Fmr1 cell autonomously promotes maturation of callosal excitatory synapses between somatosensory barrel cortices. Postnatal deletion of Fmr1 in postsynaptic Layer (L) 2/3 or L5 neurons results in a selective weakening of AMPA receptor- mediated function.
Reduced structural and functional interhemispheric connectivity correlates with the severity Autism Spectrum Disorder (ASD) behaviors in humans. Little is known of how ASD-risk genes regulate callosal connectivity. Here we show that Fmr1, whose loss-of-function leads to Fragile X Syndrome (FXS), cell autonomously promotes maturation of callosal excitatory synapses between somatosensory barrel cortices. Postnatal, cell-autonomous deletion of Fmr1 in postsynaptic Layer (L) 2/3 or L5 neurons results in a selective weakening of AMPA receptor- (R), but not NMDA receptor-, mediated synaptic function, indicative of increased 'silent', or immature, callosal synapses. Sensory deprivation by contralateral whisker trimming normalizes callosal input strength, suggesting that experience-driven activity of postsynaptic Fmr1 KO L2/3 neurons weakens callosal synapses. In contrast to callosal inputs, synapses originating from local L4 and L2/3 circuits are normal, revealing an input-specific role for postsynaptic Fmr1 in promoting callosal synaptic connectivity. Multielectrode EEG recordings in awake Fmr1 KO mice find reduced coherence of bilateral neural oscillations in beta and gamma frequencies during rest and sensory stimulation, reflecting reduced interhemispheric integration. Our findings reveal the cellular and synaptic mechanisms by which loss of Fmr1 leads to reduced interhemispheric connectivity and suggests a novel biomarker of brain dysfunction in FXS.