apterous


DEVELOPMENTAL BIOLOGY

Embryonic

apterous is first expressed at 6 hours in a segmentally repeated subset of mesodermal cells in postions where ventral and mediolateral muscle precursors arise. The expression later disappears. apterous plays a role in embryonic muscle patterning, and mutations lead to a loss of a specific set of muscles. Misexpression of apterous causes development of ectopic muscles (Bourgouin, 1992). Many neural cells express apterous, including cells of the brain. In the ventral nerve cord (CNS), Apterous is detected at 10 hours when neurons begin extending axons. In neuroblasts apterous expression is reduced to only three cells per segment. The growth cones of the developing interneurons, derived from the apterous expressing neuroblasts, fasiculate preferentially to one other.

In addition to the interneurons, apterous is expressed in the PNS in four lateral cells of each thoracic hemisegment. The resulting axons eventually fasiculate the with anterior/posterior bundle (Lundgren, 1995).

islet interneurons belong to several different classes based on their morphology. Class I and II interneurons project either ipsi- or contralaterally and extend axons within the connectives, forming two discrete fascicles within the longitudinal connectives. A third class is composed of local interneurons that project across the midline and terminate contralaterally within the same segment. apterous is expressed in a small subset of ipsilaterally projecting interneurons that form a single fascicle in the connective, similar to the Class I and II islet interneurons (Lundgren, 1995). AP and ISL are expressed in nonoverlapping sets of neurons. In addition, both apterous and islet expressing neuronal subsets also project axons along different pathways (Thor, 1997).

The expression patterns of the murine genes Lhx2 and Msx1 and their Drosophila orthologs apterous and muscle-segment homeobox are described and compared. Lhx2 and Msx1 show complementary patterns of expression in most tissues, including the neural and cranial epithelium, pituitary gland, olfactory organs, and neural tube; in contrast, Lhx2 and Msx1 are coexpressed in the developing limbs. Strikingly, the spatial relationship between ap and msh expression in Drosophila is very reminiscent of the expression of their murine orthologs. ap and msh show complementary expression in the leg and antennal imaginal discs, and brain and ventral ganglion of the central nervous system (CNS), but both are coexpressed in the wing imaginal disc. These observations suggest conservation in the regulation of these genes between Drosophila and mice (Lu, 2000).

Lhx2 and Msx1 are found to be coexpressed in the progress zone of the developing mouse limbs. However, Lhx2 is excluded from the tip of the limb bud corresponding to the apical ectodermal ridge (AER), whereas Msx1 is expressed in this area. The Drosophila genes msh and ap also exhibit overlapping expression patterns in the wing imaginal disc, particularly within the dorsal compartment. msh is also expressed in the anterior mesopleura where ap is not expressed. Similar expression profiles are observed in haltere discs (Lu, 2000).

The spatial relationship between Lhx2 and Msx1 expression was examined in other embryonic areas. In contrast to the limbs, Lhx2 and Msx1 have reciprocal expression patterns in other regions, including mutually exclusive domains within the same tissue and juxtaposed domains in adjacent tissues. For instance, Msx1 and Lhx2 are both expressed in the olfactory epithelium, however Msx1 is restricted to the anterior region, while Lhx2 is in the posterior region that precisely fills the Msx1-negative area. Sagittal views reveal that Msx1 is expressed in the medial and lateral nasal processes, whereas Lhx2 seems to label the epithelium of the vomeronasal organ. In addition, Msx1 and Lhx2 are both expressed in the dorsal neural tube, however, Msx1 is prominent at the roof plate, whereas Lhx2 is directly lateral to the Msx1-positive zone in the marginal layer of the dorsal commissures (Lu, 2000).

The second type of reciprocal expression is that seen in adjacent tissue layers. For instance, Msx1 is expressed in the developing anterior pituitary, called Rathke's pouch, while Lhx2 is expressed in the base of the diencephalon and its infundibular evagination, which will form the posterior pituitary. In addition, Msx1 is expressed in the cranial epithelium, whereas Lhx2 is expressed in the underlying neural epithelium. Indeed, Lhx2 is absent in the roof between the telencephalic hemispheres, which is a region strongly labeled by Msx1 (Lu, 2000).

ap and msh reciprocal expression has been found in other Drosophila tissues. ap is expressed in a ring-like domain corresponding to the presumptive fourth tarsal segment of the leg discs, whereas msh is expressed in two arc-like domains that flank the ap territory. In the eye-antennal disc, msh is broadly expressed in the eye portion of the disc and in several ring-like domains within the antenna region, the stronger of which is found in the second antennal segment. In contrast, ap is specifically expressed in the center of the antenna disc, where it shares discrete areas of overlapping and complementary expression with msh. It was also found that ap and msh have mutually exclusive domains within the brain and ventral ganglion. Thus the data provide previously unreported expression profiles for msh in Drosophila larval stages, and uncover a conserved spatial relationship between the expression of msh/ap and Msx1/Lhx2 genes during evolution (Lu, 2000).

Analysis was made of the expression and function of Apterous during embryonic brain development of Drosophila. Expression of Ap in the embryonic brain begins at early stage 12 and is subsequently found in approximately 200 protocerebral neurons and in 4 deutocerebral neurons. Brain glia do not express Ap. Most of the Ap-expressing neurons are interneurons and project their axons across the midline to the contralateral hemisphere; members of a smaller subset project their axons into the ventral nerve cord. A few Ap-expressing neurons project to the ring gland, suggesting that these neurons are neurosecretory cells. In ap loss-of-function mutants, some of the protocerebral and deutocerebral interneurons that express Ap in the wild type show axon pathfinding errors and fasciculation defects in the brain, notably in the fascicles of the brain commissure. In contrast, the interneurons that project to the ring gland do not appear to be affected in ap mutants. Thus, in brain development, Ap is required for correct axon guidance and fasciculation of interneurons, and Ap-expressing cells may also be involved in the brain neuroendocrine system (Herzig, 2001).

Expression of Ap is first seen in the embryonic protocerebrum in approximately 80 cells at early stage 12. Expression of Ap in the embryonic brain is restricted to the protocerebrum until stage 13. At stage 14, approximately 130 Ap-expressing cells are found in the anterior part of the protocerebrum. Additionally, two pairs of cells in the deutocerebrum begin to express Ap; these cells are initially located at the outer surface of the developing deutocerebrum, proximal to the developing antennal nerve. Subsequently, during the process of head involution and germband retraction, these deutocerebral Ap-expressing cells move inward and by the end of embryogenesis are located on the inner surface of the developing deutocerebrum, in close association with the developing frontal connective. Their number remains constant until the end of embryogenesis, whereas the number of Ap-expressing cells in the protocerebrum increases to approximately 200 in stage-17 embryos. In the tritocerebrum, no Ap expression is seen during embryogenesis. Thus, in contrast to the metameric expression of Ap in each of the neuromeres of the VNC, Ap expression in the anterior brain is restricted to the protocerebral and deutocerebral neuromeres (Herzig, 2001).

In order to visualize the neuronal processes of Ap-expressing cells and to further document their localization in the embryonic brain, the GAL4-UAS system consisting of an apGAL4 driver and a UAS-tau-lacZ reporter was used. Double labeling of apGAL4/UAS-tau-lacZ embryos with anti-AP and anti-TAU-ß-GAL antibodies has revealed nearly perfect coexpression of Ap and tau-ß-gal in the embryonic brain. In the protocerebrum only a few neurons ectopically express apGAL4-driven tau-ß-gal and in the deutocerebrum there is no ectopic expression at all. This indicates that within the CNS, apGAL4-driven reporter gene expression represents the endogenous Ap expression pattern (Herzig, 2001).

An analysis of the projection patterns of the Ap neurons in apGAL4/UAS-tau-lacZ embryos indicates that most of the Ap-expressing neurons in the brain are interneurons; with the exception of putative neurosecretory processes, none of the labeled axons extends out of the CNS. The majority of the labeled axons are seen within the protocerebral hemineuromeres or projecting across the primary brain commissure at the level of the protocerebrum. The labeled axons in the primary brain commissure run in several distinct fascicles including one large fascicle and at least four smaller fascicles. Descending Ap axons are also seen projecting from some of the Ap-expressing protocerebral neurons through the deutocerebrum and tritocerebrum into longitudinal pathways of the VNC connectives. In the VNC connectives, these protocerebral axons project in close association with labeled axons from Ap-expressing VNC interneurons. The two pairs of Ap-expressing neurons in the deutocerebrum send processes towards the primary brain commissure; however, these do not cross the midline of the embryonic brain. In the developing VNC, as in the brain of the embryo, most of the Ap neurons are also interneurons. However, in their axonal projection patterns, the Ap-expressing VNC interneurons differ from those in the brain in that they project ipsilaterally within the longitudinal connectives and fasciculate tightly with their ipsilateral segmental homologs without ever crossing the midline (Herzig, 2001).

Double labeling of apGAL4/UAS-tau-lacZ embryos with anti-EN and anti-TAU-ß-GAL antibodies demonstrates that the Ap-expressing protocerebral interneurons are located anterior to the protocerebral En-expressing cells; it also demonstrates that the deutocerebral Ap-expressing cells are located at the same anteroposterior level as the deutocerebral En-expressing cells. The localization of Ap-expressing neurons in the brain is characterized further by double-labeling experiments of apGAL4/UAS-tau-lacZ embryos with anti-Empty spiracles (Ems) and anti-TAU-ß-GAL antibodies. The head gap gene ems is expressed in the anterior deutocerebrum and the anterior tritocerebrum. Deutocerebral Ap expression is seen at a level anterior to the tritocerebral expression domain of Ems and posterior to the deutocerebral Ems expression, thus assigning these pairs of Ap-expressing cells to the posterior deutocerebrum (Herzig, 2001).

To characterize sub-populations of Ap-expressing cells in the proto- or deuto-cerebrum further, double-labeling experiments of apGAL4/UAS-tau-lacZ embryos were carried out with anti-Eyeless (Ey) and anti-TAU-ß-GAL antibodies. Recent studies have shown that the pax gene eyeless is expressed in specific subsets of each embryonic brain neuromere as well as in the embryonic mushroom body primordia. Only a limited set of Ap-expressing cells overlap with Ey expression in the medial and lateral parts of the anteriormost protocerebrum. In particular, the position of a cluster of cells coexpressing Ap and Ey coincides with the position of the developing mushroom bodies, suggesting that Ap is expressed in the progeny of the embryonic mushroom body neuroblasts (Herzig, 2001).

In addition to the numerous Ap-expressing interneurons, a small set of Ap-expressing cells projects axons out of the protocerebrum through the nervi corporis cardiaci into the developing ring gland. The ring gland is a neurosecretory structure that consists of the ventral corpora cardiaca, the medial thoracic glands and the dorsal corpus allatum. By combining fasciclin II immunostaining, which labels both the developing ring gland and the developing nervi corporis cardiaci, with apGAL4/UAS-tau-lacZ labeling, a small fascicle of axons deriving from Ap-expressing protocerebral neurons can be seen to project from each half of the protocerebrum along the nervi corporis cardiaci towards the developing ring gland. This axon fascicle reaches the ventral part of the developing ring gland by the end of embryogenesis. The neuronal innvervation of the neurosecretory ring gland by Ap-expressing neurons suggests that these neurons are in fact protocerebral neuroendocrine cells. Corresponding with this notion, the ap gene is also expressed in the dFMRFa-positive SP2 interneuron of the developing protocerebrum. Similarly, studies on the role of Ap in the VNC indicate that a subset of the Ap-expressing cells there are also neuroendocrine. In the VNC, Ap contributes to the initiation and maintenance of expression of the FMRF neuropeptide gene in the neuroendocrine Tv neurons, which project to the neurohemal organs (Herzig, 2001).

Larval

apterous expression is not detected in early second instar wing discs, but is activated in its apparently mature pattern in early-mid second instar discs. The early expression of apterous in a dorsally restricted pattern indicates that a D/V boundary exists in early second instar discs, even before the developing disc field contains 200 cells (Williams, 1993).

Adult

Mechanisms composing Drosophila's clock are conserved within the animal kingdom. To learn how such clocks influence behavioral and physiological rhythms, the complement of circadian transcripts in adult Drosophila heads was determined. High-density oligonucleotide arrays were used to collect data in the form of three 12-point time course experiments spanning a total of 6 days. Analyses of 24 hr Fourier components of the expression patterns revealed significant oscillations for ~400 transcripts. Based on secondary filters and experimental verifications, a subset of 158 genes showed particularly robust cycling and many oscillatory phases. Circadian expression is associated with genes involved in diverse biological processes, including learning and memory/synapse function, vision, olfaction, locomotion, detoxification, and areas of metabolism. Data collected from three different clock mutants (per0, tim01, and ClkJrk), are consistent with both known and novel regulatory mechanisms controlling circadian transcription (Claridge-Chang, 2001).

A genome-wide expression analysis was performed aimed at identifying all transcripts from the fruit fly head that exhibit circadian oscillations in their expression. By taking time points every 4 hr, a data set was obtained that has a high enough sampling rate to reliably extract 24 hr Fourier components. Time course experiments spanning a day of entrainment followed by a day of free-running were performed to take advantage of both the self-sustaining property of circadian patterns and the improved amplitude and synchrony of circadian patterns found during entrainment. 36 RNA isolates from wild-type adult fruit fly heads, representing three 2 day time courses, were analyzed on high-density oligonucleotide arrays. Each array contained 14,010 probe sets (each composed of 14 pairs of oligonucleotide features) including ~13,600 genes annotated from complete sequence determination of the Drosophila genome. To identify different regulatory patterns underlying circadian transcript oscillations, four-point time course data was colleced from three strains of mutant flies with defects in clock genes (per0, tim01, and ClkJrk) during a single day of entrainment. Because all previously known clock-controlled genes cease to oscillate in these mutants but exhibit changes in their average absolute expression levels, the analysis of the mutant data was focused on changes in absolute expression levels rather than on evaluations of periodicity (Claridge-Chang, 2001).

To organize the 158 statistically significant circadian transcripts in a way that was informed by the data, hierarchical clustering was performed. Both the log ratio wild-type data (normalized per experiment) and the log ratios for each of the three clock mutants (normalized to the entire data set) were included to achieve clusters that have both a more or less uniform phase and a uniform pattern of responses to defects in the circadian clock. One of the most interesting clusters generated by this organization is the per cluster. This cluster contains genes that have an expression peak around ZT16 and a tendency to be reduced in expression in the ClkJrk mutant. Strikingly, all genes previously known to show this pattern of oscillation (per, tim, vri) are found in this cluster (Claridge-Chang, 2001).

All genes of the apterous (ap) cluster are defined by both the oscillatory phase of their expression pattern (average phase ZT17) and by a distinct expression profile in the three clock mutants. Although the 6 hr sampling interval in the mutant data makes it difficult to reliably detect oscillations, it seems that the majority of the genes in this cluster shows some degree of periodicity in the three mutant light-dark regime (LD) time courses. Although it cannot be ruled out that there are circadian oscillations independent from the known clock genes, the hypothesis that there may be a light-driven response underlying the observed mutant expression pattern is favored. The genes in this group may, therefore, be regulated not only by the circadian clock, but also by a direct light-dependent mechanism. It should be mentioned that evidence of gene expression patterns that are purely light-driven in wild-type flies was sought, but little indication was found of such regulation. Instead, genes with both a strong light-driven oscillation and a weak circadian component were encountered. apterous (ap) encodes a LIM-homeobox transcription factor, which is known to act both in neural development and in neuropeptide expression. The ap cluster includes the genes for the transcription factor moira, the synaptic regulator syndapin, two septins (Sep1 and CG9699), and two ATP binding cassette (ABC) transporters (CG6162, CG9990). In terms of chromosomal organization, CG6166, the gene adjacent to CG6162 on chromosome 3R is homologous to CG9990 and coregulated with CG6162 and CG9990 (Claridge-Chang, 2001).

Effects of Mutation and Ectopic Expression

The developing wing disc of Drosophila is divided into distinct lineage-restricted compartments along both the anterior/posterior (A/P) and dorsal/ventral (D/V) axes. At compartment boundaries, morphogenic signals pattern the disc epithelium and direct appropriate outgrowth and differentiation of adult wing structures. However, the mechanisms by which affinity boundaries are established and maintained are not completely understood. Compartment-specific adhesive differences and inter-compartment signaling have both been implicated in this process. The selector gene apterous is expressed in dorsal cells of the wing disc and is essential for D/V compartmentalization, wing margin formation, wing outgrowth and dorsal-specific wing structures. To better understand the mechanisms of Ap function and compartment formation, aspects of the ap mutant phenotype have been rescued with genes known to be downstream of Ap. Fringe, a secreted protein involved in modulation of Notch signaling, is sufficient to rescue D/V compartmentalization, margin formation and wing outgrowth when appropriately expressed in an ap mutant background. When Fng and alphaPS1 (Multiple edematous wings, a dorsally expressed integrin subunit) are co-expressed, a nearly normal-looking wing is generated. However, these wings are entirely of ventral identity. These results demonstrate that a number of wing development features, including D/V compartmentalization and wing vein formation, can occur independently of dorsal identity and that inter-compartmental signaling, refined by Fng, plays the crucial role in maintaining the D/V affinity boundary. In addition, it is clear that key functions of the ap selector gene are mediated by only a small number of downstream effectors (O’Keefe, 2001).

These results suggest that intercompartmental signaling is sufficient to maintain the D/V affinity boundary. In the absence of dorsal identity, compartmental defects associated with ap mutant wing discs can be rescued with the molecule Fng. This argues that signaling between compartments mediated by Fng and Notch, and not the autonomous acquisition of compartment-specific affinity as an aspect of cell identity, plays the crucial role in D/V compartmentalization. Consistent with this are previous findings that both fng and Notch mutant clones generated in the dorsal compartment do not respect the D/V boundary, despite the fact that they likely retain dorsal identity. While the ap alleles used in this study are not molecularly-defined nulls, these allelic combinations clearly reduce Ap function sufficiently to eliminate dorsal identity. Based on both sensory bristle and wing vein morphologies, the Fng and Fng+alphaPS1-rescued wings consist entirely of ventral cell types. The possibility cannot be excluded that these ap allelic combinations might maintain small degrees of dorsal-specific affinity (independent of dorsal identity): the mutant phenotypes indicate that any adhesive differences are clearly not sufficient to maintain D/V compartmentalization (O’Keefe, 2001).

Prospective wing vein cells are identifiable in late third instar wing discs by molecular markers such as rhomboid. Wing disc eversion results in apposition of dorsal and ventral vein components, and interplanar signaling between the dorsal and ventral wing surfaces has been shown to play a crucial role in wing vein differentiation. Clonal analysis has demonstrated that mutations that disrupt or alter vein formation, frequently have non-autonomous effects on the opposite surface, and that these effects are particularly dramatic when the genetic clone lies on the dorsal surface. These results suggest a dorsal-specific signal that induces differentiation of ventral veins. However, when forced to differentiate without interplanar signaling, vein structures are capable of forming on both surfaces, although these veins are defective in terms of refinement and their pattern of corrugation. In the Fng+alphaPS1-rescued wing there is no dorsal identity and, therefore, no dorsal-specific signal directing ventral vein differentiation. Despite this abnormality, vein components on both surfaces differentiate appropriately based on their A/P and proximal/distal position in the wing; these vein components have an entirely ventral identity. This demonstrates that wing vein refinement, alignment and pattern of corrugation can occur independently of dorsal cell types. Although interplanar signaling is certainly essential for proper wing vein differentiation, it is clear that a dorsal-to-ventral signal is not required, and that ventral cell types autonomously contain all the information necessary for wing vein development (O’Keefe, 2001).

An emerging view of selector gene function is that these genes may regulate large numbers of effector genes involved in particular morphogenetic processes. For example, in the differentiation of Drosophila haltere from wing, the transcription factor Ultrabithorax regulates genes at many levels of the wing patterning genetic cascade. So too, the selector homeoproteins Even-skipped and Fushi tarazu (Ftz) have been shown to regulate either directly or indirectly most genes during embryogenesis. However, fusion of the VP16 activation domain to Ftz has suggested that Ftz binds to and regulates only a small number of target genes. The question is therefore unanswered as to whether the number of genes regulated by selectors is large or small. In the absence of normal ap selector gene function, the expression of only two downstream effectors is sufficient to rescue wing structures to a remarkable degree. This result suggests that the compartment-specific selector gene ap regulates only a small number of target genes during wing development. It will be interesting to determine whether selector genes with broader scopes of activities function in a similar manner. Selectors that control the formation of entire structures (such as eyeless) or entire body regions (such as the Hox genes) presumably sit at the top of larger genetic hierarchies than ap, and may control larger sets of target genes to fulfill their developmental roles (O’Keefe, 2001).

Finally, although ap regulates only a small number of downstream effectors to generate the overall morphology of the wing, it may indeed regulate many genes to confer dorsal identity. It is tempting to speculate, however, that Ap may regulate only one additional gene, Dorsal wing (Tiong, 1995), in order to specify dorsal cell fate in the wing. Loss-of-function mutations in the Drosophila Dorsal wing locus result in dorsal-to-ventral transformations in the wing blade, and ventral misexpression of Dorsal wing produces ectopic dorsal structures (Tiong, 1995). While the gene corresponding to this phenotype has yet to be characterized, Dorsal wing likely forms a crucial component of Ap-dependent wing developmental processes (O’Keefe, 2001).

apterous, expressed in the ring gland (Botas, J. personal communication, 1996), has been implicated in the juvenile hormone system because mutations in apterous lead to hormone deficiency, defective histolysis of the larval fat body, arrested vitellogenesis, sterility, and aberrant sexual behavior, all of which are dependent on juvenile hormone (Shtorch, 1995).

Chip may encode an enhancer-facilitator, acting to facilitate the activity of distal enhancers. The mechanisms allowing remote enhancers to regulate promoters several kilobase pairs away are unknown but are blocked by the Drosophila suppressor of Hairy-wing protein [su(Hw)] that binds to gypsy retrovirus insertions between enhancers and promoters. su(Hw) bound to a gypsy insertion in the cut gene also appears to act interchromosomally to antagonize enhancer-promoter interactions on the homologous chromosome when activity of the Chip gene is reduced. Chip is needed for the wing margin enhancer of cut. The Chip mutation dominantly enhances the mutant phenotypes displayed by partially suppressed gypsy insertions in both cut and Ultrabithorax and is a homozygous larval lethal, indicating that Chip regulates multiple genes. Chip is normally required for wing margin enhancer function of cut because Chip mutations also enhance the cut wing phenotype of a cut mutation and heterozygotes for Chip display cut wing phenotypes when either scalloped or mastermind (mam) are also heterozygous mutant. Both Sc and Mam are known to regulate the cut distal enhancer, but in contrast to sd and mam mutants, Chip mutants display stronger genetic interactions with gypsy insertions than with wing margin enhancer deletions. Thus, in a heterozygous Chip mutant, a heterozygous gypsy insertion in cut displays a cut wing phenotype, whereas a heterozygous enhancer deletion does not. Dependence on the nature of the heterozygous lesion in the regulatory region strongly suggests that Chip directly regulates cut. More strikingly, it indicates that in a Chip heterozygote, a gypsy insertion is more deleterious to enhancer function than deletion of the enhancer. The simplest explanation is that su(Hw) bound to gypsy in one cut allele acts in a transvection-like manner (interchromosomally) to block the wing enhancer in the wild-type cut allele on a second chromosome. This implicates Chip in enhancer-promoter communication (Morcillo, 1997 and references).

Chip was cloned and found to encode a homolog of the recently discovered mouse Nli/Ldb1/Clim-2 and Xenopus Xldb1 proteins, which bind nuclear LIM domain proteins. Chip protein interacts with the LIM domains in the Apterous homeodomain protein, and Chip interacts genetically with apterous, showing that these interactions are important for Apterous function in vivo. Importantly, Chip also appears to have broad functions beyond interactions with LIM domain proteins. Chip is a ubiquitous chromosomal factor required for normal expression of diverse genes at many stages of development. It is suggested that Chip cooperates with different LIM domain proteins and other factors to structurally support remote enhancer-promoter interactions (Morcillo, 1997).

LIM domains are found in a variety of proteins, including cytoplasmic and nuclear LIM-only proteins, LIM-homeodomain (LIM-HD) transcription factors and LIM-kinases. Although the ability of LIM domains to interact with other proteins has been clearly established in vitro and in cultured cells, their in vivo function is unknown. Drosophila was used to test the roles of the LIM domains of the LIM-HD family member Apterous (Ap) in wing and nervous system development. Within the embryonic ventral nerve cord (VNC), ap is expressed by three of the approximately 200 neurons in each abdominal hemisegment. Using promoter fusions to the axon-targeting tau-lacZ reporter, it has been shown that the ap-expressing neurons are interneurons that extend axons ipsilaterally and anteriorly along a single pathway within each longitudinal connective. Upon reaching the adjacent anterior segment the Ap neurons tightly fasciculate with their homologs, forming a discrete axon bundle running the length of the VNC. In ap P44 mutant embryos, the ap neurons fail to recognize their appropriate pathway and instead wander within the connective, failing to fasciculate with one another (O'Keefe, 1998).

Using a rescuing assay of the ap mutant phenotype, the LIM domains were found to be essential for Ap function. Expression of LIM domains alone can act in a dominant-negative fashion to disrupt Ap function. The Ap LIM domains can be replaced by those of another family member to generate normal wing structure, but LIM domains are not interchangeable during axon pathfinding of the Ap neurons. ap GAL4 /UAS-ap-mediated phenotypic rescue, in which ap promoter is used to express the UAS-ap transgene in wings and CNS, was used to ask whether the Ap LIM domains are required for function. To generate ApdeltaLIM, the LIM domains were specifically delimited, and the rest of the protein left intact. This Ap derivative is unable to rescue any element of the ap phenotype when expressed using ap GAL4 in ap mutant cells, although ApdeltaLIM protein is present at high levels and is properly localized to the nucleus as assayed with the anti-Ap antibody. In ap GAL4 /ap P44 ; UAS-ap deltaLIM/+ adults, the wings remain ribbon-like outgrowths, devoid of any identifiable structures. Using the UAS-tau-lacZ transgene, the ap neurons were found to remain defasciculated, indistinguishable from those of ap GAL4 /ap P44 mutant individuals. Therefore, the LIM domains are essential for ap function. The homeodomain is also shown to be required for ap function. The ApdeltaHD protein, lacking the homeodomain, acts in a dominant-negative fashion to disrupt residual ap function. The ability to act as a dominant-negative inhibitor of Ap function in the wing is not restricted to the Ap LIM domains. In contrast to the wing, neither ApdeltaHD nor IsletdeltaHD has any dominant effects on the development of the ap neurons within the CNS. This suggests that for these two developmental processes, generation of wing structures and axon pathfinding, there are differences in the protein interactions involving the Ap LIM domains. A test was performed to determine whether the LIM domains of a different LIM-HD family member might be interchangeable with those of Ap. Drosophila Lim3 (S. Thomas, S. G. E. Andersson, A. Tomlinson and J. B. Thomas, unpublished), which is normally expressed in subsets of post-mitotic neurons, none of which co-express Ap, was chosen. Conservation of LIM domain sequence within the LIM-HD family ranges from 25% to 86% aa identity. Ap and Lim3 are relatively divergent, sharing only 37% identity within the LIM domains. Expression of Lim3 in Ap cells results in lethality during larval or early pupal stages. Lim3 can partially rescue the ap wing phenotype. Lim3 also partially rescues the ap neuronal pathfinding defects. The axons are more highly fasciculated than those in ap mutants, but 74% of the segments still display clear pathfinding errors. Thus, although Lim3 promotes some degree of axon fasciculation and the formation of a rudimentary wing margin in ap mutants, it is not interchangeable with Ap (O'Keefe, 1998).

To determine whether LIM domains are interchangeable between Lim3 and Ap, a fusion was created between the N-terminal half of Lim3, including the LIM domains, to the C-terminal half of Ap, containing the homeodomain. This Lim3:Ap chimera rescues the ap wing phenotype to the same extent as full-length Ap. Thus, interchanging the LIM domains has no effect on formation of the Ap-Chip complex in the generation of wing structure. During pathfinding of the Ap neurons, the Ap LIM domains are involved in additional protein interactions independent of Chip. These interactions are specific to the Ap LIM domains and cannot be mediated by the Lim3 LIM domains. Taken together, this data suggests that LIM domains mediate different types of protein interactions in different developmental processes and that LIM domains can participate in conferring specificity of target gene selection (O'Keefe, 1998).

The unique expression of Lim1 in a subset of motoneurons and interneurons led to an examination of whether this expression overlaps with other LIM homeodomain members. Characterization of the expression of a group of vertebrate LIM homeodomain genes (Isl-1, Isl-2, Lim-1 and Lim-3) along the chick spinal column has demonstrated that these genes overlap with one another in very distinct patterns (Tsuchida, 1994). Their spatial overlap demarcates regions of motorneuron subclasses, suggesting that the LIM homeodomain genes confer an identity to pools of motorneurons by their combinatorial expression. More recently, a combinatorial code for motorneuron pathway selection has been demonstrated for isl and lim3 in Drosophila. In order to evaluate the expression of Lim1 with respect to its LIM homeodomain relatives, a series of double labeling experiments were carried out using late stage embryos. These embryos carried enhancers from either islet, lim3 or apterous that recapitulated their expression using a tau-LacZ or tau-c-myc fusion construct as a reporter. By double staining for enhancer expression and the Lim1 protein, no overlap of expression was observed between Lim1 and Isl, Ap or Lim3. All neurons that stain positively for Lim1 in the nuclei lack enhancer expression within their cell bodies. Similar to what is observed in vertebrates, the LIM homeodomain genes that were analyzed in Drosophila are expressed in distinct subclasses of neurons within the ventral nerve cord. The expression of Lim1 is confined to a subset of motorneurons and interneurons that are lacking the other LIM homeodomain genes tested. To assess the possibility that the absence of Lim1 in cells expressing other LIM homeodomain proteins was due to repression by these family members, the expression of Lim1 was analyzed in ap and lim3 mutant embryos. In embryos with a null mutation in ap, the expression of Lim1 remains unchanged. Likewise, analysis of the Lim3-expressing RP neurons in lim3 mutants shows no upregulation of Lim1. These results indicate that the exclusion of Lim1 from cells expressing other LIM homeodomain proteins is not a result of repression by these LIM homeodomain family members. In addition to the exclusive expression of Lim1, Ap and Isl do not overlap, while Lim3 fails to overlap with Ap, but is found in a subset of Isl positive cells. Thus, as in vertebrates, the expression of these genes in the Drosophila nerve cord may provide instructional cues for proper pathfinding and target identity in the embryo (Lilly, 1999 and references therein).

LIM-homeodomain transcription factors are expressed in subsets of neurons and are required for correct axon guidance and neurotransmitter identity. The LIM-homeodomain family member Apterous requires the LIM-binding protein Chip to execute patterned outgrowth of the Drosophila wing. To determine whether Chip is a general cofactor for diverse LIM-homeodomain functions in vivo, its role in the embryonic nervous system was studied. Loss-of-function Chip mutations cause defects in neurotransmitter production that mimic apterous and islet mutants. Chip is also required cell-autonomously by Apterous-expressing neurons for proper axon guidance, and requires both a homodimerization domain and a LIM interaction domain to function appropriately. Using a Chip/Apterous chimeric molecule lacking domains normally required for their interaction, the complex was reconstituted and the axon guidance defects of apterous mutants, of Chip mutants and of embryos doubly mutant for both apterous and Chip were rescued. These results indicate that Chip participates in a range of developmental programs controlled by LIM-homeodomain proteins and that a tetrameric complex comprising two Apterous molecules bridged by a Chip homodimer is the functional unit through which Apterous acts during neuronal differentiation (van Meyel, 2000).

Chip is expressed in most, if not all, embryonic and larval tissues. In wild-type embryos, strong, nuclear Chip expression is found throughout the developing VNC with no apparent subclasses of neurons excluded. A substantial fraction of embryonic Chip is contributed maternally during oogenesis, and this maternally derived expression is required for early embryonic segmentation. To estimate the relative contribution of zygotic and maternally derived Chip to the embryonic VNC, homozygous embryos mutant for a Chip null allele were examined. Derived from an intercross of heterozygous parents, mutants are expected to retain half the maternal and not any zygotic Chip expression. Little reduction of staining in mutant embryos is observed, relative to Chip/+ heterozygotes. Thus it appears a substantial fraction of Chip in the VNC is provided maternally. Co-labelling embryos with anti-Ap and anti-Chip antibodies reveals that Chip expression overlaps with all the Ap neurons of the developing VNC (van Meyel, 2000).

If Chip were required for Ap function, elimination of Chip might be expected to result in an ap-like phenotype. The requirement of maternally supplied Chip in segmentation precluded an examination of the effects of eliminating both maternal and zygotic Chip on neuronal development. Thus, neurotransmitter expression and axon guidance were examined in Chip mutants in which half of the maternal and all of the zygotic Chip expression were absent. In each thoracic hemisegment of the VNC, ap is expressed in a lateral cluster of four neurons, one of which is the Tv neuroendocrine cell that expresses the neurotransmitter dFMRFa. In wild-type embryos, there are a total of six Tv cells, one in each thoracic hemisegment. In ap mutants, the Tv neurons are present, but half of all Tv neurons stochastically fail to express dFMRFa. This regulation of dFMRFa by ap is transcriptional, since expression of a fusion transgene comprising a 446 bp Tv neuron-specific enhancer of the dFMRFa gene driving beta-galactosidase (Tv-lacZ) is similarly reduced in ap mutants. Ap binds in vitro to each of three sequences within the enhancer, and mutagenesis of these sites has confirmed that these sequences are important for Tv-lacZ expression in vivo (van Meyel, 2000).

To determine whether reduction of Chip results in an ap-like reduction in transcriptional activation of dFMRFa, expression of the Tv-lacZ reporter transgene was assayed in wild-type, ap and Chip mutant embryos. Both ap and Chip mutant embryos show decreased Tv-lacZ activity in Tv neurons relative to wild-type controls, implicating Chip in the establishment of this Ap-regulated aspect of neuronal differentiation. The reduction of Tv-lacZ activity is less severe in Chip null mutants than ap null mutants, probably because of the maternally supplied Chip remaining in Chip mutants. In embryos homozygous for an antimorphic Chip mutation, Tv-lacZ expression is reduced further than Chip null mutants but not to the level of ap mutants (van Meyel, 2000).

Like Ap, the LIM HD protein Isl also regulates neurotransmitter identity of embryonic neurons. There are three dopaminergic cells per segment of the VNC, one unpaired midline cell and a pair of dorsal lateral cells, all of which express Isl protein and thus represent a subset of the isl interneurons. isl mutants show loss of expression of tyrosine hydroxylase (TH), a rate-limiting enzyme in the synthesis of dopamine. To test the role of Chip in the expression of TH, late-stage wild-type and Chip mutant embryos were stained with anti-TH antibodies. Homozygous Chip mutant embryos retain TH expression in the ventral unpaired midline cells, but few of the dorsal lateral cells express TH, and in those that do, TH levels are significantly reduced relative to wild-type. In embryos homozygous for a Chip antimorph, TH expression is greatly diminished in both the ventral midline and dorsal lateral dopaminergic neurons. While it is clear that the paired dorsal TH cells are more sensitive to the reduction in Chip dosage than the unpaired ventral cells, the effects of the antimorphic Chip allele suggest that TH production in the latter cells is also dependent on Chip. From these results, together with the above results on the expression of FMRFamide, it is concluded that Chip is required for both Ap- and Islet-regulated neurotransmitter production in the CNS (van Meyel, 2000).

The Notch pathway plays a crucial and universal role in the assignation of cell fates during development. In Drosophila, Notch is a transmembrane protein that acts as a receptor of two ligands, Serrate and Delta. The current model of Notch signal transduction proposes that Notch is activated upon binding its ligands and that this leads to the cleavage and release of its intracellular domain (also called Nintra). Nintra translocates to the nucleus where it forms a dimeric transcription activator with the Su(H) protein. In contrast with this activation model, experiments with the vertebrate homolog of Su(H), CBF1, suggest that, in vertebrates, Nintra converts CBF1 from a repressor into an activator. The role of Su(H) in Notch signaling during the development of the wing of Drosophila has been assessed. The results show that, during this process, Su(H) can activate the expression of some Notch target genes and that it can do so without the activation of the Notch pathway or the presence of Nintra. In contrast, the activation of other Notch target genes requires both Su(H) and Nintra, and, in the absence of Nintra, Su(H) acts as a repressor. The Hairless protein interacts with Notch signaling during wing development and inhibits the activity of Su(H). These results suggest that, in Drosophila, the activation of Su(H) by Notch involves the release of Su(H) from an inhibitory complex, which contains the Hairless protein. After its release Su(H) can activate gene expression in the absence of Nintra (Klein, 2000).

An examination was performed to see whether the degree of endogenous Su(H) activation that results from the removal of H is sufficient to elicit a biological effect. To assay this, it was asked whether or not removal of H activity can induce Su(H)-dependent development of the pouch in wing discs in which Notch signaling is absent, such as apterous and Presenilin mutant wing discs. Loss of H function rescues the loss of wing development of ap mutants: whereas ap mutants have no wing pouch, ap;H double mutants have large wing pouches with no margin structures. The enlarged pouch of the double mutant discs expresses spalt (sal) and the two vg reporters, vgQE and vgBE, all of which are expressed specifically in the wing pouch in a Notch/Su(H)-dependent manner and are not expressed in ap mutants. In contrast, no wg expression is induced in these double mutant discs, suggesting that the observed rescue is likely to be due to the activation of Su(H) in the double mutants. This is strongly supported by the fact that Su(H);H double mutants exhibit a small wing rudiment identical to that of Su(H) mutants. Expression of UAS-vg by dpp-Gal4 in ap mutant discs can recover the pouch-specific expression domain of sal, suggesting that the activation of vg expression by Su(H) is responsible for the recovered sal expression in the ap;H double mutant wing discs. Similar to overexpression of UAS-Su(H) in ap mutant wing discs, the pouch in ap;H mutant discs develops near the residual wg expression in the remaining hinge. As expected from the analysis of the wing discs, the pharate adult ap;H double mutants have large wing pouches, which are devoid of any margin like structure such as innervated bristles (Klein, 2000).

The effects on wing development of removing H in Psn mutants were examined. As in the case of ap, loss of function of H effects a strong rescue of the wing pouch in the Psn;H mutant discs in comparison to the Psn mutant discs. However, in this case, the morphology of the discs is more like wild type and, in contrast to ap;H mutant discs, the pouch develops at its normal place. Closer monitoring of double mutant discs reveals some expression of wg and the vgBE along the DV boundary. This suggests that, in contrast to the situation of ap mutants, in Psn mutants, there is some activation of Notch and it seems that the lack of H activity can enhance this residual signaling of Notch at the DV boundary. This is remarkable considering that the wing phenotype caused by the loss of Psn is stronger than that caused by loss of Su(H) function. Taken together, these results provide further evidence for a positive transcriptional activity of Su(H). They further show that H is an antagonist of Su(H) during early wing development and that it suppresses the activity of Su(H) in the absence of Notch signaling. The results also suggest that the inactivation of H is sufficient to activate Su(H) and that the activity of Notch is required to inactivate H during normal development (Klein, 2000).

Compartment formation is a developmental process that requires the existence of barriers against intermixing between cell groups. In the Drosophila wing disc, the dorso-ventral (D/V) compartment boundary is defined by the expression of the apterous selector gene in the dorsal compartment. Ap activity is under control of dLMO (Beadex) which destabilizes the formation of the Ap-Chip complex. D/V boundary formation in the wing disc also depends on early expression of vestigial. These data suggest that vg is already required for wing cell proliferation before D/V compartmentalization. In addition, over-expression of vg can, to some extent, rescue the effect of the absence of ap on D/V boundary formation. Early Vg product regulates Ap activity by inducing dLMO and thus indirectly regulating ap target genes such as fringe and the PSalpha1 and PSalpha2 integrins. It is concluded that normal cell proliferation is necessary for ap expression at the level of the D/V boundary. This would be mediated by vg, which interacts in a dose-dependent way with ap (Delanoue, 2002).

Short-range cell interactions and cell survival in the Drosophila wing

During development of multicellular organisms, cells are often eliminated by apoptosis if they fail to receive appropriate signals from their surroundings. Short-range cell interactions support cell survival in the Drosophila wing imaginal disc. Evidence is presented showing that cells incorrectly specified for their position undergo apoptosis because they fail to express specific proteins that are found on surrounding cells, including the LRR transmembrane proteins Capricious and Tartan. Interestingly, only the extracellular domains of Capricious and Tartan are required, suggesting that a bidirectional process of cell communication is involved in triggering apoptosis. Evidence showing that activation of the Notch signal transduction pathway is involved in triggering apoptosis of cells misspecified for their dorsal-ventral position (Milán, 2002).

To determine whether apoptosis might be a general response of cells unable to engage in normal interactions with their neighbors, the effects caused by producing cells with inappropriate dorsal-ventral compartment identity were examined. Clones of cells expressing Apterous (Ap) were produced to examine the survival of D cells in the V compartment. Fewer than 20% of surviving Ap-expressing clones were of V compartment origin. Half of these had sorted out into the D compartment and so were in contact with other Ap-expressing cells. The remaining ~10% of clones were recovered in the V compartment. Ventral Apterous-expressing clones were round in shape and induced Wg expression at their borders. Expression of the Apterous inhibitor dLMO was used to produce cells with V identity in the D compartment. Only 30% of dLMO-expressing clones were of D compartment origin. Most of these had sorted out into the V compartment. Fewer than 5% of dLMO-expressing clones were recovered in the D compartment. These were round in shape and induced Wg expression at their borders. These observations suggested that dLMO-expressing clones are preferentially lost from the D compartment if they are unable to make contact with V cells. Likewise, Ap-expressing clones are preferentially lost from the V compartment if they are unable to make contact with D cells. Loss of the inappropriately specified cells was suppressed by coexpression of p35. Under these conditions 48% of dLMO and p35-expressing clones were of D origin, and 51% of clones expressing Ap and p35 were of V origin. This indicates that inappropriately positioned cells are lost by apoptosis. Apoptosis of these cells occurs when clones were induced in second instar. Clones induced during third instar survive equally in both compartments. Caps and Tartan are expressed in D cells under Ap control in second instar wing discs. Ectopic expression of Caps or Tartan cause clones to sort out toward the D compartment, suggesting that these proteins may confer a preferential affinity for D compartment cells. To test whether loss of Caps or Tartan expression contributes to the poor survival of dorsal dLMO-expressing clones, the recovery was measured of clones coexpressing dLMO with Caps or with Tartan. When coexpressed with Caps, 58% of dLMO-expressing clones were of dorsal origin and were recovered in the D compartment, compared to 30% when dLMO was expressed alone. Coexpression with Tartan yielded 54% dorsal dLMO-expressing clones. Expression of CapsDeltaC and TrnDeltaC is able to support survival of dLMO-expressing clones in the D compartment almost as effectively as the full-length proteins (Milán, 2002).

Neurosecretory identity conferred by the apterous gene: lateral horn leucokinin neurons in Drosophila

The LIM-HD protein Apterous has been shown to regulate expression of the FMRFamide neuropeptide in Drosophila neurons. To test whether Apterous has a broader role in controlling neurosecretory identity, the expression of several neuropeptides was examined in apterous (ap) mutants. Apterous was shown to be necessary for expression of the Leucokinin neuropeptide in a pair of brain neurons located in the lateral horn region of the protocerebrum (LHLK neurons). ap null mutants are depleted of Leucokinin in these cells, whereas hypomorphic mutants show reduced Leucokinin expression. Other Leucokinin-containing neurons are not affected by mutations in ap gene. Co-expression of apterous and Leucokinin is observed exclusively in the LHLK neurons, from larval stages to adulthood. Rescue assays performed in null ap mutants, by expressing Apterous protein under apGAL4 and elavGAL4 drivers, demonstrate the recovery of Leucokinin in the LHLK neurons. These results reinforce the emerging role of the LIM-HD proteins in determining neuronal identity. They also clarify the neuroendocrine phenotype of apterous mutants (Herrero, 2003).

A re-evaluation of the contributions of Apterous and Notch to the dorsoventral lineage restriction boundary in the Drosophila wing

The Drosophila limb primordia are subdivided into compartments -- cell populations that do not mix during development. The wing is subdivided into dorsal (D) and ventral (V) compartments by the activity of the selector gene apterous in D cells. Apterous causes segregation of D and V cell populations by at least two distinct mechanisms. The LRR transmembrane proteins Capricious and Tartan are transiently expressed in D cells and contribute to initial segregation of D and V cells. Signaling between D and V cells mediated by Notch and Fringe contributes to the maintenance of the DV affinity boundary. Given that Notch is activated symmetrically, in D and V cells adjacent to the boundary, its role in boundary formation remains somewhat unclear. The roles of Apterous and Fringe activities in DV boundary formation have been re-examined and evidence is presented that Fringe cannot, by itself, generate an affinity difference between D and V cells. Although not sufficient, Fringe is required via Notch activation for expression of an Apterous-dependent affinity difference. It is proposed that Apterous controls expression of surface proteins that confer an affinity difference in conjunction with activated Notch. Thus, Apterous is viewed as instructive and Notch activity as essential, but permissive (Milán, 2003).

The LRR transmembrane proteins Capricious and Tartan contribute to DV boundary formation, but their role is transient. Maintenance of the boundary requires an additional mechanism. Notch activity has been implicated in this process, but its role has been questioned. Models for maintenance of the DV boundary must take into account the fact that Notch is activated symmetrically in cells on either side of the DV boundary. Therefore, an Ap-dependent process must be invoked to confer a DV difference. One proposal is that Fringe mediates the required Ap-dependent activity by acting in a Notch-independent manner, in addition to its role in Notch signaling. According to this view, confrontation of Fringe-expressing and non-expressing cells should induce a cell affinity difference. Increasing or decreasing Fringe activity has some effect, but does not produce affinity differences comparable with those produced by manipulating Apterous activity. Furthermore, the effects of restoring Fringe in D cells that lack Apterous activity can be reproduced independently by blocking Notch activation using Necd. Thus, it is unlikely that Fringe has a Notch-independent role in DV cell interactions (Milán, 2003).

A second, very different, model proposes that Notch activation confers a boundary-specific affinity state and that this is modulated into D and V states by Apterous expression. According to this model, there should be an affinity difference between boundary cells and internal cells within a compartment but not between D and V cells in the absence of Notch activity. This model proposes that Notch activity is sufficient to produce an affinity difference and hence smooth clone borders. However, clones of cells expressing the activated Notch receptor do not exhibit this property. This model is also difficult to reconcile with the observation that the borders of fringe mutant clones in the D compartment are highly irregular. It is also incompatible with the finding that restoring Notch activity in the absence of Apterous function is not sufficient to generate a smooth DV boundary and prevent mixing of D and V cells (Milán, 2003).

The results reported here support the view that Notch activity is needed for cell affinity differences between D and V cells, but indicate that Notch activation is not sufficient to cause these differences. A new model is proposed that differs in one crucial respect from the model discussed above. The role of Notch activation is considered to be permissive rather than instructive, and it is suggested that Apterous controls expression of surface proteins in D and V cells. It is envisaged that Notch activity is an essential co-factor in allowing cells to convert this into an affinity state. In molecular terms, one possibility is that D and V surface proteins form complexes with activated Notch (N*). In this scenario D+N* and V+N* are the active components, D and V are needed and instructive but have no activity alone. Interestingly, it has been observed that loss of Notch activation only in one compartment does not alter the DV affinity boundary. Thus, production of either the dorsal (D+N*) or the ventral (V+N*) boundary-specific cell state is sufficient to induce an affinity difference with cells of the opposite compartment. Another plausible molecular scenario is that Notch activity might control the subcellular localization of the predicted D and V proteins (Milán, 2003).

These examples are presented to illustrate how Notch activity can be seen as a permissive co-factor rather than as an instructive principle defining cell affinity. Many other molecular explanations are possible. This model provides a satisfactory explanation for how Notch can be required, but not sufficient for boundary maintenance. The essential difference between the permissive and instructive models for Notch function lies in the observation that Notch activation leads to an affinity difference only in the context of juxtaposition of cells with opposite DV identity. Notch activation per se does not induce a robust affinity boundary, whereas clones expressing dLMO and Necd do so only when Notch is not blocked in the cells outside the clone. Comparable results have been obtained with clones expressing Apterous and Necd (Milán, 2003).

Are the transmembrane proteins Serrate and Delta the D and V proteins, respectively? Early in development, Serrate is expressed in D cells and Delta in V cells. Late in development, both genes are regulated by Wg and are expressed in cells adjacent to the Wg-expressing cells at the DV boundary. Given that the Serrate- and Delta-expressing cells are offset from the DV boundary, it is considered unlikely that they confer the D* and V* activities. However, the possibility that they might contribute to the establishment of the DV affinity boundary in collaboration with Caps and Tartan cannot be excluded (Milán, 2003).

The interface between D and V cells behaves as an affinity boundary and as a signaling center where Notch activation is required for the growth of the wing disc. Clones of cells can be induced to sort into the opposite compartment by manipulating Apterous or Fringe activities. A distinction can be made between crossing and pushing the DV boundary as possible mechanisms. Cells with altered Apterous activity also have altered Fringe activity. It is suggested that these clones can cross the boundary and mix freely with cells in the opposite compartment because they change both their affinity state and signaling properties. Clones in which only Fringe activity is altered adopt signaling properties of the opposite compartment and displace the signaling center relative to the endogenous compartment boundary. In wild-type discs, symmetric activation of Notch and its targets leads to symmetric growth of D and V compartments. If growth is symmetric with respect to the displaced signaling center, the clone could be pushed into the opposite compartment by growth of the surrounding tissue (Milán, 2003).

At first glance, differential growth might explain how cells could be pushed to the interface between compartments. Can the model presented in the preceding section explain why some dorsal fringe mutant clones become able to mix with cells of the opposite compartment? Notch is not activated in V cells adjacent to fringe mutant clones abutting the boundary. The model presented here suggests that these cells would become V instead of V+N*; hence, there would not be a sustained affinity difference between fringe mutant D cell and the adjacent V cells. This may explain why fringe mutant D cells can sometimes mix with V cells when they are pushed into the V compartment. A similar case can be made to explain how V cells expressing Fringe can be pushed into the D compartment and mix with D cells. In both situations, it is noted that these clones form smooth borders with the cells of the compartment of origin, suggesting symmetric growth induced by Notch may contribute to the smoothness of the affinity boundary. This type of 'pushing' mechanism provides a useful explanation for the behavior of clones of cells that contact the DV boundary. It is noted that the behavior of cells expressing Apterous and Fringe was not the same when the entire P compartment was involved. P cells of ventral origin expressing Apterous were able to sort into the dorsal posterior quadrant, but cells expressing Fringe were not. It is suggested that this reflects an underlying difference between cells that have acquired a fully dorsal affinity state from those in which only the signaling properties have been altered. Fringe activity clearly plays an important role in the maintaining the segregation of D and V cells, but it is not the sole mediator of Apterous activity in this process (Milán, 2003).

Apterous and Myogenesis

Two physiologically distinct types of muscles, the direct and indirect flight muscles, develop from myoblasts associated with the Drosophila wing disc. The direct flight muscles (DFMs) are specified by the expression of Apterous, a Lim homeodomain protein, in groups of myoblasts. This suggests a mechanism of cell-fate specification by labelling groups of fusion competent myoblasts, in contrast to mechanisms in the embryo, where muscle cell fate is specified by single founder myoblasts. In addition, Apterous is expressed in the developing adult epidermal muscle attachment sites. Here, it functions to regulate the expression of stripe, a gene that is an important element of early patterning of muscle fibers, from the epidermis. These results, which may have broad implications, suggest novel mechanisms of muscle patterning in the adult, in contrast to embryonic myogenesis (Ghazi, 2000).

In examining the adult expression pattern of embryonic muscle founder markers, expression of an ap reporter was observed in a pattern that suggested specific roles in myogenesis. The DFMs, located dorsolaterally in each hemisegment of the adult mesothorax, show reporter gene activity. No staining was seen in the indirect flight muscles (IFMs), which constitute the bulk of the muscles of the dorsal mesothorax. Amongst DFMs, the most conveniently identifiable ones are muscles 49-58. Muscles 49 and 51-55, show staining for the ap lacZ reporter gene in different planes of foci (Ghazi, 2000).

Epidermal attachment sites for muscles in the adult fly are identifiable by anatomical examination and by expression of stripe (sr). sr marks all muscle attachment cells, both in the embryo and the adult. The pattern of sr expression during pupal development has been studied and the attachment sites for the IFMs identified. ap lacZ adult expression is also seen in the thoracic epidermis in the regions where muscles attach (Ghazi, 2000).

On the third instar wing imaginal disc the presumptive notum shows low ap lacZ and Ap protein expression distinct from the high levels seen in the presumptive dorsal wing. Although the presumptive notum is a dorsal structure, ap does not seem to have a selector function to define 'dorsalness' in the mesothoracic trunk as it does in the dorsal wing blade. ap expression in the pupal epidermis changes temporally beginning with an early expression in broad regions of the dorsal notal epidermis and a subsequent localization to restricted domains, including the attachment sites of the DLMs. At 18 hours after puparium formation (APF), ap lacZ expression is seen in regions that included the developing anterior attachment sites of the DLMs. This expression eventually narrows down to the anterior attachment sites and very closely abuts the posterior attachment sites of the DLMs. This can be observed by simultaneous labelling of developing pupae for ap and sr. By 36 hours APF, when a complete set of DLM fibers is in place, ap co-localization with individual muscle-attached sr-expressing tendon cells is clearly seen. This attachment site expression continues in the adult. The same pattern is seen on labelling with Ap-specific antibodies (Ghazi, 2000).

The developmental origins of the adult DFM-specific expression of ap were examined. Three simple possibilities suggest themselves: (1) ap is expressed in all myoblasts on the third larval instar wing disc and later, perhaps during metamorphosis, becomes 'switched off' in IFMs or their progenitors; (2) ap is expressed in a subset of third larval instar wing disc associated myoblasts destined to form DFMs; (3) ap expression is absent from all the myoblasts on the third instar wing disc and begins later, during pupal development, in the developing DFMs. No evidence is found of ap expression in the wing disc myoblasts. Several lines of evidence substantiate this. ap lacZ wing imaginal discs were double labelled with antibodies against the beta-galactosidase protein and against Twist, which marks all myoblasts. Although epidermal expression of ap is clear, no co-localization of ap lacZ expression with Twist is observed. These data suggest that ap expression in the DFMs or their progenitors begins during pupal development (Ghazi, 2000).

The expression of an embryonic muscle (lateral transverse muscles LT1-4) specific ap reporter strain was examined during adult muscle development and it was compared with the expression of the ap lacZ strain and with Ap antibody labelling. The embryonic muscle-specific ap lacZ (apMS lacZ) showa an adult DFM-restricted pattern of expression in a manner similar to reporter insertions in the ap locus. However, there is no detectable expression in the presumptive dorsal wing blade or in the presumptive notum. The pupal attachment site expression observed with ap lacZ and Ap antibody staining is not seen (however, in the adult, staining is seen in the dorsocentral bristles). As in the embryo, the 'muscle-specific enhancer' shows an ap expression in the developing mesoderm but, unlike the embryo, this appears in clusters of myoblasts and not in single 'founder' cells (Ghazi, 2000).

This mesodermal expression of apMS lacZ was used to follow ap expression during adult development and this allowed a close observation, uncluttered by epidermal staining, of the dynamic expression pattern of ap in the developing DFMs. It also provided for a broad developmental analysis of DFMs (Ghazi, 2000).

Myoblast expression of apMS lacZ is seen at 12-14 hours APF in clusters of cells all over the dorsal notum. The three larval muscles that escape histolysis and serve as templates for DLM formation do not express the reporter gene or Ap protein at detectable levels in these assays. The extent of ap myoblast expression as seen by reporter gene activity increases from 19-21 hours APF onwards, until about 24-26 hours APF when a number of clusters of myoblasts are seen. Between 26-28 hours APF to 34-36 hours APF, these clusters begin to arrange themselves into distinct fibers and by 36 hours APF completely formed DFMs are in place. fibers increase in size considerably after 36 hours APF until about 48 hours APF at which time the complete complement of DFMs can be identified. Adults continue to express apMS lacZ in the DFMs . Immunohistochemistry with anti-Ap antibodies confirms this (Ghazi, 2000).

Many of the clusters of myoblasts that express ap can be identified as progenitors of specific DFMs. These inferences are based on the positions of these clusters and their correlation with the muscle numbering scheme. One cluster is destined to form muscle 55, based on position and orientation, since the developing fiber in this region is always noticed at the lateral edge of the last DLM fiber and corresponds to the position occupied by DFM 55 in the adult. Another cluster that is consistently noticeable is present below the DLMs and prefigures muscle 52 (Ghazi, 2000).

To decipher the functional significance of the ap expression pattern, flight muscles of several viable ap alleles were studied. Homozygous ap4 animals show the strongest defects in the thoracic musculature. In particular, the DFMs are severely affected. For this study, concentration was placed on four of the DFMs: 51, 52, 53 and 54. Most ap4 mutants show a sliver of a fiber instead of four distinct muscles. Defects in ap lacZ homozygotes are less severe. IFM defects include a characteristic reduction in the width of the posterior attachment sites of the DLMs, giving them a thin and 'tapering' appearance. This is consistent with prominent ap expression closely abutting the posterior attachment sites of the DLMs. Another IFM defect is the incomplete splitting of templates for DLM formation, resulting in three instead of six fibers. A third phenotype is formation of a single fiber. The DVMs also show defects and are either absent or reduced in size. The severity of phenotypes can be ordered as DFMs>DLMs>DVMs. A common feature of all mutants is a striking decrease in overall muscle size and volume. It has been shown that DFM defects in ap mutants can be rescued by muscle-specific expression of ap (Ghazi, 2000).

What happens when ap is provided epidermally? ap was expressed epidermally in the ap4 mutant background using the pannier (pnr) GAL4 driver. pnr is expressed in dorsal cells of the presumptive notum on the disc and the adult expression continues in the same region of the dorsal notum. No mesodermal expression is observed. pnr is also required for normal fusion of the two heminota -- high levels of ap produced at 25°C, in the midline, causes a mid-notal cleft and IFM abnormalities. To circumvent this and still produce ap in sufficient amounts to mediate a rescue, animals were shifted to 18°C during second instar stages. This resulted in eclosion of flies with a normal notum that could be screened for rescue. The DLMs were screened for morphology and muscle size. There is a significant rescue of the DLM defects in ap mutants upon epidermal expression of ap, while DFM fibers continue to remain disorganized. There is also a very striking restoration of muscle size. A sizeable population of rescued progeny shows DLMs arranged in one large mass instead of clearly separated fibers. These results substantiate the epidermal requirement of ap for patterning IFMs (Ghazi, 2000).

Expression of ap in epidermal attachment sites of muscles and the phenotypes noticed in ap mutants suggests a regulatory role for the gene in the development of muscle attachment sites. stripe (sr), the earliest known marker for epidermal attachment sites, was chosen as a potential target for regulation by ap in mediating its epidermal function, and its expression was examined in wing discs of ap4 homozygotes. sr is crucial for differentiation of epidermal cells into muscle-attached tendon cells. sr is expressed in discrete domains in the wing disc, which will form the attachment sites of adult thoracic muscles. sr expression in the disc commences very late in third larval instar and is consistently seen as pupation is initiated. Hence the 0 hours APF white prepupal stage was chosen for examination of sr expression in ap mutants. sr expression is either completely lost or drastically reduced in ap4 wing discs. Animals that reach pupal stages show severe reduction in sr levels. These results are further strengthened by the observation that ap and sr interact with each other genetically to affect IFM development. The ap4 mutation is completely recessive, as is a sr recessive lethal. In a transheterozygous combination, the two alleles show defective IFMs in a significant population of animals. Further, an enhancer trap insertion at the sr locus that shows a very mild recessive phenotype, enhances the IFM defects of ap4 and such animals also show a dark midnotal stripe that is characteristic of strong, viable sr alleles. This suggests that ap functions in IFM patterning by influencing attachment site development by the regulation of sr (Ghazi, 2000).

Apterous and leg development

Proximal-distal leg development in Drosophila involves a battery of genes expressed and required in specific proximal-distal (PD) domains of the appendage. apterous is required for PD leg development, and the functional interactions between ap, Lim1 and other PD genes during leg development have been explored. A regulatory network formed by ap and Lim1 plus the homeobox genes aristaless and Bar specify distal leg cell fates in Drosophila (Pueyo, 2000).

Lim homeobox (Lhx) genes have been shown to interact functionally in the nervous systems of Drosophila and vertebrates. It has been suggested that different combinations of Lhx proteins shunt cells into different cell fates, and this model predicts that Lhx proteins can act combinatorially, possibly forming complexes to activate target genes. In appropriate experiments, ectopic generation of a given combination of Lhx proteins shunts cells into an ectopic, but coherent and predictable, cell fate. This study has explored the possibility of similar interactions between Lim1 and other Lhx genes in the appendages of Drosophila. A computer search of the Drosophila genome has identified four other Lhx genes. The Lhx genes Lim3 and Islet (tailup) have been characterized previously in Drosophila, but their mutant phenotypes and patterns of expression do not involve the appendages. A search has identified a new putative Lhx gene, homologous to vertebrate Lmx1, which is not expressed in legs either. The only other Lhx gene identified is the apterous (ap) gene, which is homologous to vertebrate Lhx2. ap is expressed in the leg in the presumptive tarsal segment four, near the tip of the leg close to where Lim1 is expressed. A mutant phenotype for ap in legs has not been described, but using allelic mutant combinations that produce extreme loss of function of ap the following phenotypes were observed: either fusion of tarsus four and five, reduction and deformities in tarsus four, or complete loss of tarsus four, the latter producing legs with only four tarsi but looking otherwise normal. Lim1 and ap expression was combined using UAS constructs to express ap and Lim1 ectopically in legs (Pueyo, 2000).

Expression of UASap over the presumptive claw region using several different Gal4 lines produces no discernible phenotype. In contrast, expression of UASLim1 driven by apGal4, which faithfully reproduces ap expression, produces complete absence of tarsus four, thus mimicking extreme loss of function of ap. This loss of tarsus four fates is specific, since it is also accomplished by ectopic expression of Lim3, a close sequence paralogue of Lim1, but not by other, unrelated proteins, and it is accompanied by the loss of ap expression. However, this apparent dominant negative effect of Lim1 on ap was not rescued by simultaneous co-expression of extra ap in apGal4;UASLim1;UASap flies, as would be expected if the phenotype of UASLim1 were due to either loss of ap expression or competition with the Ap protein. Furthermore, mild tarsal fusions produced by expressing UASLim1 under the control of the weak line 30AGal4 were not made worse by simultaneous reduction of endogenous ap function in ap minus 30AGal4;UASLim1 flies. Altogether these results suggest that, although there exists an effect of ectopic Lim1 on ap expression, the Lim1 and Ap proteins are not interfering directly with each other. Rather, Lim1 must interact with another element involved in tarsus four development and related to ap function (Pueyo, 2000).

The Drosophila Chip gene has been shown to encode a ubiquitous transcriptional cofactor. Chip proteins bind to the Lim domains of Ap and the ap and Chip genes have to be present in similar doses to ensure normal wing development. Interestingly, Chip has been shown to bind the Lim domains of other Lhx proteins, among them Lim1. However, no such dose relationships were found, between chip and Lim1 or between chip and ap in the leg, or in flies expressing UASChip and UASLim1. Furthermore, intermediate ap or Lim1 mutants are not rescued by UASChip, and co-expression of UASChip together with UASLim1 in the ap domain does not rescue the dominant-negative effect of UASLim1 on ap. Therefore, it is concluded that Chip is either not required for Lhx protein function in leg development, or not present in either limited amounts or stoichiometric doses. Thus Chip is unlikely to be the putative ap partner affected by Lim1 (Pueyo, 2000).

The expression of Lim1 is very similar to that of the PD gene aristaless. al is expressed in the most distal part of the leg discs, and in two rings in the peripheral and medial regions. Confocal immunofluorescence staining has shown that the expression of Lim1 and al are coincident in the most distal parts of the leg and antenna disc. Furthermore, there are similarities between the al and Lim1 mutant phenotypes, since in al mutants the claw organ, the sternopleural bristles in the leg and the arista are missing or reduced. A possible functional relationship between the two genes was explored. Since al is expressed earlier than Lim1, one possibility is that Lim1 expression is regulated by al. The expression of Lim1 was examined in al mutants, and Lim1 was found to be lost in the presumptive tip of the leg where both genes are co-expressed. In contrast, al expression is normal in Lim1 mutants. These results suggest that the Lim1 locus is regulated by the al gene, one possibility being that the Lim1 gene is a direct downstream target of the homeodomain Al protein (Pueyo, 2000).

In both al and Lim1 mutants, the strongest pupal lethal alleles eliminate the claw and reduced the rest of the pretarsal organs, but do not eliminate the whole pretarsus in all legs. This could be due to a functional cooperation between al and Lim1, such that the presence of any one product in the absence of the other would still provide enough function for some organs of the pretarsus to occasionally develop. Alternatively, the remnant pretarsal organs observed in al and Lim1 mutants could be due to hypomorphy of the mutants available and no functional relationship needs to be implied between the two genes. It was reasoned that if a functional relationship does exist, a double mutant should show an enhanced phenotype: a double mutant Lim1R12.4;alice was generated, and found to be embryonic lethal. Therefore, al and Lim1 double mutants show a synergistic effect that might betray a functional relationship. This synergistic relationship is also shown in ectopic expression experiments. Ectopic expression of either Lim1 or al driven by the 30AGal4 line produces only small defects in the joint between tarsus four and five. However, simultaneous expression of both al and Lim1 in 30AGal4;UASLim1;UASal flies produces stronger defects, including partial or complete fusion of tarsus four and five (Pueyo, 2000).

Ectopic expression of Lim1 leads to loss of ap expression and of tarsus four, but a direct regulatory relationship between ap and Lim1 in the wild type does not need to exist, since they are never expressed in the same cells. Furthermore, expression of ap in Lim1 mutants, and of Lim1 in ap mutants is normal, which indicates the absence of long-range regulatory cell signals between these genes. However, expression of Bar in the presumptive tarsus abuts that of Lim1 and al. Bar encodes two redundant Hox proteins expressed in tarsus four and five, which are required for the development of these structures and for the expression of ap in tarsus four. When Lim1 is ectopically expressed in the Bar territory, a reduction of Bar expression occurs. This loss of Bar expression could explain the loss of ap expression seen in apGal4;UASLim1 flies and suggests that in the wild type, an important regulatory role of Lim1 is to restrict Bar expression to the presumptive tarsus five. The apparent paradox that apGal4;UASLim1;UASap flies still show a mutant phenotype can be understood if Bar also has a direct requirement for tarsus four development, one beyond simply activating ap expression (Pueyo, 2000).

PD patterning in Drosophila legs seems to proceed stepwise after it is initiated by the Wg- and Dpp-mediated activation of Dll, dac and al. Later on, these genes interact among themselves and with Hth to activate, in a Wg- and Dpp-independent phase, the expression of further PD genes in new domains of expression. Similar interactions of this kind must lead to the eventual allocation of all different PD fates. The genes downstream of the initial PD genes are still to be identified but Lim1 and ap may serve downstream functions. In early third instar, shortly after 72 hours AEL, al expression at the presumptive leg tip is possibly initiated by a combination of Wg and Dpp signaling, with a requirement for Dll. Around mid-third instar, al-expressing cells in the presumptive tip of the leg activate the expression of Lim1. At this time, the expression of Bar is present in a ring in the presumptive distal tarsal region, partially overlapping that of al and Lim1. This overlap then resolves into an abutment by late third instar. This refinement is important for proper development of the claw organ and tarsus five, and could be based on direct repressory action between the Hox transcription factors Bar and al. However, whereas ectopic Bar expression represses al expression, in the reciprocal experiment ectopic al does not repress Bar. Interestingly, ectopic expression of lim1 results in a reduction of Bar expression. It is concluded that al and Bar do have a mutual repressory relationship that involves Lim1 (Pueyo, 2000).

Whereas Bar might repress al expression directly, the repressory effect of al on Bar is mediated by Lim1. This regulatory circuit between Bar, al and Lim1 establishes the abutting fields of tarsus five cells expressing Bar, and claw organ cells expressing al and Lim1. This circuit also explains why although al mutants lead to an expansion of Bar expression, ectopic al does not reduce Bar expression. Whereas loss of al produces loss of Lim1 and hence leads to ectopic Bar expression, ectopic al on its own is not able to repress Bar. The final element in the determination of distal leg fates is the expression of ap, which is activated in the presumptive tarsus four around mid-third instar. Although ap expression is reduced by ectopic Lim1, this is probably an indirect consequence of the loss of Bar, because appropriate levels of Bar are responsible for the activation of ap. Whereas low levels of Bar are needed for ap expression in tarsus four, high levels of Bar in tarsus five prevent it. Thus, the tip of the leg gets divided into its three final domains during the second half of the third instar: the presumptive claw organ or pretarsus, defined by the expression of al and Lim1; the presumptive tarsus five, defined by the expression of high levels of Bar; and the presumptive tarsus four, defined by the expression of ap and low levels of Bar. During the subsequent pupal metamorphosis into an adult fly, these transcription factors must control the expression of appropriate downstream genes, leading to the differentiation of appropriate structures in each of these presumptive leg segments (Pueyo, 2000).

Apterous and brain development

To assess the functional role of Ap during embryonic brain development, apP44 null mutants were first analyzed using immunocytochemical markers such as anti-HRP, anti-ELAV, anti-RK2, and anti FASII, which label general neuronal (or glial) domains and tracts in the developing brain. With these markers, no obvious gross morphological defects were seen in the embryonic brains of ap mutants. To analyze more specifically the location and projections of the mutant Ap neurons in ap loss-of-function mutants, apGAL4/apP44,UAS-tau-lacZ individuals were stained with antibodies to ß-gal. By several criteria, apGAL4 acts as a strong mutant allele of ap. In apGAL4/apP44 individuals, Ap protein levels are undetectable, wings are absent, and axon guidance defects within the VNC, as assayed with UAS-tau-lacZ, are indistinguishable from those of apP44 homozygotes. This labeling procedure reveals that the Ap neurons are generated correctly and are still present, and indeed extend axons in the embryonic brain, demonstrating that the Ap protein is not required for the generation of these neurons. However, the Ap brain interneurons display a number of defasciculation and pathfinding defects. The most severe defects are seen in the interneurons of the deutocerebrum. There, the two pairs of ap mutant neuronal cell bodies are positioned incorrectly in the deutocerebrum. These neurons do not appear to undergo the movements seen in the wild type and, in all cases examined, never become localized on the inner surface of the developing deutocerebrum. Instead, these cell bodies remain on the outer surface of the developing deutocerebrum and their axons project aberrantly along the frontal commissure of the stomatogastric nervous system; in rare cases their axons even grow along the recurrent nerve (Herzog, 2001).

Fasciculation and projection defects are also seen in the protocerebrum. Although the cell bodies of the protocerebral ap mutant neurons appear to be positioned properly and have a normal size and shape, many of their commissural axons fail to form commissural fascicles correctly, and often leave their commissural axon bundle and cross over to grow along a neighboring fascicle. This phenotype is less penetrant than for the deutocerebral ap mutant cells, since it is observed only in 30% of the cases. In contrast to the fascicles which contain the axons of the ap mutant neurons, other axon fascicles in the brain appear to be unaltered in the ap mutant. For example, the fasciclin II-expressing commissural fascicles in the ap mutant are indistinguisable from those in the wild type. Moreover, the ap mutant neurons that innervate the ring gland also have normal axonal projection patterns (Herzog, 2001).

These results demonstrate that Ap function is important for correct axonal guidance and fasciculation of some of the Ap-expressing brain interneurons. Comparable findings have been reported for the role of Ap in VNC development. In the embryonic VNC, the ap mutant interneurons are generated appropriately, but these interneurons manifest pathfinding and fasciculation defects. Remarkably similar results have been reported for the functional role of ttx-3, the Caenorhabditis elegans homolog of ap, which is not required for interneuron generation, but is necessary for normal axonal outgrowth. In contrast, the vertebrate ortholog of ap, Lhx2, has an earlier function in neurogenesis of the brain, but whether it also functions later in axon guidance is not known (Herzog, 2001).

Developmental transcriptional networks are required to maintain neuronal subtype identity in the mature nervous system

During neurogenesis, transcription factors combinatorially specify neuronal fates and then differentiate subtype identities by inducing subtype-specific gene expression profiles. But how is neuronal subtype identity maintained in mature neurons? Modeling this question in two Drosophila neuronal subtypes (Tv1 and Tv4), tests were performed to see whether the subtype transcription factor networks that direct differentiation during development are required persistently for long-term maintenance of subtype identity. By conditional transcription factor knockdown in adult Tv neurons after normal development, it was found that most transcription factors within the Tv1/Tv4 subtype transcription networks are indeed required to maintain Tv1/Tv4 subtype-specific gene expression in adults. Thus, gene expression profiles are not simply 'locked-in,' but must be actively maintained by persistent developmental transcription factor networks. The cross-regulatory relationships were examined between all transcription factors that persisted in adult Tv1/Tv4 neurons. Certain critical cross-regulatory relationships that had existed between these transcription factors during development are no longer present in the mature adult neuron. This points to key differences between developmental and maintenance transcriptional regulatory networks in individual neurons. Together, these results provide novel insight showing that the maintenance of subtype identity is an active process underpinned by persistently active, combinatorially-acting, developmental transcription factors. These findings have implications for understanding the maintenance of all long-lived cell types and the functional degeneration of neurons in the aging brain (Eade, 2012).

The data provide novel insight supporting the view of Blau and Baltimore (1991) that cellular differentiation is a persistent process that requires active maintenance, rather than being passively 'locked-in' or unalterable. Two primary findings are made in this study regarding the long-term maintenance of neuronal identity. First, all known developmental transcription factors acting in postmitotic Tv1 and Tv4 neurons to initiate the expression of subtype terminal differentiation genes are then persistently required to maintain their expression. Second, it was found that key developmental cross-regulatory relationships that initiated the expression of certain transcription factors were no longer required for their maintained expression in adults. Notably, this was found to be the case even between transcription factors whose expression persists in adults (Eade, 2012).

In this study, all transcription factors implicated in the initiation of subtype-specific neuropeptide expression in Tv1 and Tv4 neurons were found to maintain subtype terminal differentiation gene expression in adults (see Summary of changes in subtype transcription network configuration between initiation and maintenance of subtype identity). In Tv1, col, eya, ap and dimm are required for Nplp1 initiation during development. In this study, knockdown of each transcription factor in adult Tv1 neurons was shown to dramatically downregulate Nplp1. In Tv4 neurons, FMRFa initiation during development requires eya, ap, sqz, dac, dimm and retrograde BMP signaling. Together with previous work showing that BMP signaling maintains FMRFa expression in adults (Eade, 2009), this study now demonstrates that all six regulatory inputs are required for FMRFa maintenance. Most transcription factors, except for dac, also retained their relative regulatory input for FMRFa and Nplp1 expression. In addition, individual transcription factors also retained their developmental subroutines. For example, as found during development, dimm was required in adults to maintain PHM (independently of other regulators) and FMRFa/Nplp1 expression (combinatorially with other regulators) (Eade, 2012).

The few genetic studies that test a persistent role for developmental transcription factors support their role in initiating and maintaining terminal differentiation gene expression. In C. elegans, where just one or two transcription factors initiate most neuronal subtype-specific terminal differentiation genes, they then also appear to maintain their target terminal differentiation genes. In ASE and dopaminergic neurons respectively, CHE-1 and AST-1 initiate and maintain expression of pertinent subtype-specific terminal differentiation genes. In vertebrate neurons, where there is increased complexity in the combinatorial activity of transcription factors in subtype-specific gene expression, certain transcription factors have been demonstrated to be required for maintenance of subtype identity. These are Hand2 that initiates and maintains tyrosine hydroxylase and dopa ß-hydroxylase expression in mouse sympathetic neurons, Pet-1, Gata3 and Lmx1b for serotonergic marker expression in mouse serotonergic neurons, and Nurr1 for dopaminergic marker expression in murine dopaminergic neurons (Eade, 2012).

However, while these studies confirm a role for certain developmental transcription factors in subtype maintenance, it had remained unclear whether the elaborate developmental subtype transcription networks, that mediate neuronal differentiation in Drosophila and vertebrates, are retained in their entirety for maintenance, or whether they become greatly simplified. This analysis of all known subtype transcription network factors in Tv1 and Tv4 neurons now indicates that the majority of a developmental subtype transcription network is indeed retained and required for maintenance. Why would an entire network of transcription factors be required to maintain subtype-specific gene expression? The combinatorial nature of subtype-specific gene expression entails cooperative transcription factor binding at clustered cognate DNA sequences and/or synergism in their activation of transcription. In such cases, the data would indicate that this is not dispensed with for maintaining terminal differentiation gene expression in mature neurons (Eade, 2012).

How the transcription factors of the subtype transcription networks are maintained is less well understood. An elegant model has emerged from studies in C. elegans, wherein transcription factors stably auto-maintain their own expression and can then maintain the expression of subtype terminal differentiation genes. The transcription factor CHE-1 is a key transcription factor that initiates and maintains subtype identity in ASE neurons. CHE-1 binds to a cognate DNA sequence motif (the ASE motif) in most terminal differentiation genes expressed in ASE neurons, as well as in its own cis-regulatory region. Notably, a promoter fusion of the che-1 transcription factor failed to express in che-1 mutants, indicative of CHE-1 autoregulation, and for the cooperatively-acting TTX-3 and CEH-10 transcription factors in AIY neurons. Thus, subtype maintenance in C. elegans is anchored by auto-maintenance of the transcription factors that initiate and maintain terminal differentiation gene expression (Eade, 2012).

In contrast, all available evidence in Tv1 and Tv4 neurons fails to support such an autoregulatory mechanism. An ap reporter (apC-t-lacZ) is expressed normally in ap mutants, and in this study apdsRNAi was not found to alter apGAL4 reporter activity. Moreover, col transcription was unaffected in col mutants that express a non-functional Col protein. This leaves unresolved the question of how the majority of the transcription factors are stably maintained. For transcription factors that are initiated by transiently expressed inputs, a shift to distinct maintenance mechanisms have been invoked and in certain cases shown. In this study, this was found for the loss of cas expression in Tv1 (required for col initiation) and the loss of cas, col and grh in Tv4 (required for eya, ap, dimm, sqz, dac initiation). However, it was surprising to find that the cross-regulatory relationships between persistently-expressed transcription factors were also significantly altered in adults. Notably, eya initiated but did not maintain dimm in Tv4. In Tv1, col initiated but did not maintain eya, ap or dimm. This was particularly unexpected as eya remained critical for FMRFa maintenance and col remained critical for Nplp1 maintenance. Indeed, although tests were performed for cross-regulatory interactions between all transcription factors in both the Tv1 and Tv4 subtype transcription networks, only Dimm was found to remain dependent upon its developmental input; Eya and Ap in Tv1 as well as Ap in Tv4. However, even in this case, the regulation of Dimm was changed; it no longer required eya in Tv4, and in Tv1 it no longer required col, in spite of the fact that both col and eya are retained in these neurons. It is anticipated that such changes in transcription factor cross-regulatory relationships will be found in other Drosophila and vertebrate neurons, which exhibit high complexity in their subtype transcription networks. Indeed, recent evidence has found that in murine serotonergic neurons, the initiation of Pet-1 requires Lmx-1b, but ablation of Lmx-1b in adults did not perturb the maintenance of Pet-1 expression (Eade, 2012).

The potential role of autoregulation for the other factors in the Tv1/Tv4 subtype transcription networks is being pursued. However, there are three additional, potentially overlapping, models for subtype transcription network maintenance. First, regulators may act increasingly redundantly upon one another. Second, unknown regulators may become increasingly sufficient for transcription factor maintenance. Third, transcription factors may be maintained by dedicated maintenance mechanisms, as has been shown for the role of trithorax group genes in the maintenance of Hox genes and Engrailed. Moreover, chromatin modification is undoubtedly involved and likely required to maintain high-level transcription of Tv transcription factors as well as FMRFa, Nplp1 and PHM. However, the extent to which these are instructive as opposed to permissive has yet to be established. In this light, it is intriguing that MYST-HAT complexes, in addition to the subtype transcription factors Che-1 and Die-1, are required for maintenance of ASE-Left subtype identity in C. elegans (Eade, 2012).

Taken together, these studies have identified two apparent types of maintenance mechanism that are operational in adult neurons. On one hand, there are sets of genes that are maintained by their initiating set of transcription factors. These include the terminal differentiation genes and the transcription factor dimm. On the other, most transcription factors appear to no longer require regulatory input from their initiating transcription factor(s). Further work will be required to better understand whether these differences represent truly distinct modes of gene maintenance or reflect the existence of yet unidentified regulatory inputs onto these transcription factors. One issue to consider here is that the expression of certain terminal differentiation genes in neurons, but perhaps not subtype transcription factors, can be plastic throughout life, with changes commonly occurring in response to a developmental switch or physiological stimulus. Thus, terminal differentiation genes may retain complex transcriptional control in order to remain responsive to change. It is notable, however, that FMRFa, Nplp1 and PHM appear to be stably expressed at high levels in Tv1/4 neurons, and no conditions were found that alter their expression throughout life. Thus, these are considered to be stable terminal differentiation genes akin to serotonergic or dopaminergic markers in their respective neurons that define those cells' functional identity and, where tested, are actively maintained by their developmental inputs. Tv1/4 neurons undoubtedly express a battery of terminal differentiation genes, and sets of unknown transcription factors are likely required for their subtype-specific expression. Subtype transcription networks are considered to encompass all regulators required for differentiating the expression of all subtype-specific terminal differentiation genes. Further, differentiation of subtype identity is viewed as the completion of a multitude of distinct gene regulatory events in which each gene is regulated by a subset of the overall subtype transcription network. As highly restricted terminal differentiation genes expressed in Tv1 and Tv4 neurons, it is believed that Nplp1, FMRFa and PHM provide a suitable model for the maintenance of overall identity, with the understanding that other unknown terminal differentiation genes expressed in Tv1 and Tv4 may not be perturbed by knockdown of the transcription factors tested in this study. In the future, it will be important to incorporate a more comprehensive list of regulators and terminal differentiation genes for each neuronal subtype. However, it is believed that the principles uncovered in this study for FMRFa, Nplp1 and PHM maintenance will hold for other terminal differentiation genes (Eade, 2012).

Finally, it is proposed that the active mechanisms utilized for maintenance of subtype differentiation represent an Achilles heel that renders long-lived neurons susceptible to degenerative disorders. Nurr1 ablation in adult mDA neurons reduced dopaminergic markers and promoted cell death. Notably, Nurr1 mutation is associated with Parkinson's disease, and its downregulation is observed in Parkinson's disease mDA neurons. Adult mDA are also susceptible to degeneration in foxa2 heterozygotes, another regulator of mDA neuron differentiation that is maintained in adult mDA neurons. Studies in other long-lived cell types draw similar conclusions. Adult conditional knockout of Pdx1 reduced insulin and ß-cell mass and, importantly, heterozygosity for Pdx1 leads to a rare monogenic form of non-immune diabetes, MODY4. Similarly, NeuroD1 haploinsufficiency is linked to MODY6 and adult ablation of NeuroD in β-islet cells results in β-cell dysfunction and diabetes. These data, together with current results, underscore the need to further explore the transcriptional networks that actively maintain subtype identity, and hence the function, of adult and aging cells (Eade, 2012).


apterous: Biological Overview | Evolutionary Homologs | Regulation | Protein Interactions | Developmental Biology | References

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