Impact of NRSN2 Deficiency on Memory: Disruption of Excitatory Synaptic Plasticity Linked to Reduced Expression of NMDA Receptor Subunits and impaired LTP in the hippocampus

Our earlier human studies identified NRSN2 (Neurensin-2), a neuronal-specific vesicular protein, as a candidate gene contributing to 20p13 microdeletion syndrome, yet the mechanisms linking NRSN2 deficiency to the core phenotypes remain unknown. To elucidate the function of Nrsn2 in neurodevelopment and cognitive function, we generated homozygous Nrsn2 knockout mice ( Nrsn2 ⁻/⁻) and performed a series of behavioral, morphological, and electrophysiological analyses. Behaviorally, Nrsn2⁻/⁻ mice exhibited significant impairments in spatial learning and memory (Morris water maze) and fear memory (passive avoidance test). Morphometric analysis revealed no alterations in dendritic complexity or spine density in hippocampal CA1 pyramidal neurons or cerebellar Purkinje cells, ruling out developmental malformation. Electrophysiology and immunoblotting uncovered region-specific synaptic alterations. Collectively, Nrsn2 deficiency does not affect basal synaptic transmission through presynaptic release probability (PRP) but impairs excitatory synaptic transmission via downregulation of GluN1- and GluN2A-containing NMDA receptors and causes a frequency-specific decrease in sEPSCs amplitude in the hippocampus, as well as impaired hippocampal synaptic plasticity, evidenced by reduced long-term potentiation (LTP). In addition, we observed the disruption of hippocampal excitatory/inhibitory balance, indicated by altered sEPSC frequency but unchanged sIPSC frequency. In cerebellar Purkinje cells, GluA1-containing AMPA receptors were downregulated, accompanied by reduced frequency and amplitude of sEPSCs and a selective reduction in sIPSCs frequency. These findings demonstrate that NRSN2 is essential for maintaining synaptic receptor composition and functional plasticity, particularly within excitatory circuits. Consequently, NRSN2 deficiency disrupts synaptic transmission, rather than structural neuronal integrity, and likely underlies the observed cognitive and motor phenotypes. This study provides novel mechanistic insights into the neurobiological role of NRSN2 and its contribution to neurodevelopment, learning, and memory.