Myostatin

Myostatin
Identifiers
SymbolsMSTN ; GDF8; MSLHP
External IDsOMIM: 601788 MGI: 95691 HomoloGene: 3850 GeneCards: MSTN Gene
Orthologs
SpeciesHumanMouse
Entrez266017700
EnsemblENSG00000138379ENSMUSG00000026100
UniProtO14793O08689
RefSeq (mRNA)NM_005259NM_010834
RefSeq (protein)NP_005250NP_034964
Location (UCSC)Chr 2:
190.92 – 190.93 Mb		Chr 1:
53.06 – 53.07 Mb
PubMed search

Myostatin (also known as growth differentiation factor 8, abbreviated GDF-8) is a protein that in humans is encoded by the MSTN gene.[1] Myostatin is a secreted growth differentiation factor that is a member of the TGF beta protein family that inhibits muscle differentiation and growth in the process known as myogenesis. Myostatin is produced primarily in skeletal muscle cells, circulates in the blood and acts on muscle tissue, by binding a cell-bound receptor called the activin type II receptor.[2][3]

Animals lacking myostatin or animals treated with substances that block the activity of myostatin have significantly larger muscles. This could be of economic benefit to the livestock industry. However these animals require special care and feeding which offsets the potential economic advantage.

Mutations in both copies of the human myostatin gene results in individuals that have significantly more muscle mass and hence are considerably stronger than normal. However, because this added muscle mass is not necessarily the product of exercise-mediated growth, it may not form with the same level of useful and purposeful tone and structure as normal muscle growth, as is seen in the relationship between cardiac muscle growth and cardiomyopathy. Furthermore, blocking the activity of myostatin may have therapeutic application in treating muscle wasting diseases such as muscular dystrophy.

Discovery and sequencing

The gene encoding myostatin was discovered in 1997 by geneticists Dr. Se-Jin Lee and Alexandra McPherron who also produced a strain of mutant mice that lack the gene. These myostatin "knockout" mice have approximately twice as much muscle as normal mice.[4] These mice were subsequently named "mighty mice".

Naturally occurring lacks of myostatin have been identified in cattle by Dr. Ravi Kambadur,[5] whippets, and humans; in each case the result is a dramatic increase in muscle mass. A mutation in the 3' UTR of the myostatin gene in Texel sheep creates target sites for the microRNAs miR-1 and miR-206. This is likely to cause the muscular phenotype of this breed of sheep.[6]

Structure and mechanism of action

Myostatin is a member of the TGF beta superfamily of proteins.

Human myostatin consists of two identical subunits, each consisting of 109 (NCBI database claims human myostatin is 375 residues long) amino acid residues. Its total molecular weight is 25.0 kDa. The protein is made in an inactive form. For it to be activated, a protease cleaves the NH2-terminal, or "pro-domain" portion of the molecule, resulting in the now-active COOH-terminal dimer.

Myostatin binds to the activin type II receptor, resulting in a recruitment of a coreceptor called Alk-3 or Alk-4. This coreceptor then initiates a cell signaling cascade in the muscle, which includes the activation of transcription factors in the SMAD family - SMAD2 and SMAD3. These factors then induce myostatin-specific gene regulation. When applied to myoblasts, myostatin inhibits their differentiation into mature muscle fibers.

Recently, myostatin has also been shown to inhibit Akt, a kinase that is sufficient to cause muscle hypertrophy, in part through the activation of protein synthesis. However, it is notable that Akt is not responsible for all of the observed muscle hyperthrophic effects which are mediated by myostatin inhibition[7]

Therefore, myostatin acts in two ways, by inhibiting muscle differentiation and by inhibiting Akt-induced protein synthesis.

Myostatin is also classed as a myokine, and is in fact considered the first myokine to have been characterized.

Effects in animals

Double muscled cattle

After that discovery, several laboratories cloned and established the nucleotide sequence of a myostatin gene in two breeds of cattle Belgian Blue and Piedmontese, and found that these animals have mutations in that myostatin gene (various mutations in each breed) which in one way or another lead to absence of functional myostatin.[4][5][8] Unlike mice with a damaged myostatin gene, in these cattle breeds the muscle cells multiply rather than enlarge. People describe these cattle breeds as "double muscled", but the total increase in all muscles is no more than 40%.[5][9][10]

Animals lacking myostatin or animals treated with substances such as follistatin that block the binding of myostatin to its receptor have significantly larger muscles. Thus, reduction of myostatin could potentially benefit the livestock industry, with even a 20 percent reduction in myostatin levels potentially having a large effect on the development of muscles.[11][12]

However, the animal breeds developed as homozygous for myostatin deficiency have reproduction issues due to their unusually heavy and bulky offspring, and require special care and a more expensive diet to achieve a superior yield. This negatively affects economics of myostatin-deficient breeds to the point where they do not usually offer an obvious advantage. While hypertrophic meat (e.g. from Piedmontese beef) has a place on the specialist market due to its unusual properties, at least for purebred myostatin-deficient strains the expenses and (especially in cattle) necessity of veterinary supervision place them at a disadvantage in the bulk market.[13]

Performance enhancement in dogs

A "bully whippet" with a homozygous mutation in myostatin.[14]

A 2007 NIH study in PLOS Genetics[14] found a significant relationship in whippets between a myostatin mutation and racing performance. Whippets that were heterozygous for a 2 base pair deletion in the myostatin gene were significantly over-represented in the top racing classes. The mutation resulted in a truncated myostatin protein, likely resulting in an inactive form of myostatin.

Whippets with a homozygous deletion were apparently less able runners although their overall appearance was significantly more muscular. Whippets with the homozygous deletion also had an unusual body shape, with a broader head, pronounced overbite, shorter legs, and thicker tails. These whippets have also been called "bully whippets" by the breeding community due to their size. Despite the name "bully", these dogs tend to have a friendly and positive demeanour towards people as usual for whippets.

This particular mutation was not found in other muscular dog breeds such as boxers and mastiffs, nor was it found in other sighthounds such as greyhounds, Italian greyhounds, or Afghan hounds. The authors of the study suggest that myostatin mutation may not be desirable in greyhounds, the whippets' nearest relative, because greyhound racing requires more significant endurance due to the longer races (900 meters for greyhounds vs. 300 meters for whippets).

Clinical significance

Effect in humans

Myostatin is active in muscles used for movement (skeletal muscles) both before and after birth. It normally restrains muscle growth, ensuring that muscles do not grow too large. Mutations that reduce the production of functional myostatin lead to an overgrowth of muscle tissue. Myostatin-related muscle hypertrophy has a pattern of inheritance known as incomplete autosomal dominance. People with a mutation in both copies of the MSTN gene in each cell (homozygotes) have significantly increased muscle mass and strength. People with a mutation in one copy of the MSTN gene in each cell (heterozygotes) also have increased muscle bulk, but to a lesser degree.

In 2004, a German boy was diagnosed with a mutation in both copies of the myostatin-producing gene, making him considerably stronger than his peers. His mother has a mutation in one copy of the gene.[15][16][17][18]

An American boy born in 2005, Liam Hoekstra, was diagnosed with a clinically similar condition but with a somewhat different cause:[19] his body produces a normal level of functional myostatin; but, because he is stronger and more muscular than most others his age, his doctor believes that a defect in his myostatin receptors prevents his muscle cells from responding normally to myostatin. Liam appeared on the television show World's Strongest Toddler.

A technique for detecting mutations in myostatin variants has been developed.[20]

Therapeutic potential

Further research into myostatin and the myostatin gene may lead to therapies for muscular dystrophy.[21][22] The idea is to introduce substances that block myostatin. A monoclonal antibody specific to myostatin increases muscle mass in mice.[23] Similar results in monkeys were obtained.[12]

A two-week treatment of normal mice with soluble activin type IIB receptor, a molecule that is normally attached to cells and binds to myostatin, leads to a significantly increased muscle mass (up to 60%).[24] It is thought that binding of myostatin to the soluble activin receptor prevents it from interacting with the cell-bound receptors.

It remains unclear as to whether long-term treatment of muscular dystrophy with myostatin inhibitors is beneficial, as the depletion of muscle stem cells could worsen the disease later on. As of 2012, no myostatin-inhibiting drugs for humans are on the market, but an antibody genetically engineered to neutralize myostatin was developed by New Jersey pharmaceutical company Wyeth.[25] The inhibitor is called stamulumab, but, after an initial clinical trial, Wyeth says they will not be developing the drug.[26] Some athletes, eager to get their hands on such drugs, turn to the internet, where fake "myostatin blockers" are being sold.[12]

Myostatin levels are effectively decreased by creatine supplementation.[27]

Gene doping

Inhibition of myostatin leads to muscle hyperplasia and hypertrophy. Myostatin inhibitors can improve athletic performance and therefore there is a concern these inhibitors might be abused in the field of sports.[28] However, studies in mice suggest that myostatin inhibition does not directly increase the strength of individual muscle fibers.[29]

Myostatin in the heart

Myostatin is expressed at very low levels in cardiac myocytes.[30][31] Although its presence has been noted in cardiomyocytes of both fetal and adult mice,[32] its physiological function remains uncertain.[31] However, it has been suggested that fetal cardiac myostatin may play a role in early heart development.[32]

Myostatin is produced as promyostatin, a precursor protein kept inactive by the latent TGF-β binding protein 3 (LTBP3).[30] Pathological cardiac stress promotes N-terminal cleavage by furin convertase to create a biologically active C-terminal fragment. The mature myostatin is then segregated from the latent complex via proteolytic cleavage by BMP-1 and tolloid metallopreoteinases.[30] Free myostatin is able to bind its receptor, ActRIIB, and increase SMAD2/3 phosphorylation.[30] The latter produces a heteromeric complex with SMAD4, inducing myostatin translocation into the cardiomyocyte nucleus to modulate transcription factor activity.[33] Manipulating the muscle creatinine kinase promoter can modulate myostatin expression, though it has only been observed in male mice thus far.[30][31]

Myostatin may inhibit cardiomyocyte proliferation and differentiation by manipulating cell cycle progression.[32] This argument is supported by the fact that myostatin mRNA is poorly expressed in proliferating fetal cardiomyocytes.[30][33] In vitro studies indicate that myostatin promotes SMAD2 phosphorylation to inhibit cardiomyocyte proliferation.[33] Furthermore, myostatin has been shown to directly prevent cell cycle G1 to S phase transition by decreasing levels of cyclin-dependent kinase complex 2 (CDK2) and by increasing p21 levels.[33]

Growth of cardiomyocytes may also be hindered by myostatin-regulated inhibition of protein kinase p38 and the serine-threonine protein kinase Akt, which typically promote cardiomyocyte hypertrophy.[34] However, increased myostatin activity only occurs in response to specific stimuli,[30][34] such as in pressure stress models, in which cardiac myostatin induces whole-body muscular atrophy.[30][32]

Physiologically, minimal amounts of cardiac myostatin are secreted from the myocardium into serum, having a limited effect on muscle growth.[31] However, increases in cardiac myostatin can increase its serum concentration, which may cause skeletal muscle atrophy.[30][31] Pathological states that increase cardiac stress and promote heart failure can induce a rise in both cardiac myostatin mRNA and protein levels within the heart.[30][31] In ischemic or dilated cardiomyopathy, increased levels of myostatin mRNA have been detected within the left ventricle.[30][35] As a member of the TGF-β family, myostatin may play a role in post-infarct recovery.[31][32] It has been hypothesized that hypertrophy of the heart induces an increase in myostatin as a negative feedback mechanism in an attempt to limit further myocyte growth.[36][37] This process includes mitogen-activated protein kinases and binding of the MEF2 transcription factor within the promoter region of the myostatin gene.[30] Increases in myostatin levels during chronic heart failure have been shown to cause cardiac cachexia.[30][31][38] Systemic inhibition of cardiac myostatin with the JA-16 antibody maintains overall muscle weight in experimental models with pre-existing heart failure.[31]

Myostatin also alters excitation-contraction (EC) coupling within the heart.[39] A reduction in cardiac myostatin induces eccentric hypertrophy of the heart, and increases its sensitivity to beta-adrenergic stimuli by enhancing Ca2+ release from the SR during EC coupling. Also, phospholamban phosphorylation is increased in myostatin-knockout mice, leading to an increase in Ca2+ release into the cytosol during systole.[30] Therefore, minimizing cardiac myostatin may improve cardiac output.[39]

See also

References

  1. ↑ Gonzalez-Cadavid NF, Taylor WE, Yarasheski K, Sinha-Hikim I, Ma K, Ezzat S et al. (December 1998). "Organization of the human myostatin gene and expression in healthy men and HIV-infected men with muscle wasting". Proc. Natl. Acad. Sci. U.S.A. 95 (25): 14938–43. Bibcode:1998PNAS...9514938G. doi:10.1073/pnas.95.25.14938. PMC 24554. PMID 9843994.
  2. ↑ Carnac G, Ricaud S, Vernus B, Bonnieu A (July 2006). "Myostatin: biology and clinical relevance". Mini Rev Med Chem 6 (7): 765–70. doi:10.2174/138955706777698642. PMID 16842126.
  3. ↑ Joulia-Ekaza D, Cabello G (June 2007). "The myostatin gene: physiology and pharmacological relevance". Curr Opin Pharmacol 7 (3): 310–5. doi:10.1016/j.coph.2006.11.011. PMID 17374508.
  4. ↑ 4.0 4.1 McPherron AC, Lawler AM, Lee SJ (May 1997). "Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member". Nature 387 (6628): 83–90. Bibcode:1997Natur.387...83M. doi:10.1038/387083a0. PMID 9139826.
  5. ↑ 5.0 5.1 5.2 Kambadur R, Sharma M, Smith TP, Bass JJ (September 1997). "Mutations in myostatin (GDF8) in double-muscled Belgian Blue and Piedmontese cattle". Genome Res. 7 (9): 910–6. doi:10.1101/gr.7.9.910. PMID 9314496.
  6. ↑ Clop A, Marcq F, Takeda H, Pirottin D, Tordoir X, Bibé B et al. (July 2006). "A mutation creating a potential illegitimate microRNA target site in the myostatin gene affects muscularity in sheep". Nat. Genet. 38 (7): 813–8. doi:10.1038/ng1810. PMID 16751773.
  7. ↑ Sartori, Roberta; Gregorevic, Paul; Sandri, Marco (2014). "TGFb and BMP signaling in skeletal muscle: potential significance for muscle-related disease". Trends in Endocrinology and Metabolism 25 (9): 464–471. doi:10.1016/j.tem.2014.06.002.
  8. ↑ Grobet L, Martin LJ, Poncelet D, Pirottin D, Brouwers B, Riquet J et al. (September 1997). "A deletion in the bovine myostatin gene causes the double-muscled phenotype in cattle". Nat. Genet. 17 (1): 71–4. doi:10.1038/ng0997-71. PMID 9288100.
  9. ↑ Photos of double muscled Myostatin, inhibited Belgian Blue Bulls
  10. ↑ McPherron AC, Lee SJ (1997). "Double muscling in cattle due to mutations in the myostatin gene". Proc Natl Acad Sci USA 94 (23): 12457–61. Bibcode:1997PNAS...9412457M. doi:10.1073/pnas.94.23.12457. PMC 24998. PMID 9356471.
  11. ↑ "Myostatin – The Genetic Factor In Muscle Building". Myostatin Info. Retrieved 5 January 2012.
  12. ↑ 12.0 12.1 12.2 Kota J, Handy CR, Haidet AM, Montgomery CL, Eagle A, Rodino-Klapac LR et al. (November 2009). "Follistatin gene delivery enhances muscle growth and strength in nonhuman primates". Sci Transl Med 1 (6): 6ra15. doi:10.1126/scitranslmed.3000112. PMC 2852878. PMID 20368179. Lay summary – National Public Radio.
  13. ↑ De Smet S (2004). Jensen WK, ed. "Double- Muscled Animals". Encyclopedia of Meat Sciences (Oxford: Elsevier): 396–402. doi:10.1016/B0-12-464970-X/00260-9.
  14. ↑ 14.0 14.1 Mosher DS, Quignon P, Bustamante CD, Sutter NB, Mellersh CS, Parker HG et al. (May 2007). "A Mutation in the Myostatin Gene Increases Muscle Mass and Enhances Racing Performance in Heterozygote Dogs". PLoS Genet. 3 (5): e79. doi:10.1371/journal.pgen.0030079. PMC 1877876. PMID 17530926.
  15. ↑ cevgenetica: Gene Mutation Makes German Boy Extra Strong Muscle Baby
  16. ↑ Gina Kolota: A Very Muscular Baby Offers Hope Against Diseases, The New York Times, June 24, 2004. (Requires login)
  17. ↑ Genetic mutation turns tot into superboy
  18. ↑ Schuelke M, Wagner KR, Stolz LE, Hübner C, Riebel T, Kömen W et al. (2004). "Myostatin mutation associated with gross muscle hypertrophy in a child". N Engl J Med 350 (26): 2682–8. doi:10.1056/NEJMoa040933. PMID 15215484.
  19. ↑ Associated Press (2007-05-30). "CTV.ca | Rare condition gives toddler super strength". CTVglobemedia. Retrieved 2009-01-21.
  20. ↑ US patent 6673534, Lee S-J, McPherron AC, "Methods for detection of mutations in myostatin variants", issued 2004-01-06, assigned to The Johns Hopkins University School of Medicine
  21. ↑ Schuelke M, Wagner KR, Stolz LE, Hübner C, Riebel T, Kömen W et al. (June 2004). "Myostatin mutation associated with gross muscle hypertrophy in a child". N. Engl. J. Med. 350 (26): 2682–8. doi:10.1056/NEJMoa040933. PMID 15215484. Lay summary – Genome News Network.
  22. ↑ Tsuchida K (July 2008). "Targeting myostatin for therapies against muscle-wasting disorders". Curr Opin Drug Discov Devel 11 (4): 487–94. PMID 18600566.
  23. ↑ Whittemore LA, Song K, Li X, Aghajanian J, Davies M, Girgenrath S et al. (January 2003). "Inhibition of myostatin in adult mice increases skeletal muscle mass and strength". Biochem. Biophys. Res. Commun. 300 (4): 965–71. doi:10.1016/s0006-291x(02)02953-4. PMID 12559968.
  24. ↑ Lee SJ, Reed LA, Davies MV, Girgenrath S, Goad ME, Tomkinson KN et al. (December 2005). "Regulation of muscle growth by multiple ligands signaling through activin type II receptors". Proc. Natl. Acad. Sci. U.S.A. 102 (50): 18117–22. Bibcode:2005PNAS..10218117L. doi:10.1073/pnas.0505996102. PMC 1306793. PMID 16330774.
  25. ↑ 2/23/05 Wyeth MYO-029 press release
  26. ↑ 3/11/2008 Wyeth Won't Develop MYO-029 for MD
  27. ↑ Saremi A, Gharakhanloo R, Sharghi S, Gharaati MR, Larijani B, Omidfar K (April 2010). "Effects of oral creatine and resistance training on serum myostatin and GASP-1". Mol. Cell. Endocrinol. 317 (1–2): 25–30. doi:10.1016/j.mce.2009.12.019. PMID 20026378.
  28. ↑ Haisma HJ, de Hon O (2006). "Gene Doping". International Journal of Sports Medicine 27 (4): 257–266. doi:10.1055/s-2006-923986. PMID 16572366.
  29. ↑ Mendias CL, Kayupov E, Bradley JR, Brooks SV, Claflin DR (July 2011). "Decreased specific force and power production of muscle fibers from myostatin-deficient mice are associated with a suppression of protein degradation". J. Appl. Physiol. 111 (1): 185–91. doi:10.1152/japplphysiol.00126.2011. PMC 3137541. PMID 21565991.
  30. ↑ 30.0 30.1 30.2 30.3 30.4 30.5 30.6 30.7 30.8 30.9 30.10 30.11 30.12 30.13 Breitbart A, Auger-Messier M, Molkentin JD, Heineke J (2011). "Myostatin from the heart: local and systemic actions in cardiac failure and muscle wasting". Am. J. Physiol. Heart Circ. Physiol. 300 (6): H1973–82. doi:10.1152/ajpheart.00200.2011. PMC 3119101. PMID 21421824.
  31. ↑ 31.0 31.1 31.2 31.3 31.4 31.5 31.6 31.7 31.8 Heineke J, Auger-Messier M, Xu J, Sargent M, York A, Welle S et al. (2010). "Genetic deletion of myostatin from the heart prevents skeletal muscle atrophy in heart failure". Circulation 121 (3): 419–25. doi:10.1161/CIRCULATIONAHA.109.882068. PMC 2823256. PMID 20065166.
  32. ↑ 32.0 32.1 32.2 32.3 32.4 Sharma M, Kambadur R, Matthews KG, Somers WG, Devlin GP, Conaglen JV et al. (1999). "Myostatin, a transforming growth factor-beta superfamily member, is expressed in heart muscle and is upregulated in cardiomyocytes after infarct". J. Cell. Physiol. 180 (1): 1–9. doi:10.1002/(SICI)1097-4652(199907)180:1<1::AID-JCP1>3.0.CO;2-V. PMID 10362012.
  33. ↑ 33.0 33.1 33.2 33.3 McKoy G, Bicknell KA, Patel K, Brooks G (2007). "Developmental expression of myostatin in cardiomyocytes and its effect on foetal and neonatal rat cardiomyocyte proliferation". Cardiovasc. Res. 74 (2): 304–12. doi:10.1016/j.cardiores.2007.02.023. PMID 17368590.
  34. ↑ 34.0 34.1 Morissette MR, Cook SA, Foo S, McKoy G, Ashida N, Novikov M et al. (2006). "Myostatin regulates cardiomyocyte growth through modulation of Akt signaling". Circ. Res. 99 (1): 15–24. doi:10.1161/01.RES.0000231290.45676.d4. PMC 2901846. PMID 16763166.
  35. ↑ Torrado M, Iglesias R, Nespereira B, Mikhailov AT (2010). "Identification of candidate genes potentially relevant to chamber-specific remodeling in postnatal ventricular myocardium". J. Biomed. Biotechnol. 2010: 603159. doi:10.1155/2010/603159. PMC 2846348. PMID 20368782.
  36. ↑ Wang BW, Chang H, Kuan P, Shyu KG (2008). "Angiotensin II activates myostatin expression in cultured rat neonatal cardiomyocytes via p38 MAP kinase and myocyte enhance factor 2 pathway". J. Endocrinol. 197 (1): 85–93. doi:10.1677/JOE-07-0596. PMID 18372235.
  37. ↑ Shyu KG, Ko WH, Yang WS, Wang BW, Kuan P (2005). "Insulin-like growth factor-1 mediates stretch-induced upregulation of myostatin expression in neonatal rat cardiomyocytes". Cardiovasc. Res. 68 (3): 405–14. doi:10.1016/j.cardiores.2005.06.028. PMID 16125157.
  38. ↑ Anker SD, Negassa A, Coats AJ, Afzal R, Poole-Wilson PA, Cohn JN et al. (March 2003). "Prognostic importance of weight loss in chronic heart failure and the effect of treatment with angiotensin-converting-enzyme inhibitors: an observational study". Lancet 361 (9363): 1077–83. doi:10.1016/S0140-6736(03)12892-9. PMID 12672310.
  39. ↑ 39.0 39.1 Rodgers BD, Interlichia JP, Garikipati DK, Mamidi R, Chandra M, Nelson OL et al. (2009). "Myostatin represses physiological hypertrophy of the heart and excitation-contraction coupling". J. Physiol. (Lond.) 587 (Pt 20): 4873–86. doi:10.1113/jphysiol.2009.172544. PMC 2770153. PMID 19736304.

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