A schizophrenia risk gene, NRGN, bidirectionally modulates synaptic plasticity via regulating the neuronal phosphoproteome

NRGN is a schizophrenia risk gene identified in recent genetic studies, encoding a small neuronal protein, neurogranin (Ng). Individuals carrying a risk variant of NRGN showed decreased hippocampal activation during contextual fear conditioning. Furthermore, the expression of Ng was reduced in the post-mortem brains of schizophrenic patients. Using the mouse model, we found that the translation of Ng in hippocampus is rapidly increased in response to novel context exposure, and this up-regulation is required for encoding contextual memory. The extent and degree of the effect that altered Ng expression has on neuronal cellular functions are largely unknown. Here, we found that Ng bidirectionally regulates synaptic plasticity in the hippocampus. Elevated Ng levels facilitated long-term potentiation (LTP), whereas decreased Ng levels impaired LTP. Quantitative phosphoproteomic analysis revealed that decreasing Ng caused a significant shift in the phosphorylation status of postsynaptic density proteins, highlighting clusters of schizophrenia- and autism-related genes. In particular, decreasing Ng led to the hypo-phosphorylation of NMDAR subunit Grin2A at newly identified sites, resulting in accelerated decay of NMDAR-mediated channel currents. blocking protein phosphatase PP2B activity rescued the accelerated synaptic NMDAR current decay and the impairment of LTP caused by decreased Ng levels, suggesting that enhanced synaptic PP2B activity led to the deficits. Taken together, our work suggests that altered Ng levels under pathological conditions affect the phosphorylation status of neuronal proteins by tuning PP2B activity and thus the induction of synaptic plasticity, revealing a novel mechanistic link of a schizophrenia risk gene to cognitive deficits.