Altered grin1 splicing variants after GluN2A reduced expression in vivo

Hippocampal GluN2A knockdown after the developmental switch has been shown to induce a phenotype in rats that resembles that observed in some patients carrying grin2a mutations (the gene that codifies for GluN2A). GluN2A is one of the regulatory subunits of NMDA receptors (NMDARs), key glutamatergic receptors composed of four subunits: two obligatory GluN1 and two regulatory subunits. In the brain, GluN2A predominates in mature circuits, whereas GluN2B is more abundant in immature structures. The grin1 gene, which encodes the GluN1 subunit, generates four splicing variants in the C-terminal region, depending on the combination of three exons. These sequences are involved in membrane trafficking, retention in the endoplasmic reticulum, and interaction with different proteins. In our model, grin1 total expression remained unchanged, although total and synaptic GluN1 protein levels were reduced. However, synaptic GluN2A-containing NMDARs remained unchanged. Based on these results, we aimed to assess grin1 splice variant expression and ribosomal distribution. We found that GluN2A-kd induces a reduction in isoform 1 expression with a concomitant increase in isoform 4 levels, compatible with enhanced trafficking toward synapses. Additionally, ribosomal profiling revealed a shift toward lighter polysomes, suggesting increased translation velocity. Altogether, these findings strongly indicate that GluN2A knockdown triggers compensatory mechanisms to maintain synaptic functionality.