Mechanisms of myosin II force generation: insights from novel experimental techniques and approaches

Myosin II is a molecular motor that converts chemical energy derived from ATP hydrolysis into mechanical work. Myosin II isoforms are responsible for muscle contraction and a range of cell functions relying on the development of force and motion. When the motor attaches to actin, ATP is hydrolyzed and inorganic phosphate (Pi) and ADP are released from its active site. These reactions are coordinated with changes in the structure of myosin, promoting the so-called "power stroke" that causes the sliding of actin filaments. The general features of the myosin-actin interactions are well accepted, but there are critical issues that remain poorly understood, mostly due to technological limitations. In recent years, there has been a significant advance in structural, biochemical, and mechanical methods that have advanced the field considerably. New modeling approaches have also allowed researchers to understand actomyosin interactions at different levels of analysis. This paper reviews recent studies looking into the interaction between myosin II and actin filaments, which leads to power stroke and force generation. It reviews studies conducted with single myosin molecules, myosins working in filaments, muscle sarcomeres, myofibrils, and fibers. It also reviews the mathematical models that have been used to understand the mechanics of myosin II in approaches focusing on single molecules to ensembles. Finally, it includes brief sections on translational aspects, how changes in the myosin motor by mutations and/or posttranslational modifications may cause detrimental effects in diseases and aging, among other conditions, and how myosin II has become an emerging drug target.

Urinary titin as an early biomarker of skeletal muscle proteolysis and atrophy in various catabolic conditions

Skeletal muscle atrophy impairs quality of life and increases the risk of disease, but current methods for assessment of muscle mass have several limitations. We here investigated the urinary concentration of a fragment of the muscle protein titin as a potential biomarker for the early detection of skeletal muscle atrophy. Four mouse models with different atrophy pathways were studied: those of cardiotoxin-induced acute muscle injury, cast-induced muscle immobilization, lipopolysaccharide-induced sepsis, and streptozotocin-induced diabetes. In all four models, urinary titin levels increased early, concurrent with or preceding upregulation of the atrophy-related genes for atrogin-1 and MuRF-1. The increase in the urinary titin concentration was thus associated with initial muscle damage and the onset of proteolysis, rather than with late-stage muscle wasting. Our findings suggest that urinary titin is a promising biomarker for detection of the onset of skeletal muscle catabolism and prediction of the subsequent development of atrophy in different catabolic states. Noninvasive measurement of urinary titin may therefore allow the earlier detection of skeletal muscle proteolysis compared with conventional techniques.

Impact of lengthening velocity on the generation of eccentric force by slow-twitch muscle fibers in long stretches

After an initial increase, isovelocity elongation of a muscle fiber can lead to diminishing (referred to as Give in the literature) and subsequently increasing force. How the stretch velocity affects this behavior in slow-twitch fibers remains largely unexplored. Here, we stretched fully activated individual rat soleus muscle fibers from 0.85 to 1.3 optimal fiber length at stretch velocities of 0.01, 0.1, and 1 maximum shortening velocity, vmax, and compared the results with those of rat EDL fast-twitch fibers obtained in similar experimental conditions. In soleus muscle fibers, Give was 7%, 18%, and 44% of maximum isometric force for 0.01, 0.1, and 1 vmax, respectively. As in EDL fibers, the force increased nearly linearly in the second half of the stretch, although the number of crossbridges decreased, and its slope increased with stretch velocity. Our findings are consistent with the concept of a forceful detachment and subsequent crossbridge reattachment in the stretch's first phase and a strong viscoelastic titin contribution to fiber force in the second phase of the stretch. Interestingly, we found interaction effects of stretch velocity and fiber type on force parameters in both stretch phases, hinting at fiber type-specific differences in crossbridge and titin contributions to eccentric force. Whether fiber type-specific combined XB and non-XB models can explain these effects or if they hint at some not fully understood properties of muscle contraction remains to be shown. These results may stimulate new optimization perspectives in sports training and provide a better understanding of structure-function relations of muscle proteins.

Ventilator-induced diaphragm dysfunction: phenomenology and mechanism(s) of pathogenesis

Mechanical ventilation (MV) is used to support ventilation and pulmonary gas exchange in patients during critical illness and surgery. Although MV is a life-saving intervention for patients in respiratory failure, an unintended side-effect of MV is the rapid development of diaphragmatic atrophy and contractile dysfunction. This MV-induced diaphragmatic weakness is labelled as 'ventilator-induced diaphragm dysfunction' (VIDD). VIDD is an important clinical problem because diaphragmatic weakness is a risk factor for the failure to wean patients from MV. Indeed, the inability to remove patients from ventilator support results in prolonged hospitalization and increased morbidity and mortality. The pathogenesis of VIDD has been extensively investigated, revealing that increased mitochondrial production of reactive oxygen species within diaphragm muscle fibres promotes a cascade of redox-regulated signalling events leading to both accelerated proteolysis and depressed protein synthesis. Together, these events promote the rapid development of diaphragmatic atrophy and contractile dysfunction. This review highlights the MV-induced changes in the structure/function of diaphragm muscle and discusses the cell-signalling mechanisms responsible for the pathogenesis of VIDD. This report concludes with a discussion of potential therapeutic opportunities to prevent VIDD and suggestions for future research in this exciting field.

Association of Titin Polymorphisms with the Progression of Oral Squamous Cell Carcinoma and Its Clinicopathological Characteristics

This study examined the correlation of titin (TTN) polymorphisms with the sensitivity of oral squamous cell cancer (OSCC) and clinical characteristics. Six TTN SNPs, including rs10497520, rs12463674, rs12465459, rs2042996, rs2244492, and rs2303838, were evaluated in 322 control groups and 606 patients with oral cancer. We then investigated whether the SNP genotypes rs10497520 had associations with clinical pathological categories. Our data showed that the TC + CC genotype of rs10497520 was associated with moderate/poor tumor cell differentiation. The carriers of TTN rs10497520 polymorphic variant "TC + CC" in OSCC patients with cigarette smoking were linked with poor tumor differentiation (p = 0.008). Our results suggest that the TTN SNP rs10497520 is a possible genetic marker for oral cancer patients in the cigarette-smoking population. The TTN rs10497520 polymorphisms may be essential biomarkers to predict the onset and prognosis of oral cancer disease.

Force re-development after shortening reveals a role for titin in stretch-shortening performance enhancement in skinned muscle fibres

Stretch-shortening cycles (SSCs) involve muscle lengthening (eccentric contractions) instantly followed by shortening (concentric contractions). This combination enhances force, work and power output compared with pure shortening contractions, which is known as the SSC effect. Recent evidence indicates both cross-bridge (XB)-based and non-XB-based (e.g. titin) structures contribute to this effect. This study analysed force re-development following SSCs and pure shortening contractions to gain further insight into the roles of XB and non-XB structures regarding the SSC effect. Experiments were conducted on rat soleus muscle fibres (n=16) with different SSC velocities (30%, 60% and 85% of maximum shortening velocity) and constant stretch-shortening magnitudes (18% of optimum length). The XB inhibitor blebbistatin was used to distinguish between XB and non-XB contributions to force generation. The results showed SSCs led to significantly greater [mean±s.d. 1.02±0.15 versus 0.68±0.09 (ΔF/Δt); t62=8.61, P<0.001, d=2.79) and faster (75 ms versus 205 ms; t62=-6.37, P<0.001, d=-1.48) force re-development compared with pure shortening contractions in the control treatment. In the blebbistatin treatment, SSCs still resulted in greater [0.11±0.03 versus 0.06±0.01 (ΔF/Δt); t62=8.00, P<0.001, d=2.24) and faster (3010±1631 versus 7916±3230 ms; t62=-8.00, P<0.001, d=-1.92) force re-development compared with pure shortening contractions. These findings deepen our understanding of the SSC effect, underscoring the involvement of non-XB structures such as titin in modulating force production. This modulation is likely to involve complex mechanosensory coupling from stretch to signal transmission during muscle contraction.

Comparative analysis of phosphorylated proteomes between plerocercoid and adult Spirometra mansoni reveals phosphoproteomic profiles of the medical tapeworm

BACKGROUND

Plerocercoid larvae of the tapeworm Spirometra mansoni can infect both humans and animals, leading to severe parasitic zoonosis worldwide. Despite ongoing research efforts, our understanding of the developmental process of S. mansoni remains inadequate. To better characterize posttranslational regulation associated with parasite growth, development, and reproduction, a comparative phosphoproteomic study was conducted on the plerocercoid and adult stages of S. mansoni.

METHODS

In this study, site-specific phosphoproteomic analysis was conducted via 4D label-free quantitative analysis technology to obtain primary information about the overall phosphorylation status of plerocercoids and adults.

RESULTS

A total of 778 differentially abundant proteins (DAPs) were detected between adults and plerocercoids, of which 704 DAPs were upregulated and only 74 were downregulated. DAPs involved in metabolic activity were upregulated in plerocercoid larvae compared with adults, whereas DAPs associated with binding were upregulated in adults. Gene Ontology (GO) and Kyoto Encyclopedia of Genes (KEGG) analyses indicated that most DAPs involved in signal transduction and environmental information processing pathways were highly active in adults. DAPs upregulated in the plerocercoid group were enriched mainly in metabolic activities. The kinases PKACA, GSK3B, and smMLCK closely interact, suggesting potential active roles in the growth and development of S. mansoni.

CONCLUSIONS

The dataset presented in this study offers a valuable resource for forthcoming research on signaling pathways as well as new insights into functional studies on the molecular mechanisms of S. mansoni.

Differential analysis of ubiquitin-proteomics in skeletal muscle of Duroc pigs and Tibetan fragrant pigs

Understanding the differences in ubiquitination-modified proteins between Duroc pigs and Tibetan fragrant pigs is crucial for comprehending the growth and development of their skeletal muscles. In this study, skeletal muscle samples from 30-day-old Duroc pigs and Tibetan fragrant pigs were collected. Using ubiquitination 4D-Label free quantitative proteomics, we analyzed and identified ubiquitination-modified peptides, screening out 109 differentially expressed ubiquitination-modified peptides. Further enrichment analysis was conducted on the proteins associated with these differential peptides. GO analysis results indicated that the differential genes were primarily enriched in processes such as regulation of protein transport, motor activity, myosin complex, and actin cytoskeleton. KEGG pathway analysis revealed significant enrichment in pathways such as Glycolysis/Gluconeogenesis and Hippo signaling pathway. The differentially expressed key ubiquitinated proteins, including MYL1, MYH3, TNNC2, TNNI1, MYLPF, MYH1, MYH7, TNNT2, TTN, and TNNC1, were further identified. Our analysis demonstrates that these genes play significant roles in skeletal muscle protein synthesis and degradation, providing new insights into the molecular mechanisms of muscle development in Duroc pigs and Tibetan fragrant pigs, and offering theoretical support for breeding improvements in the swine industry.

Levosimendan's Effects on Length-Dependent Activation in Murine Fast-Twitch Skeletal Muscle

Levosimendan's calcium sensitizing effects in heart muscle cells are well established; yet, its potential impact on skeletal muscle cells has not been evidently determined. Despite controversial results, levosimendan is still expected to interact with skeletal muscle through off-target sites (further than troponin C). Adding to this debate, we investigated levosimendan's acute impact on fast-twitch skeletal muscle biomechanics in a length-dependent activation study by submersing single muscle fibres in a levosimendan-supplemented solution. We employed our MyoRobot technology to investigate the calcium sensitivity of skinned single muscle fibres alongside their stress-strain response in the presence or absence of levosimendan (100 µM). While control data are in agreement with the theory of length-dependent activation, levosimendan appears to shift the onset of the 'descending limb' of active force generation to longer sarcomere lengths without notably improving myofibrillar calcium sensitivity. Passive stretches in the presence of levosimendan yielded over twice the amount of enlarged restoration stress and Young's modulus in comparison to control single fibres. Both effects have not been described before and may point towards potential off-target sites of levosimendan.

Translating myosin-binding protein C and titin abnormalities to whole-heart function using a novel calcium-contraction coupling model

Mutations in cardiac myosin-binding protein C (cMyBP-C) or titin may respectively lead to hypertrophic (HCM) or dilated (DCM) cardiomyopathies. The mechanisms leading to these phenotypes remain unclear because of the challenge of translating cellular abnormalities to whole-heart and system function. We developed and validated a novel computer model of calcium-contraction coupling incorporating the role of cMyBP-C and titin based on the key assumptions: 1) tension in the thick filament promotes cross-bridge attachment mechanochemically, 2) with increasing titin tension, more myosin heads are unlocked for attachment, and 3) cMyBP-C suppresses cross-bridge attachment. Simulated stationary calcium-tension curves, isotonic and isometric contractions, and quick release agreed with experimental data. The model predicted that a loss of cMyBP-C function decreases the steepness of the calcium-tension curve, and that more compliant titin decreases the level of passive and active tension and its dependency on sarcomere length. Integrating this cellular model in the CircAdapt model of the human heart and circulation showed that a loss of cMyBP-C function resulted in HCM-like hemodynamics with higher left ventricular end-diastolic pressures and smaller volumes. More compliant titin led to higher diastolic pressures and ventricular dilation, suggesting DCM-like hemodynamics. The novel model of calcium-contraction coupling incorporates the role of cMyBP-C and titin. Its coupling to whole-heart mechanics translates changes in cellular calcium-contraction coupling to changes in cardiac pump and circulatory function and identifies potential mechanisms by which cMyBP-C and titin abnormalities may develop into HCM and DCM phenotypes. This modeling platform may help identify distinct mechanisms underlying clinical phenotypes in cardiac diseases.

Bruno 1/CELF regulates splicing and cytoskeleton dynamics to ensure correct sarcomere assembly in Drosophila flight muscles

Muscles undergo developmental transitions in gene expression and alternative splicing that are necessary to refine sarcomere structure and contractility. CUG-BP and ETR-3-like (CELF) family RNA-binding proteins are important regulators of RNA processing during myogenesis that are misregulated in diseases such as Myotonic Dystrophy Type I (DM1). Here, we report a conserved function for Bruno 1 (Bru1, Arrest), a CELF1/2 family homolog in Drosophila, during early muscle myogenesis. Loss of Bru1 in flight muscles results in disorganization of the actin cytoskeleton leading to aberrant myofiber compaction and defects in pre-myofibril formation. Temporally restricted rescue and RNAi knockdown demonstrate that early cytoskeletal defects interfere with subsequent steps in sarcomere growth and maturation. Early defects are distinct from a later requirement for bru1 to regulate sarcomere assembly dynamics during myofiber maturation. We identify an imbalance in growth in sarcomere length and width during later stages of development as the mechanism driving abnormal radial growth, myofibril fusion, and the formation of hollow myofibrils in bru1 mutant muscle. Molecularly, we characterize a genome-wide transition from immature to mature sarcomere gene isoform expression in flight muscle development that is blocked in bru1 mutants. We further demonstrate that temporally restricted Bru1 rescue can partially alleviate hypercontraction in late pupal and adult stages, but it cannot restore myofiber function or correct structural deficits. Our results reveal the conserved nature of CELF function in regulating cytoskeletal dynamics in muscle development and demonstrate that defective RNA processing due to misexpression of CELF proteins causes wide-reaching structural defects and progressive malfunction of affected muscles that cannot be rescued by late-stage gene replacement.

Myosin-binding protein C regulates the sarcomere lattice and stabilizes the OFF states of myosin heads

Muscle contraction is produced via the interaction of myofilaments and is regulated so that muscle performance matches demand. Myosin-binding protein C (MyBP-C) is a long and flexible protein that is tightly bound to the thick filament at its C-terminal end (MyBP-CC8C10), but may be loosely bound at its middle- and N-terminal end (MyBP-CC1C7) to myosin heads and/or the thin filament. MyBP-C is thought to control muscle contraction via the regulation of myosin motors, as mutations lead to debilitating disease. We use a combination of mechanics and small-angle X-ray diffraction to study the immediate and selective removal of the MyBP-CC1C7 domains of fast MyBP-C in permeabilized skeletal muscle. We show that cleavage leads to alterations in crossbridge kinetics and passive structural signatures of myofilaments that are indicative of a shift of myosin heads towards the ON state, highlighting the importance of MyBP-CC1C7 to myofilament force production and regulation.

Titin copy number variations associated with dominant inherited phenotypes

BACKGROUND

Titinopathies are caused by mutations in the titin gene (TTN). Titin is the largest known human protein; its gene has the longest coding phase with 364 exons. Titinopathies are very complex neuromuscular pathologies due to the variable age of onset of symptoms, the great diversity of pathological and muscular impairment patterns (cardiac, skeletal muscle or mixed) and both autosomal dominant and recessive modes of transmission. Until now, only few CNVs in TTN have been reported without clear genotype-phenotype associations.

METHODS

Our study includes eight families with dominant titinopathies. We performed next-generation sequencing or comparative genomic hybridisation array analyses and found CNVs in the TTN gene. We characterised these CNVs by RNA sequencing (RNAseq) analyses in six patients' muscles and performed genotype-phenotype inheritance association study by combining the clinical and biological data of these eight families.

RESULTS

Seven deletion-type CNVs in the TTN gene were identified among these families. Genotype and RNAseq results showed that five deletions do not alter the reading frame and one is out-of-reading frame. The main phenotype identified was distal myopathy associated with contractures. The analysis of morphological, clinical and genetic data and imaging let us draw new genotype-phenotype associations of titinopathies.

CONCLUSION

Identifying TTN CNVs will further increase diagnostic sensitivity in these complex neuromuscular pathologies. Our cohort of patients enabled us to identify new deletion-type CNVs in the TTN gene, with unexpected autosomal dominant transmission. This is valuable in establishing new genotype-phenotype associations of titinopathies, mainly distal myopathy in most of the patients.

Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Fundamentally, the heart needs to generate sufficient force and power output to dynamically meet the needs of the body. Cardiomyocytes contain specialized structures referred to as sarcomeres that power and regulate contraction. Disruption of sarcomeric function or regulation impairs contractility and leads to cardiomyopathies and heart failure. Basic, translational, and clinical studies have adapted numerous methods to assess cardiac contraction in a variety of pathophysiological contexts. These tools measure aspects of cardiac contraction at different scales ranging from single molecules to whole organisms. Moreover, these studies have revealed new pathogenic mechanisms of heart disease leading to the development of novel therapies targeting contractility. In this review, the authors explore the breadth of tools available for studying cardiac contractile function across scales, discuss their strengths and limitations, highlight new insights into cardiac physiology and pathophysiology, and describe how these insights can be harnessed for therapeutic candidate development and translational.

Regulating Striated Muscle Contraction: Through Thick and Thin

Force generation in striated muscle is primarily controlled by structural changes in the actin-containing thin filaments triggered by an increase in intracellular calcium concentration. However, recent studies have elucidated a new class of regulatory mechanisms, based on the myosin-containing thick filament, that control the strength and speed of contraction by modulating the availability of myosin motors for the interaction with actin. This review summarizes the mechanisms of thin and thick filament activation that regulate the contractility of skeletal and cardiac muscle. A novel dual-filament paradigm of muscle regulation is emerging, in which the dynamics of force generation depends on the coordinated activation of thin and thick filaments. We highlight the interfilament signaling pathways based on titin and myosin-binding protein-C that couple thin and thick filament regulatory mechanisms. This dual-filament regulation mediates the length-dependent activation of cardiac muscle that underlies the control of the cardiac output in each heartbeat.

hsa-miR-548v controls the viscoelastic properties of human cardiomyocytes and improves their relaxation rates

The impairment of left ventricular (LV) diastolic function with an inadequate increase in myocardial relaxation velocity directly results in lower LV compliance, increased LV filling pressures, and heart failure symptoms. The development of agents facilitating the relaxation of human cardiomyocytes requires a better understanding of the underlying regulatory mechanisms. We performed a high-content microscopy-based screening in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) using a library of 2,565 human miRNA mimics and measured relaxation kinetics via high-computing analyses of motion movies. We identified hsa-miR-548v, a primate-specific miRNA, as the miRNA producing the largest increase in relaxation velocities. This positive lusitropic effect was reproduced in engineered cardiac tissues generated with healthy and BRAF T599R mutant hiPSC-CMs and was independent of changes in calcium transients. Consistent with improvements in viscoelastic responses to mechanical stretch, RNA-Seq showed that hsa-miR-548v downregulated multiple targets, especially components of the mechanosensing machinery. The exogenous administration of hsa-miR-548v in hiPSC-CMs notably resulted in a significant reduction of ANKRD1/CARP1 expression and localization at the sarcomeric I-band. This study suggests that the sarcomere I-band is a critical control center regulating the ability of cardiomyocytes to relax and is a target for improving relaxation and diastolic dysfunction.

Molecular insights into titin's A-band

The thick filament-associated A-band region of titin is a highly repetitive component of the titin chain with important scaffolding properties that support thick filament assembly. It also has a demonstrated link to human disease. Despite its functional significance, it remains a largely uncharacterized part of the titin protein. Here, we have performed an analysis of sequence and structure conservation of A-band titin, with emphasis on poly-FnIII tandem components. Specifically, we have applied multi-dimensional sequence pairwise similarity analysis to FnIII domains and complemented this with the crystallographic elucidation of the 3D-structure of the FnIII-triplet A84-A86 from the fourth long super-repeat in the C-zone (C4). Structural models serve here as templates to map sequence conservation onto super-repeat C4, which we show is a prototypical representative of titin's C-zone. This templating identifies positionally conserved residue clusters in C super-repeats with the potential of mediating interactions to thick-filament components. Conservation localizes to two super-repeat positions: Ig domains in position 1 and FnIII domains in position 7. The analysis also allows conclusions to be drawn on the conserved architecture of titin's A-band, as well as revisiting and expanding the evolutionary model of titin's A-band.

Effects of exercise prehabilitation on muscle atrophy and contractile properties in hindlimb-unloaded rats

INTRODUCTION/AIMS

Effective strategies for rapid recovery after surgery are needed. Therefore, we investigated the effects of exercise prehabilitation (EP) and hindlimb unloading (HU) on muscle loss and contractility.

METHODS

Twenty-two Sprague-Dawley rats (12 wk old) were divided into normal control (NCON, n = 5), hindlimb unloading control (HCON, n = 10), and exercise prehabilitation followed by hindlimb unloading (Ex-preH, n = 7) groups. Ex-PreH performed exercise training for 14 days before hindlimb unloading for 14 days. Body composition was evaluated, along with muscle strength and function. The soleus (SOL) and extensor digitorum longus (EDL) muscle contractile properties were analyzed at the whole-muscle level. The titin concentration and myosin heavy chain (MHC) type composition were analyzed.

RESULTS

There were no effects of Ex-preH on total mass, lean mass, or muscle weight. Physical function was significantly higher in the Ex-preH group than in the HCON group (39.5° vs. 35.7°). The SOL twitch force (19.6 vs. 7.1 mN/m2 ) and specific force (107.3 vs. 61.2 mN/m2 ) were greater in Ex-preH group than in HCON group. EDL shortening velocity was higher in Ex-preH group than in HCON group (13.2 vs. 5.0 FL/s). The SOL full-length titin level was higher in Ex-preH group than in HCON group.

DISCUSSION

Exercise prehabilitation did not prevent muscle mass loss followed by muscle wasting, although it minimized the reduction of physical function. Therefore, exercise prehabilitation should be considered for rapid functional recovery after disuse due to surgery and injuries.

Titin UN2A Acts as a Stable, Non-Polymorphic Scaffold in its Binding to CARP

The N2A segment of titin functions as a pivotal hub for signal transduction and interacts with various proteins involved in structural support, chaperone activities, and transcriptional regulation. Notably, the "unique N2A" (UN2A) subdomain has been shown to interact with the stress-regulated cardiac ankyrin repeat protein (CARP), which contributes to the regulation of sarcomeric stiffness. Previously, the UN2A domain's three-dimensional structure was modelled based on its secondary structure content identified by NMR spectroscopy, considering the domain in isolation. In this study, we report experimental long-range distance distributions by electron paramagnetic resonance (EPR) spectroscopy between the three helixes within the UN2A domain linked to the immunoglobulin domain I81 in the presence and absence of CARP. The data confirm the central three-helix bundle fold of UN2A and show that this adopts a compact and stable conformation in absence of CARP. After binding to CARP, no significant conformational change was observed, suggesting that the UN2A domain retains its structure upon binding to CARP thereby, mediating the interaction approximately as a rigid-body.

Titin as a potential novel therapeutic target in colorectal cancer

Colorectal cancer (CRC) is identified as a primary cause of death around the world. The current chemotherapies are not cost-effective. Therefore, finding novel potential therapeutic target is urgent. Titin (TTN) is a muscle protein that is critical in hypertrophic cardiomyopathy. However, its role in CRC is not well understood. The study focused on exploring the possible role of TTN in CRC carcinogenesis. TTN mRNA and protein expression levels presented an obvious downregulation in CRC tissue samples, relative to normal control (p < 0.05). TTN expression significantly correlated with the clinical stage (normal vs. Stage 1, p < 0.05; normal vs. Stage 4, p < 0.05), node metastasis (normal vs. N1, p < 0.05; N1 vs. N2, p < 0.05), histological type (normal vs. adenocarcinoma, p < 0.05), race (Caucasian vs. Asian, p < 0.05; African-American vs. Asian, p < 0.05) and TP53 mutation (normal vs. TP53 mutation, p < 0.05), considering The Cancer Genome Atlas database. However, for patients who had higher TTN expression, the overall survival was remarkably shorter than patients who had low TTN expression. Furthermore, TTN was lowly expressed in four CRC cell lines. TTN overexpression facilitated CRC cells in terms of the proliferation, metastasis and invasion. Based on gene set enrichment analysis, the ERB pathway might be responsible for TTN-related CRC. Besides, TTN was involved in the response to azacitidine. Overall, TTN might serve as a potential novel therapeutic target for treating and overcoming chemotherapy resistance in CRC.

Myosin-binding protein C forms C-links and stabilizes OFF states of myosin

Contraction force in muscle is produced by the interaction of myosin motors in the thick filaments and actin in the thin filaments and is fine-tuned by other proteins such as myosin-binding protein C (MyBP-C). One form of control is through the regulation of myosin heads between an ON and OFF state in passive sarcomeres, which leads to their ability or inability to interact with the thin filaments during contraction, respectively. MyBP-C is a flexible and long protein that is tightly bound to the thick filament at its C-terminal end but may be loosely bound at its middle- and N-terminal end (MyBP-CC1C7). Under considerable debate is whether the MyBP-CC1C7 domains directly regulate myosin head ON/OFF states, and/or link thin filaments ("C-links"). Here, we used a combination of mechanics and small-angle X-ray diffraction to study the immediate and selective removal of the MyBP-CC1C7 domains of fast MyBP-C in permeabilized skeletal muscle. After cleavage, the thin filaments were significantly shorter, a result consistent with direct interactions of MyBP-C with thin filaments thus confirming C-links. Ca2+ sensitivity was reduced at shorter sarcomere lengths, and crossbridge kinetics were increased across sarcomere lengths at submaximal activation levels, demonstrating a role in crossbridge kinetics. Structural signatures of the thick filaments suggest that cleavage also shifted myosin heads towards the ON state - a marker that typically indicates increased Ca2+ sensitivity but that may account for increased crossbridge kinetics at submaximal Ca2+ and/or a change in the force transmission pathway. Taken together, we conclude that MyBP-CC1C7 domains play an important role in contractile performance which helps explain why mutations in these domains often lead to debilitating diseases.

Effects of eccentric contraction on force enhancement in rat fast-twitch muscle

The aim of this study was to elucidate the effects of eccentric contraction (ECC) on force enhancement in rat fast-twitch skeletal muscle. Gastrocnemius (GAS) muscles were subjected to 200 ECCs in situ by electrical stimulation. Immediately before and after the stimulation, isometric torque produced by ankle flexion was measured at an ankle angle of 90°. After the second torque measurement, the superficial regions of the muscles were dissected and subjected to biochemical and skinned fiber analysis. ECC did not induce changes in the amount of degraded titin. After ECC, isometric torques in the GAS muscles were markedly reduced, especially at low stimulation frequency. ECC increased passive torque in whole muscle and passive force in skinned fibers. Passive force enhancement and the ratio of passive force to the maximal Ca2+ -activated force, but not residual force enhancement, were augmented in the skinned fibers subjected to ECC. An ECC-induced increase in titin-based stiffness may contribute to the increased PFE. These results suggest that skeletal muscle is endowed with a force potentiation system that can attenuate ECC-induced force reductions.

Torsional and strain dysfunction precede overt heart failure in a mouse model of dilated cardiomyopathy pathogenesis

Detailed assessments of whole heart mechanics are crucial for understanding the consequences of sarcomere perturbations that lead to cardiomyopathy in mice. Echocardiography offers an accessible and cost-effective method of obtaining metrics of cardiac function, but the most routine imaging and analysis protocols might not identify subtle mechanical deficiencies. This study aims to use advanced echocardiography imaging and analysis techniques to identify previously unappreciated mechanical deficiencies in a mouse model of dilated cardiomyopathy (DCM) before the onset of overt systolic heart failure (HF). Mice lacking muscle LIM protein expression (MLP-/-) were used to model DCM-linked HF pathogenesis. Left ventricular (LV) function of MLP-/- and wild-type (WT) controls were studied at 3, 6, and 10 wk of age using conventional and four-dimensional (4-D) echocardiography, followed by speckle-tracking analysis to assess torsional and strain mechanics. Mice were also studied with RNA-seq. Although 3-wk-old MLP-/- mice showed normal LV ejection fraction (LVEF), these mice displayed abnormal torsional and strain mechanics alongside reduced β-adrenergic reserve. Transcriptome analysis showed that these defects preceded most molecular markers of HF. However, these markers became upregulated as MLP-/- mice aged and developed overt systolic dysfunction. These findings indicate that subtle deficiencies in LV mechanics, undetected by LVEF and conventional molecular markers, may act as pathogenic stimuli in DCM-linked HF. Using these analyses in future studies will further help connect in vitro measurements of the sarcomere function to whole heart function.NEW & NOTEWORTHY A detailed study of how perturbations to sarcomere proteins impact whole heart mechanics in mouse models is a major yet challenging step in furthering our understanding of cardiovascular pathophysiology. This study uses advanced echocardiographic imaging and analysis techniques to reveal previously unappreciated subclinical whole heart mechanical defects in a mouse model of cardiomyopathy. In doing so, it offers an accessible set of measurements for future studies to use when connecting sarcomere and whole heart function.

Nucleus Mechanosensing in Cardiomyocytes

Cardiac muscle contraction is distinct from the contraction of other muscle types. The heart continuously undergoes contraction-relaxation cycles throughout an animal's lifespan. It must respond to constantly varying physical and energetic burdens over the short term on a beat-to-beat basis and relies on different mechanisms over the long term. Muscle contractility is based on actin and myosin interactions that are regulated by cytoplasmic calcium ions. Genetic variants of sarcomeric proteins can lead to the pathophysiological development of cardiac dysfunction. The sarcomere is physically connected to other cytoskeletal components. Actin filaments, microtubules and desmin proteins are responsible for these interactions. Therefore, mechanical as well as biochemical signals from sarcomeric contractions are transmitted to and sensed by other parts of the cardiomyocyte, particularly the nucleus which can respond to these stimuli. Proteins anchored to the nuclear envelope display a broad response which remodels the structure of the nucleus. In this review, we examine the central aspects of mechanotransduction in the cardiomyocyte where the transmission of mechanical signals to the nucleus can result in changes in gene expression and nucleus morphology. The correlation of nucleus sensing and dysfunction of sarcomeric proteins may assist the understanding of a wide range of functional responses in the progress of cardiomyopathic diseases.

Stretch Harmonizes Sarcomere Strain Across the Cardiomyocyte

BACKGROUND

Increasing cardiomyocyte contraction during myocardial stretch serves as the basis for the Frank-Starling mechanism in the heart. However, it remains unclear how this phenomenon occurs regionally within cardiomyocytes, at the level of individual sarcomeres. We investigated sarcomere contractile synchrony and how intersarcomere dynamics contribute to increasing contractility during cell lengthening.

METHODS

Sarcomere strain and Ca2+ were simultaneously recorded in isolated left ventricular cardiomyocytes during 1 Hz field stimulation at 37 °C, at resting length and following stepwise stretch.

RESULTS

We observed that in unstretched rat cardiomyocytes, differential sarcomere deformation occurred during each beat. Specifically, while most sarcomeres shortened during the stimulus, ≈10% to 20% of sarcomeres were stretched or remained stationary. This nonuniform strain was not traced to regional Ca2+ disparities but rather shorter resting lengths and lower force production in systolically stretched sarcomeres. Lengthening of the cell recruited additional shortening sarcomeres, which increased contractile efficiency as less negative, wasted work was performed by stretched sarcomeres. Given the known role of titin in setting sarcomere dimensions, we next hypothesized that modulating titin expression would alter intersarcomere dynamics. Indeed, in cardiomyocytes from mice with titin haploinsufficiency, we observed greater variability in resting sarcomere length, lower recruitment of shortening sarcomeres, and impaired work performance during cell lengthening.

CONCLUSIONS

Graded sarcomere recruitment directs cardiomyocyte work performance, and harmonization of sarcomere strain increases contractility during cell stretch. By setting sarcomere dimensions, titin controls sarcomere recruitment, and its lowered expression in haploinsufficiency mutations impairs cardiomyocyte contractility.

CCND1 Overexpression in Idiopathic Dilated Cardiomyopathy: A Promising Biomarker?

Cardiomyopathy, a disorder of electrical or heart muscle function, represents a type of cardiac muscle failure and culminates in severe heart conditions. The prevalence of dilated cardiomyopathy (DCM) is higher than that of other types (hypertrophic cardiomyopathy and restrictive cardiomyopathy) and causes many deaths. Idiopathic dilated cardiomyopathy (IDCM) is a type of DCM with an unknown underlying cause. This study aims to analyze the gene network of IDCM patients to identify disease biomarkers. Data were first extracted from the Gene Expression Omnibus (GEO) dataset and normalized based on the RMA algorithm (Bioconductor package), and differentially expressed genes were identified. The gene network was mapped on the STRING website, and the data were transferred to Cytoscape software to determine the top 100 genes. In the following, several genes, including VEGFA, IGF1, APP, STAT1, CCND1, MYH10, and MYH11, were selected for clinical studies. Peripheral blood samples were taken from 14 identified IDCM patients and 14 controls. The RT-PCR results revealed no significant differences in the expression of the genes APP, MYH10, and MYH11 between the two groups. By contrast, the STAT1, IGF1, CCND1, and VEGFA genes were overexpressed in patients more than in controls. The highest expression was found for VEGFA, followed by CCND1 (p < 0.001). Overexpression of these genes may contribute to disease progression in patients with IDCM. However, more patients and genes need to be analyzed in order to achieve more robust results.

Changes in passive stiffness and length of the semitendinosus muscles in rats with arthritis-induced knee flexion contracture

BACKGROUND

Arthritis-induced joint contracture is caused by arthrogenic and myogenic factors. The arthrogenic factor, localized within the joint, is naturally accepted as the cause of contracture. However, the detailed mechanisms underlying arthritis-induced myogenic contracture are largely unknown. We aimed to elucidate the mechanisms of arthritis-induced myogenic contracture by examining the muscle mechanical properties.

METHODS

Knee arthritis was induced in rats by injecting complete Freund's adjuvant into the right knees, while the untreated contralateral knees were used as controls. After one or four weeks of injection, passive stiffness, length, and collagen content of the semitendinosus muscles were assessed, along with passive knee extension range of motion.

FINDINGS

After one week of injection, flexion contracture formation was confirmed by a decreased range of motion. Range of motion restriction was partially relieved by myotomy, but still remained even after myotomy, indicating the contribution of both myogenic and arthrogenic factors to contracture formation. After one week of injection, the stiffness of the semitendinosus muscle was significantly higher in the injected side than in the contralateral side. After four weeks of injection, the stiffness of the semitendinosus muscle in the injected side returned to levels comparable to the contralateral side, parallel to partial improvement of flexion contracture. Muscle length and collagen content did not change due to arthritis at both time points.

INTERPRETATION

Our results suggest that increased muscle stiffness, rather than muscle shortening, contributes to myogenic contracture detected during the early stage of arthritis. The increased muscle stiffness cannot be explained by excess collagen.

Stretching the story of titin and muscle function

The discovery of the giant protein titin, also known as connectin, dates almost half a century back. In this review, I recapitulate major advances in the discovery of the titin filaments and the recognition of their properties and function until today. I briefly discuss how our understanding of the layout and interactions of titin in muscle sarcomeres has evolved and review key facts about the titin sequence at the gene (TTN) and protein levels. I also touch upon properties of titin important for the stability of the contractile units and the assembly and maintenance of sarcomeric proteins. The greater part of my discussion centers around the mechanical function of titin in skeletal muscle. I cover milestones of research on titin's role in stretch-dependent passive tension development, recollect the reasons behind the enormous elastic diversity of titin, and provide an update on the molecular mechanisms of titin elasticity, details of which are emerging even now. I reflect on current knowledge of how muscle fibers behave mechanically if titin stiffness is removed and how titin stiffness can be dynamically regulated, such as by posttranslational modifications or calcium binding. Finally, I highlight novel and exciting, but still controversially discussed, insight into the role titin plays in active tension development, such as length-dependent activation and contraction from longer muscle lengths.

Eccentric muscle contractions: from single muscle fibre to whole muscle mechanics

Eccentric muscle loading encompasses several unique features compared to other types of contractions. These features include increased force, work, and performance at decreased oxygen consumption, reduced metabolic cost, improved energy efficiency, as well as decreased muscle activity. This review summarises explanatory approaches to long-standing questions in terms of muscular contraction dynamics and molecular and cellular mechanisms underlying eccentric muscle loading. Moreover, this article intends to underscore the functional link between sarcomeric components, emphasising the fundamental role of titin in skeletal muscle. The giant filament titin reveals versatile functions ranging from sarcomere organisation and maintenance, providing passive tension and elasticity, and operates as a mechanosensory and signalling platform. Structurally, titin consists of a viscoelastic spring segment that allows activation-dependent coupling to actin. This titin-actin interaction can explain linear force increases in active lengthening experiments in biological systems. A three-filament model of skeletal muscle force production (mediated by titin) is supposed to overcome significant deviations between experimental observations and predictions by the classic sliding-filament and cross-bridge theories. Taken together, this review intends to contribute to a more detailed understanding of overall muscle behaviour and force generation-from a microscopic sarcomere level to a macroscopic multi-joint muscle level-impacting muscle modelling, the understanding of muscle function, and disease.

Phosphoproteomics Profiling Defines a Target Landscape of the Basophilic Protein Kinases AKT, S6K, and RSK in Skeletal Myotubes

Phosphorylation-dependent signal transduction plays an important role in regulating the functions and fate of skeletal muscle cells. Central players in the phospho-signaling network are the protein kinases AKT, S6K, and RSK as part of the PI3K-AKT-mTOR-S6K and RAF-MEK-ERK-RSK pathways. However, despite their functional importance, knowledge about their specific targets is incomplete because these kinases share the same basophilic substrate motif RxRxxp[ST]. To address this, we performed a multifaceted quantitative phosphoproteomics study of skeletal myotubes following kinase inhibition. Our data corroborate a cross talk between AKT and RAF, a negative feedback loop of RSK on ERK, and a putative connection between RSK and PI3K signaling. Altogether, we report a kinase target landscape containing 49 so far unknown target sites. AKT, S6K, and RSK phosphorylate numerous proteins involved in muscle development, integrity, and functions, and signaling converges on factors that are central for the skeletal muscle cytoskeleton. Whereas AKT controls insulin signaling and impinges on GTPase signaling, nuclear signaling is characteristic for RSK. Our data further support a role of RSK in glucose metabolism. Shared targets have functions in RNA maturation, stability, and translation, which suggests that these basophilic kinases establish an intricate signaling network to orchestrate and regulate processes involved in translation.

The distinctive mechanical and structural signatures of residual force enhancement in myofibers

In muscle, titin proteins connect myofilaments together and are thought to be critical for contraction, especially during residual force enhancement (RFE) when force is elevated after an active stretch. We investigated titin's function during contraction using small-angle X-ray diffraction to track structural changes before and after 50% titin cleavage and in the RFE-deficient, mdm titin mutant. We report that the RFE state is structurally distinct from pure isometric contractions, with increased thick filament strain and decreased lattice spacing, most likely caused by elevated titin-based forces. Furthermore, no RFE structural state was detected in mdm muscle. We posit that decreased lattice spacing, increased thick filament stiffness, and increased non-crossbridge forces are the major contributors to RFE. We conclude that titin directly contributes to RFE.

A spectrum of clinical severity of recessive titinopathies in prenatal

Variants in TTN are associated with a broad range of clinical phenotypes, from dominant adult-onset dilated cardiomyopathy to recessive infantile-onset myopathy. However, few foetal cases have been reported for multiple reasons. Next-generation sequencing has facilitated the prenatal identification of a growing number of suspected titinopathy variants. We investigated six affected foetuses from three families, completed the intrauterine course of the serial phenotypic spectrum of TTN, and discussed the genotype-phenotype correlations from a broader perspective. The recognizable prenatal feature onset at the second trimester was started with reduced movement, then contracture 3-6 weeks later, followed with/without hydrops, finally at late pregnancy was accompanied with polyhydramnio (major) or oligohydramnios. Two cases with typical arthrogryposis-hydrops sequences identified a meta-only transcript variant c.36203-1G>T. Deleterious transcriptional consequences of the substitution were verified by minigene splicing analysis. Case 3 identified a homozygous splicing variant in the constitutively expressed Z-disc. It presented a milder phenotype than expected, which was presumably saved by the isoform of corons. A summary of the foetal-onset titinopathy cases implied that variants in TTN present with a series of signs and a spectrum of clinical severity, which followed the dosage/positional effect; the meta-only transcript allele involvement may be a prerequisite for the development of fatal hydrops.

A nanobody toolbox to investigate localisation and dynamics of Drosophila titins and other key sarcomeric proteins

Measuring the positions and dynamics of proteins in intact tissues or whole animals is key to understanding protein function. However, to date, this is challenging, as the accessibility of large antibodies to dense tissues is often limited, and fluorescent proteins inserted close to a domain of interest may affect protein function. These complications apply in particular to muscle sarcomeres, arguably one of the most protein-dense assemblies in nature, which complicates studying sarcomere morphogenesis at molecular resolution. Here, we introduce a toolbox of nanobodies recognising various domains of the two Drosophila titin homologs, Sallimus and Projectin, as well as the key sarcomeric proteins Obscurin, α-Actinin, and Zasp52. We verified the superior labelling qualities of our nanobodies in muscle tissue as compared to antibodies. By applying our toolbox to larval muscles, we found a gigantic Sallimus isoform stretching more than 2 µm to bridge the sarcomeric I-band, while Projectin covers almost the entire myosin filaments in a polar orientation. Transgenic expression of tagged nanobodies confirmed their high affinity-binding without affecting target protein function. Finally, adding a degradation signal to anti-Sallimus nanobodies suggested that it is difficult to fully degrade Sallimus in mature sarcomeres; however, expression of these nanobodies caused developmental lethality. These results may inspire the generation of similar toolboxes for other large protein complexes in Drosophila or mammals.

Nanobodies combined with DNA-PAINT super-resolution reveal a staggered titin nanoarchitecture in flight muscles

Sarcomeres are the force-producing units of all striated muscles. Their nanoarchitecture critically depends on the large titin protein, which in vertebrates spans from the sarcomeric Z-disc to the M-band and hence links actin and myosin filaments stably together. This ensures sarcomeric integrity and determines the length of vertebrate sarcomeres. However, the instructive role of titins for sarcomeric architecture outside of vertebrates is not as well understood. Here, we used a series of nanobodies, the Drosophila titin nanobody toolbox, recognising specific domains of the two Drosophila titin homologs Sallimus and Projectin to determine their precise location in intact flight muscles. By combining nanobodies with DNA-PAINT super-resolution microscopy, we found that, similar to vertebrate titin, Sallimus bridges across the flight muscle I-band, whereas Projectin is located at the beginning of the A-band. Interestingly, the ends of both proteins overlap at the I-band/A-band border, revealing a staggered organisation of the two Drosophila titin homologs. This architecture may help to stably anchor Sallimus at the myosin filament and hence ensure efficient force transduction during flight.

Vascular compliance and left ventricular compliance cross talk: Implications for using long-term heat acclimation in cardiac care

1) The first evidence of the beneficial impact of Long-Term-Heat-Acclimation (LTHA) on cardio-vascular compliance was the positive inotropic response and improved left ventricular (LV) compliance noted when isolated hearts from LTHA rats were studied. Human echo study demonstrates that passive HA affects the right ventricle and the atria as well. 2) There is a cross-talk between vascular and cardiac compliance. Vascular compliance per se is defined by central venous pressure-Blood volume relationship-Global Vascular Compliance (GVC). It is determined by the sum of the vascular compliance of the vessels in every organ in any physiological state, varies with LTHA and thus influences cardiac performance. LTHA improves endothelial function, increases NO (nitric oxide) production, in-turn stimulating alterations in ECM (extracellular matrix) via the TGF β1-SMAD pathway. 3) LTHA is associated with transformation from fast to slow myosin, heat acclimation ischemic/hypoxic cross-tolerance and alterations in the extracellular matrix. 4) A human translational study demonstrated improved LV compliance following bypass surgery in LTHA subjects compared to controls. 5) Diastolic dysfunction and the impact of comorbidities with vascular and non- vascular origins are major contributors to the syndrome of heart failure with preserved ejection function (HFPEF). Unfortunately, there is a paucity of treatment modalities that improve diastolic dysfunction. 6) In the current mini-review we suggest that LTHA may be beneficial to HFPEF patients by remodeling cardiac compliance and vascular response.

Cardiac Sarcomere Signaling in Health and Disease

The cardiac sarcomere is a triumph of biological evolution wherein myriad contractile and regulatory proteins assemble into a quasi-crystalline lattice to serve as the central point upon which cardiac muscle contraction occurs. This review focuses on the many signaling components and mechanisms of regulation that impact cardiac sarcomere function. We highlight the roles of the thick and thin filament, both as necessary structural and regulatory building blocks of the sarcomere as well as targets of functionally impactful modifications. Currently, a new focus emerging in the field is inter-myofilament signaling, and we discuss here the important mediators of this mechanism, including myosin-binding protein C and titin. As the understanding of sarcomere signaling advances, so do the methods with which it is studied. This is reviewed here through discussion of recent live muscle systems in which the sarcomere can be studied under intact, physiologically relevant conditions.

The insect perspective on Z-disc structure and biology

Myofibrils are the intracellular structures formed by actin and myosin filaments. They are paracrystalline contractile cables with unusually well-defined dimensions. The sliding of actin past myosin filaments powers contractions, and the entire system is held in place by a structure called the Z-disc, which anchors the actin filaments. Myosin filaments, in turn, are anchored to another structure called the M-line. Most of the complex architecture of myofibrils can be reduced to studying the Z-disc, and recently, important advances regarding the arrangement and function of Z-discs in insects have been published. On a very small scale, we have detailed protein structure information. At the medium scale, we have cryo-electron microscopy maps, super-resolution microscopy and protein-protein interaction networks, while at the functional scale, phenotypic data are available from precise genetic manipulations. All these data aim to answer how the Z-disc works and how it is assembled. Here, we summarize recent data from insects and explore how it fits into our view of the Z-disc, myofibrils and, ultimately, muscles.

Eccentric muscle contraction potentiates titin stiffness-related contractile properties in rat fast-twitch muscles

This study was conducted to examine the effects of an acute bout of eccentric muscle contraction (ECC) on titin stiffness-related contractile properties in rat fast-twitch skeletal muscles. Intact gastrocnemius muscles were electrically stimulated in situ to undergo 200-repeated ECCs. Immediately after cessation of the stimulation, the superficial regions of the muscles were dissected and subjected to biochemical and skinned fiber analyses. Small heat shock protein αB-crystallin in the muscle fraction enriched for myofibrillar proteins was increased by ECC. ECC resulted in an increase in the titin-based passive force. Protein kinase A-treatment decreased the passive force only in ECC-subjected but not in rested fibers. ECC decreased the maximum Ca²⁺-activated force at a sarcomere length (SL) of 2.4 μm and had no effect on myofibrillar-Ca²⁺ sensitivity at 2.6-μm SL. In both rested and ECC-subjected fibers, these two variables were higher at 3.0-μm SL than at 2.4- or 2.6-μm SL. The differences in the two variables between the short and long SLs were greater in ECC-subjected than in rested fibers. These results indicate that an acute bout of ECC potentiates titin-based passive force, maximum active force at long SLs, and length-dependent activation and suggest that this potentiation may resist muscle fatigue in the muscles of the exercising body.

TTN Variants Are Associated with Physical Performance and Provide Potential Markers for Sport-Related Phenotypes

TTN encodes the third myofilament, titin, which plays structural, mechanical, regulatory, and developmental roles in sarcomeres. The aim of this research was to determine the interaction between novel and previously described TTN variants and athletic performance, as well as competition level, in Caucasians. Firstly, 100 athletes and 47 controls were recruited, and whole-genome sequencing was performed. Secondly, 348 athletes (108 endurance, 100 sprint/power, 140 mixed-sport athletes) and 403 volunteers were included, and real-time PCR was performed. We found a significant overrepresentation of the rs10497520 CT and TT genotypes in the sprint/power athlete group (95% CI, 1.41-3.66, p = 0.0013). The rs10497520 T carriers were 2.17 times more likely to become sprint/power athletes (95% CI 1.35-3.49, p = 0.0021). We also found that the likelihood of having the TT genotype was higher for the highly elite and sub-elite sprint/power athletes. Possessing at least one TAA (rs10497520, rs55837610, rs72648256) haplotype resulted in an increase in the log-odds ratio by 0.80 (p = 0.0015), 1.42 (p = 0.003), and 0.77 (p = 0.044) for all, highly elite, and sub-elite sprint/power athletes, respectively. We demonstrated that harbouring the rs10497520 T allele, individually and in a haplotype combination, increased the chance of being an elite sprint/power athlete, indicating that this allele may be favourable for sprint/power performance.

Calpain-dependent degradation of cytoskeletal proteins as a key mechanism for a reduction in intrinsic passive stiffness of unloaded rat postural muscle

In mammals, prolonged mechanical unloading results in a significant decrease in passive stiffness of postural muscles. The nature of this phenomenon remains unclear. The aim of the present study was to investigate possible causes for a reduction in rat soleus passive stiffness after 7 and 14 days of unloading (hindlimb suspension, HS). We hypothesized that HS-induced decrease in passive stiffness would be associated with calpain-dependent degradation of cytoskeletal proteins or a decrease in actomyosin interaction. Wistar rats were subjected to HS for 7 and 14 days with or without PD150606 (calpain inhibitor) treatment. Soleus muscles were subjected to biochemical analysis and ex vivo measurements of passive tension with or without blebbistatin treatment (an inhibitor of actomyosin interactions). Passive tension of isolated soleus muscle was significantly reduced after 7- and 14-day HS compared to the control values. PD150606 treatment during 7- and 14-day HS induced an increase in alpha-actinin-2 and -3, desmin contents compared to control, partly prevented a decrease in intact titin (T1) content, and prevented a decrease in soleus passive tension. Incubation of soleus muscle with blebbistatin did not affect HS-induced reductions in specific passive tension in soleus muscle. Our study suggests that calpain-dependent breakdown of cytoskeletal proteins, but not a change in actomyosin interaction, significantly contributes to unloading-induced reductions in intrinsic passive stiffness of rat soleus muscle.

Tension-driven multi-scale self-organisation in human iPSC-derived muscle fibers

Human muscle is a hierarchically organised tissue with its contractile cells called myofibers packed into large myofiber bundles. Each myofiber contains periodic myofibrils built by hundreds of contractile sarcomeres that generate large mechanical forces. To better understand the mechanisms that coordinate human muscle morphogenesis from tissue to molecular scales, we adopted a simple in vitro system using induced pluripotent stem cell-derived human myogenic precursors. When grown on an unrestricted two-dimensional substrate, developing myofibers spontaneously align and self-organise into higher-order myofiber bundles, which grow and consolidate to stable sizes. Following a transcriptional boost of sarcomeric components, myofibrils assemble into chains of periodic sarcomeres that emerge across the entire myofiber. More efficient myofiber bundling accelerates the speed of sarcomerogenesis suggesting that tension generated by bundling promotes sarcomerogenesis. We tested this hypothesis by directly probing tension and found that tension build-up precedes sarcomere assembly and increases within each assembling myofibril. Furthermore, we found that myofiber ends stably attach to other myofibers using integrin-based attachments and thus myofiber bundling coincides with stable myofiber bundle attachment in vitro. A failure in stable myofiber attachment results in a collapse of the myofibrils. Overall, our results strongly suggest that mechanical tension across sarcomeric components as well as between differentiating myofibers is key to coordinate the multi-scale self-organisation of muscle morphogenesis.

The Diabetic Cardiorenal Nexus

The end-stage of the clinical combination of heart failure and kidney disease has become known as cardiorenal syndrome. Adverse consequences related to diabetes, hyperlipidemia, obesity, hypertension and renal impairment on cardiovascular function, morbidity and mortality are well known. Guidelines for the treatment of these risk factors have led to the improved prognosis of patients with coronary artery disease and reduced ejection fraction. Heart failure hospital admissions and readmission often occur, however, in the presence of metabolic, renal dysfunction and relatively preserved systolic function. In this domain, few advances have been described. Diabetes, kidney and cardiac dysfunction act synergistically to magnify healthcare costs. Current therapy relies on improving hemodynamic factors destructive to both the heart and kidney. We consider that additional hemodynamic solutions may be limited without the use of animal models focusing on the cardiomyocyte, nephron and extracellular matrices. We review herein potential common pathophysiologic targets for treatment to prevent and ameliorate this syndrome.

Titin-related Cardiomyopathy: Is it a Distinct Disease?

PURPOSE OF REVIEW

Truncating TTN variants (TTNtv) are the most common genetic cause of dilated cardiomyopathy (DCM), but the underlying mechanisms are incompletely understood and effective therapeutic strategies are lacking. Here we review recent data that shed new light on the functional consequences of TTNtv and how these effects may vary with mutation location.

RECENT FINDINGS

Whether TTNtv act by haploinsufficiency or dominant negative effects has been hotly debated. New evidence now implicates both mechanisms in TTNtv-related DCM, showing reduced titin content and persistent truncated titin that may be incorporated into protein aggregates. The extent to which aggregate formation and protein quality control defects differ with TTNtv location and contribute to contractile dysfunction is unresolved. TTNtv-associated DCM has a complex etiology that involves varying combinations of wild-type titin deficiency and dominant negative effects of truncated mutant titin. Therapeutic strategies to improve protein handling may be beneficial in some cases.

Exploring the Potential of Symmetric Exon Deletion to Treat Non-Ischemic Dilated Cardiomyopathy by Removing Frameshift Mutations in TTN

Non-ischemic dilated cardiomyopathy (DCM) is one of the most frequent pathologies requiring cardiac transplants. Even though the etiology of this disease is complex, frameshift mutations in the giant sarcomeric protein Titin could explain up to 25% of the familial and 18% of the sporadic cases of DCM. Many studies have shown the potential of genome editing using CRISPR/Cas9 to correct truncating mutations in sarcomeric proteins and have established the grounds for myoediting. However, these therapies are still in an immature state, with only few studies showing an efficient treatment of cardiac diseases. This publication hypothesizes that the Titin (TTN)-specific gene structure allows the application of myoediting approaches in a broad range of locations to reframe TTNtvvariants and to treat DCM patients. Additionally, to pave the way for the generation of efficient myoediting approaches for DCM, we screened and selected promising target locations in TTN. We conceptually explored the deletion of symmetric exons as a therapeutic approach to restore TTN’s reading frame in cases of frameshift mutations. We identified a set of 94 potential candidate exons of TTN that we consider particularly suitable for this therapeutic deletion. With this study, we aim to contribute to the development of new therapies to efficiently treat titinopathies and other diseases caused by mutations in genes encoding proteins with modular structures, e.g., Obscurin.

RBM20, a Therapeutic Target to Alleviate Myocardial Stiffness via Titin Isoforms Switching in HFpEF

Increased myocardial stiffness is critically involved in heart diseases with impaired cardiac compliance, especially heart failure with preserved ejection fraction (HFpEF). Myocardial stiffness mainly derives from cardiomyocyte- and extracellular matrix (ECM)-derived passive stiffness. Titin, a major component of sarcomeres, participates in myocardial passive stiffness and stress-sensitive signaling. The ratio of two titin isoforms, N2BA to N2B, was validated to influence diastolic dysfunction via several pathways. RNA binding motif protein 20 (RBM20) is a well-studied splicing factor of titin, functional deficiency of RBM20 in mice profile improved cardiac compliance and function, which indicated that RBM20 functions as a potential therapeutic target for mitigating myocardial stiffness by modulating titin isoforms. This minor review summarized how RBM20 and other splicing factors modify the titin isoforms ratio, therefore providing a promising target for improving the myocardial compliance of HFpEF.

How velocity impacts eccentric force generation of fully activated skinned skeletal muscle fibers in long stretches

Eccentric muscle contractions are fundamental to everyday life. They occur markedly in jumping, running, and accidents. Following an initial force rise, stretching of a fully activated muscle can result in a phase of decreasing force ('Give') followed by force redevelopment. However, how the stretch velocity affects 'Give' and force redevelopment remains largely unknown. We investigated the force produced by fully activated single skinned fibers of rat extensor digitorum longus muscles during long stretches. Fibers were pulled from length .85 to 1.3 optimal fiber length at a rate of 1, 10 and 100% of the estimated maximum shortening velocity. 'Give' was absent in slow stretches. Medium and fast stretches yielded a clear 'Give'. After the initial force peak, forces decreased by 11.2% and 27.8% relative to the initial peak force before rising again. During the last half of the stretch (from 1.07 to 1.3 optimal fiber length, which is within the range of the expected descending limb of the force-length relationship), the linear force slope tripled from slow to medium stretch and increased further by 60% from medium to fast stretch. These results are compatible with forcible cross-bridge detachment and re-development of a cross-bridge distribution, and a viscoelastic titin contribution to fiber force. Accounting for these results can improve muscle models and predictions of multi-body simulations.

What Can We Learn from Single Sarcomere and Myofibril Preparations?

Sarcomeres are the smallest functional contractile unit of muscle, and myofibrils are striated muscle organelles that are comprised of sarcomeres that are strictly aligned in series. Furthermore, passive forces in sarcomeres and myofibrils are almost exclusively produced by the structural protein titin, and all contractile, regulatory, and structural proteins are in their natural configuration. For these mechanical and structural reasons single sarcomere and myofibril preparations are arguably the most powerful to answer questions on the mechanisms of striated muscle contraction. We developed and optimized single myofibril research over the past 20 years and were the first to mechanically isolate and test single sarcomeres. The results from this research led to the uncovering of the crucial role of titin in muscle contraction, first molecular explanations for the origins of the passive and the residual force enhancement properties of skeletal and cardiac muscles, the discovery of sarcomere length stability on the descending limb of the force-length relationship, and culminating in the formulation of the three-filament theory of muscle contraction that, aside from actin and myosin, proposes a crucial role of titin in active force production. Aside from all the advantages and possibilities that single sarcomere and myofibril preparations offer, there are also disadvantages. These include the fragility of the preparation, the time-consuming training to master these preparations, the limited spatial resolution for length and force measurements, and the unavailability of commercial systems for single sarcomere/myofibril research. Ignoring the mechanics that govern serially linked systems, not considering the spatial resolution and associated accuracies of myofibril systems, and neglecting the fragility of myofibril preparations, has led to erroneous interpretations of results and misleading conclusions. Here, we will attempt to describe the methods and possible applications of single sarcomere/myofibril research and discuss the advantages and disadvantages by focusing on specific applications. It is hoped that this discussion may contribute to identifying the enormous potential of single sarcomere/myofibril research in discovering the secrets of muscle contraction.

HDAC6 modulates myofibril stiffness and diastolic function of the heart

Passive stiffness of the heart is determined largely by extracellular matrix and titin, which functions as a molecular spring within sarcomeres. Titin stiffening is associated with the development of diastolic dysfunction (DD), while augmented titin compliance appears to impair systolic performance in dilated cardiomyopathy. We found that myofibril stiffness was elevated in mice lacking histone deacetylase 6 (HDAC6). Cultured adult murine ventricular myocytes treated with a selective HDAC6 inhibitor also exhibited increased myofibril stiffness. Conversely, HDAC6 overexpression in cardiomyocytes led to decreased myofibril stiffness, as did ex vivo treatment of mouse, rat, and human myofibrils with recombinant HDAC6. Modulation of myofibril stiffness by HDAC6 was dependent on 282 amino acids encompassing a portion of the PEVK element of titin. HDAC6 colocalized with Z-disks, and proteomics analysis suggested that HDAC6 functions as a sarcomeric protein deacetylase. Finally, increased myofibril stiffness in HDAC6-deficient mice was associated with exacerbated DD in response to hypertension or aging. These findings define a role for a deacetylase in the control of myofibril function and myocardial passive stiffness, suggest that reversible acetylation alters titin compliance, and reveal the potential of targeting HDAC6 to manipulate the elastic properties of the heart to treat cardiac diseases.