Apical complex protein Pals1 is required to maintain cerebellar progenitor cells in a proliferative state

Through their biased localization and function within the cell, polarity complex proteins are necessary to establish the cellular asymmetry required for tissue organization. Well-characterized germinal zones, mitogenic signals, and cell types make the cerebellum an excellent model for addressing the critical function of polarity complex proteins in the generation and organization of neural tissues. Deleting apical polarity complex protein Pals1 in the developing cerebellum results in a remarkably undersized cerebellum with disrupted layers in poorly formed folia and strikingly reduced granule cell production. We demonstrate that Pals1 is not only essential for cerebellum organogenesis, but also for preventing premature differentiation and thus maintaining progenitor pools in cerebellar germinal zones, including cerebellar granule neuron precursors (CGNP) in the external granule layer (EGL). In the Pals1 mutants, expression of genes that regulate cell cycle were diminished, correlating with the loss of proliferating population of germinal zones. Furthermore, enhanced Shh signaling through activated Smoothened (Smo) cannot overcome impaired cerebellar cell generation, arguing for an epistatic role of Pals1 in proliferation capacity. Our study identifies Pals1 as a new intrinsic factor that regulates the generation of cerebellar cells and Pals1 deficiency as a potential inhibitor of overactive mitogenic signaling.