Restorative Mechanisms Regulating Protein Balance in Skeletal Muscle During Recovery From Sepsis

Abstract

Muscle deconditioning is commonly observed in patients surviving sepsis. Little is known regarding the molecular mechanisms regulating muscle protein homeostasis during the recovery or convalescence phase. We adapted a sepsis-recovery mouse model that uses cecal ligation and puncture (CLP), followed 24 h later by cecal resection and antibiotic treatment, to identify putative cellular pathways regulating protein synthesis and breakdown in skeletal muscle. Ten days after CLP, body weight and food consumption did not differ between control and sepsis-recovery mice, butgastrocnemius weight was reduced. During sepsis-recovery, muscle protein synthesis was increased 2-fold and associated with enhanced mTOR kinase activity (4E-BP1 and S6K1 phosphorylation). The sepsis-induced increase in 4E-BP1 was associated with enhanced formation of the eIF4E-eIF4G active cap-dependent complex, while the increased S6K1 was associated with increased phosphorylation of downstream targets S6 and eIF4B. Proximal to mTOR, sepsis-recovery increased Akt and TSC2 phosphorylation, did not alter AMPK phosphorylation, and decreased REDD1 protein content. Despite the decreased mRNA content for the E3 ubiquitin ligases atrogin-1 and MuRF1, proteasomal activity was increased 50%. In contrast, sepsis-recovery was associated with an apparent decrease in autophagy (e.g., increased ULK-1 phosphorylation, decreased LCB3-II and increased p62). The mRNA content for IL-1β, IL-18, TNFα and IL-6 in muscle was elevated in sepsis-recovery. During recovery after sepsis skeletal muscle responds with an increase in Akt-TSC2-mTOR-dependent protein synthesis and decreased autophagy, but full restoration of muscle protein content may be slowed by the continued stimulation of ubiquitin-proteasome activity.

Inability to replete white adipose tissue during recovery phase of sepsis is associated with increased autophagy, apoptosis, and proteasome activity

“…For the septic mice (n ϭ 30), there were no deaths within the first 24 h or after day 5; overall 10-day mortality was 43%. Other general characteristics of this sepsis-recovery model have been previously reported (12).…”

“…In vivo protein synthesis was determined by the incorporation of puromycin into tissue protein, as previously described (12,13). Thirty minutes before euthanasia and tissue excision, mice received an intraperitoneal injection of puromycin (0.04 mol/g body wt dissolved in sterile saline).…”

Impact of Very Early Physical Therapy During Septic Shock on Skeletal Muscle: A Randomized Controlled Trial

“…In an animal model of sepsis, REDD1 has been shown to play a central role in regulating both synthesis and autophagy in skeletal muscle ( 25 ). In a previous report during recovery from sepsis, mTOR activity was increased together with lower REDD1 expression and autophagy, without changes in UPP E3-ligases ( 39 ). Here, we confirmed those observations on REDD1 and several autophagy markers in the intervention group.…”

Systemic bioinformatics analysis of skeletal muscle gene expression profiles of sepsis

“…Severe sepsis induces skeletal muscle dysfunction ( 24 , 25 ) and, in turn, skeletal muscle dysfunction is associated with the outcome of sepsis ( 12 ). Identifying biomarkers associated with this process may increase our understanding of the underlying mechanisms and allow for the development of therapeutic targets and novel treatment methods.…”