TLR4-defective (C3H/HeJ) mice are not protected from cast immobilization-induced muscle atrophy

Progressive Skeletal Muscle Atrophy in Muscular Dystrophies: A Role for Toll-like Receptor-Signaling in Disease Pathogenesis

Muscle atrophy is an active process controlled by specific transcriptional programs, in which muscle mass is lost by increased protein degradation and/or decreased protein synthesis. This review explores the involvement of Toll-like receptors (TLRs) in the muscle atrophy as it is observed in muscular dystrophies, disorders characterized by successive bouts of muscle fiber degeneration and regeneration in an attempt to repair contraction-induced damage. TLRs are defense receptors that detect infection and recognize self-molecules released from damaged cells. In muscular dystrophies, these receptors become over-active, and are firmly involved in the sustained chronic inflammation exhibited by the muscle tissue, via their induction of pro-inflammatory cytokine expression. Taming the exaggerated activation of TLR2/4 and TLR7/8/9, and their downstream effectors in particular, comes forward as a therapeutic strategy with potential to slow down disease progression.

Cistanche tubulosa (Schenk) Wight Extract Enhances Hindlimb Performance and Attenuates Myosin Heavy Chain IId/IIx Expression in Cast-Immobilized Mice

Skeletal muscle atrophy is encountered in many clinical conditions, but a pharmacological treatment has not yet been established. Cistanche tubulosa (Schenk) Wight is an herbal medicine used in traditional Japanese and Chinese medicine. In the current study, we investigated the effect of C. tubulosa extract (CTE) on atrophied muscle in vivo. We also investigated hindlimb cast immobilization in mice and devised a novel type of hindlimb-immobilizing cast, consisting of sponge-like tape and a thin plastic tube. Using this method, 3 out of 4 groups of mice (n = 11 for each group) were cast-immobilized in the hindlimbs and administered CTE or vehicle for 13 days. A sham procedure was performed in the mice of the fourth group to which the vehicle was administered. Next, the triceps surae muscles (TS) were excised. To analyze the effect of the novel cast system and CTE administration on muscle atrophy, we evaluated TS wet weight and myofiber cross-sectional area (CSA). We also determined MyHC IId/IIx expression levels by western blotting, since their increase is a hallmark of disuse muscle atrophy, suggesting slow-to-fast myofiber type shift. Moreover, we performed two tests of hindlimb performance. The novel cast immobilization method significantly reduced TS wet weight and myofiber CSA. This was accompanied by deterioration of hindlimb function and an increase in MyHC IId/IIx expression. CTE administration did not alter TS wet weight or myofiber CSA; however, it showed a trend of amelioration of the loss of hindlimb function and of suppression of the increased MyHC IId/IIx expression in cast-immobilized mice. Our novel hindlimb cast immobilization method effectively induced muscle atrophy. CTE did not affect muscle mass, but suppressed the shift from slow to fast myofiber type in cast-immobilized mice, ameliorating hindlimb function deterioration.

Cancer Takes a Toll on Skeletal Muscle by Releasing Heat Shock Proteins-An Emerging Mechanism of Cancer-Induced Cachexia

Cancer-associated cachexia (cancer cachexia) is a major contributor to the modality and mortality of a wide variety of solid tumors. It is estimated that cachexia inflicts approximately ~60% of all cancer patients and is the immediate cause of ~30% of all cancer-related death. However, there is no established treatment of this disorder due to the poor understanding of its underlying etiology. The key manifestations of cancer cachexia are systemic inflammation and progressive loss of skeletal muscle mass and function (muscle wasting). A number of inflammatory cytokines and members of the TGFβ superfamily that promote muscle protein degradation have been implicated as mediators of muscle wasting. However, clinical trials targeting some of the identified mediators have not yielded satisfactory results. Thus, the root cause of the muscle wasting associated with cancer cachexia remains to be identified. This review focuses on recent progress of laboratory studies in the understanding of the molecular mechanisms of cancer cachexia that centers on the role of systemic activation of Toll-like receptor 4 (TLR4) by cancer-released Hsp70 and Hsp90 in the development and progression of muscle wasting, and the downstream signaling pathways that activate muscle protein degradation through the ubiquitin-proteasome and the autophagy-lysosome pathways in response to TLR4 activation. Verification of these findings in humans could lead to etiology-based therapies of cancer cachexia by targeting multiple steps in this signaling cascade.

TLR4-defective (C3H/HeJ) mice are not protected from cast immobilization-induced muscle atrophy

Recent studies have shown that activation of Toll‐like receptor (TLR)4 signaling may be an important factor in muscle atrophy and excessive inflammatory response associated with immobilization. To examine the role of TLR4 signaling on cast immobilization‐induced skeletal muscle atrophy, we tested the hypothesis that muscle atrophy and inflammation after cast immobilization is reduced in TLR4‐defective mice. TLR4‐defective (C3H/HeJ) and wild type (C3H/HeN) mice were divided into control and cast‐immobilization groups. Cast immobilization was imposed for 14 days. Cast immobilization increased TLR4 mRNA expression in the gastrocnemius and decreased muscle mass and cross‐sectional area (CSA) of the gastrocnemius fibers. However, there was no difference in the gastrocnemius muscle mass and CSA between TLR4‐defective and wild type mice. Cast immobilization‐induced increase in ubiquitin E3 ligases (MAFbx/Atrogin‐1 and MuRF1), inflammatory cytokines, and macrophage/monocyte marker mRNAs were unaffected by defective TLR4. Our findings in C3H/HeJ mice suggested that TLR4 signaling might not play an essential role in immobilization‐induced muscle atrophy.