Susceptibility of isolated myofibrils to in vitro glutathionylation: Potential relevance to muscle functions

Implications of S-glutathionylation of sarcomere proteins in cardiac disorders, therapies, and diagnosis

The discovery that cardiac sarcomere proteins are substrates for S-glutathionylation and that this post-translational modification correlates strongly with diastolic dysfunction led to new concepts regarding how levels of oxidative stress affect the heartbeat. Major sarcomere proteins for which there is evidence of S-glutathionylation include cardiac myosin binding protein C (cMyBP-C), actin, cardiac troponin I (cTnI) and titin. Our hypothesis is that these S-glutathionylated proteins are significant factors in acquired and familial disorders of the heart; and, when released into the serum, provide novel biomarkers. We consider the molecular mechanisms for these effects in the context of recent revelations of how these proteins control cardiac dynamics in close collaboration with Ca2+ fluxes. These revelations were made using powerful approaches and technologies that were focused on thin filaments, thick filaments, and titin filaments. Here we integrate their regulatory processes in the sarcomere as modulated mainly by neuro-humoral control of phosphorylation inasmuch evidence indicates that S-glutathionylation and protein phosphorylation, promoting increased dynamics and modifying the Frank-Starling relation, may be mutually exclusive. Earlier studies demonstrated that in addition to cTnI as a well-established biomarker for cardiac disorders, serum levels of cMyBP-C are also a biomarker for cardiac disorders. We describe recent studies approaching the question of whether serum levels of S-glutathionylated-cMyBP-C could be employed as an important clinical tool in patient stratification, early diagnosis in at risk patients before HFpEF, determination of progression, effectiveness of therapeutic approaches, and as a guide in developing future therapies.

S-glutathionylation, friend or foe in cardiovascular health and disease

Glutathione is a low molecular weight thiol that is present at high levels in the cell. The high levels of glutathione in the cell make it one of the most abundant antioxidants contributing to cellular redox homeostasis. As a general rule, throughout cardiovascular disease and progression there is an imbalance in redox homeostasis characterized by reactive oxygen species overproduction and glutathione underproduction. As research into these imbalances continues, glutathione concentrations are increasingly being observed to drive various physiological and pathological signaling responses. Interestingly in addition to acting directly as an antioxidant, glutathione is capable of post translational modifications (S-glutathionylation) of proteins through both chemical interactions and enzyme mediated events. This review will discuss both the chemical and enzyme-based S-glutathionylation of proteins involved in cardiovascular pathologies and angiogenesis.

Mechanotransduction and Metabolism in Cardiomyocyte Microdomains

Efficient contractions of the left ventricle are ensured by the continuous transfer of adenosine triphosphate (ATP) from energy production sites, the mitochondria, to energy utilization sites, such as ionic pumps and the force-generating sarcomeres. To minimize the impact of intracellular ATP trafficking, sarcomeres and mitochondria are closely packed together and in proximity with other ultrastructures involved in excitation-contraction coupling, such as t-tubules and sarcoplasmic reticulum junctions. This complex microdomain has been referred to as the intracellular energetic unit. Here, we review the literature in support of the notion that cardiac homeostasis and disease are emergent properties of the hierarchical organization of these units. Specifically, we will focus on pathological alterations of this microdomain that result in cardiac diseases through energy imbalance and posttranslational modifications of the cytoskeletal proteins involved in mechanosensing and transduction.

S-glutathiolation impairs phosphoregulation and function of cardiac myosin-binding protein C in human heart failure

Cardiac myosin-binding protein C (cMyBP-C) regulates actin-myosin interaction and thereby cardiac myocyte contraction and relaxation. This physiologic function is regulated by cMyBP-C phosphorylation. In our study, reduced site-specific cMyBP-C phosphorylation coincided with increased S-glutathiolation in ventricular tissue from patients with dilated or ischemic cardiomyopathy compared to nonfailing donors. We used redox proteomics, to identify constitutive and disease-specific S-glutathiolation sites in cMyBP-C in donor and patient samples, respectively. Among those, a cysteine cluster in the vicinity of the regulatory phosphorylation sites within the myosin S2 interaction domain C1-M-C2 was identified and showed enhanced S-glutathiolation in patients. In vitro S-glutathiolation of recombinant cMyBP-C C1-M-C2 occurred predominantly at Cys(249), which attenuated phosphorylation by protein kinases. Exposure to glutathione disulfide induced cMyBP-C S-glutathiolation, which functionally decelerated the kinetics of Ca(2+)-activated force development in ventricular myocytes from wild-type, but not those from Mybpc3-targeted knockout mice. These oxidation events abrogate protein kinase-mediated phosphorylation of cMyBP-C and therefore potentially contribute to the reduction of its phosphorylation and the contractile dysfunction observed in human heart failure.-Stathopoulou, K., Wittig, I., Heidler, J., Piasecki, A., Richter, F., Diering, S., van der Velden, J., Buck, F., Donzelli, S., Schröder, E., Wijnker, P. J. M., Voigt, N., Dobrev, D., Sadayappan, S., Eschenhagen, T., Carrier, L., Eaton, P., Cuello, F. S-glutathiolation impairs phosphoregulation and function of cardiac myosin-binding protein C in human heart failure.

A change of heart: oxidative stress in governing muscle function?

Redox/cysteine modification of proteins that regulate calcium cycling can affect contraction in striated muscles. Understanding the nature of these modifications would present the possibility of enhancing cardiac function through reversible cysteine modification of proteins, with potential therapeutic value in heart failure with diastolic dysfunction. Both heart failure and muscular dystrophy are characterized by abnormal redox balance and nitrosative stress. Recent evidence supports the synergistic role of oxidative stress and inflammation in the progression of heart failure with preserved ejection fraction, in concert with endothelial dysfunction and impaired nitric oxide-cyclic guanosine monophosphate-protein kinase G signalling via modification of the giant protein titin. Although antioxidant therapeutics in heart failure with diastolic dysfunction have no marked beneficial effects on the outcome of patients, it, however, remains critical to the understanding of the complex interactions of oxidative/nitrosative stress with pro-inflammatory mechanisms, metabolic dysfunction, and the redox modification of proteins characteristic of heart failure. These may highlight novel approaches to therapeutic strategies for heart failure with diastolic dysfunction. In this review, we provide an overview of oxidative stress and its effects on pathophysiological pathways. We describe the molecular mechanisms driving oxidative modification of proteins and subsequent effects on contractile function, and, finally, we discuss potential therapeutic opportunities for heart failure with diastolic dysfunction.

Impact of anesthesia and storage on posttranslational modifications of cardiac myofilament proteins

Although high fidelity measurements of posttranslational modifications (PTMs) of cardiac myofilament proteins exist, important issues remain regarding basic techniques of sample acquisition and storage. We investigated the effects of anesthetic regimen and sample storage conditions on PTMs of major ventricular sarcomeric proteins. Mice were anesthetized with pentobarbital (Nembutal), ketamine/xylazine mixture (Ket/Xyl), or tribromoethanol (Avertin), and the ventricular tissue was prepared and stored for 1, 7, 30, 60, or 90 days at −80°C. Myofilament protein phosphorylation and glutathionylation were analyzed by Pro-Q Diamond stain and Western blotting, respectively. With up to 7 days of storage, phosphorylation of troponin T was stable for samples from mice anesthetized with either Nembutal or Ket/Xyl but not Avertin; while myosin-binding protein C (MyBP-C) phosphorylation was reduced at 7 days with Nembutal and Ket/Xyl, though generally not significant until 90 days. Tropomyosin and regulatory myosin light chain phosphorylation were stable for up to 7 days regardless of the anesthetic regimen employed. In the case of Troponin I, by 7 days of storage there was a significant fall in phosphorylation across all anesthetics. Storage of samples from 30 to 90 days resulted in a general decrease in myofilament phosphorylation independent of the anesthetic. S-glutathionylation of MyBP-C presented a trend in reduced glutathionylation from days 30–90 in all anesthetics, with only day 90 being statistically significant. Our findings suggest that there are alterations in PTMs of major myofilament proteins from both storage and anesthetic selection, and that storage beyond 30 days will likely result in distortion of data.

S-glutathionylation reactions in mitochondrial function and disease

Mitochondria are highly efficient energy-transforming organelles that convert energy stored in nutrients into ATP. The production of ATP by mitochondria is dependent on oxidation of nutrients and coupling of exergonic electron transfer reactions to the genesis of transmembrane electrochemical potential of protons. Electrons can also prematurely “spin-off” from prosthetic groups in Krebs cycle enzymes and respiratory complexes and univalently reduce di-oxygen to generate reactive oxygen species (ROS) superoxide (O₂•⁻) and hydrogen peroxide (H₂O₂), important signaling molecules that can be toxic at high concentrations. Production of ATP and ROS are intimately linked by the respiratory chain and the genesis of one or the other inherently depends on the metabolic state of mitochondria. Various control mechanisms converge on mitochondria to adjust ATP and ROS output in response to changing cellular demands. One control mechanism that has gained a high amount of attention recently is S-glutathionylation, a redox sensitive covalent modification that involves formation of a disulfide bridge between glutathione and an available protein cysteine thiol. A number of S-glutathionylation targets have been identified in mitochondria. It has also been established that S-glutathionylation reactions in mitochondria are mediated by the thiol oxidoreductase glutaredoxin-2 (Grx2). In the following review, emerging knowledge on S-glutathionylation reactions and its importance in modulating mitochondrial ATP and ROS production will be discussed. Major focus will be placed on Complex I of the respiratory chain since (1) it is a target for reversible S-glutathionylation by Grx2 and (2) deregulation of Complex I S-glutathionylation is associated with development of various disease states particularly heart disease. Other mitochondrial enzymes and how their S-glutathionylation profile is affected in different disease states will also be discussed.

Emerging importance of oxidative stress in regulating striated muscle elasticity

The contractile function of striated muscle cells is altered by oxidative/nitrosative stress, which can be observed under physiological conditions but also in diseases like heart failure or muscular dystrophy. Oxidative stress causes oxidative modifications of myofilament proteins and can impair myocyte contractility. Recent evidence also suggests an important effect of oxidative stress on muscle elasticity and passive stiffness via modifications of the giant protein titin. In this review we provide a short overview of known oxidative modifications in thin and thick filament proteins and then discuss in more detail those oxidative stress-related modifications altering titin stiffness directly or indirectly. Direct modifications of titin include reversible disulfide bonding within the cardiac-specific N2-Bus domain, which increases titin stiffness, and reversible S-glutathionylation of cryptic cysteines in immunoglobulin-like domains, which only takes place after the domains have unfolded and which reduces titin stiffness in cardiac and skeletal muscle. Indirect effects of oxidative stress on titin can occur via reversible modifications of protein kinase signalling pathways (especially the NO-cGMP-PKG axis), which alter the phosphorylation level of certain disordered titin domains and thereby modulate titin stiffness. Oxidative stress also activates proteases such as matrix-metalloproteinase-2 and (indirectly via increasing the intracellular calcium level) calpain-1, both of which cleave titin to irreversibly reduce titin-based stiffness. Although some of these mechanisms require confirmation in the in vivo setting, there is evidence that oxidative stress-related modifications of titin are relevant in the context of biomarker design and represent potential targets for therapeutic intervention in some forms of muscle and heart disease.

Redox-sensitive residue in the actin-binding interface of myosin

We have examined the chemical and functional reversibility of oxidative modification in myosin. Redox regulation has emerged as a crucial modulator of protein function, with particular relevance to aging. We previously identified a single methionine residue in Dictyostelium discoideum (Dicty) myosin II (M394, near the myosin cardiomyopathy loop in the actin-binding interface) that is functionally sensitive to oxidation. We now show that oxidation of M394 is reversible by methionine sulfoxide reductase (Msr), restoring actin-activated ATPase activity. Sequence alignment reveals that M394 of Dicty myosin II is a cysteine residue in all human isoforms of skeletal and cardiac myosin. Using Dicty myosin II as a model for site-specific redox sensitivity of this Cys residue, the M394C mutant can be glutathionylated in vitro, resulting in reversible inhibition of actin-activated ATPase activity, with effects similar to those of methionine oxidation at this site. This work illustrates the potential for myosin to function as a redox sensor in both non-muscle and muscle cells, modulating motility/contractility in response to oxidative stress.

Protein glutathionylation in cardiovascular diseases

The perturbation of thiol-disulfide homeostasis is an important consequence of many diseases, with redox signals implicated in several physio-pathological processes. A prevalent form of cysteine modification is the reversible formation of protein mixed disulfides with glutathione (S-glutathionylation). The abundance of glutathione in cells and the ready conversion of sulfenic acids to S-glutathione mixed disulfides supports the reversible protein S-glutathionylation as a common feature of redox signal transduction, able to regulate the activities of several redox sensitive proteins. In particular, protein S-glutathionylation is emerging as a critical signaling mechanism in cardiovascular diseases, because it regulates numerous physiological processes involved in cardiovascular homeostasis, including myocyte contraction, oxidative phosphorylation, protein synthesis, vasodilation, glycolytic metabolism and response to insulin. Thus, perturbations in protein glutathionylation status may contribute to the etiology of many cardiovascular diseases, such as myocardial infarction, cardiac hypertrophy and atherosclerosis. Various reports show the importance of oxidative cysteine modifications in modulating cardiovascular function. In this review, we illustrate tools and strategies to monitor protein S-glutathionylation and describe the proteins so far identified as glutathionylated in myocardial contraction, hypertrophy and inflammation.

Oxidative stress and sarcomeric proteins

Oxidative stress accompanies a wide spectrum of clinically important cardiac disorders, including ischemia/reperfusion, diabetes mellitus, and hypertensive heart disease. Although reactive oxygen species (ROS) can activate signaling pathways that contribute to ischemic preconditioning and cardioprotection, high levels of ROS induce structural modifications of the sarcomere that impact on pump function and the pathogenesis of heart failure. However, the precise nature of the redox-dependent change in contractility is determined by the source/identity of the oxidant species, the level of oxidative stress, and the chemistry/position of oxidant-induced posttranslational modifications on individual proteins within the sarcomere. This review focuses on various ROS-induced posttranslational modifications of myofilament proteins (including direct oxidative modifications of myofilament proteins, myofilament protein phosphorylation by ROS-activated signaling enzymes, and myofilament protein cleavage by ROS-activated proteases) that have been implicated in the control of cardiac contractility.

Redox Regulation of Nonmuscle Myosin Heavy Chain during Integrin Engagement

On the basis of our findings reporting that cell adhesion induces the generation of reactive oxygen species (ROS) after integrin engagement, we were interested in identifying redox-regulated proteins during this process. Mass spectrometry analysis led us to identify nonmuscle myosin heavy chain (nmMHC) as a target of ROS. Our results show that, while nmMHC is reduced in detached/rounded cells, it turns towards an oxidized state in adherent/spread cells due to the integrin-engaged ROS machinery. The functional role of nmMHC redox regulation is suggested by the redox sensitivity of its association with actin, suggesting a role of nmMHC oxidation in cytoskeleton movement. Analysis of muscle MHC (mMHC) redox state during muscle differentiation, a process linked to a great and stable decrease of ROS content, shows that the protein does not undergo a redox control. Hence, we propose that the redox regulation of MHC in nonprofessional muscle cells is mandatory for actin binding during dynamic cytoskeleton rearrangement, but it is dispensable for static and highly organized cytoskeletal contractile architecture in differentiating myotubes.

Myocardial infarction in mice alters sarcomeric function via post-translational protein modification

Myocardial physiology in the aftermath of myocardial infarction (MI) before remodeling is an under-explored area of investigation. Here, we describe the effects of MI on the cardiac sarcomere with focus on the possible contributions of reactive oxygen species. We surgically induced MI in 6-7-month-old female CD1 mice by ligation of the left anterior descending coronary artery. Data were collected 3-4 days after MI or sham (SH) surgery. MI hearts demonstrated ventricular dilatation and systolic dysfunction upon echo cardiographic analysis. Sub-maximum Ca-activated tension in detergent-extracted fiber bundles from papillary muscles increased significantly in the preparations from MI hearts. Ca(2+) sensitivity increased after MI, whereas cooperativity of activation decreased. To assess myosin enzymatic integrity we measured splitting of Ca-ATP in myofibrillar preparations, which demonstrated a decline in Ca-ATPase activity of myofilament myosin. Biochemical analysis demonstrated post-translational modification of sarcomeric proteins. Phosphorylation of cardiac troponin I and myosin light chain 2 was reduced after MI in papillary samples, as measured using a phospho-specific stain. Tropomyosin was oxidized after MI, forming disulfide products detectable by diagonal non-reducing-reducing SDS-PAGE. Our analysis of myocardial protein oxidation post-MI also demonstrated increased S-glutathionylation. We functionally linked protein oxidation with sarcomere function by treating skinned fibers with the sulfhydryl reducing agent dithiothreitol, which reduced Ca(2+) sensitivity in MI, but not SH, samples. Our data indicate important structural and functional alterations to the cardiac sarcomere after MI, and the contribution of protein oxidation to this process.

Structural and functional impact of site-directed methionine oxidation in myosin

We have examined the structural and functional effects of site-directed methionine oxidation in Dictyostelium (Dicty) myosin II using mutagenesis, in vitro oxidation, and site-directed spin-labeling for electron paramagnetic resonance (EPR). Protein oxidation by reactive oxygen and nitrogen species is critical for normal cellular function, but oxidative stress has been implicated in disease progression and biological aging. Our goal is to bridge understanding of protein oxidation and muscle dysfunction with molecular-level insights into actomyosin interaction. In order to focus on methionine oxidation and to facilitate site-directed spectroscopy, we started with a Cys-lite version of Dicty myosin II. For Dicty myosin containing native methionines, peroxide treatment decreased actin-activated myosin ATPase activity, consistent with the decline in actomyosin function previously observed in biologically aged or peroxide-treated muscle. Methionine-to-leucine mutations, used to protect specific sites from oxidation, identified a single methionine that is functionally sensitive to oxidation: M394, near the myosin cardiomyopathy loop in the actin-binding interface. Previously characterized myosin labeling sites for spectroscopy in the force-producing region and actin-binding cleft were examined; spin-label mobility and distance measurements in the actin-binding cleft were sensitive to oxidation, but particularly in the presence of actin. Overall secondary structure and thermal stability were unaffected by oxidation. We conclude that the oxidation-induced structural change in myosin includes a redistribution of existing structural states of the actin-binding cleft. These results will be applicable to the many biological and therapeutic contexts in which a detailed understanding of protein oxidation as well as function and structure relationships is sought.

Reversible and irreversible protein glutathionylation: biological and clinical aspects

INTRODUCTION

Depending in part on the glutathione:glutathione disulfide ratio, reversible protein glutathionylation to a mixed disulfide may occur. Reversible glutathionylation is important in protecting proteins against oxidative stress, guiding correct protein folding, regulating protein activity and modulating proteins critical to redox signaling. The potential also exists for irreversible protein glutathionylation via Michael addition of an -SH group to a dehydroalanyl residue, resulting in formation of a stable, non-reducible thioether linkage.

AREAS COVERED

This article reviews factors contributing to reversible and irreversible protein glutathionylation and their biomedical implications. It also examines the possibility that certain drugs such as busulfan may be toxic by promoting irreversible glutathionylation. The reader will gain an appreciation of the protective nature and control of function resulting from reversible protein glutathionylation. The reader is also introduced to the recently identified phenomenon of irreversible protein glutathionylation and its possible deleterious effects.

EXPERT OPINION

The process of reversible protein glutathionylation is now well established but these findings need to be substantiated at the tissue and organ levels, and also with disease state. That being said, irreversible protein glutathionylation can also occur and this has implications in disease and aging. Toxicologists should consider this when evaluating the possible side effects of certain drugs such as busulfan that may generate a glutathionylating species in vivo.

Cys-141 glutathionylation of human p53: Studies using specific polyclonal antibodies in cancer samples and cell lines

Previously, we reported that human p53 is functionally inactivated by S-glutathionylation at Cys-141 during oxidative and DNA-damaging treatments. Here, we describe the presence of thiolated p53 and the dynamic nature of this modification in human tissues using unique and specific polyclonal antibodies raised against a 12-residue p53 peptide bearing a mixed disulfide at Cys-141. The affinity- purified antibodies (glut-p53) were sequence-specific in that they recognized the antigenic peptide but not the unthiolated peptide or a scrambled glutathionylated peptide in ELISAs. On immunoblots, the purified antibodies did not react with native p53 or recombinant p53 (rp53), but readily detected the glutathionylated or cysteinylated or ethanethiol-treated rp53 only under nonreducing conditions. Untreated HCT116 cells showed low levels of glut-p53, which increased markedly after H(2)O(2), diamide, cisplatin, and doxorubicin treatments. Glut-p53 levels decreased sharply after cells were passed into oxidant-free medium, suggesting efficient dethiolation. The mutant p53 present in HT29 and T47D human cancer cells was also recognized. In vitro, the glut-p53 was rapidly degraded by rabbit reticulocyte lysates. Human prostate and prostate cancer tissues showed an abundant presence of glut-p53 in luminal epithelium, a site well known to generate ROS. Melanoma and colon cancer samples were also positive for glut-p53. The availability of the thiolation-specific antibodies should enhance our knowledge of p53 regulation in redox-perturbed states found in various diseases including cancer.