Role of aquaporin-4 water channel in the development and integrity of the blood-brain barrier.
In this study, we have investigated the expression of aquaporin 4 during blood-brain barrier development in the optic tectum of chick embryos and newly hatched chicks, by means of western-blot, reverse transcriptase-polymerase chain reaction, immunohistochemistry, and freeze-fracture and high-resolution immunogold electron microscopy. In the optic tecta of day-14 embryos, western blot analysis revealed an approx. 30 kDa band, immunoreactive for aquaporin-4, which was increased in day-20 embryos and in chicks. Semi-quantitative reverse transcriptase chain reaction experiments showed that there was already a high level of aquaporin-4 mRNA in day-9 embryos as well as in the subsequent stages and in newly hatched chicks. Immunohistochemically, reactivity for aquaporin-4 was detected in the optic tectum of day-14 embryos; similar results were obtained in telencephalon and cerebellum. Ultrastructurally, the microvessels of the tectum showed immunoreactivity for aquaporin-4 on the astroglial endfeet, which discontinuously surrounded endothelial cells joined by immature tight junctions. In the tectum, telencephalon and cerebellum of 20-day embryos and chicks, aquaporin-4 strongly labeled the ependymal cells and the subpial glial membranes, as well as the bodies and processes of astroglial cells. A continuous aquaporin-4 staining was found around the microvessel endothelial cells, which were sealed off from one another by extensive tight junctions. A complete astrocytic sheath, labeled by anti-aquaporin-4 gold particles, enveloped the endothelium-pericyte layer. Orthogonal arrays of particles were observed on fractured astrocytic membranes, starting from embryonic day 14 when the aquaporin-4 immunogold staining revealed clusters of gold particles, often forming square or rectangular clusters. The results showed that aquaporin-4 expression and organization of the intramembrane particles in orthogonal arrays followed the same temporal sequence. Finally, the lipopolysaccharide, a substance that induces blood-brain barrier disruption, determines a remarkable reduction in aquaporin-4 labeling, expressed by a few aquaporin-4 gold particles attached on swollen perivascular glial membranes. All these data show that aquaporin-4 expression occurs in the chick embryonic brain, in parallel with maturation and functioning of the blood-brain barrier and suggest that there is a close relationship between water transport regulation and brain development.
Department of Human Anatomy and Histology, University of Bari Medical School, I-70124 Bari, Italy. nico@histology.uniba.it
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Nico
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Frigeri
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Nicchia
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Quondamatteo
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Herken
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Errede
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Ribatti
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Svelto
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Roncali
D000818:Animals; D051401:Aquaporin 4; D020346:Aquaporins / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D001812:Blood-Brain Barrier / Q000502:physiology*; D015153:Blotting, Western / Q000379:methods; D002642:Chick Embryo; D018507:Gene Expression Regulation, Developmental; D008070:Lipopolysaccharides / Q000494:pharmacology; D009457:Neuroglia / Q000187:drug effects / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors
D051401:Aquaporin 4; D020346:Aquaporins; D008070:Lipopolysaccharides