Caspase-3, myogenic transcription factors and cell cycle inhibitors are regulated by leukemia inhibitory factor to mediate inhibition of myogenic differentiation

Mitochondrial Apoptotic Signaling Involvement in Remodeling During Myogenesis and Skeletal Muscle Atrophy

Mitochondria play a major role in apoptotic signaling. In addition to its role in eliminating dysfunctional cells, mitochondrial apoptotic signaling is implicated as a key component of myogenic differentiation and skeletal muscle atrophy. For example, the activation of cysteine-aspartic proteases (caspases; CASP's) can aid in the initial remodeling stages of myogenic differentiation by cleaving protein kinases, transcription factors, and cytoskeletal proteins. Precise regulation of these signals is needed to prevent excessive cell disassemble and subsequent cell death. During skeletal muscle atrophy, the activation of CASP's and mitochondrial derived nucleases participate in myonuclear fragmentation, a potential loss of myonuclei, and cleavage of contractile structures within skeletal muscle. The B cell leukemia/lymphoma 2 (BCL2) family of proteins play a significant role in regulating myogenesis and skeletal muscle atrophy by governing the initiating steps of mitochondrial apoptotic signaling. This review discusses the role of mitochondrial apoptotic signaling in skeletal muscle remodeling during myogenic differentiation and skeletal muscle pathological states, including aging, disuse, and muscular dystrophy.

Myeloid cell-mediated targeting of LIF to dystrophic muscle causes transient increases in muscle fiber lesions by disrupting the recruitment and dispersion of macrophages in muscle

Leukemia inhibitory factor (LIF) can influence development by increasing cell proliferation and inhibiting differentiation. Because of its potency for expanding stem cell populations, delivery of exogenous LIF to diseased tissue could have therapeutic value. However, systemic elevations of LIF can have negative, off-target effects. We tested whether inflammatory cells expressing a LIF transgene under control of a leukocyte-specific, CD11b promoter provide a strategy to target LIF to sites of damage in the mdx mouse model of Duchenne muscular dystrophy, leading to increased numbers of muscle stem cells and improved muscle regeneration. However, transgene expression in inflammatory cells did not increase muscle growth or increase numbers of stem cells required for regeneration. Instead, transgene expression disrupted the normal dispersion of macrophages in dystrophic muscles, leading to transient increases in muscle damage in foci where macrophages were highly concentrated during early stages of pathology. The defect in inflammatory cell dispersion reflected impaired chemotaxis of macrophages to C-C motif chemokine ligand-2 and local increases of LIF production that produced large aggregations of cytolytic macrophages. Transgene expression also induced a shift in macrophage phenotype away from a CD206+, M2-biased phenotype that supports regeneration. However, at later stages of the disease when macrophage numbers declined, they dispersed in the muscle, leading to reductions in muscle fiber damage, compared to non-transgenic mdx mice. Together, the findings show that macrophage-mediated delivery of transgenic LIF exerts differential effects on macrophage dispersion and muscle damage depending on the stage of dystrophic pathology.

Novel in vivo potential of trifluoperazine to ameliorate doxorubicin-induced cardiotoxicity involves suppression of NF-κB and apoptosis

AIMS

Cardiotoxicity of doxorubicin frequently complicates treatment outcome. Aberrantly activated calcium/calmodulin pathway can eventually trigger signaling cascades that mediate cardiotoxicity. Therefore, we tested the hypothesis that trifluoperazine, a strong calmodulin antagonist, may alleviate this morbidity.

MATERIALS AND METHODS

Heart failure and cardiotoxicity were assessed via echocardiography, PCR, immunohistochemistry, histopathology, Masson's trichrome staining and transmission electron microscopy. Whereas liver and kidney structural and functional alterations were evaluated histopathologically and biochemically.

KEY FINDINGS

Results revealed that combination treatment with trifluoperazine could overcome doxorubicin-induced heart failure with reduced ejection fraction. Moreover, heart weight/body weight ratio and histopathological examination showed that trifluoperazine mitigated doxorubicin-induced cardiac atrophy, inflammation and myofibril degeneration. Transmission electron microscopy further confirmed the marked restoration of the left ventricular ultrastructures by trifluoperazine pretreatment. In addition, Masson's trichrome staining revealed that trifluoperazine could significantly inhibit doxorubicin-induced left ventricular remodeling by fibrosis. Of note, doxorubicin induced the expression of myocardial nuclear NF-κB-p65 and caspase-3 which were markedly inhibited by trifluoperazine, suggesting that cardioprotection conferred by trifluoperazine involved, at least in part, suppression of NF-κB and apoptosis. Furthermore, biochemical and histopathological examinations showed that trifluoperazine improved doxorubicin-induced renal and hepatic impairments both functionally and structurally.

SIGNIFICANCE

In conclusion, the present in vivo study is the first to provide evidences underscoring the protective effects of trifluoperazine that may pave the way for repurposing this calmodulin antagonist in ameliorating organ toxicity by doxorubicin.

Resveratrol Promotes Hypertrophy in Wildtype Skeletal Muscle and Reduces Muscle Necrosis and Gene Expression of Inflammatory Markers in Mdx Mice

Duchenne muscular dystrophy (DMD) is a progressive fatal neuromuscular disorder with no cure. Therapies to restore dystrophin deficiency have been approved in some jurisdictions but long-term effectiveness is yet to be established. There is a need to develop alternative strategies to treat DMD. Resveratrol is a nutraceutical with anti-inflammatory properties. Previous studies have shown high doses (100–400 mg/kg bodyweight/day) benefit mdx mice. We treated 4-week-old mdx and wildtype mice with a lower dose of resveratrol (5 mg/kg bodyweight/day) for 15 weeks. Voluntary exercise was used to test if a lower dosage than previously tested could reduce exercise-induced damage where a greater inflammatory infiltrate is present. We found resveratrol promoted skeletal muscle hypertrophy in wildtype mice. In dystrophic muscle, resveratrol reduced exercise-induced muscle necrosis. Gene expression of immune cell markers, CD86 and CD163 were reduced; however, signalling targets associated with resveratrol’s mechanism of action including Sirt1 and NF-κB were unchanged. In conclusion, a lower dose of resveratrol compared to the dosage used by other studies reduced necrosis and gene expression of inflammatory cell markers in dystrophic muscle suggesting it as a therapeutic candidate for treating DMD.

Enhanced in vivo targeting of estrogen receptor alpha signaling in murine mammary adenocarcinoma by nilotinib/rosuvastatin novel combination

The development of resistance to endocrine therapy of estrogen receptor alpha (ERα)-positive breast cancer is inevitable, necessitating the introduction of alternative treatment strategies. Therefore, the current study was carried out to investigate the in vivo efficacy and tolerability of nilotinib/rosuvastatin novel combination against ERα-positive breast carcinoma. Results showed that treatment of tumor-bearing mice with nilotinib/rosuvastatin exerted a significant antitumor activity. Mechanistically, the combination treatment efficiently inhibited the in vivo ERα protein expression, whereas ERα mRNA levels were unaffected suggesting a posttranslational regulation. In addition, the combination treatment markedly downregulated the expression of two ERα downstream target genes: C3 and pS2 confirming the inhibition of ERα signaling in vivo. Further, nilotinib/rosuvastatin combination strongly induced apoptosis evidenced by a marked caspase-3 cleavage and downregulation of tumor nitric oxide levels. Moreover, histopathology showed significant declines in mitotic figures and tumor giant cells implying the in vivo capability of the combination treatment to interfere with cancer cell proliferation and persistence. Of note, the combination treatment abrogated nilotinib-induced hypercholesterolemia and did not adversely affect the liver function or body weight. Overall, the present study provided evidences that warrant further assessment of nilotinib/rosuvastatin combination as an alternative therapeutic modality for ERα-positive breast cancer.

Exercise as an anti-inflammatory therapy for cancer cachexia: a focus on interleukin-6 regulation

Cancer cachexia is a complicated disorder of extreme, progressive skeletal muscle wasting. It is directed by metabolic alterations and systemic inflammation dysregulation. Numerous studies have demonstrated that increased systemic inflammation promotes this type of cachexia and have suggested that cytokines are implicated in the skeletal muscle loss. Exercise is firmly established as an anti-inflammatory therapy that can attenuate or even reverse the process of muscle wasting in cancer cachexia. The interleukin IL-6 is generally considered to be a key player in the development of the microenvironment of malignancy; it promotes tumor growth and metastasis by acting as a bridge between chronic inflammation and cancerous tissue and it also induces skeletal muscle atrophy and protein breakdown. Paradoxically, a beneficial role for IL-6 has also been identified recently, and that is its status as a “founding member” of the myokine class of proteins. Skeletal muscle is an important source of circulating IL-6 in people who participate in exercise training. IL-6 acts as an anti-inflammatory myokine by inhibiting TNFα and improving glucose uptake through the stimulation of AMPK signaling. This review discusses the action of IL-6 in skeletal muscle tissue dysfunction and the role of IL-6 as an “exercise factor” that modulates the immune system. This review also sheds light on the main considerations related to the treatment of muscle wasting in cancer cachexia.

The glucose-sensing transcription factor MLX promotes myogenesis via myokine signaling

In this study, Hunt et. al. provide novel insight into the regulation of glucose-induced myogenesis. They demonstrate that changes in glucose levels regulate myogenesis by increasing the activity of the glucose-responsive transcription factor MLX, which is necessary and sufficient for myoblast fusion and differentiation.

Metabolic stress and changes in nutrient levels modulate many aspects of skeletal muscle function during aging and disease. Growth factors and cytokines secreted by skeletal muscle, known as myokines, are important signaling factors, but it is largely unknown whether they modulate muscle growth and differentiation in response to nutrients. Here, we found that changes in glucose levels increase the activity of the glucose-responsive transcription factor MLX (Max-like protein X), which promotes and is necessary for myoblast fusion. MLX promotes myogenesis not via an adjustment of glucose metabolism but rather by inducing the expression of several myokines, including insulin-like growth factor 2 (IGF2), whereas RNAi and dominant-negative MLX reduce IGF2 expression and block myogenesis. This phenotype is rescued by conditioned medium from control muscle cells and by recombinant IGF2, which activates the myogenic kinase Akt. Importantly, MLX-null mice display decreased IGF2 induction and diminished muscle regeneration in response to injury, indicating that the myogenic function of MLX is manifested in vivo. Thus, glucose is a signaling molecule that regulates myogenesis and muscle regeneration via MLX/IGF2/Akt signaling.

NDRG2 promotes myoblast proliferation and caspase 3/7 activities during differentiation, and attenuates hydrogen peroxide - But not palmitate-induced toxicity

The function of the stress-responsive N-myc downstream-regulated gene 2 (NDRG2) in the control of myoblast growth, and the amino acids contributing to its function, are not well characterized. Here, we investigated the effect of increased NDRG2 levels on the proliferation, differentiation and apoptosis in skeletal muscle cells under basal and stress conditions. NDRG2 overexpression increased C2C12 myoblast proliferation and the expression of positive cell cycle regulators, cdk2, cyclin B and cyclin D, and phosphorylation of Rb, while the serine/threonine-deficient NDRG2, 3A-NDRG2, had less effect. The onset of differentiation was enhanced by NDRG2 as determined through the myogenic regulatory factor expression profiles and myocyte fusion index. However, the overall level of differentiation in myotubes was not different. While NDRG2 up-regulated caspase 3/7 activities during differentiation, no increase in apoptosis was measured by TUNEL assay or through cleavage of caspase 3 and PARP proteins. During H2O2 treatment to induce oxidative stress, NDRG2 helped protect against the loss of proliferation and ER stress as measured by GRP78 expression with 3A-NDRG2 displaying less protection. NDRG2 also attenuated apoptosis by reducing cleavage of PARP and caspase 3 and expression of pro-apoptotic Bax while enhancing the pro-survival Bcl-2 and Bcl-xL levels. In contrast, Mcl-1 was not altered, and NDRG2 did not protect against palmitate-induced lipotoxicity. Our findings show that NDRG2 overexpression increases myoblast proliferation and caspase 3/7 activities without increasing overall differentiation. Furthermore, NDRG2 attenuates H2O2-induced oxidative stress and specific serine and threonine amino acid residues appear to contribute to its function in muscle cells.

Xin-deficient mice display myopathy, impaired contractility, attenuated muscle repair and altered satellite cell functionality

AIM

Xin is an F-actin-binding protein expressed during development of cardiac and skeletal muscle. We used Xin-/- mice to determine the impact of Xin deficiency on different aspects of skeletal muscle health, including functionality and regeneration.

METHODS

Xin-/- skeletal muscles and their satellite cell (SC) population were investigated for the presence of myopathic changes by a series of histological and immunofluorescent stains on resting uninjured muscles. To further understand the effect of Xin loss on muscle health and its SCs, we studied SCs responses following cardiotoxin-induced muscle injury. Functional data were determined using in situ muscle stimulation protocol.

RESULTS

Compared to age-matched wild-type (WT), Xin-/- muscles exhibited generalized myopathy and increased fatigability with a significantly decreased force recovery post-fatiguing contractions. Muscle regeneration was attenuated in Xin-/- mice. This impaired regeneration prompted an investigation into SC content and functionality. Although SC content was not different, significantly more activated SCs were present in Xin-/- vs. WT muscles. Primary Xin-/- myoblasts displayed significant reductions (approx. 50%) in proliferative capacity vs. WT; a finding corroborated by significantly decreased MyoD-positive nuclei in 3 days post-injury Xin-/- muscle vs. WT. As more activated SCs did not translate to more proliferating myoblasts, we investigated whether Xin-/- SCs displayed an exaggerated loss by apoptosis. More apoptotic SCs (TUNEL+/Pax7+) were present in Xin-/- muscle vs. WT. Furthermore, more Xin-/- myoblasts were expressing nuclear caspase-3 compared to WT at 3 days post-injury.

CONCLUSION

Xin deficiency leads to a myopathic condition characterized by increased muscle fatigability, impaired regeneration and SC dysfunction.

The leukemia inhibitory factor (LIF) and p21 mediate the TGFβ tumor suppressive effects in human cutaneous melanoma

BACKGROUND

Cutaneous melanoma is the most lethal skin cancer and its incidence in developed countries has dramatically increased over the past decades. Localized tumors are easily treated by surgery, but advanced melanomas lack efficient treatment and are associated with very poor outcomes. Thus, understanding the processes underlying melanoma development and progression is critical. The Transforming Growth Factor beta (TGFβ) acts as a potent tumor suppressor in human melanoma, by inhibiting cell growth and preventing cellular migration and invasion.

METHODS

In this study, we aimed at elucidating the molecular mechanisms underlying TGFβ-mediated tumor suppression. Human cutaneous melanoma cell lines, derived from different patients, were used to assess for cell cycle analysis, apoptosis/caspase activity and cell migration. Techniques involved immunoblotting, immunohistochemistry, real time PCR and luciferase reporter assays.

RESULTS

We found the leukemia inhibitory factor (LIF) to be strongly up-regulated by TGFβ in melanoma cells, defining LIF as a novel TGFβ downstream target gene in cutaneous melanoma. Interestingly, we also showed that TGFβ-mediated LIF expression is required for TGFβ-induced cell cycle arrest and caspase-mediated apoptosis, as well as for TGFβ-mediated inhibition of cell migration. Moreover, we found that TGFβ-mediated LIF expression leads to activation of transcription of the cell cycle inhibitor p21 in a STAT3-dependent manner, and further showed that p21 is required for TGFβ/LIF-mediated cell cycle arrest and TGFβ-induced gene activation of several pro-apoptotic genes.

CONCLUSIONS

Together, our results define the LIF/p21 signaling cascade as a novel tumor suppressive-like pathway in melanoma, acting downstream of TGFβ to regulate cell cycle arrest and cell death, further highlight new potential therapeutic strategies for the treatment of cutaneous melanoma.

Internalization and fate of silica nanoparticles in C2C12 skeletal muscle cells: evidence of a beneficial effect on myoblast fusion

The use of silica nanoparticles for their cellular uptake capability opens up new fields in biomedical research. Among the toxicological effects associated with their internalization, silica nanoparticles induce apoptosis that has been recently reported as a biochemical cue required for muscle regeneration. To assess whether silica nanoparticles could affect muscle regeneration, we used the C2C12 muscle cell line to study the uptake of fluorescently labeled NPs and their cellular trafficking over a long period. Using inhibitors of endocytosis, we determined that the NP uptake was an energy-dependent process mainly involving macropinocytosis and clathrin-mediated pathway. NPs were eventually clustered in lysosomal structures. Myoblasts containing NPs were capable of differentiation into myotubes, and after 7 days, electron microscopy revealed that the NPs remained primarily within lysosomes. The presence of NPs stimulated the formation of myotubes in a dose-dependent manner. NP internalization induced an increase of apoptotic myoblasts required for myoblast fusion. At noncytotoxic doses, the NP uptake by skeletal muscle cells did not prevent their differentiation into myotubes but, instead, enhanced the cell fusion.

Mitochondrial pro-apoptotic indices do not precede the transient caspase activation associated with myogenesis

Skeletal muscle differentiation requires activity of the apoptotic protease caspase-3. We attempted to identify the source of caspase activation in differentiating C2C12 skeletal myoblasts. In addition to caspase-3, caspase-2 was transiently activated during differentiation; however, no changes were observed in caspase-8 or -9 activity. Although mitochondrial Bax increased, this was matched by Bcl-2, resulting in no change to the mitochondrial Bax:Bcl-2 ratio early during differentiation. Interestingly, mitochondrial membrane potential increased on a timeline similar to caspase activation and was accompanied by an immediate, temporary reduction in cytosolic Smac and cytochrome c. Since XIAP protein expression dramatically declined during myogenesis, we investigated whether this contributes to caspase-3 activation. Despite reducing caspase-3 activity by up to 57%, differentiation was unaffected in cells overexpressing normal or E3-mutant XIAP. Furthermore, a XIAP mutant which can inhibit caspase-9 but not caspase-3 did not reduce caspase-3 activity or affect differentiation. Administering a chemical caspase-3 inhibitor demonstrated that complete enzyme inhibition was required to impair myogenesis. These results suggest that neither mitochondrial apoptotic signaling nor XIAP degradation is responsible for transient caspase-3 activation during C2C12 differentiation.

Regulation of nonmuscle myosin II during 3-methylcholanthrene induced dedifferentiation of C2C12 myotubes

3-Methylcholanthrene (3MC) induces tumor formation at the site of injection in the hind leg of mice within 110 days. Recent reports reveal that the transformation of normal muscle cells to atypical cells is one of the causes for tumor formation, however the molecular mechanism behind this process is not well understood. Here, we show in an in vitro study that 3MC induces fragmentation of multinucleate myotubes into viable mononucleates. These mononucleates form colonies when they are seeded into soft agar, indicative of cellular transformation. Immunoblot analysis reveals that phosphorylation of myosin regulatory light chain (RLC20) is 5.6±0.5 fold reduced in 3MC treated myotubes in comparison to vehicle treated myotubes during the fragmentation of myotubes. In contrast, levels of myogenic factors such as MyoD, Myogenin and cell cycle regulators such as Cyclin D, Cyclin E1 remain unchanged as assessed by real-time PCR array and reverse transcriptase PCR analysis, respectively. Interestingly, addition of the myosin light chain kinase inhibitor, ML-7, enhances the fragmentation, whereas phosphatase inhibitor perturbs the 3MC induced fragmentation of myotubes. These results suggest that decrease in RLC20 phosphorylation may be associated with the fragmentation step of dedifferentiation.

The Polycomb group (PcG) protein EZH2 supports the survival of PAX3-FOXO1 alveolar rhabdomyosarcoma by repressing FBXO32 (Atrogin1/MAFbx)

The Polycomb group (PcG) proteins regulate stem cell differentiation via the repression of gene transcription, and their deregulation has been widely implicated in cancer development. The PcG protein Enhancer of Zeste Homolog 2 (EZH2) works as a catalytic subunit of the Polycomb Repressive Complex 2 (PRC2) by methylating lysine 27 on histone H3 (H3K27me3), a hallmark of PRC2-mediated gene repression. In skeletal muscle progenitors, EZH2 prevents an unscheduled differentiation by repressing muscle-specific gene expression and is downregulated during the course of differentiation. In rhabdomyosarcoma (RMS), a pediatric soft-tissue sarcoma thought to arise from myogenic precursors, EZH2 is abnormally expressed and its downregulation in vitro leads to muscle-like differentiation of RMS cells of the embryonal variant. However, the role of EZH2 in the clinically aggressive subgroup of alveolar RMS, characterized by the expression of PAX3-FOXO1 oncoprotein, remains unknown. We show here that EZH2 depletion in these cells leads to programmed cell death. Transcriptional derepression of F-box protein 32 (FBXO32) (Atrogin1/MAFbx), a gene associated with muscle homeostasis, was evidenced in PAX3-FOXO1 RMS cells silenced for EZH2. This phenomenon was associated with reduced EZH2 occupancy and H3K27me3 levels at the FBXO32 promoter. Simultaneous knockdown of FBXO32 and EZH2 in PAX3-FOXO1 RMS cells impaired the pro-apoptotic response, whereas the overexpression of FBXO32 facilitated programmed cell death in EZH2-depleted cells. Pharmacological inhibition of EZH2 by either 3-Deazaneplanocin A or a catalytic EZH2 inhibitor mirrored the phenotypic and molecular effects of EZH2 knockdown in vitro and prevented tumor growth in vivo. Collectively, these results indicate that EZH2 is a key factor in the proliferation and survival of PAX3-FOXO1 alveolar RMS cells working, at least in part, by repressing FBXO32. They also suggest that the reducing activity of EZH2 could represent a novel adjuvant strategy to eradicate high-risk PAX3-FOXO1 alveolar RMS.

Interleukin-6 myokine signaling in skeletal muscle: a double-edged sword?

Interleukin (IL)-6 is a cytokine with pleiotropic functions in different tissues and organs. Skeletal muscle produces and releases significant levels of IL-6 after prolonged exercise and is therefore considered as a myokine. Muscle is also an important target of the cytokine. IL-6 signaling has been associated with stimulation of hypertrophic muscle growth and myogenesis through regulation of the proliferative capacity of muscle stem cells. Additional beneficial effects of IL-6 include regulation of energy metabolism, which is related to the capacity of actively contracting muscle to synthesize and release IL-6. Paradoxically, deleterious actions for IL-6 have also been proposed, such as promotion of atrophy and muscle wasting. We review the current evidence for these apparently contradictory effects, the mechanisms involved and discuss their possible biological implications.

Role of prolactin-like protein (PRL-L) in cold-induced increase of muscle mass in chicks

This study examined the hypothesis that a novel prolactin-like protein gene (PRL-L) is involved in cold-induced growth of skeletal muscle in chicks. Six-day-old chicks (Gallus gallus domesticus) were exposed to cold at 4°C or kept warm at 30°C for 24h. Cold exposure induced significant increases in PRL-L expression that coincided with increases in the weight of the sartorius muscle, which comprises both fast- and slow-twitch fibers. Meanwhile, no induction of PRL-L mRNA was observed in the heart, liver, kidney, brain, or fat. Myoblast cells that expressed PRL-L mRNA grew faster than untransduced cells in media containing 2% serum. These results suggested that PRL-L might be involved in in controlling cold-induced muscle growth of chicks.

Nek5, a novel substrate for caspase-3, promotes skeletal muscle differentiation by up-regulating caspase activity

Accumulating evidence suggests that caspase-3-mediated cleavage of protein kinase could be a key event to regulate cell differentiation. In this study, we investigated the role of Nek5 kinase, identified as a novel substrate for caspase-3, in skeletal muscle differentiation. Up-regulation of Nek5 mRNA expression was accompanied by cell differentiation. Myotube formation was promoted in Nek5 expressing cells, and was conversely inhibited in Nek5 knockdown cells. Furthermore, we found that caspase-3 activity, an important factor for myogenic differentiation, was enhanced by Nek5 cleavage. Although caspase-3-cleaved Nek5 partially exerted a promyogenic effect, it tended to induce apoptotic cell death. In summary, our findings suggest that Nek5 promotes myogenic differentiation through up-regulation of caspase activity.

Hyaluronan synthesis and myogenesis: a requirement for hyaluronan synthesis during myogenic differentiation independent of pericellular matrix formation

Exogenous hyaluronan is known to alter muscle precursor cell proliferation, migration, and differentiation, ultimately inhibiting myogenesis in vitro. The aim of the current study was to investigate the role of endogenous hyaluronan synthesis during myogenesis. In quantitative PCR studies, the genes responsible for synthesizing hyaluronan were found to be differentially regulated during muscle growth, repair, and pathology. Although all Has genes (Has1, Has2, and Has3) were differentially regulated in these models, only Has2 gene expression consistently associated with myogenic differentiation. During myogenic differentiation in vitro, Has2 was the most highly expressed of the synthases and increased after induction of differentiation. To test whether this association between Has2 expression and myogenesis relates to a role for Has2 in myoblast differentiation and fusion, C2C12 myoblasts were depleted of Has2 by siRNA and induced to differentiate. Depletion of Has2 inhibited differentiation and caused a loss of cell-associated hyaluronan and the hyaluronan-dependent pericellular matrix. The inhibition of differentiation caused by loss of hyaluronan was confirmed with the hyaluronan synthesis inhibitor 4-methylumbelliferone. In hyaluronan synthesis-blocked cultures, restoration of the pericellular matrix could be achieved through the addition of exogenous hyaluronan and the proteoglycan versican, but this was not sufficient to restore differentiation to control levels. These data indicate that intrinsic hyaluronan synthesis is necessary for myoblasts to differentiate and form syncytial muscle cells, but the hyaluronan-dependent pericellular matrix is not sufficient to support differentiation alone; additional hyaluronan-dependent cell functions that are yet unknown may be required for myogenic differentiation.

Deep RNA sequencing of the skeletal muscle transcriptome in swimming fish

Deep RNA sequencing (RNA-seq) was performed to provide an in-depth view of the transcriptome of red and white skeletal muscle of exercised and non-exercised rainbow trout (Oncorhynchus mykiss) with the specific objective to identify expressed genes and quantify the transcriptomic effects of swimming-induced exercise. Pubertal autumn-spawning seawater-raised female rainbow trout were rested (n = 10) or swum (n = 10) for 1176 km at 0.75 body-lengths per second in a 6,000-L swim-flume under reproductive conditions for 40 days. Red and white muscle RNA of exercised and non-exercised fish (4 lanes) was sequenced and resulted in 15–17 million reads per lane that, after de novo assembly, yielded 149,159 red and 118,572 white muscle contigs. Most contigs were annotated using an iterative homology search strategy against salmonid ESTs, the zebrafish Danio rerio genome and general Metazoan genes. When selecting for large contigs (>500 nucleotides), a number of novel rainbow trout gene sequences were identified in this study: 1,085 and 1,228 novel gene sequences for red and white muscle, respectively, which included a number of important molecules for skeletal muscle function. Transcriptomic analysis revealed that sustained swimming increased transcriptional activity in skeletal muscle and specifically an up-regulation of genes involved in muscle growth and developmental processes in white muscle. The unique collection of transcripts will contribute to our understanding of red and white muscle physiology, specifically during the long-term reproductive migration of salmonids.

Novel contrasting and labeling procedures for correlative microscopy of thawed cryosections

One of the major challenges for correlative microscopy is the preparation of the sample; the protocols for transmission electron microscopy (TEM) and fluorescence microscopy (FM) often prove to be incompatible. Here, we introduce 2+Staining: an improved contrasting procedure for Tokuyasu sections that yields both excellent positive membrane contrast in the TEM and bright fluorescence of the probe labeled on the section. 2+Staining involves the contrasting of the immunolabeled sections with 1% osmium tetroxide, 2% uranyl acetate and lead citrate in sequential steps, followed by embedding in 1.8% methyl cellulose. In addition, we demonstrate an amplification of the fluorescent signal by introducing additional antibody incubation steps to the immunolabeling procedure. The methods were validated using the integrated laser and electron microscope (iLEM), a novel tool for correlative microscopy combining FM and TEM in a single setup. The approaches were tested on HL-60 cells labeled for lysosomal-associated membrane protein 2 (LAMP-2) and on sections of muscle from a facioscapulohumeral dystrophy mouse model. Yielding excellent results and greatly expediting the workflow, the methods are of great value for those working in the field of correlative microscopy and indispensible for future users of integrated correlative microscopy.

An anti-inflammatory role for leukemia inhibitory factor receptor signaling in regenerating skeletal muscle

Skeletal muscle regeneration in pathology and following injury requires the coordinated actions of inflammatory cells and myogenic cells to remove damaged tissue and rebuild syncytial muscle cells, respectively. Following contusion injury to muscle, the cytokine leukemia inhibitor factor (LIF) is up-regulated and knockout of Lif negatively impacts on morphometric parameters of muscle regeneration. Although it was speculated that LIF regulates muscle regeneration through direct effects on myogenic cells, the inflammatory effects of LIF have not been examined in regenerating skeletal muscle. Therefore, the expression and function of LIF was examined using the antagonist MH35-BD during specific inflammatory and myogenic stages of notexin-induced muscle regeneration in mice. LIF protein and mRNA were up-regulated in two distinct phases following intramuscular injection of notexin into tibialis anterior muscles. The first phase of LIF up-regulation coincided with the increased expression of pro-inflammatory cytokines; the second phase coincided with myogenic differentiation and formation of new myotubes. Administration of the LIF receptor antagonist MH35-BD during the second phase of LIF up-regulation had no significant effects on transcript expression of genes required for myogenic differentiation or associated with inflammation; there were no significant differences in morphometric parameters of the regenerating muscle. Conversely, when MH35-BD was administered during the acute inflammatory phase, increased gene transcripts for the pro-inflammatory cytokines Tnf (Tumor necrosis factor), Il1b (Interleukin-1β) and Il6 (Interleukin-6) alongside an increase in the number of Ly6G positive neutrophils infiltrating the muscle were observed. This was followed by a reduction in Myog (Myogenin) mRNA, which is required for myogenic differentiation, and the subsequent number of myotubes formed was significantly decreased in MH35-BD-treated groups compared to sham. Thus, antagonism of the LIF receptor during the inflammatory phase of skeletal muscle regeneration appeared to induce an inflammatory response that inhibited subsequent myotube formation. We propose that the predominant role of LIF in skeletal muscle regeneration appears to be in regulating the inflammatory response rather than directly effecting myogenic cells.

Deficient leukemia inhibitory factor signaling in muscle precursor cells from patients with type 2 diabetes

The cytokine leukemia-inhibitory factor (LIF) is expressed by skeletal muscle and induces proliferation of muscle precursor cells, an important feature of skeletal muscle maintenance and repair. We hypothesized that muscle precursor cells from patients with type 2 diabetes had a deficient response to LIF. The mRNA and protein expressions of LIF and its receptor (LIFR) were measured in skeletal muscle biopsies from healthy individuals and patients with type 2 diabetes by use of qPCR and Western blot. LIF signaling and response were studied following administration of recombinant LIF and siRNA knockdown of suppressor of cytokine signaling (SOCS)3 in myoblast cultures established from healthy individuals and patients with type 2 diabetes. Myoblast proliferation rate was assessed by bromodeoxyuridine incorporation. LIF and LIFR proteins were increased in both muscle tissue and cultured myoblasts from diabetic patients. Nonetheless, in the diabetic myoblasts, LIF-induced phosphorylation of signal transducer and activator of transcription (STAT)1 and STAT3 was impaired. The deficient response to LIF administration in the diabetic myoblasts was further emphasized by a lack of increase in LIF-stimulated cell proliferation and a decreased LIF-stimulated induction of the proliferation-promoting factors cyclin D1, JunB, and c-myc. SOCS3 protein was upregulated in diabetic myoblasts, and knockdown of SOCS3 rescued LIF-induced gene expression in diabetic myoblasts, whereas neither STAT1 or STAT3 signaling nor proliferation rate was affected. In conclusion, although LIF and LIFR proteins were increased in muscle tissue and myoblasts from diabetic patients, LIF signaling and LIF-stimulated cell proliferation were impaired in diabetic myoblasts, suggesting a novel mechanism by which muscle function is compromised in diabetes.

Alterations in the expression of leukemia inhibitory factor following exercise: comparisons between wild-type and mdx muscles

BACKGROUND

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine, belonging to the interleukin-6 family of cytokines, that has been suggested to have positive effects on myogenesis following injury and to minimise dystrophic pathology in mdx mice. Previous reports have suggested that Lif mRNA is up-regulated in the limb and diaphragm muscles of mdx mice, in human cases of dystrophy and acutely following exercise. This study examined expression of Lif mRNA in the quadriceps muscles of mdx and wild-type mice that were either sedentary or allowed to exercise voluntarily for two weeks.

RESULTS

Exercise caused a decrease in Lif mRNA expression in wild-type muscle, but this was not the case in mdx muscle. Lif mRNA levels in sedentary mdx mice were similar to those in exercised wild type muscles, and in mdx mice there was no further decrease in levels following exercise. Similar down-regulation of Lif mRNA was observed in the tibialis anterior and diaphragm muscles of mdx mice at three and six weeks of age respectively, compared with wild-type controls. Transcripts for the LIF receptor (Lifr) were also down-regulated in these mdx muscles, suggesting LIF activity may be minimised in dystrophic muscle. However fluorescent immunohistochemical labeling of LIF did not correlate with transcript expression data, as LIF immunoreactivity could not be detected in wild-type muscle, where mRNA expression was high, but was present in dystrophic muscle where mRNA expression was low. This study also described the translocation of membrane proteins, including LIFR, to the nuclei of syncytial muscle cells during differentiation and fusion. In addition this study demonstrates that survival of donor myoblasts injected into dystrophic muscle was enhanced by co-administration of recombinant LIF.

CONCLUSIONS

This study provides new evidence to support a role for LIF in normal muscle biology in response to exercise. Although expression levels of Lif transcript in mdx muscles were not consistent with previous studies, the detection of LIF protein in mdx muscle but not wild-type muscle supports a role for LIF in dystrophy. This study also provides evidence of the differential localisation of the LIFR, and the potential for anti-inflammatory actions of LIF that promote survival of transplanted myoblasts in dystrophic muscle.*corresponding author: Jason White, Muscular Dystrophy Research Group, Murdoch Childrens Research Institute; email: jasondw@unimelb.edu.au.