Sonic hedgehog promotes G(1) cyclin expression and sustained cell cycle progression in mammalian neuronal precursors

Sonic hedgehog (Shh) signal transduction via the G-protein-coupled receptor , Smoothened, is required for proliferation of cerebellar granule neuron pr ecursors (CGNPs) during development. Activating mutations in the Hedgehog p athway are also implicated in basal cell carcinoma and medulloblastoma, a t umor of the cerebellum in humans. However, Shh signaling interactions with cell cycle regulatory components in neural precursors are poorly understood , in part because appropriate immortalized cell lines are not available. We have utilized primary cultures from neonatal mouse cerebella in order to d etermine (i) whether Shh initiates or maintains cell cycle progression in C GNPs, (ii) if G(1) regulation by Shh resembles that of classical mitogens, and (iii) whether individual D-type cyclins are essential components of Shh proliferative signaling in CGNPs. Our results indicate that Shh can drive continued cycling in immature, proliferating CGNPs. Shh treatment resulted in sustained activity of the G(1) cyclin-Rb axis by regulating levels of cy clinD1, cyclinD2, and cyclinE mRNA transcripts and proteins. Analysis of CG NPs from cyclinD1(-/-) or cyclinD2(-/-) mice demonstrates that the Shh prol iferative pathway does not require unique functions of cyclinD1 or cyclinD2 and that D-type cyclins overlap functionally in this regard. In contrast t o many known mitogenic pathways, we show that Shh proliferative signaling i s mitogen-activated protein kinase independent. Furthermore, protein synthe sis is required for early effects on cyclin gene expression. Together, our results suggest that Shh proliferative signaling promotes synthesis of regu latory factor intermediates that upregulate or maintain cyclin gene express ion and activity of the G(1) cyclin-Rb axis in proliferating granule neuron precursors.