Background/Purpose: To investigate the impact of systemic inflammation on skeletal muscles in human tumor necrosis factor transgenic (hTNFtg) animals.

Methods: We isolated triceps surae, quadriceps, tibialis anterior as well as rectus abdominis muscles from hTNFtg animals at different time-points of disease starting at week 4 after birth. Muscle weight and body weight were assessed from these animals. Age and sex-matched wildtype (wt) animals served as controls. We performed quantitative real-time PCR for Cathepsin L, B, S, H, D, LC3-B, MMP-9 and Interleukin (IL)-1 and IL-6 from mRNA isolated from muscle tissues of hTNFtg and wt animals. Moreover, hTNFtg animals were treated for 5 weeks with anti-TNF ab (Infliximab, 10mg/kg, 3x per week, i.p.) starting either at week 6 or week 10 after birth. Muscle tissue sections were also stained for macrophages, neutrophils, T cells and B cells. Mobility of animals was assessed by video-analysis using Ethovision Software (from Noldus, The Netherlands). Functionality of triceps surae muscle was evaluated by electro-stimulation.

Results: We could demonstrate that hTNFtg mice significantly lost muscle weight when compared to sex- and age matched wt animals. Reductions in muscle weight became already manifest at early stages of the disease, at week 4, and continuously progressed until week 16. Bodyweight was also significantly lower in hTNFtg animals compared to their wt littermates. Next, we found significantly increased mRNA expression levels of Cathepsin L, a lysosomal endopeptidase responsible for muscle protein degradation, in muscles from hTNFtg compared to their wt littermates. In contrast, other proteases such as cathepsin B, S, H, D did not significantly increased differ. In addition, we also found LC3B, an enzyme for autophagy-lysosome-mediated proteolysis, to be upregulated in hTNFtg mice compared to wt animals. Moreover, proinflammatory cytokines such as IL-1 and IL-6 were also significantly upregulated in muscles from hTNFtg mice. Interestingly, we observed an increased presence of macrophages and granulocytes in the muscle vascular system but no accumulation of inflammatory cells into the muscle tissue. We also investigated the rectus abdominis muscle, which is not located between inflamed articular joints, and found elevated levels of Cathepsin L and LC3B in this muscle, indicating that hTNFtg mice suffer from a systemic muscle proteolysis due to systemic inflammation. In addition, functionality of muscles was impaired as observed by markedly reduced maximal strength and faster fatigue in triceps surae from hTNFtg mice compared to wt animals. Remarkably, mobility of hTNFtg animals was already significantly reduced at week 6 indicating reduced use of muscles at early stage of TNF driven disease. TNF blockade at both early and late time points could completely rescue mobility to wildtype levels, whereas muscle atrophy could not be prevented by treatment at late stage of arthritis.

Conclusion: Despite spontaneous development of chronic inflamed, erosive arthritis, chronic overexpression of TNF leads to skeletal muscle atrophy due to increased tissue-degrading cathepsin L and LC3B and reduced mobility starting at early phase of arthritis disease in hTNFtg animals.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/muscle-wasting-in-htnftg-mice-an-animal-model-for-rheumatoid-arthritis-due-to-increased-cathepsin-l-and-lc3b-expression/