Differential effect of alpha-syntrophin knockout on aquaporin-4 and Kir4.1 expression in retinal macroglial cells in mice.

Aquaporin-4 water channels and the inwardly rectifying potassium channels Kir4.1 are coexpressed in a highly polarized manner at the perivascular and subvitreal endfeet of retinal Müller cells and astrocytes. The present study was aimed at resolving the anchoring mechanisms responsible for the coexpression of these molecules. Both aquaporin-4 and Kir4.1 contain PDZ-domain binding motifs at their C-termini and it was recently shown that mice with targeted disruption of the dystrophin gene display altered distribution of aquaporin-4 and Kir4.1 in the retina. To test our hypothesis that alpha-syntrophin (a PDZ-domain containing protein of the dystrophin associated protein complex) is involved in aquaporin-4 and Kir4.1 anchoring in retinal cells, we studied the expression pattern of these molecules in alpha-syntrophin null mice. Judged by quantitative immunogold cytochemistry, deletion of the alpha-syntrophin gene causes a partial loss (by 70%) of aquaporin-4 labeling at astrocyte and Müller cell endfeet but no decrease in Kir4.1 labeling at these sites. These findings suggest that alpha-syntrophin is not involved in the anchoring of Kir4.1 and only partly responsible for the anchoring of aquaporin-4 in retinal endfeet membranes. Furthermore we show that wild type and alpha-syntrophin null mice exhibit strong beta1 syntrophin labeling at perivascular and subvitreal Müller cell endfeet, raising the possibility that beta1 syntrophin might be involved in the anchoring of Kir4.1 and the alpha-syntrophin independent pool of aquaporin-4.

Anchoring of aquaporin-4 in brain: molecular mechanisms and implications for the physiology and pathophysiology of water transport.

Astrocytes show an enrichment of aquaporin-4 (AQP4) in those parts of the plasma membrane that are apposed to pial or perivascular basal laminae. This observation begged the following questions: 1, What are the molecular mechanisms that are responsible for the site specific anchoring of AQP4? 2, What are the physiological and pathophysiological roles of the AQP4 pools at these specialized membrane domains? Recent studies suggest that the site specific anchoring depends on the dystrophin complex. Further, alpha-syntrophin (a member of the dystrophin complex) is required to maintain a polarized expression of AQP4 in the perivascular membranes. Hence transgenic mice deficient in alpha-syntrophin provided a model where the perivascular pool of AQP4 could be removed for assessment of its functional roles. Data suggest that the perivascular pool of AQP4 plays a role in edema formation and that this pool (through its serial coupling with the AQP4 pools in other astrocyte membranes) is involved in K(+) siphoning. In the cerebral cortex, the astrocyte membrane domain contacting the pial basal lamina differs from the perivascular membrane domain in regard to the mechanisms for AQP anchoring. Thus deletion of alpha-syntrophin causes only a 50% loss of AQP4 from the former membrane (compared with a 90% loss in the latter), pointing to the existence of additional anchoring proteins. We will also discuss the subcellular distribution and anchoring of AQP4 in the other cell types that express this protein: endothelial cells, ependymal cells, and the specialized astrocytes of the osmosensitive organs.