BIOLOGICAL OVERVIEW

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 ëkeyholeí 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).

GENE STRUCTURE

Exons - 6

PROTEIN STRUCTURE

Amino Acids - 1019

Structural Domains

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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