Fundamentals of redox regulation in biology.

Oxidation-reduction (redox) reactions are central to the existence of life. Reactive species of oxygen, nitrogen and sulfur mediate redox control of a wide range of essential cellular processes. Yet, excessive levels of oxidants are associated with ageing and many diseases, including cardiological and neurodegenerative diseases, and cancer. Hence, maintaining the fine-tuned steady-state balance of reactive species production and removal is essential. Here, we discuss new insights into the dynamic maintenance of redox homeostasis (that is, redox homeodynamics) and the principles underlying biological redox organization, termed the 'redox code'. We survey how redox changes result in stress responses by hormesis mechanisms, and how the lifelong cumulative exposure to environmental agents, termed the 'exposome', is communicated to cells through redox signals. Better understanding of the molecular and cellular basis of redox biology will guide novel redox medicine approaches aimed at preventing and treating diseases associated with disturbed redox regulation.

Defining roles of specific reactive oxygen species (ROS) in cell biology and physiology.

'Reactive oxygen species' (ROS) is a generic term that defines a wide variety of oxidant molecules with vastly different properties and biological functions that range from signalling to causing cell damage. Consequently, the description of oxidants needs to be chemically precise to translate research on their biological effects into therapeutic benefit in redox medicine. This Expert Recommendation article pinpoints key issues associated with identifying the physiological roles of oxidants, focusing on H2O2 and O2.-. The generic term ROS should not be used to describe specific molecular agents. We also advocate for greater precision in measurement of H2O2, O2.- and other oxidants, along with more specific identification of their signalling targets. Future work should also consider inter-organellar communication and the interactions of redox-sensitive signalling targets within organs and whole organisms, including the contribution of environmental exposures. To achieve these goals, development of tools that enable site-specific and real-time detection and quantification of individual oxidants in cells and model organisms are needed. We also stress that physiological O2 levels should be maintained in cell culture to better mimic in vivo redox reactions associated with specific cell types. Use of precise definitions and analytical tools will help harmonize research among the many scientific disciplines working on the common goal of understanding redox biology.

Oxidative Stress.

Oxidative stress is two sided: Whereas excessive oxidant challenge causes damage to biomolecules, maintenance of a physiological level of oxidant challenge, termed oxidative eustress, is essential for governing life processes through redox signaling. Recent interest has focused on the intricate ways by which redox signaling integrates these converse properties. Redox balance is maintained by prevention, interception, and repair, and concomitantly the regulatory potential of molecular thiol-driven master switches such as Nrf2/Keap1 or NF-κB/IκB is used for system-wide oxidative stress response. Nonradical species such as hydrogen peroxide (H2O2) or singlet molecular oxygen, rather than free-radical species, perform major second messenger functions. Chemokine-controlled NADPH oxidases and metabolically controlled mitochondrial sources of H2O2 as well as glutathione- and thioredoxin-related pathways, with powerful enzymatic back-up systems, are responsible for fine-tuning physiological redox signaling. This makes for a rich research field spanning from biochemistry and cell biology into nutritional sciences, environmental medicine, and molecular knowledge-based redox medicine.

Reactive oxygen species (ROS) as pleiotropic physiological signalling agents.

'Reactive oxygen species' (ROS) is an umbrella term for an array of derivatives of molecular oxygen that occur as a normal attribute of aerobic life. Elevated formation of the different ROS leads to molecular damage, denoted as 'oxidative distress'. Here we focus on ROS at physiological levels and their central role in redox signalling via different post-translational modifications, denoted as 'oxidative eustress'. Two species, hydrogen peroxide (H2O2) and the superoxide anion radical (O2·-), are key redox signalling agents generated under the control of growth factors and cytokines by more than 40 enzymes, prominently including NADPH oxidases and the mitochondrial electron transport chain. At the low physiological levels in the nanomolar range, H2O2 is the major agent signalling through specific protein targets, which engage in metabolic regulation and stress responses to support cellular adaptation to a changing environment and stress. In addition, several other reactive species are involved in redox signalling, for instance nitric oxide, hydrogen sulfide and oxidized lipids. Recent methodological advances permit the assessment of molecular interactions of specific ROS molecules with specific targets in redox signalling pathways. Accordingly, major advances have occurred in understanding the role of these oxidants in physiology and disease, including the nervous, cardiovascular and immune systems, skeletal muscle and metabolic regulation as well as ageing and cancer. In the past, unspecific elimination of ROS by use of low molecular mass antioxidant compounds was not successful in counteracting disease initiation and progression in clinical trials. However, controlling specific ROS-mediated signalling pathways by selective targeting offers a perspective for a future of more refined redox medicine. This includes enzymatic defence systems such as those controlled by the stress-response transcription factors NRF2 and nuclear factor-κB, the role of trace elements such as selenium, the use of redox drugs and the modulation of environmental factors collectively known as the exposome (for example, nutrition, lifestyle and irradiation).

Homeostatic control of redox status and health.

Research on oxidants and electrophiles has shifted from focusing on damage to biomolecules to the more fine-grained physiological arena. Redox transitions as excursions from a steady-state redox set point are continually ongoing in maintenance of redox balance. Current excitement on these topics results from the fact that recent research provided mechanistic insight, which gives rise to more concrete and differentiated questions. This Commentary focuses on redox eustress and the feedback restoration of steady state as concepts in active maintenance of physiological health, with brief discussion of redox stress response to viral infection, exemplified by COVID-19.

Findings in redox biology: From H2O2 to oxidative stress.

My interest in biological chemistry proceeded from enzymology in vitro to the study of physiological chemistry in vivo Investigating biological redox reactions, I identified hydrogen peroxide (H2O2) as a normal constituent of aerobic life in eukaryotic cells. This finding led to developments that recognized the essential role of H2O2 in metabolic redox control. Further research included studies on GSH, toxicological aspects (the concept of "redox cycling"), biochemical pharmacology (ebselen), nutritional biochemistry and micronutrients (selenium, carotenoids, flavonoids), and the concept of "oxidative stress." Today, we recognize that oxidative stress is two-sided. It has its positive side in physiology and health in redox signaling, "oxidative eustress," whereas at higher intensity, there is damage to biomolecules with potentially deleterious outcome in pathophysiology and disease, "oxidative distress." Reflecting on these developments, it is gratifying to witness the enormous progress in redox biology brought about by the science community in recent years.

German-Japanese relationships in biochemistry: a personal perspective.

The first Institute of Biochemistry in Japan was founded by Leonor Michaelis from Berlin at Nagoya in 1922, and there have been numerous interrelations between Japanese and German biochemists since. Some such relationships are presented here from a personal point of view as one illustrative example, which could be extended amply by the experience of many other scientists from the two countries. Fruitful exchanges are facilitated by organisations such as the Alexander von Humboldt Foundation (AvH) and the Deutscher Akademischer Austauschienst (DAAD) or the Japanese Society for the Promotion of Science (JSPS) and by the many bilateral agreements between universities and research institutions.

Role of metabolic H2O2 generation: redox signaling and oxidative stress.

Hydrogen peroxide, the nonradical 2-electron reduction product of oxygen, is a normal aerobic metabolite occurring at about 10 nm intracellular concentration. In liver, it is produced at 50 nmol/min/g of tissue, which is about 2% of total oxygen uptake at steady state. Metabolically generated H2O2 emerged from recent research as a central hub in redox signaling and oxidative stress. Upon generation by major sources, the NADPH oxidases or Complex III of the mitochondrial respiratory chain, H2O2 is under sophisticated fine control of peroxiredoxins and glutathione peroxidases with their backup systems as well as by catalase. Of note, H2O2 is a second messenger in insulin signaling and in several growth factor-induced signaling cascades. H2O2 transport across membranes is facilitated by aquaporins, denoted as peroxiporins. Specialized protein cysteines operate as redox switches using H2O2 as thiol oxidant, making this reactive oxygen species essential for poising the set point of the redox proteome. Major processes including proliferation, differentiation, tissue repair, inflammation, circadian rhythm, and aging use this low molecular weight oxygen metabolite as signaling compound.

Upregulation of the thioredoxin-dependent redox system during differentiation of 3T3-L1 cells to adipocytes.

Hydrogen peroxide acts as a signaling molecule in early adipogenesis. In differentiating adipocytes, elevated hydrogen peroxide generation is balanced through induction of antioxidant enzymes such as catalase and peroxiredoxins. Thioredoxin reductases (TrxR) and glutathione peroxidases (GPx) are selenoenzymes that constitute part of the major thiol-dependent antioxidant systems in cells. Here we show that the protein levels of cytoplasmic/nuclear TrxR1 and mitochondrial TrxR2 increase in the course of adipocyte differentiation of 3T3-L1 cells together with the TrxR2 substrate thioredoxin 2 (Trx2), resulting in elevated TrxR activity in mature adipocytes. Gene and protein expression of the GPx isoenzyme GPx4 was also stimulated during adipogenesis. Chronic exposure of 3T3-L1 cells to the anti-adipogenic factors tumor necrosis factor α (TNF-α) or rapamycin during differentiation suppressed TrxR1 and Trx2 upregulation, concomitantly with inhibition of adipogenesis and lipogenesis. In contrast, TNF-α or rapamycin did not affect expression of TrxRs and their Trx substrates in mature adipocytes. These results indicate that upregulation of the thioredoxin-dependent redox system is linked to the development of an adipocyte phenotype.

Intestinal selenoprotein P in epithelial cells and in plasma cells.

The micronutrient selenium and selenium-containing selenoproteins are involved in prevention of inflammation and carcinogenesis in the gut. Selenoprotein P (Sepp1), the plasma selenium transport protein, is secreted primarily from hepatocytes, but Sepp1 mRNA is also abundant in the intestine. By immunofluorescence analysis, we show that Sepp1 levels in epithelial cells of the rat jejunum increase along the crypt-to-villus axis. A different Sepp1 distribution pattern was observed in the rat colon, where the epithelial cells located at the base and at the top of the crypts were similarly positive for Sepp1. In addition, we found pronounced Sepp1 immunoreactivity in CD138-positive plasma cells scattered within the lamina propria of the colon. This hitherto unrecognized presence in terminally differentiated B-cells was corroborated by detection of Sepp1 in plasma cells residing in the rat spleen. Following supplementation with dietary selenium compounds, polarized intestinal epithelial Caco-2 cells secreted Sepp1 into the culture medium across the basolateral membrane. Our data suggest that Sepp1 secreted from epithelial cells may support the intestinal immune system by providing immune cells (including plasma cells) with selenium for the biosynthesis of endogenous selenoproteins.

Towards identifying novel anti-Eimeria agents: trace elements, vitamins, and plant-based natural products.

Eimeriosis, a widespread infectious disease of livestock, is caused by coccidian protozoans of the genus Eimeria. These obligate intracellular parasites strike the digestive tract of their hosts and give rise to enormous economic losses, particularly in poultry, ruminants including cattle, and rabbit farming. Vaccination, though a rational prophylactic measure, has not yet been as successful as initially thought. Numerous broad-spectrum anti-coccidial drugs are currently in use for treatment and prophylactic control of eimeriosis. However, increasing concerns about parasite resistance, consumer health, and environmental safety of the commercial drugs warrant efforts to search for novel agents with anti-Eimeria activity. This review summarizes current approaches to prevent and treat eimeriosis such as vaccination and commercial drugs, as well as recent attempts to use dietary antioxidants as novel anti-Eimeria agents. In particular, the trace elements selenium and zinc, the vitamins A and E, and natural products extracted from garlic, barberry, pomegranate, sweet wormwood, and other plants are discussed. Several of these novel anti-Eimeria agents exhibit a protective role against oxidative stress that occurs not only in the intestine of Eimeria-infected animals, but also in their non-parasitized tissues, in particular, in the first-pass organ liver. Currently, it appears to be promising to identify safe combinations of low-cost natural products with high anti-Eimeria efficacy for a potential use as feed supplementation in animal farming.

Dietary selenium affects intestinal development of Eimeria papillata in mice.

Here, we investigated the effect of the trace element selenium (Se) on course and outcome of Eimeria-paplllata-induced coccidiosis in mice. Male mice were fed on Se-adequate (0.15 ppm), Se-deficient, and Se-high diets (1.0 ppm) for 6 weeks. Mice were orally infected with 1,000 oocysts. The prepatent period lasts for 3 days, but the course of infections varied. At Se-adequate diet, the maximum fecal output of oocysts amounted to 68,300 ooccysts/g feces on day 5 p.i.. However, fecal shedding of oocysts was accelerated in mice on Se-deficient diet and occurred already on day 4 p.i.. By contrast, maximal shedding is impaired in mice on high-Se diet, which takes place on day 5 p.i., but with a decreased output of only 7,300 oocysts/g feces. Light microscopy reveals that all developmental stages are affected: meronts, micro- and macrogamonts, and developing oocysts are increased in comparison with mice fed on selenium-adequate diet. At high Se, the number of parasitic stages in the jejunum is substantially higher than at Se-deficient diet. Se does not affect the number of jejunal Alcian blue-stained goblet cells. Se deficiency increased the number of apoptotic cells in the jejunum. Substantially increased histological injury scores reveal more injuries in jejunum tissue infected by E. papillata. Our data indicate that high dietary Se exerts potential anticoccidial activity. This may be taken advantage of in control measures towards Eimeriosis as a feed additive, potentially alleviating the need for concomitantly utilized anti-coccidial drugs in the feed.

Was hydrogen peroxide present before the arrival of oxygenic photosynthesis? The important role of iron(II) in the Archean ocean.

We address the chemical/biological history of H2O2 back at the times of the Archean eon (2.5-3.9 billion years ago (Gya)). During the Archean eon the pO2 was million-fold lower than the present pO2, starting to increase gradually from 2.3 until 0.6 Gya, when it reached ca. 0.2 bar. The observation that some anaerobic organisms can defend themselves against O2 has led to the view that early organisms could do the same before oxygenic photosynthesis had developed at about 3 Gya. This would require the anaerobic generation of H2O2, and here we examine the various mechanisms which were suggested in the literature for this. Given the concentration of Fe2+ at 20-200 µM in the Archean ocean, the estimated half-life of H2O2 is ca. 0.7 s. The oceanic H2O2 concentration was practically zero. We conclude that early organisms were not exposed to H2O2 before the arrival of oxygenic photosynthesis.

Was H2O2 generated before oxygenic photosynthesis?

We obviously agree with Wu et al. that H2O2 might accumulate in the Archean land waters devoid of Fe2+. We do disagree on the topic of the half-life of H2O2, as the work cited in support for a longer half-live is not relevant to the conditions in the Archean ocean. While the existence of radicals in quartz is not in doubt, we do question the hypothesis that these radicals oxidize water to HO• and H2O2.

Health position paper and redox perspectives - Disease burden by transportation noise.

Transportation noise is a ubiquitous urban exposure. In 2018, the World Health Organization concluded that chronic exposure to road traffic noise is a risk factor for ischemic heart disease. In contrast, they concluded that the quality of evidence for a link to other diseases was very low to moderate. Since then, several studies on the impact of noise on various diseases have been published. Also, studies investigating the mechanistic pathways underlying noise-induced health effects are emerging. We review the current evidence regarding effects of noise on health and the related disease-mechanisms. Several high-quality cohort studies consistently found road traffic noise to be associated with a higher risk of ischemic heart disease, heart failure, diabetes, and all-cause mortality. Furthermore, recent studies have indicated that road traffic and railway noise may increase the risk of diseases not commonly investigated in an environmental noise context, including breast cancer, dementia, and tinnitus. The harmful effects of noise are related to activation of a physiological stress response and nighttime sleep disturbance. Oxidative stress and inflammation downstream of stress hormone signaling and dysregulated circadian rhythms are identified as major disease-relevant pathomechanistic drivers. We discuss the role of reactive oxygen species and present results from antioxidant interventions. Lastly, we provide an overview of oxidative stress markers and adverse redox processes reported for noise-exposed animals and humans. This position paper summarizes all available epidemiological, clinical, and preclinical evidence of transportation noise as an important environmental risk factor for public health and discusses its implications on the population level.

Selenium homeostasis and antioxidant selenoproteins in brain: implications for disorders in the central nervous system.

The essential trace element selenium, as selenocysteine, is incorporated into antioxidant selenoproteins such as glutathione peroxidases (GPx), thioredoxin reductases (TrxR) and selenoprotein P (Sepp1). Although comparatively low in selenium content, the brain exhibits high priority for selenium supply and retention under conditions of dietary selenium deficiency. Liver-derived Sepp1 is the major transport protein in plasma to supply the brain with selenium, serving as a "survival factor" for neurons in culture. Sepp1 expression has also been detected within the brain. Presumably, astrocytes secrete Sepp1, which is subsequently taken up by neurons via the apolipoprotein E receptor 2 (ApoER2). Knock-out of Sepp1 or ApoER2 as well as neuron-specific ablation of selenoprotein biosynthesis results in neurological dysfunction in mice. Astrocytes, generally less vulnerable to oxidative stress than neurons, are capable of up-regulating the expression of antioxidant selenoproteins upon brain injury. Occurrence of neurological disorders has been reported occasionally in patients with inadequate nutritional selenium supply or a mutation in the gene encoding selenocysteine synthase, one of the enzymes involved in selenoprotein biosynthesis. In three large trials carried out among elderly persons, a low selenium status was associated with faster decline in cognitive functions and poor performance in tests assessing coordination and motor speed. Future research is required to better understand the role of selenium and selenoproteins in brain diseases including hepatic encephalopathy.