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

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 linked symptoms to mGlu₁ mutations. However, how mutations alter mGlu₁ function remains unknown, as does amenability of receptor function to pharmacological rescue. Here, we explored SCA-associated mutation effects on mGlu₁ cell surface expression, canonical signal transduction and allosteric ligand pharmacology.

Orthosteric agonists, positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) were assessed at two functional endpoints (iCa²⁺ mobilisation and inositol 1-phosphate [IP₁] accumulation) in FlpIn Trex HEK293A cell lines expressing five mutant mGlu₁ subtypes. Key pharmacological parameters including ligand potency, affinity and cooperativity were derived using operational models of agonism and allostery.

mGlu₁ mutants exhibited differential impacts on mGlu₁ expression, with a C-terminus truncation significantly reducing surface 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. A gain-of-function Y792C mutant mGlu₁ displayed enhanced constitutive activity in IP₁ assays, which manifested as reduced orthosteric agonist activity. The 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 therapeutic strategy to restore aberrant mGlu₁ function in rare SCA subtypes.