Homeostatic Roles for Adult Intestinal Hedgehog Signaling: Insight into Smooth Muscle Maintenance and Inflammatory Control.

Abstract

The Hedgehog (Hh) signaling pathway is a critical regulator of cell migration, proliferation, and fate specification during mammalian development. In the gastrointestinal tract, Hh signals emanating from the endoderm play an important role in organogenesis, radial patterning, and mesodermal specification. Recent data have demonstrated that during development of the intestine and colon, Hh signals are required for proper muscle specification and villus development. In contrast, very little is known about the role of Hh signaling during adulthood. The studies in this thesis are designed to specifically interrogate the role of Hh signals during adult intestinal homeostasis. To this end, we investigated novel mouse models of Hh overexpression and inhibition, and now demonstrate that Hh modifies the fate or activity of three distinct cell populations: intestinal subepithelial myofibroblasts, villus smooth muscle, and myeloid immune lineages. Inhibition of Hh signaling leads to mislocalization of myofibroblasts and loss of villus smooth muscle, while overexpression of Hh causes expansion of muscle in the core of the villi. Interrogation of the pathway underlying this activity demonstrates that smooth muscle cells respond directly to Hh, and that Hh signaling can drive cell autonomous differentiation of smooth muscle cells through regulation of the myogenic transcription factor Myocardin. In the context of chronic Hh inhibition, loss of villus smooth muscle leads to atrophy of intestinal absorptive villi, and areas of villus atrophy become complicated by spontaneous inflammation. We also show that mice with a reduced capacity to respond to Hh signaling display increased susceptibility to pro-inflammatory stress. Microarray studies in isolated intestinal mesenchyme demonstrate that Hh acts as a novel epithelial anti-inflammatory signal in the small intestine, and careful examination of Hh-responsive populations demonstrates that this effect may be mediated through signaling directly to myofibroblasts and myeloid cells. These studies reveal a critical role for Hh signaling in the regulation of villus smooth muscle populations, villus structure, and inflammation during adult intestinal homeostasis. Together, our data suggest that precise control of Hh signaling level is important in the intestine, and that dysregulation of Hh signals may predispose to the development of intestinal pathology.