Calpain and Cardiometabolic Diseases

Abstract

Calpain is defined as a member of the superfamily of cysteine proteases possessing the CysPC motif within the gene. Calpain-1 and -2, which are categorized as conventional isozymes, execute limited proteolysis in a calcium-dependent fashion. Accordingly, the calpain system participates in physiological and pathological phenomena, including cell migration, apoptosis, and synaptic plasticity. Recent investigations have unveiled the contributions of both conventional and unconventional calpains to the pathogenesis of cardiometabolic disorders. In the context of atherosclerosis, overactivation of conventional calpain attenuates the barrier function of vascular endothelial cells and decreases the immunosuppressive effects attributed to lymphatic endothelial cells. In addition, calpain-6 induces aberrant mRNA splicing in macrophages, conferring atheroprone properties. In terms of diabetes, polymorphisms of the calpain-10 gene can modify insulin secretion and glucose disposal. Moreover, conventional calpain reportedly participates in amino acid production from vascular endothelial cells to induce alteration of amino acid composition in the liver microenvironment, thereby facilitating steatohepatitis. Such multifaceted functionality of calpain underscores its potential as a promising candidate for pharmaceutical targets for the treatment of cardiometabolic diseases. Consequently, the present review highlights the pivotal role of calpains in the complications of cardiometabolic diseases and embarks upon a characterization of calpains as molecular targets.

Keywords: alternative mRNA splicing; amino acids; dyslipidemia; inflammation; macrophages; non-alcoholic steatohepatitis (NASH); nonalcoholic fatty liver disease (NAFLD); regulatory T cells; vascular endothelial cells.

Figure 1

Cardiometabolic calpains. Calpains are a superfamily of cysteine proteases with a CysPc domain. Two types of conventional isozymes and two types of unconventional calpains are thought to be involved in cardiometabolic diseases. Conventional calpains are formed when the subtype-specific catalytic subunits calpain-1 and calpain-2 form heterodimers with the common regulatory subunit, calpain-s1, respectively. On the other hand, no quaternary structure has been identified for calpain-6 and calpain-10, which are unconventional isoforms. Calpastatin is known as a specific endogenous inhibitor, and its increased expression in cells may downregulate the activity of conventional isozyme and calpain-10. Notably, a cysteine residue in the active core of calpain-6 is substituted with lysine and is thought to lack protease activity. CBSW, calpain-type β-sandwich domain; CysPc, cysteine protease domain, calpain-type; GR, glycine-rich domain; PC, protease core; PEF, penta-EF-hand domain.

Figure 2

Conventional and unconventional calpains potentiate atherosclerosis. (a) The mechanism involves calpain-induced cleavage of VE-cadherin between vascular endothelial cells, which destabilizes the adherence junction and promotes the recruitment of monocytes to the vascular intima. (b) Conventional calpain is concomitantly activated in lymphatic endothelial cells in the presence of dyslipidemia, resulting in reduced stability of regulatory T cells via TGF-β1 production and reduced immunosuppression in arteries, contributing to atherosclerosis development. (c) Calpain-6 is induced in macrophages in atherosclerotic lesions and promotes pinocytosis-mediated LDL cholesterol accumulation by modifying CWC22-mediated alternative splicing. CWC22: complexed with CEF1 protein 22; eIF4A3: eukaryotic translation initiation factor 4A3; ERK: extracellular signal-regulated kinase; IL-10: interleukin-10; JNK: c-Jun N-terminal kinase; LDL: low-density lipoprotein; LPA: lysophosphatidic acid; LPC: lysophosphatidylcholine; M-CSF: macrophage colony-stimulating factor; MEKK1: mitogen-activated protein kinase kinase kinase 1; Rac1: Ras-related C3 botulinum toxin substrate 1; TGF-β1: transforming growth factor-β1; TNF-α: tumor necrosis factor-α.

Figure 3

Conventional and unconventional calpains are involved in the pathogenesis and comorbidities of diabetes. (a) Calpain-10 is known to contribute to insulin release in pancreatic islet β-cells, but this function is impaired by single nucleotide polymorphisms (SNPs) including SNP-43 and/or haplotype on CAPN10 gene. (b) Extracellular calpain-2, released through exosomes, orchestrates the direct cleavage of the IRβ subunit ectodomain. This cleavage hinders insulin signaling transduction through IRS-1/Akt/ERK. Individuals with type 2 diabetes show an inverse correlation between plasma sIR levels and insulin sensitivity. (c) Conventional calpain also contributes to ischemic myocardial injury in diabetes. In this case, mitochondrial calpain degrades ATP5A1 to reduce ATP production and downregulates autophagy through LAMP2 and Atg5 degradation to promote cell death. Atg5: autophagy related 5; ATP: adenosine triphosphate; ATP5A1: ATP synthase, H+ transporting, mitochondrial F1 complex, alpha subunit 1; ERK: extracellular signal-regulated kinase; IR: insulin receptor; IRS-1: insulin receptor substrate-1; LAMP2: lysosome-associated membrane protein 2; sIR: soluble insulin receptor; SNP: single nucleotide polymorphism.

Figure 4

Conventional calpains contribute to liver disease. (a) In the capillary-like endothelial cells found in parenchyma in NAFLD liver, conventional calpains are over-activated and execute excessive proteolysis resulting in an increase of amino acids, including leucine, in the liver microenvironment. This increase in amino acids promotes de novo lipogenesis in the surrounding hepatocytes and accelerates the formation of fatty liver. (b) the oxidative adduct 4-HNE activates hepatocyte calpain in NASH via GPR120, which permeabilizes the lysosomal membrane and increases cytosolic cathepsins. As a result, hepatocytes are thought to undergo cell death. GPR120: G-protein coupled receptor 120; 4-HNE: 4-Hydroxynonenal; mTORC1: mammalian target of rapamycin complex 1; NAFLD: non-alcoholic fatty liver disease; NASH: non-alcoholic steatohepatitis; SREBP1: sterol regulatory element-binding protein 1; S6K: ribosomal protein S6 kinase B1.