Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. II. Urodelan amphibians

The Role of Autophagy in Skeletal Muscle Diseases

Skeletal muscle is the most abundant type of tissue in human body, being involved in diverse activities and maintaining a finely tuned metabolic balance. Autophagy, characterized by the autophagosome–lysosome system with the involvement of evolutionarily conserved autophagy-related genes, is an important catabolic process and plays an essential role in energy generation and consumption, as well as substance turnover processes in skeletal muscles. Autophagy in skeletal muscles is finely tuned under the tight regulation of diverse signaling pathways, and the autophagy pathway has cross-talk with other pathways to form feedback loops under physiological conditions and metabolic stress. Altered autophagy activity characterized by either increased formation of autophagosomes or inhibition of lysosome-autophagosome fusion can lead to pathological cascades, and mutations in autophagy genes and deregulation of autophagy pathways have been identified as one of the major causes for a variety of skeleton muscle disorders. The advancement of multi-omics techniques enables further understanding of the molecular and biochemical mechanisms underlying the role of autophagy in skeletal muscle disorders, which may yield novel therapeutic targets for these disorders.

Differential muscle regulatory factor gene expression between larval and adult myogenesis in the frog Xenopus laevis: adult myogenic cell-specific myf5 upregulation and its relation to the notochord suppression of adult muscle differentiation

During Xenopus laevis metamorphosis, larval-to-adult muscle conversion depends on the differential responses of adult and larval myogenic cells to thyroid hormone. Essential differences in cell growth, differentiation, and hormone-dependent life-or-death fate have been reported between cultured larval (tail) and adult (hindlimb) myogenic cells. A previous study revealed that tail notochord cells suppress terminal differentiation in adult (but not larval) myogenic cells. However, little is known about the differences in expression patterns of myogenic regulatory factors (MRF) and the satellite cell marker Pax7 between adult and larval myogenic cells. In the present study, we compared mRNA expression of these factors between the two types. At first, reverse transcription polymerase chain reaction analysis of hindlimb buds showed sequential upregulation of myf5, myogenin, myod, and mrf4 during stages 50-54, when limb buds elongate and muscles begin to form. By contrast, in the tail, there was no such increase during the same period. Secondary, these results were duplicated in vitro: adult myogenic cells upregulated myf5, myod, and pax7 in the early culture period, followed by myogenin upregulation and myotube differentiation, while larval myogenic cells did not upregulate these genes and precociously started myotube differentiation. Thirdly, myf5 upregulation and early-phase proliferation in adult myogenic cells were potently inhibited by the presence of notochord cells, suggesting that notochord cells suppress adult myogenesis through inhibiting the transition from Myf5(-) stem cells to Myf5(+) committed myoblasts. All of the data presented here suggest that myf5 upregulation can be a good criterion for the activation of adult myogenesis during X. laevis metamorphosis.

Xenopus muscle development: from primary to secondary myogenesis

Xenopus myogenesis is characterized by specific features, different from those of mammalian and avian systems both at the cellular level and in gene expression patterns. During early embryogenesis, after the initial molecular signals inducing mesoderm, the myogenic determination factors XMyoD and XMyf-5 are activated in presomitic mesoderm in response to mesoderm-inducing factors. After these first inductions of the myogenic program, forming muscles in Xenopus can have different destinies, some of these resulting in cell death before adulthood. In particular, it is quite characteristic of this species that, during metamorphosis, the primary myotomal myofibers completely die and are progressively replaced by secondary "adult" multinucleated myofibers. This feature offers the unique opportunity to totally separate the molecular analysis of these two distinct types of myogenesis. The aim of this review is to summarize our knowledge on the cellular and molecular events as well as the epigenetic regulations involved in the construction of Xenopus muscles during development.

Effects of locomotor training on hindlimb regeneration in the urodele amphibian Pleurodeles waltlii

1. The effects of locomotor training on hindlimb regeneration were studied in the urodele amphibian Pleurodeles waltlii. 2. After amputation of one hindlimb at mid-femur, adult animals were subjected to regular training sessions (1 h daily, 5 days a week, over 8 months) of terrestrial stepping. 3. Eight months post-amputation, trained animals exhibited regenerated limbs of reduced size as compared to animals kept in their aquaria. Histological data showed an abnormal regeneration of both the femur and distal structures (e.g. digit muscles, metatarsi and phalanges) while medial structures (e.g. tibia and fibula) were totally re-formed. The study of the electromyographical activity in regenerated limbs during stepping and that of their reflex responsiveness to electrical stimulation showed that both motor and sensory innervations were functional in the limb stump of trained animals. 4. The regenerative capacity of the abnormal stumps was preserved since following a second amputation a quite normal hindlimb was regenerated in 3 months, provided the re-amputated animals were not trained to terrestrial stepping. 5. The stress due to handling, change in locomotor medium (aquatic vs. terrestrial) and the friction of the wound epidermis with the ground were not involved in the disruption of limb regeneration. 6. The locomotor pattern, the reflex responsiveness and the muscle fibre composition were similar in supernumerary forelimbs grafted on the back and in normal forelimbs. However, the supernumerary forelimbs regenerated normally even in animals subjected to locomotor training while the hindlimb did not. It is concluded that the disrupting effects of locomotor training on limb regeneration were localized to the the limb directly involved in locomotion. 7. The mechanisms underlying abnormal limb regeneration in animals subjected to locomotor training are discussed.

Neural and hormonal control of expression of myogenic regulatory factor genes during regeneration of Xenopus fast muscles: myogenin and MRF4 mRNA accumulation are neurally regulated oppositely

With the aim to investigate the influence of both innervation and thyroid hormone, on the expression of the MRFs during muscle regeneration, we performed cardiotoxin injury-induced regeneration experiments on fast muscles of adult Xenopus laevis subjected to different experimental conditions, including denervation and T3 treatment, and analyzed the accumulation of the four myogenic regulatory factors (MRFs) using RT-PCR and in situ hybridization. We show here that manipulation of hormone levels or innervation resulted in differential alterations of MRF expression. Denervation and T3 treatment transiently down-regulated Myf-5 mRNA levels at the beginning of the regeneration process. Myf-5 was the only myogenic factor subject to thyroid hormone influence. Muscle denervation persistently reduces the levels of MRF4 transcripts as early as the first stages of regeneration, whereas the levels of myogenin mRNA were increased in the late stages of regeneration. This suggests that MRF4 expression may be induced by innervation and hence may be involved in mediating transcriptional responses to innervation and that myogenin expression may compensate for the down-regulation of MRF4 gene. This switch in MRF gene expression following denervation could have important consequences for the ability of Xenopus regenerating muscles to recover function after denervation.

Expression of myogenic regulatory factors during muscle development of Xenopus: myogenin mRNA accumulation is limited strictly to secondary myogenesis

To clarify the acquisition of the adult muscle pattern in Xenopus laevis, in situ hybridization and reverse transcriptase-polymerase chain reaction were used to correlate the time course of gene expression for myogenic regulatory factors (Myf-5, MyoD, and myogenin) with the expression of contractile protein (myosin heavy chain; MHC) genes during hindlimb formation compared with their expression in dorsal body muscles. After the precocious expression of Myf-5 and MyoD mRNA in limb bud (stage 50), myogenin mRNA strongly accumulated later at paddle stages (stages 52/53) concomitantly with the accumulation of both the larval and the adult MHC mRNAs. In dorsal body muscles, as early as stage 52, myogenin transcripts accumulated in a few small, secondary myofibers expressing the adult MHC mRNA that were located along the dorsomedial edge, but they were never detected in the large, primary myofibers of the body expressing the larval MHC mRNA. During metamorphosis, the areas expressing both the adult MHC and the myogenin transcripts gradually expanded from the dorsomedial edge to the ventral side of the dorsal body muscles, accounting for the progression of the secondary "adult" myogenesis described previously (Nishikawa and Hayashi [1994] Dev. Biol. 165:86-94). This work shows that, in Xenopus, the accumulation of myogenin mRNA is restricted to secondary myogenesis, including the formation of new muscles in developing limbs as well as in dorsal muscles during body remodeling. This shows that myogenin is not required for primary myogenesis, and it suggests a crucial role for myogenin in the terminal differentiation program, including myoblast fusion and the activation of adult-type muscle genes.

Analysis of MyoD, myogenin, and muscle-specific gene mRNAs in regenerating Xenopus skeletal muscle

We have analyzed in adult Xenopus laevis, using in situ hybridization, the spatial and temporal expression patterns of MyoD, myogenin, and alpha-skeletal actin and fast myosin heavy chain mRNAs during muscle regeneration following cardiotoxin injury. MyoD transcripts could be detected in the satellite cells as early as the first stage of regeneration and were expressed persistently throughout the regeneration process. Myogenin mRNAs were transiently expressed in forming myotubes. alpha-Skeletal actin and fast myosin heavy chain mRNAs were detected precociously, before the young myotube stage. This work has shown, for the first time, the presence of myogenin transcripts during Xenopus myogenesis.

Myosin isoforms and their light chains from the ventricular muscle of the urodelan amphibian Pleurodeles waltlii: comparison with myosin from skeletal muscles

Myosin extracted from ventricular muscle of the urodelan amphibian Pleurodeles waltlii was analyzed in comparison with myosin extracted from skeletal muscles by native, one-dimensional SDS gel electrophoresis and two-dimensional gel electrophoresis. Two myosin isoforms were detected in ventricular muscle using pyrophosphate gel electrophoresis. These isomyosins contained two types of light chain subunits, LC1v and LC2v. Two-dimensional gel electrophoresis showed that LC1v comigrated with the slow light chain LC1s, whereas LC2v was characterized by a specific mobility, distinct from LC2s and LC2f. Diaphragm muscle was characterized by the coexistence of larval and adult myosin isoforms.

A new approach of urodele amphibian limb regeneration: study of myosin isoforms and their control by thyroid hormone

In P. waltlii, an urodele amphibian species which undergoes spontaneous metamorphosis, study of native myosin in pyrophosphate gels at various stages of normal development demonstrates a complete larval to fast myosin isoforms transition, which occurs more precociously in forelimb muscles than in the dorsal and ventral muscles. In the neotenic species A. mexicanum, forelimb muscles development also presents a complete myosin isoforms transition which is in contrast with the partial myosin isoforms transition observed in the dorsal muscle. In metamorphosed or neotenic animals of both species aged 1 year, forelimb regeneration is characterized by a complete transition from larval to fast myosin isoforms, that occurs earlier and more rapidly than in normal forelimb development. When forelimb regeneration is studied in P. waltlii aged 4 years, the adult fast and slow isomyosins are expressed very early in the regeneration process. In experimental hypothyroidian P. waltlii, the larval to fast isoforms transition in regenerating forelimb muscles is slightly delayed. Experimental hyperthyroidism accelerates the disappearance of larval isomyosins in regenerating forelimb muscles, both in P. waltlii and A. mexicanum aged 1 year. This work demonstrates that changes in myosin isoform pattern during forelimb regeneration in adult urodele amphibians are different from changes occurring in the normal forelimb development. They take place without any thyroid hormone influence, as opposed to normal development, and appear to be age-dependent.

Developmental modulation of myosin expression by thyroid hormone in avian skeletal muscle

It is well established that a rise in circulating thyroid hormone during the second half of chick embryo development significantly influences muscle weight gain and bone growth. We studied thyroid influence on differentiation in slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of embryos rendered hypothyroid by hypophysectomy or administration of an anti-thyroid drug. The expression of native myosins and myosin light chains (MLCs) was studied by electrophoretic analysis, and the myosin heavy chain (MHC) was characterized by immunohistochemistry. The first effects of hypothyroid status were observed at day 21 of embryonic development (stage 46 according to Hamburger and Hamilton). Analysis of myosin isoform expression in PLD muscles of hypothyroid embryos showed persistence of slow migrating native myosins and slow MLCs as well as inhibition of neonatal fast MHC expression, indicating retarded differentiation of this muscle. In ALD muscle, hypothyroidism maintained fast embryonic MHC and induced noticeable amounts of fast MLCs, thus delaying slow muscle differentiation. Our results suggest that thyroid hormones play a role in modulating the appearance of neonatal fast MHC and the disappearance of isomyosins transiently present during embryogenesis. However, T3 supplemental treatment would seem to compensate in part for the effects of hypothyroidism induced by hypophysectomy, suggesting that thyroid hormone might interfere with other factors also accounting for the observed effects.

White muscle differentiation in the eel (Anguilla anguilla L.): changes in the myosin isoforms pattern and ATPase profile during post-metamorphic development

Myosin isoforms and their light and heavy chains subunits were studied in the white lateral muscle of the eel during the post metamorphic development, in relation with the myosin ATPase profile. At elver stage VI A1 the myosin isoforms pattern was characterized by at least two isoforms, FM3 and FM2. The fast isomyosin type 1 (FM1) appeared during subsequent development. It increased progressively in correlation with the increase in the level of the light chain LC3f. FM1 became predominant at stage VI A4. At the elver stage VI A1, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed at least two heavy chains, namely type II-1 and II-2. The type II-1 heavy chain disappeared in the yellow eel white muscle, and V8-protease peptide map showed the appearance of a minor heavy chain type II-3 as early as stage VI B. Comparison of myosin heavy chains and myosin isoforms patterns showed the comigration of different myosin isoforms during white muscle development. The myosin ATPase profile was characterized by a uniform pattern as far as stage VI A4. A mosaic aspect in white muscle was observed as early as stage VI B, showing the appearance of small acid labile fibers. This observation suggests that the type II-3 heavy chain is specific to the small fibers.

Comparison of myosin isoenzymes present in skeletal and cardiac muscles of the Arctic charr Salvelinus alpinus (L.). Sequential expression of different myosin heavy chains during development of the fast white skeletal muscle

The expression of myosin isoforms and their subunit composition in the white skeletal body musculature of Arctic charr (Salvelinus alpinus) of different ages (from 77-day embryos until about 5 years old) was studied at the protein level by means of electrophoretic techniques. Myosin from the white muscle displayed three types of light chain during all the developmental stages examined: two myosin light chains type 1 (LC1F) differing in both apparent molecular mass and pI, one myosin light chain type 2 (LC2F) and one myosin light chain type 3 (LC3F). The fastest-migrating form of LC1F seemed to be predominant during the embryonic and eleutheroembryonic periods. The slowest-migrating form of LC1F was predominant in the 5-year-old fish. Between 1 year and 4 years, both types of LC1F were present in similar amounts. Cardiac as well as red muscle myosin from 3-year-old fish had two types of light chain. The myosin light chains from atria and ventriculi were indistinguishable by two-dimensional electrophoresis, but were different from the myosin light chains from red muscle. Neither the light chains from cardiac nor red muscle were coexpressed with the myosin light chains of white muscle at any of the developmental stages examined. Two myosin heavy chain bands were resolved by SDS/glycerol/polyacrylamide gel electrophoresis of the extract from embryos. One of the bands was present in minor amounts. The other, and most abundant, band comigrated with the only band found in the extracts of white muscle myosin from older fish. One-dimensional Staphylococcus aureus V8 protease peptide mapping of these bands revealed some differences during development of the white muscle tentatively interpreted as follows. The myosin heavy chain band present in minor amounts in the embryos may represent an early embryonic form that is replaced by a late embryonic or foetal form in the eleutheroembryos. The foetal myosin heavy chain appears to be present until the resorption of the yolk sack and beginning of the free-swimming stage. A new form of myosin heavy chain, termed neonatal and probably expressed around hatching, is present until about 1 year of age.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of motor units in the axolotl: a continuously growing animal

The characteristics of motor units in the iliotibialis posterior muscle of the axolotl hindlimb are described. Tension recording and intracellular electrophysiological methods demonstrate that the physiological properties of the population of motor units are continuously distributed rather than grouped into a series of discrete types. Overlap between motor units occurs and this is positively correlated with motor unit size but negatively correlated with differences in time to peak tension. Immunocytochemical staining with antimyosin antibodies combined with histochemical demonstration of actomyosin ATPase activity revealed at least four types of muscle fibre which were distributed asymmetrically within iliotibialis posterior. The results are discussed in terms of the continuous growth of the muscle and the interactions between muscle and nerve in the formation of the axolotl motor system.

Muscle-specific response to thyroid hormone of myosin isoform transitions during rat postnatal development

Transitions from embryonic and neonatal to adult-type-II isomyosins are known to be related to the increase in the thyroid hormone plasma concentration during postnatal development. These transitions have been shown, however, to occur at different times, depending on the muscle, suggesting that each muscle responds differently to the thyroid hormone. We have investigated quantitatively the effects of experimental hypothyroidism and hyperthyroidism on isomyosin transitions from birth until the 45th postnatal day in eight rat muscles: diaphragm, intercostals, gastrocnemius medialis, soleus, plantar muscles of the foot, tongue muscle, levator ani and bulbocavernosus complex, and masseter. Hypothyroidism delayed the isomyosin transitions in all the muscles examined, particularly in the sexually dimorphic muscles (levator ani and bulbocavernosus complex and masseter). However, it did not eventually inhibit isomyosin transitions, indicating that the thyroid hormone was not an absolute requirement for these to occur. Hyperthyroidism had only a slight effect on isomyosin transition in the diaphragm, and accelerated such transitions in the other muscles. The transition curves of all the muscles investigated, except those of the sexually dimorphic muscles, became similar to that of the diaphragm, demonstrating that the various muscles did not display the same sensitivity to the thyroid hormone but were regulated by it in the same way. The isomyosin transitions in the sexually dimorphic muscles remained late in comparison to that in the diaphragm, which suggests a more complex regulation. The effect of hyperthyroidism was not permanent and could be reversed, by interruption of the treatment, to a greater or lesser extent depending on the muscle. In all muscles containing slow-type-I isomyosin, hypothyroidism had no effect on this isomyosin synthesis, whereas hyperthyroidism inhibited it. This inhibition ceased rapidly after the interruption of the treatment.

Thyroidal status and myosin isoenzymic pattern in the skeletal dorsal muscle of urodelan amphibians--the perennibranchiate Proteus anguinus

In the perennibranchiate Proteus anguinus, larval myosin isoforms were shown to coexist for life with the adult isomyosins that appeared at the end of the larval stage. Analysis of the myofibrillar ATPase profile also revealed that a high percentage of immature fibers persisted in adults. A long-term treatment with large amounts of T3 had no effect on juvenile individuals. Applied to subadult animals it promoted a regression of larval myosin isoforms and a reduction in the percentage of immature fiber types. The regulative effect of T3 in the myosin isoenzymic transition may be delayed and depends on metabolic conditions, which suggests it is indirect.

Specific programs of myosin expression in the postnatal development of rat muscles

The expression of myosin during postnatal development was studied in a dozen muscles of the rat. All muscles displayed the usual sequential transitions from embryonic to neonatal and to adult isomyosins. However, we observed that these transitions did not take place uniformly. Thus, half-transition times for the appearance of the adult intermediate and fast myosin extended from seven days for diaphragm, the most precocious muscle of all those examined, to 23 days for male rat masseter. Besides the large differences between their half-transition times, we noticed that the transition curves displayed different slopes, covering different periods. Differences between muscles mainly affected the neonatal-to-adult transition rather than the embryonic-to-neonatal transition, since the embryonic-type myosin disappeared from all muscles examined except for one, at about the same time, by the end of the first week after birth. In addition, the appearance of slow myosin varied for each muscle and did not follow curves parallel to those for intermediate and fast myosins. These results indicate that each muscle of the rat is subjected to a specific program of myosin isoform transitions during postnatal development.

Histochemistry and isomyosins of tail musculature in Xenopus

The main bulk of the larval tail in Xenopus laevis is composed of thick muscle fibres which are succinate dehydrogenase (SDH) negative and show strong positive ATPase activity only at alkaline preincubation (pH 10.4). The thin muscle fibres (which cover the surface of the myotomes and extend to the tail tip) show positive SDH activity as well as strong positive ATPase activity after both alkaline and acid preincubations (pH 10.4, 4.5, 4.4, and 4.3). The pattern of myosin isoenzymes does not change in the tail muscle in the course of development of Xenopus: the same three bands of larval isomyosins were found in all the examined developmental stages: 40, 47, 50 and 56. However, the larval bands were distinct from three bands of myosin isoforms in the musculus longissimus dorsi (MLD) and musculus gastrocnemius (MG) of the metamorphosed Xenopus.

Myosin isoenzymes and their subunits in urodelan amphibian fast skeletal muscle. Coexistence of larval and adult heavy chains in neotenic individuals

The distributions of native myosin isoforms were examined by electrophoresis under non-dissociating conditions, in the fast twitch dorsal skeletal muscle of young larvae, neotenic adults and metamorphosed adults of urodelan amphibians. Both heavy and light chains of myosin isoenzymes were analysed. In pyrophosphate acrylamide gel electrophoresis three isoenzymes were demonstrated in larval myosin; other isoforms of lower electrophoretic mobility were observed in metamorphosed adults myosin. Larval and adult isoenzymes were shown to coexist in myosin from neotenic adults. Analysis of heavy chains in denaturing conditions and proteolytic digestion revealed the sequential occurrence during development of two types of heavy chains, one larval and one adult, that coexist in the myosin of neotenic adults only. Analysis of light chain patterns under denaturing conditions revealed the existence of three fast light chains which displayed no modification during the course of development. The neotenic urodelan amphibian species model represents actually the only model in which the coexistence of larval (or neonatal) and adult heavy chains is maintained throughout life in adults.

Regeneration after cardiotoxin injury of innervated and denervated slow and fast muscles of mammals. Myosin isoform analysis

The regeneration of adult rat and mouse slow (soleus) and fast (sternomastoid) muscles was examined after the degeneration of myofibers had been achieved by a snake venom cardiotoxin, under experimental conditions devised to spare as far as possible the satellite cells, the nerves, and the blood vessels of the muscles. Three days after the injury, no myosin was detectable in selected portions of the muscles. New myosins of embryonic, neonatal, and adult types started to be synthesized during the following two days. Adult myosins thus appeared more precociously than in development, which implies that the synthesis of myosin isoforms during regeneration does not entirely 'recapitulate' the sequence of myosin transitions observed during normal development. Two weeks after the injury, the isomyosin electrophoretic pattern displayed by regenerated muscles was already the same as that of control muscles; the normal adult pattern was therefore expressed more rapidly in regenerating than in developing muscles. Except for the synthesis of the slow isoform which was generally inhibited in denervated muscles, the same types of myosins were expressed during the early stages of regeneration in denervated as in innervated muscles; long-term denervation prevented however the qualitative and quantitative recovery of the normal myosin pattern.