Myostatin represses physiological hypertrophy of the heart and excitation-contraction coupling

A combinatorial approach for achieving CNS-selective RNAi

RNA interference (RNAi) is an endogenous process that can be harnessed using chemically modified small interfering RNAs (siRNAs) to potently modulate gene expression in many tissues. The route of administration and chemical architecture are the primary drivers of oligonucleotide tissue distribution, including siRNAs. Independently of the nature and type, oligonucleotides are eliminated from the body through clearance tissues, where their unintended accumulation may result in undesired gene modulation. Divalent siRNAs (di-siRNAs) administered into the CSF induce robust gene silencing throughout the central nervous system (CNS). Upon clearance from the CSF, they are mainly filtered by the kidneys and liver, with the most functionally significant accumulation occurring in the liver. siRNA- and miRNA-induced silencing can be blocked through substrate inhibition using single-stranded, stabilized oligonucleotides called antagomirs or anti-siRNAs. Using APOE as a model target, we show that undesired di-siRNA-induced silencing in the liver can be mitigated through administration of liver targeting GalNAc-conjugated anti-siRNAs, without impacting CNS activity. Blocking unwanted hepatic APOE silencing achieves fully CNS-selective silencing, essential for potential clinical translation. While we focus on CNS/liver selectivity, coadministration of differentially targeting siRNA and anti-siRNAs can be adapted as a strategy to achieve tissue selectivity in different organ combinations.

Circulating GDF11 exacerbates myocardial injury in mice and associates with increased infarct size in humans

AIMS

The heart rejuvenating effects of circulating growth differentiation factor 11 (GDF11), a transforming growth factor-β superfamily member that shares 90% homology with myostatin (MSTN), remains controversial. Here, we aimed to probe the role of GDF11 in acute myocardial infarction (MI), a frequent cause of heart failure and premature death during ageing.

METHODS AND RESULTS

In contrast to endogenous Mstn, myocardial Gdf11 declined during the course of ageing and was particularly reduced following ischaemia/reperfusion (I/R) injury, suggesting a therapeutic potential of GDF11 signalling in MI. Unexpectedly, boosting systemic Gdf11 by recombinant GDF11 delivery (0.1 mg/kg body weight over 30 days) prior to myocardial I/R augmented myocardial infarct size in C57BL/6 mice irrespective of their age, predominantly by accelerating pro-apoptotic signalling. While intrinsic cardioprotective signalling pathways remained unaffected by high circulating GDF11, targeted transcriptomics and immunomapping studies focusing on GDF11-associated downstream targets revealed attenuated Nkx2-5 expression confined to CD105-expressing cells, with pro-apoptotic activity, as assessed by caspase-3 levels, being particularly pronounced in adjacent cells, suggesting an indirect effect. By harnessing a highly specific and validated liquid chromatography-tandem mass spectrometry-based assay, we show that in prospectively recruited patients with MI circulating GDF11 but not MSTN levels incline with age. Moreover, GDF11 levels were particularly elevated in those at high risk for adverse outcomes following the acute event, with circulating GDF11 emerging as an independent predictor of myocardial infarct size, as estimated by standardized peak creatine kinase-MB levels.

CONCLUSION

Our data challenge the initially reported heart rejuvenating effects of circulating GDF11 and suggest that high levels of systemic GDF11 exacerbate myocardial injury in mice and humans alike. Persistently high GDF11 levels during ageing may contribute to the age-dependent loss of cardioprotective mechanisms and thus poor outcomes of elderly patients following acute MI.

Myostatin and the Heart

Myostatin (growth differentiation factor 8) is a member of the transforming growth factor-β superfamily. It is secreted mostly by skeletal muscles, although small amounts of myostatin are produced by the myocardium and the adipose tissue as well. Myostatin binds to activin IIB membrane receptors to activate the downstream intracellular canonical Smad2/Smad3 pathway, and additionally acts on non-Smad (non-canonical) pathways. Studies on transgenic animals have shown that overexpression of myostatin reduces the heart mass, whereas removal of myostatin has an opposite effect. In this review, we summarize the potential diagnostic and prognostic value of this protein in heart-related conditions. First, in myostatin-null mice the left ventricular internal diameters along with the diastolic and systolic volumes are larger than the respective values in wild-type mice. Myostatin is potentially secreted as part of a negative feedback loop that reduces the effects of the release of growth-promoting factors and energy reprogramming in response to hypertrophic stimuli. On the other hand, both human and animal data indicate that myostatin is involved in the development of the cardiac cachexia and heart fibrosis in the course of chronic heart failure. The understanding of the role of myostatin in such conditions might initiate a development of targeted therapies based on myostatin signaling inhibition.

A Prospective Case-Control Study Examining the Relationship Between Frailty and Serum Myostatin in Older Persons with Chronic Heart Failure

Background

Frailty affects the prognosis and management of patients with heart failure, and is often related with sarcopenia. Also, the serum myostatin (MSTN) involved in the development of sarcopenia and frailty. This study aimed to determine the connection between MSTN level and frailty in older adults with chronic heart failure (CHF).

Methods

This prospective case-control study enrolled older adult patients with CHF between May 2019 and May 2021, and analyzed their clinical data.

Results

In this study 75 older adults with CHF were included, 29 of whom were frail. The B-type natriuretic peptide (BNP) levels were significantly higher in frail older adults with CHF than in older adults with CHF who were not frail (316.82 ± 235.64 pg/mL vs 198.61 ± 112.58 pg/mL; P = 0.016). The MSTN levels were significantly higher in frail participants than in participants who were not frail (2.93 ± 1.35 ng/mL vs 2.24 ± 0.84 ng/mL; P = 0.018). Based on multivariable analysis the BNP (odds ratio [OR] = 1.004, 95% confidence interval [CI] = 1 0.001-1.008; P = 0.018) and MSTN (OR = 1.772, 95% CI = 1.079-2.912; P =0 0.024) levels were independently associated with frailty in older adults with CHF.

Conclusion

MSTN is a promising biomarker of frailty in elderly patients with CHF.

Myostatin: a multifunctional role in human female reproduction and fertility - a short review

Myostatin (MSTN) is member of the transforming growth factor β (TGF-β) superfamily and was originally identified in the musculoskeletal system as a negative regulator of skeletal muscle growth. The functional roles of MSTN outside of the musculoskeletal system have aroused researchers' interest in recent years, with an increasing number of studies being conducted in this area. Notably, the expression of MSTN and its potential activities in various reproductive organs, including the ovary, placenta, and uterus, have recently been examined. Numerous studies published in the last few years demonstrate that MSTN plays a critical role in human reproduction and fertility, including the regulation of follicular development, ovarian steroidogenesis, granule-cell proliferation, and oocyte maturation regulation. Furthermore, findings from clinical samples suggest that MSTN may play a key role in the pathogenesis of several reproductive disorders such as uterine myoma, preeclampsia (PE), ovary hyperstimulation syndrome (OHSS), and polycystic ovarian syndrome (PCOS). There is no comprehensive review regarding to MSTN related to the female reproductive system in the literature. This review serves as a summary of the genes in reproductive medicine and their potential influence. We summarized MSTN expression in different compartments of the female reproductive system. Subsequently, we discuss the role of MSTN in both physiological and several pathological conditions related to the female fertility and reproduction-related diseases.

Inhibition of activin A receptor signalling attenuates age-related pathological cardiac remodelling

In the heart, ageing is associated with DNA damage, oxidative stress, fibrosis and activation of the activin signalling pathway, leading to cardiac dysfunction. The cardiac effects of activin signalling blockade in progeria are unknown. This study investigated the cardiac effects of progeria induced by attenuated levels of Ercc1, which is required for DNA excision and repair, and the impact of activin signalling blockade using a soluble activin receptor type IIB (sActRIIB). DNA damage and oxidative stress were significantly increased in Ercc1^Δ/− hearts, but were reduced by sActRIIB treatment. sActRIIB treatment improved cardiac systolic function and induced cardiomyocyte hypertrophy in Ercc1^Δ/− hearts. RNA-sequencing analysis showed that in Ercc1^Δ/− hearts, there was an increase in pro-oxidant and a decrease in antioxidant gene expression, whereas sActRIIB treatment reversed this effect. Ercc1^Δ/− hearts also expressed higher levels of anti-hypertrophic genes and decreased levels of pro-hypertrophic ones, which were also reversed by sActRIIB treatment. These results show for the first time that inhibition of activin A receptor signalling attenuates cardiac dysfunction, pathological tissue remodelling and gene expression in Ercc1-deficient mice and presents a potentially novel therapeutic target for heart diseases.

Summary: Attenuated DNA repair is associated with pathological cardiac remodelling and gene expression. Much of this phenotype is attenuated by inhibition of the activin signalling pathway using soluble activin receptor treatment.

Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

A systems approach using Diversity Outbred mice distinguishes the cardiovascular effects and genetics of circulating GDF11 from those of its homolog, myostatin

Growth differentiation factor 11 (GDF11) is a member of the TGF-β protein family that has been implicated in the development of cardiac hypertrophy. While some studies have suggested that systemic GDF11 protects against cardiomyocyte enlargement and left ventricular wall thickening, there remains uncertainty about the true impact of GDF11 and whether its purported effects are actually attributable to its homolog myostatin. This study was conducted to resolve the statistical and genetic relationships among GDF11, myostatin, and cardiac hypertrophy in a mouse model of human genetics, the Diversity Outbred (DO) stock. In the DO population, serum GDF11 concentrations positively correlated with cardiomyocyte cross-sectional area, while circulating myostatin levels were negatively correlated with body weight, heart weight, and left ventricular wall thickness and mass. Genetic analyses revealed that serum GDF11 concentrations are modestly heritable (0.23) and identified a suggestive peak on murine chromosome 3 in close proximity to the gene Hey1, a transcriptional repressor. Bioinformatic analyses located putative binding sites for the HEY1 protein upstream of the Gdf11 gene in the mouse and human genomes. In contrast, serum myostatin concentrations were more heritable (0.57) than GDF11 concentrations, and mapping identified a significant locus near the gene FoxO1, which has binding motifs within the promoter regions of human and mouse myostatin genes. Together, these findings more precisely define the independent cardiovascular effects of GDF11 and myostatin, as well as their distinct regulatory pathways. Hey1 is a compelling candidate for the regulation of GDF11 and will be further evaluated in future studies.

Docosanoic acid conjugation to siRNA enables functional and safe delivery to skeletal and cardiac muscles

Oligonucleotide therapeutics hold promise for the treatment of muscle- and heart-related diseases. However, oligonucleotide delivery across the continuous endothelium of muscle tissue is challenging. Here, we demonstrate that docosanoic acid (DCA) conjugation of small interfering RNAs (siRNAs) enables efficient (~5% of injected dose), sustainable (>1 month), and non-toxic (no cytokine induction at 100 mg/kg) gene silencing in both skeletal and cardiac muscles after systemic injection. When designed to target myostatin (muscle growth regulation gene), siRNAs induced ~55% silencing in various muscle tissues and 80% silencing in heart, translating into a ~50% increase in muscle volume within 1 week. Our study identifies compounds for RNAi-based modulation of gene expression in skeletal and cardiac muscles, paving the way for both functional genomics studies and therapeutic gene modulation in muscle and heart.

Decorin Protects Cardiac Myocytes against Simulated Ischemia/Reperfusion Injury

Search for new cardioprotective therapies is of great importance since no cardioprotective drugs are available on the market. In line with this need, several natural biomolecules have been extensively tested for their potential cardioprotective effects. Previously, we have shown that biglycan, a member of a diverse group of small leucine-rich proteoglycans, enhanced the expression of cardioprotective genes and decreased ischemia/reperfusion-induced cardiomyocyte death via a TLR-4 dependent mechanism. Therefore, in the present study we aimed to test whether decorin, a small leucine-rich proteoglycan closely related to biglycan, could exert cardiocytoprotection and to reveal possible downstream signaling pathways. Methods: Primary cardiomyocytes isolated from neonatal and adult rat hearts were treated with 0 (Vehicle), 1, 3, 10, 30 and 100 nM decorin as 20 h pretreatment and maintained throughout simulated ischemia and reperfusion (SI/R). In separate experiments, to test the mechanism of decorin-induced cardio protection, 3 nM decorin was applied in combination with inhibitors of known survival pathways, that is, the NOS inhibitor L-NAME, the PKG inhibitor KT-5823 and the TLR-4 inhibitor TAK-242, respectively. mRNA expression changes were measured after SI/R injury. Results: Cell viability of both neonatal and adult cardiomyocytes was significantly decreased due to SI/R injury. Decorin at 1, 3 and 10 nM concentrations significantly increased the survival of both neonatal and adult myocytes after SI/R. At 3nM (the most pronounced protective concentration), it had no effect on apoptotic rate of neonatal cardiac myocytes. No one of the inhibitors of survival pathways (L-NAME, KT-5823, TAK-242) influenced the cardiocytoprotective effect of decorin. MYND-type containing 19 (Zmynd19) and eukaryotic translation initiation factor 4E nuclear import factor 1 (Eif4enif1) were significantly upregulated due to the decorin treatment. In conclusion, this is the first demonstration that decorin exerts a direct cardiocytoprotective effect possibly independent of NO-cGMP-PKG and TLR-4 dependent survival signaling.

Structural basis of specific inhibition of extracellular activation of pro- or latent myostatin by the monoclonal antibody SRK-015

Myostatin (or growth/differentiation factor 8 (GDF8)) is a member of the transforming growth factor β superfamily of growth factors and negatively regulates skeletal muscle growth. Its dysregulation is implicated in muscle wasting diseases. SRK-015 is a clinical-stage mAb that prevents extracellular proteolytic activation of pro- and latent myostatin. Here we used integrated structural and biochemical approaches to elucidate the molecular mechanism of antibody-mediated neutralization of pro-myostatin activation. The crystal structure of pro-myostatin in complex with 29H4-16 Fab, a high-affinity variant of SRK-015, at 2.79 Å resolution revealed that the antibody binds to a conformational epitope in the arm region of the prodomain distant from the proteolytic cleavage sites. This epitope is highly sequence-divergent, having only limited similarity to other closely related members of the transforming growth factor β superfamily. Hydrogen/deuterium exchange MS experiments indicated that antibody binding induces conformational changes in pro- and latent myostatin that span the arm region, the loops contiguous to the protease cleavage sites, and the latency-associated structural elements. Moreover, negative-stain EM with full-length antibodies disclosed a stable, ring-like antigen-antibody structure in which the two Fab arms of a single antibody occupy the two arm regions of the prodomain in the pro- and latent myostatin homodimers, suggesting a 1:1 (antibody:myostatin homodimer) binding stoichiometry. These results suggest that SRK-015 binding stabilizes the latent conformation and limits the accessibility of protease cleavage sites within the prodomain. These findings shed light on approaches that specifically block the extracellular activation of growth factors by targeting their precursor forms.

Activin type II receptor ligand signaling inhibition after experimental ischemic heart failure attenuates cardiac remodeling and prevents fibrosis

Myostatin (MSTN) is a transforming growth factor (TGF)-β superfamily member that acts as a negative regulator of muscle growth and may play a role in cardiac remodeling. We hypothesized that inhibition of activin type II receptors (ACTRII) to reduce MSTN signaling would reduce pathological cardiac remodeling in experimental heart failure (HF). C57BL/6J mice underwent left anterior descending coronary artery ligation under anesthesia to induce myocardial infarction (MI) or no ligation (sham). MI and sham animals were each randomly divided into groups (n ≥ 10 mice/group) receiving an ACTRII or ACTRII/TGFβ receptor-signaling inhibiting strategy: 1) myo-Fc group (weekly 10 mg/kg Myo-Fc) or 2) Fol + TGFi group (daily 12 µg/kg follistatin plus 2 mg/kg TGFβ receptor inhibitor), versus controls. ACTRII/TGFBR signaling inhibition preserved cardiac function by echocardiography and prevented an increase in brain natriuretic peptide (BNP). ACTRII/TGFBR inhibition resulted in increased phosphorylation (P) of Akt and decreased P-p38 mitogen-activated protein kinase (MAPK) in MI mice. In vitro, Akt contributed to P-SMAD2,3, P-p38, and BNP regulation in cardiomyocytes. ACTRII/TGFBR inhibition increased sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2a) levels and decreased unfolded protein response (UPR) markers in MI mice. ACTRII/TGFBR inhibition was associated with a decrease in cardiac fibrosis and fibrosis markers, connective tissue growth factor (CTGF), type I collagen, fibronectin, α-smooth muscle actin, and matrix metalloproteinase (MMP)-12 in MI mice. MSTN exerted a direct regulation on the UPR marker eukaryotic translation initiation factor-2α (eIf2α) in cardiomyocytes. Our study suggests that ACTRII ligand inhibition has beneficial effects on cardiac signaling and fibrosis after ischemic HF.NEW & NOTEWORTHY Activin type II receptor ligand inhibition resulted in preserved cardiac function, a decrease in cardiac fibrosis, improved SERCA2a levels, and a prevention of the unfolded protein response in mice with myocardial infarction.

A Sensitive and Selective Immunoassay for the Quantitation of Serum Latent Myostatin after In Vivo Administration of SRK-015, a Selective Inhibitor of Myostatin Activation

Myostatin, a member of the transforming growth factor β (TGFβ) superfamily, is a key regulator of skeletal muscle mass and a therapeutic target for muscle wasting diseases. We developed a human monoclonal antibody, SRK-015, that selectively binds to and inhibits proteolytic processing of myostatin precursors, thereby preventing growth factor release from the latent complex. As a consequence of antibody binding, latent myostatin accumulates in the circulation of animals treated with SRK-015 or closely related antibodies, suggesting that quantitation of latent myostatin in serum may serve as a biomarker for target engagement. To accurately measure SRK-015 target engagement, we developed a sensitive plate-based electrochemiluminescent immunoassay to quantitate latent myostatin in serum samples. The assay selectively recognizes latent myostatin without cross-reactivity to promyostatin, mature myostatin, or closely related members of the TGFβ superfamily. To enable use of the assay in samples from animals dosed with SRK-015, we incorporated a low-pH step that dissociates SRK-015 from latent myostatin, improving drug tolerance of the assay. The assay meets inter- and intra-assay accuracy and precision acceptance criteria, and it has a lower limit of quantitation (LLOQ) of 10 ng/mL. We then tested serum samples from a pharmacology study in cynomolgus monkeys treated with SRK-015. Serum latent myostatin increases after treatment with SRK-015, reaches a dose-dependent plateau approximately 20 days after dosing, and trends back toward baseline after cessation of antibody dosing. Taken together, these data suggest that this assay can be used to accurately measure levels of the primary circulating form of myostatin in population-based or pharmacodynamic studies.

Construction of cardiomyoblast sheets for cardiac tissue repair: comparison of three different approaches

Recently, cell sheet engineering has emerged as one of the most accentuated approaches of tissue engineering and cardiac tissue is the pioneering application area of cell sheets with clinical use. In this study, we cultured rat cardiomyoblasts (H9C2 cell line) to obtain cell sheets by using three different approaches; using (1) thermo-responsive tissue culture plates, (2) high cell seeding density/high serum content and (3) ascorbic acid treatment. To compare the outcomes of three methods, morphologic examination, immunofluorescent stainings and live/dead cell assay were performed and the effects of serum concentration and ascorbic acid treatment on cardiac gene expressions were examined. The results showed that cardiomyoblast sheets were successfully obtained in all approaches without losing their integrity and viability. Also, the results of RT-PCR analysis showed that the types of tissue culture surface, cell seeding density, serum concentration and ascorbic acid treatment affect cardiac gene expressions of cells in cell sheets. Although three methods were succeeded, ascorbic acid treatment was found as the most rapid and effective method to obtain cell sheets with cardiac characteristics.

Myostatin regulates pituitary development and hepatic IGF1

Circulating myostatin-attenuating agents are being developed to treat muscle-wasting disease despite their potential to produce serious off-target effects, as myostatin/activin receptors are widely distributed among many nonmuscle tissues. Our studies suggest that the myokine not only inhibits striated muscle growth but also regulates pituitary development and growth hormone (GH) action in the liver. Using a novel myostatin-null label-retaining model (Jekyll mice), we determined that the heterogeneous pool of pituitary stem, transit-amplifying, and progenitor cells in Jekyll mice depletes more rapidly after birth than the pool in wild-type mice. This correlated with increased levels of GH, prolactin, and the cells that secrete these hormones, somatotropes and lactotropes, respectively, in Jekyll pituitaries. Recombinant myostatin also stimulated GH release and gene expression in pituitary cell cultures although inhibiting prolactin release. In primary hepatocytes, recombinant myostatin blocked GH-stimulated expression of two key mediators of growth, insulin-like growth factor (IGF)1 and the acid labile subunit and increased expression of an inhibitor, IGF-binding protein-1. The significance of these findings was demonstrated by smaller muscle fiber size in a model lacking myostatin and liver IGF1 expression (LID-o-Mighty mice) compared with that in myostatin-null (Mighty) mice. These data together suggest that myostatin may regulate pituitary development and function and that its inhibitory actions in muscle may be partly mediated by attenuating GH action in the liver. They also suggest that circulating pharmacological inhibitors of myostatin could produce unintended consequences in these and possibly other tissues.

Centronuclear myopathies under attack: A plethora of therapeutic targets

Centronuclear myopathies are a group of congenital myopathies characterized by severe muscle weakness, genetic heterogeneity, and defects in the structural organization of muscle fibers. Their names are derived from the central position of nuclei on biopsies, while they are at the fiber periphery under normal conditions. No specific therapy exists yet for these debilitating diseases. Mutations in the myotubularin phosphoinositides phosphatase, the GTPase dynamin 2, or amphiphysin 2 have been identified to cause respectively X-linked centronuclear myopathies (also called myotubular myopathy) or autosomal dominant and recessive forms. Mutations in additional genes, as RYR1, TTN, SPEG or CACNA1S, were linked to phenotypes that can overlap with centronuclear myopathies. Numerous animal models of centronuclear myopathies have been studied over the last 15 years, ranging from invertebrate to large mammalian models. Their characterization led to a partial understanding of the pathomechanisms of these diseases and allowed the recent validation of therapeutic proof-of-concepts. Here, we review the different therapeutic strategies that have been tested so far for centronuclear myopathies, some of which may be translated to patients.

Effect of myostatin deletion on cardiac and microvascular function

The objective of this study is to test the hypothesis that increased muscle mass has positive effects on cardiovascular function. Specifically, we tested the hypothesis that increases in lean body mass caused by deletion of myostatin improves cardiac performance and vascular function. Echocardiography was used to quantify left ventricular function at baseline and after acute administration of propranolol and isoproterenol to assess β‐adrenergic reactivity. Additionally, resistance vessels in several beds were removed, cannulated, pressurized to 60 mmHg and reactivity to vasoactive stimuli was assessed. Hemodynamics were measured using in vivo radiotelemetry. Myostatin deletion results in increased fractional shortening at baseline. Additionally, arterioles in the coronary and muscular microcirculations are more sensitive to endothelial‐dependent dilation while nonmuscular beds or the aorta were unaffected. β‐adrenergic dilation was increased in both coronary and conduit arteries, suggesting a systemic effect of increased muscle mass on vascular function. Overall hemodynamics and physical characteristics (heart weight and size) remained unchanged. Myostatin deletion mimics in part the effects of exercise on cardiovascular function. It significantly increases lean muscle mass and results in muscle‐specific increases in endothelium‐dependent vasodilation. This suggests that increases in muscle mass may serve as a buffer against pathological states that specifically target cardiac function (heart failure), the β‐adrenergic system (age), and nitric oxide bio‐availability (atherosclerosis). Taken together, pharmacological inhibition of the myostatin pathway could prove an excellent mechanism by which the benefits of exercise can be conferred in patients that are unable to exercise.

Effects of an ActRIIB.Fc Ligand Trap on Cardiac Function in Simian Immunodeficiency Virus-Infected Male Rhesus Macaques

An important safety consideration in the use of antagonists of myostatin and activins is whether these drugs induce myocardial hypertrophy and impair cardiac function. The current study evaluated the effects of a soluble ActRIIB receptor Fc fusion protein (ActRIIB.Fc), a ligand trap for TGF-β/activin family members including myostatin, on myocardial mass and function in simian immunodeficiency virus (SIV)-infected juvenile rhesus macaques (Macaca mulatta). Fourteen pair-housed, juvenile male rhesus macaques were inoculated with SIVmac239; 4 weeks postinoculation, they were treated with weekly injections of 10 mg/kg ActRIIB.Fc or saline for 12 weeks. Myocardial mass and function were evaluated using two-dimensional echocardiography at baseline and after 12 weeks. The administration of ActRIIB.Fc was associated with a significantly greater increase in thickness of left ventricular posterior wall and interventricular septum both in diastole and systole. Cardiac output and cardiac index increased with time, more in animals treated with ActRIIB.Fc than in those treated with saline, but the difference was not statistically significant. The changes in ejection fraction, fractional shortening, and stroke volume did not differ significantly between groups. The changes in end-diastolic and end-systolic volumes did not differ between groups. In addition to a large reduction in IGF1 mRNA expression in the ActRIIB.Fc-treated animals, complex changes were detected in the myocardial expression of proteins related to calcium transport and storage. In conclusion, ActRIIB.Fc administration for 12 weeks was associated with increased myocardial mass but did not adversely affect myocardial function in juvenile SIV-infected rhesus macaques. Further studies are necessary to establish long-term cardiac safety.

The TGFβ superfamily in cardiac dysfunction

TGFβ superfamily includes the transforming growth factor βs (TGFβs), bone morphogenetic proteins (BMPs), growth and differentiation factors (GDFs) and Activin/Inhibin families of ligands. Among the 33 members of TGFβ superfamily ligands, many act on multiple types of cells within the heart, including cardiomyocytes, cardiac fibroblasts/myofibroblasts, coronary endothelial cells, smooth muscle cells, and immune cells (e.g. monocytes/macrophages and neutrophils). In this review, we highlight recent discoveries on TGFβs, BMPs, and GDFs in different cardiac residential cellular components, in association with functional impacts in heart development, injury repair, and dysfunction. Specifically, we will review the roles of TGFβs, BMPs, and GDFs in cardiac hypertrophy, fibrosis, contractility, metabolism, angiogenesis, and regeneration.

Blocking extracellular activation of myostatin as a strategy for treating muscle wasting

Many growth factors are intimately bound to the extracellular matrix, with regulated processing and release leading to cellular stimulation. Myostatin and GDF11 are closely related members of the TGFβ family whose activation requires two proteolytic cleavages to release the growth factor from the prodomain. Specific modulation of myostatin and GDF11 activity by targeting growth factor-receptor interactions has traditionally been challenging. Here we demonstrate that a novel strategy for blocking myostatin and GDF11, inhibition of growth factor release, specifically and potently inhibits signaling both in vitro and in vivo. We developed human monoclonal antibodies that selectively bind the myostatin and GDF11 precursor forms, including a subset that inhibit myostatin proteolytic activation and prevent muscle atrophy in vivo. The most potent myostatin activation-blocking antibodies promoted robust muscle growth and resulted in significant gains in muscle performance in healthy mice. Altogether, we show that blocking the extracellular activation of growth factors is a potent method for preventing signaling, serving as proof of concept for a novel therapeutic strategy that can be applied to other members of the TGFβ family of growth factors.

Smad7 gene delivery prevents muscle wasting associated with cancer cachexia in mice

Patients with advanced cancer often succumb to complications arising from striated muscle wasting associated with cachexia. Excessive activation of the type IIB activin receptor (ActRIIB) is considered an important mechanism underlying this wasting, where circulating procachectic factors bind ActRIIB and ultimately lead to the phosphorylation of SMAD2/3. Therapeutics that antagonize the binding of ActRIIB ligands are in clinical development, but concerns exist about achieving efficacy without off-target effects. To protect striated muscle from harmful ActRIIB signaling, and to reduce the risk of off-target effects, we developed an intervention using recombinant adeno-associated viral vectors (rAAV vectors) that increase expression of Smad7 in skeletal and cardiac muscles. SMAD7 acts as an intracellular negative regulator that prevents SMAD2/3 activation and promotes degradation of ActRIIB complexes. In mouse models of cachexia, rAAV:Smad7 prevented wasting of skeletal muscles and the heart independent of tumor burden and serum levels of procachectic ligands. Mechanistically, rAAV:Smad7 administration abolished SMAD2/3 signaling downstream of ActRIIB and inhibited expression of the atrophy-related ubiquitin ligases MuRF1 and MAFbx. These findings identify muscle-directed Smad7 gene delivery as a potential approach for preventing muscle wasting under conditions where excessive ActRIIB signaling occurs, such as cancer cachexia.

Opposite effects of Activin type 2 receptor ligands on cardiomyocyte proliferation during development and repair

Zebrafish regenerate damaged myocardial tissue very effectively. Hence, insights into the molecular networks underlying zebrafish heart regeneration might help develop alternative strategies to restore human cardiac performance. While TGF-β signaling has been implicated in zebrafish cardiac regeneration, the role of its individual ligands remains unclear. Here, we report the opposing expression response during zebrafish heart regeneration of two genes, mstnb and inhbaa, which encode TGF-β family ligands. Using gain-of-function (GOF) and loss-of-function (LOF) approaches, we show that these ligands mediate inverse effects on cardiac regeneration and specifically on cardiomyocyte (CM) proliferation. Notably, we find that Inhbaa functions as a CM mitogen and that its overexpression leads to accelerated cardiac recovery and scar clearance after injury. In contrast, mstnb GOF and inhbaa LOF both lead to unresolved scarring after cardiac injury. We further show that Mstnb and Inhbaa inversely control Smad2 and Smad3 transcription factor activities through alternate Activin type 2 receptors.

Zebrafish can regenerate damaged myocardial tissue but it is unclear how this is regulated. Here, the authors show that two TGF-β family members, Mstnb and Inhbaa, have opposite effects in regeneration, with mstnb overexpression or inhbaa loss-of-function causing cardiac scarring after injury.

Absence of Myostatin Improves Cardiac Function Following Myocardial Infarction

BACKGROUND

Myostatin inhibits the development of skeletal muscle and regulates the proliferation of skeletal muscle fibroblasts. However, the role of myostatin in regulating cardiac muscle or myofibroblasts, specifically in acute myocardial infarction (MI), is less clear. This study sought to determine whether absence of myostatin altered left ventricular function post-MI.

METHODS

Myostatin-null mice (Mstn^-/-) and wild-type (WT) mice underwent ligation of the left anterior descending artery to induce MI. Left ventricular function was measured at baseline, days 1 and 28 post-MI. Immunohistochemistry and immunofluorescence were obtained at day 28 for cellular proliferation, collagen deposition, and myofibroblastic activity.

RESULTS

Whilst left ventricular function at baseline and size of infarct were similar, significant differences in favour of Mstn^-/- compared to WT mice post-MI include a greater recovery of ejection fraction (61.8±1.1% vs 57.1±2.3%, p<0.01), less collagen deposition (41.9±2.8% vs 54.7±3.4%, p<0.05), and lower mortality (0 vs. 20%, p<0.05). There was no difference in the number of BrdU positive cells, percentage of apoptotic cardiomyocytes, or size of cardiomyocytes post-MI between WT and Mstn^-/- mice.

CONCLUSIONS

Absence of myostatin potentially protects the function of the heart post-MI with improved survival, possibly by limiting extent of fibrosis.

Myostatin mediates abdominal aortic atherosclerosis progression by inducing vascular smooth muscle cell dysfunction and monocyte recruitment

Myostatin (Mstn) is a skeletal muscle growth inhibitor involved in metabolic disorders and heart fibrosis. In this study we sought to verify whether Mstn is also operative in atherosclerosis of abdominal aorta. In human specimens, Mstn expression was almost absent in normal vessels, became detectable in the media of non-progressive lesions and increased with the severity of the damage. In progressive atherosclerotic lesions, Mstn was present in the media, neointima, plaque shoulder and in infiltrating macrophages. Mstn co-localized with α-smooth muscle actin (α-SMA) staining and with some CD45+ cells, indicating Mstn expression in VSMCs and bloodstream-derived leukocytes. In vitro, Mstn was tested in VSMCs and monocytes. In A7r5 VSMCs, Mstn downregulated proliferation and Smoothelin mRNA, induced cytoskeletal rearrangement, increased migratory rate and MCP-1/CCR2 expression. In monocytes (THP-1 cells and human monocytes), Mstn acted as a chemoattractant and increased the MCP-1-dependent chemotaxis, F-actin, α-SMA, MCP-1 and CCR2 expression; in turn, MCP-1 increased Mstn mRNA. Mstn induced JNK phosphorylation both in VSMCs and monocytes. Our results indicate that Mstn is overexpressed in abdominal aortic wall deterioration, affects VSMCs and monocyte biology and sustains a chronic inflammatory milieu. These findings propose to consider Mstn as a new playmaker in atherosclerosis progression.

Cardiokines as Modulators of Stress-Induced Cardiac Disorders

Almost 30 years ago, the protein, atrial natriuretic peptide, was identified as a heart-secreted hormone that provides a peripheral signal from the myocardium that communicates to the rest of the organism to modify blood pressure and volume under conditions of heart failure. Since then, additional peripheral factors secreted by the heart, termed cardiokines, have been identified and shown to coordinate this interorgan cross talk. In addition to this interorgan communication, cardiokines also act in an autocrine/paracrine manner to play a role in intercellular communication within the myocardium. This review focuses on the roles of newly emerging cardiokines that are mainly increased in stress-induced cardiac diseases. The potential of these cardiokines as clinical biomarkers for diagnosis and prognosis of cardiac disorders is also discussed.

Biochemistry and Biology of GDF11 and Myostatin: Similarities, Differences, and Questions for Future Investigation

Growth differentiation factor 11 (GDF11) and myostatin (or GDF8) are closely related members of the transforming growth factor β superfamily and are often perceived to serve similar or overlapping roles. Yet, despite commonalities in protein sequence, receptor utilization and signaling, accumulating evidence suggests that these 2 ligands can have distinct functions in many situations. GDF11 is essential for mammalian development and has been suggested to regulate aging of multiple tissues, whereas myostatin is a well-described negative regulator of postnatal skeletal and cardiac muscle mass and modulates metabolic processes. In this review, we discuss the biochemical regulation of GDF11 and myostatin and their functions in the heart, skeletal muscle, and brain. We also highlight recent clinical findings with respect to a potential role for GDF11 and/or myostatin in humans with heart disease. Finally, we address key outstanding questions related to GDF11 and myostatin dynamics and signaling during development, growth, and aging.

Silencing Myostatin Using Cholesterol-conjugated siRNAs Induces Muscle Growth

Short interfering RNAs (siRNAs) are a valuable tool for gene silencing with applications in both target validation and therapeutics. Many advances have recently been made to improve potency and specificity, and reduce toxicity and immunostimulation. However, siRNA delivery to a variety of tissues remains an obstacle for this technology. To date, siRNA delivery to muscle has only been achieved by local administration or by methods with limited potential use in the clinic. We report systemic delivery of a highly chemically modified cholesterol-conjugated siRNA targeting muscle-specific gene myostatin (Mstn) to a full range of muscles in mice. Following a single intravenous injection, we observe 85–95% knockdown of Mstn mRNA in skeletal muscle and >65% reduction in circulating Mstn protein sustained for >21 days. This level of Mstn knockdown is also accompanied by a functional effect on skeletal muscle, with animals showing an increase in muscle mass, size, and strength. The cholesterol-conjugated siRNA platform described here could have major implications for treatment of a variety of muscle disorders, including muscular atrophic diseases, muscular dystrophy, and type II diabetes.

The Role of Exercise in Cardiac Aging: From Physiology to Molecular Mechanisms

Aging induces structural and functional changes in the heart that are associated with increased risk of cardiovascular disease and impaired functional capacity in the elderly. Exercise is a diagnostic and therapeutic tool, with the potential to provide insights into clinical diagnosis and prognosis, as well as the molecular mechanisms by which aging influences cardiac physiology and function. In this review, we first provide an overview of how aging impacts the cardiac response to exercise, and the implications this has for functional capacity in older adults. We then review the underlying molecular mechanisms by which cardiac aging contributes to exercise intolerance, and conversely how exercise training can potentially modulate aging phenotypes in the heart. Finally, we highlight the potential use of these exercise models to complement models of disease in efforts to uncover new therapeutic targets to prevent or treat heart disease in the aging population.

Myostatin Attenuation In Vivo Reduces Adiposity, but Activates Adipogenesis

A potentially novel approach for treating obesity includes attenuating myostatin as this increases muscle mass and decreases fat mass. Notwithstanding, conflicting studies report that myostatin stimulates or inhibits adipogenesis and it is unknown whether reduced adiposity with myostatin attenuation results from changes in fat deposition or adipogenesis. We therefore quantified changes in the stem, transit amplifying and progenitor cell pool in white adipose tissue (WAT) and brown adipose tissue (BAT) using label-retaining wild-type and mstn(-/-) (Jekyll) mice. Muscle mass was larger in Jekyll mice, WAT and BAT mass was smaller and label induction was equal in all tissues from both wild-type and Jekyll mice. The number of label-retaining cells, however, dissipated quicker in WAT and BAT of Jekyll mice and was only 25% and 17%, respectively, of wild-type cell counts 1 month after induction. Adipose cell density was significantly higher in Jekyll mice and increased over time concomitant with label-retaining cell disappearance, which is consistent with enhanced expansion and differentiation of the stem, transit amplifying and progenitor pool. Stromal vascular cells from Jekyll WAT and BAT differentiated into mature adipocytes at a faster rate than wild-type cells and although Jekyll WAT cells also proliferated quicker in vitro, those from BAT did not. Differentiation marker expression in vitro, however, suggests that mstn(-/-) BAT preadipocytes are far more sensitive to the suppressive effects of myostatin. These results suggest that myostatin attenuation stimulates adipogenesis in vivo and that the reduced adiposity in mstn(-/-) animals results from nutrient partitioning away from fat and in support of muscle.

Reduced Circulating GDF11 Is Unlikely Responsible for Age-Dependent Changes in Mouse Heart, Muscle, and Brain

Recent high-profile studies report conflicting data on the age-related change in circulating growth/differentiation factor 11 (GDF11) and myostatin as well as the former's influence on muscle regeneration. Both ligands bind and activate ActRIIB receptors with similar affinities and should therefore have similar actions, yet these studies suggest that GDF11 activates muscle regeneration whereas myostatin is well known to inhibit it. They also suggest that circulating GDF11 levels, but not those of myostatin, decline with age. We performed a careful assessment of the ELISA used to quantify circulating myostatin in these studies and determined that assay reagents significantly cross react with each protein, each of which is highly homologous. Circulating myostatin levels decreased with age and estimates of GDF11 levels using myostatin null mice indicate that they were almost 500 times lower than those for myostatin. This suggests that circulating GDF11 has little physiological relevance as it could not outcompete myostatin for ActRIIB binding sites. Together, these results further suggest that the previously reported aging muscle, heart, and brain phenotypes attributed to reduced circulating GDF11 should be reconsidered.

Rat cardiac troponin T mutation (F72L)-mediated impact on thin filament cooperativity is divergently modulated by α- and β-myosin heavy chain isoforms

The primary causal link between disparate effects of human hypertrophic cardiomyopathy (HCM)-related mutations in troponin T (TnT) and α- and β-myosin heavy chain (MHC) isoforms on cardiac contractile phenotype remains poorly understood. Given the divergent impact of α- and β-MHC on the NH2-terminal extension (44-73 residues) of TnT, we tested if the effects of the HCM-linked mutation (TnTF70L) were differentially altered by α- and β-MHC. We hypothesized that the emergence of divergent thin filament cooperativity would lead to contrasting effects of TnTF70L on contractile function in the presence of α- and β-MHC. The rat TnT analog of the human F70L mutation (TnTF72L) or the wild-type rat TnT (TnTWT) was reconstituted into demembranated muscle fibers from normal (α-MHC) and propylthiouracil-treated (β-MHC) rat hearts to measure steady-state and dynamic contractile function. TnTF72L-mediated effects on tension, myofilament Ca(2+) sensitivity, myofilament cooperativity, rate constants of cross-bridge (XB) recruitment dynamics, and force redevelopment were divergently modulated by α- and β-MHC. TnTF72L increased the rate of XB distortion dynamics by 49% in α-MHC fibers but had no effect in β-MHC fibers; these observations suggest that TnTF72L augmented XB detachment kinetics in α-MHC, but not β-MHC, fibers. TnTF72L increased the negative impact of strained XBs on the force-bearing XBs by 39% in α-MHC fibers but had no effect in β-MHC fibers. Therefore, TnTF72L leads to contractile changes that are linked to dilated cardiomyopathy in the presence of α-MHC. On the other hand, TnTF72L leads to contractile changes that are linked to HCM in the presence of β-MHC.

Myostatin and IGF-I signaling in end-stage human heart failure: a qRT-PCR study

Background

Myostatin (Mstn) is a key regulator of heart metabolism and cardiomyocyte growth interacting tightly with insulin-like growth factor I (IGF-I) under physiological conditions. The pathological role of Mstn has also been suggested since Mstn protein was shown to be upregulated in the myocardium of end-stage heart failure. However, no data are available about the regulation of gene expression of Mstn and IGF-I in different regions of healthy or pathologic human hearts, although they both might play a crucial role in the pathomechanism of heart failure.

Methods

In the present study, heart samples were collected from left ventricles, septum and right ventricles of control healthy individuals as well as from failing hearts of dilated (DCM) or ischemic cardiomyopathic (ICM) patients. A comprehensive qRT-PCR analysis of Mstn and IGF-I signaling was carried out by measuring expression of Mstn, its receptor Activin receptor IIB (ActRIIB), IGF-I, IGF-I receptor (IGF-IR), and the negative regulator of Mstn miR-208, respectively. Moreover, we combined the measured transcript levels and created complex parameters characterizing either Mstn- or IGF-I signaling in the different regions of healthy or failing hearts.

Results

We have found that in healthy control hearts, the ratio of Mstn/IGF-I signaling was significantly higher in the left ventricle/septum than in the right ventricle. Moreover, Mstn transcript levels were significantly upregulated in all heart regions of DCM but not ICM patients. However, the ratio of Mstn/IGF-I signaling remained increased in the left ventricle/septum compared to the right ventricle of DCM patients (similarly to the healthy hearts). In contrast, in ICM hearts significant transcript changes were detected mainly in IGF-I signaling. In paralell with these results miR-208 showed mild upregulation in the left ventricle of both DCM and ICM hearts.

Conclusions

This is the first demonstration of a spatial asymmetry in the expression pattern of Mstn/IGF-I in healthy hearts, which is likely to play a role in the different growth regulation of left vs. right ventricle. Moreover, we identified Mstn as a massively regulated gene in DCM but not in ICM as part of possible compensatory mechanisms in the failing heart.

Cardiac myostatin upregulation occurs immediately after myocardial ischemia and is involved in skeletal muscle activation of atrophy

Myostatin (MSTN), a negative regulator of muscle growth and size, is increased after acute myocardial infarction (AMI) but timing of upregulation after injury is not known. In this study, we investigated the timing of the MSTN/AKT/p38 pathway activation in heart and skeletal muscle after AMI, as well as the potential effect of cardiac injury-related MSTN endocrine signaling on skeletal muscle and other circulating growth factors.

METHODS

Coronary artery ligation was performed in C57BL/6 mice at age 8 weeks to induce AMI. Mice were sacrificed at different time points (10 m, 1 h, 2 h, 6 h, 12 h, 24 h, 1 week, 2 weeks, 1 months and 2 months) after surgery (n=3 per time point, n=18 total).

RESULTS

Cardiac and circulating MSTN upregulation occurred as early as 10 min after AMI. Two months after AMI, increased cardiac MSTN/SMAD2,3 and p38 together with decreased IGF-1/AKT signaling suggest an anti-hypertrophic profile. In skeletal muscle, an absence of local MSTN increase was accompanied by increased MSTN-dependent SMAD2,3 signaling, suggestive of paracrine effects due to cardiac-derived MSTN. Protein degradation by the ubiquitin-proteasome system in the skeletal muscle was also evident. Serum from 24h post-MI mice effectively induced a MSTN-dependent increase in atrogin1 and MuRF1.

CONCLUSION

Our study shows that cardiac MTSN activation occurs rapidly after cardiac ischemia and may be involved in peripheral protein degradation in the skeletal muscle by activating atrogin1 and MuRF1.

Instability in the central region of tropomyosin modulates the function of its overlapping ends

The causal link between disparate tropomyosin (Tm) functions and the structural instability in Tm is unknown. To test the hypothesis that the structural instability in the central region of Tm modulates the function of the overlapping ends of contiguous Tm dimers, we used transgenic mice (Tm(DM)) that expressed a mutant α-Tm in the heart; S229E and H276N substitutions induce structural instability in the central region and the overlapping ends of Tm, respectively. In addition, two mouse cardiac troponin T mutants (TnT(1-44Δ) and TnT(45-74Δ)) that have a divergent effect on the overlapping ends of Tm were employed. The S229E-induced instability in the central region of Tm(DM) altered the overlapping ends of Tm(DM), thereby it negated the attenuating effect of H276N on Ca(2+)-activated maximal tension. The rate of cross-bridge detachment (g) decreased in Tm(DM)+TnT(WT) and Tm(H276N)+TnT(WT) fibers but increased in Tm(DM)+TnT(45-74Δ) fibers; however, TnT(45-74Δ) did not alter g, demonstrating that S229E in Tm(DM) had divergent effects on g. The S229E substitution in Tm(DM) ablated the H276N-induced desensitization of myofilament Ca(2+) sensitivity in Tm(DM)+TnT(1-44Δ) fibers. To our knowledge, novel findings from this study show that the structural instability in the central region of Tm modifies cardiac contractile function via its effect on the overlapping ends of contiguous Tm.

Myostatin regulates energy homeostasis in the heart and prevents heart failure

RATIONALE

Myostatin is a major negative regulator of skeletal muscle mass and initiates multiple metabolic changes, including enhanced insulin sensitivity. However, the function of myostatin in the heart is barely understood, although it is upregulated in the myocardium under several pathological conditions.

OBJECTIVE

Here, we aimed to decipher the role of myostatin and myostatin-dependent signaling pathways for cardiac function and cardiac metabolism in adult mice. To avoid potential counterregulatory mechanisms occurring in constitutive and germ-line-based myostatin mutants, we generated a mouse model that allows myostatin inactivation in adult cardiomyocytes.

METHODS AND RESULTS

Cardiac MRI revealed that genetic inactivation of myostatin signaling in the adult murine heart caused cardiac hypertrophy and heart failure, partially recapitulating effects of the age-dependent decline of the myostatin paralog growth and differentiation factor 11. We found that myostatin represses AMP-activated kinase activation in the heart via transforming growth factor-β-activated kinase 1, thereby preventing a metabolic switch toward glycolysis and glycogen accumulation. Furthermore, myostatin stimulated expression of regulator of G-protein signaling 2, a GTPase-activating protein that restricts Gaq and Gas signaling and thereby protects against cardiac failure. Inhibition of AMP-activated kinase in vivo rescued cardiac hypertrophy and prevented enhanced glycolytic flow and glycogen accumulation after inactivation of myostatin in cardiomyocytes.

CONCLUSIONS

Our results uncover an important role of myostatin in the heart for maintaining cardiac energy homeostasis and preventing cardiac hypertrophy.

Myostatin regulates tissue potency and cardiac calcium-handling proteins

Attenuating myostatin enhances striated muscle growth, reduces adiposity, and improves cardiac contractility. To determine whether myostatin influences tissue potency in a manner that could control such pleiotropic actions, we generated label-retaining mice with wild-type and mstn(-/-) (Jekyll) backgrounds in which slow-cycling stem, transit-amplifying, and progenitor cells are preferentially labeled by histone 2B/green fluorescent protein. Jekyll mice were born with fewer label-retaining cells (LRCs) in muscle and heart, consistent with increased stem/progenitor cell contributions to embryonic growth of both tissues. Cardiac LRC recruitment from noncardiac sources occurred in both groups, but lasted longer in Jekyll hearts, whereas heightened β-adrenergic sensitivity of mstn(-/-) hearts was explained by elevated SERCA2a, phospholamban, and β2-adrenergic receptor levels. Jekyll mice were also born with more adipose LRCs despite significantly smaller tissue weights. Reduced adiposity in mstn(-/-) animals is therefore due to reduced lipid deposition as adipoprogenitor pools appear to be enhanced. By contrast, increased bone densities of mstn(-/-) mice are likely compensatory to hypermuscularity because LRC counts were similar in Jekyll and wild-type tibia. Myostatin therefore significantly influences the potency of different tissues, not just muscle, as well as cardiac Ca²⁺-handling proteins. Thus, the pleiotropic phenotype of mstn(-/-) animals may not be due to enhanced muscle development per se, but also to altered stem/progenitor cell pools that ultimately influence tissue potency.

Through thick and thin: a circulating growth factor inhibits age-related cardiac hypertrophy

In an intriguing new study, Loffredo et al report that joining the circulation of old mice with that of young mice reduces age-related cardiac hypertrophy. They also found that the growth factor growth/differentiation factor 11 is a circulating negative regulator of cardiac hypertrophy which suggests that raising growth/differentiation factor 11 levels may be useful to treat cardiac hypertrophy associated with aging.

Divergent effects of α- and β-myosin heavy chain isoforms on the N terminus of rat cardiac troponin T

Divergent effects of α– and β–myosin heavy chain (MHC) isoforms on contractile behavior arise mainly because of their impact on thin filament cooperativity. The N terminus of cardiac troponin T (cTnT) also modulates thin filament cooperativity. Our hypothesis is that the impact of the N terminus of cTnT on thin filament activation is modulated by a shift from α- to β-MHC isoform. We engineered two recombinant proteins by deleting residues 1–43 and 44–73 in rat cTnT (RcTnT): RcTnT_1–43Δ and RcTnT_44–73Δ, respectively. Dynamic and steady-state contractile parameters were measured at sarcomere length of 2.3 µm after reconstituting proteins into detergent-skinned muscle fibers from normal (α-MHC) and propylthiouracil-treated (β-MHC) rat hearts. α-MHC attenuated Ca²⁺-activated maximal tension (∼46%) in RcTnT_1–43Δ fibers. In contrast, β-MHC decreased tension only by 19% in RcTnT_1–43Δ fibers. Both α- and β-MHC did not affect tension in RcTnT_44–73Δ fibers. The instantaneous muscle fiber stiffness measurements corroborated the divergent impact of α- and β-MHC on tension in RcTnT_1–43Δ fibers. pCa₅₀ (-log of [Ca²⁺]_free required for half-maximal activation) decreased significantly by 0.13 pCa units in α-MHC + RcTnT_1–43Δ fibers but remained unaltered in β-MHC + RcTnT_1–43Δ fibers, demonstrating that β-MHC counteracted the attenuating effect of RcTnT_1–43Δ on myofilament Ca²⁺ sensitivity. β-MHC did not alter the sudden stretch–mediated recruitment of new cross-bridges (E_R) in RcTnT_1–43Δ fibers, but α-MHC attenuated E_R by 36% in RcTnT_1–43Δ fibers. The divergent impact of α- and β-MHC on how the N terminus of cTnT modulates contractile dynamics has implications for heart disease; alterations in cTnT and MHC are known to occur via changes in isoform expression or mutations.

Myostatin inhibits myosatellite cell proliferation and consequently activates differentiation: evidence for endocrine-regulated transcript processing

Myostatin is a potent negative regulator of muscle growth in mammals. Despite high structural conservation, functional conservation in nonmammalian species is only assumed. This is particularly true for fish due to the presence of several myostatin paralogs: two in most species and four in salmonids (MSTN-1a, -1b, -2a, and -2b). Rainbow trout are a rich source of primary myosatellite cells as hyperplastic muscle growth occurs even in adult fish. These cells were therefore used to determine myostatin's effects on proliferation whereas our earlier studies reported its effects on quiescent cells. As in mammals, recombinant myostatin suppressed proliferation with no changes in cell morphology. Expression of MSTN-1a was several fold higher than the other paralogs and was autoregulated by myostatin, which also upregulated the expression of key differentiation markers: Myf5, MyoD1, myogenin, and myosin light chain. Thus, myostatin-stimulated cellular growth inhibition activates rather than represses differentiation. IGF-1 stimulated proliferation but had minimal and delayed effects on differentiation and its actions were suppressed by myostatin. However, IGF-1 upregulated MSTN-2a expression and the processing of its transcript, which is normally unprocessed. Myostatin therefore appears to partly mediate IGF-stimulated myosatellite differentiation in rainbow trout. This also occurs in mammals, although the IGF-stimulated processing of MSTN-2a transcripts is highly unique and is indicative of subfunctionalization within the gene family. These studies also suggest that the myokine's actions, including its antagonistic relationship with IGF-1, are conserved and that the salmonid gene family is functionally diverging.

The aging myostatin null phenotype: reduced adiposity, cardiac hypertrophy, enhanced cardiac stress response, and sexual dimorphism

The natural aging process results in the physiological decline of multiple tissues and organ systems. Changes commonly occur with middle age and include decreased skeletal muscle mass, bone mineral density, cardiac output, and insulin sensitivity, and increased adiposity, all of which can contribute to the onset of sarcopenia, osteoporosis, heart failure, or type 2 diabetes. Recent studies suggest that myostatin may influence many of these systems. We therefore sought to determine whether they are affected by aging, especially in 'middle-aged' Mstn-/- mice (12-20 months old (m.o.)). Although body weights were similar in wild-type (WT) and Mstn-/- mice, lean fat-free mass and skeletal muscles composed of predominantly type I, II, and mixed fibers were significantly heavier in Mstn-/- mice. These differences were accompanied by lower total adiposity, especially in female mice, white and brown fat pad weights, and adipocyte size. Hearts were heavier in Mstn-/- mice across a large age range (3-24 m.o.) and exhibited signs of dilated cardiomyopathy at rest, which include lower strain measurements compared with WT myocardium. However, Mstn-/- mice responded better to isoproterenol stress tests with greater increases in fractional shortening and ejection fraction-differences that were again more apparent in females and which are consistent with physiological cardiac hypertrophy. Spleens and kidneys were also smaller, although histologically normal, in Mstn-/- mice. These data together suggest that attenuating myostatin could potentially prevent or possibly treat pathological conditions that develop with age. Additional studies are nevertheless needed to definitively assess potential risks to cardiac function.

Myostatin stimulates myosatellite cell differentiation in a novel model system: evidence for gene subfunctionalization

Myosatellite cells play an important role in mammalian muscle regeneration as they differentiate and fuse with mature fibers. In fish, they also contribute to postnatal growth and the formation of new fibers. The relative conservation of fish systems, however, is not well known nor are the underlying mechanisms that control myosatellite cell differentiation. We therefore characterized this process in primary cells from rainbow trout and determined the effects of two known regulators in mammalian systems: IGF-I and myostatin. Unlike mammalian cell lines, subconfluent and proliferating trout myosatellite cells differentiated spontaneously and at a rate proportional to serum concentration. The expression of key myogenic markers (Myf5, MyoD1, myogenin, and MLC) and of the different myostatin paralogs (MSTN-1a/1b/2a) increased with serum-stimulated differentiation, although MSTN-1a expression was consistently higher than that of the other paralogs. In addition, MSTN-2a was only expressed as an unprocessed transcript. In low serum, where differentiation is normally suppressed, recombinant myostatin stimulated myogenic marker expression over time. The opposite was true for IGF-I as it stimulated proliferation, not differentiation, and additionally antagonized myostatin. This includes myostatin's effects on marker expression and on the autoregulation of MSTN-1a and -1b expression. These results conflict with studies using mammalian cell lines and suggest, alternatively, that myostatin is a positive, not negative, regulator of myosatellite cell differentiation. Mammalian myoblasts differentiate when confluent and with serum withdrawal, which differs considerably from how myosatellite cells differentiate in vivo. Thus the primary rainbow trout myosatellite cell culture system appears to be more physiologically relevant.

Diagnosing destabilized heart failure in the emergency setting: current and future biomarker tests

Acute or destabilized heart failure (DHF) is characterized by new or worsening signs and symptoms of heart failure leading to admission to an emergency department. Biomarkers may support the diagnosis, the prognosis and the management of DHF patients. The aim of this review article is to discuss and evaluate the clinical usefulness of both recognized and potential new biomarker tests for use in heart failure.

Telethonin deficiency is associated with maladaptation to biomechanical stress in the mammalian heart

RATIONALE

Telethonin (also known as titin-cap or t-cap) is a 19-kDa Z-disk protein with a unique β-sheet structure, hypothesized to assemble in a palindromic way with the N-terminal portion of titin and to constitute a signalosome participating in the process of cardiomechanosensing. In addition, a variety of telethonin mutations are associated with the development of several different diseases; however, little is known about the underlying molecular mechanisms and telethonin's in vivo function.

OBJECTIVE

Here we aim to investigate the role of telethonin in vivo and to identify molecular mechanisms underlying disease as a result of its mutation.

METHODS AND RESULTS

By using a variety of different genetically altered animal models and biophysical experiments we show that contrary to previous views, telethonin is not an indispensable component of the titin-anchoring system, nor is deletion of the gene or cardiac specific overexpression associated with a spontaneous cardiac phenotype. Rather, additional titin-anchorage sites, such as actin-titin cross-links via α-actinin, are sufficient to maintain Z-disk stability despite the loss of telethonin. We demonstrate that a main novel function of telethonin is to modulate the turnover of the proapoptotic tumor suppressor p53 after biomechanical stress in the nuclear compartment, thus linking telethonin, a protein well known to be present at the Z-disk, directly to apoptosis ("mechanoptosis"). In addition, loss of telethonin mRNA and nuclear accumulation of this protein is associated with human heart failure, an effect that may contribute to enhanced rates of apoptosis found in these hearts.

CONCLUSIONS

Telethonin knockout mice do not reveal defective heart development or heart function under basal conditions, but develop heart failure following biomechanical stress, owing at least in part to apoptosis of cardiomyocytes, an effect that may also play a role in human heart failure.

Myostatin from the heart: local and systemic actions in cardiac failure and muscle wasting

A significant proportion of heart failure patients develop skeletal muscle wasting and cardiac cachexia, which is associated with a very poor prognosis. Recently, myostatin, a cytokine from the transforming growth factor-β (TGF-β) family and a known strong inhibitor of skeletal muscle growth, has been identified as a direct mediator of skeletal muscle atrophy in mice with heart failure. Myostatin is mainly expressed in skeletal muscle, although basal expression is also detectable in heart and adipose tissue. During pathological loading of the heart, the myocardium produces and secretes myostatin into the circulation where it inhibits skeletal muscle growth. Thus, genetic elimination of myostatin from the heart reduces skeletal muscle atrophy in mice with heart failure, whereas transgenic overexpression of myostatin in the heart is capable of inducing muscle wasting. In addition to its endocrine action on skeletal muscle, cardiac myostatin production also modestly inhibits cardiomyocyte growth under certain circumstances, as well as induces cardiac fibrosis and alterations in ventricular function. Interestingly, heart failure patients show elevated myostatin levels in their serum. To therapeutically influence skeletal muscle wasting, direct inhibition of myostatin was shown to positively impact skeletal muscle mass in heart failure, suggesting a promising strategy for the treatment of cardiac cachexia in the future.

Endocrine actions of myostatin: systemic regulation of the IGF and IGF binding protein axis

Myostatin's inhibitory actions on striated muscle growth are believed to be directly mediated by locally produced myostatin and possibly by IGF binding proteins (IGFBPs). We therefore measured skeletal muscle, heart, and liver expression, in neonates and adults, and circulating levels of various IGF axis components (IGF-I, IGFBP-1 to IGFBP-3, and acid labile subunit) in wild-type and mstn-/- mice. Compared with wild type, differences in muscle expression were tissue specific, although IGF-I receptor expression was higher in all mstn-/- neonatal tissues and in adult gastrocnemius. Liver expression of several components also differed between genotype as IGF-I receptor, IGFBP-3 and IGFBP-5 expression was higher in mstn-/- neonates and IGF-I and IGFBP-3 in adults. Circulating IGF-I levels were also higher in mstn-/- adults, whereas IGFBP-1 and IGFBP-2 levels were lower. Comparing IGF-I:IGFBP molar ratios suggested that the relative IGF-binding capacity was potentially lower in mstn-/- mice, and thus, total and "free" IGF-I levels may be elevated. This in turn may increase negative feedback control on GH, because mstn-/- liver weights were lower. Bone growth was similar in both genotypes, suggesting that changes in circulating IGF-I may be more important to muscle, whose mass is enhanced in mstn-/- mice, than to bone. Myostatin receptors, but not myostatin itself, are expressed in the liver. Changes in hepatic production of circulating IGF axis components could therefore result from the loss of endocrine myostatin. Thus, myostatin may inhibit striated muscle growth directly at the cellular level and indirectly through systemic effects on the IGF axis.

Reversal of cancer cachexia and muscle wasting by ActRIIB antagonism leads to prolonged survival

Muscle wasting and cachexia have long been postulated to be key determinants of cancer-related death, but there has been no direct experimental evidence to substantiate this hypothesis. Here, we show that in several cancer cachexia models, pharmacological blockade of ActRIIB pathway not only prevents further muscle wasting but also completely reverses prior loss of skeletal muscle and cancer-induced cardiac atrophy. This treatment dramatically prolongs survival, even of animals in which tumor growth is not inhibited and fat loss and production of proinflammatory cytokines are not reduced. ActRIIB pathway blockade abolished the activation of the ubiquitin-proteasome system and the induction of atrophy-specific ubiquitin ligases in muscles and also markedly stimulated muscle stem cell growth. These findings establish a crucial link between activation of the ActRIIB pathway and the development of cancer cachexia. Thus ActRIIB antagonism is a promising new approach for treating cancer cachexia, whose inhibition per se prolongs survival.