Elucidating a novel WNT/?-CATENIN Signaling-Independent Environment that Precedes Villus Morphogenesis in the Embryonic Intestine

[摘要] The intestine is a vital organ responsible for several functions, including excretion of waste, acting as a major site of host immunity, and most importantly, absorption of nutrients. In order to fulfill the body’s daily demands for energy and nutrients, the intestine evolved to expand the absorptive surface area by undergoing a morphogenetic process that generates finger-like units called villi. These villi house specialized cell types critical for both absorbing nutrients from food and for protecting the host from commensal and pathogenic microbes. Villus morphogenesis occurs in the developing embryo and is a complex process that requires the successful coordination of many events, during which the epithelium, initially a tube with a flat apical surface, remodels into a complex structure with stereotypical villus units. During this process, the epithelium and mesenchyme undergo rapid cell division to support tissue expansion. Concomitantly, cell signaling crosstalk between the epithelium and mesenchyme drives the formation and patterning of regularly distributed mesenchymal cell clusters, which aggregate adjacent to the pseudostratified epithelium and demarcate nascent villi.Currently, our understanding of the molecular mechanisms regulating these processes is incomplete. This thesis work focuses on one part of villus morphogenesis, specifically the propagation of epithelial progenitors before and during the remodeling of the flat epithelia into protruding villus structures. It is unclear what signaling pathway drives epithelial proliferation before villus formation. One candidate pathway is canonical WNT/β-CATENIN signaling, as WNT/β-CATENIN signaling is essential for maintenance of the adult intestinal stem cell population and loss of WNT/β-CATENIN signal transduction results in the collapse of villus and crypt structures. In this work, I elucidated the role of WNT/β-CATENIN signaling before and during villus morphogenesis. First, I characterized WNT/β-CATENIN signal transduction using a reporter mouse model and found that before villus formation, reporter activity was very low in the pseudostratified epithelium, but after the emergence of nascent villi, reporter activity was robust and restricted to the intervillus domains. Next I conducted loss-of-function studies with two genetic mouse models that perturb WNT/β-CATENIN signal transduction in the epithelium and observed defects in epithelial proliferation and villus formation at E15.5, while mutant animals displayed no defects at earlier time points (prior to E14.5). Additionally, I found that secretion of mesenchymal WNT ligands, possibly WNT3 and WNT7b, were required for normal epithelial proliferation. Together, these data indicate that there are two phases of growth during villus morphogenesis: One before villus morphogenesis, in which WNT/β-CATENIN signal transduction is low and dispensable for epithelial proliferation, and another after villus emergence, characterized by robust and patterned epithelial WNT/β-CATENIN signal transduction (requiring mesenchymal WNT ligands) that is critical for epithelial proliferation. In sum, this dissertation provides novel insight into the role of WNT/β-CATENIN signaling to drive proliferation of epithelial progenitors during villus morphogenesis.

[发布机构] University of Michigan

[效力级别] villus morphogenesis

[关键词] intestinal development;villus morphogenesis;cell signaling;Wnt/beta-catenin signaling pathway;Molecular;Cellular and Developmental Biology;Health Sciences;Cell and Developmental Biology