Aquaporin-1 Water Channel Expression in Human Kidney
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The pattern of aquaporin-1 water channel protein (AQP1) expression in the human kidney was analyzed by immunocytochemistry using semi-thin and optimized high-resolution immunoelectron microscopy based on freeze-substituted and Lowicryl HM20 embedded tissue. In addition, in situ hybridization was used to determine AQP1 mRNA distribution. Immunoblots revealed a 28-kd band and a 35- to 45-kd band corresponding to unglycosylated and glycosylated AQP1. Glomerular capillary endothelium exhibited extensive AQP1 labeling, whereas glomerular podocytes and Bowman's capsule epithelium were unlabeled. AQP1 was localized in the proximal tubule, including the neck region directly connected to the glomerulus. However, there was a marked difference in the level of expression between cross-sections of the convoluted part and the proximal straight tubules, the latter displaying the most intense labeling. AQP1 labeling continued uninterrupted from the proximal straight tubule into descending thin limbs in outer medulla. Abrupt transitions from heavily labeled to unlabeled segments of thin limbs were observed, primarily in the inner medulla. This may represent the transition from the water-permeable thin descending limb to the water-impermeable thin ascending limb. In addition, heavy labeling of fenestrated endothelium was also observed in peritubular capillaries in cortex, outer medulla, and inner medulla. Immunolabeling controls were negative. In situ hybridization documented a marked difference in AQP1 mRNA levels within the proximal tubule, with the greatest AQP1 mRNA expression in straight proximal tubules. Glomeruli also showed marked signals, and descending thin limbs exhibited extensive expression in exact concordance with the immunocytochemical results. It was concluded that: (1) AQP1 is present in all proximal tubule segments, including segment 1 and the neck region, but there is a pronounced difference in expression levels with respect to both protein and mRNA levels; (2) AQP1 labeling is observed in the endothelium of fenestrated peritubular capillaries, as well as fenestrated glomerular capillaries; (3) AQP1 labeling continues directly from proximal tubules to descending thin limbs; and (4) abrupt transitions from labeled to unlabeled thin limb epithelium are noted.