The Plasma Membrane: A Platform for Intra- and Intercellular Redox Signaling

Abstract

Membranes are of outmost importance to allow for specific signal transduction due to their ability to localize, amplify, and direct signals. However, due to the double-edged nature of reactive oxygen species (ROS)-toxic at high concentrations but essential signal molecules-subcellular localization of ROS-producing systems to the plasma membrane has been traditionally regarded as a protective strategy to defend cells from unwanted side-effects. Nevertheless, specialized regions, such as lipid rafts and caveolae, house and regulate the activated/inhibited states of important ROS-producing systems and concentrate redox targets, demonstrating that plasma membrane functions may go beyond acting as a securing lipid barrier. This is nicely evinced by nicotinamide adenine dinucleotide phosphate (NADPH)-oxidases (NOX), enzymes whose primary function is to generate ROS and which have been shown to reside in specific lipid compartments. In addition, membrane-inserted bidirectional H₂O₂-transporters modulate their conductance precisely during the passage of the molecules through the lipid bilayer, ensuring time-scaled delivery of the signal. This review aims to summarize current evidence supporting the role of the plasma membrane as an organizing center that serves as a platform for redox signal transmission, particularly NOX-driven, providing specificity at the same time that limits undesirable oxidative damage in case of malfunction. As an example of malfunction, we explore several pathological situations in which an inflammatory component is present, such as inflammatory bowel disease and neurodegenerative disorders, to illustrate how dysregulation of plasma-membrane-localized redox signaling impacts normal cell physiology.

Keywords: NADPH oxidase; aquaporin; inflammation; inflammatory bowel disease; lipid rafts; neurodegenerative disorders; plasma membrane; redox signaling; redoxosome.

Figure 1

Major producing and removal pathways for reactive oxygen species (ROS). The sequential steps of the univalent reduction of molecular oxygen (O₂) to water (H₂O) leading to the generation of several ROS intermediates are shown. Diverse redox enzymatic systems, mainly mitochondrial respiration complexes and membrane-residing NADPH oxidases (NOXes), can convert O₂ into superoxide (O₂^•−). Superoxide dismutases (SOD) catalyze the dismutation of superoxide (O₂^•−) into H₂O₂ and O₂. H₂O₂ can be reduced directly to water by peroxiredoxins (Prx), glutathione peroxidases (GPX), or catalases (CAT). Alternatively, hydroxyl radicals (OH^•) are generated from H₂O₂ in the presence of reduced transition metals, such as Fe²⁺ (Fenton reaction). Red squares and green squares represent paired and unpaired electrons, respectively, in the oxygen atom. White squares represent the electron provided by hydrogen atoms.

Figure 2

Main models for ROS signal transmission to specific cysteines. (A) The direct model presumes that redox targets (depicted as a blue teardrop) are constituted by proteins whose rate constant of oxidation is higher than that of the reaction of ROS with its major cellular scavengers, particularly peroxiredoxins. Direct chemical reaction of ROS with those targets is thus kinetically possible and leads to their oxidation. (B) The binding hypothesis proposes that H₂O₂ sources and targets are bound or in close proximity, allowing for site-localized oxidation to occur. Also in this case, ROS will directly oxidize targets but as a function of their relative proximity to the source instead of depending on their rate constant of reaction. (C) The floodgate model overcomes the apparent kinetic limitation of target oxidation by suggesting that overoxidation of peroxiredoxins (represented by green hexagons) permits further oxidation of other proteins with slower reaction rates. (D) Peroxiredoxins have been already shown to behave as relays, transmitting redox equivalents to targets during their oxidation–reduction cycle, and thus allowing for signal transduction. This model implies an indirect ROS effect on targets, as ROS will oxidize peroxiredoxins and will not chemically react with targets. In all cases, a plasma-membrane-bound NADPH oxidase (NOX, in dark pink) has been chosen as a representative source of ROS acting at the cellular surface. Note that the four possibilities are not mutually excluding.

Figure 3

Cellular factors of plasma membrane redox signaling. As integral membrane proteins, NADPH oxidases (NOX, dark pink) are present either on lipid rafts (dashed blue line) or in caveolae, releasing their product superoxide (O₂^•−) into the extracellular space. Upon activation of NOXes, these enzymes can be also internalized into forming redoxosomes where they are still functional. Besides, extracellular O₂^•− can be generated by the activity of an extracellularly linked xanthine oxidase (XO, blue). In its uncoupled state, NOS enzymes also produce O₂^•− in the vicinity of caveolae, but since there is a certain controversy on whether this happens in non-pathological conditions, the fact is indicated with a question mark. Nevertheless, extracellular (SODex, yellow) or intracellular (SODin, brown) superoxide dismutases convert O₂^•− into hydrogen peroxide (H₂O₂) depending on the localization of the sources. H₂O₂ is transported inside or outside the cell by specialized aquaporins (AQP, orange channel), following cellular needs.

Figure 4

Putative mechanisms adopted by cells to achieve redox signal transmission after peroxiporin-mediated routing of ROS through the plasma membrane. The NOX/AQP system, situated in privileged lipid platforms for signaling (blue dashed line), offers a new perspective to re-interpret the models for redox signal transmission to intracellular targets (depicted as blue teardrops): site-localized oxidation of vicinal proteins will be favored by strategical positioning near the cytosolic mouth of the channel, possibly by localizing them to lipid raft domains, while further proteins would be reached either directly by overoxidation of peroxiredoxins (represented as green hexagons) or indirectly via peroxiredoxin-mediated relays. The particular disposition of NOXes releasing ROS to the extracellular space will also induce similar pathways in a peroxiporin-equipped adjacent cell (upper part of the scheme), thus allowing for a coordinated response. The figure has been organized as a cascade for clarity. However, all modes of signal transduction may be not hierarchical and concur simultaneously.