Bladder permeability barrier: recovery from selective injury of surface epithelial cells

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Abstract
The mammalian bladder maintains high electrochemical gradients between urine and blood, permitting the kidney to modify body chemistries through urinary excretion. To perform this function, the urothelium maintains a tight permeability barrier. When this barrier is damaged, leakage of urine components into the underlying bladder layers results, with symptoms of cystitis. In these studies, we develop a model of selective urothelial injury using protamine sulfate (PS) and define the process by which this epithelium recovers from damage. Exposure to PS (10 mg/ml), but not vehicle, caused a rapid fall in transepithelial resistance as well as striking increases in water and urea permeabilities. These changes were accompanied by necrosis and sloughing of sheets of umbrella cells, as seen by scanning and transmission electron microscopy. Over the 72 h after PS exposure, barrier function recovered, with transepithelial resistance and water and urea permeabilities returning to normal values. After loss of umbrella cells, the underlying intermediate cells underwent rapid maturation, as evidenced by increased expression of uroplakins and gradual formation of well-defined tight junctions. At day 5 after PS exposure, barrier function was restored and the surface cells exhibited normal-appearing tight junctions and normal labeling for uroplakins and zonula occludens 1. However, the cells remained smaller than umbrella cells until day 10 after exposure, when normal size was restored. These studies develop for the first time a controlled model of selective urothelial damage and demonstrate a characteristic process by which barrier function is restored and underlying intermediate cells develop into mature umbrella cells. This model will be useful in defining the mechanisms that regulate repair of urothelial damage.