Mesenchyme-epithelial interactions in gastrointestinal development and tumorigenesis

Non-cell autonomous control from mesenchymal cells profoundly affects fundamental biological processes in development and tumorigenesis. The following chapters will investigate how a single mesenchymal transcriptional factor, Barx1, can drive mouse gut development and how MMP9 in mouse intestinal mesenchyme can alter subsequent intestinal tumorigenesis.;Early in gastrointestinal development, the homeobox gene Barx1 directs gut endoderm to differentiate to stomach epithelial cells through Wnt antagonism. Expression of the homeobox gene Barx1 is confined to the stomach mesenchyme and the levels drop steadily after ∼E12 until it is barely detectable in older embryos. To extend our understanding of an inductive role for Barx1 in development, I investigated an unanticipated role of Barx1 in development of the proximal (thoracic) foregut. As explained in Chapter 2, I identified severe tracheo-esophageal defects in Barx1-/- embryos and established the role of Barx1 as a negative regulator of Wnt signaling necessary for specification of esophageal epithelium. In Chapter 3, I question a specific regulation mechanism for Barx1 expression in mouse stomach mesenchyme: miRNA-associated regulation. Depletion of the miRNA-processing enzyme Dicer in cultured stomach mesenchyme and conditional Dicer gene deletion in mice significantly increased Barx1 levels, disrupted stomach and intestine development. Computational and experimental studies identified miR-7a and miR-203 repress Barx1 expression in stomach mesenchymal cells and its function in inducing gastric epithelium.;Subepithelial myofibroblasts (SEMFs) reside just below the basement membrane in the intestinal lamina propria, adjacent to epithelial cells. In Chapter 4, I hypothesized that SEMFs can differentially regulate epithelial growth depending on their position along the rostral-caudal axis (small intestine versus colon). Murine ileal SEMFs enhance epithelial proliferation through secretion of MMP9. MMP9 regulates proliferation of intestinal progenitors and tumorigenesis in ApcMin mice. Extracellular MMP9 activates epithelial integrin and RTKs through laminin cleavage. Stromal MMP9, highly expressed in 16% of 642 human CRC samples, correlated with tumoral CpG island methylation and absence of KRAS mutations Properties of tumors expressing stromal MMP9 and associated growth signal activation hence identify non-cell autonomous mechanisms that may bypass or complement common CRC mutations early in intestinal tumorigenesis.