SCA44- and SCAR13-associated GRM1 mutations affect metabotropic glutamate receptor 1 function through distinct mechanisms

Metabotropic glutamate receptor 1 (mGlu1) is a promising therapeutic target for neurodegenerative CNS disorders including spinocerebellar ataxias (SCAs). Clinical reports have identified naturally-occurring mGlu1 mutations in rare SCA subtypes and clinical symptoms of mGlu1 mutations have been described. However, how mutations alter mGlu1 function remains unknown. We explored SCA-associated mutation effects on mGlu1 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 (iCa2+ mobilisation and IP1 accumulation). mGlu1 mutants exhibited differential impacts on receptor expression, with a truncating C-terminus mutation significantly reducing mGlu1 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 mGlu1 displayed enhanced constitutive activity in IP1 assays, which manifested as reduced orthosteric agonist activity. mGlu1 PAMs restored glutamate potency in iCa2+ mobilisation for loss-of-function mutations, and mGlu1 NAMs displayed enhanced inverse agonist activity at Y792C relative to wild-type mGlu1. Collectively, these data highlight distinct mechanisms by which mGlu1 mutations affect receptor function and show allosteric modulators may present a means to restore aberrant mGlu1 function in rare SCA subtypes.