In this study we identified cardiac myostatin as an important mediator of muscle atrophy in heart failure. Myostatin is highly and almost exclusively expressed in skeletal muscle where it strongly inhibits myoblast proliferation before birth, as well as muscle fiber hypertrophy before birth and in adulthood.6,7
While myostatin is generated in skeletal muscle where it acts in an autocrine manner, it is also present in the plasma, implying a systemic role of this TGFβ-family member protein, and the possibility that it is made and secreted by other organs.15
Indeed, myostatin is also expressed in heart and fat tissue, which could serve as another source of this factor and contribute to total plasma levels.7,9,10,13
Our analysis of gene-targeted mice suggests that it is the local production of myostatin within skeletal muscle itself that dominantly regulates developmental growth and hypertrophy in adulthood, which is a novel finding not previously reported. For example, deletion of Mstn
only in skeletal muscle with the MLC1f-cre
knock-in allele resulted in a robust 72% increase in the weight of the quadriceps at 2 months of age. Prior to this work, another group crossed Mstn
-loxP targeted mice with transgenic mice expressing cre under the muscle creatine kinase (MCK) promoter, which similarly induced skeletal muscle hypertrophy.21
However, the MCK promoter is also expressed in the heart so this approach did not unequivocally prove that deletion of Mstn
from skeletal muscle is the reason for hypertrophy induction.
Our data suggest that heart myostatin expression at baseline does not dominantly affect skeletal muscle mass. For example, cardiomyocyte-specific deletion of Mstn
with the Nkx2.5-cre
allele did not increase skeletal muscle mass in adult mice, suggesting that local production of myostatin within skeletal muscle is the primary regulator of myofiber growth, and that the heart does not secrete enough myostatin under normal physiologic conditions to impact skeletal muscle. However, myostatin production is induced in the heart by pathologic insults, which may then significantly contribute to plasma levels to secondarily impact skeletal muscle mass.8–12
Indeed, TAC stimulation in wildtype mice, but not in heart-specific Mstn
-deleted mice, enhanced circulating myostatin levels. The simplest interpretation of this observation is that the heart secretes myostatin, causing/contributing to an increase in total plasma levels after injury or prolonged cardiac stress stimulation. Myostatin protein expression is also induced in cultured cardiomyocytes in response to cyclic stretching.22
Thus, cardiac stress likely induces physiologically meaningful myostatin expression or release, which can have an effect on skeletal muscle. Indeed, α-MHC-myostatin transgenic mice showed skeletal muscle wasting and atrophy, indicating that cardiac production is competent to regulate muscle mass through an endocrine-like mechanism. From a teleological perspective, it might be beneficial to rarify skeletal muscle during heart failure to secondarily reduce total circulatory burden on the heart, although too much rarefaction likely becomes maladaptive, leading to excessive morbidity.
Many patients with heart failure present with skeletal muscle atrophy, reductions in fiber strength normalized to cross-sectional area, and a disproportionate loss of exercise tolerance.3,23
Indeed, patients in advanced heart failure showed an approximate 10–30% reduction in limb muscle weight estimated by dual x-ray absorptiometry.23
Here we showed that induction of heart failure in mice, by applying long-term pressure overload, similarly led to muscle wasting. More specifically, we observed a 10%, 13% and 26% reduction in the weight of the gastrocnemius, quadriceps and soleus muscles, respectively, in wildtype mice 12 weeks after TAC. Thus, while this degree of muscle wasting is not highly robust, it is similar to clinically observed cachexia in heart failure. Remarkably, cardiomyocyte-specific deletion of Mstn
completely prevented skeletal muscle atrophy after 12 weeks of TAC, indicating that cardiac-generated myostatin influences skeletal muscle mass in heart failure. However, the function of myostatin proposed here does not preclude a role for inflammatory cytokines like TNF-α or neurohormones like epinephrine, norepinephrine and cortisol in also contributing to muscle wasting in heart failure.5
Indeed, we identified a significant increase in local TNF-α from the quadriceps after TAC-induced heart failure, although JA-16 antibody treatment did not reduce this expression (data not shown).
Beside its inhibitory effect on skeletal muscle growth, Mstn−/−
mice were shown to develop significantly more cardiac hypertrophy in response to phenylephrine infusion, suggesting that it can also mildly affect heart growth as well.8
Consistent with these results, we observed that cardiac overexpression of myostatin results in a small, but significant reduction of heart weight. Similarly, transgenic mice with MCK promoter-driven myostatin overexpression also showed a small reduction in heart weight.24
Together these data indicate that while myostatin is primarily a regulator of skeletal muscle mass, it can negatively influence cardiac muscle hypertrophy as well.
Genetic deletion of Mstn
from the heart appeared to be more effective in combating muscle wasting in heart failure than JA-16 antibody treatment (compare vs ). However, the genetic approach rendered the heart without Mstn
from the onset of the TAC experiment, while JA-16 antibody treatment began after the induction of heart failure by TAC. This might suggest that early inhibition of myostatin is more effective, or simply that JA-16 antibody results in only partial inhibition of myostatin, especially given the increased plasma levels that are produced in heart failure. Regardless of this issue, we demonstrate that systemic inhibition of myostatin with a blocking antibody in preexisting heart failure in mice can maintain overall muscle weights to values of sham operated control mice. The effectiveness of JA-16 had been previously reported in the treatment of some forms of muscular dystrophy, where antibody mediated inhibition of myostatin increased muscle weights, strength, and also reduced fibrosis.20,25
Although drugs commonly used for the treatment of heart failure can reduce muscle atrophy (ACE inhibitors, for example, reduce the risk of cachexia in heart failure by 19%), a targeted and more efficient treatment for cardiac cachexia is needed.26,27
Interestingly, although muscle weights were increased in our study, this effect did not correlate with an improvement in mortality or cardiac function after long-term TAC. Thus, while combating muscle atrophy in heart failure may improve the quality of life in patients, it remains to be determined if it will ultimately extend lifespan.