Effects of age and diet consistency on the expression of myosin heavy-chain isoforms on jaw-closing and jaw-opening muscles in a rat model

BACKGROUND

Skeletal craniofacial morphology can be influenced by changes in masticatory muscle function, which may also change the functional profile of the muscles.

OBJECTIVES

To investigate the effects of age and functional demands on the expression of Myosin Heavy-Chain (MyHC) isoforms in representative jaw-closing and jaw-opening muscles, namely the masseter and digastric muscles respectively.

METHODS

Eighty-four male Wistar rats were divided into four age groups, namely an immature (n = 12; 4-week-old), early adult (n = 24; 16-week-old), adult (n = 24; 26-week-old) and mature adult (n = 24; 38-week-old) group. The three adult groups were divided into two subgroups each based on diet consistency; a control group fed a standard (hard) diet, and an experimental group fed a soft diet. Rats were sacrificed, and masseter and digastric muscles dissected. Real-time quantitative polymerase chain reaction was used to compare the mRNA transcripts of the MyHC isoforms-Myh7 (MyHC-I), Myh2 (MyHC-IIa), Myh4 (MyHC-IIb) and Myh1 (MyHC-IIx)-of deep masseter and digastric muscles.

RESULTS

In the masseter muscle, hypofunction increases Myh1 (26, 38 weeks; p < .0001) but decreases Myh4 (26 weeks; p = .046) and Myh2 (26 weeks; p < .0001) expression in adult rats. In the digastric muscle, hypofunction increases Myh1 expression in the mature adult rats (38 weeks; p < .0001), while Myh2 expression decreases in adult rats (26 weeks; p = .021) as does Myh4 (26 weeks; p = .001). Myh7 expression is increased in the digastric muscle of mature adult rats subjected to hypofunction (38 weeks; p = <.0001), while it is very weakly expressed in the masseter.

CONCLUSION

In jaw-opening and jaw-closing muscles, differences in myosin expression between hard- and soft-diet-fed rats become evident in adulthood, suggesting that long-term alteration of jaw function is associated with changes in the expression of MyHC isoforms and potential fibre remodelling. This may give insight into the role of function on masticatory muscles and the resultant craniofacial morphology.

MDM2 Regulation of HIF Signaling Causes Microvascular Dysfunction in Hypertrophic Cardiomyopathy

BACKGROUND

Microvasculature dysfunction is a common finding in pathologic remodeling of the heart and is thought to play an important role in the pathogenesis of hypertrophic cardiomyopathy (HCM), a disease caused by sarcomere gene mutations. We hypothesized that microvascular dysfunction in HCM was secondary to abnormal microvascular growth and could occur independent of ventricular hypertrophy.

METHODS

We used multimodality imaging methods to track the temporality of microvascular dysfunction in HCM mouse models harboring mutations in the sarcomere genes Mybpc3 (cardiac myosin binding protein C3) or Myh6 (myosin heavy chain 6). We performed complementary molecular methods to assess protein quantity, interactions, and post-translational modifications to identify mechanisms regulating this response. We manipulated select molecular pathways in vivo using both genetic and pharmacological methods to validate these mechanisms.

RESULTS

We found that microvascular dysfunction in our HCM models occurred secondary to reduced myocardial capillary growth during the early postnatal time period and could occur before the onset of myocardial hypertrophy. We discovered that the E3 ubiquitin protein ligase MDM2 (murine double minute 2) dynamically regulates the protein stability of both HIF1α (hypoxia-inducible factor 1 alpha) and HIF2α (hypoxia-inducible factor 2 alpha)/EPAS1 (endothelial PAS domain protein 1) through canonical and noncanonical mechanisms. The resulting HIF imbalance leads to reduced proangiogenic gene expression during a key period of myocardial capillary growth. Reducing MDM2 protein levels by genetic or pharmacological methods normalized HIF protein levels and prevented the development of microvascular dysfunction in both HCM models.

CONCLUSIONS

Our results show that sarcomere mutations induce cardiomyocyte MDM2 signaling during the earliest stages of disease, and this leads to long-term changes in the myocardial microenvironment.

Skeletal Muscle Function Is Altered in Male Mice on Low-Dose Androgen Receptor Antagonist or Estrogen Receptor Agonist

In males, skeletal muscle function may be altered by shifts in either circulating testosterone or estrogen. We examined the effect of acute (2-week) exposures to 17α-ethinyl estradiol (EE2), an estrogen receptor (ER) agonist, or flutamide, an androgen receptor (AR) antagonist, on the contractile function of individual skeletal muscle fibers from slow-contracting soleus and fast-contracting extensor digitorum longus muscles from adult male mice. Single fiber specific tension (force divided by cross-sectional area) was decreased with flutamide treatment in all myosin heavy chain (MHC) fiber types examined (I, IIA, and IIB); similar effects were observed with EE2 treatment but only in the fastest-contracting MHC IIB fibers. The decreases in maximally Ca2+-activated specific tension were primarily a result of fewer strongly bound myosin-actin cross-bridges, with flutamide treatment also showing lower myofilament lattice stiffness. Myosin-actin cross-bridge kinetics were slower in MHC IIA fibers in flutamide-treated mice, but faster in EE2-treated mice, indicating that contractile velocity may be affected differently in this fiber type, which is commonly expressed in human skeletal muscle. Importantly, these effects were observed in the absence of outcomes previously used to evaluate ER agonists or AR antagonists in rodents including weight of reproductive organs or mammary gland morphology. Our findings indicate that substantial shifts in skeletal muscle function occur in male mice following acute exposures to low doses of a pharmacological ER agonist and an AR antagonist. These results suggest that countermeasures to maintain physical function may be needed early in situations that induce similar ER agonist and AR antagonist conditions.

A New Autosomal Myh11-CreERT2 Smooth Muscle Cell Lineage Tracing and Gene Knockout Mouse Model-Brief Report

BACKGROUND

The Myh11 promoter is extensively used as a smooth muscle cell (SMC) Cre-driver and is regarded as the most restrictive and specific promoter available to study SMCs. Unfortunately, in the existing Myh11-CreERT2 mouse, the transgene was inserted on the Y chromosome precluding the study of female mice. Given the importance of including sex as a biological variable and that numerous SMC-based diseases have a sex-dependent bias, the field has been tremendously limited by the lack of a model to study both sexes. Here, we describe a new autosomal Myh11-CreERT2 mouse (referred to as Myh11-CreERT2-RAD), which allows for SMC-specific lineage tracing and gene knockout studies in vivo using both male and female mice.

METHODS

A Myh11-CreERT2-RAD transgenic C57BL/6 mouse line was generated using bacterial artificial chromosome clone RP23-151J22 modified to contain a Cre-ERT2 after the Myh11 start codon. Myh11-CreERT2-RAD mice were crossed with 2 different fluorescent reporter mice and tested for SMC-specific labeling by flow cytometric and immunofluorescence analyses.

RESULTS

Myh11-CreERT2-RAD transgene insertion was determined to be on mouse chromosome 2. Myh11-CreERT2-RAD fluorescent reporter mice showed Cre-dependent, tamoxifen-inducible labeling of SMCs equivalent to the widely used Myh11-CreERT2 mice. Labeling was equivalent in both male and female Cre+ mice and was limited to vascular and visceral SMCs and pericytes in various tissues as assessed by immunofluorescence.

CONCLUSIONS

We generated and validated the function of an autosomal Myh11-CreERT2-RAD mouse that can be used to assess sex as a biological variable with respect to the normal and pathophysiological functions of SMCs.

Myosin heavy chains in extraocular muscle fibres: Distribution, regulation and function

This review examines kinetic properties and distribution of the 11 isoforms of myosin heavy chain (MyHC) expressed in extraocular muscle (EOM) fibre types and the regulation and function of these MyHCs. Although recruitment and discharge characteristics of ocular motoneurons during fixation and eye movements are well documented, work directly linking these properties with motor unit contractile speed and MyHC composition is lacking. Recruitment of motor units according to Henneman's size principle has some support in EOMs but needs consolidation. Both neurogenic and myogenic mechanisms regulate MyHC expression as in other muscle allotypes. Developmentally, multiply-innervated (MIFs) and singly-innervated fibres (SIFs) are derived presumably from distinct myoblast lineages, ending up expressing MyHCs in the slow and fast ends of the kinetic spectrum respectively. They modulate the synaptic inputs of their motoneurons through different retrogradely transported neurotrophins, thereby specifying their tonic and phasic impulse patterns. Immunohistochemical analyses of EOMs regenerating in situ and in limb muscle beds suggest that the very impulse patterns driving various ocular movements equip effectors with appropriate MyHC compositions and speeds to accomplish their tasks. These experiments also suggest that satellite cells of SIFs and MIFs are distinct lineages expressing different MyHCs during regeneration. MyHC compositions and functional characteristics of orbital fibres show longitudinal variations that facilitate linear ocular rotation during saccades. Palisade endings on global MIFs are postulated to respond to active and passive tensions by triggering axon reflexes that play important roles during fixation, saccades and vergence. How EOMs implement Listings law during ocular rotation is discussed.

Sildenafil reverses the hypertrophy of mice right ventricle caused by hypoxia but does not reverse the changes in the myosin heavy chain isoforms

In this study, we investigated the effect of hypoxia and concomitant sildenafil treatment on MHC isoforms in hypoxia-induced hypertrophied right ventricles. Right ventricular hypertrophy was induced in mice by exposing them to hypoxic stimulus (11% ambient oxygen) in a normobaric chamber for 20 days. 45 mice were used in this study, distributed randomly into three groups: the first group served as a control (CO), the second group was exposed to hypoxia for 20 days without sildenafil treatment (HY), and the third group was given sildenafil orally at a dose of 30 mg.kg^-1.day^-1 plus exposure to hypoxia for 20 days (HS). Relative amounts of MHC isoforms were calculated using two ELISA kits containing antibodies against α and β MHC, and by SDS-PAGE. Compared with the CO group, the HY group showed a significant increase in right ventricle weight/left ventricle plus septum ratio (Fulton's ratio). The HS group showed a significant decrease in Fulton's ratio compared with the HY group, but not with the CO group. Expression of the MHC-β isoform was significantly increased in the HY group compared with the CO group. There was no significant difference in MHC-β between the HY group and the HS group. Plasma atrial natriuretic peptide level was significantly higher in HY group than HS group and did not return to normal after sildenafil treatment. Conclusion: sildenafil reversed the right ventricular hypertrophy induced by hypoxia but did not decrease the expression of MHC-β to normal levels.

Muscle Fiber Properties in Cattle and Their Relationships with Meat Qualities: An Overview

The control of meat quality traits constitutes an important target for any farm animal production, including cattle. Therefore, better understanding of the biochemical properties that drive muscle development and final outcomes constitutes one of the main challenging topics of animal production and meat science. Accordingly, this review has focused on skeletal muscle fibers in cattle and their relationships with beef qualities. It aimed to describe the chemical and structural properties of muscle fibers as well as a comprehensive review of their contractile and metabolic characteristics during the life of the animal. The existing methods for the classification of muscle fibers were reviewed, compared, and discussed. Then, the different stages of myogenesis in cattle were defined. The main factors regulating fetal and postnatal growth and the plasticity of muscle fibers were evidenced, especially the role of myostatin growth factor and the impact of nutritional factors. This review highlights that the knowledge about muscle fibers is paramount for a better understanding of how to control the muscle properties throughout the life of the animal for better management of the final eating qualities of beef. Accordingly, the associations between bovine muscle fibers and different meat eating qualities such as tenderness, pH decline, and color traits were further presented.

Role of Akirin1 in the regulation of skeletal muscle fiber-type switch

Akirin1 is a highly conserved ubiquitously expressed nuclear protein. Owing to its strong nuclear localization signal and protein-protein interaction properties, Akirin1 has been speculated to regulate transcription of target genes as a cofactor. Previous studies have reported Akirin1 as a downstream target of myostatin, a potent negative regulator of myogenesis. Mice lacking myostatin displayed enhanced Akirin1 gene expression. Further, in vitro evidence has shown Akirin1 overexpression leads to hypertrophy in C₂ C ₁₂ myotubes. In this study, we used Akirin1 knockout mice as a model system to further elucidate the function of Akirin1 in fully differentiated skeletal muscle. Akirin1 knockout mice did not show any obvious phenotypic difference when compared with wild type. However, promoter-reporter assay suggested that Akirin1 regulated the transcription of muscle-specific RING finger 1 (MuRF-1), an important E3 ubiquitin ligase in skeletal muscle. Furthermore, ablation of Akirin1 resulted in increased type IIa and decreased type I muscle fibers, which was further supported by an increase in Myh2 and decrease in Myh7 gene expression. Also, histochemical studies for succinate dehydrogenase activity revealed a less oxidative muscle in the absence of Akirin1. Together, our study suggests a novel role of Akirin1 in maintaining the muscle fiber type and regulation of the metabolic activity of the skeletal muscle.

Overnight Steady-State Infusions of Parenteral Nutrition on Myosin Heavy Chain Transcripts in Rectus Abdominis Muscle Related to Amino Acid Transporters, Insulin-like Growth Factor 1, and Blood Amino Acids in Patients Aimed at Major Surgery

BACKGROUND

Evaluation of improvements by nutrition support to severely ill patients requires sensitive methods to demonstrate activation of protein synthesis in various tissues from groups with a limited number of patients to be statistically efficient. This study examines effects of standard parenteral nutrition (PN) on abdominal muscle transcripts of amino acid (AA) transporters, myosin heavy chains (MHCs), and the insulin-like growth factor 1 and its receptor (IGF-1/IGF-1R) in patients aimed at major surgery.

METHODS

Twenty-two randomized patients received steady-state PN (0.16 gN/kg/d, 30 kcal/kg/d) or saline infusions for 12 hours before operation. Blood samples and muscle biopsies were obtained at operation start. Muscle messenger RNA (mRNA) levels of AA transporters (solute carrier family members SNAT2, LAT1, LAT3, LAT4, TAUT, PAT1, CD98), IGF-1, IGF-1R, MHC isoforms (MHC1, MHC2A, MHC2X), and LAT3 protein were quantified and related to concentrations of AA, IGF-1, insulin, and metabolic substrates in blood.

RESULTS

Muscle mRNA LAT3, LAT4, IGF-1R, and MHC2A increased by PN infusion, with correlations to specific AA transporters and MHC isoforms (P < .01-.05). TAUT and LAT3 correlated to slow (MHC1) and fast (MHC2A, MHC2X) isoforms (P < .001-.02). Muscle IGF-1 mRNA correlated to plasma essential AAs, whereas IGF-1R mRNA was related to LAT3, MHC2A, and serum IGF-1 (P < .001-.03).

CONCLUSIONS

The results confirm that short-term preoperative PN activates transcription of AA transporters and myosin isoforms. Thus, combinations of methods on gene transcription and translation of muscle proteins can be applied to define efficient combinations of nutrition and hormones to catabolic patients in preoperative and postoperative settings.

Muscle fiber type diversity revealed by anti-myosin heavy chain antibodies

Different forms of myosin heavy chains (MyHCs), coded by a large family of sarcomeric MYH genes, are expressed in striated muscles. The generation of specific anti-MyHC antibodies has provided a powerful tool to define the fiber types present in skeletal muscles, their functional properties, their response to conditions that affect muscle plasticity and their changes in muscle disorders. Cardiomyocyte heterogeneity has been revealed by the serendipitous observation that different MyHCs are present in atrial and ventricular myocardium and in heart conduction tissue. Developmental MyHCs present in embryonic and fetal/neonatal skeletal muscle are re-expressed during muscle regeneration and can be used to identify regenerating fibers in muscle diseases. MyHC isoforms provide cell type-specific markers to identify the signaling pathways that control muscle cell identity and are an essential reference to interpret the results of single-cell transcriptomics and proteomics.

Hypertrophic Cardiomyopathy: Genetics, Pathogenesis, Clinical Manifestations, Diagnosis, and Therapy

Hypertrophic cardiomyopathy (HCM) is a genetic disorder that is characterized by left ventricular hypertrophy unexplained by secondary causes and a nondilated left ventricle with preserved or increased ejection fraction. It is commonly asymmetrical with the most severe hypertrophy involving the basal interventricular septum. Left ventricular outflow tract obstruction is present at rest in about one third of the patients and can be provoked in another third. The histological features of HCM include myocyte hypertrophy and disarray, as well as interstitial fibrosis. The hypertrophy is also frequently associated with left ventricular diastolic dysfunction. In the majority of patients, HCM has a relatively benign course. However, HCM is also an important cause of sudden cardiac death, particularly in adolescents and young adults. Nonsustained ventricular tachycardia, syncope, a family history of sudden cardiac death, and severe cardiac hypertrophy are major risk factors for sudden cardiac death. This complication can usually be averted by implantation of a cardioverter-defibrillator in appropriate high-risk patients. Atrial fibrillation is also a common complication and is not well tolerated. Mutations in over a dozen genes encoding sarcomere-associated proteins cause HCM. MYH7 and MYBPC3, encoding β-myosin heavy chain and myosin-binding protein C, respectively, are the 2 most common genes involved, together accounting for ≈50% of the HCM families. In ≈40% of HCM patients, the causal genes remain to be identified. Mutations in genes responsible for storage diseases also cause a phenotype resembling HCM (genocopy or phenocopy). The routine applications of genetic testing and preclinical identification of family members represents an important advance. The genetic discoveries have enhanced understanding of the molecular pathogenesis of HCM and have stimulated efforts designed to identify new therapeutic agents.

Does the injection of platelet-rich plasma induce changes in the gene expression and morphology of intact Thoroughbred skeletal muscle?

Platelet-rich plasma (PRP) therapy is promising for treating skeletal muscle injuries in human athletes by promoting muscle regeneration. It might also be useful for treating muscle injuries in equine athletes. In the present study, muscle regeneration induced by injection of PRP into intact muscle of Thoroughbred was investigated. Autologous PRP and saline were injected twice into intact left and right gluteus medius muscles of seven clinically healthy Thoroughbreds. Muscle samples were collected from the injection sites by needle biopsy at 2 and 7 days after PRP injection. Immunohistochemical staining to identify the types of myosin heavy chains (MHCs) and satellite cells was performed to compare morphological changes among intact (pre-injection), saline-, and PRP-injected muscles. The expression of marker genes related to muscle regeneration (MHC-I, MHC-II, and embryonic MHC [MHC-e]), satellite cell activity (CK, Pax7, MyoD, and myogenin), and proinflammatory and promyogenic cytokines (IL-6, IGF-1, and HGF) was analyzed and compared between saline- and PRP-injected muscles. There were no obvious morphological differences among the three treatments. There were no significant differences in gene expression associated with satellite cell activity between saline and PRP injection at 7 days after injection. MHC genes showed significantly higher expression levels with PRP than with saline, including MHC-e at 2 days and MHC-I at 7 days after injection. It is suggested that injection of PRP into intact skeletal muscle does not induce specific morphological changes, but upregulate the expression of genes related to muscle regeneration.

Polyamines, myosin heavy chains, and collagen specific amino acids after a repeated bout of eccentric exercise

We investigated alternatives to commonly used biomarkers of exercise-induced tissue damage. Over 5 days following two bouts of 100 drop-to-vertical jumps (inter-bout rest period of 3 weeks), myosin heavy chain 1, hydroxylysine (HYL), hydroxyproline (HYP), spermine (SPM) and spermine synthase (SMS) were measured in the serum of 10 participants. HYL significantly increased from 5.92 ± 1.49 ng/mL to 6.48 ± 1.47 ng/mL at 24 h. A similar trend was observed for bout 2, but without reaching significance. SPM significantly increased only after bout 1 from 0.96 ± 0.19 ng/mL at pretest to a peak level of 1.12 ± 0.26 ng/mL at 24 h, while B2 increments remained non-significant. Myosin heavy chain 1, HYP and SMS values remained below the detection limit of the applied enzyme-linked immunosorbent assay (ELISA) kit. Though HYL and SM increased after the intervention, both markers showed a large standard deviation (SD) combined with small increments. Therefore, none of the investigated biomarkers provides a meaningful alternative to commonly used damage markers.

Dynamics of myosin heavy chain isoform transition in the longissimus muscle of domestic and wild pigs during growth: a comparative study

Dynamics of myofiber differentiation/maturation in porcine skeletal muscle is associated with domestication, breeding and rearing conditions. This study was aimed to comparatively elucidate the age-dependent myosin heavy chain (MyHC) isoform expression and transition pattern in domestic and wild pig (WP) skeletal muscle from birth until adulthood. Domestic pigs (DPs) of Large White breed raised in conventional production system were compared with WPs reared in a large hunting enclosure. Muscle samples for immuno/enzyme histochemistry were taken from the longissimus dorsi muscle within 24 h postmortem at 24 to 48 h, 21 to 23 days, 7 months and ~2 years postpartum. Based on the antibody reactivity to MyHCs (NCL-MHCs, A4.74, BF-F3) and succinate dehydrogenase activity, myofibers were classified into I, I/IIa, IIa, IIx and IIb types. In addition, foetal MyHC expression was determined with the use of F158.4C10 antibody. Maturation of the longissimus dorsi muscle in the WP was characterized by an accelerated transformation of the fast to slow MyHC during the first hours postpartum, followed by differentiation towards oxidative myofibers in which type I, IIa and IIx MyHCs predominated. In the DP, the transformation shifted towards glycolytic myofibers that expressed MyHC-IIb. The expression of foetal MyHC was higher in the DP than in the WP at 1 day of age, and the decline in the foetal MyHC during the first 3 weeks was more rapid in the WP than in the DP denoting an accelerated early postnatal muscle maturation in WP than DP piglets. All foetal MyHC-positive myofibers co-expressed IIa isoform, but not vice versa. The intense myofiber hypertrophy was evident from 3 weeks until 7 months of age. In this period, the myofiber cross-sectional area increased up to 10- and 20-fold in the WP and the DP, respectively. In the DP, the hypertrophy of all myofiber types was more pronounced than in the WP, particularly the hypertrophy of IIx and IIb myofibers. To summarize, the comparison between growing DP with wild ancestors showed that genetic selection and rearing conditions lead to substantial changes in the direction and intensity of postnatal MyHC transformation as evidenced by different proportion of individual myofiber types and differences in their hypertrophic potential.

Myosin content of individual human muscle fibers isolated by laser capture microdissection

Muscle fiber composition correlates with insulin resistance, and exercise training can increase slow-twitch (type I) fibers and, thereby, mitigate diabetes risk. Human skeletal muscle is made up of three distinct fiber types, but muscle contains many more isoforms of myosin heavy and light chains, which are coded by 15 and 11 different genes, respectively. Laser capture microdissection techniques allow assessment of mRNA and protein content in individual fibers. We found that specific human fiber types contain different mixtures of myosin heavy and light chains. Fast-twitch (type IIx) fibers consistently contained myosin heavy chains 1, 2, and 4 and myosin light chain 1. Type I fibers always contained myosin heavy chains 6 and 7 (MYH6 and MYH7) and myosin light chain 3 (MYL3), whereas MYH6, MYH7, and MYL3 were nearly absent from type IIx fibers. In contrast to cardiomyocytes, where MYH6 (also known as α-myosin heavy chain) is seen solely in fast-twitch cells, only slow-twitch fibers of skeletal muscle contained MYH6. Classical fast myosin heavy chains (MHC1, MHC2, and MHC4) were present in variable proportions in all fiber types, but significant MYH6 and MYH7 expression indicated slow-twitch phenotype, and the absence of these two isoforms determined a fast-twitch phenotype. The mixed myosin heavy and light chain content of type IIa fibers was consistent with its role as a transition between fast and slow phenotypes. These new observations suggest that the presence or absence of MYH6 and MYH7 proteins dictates the slow- or fast-twitch phenotype in skeletal muscle.

Genotype-phenotype relationships in Freeman-Sheldon syndrome

Distal arthrogryposis (DA) syndromes are a group of disorders characterized by multiple congenital contractures. DA type 2A (DA2A or Freeman-Sheldon syndrome), caused by mutations in MYH3, is typically considered the most severe of the DA syndromes. However, there is wide phenotypic variability among individuals with DA2A. We characterized genotype-phenotype relationships in 46 families with DA2A. MYH3 mutations were found in 43/46 (93%) kindreds, with three mutations (p.T178I, p.R672C, and p.R672H) explaining 39/43 (91%) of cases. Phenotypic severity varied significantly by genotype (P=0.0055). Individuals with p.T178I were the most severely affected with both facial contractures and congenital scoliosis. Classification of individuals with DA2A into phenotypic groups of varying severity should facilitate providing families with more accurate information about natural history and suggests that individuals might benefit from personalized medical management motivated by MYH3 genotype.

Effect of diet-induced obesity and metabolic syndrome on skeletal muscles of Ossabaw miniature swine

Ossabaw swine fed excess kilocalorie diet develop metabolic syndrome (MS) characterized by obesity, hypertension, insulin resistance, and glucose intolerance with/without dyslipidemia. The purpose of this study was to test the hypothesis that MS would have a detrimental effect on skeletal muscle structure and cause changes in the expression of myosin heavy chains (MHCs). Adult male Ossabaw swine were fed for 24 wk high-fructose or high-fat/cholesterol/fructose diets to induce normolipidemic MS (MetS) or dyslipidemic MS (DMetS), respectively, and were compared with the lean swine on control diet. MetS swine showed mild MS, lacking increases in total and low density lipoprotein (LDL) cholesterol, both of which were highly upregulated in DMetS swine. There was an ∼1.2-fold increase in the cross-sectional areas of muscle fibers in MetS and DMetS groups compared with control for biceps femoris and plantaris muscles. In plantaris muscles, DMetS diet caused an ∼2-fold decrease in slow MHC mRNA and protein expression and an ∼1.2- to 1.8-fold increase in the number of intramyocellular lipid (IMCL) droplets without large changes in the size of the droplets. There was a trend to the decrease in slow MHC expression in muscles of swine on MetS diet. The number of IMCL droplets in muscle fibers of the MetS group was comparable to controls. These data correlate well with the data on total plasma cholesterol (control = 60, MetS = 70, and DMetS = 298 mg/dl) and LDL (control = 29, MetS = 30, and DMetS = 232 mg/dl). We conclude that structural changes observed in skeletal muscle of obese Ossabaw swine correlate with those previously reported for obese humans.