Identification and functional significance of a brain aquaporin‐4/Na+, K+‐ATPase/mGluR5 macromolecular complex

Abstract

The water channel aquaporin‐4 (AQP4) is expressed in astrocytes, which play a major role in clearing the narrow extracellular space (ES) between neurons and astrocyte endfeet from K+ and glutamate released during neuronal activity. Compelling evidence suggests that AQP4 may play a role in the K+ clearance. Here we show that AQP4 co‐immunoprecipitates with Na+, K+‐ATPase and inwardly rectifying K+ channel Kir4.1; transporters of key importance for the K+ clearance. Using GST pull down assay, we identified the MVAFKGVWTQ of the AQP4 N‐terminus as the site for interaction with Na+,K+‐ATPase. The metabotropic glutamate receptor mGluR5 was also pulled down by AQP4. Using rubidium uptake and astrocyte water permeability measurements, we found that the mGluR agonist DHPG increased both Na+,K+‐ATPase‐mediated K+ transport and AQP4‐mediated water transport. Finally, we created a mathematical model, of K+ transport across the plasma membrane in the ES. This model predicts that AQP4‐driven water transport will facilitate K+ clearance and stabilize the membrane potential. Thus we have demonstrated a multiprotein K+‐transporting microdomain in the astrocyte that includes AQP4. This microdomain is functionally regulated by glutamate and serves to optimize the clearance of K+ from the ES.