Importin-8 Modulates Division of Apical Progenitors, Dendritogenesis and Tangential Migration During Development of Mouse Cortex.

Gerry Nganou,Carla G Silva,Bernard Lakaye,Dominique Engel,Ivan Gladwyn-Ng,Laurent Nguyen,Antonio V Delgado-Escueta,Bernard Coumans,Laurence De Nijs,Thierry Grisar,Miyabi Tanaka

doi:10.3389/fnmol.2018.00234

Abstract

The building of the brain is a multistep process that requires the coordinate expression of thousands of genes and an intense nucleocytoplasmic transport of RNA and proteins. This transport is mediated by karyopherins that comprise importins and exportins. Here, we investigated the role of the ß-importin, importin-8 (IPO8) during mouse cerebral corticogenesis as several of its cargoes have been shown to be essential during this process. First, we showed that Ipo8 mRNA is expressed in mouse brain at various embryonic ages with a clear signal in the sub-ventricular/ventricular zone (SVZ/VZ), the cerebral cortical plate (CP) and the ganglionic eminences. We found that acute knockdown of IPO8 in cortical progenitors reduced both their proliferation and cell cycle exit leading to the increase in apical progenitor pool without influencing the number of basal progenitors (BPs). Projection neurons ultimately reached their appropriate cerebral cortical layer, but their dendritogenesis was specifically affected, resulting in neurons with reduced dendrite complexity. IPO8 knockdown also slowed the migration of cortical interneurons. Together, our data demonstrate that IPO8 contribute to the coordination of several critical steps of cerebral cortex development. These results suggest that the impairment of IPO8 function might be associated with some diseases of neuronal migration defects.

Highlights

The mouse cerebral cortex is comprised of excitatory projection and inhibitory GABAergic interneurons −85% and 15% respectively, which arise from different progenitor zones during development (Anderson and Vanderhaeghen, 2015)
The mRNA was detected in the LGE/Median Ganglionic Eminence (MGE) and ventricular zone (VZ)/SVZ of E12 as well as in the VZ/SVZ, cortical plate (CP) and LGE/MGE of E14 embryos (Figure 1B magnified)
We studied the cell cycle exit of APs in time-mated E13.5 embryos that were In Utero Electroporation (IUE) with either control short hairpin RNA (shRNA) or mouse Ipo8 (mIPO8) shRNA-2 followed by i.p. injection of EdU 24 h before sacrifice at E15.5 to analyze EGFP, EdU and Ki67

Summary

Introduction

The mouse cerebral cortex is comprised of excitatory projection and inhibitory GABAergic interneurons −85% and 15% respectively, which arise from different progenitor zones during development (Anderson and Vanderhaeghen, 2015). Apical radial glial cells (aRGCs, known as apical progenitors (APs)) undergo mitosis at the apical surface of the VZ, where they initially undertake symmetric divisions to expand their population, but progressively switch to asymmetric division during the progression of corticogenesis (Chou and O’Leary, 2013; Daviaud et al, 2016). The latter process generates one self-renewing AP and either a basal progenitor (BP) or an excitatory projection neuron. The neurons convert to a bipolar morphology and, guided by the basal processes of aRGCs, migrate out of the IZ and pass the existing neuronal layers of the cortical plate (CP) to settle at their final destination (Luhmann et al, 2014)

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