Plasma mitochondrial DNA levels are associated with acute lung injury and mortality in septic patients

The emerging role of mtDNA release in sepsis: Current evidence and potential therapeutic targets

Sepsis is a systemic inflammatory reaction caused by infection, and severe sepsis can develop into septic shock, eventually leading to multiorgan dysfunction and even death. In recent years, studies have shown that mitochondrial damage is closely related to the occurrence and development of sepsis. Recent years have seen a surge in concern over mitochondrial DNA (mtDNA), as anomalies in this material can lead to cellular dysfunction, disruption of aerobic respiration, and even death of the cell. In this review, we discuss the latest findings on the mechanisms of mitochondrial damage and the molecular mechanisms controlling mitochondrial mtDNA release. We also explored the connection between mtDNA misplacement and inflammatory activation. Additionally, we propose potential therapeutic targets of mtDNA for sepsis treatment.

Cell-free DNA in patients with sepsis: long term trajectory and association with 28-day mortality and sepsis-associated acute kidney injury

Introduction

Outcome-prediction in patients with sepsis is challenging and currently relies on the serial measurement of many parameters. Standard diagnostic tools, such as serum creatinine (SCr), lack sensitivity and specificity for acute kidney injury (AKI). Circulating cell-free DNA (cfDNA), which can be obtained from liquid biopsies, can potentially contribute to the quantification of tissue damage and the prediction of sepsis mortality and sepsis-associated AKI (SA-AKI).

Methods

We investigated the clinical significance of cfDNA levels as a predictor of 28-day mortality, the occurrence of SA-AKI and the initiation of renal replacement therapy (RRT) in patients with sepsis. Furthermore, we investigated the long-term course of cfDNA levels in sepsis survivors at 6 and 12 months after sepsis onset. Specifically, we measured mitochondrial DNA (mitochondrially encoded NADH-ubiquinone oxidoreductase chain 1, mt-ND1, and mitochondrially encoded cytochrome C oxidase subunit III, mt-CO3) and nuclear DNA (nuclear ribosomal protein S18, n-Rps18) in 81 healthy controls and all available samples of 150 intensive care unit patients with sepsis obtained at 3 ± 1 days, 7 ± 1 days, 6 ± 2 months and 12 ± 2 months after sepsis onset.

Results

Our analysis revealed that, at day 3, patients with sepsis had elevated levels of cfDNA (mt-ND1, and n-Rps18, all p<0.001) which decreased after the acute phase of sepsis. 28-day non-survivors of sepsis (16%) had higher levels of cfDNA (all p<0.05) compared with 28-day survivors (84%). Patients with SA-AKI had higher levels of cfDNA compared to patients without AKI (all p<0.05). Cell-free DNA was also significantly increased in patients requiring RRT (all p<0.05). All parameters improved the AUC for SCr in predicting RRT (AUC=0.88) as well as APACHE II in predicting mortality (AUC=0.86).

Conclusion

In summary, cfDNA could potentially improve risk prediction models for mortality, SA-AKI and RRT in patients with sepsis. The predictive value of cfDNA, even with a single measurement at the onset of sepsis, could offer a significant advantage over conventional diagnostic methods that require repeated measurements or a baseline value for risk assessment. Considering that our data show that cfDNA levels decrease after the first insult, future studies could investigate cfDNA as a "memoryless" marker and thus bring further innovation to the complex field of SA-AKI diagnostics.

The mitochondrial signature of cultured endothelial cells in sepsis: Identifying potential targets for treatment

Sepsis is the most common cause of death from infection in the world. Unfortunately, there is no specific treatment for patients with sepsis, and management relies on infection control and support of organ function. A better understanding of the underlying pathophysiology of this syndrome will help to develop innovative therapies. In this regard, it has been widely reported that endothelial cell activation and dysfunction are major contributors to the development of sepsis. This review aims to provide a comprehensive overview of emerging findings highlighting the prominent role of mitochondria in the endothelial response in in vitro experimental models of sepsis. Additionally, we discuss potential mitochondrial targets that have demonstrated protective effects in preclinical investigations against sepsis. These promising findings hold the potential to pave the way for future clinical trials in the field.

Low mitochondrial DNA copy number in peripheral blood mononuclear cells is associated with future mortality risk: a long-term follow-up study from Japan

OBJECTIVES

The mitochondrial DNA (mtDNA) is unique and circular with multiple copies of the genome. The lower mtDNA copy number (mtDNA-CN) in leukocytes is associated with the risk of all-cause mortality. However, its long-term association is unknown. Thus, the study examined the association between mtDNA-CN and the risk of all-cause mortality in a long-term follow-up study in the Japanese population.

DESIGN

This longitudinal study included the study cohort from an annual, population-based health checkup in the town of Yakumo, Hokkaido, Japan.

SETTING AND PARTICIPANTS

814 participants (baseline age range: 38-80 years, mean: 56.3 years) were included in this study in 1990. They were followed-up regarding mortality for about 30 years (median: 28.1 years) till 2019.

MEASURES

The genomic DNA was extracted from peripheral blood mononuclear cells and the mtDNA-CN was measured using real-time polymerase chain reaction. The level of the mtDNA-CN was divided into tertiles (low, middle, and high). The participants were categorized based on their age into middle-aged (<60 years old) or old-aged (≥60 years old). Survival analysis was performed for tertile of mtDNA-CN and compared using the log-rank test. Univariate and multivariable Cox proportional hazard regression analyses were performed to assess the association between mtDNA-CN and all-cause mortality. The model adjusted with age, sex, body mass index, systolic blood pressure, smoking habit, alcohol consumption, exercise habit, and education level.

RESULTS

The low levels of mtDNA-CN resulted in a significant decrease in cumulative survival rate (P <  0.05). The risk of mortality was significantly higher in the middle-aged cohort when mtDNA-CN levels were low (hazard ratios [95% confidence intervals]: 1.98 [1.10-3.56]).

CONCLUSION

This study demonstrated that leukocyte mtDNA-CN is associated with future mortality risk. Our study findings may lead to further research on the early prediction of mortality and its underlying mechanisms.

Plasma biomarkers for brain injury in extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO) is a life-saving intervention for patients with refractory cardiorespiratory failure. Despite its benefits, ECMO carries a significant risk of neurological complications, including acute brain injury (ABI). Although standardized neuromonitoring and neurological care have been shown to improve early detection of ABI, the inability to perform neuroimaging in a timely manner is a major limitation in the accurate diagnosis of neurological complications. Therefore, blood-based biomarkers capable of detecting ongoing brain injury at the bedside are of great clinical significance. This review aims to provide a concise review of the current literature on plasma biomarkers for ABI in patients on ECMO support.

The Anti-Oxidative, Anti-Inflammatory, Anti-Apoptotic, and Anti-Necroptotic Role of Zinc in COVID-19 and Sepsis

Zinc is a structural component of proteins, functions as a catalytic co-factor in DNA synthesis and transcription of hundreds of enzymes, and has a regulatory role in protein-DNA interactions of zinc-finger proteins. For many years, zinc has been acknowledged for its anti-oxidative and anti-inflammatory functions. Furthermore, zinc is a potent inhibitor of caspases-3, -7, and -8, modulating the caspase-controlled apoptosis and necroptosis. In recent years, the immunomodulatory role of zinc in sepsis and COVID-19 has been investigated. Both sepsis and COVID-19 are related to various regulated cell death (RCD) pathways, including apoptosis and necroptosis. Lack of zinc may have a negative effect on many immune functions, such as oxidative burst, cytokine production, chemotaxis, degranulation, phagocytosis, and RCD. While plasma zinc concentrations decline swiftly during both sepsis and COVID-19, this reduction is primarily attributed to a redistribution process associated with the inflammatory response. In this response, hepatic metallothionein production increases in reaction to cytokine release, which is linked to inflammation, and this protein effectively captures and stores zinc in the liver. Multiple regulatory mechanisms come into play, influencing the uptake of zinc, the binding of zinc to blood albumin and red blood cells, as well as the buffering and modulation of cytosolic zinc levels. Decreased zinc levels are associated with increasing severity of organ dysfunction, prolonged hospital stay and increased mortality in septic and COVID-19 patients. Results of recent studies focusing on these topics are summarized and discussed in this narrative review. Existing evidence currently does not support pharmacological zinc supplementation in patients with sepsis or COVID-19. Complementation and repletion should follow current guidelines for micronutrients in critically ill patients. Further research investigating the pharmacological mechanism of zinc in programmed cell death caused by invasive infections and its therapeutic potential in sepsis and COVID-19 could be worthwhile.

Plasma Mitochondrial DNA and Necroptosis as Prognostic Indicators in Critically Ill Patients with Sepsis

Mitochondrial DNA (mtDNA) has been identified as a biomarker for predicting sepsis mortality. Although preclinical studies suggested that necroptosis could explain the mechanistic link of mtDNA in sepsis, this is not yet evident in patients with sepsis. This study evaluated the association between mtDNA and essential necroptosis mediators in prospectively enrolled patients with sepsis. Plasma mtDNA copy number was measured using quantitative PCR assay and necroptosis mediators, including receptor-interacting protein kinase-3 (RIPK3), mixed lineage domain-like pseudokinase (MLKL), and high-mobility group box 1 (HMGB1), were measured by ELISA. Receiver operating characteristic (ROC) analysis was conducted to evaluate the predictive ability of mtDNA copy number as a predictor of hospital mortality. Among the 142 patients with sepsis, the mtDNA copy number was significantly higher in non-survivors than in survivors (median, 4040 copies/µL vs. 2585 copies/µL; p < 0.001), and the area under the ROC curve was 0.73 (95% CI, 0.64−0.82) for the relationship between mtDNA and hospital mortality. Furthermore, the correlation between mtDNA copy number and each necroptosis mediator was excellent (p < 0.001 for all): RIPK3 (r = 0.803), MLKL (r = 0.897), and HMGB1 (r = 0.603). The plasma mtDNA copy number was highly correlated with essential necroptosis mediators, suggesting that mtDNA propagates necroptosis and increases sepsis mortality.

Deciphering the role of damage-associated molecular patterns and inflammatory responses in acute lung injury

Acute lung injury (ALI) is characterized by diffuse pulmonary infiltrates and causes great mortality. ALI presents with overproduction of proinflammatory cytokines, cell death, destruction of alveoli-endothelial barriers, and neutrophil infiltration in lung tissues. Damage-associated molecular patterns (DAMPs) are molecules released from damaged cells due to infection, trauma, etc. DAMPs activate innate and adaptive immunity, trigger inflammatory responses, and are important in the initiation and development of ALI. We reviewed the literatures on DAMPs in ALI. Alveolar macrophages (AMs), neutrophils, and epithelial cells (AECs) are important in the pathogenesis of ALI. We comprehensively analyzed the interaction between DAMPs and AMs, alveolar neutrophils, and AECs. During the initial stage of ALI, ruptured cell membranes or destroyed mitochondria release DAMPs. DAMPs activate the inflammasome in nearby sentinel immune cells, such as AMs. AMs produce IL-1β and other cytokines. These mediators upregulate adhesion molecules of the capillary endothelium that facilitate neutrophil recruitment. The recruited neutrophils detect DAMPs using formyl peptide receptors on the membrane, guiding their migration to the injured site. The accumulation of immune cells, cytokines, chemokines, proteases, etc., results in diffuse alveolar damage and pulmonary hyperpermeability with protein-rich fluid retention. Some clinical studies have shown that patients with ALI with higher circulating DAMPs have higher mortality rates. In conclusion, DAMPs are important in the initiation and progression of ALI. The interactions between DAMPs and AMs, neutrophils, and AECs are important in ALI. This review comprehensively addresses the mechanisms of DAMPs and their interactions in ALI.

A sesquiterpene isolated from the stems and leaves of Dioscorea opposita thunb. Transforms the composition of immune cells through ERβ in a mouse model of LPS-induced lung injury

Acute lung injury (ALI) is a common critical disease with a high mortality rate. Natural products have marked efficacy in the prevention and treatment of ALI, in addition, estrogen and its receptors are involved in the pathogenesis and development of lung injury. Our previous research shows that sesquiterpenes isolated from the stems and leaves of Dioscorea opposita Thunb. have anti-inflammatory and estrogenic-like activity. In the present study, sesquiterpene (A1) is a natural extract from the stems and leaves of Dioscorea opposita Thunb. with a view to determining whether A1 can improve lung function in a mouse model of LPS-induced ALI and exploring the involvement of the estrogen receptor β (ERβ) pathway. A1 (20 or 40 mg/kg, i. g., 2 times/day) was administered for 3 d, followed by the induction of ALI via an intratracheal LPS drip (5 mg/kg/2 h). The lung function and levels of inflammation, immune cells, apoptosis, and ERβ expression were examined. The antagonistic activity of specific ERβ blocker (THC, 1 μM) against A1 (20 μM) in co-cultured BEAS-2B cells and splenic lymphocytes induced with LPS (1 μg/mL, 24 h) was also investigated to assess whether the observed effects of A1 were mediated by ERβ. A1 improved lung function, regulated the immune system, and decreased inflammation and apoptosis. Moreover, A1 increased the expression of ERβ in LPS-induced mice, and antagonism of ERβ decreased the protective effects of A1 in a co-culture system. A1 had anti-ALI effects that might partially mediated through ERβ signaling. Our data provide molecular justification for the use of A1 in the treatment of ALI.

Role of released mitochondrial DNA in acute lung injury

Acute lung injury(ALI)/acute respiratory distress syndrome(ARDS) is a form of acute-onset hypoxemic respiratory failure characterised by an acute, diffuse, inflammatory lung injury, and increased alveolar-capillary permeability, which is caused by a variety of pulmonary or nonpulmonary insults. Recently, aberrant mitochondria and mitochondrial DNA(mtDNA) level are associated with the development of ALI/ARDS, and plasma mtDNA level shows the potential to be a promising biomarker for clinical diagnosis and evaluation of lung injury severity. In mechanism, the mtDNA and its oxidised form, which are released from impaired mitochondria, play a crucial role in the inflammatory response and histopathological changes in the lung. In this review, we discuss mitochondrial outer membrane permeabilisation (MOMP), mitochondrial permeability transition pore(mPTP), extracellular vesicles (EVs), extracellular traps (ETs), and passive release as the principal mechanisms for the release of mitochondrial DNA into the cytoplasm and extracellular compartments respectively. Further, we explain how the released mtDNA and its oxidised form can induce inflammatory cytokine production and aggravate lung injury through the Toll-like receptor 9(TLR9) signalling, cytosolic cGAS-stimulator of interferon genes (STING) signalling (cGAS-STING) pathway, and inflammasomes activation. Additionally, we propose targeting mtDNA-mediated inflammatory pathways as a novel therapeutic approach for treating ALI/ARDS.

The Role of Mitochondria in the Immune Response in Critical Illness

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency Medicine 2022. Other selected articles can be found online at https://www.biomedcentral.com/collections/annualupdate2022 . Further information about the Annual Update in Intensive Care and Emergency Medicine is available from https://link.springer.com/bookseries/8901 .

Role of Mitochondrial Nucleic Acid Sensing Pathways in Health and Patho-Physiology

Mitochondria, in symbiosis with the host cell, carry out a wide variety of functions from generating energy, regulating the metabolic processes, cell death to inflammation. The most prominent function of mitochondria relies on the oxidative phosphorylation (OXPHOS) system. OXPHOS heavily influences the mitochondrial-nuclear communication through a plethora of interconnected signaling pathways. Additionally, owing to the bacterial ancestry, mitochondria also harbor a large number of Damage Associated Molecular Patterns (DAMPs). These molecules relay the information about the state of the mitochondrial health and dysfunction to the innate immune system. Consequently, depending on the intracellular or extracellular nature of detection, different inflammatory pathways are elicited. One group of DAMPs, the mitochondrial nucleic acids, hijack the antiviral DNA or RNA sensing mechanisms such as the cGAS/STING and RIG-1/MAVS pathways. A pro-inflammatory response is invoked by these signals predominantly through type I interferon (T1-IFN) cytokines. This affects a wide range of organ systems which exhibit clinical presentations of auto-immune disorders. Interestingly, tumor cells too, have devised ingenious ways to use the mitochondrial DNA mediated cGAS-STING-IRF3 response to promote neoplastic transformations and develop tumor micro-environments. Thus, mitochondrial nucleic acid-sensing pathways are fundamental in understanding the source and nature of disease initiation and development. Apart from the pathological interest, recent studies also attempt to delineate the structural considerations for the release of nucleic acids across the mitochondrial membranes. Hence, this review presents a comprehensive overview of the different aspects of mitochondrial nucleic acid-sensing. It attempts to summarize the nature of the molecular patterns involved, their release and recognition in the cytoplasm and signaling. Finally, a major emphasis is given to elaborate the resulting patho-physiologies.