AQP11 mediates H2O2 fluxes to regulate cellular redox homeostasis and signalling

Like many reactive oxygen species (ROS), H2O2 is an essential second messenger at low levels, but toxic at high concentrations. For this reason, sophisticated mechanisms evolved to channel useful redox signals limiting unwanted effects. Thus, proteinaceous channels are needed to regulate H2O2 transport across lipid bilayers. In principle, therefore, there should be an H2O2 conduit (hence endowed with peroxiporin activity) in all organelles that produce it (plasma membrane., ER and mitochondria). Members of the aquaporin family have been shown to transport H2O2, in addition to water. Of these, AQP3, 8, 9 are present in the plasma membrane. We investigated whether additional family member(s) reside(s) primarily in the ER. This compartment hosts oxidative folding: as such it is a source of H2O2. Using imaging and biochemical techniques, we demonstrated that AQP11 is an ER resident protein, partly accumulating in mitochondrial associated membranes (MAM). Its silencing inhibits the entry of exogenous H2O2 into the ER, confirming that AQP11 acts as a peroxiporin. It also favors the accumulation of H2O2 in the ER lumen, suggesting that AQP11 mediates a constitutive ER-to-cytosol flux. To investigate the source(s) and significance of this flux, I knocked-down Ero1α and NADPH oxidase 4, two ER-resident enzymes known to produce H2O2. Unexpectedly, however, even higher concentrations of H2O2 accumulated in the ER of Ero1αKD cells. The simultaneous silencing of AQP11 prevented H2O2 accumulation, suggesting that AQP11 channels can internalize H2O2 originating from a different compartment. By using a combination of techniques, I identified complex III in the inner mitochondrial membrane as an external source capable of insufflating H2O2 into the ER through AQP11. These results identify a novel pathway regulating interorganellar redox homeostasis and pave the way for the identification of the underlying signals and functional role.