Epithelial cells provide specific barrier properties which are based on the tight junction. Main component of the tight junction is the family of claudins. Many of the 24 different claudins strenghten the barrier of the tight junction, while a few others, in contrast, form paracellular channels for cations, anions, or water. One of the claudins with unknown function is claudin-17 which is suggested to be expressed in kidney or skin tissue.

Methods: 

Expression and localization of claudin-17 was analyzed in different tissues, and functional characterization was done in overexpression clones of MDCK C7 cells. Two-path impedance spectroscopy, permeability measurements for ions and paracellular markers, confocal microscopy, and freeze fracture EM were performed to characterize claudin-17 in detail.

Results: 

Claudin-17 was detected sparsely in brain tissue, but primarily in kidney tissue, There it was found mainly in proximal segments of the nephron and was gradually decreased to more distal segments. Overexpression resulted in decrease of paracellular resistance due to increased ion permeability. Dilution and biionic potential measurements revealed this decrease to be predominantly based on an increased anion permeability. Larger anionic solutes, e.g. fluorescein, remained unaffected, as well as no changes of ultrastructure were observable in freeze fracture EM.

Conclusion: 

Claudin-17 was functionally characterized to be a channel-forming TJ protein of the proximal segments of the kidney which exhibits preference for anions. (Supported by DFG FOR 721)