Cytoplasmic cyclin D1 modulates brain cortex development

During nervous system development, the interplay between cell cycle regulation and neurogenesis is fundamental to achieving the correct timing for neuronal differentiation. However, the molecules regulating this transition are poorly understood. Among these, the cell-cycle regulatory cyclins and their cyclin-dependent kinases (CDKs) play a pivotal role. In the present work we uncover an unknown function of cyclin D1 (CCND1) during cortex development which is independent of cell cycle regulation and that relies on its cytoplasmic localization and membrane association. We show that CCND1 is localized in the cytoplasm of the radial glial process (RGP) of neuron progenitors in different regions of the developing brain, including the pallium. Cytoplasmic CCND1 is enriched at the distal tip of the RGP, adjacent to the meningeal basement membrane, and overlaps with β1-integrin at the plasma membrane. Ccnd1 knock-out embryos show an abnormal cortical layering in which the distribution of TBR2 + and CTIP2 + cells are affected without displaying proliferation defects. This is consistent with a cytoplasmic function of CCND1 as overexpression by in utero electroporation of a dominant negative CCND1, unable to activate CDKs, and targeted to the cytoplasmic membranes, reproduces some of these TBR2 and CTIP2 defects. Finally, we provide evidence that cytoplasmic CCND1 affects neuron morphology and that it is required for the proper detachment of the RGP from the meningeal basement membrane by a mechanism involving the phosphorylation of the integrin effector protein paxillin. Hence, we propose that CCND1 has an important cytoplasmic function for cortical development independently of cell cycle regulation.