Expression of mRNA for specific fibroblast growth factors associates with that of the myogenic markers MyoD and proliferating cell nuclear antigen in regenerating and overloaded rat plantaris muscle

Androgen action on myogenesis throughout the lifespan; comparison with neurogenesis

Androgens' pleiotropic actions in promoting sex differences present not only a challenge to providing a comprehensive account of their function, but also an opportunity to gain insights by comparing androgenic actions across organ systems. Although often overlooked by neuroscientists, skeletal muscle is another androgen-responsive organ system which shares with the nervous system properties of electrochemical excitability, behavioral relevance, and remarkable capacity for adaptive plasticity. Here we review androgenic regulation of mitogenic plasticity in skeletal muscle with the goal of identifying areas of interest to those researching androgenic mechanisms mediating sexual differentiation of neurogenesis. We use an organizational-activational framework to relate broad areas of similarity and difference between androgen effects on mitogenesis in muscle and brain throughout the lifespan, from early organogenesis, through pubertal organization, adult activation, and aging. The focus of the review is androgenic regulation of muscle-specific stem cells (satellite cells), which share with neural stem cells essential functions in development, plasticity, and repair, albeit with distinct, muscle-specific features. Also considered are areas of paracrine and endocrine interaction between androgen action on muscle and nervous system, including mediation of neural plasticity of innervating and distal neural populations by muscle-produced trophic factors.

Human skeletal muscle macrophages increase following cycle training and are associated with adaptations that may facilitate growth

Skeletal muscle macrophages participate in repair and regeneration following injury. However, their role in physiological adaptations to exercise is unexplored. We determined whether endurance exercise training (EET) alters macrophage content and characteristics in response to resistance exercise (RE), and whether macrophages are associated with other exercise adaptations. Subjects provided vastus lateralis biopsies before and after one bout of RE, after 12 weeks of EET (cycling), and after a final bout of RE. M2 macrophages (CD11b+/CD206+) did not increase with RE, but increased in response to EET (P < 0.01). Increases in M2 macrophages were positively correlated with fiber hypertrophy (r = 0.49) and satellite cells (r = 0.47). M2c macrophages (CD206+/CD163+) also increased following EET (P < 0.001), and were associated with fiber hypertrophy (r = 0.64). Gene expression was quantified using NanoString. Following EET, the change in M2 macrophages was positively associated with changes in HGF, IGF1, and extracellular matrix genes. EET decreased expression of IL6 (P < 0.05), C/EBPβ (P < 0.01), and MuRF (P < 0.05), and increased expression of IL-4 (P < 0.01), TNFα (P < 0.01) and the TWEAK receptor FN14 (P < 0.05). The change in FN14 gene expression was inversely associated with changes in C/EBPβ (r = -0.58) and MuRF (r = -0.46) following EET. In cultured human myotubes, siRNA inhibition of FN14 increased expression of C/EBPβ (P < 0.05) and MuRF (P < 0.05). Our data suggest that macrophages contribute to the muscle response to EET, potentially including modulation of TWEAK-FN14 signaling.

Myogenic potential of mouse embryonic stem cells lacking functional Pax7 tested in vitro by 5-azacitidine treatment and in vivo in regenerating skeletal muscle

Regeneration of skeletal muscle relies on the presence of satellite cells. Satellite cells deficiency accompanying some degenerative diseases is the reason for the search for the "replacement cells" that can be used in the muscle therapies. Due to their unique properties embryonic stem cells (ESCs), as well as myogenic cells derived from them, are considered as a promising source of therapeutic cells. Among the factors crucial for the specification of myogenic precursor cells is Pax7 that sustains proper function of satellite cells. In our previous studies we showed that ESCs lacking functional Pax7 are able to form myoblasts in vitro when differentiated within embryoid bodies and their outgrowths. In the current study we showed that ESCs lacking functional Pax7, cultured in vitro in monolayer in the medium supplemented with horse serum and 5azaC, expressed higher levels of factors associated with myogenesis, such as Pdgfra, Pax3, Myf5, and MyoD. Importantly, skeletal myosin immunolocalization confirmed that myogenic differentiation of ESCs was more effective in case of cells lacking Pax7. Our in vivo studies showed that ESCs transplanted into regenerating skeletal muscles were detectable at day 7 of regeneration and the number of Pax7-/- ESCs detected was significantly higher than of control cells. Our results support the concept that lack of functional Pax7 promotes proliferation of differentiating ESCs and for this reason more of them can turn into myogenic lineage.

Large Polyglutamine Repeats Cause Muscle Degeneration in SCA17 Mice

In polyglutamine (polyQ) diseases, large polyQ repeats cause juvenile cases with different symptoms than those of adult-onset patients, who carry smaller expanded polyQ repeats. The mechanisms behind the differential pathology mediated by different polyQ repeat lengths remain unknown. By studying knockin mouse models of spinal cerebellar ataxia-17 (SCA17), we found that a large polyQ (105 glutamines) in the TATA-box-binding protein (TBP) preferentially causes muscle degeneration and reduces the expression of muscle-specific genes. Direct expression of TBP with different polyQ repeats in mouse muscle revealed that muscle degeneration is mediated only by the large polyQ repeats. Different polyQ repeats differentially alter TBP's interaction with neuronal and muscle-specific transcription factors. As a result, the large polyQ repeat decreases the association of MyoD with TBP and DNA promoters. Our findings suggest that specific alterations in protein interactions by large polyQ repeats may account for the unique pathology in juvenile polyQ diseases.

Myogenic cell response to muscle contraction with short electrical stimulation

[Purpose] The present study aimed to determine the effects of short muscle strength exercise on hepatocyte growth factor expression and satellite cell activation. [Subjects] The study included 72 2-12-week-old male Sprague-Dawley rats. [Methods] The rat plantaris muscle was contracted with a 5-min electrical stimulation of the sciatic nerve, and then, the mRNA expressions of hepatocyte growth factor and myogenic regulatory factors in the plantaris muscle were determined, and the phosphorylation of the hepatocyte growth factor receptor (c-Met) was examined. [Results] The mRNA expressions of hepatocyte growth factor and myogenic regulatory factors increased after a short muscle contraction compared to that un-contraction. Immunofluorescence analysis showed the expression of hepatocyte growth factor protein and the possibility that downstream biological changes occurred in the hepatocyte growth factor-bound c-Met. [Conclusion] Our results demonstrated that activation of satellite cells induced hepatocyte growth factor expression during muscle contraction with a short 5-min electrical stimulation, which simulates short muscle strength exercise in physical therapy. The present study provides evidence for the use of short muscle strength exercise in physical therapy.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

Time-dependent expression and distribution of Egr-1 during skeletal muscle wound healing in rats

Recent studies have shown that early growth response factor-1 (Egr-1) plays an important role in regulation of inflammation and tissue repair, but little is known about its expression after trauma to skeletal muscles. A preliminary study on time-dependent expression and distribution of Egr-1 was performed by immunohistochemistry, immunofluorescence and Western blotting during skeletal muscle wound healing in rats. An animal model of skeletal muscle contusion was established in 45 Sprague-Dawley male rats. Samples were taken at 6 h, 12 h, 1 day, 3 days, 5 days, 7 days, 10 days, 14 days and 21 days post-injury, respectively (5 rats in each posttraumatic interval). 5 rats were employed as control. In the uninjured controls, Egr-1 positive staining was observed in the sarcoplasm and nuclei of normal myofibers. In wounded specimens, a small number of polymorphonuclear cells (PMNs), a number of mononuclear cells (MNCs), fibroblastic cells (FBCs) and regenerated multinucleated myotubes showed positive reaction for Egr-1 in contused zones. By morphometric analysis, an increase in Egr-1 expression was verified at inflammatory phase after contusion, which reached a peak in the regenerated phase overlapping with the fibrotic phase during skeletal muscle wound healing. The expression tendency was further confirmed by Western blotting assay. By immunofluorescent staining for co-localization, the Egr-1-positive MNCs and FBCs in wounds were identified as macrophages and myofibroblasts. The results demonstrate that the expression of Egr-1 is up-regulated and temporally distributed in certain cell types after trauma to skeletal muscles, which may be closely involved in inflammatory response, fibrotic repair and muscle regeneration during skeletal muscle wound healing.

Prenatal and early postnatal exposure to high-saturated-fat diet represses Wnt signaling and myogenic genes in offspring rats

The prenatal and early postnatal period is a key developmental window for nutrition status, and high-fat exposure in this period has been shown to be associated with type 2 diabetes, obesity and other features of metabolic disorders later in life. The present study was designed to investigate the underlying molecular mechanisms and role of relative genes involved in this process. We investigated the impact of prenatal and early postnatal exposure to a high-saturated-fat diet on the regulation of the Wnt signaling pathway and myogenic genes in skeletal muscle of rat offspring as well as the serum and muscle physiological outcomes. Timed-pregnant Sprague-Dawley rats were fed either a control (C, 16% kcal fat) or high-saturated-fat diet (HF, 45% kcal fat) throughout gestation and lactation. After weaning, female offspring were fed a control diet to generate two offspring groups: control diet-fed offspring of control diet-fed dams (C/C) and control diet-fed offspring of HF diet-fed dams (HF/C). The serum glucose of the HF/C offspring (5.58 ± 0.26 mmol/L) was significantly higher than that of C/C offspring (4.97 ± 0.28 mmol/L), and the Homeostasis Model Assessment-Insulin Resistance of HF/C offspring (2.00 ± 0.11) was also significantly higher when compared with C/C (1.84 ± 0.09). Furthermore, HF/C offspring presented excessive intramuscular fat accumulation (1.8-fold, P < 0.05) and decreased muscle glycogen (1.3-fold, P < 0.05), as well as impairment of muscle development at the age of 12 weeks. Meanwhile, we observed the repression of Wnt/β-catenin signaling and myogenic genes in HF/C offspring. The present study indicates that prenatal and early postnatal exposure to a high-saturated-fat diet suppresses the development of skeletal muscle and myogenic genes via Wnt/β-catenin signaling, and the inappropriate muscle development could potentially contribute to the predisposition of offspring to develop metabolic-syndrome-like phenotype in adulthood.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

Pitfalls in target mRNA quantification for real-time quantitative RT-PCR in overload-induced skeletal muscle hypertrophy

Quantifying target mRNA using real-time quantitative reverse transcription-polymerase chain reaction requires an accurate normalization method. Determination of normalization factors (NFs) based on validated reference genes according to their relative stability is currently the best standard method in most usual situations. This method controls for technical errors, but its physiological relevance requires constant NF values for a fixed weight of tissue. In the functional overload model, the increase in the total RNA concentration must be considered in determining the NF values. Here, we pointed out a limitation of the classical geNorm-derived normalization. geNorm software selected reference genes despite that the NF values extensively varied under experiment. Only the NF values calculated from four intentionally selected genes were constant between groups. However, a normalization based on these genes is questionable. Indeed, three out of four genes belong to the same functional class (negative regulator of muscle mass), and their use is physiological nonsense in a hypertrophic model. Thus, we proposed guidelines for optimizing target mRNA normalization and quantification, useful in models of muscle mass modulation. In our study, the normalization method by multiple reference genes was not appropriate to compare target mRNA levels between overloaded and control muscles. A solution should be to use an absolute quantification of target mRNAs per unit weight of tissue, without any internal normalization. Even if the technical variations will stay present as a part of the intergroup variations, leading to less statistical power, we consider this method acceptable because it will not generate misleading results.

Overexpression of NUDT7, a candidate quantitative trait locus for pork color, downregulates heme biosynthesis in L6 myoblasts

While testing a quantitative trait locus (QTL) for pork color in a cross population of pigs from the mating of Large White dams to a Japanese wild boar, our laboratory discovered a candidate gene (NUDT7) that might affect heme biosynthesis in porcine muscle. Therefore, this experiment was designed to test the effect of NUDT7 on heme biosynthesis in cultured myoblasts. Rat L6 myoblasts were transfected with a mammalian expression vector for pig NUDT7 immediately after the induction of cell differentiation, and samples were harvested at 2, 4, 6, and 8 days. Expression of exogenous NUDT7 mRNA was highest on day 4, when the heme content was substantially lower (P<0.01) than that of the control (14.2 vs. 63.9 pmol/10(5) cells). These results suggest that overexpression of pig NUDT7 may be associated with heme biosynthesis downregulation in skeletal muscle, which may partially explain differences in meat color among breeds of livestock.

Notch and Wnt signaling, physiological stimuli and postnatal myogenesis

Adult skeletal muscle stem cells, termed satellite cells are imperative to muscle regeneration. Much work has been performed on satellite cell identification and the subsequent activation of the myogenic response but the regulation of satellite cells including its activation is not well elucidated. The purpose of this review article is to synthesize what the literature reveals in regards to the current understanding of satellite cells including their contribution to muscle repair and growth following physiological stimuli. In addition, this review article will describe the recent findings on the roles of the classic developmental signaling pathways, Notch and Wnt, to the myogenic response in various muscle injury models. This purpose of this summary is to bring awareness of the impact that muscle contraction models have on the local and systemic environment of adult muscle stem cells which will be beneficial for comprehending and treatment development for muscle -associated ailments and other organ diseases.