Interaction between myosin heavy chain and troponin isoforms modulate cardiac myofiber contractile dynamics

Abstract

Chandra M, Tschirgi ML, Ford SJ, Slinker BK, Campbell KB. Interaction between myosin heavy chain and troponin isoforms modulate cardiac myofiber contractile dynamics. Am J Physiol Regul Integr Comp Physiol 293: R1595-R1607, 2007. First published July 11, 2007; doi:10.1152/ajpregu.00157.2007.-Coordinated expression of species-specific myosin heavy chain (MHC) and troponin (Tn) isoforms may bring about a dynamic complementarity to match muscle contraction speed with species-specific heart rates. Contractile system function and dynamic force-length measurements were made in muscle fibers from mouse and rat hearts and in muscle fibers after reconstitution with either recombinant homologous Tn or orthologous Tn. The rate constants of length-mediated cross-bridge (XB) recruitment (b) and tension redevelopment (k tr) of mouse fibers were significantly faster than those of rat fibers. Both the tension cost (ATPase/tension) and rate constant of length-mediated XB distortion (c) were higher in the mouse than in the rat. Thus the mouse fiber was faster in all dynamic and functional aspects than the rat fiber. Mouse Tn significantly increased b and k tr in rat fibers; conversely, rat Tn significantly decreased b and k tr in mouse fibers. Thus the lengthmediated recruitment of force-bearing XB occurs much more rapidly in the presence of mouse Tn than in the presence of rat Tn, demonstrating that the speed of XB recruitment is regulated by Tn. There was a significant interaction between Tn and MHC such that changes in either Tn or MHC affected the speed of XB recruitment. Our data demonstrate that the dynamics of myocardial contraction are different in the mouse and rat hearts because of sequence heterogeneity in MHC and Tn. At the myofilament level, coordinated expression of complementary regulatory contractile proteins produces a functional dynamic phenotype that allows the cardiovascular systems to function effectively at different heart rates. myofiber dynamics; contraction speed; heart rate THERE IS SUBSTANTIAL PROTEIN sequence heterogeneity among orthologous cardiac myosin heavy chain (MHC) and troponin (Tn) isoforms across different animal species (30). This sequence heterogeneity in regulatory contractile proteins significantly affects myofilament dynamics, as assessed by the force response to muscle length change in constantly activated cardiac myofibers, which exhibits two clearly separable processes (3,5,20,30,35): 1) a relatively fast force dynamic associated with myosin cross-bridge (XB) distortion and 2) a relatively slow force dynamic associated with recruitment of additional XB into force-bearing states. The dynamics of XB distortion are principally determined by the enzymatic kinetics of MHC, and the dynamics of XB recruitment are affected greatly by cooperative interactions between Tn actions and XB cycling kinetics (3, 5, 6, 30).Our group (9) recently showed that differences in troponin T (TnT), a subunit of the Tn regulatory protein complex, affected the slow XB recruitment dynamic (9), whereas a shift f...

Muscular tissues of the squid Doryteuthis pealeii express identical myosin heavy chain isoforms: an alternative mechanism for tuning contractile speed

“…Variations in troponin isoforms may also influence contractile dynamics. Reconstitution of mouse and rat cardiac fibres with the other species' recombinant troponin complexes resulted in differences in contractile kinetics (Chandra et al, 2007). It is unknown whether differences in light chain and troponin isoforms exist in cephalopod muscle.…”

The functional effect of dilated cardiomyopathy mutation (R144W) in mouse cardiac troponin T is differently affected by α- and β-myosin heavy chain isoforms

“…Left ventricular papillary muscle fibers from mouse hearts were isolated and detergent skinned, as described previously (4,5). Briefly, mice were deeply anesthetized using isoflurane, and their hearts were quickly excised and placed into an ice-cold high-relaxing (HR) solution of pCa 9.0 (pH of 7.0).…”

“…Recombinant Tn subunits were reconstituted into skinned fibers, as described previously (4,5,13). Briefly, excess amounts of TnT (1.5 mg/ml, wt/vol) and TnI (1.0 mg/ml) were dissolved in a buffer containing 50 mM Tris·HCl (pH 8.0), 6 M urea, 1.0 M KCl, and 1 mM DTT.…”

“…The ability of a given MHC isoform to alter the TnT-mediated function is related to its innate kinetic properties. Due to the longer XB dwell time, a slower cycling ␤-MHC isoform (10,41,43) may modify thin filament-based cooperative mechanisms and dynamic contractile behavior (4,10,29,30,43). Because the heart muscle comprises dynamically interacting components, it is expected that the mutant TnT-and the MHC-induced changes in the thin filament will interact to yield properties that are abnormal.…”

Updating the Physiology and Pathophysiology of Cardiac Myosin-Binding Protein-C

“…If reduced MyBP-C phosphorylation plays a significant role in impaired inotropic responses and slowed/incomplete relaxation in human DCM and HFpEF, it will be important to quantify sitespecific phosphorylation in larger numbers of patients and attempt to establish cause and effect. Currently, mass spectroscopic methods offer the most precise quantitative estimates of site-specific phosphorylation, 46 whereas myofilament/ myofibril protein reconstitution methods using genetically modified proteins 47 can provide in vitro mechanistic insights in human myocardium analagous to those in transgenic animals. Because TnI and other myofilament proteins in addition to MyBP-C are targets for PKA and various other kinases, changes in myofilament function in acquired human disease are undoubtedly exceedingly complex and, a priori, unlikely to be dominated by phosphorylation of a single protein.…”