Calpain in Traumatic Brain Injury: From Cinderella to Central Player.

Traumatic Brain Injury (TBI) is a major global health concern and a leading cause of death and disability, especially in young adults. It triggers complex secondary injury cascades, e.g., calcium dysregulation, mitochondrial dysfunction and protease activation, that extend well beyond the initial mechanical insult to drive ongoing neurodegeneration. The calcium-dependent protease calpain has emerged as a central mediator of TBI cellular pathology. Calpain cleaves a broad range of cytoskeletal and regulatory proteins across neuronal compartments, disrupting axonal integrity, synaptic function and calcium homeostasis. Despite decades of research, calpain remains an elusive therapeutic target. In this review, we examine the spatial and temporal patterns of calpain activation in the traumatically injured brain, categorize key calpain substrates by structure and location, and assess their mechanistic roles in TBI pathology. We also review recent advances in next-generation calpain-2 selective inhibitors with enhanced specificity and preclinical efficacy and discuss the emerging use of calpain-cleaved protein fragments such as SBDP145 and SNTF as candidate biomarkers for TBI diagnosis and progression. Drawing on molecular, preclinical, and clinical data, we argue that calpain warrants renewed attention as both a therapeutic target and mechanistic biomarker in TBI. It may be time for Cinderella to leave the basement.