Forces shaping a Hox morphogenetic gene network

A conserved function of Human DLC3 and Drosophila Cv-c in testis development

The identification of genes affecting gonad development is essential to understand the mechanisms causing Variations/Differences in Sex Development (DSD). Recently, a DLC3 mutation was associated with male gonadal dysgenesis in 46,XY DSD patients. We have studied the requirement of Cv-c, the Drosophila ortholog of DLC3, in Drosophila gonad development, as well as the functional capacity of DLC3 human variants to rescue cv-c gonad defects. We show that Cv-c is required to maintain testis integrity during fly development. We find that Cv-c and human DLC3 can perform the same function in fly embryos, as flies carrying wild type but not patient DLC3 variations can rescue gonadal dysgenesis, suggesting functional conservation. We also demonstrate that the StART domain mediates Cv-c's function in the male gonad independently from the GAP domain's activity. This work demonstrates a role for DLC3/Cv-c in male gonadogenesis and highlights a novel StART domain mediated function required to organize the gonadal mesoderm and maintain its interaction with the germ cells during testis development.

Functional analysis of the Drosophila RhoGAP Cv-c protein and its equivalence to the human DLC3 and DLC1 proteins

RhoGAP proteins control the precise regulation of the ubiquitous small RhoGTPases. The Drosophila Crossveinless-c (Cv-c) RhoGAP is homologous to the human tumour suppressor proteins Deleted in Liver Cancer 1-3 (DLC1-3) sharing an identical arrangement of SAM, GAP and START protein domains. Here we analyse in Drosophila the requirement of each Cv-c domain to its function and cellular localization. We show that the basolateral membrane association of Cv-c is key for its epithelial function and find that the GAP domain targeted to the membrane can perform its RhoGAP activity independently of the rest of the protein, implying the SAM and START domains perform regulatory roles. We propose the SAM domain has a repressor effect over the GAP domain that is counteracted by the START domain, while the basolateral localization is mediated by a central, non-conserved Cv-c region. We find that DLC3 and Cv-c expression in the Drosophila ectoderm cause identical effects. In contrast, DLC1 is inactive but becomes functional if the central non-conserved DLC1 domain is substituted for that of Cv-c. Thus, these RhoGAP proteins are functionally equivalent, opening up the use of Drosophila as an in vivo model to analyse pharmacologically and genetically the human DLC proteins.

JAK/STAT and Hox Dynamic Interactions in an Organogenetic Gene Cascade

Organogenesis is controlled by gene networks activated by upstream selector genes. During development the gene network is activated stepwise, with a sequential deployment of successive transcription factors and signalling molecules that modify the interaction of the elements of the network as the organ forms. Very little is known about the steps leading from the early specification of the cells that form the organ primordium to the moment when a robust gene network is in place. Here we study in detail how a Hox protein induces during early embryogenesis a simple organogenetic cascade that matures into a complex gene network through the activation of feedback and feed forward interaction loops. To address how the network organization changes during development and how the target genes integrate the genetic information it provides, we analyze in Drosophila the induction of posterior spiracle organogenesis by the Hox gene Abdominal-B (Abd-B). Initially, Abd-B activates in the spiracle primordium a cascade of transcription factors and signalling molecules including the JAK/STAT signalling pathway. We find that at later stages STAT activity feeds back directly into Abd-B, initiating the transformation of the Hox cascade into a gene-network. Focusing on crumbs, a spiracle downstream target gene of Abd-B, we analyze how a modular cis regulatory element integrates the dynamic network information set by Abd-B and the JAK/STAT signalling pathway during development. We describe how a Hox induced genetic cascade transforms into a robust gene network during organogenesis due to the repeated interaction of Abd-B and one of its targets, the JAK/STAT signalling cascade. Our results show that in this network STAT functions not just as a direct transcription factor, but also acts as a "counter-repressor", uncovering a novel mode for STAT directed transcriptional regulation.

Organogenesis is controlled by gene networks activated by upstream selector genes. To address how the network organization changes during development and how the target genes integrate the genetic information it provides, we analyze in Drosophila the induction of posterior spiracle organogenesis by the Hox gene Abdominal-B (Abd-B). Initially, Abd-B activates in the spiracle primordium a cascade of transcription factors and signalling molecules including the JAK/STAT pathway. We find that at later stages STAT activity feeds back into Abd-B, initiating the transformation of the Hox cascade into a gene-network. Focusing on a spiracle downstream target gene of Abd-B, we analyze how its cis regulatory elements integrate the dynamic network information set by Abd-B and the JAK/STAT signalling pathway during development. Our results also show that the well known transcription factor STAT can control gene expression as a “counter-repressor”, uncovering an alternative novel mode for STAT directed transcriptional regulation.

Hox targets and cellular functions

Hox genes are a group of genes that specify structures along the anteroposterior axis in bilaterians. Although in many cases they do so by modifying a homologous structure with a different (or no) Hox input, there are also examples of Hox genes constructing new organs with no homology in other regions of the body. Hox genes determine structures though the regulation of targets implementing cellular functions and by coordinating cell behavior. The genetic organization to construct or modify a certain organ involves both a genetic cascade through intermediate transcription factors and a direct regulation of targets carrying out cellular functions. In this review I discuss new data from genome-wide techniques, as well as previous genetic and developmental information, to describe some examples of Hox regulation of different cell functions. I also discuss the organization of genetic cascades leading to the development of new organs, mainly using Drosophila melanogaster as the model to analyze Hox function.

JAK-STAT pathway in Drosophila morphogenesis: From organ selector to cell behavior regulator

One of the main contributions of Drosophila to the JAK-STAT field is the study of morphogenesis. JAK-STAT signaling controls the formation of many different structures through surprisingly different morphogenetic behaviors that include induction of cell rearrangements, invagination, folding of tissues, modulation of cell shape, and migration. This variability may be explained by the many transcription factors and signaling molecules STAT regulates at early stages of development. But is STAT just acting as an upstream inducer of morphogenesis or does it have a more direct role in controlling cell behaviors? Here we review what is known about how the canonical phosphorylation of STAT contributes to shaping the embryonic and imaginal structures.