Multiple requirements for the receptor serine/threonine kinase thick veins reveal novel functions of TGF beta homologs during Drosophila embryogenesis

Binding sites for transcription factor NTF-1/Elf-1 contribute to the ventral repression of decapentaplegic

The Dorsal morphogen is a transcription factor that activates some genes and represses others to establish multiple domains of gene expression along the dorsal/ventral axis of the early Drosophila embryo. Repression by Dorsal appears to require accessory proteins that bind to corepression elements in Dorsal-dependent regulatory modules called ventral repression regions (VRRs). We have identified a corepression element in decapentaplegic (dpp), a zygotically active gene that is repressed by the Dorsal morphogen. This dpp repression element (DRE) is located within a previously identified VRR and close to essential Dorsal-binding sites. We have purified a factor from Drosophila embryo extracts that binds to the DRE but not to mutant forms of the DRE that fail to support efficient repression. This protein also binds to an apparently essential region in a VRR associated with the zerknüllt (zen) gene. One of the DREs in the dpp VRR overlaps the binding site for a potential activator protein suggesting that one mechanism of ventral repression may be the mutually exclusive binding of repressor and activator proteins. We have found the DRE-binding protein to be identical to NTF-1 (equivalent to Elf-1, the product of the grainyhead gene), a factor originally identified as an activator of the Ultrabithorax and Dopa decarboxylase promoters. NTF-1 mRNA is synthesized during oogenesis and deposited in the developing oocyte where it is available to contribute to ventral repression during early embryogenesis. Previous studies have shown that overexpression of NTF-1 in the postblastoderm embryo results in a phenotype that is consistent with a role for this factor in the repression of dpp later in embryogenesis.

Ventral veinless, the gene encoding the Cf1a transcription factor, links positional information and cell differentiation during embryonic and imaginal development in Drosophila melanogaster

The ventral veinless gene (vvl) encodes the previously identified Cf1a protein, a transcription factor containing a POU-domain. During embryonic development vvl function is required for the formation of the tracheal tree and in the patterning of the ventral ectoderm. During imaginal development vvl is required for cell proliferation and the differentiation of the wing veins. vvl expression is restricted to the regions where its function is required, and is dependent on the coordinate activities of signalling molecules such as decapentaplegic, wingless and hedgehog. vvl interacts with other genes involved in vein differentiation, including veinlet, thick veins, torpedo, decapentaplegic and Notch suggesting that vvl function may affect several cell-to-cell communication pathways. We propose that the gene vvl integrates information from different signalling molecules and regulates the expression of specific cell differentiation genes during tracheal development and vein differentiation.

The Drosophila schnurri gene acts in the Dpp/TGF beta signaling pathway and encodes a transcription factor homologous to the human MBP family

Decapentaplegic (dpp), a TGF beta-related ligand, plays a key role in Drosophila development. Although dpp receptors have been isolated, the downstream components of the signaling pathway remain to be identified. We have cloned the schnurri (shn) gene and show that it encodes a putative zinc finger transcription factor homologous to the human major histocompatibility complex-binding proteins 1 and 2. Mutations in shn affect multiple events that require dpp signaling as well as the transcription of dpp-responsive genes. Genetic interactions and the strikingly similar phenotypes of mutations in shn and the dpp receptors encoded by thick veins and punt suggest that shn plays a downstream role in dpp signaling.

Schnurri is required for Drosophila Dpp signaling and encodes a zinc finger protein similar to the mammalian transcription factor PRDII-BF1

Cytokines of the TGF beta superfamily regulate many aspects of cellular function by activating receptor complexes consisting of two distantly related serine/threonine kinases. Previous studies have indicated that Drosophila dpp uses similar signaling complexes and strictly requires the punt and thick veins receptors to transduce the signal across the membrane. Here, we show that the schnurri (shn) gene is required for many aspects of dpp signaling. Genetic epistasis experiments indicate that shn functions downstream of the dpp signal and its receptors. The shn gene encodes a large protein similar to a family of mammalian zinc finger transcription factors. The shn protein might therefore act as a nuclear target in the dpp signaling pathway directly regulating the expression of dpp-responsive genes.

Control of epithelial morphogenesis by cell signaling and integrin molecules in the Drosophila foregut

Coordinated cell movements are critical for tissue and organ morphogenesis in animal development. We show that the Drosophila genes hedgehog and wingless, which encode signaling molecules, and the gene myospheroid, which encodes a beta subunit of the integrins, are required for epithelial morphogenesis during proventriculus development. In contrast, this morphogenetic process is suppressed by the decapentaplegic gene, which encodes a member of the TGF beta family of growth factors. These results identify a novel cell signaling center in the foregut that directs the formation of a multiply folded organ from a simple epithelial tube.

Drosophila Dpp signaling is mediated by the punt gene product: a dual ligand-binding type II receptor of the TGF beta receptor family

Signaling by TGF beta-related factors requires ligand-induced association between type I and type II transmembrane serine/threonine kinases. In Drosophila, the saxophone (sax) and thick veins (tkv) genes encode type I receptors that mediate signaling by decapentaplegic (dpp), a member of the bone morphogenetic protein (BMP) subgroup of TGF beta-type factors. In this report, we demonstrate that the Drosophila punt gene encodes atr-II, a previously described type II receptor that on its own is able to bind activin but not BMP2, a vertebrate ortholog of dpp. Mutations in punt produce phenotypes similar to those exhibited by tkv, sax, and dpp mutants. Furthermore, punt will bind BMP2 in concert with tkv or sax, forming complexes with these receptors. We suggest that punt functions as a type II receptors for dpp and propose that BMP signaling in vertebrates may also involve sharing of type II receptors by diverse ligands.

An absolute requirement for both the type II and type I receptors, punt and thick veins, for dpp signaling in vivo

TGF beta elicits diverse cellular responses by signaling through receptor complexes formed by two distantly related transmembrane serine/threonine kinases called type II and type I receptors. Previous studies have indicated that the product of the Drosophila thick veins (tkv) gene is a type I receptor for decapentaplegic (dpp). Here, we show that the Drosophila gene punt encodes a homolog of a vertebrate type II receptor, and we demonstrate that punt, like tkv, is essential in vivo for dpp-dependent patterning processes. Because no dpp-related signalling is apparent in the absence of either the punt or tkv receptor, we infer that both receptors act in concert to transduce the dpp signal and that their functions cannot be replaced by the other extant type II and I receptors.