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.