Impact of titin strain on the cardiac slow force response

Mechanobiology in cardiac mechanics

The contraction-relaxation cycle of the heart is one of the most robust mechanical systems in the body that adapts rapidly to the body's needs by changing mechanical parameters. In many respects, we can consider the cardiac system as a complex machine and can use engineering approaches to describe its function. The classical physiology of the heart also focused on understanding function but the new molecular level tools in light microscopy and nanoengineering now enable a deeper understanding of the physiology. The field of mechanobiology has emerged with a focus on how mechanical activity alters biological systems at the molecular level and how those systems in turn control mechanical parameters. In the case of mechanical activity, there are clearly benefits of exercise for the heart, for cancer patients, and for aging but we do not understand the links at a molecular level. Why does regular exercise benefit the heart? We have some preliminary clues at a molecular level about the benefits of physical activity in the cases of cancer and aging; however, there is less known about how exercise affects cardiovascular performance. Unlike the omics approaches which generally link proteins to processes, a mechanobiological understanding of a process explains how forces and mechanical activity will regulate the process through modifications of protein activities. In other words, mechanical activity is an essential component of most biological systems that is transduced into biochemical changes in protein activity. Further, it follows logically that if a mechanical parameter of the cardiac system is typically controlled, then cellular mechanosensing systems must be able to directly or indirectly measure that parameter. The challenge is to understand how changes in activity of the heart are controlled in the short term and then how the system adapts to the integrated level of activity over the longer term. By way of introduction to molecular mechanobiology, I will present examples of mechanosensing from the molecular to the cellular scale and how they may be integrated at the cell and tissue levels. An important element of Mechanobiology at the system level is the physiological state of the cell: i.e., the cell in a senescent state, a cancer state, or a normal cell state (Sheetz 2019). The background for the mechanobiological approach is discussed in "The Cell as a Machine" (Sheetz and Yu, Cambridge Univ Press, 2018), which considers cell states and the molecular systems underlying the important cellular functions. A major challenge in mechanobiology is the understanding of the transduction of mechanical activity into changes in cell function. Of particular relevance here is the benefit of exercise to cardiac performance. This has been seen in many cases and there are a variety of factors that contribute. Further, exercise will benefit cancer patients and will reverse some of the adverse effects of aging. Exercise will cause increased cardiac activity that will be sensed by many mechanosensory systems from a molecular to a cellular level both in the heart and in the vasculature. At a molecular level in cardiac systems, proteins are able to measure stress and strain and to generate appropriate signals of the magnitude of stress and strain that can regulate the cellular contractility and other parameters. The protein sensors are generally passive systems that give a transient measure of local parameters such as the stress at cell-cell junctions during contraction and the strain of the sarcomeres during relaxation. Large stresses at the junctions can activate signaling systems that can reduce contractility or over time activate remodeling of the junctions to better support larger stresses. The proteins involved and their sensory mechanisms are not known currently; however, the mechanosensitive channel, Piezo1, has been implicated in the transduction process in the vasculature (Beech 2018). In the case of strain sensors, large stretches of titin during relaxation can unfold more titin domains that can send signals to the cell. Two different mechanisms of strain sensing are likely in titin. The titin kinase domain is activated by strain but the substrates of the kinase are not know in vivo (Linke 2018). In the backbone of titin are many Ig domains that unfold at different forces and unfolding could cause the binding of proteins that would then activate enzymatic pathways to alter the contractile cycle to give the proper level of strain (Ait-Mou et al. 2017; Granzier et al. 2014; Granzier et al. 2009). The cell-matrix adhesion protein, talin, has eleven cryptic binding sites for another adhesion protein, vinculin, that are revealed by the unfolding of domains in the talin molecule (Yao et al. 2016). Since some domains unfold at lower forces than others, small strains will preferentially unfold those domains, making the system an excellent sensor of the extent of stretch as expected for titin. Because there is an ordered array of many titin molecules, the sensing of strain can be very sensitive to small changes in sarcomere length. Needless to say, titin is only one part of the regulatory system that controls sarcomere length. As one goes more deeply into the working of the system, it is evident that many additional mechanosensory elements are involved in maintaining a functioning cardiac system.

Reduced preload increases Mechanical Control (strain-rate dependence) of Relaxation by modifying myosin kinetics

Rapid myocardial relaxation is essential in maintaining cardiac output, and impaired relaxation is an early indicator of diastolic dysfunction. While the biochemical modifiers of relaxation are well known to include calcium handling, thin filament activation, and myosin kinetics, biophysical and biomechanical modifiers can also alter relaxation. We have previously shown that the relaxation rate is increased by an increasing strain rate, not a reduction in afterload. The slope of the relaxation rate to strain rate relationship defines Mechanical Control of Relaxation (MCR). To investigate MCR further, we performed in vitro experiments and computational modeling of preload-adjustment using intact rat cardiac trabeculae. Trabeculae studies are often performed using isometric (fixed-end) muscles at optimal length (Lo, length producing maximal developed force). We determined that reducing muscle length from Lo increased MCR by 20%, meaning that reducing preload could substantially increase the sensitivity of the relaxation rate to the strain rate. We subsequently used computational modeling to predict mechanisms that might underlie this preload-dependence. Computational modeling was not able to fully replicate experimental data, but suggested that thin-filament properties are not sufficient to explain preload-dependence of MCR because the model required the thin-filament to become more activated at reduced preloads. The models suggested that myosin kinetics may underlie the increase in MCR at reduced preload, an effect that can be enhanced by force-dependence. Relaxation can be modified and enhanced by reduced preload. Computational modeling implicates myosin-based targets for treatment of diastolic dysfunction, but further model refinements are needed to fully replicate experimental data.

Extensive eccentric contractions in intact cardiac trabeculae: revealing compelling differences in contractile behaviour compared to skeletal muscles

Force enhancement (FE) is a phenomenon that is present in skeletal muscle. It is characterized by progressive forces upon active stretching-distinguished by a linear rise in force-and enhanced isometric force following stretching (residual FE (RFE)). In skeletal muscle, non-cross-bridge (XB) structures may account for this behaviour. So far, it is unknown whether differences between non-XB structures within the heart and skeletal muscle result in deviating contractile behaviour during and after eccentric contractions. Thus, we investigated the force response of intact cardiac trabeculae during and after isokinetic eccentric muscle contractions (10% of maximum shortening velocity) with extensive magnitudes of stretch (25% of optimum muscle length). The different contributions of XB and non-XB structures to the total muscle force were revealed by using an actomyosin inhibitor. For cardiac trabeculae, we found that the force-length dynamics during long stretch were similar to the total isometric force-length relation. This indicates that no (R)FE is present in cardiac muscle while stretching the muscle from 0.75 to 1.0 optimum muscle length. This finding is in contrast with the results obtained for skeletal muscle, in which (R)FE is present. Our data support the hypothesis that titin stiffness does not increase with activation in cardiac muscle.

The slow force response to stretch: Controversy and contradictions

When exposed to an abrupt stretch, cardiac muscle exhibits biphasic active force enhancement. The initial, instantaneous, force enhancement is well explained by the Frank-Starling mechanism. However, the cellular mechanisms associated with the second, slower phase remain contentious. This review explores hypotheses regarding this "slow force response" with the intention of clarifying some apparent contradictions in the literature. The review is partitioned into three sections. The first section considers pathways that modify the intracellular calcium handling to address the role of the sarcoplasmic reticulum in the mechanism underlying the slow force response. The second section focuses on extracellular calcium fluxes and explores the identity and contribution of the stretch-activated, non-specific, cation channels as well as signalling cascades associated with G-protein coupled receptors. The final section introduces promising candidates for the mechanosensor(s) responsible for detecting the stretch perturbation.

The lack of slow force response in failing rat myocardium: role of stretch-induced modulation of Ca-TnC kinetics

The slow force response (SFR) to stretch is an important adaptive mechanism of the heart. The SFR may result in ~ 20-30% extra force but it is substantially attenuated in heart failure. We investigated the relation of SFR magnitude with Ca2+ transient decay in healthy (CONT) and monocrotaline-treated rats with heart failure (MCT). Right ventricular trabeculae were stretched from 85 to 95% of optimal length and held stretched for 10 min at 30 °C and 1 Hz. Isometric twitches and Ca2+ transients were collected on 2, 4, 6, 8, 10 min after stretch. The changes in peak tension and Ca2+ transient decay characteristics during SFR were evaluated as a percentage of the value measured immediately after stretch. The amount of Ca2+ utilized by TnC was indirectly evaluated using the methods of Ca2+ transient "bump" and "difference curve." The muscles of CONT rats produced positive SFR and they showed prominent functional relation between SFR magnitude and the magnitude (amplitude, integral intensity) of Ca2+ transient "bump" and "difference curve." The myocardium of MCT rats showed negative SFR to stretch (force decreased in time) which was not correlated well with the characteristics of Ca2+ transient decay, evaluated by the methods of "bump" and "difference curve." We conclude that the intracellular mechanisms of Ca2+ balancing during stretch-induced slow adaptation of myocardial contractility are disrupted in failing rat myocardium. The potential significance of our findings is that the deficiency of slow force response in diseased myocardium may be diminished under augmented kinetics of Ca-TnC interaction.

Abnormal contractility in human heart myofibrils from patients with dilated cardiomyopathy due to mutations in TTN and contractile protein genes

Dilated cardiomyopathy (DCM) is an important cause of heart failure. Single gene mutations in at least 50 genes have been proposed to account for 25–50% of DCM cases and up to 25% of inherited DCM has been attributed to truncating mutations in the sarcomeric structural protein titin (TTNtv). Whilst the primary molecular mechanism of some DCM-associated mutations in the contractile apparatus has been studied in vitro and in transgenic mice, the contractile defect in human heart muscle has not been studied. In this study we isolated cardiac myofibrils from 3 TTNtv mutants, and 3 with contractile protein mutations (TNNI3 K36Q, TNNC1 G159D and MYH7 E1426K) and measured their contractility and passive stiffness in comparison with donor heart muscle as a control. We found that the three contractile protein mutations but not the TTNtv mutations had faster relaxation kinetics. Passive stiffness was reduced about 38% in all the DCM mutant samples. However, there was no change in maximum force or the titin N2BA/N2B isoform ratio and there was no titin haploinsufficiency. The decrease in myofibril passive stiffness was a common feature in all hearts with DCM-associated mutations and may be causative of DCM.