Populations of Radial Glial Cells Respond Differently to Reelin and Neuregulin1 in a Ferret Model of Cortical Dysplasia

Radial glial cells play an essential role during corticogenesis through their function as neural precursors and guides of neuronal migration. Both reelin and neuregulin1 (NRG1) maintain the radial glial scaffold; they also induce expression of Brain Lipid Binding Protein (BLBP), a well known marker of radial glia. Although radial glia in normal ferrets express both vimentin and BLBP, this coexpression diverges at P3; vimentin is expressed in the radial glial processes, while BLBP appears in cells detached from the ventricular zone. Our lab developed a model of cortical dysplasia in the ferret, resulting in impaired migration of neurons into the cortical plate and disordered radial glia. This occurs after exposure to the antimitotic methylazoxymethanol (MAM) on the 24th day of development (E24). Ferrets treated with MAM on E24 result in an overall decrease of BLBP expression; radial glia that continue to express BLBP, however, show only mild disruption compared with the strongly disrupted vimentin expressing radial glia. When E24 MAM-treated organotypic slices are exposed to reelin or NRG1, the severely disrupted vimentin+ radial glial processes are repaired but the slightly disordered BLBP+ processes are not. The realignment of vimentin+ processes was linked with an increase of their BLBP expression. BLBP expressing radial glia are distinguished by being both less affected by MAM treatment and by attempts at repair. We further investigated the effects induced by reelin and found that signaling was mediated via VLDLR/Dab1/Pi3K activation while NRG1 signaling was mediated via erbB3/erbB4/Pi3K. We then tested whether radial glial repair correlated with improved neuronal migration. Repairing the radial glial scaffold is not sufficient to restore neuronal migration; although reelin improves migration of neurons toward the cortical plate signaling through ApoER2/Dab1/PI3K activation, NRG1 does not

Patterns of Neurogenesis and Amplitude of Reelin Expression Are Essential for Making a Mammalian-Type Cortex

The mammalian neocortex is characterized as a six-layered laminar structure, in which distinct types of pyramidal neurons are distributed coordinately during embryogenesis. In contrast, no other vertebrate class possesses a brain region that is strictly analogous to the neocortical structure. Although it is widely accepted that the pallium, a dorsal forebrain region, is specified in all vertebrate species, little is known of the differential mechanisms underlying laminated or non-laminated structures in the pallium. Here we show that differences in patterns of neuronal specification and migration provide the pallial architectonic diversity. We compared the neurogenesis in mammalian and avian pallium, focusing on subtype-specific gene expression, and found that the avian pallium generates distinct types of neurons in a spatially restricted manner. Furthermore, expression of Reelin gene is hardly detected in the developing avian pallium, and an experimental increase in Reelin-positive cells in the avian pallium modified radial fiber organization, which resulted in dramatic changes in the morphology of migrating neurons. Our results demonstrate that distinct mechanisms govern the patterns of neuronal specification in mammalian and avian pallial development, and that Reelin-dependent neuronal migration plays a critical role in mammalian type corticogenesis. These lines of evidence shed light on the developmental programs underlying the evolution of the mammalian specific laminated cortex

Role of cytoskeletal abnormalities in the neuropathology and pathophysiology of type I lissencephaly

Type I lissencephaly or agyria-pachygyria is a rare developmental disorder which results from a defect of neuronal migration. It is characterized by the absence of gyri and a thickening of the cerebral cortex and can be associated with other brain and visceral anomalies. Since the discovery of the first genetic cause (deletion of chromosome 17p13.3), six additional genes have been found to be responsible for agyria–pachygyria. In this review, we summarize the current knowledge concerning these genetic disorders including clinical, neuropathological and molecular results. Genetic alterations of LIS1, DCX, ARX, TUBA1A, VLDLR, RELN and more recently WDR62 genes cause migrational abnormalities along with more complex and subtle anomalies affecting cell proliferation and differentiation, i.e., neurite outgrowth, axonal pathfinding, axonal transport, connectivity and even myelination. The number and heterogeneity of clinical, neuropathological and radiological defects suggest that type I lissencephaly now includes several forms of cerebral malformations. In vitro experiments and mutant animal studies, along with neuropathological abnormalities in humans are of invaluable interest for the understanding of pathophysiological mechanisms, highlighting the central role of cytoskeletal dynamics required for a proper achievement of cell proliferation, neuronal migration and differentiation

Identification of Stk25 as a genetic modifier of Tau phosphorylation in Dab1-mutant mice.

Hyperphosphorylation of the microtubule binding protein Tau is a feature of a number of neurodegenerative diseases, including Alzheimer's disease. Tau is hyperphosphorylated in the hippocampus of dab1-null mice in a strain-dependent manner; however, it has not been clear if the Tau phosphorylation phenotype is a secondary effect of the morbidity of these mutants. The dab1 gene encodes a docking protein that is required for normal brain lamination and dendritogenesis as part of the Reelin signaling pathway. We show that dab1 gene inactivation after brain development leads to Tau hyperphosphorylation in anatomically normal mice. Genomic regions that regulate the phospho Tau phenotype in dab1 mutants have previously been identified. Using a microarray gene expression comparison between dab1-mutants from the high-phospho Tau expressing and low-phospho Tau expressing strains, we identified Stk25 as a differentially expressed modifier of dab1-mutant phenotypes. Stk25 knockdown reduces Tau phosphorylation in embryonic neurons. Furthermore, Stk25 regulates neuronal polarization and Golgi morphology in an antagonistic manner to Dab1. This work provides insights into the complex regulation of neuronal behavior during brain development and provides insights into the molecular cascades that regulate Tau phosphorylation

Reelin, Rap1 and N-cadherin orient the migration of multipolar neurons in the developing neocortex.

Projection neurons migrate from the ventricular zone to the neocortical plate during the development of the mouse brain. Their overall movement is radial, but they become multipolar and move nonradially in the intermediate zone. Here we show that Reelin, the Rap1 GTPase and N-cadherin (NCad) are important for multipolar neurons to polarize their migration toward the cortical plate. Inhibition and rescue experiments indicated that Reelin regulates migration through Rap1 and Akt, and that the Rap1-regulated GTPases RalA, RalB, Rac1 and Cdc42 are also involved. We found that Rap1 regulated the plasma membrane localization of NCad and NCad rescued radial polarization when Rap1 was inhibited. However, inhibition of Rap1 or NCad had little effect on glia-dependent locomotion. We propose a multistep mechanism in which Reelin activates Rap1, Rap1 upregulates NCad, and NCad is needed to orient cell migration

The development of cortical connections.

The cortex receives its major sensory input from the thalamus via thalamocortical axons, and cortical neurons are interconnected in complex networks by corticocortical and callosal axons. Our understanding of the mechanisms generating the circuitry that confers functional properties on cortical neurons and networks, although poor, has been advanced significantly by recent research on the molecular mechanisms of thalamocortical axonal guidance and ordering. Here we review recent advances in knowledge of how thalamocortical axons are guided and how they maintain order during that process. Several studies have shown the importance in this process of guidance molecules including Eph receptors and ephrins, members of the Wnt signalling pathway and members of a novel planar cell polarity pathway. Signalling molecules and transcription factors expressed with graded concentrations across the cortex are important in establishing cortical maps of the topography of sensory surfaces. Neural activity, both spontaneous and evoked, plays a role in refining thalamocortical connections but recent work has indicated that neural activity is less important than was previously thought for the development of some early maps. A strategy used widely in the development of corticocortical and callosal connections is the early overproduction of projections followed by selection after contact with the target structure. Here we discuss recent work in primates indicating that elimination of juvenile projections is not a major mechanism in the development of pathways feeding information forward to higher levels of cortical processing, although its use is common to developing feedback pathways