Pneumolysin induced mitochondrial dysfunction leads to release of mitochondrial DNA

Neuronal control of microglia through the mitochondria

The microbial toxin β-N-methylamino-L-alanine (BMAA), which is derived from cyanobacteria, targets neuronal mitochondria, leading to the activation of neuronal innate immunity and, consequently, neurodegeneration. Although known to modulate brain inflammation, the precise role of aberrant microglial function in the neurodegenerative process remains elusive. To determine if neurons signal microglial cells, we treated primary cortical neurons with BMAA and then co-cultured them with the N9 microglial cell line. Our observations indicate that microglial cell activation requires initial neuronal priming. Contrary to what was observed in cortical neurons, BMAA was not able to activate inflammatory pathways in N9 cells. We observed that microglial activation is dependent on mitochondrial dysfunction signaled by BMAA-treated neurons. In this scenario, the NLRP3 pro-inflammatory pathway is activated due to mitochondrial impairment in N9 cells. These results demonstrate that microglia activation in the presence of BMAA is dependent on neuronal signaling. This study provides evidence that neurons may trigger microglia activation and subsequent neuroinflammation. In addition, we demonstrate that microglial activation may have a protective role in ameliorating neuronal innate immune activation, at least in the initial phase. This work challenges the current understanding of neuroinflammation by assigning the primary role to neurons.

Bacterial cholesterol-dependent cytolysins and their interaction with the human immune response

PURPOSE OF REVIEW

Many cholesterol-dependent cytolysin (CDC)-producing pathogens pose a significant threat to human health. Herein, we review the pore-dependent and -independent properties CDCs possess to assist pathogens in evading the host immune response.

RECENT FINDINGS

Within the last 5 years, exciting new research suggests CDCs can act to inhibit important immune functions, disrupt critical cell signaling pathways, and have tissue-specific effects. Additionally, recent studies have identified a key region of CDCs that generates robust immunity, providing resources for the development of CDC-based vaccines.

SUMMARY

This review provides new information on how CDCs alter host immune responses to aid bacteria in pathogenesis. These studies can assist in the design of more efficient vaccines and therapeutics against CDCs that will enhance the immune response to CDC-producing pathogens while mitigating the dampening effects CDCs have on the host immune response.

Immunomodulatory role of mitochondrial DAMPs: a missing link in pathology?

In accordance with the endosymbiotic theory, mitochondrial components bear characteristic prokaryotic signatures, which act as immunomodulatory molecules when released into the extramitochondrial compartment. These endogenous immune triggers, called mitochondrial damage-associated molecular patterns (mtDAMPs), have been implicated in the pathogenesis of various diseases, yet their role remains largely unexplored. In this review, we summarise the available literature on mtDAMPs in diseases, with a special focus on respiratory diseases. We highlight the need to bolster mtDAMP research using a multipronged approach, to study their effect on specific cell types, receptors and machinery in pathologies. We emphasise the lacunae in the current understanding of mtDAMPs, particularly in their cellular release and the chemical modifications they undergo. Finally, we conclude by proposing additional effects of mtDAMPs in diseases, specifically their role in modulating the immune system.

Mitochondrial Dysfunction in Bacterial Infections

Mitochondria are critical in numerous cellular processes, including energy generation. Bacterial pathogens target host cell mitochondria through various mechanisms to disturb the host response and improve bacterial survival. We review recent advances in the understanding of how bacteria cause mitochondrial dysfunction through perturbations in mitochondrial cell-death pathways, energy production, mitochondrial dynamics, mitochondrial quality control, DNA repair, and the mitochondrial unfolded protein response. We also briefly highlight possible therapeutic approaches aimed at restoring the host mitochondrial function as a novel strategy to enhance the host response to bacterial infection.

MitoEVs: A new player in multiple disease pathology and treatment

Mitochondrial damage plays vital roles in the pathology of many diseases, such as cancers, neurodegenerative diseases, aging, metabolic diseases and many types of organ injury. However, the regulatory mechanism of mitochondrial functions among different cells or organs in vivo is still unclear, and efficient therapies for attenuating mitochondrial damage are urgently needed. Extracellular vesicles (EVs) are cell-derived nanovesicles that can deliver bioactive cargoes among cells or organs. Interestingly, recent evidence shows that diverse mitochondrial contents are enriched in certain EV subpopulations, and such mitoEVs can deliver mitochondrial components to affect the functions of recipient cells under different conditions, which has emerged as a hot topic in this field. However, the overview and many essential questions with respect to this event remain elusive. In this review, we provide a global view of mitoEVs biology and mainly focus on the detailed sorting mechanisms, functional mitochondrial contents, and diverse biological effects of mitoEVs. We also discuss the pathogenic or therapeutic roles of mitoEVs in different diseases and highlight their potential as disease biomarkers or therapies in clinical translation. This review will provide insights into the pathology and drug development for various mitochondrial injury-related diseases.

Melatonin Suppresses Cyclic GMP-AMP Synthase-Stimulator of Interferon Genes Signaling and Delays the Development of Hearing Loss in the C57BL/6J Presbycusis Mouse Model

Melatonin supplementation has been shown to delay age-related hearing loss (ARHL) progression. Previously, melatonin was found to inhibit neuronal mitochondrial DNA (mtDNA) release, as well as inhibit cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling, thereby delaying the onset of central nervous system diseases. Therefore, we hypothesized that melatonin may delay the progression of hearing loss in the C57BL/6J presbycusis mouse model by inhibiting cGAS-STING signaling in the auditory pathway. Oral melatonin at 10 mg/kg/d was administered to 3-month-old C57BL/6J mice until 12 months of age. The auditory brainstem response (ABR) threshold was used to assess their hearing ability. By real-time polymerase chain reaction and Western blot analysis, the levels of cytosolic mtDNA, cGAS/STING, and cytokines were examined in the mouse cochlea, inferior colliculus, and auditory cortex. We found that the 12-month-old control mice exhibited significant hearing loss, increased cytosolic mtDNA, increased expression of inflammatory factors TNF-α, IL-6, IFN-β, Cxcl10, and Ifit3, up-regulated cGAS and STING expression, and enhanced interferon regulatory factor 3 (IRF3) phosphorylation in the C57BL/6J mouse cochlea, inferior colliculus, and auditory cortex. Melatonin treatment significantly improved hearing, decreased cytosolic mtDNA, suppressed the expression of inflammatory cytokines TNF-α, IL-6, IFN-β, Ifit3, and Cxcl10, down-regulated cGAS and STING expression, and attenuated IRF3 phosphorylation in the C57BL/6J mouse cochlea, inferior colliculus, and auditory cortex. This study suggested that melatonin had a protective effect on auditory function in the C57BL/6J presbycusis mouse model, which may be mediated through reducing mtDNA release, inhibiting the cGAS-STING signaling pathway in the auditory pathway.

Pneumolysin as a target for new therapies against pneumococcal infections: A systematic review

BACKGROUND

This systematic review evaluates pneumolysin (PLY) as a target for new treatments against pneumococcal infections. Pneumolysin is one of the main virulence factors produced by all types of pneumococci. This toxin (53 kDa) is a highly conserved protein that binds to cholesterol in eukaryotic cells, forming pores that lead to cell destruction.

METHODS

The databases consulted were MEDLINE, Web of Science, and Scopus. Articles were independently screened by title, abstract, and full text by two researchers, and using consensus to resolve any disagreements that occurred. Articles in other languages different from English, patents, cases report, notes, chapter books and reviews were excluded. Searches were restricted to the years 2000 to 2021. Methodological quality was evaluated using OHAT framework.

RESULTS

Forty-one articles describing the effects of different molecules that inhibit PLY were reviewed. Briefly, the inhibitory molecules found were classified into three main groups: those exerting a direct effect by binding and/or blocking PLY, those acting indirectly by preventing its effects on host cells, and those whose mechanisms are unknown. Although many molecules are proposed as toxin blockers, only some of them, such as antibiotics, peptides, sterols, and statins, have the probability of being implemented as clinical treatment. In contrast, for other molecules, there are limited studies that demonstrate efficacy in animal models with sufficient reliability.

DISCUSSION

Most of the studies reviewed has a good level of confidence. However, one of the limitations of this systematic review is the lack of homogeneity of the studies, what prevented to carry out a statistical comparison of the results or meta-analysis.

CONCLUSION

A panel of molecules blocking PLY activity are associated with the improvement of the inflammatory process triggered by the pneumococcal infection. Some molecules have already been used in humans for other purposes, so they could be safe for use in patients with pneumococcal infections. These patients might benefit from a second line treatment during the initial stages of the infection preventing acute respiratory distress syndrome and invasive pneumococcal diseases. Additional research using the presented set of compounds might further improve the clinical management of these patients.

The Caenorhabditis elegans ARIP-4 DNA helicase couples mitochondrial surveillance to immune, detoxification, and antiviral pathways

Surveillance of Caenorhabditis elegans mitochondrial status is coupled to defense responses such as drug detoxification, immunity, antiviral RNA interference (RNAi), and regulation of life span. A cytochrome p540 detoxification gene, cyp-14A4, is specifically activated by mitochondrial dysfunction. The nuclear hormone receptor NHR-45 and the transcriptional Mediator component MDT-15/MED15 are required for the transcriptional activation of cyp-14A4 by mitochondrial mutations, gene inactivations, or toxins. A genetic screen for mutations that fail to activate this cytochrome p450 gene upon drug or mutation-induced mitochondrial dysfunction identified a DNA helicase ARIP-4 that functions in concert with the NHR-45 transcriptional regulatory cascade. In response to mitochondrial dysfunction, ARIP-4 and NHR-45 protein interaction is enhanced, and they relocalize from the nuclear periphery to the interior of intestinal nuclei. NHR-45/ARIP-4 also regulates the transcriptional activation of the eol-1 gene that encodes a decapping enzyme required for enhanced RNAi and transgene silencing of mitochondrial mutants. In the absence of arip-4, animals were more susceptible to the mitochondrial inhibitor antimycin. Thus, ARIP-4 serves as a transcriptional coactivator of NHR-45 to promote this defense response. A null mutation in arip-4 extends the life span and health span of both wild type and a mitochondrial mutant, suggesting that the activation of detoxification pathways is deleterious to health when the mitochondrial dysfunction is caused by mutation that cannot be cytochrome p450-detoxified. Thus, arip-4 acts in a pathway that couples mitochondrial surveillance to the activation of downstream immunity, detoxification, and RNAi responses.

Mitochondrial ROS production by neutrophils is required for host antimicrobial function against Streptococcus pneumoniae and is controlled by A2B adenosine receptor signaling

Polymorphonuclear cells (PMNs) control Streptococcus pneumoniae (pneumococcus) infection through various antimicrobial activities. We previously found that reactive oxygen species (ROS) were required for optimal antibacterial function, however, the NADPH oxidase is known to be dispensable for the ability of PMNs to kill pneumococci. In this study, we explored the role of ROS produced by the mitochondria in PMN antimicrobial defense against pneumococci. We found that the mitochondria are an important source of overall intracellular ROS produced by murine PMNs in response to infection. We investigated the host and bacterial factors involved and found that mitochondrial ROS (MitROS) are produced independent of bacterial capsule or pneumolysin but presence of live bacteria that are in direct contact with PMNs enhanced the response. We further found that MyD88-/- PMNs produced less MitROS in response to pneumococcal infection suggesting that released bacterial products acting as TLR ligands are sufficient for inducing MitROS production in PMNs. To test the role of MitROS in PMN function, we used an opsonophagocytic killing assay and found that MitROS were required for the ability of PMNs to kill pneumococci. We then investigated the role of MitROS in host resistance and found that MitROS are produced by PMNs in response to pneumococcal infection. Importantly, treatment of mice with a MitROS scavenger prior to systemic challenge resulted in reduced survival of infected hosts. In exploring host pathways that control MitROS, we focused on extracellular adenosine, which is known to control PMN anti-pneumococcal activity, and found that signaling through the A2B adenosine receptor inhibits MitROS production by PMNs. A2BR-/- mice produced more MitROS and were significantly more resistant to infection. Finally, we verified the clinical relevance of our findings using human PMNs. In summary, we identified a novel pathway that controls MitROS production by PMNs, shaping host resistance against S. pneumoniae.

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.

Cigarette Smoke Extract Disturbs Mitochondria-Regulated Airway Epithelial Cell Responses to Pneumococci

Mitochondrial functionality is crucial for the execution of physiologic functions of metabolically active cells in the respiratory tract including airway epithelial cells (AECs). Cigarette smoke is known to impair mitochondrial function in AECs. However, the potential contribution of mitochondrial dysfunction in AECs to airway infection and airway epithelial barrier dysfunction is unknown. In this study, we used an in vitro model based on AECs exposed to cigarette smoke extract (CSE) followed by an infection with Streptococcus pneumoniae (Sp). The levels of oxidative stress as an indicator of mitochondrial stress were quantified upon CSE and Sp treatment. In addition, expression of proteins associated with mitophagy, mitochondrial content, and biogenesis as well as mitochondrial fission and fusion was quantified. Transcriptional AEC profiling was performed to identify the potential changes in innate immune pathways and correlate them with indices of mitochondrial function. We observed that CSE exposure substantially altered mitochondrial function in AECs by suppressing mitochondrial complex protein levels, reducing mitochondrial membrane potential and increasing mitochondrial stress and mitophagy. Moreover, CSE-induced mitochondrial dysfunction correlated with reduced enrichment of genes involved in apical junctions and innate immune responses to Sp, particularly type I interferon responses. Together, our results demonstrated that CSE-induced mitochondrial dysfunction may contribute to impaired innate immune responses to Sp.

The Yin and Yang of Pneumolysin During Pneumococcal Infection

Pneumolysin (PLY) is a pore-forming toxin produced by the human pathobiont Streptococcus pneumoniae, the major cause of pneumonia worldwide. PLY, a key pneumococcal virulence factor, can form transmembrane pores in host cells, disrupting plasma membrane integrity and deregulating cellular homeostasis. At lytic concentrations, PLY causes cell death. At sub-lytic concentrations, PLY triggers host cell survival pathways that cooperate to reseal the damaged plasma membrane and restore cell homeostasis. While PLY is generally considered a pivotal factor promoting S. pneumoniae colonization and survival, it is also a powerful trigger of the innate and adaptive host immune response against bacterial infection. The dichotomy of PLY as both a key bacterial virulence factor and a trigger for host immune modulation allows the toxin to display both "Yin" and "Yang" properties during infection, promoting disease by membrane perforation and activating inflammatory pathways, while also mitigating damage by triggering host cell repair and initiating anti-inflammatory responses. Due to its cytolytic activity and diverse immunomodulatory properties, PLY is integral to every stage of S. pneumoniae pathogenesis and may tip the balance towards either the pathogen or the host depending on the context of infection.

Visualizing physiological parameters in cells and tissues using genetically encoded indicators for metabolites

The study of metabolism is undergoing a renaissance. Since the year 2002, over 50 genetically-encoded fluorescent indicators (GEFIs) have been introduced, capable of monitoring metabolites with high spatial/temporal resolution using fluorescence microscopy. Indicators are fusion proteins that change their fluorescence upon binding a specific metabolite. There are indicators for sugars, monocarboxylates, Krebs cycle intermediates, amino acids, cofactors, and energy nucleotides. They permit monitoring relative levels, concentrations, and fluxes in living systems. At a minimum they report relative levels and, in some cases, absolute concentrations may be obtained by performing ad hoc calibration protocols. Proper data collection, processing, and interpretation are critical to take full advantage of these new tools. This review offers a survey of the metabolic indicators that have been validated in mammalian systems. Minimally invasive, these indicators have been instrumental for the purposes of confirmation, rebuttal and discovery. We envision that this powerful technology will foster metabolic physiology.

Control of host mitochondria by bacterial pathogens

Mitochondria control various processes that are integral to cellular and organismal homeostasis, including Ca²⁺ fluxes, bioenergetic metabolism, and cell death. Perhaps not surprisingly, multiple pathogenic bacteria have evolved strategies to subvert mitochondrial functions in support of their survival and dissemination. Here, we discuss nonimmunological pathogenic mechanisms that converge on the ability of bacteria to control the mitochondrial compartment of host cells.

Mitochondrial DNA in innate immune responses against infectious diseases

Mitochondrial DNA (mtDNA) can initiate an innate immune response when mislocalized in a compartment other than the mitochondrial matrix. mtDNA plays significant roles in regulating mitochondrial dynamics as well as mitochondrial unfolded protein response (UPR). The mislocalized extra-mtDNA can elicit innate immune response via cGAS-STING (cyclic GMP-AMP synthase-stimulator of interferon genes) pathway, inducing the expression of the interferon-stimulated genes (ISGs). Also, cytosolic damaged mtDNA is cleared up by various pathways which are responsible for participating in the activation of inflammatory responses. Four pathways of extra-mitochondrial mtDNA clearance are highlighted in this review - the inflammasome activation mechanism, neutrophil extracellular traps formation, recognition by Toll-like receptor 9 and transfer of mtDNA between cells packaged into extracellular vesicles. Anomalies in these pathways are associated with various diseases. We posit our review in the present pandemic situation and discuss how mtDNA elicits innate immune responses against different viruses and bacteria. This review gives a comprehensive picture of the role of extra-mitochondrial mtDNA in infectious diseases and speculates that research towards its understanding would help establish its therapeutic potential.

Gasdermins mediate cellular release of mitochondrial DNA during pyroptosis and apoptosis

Pyroptosis and intrinsic apoptosis are two forms of regulated cell death driven by active caspases where plasma membrane permeabilization is induced by gasdermin pores. Caspase-1 induces gasdermin D pore formation during pyroptosis, whereas caspase-3 promotes gasdermin E pore formation during apoptosis. These two types of cell death are accompanied by mitochondrial outer membrane permeabilization due to BAK/BAX pore formation in the external membrane of mitochondria, and to some extent, this complex also affects the inner mitochondrial membrane facilitating mitochondrial DNA relocalization from the matrix to the cytosol. However, the detailed mechanism responsible for this process has not been investigated. Herein, we reported that gasdermin processing is required to induce mitochondrial DNA release from cells during pyroptosis and apoptosis. Gasdermin targeted at the plasma membrane promotes a fast mitochondrial collapse along with the initial accumulation of mitochondrial DNA in the cytosol and then facilitates the DNA's release from the cell when the plasma membrane ruptures. These findings demonstrate that gasdermin action has a critical effect on the plasma membrane and facilitates the release of mitochondrial DNA as a damage-associated molecular pattern.

Microvesicles released from pneumolysin-stimulated lung epithelial cells carry mitochondrial cargo and suppress neutrophil oxidative burst

Microvesicles (MVs) are cell-derived extracellular vesicles that have emerged as markers and mediators of acute lung injury (ALI). One of the most common pathogens in pneumonia-induced ALI is Streptococcus pneumoniae (Spn), but the role of MVs during Spn lung infection is largely unknown. In the first line of defense against Spn and its major virulence factor, pneumolysin (PLY), are the alveolar epithelial cells (AEC). In this study, we aim to characterize MVs shed from PLY-stimulated AEC and explore their contribution in mediating crosstalk with neutrophils. Using in vitro cell and ex vivo (human lung tissue) models, we demonstrated that Spn in a PLY-dependent manner stimulates AEC to release increased numbers of MVs. Spn infected mice also had higher levels of epithelial-derived MVs in their alveolar compartment compared to control. Furthermore, MVs released from PLY-stimulated AEC contain mitochondrial content and can be taken up by neutrophils. These MVs then suppress the ability of neutrophils to produce reactive oxygen species, a critical host-defense mechanism. Taken together, our results demonstrate that AEC in response to pneumococcal PLY release MVs that carry mitochondrial cargo and suggest that these MVs regulate innate immune responses during lung injury.

Dryocrassin ABBA ameliorates Streptococcus pneumoniae-induced infection in vitro through inhibiting Streptococcus pneumoniae growth and neutralizing pneumolysin activity

To explore the role of dryocrassin ABBA (ABBA) in the prevention and treatment of Streptococcus pneumoniae (S. pneumoniae) infections in vitro, a minimal inhibitory concentration test, growth curve assay, hemolysis assay, BacLight LIVE/DEAD staining experiments, oligomerization inhibition assay, time-killing test, LDH release detection assay and cytotoxicity test were performed to evaluate the efficacy of ABBA against S. pneumoniae infections in vitro. The results indicated that ABBA treatment exists bactericidal effect on S. pneumoniae at a concentration of less than 8 μg/ml. Furthermore, ABBA was effective at inhibiting the oligomerization of pneumolysin (PLY) from reducing its hemolytic activity. Meanwhile, ABBA could ameliorate cell injury by neutralizing the biological activity of PLY without cytotoxicity. In summary, ABBA was a leading compound against S. pneumoniae infections through bactericidal effect and neutralizing PLY activity.

Mitochondrial DNA drives noncanonical inflammation activation via cGAS-STING signaling pathway in retinal microvascular endothelial cells

BACKGROUND

Pathological stimuli cause mitochondrial damage and leakage of mitochondrial DNA (mtDNA) into the cytosol, as demonstrated in many cell types. The cytosolic mtDNA then drives the activation of noninfectious inflammation. Retinal microvascular endothelial cells (RMECs) play an important role in the inner endothelial blood-retinal barrier (BRB). RMEC dysfunction frequently occurs in posterior-segment eye diseases, causing loss of vision. In this study, we investigated the involvement of cytosolic mtDNA in noninfectious immune inflammation in RMECs under pathological stimuli.

METHODS

RMECs were stimulated with 100 ng/ml lipopolysaccharide (LPS), 200 μM hydrogen peroxide (H₂O₂), or 25 mM D-glucose. After 24 h, immunofluorescent staining was used to detect the opening of the mitochondrial permeability transition pore (MPTP). Cytosolic mtDNA was detected with immunofluorescent staining and PCR after stimulation. mtDNA was then isolated and used to transfect RMECs in vitro, and the protein levels of cGAS were evaluated with western blotting. Real-time PCR was used to examine cGAS mRNA expression levels at different time points after mtDNA stimulation. The activation of STING was detected with immunofluorescent staining 6 h after mtDNA stimulation. Western blotting was used to determine the expression of STING and IFNβ, the phosphorylation status of TBK1, IRF3, and nuclear factor-κB (NF-κB) P65, and the nuclear translocation of IRF3 and NF-κB P65 at 0, 3, 6, 12, and 24 h. The mRNA expression of proinflammatory cytokines CCL4, CXCL10, and IFNB1, and transcription factor IRF1 were determined with real-time PCR, together with the concentrations of intercellular adhesion molecule 1 (ICAM-1) mRNA.

RESULTS

Pathological stimuli caused mtDNA to leak into the cytosol by opening the MPTP in RMECs after 24 h. Cytosolic mtDNA regulated the expression of cGAS and the distribution of STING in RMECs. It promoted ICAM-1, STING and IFNβ expression, TBK1, IRF3, and NF-κB phosphorylation and the nuclear translocation in RMECs at 12 and 24 h after its transfection. The mRNAs of proinflammatory cytokines CCL4, CXCL10, and IFNB1, and transcription factor IRF1 were significantly elevated at 12 and 24 h after mtDNA stimulation.

CONCLUSIONS

Pathological stimulation induces mtDNA escape into the cytosol of RMECs. This cytoplasmic mtDNA is recognized by the DNA sensor cGAS, increasing the expression of inflammatory cytokines through the STING-TBK1 signaling pathway. Video Abstract. (MP4 37490 kb).

Mitochondrial dysfunction in lung ageing and disease

Mitochondrial biology has seen a surge in popularity in the past 5 years, with the emergence of numerous new avenues of exciting mitochondria-related research including immunometabolism, mitochondrial transplantation and mitochondria-microbe biology. Since the early 1960s mitochondrial dysfunction has been observed in cells of the lung in individuals and in experimental models of chronic and acute respiratory diseases. However, it is only in the past decade with the emergence of more sophisticated tools and methodologies that we are beginning to understand how this enigmatic organelle regulates cellular homeostasis and contributes to disease processes in the lung. In this review, we highlight the diverse role of mitochondria in individual lung cell populations and what happens when these essential organelles become dysfunctional with ageing and in acute and chronic lung disease. Although much remains to be uncovered, we also discuss potential targeted therapeutics for mitochondrial dysfunction in the ageing and diseased lung.

Reversion of Pneumolysin-Induced Executioner Caspase Activation Redirects Cells to Survival

Apoptosis is an indispensable mechanism for eliminating infected cells and activation of executioner caspases is considered to be a point of no return. Streptococcus pneumoniae, the most common bacterial pathogen causing community-acquired pneumonia, induces apoptosis via its pore-forming toxin pneumolysin, leading to rapid influxes of mitochondrial calcium [Ca2+]m as well as fragmentation, and loss of motility and membrane potential, which is accompanied by caspase-3/7 activation. Using machine-learning and quantitative live-cell microscopy, we identified a significant number of alveolar epithelial cells surviving such executioner caspase activation after pneumolysin attack. Precise single-cell analysis revealed the [Ca2+]m amplitude and efflux rate as decisive parameters for survival and death, which was verified by pharmacological inhibition of [Ca2+]m efflux shifting the surviving cells towards the dying fraction. Taken together, we identified the regulation of [Ca2+]m as critical for controlling the cellular fate under pneumolysin attack, which might be useful for therapeutic intervention during pneumococcal infection.

Betulin attenuates pneumolysin-induced cell injury and DNA damage

AIMS

Pneumolysin, a pore-forming toxin, is an important virulence factor of Streptococcus pneumoniae with multiple biological activity, such as cell lysis and DNA damage. Thus, targeting this toxin is alternative strategy for the treatment of S. pneumoniae infection.

METHODS AND RESULTS

Haemolysin assay was performed to identify the potential PLY inhibitor. The mechanism by which betulin, a natural compound from birch bark, against PLY was determined via MICs determination, western blot analysis and oligomerization analysis. Cytotoxicity and Immunofluorescence assays were further used to evaluate the protection of betulin against PLY-induced cell injury and DNA damage. Here, betulin, a natural compound from birch bark, was indentified as an effective inhibitor of PLY. Importantly, at the concentrations required for such inhibition, betulin has no influence on S. pneumoniae viability or PLY production. The interaction of betulin with PLY restrict the olgomerizaiton of this toxin and, thus, directly neutralizing the activity of PLY. Additionally, betulin treatment alleviate PLY induced cells injury and DNA damage in the co-culture system of PLY and A549 cells.

CONCLUSIONS

Betulin could be used as a promising leading compound against S. pneumoniae virulence by directly targeting PLY without antibacterial activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results presented in this work provided a novel strategy and candidate for S. pneumoniae infection.

Parkin, an E3 ubiquitin ligase, enhances airway mitochondrial DNA release and inflammation

INTRODUCTION

Parkin (Park2), an E3 ubiquitin ligase, is critical to maintain mitochondrial function by regulating mitochondrial biogenesis and degradation (mitophagy), but recent evidence suggests the involvement of Parkin in promoting inflammation. In the present study, we determined if Parkin regulates airway mitochondrial DNA (mtDNA) release and inflammatory responses to type 2 cytokine interleukin (IL)-13 and allergens.

METHODS

We measured Parkin mRNA expression in brushed bronchial epithelial cells and mtDNA release in the paired bronchoalveolar lavage fluid (BALF) from normal subjects and asthmatics. Parkin-deficient primary human tracheobronchial epithelial (HTBE) cells generated using the CRISPR-Cas9 system were stimulated with IL-13. To determine the in vivo function of Parkin, Parkin knockout (PKO) and wild-type (WT) mice were treated with IL-13 or allergen (house dust mite, HDM) in the presence or absence of mtDNA isolated from normal mouse lungs.

RESULTS

Parkin mRNA expression in asthmatic airway epithelium was upregulated, which positively correlated with the levels of released mtDNA in BALF. IL-13-stimulated HTBE cells increased Parkin expression. Moreover, IL-13 induced mtDNA release in Parkin-sufficient, but not in Parkin-deficient HTBE cells. PKO (vs WT) mice attenuated airway mtDNA release and inflammation following IL-13 or HDM treatments. mtDNA amplified airway inflammation in mice treated with IL-13 or HDM. Notably, Parkin also mediated mtDNA-induced exacerbation of airway inflammation.

CONCLUSION

Our research findings suggest that Parkin promotes mtDNA release and inflammation in airways, thus improving our understanding of the complex role of Parkin and mitochondrial dysfunction in asthma pathogenesis.

VDAC oligomer pores: A mechanism in disease triggered by mtDNA release

A recent study suggests that voltage-dependent anion channel (VDAC) oligomer pores promote mitochondrial outer membrane permeabilization and allow mitochondrial DNA (mtDNA) to be released into the cytosol in live cells. It challenges the notion that only occurs in apoptotic cells via BAX/BAK macropores. Cytosolic mtDNA activates cyclic GMP-AMP synthase (cGAS)-stimulator of IFN gene (STING) pathway and triggers type I interferon (IFN) response thereafter, which ultimately causes systemic lupus erythematosus. Mechanistically, mtDNA can interact with three positively charged residues (Lys12, Arg15, and Lys20) at the N-terminus of VDAC1, thereby strengthening VDAC1 oligomerization and facilitating mtDNA release. In addition, there are other pathways that can mediate mtDNA release, such as BAX/BAK macropores and virus-derived pores. The mtDNA released into the cytosol also triggers type I IFN response via the generally accepted cGAS-STING-TANK-binding kinase 1-IFN regulatory factor 3 axis. Collectively, VDAC oligomer pores provide us an attractive direction for us to understand mtDNA release-related diseases.

Pneumolysin: Pathogenesis and Therapeutic Target

Streptococcus pneumoniae is an opportunistic pathogen responsible for widespread illness and is a major global health issue for children, the elderly, and the immunocompromised population. Pneumolysin (PLY) is a cholesterol-dependent cytolysin (CDC) and key pneumococcal virulence factor involved in all phases of pneumococcal disease, including transmission, colonization, and infection. In this review we cover the biology and cytolytic function of PLY, its contribution to S. pneumoniae pathogenesis, and its known interactions and effects on the host with regard to tissue damage and immune response. Additionally, we review statins as a therapeutic option for CDC toxicity and PLY toxoid as a vaccine candidate in protein-based vaccines.

Targeted antioxidants as therapeutics for treatment of pneumonia in the elderly

Community acquired pneumonia is a leading cause of mortality in the United States. Along with predisposing comorbid health status, age is an independent risk factor for determining the outcome of pneumonia. Research over the last few decades has contributed to better understanding the underlying immunodysregulation and imbalanced redox homeostasis tied to this aged population group that increases susceptibility to a wide range of pathologies. Major approaches include targeting oxidative stress by reducing ROS generation at its main sources of production which includes the mitochondrion. Mitochondria-targeted antioxidants have a number of molecular strategies that include targeting the biophysical properties of mitochondria, mitochondrial localization of catalytic enzymes, and mitigating mitochondrial membrane potential. Results of several antioxidant studies both in vitro and in vivo have demonstrated promising potential as a therapeutic in the treatment of pneumonia in the elderly. More human studies will need to be conducted to evaluate its efficacy in this clinical setting.

Mitochondrial DNA, oxidants, and innate immunity

Mitochondrial oxidant damage, including damage to mitochondrial DNA (mtDNA) is a feature of both severe microbial infections and inflammation arising from sterile (non-infectious) sources such as tissue trauma. Damaged mitochondria release intact or oxidized fragments of mtDNA into the cytoplasm, which represent oxidant injury, and the fragments promote a spontaneous innate immune response, exemplifying a modern frontier of immunological research. MtDNA and mitochondrial-derived oxidants are central factors in activating at least three innate immune pathways involving the TLR9 (Toll-like receptor 9), the NLRP3 (NACHT, LRR and PYD domains-containing protein-3) inflammasome, and the cGAS (cyclic AMP-GMP synthase) pathway. The events that allow mtDNA to escape from damaged mitochondria and from damaged cells are incompletely known, but the presence of cytoplasmic mtDNA and cell-free mtDNA as immune regulators are important for understanding the cell's capacity for protecting mitochondrial quality control (MQC) and cell viability during inflammatory states.

Cytosolic mtDNA released from pneumolysin-damaged mitochondria triggers IFN-β production in epithelial cells

Pneumolysin (Ply) is a major virulence factor of Streptococcus pneumoniae. Ply-induced interferon-β (IFN-β) expression in host macrophages has been shown to be due to the accumulation of mitochondrial deoxyribonucleic acid (mtDNA) in the cytoplasm during S. pneumoniae infection. Our findings extend this work to show human bronchial epithelial cells that reside at the interface of inflammatory injury, BEAS-2B, adapt to local cues by altering mitochondrial states and releasing excess mtDNA. The results in this research showed that purified Ply induced the expression of IFN-β in human epithelial cells, which was accompanied by mitochondrial damage both in vivo and in vitro. The observations also were supported by the increased mtDNA concentrations in the bronchial lavage fluid of mice infected with S. pneumoniae. In summary, our study demonstrated that Ply triggered the production of IFN-β in epithelial cells, and this response was mediated by mtDNA released from Ply-damaged mitochondria. It displayed an impressive modulation of IFN-β response to S. pneumoniae in epithelial cells.

Cardiovascular complications following pneumonia: focus on pneumococcus and heart failure

PURPOSE OF REVIEW

Pneumonia, an inflammatory disease, is the single largest infectious cause of death. Pneumonia has recently been established as an important contributing factor to major adverse cardiovascular events including heart failure. Developing an intermechanistic understanding of pneumonia and cardiovascular disease is crucial for successful future drug therapy and reducing healthcare expenditure.

RECENT FINDINGS

Up to 30% of patients admitted with pneumonia develop cardiovascular complications such as heart failure within 10 years of hospital discharge. Recent mechanistic studies have identified inflammation, pneumolysin, platelet activation, and thrombus formation at the center of cardiovascular disease progression.

SUMMARY

In this review, we will detail current knowledge of the mechanistic interaction between pneumonia and development of cardiovascular disease as well as discuss the current and potential drug therapy targets.

Mitochondrial DNA: A Key Regulator of Anti-Microbial Innate Immunity

During the last few years, mitochondrial DNA has attained much attention as a modulator of immune responses. Due to common evolutionary origin, mitochondrial DNA shares various characteristic features with DNA of bacteria, as it consists of a remarkable number of unmethylated DNA as 2′-deoxyribose cytidine-phosphate-guanosine (CpG) islands. Due to this particular feature, mitochondrial DNA seems to be recognized as a pathogen-associated molecular pattern by the innate immune system. Under the normal physiological situation, mitochondrial DNA is enclosed in the double membrane structure of mitochondria. However, upon pathological conditions, it is usually released into the cytoplasm. Growing evidence suggests that this cytosolic mitochondrial DNA induces various innate immune signaling pathways involving NLRP3, toll-like receptor 9, and stimulator of interferon genes (STING) signaling, which participate in triggering downstream cascade and stimulating to produce effector molecules. Mitochondrial DNA is responsible for inflammatory diseases after stress and cellular damage. In addition, it is also involved in the anti-viral and anti-bacterial innate immunity. Thus, instead of entire mitochondrial importance in cellular metabolism and energy production, mitochondrial DNA seems to be essential in triggering innate anti-microbial immunity. Here, we describe existing knowledge on the involvement of mitochondrial DNA in the anti-microbial immunity by modulating the various immune signaling pathways.

Pathogenesis and prevention of risk of cardiovascular events in patients with pneumococcal community-acquired pneumonia

It is now well recognized that cardiovascular events (CVE) occur quite commonly, both in the acute phase and in the long-term, in patients with community-acquired pneumonia (CAP). CVE have been noted in up to 30% of patients hospitalized with all-cause CAP. One systematic review and meta-analysis of hospitalized patients with all-cause CAP noted that the incidence rates for overall cardiac events were 17.7%, for incident heart failure were 14.1%, for acute coronary syndromes were 5.3% and for incident cardiac arrhythmias were 4.7%. In the case of pneumococcal CAP, almost 20% of patients studied had one or more of these cardiac events. Recent research has provided insights into the pathogenesis of the acute cardiac events occurring in pneumococcal infections. With respect to the former, key involvements of the major pneumococcal protein virulence factor, pneumolysin, are now well documented, whilst systemic platelet-driven neutrophil activation may also contribute. However, events involved in the pathogenesis of the long-term cardiovascular sequelae remain largely unexplored. Emerging evidence suggests that persistent antigenaemia may predispose to the development of a systemic pro-inflammatory/prothrombotic phenotype underpinning the risk of future cardiovascular events. The current manuscript briefly reviews the occurrence of cardiovascular events in patients with all-cause CAP, as well as in pneumococcal and influenza infections. It highlights the close interaction between influenza and pneumococcal pneumonia. It also includes a brief discussion of mechanisms of the acute cardiac events in CAP. However, the primary focus is on the prevalence, pathogenesis and prevention of the longer-term cardiac sequelae of severe pneumococcal disease, particularly in the context of persistent antigenaemia and associated inflammation.

Mitochondrial damage-associated molecular patterns released by lung transplants are associated with primary graft dysfunction

Primary graft dysfunction (PGD) is a major limitation in short- and long-term lung transplant survival. Recent work has shown that mitochondrial damage-associated molecular patterns (mtDAMPs) can promote solid organ injury, but whether they contribute to PGD severity remains unclear. We quantitated circulating plasma mitochondrial DNA (mtDNA) in 62 patients, before lung transplantation and shortly after arrival to the intensive care unit. Although all recipients released mtDNA, high levels were associated with severe PGD development. In a mouse orthotopic lung transplant model of PGD, we detected airway cell-free damaged mitochondria and mtDNA in the peripheral circulation. Pharmacologic inhibition or genetic deletion of formylated peptide receptor 1 (FPR1), a chemotaxis sensor for N-formylated peptides released by damaged mitochondria, inhibited graft injury. An analysis of intragraft neutrophil-trafficking patterns reveals that FPR1 enhances neutrophil transepithelial migration and retention within airways but does not control extravasation. Using donor lungs that express a mitochondria-targeted reporter protein, we also show that FPR1-mediated neutrophil trafficking is coupled with the engulfment of damaged mitochondria, which in turn triggers reactive oxygen species (ROS)-induced pulmonary edema. Therefore, our data demonstrate an association between mtDAMP release and PGD development and suggest that neutrophil trafficking and effector responses to damaged mitochondria are drivers of graft damage.

Mechanisms protecting host cells against bacterial pore-forming toxins

Pore-forming toxins (PFTs) are key virulence determinants produced and secreted by a variety of human bacterial pathogens. They disrupt the plasma membrane (PM) by generating stable protein pores, which allow uncontrolled exchanges between the extracellular and intracellular milieus, dramatically disturbing cellular homeostasis. In recent years, many advances were made regarding the characterization of conserved repair mechanisms that allow eukaryotic cells to recover from mechanical disruption of the PM membrane. However, the specificities of the cell recovery pathways that protect host cells against PFT-induced damage remain remarkably elusive. During bacterial infections, the coordinated action of such cell recovery processes defines the outcome of infected cells and is, thus, critical for our understanding of bacterial pathogenesis. Here, we review the cellular pathways reported to be involved in the response to bacterial PFTs and discuss their impact in single-cell recovery and infection.

Mitochondrial Dysfunction in Aged Macrophages and Lung during Primary Streptococcus pneumoniae Infection is Improved with Pirfenidone

Pneumococcal infections remain a leading cause of death in older adults, with the most serious cases occurring in persons ≥65 years of age. There is an urgent need to investigate molecular pathways underlying these impairments and devise new therapeutics to modulate innate immunity. The goal of our current study is to understand the impact of chronological aging on mitochondrial function in response to Streptococcus pneumoniae, a causative agent of bacterial pneumonia. Using chronologically aged murine models, our findings demonstrate that decreased ATP production is associated with dysregulated mitochondrial complex expression, enhanced oxidative stress, diminished antioxidant responses, and decreased numbers of healthy mitochondria in aged adult macrophages and lung in response to S. pneumoniae. Pre-treatment of aged macrophages with pirfenidone, an anti-fibrotic drug with antioxidant and anti-inflammatory properties, improved mitochondrial function and decreased cellular oxidative stress responses. In vivo administration of pirfenidone decreased superoxide formation, increased healthy mitochondria number, improved ATP production, and decreased inflammatory cell recruitment and pulmonary oedema in aged mouse lung during infection. Taken together, our data shed light on the susceptibility of older persons to S. pneumoniae and provide a possible therapeutic to improve mitochondrial responses in this population.

Mitochondrial calcium transport and the redox nature of the calcium-induced membrane permeability transition

Mitochondria possess a Ca²⁺ transport system composed of separate Ca²⁺ influx and efflux pathways. Intramitochondrial Ca²⁺ concentrations regulate oxidative phosphorylation, required for cell function and survival, and mitochondrial redox balance, that participates in a myriad of signaling and damaging pathways. The interaction between Ca²⁺ accumulation and redox imbalance regulates opening and closing of a highly regulated inner membrane pore, the membrane permeability transition pore (PTP). In this review, we discuss the regulation of the PTP by mitochondrial oxidants, reactive nitrogen species, and the interactions between these species and other PTP inducers. In addition, we discuss the involvement of mitochondrial redox imbalance and PTP in metabolic conditions such as atherogenesis, diabetes, obesity and in mtDNA stability.

Endothelial Extracellular Vesicles in Pulmonary Function and Disease

The pulmonary vascular endothelium is involved in the pathogenesis of acute and chronic lung diseases. Endothelial cell (EC)-derived products such as extracellular vesicles (EVs) serve as EC messengers that mediate inflammatory as well as cytoprotective effects. EC-EVs are a broad term, which encompasses exosomes and microvesicles of endothelial origin. EVs are comprised of lipids, nucleic acids, and proteins that reflect not only the cellular origin but also the stimulus that triggered their biogenesis and secretion. This chapter presents an overview of the biology of EC-EVs and summarizes key findings regarding their characteristics, components, and functions. The role of EC-EVs is specifically delineated in pulmonary diseases characterized by endothelial dysfunction, including pulmonary hypertension, acute respiratory distress syndrome and associated conditions, chronic obstructive pulmonary disease, and obstructive sleep apnea.

Danger-Associated Molecular Patterns (DAMPs): the Derivatives and Triggers of Inflammation

PURPOSE OF REVIEW

Allergen is an umbrella term for irritants of diverse origin. Along with other offenders such as pathogens, mutagens, xenobiotics, and pollutants, allergens can be grouped as inflammatory agents. Danger-associated molecular patterns (DAMPs) are altered metabolism products of necrotic or stressed cells, which are deemed as alarm signals by the innate immune system. Like inflammation, DAMPs play a role in correcting the altered physiological state, but in excess, they can be lethal due to their signal transduction roles. In a vicious loop, inflammatory agents are DAMP generators and DAMPs create a pro-inflammatory state. Only a handful of DAMPs such as uric acid, mtDNA, extracellular ATP, HSPs, amyloid β, S100, HMGB1, and ECM proteins have been studied till now. A large number of DAMPs are still obscure, in need to be unveiled. The identification and functional characterization of those DAMPs in inflammation pathways can be insightful.

RECENT FINDINGS

As inflammation and immune activation have been implicated in almost all pathologies, studies on them have been intensified in recent times. Consequently, the pathologic mechanisms of various DAMPs have emerged. Following PRR ligation, the activation of inflammasome, MAPK, and NF-kB is some of the common pathways. The limited number of recognized DAMPs are only a fraction of the vast array of other DAMPs. In fact, any misplaced or abnormal level of metabolite can be a DAMP. Sophisticated analysis studies can reveal the full profile of the DAMPs. Lowering the level of DAMPs is useful therapeutic intervention but certainly not as effective as avoiding the DAMP generators, i.e., the inflammatory agents. So, rather than mitigating DAMPs, efforts should be focused on the elimination of inflammatory agents.

Mechanical ventilation and Streptococcus pneumoniae pneumonia alter mitochondrial homeostasis

Required mechanical ventilation (MV) may contribute to bacterial dissemination in patients with Streptococcus pneumoniae pneumonia. Significant variations in plasma mitochondrial DNA (mtDNA) have been reported in sepsis according to the outcome. The impact of lung stretch during MV was addressed in a model of pneumonia. Healthy or S. pneumoniae infected rabbits were submitted to MV or kept spontaneously breathing (SB). Bacterial burden, cytokines release, mitochondrial DNA levels, integrity and transcription were assessed along with 48-hour mortality. Compared with infected SB rabbits, MV rabbits developed more severe pneumonia with greater concentrations of bacteria in the lungs, higher rates of systemic dissemination, higher levels of circulating inflammatory mediators and decreased survival. Pulmonary mtDNA levels were significantly lower in infected animals as compared to non-infected ones, whenever they were SB or MV. After a significant early drop, circulating mtDNA levels returned to baseline values in the infected SB rabbits, but remained low until death in the MV ones. Whole blood ex-vivo stimulation with Streptococcus pneumoniae resulted in a reduction of polymorphonuclear leukocytes mitochondrial density and plasma mtDNA concentrations. Thus, persistent mitochondrial depletion and dysfunction in the infected animals submitted to MV could account for their less efficient immune response against S. pneumoniae.