Some properties of the millimolar Ca2+-dependent proteinase from bovine cardiac muscle

Oversupplying metabolizable protein in late gestation for beef cattle: effects on prepartum BW, ruminal fermentation, nitrogen balance, and skeletal muscle catabolism

The objective of the study was to determine the effect of oversupplying MP during late gestation on maternal BW, ruminal fermentation, nitrogen balance, and skeletal muscle catabolism. Crossbred Hereford heifers (n = 24) were assigned to a control treatment designed to meet MP requirements (CON) or a treatment providing 133% of the MP requirement (HMP). Heifers were individually fed their treatment from day -55 ± 3 relative to parturition and DMI was summarized by week. BW was measured on day -55 ± 3, -41 ± 3, -27 ± 3, and -8 ± 3. Ruminal digesta samples were collected on day -34 ± 5 and -15 ± 4 for short-chain fatty acid and ammonia-N (NH3-N) concentration. Plasma was collected the day prior to ruminal digesta samples and analyzed for plasma urea-N. Nitrogen balance was measured over a 6-d period starting on day -34 ± 4 and -15 ± 4. Following completion of the N balance periods, muscle biopsies were collected from the longissimus dorsi and analyzed for abundance of proteins relating to skeletal muscle catabolism. Data were analyzed as a randomized complete block (date of parturition) design with repeated measures using the MIXED procedure of SAS. Heifers fed HMP increased conceptus-corrected BW by a greater magnitude than CON at day -8 relative to -55 and -41 (treatment × day, P < 0.01). DMI increased (P < 0.01) by 18% on week -2 compared to -8, but then decreased (P < 0.01) by 8.0% for week -1. N-intake, apparent N digestion, N excretion, and N retention (g/d) were all greater (P < 0.01) for HMP heifers than CON but did not differ when expressed as a proportion of N intake. Ruminal NH3-N decreased (treatment × day, P < 0.01) as parturition approached for HMP (10.1 to 8.6 mg/dL); whereas, NH3-N was not affected for CON (1.0 to 1.3 mg/dL). Consequently, plasma urea-N was greater (P < 0.01) for HMP heifers (15.0 vs. 7.5 mg/dL). Heifers fed HMP had improved (P < 0.01) DM, OM, and NDF digestibility relative to CON heifers. The abundance of calpastatin was greater (P = 0.03) and calpain tended to be greater (P = 0.085) for CON cows compared to HMP. Feeding greater quantities of MP during late gestation may improve ruminal fermentation, N balance, and improve BW gain prepartum.

Psychological stress and aging: role of glucocorticoids (GCs)

Psychological stress has extreme adverse consequences on health. However, the molecular mechanisms that mediate and accelerate the process of aging due to stress hormone are not well defined. This review has focused on diverse molecular paths that come out in response to chronic psychological stress via releasing of excessive glucocorticoids (GCs), involved in the aging process. GCs suppress transcription of nuclear cell adhesion molecules which impair synaptic plasticity, memory formation, and cognitive ability. Again, GCs promote muscle atrophy by means of motivating ubiquitin proteasome system and can repress muscle protein synthesis by inhibition of PI3-kinase/Akt pathway. GCs also inhibit interleukin-2 synthesis through suppressing T cell receptor signal that leads to loss of T cell activation, proliferation, and B-cell activation. Moreover, GCs increase the expression of collagenase-3, RANK ligand, and colony stimulating factor-1 that induce bone resorption. In general, stress-induced GCs can play causal role for aging and age-related disorders.

Effect of copper and cadmium ions on heart function and calpain activity in blue mussel Mytilus edulis

The heart rate and calpain activity of blue mussels Mytilus edulis from the sublittoral zone, exposed to different levels of water-borne copper and cadmium, was investigated in a long-term experiment. The content of cadmium and copper in the blue mussel was determined using flame and graphite Atomic absorption spectroscopy. The observed concentrations ranged from 2.5 to 89.1 μg/g dry weight for cadmium and from 6.1 to 51.0 μg/g dry weight for copper in the control and highest concentration, respectively. Initially, increase in cardiac activity in response to copper and Cadmium exposure was observed under all pollutant concentrations (5-250 and 10-500 μg/L, respectively). The calpain-like activity in gills and hepatopancreas of the mussels treated with metals changed in dose- and time-dependent manner: from a sharp rise at the 250 μg/L concentration of copper on the first day to a significant decrease under the effect of Cadmium in the concentration of 500 μg/L on the third day of the experiment. These results suggest that: (i) heart rate oscillation may reflect active adaptation of blue mussels to contamination and (ii) animals have different sensitivity to copper and Cadmium according to the role of the metals in the mussels' life activity.

Calpain activation is involved in acute manganese neurotoxicity in the rat striatum in vivo

Manganese is essential for life, yet chronic exposure to this metal can cause a neurodegenerative disease named manganism that affects motor function. In the present study we have evaluated Mn neurotoxicity after its administration in the rat striatum. The participation of the calcium-dependent protease calpain and the apoptosis-related protease caspase-3, in Mn-induced cell death was monitored in the striatum and globus pallidus. Mn induced the activation of both proteases, although calpain activation seems to be an earlier event. Moreover, while the broad-spectrum caspase inhibitor QVD did not significantly prevent Mn-induced cell death, the specific calpain inhibitor MDL-28170 did. The role of NMDA glutamate receptors on calpain activity was also investigated; blockage of these receptors by MK-801 and memantine did not prevent calpain activation, nor Mn-induced cell death. Finally, studies in striatal homogenates suggest a direct activation of calpain by Mn ions. Altogether the present study suggests that additional mechanisms to excitotoxicity are involved in Mn-induced cell death, placing calpain as an important mediator of acute Mn neurotoxicity in vivo.

Protein and Amino Acid Supplementation Does Not Alter Proteolytic Gene Expression following Immobilization

Objective. To determine if supplementation of protein and amino acids (PAA) decreases skeletal muscle expression of atrophy-related genes, muscle mass, and strength during immobilization in humans. Methods. Twenty males wore a lower-limb immobilization boot for 28 days and consumed either a PAA supplement (28 g protein) or carbohydrate placebo (28 g maltodextrose), while consuming their normal daily diet. Testing sessions included dietary analysis, lower-leg girth and body composition measurements, strength testing, and gastrocnemius muscle biopsies. Muscle was analyzed for mRNA expression of markers in the ubiquitin and calpain systems, myostatin, TNF-α, and NF-κB. Results. All genes of interest increased over time (P < .05), but there was no difference between groups. Lower-leg girth decreased over time (P = 0.02); however, there were no significant changes in body composition or strength. Conclusion. Short-term lower-limb disuse, despite the absence of significant muscle atrophy, is associated with increases in skeletal muscle gene expression of several proteolysis-related genes. These changes do not appear to be altered by oral PAA supplementation.

Intensive care unit-acquired weakness (ICUAW) and muscle wasting in critically ill patients with severe sepsis and septic shock

Sepsis presents a major health care problem and remains one of the leading causes of death within the intensive care unit (ICU). Therapeutic approaches against severe sepsis and septic shock focus on early identification. Adequate source control, administration of antibiotics, preload optimization by fluid resuscitation and further hemodynamic stabilisation using vasopressors whenever appropriate are considered pivotal within the early-golden-hours of sepsis. However, organ dysfunction develops frequently in and represents a significant comorbidity of sepsis. A considerable amount of patients with sepsis will show signs of severe muscle wasting and/or ICU-acquired weakness (ICUAW), which describes a frequently observed complication in critically ill patients and refers to clinically weak ICU patients in whom there is no plausible aetiology other than critical illness. Some authors consider ICUAW as neuromuscular organ failure, caused by dysfunction of the motor unit, which consists of peripheral nerve, neuromuscular junction and skeletal muscle fibre. Electrophysiologic and/or biopsy studies facilitate further subclassification of ICUAW as critical illness myopathy, critical illness polyneuropathy or critical illness myoneuropathy, their combination. ICUAW may protract weaning from mechanical ventilation and impede rehabilitation measures, resulting in increased morbidity and mortality. This review provides an insight on the available literature on sepsis-mediated muscle wasting, ICUAW and their potential pathomechanisms.

Cloning, expression, and polymorphism of the porcine calpain10 gene

Calpains are calcium-regulated proteases involved in cellular functions that include muscle proteolysis both ante- and post-mortem. This study was designed to clone the complete coding sequence of the porcine calpain10 gene, CAPN10, to analyze its expression characteristics and to investigate its polymorphism. Two isoforms of the CAPN10 gene, CAPN10A and CAPN10B, were obtained by reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends methods combined with in silico cloning. RT-PCR results indicated that CAPN10 mRNA was ubiquitously expressed in all tissues examined and, with increasing age, the expression level increased in muscles at six different growth points. In the same tissues, the expression level of CAPN10A was higher than that of CAPN10B. In addition, three single nucleotide polymorphisms were detected by the PCR-single-stranded conformational polymorphism method and by comparing the sequences of Chinese Min pigs with those of Yorkshire pigs. C527T mutation was a missense mutation and led to transforming Pro into Leu at the 176 th amino acid. The results of the current study provided basic molecular information for further study of the function of the porcine CAPN10 gene.

Isolation and characterization of mu-calpain, m-calpain, and calpastatin from postmortem muscle. I. Initial steps

Evidence has indicated that mu-calpain, m-calpain, and calpastatin have important roles in the proteolytic degradation that results in postmortem tenderization. Simple assays of these 3 proteins at different times postmortem, however, has shown that calpastatin and mu-calpain both rapidly lose their activity during postmortem storage, so that proteolytic activity of mu-calpain is nearly zero after 3 d postmortem, even when assayed at pH 7.5 and 25 degrees C, and ability of calpastatin to inhibit the calpains is 30% or less of its ability when assayed at death. m-Calpain, however, retains much of its proteolytic activity during postmortem storage, but the Ca(2+) requirement of m-calpain is much higher than that reported to exist in postmortem muscle. Consequently, it is unclear how the calpain system functions in postmortem muscle. To clarify this issue, we have initiated attempts to purify the 2 calpains and calpastatin from bovine semitendinosus muscle after 11-13 d postmortem. The known properties of the calpains and calpastatin in postmortem muscle have important effects on approaches that can be used to purify them. A hexyl-TSK hydrophobic interaction column is a critical first step in separating calpastatin from the 2 calpains in postmortem muscle. Dot-blot assays were used to detect proteolytically inactive mu-calpain. After 2 column chromatographic steps, 5 fractions can be identified: 1) calpastatin I that does not bind to an anion-exchange matrix, that does not completely inhibit the calpains, and that consists of small polypeptides <60 kDa; 2) calpastatin II that binds weakly to an anion-exchange matrix and that contains polypeptides <60 kDa; all these polypeptides are smaller than the native 115- to 125-kDa skeletal muscle calpastatin; 3) proteolytically active mu-calpain even though very little mu-calpain activity can be detected in zymogram assays of muscle extracts from 11- to 13-d postmortem muscle; this mu-calpain has an autolyzed 76-kDa large subunit but the small subunit consists of 24-, 26- and a small amount of unautolyzed 28-kDa polypeptides; 4) proteolytically active m-calpain that is not autolyzed; and 5) proteolytically inactive mu-calpain whose large subunit is autolyzed to a 76-kDa polypeptide and whose small subunit contains polypeptides similar to the proteolytically active mu-calpain. Hence, loss of calpastatin activity in postmortem muscle is due to its degradation, but the cause of the loss of mu-calpain activity remains unknown.

Myofibrillar protein turnover: the proteasome and the calpains

Metabolic turnover of myofibrillar proteins in skeletal muscle requires that, before being degraded to AA, myofibrillar proteins be removed from the myofibril without disrupting the ability of the myofibril to contract and develop tension. Skeletal muscle contains 4 proteolytic systems in amounts such that they could be involved in metabolic protein turnover: 1) the lysosomal system, 2) the caspase system, 3) the calpain system, and 4) the proteasome. The catheptic proteases in lysosomes are not active at the neutral pH of the cell cytoplasm, so myofibrillar proteins would have to be degraded inside lysosomes if the lysosomal system were involved. Lysosomes could not engulf a myofibril without destroying it, so the lysosomal system is not involved to a significant extent in metabolic turnover of myofibrillar proteins. The caspases are not activated until initiation of apoptosis, and, therefore, it is unlikely that the caspases are involved to a significant extent in myofibrillar protein turnover. The calpains do not degrade proteins to AA or even to small peptides and do not catalyze bulk degradation of the sarcoplasmic proteins, so they cannot be the only proteolytic system involved in myofibrillar protein turnover. Research during the past 20 yr has shown that the proteasome is responsible for 80 to 90% of total intracellular protein turnover, but the proteasome degrades peptide chains only after they have been unfolded, so that they can enter the catalytic chamber of the proteasome. Thus, although the proteasome can degrade sarcoplasmic proteins, it cannot degrade myofibrillar proteins until they have been removed from the myofibril. It remains unclear how this removal is done. The calpains degrade those proteins that are involved in keeping the myofibrillar proteins assembled in myofibrils, and it was proposed over 30 yr ago that the calpains initiated myofibrillar protein turnover by disassembling the outer layer of proteins from the myofibril and releasing them as myofilaments. Such myofilaments have been found in skeletal muscle. Other studies have indicated that individual myofibrillar proteins can exchange with their counterparts in the cytoplasm; it is unclear whether this can be done to an extent that is consistent with the rate of myofibrillar protein turnover in living muscle. It seems that both the calpains and the proteasome are responsible for myofibrillar protein turnover, but the mechanism is still unknown.

Identification and molecular characterization of the rainbow trout calpains (Capn1 and Capn2): their expression in muscle wasting during starvation

Calpains are calcium regulated proteases involved in cellular functions that include muscle proteolysis both ante- and postmortem. Here, we describe the molecular characterization of the rainbow trout catalytic subunits of the mu- and m-calpains, respectively. The cDNA sequence for Capn1 encodes a protein of 704 amino acids with a calculated molecular mass of 79.9 kDa. The amino acid sequence shows 66% and 86% identity with the mouse and zebrafish Capn1, respectively. The Capn2 cDNA codes for a protein consisting of 701 amino acid residues with a calculated molecular mass of 78.2 kDa. The protein shows 65% amino acid sequence identity with the mouse and chicken Capn2. The two isozymes of rainbow trout have the characteristic domains: I (propeptide), II (cysteine catalytic site), III (electrostatic switch), and IV (contains five EF-hands). Because starvation induces muscle wasting, the hypothesis of this study was that starvation could affect regulation of the calpain system in muscle. Starvation of rainbow trout fingerlings (15-20 g) for 35 days stimulated the expression of Capn1 (2.2-fold increase, P < 0.01), Capn2 (6.0-fold increase, P < 0.01), and calpastatins (1.6-fold increase, P < 0.05) as measured by quantitative real-time RT-PCR. The mRNA changes led to a 1.23-fold increase in the calpain catalytic activity. The results suggest a potential role of calpains in protein mobilization as a source of energy under fasting condition.

Effects of autolysis on properties of mu- and m-calpain

Although the biochemical changes that occur during autolysis of mu- and m-calpain are well characterized, there have been few studies on properties of the autolyzed calpain molecules themselves. The present study shows that both autolyzed mu- and m-calpain lose 50-55% of their proteolytic activity within 5 min during incubation at pH 7.5 in 300 mM or higher salt and at a slower rate in 100 mM salt. This loss of activity is not reversed by dialysis for 18 h against a low-ionic-strength buffer at pH 7.5. Proteolytic activity of the unautolyzed calpains is not affected by incubation for 45 min at ionic strengths up to 1000 mM. Size-exclusion chromatography shows that ionic strengths of 100 mM or above cause dissociation of the two subunits of autolyzed calpains and that the dissociated large subunits (76- or 78-kDa) aggregate to form dimers and trimers, which are proteolytically inactive. Hence, instability of autolyzed calpains is due to aggregation of dissociated heavy chains. Autolysis removes the N-terminal 19 (m-calpain) or 27 (mu-calpain) amino acids from the large subunit and approximately 90 amino acids from the N-terminus of the small subunit. These regions form contacts between the two subunits in unautolyzed calpains, and their removal leaves only contacts between domain IV in the large subunit and domain VI in the small subunit. Although many of these contacts are hydrophobic in nature, ionic-strength-induced dissociation of the two subunits in the autolyzed calpains indicates that salt bridges have an important, possibly indirect, role in the domain IV/domain VI interaction.

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.

Calmodulin-dependent cyclic nucleotide phosphodiesterase in an experimental rat model of cardiac ischemia-reperfusion

In the present study, we investigated the activity and expression of calmodulin-dependent cyclic nucleotide phosphodiesterase (CaMPDE) and the effects of calpains in rat heart after ischemia and reperfusion. Immunohistochemical studies indicated that CaMPDE in normal heart is localized in myocardial cells. Rat ischemic heart showed a decrease in CaMPDE activity in the presence of Ca2+ and calmodulin; however, in ischemic-reperfusion tissue a progressive increase in Ca2+ and calmodulin-independent cyclic nucleotide phosphodiesterase (CaM-independent PDE) activity was observed. Perfusion of hearts with cell-permeable calpain inhibitor suppressed the increase of Ca2+ and CaM-independent PDE activity. Protein expression of CaMPDE was uneffected by hypoxic injury to rat myocardium. The purified heart CaMPDE was proteolyzed by calpains into a 45 kDa immunoreactive fragment in vitro. Based on these results, we propose that hypoxic injury to rat myocardium results in the generation of CaM-independent PDE by calpain mediated proteolysis, allowing the maintenance of cAMP concentrations within the physiological range.

Capillary electrophoretic analysis of mu- and m-calpain using fluorescently labeled casein substrates

Calpains are unique calcium-dependent thiol proteases that have been proposed to participate in a number of physiological processes including signal transduction and protein turnover in skeletal muscle. Calpains exist in two major forms. Interestingly, the two forms of protease show no significant difference in their action on various substrates. The only demonstrable difference in their activity involves the concentration of calcium required for activation. Both mu- and m-calpains typically achieve half maximal activation at 50 microM and 0.7 mM calcium, respectively. The focus of this study was to examine the action of both forms of calpain on casein substrates and assess whether any differences could be observed in the resulting peptide finger print using capillary electrophoresis. Purified mu- and m-calpain were incubated for various lengths of time with Oregon Green labeled alphas- and beta-casein. The reactions were stopped with sodium dodecyl sulfate (SDS) and products separated by capillary electrophoresis in micellar electrokinetic capillary chromatography (MEKC) mode using laser-induced fluorescence (LIF) detection. Comparison of the electropherograms showed no difference in the peptide profile for either enzyme. However, it was found that beta-casein was hydrolyzed more extensively than alphas-casein, by both enzymes. Capillary electrophoresis was found to be a very sensitive technique for detection of calpain activity. Using beta-casein as substrate, the CE approach was able to detect 2-3 ng of calpain activity. The results also suggest that capillary electrophoresis is a useful tool for proteolytic investigations of protein structure.

Decreased expression of high-molecular-weight calmodulin-binding protein and its correlation with apoptosis in ischemia-reperfused rat heart

A cardiac high-molecular-weight calmodulin-binding protein (HMWCaMBP) was previously identified as a homologue of the calpain inhibitor, calpastatin. In the present study, we investigated the expression of HMWCaMBP and calpains in rat heart after ischemia and reperfusion. Western blot analysis of normal rat heart extract with a polyclonal antibody raised against bovine HMWCaMBP indicated a prominent immunoreactive band of 140kDa. Both the expression and the activity of HMWCaMBP were decreased by ischemia reperfusion. Immunohistochemical studies showed strong-to-moderate HMWCaMBP immunoreactivity in normal heart and poor immunoreactivity in ischemia-reperfused heart muscle. However, the expression of micro-calpain and m-calpain in ischemia-reperfused heart was increased as compared to normal heart. The calpain inhibitory activity of ischemia-reperfused heart tissues was significantly lower as compared to normal heart tissues. The pre-ischemic and post-ischemic perfusion of hearts with a cell-permeable calpain inhibitor suppressed the increase in calpain expression but increased the HMWCaMBP expression. In-vitro HMWCaMBP was proteolyzed by micro-calpain and m-calpain. We also measured apoptosis in normal and ischemia-reperfused tissues. An increase in the number of apoptotic bodies was observed with increased duration of ischemia and reperfusion. Bcl-2 expression did not change in any of the groups, whereas Bax expression increased with ischemia-reperfusion and correlated well with the degree of apoptosis. Our findings suggest that HMWCaMBP may sequester calpains from its substrates in the normal myocardium, but it is susceptible to proteolysis by calpains during ischemia-reperfusion. Thus, decreased expression of HMWCaMBP may play an important role in myocardial injury.

Time course of diaphragm injury and calpain activity during resistive loading

The purpose of this study was to determine the time course of arterial blood gas (ABG) deterioration, increased calpain activity, and diaphragm injury during 4 d of resistive loading. Adult Sprague- Dawley rats were divided into control (C) animals and groups that were tracheally banded (TB) for 1 d (TB1), 2 d (TB2), 3 d (TB3), and 4 d (TB4). In TB rats, the carotid artery was cannulated and the trachea was banded during anesthesia. TB groups (TB1, TB2, TB3, and TB4) had a 67% smaller internal cross-sectional area of the trachea than did C animals. ABG samples from awake rats showed a decreased arterial oxygen tension (Pa(O(2))) and a respiratory acidosis in the TB1, TB2, and TB3 groups. Calpain activity was higher in the diaphragm of TB than of C rats; calpainlike activities in soluble fractions of diaphragm tissue were greater in all TB groups than in C rats, whereas those in bound fractions were greater in the TB2 and TB3 groups. Point counting of hematoxylin and eosin-stained cross-sections showed that the area fraction (A(A)) of normal diaphragm was lower and the A(A) of abnormal muscle and connective tissue was higher in TB3 than in C rats. Increased resistive loading induced by tracheal banding was associated with hypercapnic ventilatory failure, increased calpain activity, and diaphragm injury. Ventilatory failure in response to resistive loading may be due to diaphragm injury and/or to decreased minute ventilation.

Altered expression of high-molecular-weight calmodulin-binding protein in human ischaemic myocardium

A high-molecular-weight calmodulin-binding protein (HMWCaMBP) was previously identified and purified from the cytosolic fraction of bovine heart. Based on the sequence homology, amino acid analysis, antibody reactivity, and calpain inhibition, HMWCaMBP has been identified as a homologue of the calpain inhibitor calpastatin. In the present study the expression of HMWCaMBP was investigated in normal and ischaemic human myocardium. Western blot analysis of normal human cardiac muscle extract with the polyclonal antibody raised against bovine HMWCaMBP indicated a prominent immunoreactive band with a molecular mass of 140 kD. HMWCaMBP was localized in the cytoplasm and myofilaments of cardiac myocytes. Furthermore, Western blot analysis of normal and ischaemic cardiac tissues indicated a decrease in the expression of HMWCaMBP in ischaemic tissues. These studies were further substantiated by immunohistochemical studies, indicating strong to moderate HMWCaMBP immunoreactivity in normal cardiac muscle and poor to negative immunoreactivity in ischaemic muscle. The results obtained from the rat ischaemic model suggested that the expression of cardiac HMWCaMBP was significantly decreased during ischaemia/reperfusion. In addition, micro-calpain and m-calpain expression was higher in ischaemic cardiac tissue samples than in normal controls. The calpain inhibitory activity of ischaemic cardiac tissues was significantly lower than normal cardiac tissue samples. In some cases of cardiac ischaemia, HMWCaMBP highlighted the contraction band necrosis seen at the margins of a myocardial infarct. In vitro, HMWCaMBP was proteolysed by micro-calpain and m-calpain. These results indicate that HMWCaMBP could be susceptible to proteolysis by calpains during ischaemia or reperfusion and may play a contributory role in myocardial injury.

A BODIPY fluorescent microplate assay for measuring activity of calpains and other proteases

The use of 4,4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacene-3-propionic acid (BODIPY-FL) labeled casein in autoquenching assays of proteolytic activity has been recently described, and we have adapted this assay to measurement of calpain activity. BODIPY-FL coupled to casein at a ratio of 8 mol of BODIPY-FL/mol of casein or higher produces a BODIPY-FL-casein substrate that can be used in an autoquenching assay of calpain proteolytic activity. This assay has a number of advantages for measuring calpain activity. (1) The procedure does not require precipitation and removal of undegraded protein, so it is much faster than other procedures that require a precipitation step, and it can be used directly in kinetic assays of proteolytic activity. (2) The BODIPY-FL-casein assay is easily adapted to a microtiter plate format, so it can be used to screen large numbers of samples. (3) Casein is an inexpensive and readily available protein substrate that more closely mimics the natural substrates of endoproteinases, such as the calpains, than synthetic peptide substrates do. Casein has K(m) values for micro- and m-calpain that are similar to those of other substrates such as fodrin or MAP2 that may be "natural" substrates for the calpains, and there is no reason to believe that calpain hydrolysis of casein is inherently different from hydrolysis of fodrin or MAP2, which are much less accessible as substrates for protease assays. (4) The BODIPY-FL-casein assay is capable of detecting 10 ng ( approximately 5 nM) of calpain and is nearly as sensitive as the most sensitive calpain assay reported thus far. (5) The BODIPY-FL-casein assay is as reproducible as the FITC-casein assay, whose reproducibility is comparable to or better than the reproducibility of other methods used to assay calpain activity. The BODIPY-FL-casein assay is a general assay for proteolytic activity and can be used with any protease that cleaves casein.

Role of calpain in skeletal-muscle protein degradation

Although protein degradation is enhanced in muscle-wasting conditions and limits the rate of muscle growth in domestic animals, the proteolytic system responsible for degrading myofibrillar proteins in skeletal muscle is not well defined. The goals of this study were to evaluate the roles of the calpains (calcium-activated cysteine proteases) in mediating muscle protein degradation and the extent to which these proteases participate in protein turnover in muscle. Two strategies to regulate intracellular calpain activities were developed: overexpression of dominant-negative m-calpain and overexpression of calpastatin inhibitory domain. To express these constructs, L8 myoblast cell lines were transfected with LacSwitch plasmids, which allowed for isopropyl beta-D-thiogalactoside-dependent expression of the gene of interest. Inhibition of calpain stabilized fodrin, a well characterized calpain substrate. Under conditions of accelerated degradation (serum withdrawal), inhibition of m-calpain reduced protein degradation by 30%, whereas calpastatin inhibitory domain expression reduced degradation by 63%. Inhibition of calpain also stabilized nebulin. These observations indicate that calpains play key roles in the disassembly of sarcomeric proteins. Inhibition of calpain activity may have therapeutic value in treatment of muscle-wasting conditions and may enhance muscle growth in domestic animals.

Calpain activity alters in rat myocardial subfractions after ischemia or reperfusion

To examine whether calpain is activated during ischemic or reperfusion injury, we measured calpain activity of the subfractions of rat myocardia after global ischemia for 60 min or the ischemia followed by 30 min reperfusion by the Langendorff procedure. The myocardial homogenate was fractionated into 600 x g, 10,000 x g and 100,000 x g pellet fractions as well as 10,000 x g supernatant fraction. The supernatant fraction was further subjected to DEAE cellulose and phenyl-Sepharose chromatographies to separate mu- and m-calpains. The m-calpain activity of the DEAE fractions after global ischemia for 60 min was higher but that after ischemia-reperfusion was lower than that of the control. On the other hand, the ischemia-reperfusion but not ischemia by itself raised the calpain activity of the phenyl-Sepharose fraction (mu-calpain) and the 10,000 x g pellet measured at 100 microM and 5 mM Ca2+. Treatment with verapamil but not with ryanodine during ischemia attenuated the increase in m-calpain activity. A dot-blotting analysis of calpain antigenicity showed a decrease in soluble but no change in the particulate fractions after ischemia-reperfusion. An immunoblotting technique did not detect proteolysis of the calpain 80-kDa subunit. These observations suggest that calpain is activated by Ca2+ influx during ischemia and reperfusion without gross changes in its amount. Some unknown processes other than translocation or autolysis are thought to be involved in the alterations.

In vitro digestion of dystrophin by calcium-dependent proteases, calpains I and II

Dystrophin is a cytoskeletal protein which is thought to play an important role in membrane physiology since its absence (due to gene deficiency) leads to the symptoms of Duchenne muscular dystrophy (DMD). Some disruption in the regulation of intracellular free Ca2+ levels could lead to DMD-like symptoms. In this study, calpains, which are very active calcium-dependent proteases, were examined for their capacity to hydrolyse dystrophin in vitro. The results show that calpains are able to split dystrophin and produce breakdown products of different sizes (the degree of cleavage being dependent on the incubation time with proteases). The time-course of protease degradation was examined by Western immunoblot using three polyclonal sera which were characterized as being specific to the central (residues 1173-1728) and two distal parts of the molecule ie specific to the N-terminal (residues 43-760) or the C-terminal (residues 3357-3660) extremities of the dystrophin molecule. The cleavage patterns of dystrophin showed an accumulation of some major protease-resistant fragments of high relative molecular mass (250-370 kDa). These observations demonstrate that calpains digest dystrophin very rapidly when the calcium concentration is compatible with their activation. For instance, it is clear that calpains first give rise to large dystrophin products in which the C-terminal region is lacking. These observations suggest that dystrophin antibodies specific to the central domain of the molecule should be used to detect dystrophin for diagnostic purposes and before any conclusion as to the presence or absence of dystrophin can be deduced from results obtained using immunoanalyses of muscle biopsies.

Role of the calpain system in muscle growth

Muscle protein degradation has an important role in rate of muscle growth. It has been difficult to develop procedures for measuring rate of muscle protein degradation in living animals, and most studies have used in vitro systems and muscle strips to determine rate of protein degradation. The relationship between results obtained by using muscle strips and rate of muscle protein turnover in living animals is unclear because these strips are in negative nitrogen balance and often develop hypoxic cores. Also, rate of protein degradation is usually estimated by release of labeled amino acids, which reflects an average rate of degradation of all cellular proteins and does not distinguish between rates of degradation of different groups of proteins such as the sarcoplasmic and the myofibrillar proteins in muscle. A number of studies have suggested that the calpain system initiates turnover of myofibrillar proteins, which are the major group of proteins in striated muscle, by making specific cleavages that release thick and thin filaments from the surface of the myofibril and large polypeptide fragments from some of the other myofibrillar proteins. The calpains do not degrade myofibrillar proteins to small peptides or to amino acids, and they cause no bulk degradation of sarcoplasmic proteins. Hence, the calpains are not directly responsible for release of amino acids during muscle protein turnover. Activity of the calpains in living cells is regulated by calpastatin and Ca2+, but the nature of this regulation is still unclear.

Comparison of the autolyzed and unautolyzed forms of mu- and m-calpain from bovine skeletal muscle

Bovine skeletal muscle mu- and m-calpain autolyze when incubated with Ca2+. During the first 30 to 300 s, autolysis: (1) has little effect on the specific proteolytic activity of either mu- or m-calpain when assayed at 5 mM Ca2+; and (2) produces two new proteolytically active forms of calpain in addition to the original mu- and m-calpain. The four proteolytically active forms of calpain are: (1) autolyzed mu-calpain, having polypeptide subunits of 76 and 18 kDa and requiring 0.60 microM Ca2+ for half-maximal activity; (2) mu-calpain with 80- and 28-kDa subunits and requiring 7.1 microM Ca2+ for half-maximal activity; (3) autolyzed m-calpain with 78- and 18-kDa subunits and requiring 180 microM Ca2+ for half-maximal activity; and (4) m-calpain with 80- and 28-kDa subunits and requiring 1000 microM Ca2+ for half-maximal activity. All four forms of the calpains have similar pH optima (7.4 to 7.6) and almost identical circular dichroism spectra in the far ultraviolet (all four have little secondary structure with 26-30% alpha-helix and less than 10% beta-sheet structure). Autolyzed mu- and unautolyzed mu-calpain are fully activated proteolytically by Mn2+ with activity starting at 125 microM Mn2+. Autolyzed m-calpain is also activated by Mn2+ up to 80% of the maximum proteolytic activity obtained with Ca2+; Mn2+ activation begins at 320 microM Mn2+. Unautolyzed m-calpain has only 6 to 8% as much activity in the presence of Mn2+ as it does in the presence of Ca2+. Autolysis increases the axial ratios of the calpains from 3.5 to 4.6 for mu-calpain and from 3.7 to 5.0 for m-calpain (assuming 20% hydration). The estimated length of the calpain molecules increases by 13% upon autolysis from 73 to 84 A for mu-calpain and from 76 to 90 A for m-calpain (assuming 20% hydration). The autolyzed calpains elute after their unautolyzed counterparts off a DEAE-ion exchange column. Because autolyzed forms of the calpains are not found in DEAE elution profiles of cell extracts, bovine skeletal muscle cells must contain very little (less than 5% of total calpain) or none of the autolyzed form of the calpains.

Effect of substrate on Ca2(+)-concentration required for activity of the Ca2(+)-dependent proteinases, mu- and m-calpain

The Ca2+ concentrations required for half-maximal activity of mu- and m-calpain purified from bovine skeletal muscle were tested using four different protein substrates and three different synthetic peptide substrates. Hammersten casein, the commonly used substrate for measuring mu- and m-calpain activity, required 2.5 microM Ca2+ for half-maximal activity of mu-calpain and 290 microM Ca2+ for half-maximal activity of m-calpain. When Hammersten casein was dialyzed against 8 M urea and 10 mM EDTA to remove all endogenous Ca2+, it required 1.9 and 290 microM Ca2+ for half-maximal activity of mu- and m-calpain, respectively. Rabbit skeletal muscle myofibrils and rabbit skeletal muscle troponin required 65 microM and 24 microM Ca2+ for half-maximal activity of mu-calpain and 380 microM and 580 microM Ca2+ for half-maximal activity of m-calpain, respectively. The three synthetic substrates tested, Suc-Leu-Tyr-MCA, Boc-Leu-Thr-Arg-MCA, and Suc-Leu-Leu-Val-Tyr-MCA, required 1.6 microM to 3.7 microM Ca2+ for half-maximal activity of mu-calpain and 200 to 560 microM Ca2+ for half-maximal activity of m-calpain.