The contraction of skeletal and cardiac muscle (striated muscle) is enabled when calcium ions bind to troponin, which causes a conformational change and pulls the tropomyosin off the myosin-binding sites on the actin filaments. The uncovering of the binding sites allows the myosin heads to bind the actin, forming a cross-bridge. Once ATP hydrolysis occurs, the power stroke needed for a muscle contraction pulls the actin and myosin filaments closer to the M line, shortening the sarcomere. Troponin is a trimeric complex of three proteins (I, T, and C), each with a different function that allows troponin to perform its role relating to muscle contraction.
Function
Each of the protein subunits has an individualized function related to troponin’s role in muscle contraction. Troponin I (TnI) binds to the actin filament, inhibiting the ATPase activity from the actin-myosin binding.[1] Troponin T (TnT) attaches to tropomyosin, anchoring it to the actin and forming the Tn-tropomyosin complex.[1] Troponin C (TnC) binds to calcium ions, inducing the conformational changes in TnI and uncovering the myosin-binding sites blocked by the tropomyosin.[1] Through this process, cross-bridge cycling occurs so that a power stroke can activate the muscle contraction.
Coinciding with different types of muscle tissue in the body, the troponin subunits have various isoforms. TnI has three different isoforms: cardiac, slow skeletal, and fast skeletal muscle.[2] For the most part, each isoform is found exclusively in its respective muscle tissue (with one exception). During embryonic development, the slow skeletal muscle TnI isoform is expressed in the heart; however, following birth, that isoform is replaced by cardiac TnI.[2] Within the heart, the troponin complex controls cardiac output through its involuntary regulation of muscle contraction. Specifically, the diastolic relaxation and systolic contraction in the myocardium of the heart are controlled by the cardiac troponin complex and the interaction with Ca2+, which modulates the cardiac stroke volume.[3] When the heart increases the end-diastolic volume, the stroke volume also increases, meaning that more blood is ejected from the heart with every contraction. The increase in stroke volume is done by following the Frank-Starling law, which states that an increase in sarcomere length enhances the contractile force of the myocyte.[3]
Disease
Cardiovascular disease (CVD) is one of the leading causes of death globally, with myocardial infarctions (MI) being one of the most life-threatening events.[1] With CVD, plaque builds up in the arteries (atherosclerosis), thinning or even completely blocking them. When the arteries of the heart become blocked entirely, a MI results, which can lead to permanent cardiac tissue damage. Once cardiac muscle tissue (myocardium) dies, the body replaces it with scar tissues that do not have the same properties or functioning as the myocardium. Early diagnosis of a MI is critical to the patient's prognosis and the ability to save as much of the myocardium as possible.4 During a MI, cTnT and cTnI are released into the bloodstream, making them a biomarker for a recent or ongoing heart attack. Electrocardiography (ECG) is usually the diagnostic tool used for MI’s; however, often, the results can be inconclusive even when the patient is symptomatic, with more than 40% of MI cases showing a normal ECG when admitted to the emergency room.1 Over the past couple of decades, point-of-care diagnostic testing and assay development for MI’s has shifted to focus on using cardiac troponin I and T as biomarkers.
Most troponin is found in the body as the trimeric complex bound to tropomyosin and actin; however, there is a small portion (<2-8%) of unbound troponin subunits that reside in the cytoplasm of cardiac muscle.5 Upon damage to muscle tissue, the free troponin subunits are released into the bloodstream. Typical concentrations of cardiac troponin I (cTnI) in the blood are low (≤1-2 ng/mL) but can reach as high as 500 ng/mL after the onset of a MI.[1] Injury to noncardiac tissue does not demonstrate an increase of cTnI. In contrast, some cTnT assays detect proteins in the blood following skeletal muscle damage.6
Myocardial injury is indicated by cardiac troponin levels exceeding the 99th percentile upper reference limit (URL) for the assay.6 The upper reference limit varies between assays and is determined by the manufacturer of the assay due to the lack of international standardization in assay development. The injury is considered acute when there is a significant change (either rise or fall) between two serial measurements.6 Myocardial ischemia occurs when blood flow to the heart muscle becomes obstructed, either partially or entirely, by plaque buildup in a coronary artery. Prolonged or untreated myocardial ischemia can lead to a myocardial infarction, also known as a heart attack, where the heart tissue dies due to a lack of blood supply. The diagnosis of a MI is made when there is myocardial injury present with evidence of myocardial ischemia, which can be determined through patient history, ECGs, and/or symptom analysis.6 In regards to any type of myocardial injury, not just MI’s, cardiac troponin levels prove to be the most useful diagnostic tool.
Relevance
Aside from the medical importance of cTnI as a biomarker of myocardial infarction, cTnI levels have further applications in the clinical setting. cTnI levels serve as a biomarker for other health conditions as well. Abnormal cTnI levels are observed in most forms of cardiac injury or dysfunction: including heart failure, myocarditis, pericarditis, pulmonary embolism, stroke, severe renal dysfunction, and septic shock.5 Fluctuations in the cTnI and cTnT levels also occur with chronic kidney disease.[3] Identifying abnormal cTnI levels and their cause in patients aids medical professionals in providing rapid, quality care. Circulating cTnI levels can triage low-risk patients and preserve vital medical resources by providing more timely care, avoiding unnecessary care and hospitalization, and minimizing hospital stays.[3]
Within the critical care setting, patients’ cardiac troponin levels have the ability to provide prognostic information. Studies repeatedly linked elevated cTns with increased mortality, morbidity, and longer hospital.5 In a 2005 study, elevated cTnI levels found in critically ill patients in the ICU were associated with mortality.5 Similar results were also observed regarding septic patients with raised troponin levels being linked with increased mortality rates.5 The prognostic value of troponin levels should not be overlooked in the critical care setting when assessing the risk of mortality even in non-cardiac critically ill patients.
Structural highlights
Just as the other subunits of the trimeric complex, cTnI has a unique structure specific to its function. This variation between isoforms is what has allowed the development of highly specific detection assays. The overall structure of cTnI can be identified by a 210-residue long protein with a molecular weight of 24 kDa.1 The protein contains four α helices intercalated by flexible disordered regions.[2] The structure domains include the cardiac-exclusive N terminal domain (NcTnI), the structurally rigid IT arm, the inhibitory-peptide, the switch-peptide, and the C-terminal domain.[2]
The cardiac isoform of TnI can be distinguished from the skeletal isoforms (both fast and slow) by the presence of an additional thirty-one N-terminal amino acids.[3] The NcTnI terminal domain contains two protein kinase A (PKA)-dependent phosphorylation sites.7 The targets of PKA-mediated phosphorylation are two adjacent Serine residues (Ser 22 and Ser 23) within the N-terminus.[2] Phosphorylation of these residues stabilizes the C-terminal α–helix through the electrostatic interactions between the phosphorylated serine residues and the neighboring basic residues.7 The structurally rigid IT arm serves a more significant structural function than regulatory, anchoring the trimeric troponin complex to the thin actin filament.7 Another structural difference between the cardiac and skeletal isoform lies in the location of the regulatory head, with a smaller angle being formed between the IT arm and sTn isoform than the cTn isoform.[2] The inhibitory-peptide region, as the name eludes, is a crucial region in the inhibitory role of cTnI. The region strongly interacts with the actin filament in the absence of Ca2+ and stabilizes the tropomyosin on the myosin-binding site, preventing muscle contraction.7 Adjacent to the inhibitory-peptide region, the switch-peptide region has a crucial role in inducing muscle contraction. Once cTnC binds to Ca2+, the switch-peptide region is required to stabilize the N-terminus of cTnC in the “open” conformation by binding to the hydrophobic patch within the terminus, leading to the detachment from the actin filament.7 The C-terminal region is also known as the mobile region and acts as a second actin-tropomyosin binding site.7 Of all the TnI regions, the C-terminal region is considered the most conserved among different species and isoforms.7
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