Autosomal recessive primary microcephaly (MCPH) is a genetically and clinically heterogeneous disease defined by a decrease in head circumference at birth. Patients often have a broad spectrum of neurological problems, including mental retardation, focal or generalized seizures, hyperactivity, and attention deficit disorder. The decrease in brain volume without major architectonic abnormalities most likely stems from a primary defect in neurogenesis and or neuronal migration. Five of eight MCPH genes localize to the centrosome during all or part of the cell cycle. In vitro studies provide evidence that these genes play roles in essential centrosomal functions such as cell cycle regulation. Nonetheless, the mechanism of MCPH in brain development is still poorly understood. The long-term goal of this project is to profile the role of MCPH genes in neocortical development and disease pathogenesis. The objectives are to uncover the molecular and cellular controls of MCPH genes on neurogenesis and to define how centrosomal proteins regulate the mode of division (symmetric or asymmetric), neuronal migration and differentiation. Recent studies from our lab and others have demonstrated that radial glial cells are a major population of neuronal progenitor cells. They divide asymmetrically to self-renew and give rise to cortical neurons. Asymmetric centrosome inheritance is believed to regulate the differential behavior of self-renewing progenitors versus differentiating progeny in the embryonic mouse neocortex. Centrosome defects in Drosophila do not dramatically perturb mitosis in most somatic cells, but the asymmetric division of larval neuroblasts is noticeably disrupted, underscoring the particular significance of centrosome behavior for asymmetric cell division of progenitor cells and determination of daughter cell fate. Furthermore, the centrosome is the primary anchor for microtubules, enabling the differentiating neuron to initiate and extend an axon, a key process of neuron differentiation. Based on these observations, the central hypothesis of this application is that the MCPH genes control neurogenesis, neuronal migration, and differentiation in the developing cortex. Guided by strong preliminary data this hypothesis will be tested by pursuing four specific aims: 1) To determine the molecular and cellular mechanism by which MCPH genes regulate radial glial cell division; 2) To explore the function of MCPH genes in regulating asymmetric inheritance of mother versus daughter centrosomes and daughter cell fate; 3) To define the function of MCPH genes in regulating neuronal migration and differentiation in the developing cortex; and 4) To validate the relevance of findings in the mouse to the pathogenesis of human MCPH using patient induced pluripotent stem (iPS) cells. With innovative approaches including high-temporal time-lapse imaging and molecular genetic techniques, the proposed research will provide new insights into the pathogenesis of MCPH and expand our knowledge of brain development. Moreover, the results of this study may shed light on mechanisms relevant to the etiology of many neurological and psychiatric disorders related to cortical function.