Potential phasic and tonic muscles express a common set of fast and slow myosin light chains and fast tropomyosin during early development of chick embryo

Promoter elements and transcriptional control of the chicken tropomyosin gene [corrected]

The chicken beta tropomyosin (beta TM) gene has two alternative transcription start sites (sk and nmCAP sites) which are used in muscle or non muscle tissues respectively. In order to understand the mechanisms involved in the tissue-specific and developmentally-regulated expression of the beta TM gene, we have analyzed the 5' regions associated with each CAP site. Truncated regions 5' to the nmCAP site were inserted upstream to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene and these constructs were transfected into avian myogenic and non myogenic cells. The maximum transcription is driven by the CAT construct (-168/ + 216 nt) in all cell types. Previous deletion analysis of the region 5' to the beta TMskCAP site has indicated that 805 nt confer myotube-specific transcription. In this work, we characterized an enhancer element (-201/-68 nt) which contains an E box (-177), a variant CArG box (-104) and a stretch of 7Cs (-147). Mutation of any of these motifs results in a decrease of the myotube-specific transcriptional activity. Electrophoretic mobility shift assays indicate that these cis-acting sequences specifically bind nuclear proteins. This enhancer functions in an orientation-dependent manner.

Denervated chicken breast muscle displays discoordinate regulation and differential patterns of expression of alpha f and beta tropomyosin genes

The expression of the alpha fast (alpha f) and beta tropomyosin (TM) genes has been analysed with muscle-specific and common cDNA probes after unilateral nerve section of the pectoralis major muscle (PM) in 4-week-old chickens. The following were observed in denervated muscles. (1) The beta TM mRNA, which was repressed during development, reaccumulates in a biphasic curve with the increase in the beta TM protein lagging behind the changes in its mRNA. Accordingly, no beta TM is seen in products translated in vitro from total and polyA+ RNA obtained 1 week after denervation. No such translation block is seen with RNA obtained from control or muscles denervated for 6 weeks. (2) No changes in the alpha fTM mRNA and corresponding protein are observed. (3) RNA processing of the two genes is not changed. (4) In the contralateral muscles, transitory increases in alpha f and beta TM mRNAs are observed while the corresponding proteins remain unchanged. Our data suggest that muscle fibres display early and long-term responses to the loss of neural input which might result from a combination of changes produced by regenerative processes and reprogramming of existing fibres. Moreover, in contrast to normal development, no reciprocal changes of alpha f and beta TM expression are seen in denervated muscles.

Myosin heavy chain composition of single cells from avian slow skeletal muscle is strongly correlated with velocity of shortening during development

We have determined the myosin heavy chain (MHC) composition (using a sensitive sodium dodecyl sulfate-polyacrylamide gel electrophoresis system) and the maximal velocity of shortening (Vmax) of single cells from neonatal and adult chicken anterior latissimus dorsi (ALD) muscles. In addition, the MHC, myosin light chain, and regulatory protein (i.e., troponin and tropomyosin subunits) compositions of bundles of ALD fibers were determined at late embryonic, neonatal, and adult ages. At young ages, there are two MHCs in ALD muscle, SM1 and SM2, with SM1 decreasing in relative amount with increasing age, as shown previously by others. The mean Vmax of single fibers also decreases from neonatal to adult ages. A strong quantitative correlation is demonstrated between the specific MHC composition and Vmax among individual cells of the ALD muscle at several ages. Since virtually no changes occur in the regulatory protein and myosin light chain compositions of the ALD muscle between late embryonic and adult ages, it appears that the MHC composition of an individual cell in this muscle is the primary determinant of the maximal shortening velocity. These results are the first to illustrate the functional significance of the developmental transition in myosin heavy chain composition of an avian slow skeletal muscle, consistent with our previous findings on mammalian muscle.

Chick and quail limb bud myoblasts, isolated at different times during muscle development, express stage-specific phenotypes when differentiated in culture

Evidence is presented which shows that myoblasts, isolated at different stages during chick and quail limb bud development, will form, in culture, myotubes which can be distinguished with a combination of morphological as well as biochemical criteria. Hind limb bud myoblasts isolated from 5-day-old embryos form very short myotubes which synthesize a myosin, the light chains of which are predominantly LC1F and LC2S. Myoblasts isolated from the limb buds of 7-8-day-old embryos form large myotubes which synthesize a myosin the light chains of which are predominantly LC1F, LC2S and LC2F. Myoblasts isolated from the thigh muscle of embryos older than 10 days form large myotubes which synthesize a myosin the light chains of which are predominantly LC1F and LC2F. These results have been confirmed by hybridization of the cellular mRNA with a molecular probe specific for LC2F. These results lead us to suggest the existence of at least two classes of myoblasts which appear at different times during limb bud development. The first class, or 'early' myoblasts, is present in the limb buds of 5-day-old embryos, whereas the second class, or 'late' myoblasts, is present in the muscles of embryos older than 8 days. This result, however, is also compatible with the hypothesis that all muscle cells are the same at all times during development, and that the different phenotypes simply reflect differences in the environmental conditions.

On the histochemical characterization and distribution of fast and slow muscle fibers in certain avian skeletal muscles

Histochemical techniques based upon the pH sensitivity of myofibrillar adenosine triphosphatase are used to differentiate avian skeletal muscle fibers as either fast or slow. Pars thoracica m. pectoralis (PM) of several avian species, Pars cranialis m. latissimi dorsi (LDCR) of the Japanese quail, and M. tensor propatagialis (TP) of the domestic pigeon are examined. Fast fibers predominate in the PM, and slow fibers in the LDCR. The TP shows marked internal variation in the distribution of muscle fibers. The occurrence of fast and slow muscle fibers, both intra- and inter-muscularly, is correlated with their functional adaptations.

Influence of spinal cord stimulation upon myosin light chain and tropomyosin subunit expression in a fast muscle (posterior latissimus dorsi) of the chick embryo

Latissimus dorsi muscles of the chick consist of a slow (ALD) and a fast (PLD) muscle. The influence of chronic spinal cord stimulation in the chick embryo upon the expression of myosin light chains and tropomyosin subunits was investigated. Early in development the two muscles exhibited the same ratio of alpha- and beta-tropomyosin subunits. Later, in the slow muscle the ratio beta:alpha decreased and in chicken the amounts of the two components were about the same. In the fast muscle, the alpha-subunit increased and reached 66% in young chicken. In the fast muscle, the alpha-subunit increased and reached 66% in young chicken. In the In the early stages of embryonic development, both muscles accumulated slow and fast light chains. However, in ALD the amount of slow light chains was greater than that of fast light chains and the reverse was observed in PLD muscle. Later during development, the slow components decreased in PLD while the fast components increased; the reverse was observed in ALD muscle. The fast myosin LC3f has been detected in 18-day-old embryonic PLD. Chronic spinal cord stimulation at a low rhythm was performed from day 10 of embryonic development to day 15 or 16. In both muscles from spinal cord-stimulated embryos, the beta-tropomyosin subunit was lower than in control embryos. In ALD, the pattern of light chains was unaffected by chronic stimulation while in PLD muscle the slow and fast components were modified. In particular the ratio LCs:LCf was increased in spinal cord-stimulated embryos with regard to controls.