Protein kinase A increases the rate of relaxation but not the rate of tension development in skinned rat cardiac muscle

Phosphorylation of cardiac troponin I by mammalian sterile 20-like kinase 1

Mst1 (mammalian sterile 20-like kinase 1) is a ubiquitously expressed serine/threonine kinase and its activation in the heart causes cardiomyocyte apoptosis and dilated cardiomyopathy. Its myocardial substrates, however, remain unknown. In a yeast two-hybrid screen of a human heart cDNA library with a dominant-negative Mst1 (K59R) mutant used as bait, cTn [cardiac Tn (troponin)] I was identified as an Mst1-interacting protein. The interaction of cTnI with Mst1 was confirmed by co-immunoprecipitation in both co-transfected HEK-293 cells (human embryonic kidney cells) and native cardiomyocytes, in which cTnI interacted with full-length Mst1, but not with its N-terminal kinase fragment. in vitro phosphorylation assays demonstrated that cTnI is a sensitive substrate for Mst1. In contrast, cTnT was phosphorylated by Mst1 only when it was incorporated into the Tn complex. MS analysis indicated that Mst1 phosphorylates cTnI at Thr(31), Thr(51), Thr(129) and Thr(143). Substitution of Thr(31) with an alanine residue reduced Mst1-mediated cTnI phosphorylation by 90%, whereas replacement of Thr(51), Thr(129) or Thr(143) with alanine residues reduced Mst1-catalysed cTnI phosphorylation by approx. 60%, suggesting that Thr(31) is a preferential phosphorylation site for Mst1. Furthermore, treatment of cardiomyocytes with hydrogen peroxide rapidly induced Mst1-dependent phosphorylation of cTnI at Thr(31). Protein epitope analysis and binding assays showed that Mst1-mediated phosphorylation modulates the molecular conformation of cTnI and its binding affinity to TnT and TnC, thus indicating functional significances. The results of the present study suggest that Mst1 is a novel mediator of cTnI phosphorylation in the heart and may contribute to the modulation of myofilament function under a variety of physiological and pathophysiological conditions.

Ca(2+) exchange with troponin C and cardiac muscle dynamics

Controversy abounds in the cardiac muscle literature over the rate-limiting steps of cardiac muscle contraction and relaxation. However, the idea of a single biochemical mechanism being the all-inclusive rate-limiting step for cardiac muscle contraction and relaxation may be oversimplified. There is ample evidence that Ca(2+) concentration and dynamics, intrinsic cross-bridge properties, and even troponin C (TnC) Ca(2+) binding and dissociation can all modulate the mechanical events of cardiac muscle contraction and relaxation. However, TnC has generally been thought to play no role in influencing cardiac muscle dynamics due to the idea that Ca(2+) exchange with TnC is very rapid. This definitely is the case for isolated TnC, but not for the more sophisticated biochemical systems of reconstituted thin filaments and myofibrils. This review will discuss the biochemical influences on Ca(2+) exchange with TnC and their physiological implications.

Effects of thin and thick filament proteins on calcium binding and exchange with cardiac troponin C

Understanding the effects of thin and thick filament proteins on the kinetics of Ca(2+) exchange with cardiac troponin C is essential to elucidating the Ca(2+)-dependent mechanisms controlling cardiac muscle contraction and relaxation. Unlike labeling of the endogenous Cys-84, labeling of cardiac troponin C at a novel engineered Cys-53 with 2-(4'-iodoacetamidoanilo)napthalene-6-sulfonic acid allowed us to accurately measure the rate of calcium dissociation from the regulatory domain of troponin C upon incorporation into the troponin complex. Neither tropomyosin nor actin alone affected the Ca(2+) binding properties of the troponin complex. However, addition of actin-tropomyosin to the troponin complex decreased the Ca(2+) sensitivity ( approximately 7.4-fold) and accelerated the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 2.5-fold). Subsequent addition of myosin S1 to the reconstituted thin filaments (actin-tropomyosin-troponin) increased the Ca(2+) sensitivity ( approximately 6.2-fold) and decreased the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 8.1-fold), which was completely reversed by ATP. Consistent with physiological data, replacement of cardiac troponin I with slow skeletal troponin I led to higher Ca(2+) sensitivities and slower Ca(2+) dissociation rates from troponin C in all the systems studied. Thus, both thin and thick filament proteins influence the ability of cardiac troponin C to sense and respond to Ca(2+). These results imply that both cross-bridge kinetics and Ca(2+) dissociation from troponin C work together to modulate the rate of cardiac muscle relaxation.

A quantitative analysis of cardiac myocyte relaxation: a simulation study

The determinants of relaxation in cardiac muscle are poorly understood, yet compromised relaxation accompanies various pathologies and impaired pump function. In this study, we develop a model of active contraction to elucidate the relative importance of the [Ca2+]i transient magnitude, the unbinding of Ca2+ from troponin C (TnC), and the length-dependence of tension and Ca2+ sensitivity on relaxation. Using the framework proposed by one of our researchers, we extensively reviewed experimental literature, to quantitatively characterize the binding of Ca2+ to TnC, the kinetics of tropomyosin, the availability of binding sites, and the kinetics of crossbridge binding after perturbations in sarcomere length. Model parameters were determined from multiple experimental results and modalities (skinned and intact preparations) and model results were validated against data from length step, caged Ca2+, isometric twitches, and the half-time to relaxation with increasing sarcomere length experiments. A factorial analysis found that the [Ca2+]i transient and the unbinding of Ca2+ from TnC were the primary determinants of relaxation, with a fivefold greater effect than that of length-dependent maximum tension and twice the effect of tension-dependent binding of Ca2+ to TnC and length-dependent Ca2+ sensitivity. The affects of the [Ca2+]i transient and the unbinding rate of Ca2+ from TnC were tightly coupled with the effect of increasing either factor, depending on the reference [Ca2+]i transient and unbinding rate.

Expression and purification of human cardiac troponin subunits and their functional incorporation into isolated cardiac mouse myofibrils

The three subunits of the human cardiac troponin complex (hcTnC, hcTnI, hcTnT) were overexpressed in E. coli, purified and reconstituted to form the hcTn complex. This complex was then incorporated into subcellular bundles of mouse cardiac myofibrils whereby the native mcTn complex was replaced. On thus exchanged myofibrils, isometric force kinetics following sudden changes in free Ca(2+) concentration were measured using atomic force cantilevers. Following the exchange, the myofibrillar force remained fully Ca(2+) regulated, i.e. myofibrils were completely relaxed at pCa 7.5 and developed the same maximum Ca(2+)-activated isometric force upon increasing the pCa to 4.5 as unexchanged myofibrils. The replacement of endogenous mcTn by wild-type hcTn neither altered the kinetics of Ca(2+)-induced force development of the mouse myofibrils nor the kinetics of force relaxation induced by the sudden, complete removal of Ca(2+). Preparations of functional Tn reconstituted myofibrils provide a promising model to study the role of Tn in kinetic mechanisms of cardiac myofibrillar contraction and relaxation.