Disruption of the Mouse μ-Calpain Gene Reveals an Essential Role in Platelet Function

Abstract

Conventional calpains are ubiquitous calcium-regulated cysteine proteases that have been implicated in cytoskeletal organization, cell proliferation, apoptosis, cell motility, and hemostasis. There are two forms of conventional calpains: the -calpain, or calpain I, which requires micromolar calcium for half-maximal activation, and the m-calpain, or calpain II, which functions at millimolar calcium concentrations. We evaluated the functional role of the 80-kDa catalytic subunit of -calpain by genetic inactivation using homologous recombination in embryonic stem cells. The -calpain-deficient mice are viable and fertile. The complete deficiency of -calpain causes significant reduction in platelet aggregation and clot retraction but surprisingly the mutant mice display normal bleeding times. No detectable differences were observed in the cleavage pattern and kinetics of calpain substrates such as the ␤3 subunit of ␣IIb␤3 integrin, talin, and ABP-280 (filamin). However, -calpain null platelets exhibit impaired tyrosine phosphorylation of several proteins including the ␤3 subunit of ␣IIb␤3 integrin, correlating with the agonist-induced reduction in platelet aggregation. These results provide the first direct evidence that -calpain is essential for normal platelet function, not by affecting the cleavage of cytoskeletal proteins but by potentially regulating the state of tyrosine phosphorylation of the platelet proteins.The calpains are a family of calcium-dependent neutral cysteine proteases present in essentially all tissues of higher animals (8,34,37). Calpain homologues distantly related to the catalytic subunits of conventional calpains are also found in lower organisms such as parasites, insects, nematodes, fungi, and yeast (34). They are believed to play functionally important roles in diverse biological processes such as reorganization of cortical cytoskeleton, cell motility, cell proliferation, apoptosis, and hemostasis (9,27,31,39). Calpains are divided into two broad classes, ubiquitous and tissue specific. Calpain I (also referred to as -calpain) and calpain II (also referred to as m-calpain) are expressed in all tissues in varying amounts and share ϳ61% sequence identity (20). Both the -and m-calpains contain an 80-kDa catalytic subunit that forms a heterodimer with the regulatory 30-kDa subunit (34). The 80-kDa catalytic subunits of the -and m-calpains are products of separate but closely related genes (referred to as Capn1 and Capn2, respectively), while the 30-kDa subunit (encoded by the Capn4 gene) is common to both (34). The -calpain is fully active in micromolar concentrations of calcium, while the mcalpain requires millimolar calcium concentrations for full activation. Larger tissue-specific calpains have been cloned from stomach and smooth muscle tissues (35, 37). Mutations of the muscle-specific Capn3 (calpain 3 gene) have been shown to cause one form of limb-girdle muscular dystrophy type 2A (30). More recently, several groups have identified CAPN10 (calpain 10) as the target gene for mutations in...

Myoblast migration is prevented by a calpain‐dependent accumulation of MARCKS

“…However, in most of these studies, both µ-and m-calpain activities were inhibited, using calpastatin overexpression, general pharmacological inhibitors or by targeting the gene encoding the common regulatory subunit (Huttenlocher et al, 1997;Potter et al, 1998;Croce et al, 1999;Dourdin et al, 1999). Other authors have focused only on µ-calpain particularly by means of a dominant negative µ-calpain or by using a cell type that presents a negligible level of m-calpain (Kulkarni et al, 1999;Azam et al, 2001). For the first time, we have employed a strategy that targets separately, or concomitantly, the two isoenzymes, thus shedding light on the importance of m-calpain during myoblast migration.…”

Death penalty for keratinocytes: apoptosis versus cornification

“…93 m-Calpain-null mice have no apparent embryological or epidermal defects, suggesting that m-calpain compensates for m-calpain during development. 94 m-Calpain-deficient mice have not been generated so far.…”

Regulating cell migration: calpains make the cut

“…However, this cell line cannot reveal which calpain isoform is responsible in each case, since the activities of both calpain 1 and calpain 2 are reduced in these cells. Studies of cells isolated from calpain-1-knockout mice reveal that, despite the absence of calpain 1, these cells can proteolyze many substrates normally, including FAK, paxillin, spectrin and talin 1 (Azam et al, 2001). However, other calpain isoforms could be compensating for calpain 1 deficiency in these cells.…”