Satellite cell proliferation in low frequency-stimulated fast muscle of hypothyroid rat

Satellite cell proliferation was assessed in low-frequency-stimulated hypothyroid rat fast-twitch muscle by 5-bromo-2'-deoxyuridine (BrdU) labeling and subsequent staining of labeled muscle nuclei, and by staining for proliferating cell nuclear antigen (PCNA). BrdU labeling and PCNA staining were highly correlated and increased approximately fourfold at 5 days of stimulation, decayed thereafter, but remained elevated over control in 10- and 20-day stimulated muscles. Myogenin mRNA was approximately 4-fold elevated at 5 days and 1.5-fold at 10 days. Staining for myogenin protein yielded results similar to that for PCNA and BrdU. Furthermore, a detailed examination of the pattern of myogenin staining revealed that the number of myogenin-positive nuclei was elevated in the fast pure IIB fiber population at 5 and 10 days of chronic low-frequency stimulation. By 20 days, myogenin staining was observed in transforming fast fibers that coexpressed embryonic and adult myosin heavy chain isoforms. In the slower fiber populations (i.e., IIA and I), myogenin-positive transforming fibers that coexpressed embryonic myosin heavy chain, appeared already at 5 days. Thus the satellite cell progeny on slower fibers seemed to proliferate less and to fuse earlier to their associated fibers than the satellite cell progeny on fast fibers. We suggest that the increase in muscle nuclei of the fast fibers might be a prerequisite for fast-to-slow fiber type transitions.

Changes in FGF and FGF receptor expression in low-frequency-stimulated rat muscles and rat satellite cell cultures

This study compares effects of chronic electrical stimulation on the expression levels of FGF-1, FGF-2 and their receptors (FGFRI, FGFR4) in rat tibialis anterior (TA) muscle of hypothyroid rat, as well as in satellite cell cultures derived from normal rat TA and soleus (SOL) muscles. In 5-day (5-d)-stimulated hypothyroid TA muscle, FGF-1 and FGF-2 mRNA levels were threefold elevated over control. FGFR1 and FGFR4 mRNAs were twofold and 1.5-fold elevated, respectively. In longer stimulated muscles, FGF-1 and FGFR4 mRNAs returned to basal levels, whereas FGF-2 mRNA remained elevated. FGFR1 mRNA decreased to control levels in 10-d stimulated muscles, but increased again after 20 days of stimulation. SOL- and TA-derived satellite cell cultures were stimulated for 5 days. At this time point, changes in myosin heavy chain isoforms were detectable consisting of increases in MHCI mRNA and decreases in MHCIIb and MHCIId mRNA. The comparison between 5-d-stimulated hypothyroid TA muscle and 5-d-stimulated TA- and SOL-derived satellite cell cultures revealed differences in the expression of FGF-1 and FGF-2, but similar expression levels of FGFR1 and FGFR4. Even though FGF-1 and FGF-2 mRNAs were elevated in the satellite cell cultures, their increases were less pronounced than in the stimulated hypothyroid muscle. Taking into consideration that skeletal muscle contains muscle fibres and various non-muscle tissues, e.g. blood vessels, these results suggest that the latter contribute to the observed increases in FGF-1 and FGF-2 expression in stimulated muscle.

Evidence that acidic fibroblast growth factor promotes maturation of rat satellite-cell-derived myotubes in vitro

Satellite cells isolated from fast tibialis anterior (TA) and slow soleus (SOL) rat muscles were cultivated on matrigel, and treated with acidic fibroblast growth factor (aFGF). The following observations were made: 1) aFGF-treated cultures exhibited enhanced proliferation as mirrored by a twofold increase in DNA content. 2) Compared to the untreated cultures, myotubes in the aFGF cultures were larger; 3) Using reverse transcriptase polymerase chain reaction (RT-PCR) and northern blot analyses, we observed enhanced expression of all adult myosin heavy chain (MHC) isoforms, as well as of myogenin. These findings indicate that, under the culture conditions used, aFGF has a stimulatory effect on proliferation but also on maturation and differentiation of satellite cells. Furthermore, transcript levels of FGF receptor 1 (FGFR1) and 4 (FGFR4) isoforms, as well as of aFGF and bFGF were assessed by RT-PCR. aFGF-treated myotubes displayed increased expression of aFGF and bFGF, suggesting a paracrine effect of exogenous aFGF. In this regard, SOL-derived cultures responded more strongly than TA-derived cultures. The effects of aFGF treatment on the two receptors consisted of a decrease in FGFR1 and an increase in FGFR4 mRNA levels in 5-day-old cultures. In 8-day-old TA cultures, effects of FGF were similar to those in 5-day-old cultures. 8-day FGF-treated SOL cultures treated with FGF for 8 days exhibited higher FGFR1 and FGFR4 mRNA levels than the respective untreated cultures. Compared to 5 day-treated cultures, FGFR1 increased and FGFR4 decreased. This led to a shift in the ratio of FGFR1 to FGFR4 in the FGF-treated cultures which may explain the ability of satellite cells to differentiate under the influence of aFGF.

Changes in satellite cell content and myosin isoforms in low-frequency-stimulated fast muscle of hypothyroid rat

Chronic low-frequency stimulation was used to study the effects of enhanced contractile activity on satellite cell content and myosin isoform expression in extensor digitorum longus muscles from hypothyroid rats. As verified by immunohistochemical staining for desmin, vimentin, and myosin heavy chain (MHC) isoforms and by histological analysis, stimulation induced a transformation of existing fast fibers toward slower fibers without signs of fiber deterioration or regeneration. Immunohistochemically detected increases in MHC I and MHC IIa isoforms, as well as reduced numbers of fibers expressing the faster MHC isoforms, mirrored the rearrangement of the thick-filament composition. These changes, especially the upregulation of MHC IIa, were accompanied by an induction of developmental MHC isoforms in the transforming adult fibers. Satellite cell content rose 2.6-, 3.0-, and 3.7-fold over that of corresponding controls (P < 0.05 in all cases) in 5-, 10-, and 20-day-stimulated muscles, respectively. Hypothyroidism alone had no effect on satellite cell content but resulted in a significant reduction in fiber size. The relative satellite cell contents increased (P < 0.05) from 3.8% in euthyroid control muscles to 7.9, 11.5, and 13.8% in the 5-, 10-, and 20-day-stimulated hypothyroid muscles, respectively. In 20-day-stimulated muscles, the relative satellite cell content reached an almost twofold higher level than that of normal slow-twitch soleus muscle. This increase occurred concomitantly with a rise in myonuclear density, most probably because of the fusion of satellite cells with existing fibers.

Effects of chronic electrical stimulation on myosin heavy chain expression in satellite cell cultures derived from rat muscles of different fiber-type composition

Myotube cultures were established from satellite cells of three rat muscles of different fiber-type composition, slow-twitch soleus, diaphragm, and fast-twitch tibialis anterior (TA). Effects of chronic electrical stimulation were studied by exposing these cultures for up to 13 days to a stimulus pattern consisting of 250 ms impulse trains of 40 Hz, repeated every 4 s. Changes in myosin expression were assessed at the mRNA level by Northern blotting and in situ hybridization. Expression of slow myosin at the protein level was analysed by immunoblotting and immunohistochemistry with two antibodies, one specific to adult slow myosin, the other reacting with developmental and adult slow myosin heavy chain (MHCI) isoforms. In all three myotube cultures stimulation enhanced the mRNA and protein expression of a developmental isoform of slow myosin (MHCI). However, the three myotube cultures differed in the extent of the increase in MHCI. It was greatest in soleus-derived myotubes, least in TA-derived myotubes, and intermediate in diaphragm-derived myotubes. In addition to the increase in slow myosin, long-term stimulation led to an isoform switch, as indicated by an increase in myotubes reacting with the antibody specific for the adult MHCI. Our results suggest that enhanced contractile activity promotes the expression of the slow phenotype predetermined in satellite cells of slow-twitch, type I fibers. The different extents of increased slow myosin expression may thus be explained as reflecting different percentages of type I fibers and consequently of slow-type satellite cells in the corresponding donor muscles.

Satellite cells from slow rat muscle express slow myosin under appropriate culture conditions

Satellite cells were isolated at high yields from slow-twitch soleus and fast-twitch tibialis anterior (TA) muscles of adult male Wistar rats. The number of satellite cells isolated from soleus muscle exceeded that from TA muscles by a factor of three. A comparison of satellite cells grown on gelatin- or Matrigel-coated dishes revealed that Matrigel greatly enhances the maturation of the satellite-cell-derived myotubes. As judged from immunohistochemistry, myosin heavy chain electrophoresis and immunoblot analyses, only cells grown on Matrigel, but not on gelatin, expressed adult myosin isoforms. Slow myosin expression was only detected in Matrigel cultures. Soleus cultures contained, in addition to the majority of myotubes expressing fast myosin, a small fraction (maximally 10%) of myotubes coexpressing fast and slow myosins. The number of fast/slow myosin-containing myotubes was negligible in TA cultures. The expression of slow myosin increased with age. Slow myosin was nonuniformly distributed along the length of specific myotubes and accumulated around some myonuclei. These results point to the existence of myotubes with a heterogeneous population of myonuclei, probably resulting from fusion of differently preprogrammed satellite cells. We suggest that the patch-like expression of slow myosin results from local accumulation of myonuclei of slow-type satellite cells.

Metabolite patterns related to exhaustion, recovery and transformation of chronically stimulated rabbit fast-twitch muscle

Rabbit fast-twitch tibialis anterior muscle was subjected to chronic low-frequency stimulation (10 Hz, 24 h/day). Measurements of the time course of changes in the concentration of metabolites of energy metabolism were performed in order to test the hypothesis whether or not alterations in the metabolite profile might represent possible signals for triggering muscle fibre type transformation. Most of the investigated metabolites displayed triphasic changes in response to persistently increased contractile activity. During the first 15 min of stimulation, drastic reductions were observed for adenosine triphosphate (ATP, 56%), phosphocreatine (PCr, 60%) and glycogen (76%), as well as 3- to 4-fold and 10-fold increases for glucose and lactate, respectively. This early metabolic perturbance coincided with a rapid reduction of isometric force. The next phase, extending to 4 days of stimulation, was characterized by a nearly complete recovery of ATP and PCr, and an overshoot in glycogen. The first signs of metabolic recovery were already detectable in 60-min-stimulated muscle when isometric force was still markedly depressed. These results demonstrated an impressive capability of the muscle to recover with ongoing stimulation from an initial, dramatic disturbance in energy metabolism. During the final phase, extending to 50 days, the metabolite profile approached that of a slow-twitch muscle with moderate reductions in total adenine nucleotides, ATP, total creatine, PCr and glycogen. A conspicuous result was the finding that, contrary to the recovery of most metabolites, the ratio of ATP to the product of free adenosine diphosphate and resting free inorganic phosphate was persistently depressed with ongoing stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered gene expression in fast-twitch muscle induced by chronic low-frequency stimulation

Increased neuromuscular activity via chronic low-frequency stimulation induces multiple fast-to-slow transitions in phenotypic properties that ultimately lead to fiber type conversions in the fast-twitch muscle of small mammals. Most of these alterations occur in an ordered sequence and result from the sequentially altered expression of myofibrillar and other protein isoforms. These changes relate to altered levels of specific mRNAs, followed by alterations in protein synthesis. As shown by the exchange of myosin heavy chain isoforms, protein degradation may be an additional control factor involved in the rearrangement of the myofibrillar apparatus. The degree of the various fast-to-slow transitions is species dependent and may be related to differences in thyroid hormone levels. It is suggested that the drastically and persistently depressed phosphorylation potential of the ATP system possibly serves to trigger the transformation process.

Characterization of myosin isoforms in satellite cell cultures from adult rat diaphragm, soleus and tibialis anterior muscles

Satellite cells were isolated by enzymatic dissociation and Percoll gradient centrifugation from adult rat diaphragm, soleus, and tibialis anterior muscles with fairly reproducible yields. Diaphragm and soleus muscle yielded approximately five times more satellite cells than tibialis anterior muscle. According to light microscopic criteria, no morphological differences existed between the satellite cell cultures of different origin. Contrary to the donor muscles, myotubes from the 10-day-cultured satellite cells contained a uniform myosin heavy chain (MHC) pattern with predominance of an immunochemically identified embryonic heavy chain. The three types of cultures displayed a typical embryonic light chain (LC) pattern with LC1emb, LC1f, LC2f, and traces of LC3f. The isomyosin pattern was characterized by four embryonic isomyosins, eM1-eM4, with similar distributions in the three cultures. In summary, these myosin analyses provide no evidence for the existence of satellite cell diversity among three rat muscles of different fiber-type composition, at least not under the applied in vitro conditions.

Effects of electrically induced contractile activity on cultured embryonic chick breast muscle cells

Development of chicken breast muscle is characterized by the sequential appearance of six electrophoretically distinct myosin heavy chain (HC) isoforms. Cultured secondary myotubes, derived from 12-day embryonic chick breast muscle, mainly express the early embryonic HC isoform HCemb/e, normally present in 8-day embryonic breast muscle, and the two fast light chain isoforms LC1f and LC2f. Direct low-frequency (2.5 Hz) stimulation of these myotubes via platinum electrodes leads to a shift in myosin HC expression with increases in the late embryonic HC isoform HCemb/l amounting to 35% of total HC in 19-day-stimulated cultures. Measurements of 35S-methionine incorporation and immunohistochemical analyses demonstrate increases in LC3f. This increase is also seen at the mRNA level. These results indicate that induced contractile activity promotes myotube maturation in vitro. The observation that chronic stimulation enhances the expression of the slow isoform LC2s at the RNA, as well as the protein level, suggests an additional effect consisting of a fast-to-slow change in phenotype expression. In view of the fact that muscle maturation and phenotype expression is under neural control during development in vivo, our results on directly stimulated, aneural myotubes indicate that neurally transmitted contractile activity may be an important factor in modulating phenotype expression of secondary myotubes.

Six myosin heavy chain isoforms are expressed during chick breast muscle development

Two major embryonic myosin heavy chains are expressed in embryonic chick breast muscle until the first week after hatching. Of these, one is already detected in the 8-day-old embryo. The other appears on day 12. Two putative slow embryonic isoforms represent minor components transiently expressed between days 8 and 12. A neonatal heavy chain is expressed at low concentrations on day 8 and increases with development. It is the only isoform two weeks after hatching, and is ultimately replaced by the fast myosin heavy chain in the adult muscle.