Transcriptional control of epithelial morphogenesis

How tissues and organs develop into their final shape during embryogenesis is a fascinating and long-standing question in developmental biology. Tissue morphogenesis is driven by a variety of events at the cellular level and individual cell shape change is one of the central morphogenetic engines. Thus, it is crucial to understand what signals specify the correct cell behavior in specific groups of cells during development. For my doctoral studies, I have focused on two cell shape change events, apical constriction and cilia assembly. First, we present data demonstrating that Shroom3 is essential for cell shape changes and morphogenesis in the developing vertebrate gut, where Shroom3 transcription requires the Pitx1 transcription factor. We identified a Pitx-responsive regulatory element in the genomic DNA upstream of Shroom3, and showed that Pitx proteins directly activated Shroom3 transcription in Xenopus. Moreover, we showed that ectopic expression of Pitx proteins was sufficient to induce Shroom3-dependent cytoskeletal reorganization and epithelial cell shape change. These data demonstrated new breadth to the requirements for Shroom3 in morphogenesis, and also provided a cell-biological mechanism for Pitx transcription factors action during morphogenesis. Next, we focused on understanding the transcriptional regulation of ciliogenesis. We first showed that Rfx2 transcription factor broadly controlled ciliogenesis, and by RNA- and ChIP-sequencing, we showed that Rfx2 directly regulated a wide range of genes encoding diverse ciliogenic machinery. Finally, in addition to ciliogenesis regulation, a large number of non-ciliary genes in our Rfx2 dataset led us to identify a novel role of Rfx2 in controlling insertion of multi-ciliated cells into the overlying mucociliary epithelium. Moreover, we showed here that Slit2, a target of Rfx2, was involved in multi-ciliated cell movements, possibly through mediating cortical E-cadherin level. This work allowed us to begin building a genetic network controlling multi-ciliated cells in mucociliary epithelium. Together, we showed a transcriptional regulation of apical constriction driving gut morphogenesis and a comprehensive transcriptional network that governs multi-ciliated cell development.