Myoblast fusion is an indispensable step for skeletal muscle development, postnatal growth, and regeneration. Myeloid differentiation primary response gene 88 (MyD88) is an adaptor protein that mediates Toll-like receptors and interleukin-1 receptor signaling. Here we report a cell-autonomous role of MyD88 in the regulation of myoblast fusion. MyD88 protein levels are increased during in vitro myogenesis and in conditions that promote skeletal muscle growth in vivo. Deletion of MyD88 impairs fusion of myoblasts without affecting their survival, proliferation, or differentiation. MyD88 regulates non-canonical NF-κB and canonical Wnt signaling during myogenesis and promotes skeletal muscle growth and overload-induced myofiber hypertrophy in mice. Ablation of MyD88 reduces myofiber size during muscle regeneration, whereas its overexpression promotes fusion of exogenous myoblasts to injured myofibers. Our study shows that MyD88 modulates myoblast fusion and suggests that augmenting its levels may be a therapeutic approach to improve skeletal muscle formation in degenerative muscle disorders.

Skeletal muscle mass is regulated by the coordinated activation of several anabolic and catabolic pathways. The endoplasmic reticulum (ER) is a major site of protein folding and a reservoir for calcium ions. Accretion of misfolded proteins or depletion in calcium concentration causes stress in the ER, which leads to the activation of a signaling network known as the unfolded protein response (UPR). In the present study, we investigated the role of the protein kinase R-like endoplasmic reticulum kinase (PERK) arm of the UPR in the regulation of skeletal muscle mass and function in naive conditions and in a mouse model of cancer cachexia. Our results demonstrate that the targeted inducible deletion of PERK reduces skeletal muscle mass, strength, and force production during isometric contractions. Deletion of PERK also causes a slow-to-fast fiber type transition in skeletal muscle. Furthermore, short hairpin RNA-mediated knockdown or pharmacologic inhibition of PERK leads to atrophy in cultured myotubes. While increasing the rate of protein synthesis, the targeted deletion of PERK leads to the increased expression of components of the ubiquitin-proteasome system and autophagy in skeletal muscle. Ablation of PERK also increases the activation of calpains and deregulates the gene expression of the members of the FGF19 subfamily. Furthermore, the targeted deletion of PERK increases muscle wasting in Lewis lung carcinoma tumor-bearing mice. Our findings suggest that the PERK arm of the UPR is essential for the maintenance of skeletal muscle mass and function in adult mice.-Gallot, Y. S., Bohnert, K. R., Straughn, A. R., Xiong, G., Hindi, S. M., Kumar, A. PERK regulates skeletal muscle mass and contractile function in adult mice.

BackgroundOvarian cancer is the fifth leading cause of cancer-related deaths amongst women in the United States. Cachexia is the primary cause of death in approximately 30% of cancer patients, and is often evidenced in ovarian cancer patients. We tested the steroidal lactone Withaferin A to examine if it could ameliorate ovarian cancer-induced cachexia.MethodsSix-week-old severely immunodeficient female mice were xenografted with the ovarian cancer cell line A2780 followed by treatment with Withaferin A or vehicle. Changes in functional grip strength were assessed on a weekly basis. Postmortem, H&E staining was performed on skeletal muscle sections and immunofluorescent immunohistochemistry was performed on skeletal muscle and tumor sections. The levels of NF-κB-related proinflammatory cytokines were assessed in the xenografted tumors and in resident host skeletal muscle.ResultsXenografting of the A2780 cell line resulted in a significant rate of mortality, which was attenuated by a therapeutic dosage of Withaferin A. Mice that received vehicle treatment following xenografting exhibited functional muscle decline over the course of the study. The therapeutic dosage Withaferin A treatment attenuated this reduction in grip strength, whereas the supratherapeutic dosage of Withaferin A was found to be toxic/lethal and demonstrated a further decline in functional muscle strength and an increased rate of mortality on par with vehicle treatment. At a histological level, the vehicle treated tumor-bearing mice exhibited a profound reduction in myofibrillar cross-sectional area compared to the vehicle treated tumor-free control group. The atrophic changes induced by the xenografted tumor were significantly ameliorated by treatment with Withaferin A. The combination of functional muscle weakening and induction of myofibrillar atrophy corroborate a cachectic phenotype, which was functionally rescued by Withaferin A. Further, treatment completely abolished the slow-to-fast myofiber type conversion observed in the settings of cancer-induced cachexia. In both host resident skeletal muscle and the xenografted tumors, we report an increase in NF-κB-related proinflammatory cytokines that was reversed by Withaferin A treatment. Finally, we demonstrated that Withaferin A significantly downregulates cytosolic and nuclear levels of phospho-p65, the active canonical NF-κB transcription factor, in xenografted tumors.ConclusionsCumulatively, our results demonstrate a previously overlooked role of Withaferin A in a xenograft model of ovarian cancer. We propose mechanisms by which Withaferin A reduces NF-κB-dependent pro-inflammatory cytokine production leading to an attenuation of the cachectic phenotype in an i.p. xenograft model of ovarian cancer.

Cachexia is a common multifactorial syndrome in the advanced stages of cancer and accounts for approximately 20–30% of all cancer-related fatalities. In addition to the progressive loss of skeletal muscle mass, cancer results in impairments in cardiac function. We recently demonstrated that WFA attenuates the cachectic skeletal muscle phenotype induced by ovarian cancer. The purpose of this study was to investigate whether ovarian cancer induces cardiac cachexia, the possible pathway involved, and whether WFA attenuates cardiac cachexia. Xenografting of ovarian cancer induced cardiac cachexia, leading to the loss of normal heart functions. Treatment with WFA rescued the heart weight. Further, ovarian cancer induced systolic dysfunction and diastolic dysfunction Treatment with WFA preserved systolic function in tumor-bearing mice, but diastolic dysfunction was partially improved. In addition, WFA abrogated the ovarian cancer-induced reduction in cardiomyocyte cross-sectional area. Finally, treatment with WFA ameliorated fibrotic deposition in the hearts of tumor-bearing animals. We observed a tumor-induced MHC isoform switching from the adult MHCα to the embryonic MHCβ isoform, which was prevented by WFA treatment. Circulating Ang II level was increased significantly in the tumor-bearing, which was lowered by WFA treatment. Our results clearly demonstrated the induction of cardiac cachexia in response to ovarian tumors in female NSG mice. Further, we observed induction of proinflammatory markers through the AT 1 R pathway, which was ameliorated by WFA, in addition to amelioration of the cachectic phenotype, suggesting WFA as a potential therapeutic agent for cardiac cachexia in oncological paradigms.