Effect of different troponin T-tropomyosin combinations on thin filament activation

Troponin Variants as Markers of Skeletal Muscle Health and Diseases

Ca2 +-regulated contractility is a key determinant of the quality of muscles. The sarcomeric myofilament proteins are essential players in the contraction of striated muscles. The troponin complex in the actin thin filaments plays a central role in the Ca2+-regulation of muscle contraction and relaxation. Among the three subunits of troponin, the Ca2+-binding subunit troponin C (TnC) is a member of the calmodulin super family whereas troponin I (TnI, the inhibitory subunit) and troponin T (TnT, the tropomyosin-binding and thin filament anchoring subunit) are striated muscle-specific regulatory proteins. Muscle type-specific isoforms of troponin subunits are expressed in fast and slow twitch fibers and are regulated during development and aging, and in adaptation to exercise or disuse. TnT also evolved with various alternative splice forms as an added capacity of muscle functional diversity. Mutations of troponin subunits cause myopathies. Owing to their physiological and pathological importance, troponin variants can be used as specific markers to define muscle quality. In this focused review, we will explore the use of troponin variants as markers for the fiber contents, developmental and differentiation states, contractile functions, and physiological or pathophysiological adaptations of skeletal muscle. As protein structure defines function, profile of troponin variants illustrates how changes at the myofilament level confer functional qualities at the fiber level. Moreover, understanding of the role of troponin modifications and mutants in determining muscle contractility in age-related decline of muscle function and in myopathies informs an approach to improve human health.

Parasitic gut infection in Libellula pulchella causes functional and molecular resemblance of dragonfly flight muscle to skeletal muscle of obese vertebrates

We previously demonstrated the existence of a naturally occurring metabolic disease phenotype in Libellula pulchella dragonflies that shows high similarity to vertebrate obesity and type II diabetes, and is caused by a protozoan gut parasite. To further mechanistic understanding of how this metabolic disease phenotype affects fitness of male L. pulchella in vivo, we examined infection effects on in situ muscle performance and molecular traits relevant to dragonfly flight performance in nature. Importantly, these traits were previously shown to be affected in obese vertebrates. Similarly to obesity effects in rat skeletal muscle, dragonfly gut infection caused a disruption of relationships between body mass, flight muscle power output and alternative pre-mRNA splicing of troponin T, which affects muscle calcium sensitivity and performance in insects and vertebrates. In addition, when simulated in situ to contract at cycle frequencies ranging from 20 to 45 Hz, flight muscles of infected individuals displayed a left shift in power-cycle frequency curves, indicating a significant reduction in their optimal cycle frequency. Interestingly, these power-cycle curves were similar to those produced by flight muscles of non-infected teneral (i.e. physiologically immature) adult L. pulchella males. Overall, our results indicate that the effects of metabolic disease on skeletal muscle physiology in natural insect systems are similar to those observed in vertebrates maintained in laboratory settings. More generally, they indicate that study of natural, host-parasite interactions can contribute important insight into how environmental factors other than diet and exercise may contribute to the development of metabolic disease phenotypes.

Effects of age and hindlimb immobilization and remobilization on fast troponin T precursor mRNA alternative splicing in rat gastrocnemius muscle

Fast skeletal muscle troponin T (TNNT3) is an important component of the skeletal muscle contractile machinery. The precursor mRNA (pre-mRNA) encoding TNNT3 is alternatively spliced, and changes in the pattern of TNNT3 splice form expression are associated with alterations in thin-filament calcium sensitivity and force production during muscle contraction and thereby regulate muscle function. Interestingly, during aging, the muscle force/cross-sectional area is reduced, suggesting that loss of mass does not completely account for the impaired muscle function that develops during the aging process. Therefore, in this study, we tested the hypothesis that age and changes in muscle loading are associated with alterations in Tnnt3 alternative splicing in the rat gastrocnemius muscle. We found that the relative abundance of several Tnnt3 splice forms varied significantly with age among 2-, 9-, and 18-month-old rats and that the pattern correlated with changes in body mass rather than muscle mass. Hindlimb immobilization for 7 days resulted in dramatic alterations in splice form relative abundance such that the pattern was similar to that observed in lighter animals. Remobilization for 7 days restored the splicing pattern toward that observed in the nonimmobilized limb, even though muscle mass had not yet begun to recover. In conclusion, the results suggest that Tnnt3 pre-mRNA alternative splicing is modulated rapidly (i.e., within days) in response to changes in the load placed on the muscle. Moreover, the results show that restoration of Tnnt3 alternative splicing to control patterns is initiated prior to an increase in muscle mass.

Human slow troponin T (TNNT1) pre-mRNA alternative splicing is an indicator of skeletal muscle response to resistance exercise in older adults

Slow skeletal muscle troponin T (TNNT1) pre-messenger RNA alternative splicing (AS) provides transcript diversity and increases the variety of proteins the gene encodes. Here, we identified three major TNNT1 splicing patterns (AS1-3), quantified their expression in the vastus lateralis muscle of older adults, and demonstrated that resistance training modifies their relative abundance; specifically, upregulating AS1 and downregulating AS2 and AS3. In addition, abundance of TNNT1 AS2 correlated negatively with single muscle fiber-specific force after resistance training, while abundance of AS1 correlated negatively with V max. We propose that TNNT1 AS1, AS2 and the AS1/AS2 ratio are potential quantitative biomarkers of skeletal muscle adaptation to resistance training in older adults, and that their profile reflects enhanced single fiber muscle force in the absence of significant increases in fiber cross-sectional area.

The functional properties of human slow skeletal troponin T isoforms in cardiac muscle regulation

Human slow skeletal troponin T (HSSTnT) shares a high degree of homology with cardiac TnT (CTnT). Although the presence of HSSTnT has not been confirmed in the heart at the protein level, detectable levels of HSSTnT mRNA have been found. Whether HSSTnT isoforms are expressed transiently remains unknown. Because transient re-expression of HSSTnT may be a potential mechanism of regulating function, we explored the effect of HSSTnT on the regulation of cardiac muscle. At least three HSSTnT isoforms have been found to exist in slow skeletal muscle: HSSTnT1 (+exons 5 and 12), HSSTnT2 (+exon 5, -exon 12), and HSSTnT3 (-exons 5 and 12). Another isoform, HSSTnT hypothetical (Hyp) (-exon 5, +exon 12), has only been found at the mRNA level. Compared with HCTnT3 (adult isoform), Tn complexes containing HSSTnT1, -2, and -3 did not alter the actomyosin ATPase activation and inhibition in the presence and absence of Ca(2+), respectively. HSSTnTHyp was not evaluated as it did not form a Tn complex under a variety of conditions. Porcine papillary skinned fibers displaced with HSSTnT1, -2, or -3 and reconstituted with human cardiac troponin I and troponin C (HCTnI·TnC) complex showed a decrease in the Ca(2+) sensitivity of force development and an increase in maximal recovered force (HSSTnT1 and -3) compared with HCTnT3. In contrast, HSSTnTHyp showed an increase in the Ca(2+) sensitivity of force development. This suggests that re- or overexpression of specific SSTnT isoforms might have therapeutic potential in the failing heart because they increase the maximal force of contraction. In addition, circular dichroism and proteolytic digestion experiments revealed structural differences between HSSTnT isoforms and HCTnT3 and that HSSTnT1 is more susceptible to calpain and trypsin proteolysis than the other HSSTnTs. Overall, HSSTnT isoforms despite being homologues of CTnT may display distinct functional properties in muscle regulation.

Cell-autonomous regulation of fast troponin T pre-mRNA alternative splicing in response to mechanical stretch

How mechanochemical signals induced by the amount of weight borne by the skeletal musculature are translated into modifications to muscle sarcomeres is poorly understood. Our laboratory recently demonstrated that, in response to experimentally induced increases in the weight load borne by a rat, alternative splicing of the fast skeletal muscle troponin T (Tnnt3) pre-mRNA in gastrocnemius was adjusted in a correlated fashion with the amount of added weight. (Schilder RJ, Kimball SR, Marden JH, Jefferson LS. J Exp Biol 214: 1523-1532, 2011). Thus muscle load is perceived quantitatively by the body, and mechanisms that sense it appear to control processes that generate muscle sarcomere composition plasticity, such as alternative pre-mRNA splicing. Here we demonstrate how mechanical stretch (see earlier comment) of C2C12 muscle cells in culture results in changes to Tnnt3 pre-mRNA alternative splicing that are qualitatively similar to those observed in response to added weight in rats. Moreover, inhibition of Akt signaling, but not that of ERK1/2, prevents the stretch-induced effect on Tnnt3 pre-mRNA alternative splicing. These findings suggest that effects of muscle load on Tnnt3 pre-mRNA alternative splicing are controlled by a cell-autonomous mechanism, rather than systemically. They also indicate that, in addition to its regulatory role in protein synthesis and muscle mass plasticity, Akt signaling may regulate muscle sarcomere composition by modulating alternative splicing events in response to load. Manipulation of Tnnt3 pre-mRNA alternative splicing by mechanical stretch of cells in culture provides a model to investigate the biology of weight sensing by skeletal muscles and facilitates identification of mechanisms through which skeletal muscles match their performance and experienced load.

Contribution of stretch to the change of activation properties of muscle fibers in the diaphragm at the transition from fetal to neonatal life

The transition from fetal to postnatal life involves clearance of liquid from the lung and airways, and rapid formation of a functional residual capacity. Despite the importance of the diaphragm in this process, the impact of birth on the mechanical and functional activity of its muscle fibers is not known. This study determined the contractile characteristics of individual “skinned” diaphragm fibers from 70 days (0.47) gestation to after birth in sheep. Based on differential sensitivity to the divalent ions calcium (Ca²⁺) and strontium (Sr²⁺), all fibers in the fetal diaphragm were classified as “fast,” whereas fibers from the adult sheep diaphragm exhibited a “hybrid” phenotype where both “fast” and “slow” characteristics were present within each single fiber. Transition to the hybrid phenotype occurred at birth, was evident after only 40 min of spontaneous breathing, and could be induced by simple mechanical stretch of diaphragm fibers from near-term fetuses (∼147 days gestation). Both physical stretch of isolated fibers, and mechanical ventilation of the fetal diaphragm in situ, significantly increased sensitivity to Ca²⁺ and Sr²⁺, maximum force generating capacity, and decreased passive tension in near-term and preterm fetuses; however, only fibers from near-term fetuses showed a complete transition to a “hybrid” activation profile. These findings suggest that stretch associated with the transition from a liquid to air-filled lung at birth induces physical changes of proteins determining the activation and elastic properties of the diaphragm. These changes may allow the diaphragm to meet the increased mechanical demands of breathing immediately after birth.

Body weight-dependent troponin T alternative splicing is evolutionarily conserved from insects to mammals and is partially impaired in skeletal muscle of obese rats

Do animals know at a physiological level how much they weigh, and, if so, do they make homeostatic adjustments in response to changes in body weight? Skeletal muscle is a likely tissue for such plasticity, as weight-bearing muscles receive mechanical feedback regarding body weight and consume ATP in order to generate forces sufficient to counteract gravity. Using rats, we examined how variation in body weight affected alternative splicing of fast skeletal muscle troponin T (Tnnt3), a component of the thin filament that regulates the actin-myosin interaction during contraction and modulates force output. In response to normal growth and experimental body weight increases, alternative splicing of Tnnt3 in rat gastrocnemius muscle was adjusted in a quantitative fashion. The response depended on weight per se, as externally attached loads had the same effect as an equal change in actual body weight. Examining the association between Tnnt3 alternative splicing and ATP consumption rate, we found that the Tnnt3 splice form profile had a significant association with nocturnal energy expenditure, independently of effects of weight. For a subset of the Tnnt3 splice forms, obese Zucker rats failed to make the same adjustments; that is, they did not show the same relationship between body weight and the relative abundance of five Tnnt3 β splice forms (i.e. Tnnt3 β2-β5 and β8), four of which showed significant effects on nocturnal energy expenditure in Sprague-Dawley rats. Heavier obese Zucker rats displayed certain splice form relative abundances (e.g. Tnnt3 β3) characteristic of much lighter, lean animals, resulting in a mismatch between body weight and muscle molecular composition. Consequently, we suggest that body weight-inappropriate skeletal muscle Tnnt3 expression in obesity is a candidate mechanism for muscle weakness and reduced mobility. Weight-dependent quantitative variation in Tnnt3 alternative splicing appears to be an evolutionarily conserved feature of skeletal muscle and provides a quantitative molecular marker to track how an animal perceives and responds to body weight.

Patterns of tropomyosin and troponin-T isoform expression in jaw-closing muscles of mammals and reptiles that express masticatory myosin

We recently reported that masticatory ('superfast') myosin is expressed in jaw-closing muscles of some rodent species. Most mammalian limb muscle fibers express tropomyosin-β (Tm-β), along with fast-type or slow-type tropomyosin-β (Tm-β), but jaw-closing muscle fibers in members of Carnivora express a unique isoform of Tm [Tm-masticatory (Tm-M)] and little or no Tm-β. The goal of this study was to determine patterns of Tm and troponin-T (TnT) isoform expression in the jaw-closing muscles of rodents and other vertebrate species that express masticatory myosin, and compare the results to those from members of Carnivora. Comparisons of electrophoretic mobility, immunoblotting and mass spectrometry were used to probe the Tm and fast-type TnT isoform composition of jaw-closing and limb muscles of six species of Carnivora, eight species of Rodentia, five species of Marsupialia, big brown bat, long-tailed macaque and six species of Reptilia. Extensive heterogeneity exists in Tm and TnT isoform expression in jaw-closing muscles between phylogenetic groups, but there are fairly consistent patterns within each group. We propose that the differences in Tm and TnT isoform expression patterns between phylogenetic groups, which share the expression of masticatory myosin, may impart fundamental differences in thin-filament-mediated muscle activation to accommodate markedly different feeding styles that may require high force generation in some species (e.g. many members of Carnivora) and high speed in others (e.g. Rodentia).

Mapping of O-linked beta-N-acetylglucosamine modification sites in key contractile proteins of rat skeletal muscle

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a widespread modification of serine/threonine residues of nucleocytoplasmic proteins. Recently, several key contractile proteins in rat skeletal muscle (i.e., myosin heavy and light chains and actin) were identified as O-GlcNAc modified. Moreover, it was demonstrated that O-GlcNAc moieties involved in contractile protein interactions could modulate Ca(2+) activation parameters of contraction. In order to better understand how O-GlcNAc can modulate the contractile activity of muscle fibers, we decided to identify the sites of O-GlcNAc modification in purified contractile protein homogenates. Using an MS-based method that relies on mild beta-elimination followed by Michael addition of DTT (BEMAD), we determined the localization of one O-GlcNAc site in the subdomain four of actin and four O-GlcNAc sites in the light meromyosin region of myosin heavy chains (MHC). According to previous reports concerning the role of these regions, our data suggest that O-GlcNAc sites might modulate the actin-tropomyosin interaction, and be involved in MHC polymerization or interactions between MHC and other contractile proteins. Thus, the results suggest that this PTM might be involved in protein-protein interactions but could also modulate the contractile properties of skeletal muscle.

Altered fast- and slow-twitch muscle fibre characteristics in female mice with a (S248F) knock-in mutation of the brain neuronal nicotinic acetylcholine receptor

We generated a mouse line with a missense mutation (S248F) in the gene (CHRNA4) encoding the alpha4 subunit of neuronal nicotinic acetylcholine receptor (nAChR). Mutant mice demonstrate brief nicotine induced dystonia that resembles the clinical events seen in patients with the same mutation. Drug-induced dystonia is more pronounced in female mice, thus our aim was to determine if the S248F mutation changed the properties of fast- and slow-twitch muscle fibres from female mutant mice. Reverse transcriptase-PCR confirmed CHRNA4 gene expression in the brain but not skeletal muscles in normal and mutant mice. Ca(2+) and Sr(2+) force activation curves were obtained using skinned muscle fibres prepared from slow-twitch (soleus) and fast-twitch (EDL) muscles. Two significant results were found: (1) the (pCa(50) - pSr(50)) value from EDL fibres was smaller in mutant mice than in wild type (1.01 vs. 1.30), (2) the percentage force produced at pSr 5.5 was larger in mutants than in wild type (5.76 vs. 0.24%). Both results indicate a shift to slow-twitch characteristics in the mutant. This conclusion is supported by the identification of the myosin heavy chain (MHC) isoforms. Mutant EDL fibres expressed MHC I (usually only found in slow-twitch fibres) as well as MHC IIa. Despite the lack of spontaneous dystonic events, our findings suggest that mutant mice may be having subclinical events or the mutation results in a chronic alteration to muscle neural input.

Phenotypical transitions and Ca2+activation properties in human muscle fibers: effects of a 60-day bed rest and countermeasures

Muscle biopsies were taken from soleus and vastus lateralis before and after a 60-day bed rest (BR) to examine expression changes in the regulatory proteins of the thin filament and in contractile function. Twenty-four women separated in three groups were submitted to BR or a combined protocol of resistance and aerobic exercises during BR or received a supplementation of amino acids during BR. Ca2+-tension relationships were established in single skinned fibers identified by their myosin heavy chain and troponin C isoform expressions. Expression patterns of regulatory proteins were analyzed on muscle pieces. For both muscles, BR produced similar decreases in slow and fast fiber diameters but larger decreases in P0maximal forces in slow than in fast fibers. Specific forces were decreased in slow soleus and vastus fibers, which displayed a reduction in Ca2+affinity. These changes were accompanied by slow-to-fast transitions in regulatory proteins, with troponins C and T appearing as sensitive markers of unloading. Exercises prevented the changes in fiber diameters and forces and counteracted most of the slow-to-fast transitions. The nutrition program had a morphological beneficial effect on slow fibers. However, these fibers still presented decreases in specific P0after BR. Phenotypical transitions due to BR were not prevented by amino acids. Finally, in vastus lateralis muscle, BR induced a decrease in O-glycosylation level that was prevented by exercise and attenuated by nutrition. In conclusion, this study has addressed for the first time in women the respective efficiencies of two countermeasures associated with BR on muscle properties and regulatory protein expression.

Defective regulation of contractile function in muscle fibres carrying an E41K beta-tropomyosin mutation

A novel E41K beta-tropomyosin (beta-Tm) mutation, associated with congenital myopathy and muscle weakness, was recently identified in a woman and her daughter. In both patients, muscle weakness was coupled with muscle fibre atrophy. It remains unknown, however, whether the E41K beta-Tm mutation directly affects regulation of muscle contraction, contributing to the muscle weakness. To address this question, we studied a broad range of contractile characteristics in skinned muscle fibres from the two patients and eight healthy controls. Results showed decreases (i) in speed of contraction at saturated Ca(2+) concentration (apparent rate constant of force redevelopment (k(tr)) and unloaded shortening speed (V(0))); and (ii) in contraction sensitivity to Ca(2+) concentration, in fibres from patients compared with controls, suggesting that the mutation has a negative effect on contractile function, contributing to the muscle weakness. To investigate whether these negative impacts are reversible, we exposed skinned muscle fibres to the Ca(2+) sensitizer EMD 57033. In fibres from patients, 30 mum of EMD 57033 (i) had no effect on speed of contraction (k(tr) and V(0)) at saturated Ca(2+) concentration but (ii) increased Ca(2+) sensitivity of contraction, suggesting a potential therapeutic approach in patients carrying the E41K beta-Tm mutation.

Muscle specialization in the squid motor system

Although muscle specialization has been studied extensively in vertebrates, less is known about the mechanisms that have evolved in invertebrate muscle that modulate muscle performance. Recent research on the musculature of squid suggests that the mechanisms of muscle specialization in cephalopods may differ from those documented in vertebrates. Muscle diversity in the development and the evolution of cephalopods appears to be characterized by modulation of the dimensions of the myofilaments, in contrast to the relatively fixed myofilament dimensions of vertebrate muscle. In addition, the arrangement of the myofilaments may also be altered, as has been observed in the extensor muscle fibres of the prey capture tentacles of squid and cuttlefish, which show cross-striation and thus differ from the obliquely striated pattern of most cephalopod locomotor muscle fibres. Although biochemical specializations that reflect differences in aerobic capacity have been documented previously for specific layers of the mantle muscle of squid, comparison of protein profiles of myofilament preparations from the fast cross-striated tentacle fibres and slow obliquely striated fibres from the arms has revealed remarkably few differences in myofilament lattice proteins. In particular, previous studies using a variety of SDS-PAGE techniques and peptide mapping of the myosin heavy chain were unable to resolve differences in the myosin light and heavy chains. Since these techniques cannot exclude the presence of a highly conserved variant that differs in only a few amino acids, in this study semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) analysis of myosin heavy chain messenger RNAs (mRNAs) from the cross-striated tentacle and obliquely striated arm muscle fibres was conducted. This analysis showed that a previously reported alternatively spliced isoform of the squid myosin motor domain is present only in low abundance in both muscle types and therefore differential expression of the two myosins cannot explain the difference in contractile properties. It thus appears that modulation of the contractile properties of the musculature of squid and other cephalopods occurs primarily through variation in the arrangement and dimensions of the myofilaments.

Co-expression of skeletal and cardiac troponin T decreases mouse cardiac function

In contrast to skeletal muscles that simultaneously express multiple troponin T (TnT) isoforms, normal adult human cardiac muscle contains a single isoform of cardiac TnT. To understand the significance of myocardial TnT homogeneity, we examined the effect of TnT heterogeneity on heart function. Transgenic mouse hearts overexpressing a fast skeletal muscle TnT together with the endogenous cardiac TnT was investigated in vivo and ex vivo as an experimental system of concurrent presence of two classes of TnT in the adult cardiac muscle. This model of myocardial TnT heterogeneity produced pathogenic phenotypes: echocardiograph imaging detected age-progressive reductions of cardiac function; in vivo left ventricular pressure analysis showed decreased myocardial contractility; ex vivo analysis of isolated working heart preparations confirmed an intrinsic decrease of cardiac function in the absence of neurohumoral influence. The transgenic mice also showed chronic myocardial hypertrophy and degeneration. The dominantly negative effects of introducing a fast TnT into the cardiac thin filaments to produce two classes of Ca(2+) regulatory units in the adult myocardium suggest that TnT heterogeneity decreases contractile function by disrupting the synchronized action during ventricular contraction that is normally activated as an electrophysiological syncytium.

Effects of a R133W beta-tropomyosin mutation on regulation of muscle contraction in single human muscle fibres

A novel R133W beta-tropomyosin (beta-Tm) mutation, associated with muscle weakness and distal limb deformities, has recently been identified in a woman and her daughter. The muscle weakness was not accompanied by progressive muscle wasting or histopathological abnormalities in tibialis anterior muscle biopsy specimens. The aim of the present study was to explore the mechanisms underlying the impaired muscle function in patients with the beta-Tm mutation. Maximum force normalized to fibre cross-sectional area (specific force, SF), maximum velocity of unloaded shortening (V0), apparent rate constant of force redevelopment (ktr) and force-pCa relationship were evaluated in single chemically skinned muscle fibres from the two patients carrying the beta-Tm mutation and from healthy control subjects. Significant differences in regulation of muscle contraction were observed in the type I fibres: a lower SF (P<0.05) and ktr (P<0.01), and a faster V0 (P<0.05). The force-pCa relationship did not differ between patient and control fibres, indicating an unaltered Ca2+ activation of contractile proteins. Collectively, these results indicate a slower cross-bridge attachment rate and a faster detachment rate caused by the R133W beta-Tm mutation. It is suggested that the R133W beta-Tm mutation induces alteration in myosin-actin kinetics causing a reduced number of myosin molecules in the strong actin-binding state, resulting in overall muscle weakness in the absence of muscle wasting.

Isoform switching in myofibrillar and excitation-contraction coupling proteins contributes to diminished contractile function in regenerating rat soleus muscle

Postnatal development of skeletal muscle occurs through the progressive transformation of diverse biochemical, metabolic, morphological, and functional characteristics from the embryonic to the adult phenotype. Since muscle regeneration recapitulates postnatal development of muscle fiber, it offers an appropriate experimental model to investigate the existing relationships between diverse muscle functions and the expression of key protein isoforms, particularly at the single-fiber level. This study was carried out in regenerating soleus muscle 14 days after injury. At this intermediate stage, the regenerating muscle exhibited a recovery of mass greater than its force generation capacity. The lower specific tension of regenerating muscle suggested intrinsic defective excitation-contraction coupling and/or contractility processes. The presence of developmental isoforms of both the voltage-gated Ca2+channel (α1C) and of ryanodine receptor 3, paralleled by an abnormal caffeine contracture development, confirms the immature excitation-contraction coupling of the regenerating muscle. The defective Ca2+handling could also be confirmed by the lower sarcoplasmic reticulum caffeine sensitivity of regenerating single fibers. Also, regenerating single fibers revealed a lower maximal specific tension, which was associated with the residual presence of embryonic myosin heavy chains. Moreover, the fibers showed a reduced Ca2+sensitivity of myofibrillar proteins, particularly those simultaneously expressing the slow and fast isoforms of troponin C. The present results indicate that the expression of developmental proteins determines the incomplete functional recovery of regenerating soleus.

Muscle protein expression of pikeperches (Stizostedion lucioperca and S. volgense)

The purpose of the present study was to investigate the muscle protein expression in two pikeperches (Stizostedion lucioperca and S. volgense) through intra- and intermyomeric composition of white muscles. Using denaturing 10% sodium dodecylsulfate-polyacrylamide gel electrophoresis, muscle protein expression was studied in relation to within- and between-species morphological development, sex, maturity and age of pikeperches. Myosin, actin and troponin have a distinct role in the contraction and length tension of muscle fibers of these species. No obvious intramyomeric differences were found in the myosin heavy chain of both species. Myosin light chains (15-38 kDa) have different expression in different age groups. The muscle protein of the fingerling and adult S. lucioperca had high molecular weight (50 kDa) myosin in contrast to the other Percid species. The molecular weight of actins increased comparatively in low-age-group fish. ATP is stored in myosin and released to cause contraction when myosin comes in contact with actin of the experimental fish. Troponin regulates increasing concentration of light-chain myosin in mature fish. Because troponin T has been implicated in the regulation of skeletal muscle kinetics, muscle contraction kinetics was predicted in different age groups. The muscle proteins of both sexes of these species have polymorphism in various age groups but have no difference in similar aged fish. No muscle protein dimorphism was found in these Percid species. The white muscle protein composition and contractile properties affect power production during fast, unsteady movement and swimming.

Role of the fetal and alpha/beta exons in the function of fast skeletal troponin T isoforms: correlation with altered Ca2+ regulation associated with development

In mammalian fast skeletal muscle, constitutive and alternative splicing from a single troponin T (TnT) gene produce multiple developmentally regulated and tissue specific TnT isoforms. Two exons, alpha (exon 16) and beta (exon 17), located near the 3' end of the gene and coding for two different 14 amino acid residue peptides are spliced in a mutually exclusive manner giving rise to the adult TnTalpha and the fetal TnTbeta isoforms. In addition, an acidic peptide coded by a fetal (f) exon located between exons 8 and 9 near the 5' end of the gene, is specifically present in TnTbeta and absent in the adult isoforms. To define the functional role of the f and alpha/beta exons, we constructed combinations of TnT cDNAs from a single human fetal fast skeletal TnTbeta cDNA clone in order to circumvent the problem of N-terminal sequence heterogeneity present in wild-type TnT isoforms, irrespective of the stage of development. Nucleotide sequences of these constructs, viz. TnTalpha, TnTalpha + f, TnTbeta - f and TnTbeta are identical, except for the presence or absence of the alpha or beta and f exons. Our results, using the recombinant TnT isoforms in different functional in vitro assays, show that the presence of the f peptide in the N-terminal T1 region of TnT, has a strong inhibitory effect on binary interactions between TnT and other thin filament proteins, TnI, TnC and Tm. The presence of the f peptide led to reduced Ca2+-dependent ATPase activity in a reconstituted thin filament, whereas the contribution of the alpha and beta peptides in the biological activity of TnT was primarily modulatory. These results indicate that the f peptide confers an inhibitory effect on the biological function of fast skeletal TnT and this can be correlated with changes in the Ca2+ regulation associated with development in fast skeletal muscle.

Electrophoretic and functional identification of two troponin C isoforms in toad skeletal muscle fibers

The differential sensitivity of frog twitch and slow-tonic fibers to Ca(2+) and Sr(2+) suggests that these two fiber types express different troponin C (TnC) isoforms. To date, only one TnC isoform from anurans (resembling the mammalian fast-twitch isoform) has been isolated and characterized. In this study, we examined the possibility that anuran striated muscle contains more than one TnC isoform. Toward this end, we determined the TnC isoform composition of 198 single fibers from the rectus abdominis of the cane toad (a mixed slow-tonic and twitch muscle) and of toad cardiac muscle using a method that enables the identification of TnC isoforms on the basis of the effect of Ca(2+) on their electrophoretic mobility. The fibers were typed according to their myosin heavy chain (MHC) isoform composition. The data indicate that striated muscle of the cane toad contains two TnC isoforms, one of which (TnC-t) is present in all fibers displaying only twitch MHC isoforms and the other of which (TnC-T/c) is present in fibers displaying the tonic MHC isoform and in cardiac muscle. For a subpopulation of 15 fibers, the TnC isoform composition was also compared with Ca(2+) and Sr(2+) activation characteristics. Fibers containing the TnC-T/c isoform were approximately 3-fold more sensitive to Ca(2+), approximately 40-fold more sensitive to Sr(2+), and responded to a approximately 4.6-fold broader range of [Ca(2+)] than did fibers containing the TnC-t isoform. The Ca(2+) activation properties of toad fibers containing the TnC-T/c isoform appear to be consistent with the previously reported physiological characteristics of amphibian slow-tonic muscle fibers.

Expression and functional properties of four slow skeletal troponin T isoforms in rat muscles

We investigated the expression and functional properties of slow skeletal troponin T (sTnT) isoforms in rat skeletal muscles. Four sTnT cDNAs were cloned from the slow soleus muscle. Three isoforms were found to be similar to sTnT1, sTnT2, and sTnT3 isoforms described in mouse muscles. A new rat isoform, with a molecular weight slightly higher than that of sTnT3, was discovered. This fourth isoform had never been detected previously in any skeletal muscle and was therefore called sTnTx. From both expression pattern and functional measurements, it appears that sTnT isoforms can be separated into two classes, high-molecular-weight (sTnT1, sTnT2) and low-molecular-weight (sTnTx, sTnT3) isoforms. By comparison to the apparent migration pattern of the four recombinant sTnT isoforms, the newly described low-molecular-weight sTnTx isoform appeared predominantly and typically expressed in fast skeletal muscles, whereas the higher-molecular-weight isoforms were more abundant in slow soleus muscle. The relative proportion of the sTnT isoforms in the soleus was not modified after exposure to hindlimb unloading (HU), known to induce a functional atrophy and a slow-to-fast isoform transition of several myofibrillar proteins. Functional data gathered from replacement of endogenous troponin complexes in skinned muscle fibers showed that the sTnT isoforms modified the Ca2+activation characteristics of single skeletal muscle fibers, with sTnT2 and sTnT1 conferring a similar increase in Ca2+affinity higher than that caused by low-molecular-weight isoforms sTnTx and sTnT3. Thus we show for the first time the presence of sTnT in fast muscle fibers, and our data show that the changes in neuromuscular activity on HU are insufficient to alter the sTnT expression pattern.

Alterations in the proportions of skeletal muscle proteins following a unilateral lesion to the substantia nigra pars compacta of rats

It is well established that mammalian skeletal muscles exhibit a considerable degree of plasticity and one of the main determining factors of this plasticity is the activity pattern and duration of motoneurone discharge. Lesions to the right substantia nigra pars compacta (SNpc) of six adult rats were made to determine whether altered output from the SNpc ultimately leads to a change in the expression of proteins in contralateral skeletal muscles. After 4 months, altered motor performance was identified by the administration of amphetamine. After 7 months, 30-70% of dopaminergic cells in the SNpc had been destroyed. The protein content of muscles was then quantified from densitometric scans of gels, and expressed as a % of the amount of actin (the protein used as a reference in this study). The lesion affected the expression of different protein isoforms in the fast- and slow-twitch muscles. In slow-twitch soleus muscles, the lesion decreased the proportion of alpha-tropomyosin and increased the proportion of beta-tropomyosin. In the fast-twitch extensor digitorum longus muscles, the lesion increased the proportion of the fast isoform of troponin-T1f, and decreased the proportions of the two isoforms of myosin light chain. This study establishes a connection between the chronic effects of a lesion to the SNpc, with a loss of dopaminergic neurones, impaired motor performance, and altered expression of proteins in skeletal muscle. The implication of these results is that the altered motor function observed in Parkinson's disease may be associated with alterations to the expression of skeletal muscle proteins.

Troponin T expression in trout red muscle correlates with muscle activation

Red or aerobic muscle from the anterior of rainbow trout Oncorhynchus mykiss activates (generates force) more quickly than that from the posterior. TnT is a component of the troponin complex that modulates muscle activation once Ca2+ is bound. Since trout express at least two forms of TnT in their red muscle (S1 and S2), the differential expression of these two forms was predicted to explain variations in contractile properties. TnT isoforms from trout muscle were identified through hydroxy-apatite chromatography of purified myofibrillar proteins followed by SDS-PAGE. Western blots employing a mammalian anti-troponin T monoclonal antibody were used to identify TnT isoforms. The relative expression of the two isoforms of TnT was then examined at seven longitudinal positions from each of three fish using SDS-PAGE and densitometry on the silver-stained TnT bands. A significant shift in expression was detected from anterior to posterior in all three fish with TnT S1 becoming more dominant in the posterior red muscle. As predicted, a shift in TnT expression was associated with the decrease in activation rate along the length of the fish. This study was then extended to include a different species of salmonid, brook trout Salvelinus fontinalis, to explore the generality of TnT modulation of muscle activation. Muscle contractile properties were determined from anterior and posterior muscle, and relative expression of S1 and S2 was determined. Unlike rainbow trout, there is no consistent longitudinal pattern of muscle activation in brook trout:some fish have kinetically faster muscle in the anterior, some in the posterior. Similarly, there is no consistent pattern of TnT expression. Individual analysis of muscle activation and TnT expression in brook trout provides insight into the role of TnT in modulating muscle activation in slow fish muscle.

Cardiac Troponin T Isoforms Affect the Ca2+ Sensitivity of Force Development in the Presence of Slow Skeletal Troponin I

In this study we investigated the physiological role of the cardiac troponin T (cTnT) isoforms in the presence of human slow skeletal troponin I (ssTnI). ssTnI is the main troponin I isoform in the fetal human heart. In reconstituted fibers containing the cTnT isoforms in the presence of ssTnI, cTnT1-containing fibers showed increased Ca(2+) sensitivity of force development compared with cTnT3- and cTnT4-containing fibers. The maximal force in reconstituted skinned fibers was significantly greater for the cTnT1 (predominant fetal cTnT isoform) when compared with cTnT3 (adult TnT isoform) in the presence of ssTnI. Troponin (Tn) complexes containing ssTnI and reconstituted with cTnT isoforms all yielded different maximal actomyosin ATPase activities. Tn complexes containing cTnT1 and cTnT4 (both fetal isoforms) had a reduced ability to inhibit actomyosin ATPase activity when compared with cTnT3 (adult isoform) in the presence of ssTnI. The rate at which Ca(2+) was released from site II of cTnC in the cTnI.cTnC complex (122/s) was 12.5-fold faster than for the ssTnI.cTnC complex (9.8/s). Addition of cTnT3 to the cTnI.cTnC complex resulted in a 3.6-fold decrease in the Ca(2+) dissociation rate from site II of cTnC. Addition of cTnT3 to the ssTnI.cTnC complex resulted in a 1.9-fold increase in the Ca(2+) dissociation rate from site II of cTnC. The rate at which Ca(2+) dissociated from site II of cTnC in Tn complexes also depended on the cTnT isoform present. However, the TnI isoforms had greater effects on the Ca(2+) dissociation rate of site II than the cTnT isoforms. These results suggest that the different N-terminal TnT isoforms would produce distinct functional properties in the presence of ssTnI when compared with cTnI and that each isoform would have a specific physiological role in cardiac muscle.

Hypergravity from conception to adult stage: effects on contractile properties and skeletal muscle phenotype

This study examined the effects of an elevation of the gravity factor(hypergravity – 2 g) on the molecular and functional characteristics of rat soleus and plantaris muscles. Long Evans rats were conceived, born and reared (CBR) continuously in hypergravity conditions until the age of 100 days. Whole muscle morphological parameters, Ca2+activation characteristics from single skinned fibers, troponin (Tn) subunit and myosin heavy (MHC) and light (MLC) chains isoform compositions were examined in CBR and control muscles from age-paired terrestrial rats. Decreases in body and muscle mass in soleus and plantaris muscles were observed and associated, in the soleus, with a decrease in fiber diameter. The specific force of CBR soleus fibers was increased, and correlated with the elevation of Ca2+ affinity. This was accompanied by slow-to-slower TnC and TnI isoform transitions and a rearrangement in TnT fast isoform content. The MHC transformations of the soleus after hypergravity were associated with the up (down)-regulation of the MHCI (MHCIIa) mRNA isoforms. The MLC2 phosphorylation state remained unchanged in the soleus muscle. The results suggested that the gravity factor could interact with rat muscle development and that hypergravity experiments could provide good tools for the study of myofibrillar protein plasticity and their associated pathways of regulation.

Charged residue changes in the carboxy-terminus of alpha-tropomyosin alter mouse cardiac muscle contractility

Striated muscle tropomyosin (TM) is an essential thin filament protein that is sterically and allosterically involved in calcium-mediated cardiac contraction. We have previously shown that overexpressing the beta-TM isoform in mouse hearts leads to physiological changes in myocardial relaxation and Ca(2+) handling of myofilaments. Two important charge differences in beta-TM compared to alpha-TM are the exchange of serine and histidine at positions 229 and 276 with glutamic acid and asparagine, respectively, imparting a more negative charge to beta-TM relative to alpha-TM. Our hypothesis is that the net charge at specific sites on TM might be a major determinant of its role in modulating cardiac muscle performance and in regulating Ca(2+) sensitivity of the myofilaments. To address this, we generated transgenic (TG) double mutation mouse lines (alpha-TM DM) expressing mutated alpha-TM at the two residues that differ between alpha- and beta-TM (Ser229Glu + His276Asn). Molecular analyses show 60-88% of the native TM is replaced with alpha-TM DM in the different TG lines. Work-performing heart analyses show that alpha-TM DM mouse hearts exhibit decreased rates of pressure development and relaxation (+dP/dt and -dP/dt). Skinned myofibre preparations from the TG hearts indicate a decrease in calcium sensitivity of steady state force. Protein modelling studies show that these two charge alterations in alpha-TM cause a change in the surface charges of the molecule. Our results provide the first evidence that charge changes at the carboxy-terminal of alpha-TM alter the functional characteristics of the heart at both the whole organ and myofilament levels.

Effects of glycine and proline on the calcium activation properties of skinned muscle fibre segments from crayfish and rat

The effects of the polar amino acid glycine (20 mmol l(-1)) and the non-polar amino acid proline (20 mmol l(-1)) on Ca(2+)-activated contraction have been examined in four types of striated muscle fibres. Single fibres dissected from the claw muscle of a crustacean (long- and short-sarcomere) and the hindlimb muscles of the rat (slow-twitch from soleus and fast-twitch from extensor digitorum longus) were activated in matched solutions that either contained the amino acid ('test') or not ('control'). The steady-state force produced in these solutions was used to determine the relation between force production and pCa (-log10[Ca2+]). The results show that in the concentrations used, glycine and proline had only small effects on the maximum Ca(2+)-activated force, pCa corresponding to 10, 50 and 90% maximum force (pCa10, pCa50, pCa90, respectively) or on the slope of the force-pCa curves in the four different fibre types. The relative lack of effects of glycine and proline on contractile activation would confer a distinct physiological advantage to force production of muscle of Cherax, where the concentrations of glycine and proline vary considerably. Finally, the results show that glycine and proline may be useful to balance control solutions when the effects of other amino acids or zwitterions on contractile activation are examined.

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.

Contractile properties of rat single muscle fibers and myosin and troponin isoform expression after hypergravity

The effects of 19 days of hypergravity (HG) were investigated on the biochemical and physiological properties of the slow soleus muscle and its fast agonist, the plantaris. HG was induced by rotational centrifugation that led to a 2-G gravity level. The HG rats were characterized by a slower body growth than control, whereas the soleus muscle mass was increased by 15%. Using electrophoretic techniques, we showed that the distribution of myosin heavy chain and troponin T isoforms was not modified after HG in both soleus and plantaris. In contrast, the isoform expression pattern of two troponin subunits, troponin I and troponin C, was changed in a slow-to-fast manner only in the soleus. From tension-pCa relationships, changes in Ca(2+) activation threshold by 0.18 pCa unit indicated a decrease in Ca(2+) sensitivity and an increase in the slope of the curve, attesting to a higher cooperativity along the thin filament after HG. Comparison of our HG data with previous results in microgravity conditions indicated that muscle characteristics, except muscle mass, did not evolve linearly from 0 to 2 G.

Sarcomere thin filament regulatory isoforms. Evidence of a dominant effect of slow skeletal troponin I on cardiac contraction

Thin filament proteins tropomyosin (Tm), troponin T (TnT), and troponin I (TnI) form an allosteric regulatory complex that is required for normal cardiac contraction. Multiple isoforms of TnT, Tm, and TnI are differentially expressed in both cardiac development and disease, but concurrent TnI, Tm, and TnT isoform switching has hindered assignment of cellular function to these transitions. We systematically incorporated into the adult sarcomere the embryonic/fetal isoforms of Tm, TnT, and TnI by using gene transfer. In separate experiments, greater than 90% of native TnI and 40-50% of native Tm or TnT were specifically replaced. The Ca(2+) sensitivity of tension development was markedly enhanced by TnI replacement but not by TnT or Tm isoform replacement. Titration of TnI replacement from >90% to <30% revealed a dominant functional effect of slow skeletal TnI to modulate regulation. Over this range of isoform replacement, TnI, but not Tm or TnT embryonic isoforms, influenced calcium regulation of contraction, and this identifies TnI as a potential target to modify contractile performance in normal and diseased myocardium.

The superfast extraocular myosin (MYH13) is localized to the innervation zone in both the global and orbital layers of rabbit extraocular muscle

Extraocular muscles (EOMs) are the most molecularly heterogeneous and physiologically diverse mammalian striated muscles. They express the entire array of striated muscle myosins, including a specialized myosin heavy chain MYH13, which is restricted to extraocular and laryngeal muscles. EOMs also exhibit a breadth of contractile activity, from superfast saccades to slow tracking and convergence movements. These movements are accomplished by the action of six ultrastructurally defined fiber types that differ from the type IIa, IIb, IIx and I fibers found in other skeletal muscles. Attempts to associate different eye movements with either the expression of different myosins or the activity of particular EOM fiber types are complicated by the molecular heterogeneity of several of the fiber types, and by electromyography studies showing that the majority of extraocular motor units participate in both fast and slow eye movements. To better understand the role of MYH13 in ocular motility, we generated MYH13-sequence-specific antibodies and used SDS-PAGE to quantify the regional distribution of myosin in EOM and to characterize its heterogeneity in single fibers. These studies demonstrate that MYH13 is preferentially expressed in the majority of orbital and global fibers in the central innervation zone of rabbit EOM. Many individual fibers express MYH13 with the fast IIb myosin and varying amounts of IIx myosin. The differential localization of MYH13, coupled with specialization of the sarcoplasmic reticulum and thin filament systems, probably explains how activation of the endplate band region enables the majority of EOM fibers to contribute to superfast contractions.

Tropomyosin 3 expression leads to hypercontractility and attenuates myofilament length-dependent Ca(2+) activation

Tropomyosin (TM), an integral component of the thin filament, is encoded by three striated muscle isoforms: alpha-TM, beta-TM, and TPM 3. Although the alpha-TM and beta-TM isoforms are well characterized, less is known about the function of the TPM 3 isoform, which is predominantly found in the slow-twitch musculature of mammals. To determine its functional significance, we ectopically expressed this isoform in the hearts of transgenic mice. We generated six transgenic mouse lines that produce varying levels of TPM 3 message with ectopic TPM 3 protein accounting for 40-60% of the total striated muscle tropomyosin. The transgenic mice have normal life spans and exhibit no morphological abnormalities in their sarcomeres or hearts. However, there are significant functional alterations in cardiac performance. Physiological assessment of these mice by using closed-chest analyses and a work-performing model reveals a hyperdynamic effect on systolic and diastolic function. Analysis of detergent-extracted fiber bundles demonstrates a decreased sensitivity to Ca(2+) in force generation and a decrease in length-dependent Ca(2+) activation with no detectable change in interfilament spacing as determined by using X-ray diffraction. Our data are the first to demonstrate that TM isoforms can affect sarcomeric performance by decreasing sensitivity to Ca(2+) and influencing the length-dependent Ca(2+) activation.

Cardiac troponin T isoforms affect the Ca2+ sensitivity and inhibition of force development. Insights into the role of troponin T isoforms in the heart

At least four isoforms of troponin T (TnT) exist in the human heart, and they are expressed in a developmentally regulated manner. To determine whether the different N-terminal isoforms are functionally distinct with respect to structure, Ca(2+) sensitivity, and inhibition of force development, the four known human cardiac troponin T isoforms, TnT1 (all exons present), TnT2 (missing exon 4), TnT3 (missing exon 5), and TnT4 (missing exons 4 and 5), were expressed, purified, and utilized in skinned fiber studies and in reconstituted actomyosin ATPase assays. TnT3, the adult isoform, had a slightly higher alpha-helical content than the other three isoforms. The variable region in the N terminus of cardiac TnT was found to contribute to the determination of the Ca(2+) sensitivity of force development in a charge-dependent manner; the greater the charge the higher the Ca(2+) sensitivity, and this was primarily because of exon 5. These studies also demonstrated that removal of either exon 4 or exon 5 from TnT increased the cooperativity of the pCa force relationship. Troponin complexes reconstituted with the four TnT isoforms all yielded the same maximal actin-tropomyosin-activated myosin ATPase activity. However, troponin complexes containing either TnT1 or TnT2 (both containing exon 5) had a reduced ability to inhibit this ATPase activity when compared with wild type troponin (which contains TnT3). Interestingly, fibers containing these isoforms also showed less relaxation suggesting that exon 5 of cardiac TnT affects the ability of Tn to inhibit force development and ATPase activity. These results suggest that the different N-terminal TnT isoforms would produce different functional properties in the heart that would directly affect myocardial contraction.

Differential susceptibility of the ocular motor system to disease

This review summarizes an alternative approach to the understanding of neuromuscular disease. By contrasting disease susceptibility of extraocular muscle and ocular motor neurons, it is hoped that unique insights into disease mechanisms may be identified. Disorder of eye movements leads to dramatic symptoms for patients and the ocular motor system is relatively limited in its ability to compensate rapidly for such disruptions. However, more profound reasons exist as to why myasthenia gravis compromises neuromuscular transmission at ocular muscle synapses as well as why Graves' ophthalmopathy exists. In contrast, muscular dystrophies spare the eye muscles while devastating all other skeletal muscles; the same is true for motor neuron diseases. It is hoped that this review will encourage others to view the world of neuromuscular diseases as delineated into those that spare the ocular motor system and those that do not.

Ca(2+)-desensitizing effect of a deletion mutation Delta K210 in cardiac troponin T that causes familial dilated cardiomyopathy

A deletion mutation Delta K210 in cardiac troponin T (cTnT) was recently found to cause familial dilated cardiomyopathy (DCM). To explore the effect of this mutation on cardiac muscle contraction under physiological conditions, we determined the Ca(2+)-activated force generation in permeabilized rabbit cardiac muscle fibers into which the mutant and wild-type cTnTs were incorporated by using our TnT exchange technique. The free Ca(2+) concentrations required for the force generation were higher in the mutant cTnT-exchanged fibers than in the wild-type cTnT-exchanged ones, with no statistically significant differences in maximal force-generating capability and cooperativity. Exchanging the mutant cTnT into isolated cardiac myofibrils also increased the free Ca(2+) concentrations required for the activation of ATPase. In contrast, a deletion mutation Delta E160 in cTnT that causes familial hypertrophic cardiomyopathy (HCM) decreased the free Ca(2+) concentrations required for force generation, just as in the case of the other HCM-causing mutations in cTnT. The results indicate that cTnT mutations found in the two distinct forms of cardiomyopathy (i.e., HCM and DCM) change the Ca(2+) sensitivity of cardiac muscle contraction in opposite directions. The present study strongly suggests that Ca(2+) desensitization of force generation in sarcomere is a primary mechanism for the pathogenesis of DCM associated with the deletion mutation Delta K210 in cTnT.

Longitudinal variation in muscle protein expression and contraction kinetics of largemouth bass axial muscle

The present study investigates muscle protein expression in largemouth bass Micropterus salmoides through intra- and intermyomeric comparisons of white muscle. Using denaturing SDS-polyacrylamide gel electrophoresis, muscle protein expression in the arm and cone regions of sequential myomeres was compared for three bass. Low percentage (4.75 %) polyacrylamide-SDS gels and cyanogen bromide (CNBr) peptide mapping revealed no obvious intramyomeric differences between the myosin heavy chains of the arm and cone regions. Electrophoresis of myofibrils and muscle homogenates on higher percentage gels also failed to demonstrate any significant differences between arm and cone regions in either the myosin light chains or any of the major insoluble and soluble contractile proteins. Two differences were discovered intermyomerically: (i) the ratio of two troponin T isoforms changed from head to tail and (ii) caudal muscle had a lower total parvalbumin content than rostral muscle. Since troponin T and parvalbumin have been implicated in the regulation of skeletal muscle kinetics, longitudinal variation in muscle contraction kinetics was predicted. Subsequent experiments revealed that bass rostral white muscle showed faster rates of activation and relaxation than more caudal muscle, as has been observed in white muscle of other fish species. Rostral–caudal variations in white muscle protein composition and contractile properties are predicted to affect patterns of power production during fast, unsteady swimming.

Compensatory changes in Ca(2+) and myocardial O(2) consumption in beta-tropomyosin transgenic hearts

Transgenic mice overexpressing beta-tropomyosin have increased myofilament Ca(2+) sensitivity that we hypothesized would result in altered relationships among pressure and heart rates, intracellular Ca(2+), and myocardial O(2) consumption. In perfused hearts from transgenic mice there was a marked negative force-frequency response between 6 and 10 Hz with a 30 +/- 3% reduction in peak-positive first derivative of pressure development over time (dP/dt) compared with 14 +/- 2% in wild-type mice (P < 0.001). At 8 Hz systolic pressures were normal, though peak systolic intracellular Ca(2+) was significantly reduced in transgenic mice versus wild type (726 +/- 61 vs. 936 +/- 67 nM, P < 0.05) indicating an alteration in the pressure-Ca(2+) relationship. Over a wide range of positive and negative inotropic interventions there were normal developed pressures, though marked elevations in myocardial O(2) consumption (15-54%). Because pressures are normal and intracellular Ca(2+) decreased and myocardial O(2) consumption increased, this suggests that these abnormalities are at least in part compensatory mechanisms to the altered myofilament function.

Myosin heavy chain expression in the red, white, and ventricular muscle of juvenile stages of rainbow trout

Juvenile stages of rainbow trout, smaller parr and older juveniles, termed smolts, show differences in red muscle contractile properties: parr red muscle has faster kinetics and a faster maximum shortening velocity than smolt red muscle. A developmental reduction in the number of MHC isoforms as detected by SDS-PAGE between parr and smolt has also been observed. To investigate whether this shift in contractile kinetics results from differential gene expression, three different MHC cDNA fragments, one each from red, white, and ventricular muscle, were identified. The red muscle and ventricular forms are novel MHCs, and the white muscle form is identical to a published MHC from adult trout white muscle. Tissue and developmental stage-specific expression patterns of these MHC isoforms were examined using isoform-specific RT-PCR. Ventricular muscle typically showed only the ventricular form; 60% parr and 80% smolts expressed the ventricular form only. Approximately half of the white muscle samples of either parr or smolts, 58% and 50%, respectively, expressed only white muscle MHC. Red muscle samples were the most heterogeneous, with red muscle MHC found in combination with either the white or ventricular form or both. Combining samples from the anterior and posterior, 8% of parr red muscle samples expressed solely the red muscle MHC form, and 30% of smolt red muscle samples expressed the red muscle form alone. Variations in the relative contribution of each MHC to the red muscle of parr and smolt may explain observed differences in protein composition and contractile properties. J. Exp. Zool. 290:751-758, 2001.

Comparative studies on the expression patterns of three troponin T genes during mouse development

In vertebrates, three troponin T (TnT) genes, cardiac TnT (cTnT), skeletal muscle fast-twitch TnT (fTnT), and slow-twitch TnT (sTnT), have evolved for the regulation of striated muscle contraction. To understand the mechanism for muscle fiber-specific expression of the TnT genes, we compared their expression patterns during mouse development. Our data revealed that the TnT expression in the developing embryo was not as restricted as that in the adult. In addition to a strong expression in the developing heart beginning at day 7.5 p.c (postcoitum), the cTnT transcript was detected at later stages in some skeletal muscles, where beginning at day 11.75 p.c. the fTnT and sTnT genes were also expressed. Only sTnT but not fTnT was found transiently in the developing heart. At day 13.5 p.c., expressions of all three genes were detected in the developing tongue and this co-expression continued to day 16.5 p.c. with the fTnT isoform being predominant. At this stage, overlapping and distinct expression patterns of both sTnT and fTnT genes were also evident in many developing skeletal muscles. These data suggest that different muscles during development undergo a complex change in TnT isoforms resulting in different contractile properties. Unexpectedly, the cTnT transcript was persistently found in the developing bladder, where presumably smooth muscle is present. In transgenic mice, expression of a LacZ gene driven by a rat cTnT promoter (-497 to +192 bp) was very similar to that of the endogenous cTnT gene, suggesting that this promoter contained regulatory elements sufficient for the control of tissue-specific cTnT expression during development.

Plasticity of monkey triceps muscle fibers in microgravity conditions

We examined the changes in functional properties of triceps brachii skinned fibers from monkeys flown aboard the BION 11 satellite for 14 days and after ground-based arm immobilization. The composition of myosin heavy chain (MHC) isoforms allowed the identification of pure fibers containing type I (slow) or type IIa (fast) MHC isoforms or hybrid fibers coexpressing predominantly slow (hybrid slow; HS) or fast (hybrid fast) MHC isoforms. The ratio of HS fibers to the whole slow population was higher after flight (28%) than in the control population (7%), and the number of fast fibers was increased (up to 86% in flight vs. 12% in control). Diameters and maximal tensions of slow fibers were decreased after flight. The tension-pCa curves of slow and fast fibers were modified, with a decrease in pCa threshold and an increase in steepness. The proper effect of microgravity was distinguishable from that of immobilization, which induced less marked slow-to-fast transitions (only 59% of fast fibers) and changed the tension-pCa relationships.

Expression and functional behavior of troponin C in soleus muscle fibers of rat after hindlimb unloading

Troponin C (TnC) plays a key role in the regulation of muscle contraction, thereby modulating the Ca(2+)-activation characteristics of skinned muscle fibers. This study was performed to assess the effects of a 15-day hindlimb unloading (HU) period on TnC expression and its functional behavior in the slow postural muscles of the rat. We investigated the TnC isoform expression in whole soleus muscles and in single fibers. The latter were also checked for their Ca(2+) activation characteristics and sensitivity to bepridil, a Ca(2+) sensitizer molecule. This drug has been described as exerting a differential effect on slow and fast fibers, depending on the TnC isoform. With regard to TnC expression, three populations were found in control muscle fibers: slow, hybrid slow, and hybrid fast fibers, with the TnC fast being always coexpressed with TnC slow. In the whole muscle, TnC fast expression increased after HU because of the increase in the proportion of hybrid fast fibers. The HU hybrid fast fibers had properties similar to those of control hybrid fast fibers. The fibers that remained slow after HU exhibited similar bepridil and Sr(2+) properties as control slow fibers. Therefore, in these fibers, the changes could not be related to the TnC molecule.

Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been studied for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performance. Recently it has become possible to dissect short segments of single human muscle fibres from biopsy samples and make them work in nearly physiologic conditions in vitro. At the same time, the development of molecular biology has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform composition of the same fibre segments used for functional studies. In this way the histological identification of three main human muscle fibre types (I, IIA and IIX, previously called IIB) has been followed by a precise description of molecular composition and functional and biochemical properties. It has become apparent that the expression of different protein isoforms and therefore the existence of distinct muscle fibre phenotypes is one of the main determinants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functional characteristics. Then the molecular and functional diversity will be examined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to producing ATP. The last section of the review will discuss the advantage that fibre diversity can offer in optimizing muscle contractile performance.

Early specialization of the superfast myosin in extraocular and laryngeal muscles

Extraocular muscle (EOM) exhibits high-velocity, low-tension contractions compared with other vertebrate striated muscles. These distinctive properties have been associated with a novel myosin heavy chain (MyHC) isoform, MyHC-EO. An atypical MyHC, MyHC IIL, has also been identified in laryngeal muscles that have similarly fast contractile properties. It co-migrates with MyHC-EO on high-resolution SDS gels, but appeared to be encoded by a different mRNA. We combined CNBr peptide maps and full-length cDNA sequences to show that rabbit muscle EO and IIL MyHCs are identical. Analysis of the 5; untranslated region (5;UTR) of the mRNAs identified three variants that result from a combination of alternative splicing and multiple transcription initiation sites. This complex pattern of 5;UTRs has not been reported previously for MyHC genes. We identified the human homologue of the MyHC-EO gene in GenBank, and analyzed the 5; upstream region, which revealed a paucity of muscle-specific transcription factor binding sites compared with the other MyHC genes. These features are likely to be critical to the unique regulation and tissue-specific expression of the MyHC-EO/IIL gene.Phylogenetic analysis indicates that MyHC-EO/IIL diverged from an ancestral MyHC gene to generate the first specialized fast myosin. The catalytic S1 head domain is more closely related to the fast MyHCs, while the rod is more closely related to the slow/cardiac MyHCs. The exon boundaries of the MyHC-EO are identical to those of the embryonic MyHC gene and virtually identical to those of the α and (β) cardiac genes. This implies that most of the current exon boundaries were present in the ancestral gene, predating the duplications that generated the family of skeletal and cardiac myosin genes.

Actin-filament motion in the in vitro motility assay has a periodic component

The interaction between actin and myosin can be studied in the in vitro motility assay, where fluorescently labelled actin filaments are observed to move over a lawn of myosin heads. To examine details of this movement, we measured systematically the velocities of the front end, rear end, and centroid of the actin filament as the filament translated over the assay surface. We found that these velocities exhibited an unexpectedly periodic component, alternating regularly between high and low values, superimposed on the steady velocity component. The period of the oscillatory component was approximately 380 ms. When translation was stopped by an increase in osmolarity, the filaments wiggled with a periodicity similar to the translating filament, implying that wiggling and translation may be related. Rigor filaments showed no periodicity. From the frequency content of the auto- and cross-correlation functions derived from the velocities of the front end, rear end, and centroid of the actin filament, we infer a deterministic, possibly wave-like process travelling along the actin filament. Potential molecular mechanisms underlying this phenomenon are considered.