Emerging Concepts of Mechanisms Controlling Cardiac Tension: Focus on Familial Dilated Cardiomyopathy (DCM) and Sarcomere-Directed Therapies

Abstract

Novel therapies for the treatment of familial dilated cardiomyopathy (DCM) are lacking. Shaping research directions to clinical needs is critical. Triggers for the progression of the disorder commonly occur due to specific gene variants that affect the production of sarcomeric/cytoskeletal proteins. Generally, these variants cause a decrease in tension by the myofilaments, resulting in signaling abnormalities within the micro-environment, which over time result in structural and functional maladaptations, leading to heart failure (HF). Current concepts support the hypothesis that the mutant sarcomere proteins induce a causal depression in the tension-time integral (TTI) of linear preparations of cardiac muscle. However, molecular mechanisms underlying tension generation particularly concerning mutant proteins and their impact on sarcomere molecular signaling are currently controversial. Thus, there is a need for clarification as to how mutant proteins affect sarcomere molecular signaling in the etiology and progression of DCM. A main topic in this controversy is the control of the number of tension-generating myosin heads reacting with the thin filament. One line of investigation proposes that this number is determined by changes in the ratio of myosin heads in a sequestered super-relaxed state (SRX) or in a disordered relaxed state (DRX) poised for force generation upon the Ca²⁺ activation of the thin filament. Contrasting evidence from nanometer-micrometer-scale X-ray diffraction in intact trabeculae indicates that the SRX/DRX states may have a lesser role. Instead, the proposal is that myosin heads are in a basal OFF state in relaxation then transfer to an ON state through a mechano-sensing mechanism induced during early thin filament activation and increasing thick filament strain. Recent evidence about the modulation of these mechanisms by protein phosphorylation has also introduced a need for reconsidering the control of tension. We discuss these mechanisms that lead to different ideas related to how tension is disturbed by levels of mutant sarcomere proteins linked to the expression of gene variants in the complex landscape of DCM. Resolving the various mechanisms and incorporating them into a unified concept is crucial for gaining a comprehensive understanding of DCM. This deeper understanding is not only important for diagnosis and treatment strategies with small molecules, but also for understanding the reciprocal signaling processes that occur between cardiac myocytes and their micro-environment. By unraveling these complexities, we can pave the way for improved therapeutic interventions for managing DCM.

Keywords: TTI; myosin control mechanisms; precision medicine; sarcomere activators; sarcomere disease genes.

Figure 1

Near-neighbor C-zone cardiac regulatory units in OFF and ON states. The left regulatory unit is in an end-diastolic state (relaxing levels of cellular Ca²⁺) with low levels of the phosphorylation of key control elements in cTnI, MyBP-C3, and titin. See the text and the legend and diagram in Figure 2 for further discussion. Relaxed myosin heads are folded close to the thick filament in an OFF state with force-generating reactions with actin impeded by Tm. This Tm steric block is by the cTn complex extending along the functional unit with multiple inhibitory interactions involving N- and C-terminal regions of cTnI and cTnT interacting with cTnC, actin, and Tm. Head-to-tail overlap of contiguous Tms interacts with regions of the cTnT N-terminus reducing the possibility of cooperative activation along the sarcomere strand. The low-Ca²⁺ blocked state of the thin filament is tempered by interactions of N-terminal domains of MyBP-C3 with actin–Tm that partially reduce the steric block. This interaction is reduced with elevated Ca²⁺ and abolished with the phosphorylation of MyBP-C3. Not shown is the possibility for direct interactions of MyBP-C3 with cTnI, proposed to affect coordinated effects of the phosphorylation of these proteins. The stretch of titin elastic domains induces resting tension and diastolic pressure; titin C-zone interactions with MyBP-C3 and myosin potentially modulate the OFF state. There are also interactions of MyBP-C3 domains with the regulatory light chain (RLC) of myosin in a phosphorylation-dependent manner. Complex allosteric, steric, and cooperative mechano-sensing mechanisms involving all the major sarcomere proteins activate sarcomere force and shortening. Acting allosterically, elevations in Ca²⁺ binding to cTnC trigger the modification of protein–protein interactions releasing a nearly complete Tm steric block and inducing the reaction of force-generating cross-bridges with the thin filament. According to the ON/OFF theory of tension control, these initial cross-bridge reactions induce a mechanical strain on the thick filament promoting more cross-bridges to engage and develop tension and further activate the thin filament. The activation of a near-neighbor functional unit occurs by a cooperative spread along the strands of sarcomeres aided by effects’ cross-bridge-dependent thin filament activation mechanisms and by Tm-Tm interactions between contiguous sarcomeres. The reactions responsible for the level of the relaxed state and the intensity of the active state are variables controlled by the gain in mechano-sensing by load and length and by neuro-humoral-induced post-translational mechanisms. These homoeostatic mechanisms are exquisitely sensitive to mutations triggering DCM.

Figure 2

Sarcomere protein–protein interactions in the transition between diastole and systole. The forward (green arrows) and reverse (red arrows) flow of reactions according to the ON/OFF theory of tension generation is initially triggered by Ca²⁺ binding to cTnC with the generation of tension by a few cross-bridges promoting mechano-transduction. The increased strain in the thick filament leads to more force-generating cross-bridges and establishing tension during the heartbeat. The modulation of this process is pictured to occur downstream of these interactions that alter the gain in reactions. This variation in gain occurs with alterations in protein–protein interactions in the biochemical environment, changes in sarcomere length, signaling via post-translational mechanisms, and the expression of natural and DCM-linked isoforms. Small molecules that activate tension are also thought to modify this gain. See text and Figure 1 for further discussion and information.