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

Abstract

AbstractMetabotropic glutamate receptor 1 (mGlu1) is a promising therapeutic target for neurodegenerative CNS disorders including spinocerebellar ataxias (SCAs). Clinical reports have identified naturally-occurring mGlu1mutations in rare SCA subtypes and clinical symptoms of mGlu1mutations have been described. However, how mutations alter mGlu1function remains unknown. We explored SCA-associated mutation effects on mGlu1cell 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 IP1accumulation). mGlu1mutants exhibited differential impacts on receptor expression, with a truncating C-terminus mutation significantly reducing mGlu1expression. 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 mGlu1displayed enhanced constitutive activity in IP1assays, which manifested as reduced orthosteric agonist activity. mGlu1PAMs restored glutamate potency in iCa2+mobilisation for loss-of-function mutations, and mGlu1NAMs displayed enhanced inverse agonist activity at Y792C relative to wild-type mGlu1. Collectively, these data highlight distinct mechanisms by which mGlu1mutations affect receptor function and show allosteric modulators may present a means to restore aberrant mGlu1function in rare SCA subtypes.