Calcineurin differentially regulates maintenance and growth of phenotypically distinct muscles

Transcriptome Profiling Identifies Differentially Expressed Genes in Skeletal Muscle Development in Native Chinese Ducks

China boasts a rich diversity of indigenous duck species, some of which exhibit desirable economic traits. Here, we generated transcriptome sequencing datasets of breast muscle tissue samples from 1D of four groups: Pekin duck pure breeding group (P), Jinling White duck breeding group (J), P ♂ × J ♀ orthogonal group (PJ) and J ♂ × P ♀ reciprocal-cross group (JP) (n = 3), chosen based on the distinctive characteristics of duck muscle development during the embryonic period. We identified 5053 differentially expressed genes (DEGs) among the four groups. Network prediction analysis showed that ribosome and oxidative phosphorylation-related genes were the most enriched, and muscular protein-related genes were found in the 14-day-old embryonic group. We found that previously characterized functional genes, such as FN1, AGRN, ADNAMST3, APOB and FGF9, were potentially involved in muscle development in 14-day-old embryos. Functional enrichment analysis suggested that genes that participated in molecular function and cell component and key signaling pathways (e.g., hippo, ribosome, oxidative phosphorylation) were significantly enriched in the development of skeletal muscle at 14 days of embryonic age. These results indicate a possible role of muscle metabolism and myoglobin synthesis in skeletal muscle development in both duck parents and hybrids.

Analysis of mRNA and lncRNA Expression Profiles of Breast Muscle during Pigeon (Columbalivia) Development

The breast muscle is essential for flight and determines the meat yield and quality of the meat type in pigeons. At present, studies about long non-coding RNA (lncRNA) expression profiles in skeletal muscles across the postnatal development of pigeons have not been reported. Here, we used transcriptome sequencing to examine the White-King pigeon breast muscle at four different ages (1 day, 14 days, 28 days, and 2 years old). We identified 12,918 mRNAs and 9158 lncRNAs (5492 known lncRNAs and 3666 novel lncRNAs) in the breast muscle, and 7352 mRNAs and 4494 lncRNAs were differentially expressed in the process of development. We found that highly expressed mRNAs were mainly related to cell-basic and muscle-specific functions. Differential expression and time-series analysis showed that differentially expressed genes were primarily associated with muscle development and functions, blood vessel development, cell cycle, and energy metabolism. To further predict the possible role of lncRNAs, we also conducted the WGCNA and trans/cis analyses. We found that differentially expressed lncRNAs such as lncRNA-LOC102093252, lncRNA-G12653, lncRNA-LOC110357465, lncRNA-G14790, and lncRNA-LOC110360188 might respectively target UBE2B, Pax7, AGTR2, HDAC1, Sox8 and participate in the development of the muscle. Our study provides a valuable resource for studying the lncRNAs and mRNAs of pigeon muscles and for improving the understanding of molecular mechanisms in muscle development.

Cellular and molecular pathways controlling muscle size in response to exercise

From the discovery of ATP and motor proteins to synaptic neurotransmitters and growth factor control of cell differentiation, skeletal muscle has provided an extreme model system in which to understand aspects of tissue function. Muscle is one of the few tissues that can undergo both increase and decrease in size during everyday life. Muscle size depends on its contractile activity, but the precise cellular and molecular pathway(s) by which the activity stimulus influences muscle size and strength remain unclear. Four correlates of muscle contraction could, in theory, regulate muscle growth: nerve-derived signals, cytoplasmic calcium dynamics, the rate of ATP consumption and physical force. Here, we summarise the evidence for and against each stimulus and what is known or remains unclear concerning their molecular signal transduction pathways and cellular effects. Skeletal muscle can grow in three ways, by generation of new syncytial fibres, addition of nuclei from muscle stem cells to existing fibres or increase in cytoplasmic volume/nucleus. Evidence suggests the latter two processes contribute to exercise-induced growth. Fibre growth requires increase in sarcolemmal surface area and cytoplasmic volume at different rates. It has long been known that high-force exercise is a particularly effective growth stimulus, but how this stimulus is sensed and drives coordinated growth that is appropriately scaled across organelles remains a mystery.

The role of SERCA and sarcolipin in adaptive muscle remodeling

Sarcolipin (SLN) is a small integral membrane protein that regulates the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pump. When bound to SERCA, SLN reduces the apparent Ca²⁺ affinity of SERCA and uncouples SERCA Ca²⁺ transport from its ATP consumption. As such, SLN plays a direct role in altering skeletal muscle relaxation and energy expenditure. Interestingly, the expression of SLN is dynamic during times of muscle adaptation, where large increases in SLN content are found in response to development, atrophy, overload and disease. Several groups have suggested that increases in SLN, especially in dystrophic muscle, are deleterious to muscle function and exacerbate already abhorrent intracellular Ca²⁺ levels. However, there is also significant evidence to show that increased SLN content is a beneficial adaptive mechanism which protects the SERCA pump and activates Ca²⁺ signaling and adaptive remodeling during times of cell stress. In this review, we first discuss the role for SLN in healthy muscle during both development and overload, where SLN has been shown to activate Ca²⁺ signaling to promote mitochondrial biogenesis, fibre type shifts and muscle hypertrophy. Then, with respect to muscle disease, we summarize the discrepancies in the literature as to whether SLN upregulation is adaptive or maladaptive in nature. This review is the first to offer the concept of SLN hormesis in muscle disease, wherein both too much and too little SLN are detrimental to muscle health. Finally, the underlying mechanisms which activate SLN upregulation are discussed, specifically acknowledging a potential positive feedback loop between SLN and Ca²⁺ signaling molecules.

Analysis of potential regulatory LncRNAs and CircRNAs in the oxidative myofiber and glycolytic myofiber of chickens

SART and PMM are mainly composed of oxidative myofibers and glycolytic myofibers, respectively, and myofiber types profoundly influence postnatal muscle growth and meat quality. SART and PMM are composed of lncRNAs and circRNAs that participate in myofiber type regulation. To elucidate the regulatory mechanism of myofiber type, lncRNA and circRNA sequencing was used to systematically compare the transcriptomes of the SART and PMM of Chinese female Qingyuan partridge chickens at their marketing age. The luminance value (L*), redness value (a*), average diameter, cross-sectional area, and density difference between the PMM and SART were significant (p < 0.05). ATPase staining results showed that PMMs were all darkly stained and belonged to the glycolytic type, and the proportion of oxidative myofibers in SART was 81.7%. A total of 5 420 lncRNAs were identified, of which 365 were differentially expressed in the SART compared with the PMM (p < 0.05). The cis-regulatory analysis identified target genes that were enriched for specific GO terms and KEGG pathways (p < 0.05), including striated muscle cell differentiation, regulation of cell proliferation, regulation of muscle cell differentiation, myoblast differentiation, regulation of myoblast differentiation, and MAPK signaling pathway. Pathways and coexpression network analyses suggested that XR_003077811.1, XR_003072304.1, XR_001465942.2, XR_001465741.2, XR_001470487.1, XR_003077673.1 and XR_003074785.1 played important roles in regulating oxidative myofibers by TBX3, QKI, MYBPC1, CALM2, and PPARGC1A expression. A total of 10 487 circRNAs were identified, of which 305 circRNAs were differentially expressed in the SART compared with the PMM (p < 0.05). Functional enrichment analysis showed that differentially expressed circRNAs were involved in host gene expression and were enriched in the AMPK, calcium signaling pathway, FoxO signaling pathway, p53 signaling pathway, and cellular senescence. Novel_circ_004282 and novel_circ_002121 played important roles in regulating oxidative myofibers by PPP3CA and NFATC1 expression. Using lncRNA-miRNA/circRNA-miRNA integrated analysis, we identified many candidate interaction networks that might affect muscle fiber performance. Important lncRNA-miRNA-mRNA networks, such as lncRNA-XR_003074785.1/miR-193-3p/PPARGC1A, regulate oxidative myofibers. This study reveals that lncXR_003077811.1, lncXR_003072304.1, lncXR_001465942.2, lncXR_001465741.2, lncXR_001470487.1, lncXR_003077673.1, XR_003074785.1, novel_circ_004282 and novel_circ_002121 might regulate oxidative myofibers. The lncRNA-XR_003074785.1/miR-193-3p/PPARGC1A pathway might regulate oxidative myofibers. All these findings provide rich resources for further in-depth research on the regulatory mechanism of lncRNAs and circRNAs in myofibers.

Changes in Body Composition, Muscle Strength, and Fat Distribution Following Kidney Transplantation

RATIONALE AND OBJECTIVE

Low muscle mass relative to fat mass (relative sarcopenia) has been associated with mortality and disability but has not been examined following transplantation. We studied how measures of body composition change after receipt of a kidney allograft.

STUDY DESIGN

Prospective longitudinal cohort study.

SETTING AND PARTICIPANTS

60 kidney transplant recipients (ages 20-60 years) at the University of Pennsylvania.

EXPOSURE

Kidney transplantation.

OUTCOMES

DXA measures of fat mass index (FMI) and appendicular lean mass index (ALMI; representing muscle mass), CT measures of muscle density (low density represents increased intramuscular adipose tissue), dynamometer measures of leg muscle strength, and physical activity. ALMI relative to FMI (ALM_FMI) is an established index of relative sarcopenia.

ANALYTICAL APPROACH

Measures expressed as age, sex, and race-specific Z-scores for transplant recipients were compared to 327 healthy controls. Regression models were used to identify correlates of change in outcome Z-scores and compare transplant recipients to controls.

RESULTS

At transplantation, ALMI, ALMI_FMI, muscle strength and muscle density Z-scores were lower vs. controls (all p≤0.001). Transplant recipients received glucocorticoids throughout. The prevalence of obesity increased from 18 to 45%. Although ALMI increased following transplantation (p<0.001) and was comparable to controls from 6 months onward, gains were outpaced by increases in FMI, resulting in persistent ALMI_FMI deficits (mean Z-score -0.31 at 24 months, p=0.02 vs controls). Muscle density improved following transplantation despite gains in FMI (p = 0.02). Muscle strength relative to ALMI also improved (p = 0.04) but remained low compared with controls (p=0.01). Exercise increased in the early months following transplantation (p<0.05) but remained lower than controls (p=0.02).

LIMITATIONS

Lack of muscle biopsies precluded assessment of muscle histology and metabolism.

CONCLUSIONS

The two-year interval following kidney transplantation was characterized by gains in muscle mass and strength that were outpaced by gains in fat mass resulting in persistent relative sarcopenia.

Expression of FKBP prolyl isomerase 5 gene in tissues of muscovy duck at different growth stages and its association with muscovy duck weight

Objective

FKBP prolyl isomerase 5 (FKBP5) has been shown to play an important role in metabolically active tissues such as skeletal muscle. However, the expression of FKBP5 in Muscovy duck tissues and its association with body weight are still unclear.

Methods

In this study, real-time quantitative polymerase chain reaction was used to detect the expression of FKBP5 in different tissues of Muscovy duck at different growth stages. Further, single nucleotide polymorphisms (SNPs) were detected in the exon region of FKBP5 and were combined analyzed with the body weight of 334 Muscovy ducks.

Results

FKBP5 was highly expressed in various tissues of Muscovy duck at days 17, 19, 21, 24, and 27 of embryonic development. In addition, the expression of FKBP5 in the tissues of female adult Muscovy ducks was higher than that of male Muscovy ducks. Besides, an association analysis indicated that 3 SNPs were related to body weight trait. At the g.4819252 A>G, the body weight of AG genotype was significantly higher than that of the AA and the GG genotype. At the g.4821390 G>A, the genotype GA was extremely significantly related to body weight. At the g.4830622 T>G, the body weight of TT was significantly higher than GG and TG.

Conclusion

These findings indicate the possible effects of expression levels in various tissues and the SNPs of FKBP5 on Muscovy duck body weight trait. FKBP5 could be used as molecular marker for muscle development trait using early marker-assisted selection of Muscovy ducks.

Skeletal Muscle Transcriptome Analysis of Hanzhong Ma Duck at Different Growth Stages Using RNA-Seq

As one of the most important poultry worldwide, ducks (Anas platyrhynchos) are raised mainly for meat and egg products, and muscle development in ducks is important for meat production. Therefore, an investigation of gene expression in duck skeletal muscle would significantly contribute to our understanding of muscle development. In this study, twenty-four cDNA libraries were constructed from breast and leg muscles of Hanzhong Ma ducks at day 17, 21, 27 of the embryo and postnatal at 6-month-old. High-throughput sequencing and bioinformatics were used to determine the abundances and characteristics of transcripts. A total of 632,172,628 (average 52,681,052) and 637,213,938 (average 53,101,162) reads were obtained from the sequencing data of breast and leg muscles, respectively. Over 71.63% and 77.36% of the reads could be mapped to the Anas platyrhynchos genome. In the skeletal muscle of Hanzhong duck, intron variant (INTRON), synonymous variant (SYNONYMOUS_CODING), and prime 3' UTR variant (UTR_3_PRIME) were the main single nucleotide polymorphisms (SNP) annotation information, and "INTRON", "UTR_3_PRIME", and downstream-gene variant (DOWNSTREAM) were the main insertion-deletion (InDel) annotation information. The predicted number of alternative splicing (AS) in all samples were mainly alternative 5' first exon (transcription start site)-the first exon splicing (TSS) and alternative 3' last exon (transcription terminal site)-the last exon splicing (TTS). Besides, there were 292 to 2801 annotated differentially expressed genes (DEGs) in breast muscle and 304 to 1950 annotated DEGs in leg muscle from different databases. It is worth noting that 75 DEGs in breast muscle and 49 DEGs in leg muscle were co-expressed at all developmental points of comparison, respectively. The RNA-Seq data were confirmed to be reliable by qPCR. The identified DEGs, such as CREBL2, RHEB, GDF6, SHISA2, MYLK2, ACTN3, RYR3, and STMN1, were specially highlighted, indicating their strong associations with muscle development in the Hanzhong Ma duck. KEGG pathway analysis suggested that regulation of actin cytoskeleton, oxidative phosphorylation, and focal adhesion were involved in the development of skeletal muscle. The findings from this study can contribute to future investigations of the growth and development mechanism in duck skeletal muscle.

Comparative Transcriptome Profiling of Skeletal Muscle from Black Muscovy Duck at Different Growth Stages Using RNA-seq

In China, the production for duck meat is second only to that of chicken, and the demand for duck meat is also increasing. However, there is still unclear on the internal mechanism of regulating skeletal muscle growth and development in duck. This study aimed to identity candidate genes related to growth of duck skeletal muscle and explore the potential regulatory mechanism. RNA-seq technology was used to compare the transcriptome of skeletal muscles in black Muscovy ducks at different developmental stages (day 17, 21, 27, 31, and 34 of embryos and postnatal 6-month-olds). The SNPs and InDels of black Muscovy ducks at different growth stages were mainly in “INTRON”, “SYNONYMOUS_CODING”, “UTR_3_PRIME”, and “DOWNSTREAM”. The average number of AS in each sample was 37,267, mainly concentrated in TSS and TTS. Besides, a total of 19 to 5377 DEGs were detected in each pairwise comparison. Functional analysis showed that the DEGs were mainly involved in the processes of cell growth, muscle development, and cellular activities (junction, migration, assembly, differentiation, and proliferation). Many of DEGs were well known to be related to growth of skeletal muscle in black Muscovy duck, such as MyoG, FBXO1, MEF2A, and FoxN2. KEGG pathway analysis identified that the DEGs were significantly enriched in the pathways related to the focal adhesion, MAPK signaling pathway and regulation of the actin cytoskeleton. Some DEGs assigned to these pathways were potential candidate genes inducing the difference in muscle growth among the developmental stages, such as FAF1, RGS8, GRB10, SMYD3, and TNNI2. Our study identified several genes and pathways that may participate in the regulation of skeletal muscle growth in black Muscovy duck. These results should serve as an important resource revealing the molecular basis of muscle growth and development in duck.

The involvement of transient receptor potential canonical type 1 in skeletal muscle regrowth after unloading-induced atrophy

KEY POINTS

Decreased mechanical loading results in skeletal muscle atrophy. The transient receptor potential canonical type 1 (TRPC1) protein is implicated in this process. Investigation of the regulation of TRPC1 in vivo has rarely been reported. In the present study, we employ the mouse hindlimb unloading and reloading model to examine the involvement of TRPC1 in the regulation of muscle atrophy and regrowth, respectively. We establish the physiological relevance of the concept that manipulation of TRPC1 could interfere with muscle regrowth processes following an atrophy-inducing event. Specifically, we show that suppressing TRPC1 expression during reloading impairs the recovery of the muscle mass and slow myosin heavy chain profile. Calcineurin appears to be part of the signalling pathway involved in the regulation of TRPC1 expression during muscle regrowth. These results provide new insights concerning the function of TRPC1. Interventions targeting TRPC1 or its downstream or upstream pathways could be useful for promoting muscle regeneration.

ABSTRACT

Decreased mechanical loading, such as bed rest, results in skeletal muscle atrophy. The functional consequences of decreased mechanical loading include a loss of muscle mass and decreased muscle strength, particularly in anti-gravity muscles. The purpose of this investigation was to clarify the regulatory role of the transient receptor potential canonical type 1 (TRPC1) protein during muscle atrophy and regrowth. Mice were subjected to 14 days of hindlimb unloading followed by 3, 7, 14 and 28 days of reloading. Weight-bearing mice were used as controls. TRPC1 expression in the soleus muscle decreased significantly and persisted at 7 days of reloading. Small interfering RNA (siRNA)-mediated downregulation of TRPC1 in weight-bearing soleus muscles resulted in a reduced muscle mass and a reduced myofibre cross-sectional area (CSA). Microinjecting siRNA into soleus muscles in vivo after 7 days of reloading provided further evidence for the role of TRPC1 in regulating muscle regrowth. Myofibre CSA, as well as the percentage of slow myosin heavy chain-positive myofibres, was significantly lower in TRPC1-siRNA-expressing muscles than in control muscles after 14 days of reloading. Additionally, inhibition of calcineurin (CaN) activity downregulated TRPC1 expression in both weight-bearing and reloaded muscles, suggesting a possible association between CaN and TRPC1 during skeletal muscle regrowth.

Expression profile of IGF-I-calcineurin-NFATc3-dependent pathway genes in skeletal muscle during early development between duck breeds differing in growth rates

The insulin-like growth factor I (IGF-I)-calcineurin (CaN)-NFATc signaling pathways have been implicated in the regulation of myocyte hypertrophy and fiber-type specificity. In the present study, the expression of the CnAα, NFATc3, and IGF-I genes was quantified by RT-PCR for the first time in the breast muscle (BM) and leg muscle (LM) on days 13, 17, 21, 25, and 27 of embryonic development, as well as at 7 days posthatching (PH), in Gaoyou and Jinding ducks, which differ in their muscle growth rates. Consistent expression patterns of CnAα, NFATc3, and IGF-I were found in the same anatomical location at different development stages in both duck breeds, showing significant differences in an age-specific fashion. However, the three genes were differentially expressed in the two different anatomical locations (BM and LM). CnAα, NFATc3, and IGF-I messenger RNA (mRNA) could be detected as early as embryonic day 13 (ED13), and the highest level appeared at this stage in both BM and LM. Significant positive relationships were observed in the expression of the studied genes in the BM and LM of both duck breeds. Also, the expression of these three genes showed a positive relationship with the percentage of type IIb fibers and a negative relationship with the percentage of type I fibers and type IIa fibers. Our data indicate differential expression and coordinated developmental regulation of the selected genes involved in the IGF-I-calcineurin-NFATc3 pathway in duck skeletal muscle during embryonic and early PH growth and development; these data also indicate that this signaling pathway might play a role in the regulation of myofiber type transition.

Anabolic steroids activate calcineurin-NFAT signaling and thereby increase myotube size and reduce denervation atrophy

Anabolic androgens have been shown to reduce muscle loss due to immobilization, paralysis and many other medical conditions, but the molecular basis for these actions is poorly understood. We have recently demonstrated that nandrolone, a synthetic androgen, slows muscle atrophy after nerve transection associated with down-regulation of regulator of calcineurin 2 (RCAN2), a calcineurin inhibitor, suggesting a possible role of calcineurin-NFAT signaling. To test this possibility, rat gastrocnemius muscle was analyzed at 56 days after denervation. In denervated muscle, calcineurin activity declined and NFATc4 was excluded from the nucleus and these effects were reversed by nandrolone. Similarly, nandrolone increased calcineurin activity and nuclear NFATc4 levels in cultured L6 myotubes. Nandrolone also induced cell hypertrophy that was blocked by cyclosporin A or overexpression of RCAN2. Finally protection against denervation atrophy by nandrolone in rats was blocked by cyclosporin A. These results demonstrate for the first time that nandrolone activates calcineurin-NFAT signaling, and that such signaling is important in nandrolone-induced cell hypertrophy and protection against paralysis-induced muscle atrophy.

Adaptive responses of TRPC1 and TRPC3 during skeletal muscle atrophy and regrowth

INTRODUCTION

We assessed the time-dependent changes of transient receptor potential canonical type 1 (TRPC1) and TRPC3 expression and localization associated with muscle atrophy and regrowth in vivo.

METHODS

Mice were subjected to hindlimb unloading for 7 or 14 days (7U, 14U) followed by 3, 7, or 14 days of reloading (3R, 7R, 14R).

RESULTS

Soleus muscle mass and tetanic force were reduced significantly at 7U and 14U and recovered by 14R. Recovery of muscle fiber cross-sectional area was observed by 28R. TRPC1 mRNA was unaltered during the unloading-reloading period. However, protein expression remained depressed through 14R. Decreased localization of TRPC1 to the sarcolemma was observed. TRPC3 mRNA and protein expression levels were decreased significantly during the early phase of reloading.

CONCLUSIONS

Given the known role of these channels in muscle development, changes observed in TRPC1 and TRPC3 may relate closely to muscle atrophy and remodeling processes.

Molecular cloning, structural analysis and tissue expression of protein phosphatase 3 catalytic subunit alpha isoform (PPP3CA) gene in Tianfu goat muscle

Calcineurin, a Ca(2+)/calmodulin-dependent protein phosphatase, plays a critical role in controlling skeletal muscle fiber type. However, little information is available concerning the expression of calcineurin in goat. Therefore, protein phosphatase 3 catalytic subunit alpha isoform (PPP3CA) gene, also called calcineurin Aα, was cloned and its expression characterized in Tianfu goat muscle. Real time quantitative polymerase chain reaction (RT-qPCR) analyses revealed that Tianfu goat PPP3CA was detected in cardiac muscle, biceps femoris muscle, abdominal muscle, longissimus dors muscle, and soleus muscle. High expression levels were found in biceps femoris muscle, longissimus muscle and abdominal muscle (p < 0.01), and low expression levels were seen in cardiac muscle and soleus muscle (p > 0.05). In addition, the spatial-temporal mRNA expression levels showed different variation trends in different muscles with the age of the goats. Western blotting further revealed that PPP3CA protein was expressed in the above-mentioned tissues, with the highest level in biceps femoris muscle, and the lowest level in soleus muscle. In this study, we isolated the full-length coding sequence of Tianfu goat PPP3CA gene, analyzed its structure, and investigated its expression in different muscle tissues from different age stages. These results provide a foundation for understanding the function of the PPP3CA gene in goats.

Calcineurin: a poorly understood regulator of muscle mass

This review will discuss the existing literature that has examined the role of calcineurin (CnA) in the regulation of skeletal muscle mass in conditions associated with hypertrophic growth or atrophy. Muscle mass is determined by the balance between protein synthesis and degradation which is controlled by a number of intracellular signaling pathways, most notably the insulin/IGF/phosphatidylinositol 3-kinase (PI3K)/Akt system. Despite being activated by IGF-1 and having well-described functions in the determination of muscle fiber phenotypes, calcineurin (CnA), a Ca(2+)-activated serine/threonine phosphatase, and its downstream signaling partners have garnered little attention as a regulator of muscle mass. Compared to other signaling pathways, the relatively few studies that have examined the role of CnA in the regulation of muscle size have produced discordant results. The reasons for these differences is not obvious but may be due to the selective nature of the genetic models studied, fluctuations in the endogenous level of CnA activity in various muscles, and the variable use of CnA inhibitors to inhibit CnA signaling. Despite the inconsistent nature of the outcomes, there is sufficient direct and indirect evidence to conclude that CnA plays a role in the regulation of skeletal muscle mass. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Localized calcineurin confers Ca2+-dependent inactivation on neuronal L-type Ca2+ channels

Excitation-driven entry of Ca(2+) through L-type voltage-gated Ca(2+) channels controls gene expression in neurons and a variety of fundamental activities in other kinds of excitable cells. The probability of opening of Ca(V)1.2 L-type channels is subject to pronounced enhancement by cAMP-dependent protein kinase (PKA), which is scaffolded to Ca(V)1.2 channels by A-kinase anchoring proteins (AKAPs). Ca(V)1.2 channels also undergo negative autoregulation via Ca(2+)-dependent inactivation (CDI), which strongly limits Ca(2+) entry. An abundance of evidence indicates that CDI relies upon binding of Ca(2+)/calmodulin (CaM) to an isoleucine-glutamine motif in the carboxy tail of Ca(V)1.2 L-type channels, a molecular mechanism seemingly unrelated to phosphorylation-mediated channel enhancement. But our work reveals, in cultured hippocampal neurons and a heterologous expression system, that the Ca(2+)/CaM-activated phosphatase calcineurin (CaN) is scaffolded to Ca(V)1.2 channels by the neuronal anchoring protein AKAP79/150, and that overexpression of an AKAP79/150 mutant incapable of binding CaN (ΔPIX; CaN-binding PXIXIT motif deleted) impedes CDI. Interventions that suppress CaN activity-mutation in its catalytic site, antagonism with cyclosporine A or FK506, or intracellular perfusion with a peptide mimicking the sequence of the phosphatase's autoinhibitory domain-interfere with normal CDI. In cultured hippocampal neurons from a ΔPIX knock-in mouse, CDI is absent. Results of experiments with the adenylyl cyclase stimulator forskolin and with the PKA inhibitor PKI suggest that Ca(2+)/CaM-activated CaN promotes CDI by reversing channel enhancement effectuated by kinases such as PKA. Hence, our investigation of AKAP79/150-anchored CaN reconciles the CaM-based model of CDI with an earlier, seemingly contradictory model based on dephosphorylation signaling.

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.

Differential alterations in gene expression profiles contribute to time-dependent effects of nandrolone to prevent denervation atrophy

BACKGROUND

Anabolic steroids, such as nandrolone, slow muscle atrophy, but the mechanisms responsible for this effect are largely unknown. Their effects on muscle size and gene expression depend upon time, and the cause of muscle atrophy. Administration of nandrolone for 7 days beginning either concomitantly with sciatic nerve transection (7 days) or 29 days later (35 days) attenuated denervation atrophy at 35 but not 7 days. We reasoned that this model could be used to identify genes that are regulated by nandrolone and slow denervation atrophy, as well as genes that might explain the time-dependence of nandrolone effects on such atrophy. Affymetrix microarrays were used to profile gene expression changes due to nandrolone at 7 and 35 days and to identify major gene expression changes in denervated muscle between 7 and 35 days.

RESULTS

Nandrolone selectively altered expression of 124 genes at 7 days and 122 genes at 35 days, with only 20 genes being regulated at both time points. Marked differences in biological function of genes regulated by nandrolone at 7 and 35 days were observed. At 35, but not 7 days, nandrolone reduced mRNA and protein levels for FOXO1, the mTOR inhibitor REDD2, and the calcineurin inhibitor RCAN2 and increased those for ApoD. At 35 days, correlations between mRNA levels and the size of denervated muscle were negative for RCAN2, and positive for ApoD. Nandrolone also regulated genes for Wnt signaling molecules. Comparison of gene expression at 7 and 35 days after denervation revealed marked alterations in the expression of 9 transcriptional coregulators, including Ankrd1 and 2, and many transcription factors and kinases.

CONCLUSIONS

Genes regulated in denervated muscle after 7 days administration of nandrolone are almost entirely different at 7 versus 35 days. Alterations in levels of FOXO1, and of genes involved in signaling through calcineurin, mTOR and Wnt may be linked to the favorable action of nandrolone on denervated muscle. Marked changes in the expression of genes regulating transcription and intracellular signaling may contribute to the time-dependent effects of nandrolone on gene expression.

The functional role of calcineurin in hypertrophy, regeneration, and disorders of skeletal muscle

Skeletal muscle uses calcium as a second messenger to respond and adapt to environmental stimuli. Elevations in intracellular calcium levels activate calcineurin, a serine/threonine phosphatase, resulting in the expression of a set of genes involved in the maintenance, growth, and remodeling of skeletal muscle. In this review, we discuss the effects of calcineurin activity on hypertrophy, regeneration, and disorders of skeletal muscle. Calcineurin is a potent regulator of muscle remodeling, enhancing the differentiation through upregulation of myogenin or MEF2A and downregulation of the Id1 family and myostatin. Foxo may also be a downstream candidate for a calcineurin signaling molecule during muscle regeneration. The strategy of controlling the amount of calcineurin may be effective for the treatment of muscular disorders such as DMD, UCMD, and LGMD. Activation of calcineurin produces muscular hypertrophy of the slow-twitch soleus muscle but not fast-twitch muscles.

Effects of aging and gender on muscle mass and regulation of Akt-mTOR-p70s6k related signaling in the F344BN rat model

Sarcopenia is the loss of muscle mass and strength which occurs with aging. Whether the molecular basis of sarcopenia differs with muscle type and across sex is not well understood. Here we examine how aging affects the regulation of protein kinase B (Akt), the mammalian target of rapamycin (mTOR), AMP activated kinase (AMPK), p70 ribosomal S6 kinase (p70s6k), S6 ribosomal protein (rps6) and calcineurin (CaN) in the slow soleus and fast extensor digitorum longus (EDL) muscles of 6- (adult), 30- (aged), and 36-month (very aged) male and 6- (adult), 26- (aged), and 30-month (very aged) female Fischer 344xBrown Norway (F344BN) rats. In male animals, soleus and EDL muscle to body weight ratios decreased steadily with age while in the females, losses remained unchanged after 26 months. These age-related changes in the degree of muscle atrophy across sex were associated with differences in the regulation of Akt, mTOR, and p70s6k in the slow-twitch soleus and the regulation of AMPK, 4EBP1, p70s6k and rpS6 in the fast-twitch EDL. Irrespective of muscle type, aging in both the genders was associated with increased calcineurin expression. Taken together, these data suggest that indices of protein synthesis and muscle adaptation are regulated differently with aging in different muscle types and sex.

Calcium-dependent signaling mechanisms and soleus fiber remodeling under gravitational unloading

The decrease in postural muscle fiber size, diminishing of their contractile properties, slow-to-fast shift in myosin heavy chain expression pattern are known to be the main consequences of gravitational unloading. The Ca(2+) role in these processes has been studied for about 20 years. Ingalls et al. [J Appl Physiol 87(1):382-390, 1999] found the resting Ca(2+) level increase in soleus fibers of hindlimb unloaded mice. Results obtained in our laboratory showed that systemic or local application of nifedipine (L-type Ca(2+) channels' blocker) prevents Ca(2+) accumulation in fibers. Thus, activation of dihydropyridine calcium channels can be supposed to promote resting Ca(2+) loading under disuse. So, calcium-dependent signaling pathways may play an important role in the development of some key events observed under unloading. Since 90th the increased activities of Ca(2+)-dependent proteases (calpains) were considered as the crucial effect of hypogravity-induced muscle atrophy, which was proved later. We observed maintenance of titin and nebulin relative content in soleus muscle under unloading combined with Ca(2+) chelators administration. Nifedipine administration was shown to considerably restrict the slow-to-fast transition of myosin heavy chains (MHC) under unloading (at the RNA level and at the protein level as well). To clarify the role of calcineurin/NFAT signaling system in MHC pattern transition under unloading, we blocked this pathway by cyclosporine A application. Hereby, we demonstrated that calcineurin/NFAT pathway possesses a stabilizing function counteracting the myosin phenotype transformation under gravitational unloading.

Slow myosin heavy chain expression in the absence of muscle activity

Innervation has been generally accepted to be a major factor involved in both triggering and maintaining the expression of slow myosin heavy chain (MHC-1) in skeletal muscle. However, previous findings from our laboratory have suggested that, in the mouse, this is not always the case (30). Based on these results, we hypothesized that neurotomy would not markedly reduced the expression of MHC-1 protein in the mouse soleus muscles. In addition, other cellular, biochemical, and functional parameters were also studied in these denervated soleus muscles to complete our study. Our results show that denervation reduced neither the relative amount of MHC-1 protein, nor the percentage of muscle fibers expressing MHC-1 protein (P > 0.05). The fact that MHC-1 protein did not respond to muscle inactivity was confirmed in three different mouse strains (129/SV, C57BL/6, and CD1). In contrast, all of the other histological, biochemical, and functional muscle parameters were markedly altered by denervation. Cross-sectional area (CSA) of muscle fibers, maximal tetanic isometric force, maximal velocity of shortening, maximal power, and citrate synthase activity were all reduced in denervated muscles compared with innervated muscles (P < 0.05). Contraction and one-half relaxation times of the twitch were also increased by denervation (P < 0.05). Addition of tenotomy to denervation had no further effect on the relative expression of MHC-1 protein (P > 0.05), despite a greater reduction in CSA and citrate synthase activity (P < 0.05). In conclusion, a deficit in neural input leads to marked atrophy and reduction in performance in mouse soleus muscles. However, the maintenance of the relative expression of slow MHC protein is independent of neuromuscular activity in mice.

dnRas stimulates autocrine-paracrine growth of regenerating muscle via calcineurin-NFAT-IL-4 pathway

Ras and calcineurin are members of two independent pathways in muscle growth but their interaction is not known. This work shows that the transfection of about 1% of the muscle fibers with dominant negative Ras (dnRas) shows a wilder effect; it stimulates the fiber growth in the entire regenerating soleus muscle, including the nontransfected fibers. Co-transfection with the calcineurin inhibitor cain/cabin prevented the growth stimulation. Injection of antibody for interleukin-4 (IL-4) also abolished the growth ameliorating effect. These results suggest that the inactivation of Ras in 1% of the fibers upregulates the calcineurin-NFAT-IL-4 pathway and the secreted IL-4 triggers fiber growth stimulation in the whole regenerating soleus muscle of the rat. The results highlight the importance of the autocrine-paracrine regulation in muscle regeneration and hint to a novel method of gene theraphy of degenerative-regenerative muscle dystrophies.

Cellular adaptations in soleus muscle during recovery after hindlimb unloading

AIM

We used a model of chronic unloading followed by reloading to examine the apoptotic responses associated with soleus muscle atrophy and subsequent recovery.

METHODS

Male Wistar rats were subjected to hindlimb unloading (HU) for 2 weeks and subsequent reloading for 0, 3, 7 and 14 days. One-half of the HU-reloaded rats were administered cyclosporine A (CsA), a calcineurin (CaN) inhibitor.

RESULTS

There was fibre atrophy (73%) and a decrease in slow type I fibre/myosin heavy chain (MyHC) composition in the soleus muscle after 2 weeks of HU. Fibre size and type I MyHC composition recovered to near the age-matched control levels by recovery day 14 in non-treated, but not in CsA-treated, rats. Myonuclear number was lower and the number of apoptotic nuclei higher in 2-week HU than control rats. These values returned to control levels after 7 and 14 days of recovery, respectively, in both HU-recovery groups. After 2 weeks of HU, the levels of heat shock proteins (Hsp) 60 and 72, mitochondrial cytochrome c oxidase subunit IV (Cox IV), and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1) proteins were lower than control. The levels of all of these proteins gradually increased to or above the control levels during cage recovery in both groups.

CONCLUSION

Our results indicate that apoptotic mechanisms are involved in the modulation of myonuclear number during chronic unloading and subsequent reloading. Furthermore, it appears that CaN is related to fibre size and phenotype adaptations, but not to apoptotic responses.

Ca2+/calmodulin-based signalling in the regulation of the muscle fibre phenotype and its therapeutic potential via modulation of utrophin A and myostatin expression

Ca2+ signalling plays an important role in excitation-contraction coupling and the resultant force output of skeletal muscle. It is also known to play a crucial role in modulating both short- and long-term muscle cellular phenotypic adaptations associated with these events. Ca2+ signalling via the Ca2+/calmodulin (CaM)-dependent phosphatase calcineurin (CnA) and via Ca2+/CaM-dependent kinases, such as CaMKI and CaMKII, is known to regulate hypertrophic growth in response to overload, to direct slow versus fast fibre gene expression, and to contribute to mitochondrial biogenesis. The CnA- and CaMK-dependent regulation of the downstream transcription factors nuclear factor of activated T cells (NFAT) and myocyte-specific enhancer factor 2 are known to activate muscle-specific genes associated with a slower, more oxidative fibre phenotype. We have also recently shown the expression of utrophin A, a cytoskeletal protein that accumulates at the neuromuscular junction and plays a role in maturation of the postsynaptic apparatus, to be regulated by CnA-NFAT and Ca2+/CaM signalling. This regulation is fibre-type specific and potentiated by interactions with the transcriptional regulators and coactivators GA binding protein (also known as nuclear respiratory factor 2) and peroxisome proliferator-activated receptor-gamma coactivator 1 alpha. Another downstream target of CnA signalling may be myostatin, a transforming growth factor-beta family member that is a negative regulator of muscle growth. While the list of the downstream targets of CnA/NFAT- and Ca2+/CaM-dependent signalling is emerging, the precise interaction of these pathways with the Ca2+-independent pathways p38 mitogen-activated protein kinase, extracellular signal-regulated kinases 1 and 2, phosphoinositide-3 kinase, and protein kinase B (Akt/PKB) must also be considered when deciphering fibre responses and plasticity to altered contractile load.

Calcineurin-A alpha activation enhances the structure and function of regenerating muscles after myotoxic injury

Calcineurin signaling is essential for successful muscle regeneration. Although calcineurin inhibition compromises muscle repair, it is not known whether calcineurin activation can enhance muscle repair after injury. Tibialis anterior (TA) muscles from adult wild-type (WT) and transgenic mice overexpressing the constitutively active calcineurin-A alpha transgene under the control of the mitochondrial creatine kinase promoter (MCK-CnA alpha*) were injected with the myotoxic snake venom Notexin to destroy all muscle fibers. The TA muscle of the contralateral limb served as the uninjured control. Muscle structure was assessed at 5 and 9 days postinjury, and muscle function was tested in situ at 9 days postinjury. Calcineurin stimulation enhanced muscle regeneration and altered levels of myoregulatory factors (MRFs). Recovery of myofiber size and force-producing capacity was hastened in injured muscles of MCK-CnA alpha* mice compared with control. Myogenin levels were greater 5 days postinjury and myocyte enhancer factor 2a (MEF2a) expression was greater 9 days postinjury in muscles of MCK-CnA alpha* mice compared with WT mice. Higher MEF2a expression in regenerating muscles of MCK-CnA alpha* mice 9 days postinjury may be related to an increase of slow fiber genes. Calcineurin activation in uninjured and injured TA muscles slowed muscle contractile properties, reduced fatigability, and enhanced force recovery after 4 min of intermittent maximal stimulation. Therefore, calcineurin activation can confer structural and functional benefits to regenerating skeletal muscles, which may be mediated in part by differential expression of MRFs.

Modulations of the calcineurin/NF-AT pathway in skeletal muscle atrophy

Calcineurin has been proposed to regulate skeletal muscle hypertrophy, while its relevance to the pathogenesis of muscle atrophy is unknown. The present study was aimed to investigate if perturbations of the calcineurin pathway may be involved in causing skeletal muscle atrophy in two different experimental conditions: cancer cachexia (rats bearing the AH-130 hepatoma), and hyperglycemia (rats treated with streptozotocin). Calcineurin expression in the gastrocnemius was comparable between tumor hosts and controls. By contrast, besides unchanged calcineurin mRNA levels, those of protein were lower in diabetic animals than in controls. The DNA-binding activity of the transcription factors NF-AT and MEF-2 was analysed as an indirect measure of calcineurin activity in vivo. The nuclear translocation of both factors was similar in tumor hosts and controls. Consistently with the reduced calcineurin protein levels, NF-AT DNA-binding activity significantly decreased in the gastrocnemius of diabetic rats compared to controls. Finally, muscle wasting correction afforded in the AH-130 hosts by pentoxifylline or interleukin-15 was not paralleled by changes of calcineurin mRNA levels, while treatment of diabetic animals with dehydroepiandrosterone partially prevented calcineurin down-regulation. These results suggest that modulations of calcineurin activity may be involved in the pathogenesis of muscle wasting in diabetes though not in cancer cachexia.

Calcineurin and skeletal muscle growth

Recruitment determines the profile of fibre-type-specific genes expressed across the range of muscle fibres associated with slow, fast fatigue-resistant and fast fatiguable motor units. Downstream signalling pathways activated by neural signalling and mechanical load have been the focus of intensive research in past years. It is now known that Ca2+-dependent calcineurin–nuclear factor of activated T cells and insulin-like growth factor 1 pathways and their downstream mediators contribute to these adaptive responses. These pathways regulate gene expression through muscle-specific (myocyte-enhancing factor 2, myoblast determination protein) and non-specific (nuclear factor of activated T cell 2, GATA-2) transcription factors. Transcriptional signals activated with increased contractile activity result in altered expression of fibre-type specific genes, including the myosin heavy chain isoforms and oxidative and glycolytic enzymes and a net change in muscle fibre-type composition. In contrast, transcriptional signals activated by increased load bearing result in hypertrophy or a growth response, a component of which involves satellite cell recruitment and fusion with existing adult myofibres. Calcineurin has been identified as a key mediator in the hypertrophic response, and the current challenge has been to determine the downstream target genes of this pathway. Exciting new data have emerged, showing that myostatin, a negative regulator of muscle growth, and utrophin, a cytoskeletal protein important in maintaining membrane integrity, are downstream targets of calcineurin signalling. Increased understanding of these mediators of muscle growth may provide strategies for the development of effective therapeutics to counter muscle weakness and muscular dystrophy.

The role of muscle activation pattern and calcineurin in acetylcholinesterase regulation in rat skeletal muscles

Acetylcholinesterase (AChE) expression in fast rat muscles is approximately fourfold higher than in slow muscles. We examined whether different muscle activation patterns are responsible for this difference and whether the calcineurin signaling pathway is involved in AChE regulation. The slow soleus and fast extensor digitorum longus (EDL) muscles were directly or indirectly stimulated by a tonic low-frequency or a phasic high-frequency pattern of electric impulses. The phasic, but not tonic, stimulation increased the AChE mRNA levels in denervated soleus muscles to those in the normal EDL and maintained high levels of AChE mRNA in denervated EDL muscles. Therefore, muscle activation pattern is the predominant regulator of extrajunctional AChE expression in rat muscles. Indirect phasic stimulation of innervated muscles, imposed on their natural pattern of neural activation, did not increase the AChE transcript levels in the soleus, whereas a 30% reduction was observed in the EDL muscles. A low number of impulses per day is therefore prerequisite for high AChE expression. Treatment by tacrolimus and cyclosporin A, two inhibitors of calcineurin (but not by a related substance rapamycin, which does not inhibit calcineurin), increased the levels of AChE transcripts in the control soleus muscles and in tonically electrically stimulated soleus and EDL muscles, even to reach those in the control EDL muscles. Therefore, tonic muscle activation reduces the extrajunctional levels of AChE transcripts by activating the calcineurin signaling pathway. In denervated soleus and EDL muscles, tacrolimus did not prevent the reduction of AChE mRNA levels, indicating that a calcineurin-independent suppressive mechanism was involved.

Differential Expression of Calcineurin and SR Ca2+ Handling Proteins in Equine Muscle Fibers During Early Postnatal Growth

During early postnatal development, the myosin heavy chain (MyHC) expression pattern in equine gluteus medius muscle shows adaptation to movement and load, resulting in a decrease in the number of fast MyHC fibers and an increase in the number of slow MyHC fibers. In the present study we correlated the expression of MyHC isoforms to the expression of sarcoplasmic(endo)reticulum Ca2+-ATPase 1 and 2a (SERCA), phospholamban (PLB), calcineurin A (CnA), and calcineurin B (CnB). Gluteus medius muscle biopsies were taken at 0, 2, 4, and 48 weeks and analyzed using immunofluorescence. Both SERCA isoforms and PLB were expressed in almost all fiber types at birth. From 4 weeks of age onward, SERCA1 was exclusively expressed in fast MyHC fibers and SERCA2a and PLB in slow MyHC fibers. At all time points, CnA and CnB proteins were expressed at a basal level in all fibers, but with a higher expression level in MyHC type 1 fibers. From 4 weeks onward, expression of only CnA was also higher in MyHC type 2a and 2ad fibers. We propose a double function of calcineurin in calcium homeostasis and maintenance of slow MyHC fiber type identity. Although equine muscle is already functional at birth, expression patterns of the monitored proteins still show adaptation, depending on the MyHC fiber type.

Calcineurin-mediated slow-type fiber expression and growth in reloading condition

PURPOSE

Calcineurin (CaN) signaling pathway has been implicated in the transcriptional regulation of slow muscle fiber genes and in muscle hypertrophy. Our aim was to investigate the functional role of CaN as a regulator of muscle growth and/or muscle fiber type under conditions of recovery from inactivity.

METHODS

Female ICR mice (8 wk of age, 28-32 g) were used. To examine the effects of hindlimb suspension (HS) and reloading on skeletal muscle fiber size and muscle fiber type, animals were designated to 8 wk of HS and subsequent reloading for 4 wk. During reloading, animals were treated with pharmacological inhibitors for CaN (FK506) by intraperitoneal administration (3-5 mg.kg.d). After each experimental period, antigravitational soleus muscle was analyzed.

RESULTS

HS treatment resulted in obvious muscle atrophy and slow-to-fast fiber-type transformation in the soleus muscle. Subsequent reloading for 4 wk following HS induced muscle regrowth and fiber-type reversion toward a slow profile. FK506 administration prevented this kind of reloading-induced transformation of muscle fiber type. Furthermore, it was confirmed that FK506 administration attenuated maintenance of fiber cross-sectional area and reloading-induced fiber regrowth, specifically in slow-type muscle fibers.

CONCLUSION

Reloading-induced fiber-type reversion toward a slow profile is prevented by the pharmacological inhibition of CaN. Additionally, inhibition of CaN prevented maintenance and regrowth of slow-type muscle fibers. These results implicate that the CaN signaling pathway is required in the slow-type muscle fiber program under maintenance and suspension-reloading conditions.

The responsiveness of regenerated soleus muscle to pharmacological calcineurin inhibition

The responsiveness of mature regenerated soleus (SOL) muscles to cyclosporin A (CsA) administration was studied in rats. Forty-two days after notexin-induced degeneration of left SOL muscles, rats were treated with CsA (25 mg/kg x day) or vehicle daily for 3 weeks. CsA administration decreased by eightfold the level of transcription of MCIP-1, a well-known calcineurin-induced gene, in intact as well as in regenerated muscles (P < 0.001). In response to CsA-administration we observed a slow-to-fast transition in the MHC profile, more marked in regenerated than in intact muscles (P < 0.05), but mainly restricted to MHC-Ibeta toward MHC-IIA. Immunohistochemical analysis showed that MHC-IIA was often co-expressed with MHC-Ibeta within myofibers of intact muscles, whereas it was mainly expressed within pure fast fibers of regenerated muscles. MHC-Ibeta mRNA levels were lower in regenerated than in intact muscles, but did not change in response to CsA-administration. CsA administration induced a significant increase in MHC-IIA mRNA levels (P < 0.001) similar in both intact and regenerated muscles. Present results suggest that in vivo in intact SOL muscles, calcineurin blocks the upregulation of the MHC-IIA isoform at the transcriptional level. On the other hand, the higher response of regenerated muscles to CsA administration cannot be explained by transcriptional events, and may result from either a more rapid turnover of MHC proteins in regenerated muscles than in intact ones, or translational events. This study further suggests that the developmental history of myofibers could play a role in the adaptability of skeletal muscle to variations in neuromuscular activity.

Effects of Cyclosporine-A on Rat Soleus Muscle Fiber Size and Phenotype

PURPOSE

Organ transplant patients treated with cyclosporine-A (CsA) often exhibit weight loss and muscle weakness. The cellular target of CsA, calcineurin, has been implicated in maintenance of muscle fiber size and in expression of the type I skeletal muscle phenotype. We hypothesized that CsA treatment would cause fiber atrophy, as well as increase type IIa myosin heavy chain (MHC) content and oxidative enzyme activities in the soleus muscle.

METHODS

Rats were treated with CsA for 21 d (20 mg.kg(-1).d(-1); N = 16) and compared with control rats given olive oil vehicle (Veh; N = 16). Soleus muscles were excised bilaterally. MHC content was determined by gel electrophoresis, oxidative enzyme activities by spectrophotometric methods, and fiber type and size by histochemistry.

RESULTS

Lymphocyte count was depressed in CsA rats (P < 0.05), indicating treatment efficacy. Type IIa MHC content was increased in the soleus muscle with CsA (Veh, 10.4 +/- 1.7%; CsA, 15.1 +/- 2.0; P < 0.05) at the expense of type I MHC. Soleus muscle oxidative enzyme activities were also increased with CsA treatment (P < 0.05). Soleus muscle atrophy occurred, reflected by a 22% decrease in fiber cross-sectional area (Veh, 3255 +/- 105 microm(2); CsA, 2533 +/- 125; P < 0.05).

CONCLUSION

These findings indicate that CsA treatment is associated with changes in skeletal muscle fiber size and phenotype. The former may underlie clinical symptoms of transplant patients treated with CsA.

Differential calcineurin signalling activity and regeneration efficacy in diaphragm and limb muscles of dystrophic mdx mice

Calcineurin activity is essential for successful skeletal muscle regeneration in young mdx mice and in wild type mice following myotoxic injury and cryodamage. In mature myofibres of adult mdx mice, calcineurin stimulation can ameliorate the dystrophic pathology. The aim of this study was to test the hypothesis that the more severe dystrophic pathology of the diaphragm compared with hindlimb muscles of mdx mice could be attributed to aberrant calcineurin signalling and that due to ongoing regeneration calcineurin activity would be greater in muscles of adult mdx than wild type mice. Differences in markers of regeneration between tibialis anterior and diaphragm muscles were also characterised, to determine whether there was an association between regeneration efficacy and calcineurin activity in dystrophic muscles. In diaphragm muscles of adult mdx mice, the proportion of centrally nucleated fibres and developmental myosin heavy chain protein expression was lower and myogenin protein expression was higher than in tibialis anterior muscles. Calcineurin and activated NFATc1 protein content and calcineurin phosphatase activity were higher in muscles from mdx than wild type mice and calcineurin activation was greater in diaphragm than tibialis anterior muscles of mdx mice. Thus, despite greater calcineurin activity in diaphragm compared to hindlimb muscles, regeneration events downstream of myoblast differentiation and mediated by the injured myofibre were severely compromised.

The COX-2 pathway regulates growth of atrophied muscle via multiple mechanisms

Loss of muscle mass occurs with disease, injury, aging, and inactivity. Restoration of normal muscle mass depends on myofiber growth, the regulation of which is incompletely understood. Cyclooxygenase (COX)-2 is one of two isoforms of COX that catalyzes the synthesis of prostaglandins, paracrine hormones that regulate diverse physiological and pathophysiological processes. Previously, we demonstrated that the COX-2 pathway regulates early stages of myofiber growth during muscle regeneration. However, whether the COX-2 pathway plays a common role in adult myofiber growth or functions specifically during muscle regeneration is unknown. Therefore, we examined the role of COX-2 during myofiber growth following atrophy in mice. Muscle atrophy was induced by hindlimb suspension (HS) for 2 wk, followed by a reloading period, during which mice were treated with either the COX-2-selective inhibitor SC-236 (6 mg x kg(-1) x day(-1)) or vehicle. COX-2 protein was expressed and SC-236 attenuated myofiber growth during reloading in both soleus and plantaris muscles. Attenuated myofiber growth in the soleus was associated with both decreased myonuclear addition and decreased inflammation, whereas neither of these processes mediated the effects of SC-236 on plantaris growth. In addition, COX-2(-/-) satellite cells exhibited impaired activation/proliferation in vitro, suggesting direct regulation of muscle cell activity by COX-2. Together, these data suggest that the COX-2 pathway plays a common regulatory role during various types of muscle growth via multiple mechanisms.

Cyclosporin-A does not affect skeletal muscle mass during disuse and recovery

Cyclosporin-A (CsA) is an immunosuppressive drug that acts as an inhibitor of calcineurin, a calcium phosphatase that has been suggested to play a role in skeletal muscle hypertrophy. The aim of the present study was to determine the effect of CsA administration (25 mg kg(-1) day(-1)) on skeletal muscle mass and phenotype during disuse and recovery. Male Wistar rats received vehicle (N = 8) or CsA (N = 8) during hind limb immobilization (N = 8) and recovery (N = 8). Muscle weight (dry/wet) and cross-sectional area were evaluated to verify the effect of CsA treatment on muscle mass. Muscle phenotype was assessed by histochemistry of myosin ATPase. CsA administration during immobilization and recovery did not change muscle/body weight ratio in the soleus (SOL) or plantaris (PL). Regarding muscle phenotype, we observed a consistent slow-to-fast shift in all experimental groups (immobilized only, receiving CsA only, and immobilized receiving CsA) as compared to control in both SOL and PL (P < 0.05). During recovery, no difference was observed in SOL or PL fiber type composition between the experimental recovered group and recovered group receiving CsA compared to their respective controls. Considering the muscle/body weight ratio, CsA administration does not maximize muscle mass loss induced by immobilization. Our results also indicate that CsA fails to block skeletal muscle regrowth after disuse. The present data suggest that calcineurin inhibition by CsA modulates muscle phenotype rather than muscle mass.

Presence of SERCA and calcineurin during fetal development of porcine skeletal muscle

Mechanisms involved in skeletal myofiber differentiation during fetal development of large animals are poorly understood. Studies in small animals suggest that the calcineurin (Cn) pathway is involved in myofiber differentiation. Neural activity is a prerequisite for Cn activity, implying maintenance of sustained low intracellular Ca(2+) concentrations. To study the role of Cn in fetal myofiber differentiation, we monitored the temporal and spatial distribution of Cn subunits, sarcoplasmic reticulum Ca(2+) ATPase (SERCA), phospholamban (PLB), and myosin heavy chain (MyHC) isoforms in relation to ingrowing nerves in porcine semitendinosus muscle (m. semitendinosus) at 55 and 75 days of gestation (dg) and at term. Immunofluorescence analysis revealed the presence of Cn subunits and SERCA isoforms at all analyzed stages. Cn distribution was not fiber-type specific, but expression became more prominent at term. At 75 dg, differential SERCA2 expression was accompanied by perinuclear PLB in primary fibers. SERCA1 was expressed in all fiber types at all stages. No specific MyHC isoform distribution was seen in relation to neuromuscular contacts, although neuromuscular contacts were present. From these results we speculate that in porcine m. semitendinosus differential SERCA2 expression precedes differential Cn expression. The question whether the Cn pathway is involved in prenatal myofiber differentiation needs further studies.

Effects of Clenbuterol and Cyclosporin A on the Myosin Heavy Chain mRNA Level and the Muscle Mass in Rat Masseter

To gain more insight into the molecular mechanism of muscle growth and fiber-type transformations, we analyzed the effects of beta(2)-adrenergic agonist clenbuterol (CB) and/or cyclosporin A (CsA), a potent inhibitor of calcineurin (CaN), on the muscle mass as well as on the mRNA levels of myosin heavy chains (MHC I, IIa, IId/x, IIb), using a real-time RT-PCR with specific primers in rat masseter. In comparison with control, the CB treatment significantly decreased the MHC I mRNA level (p < 0.01), but increased the MHC IId/x mRNA level (p < 0.01), and the CsA treatment significantly decreased the MHC I mRNA level (p < 0.05) in association with the significant decrease in MHC IIb mRNA level (p < 0.05). The CB+CsA treatment significantly decreased the levels of MHC I (p < 0.01) and IIa mRNAs (p < 0.05), but increased the MHC IId/x mRNA level (p < 0.001) in association with a significant decrease in MHC IIb mRNA level (p < 0.01), in comparison with control. The masseter muscle mass was significantly (p < 0.001) increased by either the CB or the CB + CsA treatment, but decreased with the CsA treatment (p < 0.01). These results suggest that in rat masseter muscle, CB has an anabolic action accompanying MHC mRNA I IIa IId/x sequence transition independently of CaN-signaling pathways, and CaN is involved in the type I fiber gene expression and the muscle mass maintenance of type IIb fiber.

Adaptation of muscle size and myofascial force transmission: a review and some new experimental results

This paper considers the literature and some new experimental results important for adaptation of muscle fiber cross-sectional area and serial sarcomere number. Two major points emerge: (1) general rules for the regulation of adaptation (for in vivo immobilization, low gravity conditions, synergist ablation, tenotomy and retinaculum trans-section experiments) cannot be derived. As a consequence, paradoxes are reported in the literature. Some paradoxes are resolved by considering the interaction between different levels of organization (e.g. muscle geometrical effects), but others cannot. (2) An inventory of signal transduction pathways affecting rates of muscle protein synthesis and/or degradation reveals controversy concerning the pathways and their relative contributions. A major explanation for the above is not only the inherently limited control of the experimental conditions in vivo, but also of in situ experiments. Culturing of mature single Xenopus muscle fibers at high and low lengths (allowing longitudinal study of adaptation for periods up to 3 months) did not yield major changes in the fiber cross-sectional area or the serial sarcomere number. This is very different from substantial effects (within days) of immobilization in vivo. It is concluded that overall strain does not uniquely regulate muscle fiber size. Force transmission, via pathways other than the myotendinous junctions, may contribute to the discrepancies reported: because of substantial serial heterogeneity of sarcomere lengths within muscle fibers creating local variations in the mechanical stimuli for adaptation. For the single muscle fiber, mechanical signalling is quite different from the in vivo or in vitro condition. Removal of tensile and shear effects of neighboring tissues (even of antagonistic muscle) modifies or removes mechanical stimuli for adaptation. It is concluded that the study of adaptation of muscle size requires an integrative approach taking into account fundamental mechanisms of adaptation, as well as effects of higher levels of organization. More attention should be paid to adaptation of connective tissues within and surrounding the muscle and their effects on muscular properties.

Nerve influence on myosin light chain phosphorylation in slow and fast skeletal muscles

Neural stimulation controls the contractile properties of skeletal muscle fibres through transcriptional regulation of a number of proteins, including myosin isoforms. To study whether neural stimulation is also involved in the control of post-translational modifications of myosin, we analysed the phosphorylation of alkali myosin light chains (MLC1) and regulatory myosin light chains (MLC2) in rat slow (soleus) and fast (extensor digitorum longus EDL) muscles using 2D-gel electrophoresis and mass spectrometry. In control rats, soleus and EDL muscles differed in the proportion of the fast and slow isoforms of MLC1 and MLC2 that they contained, and also in the distribution of the variants with distinct isoelectric points identified on 2D gels. Denervation induced a slow-to-fast transition in myosin isoforms and increased MLC2 phosphorylation in soleus, whereas the opposite changes in myosin isoform expression and MLC2 phosphorylation were observed in EDL. Chronic low-frequency stimulation of EDL, with a pattern mimicking that of soleus, induced a fast-to-slow transition in myosin isoforms, accompanied by a decreased MLC2 phosphorylation. Chronic administration (10 mg x kg(-1) x d(-1) intraperitoneally) of cyclosporin A, a known inhibitor of calcineurin, did not change significantly the distribution of fast and slow MLC2 isoforms or the phosphorylation of MLC2. All changes in MLC2 phosphorylation were paralleled by changes in MLC kinase expression without any variation of the phosphatase subunit, PP1. No variation in MLC1 phosphorylation was detectable after denervation or cyclosporin A administration. These results suggest that the low-frequency neural discharge, typical of soleus, determines low levels of MLC2 phosphorylation together with expression of slow myosin, and that MLC2 phosphorylation is regulated by controlling MLC kinase expression through calcineurin-independent pathways.

Cyclosporin A treatment upregulates Id1 and Smad3 expression and delays skeletal muscle regeneration

The molecular signaling pathway linked to muscle regeneration has not yet been identified. Previously, we demonstrated that mice treated with cyclosporin A (CsA), a calcineurin inhibitor, failed to regenerate normally after muscle damage. Using reverse transcription (RT)-PCR, Western blot and immunohistochemical analysis, we investigated whether the amounts of nuclear factor of activated T cells (NFAT), myocyte-enhancer factor 2 (MEF2), the MyoD family, Id-1, and Smad3 change in the regenerating muscle after CsA treatment. Adult male ICR mice were subjected to a bupivacaine injection into the tibialis anterior muscle, and were treated with either CsA (25 mg/kg) or vehicle once daily. They were killed at 1, 2, 4, 6, 9 and 14 days post injury. RT-PCR analysis did not show a significant difference in MEF2s, MyoD and myogenin mRNA levels in the regenerating muscle in either placebo- and CsA-administered mice. In contrast, a significant increase in MRF4 mRNA was seen in CsA-administered mice compared to the placebo-treated mice at 4 and 9 days post surgery. In CsA-treated mice, the level of Id1 mRNA was elevated at day 9 relative to the placebo-treated mice. After 6 days, the CsA-treated mice possessed more abundant proliferating cell nuclear antigen (PCNA) and cyclin D1 protein in many satellite cells and/or myoblast-like cells in the regenerating muscle. The amount of myostatin, TGF-beta2 and Smad3 mRNA and proteins was increased more markedly in the mice treated with CsA. After 9 days, many satellite cells and/or myoblasts showed apparent co-localization of both MyoD and Smad3 in CsA-, but not in placebo-, treated mice. Our results demonstrated that CsA treatment upregulates Id1 and Smad3 expression and delays skeletal muscle regeneration in vivo.

Calcineurin and heat shock protein 72 in functionally overloaded rat plantaris muscle

The involvement of calcineurin (CaN) and heat shock protein (Hsp) 72 in the regulation of fiber size and/or phenotype in response to functional overload (FO) was investigated. In one FO group, the plantaris muscle was overloaded by cutting the distal tendons (5-10 mm length) of the soleus and gastrocnemius of 3-week-old male Wistar rats. Cyclosporin A (CsA), a CaN inhibitor, was injected daily (5 mg/kg body weight, i.p.) in a second group of FO rats (FO+CsA group) for a 2-week period. Compared to age-matched controls (Con), the absolute and relative plantaris weights were increased in both FO groups: the hypertrophic response was attenuated in FO+CsA rats. The mean cross-sectional area of each fiber type was increased (approximately 2.0-fold) in the plantaris of FO rats: CsA treatment attenuated this effect, although the fibers were still larger than in Con rats. The percent composition of myosin heavy chain (MHC) IIb decreased from 54% in Con to 19% in FO rats, whereas types I, IIa, and IIx MHC increased in the FO rats. CsA treatment blunted the shifts in MHC isoforms: the FO+CsA group showed a smaller decrease in type IIb and a smaller increase in type IIx MHC than the FO group. The levels of CaN-A and -B proteins were higher (approximately 2.5-fold) in FO than Con rats, whereas these values were similar in Con and FO+CsA rats. Hsp72 protein levels were higher in FO (3.6-fold) and FO+CsA (5.2-fold) than Con rats, with the values being significantly higher in the FO+CsA than FO rats. CsA treatment in Con rats had no effects on muscle mass, fiber size, MHC composition, and Hsp72 or CaN levels. Combined, these results suggest that CaN levels are related to changes in both fiber size and phenotype, and that Hsp72 levels are more related to the levels of stress added to the muscle rather than to increases in the slow fiber phenotype in functionally overloaded rat plantaris muscles.

Possible role of calcineurin in heating-related increase of rat muscle mass

The purpose of this study was to investigate the contribution of calcineurin-related intracellular signal for heat-stress-associated muscle hypertrophy. Wistar strain male rats (7-week-old) were randomly divided into four groups: (1) control (CC, n=15), (2) control with the injection of cyclosporine A (CsA) (CA, n=15), (3) heat-stressed (HC, n=15), and (4) heat-stressed with the injection of CsA (HA, n=15). The heat-stress groups (HC and HA) were exposed to heat (41 degrees C for 60 min) in a controlled heat chamber without anesthesia. Soleus and extensor digitorum longus (EDL) muscles were dissected and weighed 1, 7, and 14 days after the exposure. Wet and dry weights of soleus were increased 7 days following heat exposure. The expressions of heat shock protein 72 (HSP72) and calcineurin in both muscles were also increased within 1 and 7 days following heat-stress, respectively. Administration of CsA, a specific inhibitor for calcineurin, depressed heat-stress-associated increase of muscle weight and calcineurin expression, especially in soleus. These observations suggest that a calcineurin-dependent signaling pathway may play an important role in the heat-stress-related skeletal muscular hypertrophy. Application of heat-stress to skeletal muscles may be a useful tool to gain muscular mass and force generation not only in athletes, but also in patients during rehabilitation.

Changes in PKB/Akt and calcineurin signaling during recovery in atrophied soleus muscle induced by unloading

Protein kinase B [PKB, also known as Akt (PKB/Akt)] and calcineurin (CaN) are postulated to play important roles in integrating intracellular signaling in skeletal muscle in response to disuse and increased muscle loading. These experiments investigated changes in signal transduction of the downstream pathways of PKB/Akt and CaN during recovery following disuse-induced muscle atrophy. A 10-day period of hindlimb unloading (HLU) via tail suspension (male rats) was used to produce soleus muscle atrophy. Muscle recovery was achieved by returning animals to normal ambulation for 3-10 days. HLU resulted in significant muscle atrophy and a slow-to-fast fiber transition as revealed by appearance of type IId/x and IIb myosin heavy chain (MHC) isoforms. Muscle mass in HLU animals recovered to control (Con) levels after 10 days of reloading, but the fast-to-slow shift in muscle MHC was incomplete, as indicated by the continued presence of type IId/x MHC. Ten days of HLU resulted in a significant decrease (-43%) in muscle levels of phosphorylated PKB/Akt. In contrast, muscle levels of phosphorylated PKB/Akt were greater (+56%) in HLU than in Con animals early after the onset of reloading (3 days). Soleus levels of phosphorylated p70S6K were significantly higher (+26%) in HLU animals after 3 days of muscle reloading. Muscle levels of phosphorylated PKB/Akt and phosphorylated p70S6K returned to Con levels by day 10 of recovery. Moreover, muscle CaN levels were significantly higher than Con levels after 10 days of muscle reloading. These findings are consistent with the hypothesis that PKB/Akt and its downstream mediators are active in the regrowth of muscle mass during the early periods of recovery from muscle atrophy. Our data support the concept that CaN is involved in muscle remodeling during the later phases of recovery from disuse muscle atrophy.