Disease relevance of Tnni3

• To confirm the role of p90RSK in cTnI phosphorylation in vivo, we generated adenovirus containing a dominant negative form of p90RSK (Ad-DN-p90RSK) [1].
• Gene mutations in cardiac troponin I (cTnI) account for up to 5% of genotyped families with familial hypertrophic cardiomyopathy (FHC) [2].
• These data indicate that individual genetic conditions and environmental factors participate together in the development of the cTnI mutation based-cardiac muscle disorders [3].
• Cardiac troponin I (cTnI) mutations have been linked to the development of restrictive cardiomyopathy (RCM) in human patients [3].
• To address this question, the following recombinant proteins were expressed in Escherichia coli and purified: mouse wild-type cTnI (WT cTnI; 211 residues), cTnI-(1-199) (missing 12 residues), cTnI-(1-188) (missing 23 residues), and cTnI-(1-151) (missing 60 residues) [4].

High impact information on Tnni3

• Unexpectedly, in addition to loss of Tnnt2 expression in sih mutant hearts, we observed a significant reduction in Tpma and Tnni3, and consequently, severe sarcomere defects [5].
• In this study, we purified the 30-kDa protein from heart extract and identified it as cardiac troponin I (cTnI), encoded by a gene in which mutations can cause familial hypertrophic cardiomyopathy (HCM) [6].
• Furthermore, this fragment is able to repress cardiac troponin I (cTnI) promoter activity selectively in the embryonic myocardium of the atrioventricular canal (AVC) [7].
• Importantly, elevations of cTnI in patients with myocarditis were significantly correlated with < or = 1 month duration of heart failure symptoms (P = .02), suggesting that the majority of myocyte necrosis occurs early, and thus the window for diagnosis and treatment may be relatively brief [8].
• Because the histological diagnosis of myocarditis requires the presence of myocyte injury, we sought to determine whether measurement of cardiac troponin I (cTnI), which is a serum marker with high sensitivity and specificity for cardiac myocyte injury, could aid in the diagnosis of myocarditis [8].

Chemical compound and disease context of Tnni3

• Histopathological and pathophysiological cardiac changes in dogs, rats and mice correlated with increased serum cTnI with various cardiac inotropic agents, and cardiotoxic drugs and with cardiac arrhythmias, tachycardia, cardiac effusion with dyspnoea, and ageing [9].

Biological context of Tnni3

• The cardiac troponin I locus (Tnni3) also mapped to Chr 7, approximately 5-10 cM from the centromere and unlinked to the fast skeletal muscle troponin I locus [10].
• No differences in isoform expression of tropomyosin, myosin heavy chain, essential and regulatory myosin light chains (MLC), TnI, or in posttranslational modifications of mouse cTnT, cTnI, or regulatory MLC were observed [11].
• Protein kinase C (PKC)-induced phosphorylation of cardiac troponin I (cTnI) depresses the acto-myosin interaction and may be important during the progression of heart failure [1].
• The observed Ca(2+)-induced conformational change may be a switch mechanism by which movement of the regulatory region of cTnI to the exposed hydrophobic patch of the open regulatory N-domain of cTnC pulls the inhibitory region away from actin upon Ca(2+) activation in cardiac muscle [12].
• To approach this question we have used molecular cloning, mutagenesis, and bacterial synthesis of a full-length cTnI and a truncated mutant (cTnI/NH2) missing the 32 amino acids [13].

Anatomical context of Tnni3

• Although both PKCbetaII and PKCepsilon can phosphorylate cTnI, only PKCbeta expression and activity are elevated in failing human myocardium during end-stage heart failure [1].
• Although the C terminus of troponin I is known to be important in myofilament Ca2+ regulation in skeletal muscle, the regulatory function of this region of cardiac troponin I (cTnI) has not been defined [4].
• We also formed a complex of either WT cTnI or each of the mutants with cTnC, reconstituted the complex into the cTnT-treated myofibrils, and measured the Mg2+-ATPase activity as a function of pCa [4].
• We studied intact and detergent-extracted papillary muscles from nontransgenic (NTG) and transgenic (TG) mouse hearts that express a mutant cTnI (Ser43Ala, Ser45Ala) that lacks specific PKC-dependent phosphorylation sites [14].
• Here, we report that regulation of myocyte twitch kinetics by beta-stimulation and by endothelin-1 was altered in myocytes containing mutant cTnI [15].

Associations of Tnni3 with chemical compounds

• An important role for these cTnT sites is indicated by results demonstrating that ROCK-II induced a depression in tension and ATPase activity in skinned fiber bundles from a TG model in which cTnI is replaced by slow skeletal TnI, which lacks S23 and S24 and in which T144 is replaced by proline [16].
• Phosphorylation of Ser residues in the NH2-terminal extension unique to cardiac troponin I (cTnI) is known to occur through protein kinase A and to alter myofilament Ca2+ activation (Robertson, S [13].
• To investigate the effect(s) of complete and chronic cTnI phosphorylation on cardiac function, we generated transgenic animals in which the five possible phosphorylation sites were replaced with aspartic acid, mimicking a constant state of complete phosphorylation (cTnI-AllP) [17].
• We studied the contractile response to isoprenaline (10 nm) in isolated hearts and isolated cardiomyocytes from transgenic mice with cardiac-specific expression of slow skeletal TnI (ssTnI, which lacks the N-terminal protein extension containing PKA-sensitive phosphorylation sites in cTnI) and matched wild-type littermate controls [18].
• To investigate the contributions of cTnI phosphorylation to cardiac regulation, transgenic mice were created with the phosphorylation sites of cTnI mutated to alanine [19].

Physical interactions of Tnni3

• These data provide the first evidence of a significant function of a cTnT-binding domain on cTnI [20].

Regulatory relationships of Tnni3

• PKC activation no longer altered cTnI phosphorylation, depressed ATPase rates or enhanced myofilament Ca2+ sensitivity in transgenic mice expressing cTnI that could not be phosphorylated on serines43/45 and threonine144 (PKC sites) [19].
• We propose that beta-agonists and endothelin-1 regulate cardiac twitch dynamics in opposite directions in part through phosphorylation of the myofilament protein cTnI on distinct sites [15].

Other interactions of Tnni3

• The beating cells of both groups, stained positive with anti alpha-actinin, desmin, cardiac troponin I and connexin antibodies, and revealed similar ultrastructural features [21].
• These findings indicate that removal of the N-terminal extension of cTnI via restricted proteolysis enhances cardiac function by increasing the rate of myocardial relaxation and lowering left ventricular end diastolic pressure to facilitate ventricular filling, thus resulting in better utilization of the Frank-Starling mechanism [22].
• Modest changes in myosin regulatory light chain phosphorylation occurred in all mouse lines, but increases in myofilament Ca2+ sensitivity required the presence of phosphorylatable cTnI [19].
• These observations are consistent with important roles for both cTnI and phospholamban phosphorylation in accelerating relaxation after beta-adrenergic stimulation [15].
• The cardiac myofilament protein troponin I (cTnI) is phosphorylated by protein kinase C (PKC), a family of serine/threonine kinases activated within heart muscle by a variety of agonists. cTnI is also a substrate for cAMP-dependent protein kinase (PKA) activated during beta-adrenergic signaling [15].

Analytical, diagnostic and therapeutic context of Tnni3

• Western blot analysis of human or mouse homogenized muscle specimens showed no evidence for cardiac TnT and cTnI expression, despite strong signals for skeletal muscle troponin isoforms [23].
• Each cTnI C-terminal deletion mutant was able to bind to cTnC, as shown by urea-polyacrylamide gel-shift analysis and size exclusion chromatography [4].
• Two of the five cTnI-specific monoclonal antibodies were utilized in an immunoassay [24].
• However, during ISO perfusion, when cTnI was phosphorylated, the rate of relaxation was significantly slower in PLBKO/ssTnI compared to PLBKO/cTnI hearts [25].
• We conclude that cTnI is a powerful candidate in mammals, a possible candidate in birds, but unlikely to be of use in fish as a sensitive and tissue-selective diagnostic test for cardiac injury [26].

References