MicroRNAs (miRNAs) are a class of small non-coding RNAs that are widely involved in a variety of biological processes. Different skeletal muscle fiber type composition exhibits characteristic ...differences in functional properties and energy metabolism of skeletal muscle. However, the molecular mechanism by which miRNAs control the different type of muscle fiber formation is still not fully understood. In the present study, we characterized the role of microRNA-139-5p (miR-139-5p) in the regulation of myosin heavy chain (MyHC) isoform expression and its underlying mechanisms. Here we found that the expression of miR-139-5p was significantly higher in mouse slow-twitch muscle than in fast-twitch muscle. Overexpression of miR-139-5p downregulated the expression of MyHC I and MyHC IIa, whereas inhibition of miR-139-5p upregulated them. We also found that the levels of calcineurin (CaN), NFATc1, MEF2C and MCIP1.4, which are the components of CaN/NFAT signaling pathway that has shown to positively regulate slow fiber-selective gene expression, were notably inhibited by miR-139-5p overexpression. Furthermore, treatment of phenylephrine (PE), a α1-adrenoceptor agonist, abolished the inhibitory effect of miR-139-5p on MyHC I and MyHC IIa expression. Together, our findings indicated that the role of miR-139-5p in regulating the MyHC isoforms, especially MyHC I and MyHC IIa, may be achieved through inhibiting CaN/NFAT signaling pathway.
•MiR-139-5p was higher in mouse slow-twitch than in fast-twitch muscle.•MiR-139-5p overexpression downregulated the expression of MyHC I and MyHC IIa.•CaN/NFAT signaling was notably inhibited by miR-139-5p overexpression.•MiR-139-5p suppressed MyHC I and IIa expression via inhibiting CaN/NFAT signaling.
Context:
Studies examining whether vitamin D supplementation increases muscle mass or muscle-specific vitamin D receptor (VDR) concentration are lacking.
Objective:
Our objective was to determine ...whether vitamin D3 4000 IU/d alters muscle fiber cross-sectional area (FCSA) and intramyonuclear VDR concentration over 4 months.
Design and Setting:
This was a randomized, double-blind, placebo-controlled study in a single center.
Participants:
Participants were 21 mobility-limited women (aged ≥65 years) with serum 25-hydroxyvitamin D (25OHD) levels of 22.5 to 60 nmol/L.
Main Outcome Measures:
Baseline and 4-month FCSA and intramyonuclear VDR were measured from vastus lateralis muscle cross-sections probed for muscle fiber type (I/IIa/IIx) and VDR using immunofluorescence.
Results:
At baseline, mean (±SD) age was 78 ± 5 years; body mass index was 27 ± 5 kg/m2, 25OHD was 46.3 ± 9.5 nmol/L, and a short physical performance battery score was 7.95 ± 1.57 out of 12. At 4 months, 25OHD level was 52.5 ± 17.1 (placebo) vs 80.0 ± 11.5 nmol/L (vitamin D VD; P < .01), and change in 25OHD level was strongly associated with percent change in intramyonuclear VDR concentration-independent of group (r = 0.87, P < .001). By treatment group, percent change in intramyonuclear VDR concentration was 7.8% ± 18.2% (placebo) vs 29.7% ± 11.7% (VD; P = .03) with a more pronounced group difference in type II vs I fibers. Percent change in total (type I/II) FCSA was −7.4% ± 18.9% (placebo) vs 10.6% ± 20.0% (VD; P = .048).
Conclusion:
Vitamin D3 supplementation increased intramyonuclear VDR concentration by 30% and increased muscle fiber size by 10% in older, mobility-limited, vitamin D-insufficient women. Further work is needed to determine whether the observed effect of vitamin D on fiber size is mediated by the VDR and to identify which signaling pathways are involved.
Different patterns of motor nerve activity drive distinctive programs of gene transcription in skeletal muscles, thereby establishing a high degree of metabolic and physiological specialization among ...myofiber subtypes. Recently, we proposed that the influence of motor nerve activity on skeletal muscle fiber type is transduced to the relevant genes by calcineurin, which controls the functional activity of NFAT (nuclear family of activated T cell) proteins. Here we demonstrate that calcineurin‐dependent gene regulation in skeletal myocytes is mediated also by MEF2 transcription factors, and is integrated with additional calcium‐regulated signaling inputs, specifically calmodulin‐dependent protein kinase activity. In skeletal muscles of transgenic mice, both NFAT and MEF2 binding sites are necessary for properly regulated function of a slow fiber‐specific enhancer, and either forced expression of activated calcineurin or motor nerve stimulation up‐regulates a MEF2‐dependent reporter gene. These results provide new insights into the molecular mechanisms by which specialized characteristics of skeletal myofiber subtypes are established and maintained.
Our primary goal was to determine the effects of 6-mo flight on the International Space Station (ISS) on selected anaerobic and aerobic enzymes, and the content of glycogen and lipids in slow and ...fast fibers of the soleus and gastrocnemius. Following local anesthesia, biopsies were obtained from nine ISS crew members ∼45 days preflight and on landing day (R+0) postflight. We subdivided the crew into those who ran 200 min/wk or more (high treadmill, HT) in-flight from those who ran <100 min/wk (low treadmill, LT). In the LT group, there was a loss of lipid in soleus type I fibers, and muscle glycogen significantly increased in soleus fiber types postflight. Soleus cytochrome oxidase (CO) activity was significantly depressed postflight in the type I fiber. This was attributed to the LT group where CO activity was reduced 59%. Otherwise, there was no change in the crew mean for type I or IIa fiber glycolytic or mitochondrial enzyme activities pre- vs. postflight in either muscle. However, two of the three HT subjects (Subjects E and H) showed significant increases in both β-hydroxyacyl-CoA dehydrogenase and citrate synthase in the soleus type I fibers, and Subject E, exhibiting the largest increase in soleus oxidative enzymes, was the only subject to show a significant decrease in glycolytic enzyme activity. It is apparent that crew members performing adequate treadmill running can maintain calf muscle enzymes, which suggests that increased fatigue with weightlessness cannot be directly caused by a decline in muscle enzyme capacity.
PURPOSE OF REVIEWThere are a variety of pathophysiologic conditions that are known to induce skeletal muscle atrophy. However, muscle wasting can occur through multiple distinct signaling pathways ...with differential sensitivity between selective skeletal muscle fiber subtypes. This review summarizes some of the underlying molecular mechanisms responsible for fiber-specific muscle mass regulation.
RECENT FINDINGSPeroxisome proliferator-activated receptor gamma coactivator 1-alpha protects slow-twitch oxidative fibers from denervation/immobilization (disuse)-induced muscle atrophies. Nutrient-related muscle atrophies, such as those induced by cancer cachexia, sepsis, chronic heart failure, or diabetes, are largely restricted to fast-twitch glycolytic fibers, of which the underlying mechanism is usually related to abnormality of protein degradation, including proteasomal and lysosomal pathways. In contrast, nuclear factor kappaB activation apparently serves a dual function by inducing both fast-twitch fiber atrophy and slow-twitch fiber degeneration.
SUMMARYFast-twitch glycolytic fibers are more vulnerable than slow-twitch oxidative fibers under a variety of atrophic conditions related to signaling transduction of Forkhead box O family, autophagy inhibition, transforming growth factor beta family, and nuclear factor-kappaB. The resistance of oxidative fibers may result from the protection of peroxisome proliferator-activated receptor gamma coactivator 1-alpha.
Skeletal muscle excitation-contraction (EC) coupling is independent of calcium influx. In fact, alternative splicing of the voltage-gated calcium channel CaV1.1 actively suppresses calcium currents ...in mature muscle. Whether this is necessary for normal development and function of muscle is not known. However, splicing defects that cause aberrant expression of the calcium-conducting developmental CaV1.1e splice variant correlate with muscle weakness in myotonic dystrophy. Here, we deleted CaV1.1 (Cacna1s) exon 29 in mice. These mice displayed normal overall motor performance, although grip force and voluntary running were reduced. Continued expression of the developmental CaV1.1e splice variant in adult mice caused increased calcium influx during EC coupling, altered calcium homeostasis, and spontaneous calcium sparklets in isolated muscle fibers. Contractile force was reduced and endurance enhanced. Key regulators of fiber type specification were dysregulated and the fiber type composition was shifted toward slower fibers. However, oxidative enzyme activity and mitochondrial content declined. These findings indicate that limiting calcium influx during skeletal muscle EC coupling is important for the secondary function of the calcium signal in the activity-dependent regulation of fiber type composition and to prevent muscle disease.
The morphological appearance of the vastus lateralis (VL) muscle from high-level power-lifters on long-term anabolic steroid supplementation (PAS) and power-lifters never taking anabolic steroids (P) ...was compared. The effects of long- and short-term supplementation were compared. Enzyme-immunohistochemical investigations were performed to assess muscle fiber type composition, fiber area, number of myonuclei per fiber, internal myonuclei, myonuclear domains and proportion of satellite cells. The PAS group had larger type I, IIA, IIAB and IIC fiber areas (p<0.05). The number of myonuclei/fiber and the proportion of central nuclei were significantly higher in the PAS group (p<0.05). Similar results were seen in the trapezius muscle (T) but additionally, in T the proportion of fibers expressing developmental myosin isoforms was higher in the PAS group compared to the P group. Further, in VL, the PAS group had significantly larger nuclear domains in fibers containing > or = 5 myonuclei. The results of AS on VL morphology in this study were similar to previously reported short-term effects of AS on VL. The initial effects from AS appear to be maintained for several years.
1 Division of Endocrinology, Metabolism, and
Molecular Medicine, Charles R. Drew University of Medicine and
Science, Los Angeles, 90059; and 2 Division of
Respiratory and Critical Care Physiology ...and Medicine,
Harbor-University of California at Los Angeles Medical Center,
Torrance, California 90509
Administration
of replacement doses of testosterone to healthy hypogonadal men and
supraphysiological doses to eugonadal men increases muscle size. To
determine whether testosterone-induced increase in muscle size is due
to muscle fiber hypertrophy, 61 healthy men, 18-35 yr of age,
received monthly injections of a long-acting gonadotropin-releasing
hormone (GnRH) agonist to suppress endogenous testosterone secretion
and weekly injections of 25, 50, 125, 300, or 600 mg testosterone
enanthate (TE) for 20 wk. Thigh muscle volume was measured by magnetic
resonance imaging (MRI) scan, and muscle biopsies were obtained from
vastus lateralis muscle in 39 men before and after 20 wk of combined
treatment with GnRH agonist and testosterone. Administration of GnRH
agonist plus TE resulted in mean nadir testosterone concentrations of 234, 289, 695, 1,344, and 2,435 ng/dl at the 25-, 50-, 125-, 300-, and
600-mg doses, respectively. Graded doses of testosterone administration were associated with testosterone dose and concentration-dependent increase in muscle volume measured by MRI (changes in vastus lateralis volume, 4, +7, +15, +32, and +48 ml at 25-, 50-, 125-, 300-, and
600-mg doses, respectively). Changes in cross-sectional areas of both
type I and II fibers were dependent on testosterone dose and
significantly correlated with total ( r = 0.35, and
0.44, P < 0.0001 for type I and II fibers,
respectively) and free ( r = 0.34 and 0.35, P < 0.005) testosterone concentrations during
treatment. The men receiving 300 and 600 mg of TE weekly experienced
significant increases from baseline in areas of type I (baseline vs. 20 wk, 3,176 ± 186 vs. 4,201 ± 252 µm 2 ,
P < 0.05 at 300-mg dose, and 3,347 ± 253 vs.
4,984 ± 374 µm 2 , P = 0.006 at
600-mg dose) muscle fibers; the men in the 600-mg group also had
significant increments in cross-sectional area of type II (4,060 ± 401 vs. 5,526 ± 544 µm 2 , P = 0.03) fibers. The relative proportions of type I and type II fibers did
not change significantly after treatment in any group. The myonuclear
number per fiber increased significantly in men receiving the 300- and
600-mg doses of TE and was significantly correlated with testosterone
concentration and muscle fiber cross-sectional area. In conclusion, the
increases in muscle volume in healthy eugonadal men treated with graded
doses of testosterone are associated with concentration-dependent
increases in cross-sectional areas of both type I and type II muscle
fibers and myonuclear number. We conclude that the testosterone induced
increase in muscle volume is due to muscle fiber hypertrophy.
androgen; muscle hypertrophy; satellite cells; mechanism of
action
Androgen deprivation therapy (ADT) improves life expectancy in prostate cancer (PCa) patients, but is associated with adverse effects on muscle mass. Here, we investigated the effects of strength ...training during ADT on muscle fiber cross‐sectional area (CSA) and regulators of muscle mass. PCa patients on ADT were randomized to 16 weeks of strength training (STG) (n = 12) or a control group (CG; n = 11). Muscle biopsies were obtained from m. vastus lateralis and analyzed by immunohistochemistry and western blot. Muscle fiber CSA increased with strength training (898 μm2, P = 0.04), with the only significant increase observed in type II fibers (1076 μm2, P = 0.03). There was a trend toward a difference in mean change between groups myonuclei number (0.33 nuclei/fiber, P = 0.06), with the only significant increase observed in type I fibers, which decreased the myonuclear domain size of type I fibers (P = 0.05). Satellite cell numbers and the content of androgen receptor and myostatin remained unchanged. Sixteen weeks of strength training during ADT increased type II fiber CSA and reduced myonuclear domain in type I fibers in PCa patients. The increased number of satellite cells normally seen following strength training was not observed.
The cancerogen 1,2-dimethylhydrazine (DMH), widely used in the experimental animal model of carcinogenesis, affects various organs, but its effect on muscle fibers is unknown. To evaluate the effect ...of 15-week DMH treatment on the fiber size and myosin heavy chain (MyHC) isoforms, which substantially determine fiber types and their contractile characteristics, pure and hybrid fiber types were immunohistochemically determined according to the MyHC isoform expression in soleus, extensor digitorum longus, gastrocnemius medialis and lateralis muscles of DMH-treated and control male Wistar rats. Whereas the size of fibers was mostly unaffected, the MyHC isoform expression was partially affected in both gastrocnemius samples, but not in the soleus and extensor digitorum longus of DMH-treated rats. The lower proportions of hybrid fiber types and especially that of type 1/2x in most gastrocnemius samples of DMH-treated rats resulted in a shift towards a single MyHC isoform expression, but the extent and pattern of the MyHC isoform shift varied across the different gastrocnemius samples. Such variable response to DMH treatment across muscles indicates that each muscle possesses its own adaptive range. These findings are essential for an accurate evaluation of skeletal muscle characteristics in DMH animal model.
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