Isoforms of contractile proteins

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.

Developmental expression of the alpha-skeletal actin gene

Background

Actin is a cytoskeletal protein which exerts a broad range of functions in almost all eukaryotic cells. In higher vertebrates, six primary actin isoforms can be distinguished: alpha-skeletal, alpha-cardiac, alpha-smooth muscle, gamma-smooth muscle, beta-cytoplasmic and gamma-cytoplasmic isoactin. Expression of these actin isoforms during vertebrate development is highly regulated in a temporal and tissue-specific manner, but the mechanisms and the specific differences are currently not well understood. All members of the actin multigene family are highly conserved, suggesting that there is a high selective pressure on these proteins.

Results

We present here a model for the evolution of the genomic organization of alpha-skeletal actin and by molecular modeling, illustrate the structural differences of actin proteins of different phyla. We further describe and compare alpha-skeletal actin expression in two developmental stages of five vertebrate species (mouse, chicken, snake, salamander and fish). Our findings confirm that alpha-skeletal actin is expressed in skeletal muscle and in the heart of all five species. In addition, we identify many novel non-muscular expression domains including several in the central nervous system.

Conclusion

Our results show that the high sequence homology of alpha-skeletal actins is reflected by similarities of their 3 dimensional protein structures, as well as by conserved gene expression patterns during vertebrate development. Nonetheless, we find here important differences in 3D structures, in gene architectures and identify novel expression domains for this structural and functional important gene.

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.

Differential effects of arginine, glutamate and phosphoarginine on Ca(2+)-activation properties of muscle fibres from crayfish and rat

The effects of two amino acids, arginine which has a positively charged side-chain and glutamate which has a negatively charged side-chain on the Ca2+-activation properties of the contractile apparatus were examined in four structurally and functionally different types of skeletal muscle; long- and short-sarcomere fibres from the claw muscle of the yabby (a freshwater decapod crustacean), and fast- and slow-twitch fibres from limb muscles of the rat. Single skinned fibres were activated in carefully balanced solutions of different pCa (-log10[Ca2+]) that either contained the test solute ("test") or not ("control"). The effect of phosphoarginine, a phosphagen that bears a nett negative charge, was also compared to the effects of arginine. Results show that (i) arginine (33-36 mmol l(-1)) significantly shifted the force-pCa curve by 0.08-0.13 pCa units in the direction of increased sensitivity to Ca2+-activated contraction in all fibre types; (ii) phosphoarginine (9-10 mmol l(-1)) induced a significant shift of the force-pCa curve by 0.18-0.24 pCa units in the direction of increased sensitivity to Ca2+ in mammalian fast- and slow-twitch fibres, but had no significant effects on the force-pCa relation in either long- or short-sarcomere crustacean fibres; (iii) glutamate (36-40 mmol l(-1)), like arginine affected the force-pCa relation of all fibre types investigated, but in the opposite direction, causing a significant decrease in the sensitivity to Ca2+-activated contraction by 0.08-0.19 pCa units; (iv) arginine, phosphoarginine and glutamate had little or no effect on the maximum Ca2+-activated force of crustacean and mammalian fibres. The results suggest that the opposing effects of glutamate and arginine are not related to simply their charge structure, but must involve complex interactions between these molecules, Ca2+ and the regulatory and other myofibrillar proteins.

Effect of tibial bone resection on the development of fast- and slow-twitch skeletal muscles in foetal sheep

To determine if longitudinal bone growth affects the differentiation of fast- and slow-twitch muscles, the tibial bone was sectioned at 90 days gestation in foetal sheep so that the lower leg was permanently without structural support. At 140 days (term is approximately 147 days) the contractile properties of whole muscles, activation profiles of single fibres and ultrastructure of fast- and slow-twitch muscles from the hindlimbs were studied. The contractile and activation profiles of the slow-twitch soleus muscles were significantly affected by tibial bone resection (TIBX). The soleus muscles from the TIBX hindlimbs showed: (1) a decrease in the time to peak of the twitch responses from 106.2 +/- 10.7 ms (control, n = 4) to 65.1 +/- 2.48 ms (TIBX, n = 5); (2) fatigue profiles more characteristic of those observed in the fast-twitch muscles: and (3) Ca2+ - and Sr2+ -activation profiles of skinned fibres similar to those from intact hindlimbs at earlier stages of gestation. In the FDL, TIBX did not significantly change whole muscle twitch contraction time, the fatigue profile or the Ca2+ - and Sr2+ -activation profiles of skinned fibres. Electron microscopy showed an increased deposition of glycogen in both soleus and FDL muscles. This study shows that the development of the slow-twitch phenotype is impeded in the absence of the physical support normally provided by the tibial bone. We suggest that longitudinal stretch is an important factor in allowing full expression of the slow-twitch phenotype.

Developmental changes in the activation properties and ultrastructure of fast- and slow-twitch muscles from fetal sheep

At early stages of muscle development, skeletal muscles contract and relax slowly, regardless of whether they are destined to become fast- or slow-twitch. In this study, we have characterised the activation profiles of developing fast- and slow-twitch muscles from a precocial species, the sheep, to determine if the activation profiles of the muscles are characteristically slow when both the fast- and slow-twitch muscles have slow isometric contraction profiles. Single skinned muscle fibres from the fast-twitch flexor digitorum longus (FDL) and slow-twitch soleus muscles from fetal (gestational ages 70, 90, 120 and 140 days; term 147 days) and neonatal (8 weeks old) sheep were used to determine the isometric force-pCa (pCa = -log10[Ca2+]) and force-pSr relations during development. Fast-twitch mammalian muscles generally have a greatly different sensitivity to Ca2+ and Sr2+ whereas slow-twitch muscles have a similar sensitivity to these divalent cations. At all ages studied, the force-pCa and force-pSr relations of the FDL muscle were widely separated. The mean separation of the mid-point of the curves (pCa50-pSr50) was approximately 1.1. This is typical of adult fast-twitch muscle. The force-pCa and force-pSr curves for soleus muscle were also widely separated at 70 and 90 days gestation (pCa50-pSr50 approximately 0.75); between 90 days and 140 days this separation decreased significantly to approximately 0.2. This leads to a paradoxical situation whereby at early stages of muscle development the fast muscles have contraction dynamics of slow muscles but the slow muscles have activation profiles more characteristic of fast muscles. The time course for development of the FDL and soleus is different, based on sarcomere structure with the soleus muscle developing clearly defined sarcomere structure earlier in gestation than the FDL. At 70 days gestation the FDL muscle had no clearly defined sarcomeres. Force (N cm-2) increased almost linearly between 70 and 140 days gestation in both muscle types and there was no difference between the Ca(2+)- and Sr(2+)-activated force throughout development.

Expression and epitopic conservation of calponin in different smooth muscles and during development

Calponin is a thin filament associated protein found in smooth muscle as a potential modulator of contraction. Five mouse monoclonal antibodies (mAbs CP1, CP3, CP4, CP7, and CP8) were prepared against chicken gizzard alpha-calponin. The CP1 epitopic structure is conserved in smooth muscles across vertebrate phyla and is highly sensitive to CNBr cleavage in contrast with the chicken-specific CP4 and the avian-mammalian-specific CP8 epitopes that are resistant to CNBr fragmentation. Using this panel of mAbs against multiple epitopes, only alpha-calponin was detected in adult chicken smooth muscles and throughout development of the gizzard. Western blotting showed that the calponin content varied among different smooth muscle tissues and correlated with that of h-caldesmon. In contrast with the constitutive expression of calponin in phasic smooth muscle of the digestive tract, very low levels of calponin were detected in adult avian tracheas and no calponin expression was detected in embryonic and young chick tracheas. These results provide information on the structural conservation of calponins and suggest a relationship between calponin expression and smooth muscle functional states.

Characterisation of fast, slow and cardiac muscle tropomyosins from salmonid fish

Tropomyosin (TM) has been isolated from the cardiac muscle, and fast and slow trunk (myotomal) muscles of the mature salmonid fish Atlantic salmon (Salmo salar) and rainbow trout (Salmo gairdneri). When examined electrophoretically, isoforms of TM were detected which were specific, and exclusive, to each type of muscle. Cardiac and fast muscles contained single and distinct isoforms, while slow muscle contained two distinct isoforms, closely related in terms of apparent M(r), and pI. There was no detectable difference between the same TM type from either salmon or trout. On a variety of gel systems, the cardiac and slow isoforms migrated in close proximity to each other and to rabbit alpha-TM. The fast isoform comigrated with rabbit beta-TM. In developing salmon fry, a more acidic (unphosphorylated) variant of TM was present in addition to, and of similar M(r) to, the fast adult isoform. This TM declined in steady-state level during maturation and was virtually undetected in adult muscle. All of the isolated TMs contained little or no covalently bound phosphate and were blocked at the N-terminus. The amino acids released by carboxypeptidase A, when ordered to give maximal similarity to other muscle TMs, were consistent with the following sequences: fast (LDNALNDMTSI) and cardiac (LDHALNDMTSL). The C-terminal region of the slow TM contained His but was heterogeneous. In viscosity measurements, performed as a function of increasing protein concentration, at low ionic strength (t = 5 degrees C, pH 7.00), fast TM exhibited the highest relative viscosity values. Lower and equivalent levels of polymerisation occurred with the cardiac and slow TMs. Polymerisation of all three isoforms was temperature-dependent, with cardiac TM being least sensitive and fast TM being most sensitive. Determination of the complete coding sequence of adult fast TM confirmed the findings of the carboxypeptidase analysis, but the remainder of the sequence more closely resembled alpha-type TMs than beta-type TMs. Overall, salmon fast TM contains 20 (mostly conservative) substitutions compared to rabbit striated muscle alpha-TM and 40 (mostly conservative) substitutions compared to rabbit striated muscle beta-TM. This demonstrates that electrophoretic mobility is not, in all instances, a suitable method to assess the isomorphic nature of striated muscle TMs.

Analysis of Ca2+ and Sr2+ activation characteristics in skinned muscle fibre preparations with different proportions of myofibrillar isoforms

To understand how the coexistence of fast and slow contractile and regulatory systems within single skeletal muscle fibres might affect contractile behaviour, fibre segments from the fast-twitch extensor digitorum longus and predominantly slow-twitch soleus muscle of the adult rat were tied together, either in parallel or in series, and then activated in Ca(2+)- and Sr(2+)-buffered solutions. Experimental force-pCa and force-pSr relations were compared with theoretical force-pCa and force-pSr curves predicted by a model for composite fibres, which accounted for the coexistence of fast and slow myosin within the contractile unit and enabled an estimate to be made of the relative contribution of fast- and slow-twitch elements within the tied-fibre combinations. The contractile behaviour of a fast-twitch and a slow-twitch muscle fibre tied either in series or in parallel, were compared with the force-pCa and force-pSr data predicted from the composite fibre model. Interestingly, the resultant force-pCa(-pSr) curves of the parallel-tied fibre combinations were well fitted with those predicted by the composite model. However, the experimental force-pCa(-pSr) curves of the series-tied fibres were not well fitted by a composite curve based on the known proportion of fast- and slow-twitch fibre components. A total force-length diagram was devised to take into account changes in the length of the fibre segments tied in series during activation, as well as possible differences in fibre diameter. Using this diagram it was possible to explain accurately the Ca2+ and Sr2+ activation curves of known fast- and slow-twitch segments tied in series. The results from this study are important for the interpretation of contractile data obtained from single muscle fibres exhibiting mixed fast- and slow-twitch contractile characteristics. Such muscle fibres have previously been identified in animals affected by muscular diseases (e.g. dystrophy), in mammalian extraocular muscles and in animals subjected to long-term exercise training.

Myosin polymorphism and differential expression in adult human skeletal muscle

1. Myosin heavy chain (HC) and light chain (LC) isoforms are expressed in a tissue-specific and developmentally-regulated manner in human skeletal muscle. 2. At least seven myosin HC isoforms are expressed in skeletal muscle of the adult. 3. Histochemically-delineated fibre types (based on the stability of myofibrillar actomyosin adenosine triphosphatase activity) in limb muscles correlate with the myosin HC content. 4. Alterations in the phenotypic expression of myosin provides a mechanism of adaptation to stresses placed upon the muscle (e.g. increased and decreased usage).

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

Complete nucleotide sequence and structural organization of rat cardiac troponin T gene. A single gene generates embryonic and adult isoforms via developmentally regulated alternative splicing

We have previously demonstrated that rat cardiac troponin T (TnT) is expressed as two different isoforms during development, the larger, more acidic embryonic isoform and the smaller, more basic adult isoform, which appear to be generated from a common transcript of the cardiac TnT gene by alternative RNA splicing. In this study, Southern blot analysis confirmed the existence of a single copy of cardiac TnT gene in the rat genome. For investigation of the molecular mechanism of isoform switch and the control of this gene expression in myocardial development, several overlapping genomic clones were isolated from a rat genomic library. Complete nucleotide sequences were determined from these genomic clones and revealed a 19,186 base-pair DNA fragment containing 16 exons of rat cardiac TnT gene. Its DNA sequence and exon organization appeared to differ from that of the rat fast skeletal muscle TnT gene or chicken cardiac TnT gene. Comparison of genomic and cDNA clones also confirmed that the cardiac TnT isoform switching was due to the inclusion or exclusion of exon 4 during RNA processing. Sequence analysis allowed us to further identify the other alternatively spliced exon containing only nine nucleotides in size (exon 12). The inclusion and complete or partial exclusion of this exon may be responsible for generating three classes of mRNAs detected by our cDNA clones. The functional significance of this variation in TnT isoforms remained unknown, but its splicing pattern did not appear to link to the developmental changes. The 5' upstream structure was very similar to that in chicken cardiac TnT gene but differed from that in the rat fast skeletal muscle TnT gene, suggesting a similar regulatory mechanism for mammalian and avian cardiac TnT expression.

Isomyosin expression pattern during formation of the tubular chicken heart: a three-dimensional immunohistochemical analysis

Three-dimensional (3-D) distribution of atrial and ventricular isomyosins is analysed immunohistochemically during the formation of the tubular chicken heart (stage 7 to 12 [H/H]) using antibodies specific for adult chicken atrial and ventricular myosin heavy chains, respectively. This analysis revealed that both types of isomyosins can be first detected at stage 8 (H/H, possessing four pairs of somites), i.e., when the heart primordium still exists as two separate cardiogenic plates. The ventricular type of isomyosin is initially expressed in those areas of cardiogenic plates in the vicinity of the anterior intestinal portal. The atrial type of isomyosin is initially expressed in zones caudal and lateral to the areas of ventricular isomyosin expression. Medial to the atrial isomyosin-expressing areas, cardiogenic plate areas exist that initially lack myosin expression. Those parts of the cardiogenic plates that fuse in front of the anterior intestinal portal, thereby forming the heart tube, are characterized by the expression of both isomyosins; however, the caudolateral parts of the heart primordium maintain their single atrial isomyosin expression during further development. Cardiac contractions are therefore first observed at stage 10 (H/H, possessing ten pairs of somites) in myocardium that coexpresses both isomyosins.

Shortening velocity and myosin and myofibrillar ATPase activity related to myosin isoenzyme composition during postnatal development in rat myocardium

The relation between functional properties of the contractile apparatus, such as shortening velocity and ATPase activity, and myosin isoenzyme composition was studied in ventricular myocardium of adult (60-90-day-old) rats and of newborn (3-day-old) and young (10- and 20-day-old) rats. In adult animals, variations of isomyosin pattern were produced by reducing food intake and by changing the thyroid state. Hyperthyroidism was induced with triiodothyronine daily injection for 15 days; hypothyroidism was induced with iodine-free diet and KClO4 in drinking water for 50-60 days. The following parameters were studied: 1) calcium-magnesium-activated and magnesium-activated ATPase activity of washed and purified myofibrils, 2) calcium-activated ATPase activity of purified myosin, 3) isomyosin composition and relative content of alpha-myosin heavy chains (alpha-MHCs), and 4) force-velocity curve of left and right ventricle papillary muscles. To take into account the difference in excitation-contraction coupling between newborn and adult myocardium, the determination of the force-velocity curve was repeated in Krebs' solution with normal [CaCl2] (2.5 mM) and in Krebs' solution with high [CaCl2] (10 mM). During postnatal growth, the relative content of alpha-MHC increased and reached a maximum at about 20 days. Pronounced increases of myofibrillar and myosin ATPase activity and in shortening velocity occurred during the same period. In adult hyperthyroid rats, alpha-MHC content as well as enzymatic activity and shortening velocity were higher than in control adult animals. Hypothyroidism and food deprivation caused a decrease of alpha-MHC content and a reduction of both enzymatic activities and shortening velocity. The study of the relations between alpha-MHC relative content and functional parameters showed that 1) in ventricular myocardium of adult rats a linear relation existed between alpha-MHC content and myosin and myofibrillar ATPase activity and shortening velocity, and 2) in newborn and young rat ventricular myocardium, both enzymatic activities and shortening velocity were lower than would have been expected on the basis of the linear relation described above. This latter observation could be accounted for by a variation in specific activity of myosin during postnatal development or by the presence of peculiar isomyosins that cannot be detected with usual electrophoretic techniques.

Variations in contractile properties of rabbit single muscle fibres in relation to troponin T isoforms and myosin light chains

1. The maximal velocity of shortening (Vmax), tension-pCa relationships and the contractile and regulatory protein composition were determined in single, chemically skinned fibres from adult rabbit plantaris muscles. 2. Three groups of fibres were identified based on their protein compositions. One group had exclusively the slow-type myosin heavy chain (MHC) and myosin light chains (LC) and had low velocities. Another group of fibres had mixtures of fast-type and slow-type MHCs and LCs and had intermediate shortening velocities. The third group of fibres had fast-type myosin heavy and light chains and high velocities. 3. The low-velocity fibres had a mean velocity (+/- S.E.M.) of 0.86 +/- 0.03 muscle lengths/s (ML/s) at 15 degrees C. The remaining fibres formed a continuum with respect to Vmax from 1.37 to 3.94 ML/s. These results indicate that a much greater diversity exists among single fibres from adult mammalian skeletal muscle than previously recognized. The intermediate- and high-velocity fibres formed a continuum (from slow to fast) with respect to the amount of myosin light chain 3 (LC3). That is, Vmax increased with the relative LC3 content in single fibres in the intermediate- and high-velocity groups in a quantitative, statistically significant manner. 4. Three isoforms of fast-type troponin T were identified among the intermediate- and high-velocity fibres. These fibres also contained fast-type troponin C and troponin I. As was the case with the relative LC3 content, these fibres also formed a continuum with respect to the relative proportions of the three isoforms of fast-type troponin T. It appears that different isoforms of troponin T are responsible for a slightly higher Ca2+ sensitivity of tension development in the high-velocity fibres compared to the intermediate fibres. The continuum in troponin T isoform composition paralleled an increase in Vmax among these fibres. 5. The low-velocity fibres had the highest Ca2+ sensitivity of the three groups and had exclusively the slow-type isoforms of the regulatory proteins in the troponin complex. 6. The co-ordinated variations in troponin T and LC3 compositions among the intermediate- and high-velocity fibres are discussed as a possible means for the further differentiation of the contractile properties of the fibres in these two groups, beyond that provided by myosin heavy chain isoforms alone.

Neonatal myosin heavy chains are not expressed in Ni-induced rat rhabdomyosarcoma

Myosin heavy chain (MHC) composition of chemically-induced rhabdomyosarcoma (RMS) was analyzed by gel electrophoresis and Western blotting using a panel of monoclonal antimyosin antibodies specific for embryonic-, neonatal-, slow- and adult fast-type MHC isoforms. Myosin extracted from tumours and electrophoresed on 6%-sodium dodecyl sulfate (SDS)glycerol gels was found to migrate as three distinct MHC components. These polypeptides were present in different relative amounts in the five RMS studied. Western blotting experiments revealed that variable proportions of embryonic-, slow- and adult fast-, but not neonatal-type, MHC isoforms are consistently expressed in RMS. Indirect and double immunofluorescence procedures applied to cryosections of tumoral tissue showed that: (a) RMS cells were unreactive with antineonatal-type-MHC antibody, (b) the majority of neoplastic, desmin-positive, cells contained embryonic- as well as adult fast-type MHCs and (c) a minority of cells were labelled by anti-slow MHC antibody. The results of this study indicate that there is no obligatory sequence of MHC isoform expression in the molecular transition (emb----neo----adult) which occurs during rat skeletal myogenesis.