1998 Global Conference Proceeding

March 02 - 04, 1998

Conference: 1998 Global Conference
Title: Aquaporin 2-containing Apical Membrane Endosomes (AQP-2 Endosomes) Possess a Multiprotein Signaling Complex
Author: Harris, H. William
Institution: Aqua Bio Products Sciences Llc
William Harris Our laboratory has previously purified endosomes derived from the apical membrane of rat kidney inner medullary collecting ducts (IMCD) containing abundant functional AQP-2 protein. Phosphorylation of AQP-2 by cAMP-dependent protein kinase (PKA) is believed to regulate the membrane trafficking of AQP-2 within IMCD cells. Purified apical membrane endosomes contain AQP-2 that is phosphorylated by endogenous PKA and dephosphorylated by a phosphatase (Lande, MB et al. J. Biol. Chem. 271:5552, 1996. ). A combination of immunoblotting and overlay assays reveal that AQP-2 endosomes contain the regulatory subunit of type II PKA (RII), protein phosphatase 2B (PP2B or calcineurin), the § isoform of protein kinase C together with a 95 kDa PKA anchoring protein or AKAP. Affinity chromatography of Triton X-100 solubilized AQP-2 endosomal proteins on cAMP agarose permits recovery of these 4 proteins as a complex similar to a AKAP 79 signaling complex present in postsynaptic densities of hippocampal neurons (Coghlan, VM et al. Science 267:108, 1995). In similar studies (Sands, JM et al. J. Clin. Invest. 99:1399, 1997) we have identified another signaling protein, the extracellular calcium polyvalent cation sensing receptor (CaR) protein and its associated signal transduction components that are also present in AQP-2 endosomes. By sensing the ionic composition of the extracellular fluid bathing the IMCD apical membrane, CaR may regulate apical membrane trafficking in IMCD in response to alterations in AVP-elicited water transport. We conclude these data provide evidence for the existence of an apical membrane signaling complex that may regulate the apical membrane trafficking of the vasopressin-elicited water channel AQP-2 and thus modulate IMCD water permeability. Pharmacological manipulation of these apical membrane signaling components may prove of value as new therapies for nephrogenic diabetes insipidus.
Water-transporting proteins such as aquaporin 2 (AQP2) help water permeate cell membranes. To do this they must be able to travel from inside cells to the cell surface. Phosphorylation, a metabolic process that introduces a phosphate group into AQP2, is believed to regulate AQP2 movement to and from the cell surface. Harris, et. al., have identified a protein called the extracellular calcium polyvalent cation sensing receptor protein (CaR) in the AQP2. They conclude CaR is an integral part of a membrane signaling complex within AQP2 that may help AQP2 travel to cell membranes, thus regulating the degree to which water may permeate the membranes of water collecting ducts within the kidney. The authors' believe these membrane signaling components may be manipulated through drugs, and that this could prove to be a valuable new therapy in treating nephrogenic diabetes insipidus (NDI).