Fibroblast growth factor inducible 14 (Fn14) is required for the expression of myogenic regulatory factors and differentiation of myoblasts into myotubes. Evidence for TWEAK-independent functions of Fn14 during myogenesis

Fibroblast growth factor-inducible 14 regulates satellite cell self-renewal and expansion during skeletal muscle repair

Skeletal muscle regeneration in adults is predominantly driven by satellite cells. Loss of satellite cell pool and function leads to skeletal muscle wasting in many conditions and disease states. Here, we demonstrate that the levels of fibroblast growth factor-inducible 14 (Fn14) were increased in satellite cells after muscle injury. Conditional ablation of Fn14 in Pax7-expressing satellite cells drastically reduced their expansion and skeletal muscle regeneration following injury. Fn14 was required for satellite cell self-renewal and proliferation as well as to prevent precocious differentiation. Targeted deletion of Fn14 inhibited Notch signaling but led to the spurious activation of STAT3 signaling in regenerating skeletal muscle and in cultured muscle progenitor cells. Silencing of STAT3 improved proliferation and inhibited premature differentiation of Fn14-deficient satellite cells. Furthermore, conditional ablation of Fn14 in satellite cells exacerbated myopathy in the mdx mouse model of Duchenne muscular dystrophy (DMD), whereas its overexpression improved the engraftment of exogenous muscle progenitor cells into the dystrophic muscle of mdx mice. Altogether, our study highlights the crucial role of Fn14 in the regulation of satellite cell fate and function and suggests that Fn14 can be a potential molecular target to improve muscle regeneration in muscular disorders.

Fibroblast growth factor-inducible 14 regulates satellite cell self-renewal and expansion during skeletal muscle repair

Skeletal muscle regeneration in adults is predominantly driven by satellite cells. Loss of satellite cell pool and function leads to skeletal muscle wasting in many conditions and disease states. Here, we demonstrate that the levels of fibroblast growth factor-inducible 14 (Fn14) are increased in satellite cells after muscle injury. Conditional ablation of Fn14 in Pax7-expressing satellite cells drastically reduces their expansion and skeletal muscle regeneration following injury. Fn14 is required for satellite cell self-renewal and proliferation as well as to prevent precocious differentiation. Targeted deletion of Fn14 inhibits Notch signaling but leads to the spurious activation of STAT3 signaling in regenerating skeletal muscle and in cultured muscle progenitor cells. Silencing of STAT3 improves proliferation and inhibits premature differentiation of Fn14-deficient satellite cells. Furthermore, conditional ablation of Fn14 in satellite cells exacerbates myopathy in the mdx mouse model of Duchenne muscular dystrophy (DMD) whereas its overexpression improves the engraftment of exogenous muscle progenitor cells into the dystrophic muscle of mdx mice. Altogether, our study highlights the crucial role of Fn14 in the regulation of satellite cell fate and function and suggests that Fn14 can be a potential molecular target to improve muscle regeneration in muscular disorders.

Exercise, disease state and sex influence the beneficial effects of Fn14-depletion on survival and muscle pathology in the SOD1G93A amyotrophic lateral sclerosis (ALS) mouse model

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a devastating and incurable neurodegenerative disease. Accumulating evidence strongly suggests that intrinsic muscle defects exist and contribute to disease progression, including imbalances in whole-body metabolic homeostasis. We have previously reported that tumour necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and fibroblast growth factor inducible 14 (Fn14) are significantly upregulated in skeletal muscle of the SOD1G93A ALS mouse model. While antagonising TWEAK did not impact survival, we did observe positive effects in skeletal muscle. Given that Fn14 has been proposed as the main effector of the TWEAK/Fn14 activity and that Fn14 can act independently from TWEAK in muscle, we suggest that manipulating Fn14 instead of TWEAK in the SOD1G93A ALS mice could lead to differential and potentially improved benefits.

METHODS

We thus investigated the contribution of Fn14 to disease phenotypes in the SOD1G93A ALS mice. To do so, Fn14 knockout mice (Fn14-/-) were crossed onto the SOD1G93A background to generate SOD1G93A;Fn14-/- mice. Investigations were performed on both unexercised and exercised (rotarod and/or grid test) animals (wild type (WT), Fn14-/-, SOD1G93A and SOD1G93A;Fn14-/-).

RESULTS

Here, we firstly confirm that the TWEAK/Fn14 pathway is dysregulated in skeletal muscle of SOD1G93A mice. We then show that Fn14-depleted SOD1G93A mice display increased lifespan, myofiber size, neuromuscular junction endplate area as well as altered expression of known molecular effectors of the TWEAK/Fn14 pathway, without an impact on motor function. Importantly, we also observe a complex interaction between exercise (rotarod and grid test), genotype, disease state and sex that influences the overall effects of Fn14 deletion on survival, expression of known molecular effectors of the TWEAK/Fn14 pathway, expression of myosin heavy chain isoforms and myofiber size.

CONCLUSIONS

Our study provides further insights on the different roles of the TWEAK/Fn14 pathway in pathological skeletal muscle and how they can be influenced by age, disease, sex and exercise. This is particularly relevant in the ALS field, where combinatorial therapies that include exercise regimens are currently being explored. As such, a better understanding and consideration of the interactions between treatments, muscle metabolism, sex and exercise will be of importance in future studies.

TWEAK/Fn14 signalling driven super-enhancer reprogramming promotes pro-metastatic metabolic rewiring in triple-negative breast cancer

Triple Negative Breast Cancer (TNBC) is the most aggressive breast cancer subtype suffering from limited targeted treatment options. Following recent reports correlating Fibroblast growth factor-inducible 14 (Fn14) receptor overexpression in Estrogen Receptor (ER)-negative breast cancers with metastatic events, we show that Fn14 is specifically overexpressed in TNBC patients and associated with poor survival. We demonstrate that constitutive Fn14 signalling rewires the transcriptomic and epigenomic landscape of TNBC, leading to enhanced tumour growth and metastasis. We further illustrate that such mechanisms activate TNBC-specific super enhancers (SE) to drive the transcriptional activation of cancer dependency genes via chromatin looping. In particular, we uncover the SE-driven upregulation of Nicotinamide phosphoribosyltransferase (NAMPT), which promotes NAD+ and ATP metabolic reprogramming critical for filopodia formation and metastasis. Collectively, our study details the complex mechanistic link between TWEAK/Fn14 signalling and TNBC metastasis, which reveals several vulnerabilities which could be pursued for the targeted treatment of TNBC patients.

Fn14 promotes myoblast fusion during regenerative myogenesis

Skeletal muscle regeneration involves coordinated activation of an array of signaling pathways. Fibroblast growth factor-inducible 14 (Fn14) is a bona fide receptor for the TWEAK cytokine. Levels of Fn14 are increased in the skeletal muscle of mice after injury. However, the cell-autonomous role of Fn14 in muscle regeneration remains unknown. Here, we demonstrate that global deletion of the Fn14 receptor in mice attenuates muscle regeneration. Conditional ablation of Fn14 in myoblasts but not in differentiated myofibers of mice inhibits skeletal muscle regeneration. Fn14 promotes myoblast fusion without affecting the levels of myogenic regulatory factors in the regenerating muscle. Fn14 deletion in myoblasts hastens initial differentiation but impairs their fusion. The overexpression of Fn14 in myoblasts results in the formation of myotubes having an increased diameter after induction of differentiation. Ablation of Fn14 also reduces the levels of various components of canonical Wnt and calcium signaling both in vitro and in vivo. Forced activation of Wnt signaling rescues fusion defects in Fn14-deficient myoblast cultures. Collectively, our results demonstrate that Fn14-mediated signaling positively regulates myoblast fusion and skeletal muscle regeneration.

TWEAK and Fn14 are overexpressed in immune-mediated necrotizing myopathy: implications for muscle damage and repair

OBJECTIVES

TNF-like weak inducer of apoptosis (TWEAK) and its sole receptor fibroblast growth factor-inducible 14 (Fn14) are involved in various inflammatory conditions. This study was performed to investigate the potential role of TWEAK/Fn14 in immune-mediated necrotizing myopathy (IMNM).

METHODS

Muscle biopsies from patients with IMNM (n = 37) and controls (n = 11) were collected. Human muscle cells were treated with TWEAK in vitro. Muscle biopsies and cultured muscle cells were analysed by immunostaining and quantitative PCR. Serum levels of TWEAK and Fn14 were detected by ELISA.

RESULTS

TWEAK and Fn14 were overexpressed in IMNM muscle biopsies. The percentage of Fn14-positive myofibers correlated with disease severity, myonecrosis, regeneration and inflammation infiltrates. Fn14-positive myofibers tended to be surrounded or invaded by CD68+ macrophages. TWEAK treatment had a harmful effect on cultured muscle cells by inducing the production of multiple chemokines and pro-inflammatory cytokines. Serum Fn14 levels were increased in patients with IMNM and correlated with muscle weakness.

CONCLUSIONS

TWEAK/Fn14 signalling was activated in IMNM, most likely aggravating muscle damage via amplifying inflammatory response and macrophages chemotaxis. Fn14 seems to be a biomarker for assessing disease severity in IMNM. In addition, Fn14 may also contribute to muscle injury repair.

Protein therapy of skeletal muscle atrophy and mechanism by angiogenic factor AGGF1

BACKGROUND

Skeletal muscle atrophy is a common condition without a pharmacologic therapy. AGGF1 encodes an angiogenic factor that regulates cell differentiation, proliferation, migration, apoptosis, autophagy and endoplasmic reticulum stress, promotes vasculogenesis and angiogenesis and successfully treats cardiovascular diseases. Here, we report the important role of AGGF1 in the pathogenesis of skeletal muscle atrophy and attenuation of muscle atrophy by AGGF1.

METHODS

In vivo studies were carried out in impaired leg muscles from patients with lumbar disc herniation, two mouse models for skeletal muscle atrophy (denervation and cancer cachexia) and heterozygous Aggf1^+/- mice. Mouse muscle atrophy phenotypes were characterized by body weight and myotube cross-sectional areas (CSA) using H&E staining and immunostaining for dystrophin. Molecular mechanistic studies include co-immunoprecipitation (Co-IP), western blotting, quantitative real-time PCR analysis and immunostaining analysis.

RESULTS

Heterozygous Aggf1^+/- mice showed exacerbated phenotypes of reduced muscle mass, myotube CSA, MyHC (myosin heavy chain) and α-actin, increased inflammation (macrophage infiltration), apoptosis and fibrosis after denervation and cachexia. Intramuscular and intraperitoneal injection of recombinant AGGF1 protein attenuates atrophy phenotypes in mice with denervation (gastrocnemius weight 81.3 ± 5.7 mg vs. 67.3 ± 5.1 mg for AGGF1 vs. buffer; P < 0.05) and cachexia (133.7 ± 4.7 vs. 124.3 ± 3.2; P < 0.05). AGGF1 expression undergoes remodelling and is up-regulated in gastrocnemius and soleus muscles from atrophy mice and impaired leg muscles from patients with lumbar disc herniation by 50-60% (P < 0.01). Mechanistically, AGGF1 interacts with TWEAK (tumour necrosis factor-like weak inducer of apoptosis), which reduces interaction between TWEAK and its receptor Fn14 (fibroblast growth factor-inducing protein 14). This leads to inhibition of Fn14-induced NF-kappa B (NF-κB) p65 phosphorylation, which reduces expression of muscle-specific E3 ubiquitin ligase MuRF1 (muscle RING finger 1), resulting in increased MyHC and α-actin and partial reversal of atrophy phenotypes. Autophagy is reduced in Aggf1^+/- mice due to inhibition of JNK (c-Jun N-terminal kinase) activation in denervated and cachectic muscles, and AGGF1 treatment enhances autophagy in two atrophy models by activating JNK. In impaired leg muscles of patients with lumbar disc herniation, MuRF1 is up-regulated and MyHC and α-actin are down-regulated; these effects are reversed by AGGF1 by 50% (P < 0.01).

CONCLUSIONS

These results indicate that AGGF1 is a novel regulator for the pathogenesis of skeletal muscle atrophy and attenuates skeletal muscle atrophy by promoting autophagy and inhibiting MuRF1 expression through a molecular signalling pathway of AGGF1-TWEAK/Fn14-NF-κB. More importantly, the results indicate that AGGF1 protein therapy may be a novel approach to treat patients with skeletal muscle atrophy.

Dysregulation of Tweak and Fn14 in skeletal muscle of spinal muscular atrophy mice

BACKGROUND

Spinal muscular atrophy (SMA) is a childhood neuromuscular disorder caused by depletion of the survival motor neuron (SMN) protein. SMA is characterized by the selective death of spinal cord motor neurons, leading to progressive muscle wasting. Loss of skeletal muscle in SMA is a combination of denervation-induced muscle atrophy and intrinsic muscle pathologies. Elucidation of the pathways involved is essential to identify the key molecules that contribute to and sustain muscle pathology. The tumor necrosis factor-like weak inducer of apoptosis (TWEAK)/TNF receptor superfamily member fibroblast growth factor-inducible 14 (Fn14) pathway has been shown to play a critical role in the regulation of denervation-induced muscle atrophy as well as muscle proliferation, differentiation, and metabolism in adults. However, it is not clear whether this pathway would be important in highly dynamic and developing muscle.

METHODS

We thus investigated the potential role of the TWEAK/Fn14 pathway in SMA muscle pathology, using the severe Taiwanese Smn^-/-; SMN2 and the less severe Smn^2B/- SMA mice, which undergo a progressive neuromuscular decline in the first three post-natal weeks. We also used experimental models of denervation and muscle injury in pre-weaned wild-type (WT) animals and siRNA-mediated knockdown in C2C12 muscle cells to conduct additional mechanistic investigations.

RESULTS

Here, we report significantly dysregulated expression of Tweak, Fn14, and previously proposed downstream effectors during disease progression in skeletal muscle of the two SMA mouse models. In addition, siRNA-mediated Smn knockdown in C2C12 myoblasts suggests a genetic interaction between Smn and the TWEAK/Fn14 pathway. Further analyses of SMA, Tweak^-/-, and Fn14^-/- mice revealed dysregulated myopathy, myogenesis, and glucose metabolism pathways as a common skeletal muscle feature, providing further evidence in support of a relationship between the TWEAK/Fn14 pathway and Smn. Finally, administration of the TWEAK/Fn14 agonist Fc-TWEAK improved disease phenotypes in the two SMA mouse models.

CONCLUSIONS

Our study provides mechanistic insights into potential molecular players that contribute to muscle pathology in SMA and into likely differential responses of the TWEAK/Fn14 pathway in developing muscle.

A potential fate decision landscape of the TWEAK/Fn14 axis on stem and progenitor cells: a systematic review

Stem and progenitor cells (SPCs) possess self-remodeling ability and differentiation potential and are responsible for the regeneration and development of organs and tissue systems. However, the precise mechanisms underlying the regulation of SPC biology remain unclear. Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) acts on miscellaneous cells via binding to fibroblast growth factor-inducible 14 (Fn14) and exerts pleiotropic functions in the regulation of divergent stem cell fates. TWEAK/Fn14 signaling can regulate the proliferation, differentiation, and migration of multiple SPCs as well as tumorigenesis in certain contexts. Although TWEAK’s roles in modulating multiple SPCs are sparsely reported, the systemic effector functions of this multifaceted protein have not been fully elucidated. In this review, we summarized the fate decisions of TWEAK/Fn14 signaling on multiple stem cells and characterized its potential in stem cell therapy.

BMP2-induction of FN14 promotes protumorigenic signaling in gynecologic cancer cells

We previously reported that bone morphogenetic protein (BMP) signaling promotes tumorigenesis in gynecologic cancer cells. BMP2 enhances proliferation of ovarian and endometrial cancer cells via c-KIT induction, and triggers epithelial-mesenchymal transition (EMT) by SNAIL and/or SLUG induction, leading to increased cell migration. However, the downstream effectors of BMP signaling in gynecological cancer cells have not been clearly elucidated. In this study, we performed RNA-sequencing of Ishikawa endometrial and SKOV3 ovarian cancer cells after BMP2 stimulation, and identified TNFRSF12A, encoding fibroblast growth factor-inducible 14 (FN14) as a common BMP2-induced gene. FN14 knockdown suppressed BMP2-induced cell proliferation and migration, confirmed by MTS and scratch assays, respectively. In addition, FN14 silencing augmented chemosensitivity of SKOV3 cells. As a downstream effector of BMP signaling, FN14 modulated both c-KIT and SNAIL expression, which are important for growth and migration of ovarian and endometrial cancer cells. These results support the notion that the tumor promoting effects of BMP signaling in gynecological cancers are partially attributed to FN14 induction.

Efficient and modified 2-NBDG assay to measure glucose uptake in cultured myotubes

Under type-2 diabetes, insulin resistance develops in skeletal muscles as a key defect and to study the disorder, its manifestation, and possible solution, measurement of glucose uptake is a fundamental necessity. Of various approaches (i.e. scintillation counting, flow cytometry, fluorometry and spectrophotometry) fluorescent labelled glucose analogue, 2-NBDG solution is the most popular one. Although 2-NBDG based assay is the most widely used approach in various cells including skeletal muscle, even then all available protocols possess huge variability which impacts the overall data reproducibility. Moreover, starvation (use of glucose/serum free medium), one of the prerequisite condition for glucose uptake assay, itself induces stress specifically during longer pre-incubation periods and alters muscle cell metabolism and morphology, but the fact has not been duly considered. Therefore in the present article, using specific skeletal muscle cells i.e. C2C12 myotubes, we have re-established the conditions like pre-incubation time period, concentrations of insulin, glucose and serum/BSA while maintaining the cultured myotubes in morphologically healthy state. Our lab standardized protocols were observed to be effective in studying insulin resistance condition induced by diverse stresses (oxidative & inflammation) in myotubes. Comparative study conducted with already established protocols demonstrates that the present method is more efficient, effective and better improvised for studying glucose uptake in C2C12.

Effect of Photobiomodulation in Lipopolysaccharide-Treated Myoblasts

Objective: To evaluate the effect of photobiomodulation (PBM) on cell viability, synthesis of nitric oxide (NO), and interleukin (IL)-6 inflammatory cytokine production in myoblasts cultured in the presence of lipopolysaccharides (LPSs). Methods: C2C12 myoblasts were treated with LPS and PBM using different parameters (wavelength: 780 nm; beam spot: 0.04 cm²; power output: 10 or 40 mW; energy density: 5 or 20 J/cm²; and 20-sec exposure time). Nonirradiated cells were used to the control group. Results: An increase in cell viability was found in both LPS groups in comparison with the control. PBM with the higher power output (40 mW) induced a reduction in cell viability. PBM also modulated the synthesis of NO in the myoblasts, but did not alter the expression of IL-6. Conclusions: Based on these findings, PBM is capable of modulating the cell viability and the production of NO in LPS-treated myoblasts and it is, therefore, a possible tool for the treatment of muscle injury caused by infection.

Cancer-Associated Muscle Wasting-Candidate Mechanisms and Molecular Pathways

Excessive muscle loss is commonly observed in cancer patients and its association with poor prognosis has been well-established. Cancer-associated sarcopenia differs from age-related wasting in that it is not responsive to nutritional intervention and exercise. This is related to its unique pathogenesis, a result of diverse and interconnected mechanisms including inflammation, disordered metabolism, proteolysis and autophagy. There is a growing body of evidence that suggests that the tumor is the driver of muscle wasting by its elaboration of mediators that influence each of these pro-sarcopenic pathways. In this review, evidence for these tumor-derived factors and putative mechanisms for inducing muscle wasting will be reviewed. Potential targets for future research and therapeutic interventions will also be reviewed.

Controversies in TWEAK-Fn14 signaling in skeletal muscle atrophy and regeneration

Skeletal muscle is one of the largest functional tissues in the human body; it is highly plastic and responds dramatically to anabolic and catabolic stimuli, including weight training and malnutrition, respectively. Excessive loss of muscle mass, or atrophy, is a common symptom of many disease states with severe impacts on prognosis and quality of life. TNF-like weak inducer of apoptosis (TWEAK) and its cognate receptor, fibroblast growth factor-inducible 14 (Fn14) are an emerging cytokine signaling pathway in the pathogenesis of muscle atrophy. Upregulation of TWEAK and Fn14 has been described in a number of atrophic and injured muscle states; however, it remains unclear whether they are contributing to the degenerative or regenerative aspect of muscle insults. The current review focuses on the expression and apparent downstream outcomes of both TWEAK and Fn14 in a range of catabolic and anabolic muscle models. Apparent changes in the signaling outcomes of TWEAK-Fn14 activation dependent on the relative expression of both the ligand and the receptor are discussed as a potential source of divergent TWEAK-Fn14 downstream effects. This review proposes both a physiological and pathological model of TWEAK-Fn14 signaling. Further research is needed on the switch between these states to develop therapeutic interventions for this pathway.

Tinospora cordifolia protects from skeletal muscle atrophy by alleviating oxidative stress and inflammation induced by sciatic denervation

ETHANOPHARMACOLOGICAL RELEVANCE

Tinospora cordifolia (TC) is widely being used as immunomodulatory and re-juvenile drug and well described in Indian Ayurveda system of medicine. Rejuvenation also means the fine tuning of the skeletal muscles. Skeletal muscle related disorder, i.e. atrophy is major problem which arise due to cachexia, sarcopenia and immobilization. However, despite of the great efforts, there is scarcity of FDA approved drugs in the market to treat skeletal muscle atrophy.

AIM OF THE STUDY

The current study was aimed to explore the in-vitro and in-vivo efficacy and mechanism of TC in myogenic differentiation and skeletal muscle atrophy to establish the possibility of its usage to counteract skeletal muscle atrophy.

MATERIALS AND METHODS

C2C12 cell lines were used to determine myogenic potential and anti-atrophic effects of T. cordifolia water extract (TCE). Its in-vitro efficacy was re-validated in vivo by supplementation of TCE at a dose of 200 mg/kg/p.o. for 30 days in denervated mice model of skeletal muscle atrophy. Effects of TCE administration on levels of oxidative stress, inflammatory markers and proteolysis were determined.

RESULTS

TCE supplementation displayed increased lymphocyte proliferation and induced myogenic differentiation of C2C12 myoblasts by significantly increasing myocytes length and thickness, in comparison to control (p < 0.05). TCE supplementation decreased oxidative stress and inflammatory response by significantly modulating activities of catalase, glutathione peroxidase, lipid peroxidase, superoxide dismutase and β-glucuronidase (p < 0.05). It increased MF-20c expression and ameliorated degradation of muscle protein by down-regulating MuRF-1 and calpain activity.

CONCLUSION

TCE supplementation promotes myogenic differentiation in C2C12 cell lines and prevents denervation induced skeletal muscle atrophy by antagonizing the proteolytic systems (calpain and UPS) and maintaining the oxidative defense mechanism of the cell. Hence, TCE can be used as a protective agent against muscle atrophy.

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.

Tumor necrosis factor-like weak inducer of apoptosis induces inflammation in Graves' orbital fibroblasts

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK), along with its receptor fibroblast growth factor-inducible (Fn)14, is associated with various biological activities including inflammation. However, its role in the pathogenesis of Graves’ orbitopathy (GO) is unknown. In this study, we investigated the mechanism by which TWEAK regulates inflammatory signaling in orbital fibroblasts from GO patients. We found that TWEAK and tumor necrosis factor-α (TNFA) mRNA levels were upregulated in GO as compared to non-GO tissue samples. TWEAK, TNF receptor (TNFR)1, TNFR2, and TNFR superfamily member 12A mRNA, and TWEAK and Fn14 protein levels were increased by interleukin (IL)-1β and TNF-α treatment. Treatment with exogenous recombinant TWEAK increased the transcript and protein expression of the pro-inflammatory cytokines IL-6, IL-8, and monocyte chemoattractant protein-1 to a greater extent in GO than in non-GO cells, while treatment with the anti-Fn14 antibody ITEM4 suppressed TWEAK-induced pro-inflammatory cytokine release and hyaluronan production. Additionally, the serum level of TWEAK was higher in Graves’ disease patients with (341.86 ± 86.3 pg/ml) as compared to those without (294.09 ± 41.44 pg/ml) GO and healthy subjects (255.33 ± 39.38 pg/ml), and was positively correlated with clinical activity score (r = 0.629, P < 0.001) and thyroid binding immunoglobulin level (r = 0.659, P < 0.001). These results demonstrate that TWEAK/Fn14 signaling contributes to GO pathogenesis. Moreover, serum TWEAK level is a potential diagnostic biomarker for inflammatory GO, and modulating TWEAK activity may be an effective therapeutic strategy for suppressing inflammation and tissue remodeling in GO.

Muscle Fn14 gene expression is associated with fat-free mass retention during energy deficit at high altitude

Intramuscular factors that modulate fat-free mass (FFM) loss in lowlanders exposed to energy deficit during high-altitude (HA) sojourns remain unclear. Muscle inflammation may contribute to FFM loss at HA by inducing atrophy and inhibiting myogenesis via the tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and its receptor, fibroblast growth factor-inducible protein 14 (Fn14). To explore whether muscle inflammation modulates FFM loss reportedly developing during HA sojourns, muscle inflammation, myogenesis, and proteolysis were assessed in 16 men at sea level (SL) and following 21 days of energy deficit (-1862 ± 525 kcal/days) at high altitude (HA, 4300 m). Total body mass (TBM), FFM, and fat mass (FM) were assessed using DEXA. Gene expression and proteolytic enzymatic activities were assessed in muscle samples collected at rest at SL and HA. Participants lost 7.2 ± 1.8 kg TBM (P < 0.05); 43 ± 30% and 57 ± 30% of the TBM lost was FFM and FM, respectively. Fn14, TWEAK, TNF alpha-receptor (TNFα-R), TNFα, MYOGENIN, and paired box protein-7 (PAX7) were upregulated (P < 0.05) at HA compared to SL. Stepwise linear regression identified that Fn14 explained the highest percentage of variance in FFM loss (r2  = 0.511, P < 0.05). Dichotomization of volunteers into HIGH and LOW Fn14 gene expression indicated HIGH lost less FFM and more FM (28 ± 28% and 72 ± 28%, respectively) as a proportion of TBM loss than LOW (58 ± 26% and 42 ± 26%; P < 0.05) at HA. MYOGENIN gene expression was also greater for HIGH versus LOW (P < 0.05). These data suggest that heightened Fn14 gene expression is not catabolic and may protect FFM during HA sojourns.

Pharmacology of manipulating lean body mass

Dysfunction and wasting of skeletal muscle as a consequence of illness decreases the length and quality of life. Currently, there are few, if any, effective treatments available to address these conditions. Hence, the existence of this unmet medical need has fuelled large scientific efforts. Fortunately, these efforts have shown many of the underlying mechanisms adversely affecting skeletal muscle health. With increased understanding have come breakthrough disease-specific and broad spectrum interventions, some progressing through clinical development. The present review focuses its attention on the role of the antagonistic process regulating skeletal muscle mass before branching into prospective promising therapeutic targets and interventions. Special attention is given to therapies in development against cancer cachexia and Duchenne muscular dystrophy before closing remarks on design and conceptualization of future therapies are presented to the reader.

TWEAK-Fn14 pathway activation after exercise in human skeletal muscle: insights from two exercise modes and a time course investigation

The cell surface receptor Fn14/TWEAKR was recently reported by our laboratory to be a prominent marker in the resistance exercise (RE) induced Transcriptome. The purpose of the present study was to extend our Transcriptome findings and investigate the gene and protein expression time course of markers in the TWEAK-Fn14 pathway following RE or run exercise (RUN). Vastus lateralis muscle biopsies were obtained from 6 RE subjects [25 ± 4 yr, 1-repetition maximum (RM): 99 ± 27 kg] pre- and 0, 1, 2, 4, 8, 12, and 24 h post RE (3 × 10 at 70% 1-RM). Lateral gastrocnemius biopsies were obtained from 6 RUN subjects [25 ± 4 yr, maximum oxygen uptake (V̇O2max): 63 ± 8 ml·kg(-1)·min(-1)] pre- and 0, 1, 2, 4, 8, 12, and 24 h after a 30-min RUN (75% V̇O2max). After RE, Fn14 gene and protein expression were induced (P < 0.05) and peaked at 8 and 12 h, respectively. Downstream markers analyzed showed evidence of TWEAK-Fn14 signaling through the alternative NF-κB pathway after RE. After RUN, Fn14 gene expression was induced (P < 0.05) to a much lesser extent and peaked at 24 h. Fn14 protein expression was only measurable on a sporadic basis, and there was weak evidence of alternative NF-κB pathway signaling after RUN. TWEAK gene and protein expression were not influenced by either exercise mode. These are the first human data to show a transient activation of the TWEAK-Fn14 axis in the recovery from exercise, and our data suggest the level of activation is exercise mode dependent. Furthermore, our collective data support a myogenic role for TWEAK-Fn14 through the alternative NF-κB pathway in human skeletal muscle.

Single muscle fiber gene expression with run taper

This study evaluated gene expression changes in gastrocnemius slow-twitch myosin heavy chain I (MHC I) and fast-twitch (MHC IIa) muscle fibers of collegiate cross-country runners (n = 6, 20±1 y, VO_2max = 70±1 ml•kg⁻¹•min⁻¹) during two distinct training phases. In a controlled environment, runners performed identical 8 kilometer runs (30∶18±0∶30 min:s, 89±1% HR_max) while in heavy training (∼72 km/wk) and following a 3 wk taper. Training volume during the taper leading into peak competition was reduced ∼50% which resulted in improved race times and greater cross-section and improved function of MHC IIa fibers. Single muscle fibers were isolated from pre and 4 hour post run biopsies in heavily trained and tapered states to examine the dynamic acute exercise response of the growth-related genes Fibroblast growth factor-inducible 14 (FN14), Myostatin (MSTN), Heat shock protein 72 (HSP72), Muscle ring-finger protein-1 (MURF1), Myogenic factor 6 (MRF4), and Insulin-like growth factor 1 (IGF1) via qPCR. FN14 increased 4.3-fold in MHC IIa fibers with exercise in the tapered state (P<0.05). MSTN was suppressed with exercise in both fiber types and training states (P<0.05) while MURF1 and HSP72 responded to running in MHC IIa and I fibers, respectively, regardless of training state (P<0.05). Robust induction of FN14 (previously shown to strongly correlate with hypertrophy) and greater overall transcriptional flexibility with exercise in the tapered state provides an initial molecular basis for fast-twitch muscle fiber performance gains previously observed after taper in competitive endurance athletes.

The TWEAK-Fn14 system as a potential drug target

Fibroblast growth factor-inducible 14 (Fn14) is a member of the tumour necrosis factor (TNF) receptor family that is induced in a variety of cell types in situations of tissue injury. Fn14 becomes activated by TNF-like weak inducer of apoptosis (TWEAK), a typical member of the TNF ligand family. TWEAK is constitutively expressed by monocytes and some tumour cell lines and also shows cytokine inducible expression in various other cell types. Fn14 activation results in stimulation of signalling pathways culminating in the activation of NFκB transcription factors and various MAPKs but might also trigger the PI3K/Akt pathway and GTPases of the Rho family. In accordance with its tissue damage-associated expression pattern and its pleiotropic proinflammatory signalling capabilities, the TWEAK-Fn14 system has been implicated in a huge number of pathologies. The use of TWEAK- and Fn14-knockout mice identified the TWEAK-Fn14 system as a crucial player in muscle atrophy, cerebral ischaemia, kidney injury, atherosclerosis and infarction as well as in various autoimmune scenarios including experimental autoimmune encephalitis, rheumatoid arthritis and inflammatory bowel disease. Moreover, there is increasing preclinical evidence that Fn14 targeting is a useful option in tumour therapy. Based on a discussion of the signalling capabilities of TWEAK and Fn14, this review is focused on two major issues. On the one hand, on the molecular and cellular basis of the TWEAK/Fn14-related pathological outcomes in the aforementioned diseases and on the other hand, on the preclinical experience that have been made so far with TWEAK and Fn14 targeting drugs.

DNA methyltransferase 3a and mitogen-activated protein kinase signaling regulate the expression of fibroblast growth factor-inducible 14 (Fn14) during denervation-induced skeletal muscle atrophy

The TWEAK-fibroblast growth factor-inducible 14 (Fn14) system is a critical regulator of denervation-induced skeletal muscle atrophy. Although the expression of Fn14 is a rate-limiting step in muscle atrophy on denervation, mechanisms regulating gene expression of Fn14 remain unknown. Methylation of CpG sites within promoter region is an important epigenetic mechanism for gene silencing. Our study demonstrates that Fn14 promoter contains a CpG island close to transcription start site. Fn14 promoter also contains multiple consensus DNA sequence for transcription factors activator protein 1 (AP1) and specificity protein 1 (SP1). Denervation diminishes overall genomic DNA methylation and causes hypomethylation at specific CpG sites in Fn14 promoter leading to the increased gene expression of Fn14 in skeletal muscle. Abundance of DNA methyltransferase 3a (Dnmt3a) and its interaction with Fn14 promoter are repressed in denervated skeletal muscle of mice. Overexpression of Dnmt3a inhibits the gene expression of Fn14 and attenuates skeletal muscle atrophy upon denervation. Denervation also causes the activation of ERK1/2, JNK1/2, and ERK5 MAPKs and AP1 and SP1, which stimulate the expression of Fn14 in skeletal muscle. Collectively, our study provides novel evidence that Dnmt3a and MAPK signaling regulate the levels of Fn14 in skeletal muscle on denervation.

Increased expression of atrogenes and TWEAK family members after severe burn injury in nonburned human skeletal muscle

Severe burn induces rapid skeletal muscle proteolysis after the injury, which persists for up to 1 year and results in skeletal muscle atrophy despite dietary and rehabilitative interventions. The purpose of this research was to determine acute changes in gene expression of skeletal muscle mass regulators postburn injury. Specimens were obtained for biopsy from the vastus lateralis of a nonburned leg of eight burned subjects (6M, 2F: 34.8 ± 2.7 years: 29.9 ± 3.1% TBSA burn) at 5.1 ± 1.1 days postburn injury and from matched controls. mRNA expression of cytokines and receptors in the tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) families, and the ubiquitin proteasome E3 ligases, atrogin-1 and MuRF-1, was determined. TNF receptor 1A was over 3.5-fold higher in burn. Expression of TNF-like weak inducer of apoptosis and its receptor were over 1.6 and 6.0-fold higher in burn. IL-6, IL-6 receptor, and glycoprotein 130 were elevated in burned subjects with IL-6 receptor over 13-fold higher. The level of suppressor of cytokine signaling-3 was also increased nearly 6-fold in burn. Atrogin-1 and MuRF-1 were more than 4- and 3-fold higher in burn. These results demonstrate for the first time that severe burn in humans has a remarkable impact on gene expression in skeletal muscle of a nonburned limb of genes that promote inflammation and proteolysis. Because these changes likely contribute to the acute skeletal muscle atrophy in areas not directly affected by the burn, in the future it will be important to determine the responsible systemic cues.

EPO-receptor is present in mouse C2C12 and human primary skeletal muscle cells but EPO does not influence myogenesis

The role and regulation of the pleiotropic cytokine erythropoietin (EPO) in skeletal muscle are controversial. EPO exerts its effects by binding its specific receptor (EPO‐R), which activates intracellular signaling and gene transcription in response to internal and external stress signals. EPO is suggested to play a direct role in myogenesis via the EPO‐R, but several studies have questioned the effect of EPO treatment in muscle in vitro and in vivo. The lack of certainty surrounding the use of nonspecific EPO‐R antibodies contributes to the ambiguity of the field. Our study demonstrates that the EPO‐R gene and protein are expressed at each stage of mouse C2C12 and human skeletal muscle cell proliferation and differentiation and validates a specific antibody for the detection of the EPO‐R protein. However, in our experimental conditions, EPO treatment had no effect on mouse C2C12 and human muscle cell proliferation, differentiation, protein synthesis or EPO‐R expression. While an increase in Akt and MAPK phosphorylation was observed, we demonstrate that this effect resulted from the stress caused by changing medium and not from EPO treatment. We therefore suggest that skeletal muscle EPO‐R might be present in a nonfunctional form, or too lowly expressed to play a role in muscle cell function.

The EPO‐R is expressed at the gene and protein level in mouse and human myoblasts and myotubes. However, EPO treatment does not seem to activate the EPO‐R and its downstream signaling pathways in skeletal muscle cells, questioning its functionality.

TNF-like weak inducer of apoptosis aggravates left ventricular dysfunction after myocardial infarction in mice

Background. TNF-like weak inducer of apoptosis (TWEAK) has recently been shown to be potentially involved in adverse cardiac remodeling. However, neither the exact role of TWEAK itself nor of its receptor Fn14 in this setting is known. Aim of the Study. To analyze the effects of sTWEAK on myocardial function and gene expression in response to experimental myocardial infarction in mice. Results. TWEAK directly suppressed the expression of PGC-1α and genes of oxidative phosphorylation (OXPHOS) in cardiomyocytes. Systemic sTWEAK application after MI resulted in reduced left ventricular function and increased mortality without changes in interstitial fibrosis or infarct size. Molecular analysis revealed decreased phosphorylation of PI3K/Akt and ERK1/2 pathways associated with reduced expression of PGC-1α and PPARα. Likewise, expression of OXPHOS genes such as atp5O, cycs, cox5b, and ndufb5 was also reduced. Fn14 −/− mice showed significantly improved left ventricular function and PGC-1α levels after MI compared to their respective WT littermates (Fn14 +/+). Finally, inhibition of intrinsic TWEAK with anti-TWEAK antibodies resulted in improved left ventricular function and survival. Conclusions. TWEAK exerted maladaptive effects in mice after myocardial infarction most likely via direct effects on cardiomyocytes. Analysis of the potential mechanisms revealed that TWEAK reduced metabolic adaptations to increased cardiac workload by inhibition of PGC-1α.

Role of the TWEAK-Fn14-cIAP1-NF-κB Signaling Axis in the Regulation of Myogenesis and Muscle Homeostasis

Mammalian skeletal muscle maintains a robust regenerative capacity throughout life, largely due to the presence of a stem cell population known as “satellite cells” in the muscle milieu. In normal conditions, these cells remain quiescent; they are activated upon injury to become myoblasts, which proliferate extensively and eventually differentiate and fuse to form new multinucleated muscle fibers. Recent findings have identified some of the factors, including the cytokine TNFα-like weak inducer of apoptosis (TWEAK), which govern these cells’ decisions to proliferate, differentiate, or fuse. In this review, we will address the functions of TWEAK, its receptor Fn14, and the associated signal transduction molecule, the cellular inhibitor of apoptosis 1 (cIAP1), in the regulation of myogenesis. TWEAK signaling can activate the canonical NF-κB signaling pathway, which promotes myoblast proliferation and inhibits myogenesis. In addition, TWEAK activates the non-canonical NF-κB pathway, which, in contrast, promotes myogenesis by increasing myoblast fusion. Both pathways are regulated by cIAP1, which is an essential component of downstream signaling mediated by TWEAK and similar cytokines. This review will focus on the seemingly contradictory roles played by TWEAK during muscle regeneration, by highlighting the interplay between the two NF-κB pathways under physiological and pathological conditions. We will also discuss how myogenesis is negatively affected by chronic conditions, which affect homeostasis of the skeletal muscle environment.

Expression of tumor necrosis factor-like weak inducer of apoptosis and fibroblast growth factor-inducible 14 in patients with polymyositis and dermatomyositis

INTRODUCTION

The aim of this study was to investigate the expression of tumor necrosis factor-like weak inducer of apoptosis (TWEAK) and its receptor fibroblast growth factor-inducible 14 (Fn14) in patients with polymyositis (PM) and dermatomyositis (DM), and their relation to clinical manifestations.

METHODS

Serum levels of TWEAK were detected in 98 PM/DM patients and 37 healthy controls by using the ELISA method. Total RNA isolated from fresh-frozen muscle tissue samples of 36 PM/DM patients and 10 healthy controls were used for analyzing the mRNA levels of TWEAK and Fn14 by quantitative reverse transcription polymerase chain reaction (RT-PCR). Immunofluorescence staining of TWEAK and Fn14 was conducted on muscle biopsy specimens from 23 PM/DM patients and seven healthy controls.

RESULTS

Serum levels of TWEAK were significantly decreased in the PM/DM patients compared to those in the healthy controls (P < 0.001), and serum TWEAK levels negatively correlated with serum CD163 levels in PM/DM patients (r = -0.49, P < 0.001). The expression of Fn14 mRNA was significantly increased in the muscle tissue of PM/DM patients than in the muscle tissue of healthy controls (P < 0.01), whereas the expression of TWEAK mRNA in PM/DM patients was not statistically different from that of the healthy controls (P > 0.05). Fn14 mRNA levels in muscle tissue positively correlated with muscle disease activity (r = 0.512, P < 0.01). Patients with oropharyngeal dysphagia had significantly higher Fn14 mRNA levels than patients without oropharyngeal dysphagia (P < 0.05). The results of immunofluorescence staining showed that 19 out of 23 PM/DM patients were TWEAK-positive, and 20 out of 23 PM/DM patients were Fn14-positive. No detectable expressions of TWEAK or Fn14 were observed in the healthy controls.

CONCLUSIONS

TWEAK-Fn14 axis may be involved in the pathogenesis of PM/DM. Further understanding of TWEAK-Fn14 function in PM/DM may help to define therapeutic targets for PM/DM.

TWEAK/Fn14 Signaling Axis Mediates Skeletal Muscle Atrophy and Metabolic Dysfunction

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) through binding to its receptor fibroblast growth factor inducible 14 (Fn14) has been shown to regulate many cellular responses including proliferation, differentiation, apoptosis, inflammation, and fibrosis, under both physiological and pathological conditions. Emerging evidence suggests that TWEAK is also a major muscle wasting cytokine. TWEAK activates nuclear factor-κB signaling and proteolytic pathways such as ubiquitin–proteasome system, autophagy, and caspases to induce muscle proteolysis in cultured myotubes. Fn14 is dormant or expressed in minimal amounts in normal healthy muscle. However, specific atrophic conditions, such as denervation, immobilization, and starvation stimulate the expression of Fn14 leading to activation of TWEAK/Fn14 signaling and eventually skeletal muscle atrophy. TWEAK also causes slow- to fast-type fiber transition in skeletal muscle. Furthermore, recent studies suggest that TWEAK diminishes mitochondrial content and represses skeletal muscle oxidative phosphorylation capacity. TWEAK mediates these effects through affecting the expression of a number of genes and microRNAs. In this review article, we have discussed the recent advancements toward understanding the role and mechanisms of action of TWEAK/Fn14 signaling in skeletal muscle with particular reference to different models of atrophy and oxidative metabolism.

TWEAK/Fn14 Axis: A Promising Target for the Treatment of Cardiovascular Diseases

Cardiovascular diseases (CVD) are the first cause of mortality in Western countries. CVD include several pathologies such as coronary heart disease, stroke or cerebrovascular accident, congestive heart failure, peripheral arterial disease, and aortic aneurysm, among others. Interaction between members of the tumor necrosis factor (TNF) superfamily and their receptors elicits several biological actions that could participate in CVD. TNF-like weak inducer of apoptosis (TWEAK) and its functional receptor and fibroblast growth factor-inducible molecule 14 (Fn14) are two proteins belonging to the TNF superfamily that activate NF-κB by both canonical and non-canonical pathways and regulate several cell functions such as proliferation, migration, differentiation, cell death, inflammation, and angiogenesis. TWEAK/Fn14 axis plays a beneficial role in tissue repair after acute injury. However, persistent TWEAK/Fn14 activation mediated by blocking experiments or overexpression experiments in animal models has shown an important role of this axis in the pathological remodeling underlying CVD. In this review, we summarize the role of TWEAK/Fn14 pathway in the development of CVD, focusing on atherosclerosis and stroke and the molecular mechanisms by which TWEAK/Fn14 interaction participates in these pathologies. We also review the role of the soluble form of TWEAK as a biomarker for the diagnosis and prognosis of CVD. Finally, we highlight the results obtained with other members of the TNF superfamily that also activate canonical and non-canonical NF-κB pathway.

Pericytes and adaptive angioplasticity: the role of tumor necrosis factor-like weak inducer of apoptosis (TWEAK)

The TNF superfamily member TWEAK has emerged as a pleiotropic cytokine that regulates many cellular functions that include immune/inflammatory activity, angiogenesis, cell proliferation, and fate. TWEAK through its inducible receptor, FGF-inducible molecule 14 (Fn14), can induce both beneficial and deleterious activity that has a profound effect on cell survival. Thus it is highly likely that TWEAK and Fn14 expressed by cells of the neurovascular unit help regulate and maintain vascular and tissue homeostasis. In this chapter we discuss the expression of TWEAK and Fn14 signaling in the cerebral microvascular pericyte. Pericytes are a highly enigmatic population of microvascular cells that are important in regulatory pathways that modulate physiological angiogenesis in response to chronic mild hypoxic stress. A brief introduction will identify the microvascular pericyte. A more detailed discussion of pericyte TWEAK signaling during adaptive angioplasticity will follow.

The TWEAK-Fn14 pathway: a potent regulator of skeletal muscle biology in health and disease

TNF-like weak inducer of apoptosis (TWEAK), a TNF superfamily ligand, and its bona fide receptor, the TNF receptor superfamily member fibroblast growth factor-inducible 14 (Fn14), represent a pivotal axis for shaping both physiological and pathological tissue responses to acute or chronic injury and disease. In recent years significant advances have been made in delineating the prominent role of TWEAK-Fn14 dyad in regulating skeletal muscle mass and metabolism. Also emerging from the broad study of tissue injury in skeletal muscle and other organs is the role of the TWEAK-Fn14 pathway in promoting fibrosis. This review article highlights recent advancements toward understanding how the TWEAK-Fn14 pathway regulates the response to various skeletal muscle insults and, more broadly, engages multiple mechanisms to drive tissue fibrosis.

Proinflammatory cytokine tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) suppresses satellite cell self-renewal through inversely modulating Notch and NF-κB signaling pathways

Satellite cell self-renewal is an essential process to maintaining the robustness of skeletal muscle regenerative capacity. However, extrinsic factors that regulate self-renewal of satellite cells are not well understood. Here, we demonstrate that TWEAK cytokine reduces the proportion of Pax7(+)/MyoD(-) cells (an index of self-renewal) on myofiber explants and represses multiple components of Notch signaling in satellite cell cultures. The number of Pax7(+) cells is significantly increased in skeletal muscle of TWEAK knock-out (KO) mice compared with wild-type in response to injury. Furthermore, Notch signaling is significantly elevated in cultured satellite cells and in regenerating myofibers of TWEAK-KO mice. Forced activation of Notch signaling through overexpression of the Notch1 intracellular domain (N1ICD) rescued the TWEAK-mediated inhibition of satellite cell self-renewal. TWEAK also activates the NF-κB transcription factor in satellite cells and inhibition of NF-κB significantly improved the number of Pax7(+) cells in TWEAK-treated cultures. Furthermore, our results demonstrate that a reciprocal interaction between NF-κB and Notch signaling governs the inhibitory effect of TWEAK on satellite cell self-renewal. Collectively, our study demonstrates that TWEAK suppresses satellite cell self-renewal through activating NF-κB and repressing Notch signaling.

Characterization of primary human skeletal muscle cells from multiple commercial sources

There is a significant unmet need for safe, anabolic muscle therapies to treat diseases and conditions associated with severe muscle weakness and frailty. The identification of such therapies requires appropriate cell-based screening assays to select compounds for further development using animal models. Primary human skeletal muscle cells have recently become available from a number of commercial vendors. Such cells may be valuable for studying the mechanisms that direct muscle differentiation, and for identifying and characterizing novel therapeutic approaches for the treatment of age- and injury-induced muscle disorders. However, only limited characterization of these cells has been reported to date. Therefore, we have examined four primary human muscle cell preparations from three different vendors for their capacity to differentiate into multinucleated myotubes. Two of the preparations demonstrated robust myotube formation and expressed characteristic markers of muscle differentiation. Furthermore, these myotubes could be induced to undergo morphological atrophy- and hypertrophy-like responses, and atrophy could be blocked with an inhibitor of myostatin signaling, a pathway that is known to negatively regulate muscle mass. Finally, the myotubes were efficiently infected with recombinant adenovirus, providing a tool for genetic modification. Taken together, our results indicate that primary human muscle cells can be a useful system for studying muscle differentiation, and may also provide tools for studying new therapeutic molecules for the treatment of muscle disease.

Heightened muscle inflammation susceptibility may impair regenerative capacity in aging humans

The regenerative response of skeletal muscle to mechanically induced damage is impaired with age. Previous work in our laboratory suggests this may result from higher proinflammatory signaling in aging muscle at rest and/or a greater inflammatory response to damage. We, therefore, assessed skeletal muscle proinflammatory signaling at rest and 24 h after unaccustomed, loaded knee extension contractions that induced modest muscle damage (72% increase in serum creatine kinase) in a cohort of 87 adults across three age groups (AGE40, AGE61, and AGE76). Vastus lateralis muscle gene expression and protein cell signaling of the IL-6 and TNF-α pathways were determined by quantitative PCR and immunoblot analysis. For in vitro studies, cell signaling and fusion capacities were compared among primary myoblasts from young (AGE28) and old (AGE64) donors treated with TNF-α. Muscle expression was higher (1.5- to 2.1-fold) in AGE76 and AGE61 relative to AGE40 for several genes involved in IL-6, TNF-α, and TNF-like weak inducer of apoptosis signaling. Indexes of activation for the proinflammatory transcription factors signal transducer and activator of transcription-3 and NF-κB were highest in AGE76. Resistance loading reduced gene expression of IL-6 receptor, muscle RING finger 1, and atrogin-1, and increased TNF-like weak inducer of apoptosis receptor expression. Donor myoblasts from AGE64 showed impaired differentiation and fusion in standard media and greater NF-κB activation in response to TNF-α treatment (compared with AGE28). We show for the first time that human aging is associated with muscle inflammation susceptibility (i.e., higher basal state of proinflammatory signaling) that is present in both tissue and isolated myogenic cells and likely contributes to the impaired regenerative capacity of skeletal muscle in the older population.

Crystal structure of human TWEAK in complex with the Fab fragment of a neutralizing antibody reveals insights into receptor binding

The tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a multifunctional cytokine playing a key role in tissue regeneration and remodeling. Dysregulation of TWEAK signaling is involved in various pathological processes like autoimmune diseases and cancer. The unique interaction with its cognate receptor Fn14 makes both ligand and receptor promising targets for novel therapeutics. To gain insights into this important signaling pathway, we determined the structure of soluble human TWEAK in complex with the Fab fragment of an antibody selected for inhibition of receptor binding. In the crystallized complex TWEAK is bound by three Fab fragments of the neutralizing antibody. Homology modeling shows that Fab binding overlaps with the putative Fn14 binding site of TWEAK. Docking of the Fn14 cysteine rich domain (CRD) to that site generates a highly complementary interface with perfectly opposing charged and hydrophobic residues. Taken together the presented structure provides new insights into the biology of TWEAK and the TWEAK/Fn14 pathway, which will help to optimize the therapeutic strategy for treatment of related cancer types and autoimmune diseases.

Signaling mechanisms in mammalian myoblast fusion

Myoblast fusion is a critical process that contributes to the growth of muscle during development and to the regeneration of myofibers upon injury. Myoblasts fuse with each other as well as with multinucleated myotubes to enlarge the myofiber. Initial studies demonstrated that myoblast fusion requires extracellular calcium and changes in cell membrane topography and cytoskeletal organization. More recent studies have identified several cell-surface and intracellular proteins that mediate myoblast fusion. Furthermore, emerging evidence suggests that myoblast fusion is also regulated by the activation of specific cell-signaling pathways that lead to the expression of genes whose products are essential for the fusion process and for modulating the activity of molecules that are involved in cytoskeletal rearrangement. Here, we review the roles of the major signaling pathways in mammalian myoblast fusion.

Thrombospondin-4, tumour necrosis factor-like weak inducer of apoptosis (TWEAK) and its receptor Fn14: novel extracellular matrix modulating factors in cardiac remodelling

Cardiac remodelling is defined as changes in the size, shape, and function of the heart, which are most commonly caused by hypertension-induced left ventricular hypertrophy and myocardial infarction. Both neurohumoral and inflammatory factors have critical roles in the regulation of cardiac remodelling. A characteristic feature of cardiac remodelling is modification of the extracellular matrix (ECM), often manifested by fibrosis, a process that has vital consequences for the structure and function of the myocardium. In addition to established modulators of the ECM, the matricellular protein thrombospondin-4 (TSP-4) as well as the tumour necrosis factor-like weak inducer of apoptosis (TWEAK) and its receptor Fn14 has been recently shown to modulate cardiac ECM. TSP-4 null mice develop pronounced cardiac hypertrophy and fibrosis with defects in collagen maturation in response to pressure overload. TWEAK and Fn14 belong to the tumour necrosis factor superfamily of proinflammatory cytokines. Recently it was shown that elevated levels of circulating TWEAK via Fn14 critically affect the cardiac ECM, characterized by increasing fibrosis and cardiomyocyte hypertrophy in mice. Here we review the literature concerning the role of matricellular proteins and inflammation in cardiac ECM remodelling, with a special focus on TSP-4, TWEAK, and its receptor Fn14.

Efeito da natação na expressão de fatores regulatórios miogênicos durante o reparo do musculoesquelético de rato

CONTEXTUALIZAÇÃO: O músculo esquelético tem a capacidade de adaptação frente a estímulos variados, tais como atividade contrátil, danos diretos e indiretos. Uma das modalidades terapêuticas utilizadas na reabilitação de disfunções musculoesqueléticas que vem demonstrando resultados positivos no tratamento e na prevenção de várias patologias é a terapia aquática. OBJETIVO: Analisar o efeito da natação na expressão dos fatores regulatórios miogênicos MyoD e miogenina durante o reparo do músculo esquelético de rato após criolesão. MÉTODOS: Foram utilizados 40 ratos Wistar, divididos em 04 grupos: (1) Controle; (2) "Sham" (sem lesão, submetido a exposição do músculo tibial anterior (TA); (3) Criolesionado e (4) Criolesionado e submetido à natação, analisados em 7, 14 e 21 dias. A criolesão foi realizada por meio de duas aplicações, utilizando um bastão metálico de extremidade plana, resfriado em nitrogênio líquido diretamente no ventre muscular. O protocolo consistiu de sessões de natação com duração de 90 minutos, realizadas 6 vezes por semana. Ao término do protocolo os animais foram eutanasiados, os músculos TA foram removidos e o RNA total foi extraído. Em seguida, foi obtido o cDNA para a realização do PCR em tempo real utilizando primers específicos para MyoD e miogenina. RESULTADOS: Os resultados evidenciaram uma redução na expressão de miogenina após 7 dias nos grupos criolesionado com (p<0.01) e sem (p<0.01) natação e após 14 no grupo criolesionado com natação (p<0.05) com relação aos grupos controle e "sham", respectivamente. Não encontramos diferenças entre os grupos criolesionados com (p>0.05) e sem natação (p>0.05). Com relação à expressão de MyoD não houve diferença entre os grupos avaliados. CONCLUSÃO: A natação não influenciou a expressão dos fatores regulatórios miogênicos durante o processo de reparo de músculo esquelético de rato após criolesão.

TWEAK and cIAP1 regulate myoblast fusion through the noncanonical NF-κB signaling pathway

The fusion of mononucleated muscle progenitor cells (myoblasts) into multinucleated muscle fibers is a critical aspect of muscle development and regeneration. We identified the noncanonical nuclear factor κB (NF-κB) pathway as a signaling axis that drives the recruitment of myoblasts into new muscle fibers. Loss of cellular inhibitor of apoptosis 1 (cIAP1) protein led to constitutive activation of the noncanonical NF-κB pathway and an increase in the number of nuclei per myotube. Knockdown of essential mediators of NF-κB signaling, such as p100, RelB, inhibitor of κB kinase α, and NF-κB-inducing kinase, attenuated myoblast fusion in wild-type myoblasts. In contrast, the extent of myoblast fusion was increased when the activity of the noncanonical NF-κB pathway was enhanced by increasing the abundance of p52 and RelB or decreasing the abundance of tumor necrosis factor (TNF) receptor-associated factor 3, an inhibitor of this pathway. Low concentrations of the cytokine TNF-like weak inducer of apoptosis (TWEAK), which preferentially activates the noncanonical NF-κB pathway, also increased myoblast fusion, without causing atrophy or impairing myogenesis. These results identify roles for TWEAK, cIAP1, and noncanonical NF-κB signaling in the regulation of myoblast fusion and highlight a role for cytokine signaling during adult skeletal myogenesis.

Role of TWEAK in lupus nephritis: a bench-to-bedside review

There is significant unmet need in the treatment of lupus nephritis (LN) patients. In this review, we highlight the role of the TWEAK/Fn14 pathway in mediating key pathologic processes underlying LN involving both glomerular and tubular injury, and thus the potential for renal protection via blockade of this pathway. The specific pathological mechanisms of TWEAK - namely promoting inflammation, renal cell proliferation and apoptosis, vascular activation and fibrosis - are described, with supporting data from animal models and in vitro systems. Furthermore, we detail the translational relevance of these mechanisms to clinical readouts in human LN. We present the opportunity for an anti-TWEAK therapeutic as a renal protective agent to improve efficacy relative to current standard of care treatments hopefully without increased safety risk, and highlight a phase II trial with BIIB023, an anti-TWEAK neutralizing antibody, designed to assess efficacy in LN patients. Taken together, targeting the TWEAK/Fn14 axis represents a potential new therapeutic paradigm for achieving renal protection in LN patients.

The TWEAK-Fn14 system: breaking the silence of cytokine-induced skeletal muscle wasting

The occurrence of skeletal muscle atrophy, a devastating complication of a large number of disease states and inactivity/disuse conditions, provides a never ending quest to identify novel targets for its therapy. Proinflammatory cytokines are considered the mediators of muscle wasting in chronic diseases; however, their role in disuse atrophy has just begun to be elucidated. An inflammatory cytokine, tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK), has recently been identified as a potent inducer of skeletal muscle wasting. TWEAK activates various proteolytic pathways and stimulates the degradation of myofibril protein both in vitro and in vivo. Moreover, TWEAK mediates the loss of skeletal muscle mass and function in response to denervation, a model of disuse atrophy. Adult skeletal muscle express very low to minimal levels of TWEAK receptor, Fn14. Specific catabolic conditions such as denervation, immobilization, or unloading rapidly increase the expression of Fn14 in skeletal muscle which in turn stimulates the TWEAK activation of various catabolic pathways leading to muscle atrophy. In this article, we have discussed the emerging roles and the mechanisms of action of TWEAK-Fn14 system in skeletal muscle with particular reference to different models of muscle atrophy and injury and its potential to be used as a therapeutic target for prevention of muscle loss.

TWEAK/Fn14 promotes the proliferation and collagen synthesis of rat cardiac fibroblasts via the NF-кB pathway

We wished to elucidate a potential role of the tumor necrosis factor-like weak inducer of apoptosis (TWEAK)/fibroblast growth factor-inducible molecule 14 (Fn14) axis in myocardial fibrosis. Stimulation of neonatal rat cardiac fibroblasts (CFs) with TWEAK could increase CFs numbers and collagen synthesis. Conversely, when CFs were pretreated with siRNA against Fn14, induction of cell proliferation and collagen synthesis by TWEAK were inhibited. Pretreatment with TWEAK on CFs induced activation of the nuclear factor-kappaB (NF-кB) pathway and subsequently increased the production of metalloproteinase-9 (MMP-9). Cell treatment with siRNA against Fn14 led to inhibition of the NF-кB pathway. Additionally, after stimulation of cell with ammonium pyrrolidine dithiocarbamate, cell proliferation and collagen synthesis induced by NF-кB and the upregulation of MMP-9 production were inhibited. The present study suggested that the TWEAK/Fn14 axis increased cell proliferation and collagen synthesis by activating the NF-кB pathway and increasing MMP-9 activity. This axis may be important for regulating myocardial fibrosis.

TWEAK and TRAF6 regulate skeletal muscle atrophy

PURPOSE OF REVIEW

To discuss the roles and mechanisms of action of tumor necrosis factor-like weak inducer of apoptosis (TWEAK) and tumor necrosis factor receptor-associated factor 6 (TRAF6) in skeletal muscle atrophy.

RECENT FINDINGS

Proinflammatory cytokines are known to mediate muscle atrophy in many chronic disease states. However, their role in the loss of skeletal muscle mass in disuse conditions has just begun to be elucidated. Further, the initial signaling events leading to the activation of various catabolic pathways in skeletal muscle under different atrophic conditions are also less well understood. The TWEAK-Fn14 system has now been identified as a novel inducer of skeletal muscle wasting. Adult skeletal muscles express minimal levels of Fn14, the bona fide TWEAK receptor. Specific conditions of atrophy such as denervation, immobilization, or unloading rapidly induce the expression of Fn14 leading to TWEAK-induced activation of various proteolytic pathways in skeletal muscle. Recent studies have also demonstrated that the expression and activity of TRAF6 are increased in distinct models of muscle atrophy. Muscle-specific ablation of TRAF6 inhibits the induction of atrophy program in response to starvation, denervation, or cancer cachexia. Moreover, TWEAK also appears to activate some catabolic signaling through TRAF6-dependent mechanisms.

SUMMARY

Recent findings have uncovered TWEAK and TRAF6 as novel regulators of skeletal muscle atrophy. These proteins should potentially be used as molecular targets for prevention and/or treatment of muscular atrophy in future therapies.