Gene name - defective proventriculus
Cytological map position - 58D1-2
Function - transcription factor
Symbol - dve
FlyBase ID: FBgn0020307
Genetic map position -
Classification - homeodomain protein
Cellular location - nuclear
|Recent literature||Yan, J., Anderson, C., Viets, K., Tran, S., Goldberg, G., Small, S. and Johnston, R. J., (2017). Regulatory logic driving stable levels of defective proventriculus expression during terminal photoreceptor specification in flies. Development [Epub ahead of print]. PubMed ID: 28126841
How differential levels of gene expression are controlled in post-mitotic neurons is poorly understood. In the Drosophila retina, expression of the transcription factor Defective Proventriculus (Dve) at distinct cell-type-specific levels is required for terminal differentiation of color- and motion-detecting photoreceptors. This study found that the activities of two cis-regulatory enhancers are coordinated to drive dve expression in the fly eye. Three transcription factors act on these enhancers to determine cell-type-specificity. Negative autoregulation by Dve maintains expression from each enhancer at distinct homeostatic levels. One enhancer acts as an inducible backup ("dark" shadow enhancer) that is normally repressed but becomes active in the absence of the other enhancer. Thus, two enhancers integrate combinatorial transcription factor input, feedback, and redundancy to generate cell-type specific levels of dve expression and stable photoreceptor fate. This regulatory logic may represent a general paradigm for how precise levels of gene expression are established and maintained in post-mitotic neurons.
The gut epithelium of Drosophila is derived from the anterior and posterior primordia at both ends of the blastoderm embryo. These primordia are initially nonsegmental and fused into a single continuum. Secreted molecules, such as Decapentaplegic and Wingless, induce subsequent morphogenetic events that ultimately compartmentalize the primordia into morphologically distinct sectors. During this process, these signals also direct cells to distinct developmental paths thus establishing the functional organization of the midgut. The gene defective proventriculus (dve) is a target of Dpp and Wg during the establishment of two gut structures: the proventriculus (a foregut structure formed at the junction of the foregut and the midgut), and the central midgut (Nakagoshi, 1998). This overview will consider the roles of dve in the formation of these two structures.
The proventriculus is located at the caudal end of the esophagus and serves as a valve, regulating food passage into the midgut. It is composed of three tissue layers. The inner and outer layers consist of an ectodermal epithelial layer and the ensheathing visceral mesoderm, respectively. The third layer, intervening between the first two, is completely free of mesodermal tissues. This internal portion, called the cardiac valve (the proventriculus is also referred to as the cardia), is innervated by three axons from the proventricular ganglion, one of four major interconnected ganglia that together constitute the stomatogastric nervous system. The proventriculus develops at the junction of the foregut and the midgut. Initially, there is an outward buckling of the foregut tube, in a region that is free of visceral mesoderm, to form what is referred to as the keyhole structure. This area then undergoes further outward movement and will then fold back on itself and move inward to form the mature, multi-layered proventriculus (Pankratz, 1995).
The keyhole region expresses hh and wg: the activities of both these genes are essential for the subsequent posterior migration of the foregut, both toward and into the foregut, the anterior-most region of midgut (Pankratz, 1995). The anterior-most midgut, consisting of visceral mesoderm and endoderm (Pankratz, 1995), contributes to the outer layer of the proventriculus after stage 16 (Nakagoshi, 1998 and references). Thus the outer wall of the proventriculus is composed of inner endodermal and outer mesodermal tissues. The outer wall of the proventriculus, in particular the endodermal component, expresses dve (Nakagoshi, 1998).
The reduced body sizes of dve mutant larvae suggest that something in the way food is utilized is affected by the dve mutation. Colored yeast fed to heterozygous larvae stains red throughout the length of the gut, but the same colored yeast accumulates only in the proventriculus in dve mutant larvae. Consistent with this observation, the proventriculus is found to form aberrently in dve mutant larvae. In the wild type, cell movement leads to formation of the internal portion of the proventriculus during embryonic stages 16-17; cells of the foregut epithelium invaginate into the anterior-most midgut that normally expresses dve. In dve embryos, the cell migration is greatly delayed and the internalization is only temporary. As a result, dve larvae cannot form the three-layered structure of the proventriculus, the same failure that is observed in hh or wg mutant embryos. Since the dve-expressing anterior-most midgut constitutes the outer layer of the proventriculus, this dve phenotype suggests that dve activity is required for the functional development of outer layer cells and the consequent retention of the internal portion of the proventriculus. dve expression in the outer layer of the proventriculus is dependent on wingless expression in the keyhole structure. It is concluded that the Wg signal regulates dve expression during proventriculus development (Nakagoshi, 1998).
This discussion now turns from the role of dve in proventriculus development to dve's function in midgut development. The midgut consists of two germ layers: the visceral mesoderm and the endoderm. Cells of the portion of the middle midgut that are derived from the endoderm differentiate into four distinct cell types: copper, interstitial, large flat, and iron cells. These endodermal cell types are specified by Dpp and Wg, which are expressed in the adhering visceral mesoderm of the parasegments (PS) 7 and 8, respectively. Copper cells exhibit a unique morphology with banana shapes and exhibit UV light-induced fluorescence after copper feeding. These characteristics are specified by a homeotic gene, labial (lab), which is activated by the Dpp signal in the midgut. Two different thresholds of Wg define copper and large flat cells. However, it has been unclear how Lab confers the transcriptional regulation to specify copper cells. In the middle midgut, the dve gene is expressed in all precursors of the four distinct cell types; subsequent to this broad expression, dve is repressed only in copper cells. This repression is mediated by two Dpp target genes, lab and dve itself, and is also essential for the functional specification of copper cells. Thus, dve is involved in different developmental aspects of the midgut under the control of different extracellular signals (Nakagoshi, 1998).
The expression domains and regulation of labial and dve in the middle midgut were compared. It was found that there are two endodermal expression domains: one is located immediately adjacent to the visceral mesoderm, and the other in a more interior inner endoderm layer. Dpp has been shown to be sufficient to induce dve expression in the midgut without Wg. These results indicate that dve expression in the middle midgut does not depend on Wg but on Dpp. This is in contrast to dve expression during proventriculus development. lab is expressed under the control of Dpp as is dve, however, lab is regulated negatively by the Wg signal to generate a sharp posterior border. The expression of lab is observed in the endoderm just beneath the dpp-expressing visceral mesoderm of PS 7, but not in the inner endodermal cells. In contrast, dve is expressed more broadly throughout the inner endodermal layers, including presumptive interstitial cell precursors. Another difference in lab and dve expression is that dve expression is subsequently repressed in lab-expressing cells that become copper cells. The possibility that Lab might be involved in the repression of dve in copper cells was examined. In lab mutants, dve expression is not repressed in presumptive copper cells. This pattern of gene expression is similar to that of neighboring interstitial cells, which express dve continuously without lab expression in the wild type. Evidence is presented that it is unlikely that the lab mutation causes the transformation of copper cells into interstitial cells. Taken together, the repression of dve requires the activities of both Lab and Dve itself (Nakagoshi, 1998).
To determine whether the dve repression in copper cells is essential for the establishment of their correct identities, dve was overexpressed ubiquitously at stage 17. Strong heat shock-induced dve expression results in an abnormal morphology for copper cells. The typical banana shape is frequently lost, and the cells becoming circular, suggesting an abnormal cytoskeletal organization. To determine the effect of ectopic dve on the copper cell function, ubiquitous dve expression was induced using a milder heat shock. Under this condition, the copper cells appear to retain their normal morphology, however, the typical character of copper cells, UV light-induced orange fluorescence on copper feeding, is greatly reduced. This mild heat shock does not affect the posterior fluorescence attributable to iron cells, suggesting that the function of copper cells is specifically impaired by this treatment. Taken together with the results for dve mutants described above, both the loss of function and ectopic expression of the dve gene affect the morphology of copper cells, and ectopic dve expression impairs the function of copper cells without affecting their morphology. These results indicate that temporally restricted dve repression is essential for this functional specification, in addition to the dve gene, which is indispensable for copper cell development. This repression depends on Lab and Dve itself. Thus, the cross-regulation of the two Dpp target genes (dve and lab) specifies the functional identity of copper cells (Nakagoshi, 1998).
A pair of the Drosophila eye-antennal disc gives rise to four distinct organs (eyes, antennae, maxillary palps, and ocelli) and surrounding head cuticle. Developmental processes of this imaginal disc provide an excellent model system to study the mechanism of regional specification and subsequent organogenesis. The dorsal head capsule (vertex) of adult Drosophila is divided into three morphologically distinct subdomains: ocellar, frons, and orbital. The homeobox gene orthodenticle (otd) is required for head vertex development, and mutations that reduce or abolish otd expression in the vertex primordium lead to ocelliless flies. The homeodomain-containing transcriptional repressor Engrailed (En) is also involved in ocellar specification, and the En expression is completely lost in otd mutants. However, the molecular mechanism of ocellar specification remains elusive. This study provides evidence that the homeobox gene defective proventriculus (dve) is a downstream effector of Otd, and also that the repressor activity of Dve is required for en activation through a relief-of-repression mechanism. Furthermore, the Dve activity is involved in repression of the frons identity in an incoherent feedforward loop of Otd and Dve (Yorimitsu, 2011).
This study presents evidence that Dve is a new member involved in ocellar specification and acts as a downstream effector of Otd. The results also revealed a complicated pathway of transcriptional regulators, Otd-Dve-Ara-Ci-En, for ocellar specification (Yorimitsu, 2011).
Transcription networks contain a small set of recurring regulation patterns called network motifs. A feedforward loop (FFL) consists of three genes, two input transcription factors and a target gene, and their regulatory interactions generate eight possible structures of feedforward loop (FFL). When a target gene is suppressed by a repressor 1 (Rep1), relief of this repression by another repressor 2 (Rep2) can induce the target gene expression. When Rep2 also acts as an activator of the target gene, this relief of repression mechanism is classified as a coherent type-4 feedforward loop (c-FFL). During vertex development, Ara is involved in hh repression, and the Dve-mediated ara repression is crucial for hh expression and subsequent ocellar specification. However, the cascade of dve-ara-hh seems to be a relief of repression rather than a cFFL, because Dve is not a direct activator of the hh gene. Furthermore, dve RNAi phenotypes were rescued in the ara mutant background, suggesting that a linear relief of repression mechanism is crucial for hh maintenance (Yorimitsu, 2011).
In photoreceptor R7, Dve acts as a key molecule in a cFFL. Dve (as a Rep1) represses rh3, and the transcription factor Spalt (Sal) (as a Rep2) represses dve and also activates rh3 in parallel to induce rh3 expression. Interestingly, Notch signaling is closely associated with the relief of Dve-mediated transcriptional repression in wing and leg disks. These regulatory networks may also be cFFLs in which Dve acts as a Rep1, although repressors involved in dve repression are not yet identified. In wing disks, expression of wg and ct are repressed by Dve, and Notch signaling represses dve to induce these genes at the dorso-ventral boundary. The Dve activity adjacent to the dorso-ventral boundary still represses wg to refine the source of morphogen. In leg disks, Dve represses expression of dAP-2, and Notch signaling represses dve to induce dAP-2 at the presumptive joint region. The Dve activity distal to the segment boundary still represses dAP-2 to prevent ectopic joint formation. Taken together, these results suggest that Dve plays a critical role as a Rep1 in cFFLs in different tissues. In the head vertex region, it is likely that the repressor activity of Dve is repressed in a cFFL to induce frons identity (Yorimitsu, 2011).
The homeodomain protein Otd is the most upstream transcription factor required for establishment of the head vertex. During second larval instar, Otd is ubiquitously expressed in the eye-antennal disk and it is gradually restricted in the vertex primordium until early third larval instar. Expression of an Otd-target gene, dve, is also detected in the same vertex region at early third larval instar. Otd is required for Dve expression, and the Otd-induced Dve is required for repression of frons identity through the Hh signaling pathway in the medial region. However, Otd is also required for the frons identity in both the medial and mediolateral regions (Yorimitsu, 2011).
This regulatory network is quite similar to the incoherent type-1 feedforward loop (iFFL) in photoreceptor R7. Otd-induced Dve is involved in rh3 repression, whereas Otd is also required for rh3 activation. iFFLs have been known to generate pulse-like dynamics and response acceleration if Rep1 does not completely represses its target gene expression. However, the repressor activity of Dve supersedes the Otd-dependent rh3 activation, resulting in complete rh3 repression in yR7. In pR7, Dve is repressed by Sal, resulting in rh3 expression through the Otd- and Sal-dependent rh3 activation. Thus, Dve serves as a common node that integrates the two loops, the Otd-Dve-Rh3 iFFL and the Sal-Dve-Rh3 cFFL (Yorimitsu, 2011).
In the head vertex region, Otd and Dve are expressed in a graded fashion along the mediolateral axis with highest concentration in the medial region. It is assumed that Otd determines the default state for frons development through restricting the source of morphogens Hh and Wg, and also that high level of Dve expression in the medial ocellar region represses the frons identity through an iFFL. It is likely that repression of dve by an unknown repressor X occurs in a cFFL and induces the frons identity in the mediolateral region (Yorimitsu, 2011).
Interlocked FFLs including Otd and Dve appear to be a common feature in the eye and the head vertex. However, other factors are not shared between two tissues. In R7, a default state is the Otd-dependent Rh3 activation, an acquired state is (1) Rh3 repression through the Otd-Dve iFFL and (2) Spineless-dependent Rh4 expression. In the vertex, a default state is Otd-dependent frons formation, an acquired state is (1) frons repression through the Otd-Dve iFFL and (2) Hh-dependent ocellar specification associated with En and Eya activation (Yorimitsu, 2011).
Both Otd and Dve are K50-type homeodomain transcription factors, and they bind to the rh3 promoter via canonical K50 binding sites (TAATCC). The Otd-Dve iFFL in the eye depends on direct binding activities to these K50 binding sites, but the iFFL in the vertex seems to be more complex. Although target genes for frons determination are not identified, the iFFL in the vertex includes some additional network motifs. For instance, in the downstream of Dve, Hh signaling is critically required for repression of the frons identity (Yorimitsu, 2011).
Since iFFLs also act as fold-change detection to normalize noise in inputs, interlocked FFLs of Dve-mediated transcriptional repression may contribute to robustness of gene expression by preventing aberrant activation. It is an intriguing possibility that, in wing and leg disks, Dve also serves as a common node that integrates the two loops as observed in the eye and the vertex. Further characterization of regulatory networks including Dve will clarify molecular mechanisms of cell specification (Yorimitsu, 2011).
Exons - 6
The homeodomain is found near the Dve carboxy-terminal region. The Dve homeodomain contains all four invariant amino acids located within helix 3, and matches well with other highly conserved residues. However, the Dve homeodomain is unusual: it has a 10-amino-acid insertion between helices 2 and 3. The ninth amino acid of helix 3 confers the recognition specificity for its binding sequences, and the recognition helix of Dve is closest to the Orthodenticle (Otd) class homeodomain. In contrast, Dve helices 1 and 2 exhibit homology with POU homeodomains rather than Otd class homeodomains. Therefore, the Dve homeodomain seems to be a novel class of homeodomain that is intermediate between POU and Otd class homeodomains (Nakagoshi, 1998).
date revised: 21 September 98
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