Haploinsufficiency of Cyfip2 Causes Lithium-Responsive Prefrontal Dysfunction.

OBJECTIVE: Genetic variants of the cytoplasmic FMR1-interacting protein 2 (CYFIP2) encoding an actin-regulatory protein are associated with brain disorders, including intellectual disability and epilepsy. However, specific in vivo neuronal defects and potential treatments for CYFIP2-associated brain disorders remain largely unknown. Here, we characterized Cyfip2 heterozygous (Cyfip2+/- ) mice to understand their neurobehavioral phenotypes and the underlying pathological mechanisms. Furthermore, we examined a potential treatment for such phenotypes of the Cyfip2+/- mice and specified a neuronal function mediating its efficacy.

METHODS: We performed behavioral analyses of Cyfip2+/- mice. We combined molecular, ultrastructural, and in vitro and in vivo electrophysiological analyses of Cyfip2+/- prefrontal neurons. We also selectively reduced CYFIP2 in the prefrontal cortex (PFC) of mice with virus injections.

RESULTS: Adult Cyfip2+/- mice exhibited lithium-responsive abnormal behaviors. We found increased filamentous actin, enlarged dendritic spines, and enhanced excitatory synaptic transmission and excitability in the adult Cyfip2+/- PFC that was restricted to layer 5 (L5) neurons. Consistently, adult Cyfip2+/- mice showed increased seizure susceptibility and auditory steady-state responses from the cortical electroencephalographic recordings. Among the identified prefrontal defects, lithium selectively normalized the hyperexcitability of Cyfip2+/- L5 neurons. RNA sequencing revealed reduced expression of potassium channel genes in the adult Cyfip2+/- PFC. Virus-mediated reduction of CYFIP2 in the PFC was sufficient to induce L5 hyperexcitability and lithium-responsive abnormal behavior.

INTERPRETATION: These results suggest that L5-specific prefrontal dysfunction, especially hyperexcitability, underlies both the pathophysiology and the lithium-mediated amelioration of neurobehavioral phenotypes in adult Cyfip2+/- mice, which can be implicated in CYFIP2-associated brain disorders. ANN NEUROL 2020;88:526-543.