Aging Alters Inflammatory Gene Expression in Acute Lung Injury-associated Skeletal Muscle Wasting

Abstract RATIONALE Muscle weakness in critical illnesses such as acute respiratory distress syndrome (ARDS) is a common and significant complication that disproportionately affects older adults. ICU-acquired weakness (ICUAW) in older adults is associated with delayed recovery as well as impaired long term disability. The mechanisms underlying ICUAW and aging remain unclear. Aging in the general population affects both skeletal muscle and the immune system with alterations in baseline immune homeostasis and impaired immune responses to acute injury (“inflammaging”). We have previously found in adult mice, ablation of macrophages during muscle recovery increase muscle fiber size and function. We aim to explore this response in older mice. We hypothesize that in a pre-clinical model of ARDS-associated muscle wasting (acute lung injury [ALI] mice), aging alters inflammatory gene expression in skeletal muscle. METHODS Our lab has previously demonstrated aging is associated with increased susceptibility to ALI induced muscle wasting, mortality, and skeletal muscle dysfunction in aged (20 month) versus adult (6 month) ALI mice. In the present study, ALI was induced in male aged and adult C57BL/6J mice using intratracheal Escherichia coli LPS (IT-LPS) or sterile water (sham). RNA was isolated from gastrocnemius muscles homogenate from sham and ALI day 3, and we performed quantitative reverse transcription PCR. RESULTS There is increased transcription of proinflammatory cytokine IL1β in skeletal muscle of aged ALI mice at ALI day 3, a peak injury timepoint. TNFα was higher in aged ALI mice at this time point as well; however, it was similar to sham aged mice, possibly reflecting chronic inflammation in older mice. There is no significant difference in gene expression of F4/80, a marker of macrophages. There is increased transcription of ARG1, a metabolic enzyme associated with alternatively (M2) polarized macrophages. Cytokines IL6, MCP-1 and IL10 were similar between groups. CONCLUSION Aging is associated with altered inflammatory gene expression in ALI associated skeletal muscle. This suggests that aged lung injured mice have an exaggerated or ineffective muscle cytokine response, which could be from either myofibers or leukocytes. Identifying the origin of the altered cytokine response in aging could lead to a better understanding of heterogeneity in ICUAW and targeted treatment.