Calpain in Traumatic Brain Injury: From Cinderella to Central Player

Abstract

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.

Keywords: SNTF; Traumatic Brain Injury; axon degeneration; calcium dysregulation; calpain-2 inhibitors; excitotoxicity; neurodegeneration; neuropathology; spectrin; therapeutics.

Figure 1

Schematic overview of calpain’s role in TBI pathophysiology and its translational applications. TBI disrupts calcium homeostasis, triggering calpain activation. Calpain-specific cleavage products serve as candidate biomarkers for clinical diagnosis and disease monitoring. Red arrows illustrate inhibition, green arrows indicate the sequence from injury to molecular disruption and potential intervention. This integrative model highlights calpain’s central role as both a pathogenic effector and a potential target for therapeutic and diagnostic development. Image created with BioRender.com.

Figure 2

The Calpain-Ca²⁺-Loop is a self-perpetuating cycle triggered by TBI-induced calcium dysregulation. TBI initiates calcium influx through mechanoporation, glutamate excitotoxicity, membrane depolarization and reversal of calcium efflux pumps (A). Intracellular calcium increases rapidly (B) which leads to calpain activation (C). Calpain cleaves key regulatory proteins in the plasmalemma and ER including plasma membrane receptors, ER IP3 receptors, and calcium pumps, resulting in loss of calcium homeostasis, (D1–D3). This leads to sustained intracellular calcium elevation which drives further calpain activation, creating a feedforward cycle of progressive cellular damage. Image created with BioRender.com.

Figure 3

Subcellular localization of calpain substrates in neurons. Calpain targets are distributed across four major compartments: the cytoskeleton, membrane, synaptic region, and intracellular organelles. Proteolysis in each domain drives distinct pathological outcomes. Cytoskeletal cleavage disrupts axonal transport and structural integrity; membrane-associated substrate cleavage impairs ion channel stability and anchoring; synaptic protein degradation compromises neurotransmission and plasticity; and cleavage of organelle-associated targets perturbs calcium homeostasis and mitochondrial function. Lightning bolts denote sites of axonal injury. Image created with BioRender.com.

Figure 4

Proposed spatiotemporal model of SBDP145 in four different brain regions: cortex, thalamus, hippocampus, and subcortical white matter. Preclinical TBI models vary in methodology, limiting cross-study comparability. This schematic summarizes reported regional and temporal patterns in SBDP145 detection. Cortical accumulation is detectable by 15 min post-injury, with a distinct peak between 1–3 days. Thalamus, hippocampus and subcortical white matter exhibit slower but more sustained elevations. Arrow denotes the earliest reported detection. Question marks indicate timepoints beyond which data are not available. Chronic persistence remains largely uncharacterized. The time-axis is plotted logarithmically. Image created with BioRender.com.