Expression Of Slow Myosin Heavy Chain Research Articles

Garcinol Promotes the Formation of Slow-Twitch Muscle Fibers by Inhibiting p300-Dependent Acetylation of PGC-1α.

The conversion of skeletal muscle fiber from fast-twitch to slow-twitch is crucial for sustained contractile and stretchable events, energy homeostasis, and anti-fatigue ability. The purpose of our study was to explore the mechanism and effects of garcinol on the regulation of skeletal muscle fiber type transformation. Forty 21-day-old male C57/BL6J mice (n = 10/diet) were fed a control diet or a control diet plus garcinol at 100 mg/kg (Low Gar), 300 mg/kg (Mid Gar), or 500 mg/kg (High Gar) for 12 weeks. The tibialis anterior (TA) and soleus muscles were collected for protein and immunoprecipitation analyses. Dietary garcinol significantly downregulated (p < 0.05) fast myosin heavy chain (MyHC) expression and upregulated (p < 0.05) slow MyHC expression in the TA and soleus muscles. Garcinol significantly increased (p < 0.05) the activity of peroxisome proliferator-activated receptor gamma co-activator 1α (PGC-1α) and markedly decreased (p < 0.05) the acetylation of PGC-1α. In vitro and in vivo experiments showed that garcinol decreased (p < 0.05) lactate dehydrogenase activity and increased (p < 0.05) the activities of malate dehydrogenase and succinic dehydrogenase. In addition, the results of C2C12 myotubes showed that garcinol treatment increased (p < 0.05) the transformation of glycolytic muscle fiber to oxidative muscle fiber by 45.9%. Garcinol treatment and p300 interference reduced (p < 0.05) the expression of fast MyHC but increased (p < 0.05) the expression of slow MyHC in vitro. Moreover, the acetylation of PGC-1α was significantly decreased (p < 0.05). Garcinol promotes the transformation of skeletal muscle fibers from the fast-glycolytic type to the slow-oxidative type through the p300/PGC-1α signaling pathway in C2C12 myotubes.

HDAC4 Is Indispensable for Reduced Slow Myosin Expression at the Early Stage of Hindlimb Unloading in Rat Soleus Muscle.

It is well known that reduced contractile activity of the main postural soleus muscle during long-term bedrest, immobilization, hindlimb unloading, and space flight leads to increased expression of fast isoforms and decreased expression of the slow isoform of myosin heavy chain (MyHC). The signaling cascade such as HDAC4/MEF2-D pathway is well-known to take part in regulating MyHC I gene expression. Earlier, we found a significant increase of HDAC4 in myonuclei due to AMPK dephosphorylation during 24 h of hindlimb unloading via hindlimb suspension (HU) and it had a significant impact on the expression of MyHC isoforms in rat soleus causing a decrease in MyHC I(β) pre-mRNA and mRNA expression as well as MyHC IIa mRNA expression. We hypothesized that dephosphorylated HDAC4 translocates into the nuclei and can lead to a reduced expression of slow MyHC. To test this hypothesis, Wistar rats were treated with HDAC4 inhibitor (Tasquinimod) for 7 days before HU as well as during 24 h of HU. We discovered that Tasquinimod treatment prevented a decrease in pre-mRNA expression of MyHC I. Furthermore, 24 h of hindlimb suspension resulted in HDAC4 nuclear accumulation of rat soleus but Tasquinimod pretreatment prevented this accumulation. The results of the study indicate that HDAC4 after 24 h of HU had a significant impact on the precursor MyHC I mRNA expression in rat soleus.

Arginine induces skeletal muscle fiber type conversion by upregulating Akirin2 and AMPK/PGC-1α in mice

We have previously reported that arginine promotes slow myosin heavy chain expression in cell culture via Akirin2 and the AMP-activated protein kinase (AMPK) signaling pathway. Our goal in the present study was to investigate whether arginine has similar functions and molecular mechanisms in vivo. Kunming mice were fed a diet supplemented with arginine and actively immunized with Akirin2. The study lasted for 40 d. Tibialis anterior (TA) muscle was harvested for analysis. We found that Kunming mice supplemented with arginine significantly increased the oxidative fibers expression and decreased the expression of glycolytic fiber. Expressions of peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) and AMPK were increased in the TA muscle of arginine-treated Kunming mice. We also find that dietary supplementation of arginine increased the succinate dehydrogenase and malate dehydrogenase activities and decreased the lactate dehydrogenase activity. But active immunization against Akirin2 got opposite results. These results indicated that arginine induced muscle fiber transition from type II to type I, which may be mediated by Akirin2 and AMPK//PGC-1α signaling pathway.

Rapid decline in MyHC I(β) mRNA expression in rat soleus during hindlimb unloading is associated with AMPK dephosphorylation.

Inactivation of a skeletal muscle results in slow to fast myosin heavy chain (MyHC) shift. AMP-activated protein kinase (AMPK) can be implicated in the regulation of genes encoding the slow MyHC isoform. Here we report that AMPK dephosphorylation after 24h of mechanical unloading can contribute to histone deacetylase (HDAC) nuclear translocation; activation of AMPK prevents HDAC4 nuclear accumulation after 24h of unloading and AMPK dephosphorylation inhibits slow MyHC expression following 24h of unloading. Our data indicate that AMPK dephosphorylation during the first 24h of mechanical unloading has a significant impact on the expression of MyHC isoforms in rat soleus causing a decrease in MyHC I(β) pre-mRNA and mRNA expression as well as MyHC IIa mRNA expression. One of the key events that occurs during skeletal muscle inactivation is a change in myosin phenotype, i.e. increased expression of fast isoforms and decreased expression of the slow isoform of myosin heavy chain (MyHC). It is known that calcineurin/nuclear factor of activated T-cells and AMP-activated protein kinase (AMPK) can regulate the expression of genes encoding MyHC slow isoform. Earlier, we found a significant decrease in phosphorylated AMPK in rat soleus after 24h of hindlimb unloading (HU). We hypothesized that a decrease in AMPK phosphorylation and subsequent histone deacetylase (HDAC) nuclear translocation can be one of the triggering events leading to a reduced expression of slow MyHC. To test this hypothesis, Wistar rats were treated with AMPK activator (AICAR) for 6days before HU as well as during 24h of HU. We discovered that AICAR treatment prevented a decrease in pre-mRNA and mRNA expression of MyHC I as well as MyHC IIa mRNA expression. Twenty-four hours of hindlimb suspension resulted in HDAC4 accumulation in the nuclei of rat soleus but AICAR pretreatment prevented this accumulation. The results of the study indicate that AMPK dephosphorylation after 24h of HU had a significant impact on the MyHC I and MyHC IIa mRNA expression in rat soleus. AMPK dephosphorylation also contributed to HDAC4 translocation to the nuclei of soleus muscle fibres, suggesting an important role of HDAC4 as an epigenetic regulator in the process of myosin phenotype transformation.

Akirin2 promotes slow myosin heavy chain expression by CaN/NFATc1 signaling in porcine skeletal muscle satellite cells.

The objective of this study was to evaluate the effect of Akirin2 on slow myosin heavy chain (slow MyHC, MyHC I) gene expression and its molecular mechanisms. In this study, we showed that the protein expression of Akirin2 in pig slow oxidative Psoas major muscle is higher than that in fast glycolytic tibialis anterior muscle, suggesting that Akirin2 may play a role in myofiber typing. Knockdown of Akirin2 decreased the MyHC I expression and the calcineurin (CaN) activity, and also decreased the expressions of NFATc1 and MCIP1.4. Conversely, overexpression of Akirin2 got the opposite results. Furthermore, inhibition of CaN or knockdown of NFATc1 attenuated Akirin2 overexpression-induced upregulation of MyHC I. Together, these results demonstrate that Akirin2 promotes MyHC I expression via CaN/NFATc1 signaling pathway in porcine skeletal muscle satellite cells.

Heat shock transcription factor 1-associated expression of slow myosin heavy chain in mouse soleus muscle in response to unloading with or without reloading.

The effects of heat shock transcription factor 1 (HSF1) deficiency on the fibre type composition and the expression level of nuclear factor of activated T cells (NFAT) family members (NFATc1, NFATc2, NFATc3 and NFATc4), phosphorylated glycogen synthase kinase 3α (p-GSK3α) and p-GSK3β, microRNA-208b (miR-208b), miR-499 and slow myosin heavy chain (MyHC) mRNAs (Myh7 and Myh7b) of antigravitational soleus muscle in response to unloading with or without reloading were investigated. HSF1-null and wild-type mice were subjected to continuous 2-week hindlimb suspension followed by 2- or 4-week ambulation recovery. In wild-type mice, the relative population of slow type I fibres, the expression level of NFATc2, p-GSK3 (α and β), miR-208b, miR-499 and slow MyHC mRNAs (Myh7 and Myh7b) were all decreased with hindlimb suspension, but recovered after it. Significant interactions between train and time (the relative population of slow type I fibres; P=0.01, the expression level of NFATc2; P=0.001, p-GSKβ; P=0.009, miR-208b; P=0.002, miR-499; P=0.04) suggested that these responses were suppressed in HSF1-null mice. HSF1 may be a molecule in the regulation of the expression of slow MyHC as well as miR-208b, miR-499, NFATc2 and p-GSK3 (α and β) in mouse soleus muscle.

KLF15 regulates slow myosin heavy chain expression through NFATc1 in C2C12 myotubes

A comprehensive understanding of genetic and environmental factors that control skeletal muscle fiber type specification and transformation is essential not only in sports science, but also in myopathy and metabolic disorders. Krüppel-like factors (KLFs) are a subfamily of the zinc-finger class of transcription factors, which are involved in the development, homeostasis, and pathology of cardiovascular systems. Compared to cardiac and smooth muscles, the role of KLFs in skeletal muscle is much less understood. In this study, the endogenous expression of KLF15 was analyzed in differentiating C2C12 muscle cells and mouse skeletal muscle. Our data indicated that Klf15 was upregulated during myogenic differentiation and higher levels of Klf15 mRNA were detected in mouse slow, oxidative soleus muscle (SL) compared to that in fast, glycolytic tibialis anterior muscle (TA), indicating that KLF15 may play a role in myogenesis or myofiber typing. Additional studies revealed that KLF15 regulated the expression of MHC-β/slow rather than muscle cell differentiation. Gene silencing, overexpression, and luciferase reporter assay showed that KLF15 regulated MHC-β/slow by binding to Nfatc1 promoter, inducing its activity, therefore mediating calcineurin/NFAT signaling. Our study contributed to the current knowledge on KLFs in skeletal muscle, and it indicated a need for further intensive studies on the redundant and divergent functions of KLFs.

P.8.8 Basement membrane deposition during muscle development in the FKRP Deficient Mouse

The defective glycosylation of <i>α</i>-dystroglycan is associated with a group of muscular dystrophies collectively referred to as the dystroglycanopathies. We previously generated a FKRP deficient mouse (FKRPKD) which recapitulates the severe end of the clinical spectrum and displays a muscle/eye/brain phenotype and dies soon after birth. In view of the well documented role of <i>α</i>-dystroglycan in basement membrane deposition we sought to determine if secondary myogenesis was altered in the FKRPKD mice. Here we show that the laminin binding epitope of <i>α</i>-dystroglycan is present from the earliest stages of secondary myotube formation at E15.5 in wild type mice. FKRPKD mice show a mild reduction in laminin <i>α</i>-2 at this time point. However, this reduction is not apparent using an antibody to pan laminin (identifies several laminin chains). However, at P0, a marked reduction of laminin <i>α</i>2 was noted in FKRPKD compared to wild type, together with a moderate reduction in the pan laminin, suggesting that the initial reduction in <i>α</i>2 subsequently alters the deposition of other laminin chains. In order to further examine this we have now examined the deposition of other laminin chains (<i>α</i>1, <i>α</i>4, <i>α</i>1, and ϒ1). There was no alteration in laminin <i>α</i>1, <i>α</i>4 and ϒ1 expression between wild type and mutant at either E15.5 or P0. Laminin <i>α</i>1 however, showed a subtle reduction in FKRPKD compared to wild type at E15.5 which became more apparent at P0. To determine the effect of these alterations on fibre formation we counted muscle fibres at E15.5 and P0 and showed that neither the number of primary myotubes (as indicated by slow myosin heavy chain expression) nor the total number of fibres at P0 was significantly different between FKRPKD and wild type mice. These observations suggest that alterations in basement membrane deposition during myogenesis do influence myoblast alignment and fusion.

Regulation of skeletal muscle oxidative phenotype by classical NF-κB signalling

BackgroundImpairments in skeletal muscle oxidative phenotype (OXPHEN) have been linked to the development of insulin resistance, metabolic inflexibility and progression of the metabolic syndrome and have been associated with progressive disability in diseases associated with chronic systemic inflammation. We previously showed that the inflammatory cytokine tumour necrosis factor-α (TNF-α) directly impairs muscle OXPHEN but underlying molecular mechanisms remained unknown. Interestingly, the inflammatory signalling pathway classical nuclear factor-κB (NF-κB) is activated in muscle in abovementioned disorders. Therefore, we hypothesised that muscle activation of classical NF-κB signalling is sufficient and required for inflammation-induced impairment of muscle OXPHEN. MethodsMyotubes from mouse and human muscle cell lines were subjected to activation or blockade of the classical NF-κB pathway. In addition, wild-type and MISR (muscle-specific inhibition of classical NF-κB) mice were injected intra-muscularly with TNF-α. Markers and key regulators of muscle OXPHEN were investigated. ResultsClassical NF-κB activation diminished expression of oxidative phosphorylation (OXPHOS) sub-units, slow myosin heavy chain expression, activity of mitochondrial enzymes and potently reduced intra-cellular ATP levels. Accordingly, PGC-1/PPAR/NRF-1/Tfam signalling, the main pathway controlling muscle OXPHEN, was impaired upon classical NF-κB activation which required intact p65 trans-activation domains and depended on de novo gene transcription. Unlike wild-type myotubes, IκBα-SR myotubes (blocked classical NF-κB signalling) were refractory to TNF-α-induced impairments in OXPHEN and its regulation by the PGC-1/PPAR/NRF-1/Tfam cascade. In line with in vitro data, NF-κB blockade in vivo abrogated TNF-α-induced reductions in PGC-1α expression. ConclusionClassical NF-κB activation impairs skeletal muscle OXPHEN.

Canonical Wnt signaling induces BMP-4 to specify slow myofibrogenesis of fetal myoblasts

BackgroundThe Wnts are secreted proteins that play important roles in skeletal myogenesis, muscle fiber type diversification, neuromuscular junction formation and muscle stem cell function. How Wnt proteins orchestrate such diverse activities remains poorly understood. Canonical Wnt signaling stabilizes β-catenin, which subsequently translocate to the nucleus to activate the transcription of TCF/LEF family genes.MethodsWe employed TCF-reporter mice and performed analysis of embryos and of muscle groups. We further isolated fetal myoblasts and performed cell and molecular analyses.ResultsWe found that canonical Wnt signaling is strongly activated during fetal myogenesis and weakly activated in adult muscles limited to the slow myofibers. Muscle-specific transgenic expression of a stabilized β-catenin protein led to increased oxidative myofibers and reduced muscle mass, suggesting that canonical Wnt signaling promotes slow fiber types and inhibits myogenesis. By TCF-luciferase reporter assay, we identified Wnt-1 and Wnt-3a as potent activators of canonical Wnt signaling in myogenic progenitors. Consistent with in vivo data, constitutive overexpression of Wnt-1 or Wnt-3a inhibited the proliferation of both C2C12 and primary myoblasts. Surprisingly, Wnt-1 and Wnt-3a overexpression up-regulated BMP-4, and inhibition of BMP-4 by shRNA or recombinant Noggin protein rescued the myogenic inhibitory effect of Wnt-1 and Wnt-3a. Importantly, Wnt-3a or BMP-4 recombinant proteins promoted slow myosin heavy chain expression during myogenic differentiation of fetal myoblasts.ConclusionsThese results demonstrate a novel interaction between canonical Wnt and BMP signaling that induces myogenic differentiation towards slow muscle phenotype.

P43-dependent mitochondrial activity regulates myoblast differentiation and slow myosin isoform expression by control of Calcineurin expression

We have previously shown that mitochondrial protein synthesis regulates myoblast differentiation, partly through the control of c-Myc expression, a cellular oncogene regulating myogenin expression and myoblast withdrawal from the cell cycle. In this study we provide evidence of the involvement of Calcineurin in this regulation. In C2C12 myoblasts, inhibition of mitochondrial protein synthesis by chloramphenicol decreases Calcineurin expression. Conversely, stimulation of this process by overexpressing the T3 mitochondrial receptor (p43) increases Calcineurin expression. Moreover, expression of a constitutively active Calcineurin (ΔCN) stimulates myoblast differentiation, whereas a Calcineurin antisense has the opposite effect. Lastly, ΔCN expression or stimulation of mitochondrial protein synthesis specifically increases slow myosin heavy chain expression. In conclusion, these data clearly suggest that, partly via Calcineurin expression, mitochondrial protein synthesis is involved in muscle development through the control of myoblast differentiation and probably the acquisition of the contractile and metabolic phenotype of muscle fibres.

Changing sexes in a complex environment ‐ shifts in skeletal myosin isoforms in coastal populations of California sheephead fish

Myosin heavy chain (MyHC) isoform expression was investigated in California sheephead, Semicossyphus pulcher, a protogynous hermaphroditic fish found in coastal Southern California and Northern Mexico. This species was collected from nine capture sites near the Channel Islands where both overall size, and size of individuals at the transitional sexual stage, varies considerably. We therefore cloned MyHC isoforms and analyzed proteins by SDS‐PAGE to determine if gender, size, water temperature, or geographic location influenced contractile protein expression. The relative percentage of the fast MyHC isoform decreased as sheephead fish transitioned from female to male. The highest expression of slow MyHC isoform was observed in male sheephead. The lowest expression of slow MyHC was seen in Northern and off‐shore female populations, when compared to Southern and near‐shore female populations. Relative percentage of slow MyHC isoform positively correlated with standard length. Thus, in this interesting system where muscle mass undergoes dramatic growth accompanying gender transitions, MyHC isoforms are likely influenced by both sexual development and environment.

Are calcineurin genes associated with endurance phenotype traits?

Polymorphisms in calcineurin genes are candidates to explain individual variations in endurance phenotype traits owing to the pivotal role that the calcineurin signaling pathway plays in the regulation of important cardiac and skeletal muscle phenotypes such as slow myosin heavy chain expression, skeletal muscle oxidative capacity or cardiac hypertrophy. We studied the possible association of 55 polymorphisms in the calcineurin gene isoforms PPP3CA, PPP3CB, PPP3CC, PPP3R1 and PPP3R2 with both baseline levels and responsiveness to a 18-week endurance training program of maximal oxygen uptake (VO(2)max) and running economy (RE) in healthy young Chinese men [n = 102; mean (SD) age 19 +/- 1 years] of Han origin. We used two-way (genotype, training) ANOVA for repeated measures to compare baseline and trainability of VO(2)max and RE among genotypes of the aforementioned polymorphisms. We found a significant association between (a) baseline VO(2)max and the rs2850965 and rs3804423 polymorphisms in the PPP3CA gene; (b) training responsiveness of VO(2)max and both the rs3804358 polymorphism (PPP3CA) and the rs4671887 polymorphism (PPP3R1); and (c) training responsiveness of RE and rs3739723 (PPP3R2). Though more research is needed, our findings suggest that polymorphisms in the calcineurin genes might be among the numerous potential genetic variant candidates that, together with other factors (e.g., environment, complex genetics interactions), can help explaining human variations in endurance exercise phenotype traits.

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.

Slow myosin heavy chain expression in the absence of muscle activity

Innervation has been generally accepted to be a major factor involved in both triggering and maintaining the expression of slow myosin heavy chain (MHC-1) in skeletal muscle. However, previous findings from our laboratory have suggested that, in the mouse, this is not always the case (30). Based on these results, we hypothesized that neurotomy would not markedly reduced the expression of MHC-1 protein in the mouse soleus muscles. In addition, other cellular, biochemical, and functional parameters were also studied in these denervated soleus muscles to complete our study. Our results show that denervation reduced neither the relative amount of MHC-1 protein, nor the percentage of muscle fibers expressing MHC-1 protein (P > 0.05). The fact that MHC-1 protein did not respond to muscle inactivity was confirmed in three different mouse strains (129/SV, C57BL/6, and CD1). In contrast, all of the other histological, biochemical, and functional muscle parameters were markedly altered by denervation. Cross-sectional area (CSA) of muscle fibers, maximal tetanic isometric force, maximal velocity of shortening, maximal power, and citrate synthase activity were all reduced in denervated muscles compared with innervated muscles (P < 0.05). Contraction and one-half relaxation times of the twitch were also increased by denervation (P < 0.05). Addition of tenotomy to denervation had no further effect on the relative expression of MHC-1 protein (P > 0.05), despite a greater reduction in CSA and citrate synthase activity (P < 0.05). In conclusion, a deficit in neural input leads to marked atrophy and reduction in performance in mouse soleus muscles. However, the maintenance of the relative expression of slow MHC protein is independent of neuromuscular activity in mice.

Early mechanical dysfunction of the diaphragm in the muscular dystrophy with myositis (Ttnmdm) model

A complex rearrangement mutation in the mouse titin gene leads to an in-frame 83-amino acid deletion in the N2A region of titin. Autosomal recessive inheritance of the titin muscular dystrophy with myositis (Ttn(mdm/mdm)) mutation leads to a severe early-onset muscular dystrophy and premature death. We hypothesized that the N2A deletion would negatively impact the force-generating capacity and passive mechanical properties of the mdm diaphragm. We measured in vitro active isometric contractile and passive length-tension properties to assess muscle function at 2 and 6 wk of age. Micro-CT, myosin heavy chain Western blotting, and histology were used to assess diaphragm structure. Marked chest wall distortions began at 2 wk and progressively worsened until 5 wk. The percentage of myofibers with centrally located nuclei in mdm mice was significantly (P < 0.01) increased at 2 and 6 wk by 4% and 17%, respectively, compared with controls. At 6 wk, mdm diaphragm twitch stress was significantly (P < 0.01) reduced by 71%, time to peak twitch was significantly (P < 0.05) reduced by 52%, and half-relaxation time was significantly (P < 0.05) reduced by 57%. Isometric tetanic stress was significantly (P < 0.05) depressed in 2- and 6-wk mdm diaphragms by as much as 64%. Length-tension relationships of the 2- and 6-wk mdm diaphragms showed significantly (P < 0.05) decreased extensibility and increased stiffness. Slow myosin heavy chain expression was aberrantly favored in the mdm diaphragm at 6 wk. Our data strongly support early contractile and passive mechanical aberrations of the respiratory pump in mdm mice.

Myofibrillar gene expression in differentiating lobster claw muscles

Lobster claw muscles undergo a process of fiber switching during development, where isomorphic muscles containing a mixture of both fast and slow fibers, become specialized into predominantly fast, or exclusively slow, muscles. Although this process has been described using histochemical methods, we lack an understanding of the shifts in gene expression that take place. In this study, we used several complementary techniques to follow changes in the expression of a number of myofibrillar genes in differentiating juvenile lobster claw muscles. RNA probes complementary to fast and slow myosin heavy chain (MHC) mRNA were used to label sections of 7th stage (approximately 3 months old) juvenile claw muscles from different stages of the molt cycle. Recently molted animals (1-5 days postmolt) had muscles with distinct regions of fast and slow gene expression, whereas muscles from later in the molt cycle (7-37 days postmolt) had regions of fast and slow MHC expression that were co-mingled and indistinct. Real-time PCR was used to quantify several myofibrillar genes in 9th and 10th stages (approximately 6 months old) juvenile claws and showed that these genes were expressed at significantly higher levels in the postmolt claws, as compared with the intermolt and premolt claws. Finally, Western blot analyses of muscle fibers from juvenile lobsters approximately 3 to 30 months in age showed a shift in troponin-I (TnI) isoform expression as the fibers differentiated into the adult phenotypes, with expression of the adult fast fiber TnI pattern lagging behind the adult slow fiber TnI pattern. Collectively, these data show that juvenile and adult fibers differ both qualitatively and quantitative in the expression of myofibrillar proteins and it may take as much as 2 years for juvenile fibers to achieve the adult phenotype.

Distribution of Slow Muscle Fiber of Muscle Spindle in Postnatal Rat Masseter Muscle

We investigated the properties of the muscle spindle in the masseter muscle at an immunohistochemical level in rats fed for 6 weeks. Slow myosin heavy chain (MyHC) isoforms were measured and intrafusal fibers in the muscle spindle were studied to determine the relationship between the superficial and deep regions of rat masseter muscle after alternated feeding pattern. However, muscle spindles were found in both regions, mainly in the deep region of the posterior superficial region of masseter muscle. The total number of the slow fiber in the intrafusal fiber and number of muscle spindle in the deep region were high from 5 to 8 weeks old in spite of various dimensions of data such as diameter and the compositions of the intrafusal fiber. The relationship of the protein expression of slow MyHC in the two regions at 5 weeks old reversed five weeks later (10 weeks old). This period is an important stage because the mastication system in masseter muscle with muscle spindle may be changed during the alternated feeding pattern of suckling to mastication. The changes may be a marker of the feeding system and of the control by the tension receptor of muscle spindle in this stage of masseter muscle after postnatal development.

Fiber Content and Myosin Heavy Chain Composition of Muscle Spindles in Aged Human Biceps Brachii

The present study investigated potential age-related changes in human muscle spindles with respect to the intrafusal fiber-type content and myosin heavy chain (MyHC) composition in biceps brachii muscle. The total number of intrafusal fibers per spindle decreased significantly with aging, due to a significant reduction in the number of nuclear chain fibers. Nuclear chain fibers in old spindles were short and some showed novel expression of MyHC alpha-cardiac. The expression of MyHC alpha-cardiac in bag1 and bag2 fibers was greatly decreased in the A region. The expression of slow MyHC was increased in nuclear bag1 fibers and that of fetal MyHC decreased in bag2 fibers whereas the patterns of distribution of the remaining MyHC isoforms were generally not affected by aging. We conclude that aging appears to have an important impact on muscle spindle composition. These changes in muscle spindle phenotype may reflect an age-related deterioration in sensory and motor innervation and are likely to have an impact in motor control in the elderly.

Gene expression in muscle in response to exercise.

Muscle has an intrinsic ability to adapt to different types of work by changing fibre type and muscle mass. This process involves quantitative and qualitative changes in gene expression including those of the myosin heavy chain (MyHC) isogenes that encode different types of molecular motors. Increased expression of slow MyHC and of metabolic genes result in increased fatigue resistance. Recently, there has been some insight into how oxidative metabolism, as well as slow myosin expression, is regulated and the role of calcium in initiating switches in gene expression. In relation to muscle mass and power output it has been appreciated that local as well as systemic factors are important. Our group have cloned three types of IGF-I in human muscle which are derived from the IGF-I gene by alternative splicing. The expression of one of these that appears to be an autocrine/paracrine splice variant is only detectable after mechanical stimulation (MGF) and a systemic type (IGF-IEa) that is produced by the liver and other tissue including muscle. As the result of a reading frame shift, the MGF peptide has a different C terminal sequence to IGF-IEa. Interestingly, the MGF C terminal peptide has been found to act as a separate growth factor and to initially activate mononluceated myoblasts (satellite cells). MGF also responds to different signals and has different expression kinetics to IGF-IEa. The mechanotransduction mechanism for this signalling may directly or indirectly involve the dystrophin complex as dystrophic muscle, unlike normal muscle, is unable to express MGF in response to overload. Also the ability to express MGF has been found to decline markedly during ageing. The deficiency in expressing MGF and activating satellite cells in dystrophic and aged muscles may explain why muscle mass is not maintained in these situations. However, in normal muscle MGF appears to initiate local muscle repair with its over expression resulting in hypertrophy.