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

Functional and molecular outcomes of the human masticatory muscles

The masticatory muscles achieve a broad range of different activities such as chewing, sucking, swallowing, and speech. In order to accomplish these duties, masticatory muscles have a unique and heterogeneous structure and fiber composition, enabling them to produce their strength and contraction speed largely dependent on their motor units and myosin proteins that can change in response to genetic and environmental factors. Human masticatory muscles express unique myosin isoforms, including a combination of thick fibers, expressing myosin light chains (MyLC) and myosin class I and II heavy chains (MyHC) -IIA, -IIX, α-cardiac, embryonic and neonatal and thin fibers, respectively. In this review, we discuss the current knowledge regarding the importance of fiber-type diversity in masticatory muscles versus supra- and infrahyoid muscles, and versus limb and trunk muscles. We also highlight new information regarding the adaptive response and specific genetic variations of muscle fibers on the functional significance of the masticatory muscles, which influences craniofacial characteristics, malocclusions, or asymmetry. These findings may offer future possibilities for the prevention of craniofacial growth disturbances.

Effects of prolonged exposure to CO2 on behaviour, hormone secretion and respiratory muscles in young female rats

Atmospheric CO₂ concentrations increased significantly over the last century and continuing increases are expected to have significant effects on current ecosystems. This study evaluated the behavioural and physiological (hormone status, muscle structure) effects of prolonged CO₂ exposure in young female Wistar rats exposed at 700ppm of CO₂ during 6h a day for 15days. Prolonged CO₂ exposure, though not continuous, produced significant disturbances in behaviour with an increase in drinking, grooming and resting, and a reduction in rearing, jumping-play and locomotor activity. Furthermore, CO₂ exposure was accompanied by increased plasma levels of corticosterone, suggesting that prolonged exposure to CO₂ was stressful. The muscular structure can also be modified also when respiratory working conditions change. The expression of myosin heavy chain was significantly affected in the diaphragm and oral respiratory muscles: Masseter Superficialis and Anterior Digastric. Modified behaviour and hormonal changes both appear to be at the origin of the observed muscular adaptation.

Six1 homeoprotein drives myofiber type IIA specialization in soleus muscle

BACKGROUND

Adult skeletal muscles are composed of slow and fast myofiber subtypes which each express selective genes required for their specific contractile and metabolic activity. Six homeoproteins are transcription factors regulating muscle cell fate through activation of myogenic regulatory factors and driving fast-type gene expression during embryogenesis.

RESULTS

We show here that Six1 protein accumulates more robustly in the nuclei of adult fast-type muscles than in adult slow-type muscles, this specific enrichment takes place during perinatal growth. Deletion of Six1 in soleus impaired fast-type myofiber specialization during perinatal development, resulting in a slow phenotype and a complete lack of Myosin heavy chain 2A (MyHCIIA) expression. Global transcriptomic analysis of wild-type and Six1 mutant myofibers identified the gene networks controlled by Six1 in adult soleus muscle. This analysis showed that Six1 is required for the expression of numerous genes encoding fast-type sarcomeric proteins, glycolytic enzymes and controlling intracellular calcium homeostasis. Parvalbumin, a key player of calcium buffering, in particular, is a direct target of Six1 in the adult myofiber.

CONCLUSIONS

This analysis revealed that Six1 controls distinct aspects of adult muscle physiology in vivo, and acts as a main determinant of fast-fiber type acquisition and maintenance.

Thyroid hormone regulates muscle fiber type conversion via miR-133a1

Thyroid hormone promotes slow-to-fast muscle fiber type conversion by inducing miR-133a1 and thereby repressing the expression of the slow muscle determinant TEAD1.

It is known that thyroid hormone (TH) is a major determinant of muscle fiber composition, but the molecular mechanism by which it does so remains unclear. Here, we demonstrated that miR-133a1 is a direct target gene of TH in muscle. Intriguingly, miR-133a, which is enriched in fast-twitch muscle, regulates slow-to-fast muscle fiber type conversion by targeting TEA domain family member 1 (TEAD1), a key regulator of slow muscle gene expression. Inhibition of miR-133a in vivo abrogated TH action on muscle fiber type conversion. Moreover, TEAD1 overexpression antagonized the effect of miR-133a as well as TH on muscle fiber type switch. Additionally, we demonstrate that TH negatively regulates the transcription of myosin heavy chain I indirectly via miR-133a/TEAD1. Collectively, we propose that TH inhibits the slow muscle phenotype through a novel epigenetic mechanism involving repression of TEAD1 expression via targeting by miR-133a1. This identification of a TH-regulated microRNA therefore sheds new light on how TH achieves its diverse biological activities.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Differential developmental programming by early protein restriction of rat skeletal muscle according to its fibre-type composition

AIMS

Differences in fibre-type composition of skeletal muscle have been associated with obesity and insulin resistance. As a poor nutrient environment early in life is a predisposing factor for the development of obesity and related metabolic diseases at adulthood, this study aimed at determining the long-term consequences of maternal undernutrition on the structural and metabolic properties of two skeletal muscles characterized by their different fibre-type composition and metabolic properties.

METHODS

The fibre-type composition and enzymatic activities of hexokinase (HK), beta-hydroxyacyl-CoA dehydrogenase (β-HAD) and citrate synthase (CS) were measured in soleus and extensor digitorum longus (EDL) muscles from adult rats born to dams fed a control (17% protein) or a low-protein [8% protein (PR)] diet throughout pregnancy and lactation. In addition, the expression levels of several genes regulating glycolysis, fatty acid oxidation and mitochondrial biogenesis were determined by real-time PCR.

RESULTS

Protein rats exhibited enhanced density of type II fibres along with decreased rate of fatty acid oxidation and glycolysis in soleus but not EDL. Malnourished rats exhibited also a different gene expression profile in soleus and EDL. Altogether, these alterations correspond to a state of energy deficiency and are present in animals which do not show yet any sign of obesity or glucose intolerance.

CONCLUSION

We conclude that maternal protein restriction alters in the long term the structural and enzymatic properties of offspring skeletal muscle in a fibre-type-dependent manner. These alterations might have a causative role in the development of obesity and related metabolic disorders later in life.

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.

Effects of short term forced oral breathing: physiological changes and structural adaptation of diaphragm and orofacial muscles in rats

OBJECTIVE

We studied adaptation of diaphragm and orofacial muscles as well as hormonal responses to forced oral breathing (lasting for only 4 days) following reversible bilateral nasal obstruction performed on day 8 post-natal male rats.

DESIGN

Muscle myosin heavy chain (MHC) composition and hormone levels were analysed during two periods: 1 and 3 days after obstruction (days 9 and 11 post-natal), and following 3 months recovery with nasal breathing (90 days, adult).

RESULTS

Diaphragm muscle showed significant increases in adult isoforms (MHC 1, 2a) in oral breathing group versus control. We observed increases in MHC neonatal and adult type 1 isoforms in muscles involved with oral breathing, masseter superficialis and anterior digastric. No changes were observed in the levator nasolabialis muscle involved with nasal breathing. Reversible nasal obstruction was associated with reduced growth of the olfactory bulbs lasting into adulthood, and an initial decrease in lung growth followed by recovery at 90 days. Adrenal hypertrophy was observed after 1 day of nasal obstruction and lasted into adulthood. The "stress" hormone response was variable, increased (over 1000%) during the obstruction but normal by adulthood. An increase in plasma testosterone was observed during the obstruction, and a decrease in thyroid hormone levels throughout.

CONCLUSIONS

Very short term nasal obstruction, i.e. forced oral breathing, leads to long term hormonal changes and respiratory muscle fibre adaptation.

Effect of triiodothyronine on the maxilla and masseter muscles of the rat stomatognathic system

The maxilla and masseter muscles are components of the stomatognathic system involved in chewing, which is frequently affected by physical forces such as gravity, and by dental, orthodontic and orthopedic procedures. Thyroid hormones (TH) are known to regulate the expression of genes that control bone mass and the oxidative properties of muscles; however, little is known about the effects of TH on the stomatognathic system. This study investigated this issue by evaluating: i) osteoprotegerin (OPG) and osteopontine (OPN) mRNA expression in the maxilla and ii) myoglobin (Mb) mRNA and protein expression, as well as fiber composition of the masseter. Male Wistar rats (~250 g) were divided into thyroidectomized (Tx) and sham-operated (SO) groups (N = 24/group) treated with T3 or saline (0.9%) for 15 days. Thyroidectomy increased OPG (~40%) and OPN (~75%) mRNA expression, while T3 treatment reduced OPG (~40%) and OPN (~75%) in Tx, and both (~50%) in SO rats. Masseter Mb mRNA expression and fiber type composition remained unchanged, despite the induction of hypo- and hyperthyroidism. However, Mb content was decreased in Tx rats even after T3 treatment. Since OPG and OPN are key proteins involved in the osteoclastogenesis inhibition and bone mineralization, respectively, and that Mb functions as a muscle store of O2 allowing muscles to be more resistant to fatigue, the present data indicate that TH also interfere with maxilla remodeling and the oxidative properties of the masseter, influencing the function of the stomatognathic system, which may require attention during dental, orthodontic and orthopedic procedures in patients with thyroid diseases.

Is fiber-type composition related to daily jaw muscle activity during postnatal development?

AIM

Muscles containing large numbers of slow-contracting fibers are generally more active than muscles largely composed of fast fibers. This relationship between muscle activity and phenotype suggests that (1) changes in fiber-type composition during postnatal development are accompanied by changes in daily activity and (2) individual variations in fiber-type composition are related to similar variations in daily muscle activity.

METHODS

The masseter and digastric muscles of 23 New Zealand White rabbits (young, juvenile and adult) were examined for their phenotype (myosin heavy chain content) and their daily activity (total daily number of activity bursts).

RESULTS

During development, the masseter showed a strong increase in the number of fast-type fibers compared to the number of slow-type fibers. During development, also the number of powerful bursts in the masseter increased. The digastric showed no significant changes in fiber types or burst numbers. Within each muscle, across individual animals, no significant correlations (R < 0.70) were found between any of the fiber types and daily burst numbers in any of the age groups.

CONCLUSIONS

The results suggest that activity-related influences are of relatively minor importance during development and that other factors are dominant in determining fiber-type composition.

Expression of nuclear and mitochondrial thyroid hormone receptors in postnatal rat tongue muscle

In this quantitative study, a competitive RT-PCR analysis was used to measure the level of the thyroid hormone receptors (TRs) in rat tongue muscle during the development of male Wistar rats aged 0, 5, 10, 15 and 21 postnatal days. There were differences between the expression of TR-alpha1 mRNA and the mRNAs for TR-beta1 and TR-beta2 in rat tongue muscle. Using Western blot analysis, a difference in expression between TR-alpha1 protein (c-ErbAalpha1 protein) and 43-kD c-ErbAalpha1 protein (T(3)-binding 43-kD mitochondrial protein) was detected during the development of the rat tongue muscle. Immunohistochemical examination using electron microscopy showed that TR-alpha1 was found in the mitochondria and nuclei in contrast to TR-beta1 detected in rat tongue muscle. In mitochondrial fractions from rat tongue muscle, the expression of 43-kD c-ErbAalpha1 protein was increased dramatically at 15 and 21 days, and a similar tendency was seen in cytochrome c proteins using Western blot analysis. We presume that the 43-kD c-ErbAalpha1 protein plays a role in regulating mitochondrial RNA synthesis during the postnatal development of rat tongue. The mRNA and protein myosin heavy chain isoforms of muscle also had a different expression during development. The slow myosin isoform protein was not found from day 10 in contrast to fast myosin isoforms. It is likely that the expression of TR-alpha1 mRNA from the rat tongue muscle may be related to a specific phase in muscle phenotype during the development.

Postnatal development of masseteric motoneurons in congenital hypothyroid rats

It has been known that an intact thyroid hormone is obligatory for the attainment of the normal masticatory function at the time of weaning. Following induced maternal thyroid hypo-function, the development of masseter motoneurons was determined at postnatal days 1, 7, 15 and 23 (weaning time), using retrograde transport of horseradish peroxidase (HRP) in the normal and hypothyroid pups. Based on the HRP labeling profile (strong and weak), the soma area of the masseteric labeled motoneurons was measured in each group. No significant morphological differences were observed at the end of the first week of life. On day 15, hypothyroid masseteric labeled motoneurons consisted of 76% small and 24% medium-sized neurons compared to 58% and 42% in normal pups, respectively. At the time of weaning (i.e., day 23) the number of large masseter motoneurons reached to 1/3 of normal value with few, short and disoriented dendrites in the hypothyroid pup. There was no statistically significant difference in the uptake of HRP from the neuromuscular junction. These results suggest that neonatal thyroid hormone deficiency considerably postponed the development of feeding behavior from sucking to chewing and biting.

Myosin heavy chain expression and muscle adaptation to chronic oral breathing in rat

The purpose of this study was to establish if early chronic oral breathing could induce an ultra-structural adaptation of the diaphragm and orofacial muscles related to oral or nasal breathing. Therefore, we performed a bilateral nasal obstruction at day 8 on rat pups and the myosin heavy chain (MHC) composition of the muscles was analyzed at day 21. Nasal obstruction and the related switch to chronic oral breathing were associated with impaired growth, atrophy of olfactory bulbs, hypertrophy of adrenal glands and reduced muscle growth for all muscles studied except the diaphragm. Furthermore, we detected a smaller decrease of MHC 2b compared to MHC 2a and 2x in levator nasolabialis, a muscle involved with nasal breathing. In masseter superficialis and anterior digastric involved with oral breathing, we observed a smaller decrease of MHC 2a compared to MHC 2b or 2x, respectively. No difference was detected in the diaphragm MHC expression of oral breathing animals. Since the relative expression of fatigue resistant MHC fiber types increased in muscles involved with oral breathing, orofacial muscles seem to present a profile in MHC adapted to the transition from nasal to oral breathing, facilitating respiration.

Fiber-type composition of the human jaw muscles--(part 2) role of hybrid fibers and factors responsible for inter-individual variation

This is the second of two articles about fiber-type composition of the human jaw muscles. It reviews the functional relationship of hybrid fibers and the adaptive properties of jaw-muscle fibers. In addition, to explain inter-individual variation in fiber-type composition, we discuss these adaptive properties in relation to environmental stimuli or perturbations. The fiber-type composition of the human jaw muscles is very different from that of limb and trunk muscles. Apart from the presence of the usual type I, IIA, and IIX myosin heavy-chains (MyHC), human jaw-muscle fibers contain MyHCs that are typical for developing or cardiac muscle. In addition, much more frequently than in limb and trunk muscles, jaw-muscle fibers are hybrid, i.e., they contain more than one type of MyHC isoform. Since these fibers have contractile properties that differ from those of pure fibers, this relatively large quantity of hybrid fibers provides a mechanism that produces a very fine gradation of force and movement. The presence of hybrid fibers might also reflect the adaptive capacity of jaw-muscle fibers. The capacity for adaptation also explains the observed large inter-individual variability in fiber-type composition. Besides local influences, like the amount of muscle activation and/or stretch, more general influences, like aging and gender, also play a role in the composition of fiber types.

Effect of glucocorticoïd receptor ligands on myosin heavy chains expression in rat skeletal muscles during controllable stress

The influence of agonist (dexamethasone) and antagonist (mifepristone) of glucocorticoïd receptor during controllable painless stress was evaluated on myosin heavy chains expression in three masticatory and two nape rat muscles: anterior digastric (AD), anterior temporalis (AT), masseter superficialis (MS), longissimus capitis (L) and rectus capitis dorsalis major (R). The relative amounts of myosin heavy chain (MHC) protein isoform contained were significantly affected in four muscles studied by dexamethasone and in three muscles studied under mifepristone, versus control during the stress procedure, after only 1 week of treatment. The control group AT muscles contained respectively 18.2% of MHC 2A, 34.5% of MHC 2X and 47.4% of MHC 2B. The effects of dexamethasone and mifepristone were opposite in this muscle: under dexamethasone, the relative proportions of the three isoforms were 14.2, 31.0 and 54.8%: consequently, MHC 2A and 2X decreased with the profit of 2B. Under mifepristone, the relative proportions were 21.1, 36.6 and 42.3% (MHC 2A and 2X increased to the detriment of 2B). The L muscle was not affected by the two treatments and MS muscle was only affected by dexamethasone. Dexamethasone increased MHC 2B to the detriment of MHC 2A in MS, AD and R. Mifepristone and dexamethasone induced the same changes in AD. The mifepristone treatment decreased the MHC 2X profile in R. Under dexamethasone, four muscles exhibited a significantly higher proportion of the more rapid isoforms than under mifepristone. A previous work showed that controllable stress induced a marked increase in the relative expression of MHC 2B in the same skeletal muscles (Martrette et al. , 1998). Our results confirm then a significant participation of glucocorticoïd in MHC isoform expression during controllable stress.

Myosin heavy chain isoforms of the murine masseter muscle during pre- and post-natal development

Masticatory muscles that are derived from the branchial arches express different compositions of myosin heavy chain (MHC) isoforms during the transitional phase from suckling to mastication. To clarify the developmental changes of murine masseter muscle, the composition of MHC isoforms was examined using immunohistochemical staining and competitive reverse transcription PCR. We found that MHC1 was expressed transiently in the pre and post-natal stages. In the compositional change of isoforms, the embryonic type MHCp was expressed consistently, whereas the adult isoforms increased with the developmental process. In particular, a significant change was observed between embryonic days 14 and 16, a stage when murine facial development is conspicuous. This suggests that the development of murine masseter muscle is closely associated with facial development.

Effect of thyroid hormones on acetylcholinesterase mRNA levels in the slow soleus and fast extensor digitorum longus muscles of the rat

In the rat, the level of acetylcholinesterase messenger RNA in the typical slow soleus muscles is only about 20-30% of that in the fast extensor digitorum longus muscles. The expression of contractile proteins in muscles is influenced by thyroid hormones and hyperthyroidism makes the slow soleus muscle faster. The influence of thyroid hormones on the levels of acetylcholinesterase messenger RNA level in the slow soleus and fast extensor digitorum longus muscle of the rat was studied in order to examine the effect of thyroid hormones on muscle acetylcholinesterase expression. Hyperthyroidism was induced in rats by daily thyroid hormone injection or thyroid hormone releasing tablet implantation. Hind-limb suspension was applied to produce muscle unloading. Muscle denervation or reinnervation was achieved by sciatic nerve transection or crush. Acetylcholinesterase messenger RNA levels were analyzed by Northern blots and evaluated densitometrically. Hyperthyroidism increased the levels of acetylcholinesterase messenger RNA in the slow soleus muscles close to the levels in the fast extensor digitorum longus. The effect was the same in the unloaded soleus muscles. Acetylcholinesterase expression increased also in the absence of innervation (denervation), in the presence of changed nerve activation pattern (reinnervation), and under enhanced tonic neural activation of the soleus muscle (electrical stimulation). However, the changes were substantially smaller than those observed in the control soleus muscles. Enhancement of acetylcholinesterase expression in the soleus muscles by the thyroid hormones is, therefore, at last in part due to hormonal effect on the muscle itself. On the contrary, increased level of the thyroid hormones had no influence on acetylcholinesterase expression in the normal fast extensor digitorum longus muscles. However, some enhancing influence was apparent whenever the total number of nerve-induced muscle activations per day in the extensor digitorum longus muscle was increased. Thyroid hormones seem to be an independent extrinsic factor of acetylcholinesterase regulation in the slow soleus muscle.

Early postnatal food intake alters myofiber maturation in pig skeletal muscle

We examined the effects of undernutrition on muscle development during the first postnatal week in pigs. Eighteen piglets were subjected to three nutritional levels (300, 200 or 100 g/(kg body. d) of colostrum then milk) between birth and slaughter at 7 d of age. Longissimus lumborum (LL), a fast-twitch glycolytic muscle, and rhomboideus (RH), a mixed slow- and fast-twitch oxido-glycolytic muscle, were taken for myofiber typing and biochemical analyses. Enzyme activities of lactate dehydrogenase (LDH), citrate synthase (CS) and beta-hydroxy-acyl-CoA-dehydrogenase (HAD) were used as markers of glycolytic, oxidative and lipid beta-oxidation capacities, respectively. Undernutrition selectively decreased (P < 0.001) hypertrophy of the future fast-twitch glycolytic fibers in LL. Contractile and metabolic maturation was delayed in the later maturing LL, as reflected by a decrease in muscle protein concentration (P < 0.01), an increase (P < 0.05) in the percentage of myofibers still expressing the fetal myosin heavy chain (MyHC), a lower postnatal increase in LDH activity (P < 0.001) and a delayed decrease in the percentage of IIa MyHC positive fibers (P < 0.001). Otherwise, restriction tended (P < 0.10) to increase the percentage of slow type I MyHC containing fibers in both muscles and of alpha-cardiac MyHC positive fibers in RH (P < 0.05). The LDH/CS ratio decreased dramatically (P < 0.001) after restriction, to a greater extent in LL than in RH. These changes denoted a more oxidative metabolism using fewer carbohydrates and more lipids in restricted pigs, as suggested by the increased activity of HAD (P < 0.001) and decreased respiratory quotient (P < 0.001).

Influence of early postnatal cold exposure on myofiber maturation in pig skeletal muscle

Early after birth, piglets rely almost exclusively on muscular shivering thermogenesis to produce heat in the cold and this can possibly modulate skeletal muscle development. An experiment involving 10 individually housed piglets was conducted to determine the influence of cold (24-15 degrees C, D5C group) vs. thermoneutrality (34-30 degrees C, D5TN group) between birth and 5 days on myosin heavy chain (MyHC) polymorphism and metabolic characteristics of longissimus lumborum (LL) and rhomboideus (RH) muscles. Five additional piglets were sacrificed at birth. Piglets exposed to cold received 43% more artificial milk on a liveweight basis in order to achieve similar growth rates. D5C piglets produced 93% more heat and exhibited intense shivering during the whole experiment. Contractile and metabolic characteristics of muscles were determined by immunocytochemistry, electrophoresis and enzyme activities. At least eight MyHC isoforms were detected, including atypical expressions of the alpha-cardiac and extraocular isoforms. Dramatic changes in MyHC composition, myofiber cross-sectional area (CSA) and energy metabolism occurred between birth and 5 days. Cold exposure did not affect either the total number of fibers or the CSA, but it did influence muscle maturation. In particular, it increased the expression of alpha-cardiac and type I MyHC, and decreased that of fetal MyHC, confirming an acceleration in the rate of postnatal maturation. An increase in oxidative enzyme activities was observed in both muscles in the cold, whereas the activity of a glycolytic enzyme, lactate dehydrogenase, remained unchanged. Cold exposure also induced an increase in T3 plasma levels. The extent to which these changes are the result of sustained shivering or are due to the action of hormonal factors, such as thyroid hormones, are discussed.

Myosin isoforms, muscle fiber types, and transitions

Skeletal muscle is an extremely heterogeneous tissue composed of a variety of fast and slow fiber types and subtypes. Moreover, muscle fibers are versatile entities capable of adjusting their phenotypic properties in response to altered functional demands. Major differences between muscle fiber types relate to their myosin complement, i.e., isoforms of myosin light and heavy chains. Myosin heavy chain (MHC) isoforms appear to represent the most appropriate markers for fiber type delineation. On this basis, pure fiber types are characterized by the expression of a single MHC isoform, whereas hybrid fiber type express two or more MHC isoforms. Hybrid fibers bridge the gap between the pure fiber types. The fiber population of skeletal muscles, thus, encompasses a continuum of pure and hybrid fiber types. Under certain conditions, changes can be induced in MHC isoform expression heading in the direction of either fast-to-slow or slow-to-fast. Increased neuromuscular activity, mechanical loading, and hypothyroidism are conditions that induce fast-to-slow transitions, whereas reduced neuromuscular activity, mechanical unloading, and hyperthyroidism cause transitions in the slow-to-fast direction.

Effects of thyroid hormone receptor gene disruption on myosin isoform expression in mouse skeletal muscles

Skeletal muscle is known to be a target for the active metabolite of thyroid hormone, i.e., 3,5,3'-triiodothyronine (T(3)). T(3) acts by repressing or activating genes coding for different myosin heavy chain (MHC) isoforms via T(3) receptors (TRs). The diverse function of T(3) is presumed to be mediated by TR-alpha(1) and TR-beta, but the function of specific TRs in regulating MHC isoform expression has remained undefined. In this study, TR-deficient mice were used to expand our knowledge of the mechanisms by which T(3) regulates the expression of specific MHC isoforms via distinct TRs. In fast-twitch extensor digitorum longus (EDL) muscle, TR-alpha(1)-, TR-beta-, or TR-alpha(1)beta-deficient mice showed a small but statistically significant decrease (P < 0.05) of type IIB MHC content and an increased number of type I fibers. In the slow-twitch soleus, the beta/slow MHC (type I) isoform was significantly (P < 0. 001) upregulated in the TR-deficient mice, but this effect was highly dependent on the type of receptor deleted. The lack of TR-beta had no significant effect on the expression of MHC isoforms. An increase (P < 0.05) of type I MHC was observed in the TR-alpha(1)-deficient muscle. A dramatic overexpression (P < 0.001) of the slow type I MHC and a corresponding downregulation of the fast type IIA MHC (P < 0.001) was observed in TR-alpha(1)beta-deficient mice. The muscle- and fiber-specific differences in MHC isoform expression in the TR-alpha(1)beta-deficient mice resembled the MHC isoform transitions reported in hypothyroid animals, i.e., a mild MHC transition in the EDL, a dramatic but not complete upregulation of the beta/slow MHC isoform in the soleus, and a variable response to TR deficiency in different soleus muscle fibers. Thus the consequences on muscle are similar in the absence of thyroid hormone or absence of thyroid hormone receptors, indicating that TR-alpha(1) and TR-beta together mediate the known actions of T(3). However, it remains unknown how thyroid hormone exerts muscle- and muscle fiber-specific effects in its action. Finally, although developmental MHC transitions were not studied specifically in this study, the absence of embryonic and fetal MHC isoforms in the TR-deficient mice indicates that ultimately the transition to the adult MHC isoforms is not solely mediated by TRs.

Single-fiber myosin heavy chain polymorphism during postnatal development: modulation by hypothyroidism

The primary objective of this study was to follow the developmental time course of myosin heavy chain (MHC) isoform transitions in single fibers of the rodent plantaris muscle. Hypothyroidism was used in conjunction with single-fiber analyses to better describe a possible linkage between the neonatal and fast type IIB MHC isoforms during development. In contrast to the general concept that developmental MHC isoform transitions give rise to muscle fibers that express only a single MHC isoform, the single-fiber analyses revealed a very high degree of MHC polymorphism throughout postnatal development. In the adult state, MHC polymorphism was so pervasive that the rodent plantaris muscles contained approximately 12-15 different pools of fibers (i.e., fiber types). The degree of polymorphism observed at the single-fiber level made it difficult to determine specific developmental schemes analogous to those observed previously for the rodent soleus muscle. However, hypothyroidism was useful in that it confirmed a possible link between the developmental regulation of the neonatal and fast type IIB MHC isoforms.

Effects of hindlimb unloading on neuromuscular development of neonatal rats

We hypothesized that hindlimb suspension unloading of 8-day-old neonatal rats would disrupt the normal development of muscle fiber types and the motor innervation of the antigravity (weightbearing) soleus muscles but not extensor digitorum longus (EDL) muscles. Five rats were suspended 4.5 h and returned 1.5 h to the dam for nursing on a 24 h cycle for 9 days. To control for isolation from the dam, the remaining five littermates were removed on the same schedule but not suspended. Another litter of 10 rats housed in the same room provided a vivarium control. Fibers were typed by myofibrillar ATPase histochemistry and immunostaining for embryonic, slow, fast IIA and fast IIB isomyosins. The percentage of multiple innervation and the complexity of singly-innervated motor terminal endings were assessed in silver/cholinesterase stained sections. Unique to the soleus, unloading accelerated production of fast IIA myosin, delayed expression of slow myosin and retarded increases in standardized muscle weight and fiber size. Loss of multiple innervation was not delayed. However, fewer than normal motor nerve endings achieved complexity. Suspended rats continued unloaded hindlimb movements. These findings suggest that motor neurons resolve multiple innervation through nerve impulse activity, whereas the postsynaptic element (muscle fiber) controls endplate size, which regulates motor terminal arborization. Unexpectedly, in the EDL of unloaded rats, transition from embryonic to fast myosin expression was retarded. Suspension-related foot drop, which stretches and chronically loads EDL, may have prevented fast fiber differentiation. These results demonstrate that neuromuscular development of both weightbearing and non-weightbearing muscles in rats is dependent upon and modulated by hindlimb loading.

Thyroid hormones differentially modulate enolase isozymes during rat skeletal and cardiac muscle development

During muscle development, an isozymic transition of the glycolytic enzyme enolase occurs from the embryonic and ubiquitous alphaalpha-isoform to the muscle-specific betabeta-isoform. Here, we demonstrate a stimulatory role of thyroid hormones on these two enolase genes during rat development in hindlimb muscles and an inhibitory effect on the muscle-specific enolase gene in cardiac muscle. In hindlimb muscles the ubiquitous alpha-transcript level is diminished by hypothyroidism, starting at birth. On the contrary, the more abundant muscle-specific beta-transcript is insensitive to hypothyroidism before establishment of the functional diversification of fibers and is greatly decreased thereafter. Our data support the hypothesis of a role of thyroid hormones in coordinating the expressions of contractile proteins and metabolic enzymes during muscle development. The subcellular localization of isoenolases, established here, is not modified by hypothyroidism. Our results underline the specificity of action of thyroid hormones, which modulate differentially two isozymes in the same muscle and regulate, in opposite directions, the expression of the same gene in two different muscles.

The influence of thyroid hormone on myosin isoform composition and shortening velocity of single skeletal muscle fibres with special reference to ageing and gender

This review summarizes the effects of altered thyroid hormone levels on the expression of myosin isoforms and contractility in single muscle fibres from fast- and slow-twitch muscles from young and old male and female rats. The differences between male and female hyperthyroid soleus muscles are suggested to be related to an interaction of thyroid hormones and sex hormones in the regulation of myosin gene expression. Additionally, the mismatch between the protein and mRNA levels of MyHCs between male and female hyperthyroid extensor digitorum longus (EDL) muscles raises the possibility of a gender-related difference in post-transcriptional, translational or post-translational regulation of MyHC isoforms by T3.

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.

Thyroid hormone regulation of MHC isoform composition and myofibrillar ATPase activity in rat skeletal muscles

Myosin heavy chain (MHC) isoform composition and Ca2+ Mg2+ dependent ATPase activity were determined in myofibrils prepared from skeletal muscles (diaphragm, soleus, plantaris and tibialis anterior) of euthyroid (C), hypothyroid (Tx) and hyperthyroid (T3) rats. Direct comparison between T3 and Tx gave an indication of the maximal effect of thyroid hormones. Significant differences in MHC-1 and MHC-2B proportions and in ATPase activity were found in all muscles. The difference in MHC-2A/X proportion was significant only in soleus, diaphragm and plantaris. When T3 and C were compared, significant variations in MHC isoform composition were found only in plantaris and diaphragm. The comparison between Tx and C showed significant differences in MHC isoform distribution and in ATPase activity in most muscles. The differences in ATPase activity among muscles and among thyroid states were consistent with those in MHC isoform distribution. From the correlations between ATPase activity and MHC isoform distribution the enzymatic activities of individual MHC isoforms were calculated. The results indicate that MHC isoform distribution is controlled by thyroid state in all skeletal muscles and that changes in MHC isoforms distribution are accompanied by proportional changes in ATPase activity.

Effect of controllable stress on myosin heavy chain expression and muscle-specific protection by clomipramine

This study evaluated the influence of a controllable and painless stress, conditioned bright-light active-avoidance, on the expression of myosin heavy chain (MHC) protein isoforms in two nape and three masticatory rat muscles: longissimus capitis (L), rectus capitis dorsalis major (R), anterior digastric (AD), anterior temporalis (AT) and masseter superficialis (MS). The effects of a concomitant antidepressant treatment with clomipramine (CMI) on the muscle structure were also investigated. The three adult fast MHC isoforms were detected in all muscles studied: MHC 2A, 2X and 2B. The AT structure was not significantly modified by stress either under saline or under CMI treatment. In the other muscles studied, the stress situation induced a marked increase in the relative expression of MHC 2B and a decrease in MHCs 2X and 2A, except in L in which the MHC 2A decrease did not reach a statistically significant level. Under controllable stress, the CMI treatment led to the same MHC profile in AT, L, R and AD as saline, except in L where the MHC 2X decrease was no longer statistically significant. However, in MS, under controllable stress and CMI treatment, the MHC distribution was significantly different from the stressed saline-treated group and became comparable to the control again. MHC 2B has a higher shortening velocity than MHC 2X, which has a higher one than MHC 2A. According to total MHC isoform expression, the controllable stress-induced transformations would thus lead to increased velocity of all five muscles studied except in AT. This latter seems, therefore, not very sensitive to environmental requirements. Our results indicate that controllable stress produces important changes in the contractile properties of nape and masticatory muscles. Furthermore, this study demonstrates the protective effect of CMI against muscle structure transformations induced by controllable stress in MS, and that these effects are muscle type-specific.

The final common pathway in postural control--developmental perspective

A brief review is given concerning postural specialisations among mammalian muscle fibres and motor units. Most skeletal muscles contain a mixture of fibres with different characteristics, and their slow-twitch (S) units are well-known to possess properties suitable for postural tasks: they are highly fatigue-resistant, well equipped for oxidative metabolism, and their slowness makes them energetically cheap in (semi-)isometric contractions. These features are adequately employed in motor behaviour owing to characteristics of the associated motoneurones. In adult mammals, the way in which a muscle is used can influence its proportion of S units. This adjustment occurs within a restricted 'adaptive range' which differs between muscles and animal species, presumably being preset at an early age. In the course of early foetal development, part of the slow vs. fast differentiation of muscle fibre properties can take place independently of innervation. Once innervation has taken place, however, motoneurones influence the differentiation in various ways. On the whole, a well coordinated timing seems to exist between the early differentiation of central motor mechanisms and of the peripheral machinery, largely causing the neuromuscular system to be/become ready for use when the brain needs it.

Myosin heavy chain transitions during development. Functional implications for the respiratory musculature

The myosin heavy chain (MHC) exists as multiple isoforms that are encoded for by a family of genes. The respiratory musculature demonstrates muscle-specific and temporally-dependent changes in MHC isoform expression during maturation. Developmental expression of MHC isoforms correlate well with postnatal changes in actomyosin ATPase activity, specific force generation (P0/CSA), maximum unloaded velocity of shortening (V0) and and fatigue resistance. More specifically, as the expression of MHCneonatal declines and MHC2A, MHC2X, and MHC2B increase, actomyosin ATPase activity, P0/CSA, V0, and muscle fatigability increase. The increase in actomyosin ATPase activity with maturation is partially offset by a postnatal increase in oxidative capacity; however, as fatigue resistance declines with development it is apparent that the energy costs of contraction are not fully matched by an increase in energy production. Developmental transitions in smooth muscle MHC phenotype also occur although their functional importance remains unclear.

Levels of MyoD protein expression following injury of mdx and normal limb muscle are modified by thyroid hormone

Thyroid hormone (T3) affects muscle development and muscle regeneration. It also interacts with the muscle regulatory gene MyoD in culture and affects myoblast proliferation. We studied the localization of MyoD protein using a well-characterized polyclonal antibody for immunohistochemistry. Relative numbers of myogenic precursor cells per field were identified by their MyoD expression during muscle regeneration in normal and mdx dystrophic mice, with particular reference to the expression in mononuclear cells and myotubes at various T3 levels. In regeneration by normal muscles, relatively few MyoD+ nuclei per field were present in mononuclear cells of euthyroid and hypothyroid mice. MyoD staining of mononuclear cell nuclei was approximately doubled in fields of regenerating muscles of normal hyperthyroid compared to euthyroid mice, and was observed in precursors that appeared to be aligned before fusion into myotubes. In mdx regenerating muscle, twofold more mononuclear cells positive for MyoD were present in all three treatment groups compared to normal muscles regenerating under the same conditions. Localization was similar to the pattern in normal euthyroid mice. However, in muscles regenerating in hyperthyroid mdx mice, both mononuclear cell nuclei and centrally located nuclei in a subpopulation (about 15%) of new myotubes formed after the crush injury were intensely stained for MyoD protein. The changes observed are consistent with reports on T3-induced alteration of muscle repair, and propose a link between MyoD regulation and the accelerated differentiation during regeneration under high T3 conditions. (J Histochem Cytochem 46:59-67, 1998)

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.

Characterization of the promoter of the rat sarcoplasmic endoplasmic reticulum Ca2+-ATPase 1 gene and analysis of thyroid hormone responsiveness

Relaxation of skeletal muscle requires the re-uptake of Ca2+, which is mediated by the sarcoplasmic reticulum Ca2+-ATPase (SERCA). Thyroid hormone (T3) stimulates the expression of the SERCA1 isoform, which is essential for fast skeletal muscle fiber phenotype. We have cloned and studied the first 962 base pairs of the 5'-flanking region of the rat SERCA1 gene. This sequence was tested for T3-regulated expression in transient transfection experiments using COS7 cells and for binding of thyroid hormone receptor (TR) alpha in mobility shift assays. A construct of the 5'-flanking region and a reporter gene was unresponsive to T3 in the absence of co-transfected thyroid hormone receptor. In the presence of TRalpha, a T3 induction ratio of almost 4.0 was found, and this induction ratio was doubled with co-transfection of an RXR expression plasmid. Analysis of progressive 5'-deletion fragments of the sequence indicated multiple regions involved in T3 responsiveness. Three regions, R1, R2, and R3, were identified that bound TR complexes in mobility shift assays and conferred T3 responsiveness to a heterologous promoter. The most potent of these thyroid hormone response elements, R3, increased the 2-fold background T3 stimulation of the thymidine kinase promoter to nearly 6-fold. Detailed analysis of this element showed that four TR-binding half-sites, comprising two independent thyroid hormone response elements, interact cooperatively to give the maximal T3 response. T3 regulation of SERCA1 expression is mediated by a complex thyroid hormone response element that may serve to provide a greater range of response in interaction with nuclear receptor partners or cell-specific transcription factors.

Myosin isoform transitions in four rabbit muscles during postnatal growth

Four rabbit muscles (i.e. semimembranosus proprius, psoas major, biceps femoris and longissimus lumborum), differing in their fibre type composition in the adult, were investigated during postnatal development. Muscle samples were taken at 1, 7, 14, 21, 28, 35, 49 and 77 days of age. Complementary techniques were used to characterize myosin heavy chain (MHC) isoform transitions, i.e. SDS-PAGE, immunocytochemistry and conventional histochemistry. Good accordance was found between electrophoretic and immunocytochemical techniques. Our results show that rabbit muscles were phenotypically immature at birth. At 1 day of age, perinatal isoform represented 70-90% of the total isoform content of the muscles. Two generations of myofibres could be observed on the basis of their morphology and reaction to specific antibodies. In all muscles, primary fibres expressed slow MHC. In contrast, secondary generation of fibres never expressed slow MHC in future fast muscles, while half of them expressed slow MHC in the future slow-twitch muscle, the semimembranosus proprius. During the postnatal period, all muscles displayed a transition from embryonic to perinatal MHC isoforms, followed by a transition from perinatal to adult MHC isoforms. These transitions occured mainly during the first postnatal month. The embryonic isoform was no longer expressed after 14 days, except in longissimus where it disappeared after 28 days. On the contrary, large differences were found in the timing of disappearance of the perinatal isoform between the four muscles. The perinatal isoform disappeared between 28 and 35 days in semimembranosus proprius and 35 and 49 days in psoas and biceps femoris. Interestingly, the perinatal isoform was still present in 6% of the fibres in longissimus at 77 days, the commercial slaughter age, denoting a great delay in the maturation. Fate of each generation of fibres differed between muscles.

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.

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)

Effect of hypothyroidism on the expression of cytochrome c and cytochrome c oxidase in heart and muscle during development

The effect of thyroid hormone on the expression of mitochondrial proteins was evaluated during development by measuring cytochrome c oxidase (CYTOX) activity and cytochrome c protein and mRNA levels in heart and skeletal muscle of control and hypothyroid rats. Animals were killed at the late fetal, early, and late postnatal stages up to 56 days of age. In heart, CYTOX activity increased 2.3-fold above the fetal level throughout development, most of which occurred prior to 2 days of age. No increase was observed in muscle. CYTOX activity was reduced in hypothyroid animals throughout development in heart compared to controls (by 50% at 56 days), but in muscle no effect of hypothyroidism was observed. In muscle and heart 4- and 1.5-fold increases in cytochrome c above the fetal level were evident by 1 day of age, with further increases to 8.5- and 2.7-fold by 56 days, respectively. The increase in cytochrome c differed from the increase in CYTOX, indicating changes in mitochondrial composition. Hypothyroidism reduced cytochrome c in muscle by 30-35% at 56 days, but had no effect in heart, indicating a muscle type-specific effect of thyroid hormone on cytochrome c protein expression. Cytochrome c mRNA increased rapidly to 4-5 fold above the fetal level in both heart and muscle by 6 h post-partum. Between 7 and 56 days of age, further increases to 6- and 25-fold were observed in muscle and heart, respectively. In muscle, the 6-fold developmental increase in mRNA paralleled that of the protein, suggesting transcriptional regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Effects of chronic stimulation with different impulse patterns on the expression of myosin isoforms in rat myotube cultures

In order to study maturation and differentiation of aneural myotubes in vitro, long-term myotube cultures were established from hindlimb musculature of newborn rats. The developmental state of the myotubes was judged by their myosin heavy chain (HC) patterns. Newly formed myotubes only expressed the embryonic isoform, HCemb, older myotubes expressed the neonatal isoform HCneo, as well as the fast adult isoforms HCIIb and HCIId. HCIId increased continuously, reaching a relative concentration of 47% in 37-day-old cultures. The third fast isoform, HCIIa, was not detected and also the slow isoform HCI was absent. Effects of chronic (20 days) electrostimulation were studied by exposing the cultures to various stimulus patterns. Bursts of 250 ms duration at various pulse frequencies were applied at low and high burst frequencies. Although HCemb remained the predominant isoform under all conditions, different stimulus patterns induced specific changes in the patterns of fast and slow HC isoforms. Bursts of 250 ms duration at 15 Hz, 40 Hz, or 100 Hz, repeated every second or every 4 s, induced the expression of slow myosin, i.e., HCl. Bursts of 250 ms duration at 100 Hz, repeated every 100 s, enhanced the expression of HCIId, but not of HCI. Because slow myosin was induced at high burst frequency with low and high pulse rates, we suggest that burst frequency rather than pulse frequency has a specifying effect on myosin expression. Our results show that the basal program of myosin expression during myogenesis in vitro can be modulated by electrostimulation, suggesting a possible influence of neuromuscular activity on the development of adult fiber types.

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.

Single fiber analyses of type IIA myosin heavy chain distribution in hyper- and hypothyroid soleus

The objectives of this study were to 1) examine the effect of hypo- and hyperthyroidism (triiodothyronine treatment) on the distribution of type IIA myosin heavy chain (MHC) in the soleus at the single fiber level and 2) correlate changes in the single fiber distribution of type IIA MHC with the maximal shortening velocity of whole skeletal muscle. The presence of the type IIA MHC in single fibers was determined using a monoclonal antibody reactive to the type IIA MHC and quantified with a Meridian ACAS 570 interactive laser cytometer. The findings of this study demonstrate that 1) hyperthyroidism significantly increases the relative number of muscle fibers that express type IIA MHC, 2) not all type I fibers are capable of expressing fast type IIA MHC under hyperthyroid conditions, and 3) there is a high correlation between maximal shortening velocity and the relative number of type IIA fibers. This latter observation suggests that the maximal shortening velocity of whole skeletal muscle may not be solely determined by its fastest fiber(s) but rather by the relative proportion of fibers expressing fast type IIA MHC.

Opposite regulations by androgenic and thyroid hormones of V1 myosin expression in the two types of rabbit striated muscle: skeletal and cardiac

The finding that V1 cardiac myosin is expressed in masticatory skeletal muscles of the rabbit provided a unique opportunity for comparing the hormonal regulation of V1 in skeletal and cardiac muscles. Thyroid hormones had no significant effect on the postnatal expression of V1 in masticatory muscles, but increased this expression in cardiac ventricles. In contrast, androgenic hormones reduced V1 expression in masticatory muscles, but did not affect it significantly in cardiac ventricles. Modulation of V1 gene transcription in striated muscle is thus shown here to depend both on the target muscle and on the hormone.