The Special Structure and Function of Troponin I in Regulation of Cardiac Contraction and Relaxation

Molecular evolution of troponin I and a role of its N-terminal extension in nematode locomotion

The troponin complex, composed of troponin T (TnT), troponin I (TnI), and troponin C (TnC), is the major calcium-dependent regulator of muscle contraction, which is present widely in both vertebrates and invertebrates. Little is known about evolutionary aspects of troponin in the animal kingdom. Using a combination of data mining and functional analysis of TnI, we report evidence that an N-terminal extension of TnI is present in most of bilaterian animals as a functionally important domain. Troponin components have been reported in species in most of representative bilaterian phyla. Comparison of TnI sequences shows that the core domains are conserved in all examined TnIs, and that N- and C-terminal extensions are variable among isoforms and species. In particular, N-terminal extensions are present in all protostome TnIs and chordate cardiac TnIs but lost in a subset of chordate TnIs including vertebrate skeletal-muscle isoforms. Transgenic rescue experiments in Caenorhabditis elegans striated muscle show that the N-terminal extension of TnI (UNC-27) is required for coordinated worm locomotion but not in sarcomere assembly and single muscle-contractility kinetics. These results suggest that N-terminal extensions of TnIs are retained from a TnI ancestor as a functional domain.

Contractility of single cardiomyocytes differentiated from pluripotent stem cells depends on physiological shape and substrate stiffness

Single cardiomyocytes contain myofibrils that harbor the sarcomere-based contractile machinery of the myocardium. Cardiomyocytes differentiated from human pluripotent stem cells (hPSC-CMs) have potential as an in vitro model of heart activity. However, their fetal-like misalignment of myofibrils limits their usefulness for modeling contractile activity. We analyzed the effects of cell shape and substrate stiffness on the shortening and movement of labeled sarcomeres and the translation of sarcomere activity to mechanical output (contractility) in live engineered hPSC-CMs. Single hPSC-CMs were cultured on polyacrylamide substrates of physiological stiffness (10 kPa), and Matrigel micropatterns were used to generate physiological shapes (2,000-µm(2) rectangles with length:width aspect ratios of 5:1-7:1) and a mature alignment of myofibrils. Translation of sarcomere shortening to mechanical output was highest in 7:1 hPSC-CMs. Increased substrate stiffness and applied overstretch induced myofibril defects in 7:1 hPSC-CMs and decreased mechanical output. Inhibitors of nonmuscle myosin activity repressed the assembly of myofibrils, showing that subcellular tension drives the improved contractile activity in these engineered hPSC-CMs. Other factors associated with improved contractility were axially directed calcium flow, systematic mitochondrial distribution, more mature electrophysiology, and evidence of transverse-tubule formation. These findings support the potential of these engineered hPSC-CMs as powerful models for studying myocardial contractility at the cellular level.

New potential uses for cardiac troponins

This review briefly synthesizes the molecular biology of troponin, which is currently the best biochemical marker for the detection of cardiac injury and, thus, acute myocardial infarction as well. Potential new uses for the marker based on these insights, with a specific interest in cardiac troponin fragments that potentially could be linked to distinct clinical conditions, are described. Some of the clinical problems clinicians are faced with including how to use the markers in renal failure and the difficulties associated with the heterogeneity of current troponin assays are also discussed. Finally, we present the possibility of specific cardiac troponin fragments resulting from modification or degradation, associated with distinct pathological processes, as new potential uses for this biomarker.

Removing the regulatory N-terminal domain of cardiac troponin I diminishes incompatibility during bacterial expression

Troponin I (TnI) is a muscle-specific protein and plays an allosteric function in the Ca(2+) regulation of cardiac and skeletal muscle contraction. Expression of cloned cDNA in Escherichia coli is an essential approach to preparing human TnI and mutants for structural and functional studies. The expression level of cardiac TnI in E. coli is very low. To reduce the potential toxicity of cardiac TnI to the host cell, we constructed a bi-cistronic expression vector to co-express cardiac TnI and cardiac/slow troponin C (TnC), a natural binding partner of TnI and a protein that readily expresses in E. coli at high levels. The co-expression moderately increased the expression of cardiac TnI although a high amount of TnC protein was produced from the bi-cistronic mRNA. The use of an E. coli strain containing additional tRNAs for certain low bacterial usage eukaryotic codons improved the expression of cardiac TnI. Modifications of two 5'-regional codons that have predicted low usages in bacterial cells did not reproduce the improvement, indicating that not the 5' but the overall codon usage restricts the translational efficiency of cardiac TnI mRNA in E. coli. However, deletion of the cardiac TnI-specific N-terminal 28 amino acids significantly improved the protein expression independent of the host cell tRNA modifications. The results suggest that the regulatory N-terminal domain of cardiac TnI is a dominant factor for the incompatibility in bacterial cells, supporting its role in modulating the overall molecular conformation.

Specific enhancement of sarcomeric response to Ca2+protects murine myocardium against ischemia-reperfusion dysfunction

Alteration in myofilament response to Ca2+is a major mechanism for depressed cardiac function after ischemia-reperfusion (I/R) dysfunction. We tested the hypothesis that hearts with increased myofilament response to Ca2+are less susceptible to I/R. In one approach, we studied transgenic (TG) mice with a constitutive increase in myofilament Ca2+sensitivity in which the adult form of cardiac troponin I (cTnI) is stoichiometrically replaced with the embryonic/neonatal isoform, slow skeletal TnI (ssTnI). We also studied mouse hearts with EMD-57033, which acts specifically to enhance myofilament response to Ca2+. We subjected isolated, perfused hearts to an I/R protocol consisting of 25 min of no-flow ischemia followed by 30 min of reperfusion. After I/R, developed pressure and rates of pressure change were significantly depressed and end-diastolic pressure was significantly elevated in nontransgenic (NTG) control hearts. These changes were significantly blunted in TG hearts and in NTG hearts perfused with EMD-57033 during reperfusion, with function returning to nearly baseline levels. Ca2+- and cross bridge-dependent activation, protein breakdown, and phosphorylation in detergent-extracted fiber bundles were also investigated. After I/R NTG fiber bundles exhibited a significant depression of cross bridge-dependent activation and Ca2+-activated tension and length dependence of activation that were not evident in TG preparations. Only NTG hearts demonstrated a significant increase in cTnI phosphorylation. Our results support the hypothesis that specific increases in myofilament Ca2+sensitivity are able to diminish the effect of I/R on cardiac function.

Developing the next generation of cardiac markers: Disease-induced modifications of troponin I

Troponin I (TnI) and Troponin T (TnT) have evolved into arguably the two most important diagnostic markers for acute myocardial injury. Part of their diagnostic utility lies in the uniquely important roles that both TnI and TnT play in the calcium-dependent regulation of cardiac muscle contraction. Both proteins undergo extensive physiologic regulation, principally through phosphorylation, as well as specific disease-induced pathologic modifications, including phosphorylation, oxidation, and proteolysis. Many, if not all, of these protein modifications in some way modulate contractility, and when detected in serum may therefore provide important information about both the disease state and functional status of the heart. However, the complexity of the TnI (and TnT) forms in the serum is large, which leads to difficulty in detecting all of the Tn subunits in serum, and hence interpreting the biologic significance of each modified product. But, as diagnostic tools and modalities improve, our ability to monitor and detect specific disease-induced modified forms of proteins will inevitably lead to better and more specific diagnoses and therapies.