CELL TYPE-SPECIFIC SYNAPTIC PHENOTYPES OF CLINICALLY RELEVANT <EM>GRIN2B</EM> GENE VARIANTS ASSOCIATED WITH NEURODEVELOPMENTAL DISORDERS

N-methyl-D-aspartate receptors (NMDARs) play a key role in excitatory synaptic transmission, plasticity and neurodevelopment. NMDARs are heterotetrameric complexes of two obligatory GluN1 subunits and two GluN2(A–D) or GluN3(A–B) subunits. The expression of NMDAR subunits is spatially and developmentally regulated, with GluN1, GluN2A and GluN2B predominant in the forebrain, and GluN2B expressed earlier in development than GluN2A. Variants within the GRIN2B gene encoding the GluN2B subunit are associated with neurodevelopmental disorders characterized by developmental delay and intellectual disability, but the disease mechanisms involved are poorly understood. In particular, the impact of patient GRIN2B variants in different cell types has not been systematically studied to date.

We used two mouse models with clinically relevant variants in the Grin2b gene: a strain carrying a loss-of-function missense variant (GluN2B-L825V) found in a patient with autism and severe intellectual disability, and a strain carrying a protein-truncating frameshift variant in the same location (GluN2B-L825fs). We studied excitatory synaptic input to excitatory and inhibitory neurons using patch-clamp electrophysiology in synaptically connected neuron pairs in primary hippocampal cultures prepared from Grin2b^+/L825V or Grin2b^+/L825fs pups of both sexes. As a result of the incorporation of the loss-of-function GluN2B-L825V subunit into synaptic NMDARs (Grin2b^+/L825V strain) or reduced GluN2B proteins levels (Grin2b^+/L825fs strain) we observed a decreased functional contribution of GluN2B to synaptic NMDAR signaling in both strains, in excitatory as well as inhibitory neurons. Interestingly, our results suggest that the loss of GluN2B signaling is particularly profound in inhibitory neurons, with important implications for circuit function.