GRIN2B-related neurodevelopmental disorder: current understanding of pathophysiological mechanisms

Abstract

The GRIN2B-related neurodevelopmental disorder is a rare disease caused by mutations in the GRIN2B gene, which encodes the GluN2B subunit of NMDA receptors. Most individuals with GRIN2B-related neurodevelopmental disorder present with intellectual disability and developmental delay. Motor impairments, autism spectrum disorder, and epilepsy are also common. A large number of pathogenic de novo mutations have been identified in GRIN2B. However, it is not yet known how these variants lead to the clinical symptoms of the disease. Recent research has begun to address this issue. Here, we describe key experimental approaches that have been used to better understand the pathophysiology of this disease. We discuss the impact of several distinct pathogenic GRIN2B variants on NMDA receptor properties. We then critically review pivotal studies examining the synaptic and neurodevelopmental phenotypes observed when disease-associated GluN2B variants are expressed in neurons. These data provide compelling evidence that various GluN2B mutants interfere with neuronal differentiation, dendrite morphogenesis, synaptogenesis, and synaptic plasticity. Finally, we identify important open questions and considerations for future studies aimed at understanding this complex disease. Together, the existing data provide insight into the pathophysiological mechanisms that underlie GRIN2B-related neurodevelopmental disorder and emphasize the importance of comparing the effects of individual, disease-associated variants. Understanding the molecular, cellular and circuit phenotypes produced by a wide range of GRIN2B variants should lead to the identification of core neurodevelopmental phenotypes that characterize the disease and lead to its symptoms. This information could help guide the development and application of effective therapeutic strategies for treating individuals with GRIN2B-related neurodevelopmental disorder.

Keywords: GRIN2B; GluN2B (NMDA receptor subunit NR2B); NMDAR (NMDA receptor); autism (ASD); dendrite development; disease variants; neuron development; synapse development.

Figure 1

Neuronal development progresses through several steps, coinciding with the period of high expression of GluN2B. During the early stages of development, GluN2B levels progressively increase, while GluN2A levels are low (yellow). As neurons mature, GluN2A levels increase and ultimately surpass GluN2B expression (blue). GluN2B has been proposed to play a role in neuronal differentiation, dendrite morphogenesis, synaptogenesis, circuit refinement, and synaptic plasticity.

Figure 2

Expression of GluN2B variants interferes with neuronal differentiation. Induced pluripotent stem cells (iPSCs) were generated with either: (1) GluN2B harboring the disease-associated missense mutation, E413G, within the ligand-binding domain (LBD); (2) deletion of a section of the GluN2B LBD, or (3) GluN2B haploinsufficiency (Bell et al., 2018). Neural progenitor cells were generated from iPSCs, then differentiated into forebrain neurons. In differentiated cells with GluN2B variants, expression of genes related to cell proliferation and pluripotency was elevated, while genes associated with neuronal differentiation were reduced, when compared to cells with two alleles of wild-type GluN2B. Illustrations were partially created with BioRender.com.

Figure 3

Expression of GluN2B variants interferes with dendrite morphogenesis. (A) Primary neuronal cultures were prepared from wild-type rat cortex, at postnatal day 0–1 (P0–1), then transfected at 2 days in vitro (DIV) with either wild-type GluN2B or GluN2B bearing a truncation at amino acid 724 (724t), within the LBD (Sceniak et al., ; Bahry et al., 2021). In parallel, GluN2B with a large depletion in the LBD was also examined, producing similar results. (B) In contrast to neurons expressing only wild-type GluN2B, neurons expressing mutant GluN2B had impaired dendrite development, with some neurons appearing under-developed and others appearing dysmorphic (displaying unusual dendritic structures). Overall, GluN2B variants produced fewer branches and reduced branch length. Illustrations were partially created with BioRender.com. Images in (B) are from Sceniak et al. (2019).

Figure 4

Expression of GluN2B variants does not affect synapse number but reduces NMDARs at synapses. Primary neuronal cultures were generated from P0 rat hippocampus, then transfected at 7–9 DIV with disease-associated GluN2B variants, G689C or G689S, in the LBD (Kellner et al., 2021). After 3–4 days, miniature excitatory postsynaptic currents (mEPSCs) were subjected to patch clamp recording, separating NMDAR- and AMPAR-mediated components of mEPSCs. AMPAR-mEPSC frequency was unchanged, suggesting no change in the number of synapses. However, the frequency of NMDAR-mEPSCs was reduced, likely due to an increase in synapses that lack NMDARs. Illustrations were partially created with BioRender.com. Recordings are from Kellner et al. (2021).

Figure 5

Expression of a GluN2B variant reduces LTD. Mice were generated that were heterozygous for the disease-associated missense variant, C456Y, in the LBD (Shin et al., 2020). At postnatal day 19–23, field EPSPs (fEPSPs) were recorded from CA1 neurons in acute hippocampal slices. Neurons expressing the C456Y variant had impaired NMDAR-dependent LTD, induced by low frequency stimulation (LFS). Synapse density and postsynaptic ultrastructure were unaffected. Illustrations were partially created with BioRender.com. Recordings are from Shin et al. (2020).