Reactive Oxygen Species: Not Omnipresent but Important in Many Locations

Abstract

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.

Keywords: NADPH oxidases; ROS inhibitors; ROS probes; ROS sources; mitochondria; oxidative stress; reactive oxygen species; redox balance.

FIGURE 1

Several studies suggested that ROS are produced in excess, saturate the cell and find their redox-sensitive targets at random. Usage of diffusible ROS probes, globally working ROS scavengers and unspecific inhibitors often place the suggested ROS source at a completely different location than the redox-sensitive target, which might lead to the interpretation that cells “take into account” the damage that ROS can inflict on their way to the target molecule. The importance of ROS in general for various cellular processes was shown by many excellent studies (Bulua et al., 2011; Nazarewicz et al., 2013; Kelly et al., 2015; Garaude et al., 2016; Kim et al., 2017), however, diffusible ROS probes or only one ROS probe are often used to determine ROS production in cells, which might lead to the suggestions, e.g., that (1) ROS escape from the mitochondrial matrix and regulate expression and secretion of cytokines (Bulua et al., 2011; Kelly et al., 2015), (2) extracellular Nox2-derived ROS modify enzyme activity in the mitochondrial matrix or matrix-derived ROS modulate Nox2 activity (Nazarewicz et al., 2013; Garaude et al., 2016) or (3) ROS produced by ER-located Nox4 reach the phagosome for inactivation of phagocytosed parasites (Kim et al., 2017).

FIGURE 2

Growing evidence supports the hypothesis that cellular compartments show big differences and tight regulation of their redox status. The induction of ROS production is controlled by the cell in terms of location, source, duration and amount. The localized and timely controlled ROS production in the direct vicinity of the redox-sensitive target reduces the induced damage to cellular components and results in beneficial consequences for the cell, a condition termed as oxidative eustress (Sies, 2021). Examples of localized ROS production are (1) the production of antimicrobial ROS by Nox2 (Craig and Slauch, 2009; Gluschko et al., 2018) or mitochondria, which are recruited to pathogen-containing phagosomes (West et al., 2011a; Geng et al., 2015), (2) the recruitment of the redox-regulated target to ROS-producing mitochondria for NLRP3 inflammasome activation (Zhou et al., 2011) or the relocation of mitochondria to the nucleus for ROS-mediated nuclear signaling (Al-Mehdi et al., 2012) and (3) ROS production by ER-localized Nox4 during formation of mitochondria-associated membranes for regulation of calcium signaling (Beretta et al., 2020).