Obesity-associated low-grade inflammation in metabolically relevant tissues contributes to insulin resistance. We recently reported monocyte/macrophage infiltration in mouse and human skeletal muscles. However, the molecular triggers of this infiltration are unknown, and the role of muscle cells in this context is poorly understood. Animal studies are not amenable to the specific investigation of this vectorial cellular communication. Using cell cultures, we investigated the crosstalk between myotubes and monocytes exposed to physiological levels of saturated and unsaturated fatty acids. Media from L6 myotubes treated with palmitate—but not palmitoleate—induced THP1 monocyte migration across transwells. Palmitate activated the Toll-like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) pathway in myotubes and elevated cytokine expression, but the monocyte chemoattracting agent was not a polypeptide. Instead, nucleotide degradation eliminated the chemoattracting properties of the myotube-conditioned media. Moreover, palmitate-induced expression and activity of pannexin-3 channels in myotubes were mediated by TLR4-NF-κB, and TLR4-NF-κB inhibition or pannexin-3 knockdown prevented monocyte chemoattraction. In mice, the expression of pannexin channels increased in adipose tissue and skeletal muscle in response to high-fat feeding. These findings identify pannexins as new targets of saturated fatty acid–induced inflammation in myotubes, and point to nucleotides as possible mediators of immune cell chemoattraction toward muscle in the context of obesity.

-induced inflammatory pathways in human adipose microvascular endothelial cells promote monocyte adhesion and impair insulin transcytosis.

The mechanisms underpinning decreased skeletal muscle strength and slowing of movement during aging are ill-defined. "Inflammaging," increased inflammation with advancing age, may contribute to aspects of sarcopenia, but little is known about the participatory immune components. We discovered that aging was associated with increased caspase-1 activity in mouse skeletal muscle. We hypothesized that the caspase-1-containing NLRP3 inflammasome contributes to sarcopenia in mice. Male C57BL/6J wild-type (WT) and NLRP3 mice were aged to 10 (adult) and 24 mo (old). NLRP3 mice were protected from decreased muscle mass (relative to body mass) and decreased size of type IIB and IIA myofibers, which occurred between 10 and 24 mo of age in WT mice. Old NLRP3 mice also had increased relative muscle strength and endurance and were protected from age-related increases in the number of myopathic fibers. We found no evidence of age-related or NLRP3-dependent changes in markers of systemic inflammation. Increased caspase-1 activity was associated with GAPDH proteolysis and reduced GAPDH enzymatic activity in skeletal muscles from old WT mice. Aging did not alter caspase-1 activity, GAPDH proteolysis, or GAPDH activity in skeletal muscles of NLRP3 mice. Our results show that the NLRP3 inflammasome participates in age-related loss of muscle glycolytic potential. Deletion of NLRP3 mitigates both the decline in glycolytic myofiber size and the reduced activity of glycolytic enzymes in muscle during aging. We propose that the etiology of sarcopenia involves direct communication between immune responses and metabolic flux in skeletal muscle.

body size without altering the balance between bone formation and bone resorption. Interestingly, loss of JAK2 impaired expression of IGF1 in osteoclasts, suggesting an important role for osteoclast-derived IGF1 in determination of body size. We also show that overexpression of circulating IGF1 in Oc-JAK2-KO mice rescued their growth defects, supporting a causal role between JAK2-mediated IGF1 expression in osteoclasts and postnatal growth. Results Generation of Oc-JAK2-KO mice. To validate the osteoclast specificity of the Cathepsin K-Cre (Ctsk-Cre) mouse, we bred Ctsk-Cre + mice to mTmG +/reporter mice whose tissue expressed either fluorescence TdTomato in the absence of Cre-mediated recombination or enhanced GFP (EGFP) in its presence. Sections of the knee joint showed robust expression of EGFP along the surface of the trabecular bone in Ctsk-Cre + mT-mG + mice (Figure 1A). Staining for tartrate resistant alkaline phosphatase (TRAP) on the adjacent serial section confirmed that EGFP was present in the expected distribution for osteoclasts. EGFP was not detected in Crecontrols. Sections of other tissues, including growth plate cartilage, brain, hypothalamus, testis, ovary, pancreas, liver, small intestine, kidney, and thyroid did not express EGFP (Figure 1B). Next, to generate Oc-JAK2-KO mice, Ctsk-Cre + mice were bred to Jak2 floxed mice (Jak2 fl/fl) and resultant Ctsk-Cre +-Jak2 fl/+ mice were crossed to Jak2 fl/fl mice to generate KO mice (Ctsk-Cre + Jak2 fl/fl) and littermate control mice (Jak2 fl/fl and Ctsk-Cre + Jak2 fl/+). To validate osteoclast-specific deficiency of JAK2 in our Oc-JAK2-KO mouse, we measured Jak2 mRNA levels in tissues and in BM-derived osteoclasts from Ctsk-Cre + Jak2 fl/fl and Jak fl/fl mice. JAK2 mRNA expression was reduced by 26% in bone and by 70% in osteoclast cultures from Ctsk-Cre + Jak2 fl/fl mice (Figure 1C). Importantly, Jak2 mRNA expression was unchanged in BM-derived macrophages, liver, testis, ovary, or hypothalamus. There have been previous reports of Ctsk-Cre mediating germline gene deletion of certain floxed alleles (12); however, this did not occur in Ctsk-Cre + Jak2 fl/fl mice, since Jak2 expression was preserved in nonosteoclast cells and tissues. Lastly, we performed IHC for JAK2 on femur sections with TRAP staining on the adjacent serial section to identify TRAP + multinucleated giant cells. In control mice, there was weak to moderate immunostaining for JAK2 in TRAP + multinucleated giant cells (Figure 1D). In Ctsk-Cre + Jak2 fl/fl mice, immunoreactivity for JAK2 was absent in TRAP + multinucleated giant cells, with retained expression in other BM cells. Overall, these results indicate that Ctsk-Cre + Jak2 fl/fl mice represent a rigorous experimental model to study the role of osteoclast JAK2 in vivo. For all experiments, both Jak2 fl/fl and Ctsk-Cre + Jak2 fl/+ mice were used as controls in approximately equal proportions unless otherwise specified. There were no significant differences in any measured outcomes between Jak2 fl/fl and Ctsk-Cre + Jak2 fl/+ mice, in...