Single Residue Variation in Skeletal Muscle Myosin Enables Direct and Selective Drug Targeting for Spasticity and Muscle Stiffness.

Muscle spasticity after nervous system injuries and painful low back spasm affect more than 10% of global population. Current medications are of limited efficacy and cause neurological and cardiovascular side effects because they target upstream regulators of muscle contraction. Direct myosin inhibition could provide optimal muscle relaxation; however, targeting skeletal myosin is particularly challenging because of its similarity to the cardiac isoform. We identified a key residue difference between these myosin isoforms, located in the communication center of the functional regions, which allowed us to design a selective inhibitor, MPH-220. Mutagenic analysis and the atomic structure of MPH-220-bound skeletal muscle myosin confirmed the mechanism of specificity. Targeting skeletal muscle myosin by MPH-220 enabled muscle relaxation, in human and model systems, without cardiovascular side effects and improved spastic gait disorders after brain injury in a disease model. MPH-220 provides a potential nervous-system-independent option to treat spasticity and muscle stiffness.

X-ray Crystallographic and Molecular Dynamic Analyses of Drosophila melanogaster Embryonic Muscle Myosin Define Domains Responsible for Isoform-Specific Properties.

Drosophila melanogaster is a powerful system for characterizing alternative myosin isoforms and modeling muscle diseases, but high-resolution structures of fruit fly contractile proteins have not been determined. Here we report the first x-ray crystal structure of an insect myosin: the D melanogaster skeletal muscle myosin II embryonic isoform (EMB). Using our system for recombinant expression of myosin heavy chain (MHC) proteins in whole transgenic flies, we prepared and crystallized stable proteolytic S1-like fragments containing the entire EMB motor domain bound to an essential light chain. We solved the x-ray crystal structure by molecular replacement and refined the resulting model against diffraction data to 2.2 Å resolution. The protein is captured in two slightly different renditions of the rigor-like conformation with a citrate of crystallization at the nucleotide binding site and exhibits structural features common to myosins of diverse classes from all kingdoms of life. All atom molecular dynamics simulations on EMB in its nucleotide-free state and a derivative homology model containing 61 amino acid substitutions unique to the indirect flight muscle isoform (IFI) suggest that differences in the identity of residues within the relay and the converter that are encoded for by MHC alternative exons 9 and 11, respectively, directly contribute to increased mobility of these regions in IFI relative to EMB. This suggests the possibility that alternative folding or conformational stability within these regions contribute to the observed functional differences in Drosophila EMB and IFI myosins.

Knockdown of fast skeletal myosin-binding protein C in zebrafish results in a severe skeletal myopathy.

Myosin-binding protein C (MyBPC) in the muscle sarcomere interacts with several contractile and structural proteins. Mutations in the cardiac isoform (MyBPC-3) in humans, or animal knockout, are associated with cardiomyopathy. Function of the fast skeletal isoform (MyBPC-2) in living muscles is less understood. This question was addressed using zebrafish models, combining gene expression data with functional analysis of contractility and small-angle x-ray diffraction measurements of filament structure. Fast skeletal MyBPC-2B, the major isoform, was knocked down by >50% using morpholino antisense nucleotides. These morphants exhibited a skeletal myopathy with elevated apoptosis and up-regulation of factors associated with muscle protein degradation. Morphant muscles had shorter sarcomeres with a broader length distribution, shorter actin filaments, and a wider interfilament spacing compared with controls, suggesting that fast skeletal MyBPC has a role in sarcomere assembly. Active force was reduced more than expected from the decrease in muscle size, suggesting that MyBPC-2 is required for optimal force generation at the cross-bridge level. The maximal shortening velocity was significantly increased in the MyBPC-2 morphants, but when related to the sarcomere length, the difference was smaller, reflecting that the decrease in MyBPC-2B content and the resulting myopathy were accompanied by only a minor influence on filament shortening kinetics. In the controls, equatorial patterns from small-angle x-ray scattering revealed that comparatively few cross-bridges are attached (as evaluated by the intensity ratio of the 11 and 10 equatorial reflections) during active contraction. X-ray scattering data from relaxed and contracting morphants were not significantly different from those in controls. However, the increase in the 11:10 intensity ratio in rigor was lower compared with that in controls, possibly reflecting effects of MyBPC on the cross-bridge interactions. In conclusion, lack of MyBPC-2 results in a severe skeletal myopathy with structural changes and muscle weakness.

The Relay/Converter Interface Influences Hydrolysis of ATP by Skeletal Muscle Myosin II.

The interface between relay and converter domain of muscle myosin is critical for optimal myosin performance. Using Drosophila melanogaster indirect flight muscle S1, we performed a kinetic analysis of the effect of mutations in the converter and relay domain. Introduction of a mutation (R759E) in the converter domain inhibits the steady-state ATPase of myosin S1, whereas an additional mutation in the relay domain (N509K) is able to restore the ATPase toward wild-type values. The R759E S1 construct showed little effect on most steps of the actomyosin ATPase cycle. The exception was a 25-30% reduction in the rate constant of the hydrolysis step, the step coupled to the cross-bridge recovery stroke that involves a change in conformation at the relay/converter domain interface. Significantly, the double mutant restored the hydrolysis step to values similar to the wild-type myosin. Modeling the relay/converter interface suggests a possible interaction between converter residue 759 and relay residue 509 in the actin-detached conformation, which is lost in R759E but is restored in N509K/R759E. This detailed kinetic analysis of Drosophila myosin carrying the R759E mutation shows that the interface between the relay loop and converter domain is important for fine-tuning myosin kinetics, in particular ATP binding and hydrolysis.

Dietary resveratrol supplementation improves meat quality of finishing pigs through changing muscle fiber characteristics and antioxidative status.

This study investigated the effects of resveratrol (0, 300, 600 mg/kg) on meat quality, muscle fiber characteristics and antioxidative capacity of finishing pigs. The results showed that resveratrol not only increased m. longissimus dorsi (LM) pH(24 h), a*, crude protein and myoglobin content but also decreased L*(24 h), shear force, drip loss, glycolytic potential, as well as backfat depth, LM lactate dehydrogenase activity and mRNA level. Meanwhile, LM total antioxidative capacity, glutathione peroxidase activity and its mRNA level were increased by resveratrol, while malonaldehyde content was decreased. In addition, resveratrol increased myosin heavy chain (MyHC)IIa mRNA level and decreased MyHCIIb mRNA level, along with decreased myofiber cross-sectional area. In conclusion, these results suggest that resveratrol is an effective feed additive to improve pork quality, and the underlying mechanism may be partly due to the changed muscle fiber characteristics and antioxidative capacity induced by resveratrol.

The superfast human extraocular myosin is kinetically distinct from the fast skeletal IIa, IIb, and IId isoforms.

Humans express five distinct myosin isoforms in the sarcomeres of adult striated muscle (fast IIa, IId, the slow/cardiac isoform I/ß, the cardiac specific isoform α, and the specialized extraocular muscle isoform). An additional isoform, IIb, is present in the genome but is not normally expressed in healthy human muscles. Muscle fibers expressing each isoform have distinct characteristics including shortening velocity. Defining the properties of the isoforms in detail has been limited by the availability of pure samples of the individual proteins. Here we study purified recombinant human myosin motor domains expressed in mouse C2C12 muscle cells. The results of kinetic analysis show that among the closely related adult skeletal isoforms, the affinity of ADP for actin·myosin (K(AD)) is the characteristic that most readily distinguishes the isoforms. The three fast muscle myosins have K(AD) values of 118, 80, and 55 µM for IId, IIa, and IIb, respectively, which follows the speed in motility assays from fastest to slowest. Extraocular muscle is unusually fast with a far weaker K(AD) = 352 µM. Sequence comparisons and homology modeling of the structures identify a few key areas of sequence that may define the differences between the isoforms, including a region of the upper 50-kDa domain important in signaling between the nucleotide pocket and the actin-binding site.

CL316,243, a selective ß3-adrenoceptor agonist, activates protein translation through mTOR/p70S6K signaling pathway in rat skeletal muscle cells.

Functional ß3-adrenoceptors have been found in skeletal muscle where they mediate metabolic oxidation and glucose utilization. Whether ß3-adrenoceptors (ARs) also play any role in muscle protein metabolism still remains uncertain. By using rat L6 myocyte cultures, we found that CL316,243, a ß3-AR selective agonist, at the concentration of 10(-6) M for 24 h, induced a significant increase of skeletal muscle constitutive proteins such as H- and L-myosin and ß-actin. Such effect was correlated to an increased expression of phosphorylated p70(S6K) that was significantly inhibited by ß3-AR antagonist, SR 59230A, but not by ß2-AR antagonist, ICI-118,551. The CL316,243-induced activation of p70(S6K) was markedly inhibited by wortmannin, a PI3K inhibitor, and rapamycin, a specific inhibitor of mTOR, suggesting a critical involvement of the PI3K-mTOR-p70(S6K) signaling cascade in the anabolic response of L6 cells to ß3-AR agonist. Taken together, these results suggest that stimulation of ß3-AR in skeletal muscle cells activates a specific signaling pathway leading to protein synthesis and, eventually, muscle growth.

Aspects of molecular mechanisms in myocardial hypertrophy, particular morphological changes and cell bioenergetic characteristics in patients with dilated cardiomyopathy.

AIM: To present the particular morphological aspects of hypertrophied myocardium by determining protein expression profile, the role of creatine kinase in cardiac cell function and mitochondrial respiration using endomyocardial biopsy samples obtained at cardiac catheterization in patients with dilated cardiomyopathy (DCM). MATERIAL AND METHODS: The study included 43 DCM patients (37 men and 6 women, mean age 34 +/- 8.6 years). Protein content in myocardial samples was determined by quantitative immunoblotting and electrophoresis, morphometric indices were evaluated with a JEM-100C electron microscope, and mitochondrial respiratory parameters were determined with a Clark electrode in an oxigrtaphic cell. RESULTS: The study of the expression of hypertrophic protein markers showed increases in the amount of tubulin on average by 42%,desmin by 33%, and skeletal myosin beta-isoform by 20%, and of smooth muscle proteins--alpha-smooth muscle actin and SM22 on average by 1.5 times. In all DCM patients C-reactive protein (CRP) level was on average 52% and that of myosin light-chain kinase (MLCK) 70% higher. The morphometric study showed variable characteristics of cardiomyocytes in the entire patient group, particularly bizarre and oversized nuclei, determining us to divide the patients into two groups. The electrophoresis analysis of total creatine kinase fraction revealed significant alterations in the expression of creatine kinase isoenzymes. Mitochondrial respiration rate after addition of creatine was very low. CONCLUSIONS: We found a high content of myosin light-chain kinase and CRP in the myocardium of patients with DCM, proving their role in hypertrophy; and this can be used in the differential diagnosis of myocardial hypertrophy as a consequence of cardiomyocyte hypertrophy and a result of the fibrotic process. Energy disturbances are significantly more pronounced in patients with DCM, with a more significant cardiomyocyte hypertrophy and an obvious increase in the size of nuclei.

Histological changes and changes in the myosin mRNA content of the porcine masticatory muscles after masseter treatment with botulinum toxin A.

OBJECTIVES: Botulinum toxin A (Botox) is increasingly used for treatment of muscle hyperfunction. For a better understanding of the possible morphologic and chewing changes in patients induced by a therapy with Botox, muscle fiber and myosin heavy chain (MyHC) mRNA alterations were examined in this animal study. MATERIALS AND METHODS: The investigation was carried out on 14-week-old pigs (seven treated animals, eight controls; calculated animal size with a power of 0.5). To initialise the total immobilisation of the right masseter, the Botox injection was distributed into ten areas. After a 56-day period, muscle tissue was taken from the left and right side of the masseter (three regions), temporal (two regions), medial pterygoid and geniohyoid muscles using a standardized method. The muscle fiber cross sections were examined immunohistochemically. Fiber staining was accomplished with antibodies to specific MyHC isoforms. The MyHC mRNA changes were analysed using real-time RT-PCR. RESULTS: Muscles adapt to such stress by changing fiber types and MyHC mRNA content. Paralysed masseters display atrophic changes while other masticatory muscles show hypertrophic changes. The results indicated that the typical distributions of type IIa und IIb fiber types in masticatory muscles were increased in the masseter muscles due to Botox application. On the other hand, the masseters without Botox in the treated group showed a significant increase of type I MyHC. CONCLUSIONS: Application of Botox may lead to uncontrolled structural changes in affected and unaffected muscles. CLINICAL RELEVANCE: Treatment of muscle hypertrophy with Botox may cause muscle imbalance.

Feed restriction delays developmental fast skeletal muscle myosin heavy chain isoforms in turkey poults selected for differential growth.

Genetic selection has been very successful at significantly increasing BW and breast muscle proportion in commercial broiler and turkey strains. The mechanisms of breast muscle growth in poultry and the interactive effects of nutritional status and selection are not fully understood. The hypothesis underlying the current study is that feed restriction, simply as a vehicle for controlling early growth, would delay the temporal expression pattern of neonatal (nMyHC) and adult (aMyHC) fast skeletal muscle myosin heavy chain (MyHC) isoforms in the pectoralis major muscle of turkey poults. The poultry growth model used to evaluate this hypothesis consisted of a randombred control turkey line (RBC2) that represents commercial turkeys of the 1960s and a line developed from the RBC2 by selection for BW at 16 wk of age (F line). The F line has significantly heavier breast muscles than the RBC2 concomitant with increased BW, but the proportion of breast muscle relative to BW is similar. A quantitative indirect ELISA using fast skeletal MyHC isoform specific monoclonal antibodies revealed no significant line differences in the temporal expression of posthatch fast skeletal muscle MyHC in ad libitum fed poults. Feed restriction, however, altered the temporal expression patterns of nMyHC and aMyHC in both F line and RBC2 poults compared with the poults fed ad libitum.

Transcriptome signature of resistance exercise adaptations: mixed muscle and fiber type specific profiles in young and old adults.

This investigation examined the effects of acute resistance exercise (RE), progressive resistance training (PRT), and age on the human skeletal muscle Transcriptome. Two cohorts of young and old adults [study A: 24 yr, 84 yr (n = 28); study B: 25 yr, 78 yr (n = 36)] were studied. Vastus lateralis biopsies were obtained pre- and 4 h post-RE in conjunction with the 1st and 36th (last) training session as part of a 12-wk PRT program in study A, whereas biopsies were obtained in the basal untrained state in study B. Additionally, the muscle fiber type specific (MHC I and MHC IIa) Transcriptome response to RE was examined in a subset of young and old women from study A. Transcriptome profiling was performed using HG U133 Plus 2.0 Arrays. The main findings were 1) there were 661 genes affected by RE during the 1st and 36th training bout that correlated with gains in muscle size and strength with PRT (termed the Transcriptome signature of resistance exercise adaptations); 2) the RE gene response was most pronounced in fast-twitch (MHC IIa) muscle fibers and provided additional insight into the skeletal muscle biology affected by RE; 3) skeletal muscle of young adults is more responsive to RE at the gene level compared with old adults and age also affected basal level skeletal muscle gene expression. These skeletal muscle Transcriptome findings provide further insight into the molecular basis of sarcopenia and the impact of resistance exercise at the mixed muscle and fiber type specific level.

Preferential skeletal muscle myosin loss in response to mechanical silencing in a novel rat intensive care unit model: underlying mechanisms.

The muscle wasting and impaired muscle function in critically ill intensive care unit (ICU) patients delay recovery from the primary disease, and have debilitating consequences that can persist for years after hospital discharge. It is likely that, in addition to pernicious effects of the primary disease, the basic life support procedures of long-term ICU treatment contribute directly to the progressive impairment of muscle function. This study aims at improving our understanding of the mechanisms underlying muscle wasting in ICU patients by using a unique experimental rat ICU model where animals are mechanically ventilated, sedated and pharmacologically paralysed for duration varying between 6 h and 14 days. Results show that the ICU intervention induces a phenotype resembling the severe muscle wasting and paralysis associated with the acute quadriplegic myopathy (AQM) observed in ICU patients, i.e. a preferential loss of myosin, transcriptional down-regulation of myosin synthesis, muscle atrophy and a dramatic decrease in muscle fibre force generation capacity. Detailed analyses of protein degradation pathways show that the ubiquitin proteasome pathway is highly involved in this process. A sequential change in localisation of muscle-specific RING finger proteins 1/2 (MuRF1/2) observed during the experimental period is suggested to play an instrumental role in both transcriptional regulation and protein degradation. We propose that, for those critically ill patients who develop AQM, complete mechanical silencing, due to pharmacological paralysis or sedation, is a critical factor underlying the preferential loss of the molecular motor protein myosin that leads to impaired muscle function or persisting paralysis.

Differential effects of maternal nutrient restriction through pregnancy on kidney development and later blood pressure control in the resulting offspring.

The mechanisms whereby maternal nutritional manipulation through pregnancy result in altered blood pressure in the offspring may include changes in fetal and newborn and adult renal prostaglandin (PG) synthesis, metabolism, and receptor expression. Since the postnatal effects of nutrient restriction on the renal PG synthesis and receptor system during nephrogenesis in conjunction with nephron numbers and blood pressure have not been evaluated in the rat, the present study examined the effect of reducing maternal food intake by 50% of ad libitum through pregnancy on young male rats. Six control-fed mothers and eight nutrient-restricted pregnant rats with single litter mates were used at each sampling time point, most of which occurred during nephrogenesis. Offspring of nutrient-restricted dams were lighter from birth to 3 days. This was accompanied by reduced PGE2, with smaller kidneys up to 14 days. Nutrient restriction also decreased mRNA expression of the PG synthesis enzyme, had little effect on the PG receptors, and increased mRNA expression of the degradation enzyme during nephrogenesis and the glucocorticoid receptor in the adult kidney. These mRNA changes were normally accompanied by similar changes in protein. Nephron number was also reduced from 7 days up to adulthood when blood pressure (measured by telemetry) did not increase as much as in control offspring during the dark, active period. In conclusion, maternal nutrient restriction suppressed renal PG concentrations in the offspring, and this was associated with suppressed kidney growth and development and decreased blood pressure.

IIx myosin heavy chain promoter regulation cannot be characterized in vivo by direct gene transfer.

In skeletal muscle of the adult mammal IIx is a pivotal myosin heavy chain (MHC) isoform that can be either up- or downregulated depending on both the fiber type of the target muscle and the type of external stimulus imposed. Since little is known about promoter elements of the IIx MHC gene that are important for its transcriptional regulation in vivo,the main goal of this study was to characterize IIx MHC promoter activity and identify potential regulatory elements on the IIx MHC promoter. A direct gene transfer approach was used, and this approach involved transfection of promoter-reporter constructs into intact rat soleus and plantaris muscle under control and denervated conditions, as well as hindlimb suspension (i.e., models to upregulate IIx MHC transcription). Fast-twitch (plantaris) muscle fibers were confirmed to have significantly greater IIx MHC transcriptional products (pre-mRNA and mRNA) than slow-twitch (soleus) muscle fibers. However, promoter sequences corresponding to -2671 to +1720, -1000 to +392, and -605/+392 relative to the IIx MHC transcription start site, plus an additional construct ligated to a putative embryonic MHC enhancer, failed to produce a fiber type-specific response that is characteristic of the endogenous IIx MHC promoter. Furthermore, the activity of these promoter constructs did not demonstrate the expected response to denervation or hindlimb suspension (i.e., marked upregulation), despite normal uptake and activity of a coinjected alpha-actin reference promoter. On the basis of these findings with IIx MHC promoter-reporters we conclude that the loss of the native chromatin environment as well as other necessary cis elements may preclude use of the gene transfer approach, thereby suggesting that there are hidden layers of regulation for the IIx MHC gene.

Fiber types in canine muscles: myosin isoform expression and functional characterization.

This study was aimed to achieve a definitive and unambiguous identification of fiber types in canine skeletal muscles and of myosin isoforms that are expressed therein. Correspondence of canine myosin isoforms with orthologs in other species as assessed by base sequence comparison was the basis for primer preparation and for expression analysis with RT-PCR. Expression was confirmed at protein level with histochemistry, immunohistochemistry, and SDS-PAGE combined together and showed that limb and trunk muscles of the dog express myosin heavy chain (MHC) type 1, 2A, and 2X isoforms and the so-called "type 2dog" fibers express the MHC-2X isoform. MHC-2A was found to be the most abundant isoform in the trunk and limb muscle. MHC-2X was expressed in most but not all muscles and more frequently in hybrid 2A-2X fibers than in pure 2X fibers. MHC-2B was restricted to specialized extraocular and laryngeal muscles, although 2B mRNA, but not 2B protein, was occasionally detected in the semimembranosus muscle. Isometric tension (P(o)) and maximum shortening velocity (V(o)) were measured in single fibers classified on the basis of their MHC isoform composition. Purified myosin isoforms were extracted from single muscle fibers and characterized by the speed (V(f)) of actin filament sliding on myosin in an in vitro motility assay. A close proportionality between V(o) and V(f) indicated that the diversity in V(o) was due to the different myosin isoform composition. V(o) increased progressively in the order 1/slow < 2A < 2X < 2B, thus confirming the identification of the myosin isoforms and providing their first functional characterization of canine muscle fibers.

Oxandrolone enhances skeletal muscle myosin synthesis and alters global gene expression profile in Duchenne muscular dystrophy.

Earlier studies have shown that the progressive, unrelenting muscle loss associated with Duchenne muscular dystrophy (DMD) involves an imbalance between the rates of synthesis and degradation of muscle proteins. Although previous studies have suggested that oxandrolone may be beneficial in DMD, the mechanism of action of oxandrolone on muscle in DMD remains unclear. To address these issues, we combined stable isotope studies and gene expression analysis to measure the fractional synthesis rate of myosin heavy chain (MHC), the key muscle contractile protein, the transcript levels of the isoforms of MHC, and global gene expression profiles in four children with DMD before and after 3 mo of treatment with oxandrolone. Gastrocnemius muscle biopsies and blood samples were collected during the course of a primed 6-h continuous infusion of l-[U-(13)C]leucine on two separate occasions, before and after the 3-mo treatment with oxandrolone (0.1 mg.kg(-1).day(-1)). Gene expression analysis was done with microarrays and RT-qPCR. In response to the treatment, MHC synthesis rate increased 42%, and this rise was accounted for, at least in part, by an upregulation of the transcript for MHC8 (perinatal MHC). Gene expression data suggested a decrease in muscle regeneration as a consequence of oxandrolone therapy, presumably because of a decrease in muscle degeneration. These findings suggest that 1) oxandrolone has a powerful protein anabolic effect on a key contractile protein and 2) larger and longer-term studies are warranted to determine whether these changes translate into meaningful therapy for these patients.

Myosin heavy chain 2B isoform is expressed in specialized eye muscles but not in trunk and limb muscles of cattle.

Myosin heavy chain isoforms (MHC) of adult skeletal muscles are codified by four genes named: slow, or type 1, and fast types 2A, 2X and 2B. The slow, 2A and 2X isoforms have been found expressed in all mammalian species studied so far whereas there is a large inter-species variability in the expression of MHC-2B. In this study histochemistry (m-ATPase), immunohistochemistry with the use of specific monoclonal antibodies and RT-PCR were combined together to assess whether the MHC-2B gene is expressed in bovine muscles. ATPase staining and RT-PCR experiments showed that three MHC isoforms (1, 2A, 2X) were expressed in trunk and limb muscles. Slow or type 1 expression was confirmed using a specific antibody (BA-F8) whereas the detection of fast MHC isoforms were validate by means of BF-35 antibody although not by the SC-71 antibody. MHC-2B was absent in limb and trunk muscles, but was present in specialized eye muscles (rectus lateralis and retractor bulbi) as consistently showed by RT-PCR and reactivity with a specific antibody (BF-F3). Interestingly, a cardiac isoform, MHC-a-cardiac was found to be expressed not only in extraocular muscles but also in masticatory muscles as masseter.

Differential expression of equine myosin heavy-chain mRNA and protein isoforms in a limb muscle.

The horse is one of the few animals kept and bred for its athletic performance and is therefore an interesting model for human sports performance. The regulation of the development of equine locomotion in the first year of life, and the influence of early training on later performance, are largely unknown. The major structural protein in skeletal muscle, myosin heavy-chain (MyHC), is believed to be primarily transcriptionally controlled. To investigate the expression of the MyHC genes at the transcriptional level, we isolated cDNAs encoding the equine MyHC isoforms type 1 (slow), type 2a (fast oxidative), and type 2d/x (fast glycolytic). cDNAs encoding the 2b gene were not identified. The mRNA expression was compared to the protein expression on a fiber-to-fiber basis using in situ hybridization (non-radioactive) and immunohistochemistry. Marked differences were detected between the expression of MyHC transcripts and MyHC protein isoforms in adult equine gluteus medius muscle. Mismatches were primarily due to the presence of hybrid fibers expressing two fast (2ad) MyHC protein isoforms, but only one fast (mainly 2a) MyHC RNA isoform. This discrepancy was most likely not due to differential mRNA expression of myonuclei.