Mitochondrial reactive oxygen species enable proinflammatory signaling through disulfide linkage of NEMO

Mitochondrial Reactive Oxygen Species in Infection and Immunity

Reactive oxygen species (ROS) contain at least one oxygen atom and one or more unpaired electrons and include singlet oxygen, superoxide anion radical, hydroxyl radical, hydroperoxyl radical, and free nitrogen radicals. Intracellular ROS can be formed as a consequence of several factors, including ultra-violet (UV) radiation, electron leakage during aerobic respiration, inflammatory responses mediated by macrophages, and other external stimuli or stress. The enhanced production of ROS is termed oxidative stress and this leads to cellular damage, such as protein carbonylation, lipid peroxidation, deoxyribonucleic acid (DNA) damage, and base modifications. This damage may manifest in various pathological states, including ageing, cancer, neurological diseases, and metabolic disorders like diabetes. On the other hand, the optimum levels of ROS have been implicated in the regulation of many important physiological processes. For example, the ROS generated in the mitochondria (mitochondrial ROS or mt-ROS), as a byproduct of the electron transport chain (ETC), participate in a plethora of physiological functions, which include ageing, cell growth, cell proliferation, and immune response and regulation. In this current review, we will focus on the mechanisms by which mt-ROS regulate different pathways of host immune responses in the context of infection by bacteria, protozoan parasites, viruses, and fungi. We will also discuss how these pathogens, in turn, modulate mt-ROS to evade host immunity. We will conclude by briefly giving an overview of the potential therapeutic approaches involving mt-ROS in infectious diseases.

The role of mitochondrial damage-associated molecular patterns in acute pancreatitis

Acute pancreatitis (AP) is one of the most common gastrointestinal tract diseases with significant morbidity and mortality. Current treatments remain unspecific and supportive due to the severity and clinical course of AP, which can fluctuate rapidly and unpredictably. Mitochondria, cellular power plant to produce energy, are involved in a variety of physiological or pathological activities in human body. There is a growing evidence indicating that mitochondria damage-associated molecular patterns (mtDAMPs) play an important role in pathogenesis and progression of AP. With the pro-inflammatory properties, released mtDAMPs may damage pancreatic cells by binding with receptors, activating downstream molecules and releasing inflammatory factors. This review focuses on the possible interaction between AP and mtDAMPs, which include cytochrome c (Cyt c), mitochondrial transcription factor A (TFAM), mitochondrial DNA (mtDNA), cardiolipin (CL), adenosine triphosphate (ATP) and succinate, with focus on experimental research and potential therapeutic targets in clinical practice. Preventing or diminishing the release of mtDAMPs or targeting the mtDAMPs receptors might have a role in AP progression.

Redox regulation of macrophages

Redox signaling, a mode of signal transduction that involves the transfer of electrons from a nucleophilic to electrophilic molecule, has emerged as an essential regulator of inflammatory macrophages. Redox reactions are driven by reactive oxygen/nitrogen species (ROS and RNS) and redox-sensitive metabolites such as fumarate and itaconate, which can post-translationally modify specific cysteine residues in target proteins. In the past decade our understanding of how ROS, RNS, and redox-sensitive metabolites control macrophage function has expanded dramatically. In this review, we discuss the latest evidence of how ROS, RNS, and metabolites regulate macrophage function and how this is dysregulated with disease. We highlight the key tools to assess redox signaling and important questions that remain.

Multifaceted mitochondria in innate immunity

The ability of mitochondria to transform the energy we obtain from food into cell phosphorylation potential has long been appreciated. However, recent decades have seen an evolution in our understanding of mitochondria, highlighting their significance as key signal-transducing organelles with essential roles in immunity that extend beyond their bioenergetic function. Importantly, mitochondria retain bacterial motifs as a remnant of their endosymbiotic origin that are recognised by innate immune cells to trigger inflammation and participate in anti-microbial defence. This review aims to explore how mitochondrial physiology, spanning from oxidative phosphorylation (OxPhos) to signalling of mitochondrial nucleic acids, metabolites, and lipids, influences the effector functions of phagocytes. These myriad effector functions include macrophage polarisation, efferocytosis, anti-bactericidal activity, antigen presentation, immune signalling, and cytokine regulation. Strict regulation of these processes is critical for organismal homeostasis that when disrupted may cause injury or contribute to disease. Thus, the expanding body of literature, which continues to highlight the central role of mitochondria in the innate immune system, may provide insights for the development of the next generation of therapies for inflammatory diseases.

Translocator protein (18 kDa) (Tspo) in the retina and implications for ocular diseases

Translocator protein (18 kDa) (Tspo), formerly known as peripheral benzodiazepine receptor is a highly conserved transmembrane protein primarily located in the outer mitochondrial membrane. In the central nervous system (CNS), especially in glia cells, Tspo is upregulated upon inflammation. Consequently, Tspo was used as a tool for diagnostic in vivo imaging of neuroinflammation in the brain and as a potential therapeutic target. Several synthetic Tspo ligands have been explored as immunomodulatory and neuroprotective therapy approaches. Although the function of Tspo and how its ligands exert these beneficial effects is not fully clear, it became a research topic of interest, especially in ocular diseases in the past few years. This review summarizes state-of-the-art knowledge of Tspo expression and its proposed functions in different cells of the retina including microglia, retinal pigment epithelium and Müller cells. Tspo is involved in cytokine signaling, oxidative stress and reactive oxygen species production, calcium signaling, neurosteroid synthesis, energy metabolism, and cholesterol efflux. We also highlight recent developments in preclinical models targeting Tspo and summarize the relevance of Tspo biology for ocular and retinal diseases. We conclude that glial upregulation of Tspo in different ocular pathologies and the use of Tspo ligands as promising therapeutic approaches in preclinical studies underline the importance of Tspo as a potential disease-modifying protein.

Pathological high intraocular pressure induces glial cell reactive proliferation contributing to neuroinflammation of the blood-retinal barrier via the NOX2/ET-1 axis-controlled ERK1/2 pathway

BACKGROUND

NADPH oxidase (NOX), a primary source of endothelial reactive oxygen species (ROS), is considered a key event in disrupting the integrity of the blood-retinal barrier. Abnormalities in neurovascular-coupled immune signaling herald the loss of ganglion cells in glaucoma. Persistent microglia-driven inflammation and cellular innate immune system dysregulation often lead to deteriorating retinal degeneration. However, the crosstalk between NOX and the retinal immune environment remains unresolved. Here, we investigate the interaction between oxidative stress and neuroinflammation in glaucoma by genetic defects of NOX2 or its regulation via gp91ds-tat.

METHODS

Ex vivo cultures of retinal explants from wildtype C57BL/6J and Nox2 -/- mice were subjected to normal and high hydrostatic pressure (Pressure 60 mmHg) for 24 h. In vivo, high intraocular pressure (H-IOP) was induced in C57BL/6J mice for two weeks. Both Pressure 60 mmHg retinas and H-IOP mice were treated with either gp91ds-tat (a NOX2-specific inhibitor). Proteomic analysis was performed on control, H-IOP, and treatment with gp91ds-tat retinas to identify differentially expressed proteins (DEPs). The study also evaluated various glaucoma phenotypes, including IOP, retinal ganglion cell (RGC) functionality, and optic nerve (ON) degeneration. The superoxide (O2-) levels assay, blood-retinal barrier degradation, gliosis, neuroinflammation, enzyme-linked immunosorbent assay (ELISA), western blotting, and quantitative PCR were performed in this study.

RESULTS

We found that NOX2-specific deletion or activity inhibition effectively attenuated retinal oxidative stress, immune dysregulation, the internal blood-retinal barrier (iBRB) injury, neurovascular unit (NVU) dysfunction, RGC loss, and ON axonal degeneration following H-IOP. Mechanistically, we unveiled for the first time that NOX2-dependent ROS-driven pro-inflammatory signaling, where NOX2/ROS induces endothelium-derived endothelin-1 (ET-1) overexpression, which activates the ERK1/2 signaling pathway and mediates the shift of microglia activation to a pro-inflammatory M1 phenotype, thereby triggering a neuroinflammatory outburst.

CONCLUSIONS

Collectively, we demonstrate for the first time that NOX2 deletion or gp91ds-tat inhibition attenuates iBRB injury and NVU dysfunction to rescue glaucomatous RGC loss and ON axon degeneration, which is associated with inhibition of the ET-1/ERK1/2-transduced shift of microglial cell activation toward a pro-inflammatory M1 phenotype, highlighting NOX2 as a potential target for novel neuroprotective therapies in glaucoma management.

Environmental Chemical-Induced Reactive Oxygen Species Generation and Immunotoxicity: A Comprehensive Review

Significance: Reactive oxygen species (ROS), the reactive oxygen-carrying chemicals moieties, act as pleiotropic signal transducers to maintain various biological processes/functions, including immune response. Increased ROS production leads to oxidative stress, which is implicated in xenobiotic-induced adverse effects. Understanding the immunoregulatory mechanisms and immunotoxicity is of interest to developing therapeutics against xenobiotic insults. Recent Advances: While developmental studies have established the essential roles of ROS in the establishment and proper functioning of the immune system, toxicological studies have demonstrated high ROS generation as one of the potential mechanisms of immunotoxicity induced by environmental chemicals, including heavy metals, pesticides, aromatic hydrocarbons (benzene and derivatives), plastics, and nanoparticles. Mitochondrial electron transport and various signaling components, including NADH oxidase, toll-like receptors (TLRs), NF-κB, JNK, NRF2, p53, and STAT3, are involved in xenobiotic-induced ROS generation and immunotoxicity. Critical Issues: With many studies demonstrating the role of ROS and oxidative stress in xenobiotic-induced immunotoxicity, rigorous and orthogonal approaches are needed to achieve in-depth and precise understanding. The association of xenobiotic-induced immunotoxicity with disease susceptibility and progression needs more data acquisition. Furthermore, the general methodology needs to be possibly replaced with high-throughput precise techniques. Future Directions: The progression of xenobiotic-induced immunotoxicity into disease manifestation is not well documented. Immunotoxicological studies about the combination of xenobiotics, age-related sensitivity, and their involvement in human disease incidence and pathogenesis are warranted. Antioxid. Redox Signal. 40, 691-714.

NADPH Oxidase 3: Beyond the Inner Ear

Reactive oxygen species (ROS) were formerly known as mere byproducts of metabolism with damaging effects on cellular structures. The discovery and description of NADPH oxidases (Nox) as a whole enzyme family that only produce this harmful group of molecules was surprising. After intensive research, seven Nox isoforms were discovered, described and extensively studied. Among them, the NADPH oxidase 3 is the perhaps most underrated Nox isoform, since it was firstly discovered in the inner ear. This stigma of Nox3 as "being only expressed in the inner ear" was also used by me several times. Therefore, the question arose whether this sentence is still valid or even usable. To this end, this review solely focuses on Nox3 and summarizes its discovery, the structural components, the activating and regulating factors, the expression in cells, tissues and organs, as well as the beneficial and detrimental effects of Nox3-mediated ROS production on body functions. Furthermore, the involvement of Nox3-derived ROS in diseases progression and, accordingly, as a potential target for disease treatment, will be discussed.

Mitochondrial Regulation of Macrophages in Innate Immunity and Diverse Roles of Macrophages During Cochlear Inflammation

Macrophages are essential components of the innate immune system and constitute a non-specific first line of host defense against pathogens and inflammation. Mitochondria regulate macrophage activation and innate immune responses in various inflammatory diseases, including cochlear inflammation. The distribution, number, and morphological characteristics of cochlear macrophages change significantly across different inner ear regions under various pathological conditions, including noise exposure, ototoxicity, and age-related degeneration. However, the exact mechanism underlying the role of mitochondria in macrophages in auditory function remains unclear. Here, we summarize the major factors and mitochondrial signaling pathways (e.g., metabolism, mitochondrial reactive oxygen species, mitochondrial DNA, and the inflammasome) that influence macrophage activation in the innate immune response. In particular, we focus on the properties of cochlear macrophages, activated signaling pathways, and the secretion of inflammatory cytokines after acoustic injury. We hope this review will provide new perspectives and a basis for future research on cochlear inflammation.

First transcriptomic insight into the reprogramming of human macrophages by levan-type fructans

Based on stimuli in the biological milieu, macrophages can undergo classical activation into the M1 pro-inflammatory (anti-cancer) phenotype or to the alternatively activated M2 anti-inflammatory one. Drug-free biomaterials have emerged as a new therapeutic strategy to modulate macrophage phenotype. Among them, polysaccharides polarize macrophages to M1 or M2 phenotypes based on the surface receptors they bind. Levan, a fructan, has been proposed as a novel biomaterial though its interaction with macrophages has been scarcely explored. In this study, we investigate the interaction of non-hydrolyzed and hydrolyzed Halomonas levan and its sulfated derivative with human macrophages in vitro. Viability studies show that these levans are cell compatible. In addition, RNA-sequencing analysis reveals the upregulation of pro-inflammatory pathways. These results are in good agreement with real time-quantitative polymerase chain reaction that indicates higher expression levels of C-X-C Motif Chemokine Ligand 8 and interleukin-6 genes and the M2-to-M1 reprogramming of these cells upon levan treatment. Finally, cytokine release studies confirm that hydrolyzed levans increase the secretion of pro-inflammatory cytokines and reprogram IL-4-polarized macrophages to the M1 state. Overall findings indicate that Halomonas levans trigger a classical macrophage activation and pave the way for their application in therapeutic interventions requiring a pro-inflammatory phenotype.

Engineered Bio-Heterojunction with Infection-Primed H2 S Liberation for Boosted Angiogenesis and Infectious Cutaneous Regeneration

Photodynamic therapy (PDT) acts as a powerful weapon against infectious diseases for its enormous antimicrobial activity that quickly elicits storms of reactive oxygen species (ROS). Nevertheless, redundant ROS during treatment inevitably bring detriments in revascularization. To address this dilemma, an innovative P-N bio-heterojunction (bio-HJ) material consisting of p-type copper sulfide (p-CuS), n-type bismuth sulfide (n-Bi2 S3 ), and lactate oxidase (LOx) for effective treatment of recalcitrant infectious wounds by promoting angiogenesis is devised. LOx exhausts lactic acid accumulated in infection environment and converts it to hydrogen peroxide (H2 O2 ), which subsequently yields bactericidal hydroxyl radicals (·OH) via Fenton-like reactions. Ultimately, the P-N bio-HJs exert synergistic photothermal, photodynamic, and chemodynamic effects for rapid bacterial annihilation. Moreover, in vitro and RNA-seq analyses reveal that the crafted bio-HJs dramatically expedite the proliferation of L929 cells and promote angiogenesis by up-regulating angiogenic gene expression in hypoxia-inducible factor-1 (HIF-1) signaling pathway, which may ascribe to the evolution of H2 S in response to the infection microenvironment. Critically, results of in vivo experiments have authenticated that the bio-HJs significantly boost healing rates of full-thickness wounds by slaughtering bacteria, elevating angiogenesis, and promoting cytothesis. As envisioned, this work furnishes a novel tactic for the effective treatment of bacteria-invaded wound using H2 S-liberating P-N bio-HJs.

Amygdalin Reverses Macrophage PANoptosis Induced by Drug-Resistant Escherichia coli

Infectious diseases caused by drug-resistant Escherichia coli (E. coli) pose a critical concern for medical institutions as they can lead to high morbidity and mortality rates. In this study, amygdalin exhibited anti-inflammatory and antioxidant activities, as well as other potentials. However, whether it could influence the drug-resistant E. coli-infected cells remained unanswered. Amygdalin was therefore tested in a cellular model in which human macrophages were exposed to resistant E. coli. Apoptosis was measured by flow cytometry and the lactate dehydrogenase (LDH) assay. Western immunoblotting and quantitative reverse-transcription polymerase chain reaction (qRT-PCR) were used to quantify interleukin-18 (IL-18), interleukin-1β (IL-1β), and interleukin-6 (IL-6). The production of reactive oxygen species (ROS) in macrophages was detected by ROS kit. The expression of panapoptotic proteins in macrophages was measured by qRT-PCR and Western immunoblotting. Drug-Resistant E. coli inhibited cell viability and enhanced apoptosis in the cellular model. In cells treated with amygdalin, this compound can inhibit cell apoptosis and reduce the expression of pro - inflammatory cytokines such as IL-1β, IL-18 and IL-6. Additionally, it decreases the production of PANoptosis proteins, Furthermore, amygdalin lowered the levels of reactive oxygen species induced by drug-resistant E. coli, in cells, demonstrating its antioxidant effects. Amygdalin, a drug with a protective role, alleviated cell damage caused by drug-resistant E. coli in human macrophages by inhibiting the PANoptosis signaling pathway.

18 kDa Translocator Protein TSPO Is a Mediator of Astrocyte Reactivity

An increase in astrocyte reactivity has been described in Alzheimer's disease and seems to be related to the presence of a pro-inflammatory environment. Reactive astrocytes show an increase in the density of the 18 kDa translocator protein (TSPO), but TSPO involvement in astrocyte functions remains poorly understood. The goal of this study was to better characterize the mechanisms leading to the increase in TSPO under inflammatory conditions and the associated consequences. For this purpose, the C6 astrocytic cell line was used in the presence of lipopolysaccharide (LPS) or TSPO overexpression mediated by the transfection of a plasmid encoding TSPO. The results show that nonlethal doses of LPS induced TSPO expression at mRNA and protein levels through a STAT3-dependent mechanism and increased the number of mitochondria per cell. LPS stimulated reactive oxygen species (ROS) production and decreased glucose consumption (quantified by the [18F]FDG uptake), and these effects were diminished by FEPPA, a TSPO antagonist. The transfection-mediated overexpression of TSPO induced ROS production, and this effect was blocked by FEPPA. In addition, a synergistic effect of overexpression of TSPO and LPS on ROS production was observed. These data show that the increase of TSPO in astrocytic cells is involved in the regulation of glucose metabolism and in the pro-inflammatory response. These data suggest that the overexpression of TSPO by astrocytes in Alzheimer's disease would have rather deleterious effects by promoting the pro-inflammatory response.

Unlocking the power of NOX2: A comprehensive review on its role in immune regulation

Reactive oxygen species (ROS) are a family of highly reactive molecules with numerous, often pleiotropic functions within the cell and the organism. Due to their potential to destroy biological structures such as membranes, enzymes and organelles, ROS have long been recognized as harmful yet unavoidable by-products of cellular metabolism leading to "oxidative stress" unless counterbalanced by cellular anti-oxidative defense mechanisms. Phagocytes utilize this destructive potential of ROS released in high amounts to defend against invading pathogens. In contrast, a regulated and fine-tuned release of "signaling ROS" (sROS) provides essential intracellular second messengers to modulate central aspects of immunity, including antigen presentation, activation of antigen presenting cells (APC) as well as the APC:T cell interaction during T cell activation. This regulated release of sROS is foremost attributed to the specialized enzyme NADPH-oxidase (NOX) 2 expressed mainly in myeloid cells such as neutrophils, macrophages and dendritic cells (DC). NOX-2-derived sROS are primarily involved in immune regulation and mediate protection against autoimmunity as well as maintenance of self-tolerance. Consequently, deficiencies in NOX2 not only result in primary immune-deficiencies such as Chronic Granulomatous Disease (CGD) but also lead to auto-inflammatory diseases and autoimmunity. A comprehensive understanding of NOX2 activation and regulation will be key for successful pharmaceutical interventions of such ROS-related diseases in the future. In this review, we summarize recent progress regarding immune regulation by NOX2-derived ROS and the consequences of its deregulation on the development of immune disorders.

Ginsenosides for therapeutically targeting inflammation through modulation of oxidative stress

Ginsenosides are steroid glycosides derived from ginseng plants such as Panax ginseng, Panax quinquefolium, and Panax notoginseng. Advances in recent studies have identified numerous physiological functions of each type of ginsenoside, i.e., immunomodulatory, antioxidative, and anti-inflammatory functions, in the context of inflammatory diseases. Accumulating evidence has revealed the molecular mechanisms by which the single or combined ginsenoside(s) exhibit anti-inflammatory effects, although it remains largely unclear. It is well known that excessive production of reactive oxygen species (ROS) is associated with pathological inflammation and cell death in a variety of cells, and that inhibition of ROS generation ameliorates the local and systemic inflammatory responses. The mechanisms by which ginsenosides attenuate inflammation are largely unknown; however, targeting ROS is suggested as one of the crucial mechanisms for the ginsenosides to control the pathological inflammation in the immune and non-immune cells. This review will summarize the latest progress in ginsenoside studies, particularly in the context of antioxidant mechanisms for its anti-inflammatory effects. A better understanding of the distinct types and the combined action of ginsenosides will pave the way for developing potential preventive and therapeutic modalities in treating various inflammation-related diseases.

TXNIP contributes to induction of pro-inflammatory phenotype and caspase-3 activation in astrocytes during Alzheimer's diseases

Neuroinflammation and oxidative stress have been implicated in the pathogenesis of Alzheimer's disease (AD). Neuroinflammation and oxidative stress are associated with neuronal death in AD. Astrocytes are linked to neuroinflammation during AD. Astrocytes are important contributors to AD progression. Although the role of thioredoxin-interacting protein (TXNIP) has been identified in inflammation and oxidative stress, the mechanism by which TXNIP regulates inflammation and oxidative stress in astrocytes during AD remains unclear. In the present study, we found that TXNIP gene levels were elevated in cerebral cortex of patients with AD. The protein levels of TXNIP were elevated in GFAP-positive astrocytes of cerebral cortex from patients with AD and APP/PS1 double-transgenic mouse model of AD. Our results showed that TXNIP increased expression of genes related to pro-inflammatory reactive astrocytes and pro-inflammatory cytokines and chemokines in human astrocytes. Moreover, TXNIP increased production of pro-inflammatory cytokines and chemokines in human astrocytes. TXNIP induced activation of NK-kB signaling and over-production of mitochondrial reactive oxygen species (mtROS) in human astrocytes. TXNIP also induced mitochondrial oxidative stress by reduction of mitochondrial respiration and ATP production in human astrocytes. Furthermore, elevated TXNIP levels are correlated with caspase-3 activation of GFAP-positive astrocytes in patients with AD and mouse AD. TXNIP induced mitochondria-dependent apoptosis via caspase-9 and caspase-3 activation in human astrocytes. These results suggest that TXNIP contributes to induction of pro-inflammatory phenotype and caspase-3 activation in astrocytes during AD.

Methylprednisolone Promotes Mycobacterium smegmatis Survival in Macrophages through NF-κB/DUSP1 Pathway

Background: Mycobacterium tuberculosis (M. tuberculosis) is the causative agent of tuberculosis. As an important component of host immunity, macrophages are not only the first line of defense against M. tuberculosis but also the parasitic site of M. tuberculosis in the host. Glucocorticoids can cause immunosuppression, which is considered to be one of the major risk factors for active tuberculosis, but the mechanism is unclear. Objective: To study the effect of methylprednisolone on the proliferation of mycobacteria in macrophages and try to find key molecules of this phenomenon. Methods: The macrophage line RAW264.7 infected by M. smegmatis was treated with methylprednisolone, and the intracellular bacterial CFU, Reactive Oxygen Species (ROS), cytokine secretion, autophagy, and apoptosis were measured. After the cells were treated with NF-κB inhibitor BAY 11-7082 and DUSP1 inhibitor BCI, respectively, the intracellular bacterial CFU, ROS, IL-6, and TNF-α secretion were detected. Results: After treatment with methylprednisolone, the CFU of intracellular bacteria increased, the level of ROS decreased, and the secretion of IL-6 and TNF-α decreased in infected macrophages. After BAY 11-7082 treatment, the CFU of M. smegmatis in macrophages increased, and the level of ROS production and the secretion of IL-6 by macrophages decreased. Transcriptome high-throughput sequencing and bioinformatics analysis suggested that DUSP1 was the key molecule in the above phenomenon. Western blot analysis confirmed that the expression level of DUSP1 was increased in the infected macrophages treated with methylprednisolone and BAY 11-7082, respectively. After BCI treatment, the level of ROS produced by infected macrophages increased, and the secretion of IL-6 increased. After the treatment of BCI combined with methylprednisolone or BAY 11-7082, the level of ROS produced and the secretion of IL-6 by macrophages were increased. Conclusion: methylprednisolone promotes the proliferation of mycobacteria in macrophages by suppressing cellular ROS production and IL-6 secretion through down-regulating NF-κB and up-regulating DUSP1 expression. BCI, an inhibitor of DUSP1, can reduce the level of DUSP1 in the infected macrophages and inhibit the proliferation of intracellular mycobacteria by promoting cellular ROS production and IL-6 secretion. Therefore, BCI may become a new molecule for host-directed therapy of tuberculosis, as well as a new strategy for the prevention of tuberculosis when treated with glucocorticoids.

Innate sensing and cellular metabolism: role in fine tuning antiviral immune responses

Several studies over the last decade have identified intimate links between cellular metabolism and macrophage function. Metabolism has been shown to both drive and regulate macrophage function by producing bioenergetic and biosynthetic precursors as well as metabolites (and other bioactive molecules) that regulate gene expression and signal transduction. Many studies have focused on lipopolysaccharide-induced reprogramming, assuming that it is representative of most inflammatory responses. However, emerging evidence suggests that diverse pathogen-associated molecular patterns (PAMPs) are associated with unique metabolic profiles, which may drive pathogen specific immune responses. Further, these metabolic pathways and processes may act as a rheostat to regulate the magnitude of an inflammatory response based on the biochemical features of the local microenvironment. In this review, we will discuss recent work examining the relationship between cellular metabolism and macrophage responses to viral PAMPs and describe how these processes differ from lipopolysaccharide-associated responses. We will also discuss how an improved understanding of the specificity of these processes may offer new insights to fine-tune macrophage function during viral infections or when using viral PAMPs as therapeutics.

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.

Molecular and Cellular Interactions in Pathogenesis of Sporadic Parkinson Disease

An increasing number of the population all around the world suffer from age-associated neurodegenerative diseases including Parkinson’s disease (PD). This disorder presents different signs of genetic, epigenetic and environmental origin, and molecular, cellular and intracellular dysfunction. At the molecular level, α-synuclein (αSyn) was identified as the principal molecule constituting the Lewy bodies (LB). The gut microbiota participates in the pathogenesis of PD and may contribute to the loss of dopaminergic neurons through mitochondrial dysfunction. The most important pathogenetic link is an imbalance of Ca²⁺ ions, which is associated with redox imbalance in the cells and increased generation of reactive oxygen species (ROS). In this review, genetic, epigenetic and environmental factors that cause these disorders and their cause-and-effect relationships are considered. As a constituent of environmental factors, the example of organophosphates (OPs) is also reviewed. The role of endothelial damage in the pathogenesis of PD is discussed, and a ‘triple hit hypothesis’ is proposed as a modification of Braak’s dual hit one. In the absence of effective therapies for neurodegenerative diseases, more and more evidence is emerging about the positive impact of nutritional structure and healthy lifestyle on the state of blood vessels and the risk of developing these diseases.

A bacterial phospholipid phosphatase inhibits host pyroptosis by hijacking ubiquitin

The inflammasome-mediated cleavage of gasdermin D (GSDMD) causes pyroptosis and inflammatory cytokine release to control pathogen infection, but how pathogens evade this immune response remains largely unexplored. Here we identify the known protein phosphatase PtpB from Mycobacterium tuberculosis as a phospholipid phosphatase inhibiting the host inflammasome-pyroptosis pathway. Mechanistically, PtpB dephosphorylated phosphatidylinositol-4-monophosphate and phosphatidylinositol-(4,5)-bisphosphate in host cell membrane, thus disrupting the membrane localization of the cleaved GSDMD to inhibit cytokine release and pyroptosis of macrophages. Notably, this phosphatase activity requires PtpB binding to ubiquitin. Disrupting phospholipid phosphatase activity or the ubiquitin-interacting motif of PtpB enhanced host GSDMD-dependent immune responses and reduced intracellular pathogen survival. Thus, pathogens inhibit pyroptosis and counteract host immunity by altering host membrane composition.

PINK1/parkin-mediated mitophagy alleviates Staphylococcus aureus-induced NLRP3 inflammasome and NF-κB pathway activation in bovine mammary epithelial cells

Staphylococcus aureus (S. aureus) is known to induce chronic and persistent bovine mammary infection, which affects milk quality and leads to premature culling. The ability of S. aureus to invade mammalian cells protects it from clearance by the immune system. Mitophagy is important in cell homeostasis, and can be utilized by pathogens for immune escape. However, mitophagy's role in S. aureus-associated bovine mastitis remains unclear. Here, S. aureus infection induced mitophagy and enhanced mitochondrial translocation of parkin in MAC-T cells. After mitophagy inhibition by Mdivi-1 treatment or PTEN-induced putative kinase 1 (PINK1) silencing in MAC-T cells infected with S. aureus, NOD-like receptor protein 3 (NLRP3) inflammasome activation and p65 and IκBα phosphorylation were increased. Meanwhile, PINK1 overexpression had the opposite effects. In addition, NLRP3 inflammasome overactivation and enhanced p65 and IκBα phosphorylation caused by PINK1 silencing were reversed by MitoTEMPO. Furthermore, PINK1/parkin-mediated mitophagy promoted S. aureus survival and contributed to persistent S. aureus infection. These findings provide new insights into S. aureus invasion in bovine mastitis.

Pros and cons of NRF2 activation as adjunctive therapy in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease with an important inflammatory component accompanied by deregulated redox-dependent signaling pathways that are feeding back into inflammation. In this context, we bring into focus the transcription factor NRF2, a master redox regulator that exerts exquisite antioxidant and anti-inflammatory effects. The review does not intend to be exhaustive, but to point out arguments sustaining the rationale for applying an NRF2-directed co-treatment in RA as well as its potential limitations. The involvement of NRF2 in RA is emphasized through an analysis of publicly available transcriptomic data on NRF2 target genes and the findings from NRF2-knockout mice. The impact of NRF2 on concurrent pathologic mechanisms in RA is explained by its crosstalk with major redox-sensitive inflammatory and cell death-related pathways, in the context of the increased survival of pathologic cells in RA. The proposed adjunctive therapy targeted to NRF2 is further sustained by the existence of promising NRF2 activators that are in various stages of drug development. The interference of NRF2 with conventional anti-rheumatic therapies is discussed, including the cytoprotective effects of NRF2 for alleviating drug toxicity. From another perspective, the review presents how NRF2 activation would be decreasing the efficacy of synthetic anti-rheumatic drugs by increasing drug efflux. Future perspectives regarding pharmacologic NRF2 activation in RA are finally proposed.

Anti-Inflammatory Effect of Resveratrol Derivatives via the Downregulation of Oxidative-Stress-Dependent and c-Src Transactivation EGFR Pathways on Rat Mesangial Cells

In Taiwan, the root extract of Vitis thunbergii Sieb. et Zucc. (Vitaceae, VT) is rich in stilbenes, with resveratrol (Res) and its derivatives being the most abundant. Previously, we showed that the effect of Res derivatives against tumor necrosis factor-α (TNF-α)-stimulated inflammatory responses occurs via cPLA2/COX-2/PGE2 inhibition. This study compared and explored the underlying anti-inflammatory pharmacological mechanisms. Before stimulation with TNF-α, RMCs were treated with/without pharmacological inhibitors of specific protein kinases. The expression of inflammatory mediators was determined by Western blotting, gelatin zymography, real-time PCR, and luciferase assay. Cellular and mitochondrial ROS were measured by H₂DHFDA or DHE and MitoSOX™ Red staining, respectively. The RNS level was indirectly measured by Griess reagent assay. Kinase activation and association were assayed by immunoprecipitation followed by Western blotting. TNF-α binding to TNFR recruited Rac1 and p47^phox, thus activating the NAPDH oxidase-dependent MAPK and NF-κB pathways. The TNF-α-induced NF-κB activation via c-Src-driven ROS was independent from the EGFR signaling pathway. The anti-inflammatory effects of Res derivatives occurred via the inhibition of ROS derived from mitochondria and NADPH oxidase; RNS derived from iNOS; and the activation of the ERK1/2, JNK1/2, and NF-κB pathways. Overall, this study provides an understanding of the various activities of Res derivatives and their pharmacological mechanisms. In the future, the application of the active molecules of VT to health foods and medicine in Taiwan may increase.

Redox Activation of Mitochondrial DAMPs and the Metabolic Consequences for Development of Autoimmunity

Significance: Reactive oxygen species (ROS) are well known to promote innate immune responses during and in the absence of microbial infections. However, excessive or prolonged exposure to ROS provokes innate immune signaling dysfunction and contributes to the pathogenesis of many autoimmune diseases. The relatively high basal expression of pattern recognition receptors (PRRs) in innate immune cells renders them prone to activation in response to minor intrinsic or extrinsic ROS misbalances in the absence of pathogens. Critical Issues: A prominent source of ROS are mitochondria, which are also major inter-organelle hubs for innate immunity activation, since most PRRs and downstream receptor molecules are directly located either at mitochondria or at mitochondria-associated membranes. Due to their ancestral bacterial origin, mitochondria can also act as quasi-intrinsic self-microbes that mimic a pathogen invasion and become a source of danger-associated molecular patterns (DAMPs) that triggers innate immunity from within. Recent Advances: The release of mitochondrial DAMPs correlates with mitochondrial metabolism changes and increased generation of ROS, which can lead to the oxidative modification of DAMPs. Recent studies suggest that ROS-modified mitochondrial DAMPs possess increased, persistent immunogenicity. Future Directions: Herein, we discuss how mitochondrial DAMP release and oxidation activates PRRs, changes cellular metabolism, and causes innate immune response dysfunction by promoting systemic inflammation, thereby contributing to the onset or progression of autoimmune diseases. The future goal is to understand what the tipping point for DAMPs is to become oxidized, and whether this is a road without return. Antioxid. Redox Signal. 36, 441-461.

mtROS Induced via TLR-2-SOCE Signaling Plays Proapoptotic and Bactericidal Role in Mycobacterium fortuitum-Infected Head Kidney Macrophages of Clarias gariepinus

The mechanisms underlying Mycobacterium fortuitum-induced mycobacteriosis remain unexplored. Using head kidney macrophages (HKM) from catfish (Clarias gariepinus), we report that Ca²⁺ surge across mitochondrial-Ca²⁺ uniporter (MICU), and consequent mitochondrial ROS (mtROS) production, is imperative for mycobactericidal activity. Inhibition of mtROS alleviated HKM apoptosis and enhanced bacterial survival. Based on RNA interference (RNAi) and inhibitor studies, we demonstrate that the Toll-like receptor (TLR)-2–endoplasmic reticulum (ER) stress–store-operated calcium entry (SOCE) axis is instrumental for activating the mt-Ca²⁺/mtROS cascade in M. fortuitum-infected HKM. Additionally, pharmacological inhibition of mtROS attenuated the expression of CHOP, STIM1, and Orai1, which suggests a positive feedback loop between ER-stress-induced SOCE and mtROS production. Elevated tumor necrosis factor alpha (TNF-α) levels and caspase-8 activity were observed in HKM consequent to M. fortuitum infection, and our results implicate that mtROS is crucial in activating the TNF-mediated caspase-8 activation. Our results for the first time demonstrate mitochondria as an innate immune signaling center regulating mycobacteriosis in fish. We conclude that M. fortuitum-induced persistent SOCE signaling leads to mtROS production, which in turn activates the TNF-α/caspase-8 axis culminating in HKM apoptosis and bacterial clearance.

Redox Homeostasis and Immune Alterations in Coronavirus Disease-19

The global Coronavirus Disease 2019 (COVID-19) pandemic is characterized by a wide variety of clinical features, from no or moderate symptoms to severe illness. COVID-19 is caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) that first affects the respiratory tract. Other than being limited to lungs, SARS-CoV-2 may lead to a multisystem disease that can even be durable (long COVID). The clinical spectrum of COVID-19 depends on variability in the immune regulation. Indeed, disease progression is consequent to failure in the immune regulation, characterized by an intensification of the pro-inflammatory response. Disturbance of systemic and organ-related redox balance may be a further mechanism underlying variability in COVID-19 severity. Other than being determinant for SARS-CoV-2 entry and fusion to the host cell, reactive species and redox signaling are deeply involved in the immune response. This review sums up the present knowledge on the role of redox balance in the regulation of susceptibility to SARS-CoV-2 infection and related immune response, debating the effectiveness of antioxidant compounds in the management of COVID-19.

Prototheca spp. induce an inflammatory response via mtROS-mediated activation of NF-κB and NLRP3 inflammasome pathways in bovine mammary epithelial cell cultures

Emergence of bovine mastitis caused by Prototheca algae is the impetus to better understand these infections. Both P. bovis and P. ciferrii belong to Prototheca algae, but they differ in their pathogenicity to induce inflammatory responses. The objective was to characterize and compare pathogenesis of inflammatory responses in bMECs induced by P. bovis versus P. ciferrii. Mitochondrial ultrastructure, activity and mtROS in bMECs were assessed with transmission electron microscopy and laser scanning confocal microscopy. Cytokines, including TNF-α, IL-1β and IL-18, were measured by ELISA and real-time PCR, whereas expressions of various proteins in the NF-κB and NLRP3 inflammasome pathways were detected with immunofluorescence or Western blot. Infection with P. bovis or P. ciferrii damaged mitochondria, including dissolution and vacuolation of cristae, and decreased mitochondrial activity, with P. bovis being more pathogenic and causing greater destruction. There were increases in NADPH production and mtROS accumulation in infected bMECs, with P. bovis causing greater increases and also inducing higher cytokine concentrations. Expressions of NF-κB-p65, p-NF-κB-p65, IκBα and p-IκBα proteins in the NF-κB pathway, as well as NLRP3, Pro Caspase1, Caspase1 p20, ASC, Pro IL-1β, and IL-1β proteins in the NLRP3 inflammasome pathway, were significantly higher in P. bovis-infected bMECs. However, mito-TEMPO significantly inhibited production of cytokines and decreased expression of proteins in NF-κB and NLRP3 inflammasome pathways in bMECs infected with either P. bovis or P. ciferrii. In conclusion, P. bovis or P. ciferrii infections induced inflammatory responses in bMECs, with increased mtROS in damaged mitochondria and activated NF-κB and NLRP3 inflammasome pathways, with P. bovis causing a more severe reaction.

Oxidative Stress in the Brain: Basic Concepts and Treatment Strategies in Stroke

The production of free radicals is inevitably associated with metabolism and other enzymatic processes. Under physiological conditions, however, free radicals are effectively eliminated by numerous antioxidant mechanisms. Oxidative stress occurs due to an imbalance between the production and elimination of free radicals under pathological conditions. Oxidative stress is also associated with ageing. The brain is prone to oxidative damage because of its high metabolic activity and high vulnerability to ischemic damage. Oxidative stress, thus, plays a major role in the pathophysiology of both acute and chronic pathologies in the brain, such as stroke, traumatic brain injury or neurodegenerative diseases. The goal of this article is to summarize the basic concepts of oxidative stress and its significance in brain pathologies, as well as to discuss treatment strategies for dealing with oxidative stress in stroke.

Modulation of Macrophage Activity by Pulsed Electromagnetic Fields in the Context of Fracture Healing

Delayed fracture healing and fracture non-unions impose an enormous burden on individuals and society. Successful healing requires tight communication between immune cells and bone cells. Macrophages can be found in all healing phases. Due to their high plasticity and long life span, they represent good target cells for modulation. In the past, extremely low frequency pulsed electromagnet fields (ELF-PEMFs) have been shown to exert cell-specific effects depending on the field conditions. Thus, the aim was to identify the specific ELF-PEMFs able to modulate macrophage activity to indirectly promote mesenchymal stem/stromal cell (SCP-1 cells) function. After a blinded screening of 22 different ELF-PEMF, two fields (termed A and B) were further characterized as they diversely affected macrophage function. These two fields have similar fundamental frequencies (51.8 Hz and 52.3 Hz) but are emitted in different groups of pulses or rather send-pause intervals. Macrophages exposed to field A showed a pro-inflammatory function, represented by increased levels of phospho-Stat1 and CD86, the accumulation of ROS, and increased secretion of pro-inflammatory cytokines. In contrast, macrophages exposed to field B showed anti-inflammatory and pro-healing functions, represented by increased levels of Arginase I, increased secretion of anti-inflammatory cytokines, and growth factors are known to induce healing processes. The conditioned medium from macrophages exposed to both ELF-PEMFs favored the migration of SCP-1 cells, but the effect was stronger for field B. Furthermore, the conditioned medium from macrophages exposed to field B, but not to field A, stimulated the expression of extracellular matrix genes in SCP-1 cells, i.e., COL1A1, FN1, and BGN. In summary, our data show that specific ELF-PEMFs may affect immune cell function. Thus, knowing the specific ELF-PEMFs conditions and the underlying mechanisms bears great potential as an adjuvant treatment to modulate immune responses during pathologies, e.g., fracture healing.

Reactive Oxygen Species in Macrophages: Sources and Targets

Reactive oxygen species (ROS) are fundamental for macrophages to eliminate invasive microorganisms. However, as observed in nonphagocytic cells, ROS play essential roles in processes that are different from pathogen killing, as signal transduction, differentiation, and gene expression. The different outcomes of these events are likely to depend on the specific subcellular site of ROS formation, as well as the duration and extent of ROS production. While excessive accumulation of ROS has long been appreciated for its detrimental effects, there is now a deeper understanding of their roles as signaling molecules. This could explain the failure of the “all or none” pharmacologic approach with global antioxidants to treat several diseases. NADPH oxidase is the first source of ROS that has been identified in macrophages. However, growing evidence highlights mitochondria as a crucial site of ROS formation in these cells, mainly due to electron leakage of the respiratory chain or to enzymes, such as monoamine oxidases. Their role in redox signaling, together with their exact site of formation is only partially elucidated. Hence, it is essential to identify the specific intracellular sources of ROS and how they influence cellular processes in both physiological and pathological conditions to develop therapies targeting oxidative signaling networks. In this review, we will focus on the different sites of ROS formation in macrophages and how they impact on metabolic processes and inflammatory signaling, highlighting the role of mitochondrial as compared to non-mitochondrial ROS sources.

Decursinol Angelate Mitigates Sepsis Induced by Methicillin-Resistant Staphylococcus aureus Infection by Modulating the Inflammatory Responses of Macrophages

The herbal plant Angelica gigas (A. gigas) has been used in traditional medicine in East Asian countries, and its chemical components are reported to have many pharmacological effects. In this study, we showed that a bioactive ingredient of A. gigas modulates the functional activity of macrophages and investigated its effect on inflammation using a sepsis model. Among 12 different compounds derived from A. gigas, decursinol angelate (DA) was identified as the most effective in suppressing the induction of TNF-α and IL-6 in murine macrophages. When mice were infected with a lethal dose of methicillin-resistant Staphylococcus aureus (MRSA), DA treatment improved the mortality and bacteremia, and attenuated the cytokine storm, which was associated with decreased CD38⁺ macrophage populations in the blood and liver. In vitro studies revealed that DA inhibited the functional activation of macrophages in the expression of pro-inflammatory mediators in response to microbial infection, while promoting the bacterial killing ability with an increased production of reactive oxygen species. Mechanistically, DA treatment attenuated the NF-κB and Akt signaling pathways. Intriguingly, ectopic expression of an active mutant of IKK2 released the inhibition of TNF-α production by the DA treatment, whereas the inhibition of Akt resulted in enhanced ROS production. Taken together, our experimental evidence demonstrated that DA modulates the functional activities of pro-inflammatory macrophages and that DA could be a potential therapeutic agent in the management of sepsis.

Reactive Oxygen Species: Not Omnipresent but Important in Many Locations

Reactive oxygen species (ROS), such as the superoxide anion or hydrogen peroxide, have been established over decades of research as, on the one hand, important and versatile molecules involved in a plethora of homeostatic processes and, on the other hand, as inducers of damage, pathologies and diseases. Which effects ROS induce, strongly depends on the cell type and the source, amount, duration and location of ROS production. Similar to cellular pH and calcium levels, which are both strictly regulated and only altered by the cell when necessary, the redox balance of the cell is also tightly regulated, not only on the level of the whole cell but in every cellular compartment. However, a still widespread view present in the scientific community is that the location of ROS production is of no major importance and that ROS randomly diffuse from their cellular source of production throughout the whole cell and hit their redox-sensitive targets when passing by. Yet, evidence is growing that cells regulate ROS production and therefore their redox balance by strictly controlling ROS source activation as well as localization, amount and duration of ROS production. Hopefully, future studies in the field of redox biology will consider these factors and analyze cellular ROS more specifically in order to revise the view of ROS as freely flowing through the cell.

Essential role for paxillin tyrosine phosphorylation in LPS-induced mitochondrial fission, ROS generation and lung endothelial barrier loss

We have shown that both reactive oxygen species (ROS) and paxillin tyrosine phosphorylation regulate LPS-induced human lung endothelial permeability. Mitochondrial ROS (mtROS) is known to increase endothelial cell (EC) permeability which requires dynamic change in mitochondrial morphology, events that are likely to be regulated by paxillin. Here, we investigated the role of paxillin and its tyrosine phosphorylation in regulating LPS-induced mitochondrial dynamics, mtROS production and human lung microvascular EC (HLMVEC) dysfunction. LPS, in a time-dependent manner, induced higher levels of ROS generation in the mitochondria compared to cytoplasm or nucleus. Down-regulation of paxillin expression with siRNA or ecto-expression of paxillin Y31F or Y118F mutant plasmids attenuated LPS-induced mtROS in HLMVECs. Pre-treatment with MitoTEMPO, a scavenger of mtROS, attenuated LPS-induced mtROS, endothelial permeability and VE-cadherin phosphorylation. Further, LPS-induced mitochondrial fission in HLMVECs was attenuated by both a paxillin siRNA, and paxillin Y31F/Y118F mutant. LPS stimulated phosphorylation of dynamin-related protein (DRP1) at S616, which was also attenuated by paxillin siRNA, and paxillinY31/Y118 mutants. Inhibition of DRP1 phosphorylation by P110 attenuated LPS-induced mtROS and endothelial permeability. LPS challenge of HLMVECs enhanced interaction between paxillin, ERK, and DRP1, and inhibition of ERK1/2 activation with PD98059 blocked mitochondrial fission. Taken together, these results suggest a key role for paxillin tyrosine phosphorylation in LPS-induced mitochondrial fission, mtROS generation and EC barrier dysfunction.

Mechanical competition triggered by innate immune signaling drives the collective extrusion of bacterially infected epithelial cells

Intracellular pathogens alter their host cells' mechanics to promote dissemination through tissues. Conversely, host cells may respond to the presence of pathogens by altering their mechanics to limit infection. Here, we monitored epithelial cell monolayers infected with intracellular bacterial pathogens, Listeria monocytogenes or Rickettsia parkeri, over days. Under conditions in which these pathogens trigger innate immune signaling through NF-κB and use actin-based motility to spread non-lytically intercellularly, we found that infected cell domains formed three-dimensional mounds. These mounds resulted from uninfected cells moving toward the infection site, collectively squeezing the softer and less contractile infected cells upward and ejecting them from the monolayer. Bacteria in mounds were less able to spread laterally in the monolayer, limiting the growth of the infection focus, while extruded infected cells underwent cell death. Thus, the coordinated forceful action of uninfected cells actively eliminates large domains of infected cells, consistent with this collective cell response representing an innate immunity-driven process.

Mitochondrial Redox Signaling and Oxidative Stress in Kidney Diseases

Mitochondria are essential organelles in physiology and kidney diseases, because they produce cellular energy required to perform their function. During mitochondrial metabolism, reactive oxygen species (ROS) are produced. ROS function as secondary messengers, inducing redox-sensitive post-translational modifications (PTM) in proteins and activating or deactivating different cell signaling pathways. However, in kidney diseases, ROS overproduction causes oxidative stress (OS), inducing mitochondrial dysfunction and altering its metabolism and dynamics. The latter processes are closely related to changes in the cell redox-sensitive signaling pathways, causing inflammation and apoptosis cell death. Although mitochondrial metabolism, ROS production, and OS have been studied in kidney diseases, the role of redox signaling pathways in mitochondria has not been addressed. This review focuses on altering the metabolism and dynamics of mitochondria through the dysregulation of redox-sensitive signaling pathways in kidney diseases.

Selective mitochondrial antioxidant MitoTEMPO reduces renal dysfunction and systemic inflammation in experimental sepsis

BACKGROUND

Excess mitochondrial reactive oxygen species (mROS) in sepsis is associated with organ failure, in part by generating inflammation through the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome. We determined the impact of a mitochondrial-targeted antioxidant (MitoTEMPO) on mitochondrial dysfunction in renal proximal tubular epithelial cells, peritoneal immune cell function ex vivo, and organ dysfunction in a rat model of sepsis.

METHODS

The effects of MitoTEMPO were assessed ex vivo using adenosine triphosphate and lipopolysaccharide-stimulated rat peritoneal immune cells and fresh rat kidney slices exposed to serum from septic rats. We assessed mROS production and phagocytotic capacity (flow cytometry), mitochondrial functionality (multiphoton imaging, respirometry), and NLRP3 inflammasome activation in cell culture. The effect of MitoTEMPO on organ dysfunction was evaluated in a rat model of faecal peritonitis.

RESULTS

MitoTEMPO decreased septic serum-induced mROS (P<0.001) and maintained normal reduced nicotinamide adenine dinucleotide redox state (P=0.02) and mitochondrial membrane potential (P<0.001) in renal proximal tubular epithelial cells ex vivo. In lipopolysaccharide-stimulated peritoneal immune cells, MitoTEMPO abrogated the increase in mROS (P=0.006) and interleukin-1β (IL-1β) (P=0.03) without affecting non-mitochondrial oxygen consumption or the phagocytotic-induced respiratory burst (P>0.05). In vivo, compared with untreated septic animals, MitoTEMPO reduced systemic IL-1β (P=0.01), reduced renal oxidative stress as determined by urine isoprostane levels (P=0.04), and ameliorated renal dysfunction (reduced serum urea (P<0.001) and creatinine (P=0.05).

CONCLUSIONS

Reduction of mROS by a mitochondria-targeted antioxidant reduced IL-1β, and protected mitochondrial, cellular, and organ functionality after septic insults.

TLR2 Regulates Mast Cell IL-6 and IL-13 Production During Listeria monocytogenes Infection

Listeria monocytogenes (L.m) is efficiently controlled by several cells of the innate immunity, including the Mast Cell (MC). MC is activated by L.m inducing its degranulation, cytokine production and microbicidal mechanisms. TLR2 is required for the optimal control of L.m infection by different cells of the immune system. However, little is known about the MC receptors involved in recognizing this bacterium and whether these interactions mediate MC activation. In this study, we analyzed whether TLR2 is involved in mediating different MC activation responses during L.m infection. We found that despite MC were infected with L.m, they were able to clear the bacterial load. In addition, MC degranulated and produced ROS, TNF-α, IL-1β, IL-6, IL-13 and MCP-1 in response to bacterial infection. Interestingly, L.m induced the activation of signaling proteins: ERK, p38 and NF-κB. When TLR2 was blocked, L.m endocytosis, bactericidal activity, ROS production and mast cell degranulation were not affected. Interestingly, only IL-6 and IL-13 production were affected when TLR2 was inhibited in response to L.m infection. Furthermore, p38 activation depended on TLR2, but not ERK or NF-κB activation. These results indicate that TLR2 mediates only some MC activation pathways during L.m infection, mainly those related to IL-6 and IL-13 production.

Reactive oxygen species: The Yin and Yang in (auto-)immunity

Reactive oxygen species (ROS) are produced by immune cells in response to antigens. They are produced mostly in the mitochondria and their levels are tightly controlled by a series of metabolic processes. ROS are necessary for the development of the immune response but the role of ROS in the development of autoimmune disease needs further clarification. Early clinical information points to the beneficial role of supplementation of antioxidant agents or the reduction of ROS production. We review recent information in the field in an effort to identify areas more studies are needed.

Raising the 'Good' Oxidants for Immune Protection

Redox medicine is a new therapeutic concept targeting reactive oxygen species (ROS) and secondary reaction products for health benefit. The concomitant function of ROS as intracellular second messengers and extracellular mediators governing physiological redox signaling, and as damaging radicals instigating or perpetuating various pathophysiological conditions will require selective strategies for therapeutic intervention. In addition, the reactivity and quantity of the oxidant species generated, its source and cellular location in a defined disease context need to be considered to achieve the desired outcome. In inflammatory diseases associated with oxidative damage and tissue injury, ROS source specific inhibitors may provide more benefit than generalized removal of ROS. Contemporary approaches in immunity will also include the preservation or even elevation of certain oxygen metabolites to restore or improve ROS driven physiological functions including more effective redox signaling and cell-microenvironment communication, and to induce mucosal barrier integrity, eubiosis and repair processes. Increasing oxidants by host-directed immunomodulation or by exogenous supplementation seems especially promising for improving host defense. Here, we summarize examples of beneficial ROS in immune homeostasis, infection, and acute inflammatory disease, and address emerging therapeutic strategies for ROS augmentation to induce and strengthen protective host immunity.

Doxorubicin-Induced Cognitive Impairment: The Mechanistic Insights

Chemotherapy can significantly prolong the survival of patients with breast cancer; Nevertheless, the majority of patients receiving chemotherapy such as doxorubicin may have cognitive deficits that manifest as impairments in learning, reasoning, attention, and memory. The phenomenon of chemotherapy-induced cognitive decline is termed as chemotherapy-related cognitive impairment (CRCI) or chemo-brain. Doxorubicin (DOX), a commonly used drug in adjuvant chemotherapy for patients with breast cancer, has been reported to induce chemo-brain through a variety of mechanisms including DNA damage, oxidative stress, inflammation, dysregulation of apoptosis and autophagy, changes in neurotransmitter levels, mitochondrial dysfunction, glial cell interactions, neurogenesis inhibition, and epigenetic factors. These mechanisms do not operate independently but are inter-related, coordinately contributing to the development of chemo-brain. Here we review the relationships of these mechanisms and pathways in attempt to provide mechanistic insights into the doxorubicin-induced cognitive impairment.

Mitohormesis reprogrammes macrophage metabolism to enforce tolerance

Macrophages generate mitochondrial reactive oxygen species and mitochondrial reactive electrophilic species as antimicrobials during Toll-like receptor (TLR)-dependent inflammatory responses. Whether mitochondrial stress caused by these molecules impacts macrophage function is unknown. Here, we demonstrate that both pharmacologically driven and lipopolysaccharide (LPS)-driven mitochondrial stress in macrophages triggers a stress response called mitohormesis. LPS-driven mitohormetic stress adaptations occur as macrophages transition from an LPS-responsive to LPS-tolerant state wherein stimulus-induced pro-inflammatory gene transcription is impaired, suggesting tolerance is a product of mitohormesis. Indeed, like LPS, hydroxyoestrogen-triggered mitohormesis suppresses mitochondrial oxidative metabolism and acetyl-CoA production needed for histone acetylation and pro-inflammatory gene transcription, and is sufficient to enforce an LPS-tolerant state. Thus, mitochondrial reactive oxygen species and mitochondrial reactive electrophilic species are TLR-dependent signalling molecules that trigger mitohormesis as a negative feedback mechanism to restrain inflammation via tolerance. Moreover, bypassing TLR signalling and pharmacologically triggering mitohormesis represents a new anti-inflammatory strategy that co-opts this stress response to impair epigenetic support of pro-inflammatory gene transcription by mitochondria.

From Flies to Men: ROS and the NADPH Oxidase in Phagocytes

The cellular formation of reactive oxygen species (ROS) represents an evolutionary ancient antimicrobial defense system against microorganisms. The NADPH oxidases (NOX), which are predominantly localized to endosomes, and the electron transport chain in mitochondria are the major sources of ROS. Like any powerful immunological process, ROS formation has costs, in particular collateral tissue damage of the host. Moreover, microorganisms have developed defense mechanisms against ROS, an example for an arms race between species. Thus, although NOX orthologs have been identified in organisms as diverse as plants, fruit flies, rodents, and humans, ROS functions have developed and diversified to affect a multitude of cellular properties, i.e., far beyond direct antimicrobial activity. Here, we focus on the development of NOX in phagocytic cells, where the so-called respiratory burst in phagolysosomes contributes to the elimination of ingested microorganisms. Yet, NOX participates in cellular signaling in a cell-intrinsic and -extrinsic manner, e.g., via the release of ROS into the extracellular space. Accordingly, in humans, the inherited deficiency of NOX components is characterized by infections with bacteria and fungi and a seemingly independently dysregulated inflammatory response. Since ROS have both antimicrobial and immunomodulatory properties, their tight regulation in space and time is required for an efficient and well-balanced immune response, which allows for the reestablishment of tissue homeostasis. In addition, distinct NOX homologs expressed by non-phagocytic cells and mitochondrial ROS are interlinked with phagocytic NOX functions and thus affect the overall redox state of the tissue and the cellular activity in a complex fashion. Overall, the systematic and comparative analysis of cellular ROS functions in organisms of lower complexity provides clues for understanding the contribution of ROS and ROS deficiency to human health and disease.

Mitochondrial Reactive Oxygen Species Regulate Immune Responses of Macrophages to Aspergillus fumigatus

Reactive Oxygen Species (ROS) are highly reactive molecules that can induce oxidative stress. For instance, the oxidative burst of immune cells is well known for its ability to inhibit the growth of invading pathogens. However, ROS also mediate redox signalling, which is important for the regulation of antimicrobial immunity. Here, we report a crucial role of mitochondrial ROS (mitoROS) in antifungal responses of macrophages. We show that mitoROS production rises in murine macrophages exposed to swollen conidia of the fungal pathogen Aspergillus fumigatus compared to untreated macrophages, or those treated with resting conidia. Furthermore, the exposure of macrophages to swollen conidia increases the activity of complex II of the respiratory chain and raises mitochondrial membrane potential. These alterations in mitochondria of infected macrophages suggest that mitoROS are produced via reverse electron transport (RET). Significantly, preventing mitoROS generation via RET by treatment with rotenone, or a suppressor of site IQ electron leak, S1QEL1.1, lowers the production of pro-inflammatory cytokines TNF-α and IL-1β in macrophages exposed to swollen conidia of A. fumigatus. Rotenone and S1QEL1.1 also reduces the fungicidal activity of macrophages against swollen conidia. Moreover, we have established that elevated recruitment of NADPH oxidase 2 (NOX2, also called gp91phox) to the phagosomal membrane occurs prior to the increase in mitoROS generation. Using macrophages from gp91phox-/- mice, we have further demonstrated that NOX2 is required to regulate cytokine secretion by RET-associated mitoROS in response to infection with swollen conidia. Taken together, these observations demonstrate the importance of RET-mediated mitoROS production in macrophages infected with A. fumigatus.

Functions of ROS in Macrophages and Antimicrobial Immunity

Reactive oxygen species (ROS) are a chemically defined group of reactive molecules derived from molecular oxygen. ROS are involved in a plethora of processes in cells in all domains of life, ranging from bacteria, plants and animals, including humans. The importance of ROS for macrophage-mediated immunity is unquestioned. Their functions comprise direct antimicrobial activity against bacteria and parasites as well as redox-regulation of immune signaling and induction of inflammasome activation. However, only a few studies have performed in-depth ROS analyses and even fewer have identified the precise redox-regulated target molecules. In this review, we will give a brief introduction to ROS and their sources in macrophages, summarize the versatile roles of ROS in direct and indirect antimicrobial immune defense, and provide an overview of commonly used ROS probes, scavengers and inhibitors.

Reactive Oxygen Species in Pathogen Clearance: The Killing Mechanisms, the Adaption Response, and the Side Effects

Reactive oxygen species (ROS) are attractive weapons in both antibiotic-mediated killing and host-mediated killing. However, the involvement of ROS in antibiotic-mediated killing and complexities in host environments challenge the paradigm. In the case of bacterial pathogens, the examples of some certain pathogens thriving under ROS conditions prompt us to focus on the adaption mechanism that pathogens evolve to cope with ROS. Based on these, we here summarized the mechanisms of ROS-mediated killing of either antibiotics or the host, the examples of bacterial adaption that successful pathogens evolved to defend or thrive under ROS conditions, and the potential side effects of ROS in pathogen clearance. A brief section for new antibacterial strategies centered around ROS was also addressed.

Corrigendum: The Role Played by Mitochondria in FcϵRI-Dependent Mast Cell Activation

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Age-related mitochondrial dysfunction as a key factor in COVID-19 disease

SARS-CoV-2 causes a severe pneumonia (COVID-19) that affects essentially elderly people. In COVID-19, macrophage infiltration into the lung causes a rapid and intense cytokine storm leading finally to a multi-organ failure and death. Comorbidities such as metabolic syndrome, obesity, type 2 diabetes, lung and cardiovascular diseases, all of them age-associated diseases, increase the severity and lethality of COVID-19. Mitochondrial dysfunction is one of the hallmarks of aging and COVID-19 risk factors. Dysfunctional mitochondria is associated with defective immunological response to viral infections and chronic inflammation. This review discuss how mitochondrial dysfunction is associated with defective immune response in aging and different age-related diseases, and with many of the comorbidities associated with poor prognosis in the progression of COVID-19. We suggest here that chronic inflammation caused by mitochondrial dysfunction is responsible of the explosive release of inflammatory cytokines causing severe pneumonia, multi-organ failure and finally death in COVID-19 patients. Preventive treatments based on therapies improving mitochondrial turnover, dynamics and activity would be essential to protect against COVID-19 severity.