Denervation of newborn rat muscle does not block the appearance of adult fast myosin heavy chain

Our journey with François Gros

Gillian Butler-Browne began working on muscle at the Institut Pasteur in the laboratory of François Gros in 1978. She characterized the expression profile of different myosin isoforms during both human and rodent development. Vincent Mouly joined this laboratory for his PhD in 1982, and defined the different populations of myoblasts appearing during development in birds and then in humans. Together, they demonstrated the impact of the limit in proliferation of the precursor cells on the regenerative capacity of human skeletal muscle, and their group developed models to evaluate the regenerative potential of skeletal muscle in vitro, measuring the telomeric erosion, and identified the involvement of a stress pathway in the proliferative arrest of muscle progenitors. A platform to produce human immortalized muscle cell lines was the successful result of this research, initiated with François Gros and W. E. Wright. The in vivo regenerative potential of human muscle cells was evaluated by injection into muscles of immunodeficient mice. Their group in collaboration with the clinical team of Professor Jean Lacau St-Guily and Professor Sophie Perié completed a successful autologous myoblast transplantation clinical trial for Oculo-pharyngeal muscular dystrophy. This common scientific career was made possible thanks to the precious and always benevolent support of François Gros.

Memories of Professor François Gros

I joined the laboratory of Professor Francois Gros in 1987 and worked there as a postdoc with Robert Whalen until 1992. I recount the research we carried out and mention that of the other scientists also working on skeletal muscle on the 6th floor of the Molecular Biology Department of the Institut Pasteur at that time. I then present my subsequent research when I returned to Japan. I pay tribute to the influence of Professor Gros and to his support in establishing Japanese/French meetings on muscle biology and muscular dystrophy. I also invoke personal memories of Robert Whalen and Margaret Buckingham and remember the occasions when I returned to Paris to honour François Gros.

Muscle-nerve communication and the molecular assessment of human skeletal muscle denervation with aging

Muscle fiber denervation is a major contributor to the decline in physical function observed with aging. Denervation can occur through breakdown of the neuromuscular junctions (NMJ) itself, affecting only that particular fiber, or through the death of a motor neuron, which can lead to a loss of all the muscle fibers in that motor unit. In this review, we discuss the muscle-nerve relationship, where signaling from both the motor neuron and the muscle fiber is required for maximal preservation of neuromuscular function in old age. Physical activity is likely to be the most important single factor that can contribute to this preservation. Furthermore, we propose that inactivity is not an innocent bystander, but plays an active role in denervation through the production of signals hostile to neuron survival. Investigating denervation in human muscle tissue samples is challenging due to the shared protein profile of regenerating and denervated muscle fibers. In this review, we provide a detailed overview of the key traits observed in immunohistochemical preparations of muscle biopsies from healthy, young, and elderly individuals. Overall, a combination of assessing tissue samples, circulating biomarkers, and electrophysiological assessments in humans will prove fruitful in the quest to gain more understanding of denervation of skeletal muscle. In addition, cell culture models represent a valuable tool in the search for key signaling factors exchanged between muscle and nerve, and which exercise has the capacity to alter.

Nfix Induces a Switch in Sox6 Transcriptional Activity to Regulate MyHC-I Expression in Fetal Muscle

Sox6 belongs to the Sox gene family and plays a pivotal role in fiber type differentiation, suppressing transcription of slow-fiber-specific genes during fetal development. Here, we show that Sox6 plays opposite roles in MyHC-I regulation, acting as a positive and negative regulator of MyHC-I expression during embryonic and fetal myogenesis, respectively. During embryonic myogenesis, Sox6 positively regulates MyHC-I via transcriptional activation of Mef2C, whereas during fetal myogenesis, Sox6 requires and cooperates with the transcription factor Nfix in repressing MyHC-I expression. Mechanistically, Nfix is necessary for Sox6 binding to the MyHC-I promoter and thus for Sox6 repressive function, revealing a key role for Nfix in driving Sox6 activity. This feature is evolutionarily conserved, since the orthologs Nfixa and Sox6 contribute to repression of the slow-twitch phenotype in zebrafish embryos. These data demonstrate functional cooperation between Sox6 and Nfix in regulating MyHC-I expression during prenatal muscle development.

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Sox6 has opposite roles in MyHC-I regulation during embryonic and fetal myogenesis

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In embryonic muscle, Sox6 enhances MyHC-I expression via regulation of Mef2C

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In fetal muscle, Nfix is required for Sox6-mediated repression of MyHC-I

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The Sox6 and Nfixa orthologs cooperate in repressing smyhc1 in zebrafish

PABPN1 gene therapy for oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is an autosomal dominant, late-onset muscle disorder characterized by ptosis, swallowing difficulties, proximal limb weakness and nuclear aggregates in skeletal muscles. OPMD is caused by a trinucleotide repeat expansion in the PABPN1 gene that results in an N-terminal expanded polyalanine tract in polyA-binding protein nuclear 1 (PABPN1). Here we show that the treatment of a mouse model of OPMD with an adeno-associated virus-based gene therapy combining complete knockdown of endogenous PABPN1 and its replacement by a wild-type PABPN1 substantially reduces the amount of insoluble aggregates, decreases muscle fibrosis, reverts muscle strength to the level of healthy muscles and normalizes the muscle transcriptome. The efficacy of the combined treatment is further confirmed in cells derived from OPMD patients. These results pave the way towards a gene replacement approach for OPMD treatment.

Oculopharyngeal muscular dystrophy is caused by trinucleotide repeat expansions in the PABPN1 gene. Here the authors use AAV-based gene therapy to knockdown the mutant gene and replace it with a wild-type allele, and show effectiveness in mice and in patient cells.

Developmental myosins: expression patterns and functional significance

Developing skeletal muscles express unique myosin isoforms, including embryonic and neonatal myosin heavy chains, coded by the myosin heavy chain 3 (MYH3) and MYH8 genes, respectively, and myosin light chain 1 embryonic/atrial, encoded by the myosin light chain 4 (MYL4) gene. These myosin isoforms are transiently expressed during embryonic and fetal development and disappear shortly after birth when adult fast and slow myosins become prevalent. However, developmental myosins persist throughout adult stages in specialized muscles, such as the extraocular and jaw-closing muscles, and in the intrafusal fibers of the muscle spindles. These myosins are re-expressed during muscle regeneration and provide a specific marker of regenerating fibers in the pathologic skeletal muscle. Mutations in MYH3 or MYH8 are responsible for distal arthrogryposis syndromes, characterized by congenital joint contractures and orofacial dysmorphisms, supporting the importance of muscle contractile activity and body movements in joint development and in shaping the form of the face during fetal development. The biochemical and biophysical properties of developmental myosins have only partially been defined, and their functional significance is not yet clear. One possibility is that these myosins are specialized in contracting against low loads, and thus, they may be adapted to the prenatal environment, when fetal muscles contract against a very low load compared to postnatal muscles.

It takes a mouth to eat and a nose to breathe: abnormal oral respiration affects neonates' oral competence and systemic adaptation

Mammalian, including human, neonates are considered to be obligate nose breathers. When constrained to breathe through their mouth in response to obstructed or closed nasal passages, the effects are pervasive and profound, and sometimes last into adulthood. The present paper briefly surveys neonates' and infants' responses to this atypical mobilisation of the mouth for breathing and focuses on comparisons between human newborns and infants and the neonatal rat model. We present the effects of forced oral breathing on neonatal rats induced by experimental nasal obstruction. We assessed the multilevel consequences on physiological, structural, and behavioural variables, both during and after the obstruction episode. The effects of the compensatory mobilisation of oral resources for breathing are discussed in the light of the adaptive development of oromotor functions.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Hypoxia inducible factor 1 links fast-patterned muscle activity and fast muscle phenotype in rats

Exercise influences muscle phenotype by the specific pattern of action potentials delivered to the muscle, triggering intracellular signalling pathways. PO2 can be reduced by an order of magnitude in working muscle. In humans, carriers of a hyperactive polymorphism of the transcription factor hypoxia inducible factor 1α (HIF-1α) have 50% more fast fibres, and this polymorphism is prevalent among strength athletes. We have investigated the putative role of HIF-1α in mediating activity changes in muscle.When rat muscles were stimulated with short high frequency bursts of action potentials known to induce a fast muscle phenotype, HIF-1α increased by about 80%. In contrast, a pattern consisting of long low frequency trains known to make fast muscles slow reduced the HIF-1α level of the fast extensor digitorum longus (EDL) muscle by 44%. Nuclear protein extracts from normal EDL contained 2.3-fold more HIF-1α and 4-fold more HIF-1β than the slow soleus muscle, while von-Hippel-Lindau protein was 4.8-fold higher in slow muscles. mRNA displayed a reciprocal pattern; thus FIH-1 mRNA was almost 2-fold higher in fast muscle, while the HIF-1α level was half, and consequently protein/mRNA ratio for HIF-1α was more than 4-fold higher in the fast muscle, suggesting that HIF-1α is strongly suppressed post-transcriptionally in slow muscles.When HIF-1α was overexpressed for 14 days after somatic gene transfer in adult rats, a slow-to-fast transformation was observed, encompassing an increase in fibre cross sectional area, oxidative enzyme activity and myosin heavy chain. The latter was shown to be regulated at the mRNA level in C2C12 myotubes.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Fibre types in skeletal muscle: a personal account

Muscle performance is in part dictated by muscle fibre composition and a precise understanding of the genetic and acquired factors that determine the fibre type profile is important in sport science, but is also relevant to neuromuscular diseases and to metabolic diseases, such as type 2 diabetes. The dissection of the signalling pathways that determine or modulate the muscle fibre phenotype has thus potential clinical significance. In this brief review, I examine the evolution of the notion of muscle fibre types, discuss some aspects related to species differences, point at problems in the interpretation of transgenic and knockout models and show how in vivo transfection can be used to identify regulatory factors involved in fibre type diversification, focusing on the calcineurin-nuclear factor of activated T cells (NFAT) pathway.

The ups and downs of gene regulation by electrical activity in skeletal muscles

Adult skeletal muscles retain an adaptive capacity to switch between slow- and fast-twitch properties that are largely dependent on motoneuron activity. Our studies on the transcriptional regulation of the Troponin I slow (TnIs) and fast (TnIf) genes uncovered a dual mechanism of transcriptional enhancement and repression by a single activity pattern, that promotes the phenotypic differences among myofibers while preserving their adaptive capacity. Using the Tnf Fast Intronic Regulatory Element (FIRE), we initially demonstrated that fast-patterned activity (infrequent, high frequency depolarization) is necessary to up-regulate FIRE-dependent transcription and that its effect differs dramatically from muscle denervation. Hence, the "fast muscle program" is not a default state mimicked simply by denervation or muscle inactivity. Next, we found that slow-patterned activity (tonic, slow frequency stimulation) selectively represses FIRE-dependent transcription while enhancing transcription from the TnIs Slow Upstream Regulatory Element. Unexpectedly, repression of the TnIf FIRE by slow-patterned activity is mediated by an NFAT element that directly binds NFATc1, a transcription factor that translocates to the nucleus selectively by slow-pattern depolarization and has been implicated in the up-regulation of the slow muscle program. Transfection of siRNAs targeting NFATc1 or mutation of the TnIFIRE NFAT site result in the upregulation of FIRE-dependent transcription in slow muscle, but have no effect in fast muscle. These findings demonstrate a novel function of NFAT as a repressor of transcription of fast contractile genes in slow muscles and, more importantly, they illustrate how specific activity patterns can enhance the phenotypic differences among fibre-types by differentially regulating transcription in a use-dependent manner while retaining the adaptive properties of adult muscles.

Effects of hypothyroidism on myosin heavy chain composition and fibre types of fast skeletal muscles in a small marsupial, Antechinus flavipes

Effects of drug-induced hypothyroidism on myosin heavy chain (MyHC) content and fibre types of fast skeletal muscles were studied in a small marsupial, Antechinus flavipes. SDS-PAGE of MyHCs from the tibialis anterior and gastrocnemius revealed four isoforms, 2B, 2X, 2A and slow, in that order of decreasing abundance. After 5 weeks treatment with methimazole, the functionally fastest 2B MyHC significantly decreased, while 2X, 2A and slow MyHCs increased. Immunohistochemistry using monospecific antibodies to each of the four MyHCs revealed decreased 2b and 2x fibres, and increased 2a and hybrid fibres co-expressing two or three MyHCs. In the normally homogeneously fast superficial regions of these muscles, evenly distributed slow-staining fibres appeared, resembling the distribution of slow primary myotubes in fast muscles during development. Hybrid fibres containing 2A and slow MyHCs were virtually absent. These results are more detailed but broadly similar to the earlier studies on eutherians. We hypothesize that hypothyroidism essentially reverses the effects of thyroid hormone on MyHC gene expression of muscle fibres during myogenesis, which differ according to the developmental origin of the fibre: it induces slow MyHC expression in 2b fibres derived from fast primary myotubes, and shifts fast MyHC expression in fibres of secondary origin towards 2A, but not slow, MyHC.

NFAT isoforms control activity-dependent muscle fiber type specification

The intracellular signals that convert fast and slow motor neuron activity into muscle fiber type specific transcriptional programs have only been partially defined. The calcium/calmodulin-dependent phosphatase calcineurin (Cn) has been shown to mediate the transcriptional effects of motor neuron activity, but precisely how 4 distinct muscle fiber types are composed and maintained in response to activity is largely unknown. Here, we show that 4 nuclear factor of activated T cell (NFAT) family members act coordinately downstream of Cn in the specification of muscle fiber types. We analyzed the role of NFAT family members in vivo by transient transfection in skeletal muscle using a loss-of-function approach by RNAi. Our results show that, depending on the applied activity pattern, different combinations of NFAT family members translocate to the nucleus contributing to the transcription of fiber type specific genes. We provide evidence that the transcription of slow and fast myosin heavy chain (MyHC) genes uses different combinations of NFAT family members, ranging from MyHC-slow, which uses all 4 NFAT isoforms, to MyHC-2B, which only uses NFATc4. Our data contribute to the elucidation of the mechanisms whereby activity can modulate the phenotype and performance of skeletal muscle.

Role of regulatory T cells in a new mouse model of experimental autoimmune myositis

Polymyositis is a rare and severe inflammatory muscle disorder. Treatments are partially efficacious but have many side effects. New therapeutic approaches must be first tested in a relevant animal model. Regulatory CD4+CD25+ T cells (Tregs) have been rediscovered as a pivotal cell population in the control of autoimmunity, but the connection between polymyositis and Tregs is currently unknown. To develop a reproducible experimental autoimmune myositis model of polymyositis, mice were immunized once a week for 3 weeks with 1 mg of partially purified myosin emulsified in complete Freund's adjuvant. All mice injected with myosin and complete Freund's adjuvant developed myositis. The infiltrates were composed of CD4(+) and CD8(+) cells, as well as macrophages, but did not contain B lymphocytes. In mice that were depleted of Tregs, the myositis was more severe, as determined by quantitative scoring of muscle inflammation (2.36 +/- 0.9 vs. 1.64 +/- 0.8, P = 0.019). In contrast, injection of in vitro expanded polyclonal Tregs at the time of immunization significantly improved the disease (quantitative score of inflammation 0.87 +/- 1.06 vs. 2.4 +/- 0.67, P = 0.047). Transfer of sensitized or CD4(+)-sorted cells from the lymph nodes of experimental autoimmune myositis mice induced myositis in naïve, irradiated, recipient mice. Thus, experimental autoimmune myositis is a reproducible, transferable disease in mice, both aggravated by Treg depletion and improved by polyclonal Treg injection.

Nerve-dependent changes in skeletal muscle myosin heavy chain after experimental denervation and cross-reinnervation and in a demyelinating mouse model of Charcot-Marie-Tooth disease type 1A

Innervation regulates the contractile properties of vertebrate muscle fibers, in part through the effect of electrical activity on expression of distinct myosins. Herein we analyze the role of innervation in regulating the accumulation of the general, maturational, and adult forms of rodent slow myosin heavy chain (MyHC) that are defined by the presence of distinct antigenic epitopes. Denervation increases the number of fibers that express general slow MyHC, but it decreases the adult slow MyHC epitope. Cross-reinnervation of slow muscle by a fast nerve leads to an increase in the number of fibers that express fast MyHC. In both cases, there is an increase in the number of fibers that express slow and fast IIA MyHCs, but without the adult slow MyHC epitope. The data suggest that innervation is required for maturation and maintenance of diversity of both slow and fast fibers. The sequence of slow MyHC epitope transitions is a useful biomarker, and it may play a significant role during nerve-dependent changes in muscle fiber function. We applied this detailed muscle analysis to a transgenic mouse model of human motor and sensory neuropathy IA, also known as Charcot-Marie-Tooth disease type 1A (CMT1A), in which electrical conduction in some motor nerves is poor due to demyelination. The mice display atrophy of some muscle fibers and changes in slow and fast MyHC epitope expression, suggestive of a progressive increase in innervation of muscle fibers by fast motor neurons, even at early stages. The potential role of these early changes in disease pathogenesis is assessed.

Regeneration of reinnervated rat soleus muscle is accompanied by fiber transition toward a faster phenotype

The functional recovery of skeletal muscles after peripheral nerve transection and microsurgical repair is generally incomplete. Several reinnervation abnormalities have been described even after nerve reconstruction surgery. Less is known, however, about the regenerative capacity of reinnervated muscles. Previously, we detected remarkable morphological and motor endplate alterations after inducing muscle necrosis and subsequent regeneration in the reinnervated rat soleus muscle. In the present study, we comparatively analyzed the morphometric properties of different fiber populations, as well as the expression pattern of myosin heavy chain isoforms at both immunohistochemical and mRNA levels in reinnervated versus reinnervated-regenerated muscles. A dramatic slow-to-fast fiber type transition was found in reinnervated soleus, and a further change toward the fast phenotype was observed in reinnervated-regenerated muscles. These findings suggest that the (fast) pattern of reinnervation plays a dominant role in the specification of fiber phenotype during regeneration, which can contribute to the long-lasting functional impairment of the reinnervated muscle. Moreover, because the fast II fibers (and selectively, a certain population of the fast IIB fibers) showed better recovery than did the slow type I fibers, the faster phenotype of the reinnervated-regenerated muscle seems to be actively maintained by selective yet undefined cues.

Atrophy, but not necrosis, in rabbit skeletal muscle denervated for periods up to one year

Our understanding of the effects of long-term denervation on skeletal muscle is heavily influenced by an extensive literature based on the rat. We have studied physiological and morphological changes in an alternative model, the rabbit. In adult rabbits, tibialis anterior muscles were denervated unilaterally by selective section of motor branches of the common peroneal nerve and examined after 10, 36, or 51 wk. Denervation reduced muscle mass and cross-sectional area by 50-60% and tetanic force by 75%, with no apparent reduction in specific force (force per cross-sectional area of muscle fibers). The loss of mass was associated with atrophy of fast fibers and an increase in fibrous and adipose connective tissue; the diameter of slow fibers was preserved. Within fibers, electron microscopy revealed signs of ultrastructural disorganization of sarcomeres and tubular systems. This, rather than the observed transformation of fiber type from IIx to IIa, was probably responsible for the slow contractile speed of the muscles. The muscle groups denervated for 10, 36, or 51 wk showed no significant differences. At no stage was there any evidence of necrosis or regeneration, and the total number of fibers remained constant. These changes are in marked contrast to the necrotic degeneration and progressive decline in mass and force that have previously been found in long-term denervated rat muscles. The rabbit may be a better choice for a model of the effects of denervation in humans, at least up to 1 yr after lesion.

The denervated muscle: facts and hypotheses. A historical review

Denervation changes in skeletal muscle (atrophy; alterations of myofibrillar expression, muscle membrane electrical properties, ACh sensitivity and excitation-contraction coupling process; fibrillation), and their possible causes are reviewed. All changes can be counteracted by muscle electrostimulation, while denervation-like effects can be caused by the complete conduction block in muscle nerve. These results do not support the hypothesis that the lack of neurotrophic, non-motor factors plays a role in denervation phenomena. Instead they support the view that the lack of neuromotor discharge is the only cause of the phenomena and that neuromotor activity is an essential factor in regulating muscle properties. However, some experimental results cannot apparently be explained by the lack of neuromotor impulses, and may still suggest that neurotrophic influences exist. A hypothesis is that neurotrophic factors, too feeble to maintain a role in completely differentiated, adult muscles, can concur with neuromotor activity in the differentiation of immature, developing muscles.

Mechanisms underlying myosin heavy chain expression during development of the rat diaphragm muscle

During early postnatal development in rat diaphragm muscle (Dia(m)), significant transitions in myosin heavy chain (MHC) isoform expression occur that are associated with fiber growth and increased MHC protein. At present, there is no direct information regarding the transcriptional regulation of MHC isoform expression during postnatal Dia(m) development. We hypothesized postnatal changes in MHC isoform mRNA expression are followed by concomitant changes in MHC protein expression. The Dia(m) was removed at postnatal days 0, 14, 28, and 84 (adult). MHC mRNA expression was determined by real-time RT-PCR. MHC protein expression was determined by SDS-PAGE. There was a significant effect of postnatal age on MHC isoform mRNA and protein expression. At birth, the MHC(Neo) isoform accounted for 28% of MHC mRNA and 54% of total MHC protein. By postnatal day 14, MHC(Neo) mRNA and protein increased significantly, and both decreased significantly by day 28, consistent with transcriptional control of the expression of this developmental isoform. By postnatal day 28, there were minimal changes in mRNA expression for MHC(Slow) and MHC(2X), yet protein expression increased significantly. MHC(2A) mRNA and protein expression did not change during this time. Thus changes in MHC protein expression did not follow (or parallel) changes in MHC mRNA for the adult MHC isoforms. The present findings indicate that changes in MHC expression in the developing rat Dia(m) are not driven solely by changes in mRNA expression. Knowledge of isoform-specific MHC mRNA expression only yields predictive information on MHC protein expression for the MHC(Neo) isoform.

Imaging transcription in vivo: distinct regulatory effects of fast and slow activity patterns on promoter elements from vertebrate troponin I isoform genes

Firing patterns typical of slow motor units activate genes for slow isoforms of contractile proteins, but it remains unclear if there is a distinct pathway for fast isoforms or if their expression simply occurs in the absence of slow activity. Here we first show that denervation in adult soleus and EDL muscles reverses the postnatal increase in expression of troponin I (TnI) isoforms, suggesting that high-level transcription of both genes in mature muscles is under neural control. We then use a combination of in vivo transfection, live muscle imaging and fluorescence quantification to investigate the role of patterned electrical activity in the transcriptional control of troponin I slow (TnIs) and fast (TnIf) regulatory sequences by directly stimulating denervated muscles with pattern that mimic fast and slow motor units. Rat soleus muscles were electroporated with green fluorescent protein (GFP) reporter constructs harbouring 2.7 and 2.1 kb of TnIs and TnIf regulatory sequences, respectively. One week later, electrodes were implanted and muscles stimulated for 12 days. The change in GFP fluorescence of individual muscle fibres before and after the stimulation was used as a measure for transcriptional responses to different patterns of action potentials. Our results indicate that the response of TnI promoter sequences to electrical stimulation is consistent with the regulation of the endogenous genes. The TnIf and TnIs enhancers were activated by matching fast and slow activity patterns, respectively. Removal of nerve-evoked activity by denervation, or stimulation with a mismatching pattern reduced transcriptional activity of both enhancers. These results strongly suggest that distinct signalling pathways couple both fast and slow patterns of activity to enhancers that regulate transcription from the fast and slow troponin I isoforms.

Differentiation of rat skeletal muscle fibres during development and ageing

The purpose of the present study was to determine at which point in the period from embryonic day 21 up to postnatal day (PD) 75, the different fibre types and subtypes are detectable in rat extensor digitorum longus, soleus and gastrocnemius muscles using immunohistochemical, enzyme histochemical and cytophotometrical methods. Moreover, fibre type-specific changes in metabolic profile and changes in fibre type population during postnatal development were analysed. Before birth, no clear differentiation of fibre types was found. At PD 1, slow and fast fibres were typed by antibodies against neonatal, slow and fast myosin heavy chains (MHCs). At PD 8, the different ATPase activities of slow and fast MHCs after alkaline preincubation were detected histochemically. At PD 21, differences in acid stability of ATPase activity of fast MHC isoforms revealed the fast subtypes IIA and IIB (including IIX). At this age, also differences in metabolic properties (oxidative and glycolytic enzyme activities) of fibres were detected for the first time by cytophotometry classifying the fibres into SO, FOG I, FOG II and FG. Before the age of 21 days, the fast fibres were metabolically undifferentiated. During further development and ageing, the population of FG fibres with high glycolytic activity increased at the expense of FOG fibres suggesting FOG to FG transformation. Cytophotometrical measurements revealed that the muscle fibres developed their highest contractile, oxidative and glycolytic activity at PD 21, the time of weaning. In this way, muscle fibres may be prepared for the higher demands for posture and mobility after leaving the nest.

Myotoxic phospholipases A2 and the regeneration of skeletal muscles

The review explains why the myotoxic phospholipases A2 and cardiotoxins are such important tools in the study of the regeneration and maturation of mammalian skeletal muscle. The role of satellite cells as precursors of cell-based regeneration is discussed and recent controversies on the origin of myogenic cells involved in the regeneration of mature skeletal muscle are addressed. This is followed by discussions of sarcomere reconstruction, myosin and sarcoplasmic reticulum ATPase expression, the electrophysiological properties of regenerating muscle, and the reconstruction of the neuromuscular junction. The emphasis throughout is on the plastic changes of major structural and functional proteins that occur during regeneration, and on other influences that determine the final outcome of regenerative activity such as innervation, thyroid status, mechanical work and the functional integrity of the microcirculation. The review closes with a discussion of some of the factors--such as active regeneration--that influence the success of gene-based therapies applied to inherited muscle disease.

Changes in myotonic dystrophy protein kinase levels and muscle development in congenital myotonic dystrophy

Myotonic dystrophy (DM1) is caused by the expansion of a CTG repeat in the noncoding region of a protein kinase, DMPK, expressed in skeletal and cardiac muscles. The aim of the present study was to determine the effects of very large CTG expansions on DMPK expression and skeletal muscle development. In fetuses suffering from the severe congenital form of DM1 with large CTG expansions (1800 to 3700 repeats), the skeletal muscle level of DMPK was reduced to 57% of control levels and a similar reduction was observed in cultured DM1 muscle cells relative to control cultures. These results are consistent with greatly reduced DMPK expression from the mutant allele and normal expression from the unaffected allele in this autosomal dominant disorder. In normal fetuses, DMPK protein levels increased dramatically between 9 and 16 weeks and remained high throughout the remaining gestation period. DM1 fetuses showed impaired skeletal muscle development, characterized by a persistence of embryonic and fetal myosin heavy chains and almost total absence of slow myosin heavy chains at the end of gestation. DMPK expression, however, was similar in both fast and slow fibers from normal adult muscle. The reduced DMPK and the delayed slow fiber maturation in congenital DM1 may be two separate consequences of nuclear retention of DMPK RNA transcripts with expanded CUG repeats.

Embryonic and fetal rat myoblasts form different muscle fiber types in an ectopic in vivo environment

Limb muscle development is characterized by the migration of muscle precursor cells from the somite followed by myoblast differentiation and the maturation of myotubes into distinct muscle fiber types. Previous in vitro experiments have suggested that rat limb myoblasts are composed of at least two distinct myoblast subpopulations that appear in the developing hindlimb at different developmental stages. These embryonic and fetal myoblast subpopulations are believed to generate primary and secondary myotubes, respectively. To test this hypothesis, cells obtained from embryonic day 14 (ED 14) and ED 20 rat hindlimbs were analyzed for myosin heavy chain expression after long-term differentiation in adult rat brains. Fetal myoblasts from ED 20 hindlimbs produced muscle fibers with a phenotype similar to that seen in tissue culture--predominantly fast myosin with a small proportion also coexpressing slow myosin. However, injection sites populated by embryonic myoblasts from ED 14 hindlimbs produced a different phenotype from that previously reported in culture, with fibers expressing an entire array of myosin isoforms. In addition, a subpopulation of fibers expressing exclusively slow myosin was found only in the embryonic injection sites. Our results support the existence of at least three myogenic subpopulations in early rat limb buds with only one exhibiting the capability to differentiate in vitro. These findings are consistent with a model of muscle fiber type development in which the fiber type potential of myoblast populations is established before differentiation into myotubes. This process establishes myogenic subpopulations that have restricted adaptive ranges regulated by both intrinsic and extrinsic factors.

Early changes of type 2B fibers after denervation of rat EDL skeletal muscle

Skeletal muscle type 2B fibers normally receive a moderate level of motoneuron discharge. As a consequence, we hypothesize that type 2B fiber properties should be less sensitive to the absence of the nerve. Therefore, we have investigated the response of sarcoplasmic reticulum and myofibrillar proteins of type 2B fibers isolated from rat extensor digitorum longus muscle after denervation (2 and 7 days). Single fibers were identified by SDS-PAGE of myosin heavy chain isoforms. Electrophysiological and isometric contractile properties of the whole muscle were also analyzed. The pCa-tension relationship of type 2B single fibers was shifted to the left at 2 days and to right at 7 days after denervation, with significant differences in the Hill coefficients and pCa threshold values in 2- vs. 7-day-denervated fibers. The sarcoplasmic reticulum Ca2+ uptake capacity and rate significantly decreased after 2 days of denervation, whereas both increased at 7 days. Caffeine sensitivity of sarcoplasmic reticulum Ca2+ release was transitory and markedly increased in 2-day-denervated fibers. Our results indicate that type 2B fiber functional properties are highly sensitive to the interruption of nerve supply. Moreover, most of 2-day-denervated changes were reverted at 7 days.

The expression of the myogenic regulatory factors in denervated and normal muscles of different phenotypes

The nerve is known to play a pivotal role in the diversification of muscle fibre types postnatally. Reducing neuronal activity in a slow muscle such as the soleus by denervation, switches on genes associated with a fast muscle phenotype. On the other hand, denervating a fast muscle such as the extensor digitorum longus (EDL) induces the conversion of fast fibres to a 'slower' contractile phenotype. The myogenic regulatory factors (MRFs) are proposed as the regulators of muscle phenotype as MyoD and myogenin have been shown to differentially accumulate in fast and slow muscle upon the induction of fibre type transformation. The denervation model has been used in the present study to induce changes in MRF expression in the muscles of the lower hindlimb which have distinct phenotypic characteristics. The level of MRF expression in pairs of denervated and innervated soleus, EDL, tibialis anterior (TA), plantaris and gastrocnemius muscles has been determined by Northern analysis and compared. The present study has shown that each muscle responds differently to denervation with respect to the increases in MRF expression. Fast muscles responded very quickly to denervation by increasing the level of MRF transcripts while slow muscles did not show significant increases in expression after 48 h denervation. The innervated EDL (fast) and soleus (slow) muscle differed with respect to the level of MRF-4 expressed, MRF-4 being expressed at higher levels in the slow muscle compared to the fast, suggesting that MRF-4 is important in the maintenance of a slow muscle phenotype. Moreover, MRF-4 and myogenin show the greatest fold increases in expression in the fast muscles examined. MyoD and Myf 5 show less dramatic increase in expression in response to denervation but exhibit the greatest fold increases in the fast muscles compared to slow.

Invited Review: plasticity and energetic demands of contraction in skeletal and cardiac muscle

Numerous studies have explored the energetic properties of skeletal and cardiac muscle fibers. In this mini-review, we specifically explore the interactions between actin and myosin during cross-bridge cycling and provide a conceptual framework for the chemomechanical transduction that drives muscle fiber energetic demands. Because the myosin heavy chain (MHC) is the site of ATP hydrolysis and actin binding, we focus on the mechanical and energetic properties of different MHC isoforms. Based on the conceptual framework that is provided, we discuss possible sites where muscle remodeling may impact the energetic demands of contraction in skeletal and cardiac muscle.

Postnatal myosin heavy chain isoform expression in normal mice and mice null for IIb or IId myosin heavy chains

The patterns of myosin heavy chain (MyHC) isoform expression in the embryo and in the adult mouse are reasonably well characterized and quite distinct. However, little is known about the transition between these two states, which involves major decreases and increases in the expression of several MyHC genes. In the present study, the expression of seven sarcomeric MyHCs was analyzed in the hindlimb muscles of wild-type mice and in mice null for the MyHC IIb or IId/x genes at several time points from 1 day of postnatal life (dpn) to 20 dpn. In early postnatal life, the developmental isoforms (embryonic and perinatal) comprise >90% of the total MyHC expression, while three adult fast isoforms (IIa, IIb, and IId) comprise <1% of the total MyHC protein. However, between 5 and 20 dpn their expression increases to comprise >90% of the total MyHC. Expression of each of the three adult fast isoforms occurs in a spatially and temporally distinct manner. We also show that alpha MyHC, which is almost exclusively expressed in the heart, is expressed in scattered fibers in all hindlimb muscles during postnatal development. Surprisingly, the timing and localization of expression of the MyHC isoforms is unchanged in IIb and IId/x null mice, although the magnitude of expression is altered for some isoforms. Together these data provide a comprehensive overview of the postnatal expression pattern of the sarcomeric MyHC isoforms in the mouse hindlimb.

Denervation alters myosin heavy chain expression and contractility of developing rat diaphragm muscle

We hypothesized that unilateral denervation (DNV) of the rat diaphragm muscle (Dia(m)) in neonates at postnatal day 7 (D-7) alters normal transitions of myosin heavy chain (MHC) isoform expression and thereby affects postnatal changes in maximum specific force (P(o)) and maximum unloaded shortening velocity (V(o)). The relative expression of different MHC isoforms was analyzed electrophoretically. With DNV at D-7, expression of MHC(neo) in the Dia(m) persisted, and emergence of MHC(2X) and MHC(2B) was delayed. By D-21 and D-28, relative expression of MHC(2A) and MHC(2B) was reduced in DNV compared with control (CTL) animals. Expression of MHC(neo) also reappeared in adult Dia(m) by 2-3 wk after DNV, and relative expression of MHC(2B) was reduced. At each age, P(o) was reduced and V(o) was slowed by DNV, compared with CTL. In CTL Dia(m), postnatal changes in P(o) and V(o) were associated with an increase in fast MHC isoform expression. In DNV Dia(m), no such association existed. We conclude that, in the Dia(m), DNV induces alterations in both MHC isoform expression and contractile properties, which are not necessarily causally linked.

Developmental changes in histochemical properties of intrinsic laryngeal muscles in rats

OBJECTIVE

Using neonatal rats, the developmental changes in muscle fiber type of the intrinsic laryngeal muscles were analyzed. The potential influence of two factors were also studied, that were predicted would influence developmental changes in muscle fiber type, denervation and hypothyroidism.

METHODS

Using the histochemical technique of myosin ATPase staining, postnatal changes in the ratio of muscle fiber types of each intrinsic laryngeal muscle were determined. In addition, to clarify factors influencing the development of the intrinsic laryngeal muscles, the same technique was employed to study recurrent laryngeal nerve-denervated rats and rats with experimentally-induced hypothyroidism.

RESULTS

In normal pups, type 2C fibers had almost disappeared by postnatal day (PND) 14. In denervated pups, differentiation to type 1 and 2A muscle fibers was not observed. In contrast, differentiation to type 2B muscle fibers was impaired in the hypothyroid pups.

CONCLUSION

The differentiation of intrinsic laryngeal muscles occur earlier than that of hindlimb muscles. Each intrinsic laryngeal muscle exhibits a particular pattern of developmental changes in normal pups. The developmental changes in the intrinsic laryngeal muscles are affected by recurrent laryngeal nerve innervation and thyroid hormonal control. The findings suggest that both recurrent laryngeal nerve innervation and thyroid hormone play important roles in the differentiation of the intrinsic laryngeal muscles.

Neural regulation of myosin gene expression in regenerating skeletal muscle

Excitation-transcription coupling, namely the process whereby plasma membrane depolarization leads to gene activation or inactivation, is still a black box for most muscle genes. Muscle regeneration is a useful model system to ask basic questions concerning the triggering signals and the transduction pathways involved in activity-dependent gene regulation. We report ongoing research in our laboratory concerning (1) myosin heavy chain changes in regenerating muscle in the presence and absence of the nerve, as well as changes induced by electrical stimulation, (2) identification of activity response elements in the promoter of a slow myosin light chain gene, and (3) potential approaches to define the transduction pathways induced by neural or electrical activity and implicated in muscle gene regulation.

MEF-2 and Oct-1 bind to two homologous promoter sequence elements and participate in the expression of a skeletal muscle-specific gene

The murine adult IIB myosin heavy chain (IIB MyHC) gene is expressed only in certain skeletal muscle fibers. Within the proximal promoter are two A + T-rich motifs, mAT1 and mAT2, which greatly enhance muscle-specific transcription; myogenic cells contain proteins that bind to these sequences. MEF-2 binds to both mAT1 and mAT2; a mutation abolishing its binding to mAT1 greatly diminishes the activity of the promoter. Both mAT motifs also form complexes with a protein requiring a target sequence typical of POU domain proteins, which migrate in electrophoretic mobility shift assays to the same position as a complex containing purified Oct-1 and which are supershifted by an antibody specific to Oct-1; this protein is therefore probably Oct-1. Footprinting experiments demonstrate that mAT1 is preferentially occupied by MEF-2 and mAT2 by Oct-1 and that these two proteins appear to bind cooperatively to their respective sites. Although the two mAT motifs have sequences that are very similar, they nonetheless exhibit distinct behaviors and perform differently in the activation of the promoter. The contribution of the IIB MyHC gene to specification of the myogenic phenotype is thus at least in part regulated by MEF-2 and Oct-1.

Regulation of myosin heavy chain gene expression after short-term diaphragm inactivation

Although prolonged diaphragm denervation (DNV) produces myofiber atrophy and a loss of type I myosin heavy chain (MHC) expression, short-term DNV leads to significant diaphragm hypertrophy. The purpose of this study was to explore the regulation of MHC isoform expression and muscle remodeling during DNV hypertrophy of the diaphragm. Both unilateral and bilateral DNV led to similar changes, with a significant increase in total RNA content and muscle mass but no change in dry-to-wet weight ratio. Sarcomere number was also increased in diaphragm myofibers after DNV ( approximately 20%), suggesting an adaptive response to muscle stretch. There was hypertrophy of type I myofibers and increased coexpression of type I and type II MHCs within single myofibers by immunocytochemistry as well as increased type I MHC (25-46%) and decreased type IIb MHC (14-39%) by SDS-PAGE. Contractility parameters were also consistent with a type II-to-type I MHC phenotype transformation. Importantly, DNV-induced modulation of MHC isoform mRNA transcript levels did not correspond to changes in their cognate proteins, suggesting a major degree of posttranscriptional control. We conclude that DNV hypertrophy of the diaphragm is associated with reciprocal changes in type I and type II MHC isoforms that are directly opposed to the type I-to-type II MHC phenotype transformation reported in the diaphragm DNV atrophy model. Furthermore, in contradistinction to most hypertrophy models, control of MHC gene expression and myofibrillar remodeling after short-term DNV appears to entail major involvement of posttranscriptional regulatory mechanisms.

Myosin heavy chain expression in skeletal muscle autografts under neural or aneural conditions

BACKGROUND

Our purpose was to investigate (1) the heterogeneity of satellite cells derived from adult fast-twitch and slow-twitch skeletal muscles, (2) the influence of innervation on muscle regeneration, and (3) the differences between developmental myoblasts and satellite cells with regard to myosin heavy chain (MHC) expression.

MATERIALS AND METHODS

Autografts under neural (nerve-intact graft; brief denervation interval) or aneural (aneural graft; prolonged denervation interval) conditions of the fast-twitch extensor digitorum longus (EDL) muscle or the slow-twitch soleus muscle were performed in adult rat hindlimbs. MHC expression during skeletal muscle regeneration was determined sequentially using immunocytochemistry.

RESULTS

After grafting, most muscle fibers in the EDL and soleus underwent ischemic degeneration and regeneration; at the periphery of each muscle, a few adult fibers survived. All regenerating fibers initially expressed embryonic/fetal (developmental) MHC alone, and subsequently both developmental and fast MHC. During the first week, no expression of slow MHC was observed in regenerating fibers in either the EDL or the soleus. In nerve-intact grafts, regenerating fibers expressed slow MHC as early as the second week; under aneural conditions, no regenerating fibers expressed slow MHC even 4 weeks after grafting. On the other hand, some persisting fibers in aneural grafts could maintain expression of slow MHC 4 weeks after grafting; other fibers underwent MHC transformation induced by denervation. No significant difference in MHC expression during regeneration was observed for slow compared with fast muscles, under either neural or aneural condition.

CONCLUSIONS

These data suggest that regenerating adult skeletal muscle fibers, derived only from satellite cells, cannot express slow MHC without motor innervation, and that persisting muscle fibers, derived from both myoblasts in fetal development and satellite cells, may be intrinsically distinct from regenerating fibers. Satellite cells derived from slow and from fast muscles may be a single, homogenous population and may be the same population as fetal (secondary) myoblasts with regard to MHC expression.

Expression of the potassium channel KV3.4 in mouse skeletal muscle parallels fiber type maturation and depends on excitation pattern

We report the detailed expression pattern of the voltage-dependent potassium channel KV3.4 (rat homologue, Raw3) in mouse skeletal muscle. Using semi-quantitative RT-PCR, we show that its expression is detectable at embryonic day 17 and rises to adult levels within 2 weeks after birth. Expression is fiber type-dependent, with mRNA levels being 5-6-fold lower in the mixed slow/fast soleus muscle than in the fast tibialis anterior and extensor digitorum longus muscles. Fast muscles from myotonic mice exhibit low KV3.4 mRNA levels similar to those of wild-type soleus. In denervated extensor digitorum longus, KV3.4 expression declines to perinatal levels. We conclude that KV3.4 expression in mouse skeletal muscle is regulated by the pattern of excitation.

Transitory expression of alpha cardiac myosin heavy chain in a subpopulation of secondary generation muscle fibers in the pig

Unlike the random distribution of fiber types seen in skeletal muscles of most mammals, pig muscle exhibits a rosette pattern consisting of islets of slow fibers surrounded by concentric circles of type IIA and IIB fibers. Within each islet of slow fibers, one of the central fibers is a primary myofiber, whereas all others are secondary fibers. The present study demonstrates that a subpopulation of the slow secondary fibers transiently expresses alpha-myosin heavy chain (MHC). Two cDNA libraries were made from longissimus dorsi skeletal muscle of 14-day-old piglet and adult pig atrium; the latter muscle is mainly composed of alpha-MHC. Screening of the libraries with a human anti-alpha-MHC mAb (F8812F8) demonstrated the presence of positive MHC clones in both libraries; the nucleotide sequence of the 3'-untranslated region (3'-UTR) was identical in both libraries. As this MHC 3'-UTR had 75% homology with the human alpha-MHC, it was identified as pig alpha-MHC. Using specific cRNA probes and mAbs against pig alpha-cardiac and beta/slow/type I MHC, we studied the expression of these MHCs in developing pig semitendinosus muscle by combining in situ hybridization and immunocytochemistry on serial sections at 90 days of gestation, and at 1, 6, 35 days and 6 months of age. The results showed that a subpopulation of secondary fibers that directly abut primary fibers, transiently produced alpha-MHC, both at the levels of the protein and its transcript. Subsequently, these fibres expressed beta-MHC. At 1 day, immunocytochemistry showed that 16% of the secondary fibers expressed alpha-MHC, among which 20% did not yet express beta-MHC. At 6 days, alpha- and beta-MHCs were mostly present in the same fibers, i.e., 23% of the secondary fibers. Thereafter, the proportion of secondary fibers reacting with alpha-MHC mAb decreased to 10% at 5 weeks and 0% at 6 months, whereas beta-MHC was still accumulating in about 38% of the secondary fibers. During the period studied, the distribution of alpha- and beta-MHC transcripts closely matched that of the corresponding proteins. Expression of alpha-MHC was not detected in primary type I muscle fibers and slow type I secondary fibers at the periphery of the rosettes of slow fibers. This study is the first unequivocal demonstration of a transitory expression of alpha-MHC in a subpopulation of secondary fibers in a limb skeletal muscle during mammalian development.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Specific changes in skeletal muscle myosin heavy chain composition in cardiac failure: differences compared with disuse atrophy as assessed on microbiopsies by high resolution electrophoresis

OBJECTIVE

In congestive heart failure (CHF) the skeletal muscle of the lower limbs develops a myopathy with atrophy and shift from the slow type to the fast type fibres. The aim was to test the hypothesis that this myopathy is specific and not simply related to detraining, by comparing patients with different degrees of CHF with patients with severe muscle atrophy due to disuse.

DESIGN

Case-control study involving 50-150 micrograms needle biopsies of the gastrocnemius muscle. By an electrophoretic micromethod, the three isoforms of myosin heavy chains (MHC) were separated.

PATIENTS

Five patients restricted to bed for more than one year because of stroke with disuse atrophy and normal ventricular function, and 19 with CHF were studied. There were seven age matched controls.

MAIN OUTCOME MEASURES

The percentage of MHC1 (slow isoform), MHC2a (fast oxidative), and MHC2b (fast glycolytic) was determined by densitometric scan and correlated with indices of severity of cardiac failure.

RESULTS

Ejection fraction was 42.5 (SD 15.2)% in CHF, 59.5 (1.0)% in disuse atrophy and 60.3 (1.4)% in controls (P < 0.001 v both). The degree of muscle atrophy as calculated by the body mass index/gastrocnemius cross sectional area, showed a profound degree of atrophy in patients with muscle disuse [0.94 (0.39)]. This was worse than in the controls [4.27 (0.16), P < 0.0005] and the CHF patients [2.60 (1.10), P < 0.005]. Atrophy in CHF patients was also greater than in controls (P < 0.005). MHC1 was lower in CHF than in disuse atrophy [51.83 (15.04) v 84.5 (17.04), P < 0.01] while MHC2b was higher [23.5 (7.4) v 7.25 (7.92), P < 0.001]. There was a similar trend for MHC2a [24.83 (15.01) v 8.25 (9.12), P < 0.05]. Within the CHF group there was a positive correlation between NYHA class and MHC2a (r = 0.47, P < 0.05) and MHC2b (r = 0.55, P < 0.01) and a negative correlation between NYHA class and MHC1 (r = -0.74, P < 0.001). Similarly, significant correlations were found for ejection fraction, diuretic consumption score, exercise test tolerance, and degree of muscle atrophy.

CONCLUSIONS

The CHF myopathy appears to be specific and not related to detraining. The magnitude of MCH redistribution correlates with the severity of the disease. The electrophoretic micromethod used is very sensitive and reproducible. Biopsies are so well tolerated that can be repeated frequently, allowing thorough follow up.

Changes in some troponin and insulin-like growth factor messenger ribonucleic acids in regenerating and denervated skeletal muscles

To investigate the role of innervation and to determine if the process of muscle differentiation is preprogrammed, the expression of insulin-like growth factors (IGF-I and IGF-II), troponin I and troponin T mRNAs was studied in regenerating transplants of rat Extensor digitorum longus muscle in the presence and absence of nerve. The role of innervation was further investigated by denervating some adult fast (Gastrocnemius and Plantaris) and slow (Soleus) skeletal muscles. In normal adult skeletal muscles, IGF-I, IGF-II and developmental fast troponin T mRNA containing exon y, are undetectable or present at very low levels. Induction of all these mRNAs was observed in regenerating muscles in both the presence and absence of nerve as well as following denervation of adult fast and slow skeletal muscles. Their low level expression was maintained in adult denervated skeletal muscles but gradually suppressed in both innervated and noninnervated regenerating extensor digitorum longus muscle transplants after 2 months. Fast troponin T mRNA was synthesized in both innervated and noninnervated EDL transplants although the level of this transcript changed markedly in response to denervation of both adult fast and slow skeletal muscles. The fast troponin T mRNA containing exon 17 was also initially expressed in both regenerating muscles but its level was reduced with time in both transplants and in all adult denervated skeletal muscles. Fast and slow troponin I mRNAs were synthesised during EDL muscle regeneration in both the presence and absence of nerve but the slow troponin I expression was not maintained in noninnervated transplants. The level of fast troponin I mRNA decreased in denervated fast skeletal muscles but markedly increased in denervated Soleus. The level of slow troponin I mRNA was slightly increased in denervated fast skeletal muscles but considerably reduced in denervated Soleus.

Pattern of muscle fiber type formation in the pig

The aim of this study was to analyze the temporal sequence of expression of the myosin isoforms in the populations of muscle fibers in the pig and to bring more information on the origin of the strikingly different pattern of fiber composition and distribution between the deep medial red (oxido-glycolytic) and superficial white (glycolytic) portions of semitendinosus (ST) muscle. Muscle samples were taken from 49-, 55-, 75-, 90-, 103-, and 113- (birth) day-old fetuses, from 6-, 11-, 21-, 35-, 50-, and 80-day-old piglets, and from a 3-year-old pig. Our results confirm the sequential formation of primary and secondary generation fibers. The use of immunohistochemistry and heterologous monoclonal antibodies (mAb) directed against specific myosin heavy chain (MHC) isoforms revealed a different pattern of gene expression between the two portions of the ST muscle for both generations of fibers. By 75 days of gestation (dg), primary myotubes from the deep medial portion stained positively for the anti-slow MHC mAb and negatively for the adult anti-fast MHC, whereas the opposite was observed in the superficial portion. Secondary fibers never expressed slow MHC until late gestation. Instead, they expressed an adult fast MHC isoform as soon as they formed in the deep medial portion and later on in the superficial portion. From late gestation to the first 3 postnatal weeks, slow MHC began to be expressed in a subpopulation of secondary fibers. These fibers were in the direct vicinity of primary myotubes in the deep medial portion, whereas their location could not be established in the superficial portion. The remaining secondary fibers matured to type IIA in the direct vicinity of these type I fibers and to type IIB at the periphery of the islets. In both portions of the muscle, a subpopulation of secondary fibers, the first ones to express slow MHC, also transitorily expressed a MHC that was identical or closely related to the alpha-cardiac MHC during the early postnatal period. A third generation of small diameter fibers was observed shortly after birth and reacted with the anti-fetal MHC mAb; their destiny remains to be established.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of vertebrate muscle differentiation by thyroid hormone: the role of the myoD gene family

Skeletal myoblasts have their origin early in embryogenesis within specific somites. Determined myoblasts are committed to a myogenic fate; however, they only differentiate and express a muscle-specific phenotype after they have received the appropriate environmental signals. Once proliferating myoblasts enter the differentiation programme they withdraw from the cell cycle and form post-mitotic multinucleated myofibres (myogenesis); this transformation is accompanied by muscle-specific gene expression. Muscle development is associated with complex and diverse protein isoform transitions, generated by differential gene expression and mRNA splicing. The myofibres are in a state of dynamic adaptation in response to hormones, mechanical activity and motor innervation, which modulate differential gene expression and splicing during this functional acclimatisation. This review will focus on the profound effects of thyroid hormone on skeletal muscle, which produce alterations in gene and isoform expression, biochemical properties and morphological features that precipitate in modified contractile/mechanical characteristics. Insight into the molecular events that control these events was provided by the recent characterisation of the MyoD gene family, which encodes helix-loop-helix proteins; these activate muscle-specific transcription and serve as targets for a variety of physiological stimuli. The current hypothesis on hormonal regulation of myogenesis is that thyroid hormones (1) directly regulate the myoD and contractile protein gene families, and (2) induce thyroid hormone receptor-transcription factor interactions critical to gene expression.

Nerve-dependent recovery of metabolic pathways in regenerating soleus muscles

The metabolic recovery potential of muscle was studied in regenerating soleus muscles of young adult rats. Degeneration was induced by subfascial injection of a myotoxic snake venom. After regeneration for selected periods up to 2 weeks, samples of whole muscle were analysed for hexokinase (EC 2.7.1.1), phosphofructokinase (EC 2.7.1.11), lactate dehydrogenase (EC 1.1.11.27), adenylokinase (EC 2.7.4.3), creatine kinase (EC 2.7.3.2), malate dehydrogenase (EC 1.1.11.37), citrate synthase (EC 4.1.3.7) and beta-hydroxyacyl CoA dehydrogenase (EC 1.1.1.35). Lactate dehydrogenase, adenylokinase, malate dehydrogenase and beta-hydroxyacyl CoA dehydrogenase were also measured in individual fibres of muscle regenerating up to 4 weeks. We found that in the presence of nerve there was complete recovery of muscle metabolic capacity. However, there were differences in the rate of recovery of the activity of enzymes belonging to different energy-generating pathways. Lactate dehydrogenase, an enzyme representing glycolytic metabolism, reached normal activity immediately upon myofibre formation, only 3 days after venom injection, while oxidative enzymes required a week or more to reach normal activity levels. The delay in oxidative enzyme recovery coincided with physiological parameters of reinnervation. Therefore, to further test the role of nerve on the metabolic recovery process, muscle regeneration was studied following venom-induced degeneration coupled with denervation. In the absence of innervation, most enzymes failed to recover to normal activity levels. Lactate dehydrogenase was the only enzyme to achieve normal levels, and it did so as rapidly as in innervated-regenerating soleus muscles. The remainder of the glycolytic enzymes and the high energy phosphate enzymes recovered only partially. Oxidative enzymes showed no recovery and were severely reduced in the absence of reinnervation.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of denervation on myosin isoform synthesis in rabbit slow-type and fast-type muscles during terminal differentiation. Denervation induces differentiation into slow-type muscles

The soleus and gastrocnemius medialis of eight-day-old rabbits were denervated and the effects were examined after fifty-two days by biochemical, cytochemical and mechanical methods. The contralateral soleus exhibited the properties of slow-type muscle, namely a predominance of slow-type myosin isoforms and slow-type oxidative fibers, slow twitch and low maximal velocity for shortening. The contralateral gastrocnemius exhibited the properties of fast-type muscle, namely a predominance of fast-type myosin isoforms and fast-type non-oxidative fibers, fast twitch and high maximal velocity of shortening. Denervation of muscles caused the differentiation of the two muscles towards slow-type muscles. Both denervated soleus and gastrocnemius muscles exhibited a predominance of slow-type myosins (either the normal type, made up of slow heavy and light chains, or the hybrid type, made up of slow heavy and regulatory light chains and fast essential light chains), a predominance of slow-type fibers, and slow mechanical properties. Thus, innervation in rabbit appears to be a determining factor for differentiation into fast-type muscle, but it is not necessary for differentiation into slow-type muscle. This conclusion contradicts the findings of previous studies in rat and thus raises new questions concerning the role of nerves in controlling the expression of myosin isoforms and the differentiation of muscle fibers.

Barrier inhibition of a temporal neuraxial influence on early chick somitic myogenesis

Skeletal myogenesis in the chick embryo first occurs in the somite. Somites are transient, paired mesodermal structures adjacent to the neural tube. Somites form from the segmental plate mesenchyme at approximately 90-min intervals. We identify somitic myogenic cells by using confocal microscopy to detect the muscle specific intermediate filament protein, desmin, in whole mount chick embryo preparations. The appearance of desmin in somitic cells does not occur at a constant interval after the somite has formed. The rate of chick somitic myogenic onset, as evidenced by detection of desmin, is approximately 1.5 times faster than the rate of somitogenesis (Borman and Yorde [1994] J. Histochem. Cytochem. 42:265-272). Somitic myogenesis does not appear to be directly linked to somitogenesis but instead may be regulated by some influence external to the somite. Here we have specifically addressed the issue of whether an impermeable barrier placed between the neuraxis and the somites can prevent the onset of somitic myogenesis. When tantalum foil barriers are placed medial to the caudalmost 3-5 somites of embryos having up to 20 somites total (stage 13), the predominant result is an inhibition of myogenic cells lateral to the barrier. Conversely, when the tantalum foil is placed medial to the caudal somites of an embryo having 21 somites (stage 14) or more, desmin is detected lateral to the barrier in most cases. There is a temporal influence originating in the neuraxis which plays a role in the onset of somitic myogenesis. Although the nature of this interaction between the neuraxis and the somites is not yet clear, we have defined a precise temporal location within the developing embryo at which this tissue interaction is taking place.

Biochemical and immunocytochemical analysis in chronic proximal spinal muscular atrophy

Immunocytochemical and biochemical analyses were carried out on patients affected by chronic SMA. Three groups of patients were identified. In group I, the muscle presented a fascicular atrophy; a high percentage of atrophic type II fibers; and fibers expressing fast, slow, embryonic, and fetal myosin isoforms. In group II, the muscle was characterized by atrophic fibers and normal/hypertrophic fibers expressing only slow myosin isoforms. In group III, the muscle was characterized by fiber type grouping and fibers coexpressing fast and slow myosin isoforms but never embryonic or fetal MHC isoforms. The muscles of groups I and III contained both fast and slow myosins whereas group II muscles were predominantly slow by immunocytochemical analysis or only slow by biochemical analysis. In view of these results, immunocytochemical and histochemical analyses could help to classify chronic SMA and help to understand the different pathogenic processes which seem to be related to the maturational stage of the muscle at the age of onset of the disease.

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.

Neural regulation of differentiation of rat skeletal muscle cell types

Three monoclonal antibodies (LM5, F2 and F39) to the fast class of myosin heavy chain (MHC) were used to study the effect of denervation on the differentiation of muscle cell types in some rat skeletal muscles. Antibody LM5 in immunocytochemical investigations did not stain any myotubes during early fetal development but presumptive fast muscle cells started to stain during later fetal development. Unlike antibody LM5, antibodies F2 and F39 stained all myotubes during fetal development. The suppression of fast myosin heavy chains recognised in presumptive slow muscle cells was observed within 1-2 days after birth with antibody F39 but not until 10-14 days after birth with antibody F2. The emergence of subsets of fast muscle fibre types in rat extensor digitorum longus (EDL) and tibialis anteri (TA) detectable by F39 and F2 antibodies was not observed until 2-3 weeks after birth. Denervation of developing muscles led to marked changes in the expression of myosins identified by these antibodies.

Evolution of muscle specific proteins in Werdnig-Hoffman's disease

The pattern of expression of desmin, vimentin, titin and different myosin isoforms expressed in atrophic and hypertrophic type I and type II muscle fibers was investigated in 7 biopsies from patients of various ages all diagnosed as suffering from Werdnig-Hoffman's disease. The results revealed that there was a progressive atrophy affecting both type I and type II muscle fibers. The proportion of atrophic type II fibers increased with age. These atrophic fibers expressed predominantly fast MHC together with variable amounts of embryonic and fetal abnormal concentrations of desmin, vimentin and titin were also observed in some of these fibers. Hypertrophic type I fibers expressed exclusively slow MHC. These results are in good agreement with the hypothesis that Werdnig-Hoffman's disease is associated with a persistence of slow twitch type I motor units and a loss of phasic type II motor units. They also confirm that the atrophic fibers were frequently immature although embryonic MLC was never detected in these muscles. In addition we have demonstrated that the hypertrophic fibers were not completely normal since they frequently contained abnormal concentrations of desmin and titin at their periphery.