TWEAK Regulates Muscle Functions in a Mouse Model of RNA Toxicity

Sarcopenia is associated with hypomethylation of TWEAK and increased plasma levels of TWEAK and its downstream inflammatory factor TNF-α in older adults: A case-control study

BACKGROUND

Sarcopenia is a harmful condition common among older adults for which no treatment is available. Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and its receptor fibroblast growth factor inducible 14 (FN14) are known to play important roles in the pathogenesis of sarcopenia. This study investigated alterations in methylation in TWEAK and Fn14 to identify potential targets for the managing sarcopenia.

MATERIALS AND METHODS

Through an epidemiological investigation, we detected methylation of CpG islands (CpGs) in TWEAK and Fn14 in community-dwelling older adult of Xinjiang by bisulfite sequencing. Significant CpGs associated with sarcopenia were selected for detection in 152 older individuals by pyrosequencing. Associations between CpG methylation, plasma inflammatory marker levels, and sarcopenia were analyzed.

RESULTS

Of 38 CpGs in TWEAK and 30 CpGs in Fn14 detected in 60 individuals, 6 CpGs showed lower methylation in sarcopenia patients compared with control individuals. In 152 older adults, covariance analysis with adjustment for age, gender, triglyceride level, obesity, diabetes, and hypertension showed that the methylation levels of 6 CpGs (CpG8, CpG12, CpG13, CpG20 and CpG21of TWEAK, and CpG24 of Fn14) were significantly lower in sarcopenia patients than in control individuals. With adjustment for additional confounding factors, covariate variance analysis showed that plasma TWEAK, TNF-α and IL-10 levels in the sarcopenia group were significant higher than those in the control group (P = 0.007, P < 0.001, P = 0.003). Multivariate logistic regression analysis showed that CpG8, CpG13, CpG21, and total methylation of TWEAK (OR = 0.767, 95 % CI = 0.622-0.947; OR = 0.740, 95 % CI = 0.583-0.941; OR = 0.734, 95 % CI = 0.561-0.958; OR = 0.883, 95 % CI = 0.795-0.980) as well as CpG22 and total methylation of Fn14 were significantly associated with sarcopenia (OR = 826, 95 % CI = 0.704-0.968; OR = 0.918, 95 % CI = 0.852-0.989). From partial correlation analysis, plasma TWEAK was correlated with plasma TNF-α (r = 0.172, P = 0.042).

CONCLUSION

Sarcopenia is associated with hypomethylation of TWEAK and increased plasma levels of TWEAK and its downstream inflammatory factor TNF-α in a community-dwelling population of older adults in Xinjiang.

Studying the Effect of MBNL1 and MBNL2 Loss in Skeletal Muscle Regeneration

Loss of function of members of the muscleblind-like (MBNL) family of RNA binding proteins has been shown to play a key role in the spliceopathy of RNA toxicity in myotonic dystrophy type 1 (DM1), the most common muscular dystrophy affecting adults and children. MBNL1 and MBNL2 are the most abundantly expressed members in skeletal muscle. A key aspect of DM1 is poor muscle regeneration and repair, leading to dystrophy. We used a BaCl2-induced damage model of muscle injury to study regeneration and effects on skeletal muscle satellite cells (MuSCs) in Mbnl1∆E3/∆E3 and Mbnl2∆E2/∆E2 knockout mice. Similar experiments have previously shown deleterious effects on these parameters in mouse models of RNA toxicity. Muscle regeneration in Mbnl1 and Mbnl2 knockout mice progressed normally with no obvious deleterious effects on MuSC numbers or increased expression of markers of fibrosis. Skeletal muscles in Mbnl1∆E3/∆E3/ Mbnl2∆E2/+ mice showed increased histopathology but no deleterious reductions in MuSC numbers and only a slight increase in collagen deposition. These results suggest that factors beyond the loss of MBNL1/MBNL2 and the associated spliceopathy are likely to play a key role in the defects in skeletal muscle regeneration and deleterious effects on MuSCs that are seen in mouse models of RNA toxicity due to expanded CUG repeats.

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.

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.

Modeling muscle regeneration in RNA toxicity mice

RNA toxicity underlies the pathogenesis of disorders such as myotonic dystrophy type 1 (DM1). Muscular dystrophy is a key element of the pathology of DM1. The means by which RNA toxicity causes muscular dystrophy in DM1 is unclear. Here, we have used the DM200 mouse model of RNA toxicity due to the expression of a mutant DMPK 3′UTR mRNA to model the effects of RNA toxicity on muscle regeneration. Using a BaCl₂-induced damage model, we find that RNA toxicity leads to decreased expression of PAX7, and decreased numbers of satellite cells, the stem cells of adult skeletal muscle (also known as MuSCs). This is associated with a delay in regenerative response, a lack of muscle fiber maturation and an inability to maintain a normal number of satellite cells. Repeated muscle damage also elicited key aspects of muscular dystrophy, including fat droplet deposition and increased fibrosis, and the results represent one of the first times to model these classic markers of dystrophic changes in the skeletal muscles of a mouse model of RNA toxicity. Using a ligand-conjugated antisense (LICA) oligonucleotide ASO targeting DMPK sequences for the first time in a mouse model of RNA toxicity in DM1, we find that treatment with IONIS 877864, which targets the DMPK 3′UTR mRNA, is efficacious in correcting the defects in regenerative response and the reductions in satellite cell numbers caused by RNA toxicity. These results demonstrate the possibilities for therapeutic interventions to mitigate the muscular dystrophy associated with RNA toxicity in DM1.

Uncovering the dosage-dependent roles of Arid1a in gastric tumorigenesis for combinatorial drug therapy

Gastric cancer (GC) is one of the most common deadly cancers in the world. Although patient genomic data have identified AT-rich interaction domain 1A (ARID1A), a key chromatin remodeling complex subunit, as the second most frequently mutated gene after TP53, its in vivo role and relationship to TP53 in gastric tumorigenesis remains unclear. Establishing a novel mouse model that reflects the ARID1A heterozygous mutations found in the majority of human GC cases, we demonstrated that Arid1a heterozygosity facilitates tumor progression through a global loss of enhancers and subsequent suppression of the p53 and apoptosis pathways. Moreover, mouse genetic and single-cell analyses demonstrated that the homozygous deletion of Arid1a confers a competitive disadvantage through the activation of the p53 pathway, highlighting its distinct dosage-dependent roles. Using this unique vulnerability of Arid1a mutated GC cells, our combined treatment with the epigenetic inhibitor, TP064, and the p53 agonist, Nutlin-3, inhibited growth of Arid1a heterozygous tumor organoids, providing a novel therapeutic option for GC.

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.