IIb Myosin Heavy Chain Isoforms Research Articles

Diet and Exercise Restores Muscle Fiber Phenotype Following Long Term High Fat/High Sugar Diet Yet mRNA Expression Remains Unchanged

Obesity affects roughly 42% of the US population. High fat/high sugar diets (HFHS) often referred as “western diet” contributes to this prevalence. Diet‐induced obesity (DIO) results in impaired metabolic responses and associated disease states (i.e., Type 2 Diabetes). Metabolic impairments in DIO are a result of changes in muscle metabolism and muscle fiber phenotype. This is determined by both mRNA expression and protein isoform‐content of myosin heavy chain (MHC). Fast muscle fiber phenotype (i.e., type IIb in mice) is characterized by lower capacity for lipid utilization, implicated in the pathogenesis of Type 2 Diabetes. Regular exercise expresses mRNA in a fiber type specific manner, shifting MHC proportions under healthy circumstances. However, diet and exercise‐driven fiber type shifts in DIO are less understood. We aimed to determine the impact of exercise and diet on fiber‐type proportions in mice with DIO. We hypothesized exercise shifts mouse gastroc muscle phenotype induced by HFHS diet away from IIb fast fiber types. 49 C57BL/6 mice were split into 4 groups: 1) Lean Control (n = 9) fed standard chow and water for 24 wks, 2) HFHS (n = 10), fed HFHS diet (60% of calories from fat, high sugar/fructose: 42 g/L in drinking water) for 24 wks, 3) HFHS Control (n = 10) fed HFHS diet for 12 wks followed by standard chow and water for 12 wks (i.e., simulating traditional diet approach), and 4) HFHS + exercise group (n = 10) fed HFHS diet for 24 wks and performed aerobic exercise (30 min of treadmill running 5 d/wk) in the last 12 wks. Gastroc muscles were collected, homogenized, and analyzed for MHC mRNA expression of MYH 1 (type IIx), MYH 2 (type IIa), MYH 4 (type IIb), and protein isoforms via q‐PCR and SDS‐PAGE respectively. Intensity of bands corresponding to MHC isoforms were quantified using Image J. mRNA expression was calculated using comparative CT analysis (2‐∆∆CT). Paired sample t‐tests were conducted to test differences between MHC isoforms across groups. Proportions of MHC IIb isoform increased (91 ± 3%) in HFHS compared to Control (81 ± 6.2%, p>0.05), HFHS Control (77 ± 3%, p=0.004), and HFHS + exercise groups (79 ± 5%, p=0.057). Additionally, MHC IIa/x proportions in the HFHS (8 ± 3%) compared to Control (17 ± 5.9%, p>0.05), HFHS Control (20 ± 2%, p=0.004), and HFHS + exercise (18 ± 4%, p=0.054) groups were reduced. There were no differences in mRNA expression data. These data suggest performing aerobic exercise with HFHS diet or switching to a standard diet restores muscle fiber phenotype in mouse gastroc. Yet, this was not reflected in mRNA expression data. Changes in MHC isoforms are likely better detected at the protein level rather than mRNA expression. We conclude that exercise and dietary intervention may be a good strategy to shift MHC isoforms away from extreme fast fiber phenotype with lower capacity for lipid utilization. Future research should aim to determine single muscle fiber phenotype shifts with long‐term diet and exercise in humans with obesity to better understand regulation of muscle fiber phenotypes impact on human metabolism.

Age-related changes in histone modification in rat gastrocnemius muscle.

Changes in histone acetylation and methylation status with aging affect gene expression and phenotype in several tissues; however, age-related changes in histone modification in the skeletal muscle have not been elucidated yet. This study investigated age-related global changes in histone modification in rat gastrocnemius muscle. Male Wistar rats (n = 28) were assigned to one of four age groups (n = 7 per group) corresponding to different life stages: 3 months old (3-mo; young), 6 months old (adult), 12 months old (12-mo; middle-aged), and 24 months old (24-mo; old). The gastrocnemius muscle was removed and global histone modification (acetylation and tri-methylation) at K9 and K27 was evaluated by western blotting. Relative muscle mass decreased in the 12- and 24-mo rats accompanied with reduction in type IIb myosin heavy chain isoforms and Myh4 (MHC IIB) mRNA expression. Histone H3 acetylation decreased in an age-dependent manner, with lower levels in 12- and 24-mo groups than in the 3-mo group. K9 and K27 acetylation decreased with age. Although there was no significant change in K27 tri-methylation, K9 tri-methylation showed an age-dependent decline. Histone modification status (acetylation at K9 and K27 and tri-methylation at K9) was positively associated with relative gastrocnemius muscle weight, the percentage of type IIb myosin heavy chain isoform, myosin heavy chain type IIb protein expression, and the level of Myh4 mRNA. Thus, global histone H3 methylation and acetylation decrease with age, and the latter might be associated with age-related muscle atrophy of rat gastrocnemius muscle.

Negative Impact of Icing Treatment on Qualitative Recovery of Injured Soleus Muscle in Rats

PURPOSE: Although the RICE (Rest, Ice, Compression, and Elevation) treatment has been recognized as the gold standard of first aid treatment for sports injuries, we and the others previously demonstrated that a transient icing treatment immediately after skeletal muscle injury impaired muscle regeneration. However, the information about the influence of icing treatment following skeletal muscle injury remains limited. This study was, therefore, to investigate the impact of icing as a first-aid treatment on qualitative recovery of damaged muscle in terms of fibrosis and myosin heavy chain (MyHC) profile. METHODS: Male Wistar rats (9-10 weeks of age) were randomly assigned to control (Con), injured, and injured with icing groups (Ice). Bupivacaine (BPVC) was injected into slow soleus muscles bilaterally in order to induce muscle injury in the two injury groups. Application of icing treatment (ice pack, 0°C for 20 min) to the icing group was carried out immediately after the BPVC injection. At 3 days-4 weeks after BPVC injection, soleus muscles were removed and analyzed. RESULTS: Compared with the Con group, a significant increase in fibrotic area was observed after 2 weeks following injury in the injured groups, but after 1 week following injury in the Ice group (P < 0.05). This area was also tended to be higher in Ice than in injured animals during 1-4 weeks of recovery period. In addition, the number of Tcf4-positive nuclei, a fibroblast marker, located in interstitial spaces in both injured groups markedly increased 1 week after BPVC injection. The numbers were tended to be more magnified in the Ice group than in the injured group, then their number in both injured groups gradually degreased thereafter. Injury-related de novo appearance of embryonic, neonatal, IId/x, and IIb MyHC isoforms was noted in both injured groups 1 week after BPVC injection. These MyHC isoforms were significantly decreased toward the undetectable level thereafter. However, the embryonic MyHC isoform was still detectable in icing, but not in injured, animals 4 weeks after BPVC injection. CONCLUSIONS: Our results suggested that icing treatment following skeletal muscle injury will have a negative impact on recovery process (fibrosis and normalization of MyHC profile) of regenerating muscle.

Impact of different temperature stimuli on the expression of myosin heavy chain isoforms during recovery from bupivacaine-induced muscle injury in rats.

Limited information exists regarding the impact of different temperature stimuli on myosin heavy chain (MyHC) expression in skeletal muscle during recovery from injury. Therefore, this experiment investigated the impact of both cold and heat exposure on the MyHC isoform profile in the rat soleus during recovery from injury. Male Wistar rats were randomly divided into control, bupivacaine-injected (BPVC), BPVC with icing, and BPVC with heat stress groups. Muscle injury was induced by intramuscular injection of bupivacaine into soleus muscles of male Wistar rats. Icing treatment (0°C for 20 min) was performed immediately after the injury. Intermittent heat stress (42°C for 30 min on alternating days) was carried out during 2-14 days after bupivacaine injection. In response to injury, a transient increase in developmental, IId/x, and IIb MyHC isoforms, as well as various types of hybrid fibers, followed by the recovery of the MyHC profile toward the control level, was noted in the regeneration of the soleus. The restoration of the MyHC profile in the regenerating muscle at whole-muscle and individual myofiber levels was partially delayed by icing but facilitated by heat stress. In addition, the application of repeated heat stress promoted the recovery of soleus muscle mass toward the control level following injury. We conclude that compared with acute and immediate cold (icing) treatment, chronic and repeated heat stress may be a more appropriate treatment for the enhancement of both normalization of the MyHC profile and restoration of muscle mass following injury. NEW & NOTEWORTHY Cold exposure (icing), but not heat exposure, has been well accepted as a first-aid treatment for accidental and/or sports-related injuries. However, recent evidence suggests the negative impact of icing treatment on skeletal muscle regeneration following injury. Here, we demonstrated that acute/immediate icing treatment delayed the restoration of the myosin heavy chain (MyHC) profile, but intermittent hyperthermia, repeated for several days, facilitated the recovery of both muscle mass and the MyHC profile in the regeneration of skeletal muscle following injury.

AMPK Mediates Muscle Mass Change But Not the Transition of Myosin Heavy Chain Isoforms during Unloading and Reloading of Skeletal Muscles in Mice.

5′AMP-activated protein kinase (AMPK) plays an important role in the regulation of skeletal muscle mass and fiber-type distribution. However, it is unclear whether AMPK is involved in muscle mass change or transition of myosin heavy chain (MyHC) isoforms in response to unloading or increased loading. Here, we checked whether AMPK controls muscle mass change and transition of MyHC isoforms during unloading and reloading using mice expressing a skeletal-muscle-specific dominant-negative AMPKα1 (AMPK-DN). Fourteen days of hindlimb unloading reduced the soleus muscle weight in wild-type and AMPK-DN mice, but reduction in the muscle mass was partly attenuated in AMPK-DN mice. There was no difference in the regrown muscle weight between the mice after 7 days of reloading, and there was concomitantly reduced AMPKα2 activity, however it was higher in AMPK-DN mice after 14 days reloading. No difference was observed between the mice in relation to the levels of slow-type MyHC I, fast-type MyHC IIa/x, and MyHC IIb isoforms following unloading and reloading. The levels of 72-kDa heat-shock protein, which preserves muscle mass, increased in AMPK-DN-mice. Our results indicate that AMPK mediates the progress of atrophy during unloading and regrowth of atrophied muscles following reloading, but it does not influence the transition of MyHC isoforms.

IGF1 stimulates greater muscle hypertrophy in the absence of myostatin in male mice.

Insulin-like growth factors (IGFs) and myostatin have opposing roles in regulating the growth and size of skeletal muscle, with IGF1 stimulating, and myostatin inhibiting, growth. However, it remains unclear whether these proteins have mutually dependent, or independent, roles. To clarify this issue, we crossed myostatin null (Mstn-/-) mice with mice overexpressing Igf1 in skeletal muscle (Igf1+) to generate six genotypes of male mice; wild type (Mstn+/+ ), Mstn+/-, Mstn-/-, Mstn+/+:Igf1+, Mstn+/-:Igf1+ and Mstn-/-:Igf1+ Overexpression of Igf1 increased the mass of mixed fibre type muscles (e.g. Quadriceps femoris) by 19% over Mstn+/+ , 33% over Mstn+/- and 49% over Mstn-/- (P < 0.001). By contrast, the mass of the gonadal fat pad was correspondingly reduced with the removal of Mstn and addition of Igf1 Myostatin regulated the number, while IGF1 regulated the size of myofibres, and the deletion of Mstn and Igf1+ independently increased the proportion of fast type IIB myosin heavy chain isoforms in T. anterior (up to 10% each, P < 0.001). The abundance of AKT and rpS6 was increased in muscles of Mstn-/-mice, while phosphorylation of AKTS473 was increased in Igf1+mice (Mstn+/+:Igf1+, Mstn+/-:Igf1+ and Mstn-/-:Igf1+). Our results demonstrate that a greater than additive effect is observed on the growth of skeletal muscle and in the reduction of body fat when myostatin is absent and IGF1 is in excess. Finally, we show that myostatin and IGF1 regulate skeletal muscle size, myofibre type and gonadal fat through distinct mechanisms that involve increasing the total abundance and phosphorylation status of AKT and rpS6.

Dietary supplementation with β-hydroxy-β-methylbutyrate calcium during the early postnatal period accelerates skeletal muscle fibre growth and maturity in intra-uterine growth-retarded and normal-birth-weight piglets.

Intra-uterine growth restriction (IUGR) impairs postnatal growth and skeletal muscle development in neonatal infants. This study evaluated whether dietary β-hydroxy-β-methylbutyrate Ca (HMB-Ca) supplementation during the early postnatal period could improve muscle growth in IUGR neonates using piglets as a model. A total of twelve pairs of IUGR and normal-birth-weight (NBW) male piglets with average initial weights (1·85 (sem 0·36) and 2·51 (sem 0·39) kg, respectively) were randomly allotted to groups that received milk-based diets (CON) or milk-based diets supplemented with 800 mg/kg HMB-Ca (HMB) during days 7-28 after birth. Blood and longissimus dorsi (LD) samples were collected and analysed for plasma amino acid content, fibre morphology and the expression of genes related to muscle development. The results indicate that, regardless of diet, IUGR piglets had a significantly decreased average daily weight gain (ADG) compared with that of NBW piglets (P<0·05). However, IUGR piglets fed HMB-Ca had a net weight and ADG similar to that of NBW piglets fed the CON diet. Irrespective of body weight (BW), HMB-Ca supplementation markedly increased the type II fibre cross-sectional area and the mRNA expression of mammalian target of rapamycin (mTOR), insulin-like growth factor-1 and myosin heavy-chain isoform IIb in the LD of piglets (P<0·05). Moreover, there was a significant interaction between the effects of BW and HMB on mTOR expression in the LD (P<0·05). In conclusion, HMB-Ca supplementation during the early postnatal period could improve skeletal muscle growth and maturity by accelerating fast-twitch glycolytic fibre development in piglets.

Myosin heavy chain changes in heart failure‐rat diaphragm muscle are not related to myogenic regulatory factors expression

ObjectiveIn this study we tested the hypothesis that diaphragm myosin heavy chain (MyHC) isoform changes are related to alterations of myogenic regulatory factors (MRF) expression in chronic heart failure.MethodsWe studied Sham (n=10) and infarcted rats with heart failure (MI/HF, n=10). Cardiac structures and ventricular function were assessed by echocardiogram. MyHC isoforms were analyzed by electrophoresis. MRF myogenin, MyoD, and MRF4 gene expression was evaluated by real time RT‐PCR and protein expression was analyzed by Western‐blot. Statistical analysis was performed by Student's t test and Pearson correlation.ResultsAll MI/HF rats had pulmonary congestion and right ventricular hypertrophy. In echocardiographic evaluation, MI/HF presented left cardiac chambers dilation and left ventricular systolic and diastolic dysfunction. The percentage of MyHC I isoform was higher and IIb isoform was lower in MI/HF. Left atrial diameter‐to‐body weight ratio positively correlated with MyHC I and negatively correlated with MyHC IIb isoform percentage. No differences were observed in MRF gene and protein expressions between groups.ConclusionMRF expression does not modulate diaphragm MyHC isoforms changes in rats with chronic heart failure. Diaphragm MyHC changes are related to ventricular dysfunction severity. Financial support: Fapesp.

Muscle fiber type characterization and myosin heavy chain (MyHC) isoform expression in Mediterranean buffaloes

This study aimed to evaluate myosin heavy chain (MyHC) isoform expression and muscle fiber types of Longissimus dorsi (LD) and Semitendinosus (ST) in Mediterranean buffaloes and possible fibers muscles modulation according to different slaughter weights. The presence of MyHC IIb isoforms was not found. Only three isoforms of MyHC (IIa, IIx/d and I) were observed and their percentages did not vary significantly among slaughter weights. The confirmation of the presence of hybrid muscles fibers (IIA/X) in LD and ST muscles necessitated classifying the fiber types into fast and slow according to their contractile activity, by m-ATPase assay. For both muscles, the muscle fiber frequency was higher for fast than for slow fibers in all weight groups. There was a difference (P<0.05) in the frequency of LD and ST muscle fiber types according to slaughter weights, which demonstrate that the slaughter weight influences the profile of muscle fibers from buffaloes.

IIb or not IIb? Regulation of myosin heavy chain gene expression in mice and men

BackgroundWhile the myosin heavy chain IIb isoform (MyHC-IIb) is the predominant motor protein in most skeletal muscles of rats and mice, the messenger RNA (mRNA) for this isoform is only expressed in a very small subset of specialized muscles in adult large mammals, including humans.ResultsWe identify the DNA sequences limiting MyHC-IIb expression in humans and explore the activation of this gene in human skeletal muscle. We demonstrate that the transcriptional activity of ~1.0 kb of the human MyHC-IIb promoter is greatly reduced compared to that of the corresponding mouse sequence in both mouse and human myotubes in vitro and show that nucleotide differences that eliminate binding sites for myocyte enhancer factor 2 (MEF2) and serum response factor (SRF) account for this difference. Despite these differences, we show that MyHC-IIb mRNA is expressed in fetal human muscle cells and that MyHC-IIb mRNA is significantly up-regulated in the skeletal muscle of Duchene muscular dystrophy patients.ConclusionsThese data identify the genetic basis for a key phenotypic difference between the muscles of large and small mammals, and demonstrate that mRNA expression of the MyHC-IIb gene can be re-activated in human limb muscle undergoing profound degeneration/regeneration.

Novel epigenetic regulation of skeletal muscle myosin heavy chain genes. Focus on “Differential epigenetic modifications of histones at the myosin heavy chain genes in fast and slow skeletal muscle fibers and in response to muscle unloading”

skeletal muscle is a highly plastic tissue, capable of responding and adapting to a variety of physiological stimuli, e.g., muscle loading and unloading. Multiple functional phenotypes of skeletal muscle exist throughout the body at the same time. For example, muscles required for antigravity support of the skeleton or sustained locomotion are generally slow-contracting, low force-generating, and relatively resistant to fatigue, while other muscles required for quick, powerful movements are generally fast-contracting, high force-generating, and easily fatigued. In fact, skeletal muscle phenotypes have been characterized using combinations of muscle properties, including metabolic, fatigability, color, innervation, or predominant contractile protein expression. Myosin heavy chain (MHC) is the most abundant protein in skeletal muscle, comprising ∼25% of the total protein content, and is a major component of the complex responsible for generating contractile force in skeletal muscle. At least nine MHC isoforms, each transcribed from their own gene, exist in mammalian striated muscle; but only slow type I MHC and various isoforms of fast type II (IIa, IIx/d, and IIb MHC isoforms) are present in adult limb skeletal muscle that is not undergoing regeneration (16). It is well established that skeletal muscle phenotype, particularly MHC gene expression, can be altered by different states of muscular activity or inactivity, as well as various hormonal and metabolic factors (1).

Correlation of dystrophin–glycoprotein complex and focal adhesion complex with myosin heavy chain isoforms in rat skeletal muscle

The dystrophin-glycoprotein complex (DGC) and focal adhesion complex (FAC) are transmembrane structures in muscle fibres that link the intracellular cytoskeleton to the extracellular matrix. DGC and FAC proteins are abundant in slow-type muscles, indicating the structural reinforcement which play a pivotal role in continuous force output to maintain posture for long periods. The aim of the present study was to examine the expression of these structures across fast-type muscles containing different myosin heavy chain (MHC) isoform patterns which reflect the fatigue-resistant characteristics of skeletal muscle. We measured the expression of dystrophin and beta1 integrin (representative proteins of DGC and FAC respectively) in plantaris, extensor digitorum longus, tibialis anterior, red and white portions of gastrocnemius, superficial portion of vastus lateralis and diaphragm, in comparison with soleus (SOL) and cardiac muscle from rats. The expression of dystrophin and beta1 integrin correlated positively with the percentage of type I, IIa and IIx MHC isoforms and negatively with that of type IIb MHC isoform in fast-type skeletal muscles, and their expression was abundant in SOL and cardiac muscle. Our results support the idea that DGC and FAC are among the factors that explain the fatigue-resistant property not only of slow-type but also of fast-type skeletal muscles.

Regional specialization of rat quadriceps myosin heavy chain isoforms occurring in distal to proximal parts of middle and deep regions is not mirrored by citrate synthase activity

Myosin heavy chain (MHC) isoform content and citrate synthase (CS) activities were measured in the Quadriceps femoris (QF) muscle of 18 female rats. The muscle group was divided into superficial, middle and deep, distal, central and proximal parts. MHC IIb and IIx were more abundant in superficial regions (P<0.05) with low CS activities compared with deeper parts. The deeper parts expressed all four isoforms (MHC IIb, MHC IIx, MHC IIa and MHC I), with a concomitantly higher CS activity. MHC I, MHC IIa and MHC IIb isoform content varied significantly along the length of the deep regions. Only MHC IIb and CS activity in the proximal middle part correlated (negatively) with each other. This study showed that the QF has regional specialization and that standardization of sampling site is important. Furthermore, CS activity and MHC isoforms are only loosely associated, or not at all.

Composition and turnover of contractile proteins in volume-overtrained skeletal muscle.

The purpose of this study was to find the composition shift of myosin heavy chain (MyHC) isoforms in overtraining in fast- and slow-twitch skeletal muscles and different changes in MyHC isofom composition, synthesis and turnover rate between 4-week and 6-week overtraining. Male Wistar rats were randomly assigned to 4-week and 6-week endurance training, 4-week and 6-week overtraining groups. Plantaris (Pla), extensor digitorum longus (EDL), and soleus (Sol) muscles were studied. Daily excretion of 3-methylhistidine (3-MeHis) pool as an indicator for protein degradation increased in the 4-week and 6-week overtraining group to 4.04 +/- 0.21 and 4.32 +/- 0.23 %/day subsequently in comparison with the control group (2.16 +/- 14 %/day, p < 0.001). In Pla muscle MyHC I isoform synthesis rate was 33 200 +/- 2150 (after 6-week overtraining 27 100 +/- 1800, p < 0.05), IIa 32 600 +/- 2100; IId 27 300 +/- 1890 and IIb isoform 20 100 +/- 1600 (after 6-week overtraining 15 500 +/- 1400, p < 0.05) dpm/M leucine/min. Actin synthesis rate increased in fast-twitch muscles during 4- and 6-week overtraining, and in soleus muscle during 6-week overtraining. In EDL and Sol muscle MyHC isoform composition during 6-week overtraining did not change significantly. During the 6-week overtraining the relative content of MyHC I and IIb isoforms decreased and IIa and IId isoforms increased in Pla muscle. The initial increase of MyHC IIb isoform after 4-week overtraining shows the higher stability of this isoform in comparison with MyHC I isoform in fast-twitch muscles during high volume exercise.

The effect of mechanical loading on the MyHC synthesis rate and composition in rat plantaris muscle.

The aim of this study was to investigate the response of protein synthesis rate, particularly myosin heavy chain (MyHC) isoforms synthesis and the magnitude of its isoform transformation in fast-twitch plantaris muscle, to different modes of prolonged mechanical loading. Different protocols of mechanical loading were used: resistance training (RT), compensatory hypertrophy (CH) of m. plantaris after tenotomy of m. gastrocnemius and a combination of the two previous loadings (RT + CH). During the different modes of loading, plantaris muscle hypertrophy in RT group was approximately 10 %, CH approximately 40 % and CH + RT approximately 44 %. MyHC I and IID isoform synthesis rate increased in all experimental groups, as well as their relative content. MyHC IIA relative content decreased during RT and RT + CH and increased during CH. MHC IIB isoform relative content decreased in all experimental groups, but compared with CH in CH + RT MyHC IIB isoform content increased in plantaris muscle. These results demonstrate that different modes of mechanical loading resulted in the selective up- and down-regulation of MyHC isoforms in fast-twitch skeletal muscle. The synthesis rate and relative content of the two fastest isoforms of MyHC IIB and IID are regulated to different directions during mechanical loading.

Transcriptional Control of Myosin Heavy Chain IIB Gene Expression in Normal and Overloaded Rat Plantaris

1038 The rat plantaris (P) muscle predominantly expresses fast-contracting isoforms of myosin heavy chain (MHC) proteins. During hypertrophy induced by chronic mechanical overload, the expression of the fastest type IIb MHC isoform decreases as the muscle shifts phenotype toward expression of slower-contracting MHC isoforms. PURPOSE: To determine the transcriptional control elements that reside downstream of the transcription start site (tss) and regulate MHC IIb expression in normal (Con) and functional overload (FO) rat P. METHODS: Female SD rats (N = 8/group) underwent 7 d FO of the left P. MHC IIb pre-mRNA and mRNA expression were measured by RT-PCR. The transcriptional activities of five MHC IIb promoters were tested by direct gene transfer into the P. Each promoter fragment began −1.431 kb upstream of the tss, and extended into either exon 1 (+0.013 kb), intron 2 (+1.008, +1.568, +2.054 kb), or exon 3 (+2.814 kb). Promoter activity was measured by a firefly luciferase (fLuc) reporter assay 7 d after bilateral intramuscular injection of plasmid constructs containing the fLuc gene controlled by the inserted IIb promoter fragment. To control for plasmid uptake efficiency, the fLuc reporter activity was corrected to the renilla luciferase activity of a co-injected alpha-actin promoter-reporter plasmid. RESULTS: After 7 d FO, the wet weight of the P increased ∼31% (P < 0.05). IIb pre-mRNA and mRNA each decreased by ∼60% after FO (P < 0.05). Activities of the +2.054 and +2.814 kb promoters were similar, and decreased ∼50% after FO (P < 0.05). The three promoters that extended downstream between +0.013 and +1.568 kb were 80–99% less active (P < 0.05) than the +2.814 kb promoter, and did not decrease after FO. CONCLUSION: MHC IIb expression in Con and FO P is controlled by transacting factor(s) located downstream of the tss, and specifically within the intron 2 region between +1.568 and +2.054 kb. Further studies are aimed at identifying the trans-acting factors that bind this region of DNA to control transcription of the IIb gene in the normal rat P and its down-regulation during hypertrophy. Supported by NIH grant AR30346

Effect of Dietary Creatine on Skeletal Muscle Myosin Heavy Chain Isoform Expression in an Animal Model of Uremia

Background: Chronic renal failure is accompanied with muscle dysfunction and myopathy, characterized by muscle weakness and increased fatigue. Myosin heavy chain (MHC) is the principal structural protein that controls the intrinsic contractile properties of striated muscle. Creatine is widely used as an ergogenic nutritional supplement in sportsmen. This study investigates the effect of creatine supplementation on MHC expression in the setting of uremic myopathy. Methods: Male Wistar rats were either sham operated or subtotally nephrectomized and received a control diet or creatine (2% w/w)-supplemented diet. After 4 weeks of treatment, serum creatinine, creatine, urea and creatinine clearances were determined. MHC isoforms were determined electrophoretically in the extensor digitorum longus and soleus muscles. Results: Creatinine clearances were lower in nephrectomized animals, but similar in creatine-supplemented and control diet animals. Nephrectomized animals had significantly higher MHC IIb and lower MHC IIx isoform expression in the extensor digitorum longus muscle compared to sham-operated animals. In the soleus muscle, MHC IIb expression was increased in nephrectomized animals. Creatine supplementation reversed the MHC transitions observed in uremia in the soleus muscle, but not in the extensor digitorum longus muscle. Conclusion: We observed altered expression of MHC isoforms in uremia. In uremic animals, fast MHC IIb isoforms were increased, whereas MHC I and IIx isoforms predominate in control animals. Dietary creatine supplementation reversed the altered MHC expression during uremia in slow-twitch, but not in fast-twitch muscles.

Early postmortem biochemical factors influence tenderness and water-holding capacity of three porcine muscles.

The objective of this study was to determine whether differences in pork tenderness and water-holding capacity could be explained by factors influencing calpain activity and proteolysis. Halothane-negative (HAL-1843 normal) Duroc pigs (n = 16) were slaughtered, and temperature and pH of the longissimus dorsi (LD), semimembranosus (SM), and psoas major (PM) were measured at 30 and 45 min and 1, 6, 12, and 24 h postmortem. Calpastatin activity; mu-calpain activity; and autolysis and proteolysis of titin, nebulin, desmin, and troponin-T were determined on muscle samples from the LD, SM, and PM at early times postmortem. Myofibrils from each muscle were purified to assess myofibril-bound (mu-calpain. Percentage drip loss was determined, and Warner-Bratzler shear (WBS) force was analyzed. Myosin heavy-chain (MHC) isoforms were examined using SDS-PAGE. The pH of PM was lower (P < 0.01) than the pH of LD and SM at 30 and 45 min and 1 h postmortem. The PM had a higher (P < 0.01) percentage of the MHC type IIa/IIx isoforms than the LD. The-LD had the greatest proportion of (P < 0.01) MHC IIb isoforms of any of the muscles. The PM had the lowest (P < 0.01) percentage of MHC IIb isoforms and a greater (P < 0.05) percentage of type I MHC isoforms than the LD and SM. The PM had less (P < 0.01) drip loss after 96 h of storage than the SM and LD. The PM had more desmin degradation (P < 0.01) than the LD and SM at 45 min and 6 h postmortem. Degradation of titin occurred earlier in the PM than the LD and SM. At 45 min postmortem, the PM consistently had some autolysis of mu-calpain, whereas the LD and SM did not. At 6 h postmortem, some autolysis of mu-calpain (80-kDa subunit) was observed in all three muscles. The rapid pH decline and increased rate of autolysis in the PM paralleled an earlier appearance of myofibril-bound mu-calpain. The SM had higher calpastatin activity (P < 0.05) at 45 min, 6 h, and 24 h and had higher WBS values at 48 h (P < 0.01) and 120 h (P < 0.05) postmortem than the LD. At 48 and 120 h postmortem, more degradation of desmin, titin, and nebulin were observed in the LD than in the SM. These results show that mu-calpain activity, mu-calpain autolysis, and protein degradation are associated with differences in pork tenderness and water-holding capacity observed in different muscles.

The effect of glucocorticoids on the myosin heavy chain isoforms’ turnover in skeletal muscle

The purpose of this study was to find the effect of dexamethasone on the myosin heavy chain (MyHC) isoforms’ composition in different skeletal muscles and glycolytic (G) fibres in relation with their synthesis rate and degradation of MyHC isoforms by alkaline proteinases. Eighteen-week-old male rats of the Wistar strain were treated with dexamethasone (100 μg/100 g bwt) during10 days. The forelimb strength decreased from 9.52 to 6.19 N ( P<0.001) and hindlimb strength from 15.54 to 8.55 N ( P<0.001). Daily motor activity decreased (total activity from 933 to 559 and ambulatory activity from 482 to 226 movements/h, P<0.001). The degradation rate of muscle contractile proteins increased from 2.0 to 5.9% per day ( P<0.001), as well as the myosin heavy chain IIB isoform degradation with alkaline proteinase in fast-twitch (F-T) muscles (12±0.9%; P<0.05) and glycolytic muscle fibres (15±1.1%; P<0.001). The synthesis rate of MyHC type II isoforms decreased in Pla muscles ( P<0.05) and MyHC IIA ( P<0.05) and IIB in EDL muscle and G fibres ( P<0.001). The relative content of MyHC IIB isoform decreased in F-T muscles ( P<0.001) and in G fibres ( P<0.01), and the relative content of IIA and IID isoforms increased simultaneously. Dexamethasone decreased the MyHC IIB isoform synthesis rate and increased the sensibility of MyHC IIB isoform to alkaline proteinase, which in its turn led to the decrease of MyHC IIB isoform relative content in F-T muscles with low oxidative potential and G muscle fibres.

Bilateral effect of a unilateral occlusal splint on the expression of myosin heavy-chain isoforms in rat deep masseter muscle

Many studies have shown that various myosin isoforms are involved in muscle contraction. A search for specific antibodies directed against the myosin heavy chain (MHC) resulted in the identification of at least two main classes, referred to as MHC type I and type II. In this study, immunohistology and gel electrophoresis were used to determine the proportion of MHC isoforms in rat deep masseter muscle at different times after the insertion of an unilateral occlusal splint. An increasing proportion of MHC type I isoforms was found in both deep masseters soon after splinting, and this trend continued until 7 days after splint insertion. The type I fibres were clearly distributed on either side of the central axis of the muscle. At 15 days, a significant decrease in the percentage of the type IIb MHC isoform was observed on the occlusal splint side compared to the contralateral side. After 30 days of unilateral splinting, the proportion of type IIb fibres on the splint side returned to baseline whereas on the contralateral side there was an increase in the proportion of this type. The results suggest an initial adaptation after the unilateral occlusal disturbance in which muscles of both sides react in the same way; later, the muscles of each side adapt their expression of MHC isoforms according to altered functional demand.