Spatiotemporal differential regulation of extrasynaptic GluN2B receptor subunits and PSA-NCAM in brain aging and Alzheimer's disease

Oghenetega Edward Imiruaye¹Isis G Perez¹Brian C Carson¹Christian Crouzet²Jerome Garcia³Derick Han^1*Subhrajit Bhattacharya^1*

• ¹Keck Graduate Institute, Claremont, United States
• ²University of California Irvine Beckman Laser Institute & Medical Clinic, Irvine, United States
• ³University of La Verne, La Verne, United States

Extrasynaptic GluN2B N-methyl-D-aspartate receptors (ES-GluN2B) are localized outside synapses and promote excitotoxic signaling, apoptosis, and long-term depression (LTD) in Alzheimer's disease (AD). Polysialylated neural cell adhesion molecule (PSA-NCAM) physiologically inhibits ES-GluN2B activity, and its downregulation is associated with impaired synaptic plasticity. However, the spatiotemporal changes of ES-GluN2B and PSA-NCAM during brain aging versus AD remain poorly understood. We investigated GluN2A, GluN2B, ES-GluN2B, and PSA-NCAM expression across brain regions in young and old Tg2576 AD and wild-type (WT) mice. Normal aging was associated with decreased GluN2B, increased GluN2A, stable ES-GluN2B, and elevated PSA-NCAM levels. In contrast, AD aging showed elevated ES-GluN2B and reduced PSA-NCAM, particularly in the hippocampus and cortex, with no change in total NCAM expression. Pull-down of PSD-95 fragments revealed increased extrasynaptic GluN2B in aged AD brains. Mechanistically, AD aging was associated with elevated phosphorylation of GluN2B at Ser1480, mediated by casein kinase 2 (CK2), promoting GluN2B detachment from synaptic scaffolds and redistribution to extrasynaptic sites. In vitro, CRISPRa-driven ST8Sia4 upregulation in IMR-32 neuroblastoma cells enhanced PSA-NCAM levels and reduced pGluN2B expression, supporting a direct regulatory role for PSA-NCAM in GluN2B trafficking. Furthermore, Aβ treatment suppressed PSA-NCAM biosynthetic enzymes (ST8Sia4 and UDP-E), linking Aβ to impaired polysialylation. Together, our findings highlight distinct regulatory patterns of ES-GluN2B and PSA-NCAM in AD versus normal aging and support a model in which impaired PSA-NCAM buffering facilitates pathological ES-GluN2B signaling and plasticity loss in AD progression.