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Homer Protein–Metabotropic Glutamate Receptor Binding Regulates Endocannabinoid Signaling and Affects Hyperexcitability in a Mouse Model of Fragile X Syndrome

The Fmr1 knock-out mouse model of fragile X syndrome (Fmr1−/y) has an epileptogenic phenotype that is triggered by group I metabotropic glutamate receptor (mGluR) activation. We found that a membrane-permeable peptide that disrupts mGluR5 interactions with long-form Homers enhanced mGluR-induced epileptiform burst firing in wild-type (WT) animals, replicating the early stages of hyperexcitability in Fmr1−/y. The peptide enhanced mGluR-evoked endocannabinoid (eCB)-mediated suppression of inhibitory synapses, decreased it at excitatory synapses in WTs, but had no effect on eCB actions in Fmr1−/y. At a low concentration, the mGluR agonist did not generate eCBs at excitatory synapses but nevertheless induced burst firing in both Fmr1−/y and peptide-treated WT slices. This burst firing was suppressed by a cannabinoid receptor antagonist. We suggest that integrity of Homer scaffolds is essential for normal mGluR–eCB functioning and that aberrant eCB signaling resulting from disturbances of this molecular structure contributes to the epileptic phenotype of Fmr1−/y.