Functional and structural basis of a hypermorphic TRPC3 variant

Cerebellar ataxias are characterized by impaired motor coordination resulting from neuronal dysfunction within the cerebellum. The mechanisms underlying this pathology and its cerebellar-specific neurodegeneration remain unknown. We uncover how a gain-of-function canonical transient receptor potential member 3 (TRPC3) mutation, coupled with a cerebellum-specific isoform, stabilizes the channel's open state, resists the leading inhibitor Pyr3, and drives calcium-dependent cell death. Restoring calcium homeostasis by expressing a Purkinje cell calcium pump improves cell viability. Transgenic expression of the TRPC3 hypermorphic variant in Caenorhabditis elegans induces neurodegeneration, confirming its pathogenicity across species. Cryo-electron microscopy and molecular simulations reveal the structural basis for the stabilization of the cerebellar-specific TRPC3 variant in its open state and uncover a druggable allosteric inhibitory binding site. These findings provide an explanation for the vulnerability of cerebellar neurons in TRPC3-associated ataxias and highlight a site for therapeutic intervention.