Question:

In the recent years the perception of the paracellular pathway changed from a passive diffusion route to the concept of a highly regulated and selective pathway. The properties of the paracellular pathway are mainly determined by the tight junctions (TJ), which are formed by neighboring cells, and are located close to the luminal side. TJ consist of a multitude of proteins. For the selectivity of the paracellular pathway claudins are the most important ones. So far the family of claudin proteins is still growing and characterization of the claudins is revealing specialized functions. We analysed the expression pattern of different claudins along the nephron and the segment specific TJ morphology.

Methods:

Immunofluorescence staining on perfusion fixed cryosections of murine kidneys was performed. Antibodies for claudin-10, -16, and -19 were used. Co-stainings for segment specific transporters were performed for proper classification of tubular segments. Epifluorescence microscopy was used to evaluate the expression pattern along the nephron.

Results:

Claudin-10 staining of TJ was found in the proximal tubule (PT), the thin limbs (TL), and the thick ascending limbs (TAL). TJ staining of claudin-16 was restricted to TL, and TAL. TJ staining of claudin-19 was most prominent in the TL, and the TAL. The distal convoluted tubule (DCT) and the collecting duct (CD) showed additional focal claudin-19 positive TJ. Staining patterns revealed a changing TJ morphology along the nephron. In the PT TJ appeared as thin lines and meandering. In the TL and the TAL these lines were more prominent, and meandering was even more pronounced. In contrast we found a linear TJ appearance in the DCT and CD. The CD showed a dot-like and discontinuous staining for Claudin-19. Within each tubular segment, morphology of TJ was consistent.

Conclusions:

The length of the paracellular cleft seems to be enlarged by the meandering in PT, TL, and TAL, reflecting the respective tubular function.