Redox crosstalk at endoplasmic reticulum (ER) membrane contact sites (MCS) uses toxic waste to deliver messages

Abstract

Many cellular redox reactions housed within mitochondria, peroxisomes and the endoplasmic reticulum (ER) generate hydrogen peroxide (H₂O₂) and other reactive oxygen species (ROS). The contribution of each organelle to the total cellular ROS production is considerable, but varies between cell types and also over time. Redox-regulatory enzymes are thought to assemble at a "redox triangle" formed by mitochondria, peroxisomes and the ER, assembling "redoxosomes" that sense ROS accumulations and redox imbalances. The redoxosome enzymes use ROS, potentially toxic by-products made by some redoxosome members themselves, to transmit inter-compartmental signals via chemical modifications of downstream proteins and lipids. Interestingly, important components of the redoxosome are ER chaperones and oxidoreductases, identifying ER oxidative protein folding as a key ROS producer and controller of the tri-organellar membrane contact sites (MCS) formed at the redox triangle. At these MCS, ROS accumulations could directly facilitate inter-organellar signal transmission, using ROS transporters. In addition, ROS influence the flux of Ca²⁺ ions, since many Ca²⁺ handling proteins, including inositol 1,4,5 trisphosphate receptors (IP₃Rs), SERCA pumps or regulators of the mitochondrial Ca²⁺ uniporter (MCU) are redox-sensitive. Fine-tuning of these redox and ion signaling pathways might be difficult in older organisms, suggesting a dysfunctional redox triangle may accompany the aging process.

Fig. 1. The ER–mitochondria–peroxisome redox triangle

The endoplasmic reticulum (ER), mitochondria and peroxisomes are three important redox-sensitive organelles. All three house biochemical reactions that produce reactive oxygen species (ROS), for the list of ROS producers see Table 1. ROS can be released by all three organelles through aquaporins or as of yet unknown proteinaceous channels. Accumulated ROS within the redox triangle affect the functioning of ER–mitochondria Ca²⁺ exchange, oxidative phosphorylation, and especially oxidative protein folding within the ER

Fig. 2. Overview of the ER redox processes

The oxidation of Ero1 by O₂ initiates disulfide relays leading to the insertion of disulfide bonds into proteins as they fold in the ER (here, the protein “Y”) with oxidoreductases of the PDI family playing a key intermediary role (see main text for details). H₂O₂ produced during Ero1 oxidation can be scavenged by the peroxidases Gpx7, Gpx8 and Prx4. H₂O₂ is also formed in the ER NOX4. Upon H₂O₂ scavenging, oxidized Prx4 and Gpx7 transfer a disulfide bond to PDI, contributing to oxidative protein folding. Intermolecular disulfide bonds are also formed into the ER for establishing oligomeric covalent structures or for example retaining misfolded proteins

Fig. 3. Overview of redox post-translational modifications of cysteines

Oxidation by ROS (like H₂O₂) initially leads to sulfenylation (SOH). Sulfenylated cysteine can additionally react with ROS leading first to sulfinylation (SO₂H) and then to sulfonylation (SO₃H). While sulfonylation is so far considered irreversible, sulfinylation can be reversed through the catalytic activity of the cytoplasmic enzyme sulfiredoxin-1 (SRXN-1). Many reactions can lead to disulfide bond formation: (i) intermolecular disulfide bonds can be formed with another protein or low molecular weight thiols (glutathione for example), (ii) intramolecular disulfide bonds are often inserted into a reduced protein by disulfide exchange (via formation of mixed disulfides) with GSSG or another oxidized protein (e.g., PDI) or through reaction of the relative instable sulfenylated cysteine. Please note that reactions involving thiol groups (SH) implies the formation of thiolate (_-S^-) through deprotonation and so are strongly dependent on the local pKa