Structural dynamics of troponin I during Ca2+-activation of cardiac thin filaments: a multi-site Förster resonance energy transfer study

C-Terminal Basic Region of Troponin T Alters the Ca2+-Dependent Changes in Troponin I Interactions

The C-terminal 14-16 residues of human troponin T are required for full inactivation, and they prevent full activation at saturating Ca²⁺. Basic residues within that C-terminal region of TnT are essential for its function, but the mechanism of action is unknown. That region of TnT is natively disordered and does not appear in reconstructions of the troponin structure. We used Förster resonance energy transfer to determine if the C-terminal basic region of TnT alters transitions of TnI or if it operates independently. We also examined Ca²⁺-dependent changes in the C-terminal region of TnT itself. Probes on TnI-143 (inhibitory region) and TnI-159 (switch region) moved away from sites on actin and tropomyosin and toward TnC-84 at high Ca²⁺. Ca²⁺ also displaced C-terminal TnT from actin-tropomyosin but without movement toward TnC. Deletion of C-terminal TnT produced changes in TnI-143 like those effected by Ca²⁺, but effects on TnI-159 were muted; there was no effect on the distance of the switch region to TnC-84. Substituting Ala for basic residues within C-terminal TnT displaced C-terminal TnT from actin-tropomyosin. The results suggest that C-terminal TnT stabilizes tropomyosin in the inactive position on actin. Removal of basic residues from C-terminal TnT produced a Ca²⁺-like state except that the switch region of TnI was not bound to TnC. Addition of Ca²⁺ caused more extreme displacement from actin-tropomyosin as the active state became more fully occupied as in the case of wild-type TnT in the presence of both Ca²⁺ and bound rigor myosin S1.

Order-Disorder Transitions in the Cardiac Troponin Complex

The troponin complex is a molecular switch that ties shifting intracellular calcium concentration to association and dissociation of actin and myosin, effectively allowing excitation-contraction coupling in striated muscle. Although there is a long history of muscle biophysics and structural biology, many of the mechanistic details that enable troponin's function remain incompletely understood. This review summarizes the current structural understanding of the troponin complex on the muscle thin filament, focusing on conformational changes in flexible regions of the troponin I subunit. In particular, we focus on order-disorder transitions in the C-terminal domain of troponin I, which have important implications in cardiac disease and could also have potential as a model system for the study of coupled binding and folding.

The functional significance of the last 5 residues of the C-terminus of cardiac troponin I

The C-terminal region of cardiac troponin I (cTnI) is known to be important in cardiac function, as removal of the last 17 C-terminal residues of human cTnI has been associated with myocardial stunning. To investigate the C-terminal region of cTnI, three C-terminal deletion mutations in human cTnI were generated: Δ1 (deletion of residue 210), Δ3 (deletion of residues 208-210), and Δ5 (deletion of residues 206-210). Mammalian two-hybrid studies showed that the interactions between cTnI mutants and cardiac troponin C (cTnC) or cardiac troponin T (cTnT) were impaired in Δ3 and Δ5 mutants when compared to wild-type cTnI. Troponin complexes containing 2-[4'-(iodoacetamido) anilino] naphthalene-6-sulfonic acid (IAANS) labeled cTnC showed that the troponin complex containing cTnI Δ5 had a small increase in Ca(2+) affinity (P < 0.05); while the cTnI Δ1- and Δ3 troponin complexes showed no difference in Ca(2+) affinity when compared to wild-type troponin. In vitro motility assays showed that all truncation mutants had increased Ca(2+) dependent motility relative to wild-type cTnI. These results suggest that the last 5 C-terminal residues of cTnI influence the binding of cTnI with cTnC and cTnT and affect the Ca(2+) dependence of filament sliding, and demonstrate the importance of this region of cTnI.

High-contrast grating resonators for label-free detection of disease biomarkers

A label-free optical biosensor is described that employs a silicon-based high-contrast grating (HCG) resonator with a spectral linewidth of ~500 pm that is sensitive to ligand-induced changes in surface properties. The device is used to generate thermodynamic and kinetic data on surface-attached antibodies with their respective antigens. The device can detect serum cardiac troponin I, a biomarker of cardiac disease to 100 pg/ml within 4 mins, which is faster, and as sensitive as current enzyme-linked immuno-assays for cTnI.

Identification of rare variants in TNNI3 with atrial fibrillation in a Chinese GeneID population

Despite advances by genome-wide association studies (GWAS), much of heritability of common human diseases remains missing, a phenomenon referred to as 'missing heritability'. One potential cause for 'missing heritability' is the rare susceptibility variants overlooked by GWAS. Atrial fibrillation (AF) is the most common arrhythmia seen at hospitals and increases risk of stroke by fivefold and doubles risk of heart failure and sudden death. Here, we studied one large Chinese family with AF and hypertrophic cardiomyopathy (HCM). Whole-exome sequencing analysis identified a mutation in TNNI3, R186Q, that co-segregated with the disease in the family, but did not exist in >1583 controls, suggesting that R186Q causes AF and HCM. High-resolution melting curve analysis and direct DNA sequence analysis were then used to screen mutations in all exons and exon-intron boundaries of TNNI3 in a panel of 1127 unrelated AF patients and 1583 non-AF subjects. Four novel missense variants were identified in TNNI3, including E64G, M154L, E187G and D196G in four independent AF patients, but no variant was found in 1583 non-AF subjects. All variants were not found in public databases, including the ExAC Browser database with 60,706 exomes. These data suggest that rare TNNI3 variants are associated with AF (P = 0.03). TNNI3 encodes troponin I, a key regulator of the contraction-relaxation function of cardiac muscle and was not previously implicated in AF. Thus, this study may identify a new biological pathway for the pathogenesis of AF and provides evidence to support the rare variant hypothesis for missing heritability.

TNNI1, TNNI2 and TNNI3: Evolution, regulation, and protein structure-function relationships

Troponin I (TnI) is the inhibitory subunit of the troponin complex in the sarcomeric thin filament of striated muscle and plays a central role in the calcium regulation of contraction and relaxation. Vertebrate TnI has evolved into three isoforms encoded by three homologous genes: TNNI1 for slow skeletal muscle TnI, TNNI2 for fast skeletal muscle TnI and TNNI3 for cardiac TnI, which are expressed under muscle type-specific and developmental regulations. To summarize the current knowledge on the TnI isoform genes and products, this review focuses on the evolution, gene regulation, posttranslational modifications, and structure-function relationship of TnI isoform proteins. Their physiological and medical significances are also discussed.

In situ time-resolved FRET reveals effects of sarcomere length on cardiac thin-filament activation

During cardiac thin-filament activation, the N-domain of cardiac troponin C (N-cTnC) binds to Ca(2+) and interacts with the actomyosin inhibitory troponin I (cTnI). The interaction between N-cTnC and cTnI stabilizes the Ca(2+)-induced opening of N-cTnC and is presumed to also destabilize cTnI-actin interactions that work together with steric effects of tropomyosin to inhibit force generation. Recently, our in situ steady-state FRET measurements based on N-cTnC opening suggested that at long sarcomere length, strongly bound cross-bridges indirectly stabilize this Ca(2+)-sensitizing N-cTnC-cTnI interaction through structural effects on tropomyosin and cTnI. However, the method previously used was unable to determine whether N-cTnC opening depends on sarcomere length. In this study, we used time-resolved FRET to monitor the effects of cross-bridge state and sarcomere length on the Ca(2+)-dependent conformational behavior of N-cTnC in skinned cardiac muscle fibers. FRET donor (AEDANS) and acceptor (DDPM)-labeled double-cysteine mutant cTnC(T13C/N51C)AEDANS-DDPM was incorporated into skinned muscle fibers to monitor N-cTnC opening. To study the structural effects of sarcomere length on N-cTnC, we monitored N-cTnC opening at relaxing and saturating levels of Ca(2+) and 1.80 and 2.2-μm sarcomere length. Mg(2+)-ADP and orthovanadate were used to examine the structural effects of noncycling strong-binding and weak-binding cross-bridges, respectively. We found that the stabilizing effect of strongly bound cross-bridges on N-cTnC opening (which we interpret as transmitted through related changes in cTnI and tropomyosin) become diminished by decreases in sarcomere length. Additionally, orthovanadate blunted the effect of sarcomere length on N-cTnC conformational behavior such that weak-binding cross-bridges had no effect on N-cTnC opening at any tested [Ca(2+)] or sarcomere length. Based on our findings, we conclude that the observed sarcomere length-dependent positive feedback regulation is a key determinant in the length-dependent Ca(2+) sensitivity of myofilament activation and consequently the mechanism underlying the Frank-Starling law of the heart.

Citrullination of myofilament proteins in heart failure

AIMS

Citrullination, the post-translational conversion of arginine to citrulline by the enzyme family of peptidylarginine deiminases (PADs), is associated with several diseases, and specific citrullinated proteins have been shown to alter function while others act as auto-antigens. In this study, we identified citrullinated proteins in human myocardial samples, from healthy and heart failure patients, and determined several potential functional consequences. Further we investigated PAD isoform cell-specific expression in the heart.

METHODS AND RESULTS

A citrullination-targeted proteomic strategy using data-independent (SWATH) acquisition method was used to identify the modified cardiac proteins. Citrullinated-induced sarcomeric proteins were validated using two-dimensional gel electrophoresis and investigated using biochemical and functional assays. Myocardial PAD isoforms were confirmed by RT-PCR with PAD2 being the major isoform in myocytes. In total, 304 citrullinated sites were identified that map to 145 proteins among the three study groups: normal, ischaemia, and dilated cardiomyopathy. Citrullination of myosin (using HMM fragment) decreased its intrinsic ATPase activity and inhibited the acto-HMM-ATPase activity. Citrullinated TM resulted in stronger F-actin binding and inhibited the acto-HMM-ATPase activity. Citrullinated TnI did not alter the binding to F-actin or acto-HMM-ATPase activity. Overall, citrullination of sarcomeric proteins caused a decrease in Ca(2+) sensitivity in skinned cardiomyocytes, with no change in maximal calcium-activated force or hill coefficient.

CONCLUSION

Citrullination unique to the cardiac proteome was identified. Our data indicate important structural and functional alterations to the cardiac sarcomere and the contribution of protein citrullination to this process.