IMPACTS OF <EM>GRIN1</EM> MUTATIONS ON NMDA RECEPTOR LOCALIZATION AND CORTICAL DEVELOPMENT

Polymicrogyria (PMG) is a severe cortical malformation characterized by overfolded, disordered lamination that leads to intellectual disability and refractory epilepsy. While typically associated with cytoskeletal defects, recent clinical evidence has identified de novo mutations in GRIN1, which encodes the essential GluN1 subunit of the NMDAR, as a novel cause of extensive bilateral PMG. This finding is intriguing, as GRIN1 mutations are traditionally associated with functional encephalopathies without structural malformations. Notably, PMG-associated GRIN1 variants cluster within evolutionarily conserved transmembrane and ligand-binding domains, suggesting a distinct pathogenic mechanism affecting cortical development.
We aim to characterize the functional consequences of these variants, hypothesizing that they may exert distinct effects, such as a Gain-of-Function mechanism, compared to the Loss-of-Function typically observed in epilepsy cases. We transfected mutant GRIN1 constructs into cultured cells to assess GluN1 distribution and membrane localization. Confocal analyses show that wild-type GRIN1 localizes to the plasma membrane, whereas variants exhibit altered distribution patterns.
Additionally, we introduce these mutations into the embryonic mouse cortex via in utero electroporation (IUE) to evaluate their effects on cortical development in vivo. Our preliminary data demonstrate that Grin1 knockdown transiently delays migration, followed by superficial overmigration of upper-layer cortical neurons and abnormal callosal projections. In contrast, PMG-associated GRIN1 variants cause severe migration arrest and pronounced laminar mispositioning.
This study highlights a previously unrecognized role of GRIN1 as both an ionotropic receptor component and a developmental regulator during cortical organization. Identifying PMG-associated GRIN1 gating defects advances our understanding of the molecular mechanisms underlying neuronal migration disorders.