Genetic ablation of TWEAK augments regeneration and post-injury growth of skeletal muscle in mice

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.

Cytokine expression and cytokine-mediated cell-cell communication during skeletal muscle regeneration revealed by integrative analysis of single-cell RNA sequencing data

Skeletal muscles undergo self-repair upon injury, owing to the resident muscle stem cells and their extensive communication with the microenvironment of injured muscles. Cytokines play a critical role in orchestrating intercell communication to ensure successful regeneration. Immune cells as well as other types of cells in the injury site, including muscle stem cells, are known to secret cytokines. However, the extent to which various cell types express distinct cytokines and how the secreted cytokines are involved in intercell communication during regeneration are largely unknown. Here we integrated 15 publicly available single-cell RNA-sequencing (scRNA-seq) datasets of mouse skeletal muscles at early regeneration timepoints (0, 2, 5, and 7 days after injury). The resulting dataset was analyzed for the expression of 393 annotated mouse cytokines. We found widespread and dynamic cytokine expression by all cell types in the regenerating muscle. Interrogating the integrated dataset using CellChat revealed extensive, bidirectional cell-cell communications during regeneration. Our findings provide a comprehensive view of cytokine signaling in the regenerating muscle, which can guide future studies of ligand-receptor signaling and cell-cell interaction to achieve new mechanistic insights into the regulation of muscle regeneration.

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.

Antibody-based soluble and membrane-bound TWEAK mimicking agonists with FcγR-independent activity

Fibroblast growth factor (FGF)-inducible 14 (Fn14) activates the classical and alternative NFκB (nuclear factor 'kappa-light-chain-enhancer' of activated B-cells) signaling pathway but also enhances tumor necrosis factor (TNF)-induced cell death. Fn14 expression is upregulated in non-hematopoietic cells during tissue injury and is also often highly expressed in solid cancers. In view of the latter, there were and are considerable preclinical efforts to target Fn14 for tumor therapy, either by exploiting Fn14 as a target for antibodies with cytotoxic activity (e.g. antibody-dependent cellular cytotoxicity (ADCC)-inducing IgG variants, antibody drug conjugates) or by blocking antibodies with the aim to interfere with protumoral Fn14 activities. Noteworthy, there are yet no attempts to target Fn14 with agonistic Fc effector function silenced antibodies to unleash the proinflammatory and cell death-enhancing activities of this receptor for tumor therapy. This is certainly not at least due to the fact that anti-Fn14 antibodies only act as effective agonists when they are presented bound to Fcγ receptors (FcγR). Thus, there are so far no antibodies that robustly and selectively engage Fn14 signaling without triggering unwanted FcγR-mediated activities. In this study, we investigated a panel of variants of the anti-Fn14 antibody 18D1 of different valencies and domain architectures with respect to their inherent FcγR-independent ability to trigger Fn14-associated signaling pathways. In contrast to conventional 18D1, the majority of 18D1 antibody variants with four or more Fn14 binding sites displayed a strong ability to trigger the alternative NFκB pathway and to enhance TNF-induced cell death and therefore resemble in their activity soluble (TNF)-like weak inducer of apoptosis (TWEAK), one form of the natural occurring ligand of Fn14. Noteworthy, activation of the classical NFκB pathway, which naturally is predominately triggered by membrane-bound TWEAK but not soluble TWEAK, was preferentially observed with a subset of constructs containing Fn14 binding sites at opposing sites of the IgG scaffold, e.g. IgG1-scFv fusion proteins. A superior ability of IgG1-scFv fusion proteins to trigger classical NFκB signaling was also observed with the anti-Fn14 antibody PDL192 suggesting that we identified generic structures for Fn14 antibody variants mimicking soluble and membrane-bound TWEAK.

Targeted ablation of Fn14 receptor improves exercise capacity and inhibits neurogenic muscle atrophy

Skeletal muscle atrophy is a prevalent complication in multiple chronic diseases and disuse conditions. Fibroblast growth factor-inducible 14 (Fn14) is a member of the TNF receptor superfamily and a bona fide receptor of the TWEAK cytokine. Accumulating evidence suggests that Fn14 levels are increased in catabolic conditions as well as during exercise. However, the role of Fn14 in the regulation of skeletal muscle mass and function remains poorly understood. In this study, through the generation of novel skeletal muscle-specific Fn14-knockout mice, we have investigated the muscle role of Fn14 in the regulation of exercise capacity and denervation-induced muscle atrophy. Our results demonstrate that there was no difference in skeletal muscle mass between control and muscle-specific Fn14-knockout mice. Nevertheless, the deletion of Fn14 in skeletal muscle significantly improved exercise capacity and resistance to fatigue. This effect of Fn14 deletion is associated with an increased proportion of oxidative myofibers and higher capillaries number per myofiber in skeletal muscle. Furthermore, our results demonstrate that targeted deletion of Fn14 inhibits denervation-induced muscle atrophy in adult mice. Deletion of Fn14 reduced the expression of components of the ubiquitin-proteasome system and non-canonical NF-kappa B signaling in denervated skeletal muscle, as well as increased the phosphorylation of Akt kinase and FoxO3a transcription factor. Collectively, our results demonstrate that targeted inhibition of Fn14 improves exercise tolerance and inhibits denervation-induced muscle atrophy in adult mice.

Targeting fibroblast growth factor (FGF)-inducible 14 (Fn14) for tumor therapy

Fibroblast growth factor-inducible 14 (Fn14) is a member of the tumor necrosis factor (TNF) receptor superfamily (TNFRSF) and is activated by its ligand TNF-like weak inducer of apoptosis (TWEAK). The latter occurs as a homotrimeric molecule in a soluble and a membrane-bound form. Soluble TWEAK (sTWEAK) activates the weakly inflammatory alternative NF-κB pathway and sensitizes for TNF-induced cell death while membrane TWEAK (memTWEAK) triggers additionally robust activation of the classical NF-κB pathway and various MAP kinase cascades. Fn14 expression is limited in adult organisms but becomes strongly induced in non-hematopoietic cells by a variety of growth factors, cytokines and physical stressors (e.g., hypoxia, irradiation). Since all these Fn14-inducing factors are frequently also present in the tumor microenvironment, Fn14 is regularly found to be expressed by non-hematopoietic cells of the tumor microenvironment and most solid tumor cells. In general, there are three possibilities how the tumor-Fn14 linkage could be taken into consideration for tumor therapy. First, by exploitation of the cancer associated expression of Fn14 to direct cytotoxic activities (antibody-dependent cell-mediated cytotoxicity (ADCC), cytotoxic payloads, CAR T-cells) to the tumor, second by blockade of potential protumoral activities of the TWEAK/Fn14 system, and third, by stimulation of Fn14 which not only triggers proinflammtory activities but also sensitizes cells for apoptotic and necroptotic cell death. Based on a brief description of the biology of the TWEAK/Fn14 system and Fn14 signaling, we discuss the features of the most relevant Fn14-targeting biologicals and review the preclinical data obtained with these reagents. In particular, we address problems and limitations which became evident in the preclinical studies with Fn14-targeting biologicals and debate possibilities how they could be overcome.

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.

Role of Protein Arginine Methyltransferases and Inflammation in Muscle Pathophysiology

Arginine methylation mediated by protein arginine methyltransferases (PRMTs) is a post-translational modification of both histone and non-histone substrates related to diverse biological processes. PRMTs appear to be critical regulators in skeletal muscle physiology, including regeneration, metabolic homeostasis, and plasticity. Chronic inflammation is commonly associated with the decline of skeletal muscle mass and strength related to aging or chronic diseases, defined as sarcopenia. In turn, declined skeletal muscle mass and strength can exacerbate chronic inflammation. Thus, understanding the molecular regulatory pathway underlying the crosstalk between skeletal muscle function and inflammation might be essential for the intervention of muscle pathophysiology. In this review, we will address the current knowledge on the role of PRMTs in skeletal muscle physiology and pathophysiology with a specific emphasis on its relationship with inflammation.

Divergent Roles of Inflammation in Skeletal Muscle Recovery From Injury

A transient increase in local pro-inflammatory cytokine expression following skeletal muscle injury mediates the repair and regeneration of damaged myofibers through myogenesis. Regenerative capacity is diminished and muscle wasting occurs, however, when intramuscular inflammatory signaling is exceedingly high or persists chronically. An excessive and persistent inflammatory response to muscle injury may therefore impair recovery by limiting the repair of damaged tissue and triggering muscle atrophy. The concentration-dependent activation of different downstream signaling pathways by several pro-inflammatory cytokines in cell and animal models support these opposing roles of post-injury inflammation. Understanding these molecular pathways is essential in developing therapeutic strategies to attenuate excessive inflammation and accelerate functional recovery and muscle mass accretion following muscle damage. This is especially relevant given the observation that basal levels of intramuscular inflammation and the inflammatory response to muscle damage are not uniform across all populations, suggesting certain individuals may be more susceptible to an excessive inflammatory response to injury that limits recovery. This narrative review explores the opposing roles of intramuscular inflammation in muscle regeneration and muscle protein turnover. Factors contributing to an exceedingly high inflammatory response to damage and age-related impairments in regenerative capacity are also considered.

Analysis of Fn14-NF-κB signaling response dynamics using a mechanistic model

Fn14 is a transmembrane receptor protein belonging to the tumor necrosis factor receptor (TNFR) superfamily. Many experimental reports have shown that crosslinking of the receptor by its extracellular ligand TWEAK induces prolonged activation of transcription factor NF-κB. This behavior is distinct from TNF-α receptor, which is a more well-characterized member of the TNFR family. TNF-α receptor, despite sharing many similar molecular interactions with Fn14, only transiently activates NF-κB in response to TNF-α stimulation. Here, we investigate molecular mechanisms that enable Fn14 to display such distinctive behavior. In particular, we focus on two specific features of the Fn14 pathway that potentially give rise to a positive feedback regulation and differentiate it from the TNF-α receptor signaling. By developing a mechanistic model, we analyze how these features may determine the dynamics of an Fn14-NF-κB response. Our analysis reveals that stimulation of Fn14 by TWEAK may generate highly non-linear dynamics, including stable limit cycles and bistable responses. The type of response depends both on the strength and duration of a TWEAK signal. Our predictions and analyses also show that the molecular interactions underlying the positive feedback explain the prolonged activation of NF-κB under certain parameter regimes. In light of the model predictions, we propose possible deregulations of Fn14 leading to its overexpression in solid tumors and tissue injuries.

Long-term exercise training prevents mammary tumorigenesis-induced muscle wasting in rats through the regulation of TWEAK signalling

AIM

Exercise training has been suggested as a non-pharmacological approach to prevent skeletal muscle wasting and improve muscle function in cancer cachexia. However, little is known about the molecular mechanisms underlying such beneficial effect. In this study, we aimed to, firstly, examine the contribution of TWEAK signalling to cancer-induced skeletal muscle wasting and, secondly, evaluate whether long-term exercise alters TWEAK signalling and prevents muscle wasting.

METHODS

Female Sprague-Dawley rats were randomly assigned to control and exercise groups. Fifteen animals from each group were exposed to N-Methyl-N-nitrosourea carcinogen. Animals in exercise groups were submitted to moderate treadmill exercise for 35 weeks. After the experimental period, animals were killed and gastrocnemius muscles were harvested for morphological and biochemical analysis.

RESULTS

We verified that exercise training prevented tumour-induced TWEAK/NF-κB signalling in skeletal muscle with a beneficial impact in fibre cross-sectional area and metabolism. Indeed, 35 weeks of exercise training promoted the upregulation of PGC-1α and oxidative phosphorylation complexes. This exercise-induced muscle remodelling in tumour-bearing animals was associated with less malignant mammary lesions.

CONCLUSION

Data support the benefits of an active lifestyle for the prevention of muscle wasting secondary to breast cancer, highlighting TWEAK/NF- κB signalling as a potential therapeutic target for the preservation of muscle mass.

Regulation of muscle growth and regeneration by the immune system

Diseases of muscle that are caused by pathological interactions between muscle and the immune system are devastating, but rare. However, muscle injuries that involve trauma and regeneration are fairly common, and inflammation is a clear feature of the regenerative process. Investigations of the inflammatory response to muscle injury have now revealed that the apparently nonspecific inflammatory response to trauma is actually a complex and coordinated interaction between muscle and the immune system that determines the success or failure of tissue regeneration.

Cytokine Signaling in Skeletal Muscle Wasting

Skeletal muscle wasting occurs in a variety of diseases including diabetes, cancer, Crohn's disease, chronic obstructive pulmonary disease (COPD), disuse, and denervation. Tumor necrosis factor α (TNF-α) is involved in mediating the wasting effect. To date, a causal relationship between TNF-α signaling and muscle wasting has been established in animal models. However, results from clinical trials are conflicting. This is partly due to the fact that other factors such as TNF-like weak inducer of apoptosis (TWEAK) and interleukin 6 (IL-6) are also involved in skeletal muscle wasting. Because muscle wasting is often associated with physical inactivity and reduced food intake, therapeutic interventions will be most effective when multiple approaches are used in conjunction with nutritional support and exercise.

Heightened TWEAK-NF-κB signaling and inflammation-associated fibrosis in paralyzed muscles of men with chronic spinal cord injury

Individuals with long-standing spinal cord injury (SCI) often present with extreme muscle atrophy and impaired glucose metabolism at both the skeletal muscle and whole body level. Persistent inflammation and increased levels of proinflammatory cytokines in the skeletal muscle are potential contributors to dysregulation of glucose metabolism and atrophy; however, to date no study has assessed the effects of long-standing SCI on their expression or intracellular signaling in the paralyzed muscle. In the present study, we assessed the expression of genes (TNFαR, TNFα, IL-6R, IL-6, TWEAK, TWEAK R, atrogin-1, and MuRF1) and abundance of intracellular signaling proteins (TWEAK, TWEAK R, NF-κB, and p-p65/p-50/105) that are known to mediate inflammation and atrophy in skeletal muscle. In addition, based on the effects of muscle inflammation on promotion of skeletal muscle fibrosis, we assessed the degree of fibrosis between myofibers and fascicles in both groups. For further insight into the distribution and variability of muscle fiber size, we also analyzed the frequency distribution of SCI fiber size. Resting vastus lateralis (VL) muscle biopsy samples were taken from 11 men with long-standing SCI (≈22 yr) and compared with VL samples from 11 able-bodied men of similar age. Our results demonstrated that chronic SCI muscle has heightened TNFαR and TWEAK R gene expression and NF-κB signaling (higher TWEAK R and phospho-NF-κB p65) and fibrosis, along with substantial myofiber size heterogeneity, compared with able-bodied individuals. Our data suggest that the TWEAK/TWEAK R/NF-κB signaling pathway may be an important mediator of chronic inflammation and fibrotic adaptation in SCI muscle.

TWEAK Regulates Muscle Functions in a Mouse Model of RNA Toxicity

Myotonic dystrophy type 1 (DM1), the most common form of muscular dystrophy in adults, is caused by toxic RNAs produced from the mutant DM protein kinase (DMPK) gene. DM1 is characterized by progressive muscle wasting and weakness. Therapeutic strategies have mainly focused on targeting the toxic RNA. Previously, we found that fibroblast growth factor-inducible 14 (Fn14), the receptor for TWEAK, is induced in skeletal muscles and hearts of mouse models of RNA toxicity and that blocking TWEAK/Fn14 signaling improves muscle function and histology. Here, we studied the effect of Tweak deficiency in a RNA toxicity mouse model. The genetic deletion of Tweak in these mice significantly reduced muscle damage and improved muscle function. In contrast, administration of TWEAK in the RNA toxicity mice impaired functional outcomes and worsened muscle histopathology. These studies show that signaling via TWEAK is deleterious to muscle in RNA toxicity and support the demonstrated utility of anti-TWEAK therapeutics.

Targeting of Fn14 Prevents Cancer-Induced Cachexia and Prolongs Survival

The cytokine TWEAK and its cognate receptor Fn14 are members of the TNF/TNFR superfamily and are upregulated in tumors. We found that Fn14, when expressed in tumors, causes cachexia and that antibodies against Fn14 dramatically extended lifespan by inhibiting tumor-induced weight loss although having only moderate inhibitory effects on tumor growth. Anti-Fn14 antibodies prevented tumor-induced inflammation and loss of fat and muscle mass. Fn14 signaling in the tumor, rather than host, is responsible for inducing this cachexia because tumors in Fn14- and TWEAK-deficient hosts developed cachexia that was comparable to that of wild-type mice. These results extend the role of Fn14 in wound repair and muscle development to involvement in the etiology of cachexia and indicate that Fn14 antibodies may be a promising approach to treat cachexia, thereby extending lifespan and improving quality of life for cancer patients.

CD163 interacts with TWEAK to regulate tissue regeneration after ischaemic injury

Macrophages are an essential component of the immune response to ischaemic injury and play an important role in promoting inflammation and its resolution, which is necessary for tissue repair. The type I transmembrane glycoprotein CD163 is exclusively expressed on macrophages, where it acts as a receptor for haemoglobin:haptoglobin complexes. An extracellular portion of CD163 circulates in the blood as a soluble protein, for which no physiological function has so far been described. Here we show that during ischaemia, soluble CD163 functions as a decoy receptor for TWEAK, a secreted pro-inflammatory cytokine of the tumour necrosis factor family, to regulate TWEAK-induced activation of canonical nuclear factor-κB (NF-κB) and Notch signalling necessary for myogenic progenitor cell proliferation. Mice with deletion of CD163 have transiently elevated levels of TWEAK, which stimulate muscle satellite cell proliferation and tissue regeneration in their ischaemic and non-ischaemic limbs. These results reveal a role for soluble CD163 in regulating muscle regeneration after ischaemic injury.

CD163 is a glycoprotein receptor expressed on the surface of macrophages. Here, the authors demonstrate that a soluble form of CD163 can act as a decoy receptor for the pro inflammatory cytokine TWEAK, thereby revealing a new mechanism for the regulation of tissue repair after ischaemic injury.

Conditional Cripto overexpression in satellite cells promotes myogenic commitment and enhances early regeneration

Skeletal muscle regeneration mainly depends on satellite cells, a population of resident muscle stem cells. Despite extensive studies, knowledge of the molecular mechanisms underlying the early events associated with satellite cell activation and myogenic commitment in muscle regeneration remains still incomplete. Cripto is a novel regulator of postnatal skeletal muscle regeneration and a promising target for future therapy. Indeed, Cripto is expressed both in myogenic and inflammatory cells in skeletal muscle after acute injury and it is required in the satellite cell compartment to achieve effective muscle regeneration. A critical requirement to further explore the in vivo cellular contribution of Cripto in regulating skeletal muscle regeneration is the possibility to overexpress Cripto in its endogenous configuration and in a cell and time-specific manner. Here we report the generation and the functional characterization of a novel mouse model for conditional expression of Cripto, i.e., the Tg:DsRed^loxP/loxPCripto-eGFP mice. Moreover, by using a satellite cell specific Cre-driver line we investigated the biological effect of Cripto overexpression in vivo, and provided evidence that overexpression of Cripto in the adult satellite cell compartment promotes myogenic commitment and differentiation, and enhances early regeneration in a mouse model of acute injury.

TWEAK/Fn14, a pathway and novel therapeutic target in myotonic dystrophy

Myotonic dystrophy type 1 (DM1), the most prevalent muscular dystrophy in adults, is characterized by progressive muscle wasting and multi-systemic complications. DM1 is the prototype for disorders caused by RNA toxicity. Currently, no therapies exist. Here, we identify that fibroblast growth factor-inducible 14 (Fn14), a member of the tumor necrosis factor receptor super-family, is induced in skeletal muscles and hearts of mouse models of RNA toxicity and in tissues from DM1 patients, and that its expression correlates with severity of muscle pathology. This is associated with downstream signaling through the NF-κB pathways. In mice with RNA toxicity, genetic deletion of Fn14 results in reduced muscle pathology and better function. Importantly, blocking TWEAK/Fn14 signaling with an anti-TWEAK antibody likewise improves muscle histopathology and functional outcomes in affected mice. These results reveal new avenues for therapeutic development and provide proof of concept for a novel therapeutic target for which clinically available therapy exists to potentially treat muscular dystrophy in DM1.

Elevated levels of TWEAK in skeletal muscle promote visceral obesity, insulin resistance, and metabolic dysfunction

Skeletal muscle is responsible for the majority of glucose disposal in body. Impairment in skeletal muscle glucose handling capacity leads to the state of insulin resistance. The TNF-like weak inducer of apoptosis (TWEAK) cytokine has now emerged as a major regulator of skeletal muscle mass and function. However, the role of TWEAK in skeletal muscle metabolic function remains less understood. Here, we demonstrate that with progressive age, skeletal muscle-specific TWEAK-transgenic (TWEAK-Tg) mice gain increased body weight (∼16%) and fat mass (∼64%) and show glucose intolerance and insulin insensitivity. TWEAK-Tg mice also exhibit adipocyte hypertrophy in the epididymal fat. Oxygen uptake, voluntary physical activity, and exercise capacity were significantly reduced in TWEAK-Tg mice compared with controls. Overexpression of TWEAK inhibited (∼31%) 5' AMP-activated protein kinase (AMPK) and reduced (∼31%) the levels of glucose transporter type 4 (GLUT4) without affecting the Akt pathway. TWEAK also inhibited insulin-stimulated glucose uptake (∼32%) and repressed the levels of GLUT4 (∼50%) in cultured myotubes from C57BL6 mice. TWEAK represses the levels of Krüppel-like factor 15; myocyte enhancer factor 2, and peroxisome proliferator-activated receptor-γ coactivator-1α, which are required for the activation of the GLUT4 locus. Collectively our study demonstrates that elevated levels of TWEAK in skeletal muscle cause metabolic abnormalities. Inhibition of TWEAK could be a potential approach to prevent weight gain and type 2 diabetes.

A phase I monotherapy study of RG7212, a first-in-class monoclonal antibody targeting TWEAK signaling in patients with advanced cancers

PURPOSE

Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and fibroblast growth factor-inducible molecule 14 (Fn14) are a ligand-receptor pair frequently overexpressed in solid tumors.

TWEAK

Fn14 signaling regulates multiple oncogenic processes through MAPK, AKT, and NFκB pathway activation. A phase I study of RG7212, a humanized anti-TWEAK IgG1κ monoclonal antibody, was conducted in patients with advanced solid tumors expressing Fn14.

EXPERIMENTAL DESIGN

Dose escalations, over a 200- to 7,200-mg range, were performed with patients enrolled in weekly (QW), bi-weekly (Q2W), or every-three-week (Q3W) schedules. Primary objectives included determination of dose and safety profile. Secondary endpoints included assessments related to inhibition of

TWEAK

Fn14 signaling, tumor proliferation, tumor immune cell infiltration, and pharmacokinetics.

RESULTS

In 192 treatment cycles administered to 54 patients, RG7212 was well-tolerated with no dose-limiting toxicities observed. More than 95% of related adverse events were limited to grade 1/2. Pharmacokinetics were dose proportional for all cohorts, with a t1/2 of 11 to 12 days. Pharmacodynamic changes included clearance of free and total TWEAK ligand and reductions in tumor Ki-67 and TRAF1. A patient with BRAF wild-type melanoma who received 36 weeks of RG7212 therapy had tumor regression and pharmacodynamic changes consistent with antitumor effects. Fifteen patients (28%) received 16 or more weeks of RG7212 treatment.

CONCLUSION

RG7212 demonstrated excellent tolerability and favorable pharmacokinetics. Pharmacodynamic endpoints were consistent with reduced

TWEAK

Fn14 signaling. Tumor regression was observed and prolonged stable disease was demonstrated in multiple heavily pretreated patients with solid tumors. These encouraging results support further study of RG7212. Clin Cancer Res; 21(2); 258-66. ©2014 AACR.

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.

The TWEAK-Fn14 dyad is involved in age-associated pathological changes in skeletal muscle

Progressive loss of skeletal muscle mass and strength (sarcopenia) is a major clinical problem in the elderly. Recently, proinflammatory cytokine TWEAK and its receptor Fn14 were identified as key mediators of muscle wasting in various catabolic states. However, the role of the TWEAK-Fn14 pathway in pathological changes in skeletal muscle during aging remains unknown. In this study, we demonstrate that the levels of Fn14 are increased in skeletal muscle of 18-month old (aged) mice compared with adult mice. Genetic ablation of Fn14 significantly increased the levels of specific muscle proteins and blunted the age-associated fiber atrophy in mice. While gene expression of two prominent muscle-specific E3 ubiquitin ligases MAFBx and MuRF1 remained comparable, levels of ubiquitinated proteins and the expression of autophagy-related molecule Atg12 were significantly reduced in Fn14-knockout (KO) mice compared with wild-type mice during aging. Ablation of Fn14 significantly diminished the DNA-binding activity of transcription factor nuclear factor-kappa B (NF-κB), gene expression of various inflammatory molecules, and interstitial fibrosis in skeletal muscle of aged mice. Collectively, our study suggests that the TWEAK-Fn14 signaling axis contributes to age-associated muscle atrophy and fibrosis potentially through its local activation of proteolytic systems and inflammatory pathways.

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.

Nrf2 protects against TWEAK-mediated skeletal muscle wasting

Skeletal muscle (SM) regeneration after injury is impaired by excessive inflammation. Particularly, the inflammatory cytokine tumour necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) is a potent inducer of skeletal muscle wasting and fibrosis. In this study we investigated the role of Nrf2, a major regulator of oxidative stress defence, in SM ischemia/reperfusion (I/R) injury and TWEAK induced atrophy. We explored the time-dependent expression of TWEAK after I/R in SM of Nrf2-wildtype (WT) and knockout (KO) mice. Nrf2-KO mice expressed significant higher levels of TWEAK as compared to WT mice. Consequently, Nrf2-KO mice present an insufficient regeneration as compared to Nrf2-WT mice. Moreover, TWEAK stimulation activates Nrf2 in the mouse myoblast cell line C2C12. This Nrf2 activation inhibits TWEAK induced atrophy in C2C12 differentiated myotubes. In summary, we show that Nrf2 protects SM from TWEAK-induced cell death in vitro and that Nrf2-deficient mice therefore have poorer muscle regeneration.

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.

Cytokines and chemokines as regulators of skeletal muscle inflammation: presenting the case of Duchenne muscular dystrophy

Duchenne muscular dystrophy is a severe inherited muscle disease that affects 1 in 3500 boys worldwide. Infiltration of skeletal muscle by inflammatory cells is an important facet of disease pathophysiology and is strongly associated with disease severity in the individual patient. In the chronic inflammation that characterizes Duchenne muscle, cytokines and chemokines are considered essential activators and recruiters of inflammatory cells. In addition, they provide potential beneficiary effects on muscle fiber damage control and tissue regeneration. In this review, current knowledge of cytokine and chemokine expression in Duchenne muscular dystrophy and its relevant animal disease models is listed, and implications for future therapeutic avenues are discussed.

Anti-inflammatory interventions and skeletal muscle injury: benefit or detriment?

Exercise, eccentric contractions, acute trauma, and disease are all causal mechanisms of skeletal muscle injury. After skeletal muscle is injured, it undergoes sequential phases of degeneration, inflammation, regeneration, and fibrosis. Events that occur in response to inflammation trigger regenerative processes. However, since inflammation causes pain, decreases skeletal muscle function, has a negative effect on performance, and contributes to fibrosis, which is one of the leading causes of delayed regeneration, the general practice has been to reduce inflammation. The problem with this approach is that preventing inflammation may hinder recovery. Current treatment options for inflammation are not necessarily effective and, in some cases, they may be unsafe. This review focuses on the question of whether the most beneficial course of treatment should be to block inflammation or if it is sensible to allow inflammatory processes to progress naturally. If blocking inflammation is perceived as a beneficial approach, it is not yet known at what time point during the inflammatory response it is most sensible to interfere. To address these issues, this review evaluates the effects of various anti-inflammatory agents on recovery processes in response to exercise-induced, traumatic, and disease-associated models of skeletal muscle injury. A collective analysis such as this should lay the foundation for future work that systematically manipulates the inflammatory response to most effectively promote regeneration and functional recovery in injured skeletal muscle, while reducing the negative effects of inflammatory processes such as pain and fibrosis.

TWEAK inhibits TRAF2-mediated CD40 signaling by destabilization of CD40 signaling complexes

We found recently that TNF-like weak inducer of apoptosis (TWEAK) and fibroblast growth factor-inducible-14 (Fn14) by virtue of their strong capability to reduce the freely available cytoplasmic pool of TNFR-associated factor (TRAF)2 and cellular inhibitors of apoptosis (cIAPs) antagonize the functions of these molecules in TNFR1 signaling, resulting in sensitization for apoptosis and inhibition of classical NF-κB signaling. In this study, we demonstrate that priming of cells with TWEAK also interferes with activation of the classical NF-κB pathway by CD40. Likewise, there was strong inhibition of CD40 ligand (CD40L)-induced activation of MAPKs in TWEAK-primed cells. FACS analysis and CD40L binding studies revealed unchanged CD40 expression and normal CD40L-CD40 interaction in TWEAK-primed cells. CD40L immunoprecipitates, however, showed severely reduced amounts of CD40 and CD40-associated proteins, indicating impaired formation or reduced stability of CD40L-CD40 signaling complexes. The previously described inhibitory effect of TWEAK on TNFR1 signaling has been traced back to reduced activity of the TNFR1-associated TRAF2-cIAP1/2 ubiquitinase complex and did not affect the stability of the immunoprecipitable TNFR1 receptor complex. Thus, the inhibitory effect of TWEAK on CD40 signaling must be based at least partly on other mechanisms. In line with this, signaling by the CD40-related TRAF2-interacting receptor TNFR2 was also attenuated but still immunoprecipitable in TWEAK-primed cells. Collectively, we show that Fn14 activation by soluble TWEAK impairs CD40L-CD40 signaling complex formation and inhibits CD40 signaling and thus identify the Fn14-TWEAK system as a potential novel regulator of CD40-related cellular functions.

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.

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.

Reciprocal interaction between TRAF6 and notch signaling regulates adult myofiber regeneration upon injury

Skeletal muscle is a postmitotic tissue that repairs and regenerates through activation of a population of stem-cell-like satellite cells. However, signaling mechanisms governing adult skeletal muscle regeneration remain less understood. In the present study, we have investigated the role of tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), an adaptor protein involved in receptor-mediated activation of multiple signaling pathways in regeneration of adult myofibers. Skeletal muscle-specific depletion of TRAF6 in mice (TRAF6(mko)) improved regeneration of myofibers upon injury with a concomitant increase in the number of satellite cells and activation of the Notch signaling pathway. Ex vivo cultures of TRAF6(mko) myofiber explants demonstrated an increase in the proliferative capacity of myofiber-associated satellite cells accompanied by an upregulation of Notch ligands. Deletion of TRAF6 also inhibited the activity of transcription factor NF-κB and the expression of inflammatory cytokines and augmented the M2c macrophage phenotype in injured muscle tissues. Collectively, our study demonstrates that specific inhibition of TRAF6 improves satellite cell activation and skeletal muscle regeneration through upregulation of Notch signaling and reducing the inflammatory repertoire.

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.

A Bioinformatics Resource for TWEAK-Fn14 Signaling Pathway

TNF-related weak inducer of apoptosis (TWEAK) is a new member of the TNF superfamily. It signals through TNFRSF12A, commonly known as Fn14. The TWEAK-Fn14 interaction regulates cellular activities including proliferation, migration, differentiation, apoptosis, angiogenesis, tissue remodeling and inflammation. Although TWEAK has been reported to be associated with autoimmune diseases, cancers, stroke, and kidney-related disorders, the downstream molecular events of TWEAK-Fn14 signaling are yet not available in any signaling pathway repository. In this paper, we manually compiled from the literature, in particular those reported in human systems, the downstream reactions stimulated by TWEAK-Fn14 interactions. Our manual amassment of the TWEAK-Fn14 pathway has resulted in cataloging of 46 proteins involved in various biochemical reactions and TWEAK-Fn14 induced expression of 28 genes. We have enabled the availability of data in various standard exchange formats from NetPath, a repository for signaling pathways. We believe that this composite molecular interaction pathway will enable identification of new signaling components in TWEAK signaling pathway. This in turn may lead to the identification of potential therapeutic targets in TWEAK-associated disorders.

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.

TWEAK/Fn14 pathway: an immunological switch for shaping tissue responses

Our immune system performs the vital function of recognizing and eliminating invading pathogens and malignancies. There is an increasing appreciation that the immune system also actively mediates tissue responses under both physiological and pathological conditions, significantly impacting the inflammatory, fibrogenic, and regenerative components. Likewise, there is a growing understanding of how epithelial, endothelial, and other non-hematopoietic tissue cell types actively contribute to the interplay that shapes tissue responses. While much of the molecular basis underlying the immune regulation of tissue responses remains to be delineated, the tumor necrosis factor (TNF) superfamily ligand/receptor pair of TNF-like weak inducer of apoptosis (TWEAK) and fibroblast growth factor-inducible molecule 14 (Fn14) has now emerged as a key piece of this puzzle. In this review, we first discuss how the usually 'dormant' TWEAK/Fn14 pathway becomes activated specifically in injury and disease contexts. We then summarize how TWEAK-mediated Fn14 signaling triggers a wide range of activities in tissue parenchymal and stromal cells as well as progenitor cells. Finally, we review recent experimental evidence that further supports the functional dichotomy of TWEAK/Fn14 activation in physiological versus pathological tissue responses and its potential therapeutic implications. Whereas transient TWEAK/Fn14 activation promotes productive tissue responses after injury, excessive or persistent TWEAK/Fn14 activation drives pathological tissue responses, leading to progressive damage and degeneration.

TWEAK in inclusion-body myositis muscle: possible pathogenic role of a cytokine inhibiting myogenesis

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) and its receptor Fn14 exert pleiotropic effects, including regulation of myogenesis. Sporadic inclusion-body myositis (IBM) is the most common muscle disease of the elderly population and leads to severe disability. IBM mesoangioblasts, different from mesoangioblasts in other inflammatory myopathies, display a myogenic differentiation defect. The objective of the present study was to investigate TWEAK-Fn14 expression in IBM and other inflammatory myopathies and explore whether TWEAK modulation affects myogenesis in IBM mesoangioblasts. TWEAK, Fn14, and NF-κB expression was assessed by immunohistochemistry and Western blot in cell samples from both muscle biopsies and primary cultures. Mesoangioblasts isolated from samples of IBM, dermatomyositis, polymyositis, and control muscles were treated with recombinant human TWEAK, Fn14-Fc chimera, and anti-TWEAK antibody. TWEAK-RNA interference was performed in IBM and dermatomyositis mesoangioblasts. TWEAK levels in culture media were determined by enzyme-linked immunosorbent assay. In IBM muscle, we found increased TWEAK-Fn14 expression. Increased levels of TWEAK were found in differentiation medium from IBM mesoangioblasts. Moreover, TWEAK inhibited myogenic differentiation of mesoangioblasts. Consistent with this evidence, TWEAK inhibition by Fn14-Fc chimera or short interfering RNA induced myogenic differentiation of IBM mesoangioblasts. We provide evidence that TWEAK is a negative regulator of human mesoangioblast differentiation. Dysregulation of the TWEAK-Fn14 axis in IBM muscle may induce progressive muscle atrophy and reduce activation and differentiation of muscle precursor cells.

Transcriptome signature of resistance exercise adaptations: mixed muscle and fiber type specific profiles in young and old adults

This investigation examined the effects of acute resistance exercise (RE), progressive resistance training (PRT), and age on the human skeletal muscle Transcriptome. Two cohorts of young and old adults [study A: 24 yr, 84 yr (n = 28); study B: 25 yr, 78 yr (n = 36)] were studied. Vastus lateralis biopsies were obtained pre- and 4 h post-RE in conjunction with the 1st and 36th (last) training session as part of a 12-wk PRT program in study A, whereas biopsies were obtained in the basal untrained state in study B. Additionally, the muscle fiber type specific (MHC I and MHC IIa) Transcriptome response to RE was examined in a subset of young and old women from study A. Transcriptome profiling was performed using HG U133 Plus 2.0 Arrays. The main findings were 1) there were 661 genes affected by RE during the 1st and 36th training bout that correlated with gains in muscle size and strength with PRT (termed the Transcriptome signature of resistance exercise adaptations); 2) the RE gene response was most pronounced in fast-twitch (MHC IIa) muscle fibers and provided additional insight into the skeletal muscle biology affected by RE; 3) skeletal muscle of young adults is more responsive to RE at the gene level compared with old adults and age also affected basal level skeletal muscle gene expression. These skeletal muscle Transcriptome findings provide further insight into the molecular basis of sarcopenia and the impact of resistance exercise at the mixed muscle and fiber type specific level.

Studies of binding of tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) to fibroblast growth factor inducible 14 (Fn14)

To perform highly sensitive cellular binding studies with TNF-like weak inducer of apoptosis (TWEAK), we developed a bioluminescent variant of soluble TWEAK (GpL-FLAG-TNC-TWEAK) by fusing it genetically to the C terminus of the luciferase of Gaussia princeps (GpL). Equilibrium binding studies on human (HT1080 and HT29) and murine (Renca and B16) cell lines at 37 °C revealed high affinities of human TWEAK from 53 to 112 pm. The dissociation rate constant of the TWEAK-Fn14 interaction was between 0.48×10(-3) s(-1) (HT29) and 0.58×10(-3) s(-1) (HT1080) for the human molecules, and the association rate constant obtained was 3.3×10(6) m(-1) s(-1) for both cell lines. It has been shown previously that oligomerization of soluble TWEAK trimers results in enhanced Fn14-mediated activation of the classical NFκB pathway. Binding studies with GpL-FLAG-TNC-TWEAK trimers oligomerized by help of a FLAG tag-specific antibody gave no evidence for a major increase in Fn14 occupancy by oligomerized ligand despite strongly enhanced induction of the NFκB target IL8. Thus, aggregated complexes of soluble TWEAK and Fn14 have a higher intrinsic activity to stimulate the classical NFκB pathway and qualitatively differ from isolated trimeric TWEAK-Fn14 complexes. Furthermore, determination of IL8 induction as a function of occupied activated receptors revealed that the intrinsic capability of TNFR1 to stimulate the classical NFκB pathway and IL8 production was ∼100-fold higher than Fn14. Thus, although ∼25 activated TNFR1 trimers were sufficient to trigger half-maximal IL8 production, more than 2500 cell-bound oligomerized TWEAK trimers were required to elicit a similar response.