A small-molecule stabilizer of the calpastatin–calpain-2 complex restores mitochondrial function and mitigates neurodegeneration

Mitochondrial dysfunction and dysregulated proteolysis drive Huntington's disease (HD), tauopathy, and related neurodegenerative disorders. Calpain-2, a Ca²⁺-activated protease restrained by calpastatin (CAST), is pathologically overactivated, yet no therapies directly target this axis. We identify A36, a brain-penetrant small molecule derived from CHIR99021 that selectively stabilizes the CAST-calpain-2 complex without inhibiting GSK3. A36 acts as a protein-protein interaction stabilizer, enhancing CAST-calpain-2 binding, preventing CAST degradation, and thereby limiting calpain-2 activation and mitochondrial damage. In patients with HD induced pluripotent stem cell-derived neurons and mutant mouse striatal neurons, A36 normalized mitochondrial morphology and membrane potential, reduced oxidative stress, and improved survival. In vivo, A36 displayed favorable pharmacokinetics and central nervous system exposure; treatment reduced striatal neurodegeneration, mutant huntingtin aggregation, and motor deficits in HD R6/2 mice, and lowered phosphorylated tau, neuroinflammation, and cognitive decline in tauopathy PS19 mice. These findings establish pharmacological stabilization of CAST-calpain-2 as a therapeutic strategy and position A36 as a mechanism-selective modulator with broad neurodegenerative disease potential.