The role of myostatin in chicken (Gallus domesticus) myogenic satellite cell proliferation and differentiation

Myostatin gene role in regulating traits of poultry species for potential industrial applications

The myostatin (MSTN) gene is considered a potential genetic marker to improve economically important traits in livestock, since the discovery of its function using the MSTN knockout mice. The anti-myogenic function of the MSTN gene was further demonstrated in farm animal species with natural or induced mutations. In poultry species, myogenesis in cell culture was regulated by modulation of the MSTN gene. Also, different expression levels of the MSTN gene in poultry models with different muscle mass have been reported, indicating the conserved myogenic function of the MSTN gene between mammalian and avian species. Recent advances of CRISPR/Cas9-mediated genome editing techniques have led to development of genome-edited poultry species targeting the MSTN gene to clearly demonstrate its anti-myogenic function and further investigate other potential functions in poultry species. This review summarizes research conducted to understand the function of the MSTN gene in various poultry models from cells to whole organisms. Furthermore, the genome-edited poultry models targeting the MSTN gene are reviewed to integrate diverse effects of the MSTN gene on different traits of poultry species.

Skeletal muscle and metabolic flexibility in response to changing energy demands in wild birds

Phenotypically plastic responses of animals to adjust to environmental variation are pervasive. Reversible plasticity (i.e., phenotypic flexibility), where adult phenotypes can be reversibly altered according to prevailing environmental conditions, allow for better matching of phenotypes to the environment and can generate fitness benefits but may also be associated with costs that trade-off with capacity for flexibility. Here, we review the literature on avian metabolic and muscle plasticity in response to season, temperature, migration and experimental manipulation of flight costs, and employ an integrative approach to explore the phenotypic flexibility of metabolic rates and skeletal muscle in wild birds. Basal (minimum maintenance metabolic rate) and summit (maximum cold-induced metabolic rate) metabolic rates are flexible traits in birds, typically increasing with increasing energy demands. Because skeletal muscles are important for energy use at the organismal level, especially to maximum rates of energy use during exercise or shivering thermogenesis, we consider flexibility of skeletal muscle at the tissue and ultrastructural levels in response to variations in the thermal environment and in workloads due to flight exercise. We also examine two major muscle remodeling regulatory pathways: myostatin and insulin-like growth factor -1 (IGF-1). Changes in myostatin and IGF-1 pathways are sometimes, but not always, regulated in a manner consistent with metabolic rate and muscle mass flexibility in response to changing energy demands in wild birds, but few studies have examined such variation so additional study is needed to fully understand roles for these pathways in regulating metabolic flexibility in birds. Muscle ultrastrutural variation in terms of muscle fiber diameter and associated myonuclear domain (MND) in birds is plastic and highly responsive to thermal variation and increases in workload, however, only a few studies have examined ultrastructural flexibility in avian muscle. Additionally, the relationship between myostatin, IGF-1, and satellite cell (SC) proliferation as it relates to avian muscle flexibility has not been addressed in birds and represents a promising avenue for future study.

Identification and functional characterization of Pomstna in Japanese flounder (Paralichthys olivaceus)

Myostatin (MSTN) as a negative regulator of muscle growth has been identified in Japanese flounder. Yet, most fish experienced the teleost specific genome duplication and possess at least two mstn genes. In current study, the second mstn gene named Pomstna is identified in Japanese flounder. Pomstna is clustered with other mstn2 of teleosts and owned highly conserved TGF-beta domain. In addition to muscle, Pomstna also highly expressed in brain and spleen. Using the primarily cultured muscle cells of Japanese flounder, we found that Pomstna could inhibit the proliferation and differentiation of muscle cells in vitro. As a ligand of TGF-beta signaling pathway, Pomstnb could regulate the expression of p21 and myod by activating the TGF-beta signaling pathway. Different from the function of Pomstnb, Pomstna could not activate the TGF-beta signaling pathway in vitro. During the differentiation of PoM cells, the expression of Pomstnb decreased significantly but the expression of Pomstna showed no change. Our study suggests that Pomstna could negatively regulate the growth and differentiation of muscle like Pomstnb yet through a different regulatory mechanism than Pomstnb. The present study suggests that muscle proliferation and differentiation were regulated by mstn not only through the TGF-beta signaling pathway but also other unknown mechanisms.

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.

Identification of Molecules from Coffee Silverskin That Suppresses Myostatin Activity and Improves Muscle Mass and Strength in Mice

Coffee has been shown to attenuate sarcopenia, the age-associated muscle atrophy. Myostatin (MSTN), a member of the TGF-β growth/differentiation factor superfamily, is a potent negative regulator of skeletal muscle mass, and MSTN-inhibition increases muscle mass or prevents muscle atrophy. This study, thus, investigated the presence of MSTN-inhibitory capacity in coffee extracts. The ethanol-extract of coffee silverskin (CSE) but not other extracts demonstrated anti-MSTN activity in a pGL3-(CAGA)₁₂-luciferase reporter gene assay. CSE also blocked Smad3 phosphorylation induced by MSTN but not by GDF11 or Activin A in Western blot analysis, demonstrating its capacity to block the binding of MSTN to its receptor. Oral administration of CSE significantly increased forelimb muscle mass and grip strength in mice. Using solvent partitioning, solid-phase chromatography, and reverse-phase HPLC, two peaks having MSTN-inhibitory capacity were purified from CSE. The two peaks were identified as ^βN-arachinoyl−5-hydroxytryptamide (C₂₀−5HT) and ^βN-behenoyl−5-hydroxytryptamide (C₂₂−5HT) using mass spectrometry and NMR analysis. In summary, the results show that CSE has the MSTN-inhibitory capacity, and C₂₀−5HT and C₂₂−5HT are active components of CSE-suppressing MSTN activity, suggesting the potential of CSE, C₂₀−5HT, and C₂₂−5HT being developed as agents to combat muscle atrophy and metabolic syndrome.

MicroRNA-27b-3p Targets the Myostatin Gene to Regulate Myoblast Proliferation and Is Involved in Myoblast Differentiation

microRNAs play an important role in the growth and development of chicken embryos, including the regulation of skeletal muscle genesis, myoblast proliferation, differentiation, and apoptosis. Our previous RNA-seq studies showed that microRNA-27b-3p (miR-27b-3p) might play an important role in regulating the proliferation and differentiation of chicken primary myoblasts (CPMs). However, the mechanism of miR-27b-3p regulating the proliferation and differentiation of CPMs is still unclear. In this study, the results showed that miR-27b-3p significantly promoted the proliferation of CPMs and inhibited the differentiation of CPMs. Then, myostatin (MSTN) was confirmed to be the target gene of miR-27b-3p by double luciferase reporter assay, RT-qPCR, and Western blot. By overexpressing and interfering with MSTN expression in CPMs, the results showed that overexpression of MSTN significantly inhibited the proliferation and differentiation of CPMs. In contrast, interference of MSTN expression had the opposite effect. This study showed that miR-27b-3p could promote the proliferation of CPMs by targeting MSTN. Interestingly, both miR-27b-3p and MSTN can inhibit the differentiation of CPMs. These results provide a theoretical basis for further understanding the function of miR-27b-3p in chicken and revealing its regulation mechanism on chicken muscle growth.

Transcriptomic profile of semitendinosus muscle of bulls of different breed and performance

The aim of the study was to compare the transcriptomic profiles of fully differentiated skeletal muscle derived from bulls belonging to different breeds of varying performance. Microarray analyses were performed to determine the differences in the expression profiles of genes between semitendinosus muscles of 15-month-old beef-breed bulls (Limousin—LIM and Hereford—HER) and dairy-breed bulls (Holstein Friesian—HF). These analyses allowed for the identification of those genes the expression of which is similar and characteristic of fully differentiated muscle in beef breeds, but differs in skeletal muscle of a typical dairy breed. The analysis revealed 463 transcripts showing similar expression in the semitendinosus muscle of beef breeds (LIM/HER), in comparison with the dairy breed (HF). Among the identified genes, 227 were upregulated and 236 were downregulated in beef breeds. The ontological analyses revealed that the largest group of genes similarly expressed in LIM and HER was involved in the processes of protein metabolism and development of muscle organ. In beef breeds, some genes involved in protein synthesis and proteolysis showed an upregulation, including ctsd, ctsf, fhl2, fhl3, fst, sirt1, and trim63, whereas some were downregulated, including bmpr1a, bmpr2, mstn, smad2, hspa8, gsk3β, and tgfβ2. The expression of the chosen genes was confirmed by RT-qPCR technique. Thus, it can be assumed that the identified genes involved in the regulation of growth and development of muscle tissue and the processes of protein metabolism in the examined cattle breeds may be responsible for the greater gain of muscle mass in beef-breed bulls.

Circular RNA profiling identified an abundant circular RNA circTMTC1 that inhibits chicken skeletal muscle satellite cell differentiation by sponging miR-128-3p

Scope: Myogenesis involves a series of complex cellular and developmental processes regulated by many genes, transcription factors and non-coding RNAs. Recent studies have demonstrated the involvement of circular RNAs (circRNAs) in myogenesis. While previous studies have established a role for some circRNAs, the precise functions and mechanisms of circRNAs in skeletal muscle development are still not completely understood in chicken. Methods: To identify potential circRNAs during chicken embryonic skeletal muscle development, rRNA- libraries sequencing was performed in breast muscles from 12 broilers and 12 layers at four different embryonic points, embryonic day 10 (E10), E13, E16 and E19. Through circRNA differential expression analysis and target miRNA prediction, the circTMTC1 was predicted to participate in the embryonic muscle formation by sponging miRNA, which were verified in vitro experiments. Results: We identified 228 differentially expressed circRNAs between broilers and layers (fold change >2; p-value < 0.05), and 43 circRNAs were differentially expressed at multiple embryonic days. circTMTC1, a novel circRNA transcribed from the TMTC1 gene, was expressed significantly higher in layers than in broilers at E10, E13 and E16. Furthermore, circTMTC1 knockdown accelerated proliferation and differentiation in chicken skeletal muscle satellite cells (SMSCs), besides, circTMTC1-overexpressing cells showed opposite effects. circTMTC1 functioned as a miR-128-3p sponge at the differentiation stage of SMSCs, and circTMTC1 inhibited the expression of miR-128-3p. Furthermore, miR-128-3p promoted differentiation of chicken SMSCs, and circTMTC1 inhibited the promotion effect of miR-128-3p on chicken SMSC differentiation. Conclusion: Our study revealed that circRNAs are differentially expressed during chicken embryonic development between the two chicken models, and circTMTC1 inhibits chicken SMSC differentiation by sponging miR-128-3p.

Effects of dietary vitamin B6 on the skeletal muscle protein metabolism of growing rabbits

This study aimed to evaluate the effects of dietary vitamin B6 on the skeletal muscle protein metabolism and expression of transcription and growth factor of growing rabbits. Two hundred, healthy, rabbits with similar bodyweights were randomly assigned to one of five dietary groups with 40 animals per group. The dietary groups consisted of the following different vitamin B6 supplementation levels: 0, 5, 10, 20 and 40 mg/kg. The feeding trial lasted 60 days. The results showed that dietary vitamin B6 elicited significant effects on the fore and hind leg muscle ratio (the fore and hind leg muscle weight/the liveweight; P < 0.05) and on serum total amino acids (T-AA), blood urea and insulin-like growth factor 1 (IGF1) content (P < 0.05). Additionally, expression of IGF1, myogenic determination factor (MYOD) and myogenin (MYOG), myocyte regulation factor 5 (MYF5), myostatin (MSTN) and WW domain-containing E3 proteasome ubiquitin ligase 1 (WWP1) mRNA in the loin (M. longissimus dorsi) were affected by vitamin B6 in diets (P < 0.05). The immunoblot analysis revealed that dietary vitamin B6 elicited significant effects on IGF1, MYOG and WWP1 expression in the loin (P < 0.05). Our results indicate that the addition of dietary vitamin B6 can significantly alter the protein metabolism of growing rabbits and that an appropriate vitamin B6 supplementation level is 20 mg/kg for 3–5-month-old growing rabbits (the basic diet vitamin B6 content was 4.51 mg/kg).

The effect of temperature on proliferation and differentiation of chicken skeletal muscle satellite cells isolated from different muscle types

Skeletal muscle satellite cells are a muscle stem cell population that mediate posthatch muscle growth and repair. Satellite cells respond differentially to environmental stimuli based upon their fiber-type of origin. The objective of this study was to determine how temperatures below and above the in vitro control of 38°C affected the proliferation and differentiation of satellite cells isolated from the chicken anaerobic pectoralis major (p. major) or mixed fiber biceps femoris (b.femoris) muscles. The satellite cells isolated from the p. major muscle were more sensitive to both cold and hot temperatures compared to the b.femoris satellite cells during both proliferation and differentiation. The expressions of myogenic regulatory transcription factors were also different between satellite cells from different fiber types. MyoD expression, which partially regulates proliferation, was generally expressed at higher levels in p. major satellite cells compared to the b.femoris satellite cells from 33 to 43°C during proliferation and differentiation. Similarly, myogenin expression, which is required for differentiation, was also expressed at higher levels in p. major satellite cells in response to both cold and hot temperatures during proliferation and differentiation than b. femoris satellite cells. These data demonstrate that satellite cells from the anaerobic p. major muscle are more sensitive than satellite cells from the aerobic b. femoris muscle to both hot and cold thermal stress during myogenic proliferation and differentiation.

Myostatin inhibits proliferation but not differentiation of trout myoblasts

The muscle growth in mammals is regulated by several growth factors including myostatin (MSTN), a member of the transforming growth factor-beta (TGF-beta) superfamily. To date, it is unknown in fish whether MSTN could have any effect on proliferation or differentiation of myogenic cells. Using culture of trout satellite cells, we showed that mstn1a and mstn1b mRNA are expressed in myoblasts and that their expression decreased in differentiating myoblasts. We also demonstrated that a treatment with huMSTN decreased the proliferation of IGF1-stimulated myoblasts in a dose-dependent manner. By contrast, treatment of myoblasts with 100 nM of huMSTN for three days, did not affect the percentage of positive cells for myogenin neither the percentage of nuclei in myosin positive cells. Moreover, our results clearly indicated that huMSTN treatment had no effect on MyoD and myogenin protein levels, which suggests that huMSTN did not strongly affect MyoD activity. In conclusion, we showed that huMSTN inhibited proliferation but not differentiation of trout myoblasts, probably resulting from a lack of huMSTN effect on MyoD activity. Altogether, these results show high interspecies differences in the function of MSTN.

Migration- and exercise-induced changes to flight muscle size in migratory birds and association with IGF1 and myostatin mRNA expression

Seasonal adjustments to muscle size in migratory birds may result from preparatory physiological changes or responses to changed workloads. The mechanisms controlling these changes in size are poorly understood. We investigated some potential mediators of flight muscle size (myostatin and insulin-like growth factor, IGF1) in pectoralis muscles of wild wintering or migrating white-throated sparrows (Zonotrichia albicollis), captive white-throated sparrows that were photoperiod manipulated to be in a `wintering' or `migratory' (Zugunruhe) state, and captive European starlings (Sturnus vulgaris) that were either exercised for 2 weeks in a wind tunnel or untrained. Flight muscle size increased in photo-stimulated `migrants' and in exercised starlings. Acute exercise but not long-term training caused increased expression of IGF1, but neither caused a change in expression of myostatin or its metalloprotease activator TLL1. Photo-stimulated `migrant' sparrows demonstrated increased expression of both myostatin and IGF1, but wild sparrows exhibited no significant seasonal changes in expression of either myostatin or IGF1. Additionally, in both study species we describe several splice variants of myostatin that are shared with distantly related bird species. We demonstrate that their expression patterns are not different from those of the typical myostatin, suggesting that they have no functional importance and may be mistakes of the splicing machinery. We conclude that IGF1 is likely to be an important mediator of muscle phenotypic flexibility during acute exercise and during endogenous, seasonal preparation for migration. The role of myostatin is less clear, but its paradoxical increase in photo-stimulated `migrants' may indicate a role in seasonal adjustments of protein turnover.

Sarcopenia associated with portosystemic shunting is reversed by follistatin

BACKGROUND & AIMS

The distinct role of portosystemic shunting (PSS) in the pathogenesis of sarcopenia (skeletal muscle loss) that occurs commonly in cirrhosis is unclear. We have previously shown increased expression of myostatin (inhibitor of skeletal muscle mass) in the portacaval anastamosis (PCA) rat model of sarcopenia of PSS. The present study was performed to examine the mechanisms of sarcopenia following PCA.

METHODS

In PCA and sham operated pair fed control rats, the phenylalanine flooding dose method was used to quantify the fractional and absolute protein synthesis rates in the skeletal muscle over time and in response to follistatin, a myostatin antagonist. The expression of myostatin and markers of satellite cell (myocyte precursors) proliferation and differentiation were quantified by real-time PCR and Western blot analyses.

RESULTS

The absolute synthesis rate (ASR) was lower at 2, 4, and 6 weeks (p<0.05) and the fractional synthesis rate (FSR) of skeletal muscle protein was significantly lower (p<0.05) at week 2 in the PCA rats compared to control rats. Expression of myostatin was elevated while markers of satellite cell proliferation and differentiation were lower at 4 and 6 weeks after PCA. Follistatin increased skeletal muscle mass, muscle FSR and ASR, decreased expression of myostatin protein, and increased expression of markers of satellite cell function.

CONCLUSIONS

Sarcopenia associated with PSS is caused by impaired protein synthesis and reduced satellite cell function due to increased myostatin expression. Confirming these alterations in human patients with cirrhosis will provide novel therapeutic targets for sarcopenia of liver disease.

Nampt/visfatin/PBEF affects expression of myogenic regulatory factors and is regulated by interleukin-6 in chicken skeletal muscle cells

Nicotinamide phosphoribosyltransferase (Nampt/visfatin/PBEF) has been identified as a rate-limiting NAD(+) biosynthetic enzyme and an adipokine found in the circulation. Human and chicken skeletal muscles are reported to have the highest level of Nampt expression among various tissues whose functional significance remains undetermined. Expression of Nampt is regulated by interleukin-6 (IL-6), an essential cytokine for postnatal muscle growth in mammals. The objective of the current study was to characterize expression of Nampt in chicken (Gallus gallus) myogenic cells and to determine the effect of Nampt on expression of IL-6, myogenic transcription factors, and glucose uptake. We also sought to determine the effect of IL-6 on Nampt expression in chicken myogenic cells. Nampt mRNA and protein were identified in both myoblasts and myocytes, although expression did not differ between the two cell types. Treatment with recombinant human Nampt was found to decrease myoD and mrf4 expression but to increase myf5 expression in myocytes, while glucose uptake was unaffected. In response to treatment with recombinant Nampt, IL-6 expression in myocytes was increased at 24h but decreased when treated for 48 or 72 h. Forced over-expression of chicken Nampt cDNA significantly decreased myf5 expression in myoblasts. Treatment of myogenic cells with lower levels (1 ng.mL(-1)) of recombinant IL-6 increased Nampt expression, whereas a higher IL-6 concentration (100 ng.mL(-1)) decreased Nampt mRNA abundance. Collectively, these results demonstrate that Nampt, regulated in part by IL-6, alters the expression of key myogenic transcription factors and thereby may influence postnatal myogenesis.

Overexpression of follistatin in trout stimulates increased muscling

Deletion or inhibition of myostatin in mammals has been demonstrated to markedly increase muscle mass by hyperplasia, hypertrophy, or a combination of both. Despite a remarkably high degree of conservation with the mammalian protein, the function of myostatin remains unknown in fish, many species of which continue muscle growth throughout the lifecycle by hyperplasia. Transgenic rainbow trout (Oncorhynchus mykiss) overexpressing follistatin, one of the more efficacious antagonists of myostatin, were produced to investigate the effect of this protein on muscle development and growth. P(1) transgenics overexpressing follistatin in muscle tissue exhibited increased epaxial and hypaxial muscling similar to that observed in double-muscled cattle and myostatin null mice. The hypaxial muscling generated a phenotype reminiscent of well-developed rectus abdominus and intercostal muscles in humans and was dubbed "six pack." Body conformation of the transgenic animals was markedly altered, as measured by condition factor, and total muscle surface area increased. The increased muscling was due almost exclusively to hyperplasia as evidenced by a higher number of fibers per unit area and increases in the percentage of smaller fibers and the number of total fibers. In several individuals, asymmetrical muscling was observed, but no changes in mobility or behavior of follistatin fish were observed. The findings indicate that overexpression of follistatin in trout, a species with indeterminate growth rate, enhances muscle growth. It remains to be determined whether the double muscling in trout is due to inhibition of myostatin, other growth factors, or both.

Molecular profiles of Quadriceps muscle in myostatin-null mice reveal PI3K and apoptotic pathways as myostatin targets

Background

Myostatin (MSTN), a member of the TGF-β superfamily, has been identified as a negative regulator of skeletal muscle mass. Inactivating mutations in the MSTN gene are responsible for the development of a hypermuscular phenotype. In this study, we performed transcriptomic and proteomic analyses to detect altered expression/abundance of genes and proteins. These differentially expressed genes and proteins may represent new molecular targets of MSTN and could be involved in the regulation of skeletal muscle mass.

Results

Transcriptomic analysis of the Quadriceps muscles of 5-week-old MSTN-null mice (n = 4) and their controls (n = 4) was carried out using microarray (human and murine oligonucleotide sequences) of 6,473 genes expressed in muscle. Proteomic profiles were analysed using two-dimensional gel electrophoresis coupled with mass spectrometry. Comparison of the transcriptomic profiles revealed 192 up- and 245 down- regulated genes. Genes involved in the PI3K pathway, insulin/IGF pathway, carbohydrate metabolism and apoptosis regulation were up-regulated. Genes belonging to canonical Wnt, calcium signalling pathways and cytokine-receptor cytokine interaction were down-regulated. Comparison of the protein profiles revealed 20 up- and 18 down-regulated proteins spots. Knockout of the MSTN gene was associated with up-regulation of proteins involved in glycolytic shift of the muscles and down-regulation of proteins involved in oxidative energy metabolism. In addition, an increased abundance of survival/anti-apoptotic factors were observed.

Conclusion

All together, these results showed a differential expression of genes and proteins related to the muscle energy metabolism and cell survival/anti-apoptotic pathway (e.g. DJ-1, PINK1, 14-3-3ε protein, TCTP/GSK-3β). They revealed the PI3K and apoptotic pathways as MSTN targets and are in favour of a role of MSTN as a modulator of cell survival in vivo.

Extrinsic regulation of domestic animal-derived myogenic satellite cells II

The existence of myogenic satellite cells was reported some 47 years ago, and, since that time, satellite cell research has flourished. So much new information is generated (daily) on these cells that it can be difficult for individuals to keep abreast of important issues related to their activation and proliferation, the modulation of the activity of other cell types, the differentiation of the cells to facilitate normal skeletal muscle growth and development, or to the repair of damaged myofibers. The intent of this review is to summarize new information about the extrinsic regulation of myogenic satellite cells and to provide specific mechanisms involved in altering satellite cell physiology. Where possible, examples from agriculturally important animals are used for illustrative purposes.

Clinical, agricultural, and evolutionary biology of myostatin: a comparative review

The discovery of myostatin and our introduction to the "Mighty Mouse" over a decade ago spurred both basic and applied research and impacted popular culture as well. The myostatin-null genotype produces "double muscling" in mice and livestock and was recently described in a child. The field's rapid growth is by no means surprising considering the potential benefits of enhancing muscle growth in clinical and agricultural settings. Indeed, several recent studies suggest that blocking myostatin's inhibitory effects could improve the clinical treatment of several muscle growth disorders, whereas comparative studies suggest that these actions are at least partly conserved. Thus, neutralizing myostatin's effects could also have agricultural significance. Extrapolating between studies that use different vertebrate models, particularly fish and mammals, is somewhat confusing because whole genome duplication events have resulted in the production and retention of up to four unique myostatin genes in some fish species. Such comparisons, however, suggest that myostatin's actions may not be limited to skeletal muscle per se, but may additionally influence other tissues including cardiac muscle, adipocytes, and the brain. Thus, therapeutic intervention in the clinic or on the farm must consider the potential of alternative side effects that could impact these or other tissues. In addition, the presence of multiple and actively diversifying myostatin genes in most fish species provides a unique opportunity to study adaptive molecular evolution. It may also provide insight into myostatin's nonmuscle actions as results from these and other comparative studies gain visibility in biomedical fields.