daughterless

DEVELOPMENTAL BIOLOGY

Embryonic

In the preblastoderm, maternally supplied DA is present throughout the egg and is detected in all nuclei. By early blastoderm, DA levels decline but rapidly increase again shortly before germband extention, reaching maximal levels during stages 9-11 [Images], when most neuronal precursors form. DA protein is present in ectodermal cells as well as putative neuroblasts both during and after their delamination. Neural precursors have a somewhat higher level of DA. Every type of somatic cell expresses DA, but generally higher levels are found in salivary glands, muscles and parts of the gut (Vaessin, 1994).

Larval

The level of DA is fairly uniform in all epidermal cells of the wing disc, but is elevated in many neuronal precursors. This is true in the leg and eye disc as well.

Adult

The daughterless gene is expressed in the somatic ovary during egg chamber morphogenesis. Hypomorphic da mutant genotypes exhibit dramatic defects during oogenesis, including aberrantly defined follicles and loss of interfolicular stalks. The defects are similar to those for Notch or Delta mutant ovaries. The role of Daughterless in this case is not as a transcriptional activator of either Delta or Notch (Cummings, 1994).

During Drosophila oogenesis two distinct stem cell populations produce either germline cysts or the somatic cells that surround each cyst and separate each formed follicle. From analyzing daughterless (da) loss-of-function, overexpression and genetic interaction phenotypes, several specific requirements have been identified for da⁺ in somatic cells during follicle formation. (1) da is a critical regulator of somatic cell proliferation. (2) da is required for the complete differentiation of polar and stalk cells, and elevated da levels can even drive the convergence and extension that is characteristic of interfollicular stalks. (3) da is a genetic regulator of an early checkpoint for germline cyst progression: loss of da function inhibits normally occurring apoptosis of germline cysts at the region 2a/2b boundary of the germarium, while da overexpression leads to postmitotic cyst degradation. Collectively, these da functions govern the abundance and diversity of somatic cells as they coordinate with germline cysts to form functional follicles (Smith, 2002).

Integrating this information about da activities with several published models of early oogenesis leads to the following outline for the sequential steps required for the iterative production of follicles. (1) Signaling by Hh protein from the terminal filament creates a somatic stem cell niche at the region 2a/2b boundary. (2) Somatic stem cells produce undifferentiated mesenchymal cells that surround the germline cyst and compress it into the characteristic lens shape. (3) EGFR-mediated signaling from the germline cyst provides a continuous proliferative signal to somatic cells. (4) A second germline-to-soma signal, Delta, induces the somatic cells, located between adjacent cysts in region 2b/3, to begin differentiating as polar cells. (5) Other somatic cells, which contact only a single germline cyst, differentiate as cuboidal epithelial cells. (6) The differentiating polar cells signal to neighboring cells to refine polar cell number, recruit stalk cells and promote local somatic cell proliferation. (7) The stalk cells migrate between the polar cells associated with each cyst and converge and extend to form a single column of cells as they terminally differentiate. In this scheme, da function appears to contribute to steps 2, 3, 6 and 7 (Smith, 2002).

The initial requirement for da during follicle formation is somatic cell proliferation during steps 2 and 3. The straightforward observation that one extra copy of da⁺ results in excess somatic cell production, while da loss of function leads to an insufficient number of somatic cells for germline cyst envelopment, demonstrates a role for da in proliferation. The genetic interactions between da and every component of the EGFR pathway, which is also known to regulate somatic proliferation through mid-oogenesis, suggest that da and the EGFR pathway cooperate to control cell division. Gurken has been identified as the germline-localized ligand for this proliferative function; however, grk null ovaries have a relatively low frequency of follicle formation defects. The genetic interaction observed between da and spi implicates Spi as a second ligand for EGFR in proliferation, although this must be a somatic signal, since spi expression is restricted to somatic cells. Finally, although there is no evidence for da acting during specification of the somatic stem cells, it may control their proliferation once founded. Alternatively, da control of proliferation may be limited to the progeny of the stem cells. Either possibility is consistent with suppression of the ectopic hh phenotype in da heterozygotes and the complete epistasis of da^lyh (Smith, 2002).

Although da's role in the control of somatic proliferation is unknown, it probably involves regulation of cell cycle progression. Connections between EGFR signaling and cell cycle progression have already been established in the R2-R5 photoreceptor cells in the morphogenetic furrow of the eye, where EGFR signal transduction is required for G₂/M progression, but signal inactivity is necessary for G₁/S progression. Coincidentally, Da protein levels are high in those cells, and da function is required for their G₁/S progression. A similar connection between da and cell cycle control in the ovary is implicated by the observation that da exhibits genetic interaction phenotypes with both loss of function (rl^-) and persistently activated (rl^Sem) MAPK alleles. Loss of EGFR signaling would be expected to delay cell cycle progression at the G₂/M transition, but persistent MAPK^Sem activity, being a poor substrate for the inactivating phosphatase, would delay the cell cycle at the G₁/S transition. In either situation additionally reducing the da dose (which itself would slow G₁/S progression) would lead to the mutant phenotype observed in genetic interactions, and the higher frequency of defects with rl^Sem is consistent with both da and rl^Sem impacting on the same stage of the cell cycle (Smith, 2002).

There is probably no role for da in induction of polar cells or the differentiation of follicular epithelium (steps 4 and 5). Nevertheless, complete polar cell differentiation (step 6) depends on da and is required for several non autonomous polar cell functions. The first function of differentiating polar cells is stalk cell recruitment. Clonal analysis of fng and N has shown that the function of these genes is required in the polar cells to form interfollicular stalks. In fng^- clones in which no distinct stalk is visible, the stalk-specific enhancer trap B1-93F is expressed in a peripheral cluster of cells at the junction between two incompletely separated follicles; the same clusters are observed in N^ts1 mutant ovaries. This arrangement would result if the differentiating polar cells normally recruit stalk cells from the periphery, where a population of undifferentiated somatic cells is not in contact with any germline cyst. Like N and fng, da interferes with recruitment of stalk cells from the periphery, since similar 'stalk cell' clusters were often observed in hypomorphic mutant ovarioles. In da mutant ovaries with more extreme defects, the 'stalk cells' are actually integrated within the follicular epithelium, suggesting that there are insufficient somatic cells to complete the epithelium. Thus, when there are sufficient somatic epthelial cells to cover the germline cyst completely (weak da^- phenotype), differentiating stalk cells never touch the germline; however, when there are not enough somatic cells (strong da^- phenotype), differentiating stalk cells consequently make contact with the germline and become incorporated into the epithelium. It is proposed that a second function of differentiating polar cells is the production of a 'booster' proliferative signal to ensure sufficient somatic cells to form a stalk. A number of genetic manipulations [elevated da; ectopic N^intra, Dl, unpaired (upd) or hh, and clones of patched^- or of double mutant Protein kinaseA^- (PKA/Pka-C1) Suppressor of fused^- (Su(fu))] lead to overproduction of somatic cells within the germarium, and in every case the result is excess interfollicular cells. (To what extent these interfollicular cells organize into recognizable stalks likely reflects each genotype's impact on stalk cell differentiation.) These phenotypes implicate all of these genes in proliferation control: the N pathway (N, Dl), the JAK/STAT pathway (upd), and the hh pathway [hh, ptc, PKA, Su(fu)]. A proliferative role for the N pathway is substantiated by reconsideration of the loss-of-function phenotypes (N^ts1, fng), in which 'stalk cells' are observed within the follicular epithelium; the N pathway may be generally required for somatic proliferation, like da. However, for the JAK/STAT pathway, whose only known ligand is encoded by upd, expression of the ligand is restricted to the polar cells in the ovary. Although other studies have demonstrated roles for the JAK/STAT pathway that are limited to polar and stalk cell specification and/or differentiation during follicle formation, the effects of ectopic upd, together with the genetic interaction phenotypes between da and JAK/STAT pathway mutants, suggest that JAK/STAT is also a regulator of proliferation in the ovary, as it is elsewhere. If so, this would require expression of upd in region 2b, earlier than previously detected. Finally, complete polar cell differentiation includes refinement to two polar cells; a number of markers (FasIII, A101, fng and PZ80) show that variable numbers (4-8 between adjacent germline cysts) of polar cells form but always refine to 2 per pole by the time a follicle matures to stage 4. In a number of mutants, including da, excess polar cells often persist past stage 4, suggesting a failure in refinement (Smith, 2002).

The differentiation of stalk cells (step 7) also requires da. In weak da^- phenotypes where cells expressing stalk cell markers were seen in clusters physically isolated from the germline cyst by an epithelial layer, the 'stalk cells' did not converge and extend to form a stalk, as wild-type stalk cells would. This failure to form stalks could result from defects in recruitment of stalk cells by polar cells (step 6), in stalk cell differentiation, or both. Consistent with a proactive role for da in stalk cell differentiation, genotypes with elevated da levels occasionally formed stalk-like structures at the expense of the follicular epithelium. Indeed, Da protein levels normally remain high in the stalk and polar cells, even after they have dropped in the follicular epithelium. Additionally, stalk-like differentiation of the excess somatic cells generated by ectopic hh is da-dependent: reduced da results in more aggregation (i.e. less convergence and extension) and increased da results in more convergence and extension (i.e. less aggregation). In this context, the 'lollipop' phenotype reflects the acquisition of a stalk-like characteristic by excess somatic cells; hh-induced somatic cells that rearrange in the vitellarium to form cables running along the sides of follicles converge and extend to form a 'lollipop', when da is increased. Relatively high Da levels appear to be required to drive convergence and extension in differentiating stalk cells. This da requirement may involve transcriptional activation of stalk-specific genes, since the expression of two stalk cell markers is reduced in strong da^- phenotypes. However, reduced marker expression could result indirectly from incorporation of 'stalk cells' into the follicular epithelium, where stalk-specific gene expression may be repressed (Smith, 2002).

Successful follicle formation requires the right balance of somatic cells per germline cyst, such that ratios that are too low activate germline apoptosis to abort cyst progression in the germarium; the function of this cyst progression checkpoint was first demonstrated in nutrient-deprived flies. The mechanism for assessing the soma-to-germline ratio is completely unknown; however, the relative balance of cells is evaluated as each 16-cell cyst enters region 2b of the germarium. Thus, environmental variables such as nutrition could lead to activation of the cyst progression checkpoint either by increasing germline cyst production or retarding somatic cell production. If these two cell populations (germline and soma) have different nutritional requirements, cyst apoptosis might be activated only at nutritional extremes: at one extreme (low nutritional values), slackened somatic cell proliferation does not keep pace with normal cyst production, resulting in aborted cyst progression, while at the other extreme (high nutritional values), accelerated cyst production outpaces normal somatic cell proliferation, resulting in a similar termination of cyst progression. Only in situations in which the rates of cyst production and somatic cell proliferation are balanced (e.g., intermediate nutritional values) would activation of the cyst progression checkpoint be unnecessary. Age could affect the proliferation rate of either of these two cell populations, and the frequency of cyst apoptosis in the germarium does increase with age. Other environmental conditions that have been shown to affect egg production, such as temperature, humidity, prior anesthesia, adult crowding, mate abundance and dessication state, should be examined similarly for effects on cyst progression. Checkpoint activation is also influenced by the genetic background since the frequency of apoptosis varies among wild-type strains. How a somatic cell deficit, once detected, leads to activation of apoptosis in the germline cyst is unknown; however, evidence indicates that somatic cells are involved in the process (Smith, 2002).

Numerous observations from this analyses of da mutant phenotypes identify da as a key component in the soma's regulation of the cyst progression checkpoint. The significant reduction in cyst degradation (as viewed either by Acridine Orange staining or TUNEL) in da loss-of-function mutants indicates that the checkpoint is da-dependent, and since Da protein is absent from the germline, it is the gene's somatic dose that is critical in this process. This is consistent with the phenotype caused by moderate elevations of da (by chromosomal duplications) in which the checkpoint appears to function normally; the increased somatic cell production provided by the weaker duplication only results in longer interfollicular stalks, while the stronger duplication additionally results in more cysts surviving the checkpoint and being packaged into follicles due to the further increased production of somatic cells. Additional evidence that da normally contributes to the checkpoint comes from the analysis of the effects of higher elevations of da, which can lead to ectopic cyst degradation in the germarium. The synergistic interaction between the da dose and an environmental variable (i.e. cyst degradation in flies with elevated da levels increased with age) suggests that environmental conditions can sensitize the checkpoint to activation by da. Moreover, the apoptotic checkpoint is only activated in post-mitotic cysts, since elevated da does not lead to the degradation of still-dividing cysts, even when these slip into region 2b or 3 of the germarium. Although only post-mitotic cysts appear capable of activating the apoptosis pathway, cells in the adjacent soma are responsible for monitoring the germline cyst/somatic cell balance and sending an activating signal. The role of da in those cells could entail either positive or negative regulation. For positive regulation, da⁺ would promote the generation of a proapoptotic factor as an integral part of the checkpoint. For negative regulation, da⁺ would repress an antiapoptotic (i.e. prosurvival) factor; such a factor would normally be required for the maintenance of post-mitotic cysts in the germarium. The identification of additional genetic components of the checkpoint will help distinguish between these two models (Smith, 2002).

Effects of Mutation or Deletion

The maternal effect caused by the daughterless mutation depends on the sex of the progeny. Homozygous daughterless mothers produce only males. Their eggs cannot support female development (Cline, 1976). Daughterless is required to activate Sex lethal, the key gene for sex determination (Bier, 1992).

The requirement for da function in photoreceptor cells of the developing eye was examined in clones of mutant cells for da. Mutant cells most often give rise to a narrow anterior-posterior scar across the eye. Mutant cells never express the 22C10 antigen (see Futsch), which is expressed by differentiated photoreceptor cells R1-R8. Staining with anti-DA antibody reveals that DA protein is present within all eye disc cells, but there is elevated expression in the morphogenetic furrow. At the posterior portion of the furrow, R8 cells appear to have elevated DA levels. The anterior boundary of elevated DA levels within the furrow is adjacent to but does not overlap the posterior edge of Hairy protein expression. Hairy is a negative regulator of neuronal photoreceptor development. When da mutant patches encampass the morphogenetic furrow, apical cell constrictions are lacking, suggesting that furrow progression may stop in the absence of da function. Loss of da expression results in a absence of dividing M phase cells posterior to the furrow, including the second wave of mitosis that occurs after passing of the furrow (Brown, 1996).

The expression of the MyoD gene homolog, nautilus (nau), in the Drosophila embryo defines a subset of mesodermal cells known as the muscle 'pioneer' or 'founder' cells. These cells are thought to establish the future muscle pattern in each hemisegment. Founders appear to recruit fusion-competent mesodermal cells to establish a particular muscle fiber type. In support of this concept every somatic muscle in the embryo is associated with one or more nautilus-positive cells. However, because of the lack of known (isolated) nautilus mutations, no direct test of the founder cell hypothesis has been possible. Toxin ablation and genetic interference by double-stranded RNA (RNA interference or RNA-i) have been used to determine both the role of the nautilus-expressing cells and the nautilus gene, respectively, in embryonic muscle formation. In the absence of nautilus-expressing cells muscle formation is severely disrupted or absent. A similar phenotype is observed with the elimination of the nautilus gene product by genetic interference upon injection of nautilus double-stranded RNA (Misquitta, 1999).

The results from the injection of nautilus dsRNA point to a more general approach for the analysis of gene function during Drosophila development and suggest that the RNA interference method essentially would mimic a gene knock-out in the injected generation of Drosophila embryos. To test this idea a variety of cDNA clones were obtained representing a maternal gene expressed in the embryo (daughterless); additional genes involved in myogenesis (S59, DMEF2); homeobox genes (engrailed and S59); a gene important for gastrulation (twist), and a gene expressed in the adult eye (white). This panel of genes covers most stages of Drosophila development. twist was initiatially tested because the mutant has a clear phenotype that is easy to score when compared with wild-type larva. The injection of twist dsRNA (the complete coding region) into embryos produces a twisted larval phenotype that is indistinguishable from the original twist mutation. Similarly, injection of the first 1,200 bp of engrailed dsRNA produces the compressed dentical belt pattern characteristic of an engrailed null mutant. Daughterless mRNA is both maternally loaded and expressed zygotically, and the mutant phenotype produces very characteristic disruptions in the central nervous system (CNS) and peripheral nervous system (PNS). It has been shown previously that mex3, a maternally loaded RNA in C. elegans, can be ablated by dsRNA injection into the gonads. daughterless dsRNA (complete coding region) was injected and the characteristic neuronal phenotypes were sought by using the mAb MAB 22C10. The CNS as well as the PNS were disrupted to varying degrees in the injected embryos. The severity of the phenotype consistently shows a CNS disruption with a variable PNS pattern, possibly reflecting the fact that the CNS is formed before the PNS. This result suggests that maternally loaded as well as zygotically expressed RNA can be affected by RNA-i in Drosophila. The homeobox gene S59 marks a subset of muscle founder cells for 5 of 29 muscles in each hemisegment of the embryo corresponding to muscles 5, 18, 25, 26, and 27. Embryos with an S59 lacZ transgene marking muscles 18 and 25 were injected with S59 dsRNA (complete coding region). In this case, the S59-specific lacZ antibody-staining pattern is abolished. The total muscle pattern for embryos injected with S59 dsRNA, although disrupted, still shows the presence of poorly organized muscle groups in each hemisegment. This is unlike the almost complete absence of muscle observed with the injection of nautilus dsRNA. DMEF2, a member of the MADS domain transcription factor family, is essential for muscle formation in Drosophila. The DMEF2 / embryo has no muscle and is missing the characteristic gut constrictions found in the uninjected embryo. Injection of DMEF2 dsRNA (complete coding region) results in embryos that lack any detectable muscle and an absence of gut morphology (Misquitta, 1999).

In mutants lacking both the Achaete-Scute Complex (ASC) and atonal, coding for proteins known to establish SOP cell fate, two to three neurons of the solo-MD type remain in the dorsal region of each abdominal hemisegment. The identity of these remaining neurons are controlled by absent MD neurons and olfactory sensilla (amos). Both the ASC and Atonal physically interact with the protein Daughterless (Da). Since the solo-MD neurons that exist in ASC;ato double mutants are eliminated in da mutants, this observation implies that the proneural gene for those solo-MD neurons also encodes a transcription factor of the bHLH family (Huang, 2000).

SOP formation is very sensitive to simultaneous reduction in copy number of proneural genes and da, which encodes their common heterodimer partner. For instance, simultaneously removing one copy of the AS-C and da genes (i.e., transheterozygotes) results in adults with a proportion of missing sensory bristles. Likewise, reducing the dosage of ato and da genes results in reduction in the number of chordotonal organs and sensilla coeloconica. Although there is no specific mutation of amos, lethal chromosomal deficiencies have been investigated that delete amos (Df(2L)M36F-S5 and Df(2L)M36F-S6) for genetic interaction with da. Olfactory sensillum numbers were analyzed in flies heterozygous for an amos deficiency either alone or in combination with the loss of one copy of da (Df(2R)da^KX136/+). In flies with a single copy of each gene (abbreviated as amos^+/-:da^+/-), the number of sensilla basiconica is significantly reduced (by 30%) compared with wild-type, amos^+/-, da^+/-, or ato^+/-:da^+/- flies. This genetic interaction suggests functional cooperation between da and a bHLH gene in the amos genomic region. Given that amos is the only bHLH-encoding gene in this region, these data are consistent with the function of Amos/Da heterodimers during the formation of sensilla basiconica. Sensilla trichodea are also significantly reduced in amos^+/-:da^+/- flies, but a reduction in amos^+/- flies, when compared with wild-type was also observed. Therefore, although consistent with a requirement for amos in trichodea formation, such a requirement seems to be less sensitive to da gene dosage. Although sensillum coeloconica numbers are rather variable, amos^+/-:da^+/- flies have only slightly fewer sensilla coeloconica than flies with either mutation alone, whereas there are significantly fewer sensilla coeloconica in ato^+/-:da^+/- flies as expected. In summary, these data support a role for the chromosomal region containing amos in sensillum basiconica formation and are suggestive of a role during sensillum trichodea development (Golding, 2000).

Evidence that nervy, the Drosophila homolog of ETO/MTG8, promotes mechanosensory organ development by enhancing Notch signaling: nvy interacts with daughterless

In the imaginal tissue of developing fruit flies, achaete (ac) and scute (sc) expression defines a group of neurally-competent cells called the proneural cluster (PNC). From the PNC, a single cell, the sensory organ precursor (SOP), is selected as the adult mechanosensory organ precursor. The SOP expresses high levels of ac and sc and sends a strong Delta (Dl) signal, which activates the Notch (N) receptor in neighboring cells, preventing them from also adopting a neural fate. Previous work has determined how ac and sc expression in the PNC and SOP is regulated, but less is known about SOP-specific factors that promote SOP fate. This study describes the role of nervy (nvy), the Drosophila homolog of the mammalian proto-oncogene ETO, in mechanosensory organ formation. Nvy is specifically expressed in the SOP, where it interacts with the Ac and Sc DNA binding partner Daughterless (Da) and affects the expression of Ac and Sc targets. nvy loss- and gain-of-function experiments suggest that nvy reinforces, but is not absolutely required for, the SOP fate. A model is proposed in which nvy acts downstream of ac and sc to promote the SOP fate by transiently strengthening the Dl signal emanating from the SOP (Wildonger, 2005).

These results suggest that Nvy plays a role, albeit subtle, in the SOP's ability to send a strong Dl signal to neighboring cells. Although the data demonstrate that nvy is not required for the SOP fate, it is suggested that the ability of Nervy to increase the Dl signal sent by the SOP helps to reinforce the SOP fate. When nvy is ectopically expressed it completely inhibits the formation of mechanosensory organs. Using reagents that mark the PNC and SOP, it was found that ectopic Nvy blocks the formation of the SOP, but not the PNC. In contrast, elevating Nvy levels specifically within the SOP (using neur-Gal4) does not affect sensory organ development, indicating that ectopic Nvy blocks the formation of the SOP but does not inhibit its development once it is specified. Furthermore, ectopic Nvy does not block mechanosensory organ formation when Sens is also over-expressed, suggesting that ectopic Nvy blocks SOP formation before there are high levels of Sens in the nascent SOP. Consistent with this idea, no Sens expression is observed in the pnr domain of pnr-Gal4 UAS-nvy wing discs or in clones that ectopically express Nvy. These data suggest that ectopic Nvy interferes with SOP formation at a stage before Sens is expressed, which corresponds to when the SOP is initially specified (Wildonger, 2005).

nvy is normally expressed in the SOP shortly after Ac and Sc levels increase. Given the expression of endogenous nvy within the SOP, the following two possibilities werre considered to explain the ectopic Nvy phenotype and to gain some clues about wild type function of nvy. (1) It is possible that ectopic Nvy blocks SOP formation cell autonomously by inhibiting the expression of ac, sc, or their downstream targets (such as sens) that are necessary for SOP formation. (2) It is possible that ectopic Nvy acts cell non-autonomously by enhancing Dl signaling, resulting in the 'mutual inhibition' of cells expressing precociously high levels of nvy. A closer examination of clones that ectopically express Nvy revealed that SOPs were significantly less likely to form near the borders of Nvy expressing clones than control clones. These results suggest that Nvy is acting, at least in part, cell non-autonomously, perhaps by increasing the strength of the Dl signal (the possibility that Nvy may also act cell autonomously is discussed in the following section). As a test of this idea, Nvy was ectopically expressed in clones lacking nic, which encodes a transmembrane protein required for cleaving and activating N in response to ligand binding. Ectopic Nvy was unable to block SOP formation in nic mutant clones, demonstrating that Nvy's ability to block SOP formation requires the N signaling pathway to be intact. This finding is therefore consistent with the idea that Nvy normally enhances the level of active Dl in the SOP. Importantly, loss-of-function nvy experiments are also consistent with this proposed role for Nvy. Using two different methods to remove nvy (expressing nvy RNAi or generating clones of a nvy deficiency), it was found that PNC cells that neighbor nvy⁻ clones are more likely to adopt the SOP fate than PNC cells that neighbor wild type clones. This result is similar to what was observed when the relative amount of Dl differs between neighboring PNC cells: PNC cells that neighbor cells with less Dl are more likely to differentiate as SOPs. In contrast to the Dl experiments, however, the complete absence of nvy did not cause all PNCs to become SOPs. Keeping in mind that nvy expression is restricted to the SOP (nvy is not detectably expressed in the PNC), these data suggest that nvy is not a general regulator of Dl signaling throughout the PNC, but that nvy enhances Dl activity in the SOP when it is forming (Wildonger, 2005).

Although these experiments are consistent with the idea that nvy enhances Dl signaling in the SOP, no changes in Dl protein levels were directly detected in either nvy loss- or gain-of-function situations. There are several possible explanations for this negative result: (1) it is possible that nvy does affect Dl expression levels, but that the change is too slight or brief to distinguish with the available anti-Dl antibody; (2) nvy might not affect Dl expression, but affect its localization and/or signaling ability in a manner that cannot be detected in these experiments; (3) it is also possible that nvy does not affect Dl at all, but interacts with other factors to produce the phenotypes observed. It is suggested that experiments using VP16-Nvy help to distinguish between these possibilities. Expressing VP16-Nvy produces results opposite to those resulting from expressing Nvy: VP16-Nvy enhances E-lacZ expression, which ectopic Nvy represses, and its expression results in ectopic Sens⁺ SOPs. Based on these data and the evidence that ETO, the mammalian homolog of Nvy, acts as a transcriptional repressor, it is suggested that VP16-Nvy acts as a transcriptional activator of targets that wild type Nvy normally represses. When expressed in a PNC, VP16-Nvy strongly reduces the amount of Dl observed at the cell surface and in intracellular vesicles. This result suggests that wild type Nvy has the potential to affect Dl, although the result does not distinguish an effect on expression from an effect on protein stability or trafficking. That ectopic Nvy does not inhibit the expression of Dl-lacZ suggests that Nvy may be more likely to transcriptionally regulate a factor is involved in Dl stability or trafficking. Regardless of the mechanism, the finding that VP16-Nvy reduces Dl levels suggests that wild type Nvy has the potential to increase Dl levels, a proposal that is consistent with loss- and gain-of-function experiments (Wildonger, 2005).

The VP16-Nvy results, while consistent with the idea that Nvy affects Dl, do not explain why no change in Dl levels were detected in nvy loss- and gain-of-function experiments. Thus, it is thought that Nvy causes a small and/or transient increase in Dl activity (by affecting its expression, stability or localization). Nevertheless, no change in the amount or localization of Dl in wild type SOPs has been observed, despite genetic evidence that Dl signaling is a critical step in SOP fate determination. The lack of an observable change in Dl during wild type development, in combination with the current findings, lead to a proposal that the presumptive SOP may send a transient pulse of increased Dl signal that is sufficient to bias cell fates within the PNC. Nvy may, therefore, contribute to this transient pulse of Dl (Wildonger, 2005).

The experiments described here shed some light on the molecular activities Nvy has in the SOP. (1) Based on its ability to repress well-defined lacZ reporter genes, Nvy appears to be a transcriptional repressor, as is its mammalian homolog ETO. (2) This study shows that ectopic Nvy appears to interfere with the function (as opposed to the expression) of Ac and Sc because re-supplying Ac and Sc in pnr-Gal4 UAS-nvy flies was unable to rescue the bald phenotype. In contrast, expression of Da, a bHLH DNA binding partner for Ac and Sc, was able to partially rescue the bald phenotype of pnr-Gal4 UAS-nvy flies. Moreover, nvy and da were found to genetically interact (e.g., reducing nvy levels enhanced a da gain-of-function phenotype), and Nvy and Da were found to physically interact. These findings are consistent with a recent report showing that ETO directly interacts with HEB, a bHLH factor in the same class as Da. The domain through which ETO interacts with HEB (and other mammalian class I bHLH transcription factors) is conserved in Nvy, and HEB's ETO interaction domain is found in Da. These data lead to a proposal that Nvy, a presumptive transcriptional repressor, has the ability to function with Ac/Da and Sc/Da heterodimers to repress the transcription of some target genes. In the absence of Nvy, such as in the non-SOP cells of a PNC, Ac/Da and Sc/Da may have the potential to activate these same target genes. However, these experiments also suggest that the interaction between Nvy and Da may not be required for all of Nvy's functions because VP16-Nvy is able to lower Dl levels even in da mutant clones. One potential explanation for this Da-independent function is that Nvy may be able to directly interact with DNA. In summary, it is speculated that the Nvy–Da interaction is only required for the regulation of a subset of target genes (Wildonger, 2005).

The proposal that Nvy works with Ac/Da and Sc/Da to repress target genes may on the surface seem at odds with the suggestion that Nvy can transiently increase the levels of Dl, because it is thought that Ac/Da and Sc/Da heterodimers activate Dl expression in the SOP. However, it is not known if Dl levels are in fact directly increased by Ac/Sc. It is stressed that the timing of expression of these genes is critical to understanding how they function in vivo. Based on the wild type timing of its expression, nvy is likely to be a target of Ac/Sc in the presumptive SOP. Accordingly, there will be a window of time when Ac/Sc levels are high and Nvy levels are low in the presumptive SOP. This window of time may be sufficient for Ac/Sc to affect Dl expression and initiate the bias in favor of the SOP fate. Once Nvy levels increase, it may then work with Ac/Sc to repress the expression of some target genes, some of which may cause a further increase in Dl signaling. However, it is hypothesized that nvy's role in this process is after the bias has already been initiated (Wildonger, 2005).

In summary, it is suggested that Nvy plays a subtle but observable role in the establishment of the SOP fate. Although it is not essential for the SOP fate, it may be that Nvy helps the SOP/non-SOP bias by increasing the strength of the Dl signal sent by the SOP. Because nvy is evolutionarily conserved, both in its protein sequence and nervous system expression, it is suggested that this role, although subtle, is important for the stereotyped uniformity of mechanosensory organ development. In addition, nvy may also play a role in later stages of neurogenesis, in particular axon pathfinding. Because of Nvy's role as a transcriptional repressor, it is further suggested that Nvy increases the Dl signal indirectly, by repressing a gene (factor X) that normally inhibits Dl activity. Based on Nvy's ability to interact with Da, this hypothetical target may be repressed by Nvy in combination with Ac/Da and Sc/Da heterodimers. Interestingly, it follows that in non-SOP cells of the PNC, which express ac and sc but not nvy, this hypothetical target may continue to be expressed, helping to downregulate Dl activity in these cells and thereby further increase the SOP/non-SOP bias. Clearly, the test of this proposal requires the identification of factor X as well as a more detailed understanding of how Dl levels and activity are modulated in the SOP (Wildonger, 2005).

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date revised: 20 December 2007

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