Why we need to understand the mechanics of developing cardiac sarcomeres in humans

In an article in this issue of The Journal of Physiology, Racca et al. (2016) report the first data on the mechanical/kinetic properties and response to Ca2+ of myofibril size preparations from left ventricle of human fetal hearts from 8 to 19 weeks of gestation. Taking on this project was a formidable task in view of the difficulty of sample acquisition, sample size and maintenance of function. With the possibility of high resolution echo-cardiographic assessment of regional mechanics of the developing human fetal heart by tissue Doppler myocardial velocity measurements (Pauliks, 2013), one may wonder how studies of myofibril mechanics will advance understanding. In addressing this question here, I focus on cardiac sarcomeres in human fetal development, which is the emphasis of the study by Racca et al. As with many advances in science, methodological breakthroughs are fundamental to making progress, and this is certainly the case here. The ability to rapidly obtain and to perform mechanical studies of myofibril bundles, in this case 6–7 μm in width, offers the possibility of overcoming the small sample size available from fetal human hearts. Since the initial development of this method in skeletal muscle where myofibrils are well defined and relatively stable, the approach has been continually refined and applied to the heart to produce high fidelity determinations of specific tension development. Moreover the rates of activation and relaxation of the myofibrils can be obtained by rapid solution changes delivering Ca2+ to and from the myofibrils with minimal diffusion barriers. In some cases Racca et al. were also able to quantify protein isoform expression of abundant proteins, and to relate their mechanical data to quantification of protein isoform population and to ultrastructural organization of the myocytes and sarcomeres. The measurements provide insights into the integrative biology of myofibrils operating in a phase of relatively rapid growth and remodelling. The data emphasize the significance of changes in structural order as well as abundance of the motor protein myosin and the regulatory proteins determining the myofibrillar response to Ca2+. Speculation on a role for Z-disk integrity offers a relatively new and poorly appreciated concept. Moreover, the data indicate that control of kinetics of myofibril activation and relaxation may occur with a dominant role of thin filament proteins and independently of shifts in myosin isoform population. The data on development are also relevant to emerging knowledge regarding the role of sarcomeric structure and function in signals controlling excitation–contraction coupling, myocyte growth and metabolism (Solaro et al. 2013). Apart from the contribution of these data to the advancement of understanding of the process of development in human myocardium in relation to sarcomere mechanics and dynamics, the data are relevant to an emerging arena of research involving human cardiac muscle cells derived from inducible pluripotent stem cells (hiPSC-CMs). Methods exist to determine cellular mechanics, electrophysiology, and Ca2+ transients in these cells (Sun et al. 2012). An excellent example of this approach is the use of patient-specific hiPSC-CMs from controls and from carriers of mutations in sarcomeric proteins linked to common cardiomyopathies (Sun et al. 2012). These mutations lead to sudden death in children and young adults, and non-invasive therapies are lacking. However, excitement regarding the study and utility of hiPSC-CMs has been tempered by the realization that the myocytes look and behave like immature cardiac myocytes. Thus, the challenge is to develop conditions promoting development of the hiPSC-CMs in culture to an adult phenotype, and to know what stages of cardiac development the immature hiPSC-CMs represent. Development of better culture methods is sure to come, and one has to be optimistic that the practical use of hiPSC-CMs will improve. But it appears that for some time high-throughput investigation of the cells, e.g. in their use as a platform for drug discovery, is likely to involve immature cells. In the case of the mutant forms of sarcomeric proteins, it will be of interest to know the effect of the mutation on the development and mechanics of the ihPSC-CMs in culture as a model of the processes occurring in utero. The data reported by Racca et al. provide a glimpse into this process by providing the possibility of correlating mechanical behaviour of myofibrillar preparations from hiPSC-CMs to behaviour and level of maturation of fetal myofibrils. These correlations should aid in the interpretation of the data, e.g. on the effect of a mutation on mechanical function and response to therapeutic agents found to be effective in cardiomyopathic models (Utter et al. 2015). As pointed out by the authors, more work needs to be carried out in general on the left ventricle and other cardiac chambers. The challenges are great, but the rewards are worth the effort.