Cortical expansion is an important hallmark for intellectual evolution in mammals, which involves an expansion of the neural progenitor pool during development. In the rodent brain, cortical neurons are generated directly from radial glial (RG) cells in the ventricular zone (VZ) or indirectly by intermediate progenitor (IP) cells in the subventricular zone (SVZ). In gyrencephalic brains, such as those in primates, a novel pool of neural progenitors are found in the outer subventricular zone (OSVZ) in the developing brain, termed OSVZ radial glia-like (oRG) cells during the mid-gestational period of cortical expansion. The OSVZ is a characteristic of gyrencephalic brains;therefore OSVZ progenitors are implicated in the massive cortical expansion necessary for gyrencephalic brain formation. Interestingly, mutations in several genes cause defects in gyrogenesis, leading to a relatively common (1 in ~30,000 live births) human brain developmental disease, lissencephaly (smooth brain). Using in vitro and rodent models, previous studies on these genes have uncovered their functions in neural stem cell proliferation, morphogenesis, and neuronal migration. We therefore hypothesize that lissencephaly genes play important functions in proliferation and/or motility of OSVZ progenitors and thus contribute to the massive cortical expansion that leads to gyrencephaly. The objectives of this proposal are to uncover the molecular and cellular controls of lissencephaly genes on oRG cell neurogenesis and to define how these proteins regulate oRG cell division, fate specification, and differentiation, questions that are unanswerable using rodent models. The long-term goal is to investigate the detailed roles of lissencephaly genes in oRG cell function and explore their contributions to evolutionary and developmental cortical expansion, gyrogenesis, and pathogenesis in human patients. Guided by preliminary data, we will pursue three specific aims: 1) To determine roles of lissencephaly genes in cell cycle progression and mitosis in oRG cells;2) To define novel subcellular functions of lissencephaly genes in regulating oRG cell behavior;and 3) To investigate cellular and molecular mechanisms of asymmetric cell division and cell fate determination in oRG cells. Using a human fetal tissue model with innovative approaches including RNA interference, transfection, and photoconversion of fluorescent subcellular markers, electrophysiology, and in situ time-lapse confocal and two-photon imaging techniques, the proposed research will provide new insights into the pathogenesis of lissencephaly and expand our knowledge of the mechanism of brain development and evolution. Moreover, the results of this study may shed light on mechanisms relevant to the etiology of many other neurological and psychiatric disorders related to cortical development.
We propose to investigate the cellular and molecular functions of lissencephaly genes in the largest neural progenitor zone, the outer subventricular zone, in developing human cortex during mid-gestation, and assess their contributions to cortical expansion, gyrogenesis, and the pathogenesis of lissencephaly. By using a combination of innovative genetic, molecular, and imaging techniques, we seek to improve our understanding of brain development under normal and disease conditions and provide important information for potential therapeutic strategies for lissencephaly and related neurodevelopmental disorders.