Sarcomere Length Nonuniformity and Force Regulation in Myofibrils and Sarcomeres

The smallest contractile unit in striated muscles is the sarcomere. Although some of the classic features of contraction assume a uniform behavior of sarcomeres within myofibrils, the occurrence of sarcomere length nonuniformities has been well recognized for years, but it is yet not well understood. In the past years, there has been a great advance in experiments using isolated myofibrils and sarcomeres that has allowed scientists to directly evaluate sarcomere length nonuniformity. This review will focus on studies conducted with these preparations to develop the hypotheses that 1) force production in myofibrils is largely altered and regulated by intersarcomere dynamics and that 2) the mechanical work of one sarcomere in a myofibril is transmitted to other sarcomeres in series. We evaluated studies looking into myofibril activation, relaxation, and force changes produced during activation. We conclude that force production in myofibrils is largely regulated by intersarcomere dynamics, which arises from the cooperative work of the contractile and elastic elements within a myofibril.

Sarcomere length non-uniformities dictate force production along the descending limb of the force-length relation

The force-length relation is one of the most defining features of muscle contraction, and yet a topic of debate in the literature. The sliding filament theory predicts that the force produced by muscle fibres is proportional to the degree of overlap between myosin and actin filaments, producing a linear descending limb of the active force-length relation. However, several studies have shown forces that are larger than predicted, especially at long sarcomere lengths (SLs). Studies have been conducted with muscle fibres, preparations containing thousands of sarcomeres that make measurements of individual SL challenging. The aim of this study was to evaluate force production and sarcomere dynamics in isolated myofibrils and single sarcomeres from the rabbit psoas muscle to enhance our understanding of the theoretically predicted force-length relation. Contractions at varying SLs along the plateau (SL = 2.25-2.39 µm) and the descending limb (SL > 2.39 µm) of the force-length relation were induced in sarcomeres and myofibrils, and different modes of force measurements were used. Our results show that when forces are measured in single sarcomeres, the experimental force-length relation follows theoretical predictions. When forces are measured in myofibrils with large SL dispersions, there is an extension of the plateau and forces elevated above the predicted levels along the descending limb. We also found an increase in SL non-uniformity and slowed rates of force production at long lengths in myofibrils but not in single sarcomere preparations. We conclude that the deviation of the descending limb of the force-length relation is correlated with the degree of SL non-uniformity and slowed force development.

The load dependence and the force-velocity relation in intact myosin filaments from skeletal and smooth muscles

In the present study we evaluated the load dependence of force produced by isolated muscle myosin filaments interacting with fluorescently labeled actin filaments, using for the first time whole native myosin filaments. We used a newly developed approach that allowed the use of physiological levels of ATP. Single filaments composed of either skeletal or smooth muscle myosin and single filaments of actin were attached between pairs of nano-fabricated cantilevers of known stiffness. The filaments were brought into contact to produce force, which caused sliding of the actin filaments over the myosin filaments. We applied load to the system by either pushing or pulling the filaments during interactions and observed that increasing the load increased the force produced by myosin and decreasing the load decreased the force. We also performed additional experiments in which we clamped the filaments at predetermined levels of force, which caused the filaments to slide to adjust the different loads, allowing us to measure the velocity of length changes to construct a force-velocity relation. Force values were in the range observed previously with myosin filaments and molecules. The force-velocity curves for skeletal and smooth muscle myosins resembled the relations observed for muscle fibers. The technique can be used to investigate many issues of interest and debate in the field of muscle biophysics.

A fast method to reprogram and CRISPR/Cas9 gene editing from erythroblasts

An efficient one-step procedure to reprogram fibroblasts into human induced pluripotent stem cells (hiPSC) and perform CRISPR/Cas9 gene editing simultaneously was recently reported. Here we show that such simultaneous reprogramming and gene editing can be efficiently done from erythroblasts. We successfully obtained human induced pluripotent stem cells colonies together with in frame and out of frame CAPN1 mutations in one or both alleles. We did not identify off-targets in edited cell lines. The entire process, from blood collection to mutated hiPSC took approximately 5 weeks, a much shorter period than standard multi-step methodologies using fibroblasts. Noteworthy, blood drawing is a less invasive procedure than a skin biopsy.

Prolonged force depression after mechanically demanding contractions is largely independent of Ca2+ and reactive oxygen species

Increased production of reactive oxygen/nitrogen species (ROS) and impaired cellular Ca²⁺ handling are implicated in the prolonged low-frequency force depression (PLFFD) observed in skeletal muscle after both metabolically and mechanically demanding exercise. Metabolically demanding high-intensity exercise can induce PLFFD accompanied by ROS-dependent fragmentation of the sarcoplasmic reticulum Ca²⁺ release channels, the ryanodine receptor 1s (RyR1s). We tested whether similar changes occur after mechanically demanding eccentric contractions. Human subjects performed 100 repeated drop jumps, which require eccentric knee extensor contractions upon landing. This exercise caused a major PLFFD, such that maximum voluntary and electrically evoked forces did not recover within 24 h. Drop jumps induced only minor signs of increased ROS, and RyR1 fragmentation was observed in only 3 of 7 elderly subjects. Also, isolated mouse muscle preparations exposed to drop-jump-mimicking eccentric contractions showed neither signs of increased ROS nor RyR1 fragmentation. Still, the free cytosolic [Ca²⁺] during tetanic contractions was decreased by ∼15% 1 h after contractions, which can explain the exaggerated force decrease at low-stimulation frequencies but not the major frequency-independent force depression. In conclusion, PLFFD caused by mechanically demanding eccentric contractions does not involve any major increase in ROS or RyR1 fragmentation.-Kamandulis, S., de Souza Leite, F., Hernandez, A., Katz, A., Brazaitis, M., Bruton, J. D., Venckunas, T., Masiulis, N., Mickeviciene, D., Eimantas, N., Subocius, A., Rassier, D. E., Skurvydas, A., Ivarsson, N., Westerblad, H. Prolonged force depression after mechanically demanding contractions is largely independent of Ca²⁺ and reactive oxygen species.

Residual force enhancement is regulated by titin in skeletal and cardiac myofibrils

KEY POINTS

When a skeletal muscle is stretched while it contracts, the muscle produces a relatively higher force than the force from an isometric contraction at the same length: a phenomenon referred to as residual force enhancement. Residual force enhancement is puzzling because it cannot be directly explained by the classical force-length relationship and the sliding filament theory of contraction, the main paradigms in the muscle field. We used custom-built instruments to measure residual force enhancement in skeletal myofibrils, and, for the first time, in cardiac myofibrils. Our data report that residual force enhancement is present in skeletal muscles, but not cardiac muscles, and is regulated by the different isoforms of the titin protein filaments.

ABSTRACT

When a skeletal muscle contracts isometrically, the muscle produces a force that is relative to the final isometric sarcomere length (SL). However, when the same final SL is reached by stretching the muscle while it contracts, the muscle produces a relatively higher force: a phenomenon commonly referred to as residual force enhancement. In this study, we investigated residual force enhancement in rabbit skeletal psoas myofibrils and, for the first time, cardiac papillary myofibrils. A custom-built atomic force microscope was used in experiments that stretched myofibrils before and after inhibiting myosin and actin interactions to determine whether the different cardiac and skeletal titin isoforms regulate residual force enhancement. At SLs ranging from 2.24 to 3.13 μm, the skeletal myofibrils enhanced the force by an average of 9.0%, and by 29.5% after hindering myosin and actin interactions. At SLs ranging from 1.80 to 2.29 μm, the cardiac myofibrils did not enhance the force before or after hindering myosin and actin interactions. We conclude that residual force enhancement is present only in skeletal muscles and is dependent on the titin isoforms.