Sex- and fiber-type-related contractile properties in human single muscle fiber

This study aimed to examine the distribution and contractile properties of single muscle fiber sex/myosin heavy chain (MHC) type-related differences and to evaluate the correlation of cross-sectional area (CSA) and specific force (SF) in a single muscle fiber. Six young men and six young women were participated in this study. Muscle sample was obtained from vastus lateralis muscle. To examine potential gender differences within each fiber contractile properties (CSA, maximal isometric force, SF, maximal shortening velocity) and relationship between CSA and SF of single fiber using Pearson correlation. After mechanical measurements, single muscle fiber determined MHC isoforms using silver stain. MHC isoform composition did not differ by sex (chi-square=6.978, P=0.073). There were sex-related differences in CSA and maximal isometric force (P<0.05), but no fiber type-related differences (P>0.05). Related to SF and maximal shortening velocity, there were no sex-related differences only fiber type-related differences (P<0.05). However, there were differences in SF between single fiber types in men but not in women. A negative correlation was found between CSA and SF in both men and women (P<0.05). It is suggested that there might be different mechanical properties of cross-bridges according to sex.

Natriuretic peptide signaling is involved in the expression of oxidative metabolism-related and muscle fiber constitutive genes in the gastrocnemius muscle

Natriuretic peptides regulate cyclic guanosine monophosphate (cGMP) levels via their receptors and have various physiological effects. Natriuretic peptide receptor C (NPR-C) increases cGMP signaling by functioning as a clearance receptor. We analyzed the role of natriuretic peptides in the skeletal muscle, which increases in mass with bone elongation, of NPR-C- mice. High-fat diet (HFD)-fed NPR-C- mice exhibited obesity resistance and higher oxygen consumption. PGC1α gene expression was upregulated in the gastrocnemius muscle of HFD-fed NPR-C- mice compared with HFD-fed NPR-C+ (wild-type) mice. Gene expression of proliferator-activated receptor delta and estrogen-related receptor α, which upregulate oxidative metabolism, was increased in the gastrocnemius muscle of NPR-C- mice, irrespective of diet. Expression of myosin heavy chain 7, a component of type I slow-twitch fiber, was enhanced. Natriuretic peptide signaling may influence oxidative metabolism-related and slow-twitch fiber constitutive gene expression in the fast-twitch gastrocnemius muscle but not in slow-twitch muscles such as the soleus.

Phenotypic Drug Screening for Dysferlinopathy Using Patient-Derived Induced Pluripotent Stem Cells

Dysferlinopathy is a progressive muscle disorder that includes limb‐girdle muscular dystrophy type 2B and Miyoshi myopathy (MM). It is caused by mutations in the dysferlin (DYSF) gene, whose function is to reseal the muscular membrane. Treatment with proteasome inhibitor MG‐132 has been shown to increase misfolded dysferlin in fibroblasts, allowing them to recover their membrane resealing function. Here, we developed a screening system based on myocytes from MM patient‐derived induced pluripotent stem cells. According to the screening, nocodazole was found to effectively increase the level of dysferlin in cells, which, in turn, enhanced membrane resealing following injury by laser irradiation. Moreover, the increase was due to microtubule disorganization and involved autophagy rather than the proteasome degradation pathway. These findings suggest that increasing the amount of misfolded dysferlin using small molecules could represent an effective future clinical treatment for dysferlinopathy. stem cells translational medicine2019;8:1017–1029

Obesity and aging affects skeletal muscle renin-angiotensin system and myosin heavy chain proportions in pre-diabetic Zucker rats

There is a gap in the knowledge regarding regulation of local renin-angiotensin system (RAS) in skeletal muscle during development of obesity and insulin resistance in vivo. This study evaluates the obesity- and age-related changes in the expression of local RAS components. Since RAS affects skeletal muscle remodelling, we also evaluated the muscle fibre type composition, defined by myosin heavy chain (MyHC) mRNAs and protein content. Gene expressions were determined by qPCR and/or Western blot analysis in musculus quadriceps of 3- and 8-month-old male obese Zucker rats and their lean controls. The enzymatic activity of aminopeptidase A (APA) was determined flourometrically. Activation of renin receptor (ReR)/promyelocytic leukaemia zinc finger (PLZF) negative feedback mechanism was observed in obesity. The expression of angiotensinogen and AT1 was downregulated by obesity, while neutral endopeptidase and AT2 expressions were upregulated in obese rats with aging. Skeletal muscle APA activity was decreased by obesity, which negatively correlated with the increased plasma APA activity and plasma cholesterol. The expression of angiotensin-converting enzyme (ACE) positively correlated with MyHC mRNAs characteristic for fast-twitch muscle fibres. The obesity- and age-related alterations in the expression of both classical and alternative RAS components suggest an onset of a new equilibrium between ACE/AngII/AT1 and ACE2/Ang1-7/Mas at lower level accompanied by increased renin/ReR/PLZF activation. Increased APA release from the skeletal muscle in obesity might contribute to increased plasma APA activity. There is a link between reduced ACE expression and altered muscle MyHC proportion in obesity and aging.

Differential expression of myosin heavy chain isoforms in cardiac segments of gnathostome vertebrates and its evolutionary implications

Background

Immunohistochemical studies of hearts from the lesser spotted dogfish, Scyliorhinus canicula (Chondrichthyes) revealed that the pan-myosin heavy chain (pan-MyHC) antibody MF20 homogeneously labels all the myocardium, while the pan-MyHC antibody A4.1025 labels the myocardium of the inflow (sinus venosus and atrium) but not the outflow (ventricle and conus arteriosus) cardiac segments, as opposed to other vertebrates. We hypothesized that the conventional pattern of cardiac MyHC isoform distribution present in most vertebrates, i.e. MYH6 in the inflow and MYH7 in the outflow segments, has evolved from a primitive pattern that persists in Chondrichthyes. In order to test this hypothesis, we conducted protein detection techniques to identify the MyHC isoforms expressed in adult dogfish cardiac segments and to assess the pan-MyHC antibodies reactivity against the cardiac segments of representative species from different vertebrate groups.

Results

Western and slot blot results confirmed the specificity of MF20 and A4.1025 for MyHC in dogfish and their differential reactivity against distinct myocardial segments. HPLC-ESI-MS/MS and ESI-Quadrupole-Orbitrap revealed abundance of MYH6 and MYH2 in the inflow and of MYH7 and MYH7B in the outflow segments. Immunoprecipitation showed higher affinity of A4.1025 for MYH2 and MYH6 than for MYH7 and almost no affinity for MYH7B. Immunohistochemistry showed that A4.1025 signals are restricted to the inflow myocardial segments of elasmobranchs, homogeneous in all myocardial segments of teleosts and acipenseriforms, and low in the ventricle of polypteriforms.

Conclusions

The cardiac inflow and outflow segments of the dogfish show predominance of fast- and slow-twitch MyHC isoforms respectively, what can be considered a synapomorphy of gnathostomes. The myocardium of the dogfish contains two isomyosins (MYH2 and MYH7B) not expressed in the adult heart of other vertebrates. We propose that these isomyosins lost their function in cardiac contraction during the evolution of gnathostomes, the later acquiring a regulatory role in myogenesis through its intronic miRNA. Loss of MYH2 and MYH7B expression in the heart possibly occurred before the origin of Osteichthyes, being the latter reacquired in polypteriforms. We raise the hypothesis that the slow tonic MYH7B facilitates the peristaltic contraction of the conus arteriosus of fish with a primitive cardiac anatomical design and of the vertebrate embryo.

Hybrid fiber alterations in exercising seniors suggest contribution to fast-to-slow muscle fiber shift

BACKGROUND

Human skeletal muscle is composed of a functional and metabolic continuum of slow (Type I) and fast fibers (IIa and IIx). Hybrid fibers co-expressing different myosin heavy chains are also present and seem to be more prominent in aging muscle. Their role is debated; hybrid fibers were reported either in a transitional state, between slow and fast fibers, or as fixed individual entities. This study examined the fate of hybrid fibers with an endurance exercise intervention in an elderly sedentary population.

METHODS

Twenty-two sedentary healthy elderly men and women underwent a 16-week supervised endurance exercise intervention. Eighteen endurance-trained age- and gender-matched volunteers served as controls. Fiber type distribution was determined by immunohistochemistry on vastus lateralis muscle biopsies pre-intervention and post-intervention.

RESULTS

A total of 13840 fibers were analyzed. At baseline, a Type II dominant fiber profile was observed compared with the control group, with more Type IIa (P = 0.0301) and Type IIx fibers (P = 0.0328). Hybrid fibers represented almost 5% of total muscle fibers in both groups. There was no significant difference between groups (I-IIa, P = 0.6719 and IIa-IIx, P = 0.0998). Intervention triggered qualitative dynamics towards an increase in Type I, and decrease in Type II fibers, paralleled by an increase in I-IIa hybrids (P = 0.0301).

CONCLUSIONS

The present study is, to our knowledge, the first to examine hybrid muscle fiber type adaptations to an endurance exercise intervention in the elderly. Hybrid fiber proportions did not differ between chronic sedentary state and chronic endurance-trained state. Exercise intervention increased Type I-IIa hybrid fibers along with shift dynamics in other fiber types suggesting the contribution of hybrid fiber to a fast-to-slow fiber type transition, eventually serving as intermediate reservoir from one monomorphic myosin heavy chain expressing fiber type to another. This finding favours the transitional theory regarding hybrid muscle fibers and exercise, crucial to understanding reversible mechanisms of sarcopenia and development of prevention measures.

Myosin heavy chain mutations that cause Freeman-Sheldon syndrome lead to muscle structural and functional defects in Drosophila

Missense mutations in the MYH3 gene encoding myosin heavy chain-embryonic (MyHC-embryonic) have been reported to cause two skeletal muscle contracture syndromes, Freeman Sheldon Syndrome (FSS) and Sheldon Hall Syndrome (SHS). Two residues in MyHC-embryonic that are most frequently mutated, leading to FSS, R672 and T178, are evolutionarily conserved across myosin heavy chains in vertebrates and Drosophila. We generated transgenic Drosophila expressing myosin heavy chain (Mhc) transgenes with the FSS mutations and characterized the effect of their expression on Drosophila muscle structure and function. Our results indicate that expressing these mutant Mhc transgenes lead to structural abnormalities in the muscle, which increase in severity with age and muscle use. We find that flies expressing the FSS mutant Mhc transgenes in the muscle exhibit shortening of the inter-Z disc distance of sarcomeres, reduction in the Z-disc width, aberrant deposition of Z-disc proteins, and muscle fiber splitting. The ATPase activity of the three FSS mutant MHC proteins are reduced compared to wild type MHC, with the most severe reduction observed in the T178I mutation. Structurally, the FSS mutations occur close to the ATP binding pocket, disrupting the ATPase activity of the protein. Functionally, expression of the FSS mutant Mhc transgenes in muscle lead to significantly reduced climbing capability in adult flies. Thus, our findings indicate that the FSS contracture syndrome mutations lead to muscle structural defects and functional deficits in Drosophila, possibly mediated by the reduced ATPase activity of the mutant MHC proteins.

Myricetin improves endurance capacity by inducing muscle fiber type conversion via miR-499

Background

Reprogramming of fast-to-slow myofiber switch can improve endurance capacity and alleviate fatigue. Accumulating evidence suggests that a muscle-specific microRNA, miR-499 plays a crucial role in myofiber type transition. In this study, we assessed the effects of natural flavonoid myricetin on exercise endurance and muscle fiber constitution, and further investigated the underlying mechanism of myricetin in vivo and in vitro.

Methods

A total of 66 six-week-old male Sprague Dawley rats were divided into non-exercise or exercise groups with/without orally administered myricetin (50 or 150 mg/kg) for 2 or 4 weeks. Time-to-exhaustion, blood biochemical parameters, muscle fiber type proportion, the expression of muscle type decision related genes were measured. Mimic/ inhibitor of miR-499 were transfected into cultured L6 myotubes, the expressions of muscle type decision related genes and mitochondrial respiration capacity were investigated.

Results

Myricetin treatment significantly improved the time-to-exhaustion in trained rats. The enhancement of endurance capacity was associated with an increase of the proportion of slow-twitch myofiber in both soleus and gastrocnemius muscles. Importantly, myricetin treatment amplified the expression of miR-499 and suppressed the expression of Sox6, the down-stream target gene of miR-499, both in vivo and in vitro. Furthermore, inhibition of miR-499 overturned the effects of myricetin on down-regulating Sox6.

Conclusions

Myricetin promoted the reprogramming of fast-to-slow muscle fiber type switch and reinforced the exercise endurance capacity. The precise mechanisms responsible for the effects of myricetin are not resolved but likely involve regulating miR-499/Sox6 axis.

AMPK/FOXO1 signaling pathway is indispensable in visfatin-regulated myosin heavy chain expression in C2C12 myotubes

OBJECTIVE

Few studies have addressed the effects of visfatin on skeletal muscle remodeling. The aim of the study was to investigate the effects of visfatin on the expressions of myosin heavy chain (MHC) and its isoforms, the major indicator of fiber types and contractile properties of skeletal muscle.

MATERIALS AND METHODS

Levels of MHC, MHC I, MHC IIa, MHC IIb, adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK), p-AMPK and forkhead box protein O1 (FOXO1) were tested in visfatin-treated C2C12 myotubes. C2C12 myotubes were treated with visfatin combined with AMPK inhibitor or AMPK activator to investigate the role of AMPK in visfatin-mediated MHC expression. FOXO1 was overexpressed or knocked down in C2C12 myotubes to explore the role of FOXO1 in visfatin-mediated MHC expression.

RESULTS

Compared with the vehicle group, treatment with 5 μg/ml visfatin increased the levels of total MHC and its isoforms, MHC I, MHC IIa and MHC IIb, by 1.93, 1.84, 1.80, and 1.92 folds, respectively (all p = 0,001). Visfatin suppressed AMPK phosphorylation and decreased FOXO1 expression in C2C12 myotubes. The effects of visfatin on MHC I and MHC IIa expression were canceled by AMPK activator AICAR. FOXO1 overexpression minimized the visfatin-induced upregulation of MHC I, MHC IIa and MHC IIb. The effect of AMPK activator AICAR on MHC and its isoforms expression was minimized by knockdown of FOXO1.

CONCLUSIONS

The findings revealed that visfatin promoted expressions of MHC and its isoforms in C2C12 myotubes via suppressing AMPK/FOXO1 signaling pathway.

Differential isoform expression of SERCA and myosin heavy chain in hypopharyngeal muscles

Composition of slow, fast and hybrid fibres of pharyngeal muscles, associated with pharyngeal movements and regulation, has been rarely studied. The present study aimed to identify expression of sarcoplasmic reticulum Ca2⁺ ATPase (SERCA) and myosin heavy chain (MHC) and hybrid isoforms in different pharyngeal muscles of young and aged rats as well as humans. Isoform expression profiles of SERCA, MHC and hybrid isoforms among six components of pharyngeal muscle were immunohistochemically evaluated in rat and human. The result showed that pharyngeal muscles predominantly expressed fast fibres (SERCA1 and MHCII), whereas expression of slow fibres (SERCA2 and MHCI) was low, but different depending on muscle components. Inner layer of pharyngeal muscles expressed more SERCA2 and hybrid fibres than the outer layer. Pharyngeal muscles in aged rats showed increased hybrid fibers and SERCA2. Human thyropharyngeus also showed a higher portion of fast fibres compared to cricopharyngeus. Thus, in contrast to abundance of fast fibres, slow and hybrid fibres are differentially expressed depending on muscle components and layers as well as aging. These results lead to further understanding of coordinated regulation for speech and swallowing. The unique data presented in this study on SERCA isoform expressions in both rats and human suggest an ability to handle calcium changes according functional demands.

Tooth extraction and subsequent dental implant placement in Sprague-Dawley rats induce differential changes in anterior digastric myofibre size and myosin heavy chain isoform expression

OBJECTIVE

to determine if tooth loss and dental implant placement in rats induce changes in the morphological and histochemical features of the Anterior Digastric muscle.

DESIGN

Adult male Sprague-Dawley rats had their right maxillary molar teeth extracted. 'Extraction-1' and 'Extraction-2 groups were sacrificed, respectively, 4 or 8 weeks later, and an Implant group had an implant placement 2 weeks after the molar extraction, and rats were sacrificed 3 weeks later (n = 4/group). Naive rats (n = 3) had no treatment. Morphometric and immunohistochemical techniques quantified Anterior Digastric muscle myofibres' cross-sectional area (CSA) and myosin heavy chain (MyHC) isoform proportions. Significant ANOVAs were followed by post-hoc tests; p < 0.05 and 0.1 were considered to reflect levels of statistical significance.

RESULTS

In naïve rats, the peripheral regions of the Anterior Digastric muscle was dominated by MyHC-IIx/b isoform and there were no MyHC-I isoforms; the central regions dominated by MyHC-IIx/b and MyHC-IIa isoforms. Compared with naive rats, tooth extraction produced, 8 (but not 4) weeks later, a decreased proportion of fast-contracting fatigue-resistant MyHC-IIa isoform (p = 0.08), and increased proportion of fast and intermediate fatigue-resistance MyHC-IIa/x/b isoform (p = 0.03). Dental implant placement following tooth extraction attenuated the extraction effects but produced a decreased proportion of fast-contracting fatiguable MyHC-llx/b isoform (p = 0.03) in the peripheral region, and increased inter-animal variability in myofibre-CSAs.

CONCLUSIONS

Given the crucial role that the Anterior Digastric muscle plays in many vital oral functions (e.g., chewing, swallowing), these changes may contribute to the changes in oral sensorimotor functions that occur in humans following such treatments.

Measuring Both Glucose Uptake and Myosin Heavy Chain Isoform Expression in Single Rat Skeletal Muscle Fibers

Glucose uptake by skeletal muscle is important for metabolic health. Because skeletal muscle is composed of multiple fiber types that have differing metabolic and contractile properties, studying glucose uptake in whole muscle tissue does not elucidate differences at the cellular level. Here, we describe a procedure that enables the measurement of both glucose uptake and fiber type (by myosin heavy chain isoform expression) in individual rat epitrochlearis muscle fibers.

Muscle health and performance in monozygotic twins with 30 years of discordant exercise habits

INTRODUCTION

Physical health and function depend upon both genetic inheritance and environmental factors (e.g., exercise training).

PURPOSE

To enhance the understanding of heritability/adaptability, we explored the skeletal muscle health and physiological performance of monozygotic (MZ) twins with > 30 years of chronic endurance training vs. no specific/consistent exercise.

METHODS

One pair of male MZ twins (age = 52 years; Trained Twin, TT; Untrained Twin, UT) underwent analyses of: (1) anthropometric characteristics and blood profiles, (2) markers of cardiovascular and pulmonary health, and (3) skeletal muscle size, strength, and power and molecular markers of muscle health.

RESULTS

This case study represents the most comprehensive physiological comparison of MZ twins with this length and magnitude of differing exercise history. TT exhibited: (1) lower body mass, body fat%, resting heart rate, blood pressure, cholesterol, triglycerides, and plasma glucose, (2) greater relative cycling power, anaerobic endurance, and aerobic capacity (VO₂max), but lower muscle size/strength and poorer muscle quality, (3) more MHC I (slow-twitch) and fewer MHC IIa (fast-twitch) fibers, (4) greater AMPK protein expression, and (5) greater PAX7, IGF1Ec, IGF1Ea, and FN14 mRNA expression than UT.

CONCLUSIONS

Several measured differences are the largest reported between MZ twins (TT expressed 55% more MHC I fibers, 12.4 ml/kg/min greater VO₂max, and 8.6% lower body fat% vs. UT). These data collectively (a) support utilizing chronic endurance training to improve body composition and cardiovascular health and (b) suggest the cardiovascular and skeletal muscle systems exhibit greater plasticity than previously thought, further highlighting the importance of studying MZ twins with large (long-term) differences in exposomes.

The effects of obesity on skeletal muscle contractile function

Obesity can cause a decline in contractile function of skeletal muscle, thereby reducing mobility and promoting obesity-associated health risks. We reviewed the literature to establish the current state-of-knowledge of how obesity affects skeletal muscle contraction and relaxation. At a cellular level, the dominant effects of obesity are disrupted calcium signalling and 5'-adenosine monophosphate-activated protein kinase (AMPK) activity. As a result, there is a shift from slow to fast muscle fibre types. Decreased AMPK activity promotes the class II histone deacetylase (HDAC)-mediated inhibition of the myocyte enhancer factor 2 (MEF2). MEF2 promotes slow fibre type expression, and its activity is stimulated by the calcium-dependent phosphatase calcineurin. Obesity-induced attenuation of calcium signalling via its effects on calcineurin, as well as on adiponectin and actinin affects excitation-contraction coupling and excitation-transcription coupling in the myocyte. These molecular changes affect muscle contractile function and phenotype, and thereby in vivo and in vitro muscle performance. In vivo, obesity can increase the absolute force and power produced by increasing the demand on weight-supporting muscle. However, when normalised to body mass, muscle performance of obese individuals is reduced. Isolated muscle preparations show that obesity often leads to a decrease in force produced per muscle cross-sectional area, and power produced per muscle mass. Obesity and ageing have similar physiological consequences. The synergistic effects of obesity and ageing on muscle function may exacerbate morbidity and mortality. Important future research directions include determining: the relationship between time course of weight gain and changes in muscle function; the relative effects of weight gain and high-fat diet feeding per se; the effects of obesity on muscle function during ageing; and if the effects of obesity on muscle function are reversible.

Age- and sex-related differences in myosin heavy chain isoforms and muscle strength, function, and quality: a cross sectional study

[Purpose]

Declining muscle strength and function are hallmarks of the aging process. This study aimed to determine sex-related differences in myosin heavy chain (MHC) isoforms and muscle mass, strength, and quality with aging.

[Methods]

This cross-sectional study recruited 53 healthy participants (32 men, 21 women) aged 20–85 years who were divided into four groups: young men (n=17, YM, 29.23±4.51), older men (n=15, OM, 71.87±3.42), young women (n=11, YW, 29.64±4.88), and older women (n=10, OW, 68.1±1.91). Body composition and muscle strength and quality were analyzed. Muscle specimens were obtained from the vastus lateralis in all participants to analyze the type of MHC isoforms.

[Results]

Men showed a greater age-related decline in skeletal muscle mass (18.6%, p<0.01), lean body mass (10.1%, p<0.05), grip strength (35.3%, p<0.001), isometric strength (29.6%, p<0.001), isotonic power (42.5%, p<0.001), isokinetic strength (up to 44.3%, p<0.001), and muscle quality (up to 24.8%, p<0.01). In contrast, women had significantly lower isometric strength (24.2%, p<0.05), isotonic power (36.5%, p<0.01), and upper-body muscle quality (24.7%, p<0.001) with aging. In addition, the proportion of MHC IIa was significantly lower in OM (p<0.05) and OW (p<0.05) than in YM and YW, respectively. However, the proportion of MHC I was significantly higher in OM (p<0.01) than in YM but was high in both YW and OW. MHC I and MHC IIa negatively and positively correlated, respectively, with muscle strength and function.

[Conclusion]

These results indicate the existence of sex-related differences in muscle mass, strength, and quality and MHC isoform composition with increasing age. The effects on muscle strength and function with aging were significant in men, but not in women. Higher and lower proportions of MHC I and MHC IIa fibers, respectively, were inversely associated with muscle strength and quality. In particular, Korean YW showed lower muscle strength and quality, and the proportion of MHC isoforms was similar to that in the muscles of OW.

MicroRNA-139-5p suppresses myosin heavy chain I and IIa expression via inhibition of the calcineurin/NFAT signaling pathway

MicroRNAs (miRNAs) are a class of small non-coding RNAs that are widely involved in a variety of biological processes. Different skeletal muscle fiber type composition exhibits characteristic differences in functional properties and energy metabolism of skeletal muscle. However, the molecular mechanism by which miRNAs control the different type of muscle fiber formation is still not fully understood. In the present study, we characterized the role of microRNA-139-5p (miR-139-5p) in the regulation of myosin heavy chain (MyHC) isoform expression and its underlying mechanisms. Here we found that the expression of miR-139-5p was significantly higher in mouse slow-twitch muscle than in fast-twitch muscle. Overexpression of miR-139-5p downregulated the expression of MyHC I and MyHC IIa, whereas inhibition of miR-139-5p upregulated them. We also found that the levels of calcineurin (CaN), NFATc1, MEF2C and MCIP1.4, which are the components of CaN/NFAT signaling pathway that has shown to positively regulate slow fiber-selective gene expression, were notably inhibited by miR-139-5p overexpression. Furthermore, treatment of phenylephrine (PE), a α1-adrenoceptor agonist, abolished the inhibitory effect of miR-139-5p on MyHC I and MyHC IIa expression. Together, our findings indicated that the role of miR-139-5p in regulating the MyHC isoforms, especially MyHC I and MyHC IIa, may be achieved through inhibiting CaN/NFAT signaling pathway.

Quantification of Myosin Heavy Chain Isoform mRNA Transcripts in the Supraspinatus Muscle of Vertical Clinger Primates

Vertical clinging is a specialized form of locomotion characteristic of the primate family Callitrichidae. Vertical clinging requires these pronograde primates to maintain a vertical posture, so the protraction of their forelimbs must resist gravity. Since pronograde primates usually move as horizontal quadrupeds, we hypothesized that the supraspinatus muscle of vertical clingers would present specific characteristics related to the functional requirements imposed on the shoulder area by vertical clinging. To test this hypothesis, we quantified by real-time quantitative polymerase chain reaction the mRNA transcripts of myosin heavy chain (MHC) isoforms in the supraspinatus muscle of 15 species of pronograde primates, including vertical clingers. Our results indicate that the supraspinatus of vertical clingers has a specific expression pattern of the MHC isoforms, with a low expression of the transcripts of the slow MHC-I isoform and a high expression of the transcripts of the fast MHC-II isoforms. We conclude that these differences can be related to the particular functional characteristics of the shoulder in vertical clingers, but also to other anatomical adaptations of these primates, such as their small body size.

20-Hydroxyecdysone attenuates cardiac remodeling in spontaneously hypertensive rats

BACKGROUND

Ecdysteroids, a group of steroid hormones found in insects and many plants, have been shown to prevent various changes in mammalian tissues after female sex hormone deprivation.

PURPOSE

To examine whether an ecdysteroid, 20-hydroxyecdysone (20-HE), exhibits regulatory or protective roles in the cardiovascular system.

STUDY DESIGN/METHOD

Blood pressure and cardiac function were evaluated in spontaneously hypertensive rats (SHR) during and after daily treatment with 20-HE for six weeks.

RESULTS

The progressive increase in systolic blood pressure with age in SHR rats was significantly lower in animals treated with either 5 or 10mg/kg body weight of 20-HE. However, treatment with 20-HE did not diminish the increase in diastolic pressure. Echocardiography after six weeks of treatment demonstrated that the left ventricular chamber of SHR rats treated with 20-HE was smaller than that of SHR controls, while contractility was not affected by 20-HE. Histological images also demonstrated a decrease in cardiomyocyte cross-sectional area in 20-HE treated groups. Interestingly, treatment with 20-HE caused a shift in cardiac myosin heavy chain towards more β-isoforms. SHR rats treated with 20-HE also exhibited a decrease in seminal vesicular weight and an increase in testicular weight, especially at a dose of 10mg/kg body weight. This finding suggests a possible anti-androgenic effect of 20-HE.

CONCLUSION

Our finding reveal that 20-HE has a beneficial effect on reducing blood pressure and consequently preventing dilated cardiac hypertrophy in SHR rats.

Comparison of myosin heavy chain content determined by label-free quantification between porcine longissimus thoracis, psoas major and semimembranosus muscles

The present study was conducted to quantify MHCs in porcine longissimus thoracis (LT), psoas major (PM) and semimembranosus (SM) muscles through the label-free quantification (LFQ). To estimate the accuracy of LFQ, quantitative RT-PCR (qRT-PCR), immunohistochemistry (IHC), and Western-blotting (WB) were performed. MHCs 2x, 2a, 2b and slow were identified by LC-MS/MS analysis and 279 ion spectra were selected for LFQ analysis. The content of MHC 2b was higher in LT and in SM than in PM (p<0.05), while the content of MHC slow was highest in PM among the muscles (p<0.01) regardless of LFQ types. Positive correlation coefficients of MHC 2b and MHC slow between LFQ and IHC (relative area composition) and qRT-PCR results partially supported the LFQ results. Though low-abundant peptides should be considered to estimate MHC contents via the spectral count method, LFQ enables the determination of MHC contents at protein level regardless of LFQ types.

Novel large-particle FACS purification of adult ventricular myocytes reveals accumulation of myosin and actin disproportionate to cell size and proteome in normal post-weaning development

RATIONALE

Quantifying cellular proteins in ventricular myocytes (MCs) is challenging due to tissue heterogeneity and the variety of cell sizes in the heart. In post-weaning cardiac ontogeny, rod-shaped MCs make up the majority of the cardiac mass while remaining a minority of cardiac cells in number. Current biochemical analyses of cardiac proteins do not correlate well the content of MC-specific proteins to cell type or size in normally developing tissue.

OBJECTIVE

To develop a new large-particle fluorescent-activated cell sorting (LP-FACS) strategy for the purification of adult rod-shaped MCs. This approach is developed to enable growth-scaled measurements per-cell of the MC proteome and sarcomeric proteins (i.e. myosin heavy chain (MyHC) and alpha-actin (α-actin)) content.

METHODS AND RESULTS

Individual cardiac cells were isolated from 21 to 94days old mice. An LP-FACS jet-in-air system with a 200-μm nozzle was defined for the first time to purify adult MCs. Cell-type specific immunophenotyping and sorting yielded ≥95% purity of adult MCs independently of cell morphology and size. This approach excluded other cell types and tissue contaminants from further analysis. MC proteome, MyHC and α-actin proteins were measured in linear biochemical assays normalized to cell numbers. Using the allometric coefficient α, we scaled the MC-specific rate of protein accumulation to growth post-weaning. MC-specific volumes (α=1.02) and global protein accumulation (α=0.94) were proportional (i.e. isometric) to body mass. In contrast, MyHC and α-actin accumulated at a much greater rate (i.e. hyperallometric) than body mass (α=1.79 and 2.19 respectively) and MC volumes (α=1.76 and 1.45 respectively).

CONCLUSION

Changes in MC proteome and cell volumes measured in LP-FACS purified MCs are proportional to body mass post-weaning. Oppositely, MyHC and α-actin are concentrated more rapidly than what would be expected from MC proteome accumulation, cell enlargement, or animal growth alone. LP-FACS provides a new standard for adult MC purification and an approach to scale the biochemical content of specific proteins or group of proteins per cell in enlarging MCs.

Congenital myopathy due to myosin heavy chain 2 mutation presenting as chronic aspiration pneumonia in infancy

A 7-week-old infant presented with persistent noisy breathing and aspirations during swallowing. Neurological examination and brain MRI were normal. His 12-year-old brother underwent pneumonectomy at the age of 10 years due to recurrent aspirations leading to severe lung damage. The older brother developed subsequently ophthalmoplegia and nystagmus along with mild weakness of the neck flexors and proximal muscles. Exome analysis revealed homozygosity for a novel truncating mutation p.G800fs27* in the Myosin Heavy Chain 2 (MYH2) gene in both brothers, while parents and an unaffected sibling were heterozygous. A muscle biopsy from the older brother showed absence of type-2 muscle fibers and predominance of type-1 fibers. The aspirations causing pneumonia likely result from weakness of the laryngeal muscles, normally rich in type-2 fibers. The findings expand the phenotypic spectrum of MYH2 deficiency. MYH2 mutations should be included in the differential diagnosis of infants presenting with recurrent aspirations.

Comparative physiology of vocal musculature in two odontocetes, the bottlenose dolphin (Tursiops truncatus) and the harbor porpoise (Phocoena phocoena)

The mechanism by which odontocetes produce sound is unique among mammals. To gain insight into the physiological properties that support sound production in toothed whales, we examined myoglobin content ([Mb]), non-bicarbonate buffering capacity (β), fiber-type profiles, and myosin heavy chain expression of vocal musculature in two odontocetes: the bottlenose dolphin (Tursiops truncatus; n = 4) and the harbor porpoise (Phocoena phocoena; n = 5). Both species use the same anatomical structures to produce sound, but differ markedly in their vocal repertoires. Tursiops produce both broadband clicks and tonal whistles, while Phocoena only produce higher frequency clicks. Specific muscles examined in this study included: (1) the nasal musculature around the phonic lips on the right (RNM) and left (LNM) sides of the head, (2) the palatopharyngeal sphincter (PPS), which surrounds the larynx and aids in pressurizing cranial air spaces, and (3) the genioglossus complex (GGC), a group of muscles positioned ventrally within the head. Overall, vocal muscles had significantly lower [Mb] and β than locomotor muscles from the same species. The PPS was predominately composed of small diameter slow-twitch fibers. Fiber-type and myosin heavy chain analyses revealed that the GGC was comprised largely of fast-twitch fibers (Tursiops: 88.6%, Phocoena: 79.7%) and had the highest β of all vocal muscles. Notably, there was a significant difference in [Mb] between the RNM and LNM in Tursiops, but not Phocoena. Our results reveal shared physiological characteristics of individual vocal muscles across species that enhance our understanding of key functional roles, as well as species-specific differences which appear to reflect differences in vocal capacities.

The effect of caloric restriction on the forelimb skeletal muscle fibers of the hypertrophic myostatin null mice

Skeletal muscle mass loss has a broad impact on body performance and physical activity. Muscle wasting occurs due to genetic mutation as in muscular dystrophy, age-related muscle loss (sarcopenia) as well as in chronic wasting disorders as in cancer cachexia. Food restriction reduces muscle mass underpinned by increased muscle protein break down. However the influence of dietary restriction on the morphometry and phenotype of forelimb muscles in a genetically modified myostatin null mice are not fully characterized. The effect of a five week dietary limitation on five anatomically and structurally different forelimb muscles was examined. C57/BL6 wild type (Mstn^+/+) and myostatin null (Mstn^-/-) mice were either given a standard rodent normal daily diet ad libitum (ND) or 60% food restriction (FR) for a 5 week period. M. triceps brachii Caput laterale (T.lateral), M. triceps brachii Caput longum (T.long), M. triceps brachii Caput mediale (T.medial), M. extensor carpi ulnaris (ECU) and M. flexor carpi ulnaris (FCU) were dissected, weighted and processed for immunohistochemistry. Muscle mass, fibers cross sectional areas (CSA) and myosin heavy chain types IIB, IIX, IIA and type I were analyzed. We provide evidence that caloric restriction results in muscle specific weight reduction with the fast myofibers being more prone to atrophy. We show that slow fibers are less liable to dietary restriction induced muscle atrophy. The effect of dietary restriction was more pronounced in Mstn^-/- muscles to implicate the oxidative fibers compared to Mstn^+/+. Furthermore, peripherally located myofibers are more susceptible to dietary induced reduction compared to deep fibers. We additionally report that dietary restriction alters the glycolytic phenotype of the Mstn^-/- into the oxidative form in a muscle dependent manner. In summary our study shows that calorie restriction alters muscle fiber profile of forelimb muscles of Myostatin null mice.

A Drosophila model of dominant inclusion body myopathy type 3 shows diminished myosin kinetics that reduce muscle power and yield myofibrillar defects

Individuals with inclusion body myopathy type 3 (IBM3) display congenital joint contractures with early-onset muscle weakness that becomes more severe in adulthood. The disease arises from an autosomal dominant point mutation causing an E706K substitution in myosin heavy chain type IIa. We have previously expressed the corresponding myosin mutation (E701K) in homozygous Drosophila indirect flight muscles and recapitulated the myofibrillar degeneration and inclusion bodies observed in the human disease. We have also found that purified E701K myosin has dramatically reduced actin-sliding velocity and ATPase levels. Since IBM3 is a dominant condition, we now examine the disease state in heterozygote Drosophila in order to gain a mechanistic understanding of E701K pathogenicity. Myosin ATPase activities in heterozygotes suggest that approximately equimolar levels of myosin accumulate from each allele. In vitro actin sliding velocity rates for myosin isolated from the heterozygotes were lower than the control, but higher than for the pure mutant isoform. Although sarcomeric ultrastructure was nearly wild type in young adults, mechanical analysis of skinned indirect flight muscle fibers revealed a 59% decrease in maximum oscillatory power generation and an approximately 20% reduction in the frequency at which maximum power was produced. Rate constant analyses suggest a decrease in the rate of myosin attachment to actin, with myosin spending decreased time in the strongly bound state. These mechanical alterations result in a one-third decrease in wing beat frequency and marginal flight ability. With aging, muscle ultrastructure and function progressively declined. Aged myofibrils showed Z-line streaming, consistent with the human heterozygote phenotype. Based upon the mechanical studies, we hypothesize that the mutation decreases the probability of the power stroke occurring and/or alters the degree of movement of the myosin lever arm, resulting in decreased in vitro motility, reduced muscle power output and focal myofibrillar disorganization similar to that seen in individuals with IBM3.

Summary: Reduced muscle power output and progressive myofibrillar defects in a Drosophila model of inclusion body myopathy 3 arise from the decreased rate of weak to strong actin-binding transition of myosin.

Agonist muscle adaptation accompanied by antagonist muscle atrophy in the hindlimb of mice following stretch-shortening contraction training

Background

The vast majority of dynamometer-based animal models for investigation of the response to chronic muscle contraction exposure has been limited to analysis of isometric, lengthening, or shortening contractions in isolation. An exception to this has been the utilization of a rat model to study stretch-shortening contractions (SSCs), a sequence of consecutive isometric, lengthening, and shortening contractions common during daily activity and resistance-type exercise. However, the availability of diverse genetic strains of rats is limited. Therefore, the purpose of the present study was to develop a dynamometer-based SSC training protocol to induce increased muscle mass and performance in plantarflexor muscles of mice.

Methods

Young (3 months old) C57BL/6 mice were subjected to 1 month of plantarflexion SSC training. Hindlimb muscles were analyzed for muscle mass, quantitative morphology, myogenesis/myopathy relevant gene expression, and fiber type distribution.

Results

The main aim of the research was achieved when training induced a 2-fold increase in plantarflexion peak torque output and a 19% increase in muscle mass for the agonist plantaris (PLT) muscle. In establishing this model, several outcomes emerged which raised the value of the model past that of being a mere recapitulation of the rat model. An increase in the number of muscle fibers per transverse muscle section accounted for the PLT muscle mass gain while the antagonist tibialis anterior (TA) muscle atrophied by 30% with preferential atrophy of type IIb and IIx fibers. These alterations were accompanied by distinct gene expression profiles.

Conclusions

The findings confirm the development of a stretch-shortening contraction training model for the PLT muscle of mice and demonstrate that increased cross-sectional fiber number can occur following high-intensity SSC training. Furthermore, the TA muscle atrophy provides direct evidence for the concept of muscle imbalance in phasic non-weight bearing muscles, a concept largely characterized based on clinical observation of patients. The susceptibility to this imbalance is demonstrated to be selective for the type IIb and IIx muscle fiber types. Overall, the study highlights the importance of considering muscle fiber number modulation and the effect of training on surrounding muscles in exercise comprised of SSCs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-017-1397-4) contains supplementary material, which is available to authorized users.