Project Summary: The Shh pathway plays critical roles in development, stem cell maintenance and tissue homeostasis. Deregulated Shh signaling during cerebellar development causes medulloblastoma, the most common pediatric brain tumors in childhood with presumed cellular origin in granule cell precursors (GCPs). During postnatal development, GCPs undergo rapid and transient proliferation in the outer external granule layer (EGL) in response to Shh signaling before differentiating and migrating inward to become granule neurons. Persistent Shh signaling counters this stereotypic developmental pattern, resulting in disrupted differentiation and prolonged stay of GCPs in the outer EGL where cerebellar neoplasm is thought to initiate. Therefore, elucidating the mechanism by which CGP proliferation and differentiation are regulated is important to our understanding of cerebellar tumorigenesis. Deregulated cellular growth and proliferation are hallmarks of neoplasms that entail energy-consuming anabolic processes such as protein synthesis and lipogenesis. These anabolic processes are highly regulated and subject to stringent control by cellular energy sensors. The key energy sensor that is activated under condition of energy depletion is AMP-activated protein kinase (AMPK). We discovered that AMPK is a potent inhibitor of Shh signaling in GCPs and primary medulloblastoma cells. Moreover, AMPK is selectively activated in the inner EGL of the developing cerebellum where Shh signaling is downregulated and GCPs have begun differentiating. These observations suggest that AMPK may play a critical role in modulating Shh signaling in GCPs for differentiation, proliferation and tumorigenesis. We will test this hypothesis by three aims: 1) Elucidate the mechanism by which AMPK activation antagonizes Shh pathway activity; 2) Determine the role of AMPK in modulating GCP proliferation and differentiation in vivo; 3) Determine the effect of AMPK activation in Shh-driven medulloblastoma in vivo.