Calpain inhibition reduces infarct size and improves global hemodynamics and left ventricular contractility in a porcine myocardial ischemia/reperfusion model

Calpain inhibition decreases oxidative stress via mitochondrial regulation in a swine model of chronic myocardial ischemia

INTRODUCTION

Calpain overexpression is implicated in mitochondrial damage leading to tissue oxidative stress and myocardial ischemic injury. The aim of this study was to determine the effects of calpain inhibition (CI) on mitochondrial impairment and oxidative stress in a swine model of chronic myocardial ischemia and metabolic syndrome.

METHODS

Yorkshire swine were fed a high-fat diet for 4 weeks to induce metabolic syndrome then underwent placement of an ameroid constrictor to the left circumflex artery. Three weeks later, animals received: no drug (control, "CON"; n= 7); a low-dose calpain inhibitor (0.12 mg/kg; "LCI", n= 7); or high-dose calpain inhibitor (0.25 mg/kg; "HCI", n=7). Treatment continued for 5 weeks, followed by tissue harvest. Cardiac tissue was assayed for protein carbonyl content, as well as antioxidant and mitochondrial protein expression. Reactive oxygen species (ROS) and mitochondrial respiration was measured in H9c2 cells following exposure to normoxia or hypoxia (1%) for 24 h with or without CI.

RESULTS

In ischemic myocardial tissue, CI was associated with decreased total oxidative stress compared to control. CI was also associated with increased expression of mitochondrial proteins superoxide dismutase 1, SDHA, and pyruvate dehydrogenase compared to control. 100 nM of calpain inhibitor decreased ROS levels and respiration in both normoxic and hypoxic H9c2 cardiomyoblasts.

CONCLUSIONS

In the setting of metabolic syndrome, CI improves oxidative stress in chronically ischemic myocardial tissue. Decreased oxidative stress may be via modulation of mitochondrial proteins involved in free radical scavenging and production.

Calpains as Potential Therapeutic Targets for Myocardial Hypertrophy

Despite advances in its treatment, heart failure remains a major cause of morbidity and mortality, evidencing an urgent need for novel mechanism-based targets and strategies. Myocardial hypertrophy, caused by a wide variety of chronic stress stimuli, represents an independent risk factor for the development of heart failure, and its prevention constitutes a clinical objective. Recent studies performed in preclinical animal models support the contribution of the Ca²⁺-dependent cysteine proteases calpains in regulating the hypertrophic process and highlight the feasibility of their long-term inhibition as a pharmacological strategy. In this review, we discuss the existing evidence implicating calpains in the development of cardiac hypertrophy, as well as the latest advances in unraveling the underlying mechanisms. Finally, we provide an updated overview of calpain inhibitors that have been explored in preclinical models of cardiac hypertrophy and the progress made in developing new compounds that may serve for testing the efficacy of calpain inhibition in the treatment of pathological cardiac hypertrophy.

Calpain inhibition decreases myocardial fibrosis in chronically ischemic hypercholesterolemic swine

OBJECTIVES

Calpain activation during ischemia is known to play critical roles in myocardial remodeling. We hypothesize that calpain inhibition (CI) may serve to reverse and/or prevent fibrosis in chronically ischemic myocardium.

METHODS

Yorkshire swine were fed a high-cholesterol diet for 4 weeks followed by placement of an ameroid constrictor on the left circumflex artery to induce myocardial ischemia. 3 weeks later, animals received either: no drug; high-cholesterol control group (CON; n = 8); low-dose CI (0.12 mg/kg; LCI, n = 9); or high-dose CI (0.25 mg/kg; HCI, n = 8). The high-cholesterol diet and CI were continued for 5 weeks, after which myocardial tissue was harvested. Tissue samples were analyzed by western blot for changes in protein content.

RESULTS

In the setting of hypercholesterolemia and chronic myocardial ischemia, CI decreased the expression of collagen in ischemic and nonischemic myocardial tissue. This reduced collagen content was associated with a corresponding decrease in Jak/STAT/MCP-1 signaling pathway, suggesting a role for Jak 2 signaling in calpain activity. CI also decreases the expression of focal adhesion proteins (vinculin) and stabilizes the expression of cytoskeletal and structural proteins (N-cadherin, α-fodrin, desmin, vimentin, filamin, troponin-I). CI had no significant effect on metabolic and hemodynamic parameters.

CONCLUSIONS

Calpain inhibition may be a beneficial medical therapy to decrease collagen formation in patients with coronary artery disease and associated comorbidities.

Reperfusion Cardiac Injury: Receptors and the Signaling Mechanisms

It has been documented that Ca2+ overload and increased production of reactive oxygen species play a significant role in reperfusion injury (RI) of cardiomyocytes. Ischemia/reperfusion induces cell death as a result of necrosis, necroptosis, apoptosis, and possibly autophagy, pyroptosis and ferroptosis. It has also been demonstrated that the NLRP3 inflammasome is involved in RI of the heart. An increase in adrenergic system activity during the restoration of coronary perfusion negatively affected cardiac resistance to RI. Toll-like receptors are involved in RI of the heart. Angiotensin II and endothelin-1 aggravated ischemic/reperfusion injury of the heart. Activation of neutrophils, monocytes, CD4+ T-cells and platelets contributes to cardiac ischemia/reperfusion injury. Our review outlines the role of these factors in reperfusion cardiac injury.

Spatio-temporal regulation of calpain activity after experimental myocardial infarction in vivo

Background

Calpains are calcium activated cysteine proteases that play a pivotal role in the pathophysiology of cardiac remodeling.

Methods

Here, we performed left anterior descending coronary artery ligation in rats as a model for ischemic systolic heart failure and examined the time- and region-specific regulation of calpain-1 and calpain-2 in the left ventricular myocardium.

Results

Following anterior wall myocardial infarction, calpain activity was significantly increased restricted to the ischemic anterior area at days 1, 5 and 14. No changes in calpain activity at neither time point were detected in the borderzone and remote posterior area of the left ventricle. Of note, calpain activity in the infarcted anterior myocardium was regulated differentially in the acute vs. subacute and chronic phase. In the acute phase, calpain translocation to the plasma membrane and attenuation of the expression of its endogenous inhibitor, calpastatin, were identified as the driving forces. In the subacute and chronic phase, calpain activity was regulated at the level of protein expression that was shown to be essentially independent of transcriptional activity.

Conclusions

We conclude that myocardial infarction leads to a distinct calpain regulation pattern in the left ventricular myocardium that is region specific and time dependent. Considering the results from our previous studies, a spatio-temporal interaction between calpains and calcium dependent natriuretic peptide production in the infarcted myocardium is possible.

General significance

Our results shed more light in the differential regulation of calpain activity in the myocardium and might aid in the development of targeted post-infarct and/or heart failure therapeutics.

Effect of chronic alcohol consumption on myocardial apoptosis in the rat model of isoproterenol-induced myocardial injury and investigation on the cardioprotective role of calpain inhibitor 1

We investigated the presence of myocardial apoptosis on isoproterenol (ISO)-induced myocardial injury (MI) after long-term high dose alcohol consumption and examined the antiapoptotic role of calpain inhibitor 1. Male Wistar Albino rats (n = 108) were divided into six groups: Control, alcohol (ethanol was given during 30 days for chronic alcohol consumption), MI (150 mg/kg ISO injection at last two days of alcohol consumption), alcohol + MI, alcohol + MI + calpain inhibitor 1 (10 mg/kg inhibitor was injected at 15 min before ISO injections) and Dimethyl Sulfoxide (DMSO) groups. Biochemical, histological, and morphometric methods determined apoptosis levels in the heart tissue of rats. Cytochrome c, caspase 3, and calpain levels were significantly high in alcohol, MI, and alcohol + MI groups. In contrast, mitochondrial cardiolipin content was found to be low in alcohol, MI, and alcohol + MI groups. These parameters were close to the control group in the therapy group. Histological and morphometric data have supported biochemical results. As a result of our biochemical data, myocardial apoptosis was seen in the alcohol, MI, and especially alcohol after MI groups. Calpain inhibitor 1 reduced apoptotic cell death and prevented myocardial tissue injury in these groups. The efficiency of calpain inhibitor was very marked in MI after long-term high dose alcohol consumption.

Capn4 aggravates angiotensin II-induced cardiac hypertrophy by activating the IGF-AKT signaling pathway

Capn4 belongs to a family of calpains that participate in a wide variety of biological functions, but little is known about the role of Capn4 in cardiac disease. Here, we show that the expression of Capn4 was significantly increased in Angiotensin II (Ang II)-treated cardiomyocytes and Ang II-induced cardiac hypertrophic mouse hearts. Importantly, in agreement with the Capn4 expression patterns, the maximal calpain activity measured in heart homogenates was elevated in Ang II-treated mice, and oral coadministration of SNJ-1945 (calpain inhibitor) attenuated the total calpain activity measured in vitro. Functional assays indicated that overexpression of Capn4 obviously aggravated Ang II-induced cardiac hypertrophy, whereas Capn4 knockdown resulted in the opposite phenotypes. Further investigation demonstrated that Capn4 maintained the activation of the insulin-like growth factor (IGF)-AKT signaling pathway in cardiomyocytes by increasing c-Jun expression. Mechanistic investigations revealed that Capn4 directly bound and stabilized c-Jun, and knockdown of Capn4 increased the ubiquitination level of c-Jun in cardiomyocytes. Additionally, our results demonstrated that the antihypertrophic effect of Capn4 silencing was partially dependent on the inhibition of c-Jun. Overall, these data suggested that Capn4 contributes to cardiac hypertrophy by enhancing the c-Jun-mediated IGF-AKT signaling pathway and could be a potential therapeutic target for hypertrophic cardiomyopathy.

An update on the therapeutic potential of calpain inhibitors: a patent review

INTRODUCTION

Calpain is a cytosolic proteinase that regulates of a wide range of physiological functions. The enzyme has been implicated in various pathological conditions including neurodegenerative disorders, cardiovascular disorders, cancer, and several other diseases. Therefore, calpain inhibitors are of interest as therapeutic agents and have been studied in preclinical models of several diseases in which the enzyme has been implicated.

AREAS COVERED

Calpain inhibitors that were disclosed over the last 5 years (2015-2019) include calpastatin-based peptidomimetics; thalassospiramide lipopeptides; disulfide analogs of alpha-mercaptoacrylic acids; allosteric modulators; azoloimidazolidenones; and macrocyclic/non-macrocyclic carboxamides. The effectiveness of some of the inhibitors in preclinical animal models is discussed.

EXPERT OPINION

Significant milestones that were made over this time frame include: a) disclosure of novel blood-brain barrier (BBB) permeable calpastatin analogs as calpain inhibitors; b) disclosure that potent calpain inhibitors can be obtained by targeting the hydrophobic pockets on chain A of PEF(S) of the small subunit of calpain; c) use of PEF(S) (PDB ID: 4WQ2) in virtual screening to identify novel structurally diverse calpain inhibitors; and d) mitigation of the metabolic instability of the alpha-ketoamide warhead of calpain inhibitors.

Mitochondrial inner membrane protein (mitofilin) knockdown induces cell death by apoptosis via an AIF-PARP-dependent mechanism and cell cycle arrest

Mitofilin is an inner membrane protein that has been defined as a mitochondria-shaping protein in controlling and maintaining mitochondrial cristae structure and remodeling. We determined the role of mitofilin in cell survival by investigating the mechanism underlying mitofilin knockdown-induced cell death by apoptosis. Cultured H9c2 myoblasts and HEK 293 cells were treated with mitofilin siRNA or scrambled siRNA for 24 h. Cell death (apoptosis), caspase 3 activity and cell cycle phases were assessed by flow cytometry, while cytochrome c release and intracellular ATP production were measured by ELISA. Mitofilin, apoptosis-inducing factor (AIF) and poly(ADP-ribose) polymerase (PARP) expression were measured by Western blot analysis and calpain activity was assessed using a calpain activity kit. Mitochondrial images were taken using electron microscopy. We found that mitofilin knockdown increases apoptosis mainly via activation of the AIF-PARP pathway leading to nuclear fragmentation that is correlated with S phase arrest of the cell cycle. Knockdown of mitofilin also led to mitochondrial swelling and damage of cristae that is associated with the increase in reactive oxygen species production and mitochondrial calpain activity, as well as a marked decrease in intracellular ATP production and mitochondrial membrane potential. Together, these results indicate that mitofilin knockdown by siRNA increases calpain activity that presumably leads to mitochondrial structural degradation resulting in a critical reduction of mitochondrial function that is responsible for the increase in cell death by apoptosis via an AIF-PARP mechanism and associated with nuclear fragmentation, and S phase arrest of the cell cycle.

Discovery of Novel and Highly Selective Inhibitors of Calpain for the Treatment of Alzheimer's Disease: 2-(3-Phenyl-1H-pyrazol-1-yl)-nicotinamides

Calpain overactivation has been implicated in a variety of pathological disorders including ischemia/reperfusion injury, cataract formation, and neurodegenerative diseases such as Alzheimer's disease (AD). Herein we describe our efforts leading to the identification of ketoamide-based 2-(3-phenyl-1H-pyrazol-1-yl)nicotinamides as potent and reversible inhibitors of calpain with high selectivity versus related cysteine protease cathepsins, other proteases, and receptors. Broad efficacy in a set of preclinical models relevant to AD suggests that inhibition of calpain represents an attractive approach with potential benefit for the treatment of AD.

Ischemia/Reperfusion Injury following Acute Myocardial Infarction: A Critical Issue for Clinicians and Forensic Pathologists

Acute myocardial infarction (AMI) is a leading cause of morbidity and mortality. Reperfusion strategies are the current standard therapy for AMI. However, they may result in paradoxical cardiomyocyte dysfunction, known as ischemic reperfusion injury (IRI). Different forms of IRI are recognized, of which only the first two are reversible: reperfusion-induced arrhythmias, myocardial stunning, microvascular obstruction, and lethal myocardial reperfusion injury. Sudden death is the most common pattern for ischemia-induced lethal ventricular arrhythmias during AMI. The exact mechanisms of IRI are not fully known. Molecular, cellular, and tissue alterations such as cell death, inflammation, neurohumoral activation, and oxidative stress are considered to be of paramount importance in IRI. However, comprehension of the exact pathophysiological mechanisms remains a challenge for clinicians. Furthermore, myocardial IRI is a critical issue also for forensic pathologists since sudden death may occur despite timely reperfusion following AMI, that is one of the most frequently litigated areas of cardiology practice. In this paper we explore the literature regarding the pathophysiology of myocardial IRI, focusing on the possible role of the calpain system, oxidative-nitrosative stress, and matrix metalloproteinases and aiming to foster knowledge of IRI pathophysiology also in terms of medicolegal understanding of sudden deaths following AMI.

Mitochondrial Ca2+ and regulation of the permeability transition pore

The mitochondrial permeability transition pore was originally described in the 1970's as a Ca2+ activated pore and has since been attributed to the pathogenesis of many diseases. Here we evaluate how each of the current models of the pore complex fit to what is known about how Ca2+ regulates the pore, and any insight that provides into the molecular identity of the pore complex. We also discuss the central role of Ca2+ in modulating the pore's open probability by directly regulating processes, such as ATP/ADP balance through the tricarboxylic acid cycle, electron transport chain, and mitochondrial membrane potential. We review how Ca2+ influences second messengers such as reactive oxygen/nitrogen species production and polyphosphate formation. We discuss the evidence for how Ca2+ regulates post-translational modification of cyclophilin D including phosphorylation by glycogen synthase kinase 3 beta, deacetylation by sirtuins, and oxidation/ nitrosylation of key residues. Lastly we introduce a novel view into how Ca2+ activated proteolysis through calpains in the mitochondria may be a driver of sustained pore opening during pathologies such as ischemia reperfusion injury.

Acute intermittent hypoxia in rats activates muscle proteolytic pathways through a gluccorticoid-dependent mechanism

Although it is well known that chronic hypoxia induces muscle wasting, the effects of intermittent hypoxia on skeletal muscle protein metabolism remain unclear. We hypothesized that acute intermittent hypoxia (AIH), a challenge that activates the hypothalamic-pituitary-adrenal axis, would alter muscle protein homeostasis through a glucocorticoid-dependent mechanism. Three-week-old rats were submitted to adrenalectomy (ADX) and exposed to 8 h of AIH (6% O₂ for 40 s at 9-min intervals). Animals were euthanized, and the soleus and extensor digitorum longus (EDL) muscles were harvested and incubated in vitro for measurements of protein turnover. AIH increased plasma levels of corticosterone and induced insulin resistance as estimated by the insulin tolerance test and lower rates of muscle glucose oxidation and the HOMA index. In both soleus and EDL muscles, rates of overall proteolysis increased after AIH. This rise was accompanied by an increased proteolytic activities of the ubiquitin(Ub)-proteasome system (UPS) and lysosomal and Ca²⁺-dependent pathways. Furthermore, AIH increased Ub-protein conjugates and gene expression of atrogin-1 and MuRF-1, two key Ub-protein ligases involved in muscle atrophy. In parallel, AIH increased the mRNA expression of the autophagy-related genes LC3b and GABARAPl1. In vitro rates of protein synthesis in skeletal muscles did not differ between AIH and control rats. ADX completely blocked the insulin resistance in hypoxic rats and the AIH-induced activation of proteolytic pathways and atrogene expression in both soleus and EDL muscles. These results demonstrate that AIH induces insulin resistance in association with activation of the UPS, the autophagic-lysosomal process, and Ca²⁺-dependent proteolysis through a glucocorticoid-dependent mechanism.NEW & NOTEWORTHY Since hypoxia is a condition in which the body is deprived of adequate oxygen supply and muscle wasting is induced, the present work provides evidence linking hypoxia to proteolysis through a glucocorticoid-dependent mechanism. We show that the activation of proteolytic pathways, atrophy-related genes, and insulin resistance in rats exposed to acute intermittent hypoxia was abolished by surgical removal of adrenal gland. This finding will be helpful for understanding of the muscle wasting in hypoxemic conditions.

Ischemia/Reperfusion

Ischemic disorders, such as myocardial infarction, stroke, and peripheral vascular disease, are the most common causes of debilitating disease and death in westernized cultures. The extent of tissue injury relates directly to the extent of blood flow reduction and to the length of the ischemic period, which influence the levels to which cellular ATP and intracellular pH are reduced. By impairing ATPase-dependent ion transport, ischemia causes intracellular and mitochondrial calcium levels to increase (calcium overload). Cell volume regulatory mechanisms are also disrupted by the lack of ATP, which can induce lysis of organelle and plasma membranes. Reperfusion, although required to salvage oxygen-starved tissues, produces paradoxical tissue responses that fuel the production of reactive oxygen species (oxygen paradox), sequestration of proinflammatory immunocytes in ischemic tissues, endoplasmic reticulum stress, and development of postischemic capillary no-reflow, which amplify tissue injury. These pathologic events culminate in opening of mitochondrial permeability transition pores as a common end-effector of ischemia/reperfusion (I/R)-induced cell lysis and death. Emerging concepts include the influence of the intestinal microbiome, fetal programming, epigenetic changes, and microparticles in the pathogenesis of I/R. The overall goal of this review is to describe these and other mechanisms that contribute to I/R injury. Because so many different deleterious events participate in I/R, it is clear that therapeutic approaches will be effective only when multiple pathologic processes are targeted. In addition, the translational significance of I/R research will be enhanced by much wider use of animal models that incorporate the complicating effects of risk factors for cardiovascular disease. © 2017 American Physiological Society. Compr Physiol 7:113-170, 2017.

Calpain research for drug discovery: challenges and potential

Calpains are a family of proteases that were scientifically recognized earlier than proteasomes and caspases, but remain enigmatic. However, they are known to participate in a multitude of physiological and pathological processes, performing 'limited proteolysis' whereby they do not destroy but rather modulate the functions of their substrates. Calpains are therefore referred to as 'modulator proteases'. Multidisciplinary research on calpains has begun to elucidate their involvement in pathophysiological mechanisms. Therapeutic strategies targeting malfunctions of calpains have been developed, driven primarily by improvements in the specificity and bioavailability of calpain inhibitors. Here, we review the calpain superfamily and calpain-related disorders, and discuss emerging calpain-targeted therapeutic strategies.

Mitochondrial function in hypoxic ischemic injury and influence of aging

Mitochondria are a major target in hypoxic/ischemic injury. Mitochondrial impairment increases with age leading to dysregulation of molecular pathways linked to mitochondria. The perturbation of mitochondrial homeostasis and cellular energetics worsens outcome following hypoxic-ischemic insults in elderly individuals. In response to acute injury conditions, cellular machinery relies on rapid adaptations by modulating posttranslational modifications. Therefore, post-translational regulation of molecular mediators such as hypoxia-inducible factor 1α (HIF-1α), peroxisome proliferator-activated receptor γ coactivator α (PGC-1α), c-MYC, SIRT1 and AMPK play a critical role in the control of the glycolytic-mitochondrial energy axis in response to hypoxic-ischemic conditions. The deficiency of oxygen and nutrients leads to decreased energetic reliance on mitochondria, promoting glycolysis. The combination of pseudohypoxia, declining autophagy, and dysregulation of stress responses with aging adds to impaired host response to hypoxic-ischemic injury. Furthermore, intermitochondrial signal propagation and tissue wide oscillations in mitochondrial metabolism in response to oxidative stress are emerging as vital to cellular energetics. Recently reported intercellular transport of mitochondria through tunneling nanotubes also play a role in the response to and treatments for ischemic injury. In this review we attempt to provide an overview of some of the molecular mechanisms and potential therapies involved in the alteration of cellular energetics with aging and injury with a neurobiological perspective.

Chronic heart failure: Ca(2+), catabolism, and catastrophic cell death

Robust successes have been achieved in recent years in conquering the acutely lethal manifestations of heart disease. Many patients who previously would have died now survive to enjoy happy and productive lives. Nevertheless, the devastating impact of heart disease continues unabated, as the spectrum of disease has evolved with new manifestations. In light of this ever-evolving challenge, insights that culminate in novel therapeutic targets are urgently needed. Here, we review fundamental mechanisms of heart failure, both with reduced (HFrEF) and preserved (HFpEF) ejection fraction. We discuss pathways that regulate cardiomyocyte remodeling and turnover, focusing on Ca(2+) signaling, autophagy, and apoptosis. In particular, we highlight recent insights pointing to novel connections among these events. We also explore mechanisms whereby potential therapeutic approaches targeting these processes may improve morbidity and mortality in the devastating syndrome of heart failure.

Stress response factors as hub-regulators of microRNA biogenesis: implication to the diseased heart

MicroRNAs (miRNAs) are important regulators of heart function and then an intriguing therapeutic target for plenty of diseases. The problem raised is that many data in this area are contradictory, thus limiting the use of miRNA-based therapy. The goal of this review is to describe the hub-mechanisms regulating the biogenesis and function of miRNAs, which could help in clarifying some contradictions in the miRNA world. With this scope, we analyse an array of factors, including several known agents of stress response, mediators of epigenetic changes, regulators of alternative splicing, RNA editing, protein synthesis and folding and proteolytic systems. All these factors are important in cardiovascular function and most of them regulate miRNA biogenesis, but their influence on miRNAs was shown for non-cardiac cells or some specific cardiac pathologies. Finally, we consider that studying the stress response factors, which are upstream regulators of miRNA biogenesis, in the diseased heart could help in (1) explaining some contradictions concerning miRNAs in heart pathology, (2) making the role of miRNAs in pathogenesis of cardiovascular disease more clear, and therefore, (3) getting powerful targets for its molecular therapy.

BNIP3 promotes calcium and calpain-dependent cell death

AIMS

Loss of cardiac muscle by programmed cell death contributes to the progression of ischemic heart disease. Hypoxia, metabolite waste buildup and energy depletion are components of ischemia which may initiate caspase dependent and independent cell death pathways. Previous work from our laboratory has shown that combined hypoxia with acidosis, a hallmark of ischemia promotes cardiac myocyte injury with increasing severity as the pH declines. Hypoxia-acidosis was demonstrated to activate the pro-apoptotic Bcl-2 protein BNIP3 which initiated opening of the mitochondrial permeability transition pore and cell death in the absence of caspase activation. Because calpains are known to contribute to ischemic myocardial damage in some models, we hypothesized that they are intermediates in the BNIP3-mediated death caused by hypoxia-acidosis.

MAIN METHODS

Neonatal rat cardiac myocytes were subjected to hypoxia with and without acidosis and the contribution of calpains to hypoxia-acidosis cell death determined.

KEY FINDINGS

Here we report that the death pathway activated by hypoxia-acidosis is driven by a combination of calcium-activated calpains and pro-death factors (DNases) secreted by the mitochondria. Cytochrome c accumulated in the cytoplasm during hypoxia-acidosis but caspase activity was repressed through a calpain-dependent process that prevents the cleavage of procaspase 3. Calpain inhibitors provide vigorous protection against hypoxia-acidosis-induced programmed death. Knockdown of BNIP3 with siRNA prevented calpain activation confirming a central role of BNIP3 in this pathway.

SIGNIFICANCE

The results implicate BNIP3 and calpain as dependent components of cardiac myocyte death caused by hypoxia-acidosis.

Calpains and Coronary Vascular Disease

Despite many advances in percutaneous and surgical interventions in the treatment of coronary artery disease (CAD), up to one-third of patients are still either not candidates or receive suboptimal revascularization. Calpains are a class of calcium-activated non-lysosomal cysteine proteases that serve as a proteolytic unit for cellular homeostasis. Uncontrolled activation of calpain has been found to be involved in the pathogenesis of myocardial reperfusion injury, cardiac hypertrophy, myocardial stunning and cardiac ischemia. Inhibition of calpains has been shown to significantly attenuate myocardial stunning and reduced infarct size after ischemia-reperfusion. Calpain inhibition therefore serves as a potential medical therapy for patients suffering from a number of diseases, including CAD.

PD150606 protects against ischemia/reperfusion injury by preventing μ-calpain-induced mitochondrial apoptosis

Calpain plays an important role in myocardial ischemia/reperfusion (I/R) injury. PD150606, a nonpeptide, cell-permeable and noncompetitive calpain inhibitor, has been shown to have protective properties in ischemic disease. The aims of the present study were to investigate whether PD150606 could alleviate myocardial I/R injury and to examine the possible mechanisms involved. The I/R model was established in vivo in C57BL/6 mice and in vitro using neonatal mouse cardiomyocytes, respectively. To evaluate the protective effects of PD150606 on I/R injury, we measured the myocardial infarct area, apoptosis, and expression of cleaved caspase-3. We also investigated the underlying mechanisms by examining mitochondrial function as reflected by the ATP concentration, translocation of cytochrome c, dynamics of mPTP opening, and membrane potential (ΔΨm), coupled with calpain activity. Pretreatment with PD150606 significantly reduced the infarct area and apoptosis caused by I/R. PD150606 pretreatment also reduced mitochondrial dysfunction by inhibiting calpain activation. Moreover, we found that μ-calpain is the main contributor to I/R-induced calpain activation. Knockdown of μ-calpain with siRNA significantly reversed calpain activation, mitochondrial dysfunction, and cardiomyocyte apoptosis caused by I/R in vitro. Our results suggest that PD150606 may protect against I/R injury via preventing μ-calpain-induced mitochondrial apoptosis.

Mitochondrial m-calpain opens the mitochondrial permeability transition pore in ischemia-reperfusion

BACKGROUND/OBJECTIVES

Opening of the mitochondrial permeability transition pore (mPTP) is involved in ischemia-reperfusion injury. Isoforms of Ca(2+)-activated cysteine proteases, calpains, are implicated in the development of myocardial infarction in ischemia-reperfusion. Growing evidence has revealed the presence of calpains in the mitochondria. We aimed to characterize mitochondrial calpains in the rat heart and to investigate the roles of calpains in mPTP opening after ischemia-reperfusion.

METHODS AND RESULTS

Western blotting analysis showed the expression of μ-calpain, m-calpain and calpain 10 in mitochondria isolated from male Sprague-Dawley rats, but casein zymography detected only m-calpain activity. Subcellular fractionation of mitochondria demonstrated the distribution of m-calpain to the matrix fraction. Addition of >500μM of Ca(2+) to isolated mitochondria induced mitochondrial swelling, reflecting mPTP opening, and calpain activation. Ca(2+)-induced mitochondrial swelling was inhibited partially by the calpain inhibitor calpeptin. These results support a partial contribution of calpain in the opening of the mPTP. The addition of Ca(2+) to the mitochondria induced inactivation of complex I of the electron transport chain, and cleavage of the ND6 complex I subunit, which were inhibited by calpeptin. Mitochondria isolated from rat hearts that underwent 30min of coronary occlusion followed by 30min of reperfusion showed activation of mitochondrial calpains, ND6 cleavage, complex I inactivation, and mPTP opening, which were inhibited by pretreatment with calpain inhibitor 1.

CONCLUSIONS

We demonstrated for the first time the presence of mitochondrial matrix m-calpain, and its contribution to complex I inactivation and mPTP opening after postischemic reperfusion in the rat heart.

Calpain inhibition decreases myocardial apoptosis in a swine model of chronic myocardial ischemia

INTRODUCTION

Calpain is a family of cysteine proteases that has an important role in the initiation, regulation, and execution of cell death. Our recent studies using a hypercholesterolemic swine model demonstrated that in the setting of the metabolic syndrome, calpain inhibition (CI) improved collateral-dependent perfusion and increased expression of proteins implicated in angiogenesis and vasodilation. In this study, we hypothesized that CI (by MLD28170) would decrease myocardial apoptosis in the same model.

METHODS

Yorkshire swine, all fed a high-cholesterol diet for 4 weeks underwent placement of an ameroid constrictor on the left circumflex coronary artery. Three weeks later, animals received either no drug, termed the high-cholesterol control group (HCC; n = 8); low-dose CI (0.12 mg/kg; LCI, n = 9); or high-dose CI (0.25 mg/kg; HCI, n = 8). The high-cholesterol diet and the CI were continued for 5 weeks, after which the pig was humanely killed and the left ventricular myocardium was harvested and analyzed via terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining, oxyblot analysis, and Western blots. Data were analyzed using the Kruskal-Wallis test.

RESULTS

The percentage of apoptotic cells to total cells in ischemic myocardial territory was decreased in the LCI and HCI groups compared with the HCC group as shown by TUNEL staining (P = .018). There was a decrease in proapoptotic proteins, including cleaved caspase 3, caspase 9, cleaved caspase 9, Bax, BAD, p-BAD, and Erk 1/2 (P ≤ .049 each), but no decrease in caspase 3 (P = .737). There was also an increase in antiapoptotic proteins, including BCL-2 and p-BCL2 (P ≤ .025 each). In the ischemic myocardium, several proangiogenic proteins were increased in the LCI and HCI groups compared with the HCC group, including p-AKT, p-eNOS, and eNOS (P ≤ .006 each) but there was no increase in AKT (P = .311). CI decreased tissue oxidative stress in both the LCI and HCI groups compared to the HCC group as shown by oxyblot analysis (P = .021).

CONCLUSION

In the setting of hypercholesterolemia, CI decreases apoptosis and the expression of proteins in proapoptotic signaling pathways. CI also increased expression of proteins implicated in anti apoptotic pathways and improves oxidative stress in ischemic myocardial tissue.

Myocardial matrix metalloproteinase-2: inside out and upside down

Since their inaugural discovery in the early 1960s, matrix metalloproteinases (MMPs) have been shown to mediate multiple physiological and pathological processes. In addition to their canonical function in extracellular matrix (ECM) remodeling, research in the last decade has highlighted new MMP functions, including proteolysis of novel substrates beyond ECM proteins, MMP localization to subcellular organelles, and proteolysis of susceptible intracellular proteins in those subcellular compartments. This review will provide a comparison of the extracellular and intracellular roles of MMPs, illustrating that MMPs are far more interesting than the one-dimensional view originally taken. We focus on the roles of MMP-2 in cardiac injury and repair, as this is one of the most studied MMPs in the cardiovascular field. We will highlight how understanding all dimensions, such as localization of activity and timing of interventions, will increase the translational potential of research findings. Building upon old ideas and turning them inside out and upside down will help us to better understand how to move the MMP field forward.

Roles of Calcium Regulating MicroRNAs in Cardiac Ischemia-Reperfusion Injury

Cardiac Ca2+ cycling and signaling are closely associated with cardiac function. Changes in cellular Ca2+ homeostasis may lead to aberrant cardiac rhythm and may play a critical role in the pathogenesis of cardiac diseases, due to their exacerbation of heart failure. MicroRNAs (miRNAs) play a key role in the regulation of gene expression at the post-transcriptional level and participate in regulating diverse biological processes. The emerging evidence indicates that the expression profiles of miRNAs vary among human diseases, including cardiovascular diseases. Cardiac Ca2+-handling and signaling proteins are also regulated by miRNAs. Given the relationship between cardiac Ca2+ homeostasis and signaling and miRNA, Ca2+-related miRNAs may serve as therapeutic targets during the treatment of heart failure. In this review, we summarize the knowledge currently available regarding the role of Ca2+ in cardiac function, as well as changes in Ca2+ cycling and homeostasis and the handling of these processes by miRNAs during cardiac ischemia-reperfusion injury.

Calpain system and its involvement in myocardial ischemia and reperfusion injury

Calpains are ubiquitous non-lysosomal Ca²⁺-dependent cysteine proteases also present in myocardial cytosol and mitochondria. Numerous experimental studies reveal an essential role of the calpain system in myocardial injury during ischemia, reperfusion and postischemic structural remodelling. The increasing Ca²⁺-content and Ca²⁺-overload in myocardial cytosol and mitochondria during ischemia and reperfusion causes an activation of calpains. Upon activation they are able to injure the contractile apparatus and impair the energy production by cleaving structural and functional proteins of myocytes and mitochondria. Besides their causal involvement in acute myocardial dysfunction they are also involved in structural remodelling after myocardial infarction by the generation and release of proapoptotic factors from mitochondria. Calpain inhibition can prevent or attenuate myocardial injury during ischemia, reperfusion, and in later stages of myocardial infarction.

Calpain-dependent cleavage of N-cadherin is involved in the progression of post-myocardial infarction remodeling

Enzymatic proteolysis by calpains, Ca(2+)-dependent intracellular cysteine proteases, has been implicated in pathological processes such as cellular degeneration or death. Here, we investigated the role of calpain activation in the hearts subjected to myocardial infarction. We produced myocardial infarction in Cast(-/-) mice deficient for calpastatin, the specific endogenous inhibitory protein for calpains, and Cast(+/+) mice. The activity of cardiac calpains in Cast(+/+) mice was not elevated within 1 day but showed a gradual elevation after 7 days following myocardial infarction, which was further pronounced in Cast(-/-) mice. Although the prevalence of cardiomyocyte death was indistinguishable between Cast(-/-) and Cast(+/+) mice, Cast(-/-) mice exhibited profound contractile dysfunction and chamber dilatation and showed a significant reduction in survival rate after myocardial infarction as compared with Cast(+/+) mice. Notably, immunofluorescence revealed that at 28 days after myocardial infarction, calpains were activated in cardiomyocytes exclusively at the border zone and that Cast(-/-) mice showed higher intensity and a broader extent of calpain activation at the border zone than Cast(+/+) mice. In the border zone of Cast(-/-) mice, pronounced activation of calpains was associated with a decrease in N-cadherin expression and up-regulation of molecular markers for cardiac hypertrophy and fibrosis. In cultured rat neonatal cardiomyocytes, calpain activation by treatment with ionomycin induced cleavage of N-cadherin and decreased expression levels of β-catenin and connexin 43, which was attenuated by calpain inhibitor. These results thus demonstrate that activation of calpains disassembles cell-cell adhesion at intercalated discs by degrading N-cadherin and thereby promotes left ventricular remodeling after myocardial infarction.

Molecular mechanisms of ischemic conditioning: translation into patient outcomes

Following the initiation of an ischemic insult, reperfusion injury (RI) can result in numerous deleterious cardiac effects, including cardiomyocyte death. Experimental data have suggested that ischemic conditioning, when delivered either before or after the ischemic event, can provide considerable cardioprotection against RI. Ischemic conditioning involves delivering brief repetitive cycles of ischemia to the myocardium (local) or to another distal organ or structure (remote). This review will discuss recent advances in the molecular mechanisms involved in RI, the signaling pathways recruited by ischemic conditioning and conclude with an appraisal of the evidence for the use of ischemic conditioning in current clinical practice.

Protective effects of sitagliptin on myocardial injury and cardiac function in an ischemia/reperfusion rat model

The purpose of this study is to investigate the effects and the underlying mechanisms of sitagliptin pretreatment on myocardial injury and cardiac function in myocardial ischemia/reperfusion (I/R) rat model. The rat model of myocardial I/R was constructed by coronary occlusion. Rats were pretreated with sitagliptin (300 mg/kg/day) for 2 weeks, and then subjected to 30 min ischemia and 2h reperfusion. The release of lactate dehydrogenase (LDH) and creatine kinase-MB (CK-MB), cardiac function and cardiomyocyte apoptosis were evaluated. The levels of malondialdehyde (MDA), glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) in heart and glucagon-like peptide-1 (GLP-1) level in plasma were measured. Western blot analysis was performed to detect the target proteins of sitagliptin. Our results showed that sitagliptin pretreatment decreased LDH and CK-MB release, and MDA level in I/R rats. More importantly, we revealed for the first time that sitagliptin pretreatment decreased cardiomyocyte apoptosis while increased the levels of GSH-Px and SOD in heart. Sitagliptin also increased GLP-1 level and enhanced cardiac function in I/R rats. Furthermore, sitagliptin pretreatment up-regulated Akt(serine473) and Bad(serine136) phosphorylation, reduced the ratio of Bax/Bcl-2, and decreased expression levels of cleaved caspase-3 and caspase-3. Interestingly, the above observed effects of sitagliptin were all abolished when co-administered with GLP-1 receptor antagonist exendin-(9-39) or PI3K inhibitor LY294002. Taken together, our data indicate that sitagliptin pretreatment could reduce myocardial injury and improve cardiac function in I/R rats by reducing apoptosis and oxidative damage. The underlying mechanism might be the activation of PI3K/Akt signaling pathway by GLP-1/GLP-1 receptor.

Cardioprotective effects of exenatide against oxidative stress-induced injury

Myocardial ischemia/reperfusion (MI/R) leads to oxidative stress, which may in turn lead to myocardial injury. In the present study, we investigated the effects of exenatide, a glucagon-like peptide-1 (GLP-1) analogue, on oxidative stress-induced injury in vitro and in vivo. In in vitro experiments, H9c2 cells were incubated with exenatide to determine the direct cytoprotective effects of exenatide following exposure to hydrogen peroxide (H2O2). Pre-treatment with exenatide (1 nM), prior to H2O2 exposure, increased cell viability and inhibited H2O2-induced reactive oxygen species (ROS) production. Exenatide also decreased the levels of lactate dehydrogenase (LDH) and creatine kinase-MB (CK-MB) in the cultured supernatants, as well as those of malondialdehyde (MDA) in the H9c2 cells and increased the total superoxide dismutase (T-SOD) levels in the H9c2 cells. In in vivo experiments, an animal model of MI/R was induced by coronary occlusion. Pre-treatment with exenatide (10 µg/kg/day) protected the rat hearts from MI/R-induced injury by decreasing the levels of LDH and CK-MB in plasma, increasing the levels of catalase, T-SOD and glutathione peroxidase (GSH-Px) in the heart and decreasing the MDA levels in the rats with MI/R-induced injury. Exenatide also reduced the infarct size and enhanced cardiac function in the rats with MI/R-induced injury. Moreover, pre-treatment with exenatide inhibited cardiomyocyte apoptosis, increased Aktserine473 and Badserine136 phosphorylation and decreased cleaved caspase-3 expression in vitro and in vivo; however, these effects were attenuated by the phosphoinositide 3-kinase (PI3K) inhibitor, LY294002. Our results suggest that exenatide exerts significant cardioprotective effects against oxidative stress-induced injury in vitro and in vivo. The mechanisms involved may be attributed to the scavenging of oxidative stress products, such as ROS, the increase in the concentrations of antioxidant defense enzymes and the inhibition of cardiomyocyte apoptosis. The anti-apoptotic effects of exenatide were, at least in part, associated with the activation of the PI3K/Akt signaling pathway.

Lipoxin a4 preconditioning and postconditioning protect myocardial ischemia/reperfusion injury in rats

This study aims to investigate the pre- and postconditioning effects of lipoxin A₄ (LXA₄) on myocardial damage caused by ischemia/reperfusion (I/R) injury. Seventy-two rats were divided into 6 groups: sham groups (C₁ and C₂), I/R groups (I/R₁ and I/R₂), and I/R plus LXA₄ preconditioning and postconditioning groups (LX₁ and LX₂). The serum levels of IL-1β, IL-6, IL-8, IL-10, TNF-α, and cardiac troponin I (cTnI) were measured. The content and the activity of Na⁺-K⁺-ATPase as well as the superoxide dismutase (SOD), and malondialdehyde (MDA) levels were determined. Along with the examination of myocardium ultrastructure and ventricular arrhythmia scores (VAS), connexin 43 (Cx43) expression were also detected. Lower levels of IL-1β, IL-6, IL-8, TNF-α, cTnI, MDA content, and VAS and higher levels of IL-10, SOD activity, Na⁺-K⁺-ATPase content and activity, and Cx43 expression appeared in LX groups than I/R groups. Besides, H&E staining, TEM examination as well as analysis of gene, and protein confirmed that LXA₄ preconditioning was more effective than postconditioning in preventing arrhythmogenesis via the upregulation of Cx43. That is, LXA₄ postconditioning had better protective effect on Na⁺-K⁺-ATPase and myocardial ultrastructure.

Over-expression of calpastatin aggravates cardiotoxicity induced by doxorubicin

AIMS

Doxorubicin causes damage to the heart, which may present as cardiomyopathy. However, the mechanisms by which doxorubicin induces cardiotoxicity remain not fully understood and no effective prevention for doxorubicin cardiomyopathy is available. Calpains, a family of calcium-dependent thiol-proteases, have been implicated in cardiovascular diseases. Their activities are tightly controlled by calpastatin. This study employed transgenic mice over-expressing calpastatin to investigate the role of calpain in doxorubicin-induced cardiotoxicity.

METHODS AND RESULTS

Doxorubicin treatment decreased calpain activities in cultured neonatal mouse cardiomyocytes and in vivo mouse hearts, which correlated with down-regulation of calpain-1 and calpain-2 proteins. Over-expression of calpastatin or incubation with pharmacological calpain inhibitors enhanced apoptosis in neonatal and adult cardiomyocytes induced by doxorubicin. In contrast, over-expression of calpain-2 but not calpain-1 attenuated doxorubicin-induced apoptosis in cardiomyocytes. The pro-apoptotic effects of calpain inhibition were associated with down-regulation of protein kinase B (AKT) protein and mRNA expression, and a concomitant reduction in glycogen synthase kinase-3beta (GSK-3β) phosphorylation (Ser9) in doxorubicin-treated cardiomyocytes. Blocking AKT further increased doxorubicin-induced cardiac injuries, suggesting the effects of calpain inhibition may be mediated by inactivating the AKT signalling. In an in vivo model of doxorubicin-induced cardiotoxicity, over-expression of calpastatin exacerbated myocardial dysfunction as assessed by echocardiography and haemodynamic measurement in transgenic mice 5 days after doxorubicin injection. The 5-day mortality was higher in transgenic mice (29.16%) compared with their wild-type littermates (8%) after doxorubicin treatment.

CONCLUSION

Over-expression of calpastatin enhances doxorubicin-induced cardiac injuries through calpain inhibition and thus, calpains may protect cardiomyocytes against doxorubicin-induced cardiotoxicity.

Calpain inhibitor MDL 28170 protects against the Ca2+ paradox in rat hearts

The calcium paradox represents an important model in which to study myocardial injuries due to intracellular Ca(2+) overload. In a previous study, calpain was transiently activated in Ca(2+) -paradoxic hearts. The aim of the present study was to determine the role of calpain in myocardial dysfunction in hearts subjected to the Ca(2+) paradox and to elucidate the underlying mechanisms. Rat hearts were isolated, Langendorff perfused and subjected to the Ca(2+) paradox, which was induced by 3 min Ca(2+) depletion followed by 30 min Ca(2+) repletion, in the presence or absence of the calpain inhibitor 10 umol/L MDL 28170. Cardiac function was evaluated. Furthermore, cell death and the degradation of troponin I (TnI) were assessed and calpain activity was determined by measurement of the α-fodrin fragment and confocal image analysis. Upon Ca(2+) repletion, the hearts immediately deteriorated, exhibiting a marked depression in cardiac function and an enlarged myocardial injury area. This was accompanied by significant increases in lactate dehydrogenase, mitochondrial release of cytochrome c, the apoptotic index and degraded TnI. These changes were significantly inhibited by MDL 28170, with the exception of TnI degradation. Compared with the control group, Ca(2+) -paradoxic hearts showed a marked increase in cleaved 150 kDa fragments resulting from specific calpain-mediated proteolysis of α-fodrin. This effect was attenuated by MDL 28170. Confocal image analysis revealed the translocation of both μ- and m-calpain to the sarcolemmal membrane in Ca(2+) -paradoxic hearts, indicating increased activity of both isoforms. The results suggest that the Ca(2+) paradox promotes calpain activity, leading to necrosis, apoptosis and myocardial dysfunction.

Contribution of calpains to myocardial ischaemia/reperfusion injury

Loss of calcium (Ca(2+)) homeostasis contributes through different mechanisms to cell death occurring during the first minutes of reperfusion. One of them is an unregulated activation of a variety of Ca(2+)-dependent enzymes, including the non-lysosomal cysteine proteases known as calpains. This review analyses the involvement of the calpain family in reperfusion-induced cardiomyocyte death. Calpains remain inactive before reperfusion due to the acidic pHi and increased ionic strength in the ischaemic myocardium. However, inappropriate calpain activation occurs during myocardial reperfusion, and subsequent proteolysis of a wide variety of proteins contributes to the development of contractile dysfunction and necrotic cell death by different mechanisms, including increased membrane fragility, further impairment of Na(+) and Ca(2+) handling, and mitochondrial dysfunction. Recent studies demonstrating that calpain inhibition contributes to the cardioprotective effects of preconditioning and postconditioning, and the beneficial effects obtained with new and more selective calpain inhibitors added at the onset of reperfusion, point to the potential cardioprotective value of therapeutic strategies designed to prevent calpain activation.

Deficiency of Capn4 gene inhibits nuclear factor-κB (NF-κB) protein signaling/inflammation and reduces remodeling after myocardial infarction

Calpain has been implicated in acute myocardial injury after myocardial infarction (MI). However, the causal relationship between calpain and post-MI myocardial remodeling has not been fully understood. This study examined whether deletion of Capn4, essential for calpain-1 and calpain-2 activities, reduces myocardial remodeling and dysfunction following MI, and if yes, whether these effects of Capn4 deletion are associated with NF-κB signaling and inflammatory responses in the MI heart. A novel mouse model with cardiomyocyte-specific deletion of Capn4 (Capn4-ko) was employed. MI was induced by left coronary artery ligation. Deficiency of Capn4 dramatically reduced the protein levels and activities of calpain-1 and calpain-2 in the Capn4-ko heart. In vivo cardiac function was relatively improved in Capn4-ko mice at 7 and 30 days after MI when compared with their wild-type littermates. Deletion of Capn4 reduced apoptosis, limited infarct expansion, prevented left ventricle dilation, and reduced mortality in Capn4-ko mice. Furthermore, cardiomyocyte cross-sectional areas and myocardial collagen deposition were significantly attenuated in Capn4-ko mice, which were accompanied by down-regulation of hypertrophic genes and profibrotic genes. These effects of Capn4 knock-out correlated with restoration of IκB protein and inhibition of NF-κB activation, leading to suppression of proinflammatory cytokine expression and inflammatory cell infiltration in the Capn4-ko heart after MI. In conclusion, deficiency of Capn4 reduces adverse myocardial remodeling and myocardial dysfunction after MI. These effects of Capn4 deletion may be mediated through prevention of IκB degradation and NF-κB activation, resulting in inhibition of inflammatory responses.

Calpains, mitochondria, and apoptosis

Mitochondrial activity is critical for efficient function of the cardiovascular system. In response to cardiovascular injury, mitochondrial dysfunction occurs and can lead to apoptosis and necrosis. Calpains are a 15-member family of Ca(2+)-activated cysteine proteases localized to the cytosol and mitochondria, and several have been shown to regulate apoptosis and necrosis. For example, in endothelial cells, Ca(2+) overload causes mitochondrial calpain 1 cleavage of the Na(+)/Ca(2+) exchanger leading to mitochondrial Ca(2+) accumulation. Also, activated calpain 1 cleaves Bid, inducing cytochrome c release and apoptosis. In renal cells, calpains 1 and 2 promote apoptosis and necrosis by cleaving cytoskeletal proteins, which increases plasma membrane permeability and cleavage of caspases. Calpain 10 cleaves electron transport chain proteins, causing decreased mitochondrial respiration and excessive activation, or inhibition of calpain 10 activity induces mitochondrial dysfunction and apoptosis. In cardiomyocytes, calpain 1 activates caspase 3 and poly-ADP ribose polymerase during tumour necrosis factor-α-induced apoptosis, and calpain 1 cleaves apoptosis-inducing factor after Ca(2+) overload. Many of these observations have been elucidated with calpain inhibitors, but most calpain inhibitors are not specific for calpains or a specific calpain family member, creating more questions. The following review will discuss how calpains affect mitochondrial function and apoptosis within the cardiovascular system.

Calpain inhibitors exhibit matrix metalloproteinase-2 inhibitory activity

Matrix metalloproteinase (MMP)-2 is a zinc-dependent endopeptidase which, alongside its known extracellular actions, plays fundamental roles in oxidative stress-induced injury to the heart. Intracellular cleavage targets of MMP-2 selectively mediating this injury include the sarcomeric proteins troponin I, myosin light chain-1 and titin; some of these are also targeted by calpains. In myocardial ischemia and reperfusion injury, inhibitors of MMP-2 and some calpain inhibitors were shown to improve the recovery of contractile function. We hypothesized that the protective effects of calpain inhibitors may be due in part to their ability to inhibit MMP-2. Four calpain inhibitors (calpain inhibitor III, ALLM, ALLN, and PD-150606) were tested for their ability to inhibit MMP-2 in comparison to the selective MMP inhibitor ONO-4817. At 100 μM, all calpain inhibitors, except ALLM, showed significant inhibition of MMP-2 gelatinolytic activity. When assessed by the troponin I proteolysis assay, both ALLN and PD-150606, but neither ALLM nor calpain inhibitor III (at 20 μM), significantly inhibited MMP-2 activity. Using a fluorogenic MMP substrate peptide OmniMMP in a kinetic assay the rank order of IC(50) values against MMP-2 were: PD-150606<ALLN<calpain inhibitor III <<< ALLM. These experiments show that the calpain inhibitors PD-150606 and ALLN have significant additional pharmacological activity as MMP-2 inhibitors. This suggests that the protective effect of some calpain inhibitors is due in part to their ability to inhibit MMP activity.

Regulation and physiological roles of the calpain system in muscular disorders

Calpains, a family of Ca(2+)-dependent cytosolic cysteine proteases, can modulate their substrates' structure and function through limited proteolytic activity. In the human genome, there are 15 calpain genes. The most-studied calpains, referred to as conventional calpains, are ubiquitous. While genetic studies in mice have improved our understanding about the conventional calpains' physiological functions, especially those essential for mammalian life as in embryogenesis, many reports have pointed to overactivated conventional calpains as an exacerbating factor in pathophysiological conditions such as cardiovascular diseases and muscular dystrophies. For treatment of these diseases, calpain inhibitors have always been considered as drug targets. Recent studies have introduced another aspect of calpains that calpain activity is required to protect the heart and skeletal muscle against stress. This review summarizes the functions and regulation of calpains, focusing on the relevance of calpains to cardiovascular disease.

Calcium-mediated cell death during myocardial reperfusion

Reperfusion may induce additional cell death in patients with acute myocardial infarction receiving primary angioplasty or thrombolysis. Altered intracellular Ca(2+) handling was initially considered an essential mechanism of reperfusion-induced cardiomyocyte death. However, more recent studies have demonstrated the importance of Ca(2+)-independent mechanisms that converge on mitochondrial permeability transition (MPT) and are shared by cardiomyocytes and other cell types. This article analyses the importance of Ca(2+)-dependent cell death in light of these new observations. Altered Ca(2+) handling includes increased cytosolic Ca(2+) levels, leading to activation of calpain-mediated proteolysis and sarcoplasmic reticulum-driven oscillations; this can induce hypercontracture, but also MPT due to the privileged Ca(2+) transfer between sarcoplasmic reticulum and mitochondria through cytosolic Ca(2+) microdomains. In the opposite direction, permeability transition can worsen altered Ca(2+) handling and favour hypercontracture. Ca(2+) appears to play an important role in cell death during the initial minutes of reperfusion, particularly after brief periods of ischaemia. Developing effective and safe treatments to prevent Ca(2+)-mediated cardiomyocyte death in patients with transient ischaemia, by targeting Ca(2+) influx, intracellular Ca(2+) handling, or Ca(2+)-induced cell death effectors, is an unmet challenge with important therapeutic implications and large potential clinical impact.

Activation of proteolytic enzymes and depression of the sarcolemmal Na+/K+-ATPase in ischemia-reperfused heart may be mediated through oxidative stress

We tested whether the activation of proteolytic enzymes, calpain, and matrix metalloproteinases (MMPs) during ischemia-reperfusion (I/R) is mediated through oxidative stress. For this purpose, isolated rat hearts were subjected to a 30 min global ischemia followed by a 30 min reperfusion. Cardiac function was monitored and the activities of Na(+)/K(+)-ATPase, Mg(2+)-ATPase, calpain, and MMP were measured. Depression of cardiac function and Na(+)/K(+)-ATPase activity in I/R hearts was associated with increased calpain and MMP activities. These alterations owing to I/R were similar to those observed in hearts perfused with hypoxic medium, H(2)O(2) and xanthine plus xanthine oxidase. The I/R-induced changes were attenuated by ischemic preconditioning as well as by perfusing the hearts with N-acetylcysteine or mercaptopropionylglycine. Inhibition of MMP activity in hearts treated with doxycycline depressed the I/R-induced changes in cardiac function and Na(+)/K(+)-ATPase activity without affecting the calpain activation. On the other hand, inhibition of calpain activity upon treatment with leupeptin or MDL 28170 significantly reduced the MMP activity in addition to attenuating the I/R-induced alterations in cardiac function and Na(+)/K(+)-ATPase activity. These results suggest that the I/R-induced depression in Na(+)/K(+)-ATPase and cardiac function may be a consequence of the increased activities of both calpain and MMP because of oxidative stress in the heart.

Cell biology of ischemia/reperfusion injury

Disorders characterized by ischemia/reperfusion (I/R), such as myocardial infarction, stroke, and peripheral vascular disease, continue to be among the most frequent causes of debilitating disease and death. Tissue injury and/or death occur as a result of the initial ischemic insult, which is determined primarily by the magnitude and duration of the interruption in the blood supply, and then subsequent damage induced by reperfusion. During prolonged ischemia, ATP levels and intracellular pH decrease as a result of anaerobic metabolism and lactate accumulation. As a consequence, ATPase-dependent ion transport mechanisms become dysfunctional, contributing to increased intracellular and mitochondrial calcium levels (calcium overload), cell swelling and rupture, and cell death by necrotic, necroptotic, apoptotic, and autophagic mechanisms. Although oxygen levels are restored upon reperfusion, a surge in the generation of reactive oxygen species occurs and proinflammatory neutrophils infiltrate ischemic tissues to exacerbate ischemic injury. The pathologic events induced by I/R orchestrate the opening of the mitochondrial permeability transition pore, which appears to represent a common end-effector of the pathologic events initiated by I/R. The aim of this treatise is to provide a comprehensive review of the mechanisms underlying the development of I/R injury, from which it should be apparent that a combination of molecular and cellular approaches targeting multiple pathologic processes to limit the extent of I/R injury must be adopted to enhance resistance to cell death and increase regenerative capacity in order to effect long-lasting repair of ischemic tissues.

Targeted inhibition of calpain reduces myocardial hypertrophy and fibrosis in mouse models of type 1 diabetes

OBJECTIVE

Recently we have shown that calpain-1 activation contributes to cardiomyocyte apoptosis induced by hyperglycemia. This study was undertaken to investigate whether targeted disruption of calpain would reduce myocardial hypertrophy and fibrosis in mouse models of type 1 diabetes.

RESEARCH DESIGN AND METHODS

Diabetes in mice was induced by injection of streptozotocin (STZ), and OVE26 mice were also used as a type 1 diabetic model. The function of calpain was genetically manipulated by cardiomyocyte-specific knockout Capn4 in mice and the use of calpastatin transgenic mice. Myocardial hypertrophy and fibrosis were investigated 2 and 5 months after STZ injection or in OVE26 diabetic mice at the age of 5 months. Cultured isolated adult mouse cardiac fibroblast cells were also investigated under high glucose conditions.

RESULTS

Calpain activity, cardiomyocyte cross-sectional areas, and myocardial collagen deposition were significantly increased in both STZ-induced and OVE26 diabetic hearts, and these were accompanied by elevated expression of hypertrophic and fibrotic collagen genes. Deficiency of Capn4 or overexpression of calpastatin reduced myocardial hypertrophy and fibrosis in both diabetic models, leading to the improvement of myocardial function. These effects were associated with a normalization of the nuclear factor of activated T-cell nuclear factor-κB and matrix metalloproteinase (MMP) activities in diabetic hearts. In cultured cardiac fibroblasts, high glucose-induced proliferation and MMP activities were prevented by calpain inhibition.

CONCLUSIONS

Myocardial hypertrophy and fibrosis in diabetic mice are attenuated by reduction of calpain function. Thus targeted inhibition of calpain represents a potential novel therapeutic strategy for reversing diabetic cardiomyopathy.

Calpain protects the heart from hemodynamic stress

Calpains make up a family of Ca(2+)-dependent intracellular cysteine proteases that include ubiquitously expressed μ- and m-calpains. Both are heterodimers consisting of a distinct large catalytic subunit (calpain 1 for μ-calpain and calpain 2 for m-calpain) and a common regulatory subunit (calpain 4). The physiological roles of calpain remain unclear in the organs, including the heart, but it has been suggested that calpain is activated by Ca(2+) overload in diseased hearts, resulting in cardiac dysfunction. In this study, cardiac-specific calpain 4-deficient mice were generated to elucidate the role of calpain in the heart in response to hemodynamic stress. Cardiac-specific deletion of calpain 4 resulted in decreased protein levels of calpains 1 and 2 and showed no cardiac phenotypes under base-line conditions but caused left ventricle dilatation, contractile dysfunction, and heart failure with interstitial fibrosis 1 week after pressure overload. Pressure-overloaded calpain 4-deficient hearts took up a membrane-impermeant dye, Evans blue, indicating plasma membrane disruption. Membrane repair assays using a two-photon laser-scanning microscope revealed that calpain 4-deficient cardiomyocytes failed to reseal a plasma membrane that had been disrupted by laser irradiation. Thus, the data indicate that calpain protects the heart from hemodynamic stresses, such as pressure overload.

The role of oxidized cytochrome c in regulating mitochondrial reactive oxygen species production and its perturbation in ischaemia

Oxidized cytochrome c is a powerful superoxide scavenger within the mitochondrial IMS (intermembrane space), but the importance of this role in situ has not been well explored. In the present study, we investigated this with particular emphasis on whether loss of cytochrome c from mitochondria during heart ischaemia may mediate the increased production of ROS (reactive oxygen species) during subsequent reperfusion that induces mPTP (mitochondrial permeability transition pore) opening. Mitochondrial cytochrome c depletion was induced in vitro with digitonin or by 30 min ischaemia of the perfused rat heart. Control and cytochrome c-deficient mitochondria were incubated with mixed respiratory substrates and an ADP-regenerating system (State 3.5) to mimic physiological conditions. This contrasts with most published studies performed with a single substrate and without significant ATP turnover. Cytochrome c-deficient mitochondria produced more H₂O₂ than control mitochondria, and exogenous cytochrome c addition reversed this increase. In the presence of increasing [KCN] rates of H₂O₂ production by both pre-ischaemic and end-ischaemic mitochondria correlated with the oxidized cytochrome c content, but not with rates of respiration or NAD(P)H autofluorescence. Cytochrome c loss during ischaemia was not mediated by mPTP opening (cyclosporine-A insensitive), neither was it associated with changes in mitochondrial Bax, Bad, Bak or Bid. However, bound HK2 (hexokinase 2) and Bcl-xL were decreased in end-ischaemic mitochondria. We conclude that cytochrome c loss during ischaemia, caused by outer membrane permeabilization, is a major determinant of H₂O₂ production by mitochondria under pathophysiological conditions. We further suggest that in hypoxia, production of H₂O₂ to activate signalling pathways may be also mediated by decreased oxidized cytochrome c and less superoxide scavenging.

Programmed necrosis, not apoptosis, in the heart

It is well known that apoptosis is an actively mediated cell suicide process. In contrast, necrosis, a morphologically distinct form of cell death, has traditionally been regarded as passive and unregulated. Over the past decade, however, experiments in Caenorhabditis elegans and mammalian cells have revealed that a significant proportion of necrotic death is, in fact, actively mediated by the doomed cell. Although a comprehensive understanding of necrosis is still lacking, some key molecular events have come into focus. Cardiac myocyte apoptosis and necrosis are prominent features of the major cardiac syndromes. Accordingly, the recognition of necrosis as a regulated process mandates a reexamination of cell death in the heart. This review discusses pathways that mediate programmed necrosis, how they intersect with apoptotic pathways, roles of necrosis in heart disease, and new therapeutic opportunities that the regulated nature of necrosis presents.

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.