SCA44- and SCAR13-associatedGRM1mutations affect metabotropic glutamate receptor 1 function through distinct mechanisms

Metabotropic glutamate receptor 1 (mGlu 1 ) is a promising therapeutic target for neurodegenerative CNS disorders including spinocerebellar ataxias (SCAs). Clinical reports have identified naturally-occurring mGlu 1 mutations in rare SCA subtypes and clinical symptoms of mGlu 1 mutations have been described. However, how mutations alter mGlu 1 function remains unknown. We explored SCA-associated mutation effects on mGlu 1 cell surface expression and canonical signal transduction. Orthosteric agonists and positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) were assessed at two functional endpoints (iCa 2+ mobilisation and IP 1 accumulation). mGlu 1 mutants exhibited differential impacts on receptor expression, with a truncating C-terminus mutation significantly reducing mGlu 1 expression. Mutations differentially influenced orthosteric ligand affinity, efficacy, and functional cooperativity between allosteric and orthosteric ligands. Loss-of-function mutations L454F and N885del reduced orthosteric affinity and efficacy, respectively. Gain-of-function Y792C mutant mGlu 1 displayed enhanced constitutive activity in IP 1 assays, which manifested as reduced orthosteric agonist activity. mGlu 1 PAMs restored glutamate potency in iCa 2+ mobilisation for loss-of-function mutations, and mGlu 1 NAMs displayed enhanced inverse agonist activity at Y792C relative to wild-type mGlu 1 . Collectively, these data highlight distinct mechanisms by which mGlu 1 mutations affect receptor function and show allosteric modulators may present a means to restore aberrant mGlu 1 function in rare SCA subtypes.