Dilated cardiomyopathy mutations in δ-sarcoglycan exert a dominant-negative effect on cardiac myocyte mechanical stability

Sarcospan Deficiency Increases Oxidative Stress and Arrhythmias in Hearts after Acute Ischemia-Reperfusion Injury

The protein sarcospan (SSPN) is an integral member of the dystrophin-glycoprotein complex (DGC) and has been shown to be important in the heart during the development and the response to acute stress. In this study, we investigated the role of SSPN in the cardiac response to acute ischemia-reperfusion (IR) injury in SSPN-deficient (SSPN^-/-) mice. First, the hemodynamic response of SSPN^-/- mice was tested and was similar to SSPN^+/+ (wild-type) mice after isoproterenol injection. Using the in situ Langendorff perfusion method, SSPN^-/- hearts were subjected to IR injury and found to have increased infarct size and arrhythmia susceptibility compared to SSPN^+/+. Ca²⁺ handling was assessed in single cardiomyocytes and diastolic Ca²⁺ levels were increased after acute β-AR stimulation in SSPN^+/+ but not SSPN^-/-. It was also found that SSPN^-/- cardiomyocytes had reduced Ca²⁺ SR content compared to SSPN^+/+ but similar SR Ca²⁺ release. Next, we used qRT-PCR to examine gene expression of Ca²⁺ handling proteins after acute IR injury. SSPN^-/- hearts showed a significant decrease in L-type Ca²⁺ channels and a significant increase in Ca²⁺ release channel (RyR2) expression. Interestingly, under oxidizing conditions reminiscent of IR, SSPN^-/- cardiomyocytes, had increased H₂O₂-induced reactive oxygen species production compared to SSPN^+/+. Examination of oxidative stress proteins indicated that NADPH oxidase 4 and oxidized CAMKII were increased in SSPN^-/- hearts after acute IR injury. These results suggest that increased arrhythmia susceptibility in SSPN^-/- hearts post-IR injury may arise from alterations in Ca²⁺ handling and a reduced capacity to regulate oxidative stress pathways.

The mechanical cell - the role of force dependencies in synchronising protein interaction networks

The role of mechanical signals in the proper functioning of organisms is increasingly recognised, and every cell senses physical forces and responds to them. These forces are generated both from outside the cell or via the sophisticated force-generation machinery of the cell, the cytoskeleton. All regions of the cell are connected via mechanical linkages, enabling the whole cell to function as a mechanical system. In this Review, we define some of the key concepts of how this machinery functions, highlighting the critical requirement for mechanosensory proteins, and conceptualise the coupling of mechanical linkages to mechanochemical switches that enables forces to be converted into biological signals. These mechanical couplings provide a mechanism for how mechanical crosstalk might coordinate the entire cell, its neighbours, extending into whole collections of cells, in tissues and in organs, and ultimately in the coordination and operation of entire organisms. Consequently, many diseases manifest through defects in this machinery, which we map onto schematics of the mechanical linkages within a cell. This mapping approach paves the way for the identification of additional linkages between mechanosignalling pathways and so might identify treatments for diseases, where mechanical connections are affected by mutations or where individual force-regulated components are defective.

The role of the dystrophin glycoprotein complex in muscle cell mechanotransduction

Dystrophin is the central protein of the dystrophin-glycoprotein complex (DGC) in skeletal and heart muscle cells. Dystrophin connects the actin cytoskeleton to the extracellular matrix (ECM). Severing the link between the ECM and the intracellular cytoskeleton has a devastating impact on the homeostasis of skeletal muscle cells, leading to a range of muscular dystrophies. In addition, the loss of a functional DGC leads to progressive dilated cardiomyopathy and premature death. Dystrophin functions as a molecular spring and the DGC plays a critical role in maintaining the integrity of the sarcolemma. Additionally, evidence is accumulating, linking the DGC to mechanosignalling, albeit this role is still less understood. This review article aims at providing an up-to-date perspective on the DGC and its role in mechanotransduction. We first discuss the intricate relationship between muscle cell mechanics and function, before examining the recent research for a role of the dystrophin glycoprotein complex in mechanotransduction and maintaining the biomechanical integrity of muscle cells. Finally, we review the current literature to map out how DGC signalling intersects with mechanical signalling pathways to highlight potential future points of intervention, especially with a focus on cardiomyopathies.

A review of the function of the Dystrophic Glycoprotein Complex (DGC) in mechanosignaling provides an overview of the various components of DGC and potential mechanopathogenic mechanisms, particularly as they relate to muscular dystrophy.

Implications of the complex biology and micro-environment of cardiac sarcomeres in the use of high affinity troponin antibodies as serum biomarkers for cardiac disorders

Cardiac troponin I (cTnI), the inhibitory-unit, and cardiac troponin T (cTnT), the tropomyosin-binding unit together with the Ca-binding unit (cTnC) of the hetero-trimeric troponin complex signal activation of the sarcomeres of the adult cardiac myocyte. The unique structure and heart myocyte restricted expression of cTnI and cTnT led to their worldwide use as biomarkers for acute myocardial infarction (AMI) beginning more than 30 years ago. Over these years, high sensitivity antibodies (hs-cTnI and hs-cTnT) have been developed. Together with careful determination of history, physical examination, and EKG, determination of serum levels using hs-cTnI and hs-cTnT permits risk stratification of patients presenting in the Emergency Department (ED) with chest pain. With the ability to determine serum levels of these troponins with high sensitivity came the question of whether such measurements may be of diagnostic and prognostic value in conditions beyond AMI. Moreover, the finding of elevated serum troponins in physiological states such as exercise and pathological states where cardiac myocytes may be affected requires understanding of how troponins may be released into the blood and whether such release may be benign. We consider these questions by relating membrane stability to the complex biology of troponin with emphasis on its sensitivity to the chemo-mechanical and micro-environment of the cardiac myocyte. We also consider the role determinations of serum troponins play in the precise phenotyping in personalized and precision medicine approaches to promote cardiac health.

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Serum levels of cardiac TnI and cardiac TnT permit stratification of patients with chest pain.

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Release of troponins into blood involves not only frank necrosis but also programmed necroptosis.

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Genome wide analysis of serum troponin levels in the general population may be prognostic about cardiovascular health.

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Significant levels of serum troponins with exhaustive exercise may not be benign.

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Troponin in serum can lead to important data related to personalized and precision medicine.

Advances in Stem Cell Modeling of Dystrophin-Associated Disease: Implications for the Wider World of Dilated Cardiomyopathy

Familial dilated cardiomyopathy (DCM) is mostly caused by mutations in genes encoding cytoskeletal and sarcomeric proteins. In the pediatric population, DCM is the predominant type of primitive myocardial disease. A severe form of DCM is associated with mutations in the DMD gene encoding dystrophin, which are the cause of Duchenne Muscular Dystrophy (DMD). DMD-associated cardiomyopathy is still poorly understood and orphan of a specific therapy. In the last 5 years, a rise of interest in disease models using human induced pluripotent stem cells (hiPSCs) has led to more than 50 original studies on DCM models. In this review paper, we provide a comprehensive overview on the advances in DMD cardiomyopathy disease modeling and highlight the most remarkable findings obtained from cardiomyocytes differentiated from hiPSCs of DMD patients. We will also describe how hiPSCs based studies have contributed to the identification of specific myocardial disease mechanisms that may be relevant in the pathogenesis of DCM, representing novel potential therapeutic targets.

Cyclosporine-insensitive mode of cell death after prolonged myocardial ischemia: Evidence for sarcolemmal permeabilization as the pivotal step

A prominent theory of cell death in myocardial ischemia/reperfusion (I/R) posits that the primary and pivotal step of irreversible cell injury is the opening of the mitochondrial permeability transition (MPT) pore. However, the predominantly positive evidence of protection against infarct afforded by the MPT inhibitor, Cyclosporine A (CsA), in experimental studies is in stark contrast with the overall lack of benefit found in clinical trials of CsA. One reason for the discrepancy might be the fact that relatively short experimental ischemic episodes (<1 hour) do not represent clinically-realistic durations, usually exceeding one hour. Here we tested the hypothesis that MPT is not the primary event of cell death after prolonged (60–80 min) episodes of global ischemia. We used confocal microcopy in Langendorff-perfused rabbit hearts treated with the electromechanical uncoupler, 2,3-Butanedione monoxime (BDM, 20 mM) to allow tracking of MPT and sarcolemmal permeabilization (SP) in individual ventricular myocytes. The time of the steepest drop in fluorescence of mitochondrial membrane potential (ΔΨ_m)-sensitive dye, TMRM, was used as the time of MPT (T_MPT). The time of 20% uptake of the normally cell-impermeable dye, YO-PRO1, was used as the time of SP (T_SP). We found that during reperfusion MPT and SP were tightly coupled, with MPT trending slightly ahead of SP (T_SP-T_MPT = 0.76±1.31 min; p = 0.07). These coupled MPT/SP events occurred in discrete myocytes without crossing cell boundaries. CsA (0.2 μM) did not reduce the infarct size, but separated SP and MPT events, such that detectable SP was significantly ahead of MPT (T_SP -T_MPT = -1.75±1.28 min, p = 0.006). Mild permeabilization of cells with digitonin (2.5–20 μM) caused coupled MPT/SP events which occurred in discrete myocytes similar to those observed in Control and CsA groups. In contrast, deliberate induction of MPT by titration with H₂O₂ (200–800 μM), caused propagating waves of MPT which crossed cell boundaries and were uncoupled from SP. Taken together, these findings suggest that after prolonged episodes of ischemia, SP is the primary step in myocyte death, of which MPT is an immediate and unavoidable consequence.

Analysis on sarcoglycans expression as markers of septic cardiomyopathy in sepsis-related death

The post-mortem assessment of sepsis-related death can be carry out by many methods recently suggested as microbiological and biochemical investigations. In these cases, the cause of death is a multiple organ dysfunction due to a dysregulated inflammatory response occurring after the failure of infection control process. It was highlighted also that the heart can be a target organ in sepsis which determines the so-called septic cardiomyopathy characterized by myocardial depression. Several mechanisms to explain the pathophysiology of septic cardiomyopathy were suggested, but very few studies about the structural alterations of cardiac cells responsible for myocardial depression were carried out. The aim of this study was to evaluate whether sarcoglycans (SG) were involved in septic cardiac damage analyzing their expression in sepsis-related deaths and, particularly, if these proteins can be used as markers of septic myocardial dysfunction. Cases of septic-related death confirmed by clinical and autopsy records were investigated and compared to a control group of traumatic deaths. Indirect immunofluorescence analysis was performed to analyze α-SG, β-SG, δ-SG, ζ-SG, ε-SG, and γ-SG. Decrease of fluorescence staining pattern for all tested sarcoglycans was observed in the septic-related deaths compared to normal fluorescence staining pattern of control group. These results provide new findings about the myocytes structural alterations due to sepsis and suggest that these proteins could be used in forensic assessment of septic cardiomyopathy.

Altered membrane integrity in the progression of muscle diseases

Sarcolemmal integrity is orchestrated through the interplay of preserving membrane strength and fast tracking the membrane repair process during an event of compromised membrane fragility. Several molecular players have been identified that act in a concerted fashion to maintain the barrier function of the muscle membrane. Substantial research findings in the field of muscle biology point out the importance of maintaining membrane integrity as a key contributory factor to cellular homeostasis. Innumerable data on the progression of membrane pathology associated with compromised muscle membrane integrity support targeting sarcolemmal integrity in skeletal and cardiac muscle as a model therapeutic strategy to alleviate some of the pathologic conditions. This review will discuss strategies that researchers have undertaken to compensate for an imbalance in sarcolemma membrane fragility and membrane repair to maintain muscle membrane integrity.