Cortical interneuron development: a role for small Rho GTPases

GABAergic interneurons control cortical excitation/inhibition balance and are implicated in severe neurodevelopmental disorders. In contrast to the multiplicity of signals underlying the generation and migration of cortical interneurons, the intracellular proteins mediating the response to these cues are largely unknown. We have demonstrated the unique and diverse roles of the Rho GTPases Rac1 and 3 in cell cycle and morphology in transgenic animals where Rac1 and Rac1/3 were ablated specifically in cortical interneurons. In the Rac1 mutant, progenitors delay their cell cycle exit, probably due to a prolonged G1 phase resulting in a cortex with 50% reductions in interneurons and an imbalance of excitation/inhibition in cortical circuits. This disruption in GABAergic inhibition alters glutamatergic function in the adult cortex, which could be reversed by enhancement of GABAergic functions during an early postnatal period. Furthermore, this disruption disturbs neuronal synchronization in the adult cortex. In the double mutant, additional severe cytoskeletal defects result in an 80% interneuron decrease. Both lines die postnatally from epileptic seizures. We have made progress towards characterizing the cell cycle defect in Rac1 mutant interneuron progenitors, determining the morphological and synaptic characteristics of single and double mutant interneurons and identifying some of the molecular players through which Racs exert their actions via proteomic analysis. In our present work, we review these studies and discuss open questions and future perspectives. We hope that our data will contribute to the understanding of the function of cortical interneurons, especially since preclinical models of interneuron-based cell therapies are being established.