Calcium-supported calpain degradation rates for cardiac myofibrils in diabetes. Sulfhydryl and hydrophobic interactions

Skeletal muscle atrophy in sedentary Zucker obese rats is not caused by calpain-mediated muscle damage or lipid peroxidation induced by oxidative stress

Background

Skeletal muscle undergoes significant atrophy in Type 2 diabetic patients and animal models. We aimed to determine if atrophy of Zucker rat skeletal muscle was due to the activation of intracellular damage pathways induced by excess reactive oxygen species production (specifically those associated with the peroxidation of lipid membranes) and calpain activity. 14 week old obese Zucker rats and littermate lean controls were injected with 1% Evan’s Blue Dye. Animals were anaesthetised and extensor digitorum longus and soleus muscles were dissected, snap frozen and analysed for ROS-mediated F₂-isoprostane production and calpain activation/autolysis. Contralateral muscles were histologically analysed for markers of muscle membrane permeability and atrophy.

Results

Muscle mass was lower in extensor digitorum longus and soleus of obese compared with lean animals, concomitant with reduced fibre area. Muscles from obese rats had a higher proportional area of Evan’s Blue Dye fluorescence, albeit this was localised to the interstitium/external sarcolemma. There were no differences in F₂-isoprostane production when expressed relative to arachidonic acid content, which was lower in the obese EDL and soleus muscles. There were no differences in the activation of either μ-calpain or calpain-3.

Conclusions

This study highlights that atrophy of Zucker rat skeletal muscle is not related to sarcolemmal damage, sustained hyperactivation of the calpain proteases or excessive lipid peroxidation. As such, establishing the correct pathways involved in atrophy is highly important so as to develop more specific treatment options that target the underlying cause. This study has eliminated two of the potential pathways theorised to be responsible.

Disruption of Rac1 signaling reduces ischemia-reperfusion injury in the diabetic heart by inhibiting calpain

Diabetes increases myocardial ischemia/reperfusion (I/R) injury. However, the underlying mechanisms remain incompletely understood. This study investigated the role of Rac1 signaling and calpain in exacerbated I/R injury in diabetic hearts. Mice with cardiac-specific deletion of Rac1 (Rac1-ko) and transgenic mice with cardiac-specific superoxide dismutase-2 (SOD2) or calpastatin overexpression were rendered diabetic with streptozotocin. Isolated perfused hearts were subjected to global I/R. After I/R, Rac1 activity was significantly enhanced in diabetic compared with nondiabetic hearts. Diabetic hearts displayed more severe I/R injury than nondiabetic hearts, as evidenced by more lactate dehydrogenase release and apoptosis and decreased cardiac function. These adverse impacts of diabetes were abrogated in Rac1-ko hearts or by perfusion with the Rac1 inhibitor NSC23766. In an in vivo I/R mouse model, infarct size was much smaller in diabetic Rac1-ko compared with wild-type mice. Inhibition of Rac1 signaling prevented NADPH oxidase activation, reactive oxygen species production, and protein carbonyl accumulation, leading to inhibition of calpain activation. Furthermore, SOD2 or calpastatin overexpression significantly reduced I/R injury in diabetic hearts and improved cardiac function after I/R. In summary, Rac1 activation increases I/R injury in diabetic hearts and the role of Rac1 signaling is mediated, at least in part, through calpain activation.

Fast and slow myosins as markers of muscle injury

Objective:

The diagnosis of muscular lesions suffered by athletes is usually made by clinical criteria combined with imaging of the lesion (ultrasonography and/or magnetic resonance) and blood tests to detect the presence of non-specific muscle markers. This study was undertaken to evaluate injury to fast and slow-twitch fibres using specific muscle markers for these fibres.

Methods:

Blood samples were obtained from 51 non-sports people and 38 sportsmen with skeletal muscle injury. Western blood analysis was performed to determine fast and slow myosin and creatine kinase (CK) levels. Skeletal muscle damage was diagnosed by physical examination, ultrasonography and magnetic resonance and biochemical markers.

Results:

The imaging tests were found to be excellent for detecting and confirming grade II and III lesions. However, grade I lesions were often unconfirmed by these techniques. Grade I lesions have higher levels of fast myosin than slow myosin with a very small increase in CK levels. Grade II and III lesions have high values of both fast and slow myosin.

Conclusions:

The evaluation of fast and slow myosin in the blood 48 h after the lesion occurs is a useful aid for the detection of type I lesions in particular, since fast myosin is an exclusive skeletal muscle marker. The correct diagnosis of grade I lesions can prevent progression of the injury in athletes undergoing continual training sessions and competitions, thus aiding sports physicians in their decision making.

Control of ubiquitination in skeletal muscle wasting

The ubiquitin proteasome system is now well recognized to play a role in mediating skeletal muscle protein wasting. Ubiquitin exerts its effects by covalent attachment to other proteins. Increased ubiquitination of muscle proteins has been observed in a number of conditions of atrophy suggesting that flux through the pathway may be regulated by controlling availability of ubiquitinated substrates for the proteasome. Therefore the enzymes that control ubiquitination of proteins likely play critical roles in regulating flux through the pathway, are sites of activation by catabolic stimuli and potentially good drug targets in the search for therapies for wasting disorders. In this article, the enzymes that can modulate ubiquitination are briefly reviewed and the current data regarding regulation of these enzymes in skeletal muscle are described. Physiological regulators of muscle size appear to modulate many of these enzymes and several of these regulators appear to do so via signaling pathways that involve Akt or NFkappaB. Further work needs to be done to identify all the enzymes that are involved in controlling ubiquitination in muscle, to characterize their regulation by non-transcriptional mechanisms also, and most importantly to identify their target substrates and to determine how these various pathways of ubiquitination work together to mediate the catabolic stimulus.

Mg2+-dependent ATPase activity in cardiac myofibrils from the insulin-resistant JCR:LA-cp rat

There is a great deal of information presently available documenting a cardiomyopathic condition in insulin-deficient models of diabetes. Less information is available documenting a similar status in non insulin-dependent models of diabetes. We have studied the functional integrity of the myofibrils isolated from hearts of JCR:LA rats. The JCR:LA rat is hyperinsulinemic, hyperlipidemic, glucose intolerant and obese. As such, it carries many of the characteristics found in humans with non insulin-dependent diabetes mellitus. These animals also have many indications of heart disease. However, it is not clear if the hearts suffer from vascular complications or are cardiomyopathic in nature. We examined Mg2+-dependent myofibrillar ATPase in hearts of JCR:LA-cp/cp rats and their corresponding control animals (+/?) and found no significant differences (P> 0.05). This is in striking contrast to the depression in this activity exhibited by cardiac myofibrils isolated from insulin-deficient models of diabetes. Our data demonstrate that myofibrillar functional integrity is normal in JCR:LA-cp rats and suggest that these hearts are not in a cardiomyopathic state. Insulin status may be critical in generating a cardiomyopathic condition in diabetes.

Sepsis stimulates release of myofilaments in skeletal muscle by a calcium-dependent mechanism

Sepsis is associated with a pronounced catabolic response in skeletal muscle, mainly reflecting degradation of the myofibrillar proteins actin and myosin. Recent studies suggest that sepsis-induced muscle proteolysis may reflect ubiquitin-proteasome-dependent protein breakdown. An apparently conflicting observation is that the ubiquitin-proteasome pathway does not degrade intact myofibrils. Thus, it is possible that actin and myosin need to be released from the myofibrils before they can be ubiquitinated and degraded by the proteasome. We tested the hypothesis that sepsis results in disruption of Z-bands, increased expression of calpains, and calcium-dependent release of myofilaments in skeletal muscle. Sepsis induced in rats by cecal ligation and puncture resulted in increased gene expression of micro-calpain, m-calpain, and p94 and in Z-band disintegration in the extensor digitorum longus muscle. The release of myofilaments from myofibrillar proteins was increased in septic muscle. This response to sepsis was blocked by treating the rats with dantrolene, a substance that inhibits the release of calcium from intracellular stores to the cytoplasm. The present results provide evidence that sepsis is associated with Z-band disintegration and a calcium-dependent release of myofilaments in skeletal muscle. Release of myofilaments may be an initial and perhaps rate-limiting component of sepsis-induced muscle breakdown.

Cardiac myofibrillar and sarcoplasmic reticulum function are not depressed in insulin-resistant JCR:LA-cp rats

Depressed myofibrillar Ca2+-ATPase activity and sarcoplasmic reticulum (SR) Ca2+ uptake are important mechanisms that are responsible for the cardiac dysfunction exhibited by insulin-deficient (type I) diabetic animals. The JCR:LA-cp rat is a model for type II non-insulin-dependent diabetes mellitus (NIDDM). This rat is insulin resistant, obese, and has high levels of circulating glucose, cholesterol, insulin, and triglycerides. The purpose of this study was to determine whether changes in cardiac myofibrillar, SR, and cardiomyocyte function exist in this model of type II diabetes. Myofibrils and SR were isolated from hearts by differential centrifugation. Surprisingly, we found that myofibrillar Ca2+-ATPase activities were unaltered in these animals. Ca2+ uptake in isolated SR fractions was increased in diabetic cp/cp rats, whereas Ca2+-ATPase activity and ryanodine binding were unchanged. Cardiomyocytes isolated from hearts of control and experimental animals had similar active cell shortening and intracellular Ca2+ concentration under basal conditions and in response to caffeine. Our data argue against the presence of a cardiomyopathy in this diabetic model and suggest that insulin may be an important factor in the cardiomyopathy observed in type I diabetic models.

Exercise-induced muscle injury: a calpain hypothesis

It is well established that periods of increased contractile activity result in significant changes in muscle structure and function. Such morphological changes as sarcomeric Z-line disruption and sarcoplasmic reticulum vacuolization are characteristic of exercise-induced muscle injury. While the precise mechanism(s) underlying the perturbations to muscle following exercise remains to be elucidated, it is clear that disturbances in Ca2+ homeostasis and changes in the rate of protein degradation occur. The resulting elevation in intracellular [Ca2+] activates the non-lysosomal cysteine protease, calpain. Because calpain cleaves a variety of protein substrates including cytoskeletal and myofibrillar proteins, calpain-mediated degradation is thought to contribute to the changes in muscle structure and function that occur immediately following exercise. In addition, calpain activation may trigger the adaptation response to muscle injury. The purpose of this paper is to: (i) review the chemistry of the calpain-calpastatin system; (ii) provide evidence for the involvement of the non-lysosomal, calcium-activated neutral protease (calpain) in the response of skeletal muscle protein breakdown to exercise (calpain hypothesis); and (iii) describe the possible involvement of calpain in the inflammatory and regeneration response to exercise.

Role of calcium-activated neutral protease (calpain) with diet and exercise

Although the proteolytic events accompanying acute and chronic perturbations in striated muscle protein turnover remain to be fully elucidated, the purpose of this paper is to (a) review the chemistry of the nonlysosomal calpain-calpastatin system, and (b) provide evidence for the involvement of a nonlysosomal, calcium-activated neutral protease (calpain) in the response of skeletal muscle protein breakdown to altered nutritional status (diet composition; energy restriction) and increased periods of contractile activity (exercise). In reviewing the literature, it is apparent that calpain is involved in the protein catabolism which accompanies alterations in diet composition and/or energy restriction. The precise mechanism of calpain action remains to be elucidated; however, the role of altered metabolic status contributing to calcium imbalances is discussed relative to increasing protein degradation. Hypotheses for further investigation are provided in regard to identifying the targeting of selected proteins (and organelles) for degradation by calpain.