TNNI2 explained

Troponin I, fast skeletal muscle is a protein that in humans is encoded by the TNNI2 gene.^{[1] [2]}

The TNNI2 gene is located at 11p15.5 in the human chromosomal genome, encoding the fast twitch skeletal muscle troponin I (fsTnI). fsTnI is a 21.3 kDa protein consisting of 182 amino acids including the first methionine with an isoelectric point (pI) of 8.74. It is the inhibitory subunit of the troponin complex in fast twitch skeletal muscle fibers.^[3]

Gene evolution

Three homologous genes have evolved in vertebrates, encoding three muscle type-specific isoforms of TnI.^{[4] [5] [6]} Sequence analysis, immunological distance, and examination of evolutionarily suppressed conformational states showed that the TnI genes have evolved in close linkage with the genes encoding troponin T (TnT), another subunit of the troponin complex. The fast TnI-fast TnT gene pair represents the original TnI and TnT genes (Fig. 1). The three muscle fiber type-specific TnI-TnT gene pairs were likely originated from a TnI-like ancestor gene that presumably duplicated to form a closely linked fast TnI-like and fast TnT-like gene pair. A later duplication events resulted in emergences of a slow TnI-like and cardiac TnT-like gene pair that was further duplicated to give rise of the present-day slow TnI-cardiac TnT and cardiac TnI-slow TnT gene pairs. The seemingly scrambled ssTnI and cTnI gene pair is actually functionally related as they co-express and form troponin complex in the embryonic heart. The overlapping of enhancer elements of the TnT gene promoter with the upstream TnI gene structure may be a critical factor in the preservation of the close linkage of TnI and TnT gene pairs^[7]

The phylogenetic tree in Fig. 2 summarizes the evolutionary lineage of fsTnI isoforms in vertebrate species.

Phylogenetic analysis of vertebrate TnI isoforms demonstrated that each of the muscle type-specific isoforms is more conserved across species than the three isoforms in one given species, indicating early diverged functions of the muscle fiber type-specific isoforms as well as the conservation of functions for each muscle fiber type.^[8]

Tissue distribution

Fast skeletal muscle TnI was first cloned from a skeletal muscle cDNA library.^[9] It is generally observed that fsTnI is exclusively expressed in fast twitch skeletal muscle fibers. More recent studies reported that subunits of fast skeletal muscle troponin (fsTnI, fsTnT, fsTnC) were expressed at significant levels in smooth muscle cells of mouse blood vessels,^[10] bladder and bronchi.^[11] Expression of fsTnI was also found in non-muscle cells, such as human corneal epithelial cells^[12] and cartilage.^{[13] [14]} The function of fsTnI expressed in smooth muscle and non-muscle cells is unclear.

Protein structure and function

Crystallographic structure of fsTnI in troponin complex from chicken fast skeletal muscle showed an overall structure^[15] similar to that of cardiac troponin.^[16] The inhibitory region of fsTnI was resolved in skeletal troponin whereas it was invisible in the cardiac troponin crystal structure. Based on the crystal structure, a schematic illustration (Fig. 3) was proposed to show the conformational changes in troponin during muscle activation and relaxation.

Posttranslational modifications

Phosphorylation: Ser₁₁₈ of fsTnI, equivalent to Ser₁₅₀ in cTnI, was reported as a phosphorylation substrate of AMPK.^[17] As AMPK is a key regulator of cellular energetics, phosphorylation of this site may provide an adaptive mechanism during energy deprivation in both skeletal and cardiac muscles.

S-glutathionylation: fsTnI was found to be S-glutathionylated at Cys₁₃₃ in rodent fast-twitch skeletal muscle and in human type II muscle fibers after exercise, which increased Ca²⁺ sensitivity of the contractile apparatus.^[18]

Clinical significance

A missense mutation R174Q, a nonsense mutation R156X, and three single residue deletions DE167, DK175 and DK176, all in the C-terminal actin-tropomyosin interacting domain, have been found in patients with distal arthrogryposis.^{[19] [20] [21] [22]}

Skeletal muscle TnI has been proposed as a sensitive and fast fiber-specific serum marker of skeletal muscle injury.^{[23] [24]} fsTnI concentration in increased peripheral blood when fast twitch fibers were damaged.

Further reading

• Kinoshita S, Adachi W, Sotozono C, Nishida K, Yokoi N, Quantock AJ, Okubo K . Characteristics of the human ocular surface epithelium . Progress in Retinal and Eye Research . 20 . 5 . 639–73 . Sep 2001 . 11470454 . 10.1016/S1350-9462(01)00007-6 . 25497205 .
• Wade R, Eddy R, Shows TB, Kedes L . cDNA sequence, tissue-specific expression, and chromosomal mapping of the human slow-twitch skeletal muscle isoform of troponin I . Genomics . 7 . 3 . 346–57 . Jul 1990 . 2365354 . 10.1016/0888-7543(90)90168-T .
• Huang TS, Bylund DB, Stull JT, Krebs EG . The amino acid sequences of the phosphorylated sites in troponin-I from rabbit skeletal muscle . FEBS Letters . 42 . 3 . 249–52 . Jun 1974 . 4369265 . 10.1016/0014-5793(74)80738-6 . 40748797 . free .
• Moir AJ, Ordidge M, Grand RJ, Trayer IP, Perry SV . Studies of the interaction of troponin I with proteins of the I-filament and calmodulin . The Biochemical Journal . 209 . 2 . 417–26 . Feb 1983 . 6847627 . 1154108 . 10.1042/bj2090417.
• Grand RJ, Levine BA, Perry SV . Proton-magnetic-resonance studies on the interaction of rabbit skeletal-muscle troponin I with troponin C and actin . The Biochemical Journal . 203 . 1 . 61–8 . Apr 1982 . 7103951 . 1158193 . 10.1042/bj2030061.
• Chong PC, Hodges RS . Photochemical cross-linking between rabbit skeletal troponin subunits. Troponin I-troponin T interactions . The Journal of Biological Chemistry . 257 . 19 . 11667–72 . Oct 1982 . 10.1016/S0021-9258(18)33814-6 . 7118902 . free .
• Zhu L, Perez-Alvarado G, Wade R . Sequencing of a cDNA encoding the human fast-twitch skeletal muscle isoform of troponin I . Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression . 1217 . 3 . 338–40 . Apr 1994 . 8148383 . 10.1016/0167-4781(94)90297-6 .
• Lanfranchi G, Muraro T, Caldara F, Pacchioni B, Pallavicini A, Pandolfo D, Toppo S, Trevisan S, Scarso S, Valle G . Identification of 4370 expressed sequence tags from a 3'-end-specific cDNA library of human skeletal muscle by DNA sequencing and filter hybridization . Genome Research . 6 . 1 . 35–42 . Jan 1996 . 8681137 . 10.1101/gr.6.1.35 . free .
• Jha PK, Leavis PC, Sarkar S . Interaction of deletion mutants of troponins I and T: COOH-terminal truncation of troponin T abolishes troponin I binding and reduces Ca2+ sensitivity of the reconstituted regulatory system . Biochemistry . 35 . 51 . 16573–80 . Dec 1996 . 8987992 . 10.1021/bi9622433 .
• Stefancsik R, Jha PK, Sarkar S . Identification and mutagenesis of a highly conserved domain in troponin T responsible for troponin I binding: potential role for coiled coil interaction . Proceedings of the National Academy of Sciences of the United States of America . 95 . 3 . 957–62 . Feb 1998 . 9448267 . 18637 . 10.1073/pnas.95.3.957 . free . 1998PNAS...95..957S .
• Vassylyev DG, Takeda S, Wakatsuki S, Maeda K, Maéda Y . Crystal structure of troponin C in complex with troponin I fragment at 2.3-A resolution . Proceedings of the National Academy of Sciences of the United States of America . 95 . 9 . 4847–52 . Apr 1998 . 9560191 . 20176 . 10.1073/pnas.95.9.4847 . free .
• Jha PK, Sarkar S . A recombinant monocysteine mutant (Ser to Cys-155) of fast skeletal troponin T: identification by cross-linking of a domain involved in a physiologically relevant interaction with troponins C and I . Biochemistry . 37 . 35 . 12253–60 . Sep 1998 . 9724539 . 10.1021/bi980025z .
• Moses MA, Wiederschain D, Wu I, Fernandez CA, Ghazizadeh V, Lane WS, Flynn E, Sytkowski A, Tao T, Langer R . Troponin I is present in human cartilage and inhibits angiogenesis . Proceedings of the National Academy of Sciences of the United States of America . 96 . 6 . 2645–50 . Mar 1999 . 10077564 . 15822 . 10.1073/pnas.96.6.2645 . free . 1999PNAS...96.2645M .
• Mullen AJ, Barton PJ . Structural characterization of the human fast skeletal muscle troponin I gene (TNNI2) . Gene . 242 . 1–2 . 313–20 . Jan 2000 . 10721725 . 10.1016/S0378-1119(99)00519-3 .
• Sung SS, Brassington AM, Grannatt K, Rutherford A, Whitby FG, Krakowiak PA, Jorde LB, Carey JC, Bamshad M . Mutations in genes encoding fast-twitch contractile proteins cause distal arthrogryposis syndromes . American Journal of Human Genetics . 72 . 3 . 681–90 . Mar 2003 . 12592607 . 1180243 . 10.1086/368294 .
• Li Q, Liu Y, Shen PY, Dai XQ, Wang S, Smillie LB, Sandford R, Chen XZ . Troponin I binds polycystin-L and inhibits its calcium-induced channel activation . Biochemistry . 42 . 24 . 7618–25 . Jun 2003 . 12809519 . 10.1021/bi034210a .
• Thijssen VL, Ausma J, Gorza L, van der Velden HM, Allessie MA, Van Gelder IC, Borgers M, van Eys GJ . Troponin I isoform expression in human and experimental atrial fibrillation . Circulation . 110 . 7 . 770–5 . Aug 2004 . 15289369 . 10.1161/01.CIR.0000138849.03311.C6 . 975263 .

Notes and References

1. Tiso N, Rampoldi L, Pallavicini A, Zimbello R, Pandolfo D, Valle G, Lanfranchi G, Danieli GA . Fine mapping of five human skeletal muscle genes: alpha-tropomyosin, beta-tropomyosin, troponin-I slow-twitch, troponin-I fast-twitch, and troponin-C fast . Biochemical and Biophysical Research Communications . 230 . 2 . 347–50 . Jan 1997 . 9016781 . 10.1006/bbrc.1996.5958 . 11577/2466889 . free .
2. Web site: Entrez Gene: TNNI2 troponin I type 2 (skeletal, fast).
3. Jin JP, Zhang Z, Bautista JA . Isoform diversity, regulation, and functional adaptation of troponin and calponin . Critical Reviews in Eukaryotic Gene Expression . 18 . 2 . 93–124 . 2008-01-01 . 18304026 . 10.1615/critreveukargeneexpr.v18.i2.10.
4. Hastings KE . Molecular evolution of the vertebrate troponin I gene family . Cell Structure and Function . 22 . 1 . 205–11 . Feb 1997 . 9113408 . 10.1247/csf.22.205 . free .
5. Perry SV . Troponin I: inhibitor or facilitator . Molecular and Cellular Biochemistry . 190 . 1–2 . 9–32 . Jan 1999 . 10098965 . 10.1023/A:1006939307715 . 23721684 .
6. Chong SM, Jin JP . To investigate protein evolution by detecting suppressed epitope structures . Journal of Molecular Evolution . 68 . 5 . 448–60 . May 2009 . 19365646 . 2752406 . 10.1007/s00239-009-9202-0 . 2009JMolE..68..448C .
7. Huang QQ, Jin JP . Preserved close linkage between the genes encoding troponin I and troponin T, reflecting an evolution of adapter proteins coupling the Ca(2+) signaling of contractility . Journal of Molecular Evolution . 49 . 6 . 780–8 . Dec 1999 . 10594179 . 10.1007/pl00006600. 1999JMolE..49..780H . 9839814 .
8. Jin JP, Chen A, Huang QQ . Three alternatively spliced mouse slow skeletal muscle troponin T isoforms: conserved primary structure and regulated expression during postnatal development . Gene . 214 . 1–2 . 121–9 . Jul 1998 . 9651500 . 10.1016/s0378-1119(98)00214-5.
9. Zhu L, Perez-Alvarado G, Wade R . Sequencing of a cDNA encoding the human fast-twitch skeletal muscle isoform of troponin I . Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression . 1217 . 3 . 338–40 . Apr 1994 . 8148383 . 10.1016/0167-4781(94)90297-6.
10. Moran CM, Garriock RJ, Miller MK, Heimark RL, Gregorio CC, Krieg PA . Expression of the fast twitch troponin complex, fTnT, fTnI and fTnC, in vascular smooth muscle . Cell Motility and the Cytoskeleton . 65 . 8 . 652–61 . Aug 2008 . 18548613 . 2570210 . 10.1002/cm.20291 .
11. Ju Y, Li J, Xie C, Ritchlin CT, Xing L, Hilton MJ, Schwarz EM . Troponin T3 expression in skeletal and smooth muscle is required for growth and postnatal survival: characterization of Tnnt3(tm2a(KOMP)Wtsi) mice . Genesis . 51 . 9 . 667–75 . Sep 2013 . 23775847 . 3787964 . 10.1002/dvg.22407 .
12. Kinoshita S, Adachi W, Sotozono C, Nishida K, Yokoi N, Quantock AJ, Okubo K . Characteristics of the human ocular surface epithelium . Progress in Retinal and Eye Research . 20 . 5 . 639–73 . Sep 2001 . 11470454 . 10.1016/s1350-9462(01)00007-6. 25497205 .
13. Moses MA, Wiederschain D, Wu I, Fernandez CA, Ghazizadeh V, Lane WS, Flynn E, Sytkowski A, Tao T, Langer R . Troponin I is present in human cartilage and inhibits angiogenesis . Proceedings of the National Academy of Sciences of the United States of America . 96 . 6 . 2645–50 . Mar 1999 . 10077564 . 15822 . 10.1073/pnas.96.6.2645. free . 1999PNAS...96.2645M .
14. Li Q, Shen PY, Wu G, Chen XZ . Polycystin-2 interacts with troponin I, an angiogenesis inhibitor . Biochemistry . 42 . 2 . 450–7 . Jan 2003 . 12525172 . 10.1021/bi0267792 .

15. Vinogradova MV, Stone DB, Malanina GG, Karatzaferi C, Cooke R, Mendelson RA, Fletterick RJ . Ca(2+)-regulated structural changes in troponin . Proceedings of the National Academy of Sciences of the United States of America . 102 . 14 . 5038–43 . Apr 2005 . 15784741 . 555973 . 10.1073/pnas.0408882102 . free . 2005PNAS..102.5038V .

16. Takeda S, Yamashita A, Maeda K, Maéda Y . Structure of the core domain of human cardiac troponin in the Ca(2+)-saturated form . Nature . 424 . 6944 . 35–41 . Jul 2003 . 12840750 . 10.1038/nature01780 . 2003Natur.424...35T . 2174019 .
17. Sancho Solis R, Ge Y, Walker JW . A preferred AMPK phosphorylation site adjacent to the inhibitory loop of cardiac and skeletal troponin I . Protein Science . 20 . 5 . 894–907 . May 2011 . 21416543 . 3125873 . 10.1002/pro.623 .
18. Mollica JP, Dutka TL, Merry TL, Lamboley CR, McConell GK, McKenna MJ, Murphy RM, Lamb GD . S-glutathionylation of troponin I (fast) increases contractile apparatus Ca2+ sensitivity in fast-twitch muscle fibres of rats and humans . The Journal of Physiology . 590 . Pt 6 . 1443–63 . Mar 2012 . 22250211 . 3382333 . 10.1113/jphysiol.2011.224535 .
19. Sung SS, Brassington AM, Grannatt K, Rutherford A, Whitby FG, Krakowiak PA, Jorde LB, Carey JC, Bamshad M . Mutations in genes encoding fast-twitch contractile proteins cause distal arthrogryposis syndromes . American Journal of Human Genetics . 72 . 3 . 681–90 . Mar 2003 . 12592607 . 1180243 . 10.1086/368294 .
20. Jiang M, Zhao X, Han W, Bian C, Li X, Wang G, Ao Y, Li Y, Yi D, Zhe Y, Lo WH, Zhang X, Li J . A novel deletion in TNNI2 causes distal arthrogryposis in a large Chinese family with marked variability of expression . Human Genetics . 120 . 2 . 238–42 . Sep 2006 . 16802141 . 10.1007/s00439-006-0183-4 . 6246993 .
21. Kimber E, Tajsharghi H, Kroksmark AK, Oldfors A, Tulinius M . A mutation in the fast skeletal muscle troponin I gene causes myopathy and distal arthrogryposis . Neurology . 67 . 4 . 597–601 . Aug 2006 . 16924011 . 10.1212/01.wnl.0000230168.05328.f4 . 72428621 .
22. Robinson P, Lipscomb S, Preston LC, Altin E, Watkins H, Ashley CC, Redwood CS . Mutations in fast skeletal troponin I, troponin T, and beta-tropomyosin that cause distal arthrogryposis all increase contractile function . FASEB Journal . 21 . 3 . 896–905 . Mar 2007 . 17194691 . 10.1096/fj.06-6899com . free . 25491760 .
23. Simpson JA, Labugger R, Collier C, Brison RJ, Iscoe S, Van Eyk JE . Fast and slow skeletal troponin I in serum from patients with various skeletal muscle disorders: a pilot study . Clinical Chemistry . 51 . 6 . 966–72 . Jun 2005 . 15833785 . 10.1373/clinchem.2004.042671 . free .
24. Chapman DW, Simpson JA, Iscoe S, Robins T, Nosaka K . Changes in serum fast and slow skeletal troponin I concentration following maximal eccentric contractions . Journal of Science and Medicine in Sport . 16 . 1 . 82–5 . Jan 2013 . 22795680 . 10.1016/j.jsams.2012.05.006 . free .