RNA content differs in slow and fast muscle fibers: implications for interpretation of changes in muscle gene expression

Muscle fiber phenotype: a culprit of abnormal metabolism and function in skeletal muscle of humans with obesity

The proportion of the different types of fibers in a given skeletal muscle contributes to its overall metabolic and functional characteristics. Greater proportion of type I muscle fibers is associated with favorable oxidative metabolism and function of the muscle. Humans with obesity have a lower proportion of type I muscle fibers. We discuss how lower proportion of type I fibers in skeletal muscle of humans with obesity may explain metabolic and functional abnormalities reported in these individuals. These include lower muscle glucose disposal rate, mitochondrial content, protein synthesis, and quality/contractile function, as well as increased risk for heart disease, lower levels of physical activity, and propensity for weight gain/resistance to weight loss. We delineate future research directions and the need to examine hybrid muscle fiber populations, which are indicative of a transitory state of fiber phenotype within skeletal muscle. We also describe methodologies for precisely characterizing muscle fibers and gene expression at the single muscle fiber level to enhance our understanding of the regulation of muscle fiber phenotype in obesity. By contextualizing research in the field of muscle fiber type in obesity, we lay a foundation for future advancements and pave the way for translation of this knowledge to address impaired metabolism and function in obesity.

The skeletal muscle fiber periphery: A nexus of mTOR-related anabolism

Skeletal muscle anabolism is driven by numerous stimuli such as growth factors, nutrients (i.e., amino acids, glucose), and mechanical stress. These stimuli are integrated by the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) signal transduction cascade. In recent years, work from our laboratory and elsewhere has sought to unravel the molecular mechanisms underpinning the mTOR-related activation of muscle protein synthesis (MPS), as well as the spatial regulation of these mechanisms within the skeletal muscle cell. These studies have suggested that the skeletal muscle fiber periphery is a region of central importance in anabolism (i.e., growth/MPS). Indeed, the fiber periphery is replete with the substrates, molecular machinery, and translational apparatus necessary to facilitate MPS. This review provides a summary of the mechanisms underpinning the mTOR-associated activation of MPS from cell, rodent, and human studies. It also presents an overview of the spatial regulation of mTORC1 in response to anabolic stimuli and outlines the factors that distinguish the periphery of the cell as a highly notable region of skeletal muscle for the induction of MPS. Future research should seek to further explore the nutrient-induced activation of mTORC1 at the periphery of skeletal muscle fibers.

Ribosome accumulation during early phase resistance training in humans

Aim

To describe ribosome biogenesis during resistance training, its relation to training volume and muscle growth.

Methods

A training group (n = 11) performed 12 sessions (3‐4 sessions per week) of unilateral knee extension with constant and variable volume (6 and 3‐9 sets per session respectively) allocated to either leg. Ribosome abundance and biogenesis markers were assessed from vastus lateralis biopsies obtained at baseline, 48 hours after sessions 1, 4, 5, 8, 9 and 12, and after eight days of de‐training, and from a control group (n = 8). Muscle thickness was measured before and after the intervention.

Results

Training led to muscle growth (3.9% over baseline values, 95% CrI: [0.2, 7.5] vs. control) with concomitant increases in total RNA, ribosomal RNA, upstream binding factor (UBF) and ribosomal protein S6 with no differences between volume conditions. Total RNA increased rapidly in response to the first four sessions (8.6% [5.6, 11.7] per session), followed by a plateau and peak values after session 8 (49.5% [34.5, 66.5] above baseline). Total RNA abundance was associated with UBF protein levels (5.0% [0.2, 10.2] per unit UBF), and the rate of increase in total RNA levels predicted hypertrophy (0.3 mm [0.1, 0.4] per %‐point increase in total RNA per session). After de‐training, total RNA decreased (−19.3% [−29.0, −8.1]) without muscle mass changes indicating halted biosynthesis of ribosomes.

Conclusion

Ribosomes accumulate in the initial phase of resistance training with abundances sensitive to training cessation and associated with UBF protein levels. The average accumulation rate predicts muscle training‐induced hypertrophy.

Glycine receptor subunit-ß -deficiency in a mouse model of spasticity results in attenuated physical performance, growth and muscle strength

Spasticity is the most common neurological disorder associated with increased muscle contraction causing impaired movement and gait. The aim of this study was to characterize the physical performance, skeletal muscle function, and phenotype of mice with a hereditary spastic mutation (B6.Cg-Glrb^spa/J). Motor function, gait, and physical activity of juvenile and adult spastic mice and the morphological, histological, and mechanical characteristics of their soleus and gastrocnemius medialis muscles were compared with those of their wild-type (WT) littermates. Spastic mice showed attenuated growth, impaired motor function, and low physical activity. Gait of spastic mice was characterized by a typical hopping pattern. Spastic mice showed lower muscle forces, which were related to the smaller physiological cross-sectional area of spastic muscles. The muscle-tendon complex length-force relationship of adult gastrocnemius medialis was shifted toward shorter lengths, which was explained by attenuated longitudinal tibia growth. Spastic gastrocnemius medialis was more fatigue resistant than WT gastrocnemius medialis. This was largely explained by a higher mitochondrial content in muscle fibers and relatively higher percentage of slow-type muscle fibers. Muscles of juvenile spastic mice showed similar differences compared with WT juvenile mice, but these were less pronounced than between adult mice. This study shows that in spastic mice, disturbed motor function and gait is likely to be the result of hyperactivity of skeletal muscle and impaired skeletal muscle growth, which progress with age.

Does a Hypertrophying Muscle Fibre Reprogramme its Metabolism Similar to a Cancer Cell?

In 1924, Otto Warburg asked "How does the metabolism of a growing tissue differ from that of a non-growing tissue?" Currently, we know that proliferating healthy and cancer cells reprogramme their metabolism. This typically includes increased glucose uptake, glycolytic flux and lactate synthesis. A key function of this reprogramming is to channel glycolytic intermediates and other metabolites into anabolic reactions such as nucleotide-RNA/DNA synthesis, amino acid-protein synthesis and the synthesis of, for example, acetyl and methyl groups for epigenetic modification. In this review, we discuss evidence that a hypertrophying muscle similarly takes up more glucose and reprogrammes its metabolism to channel energy metabolites into anabolic pathways. We specifically discuss the functions of the cancer-associated enzymes phosphoglycerate dehydrogenase and pyruvate kinase muscle 2 in skeletal muscle. In addition, we ask whether increased glucose uptake by a hypertrophying muscle explains why muscularity is often negatively associated with type 2 diabetes mellitus and obesity.

Evaluation of RNA quality and functional transcriptome of beef longissimus thoracis over time post-mortem

Evaluating RNA quality and transcriptomic profile of beef muscle over time post-mortem may provide insight into RNA degradation and underlying biological and functional mechanisms that accompany biochemical changes occurring post-mortem during transformation of muscle to meat. RNA was extracted from longissimus thoracis (LT) sampled from British Continental crossbred heifer carcasses (n = 7) stored at 4°C in an abattoir drip cooler at 5 time points post-mortem, i.e., 45 min (0 h), 6 h, 24 h, 48 h, and 72 h. Following RNA-Sequencing, processed reads were aligned to the ARS-UCD1.2 bovine genome assembly. Subsequent differential expression (DE) analysis identified from 51 to 1434 upregulated and 27 to 2256 downregulated DE genes at individual time points compared to time 0 h, showing a trend for increasing counts of both upregulated and downregulated genes over time. Gene ontology and biological pathway term enrichment analyses on sets of DE genes revealed several processes and their timelines of activation/deactivation that accompanied or were involved with muscle transformation to meat. Although the quality of RNA in refrigerated LT remained high for several days post-mortem, the expression levels of several known biomarker genes for meat quality began to change from 24 h onwards. Therefore, to ensure accuracy of predictions on meat quality traits based on the expression levels of those biomarker genes in refrigerated beef muscle tissue, it is crucial that those expression measurements be made on RNA sampled within 24 h post-mortem. The present study also highlighted the need for more research on the roles of mitochondrial genes and non-coding genes in orchestrating muscle tissue processes after death, and how pre-mortem immune status might influence post-mortem meat quality.

Ractopamine-induced fiber type-specific gene expression in porcine skeletal muscles is independent of growth

Feeding ractopamine (RAC), a β-adrenergic agonist (BAA), to pigs increases type IIB muscle fiber type-specific protein and mRNA expression. However, increases in the abundance of these fast-twitch fiber types occur with other forms of muscle hypertrophy and thus BAA-induced changes in myosin heavy chain (MyHC) composition may simply be associated with increased muscle growth known to occur in response to BAA feeding. The objective of this study was to determine whether RAC feeding could change the MyHC gene expression in the absence of maximal muscle growth. Pigs were fed either an adequate diet that supported maximal muscle hypertrophy or a low nutrient diet that limited muscle growth. RAC was included in diets at 0 or 20 mg/kg for 1, 2, or 4 wk. Backfat depth was less (P < 0.05) in pigs fed the low nutrient diet compared with the adequate diet but was not affected by RAC. Loin eye area was greater (P < 0.05) in pigs fed an adequate diet plus RAC at 1 wk but did not differ among remaining pigs. At 2 and 4 wk, however, pigs fed the adequate diet had greater loin eye areas (P < 0.05) than pigs fed the low nutrient diet regardless of RAC feeding. Gene expression of the MyHC isoforms, I, IIA, IIX, and IIB, as well as glycogen synthase, citrate synthase, β 1-adrenergic receptor (AR), and β 2-AR were determined in longissimus dorsi (LD) and red (RST) and white (WST) portions of the semitendinosus muscles. MyHC type I gene expression was not altered by RAC or diet. Feeding RAC decreased (P < 0.01) MyHC type IIA gene expression in all muscles, but to a greater extent in WST and LD. MyHC type IIX gene expression was lower (P < 0.05) in WST and LD muscles in response to RAC but was not altered in RST muscles. RAC increased (P < 0.05) MyHC type IIB gene expression in all muscles, but to a greater extent in RST. β 1-AR gene expression was unaffected by RAC or diet, whereas the expression of the β 2-AR gene was decreased (P < 0.001) by RAC. No significant RAC * diet interactions were observed in gene expression in this study, indicating that RAC altered MyHC and β 2-AR gene expression in porcine skeletal muscles independent of growth.

Synergist ablation-induced hypertrophy occurs more rapidly in the plantaris than soleus muscle in rats due to different molecular mechanisms

We examined molecular mechanisms that were altered during rapid soleus (type I fiber-dominant) and plantaris (type II fiber-dominant) hypertrophy in rats. Twelve Wistar rats (3.5 mo old; 6 female, 6 male) were subjected to surgical right-leg soleus and plantaris dual overload [synergist ablation (SA)], and sham surgeries were performed on left legs (CTL). At 14 days after surgery, the muscles were dissected. Plantaris mass was 27% greater in the SA than CTL leg (P < 0.001), soleus mass was 13% greater in the SA than CTL leg (P < 0.001), and plantaris mass was higher than soleus mass in the SA leg (P = 0.001). Plantaris total RNA concentrations and estimated total RNA levels (suggestive of ribosome density) were 19% and 47% greater in the SA than CTL leg (P < 0.05), protein synthesis levels were 64% greater in the SA than CTL leg (P = 0.038), and satellite cell number per fiber was 60% greater in the SA than CTL leg (P = 0.003); no differences in these metrics were observed between soleus SA and CTL legs. Plantaris, as well as soleus, 20S proteasome activity was lower in the SA than CTL leg (P < 0.05), although the degree of downregulation was greater in the plantaris than soleus muscle (-63% vs. -20%, P = 0.001). These data suggest that early-phase plantaris hypertrophy occurs more rapidly than soleus hypertrophy, which coincided with greater increases in ribosome biogenesis, protein synthesis, and satellite cell density, as well as greater decrements in 20S proteasome activity, in the plantaris muscle.

The Importance of mTOR Trafficking for Human Skeletal Muscle Translational Control

This review will critique cell, rodent, and human models of mTOR regulation to discuss why mTOR trafficking may represent a novel and physiologically relevant model of regulation in skeletal muscle.

The mechanistic target of rapamycin (mTOR) is a central regulator of muscle protein synthesis, and its activation has long been attributed to its translocation to the lysosome. Here, we present a novel model of mTOR activation in skeletal muscle where the translocation of mTOR and the lysosome toward the cell membrane is a key process in mTOR activation.

β-hydroxy-β-methyl butyrate promotes leucine metabolism and improves muscle fibre composition in growing pigs

The aim of this study was to investigate the effects of excess leucine (Leu) vs. its metabolites α-ketoisocaproate (KIC) and β-hydroxy-β-methyl butyrate (HMB) on Leu metabolism, muscle fibre composition and muscle growth in growing pigs. Thirty-two pigs with a similar initial weight (9.55 ± 0.19 kg) were fed 1 of 4 diets for 45 days: basal diet, basal diet + 1.25% L-Leu, basal diet + 1.25% KIC-Ca, basal diet + 0.62% HMB-Ca. Results indicated that relative to the basal diet and HMB groups, Leu and KIC groups exhibited increased Leu concentrations and decreased concentrations of isoleucine, valine and EAAs in selected muscle (p < 0.05) and had lower mRNA levels of MyHC I and higher expression of MyHC IIx/IIb (p < 0.05), and there was no significant difference between the basal and HMB-supplemented groups. Moreover, the mRNA expression levels of AMPKα and UCP3 were higher but the myostatin mRNA levels were lower in the soleus muscle of the HMB group than those from other groups (p < 0.05). These findings demonstrated that doubling dietary Leu content exerted growth-depressing effects in growing pigs; dietary KIC supplementation induced muscular branched-chain amino acid imbalance and promoted muscle toward a more glycolytic phenotype; while dietary HMB supplementation promoted the generation of more oxidative muscle types and increased muscle growth specially in oxidative skeletal muscle, and these effects of HMB might be associated with the AMPKα-Sirt1-PGC-1α axis and mitochondrial biogenesis.

Physiological Differences Between Low Versus High Skeletal Muscle Hypertrophic Responders to Resistance Exercise Training: Current Perspectives and Future Research Directions

Numerous reports suggest there are low and high skeletal muscle hypertrophic responders following weeks to months of structured resistance exercise training (referred to as low and high responders herein). Specifically, divergent alterations in muscle fiber cross sectional area (fCSA), vastus lateralis thickness, and whole body lean tissue mass have been shown to occur in high versus low responders. Differential responses in ribosome biogenesis and subsequent protein synthetic rates during training seemingly explain some of this individual variation in humans, and mechanistic in vitro and rodent studies provide further evidence that ribosome biogenesis is critical for muscle hypertrophy. High responders may experience a greater increase in satellite cell proliferation during training versus low responders. This phenomenon could serve to maintain an adequate myonuclear domain size or assist in extracellular remodeling to support myofiber growth. High responders may also express a muscle microRNA profile during training that enhances insulin-like growth factor-1 (IGF-1) mRNA expression, although more studies are needed to better validate this mechanism. Higher intramuscular androgen receptor protein content has been reported in high versus low responders following training, and this mechanism may enhance the hypertrophic effects of testosterone during training. While high responders likely possess “good genetics,” such evidence has been confined to single gene candidates which typically share marginal variance with hypertrophic outcomes following training (e.g., different myostatin and IGF-1 alleles). Limited evidence also suggests pre-training muscle fiber type composition and self-reported dietary habits (e.g., calorie and protein intake) do not differ between high versus low responders. Only a handful of studies have examined muscle biomarkers that are differentially expressed between low versus high responders. Thus, other molecular and physiological variables which could potentially affect the skeletal muscle hypertrophic response to resistance exercise training are also discussed including rDNA copy number, extracellular matrix and connective tissue properties, the inflammatory response to training, and mitochondrial as well as vascular characteristics.

Distinguishing States of Arrest: Genome-Wide Descriptions of Cellular Quiescence Using ChIP-Seq and RNA-Seq Analysis

Regenerative potential in adult stem cells is closely associated with the establishment of-and exit from-a temporary state of quiescence. Emerging evidence not only provides a rationale for the link between lineage determination programs and cell cycle regulation but also highlights the understanding of quiescence as an actively maintained cellular program, encompassing networks and mechanisms beyond mitotic inactivity or metabolic restriction. Interrogating the quiescent genome and transcriptome using deep-sequencing technologies offers an unprecedented view of the global mechanisms governing this reversibly arrested cellular state and its importance for cell identity. While many efforts have identified and isolated pure target stem cell populations from a variety of adult tissues, there is a growing appreciation that their isolation from the stem cell niche in vivo leads to activation and loss of hallmarks of quiescence. Thus, in vitro models that recapitulate the dynamic reversibly arrested stem cell state in culture and lend themselves to comparison with the activated or differentiated state are useful templates for genome-wide analysis of the quiescence network.In this chapter, we describe the methods that can be adopted for whole genome epigenomic and transcriptomic analysis of cells derived from one such established culture model where mouse myoblasts are triggered to enter or exit quiescence as homogeneous populations. The ability to synchronize myoblasts in G₀ permits insights into the genome in "deep quiescence." The culture methods for generating large populations of quiescent myoblasts in either 2D or 3D culture formats are described in detail in a previous chapter in this series (Arora et al. Methods Mol Biol 1556:283-302, 2017). Among the attractive features of this model are that genes isolated from quiescent myoblasts in culture mark satellite cells in vivo (Sachidanandan et al., J Cell Sci 115:2701-2712, 2002) providing a validation of its approximation of the molecular state of true stem cells. Here, we provide our working protocols for ChIP-seq and RNA-seq analysis, focusing on those experimental elements that require standardization for optimal analysis of chromatin and RNA from quiescent myoblasts, and permitting useful and revealing comparisons with proliferating myoblasts or differentiated myotubes.

Linking Cancer Cachexia-Induced Anabolic Resistance to Skeletal Muscle Oxidative Metabolism

Cancer cachexia, a wasting syndrome characterized by skeletal muscle depletion, contributes to increased patient morbidity and mortality. While the intricate balance between protein synthesis and breakdown regulates skeletal muscle mass, the suppression of basal protein synthesis may not account for the severe wasting induced by cancer. Therefore, recent research has shifted to the regulation of “anabolic resistance,” which is the impaired ability of nutrition and exercise to stimulate protein synthesis. Emerging evidence suggests that oxidative metabolism can regulate both basal and induced muscle protein synthesis. While disrupted protein turnover and oxidative metabolism in cachectic muscle have been examined independently, evidence suggests a linkage between these processes for the regulation of cancer-induced wasting. The primary objective of this review is to highlight the connection between dysfunctional oxidative metabolism and cancer-induced anabolic resistance in skeletal muscle. First, we review oxidative metabolism regulation of muscle protein synthesis. Second, we describe cancer-induced alterations in the response to an anabolic stimulus. Finally, we review a role for exercise to inhibit cancer-induced anabolic suppression and mitochondrial dysfunction.

Resistance exercise initiates mechanistic target of rapamycin (mTOR) translocation and protein complex co-localisation in human skeletal muscle

The mechanistic target of rapamycin (mTOR) is a central mediator of protein synthesis in skeletal muscle. We utilized immunofluorescence approaches to study mTOR cellular distribution and protein-protein co-localisation in human skeletal muscle in the basal state as well as immediately, 1 and 3 h after an acute bout of resistance exercise in a fed (FED; 20 g Protein/40 g carbohydrate/1 g fat) or energy-free control (CON) state. mTOR and the lysosomal protein LAMP2 were highly co-localised in basal samples. Resistance exercise resulted in rapid translocation of mTOR/LAMP2 towards the cell membrane. Concurrently, resistance exercise led to the dissociation of TSC2 from Rheb and increased in the co-localisation of mTOR and Rheb post exercise in both FED and CON. In addition, mTOR co-localised with Eukaryotic translation initiation factor 3 subunit F (eIF3F) at the cell membrane post-exercise in both groups, with the response significantly greater at 1 h of recovery in the FED compared to CON. Collectively our data demonstrate that cellular trafficking of mTOR occurs in human muscle in response to an anabolic stimulus, events that appear to be primarily influenced by muscle contraction. The translocation and association of mTOR with positive regulators (i.e. Rheb and eIF3F) is consistent with an enhanced mRNA translational capacity after resistance exercise.

GROWTH AND DEVELOPMENT SYMPOSIUM: Adenosine monophosphate-activated protein kinase and mitochondria in Rendement Napole pig growth

The Rendement Napole mutation (RN-), which is well known to influence pork quality, also has a profound impact on metabolic characteristics of muscle. Pigs with RN- possess a SNP in the γ3 subunit of adenosine monophosphate (AMP)-activated protein kinase (AMPK); AMPK, a key energy sensor in skeletal muscle, modulates energy producing and energy consuming pathways to maintain cellular homeostasis. Importantly, AMPK regulates not only acute response to energy stress but also facilitates long-term adaptation via changes in gene and protein expression. The RN- allele increases AMPK activity, which alters the metabolic phenotype of skeletal muscle by increasing mitochondrial content and oxidative capacity. Fibers with greater oxidative capacity typically exhibit increased protein turnover and smaller fiber size, which indicates that RN- pigs may exhibit decreased efficiency and growth potential. However, whole body and muscle growth of RN- pigs appear similar to that of wild-type pigs and despite increased oxidative capacity, fibers maintain the capacity for hypertrophic growth. This indicates that compensatory mechanisms may allow RN- pigs to achieve rates of muscle growth similar to those of wild-type pigs. Intriguingly, lipid oxidation and mitochondria function are enhanced in RN- pig muscle. Thus far, characteristics of RN- muscle are largely based on animals near market weight. To better understand interaction between energy signaling and protein accretion in muscle, further work is needed to define age-dependent relationships between AMPK signaling, metabolism, and muscle growth.

Effects of dietary protein restriction on muscle fiber characteristics and mTORC1 pathway in the skeletal muscle of growing-finishing pigs

Background

To investigate the effects of dietary crude protein (CP) restriction on muscle fiber characteristics and key regulators related to protein deposition in skeletal muscle, a total of 18 growing-finishing pigs (62.30 ± 0.88 kg) were allotted to 3 groups and fed with the recommended adequate protein (AP, 16 % CP) diet, moderately restricted protein (MP, 13 % CP) diet and low protein (LP, 10 % CP) diet, respectively. The skeletal muscle of different locations in pigs, including longissimus dorsi muscle (LDM), psoas major muscle (PMM) and biceps femoris muscle (BFM) were collected and analyzed.

Results

Results showed that growing-finishing pigs fed the MP or AP diet improved (P < 0.01) the average daily gain and feed: gain ratio compared with those fed the LP diet, and the MP diet tended to increase (P = 0.09) the weight of LDM. Moreover, the ATP content and energy charge value were varied among muscle samples from different locations of pigs fed the reduced protein diets. We also observed that pigs fed the MP diet up-regulated (P < 0.05) muscular mRNA expression of all the selected key genes, except that myosin heavy chain (MyHC) IIb, MyHC IIx, while mRNA expression of ubiquitin ligases genes was not affected by dietary CP level. Additionally, the activation of mammalian target of rapamycin complex 1 (mTORC1) pathway was stimulated (P < 0.05) in skeletal muscle of the pigs fed the MP or AP diet compared with those fed the LP diet.

Conclusion

The results suggest that the pigs fed the MP diet could catch up to the growth performance and the LDM weight of the pigs fed the AP diet, and the underlying mechanism may be partly due to the alteration in energy status, modulation of muscle fiber characteristics and mTORC1 activation as well as its downstream effectors in skeletal muscle of different locations in growing-finishing pigs.

Electronic supplementary material

The online version of this article (doi:10.1186/s40104-016-0106-8) contains supplementary material, which is available to authorized users.

Alteration of cathepsin-D expression in atrophied muscles and apoptotic myofibers by hindlimb unloading in a low-temperature environment

[Purpose] The purpose of this study was to elucidate the cathepsin-D involvement in signaling pathways for the survival and apoptosis of myofibers in rats with hindlimb-unloading in a low-temperature environment. [Subjects and Methods] Wistar rats were divided into two groups: a control group and a group that underwent hindlimb unloading in a low-temperature environment to induce muscle apoptosis. Cathepsin-D localization in the soleus and extensor digitorum longus muscles, along with the expression of cathepsin-D in apoptotic myofibers, was examined. Expression of the active and inactive forms of cathepsin-D was also analyzed. [Results] Cathepsin-D was mainly expressed in type I myofibers and was observed to have punctate patterns in the control group. In the hindlimb unloading in a low-temperature environment group, the type I myofiber composition ratio decreased, and caspase-3 activation and TUNEL-positive apoptotic myofibers were observed. In caspase-3-activated myofibers, cathepsin-D overexpression and leakage of it into the cytoplasm were observed. In the hindlimb unloading in a low-temperature environment group, the amount of inactive cathepsin-D decreased, whereas that of the active form increased. [Conclusion] Cathepsin-D was deduced to be indicative of a myofiber-type classification and a factor related to myofiber type maintenance. In addition, cathepsin-D leakage into the cytoplasm was appeared to be involved in caspase-3 activation in the hindlimb unloading in a low-temperature environment group.

Correction of gene expression data: Performance-dependency on inter-replicate and inter-treatment biases

This report investigates for the first time the potential inter-treatment bias source of cell number for gene expression studies. Cell-number bias can affect gene expression analysis when comparing samples with unequal total cellular RNA content or with different RNA extraction efficiencies. For maximal reliability of analysis, therefore, comparisons should be performed at the cellular level. This could be accomplished using an appropriate correction method that can detect and remove the inter-treatment bias for cell-number. Based on inter-treatment variations of reference genes, we introduce an analytical approach to examine the suitability of correction methods by considering the inter-treatment bias as well as the inter-replicate variance, which allows use of the best correction method with minimum residual bias. Analyses of RNA sequencing and microarray data showed that the efficiencies of correction methods are influenced by the inter-treatment bias as well as the inter-replicate variance. Therefore, we recommend inspecting both of the bias sources in order to apply the most efficient correction method. As an alternative correction strategy, sequential application of different correction approaches is also advised.

Measuring protein synthesis with SUnSET: a valid alternative to traditional techniques?

Protein synthesis rates commonly are measured using isotopic tracers to quantify the incorporation of a labeled amino acid into muscle proteins. Here we provide evidence supporting our hypothesis that the nonisotopic SUnSET technique is a valid and accurate method for the measurement of in vivo changes in protein synthesis at the whole-muscle and single-muscle fiber levels.

Hormone replacement therapy improves contractile function and myonuclear organization of single muscle fibres from postmenopausal monozygotic female twin pairs

Ageing is associated with a decline in muscle mass and strength leading to increased physical dependency in old age. Postmenopausal women experience a greater decline than men of similar age in parallel with the decrease in female sex steroid hormone production. We recruited six monozygous female twin pairs (55-59 years old) where only one twin pair was on hormone replacement therapy (HRT use = 7.8 ± 4.3 years) to investigate the association of HRT with the cytoplasmic volume supported by individual myonuclei (myonuclear domain (MND) size,) together with specific force at the single fibre level. HRT use was associated with a significantly smaller (∼27%; P < 0.05) mean MND size in muscle fibres expressing the type I but not the IIa myosin heavy chain (MyHC) isoform. In comparison to non-users, higher specific force was recorded in HRT users both in muscle fibres expressing type I (∼27%; P < 0.05) and type IIa (∼23%; P < 0.05) MyHC isoforms. These differences were fibre-type dependent, i.e. the higher specific force in fast-twitch muscle fibres was primarily caused by higher force per cross-bridge while slow-twitch fibres relied on both a higher number and force per cross-bridge. HRT use had no effect on fibre cross-sectional area (CSA), velocity of unloaded shortening (V0) and relative proportion of MyHC isoforms. In conclusion, HRT appears to have significant positive effects on both regulation of muscle contraction and myonuclei organization in postmenopausal women.

Muscle fiber type-dependent differences in the regulation of protein synthesis

This study examined fiber type-dependent differences in the regulation of protein synthesis in individual muscle fibers found within the same whole muscle. Specifically, the in vivo SUrface SEnsing of Translation (SUnSET) methodology was used to measure protein synthesis in type 1, 2A, 2X and 2B fibers of the mouse plantaris muscle, in response to food deprivation (FD), and mechanical overload induced by synergist ablation (SA). The results show that 48 h of FD induced a greater decrease in protein synthesis in type 2X and 2B fibers compared to type 1 and 2A fibers. Type 2X and 2B fibers also had the largest FD-induced decrease in total S6 protein and Ser(240/244) S6 phosphorylation, respectively. Moreover, only type 2X and 2B fibers displayed a FD-induced decrease in cross-sectional area (CSA). Ten days of SA also induced fiber type-dependent responses, with type 2B fibers having the smallest SA-induced increases in protein synthesis, CSA and Ser(240/244) S6 phosphorylation, but the largest increase in total S6 protein. Embryonic myosin heavy chain (MHC(Emb)) positive fibers were also found in SA muscles and the protein synthesis rates, levels of S6 Ser(240/244) phosphorylation, and total S6 protein content, were 3.6-, 6.1- and 2.9-fold greater than that found in fibers from control muscles, respectively. Overall, these results reveal differential responses in the regulation of protein synthesis and fiber size between fiber types found within the same whole muscle. Moreover, these findings demonstrate that changes found at the whole muscle level do not necessarily reflect changes in individual fiber types.

Microgenomic analysis in skeletal muscle: expression signatures of individual fast and slow myofibers

Background

Skeletal muscle is a complex, versatile tissue composed of a variety of functionally diverse fiber types. Although the biochemical, structural and functional properties of myofibers have been the subject of intense investigation for the last decades, understanding molecular processes regulating fiber type diversity is still complicated by the heterogeneity of cell types present in the whole muscle organ.

Methodology/Principal Findings

We have produced a first catalogue of genes expressed in mouse slow-oxidative (type 1) and fast-glycolytic (type 2B) fibers through transcriptome analysis at the single fiber level (microgenomics). Individual fibers were obtained from murine soleus and EDL muscles and initially classified by myosin heavy chain isoform content. Gene expression profiling on high density DNA oligonucleotide microarrays showed that both qualitative and quantitative improvements were achieved, compared to results with standard muscle homogenate. First, myofiber profiles were virtually free from non-muscle transcriptional activity. Second, thousands of muscle-specific genes were identified, leading to a better definition of gene signatures in the two fiber types as well as the detection of metabolic and signaling pathways that are differentially activated in specific fiber types. Several regulatory proteins showed preferential expression in slow myofibers. Discriminant analysis revealed novel genes that could be useful for fiber type functional classification.

Conclusions/Significance

As gene expression analyses at the single fiber level significantly increased the resolution power, this innovative approach would allow a better understanding of the adaptive transcriptomic transitions occurring in myofibers under physiological and pathological conditions.

Novel insights into the regulation of skeletal muscle protein synthesis as revealed by a new nonradioactive in vivo technique

In this study, the principles of surface sensing of translation (SUnSET) were used to develop a nonradioactive method for ex vivo and in vivo measurements of protein synthesis (PS). Compared with controls, we first demonstrate excellent agreement between SUnSET and a [(3)H]phenylalanine method when detecting synergist ablation-induced increases in skeletal muscle PS ex vivo. We then show that SUnSET can detect the same synergist ablation-induced increase in PS when used in vivo (IV-SUnSET). In addition, IV-SUnSET detected food deprivation-induced decreases in PS in the heart, kidney, and skeletal muscles, with similar changes being visualized with an immunohistochemical version of IV-SUnSET (IV-IHC-SUnSET). By combining IV-IHC-SUnSET with in vivo transfection, we demonstrate that constitutively active PKB induces a robust increase in skeletal muscle PS. Furthermore, transfection with Ras homolog enriched in brain (Rheb) revealed that a PKB-independent activation of mammalian target of rapamycin is also sufficient to induce an increase in skeletal muscle PS. Finally, IV-IHC-SUnSET exposed the existence of fiber type-dependent differences in skeletal muscle PS, with PS in type 2B and 2X fibers being significantly lower than that in type 2A fibers within the same muscle. Thus, our nonradioactive method allowed us to accurately visualize and quantify PS under various ex vivo and in vivo conditions and revealed novel insights into the regulation of PS in skeletal muscle.

The muscle fiber type-fiber size paradox: hypertrophy or oxidative metabolism?

An inverse relationship exists between striated muscle fiber size and its oxidative capacity. This relationship implies that muscle fibers, which are triggered to simultaneously increase their mass/strength (hypertrophy) and fatigue resistance (oxidative capacity), increase these properties (strength or fatigue resistance) to a lesser extent compared to fibers increasing either of these alone. Muscle fiber size and oxidative capacity are determined by the balance between myofibrillar protein synthesis, mitochondrial biosynthesis and degradation. New experimental data and an inventory of critical stimuli and state of activation of the signaling pathways involved in regulating contractile and metabolic protein turnover reveal: (1) higher capacity for protein synthesis in high compared to low oxidative fibers; (2) competition between signaling pathways for synthesis of myofibrillar proteins and proteins associated with oxidative metabolism; i.e., increased mitochondrial biogenesis via AMP-activated protein kinase attenuates the rate of protein synthesis; (3) relatively higher expression levels of E3-ligases and proteasome-mediated protein degradation in high oxidative fibers. These observations could explain the fiber type-fiber size paradox that despite the high capacity for protein synthesis in high oxidative fibers, these fibers remain relatively small. However, it remains challenging to understand the mechanisms by which contractile activity, mechanical loading, cellular energy status and cellular oxygen tension affect regulation of fiber size. Therefore, one needs to know the relative contribution of the signaling pathways to protein turnover in high and low oxidative fibers. The outcome and ideas presented are relevant to optimizing treatment and training in the fields of sports, cardiology, oncology, pulmonology and rehabilitation medicine.

Ageing influences myonuclear domain size differently in fast and slow skeletal muscle of rats

AIM

In multinucleated skeletal muscle, a myonuclear domain is the region of cytoplasm governed by one nucleus, and myofibres are mosaics of overlapping myonuclear domains. Association of ageing and myonuclear domain is important in the understanding of sarcopenia and with prevention or combating age-related muscle declines. This study examined the effects of age, fibre type and muscle on nucleo-cytoplasmic (N/C) relationships as reflecting myonuclear domain size.

METHODS

The N/C was compared in fibre types of soleus and plantaris muscles from young (n = 6) and ageing (n = 8) male Fisher 344 rats.

RESULTS

There were no significant differences in fibre type composition or cross-sectional area of the soleus across ages. The old soleus had significantly more myonuclei, resulting in a significantly smaller myonuclear domain size. The plantaris muscle showed a higher percentage of slow fibres in old compared with young fibres. There were no differences in the number of myonuclei or in myonuclear domain size between young and older animals.

CONCLUSION

We found muscle-specific differences in the effects of ageing on myonuclear domain, possibly as a result of reduced efficiency of the myonuclei in the slow muscles.

Focal adhesion kinase is a load-dependent governor of the slow contractile and oxidative muscle phenotype

Striated muscle exhibits a pronounced structural-functional plasticity in response to chronic alterations in loading. We assessed the implication of focal adhesion kinase (FAK) signalling in mechano-regulated differentiation of slow-oxidative muscle. Load-dependent consequences of FAK signal modulation were identified using a multi-level approach after electrotransfer of rat soleus muscle with FAK-expression plasmid vs. empty plasmid-transfected contralateral controls. Muscle fibre-targeted over-expression of FAK in anti-gravitational muscle for 9 days up-regulated transcript levels of gene ontologies underpinning mitochondrial metabolism and contraction in the transfected belly portion. Concomitantly, mRNA expression of the major fast-type myosin heavy chain (MHC) isoform, MHC2A, was reduced. The promotion of the slow-oxidative expression programme by FAK was abolished after co-expression of the FAK inhibitor FAK-related non-kinase (FRNK). Elevated protein content of MHC1 (+9%) and proteins of mitochondrial respiration (+165-610%) with FAK overexpression demonstrated the translation of transcript differentiation in targeted muscle fibres towards a slow-oxidative muscle phenotype. Coincidentally MHC2A protein was reduced by 50% due to protection of muscle from de-differentiation with electrotransfer. Fibre cross section in FAK-transfected muscle was elevated by 6%. The FAK-modulated muscle transcriptome was load-dependent and regulated in correspondence to tyrosine 397 phosphorylation of FAK. In the context of overload, the FAK-induced gene expression became manifest at the level of contraction by a slow transformation and the re-establishment of normal muscle force from the lowered levels with transfection. These results highlight the analytic power of a systematic somatic transgene approach by mapping a role of FAK in the dominant mechano-regulation of muscular motor performance via control of gene expression.

Effect of age on skeletal muscle myofibrillar mRNA abundance: relationship to myosin heavy chain protein synthesis rate

Age-related changes in myosin heavy chain (MHC) phenotype impact both the quantity and functional character of skeletal muscle. The present study was undertaken to examine the hypothesis that age-related changes in MHC mRNA abundance underlie alterations in protein synthesis rates and content. We measured the abundance of mRNA for MHC isoforms (MHC I, MHC IIa, IIx) and actin by RT-PCR in 6 young (mean +/- SE; 29 +/- 3) and 12 elderly (73 +/- 1 yr; P<0.01) volunteers and examined their association to MHC protein synthesis rates and their respective protein products. We found no differences between young and elderly volunteers in MHC isoform or actin transcript levels. Because of the absence of age effects, data were pooled for correlation analyses. Although total MHC mRNA levels were not related to MHC fractional synthesis rates, the relative abundance of MHC I and MHC IIa mRNA were positively (r = 0.450; P = 0.06) and negatively (r = -0.493; P<0.05) associated with MHC protein synthesis rates, respectively. MHC mRNA levels were positively correlated to their respective protein products (range of r-values: 0.551-0.727; P<0.05 to P<0.01), but were not related to skeletal muscle IGF-I mRNA abundance, circulating IGF-I or markers of immune activation. Our results argue against the notion that changes in MHC protein synthesis rates with age are related to altered MHC mRNA abundance, although our findings do suggest that possibility that individual variability in MHC protein synthesis rates is related to the relative abundance of MHC I versus MHC IIa transcripts.

Differential effects of muscle fibre length and insulin on muscle-specific mRNA content in isolated mature muscle fibres during long-term culture

The aims of this study were (1) to determine the relationship between muscle fibre cross-sectional area and cytoplasmic density of myonuclei in high- and low-oxidative Xenopus muscle fibres and (2) to test whether insulin and long-term high fibre length caused an increase in the number of myonuclei and in the expression of alpha-skeletal actin and of myogenic regulatory factors (myogenin and MyoD) in these muscle fibres. In high- and low-oxidative muscle fibres from freshly frozen iliofibularis muscles, the number of myonuclei per millimetre fibre length was proportional to muscle fibre cross-sectional area. The in vivo myonuclear density thus seemed to be strictly regulated, suggesting that the induction of hypertrophy required the activation of satellite cells. The effects of muscle fibre length and insulin on myonuclear density and myonuclear mRNA content were investigated on high-oxidative single muscle fibres cultured for 4-5 days. Muscle fibres were kept at a low length (~15% below passive slack length) in culture medium with a high insulin concentration (~6 nmol/l: "high insulin medium") or without insulin, and at a high length (~5% above passive slack length) in high insulin medium. High fibre length and high insulin medium did not change the myonuclear density of isolated muscle fibres during culture. High insulin increased the myonuclear alpha-skeletal actin mRNA content, whereas fibre length had no effect on alpha-skeletal actin mRNA content. After culture at high fibre length in high insulin medium, the myonuclear myogenin mRNA content was 2.5-fold higher than that of fibres cultured at low length in high insulin medium or in medium without insulin. Myonuclear MyoD mRNA content was not affected by fibre length or insulin. These in vitro experiments indicate that high muscle fibre length and insulin enhance muscle gene expression but that other critical factors are required to induce adaptation of muscle fibre size and performance.

Regulation of skeletal muscle mitochondrial content during aging

Mitochondrial content of skeletal muscle varies among fiber types, and changes in complex ways during aging. We evaluated the regulatory origins of differences in mitochondrial content among muscles of varied fiber type in F344xBNF1 rats, and how these regulatory patterns are altered with aging. In adult (12 month) animals we found that units citrate synthase (CS)/g tissue, a marker for mitochondrial content, varied approximately 3-fold among 10 skeletal muscles. Stoichiometric relationships between CS and isocitrate dehydrogenase, aconitase, and cytochrome c oxidase were generally preserved across fiber types. Among the 10 muscles of adult rats, CS content correlated with nuclear content (R2= 0.36). Muscles differed widely in CS messenger RNA (mRNA)/DNA (an index of variation in transcriptional regulations) and units CS/CS mRNA (an index of variation in posttranscriptional regulations). All muscles of aged rats (35 months) showed an increase in mg DNA/g, suggestive of atrophy. Age-dependent declines in units CS/DNA were accompanied by reductions in CS mRNA/DNA and/or units CS/CS mRNA, depending on muscle fiber type. Thus, declines in units CS/DNA with age appeared to be due to transcriptional as well as translational variations. Differences in mitochondrial content among muscle fiber types and age groups may arise from variations in nuclear content and posttranscriptional processes, as well as transcriptional regulation.

Identification and tissue-specific expression of amphioxus GM2 activator protein gene from amphioxus Branchiostoma belcheri

An amphioxus cDNA, AmphiGM2AP, encoding GM2 activator protein was isolated from the gut cDNA library of Branchiostoma belcheri. It is 907 bp long, and its longest open reading frame codes for a precursor protein consisting of 242 amino acid residues with a signal peptide of 14 amino acids. The deduced amino acid sequence includes a conserved domain typical of GM2APs between residues 53 and 224, a single N-linked glycosylation site at position 65 and 8 conserved cysteines. Phylogenetic analysis showed that amphiGM2AP forms a club together with invertebrate GM2APs, indicating that AmphiGM2AP is evolutionarily closely related to invertebrate GM2APs rather than vertebrate ones. Both Northern blotting and in situ hybridization histochemistry analyses revealed a tissue-specific expression pattern of AmphiGM2AP in adult amphioxus with the strongest expression in the digestive system, which is in contrast to the widespread expression pattern of human, mouse and sheep GM2AP genes. It is suggested that AmphiGM2AP is possibly involved in the take-in of digested food components like lipid molecules.

Quantification by real-time PCR of developmental and adult myosin mRNA in rat muscles

A real-time RT-PCR assay using newly designed primers was developed to analyze developmental and adult MHC mRNA expression both in skeletal muscles and single fibers. Only 4 ng of total RNA was necessary for the analysis of the relative mRNA expression of MHC genes. Different validation steps were realized concerning both specificity and sensitivity of each primer set, and linearity and efficiency of each real-time PCR amplification. Then, quantification of MHC mRNA in neonatal and adult muscles as well as in single fibers was done by the deltaC(T) method, with CycA gene as the reference gene. Due to a higher sensitivity than that of a competitive PCR method, we demonstrated that this assay is suitable to study very low level of MHC mRNA expression as developmental MHC in adult muscle and to quantify mRNA from very small samples.

Transcriptional profiling of tissue plasticity: role of shifts in gene expression and technical limitations

Reprogramming of gene expression has been recognized as a main instructive modality for the adjustments of tissues to various kinds of stress. The recent application of gene expression profiling has provided a powerful tool to elucidate the molecular pathways underlying such tissue remodeling. However, the biological interpretations of expression profiling results critically depend on normalization of transcript signals to mRNA standards before statistical evaluation. A hypothesis is proposed whereby the “fluctuating nature” of gene expression represents an inherent limitation of the test system used to quantify RNA levels. Misinterpretation of gene expression data occurs when RNA quantities are normalized to a subset of mRNAs that are subject to strong regulation. The contention of contradictory biological outcomes using different RNA-normalization schemes is demonstrated in two models of skeletal muscle plasticity with data from custom-designed microarrays and biochemical and ultrastructural evidence for correspondingly altered RNA content and nucleolar activity. The prevalence of these biological constraints is underlined by a literature survey in different models of tissue plasticity with emphasis on the unique malleability of skeletal muscle. Finally, recommendations on the optimal experimental layout are given to control biological and technical variability in microarray and RT-PCR studies. It is proposed to approach normalization of transcript signals by measuring total RNA and DNA content per sample weight and by correcting for concurrently estimated endogenous standards such as major ribosomal RNAs and spiked RNA and DNA species. This allows for later conversion to diverse tissue-relevant references and should improve the physiological interpretations of phenotypic plasticity.

Regulatory gene expression in skeletal muscle of highly endurance-trained humans

AIM AND BACKGROUND

Changes in regulatory and structural gene expression provide the molecular basis for the adaptation of human skeletal muscle to endurance exercise.

HYPOTHESIS

The steady-state levels of multiple mRNAs mainly involved in regulatory functions differ between highly endurance-trained and untrained subjects in a muscle heavily recruited during the exercise.

METHODS

Biopsies from musculus vastus lateralis of seven untrained (UT) subjects [maximal oxygen consumption (VO2max) = 39 mL kg-1 min-1] and seven trained (T) professional cyclists (VO2max = 72 mL kg-1 min-1) were analysed for the contents of 597 different mRNAs using commercially available cDNA arrays (Clontech no. 7740-1). Intra-individual expression profiles were compared by least-square linear regression analysis. Differences in gene expression between the two groups were tested for statistical significance using L1 regression analysis combined with the sign test on all permutations of scatter plots of log raw values from UT vs. T subjects.

RESULTS

Transcripts for 144 of 597 genes were sufficiently abundant to be analysed quantitatively. The expression profiles of the T group had a better intragroup correlation (R2) than those of the UT group (0.78 vs. 0.65, P < 0.05). An intergroup (T vs. UT) correlation of expression profiles gave an R2 of 0.71. Statistical analysis at a false discovery rate of 5% identified differential expression of nine cell-regulatory genes between T and UT. The mRNA levels of eight genes, including two DNA repair enzymes, transcription factors, signal transducers, a glycolytic enzyme and a factor involved in steroid hormone metabolism were increased in T vs. UT. Conversely, the mRNA of the tumour suppressor APC was downregulated with endurance training. Selective reverse-transcriptase polymerase chain reaction experiments confirmed the signal estimates from the array analysis.

CONCLUSIONS

The repetitive impact of the complex exercise stimuli in professional cyclists attenuated the interindividual differences in regulatory gene expression in skeletal muscle. Long-term nuclear reprogramming of regulatory gene expression seems to be characteristic of human musculus vastus lateralis in a highly endurance-trained steady state.

Depressed mitochondrial transcription factors and oxidative capacity in rat failing cardiac and skeletal muscles

Congestive heart failure (CHF) induces alterations in energy metabolism and mitochondrial function that span cardiac as well as skeletal muscles. Whether these defects originate from altered mitochondrial DNA copy number and/or mitochondrial gene transcription is not known at present, nor are the factors that control mitochondrial capacity in different muscle types completely understood. We used an experimental model of CHF induced by aortic banding in the rat and investigated mitochondrial respiration and enzyme activity of biochemical mitochondrial markers in cardiac, slow and fast skeletal muscles. We quantified mitochondrial DNA (mtDNA), expression of nuclear (COX IV) and mitochondrial (COX I) encoded cytochrome c oxidase subunits as well as nuclear factors involved in mitochondrial biogenesis and in the necessary coordinated interplay between nuclear and mitochondrial genomes in health and CHF. CHF induced a decrease in oxidative capacity and mitochondrial enzyme activities with a parallel decrease in the mRNA level of COX I and IV, but no change in mtDNA content. The expression of the peroxisome proliferator activated receptor gamma co-activator 1 alpha (PGC-1 alpha) gene was downregulated in CHF, as well as nuclear respiratory factor 2 and mitochondrial transcription factor A, which act downstream from PGC-1 alpha. Most interestingly, only the level of PGC-1 alpha expression was strongly correlated with muscle oxidative capacity in cardiac and skeletal muscles, both in healthy and CHF rats. Mitochondrial gene transcription is reduced in CHF, and PGC-1 alpha appears as a potential modulator of muscle oxidative capacity under these experimental conditions.

Species-specific differences of myosin content in the developing cardiac chambers of fish, birds, and mammals

Key morphogenetic events during heart ontogenesis are similar in different vertebrate species. We report that in primitive vertebrates, i.e., cartilaginous fishes, both the embryonic and the adult heart show a segmental subdivision similar to that of the embryonic mammalian heart. Early morphogenetic events during cardiac development in the dogfish are long-lasting, providing a suitable model to study changes in pattern of gene expression during these stages. We performed a comparative study among dogfish, chicken, rat, and mouse to assess whether species-specific qualitative and/or quantitative differences in myosin heavy chain (MyHC) distribution arise during development, indicative of functional differences between species. MyHC RNA content was investigated by means of in situ hybridisation using an MyHC probe specific for a highly conserved domain, and MyHC protein content was assessed by immunohistochemistry. MyHC transcripts were found to be homogeneously distributed in the myocardium of the tubular and embryonic heart of dogfish and rodents. A difference between atrial and ventricular MyHC content (mRNA and protein) was observed in the adult stage. Interestingly, differences in the MyHC content were observed at the tubular heart stage in chicken. These differences in MyHC content illustrate the distinct developmental profiles of avian and mammalian species, which might be ascribed to distinct functional requirements of the myocardial segments during ontogenesis. The atrial myocardium showed the highest MyHC content in the adult heart of all species analysed (dogfish (S. canicula), mouse (M. musculus), rat (R. norvegicus), and chicken (G. gallus)). These observations indicate that in the adult heart of vertebrates the atrial myocardium contains more myosin than the ventricular myocardium.

Effects of aging on human skeletal muscle myosin heavy-chain mRNA content and protein isoform expression

The purpose of this investigation was to determine the role played by pretranslational events in the decreased rate of myosin heavy-chain (MyHC) protein synthesis in old age. It was hypothesized that the decreased rate of MyHC protein synthesis reported in the elderly population is, at least in part, related to lower MyHC messenger RNA (mRNA) in old age. MyHC protein expression and mRNA levels for the three MyHC isoforms expressed in human muscle, that is, types I, IIa, and IIx/d, were measured in percutaneous vastus lateralis muscle biopsies from 16 young and 16 old healthy men. The MyHC isoform mRNA content was determined by quantitative, real-time reverse transcriptase polymerase chain reaction, and it was normalized to 18S ribosomal RNA; the relative MyHC protein isoform content was measured on silver-stained 7% sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels. The old men demonstrated signs of sarcopenia, such as loss of muscle force, a trend toward a loss in lean body mass, and an increased percentage of body fat. Statistically significant correlations were observed between the isoform expression of different MyHCs at the protein and mRNA levels. However, the expression of the different MyHC isoforms at the mRNA and protein levels did not differ between the young and old men. Thus, the present results do not support the hypothesis that pretranslational events in MyHC protein synthesis are playing a significant role in the development of sarcopenia.

Quantifying the temporospatial expression of postnatal porcine skeletal myosin heavy chain genes

Postnatal skeletal muscle fiber type is commonly defined by one of four major myosin heavy chain (MyHC) gene isoforms (slow/I, 2a, 2x, and 2b) that are expressed. We report on the novel use of combined TaqMan quantitative real-time RT-PCR and image analysis of serial porcine muscle sections, subjected to in situ hybridization (ISH) and immunocytochemistry (IHC), to quantify the mRNA expression of each MyHC isoform within its corresponding fiber type, termed relative fiber type-restricted expression. This versatile approach will allow quantitative temporospatial comparisons of each MyHC isoform among muscles from the same or different individuals. Using this approach on porcine skeletal muscles, we found that the relative fiber type-restricted expression of each postnatal MyHC gene showed wide spatial and temporal variation within a given muscle and between muscles. Marked differences were also observed among pig breeds. Notably, of the four postnatal MyHC isoforms, the 2a MyHC gene showed the highest relative fiber type-restricted expression in each muscle examined, regardless of age, breed, or muscle type. This suggests that although 2a fibers are a minor fiber type, they may be disproportionately more important as a determinant of overall muscle function than was previously believed.

Acute exercise induced changes in rat skeletal muscle mRNAs and proteins regulating type IV collagen content

This experiment tested the hypothesis that running-induced damage to rat skeletal muscle causes changes in synthesis and degradation of basement membrane type IV collagen and to proteins regulating its degradation. Samples from soleus muscle and red and white parts of quadriceps femoris muscle (MQF) were collected 6 h or 1, 2, 4, or 7 days after downhill running. Increased muscle beta-glucuronidase activity indicated greater muscle damage in the red part of MQF than in the white part of MQF or soleus. In the red part of MQF, type IV collagen expression was upregulated at the pretranslational level and the protein concentration decreased, whereas matrix metalloproteinase-2 (MMP-2), a protein that degrades type IV collagen, and tissue inhibitor of metalloproteinase-2 (TIMP-2), a protein that inhibits degradation, were increased in parallel both at mRNA and protein levels. Type IV collagen mRNA level increased in the white part of MQF and soleus muscle. The protein concentration increased in the white part of MQF and was unchanged in soleus muscle. MMP-2 and TIMP-2 changed only slightly in the white part of MQF and soleus muscle. The changes seem to depend on the severity of myofiber injury and thus probably reflect reorganization of basement membrane compounds.

Creatine transporter protein content, localization, and gene expression in rat skeletal muscle

The present study examined the gene expression and cellular localization of the creatine transporter (CreaT) protein in rat skeletal muscle. Soleus (SOL) and red (RG) and white gastrocnemius (WG) muscles were analyzed for CreaT mRNA, CreaT protein, and total creatine (TCr) content. Cellular location of the CreaT protein was visualized with immunohistochemical analysis of muscle cross sections. TCr was higher (P < or = 0.05) in WG than in both RG and SOL, and was higher in RG than in SOL. Total CreaT protein content was greater (P < or = 0.05) in SOL and RG than in WG. Two bands (55 and 70 kDa) of the CreaT protein were found in all muscle types. Both the 55-kDa (CreaT-55) and the 70-kDa (CreaT-70) bands were present in greater (P < or = 0.05) amounts in SOL and RG than in WG. SOL and RG had a greater amount (P < or = 0.05) of CreaT-55 than CreaT-70. Immunohistochemical analysis revealed that the CreaT was mainly associated with the sarcolemmal membrane in all muscle types. CreaT mRNA expression per microgram of total RNA was similar across the three muscle types. These data indicate that rat SOL and RG have an enhanced potential to transport Cr compared with WG, despite a higher TCr in the latter.

Chamber formation and morphogenesis in the developing mammalian heart

In this study we challenge the generally accepted view that cardiac chambers form from an array of segmental primordia arranged along the anteroposterior axis of the linear and looping heart tube. We traced the spatial pattern of expression of genes encoding atrial natriuretic factor, sarcoplasmic reticulum calcium ATPase, Chisel, Irx5, Irx4, myosin light chain 2v, and beta-myosin heavy chain and related these to morphogenesis. Based on the patterns we propose a two-step model for chamber formation in the embryonic heart. First, a linear heart forms, which is composed of "primary" myocardium that nonetheless shows polarity in phenotype and gene expression along its anteroposterior and dorsoventral axes. Second, specialized ventricular chamber myocardium is specified at the ventral surface of the linear heart tube, while distinct left and right atrial myocardium forms more caudally on laterodorsal surfaces. The process of looping aligns these primordial chambers such that they face the outer curvature. Myocardium of the inner curvature, as well as that of inflow tract, atrioventricular canal, and outflow tract, retains the molecular signature originally found in linear heart tube myocardium. Evidence for distinct transcriptional programs which govern compartmentalization in the forming heart is seen in the patterns of expression of Hand1 for the dorsoventral axis, Irx4 and Tbx5 for the anteroposterior axis, and Irx5 for the distinction between primary and chamber myocardium.