Creating microenvironments with Claudins: The proper way to build gut and liver
While identifying genes necessary for organogenesis is important, having the appropriate cellular environment for cells to undergo morphogenesis is as important. Understanding the cell biology of zebrafish gut and liver development is the main interest of my thesis work. Claudins are junction proteins that are involved in the maintenance of polarity, the regulation of paracellular transport, and providing barrier functions. As such, they play a role in the establishment and maintenance of appropriate cellular environment by regulating the localization of different proteins or substances to their appropriate places. I have identified two claudins in my thesis work that are necessary for organ development.
During gut development, endodermal cells must rearrange themselves from an initial disorganized state to an organized radial pattern. I found that regulation of the dynamic localization of the adhesion proteins Cadherins may be important for mediating this process. After cells have arranged into a radial pattern, lumen that has formed at the apical region of these cells must coalesce to form a single lumen. We showed that the homeobox gene tcf2 transcriptionally controls the expression of claudin15 and Na+/K+ ATPase to regulate single lumen formation. Together, claudin15 and Na+/K+ ATPase create the appropriate electrochemical microenvironment to drive fluid accumulation. In turn, the fluid accumulation drives lumen coalescence. Subsequently, the single lumen must expand to form a functional gut tube. We provide evidence that apical membrane biogenesis is important for lumen expansion in gut development.
Another claudin, claudin15-likeb(cldn15lb), was found to have a dynamic expression pattern in the liver during development. Its initial expression in hepatocytes and biliary epithelial cells (BECs) becomes restricted to only the BECs at later stages of development. During biliary duct morphogenesis, BECs remodel to form a mature, functional network. Mutants of cldn15lb fail to remodel and form a disorganized biliary ductal network. Interestingly, these mutants do not have a severe polarity defect and can form tight junctions. The expression data and mutant phenotype suggest that cldn15lb may play a role in directing the remodeling process during biliary duct morphogenesis.