Multiple forms of rat calpastatin cDNA in the coding region of functionally unknown amino-terminal domain

Constant expression of mouse calpastatin isoforms during differentiation in myoblast cell line, C2C12

C2C12 is a myoblast cell line which is used to studydifferentiation into multinucleated cells in vitro. Addition of calpain inhibitors, calpeptin orE-64d, to the culture medium prevented the myoblasticfusion of C2C12 cells. Immunoblot studies usingaffinity-purified antibody, revealed that the expressedlevels of mouse calpastatin remained unaltered duringC2C12 cell fusion. The detected calpastatin migratedas a protein of 130 kDa on SDS-polyacrylamide gelelectrophoresis. The estimated molecular mass wassomewhat greater than that in mouse liver anderythrocytes, and much greater than that reported inrat myoblasts. The 130 kDa isoform may contain anadditional N-terminal region designated XL domainfound in bovine calpastatin.

Association of Calpastatin with Inactive Calpain

It is generally accepted that the Ca(2+)-dependent interaction of calpain with calpastatin is the most relevant mechanism involved in the regulation of Ca(2+)-induced proteolysis. We now report that a calpain-calpastatin association can occur also in the absence of Ca(2+) or at very low Ca(2+) concentrations, reflecting the physiological conditions under which calpain retains its inactive conformational state. The calpastatin binding region is localized in the non-inhibitory L-domain containing the amino acid sequences encoded by exons 4-7. This calpastatin region recognizes a calpain sequence located near the end of the DII-domain. Interaction of calpain with calpastatins lacking these sequences becomes strictly Ca(2+)-dependent because, under these conditions, the transition to an active state of the protease is an obligatory requirement. The occurrence of the molecular association between Ca(2+)-free calpain and various recombinant calpastatin forms has been demonstrated by the following experimental results. Addition of calpastatin protected calpain from trypsin digestion. Calpain was coprecipitated when calpastatin was immunoprecipitated. The calpastatin molecular size increased following exposure to calpain. The two proteins comigrated in zymogram analysis. Furthermore, calpain-calpastatin interaction was perturbed by protein kinase C phosphorylation occurring at sites located at the exons involved in the association. At a functional level, calpain-calpastatin interaction at a physiological concentration of Ca(2+) represents a novel mechanism for the control of the amount of the active form of the protease potentially generated in response to an intracellular Ca(2+) influx.

Dysregulation of the calpain-calpastatin system plays a role in the development of cerulein-induced acute pancreatitis in the rat

Calpain, a calcium-dependent cytosolic cysteine protease, is implicated in a multitude of cellular functions but also plays a role in cell death. Recently, we have shown that two ubiquitous isoforms, termed micro-calpain and m-calpain, are expressed in rat pancreatic acinar cells and that calcium ionophore-induced calpain activation leads to acinar cell injury. On the basis of these observations, we have now investigated the role of both calpain forms and the endogenous calpain inhibitor calpastatin in acute pancreatitis. After treatment of rats either without or with calpain inhibitor Z-Val-Phe methyl ester (ZVP; 60 mg/kg i.p.), pancreatitis was induced by cerulein injections (10 microg/kg i.p.; 5 times at hourly intervals). Calpain activation and calpastatin expression in the pancreatic tissue were studied by Western blot analysis. Pancreatic injury was assessed by plasma amylase activity, pancreatic wet/dry weight ratio (edema), histological and electron-microscopic analyses, as well as fluorescence labeling of actin filaments. Cerulein caused an activation of both micro-calpain and m-calpain, accompanied by degradation of calpastatin. Prophylactic administration of ZVP reduced the cerulein-induced calpain activation but had no effect on calpastatin alterations. In correlation to the diminished calpain activity, the severity of pancreatitis decreased as indicated by a decline in amylase activity (P < 0.01), pancreatic edema formation (P < 0.05), histological score for eight parameters (P < 0.01), and actin filament alterations. Our findings support the hypothesis that dysregulation of the calpain-calpastatin system may play a role in the onset of acute pancreatitis.

The calpain system

The calpain system originally comprised three molecules: two Ca2+-dependent proteases, mu-calpain and m-calpain, and a third polypeptide, calpastatin, whose only known function is to inhibit the two calpains. Both mu- and m-calpain are heterodimers containing an identical 28-kDa subunit and an 80-kDa subunit that shares 55-65% sequence homology between the two proteases. The crystallographic structure of m-calpain reveals six "domains" in the 80-kDa subunit: 1). a 19-amino acid NH2-terminal sequence; 2). and 3). two domains that constitute the active site, IIa and IIb; 4). domain III; 5). an 18-amino acid extended sequence linking domain III to domain IV; and 6). domain IV, which resembles the penta EF-hand family of polypeptides. The single calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. Since 1989, cDNA cloning has identified 12 additional mRNAs in mammals that encode polypeptides homologous to domains IIa and IIb of the 80-kDa subunit of mu- and m-calpain, and calpain-like mRNAs have been identified in other organisms. The molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in cells is still unclear, but the calpains ostensibly participate in a variety of cellular processes including remodeling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma.

Very early activation of m-calpain in peripheral nerve during Wallerian degeneration

Peripheral nerve injury results in a series of events culminating in degradation of the axonal cytoskeleton (Wallerian degeneration). In the time period between axotomy and cytoskeletal degradation (24-48 h in rodents), there is calcium entry and activation of calpains within the axon. The precise timing of these events during this period is unknown. In the present study, antibodies were generated to three distinct peptide epitopes of m-calpain, and a fusion protein antibody was generated to the intrinsic calpain inhibitor calpastatin. These antibodies were used to measure changes in these proteins in mouse sciatic nerves during Wallerian degeneration. In sciatic nerve homogenates and cultured dorsal root ganglion (DRG) neurites, m-calpain protein was significantly reduced in transected nerves very early after nerve injury, long before axonal degeneration occurred. Levels of m-calpain protein remained low as compared to control nerves for the remainder of the 72-h time course. No changes in calpastatin protein were evident. Systemic treatment of animals with the protease inhibitor leupeptin partially prevented the rapid loss of calpain protein. Removal of calcium in DRG cultures had the same effect. These data indicate that m-calpain protein is lost very early after axonal injury, and likely reflect activation and degradation of this protein long before the cytoskeleton is degraded. Calpain activation may be an early event in a proteolytic cascade that is initiated by axonal injury and culminates with axonal degeneration.

Characterization of the calcium-dependent proteolytic system in a mouse muscle cell line

Many studies have demonstrated that the calcium-dependent proteolytic system (calpains and calpastatin) is involved in myoblast differentiation. It is also known that myogenic differentiation can be studied in vitro. In the present experiments, using a mouse muscle cell line (C2C12) we have analyzed both the sequences of appearance and the expression profiles of calpains 1, 2, 3 and calpastatin during the course of myoblast differentiation. Our results mainly show that the expression of ubiquitous calpains (calpain 1 and 2) and muscle-specific calpain (calpain 3) at the mRNAs level as well as at the protein level do not change significantly all along this biological process. In the same time, the specific inhibitor of ubiquitous calpains, calpastatin, presents a stable expression at mRNAs level as well as protein level, all along myoblast to myotube transition. A comparison with other myogenic cells is presented.

Age-dependent degradation of calpastatin in kidney of hypertensive rats

Hypertensive rats from the Milan strain show a significant decrease in calpastatin activity as compared with normotensive control animals. Calpastatin deficiency is age-related and highly relevant in kidney, heart, and erythrocytes and of minor entity in brain tissue. In normotensives the changes during aging in the levels of calpastatin activity and mRNA are consistent with an increase of calpastatin protein. In hypertensive rats such a relationship during aging is not observed, because a progressive accumulation of mRNA is accompanied by a lower amount of calpastatin protein as compared with control rats. Together with the low level of calpastatin in kidney of hypertensive rats, a progressive accumulation of an active 15-kDa calpastatin fragment, previously shown to represent a typical product of calpain-mediated calpastatin degradation, is also observed. Evidence for such intracellular proteolysis by Ca(2+)-activated calpain is provided by the normalization of the calpastatin level, up to that of control animals, in hypertensive rats treated with drugs known to reduce both blood pressure and intracellular Ca(2+) influx. Further evidence is provided by the disappearance, in these conditions, of the 15-kDa calpastatin fragment. These data allow the conclusion that calpastatin degradation is a relevant part of the overall mechanism for regulating calpain activity.

Differential degradation of calpastatin by mu- and m-calpain in Ca(2+)-enriched human neuroblastoma LAN-5 cells

In neuroblastoma LAN-5 cells during calpain activation, in addition to the two expressed 70 kDa and 30 kDa calpastatin forms, other inhibitory species are produced, having molecular masses of 50 kDa and 15 kDa. At longer times of incubation, both native and new calpastatin species disappear. The formation of these new calpastatins as well as the decrease in intracellular total calpastatin activity are mediated by calpain itself, as indicated by the effect of the synthetic calpain inhibitor I, which prevents both degradative processes. Analysis of the calcium concentrations required for the two processes indicates that the first conservative proteolytic event is mediated by micro-calpain, whereas the second one is preferentially carried out by m-calpain. The appearance of the 15 kDa form, containing only the calpastatin repetitive inhibitory domain and identified also in red cells of hypertensive rats as the major inhibitor form, can be considered a marker of intracellular calpain activation, and it can be used for the monitoring of the involvement of calpain in pathological situations.

Structure of mouse calpastatin isoforms: implications of species-common and species-specific alternative splicing

Mouse calpastatin cDNAs were cloned by the method of RT-PCR using RNA isolated from myoblast C2C12 cells. Nucleotide sequencing of the isolated clones revealed an in-frame ATG codon upstream of the previously assigned translation initiation methionine. Except for the N-terminal segment, the new translatable region (domain XL) was similar to the sequence of bovine calpastatin in which domain XL was first identified. Among the isolated mouse calpastatin cDNA clones, three isoforms (mCS-a, mCS-b, and mCS-c) were identified. In domain L, mCS-b had a deletion of the region corresponding to exon 3 of the human calpastatin gene. RT-PCR analyses of various mouse tissues revealed that mCS-b was the major form and that the content of mCS-a, nondeleted form, was 5-10% in tissues including skeletal muscle, liver, brain, etc. and about 30% in the myoblast C2C12 cells. Unlike human and rat cDNAs, no other deletions were detected in mouse calpastatin domain L. Isolation of the cDNA clone of mCS-c, which lacked regions corresponding to exons 3 and 12, was obtained by chance because its expression level was under the detectable level in the mouse tissues and even in C2C12 cells.

Properties of calpastatin forms in rat brain

Four recombinant calpastatin forms, deduced from rat brain mRNAs and differing in the number of inhibitory repetitive domains from zero to four, were expressed and characterized for their inhibitory efficiency on mu- and m-calpain. Although the most effective one is a truncated calpastatin form composed of the N-terminal region (domain L) and a single inhibitory domain, all inhibitors are more active against mu-calpain, but are preferentially degraded and inactivated by m-calpain. The protein form composed exclusively of a domain L is deprived of any inhibitory activity but prevents inhibition of calpain by the other calpastatin forms, indicating that this calpastatin region could be relevant in the recognition of the proteinase. A calpastatin form having molecular properties similar to those of the recombinant truncated calpastatin, has also been found in rat brain. It does not derive from proteolysis of a higher molecular mass precursor. The expression of multiple calpastatin forms may be relevant for the specific modulation of the different calpain isozymes normally present in a single cell type.

Rat brain contains multiple mRNAs for calpastatin

This work was undertaken to establish the forms of the calpain inhibitor, calpastatin, expressed in the brain tissue. Five cDNA clones were obtained and the corresponding amino acid sequences were deduced. Three of these proteins contain an N-terminal domain (domain L) and four inhibitory repeats typical of the calpastatin molecule. The other two are truncated forms, containing the domain L, free or associated with a single inhibitory repeat. Other differences, due to exon skipping, produce calpastatin forms with different susceptibility to posttranslational modifications. The more represented mRNA form corresponds to a calpastatin molecule containing the four inhibitory domains. These results may be useful to understand the involvement of calpain in the onset of acute and degenerative disorders of the central nervous system.

Altered gene expression for calpain/calpastatin system in motor neuron degeneration (Mnd) mutant mouse brain and spinal cord

The calcium-activated neutral proteases (CANP, calpains) have been implicated in both acute and chronic neurodegenerative processes. In the present study, we analyzed the in situ mRNA expression of calpain I and II and their endogenous inhibitor, calpastatin, in the motor neuron degeneration (Mnd) mutant mouse, which exhibits progressive dysfunction of the spinal cord and brain. As the disease progresses, the mutants show increasingly pronounced motor abnormalities which coincide with swelling of the spinal motor neurons, neocortex, hippocampal CA regions and cerebellar Purkinje cells. In situ hybridization studies show that the Mnd mice have a significantly higher level of calpain I, calpain II and calpastatin than the congenic controls in the following brain regions and cell types: hippocampal CA3 region, pyramidal cells, cerebellar Purkinje cells and spinal cord motor neurons. However, no differences in calpain or calpastatin mRNA levels are observed in glial and cerebellar granule cells of Mnd and control mice. Western blots and competitive RT-PCR analyses of brain and spinal cord homogenates are confirmative. Such altered gene expression in specific cell types of brain and spinal cord suggests the involvement of the calpain/calpastatin system.

Regional differences in gene expression for calcium activated neutral proteases (calpains) and their endogenous inhibitor calpastatin in mouse brain and spinal cord

The family of calpains (CANP or calcium activated neutral proteases) and their endogenous inhibitor calpastatin have been implicated in many neural functions; however, functional distinctions between the major calpain isoforms, calpain I and II, have not been clearly established. In the present study we analyzed the gene expression patterns for calpain I and II and calpastatin in mouse brain and spinal cord by measuring both their mRNA and protein levels. Our results show that the overall mRNA level measured by competitive reverse transcription polymerase chain reaction for calpain II is 15-fold higher and for calpastatin is three-fold higher than that for calpain I. Overall, both mRNA and protein expression levels for the calpains and calpastatin showed no significant difference between the spinal cord and the brain. The cellular distributions of mRNA for calpain I or calpastatin, measured by in situ hybridization, are relatively uniform throughout the brain. In contrast, calpain II gene expression is selectively higher in certain neuron populations including pyramidal neurons of the hippocampus and the deep neocortical layers, Purkinje cells of cerebellum, and motor neurons of the spinal cord. The motor neurons were the most enriched in calpain message. Motor neurons possessed 10-fold more calpain II mRNA than any other spinal cord cell type. The differential distribution of the two proteases in the brain and the spinal cord at the mRNA level indicates that the two calpain genes are differentially regulated, suggesting that they play different physiological roles in neuronal activities and that they may participate in the pathogenesis of certain regional neurological degenerative diseases.

Calpains are activated in necrotic fibers from mdx dystrophic mice

Death of dystrophin-deficient muscle purportedly results from increases in [Ca]in that cause the activation of calpains. We have tested whether calpains play a role in this process by assaying for changes in calpain concentration and activation in peak necrotic mdx mice (4 weeks of age) and in completely regenerated mdx mice (14 weeks of age). Biochemical fractionation and immunoblotting with epitope-specific antisera allowed measurement of the concentrations of m- and mu-calpains and the extent of autoproteolytic modification. Our findings show that total calpain concentration is elevated in both 4-week and 14-week mdx mice. This increase in concentration was shown to result primarily from a significant increase in m-calpain concentration at 4 weeks. Northern analysis demonstrated that neither m- nor mu-calpain mRNA concentrations differed between mdx and controls suggesting that the increased calpain concentration results from post-translational regulation. Immunoblotting with antibodies directed against amino-terminal peptides revealed an increase in autoproteolysis of mu-calpain, indicative of increased activation. The extent of autoproteolysis of mu-calpain returns to control levels during regeneration. This is not a consequence of increased calpastatin mRNA or protein. The findings reported here support a role for calpains in both the degenerative and regenerative aspects of mdx dystrophy.