Alterations of NMDAR Subunits in the Cerebrospinal Fluid Across Neurodegenerative and Immunological Disorders

N-methyl-D-aspartate receptors (NMDARs) are glutamate-binding calcium channels that play a key role in brain function and have been linked to many neurological disorders. NMDARs are multi-pass membrane heterotetrameric complexes composed of two compulsory GluN1 subunits and two GluN2 (A-D) or GluN3 (A-B) subunits, from which GluN1, GluN2B, GluN2A, and GluN3A are widely expressed in the adult brain. This study assesses the presence of GluN1, GluN2B, GluN2A, and GluN3A in the cerebrospinal fluid (CSF) from healthy individuals, viral and autoimmune encephalitis, Huntington's disease (HD) and Alzheimer's disease (AD) patients. Samples were run in SDS-PAGE under reducing conditions and resolved with different anti-ectodomain and anti-C-terminal antibodies that combined with immunoprecipitation analyses, served to demonstrate the presence of full-length GluN1, GluN2A, GluN2B, and GluN3A in CSF. These NMDAR subunit complexes are not associated with extracellular vesicles. As a proof of concept of the identity of NMDAR subunits in the CSF, we demonstrated reduced levels of GluN1 in the CSF from patients with autoimmune encephalitis caused by anti-GluN1 antibodies compared with other causes of encephalitis; and showed a depletion of CSF GluN3A in a Grin3a knockout mouse model. Moreover, we observed higher GluN3A levels in CSF in both asymptomatic and symptomatic HD patients; while GluN2A levels were lower in CSF from AD patients. In conclusion, here we demonstrate the presence of NMDAR full-length subunits in CSF and that changes in NMDAR subunits balance could serve to identify alterations related to pathological conditions.