Improvement of Behavioral Deficits in a Mouse Model of Fragile X Syndrome by Restoring α7 nAChR Hypofunction Associated with Aberrant Ly6H Expression

Abstract

Fragile X Syndrome (FXS) is the most common form of inherited intellectual disability and often accompanied with debilitating pathologies including seizures and hyperactivity. FXS arises from a trinucleotide repeat expansion in the 5 UTR of the FMR1 gene that silences expression of the RNA-binding protein FMRP. Despite progress in understanding FMRP functions, the identification of effective therapeutic targets has lagged and at present there are no viable treatment options. Here we identify the 7 nicotinic acetylcholine receptor (nAChR) as candidate target for intervention in FXS. In the early postnatal hippocampus of Fmr1 knockout (Fmr1KO) mice, an established pre-clinical model of FXS, the 7 nAChR accessory protein Ly6H is abnormally distributed, showing enrichment at the neuronal surface and mislocalization in dendrites. Ly6H, a GPI-anchored protein, binds 7 nAChRs with high affinity and can limit 7 nAChR surface expression and signaling. We find that 7 nAChR-evoked Ca2+ responses are dampened in immature glutamatergic and GABAergic Fmr1KO neurons compared to wild type. Knockdown of endogenous Ly6H in Fmr1KO neurons is sufficient to rescue dampened 7 nAChR Ca2+ responses in vitro, providing evidence of a cell-autonomous role for Ly6H aberrant expression in 7 nAChR hypofunction. In line with intrinsic deficits in 7 nAChR activity in Fmr1KO neurons, in vivo administration of the 7 nAChR-selective positive allosteric modulator PNU-120596 improved spatial memory and reduced hyperactivity and seizure severity in adolescent Fmr1KO mice. Taken together, our in vitro mechanistic findings and in vivo rescue studies implicate 7 nAChR hypofunction in FXS pathology.