Uncoupling of the Astrocyte Syncytium Differentially Affects AQP4 Isoforms.

Shirin Katoozi,Martine Cohen-Salmon,Mahmood Amiry-Moghaddam,Ole Petter Ottersen,Tushar Deshpande,Claudia Palazzo,Pascal Ezan,Christian Steinhäuser,Antonio Frigeri,Soulmaz Zahl,Nadia Skauli

doi:10.3390/cells9020382

Abstract

The water channel protein aquaporin-4 (AQP4) and the gap junction forming proteins connexin-43 (Cx43) and connexin-30 (Cx30) are astrocytic proteins critically involved in brain water and ion homeostasis. While AQP4 is mainly involved in water flux across the astrocytic endfeet membranes, astrocytic gap junctions provide syncytial coupling allowing intercellular exchange of water, ions, and other molecules. We have previously shown that mice with targeted deletion of Aqp4 display enhanced gap junctional coupling between astrocytes. Here, we investigate whether uncoupling of the astrocytic syncytium by deletion of the astrocytic connexins Cx43 and Cx30 affects AQP4 membrane localization and expression. By using quantitative immunogold cytochemistry, we show that deletion of astrocytic connexins leads to a substantial reduction of perivascular AQP4, concomitant with a down-regulation of total AQP4 protein and mRNA. Isoform expression analysis shows that while the level of the predominant AQP4 M23 isoform is reduced in Cx43/Cx30 double deficient hippocampal astrocytes, the levels of M1, and the alternative translation AQP4ex isoform protein levels are increased. These findings reveal a complex interdependence between AQP4 and connexins, which are both significantly involved in homeostatic functions and astrogliopathologies.

Highlights

IntroductionGap junctions and Aquaporin-4 (AQP4) water channels are prominently involved in water and ion homeostasis in the brain
Astrogliopathology is emerging as a feature common to several neurological conditions [1,2].In particular, loss or mislocalization of astrocytic transporter or channel molecules has been associated with Alzheimer’s disease, stroke, and various forms of human and experimentally induced epilepsy [3,4,5,6].The prevailing concept is that astrocytes have important homeostatic functions that critically depend on the specific localization of membrane molecules.Cells 2020, 9, 382; doi:10.3390/cells9020382 www.mdpi.com/journal/cellsGap junctions and Aquaporin-4 (AQP4) water channels are prominently involved in water and ion homeostasis in the brain
Quantitative high resolution immunogold cytochemistry using an antibody against AQP4 revealed a decrease in the density of perivascular AQP4 immunogold particles in parietal cortex and hippocampus of Cx43/30 dKO mice compared to wild type (WT) (Figure 1A–D)

Summary

Introduction

Gap junctions and Aquaporin-4 (AQP4) water channels are prominently involved in water and ion homeostasis in the brain. Gap junctions assemble into intramembranous plaques which couple adjacent astrocytes into a syncytium allowing spatial redistribution of water ions and larger molecules up to a size of 1 kD [10,11]. AQP4 would be expected to regulate the volume of the astrocytic syncytium at large. This is in line with several studies which implicate the perivascular AQP4 pool in the buildup and resolution of brain edema [15,19,20,21,22,23]

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