Troponin I - a comprehensive review of its function, structure, evolution, and role in muscle diseases

Abstract

The troponin complex is a critical component of thin filaments and plays an essential role in the calcium-mediated regulation of contraction and relaxation in striated muscles, including both cardiac and skeletal muscle. Troponin I, a subunit of this complex, inhibits actomyosin interactions during muscle relaxation. Its function is finely tuned by posttranslational modifications, particularly phosphorylation, which influence calcium sensitivity and actin affinity, thus impacting muscle contraction. Mutations in troponin I are closely associated with various human diseases. Specifically, several mutations in cardiac troponin I have been linked to cardiomyopathies, such as hypertrophic, dilated, and restrictive cardiomyopathies, which affect heart contractility and calcium handling. In this review, we explore the multifaceted aspects of troponin I, including its structure, functional role in muscle contraction, evolution, and the complex interactions between posttranslational modifications and genetic mutations that alter its function and contribute to disease progression.

Keywords: Troponin I; cardiomyopathy; evolution of troponin I; genetic mutations in troponin I; post-transcriptional modification of troponin I.

Figure 1.

Thin filament and cTnI EM structure. A. Overlay of the relaxed (top, Ca²⁺ Unbound) and excited (bottom, Ca²⁺ Bound) thin filament states, integrating the thin-filament coordinates (PDB: 6KN7/6KN8) (Yamada et al. 2020) with the myosin head (PDB: 8EFI) (Doran et al. 2023). In the red squares, we calculated the number of atomic contacts between the myosin head and tropomyosin, represented by red pseudobonds. Steric clashes between tropomyosin and myosin head are quantified as atom-atom contacts less than 0.6Å (relaxed = 467, active = 163). In the relaxed state (PDB: 6KN7), in which Ca²⁺ is unbound, the tropomyosin (magenta) blocks the myosin-binding sites on the actin filaments (light-blue), preventing interaction between myosin heads (purple) and actin filaments. In this conditions, the ‘matchmaker’ between myosin head and thin filament yields 467 atom-atom contacts between the myosin head and tropomyosin. In the Ca²⁺ Bound excited state (PDB: 6KN8), calcium ions bind to troponin C, resulting in a conformational shift of the troponin complex. This shift is reflected by a reduced number of 163 atom-atom contacts between the myosin head and tropomyosin, pulling tropomyosin away from the myosin-binding sites on actin and allowing the myosin heads to bind to these exposed sites. It is notable that the contact count does not reach zero because the myosin head and thin-filament structures are derived from independently obtained data sets. As a result of the shift, the myosin heads can now bind to these exposed sites on actin, initiating contraction through their power stroke. B. Structural representation of human cardiac troponin I (cTnI) (PDB: 6KN7) (Yamada et al. 2020) highlighting its domain-specific interactions. The cardiac-specific N-terminus (residues 1-30) is unique to cTnI and influences calcium sensitivity through phosphorylation. Helix 1 (residues 42-80) and Helix 2 (residues 89-141) interact with Troponin T(yellow). Helix 3 (residues 149-154) and Helix 4 (residues 156-184) form part of the switch and mobile regions, respectively, interacting with troponin C (green). The mobile region (residues 163-210) also interacts with tropomyosin (magenta). Tropomyosin (Tpm); Troponin T (TnT); Troponin C (TnC).

Figure 2.

Sequence alignment of troponin I proteins and reported missense and nonsense mutation in ClinVar. A. Sequence alignment of troponin I (TNNI) proteins reveals conserved domains and features across the three isoforms, with the exception of the TNNI3-specific N-terminus. In the alignment, α-helices (3₁₀-helices) and π-helices are represented as squiggles, while the η symbol denotes a 3₁₀-helix. Strict β-turns are marked as ‘TT.’ (https://espript.ibcp.fr) (Robert and Gouet 2014). B. Various mutations including missense and nonsense mutations are reported in ClinVar. The upper and lower panels are schematic representation of missense and nonsense mutations with clinical classification. Each horizontal bar indicates the amino acid sequence of troponin I(TNNI1: 1–187; TNNI2: 1–182; TNNI3: 1–210). The colored and gray boxes indicate individual variants, color-coded by clinical classification (red, pathogenic; orange, likely pathogenic; green, likely benign; blue, benign; and small gray boxes, uncertain significance/conflicting classifications). In lower panel, mutations of TNNI3 are plotted to reported diseases – restrictive cardiomyopathy (RCM), dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), and other pathogenic phenotype. Clustering of pathogenic and likely pathogenic variants within C-terminus, which is ‘mobile regions’ highlights the importance of these domains in pathogenesis and function.

Figure 3.

Hypothetical evolutionary pathways of troponin I and troponin T genes. Based on loci and sequence similarities, the evolution of human troponin I (TNNI) and troponin T (TNNT) genes appears to have been shaped by whole-genome duplications (WGDs) and multiple gene duplication events. An ancestral TNNI gene, which gave rise to the TNNI and TNNT gene families, is hypothesized to have first diverged into TNNI2-like and TNNT3-like lineages. Subsequent evolutionary events further differentiated these genes into TNNI1-like, TNNT2-like, and other specific subtypes, ultimately leading to the diversification of the modern TNNI and TNNT gene families.