Shh signaling in limb development
Sonic hedgehog (Shh), a secreted signaling molecule, is generated from the posterior limb margin and essential in regulating anterior-posterior (A-P) patterning as well as growth and differentiation of limb mesenchyme. During limb patterning, Shh acts not only upon immediate neighboring cells to induce juxtacrine signaling but also over a long distance to induce paracrine signaling. However, major gaps remain in our understanding of the molecular mechanisms by which the Shh activity gradient regulates defined patterns in the limb. Addressing this question requires a better understanding of the downstream effectors and their functions in the responsive tissue, as well as the roles of paracrine activity. As part of my thesis work, a microarray analysis is performed to identify posterior-enriched genes. Among the list, Hyaluronic Acid Synthase 2 (Has2) gene is identified as a bona fide target of Shh signaling. Two Gli-binding-sites are identified near Has2 promoter, both of which are critical for Gli transcriptional regulation during early mouse limb development. Further conditional knockout experiment in mouse embryos reveals that Has2 is critical for chondrogenesis and the positioning of the joint progenitor cells in the interzone by stabilizing chondroitin sulfate proteoglycans (CSPG) aggregate complex, providing a novel link between Shh signaling and extracellular matrix integrity during development. In addition, by generating genetic mutant of Disp1, a transmembrane protein required for Shh paracrine signaling, we observe digit loss of anterior elements, indicating that the Disp1-mediated paracrine Shh signaling is essential for digit patterning.