TLR4 Modulates Myokine Gene Transcription Following in Vivo Electrical Muscle Stimulation to Fatigue

Abstract

Muscle contraction stimulates local cytokine release suggesting that these cytokines play a modulatory role in exercise-induced adaptations. A strong link has been established between muscle-derived interleukin 6 (IL6) and exercise capacity in mice. Toll-like receptor 4 (TLR4) is a cellular immune receptor known to initiate a downstream signaling cascade, which promotes pro-inflammatory cytokine production. In skeletal muscle, however, cytokine gene expression could be triggered through multiple signaling pathways. As such, the question remains as to whether TLR4 modulates myokine responses (IL6, monocyte chemotactic protein 1 (MCP1)) in working muscles. PURPOSE: To determine the potential role of skeletal muscleTLR4 in myokine release in response to skeletal muscle contraction. METHODS: We used a muscle-specific TLR4 knock-out mouse model (mTLR4-/-) on a C57BL/6 background and stimulated plantar flexor muscles for 32 min to fatigue in vivo. Gastrocnemius muscles were collected 3 hours after the completion of the fatigue protocol. Total mRNA was extracted to quantify β-actin, IL6, and MCP1 transcriptional levels using qRT-PCR. RESULTS: Wild type (WT) mice exhibited a substantial increase in transcriptional levels of skeletal muscle IL6 (WT: 34 ± 9-fold increase vs. mTLR4-/-: 16.54 ± 4.8-fold increase, P < 0.05) and MCP1 (WT: 148 ± 73-fold increase vs. mTLR4-/-: 74.9 ± 39-fold increase, P < 0.05) following in vivo stimulation but this increase was significantly blunted in the TLR4 knockout littermates. CONCLUSION: These data suggest a link between innate immunity and muscle fatigue in mice. This link may be facilitated, in part, by skeletal muscle TLR4-mediated cytokine response to muscle contraction. Supported by National Institutes of Health-NIDDK (2RO1DK078765, M.W.H)