Effects of mechanical over-loading on the properties of soleus muscle fibers, with or without damage, in wild type and mdx mice

Effects of mechanical over-loading on the characteristics of regenerating or normal soleus muscle fibers were studied in dystrophin-deficient (mdx) and wild type (WT) mice. Damage was also induced in WT mice by injection of cardiotoxin (CTX) into soleus muscle. Over-loading was applied for 14 days to the left soleus muscle in mdx and intact and CTX-injected WT mouse muscles by ablation of the distal tendons of plantaris and gastrocnemius muscles. All of the myonuclei in normal muscle of WT mice were distributed at the peripheral region. But, central myonuclei were noted in all fibers of WT mice regenerating from CTX-injection-related injury. Further, many fibers of mdx mice possessed central myonuclei and the distribution of such fibers was increased in response to over-loading, suggesting a shift of myonuclei from peripheral to central region. Approximately 1.4% branched fibers were seen in the intact muscle of mdx mice, although these fibers were not detected in WT mice. The percentage of these fibers in mdx, not in WT, mice was increased by over-loading (～51.2%). The fiber CSA in normal WT mice was increased by over-loading (p<0.05), but not in mdx and CTX-injected WT mice. It was suggested that compensatory hypertrophy is induced in normal muscle fibers of WT mice following functional over-loading. But, it was also indicated that muscle fibers in mdx mice are susceptible to mechanical over-loading and fiber splitting and shift of myonuclei from peripheral to central region are induced.     

Compensatory adaptations of skeletal muscle fiber types to a long-term functional overload

We investigated selected histochemical and histometrical characteristics of the heterogeneous fiber types of rat skeletal muscle following long-term compensatory muscle growth. Sixty days following surgical removal of the synergistic gastrocnemius muscle, the compensated ipsilateral plantaris and soleus muscles and the corresponding control muscles from the contralateral leg were excised and stained histochemically for myofibrillar ATPase and DPNH-diaphorase activities. The number of fibers per cross-section was determined by a direct count from transverse sections taken from the midportion of the muscles. Fiber area was determined by direct planimetry. The plantaris and soleus muscles hypertrophied 103% and 45%, respectively, within 60 days. Compensatory hypertrophy of the plantaris muscle was accompanied by a significant but disproportionate increase in the cross-sectional areas of the three muscle fiber types. There was an approximate 4-fold increase in the number of slow-twitch-oxidative (SO) fibers observed per transverse section. The hypertrophied plantaris muscle exhibited a significantly greater number of fibers per cross-section (29%) than the respective control muscle. The compensated soleus muscle consisted of nearly 100% SO fibers compared to 83% for the control soleus muscle.     

Changes in total fiber numbers of disused soleus muscle on rats

In this study, by use of technique that was modified from Morey method, we discussed the histological influence on the soleus muscle of the rats caused by disuse. This study is characterized by the calculating of total numbers of muscle fibers. ST (slow-twitch) and FT (fast-twitch) fibers in total muscular cross-sectional area were classified by their difference in intensity of staining of actomyosin adenosinetriphosphatase (myosin ATPase). During the experiment, average fiber diameter of ST and FT fibers declined when compared to control group (p less than 0.01). A 54% decrease in the total number of ST fibers was observed in the experimental group (p less than 0.01). Conversely, the total number of FT fibers increased to 362% of the control value (p less than 0.01). These results of the changes evoked in ST and FT fibers indicate 34% decrease in total muscular cross-sectional area, and showed that muscular function shifted toward a faster muscle in disused soleus muscle.     

Fiber-type proportions in mammalian soleus muscle during postnatal development

We analyzed the fiber-type composition of the soleus muscle in rats and mice to determine whether the adult proportion of fiber types is fixed soon after birth or whether it changes during postnatal maturation. We examined muscles from animals varying in age from 1 week to 1 year using monoclonal antibodies that distinguish between fast and slow isoforms of myosin heavy chains. In cross sections of unfixed muscle containing profiles of all myofibers in the muscle, we counted the fibers that stained with antibodies to fast myosin, and in adjacent sections, those that stained positive with an antibody to slow myosin. We also counted the total number of fibers in each section. Rat soleus contained about 2500 myofibers, and mouse about 1000 at all ages studied, suggesting that myogenesis ceases in soleus by 1 week after birth or sooner. In mouse soleus, the relative proportions of fibers staining positive with fast and slow myosin antibodies were similar at all ages studied, about 60%–70% being fast and 30%–40% slow. In rat soleus, however, the proportions of fast antibody-positive and slow antibody-positive fibers changed dramatically during postnatal maturation. At 1 week after birth, about 50% of rat soleus fibers stained with fast myosin antibodies, whereas between 1 and 2 months this value fell to about 10%. In mouse, about 10% of fibers at 1 week, but none at 1 year, reacted with both fast and slow antibodies, whereas in rat, fewer than 3% bound both antibodies to a significant degree at 1 week. It is puzzling why, in rat soleus, the majority of apparently fast fibers present at 1 week is converted to a slow phenotype, whereas in mouse soleus the predominant change appears to be the suppression of fast myosin expression in a subset of fibers that expresses both myosin types at 1 week. It is possible that this may be related to differences in size and the amount of body growth between these two species.     

Fiber-type proportions in mammalian soleus muscle during postnatal development.

We analyzed the fiber-type composition of the soleus muscle in rats and mice to determine whether the adult proportion of fiber types is fixed soon after birth or whether it changes during postnatal maturation. We examined muscles from animals varying in age from 1 week to 1 year using monoclonal antibodies that distinguish between fast and slow isoforms of myosin heavy chains. In cross sections of unfixed muscle containing profiles of all myofibers in the muscle, we counted the fibers that stained with antibodies to fast myosin, and in adjacent sections, those that stained positive with an antibody to slow myosin. We also counted the total number of fibers in each section. Rat soleus contained about 2500 myofibers, and mouse about 1000 at all ages studied, suggesting that myogenesis ceases in soleus by 1 week after birth or sooner. In mouse soleus, the relative proportions of fibers staining positive with fast and slow myosin antibodies were similar at all ages studied, about 60%-70% being fast and 30%-40% slow. In rat soleus, however, the proportions of fast antibody-positive and slow antibody-positive fibers changed dramatically during postnatal maturation. At 1 week after birth, about 50% of rat soleus fibers stained with fast myosin antibodies, whereas between 1 and 2 months this value fell to about 10%. In mouse, about 10% of fibers at 1 week, but none at 1 year, reacted with both fast and slow antibodies, whereas in rat, fewer than 3% bound both antibodies to a significant degree at 1 week. It is puzzling why, in rat soleus, the majority of apparently fast fibers present at 1 week is converted to a slow phenotype, whereas in mouse soleus the predominant change appears to be the suppression of fast myosin expression in a subset of fibers that expresses both myosin types at 1 week. It is possible that this may be related to differences in size and the amount of body growth between these two species.     

Exercise-induced satellite cell activation in senescent soleus muscle.

The purposes of this study were 1) to determine satellite cell mitotic activity and myofiber nuclear density in the soleus muscle of aged rats and 2) to examine the effect of exercise training on these same parameters. Twenty-four-month-old specific pathogen-free female Fischer 344 rats were assigned to either a training or a control group. The trained group performed 10 wk of progressive treadmill running that resulted in a significant increase (P less than or equal to 0.05) in vastus lateralis muscle malate dehydrogenase activity compared with control rats. Training produced a doubling of soleus muscle satellite cell mitotic activity (trained 1.28 +/- 0.33, control 0.52 +/- 0.13 thymidine-labeled satellite cells per 1,000 nuclei; P less than or equal to 0.05). Training also resulted in a doubling in the number of damaged fibers in the soleus muscle (P less than or equal to 0.05). Mean myofiber nuclear density was unaltered by exercise training but varied as a function of soleus muscle fiber size. Nuclear density of a subpopulation of small fibers (cross-sectional area less than one standard deviation below the mean cross-sectional area of all fibers examined) was significantly higher (P less than or equal to 0.05) than in other fibers in the soleus muscle. A high nuclear density and small size suggest that these fibers were immature. In addition, the soleus muscle from trained rats had significantly more (P less than or equal to 0.05) small fibers with high nuclear density than muscle from control animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

High-frequency, low-magnitude vibrations suppress the number of blood vessels per muscle fiber in mouse soleus muscle.

Extremely low-magnitude (0.3 g), high-frequency (30-90 Hz), whole body vibrations can stimulate bone formation and are hypothesized to provide a surrogate for the oscillations of muscle during contraction. Little is known, however, about the potential of these mechanical signals to stimulate adaptive responses in other tissues. The objective of this study was to determine whether low-level mechanical signals produce structural adaptations in the vasculature of skeletal muscle. Eight-week-old male BALB/cByJ (BALB) mice were divided into two experimental groups: mice subjected to low-level, whole body vibrations (45 Hz, 0.3 g) superimposed on normal cage activities for 15 min/day (n = 6), and age-matched controls (n = 7). After the 6-wk experimental protocol, sections from end and mid regions of the soleus muscles were stained with lectin from Bandeiraea Simplicifolia, an endothelial cell marker, and smooth muscle (SM) alpha-actin, a perivascular cell marker. Six weeks of this low-level vibration caused a 29% decrease in the number of lectin-positive vessels per muscle fiber in the end region of the soleus muscle, indicating a significant reduction in the number of capillaries per muscle fibers. Similarly, these vibrations caused a 36% reduction in SM alpha-actin-positive vessels per muscle fiber, indicating a reduction in the number of arterioles and venules. The decreases in lectin- and SM alpha-actin-positive vessels per muscle fiber ratios were not significant in the mid muscle sections. These results demonstrate the sensitivity of the vasculature in mouse skeletal muscle to whole body, low-level mechanical signals.     

Role of gravitational loading in the development of rat soleus muscle fibers

Effects of hindlimb unloading during the first 3 months after birth on the development of soleus muscle fibers were studied in rats. The mean absolute weigh and cross-sectional area of whole soleus muscle in the unloaded rats were -1/3 and 1/4 of those in the controls, respectively. But the unloading did not affect the lengths of muscle, at 90 degrees of ankle joint angle, and of muscle fibers sampled from tendon to tendon, and the total sarcomere number. Since the total number of fibers in soleus was not affected either, the inhibited increase of muscle mass following unloading was mainly due to the smaller CSA of individual fibers. Numbers of both myonuclei and satellite cells were significantly less in unloaded than control rats. The % distribution of fibers expressing pure type I myosin heavy chain was significantly less in unloaded than controls (-23 %). Further, muscle fibers with multiple innervation were noted in the unloaded rats. It is suggested that the development and/or differentiation of soleus muscle fibers are closely associated with gravitational loading and that the growth-associated increase in fiber number may be genetically programmed.     

Effective fiber hypertrophy in satellite cell-depleted skeletal muscle

An important unresolved question in skeletal muscle plasticity is whether satellite cells are necessary for muscle fiber hypertrophy. To address this issue, a novel mouse strain (Pax7-DTA) was created which enabled the conditional ablation of >90% of satellite cells in mature skeletal muscle following tamoxifen administration. To test the hypothesis that satellite cells are necessary for skeletal muscle hypertrophy, the plantaris muscle of adult Pax7-DTA mice was subjected to mechanical overload by surgical removal of the synergist muscle. Following two weeks of overload, satellite cell-depleted muscle showed the same increases in muscle mass (approximately twofold) and fiber cross-sectional area with hypertrophy as observed in the vehicle-treated group. The typical increase in myonuclei with hypertrophy was absent in satellite cell-depleted fibers, resulting in expansion of the myonuclear domain. Consistent with lack of nuclear addition to enlarged fibers, long-term BrdU labeling showed a significant reduction in the number of BrdU-positive myonuclei in satellite cell-depleted muscle compared with vehicle-treated muscle. Single fiber functional analyses showed no difference in specific force, Ca(2+) sensitivity, rate of cross-bridge cycling and cooperativity between hypertrophied fibers from vehicle and tamoxifen-treated groups. Although a small component of the hypertrophic response, both fiber hyperplasia and regeneration were significantly blunted following satellite cell depletion, indicating a distinct requirement for satellite cells during these processes. These results provide convincing evidence that skeletal muscle fibers are capable of mounting a robust hypertrophic response to mechanical overload that is not dependent on satellite cells.     

Catalase-positive microperoxisomes in rat soleus and extensor digitorum longus muscle fiber types

The size, distribution, and content of catalase-reactive microperoxisomes were studied cytochemically in slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG), and fast-twitch glycolytic (FG) fibers of soleus and extensor digitorum longus (EDL) rat muscles. Fiber types were classified on the basis of mitochondrial content and distribution, Z-band widths, and myofibril size and shape. Microperoxisomes were generally located between myofibrils at the I-bands. The absence of crystalloid inclusions prevented positive identification of microperoxisomes in nonreacted and aminotriazole-inhibited muscles. EDL and soleus SO fibers possessed the largest microperoxisomes, whereas FOG and FG fibers of the EDL contained small- to medium-sized microperoxisomes. Comparing either microperoxisome number per muscle fiber area or microperoxisome area per fiber area revealed significant differences between fiber types with this ranking: soleus SO greater than EDL SO greater than EDL FOG greater than EDL FG. The present observations demonstrate that the content of catalase-positive microperoxisomes is greatest in the oxidative muscle fiber types. These cytochemical findings account for the higher catalase activity in homogenates of soleus muscles as compared to that of EDL muscles, because the soleus contains more oxidative fibers than EDL.     

Morphological Alterations in Gastrocnemius and Soleus Muscles in Male and Female Mice in a Fibromyalgia Model

Background

Fibromyalgia (FM) is a chronic musculoskeletal pain disorder, characterized by chronic widespread pain and bodily tenderness and is often accompanied by affective disturbances, however often with unknown etiology. According to recent reports, physical and psychological stress trigger FM. To develop new treatments for FM, experimental animal models for FM are needed to be development and characterized. Using a mouse model for FM including intermittent cold stress (ICS), we hypothesized that ICS leads to morphological alterations in skeletal muscles in mice.

Methods

Male and female ICS mice were kept under alternating temperature (4°C/room temperature [22°C]); mice constantly kept at room temperature served as control. After scarification, gastrocnemius and soleus muscles were removed and snap-frozen in liquid nitrogen–cooled isopentane or fixed for electron microscopy.

Results

In gastrocnemius/soleus muscles of male ICS mice, we found a 21.6% and 33.2% decrease of fiber cross sectional area (FCSA), which in soleus muscle concerns the loss of type IIa and IIx FCSA. This phenomenon was not seen in muscles of female ICS mice. However, this loss in male ICS mice was associated with an increase in gastrocnemius of the density of MIF⁺ (8.6%)-, MuRF⁺ (14.7%)-, Fbxo32⁺ (17.8%)-cells, a 12.1% loss of capillary contacts/muscle fiber as well as a 30.7% increase of damaged mitochondria in comparison with male control mice. Moreover, significant positive correlations exist among densities (n/mm²) of MIF⁺, MuRF⁺, Fbxo32⁺-cells in gastrocnemius/ soleus muscles of male ICS mice; these cell densities inversely correlate with FCSA especially in gastrocnemius muscle of male ICS mice.

Conclusion

The ICS-induced decrease of FCSA mainly concerns gastrocnemius muscle of male mice due to an increase of inflammatory and atrogenic cells. In soleus muscle of male ICS and soleus/gastrocnemius muscles of female ICS mice morphological alterations seem to occur not at all or delayed. The sex-specificity of findings, which is not easily reconciled with the epidemiology of FM (female predominance), implicate that gastrocnemius muscle of male ICS mice should preferentially be used for future investigations with FM. Moreover, we suggest to investigate morphological and/or molecular alterations at different time-points (up to two weeks) after ICS.     

A histochemical study of postnatal differentiation of skeletal muscle with reference to functional overload.

Postnatal differentiation and growth of the fibers comprising the tonic soleus and phasic plantaris muscles, were investigated histochemically in kittens. Compensatory hypertrophy was induced by ablation of the synergistic gastrocnemius muscle.At birth both muscles consist of relatively homogeneous fiber populations as demonstrated by myosin ATPase and succinic dehydrogenase activities and glycogen content (PAS method). Differential myosin ATPase activities become evident during the first week (soleus and plantaris), while diversification of fiber types according to SDH and glycogen develop gradually and independently during the first 2 mo of life (plantaris).Compensatory hypertrophy is associated with a substantial enlargement of both dark and light fibers (incubated for myosin ATPase) and, with increases in SDH activity which are most notable in fibers that normally are low in this enzyme. The normal growth associated reduction in the percentage of fibers with high myosin ATPase activity is significantly accelerated in the hypertrophic soleus, while the hypertrophic plantaris, which normally undergoes only a slight reduction in the percentage of such fibers, is unaffected. These results underline the paramount role of the nerve fiber in the process of differentiation but also indicate that functional overload exerts a modifying influence on this process.     

Perlecan deficiency causes muscle hypertrophy,a decrease in myostatin expression,and changes in muscle fiber composition

Perlecan is a component of the basement membrane that surrounds skeletal muscle. The aim of the present study is to identify the role of perlecan in skeletal muscle hypertrophy and myostatin signaling, with and without mechanical stress, using a mouse model (Hspg2^?/?-Tg) deficient in skeletal muscle perlecan. We found that myosin heavy chain (MHC) type IIb fibers in the tibialis anterior (TA) muscle of Hspg2^?/?-Tg mice had a significantly increased fiber cross-sectional area (CSA) compared to control (WT-Tg) mice. Hspg2^?/?-Tg mice also had an increased number of type IIx fibers in the TA muscle. Myostatin and its type I receptor (ALK4) expression was substantially decreased in the Hspg2^?/?-Tg TA muscle. Myostatin-induced Smad activation was also reduced in a culture of myotubes from the Hspg2^?/?-Tg muscle, suggesting that myostatin expression and its signaling were decreased in the Hspg2^?/?-Tg muscle. To examine the effects of mechanical overload or unload on fast and slow muscles in Hspg2^?/?-Tg mice, we performed tenotomy of the plantaris (fast) muscle and the soleus (slow) muscle. Mechanical overload on the plantaris muscle of Hspg2^?/?-Tg mice significantly increased wet weights compared to those of control mice, and unloaded plantaris muscles of Hspg2^?/?-Tg mice caused less decrease in wet weights compared to those of control mice. The decrease in myostatin expression was significantly profound in the overloaded plantaris muscle of Hspg2^?/?-Tg mice, compared with that of control mice. In contrast, overloading the soleus muscle caused no changes in either type of muscle. These results suggest that perlecan is critical for maintaining fast muscle mass and fiber composition, and for regulating myostatin signaling.     

Skeletal muscle adaptations to microgravity exposure in the mouse.

To investigate the effects of microgravity on murine skeletal muscle fiber size, muscle contractile protein, and enzymatic activity, female C57BL/6J mice, aged 64 days, were divided into animal enclosure module (AEM) ground control and spaceflight (SF) treatment groups. SF animals were flown on the space shuttle Endeavour (STS-108/UF-1) and subjected to approximately 11 days and 19 h of microgravity. Immunohistochemical analysis of muscle fiber cross-sectional area revealed that, in each of the muscles analyzed, mean muscle fiber cross-sectional area was significantly reduced (P < 0.0001) for all fiber types for SF vs. AEM control. In the soleus, immunohistochemical analysis of myosin heavy chain (MHC) isoform expression revealed a significant increase in the percentage of muscle fibers expressing MHC IIx and MHC IIb (P < 0.05). For the gastrocnemius and plantaris, no significant changes in MHC isoform expression were observed. For the muscles analyzed, no alterations in MHC I or MHC IIa protein expression were observed. Enzymatic analysis of the gastrocnemius revealed a significant decrease in citrate synthase activity in SF vs. AEM control.     

Mitochondrial abnormalities are more frequent in muscles undergoing sarcopenia.

The hypothesis that the accumulation of electron transport system (ETS) abnormalities and sarcopenia are linked was investigated. Vastus lateralis, soleus, and adductor longus muscles were studied in 5-, 18-, and 36-mo-old male Fischer 344 x Brown Norway F(1) hybrid rats. A significant decrease in soleus and vastus lateralis muscle mass was observed with age. Adductor longus was resistant to muscle mass loss. Multiple serial sections were analyzed for the activities of cytochrome-c oxidase (COX) and succinate dehydrogenase (SDH). The number of fibers exhibiting a COX(-)/SDH(++) phenotype increased with age in both vastus lateralis and soleus muscles. No ETS-abnormal fibers were identified in adductor longus at any age. Cross-sectional area of ETS-abnormal fibers decreased in the abnormal region (region displaying COX(-)/SDH(++) phenotype), whereas control fibers did not. The vastus lateralis muscle, which undergoes a high degree of sarcopenia, exhibited more ETS abnormalities and associated fiber loss than the soleus and adductor longus muscles, which are more resistant to sarcopenia, suggesting a direct association between ETS abnormalities and fiber loss.     

Repair of spinal cord transection and its effects on muscle mass and myosin heavy chain isoform phenotype.

A number of significant advances have been developed for treating spinal cord injury during the past two decades. The combination of peripheral nerve grafts and acidic fibroblast growth factor (hereafter referred to as PNG) has been shown to partially restore hindlimb function. However, very little is known about the effects of such treatments in restoring normal muscle phenotype. The primary goal of the current study was to test the hypothesis that PNG would completely or partially restore 1) muscle mass and muscle fiber cross-sectional area and 2) the slow myosin heavy chain phenotype of the soleus muscle. To test this hypothesis, we assigned female Sprague-Dawley rats to three groups: 1) sham control, 2) spinal cord transection (Tx), and 3) spinal cord transection plus PNG (Tx+PNG). Six months following spinal cord transection, the open-field test was performed to assess locomotor function, and then the soleus muscles were harvested and analyzed. SDS-PAGE for single muscle fiber was used to evaluate the myosin heavy chain (MHC) isoform expression pattern following the injury and treatment. Immunohistochemistry was used to identify serotonin (5-HT) fibers in the spinal cord. Compared with the Tx group, the Tx+PNG group showed 1) significantly improved Basso, Beattie, and Bresnahan scores (hindlimb locomotion test), 2) less muscle atrophy, 3) a higher percentage of slow type I fibers, and 4) 5-HT fibers distal to the lesion site. We conclude that the combined treatment of PNG is partially effective in restoring the muscle mass and slow phenotype of the soleus muscle in a T-8 spinal cord-transected rat model.     

Effects of exogenous growth hormone on skeletal muscle of young female rats

We examined the effects of exogenous growth hormone (GH) treatment on the soleus and rectus femoris muscles of young female rats. Rat GH (1.8 IU/mg) was administered for 3 weeks by subcutaneous injection, twice a day, at doses of 0.5, 0.6, and 0.8 mg/day during the 1st, 2nd, and 3rd week, respectively. Final body weight, as well as wet and dry weight, of the soleus and rectus femoris muscles were significantly greater in the GH-treated group, compared to controls. Muscle weight to body weight ratios did not differ between the two groups. The fiber type composition of the soleus muscle was determined by histochemical staining for myosin ATPase activity. No statistically significant difference was found between the GH-treated and the control groups in the percentages of fiber types. However, GH treatment significantly increased the cross-sectional area of type II fibers of the soleus muscle. These results suggest that, in young female rats, acceleration of body weight gain by homologous GH administration is accompanied by a proportional hypertrophy of skeletal muscle mass. Increased muscle mass is due to hypertrophy of muscle fibers. Type II muscle fibers appear to be more sensitive to GH stimulation.     

Running during recovery from hindlimb suspension induces transient muscle injury

The objectives were to study morphological adaptations of soleus muscle to decreased loading induced by hindlimb suspension and the effect of run training during the subsequent recovery period. Adult female Wistar rats were kept for 28 days with hindlimbs suspended. For the next 28 days, rats were assigned to a cage-sedentary or daily running group. Compared with control soleus muscles, 28 days of hindlimb suspension reduced the mass and fiber cross-sectional area to 58 and 53% of control values, respectively, and decreased type I fibers from 92 +/- 2 to 81 +/- 2%. During recovery, clusters of damaged fibers were observed in the soleus muscle, and this observation was more pronounced in trained animals. Type IIc fibers appeared transiently during recovery, and their presence was exacerbated with training, as IIc fibers increased to approximately 20% of the total by day 14 of recovery and were no longer evident at day 28. Although muscle wet mass does not differ as a result of mode of recovery at day 14, training transiently decreased the overall fiber area compared with sedentary recovery at this point. By day 28 of recovery the morphological characteristics of soleus muscle in the trained group did not differ from control muscle, whereas in the sedentary group muscle mass and overall fiber cross-sectional area were approximately 14% less than control values.     

Effects of Pleiotrophin Overexpression on Mouse Skeletal Muscles in Normal Loading and in Actual and Simulated Microgravity

Pleiotrophin (PTN) is a widespread cytokine involved in bone formation, neurite outgrowth, and angiogenesis. In skeletal muscle, PTN is upregulated during myogenesis, post-synaptic induction, and regeneration after crushing, but little is known regarding its effects on muscle function. Here, we describe the effects of PTN on the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles in mice over-expressing PTN under the control of a bone promoter. The mice were maintained in normal loading or disuse condition, induced by hindlimb unloading (HU) for 14 days. Effects of exposition to near-zero gravity during a 3-months spaceflight (SF) into the Mice Drawer System are also reported. In normal loading, PTN overexpression had no effect on muscle fiber cross-sectional area, but shifted soleus muscle toward a slower phenotype, as shown by an increased number of oxidative type 1 fibers, and increased gene expression of cytochrome c oxidase subunit IV and citrate synthase. The cytokine increased soleus and EDL capillary-to-fiber ratio. PTN overexpression did not prevent soleus muscle atrophy, slow-to-fast transition, and capillary regression induced by SF and HU. Nevertheless, PTN exerted various effects on sarcolemma ion channel expression/function and resting cytosolic Ca²⁺ concentration in soleus and EDL muscles, in normal loading and after HU. In conclusion, the results show very similar effects of HU and SF on mouse soleus muscle, including activation of specific gene programs. The EDL muscle is able to counterbalance this latter, probably by activating compensatory mechanisms. The numerous effects of PTN on muscle gene expression and functional parameters demonstrate the sensitivity of muscle fibers to the cytokine. Although little benefit was found in HU muscle disuse, PTN may emerge useful in various muscle diseases, because it exerts synergetic actions on muscle fibers and vessels, which could enforce oxidative metabolism and ameliorate muscle performance.     

Reduced TrkB expression results in precocious age-like changes in neuromuscular structure, neurotransmission, and muscle function

Acute blockade of signaling through the tyrosine kinase receptor B (TrkB) attenuates neuromuscular transmission and fragments postsynaptic acetylcholine receptors (AChRs) in adult mice, suggesting that TrkB signaling is a key regulator of neuromuscular function. Using immunohistochemical, histological, and in vitro muscle contractile techniques, we tested the hypothesis that constitutively reduced TrkB expression would disrupt neuromuscular pre- and postsynaptic structure, neurotransmission, muscle fiber size, and muscle function in the soleus muscle of 6- to 8-mo-old TrkB?/? mice compared with age-matched littermates. Age-like expansion of postsynaptic AChR area, AChR fragmentation, and denervation was observed in TrkB?/? mice similar to that found in 24-mo-old wild-type mice. Neurotransmission failure was increased in TrkB?/? mice, suggesting that these morphologic changes were sufficient to alter synaptic function. Reduced TrkB expression resulted in decreased muscle strength and fiber cross-sectional area. Immunohistochemical and muscle retrograde labeling experiments show that motor neuron number and size are unaffected in TrkB?/? mice. These results suggest that TrkB- signaling at the neuromuscular junction plays a role in synaptic stabilization, neurotransmission, and muscle function and may impact the aging process of sarcopenia.