Mitotic spindle orientation and plane of cleavage in mammals is a determinant of whether division yields progenitor expansion and/or birth of new neurons during radial glial progenitor cell (RGPC) neurogenesis, but its role earlier in neuroepithelial stem cells is poorly understood. Here we report that Lis1 is essential for precise control of mitotic spindle orientation in both neuroepithelial stem cells and radial glial progenitor cells. Controlled gene deletion of Lis1 in vivo in neuroepithelial stem cells, where cleavage is uniformly vertical and symmetrical, provokes rapid apoptosis of those cells, while radial glial progenitors are less affected. Impaired cortical microtubule capture via loss of cortical dynein causes astral and cortical microtubules to be greatly reduced in Lis1-deficient cells. Increased expression of the LIS/dynein binding partner NDEL1 restores cortical microtubule and dynein localization in Lis1-deficient cells. Thus, control of symmetric division, essential for neuroepithelial stem cell proliferation, is mediated through spindle orientation determined via LIS1/NDEL1/dynein-mediated cortical microtubule capture.

Summary Coordinated migration of newly born neurons to their prospective target laminae is a prerequisite for neural circuit assembly in the developing brain. The evolutionarily conserved LIS1/NDEL1 complex is essential for neuronal migration in the mammalian cerebral cortex. The cytoplasmic nature of LIS1 and NDEL1 proteins suggest that they regulate neuronal migration cell autonomously. Here, we extend mosaic analysis with double markers (MADM) to mouse chromosome 11 where Lis1, Ndel1, and 14-3-3ε (encoding a LIS1/NDEL1 signaling partner) are located. Analyses of sparse and uniquely labeled mutant cells in mosaic animals reveal distinct cell-autonomous functions for these three genes. Lis1 regulates neuronal migration efficiency in a dose-dependent manner, while Ndel1 is essential for a specific, previously uncharacterized, late step of neuronal migration: entry into the target lamina. Comparisons with previous genetic perturbations of Lis1 and Ndel1 also suggest a surprising degree of cell-nonautonomous function for these proteins in regulating neuronal migration.

Haploinsufficiency of LIS1 results in lissencephaly, a human neuronal migration disorder. LIS1 is a microtubule-(MT) and centrosome-[microtubule organizing center (MTOC)] associated protein that regulates nucleokinesis via the regulation of dynein motor function and localization. NDEL1 (NudE isoform, NudE like) interacts with LIS1/dynein complex, and is phosphorylated by CDK5/P35. Previous reports using siRNA-mediated knock-down demonstrated similar critical roles for LIS1 and NDEL1 during neuronal migration, but neuronal migration has not been studied in genetic mutants for Lis1 and Ndel1 where protein levels are uniform in all cells. Brains from mice with complete loss of Lis1 and Ndel1 displayed severe cortical layering and hippocampal defects, but Lis1 mutants had more severe defects. Neuronal migration speed was reduced and neurite lengths were elongated in proportion to the reduction of LIS1 and NDEL1 protein levels in embryonic day 14.5 mutant cortical slices compared to wild type, using two-photon confocal time lapse videomicroscopy. Additionally, mice with 35% of wild-type NDEL1 levels displayed diverse branched migration modes with multiple leading processes, suggesting defects in adhesion and/or polarity. Complete loss of Lis1 or Ndel1 resulted in the total inhibition of nuclear movement in cortical slice assays, and in neurosphere assays, the percentage of migrating neurons with correctly polarized MTOC location was significantly reduced while nuclear-centrosomal distance was extended. Neurite lengths were increased after complete loss Ndel1 but reduced after complete loss of Lis1. Thus, Lis1 and Ndel1 are essential for normal cortical neuronal migration, neurite outgrowth, and function of the MTOC in a dose-dependent manner.

The primary cilium, a sensory appendage that is present in most mammalian cells, plays critical roles in signaling pathways and cell cycle progression. Mutations that affect the structure or function of primary cilia result in ciliopathies, a group of developmental and degenerative diseases that affect almost all organs and tissues. Our understanding of the constituents, development, and function of primary cilia has advanced considerably in recent years, revealing pathogenic mechanisms that potentially underlie ciliopathies. In the brain, the primary cilia are crucial for early patterning, neurogenesis, neuronal maturation and survival, and tumorigenesis, mostly through regulating cell cycle progression, Hedgehog signaling, and WNT signaling. We review these advances in our knowledge of primary cilia, focusing on brain development, and discuss the mechanisms that may underlie brain abnormalities in ciliopathies.

Heterozygous LIS1 mutations are responsible for the human neuronal migration disorder lissencephaly. Mitotic functions of LIS1 have been suggested from many organisms throughout evolution. However, the cellular functions of LIS1 at distinct intracellular compartments such as the centrosome and the cell cortex have not been well defined especially during mitotic cell division. Here, we used detailed cellular approaches and time-lapse live cell imaging of mitosis from Lis1 mutant mouse embryonic fibroblasts to reveal critical roles of LIS1 in mitotic spindle regulation. We found that LIS1 is required for the tight control of chromosome congression and segregation to dictate kinetochore-microtubule (MT) interactions and anaphase progression. In addition, LIS1 is essential for the establishment of mitotic spindle pole integrity by maintaining normal centrosome number. Moreover, LIS1 plays crucial roles in mitotic spindle orientation by increasing the density of astral MT plus-end movements toward the cell cortex, which enhances cortical targeting of LIS1-dynein complex. Overexpression of NDEL1-dynein and MT stabilization rescues spindle orientation defects in Lis1 mutants, demonstrating that mouse LIS1 acts via the LIS1-NDEL1-dynein complex to regulate astral MT plus-ends dynamics and establish proper contacts of MTs with the cell cortex to ensure precise cell division.