Loss ofGrin2aCauses a Transient Delay in the Electrophysiological Maturation of Hippocampal Parvalbumin Interneurons: A Possible Mechanism for Transient Seizure Burden in Patients with NullGRIN2AVariants

Abstract

N-methyl-D-aspartate receptors (NMDARs) are excitatory glutamate-gated ion channels that are expressed throughout the central nervous system. NMDARs mediate calcium entry into cells, and are involved in a host of neurological functions, including neuronal development and maturation. The GluN2A subunit, encoded by the GRIN2A gene, has a slightly delayed expression pattern, with low transcript levels during embryonic development that peak in the early neonatal period. Given its unique expression pattern and ability to speed up the synaptic time course after incorporation into the postsynaptic density compared to other GluN2 subunits, the GluN2A subunit is well positioned to participate in synaptic maturation and circuit refinement. By using Grin2a knockout mice, we show that the loss of GluN2A signaling impacts parvalbumin-positive GABAergic interneuron development in the hippocampal CA1 subfield. Specifically, Grin2a knockout mice have 33% more parvalbumin-positive cells in CA1 compared to wild type controls, with no impact on cholecystokinin-positive cell density. By using immunohistochemical colocalization staining and electrophysiological recordings, we demonstrate that these excess parvalbumin cells do eventually incorporate into the hippocampal network and participate in phasic inhibition, although their presynaptic release probability may be dampened. Moreover, we show that although the morphology of Grin2a knockout parvalbumin-positive cells is unaffected, key measures of intrinsic excitability and action-potential firing properties show age-dependent alterations. Preadolescent (P20-25) parvalbumin-positive cells have an increased input resistance, longer membrane time constant, longer action-potential half-width, a lower current threshold for depolarization-induced block of action-potential firing, and a decrease in peak action-potential firing rate. Each of these electrophysiological measures becomes corrected in adulthood, reaching wild type levels, suggesting a delay of electrophysiological maturation. The circuit and behavioral implications of delayed parvalbumin-positive interneuron maturation are not known; however, we find that neonatal Grin2a knockout mice are more susceptible to lipopolysaccharide and febrile-induced seizures, consistent with a critical role for early GluN2A signaling in neuronal development and maintenance of excitatory-inhibitory balance. These results could provide insights into how loss-of-function GRIN2A human variants can generate an epileptic phenotype.