Sepsis attenuates the anabolic response to skeletal muscle contraction

Electrically stimulated muscle contraction is a potential clinical therapy to treat sepsis-induced myopathy; however, whether sepsis alters contraction-induced anabolic signaling is unknown. Polymicrobial peritonitis was produced by cecal ligation and puncture (CLP) in male C57BL/6 mice and time-matched, pair-fed controls (CON). At ∼24 h post-CLP, the right hindlimb was electrically stimulated via the sciatic nerve to evoke maximal muscle contractions, and the gastrocnemius was collected 2 h later. Protein synthesis was increased by muscle contraction in CON mice. Sepsis suppressed the rate of synthesis in both the nonstimulated (31%) and stimulated (57%) muscle versus CON. Contraction of muscle in CON mice increased the phosphorylation of mTORC1 (mammalian target of rapamycin [mTOR] complex 1) substrates S6K1 (70-kd ribosomal protein S6 kinase 1) Thr³⁸⁹ (8-fold), S6K1 Thr^421/Ser⁴²⁴ (7-fold) and 4E-BP1 Ser⁶⁵(11-fold). Sepsis blunted the contraction-induced phosphorylation of S6K1 Thr³⁸⁹ (67%), S6K1 Thr^421/Ser⁴²⁴ (46%), and 4E-BP1 Ser⁶⁵ (85%). Conversely, sepsis did not appear to modulate protein elongation as eEF2 Thr⁵⁶ phosphorylation was decreased similarly by muscle contraction in both groups. Mitogen-activated protein kinase signaling was discordant following contraction in septic muscle; phosphorylation of extracellular signal-regulated kinase Thr²⁰²/Tyr²⁰⁴ and p38 Thr¹⁸⁰/Tyr¹⁸² was increased similarly in both CON and CLP mice, while sepsis prevented the contraction-induced phosphorylation of JNK Thr¹⁸³/Tyr¹⁸⁵ and c-JUN Ser⁶³. The expression of interleukin 6 and tumor necrosis factor α (TNF-α) mRNA in muscle was increased by sepsis, and contraction increased TNF-α to a greater extent in muscle from septic than CON mice. Injection of the mTOR inhibitor Torin2 in separate mice confirmed that contraction-induced increases in S6K1 and 4E-BP1 were mTOR mediated. These findings demonstrate that resistance to contraction-induced anabolic signaling occurs during sepsis and is predominantly mTORC1-dependent.