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

## TL;DR

A new small molecule stabilizes a key protein complex to restore mitochondrial function and reduce neurodegeneration in diseases like Huntington's and tauopathy.

## Contribution

A36 is a brain-penetrant molecule that selectively stabilizes the CAST–calpain-2 complex without inhibiting GSK3, offering a novel therapeutic strategy.

## Key findings

- A36 normalizes mitochondrial function and reduces oxidative stress in patient-derived neurons and mouse models.
- In vivo, A36 reduces neurodegeneration, mutant huntingtin aggregation, and motor deficits in HD mice.
- A36 lowers phosphorylated tau and neuroinflammation in tauopathy mouse models.

## Abstract

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.

Medicinal chemistry identified a small molecule offering translational potential for neurodegenerative diseases.