is a universal master control gene for eye morphogenesis. To determine the extent of evolutionary conservation of the eye subordinate target genes of Pax-6, subordinate genes of Pax6 have been sought. Expression of two genes, sine oculis (so) and eyes absent (eya), is induced by eyeless (ey), the Pax-6 homolog of Drosophila. Evidence from ectopic expression studies in transgenic flies, from transcription activation studies in yeast, and from gel shift assays in vitro is presented supporting the notion that the EY protein activates transcription of sine oculis by direct interaction with an eye-specific enhancer in the long intron of the so gene. Sequences of the putative sites are at maximum 88% homologous (70% with the consensus PRD binding site sequence). The functional importance of the eye-specific enhancer of so has been demonstrated in vivo by means of the so1 mutant, which deletes a 1.3 kb region including the enhancer, but leaves the coding sequences intact. In so1 homozygous flies, ey is neither capable of inducing so transcription nor can it induce ectopic eyes. In contrast to ey, its paralog toy induces both ectopic so transcription and ectopic eyes in a so1 mutant background. This indicates that ey and toy regulate so by different mechanisms (Niimi, 1999).

The eyes absent gene is critical for normal eye development in Drosophila and is highly conserved to vertebrates. To define regions of the gene critical for eye function, the mutations in the four viable eya alleles have been defined. Two of these mutations are eye specific and undergo transvection with other mutations in the gene. These are deletion mutations that remove regulatory sequence critical for eye cell expression of the gene. Two other viable alleles cause a reduced eye phenotype and affect the function of the gene in additional tissues, such as the ocelli. These mutations are insertion mutations of different transposable elements within the 5' UTR of the transcript. Detailed analysis of one of these reveals that the transposable element has become subject to regulation by eye enhancer sequences of the eya gene, disrupting normal expression of Eya in the eye. More extended analysis of the deletion region in the eye-specific alleles indicatesthat the deleted region defines an enhancer that activates gene expression in eye progenitor cells. This enhancer is responsive to ectopic expression of the eyeless gene. This analysis has defined a critical regulatory region required for proper eye expression of the eya gene (Zimmerman, 2000).

Eyeless directly regulates rhodopsin 1 (rh1) expression in photoreceptor cells. rh1 is expressed specifically in photoreceptor cells R1 to R6. eyeless is expressed in both larval and adult terminally differentiated photoreceptor cells. The homeodomain of Eyeless binds to a palindromic homeodomain binding site P3/RCS1 in the rh1 proximal promoter, which is essential for rh1 expression. P3RCS1 can be replaced by binding sites specific for the Paired domain of Eyeless. P3RCS1 is conserved in the promoters of all Drosophila rhodopsin genes as well as in many opsin genes in vertebrates. Multimerized P3 sites in front of a basal promoter are able to drive the expression of a reporter gene in all photoreceptors. These results suggest that Pax-6/Ey directly regulates rh1 gene expression by binding to the conserved P3RCS1 element in the promoter (Sheng, 1997).

Eyeless initiates the expression of both sine oculis and eyes absent during Drosophila compound eye development

The development of the embryonic eye anlagen was examined making use of an eyeless-eye enhancer lacZ reporter. In wild-type embryos, the transgene drives beta-galactosidase expression in part of the morphologically distinct eye primordia. During larval stages, beta-galactosidase is continuously expressed in the eye discs and in parts of the brain. The position and number of cells that express this reporter in ey 2 mutant embryos is indistinguishable from wild-type embryos. It is concluded that the anlagen of the eye are formed in ey 2 mutant embryos. Therefore, defects in the first steps of eye development are not the major cause of the eyeless phenotype (Halder, 1998).

In contrast to the normal appearance of the eye anlagen in ey 2 embryos, the morphology of eye-antennal imaginal discs from late third instar ey 2 mutant larvae is highly abnormal, with the eye portion being strongly reduced. The antennal part is not affected. Staining for differentiating photoreceptors fails to show any evidence of ommatidial cluster formation in most ey 2 mutant eye discs. Previous work suggested that the ey 2 phenotype was a result of cell death in third instar eye discs. To assess cell death, eye discs were stained with the vital dye Acridine orange. A low level of cell death is normally observed in wild-type eye discs, mainly in the region just anterior to the furrow. In contrast, eye discs from third instar ey 2 larvae display massive cell death in the remainder of the eye discs. Eye discs with weaker phenotypes show ectopic cell death anterior to the furrow. This cell death phenotype is very similar to those observed in so 1 and eya 1 mutants (Halder, 1998).

To gain insight into the epistatic relationships among ey, so and eya, their expression patterns in eye discs were compared. Ey expression in the eye disc starts in the embryo and is later observed in the entire eye disc of late second and early third instars. During subsequent development, Ey expression is strong in the region anterior to the furrow and downregulated in differentiating cells. Very little, if any, expression posterior to the furrow or in the region of the developing ocelli in third instar eye discs is detected with polyclonal antibody or by in situ hybridisation. At the furrow, the expression patterns of Ey and Decapentaplegic (Dpp) abut each other, indicating that Ey expression is downregulated just before cells enter the furrow. Eya and So start to be expressed in eye discs later than Ey. In contrast to Ey, neither So nor Eya is expressed in the eye anlagen of stage-16 embryos. Expression of Eya and So in the eye disc starts in the late second and early third instar, respectively. At these stages, both genes are expressed in a gradient with the strongest expression at the posterior of the eye disc. Later, when the furrow moves across the eye disc, So and Eya are expressed in a graded fashion with strongest expression just anterior to the furrow. In this region the expression pattern of Ey overlaps with those of So and Eya. However, in the most anterior part of the eye disc only Ey is detected at high levels. Unlike Ey, So and Eya continue to be expressed posterior to the furrow. Both genes are also expressed in the region of the differentiating ocelli. In summary, Ey is expressed in the eye disc from embryonic stages onward, until cells enter the furrow and start to differentiate, while So and Eya start to be expressed later, and cells begin to express increasing levels of So and Eya as the furrow moves across the eye disc. These results are consistent with ey acting upstream of so and eya during eye disc development (Halder, 1998).

Gene expression was studied in ey 2, so 1 and eya 1 mutant eye discs. Genetic and molecular data indicate that the so 1 and eya 1 alleles are amorphic or severely hypomorphic in the developing eye. Because massive cell death is observed in late third instar eye discs of all three mutants, gene expression analysis at this stage is not possible. Expression patterns were therefore studied in early third instar eye discs. At this stage all three genes are expressed and cells in the so 1 and eya 1 mutant eye discs are still viable. Eye discs from ey 2 mutants, however, already show first signs of morphological abnormalities, indicating that ey function is required prior to this stage. In eye discs of so 1 and eya 1 mutants, Ey is expressed normally, indicating that the functions of so and eya are not required for Ey expression. However, neither SO nor Eya expression is observed in ey 2 mutant eye discs. This demonstrates that ey function is required for eye disc expression of So and Eya. In about half of the so 1 mutant eye discs weak Eya immunoreactivity is detected, suggesting that so may not be required for EYA expression. Expression of So is not seen in eya 1 mutant eye discs. However, because So and Eya are expressed in nearly identical patterns and because both genes are required for cell viability, these results are not conclusive. In summary, (1) ey acts earlier than and upstream of so and eya in the developing eye disc and (2) the functions of so and eya in the eye disc appear to be dispensable for ey expression (Halder, 1998).

To further investigate the epistatic relationships among ey, so and eya, gene expression was examined in the developing extra eyes induced by Gal4-directed ectopic expression of eyeless. In wild-type third instar larvae So and Eya are not expressed in the wing disc proper. However, in wing discs that develop ey-induced extra eyes, both genes are ectopically expressed in and surrounding the developing photoreceptor clusters. These results indicate that ey acts upstream of so and eya during extra eye development. In order to investigate the dynamics and the spatial restriction of the induction of so and eya expression, ey was ubiquitously expressed in a temporally controlled manner using a heat-inducible transgene. Expression of so and eya was monitored by assaying lacZ expression of so and eya enhancer-traps. Ubiquitous expression of ey was induced starting at 83 hours after egg laying during the mid third instar stage. At this time neither so nor eya are expressed in the wing disc proper and eya is not expressed in leg discs. Two heat shocks induce only weak ectopic expression of so and eya; do not induce extra eye formation in adult flies, and just barely affect their morphology. This suggests that higher or prolonged levels of Ey may be required to efficiently reprogram cells into the eye developmental pathway. Consistent with this, induction of extra eyes is efficient when larvae carrying the heat-inducible ey transgene are heat-shocked six times. Such animals readily induce ectopic expression of so and eya; nearly 100% of pharate adult flies developed extra eyes. Although Ey is expressed ubiquitously, induction of both genes is confined to regions close to the A/P boundary that do not express Wg but do express Dpp. Thus, Ey alone is not sufficient to induce so and eya, bearing in mind that only those cells that are close to a source of Dpp appear competent to express so and eya in response to Ey. The finding that ey positively regulates so and eya transcription raised the possibility that so and eya may be required downstream of ey for ectopic eye formation. Indeed, targeted expression of ey is unable to induce ectopic eye development in so 1 and eya 1 mutant backgrounds, although ectopic Ey protein is produced and functional as inferred from its deleterious effects. Consistent with the lack of ectopic eye production, no ectopic photoreceptors develop in wing discs of so 1 and eya 1 mutants following targeted expression of ey (Halder, 1998).

Advantage was taken of the ectopic induction of so and eya by Ey to find out whether Ey activates so and eya in parallel and independently of one another or whether induction of one gene depends upon the function of the other one. The cell death phenotypes observed in the eye discs of so 1 and eya 1 make such an analysis difficult in the eye discs. It was reasoned that by expressing ey ectopically those requirements for cell viability might be bypassed. However, in late third instar larvae, ectopic Ey expression in so 1 and eya 1 mutant backgrounds causes ectopic cell death in wing discs and results in strongly reduced and deformed adult structures. Apparently, Ey is able to completely reprogram wing cells into the eye developmental pathway, even if that leads to cell death, as is the case in so 1 and eya 1 mutants. Nevertheless, in early to mid third instar wing discs, Ey induces ectopic expression of eya in a so 1 mutant background and, conversely, so is induced by Ey in an eya 1 mutant background. Therefore, both genes appear to be independent targets of Ey. However, the ectopic expression is weaker than that induced in a wild-type background, suggesting that so and eya are required for efficient induction of each other's expression. In summary, these results show that Ey acts upstream of so and eya and requires their function during ectopic eye induction (Halder, 1998).

In addition to its function in the developing compound eye, so is required for the formation of the entire visual system, including the optic lobes of the brain and the larval photoreceptor organs known as Bolwig's organs. In blastoderm-stage embryos, so is expressed in a dorsal domain of the head region that gives rise to those structures. Whether this region also includes the primordia of the eye discs is unknown and no so transcripts are detected in the eye discs when they become morphologically discernible toward the end of embryogenesis. A second Pax-6 gene has been isolated from Drosophila, designated twin of eyeless (toy), which is expressed in the developing head from the blastoderm stage onward. In contrast, ey starts to be expressed at germ band extension. The early expression of toy overlaps so expression in the head and their epistatic relationship has been investigated. Cytologically, toy maps close to ey on the fourth chromosome. Since no mutations in toy have been identified thus far, advantage was taken of a compound fourth chromosome to generate nullo 4 embryos that lack both toy and ey functions. Such embryos express so at normal levels in the head, indicating that toy is not required for so expression in the embryonic head. Similarly, toy is expressed in an appropriate pattern in embryos homozygous for a null allele of so. Therefore, so and toy appear to act in parallel during the development of the embryonic head of Drosophila. Later in development, so null embryos express toy and ey in the eye anlagen indicating that so is not only dispensable for that expression but also for the initial formation of the eye anlagen (Halder, 1998).

Targeted expression of teashirt induces ectopic eyes in Drosophila

teashirt was initially identified as a gene required for the specification of the trunk segments in Drosophila embryogenesis and encodes a transcription factor with zinc finger motifs. Targeted expression of teashirt in imaginal discs is sufficient to induce ectopic eye formation in non-eye tissues, a phenotype similar to that produced from targeted expression of eyeless, dachshund, and eyes absent. The expression of so and dac are induced in the antennal disc by the ectopic expression of tsh, suggesting that tsh may act upstream of these genes in eye development. Furthermore, teashirt and eyeless induce the expression of one another, suggesting that teashirt is part of the gene network that functions to specify eye identity (Pan, 1998).

However, these results do not prove that tsh does play a role in specifying the eye identity during normal development. To address this issue, an examination was carried out to see if tsh is expressed at the right time and the right place to have a role in specifying the eye identity. Indeed, TSH mRNA is expressed in the eye disc, with the strongest expression anterior to the morphogenetic furrow. This pattern of expression is similar to that of ey, a gene that is known to play an essential role in specifying eye identity. An examination was carried out to see if loss-of-function mutations of tsh affect eye development. Several weak loss-of-function tsh alleles were examined and no eye defects were found. X-ray-induced mitotic recombination was used to generate mutant clones of a null tsh allele. tsh mutant clones were recovered at a frequency similar to the wild-type control, and sections through the mutant clones revealed a normal ommatidial organization. These data suggest that tsh may play a redundant role during normal eye development, and the requirement for tsh may be masked by other factor(s) that play a role similar to tsh (Pan, 1998).

Functional analysis of an eye enhancer of the Drosophila eyes absent gene: Differential regulation by eye specification genes

Genes involved in eye development are highly conserved between vertebrates and Drosophila. Given the complex genetic network controlling early eye development, identification of regulatory sequences controlling gene expression will provide valuable insights toward understanding central events of early eye specification. The focus of this study is the defining of regulatory elements critical for Drosophila eyes absent expression. Although eya has a complex expression pattern during development, analysis of eye-specific mutations in the gene reveal a region selectively deleted in the eye-specific alleles. Detailed analysis has been performed of a small 322 bp region immediately upstream of transcriptional start that is deleted in the eye-specific eya2 allele. This analysis shows that this region can direct early eya gene expression in a pattern consistent with that of normal eya in eye progenitor cells. Functional studies indicate that this element will restore appropriate eya transcript expression to rescue the eye-specific allele. Regulation of this element during eye specification has been examined, both in normal eye development and in ectopic eye formation. These studies demonstrate that the element is activated upon ectopic expression of the eye specification genes eyeless and dachshund, but does not respond to ectopic expression of eya or sine oculis. The differential regulation of this element by genes involved during early retinal formation reveals new aspects of the genetic hierarchy of eye development (Bui, 2000).

The eya enhancer is expressed in ey, so, and dac mutant eye discs in a pattern consistent with previous studies of Eya protein expression during normal eye development. Normally, eya expression is dependent upon ey activity, partially dependent upon so activity, and independent of dac activity. Regulation during ectopic eye formation was addressed in order to define genes that control the expression of this eya enhancer region and to observe differential activation of the eya enhancer. Activity of the enhancer was detected upon ey- and dac-induced eye formation, as anticipated by previous studies. However, enhancer activation is not apparent upon ectopic eya or so gene expression or the combination of eya and so together. Thus, this eya enhancer appears to be selectively activated during ectopic eye formation, indicating a molecular distinction in how ey and dac genes induce ectopic retinal tissue compared to induction by the eya and so genes, at least with respect to regulation revealed by this element (Bui, 2000).

The regulation of this defined eye enhancer for eya suggests that eya and so function distinctively, at least in part, from dac and ey in ectopic eye formation. Whereas ey and dac either directly activate or feedback to activate eya expression, eya and so do not participate in regulatory loops to the level of activation of eya gene expression as defined by the eya eye enhancer (Bui, 2000).

eya can also synergize with dac in ectopic eye development, and physically interact with the Dac protein. However, the loss-of-function phenotype of dac in the eye is not identical to that of eya and so. These studies also suggest that dac is not acting the same way as eya with respect the eya enhancer: dac strongly activates expression, but eya does not. Based on observations from expression studies, dac has previously been placed downstream of eya. However, Dac is reduced, but not missing from eya mutant eye discs. The reduced expression may reflect massive loss of eye progenitor cells in eya mutant eye discs; alternatively, or in addition, there may be a partial dependence of Dac expression upon eya gene function. Thus, Dac may indeed be involved normally in aspects of eya gene expression. Previous studies showing Eya expression on ectopic eye formation are confounded by the fact that Eya is expressed both prior to and after the appearance of the furrow, but this expression is likely to be under the control of different regulatory elements. The element defined here presents a probe for at least some aspects of the early regulation of eya gene expression. The functional requirement by eya for ey and dac activity (and vice versa) in ectopic eye formation may reflect concurrent roles or other, later roles of these genes in eye formation. ey clearly has multiple roles at distinct times in eye development, such as regulation of genes important for late events of photoreceptor cell differentiation, in addition to the early function stressed here (Bui, 2000).

With respect to eya enhancer activation, ey and dac may directly bind to the eya eye enhancer or the regulation may be indirect through additional, yet-to-be defined genes. It is suggested the regulation may not be direct, at least for Ey, as Ey binding sites are not clearly apparent within the element. Whether Dac protein directly binds to DNA has yet to be determined, but it likely interacts with known transcriptional regulators in addition to interacting with Eya. Yeast one-hybrid experiments have also failed to support direct activation of the eya enhancer by Dac or Ey (as well as confirmed lack of activation by Eya and So). These studies provide a framework from which to define additional molecular genetic controls on early retinal specification. Recent studies showing that the fundamentals of ey/Pax-6 regulation can cross species boundaries suggests that not only are elements of the genetic pathway controlling eye development conserved in vertebrates, but fundamental aspects of the regulatory mechanisms may also be conserved. Given that vertebrate Eya homologs display functional rescue of Drosophila eya mutants, key regulatory aspects of eya gene expression, in addition to the function of the protein, may also be conserved. Eya is a critical gene of eye formation, with complex regulation of expression as shown here, as well as complex protein interactions, and multiple downstream targets. This eye enhancer controlling early eya expression provides a molecular genetic tool to help dissect additional regulatory events of eye specification that are involved in the conserved pathways of eye formation (Bui, 2000).

A conserved regulatory element present in all Drosophila rhodopsin genes mediates Pax6 functions and participates in the fine-tuning of cell-specific expression

The Drosophila rhodopsin genes (rhs) represent a unique family of highly regulated cell-specific genes, where each member has its own expression pattern in the visual system. Extensive analysis of the rhs has revealed several functional elements that are involved in cell-specificity. The functional role of the RCSI/P3 site, which is found in the proximal promoter of all Drosophila rh genes, was investigated. This sequence is remarkably conserved in evolution and is located 15-30 bp upstream of the TATA box. In the context of the rh1 promoter, this element is recognized in vivo by a Pax6 protein, the master regulator of eye development. Thus, rh regulation might represent the ancestral function of Pax6. The role of the RCSI/P3 sequence in the other rh genes has been investigated and they have been found to also mediate Pax6 function. The potential impact of the various RCSI/P3 sequences on the precise cell-specific expression of rh genes was also investigated. Even though all RCSI/P3 sequences bind Pax6, they are clearly distinct in various rh promoters and these differences are conserved throughout evolution: RCSI/P3 appears to participate in the fine-tuning of cell-specificity. Pax6 or a related Pax protein may be involved in the regulation of olfactory genes. Therefore, in addition to performing a global photoreceptor-specific function, RCSI also appears to mediate the combined action of Pax6 and other factors and to contribute to rh regulation in subsets of photoreceptors (Papatsenko, 2001).

Based on the fact that all rhs are only expressed in photoreceptors (PRs), one might have expected to find common sequence elements among these related genes. Initial attempts to define common features of rh promoters by sequence comparison within a range of rh minimal promoters (from ~ -500 to +100) have resulted in the discovery of only two elements common to all genes: the TATA-box and the so-called RCSI sequences (Rhodopsin Conserved Sequence I). Both of these elements are necessary for transcription of all rh promoters; their mutation causes a total loss of activity. The homology between RCSI and the palindromic Paired-class homeodomain binding sites (P3 subtype) has suggested a functional link between RCSI and the critical regulator of eye development, Pax6. The direct interaction between what is believed to be a dimer of the Pax6 homeodomain (HD) and RCSI/P3 has been demonstrated genetically and molecularly for the rh1 gene (Papatsenko, 2001 and references therein).

Interestingly there are no sequences other than RCSI that are common to all rh promoters. At best, a few similar and/or evolutionary conserved regions can be detected, but only among certain subsets of the rh promoters. Moreover, in some cases, such as the rh3 and rh4 promoters, virtually no common sequences outside of the TATA box and RCSI can be observed, despite the fact that these two genes are both expressed in R7 PRs (rh3 in R7 pale, rh4 in R7 yellow). Based on these observations, a formal principle for the organization of rh promoters has been proposed: the proximal part, including RCSI, confers photoreceptor specificity. Distal upstream sequences, together with the proximal part, direct photoreceptor subtype specific expression. Although the separation of these two functions might not be absolute, it helps define the different functional units of rh promoters and facilitates their further dissection (Papatsenko, 2001).

The RCSI sequence, which contains a potential Pax6 HD recognition motif, was replaced by Pax6 PD binding sites in the rh3, rh5 and rh6 promoters. Despite the presence of ectopic expression in some cases, all target promoters maintained their original expression. The Pax6 PD and HD sites share no common sequence, but Pax6 binds both with high affinity. In many transgenic lines, the Pax6 PD binding sites have provided correct qualitative activity to replace the original RCSI HD elements. Therefore it is concluded that the RCSI site in all rh promoters contains a functional Pax6 homeodomain binding site. This site is necessary for the activation of all rh genes in the compound eye and thus, the Pax6 protein can be considered as a general activator that does not play a significant role in the extreme photoreceptor subtype specificity of the rh promoters. However, despite the fact that most modified promoters retain expression in their original location, different side effects, such as expansion into other classes of PRs, are also observed. This suggests that the presence of a functional Pax6 site alone is not sufficient to replace the RCSI sequence, since it is not sufficient for perfect promoter regulation (Papatsenko, 2001).

RCSI swap experiments among rh3, rh5 and rh6 were performed in order to investigate whether each RCSI sequence contains not only a general Pax6 site, but also other photoreceptor-specific information. This possibility is supported by the distinct structure of the RCSI sequence in each rh promoters. Conserved variations among families of RCSI sites might contribute to rh promoter specificity by either recruiting heterodimers of Pax6 with other Prd-class HD transcription factors, or some homeodomain proteins, such as K50 homeoproteins, to the RCSI element rh (Papatsenko, 2001).

The main result of the swap experiments was essentially the same as for the PD substitution: all modified promoters correctly maintain expression in their original location, but in several cases, ectopic expression patterns are observed. Another consequence of the RCSI swaps is considerable change in expression levels. This may be an indication of different affinity of the Pax6 site within different RCSI sequences. Based on the analysis of several lines (at least ten) for each construct, the strength of the different RCSI can be ordered: rh6 > rh3 > rh5. These swap and replacement tests suggest that Pax6 plays a similar activator role for the regulation of all rh promoters (Papatsenko, 2001).

The presence of similar Pax6 sites in all RCSI sites does not explain, however, the ectopic expression observed in several instances. One simple explanation might be the different strengths of Pax6 sites. Incorrect affinity of the main regulatory site might lead to such unexpected patterns. In particular, a stronger site might override other regulatory interactions. For instance, it is possible that Pax6 binding sites of strictly defined strength are required to provide the correct pattern of a given rh promoter. A second explanation for the ectopic expression is the presence of other regulatory motifs within RCSI, together with Pax6 sites. Disruption of these binding sequences in the replacements may explain the observed disruption of promoter specificity. The RCSI sequences might in fact be composite elements that differ in each rh promoter (Papatsenko, 2001).

Although Pax6 appears necessary for the activation of all rh genes through the RCSI, the results suggest that this interaction may not be sufficient for perfect rh expression. RCSI might represent a composite element that interacts with several regulatory molecules. For instance, a Pax6 homodimer might bind to its palindromic site, unless a heterodimer of Pax6 and another Prd-class HD protein bind to this site. Finally, a regulatory protein other than a homeoprotein might bind to RCSI, thus providing a third factor that either forms a complex with, or might compete with Prd-class homeoproteins (including Pax6). Among the potential candidates for the heterodimer model, the two Pax6 genes, ey and twin of eyeless (toy) might act together. It is not known which Pax6 gene, toy or ey acts on RCSI in vivo. Both Ey and Toy have similar affinity to the Pax6 HD sites (RCSI), and thus, they could be both involved in regulating rh genes through binding to RCSI as homo- or hetero-dimers. Although ey expression is turned off after the furrow in the third instar larvae, it is re-expressed during larval life. Currently there are no reports describing expression of toy in the adult. Since both Ey and Toy have a PD, the PD-replacement experiments could reflect the function of either gene. However, it has been reported that the PD of Ey binds poorly to DNA as compared to Toy, due to a mutation at a critical residue of the PD, suggesting that Toy is the protein binding to the PD replacement promoter. It must be noted, however, that the Toy binding sites found in the ey promoter are quite different from the Pax6 PD sites defined in vitro and the synthetic PD site used in this study (Papatsenko, 2001).

RCSI might also represent a site for a heterodimer of Pax6 (Toy or Ey) with another homeoprotein from the Prd-class. Binding of such heterodimers to RCSI might lead to different transcriptional outcomes: repression versus activation. This model is supported by the presence of K50 motif in the rh3 and rh6 RCSI sequences. Both RCSI sequences contain the motif CTAATCC that perfectly matches the binding consensus for a K50 subclass of homeoproteins, such as Gsc and Otd, which are known as repressors. In this case, a Pax6-K50 heterodimer might form a repressor complex on RCSI. There are also examples where K50 HD sites overlap other recognition sequences. For instance, the even-skipped stripe2 enhancer has K50 Bicoid activator binding sites (CTAATCC) overlapping binding sites for the Kruppel repressor (Papatsenko, 2001).

The swap experiments show that the Pax6 PD sites can fully replace the Pax6 HD sites only in one case out of three (rh5). In a few instances, replacement of the RCSI of one gene by another does maintain a correct expression pattern. However, it is clear that, in most cases, substitution of a native RCSI sequence by a generic Pax6 site is not fully sufficient to maintain exclusive expression. Therefore, a proper, specific structure for RCSI is required to achieve non-overlapping, specific patterns of expression. These facts, taken together with the presence of other known motifs (K50) within some RCSI sites, suggest a complex role for this potential composite element (Papatsenko, 2001).

In all the experiments, the initial specificity of the promoters was always preserved since ectopic expression was observed in addition to the expected pattern. This effect is more likely to be caused by disruption of existing repressor elements within RCSI rather than by creating new activator sites. Therefore some repressor sites might be present within at least some RCSI elements. For instance, rh5 and rh6 can be expressed in R7 cells upon changing their RCSI sequence; rh3 can also be expressed in all R7 when its RCSI is changed. Normally, rh5 and rh6 are expressed in two non-overlapping subsets of R8 PR's. However, in several cases, expression of these genes is observed in R7 cells. In one case, ectopic expression in R7 was obtained as the result of swapping the RCSI sequence of rh5 with that of rh6. In the other case, it was the result of rh6 RCSI replacement by Pax6 PD sites. It is possible that the rh5 and rh6 promoters both have an intrinsic potential to be expressed in both R7 and R8, but are selectively repressed by another factor in R7 cells. The putative R7 repressor factor might bind in the proximity or within the RCSI sequence, and its binding site might be affected by replacement of the RCSI sequence. Through extensive sequence analysis, a motif common to rh5 and rh6, GNCTAAGNC has been identified that is within the highly conserved regions of these promoters. The best match to this motif (GGCTAAGAC) is located 67bp upstream of RCSI in rh5. However, a weak site overlaps the RCSI (aAtTAAGTC). At least two putative R7 repressor sites flank/overlap the rh6 RCSI with the strongest one downstream of the third HD core (GTCTAAGAC). Some of these potential repressor sites have been disrupted in these experiments, perhaps resulting in partial derepression of the R8 promoters in R7 cells. The function of the R7 repressor sequence in both rh5 and rh6 promoters is currently being tested (Papatsenko, 2001).

An apparent derepression effect is also observed when the rh3 RCSI is replaced by either Pax6 PD sites, or by a stronger RCSI. In this case, rh3, which is normally only expressed in the pale subset of R7, exhibits equal expression in all R7, becoming a pan-R7 promoter. Some of the wild type minimal rh3 lines are also expressed in a pan-R7 pattern, but in those cases, expression in R7 yellow (rh4-specific) is much weaker than in R7 pale (rh3-specific) and is restricted to the dorsal part of the eye. This derepression cannot be linked to the disruption of the K50 site in the rh3 RCSI since the replacement by the similar RCSI from rh6 does not restore the correct pattern. It is also difficult to explain this effect by a stronger Pax6 binding site that somehow overrides R7 yellow repression, since a swap with the weaker rh5 RCSI gives the same result. Another repressor binding sequence, different from Pax6 and K50 might overlap rh3 RCSI or be placed next to it. Sequence analysis of conserved sites within the proximity of rh3 RCSI identifies the sequence ATTCCG that is unique to rh3 RCSI and is highly conserved. In all the tests with rh3, at least two mutations were introduced into that sequence. Furthermore, mutation of this sequence to ATTtgG, also result in the same very strong pan-R7 pattern (Papatsenko, 2001).

Altogether, these experiments support a functional significance for this potential repressor element. The effects caused by changes in RCSI sequences lead to the general conclusion that regulation of Drosophila rh promoters occurs through the complex interplay of activation and repression. In particular, activation by Pax6 through the RCSI composite element might be regulated by several repressors, specific for different rh promoters. Together with the effect of upstream regulatory regions of the rh promoters, this leads to the formation of highly specific, exclusive expression patterns (Papatsenko, 2001).

Differential interactions of eyeless and twin of eyeless with the sine oculis enhancer

Drosophila eye development is under the control of early eye specifying genes including eyeless (ey), twin of eyeless (toy), eyes absent (eya), dachshund (dac) and sine oculis (so). They are all conserved between vertebrates and insects and they interact in a combinatorial and hierarchical network to regulate each other's expression. so has been shown to be directly regulated by ey through an eye-specific enhancer (so10). The regulation of this element has been studied; both Drosophila Pax6 proteins, namely Ey and Toy, bind and positively regulate so10 expression through different binding sites. By targeted mutagenesis experiments, these Ey and Toy binding sites were disrupted and their functional involvement in the so10 enhancer expression in the eye progenitor cells was studied. A differential requirement has been shown for the Ey and Toy binding sites in activating so10 during the different stages of eye development. Additionally, in a rescue experiment performed in the so1 mutant, the Ey and Toy binding sites were shown to be required for compound eye and ocellus development, respectively. Altogether, these results suggest a differential requirement for Ey and Toy to specify the development of the two types of adult visual systems, namely the compound eye and the ocellus (Punzo, 2002).

All animals analyzed so far, ranging from flatworms to mammals, have a Pax6 gene that is universally required for eye specification, according to the current state of knowledge. In contrast to vertebrates, where generally a single Pax6 gene gives rise to several differentially spliced transcripts, Drosophila and other holometabolous insects have two Pax6 genes, raising the question of functional redundancy. Gene duplication and subsequent divergence of developmental control genes is a major driving force in evolution, increasing the diversity and complexity of the organisms. A second mechanism for recruiting additional genes into a developmental pathway is enhancer fusion. The acquisition of new cis-regulatory elements represents an important mechanism for functional diversification. The findings reported in this study strongly support both of these hypotheses, since toy is able to rescue the eye development in an ey mutant when expressed in the ey domain. The finding that ey and toy exhibit different expression patterns during embryogenesis might account in part for their functional biological diversity. In the eye, both genes are co-expressed, except for the ocellar territory where only toy is expressed. In addition, it has been proposed that Toy and Ey diverged to regulate different sets of target genes because of a N14G mutation that changes the DNA binding specificity of the PD domain of ey. Indeed, using the so10 regulatory element it was found that Toy does not bind to the same sequences as Ey, but interestingly, Toy and Ey regulate the same target enhancer in different cells. The phenotypes obtained in rescue experiments using either the Ey/Toy or Toy binding site mutated enhancers, nicely parallel the phenotypes observed in ey mutants. The ey null mutant still has ocelli but lacks compound eyes. Interestingly escapers from the recently isolated toy mutant (toyG7.39) exhibit no eye reduction whereas the ocelli are partially missing. Therefore, removal of the common target gene of both Pax6 proteins in the eye (e.g. so1 mutant) consequently leads to a loss of both compound eyes and ocelli. Therefore, it is proposed that one of the developmental programs of toy is in part to specify ocellar development in addition to head formation, since toy mutants generated are characterized by pupal lethality, pharate adults lacking half of the head or the entire head capsule. Thus, it is proposed that the so gene is regulated by toy to specify the ocelli and by ey to specify the compound eyes during larval development (Punzo, 2002).

The analysis of ey and toy allows for the dissection of the evolutionary changes after the gene duplication event that happened during insect evolution. (1) The cis-regulatory regions of the two genes have diverged, leading to both temporal and spatial changes of expression; toy is expressed much earlier than ey during embryogenesis, whereas ey is not expressed in the ocellar region of the larval eye disc. (2) The protein coding regions of the two genes have diverged, most importantly in the paired domain where asparagine 14 (which is present in most Pax6 homologs), has been mutated in ey to glycine, which changes the DNA binding properties of the protein significantly. (3) The positive autocatalytic feedback loop found in vertebrates for their single Pax6 gene, has evolved into a heterocatalytic control loop in which toy transcriptionally activates ey by binding to the eye-specific enhancer of ey. (4) Both toy and ey cooperate in differentially regulating the so target gene, reflecting the fact that earlier in evolution so was regulated by a single Pax6 gene. These findings strongly support the hypothesis of intercalary evolution showing that the ey gene has been intercalated into the eye developmental pathway between toy and so. The observation that toy activates ey in the eye progenitor cells of the embryo, where neither so and eya are expressed, indicates that toy and ey are acting high up in the genetic hierarchy leading to eye development (Punzo, 2002).

hh induces dpp, which in turn cooperates with ey to initiate retinal morphogenesis

Although Hedgehog (Hh) signaling is essential for morphogenesis of the Drosophila eye, its exact link to the network of tissue-specific genes that regulate retinal determination has remained elusive. In this report, it is demonstrated that the retinal determination gene eyes absent (eya) is the crucial link between the Hedgehog signaling pathway and photoreceptor differentiation. Specifically, it is shown that the mechanism by which Hh signaling controls initiation of photoreceptor differentiation is to alleviate repression of eya and decapentaplegic (dpp) expression by the zinc-finger transcription factor Cubitus interruptus (Cirep). Furthermore, the results suggest that stabilized, full length Ci (Ciact) plays little or no role in Drosophila eye development. Moreover, while the effects of Hh are primarily concentration dependent in other tissues, hh signaling in the eye acts as a binary switch to initiate retinal morphogenesis by inducing expression of the tissue-specific factor Eya (Pappu, 2003).

Misexpression of eyeless (ey) in the wing disc causes ectopic photoreceptor differentiation only in regions where both dpp and hh signaling are normally active. The simplest explanation for this effect invokes a linear regulatory hierarchy where hh induces dpp, which in turn cooperates with ey to initiate retinal morphogenesis. While misexpression of ey and dpp together does indeed lead to synergistic photoreceptor differentiation, this occurs only in the posterior compartment of the wing disc. Notably, Hh signaling is not transduced in the posterior compartment of the wing disc due to the repression of ci by En. Furthermore, dpp and ey expression does not induce Ci expression in the posterior compartment of the wing disc. Thus, it is concluded that dpp and ey can induce Eya expression and photoreceptor differentiation in the posterior compartment of the wing disc in the absence of Hh signaling and Cirep. Misexpression of hh and ey induces robust eya expression and photoreceptor differentiation in the wing disc, but only in the anterior compartment. This result is consistent with a model in which Hh signaling normally blocks the production of Cirep and converts it into an activated form, Ciact, in the anterior compartment of the wing disc. Ciact can induce dpp expression in the anterior compartment and dpp can in turn cooperate with ey to induce robust Eya expression and photoreceptor differentiation. Consistent with this model, co-expression of hh, dpp and ey leads to Eya expression and photoreceptor differentiation in both compartments of the wing disc. Taken together, these results suggest that, in the wing disc, ey and dpp can activate eya expression only in the absence of Cirep (Pappu, 2003).

Co-expression of dpp, ey and eya using the 30A-Gal4 driver induces photoreceptor differentiation in both wing compartments, albeit with low penetrance. This effect becomes stronger and more penetrant when dpp, ey, eya and so are misexpressed in a ring around the wing pouch. These results demonstrate that providing ey, dpp and eya from an exogenous source is sufficient to bypass the requirement for Hh signaling during initiation of ectopic photoreceptor differentiation. In addition, these results implicate eya as a key target for Hh signaling during the initiation of normal retinal morphogenesis, most likely by blocking Cirep (Pappu, 2003).

The hernandez and fernandez genes of Drosophila specify eye and antenna

The formation of different structures in Drosophila depends on the combined activities of selector genes and signaling pathways. For instance, the antenna requires the selector gene homothorax, which distinguishes between the leg and the antenna and can specify distal antenna if expressed ectopically. Similarly, the eye is formed by a group of 'eye-specifying' genes, among them eyeless, which can direct eye development ectopically. hernandez (distal antenna related or danr) and fernandez (distal antenna or dan) are expressed in the antennal and eye primordia of the eye-antenna imaginal disc (see Dan and Danr). Hernandez and Fernandez are the names of twin brothers in Tintin comic-books. The predicted proteins encoded by these two genes have 27% common amino acids and include a Pipsqueak domain. Reduced expression of either hernandez or fernandez mildly affects antenna and eye development, while the inactivation of both genes partially transforms distal antenna into leg. Ectopic expression of either of the two genes results in two different phenotypes: such expression can form distal antenna, activating genes like homothorax, spineless, and spalt, and can promote eye development and activates eyeless. Reciprocally, eyeless can induce hernandez and fernandez expression, and homothorax and spineless can activate both hernandez and fernandez when ectopically expressed. The formation of eye by these genes seems to require Notch signaling, since both the induction of ectopic eyes and the activation of eyeless by the hernandez gene are suppressed when the Notch function is compromised. These results show that the hernandez and fernandez genes are required for antennal and eye development and are also able to specify eye or antenna ectopically (Suzanne, 2003).

hern and fer genes are required for normal eye development and form eye tissue and activate ey when ectopically expressed. To study the role of the hern and fer genes in eye development, the eye phenotype was examined when either the hern or fer genes are inactivated by RNAi or are expressed ectopically. Expression of ds-hern or ds-fer RNA in the eye primordium with a GMR-GAL4 driver causes a slightly rough eye, with some bristles irregularly positioned. Curiously, the phenotype is not increased if the ds-hern and ds-fer RNAs are induced in the same fly. Misexpression experiments also suggest that both hern and fer are involved in eye development. Thus, the expression of either hern or fer with different GAL4 drivers causes the appearance of ectopic eye tissue in the third antennal segment or rostral membrane. These transformations are accompanied by the ectopic expression of ey, although this effect may also indicate the maintenance of a previous ey expression. Conversely, the misexpression of ey activates the hern and fer genes ectopically. Both hern and fer also activate embryonic lethal abnormal vision (elav), a marker of neuronal differentiation, when ectopically expressed. The analysis of clones expressing the fer gene in the leg, eye-antennal, or wing discs shows that elav activation is strictly nonautonomous, and only occurs in some cells adjacent to some of these clones (Suzanne, 2003).

The formation of the morphogenetic furrow in the eye is limited laterally by wg signaling. hern and fer expression within the eye primordium includes the more lateral wg-expressing regions. Interestingly, both hern and fer activate wg transcription when ectopically expressed. In ptc-GAL4/UAS-hern or dpp-GAL4/UAS-fer flies, the wings show several alterations, including the appearance of marginal bristles in the middle of the wing blade. This phenotype is characteristic of ectopic wg signaling, and in fact, wg is ectopically expressed in the wing discs of these larvae. Clones expressing the fer genes in the eye-antenna, leg, or wing discs also show induction of wg, mostly within but also outside the clone. The elav gene is also induced nonautonomously by these clones. Cells ectopically expressing elav do not coincide with those expressing wg and this reproduces the wild-type situation in the eye (Suzanne, 2003).

Signaling pathways can modify the activity of selector genes and are needed for proper organ formation. N signaling, for instance, is needed for eye formation and can activate ey when ectopically activated. Moreover, N has been implicated in the decision of making eye or antenna, directing eye development, and suppressing antenna formation. Therefore, whether N signaling could alter the ey and elav expression induced by the Tintin genes was examined. The coexpression of the hern gene and a dominant negative form of the Notch receptor substantially reduces ey and eliminates elav ectopic signals. Accordingly, no ectopic eyes are formed in this genetic combination. This indicates that the effect of hern on ey expression and eye formation requires N signaling (Suzanne, 2003).

Several eye-specifying genes have been identified, and they fulfill two conditions: they are required to make the eye and they can form ectopic eyes when expressed in different parts of the body. The hern and fer genes probably form part of this network of 'eye-specification' genes: (1) they are expressed in the eye primordium, with higher levels of expression anterior to the morphogenetic furrow; (2) they activate ey and elav and make ectopic eyes when expressed ectopically; (3) ey also activates the hern and fer genes when ectopically expressed. hern and fer genes have also been identified as downstream of ey in eye ectopic formation (Michaut, 2003). However, the inactivation of both hern and fer genes by RNAi with the GAL4 driver does not grossly affect eye development, as do mutants in the eye-specification genes. The nonautonomous induction of elav when hern or fer are ectopically expressed reproduces the wild-type situation, in which high levels of hern and fer are observed adjacent to the differentiating, elav-expressing, photoreceptor cells. Another similarity of hern and fer with some of the 'eye-specification'genes is that ectopic eye tissue is obtained in the antennae. The eye-specification genes eya and dac also form eyes predominantly, when ectopically expressed, in this same position. This is perhaps due to ey being expressed in the antennal primordium in late embryos, thus providing a favorable genetic context for eye formation. In accordance, when either the hern or the fer gene is ectopically expressed, ectopic ey expression is detected only in the antennal disc. Eyes are also obtained in the rostral membrane when ectopically expressing the fer gene. This may be due to the absence of hth, since high levels of either hern or fer repress hth and removal of this gene in the rostral membrane forms ectopic eyes (Suzanne, 2003).

The hern and fer genes can form ectopic aristae and eye tissue, but only in a limited number of regions of the adult cuticle. This is similar to what happens with other genes making ectopic antennae (hth, ss) or eye (eye-specification genes). This is due to the particular developmental context of the region where the genes are ectopically activated (Suzanne, 2003).

Transformations are observed of third antennal segment, where hern and fer are normally transcribed, to eye tissue, in Dll-GAL4/UAS-hern or dpp-Gal4/UAS-fer flies. This suggests that the levels of Hern and Fer products may be important in inducing or maintaining ey expression and distinguishing eye from antenna. Accordingly, when Hern or Fer products are increased in the antennal primordium, the expression of hth, an inhibitor of eye development, is eliminated. It is also noted that, in the wild-type eye-antennal discs, hern and fer show higher levels of expression in the eye primordium than in the antennal one, where these genes are coexpressed with hth. However, the amount of Tintin product is not the only factor in this distinction, since, for instance, in Dll-GAL4/UAS-hern eye-antennal discs, the area of ectopic ey transcription in the antenna is smaller than the area of hern overexpression. The activity of other genes will probably contribute to the formation of either eye or antenna. Thus, the ectopic expression of either hern or fer induces wg, an inhibitor of morphogenetic furrow formation, and this probably limits the places where the eye can develop (Suzanne, 2003).

Two recent models have been proposed to explain the specification of eye and antenna within the eye-antennal disc. Both models suggest that the activation of the N signaling pathway is a key element in this process. It has been suggested that N signaling activates both ey and Dll in the eye and antennal primordia; subsequently, ey represses Dll in the eye and perhaps the hth and extradenticle genes repress ey in the antenna. In this way, the exclusive expression of ey (in the eye) and Dll and hth (in the antenna) determine eye and antenna identity, respectively. It has been proposed that the N and Egfr signaling pathways (together with the hedgehog and wg genes) are instrumental in the decisions to make eye or antenna. N signaling has been proposed to promote eye development and prevents formation of the antenna, whereas Egfr signaling does the opposite. Ectopic expression of either hern or fer in the antenna induces ectopic eyes and activates ey and elav, but the coexpression of hern and an N dominant-negative protein does not result in ectopic eyes and almost eliminates ey and elav activation. This suggests that N function impinges on hern activity to form ectopic eyes. As in other cases, the combined activity of signaling pathways and selector genes determine the specification of different structures (Suzanne, 2003).

Genes induced by ectopic expression of eyelesseyeless in leg discs

Pax-6 genes encode evolutionarily conserved transcription factors capable of activating the gene-expression program required to build an eye. When ectopically expressed in Drosophila imaginal discs, Pax-6 genes induce the eye formation on the corresponding appendages of the adult fly. Two different Drosophila full-genome DNA microarrays were used to compare gene expression in wild-type leg discs versus leg discs where eyeless, one of the two Drosophila Pax-6 genes, was ectopically expressed. These data were validated by analyzing the endogenous expression of selected genes in eye discs and identified 371 genes that are expressed in the eye imaginal discs and up-regulated when an eye morphogenetic field is ectopically induced in the leg discs. These genes mainly encode transcription factors involved in photoreceptor specification, signal transducers, cell adhesion molecules, and proteins involved in cell division. As expected, genes already known to act downstream of eyeless during eye development were identified, together with a group of genes that were not yet associated with eye formation (Michaut, 2003).

Transcripts corresponding to 5,600-6,100 genes have been detected in the eye discs. These genes may act in eye development upstream or in parallel to ey, such as toy and optix, or may also be required for leg disc development (Notch, Egfr, and dpp). Therefore, despite their important role in eye development, their transcription is not significantly up-regulated by ectopic ey. The genes identified in this study are more likely to be preferentially involved in retinal differentiation rather than being required for general morphogenesis of imaginal discs. In agreement with previous findings, the DNA microarrays detect an up-regulation of eyes absent, so, and dachshund (dac), which encode evolutionarily conserved proteins functioning together with Pax-6 at the top of the eye developmental cascade. However, dac up-regulation occurs at only 74% of confidence because it is already highly expressed in leg imaginal discs in the absence of ey, consistent with its role in leg development. Because leg imaginal discs were used as the baseline for gene activity in this screen, genes more specifically required for eye rather than leg development are detected at a higher confidence level (Michaut, 2003).

Among the 38 transcription factors found to be both induced during ectopic eye formation and expressed in the eye imaginal discs, 18 were already associated with eye development. They are endogenously expressed in the vicinity of the morphogenetic furrow and known to be required during the first steps of photoreceptor differentiation. Among those, the E(spl) transcripts m delta and m gamma are expressed in the morphogenetic furrow. atonal is first broadly expressed in cells ahead of the advancing furrow and then undergoes successive refinements until it is expressed only in a single cell in each ommatidium, the R8 cell, which is the first photoreceptor to differentiate. rough controls the differentiation of the R2 and R5 cells, which are subsequently differentiating, and the bunched gene is expressed in a hedgehog-dependent stripe in the undifferentiated cells just anterior to the morphogenetic furrow. The genes pebbled and glass start to be expressed in the morphogenetic furrow, and their expression extends posteriorly in the differentiated photoreceptors. ey also induces the ectopic expression of lozenge, which is expressed in all undifferentiated cells arising from the second wave of morphogenesis that give rise to the R1/R6, R7, cone, and pigment cells (Michaut, 2003).

Among the other 20 transcription factors up-regulated during ectopic eye formation, eight have been described as being involved in other developmental processes. For instance, the roles of lola, sequoia (seq), and stich1 in embryonic nervous system development were investigated on the basis on their loss-of-function phenotypes. Similarly, loss-of-function mutations in the net gene, which encodes the Drosophila homolog of MATH6, have been described as affecting wing vein patterning. The endogenous transcription of these four genes in eye imaginal discs and their up-regulation during ectopic eye development suggest a possible role during eye development. Moreover, the transcription of these genes in the developing eye was independently confirmed by serial analysis of gene expression (SAGE) transcript imaging of purified cell populations from eye imaginal discs; SAGE tags corresponding to lola, seq, stich1, and net were indeed detected in cDNA libraries derived from sorted populations of eye disc cells (Michaut, 2003).

The fruitless (fru) and ken and barbie (ken) genes also encode transcription factors that are both expressed in the eye discs and induced by ey during ectopic eye development. Although ken transcripts are present in the eye disc in several rows of cells posterior to the morphogenetic furrow, no defects in eye development or morphology are described for viable mutant alleles. One possibility is that these mutant alleles do not affect ken function in the eye, similar to the case of the fru alleles; fru-viable mutations cause anomalies in male courtship behavior and affect the sex-specific transcripts produced under the control of a distal promoter of the gene. fru is a multifunctional gene that encodes sex-nonspecific proteins in addition to the protein involved in male behavior. One or more of these proteins could be responsible for fru function in the eye (Michaut, 2003).

In addition to transcription factors, signal transducers represent an important category of genes expressed in the eye discs and up-regulated during ectopic eye formation. The expressions in eye discs of the Ras interactors Sur-8 and sprint as well as the APPL-interacting protein 1 were confirmed. The specific expression of these three genes in the area of the morphogenetic furrow and their significant induction during ectopic eye development argues in favor of a previously uncharacterized function during eye development. Among the three Rac genes present in Drosophila, only Rac2 is up-regulated during ectopic eye formation. Rac GTPases act at various steps of development by controlling changes in cell shape. These modifications of the actin cytoskeleton are mediated by actin-binding proteins. The data show that the Quail protein, which is involved in actin bundle assembly during oogenesis is also present in the eye discs posterior to the morphogenetic furrow and is up-regulated during ectopic eye development (Michaut, 2003).

Transcription of a number of genes required at various steps of cell division is up-regulated during ectopic eye formation; twins encodes the regulatory subunit of protein phosphatase type 2A involved in regulation of mitosis and is expressed in imaginal discs. greatwall encodes a putative protein kinase required for chromosome condensation and mitotic progression, and skeletor encodes a chromosomal protein relocalizing during mitosis, which was postulated to constitute a matrix for assembly of the microtubule-based spindle during prophase. skeletor is expressed in the eye disc in a discrete row of cells posterior to the morphogenetic furrow; this row of cells could correspond to the cells undergoing the second wave of mitosis (Michaut, 2003).

The chit gene encodes a chitinase-related imaginal disc growth factor synthesized by the fat body and involved in the control of imaginal disc growth. chit is also transcribed in leg and eye imaginal discs and its transcription is increased during ectopic eye formation, indicating an autonomous role of chit in imaginal disc development and more specifically in eye differentiation. This finding is in perfect agreement with the results of a microarray analysis of genes differentially expressed in the various imaginal discs, where chit expression was found to be 2-fold higher in eye-antennal than in wing discs (Michaut, 2003).

More than half of the 371 ey-induced genes identified in this study are uncharacterized. No molecular function could be assigned to 117 of them, such as SP1173 (FBgn0035710), for which no homolog nor any functional domain could be identified clearly. Interestingly, SP1173 transcripts are present in two distinct regions of the eye discs: in a band of cells located in the area of the morphogenetic furrow and at the posterior edge of the disc. Transcription of three previously uncharacterized genes potentially encoding cell adhesion molecules is also up-regulated during ectopic eye formation: CG13532, BcDNA:gh11973, and CG9134 are expressed in the area of the morphogenetic furrow. CG12605 encodes a putative transcription factor similar to the pan-neural gene scratch and is expressed posterior to the morphogenetic furrow, where neuronal differentiation occurs. CG11849 and CG13651 encode homolog proteins containing a N-terminal pipsqueak-DNA binding domain. Both are ectopically induced by ey in the leg imaginal discs and display almost identical expression patterns in the eye discs, in nondifferentiated cells anterior to the morphogenetic furrow. These genes encode putative transcription factors that may represent previously uncharacterized, important regulators of eye development (Michaut, 2003).

BcDNA:gh11415 encodes the homolog of the evolutionarily conserved cell fate-determining protein mab-21 identified in the nematode, zebrafish mouse and human. The mouse mab-21 homolog participates in cerebellar, midbrain, and eye development. In midgestation embryogenesis, it is expressed at its highest levels in the rhombencephalon, cerebellum, midbrain, and prospective neural retina. The human mab-21 homolog, CAGR1, was detected originally in a retinal cDNA library; it is expressed in several tissues, most prominently in the cerebellum. BcDNA:gh11415 expression anterior to the morphogenetic furrow in Drosophila eye imaginal discs and its ectopic induction by ey are consistent with an evolutionarily conserved role of mab-21 in eye development (Michaut, 2003).

Eyg and Ey Pax proteins act by distinct transcriptional mechanisms in Drosophila development

Drosophila has two pairs of Pax genes, ey/toy and eyg/toe, that play different functions during eye development. ey specifies eye fate, while eyg promotes cell proliferation. This study has determined the molecular basis for the functional diversity of Eyg and Ey. Eyg and Ey act by distinct transcriptional mechanisms. They use different DNA-binding domains for target recognition. Most interestingly, Eyg acts exclusively as a repressor, whereas Ey is an activator. Several vertebrate Pax proteins are known to switch between activator and repressor activities, but none as repressors only. Eyg may be the first Pax protein as a dedicated repressor. Vertebrates produce a Pax6 isoform, Pax6-5a, differing from Pax6 in DNA-binding properties and functions and structurally similar to Eyg/Toe. Pax6-5a acts as an activator like Ey, but has DNA-binding specificity like Eyg (Yao, 2005; full text of article).

The paired domain (PD) can be further divided into two DNA-binding subdomains, PAI and RED, plus a linker (L) region linking the two. Ey specificity, compared with Eyg, is determined by its PAI and L domains. In vitro studies show that when the intact PAI is present, the DNA binding of the RED is masked. However, it was found that in vivo the RED of Ey may still play a role in target recognition or in maintaining the PD structure. The HD is not required for most of Ey functions, although it is required to suppress Dll expression. In contrast, Eyg uses the L, RED, and HD domains for target recognition. The L region is important for the functional specificity of both Ey and Eyg. Although the L region has been shown to contact DNA by X-ray crystallography, its functional role has not been previously demonstrated. The non-DNA-binding B region is not involved in target specificity, but in both Ey and Eyg it is required for the full activity. The different DNA-binding domains may be used independently or in a combinatorial fashion. For example, the Ey PD, but not the HD, is used to induce ectopic eye. The Ey HD is required for the suppression of Dll in order to block the antennal fate during eye development. For Eyg, the PD and HD are redundantly involved in wg suppression. The finding that the Eyg PD and HD could function in trans suggests that the two domains may bind independently to different target genes (Yao, 2005).

The results strongly suggest that Eyg acts exclusively as a transcriptional repressor. This is based on three sets of results: (1) Expression of the obligatory repressor Eyg-En in eyg expression domains completely rescued the eyg mutant defects in eye, head, and thorax development, while Eyg-VP16 caused dominant-negative effects. Moreover, ey>eyg554 and ey>eyg-en could rescue headless homozygous eygM3-12 to approximately 500-600 ommatidia in size. (2) Ectopic expression of Eyg-En caused phenotypes similar to those of Eyg, while Eyg-VP16 caused opposite effects. Thus, in all functional assays that were tested, Eyg-En behaved just like Eyg, while Eyg-VP16 had the opposite effects. (3) Eyg has two sets of repression domains but has no activation domain (Yao, 2005).

Ey is known to activate directly so and the rhodopsin genes. Ey-VP16 and Ey-En results suggest that, at least for inducing ectopic eye development, Ey functions as an activator. The suppression of Dll transcription by Ey is suggested to be indirect. It was found that Ey-VP16 could repress Dll, while Ey-En could not, strongly suggesting that Ey probably activates a repressor of Dll. These results suggest that Ey in general acts as an activator. Ey uses its C-terminus for the transactivation activity. It was also found that the Toy C-terminus has only weak transactivation activity, providing an explanation on why the Toy C-terminus cannot functionally substitute for the Ey C-terminus (Yao, 2005).

Both Pax6-5a and Eyg have a disrupted PAI and similar DNA-binding preference. It has been proposed that PAX6-5a is functionally equivalent to Eyg. The results ruled out this interesting possibility. This study showed that expression of Eyg and hPAX6-5a cause different effects, and that hPAX6-5a failed to rescue eyg mutant. In addition, hPAX6-5a acted as a transcriptional activator in flies. Thus, hPAX6-5a is not functionally equivalent to Eyg. In contrast, hPAX6-5a functioned like the Ey construct without an intact PAI, suggesting that hPax6-5a uses the RED for DNA binding. Thus, both hPAX6 and hPAX6-5a acted as activators, but induced different target genes through use of different DNA-binding domains (Yao, 2005).

Pax6 genes from a variety of organisms can induce ectopic eyes when expressed in Drosophila, suggesting that the functional mechanism is conserved and they likely act also as activators in flies. In vertebrate development, Pax6 and Pax6-5a can also act as transcriptional repressors. Pax2, Pax3, Pax4, Pax5, and Pax8 can also function as repressors, in addition to being activators. The switch between functioning as activators and repressors may depend on interaction with cofactors. For example, Pax3 interacts with Daxx and HIRA. Eyg is not only the first repressor Pax in Drosophila, but also the first example of a dedicated repressor Pax protein. The results also suggest that Ey may function exclusively as an activator, at least for inducing ectopic eye development. Thus, the activator and repressor roles are delegated to proteins encoded by two different genes, ey and eyg, respectively (Yao, 2005).

Genome-wide identification of direct targets of the Drosophila retinal determination protein Eyeless

A combinatorial strategy was used to conduct a genome-wide search for novel direct targets of Eyeless (Ey), a key transcription factor controlling early eye development in Drosophila. To overcome the lack of high-quality consensus binding site sequences, phylogenetic shadowing of known Ey binding sites in sine oculis (so) was used to construct a position weight matrix (PWM) of the Ey protein. This PWM was then used for in silico prediction of potential binding sites in the Drosophila melanogaster genome. To reduce the false positive rate, conservation of these potential binding sites was assessed by comparing the genomic sequences from seven Drosophila species. In parallel, microarray analysis of wild-type versus ectopic ey-expressing tissue, followed by microarray-based epistasis experiments in an atonal (ato) mutant background, identified 188 genes induced by ey. Intersection of in silico predicted conserved Ey binding sites with the candidate gene list produced through expression profiling yields a list of 20 putative ey-induced, eye-enriched, ato-independent, direct targets of Ey. The accuracy of this list of genes was confirmed using both in vitro and in vivo methods. Initial analysis reveals three genes, eyes absent, shifted, and Optix, as direct targets of Ey. These results suggest that the integrated strategy of computational biology, genomics, and genetics is a powerful approach to identify direct downstream targets for any transcription factor genome-wide (Ostrin, 2006; full text of article).

Regulation of the retinal determination gene dachshund in the embryonic head and developing eye

Drosophila eye development is controlled by a conserved network of retinal determination (RD) genes. The RD genes encode nuclear proteins that form complexes and function in concert with extracellular signal-regulated transcription factors. Identification of the genomic regulatory elements that govern the eye-specific expression of the RD genes will allow a better understanding of how spatial and temporal control of gene expression occurs during early eye development. Conserved non-coding sequences (CNCSs) between five Drosophilids were compared along the ~40 kb genomic locus of the RD gene dachshund (dac). This analysis uncovers two separate eye enhancers, in intron eight and the 3' non-coding regions of the dac locus, defined by clusters of highly conserved sequences. Loss- and gain-of-function analyses suggest that the 3' eye enhancer is synergistically activated by a combination of eya, so and dpp signaling, and only indirectly activated by ey, whereas the 5' eye enhancer is primarily regulated by ey, acting in concert with eya and so. Disrupting conserved So-binding sites in the 3' eye enhancer prevents reporter expression in vivo. These results suggest that the two eye enhancers act redundantly and in concert with each other to integrate distinct upstream inputs and direct the eye-specific expression of dac (Anderson, 2006).

The smallest fragment in the 3' dac eye enhancer that can respond to dpp, eya and so is 3EE194 bp, which is centered around two CNCS blocks of ~40 bp and 20 bp. These two CNCS blocks are also common to all active fragments of the 3' eye enhancer. These two evolutionarily conserved stretches were scanned for known, genetically upstream transcription factor binding sites. The 40 bp conserved stretch contains two putative consensus So-binding sites, S1-5'-CGATAT and S2-5'-CGATAC, compared with the consensus 5'-(C/T)GATA(C/T) described previously. Each of these putative So-binding sites in 3EE were mutated individually and in combination to test their requirement for normal enhancer activity in vivo. Mutation of individual So-binding sites causes a severe reduction, but not complete elimination, of enhancer activity in vivo. However, simultaneous mutation of both So binding sites completely abolishes enhancer activity in vivo. These results, coupled with loss-and gain-of-function analyses with dpp, eya and so, suggest that So binds to the 3' eye enhancer directly and nucleates a protein complex that includes Eya to regulate 3EE. However, despite much effort using a wide variety of binding conditions, it was not possible to demonstrate specific, direct binding of So protein to oligos that contain these So-binding sites. The 5' eye enhancer, which has four CNCS blocks, were scanned for potential upstream transcription factor binding sites and no strong candidate binding sites were found within the CNCS blocks (Anderson, 2006).

Loss- and gain-of-function analyses with the two eye enhancers suggest that each enhancer is regulated by a distinct set of protein complexes. The 5' eye enhancer is activated by a combination of ey, eya and so, but is not activated by Dpp signaling. 5EE is activated by ectopic ey expression even in eya and so mutants, suggesting that it is regulated exclusively by ey. However, somewhat paradoxically, expression of 5EE, the intron 8 enhancer, is lost in eya and so mutants even though ectopic expression of a combination of dpp, eya and so does not activate this enhancer. Furthermore, driving high levels of ey in so1 mutant eye discs restores 5EE-lacZ expression. Coupled together, these results suggest that 5EE is primarily regulated by ey but that the regulation of 5EE by ey also requires eya and so (Anderson, 2006).

By contrast, the 3' dac eye enhancer is regulated by a combination of eya, so and dpp signaling, but is not directly dependent on ey. 3EE-GFP expression is lost in eya2 and so1 mutant eye discs, and in posterior margin mad1-2 mutant clones. Furthermore, ey cannot bypass the requirement for eya and so to activate 3EE. Conversely, 3EE is strongly induced by co-expression of eya and so. Moreover, dpp signaling via the tkv receptor can synergize with eya and so to induce 3EE in ectopic expression assays. Furthermore, neither Mad nor Medea, the intracellular transducers of Dpp signaling, is sufficient to bypass the requirement for activation of the Dpp receptor Tkv in these assays. Thus, it is concluded that events downstream of Dpp-Tkv signaling, such as the phosphorylation of Mad, are essential for the synergistic activation of the 3' dac eye enhancer by eya and so. Taken together, these results suggest that there are distinct requirements for the activation of the 5' and 3' dac eye enhancers. However, the exact nature of the protein complexes that regulate 5EE and 3EE remain to be determined (Anderson, 2006).

Morphogenetic furrow (MF) initiation is completely blocked in posterior margin dac3-null mutant clones. However, dac3 clones that do not include any part of the posterior margin develop and do not prevent MF progression, but do cause defects in ommatidial cell number and organization. This dichotomy in dac function is reflected in the two eye enhancers characterized in this study. Analysis of dac7 homozygotes demonstrates that the 3' eye enhancer is dispensable for MF initiation and progression. It is proposed that in dac7 mutants, the intact 5EE enhancer is sufficiently activated by ey to drive high enough levels of dac expression to initiate and complete retinal morphogenesis. However, dac7 mutants have readily observable defects in ommatidial organization. Thus, it is further proposed that this lack of normal patterning in dac7 mutants is most likely due to the loss of 3EE, which normally acts in concert with 5EE after MF initiation, to integrate patterning inputs from extracellular signaling molecules such as Dpp with tissue-specific upstream regulators such as ey, eya and so. However, it is not known if the 3' eye enhancer is sufficient to initiate dac expression in the absence of the 5' eye enhancer (Anderson, 2006).

Based on the results, a two-step model is proposed for the regulation of dac expression in the eye. First, the initiation of dac expression in the eye disc is dependent on Ey binding to 5EE. However, Ey is fully functional only when So and Eya are present. It is possible that Ey recruits So and Eya to 5EE, but a model is favored in which Ey bound to 5EE cooperates with an So/Eya complex bound to 3EE to initiate dac expression in the eye. After initiation of the MF, dac expression is maintained by an Eya and So complex bound to 3EE. In addition, 3EE can integrate patterning information received via dpp signaling, thereby allowing the precise spatial and temporal expression of dac in the eye. This two part retinal enhancer ensures that dac expression is initiated only after ey activates eya and so expression. Thus, the dac eye enhancers provide a unique model with which the sequential activation of RD proteins allows the progressive formation of specialized protein complexes that can activate retinal specific genes (Anderson, 2006).

The redundancy in dac enhancer activity also explains the inability to isolate eye-specific alleles of dac, despite multiple genetic screens. The modular nature of the two enhancers and their potential ability to act independently or in concert suggest that both enhancers must be disrupted to block high levels of transcription of dac. Thus, two independent hits in the same generation, a phenomenon that occurs infrequently in genetic screens, would be required to obtain an eye-specific allele in dac (Anderson, 2006).

Despite much investigation, very few direct targets of RD proteins, especially for Eya and So, have been identified. One study suggests that So can bind to and regulate an eye-specific enhancer of the lz gene. However, lz is not expressed early during eye development and is required only for differentiation of individual cell types. The results suggest that regulation of dac expression occurs via the interaction of two independent eye enhancers that are likely to be bound by Ey, Eya and So, and respond to dpp signaling. This analysis of the 3' eye enhancer suggests that two putative conserved So-binding sites are essential for 3EE activity in vivo. Mutation of individual So-binding sites dramatically reduces, but does not completely eliminate, reporter expression in the eye. Mutating both predicted So-binding sites completely blocks enhancer activity in vivo. Thus, it is concluded that So binds to 3EE via these conserved binding sites. However, it has not been possible to demonstrate a direct specific interaction of either So alone or a combination of Eya and So with oligos that contain these putative So-binding sites in vitro. It is possible that other unidentified proteins are required for stabilizing the Eya and So complex. Furthermore, the 194 bp fragment that responds to ectopic expression of dpp, eya, and so contains no conserved or predicted Mad-binding sites. This raises the intriguing possibility that dpp signaling activates other genes, which then directly act with eya and so to regulate the 3' eye enhancer. Alternatively, a large complex that includes Eya, So and the intracellular transducers of dpp signaling, such as Mad and Medea, may be responsible for activation of 3EE. Similarly, the results suggest that the 5' eye enhancer is regulated primarily by ey. However, it is unclear whether Ey directly binds 5EE. Furthermore, Ey is fully functional only in the presence of Eya and So. Thus, Ey either independently recruits Eya and So into a 5' complex or is activated by virtue of its proximity to the So/Eya complex bound to the 3' enhancer or both (Anderson, 2006).

The exact order and dynamics of protein complex assembly at 5EE and 3EE requires further investigation. However, the two dac eye enhancers are extremely useful tools with which to investigate fundamental issues about the mechanism of RD protein action. One significant issue concerns the mechanism of Eya function during eye development. Eya consists of two major conserved domains, an N-terminal domain that has phosphatase activity in vitro and a C-terminal domain that can function as a transactivator in cell culture assays. So contains a conserved Six domain and a DNA binding homeodomain. However, it is unclear if Eya provides phosphatase activity, transactivator function, or both, in this complex. Characterization of the components of the protein complexes that regulates dac expression may uncover the targets of Eya phosphatase activity during eye development. Thus, the isolation of two eye enhancers with distinct regulation provides very useful tools with which to study protein complex formation and function during Drosophila retinal specification and determination (Anderson, 2006).

Direct control of neurogenesis by selector factors in the fly eye: regulation of atonal by Ey and So

During eye development, the selector factors of the Eyeless/Pax6 or Retinal Determination (RD) network control specification of organ-type whereas the bHLH-type proneural factor Atonal drives neurogenesis. Although significant progress has been made in dissecting the acquisition of 'eye identity' at the transcriptional level, the molecular mechanisms underlying the progression from neuronal progenitor to differentiating neuron remain unclear. A recently proposed model for the integration of organ specification and neurogenesis hypothesizes that atonal expression in the eye is RD-network-independent and that Eyeless works in parallel or downstream of atonal to modify the neurogenetic program. This study shows that distinct cis-regulatory elements control atonal expression specifically in the eye and that the RD factors Eyeless and Sine oculis function as direct regulators. These transcription factors interact in vitro and indirect evidence is provided that this interaction may be required in vivo. The subordination of neurogenesis to the RD pathway in the eye provides a direct mechanism for the coordination of neurogenesis and tissue specification during sensory organ formation (Zhang, 2006).

This study found that regulatory elements controlling the early phase of ato expression in the eye lie within a 1.2 kb region located 3.1 kb downstream of the ato transcription unit. The early phase of ato transcription results from the integration of multiple regulatory inputs through separate cis-regulatory modules present within the 1.2 kb region (Zhang, 2006).

Cis-regulatory elements essential for gene activation map to the last 348 bp of the 1.2 kb region and include the So- and Ey-binding sites. Interestingly, the 348 bp region contains two relatively large (A1=99 bp and A2=140 bp) DNA sequences that are highly conserved from D. melanogaster to D. virilis. Based on this observation, constructs were generated containing only A1 or A2. However, neither region alone was sufficient to drive the stripe of reporter gene expression in the eye disc. Based on these results, it is conclude that the 348 bp region constitutes a 'core' or 'minimal' enhancer region for the transcriptional activation of ato in eye progenitor cells. Other factors undoubtedly bind to sequences within A1-A2 and regulate gene expression as neither A1 nor A2 alone are sufficient to drive expression in the eye disc. Genetic evidence suggests that signaling by the Bmp4-type factor Decapentaplegic (Dpp) also contributes to ato activation and two putative binding sites for Mad (a transcription factor shown to activate Dpp pathway targets) appear to be required for ato expression in all discs. However, a Mad consensus site present in the A2 box does not correspond to either of the two elements previously identified . Moreover, both the previously identified sites lie within the L fragment well upstream of the M'-M" interval containing the eye-disc enhancers. Future analyses of 3' enhancer-promoter interactions may resolve this issue (Zhang, 2006).

Separate cis-regulatory elements located within the conserved DNA regions IC1 and IC2 (IC1=88 bp and IC2=133 bp) control initial clusters formation. This feature of ato expression has been shown to require Notch (N) function. Sequence analysis of the IC1-IC2 region does identify a binding site for the effector of N signaling Suppressor of Hairless [Su(H)]. However, contradictory reports have been published on how Notch controls ato expression. Sun and colleagues found that transcription of ato is uniformly upregulated upon inactivation of Notch in Nts1 mutant discs. By contrast, early Ato protein expression is severely reduced in null Notch mutant clones . Since these experiments made use of different genetic reagents, it is difficult to interpret these results. Notch signaling may independently regulate ato expression at the mRNA and protein levels. Alternatively, the source of the discrepancy may lie in the use of different alleles, one hypomorphic (Nts1), and the other null (N54l9) (Zhang, 2006).

Lastly, activation of the 3'ato348gal reporter (core element) occurs prematurely as compared with endogenous ato. The 3'ato348gal mRNA is also found in cells lying just anterior to the proneural domain. Eye progenitors from this region are at a developmental stage referred to as pre-proneural and are characterized by the expression of the transcription factor Hairy (H) in addition to RD proteins. In the absence of Hairy and its partner Extra Macrochaetae, neurogenesis begins precociously within the eye disc. Thus, Hairy contributes to the downregulation of ato expression and prevents precocious neurogenesis. Activation of the reporters 3'ato348gal and 3'ato488gal (but not 3'ato1.2gal or 3'ato1.2-Δ298gal) in pre-proneural cells suggests that cis-elements mediating anterior repression lie within the 1.2 kb DNA fragment but outside the IC and A boxes. Although a search for canonical Hairy-binding sites does not identify potential regulatory elements, additional short stretches of evolutionarily conserved DNA are present and may contribute to this and/or other aspects of ato regulation (Zhang, 2006).

Over the last few years, Ey and So have been shown to play a crucial role in the deployment and maintenance of the RD network by directly regulating the transcription of several eye-specification genes [ey, so, eya, dachshund (dac) and optix]. However, little is known about downstream targets of the RD cascade. Although So also activates the post-MF expression of hedgehog and lozenge, this gene regulation is likely to reflect the late, differentiation-related functions of So. Thus, it is unclear how the RD factors induce eye formation and what aspects of the morphogenetic program they control directly (Zhang, 2006).

The results strongly suggest that the transcription factors Ey and So control activation of ato expression. This is the first example of a gene required during eye morphogenesis that is directly regulated by the RD network. The direct control of ato by Ey and So is a likely reason why ectopic eye induction by Eya+So or Dac depends on the activation of their upstream regulator ey. Other downstream targets may also be similarly controlled by multiple RD factors (Zhang, 2006).

The in vitro and in vivo evidence presented in this study also suggest that Ey and So may form a complex when bound to the adjacent cis-regulatory sites in the 3'ato core element. Together with the previously reported interactions of Eya-So and Eya-Dac, this finding raises the possibility that additional multimeric complexes involving several RD factors may also be involved in driving the transcriptional program for eye development. The observation that normal eye development is severely disrupted when one or another RD factor is over-expressed suggests that the RD proteins must be present at an appropriate level relative to one another. As all four proteins, Ey, Eya, So and Dac, have now been shown to interact in various combinations, the formation of such complexes and the recruitment of additional shared co-factors are likely to be sensitive to the relative concentration of RD factors present in eye progenitor cells (Zhang, 2006).

The model of gene regulation exemplified by the control of ato transcription provides a strong rationale for the feedback regulatory loops that link late and early RD gene expression. This regulation is likely to play a crucial role in ensuring the presence of appropriate levels of all four RD factors to optimize complex formation and co-regulation of downstream targets (Zhang, 2006).

Current models for the co-ordination of organ identity and neurogenesis in the eye place the Pax6 pathway either upstream of, or in parallel to, the control of neurogenesis. The findings presented in this paper favor the former model. Separate regions have been identified for the regulation of ato transcription in the eye versus other sensory organs (JO and CH). In addition, the presence of Ey- and So-binding sites that are required in vivo for reporter gene activation strongly suggests that endogenous ato expression is directly regulated by these factors. Thus, the RD network does not merely modify sensory organ development within the eye disc, but does, in fact, directly control it. In doing so, it also contributes to the co-ordination of selector and neurogenic inputs required to generate complex sensory structures such as the eye (Zhang, 2006).

Is this regulatory relationship between Ey-So and ato ancestrally derived? That is, was the direct link between ancestral Pax- and Ath-like genes already established in the protosensory organ that gave rise to today's ato-dependent sensory structures? The association of Pax-, Six- and Ath-type factors with sensory perception is not restricted to photic sensation but extends to mechanoreception in diverse organisms including mouse, jellyfish and mollusks. In the jellyfish P. carnea, which lacks eyes but responds to a variety of environmental stimuli including light, expression of a putatively ancestral-like PaxB gene, Six1/2, Six3/6 and atonal-like 1 is associated with neuronal precursors found in the medusa tentacles. Although the studies carried out in more basal metazoa consist mostly of analyses of gene expression and not function, this evidence does suggest that the association of Pax/Six/Ath-type factors and sensory organ development is ancient and may have been retained over more than 600 million years of evolutionary history (Zhang, 2006).

It is possible that the mechanisms of transcriptional regulation uncovered between Pax and Six genes and between Pax/Six and ato may have arisen early during evolution. Such regulatory interactions may have favored the continued association of Pax/Six/Ath as various modifications of their genetic cascades led to the development of more complex and diverse sensory organs. The investigation of ato/Ath gene regulation in other sensory organs and in basal metazoans is likely to clarify the evolutionary relationship among these pathways and the sensory modalities they control (Zhang, 2006).

Insulin/insulin-like growth factor (IGF) signaling constitutes an evolutionarily conserved pathway that controls growth, energy homeostasis, and longevity. In Drosophila melanogaster, key components of this pathway are the insulin-like peptides (Dilps). The major source of Dilps is a cluster of large neurons in the brain, the insulin-producing cells (IPCs). The genetic control of IPC development and function is poorly understood. This study demonstrates that the Pax6 homolog Eyeless is required in the IPCs to control their differentiation and function. Loss of eyeless results in phenotypes associated with loss of insulin signaling, including decreased animal size and increased carbohydrate levels in larval hemolymph. Mutations in eyeless lead to defective differentiation and morphologically abnormal IPCs. Eyeless controls IPC function by the direct transcriptional control of one of the major Dilps, dilp5. It is proposed that Eyeless has an evolutionarily conserved role in IPCs with remarkable similarities to the role of vertebrate Pax6 in beta cells of the pancreas (Clements, 2008).

The results demonstrate that Eyeless controls differentiation and function of the IPCs. These data identify Eyeless as the first transcription factor involved in the independent regulation of the three dilps expressed in the IPCs, i.e., dilp5 versus dilp2 and dilp3. Finally, the data suggest a remarkable degree of evolutionary conservation in the molecular-genetic control of development and function of insulin-producing cells (Clements, 2008).

All seven Dilps in the Drosophila genome seem to be functionally equivalent agonists for the Drosophila insulin receptor. Three Dilps, dilp2, dilp3, and dilp5, are expressed in the IPCs. Expression of the dilps is precisely regulated throughout development. Beginning in the first larval instar stage, dilp2 expression is initiated in the IPCs followed by dilp5 in the second instar, and finally dilp3 expression in the third instar, providing a mechanism to increase overall dilp levels as the animal grows. It was proposed that the three Dilps in the IPCs (dilp2, -3, and -5) are independently regulated at the transcriptional level. The data reveal that Eyeless regulates dilp5 expression but not dilp2 or dilp3 in the IPCs. Consistent with this finding, eyeless mutants (with a loss of expression of only one of the three major dilps expressed in the IPCs) exhibit defects in growth and hemolymph carbohydrate levels that are intermediate to the phenotypes observed in animals where the IPCs have been ablated (and therefore a loss of all three major Dilps) (Clements, 2008).

The results also provide support for an evolutionarily ancient role of Pax6 in IPCs. In vertebrates, Pax6 is required for development and function of the pancreas. Loss of Pax6 leads to disorganization of all endocrine cells within the islets of Langerhans and a complete loss of glucagon-producing α cells and a reduction in the number of insulin-, somatostatin-, and pancreatic polypeptide-producing endocrine cells. All endocrine cells within the pancreas also continually express Pax6 throughout development and adulthood. In accordance with these observations, the tissue-specific inactivation of Pax6 in the endocrine pancreas results in a significant loss of α and β cells and a severe diabetic phenotype with hyperglycemia and hypoinsulinemia. Pax6 also controls pancreatic function by directly activating the expression of insulin and glucagon in the mouse. In humans, mutations in PAX6 have been linked to impaired insulin secretion and early-onset diabetes mellitus. The conservation of insulin function and the conservation of the characteristics of the cells involved in insulin production between flies and mammals implies the existence of a conserved mechanism required to regulate the differentiation and function of this system. It is proposed that this is a reflection of an evolutionarily old relationship between Pax6 and insulin production. This could be either through a role of Pax6 in an ancestral insulin-producing organ or cell type of neural origin or, alternatively, through an independent recruitment of a Pax6-insulin transcriptional regulatory network during evolution of the endocrine pancreas (Clements, 2008).

Onset of atonal expression in Drosophila retinal progenitors involves redundant and synergistic contributions of Ey/Pax6 and So bindings sites within two distant enhancers

Proneural transcription factors drive the generation of specialized neurons during nervous system development, and their dynamic expression pattern is critical to their function. The activation of the proneural gene atonal (ato) in the Drosophila eye disc epithelium represents a critical step in the transition from retinal progenitor cell to developing photoreceptor neuron. This study shows that the onset of ato transcription depends on two distant enhancers that function differently in subsets of retinal progenitor cells. A detailed analysis of the crosstalk between these enhancers identifies a critical role for three binding sites for the Retinal Determination factors Eyeless (Ey) and Sine oculis (So). The study shows how these sites interact to induce ato expression in distinct regions of the eye field and confirms them to be occupied by endogenous Ey and So proteins in vivo. This study suggests that Ey and So operate differently through the same 3' cis-regulatory sites in distinct populations of retinal progenitors (Zhou, 2013).

Temporal patterning of neuroblasts controls Notch-mediated cell survival through regulation of Hid or Reaper

Temporal patterning of neural progenitors is one of the core mechanisms generating neuronal diversity in the central nervous system. This study shows that, in the tips of the outer proliferation center (tOPC) of the developing Drosophila optic lobes, a unique temporal series of transcription factors not only governs the sequential production of distinct neuronal subtypes but also controls the mode of progenitor division, as well as the selective apoptosis of NotchOFF or NotchON neurons during binary cell fate decisions. Within a single lineage, intermediate precursors initially do not divide and generate only one neuron; subsequently, precursors divide, but their NotchON progeny systematically die through Reaper activity, whereas later, their NotchOFF progeny die through Hid activity. These mechanisms dictate how the tOPC produces neurons for three different optic ganglia. It is concluded that temporal patterning generates neuronal diversity by specifying both the identity and survival/death of each unique neuronal subtype (Bertet, 2014).

Although apoptosis is a common feature of neurogenesis in both vertebrates and Drosophila, the mechanisms controlling this process are still poorly understood. For instance, several studies in Drosophila have shown that, depending on the context, Notch can either induce neurons to die or allow them to survive during binary cell fate decisions. This is the case in the antennal lobes where Notch induces apoptosis in the antero-dorsal projecting neurons lineage (adpn), whereas it promotes survival in the ventral projecting neurons lineage (vPN). In both of these cases, the entire lineage makes the same decision whether the NotchON or NotchOFF cells survive or die. This suggests that, in this system, Notch integrates spatial signals to specify neuronal survival or apoptosis (Bertet, 2014).

This study shows that, during tOPC neurogenesis, neuronal survival is determined by the interplay between Notch and temporal patterning of progenitors. Indeed, within the same lineage, Notch signaling leads to two different fates: it first induces neurons to die, whereas later, it allows them to survive. This switch is due to the sequential expression of three highly conserved transcription factors-Dll/Dlx, Ey/Pax-6, and Slp/Fkh-in neural progenitors. These three factors have distinct functions, with Dll promoting survival of NotchOFF neurons, Ey inducing apoptosis of NotchOFF neurons, and Slp promoting survival of NotchON neurons. These data suggest that Ey induces death of NotchOFF neurons by activating the proapoptotic factor hid. Thus, Dll probably antagonizes Ey activity by preventing Ey from activating hid. The data also suggest that Notch signaling induces neuronal death by activating the proapoptotic gene rpr. Thus, Slp might promote survival of NotchON neurons by directly repressing rpr expression or by preventing Notch from activating it. In both cases, the interplay between Notch and Slp modifies the default fate of NotchON neurons, allowing them to survive. Further investigations will test these hypotheses and determine how Dll, Ey, Slp, and Notch differentially activate/repress hid and rpr (Bertet, 2014).

Although the tOPC and the main OPC have related temporal sequences, their neurogenesis is very different. This difference is in part due to the fact that newly specified tOPC neuroblasts express Dll, which controls neuronal survival, instead of Hth. Why do tOPC neuroblasts express Dll? The tOPC, which is defined by Wg expression in the neuroepithelium, is flanked by a region expressing Dpp. Previous studies have shown that high levels of Wg and Dpp activate Dll expression in the distal cells of the Drosophila leg disc. Wg and Dpp could therefore also activate Dll in the neuroepithelium and at the beginning of the temporal series in tOPC progenitors. Another difference between the main OPC and tOPC neurogenesis is that Ey and Slp have completely different functions in these regions. Indeed, unlike in the main OPC, Ey and Slp control the survival of tOPC neurons. This suggests that autonomous and/or nonautonomous signals interact with these temporal factors and modify their function in the tOPC (Bertet, 2014).

Finally, tOPC neuroblasts produce neurons for three different neuropils of the adult visual system, the medulla, the lobula, and the lobula plate. This ability could be due to the particular location of this region in the larval optic lobes. Indeed, the tOPC is very close to the two larval structures giving rise to the lobula and lobula plate neuropils-Dll-expressing neuroblasts are located next to the lobula plug, whereas D-expressing neuroblasts are close to the IPC. Interestingly, Dll and D neuroblasts specifically produce lobula plate neurons. This raises the possibility that these neuroblasts and/or the neurons produced by these neuroblasts receive signals from the lobula plug and the IPC, which instruct them to specifically produce lobula plate neurons. These nonautonomous signals could also modify the function of Ey and Slp in the tOPC (Bertet, 2014).

In summary, this study demonstrates that temporal patterning of progenitors, a well-conserved mechanism from Drosophila to vertebrates, generates neural cell diversity by controlling multiple aspects of neurogenesis, including neuronal identity, Notch-mediated cell survival decisions, and the mode of intermediate precursor division. In the tOPC temporal series, some factors control two of these aspects (Ey), whereas others have a specialized function (Dll, Slp, and D). This suggests that temporal patterning does not consist of a unique series of transcription factors controlling all aspects of neurogenesis but instead consists of multiple superimposed series, each with distinct functions (Bertet, 2014).

Protein Interactions

Hox genes encoding homeodomain transcriptional regulators are known to specify the body plan of multicellular organisms and are able to induce body plan transformations when misexpressed. These findings led to the hypothesis that duplication events and misexpression of Hox genes during evolution have been necessary for generating the observed morphological diversity found in metazoans. It is known that overexpressing Antennapedia in the head induces antenna-to-leg as well as head-to-thorax transformation and eye reduction. At present, little is known about the exact molecular mechanism causing these phenotypes. The aim of this study was to understand the basis of inhibition of eye development. It has been demonstrated that Antp represses the activity of the eye regulatory cascade. By ectopic expression, it has been that Antp antagonizes the activity of the eye selector gene eyeless. Using both in vitro and in vivo experiments, it has been demonstrated that this inhibitory mechanism involves direct protein-protein interactions between the DNA-binding domains of Ey and Antp, resulting in mutual inhibition (Plaza, 2001).

If the Antp protein is able to block Ey activity, this mechanism should also function in other tissues. Therefore, ectopic eye formation should also be blocked by Antp. To test this prediction, ectopic eyes were induced on wing, antennae and legs using the UAS-GAL4 system. Results show that the ectopic eye formation induced by ey is completely blocked on co-expressing ey and Antp. Moreover, the Antp induced antenna-to-leg transformation is inhibited by ey. A series of similar tests employing hs-ey and hs-Antp transgenes, singly or in combination, leads to the same conclusions. Furthermore, these tests reveal a specific requirement for the Antp homeodomain (HD), since N-terminal deletions of the Antp protein do not affect its ability to inhibit Ey activity, whereas deletion of the HD results in a protein unable to inhibit ey function. Similarly, using the UAS-GAL4 system, the Antp HD-deleted protein is unable to repress ectopic eye formation. These results made it necessary to demonstrate that both proteins co-localize in the same cells of the discs. Upon examining protein accumulation by confocal microscopy, it was found that both proteins are efficiently co-expressed in these different tissues. Furthermore, immunostaining experiments performed using the ey antibody or the neuronal marker 22C10 confirm that, despite the presence of Ey in the disc, co-expression of Antp leads to inhibition of neuronal differentiation (Plaza, 2001).

In order to test whether the DNA-binding activity of Antp is not required for the inhibition of eye development, an Antp mutant was tested in which the DNA-binding specificity was changed (Q50K). Interestingly, this mutated protein is still able to repress eye development. In addition, mutagenesis experiments were performed to convert Q50 and N51, residues shown to be crucial for DNA contacts, into alanines. This mutant is unable to bind a DNA PS2 probe containing a Hox/Exd/Hth motif, even in the presence of EXD and HTH in the bandshift assay. This A50,A51 mutant protein is still capable of inhibiting eye development when expressed in the eye disc using a strong EyE-GAL4 line, although with a lower activity than the wild-type Antp protein (Plaza, 2001).

Based on these in vivo results it was asked whether Antp and Ey might interact directly and thereby inhibit each others activities. Potential in vitro interactions between Antp and Ey were examined using glutathione S-transferase (GST)-Antp fusion proteins immobilized on glutathione-Sepharose beads. These immobilized proteins were tested for their ability to retain in vitro synthesized 35S-labeled Ey protein. Different portions of the Antp protein were produced and tested separately for their ability to interact with Ey. Only the C-terminal portion of Antp including the HD is able to interact with Ey (Plaza, 2001).

To define the regions within Ey and Antp that are required for the interaction of the two proteins, a set of deletion mutants of each protein was tested for the ability to interact in vitro. Structure-function studies of both proteins have delineated specific domains that contribute to their functions as transcription factors as well as their interactions with other proteins. The Antp HD that mediates DNA binding has also been shown to interact with other HD proteins such as Exd. The Ey protein contains two DNA-binding domains, a paired domain and an HD. The paired domain has been shown to interact with different transcription factors. These findings led to an investigation of whether Ey paired domains and HDs are involved in the interaction with Antp. Deleting either of these domains in the Ey protein results in a partial loss of the interaction with the Antp HD. Furthermore, either the Ey paired domain or the Ey HD alone is still able to interact with Antp. These experiments suggest that since each domain is able to interact with Antp, both domains might cooperate for efficient binding of Ey to Antp. Moreover, deletion of the C-terminal part of the Antp protein results in the loss of binding to both the paired domain and the HD of Ey, confirming that the Antp HD is essential for the interaction with Ey. Since the complexes were formed using Ey paired domain and Antp HD purified from bacteria, the two proteins appear to interact directly through their respective DNA-binding region. It was hypothesized that the DNA interface might be important to stabilize the interaction. Indeed, the interaction between the two proteins requires that one of them binds to DNA (Plaza, 2001).

The question of whether the ability of Antp to repress eye development and to interact with Ey can be extended to other homeotic genes. For this purpose, expression of Scr, Ubx, abdA and AbdB were targeted into the eye disc using dppblink-GAL4. Expression of these different genes also results in inhibition of eye development by inducing apoptosis. Interestingly, these different proteins are also able to interact with Ey in vitro. Deletion of the ABD-A HD region abrogates binding to Ey, suggesting that also for this protein, the HD is required for interaction with Ey (Plaza, 2001).

Combinatorial control of Drosophila eye development by Eyeless, Homothorax, and Teashirt

In Drosophila, the development of the compound eye depends on the movement of a morphogenetic furrow (MF) from the posterior (P) to the anterior (A) of the eye imaginal disc. Several subdomains along the A-P axis of the eye disc have been described that express distinct combinations of transcription factors. One subdomain, anterior to the MF, expresses two homeobox genes, eyeless (ey) and homothorax (hth), and the zinc-finger gene teashirt (tsh). Evidence suggests that this combination of transcription factors may function as a complex and that their combination plays at least two roles in eye development: it blocks the expression of later-acting transcription factors in the eye development cascade, and it promotes cell proliferation. A key step in the transition from an immature proliferative state to a committed state in eye development is the repression of hth by the BMP-4 homolog Dpp (Bessa, 2002).

Anterior to the MF, at least three cell types can be distinguished by the patterns of Hth, Ey, and Tsh expression. The most anterior domain in the eye field, which is next to the antennal portion of the eye-antennal imaginal disc, expresses Hth, but not Tsh or Ey. In a slightly more posterior domain, all three of these factors are coexpressed (region II). In a more posterior domain, Tsh and Ey, but not Hth, are coexpressed. This domain, which also expresses hairy, is equivalent to the pre-proneural (PPN) domain. The MF, marked by the expression of Dpp, is immediately posterior to the PPN domain, and therefore abuts Tsh + Ey-expressing cells (Bessa, 2002).

Domain II is the only region of the eye-antennal imaginal disc that strongly expresses all three of these transcription factors. Posterior to the MF, Hth, but not Tsh or Ey, is expressed in cells committed to become pigment cells. Hth and Ey, but not Tsh, are coexpressed in a narrow row of margin cells that frame the eye field and separate the main epithelium of the eye disc from the peripodial membrane. Finally, Hth is also strongly expressed in peripodial cells, whereas Ey and Tsh are weakly expressed in a subset of these cells (Bessa, 2002).

The expression patterns of So, Dac, and Eya were also examined in wild-type eye discs. All three of these transcription factors are expressed in the PPN domain but not in domain II. Their expression domains have the same anterior limit but different posterior limits. Furthermore, the anterior limits of their expression domains are not sharp, but instead decrease gradually as Hth levels increase. Thus, cells in the PPN domain express So, Dac, and Eya as well as Tsh, Ey, and Hairy. Anterior to the PPN domain there is a gradual transition into domain II, where cells express Hth, Ey, and Tsh, but not So, Eya, Dac, or Hairy (Bessa, 2002).

In late second/early-third-instar eye discs, before or just as the MF is initiated, most eye disc cells express tsh, hth, and ey, although the levels of Hth are lower close to the posterior margin. Therefore, at this stage of development most eye disc cells express the same combination of transcription factors as domain II of third-instar discs. In both cases, these cells are uncommitted and dividing asynchronously (Bessa, 2002).

The overlapping expression patterns of ey, hth, and tsh in domain II raised the possibility that their gene products could be functioning together. As a first test of this idea, it was determined whether their protein products could interact with each other in vitro and in vivo. Histidine (his)-tagged Hth, alone or together with its partner protein Extradenticle (Exd), specifically binds to 35S-labeled Ey and Tsh in vitro. In vivo, it was found that both Exd and Tsh could be coimmunoprecipitated (IP) from wild-type embryos with Hth. Ey and Hth could not be IPed from wild-type embryos, perhaps because the number of cells coexpressing these transcription factors is too few. These results suggest that Hth, Exd, Tsh, and Ey have the potential to interact with each other in vivo. However, additional experiments are required to definitively test this idea (Bessa, 2002).

Tests were made of the ability of these factors to regulate each other's expression in the eye disc. Clones of cells were generated that express the yeast transcription factor Gal4 in flies containing UAS-Ey, UAS-Hth, or UAS-Tsh transgenes. These clones were generated during the second instar, when all three of these genes are coexpressed throughout the eye disc, and they were analyzed during the third instar, when the Hth expression pattern is distinct from the Tsh and Ey expression patterns. Tsh or Ey overexpressing clones in the PPN domain up-regulate Hth. The ability to maintain Hth expression was limited to the PPN domain; Ey- or Tsh-expressing clones within or posterior to the MF did not alter Hth expression. Hth can maintain Ey and Tsh expression posterior to its normal expression domain. Although this effect was not limited to the PPN domain, it was only observed in ~50% of the clones generated during the second instar, suggesting that other factors or the timing of clone induction limit this response. In addition, ectopic Tsh can also induce Ey expression in a subset of the eye imaginal disc. Together with the protein interaction experiments, the ability of these transcription factors to maintain or induce each other's expression suggests that these proteins may function together in eye development (Bessa, 2002).

Expression of Tsh or Ey maintains Hth expression in the PPN domain, where Tsh and Ey are already expressed. This result is interpreted as suggesting that hth is under two competing controls: maintenance by Tsh and Ey and repression by other factors, in particular the Dpp pathway, and that expressing higher levels of Tsh or Ey can shift this balance in favor of maintenance (Bessa, 2002).

Because Hth is coexpressed and can interact in vitro with Tsh and Ey, the possibility was considered that combinations of these transcription factors might be required to repress eya and dac. Consistent with this idea, it was found that the simultaneous expression of Tsh and Hth efficiently represses eya and dac expression. Importantly, the dual expression of Tsh and Hth is maintained by Ey expression; consequently, these clones expressed all three of these transcription factors. Other pairs of these transcription factors (Hth + Ey and Tsh + Ey) were also tested, and it was found that they can also partially repress eya (Bessa, 2002).

The above results suggest that the combination of Hth + Ey + Tsh, which is normally present in domain II, is able to repress the expression of eya. To test if hth normally plays a role in the repression of these genes, hth- clones were examined. Although hth- clones anterior to the MF are rare, it was found that both dac and eya are de-repressed in anterior hth- clones (Bessa, 2002).

In summary, these data suggest that the combination of the factors expressed in domain II is necessary and sufficient to repress eya and dac. In contrast, Hth is sufficient to repress the pre-proneural gene hairy. Conversely, eya and dac, together with Dpp, repress hth as the MF advances. It is suggested that one function for this reciprocal antagonism may be to prevent premature and uncoordinated differentiation anterior to the MF. However, as the MF advances, hth must be repressed to allow differentiation to occur (Bessa, 2002).

These experiments have shed new light on the nature and function of the cells anterior to the MF. Two domains anterior to the hairy-expressing PPN domain have been defined. One of these domains (II) expresses three transcription factors: Ey, which was already known to play a central role in eye development; Hth, which also plays a role in suppressing eye development in the ventral head, and Tsh, which, because of its ability to induce ectopic eyes elsewhere in the head, has also been implicated in eye development. The results suggest that, although these cells have not committed to become a particular cell type, they are predisposed to become eye tissue. Furthermore, it is suggested that the combination of Hth, Ey, and Tsh performs at least two functions during eye development: it represses the expression of later-acting transcription factors in the eye development cascade, and it promotes cell proliferation. Each of these points is discussed and these findings are integrated with the current view of eye development (Bessa, 2002).

These experiments suggest that one of the functions mediated by Ey-Hth-Tsh is to repress eya and dac. This proposal stems from both ectopic expression experiments, showing that the coexpression of Ey, Hth, and Tsh represses these genes, and from loss-of-function experiments, showing that hth- clones anterior to the MF de-repress these genes. Similarly, hth is de-repressed in both eya- and dac- clones, suggesting that this antagonism exists in both directions. Interestingly, the antagonism between these two sets of genes is analogous to that observed in other appendages. In the leg, hth and tsh are required for the development of proximal fates, and have been shown to be mutually antagonistic with dac and Distal-less (Dll), two genes required for intermediate and distal leg fates, respectively. Similarly, in the wing, hth and tsh are required for proximal wing fates, and oppose the activity of vestigial (vg), which is required for more distal wing fates (Bessa, 2002).

It is proposed that the putative Ey-Hth-Tsh complex promotes cell proliferation in early eye discs and in cells anterior to the PPN domain in third-instar discs. This suggestion is based on three observations. (1) In young discs, when most of the growth of the eye disc occurs and before the MF initiates, all eye disc cells express all three of these transcription factors. (2) hth- clones are only rarely observed anterior to the MF. The lack of hth- clones observed in this region of the eye disc suggests that hth is playing an important role in either the survival or proliferation of these cells (Bessa, 2002).

(3) Linking this combination of transcription factors with the growth of the eye disc stems from the observation that, when coexpressed, these factors can induce cell proliferation. This was most readily observed in clones that include cells at the edge of the eye disc. These cells may be unique in the eye disc because they express wg. Interestingly, activation of the wg pathway by generating axin- clones in the eye disc also induces proliferation and the maintenance of ey, hth, and tsh expression. Thus, proliferating eye disc cells express hth, ey, and tsh and are in a state in which the wg pathway is activated. It is speculated that this state, which can be induced by the expression of Tsh, Ey, and Hth at the edge of the eye disc, mimics the normal state of eye disc cells during the second instar, when the disc is growing most rapidly. Consistent with this idea, anterior hth expression in the eye disc is autonomously lost in dishevelled- (dsh-) clones, showing that these cells require wg signaling to maintain their anterior identity (Bessa, 2002).

Transcription factors often act in unique combinations to elicit distinct biological outputs. The combination examined here is Ey-Hth-Tsh. Because Hth and Tsh are also required for leg and wing development, Ey must make this combination specific for eye development. It is suggested that this combination of factors is used transiently during eye development to promote the proliferation of eye disc cells and to prevent the premature expression of later-acting transcription factors that are required for eye development. Consistent with this second role, ectopic expression of Hth blocks eye development. Similarly, forcing the expression of Ey can also interfere with eye development. The ability of these factors to repress eye development may in part be due to the ability of the Ey-Hth-Tsh combination to repress eya and dac (Bessa, 2002).

In addition to the functions suggested here, Ey is also important for promoting eye morphogenesis and has been called the master regulator of eye development. In fact, Ey is likely to be a direct activator of so. It is speculated that the eye-activating functions of Ey may be carried out in cells that express a different combination of transcription factors from those present in domain II. Cells in the PPN domain, for example, express Ey and Tsh, but not Hth. These cells also express so. It is therefore possible that Ey activates so in the PPN domain. In contrast, Ey-Hth-Tsh appears to repress eya, dac, and, by inference, so. Since all three of these factors are DNA-binding proteins, one possibility is that they are part of a specific DNA-binding complex that directly regulates these, as well as other, target genes in domain II. A different set of target genes may be regulated by Ey (+/-Tsh) in the absence of Hth. A second possibility is that the regulation observed in this study is indirect. Finally, the results are also consistent with a model in which Hth binds to Ey and blocks its ability to bind DNA. Such a mechanism has been proposed to account for repression of eye development by the Hox protein Antennapedia (Antp). Toy, a second Pax6 family member in flies, may also be part of the combinatorial control of eye development described here. An assessment of Toy's role is not possible at present, but will be important to characterize in the future (Bessa, 2002).

The progression of the MF across the eye is an elegant mechanism for gradually changing the combination of transcription factors as development proceeds. So, Eya, and Dac also have the ability to positively activate each other's expression, as is the case with Hth, Ey, and Tsh. Thus, both ahead of and behind the MF, eye disc cells are in different, but relatively stable states, in part because the factors expressed within these regions -- Hth, Tsh, and Ey in domain II and Eya, So, and Dac posterior to the MF -- can reinforce each other's expression. These two states are important for promoting proliferation and differentiation, respectively. Signals coming from the MF convert one state into another, and a key to flipping this switch is the repression of hth. Remarkably, in the vertebrate retina, Sonic hedgehog, a homolog of Drosophila Hh, is expressed in a wave-like fashion as retina cells differentiate. Furthermore, Pax6, the vertebrate ey homolog, is required to keep retinal cells multipotent: this is reminiscent of the uncommitted state of anterior cells in the fly eye disc. Given these intriguing parallels, it will be very interesting to determine if homologs of hth and tsh play analogous roles in the vertebrate retina before the initiation of differentiation (Bessa, 2002).

Drosophila CtBP regulates proliferation and differentiation of eye precursors and complexes with Eyeless, Dachshund, Dan, and Danr during eye and antennal development

Specification factors regulate cell fate in part by interacting with transcriptional co-regulators like CtBP to regulate gene expression. This study demonstrates that CtBP forms a complex or complexes with the Drosophila Pax6 homolog Eyeless (Ey), and with Distal antenna (Dan), Distal antenna related (Danr), and Dachshund to promote eye and antennal specification. Phenotypic analysis together with molecular data indicate that CtBP interacts with Ey to prevent overproliferation of eye precursors. In contrast, CtBP;dan;danr triple mutant adult eyes have significantly fewer ommatidia than CtBP single or dan;danr double mutants, suggesting that the CtBP/Dan/Danr complex functions to recruit ommatidia from the eye precursor pool. Furthermore, CtBP single and to a greater extent CtBP;dan;danr triple mutants affect the establishment and maintenance of the R8 precursor, which is the founding ommatidial cell. Thus, CtBP interacts with different eye specification factors to regulate gene expression appropriate for proliferative vs. differentiative stages of eye development (Hoang, 2010).

The eyes of several CtBP loss-of-function mutant combinations have statistically significantly more ommatidia than wild-type eyes, and CtBP87De-10 clones ahead of the furrow show a statistically significant increase in the number of cells undergoing mitosis per unit area. These results suggest a role for CtBP in downregulating proliferation of eye precursors, although it cannot be currently exclude that additional processes such as apoptosis that might contribute to the increase in eye size in CtBP- mutants (Hoang, 2010).

The evidence suggests that CtBP is required to downregulate proliferation of eye precursors ahead of the morphogenetic furrow: CtBP- clones are larger and show more mitotic figures than wild type clones in the most anterior regions of the eye field. A number of factors are known to promote proliferation ahead of the furrow, and have been connected to CtBP in some way. Many of these factors can cause massive overgrowth when overexpressed. Thus, the role of CtBP may be to counteract the activity of one or more of these factors, to ensure that cells do not proliferate out of control (Hoang, 2010).

Factors that have previously been linked to CtBP and are known to regulate proliferation of eye precursors ahead of the furrow include the Wingless, Notch and JAK/STAT signaling pathways. This study has not ruled out a possible interaction between CtBP and these signaling pathways in the context of eye precursor proliferation (Hoang, 2010).

However, the combination of Hth, Tsh and Ey also regulates proliferation in ahead of the morphogenetic furrow, and this study has demonstrated that Ey and CtBP are part of a complex in eye-antennal disc cells, and that they interact genetically during eye development. Circumstantial evidence also suggests links between CtBP and Hth and Tsh. For instance, Drosophila and mouse Tsh homologs both contain a PxDLS motif, and have been shown to interact in vitro with Drosophila and mouse CtBP, respectively. In addition, the Drosophila Cdc25 homolog encoded by string, which triggers mitosis, appears to be a target of Hth, although it is not clear if it is a direct target. DamID experiments with CtBP in Kc cells have also identified string as a potential direct CtBP target. Based on current data, it is therefore proposed that CtBP interacts with the Hth/Tsh/Ey complex in eye precursors ahead of the furrow (Hoang, 2010).

CtBP87De-10 single mutant clones ahead of the furrow in the larval eye disc are larger than wild-type clones and show evidence of eye precursor overproliferation. Accordingly, adult eyes of CtBP transheterozygous combinations or of CtBP87De-10/M mosaic individuals have more ommatidia than wild-type eyes. In contrast, whereas CtBP87De-10,dan,danrex56 triple mutant clones are similar to CtBP87De-10 single mutant clones in being larger than wild-type clones ahead of the furrow, CtBP87De-10,dan,danrex56 adults have small rough eyes. This suggests either that the CtBP87De-10,dan,danrex56 triple mutant cells fail to be efficiently recruited into ommatidia and/or eventually undergo apoptosis. In support of the former hypothesis, the phenotypic analysis demonstrates that recruitment of the R8 photoreceptor, which is required to recruit all other ommatidial cells, is affected by loss of CtBP, and is more strongly affected in the CtBP,dan,danr triple mutant (Hoang, 2010).

Given the dynamic and overlapping expression patterns of the retinal determination factors, one intriguing hypothesis that fits the data is that a complex containing CtBP may have different members at different stages of eye development. For instance, a complex containing CtBP, Ey as well as possibly Tsh and Hth anterior to the furrow, might participate in maintaining a 'poised' chromatin structure with respect to eye specific genes, in which genes involved in eye differentiation are not yet expressed and the cells are kept in a proliferative state. It has been suggested that vertebrate Pax6 is a 'pioneering' factor for the lens lineage, and other 'pioneering' factors have been shown to promote a 'bivalent' state in which developmental genes are silenced, but 'poised' for activation. The down-regulation of Ey close to the furrow, and the initiation of Dan, Danr and Dac expression in the same region would be expected to change the composition of the complex containing CtBP and lead to changes in transcription that reflect the transition from proliferation to differentiation (Hoang, 2010).

At present it is not known what genes might be direct targets of the complexes containing CtBP and the eye specification factors. Some possibilities include the cell cycle regulator string, and the pre-proneural gene atonal, which is known to be regulated by Ey, So, Dan and Danr (Ey and So are direct regulators), and which plays a critical role in the transition from proliferation to differentiation of eye precursors. Thus, future work on Drosophila CtBP will shed light on the functions of this important transcriptional regulator, as well as on important transitions during development (Hoang, 2010).

Conserved role for the Dachshund protein with Drosophila Pax6 homolog Eyeless in insulin expression

Members of the insulin family peptides have conserved roles in the regulation of growth and metabolism in a wide variety of metazoans. The Drosophila genome encodes seven insulin-like peptide genes, dilp1-7, and the most prominent dilps (dilp2, dilp3, and dilp5) are expressed in brain neurosecretory cells known as 'insulin-producing cells' (IPCs). Although these dilps are expressed in the same cells, the expression of each dilp is regulated independently. However, the molecular mechanisms that regulate the expression of individual dilps in the IPCs remain largely unknown. This study shows that Dachshund (Dac), which is a highly conserved nuclear protein, is a critical transcription factor that specifically regulates dilp5 expression. Dac was strongly expressed in IPCs throughout development. dac loss-of-function analyses revealed a severely reduced dilp5 expression level in young larvae. Dac interacted physically with the Drosophila Pax6 homolog Eyeless (Ey), and these proteins synergistically promoted dilp5 expression. In addition, the mammalian homolog of Dac, Dach1/2, facilitated the promoting action of Pax6 on the expression of islet hormone genes in cultured mammalian cells. These observations indicate the conserved role of Dac/Dach in controlling insulin expression in conjunction with Ey/Pax6 (Okamoto, 2012).

eyeless: Biological Overview | Evolutionary Homologs | Developmental Biology | Effects of Mutation | References

Home page: The Interactive Fly © 1997 Thomas B. Brody, Ph.D.

The Interactive Fly resides on the
Society for Developmental Biology's Web server.