A novel member of the Xenopus Zic family, Zic5, mediates neural crest development

We characterized Xenopus Zic5 which belongs to a novel class of the Zic family. Zic5 is more specifically expressed in the prospective neural crest than other Zic genes. Overexpression of Zic5 in embryos led to ectopic expression of the early neural crest markers, Xsna and Xslu, with the loss of epidermal marker expression. In Zic5-overexpressing animal cap explants, there was marked induction of neural crest markers, without mesodermal and anterior neural markers. This was in contrast to other Xenopus Zic genes, which induce both anterior and the neural crest markers in the same assay. Injection of a dominant-negative form of Zic5 can block neural crest formation in vivo. These results indicate that Zic5 expression converts cells from an epidermal fate to a neural crest cell fate. This is the first evidence for neural crest tissue inductive activity separate from anterior neural tissue inductive activity in a Zic family gene.     

Hox proteins coordinate peripodial decapentaplegic expression to direct adult head morphogenesis in Drosophila

The Drosophila BMP, decapentaplegic (dpp), controls morphogenesis of the ventral adult head through expression limited to the lateral peripodial epithelium of the eye-antennal disc by a 3.5kb enhancer in the 5' end of the gene. We recovered a 15bp deletion mutation within this enhancer that identified a homeotic (Hox) response element that is a direct target of labial and the homeotic cofactors homothorax and extradenticle. Expression of labial and homothorax are required for dpp expression in the peripodial epithelium, while the Hox gene Deformed represses labial in this location, thus limiting its expression and indirectly that of dpp to the lateral side of the disc. The expression of these homeodomain genes is in turn regulated by the dpp pathway, as dpp signalling is required for labial expression but represses homothorax. This Hox-BMP regulatory network is limited to the peripodial epithelium of the eye-antennal disc, yet is crucial to the morphogenesis of the head, which fate maps suggest arises primarily from the disc proper, not the peripodial epithelium. Thus Hox/BMP interactions in the peripodial epithelium of the eye-antennal disc contribute inductively to the shape of the external form of the adult Drosophila head.     

Anchor negatively regulates BMP signalling to control Drosophila wing development

G protein-coupled receptors play particularly important roles in many organisms. The novel Drosophila gene anchor is an orthologue of vertebrate GPR155. However, the roles of anchor in molecular functions and biological processes, especially in wing development, remain unknown. Knockdown of anchor resulted in an increased wing size and additional and thickened veins. These abnormal wing phenotypes were similar to those observed in BMP signalling gain-of-function experiments. We observed that the BMP signalling indicator p-Mad was significantly increased in wing discs in which anchor RNAi was induced in larvae and accumulated abnormally in intervein regions in pupae. Furthermore, the expression of target genes of the BMP signalling pathway was examined using a lacZ reporter, and the results indicated that omb and sal were substantially increased in anchor-knockdown wing discs. An investigation of genetic interactions between Anchor and the BMP signalling pathway revealed that the thickened and ectopic vein tissues were rescued by knocking down BMP levels. These results suggested that Anchor functions to negatively regulate BMP signalling during wing development and vein formation.     

Molecular integration of wingless, decapentaplegic, and autoregulatory inputs into Distalless during Drosophila leg development

The development of the Drosophila leg requires both Decapentaplegic (Dpp) and Wingless (Wg), two signals that establish the proximo-distal (PD) axis by activating target genes such as Distalless (Dll). Dll expression in the leg depends on a Dpp- and Wg-dependent phase and a maintenance phase that is independent of these signals. Here, we show that accurate Dll expression in the leg results from the synergistic interaction between two cis-regulatory elements. The Leg Trigger (LT) element directly integrates Wg and Dpp inputs and is only active in cells receiving high levels of both signals. The Maintenance (M) element is able to maintain Wg- and Dpp-independent expression, but only when in cis to LT. M, which includes the native Dll promoter, functions as an autoregulatory element by directly binding Dll. The "trigger-maintenance" model describes a mechanism by which secreted morphogens act combinatorially to induce the stable expression of target genes.     

Dynamic decapentaplegic signaling regulates patterning and adhesion in the Drosophila pupal retina

The correct organization of cells within an epithelium is essential for proper tissue and organ morphogenesis. The role of Decapentaplegic/Bone morphogenetic protein (Dpp/BMP) signaling in cellular morphogenesis during epithelial development is poorly understood. In this paper, we used the developing Drosophila pupal retina--looking specifically at the reorganization of glial-like support cells that lie between the retinal ommatidia--to better understand the role of Dpp signaling during epithelial patterning. Our results indicate that Dpp pathway activity is tightly regulated across time in the pupal retina and that epithelial cells in this tissue require Dpp signaling to achieve their correct shape and position within the ommatidial hexagon. These results point to the Dpp pathway as a third component and functional link between two adhesion systems, Hibris-Roughest and DE-cadherin. A balanced interplay between these three systems is essential for epithelial patterning during morphogenesis of the pupal retina. Importantly, we identify a similar functional connection between Dpp activity and DE-cadherin and Rho1 during cell fate determination in the wing, suggesting a broader link between Dpp function and junctional integrity during epithelial development.     

A Drosophila growth factor homolog, decapentaplegic, regulates homeotic gene expression within and across germ layers during midgut morphogenesis

The decapentaplegic (dpp) gene product, a member of the transforming growth factor-beta family, is required in Drosophila embryos for normal gastrulation and the establishment of dorsal-ventral polarity in the embryo. dpp is also expressed at specific positions in the visceral mesoderm along the developing midgut. We find that mutations that eliminate the visceral mesoderm expression of dpp lead to defects in midgut morphogenesis and alter the spatially localized expression of the homeotic genes Sex combs reduced (Scr), Ultrabithorax (Ubx), and Antennapedia (Antp) in the visceral mesoderm. The extracellular dpp protein migrates from the visceral mesoderm across the apposing endodermal cell layer in a region of the endoderm that expresses the homeotic gene labial (lab). Mesodermal expression of dpp is required for the expression of lab in these endodermal cells indicating that dpp mediates an inductive interaction between the two germ layers. We propose that extracellular dpp protein regulates gut morphogenesis, in part, by regulating homeotic gene expression in the visceral mesoderm and endoderm of the developing midgut.     

Localized enhancement and repression of the activity of the TGF-beta family member, decapentaplegic, is necessary for dorsal-ventral pattern formation in the Drosophila embryo.

Seven zygotically active genes are required for normal patterning of the dorsal 40% of the Drosophila embryo. Among these genes, decapentaplegic (dpp) has the strongest mutant phenotype: in the absence of dpp, all cells in the dorsal and dorsolateral regions of the embryo adopt fates characteristic of more ventrally derived cells (Irish and Gelbart (1987) Genes Dev. 1, 868-879). Here we describe the phenotypes caused by alleles of another of this set of genes, tolloid, and show that tolloid is required for dorsal, but not dorsolateral, pattern. Extragenic suppressors of tolloid mutations were isolated that proved to be mutations that elevate dpp activity. We studied the relationship between tolloid and dpp by analyzing the phenotypes of tolloid embryos with elevated numbers of the dpp gene and found that doubling the dpp+ gene dosage completely suppressed weak tolloid mutants and partially suppressed the phenotypes of tolloid null mutants. We conclude that the function of tolloid is to increase dpp activity. We also examined the effect of doubling dpp+ gene dosage on the phenotypes caused by other mutations affecting dorsal development. Like tolloid, the phenotypes of mutant embryos lacking shrew gene function were suppressed by elevated dpp, indicating that shrew also acts upstream of dpp to increase dpp activity. In contrast, increasing the number of copies of the dpp gene enhanced the short gastrulation (sog) mutant phenotype, causing ventrolateral cells to adopt dorsal fates. This indicates that sog gene product normally blocks dpp activity ventrally. We propose that the tolloid, shrew and sog genes are required to generate a gradient of dpp activity, which directly specifies the pattern of the dorsal 40% of the embryo.     

Role of BMP family members during kidney development.

Members of the Bone morphogenetic protein (BMP) family have been shown to be important signaling molecules throughout mouse development. Accordingly, many BMPs are also expressed during organogenesis of the metanephric kidney. However, only BMP7 has been shown to be absolutely required for proper formation of the kidney, thus the majority of information known involves this family member. BMP7 is expressed in both the ureteric epithelium and the mesenchyme throughout embryonic development and has been shown to function as a survival factor for the nephrogenic mesenchyme. However, there has been some controversy over the role of BMP7 as an inducing molecule for the metanephric mesenchyme. Recent studies have shown that BMP7 functions as an anti-differentiation factor for this mesenchyme cell population. The function of BMPs in the ureter and in the more differentiated epithelial structures of the nephron is less well understood.     

The Drosophila tumor suppressor expanded regulates growth, apoptosis, and patterning during development

The Drosophila expanded (ex) gene encodes a protein thought to play a role in signaling at apical junctions of epithelial cells. Previous studies have characterized this gene as a tumor suppressor involved in regulating the growth of a subset of Drosophila imaginal discs (Boedigheimer, M., Laughon, A., 1993. expanded: a gene involved in the control of cell proliferation in imaginal discs, Development 118, 1291-1301); although ex negatively regulates cell proliferation in the developing wing, it appeared to have a conflicting role in the eye. In contrast, our analysis of the loss-of-function phenotype indicates that ex does, in fact, regulate growth in the eye. We also show that this gene plays a role in patterning of the eye, mainly at the level of planar polarity. Our studies further demonstrate that, contrary to what was expected based on loss-of-function data, the tissue reduction phenotypes resulting from Ex overexpression are attributable to the induction of apoptotic cell death. Taken together, our data suggest that Ex is a versatile molecule that plays a role in most of the processes that govern disc development.     

The patterns of wingless,decapentaplegic, and tinman position the Drosophila heart

Two secreted signaling molecules, wingless (wg) and decapentaplegic (dpp), are required to specify the heart in Drosophila. wg and dpp are also required to specify other cell types within the mesoderm and in many other regions of the embryo. Because the spatial patterns of wg and dpp are dynamic, different populations of mesodermal cells are exposed to different combinations of wg and/or dpp at different times. To determine whether the patterns of wg and dpp expression provide unique positional information for the specification of heart precursors, we altered these patterns. Our data suggest that wg and dpp contribute progressively to the elaboration of the expression pattern of the mesoderm-specific homeobox-containing gene tinman (tin), and that the overlap of wg and dpp at an early stage (9) as well as at a later stage (11) in the presence of tin-expressing cells directs cardiac-specific differentiation. Furthermore, ectopic tin expression in the ectoderm at wg/dpp intersects (the primordia of the thoracic imaginal disks) also leads to cardiac-specific differentiation, suggesting that tin confers mesoderm-specificity to the wg/dpp response. We conclude that ectopic heart can be generated by altering the patterns of wg and dpp within the tin-expressing mesoderm, or by ectopic induction of tin within the wg- and dpp-expressing ectoderm.     

The Drosophila segment polarity gene patched interacts with decapentaplegic in wing development.

The decapentaplegic (dpp) gene of Drosophila melanogaster encodes a polypeptide of the transforming growth factor-beta family of secreted factors. It is required for the proper development of both embryonic and adult structures, and may act as a morphogen in the embryo. In wing imaginal discs, dpp is expressed and required in a stripe of cells near the anterior-posterior compartment boundary. Here we show that viable mutations in the segment polarity genes patched (ptc) and costal-2 (cos2) cause specific alterations in dpp expression within the anterior compartment of the wing imaginal disc. The interaction between ptc and dpp is particularly interesting; both genes are expressed with similar patterns at the anterior-posterior compartment boundary of the disc, and mis-expressed in a similar way in segment polarity mutant backgrounds like ptc and cos2. This mis-expression of dpp could be correlated with some of the features of the adult mutant phenotypes. We propose that ptc controls dpp expression in the imaginal discs, and that the restricted expression of dpp near the anterior-posterior compartment boundary is essential to maintain the wild-type morphology of the wing disc.     

The centrosomal protein CP190 regulates myosin function during early Drosophila development

Centrosomes are the main microtubule (MT)-organizing centers in animal cells, but they also influence the actin/myosin cytoskeleton. The Drosophila CP190 protein is nuclear in interphase, interacts with centrosomes during mitosis, and binds to MTs directly in vitro. CP190 has an essential function in the nucleus as a chromatin insulator, but centrosomes and MTs appear unperturbed in Cp190 mutants. Thus, the centrosomal function of CP190, if any, is unclear. Here, we examine the function of CP190 in Cp190 mutant germline clone embryos. Mitosis is not perturbed in these embryos, but they fail in axial expansion, an actin/myosin-dependent process that distributes the nuclei along the anterior-to-posterior axis of the embryo. Myosin organization is disrupted in these embryos, but actin appears unaffected. Moreover, a constitutively activated form of the myosin regulatory light chain can rescue the axial expansion defect in mutant embryos, suggesting that CP190 acts upstream of myosin activation. A CP190 mutant that cannot bind to MTs or centrosomes can rescue the lethality associated with Cp190 mutations, presumably because it retains its nuclear functions, but it cannot rescue the defects in myosin organization in embryos. Thus, CP190 has distinct nuclear and centrosomal functions, and it provides a crucial link between the centrosome/MT and actin/myosin cytoskeletal systems in early embryos.