Claudin-16 affects transcellular Cl- secretion in MDCK cells.

Claudin-16 (paracellin-1) is a tight junction protein localized mainly in the thick ascending limb of Henle's loop and also in the distal nephron. Its defect causes familial hypomagnesaemia with hypercalciuria and nephrocalcinosis. This had been taken as an indication that claudin-16 conveys paracellular Mg(2+) and Ca(2+) transport; however, evidence is still conflicting. We studied paracellular ion permeabilities as well as effects of claudin-16 on the driving forces for passive ion movement. MDCK-C7 cells were stably transfected with wild-type (wt) and mutant (R146T, T233R) claudin-16. Results indicated that paracellular permeability to Mg(2+) but not to Ca(2+) is increased in cells transfected with wt compared to mutant claudin-16 and control cells. Increased basolateral Mg(2+) concentration activated a transcellular Cl(-) current which was greatly enhanced in cells transfected with wt and T233R claudin-16, as compared to R146T claudin-16-transfected or control cells. This current was triggered by the basolateral calcium-sensing receptor causing Ca(2+) release from internal stores, thus activating apical Ca(2+)-sensitive Cl(-) channels and basolateral Ca(2+)-sensitive K(+) channels. Immunohistochemical data suggest that the Cl(-) channel involved is bestrophin. We conclude that claudin-16 itself possesses only moderate paracellular Mg(2+) permeability but governs transcellular Cl(-) currents by interaction with apical Ca(2+)-activated Cl(-) channels, presumably bestrophin. As the transepithelial voltage generated by such a current alters the driving force for all ions, this may be the major mechanism to regulate Mg(2+) and Ca(2+) absorption in the kidney.

Claudin function in the thick ascending limb of Henle's loop.

During the past decade, claudins have been established as major determinants of paracellular permeablilty in epithelia. In the kidney, each nephron segment expresses a distinct pattern of claudins. Cells of the thick ascending limb of Henle's loop (TAL), which is characterized by high paracellular cation permeability, co-express an unusually large number of different claudins: claudin-10, -16, and -19 and, depending on the species, also claudin-3, -4, -8, and/or -11. The function of most of these claudins has been investigated in vitro. We present a summary of their function with special emphasis on claudin-16 and -19. Mutations in the corresponding human genes lead to severely impaired renal Ca(2+) and Mg(2+) handling. To date, 42 different claudin-16 mutations and three claudin-19 mutations have been reported. These mutations prevent the claudins from reaching the surface membrane, decrease membrane residence time, or render them functionless. In spite of the clear clinical symptoms such as hypomagnesemia, hypercalciuria, nephrocalcinosis, and renal insufficiency, mechanisms that link claudin-16 and -19 to these symptoms are still unknown. Depending on the cell type used in overexpression studies, claudin-16 appears to cause a mild increase in paracellular Mg(2+)-permeability or a pronounced increase in Na(+) permeability. Claudin-19 selectively decreases Cl(-) permeability, thus synergistically increasing relative cation permeability, or indiscriminately decreases paracellular permeability. In the light of these results it is hypothesized that the renal Mg(2+)/Ca(2+) waste may not be solely due to reduced resorption in the TAL but at least in part to paracellular back-leak of Mg(2+)/Ca(2+) into the tubular lumen of the distal convoluted tubule.