E. S. Anton

Receptor tyrosine kinase ErbB4 modulates neuroblast migration and placement in the adult forebrain.

Neural progenitor proliferation, differentiation and migration are continually active in the rostral migratory stream of the adult brain. Here, we show that the receptor tyrosine kinase ErbB4 is expressed prominently by the neuroblasts present in the subventricular zone and the rostral migratory stream. The neuregulins (NRG1–NRG3), which have been identified as ErbB4 ligands, are detected either in the stream or in adjacent regions. Mice deficient in ErbB4 expressed under the control of either the nestin or the hGFAP promoter have altered neuroblast chain organization and migration and deficits in the placement and differentiation of olfactory interneurons. These findings suggest that ErbB4 activation helps to regulate the organization of neural chains that form the rostral migratory stream and influences the differentiation of olfactory interneuronal precursors.

Essential Roles for GSK-3s and GSK-3-Primed Substrates in Neurotrophin-Induced and Hippocampal Axon Growth

Glycogen synthase kinase-3beta (GSK-3beta) is thought to mediate morphological responses to a variety of extracellular signals. Surprisingly, we found no gross morphological deficits in nervous system development in GSK-3beta null mice. We therefore designed an shRNA that targeted both GSK-3 isoforms. Strong knockdown of both GSK-3alpha and beta markedly reduced axon growth in dissociated cultures and slice preparations. We then assessed the role of different GSK-3 substrates in regulating axon morphology. Elimination of activity toward primed substrates only using the GSK-3 R96A mutant was associated with a defect in axon polarity (axon branching) compared to an overall reduction in axon growth induced by a kinase-dead mutant. Consistent with this finding, moderate reduction of GSK-3 activity by pharmacological inhibitors induced axon branching and was associated primarily with effects on primed substrates. Our results suggest that GSK-3 is a downstream convergent point for many axon growth regulatory pathways and that differential regulation of primed versus all GSK-3 substrates is associated with a specific morphological outcome.

Neuregulin 1- erbB2 signaling is required for the establishment of radial glia and their transformation into astrocytes in cerebral cortex

Radial glial cells and astrocytes function to support the construction and maintenance, respectively, of the cerebral cortex. However, the mechanisms that determine how radial glial cells are established, maintained, and transformed into astrocytes in the cerebral cortex are not well understood. Here, we show that neuregulin-1 (NRG-1) exerts a critical role in the establishment of radial glial cells. Radial glial cell generation is significantly impaired in NRG mutants, and this defect can be rescued by exogenous NRG-1. Down-regulation of expression and activity of erbB2, a member of the NRG-1 receptor complex, leads to the transformation of radial glial cells into astrocytes. Reintroduction of erbB2 transforms astrocytes into radial glia. The activated form of the Notch1 receptor, which promotes the radial glial phenotype, activates the erbB2 promoter in radial glial cells. These results suggest that developmental changes in NRG-1–erbB2 interactions modulate the establishment of radial glia and contribute to their appropriate transformation into astrocytes.

Transcriptional co-regulation of neuronal migration and laminar identity in the neocortex

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 layer-specific 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.

Neuronal migration in the adult brain : are we there yet?

The generation and targeting of appropriate numbers and types of neurons to where they are needed in the brain is essential for the establishment, maintenance and modification of neural circuitry. This review aims to summarize the patterns, mechanisms and functional significance of neuronal migration in the postnatal brain, with an emphasis on the migratory events that persist in the mature brain.

Close Homolog of L1 Modulates Area-Specific Neuronal Positioning and Dendrite Orientation in the Cerebral Cortex

We show that the neural cell recognition molecule Close Homolog of L1 (CHL1) is required for neuronal positioning and dendritic growth of pyramidal neurons in the posterior region of the developing mouse neocortex. CHL1 was expressed in pyramidal neurons in a high-caudal to low-rostral gradient within the developing cortex. Deep layer pyramidal neurons of CHL1-minus mice were shifted to lower laminar positions in the visual and somatosensory cortex and developed misoriented, often inverted apical dendrites. Impaired migration of CHL1-minus cortical neurons was suggested by strikingly slower rates of radial migration in cortical slices, failure to potentiate integrin-dependent haptotactic cell migration in vitro, and accumulation of migratory cells in the intermediate and ventricular/subventricular zones in vivo. The restriction of CHL1 expression and effects of its deletion in posterior neocortical areas suggests that CHL1 may regulate area-specific neuronal connectivity and, by extension, function in the visual and somatosensory cortex.

SPARC-like 1 Regulates the Terminal Phase of Radial Glia-Guided Migration in the Cerebral Cortex

Differential adhesion between migrating neurons and transient radial glial fibers enables the deployment of neurons into appropriate layers in the developing cerebral cortex. The identity of radial glial signals that regulate the termination of migration remains unclear. Here, we identified a radial glial surface antigen, SPARC (secreted protein acidic and rich in cysteine)-like 1, distributed predominantly in radial glial fibers passing through the upper strata of the cortical plate (CP) where neurons end their migration. Neuronal migration and adhesion assays indicate that SPARC-like 1 functions to terminate neuronal migration by reducing the adhesivity of neurons at the top of the CP. Cortical neurons fail to achieve appropriate positions in the absence of SPARC-like 1 function in vivo. Together, these data suggest that antiadhesive signaling via SPARC-like 1 on radial glial cell surfaces may enable neurons to recognize the end of migration in the developing cerebral cortex.

α3β1 integrin modulates neuronal migration and placement during early stages of cerebral cortical development

We show that α3 integrin mutation disrupts distinct aspects of neuronal migration and placement in the cerebral cortex. The preplate develops normally in α3 integrin mutant mice. However, time lapse imaging of migrating neurons in embryonic cortical slices indicates retarded radial and tangential migration of neurons, but not ventricular zone-directed migration. Examination of the actin cytoskeleton of α3 integrin mutant cortical cells reveals aberrant actin cytoskeletal dynamics at the leading edges. Deficits are also evident in the ability of developing neurons to probe their cellular environment with filopodial and lamellipodial activity. Calbindin or calretinin positive upper layer neurons as well as the deep layer neurons ofα 3 integrin mutant mice expressing EGFP were misplaced. These results suggest that α3β1 integrin deficiency impairs distinct patterns of neuronal migration and placement through dysregulated actin dynamics and defective ability to search and respond to migration modulating cues in the developing cortex.

Zika Virus Disrupts Phospho-TBK1 Localization and Mitosis in Human Neuroepithelial Stem Cells and Radial Glia.

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 as well as 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.

Arl13b in Primary Cilia Regulates the Migration and Placement of Interneurons in the Developing Cerebral Cortex

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 that migrating interneurons display highly dynamic primary cilia and we correlate cilia dynamics with the interneurons 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 in part underlie the neurological defects in Joubert syndrome patients.