Myosin essential light chain 1sa decelerates actin and thin filament gliding on β-myosin molecules

Phosphate rebinding induces force reversal via slow backward cycling of cross-bridges

Objective

Previous studies on muscle fibers, myofibrils, and myosin revealed that the release of inorganic phosphate (Pi) and the force-generating step(s) are reversible, with cross-bridges also cycling backward through these steps by reversing force-generating steps and rebinding Pi. The aim was to explore the significance of force redevelopment kinetics (rate constant k TR) in cardiac myofibrils for the coupling between the Pi binding induced force reversal and the rate-limiting transition f - for backward cycling of cross-bridges from force-generating to non-force-generating states.

Methods

k TR and force generation of cardiac myofibrils from guinea pigs were investigated at 0.015-20 mM Pi. The observed force-[Pi], force-log [Pi], k TR-[Pi], and k TR-force relations were assessed with various single-pathway models of the cross-bridge cycle that differed in sequence and kinetics of reversible Pi release, reversible force-generating step and reversible rate-limiting transition. Based on the interpretation that k TR reflects the sum of rate-limiting transitions in the cross-bridge cycle, an indicator, the coupling strength, was defined to quantify the contribution of Pi binding induced force reversal to the rate-limiting transition f - from the [Pi]-modulated k TR-force relation.

Results

Increasing [Pi] decreased force by a bi-linear force-log [Pi] relation, increased k TR in a slightly downward curved dependence with [Pi], and altered k TR almost reciprocally to force reflected by the k TR-force relation. Force-[Pi] and force-log [Pi] relations provided less selectivity for the exclusion of models than the k TR-[Pi] and k TR-force relations. The k TR-force relation observed in experiments with cardiac myofibrils yielded the coupling strength +0.84 ± 0.08 close to 1, the maximum coupling strength expected for the reciprocal k TR-force relationship. Single pathway models consisting of fast reversible force generation before or after rapid reversible Pi release failed to describe the observed k TR-force relation. Single pathway models consistent with the observed k TR-force relation had either slow Pi binding or slow force reversal, i.e., in the consistent single pathway models, f - was assigned to the rate of either Pi binding or force reversal.

Conclusion

Backward flux of cross-bridges from force-generating to non-force-generating states is limited by the rates of Pi binding or force reversal ruling out other rate-limiting steps uncoupled from Pi binding induced force reversal.

Mechanisms of myosin II force generation: insights from novel experimental techniques and approaches

Myosin II is a molecular motor that converts chemical energy derived from ATP hydrolysis into mechanical work. Myosin II isoforms are responsible for muscle contraction and a range of cell functions relying on the development of force and motion. When the motor attaches to actin, ATP is hydrolyzed and inorganic phosphate (Pi) and ADP are released from its active site. These reactions are coordinated with changes in the structure of myosin, promoting the so-called "power stroke" that causes the sliding of actin filaments. The general features of the myosin-actin interactions are well accepted, but there are critical issues that remain poorly understood, mostly due to technological limitations. In recent years, there has been a significant advance in structural, biochemical, and mechanical methods that have advanced the field considerably. New modeling approaches have also allowed researchers to understand actomyosin interactions at different levels of analysis. This paper reviews recent studies looking into the interaction between myosin II and actin filaments, which leads to power stroke and force generation. It reviews studies conducted with single myosin molecules, myosins working in filaments, muscle sarcomeres, myofibrils, and fibers. It also reviews the mathematical models that have been used to understand the mechanics of myosin II in approaches focusing on single molecules to ensembles. Finally, it includes brief sections on translational aspects, how changes in the myosin motor by mutations and/or posttranslational modifications may cause detrimental effects in diseases and aging, among other conditions, and how myosin II has become an emerging drug target.

Single-Molecule Investigation of Load-Dependent Actomyosin Dissociation Kinetics for Cardiac and Slow Skeletal Myosin

Myosins are ATP-powered, force-generating motor proteins involved in cardiac and muscle contraction. The external load experienced by the myosins modulates and coordinates their function in vivo. Here, this study investigates the tension-sensing mechanisms of rabbit native β-cardiac myosin (βM-II) and slow skeletal myosins (SolM-II) that perform in different physiological settings. Using mobile optical tweezers with a square wave-scanning mode, a range of external assisting and resisting loads from 0 to 15 pN is exerted on single myosin molecules as they interact with the actin filament. Influenced of load on specific strongly-bound states in the cross-bridge cycle is examined by adjusting the [ATP]. The results implies that the detachment kinetics of actomyosin ADP.Pi strongly-bound force-generating state are load sensitive. Low assisting load accelerates, while the resisting load hinders the actomyosin detachment, presumably, by slowing both the Pi and ADP release. However, under both high assisting and resisting load, the rate of actomyosin dissociation decelerates. The transition from actomyosin ADP.Pi to ADP state appears to occur with a higher probability for βM-II than SolM-II. This study interpret that dissociation of at least three strongly-bound actomyosin states are load-sensitive and may contribute to functional diversity among different myosins.

Cost-Efficient Expression of Human Cardiac Myosin Heavy Chain in C2C12 Cells with a Non-Viral Transfection Reagent

Production of functional myosin heavy chain (MHC) of striated muscle myosin II for studies of isolated proteins requires mature muscle (e.g., C2C12) cells for expression. This is important both for fundamental studies of molecular mechanisms and for investigations of deleterious diseases like cardiomyopathies due to mutations in the MHC gene (MYH7). Generally, an adenovirus vector is used for transfection, but recently we demonstrated transfection by a non-viral polymer reagent, JetPrime. Due to the rather high costs of JetPrime and for the sustainability of the virus-free expression method, access to more than one transfection reagent is important. Here, we therefore evaluate such a candidate substance, GenJet. Using the human cardiac β-myosin heavy chain (β-MHC) as a model system, we found effective transfection of C2C12 cells showing a transfection efficiency nearly as good as with the JetPrime reagent. This was achieved following a protocol developed for JetPrime because a manufacturer-recommended application protocol for GenJet to transfect cells in suspension did not perform well. We demonstrate, using in vitro motility assays and single-molecule ATP turnover assays, that the protein expressed and purified from cells transfected with the GenJet reagent is functional. The purification yields reached were slightly lower than in JetPrime-based purifications, but they were achieved at a significantly lower cost. Our results demonstrate the sustainability of the virus-free method by showing that more than one polymer-based transfection reagent can generate useful amounts of active MHC. Particularly, we suggest that GenJet, due to its current ~4-fold lower cost, is useful for applications requiring larger amounts of a given MHC variant.

Mechanical and signaling responses of unloaded rat soleus muscle to chronically elevated β-myosin activity

It has been reported that muscle functional unloading is accompanied by an increase in motoneuronal excitability despite the elimination of afferent input. Thus, we hypothesized that pharmacological potentiation of spontaneous contractile soleus muscle activity during hindlimb unloading could activate anabolic signaling pathways and prevent the loss of muscle mass and strength. To investigate these aspects and underlying molecular mechanisms, we used β-myosin allosteric effector Omecamtiv Mekarbil (OM). We found that OM partially prevented the loss of isometric strength and intrinsic stiffness of the soleus muscle after two weeks of disuse. Notably, OM was able to attenuate the unloading-induced decrease in the rate of muscle protein synthesis (MPS). At the same time, the use of drug neither prevented the reduction in the markers of translational capacity (18S and 28S rRNA) nor activation of the ubiquitin-proteosomal system, which is evidenced by a decrease in the cross-sectional area of fast and slow muscle fibers. These results suggest that chemically-induced increase in low-intensity spontaneous contractions of the soleus muscle during functional unloading creates prerequisites for protein synthesis. At the same time, it should be assumed that the use of OM is advisable with pharmacological drugs that inhibit the expression of ubiquitin ligases.

Virus-free transfection, transient expression, and purification of human cardiac myosin in mammalian muscle cells for biochemical and biophysical assays

Myosin expression and purification is important for mechanistic insights into normal function and mutation induced changes. The latter is particularly important for striated muscle myosin II where mutations cause several debilitating diseases. However, the heavy chain of this myosin is challenging to express and the standard protocol, using C2C12 cells, relies on viral infection. This is time and work intensive and associated with infrastructural demands and biological hazards, limiting widespread use and hampering fast generation of a wide range of mutations. We here develop a virus-free method to overcome these challenges. We use this system to transfect C2C12 cells with the motor domain of the human cardiac myosin heavy chain. After optimizing cell transfection, cultivation and harvesting conditions, we functionally characterized the expressed protein, co-purified with murine essential and regulatory light chains. The gliding velocity (1.5-1.7 µm/s; 25 °C) in the in vitro motility assay as well as maximum actin activated catalytic activity (kcat; 8-9 s-1) and actin concentration for half maximal activity (KATPase; 70-80 µM) were similar to those found previously using virus based infection. The results should allow new types of studies, e.g., screening of a wide range of mutations to be selected for further characterization.