Functional consequences of postnatal interventions in a mouse model of Fragile X syndrome

Background: Fragile X syndrome (FXS) is a leading genetic cause of autism and intellectual disability with cortical hyperexcitability and sensory hypersensitivity attributed to loss and hypofunction of inhibitory parvalbumin-expressing (PV) cells. Our studies provide novel insights into the role of excitatory neurons in abnormal development of PV cells during a postnatal period of inhibitory circuit refinement. Methods: To achieve Fragile X mental retardation gene (Fmr1) deletion and re-expression in excitatory neurons during the postnatal day (P)14-P21 period, we generated Cre(CaMKIIa)/Fmr1(Flox/y) (cOFF) and Cre(CaMKIIa)/Fmr1(FloxNeo/y) (cON) mice, respectively. Cortical phenotypes were evaluated in adult mice using biochemical, cellular, clinically relevant electroencephalogram (EEG) and behavioral tests. Results: We found that similar to global Fmr1 KO mice, the density of PV-expressing cells, their activation, and sound-evoked gamma synchronization were impaired in cOFF mice, but the phenotypes were improved in cON mice. cOFF mice also showed enhanced cortical gelatinase activity and baseline EEG gamma power, which were reduced in cON mice. In addition, TrkB phosphorylation and PV levels were lower in cOFF mice, which also showed increased locomotor activity and anxiety-like behaviors. Remarkably, when FMRP levels were restored in only excitatory neurons during the P14-P21 period, TrkB phosphorylation and mouse behaviors were also improved. Conclusions: These results indicate that postnatal deletion or re-expression of FMRP in excitatory neurons is sufficient to elicit or ameliorate structural and functional cortical deficits, and abnormal behaviors in mice, informing future studies about appropriate treatment windows and providing fundamental insights into the cellular mechanisms of cortical circuit dysfunction in FXS.