Zammou, B. | Search bioRxiv

bioRxiv2026

The atypical RHO-GTPase RND3/RHOE interacts with FLRT3 to regulate cortical migration and folding

Cortical gyrification, characterized by the formation of sulcal fissures and gyral peaks, is a distinctive feature of large mammalian brains and is associated with the emergence of higher cognitive functions. This contrasts with small mammals, like mice, which generally display smooth (lissencephalic) cortices. Gyrification results from the integration of several complex processes such as neuron progenitor amplification and migration. However, the underlying molecular mechanisms remain poorly understood. Here, we show that genetic ablation of Rnd3, encoding the atypical RHO-GTPase RND3/RHOE, induces spontaneous cortical sulci in a subset of mouse embryos without increasing neural progenitor amplification. Instead, RND3 regulates cortical neuron migration, as RND3 overexpression delays neuronal migration, whereas its loss accelerates this process. Similar phenotypes reported in the double Flrt1/3 mutants, suggested a functional interaction between the transmembrane FLRTs and RND3 during cortex development. We demonstrate that Rnd3 and Flrt3 are co-expressed in migrating cortical neurons and interact through a {beta}-strand-mediated binding mechanism, involving a conserved motif in RND3. Disruption of this motif abolishes FLRT3 binding, impairs RND3-mediated regulation of neuronal migration and promotes sulcus formation, indicating that FLRT3-RND3 suppresses cortical folding by regulating neuronal migration in mice. Consistently, Rnd3 expression is reduced in the outer subventricular zone of the prospective sulcal regions in the gyrencephalic ferret brain, supporting the notion that RND3 downregulation contributes to sulci development. Together, these findings identify FLRT3-RND3 signalling as a conserved negative regulator of cortical neuron migration and cortical folding.

neuroscience↗

bioRxiv2024

Cytoplasmic expression of the cell cycle regulator cyclin D1 in radial glial progenitor cells modulates brain cortex development

During nervous system development, the interplay between cell cycle regulation and neurogenesis is fundamental to achieve the correct timing for neuronal differentiation. However, the molecular players 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 cortex. Cytoplasmic Ccnd1 is enriched at the distal tip of the RGP, adjacent to the meningeal basement membrane, and overlaps with {beta}1-integrin at the plasma membrane. CCND1 knock-out animals 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. Significant StatementA key developmental step during nervous system formation is the transition from proliferating progenitors to postmitotic neurons. However, the molecular mechanisms regulating this process are not fully understood. Cyclin D1 (Ccnd1) is a canonical regulator of cell cycle in the cell nucleus. Surprisingly, we show that Ccnd1 is also located in the radial glial process of neuron progenitors and associated to the plasma membrane in different regions of the developing mouse brain. We uncover a novel function for this cytoplasmic Ccnd1 and show that it is required for proper cortical layering, independent of cell cycle regulation. Mechanistically, we provide evidence that this function is mediated by the integrin effector paxillin. We propose therefore that cytoplasmic Ccnd1 is important for cortex development independent of cell cycle regulation.

neuroscience↗