Bio‐Mechanical Properties of the Mammalian Urothelium

Abstract

In the bladder urothelium, the urothelial surface umbrella cells (UC's) maintain an exceptionally tight barrier between urine and blood that prevents noxious leakage. The permeability barrier has been shown to reside on the apical surface of the umbrella cells, which are covered by specialized proteins called uroplakins. Because these proteins form plaques on the surface of umbrella cell and cover up to 90% of the urothelial surface, the urothelial surface is considered to be rigid. It is not clear how a rigid surface can be flexible as the bladder undergoes stretch and relaxation cycles. To develop an understand of the mechanical properties of the urothelial surface we attached magnetic beads coated with polylysine to the apical membrane (AM) surface of intact UC's in intact urothelia and measured their elastic moduli G, by optical magnetic tracking cytometry, OMCT. Control UC's exhibited G values which tracked closely with highly deformable red blood cells (which must deform readily to traverse capillaries). Removal of surface UC's with protamine exposed underlying intermediate cells, which exhibited G values 12–15 fold higher than UC's, and similar to cultured MDCK cells. Fixation of UC's with formaldehyde raised G by two orders of magnitude. Because submembrane actin often plays a critical role in reducing deformability, we performed confocal microscopy on control UC's and found very little subapical actin. In UC's basolateral actin was abundant, and actin was abundant under apical and basolateral membranes of intermediate cells. We conclude that UC's are strikingly deformable, despite the dense array of uroplakins in their apical membranes. Whether this deformability results from the physical effect of uroplakins or the relative lack of subapical actin will be determined using uroplakin knockout mice.