CAPN1

CAPN1 Full Name

calpain 1, (mu/I) large subunit

CAPN1 Introduction

CAPN1 (calpain 1), also known as μ-calpain or calpain-1 catalytic subunit, is a calcium‑dependent cysteine protease that belongs to the calpain family. The gene encoding CAPN1 is located on human chromosome 11q13.1 and produces a protein of approximately 714 amino acids with a molecular weight of about 82 kDa. CAPN1 is ubiquitously expressed in all mammalian tissues, with particularly high levels in the brain, heart, and skeletal muscle. It exists as a heterodimer composed of a large catalytic subunit (CAPN1) and a small regulatory subunit (CAPNS1), which is shared with calpain‑2 (m-calpain). CAPN1 requires micromolar concentrations of calcium (hence the name μ-calpain) for full activation, distinguishing it from CAPN2 which requires millimolar calcium. The enzyme is involved in a wide range of physiological processes, including cytoskeletal remodeling, cell migration, signal transduction, apoptosis, and synaptic plasticity. Dysregulation of CAPN1 activity has been implicated in numerous pathological conditions, including neurodegenerative diseases, muscle dystrophies, ischemia‑reperfusion injury, and cancer.

Figure 1. Strcuture of CAPN1.

Physiological Functions and Substrates
CAPN1 is a limited protease, meaning it cleaves only a subset of specific substrates rather than degrading proteins non‑selectively. Known physiological substrates include cytoskeletal proteins (spectrin, talin, vinculin, filamin, and fodrin), membrane receptors (integrins, metabotropic glutamate receptors), signaling molecules (protein kinase C, calcineurin, G‑protein subunits), transcription factors (p53, c‑Jun, c‑Fos, NF‑κB), and caspases. Through these cleavages, CAPN1 regulates cell adhesion, migration, morphology, and signal transduction. In neurons, CAPN1 is essential for long‑term potentiation (LTP) and memory formation, as it cleaves and activates signaling molecules required for synaptic plasticity. In skeletal muscle, CAPN1 participates in sarcomere remodeling and the response to mechanical stress. During apoptosis, CAPN1 can cleave several substrates (including caspases themselves) and acts as a mediator of both caspase‑dependent and caspase‑independent cell death. CAPN1 also contributes to cell cycle progression by degrading the cyclin‑dependent kinase inhibitor p27 and activating cyclin D1.

Role in Neurodegenerative and Neurological Disorders
Excessive or dysregulated CAPN1 activity is a hallmark of several neurodegenerative diseases. In Alzheimer's disease (AD), CAPN1 is hyperactivated by calcium overload and cleaves amyloid precursor protein (APP) to produce neurotoxic fragments, as well as cleaving tau protein, promoting its aggregation into neurofibrillary tangles. In Parkinson's disease (PD), CAPN1 cleaves α‑synuclein, generating aggregation‑prone fragments, and also activates the mitochondrial apoptotic pathway. In Huntington's disease (HD), CAPN1 cleaves huntingtin protein, contributing to its toxicity. In cerebral ischemia (stroke), calcium influx through NMDA receptors activates CAPN1, leading to proteolysis of spectrin and neuronal death; calpain inhibitors have shown neuroprotective effects in animal models. In spinal cord injury and traumatic brain injury, CAPN1 activation exacerbates secondary damage. In epilepsy, prolonged seizures cause CAPN1‑mediated loss of synaptic proteins. These observations have made CAPN1 an attractive target for neuroprotective drug development, though no calpain‑specific inhibitor has yet reached clinical approval.

Alternate Names for CAPN1

CAPN1; calpain 1, (mu/I) large subunit; CANP; muCL; CANP1; CANPL1; muCANP; calpain-1 catalytic subunit; CANP 1; calpain mu-type