Multiple isoforms of myosin light chain 1 in pig diaphragm slow fibers: correlation with maximal shortening velocity and force generation

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

The β-myosin heavy chain expressed in ventricular myocardium and the myosin heavy chain (MyHC) in slow-twitch skeletal Musculus soleus (M. soleus) type-I fibers are both encoded by MYH7. Thus, these myosin molecules are deemed equivalent. However, some reports suggested variations in the light chain composition between M. soleus and ventricular myosin, which could influence functional parameters, such as maximum velocity of shortening. To test for functional differences of the actin gliding velocity on immobilized myosin molecules, we made use of in vitro motility assays. We found that ventricular myosin moved actin filaments with ∼0.9 µm/s significantly faster than M. soleus myosin (0.3 µm/s). Filaments prepared from isolated actin are not the native interaction partner of myosin and are believed to slow down movement. Yet, using native thin filaments purified from M. soleus or ventricular tissue, the gliding velocity of M. soleus and ventricular myosin remained significantly different. When comparing the light chain composition of ventricular and M. soleus β-myosin, a difference became evident. M. soleus myosin contains not only the "ventricular" essential light chain (ELC) MLC1sb/v, but also an additional longer and more positively charged MLC1sa. Moreover, we revealed that on a single muscle fiber level, a higher relative content of MLC1sa was associated with significantly slower actin gliding. We conclude that the ELC MLC1sa decelerates gliding velocity presumably by a decreased dissociation rate from actin associated with a higher actin affinity compared to MLC1sb/v. Such ELC/actin interactions might also be relevant in vivo as differences between M. soleus and ventricular myosin persisted when native thin filaments were used.

Effect of Cardiac Myosin-Binding Protein C on Tropomyosin Regulation of Actin-Myosin Interaction Using In Vitro Motility Assay

We studied the modulating role of cardiac myosin-binding protein C (cMyBP-C) in tropomyosin regulation of the actin-myosin interaction. The effect of cMyBP-C on the velocity of actin-tropomyosin filament sliding over cardiac and slow skeletal myosins was evaluated using in vitro motility assay. The effect of cMyBP-C on the actin-tropomyosin filaments sliding depended on the type of myosin. The regulatory effect of cMyBP-C differs for cardiac and slow skeletal myosin because of the presence of specific essential light chain (LC1sa) in slow skeletal myosin isoform.

Disrupted myosin cross-bridge cycling kinetics triggers muscle weakness in nebulin-related myopathy

Nebulin is a giant protein expressed at high levels in skeletal muscle. Mutations in the nebulin gene (NEB) lead to muscle weakness and various congenital myopathies. Despite increasing clinical and scientific interest, the pathogenesis of weakness remains unknown. The present study, therefore, aims at unraveling the underlying molecular mechanisms. Hence, we recorded and analyzed the mechanics as well as the X-ray diffraction patterns of human membrane-permeabilized single muscle fibers expressing nebulin mutations. Results demonstrated that, during contraction, the cycling rate of myosin heads attaching to actin is dramatically perturbed, causing a reduction in the fraction of myosin-actin interactions in the strong binding state. This phenomenon prevents complete thin-filament activation, more especially proper and full tropomyosin movement, further limiting additional binding of myosin cross-bridges. At the cell level, this reduces the force-generating capacity and, overall, provokes muscle weakness. To reverse such a negative cascade of events, future potential therapeutic interventions should, therefore, focus on the triggering component, the altered myosin cross-bridge cycling kinetics.

Chronic heart failure decreases cross-bridge kinetics in single skeletal muscle fibres from humans

Skeletal muscle function is impaired in heart failure patients due, in part, to loss of myofibrillar protein content, in particular myosin. In the present study, we utilized small-amplitude sinusoidal analysis for the first time in single human skeletal muscle fibres to measure muscle mechanics, including cross-bridge kinetics, to determine if heart failure further impairs contractile performance by altering myofibrillar protein function. Patients with chronic heart failure (n = 9) and controls (n = 6) were recruited of similar age and physical activity to diminish the potentially confounding effects of ageing and muscle disuse. Patients showed decreased cross-bridge kinetics in myosin heavy chain (MHC) I and IIA fibres, partially due to increased myosin attachment time (t(on)). The increased t(on) compensated for myosin protein loss previously found in heart failure patients by increasing the fraction of the total cycle time myosin is bound to actin, resulting in a similar number of strongly bound cross-bridges in patients and controls. Accordingly, isometric tension did not differ between patients and controls in MHC I or IIA fibres. Patients also had decreased calcium sensitivity in MHC IIA fibres and alterations in the viscoelastic properties of the lattice structure of MHC I and IIA fibres. Collectively, these results show that heart failure alters skeletal muscle contraction at the level of the myosin-actin cross-bridge, leading to changes in muscle mechanics which could contribute to impaired muscle function. Additionally, we uncovered a unique kinetic property of MHC I fibres, a potential indication of two distinct populations of cross-bridges, which may have important physiological consequences.

The myosin light chain 1 isoform associated with masticatory myosin heavy chain in mammals and reptiles is embryonic/atrial MLC1

We recently reported that masticatory myosin heavy chain (MHC-M) is expressed as the exclusive or predominant MHC isoform in masseter and temporalis muscles of several rodent species, contrary to the prevailing dogma that rodents express almost exclusively MHC isoforms that are typically found in fast limb muscles and not masticatory myosin. We also reported that the same rodent species express the embryonic/atrial isoform of myosin light chain 1 (MLC1E/A) in jaw-closing muscles and not a unique masticatory MLC1 isoform that others have reported as being expressed in jaw-closing muscles of carnivores that express MHC-M. The objective of this study was to test the hypothesis that MLC1E/A is consistently expressed in jaw-closing muscles whenever MHC-M is expressed as the predominant or exclusive MHC isoform. Jaw-closing muscles, fast and slow limb muscles, and cardiac atria and ventricles of 19 species (six Carnivora species, one Primates species, one Chiroptera species, five marsupial species, an alligator and five turtle species) were analyzed using protein gel electrophoresis, immunoblotting, mass spectrometry and RNA sequencing. Gel electrophoresis and immunoblotting indicate that MHC-M is the exclusive or predominant MHC isoform in the jaw-closing muscles of each of the studied species. The results from all of the approaches collectively show that MLC1E/A is exclusively or predominantly expressed in jaw-closing muscles of the same species. We conclude that MLC1E/A is the exclusive or predominant MLC1 isoform that is expressed in jaw-closing muscles of vertebrates that express MHC-M, and that a unique masticatory isoform of MLC1 probably does not exist.

Masticatory (;superfast') myosin heavy chain and embryonic/atrial myosin light chain 1 in rodent jaw-closing muscles

Masticatory myosin is widely expressed among several vertebrate classes. Generally, the expression of masticatory myosin has been associated with high bite force for a carnivorous feeding style (including capturing/restraining live prey), breaking down tough plant material and defensive biting in different species. Masticatory myosin expression in the largest mammalian order, Rodentia, has not been reported. Several members of Rodentia consume large numbers of tree nuts that are encased in very hard shells, presumably requiring large forces to access the nutmeat. We, therefore, tested whether some rodent species express masticatory myosin in jaw-closing muscles. Myosin isoform expression in six Sciuridae species was examined, using protein gel electrophoresis, immunoblotting, mass spectrometry and RNA analysis. The results indicate that masticatory myosin is expressed in some Sciuridae species but not in other closely related species with similar diets but having different nut-opening strategies. We also discovered that the myosin light chain 1 isoform associated with masticatory myosin heavy chain, in the same four Sciuridae species, is the embryonic/atrial isoform. We conclude that rodent speciation did not completely eliminate masticatory myosin and that its persistent expression in some rodent species might be related to not only diet but also to feeding style.

Influence of fast and slow alkali myosin light chain isoforms on the kinetics of stretch-induced force transients of fast-twitch type IIA fibres of rat

This study contributes to understand the physiological role of slow myosin light chain isoforms in fast-twitch type IIA fibres of skeletal muscle. These isoforms are often attached to the myosin necks of rat type IIA fibres, whereby the slow alkali myosin light chain isoform MLC1s is much more frequent and abundant than the slow regulatory myosin light chain isoform MLC2s. In the present study, single-skinned rat type IIA fibres were maximally Ca(2+) activated and subjected to stepwise stretches for causing a perturbation of myosin head pulling cycles. From the time course of the resulting force transients, myosin head kinetics was deduced. Fibres containing MLC1s exhibited slower kinetics independently of the presence or absence of MLC2s. At the maximal MLC1s concentration of about 75%, the slowing was about 40%. The slowing effect of MLC1s is possibly due to differences in the myosin heavy chain binding sites of the fast and slow alkali MLC isoforms, which changes the rigidity of the myosin neck. Compared with the impact of myosin heavy chain isoforms in various fast-twitch fibre types, the influence of MLC1s on myosin head kinetics of type IIA fibres is much smaller. In conclusion, the physiological role of fast and slow MLC isoforms in type IIA fibres is a fine-tuning of the myosin head kinetics.