Regulation of bovine satellite cell proliferation and differentiation by insulin and triiodothyronine

Embryonic thermal manipulation: a potential strategy to mitigate heat stress in broiler chickens for sustainable poultry production

Due to high environmental temperatures and climate change, heat stress is a severe concern for poultry health and production, increasing the propensity for food insecurity. With climate change causing higher temperatures and erratic weather patterns in recent years, poultry are increasingly vulnerable to this environmental stressor. To mitigate heat stress, nutritional, genetic, and managerial strategies have been implemented with some success. However, these strategies did not adequately and sustainably reduce the heat stress. Therefore, it is crucial to take proactive measures to mitigate the effects of heat stress on poultry, ensuring optimal production and promoting poultry well-being. Embryonic thermal manipulation (TM) involves manipulating the embryonic environment's temperature to enhance broilers' thermotolerance and growth performance. One of the most significant benefits of this approach is its cost-effectiveness and saving time associated with traditional management practices. Given its numerous advantages, embryonic TM  is a promising strategy for enhancing broiler production and profitability in the poultry industry. TM increases the standard incubation temperature in the mid or late embryonic stage to induce epigenetic thermal adaption and embryonic metabolism. Therefore, this review aims to summarize the available literature and scientific evidence of the beneficial effect of pre-hatch thermal manipulation on broiler health and performance.

A Comparative Study on the Adipogenic and Myogenic Capacity of Muscle Satellite Cells, and Meat Quality Characteristics between Hanwoo and Vietnamese Yellow Steers

Myogenesis and adipogenesis are the important processes determining the muscle growth and fat accumulation livestock, which ultimately affecting their meat quality. Hanwoo is a popular breed and its meat has been exported to other countries. The objective of this study was to compare the myogenesis and adipogenesis properties in satellite cells, and meat quality between Hanwoo and Vietnamese yellow cattle (VYC). Same 28-months old Hanwoo (body weight: 728±45 kg) and VYC (body weight: 285±36 kg) steers (n=10 per breed) were used. Immediately after slaughter, tissue samples were collected from longissimus lumborum (LL) muscles for satellite cells isolation and assays. After 24 h post-mortem, LL muscles from left carcass sides were collected for meat quality analysis. Under the same in vitro culture condition, the proliferation rate was higher in Hanwoo compared to VYC (p<0.05). Fusion index was almost 3 times greater in Hanwoo (42.17%), compared with VYC (14.93%; p<0.05). The expressions of myogenesis (myogenic factor 5, myogenic differentiation 1, myogenin, and myogenic factor 6)- and adipogenesis (peroxisome proliferator-activated receptor gamma)-regulating genes, and triglyceride content were higher in Hanwoo, compared with VYC (p<0.05). Hanwoo beef had a higher intramuscular fat and total monounsaturated fatty acids contents than VYC beef (p<0.05). Whilst, VYC meat had a higher CIE a* and total polyunsaturated fatty acids content (p<0.05). Overall, there was a significant difference in the in vitro culture characteristics and genes expression of satellite cells, and meat quality between the Hanwoo and VYC.

Effect of palmitoleic acid on the differentiation of bovine skeletal muscle satellite cells

We hypothesized that the unsaturated fatty acid palmitoleic acid (POA) could promote the expression of adipogenic/lipogenic genes in bovine skeletal muscle satellite cells (BSCs). The BSCs were cultured in a growth medium containing 10% fetal bovine serum. When the cells reached 80%-90% confluence, we used the differentiation medium with 5% horse serum for differentiation for 96 h. The differentiation medium contained 50 µM, 100 µM and 200 µM POA. Control BSC were cultured only in differentiation media. Compared with the control BSC, the POA BSC significantly up-regulated the expression of paired box 3 (Pax3) and paired box 7 (Pax7) and down-regulated myogenin gene expression (p < 0.01), which indicates a depression in muscle fiber development. However, all POA treatments up-regulated the expression of the adipocyte transcription factors peroxisome proliferator-activated receptor γ (PPARγ), CCAAT/enhancer-binding protein alpha and beta (C/EBP α and C/EBP β), and other genes (p < 0.01) and increased the expression of PAT-family proteins and the concentration of adiponectin in the media. These results indicate that POA can convert part of BSCs into adipocytes.

A Two-Week Insulin Infusion in Intrauterine Growth Restricted Fetal Sheep at 75% Gestation Increases Skeletal Myoblast Replication but Did Not Restore Muscle Mass or Increase Fiber Number

Intrauterine growth restricted (IUGR) fetuses are born with lower skeletal muscle mass, fewer proliferating myoblasts, and fewer myofibers compared to normally growing fetuses. Plasma concentrations of insulin, a myogenic growth factor, are lower in IUGR fetuses. We hypothesized that a two-week insulin infusion at 75% gestation would increase myoblast proliferation and fiber number in IUGR fetal sheep. Catheterized control fetuses received saline (CON-S, n=6), and the IUGR fetuses received either saline (IUGR-S, n=7) or insulin (IUGR-I, 0.014 ± 0.001 units/kg/hr, n=11) for 14 days. Fetal arterial blood gases and plasma amino acid levels were measured. Fetal skeletal muscles (biceps femoris, BF; and flexor digitorum superficialis, FDS) and pancreases were collected at necropsy (126 ± 2 dGA) for immunochemistry analysis, real-time qPCR, or flow cytometry. Insulin concentrations in IUGR-I and IUGR-S were lower vs. CON-S (P ≤ 0.05, group). Fetal arterial P_aO₂, O₂ content, and glucose concentrations were lower in IUGR-I vs. CON-S (P ≤ 0.01) throughout the infusion period. IGF-1 concentrations tended to be higher in IUGR-I vs. IUGR-S (P=0.06), but both were lower vs. CON-S (P ≤ 0.0001, group). More myoblasts were in S/G₂ cell cycle stage in IUGR-I vs. both IUGR-S and CON-S (145% and 113%, respectively, P ≤ 0.01). IUGR-I FDS muscle weighed 40% less and had 40% lower fiber number vs. CON-S (P ≤ 0.05) but were not different from IUGR-S. Myonuclear number per fiber and the mRNA expression levels of muscle regulatory factors were not different between groups. While the pancreatic β-cell mass was lower in both IUGR-I and IUGR-S compared to CON-S, the IUGR groups were not different from each other indicating that feedback inhibition by endogenous insulin did not reduce β-cell mass. A two-week insulin infusion at 75% gestation promoted myoblast proliferation in the IUGR fetus but did not increase fiber or myonuclear number. Myoblasts in the IUGR fetus retain the capacity to proliferate in response to mitogenic stimuli, but intrinsic defects in the fetal myoblast by 75% gestation may limit the capacity to restore fiber number.

The thickness of poly-phenoxyethyl methacrylate brush interferes with cellular behavior and function of myofibers

Introducing or grafting molecules onto biomaterial surfaces to regulate muscle cell destination via biophysical cues is one of the important steps for biomaterial design in muscle tissue engineering. Therefore, it is important to understand the interaction between myoblasts and myofibers with substrates modified by biomimetic layer with different thicknesses. In this study, we used a surface-induced atom transfer radical polymerization method to synthetize and graft poly-phenoxyethyl methacrylate (PHEMA) brushes having different lengths on the glass substrates. C2C12 myoblasts were seeded on the PHEMA brushes and differentiated using horse serum, for analyzing the sensibility of muscle cells to feel environment changing, and further investigating whether the depths of grafting layer on the biomaterial surface are important factors in regulating muscle cell behaviors. Our results demonstrated that on the thicker PHEMA brushes surface (200 and 450 nm), C2C12 myoblasts showed a better survival and proliferation and were favorable for cell fusion and myotube formation. Furthermore, myofibers survived on the thicker brushes were more functional and upregulated cytoskeleton proteins (tubulin, vimentin, and vinculin) and FAK levels, and enhanced the expression levels for mechanical stress molecules (HGF, NOS-1, and c-Met). These results suggest that grafting thickness of PHEMA layer on the substrate led to the myoblasts/myofiber behavior change, which would be valuable for the design and preparation of the modified layer on muscle tissue engineering scaffolds. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1264-1272, 2019.

PHYSIOLOGY AND ENDOCRINOLOGY SYMPOSIUM: Roles for insulin-supported skeletal muscle growth

Basic principles governing skeletal muscle growth and development, from a cellular point of view, have been realized for several decades. Skeletal muscle is marked by the capacity for rapid hypertrophy and increases in protein content. Ultimately, skeletal muscle growth is controlled by 2 basic means: 1) myonuclear accumulation stemming from satellite cell (myoblast) proliferation and 2) the balance of protein synthesis and degradation. Each process underlies the rapid changes in lean tissue accretion evident during fetal and neonatal growth and is particularly sensitive to nutritional manipulation. Although multiple signals converge to alter skeletal muscle mass, postprandial changes in the anabolic hormone insulin link feed intake with enhanced rates of protein synthesis in the neonate. Indeed, a consequence of insulin-deficient states such as malnutrition is reduced myoblast activity and a net loss of body protein. A well-characterized mechanism mediating the anabolic effect of insulin involves the phosphatidylinositol 3-kinase (PI3K)-mammalian target of rapamycin (mTOR) signaling pathway. Activation of mTOR leads to translation initiation control via the phosphorylation of downstream targets. Modulation of this pathway by insulin, as well as by other hormones and nutrients, accounts for enhanced protein synthesis leading to efficient lean tissue accretion and rapid skeletal muscle gain in the growing animal. Dysfunctional insulin activity during fetal and neonatal stages likely alters growth through cellular and protein synthetic capacities.

Chronic Hyperinsulinemia Increases Myoblast Proliferation in Fetal Sheep Skeletal Muscle

Insulin is an important fetal growth factor. However, chronic experimental hyperinsulinemia in the fetus fails to accelerate linear and lean mass growth beyond normal rates. Mechanisms preventing accelerated lean mass accretion during hyperinsulinemia are unknown. To address potential mechanisms, late-gestation fetal sheep were infused with iv insulin and glucose to produce euglycemic hyperinsulinemia (INS) or saline for 7-9 days. Fetal substrate uptake and protein metabolic rates were measured. INS fetuses had 1.5-fold higher insulin concentrations (P < .0001) and equivalent glucose concentrations. INS fetuses had 20% more Pax7(+) nuclei in the biceps femoris, which indicates the potential for hyperinsulinemia to increase the number of myoblasts within late-gestation fetal skeletal muscle. Additionally, the percentage of Pax7(+) myoblasts that expressed Ki-67 was 1.3-fold higher and expression of myogenic regulatory factors was 50% lower in INS fetuses (MYF5 and MYOG [myogenin], P < .005), which indicates a shift toward myoblast proliferation over differentiation. There were no differences for fetal body, organ, or muscle weights, although INS placentas weighed 28% less (P < .05). Protein synthesis and accretion rates did not change in INS fetuses, nor did fiber muscle size. Essential amino acid concentrations were lower in the INS group (P < .05) except for tryptophan. Umbilical blood flow, net total amino acids, and O2 uptakes rates did not differ between groups. Arterial O2 content was 33% lower (P < .005) and norepinephrine was 100% higher in the INS fetuses (P < .01), all of which are factors that may counteract fetal protein accretion during hyperinsulinemia despite an increase in myoblast proliferation.

Thermal manipulation during embryogenesis affects myoblast proliferation and skeletal muscle growth in meat-type chickens

Thermal manipulation (TM) of 39.5°C applied during mid-embryogenesis (embryonic d 7 to 16) has been proven to promote muscle development and enhance muscle growth and meat production in meat-type chickens. This study aimed to elucidate the cellular basis for this effect. Continuous TM or intermittent TM (for 12 h/d) increased myoblast proliferation manifested by higher (25 to 48%) myoblast number in the pectoral muscles during embryonic development but also during the first week posthatch. Proliferation ability of the pectoral-muscle-derived myoblasts in vitro was significantly higher in the TM treatments until embryonic d 15 (intermittent TM) or 13 (continuous TM) compared to that of controls, suggesting increased myogenic progeny reservoir in the muscle. However, the proliferation ability of myoblasts was lower in the TM treatments vs. control during the last days of incubation. This coincided with higher levels of myogenin expression in the muscle, indicating enhanced cell differentiation in the TM muscle. A similar pattern was observed posthatch: Myoblast proliferation was significantly higher in the TM chicks relative to controls during the peak of posthatch cell proliferation until d 6, followed by lower cell number 2 wk posthatch as myoblast number sharply decreases. Higher myogenin expression was observed in the TM chicks on d 6. This resulted in increased muscle growth, manifested by significantly higher relative weight of breast muscle in the embryo and posthatch. It can be concluded that temperature elevation during mid-term embryogenesis promotes myoblast proliferation, thus increasing myogenic progeny reservoir in the muscle, resulting in enhanced muscle growth in the embryo and posthatch.

Differentiation of bovine satellite cell-derived myoblasts under different culture conditions

The aim of this study was to develop adequate in vitro conditions for the differentiation of bovine skeletal muscle cells. Therefore, satellite cells isolated from the left foreleg of a Holstein-Friesian fetus at 4.5 mo of gestation were seeded on 24-well plates coated with extracellular matrix gel. Cells were cultured for 5 d in growth medium containing 10% fetal bovine serum. After reaching confluence, several differentiation media were tested for inducing myotube formation. The highest fusion rate of approximately 30% was achieved with a serum-free medium containing 1 μM dexamethasone, 1 μg/ml linoleic acid, and 0.1 μM insulin after a differentiation phase of 72 h. Two different culture conditions (serum-free and serum-containing) appropriate for bovine skeletal muscle cell differentiation are described in detail which allow the investigation of bovine skeletal muscle cell proliferation and differentiation in general as well as in response to bioactive compounds.

The NF-ҡB modulated miR-195/497 inhibit myoblast proliferation by targeting Igf1r/Insr and cyclin genes

MicroRNAs (miRNAs) play important roles in the development of skeletal muscle. In our previous study, expression of miR-195 and miR-497 were shown to be up-regulated during muscle development in pigs. In this study, we investigated the roles of these two miRNAs in myogenesis and analyzed their transcriptional regulation. Our results showed that miR-195 and miR-497 were up-regulated during muscle development and myoblast differentiation. Moreover, miR-195/497 inhibited proliferation but not differentiation in C2C12 cells. Further investigation revealed that Igf1r, Insr, Ccnd2, and Ccne1 were directly targeted by miR-195/497 in myoblasts. In addition, we confirmed that similarly expressed miR-195 and miR-497 were negatively regulated by nuclear factor-kappaB (NF-ҡB) in both myoblasts and skeletal muscle tissue. Our data illustrated that the NF-ҡB-miR-195/497-Igf1r/Insr-Ccnd2/Ccne1 signaling pathway played important roles in the myogenesis. Our study provides novel evidence for the roles of miR-195/497 in muscle development.

Endocrine regulation of fetal skeletal muscle growth: impact on future metabolic health

Establishing sufficient skeletal muscle mass is essential for lifelong metabolic health. The intrauterine environment is a major determinant of the muscle mass that is present during the life course of an individual, because muscle fiber number is set at the time of birth. Thus, a compromised intrauterine environment from maternal nutrient restriction or placental insufficiency that restricts muscle fiber number can have permanent effects on the amount of muscle an individual will live with. Reduced muscle mass due to fewer muscle fibers persists even after compensatory or 'catch-up' postnatal growth occurs. Furthermore, muscle hypertrophy can only partially compensate for this limitation in fiber number. Compelling associations link low birth weight and decreased muscle mass to future insulin resistance, which can drive the development of the metabolic syndrome and type 2 diabetes, and the risk of cardiovascular events later in life. There are gaps in knowledge about the origins of reduced muscle growth at the cellular level and how these patterns are set during fetal development. By understanding the nutrient and endocrine regulation of fetal skeletal muscle growth and development, we can direct research efforts toward improving muscle growth early in life to prevent the development of chronic metabolic diseases later in life.

Diabetic myopathy: impact of diabetes mellitus on skeletal muscle progenitor cells

Diabetes mellitus is defined as a group of metabolic diseases that are associated with the presence of a hyperglycemic state due to impairments in insulin release and/or function. While the development of each form of diabetes (Type 1 or Type 2) drastically differs, resultant pathologies often overlap. In each diabetic condition, a failure to maintain healthy muscle is often observed, and is termed diabetic myopathy. This significant, but often overlooked, complication is believed to contribute to the progression of additional diabetic complications due to the vital importance of skeletal muscle for our physical and metabolic well-being. While studies have investigated the link between changes to skeletal muscle metabolic health following diabetes mellitus onset (particularly Type 2 diabetes mellitus), few have examined the negative impact of diabetes mellitus on the growth and reparative capacities of skeletal muscle that often coincides with disease development. Importantly, evidence is accumulating that the muscle progenitor cell population (particularly the muscle satellite cell population) is also negatively affected by the diabetic environment, and as such, likely contributes to the declining skeletal muscle health observed in diabetes mellitus. In this review, we summarize the current knowledge surrounding the influence of diabetes mellitus on skeletal muscle growth and repair, with a particular emphasis on the impact of diabetes mellitus on skeletal muscle progenitor cell populations.

Early-age feed restriction affects viability and gene expression of satellite cells isolated from the gastrocnemius muscle of broiler chicks

Background

Muscle growth depends on the fusion of proliferate satellite cells to existing myofibers. We reported previously that 0–14 day intermittent feeding led to persistent retardation in myofiber hypertrophy. However, how satellite cells respond to such nutritional insult has not been adequately elucidated.

Results

One-day-old broiler chicks were allocated to control (Con, ad libitum feeding), intermittent feeding (IF, feed provided on alternate days) and re-feeding (RF, 2 days ad libitum feeding after 12 days of intermittent feeding) groups. Chickens were killed on Day 15 and satellite cells were isolated. When cultured, satellite cells from the IF group demonstrated significant retardation in proliferation and differentiation potential, while RF partly restored the proliferation rate and differentiation potential of the satellite cells. Significant up-regulation of insulin like growth factor I receptor (IGF-IR) (P<0.05) and thyroid hormone receptor α (TRα) (P<0.05), and down-regulation of growth hormone receptor (GHR) (P<0.01) and IGF-I (P<0.01) mRNA expression was observed in freshly isolated IF satellite cells when compared with Con cells. In RF cells, the mRNA expression of IGF-I was higher (P<0.05) and of TRα was lower (P<0.01) than in IF cells, suggesting that RF restored the mRNA expression of TRα and IGF-I, but not of GHR and IGF-IR. The Bax/Bcl-2 ratio tended to increase in the IF group, which was reversed in the RF group (P<0.05), indicating that RF reduced the pro-apoptotic influence of IF. Moreover, no significant effect of T₃ was detected on cell survival in IF cells compared with Con (P<0.001) or RF (P<0.05) cells.

Conclusions

These data suggest that early-age feed restriction inhibits the proliferation and differentiation of satellite cells, induces changes in mRNA expression of the GH/IGF-I and thyroid hormone receptors in satellite cells, as well as blunted sensitivity of satellite cells to T₃, and that RF partially reverses these effects. Thus, a moderate nutritional strategy for feed restriction should be chosen in early chick rearing systems.

Endocrine and metabolic regulation of muscle growth and body composition in cattle

Muscle metabolism (in interaction with other organs and tissues, including adipose tissue) plays an important role in the control of growth and body composition. Muscle ontogenesis has been described in different genotypes of cattle for myofibres, connective tissue and intramuscular depots. The ontogenesis or the action of putatively important factors controlling muscle development (IGF-II expression, IGF receptors, growth hormone (GH) receptor, myostatin, basic fibroblast growth factor, transforming growth factor-β1, insulin and thyroid hormones) has also been studied on bovine foetal muscle samples and satellite cells. The glucose/insulin axis has been specifically studied in both the bovine adipose tissue and heart. Clearly, cattle, like sheep, are mature species at birth based on their muscle characteristics compared to other mammalian or farm animal species. The different myoblast generations have been well characterised in cattle, including the second generation which is liable to be affected by foetal undernutrition at least in sheep. Interesting genotypes, for example, double-muscled genotype, have been characterised by an altered metabolic and endocrine status associated with a reduced fat mass, specific muscle traits and different foetal characteristics. Finally, the recent development of genomics in cattle has allowed the identification of novel genes controlling muscle development during foetal and postnatal life. Generally, a high muscle growth potential is associated with a reduced fat mass and a switch of muscle fibres towards the glycolytic type. The possibility and the practical consequences of manipulating muscle growth and, hence, body composition by nutritional and hormonal factors are discussed for bovines based on our current biological knowledge.

A simplified but robust method for the isolation of avian and mammalian muscle satellite cells

Background

Current methods of isolation of muscle satellite cells from different animal species are highly variable making inter-species comparisons problematic. This variation mainly stems from the use of different proteolytic enzymes to release the satellite cells from the muscle tissue (sometimes a single enzyme is used but often a combination of enzymes is preferred) and the different extracellular matrix proteins used to coat culture ware. In addition, isolation of satellite cells is frequently laborious and sometimes may require pre-plating of the cell preparation on uncoated flasks or Percoll centrifugation to remove contaminating fibroblasts. The methodology employed to isolate and culture satellite cells in vitro can critically determine the fusion of myoblasts into multi-nucleated myotubes. These terminally differentiated myotubes resemble mature myofibres in the muscle tissue in vivo, therefore optimal fusion is a keystone of in vitro muscle culture. Hence, a simple method of muscle satellite cell isolation and culture of different vertebrate species that can result in a high fusion rate is highly desirable.

Results

We demonstrate here a relatively simple and rapid method of isolating highly enriched muscle satellite cells from different avian and mammalian species. In brief, muscle tissue was mechanically dissociated, digested with a single enzyme (pronase), triturated with a 10-ml pipette, filtered and directly plated onto collagen coated flasks. Following this method and after optimization of the cell culture conditions, excellent fusion rates were achieved in the duck, chicken, horse and cow (with more than 50% cell fusion), and to a lesser extent pig, pointing to pronase as a highly suitable enzyme to release satellite cells from muscle tissue.

Conclusions

Our simplified method presents a quick and simple alternative to isolating highly enriched muscle satellite cell cultures which can subsequently rapidly differentiate into well developed primary myotubes. The use of the same isolation protocol allows better inter-species comparisons of muscle satellite cells. Of all the farm animal species investigated, harvested chicken muscle cells showed the highest percentage of muscle satellite cells, and equine muscle cells presented the highest fusion index, an impressive ≈ 77%. Porcine cells displayed the lowest amount of satellite cells but still achieved a modest fusion rate of ≈ 41%.

Production of transgenic calves expressing an shRNA targeting myostatin

Myostatin (MSTN) is a well-known negative regulator of muscle growth. Animals that possess mutations within this gene display an enhanced muscling phenotype, a desirable agricultural trait. Increased neonatal morbidity is common, however, resulting from complications arising from the birth of offspring with increased fetal muscle mass. The objective of the current research was to generate an attenuated MSTN-null phenotype in a large-animal model using RNA interference to enhance muscle development without the detrimental consequences of an inactivating mutation. To this end, we identified a series of short interfering RNAs that demonstrated effective suppression of MSTN mRNA and protein levels. To produce transgenic offspring capable of stable MSTN suppression in vivo, a recombinant lentiviral vector expressing a short hairpin RNA (shRNA) targeting MSTN for silencing was introduced into bovine fetal fibroblasts. These cells were used as nucleus donors for somatic cell nuclear transfer (SCNT). Twenty blastocysts were transferred into seven recipient cows resulting in five pregnancies. One transgenic calf developed to term, but died following delivery by Caesarean-section. As an alternative strategy, microinjection of recombinant lentiviral particles into the perivitelline space of in vitro-produced bovine zygotes was utilized to produce 40 transgenic blastocysts that were transferred into 14 recipient cows, resulting in 7 pregnancies. Five transgenic calves were produced, of which three expressed the transgene. This is the first report of transgenic livestock produced by direct injection of a recombinant lentivirus, and expressing transgenes encoding shRNAs targeting an endogenous gene (myostatin) for silencing.

Expression of masticatory-specific isoforms of myosin heavy-chain, myosin-binding protein-C and tropomyosin in muscle fibers and satellite cell cultures of cat masticatory muscle

We test the hypothesis that cat jaw satellite cells belong to a distinct lineage preprogrammed to express masticatory-specific isoforms of myosin heavy-chain (m-MyHC), myosin-binding protein-C (m-MBP-C), and tropomyosin (m-Tm) during myogenesis in vitro. A monoclonal antibody (MAb) against m-MyHC and MAbs raised here against cat m-MBP-C and m-Tm were used to stain cryostat sections of cat masseter muscle and cultured myotubes derived from satellite cells of cat temporalis and limb muscles, using peroxidase immunohistochemistry. MAbs against m-MBP-C bound purified m-MBP-C in Western blots. MAbs against m-Tm failed to react with m-Tm in Western blots, but reacted with native m-Tm in gel electrophoresis-derived ELISA. In cat masseter sections, MAbs against m-MyHC, m-MBP-C, and m-Tm stained all masticatory fibers, but not the jaw-slow fibers. Cat jaw and limb muscle cultures mature significantly more slowly relative to rodent cultures. However, at 3 weeks, all three MAbs extensively stained temporalis myotubes, whereas they apparently stained isolated myotubes weakly in cat limb and rat jaw cultures. We conclude that satellite cells of masticatory fibers are preprogrammed to express these isoforms during myogenesis in vitro. These results consolidate the notion that masticatory and limb muscle allotypes are distinct.

In vitro myogenic and adipogenic differentiation model of genetically engineered bovine embryonic fibroblast cell lines

Our current understanding of muscle and adipose tissue development has been largely restricted to the study of murine myogenic and adipogenic cell lines, since attempts to establish these cell lines from other species have met with only limited success. Here we report that a spontaneously immortalized bovine embryonic fibroblast cell line (BEFS) undergoes differentiation into adipogenic or myogenic lineages when ectopically transduced with PPARgamma2 (an adipogenic lineage determinant) or MyoD (a myogenic lineage determinant) and grown in adipogenic and myogenic differentiation culture media (ADCM and MDCM, respectively). We also found that PPARgamma2-overexpressing BEFS cells (BEFS-PPARgamma2) grown in ADCM with or without the PPARgamma2 ligand, troglitazone, preferentially differentiate into adipogenic cells in the presence of ectopic MyoD expression. Ectopic expression of PPARgamma2 in the inducible MyoD-overepxressing BEFS cells (BEFS-TetOn-MyoD) completely suppresses myogenic differentiation and leads to a significant increase in adipogenic differentiation, suggesting that the adipogenic differentiation program might be dominant. Therefore, BEFS, BEFS-PPARgamma2, and BEFS-TetOn-MyoD would be a valuable biological model for understanding a fundamental principle underlying myogenic and adipogenic development, and for isolating various genetic and chemical factors that enable muscle and adipocyte differentiation.

Effects of elevated thyroid hormone on adult rabbit extraocular muscles

PURPOSE

Human extraocular muscles (EOM) are preferentially susceptible to thyroid eye disease. Although the specific cause of this autoimmune disorder is unknown, it is often associated with elevated thyroid hormone levels. Thus, the effect of elevated thyroid hormone levels on cross-sectional area, myofiber size, satellite cells, and myosin heavy chain (MyHC) isoform expression was examined in adult rabbit EOMs, to determine how elevated thyroid hormone alters EOM biology.

METHODS

After 1 month of elevated thyroid hormone levels, the EOMs were removed and prepared for histologic examination. Total muscle mass, myofiber size, patterns of MyHC isoform expression, and the number of satellite cells were determined.

RESULTS

Elevated thyroid hormone levels significantly decreased muscle mass, total number of myofibers, and mean cross-sectional area of the myofibers. Alterations in MyHC isoform expression were extremely complex, but several basic patterns emerged. The percentages of neonatal- and developmental-positive myofibers decreased in almost all EOM regions examined, and the percentages of slow-positive myofibers significantly increased. In contrast to normal EOMs, which retain a population of activated satellite cells throughout life, elevated thyroid hormone levels resulted in the virtual disappearance of MyoD-positive cells and a decrease in Pax7-positive cells.

CONCLUSIONS

The reductions in EOM size, number of fibers expressing developmental and neonatal MyHC, and number of MyoD- and Pax7-positive satellite cells suggest that elevated thyroid hormone levels decrease the ongoing myofiber remodeling normally seen in the EOM. These catabolic changes have important implications for maintenance of function in the EOMs.

Not only insulin stimulates mitochondriogenesis in muscle cells, but mitochondria are also essential for insulin-mediated myogenesis

Viability and myogenesis from C2C12 muscle cells and L6 rat myoblasts were dose-dependently stimulated by insulin. The metabolic inhibitors of phosphatidyl-inositol-3-kinase (PI-3K, LY294002) and of MAPKK/ERK kinase (MEK, PD98059) differently affected insulin-stimulated myogenesis of the cells. After LY294002 and PD98059 treatment, viability deteriorated and apparently an additive effect of both metabolic inhibitors was observed, irrespective of the method of measurement (neutral red or MTT assay). These inhibitors were antagonistic in myogenesis. Our results confirm that insulin regulates cell viability by at least two distinct pathways, namely by PI-3K- and MEK-dependent signalling cascades. Both pathways are agonistic in cell viability, whereas PI-3K rather than MEK supports insulin-mediated myogenicity. Accordingly, inhibition of insulin action by LY294002, but not PD98059, was accompanied with a reduced level of Ser473-phosphorylated Akt with additional loss of myogenin protein. Besides, repression of insulin signalling by either PI-3K or MEK inhibitor diminished expression of selected subunits of the mitochondrial oxidative phosphorylation enzymes (OXPHOS). In turn, insulin raised and accelerated protein expression of subunits I and IV of mitochondrial cytochrome-c oxidase (COX). In addition, the level of myogenin, the molecular marker of terminal and general muscle differentiation indices decreased if selected OXPHOS enzymes were individually blocked by rotenone, myxothiazol or oligomycin. Summing up, our results pointed to mitochondria as an essential organelle for insulin-dependent myogenesis. Insulin positively affects mitochondrial function by induction of OXPHOS enzymes, which provide energy indispensable for the anabolic effect of insulin.

A method to establish co-cultures of myotubes and preadipocytes from collagenase digested neonatal pig semitendinosus muscles1

The relationships between adipocyte and muscle cell development within muscle are important in the study of factors or agents that may improve meat quality. Neonatal porcine muscle has the potential to yield both cell types for cell culture because it contains developing adipocytes and a high number of muscle satellite cells. Therefore, we modified a conventional collagenase-based procedure to digest neonatal porcine muscle and subsequently cultured the resultant muscle stromal-vascular (SV) cells on several substrata in basal and dexamethasone (DEX)-containing media. Developing myotubes and preadipocytes were present in muscle SV cell cultures on laminin substrata following seeding and plating with fetal bovine serum (FBS) with or without DEX. Myotube number was much higher (P < 0.05) on laminin substrata compared with all other substrata, whereas preadipocyte number in muscle SV cell cultures was independent of substrata, as we have shown previously. This approach can be used to establish co-cultures of differentiating adipocytes and myotubes from collagenase-digested neonatal pig muscle. Because the comparison is within the same culture dish, this method allows for a direct comparison of the responses of adipogenic and myogenic cells to growth and differentiation factors. For example, DEX did not alter myogenesis (i.e., 11 +/- 3 vs. 11 +/- 4 myotubes per unit area for control and DEX-treated cultures, respectively), but it has been shown to markedly increase preadipocyte number in muscle SV cell cultures.

Preconditioning with millimolar concentrations of vitamin C or N-acetylcysteine protects L6 muscle cells insulin-stimulated viability and DNA synthesis under oxidative stress

The effect of reactive oxygen/nitrogen species (ROS/RNS)(hydrogen peroxide -- H(2)O(2), superoxide anion radical O(2)*- and hydroxyl radical *OH -- the reaction products of hypoxanthine/xanthine oxidase system), nitric oxide (NO* from sodium nitroprusside -- SNP), and peroxynitrite (ONOO(-) from 3-morpholinosydnonimine -- SIN-1) on insulin mitogenic effect was studied in L6 muscle cells after one day pretreatment with/or without antioxidants. ROS/RNS inhibited insulin-induced mitogenicity (DNA synthesis). Insulin (0.1 microM), however, markedly improved mitogenicity in the muscle cells treated with increased concentrations (0.1, 0.5, 1 mM) of donors of H(2)O(2), O(2)*-, *OH, ONOO(-) and NO*. Cell viability assessed by morphological criteria was also monitored. Massive apoptosis was induced by 1 mM of donors of H(2)O(2) and ONOO(-), while NO* additionally induced necrotic cell death. Taken together, these results have shown that ROS/RNS provide a good explanation for the developing resistance to the growth promoting activity of insulin in myoblasts under conditions of oxidative or nitrosative stress. Cell viability showed that neither donor induced cell death when given below 0.5 mM. In order to confirm the deleterious effects of ROS/RNS prior to the subsequent treatment with ROS/RNS plus insulin one day pretreatment with selected antioxidants (sodium ascorbate - ASC (0.01, 0.1, 1 mM), or N-acetylcysteine - NAC (0.1, 1, 10 mM) was carried out. Surprisingly, at a low dose (micromolar) antioxidants did not abrogate and even worsened the concentration-dependent effects of ROS/RNS. In contrast, pretreatment with millimolar dose of ASC or NAC maintained an elevated mitogenicity in response to insulin irrespective of the ROS/RNS donor type used.

Receptors for insulin-like growth factor-I (IGF-I) predominate over insulin receptors in skeletal muscle throughout the life cycle of brown trout, Salmo trutta

Insulin and IGF-I binding has been studied in brown trout (Salmo trutta) wheat germ agglutinin semipurified receptors from embryos (organogenesis), larvae (yolk sac), juveniles (2.98 +/- 0.21 g bw) and adults (111.6 +/- 6.92 and 522 +/- 53 g bw). Embryos and larvae were sampled at 5 and 12 weeks after fertilization (December 1999 and February 2000) and juvenile and adults were taken simultaneously (July 1999) and under the same feeding conditions to minimize potential nutritional and seasonal effects. Insulin receptor number was maximal at 12 weeks (144 fmol/mg glycoprotein) and progressively decreased in subsequent samplings. No alterations in affinity were detected (K(d) range, 0.21-0.32 nM) and changes in number of receptor paralleled changes in total specific binding. IGF-I receptor number was highest at 5 weeks (1044 fmol/mg) and was significantly higher than values for insulin in all samplings. The affinity of IGF-I receptor did not change (K(d) range, 0.11-0.18 nM) but was consistently higher than that for the insulin receptor. A more rapid decrease of IGF-I binding and receptor number was found with age. However, the ratio of insulin/IGF-I binding established in 12-week-old larvae (0.18 +/- 0.01) was thereafter maintained at very similar values in juveniles and adults (0.15-0.17). Tyrosine kinase activity (TKA) for insulin receptors ranged between 136 and 183% and there were no significant changes with age. For the IGF-I receptor, TKA ranged from 174 to 281% and was significantly higher in 5-week-old larvae coincident with the highest levels of receptor number and declined gradually in parallel with binding levels. In conclusion, the greater abundance of IGF-I receptors during embryonic and larval development is maintained throughout juvenile and adult stages. This would suggest a key role for IGF-I in the growth and metabolism of trout muscle.