Aquaporins as channels for CO2, O2 and H2O2

• Applicant: Privatdozent Dr. Volker Endeward
• Subject Area: Anatomy and Physiology
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 413120011

The project "Aquaporins as channels for CO2, O2 and H2O2“ aims at characterizing the transport function of aquaporins (AQP) for the respiratory gases O2 and CO2, and for H2O2 serving as an example of a signal molecule. In the 1st of three subprojects we study the significance of AQP5 for pulmonary gas exchange. The major site of expression of AQP5 is lung alveolar epithelium. As AQP5 is an efficient channel for CO2 – and probably O2 as well – we pursue the hypothesis that pulmonary AQP5 makes a major contribution to pulmonary gas exchange. Indeed, we have shown in preliminary work that under hypoxia maximal O2 consumption is markedly reduced in AQP5-knockout mice. We will study if impaired diffusion of respiratory gases in the lung is the cause for this finding by comparing AQP5-KO and WT mice with respect to 1.) CO2 permeability of isolated alveolar type I epithelia, 2.) gas diffusion capacity of the lungs in vivo, and 3.) arterial blood gases of mice under conditions of maximal O2 consumption in hypoxia. If the hypothesis of a significant role of AQP5 can be confirmed, this would constitute a major extension of the established concept of pulmonary gas exchange.In the 2nd subproject we will test the hypothesis that a complex of AQP1 and carbonic anhydrase II (CAII), acting as a “metabolon” in the membrane, facilitates CO2 permeation by rapid formation of CO2 from HCO3- and delivering this CO2 directly to the gas pore of AQP1. Such a mechanism is plausible, because HCO3- concentration is 20 times greater than that of CO2 and thus serves as reservoir for CO2. The existence of the complex will be checked by FRET and Bimolecular Fluorescence Complementation (BiFC) techniques applied to HEK293 cells that coexpress both partner proteins. Next, we will determine whether the simultaneous presence of AQP1 and CAII in the membranes of a) HEK293 cells and b) liposomes indeed accelerates CO2 transport compared to AQP1 alone. If the hypothesis can be proven, this project will establish a novel mechanistic feature of CO2 permeation through gas channels. In the 3rd subproject we endeavour to study the basal properties of H2O2 permeation through lipid membranes and aquaporins. Membrane permeation of H2O2 is crucial for its action as a signal molecule, both when it is released from its site of production in the mitochondrium, and when it is taken up into cells inside which either its signaling action occurs or inside which it is inactivated such as in red blood cells (RBC). The influence of membrane lipid composition on H2O2 permeation will be determined by measuring H2O2 fluxes across liposomes. Using RBC including RBC from AQP1-KO mice we will characterize H2O2 permeation across this membrane and the role of AQP1 therein. Using mitochondria from normal livers and from HepG2 cell cultures with AQP8 knockdown, we will determine H2O2 permeation across the mitochondrial membrane as well as the role of the AQP8 expressed in these membranes.

• DFG Programme: Research Grants