Partial inactivation of GABAA receptors containing the α5 subunit affects the development of adult-born dentate gyrus granule cells.

Alterations of neuronal activity due to changes in GABAA receptors (GABAA R) mediating tonic inhibition influence different hippocampal functions. Gabra5-null mice and α5 subunit((H105R)) knock-in mice exhibit signs of hippocampal dysfunction, but are capable of improved performance in several learning and memory tasks. Accordingly, alleviating abnormal GABAergic tonic inhibition in the hippocampal formation by selective α5-GABAA R modulators represents a possible therapeutic approach for several intellectual deficit disorders. Adult neurogenesis in the dentate gyrus is an important facet of hippocampal plasticity; it is regulated by tonic GABAergic transmission, as shown by deficits in proliferation, migration and dendritic development of adult-born neurons in Gabra4-null mice. Here, we investigated the contribution of α5-GABAA Rs to granule cell development, using retroviral vectors expressing eGFP for labeling precursor cells in the subgranular zone. Global α5-GABAA R knockout (α5-KO) mice showed no alterations in migration and morphological development of eGFP-positive granule cells. However, upregulation of α1 subunit-immunoreactivity was observed in the hippocampal formation and cerebral cortex. In contrast, partial gene inactivation in α5-heterozygous (α5-het) mice, as well as single-cell deletion of Gabra5 in newborn granule cells from α5-floxed mice, caused severe alterations of migration and dendrite development. In α5-het mice, retrovirally mediated overexpression of Cdk5 resulted in normal migration and dendritic branching, suggesting that Cdk5 cooperates with α5-GABAA Rs to regulate neuronal development. These results show that minor imbalance of α5-GABAA R-mediated transmission may have major consequences for neuronal plasticity; and call for caution upon chronic therapeutic use of negative allosteric modulators acting at these receptors.

Postsynaptic gephyrin clustering controls the development of adult-born granule cells in the olfactory bulb.

In adult rodent olfactory bulb, GABAergic signaling regulates migration, differentiation, and synaptic integration of newborn granule cells (GCs), migrating from the subventricular zone. Here we show that these effects depend on the formation of a postsynaptic scaffold organized by gephyrin-the main scaffolding protein of GABAergic synapses, which anchors receptors and signaling molecules to the postsynaptic density-and are regulated by the phosphorylation status of gephyrin. Using lentiviral vectors to selectively transfect adult-born GCs, we observed that overexpression of the phospho-deficient gephyrin mutant eGFP-gephyrin(S270A), which facilitates the formation of supernumerary GABAergic synapses in vitro, favors dendritic branching and the formation of transient GABAergic synapses on spines, identified by the presence of α2-GABAA Rs. In contrast, overexpression of the dominant-negative eGFP-gephyrin(L2B) (a chimera that is enzymatically active but clustering defective), curtailed dendritic growth, spine formation, and long-term survival of GCs, pointing to the essential role of gephyrin cluster formation for its function. We could exclude any gephyrin overexpression artifacts, as GCs infected with eGFP-gephyrin were comparable to those infected with eGFP alone. The opposite effects induced by the two gephyrin mutant constructs indicate that the gephyrin scaffold at GABAergic synapses orchestrates signaling cascades acting on the cytoskeleton to regulate neuronal growth and synapse formation. Specifically, gephyrin phosphorylation emerges as a novel mechanism regulating morphological differentiation and long-term survival of adult-born olfactory bulb neurons.