Alpha syntrophin deletion removes the perivascular but not the endothelial pool of aquaporin‐4 at the blood‐brain barrier and delays the development of brain edema in an experimental model of acute hyponatremia

Abstract

The formation of brain edema, commonly occurring as a potentially lethal complication of acute hyponatremia, is delayed following knockout of the water channel aquaporin-4 (AQP4). Here we show by high-resolution immunogold analysis of the blood-brain-barrier that AQP4 is expressed in brain endothelial cells as well as in the perivascular membranes of astrocyte endfeet. A selective removal of perivascular AQP4 by alpha-syntrophin deletion delays the buildup of brain edema (assessed by Diffusion-weighted MRI) following water intoxication, despite the presence of a normal complement of endothelial AQP4. This indicates that the perivascular membrane domain, which is peripheral to the endothelial blood-brain barrier, may control the rate of osmotically driven water entry. This study is also the first to demonstrate that the time course of edema development differs among brain regions, probably reflecting differences in aquaporin-4 distribution. The resolution of the molecular basis and subcellular site of osmotically driven brain water uptake should help design new therapies for acute brain edema.

Glial-conditional deletion of aquaporin-4 ( Aqp4 ) reduces blood–brain water uptake and confers barrier function on perivascular astrocyte endfeet

“…We interpret this finding to suggest that the immunogold signal over endothelium [as recorded in the present study and in that of AmiryMoghaddam et al (8)] represents antibody binding to AQP4 molecules embedded in the endfoot membrane. The endfoot membrane and abluminal endothelial cell membrane are separated by a basal lamina whose width is comparable to the length of the antibody bridge between epitope and gold particle (13).…”

“…Moreover, some studies have pointed to the possible presence of an endothelial pool of AQP4 in brain (8)(9)(10). This would be missing in constitutive knockout animals along with the endfoot pool, complicating the analysis of water transport at the blood-brain interface.…”

“…This conclusion was tempered by the controversy surrounding the exact distribution of AQP4 at the blood-brain interface. Notably, some data indicated that AQP4 was also present in endothelia (8)(9)(10)). An endothelial pool of AQP4, if any, would have been eliminated after global Aqp4 deletion and could have accounted for the reduced water absorption and edema formation in Aqp4 −/− animals.…”

Critical role of aquaporin-4 (AQP4) in astrocytic Ca 2+ signaling events elicited by cerebral edema

“…Taken together, the data suggest that AQP4 not only mediates water influx but also is essential for initiating signaling events associated with edema formation. This may explain the rather pronounced protective effect of Aqp4 KO or AQP4 mislocalization in stroke models (8,20). It is well-known that water passes through the lipid bilayer of the plasma membrane (although to a limited extent compared with the water flux through aquaporin channels) and that diffusion also occurs through the thin slits that separate the astrocyte endfeet.…”

Localization of brain endothelial luminal and abluminal transporters with immunogold electron microscopy

“…99 A recent report suggests that endothelial AQP4 can be seen in ␣-syntrophin knockout mice, and that endothelial expression may be seen in these mice when the water channel protein is lost from the adjacent membrane of the glial foot process. 98 These authors further indicated that endothelial AQP4 expression was quite low and FIG. 13.…”