Redox regulation of mitophagy in the lung during murine Staphylococcus aureus sepsis

Abstract

Background Oxidative mitochondrial damage is closely linked to inflammation and to cell death, but low levels of reactive oxygen and nitrogen species serve as signals that involve mitochondrial repair and resolution of inflammation. More specifically, cytoprotection relies on the elimination of damaged mitochondria by selective autophagy (mitophagy) during mitochondrial quality control. Objective To identify and localize mitophagy in mouse lung as a potentially up-regulatable redox response to S. aureus sepsis. Methods Anesthetized C57BL/6 and B6.129X1-Nfe2l2tm1Ywk/J (Nrf2−/−) mice had fibrin clots loaded with S. aureus (1×107 CFU) implanted abdominally. At the time of implantation, mice were given Vancomycin (6mg/kg) and fluid resuscitation. Mouse lungs were harvested at 0, 6, 24, and 48 hours for bronchoalveolar lavage (BAL), Western blot analysis and qRT-PCR. To localize mitochondria with autophagy protein LC3, we used lung immunofluorescence staining in LC3-GFP transgenic mice. Results In C57BL/6 mice, sepsis-induced pulmonary inflammation was detected by significant increases in mRNA for the inflammatory markers IL-1β and TNF-α at 6h and 24h respectively hours. BAL cell count and protein increased. Sepsis suppressed lung Beclin-1 protein, but not mRNA, suggesting activation of canonical autophagy. Notably sepsis also increased the LC3-II autophagosome marker, as well as the lung’s non-canonical autophagy pathway as evidenced by loss of p62, a redox-regulated scaffolding protein of the autophagosome. In LC3-GFP mice lungs, immunofluorescence staining showed co-localization of LC3-II to mitochondria, mainly in Type 2 epithelium and alveolar macrophages. In contrast, marked accumulation of p62, as well as attenuation of LC3-II in Nrf2 KO mice supported an overall decrease in autophagic turnover. Conclusions The down-regulation of canonical autophagy during sepsis may contribute to lung inflammation while the switch to non-canonical autophagy selectively removes damaged mitochondria and accompanies tissue repair and cell survival. Furthermore, mitophagy in the alveolar region appears to depend on activation of Nrf2. Thus, efforts to promote mitophagy may be a useful therapeutic adjunct for acute lung injury in sepsis.

Mitochondria in lung disease

“…In contrast, loss of mitochondrial biogenesis is associated with COPD, which may be associated with a significantly lower body mass index and lower muscle mass (ref. 48 and (44) PGC1-α and TFAM increased after S. aureus sepsis in the distal lung (44) Increased in ALI, pneumonia, hyperoxia (43) Increased upon S. aureus-associated sepsis (44), may be associated with resolution of lung injury (42) Increased in bronchial smooth muscle remodeling in asthma (46) Increased in lung cancer (47) Reduced in COPD, which may be associated with a significantly lower body mass index and less muscle mass (48) (53), and loss of PINK1 and defective mitophagy promote pulmonary fibrosis (PF) in animal models and in human idiopathic pulmonary fibrosis (IPF) (54,55). PF is characterized by irreversible destruction of lung architecture, abnormal wound healing, and deposition of extracellular matrix (ECM) proteins, leading to disruption of gas exchange and death from respiratory failure.…”

“…PINK1-induced mitophagy triggers pulmonary vascular remodelesis can occur in distal lung cells, including type II AECs (42), in small blood vessel SMCs, and in inflammatory cells of the alveolar region (9) and is thought to arise during growth, conditions of high-energy demand, or cellular stress. In the parenchyma, type II AECs initiate mitochondrial biogenesis during acute lung injury (ALI), pneumonia, hyperoxic lung injury (43), and Staphylococcus aureus-associated sepsis (44). ALI, acute respiratory distress syndrome (ARDS), and sepsis remain significant sources of morbidity and mortality in the critically ill patient population (45).…”

“…Other mitochondrial-targeted therapeutic strategies that could potentially be used to treat lung diseases include the use of metabolic modulating compounds such as dicholoroacetate (138), histone deacetylase inhibitors (139,140), and fission inhibitors. Stimulating adaptive mitochondrial biogenesis and mitophagy may be a useful adjuvant therapy for ALI in sepsis (44), and transfer of mitochondria from bone marrowderived mesenchymal stem cells to injured alveolar epithelium may be beneficial in ARDS, asthma, or COPD (141).…”

Autophagy in sepsis: Degradation into exhaustion?

“…67 In the lungs, mitophagy in alveoli appears to be dependent on NFE2L2/ NRF2. 68 Interestingly, reduced autophagic activity may prevent exocytosis of cytosolic mitochondrial DNA, perhaps conferring protection for the host. In Atg5 C/C murine embryonic fibroblasts and rat hepatocytes, mitochondrial constituents are actively released from the cell, but not from atg5 ¡/¡ cells.…”

“…70 Interestingly, deletion of Atg16l1 in endothelial cells is more lethal than that in macrophages or dendritic cells. 68 The interaction between bacteria and autophagy, or xenophagy, has been comprehensively reviewed by Huang and colleagues. 72 Autophagy activator: An emerging therapeutic strategy for sepsis…”