Striated muscle contraction is regulated by dynamic and cooperative interactions among Ca2+, troponin, and tropomyosin on the thin filament. While Ca2+ regulation has been extensively studied, little is known about the dynamics of individual regulatory units and structural changes of individual tropomyosin molecules in relation to their mechanical properties, and how these factors are altered by cardiomyopathy mutations in the Ca2+ regulatory proteins. In this hypothesis paper, we explore how various experimental and analytical approaches could broaden our understanding of the cooperative regulation of cardiac contraction in health and disease.

α-Tropomyosin (αTm) is the predominant tropomyosin isoform in adult human heart and constitutes a major component in Ca2+-regulated systolic contraction of cardiac muscle. We present here the first direct probe images of WT human cardiac αTm by atomic force microscopy, and quantify its mechanical flexibility with three independent analysis methods. Single molecules of bacterially-expressed human cardiac αTm were imaged on poly-lysine coated mica and their contours were analyzed. Analysis of tangent-angle (θ(s)) correlation along molecular contours, second moment of tangent angles (<θ2(s)>), and end-to-end length (Le-e) distributions respectively yielded values of persistence length (Lp) of 41–46 nm, 40–45 nm, and 42–52 nm, corresponding to 1–1.3 molecular contour lengths (Lc). We also demonstrate that a sufficiently large population, with at least 100 molecules, is required for a reliable Lp measurement of αTm in single molecule studies. Our estimate that Lp for αTm is only slightly longer than Lc is consistent with a previous study showing there is little spread of cooperative activation into near-neighbor regulatory units of cardiac thin filaments. The Lp determined here for human cardiac αTm perhaps represents an evolutionarily tuned optimum between Ca2+ sensitivity and cooperativity in cardiac thin filaments and likely constitutes an essential parameter for normal function in the human heart.

α-tropomyosin (αTm) is central to Ca2+-regulation of cardiac muscle contraction. The familial hypertrophic cardiomyopathy mutation αTm E180G enhances Ca2+-sensitivity in functional assays. To investigate the molecular basis, we imaged single molecules of human cardiac αTm E180G by direct probe atomic force microscopy. Analyses of tangent angles along molecular contours yielded persistence length corresponding to ~35% increase in flexibility compared to wild-type. Increased flexibility of the mutant was confirmed by fitting end-to-end length distributions to the worm-like chain model. This marked increase in flexibility can significantly impact systolic and possibly diastolic phases of cardiac contraction, ultimately leading to hypertrophy.

Actomyosin kinetics in both skinned skeletal muscle fibers at maximum Ca2+-activation and unregulated in vitro motility assays are modulated by solvent microviscosity in a manner consistent with a diffusion limited process. Viscosity might also influence cardiac thin filament Ca2+-regulatory protein dynamics. In vitro motility assays were conducted using thin filaments reconstituted with recombinant human cardiac troponin and tropomyosin; solvent microviscosity was varied by addition of sucrose or glucose. At saturating Ca2+, filament sliding speed (s) was inversely proportional to viscosity. Ca2+-sensitivity (pCa50) of s decreased markedly with elevated viscosity (η/η0 ≥ ~1.3). For comparison with unloaded motility assays, steady-state isometric force (F) and kinetics of isometric tension redevelopment (kTR) were measured in single, permeabilized porcine cardiomyocytes when viscosity surrounding the myofilaments was altered. Maximum Ca2+-activated F changed little for sucrose ≤ 0.3 M (η/η0 ~1.4) or glucose ≤ 0.875 M (η/η0 ~1.66), but decreased at higher concentrations. Sucrose (0.3 M) or glucose (0.875 M) decreased pCa50 for F. kTR at saturating Ca2+ decreased steeply and monotonically with increased viscosity but there was little effect on kTR at sub-maximum Ca2+. Modeling indicates that increased solutes affect dynamics of cardiac muscle Ca2+-regulatory proteins to a much greater extent than actomyosin cross-bridge cycling.

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Indirect flight muscle (IFM) contracts at high frequencies at a priming level of Ca2þ that stays constant during oscillations. The muscles are stretch-activated. Alternating contraction of opposing muscles produces resonant distortions of the thorax, which results in rapid movement of the wings. The TnC isoform, F1, which binds one Ca2þ in the C-lobe, is needed for stretch-activation. The N-lobe of F1 is inactive and does not bind TnH (the TnI of IFM). The C-lobe changes from a closed to open conformation on binding Ca2þ. However, the binding of TnH to this lobe is independent of Ca2þ, and the transition may be necessary for optimum orientation of TnH. The minor TnC isoform, F2, which is needed for the development of isometric force at relatively high [Ca2þ], binds one Ca2þ in each lobe; association with TnH involves both lobes and is Ca2þ-dependent. In Lethocerus IFM, the C-terminus of TnH is close to a crossbridge, and may form part of a ''troponin bridge'' between the thick and thin filament, transmitting strain to the thin filament on stretch. The inhibitory sequence of TnH would be pulled off actin by stretching, rather than by reversibly binding to the N-lobe of TnC in the presence of Ca2þ, as occurs in skeletal muscle. We will describe the effect of the C-terminal half of F1 alone on oscillatory contraction, and the effect of adding the C-terminal region of TnH to compete with endogenous TnH.

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