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

Role of Myostatin in Rheumatoid Arthritis: A Review of the Clinical Impact

Rheumatoid arthritis (RA) is a chronic inflammatory disease that affects synovial joints and that frequently involves extra-articular organs. A multiplicity of interleukins (IL) participates in the pathogenesis of RA, including IL-6, IL-1β, transforming growth factor-beta (TGF-β), and tumor necrosis factor (TNF)-α; immune cells such as monocytes, T and B lymphocytes, and macrophages; and auto-antibodies, mainly rheumatoid factor and anti-citrullinated protein antibodies (ACPAs). Skeletal muscle is also involved in RA, with many patients developing muscle wasting and sarcopenia. Several mechanisms are involved in the myopenia observed in RA, and one of them includes the effects of some interleukins and myokines on myocytes. Myostatin is a myokine member of the TGF-β superfamily; the overproduction of myostatin acts as a negative regulator of growth and differentiates the muscle fibers, limiting their number and size. Recent studies have identified abnormalities in the serum myostatin levels of RA patients, and these have been found to be associated with muscle wasting and other manifestations of severe RA. This review analyzes recent information regarding the relationship between myostatin levels and clinical manifestations of RA and the relevance of myostatin as a therapeutic target for future research.

The Effect of MSTN Mutation on Bile Acid Metabolism and Lipid Metabolism in Cattle

Myostatin (MSTN) is a negative regulator of skeletal muscle genesis during development. MSTN mutation leads to increased lean meat production and reduced fat deposition in livestock. However, the mechanism by which MSTN promotes myogenesis by regulating metabolism is not clear. In this study, we compared the metabolomics of the livers of wild-type (WT) and MSTN mutation cattle (MT), and found changes in the content and proportion of fatty acids and bile acids in MT cattle. The differential metabolites were enriched in sterol synthesis and primary bile acid synthesis. We further analyzed the expression of genes involved in the regulation of lipid and bile acid metabolism, and found that the loss of MSTN may alter lipid synthesis and bile acid metabolism. This study provides new basic data for MSTN mutations in beef cattle breeding.

Systematic training in master swimmer athletes increases serum insulin growth factor-1 and decreases myostatin and irisin levels

During ageing, anabolic status is essential to prevent the decrease in quantity and quality of skeletal muscle mass (SMM). Exercise modulates endocrine markers of muscle status. We studied the differences of endocrine markers for muscle status in 62 non-sarcopenic Mexican swimmer adults aged 30-70 y/o, allocated into two groups: the systematic training (ST) group including master athletes with a physical activity level (PAL) >1.6, and the non-systematic training group (NST) composed by subjects with a PAL <1.5. Body composition, diet, biochemical and endocrine markers were analyzed. The ST group showed lower myostatin (MSTN) and irisin (IRI) levels, two strong regulators of SMM. The insulin growth factor-1 (IGF-1) was higher in the ST. This is consistent with most of the evidence in young athletes and resistance training programs, where IGF-1 and IRI seem to play a crucial role in maintaining anabolic status in master athletes.

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.

Muscle-bone axis in children with chronic kidney disease: current knowledge and future perspectives

Bone and muscle tissue are developed hand-in-hand during childhood and adolescence and interact through mechanical loads and biochemical pathways forming the musculoskeletal system. Chronic kidney disease (CKD) is widely considered as both a bone and muscle-weakening disease, eventually leading to frailty phenotype, with detrimental effects on overall morbidity. CKD also interferes in the biomechanical communication between two tissues. Pathogenetic mechanisms including systemic inflammation, anorexia, physical inactivity, vitamin D deficiency and secondary hyperparathyroidism, metabolic acidosis, impaired growth hormone/insulin growth factor 1 axis, insulin resistance, and activation of renin-angiotensin system are incriminated for longitudinal uncoordinated loss of bone mineral content, bone strength, muscle mass, and muscle strength, leading to mechanical impairment of the functional muscle-bone unit. At the same time, CKD may also interfere in the biochemical crosstalk between the two organs, through inhibiting or stimulating the expression of certain osteokines and myokines. This review focuses on presenting current knowledge, according to in vitro, in vivo, and clinical studies, concerning the pathogenetic pathways involved in the muscle-bone axis, and suggests approaches aimed at preventing bone loss and muscle wasting in the pediatric population. Novel therapeutic targets for preserving musculoskeletal health in the context of CKD are also discussed.

Short-Term Resistance Training Supported by Whole-Body Cryostimulation Induced a Decrease in Myostatin Concentration and an Increase in Isokinetic Muscle Strength

The study aimed to determine whether combining cryostimulation with resistance training would effectively increase muscle strength, and if so, whether this adaptation would be related to changes in circulating levels of exerkines (i.e., mediators of systemic adaptation to exercise). Twenty-five students completed 12 sessions of resistance training, each followed by either cryostimulation (n = 15, 3 min exposure at -110 °C) or passive recovery (n = 10). Prior to and post this intervention, participants performed two eccentric cycling bouts (before and after training). At these points, serum concentrations of muscle damage marker (myoglobin), exerkines (interleukin 6 (IL-6), interleukin 15 (IL-15), irisin, brain-derived neurotrophic factor), hypertrophy-related factors (myostatin, insulin-like growth factor 1), and muscle strength were measured. The applied procedure reduced the physiological burden of the second eccentric cycling bout and myoglobin concentrations only in the group subject to cryostimulation. The same group also exhibited decreased levels of myostatin (from 4.7 ± 1.7 to 3.8 ± 1.8 ng·mL^-1, p < 0.05). A significant and large interaction between the group × time was noted in IL-15 concentration (p = 0.01, ηp2=0.27). Training and cryostimulation induced a positive and likely significant improvement of isokinetic muscle strength. Altogether, obtained results support the claim that resistance training combined with cold exposure modified muscle strength through modulation of myostatin and IL-15 concentrations.

The influence of skeletal muscle on appetite regulation

INTRODUCTION

Fat-free mass, of which skeletal muscle is amajor component, correlates positively with energy intake at energy balance. This is due to the effects of metabolically active tissue on energy expenditure which in itself appears to signal to the brain adrive to eat to ensure cellular energy homeostasis. The mechanisms responsible for this drive to eat are unknown but are likely to be related to energy utilization. Here muscle imparts an indirect influence on hunger. The drive to eat is also enhanced after muscle loss secondary to intentional weight loss. The evidence suggests loss of both fat mass and skeletal muscle mass directly influences the trajectory and magnitude of weight regain highlighting their potential role in long-termappetite control. The mechanisms responsible for the potential direct drive to eat stemming from muscle loss are unknown.

AREAS COVERED

The literature pertaining to muscle and appetite at energy balance and after weight loss was examined. Aliterature search was conducted to identify studies related to appetite, muscle, exercise, and weight loss.

EXPERT OPINION

Understanding the mechanisms which link energy expenditure and muscle loss to hunger has the potential to positively impact both the prevention and the treatment of obesity.

The Dietary Lipid Content Affects the Tissue Gene Expression of Muscle Growth Biomarkers and the GH/IGF System of Pejerrey (Odontesthes bonariensis) Juveniles

Gene expression of growth hormone receptors (GHRs), insulin-like growth factors (IGFs), myostatin (MSTN) and myogenin (MyoG) was analyzed in juveniles pejerrey fed with graded levels of lipids (L): 6% (L6), 10% (L10), 25% (L25). After 14 weeks, no changes were found in liver GHR-I GHR-II and IGF-II mRNA levels whereas IGF-I decreased in L10 and L25. Muscle GHR-I gene expression increased in L25 whereas GHR-II, IGF-II and MyoG were higher in L6. IGF-I and MSTN expression was not affected by the different diets. Adipose IGF-I mRNA levels decreased in L10. Correlations between body weight and members of GH/IGF system in liver and skeletal muscle were found only in L10 group. Correlations found in L10 group between both liver and skeletal muscle GHR-I and IGF-I were lost in either L6 or L25 groups. Thus, fish fed with apparently unbalanced dietary lipid contents (6% and 25%) exhibit a compensatory regulation of systemic and local components of the GH/IGF axis. Furthermore, the marked inhibition of muscle MyoG gene expression in L25 might limit excessive lipid deposition and fish growth. Our data suggest that a dietary lipid contents of 10% would promote a particular adjustment of the endocrine and autocrine/paracrine GH/IGF system, stimulating body growth and perhaps muscle hyperplasia. On the other hand, a higher dietary lipid content would uncouple the GH/IGF system, reducing hepatic IGF-I, while slightly increasing hepatic GHR-I, probably to prompt lipolysis.

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.

Genetics of Bone and Muscle Interactions in Humans

PURPOSE OF REVIEW

To summarize the evidence from recent studies on the shared genetics between bone and muscle in humans.

RECENT FINDINGS

Genome-wide association studies (GWAS) have successfully identified a multitude of loci influencing the variability of different bone or muscle parameters, with multiple loci overlapping between the traits. In addition, joint analyses of multiple correlated musculoskeletal traits (i.e., multivariate GWAS) have underscored several genes with possible pleiotropic effects on both bone and muscle including MEF2C and SREBF1. Notably, several of the proposed pleiotropic genes have been validated using human cells or animal models. It is clear that the study of pleiotropy may provide novel insights into disease pathophysiology potentially leading to the identification of new treatment strategies that simultaneously prevent or treat both osteoporosis and sarcopenia. However, the role of muscle factors (myokines) that stimulate bone metabolism, as well as osteokines that affect muscles, is in its earliest stage of understanding.

GH replacement titrated to serum IGF-1 does not reduce concentrations of myostatin in blood or skeletal muscle

OBJECTIVE

Traditional weight-based regimens of GH replacement are more effective at reversing the loss of skeletal muscle in GH-deficient adults than currently recommended regimens, where the dose of GH is increased to restore serum concentrations of IGF-1. While weight-based regimens increase concentrations of IGF-1 and decrease concentrations of myostatin, it is not known whether the reduced effectiveness of individually titrated GH regimens is due to ongoing hypersecretion of myostatin. Consequently, the aims of this study were to determine whether concentrations of myostatin in blood and skeletal muscle are increased in GH-deficient adults, and whether these concentrations are decreased by GH replacement regimens titrated to restore serum IGF-1.

DESIGN

Twenty-six GH deficient adults (18 men and 8 women) were treated with individualised regimens of recombinant human GH aiming to achieve serum concentrations of IGF-1 within one standard deviation of the age- and gender-adjusted mean. Plasma concentrations of myostatin were measured at baseline and after 6 months of treatment were compared to fifteen healthy controls (9 men and 6 women). Skeletal muscle biopsies were performed in 19 of these GH-deficient adults (15 men and 4 women) and 10 of the healthy controls (6 men and 4 women). Expression of IGF-1 and myostatin mRNA was determined by qPCR.

RESULTS

Concentrations of IGF-1 in serum and mRNA in skeletal muscle were reduced, and concentrations of myostatin in plasma and mRNA in skeletal muscle were increased in GH-deficient adults at baseline (P < .05 versus healthy controls). Despite restoring concentrations of IGF-1, GH replacement did not reduce concentrations of myostatin in either blood or skeletal muscle. Concentrations of IGF-1 and myostatin in both blood and skeletal muscle were positively correlated in GH-deficient adults at baseline (P < .05), but not in GH-replete adults.

CONCLUSIONS

Concentrations of myostatin in blood and skeletal muscle are increased in GH-deficient adults. Despite normalising concentrations of IGF-1, individualised regimens of GH replacement do not reduce concentrations of myostatin in blood or skeletal muscle. Ongoing hypersecretion of myostatin may explain why individually titrated GH replacement regimens are less effective than higher weight-based regimens in increasing skeletal muscle mass.

Emerging role of myostatin and its inhibition in the setting of chronic kidney disease

The past two decades have witnessed tremendous progress in our understanding of the mechanisms underlying wasting and cachexia in chronic kidney disease (CKD) and in other chronic illnesses, such as cancer and heart failure. In all these conditions wasting is an effect of the activation of protein degradation in muscle, a response that increases the risk of morbidity and mortality. Major recent advances in our knowledge on how CKD and inflammation affect cellular signaling include the identification of the myostatin (MSTN)/activin system, and its related transcriptional program that promotes protein degradation. In addition, the identification of the role of MSTN/activin in the vascular wall shows premise that its inhibition can better control or prevent some effects of CKD on vessels, such as accelerated atherosclerosis and vascular calcifications. In this review, we summarize the expanding role of MSTN activation in promoting muscle atrophy and the recent clinical studies that investigated the efficacy of MSTN/activin pathway antagonism in sarcopenic patients. Moreover, we also review the utility of MSTN inhibition in the experimental models of CKD and its potential advantages in CKD patients. Lessons learned from clinical studies on MSTN antagonism in sarcopenic patients tell us that the anabolic intervention is likely better if we use a block of the two ActRII receptors. At the same time, however, it is becoming clear that MSTN-targeted therapies should not be seen as a substitute for physical activity and nutritional supplementation which are mandatory to successfully manage patients with wasting.

Role of IGF-binding proteins in regulating IGF responses to changes in metabolism

The IGF-binding protein family contains six members that share significant structural homology. Their principal function is to regulate the actions of IGF1 and IGF2. These proteins are present in plasma and extracellular fluids and regulate access of both IGF1 and II to the type I IGF receptor. Additionally, they have functions that are independent of their ability to bind IGFs. Each protein is regulated independently of IGF1 and IGF2, and this provides an important mechanism by which other hormones and physiologic variables can regulate IGF actions indirectly. Several members of the family are sensitive to changes in intermediary metabolism. Specifically the presence of obesity/insulin resistance can significantly alter the expression of these proteins. Similarly changes in nutrition or catabolism can alter their synthesis and degradation. Multiple hormones such as glucocorticoids, androgens, estrogen and insulin regulate IGFBP synthesis and bioavailability. In addition to their ability to regulate IGF access to receptors these proteins can bind to distinct cell surface proteins or proteins in extracellular matrix and several cellular functions are influenced by these interactions. IGFBPs can be transported intracellularly and interact with nuclear proteins to alter cellular physiology. In pathophysiologic states, there is significant dysregulation between the changes in IGFBP synthesis and bioavailability and changes in IGF1 and IGF2. These discordant changes can lead to marked alterations in IGF action. Although binding protein physiology and pathophysiology are complex, experimental results have provided an important avenue for understanding how IGF actions are regulated in a variety of physiologic and pathophysiologic conditions.

Downregulated Translation Initiation Signaling Predisposes Low-Birth-Weight Neonatal Pigs to Slower Rates of Muscle Protein Synthesis

Low-birth-weight (LBWT) neonates experience restricted muscle growth in their perinatal life. Our aim was to investigate the mechanisms that contribute to slower skeletal muscle growth of LBWT neonatal pigs. Twenty-four 1-day old male LBWT (816 ± 55 g) and normal-birth-weight (NBWT; 1,642 ± 55 g) littermates (n = 12) were euthanized to collect blood and longissimus dorsi (LD) muscle subsamples. Plasma glucose, insulin, and insulin-like growth factor-I (IGF-I) were lower in LBWT compared with NBWT pigs. Muscle IGF-I mRNA expression were lower in LBWT than NBWT pigs. However, IGF-I receptor mRNA and protein abundance was greater in LD of LBWT pigs. Abundance of myostatin and its receptors, and abundance and phosphorylation of smad3 were lower in LBWT LD by comparison with NBWT LD. Abundance of eukaryotic initiation factor (eIF) 4E binding protein 1 and mitogen-activated protein kinase-interacting kinases was lower in muscle of LBWT pigs compared with NBWT siblings, while eIF4E abundance and phosphorylation did not differ between the two groups. Furthermore, phosphorylation of ribosomal protein S6 kinase 1 (S6K1) was less in LBWT muscle, possibly due to lower eIF3e abundance. In addition, abundance and phosphorylation of eIF4G was reduced in LBWT pigs by comparison with NBWT littermates, suggesting translation initiation complex formation is compromised in muscle of LBWT pigs. In conclusion, diminished S6K1 activation and translation initiation signaling are likely the major contributors to impaired muscle growth in LBWT neonatal pigs. The upregulated IGF-I R expression and downregulated myostatin signaling seem to be compensatory responses for the reduction in protein synthesis signaling.

Myostatin and carbohydrate disturbances

Purpose/aim of the study: Myostatin is a myokine that has been shown to inhibit muscle growth and to have potentially deleterious effects on metabolism. The aim of the current study was to compare its circulating serum levels in subjects from the whole spectrum of carbohydrate disturbances leading to diabetes.

MATERIALS AND METHODS

A total of 159 age-, sex-, and BMI-matched subjects participated in the study - 50 had normal glucose tolerance (NGT), 60 had prediabetes (PreDM), and 49 had type 2 diabetes mellitus (T2D). Oral glucose tolerance testing was used to determine glucose tolerance. Serum myostatin was quantified by means of ELISA.

RESULTS

Circulating serum myostatin levels were highest in patients with T2D, lower in subjects with prediabetes, and lowest in subjects with normoglycemia (all p < 0.05). Myostatin was shown to be positively associated with fasting plasma glucose, HOMA-IR, hepatic enzymes, uric acid, and FINDRISC questionnaire scores in both sexes. ROC analyses determined circulating myostatin levels to be of value for differentiating subjects with T2D (AUC = 0.72, p = 0.002 in men; AUC = 0.70, p = 0.004 in women) in the study population. After adjustment for potential confounders, in a multiple binary logistic regression model, serum myostatin added further information to traditional risk estimates in distinguishing subjects with T2D.

CONCLUSIONS

Serum myostatin levels are higher with deterioration of carbohydrate tolerance. Furthermore, circulating myostatin is positively associated with traditional biochemical estimates of poor metabolic health. These data add to evidence of the involvement of this myokine in the pathogenesis of T2D.

Myostatin propeptide mutation of the hypermuscular Compact mice decreases the formation of myostatin and improves insulin sensitivity

The TGFβ family member myostatin (growth/differentiation factor-8) is a negative regulator of skeletal muscle growth. The hypermuscular Compact mice carry the 12-bp Mstn(Cmpt-dl1Abc) deletion in the sequence encoding the propeptide region of the precursor promyostatin, and additional modifier genes of the Compact genetic background contribute to determine the full expression of the phenotype. In this study, by using mice strains carrying mutant or wild-type myostatin alleles with the Compact genetic background and nonmutant myostatin with the wild-type background, we studied separately the effect of the Mstn(Cmpt-dl1Abc) mutation or the Compact genetic background on morphology, metabolism, and signaling. We show that both the Compact myostatin mutation and Compact genetic background account for determination of skeletal muscle size. Despite the increased musculature of Compacts, the absolute size of heart and kidney is not influenced by myostatin mutation; however, the Compact genetic background increases them. Both Compact myostatin and genetic background exhibit systemic metabolic effects. The Compact mutation decreases adiposity and improves whole body glucose uptake, insulin sensitivity, and ¹⁸FDG uptake of skeletal muscle and white adipose tissue, whereas the Compact genetic background has the opposite effect. Importantly, the mutation does not prevent the formation of mature myostatin; however, a decrease in myostatin level was observed, leading to altered activation of Smad2, Smad1/5/8, and Akt, and an increased level of p-AS160, a Rab-GTPase-activating protein responsible for GLUT4 translocation. Based on our analysis, the Compact genetic background strengthens the effect of myostatin mutation on muscle mass, but those can compensate for each other when systemic metabolic effects are compared.

Elevated insulin-like growth factor 2 expression may contribute to the hypermuscular phenotype of myostatin null mice

OBJECTIVES

Myostatin (Mstn) inhibits while insulin-like growth factors 1 and 2 (Igf1 and Igf2) increase skeletal muscle growth. However, there is little known regarding Mstn regulation of Igf1 and Igf2 expression. Therefore, the objective of this study was to quantify the expression of IGF family members in skeletal muscle and liver throughout the growth phase of Mstn null (MN) mice. Further, differences between male and female mice were investigated.

METHODS

Male and female wild type (WT) and MN mice were euthanized at birth (0 d), 7 days (7 d), weaning (21 d), sexual maturity (42 d), and 70 d. For the neonatal periods, 0 d and 7 d, all muscles from the hind limbs were compiled for RNA extraction. At 21 d, 42 d, and 70 d, biceps femoris (BF), tibialis anterior, triceps brachii (TB), and gastrocnemius-soleus complex were collected.

RESULTS

As expected, muscle weights were up to 90% greater in MN mice compared with WT mice at 21 d, 42 d and 70 d. However, Igf1 expression was reduced (P ≤ 0.04) at 7d and 21 d in MN mice compared to WT mice. Expression of Igf2 did not differ between genotypes at 0 d and 7d, but, at 21 d, 42 d and 70 d in BF and TB muscles, Igf2 expression was 1.9-2.9 fold greater (P<0.01) in MN compared to WT mice. Hepatic Igf1 and Igf2 levels were minimally affected by genotype; with the exception of a 1.4-fold reduction (P=0.04) in Igf1 expression in 21 d MN mice compared with WT mice. Though male mice were heavier than females starting at 21 d of age, expression differences in Igf1, Igf2, their receptors and binding proteins do not account for growth differences. In every case, when expression was different between sexes, female expression was increased despite increased growth in male mice.

CONCLUSION

This study is the first to provide evidence that Mstn may negatively regulate Igf2 expression to control postnatal skeletal muscle growth, however differences in growth between male and female mice are not readily explained by changes in expression of Igf family members.

Role of IGF-I in follistatin-induced skeletal muscle hypertrophy

Follistatin, a physiological inhibitor of myostatin, induces a dramatic increase in skeletal muscle mass, requiring the type 1 IGF-I receptor/Akt/mTOR pathway. The aim of the present study was to investigate the role of IGF-I and insulin, two ligands of the IGF-I receptor, in the follistatin hypertrophic action on skeletal muscle. In a first step, we showed that follistatin increases muscle mass while being associated with a downregulation of muscle IGF-I expression. In addition, follistatin retained its full hypertrophic effect toward muscle in hypophysectomized animals despite very low concentrations of circulating and muscle IGF-I. Furthermore, follistatin did not increase muscle sensitivity to IGF-I in stimulating phosphorylation of Akt but, surprisingly, decreased it once hypertrophy was present. Taken together, these observations indicate that increased muscle IGF-I production or sensitivity does not contribute to the muscle hypertrophy caused by follistatin. Unlike low IGF-I, low insulin, as obtained by streptozotocin injection, attenuated the hypertrophic action of follistatin on skeletal muscle. Moreover, the full anabolic response to follistatin was restored in this condition by insulin but also by IGF-I infusion. Therefore, follistatin-induced muscle hypertrophy requires the activation of the insulin/IGF-I pathway by either insulin or IGF-I. When insulin or IGF-I alone is missing, follistatin retains its full anabolic effect, but when both are deficient, as in streptozotocin-treated animals, follistatin fails to stimulate muscle growth.

Protein supplementation increases postexercise plasma myostatin concentration after 8 weeks of resistance training in young physically active subjects

Myostatin (MSTN) is a negative regulator of muscle growth even if some studies have shown a counterintuitive positive correlation between MSTN and muscle mass (MM). Our aim was to investigate the influence of 2 months of resistance training (RT) and diets with different protein contents on plasma MSTN, interleukin 1 beta (IL-1β), interleukin 6 (IL-6), tumor necrosis factor alpha (TNF-α), and insulin-like growth factor 1 (IGF-1). Eighteen healthy volunteers were randomly divided in two groups: high protein (HP) and normal protein (NP) groups. Different protein diet contents were 1.8 and 0.85 g of protein·kg bw(-1)·day(-1) for HP and NP, respectively. Subjects underwent 8 weeks of standardized progressive RT. MSTN, IGF-1, IL-1β, IL-6, and TNF-α were analyzed before and after the first and the last training sessions. Lean body mass, MM, upper-limb muscle area, and strength were measured. Plasma MSTN showed a significant increase (P<.001) after the last training in the HP group compared with NP group and with starting value. IGF-1 plasma concentration showed a positive correlation with MSTN in HP after the last training (r(2)=0.6456; P=.0295). No significant differences were found between NP and HP for IL-1β, IL-6, TNF-α, and strength and MM or area. These findings suggest a "paradoxical" postexercise increase of plasma MSTN after 8 weeks of RT and HP diets. This MSTN elevation correlates positively with IGF-1 plasma level. This double increase of opposite (catabolic/anabolic) mediators could explain the substantial overlapping of MM increases in the two groups.

Effects of sex steroids on expression of genes regulating growth-related mechanisms in rainbow trout (Oncorhynchus mykiss)

Effects of a single injection of 17β-estradiol (E2), testosterone (T), or 5β-dihydrotestosterone (DHT) on expression of genes central to the growth hormone (GH)/insulin-like growth factor (IGF) axis, muscle-regulatory factors, transforming growth factor-beta (TGFβ) superfamily signaling cascade, and estrogen receptors were determined in rainbow trout (Oncorhynchus mykiss) liver and white muscle tissue. In liver in addition to regulating GH sensitivity and IGF production, sex steroids also affected expression of IGF binding proteins, as E2, T, and DHT increased expression of igfbp2b and E2 also increased expression of igfbp2 and igfbp4. Regulation of this system also occurred in white muscle in which E2 increased expression of igf1, igf2, and igfbp5b1, suggesting anabolic capacity may be maintained in white muscle in the presence of E2. In contrast, DHT decreased expression of igfbp5b1. DHT and T decreased expression of myogenin, while other muscle regulatory factors were either not affected or responded similarly for all steroid treatments. Genes within the TGFβ superfamily signaling cascade responded to steroid treatment in both liver and muscle, suggesting a regulatory role for sex steroids in the ability to transmit signals initiated by TGFβ superfamily ligands, with a greater number of genes responding in liver than in muscle. Estrogen receptors were also regulated by sex steroids, with era1 expression increasing for all treatments in muscle, but only E2- and T-treatment in liver. E2 reduced expression of erb2 in liver. Collectively, these data identify how physiological mechanisms are regulated by sex steroids in a manner that promotes the disparate effects of androgens and estrogens on growth in salmonids.

CCN family protein 2 (CCN2) promotes the early differentiation, but inhibits the terminal differentiation of skeletal myoblasts

Many studies have reported that CCN family protein 2 (also known as connective tissue growth factor) induces fibrotic response in skeletal muscle, thus emphasizing the pathological role of CCN2 in muscle tissues. However, the physiological role of CCN2 in myogenesis is still unknown. This study clarified the CCN2 functions during myogenesis. Recombinant CCN2 (rCCN2) promoted proliferation and MyoD production in C2C12 cells and primary myoblasts, but inhibited myogenin production. In accordance with these findings, the gene expression levels of myosin heavy chain, which is a marker of terminally differentiated myoblasts and desmin, which is the main intermediate filament protein of muscle cells, were decreased by rCCN2 treatment. In vivo analyses with Ccn2-deficient skeletal muscle revealed decreased proliferating cell nuclear antigen (PCNA)/MyoD double positive cells and muscle hypoplasia. Consistent with this finding, myogenic marker genes and myotube formation were repressed in Ccn2-deficient myoblasts. The protein production of CCN2 was increased in C2C12 myoblasts treated with tumor necrosis factor-α, which is a pro-inflammatory cytokine, suggesting its role in muscle regeneration after inflammation. These findings indicate that CCN2 promotes proliferation and early differentiation but inhibits the terminal differentiation of myoblasts, thus suggesting that CCN2 plays a physiological role in myogenesis.

Lean and obese pig breeds exhibit differences in prenatal gene expression profiles of muscle development

Muscle development in domesticated animals is important for meat production. Furthermore, intramuscular fat content is an important trait of meat intended for consumption. Here, we examined differences in the expression of factors related to myogenesis, adipogenesis and skeletal muscle growth during fetal muscle development of lean (Yorkshire) and obese (Chenghua) pig breeds. At prenatal days 50 (d50) and 90 (d90), muscles and sera were collected from pig fetuses. Histology revealed larger diameters and numbers of myofibers in Chenghua pig fetuses than those in Yorkshire pig fetuses at d50 and d90. Yorkshire fetuses had higher serum concentrations of myostatin (d90), a negative regulator for muscle development, and higher mRNA expression of the growth hormone receptor Ghr (d90), myogenic MyoG (d90) and adipogenic LPL (d50). By contrast, Chenghua fetuses exhibited higher serum concentration of growth hormone (d90), and higher mRNA expression of myogenic MyoD (d90) as well as adipogenic PPARG and FABP4 (d50). Our results revealed distinct expression patterns in the two pig breeds at each developmental stage before birth. Compared with Chenghua pigs, development and maturation of fetal skeletal muscles may occur earlier in Yorkshire pigs, but the negative regulatory effects of myostatin may suppress muscle development at the later stage.

Vitamin D signaling in myogenesis: potential for treatment of sarcopenia

Muscle mass and strength progressively decrease with age, which results in a condition known as sarcopenia. Sarcopenia would lead to physical disability, poor quality of life, and death. Therefore, much is expected of an effective intervention for sarcopenia. Epidemiologic, clinical, and laboratory evidence suggest an effect of vitamin D on muscle function. However, the precise molecular and cellular mechanisms remain to be elucidated. Recent studies suggest that vitamin D receptor (VDR) might be expressed in muscle fibers and vitamin D signaling via VDR plays a role in the regulation of myoblast proliferation and differentiation. Understanding how vitamin D signaling contributes to myogenesis will provide a valuable insight into an effective nutritional strategy to moderate sarcopenia. Here we will summarize the current knowledge about the effect of vitamin D on skeletal muscle and myogenic cells and discuss the potential for treatment of sarcopenia.

Endocrine crosstalk between muscle and bone

The musculoskeletal system is a complex organ comprised of the skeletal bones, skeletal muscles, tendons, ligaments, cartilage, joints, and other connective tissue that physically and mechanically interact to provide animals and humans with the essential ability of locomotion. This mechanical interaction is undoubtedly essential for much of the diverse shape and forms observed in vertebrates and even in invertebrates with rudimentary musculoskeletal systems such as fish. It makes sense from a historical point of view that the mechanical theories of musculoskeletal development have had tremendous influence of our understanding of biology, because these relationships are clear and palpable. Less visible to the naked eye or even to the microscope is the biochemical interaction among the individual players of the musculoskeletal system. It was only in recent years that we have begun to appreciate that beyond this mechanical coupling of muscle and bones, these 2 tissues function at a higher level through crosstalk signaling mechanisms that are important for the function of the concomitant tissue. Our brief review attempts to present some of the key concepts of these new concepts and is outline to present muscles and bones as secretory/endocrine organs, the evidence for mutual genetic and tissue interactions, pathophysiological examples of crosstalk, and the exciting new directions for this promising field of research aimed at understanding the biochemical/molecular coupling of these 2 intimately associated tissues.

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.

Local overexpression of the myostatin propeptide increases glucose transporter expression and enhances skeletal muscle glucose disposal

Insulin resistance (IR) in skeletal muscle is a prerequisite for type 2 diabetes and is often associated with obesity. IR also develops alongside muscle atrophy in older individuals in sarcopenic obesity. The molecular defects that underpin this syndrome are not well characterized, and there is no licensed treatment. Deletion of the transforming growth factor-β family member myostatin, or sequestration of the active peptide by overexpression of the myostatin propeptide/latency-associated peptide (ProMyo) results in both muscle hypertrophy and reduced obesity and IR. We aimed to establish whether local myostatin inhibition would have a paracrine/autocrine effect to enhance glucose disposal beyond that simply generated by increased muscle mass, and the mechanisms involved. We directly injected adeno-associated virus expressing ProMyo in right tibialis cranialis/extensor digitorum longus muscles of rats and saline in left muscles and compared the effects after 17 days. Both test muscles were increased in size (by 7 and 11%) and showed increased radiolabeled 2-deoxyglucose uptake (26 and 47%) and glycogen storage (28 and 41%) per unit mass during an intraperitoneal glucose tolerance test. This was likely mediated through increased membrane protein levels of GLUT1 (19% higher) and GLUT4 (63% higher). Interestingly, phosphorylation of phosphoinositol 3-kinase signaling intermediates and AMP-activated kinase was slightly decreased, possibly because of reduced expression of insulin-like growth factor-I in these muscles. Thus, myostatin inhibition has direct effects to enhance glucose disposal in muscle beyond that expected of hypertrophy alone, and this approach may offer potential for the therapy of IR syndromes.

Prenatal programming of obesity in a swine model of leptin resistance: modulatory effects of controlled postnatal nutrition and exercise

The main role of early nutritional programming in the current rise of obesity and associated diseases is well known. However, translational studies are mostly based in postnatal food excess and, thus, there is a paucity of information on the phenotype of individuals with prenatal deficiencies but adequate postnatal conditions. Thus, we assessed the effects of prenatal programming (comparing descendants from females fed with a diet fulfilling 100 or only 50% of their nutritional requirements for pregnancy) on gene expression, patterns of growth and fattening, metabolic status and puberty attainment of a swine model of obesity/leptin resistance with controlled postnatal nutrition and opportunity of exercise. Maternal restriction was related to changes in the relationships among gene expression of positive (insulin-like growth factors 1 and 2) and negative (myostatin) regulators of muscle growth, with negative correlations in gilts from restricted pregnancies and positive relationships in the control group. In spite of these differences, the patterns of growth and fattening and the metabolic features during juvenile growth were similar in control gilts and gilts from restricted pregnancies. Concomitantly, there was a lack of differences in the timing of puberty attainment. However, after reaching puberty and adulthood, females from restricted pregnancies were heavier and more corpulent than control gilts, though such increases in weight and size were not accompanied by increases in adiposity. In conclusion, in spite of changes in gene expression induced by developmental programming, the propensity for higher weight and adiposity of individuals exposed to prenatal malnutrition may be modulated by controlled food intake and opportunity of physical exercise during infant and juvenile development.

Ploidy effects on genes regulating growth mechanisms during fasting and refeeding in juvenile rainbow trout (Oncorhynchus mykiss)

Diploid and triploid rainbow trout weighing approximately 3g were either fed for five weeks, or feed deprived for one week, followed by refeeding. During feed deprivation gastrointestinal somatic index decreased in diploids, but not triploids, and during refeeding, carcass growth rate recovered more quickly in triploids. Although not affected by ploidy, liver ghr2 and igfbp2b expression increased and igfbp1b decreased in fasted fish. Effects of ploidy on gene expression indicate potential mechanisms associated with improved recovery growth in triploids, which include decreased hepatic igfbp expression, which could influence IGF-I bioavailability, differences in tissue sensitivity to TGFbeta ligands due to altered tgfbr and smad expression, and differences in expression of muscle regulatory genes (myf5, mstn1a, and mstn1b). These data suggest that polyploidy influences the expression of genes critical to muscle development and general growth regulation, which may explain why triploid fish recover from nutritional insult better than diploid fish.

Myostatin signaling regulates Akt activity via the regulation of miR-486 expression

Myostatin, also known as growth and differentiation factor-8, is a pivotal negative regulator of skeletal muscle mass and reduces muscle protein synthesis by inhibiting the insulin-like growth factor-1 (IGF-1)/Akt/mammalian target of rapamycin (mTOR) pathway. However, the precise mechanism by which myostatin inhibits the IGF-1/Akt/mTOR pathway remains unclear. In this study, we investigated the global microRNA expression profile in myostatin knockout mice and identified miR-486, a positive regulator of the IGF-1/Akt pathway, as a novel target of myostatin signaling. In myostatin knockout mice, the expression level of miR-486 in skeletal muscle was significantly increased. In addition, we observed increased expression of the primary transcript of miR-486 (pri-miR-486) and Ankyrin 1.5 (Ank1.5), the host gene of miR-486, in myostatin knockout mice. In C2C12 cells, myostatin negatively regulated the expression of Ank1.5. Moreover, canonical myostatin signaling repressed the skeletal muscle-specific promoter activity of miR-486/Ank1.5. This repression was partially mediated by the E-box elements in the proximal region of the promoter. We also show that overexpression of miR-486 induced myotube hypertrophy in vitro and that miR-486 was essential to maintain skeletal muscle size both in vitro and in vivo. In addition, inhibition of miR-486 led to a decrease in Akt activity in C2C12 myotubes. Our findings indicate that miR-486 is one of the intermediary molecules connecting myostatin signaling and the IGF-1/Akt/mTOR pathway in the regulation of skeletal muscle size.

Absence of functional leptin receptor isoforms in the POUND (Lepr(db/lb)) mouse is associated with muscle atrophy and altered myoblast proliferation and differentiation

Objective

Leptin receptors are abundant in human skeletal muscle, but the role of leptin in muscle growth, development and aging is not well understood. Here we utilized a novel mouse model lacking all functional leptin receptor isoforms (POUND mouse, Lepr^db/lb) to determine the role of leptin in skeletal muscle.

Methods and Findings

Skeletal muscle mass and fiber diameters were examined in POUND mice, and primary myoblast cultures were used to determine the effects of altered leptin signaling on myoblast proliferation and differentiation. ELISA assays, integrated pathway analysis of mRNA microarrays, and reverse phase protein analysis were performed to identify signaling pathways impacted by leptin receptor deficiency. Results show that skeletal muscle mass and fiber diameter are reduced 30–40% in POUND mice relative to wild-type controls. Primary myoblast cultures demonstrate decreased proliferation and decreased expression of both MyoD and myogenin in POUND mice compared to normal mice. Leptin treatment increased proliferation in primary myoblasts from muscles of both adult (12 months) and aged (24 months) wild-type mice, and leptin increased expression of MyoD and myogenin in aged primary myoblasts. ELISA assays and protein arrays revealed altered expression of molecules associated with the IGF-1/Akt and MAPK/MEK signaling pathways in muscle from the hindlimbs of mice lacking functional leptin receptors.

Conclusion

These data support the hypothesis that the adipokine leptin is a key factor important for the regulation of skeletal muscle mass, and that leptin can act directly on its receptors in peripheral tissues to regulate cell proliferation and differentiation.

Low and Normal IGF-1 Levels in Patients with Chronic Medical Disorders (CMD) is Independent of Anterior Pituitary Hormone Deficiencies: Implications for Treating IGF-1 Abnormal Deficiencies with CMD

Over time, based on evidence-based medicine, a number of hormonal test levels including IGF-1 had been raised or lowered to meet new criteria standards. In particular, IGF-1 plasma levels have been shown in several studies to be an independent diagnostic tool in Adult Growth Hormone Deficiency (AGHD). Many endocrinology studies link low IGF-1 plasma levels with low levels of other anterior pituitary hormones (i.e., LH, FSH, and TSH). Low IGF-1 is considered by most to be between 84-100 µ/l and numerous studies recommend that raising IGF-1 to high normal range reverses Chronic Medical Diseases (CMD), improves bone mineral density (BMD), and fibromyalgia. Moreover, some studies suggest that low levels of IGF-1 by itself independent of anterior pituitary deficiencies is sufficient to determine AGHD in humans. In order to determine the relationship of low IGF-1 with that of LH, FSH, and TSH levels in subjects with CMD, we evaluated these levels (± SD) in 944 patients. Patients with IGF-1 below 84 µ/l, 100 µ/l, and 150 µ/l were accessed. 9.22% had less than 84 µ/l (SD ± 12.52); 19.9% had less than 100 µ/l (SD ± 9.54); and 51.6 had less than 150 µ/l (SD ± 26.0). Specifically, the percentages found for low LH, FSH, and TSH were only 4.2%, 4.8%, and 6.5%. We conclude that IGF-1 deficiencies occur independent of comorbid deficiencies of LH, FSH, and TSH. Finally, we propose that based on the present investigation, IGF-1 low levels between the range of 84-100 µ/l may be too low to be considered as an independent diagnostic marker to treat AGHD with CMD.

Effects of the activin A-myostatin-follistatin system on aging bone and muscle progenitor cells

The activin A-myostatin-follistatin system is thought to play an important role in the regulation of muscle and bone mass throughout growth, development, and aging; however, the effects of these ligands on progenitor cell proliferation and differentiation in muscle and bone are not well understood. In addition, age-associated changes in the relative expression of these factors in musculoskeletal tissues have not been described. We therefore examined changes in protein levels of activin A, follistatin, and myostatin (GDF-8) in both muscle and bone with age in C57BL6 mice using ELISA. We then investigated the effects of activin A, myostatin and follistatin on the proliferation and differentiation of primary myoblasts and mouse bone marrow stromal cells (BMSCs) in vitro. Myostatin levels and the myostatin:follistatin ratio increased with age in the primarily slow-twitch mouse soleus muscle, whereas the pattern was reversed with age in the fast-twitch extensor digitorum longus muscle. Myostatin levels and the myostatin:follistatin ratio increased significantly (+75%) in mouse bone marrow with age, as did activin A levels (+17%). Follistatin increased the proliferation of primary myoblasts from both young and aged mice, whereas myostatin increased proliferation of younger myoblasts but decreased proliferation of older myoblasts. Myostatin reduced proliferation of both young and aged BMSCs in a dose-dependent fashion, and activin A increased mineralization in both young and aged BMSCs. Together these data suggest that aging in mice is accompanied by changes in the expression of activin A and myostatin, as well as changes in the response of bone and muscle progenitor cells to these factors. Myostatin appears to play a particularly important role in the impaired proliferative capacity of muscle and bone progenitor cells from aged mice.

The salmonid myostatin gene family: a novel model for investigating mechanisms that influence duplicate gene fate

BACKGROUND

Most fishes possess two paralogs for myostatin, a muscle growth inhibitor, while salmonids are presumed to have four: mstn1a, mstn1b, mstn2a and mstn2b, a pseudogene. The mechanisms responsible for preserving these duplicates as well as the depth of mstn2b nonfunctionalization within the family remain unknown. We therefore characterized several genomic clones in order to better define species and gene phylogenies.

RESULTS

Gene organization and sequence conservation was particularly evident among paralog groupings and within salmonid subfamilies. All mstn2b sequences included in-frame stop codons, confirming its nonfunctionalization across taxa, although the indels and polymorphisms responsible often differed. For example, the specific indels within the Onchorhynchus tshawytscha and O. nerka genes were remarkably similar and differed equally from other mstn2b orthologs. A phylogenetic analysis weakly established a mstn2b clade including only these species, which coupled with a shared 51 base pair deletion might suggest a history involving hybridization or a shared phylogenetic history. Furthermore, mstn2 introns all lacked conserved splice site motifs, suggesting that the tissue-specific processing of mstn2a transcripts, but not those of mstn2b, is due to alternative cis regulation and is likely a common feature in salmonids. It also suggests that limited transcript processing may have contributed to mstn2b nonfunctionalization.

CONCLUSIONS

Previous studies revealed divergence within gene promoters while the current studies provide evidence for relaxed or positive selection in some coding sequence lineages. These results together suggest that the salmonid myostatin gene family is a novel resource for investigating mechanisms that regulate duplicate gene fate as paralog specific differences in gene expression, transcript processing and protein structure are all suggestive of active divergence.

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.

Double Muscling in Cattle: Genes, Husbandry, Carcasses and Meat

Molecular biology has enabled the identification of the mechanisms whereby inactive myostatin increases skeletal muscle growth in double-muscled (DM) animals. Myostatin is a secreted growth differentiation factor belonging to the transforming growth factor-β superfamily. Mutations make the myostatin gene inactive, resulting in muscle hypertrophy. The relationship between the different characteristics of DM cattle are defined with possible consequences for livestock husbandry. The extremely high carcass yield of DM animals coincides with a reduction in the size of most vital organs. As a consequence, DM animals may be more susceptible to respiratory disease, urolithiasis, lameness, nutritional stress, heat stress and dystocia, resulting in a lower robustness. Their feed intake capacity is reduced, necessitating a diet with a greater nutrient density. The modified myofiber type is responsible for a lower capillary density, and it induces a more glycolytic metabolism. There are associated changes for the living animal and post-mortem metabolism alterations, requiring appropriate slaughter conditions to maintain a high meat quality. Intramuscular fat content is low, and it is characterized by more unsaturated fatty acids, providing healthier meat for the consumer. It may not always be easy to find a balance between the different disciplines underlying the livestock husbandry of DM animals to realize a good performance and health and meat quality.

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.

The skeletal muscle secretome: an emerging player in muscle-bone crosstalk

In vitro and in vivo studies provide evidence that a variety of growth factors and cytokines are actively secreted by muscle tissue. Muscle can therefore function as an endocrine and paracrine organ. These peptides characterize the muscle secretome, and many muscle-derived factors such as insulin-like growth factor-1, basic fibroblast growth factor, interleukin-15, myostatin and secreted protein acidic and rich in cysteine (osteonectin) are also known to have significant effects on bone metabolism. The factors secreted by muscle may vary according to muscle activity, in that muscle contraction, muscle atrophy or traumatic muscle injury can alter the type and relative abundance of particular factors released from muscle cells. The molecular and cellular pathways by which muscle-derived factors affect different types of bone cells (for example, osteoblasts, osteoclasts and osteocytes) are, however, poorly understood. Nevertheless, these findings further underscore the complex nature of muscle-bone interactions, and highlight the importance of integrating muscle biology and physiology into our understanding of bone growth, development and aging.

Inherent growth hormone resistance in the skeletal muscle of the fine flounder is modulated by nutritional status and is characterized by high contents of truncated GHR, impairment in the JAK2/STAT5 signaling pathway, and low IGF-I expression

A detailed understanding of how the GH and IGF-I regulate muscle growth, especially in early vertebrates, is still lacking. The fine flounder is a flatfish species exhibiting remarkably slow growth, representing an intriguing model for elucidating growth regulatory mechanisms. Key components of the GH system were examined in groups of fish during periods of feeding, fasting, and refeeding. Under feeding conditions, there is an inherent systemic and local (muscle) GH resistance, characterized by higher levels of plasma GH than of IGF-I, skeletal muscle with a greater content of the truncated GH receptor (GHRt) than of full-length GHR (GHRfl), an impaired activation of the Janus kinase 2 (JAK2)-signal transducers and activators of transcription 5 (STAT5) signaling pathway, and low IGF-I expression. Fasting leads to further elevation of plasma GH levels concomitant with suppressed IGF-I levels. The ratio of GHRfl to GHRt in muscle decreases during fasting, causing an inactivation of the JAK2/STAT5 signaling pathway and suppressed IGF-I expression, further impairing growth. When fish are returned to nutritionally favorable conditions, plasma GH levels decrease, and the ratio of GHRfl to GHRt in muscle increases, triggering JAK2/STAT5 reactivation and local IGF-I expression, concomitant with increased growth. The study suggests that systemic IGF-I is supporting basal slow growth in this species, without ruling out that local IGF-I is participating in muscle growth. These results reveal for the first time a unique model of inherent GH resistance in the skeletal muscle of a nonmammalian species and contribute to novel insights of the endocrine and molecular basis of growth regulation in earlier vertebrates.

The type 1 insulin-like growth factor receptor (IGF-IR) pathway is mandatory for the follistatin-induced skeletal muscle hypertrophy

Myostatin inhibition by follistatin (FS) offers a new approach for muscle mass enhancement. The aim of the present study was to characterize the mediators responsible for the FS hypertrophic action on skeletal muscle in male mice. Because IGF-I and IGF-II, two crucial skeletal muscle growth factors, are induced by myostatin inhibition, we assessed their role in FS action. First, we tested whether type 1 IGF receptor (IGF-IR) is required for FS-induced hypertrophy. By using mice expressing a dominant-negative IGF-IR in skeletal muscle, we showed that IGF-IR inhibition blunted by 63% fiber hypertrophy caused by FS. Second, we showed that FS caused the same degree of fiber hypertrophy in wild-type and IGF-II knockout mice. We then tested the role of the signaling molecules stimulated by IGF-IR, in particular the Akt/mammalian target of rapamycin (mTOR)/70-kDa ribosomal protein S6 kinase (S6K) pathway. We investigated whether Akt phosphorylation is required for the FS action. By cotransfecting a dominant-negative form of Akt together with FS, we showed that Akt inhibition reduced by 65% fiber hypertrophy caused by FS. Second, we evaluated the role of mTOR in FS action. Fiber hypertrophy induced by FS was reduced by 36% in rapamycin-treated mice. Finally, because the activity of S6K is increased by FS, we tested its role in FS action. FS caused the same degree of fiber hypertrophy in wild-type and S6K1/2 knockout mice. In conclusion, the IGF-IR/Akt/mTOR pathway plays a critical role in FS-induced muscle hypertrophy. In contrast, induction of IGF-II expression and S6K activity by FS are not required for the hypertrophic action of FS.

Gene expression profiling of skeletal muscles treated with a soluble activin type IIB receptor

Inhibition of the myostatin signaling pathway is emerging as a promising therapeutic means to treat muscle wasting and degenerative disorders. Activin type IIB receptor (ActRIIB) is the putative myostatin receptor, and a soluble activin receptor (ActRIIB-Fc) has been demonstrated to potently inhibit a subset of transforming growth factor (TGF)-β family members including myostatin. To determine reliable and valid biomarkers for ActRIIB-Fc treatment, we assessed gene expression profiles for quadriceps muscles from mice treated with ActRIIB-Fc compared with mice genetically lacking myostatin and control mice. Expression of 134 genes was significantly altered in mice treated with ActRIIB-Fc over a 2-wk period relative to control mice (fold change > 1.5, P < 0.001), whereas the number of significantly altered genes in mice treated for 2 days was 38, demonstrating a time-dependent response to ActRIIB-Fc in overall muscle gene expression. The number of significantly altered genes in Mstn(-/-) mice relative to control mice was substantially higher (360), but for most of these genes the expression levels in the 2-wk treated mice were closer to the levels in the Mstn(-/-) mice than in control mice (P < 10⁻³⁰). Expression levels of 30 selected genes were further validated with quantitative real-time polymerase chain reaction (qPCR), and a correlation of ≥ 0.89 was observed between the fold changes from the microarray analysis and the qPCR analysis. These data suggest that treatment with ActRIIB-Fc results in overlapping but distinct gene expression signatures compared with myostatin genetic mutation. Differentially expressed genes identified in this study can be used as potential biomarkers for ActRIIB-Fc treatment, which is currently in clinical trials as a therapeutic agent for muscle wasting and degenerative disorders.