Myosin heavy chain gene expression changes in the diaphragm of patients with chronic lung hyperinflation

In striated muscle, chronic increases in workload result in changes in myosin phenotype. The aim of this study was to determine whether such changes occur in the diaphragm of patients with severe chronic obstructive pulmonary disease, a situation characterized by a chronic increase in respiratory load and lung volume. Diaphragm biopsies were obtained from 22 patients who underwent thoracic surgery. Myosin was characterized with electrophoresis in nondenaturing conditions, SDS-glycerol PAGE, and Western blotting with monoclonal antibodies specific for slow and fast myosin heavy chain (MHC) isoforms. Flow volume curves, total lung capacity, and functional residual capacity were measured before surgery in 20 patients. We found that the human diaphragm is composed of at least four myosin isoforms, one slow and three fast, resulting from the combination of three MHC species. Chronic overload was associated with an increase in the slow beta-MHC species at the expense of the fast species (beta-MHC, 78.2 +/- 4.6 and 50.0 +/- 6.5% in emphysematous and control patients, respectively; P < 0.005). Linear correlations were found between beta-MHC percentage and forced expiratory volume in 1 s (r = -0.52; P < 0.02), total lung capacity (r = 0.44; P < 0.05), and functional residual capacity (r = 0.65; P < 0.003). The human adult diaphragm is composed of a balanced proportion of slow and fast myosin isoforms. In patients with chronic obstructive pulmonary disease, the proportion of fast myosins decreases, whereas that of slow myosin increases. This increase appears to be closely related to lung hyperinflation and may reflect an adaptation of the diaphragm to the new functional requirements.

Isoform transitions of the myosin binding protein C family in developing human and mouse muscles: lack of isoform transcomplementation in cardiac muscle

Mutations in the gene for the cardiac isoform of myosin binding protein C (MyBP-C) have been identified as the cause of chromosome 11-associated autosomal-dominant familial hypertrophic cardiomyopathy (FHC). Most mutations produce a truncated polypeptide that lacks the sarcomeric binding region. We have now investigated the expression pattern of the cardiac and skeletal isoforms of cMyBP-C in mice and humans by in situ hybridization and immunofluorescence microscopy using specific antibodies and probes. We demonstrate that the cardiac isoform is expressed only in cardiac muscle throughout development. The slow and fast isoforms of MyBP-C remain specific for skeletal muscle, where they can be coexpressed. Immunological evidence also suggests that an embryonic isoform of MyBP-C precedes the expression of slow MyBP-C in developing skeletal muscle. This suggests that transcomplementation of MyBP-C isoforms is possible in skeletal but not cardiac muscle.

Mismatch between myosin heavy chain mRNA and protein distribution in human skeletal muscle fibers

The distribution of myosin heavy chain (MHC) isoforms was analyzed at the protein and mRNA levels in human skeletal muscle biopsies from young normal adult subjects. Using ATPase histochemical reactions, antibodies to fast- and slow-type MHCs, and in situ hybridization with probes specific for MHC-beta/slow, MHC-2A, and MHC-2X, we confirmed our previous results showing that most fibers contain either a single mRNA and isoprotein or a mixed 1/2A or 2A/2X phenotype with coexistence of two mRNAs and isoproteins. However, we also found a minor proportion of fibers showing a mismatch in the relative proportion of mRNA and protein, e.g., fibers containing MHC-2A mRNA but not the corresponding protein or fibers containing MHC-2A protein but not the corresponding transcript. These fibers were more frequent in biopsies obtained after a training or detraining period than before the training period. We propose that these fibers represent transitional fibers and that the relative content of each mRNA and isoprotein gives a clue as to the direction of change in MHC gene expression.

Fibre type-specific and nerve-dependent regulation of myosin light chain 1 slow promoter in regenerating muscle

The regulation of a slow muscle gene, the myosin light chain 1 slow/ventricular gene, has been studied by in vivo transfection into regenerating rat skeletal muscle. Constructs containing portions of the myosin light chain 1 slow/ventricular promoter linked to reporter genes were injected into fast and slow muscles 3 days after muscle injury by bupivacaine injection, and reporter gene activity was analysed after 10 days. We report that a sequence in the 5' flanking region of the myosin light chain 1 slow/ventricular gene is able to direct slow muscle-specific regulation of reporter genes, and that the expression of the transgene, like that of the corresponding endogenous gene, is dependent on intact nerve. This study validates the use of regenerating muscle as a model for studying muscle gene regulation and is the first demonstration of a myosin gene promoter regulated by nerve activity.

Early myosin switching induced by nerve activity in regenerating slow skeletal muscle

Muscle regeneration is a potentially useful model for defining the mechanisms responsible for nerve-dependent myosin isogene regulation in skeletal muscle. As a first step towards this goal we have characterized the pattern of expression of the four myosin heavy chain (MHC) genes, MHC-beta/slow, -2A, -2X and -2B isogenes, during early stages of muscle regeneration both in the presence and in the absence of the nerve. Muscle degeneration/regeneration was induced by intramuscular injection of the myotoxic drug, bupivacaine, in the rat slow soleus muscle. MHC transcripts were identified by in situ hybridization with specific riboprobes during the period from day 3 to day 7 after muscle injury. The four genes are not detected at day 3, when the regenerating muscle contains predominantly embryonic and neonatal MHC isoforms. MHC-2X and -2B transcripts are first detected at day 4 in both innervated and denervated muscles. These transcripts remain as major transcripts in denervated muscle whereas they are down-regulated by day 5 and disappear by day 6-7 in the presence of the nerve. Innervation induces strong up-regulation of MHC-2A at day 4 and MHC-beta/slow transcripts at day 5. MHC-2A transcripts are first homogeneously expressed in most fibers then become segregated in a minor population of fibers by day 6-7 while MHC-beta/slow transcripts increase in most fibers. In the absence of the nerve MHC-beta/slow transcripts are never expressed and MHC-2A transcripts are detected in rare fibers at days 5-7.

Autoimmunity in myocarditis and dilated cardiomyopathy: cardiac autoantibody frequency and clinical correlates in a patient series from Italy

BACKGROUND

Organ- and disease-specific cardiac autoantibodies, detected by indirect immunofluorescence, represent markers of autoimmunity in a subgroup (25-35%) of patients with dilated cardiomyopathy or myocarditis from Northern Europe and the United States of America. Autoantibody frequencies, as well as associations between clinical and immunological features, may vary in patients from different countries, due to ethnically related differences in genetic susceptibility to autoimmune disease.

METHODS

We assessed the frequency of cardiac autoantibodies in a series from Italy, including 91 subjects with idiopathic dilated cardiomyopathy (61 male, aged 49 +/- 11 years) and 11 with biopsy-proven (Dallas criteria) myocarditis (7 male, aged 23 +/- 16), including 2 cases of giant cell myocarditis. Controls were 160 patients with other cardiac disease, 141 with ischemic heart failure and 270 normals Cardiac antibody test was performed blindly by indirect immunofluorescence on normal human myocardium and skeletal muscle.

RESULTS

The frequency of organ-specific cardiac autoantibodies was higher (p = 0.0001) in myocarditis (45%) and in dilated cardiomyopathy (20%) than in other cardiac disease (1%), in ischemic heart failure (1%), or in normals (2.5%). Cross-reactive antibodies were detected in similar proportions of study patients and controls. Both patients with giant cell myocarditis were antibody positive. Myocarditis patients with cardiac antibodies had shorter duration of symptoms compared to those who were antibody negative (0.4 +/- 0.3 vs 4 +/- 1 months, p = 0.004). In dilated cardiomyopathy, antibody status was not associated with any clinical or diagnostic feature.

CONCLUSIONS

Autoimmunity is involved in a subset of patients with myocarditis and with dilated cardiomyopathy, regardless of their geographical origin or immunogenetic background. The antibody frequency in our dilated cardiomyopathy series from Italy tended to be lower than in other countries. This may reflect reduced antibody levels with disease progression and/or the recognised feature that Mediterranean populations are often less susceptible to autoimmune disease.

Binding of cytosolic proteins to myofibrils in ischemic rat hearts

Myofibrillar proteins (MPs) were extracted from isolated and perfused rat hearts subjected to different periods of ischemia to investigate the occurrence of protein degradation and/or the association of cytosolic proteins with the myofibrillar pellet. A 23-kD band was detected by SDS-PAGE of MPs after 5 minutes of ischemia, with its density gradually increasing to a plateau after 20 minutes. Longer periods of ischemia were associated with the appearance of a 39-kD band. Irrespective of the duration of ischemia, both these bands persisted during reperfusion. A partial proteolytic degradation of troponin T (TnT) and troponin I (TnI) has been claimed to be responsible for the generation of these peptides. However, the N-terminal sequence of the 39-kD band was identical to that of GAPDH, whereas Edman sequencing after pepsin digestion showed that the 23 kD is alpha B-crystallin. The binding of the two cytosolic proteins to myofibrils was confirmed by immunofluorescence analysis on cryosections of ischemic hearts. In vitro studies showed that acidosis was sufficient to induce the binding of alpha B-crystallin, whereas the inhibition of ATP depletion prevented the binding of GAPDH. Thiol oxidation is unlikely to promote GAPDH binding, since perfusion with iodoacetate under aerobic conditions or treatment of homogenates with N-ethylmaleimide or diamide failed to induce GAPDH association with the myofibrils. These changes of the myofibrillar proteins could be considered as intracellular markers of the evolution of the ischemic damage. In addition, the binding of the 23-kD peptide might be involved in alterations of contractility.

Molecular diversity of myofibrillar proteins: gene regulation and functional significance

Myofibrillar proteins exist as multiple isoforms that derive from multigene (isogene) families. Additional isoforms, including products of tropomyosin, myosin light chain 1 fast, troponin T, titin, and nebulin genes, can be generated from the same gene through alternative splicing or use of alternative promoters. Myofibrillar protein isogenes are differentially expressed in various muscle types and fiber types but can be coexpressed within the same fiber. Isogenes are regulated by transcriptional and posttranscriptional mechanisms; however, specific regulatory sequences and transcriptional factors have not yet been identified. The pattern of isogene expression varies during muscle development in relation to the different origin of myogenic cells and primary/secondary fiber generations and is affected by neural and hormonal influences. The variable expression of myofibrillar protein isoforms is a major determinant of the contractile properties of skeletal muscle fibers. The diversity among isomyosins is related to the differences in the parameters of chemomechanical transduction as ATP hydrolysis rate and shortening velocity. Troponin and tropomyosin isoforms determine the variable sensitivity to calcium, whereas titin isoforms dictate the elastic properties of muscle fibers at rest. Both myosin and troponin isoforms contribute to the differences in the resistance to fatigue of muscle fibers.

Cardiac and skeletal muscle troponin I isoforms are encoded by a dispersed gene family on mouse chromosomes 1 and 7

We mapped the locations of the genes encoding the slow skeletal muscle, fast skeletal muscle, and cardiac isoforms of troponin I (Tnni) in the mouse genome by interspecific hybrid backcross analysis of species-specific (C57BL/6 vs Mus spretus) restriction fragment length polymorphisms (RFLPs). The slow skeletal muscle troponin I locus (Tnni1) mapped to Chromosome (Chr) 1. The fast skeletal muscle troponin I locus (Tnni2), mapped to Chr 7, approximately 70 cM from the centromere. The cardiac troponin I locus (Tnni3) also mapped to Chr 7, approximately 5-10 cM from the centromere and unlinked to the fast skeletal muscle troponin I locus. Thus, the troponin I gene family is dispersed in the mouse genome.

Characterization of a human perinatal myosin heavy-chain transcript

Using a monoclonal antibody specific to the neonatal myosin heavy chain, we have cloned the full-length heavy chain cDNA from an 18-week human fetal cDNA library. Ribonuclease protection assays were used to survey a human muscle collection ranging from 11 weeks gestation to 16 years. Expression of the RNA encoded by this cDNA was observed at 20 and 21 weeks gestation and at 2 days after birth. No expression was observed at 13.5 weeks, before 2 years, at 2 years, or after 2 years gestation. Due to the timing of its expression, this cDNA appears to represent of the human fetal myosin heavy chain. Sequencing of the entire 6010 bases showed high similarity to the rat perinatal myosin heavy chain [Periasamy, M., Wieczorek, D. F. & Nadal-Ginard, B. (1984) J. Biol. Chem. 21, 13,573-13,578]. However, moderate divergence was observed when compared to a previously described human perinatal myosin heavy chain [Karsch-Mizrachi, I., Feghali, R., Shows, T. B. & Leinwand, L. A. (1990) Gene 89, 289-294; Feghali, R. & Leinwand, L. A. (1989) J. Cell Biol. 108, 1791-1797]. Restriction fragment-length polymorphism analyses of sites in both the S1 and rod domains showed the presence of this fetal myosin heavy chain sequence in all 27 genomic samples examined. Restriction fragment-length polymorphism analysis failed to find the previously described perinatal isoform in any sample.

Specific degradation of troponin T and I by mu-calpain and its modulation by substrate phosphorylation

The degradation of troponin (Tn) subunits by calpain was studied by incubating either isolated cardiac Tns or myocardial cryosections with two different calpain isoenzymes isolated from rat skeletal muscle. Western-blot analysis with monoclonal antibodies against TnI and TnT showed that mu-calpain was at least ten times more active than m-calpain in degrading TnI and TnT both in vitro and in situ. TnC was completely resistant to both proteinase forms. Phosphorylation by cyclic AMP-dependent protein kinase (PKA) isolated from rat skeletal muscle reduced the sensitivity of TnI to degradation. This effect in combination with an increased efficiency of the endogenous inhibitor [Salamino, De Tullio, Michetti, Mengotti, Melloni and Pontremoli (1994) Biochem. Biophys. Res. Commun. 199, 1326-1332] probably reduces the proteolytic activity of calpain in cells on PKA stimulation. Conversely, phosphorylation by protein kinase C (PKC) resulted in a twofold increase in the degradation of TnI. Degradation by m-calpain was not modified by Tn phosphorylation. The different sensitivity to mu-calpain might be related to changes in TnI oligomeric structure. Indeed, on PKC phosphorylation, the apparent molecular mass of TnI calculated from the distribution coefficient of Tn complex in Sephadex G-100 matrix was reduced from 90 to 30 kDa suggesting dissociation of the Tn complex.

[Influence of the intravenous nitroglycerin dose on the appearance of drug tolerance]

BACKGROUND

There are few studies on the influence of different doses of intravenous nitroglycerin (NTG) on the appearance of drug tolerance.

METHODS

A controlled clinical trial was performed in 40 patients admitted to an ICU with the diagnosis of acute myocardial infarction (AMI). The patients were divided into two groups: group A with a continuous perfusion of NTG at 2 mg/h, and group B with 4 mg/h. At 30 minutes of the infusion, NTG was exchanged for a placebo in half of the patients of each group (subgroups A1 and B1) with the remaining patients continuing with NTG for 24 h (A2 and B2). The hemodynamic variables studied were central venous pressure (CVP), pulmonary capillary pressure (PCP), mean pulmonary artery pressure (PAP) and mean blood pressure (BP).

RESULTS

The patients in group A showed a decrease in the hemodynamic effects in all the variables studied. At 24 h of infusion no differences were observed with respect to the previous NTG values for CVP and PCP, with significant differences being maintained for PAP and BP. In group B the hemodynamic effect was maintained for all the variables during the 24 hours studied.

CONCLUSIONS

In patients with acute myocardial infarction, the dosis of nitroglycerin administered was found to have a determined influence in the appearance of drug tolerance with this fact being more evident on evaluation of the effect of nitroglycerin on the venous system.

Thyroid hormone regulation of myosin heavy chain isoform composition in young and old rats, with special reference to IIX myosin

The effects of 4 weeks of thyroid hormone (3,5,3'-triiodothyronine, T3) treatment on the myosin heavy chain (MHC) composition were compared in the slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscles from young (3-6 months) and old (20-24 months) male albino rats. Four MHC isoforms were separated on silver-stained 6% sodium dodecyl sulphate polyacrylamide gel electrophoresis. According to immunoblotting experiments with specific MHC monoclonal antibodies, the four MHCs corresponded to types I, IIB, IIX and IIA. In the soleus, the type I MHC content was higher in the old than in the young animals, and the type IIA content lower. Type IIX myosin was observed in some young control soleus, but not in old ones. After T3 treatment, the content of type I MHC decreased substantially in both young and old animals and that of type IIA increased. After T3 treatment, type IIX myosin was observed in both young and old animals, with a slightly higher IIX myosin content in old age, but the age-related difference in the contents of types I and IIA was diminished. In EDL, the type IIX MHC content was significantly higher in the old animals, at the expense of a lower content of type IIB MHC. MHC composition was not affected significantly by T3 treatment in EDL, either in young or old animals. In conclusion, an age-related motor unit transformation is observed in both the slow-twitch soleus and the fast-twitch EDL and the capacity for MHC isoform switching in response to T3 treatment is not impaired in old age.

Type IIx myosin heavy chain transcripts are expressed in type IIb fibers of human skeletal muscle

Several members of the sarcomeric myosin heavy chain (MHC) gene family have been mapped in the human genome but many of them have not yet been identified. In this study we report the identification of two human skeletal MHC genes as fast IIa and IIx MHC based on pattern of expression and sequence homology with the corresponding rat genes in the 3'-translated and untranslated regions. The distribution of these two gene products as well as that of the beta/slow MHC gene was analyzed in human skeletal muscles by in situ hybridization. The distribution of beta/slow, IIa, and IIx MHC transcripts defines three major muscle fiber types expressing a single MHC mRNA, i.e., either beta/slow, IIa, or IIx MHC mRNA, and two populations of hybrid fibers coexpressing beta/slow with IIa or IIa with IIx MHC mRNA. Fiber typing by ATPase histochemistry shows that IIa MHC transcripts are more abundant in histochemical type IIa fibers, whereas IIx MHC transcripts are more abundant in histochemical type IIb fibers.

Differences in myosin composition between human oro-facial, masticatory and limb muscles: enzyme-, immunohisto- and biochemical studies

Immunohistochemistry was used to determine the myosin composition of defined fibre types of three embryologically different adult muscles, the oro-facial, masseter and limb muscles. In addition, the myosin composition in whole muscle specimens was analysed with biochemical methods. Both similarities and differences between muscles in the content of myosin heavy chains and myosin light chains were found. Nevertheless, each muscle had its own distinct identity. Our results indicated the presence of a previously undetected fast myosin heavy chain isoform in the oro-facial type II fibre population, tentatively termed 'fast F'. The masseter contained aberrant myosin isoforms, such as foetal myosin heavy chain and alpha-cardiac myosin heavy chain and unique combinations of myosin heavy chain isoforms which were not found in the limb or oro-facial muscles. The type IM and IIC fibres coexpressed slow and fast A myosin heavy chains in the oro-facial and limb muscles but slow and a fast B like myosin heavy chain in the masseter. While single oro-facial and limb muscle fibres contained one or two myosin heavy chain types, single masseter fibres coexpressed up to four different myosin heavy chain isoforms. Describing the fibres according to their expression of myosin heavy chain isozymes, up to five fibre types could be distinguished in the oro-facial and limb muscles and eight in the masseter. Oro-facial and limb muscles expressed five myosin light chains, MLC1S, MLC2S, MLC1F, MLC2F and MLC3F, and the masseter four, MLC1S, MLC2S, MLC1F, and, in addition, an embryonic myosin light chain, MLC1emb, which is usually not present in normal adult skeletal muscle. These results probably reflect the way the muscles have evolved to meet the specialized functional requirements imposed upon them and are in agreement with the previously proposed concept that jaw and limb muscles belong to two distinct allotypes.

Tension production and thin-filament protein isoforms in developing rat myocardium

The calcium sensitivity of tension production and the expression of troponin I (TnI) and troponin T (TnT) isoforms in skinned neonatal (7 days after birth) and adult rat myocardium were determined. Isometric tension was measured after activation at a known resting sarcomere length in ventricular trabeculae at adult and, for the first time, neonatal ages. Analysis of the tension-pCa relationships indicates a greater calcium sensitivity (approximately 0.3 pCa units) in neonatal ventricular trabeculae compared with adult trabeculae. The maximal isometric tension-generating ability (i.e., tension-tissue cross-sectional area) is threefold greater in adult compared with neonatal trabeculae. Developmental transitions in TnI and TnT isoform expression in atrial and ventricular tissue were examined simultaneously and were found to be dissimilar. Shifts in the expression of TnT isoforms precede shifts in TnI isoforms in ventricular tissue. The opposite pattern occurs in atrial tissue, with shifts in TnI preceding those in TnT. The results show that the greater calcium sensitivity of neonatal compared with adult rat ventricular tissue is associated with developmental changes in both TnT and TnI isoform expressions. These isoform expression patterns may facilitate myocardial tension production at the neonatal stage, when the tension-generating ability of individual trabeculae is much lower than that in the adult.

Myosin isoforms in mammalian skeletal muscle

Skeletal muscles of different mammalian species contain four major myosin heavy-chain (MHC) isoforms: the "slow" or beta-MHC and the three "fast" IIa-, IIx-, and IIb-MHCs; and three major myosin light-chain (MLC) isoforms, the "slow" MLC1s and the two "fast" MLC1f and MLC3f. The differential distribution of the MHCs defines four major fiber types containing a single MHC isoform and a number of intermediate hybrid fiber populations containing both beta/slow- and IIa-MHC, IIa- and IIx-MHC, or IIx- and IIb-MHC. The IIa-, IIx-, and IIb-MHCs were first detected in neonatal muscles, and their expression in developing and adult muscle is regulated by neural, hormonal, and mechanical factors. The transcriptional mechanisms responsible for the fiber type-specific regulation of MHC and MLC gene expression are not known and are presently being explored by in vivo transfection experiments. The functional role of MHC isoforms has been in part clarified by correlated biochemical-physiological studies on single skinned fibers: these studies, in agreement with results from in vitro motility assays, indicate that both MHC and MLC isoforms determine the maximum velocity of shortening of skeletal muscle fibers.

Maximum shortening velocity and coexistence of myosin heavy chain isoforms in single skinned fast fibres of rat skeletal muscle

Myosin heavy chain composition of a large number (288) of single fibres from slow (soleus), and fast (superficial part of tibialis anterior, and plantaris) muscles of adult (3-5-month-old) Wistar rats was determined. A combination of SDS-PAGE and monoclonal antibodies against myosin heavy chains allowed to identify four myosin heavy chain isoforms (1, 2A, 2X, and 2B) and to detect myosin heavy chain coexistence. Four groups of fibres containing only one myosin heavy chain (1 myosin heavy chain, 2A myosin heavy chain, 2X myosin heavy chain, and 2B myosin heavy chain), and five groups containing more than one myosin heavy chain (1 and 2A myosin heavy chains, 2A and 2X myosin heavy chains, 2X and minor amounts of 2B (2X-2B fibres), 2B and minor amounts of 2X (2B-2X fibres), and 2A, 2X, and 2B myosin heavy chain were identified and their relative percentages were assessed. Coexistence of fast myosin heavy chain isoforms was found to be very frequent (50% of the fibres in plantaris, and 30% in tibialis anterior), whereas coexistence of slow and fast (2A) myosin heavy chain was very rare. Maximum shortening velocity (V0) was determined using the slack-test procedure in a subset of 109 fast fibres from the above population. The values of V0 formed a continuum extending from 2A to 2X to 2X-2B to 2B-2X to 2B fibres. 2A fibres had the lowest value of V0 and 2B fibres the highest. Only the differences between 2A and 2B and 2A and 2B-2X fibres were statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Unloaded shortening velocity and myosin heavy chain and alkali light chain isoform composition in rat skeletal muscle fibres

1. This study aims to assess the role of myosin heavy chain (MHC) and alkali myosin light chain (MLC) isoforms in determining maximum velocity of shortening in fast skeletal muscle fibres. 2. The maximum velocity of shortening as determined by the slack test (Vo) was tested for its relationship with MHC composition and with alkali MLC isoform ratio of fast fibres of known MHC composition. 3. MHC isoform composition was determined using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and monoclonal antibodies against MHCs, and combining the results obtained using the two methods. Three groups of fast fibres containing only one MHC isoform were identified: IIA, IIX and IIB fibres containing respectively IIA MHC, IIX MHC and IIB MHC. Fibres containing more than one MHC isoform were discarded. 4. The mean Vo value of IIA fibres was 2.33 +/- 0.29 muscle lengths per second (L s-1; mean +/- S.D.), this was significantly lower than that for IIX fibres (3.07 +/- 0.70 L s-1) which in turn had a mean Vo value significantly lower than that for IIB fibres (3.69 +/- 1.01 L s-1). 5. The relative proportion of alkali MLC isoforms (MLC3f, MLC1f) was determined by means of electrophoretic separation and densitometric quantification and was expressed as MLC3f/MLC2f with reference to the dithio-nitrobenzoic acid (DTNB) light chain (MLC2f). The mean value of the MLC3f/MLC2f ratio was significantly lower in IIA than in IIX and IIB fibres. 6. Vo was found to be proportional to the relative content of MLC3f in IIA, IIX and IIB fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and regulation of the mouse cardiac troponin I gene

The gene coding for mouse cardiac troponin I (TnI) has been cloned and sequenced. The cardiac TnI gene contains 8 exons and has an exon-intron organization similar to the quail fast skeletal TnI gene except for the region of exons 1-3, which is highly divergent. Comparative analysis suggests that cardiac TnI exon 1 corresponds to fast TnI exons 1 and 2 and that cardiac exon 3, which codes for most of the cardiac-specific amino-terminal extension and has no counterpart in the fast gene, evolved by exon insertion/deletion. The amino acid sequence of cardiac TnI exon 4 shows limited homology (36% identity) with fast TnI exon 4 but is remarkably similar (79% identity) to the corresponding sequence of slow TnI, possibly reflecting an isoform-specific TnC-binding site. The cardiac TnI gene is one of the very few contractile protein genes expressed exclusively in cardiac muscle. To identify the regulatory sequences responsible for the cardiac-specific expression of this gene we transfected cultured cardiac and skeletal muscle cells with fragments up to 4.0 kilobases of the 5'-flanking region linked to a reporter gene. Deletion analysis reveals four major regions in the 5'-flanking sequence, a minimal promoter region, which directs expression at low level in cardiac and skeletal muscle cells, and two upstream cardiac-specific positive regions separated by a negative region.

Gene transfer in regenerating muscle

We have compared the efficiency of direct gene transfer in normal and regenerating rat skeletal muscle. Muscle necrosis and regeneration was induced by intramuscular injection of bupivacaine in the soleus muscle of adult rats. Plasmids containing beta-galactosidase (beta-gal) or chloramphenicol acetyltransferase (CAT) genes driven by viral promoters were injected 3 days after bupivacaine treatment into the regenerating and the contralateral uninjured muscles. Expression of CAT activity was > 80-fold higher in regenerating compared to control muscles at 7 days post-transfection, but decreased at 30 and 60 days. Southern blot analysis showed that the predominant form of CAT DNA was episomal in transfected muscles; however, CAT activity measurements performed on the same transfected muscles showed no precise correlation between enzymatic activity and amount of plasmid DNA. Expression of beta-gal was detected in numerous regenerating fibers of the injured soleus muscles at 7 days post-transfection; in contrast, only rare positive fibers were found in control muscles. Focal infiltrates of mononuclear cells, which surround and invade selectively beta-gal-positive fiber segments, were observed at 30 days post-transfection, suggesting that immune mechanisms are implicated in the progressive loss of transgenes with time. The finding that regenerating muscle fibers display a higher efficiency of transfection may be relevant to gene therapy of Duchenne muscular dystrophy, because regenerating fibers are numerous in the early stages of the disease.

Type 2X-myosin heavy chain is coded by a muscle fiber type-specific and developmentally regulated gene

We have previously reported the identification of a distinct myosin heavy chain (MyHC) isoform in a major subpopulation of rat skeletal muscle fibers, referred to as 2X fibers (Schiaffino, S., L. Gorza, S. Sartore, L. Saggin, M. Vianello, K. Gundersen, and T. Lømo. 1989. J. Muscle Res. Cell Motil. 10:197-205). However, it was not known whether 2X-MyHC is the product of posttranslational modification of other MyHCs or is coded by a distinct mRNA. We report here the isolation and characterization of cDNAs coding a MyHC isoform that is expressed in type 2X skeletal muscle fibers. 2X-MyHC transcripts differ from other MyHC transcripts in their restriction map and 3' end sequence and are thus derived from a distinct gene. In situ hybridization analyses show that 2X-MyHC transcripts are expressed at high levels in the diaphragm and fast hindlimb muscles and can be coexpressed either with 2B- or 2A-MyHC transcripts in a number of fibers. At the single fiber level the distribution of each MyHC mRNA closely matches that of the corresponding protein, determined by specific antibodies on serial sections. In hindlimb muscles 2X-, 2A-, and 2B-MyHC transcripts are first detected by postnatal day 2-5 and display from the earliest stages a distinct pattern of distribution in different muscles and different fibers. The emergence of type 2 MyHC isoforms thus defines a distinct neonatal phase of fiber type differentiation during muscle development. The functional significance of MyHC isoforms is discussed with particular reference to the velocity of shortening of skeletal muscle fibers.