Aquaporin Expression Contributes to Human Transurothelial Permeability In Vitro and Is Modulated by NaCl

Abstract

It is generally considered that the bladder is impervious and stores urine in unmodified form on account of the barrier imposed by the highly-specialised uro-epithelial lining. However, recent evidence, including demonstration of aquaporin (AQP) expression by human urothelium, suggests that urothelium may be able to modify urine content. Here we have we applied functional assays to an in vitro-differentiated normal human urothelial cell culture system and examined both whether AQP expression was responsive to changes in osmolality, and the effects of blocking AQP channels on water and urea transport. AQP3 expression was up-regulated by increased osmolality, but only in response to NaCl. A small but similar effect was seen with AQP9, but not AQP4 or AQP7. Differentiated urothelium revealed significant barrier function (mean TER 3862 Ω.cm2), with mean diffusive water and urea permeability coefficients of 6.33×10−5 and 2.45×10−5 cm/s, respectively. AQP blockade with mercuric chloride resulted in decreased water and urea flux. The diffusive permeability of urothelial cell sheets remained constant following conditioning in hyperosmotic NaCl, but there was a significant increase in water and urea flux across an osmotic gradient. Taken collectively with evidence emerging from studies in other species, our results support an active role for human urothelium in sensing and responding to hypertonic salt concentrations through alterations in AQP protein expression, with AQP channels providing a mechanism for modifying urine composition. These observations challenge the traditional concept of an impermeable bladder epithelium and suggest that the urothelium may play a modulatory role in water and salt homeostasis.

Loss of AQP3 protein expression is associated with worse progression-free and cancer-specific survival in patients with muscle-invasive bladder cancer

“…non-diseased, human urothelium expresses several AQPs, suggesting a potential role in water and urea transport across the urothelial layer 2 . Predominant expression of AQP3 is supportive of the hypothesis that the urothelium may be able to modify volume and final composition of the urine 3 . Apart from its crucial significance in human physiology, there is strong presumptive evidence that AQPs play a role in carcinogenesis, such as in tumour angiogenesis and cell migration 4 . On the basis of our findings in normal human urothelium, we have previously investigated the expression and potential significance of AQPs in UBC and showed that there is a significant correlation between AQP3 protein expression and tumour stage and grade, with AQP3 expression being reduced or lost in tumours of higher grade and stage.…”

Periodical measurement of urine volume in the bladder during sleep: Temporary volume reduction suggestive of absorption

“…AQP0, 1, 2, 5, 6, 8, 10 and 12, however, were not expressed. In a subsequent paper, those same authors suggested that the urothelium of the ureters and bladder might be responsible for serving a modulatory role in water and salt homeostasis, although they did not confirm it to be so in vivo in humans …”

Functional expression of purinergic P2 receptors and transient receptor potential channels by the human urothelium

“…After physical or other damage, the urothelium will self-repair by switching from a mitotically quiescent to a highly regenerative state (20,28). The urothelium has inbuilt mechanisms for responding to changes in urinary salt concentration (18) and for initiating the innate immune system after exposure to uropathogenic Esherichia coli (21). Thus, the urothelium may be considered a hub for sensing and transducing information to other tissue compartments and acting to coordinate physiological bladder function and responses.…”