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

Secondary Lipid Oxidation Products as Modulators of Calpain-2 Functionality In Vitro

The objective was to understand the impacts of secondary lipid oxidation products on calpain-2 activity and autolysis and, subsequently, to determine the quantity and localization of modification sites. 2-Hexenal and 4-hydroxynonenal incubation significantly decreased calpain-2 activity and slowed the progression of autolysis, while malondialdehyde had minimal impact on calpain-2 activity and autolysis. Specific modification sites were determined with LC-MS/MS, including distinct malondialdehyde modification sites on the calpain-2 catalytic and regulatory subunits. 2-Hexenal modification sites were observed on the calpain-2 catalytic subunit. Intact protein mass analysis with MALDI-MS revealed that a significant number of modifications on the calpain-2 catalytic and regulatory subunits are likely to exist. These observations confirm that specific lipid oxidation products modify calpain-2 and may affect the calpain-2 functionality. The results of these novel experiments have implications for healthy tissue metabolism, skeletal muscle growth, and post-mortem meat tenderness development.

Matrix metalloproteinase- 9 may contribute to collagen structure modification during postmortem aging of beef

Matrix metalloproteinases (MMPs) are responsible for the turnover of intramuscular connective tissue in live animals. We hypothesize that MMPs may play a role in postmortem aging of beef muscles for the degradation of connective tissues. Four different experiments were performed to: 1) characterize MMP activity during postmortem aging of beef; 2) determine if the native beef MMP can contribute to connective tissue degradation in a simulated standard industry postmortem aging condition; 3) explore approaches to improve the native beef MMP activity and 4) characterize MMP activity in beef from cattle supplemented with supranutritional level of Zn. In experiment 1, MMP was active throughout the entire aging periods (3, 21, 42 and 63 d) for beef muscles Longissimus lumborum, Gluteus medius and Gastrocnemius, and the unknown MMP responsible for the collagen degradation was identified as MMP-9 by Western Blot. In experiment 2 and 3, MMP-9 activity was noticeable in the gels after 42 d of storage in the cooler. Moreover, the addition of ZnCl2 in the model system significantly increased MMP-9 activity when compared to the control (P < 0.01). In experiment 4, Longissimus thoracis from animals supplemented with a supranutritional Zn level had increased Zn availability and MMP-9 activity than those from animals fed with a control diet (P < 0.05). Further research is needed better understand MMP-9 mechanism during postmortem aging of meat. With a better understanding of MMP-9 in the aging process, the beef industry can provide better connective tissue management strategies for lower-quality beef cuts.

The intersection between cysteine proteases, Ca2+ signalling and cancer cell apoptosis

Apoptosis is a highly complex and regulated cell death pathway that safeguards the physiological balance between life and death. Over the past decade, the role of Ca2+ signalling in apoptosis and the mechanisms involved have become clearer. The initiation and execution of apoptosis is coordinated by three distinct groups of cysteines proteases: the caspase, calpain and cathepsin families. Beyond its physiological importance, the ability to evade apoptosis is a prominent hallmark of cancer cells. In this review, we will explore the involvement of Ca2+ in the regulation of caspase, calpain and cathepsin activity, and how the actions of these cysteine proteases alter intracellular Ca2+ handling during apoptosis. We will also explore how apoptosis resistance can be achieved in cancer cells through deregulation of cysteine proteases and remodelling of the Ca2+ signalling toolkit.

Calpain Regulation and Dysregulation-Its Effects on the Intercalated Disk

The intercalated disk is a cardiac specific structure composed of three main protein complexes-adherens junctions, desmosomes, and gap junctions-that work in concert to provide mechanical stability and electrical synchronization to the heart. Each substructure is regulated through a variety of mechanisms including proteolysis. Calpain proteases, a class of cysteine proteases dependent on calcium for activation, have recently emerged as important regulators of individual intercalated disk components. In this review, we will examine how calcium homeostasis regulates normal calpain function. We will also explore how calpains modulate gap junctions, desmosomes, and adherens junctions activity by targeting specific proteins, and describe the molecular mechanisms of how calpain dysregulation leads to structural and signaling defects within the heart. We will then examine how changes in calpain activity affects cardiomyocytes, and how such changes underlie various heart diseases.

Nuclei isolation methods fail to accurately assess the subcellular localization and behaviour of proteins in skeletal muscle

AIM

Subcellular fractionation is often used to determine the subcellular localization of proteins, including whether a protein translocates to the nucleus in response to a given stimulus. Examining nuclear proteins in skeletal muscle is difficult because myonuclear proteins are challenging to isolate unless harsh treatments are used. This study aimed to determine the most effective method for isolating and preserving proteins in their native state in skeletal muscle.

METHODS

We compared the ability of detergents, commercially available kit-based and K⁺ -based physiological methodologies for isolating myonuclear proteins from resting samples of human muscle by determining the presence of marker proteins for each fraction by western blot analyses.

RESULTS

We found that following the initial pelleting of nuclei, treatment with 1% Triton-X 100, 1% CHAPS or 0.5% Na-deoxycholate under various ionic conditions resulted in the nuclear proteins being either resistant to isolation or the proteins present behaving aberrantly. The nuclear proteins in brain tissue were also resistant to 1% Triton-X 100 isolation. Here, we demonstrate aberrant behaviour and erroneous localization of proteins using the kit-based method. The aberrant behaviour was the activation of Ca²⁺ -dependent protease calpain-3, and the erroneous localization was the presence of calpain-3 and troponin I in the nuclear fraction.

CONCLUSION

Our findings indicate that it may not be possible to reliably determine the translocation of proteins between subcellular locations and the nucleus using subcellular fractionation techniques. This study highlights the importance of validating subcellular fractionation methodologies using several subcellular-specific markers and solutions that are physiologically relevant to the intracellular milieu.

Ca2+-induced binding of calpain-2 to myofibrils: Preliminary results in pork longissimus thoracis muscle supporting a role on myofibrillar protein degradation

The aim of this study was to investigate the role of Ca²⁺ in the process of calpain-2 becoming associated to myofibrils and the potential of myofibril-bound calpain to degrade myofibrillar proteins. Different Ca²⁺ concentrations were applied to myofibrils mixed with partially purified calpain-2. Ca²⁺ induced binding of calpain to myofibrils in a concentration-dependent manner. The half-maximal Ca²⁺ requirements for binding of calpain-2 to myofibrils and for calpain-2 proteolysis of myofibrils were 0.60 mM and 0.29 mM, respectively. To investigate the proteolytic activity of myofibril-bound calpain, a mixture of myofibrils and calpain-2 was briefly incubated with Ca²⁺. Unbound calpain was removed by washing with a Ca²⁺-free buffer. The myofibril-bound calpain maintained proteolytic activity and degraded desmin when re-activated with Ca²⁺. In conclusion, the results suggest that an increase in Ca²⁺ will activate and induce binding of calpain to myofibrils. Subsequently, calpain is relatively tightly bound and proteolytically active.

The ERG1a potassium channel increases basal intracellular calcium concentration and calpain activity in skeletal muscle cells

Background

Skeletal muscle atrophy is the net loss of muscle mass that results from an imbalance in protein synthesis and protein degradation. It occurs in response to several stimuli including disease, injury, starvation, and normal aging. Currently, there is no truly effective pharmacological therapy for atrophy; therefore, exploration of the mechanisms contributing to atrophy is essential because it will eventually lead to discovery of an effective therapeutic target. The ether-a-go-go related gene (ERG1A) K⁺ channel has been shown to contribute to atrophy by upregulating ubiquitin proteasome proteolysis in cachectic and unweighted mice and has also been implicated in calcium modulation in cancer cells.

Methods

We transduced C₂C₁₂ myotubes with either a human ERG1A encoded adenovirus or an appropriate control virus. We used fura-2 calcium indicator to measure intracellular calcium concentration and Calpain-Glo assay kits (ProMega) to measure calpain activity. Quantitative PCR was used to monitor gene expression and immunoblot evaluated protein abundances in cell lysates. Data were analyzed using either a Student’s t test or two-way ANOVAs and SAS software as indicated.

Results

Expression of human ERG1A in C₂C₁₂ myotubes increased basal intracellular calcium concentration 51.7% (p < 0.0001; n = 177). Further, it increased the combined activity of the calcium-activated cysteine proteases, calpain 1 and 2, by 31.9% (p < 0.08; n = 24); these are known to contribute to degradation of myofilaments. The increased calcium levels are likely a contributor to the increased calpain activity; however, the change in calpain activity may also be attributable to increased calpain protein abundance and/or a decrease in levels of the native calpain inhibitor, calpastatin. To explore the enhanced calpain activity further, we evaluated expression of calpain and calpastatin genes and observed no significant differences. There was no change in calpain 1 protein abundance; however, calpain 2 protein abundance decreased 40.7% (p < 0.05; n = 6). These changes do not contribute to an increase in calpain activity; however, we detected a 31.7% decrease (p < 0.05; n = 6) in calpastatin which could contribute to enhanced calpain activity.

Conclusions

Human ERG1A expression increases both intracellular calcium concentration and combined calpain 1 and 2 activity. The increased calpain activity is likely a result of the increased calcium levels and decreased calpastatin abundance.

A positive feedback loop involving nuclear factor IB and calpain 1 suppresses glioblastoma cell migration

Glioblastoma (GBM) is a brain tumor that remains largely incurable because of its highly-infiltrative properties. Nuclear factor I (NFI)-type transcription factors regulate genes associated with GBM cell migration and infiltration. We have previously shown that NFI activity depends on the NFI phosphorylation state and that calcineurin phosphatase dephosphorylates and activates NFI. Calcineurin is cleaved and activated by calpain proteases whose activity is, in turn, regulated by an endogenous inhibitor, calpastatin (CAST). The CAST gene is a target of NFI in GBM cells, with differentially phosphorylated NFIs regulating the levels of CAST transcript variants. Here, we uncovered an NFIB-calpain 1-positive feedback loop mediated through CAST and calcineurin. In NFI-hyperphosphorylated GBM cells, NFIB expression decreased the CAST-to-calpain 1 ratio in the cytoplasm. This reduced ratio increased autolysis and activity of cytoplasmic calpain 1. Conversely, in NFI-hypophosphorylated cells, NFIB expression induced differential subcellular compartmentalization of CAST and calpain 1, with CAST localizing primarily to the cytoplasm and calpain 1 to the nucleus. Overall, this altered compartmentalization increased nuclear calpain 1 activity. We also show that nuclear calpain 1, by cleaving and activating calcineurin, induces NFIB dephosphorylation. Of note, knockdown of calpain 1, NFIB, or both increased GBM cell migration and up-regulated the pro-migratory factors fatty acid-binding protein 7 (FABP7) and Ras homolog family member A (RHOA). In summary, our findings reveal bidirectional cross-talk between NFIB and calpain 1 in GBM cells. A physiological consequence of this positive feedback loop appears to be decreased GBM cell migration.

Effects of electrical stimulation and pre-rigor conditioning temperature on aging potential of hot-boned beef M. longissimus lumborum

The objective of this study was to create various pH/temp decline rates in hot-boned bull beef M. longissimus lumborum (LL) through a combination of electrical stimulation (ES) and pre-rigor holding temperature. The relationship between the pre-rigor interventions, the activities of µ-calpain and small heat shock proteins (sHSP), and the impacts on meat product quality were determined. Paired LL loins from 13 bulls were hot-boned within 40 min of slaughter, immediately ES and subjected to various holding temperatures (5, 15, 25, and 35°C) for 3 hr. The rate of muscle pH decline, sarcomere length, shear force, and proteolysis of muscle proteins were measured. ES-25°C had a longer sarcomere length compared to non-electrical stimulation samples. ES-25°C and ES-35°C samples had lower shear force values, higher µ-calpain activity and higher desmin, troponin-T, and sHSP degradation. The above findings suggest that pH/temp decline rates created in hot-boned muscle impacted muscle protein proteolysis by increasing the activity of proteases and degradation of sHSP.

Involvement of calpain in the neuropathogenesis of Alzheimer's disease

Alzheimer’s disease (AD) is the most common (60% to 80%) age‐related disease associated with dementia and is characterized by a deterioration of behavioral and cognitive capacities leading to death in few years after diagnosis, mainly due to complications from chronic illness. The characteristic hallmarks of the disease are extracellular senile plaques (SPs) and intracellular neurofibrillary tangles (NFTs) with neuropil threads, which are a direct result of amyloid precursor protein (APP) processing to Aβ, and τ hyperphosphorylation. However, many indirect underlying processes play a role in this event. One of these underlying mechanisms leading to these histological hallmarks is the uncontrolled hyperactivation of a family of cysteine proteases called calpains. Under normal physiological condition calpains participate in many processes of cells’ life and their activation is tightly controlled. However, with an increase in age, increased oxidative stress and other excitotoxicity assaults, this regulatory system becomes impaired and result in increased activation of these proteases involving them in the pathogenesis of various diseases including neurodegeneration like AD. Reviewed here is a pool of data on the implication of calpains in the pathogenesis of AD, the underlying molecular mechanism, and the potential of targeting these enzymes for AD therapeutics.

Postmortem protein degradation is a key contributor to fresh pork loin tenderness

The objective of this study was to determine factors that influence tenderness independent of variation in pH, color, or marbling. To achieve the objective, 2 sample groups were chosen from a population of 159 pork loins aged 11 to 16 d. Predetermined ranges (ultimate pH, 5.54 to 5.86; marbling score, 1.0 to 3.0; percent total lipid, 1.61 to 3.37%) were defined for inclusion of individual loins in the study. The pork loins with the greatest ( = 12) and least ( = 12) Instron star probe values were assigned to 2 classification groups. The high star probe group had an average star probe that was 2.8 kg greater than the low star probe group (7.75 vs. 4.95 kg). Pork quality and sensory characteristics of pH, subjective and instrumental color values, cook loss, sensory tenderness, chewiness, juiciness, pork flavor, and off flavor were determined on fresh, never frozen pork chops. Lipid content, sarcomere length, myosin heavy-chain profile, and calpain autolysis were determined. Degradation of troponin-T, desmin, filamin, and titin were evaluated on the protein extracts from each sample. Pork loin pH, subjective color scores, Minolta L values, sarcomere length, and myosin heavy-chain composition were not different across groups. Chops from the low star probe group had a significantly greater marbling score (2.3 vs. 1.9) and lipid content (2.61 vs. 2.23%). Calpain-1 was completely autolyzed in both high and low star probe samples, demonstrating that calpain-1 potentially had been active in all samples. Low star probe whole-muscle protein extracts had more troponin-T ( < 0.01), desmin ( < 0.01), and filamin degradation ( < 0.01) than high star probe samples. Both classification groups showed degradation of titin. Remarkably, some high star probe samples still had observable intact bands of titin on SDS-PAGE gels. These results demonstrate that significant variation in instrumental tenderness is observed within a moderate pH range. Lipid content and proteolysis both appear to contribute to this variation.

Bioactive peptides in beef: Endogenous generation through postmortem aging

The present research was performed to investigate endogenous release of bioactive peptides in beef during postmortem aging times (1, 10 and 20days). Gradually decreased Warner-Bratzler shear force (WBSF) values of longissimus thoracis (LT) and semitendinosus (ST) muscles were observed and the degradation of structural proteins and collagen led to release of low-molecular weight (<3kDa) peptides. These peptides exhibited 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity, ACE- and renin-inhibitory activities. The peptide sequences were identified by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS). In silico analysis (PeptideRanker and BIOPEP) of their bioactivity potentials demonstrated peptides with the predicted bioactivity scores (>0.8) as well as collagen peptides with bioactivity scores (0.6-0.8). The present findings provide insights on development of healthy beef through postmortem aging at 4°C.

Morphological and molecular aspects of immobilization-induced muscle atrophy in rats at different stages of postnatal development: the role of autophagy

Muscle loss occurs following injury and immobilization in adulthood and childhood, which impairs the rehabilitation process; however, far fewer studies have been conducted analyzing atrophic response in infants. This work investigated first the morphological and molecular mechanisms involved in immobilization-induced atrophy in soleus muscles from rats at different stages of postnatal development [i.e., weanling (WR) and adult (AR) rats] and, second, the role of autophagy in regulating muscle plasticity during immobilization. Hindlimb immobilization for 10 days reduced muscle mass and fiber cross-sectional area, with more pronounced atrophy in WR, and induced slow-to-fast fiber switching. These effects were accompanied by a decrease in markers of protein synthesis and an increase in autophagy. The ubiquitin (Ub)-ligase MuRF1 and the ubiquitinated proteins were upregulated by immobilization in AR while the autolyzed form of μ-calpain was increased in WR. To further explore the role of autophagy in muscle abnormalities, AR were concomitantly immobilized and treated with colchicine, which blocks autophagosome-lysosome fusion. Colchicine-treated immobilized muscles had exacerbated atrophy and presented degenerative features. Despite Igf1/Akt signaling was downregulated in immobilized muscles from both age groups, Foxo1 and 4 phosphorylation was increased in WR. In the same group of animals, Foxo1 acetylation and Foxo1 and 4 content was increased and decreased, respectively. Our data show that muscle disorders induced by 10-day-immobilization occur in both age-dependent and -independent manners, an understanding that may optimize treatment outcomes in infants. We also provide further evidence that the strong inhibition of autophagy may be ineffective for treating muscle atrophy.

Higher levels of phosphorylated Y1472 on GluN2B subunits in the frontal cortex of aged mice are associated with good spatial reference memory, but not cognitive flexibility

The N-methyl-D-aspartate receptor (NMDAr) is particularly vulnerable to aging. The GluN2B subunit of the NMDAr, compared to other NMDAr subunits, suffers the greatest losses of expression in the aging brain, especially in the frontal cortex. While expression levels of GluN2B mRNA and protein in the aged brain are well documented, there has been little investigation into age-related posttranslational modifications of the subunit. In this study, we explored some of the mechanisms that may promote differences in the NMDAr complex in the frontal cortex of aged animals. Two ages of mice, 3 and 24 months, were behaviorally tested in the Morris water maze. The frontal cortex and hippocampus from each mouse were subjected to differential centrifugation followed by solubilization in Triton X-100. Proteins from Triton-insoluble membranes, Triton-soluble membranes, and intracellular membranes/cytosol were examined by Western blot. Higher levels of GluN2B tyrosine 1472 phosphorylation in frontal cortex synaptic fractions of old mice were associated with better reference learning but poorer cognitive flexibility. Levels of GluN2B phosphotyrosine 1336 remained steady, but there were greater levels of the calpain-induced 115 kDa GluN2B cleavage product on extrasynaptic membranes in these old good learners. There was an age-related increase in calpain activity, but it was not associated with better learning. These data highlight a unique aging change for aged mice with good spatial learning that might be detrimental to cognitive flexibility. This study also suggests that higher levels of truncated GluN2B on extrasynaptic membranes are not deleterious to spatial memory in aged mice.

Hyperhomocysteinemia and hyperglycemia induce and potentiate endothelial dysfunction via μ-calpain activation

Plasma homocysteine (Hcy) levels are positively correlated with cardiovascular mortality in diabetes. However, the joint effect of hyperhomocysteinemia (HHcy) and hyperglycemia (HG) on endothelial dysfunction (ED) and the underlying mechanisms have not been studied. Mild (22 µmol/L) and moderate (88 µmol/L) HHcy were induced in cystathionine β-synthase wild-type (Cbs(+/+)) and heterozygous-deficient (Cbs(-/+)) mice by a high-methionine (HM) diet. HG was induced by consecutive injection of streptozotocin. We found that HG worsened HHcy and elevated Hcy levels to 53 and 173 µmol/L in Cbs(+/+) and Cbs(-/+) mice fed an HM diet, respectively. Both mild and moderate HHcy aggravated HG-impaired endothelium-dependent vascular relaxation to acetylcholine, which was completely abolished by endothelial nitric oxide synthase (eNOS) inhibitor N(G)-nitro-L-arginine methyl ester. HHcy potentiated HG-induced calpain activation in aortic endothelial cells isolated from Cbs mice. Calpain inhibitors rescued HHcy- and HHcy/HG-induced ED in vivo and ex vivo. Moderate HHcy- and HG-induced μ-calpain activation was potentiated by a combination of HHcy and HG in the mouse aorta. μ-Calpain small interfering RNA (μ-calpsiRNA) prevented HHcy/HG-induced ED in the mouse aorta and calpain activation in human aortic endothelial cells (HAECs) treated with DL-Hcy (500 µmol/L) and d-glucose (25 mmol) for 48 h. In addition, HHcy accelerated HG-induced superoxide production as determined by dihydroethidium and 3-nitrotyrosin staining and urinary 8-isoprostane/creatinine assay. Antioxidants rescued HHcy/HG-induced ED in mouse aortas and calpain activation in cultured HAECs. Finally, HHcy potentiated HG-suppressed nitric oxide production and eNOS activity in HAECs, which were prevented by calpain inhibitors or μ-calpsiRNA. HHcy aggravated HG-increased phosphorylation of eNOS at threonine 497/495 (eNOS-pThr497/495) in the mouse aorta and HAECs. HHcy/HG-induced eNOS-pThr497/495 was reversed by µ-calpsiRNA and adenoviral transduced dominant negative protein kinase C (PKC)β2 in HAECs. HHcy and HG induced ED, which was potentiated by the combination of HHcy and HG via μ-calpain/PKCβ2 activation-induced eNOS-pThr497/495 and eNOS inactivation.

Role of calpain-1 in the early phase of experimental ALS

Elevation in [Ca(2+)]i and activation of calpain-1 occur in central nervous system of SOD1(G93A) transgenic mice model of amyotrophic lateral sclerosis (ALS), but few data are available about the early stage of ALS. We here investigated the level of activation of the Ca(2+)-dependent protease calpain-1 in spinal cord of SOD1(G93A) mice to ascertain a possible role of the protease in the aetiology of ALS. Comparing the events occurring in the 120 day old mice, we found that [Ca(2+)]i and activation of calpain-1 were also increased in the spinal cord of 30 day old mice, as indicated by the digestion of some substrates of the protease such as nNOS, αII-spectrin, and the NR2B subunit of NMDA-R. However, the digestion pattern of these proteins suggests that calpain-1 may play different roles depending on the phase of ALS. In fact, in spinal cord of 30 day old mice, activation of calpain-1 produces high amounts of nNOS active species, while in 120 day old mice enhanced-prolonged activation of calpain-1 inactivates nNOS and down-regulates NR2B. Our data reveal a critical role of calpain-1 in the early phase and during progression of ALS, suggesting new therapeutic approaches to counteract its onset and fatal course.

μ-Calpain as a Novel Target for Impairment of Nitric Oxide-Mediated Vascular Relaxation in Diabetes: A Mini Review

Diabetes is one of the most prevalent metabolic disorders. In diabetes, incidence of coronary artery diseases and peripheral vascular diseases is increased 2- to 4-fold and 10-fold, respectively, compared to healthy individuals. In spite of extensive studies, the underlying mechanisms of endothelial dysfunction (ED), an early event in the development of vascular diseases, remain incompletely understood in diabetes. This mini-review discusses the role and signaling pathways of calpains - a family of Ca²⁺-sensitive intracellular proteases in nitric oxide (NO)-mediated ED in diabetes. We conclude that activation of calpains, especially μ-calpain, plays an important role in the pathogenesis of NO-mediated ED and inflammatory responses in diabetes which is mainly via endothelial Nitric Oxide Synthase (eNOS) inactivation/degradation in macro- and micro-vasculature. We review existing literature demonstrating that hyperhomocysteinemia, elevated plasma homocysteine level, potentiates hyperglycemia-induced ED via μ-calpain/PKCβ₂ activation-induced eNOS-pThr497/495 and eNOS inactivation. μ-calpain may be a critical therapeutic target for NO-mediated ED in diabetes.

Effect of short-term cold exposure on skeletal muscle protein breakdown in rats

Although it is well established that carbohydrate and lipid metabolism are profoundly altered by cold stress, the effects of short-term cold exposure on protein metabolism in skeletal muscle are still poorly understood. Because cold acclimation requires that an organism adjust its metabolic flux, and muscle amino acids may be an important energy source for heat production, we hypothesize that muscle proteolysis is increased and protein synthesis is decreased under such a stress condition. Herein, cold exposure for 24 h decreased rates of protein synthesis and increased overall proteolysis in both soleus and extensor digitorum longus (EDL) muscles, but it did not affect muscle weight. An increase in proteolysis was accompanied by hyperactivity of the ubiquitin-proteasome system (UPS) in both soleus and EDL, and Ca(2+)-dependent proteolysis in EDL. Furthermore, muscles of rats exposed to cold showed increased mRNA and protein levels of atrogin-1 and muscle RING finger enzyme-1 (MuRF1). Additionally, cold stress reduced phosphorylation of Akt and Forkhead box class O1 (FoxO1), a well-known effect that increases FoxO translocation to the nucleus and leads to activation of proteolysis. Plasma insulin levels were lower, whereas catecholamines, corticosterone, and thyroid hormones were higher in cold-exposed rats compared with control rats. The present data provide the first direct evidence that short-term cold exposure for 24 h decreases rates of protein synthesis and increases the UPS and Ca(2+)-dependent proteolytic processes, and increases expression of atrogin-1 and MuRF1 in skeletal muscles of young rats. The activation of atrophy induced by acute cold stress seems to be mediated at least in part through the inactivation of Akt/FoxO signaling and activation of AMP-activated protein kinase.

Effect of low voltage electrical stimulation on protein and quality changes in bovine muscles during postmortem aging

This experiment was conducted to determine the influence of low voltage electrical stimulation (ES) on the tenderness development of beef round muscles. Eight steers were slaughtered, and ES applied to one side of each carcass within 90 min of exsanguination. Steaks from M. longissimus dorsi, semimembranosus, adductor, and gracilis were vacuum packaged and aged at 4 °C for 9 d. Star probe, sensory evaluation, Western blot assays of troponin-T and μ-calpain autolysis and 2D-DIGE were conducted. ES resulted in accelerated (P<0.05) pH decline of the longissimus in the first 24h postmortem. ES did not influence (P>0.05) proteolysis and tenderness, but did alter the predominance of metabolic proteins in the soluble fraction of muscle. Aging for 9 d improved tenderness (P<0.05). The data confirmed that low voltage ES at 90 min of exsanguination had no effect on proteolysis and tenderness development in the longissimus dorsi, semimembranosus, adductor or gracilis in beef.

Structure-function relationships in calpains

Calpains are a family of complex multi-domain intracellular enzymes that share a calcium-dependent cysteine protease core. These are not degradative enzymes, but instead carry out limited cleavage of target proteins in response to calcium signalling. Selective cutting of cytoskeletal proteins to facilitate cell migration is one such function. The two most abundant and extensively studied members of this family in mammals, calpains 1 and 2, are heterodimers of an isoform-specific 80 kDa large subunit and a common 28 kDa small subunit. Structures of calpain-2, both Ca2+-free and bound to calpastatin in the activated Ca2+-bound state, have provided a wealth of information about the enzyme's structure-function relationships and activation. The main association between the subunits is the pairing of their C-terminal penta-EF-hand domains through extensive intimate hydrophobic contacts. A lesser contact is made between the N-terminal anchor helix of the large subunit and the penta-EF-hand domain of the small subunit. Up to ten Ca2+ ions are co-operatively bound during activation. The anchor helix is released and individual domains change their positions relative to each other to properly align the active site. Because calpains 1 and 2 require ~30 and ~350 μM Ca2+ ions for half-maximal activation respectively, it has long been argued that autoproteolysis, subunit dissociation, post-translational modifications or auxiliary proteins are needed to activate the enzymes in the cell, where Ca2+ levels are in the nanomolar range. In the absence of robust support for these mechanisms, it is possible that under normal conditions calpains are transiently activated by high Ca2+ concentrations in the microenvironment of a Ca2+ influx, and then return to an inactive state ready for reactivation.

Identification of biomarkers of meat tenderisation and its use for early classification of Asturian beef into fast and late tenderising meat

BACKGROUND

The objective of this work was to study the post-mortem evolution of potential biomarkers (µ-calpain activity and proteolytic profile) of meat tenderisation in bovine longissimus dorsi (LD) muscle from several biotypes coming from two beef breeds ('Asturiana de los Valles' and 'Asturiana de la Montaña') and showing different levels of muscular hypertrophy (mh/mh, mh/+, + /+).

RESULTS

LD samples were taken at 2, 12, 24 and 48 h and 3, 7, 14 and 21 days post-mortem. The presence of muscular hypertrophy produced a faster rate of pH decline, faster exhaustion of µ-calpain activity and earlier occurrence of proteolytic changes. Changes in the electrophoretic pattern of some peptides from sarcoplasmic (glyceraldehyde-3-phosphate dehydrogenase) and myofibrillar (troponin T and troponin I) muscle extracts within the first 24 h significantly correlated with meat toughness and allowed accurate discrimination of meat products into two groups: (1) fast tenderising meat, coming from mh-biotypes, and (2) late tenderising meat, from normal (+/+) biotypes.

CONCLUSION

Early monitoring (within 24 h after slaughter) of selected biomarkers in LD muscle allowed accurate prediction of ultimate meat toughness and could be used in the meat industry as a tool for early classification of beef into fast and late tenderising meat.

Elevated calpain activity in acute myelogenous leukemia correlates with decreased calpastatin expression

Calpains are intracellular cysteine proteases that have crucial roles in many physiological and pathological processes. Elevated calpain activity has been associated with many pathological states. Calpain inhibition can be protective or lethal depending on the context. Previous work has shown that c-myc transformation regulates calpain activity by suppressing calpastatin, the endogenous negative regulator of calpain. Here, we have investigated calpain activity in primary acute myelogenous leukemia (AML) blast cells. Calpain activity was heterogeneous and greatly elevated over a wide range in AML blast cells, with no correlation to FAB classification. Activity was particularly elevated in the CD34+CD38− enriched fraction compared with the CD34+CD38+ fraction. Treatment of the cells with the specific calpain inhibitor, PD150606, induced significant apoptosis in AML blast cells but not in normal equivalent cells. Sensitivity to calpain inhibition correlated with calpain activity and preferentially targeted CD34+CD38− cells. There was no correlation between calpain activity and p-ERK levels, suggesting the ras pathway may not be a major contributor to calpain activity in AML. A significant negative correlation existed between calpain activity and calpastatin, suggesting calpastatin is the major regulator of activity in these cells. Analysis of previously published microarray data from a variety of AML patients demonstrated a significant negative correlation between calpastatin and c-myc expression. Patients who achieved a complete remission had significantly lower calpain activity than those who had no response to treatment. Taken together, these results demonstrate elevated calpain activity in AML, anti-leukemic activity of calpain inhibition and prognostic potential of calpain activity measurement.

Relevance of calpain and calpastatin activity for texture in super-chilled and ice-stored Atlantic salmon (Salmo salar L.) fillets

The aim of the present experiment was to measure the protease activities in ice-stored and super-chilled Atlantic salmon (Salmo salar) fillets, and the effect on texture. Pre-rigour fillets of Atlantic salmon were either super-chilled to a core temperature of -1.5°C or directly chilled on ice prior to 144h of ice storage. A significantly higher calpain activity was detected in the super-chilled fillets at 6h post-treatment compared to the ice-stored fillets and followed by a significant decrease below its initial level, while the calpastatin activity was significantly lower for the super-chilled fillets at all time points. The cathepsin B+L and B activities increased significantly with time post-treatment; however, no significant differences were observed at any time points between the two treatments. For the ice stored fillets, the cathepsin L activity decreased significantly from 6 to 24h post-treatment and thereafter increased significantly to 144h post-treatment. There was also a significantly lower cathepsin L activity in the super-chilled fillets at 0h post-treatment. No significant difference in breaking force was detected; however, a significant difference in maximum compression (Fmax) was detected at 24h post-treatment with lower Fmax in the super-chilled fillets. This experiment showed that super-chilling had a significant effect on the protease activities and the ATP degradation in salmon fillets. The observed difference in Fmax may be a result of these observed differences, and may indicate a softening of the super-chilled salmon muscle at 24h post-treatment.

m-Calpain activation in vitro does not require autolysis or subunit dissociation

Calpains are Ca(2+)-dependent, intracellular cysteine proteases involved in many physiological functions. How calpains are activated in the cell is unknown because the average intracellular concentration of Ca(2+) is orders of magnitude lower than that needed for half-maximal activation of the enzyme in vitro. Two of the proposed mechanisms by which calpains can overcome this Ca(2+) concentration differential are autoproteolysis (autolysis) and subunit dissociation, both of which could release constraints on the core by breaking the link between the anchor helix and the small subunit to allow the active site to form. By measuring the rate of autolysis at different sites in calpain, we show that while the anchor helix is one of the first targets to be cut, this occurs in the same time-frame as several potentially inactivating cleavages in Domain III. Thus autolytic activation would overlap with inactivation. We also show that the small subunit does not dissociate from the large subunit, but is proteolyzed to a 40-45k heterodimer of Domains IV and VI. It is likely that this autolysis-generated heterodimer has previously been misidentified as the small subunit homodimer produced by subunit dissociation. We propose a model for m-calpain activation that does not involve either autolysis or subunit dissociation.

Calpain chronicle--an enzyme family under multidisciplinary characterization

Calpain is an intracellular Ca2+-dependent cysteine protease (EC 3.4.22.17; Clan CA, family C02) discovered in 1964. It was also called CANP (Ca2+-activated neutral protease) as well as CASF, CDP, KAF, etc. until 1990. Calpains are found in almost all eukaryotes and a few bacteria, but not in archaebacteria. Calpains have a limited proteolytic activity, and function to transform or modulate their substrates' structures and activities; they are therefore called, "modulator proteases." In the human genome, 15 genes--CAPN1, CAPN2, etc.--encode a calpain-like protease domain. Their products are calpain homologs with divergent structures and various combinations of functional domains, including Ca2+-binding and microtubule-interaction domains. Genetic studies have linked calpain deficiencies to a variety of defects in many different organisms, including lethality, muscular dystrophies, gastropathy, and diabetes. This review of the study of calpains focuses especially on recent findings about their structure-function relationships. These discoveries have been greatly aided by the development of 3D structural studies and genetic models.

Photoreceptor rescue and toxicity induced by different calpain inhibitors

Photoreceptor degeneration is the hallmark of a group of inherited blinding diseases collectively termed retinitis pigmentosa (RP); a major cause of blindness in humans. RP is at present untreatable and the underlying neurodegenerative mechanisms are largely unknown, even though the genetic causes are often established. The activation of calpain-type proteases may play an important role in cell death in various neuronal tissues, including the retina. We therefore tested the efficacy of two different calpain inhibitors in preventing cell death in the retinal degeneration (rd1) human homologous mouse model for RP. Pharmacological inhibition of calpain activity in rd1 organotypic retinal explants had ambiguous effects on photoreceptor viability. Calpain inhibitor XI had protective effects when applied for short periods of time (16 h) but demonstrated substantial levels of toxicity in both wild-type and rd1 retina when used over several days. In contrast, the highly specific calpain inhibitor calpastatin peptide reduced photoreceptor cell death in vitro after both short and prolonged exposure, an effect that was also evident after in vivo application via intravitreal injection. These findings highlight the importance of calpain activation for photoreceptor cell death but also for photoreceptor survival and propose the use of highly specific calpain inhibitors to prevent or delay RP.

Skeletal muscle-specific calpain is an intracellular Na+-dependent protease

Because intracellular [Na⁺] is kept low by Na⁺/K⁺-ATPase, Na⁺ dependence is generally considered a property of extracellular enzymes. However, we found that p94/calpain 3, a skeletal-muscle-specific member of the Ca²⁺-activated intracellular “modulator proteases” that is responsible for a limb-girdle muscular dystrophy (“calpainopathy”), underwent Na⁺-dependent, but not Cs⁺-dependent, autolysis in the absence of Ca²⁺. Furthermore, Na⁺ and Ca²⁺ complementarily activated autolysis of p94 at physiological concentrations. By blocking Na⁺/K⁺-ATPase, we confirmed intracellular autolysis of p94 in cultured cells. This was further confirmed using inactive p94:C129S knock-in (p94CS-KI) mice as negative controls. Mutagenesis studies showed that much of the p94 molecule contributed to its Na⁺/Ca²⁺-dependent autolysis, which is consistent with the scattered location of calpainopathy-associated mutations, and that a conserved Ca²⁺-binding sequence in the protease acted as a Na⁺ sensor. Proteomic analyses using Cs⁺/Mg²⁺ and p94CS-KI mice as negative controls revealed that Na⁺ and Ca²⁺ direct p94 to proteolyze different substrates. We propose three roles for Na⁺ dependence of p94; 1) to increase sensitivity of p94 to changes in physiological [Ca²⁺], 2) to regulate substrate specificity of p94, and 3) to regulate contribution of p94 as a structural component in muscle cells. Finally, this is the first example of an intracellular Na⁺-dependent enzyme.

Ca2+ signaling in airway epithelial cells facilitates leukocyte recruitment and transepithelial migration

In airway cells, TLR2 stimulation by bacterial products activates Ca2+ fluxes that signal leukocyte recruitment to the lung and facilitates transepithelial migration into the airway lumen. TLR2 is apically displayed on airway cells, where it senses bacterial stimuli. Biochemical and genetic approaches demonstrate that TLR2 ligands stimulate release of Ca2+ from intracellular stores by activating TLR2 phosphorylation by c-Src and recruiting PI3K and PLCgamma to affect Ca2+ release through IP3Rs. This Ca2+ release plays a pivotal role in signaling TLR2-dependent NF-kappaB activation and chemokine expression to recruit PMNs to the lung. In addition, TLR2-initiated Ca2+ release activates Ca2+-dependent proteases, calpains, which cleave the transmembrane proteins occludin and E-cadherin to promote PMN transmigration. This review highlights recent findings that demonstrate a central role for Ca2+ signaling in airway epithelial cells to induce proinflammatory gene transcription and to initiate junctional changes that accommodate transmigration of recruited PMNs.

Mitochondrial micro-calpain is not involved in the processing of apoptosis-inducing factor

Caspase-independent cell death, an important death pathway in many cells including neurons, is executed via apoptosis-inducing factor (AIF), an oxidoreductase, localized to the mitochondrial intermembrane space. AIF is processed and released from mitochondria following mitochondrial permeability transition pore (mPTP) formation, and translocates to the nucleus to induce DNA fragmentation and cell death. The release of AIF requires cleavage of its N-terminus anchored in the inner mitochondrial membrane. The protease responsible for this AIF truncation has not been established, although there is considerable evidence suggesting a role for micro-calpain. We previously found that a pool of micro-calpain is localized to the mitochondrial intermembrane space, the submitochondrial compartment in which AIF truncation occurs. The close submitochondrial proximity of mitochondrial micro-calpain and AIF gives support to the hypothesis that mitochondrial micro-calpain may be the protease responsible for processing AIF prior to its release. In the present study, AIF was released from rat liver mitochondria following mPTP induction by atractyloside. This release was inhibited by the cysteine protease inhibitor MDL28170, but not by more specific calpain inhibitors PD150606 and calpastatin. Atractyloside caused swelling in rat brain mitochondria, but did not induce AIF release. In a mitochondrial fraction from SH-SY5Y neuroblastoma cells, incubation with 5 mM Ca(2+) resulted in the activation of micro-calpain but not in AIF truncation. In summary, the localization of micro-calpain to the mitochondrial intermembrane space is suggestive of its possible involvement in AIF processing, but direct experimental evidence supporting such a role has been elusive.

Photoreceptor cell death mechanisms in inherited retinal degeneration

Photoreceptor cell death is the major hallmark of a group of human inherited retinal degenerations commonly referred to as retinitis pigmentosa (RP). Although the causative genetic mutations are often known, the mechanisms leading to photoreceptor degeneration remain poorly defined. Previous research work has focused on apoptosis, but recent evidence suggests that photoreceptor cell death may result primarily from non-apoptotic mechanisms independently of AP1 or p53 transcription factor activity, Bcl proteins, caspases, or cytochrome c release. This review briefly describes some animal models used for studies of retinal degeneration, with particular focus on the rd1 mouse. After outlining the major features of different cell death mechanisms in general, we then compare them with results obtained in retinal degeneration models, where photoreceptor cell death appears to be governed by, among other things, changes in cyclic nucleotide metabolism, downregulation of the transcription factor CREB, and excessive activation of calpain and PARP. Based on recent experimental evidence, we propose a putative non-apoptotic molecular pathway for photoreceptor cell death in the rd1 retina. The notion that inherited photoreceptor cell death is driven by non-apoptotic mechanisms may provide new ideas for future treatment of RP.

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.

Effect of postmortem storage on activity of mu- and m-calpain in five bovine muscles

An in situ system involving incubation of 60- to 80-g pieces of muscle at 4 degrees C under different conditions was used to determine the effects of time of postmortem storage, of pH, and of temperature on activities of mu- and m-calpain activity in bovine skeletal muscle. Casein zymograms were used to allow measurement of calpain activity with a minimum of sample preparation and to ensure that the calpains were not exposed to ionic strengths of 100 or greater before assay of their activities. In 4 of the 5 muscles (longissimus dorsi, lumbar; longissimus dorsi, thoracic; psoas major; semimembranosus; and triceps brachii) studied, mu-calpain activity decreased nearly to zero within 48 h postmortem. Activity of m-calpain also decreased in the in situ system used but at a much slower rate. Activities of both mu- and m-calpain decreased more slowly in the triceps brachii muscle than in the other 4 muscles during postmortem storage. Although previous studies have indicated that mu-calpain but not m-calpain is proteolytically active at pH 5.8, these studies have used calpains obtained from muscle at death. Both mu- and m-calpain are proteolytically inactive if their activities are measured at pH 5.8 and after incubating the muscle pieces for 24 h at pH 5.8. Western analysis suggested that neither the large 80-kDa subunit nor the small 28-kDa subunit of m-calpain was autolyzed during postmortem storage of the muscle pieces. As has been reported previously, the 80-kDa subunit of mu-calpain was autolyzed to 78- and then to a 76-kDa polypeptide after 7 d postmortem, but the 28-kDa small subunit was not autolyzed; hence, the autolyzed mu-calpain molecule in postmortem muscle is a 76-/28-kDa molecule and not a 76-/18-kDa molecule as previously assumed. Because both subunits were present in the postmortem calpains, loss of mu-calpain activity during postmortem storage is not due to dissociation of the 2 subunits and inactivation. Although previous studies have shown that the 76-/18-kDa mu-calpain molecule is completely active proteolytically, it is possible that the 76-/28-kDa mu-calpain molecule in postmortem muscle is proteolytically inactive and that this accounts for the loss of mu-calpain activity during postmortem storage. Because neither mu- nor m-calpain is proteolytically active at pH 5.8 after being incubated at pH 5.8 for 24 h, other proteolytic systems such as the caspases may contribute to postmortem proteolysis in addition to the calpains.

Calpain activity in retinal degeneration

Retinal degenerations such as retinitis pigmentosa (RP) or glaucoma are a major cause of blindness in humans. Understanding the mechanisms underlying the various types of retinal degeneration is a pre-requisite for the development of rational therapies for these diseases. Activation of the calcium dependent protease, calpain, has been suggested to play an important role in cell death in various neuronal tissues including the retina. Improved detection and analysis of calpain activity during degenerative processes is likely to expand the list of pathological conditions with calpain involvement. We give a short overview of the methods available for the detection of calpain activity, and briefly discuss properties of calpain inhibitors. We then discuss the role of calpains in different cell death mechanisms and review existing work on retinal degeneration and the possible involvement of calpains therein. The implication of calpains in retinal cell death raises the possibility to use calpain inhibitors to prevent or delay retinal degeneration.

Purification and characterization of calpain and calpastatin from rainbow trout, Oncorhynchus mykiss

Although the calpain system has been studied extensively in mammalian animals, much less is known about the properties of mu-calpain, m-calpain, and calpastatin in lower vertebrates such as fish. These three proteins were isolated and partly characterized from rainbow trout, Oncorhynchus mykiss, muscle. Trout m-calpain contains an 80-kDa large subunit, but the approximately 26-kDa small subunit from trout m-calpain is smaller than the 28-kDa small subunit from mammalian calpains. Trout mu-calpain and calpastatin were only partly purified; identity of trout mu-calpain was confirmed by labeling with antibodies to bovine skeletal muscle mu-calpain, and identity of trout calpastatin was confirmed by specific inhibition of bovine skeletal muscle mu- and m-calpain. Trout mu-calpain requires 4.4+/-2.8 microM and trout m-calpain requires 585+/-51 microM Ca(2+) for half-maximal activity, similar to the Ca(2+) requirements of mu- and m-calpain from mammalian tissues. Sequencing tryptic peptides indicated that the amino acid sequence of trout calpastatin shares little homology with the amino acid sequences of mammalian calpastatins. Screening a rainbow trout cDNA library identified three cDNAs encoding for the large subunit of a putative m-calpain. The amino acid sequence predicted by trout m-calpain cDNA was 65% identical to the human 80-kDa m-calpain sequence. Gene duplication and polyploidy occur in fish, and the amino acid sequence of the trout m-calpain 80-kDa subunit identified in this study was 83% identical to the sequence of a trout m-calpain 80-kDa subunit described earlier. This is the first report of two isoforms of m-calpain in a single species.

Flow cytometric measurement of calpain activity in living cells

BACKGROUND

Calpains are intracellular, calcium-sensitive, neutral cysteine proteases that play crucial roles in many physiological and pathological processes. Calpain regulation is complex and activity is poorly correlated with calpain protein levels. Therefore a full understanding of calpain function requires robust methods for measuring activity.

METHODS

We describe and characterize a flow cytometric method for measuring calpain activity in live cells. This method uses the BOC-LM-CMAC reagent that readily diffuses into cells where it reacts with free thiols to enhance retention.

RESULTS

We show that the reagent is cleaved specifically by calpains and follows saturation kinetics. We use the assay to measure calpain activation following PDGF stimulation of rat fibroblasts. We also show that the calpain inhibitor PD150606 inhibits calpain with a K(i) of 12.5 muM and show that Mek inhibitors PD89059 and U0126 also suppress calpain activity. We also show that the assay can measure calpain activity in subpopulations of cells present in unfractionated cord blood or in HL60 human myelomonocytic leukemia cells.

CONCLUSION

Taken together, these experiments demonstrate that this assay is a reliable and useful method for measuring calpain activity in multiple cell types.

Involvement of the ERK/MAP kinase signalling pathway in milli-calpain activation and myogenic cell migration

Recent research carried out in our laboratory has shown that IGF-1, TGF-beta1, and insulin were able to strongly stimulate myoblast migration by increasing milli-calpain expression and activity. However, the signalling pathways involved in these phenomena remain unknown. The aim of this study was to identify the signalling pathway(s) responsible for the effects of IGF-1, TGF-beta1, and insulin on myoblast migration and on milli-calpain expression and activity. For this purpose, wound healing assays were carried out in the presence of growth factors with or without specific inhibitors of ERK/MAP kinase and PI3K/Akt pathways. The results clearly showed that the inhibition of the ERK/MAP kinase pathway prevents the effects of growth factors on myoblast migration. Secondly, the expression and the activity of milli-calpain were studied in cells treated with growth factor, alone or with ERK/MAP kinase inhibitor. The results demonstrated that the up-regulation of milli-calpain expression and activity was mediated by the ERK/MAP kinase pathway. Finally, the possible implication of MyoD and myogenin, myogenic regulatory factors able to regulate milli-calpain expression, was studied. Taken together our results clearly showed that the ERK/MAP kinase signalling pathway is responsible for the effects of the three growth factors on myoblast migration and on milli-calpain expression and activity. On the opposite, the PI3K/Akt signalling pathway, MyoD and myogenin seem to be not implicated in these phenomena.

Following autolysis in proteases by NMR: Insights into multiple unfolding pathways and mutational plasticities

Biophysical studies in proteases are severely hampered due to the auto-cleavage property of these enzymes. In this context, we develop here a kinetic model and an NMR-based strategy to use this very autolytic property to derive useful insights into multiple unfolding pathways and mutational plasticities in these proteins. The basic idea lies in the interpretation of the auto-cleavage-driven decay of the folded protein peaks in the HSQC spectra as a function of time. The different peaks are seen to decay at different rates. As unfolding is the rate-determining step in the auto-cleavage reaction, the NMR spectral changes reflect on local unfolding processes at the residue level. A formalism is presented to gain insights into unfolding free energies and evaluate local perturbations due to single point mutations. The model is applied to HIV-1 protease-tethered dimer as an example, considering mutations at a particular site. Significant perturbations are seen even at very remote areas from the site of the mutation.

Activity-dependent cleavage of brain glutamic acid decarboxylase 65 by calpain

Previously, we reported that l-glutamic acid decarboxylase isoform 65 (GAD65) could be cleaved in vitro to release a stable truncated form which lacks amino acid 1-69 from the N-terminus, GAD65(Delta1-69). However, whether such a truncated form is also present under certain physiological conditions remains elusive. In the present study, we showed that, upon sustained neuronal stimulation, GAD65 could be cleaved into a truncated form in a rat synaptosomal preparation. This truncated form had similar electrophoretic mobility to purified recombinant human GAD65(Delta1-69). Furthermore, we demonstrated that this conversion was calcium dependent. Calcium-chelating reagents such as EDTA and 1,2-bis-(o-aminphenoxy)-ethane-N,N,N',N'-tetra-acetic acid tetra-acetoxy-methyl ester prevented the cleavage of GAD65. In addition, our data suggested that calpain, a calcium-dependent cysteine protease, is activated upon neuronal stimulation and could be responsible for the conversion of full-length GAD65 to truncated GAD65 in the brain. Moreover, calpain inhibitors such as calpain inhibitor I or calpastatin could block the cleavage. Results of our in vitro cleavage assay using purified calpain and immunopurified rat GAD65 also supported the idea that GAD65 could be directly cleaved by calpain.

Identification of calpain cleavage sites in the G1 cyclin-dependent kinase inhibitor p19(INK4d)

Calpains are a large family of Ca2+-dependent cysteine proteases that are ubiquitously distributed across most cell types and vertebrate species. Calpains play a role in cell differentiation, apoptosis, cytoskeletal remodeling, signal transduction and the cell cycle. The cell cycle proteins cyclin D1 and p21(KIP1), for example, have been shown to be affected by calpains. However, the rules that govern calpain cleavage specificity are poorly understood. We report here studies on the pattern of mu-calpain proteolysis of the p19(INK4d) protein, a cyclin-dependent kinase 4/6 inhibitor that negatively regulates the mammalian cell cycle. Our data show new characteristics of calpain action: mu-calpain cleaves p19(INK4d) immediately after the first and second ankyrin repeats that are structurally less stable compared to the other repeats. This is in contrast to features observed so far in the specificity of calpains for their substrates. These results imply that calpain may be involved in the cell cycle by regulating the cell cycle regulatory protein turnover through CDK inhibitors and cyclins.

Effect of oxidation, pH, and ionic strength on calpastatin inhibition of μ- and m-calpain

The objective of this study was to evaluate the effect of oxidation on mu- and m-calpain activity at varying pH and ionic strength conditions in the presence of calpastatin. In 2 separate experiments, purified porcine skeletal muscle mu- or m-calpain (0.45 units of caseinolytic activity) was incubated in the presence of calpastatin (0, 0.15, or 0.30 units) at pH 7.5, 6.5, or 6.0 with either 165 or 295 mM NaCl. The reactions were initiated with the addition of CaCl2 (100 microM for mu-calpain; 1 mM for m-calpain). In Experiment 1, mu- or m-calpain was incubated with the calpain substrate Suc-Leu-Leu-Val-Tyr-AMC (170 microM). Either 0 or 16 mu microM H2O2 was added to each assay. Activity was measured at 60 min. In Experiment 2, calpain was incubated with highly purified porcine myofibrils (4 mg/mL) under conditions described. Either 0 or 100 microM H2O2 was added immediately prior to the addition of calpain. Degradation of desmin was determined on samples collected at 2, 15, 60, and 120 min. Results from Experiment 1 indicated that oxidation decreased (P < 0.01) activity of mu-calpain. Mu-calpain had the greatest (P < 0.01) activity at pH 6.5, and m-calpain had the greatest (P < 0.01) activity at pH 7.5 at 60 min. m-Calpain activity was not detected at pH 6.0. Mu- and m-calpain activity were lower (P < 0.01) at 295 mM NaCl than at 165 mM NaCl at all pH conditions. Oxidation lowered (P < 0.01) calpastatin inhibition of mu-and m-calpain at all pH and ionic strength combinations. In Experiment 2, oxidation decreased proteolytic activity of mu-calpain against desmin at pH 6.0 (P < 0.05 at 15, 60, and 120 min) and decreased m-calpain at all pH conditions. However, desmin degradation by mu-calpain was not as efficiently inhibited by calpastatin at pH 7.5 and as at pH 6.5 (P = 0.03 at 60 min) when oxidizing conditions were created. This is consistent with the results from Experiment 1, which indicated that oxidation decreased the ability of calpastatin to inhibit mu-calpain. These studies provide evidence that oxidation influences calpain activity and inhibition of calpains by calpastatin differently under varying environmental conditions. The results suggest that, at the higher pH conditions used, calpastatin may limit the possibility of oxidation-induced inactivation of mu-calpain.

Involvement of calpains in growth factor-mediated migration

Previous research in our laboratory has already shown the importance of the role played by ubiquitous calpains during myoblast migration. The aim of this study was to investigate calpain expression during myoblast migration and, to enhance this phenomenon via calpain stimulation. Ubiquitous calpains are members of a large family of calcium-dependent cysteine proteases. They play an important role in numerous biological and pathological phenomena, such as signal transduction, apoptosis, cell-cycle regulation, cell spreading, adhesion, invasion, myogenesis, and motility. Myoblast migration is a crucial step in myogenesis, as it is necessary for myoblast alignment and fusion to form myotubes. This study started by examining changes in calpain expression during migration, then investigated the possibility of activating myoblast migration via the stimulation of calpain expression and/or activity. The migration rate of myoblasts overexpressing mu- or milli-calpain was quantified. The results showed that calpain overexpression dramatically inhibited myoblast migration. Growth-factor treatments were then used to enhance myoblast migration. The results showed that treatment with IGF-1, TGF-beta1, or insulin induced a major increase in migration and caused a significant increase in m-calpain expression and activity. The increase in migration was totally inhibited by adding calpeptin, a calpain-specific inhibitor. These findings suggest that milli-calpain is involved in growth factor-mediated migration.

μ-Calpain binds to lipid bilayers via the exposed hydrophobic surface of its Ca2+-activated conformation

Mu- and m-calpain are cysteine proteases requiring micro- and millimolar Ca2+ concentrations for their activation in vitro. Among other mechanisms, interaction of calpains with membrane phospholipids has been proposed to facilitate their activation by nanomolar [Ca2+] in living cells. Here the interaction of non-autolysing, C115A active-site mutated heterodimeric human mu-calpain with phospholipid bilayers was studied in vitro using protein-to-lipid fluorescence resonance energy transfer and surface plasmon resonance. Binding to liposomes was Ca2+-dependent, but not selective for specific phospholipid head groups. [Ca2+]0.5 for association with lipid bilayers was not lower than that required for the exposure of hydrophobic surface (detected by TNS fluorescence) or for enzyme activity in the absence of lipids. Deletion of domain V reduced the lipid affinity of the isolated small subunit (600-fold) and of the heterodimer (10- to 15-fold), thus confirming the proposed role of domain V for membrane binding. Unexpectedly, mutations in the acidic loop of the 'C2-like' domain III, a putative Ca2+ and phospholipid-binding site, did not affect lipid affinity. Taken together, these results support the hypothesis that in vitro membrane binding of mu-calpain is due to the exposed hydrophobic surface of the active conformation and does not reduce the Ca2+ requirement for activation.