Molecular development and laminar distribution of GABAergic interneurons of the cerebral cortex

Abstract

GABAergic interneurons of the cerebral cortex represent one of the most diversified populations of cells of the Central Nervous System. It is well established that different subtypes of interneurons populate and integrate into the cortical layers, where they critically modulate the firing activity of their excitatory projection neuron partners. The developmental origin of interneurons has been extensively studied, and elegant prior work has demonstrated that in rodents, the vast majority of interneurons are born in the ventral telencephalon within the medial and caudal ganglionic eminences (MGE and CGE). Although great progress has been made over the last decade in understanding the molecular diversity and fate-specification of MGE-derived interneuron subtypes, the mechanisms controlling the birth and diversification of CGE-derived interneurons, as well as their functional roles within cortical circuitry, remain poorly understood. Here, I addressed this question directly, by investigating a potential role over CGE-derived interneurons of COUP-TFI, a transcription factor highly expressed in the embryonic CGE. I generated and studied conditional, null-mutant mice where the COUP-TFI gene was selectively deleted from the ventral telencephalon (COUP-TFIfl;fl x Dlx5/6CRE-IRES-GFP). In this mouse model, all GABAergic interneurons that arise from the subventricular zone of the ventral telencephalon lack COUP-TFI, without loss of this gene from neurons within the cerebral cortex. These conditional mutants are viable and survive to adulthood, thus enabling investigation of the birth and differentiation of interneuronal subtypes through postnatal stages of development, when local cortical microcircuitry is built. I found that conditional loss-of-function of COUP-TFI in subventricular precursors and post-mitotic cells of the basal ganglia led to a decrease within the cortex of late-born, CGE-derived, VIP- and CR-expressing bipolar interneurons, which was compensated by a concurrent increase of early-born, MGE-derived, PV-expressing interneurons. Strikingly, COUP-TFI mutants were more resistant to pharmacologically induced seizures, a phenotype that we found is dependent on GABAergic signaling. Together, the data support a model by which COUP-TFI regulates the delicate balance between MGE- and CGE-derived interneurons that reach the cortex, likely by influencing intermediate progenitor cell division in the CGE. Upon fate-specification in the ventral telencephalon, interneurons travel long distances to reach and enter the cerebral cortex. Here, they precisely distribute into cortical layer and contact projection neuron partners. The developmental events governing the integration of excitatory projection neurons and inhibitory interneurons into balanced local circuitries are still poorly understood. In order to investigate the role of projection neurons in cortical interneuron lamination, I analyzed the cortex of Fezf2-/- mice, a unique mutant model in which a single population of projection neurons, subcerebral projection neurons, is absent and replaced by callosal projection neurons, without any effect on interneuron cell-autonomous fate specification and differentiation. Using this model, I found that replacement of one projection neuron type with another was sufficient to cause distinct abnormalities of interneuron lamination and altered GABAergic inhibition. The data indicate that different subtypes of projection neurons uniquely and differentially determine the laminar distribution of interneurons in the cerebral cortex. In agreement, in parallel gain-of-function experiments, I found that distinct populations of projection neurons (i.e. corticofugal projection neurons or upper layer II/III callosal projection neurons) that were experimentally generated below the cortex could recruit cortical interneurons to these ectopic locations. Strikingly, the identity of the projection neurons generated, rather than strictly their birthdate, determined the specific types of interneurons recruited. These data demonstrate that in the neocortex individual populations of projection neurons cell-extrinsically control the laminar fate of interneurons and the assembly of local inhibitory circuitry. Together, the work identifies a new transcriptional control over cell-autonomous development of CGE-derived interneurons, and provides a first demonstration that neuronal subtype-specific interactions among excitatory and inhibitory neurons is critically necessary for the building of balanced local microcircuitry in the cerebral cortex.