Search Results (67)

Small-angle X-ray fiber diffraction has informed much of what we know regarding the molecular events during muscle contraction but robust tools for predicting X-ray fiber patterns from muscle have been lacking. A complication in formulating such tools is the dynamic, stochastic nature of the sarcomere structures during contraction where individual myofilaments undergo deformations due to nonuniform strain generated by the myosin crossbridges. Here, we address this need with a “forward problem” approach using a spatially explicit model (MUSICO) to predict the molecular configurations responsible for the observed muscle force and use these configurations to predict the diffraction patterns that can be compared to experiments. We combine this with a newly developed, rigorous formulation, presented here, for the calculation of 2D diffraction patterns from actin filaments under nonuniform strain. We compare all-atom predictions to coarse-grained simulations to show how much information is lost by coarse-graining, and discuss the results in the context of diffraction patterns currently obtainable experimentally. We show that most low-resolution coarse-grained models in the literature suffice for prediction of meridional peak shapes for the purposes of estimating force distributions in the actin filaments, but accurate prediction of layer line intensities require much higher resolution models, including the all-atom models as presented here. These developments represent an important step towards our long-term goal of using molecular simulations to interpret X-ray fiber diffraction patterns from striated muscle during active contraction. Full article

Mutations in genes encoding sarcomeric proteins are a common cause of cardiomyopathy and sudden cardiac death in humans. We evaluated the hypothesis that myofilament dysfunction is coupled to morphological and functional alterations of cardiomyocyte nuclei in a Tnnc1-targeted knock-in (Tnnc1-p.A8V) mouse model of hypertrophic cardiomyopathy (HCM). Tnnc1 is the gene that codes for the isoform of the Ca²⁺-regulatory protein troponin C (cTnC) that is expressed in cardiomyocytes and slow skeletal muscle fibers and resides on thin filaments of sarcomeres in those muscles. This pathogenic mutation in a sarcomere gene alters many aspects of cardiomyocyte function, including sarcomere contractility, cytoplasmic Ca²⁺ buffering, and gene expression. Analysis of myocardial histological sections and isolated cardiomyocytes from adult Tnnc1-p.A8V mouse hearts revealed significantly smaller (cross-sectional area and volume) and rounder nuclei compared to those from age-matched, wild-type control mice. Changes in nuclear morphology could not be explained by differences in cardiomyocyte size or ploidy. Isolated wild-type and mutant cardiomyocyte nuclei, which are embedded centrally within myofibrils, undergo compression during contraction of the cardiomyocyte, indicating that during each heartbeat cardiomyocyte nuclei would be mechanically deformed as well as being exposed to elevated cytoplasmic Ca²⁺. Immunoblotting analysis indicated decreased nuclear localization of cardiac troponin C and decreased histone H4 expression in Tnnc1-p.A8V mouse hearts. Next, we investigated the influence of nucleocytoplasmic transport by immunofluorescence microscopy, and we could not confirm nuclear localization of cardiac troponin C in fixed myocardial tissue from adult mice. However, cardiac troponin C could be detected in healthy human-induced pluripotent stem cell-derived cardiomyocyte nuclei. We conclude that pathological myofilament dysfunction due to a pathogenic, cardiomyopathy-associated mutation can be linked to altered protein composition of cardiomyocyte nuclei and aberrant nuclear morphology. Full article

A subset of chondrocytes in various human cartilage tissues, including neoplastic, regenerative, and normal cartilage, expresses α-smooth muscle actin (α-SMA), a protein typically found in smooth muscle cells. These α-SMA-containing chondrocytes, termed myochondrocytes and myochondroblasts, may play important roles in cartilage physiology, regeneration, and structural integrity, particularly in auricular and articular cartilage. This review synthesizes current knowledge regarding the terminology, distribution, and biological significance of these cells across normal, osteoarthritic, transplanted, and neoplastic cartilage. We summarize key findings from immunohistochemical studies using markers such as S-100, α-SMA, and SOX9, along with ultrastructural confirmation of myofilament bundles via electron microscopy. Current evidence suggests that myochondrocytes exhibit enhanced regenerative potential and contribute to matrix remodeling. Furthermore, their presence reflects the inherent cellular heterogeneity of cartilage, potentially arising from transdifferentiation processes involving fibroblasts, mesenchymal stem cells, or chondroblasts. Finally, TGF-β1 and PDGF-BB are identified as a critical modulator of α-SMA expression and chondrocyte phenotype. A deeper understanding of nature and function of myochondrocytes and myochondroblasts may improve interpretations of cartilage pathology and inform strategies for tissue engineering and cartilage repair. This review highlights the need for further investigation into the molecular regulation and functional roles of these cells in both physiological and pathological contexts. Full article

Myosin-binding protein C (MyBP-C) comprises a family of myofilament proteins that maintain sarcomeric structure and regulate actomyosin crossbridge cycling. Pathogenic variants in MYBPC1, the gene encoding the slow skeletal isoform (sMyBP-C), lead to a dominant congenital myopathy, termed Myotrem, characterized by muscle weakness, hypotonia, and a distinctive tremor of myogenic origin, in the absence of neuropathy. However, the molecular mechanism(s) of myogenic tremorgenesis is largely unknown. One potential mechanism is aberrant myofilament stretch activation, which is defined as a delayed increase in force after a rapid stretch. We utilized the Myotrem murine model harboring the pathogenic MYBPC1 E248K variant to test the hypothesis that stretch activation is augmented in permeabilized Myotrem E248K soleus fibers. We found that stretch activation was significantly increased in E248K soleus muscle fibers. Interestingly, once submaximally Ca²⁺ activated, a subpopulation of slow-twitch E248K fibers exhibited spontaneous pulsatile sarcomere oscillations. This pulsing behavior generated a sinusoidal waveform pattern in sarcomere length, which often persisted on a timescale of minutes. These results align with sMyBP-C as key regulator of the synchronous activation of myofilaments by dampening both spontaneous oscillatory activity and stretch-dependent activation. We propose that the presence of sMyBP-C-E248K disrupts this regulation, thereby driving pathogenic myogenic tremors. Full article

In maximally Ca²⁺-activated demembranated fibres from the mammalian skeletal muscle, the depression of the force by lowering the temperature below the physiological level (~35 °C) is explained by the reduction of force in the myosin motor. Instead, cooling is reported to not affect the force per motor in Ca²⁺-activated cardiac trabeculae from the rat ventricle. Here, the mechanism of the cardiac performance depression by cooling is reinvestigated with fast sarcomere-level mechanics. We determine the changes in the half-sarcomere compliance of maximally Ca²⁺-activated demembranated rat trabeculae in the range of temperatures of 10–30 °C and analyse the data in terms of a simplified mechanical model of the half-sarcomere to extract the contribution of myofilaments and myosin motors. We find that the changes in the ensemble force are due to changes in the force per motor, while the fraction of actin-attached motors remains constant independent of temperature. The results demonstrate that in the cardiac myosin, as in the skeletal muscle myosin, the force-generating transition is endothermic. The underlying large heat absorption indicates the interaction of extended hydrophobic surfaces within the myosin motor, like those suggested by the crystallographic model of the working stroke. Full article

Spinal muscular atrophy (SMA) is caused by a deficiency of the ubiquitously expressed survival motor neuron (SMN) protein. The main pathological hallmark of SMA is the degeneration of lower motor neurons (MNs) with subsequent denervation and atrophy of skeletal muscle. However, increasing evidence indicates that low SMN levels not only are detrimental to the central nervous system (CNS) but also directly affect other peripheral tissues and organs, including skeletal muscle. To better understand the potential primary impact of SMN deficiency in muscle, we explored the cellular, ultrastructural, and molecular basis of SMA myopathy in the SMNΔ7 mouse model of severe SMA at an early postnatal period (P0-7) prior to muscle denervation and MN loss (preneurodegenerative [PND] stage). This period contrasts with the neurodegenerative (ND) stage (P8-14), in which MN loss and muscle atrophy occur. At the PND stage, we found that SMN∆7 mice displayed early signs of motor dysfunction with overt myofiber alterations in the absence of atrophy. We provide essential new ultrastructural data on focal and segmental lesions in the myofibrillar contractile apparatus. These lesions were observed in association with specific myonuclear domains and included abnormal accumulations of actin-thin myofilaments, sarcomere disruption, and the formation of minisarcomeres. The sarcoplasmic reticulum and triads also exhibited ultrastructural alterations, suggesting decoupling during the excitation–contraction process. Finally, changes in intermyofibrillar mitochondrial organization and dynamics, indicative of mitochondrial biogenesis overactivation, were also found. Overall, our results demonstrated that SMN deficiency induces early and MN loss-independent alterations in myofibers that essentially contribute to SMA myopathy. This strongly supports the growing body of evidence indicating the existence of intrinsic alterations in the skeletal muscle in SMA and further reinforces the relevance of this peripheral tissue as a key therapeutic target for the disease. Full article

Novel therapies for the treatment of familial dilated cardiomyopathy (DCM) are lacking. Shaping research directions to clinical needs is critical. Triggers for the progression of the disorder commonly occur due to specific gene variants that affect the production of sarcomeric/cytoskeletal proteins. Generally, these variants cause a decrease in tension by the myofilaments, resulting in signaling abnormalities within the micro-environment, which over time result in structural and functional maladaptations, leading to heart failure (HF). Current concepts support the hypothesis that the mutant sarcomere proteins induce a causal depression in the tension-time integral (TTI) of linear preparations of cardiac muscle. However, molecular mechanisms underlying tension generation particularly concerning mutant proteins and their impact on sarcomere molecular signaling are currently controversial. Thus, there is a need for clarification as to how mutant proteins affect sarcomere molecular signaling in the etiology and progression of DCM. A main topic in this controversy is the control of the number of tension-generating myosin heads reacting with the thin filament. One line of investigation proposes that this number is determined by changes in the ratio of myosin heads in a sequestered super-relaxed state (SRX) or in a disordered relaxed state (DRX) poised for force generation upon the Ca²⁺ activation of the thin filament. Contrasting evidence from nanometer–micrometer-scale X-ray diffraction in intact trabeculae indicates that the SRX/DRX states may have a lesser role. Instead, the proposal is that myosin heads are in a basal OFF state in relaxation then transfer to an ON state through a mechano-sensing mechanism induced during early thin filament activation and increasing thick filament strain. Recent evidence about the modulation of these mechanisms by protein phosphorylation has also introduced a need for reconsidering the control of tension. We discuss these mechanisms that lead to different ideas related to how tension is disturbed by levels of mutant sarcomere proteins linked to the expression of gene variants in the complex landscape of DCM. Resolving the various mechanisms and incorporating them into a unified concept is crucial for gaining a comprehensive understanding of DCM. This deeper understanding is not only important for diagnosis and treatment strategies with small molecules, but also for understanding the reciprocal signaling processes that occur between cardiac myocytes and their micro-environment. By unraveling these complexities, we can pave the way for improved therapeutic interventions for managing DCM. Full article

Protein kinase D (PKD) enzymes play important roles in regulating myocardial contraction, hypertrophy, and remodeling. One of the proteins phosphorylated by PKD is titin, which is involved in myofilament function. In this study, we aimed to investigate the role of PKD in cardiomyocyte function under conditions of oxidative stress. To do this, we used mice with a cardiomyocyte-specific knock-out of Prkd1, which encodes PKD1 (Prkd1^loxP/loxP; ^αMHC-Cre; PKD1 cKO), as well as wild type littermate controls (Prkd1^loxP/loxP; WT). We isolated permeabilized cardiomyocytes from PKD1 cKO mice and found that they exhibited increased passive stiffness (F_passive), which was associated with increased oxidation of titin, but showed no change in titin ubiquitination. Additionally, the PKD1 cKO mice showed increased myofilament calcium (Ca²⁺) sensitivity (pCa₅₀) and reduced maximum Ca²⁺-activated tension. These changes were accompanied by increased oxidation and reduced phosphorylation of the small myofilament protein cardiac myosin binding protein C (cMyBPC), as well as altered phosphorylation levels at different phosphosites in troponin I (TnI). The increased F_passive and pCa₅₀, and the reduced maximum Ca²⁺-activated tension were reversed when we treated the isolated permeabilized cardiomyocytes with reduced glutathione (GSH). This indicated that myofilament protein oxidation contributes to cardiomyocyte dysfunction. Furthermore, the PKD1 cKO mice exhibited increased oxidative stress and increased expression of pro-inflammatory markers interleukin (IL)-6, IL-18, and tumor necrosis factor alpha (TNF-α). Both oxidative stress and inflammation contributed to an increase in microtubule-associated protein 1 light chain 3 (LC3)-II levels and heat shock response by inhibiting the mammalian target of rapamycin (mTOR) in the PKD1 cKO mouse myocytes. These findings revealed a previously unknown role for PKD1 in regulating diastolic passive properties, myofilament Ca²⁺ sensitivity, and maximum Ca²⁺-activated tension under conditions of oxidative stress. Finally, we emphasized the importance of PKD1 in maintaining the balance of oxidative stress and inflammation in the context of autophagy, as well as cardiomyocyte function. Full article

Hypertrophic cardiomyopathy (HCM) is characterized by abnormal growth of the myocardium with myofilament disarray and myocardial hyper-contractility, leading to left ventricular hypertrophy and fibrosis. Where culprit genes are identified, they typically relate to cardiomyocyte sarcomere structure and function. Multi-modality imaging plays a crucial role in the diagnosis, monitoring, and risk stratification of HCM, as well as in screening those at risk. Following the recent publication of the first European Society of Cardiology (ESC) cardiomyopathy guidelines, we build on previous reviews and explore the roles of electrocardiography, echocardiography, cardiac magnetic resonance (CMR), cardiac computed tomography (CT), and nuclear imaging. We examine each modality’s strengths along with their limitations in turn, and discuss how they can be used in isolation, or in combination, to facilitate a personalized approach to patient care, as well as providing key information and robust safety and efficacy evidence within new areas of research. Full article

Heart failure (HF) presents a significant clinical challenge, with current treatments mainly easing symptoms without stopping disease progression. The targeting of calcium (Ca²⁺) regulation is emerging as a key area for innovative HF treatments that could significantly alter disease outcomes and enhance cardiac function. In this review, we aim to explore the implications of altered Ca²⁺ sensitivity, a key determinant of cardiac muscle force, in HF, including its roles during systole and diastole and its association with different HF types—HF with preserved and reduced ejection fraction (HFpEF and HFrEF, respectively). We further highlight the role of the two rate constants k_on (Ca²⁺ binding to Troponin C) and k_off (its dissociation) to fully comprehend how changes in Ca²⁺ sensitivity impact heart function. Additionally, we examine how increased Ca²⁺ sensitivity, while boosting systolic function, also presents diastolic risks, potentially leading to arrhythmias and sudden cardiac death. This suggests that strategies aimed at moderating myofilament Ca²⁺ sensitivity could revolutionize anti-arrhythmic approaches, reshaping the HF treatment landscape. In conclusion, we emphasize the need for precision in therapeutic approaches targeting Ca²⁺ sensitivity and call for comprehensive research into the complex interactions between Ca²⁺ regulation, myofilament sensitivity, and their clinical manifestations in HF. Full article

The maintenance of skeletal muscle mass plays a fundamental role in health and issues associated with quality of life. Mechanical signals are one of the most potent regulators of muscle mass, with a decrease in mechanical loading leading to a decrease in muscle mass. This concept has been supported by a plethora of human- and animal-based studies over the past 100 years and has resulted in the commonly used term of ‘disuse atrophy’. These same studies have also provided a great deal of insight into the structural adaptations that mediate disuse-induced atrophy. For instance, disuse results in radial atrophy of fascicles, and this is driven, at least in part, by radial atrophy of the muscle fibers. However, the ultrastructural adaptations that mediate these changes remain far from defined. Indeed, even the most basic questions, such as whether the radial atrophy of muscle fibers is driven by the radial atrophy of myofibrils and/or myofibril hypoplasia, have yet to be answered. In this review, we thoroughly summarize what is known about the macroscopic, microscopic, and ultrastructural adaptations that mediated disuse-induced atrophy and highlight some of the major gaps in knowledge that need to be filled. Full article

Smooth muscle tissue (SMT) is one of the main structural components of visceral organs, acting as a key factor in the development of adaptive and pathological conditions. Despite the crucial part of SMT in the gastrointestinal tract activity, the mechanisms of its gravisensitivity are still insufficiently studied. The study evaluated the content of smooth muscle actin (α-SMA) in the membranes of the gastric fundus and jejunum in C57BL/6N mice (30-day space flight), in Mongolian gerbils Meriones unguiculatus (12-day orbital flight) and after anti-orthostatic suspension according to E.R. Morey-Holton. A morphometric analysis of α-SMA in the muscularis externa of the stomach and jejunum of mice and Mongolian gerbils from space flight groups revealed a decreased area of the immunopositive regions, a fact indicating a weakening of the SMT functional activity. Gravisensitivity of the contractile structures of the digestive system may be due to changes in the myofilament structural components of the smooth myocytes or myofibroblast actin. A simulated antiorthostatic suspension revealed no significant changes in the content of the α-SMA expression level, a fact supporting an alteration in the functional properties of the muscularis externa of the digestive hollow organs under weightless environment. The data obtained contribute to the novel mechanisms of the SMT contractile apparatus remodeling during orbital flights and can be used to improve preventive measures in space biomedicine. Full article

Synchronized contractions of cardiomyocytes within the heart are tightly coupled to electrical stimulation known as excitation-contraction coupling. Calcium plays a key role in this process and dysregulated calcium handling can significantly impair cardiac function and lead to the development of cardiomyopathies and heart failure. Here, we describe a method and analytical technique to study myofilament-localized calcium signaling using the intensity-based fluorescent biosensor, RGECO-TnT. Dilated cardiomyopathy is a heart muscle disease that negatively impacts the heart’s contractile function following dilatation of the left ventricle. We demonstrate how this biosensor can be used to characterize 2D hiPSC-CMs monolayers generated from a healthy control subject compared to two patients diagnosed with dilated cardiomyopathy. Lastly, we provide a step-by-step guide for single-cell data analysis and describe a custom Transient Analysis application, specifically designed to quantify features of calcium transients. All in all, we explain how this analytical approach can be applied to phenotype hiPSC-CM behaviours and stratify patient responses to identify perturbations in calcium signaling. Full article

The ACTN2 gene encodes α-actinin 2, located in the Z-disc of the sarcomeres in striated muscle. In this study, we sought to investigate the effects of an ACTN2 missense variant of unknown significance (p.A868T) on cardiac muscle structure and function. Left ventricular free wall samples were obtained at the time of cardiac transplantation from a heart failure patient with the ACTN2 A868T heterozygous variant. This variant is in the EF 3–4 domain known to interact with titin and α-actinin. At the ultrastructural level, ACTN2 A868T cardiac samples presented small structural changes in cardiomyocytes when compared to healthy donor samples. However, contractile mechanics of permeabilized ACTN2 A868T variant cardiac tissue displayed higher myofilament Ca²⁺ sensitivity of isometric force, reduced sinusoidal stiffness, and faster rates of tension redevelopment at all Ca²⁺ levels. Small-angle X-ray diffraction indicated increased separation between thick and thin filaments, possibly contributing to changes in muscle kinetics. Molecular dynamics simulations indicated that while the mutation does not significantly impact the structure of α-actinin on its own, it likely alters the conformation associated with titin binding. Our results can be explained by two Z-disc mediated communication pathways: one pathway that involves α-actinin’s interaction with actin, affecting thin filament regulation, and the other pathway that involves α-actinin’s interaction with titin, affecting thick filament activation. This work establishes the role of α-actinin 2 in modulating cross-bridge kinetics and force development in the human myocardium as well as how it can be involved in the development of cardiac disease. Full article

Aerobic exercise training (AET) has been used to manage heart disease. AET may totally or partially restore the activity and/or expression of proteins that regulate calcium (Ca²⁺) handling, optimize intracellular Ca²⁺ flow, and attenuate cardiac functional impairment in failing hearts. However, the literature presents conflicting data regarding the effects of AET on Ca²⁺ transit and cardiac function in rats with heart failure resulting from aortic stenosis (AoS). This study aimed to evaluate the impact of AET on Ca²⁺ handling and cardiac function in rats with heart failure due to AoS. Wistar rats were distributed into two groups: control (Sham; n = 61) and aortic stenosis (AoS; n = 44). After 18 weeks, the groups were redistributed into: non-exposed to exercise training (Sham, n = 28 and AoS, n = 22) and trained (Sham-ET, n = 33 and AoS-ET, n = 22) for 10 weeks. Treadmill exercise training was performed with a velocity equivalent to the lactate threshold. The cardiac function was analyzed by echocardiogram, isolated papillary muscles, and isolated cardiomyocytes. During assays of isolated papillary muscles and isolated cardiomyocytes, the Ca²⁺ concentrations were evaluated. The expression of regulatory proteins for diastolic Ca²⁺ was assessed via Western Blot. AET attenuated the diastolic dysfunction and improved the systolic function. AoS-ET animals presented an enhanced response to post-rest contraction and SERCA2a and L-type Ca²⁺ channel blockage compared to the AoS. Furthermore, AET was able to improve aspects of the mechanical function and the responsiveness of the myofilaments to the Ca²⁺ of the AoS-ET animals. AoS animals presented an alteration in the protein expression of SERCA2a and NCX, and AET restored SERCA2a and NCX levels near normal values. Therefore, AET increased SERCA2a activity and myofilament responsiveness to Ca²⁺ and improved the cellular Ca²⁺ influx mechanism, attenuating cardiac dysfunction at cellular, tissue, and chamber levels in animals with AoS and heart failure. Full article