Interplay between the ABD-B Hox protein and its downstream target STAT converts the selection gene cascade into an organogenetic gene network

Abstract

Hox proteins control organogenesis by regulating downstream targets. In many cases, Hox expression is maintained during the whole organ's development. This allows a Hox protein to regulate early upstream cascade genes, as well as downstream morphogenetic processes occurring at later stages. To address how the sustained expression of Hox proteins influences development, we have studied how Abd-B induces the organogenesis of the external respiratory organs of the larva, the posterior spiracles. We show that Abd-B induces the early transcription factors required for spiracle specification. Among several targets, Abd-B activates in the spiracle primordia the transcription of unpaired (upd), the main JAK/STAT pathway ligand. We find that STAT activation facilitates, in turn, the maintenance of Abd-B expression. Moreover, STAT is also required for Abd-B to upregulate the crumbs (crb) gene through a posterior spiracle specific enhancer. Using ChIP and EMSA we show that Abd-B and STAT bind the crb-spiracle enhancer and that both are required for its activation. Dissecting the crb-spiracle enhancer we find that STAT activity is not required as a transcriptional collaborator of Abd-B, but it acts by blocking a repressive element in the enhancer. This is a novel function for STAT, as it contributes indirectly to gene expression working as a >counter-repressor> rather than as a direct transcription factor. Our results illustrate how the function of a Hox protein changes from being a selector protein that induces gene-cascades at early stages of organogenesis, into becoming an integrated element of the gene-network controlling organogenesis.