UCSF

Communication between the intestinal epithelium and the underlying mesenchyme is essential in controlling intestinal stem cell (ISC) activity, morphogenesis, differentiation, and homeostasis. These epithelial-mesenchymal interactions involve instructive signaling molecules that belong to multiple signaling families including the Wnt/&beta-catenin, BMP, and Hedgehog (Hh) pathways. How these pathways coordinate to regulate intestinal development, including ISC self-renewal and differentiation is not completely understood. Potential sources and targets of these signals exist in the microenvironment surrounding ISCs, known as the stem cell niche. Of the niche cells, intestinal subepithelial myofibroblasts (ISEMFs) are considered the main niche cell type since their location at the crypt base positions them closest to ISCs. Herein, we provide evidence that BMP antagonists, including Gremlin 1, Gremlin 2, and Chordin-like 1 are supplied by ISEMFs and smooth muscle cells surrounding the crypt. We show that Gremlin 1 activates Wnt signaling and inhibits the differentiation of intestinal epithelial cells, and thus functions as a stimulating signal that promotes ISC self-renewal.

Working in the converse direction to BMP antagonists are Hh signals, which originate in the intestinal epithelium and target mesenchymal cells, including those comprising the ISC niche. Utilizing Indian Hedgehog (Ihh) conditional knockout mice, we show that deletion of epithelial Ihh in the small and large intestine disrupts the ISC niche architecture as demonstrated by the loss of the muscularis mucosae, diminished crypt myofibroblasts, and extracellular matrix breakdown. Gene expression data suggest that Ihh mutants have an overall decrease in BMP signaling, amplified matrix metallopeptidase activity, and deficient extracellular matrix and smooth muscle development. The mesenchymal niche modifications seen in the Ihh mutants are accompanied by striking changes in the intestinal epithelium, including dilated and ectopic crypts, anatomically deranged and mislocated absorptive and secretory cell lineages, and elevated Wnt signaling. Significantly, we detected an increase in the number of ISCs in the Ihh mutants. These results demonstrate that the Ihh regulates ISCs by maintaining the mesenchymal stem cell niche. In total, the work presented in this dissertation describes previously unrecognized paracrine functions of BMP antagonists and Ihh in regulating intestinal development.