Although their physiology and functions are very different, bones, skeletal and smooth muscles, as well as the heart have the same embryonic origin. Skeletal muscles and bones interact with each other to enable breathing, kinesis, and the maintenance of posture. Often, muscle and bone tissues degenerate synchronously under various conditions such as cancers, space travel, aging, prolonged bed rest, and neuromuscular diseases. In addition, bone tissue, skeletal and smooth muscles, and the heart share common signaling pathways. The RANK/RANKL/OPG pathway, which is essential for bone homeostasis, is also implicated in various physiological processes such as sarcopenia, atherosclerosis, and cardiovascular diseases. Several studies have reported bone-skeletal muscle crosstalk through the RANK/RANKL/OPG pathway. This review will summarize the current evidence indicating that the RANK/RANKL/OPG pathway is involved in muscle function. First, we will briefly discuss the role this pathway plays in bone homeostasis. Then, we will present results from various sources indicating that it plays a physiopathological role in skeletal, smooth muscle, and cardiac functions. Understanding how the RANK/RANKL/OPG pathway interferes in several physiological disorders may lead to new therapeutic approaches aimed at protecting bones and other tissues with a single treatment.

Bone and muscle are tightly coupled and form a functional unit under normal conditions. The receptor-activator of nuclear factor κB/receptor-activator of nuclear factor κB ligand/osteoprotegerin (RANK/RANKL/OPG) triad plays a crucial role in bone remodeling. RANKL inhibition by OPG prevents osteoporosis. In contrast, the absence of OPG results in elevated serum RANKL and early onset osteoporosis. However, the impacts of OPG deletion on muscle structure and function are unknown. Our results showed that 1-, 3- and 5-month-old Opg^−/− mice have reduced tibial and femoral bone biomechanical properties and higher levels of circulating RANKL. OPG-deficient mice displayed reduced locomotor activity and signs of muscle weakness at 5 months of age. Furthermore, OPG deficiency did not affect the skeletal muscles in 1- and 3-month-old mice. However, it impaired fast-twitch EDL but not slow-twitch Sol muscles in 5-month-old Opg^−/− mice. Moreover, 5-month-old Opg^−/− mice exhibited selective atrophy of fast-twitch-type IIb myofibers, with increased expression of atrophic proteins such as NF-kB, atrogin-1 and MuRF-1. We used an in vitro model to show that RANKL-stimulated C2C12 myotubes significantly increased the expression of NF-kB, atrogin-1 and MuRF-1. A 2-month anti-RANKL treatment starting at 3 months of age in Opg^−/− mice improved voluntary activity, the ex vivo maximum specific force (sP₀) of EDL muscles, and whole limb grip force performance and rescued the biomechanical properties of bone. In conclusion, the deletion of OPG and the disruption of the RANKL/OPG balance induced osteoporosis as well as the selective weakness and atrophy of the powerful fast-twitch IIb myofibers, which was partly alleviated by an anti-RANKL treatment.

Although there is a strong association between osteoporosis and skeletal muscle atrophy/dysfunction, the functional relevance of a particular biological pathway that regulates synchronously bone and skeletal muscle physiopathology is still elusive. Receptor-activator of nuclear factor κB (RANK), its ligand RANKL and the soluble decoy receptor osteoprotegerin (OPG) are the key regulators of osteoclast differentiation and bone remodelling. We thus hypothesized that RANK/RANKL/OPG, which is a key pathway for bone regulation, is involved in Duchenne muscular dystrophy (DMD) physiopathology. Our results show that muscle-specific RANK deletion (mdx-RANK mko ) in dystrophin deficient mdx mice improves significantly specific force [54% gain in force] of EDL muscles with no protective effect against eccentric contraction-induced muscle dysfunction. In contrast, full-length OPG-Fc injections restore the force of dystrophic EDL muscles [162% gain in force], protect against eccentric contraction-induced muscle dysfunction ex vivo and significantly improve functional performance on downhill treadmill and post-exercise physical activity. Since OPG serves a soluble receptor for RANKL and as a decoy receptor for TRAIL, mdx mice were injected with anti-RANKL and anti-TRAIL antibodies to decipher the dual function of OPG. Injections of anti-RANKL and/or anti-TRAIL increase significantly the force of dystrophic EDL muscle [45% and 17% gains in force, respectively]. In agreement, truncated OPG-Fc that contains only RANKL domains produces similar gains, in terms of force production, than anti-RANKL treatments. To corroborate that full-length OPG-Fc also acts independently of RANK/RANKL pathway, dystrophin/RANK double-deficient mice were treated with full-length OPG-Fc for 10 days. Dystrophic EDL muscles exhibited a significant gain in force relative to untreated dystrophin/RANK double-deficient mice, indicating that the effect of full-length OPG-Fc is in part independent of the RANKL/RANK interaction. The sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) activity is significantly depressed in dysfunctional and dystrophic muscles and full-length OPG-Fc treatment increased SERCA activity and SERCA-2a expression. These findings demonstrate the superiority of full-length OPG-Fc treatment relative to truncated OPG-Fc, anti-RANKL, anti-TRAIL or muscle RANK deletion in improving dystrophic muscle function, integrity and protection against eccentric contractions. In conclusion, full-length OPG-Fc represents an efficient alternative in the development of new treatments for muscular dystrophy in which a single therapeutic approach may be foreseeable to maintain both bone and skeletal muscle functions.

Duchenne muscular dystrophy (DMD) is the most severe form of muscular dystrophy which leads to progressive muscle degeneration and inflammation. The receptor activator of nuclear factor NF-κB ligand (RANKL) and its receptor (RANK), which are expressed in bone and skeletal and cardiac muscles, form a signaling network upstream from nuclear factor-kappa B (NF-κB). We thus hypothesized that prolonged silencing RANKL/RANK signaling would significantly improve DMD. We showed that RANK and RANKL protein levels were increased in the microenvironment of myofibers of 5-month-old utrophin haploinsufficient mdx (mdx/utrn^+/-) mice and that a 4 mg/kg dose of anti-RANKL antibody every 3 d for 28 days is optimal and more effective than 1 mg/kg every 3 d for improving the ex vivo maximum specific force (sP₀) of dystrophic EDL muscles from mdx/utrn^+/- mice. This functional improvement was associated with a reduction in muscle edema, damage, and fibrosis and a marked reduction in serum CK levels. The anti-RANKL treatment inhibited the NF-κB pathway, increased the proportion of anti-inflammatory and non-cytotoxic M2 macrophages, and reduced the number of centrally-nucleated myofibers and the frequency of small myofibers, suggesting that anti-RANKL inhibits the cycle of degeneration/regeneration in dystrophic mice. A three-point bending test showed that a 28-d anti-RANKL treatment increases the mechanical properties of bone in mdx/utrn^+/- dystrophic mice. In conclusion, the anti-RANKL treatment protected against skeletal muscle dysfunctions while enhancing bone mechanical properties, filling two needs with one deed in the context of muscular dystrophy.

Sjögren’s syndrome (SS) is a chronic systemic autoimmune disease that affects predominately salivary and lacrimal glands. SS can occur alone or in combination with another autoimmune disease like systemic lupus erythematosus (SLE). Here we report that TLR7 signaling drives the development of SS since TLR8-deficient (TLR8ko) mice that develop lupus due to increased TLR7 signaling by dendritic cells, also develop an age-dependent secondary pathology similar to associated SS. The SS phenotype in TLR8ko mice is manifested by sialadenitis, increased anti-SSA and anti-SSB autoantibody production, immune complex deposition and increased cytokine production in salivary glands, as well as lung inflammation. Moreover, ectopic lymphoid structures characterized by B/T aggregates, formation of high endothelial venules and the presence of dendritic cells are formed in the salivary glands of TLR8ko mice. Interestingly, all these phenotypes are abrogated in double TLR7/8-deficient mice, suggesting that the SS phenotype in TLR8-deficient mice is TLR7-dependent. In addition, evaluation of TLR7 and inflammatory markers in the salivary glands of primary SS patients revealed significantly increased TLR7 expression levels compared to healthy individuals, that were positively correlated to TNF, LT-α, CXCL13 and CXCR5 expression. These findings establish an important role of TLR7 signaling for local and systemic SS disease manifestations, and inhibition of such will likely have therapeutic value.

Systemic lupus erythematosus (SLE) patients have increased prevalence of metabolic syndrome but the underlying mechanisms are unknown. Toll-like receptor 7 (TLR7) that detects single stranded-RNA plays a key role in antimicrobial host defense and also contributes to the initiation and progression of SLE both in mice and humans. Here, we report the implication of TLR7 signaling in high fat diet (HFD)-induced metabolic syndrome and exacerbation of lupus autoimmunity in TLR8-deficient (TLR8ko) mice, which develop spontaneous lupus-like disease due to increased TLR7 signaling by dendritic cells (DCs). The aggravated SLE pathogenesis in HFD-fed TLR8ko mice was characterized by increased overall immune activation, anti-DNA autoantibody production, and IgG/IgM glomerular deposition that were coupled with increased kidney histopathology. Moreover, upon HFD TLR8ko mice developed metabolic abnormalities, including liver inflammation. In contrast, upon HFD TLR7/8ko mice did not develop SLE and both TLR7ko and TLR7/8ko mice were fully protected from metabolic abnormalities, including body weight gain, insulin resistance, and liver inflammation. Interestingly, HFD led to an increase of TLR7 expression in WT mice, that was coupled with increased TNF production by DCs, and this phenotype was more profound in TLR8ko mice. Our study uncovers the implication of TLR7 signaling in the interconnection of SLE and metabolic abnormalities, indicating that TLR7 might be a novel approach as a tailored therapy in SLE and metabolic diseases.