Integrin α5β1 is necessary for regulation of radial migration of cortical neurons during mouse brain development

Giovanni Marchetti,Arjan Van Der Flier,Elisabeth Georges‐Labouesse,Sarah Escuin,Adèle De Arcangelis,Richard O Hynes

doi:10.1111/j.1460-9568.2009.07072.x

Abstract

During cerebral cortex development, post-mitotic neurons interact with radial glial fibers and the extracellular environment to migrate away from the ventricular region and form a correct laminar structure. Integrin receptors are major mediators of cell–cell and cell–extracellular matrix interactions. Several integrin heterodimers are present during formation of the cortical layers. The α5β1 receptor is expressed in the neural progenitors of the ventricular zone during cerebral cortex formation. Using in utero electroporation to introduce short hairpin RNAs in the brain at embryonic day 15.5, we were able to inhibit acutely the expression of α5 integrin in the developing cortex. The knockdown of α5 integrin expression level in neural precursors resulted in an inhibition of radial migration, without perturbing the glial scaffold. Moreover, the same inhibitory effect on neuronal migration was observed after electroporation of a Cre recombinase expression plasmid into the neural progenitors of conditional knockout mice for α5 integrin. In both types of experiments, the electroporated cells expressing reduced levels of α5 integrin accumulated in the premigratory region with an abnormal morphology. At postnatal day 2, ectopic neurons were observed in cortical layer V, while a deficit of neurons was observed in cortical layer II–IV. We show that these neurons do not express a layer V-specific marker, suggesting that they have not undergone premature differentiation. Overall, these results indicate that α5β1 integrin functions in the regulation of neural morphology and migration during cortical development, playing a role in cortical lamination.

Highlights

In the developing cerebral cortex, a large number of neurons are generated in the ventricular (VZ) and sub-ventricular zone (SVZ) of the dorsal telencephalon and migrate radially along the glial fibers, to reach their final position in the cortical plate (CP) (Nadarajah et al, 2003)
Staining experiments with antibodies at E14.5 confirmed the expression of a5 in neural progenitors and blood capillaries, but not in radial glial fibers or glial endfeet, while integrin a6 and b1 showed broader expression domains including the VZ and the CP and the nestin-positive radial glial fibers (Fig. S1)
Our results indicate that the knockdown of a5 integrin in neural progenitors at E15.5 induces a delay in radial migration in the cerebral cortex associated with lamination defects at the level of cortical layers II–IV

Summary

Introduction

In the developing cerebral cortex, a large number of neurons are generated in the ventricular (VZ) and sub-ventricular zone (SVZ) of the dorsal telencephalon and migrate radially along the glial fibers, to reach their final position in the cortical plate (CP) (Nadarajah et al, 2003). Adhesive interactions between the extracellular matrix (ECM), neurons and radial glia are likely to play important roles during this process of neuronal migration (Schmid & Anton, 2003; Sarkisian et al, 2008). Embryos lacking the a5 integrin subunit, which forms the a5b1 major FN receptor, die at embryonic day (E)10.5 before cerebral cortex development (Yang et al, 1993; Goh et al, 1997). A5b1 has been implicated in regulating morphology of dendritic spines and formation of synapses in neurons (Webb et al, 2007), as well as in mediating neurite outgrowth after injury (Gardiner et al, 2007)

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Conclusion