The cGAS-STING pathway connects mitochondrial damage to inflammation in burn-induced acute lung injury in rat

Abstract

Objective: Damage associated molecular patterns (DAMPs) are pathological mediators linking local tissue damage to systemic inflammation in various diseases. Some DAMPs, such as mitochondrial DNA (mtDNA), can be recognized by the cytoplasmic cGAS protein to trigger the activation of the stimulator of interferon genes (STING)-dependent innate immune pathway responsible for infection or sterile inflammation. The objective of our study was to evaluate the association between circulating mtDNA and cGAS-STING pathway activation in mediating inflammation following burn injury.

Methods: 48 adult Sprague-Dawley male rats were divided into eight groups (Sham, 2, 4, 8, 12, 24, 48, 72 h after burn injury). The animals underwent 40% total body surface area scald injury to produce a full-thickness burn. Plasma samples were collected via cardiac puncture under deep anesthesia. Tissues were harvested and placed in formalin, followed by paraffin embedment. Total plasma DNA was isolated followed by measurement of mtDNA using quantitative polymerase chain reaction. Haemotoxylin-Eosin stain and Western blot was used for lung histology and protein assays, respectively. Statistical analyses were performed using ANOVA and student's t-test and represented as mean ± s.d.

Results: Plasma mtDNA trended upward at early time-points following burn injury with peak levels at 8 h after burn when compared to the control group (345 ± 83.4 copies/μl vs. 239 ± 43.1 copies/μl, p = 0.07) and followed a bell-shaped distribution. Lung slices from burned rats showed acute injury marked by increased inflammatory infiltrate, with the maximum changes seen at 24 h, accompanied with significant upregulation of neutrophil elastase (p = 0.04). Compared with sham animals, cGAS and STING protein levels in lung tissue were up-regulated at 4 and 8 h after burn (p = 0.03 and p < 0.001, respectively).

Conclusion: Activation of the cGAS-STING pathway by increased plasma mtDNA is an important pathway driving neutrophil infiltration in burn-induced acute lung injury in rats. A further understanding of the STING-mediated immunopathology in lung and other susceptible organs may be important for the development of novel therapies for burn injury.

Keywords: Burn injury; DAMP; Innate immune; Lung; Mitochondrial DNA; STING; cGAS.