Tampering with springs: phosphorylation of titin affecting the mechanical function of cardiomyocytes

Differential sex-dependent susceptibility to diastolic dysfunction and arrhythmia in cardiomyocytes from obese diabetic HFpEF model

AIM

Sex-differences in heart failure with preserved ejection fraction (HFpEF) are important, but key mechanisms involved are incompletely understood. While animal models can inform about sex-dependent cellular and molecular changes, many previous preclinical HFpEF models have failed to recapitulate sex-dependent characteristics of human HFpEF. We tested for sex-differences in HFpEF using a two-hit mouse model (leptin receptor-deficient db/db mice plus aldosterone infusion for 4 weeks; db/db+Aldo).

METHODS AND RESULTS

We performed echocardiography, electrophysiology, intracellular Ca2+ imaging, and protein analysis. Female HFpEF mice exhibited more severe diastolic dysfunction in line with increased titin N2B isoform expression and PEVK element phosphorylation, and reduced troponin-I phosphorylation. Female HFpEF mice had lower BNP levels than males despite similar comorbidity burden (obesity, diabetes) and cardiac hypertrophy in both sexes. Male HFpEF mice were more susceptible to cardiac alternans. Male HFpEF cardiomyocytes (versus female) exhibited higher diastolic [Ca2+], slower Ca2+ transient decay, reduced L-type Ca2+ current, more pronounced enhancement of the late Na+ current, and increased short-term variability of action potential duration (APD). However, male and female HFpEF myocytes showed similar downregulation of inward rectifier and transient outward K+ currents, APD prolongation, and frequency of delayed afterdepolarizations. Inhibition of Ca2+/calmodulin-dependent protein kinase II (CaMKII) reversed all pathological APD changes in HFpEF in both sexes, and empagliflozin pretreatment mimicked these effects of CaMKII inhibition. Vericiguat had only slight benefits, and these effects were larger in HFpEF females.

CONCLUSION

We conclude that the db/db+Aldo preclinical HFpEF murine model recapitulates key sex-specific mechanisms in HFpEF and provides mechanistic insights into impaired excitation-contraction coupling and sex-dependent differential arrhythmia susceptibility in HFpEF with potential therapeutic implications. In male HFpEF myocytes, altered Ca2+ handling and electrophysiology aligned with diastolic dysfunction and arrhythmias, while worse diastolic dysfunction in females may depend more on altered myofilaments properties.

Titin: roles in cardiac function and diseases

The giant protein titin is an essential component of muscle sarcomeres. A single titin molecule spans half a sarcomere and mediates diverse functions along its length by virtue of its unique domains. The A-band of titin functions as a molecular blueprint that defines the length of the thick filaments, the I-band constitutes a molecular spring that determines cell-based passive stiffness, and various domains, including the Z-disk, I-band, and M-line, serve as scaffolds for stretch-sensing signaling pathways that mediate mechanotransduction. This review aims to discuss recent insights into titin's functional roles and their relationship to cardiac function. The role of titin in heart diseases, such as dilated cardiomyopathy and heart failure with preserved ejection fraction, as well as its potential as a therapeutic target, is also discussed.

Structural evidence for elastic tethers connecting separating chromosomes in crane-fly spermatocytes

Different types of anaphase bridges are reported to form between segregating chromosomes during cell division. Previous studies using laser microsurgery suggested that elastic tethers connect the telomeres of separating anaphase chromosomes in many animal meiotic and mitotic cells. However, structural evidence is lacking for their existence. In this study, by correlating live imaging with electron tomography, we examined whether visible structures connect separating telomeres in meiosis I of crane-fly primary spermatocytes. We found structures extending between separating telomeres in all stages of anaphase. The structures consist of two components: one is darkly stained, looking somewhat like chromatin, whereas the other is more lightly stained, appearing filamentous. Although in early anaphase both structures extend between telomeres, in later anaphase, the darker structure extends shorter distances from the telomeres but the lighter structure still extends between the separating telomeres. From these observations, we deduced that these structures represent the "tethers" inferred from the laser-cutting experiments. Because elastic tethers have been detected in a variety of animal cells, they probably are present during anaphase in all animal cells.

Lower heart rates and beta-blockers are associated with new-onset atrial fibrillation

Background

Lower heart rates (HRs) prolong diastole, which increases filling pressures and wall stress. As a result, lower HRs may be associated with higher brain natriuretic peptide (BNP) levels and incident atrial fibrillation (AF). Beta-blockers may increase the risk for AF due to suppression of resting HRs.

Objective

Examine the relationships of HR, BNP, beta-blockers and new-onset AF in the REVEAL-AF and SPRINT cohort of subjects at risk for developing AF.

Methods

In REVEAL-AF, 383 subjects without a history of AF and a mean CHA₂DS₂VASC score of 4.4 ± 1.3 received an insertable cardiac monitor and were followed up to 30 months. In SPRINT, 7595 patients without prior history of AF and a mean CHA₂DS₂VASC score of 2.3 ± 1.2 were followed up to 60 months.

Results

The median daytime HR in the REVEAL-AF cohort was 75bpm [IQR 68-83]. Subjects with below-median HRs had 2.4-fold higher BNP levels compared to subjects with above-median HRs (median BNP [IQR]: 62 pg/dl [37-112] vs. 26 pg/dl [13-53], p < 0.001). HRs <75bpm were associated with a higher incidence of AF: 37% vs. 27%, p < 0.05. This was validated in the SPRINT cohort after adjusting for AF risk factors. Both a HR < 75bpm and beta-blocker use were associated with a higher rate of AF: 1.9 vs 0.7% (p < 0.001) and 2.5% vs. 0.6% (p < 0.001), respectively. The hazard ratio for patients on beta-blockers to develop AF was 3.72 [CI 2.32, 5.96], p < 0.001.

Conclusions

Lower HRs are associated with higher BNP levels and incident AF, mimicking the hemodynamic effects of diastolic dysfunction. Suppression of resting HR by beta-blockers could explain their association with incident AF.

Role of Coxsackievirus B3-Induced Immune Responses in the Transition from Myocarditis to Dilated Cardiomyopathy and Heart Failure

Dilated cardiomyopathy (DCM) is a cardiac disease marked by the stretching and thinning of the heart muscle and impaired left ventricular contractile function. While most patients do not develop significant cardiac diseases from myocarditis, disparate immune responses can affect pathological outcomes, including DCM progression. These altered immune responses, which may be caused by genetic variance, can prolong cytotoxicity, induce direct cleavage of host protein, or encourage atypical wound healing responses that result in tissue scarring and impaired mechanical and electrical heart function. However, it is unclear which alterations within host immune profiles are crucial to dictating the outcomes of myocarditis. Coxsackievirus B3 (CVB3) is a well-studied virus that has been identified as a causal agent of myocarditis in various models, along with other viruses such as adenovirus, parvovirus B19, and SARS-CoV-2. This paper takes CVB3 as a pathogenic example to review the recent advances in understanding virus-induced immune responses and differential gene expression that regulates iron, lipid, and glucose metabolic remodeling, the severity of cardiac tissue damage, and the development of DCM and heart failure.

Alterations in cytoskeletal and Ca2+ cycling regulators in atria lacking the obscurin Ig58/59 module

Introduction

Obscurin (720-870 kDa) is a giant cytoskeletal and signaling protein that possesses both structural and regulatory functions in striated muscles. Immunoglobulin domains 58/59 (Ig58/59) of obscurin bind to a diverse set of proteins that are essential for the proper structure and function of the heart, including giant titin, novex-3, and phospholamban (PLN). Importantly, the pathophysiological significance of the Ig58/59 module has been further underscored by the discovery of several mutations within Ig58/59 that are linked to various forms of myopathy in humans. We previously generated a constitutive deletion mouse model, Obscn-ΔIg58/59, that expresses obscurin lacking Ig58/59, and characterized the effects of this deletion on cardiac morphology and function through aging. Our findings demonstrated that Obscn-ΔIg58/59 male animals develop severe arrhythmia, primarily manifesting as episodes of junctional escape and spontaneous loss of regular p-waves, reminiscent of human atrial fibrillation, accompanied by significant atrial enlargement that progresses in severity with aging.

Methods and Results

To comprehensively characterize the molecular alterations responsible for these pathologies, we performed proteomic and phospho-proteomic analyses in aging Obscn-ΔIg58/59 atria. Our studies revealed extensive and novel alterations in the expression and phosphorylation profile of major cytoskeletal proteins, Ca2+ regulators, and Z-disk associated protein complexes in the Obscn-ΔIg58/59 atria through aging.

Discussion

These studies implicate obscurin, particularly the Ig58/59 module, as an essential regulator of the Z-disk associated cytoskeleton and Ca2+ cycling in the atria and provide new molecular insights into the development of atrial fibrillation and remodeling.

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.

Correcting dilated cardiomyopathy with fibroblast-targeted p38 deficiency

Inherited mutations in contractile and structural genes, which decrease cardiomyocyte tension generation, are principal drivers of dilated cardiomyopathy (DCM)- the leading cause of heart failure ^1,2 . Progress towards developing precision therapeutics for and defining the underlying determinants of DCM has been cardiomyocyte centric with negligible attention directed towards fibroblasts despite their role in regulating the best predictor of DCM severity, cardiac fibrosis ^3,4 . Given that failure to reverse fibrosis is a major limitation of both standard of care and first in class precision therapeutics for DCM, this study examined whether cardiac fibroblast-mediated regulation of the heart's material properties is essential for the DCM phenotype. Here we report in a mouse model of inherited DCM that prior to the onset of fibrosis and dilated myocardial remodeling both the myocardium and extracellular matrix (ECM) stiffen from switches in titin isoform expression, enhanced collagen fiber alignment, and expansion of the cardiac fibroblast population, which we blocked by genetically suppressing p38α in cardiac fibroblasts. This fibroblast-targeted intervention unexpectedly improved the primary cardiomyocyte defect in contractile function and reversed ECM and dilated myocardial remodeling. Together these findings challenge the long-standing paradigm that ECM remodeling is a secondary complication to inherited defects in cardiomyocyte contractile function and instead demonstrate cardiac fibroblasts are essential contributors to the DCM phenotype, thus suggesting DCM-specific therapeutics will require fibroblast-specific strategies.

Signaling network model of cardiomyocyte morphological changes in familial cardiomyopathy

Familial cardiomyopathy is a precursor of heart failure and sudden cardiac death. Over the past several decades, researchers have discovered numerous gene mutations primarily in sarcomeric and cytoskeletal proteins causing two different disease phenotypes: hypertrophic (HCM) and dilated (DCM) cardiomyopathies. However, molecular mechanisms linking genotype to phenotype remain unclear. Here, we employ a systems approach by integrating experimental findings from preclinical studies (e.g., murine data) into a cohesive signaling network to scrutinize genotype to phenotype mechanisms. We developed an HCM/DCM signaling network model utilizing a logic-based differential equations approach and evaluated model performance in predicting experimental data from four contexts (HCM, DCM, pressure overload, and volume overload). The model has an overall prediction accuracy of 83.8%, with higher accuracy in the HCM context (90%) than DCM (75%). Global sensitivity analysis identifies key signaling reactions, with calcium-mediated myofilament force development and calcium-calmodulin kinase signaling ranking the highest. A structural revision analysis indicates potential missing interactions that primarily control calcium regulatory proteins, increasing model prediction accuracy. Combination pharmacotherapy analysis suggests that downregulation of signaling components such as calcium, titin and its associated proteins, growth factor receptors, ERK1/2, and PI3K-AKT could inhibit myocyte growth in HCM. In experiments with patient-specific iPSC-derived cardiomyocytes (MLP-W4R;MYH7-R723C iPSC-CMs), combined inhibition of ERK1/2 and PI3K-AKT rescued the HCM phenotype, as predicted by the model. In DCM, PI3K-AKT-NFAT downregulation combined with upregulation of Ras/ERK1/2 or titin or Gq protein could ameliorate cardiomyocyte morphology. The model results suggest that HCM mutations that increase active force through elevated calcium sensitivity could increase ERK activity and decrease eccentricity through parallel growth factors, Gq-mediated, and titin pathways. Moreover, the model simulated the influence of existing medications on cardiac growth in HCM and DCM contexts. This HCM/DCM signaling model demonstrates utility in investigating genotype to phenotype mechanisms in familial cardiomyopathy.

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.

Increased protein phosphatase 5 expression in inflammation-induced left ventricular dysfunction in rats

BACKGROUND

Titin phosphorylation contributes to left ventricular (LV) diastolic dysfunction. The independent effects of inflammation on the molecular pathways that regulate titin phosphorylation are unclear.

METHODS

We investigated the effects of collagen-induced inflammation and subsequent tumor necrosis factor-α (TNF-α) inhibition on mRNA expression of genes involved in regulating titin phosphorylation in 70 Sprague-Dawley rats. LV diastolic function was assessed with echocardiography. Circulating inflammatory markers were quantified by enzyme-linked immunosorbent assay and relative LV gene expression was assessed by Taqman® polymerase chain reaction. Differences in normally distributed variables between the groups were determined by two-way analysis of variance (ANOVA), followed by Tukey post-hoc tests. For non-normally distributed variables, group differences were determined by Kruskal-Wallis tests.

RESULTS

Collagen inoculation increased LV relative mRNA expression of vascular cell adhesion molecule 1 (VCAM1), pentraxin 3 (PTX3), and inducible nitric oxide synthase (iNOS) compared to controls, indicating local microvascular inflammation. Collagen inoculation decreased soluble guanylate cyclase alpha-2 (sGCα2) and soluble guanylate cyclase beta-2 (sGCβ2) expression, suggesting downregulation of nitric oxide-soluble guanylate cyclase-cyclic guanosine monophosphate (NO-sGC-cGMP) signaling. Inhibiting TNF-α prevented collagen-induced changes in VCAM1, iNOS, sGCα2 and sGCβ2 expression. Collagen inoculation increased protein phosphatase 5 (PP5) expression. Like LV diastolic dysfunction, increased PP5 expression was not prevented by TNF-α inhibition.

CONCLUSION

Inflammation-induced LV diastolic dysfunction may be mediated by a TNF-α-independent increase in PP5 expression and dephosphorylation of the N2-Bus stretch element of titin, rather than by TNF-α-induced downregulation of NO-sGC-cGMP pathway-dependent titin phosphorylation. The steady rise in number of patients with inflammation-induced diastolic dysfunction, coupled with low success rates of current therapies warrants a better understanding of the systemic signals and molecular pathways responsible for decreased titin phosphorylation in development of LV diastolic dysfunction. The therapeutic potential of inhibiting PP5 upregulation in LV diastolic dysfunction requires investigation.

Elamipretide effects on the skeletal muscle phosphoproteome in aged female mice

The age-related decline in skeletal muscle mass and function is known as sarcopenia. Sarcopenia progresses based on complex processes involving protein dynamics, cell signaling, oxidative stress, and repair. We have previously found that 8-week treatment with elamipretide improves skeletal muscle function, reverses redox stress, and restores protein S-glutathionylation changes in aged female mice. This study tested whether 8-week treatment with elamipretide also affects global phosphorylation in skeletal muscle consistent with functional improvements and S-glutathionylation. Using female 6-7-month-old mice and 28-29-month-old mice, we found that phosphorylation changes did not relate to S-glutathionylation modifications, but that treatment with elamipretide did partially reverse age-related changes in protein phosphorylation in mouse skeletal muscle.

Proteomic and phosphoproteomic profiling in heart failure with preserved ejection fraction (HFpEF)

Although the prevalence of heart failure with preserved ejection fraction (HFpEF) is increasing, evidence-based therapies for HFpEF remain limited, likely due to an incomplete understanding of this disease. This study sought to identify the cardiac-specific features of protein and phosphoprotein changes in a murine model of HFpEF using mass spectrometry. HFpEF mice demonstrated moderate hypertension, left ventricle (LV) hypertrophy, lung congestion and diastolic dysfunction. Proteomics analysis of the LV tissue showed that 897 proteins were differentially expressed between HFpEF and Sham mice. We observed abundant changes in sarcomeric proteins, mitochondrial-related proteins, and NAD-dependent protein deacetylase sirtuin-3 (SIRT3). Upregulated pathways by GSEA analysis were related to immune modulation and muscle contraction, while downregulated pathways were predominantly related to mitochondrial metabolism. Western blot analysis validated SIRT3 downregulated cardiac expression in HFpEF vs. Sham (0.8 ± 0.0 vs. 1.0 ± 0.0; P &lt; 0.001). Phosphoproteomics analysis showed that 72 phosphosites were differentially regulated between HFpEF and Sham LV. Aberrant phosphorylation patterns mostly occurred in sarcomere proteins and nuclear-localized proteins associated with contractile dysfunction and cardiac hypertrophy. Seven aberrant phosphosites were observed at the z-disk binding region of titin. Additional agarose gel analysis showed that while total titin cardiac expression remained unaltered, its stiffer N2B isoform was significantly increased in HFpEF vs. Sham (0.144 ± 0.01 vs. 0.127 ± 0.01; P &lt; 0.05). In summary, this study demonstrates marked changes in proteins related to mitochondrial metabolism and the cardiac contractile apparatus in HFpEF. We propose that SIRT3 may play a role in perpetuating these changes and may be a target for drug development in HFpEF.

Current Understanding of Molecular Pathophysiology of Heart Failure With Preserved Ejection Fraction

Heart Failure (HF) is the most common cause of hospitalization in the Western societies. HF is a heterogeneous and complex syndrome that may result from any dysfunction of systolic or diastolic capacity. Abnormal diastolic left ventricular function with impaired relaxation and increased diastolic stiffness is characteristic of heart failure with preserved ejection fraction (HFpEF). HFpEF accounts for more than 50% of all cases of HF. The prevalence increases with age: from around 1% for those aged &lt;55 years to &gt;10% in those aged 70 years or over. Nearly 50% of HF patients have HFrEF and the other 50% have HFpEF/HFmrEF, mainly based on studies in hospitalized patients. The ESC Long-Term Registry, in the outpatient setting, reports that 60% have HFrEF, 24% have HFmrEF, and 16% have HFpEF. To some extent, more than 50% of HF patients are female. HFpEF is closely associated with co-morbidities, age, and gender. Epidemiological evidence suggests that HFpEF is highly represented in older obese women and proposed as ‘obese female HFpEF phenotype’. While HFrEF phenotype is more a male phenotype. In addition, metabolic abnormalities and hemodynamic perturbations in obese HFpEF patients appear to have a greater impact in women then in men (Sorimachi et al., European J of Heart Fail, 2022, 22). To date, numerous clinical trials of HFpEF treatments have produced disappointing results. This outcome suggests that a “one size fits all” approach to HFpEF may be inappropriate and supports the use of tailored, personalized therapeutic strategies with specific treatments for distinct HFpEF phenotypes. The most important mediators of diastolic stiffness are the cardiomyocytes, endothelial cells, and extracellular matrix (ECM). The complex physiological signal transduction networks that respond to the dual challenges of inflammatory and oxidative stress are major factors that promote the development of HFpEF pathologies. These signalling networks contribute to the development of the diseases. Inhibition and/or attenuation of these signalling networks also delays the onset of disease. In this review, we discuss the molecular mechanisms associated with the physiological responses to inflammation and oxidative stress and emphasize the nature of the contribution of most important cells to the development of HFpEF via increased inflammation and oxidative stress.

Ca2+/calmodulin-dependent protein kinase II and protein kinase G oxidation contributes to impaired sarcomeric proteins in hypertrophy model

Aims

Volume overload (VO) induced hypertrophy is one of the hallmarks to the development of heart diseases. Understanding the compensatory mechanisms involved in this process might help preventing the disease progression.

Methods and results

Therefore, the present study used 2 months old Wistar rats, which underwent an aortocaval fistula to develop VO‐induced hypertrophy. The animals were subdivided into four different groups, two sham operated animals served as age‐matched controls and two groups with aortocaval fistula. Echocardiography was performed prior termination after 4‐ and 8‐month. Functional and molecular changes of several sarcomeric proteins and their signalling pathways involved in the regulation and modulation of cardiomyocyte function were investigated.

Results

The model was characterized with preserved ejection fraction in all groups and with elevated heart/body weight ratio, left/right ventricular and atrial weight at 4‐ and 8‐month, which indicates VO‐induced hypertrophy. In addition, 8‐months groups showed increased left ventricular internal diameter during diastole, RV internal diameter, stroke volume and velocity‐time index compared with their age‐matched controls. These changes were accompanied by increased Ca²⁺ sensitivity and titin‐based cardiomyocyte stiffness in 8‐month VO rats compared with other groups. The altered cardiomyocyte mechanics was associated with phosphorylation deficit of sarcomeric proteins cardiac troponin I, myosin binding protein C and titin, also accompanied with impaired signalling pathways involved in phosphorylation of these sarcomeric proteins in 8‐month VO rats compared with age‐matched control group. Impaired protein phosphorylation status and dysregulated signalling pathways were associated with significant alterations in the oxidative status of both kinases CaMKII and PKG explaining by this the elevated Ca²⁺ sensitivity and titin‐based cardiomyocyte stiffness and perhaps the development of hypertrophy.

Conclusions

Our findings showed VO‐induced cardiomyocyte dysfunction via deranged phosphorylation of myofilament proteins and signalling pathways due to increased oxidative state of CaMKII and PKG and this might contribute to the development of hypertrophy.

Identification and characterization of phosphoproteins in the striated and smooth adductor muscles of Yesso scallop Patinopecten yessoensis

Many proteins are known to be phosphorylated, affecting important regulatory factors of muscle quality in the aquatic animals. The striated and smooth adductor muscles of Yesso scallop Patinopecten yessoensis were used to investigate muscle texture and identify phosphoproteins by histological methods and phosphoproteomic analysis. Our present study reveals that muscle fiber density is in relation to meat texture of the striated and smooth adductor muscles. The phosphoproteomic analysis has identified 764 down-regulated and 569 up-regulated phosphosites on 743 phosphoproteins in the smooth muscle compared to the striated part. The identification of unique phosphorylation sites in glycolytic enzymes may increase the activity of glycolytic enzymes and the rate of glycolysis in the striated adductor muscle. The present findings will provide new evidences on the role of muscle structure and protein phosphorylation in scallop muscle quality and thus help to develop strategies for improving meat quality of scallop products.

The ryanodine receptor stabilizer S107 ameliorates contractility of adult Rbm20 knockout rat cardiomyocytes

RNA binding motif 20 (RBM20) cardiomyopathy has been detected in approximately 3% of populations afflicted with dilated cardiomyopathy (DCM). It is well conceived that RBM20 cardiomyopathy is provoked by titin isoform switching in combination with resting Ca2+ leaking. In this study, we characterized the cardiac function in Rbm20 knockout (KO) rats at 3-, 6-, 9-, and 12-months of age and examined the effect of the ryanodine receptor stabilizer S107 on resting intracellular levels and cardiomyocyte contractile properties. Our results revealed that even though Rbm20 depletion promoted expression of larger titin isoform and reduced myocardial stiffness in young rats (3 months of age), the established DCM phenotype required more time to embellish. S107 restored elevated intracellular Ca2+ to normal levels and ameliorated cardiomyocyte contractile properties in isolated cardiomyocytes from 6-month-old Rbm20 KO rats. However, S107 failed to preserve cardiac homeostasis in Rbm20 KO rats at 12 months of age, unexpectedly, likely due to the existence of multiple pathogenic mechanisms. Taken together, our data suggest the therapeutic promises of S107 in the management of RBM20 cardiomyopathy.

Why exercise builds muscles: titin mechanosensing controls skeletal muscle growth under load

Muscles sense internally generated and externally applied forces, responding to these in a coordinated hierarchical manner at different timescales. The center of the basic unit of the muscle, the sarcomeric M-band, is perfectly placed to sense the different types of load to which the muscle is subjected. In particular, the kinase domain of titin (TK) located at the M-band is a known candidate for mechanical signaling. Here, we develop a quantitative mathematical model that describes the kinetics of TK-based mechanosensitive signaling and predicts trophic changes in response to exercise and rehabilitation regimes. First, we build the kinetic model for TK conformational changes under force: opening, phosphorylation, signaling, and autoinhibition. We find that TK opens as a metastable mechanosensitive switch, which naturally produces a much greater signal after high-load resistance exercise than an equally energetically costly endurance effort. Next, for the model to be stable and give coherent predictions, in particular for the lag after the onset of an exercise regime, we have to account for the associated kinetics of phosphate (carried by ATP) and for the nonlinear dependence of protein synthesis rates on muscle fiber size. We suggest that the latter effect may occur via the steric inhibition of ribosome diffusion through the sieve-like myofilament lattice. The full model yields a steady-state solution (homeostasis) for muscle cross-sectional area and tension and, a quantitatively plausible hypertrophic response to training, as well as atrophy after an extended reduction in tension.

The Sarcomeric Spring Protein Titin: Biophysical Properties, Molecular Mechanisms, and Genetic Mutations Associated with Heart Failure and Cardiomyopathy

PURPOSE OF REVIEW

The giant protein titin forms the "elastic" filament of the sarcomere, essential for the mechanical compliance of the heart muscle. Titin serves a biological spring, and therefore structural modifications of titin affect function of the myocardium and are associated with heart failure and cardiomyopathy.

RECENT FINDINGS

In this review, we discuss the current understanding of titin's biophysical properties and how modifications contribute to cardiac function and heart failure. In addition, we review the most recent data on the clinical impact and phenotype heterogeneity of TTN truncating variants, including diseases involving striated muscles, and prospects for future therapies. Because of the giant structure of the titin protein and the complexity of its function, titin's role in health and disease is not yet completely understood. Future research efforts need to focus on novel therapeutic approaches able to modulate titin transcriptional and post-translational modification.

Titin N2A Domain and Its Interactions at the Sarcomere

Titin is a giant protein in the sarcomere that plays an essential role in muscle contraction with actin and myosin filaments. However, its utility goes beyond mechanical functions, extending to versatile and complex roles in sarcomere organization and maintenance, passive force, mechanosensing, and signaling. Titin’s multiple functions are in part attributed to its large size and modular structures that interact with a myriad of protein partners. Among titin’s domains, the N2A element is one of titin’s unique segments that contributes to titin’s functions in compliance, contraction, structural stability, and signaling via protein–protein interactions with actin filament, chaperones, stress-sensing proteins, and proteases. Considering the significance of N2A, this review highlights structural conformations of N2A, its predisposition for protein–protein interactions, and its multiple interacting protein partners that allow the modulation of titin’s biological effects. Lastly, the nature of N2A for interactions with chaperones and proteases is included, presenting it as an important node that impacts titin’s structural and functional integrity.

Blocking Protein Phosphatase 1 [PP1] Prevents Loss of Tether Elasticity in Anaphase Crane-Fly Spermatocytes

In normal anaphase cells, telomeres of each separating chromosome pair are connected to each other by tethers. Tethers are elastic at the start of anaphase: arm fragments cut from anaphase chromosomes in early anaphase move across the equator to the oppositely-moving chromosome, telomere moving toward telomere. Tethers become inelastic later in anaphase as the tethers become longer: arm fragments no longer move to their partners. When early anaphase cells are treated with Calyculin A (CalA), an inhibitor of protein phosphatases 1 (PP1) and 2A (PP2A), at the end of anaphase chromosomes move backward from the poles, with telomeres moving toward partner telomeres. Experiments described herein show that in cells treated with CalA, backwards movements are stopped in a variety of ways, by cutting the tethers of backwards moving chromosomes, by severing arms of backwards moving chromosomes, by severing arms before the chromosomes reach the poles, and by cutting the telomere toward which a chromosome is moving backwards. Measurements of arm-fragment velocities show that CalA prevents tethers from becoming inelastic as they lengthen. Since treatment with CalA causes tethers to remain elastic throughout anaphase and since inhibitors of PP2A do not cause the backwards movements, PP1 activity during anaphase causes the tethers to become inelastic.

The Sliding Filament Theory Since Andrew Huxley: Multiscale and Multidisciplinary Muscle Research

Two groundbreaking papers published in 1954 laid out the theory of the mechanism of muscle contraction based on force-generating interactions between myofilaments in the sarcomere that cause filaments to slide past one another during muscle contraction. The succeeding decades of research in muscle physiology have revealed a unifying interest: to understand the multiscale processes-from atom to organ-that govern muscle function. Such an understanding would have profound consequences for a vast array of applications, from developing new biomimetic technologies to treating heart disease. However, connecting structural and functional properties that are relevant at one spatiotemporal scale to those that are relevant at other scales remains a great challenge. Through a lens of multiscale dynamics, we review in this article current and historical research in muscle physiology sparked by the sliding filament theory.

Sensing and Responding of Cardiomyocytes to Changes of Tissue Stiffness in the Diseased Heart

Cardiomyocytes are permanently exposed to mechanical stimulation due to cardiac contractility. Passive myocardial stiffness is a crucial factor, which defines the physiological ventricular compliance and volume of diastolic filling with blood. Heart diseases often present with increased myocardial stiffness, for instance when fibrotic changes modify the composition of the cardiac extracellular matrix (ECM). Consequently, the ventricle loses its compliance, and the diastolic blood volume is reduced. Recent advances in the field of cardiac mechanobiology revealed that disease-related environmental stiffness changes cause severe alterations in cardiomyocyte cellular behavior and function. Here, we review the molecular mechanotransduction pathways that enable cardiomyocytes to sense stiffness changes and translate those into an altered gene expression. We will also summarize current knowledge about when myocardial stiffness increases in the diseased heart. Sophisticated in vitro studies revealed functional changes, when cardiomyocytes faced a stiffer matrix. Finally, we will highlight recent studies that described modulations of cardiac stiffness and thus myocardial performance in vivo. Mechanobiology research is just at the cusp of systematic investigations related to mechanical changes in the diseased heart but what is known already makes way for new therapeutic approaches in regenerative biology.

Arterial Stiffness in Aging: Does It Have a Place in Clinical Practice?: Recent Advances in Hypertension

Aortic stiffness increases markedly with age and is associated with excess risk for various adverse clinical outcomes, including heart disease, dementia, and kidney disease. Although evidence for adverse effects of aortic stiffening is overwhelming, integration of direct and indirect measures of aortic stiffness into routine clinical assessment has lagged behind the science. This brief review will examine recent evidence supporting the value of stiffness as an important new risk factor for hypertension and cardiovascular disease and will offer suggestions for incorporating stiffness measures into routine clinical practice.

Particulate matter inhalation and the exacerbation of cardiopulmonary toxicity due to metabolic disease

Particulate matter is a significant public health issue in the United States and globally. Inhalation of particulate matter is associated with a number of systemic and organ-specific adverse health outcomes, with the pulmonary and cardiovascular systems being particularly vulnerable. Certain subpopulations are well-recognized as being more susceptible to inhalation exposures, such as the elderly and those with pre-existing respiratory disease. Metabolic syndrome is becoming increasingly prevalent in our society and has known adverse effects on the heart, lungs, and vascular systems. The limited evaluations of individuals with metabolic syndromehave demonstrated that theymay compose a sensitive subpopulation to particulate exposures. However, the toxicological mechanisms responsible for this increased vulnerability are not fully understood. This review evaluates the currently available literature regarding how the response of an individual's pulmonary and cardiovascular systems is influenced by metabolic syndrome and metabolic syndrome-associated conditions such as hypertension, dyslipidemia, and diabetes. Further, we will discuss potential therapeutic agents and targets for the alleviation and treatment of particulate-matter induced metabolic illness. The information reviewed here may contribute to the understanding of metabolic illness as a risk factor for particulate matter exposure and further the development of therapeutic approaches to treat vulnerable subpopulations, such as those with metabolic diseases.

The role of phosphorylation in the elasticity of the tethers that connect telomeres of separating anaphase chromosomes

Elastic tethers, connecting telomeres of all separating anaphase chromosome pairs, lose elasticity when they lengthen during anaphase. Treatment with phosphatase inhibitor CalyculinA causes anaphase chromosomes to move backwards after they reach the poles, suggesting that dephosphorylation causes loss of tether elasticity. We added 50nM CalyculinA to living anaphase crane-fly spermatocytes with different length tethers. When tethers were short, almost all partner chromosomes moved backwards after nearing the poles. When tethers were longer, fewer chromosomes moved backwards. With yet longer tethers none moved backward. This is consistent with tether elasticity being lost by dephosphorylation. 50nM CalyculinA blocks both PP1 and PP2A. To distinguish between PP1 and PP2A we treated cells with short tethers with 50nM okadaic acid which blocks solely PP2A, or with 1µM okadaic acid which blocks both PP1 and PP2A. Only 1µM okadaic acid caused chromosomes to move backward. Thus, tether elasticity is lost because of dephosphorylation by PP1.

Titin-truncating mutations associated with dilated cardiomyopathy alter length-dependent activation and its modulation via phosphorylation

Aims 

Dilated cardiomyopathy (DCM) is associated with mutations in many genes encoding sarcomere proteins. Truncating mutations in the titin gene TTN are the most frequent. Proteomic and functional characterizations are required to elucidate the origin of the disease and the pathogenic mechanisms of TTN-truncating variants.

Methods and results 

We isolated myofibrils from DCM hearts carrying truncating TTN mutations and measured the Ca2+ sensitivity of force and its length dependence. Simultaneous measurement of force and adenosine triphosphate (ATP) consumption in skinned cardiomyocytes was also performed. Phosphorylation levels of troponin I (TnI) and myosin binding protein-C (MyBP-C) were manipulated using protein kinase A and λ phosphatase. mRNA sequencing was employed to overview gene expression profiles. We found that Ca2+ sensitivity of myofibrils carrying TTN mutations was significantly higher than in myofibrils from donor hearts. The length dependence of the Ca2+ sensitivity was absent in DCM myofibrils with TTN-truncating variants. No significant difference was found in the expression level of TTN mRNA between the DCM and donor groups. TTN exon usage and splicing were also similar. However, we identified down-regulation of genes encoding Z-disk proteins, while the atrial-specific regulatory myosin light chain gene, MYL7, was up-regulated in DCM patients with TTN-truncating variants.

Conclusion 

Titin-truncating mutations lead to decreased length-dependent activation and increased elasticity of myofibrils. Phosphorylation levels of TnI and MyBP-C seen in the left ventricles are essential for the length-dependent changes in Ca2+ sensitivity in healthy donors, but they are reduced in DCM patients with TTN-truncating variants. A decrease in expression of Z-disk proteins may explain the observed decrease in myofibril passive stiffness and length-dependent activation.

Cardiac Mechanoenergetics in Patients with Acute Myocardial Infarction: From Pressure-Volume Loop Diagram Related to Cardiac Oxygen Consumption

Acute myocardial infarction (AMI) results in significant changes in cardiac structure and functions, leading to left ventricular remodeling and subsequent systolic and diastolic dysfunction. To improve current approaches in diagnoses, treatments, and prevention of cardiovascular diseases, a better understanding of cardiac mechanoenergetics, including systolic performance and energy demand, becomes paramount. In this review, we summarize cardiac mechanics, cardiac energetics, and their relationship in complications related to AMI using 2 important physiologic frameworks, pressure-volume loops and the Vo₂-pressure-volume area relationship diagram, as they are powerful tools for understanding physiologic behavior and mechanoenergetics of the left ventricle.

Reaction of the Heart to High Frequency Stimulation against the Background of Acute Doxorubicin Treatment

Short-term high frequency electrostimulation (8-10 Hz) of the isolated isovolumic rat heart rapidly increased the rate of pressure rise and drop and the diastolic pressure. At the same time, the relaxation rate constant (RRC), being independent of the developed pressure, remained unaltered. These findings suggested that diastolic pressure rise was not caused by incomplete myocardial relaxation. Doxorubicin (3 μM) moderately reduced the developed pressure, but the relaxation rate constant remained unchanged. The dynamics and degree of changes in all indicators of the cardiac contractile function in high-frequency stimulation were the same as in control. It can be hypothesized that the initial effect of doxorubicin was not related to ionic transport system disturbances in cardiomyocytes.

Genetics of Muscle Stiffness, Muscle Elasticity and Explosive Strength

Muscle stiffness, muscle elasticity and explosive strength are the main components of athletes' performance and they show a sex-based as well as ethnicity variation. Muscle stiffness is thought to be one of the risk factors associated with sports injuries and is less common in females than in males. These observations may be explained by circulating levels of sex hormones and their specific receptors. It has been shown that higher levels of estrogen are associated with lower muscle stiffness responsible for suppression of collagen synthesis. It is thought that these properties, at least in part, depend on genetic factors. Particularly, the gene encoding estrogen receptor 1 (ESR1) is one of the candidates that may be associated with muscle stiffness. Muscle elasticity increases with aging and there is evidence suggesting that titin (encoded by the TTN gene), a protein that is expressed in cardiac and skeletal muscles, is one of the factors responsible for elastic properties of the muscles. Mutations in the TTN gene result in some types of muscular dystrophy or cardiomyopathy. In this context, TTN may be regarded as a promising candidate for studying the elastic properties of muscles in athletes. The physiological background of explosive strength depends not only on the muscle architecture and muscle fiber composition, but also on the central nervous system and functionality of neuromuscular units. These properties are, at least partly, genetically determined. In this context, the ACTN3 gene code for α-actinin 3 has been widely researched.

Modifications of Titin Contribute to the Progression of Cardiomyopathy and Represent a Therapeutic Target for Treatment of Heart Failure

Titin is the largest human protein and an essential component of the cardiac sarcomere. With multiple immunoglobulin(Ig)-like domains that serve as molecular springs, titin contributes significantly to the passive tension, systolic function, and diastolic function of the heart. Mutations leading to early termination of titin are the most common genetic cause of dilated cardiomyopathy. Modifications of titin, which change protein length, and relative stiffness affect resting tension of the ventricle and are associated with acquired forms of heart failure. Transcriptional and post-translational changes that increase titin’s length and extensibility, making the sarcomere longer and softer, are associated with systolic dysfunction and left ventricular dilation. Modifications of titin that decrease its length and extensibility, making the sarcomere shorter and stiffer, are associated with diastolic dysfunction in animal models. There has been significant progress in understanding the mechanisms by which titin is modified. As molecular pathways that modify titin’s mechanical properties are elucidated, they represent therapeutic targets for treatment of both systolic and diastolic dysfunction. In this article, we review titin’s contribution to normal cardiac physiology, the pathophysiology of titin truncation variations leading to dilated cardiomyopathy, and transcriptional and post-translational modifications of titin. Emphasis is on how modification of titin can be utilized as a therapeutic target for treatment of heart failure.

Mitochondrial Dysfunction in Diabetic Cardiomyopathy: The Possible Therapeutic Roles of Phenolic Acids

As the powerhouse of the cells, mitochondria play a very important role in ensuring that cells continue to function. Mitochondrial dysfunction is one of the main factors contributing to the development of cardiomyopathy in diabetes mellitus. In early development of diabetic cardiomyopathy (DCM), patients present with myocardial fibrosis, dysfunctional remodeling and diastolic dysfunction, which later develop into systolic dysfunction and eventually heart failure. Cardiac mitochondrial dysfunction has been implicated in the development and progression of DCM. Thus, it is important to develop novel therapeutics in order to prevent the progression of DCM, especially by targeting mitochondrial dysfunction. To date, a number of studies have reported the potential of phenolic acids in exerting the cardioprotective effect by combating mitochondrial dysfunction, implicating its potential to be adopted in DCM therapies. Therefore, the aim of this review is to provide a concise overview of mitochondrial dysfunction in the development of DCM and the potential role of phenolic acids in combating cardiac mitochondrial dysfunction. Such information can be used for future development of phenolic acids as means of treating DCM by alleviating the cardiac mitochondrial dysfunction.

Elastic Tethers Between Separating Anaphase Chromosomes Regulate the Poleward Speeds of the Attached Chromosomes in Crane-Fly Spermatocytes

Elastic "tethers" connect separating anaphase chromosomes in most (or all) animal cells. We tested whether tethers are involved in coordinating movements of separating anaphase chromosomes in crane-fly spermatocytes. In these cells the coupled movements of separating chromosomes become uncoupled after the tethers are severed by laser microbeam irradiation of the interzone region between the chromosomes (Sheykhani et al., 2017). While this strongly suggests that tethers are involved with coordinating the poleward chromosome movements, the experiments are open to another interpretation: laser irradiations that cut the tethers also might damage something else in the interzone, and those non-tether components might regulate chromosome movements. In the experiments reported herein we distinguish between those two possibilities by disabling the tethers without cutting the interzone. We cut the arms from individual chromosomes, thereby severing the mechanical connection between separating chromosomes, disconnecting them, without damaging components in the interzone. Disabling tethers in this way uncoupled the movements of the separating chromosomes. We thus conclude that tethers are involved in regulating the speeds of separating anaphase chromosomes in crane-fly spermatocytes.

The molecular mechanisms associated with the physiological responses to inflammation and oxidative stress in cardiovascular diseases

The complex physiological signal transduction networks that respond to the dual challenges of inflammatory and oxidative stress are major factors that promote the development of cardiovascular pathologies. These signaling networks contribute to the development of age-related diseases, suggesting crosstalk between the development of aging and cardiovascular disease. Inhibition and/or attenuation of these signaling networks also delays the onset of disease. Therefore, a concept of targeting the signaling networks that are involved in inflammation and oxidative stress may represent a novel treatment paradigm for many types of heart disease. In this review, we discuss the molecular mechanisms associated with the physiological responses to inflammation and oxidative stress especially in heart failure with preserved ejection fraction and emphasize the nature of the crosstalk of these signaling processes as well as possible therapeutic implications for cardiovascular medicine.

High-throughput proteomics fiber typing (ProFiT) for comprehensive characterization of single skeletal muscle fibers

BACKGROUND

Skeletal muscles are composed of a heterogeneous collection of fiber types with different physiological adaption in response to a stimulus and disease-related conditions. Each fiber has a specific molecular expression of myosin heavy chain molecules (MyHC). So far, MyHCs are currently the best marker proteins for characterization of individual fiber types, and several proteome profiling studies have helped to dissect the molecular signature of whole muscles and individual fibers.

METHODS

Herein, we describe a mass spectrometric workflow to measure skeletal muscle fiber type-specific proteomes. To bypass the limited quantities of protein in single fibers, we developed a Proteomics high-throughput fiber typing (ProFiT) approach enabling profiling of MyHC in single fibers. Aliquots of protein extracts from separated muscle fibers were subjected to capillary LC-MS gradients to profile MyHC isoforms in a 96-well format. Muscle fibers with the same MyHC protein expression were pooled and subjected to proteomic, pulsed-SILAC, and phosphoproteomic analysis.

RESULTS

Our fiber type-specific quantitative proteome analysis confirmed the distribution of fiber types in the soleus muscle, substantiates metabolic adaptions in oxidative and glycolytic fibers, and highlighted significant differences between the proteomes of type IIb fibers from different muscle groups, including a differential expression of desmin and actinin-3. A detailed map of the Lys-6 incorporation rates in muscle fibers showed an increased turnover of slow fibers compared to fast fibers. In addition, labeling of mitochondrial respiratory chain complexes revealed a broad range of Lys-6 incorporation rates, depending on the localization of the subunits within distinct complexes.

CONCLUSION

Overall, the ProFiT approach provides a versatile tool to rapidly characterize muscle fibers and obtain fiber-specific proteomes for different muscle groups.

Stretch-Induced Biased Signaling in Angiotensin II Type 1 and Apelin Receptors for the Mediation of Cardiac Contractility and Hypertrophy

The myocardium has an intrinsic ability to sense and respond to mechanical load in order to adapt to physiological demands. Primary examples are the augmentation of myocardial contractility in response to increased ventricular filling caused by either increased venous return (Frank-Starling law) or aortic resistance to ejection (the Anrep effect). Sustained mechanical overload, however, can induce pathological hypertrophy and dysfunction, resulting in heart failure and arrhythmias. It has been proposed that angiotensin II type 1 receptor (AT1R) and apelin receptor (APJ) are primary upstream actors in this acute myocardial autoregulation as well as the chronic maladaptive signaling program. These receptors are thought to have mechanosensing capacity through activation of intracellular signaling via G proteins and/or the multifunctional transducer protein, β-arrestin. Importantly, ligand and mechanical stimuli can selectively activate different downstream signaling pathways to promote inotropic, cardioprotective or cardiotoxic signaling. Studies to understand how AT1R and APJ integrate ligand and mechanical stimuli to bias downstream signaling are an important and novel area for the discovery of new therapeutics for heart failure. In this review, we provide an up-to-date understanding of AT1R and APJ signaling pathways activated by ligand versus mechanical stimuli, and their effects on inotropy and adaptive/maladaptive hypertrophy. We also discuss the possibility of targeting these signaling pathways for the development of novel heart failure therapeutics.

Myosin Sequestration Regulates Sarcomere Function, Cardiomyocyte Energetics, and Metabolism, Informing the Pathogenesis of Hypertrophic Cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is caused by pathogenic variants in sarcomere protein genes that evoke hypercontractility, poor relaxation, and increased energy consumption by the heart and increased patient risks for arrhythmias and heart failure. Recent studies show that pathogenic missense variants in myosin, the molecular motor of the sarcomere, are clustered in residues that participate in dynamic conformational states of sarcomere proteins. We hypothesized that these conformations are essential to adapt contractile output for energy conservation and that pathophysiology of HCM results from destabilization of these conformations.

METHODS

We assayed myosin ATP binding to define the proportion of myosins in the super relaxed state (SRX) conformation or the disordered relaxed state (DRX) conformation in healthy rodent and human hearts, at baseline and in response to reduced hemodynamic demands of hibernation or pathogenic HCM variants. To determine the relationships between myosin conformations, sarcomere function, and cell biology, we assessed contractility, relaxation, and cardiomyocyte morphology and metabolism, with and without an allosteric modulator of myosin ATPase activity. We then tested whether the positions of myosin variants of unknown clinical significance that were identified in patients with HCM, predicted functional consequences and associations with heart failure and arrhythmias.

RESULTS

Myosins undergo physiological shifts between the SRX conformation that maximizes energy conservation and the DRX conformation that enables cross-bridge formation with greater ATP consumption. Systemic hemodynamic requirements, pharmacological modulators of myosin, and pathogenic myosin missense mutations influenced the proportions of these conformations. Hibernation increased the proportion of myosins in the SRX conformation, whereas pathogenic variants destabilized these and increased the proportion of myosins in the DRX conformation, which enhanced cardiomyocyte contractility, but impaired relaxation and evoked hypertrophic remodeling with increased energetic stress. Using structural locations to stratify variants of unknown clinical significance, we showed that the variants that destabilized myosin conformations were associated with higher rates of heart failure and arrhythmias in patients with HCM.

CONCLUSIONS

Myosin conformations establish work-energy equipoise that is essential for life-long cellular homeostasis and heart function. Destabilization of myosin energy-conserving states promotes contractile abnormalities, morphological and metabolic remodeling, and adverse clinical outcomes in patients with HCM. Therapeutic restabilization corrects cellular contractile and metabolic phenotypes and may limit these adverse clinical outcomes in patients with HCM.

[Early changes of energy metabolism, isoformic content and level of titin phosphorylation at diastolic dysfunction]

RELEVANCE

 Diastolic dysfunction occurring at hypertension, obesity, diabetes, or treatment with doxorubicin tends to prevail in all patterns of chronic heart failure. Lack of effective therapy forces to look more into the metabolic processes in cardiomyocytes.

OBJECTIVE

 Assess energy metabolism in cardiomyocytes and changes in titin, a giant myofibril protein that responsible for their elasticity.

MATERIAL AND METHODS

 The study model was cardiomyopathy occurring after the 4-week administration of doxorubicin (2 mg/kg weekly). Diastolic dysfunction was identified by echocardiography and catheterization with the simultaneous measurement of pressure and volume of the left ventricle (LV).

RESULTS

 The levels of adenine nucleotides and phosphocreatine in the heart of animals treated with doxorubicin differed little from the normal values, but lactate levels were increased manifold. A 50% increase in the level of titin phosphorylation was detected, which correlated (r = 0,94) with a nearly twofold increase in the share of a more elastic N2BA-isoform of this protein.

CONCLUSION

This form of diastolic dysfunction involves the activation of anaerobic metabolism and increased stretching of myofibrils facilitating LV filling.

Calcium sensitivity during staircase with sequential incompletely fused contractions

Activity dependent potentiation is thought to result from phosphorylation of the regulatory light chains of myosin, increasing Ca²⁺ sensitivity. Yet, Ca²⁺ sensitivity decreases early in a period of intermittent contractions. The purpose of this study was to investigate the early change in Ca²⁺ sensitivity during intermittent submaximal tetanic contractions. Flexor digitorum brevis muscle fibres were dissected from mice after cervical disarticulation. Fibres were superfused with Tyrode solution at 32 °C. Length was set to yield maximal tetanic force. Indo-1 was microinjected into fibres and allowed to dissipate for 30 min. Fluorescence was measured at 405 and 495 nm wavelength and the ratio was used to estimate [Ca²⁺]. A control force-Ca²⁺ relationship was determined with stimulation over a range of frequencies, yielding constants for slope, max force, and half-maximal [Ca²⁺] (pCa^2 +₅₀). Data were collected for sequential contractions at 40 Hz at 2 s intervals. Active force decreased over the first 1-4 contractions then increased. A force-pCa²⁺ curve was fit to each contraction, using the control values for the Hill slope and max force by adjusting pCa²⁺₅₀ until the curve passed through the target contraction. Data are presented for three contractions for each fibre: first, maximum shift to the right, and last contraction. There was a significant shift to the right for pCa²⁺₅₀ (decreased Ca²⁺ sensitivity), usually early in the series of intermittent contractions, then pCa^2 +₅₀ shifted to the left, but remained significantly different from the control value. Although potentiation is associated with increased Ca²⁺ sensitivity, this increase begins only after Ca²⁺ sensitivity has decreased and, in most cases, Ca²⁺ sensitivity does not increase above the control level.

Circulating Biomarkers Predicting Longitudinal Changes in Left Ventricular Structure and Function in a General Population

Background Serial imaging studies in the general population remain important to evaluate the usefulness of pathophysiologically relevant biomarkers in predicting progression of left ventricular (LV) remodeling and dysfunction. Here, we assessed in a general population whether these circulating biomarkers at baseline predict longitudinal changes in LV structure and function. Methods and Results In 592 participants (mean age, 50.8 years; 51.4% women; 40.5% hypertensive), we derived echocardiographic indexes reflecting LV structure and function at baseline and after 4.7 years. At baseline, we measured alkaline phosphatase, markers of collagen turnover (procollagen type I, C-terminal telopeptide, matrix metalloproteinase-1) and high-sensitivity cardiac troponin T. We regressed longitudinal changes in LV indexes on baseline biomarker levels and reported standardized effect sizes as a fraction of the standard deviation of LV change. After full adjustment, a decline in LV longitudinal strain (-14.2%) and increase in E/e' ratio over time (+18.9%; P≤0.019) was associated with higher alkaline phosphatase activity at baseline. Furthermore, longitudinal strain decreased with higher levels of collagen I production and degradation at baseline (procollagen type I, -14.2%; C-terminal telopeptide, -16.4%; P≤0.029). An increase in E/e' ratio over time was borderline associated with lower matrix metalloproteinase-1 (+9.8%) and lower matrix metalloproteinase-1/tissue inhibitor of metalloproteinase-1 ratio (+11.9%; P≤0.041). Higher high-sensitivity cardiac troponin T levels at baseline correlated significantly with an increase in relative wall thickness (+23.1%) and LV mass index (+18.3%) during follow-up ( P≤0.035). Conclusions We identified a set of biomarkers predicting adverse changes in LV structure and function over time. Circulating biomarkers reflecting LV stiffness, injury, and collagen composition might improve the identification of subjects at risk for subclinical cardiac maladaptation.

[Why Myocardial Relaxation Always Slows at Cardiac Pathology?]

Chronic heart failure (CHF) in most cases is due to a decrease in myocardial contractility. In particular, this results in a reduction in the maximum rate of the pressure development in the left ventricle. At the same time the maximal rate of pressure fall at relaxation is also reduced. This is not surprising, since both depend on Ca ++ myoplasmic concentration. But most of cardiac pathologies have been associated with the impairement of myocardial relaxation to a greater extent than the contraction. In the review a new view has been proposed according to which this phenomenon is attributable to restructuring of titin, the sarcomeric protein that connects the ends of myosin filaments with the sarcomeric board, lines Z. A spring-like molecule of titin shrinks at sarcomeric contraction and straightens in parallel with removing of Ca ++ from myofibrils. A reduction of its stiffness, facilitating the filling of the left ventricle, can reduce restoring force of titin and thereby slow relaxation. The survey provides information about the functions of the calcium transport system and titin in the normal heart and in CHF observed both in experimental models and in patients.

The Giant Protein Titin's Role in Cardiomyopathy: Genetic, Transcriptional, and Post-translational Modifications of TTN and Their Contribution to Cardiac Disease

Dilated cardiomyopathy (DCM) is a leading cause of heart failure, sudden cardiac death and heart transplant. DCM is inherited in approximately 50% of cases, in which the most frequent genetic defects are truncation variants of the titin gene (TTNtv). TTN encodes titin, which is the largest protein in the body and is an essential component of the sarcomere. Titin serves as a biological spring, spanning half of the sarcomere and connecting the Z-disk to the M-line, with scaffold and signaling functions. Truncations of titin are believed to lead to either haploinsufficiency and loss-of-function, or to a “poison peptide” effect. However, other titin mechanisms are postulated to influence cardiac function including post-translational modifications, in particular changes in titin phosphorylation that alters the stiffness of the protein, and diversity of alternative splicing that generates different titin isoforms. In this article, we review the role of TTN mutations in development of DCM, how differential expression of titin isoforms relate to DCM pathophysiology, and discuss how post-translational modifications of titin can affect cardiomyocyte function. Current research efforts aim to elucidate the contribution of titin to myofibril assembly, stability, and signal transduction, and how mutant titin leads to cardiac dysfunction and human disease. Future research will need to translate this knowledge toward novel therapeutic approaches that can modulate titin transcriptional and post-translational defects to treat DCM and heart failure.

A-Band Titin Truncation in Zebrafish Causes Dilated Cardiomyopathy and Hemodynamic Stress Intolerance

Background Truncating variants in the TTN gene ( TTNtv) are common in patients with dilated cardiomyopathy (DCM) but also occur in the general population. Whether TTNtv are sufficient to cause DCM or require a second hit for DCM manifestation is an important clinical issue. Methods We generated a zebrafish model of an A-band TTNtv identified in 2 human DCM families in which early-onset disease appeared to be precipitated by ventricular volume overload. Cardiac phenotypes were serially assessed from 0 to 12 months using video microscopy, high-frequency echocardiography, and histopathologic analysis. The effects of sustained hemodynamic stress resulting from an anemia-induced hyperdynamic state were also evaluated. Results Homozygous ttna mutants had severe cardiac dysmorphogenesis and premature death, whereas heterozygous mutants ( ttna^tv/+) survived into adulthood and spontaneously developed DCM. Six-month-old ttna^tv/+ fish had reduced baseline ventricular systolic function and failed to mount a hypercontractile response when challenged by hemodynamic stress. Pulsed wave and tissue Doppler analysis also revealed unsuspected ventricular diastolic dysfunction in ttna^tv/+ fish with prolonged isovolumic relaxation and increased diastolic passive stiffness in the absence of myocardial fibrosis. These defects reduced diastolic reserve under stress conditions and resulted in disproportionately greater atrial dilation than observed in wild-type fish. Conclusions Heterozygosity for A-band titin truncation is sufficient to cause DCM in adult zebrafish. Abnormalities of systolic and diastolic reserve in titin-truncated fish reduce stress tolerance and may contribute to a substrate for atrial arrhythmogenesis. These data suggest that hemodynamic stress may be an important modifiable risk factor in human TTNtv-related DCM.

End-diastolic force pre-activates cardiomyocytes and determines contractile force: role of titin and calcium

Titin functions as a molecular spring, and cardiomyocytes are able, through splicing, to control the length of titin. We hypothesized that together with diastolic [Ca²⁺], titin‐based stretch pre‐activates cardiomyocytes during diastole and is a major determinant of force production in the subsequent contraction. Through this mechanism titin would play an important role in active force development and length‐dependent activation. Mutations in the splicing factor RNA binding motif protein 20 (RBM20) result in expression of large, highly compliant titin isoforms. We measured single cardiomyocyte work loops that mimic the cardiac cycle in wild‐type (WT) and heterozygous (HET) RBM20‐deficient rats. In addition, we studied the role of diastolic [Ca²⁺] in membrane‐permeabilized WT and HET cardiomyocytes. Intact cardiomyocytes isolated from HET left ventricles were unable to produce normal levels of work (55% of WT) at low pacing frequencies, but this difference disappeared at high pacing frequencies. Length‐dependent activation (force–sarcomere length relationship) was blunted in HET cardiomyocytes, but the force–end‐diastolic force relationship was not different between HET and WT cardiomyocytes. To delineate the effects of diastolic [Ca²⁺] and titin pre‐activation on force generation, measurements were performed in detergent‐permeabilized cardiomyocytes. Cardiac twitches were simulated by transiently exposing permeabilized cardiomyocytes to 2 µm Ca²⁺. Increasing diastolic [Ca²⁺] from 1 to 80 nm increased force development twofold in WT. Higher diastolic [Ca²⁺] was needed in HET. These findings are consistent with our hypothesis that pre‐activation increases active force development. Highly compliant titin allows cells to function at higher diastolic [Ca²⁺].

Extensive eccentric contractions in intact cardiac trabeculae: revealing compelling differences in contractile behaviour compared to skeletal muscles

Force enhancement (FE) is a phenomenon that is present in skeletal muscle. It is characterized by progressive forces upon active stretching-distinguished by a linear rise in force-and enhanced isometric force following stretching (residual FE (RFE)). In skeletal muscle, non-cross-bridge (XB) structures may account for this behaviour. So far, it is unknown whether differences between non-XB structures within the heart and skeletal muscle result in deviating contractile behaviour during and after eccentric contractions. Thus, we investigated the force response of intact cardiac trabeculae during and after isokinetic eccentric muscle contractions (10% of maximum shortening velocity) with extensive magnitudes of stretch (25% of optimum muscle length). The different contributions of XB and non-XB structures to the total muscle force were revealed by using an actomyosin inhibitor. For cardiac trabeculae, we found that the force-length dynamics during long stretch were similar to the total isometric force-length relation. This indicates that no (R)FE is present in cardiac muscle while stretching the muscle from 0.75 to 1.0 optimum muscle length. This finding is in contrast with the results obtained for skeletal muscle, in which (R)FE is present. Our data support the hypothesis that titin stiffness does not increase with activation in cardiac muscle.

Posttranslational modifications of titin from cardiac muscle: how, where, and what for?

Titin is a giant elastic protein expressed in the contractile units of striated muscle cells, including the sarcomeres of cardiomyocytes. The last decade has seen enormous progress in our understanding of how titin molecular elasticity is modulated in a dynamic manner to help cardiac sarcomeres adjust to the varying hemodynamic demands on the heart. Crucial events mediating the rapid modulation of cardiac titin stiffness are post‐translational modifications (PTMs) of titin. In this review, we first recollect what is known from earlier and recent work on the molecular mechanisms of titin extensibility and force generation. The main goal then is to provide a comprehensive overview of current insight into the relationship between titin PTMs and cardiomyocyte stiffness, notably the effect of oxidation and phosphorylation of titin spring segments on titin stiffness. A synopsis is given of which type of oxidative titin modification can cause which effect on titin stiffness. A large part of the review then covers the mechanically relevant phosphorylation sites in titin, their location along the elastic segment, and the protein kinases and phosphatases known to target these sites. We also include a detailed coverage of the complex changes in phosphorylation at specific titin residues, which have been reported in both animal models of heart disease and in human heart failure, and their correlation with titin‐based stiffness alterations. Knowledge of the relationship between titin PTMs and titin elasticity can be exploited in the search for therapeutic approaches aimed at softening the pathologically stiffened myocardium in heart failure patients.

Titin mutations and muscle disease

The introduction of next-generation sequencing technology has revealed that mutations in the gene that encodes titin (TTN) are linked to multiple skeletal and cardiac myopathies. The most prominent of these myopathies is dilated cardiomyopathy (DCM). Over 60 genes are linked to the etiology of DCM, but by far, the leading cause of DCM is mutations in TTN with truncating variants in TTN (TTNtvs) associated with familial DCM in ∼ 20% of the cases. Titin is a large (3-4 MDa) and abundant protein that forms the third myofilament type of striated muscle where it spans half the sarcomere, from the Z-disk to the M-line. The underlying mechanisms by which titin mutations induce disease are poorly understood and targeted therapies are not available. Here, we review what is known about TTN mutations in muscle disease, with a major focus on DCM. We highlight that exon skipping might provide a possible therapeutic avenue to address diseases that arise from TTNtvs.

Characterization of biventricular alterations in myocardial (reverse) remodelling in aortic banding-induced chronic pressure overload

Aortic Stenosis (AS) is the most frequent valvulopathy in the western world. Traditionally aortic valve replacement (AVR) has been recommended immediately after the onset of heart failure (HF) symptoms. However, recent evidence suggests that AVR outcome can be improved if performed earlier. After AVR, the process of left ventricle (LV) reverse remodelling (RR) is variable and frequently incomplete. In this study, we aimed at detecting mechanism underlying the process of LV RR regarding myocardial structural, functional and molecular changes before the onset of HF symptoms. Wistar-Han rats were subjected to 7-weeks of ascending aortic-banding followed by a 2-week period of debanding to resemble AS-induced LV remodelling and the early events of AVR-induced RR, respectively. This resulted in 3 groups: Sham (n = 10), Banding (Ba, n = 15) and Debanding (Deb, n = 10). Concentric hypertrophy and diastolic dysfunction (DD) were patent in the Ba group. Aortic-debanding induced RR, which promoted LV functional recovery, while cardiac structure did not normalise. Cardiac parameters of RV dysfunction, assessed by echocardiography and at the cardiomyocyte level prevailed altered after debanding. After debanding, these alterations were accompanied by persistent changes in pathways associated to myocardial hypertrophy, fibrosis and LV inflammation. Aortic banding induced pulmonary arterial wall thickness to increase and correlates negatively with effort intolerance and positively with E/e′ and left atrial area. We described dysregulated pathways in LV and RV remodelling and RR after AVR. Importantly we showed important RV-side effects of aortic constriction, highlighting the impact that LV-reverse remodelling has on both ventricles.

Cardiac Disorders and Pathophysiology of Sarcomeric Proteins

The sarcomeric proteins represent the structural building blocks of heart muscle, which are essential for contraction and relaxation. During recent years, it has become evident that posttranslational modifications of sarcomeric proteins, in particular phosphorylation, tune cardiac pump function at rest and during exercise. This delicate, orchestrated interaction is also influenced by mutations, predominantly in sarcomeric proteins, which cause hypertrophic or dilated cardiomyopathy. In this review, we follow a bottom-up approach starting from a description of the basic components of cardiac muscle at the molecular level up to the various forms of cardiac disorders at the organ level. An overview is given of sarcomere changes in acquired and inherited forms of cardiac disease and the underlying disease mechanisms with particular reference to human tissue. A distinction will be made between the primary defect and maladaptive/adaptive secondary changes. Techniques used to unravel functional consequences of disease-induced protein changes are described, and an overview of current and future treatments targeted at sarcomeric proteins is given. The current evidence presented suggests that sarcomeres not only form the basis of cardiac muscle function but also represent a therapeutic target to combat cardiac disease.