Roles of centrosome proteins associated with primary microcephaly in mammalian corticogenesis

Autosomal recessive primary microcephaly (MCPH) is a genetic disorder characterized by mental retardation, decreased head circumference, and a decrease in the size of the central nervous system with a marked effect on the cerebral cortex. The fact that MCPH is a non-progressive developmental disorder with a major impact upon development of the cerebral cortex suggests that proliferation of the neural progenitor cells that give rise to this structure are affected by the associated mutations. However, little work has been done to test the validity of this idea or to define a molecular mechanism that accounts for this defect.;In this work, I examine the function of Cyclin-dependent kinase 5 regulatory subunit associated protein 2 (Cdk5rap2), a homolog of the MCPH-associated gene CDK5RAP2, in neocortical development. Cdk5rap2 expression in the embryonic brain is present at the onset of neurogenesis and decreases as this process concludes. Knockdown of Cdk5rap2 levels in neocortical neural progenitor cells results in an early overall increase in neuronal differentiation and an enhanced contribution of Cdk5rap2-depleted cells to deep cortical layers at the expense of contribution to upper cortical layers. Cdk5rap2 depletion further results in increased cell cycle exit and decreased progenitor proliferation. Cdk5rap2 interacts with Pericentrin, a protein associated with the genetic disorder microcephalic osteodysplastic primordial dwarfism type II (MOPDII), through specific domains. Knockdown of Pericentrin phenocopies the effects of Cdk5rap2 depletion on neurogenesis, and Pericentrin is required for efficient recruitment of Cdk5rap2 to the centrosome in neural progenitors. This work demonstrates that Pericentrin is a key regulator of Cdk5rap2 recruitment to the centrosome and that acute loss of Cdk5rap2 results in depletion of the neural progenitor pool, providing a mechanism to explain the manifestation of microcephaly.;In an appendix to this work I examine the role of Abnormal spindle-like, microcephaly-associated (Aspm), a homolog of the most commonly mutated gene associated with MCPH, in neurogenesis. Depletion of Aspm levels in cortical neural progenitors also results in altered distribution of neural progenitors and their progeny across cortical layers and increased neuronal differentiation. These results demonstrate that Aspm is an important regulator of neurogenesis in the developing neocortex.