Mice that are mutant for Reelin or Dab1, or doubly mutant for the VLDL receptor (VLDLR) and ApoE receptor 2 (ApoER2), show disorders of cerebral cortical lamination. How Reelin and its receptors regulate laminar organization of cerebral cortex is unknown. We show that Reelin inhibits migration of cortical neurons and enables detachment of neurons from radial glia. Recombinant and native Reelin associate with alpha3beta1 integrin, which regulates neuron-glia interactions and is required to achieve proper laminar organization. The effect of Reelin on cortical neuronal migration in vitro and in vivo depends on interactions between Reelin and alpha3beta1 integrin. Absence of alpha3beta1 leads to a reduction of Dab1, a signaling protein acting downstream of Reelin. Thus, Reelin may arrest neuronal migration and promote normal cortical lamination by binding alpha3beta1 integrin and modulating integrin-mediated cellular adhesion.

The cerebral neocortex is segregated into six horizontal layers, each containing unique populations of molecularly and functionally distinct excitatory projection (pyramidal) neurons and inhibitory interneurons. Development of the neocortex requires the orchestrated execution of a series of crucial processes, including the migration of young neurons into appropriate positions within the nascent neocortex, and the acquisition of layer-specific neuronal identities and axonal projections. Here, we discuss emerging evidence supporting the notion that the migration and final laminar positioning of cortical neurons are also co-regulated by cell type-and layerspecific transcription factors that play concomitant roles in determining the molecular identity and axonal connectivity of these neurons. These transcriptional programs thus provide direct links between the mechanisms controlling the laminar position and identity of cortical neurons. Key words: Neuronal specification, Neuronal migration, Neuronal identity, Axon pathfinding, Pyramidal neuron, Neuronal circuits, Cerebral cortex IntroductionThe cerebral cortex, an extensive sheet of neural tissue at the most superficial part of the cerebral hemispheres, is involved in a variety of higher cognitive, emotional, sensory and motor functions. The emergence of a six-layered neocortex, the phylogenetically most recent part of the cerebral cortex, and its extensive projections to subcortical regions are key features of mammalian evolution. The ability of the neocortex to mediate complex cognitive and motor tasks depends on the accurate execution of molecular processes that control the identity and positioning of neurons and the formation of their precise synaptic connections during development. The incorrect formation of neocortical organization and circuitry might lead to cognitive impairments and increased susceptibility to major psychiatric and neurological disorders (Valiente and Marín, 2010;Liu, 2011;Manzini and Walsh, 2011;Rubenstein, 2011).The neocortex is composed of six horizontal layers (L1-L6; see Glossary, Box 1) that are cytoarchitectonically and functionally distinct. Each layer contains a unique subset of neurons ( Fig. 1), including glutamatergic excitatory projection (pyramidal) neurons (see Glossary, Box 1) and GABAergic inhibitory interneurons (see Glossary, Box 1) (Jones, 1986;DeFelipe and Farinas, 1992). Furthermore, the projection neurons of L2-L6 exhibit marked layerand subtype-specific differences in their molecular expression and axon projections (DeFelipe and Farinas, 1992;O'Leary and Koester, 1993;Molyneaux et al., 2007). For example, corticofugal axons targeting subcortical structures arise solely from deep-layer (L5 and L6) and subplate (SP; see Glossary, Box 1) neurons, whereas upper-layer (L2-L4) neurons project within the cortex, either intra-hemispherically or contralaterally, mostly via the corpus callosum (see Glossary, Box 1). In addition to projection neurons, interneurons of distinct lineages and morphological, neurochemical and electro...

Summary The mechanisms underlying Zika virus (ZIKV)-related microcephaly and other neurodevelopment defects remain poorly understood. Here, we describe the derivation and characterization, including single-cell RNA-seq, of neocortical and spinal cord neuroepithelial stem (NES) cells to model early human neurodevelopment and ZIKV-related neuropathogenesis. By analyzing human NES cells, organotypic fetal brain slices and a ZIKV-infected micrencephalic brain, we show that ZIKV infects both neocortical and spinal NES cells and their fetal homolog, radial glial cells (RGCs), causing disrupted mitoses, supernumerary centrosomes, structural disorganization and cell death. ZIKV infection of NES cells and RGCs causes centrosomal depletion and mitochondrial sequestration of phospho-TBK1 during mitosis. We also found that nucleoside analogs inhibit ZIKV replication in NES cells, protecting them from ZIKV-induced pTBK1 relocalization and cell death. We established a model system of human neural stem cells to reveal cellular and molecular mechanisms underlying neurodevelopmental defects associated with ZIKV infection and its potential treatment.

Coordinated migration and placement of interneurons and projection neurons lead to functional connectivity in the cerebral cortex; defective neuronal migration and the resultant connectivity changes underlie the cognitive defects in a spectrum of neurological disorders. Here we show that primary cilia play a guiding role in the migration and placement of postmitotic interneurons in the developing cerebral cortex, and that this process requires the ciliary protein, Arl13b. Through live imaging of interneuronal cilia we show migrating interneurons display highly dynamic primary cilia and we correlate cilia dynamics with the interneuron’s migratory state. We demonstrate that the guidance cue receptors essential for interneuronal migration localize to interneuronal primary cilia, but their concentration and dynamics are altered in the absence of Arl13b. Expression of Arl13b variants known to cause Joubert syndrome induce defective interneuronal migration, suggesting that defects in cilia-dependent interneuron migration may underlie the neurological defects in Joubert syndrome patients.

The development of the mammalian cerebral cortex depends on careful orchestration of proliferation, maturation, and migration events, ultimately giving rise to a wide variety of neuronal and non-neuronal cell types. To better understand cellular and molecular processes that unfold during late corticogenesis, we perform single-cell RNA-seq on the mouse cerebral cortex at a progenitor driven phase (embryonic day 14.5) and at birth—after neurons from all six cortical layers are born. We identify numerous classes of neurons, progenitors, and glia, their proliferative, migratory, and activation states, and their relatedness within and across age. Using the cell-type-specific expression patterns of genes mutated in neurological and psychiatric diseases, we identify putative disease subtypes that associate with clinical phenotypes. Our study reveals the cellular template of a complex neurodevelopmental process, and provides a window into the cellular origins of brain diseases.