Cerebellar granule cell precursors (GCPs) proliferate in the Outer part of the external granular layer (EGL). They begin their differentiation by exiting the cell cycle and migrating into the inner part of the EGL. Here we report that JSAP1, a scaffold protein for JNK signaling pathways, is expressed predominantly in the post-mitotic GCPs of the inner EGL. JSAP1 knockdown or treatment with a JNK inhibitor enhances the proliferation Of Cultured GCPs, but the overexpression of wild-type JSAP1 leads to increased proportions of p27(Kip1)- and NeuN-positive cells, even with Saturating concentrations of Sonic hedgehog (Shh), a potent GCP mitogen. However, these differentiation-promoting effects on GCPs are attenuated significantly in cells overexpressing a mutant JSAP1 that lacks the JNK-binding domain. Together, these data suggest that JSAP1 antagonizes the mitogenic effect of Shh on GCPs and promotes their exit front the cell cycle and differentiation, by modulating JNK activity. (c) 2008 Elsevier Inc. All rights reserved.