Two functionally distinct myosin heavy chain isoforms in slow skeletal muscle fibres

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

Fiber-type distribution in insect leg muscles parallels similarities and differences in the functional role of insect walking legs

Previous studies have demonstrated that myofibrillar ATPase (mATPase) enzyme activity in muscle fibers determines their contraction properties. We analyzed mATPase activities in muscles of the front, middle and hind legs of the orthopteran stick insect (Carausius morosus) to test the hypothesis that differences in muscle fiber types and distributions reflected differences in their behavioral functions. Our data show that all muscles are composed of at least three fiber types, fast, intermediate and slow, and demonstrate that: (1) in the femoral muscles (extensor and flexor tibiae) of all legs, the number of fast fibers decreases from proximal to distal, with a concomitant increase in the number of slow fibers. (2) The swing phase muscles protractor coxae and levator trochanteris, have smaller percentages of slow fibers compared to the antagonist stance muscles retractor coxae and depressor trochanteris. (3) The percentage of slow fibers in the retractor coxae and depressor trochanteris increases significantly from front to hind legs. These results suggest that fiber-type distribution in leg muscles of insects is not identical across leg muscles but tuned towards the specific function of a given muscle in the locomotor system.

Human skeletal muscle: transition between fast and slow fibre types

Human skeletal muscles consist of different fibre types: slow fibres (slow twitch or type I) containing the myosin heavy chain isoform (MHC)-I and fast fibres (fast twitch or type II) containing MHC-IIa (type IIA) or MHC-IId (type IID). The following order of decreasing kinetics is known: type IID > type IIA >> type I. This order is especially based on the kinetics of stretch activation, which is the most discriminative property among fibre types. In this study we tested if hybrid fibres containing both MHC-IIa and MHC-I (type C fibres) provide a transition in kinetics between fast (type IIA) and slow fibres (type I). Our data of stretch activation kinetics suggest that type C fibres, with different ratios of MHC-IIa and MHC-I, do not provide a continuous transition. Instead, a specialized group of slow fibres, which we called "transition fibres", seems to provide a transition. Apart of their kinetics of stretch activation, which is most close to that of type IIA, the transition fibres are characterized by large cross-sectional areas and low maximal tensions. The molecular cause for the mechanical properties of the transition fibres is unknown. It is possible that the transition fibres contain an unknown slow MHC isoform, which cannot be separated by biochemical methods. Alternatively, or in addition, isoforms of myofibrillar proteins, other than MHC, and posttranslational modifications of myofibrillar proteins could play a role regarding the characteristics of the transition fibres.

Prolonged space flight-induced alterations in the structure and function of human skeletal muscle fibres

The primary goal of this study was to determine the effects of prolonged space flight (180 days) on the structure and function of slow and fast fibres in human skeletal muscle. Biopsies were obtained from the gastrocnemius and soleus muscles of nine International Space Station crew members 45 days pre- and on landing day (R+0) post-flight. The main findings were that prolonged weightlessness produced substantial loss of fibre mass, force and power with the hierarchy of the effects being soleus type I > soleus type II > gastrocnemius type I > gastrocnemius type II. Structurally, the quantitatively most important adaptation was fibre atrophy, which averaged 20% in the soleus type I fibres (98 to 79 μm diameter). Atrophy was the main contributor to the loss of peak force (P(0)), which for the soleus type I fibre declined 35% from 0.86 to 0.56 mN. The percentage decrease in fibre diameter was correlated with the initial pre-flight fibre size (r = 0.87), inversely with the amount of treadmill running (r = 0.68), and was associated with an increase in thin filament density (r = 0.92). The latter correlated with reduced maximal velocity (V(0)) (r = 0.51), and is likely to have contributed to the 21 and 18% decline in V(0) in the soleus and gastrocnemius type I fibres. Peak power was depressed in all fibre types with the greatest loss (55%) in the soleus. An obvious conclusion is that the exercise countermeasures employed were incapable of providing the high intensity needed to adequately protect fibre and muscle mass, and that the crew's ability to perform strenuous exercise might be seriously compromised. Our results highlight the need to study new exercise programmes on the ISS that employ high resistance and contractions over a wide range of motion to mimic the range occurring in Earth's 1 g environment.

Sarcomeric myosin expression in the tongue body of humans, macaques and rats

Expression of developmental and unconventional myosin heavy chain (MHC) isoforms in some adult head and neck muscles is thought to reflect specific contractile demands of muscle fibers active during kinematically complex movements. Mammalian tongue muscles are active during oromotor behaviors that encompass a wide range of tongue movement speeds and tongue shape changes (e.g. respiration, oral transport, swallowing, rejection), but the extent to which tongue muscles express developmental and unconventional MHC is not known. Quantitative PCR was used to determine the mRNA content of conventional MHC-beta, MHC-2a, MHC-2b and MHC-2x, the developmental isoforms embryonic MHC and neonatal MHC and the unconventional isoforms atrial/cardiac-alpha MHC (MHC-alpha), extraocular MHC, masseter MHC and slow tonic MHC in tongue body muscles of the rat, macaque and human. In all species, conventional MHC isoforms predominate. MHC-2b and MHC-2x account for 98% of total MHC mRNA in the rat. MHC-2a, MHC-2x and MHC-beta account for 94% of total MHC mRNA in humans and 96% of total MHC mRNA in macaque. With the exception of MHC-alpha in humans (5%), developmental and unconventional MHC mRNA represents less than 0.3% of total MHC mRNA. We conclude that in these species, there is limited expression of developmental and unconventional MHC and that diversity of tongue body muscle fiber contractile properties is achieved primarily by MHC-beta, MHC-2a, MHC-2x and MHC-2b. Whether expression of MHC-alpha mRNA in tongue is unique to humans or present in other hominoids awaits further investigation.

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

Pig diaphragm slow fibers exhibit heterogeneity in myosin light chain 1 (MLC1) isoform expression, with many expressing fast-type MLC1 (MLC1F), as well as two isoforms of slow-type MLC1 (MLC1Sa and MLC1Sb). The goal of this study was to test if there is a relationship between MLC1 isoform expression and contractile properties among these fibers. Maximal shortening velocity (V(max)) and maximal isometric force generation, normalized with fiber cross-sectional area (P(o)/CSA), were measured in single fibers. V(max) was inversely related to the relative level of MLC1Sa. The level of MLC1Sa was reciprocally related to the levels of MLC1Sb and of MLC1F among individual fibers. Fibers expressing MLC1Sa and in which MLC1Sb was not detected generated greater P(o)/CSA, compared to fibers expressing MLC1Sb and not MLC1Sa. The results indicate a complex pattern of MLC1 isoform expression among pig diaphragm slow fibers and suggest that shortening velocity and force generation are modulated, in these fibers, by the MLC1 isoform composition.

Aging, muscle fiber type, and contractile function in sprint-trained athletes

Biopsy samples were taken from the vastus lateralis of 18- to 84-yr-old male sprinters (n = 91). Fiber-type distribution, cross-sectional area, and myosin heavy chain (MHC) isoform content were identified using ATPase histochemistry and SDS-PAGE. Specific tension and maximum shortening velocity (V(o)) were determined in 144 single skinned fibers from younger (18-33 yr, n = 8) and older (53-77 yr, n = 9) runners. Force-time characteristics of the knee extensors were determined by using isometric contraction. The cross-sectional area of type I fibers was unchanged with age, whereas that of type II fibers was reduced (P < 0.001). With age there was an increased MHC I (P < 0.01) and reduced MHC IIx isoform content (P < 0.05) but no differences in MHC IIa. Specific tension of type I and IIa MHC fibers did not differ between younger and older subjects. V(o) of fibers expressing type I MHC was lower (P < 0.05) in older than in younger subjects, but there was no difference in V(o) of type IIa MHC fibers. An aging-related decline of maximal isometric force (P < 0.001) and normalized rate of force development (P < 0.05) of knee extensors was observed. Normalized rate of force development was positively associated with MHC II (P < 0.05). The sprint-trained athletes experienced the typical aging-related reduction in the size of fast fibers, a shift toward a slower MHC isoform profile, and a lower V(o) of type I MHC fibers, which played a role in the decline in explosive force production. However, the muscle characteristics were preserved at a high level in the oldest runners, underlining the favorable impact of sprint exercise on aging muscle.

Human skeletal muscle myosin function at physiological and non-physiological temperatures

AIM

The aim of the study was to assess the function of human skeletal muscle myosin across a wide range of temperatures, including physiological.

METHODS

We used a single fibre in vitro motility assay. The in vitro motility speed of actin filaments propelled by myosin extracted from fibres expressing type I myosin heavy chain (MyHC; n = 9), IIa MyHC (n = 6), IIax MyHC (n = 4) and I/IIa MyHC (n = 1) was measured at 15, 20, 25, 30 and 35 degrees C.

RESULTS

The motility speed between groups of fibres expressing different MyHC differed significantly (P << 0.001). The increase in motility speed with an increase in temperature was statistically significant (P << 0.001) between all temperatures. The relative difference in motility speed between the slow type I and the fast IIax MyHC fibres decreased with increasing temperature, i.e. a 7.5-fold difference at 15 degrees C was reduced to twofold at 35 degrees C. Furthermore, the twofold difference in motility speed between type IIa and IIax MyHC at 15 degrees C disappeared completely at 35 degrees C. The activation energy, E(A), and temperature coefficient, Q(10), over the 15-35 degrees C temperature range was higher for type I MyHC, 54.47 +/- 4.37 kJ mol(-1) and 2.09 +/- 0.12, respectively, than for type IIa MyHC, 45.41 +/- 3.12 kJ mol(-1) (P < 0.001) and 1.85 +/- 0.08 (P < 0.001), or IIax MyHC, 34.71 +/- 1.75 kJ mol(-1) (P << 0.001) and 1.60 +/- 0.04 (P << 0.001).

CONCLUSION

The present results suggest a significantly reduced difference in shortening velocity between different human muscle fibre types at physiological temperature than previously reported at lower temperatures (12 or 15 degrees C) with implications for human in vivo muscle function.

Elementary steps of the cross-bridge cycle in fast-twitch fiber types from rabbit skeletal muscles

To understand the molecular mechanism underlying the diversity of mammalian skeletal muscle fibers, the elementary steps of the cross-bridge cycle were investigated in three fast-twitch fiber types from rabbit limb muscles. Skinned fibers were maximally Ca(2+)-activated at 20 degrees C and the effects of MgATP, phosphate (P, P(i)), and MgADP were studied on three exponential processes by sinusoidal analysis. The fiber types (IIA, IID, and IIB) were determined by analyzing the myosin heavy-chain isoforms after mechanical experiments using high-resolution SDS-PAGE. The results were consistent with the following cross-bridge scheme: where A is actin, M is myosin, D is MgADP, and S is MgATP. All states except for those in brackets are strongly bound states. All rate constants of elementary steps (k(2), 198-526 s(-1); k(-2), 51-328 s(-1); k(4), 13.6-143 s(-1); k(-4), 13.6-81 s(-1)) were progressively larger in the order of type IIA, type IID, and type IIB fibers. The rate constants of a transition from a weakly bound state to a strongly bound state (k(-2), k(4)) varied more among fiber types than their reversals (k(2), k(-4)). The equilibrium constants K(1) (MgATP affinity) and K(2) (=k(2)/k(-2), ATP isomerization) were progressively less in the order IIA, IID, and IIB. K(4) (=k(4)/k(-4), force generation) and K(5) (P(i) affinity) were larger in IIB than IIA and IID fibers. K(1) showed the largest variation indicating that the myosin head binds MgATP more tightly in the order IIA (8.7 mM(-1)), IID (4.9 mM(-1)), and IIB (0.84 mM(-1)). Similarly, the MgADP affinity (K(0)) was larger in type IID fibers than in type IIB fibers.

Effects of resistance training on myosin function studied by the in vitro motility assay in young and older men

It is generally believed that the maximum shortening velocity (V(o)) of a skeletal muscle fiber type does not vary unless a change in myosin heavy chain (MHC) isoform composition occurs. However, recent findings have shown that V(o) of a given fiber type can change after training, suggesting the hypothesis that the function of myosin can vary without a change in isoform. The present study addressed the latter hypothesis by studying the function of isolated myosin isoforms by the use of the in vitro motility assay (IVMA) technique. Four young (age 23-29 yr, YO) and four elderly men (age 68-82 yr, EL) underwent a 12-wk progressive resistance training program of the knee extensor muscles and to one pre- and one posttraining biopsy of the vastus lateralis muscle. The significant increase in one-repetition maximum posttraining in both YO and EL indicated that training was effective. After training, MHC isoform composition showed a shift from MHC(2X) toward MHC(2A) in YO and no shift in EL. The velocity of sliding (V(f)) of actin filaments on pure myosin isoforms extracted from single fibers was studied in IVMA. One hundred sixty IVMA samples were prepared from 480 single fibers, and at least 50 filaments were analyzed in each experiment. Whereas no training-induced change was observed in V(f) of myosin isoform 1 either in YO or in EL, a significant increase in V(f) of myosin isoform 2A after training was observed in both YO (18%) and EL (19%). The results indicate that resistance training can change the velocity of the myosin molecule.

Kinetic properties of myosin heavy chain isoforms in mouse skeletal muscle: comparison with rat, rabbit, and human and correlation with amino acid sequence

Stretch activation kinetics were investigated in skinned mouse skeletal muscle fibers of known myosin heavy chain (MHC) isoform content to assess kinetic properties of different myosin heads while generating force. The time to peak of stretch-induced delayed force increase (t(3)) was strongly correlated with MHC isoforms [t(3) given in ms for fiber types containing specified isoforms; means +/- SD with n in parentheses: MHCI 680 +/- 108 (13), MHCIIa 110.5 +/- 10.7 (23), MHCIIx(d) 46.2 +/- 5.2 (20), MHCIIb 23.5 +/- 3.3 (76)]. This strong correlation suggests different kinetics of force generation of different MHC isoforms in the following order:MHCIIb > MHCIIx(d) > MHCIIa >> MHCI. For rat, rabbit, and human skeletal muscles the same type of correlation was found previously. The kinetics decreases slightly with increasing body mass. Available amino acid sequences were aligned to quantify the structural variability of MHC isoforms of different animal species. The variation in t(3) showed a correlation with the structural variability of specific actin-binding loops (so-called loop 2 and loop 3) of myosin heads (r = 0.74). This suggests that alterations of amino acids in these loops contribute to the different kinetics of myosin heads of various MHC isoforms.

Fiber type conversion alters inactivation of voltage-dependent sodium currents in murine C2C12 skeletal muscle cells

Each skeletal muscle of the body contains a unique composition of "fast" and "slow" muscle fibers, each of which is specialized for certain challenges. This composition is not static, and the muscle fibers are capable of adapting their molecular composition by altered gene expression (i.e., fiber type conversion). Whereas changes in the expression of contractile proteins and metabolic enzymes in the course of fiber type conversion are well described, little is known about possible adaptations in the electrophysiological properties of skeletal muscle cells. Such adaptations may involve changes in the expression and/or function of ion channels. In this study, we investigated the effects of fast-to-slow fiber type conversion on currents via voltage-gated Na+ channels in the C(2)C(12) murine skeletal muscle cell line. Prolonged treatment of cells with 25 nM of the Ca2+ ionophore A-23187 caused a significant shift in myosin heavy chain isoform expression from the fast toward the slow isoform, indicating fast-to-slow fiber type conversion. Moreover, Na+ current inactivation was significantly altered. Slow inactivation less strongly inhibited the Na+ currents of fast-to-slow fiber type-converted cells. Compared with control cells, the Na+ currents of converted cells were more resistant to block by tetrodotoxin, suggesting enhanced relative expression of the cardiac Na+ channel isoform Na(v)1.5 compared with the skeletal muscle isoform Na(v)1.4. These results imply that fast-to-slow fiber type conversion of skeletal muscle cells involves functional adaptation of their electrophysiological properties.

The mechanism of the force response to stretch in human skinned muscle fibres with different myosin isoforms

Force enhancement during lengthening of an active muscle, a condition that normally occurs during locomotion in vivo, is attributed to recruitment of myosin heads that exhibit fast attachment to and detachment from actin in a cycle that does not imply ATP splitting. We investigated the kinetic and mechanical features of this cycle in Ca(2+) activated single skinned fibres from human skeletal muscles containing different myosin heavy chain (MHC) isoforms, identified with single-fibre gel electrophoresis. Fibres were activated by using a new set-up that allows development of most of the tension following a temperature jump from 0-1 degrees C to the test temperature (approximately 12 degrees C). In this way we could prevent the development of sarcomere non-uniformity and record sarcomere length changes with a striation follower in any phase of the mechanical protocol. We found that: (i) fibres with fast MHC isoforms develop 40-70% larger isometric forces than those with slow isoforms, as a result of both a larger fraction of force-generating myosin heads and a higher force per head; (ii) in both slow and fast fibres, force enhancement by stretch is due to recruitment of myosin head attachments, without increase in strain per head above the value generated by the isometric heads; and (iii) the extent of recruitment is larger in slow fibres than in fast fibres, so that the steady force and power output elicited by lengthening become similar, indicating that mechanical and kinetic properties of the actin-myosin interactions under stretch become independent of the MHC isoform.

Functional differences in type-I fibres from two slow skeletal muscles of rabbit

The present study addressed the question of whether the slow fibres of mammalian skeletal muscle, containing the myosin heavy chain MHCI (type-I fibres), are a functionally homogeneous population. We compared various properties of Ca(2+)-activated, skinned, type-I fibres from the soleus and semitendinosus muscles of a rabbit. Soleus type-I fibres showed significantly faster kinetics of stretch activation, measured as the time-to-peak of the stretch-induced, delayed force increase, t(3), than semitendinosus fibres (1239+/-438 ms, n=136, vs. 1600+/-409 ms, n=208 respectively) (means+/-SD, 22 degrees C). Similarly, the speed of unloaded shortening at 15 degrees C was faster in soleus than in semitendinosus fibres [0.79+/-0.16 fibre lengths (FL) s(-1), n=44, vs. 0.65+/-0.15 FL s(-1), n=35 respectively]. The kinetics of stretch activation were more temperature sensitive in semitendinosus than in soleus fibres. Finally, the generation of steady-state isometric force was more sensitive to Ca(2+) in semitendinosus than in soleus fibres: [pCa(50) (-log [Ca(2+)] for half-maximal activation) at 22 degrees C: 6.29+/-0.15, n=28, vs. 6.19+/-0.10, n=18 respectively]. These results suggest strongly that there is no functional homogeneity within type-I fibres of different muscles. The observed differences might reflect the existence of more than one functionally different slow myosin heavy chain isoforms or other modifications of contractile proteins.

Different effects on human skeletal myosin heavy chain isoform expression: strength vs. combination training

Myosin heavy chain (MHC) isoform expression changes with physical training. This may be one of the mechanisms for muscular adaptation to exercise. We aimed to investigate the effects of different strength-training protocols on MHC isoform expression, bearing in mind that alpha- MHC(slow) (newly identified MHC isoform) mRNA may be upregulated in response to training. Twelve volunteers performed a 6-wk strength training with maximum contractions (Max group), and another 12 of similar age performed combination training of maximum contractions and ballistic and stretch-shortening movements (Combi group). Muscle samples were taken from triceps brachii before and after training. MHC isoform composition was determined by SDS-PAGE silver staining, and mRNA levels of MHC isoforms were determined by RT-PCR. In Max group, there was an increase in MHC(2A) (49.4 to 66.7%, P < 0.01) and a decrease in MHC(2X) (33.4 to 19.5%, P < 0.01) after training, although there was no significant change in MHC(slow). In Combi group, there was also an increase in MHC(2A) (47.7 to 62.7%, P < 0.05) and a decrease in MHC(slow) (18.2 to 9.2%, P < 0.05) but no significant change in MHC(2X). An upregulation of alpha-MHC(slow) mRNA was, therefore, found in both groups as a result of training. The strength training with maximum contractions led to a shift in MHC isoform composition from 2X to 2A, whereas the combined strength training produced an MHC isoform composition shift from slow to 2A.

Effects of aging on actin sliding speed on myosin from single skeletal muscle cells of mice, rats, and humans

The effects of aging on the mechanical properties of myosin were measured in 87 fibers from muscles of humans (n = 40), rats (n = 21), and mice (n = 26) using a single fiber in vitro motility assay. Irrespective of species, an 18-25% aging-related slowing in the speed of actin filaments was observed from 62 single fibers expressing the slow (type I) beta-myosin heavy chain isoform. The mechanisms underlying the aging-related slowing of motility speed remain unknown, but it is suggested that posttranslational modifications of myosin by oxidative stress, glycation, or nitration play an important role. The aging-related slowing in the speed of actin filaments propelled by the type I myosin was confirmed in three mammalian species with an approximately 3,400-fold difference in body size. Motility speed from human myosin was 3-fold slower than from myosin of the approximately 3,400-fold smaller mouse and approximately twofold slower when compared with the approximately 130-fold smaller rat, irrespective of age. A strong correlation was observed between the log values of actin sliding speed and body mass, suggesting that the effects of scaling is, at least in part, due to altered functional properties of the motor protein itself.

Effects of unweighting and clenbuterol on myosin light and heavy chains in fast and slow muscles of rat

To investigate the plasticity of slow and fast muscles undergoing slow-to-fast transition, rat soleus (SOL), gastrocnemius (GAS), and extensor digitorum longus (EDL) muscles were exposed for 14 days to 1) unweighting by hindlimb suspension (HU), or 2) treatment with the beta(2)-adrenergic agonist clenbuterol (CB), or 3) a combination of both (HU-CB). In general, HU elicited atrophy, CB induced hypertrophy, and HU-CB partially counteracted the HU-induced atrophy. Analyses of myosin heavy (MHC) and light chain (MLC) isoforms revealed HU- and CB-induced slow-to-fast transitions in SOL (increases of MHCIIa with small amounts of MHCIId and MHCIIb) and the upregulation of the slow MHCIa isoform. The HU- and CB-induced changes in GAS consisted of increases in MHCIId and MHCIIb ("fast-to-faster transitions"). Changes in the MLC composition of SOL and GAS consisted of slow-to-fast transitions and mainly encompassed an exchange of MLC1s with MLC1f. In addition, MLC3f was elevated whenever MHCIId and MHCIIb isoforms were increased. Because the EDL is predominantly composed of type IID and IIB fibers, HU, CB, and HU-CB had no significant effect on the MHC and MLC patterns.

Aging-related changes in skeletal muscle. Mechanisms and interventions

The aging-related motor handicap and the growing population of elderly citizens have enormous socioeconomic effects on the modern healthcare system. The mechanisms underlying impaired motor performance in old age are complex and involve the central and peripheral nervous systems and the muscle tissue itself. It is widely accepted that the aging-related loss of muscle mass, strength and quality has a significant detrimental impact on motor performance in old age and on the ability to recover from falls, resulting in an increased risk of fractures and dependency. Therefore, the prevention of falls and gait instability is a very important safety issue, and different intervention strategies have been used to improve motor performance among the aging population. There is general consensus that physical exercise is a powerful intervention to obtain long term benefits on muscle function, reduce the frequency of falls, and to maintain independence and a high quality of life in older persons. The results from studies using different types of hormone supplementation therapies have shown interesting and encouraging effects on skeletal muscle mass and function. However, the potential risks with both growth hormone and androgen treatment are not known and long term clinical trials are needed to address safety concerns and the effects on skeletal muscle. Recent advancements in cellular/molecular, physiological and molecular biological techniques will significantly facilitate our understanding of aging-related impairments of muscle function and contribute to the evaluation of different intervention strategies.

Myosin isoforms, muscle fiber types, and transitions

Skeletal muscle is an extremely heterogeneous tissue composed of a variety of fast and slow fiber types and subtypes. Moreover, muscle fibers are versatile entities capable of adjusting their phenotypic properties in response to altered functional demands. Major differences between muscle fiber types relate to their myosin complement, i.e., isoforms of myosin light and heavy chains. Myosin heavy chain (MHC) isoforms appear to represent the most appropriate markers for fiber type delineation. On this basis, pure fiber types are characterized by the expression of a single MHC isoform, whereas hybrid fiber type express two or more MHC isoforms. Hybrid fibers bridge the gap between the pure fiber types. The fiber population of skeletal muscles, thus, encompasses a continuum of pure and hybrid fiber types. Under certain conditions, changes can be induced in MHC isoform expression heading in the direction of either fast-to-slow or slow-to-fast. Increased neuromuscular activity, mechanical loading, and hypothyroidism are conditions that induce fast-to-slow transitions, whereas reduced neuromuscular activity, mechanical unloading, and hyperthyroidism cause transitions in the slow-to-fast direction.

Myosin isoform composition of the human medial and lateral pterygoid muscles

The medial and lateral pterygoid muscles are different in structure as well as in function. The medial pterygoid muscle is concentrically active during jaw closing, and the superior head of the lateral pterygoid muscle is eccentrically active during jaw closing, while its inferior head is concentrically active during jaw opening. Architecturally, the medial pterygoid can deliver higher forces than the lateral pterygoid. We investigated whether these differences are reflected in the myosin heavy-chain (MyHC) composition and the fiber cross-sectional area (f-csa) of these muscles. The pterygoid muscles from eight cadavers were investigated by means of monoclonal antibodies against different isoforms of MyHC. The proportions of pure MyHC type I fibers did not differ significantly among the muscles (32% in medial pterygoid, 34% in superior head, and 36% in the inferior head of the lateral pterygoid), nor did the total proportions of pure MyHC type IIA and IIX fibers (16% in medial pterygoid, 26% in the superior head, and 19% in the inferior head of the lateral pterygoid). The mean f-csa of type I fibers was 1315 microm2, which did not differ significantly among the muscles, and was significantly larger than the f-csa of type IIA fibers. The relative proportions of hybrid fibers, which expressed more than one MyHC isoform, were 52% in the medial pterygoid, 40% in the superior head, and 45% in the inferior head of the lateral pterygoid and did not differ significantly among the muscles. The most abundant hybrid fiber types found were fibers expressing MyHCs-cardiac alpha+IIA and MyHCs-cardiac alpha+I+IIA. Significant regional differences were found in the proportions of MyHC type I fibers in the medial pterygoid and in the inferior head of the lateral pterygoid. Although the form and function of the muscles are different, we conclude that this is not reflected in their myosin isoform composition.

Evidence for differential post-translational modifications of slow myosin heavy chain during murine skeletal muscle development

The contractile properties of muscle fibres are, in part, determined by the myosin heavy chain (MyHC) isoforms they express. Using monoclonal antibodies, we show that at least three forms of slow twitch MyHC accumulate sequentially during mouse fetal development and that slow MyHC maturation in slow fibres occurs before expression of the adult fast MyHCs in fast fibres. Expression of deletion derivatives of beta-cardiac MyHC cDNA shows that the slow MyHC epitopes that are detected in adult but not in young animals are located near the N-terminus. The same N-terminal region of various fast MyHC molecules contains a conserved epitope that can, on occasions, be observed when slow MyHC cDNA is expressed in non-muscle cells. The results raise the possibility that the N-terminal epitopes result from post-translational modification of the MyHC and that a sequence of slow and fast MyHC isoform post-translational modifications plays a significant role during development of murine muscle fibres.

Time-dependent changes in myosin heavy chain mRNA and protein isoforms in unloaded soleus muscle of rat

Time-dependent changes in myosin heavy chain (MHC) isoform expression were investigated in rat soleus muscle unloaded by hindlimb suspension. Changes at the mRNA level were measured by RT-PCR and correlated with changes in the pattern of MHC protein isoforms. Protein analyses of whole muscle revealed that MHCI decreased after 7 days, when MHCIIa had increased, reaching a transient maximum by 15 days. Longer periods led to inductions and progressive increases of MHCIId(x) and MHCIIb. mRNA analyses of whole muscle showed that MHCIId(x) displayed the steepest increase after 4 days and continued to rise until 28 days, the longest time period investigated. MHCIIb mRNA followed a similar time course, although at lower levels. MHCIalpha mRNA, present at extremely low levels in control soleus, peaked after 4 days, stayed elevated until 15 days, and then decayed. Immunohistochemistry of 15-day unloaded muscles revealed that MHCIalpha was present in muscle spindles but at low amounts also in extrafusal fibers. The slow-to-fast transitions thus seem to proceed in the order MHCIbeta --> MHCIIa --> MHCIId(x) --> MHCIIb. Our findings indicate that MHCIalpha is transiently upregulated in some fibers as an intermediate step during the transition from MHCIbeta to MHCIIa.

Control of muscle fibre size: a crucial factor in ageing

Force generation by skeletal muscle declines during ageing. This change contributes substantially to increased physical dependency in the aged. The decline in muscle mass is not entirely accounted for by a fall in muscle fibre number: fibres appear to lose volume. Here we review data that address the fundamental question of how muscle fibres regulate their size. In muscles, the problem has two elements because muscle fibres are syncitia formed by the fusion of mononucleate precursor cells. Thus, fibre size appears to be regulated both by the number of nuclei incorporated into each fibre and by a second variable, the volume of cytoplasm that each nucleus supports. We conclude that understanding of the regulation of muscle cell size is in its infancy and highlight directions that might productively be pursued.

In vitro motility speed of slow myosin extracted from single soleus fibres from young and old rats

1. Isolated soleus muscle fibres from aged rats contract more slowly than those from young rats. To determine whether this effect is due to a difference between the myosin molecules, we measured the rate at which actin filaments are driven over a myosin coated surface in the presence of ATP by using a novel in vitro motility assay where myosin is extracted from single muscle fibre segments. 2. Motility was dependent on the myosin density on the coverslip. In regions of high myosin density, actin motility was orientated parallel and anti-parallel to the direction of flow during myosin adhesion to the coverslip. In contrast, in regions of lower myosin density, actin motility was more random. The speed was about 20 % higher in the high density regions (P < 0.001). Further, the speed of filaments in the high density region, moving away or towards the fibre was less variable (P < 0.05) than that of more randomly moving filaments in the low density region. 3. The speed with myosin from slow soleus fibres of young adult rats (3-6 months old; v = 1.43 +/- 0.23 microm s-1; mean +/- s.d.) was faster (P < 0.001) than with myosin from aged rats (20-24 months old; v = 1.27 +/- 0.23 microm s-1). 4. No difference in myosin isoforms between young adult and aged fibres could be detected using electrophoretic and immunocytochemical techniques. Fibres of both ages expressed the beta/slow myosin heavy chain (MyHC) isoform and slow isoforms of essential and regulatory myosin light chains (MyLCs). 5. It is concluded that an age-related alteration in myosin contributes to the slowing of the maximum shortening velocity (V0) observed in soleus muscle fibres expressing the beta/slow MyHC isoform.

Neuromuscular fatigue during repeated exhaustive submaximal static contractions of knee extensor muscles in endurance-trained, power-trained and untrained men

The neural and muscular changes during fatigue produced in repeated submaximal static contractions of knee extensors were measured. Three groups of differently adapted male subjects (power-trained, endurance-trained and untrained, 15 in each) performed the exercise that consisted of 10 trials of submaximal static contractions at the level of 40% of maximal voluntary contraction (MVC) force till exhaustion with the inter-trial rest intervals of 1 min. MVC force, reaction time and patellar reflex time components before and after the fatiguing exercise and following 5, 10 and 15 min of recovery were recorded. Endurance-trained athletes had a significantly longer holding times for all the 10 trials compared with power-trained athletes and untrained subjects. However, no significant differences in static endurance between power-trained athletes and untrained subjects were noted. The fatigue test significantly prolonged the time between onset of electrical and mechanical activity (electromechanical delay) in voluntary and reflex contractions. The electromechanical delay in voluntary contraction condition for power-trained and untrained subjects and in reflex condition for endurance-trained subjects had not recovered 15 min after cessation of exercise. No significant changes in the central component of visual reaction time (premotor time of MVC) and latency of patellar reflex were noted after fatiguing static exercise. It is concluded, that in this type of exercise the fatigue development may be largely owing to muscle contractile failure.

Kinetic properties of myosin heavy chain isoforms in single fibers from human skeletal muscle

The head portion of the myosin heavy chain is essential in force generation. As previously shown, Ca2+-activated muscle fibers from rat and rabbit display a strong correlation between their myosin heavy chain isoform composition and the kinetics of stretch activation, corresponding to an order of velocity: myosin heavy chain Ib > myosin heavy chain IId(x) > myosin heavy chain IIa >> myosin heavy chain I. Here, we show a similar correlation for human muscle fibers (myosin heavy chain IIb > myosin heavy chain IIa >> myosin heavy chain I), suggesting isoform-specific differences between the kinetics of force-generating power strokes. The kinetics of myosin heavy chain I are similar in human and rodents. This holds also true for myosin heavy chain IIa, but human myosin heavy chain IIb is slower than rodent myosin heavy chain IIb. It is similar to rodent myosin heavy chain IId(x).

Effect of a 17 day spaceflight on contractile properties of human soleus muscle fibres

1. Soleus biopsies were obtained from four male astronauts 45 days before and within 2 h after a 17 day spaceflight. 2. For all astronauts, single chemically skinned post-flight fibres expressing only type I myosin heavy chain (MHC) developed less average peak Ca2+ activated force (Po) during fixed-end contractions (0.78 +/- 0. 02 vs. 0.99 +/- 0.03 mN) and shortened at a greater mean velocity during unloaded contractions (Vo) (0.83 +/- 0.02 vs. 0.64 +/- 0.02 fibre lengths s-1) than pre-flight type I fibres. 3. The flight-induced decline in absolute Po was attributed to reductions in fibre diameter and/or Po per fibre cross-sectional area. Fibres from the astronaut who experienced the greatest relative loss of peak force also displayed a reduction in Ca2+ sensitivity. 4. The elevated Vo of the post-flight slow type I fibres could not be explained by alterations in myosin heavy or light chain composition. One alternative possibility is that the elevated Vo resulted from an increased myofilament lattice spacing. This hypothesis was supported by electron micrographic analysis demonstrating a reduction in thin filament density post-flight. 5. Post-flight fibres shortened at 30 % higher velocities than pre-flight fibres at external loads associated with peak power output. This increase in shortening velocity either reduced (2 astronauts) or prevented (2 astronauts) a post-flight loss in fibre absolute peak power (microN (fibre length) s-1). 6. The changes in soleus fibre diameter and function following spaceflight were similar to those observed after 17 days of bed rest. Although in-flight exercise countermeasures probably reduced the effects of microgravity, the results support the idea that ground-based bed rest can serve as a model of human spaceflight. 7. In conclusion, 17 days of spaceflight decreased force and increased shortening velocity of single Ca2+-activated muscle cells expressing type I MHC. The increase in shortening velocity greatly reduced the impact that impaired force production had on absolute peak power.

Regional differences in fibre type composition in the human temporalis muscle

Anatomical and electromyographic studies point to regional differences in function in the human temporalis muscle. During chewing and biting the anterior portions of the muscle are in general more intensively activated and they are capable of producing larger forces than the posterior portions. It was hypothetised that this heterogeneity in function is reflected in the fibre type composition of the muscle. The composition and surface area of different fibre types in various anteroposterior portions of the temporalis muscle were investigated in 7 cadavers employing immunohistochemistry with a panel of monoclonal antibodies against different isoforms of myosin heavy chain. Pure slow muscle fibres, type I, differed strongly in number across the muscle. In the most posterior portion of the muscle there were 24% type I fibres, in the intermediate portion 57%, and in the most anterior portion 46%. The mean fibre cross-sectional area (m-fcsa) of type I fibres was 1849 microm2, which did not differ significantly across the muscle. The proportion of pure fast muscle fibres, type IIA and IIX, remained more or less constant throughout the muscle at 13% and 11% respectively; their m-fcsa was 1309 microm2 and 1206 microm2, respectively, which did not differ significantly throughout the muscle. Pure type IIB fibres were not found. The relative proportion of hybrid fibres was 31% and did not differ significantly among the muscle portions. Fibre types I + IIA and cardiac alpha + I + IIA were the most abundant hybrid fibre types. In addition, 5% of the type I fibres had an additional myosin isoform which has only recently been described by means of electrophoresis and was named Ia. In the present study they were denoted as hybrid type I + Ia muscle fibres. It is concluded that intramuscular differences in type I fibre distribution are in accordance with regional differences in muscle function.

Bed rest increases the amount of mismatched fibers in human skeletal muscle

The effects of a 37-day period of bed rest on myosin heavy chain (MHC) expression on both mRNA and protein level in human skeletal muscle fibers were studied. Muscle biopsies from vastus lateralis muscle were obtained from seven healthy young male subjects before and after the bed-rest period. Combined in situ hybridization, immunocytochemistry, and ATPase histochemistry analysis of serial sections of the muscle biopsies demonstrated that fibers showing a mismatch between MHC isoforms at the mRNA and protein level increased significantly after the bed-rest period, suggesting an increase in the amount of muscle fibers in a transitional state. Accordingly, fibers showing a match in expression of MHC-1 and of MHC-2A at the mRNA and protein level decreased, whereas fibers showing a match between MHC-2X mRNA and protein increased after bed rest. Overall, there was an increase in fibers in a transitional state from phenotypic type 1 --> 2A and 2A --> 2X. Furthermore, a number of fibers with unusual MHC mRNA and isoprotein combinations were observed after bed rest (e.g., type 1 fibers with only mRNA for 2X and type 1 fibers negative for mRNA for MHC-beta/slow, 2A, and 2X). In contrast, no changes were revealed after an examination at the protein level alone. These data suggest that the reduced load-bearing activity imposed on the skeletal muscles through bed rest will alter MHC gene expression, resulting in combinations of mRNA and MHC isoforms normally not (or only rarely) observed in muscles subjected to load-bearing activity. On the other hand, the present data also show that 37 days of bed rest are not a sufficient stimulus to induce a similar change at the protein level, as was observed at the gene level.

Novel transitions in MHC isoforms: separate and combined effects of thyroid hormone and mechanical unloading

Single-fiber (n = 3,818 fibers) electrophoretic analyses were used to delineate the separate and combined effects of hyperthyroidism (T3) and hindlimb suspension (HS) on the myosin heavy chain (MHC) isoform composition (1-, 2-, and 4-wk time points) of the rat soleus muscle. The key findings of this study are as follows. First, T3 and HS both altered the distribution of MHC isoforms at the single-fiber level; however, the populations of fibers produced by these two interventions were clearly different from one another. Second, T3 + HS rapidly converted the soleus into a fast muscle, producing large increases in the relative contents of the fast type IIx and IIb MHC isoforms which were primarily expressed in several populations of hybrid fibers (e.g., types I/IIa/IIx, I/IIx/IIb, I/IIa/IIx/IIb). Finally, T3 + HS produced unique populations of hybrid fibers that did not adhere to the Ileft arrow over right arrow IIaleft arrow over right arrow IIxleft arrow over right arrow IIb sequential scheme of MHC plasticity. Collectively, the findings of this study demonstrate that the intervention of T3 + HS is a powerful model for manipulating and studying MHC isoform plasticity in slow skeletal muscle.

Sarcomere lesion damage occurs mainly in slow fibers of reloaded rat adductor longus muscles

Sarcomere lesions were previously observed with reloading of rat adductor longus muscles after spaceflight and hindlimb unloading (HU). Spaceflown rats displayed more lesioned fibers in the "slow-fiber" region, suggesting a damage-susceptible fiber type. Unloading induces fast myosin expression in some slow fibers, generating hybrid fibers. We examined whether lesion damage differed among slow-, hybrid-, and fast-fiber types in HU-reloaded adductor longus muscles. Temporal HU for 5, 8, 11, 14, and 17 days revealed that hybrid fiber percent, detected by antimyosin immunostaining, peaked at 29 +/- 12% by 14 days. A 14-day HU followed by 12-14 h of voluntary reloading was performed to induce lesions. chi2 analysis showed that slow fibers were preferentially damaged, accounting for 92 +/- 5% of lesioned fibers; hybrid and fast fibers accounted for 7 +/- 4 and <0.5%, respectively. Atrophy did not explain differential lesion damage across fiber types, as slow and hybrid fibers atrophied to a similar extent. Because active myofiber contractions are requisite for lesion formation, selective recruitment of slow fibers most likely explains their damage susceptibility.

Training effects on the contractile apparatus

Skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fibre types. Its dynamical nature is reflected by the ability to adapt to altered functional demands by qualitative alterations in fibre type composition. The molecular basis of this versatility is that specific myofibrillar and Ca2+-regulatory protein isoforms are assembled to functionally specialized fibre types. Based on this diversity, adult muscle fibres are capable of changing their molecular composition by altered gene expression. Myosin heavy chain (MHC) isoforms and their unique expression in 'pure' fibres, as well as their coexpression in 'hybrid' 'fibres' represent the best markers of muscle fibre diversity and adaptive changes. Chronic low-frequency stimulation (CLFS) and endurance training represent highly suitable models for studying the effects of increased neuromuscular activity on myofibrillar protein isoform expression and fibre type composition. Generally, both models induce fast-to-slow transitions in myofibrillar protein isoforms and fibre types. However, the responses to endurance training are quantitatively less pronounced than those in muscles exposed to CLFS. Parallel changes in isoforms of specific myofibrillar or Ca2+-regulatory proteins during the induced fast-to-slow transitions point to the existence of fibre type-specific patterns of gene expression. The fast-to-slow transitions do not proceed in abrupt jumps from one extreme to the other, but occur in a gradual and orderly sequential manner. Depending on the basal protein isoform profile, and hence the position within the fast-slow spectrum, the adaptive ranges of different fibre types vary. However, adaptive ranges not only depend on a particular fibre type, but also are influenced by species-specific properties.