Muscle-Specific Deletion of Toll-like Receptor 4 Impairs Metabolic Adaptation to Wheel Running in Mice.

PURPOSE: Toll-like receptor 4 (TLR4) is an inflammatory receptor expressed ubiquitously in immune cells as well as skeletal muscle and other metabolic tissues. Skeletal muscle develops favorable inflammation-mediated metabolic adaptations from exercise training. Multiple inflammatory myokines, downstream from TLR4, are proposed links to the metabolic benefits of exercise. In addition, activation of TLR4 alters skeletal muscle substrate preference. The role of skeletal muscle TLR4 (mTLR4) in exercise metabolism has not previously been investigated. Herein, we aimed to specifically test the significance of mTLR4 to exercise-induced metabolic adaptations. METHODS: We developed a novel muscle-specific TLR4 knockout (mTLR4-/-) mouse model on C57BL/6J background. Male mTLR4-/- mice and wild-type (WT) littermates were compared under sedentary (SED) and voluntary wheel running (WR) conditions for 4 wk. RESULTS: mTLR4 deletion revealed marked reductions in downstream interleukin-1 receptor-associated kinase-4 (IRAK4) phosphorylation. In addition, the disruption of mTLR4 signaling prominently blunted the metabolic adaptations in WR-mTLR4-/- mice as opposed to substantial improvements exhibited by the WT counterparts. Voluntary WR in WT mice, relative to SED, resulted in significant increases in skeletal muscle fatty acid oxidation, glucose oxidation, and associated mitochondrial enzyme activities, all of which were not significantly changed in mTLR4-/- mice. CONCLUSIONS: This study introduces a novel mTLR4-/- mouse model and identifies mTLR4 as an immunomodulatory effector of exercise-induced metabolic adaptations in skeletal muscle.

Overfeeding and Substrate Availability, But Not Age or BMI, Alter Human Satellite Cell Function.

Satellite cells (SC) aid skeletal muscle growth and regeneration. SC-mediated skeletal muscle repair can both be influenced by and exacerbate several diseases linked to a fatty diet, obesity, and aging. The purpose of this study was to evaluate the effects of different lifestyle factors on SC function, including body mass index (BMI), age, and high-fat overfeeding. For this study, SCs were isolated from the vastus lateralis of sedentary young (18-30 years) and sedentary older (60-80 years) men with varying BMIs (18-32 kg/m2), as well as young sedentary men before and after four weeks of overfeeding (OVF) (55% fat/ + 1000 kcal, n = 4). The isolated SCs were then treated in vitro with a control (5 mM glucose, 10% fetal bovine serum (FBS)) or a high substrate growth media (HSM) (10% FBS, 25 mM glucose, and 400 µM 2:1 oleate-palmitate). Cells were assessed on their ability to proliferate, differentiate, and fuel substrate oxidation after differentiation. The effect of HSM was measured as the percentage difference between SCs exposed to HSM compared to control media. In vitro SC function was not affected by donor age. OVF reduced SC proliferation rates (-19% p < 0.05) but did not influence differentiation. Cellular proliferation in response to HSM was correlated to the donor's body mass index (BMI) (r2 = 0.6121, p < 0.01). When exposed to HSM, SCs from normal weight (BMI 18-25 kg/m2) participants exhibited reduced proliferation and fusion rates with increased fatty-acid oxidation (p < 0.05), while SCs from participants with higher BMIs (BMI 25-32 kg/m2) demonstrated enhanced proliferation in HSM. HSM reduced proliferation and fusion (p < 0.05) in SCs isolated from subjects before OVF, whereas HSM exposure accelerated proliferation and fusion in SCs collected following OVF. These results indicated that diet has a greater influence on SC function than age and BMI. Though age and BMI do not influence in vitro SC function when grown in controlled conditions, both factors influenced the response of SCs to substrate challenges, indicating age and BMI may mediate responses to diet.

Selective overexpression of Toll-like receptor-4 in skeletal muscle impairs metabolic adaptation to high-fat feeding.

Toll-like receptor-4 (TLR-4) is elevated in skeletal muscle of obese humans, and data from our laboratory have shown that activation of TLR-4 in skeletal muscle via LPS results in decreased fatty acid oxidation (FAO). The purpose of this study was to determine whether overexpression of TLR-4 in skeletal muscle alters mitochondrial function and whole body metabolism in the context of a chow and high-fat diet. C57BL/6J mice (males, 6-8 mo of age) with skeletal muscle-specific overexpression of the TLR-4 (mTLR-4) gene were created and used for this study. Isolated mitochondria and whole muscle homogenates from rodent skeletal muscle (gastrocnemius and quadriceps) were investigated. TLR-4 overexpression resulted in a significant reduction in FAO in muscle homogenates; however, mitochondrial respiration and reactive oxygen species (ROS) production did not appear to be affected on a standard chow diet. To determine the role of TLR-4 overexpression in skeletal muscle in response to high-fat feeding, mTLR-4 mice and WT control mice were fed low- and high-fat diets for 16 wk. The high-fat diet significantly decreased FAO in mTLR-4 mice, which was observed in concert with elevated body weight and fat, greater glucose intolerance, and increase in production of ROS and cellular oxidative damage compared with WT littermates. These findings suggest that TLR-4 plays an important role in the metabolic response in skeletal muscle to high-fat feeding.