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

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.

Characterization of human skeletal muscle fibres according to the myosin heavy chains they express

Using a method of single muscle fibre analysis, we investigated the presence of RNA transcripts for various isoforms of the myosin heavy chain (MyoHC) gene in histochemically, immunohistochemically and electrophoretically characterized individual muscle fibres (n = 65) from adult human vastus lateralis muscle. A cDNA clone isolated in this study was shown to contain the 3' end of a previously uncharacterized human MyoHC gene which is expressed specifically in human fast IIA muscle fibres and we conclude that this clone contains part of the human fast IIA MyoHC gene. In all the fibres histochemically, immunohistochemically and electrophoretically characterized as containing the previously classified IIB MyoHC (n = 23), it was shown that the human equivalent to the rat type IIX MyoHC gene is expressed. This observation was taken to suggest that the previously classified IIB muscles fibres in human muscle express a MyoHC isoform equivalent to the rat IIX, not the IIB, and would therefore be more accurately classified as IIX fibres.

A novel synapse-associated noncoding RNA

Synaptic nuclei of innervated muscle transcribe acetylcholine receptor (AChR) genes at a much higher level than extrasynaptic nuclei. To isolate candidate synaptic regulatory molecules responsible for the unique transcriptional potential of synaptic nuclei, we have taken a subtractive hybridization approach. Here, we report the cloning and characterization of a novel synapse-associated RNA, 7H4. 7H4 is expressed selectively in the endplate zone of skeletal muscle and is upregulated during early postnatal development and after denervation. Interestingly, the 7H4 gene has no introns, and yet two different-size RNAs with identical polyadenylated 3' ends are generated. Most intriguingly, the nucleotide sequence does not contain any significant open reading frames, suggesting that 7H4 may function as a noncoding RNA.

A novel synapse-associated noncoding RNA

Synaptic nuclei of innervated muscle transcribe acetylcholine receptor (AChR) genes at a much higher level than extrasynaptic nuclei. To isolate candidate synaptic regulatory molecules responsible for the unique transcriptional potential of synaptic nuclei, we have taken a subtractive hybridization approach. Here, we report the cloning and characterization of a novel synapse-associated RNA, 7H4. 7H4 is expressed selectively in the endplate zone of skeletal muscle and is upregulated during early postnatal development and after denervation. Interestingly, the 7H4 gene has no introns, and yet two different-size RNAs with identical polyadenylated 3' ends are generated. Most intriguingly, the nucleotide sequence does not contain any significant open reading frames, suggesting that 7H4 may function as a noncoding RNA.

Identification, distribution, and myosin subunit composition of type IIX fibers in mouse muscles

The purpose of the present investigation was to study the distribution and subunit composition of type IIX fibers in mouse muscles. The existence of a population of type IIX fibers in fast-twitch muscles of the mouse was shown by mean of immunohistochemistry and gel electrophoresis. In the hindlimb muscles, tibialis anterior (TA) and extensor digitorum longus (EDL), type IIX fibers account for approximately one third of the total fiber number, with the superficial portion of the TA (TAS) being composed exclusively of type IIB and IIX fibers. A similar proportion of IIX fibers was found in diaphragm (DIA) while in tongue muscles approximately 40% of the fibers were IIX. Single fiber gel electrophoresis revealed a significant number of fibers in TAS that contain both IIB and IIX myosin heavy chain (MyHC). This was confirmed with immunohistochemistry, which revealed the presence of fibers with various degrees of staining intensity. This suggests that there may exist a degree of plasticity which results in the conversion of IIX fibers to IIB fibers and vice versa. Analysis of myosin light chain (MyLC) composition of type IIX fibers revealed that the ratio of MyLC3f to MyLC1f was significantly lower than in type IIB fibers.

Fast myosin heavy chains expressed in secondary mammalian muscle fibers at the time of their inception

Mammalian skeletal muscle is generated by two waves of fiber formation, resulting in primary and secondary fibers. These fibers mature to give rise to several classes of adult muscle fibers with distinct contractile properties. Here we describe fast myosin heavy chain (MyHC) isoforms that are expressed in nascent secondary, but not primary, fibers in the early development of rat and human muscle. These fast MyHCs are distinct from previously described embryonic and neonatal fast MyHCs. To identify these MyHCs, monoclonal antibodies were used whose specificity was determined in western blots of MyHCs on denaturing gels and reactivity with muscle tissue at various stages of development. To facilitate a comparison of our results with those of others obtained using different antibodies or species, we have identified cDNAs that encode the epitopes recognized by our antibodies wherever possible. The results suggest that epitopes characteristic of adult fast MyHCs are expressed very early in muscle fiber development and distinguish newly formed secondary fibers from primary fibers. This marker of secondary fibers, which is detectable at the time of their inception, should prove useful in future studies of the derivation of primary and secondary fibers in mammalian muscle development.

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)

Myosin heavy chain isoforms in single fibres from m. vastus lateralis of sprinters: influence of training

The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis of a group of male sprint athletes (n = 6) was analysed, before and after a three months period of intensive strength- and interval-training, using a sensitive gel electrophoretic technique. Significant improvements were observed after training in almost all of a series of performance tests. After training the sprinters revealed a decrease in fibres containing only MHC isoform I (52.0 +/- 3.0% vs. 41.2 +/- 4.7% (mean +/- SE) (P < 0.05)) and an increase in the amount of fibres containing only MHC isoform IIA (34.7 +/- 6.1% vs. 52.3 +/- 3.6% (P < 0.05)). Fibres showing co-existence of MHC isoforms IIA and IIB decreased with training (12.9 +/- 5.0% vs. 5.1 +/- 3.1% (P < 0.05)). Only one out of 1000 fibres analysed contained only MHC isoform IIB. In contrast, a higher amount of type IIB fibres (18.8 +/- 3.6% vs. 10.5 +/- 3.9%, (P < 0.05)) was observed with myofibrillar ATPase histochemistry. The majority of histochemically determined type IIB fibres of sprinters seems therefore to contain both MHC isoforms IIA and IIB. Sprint-training appears to induce an increased expression of MHC isoform IIA in skeletal muscles. This seems related to a bi-directional transformation from both MHC isoforms I and IIB towards MHC isoform IIA.