Calpain-1 (μ-Calpain) is a calcium-dependent cysteine protease composed of an 80 kDa catalytic subunit (CAPN1) and a 30 kDa regulatory subunit (CAPNS1), forming a heterodimer stabilized by domain interactions. The catalytic subunit contains four domains: domain II harbors the protease active site, while domain IV binds calcium, enabling activation at micromolar concentrations (EC₅₀ ≈ 50 μM). The regulatory subunit’s domain VI facilitates membrane association and modulates calcium sensitivity. Calpain-1 regulates cellular processes including cytoskeletal remodeling, signal transduction, and synaptic plasticity by cleaving substrates like protein kinase C (PKC), focal adhesion kinase, and β-spectrin, often altering their activity or localization.

Dysregulated calpain-1 activity is implicated in neurodegenerative diseases such as Alzheimer’s, where excessive cleavage of tau and amyloid precursor protein exacerbates neurofibrillary tangle formation. In cardiovascular pathologies, calpain-1 promotes vascular smooth muscle calcification via matrix metalloproteinase-2 activation and transforming growth factor-β1 signaling, contributing to atherosclerosis. Renal proximal tubule injury involves calpain-1-mediated degradation of cytoskeletal proteins (paxillin, talin), leading to cell necrosis. Conversely, calpain-1 activation during long-term potentiation (LTP) supports synaptic plasticity and neuroprotection by degrading inhibitors of ERK signaling, such as PHLPP1β.