Corticomedullary difference in the effects of dietary Ca2+ on tight junction properties in thick ascending limbs of Henle’s loop

Abstract

The thick ascending limb of Henle's loop (TAL) drives an important part of the reabsorption of divalent cations. This reabsorption occurs via the paracellular pathway formed by the tight junction (TJ), which in the TAL shows cation selectivity. Claudins, a family of TJ proteins, determine the permeability and selectivity of this pathway. Mice were fed with normal or high-Ca(2+) diet, and effects on the reabsorptive properties of cortical and medullary TAL segments were analysed by tubule microdissection and microperfusion. Claudin expression was investigated by immunostaining and quantitative PCR. We show that the TAL adapted to high Ca(2+) load in a sub-segment-specific manner. In medullary TAL, transcellular NaCl transport was attenuated. The transepithelial voltage decreased from 10.9 ± 0.6 mV at control diet to 8.3 ± 0.5 mV at high Ca(2+) load, thereby reducing the driving force for Ca(2+) and Mg(2+) uptake. Cortical TAL showed a reduction in paracellular Ca(2+) and Mg(2+) permeabilities from 8.2 ± 0.7 to 6.2 ± 0.5 ∙ 10(-4) cm/s and from 4.8 ± 0.5 to 3.0 ± 0.2 · 10(-4) cm/s at control and high-Ca(2+) diet, respectively. Expression, localisation and regulation of claudins 10, 14, 16 and 19 differed along the corticomedullary axis: Towards the cortex, the main site of divalent cation reabsorption in TAL, high-Ca(2+) intake led to a strong upregulation of claudin-14 within TAL TJs while claudin-16 and -19 were unaltered. Towards the inner medulla, only claudin-10 was present in TAL TJ strands. In summary, high-Ca(2+) diet induced a reduction of divalent cation reabsorption via a diminution of NaCl transport and driving force in mTAL and via decreased paracellular permeabilities in cTAL. We reveal an important regulatory pattern along the corticomedullary axis and improve the understanding how the kidney disposes of detrimental excess Ca(2+).

Mosaic expression of claudins in thick ascending limbs of Henle results in spatial separation of paracellular Na + and Mg 2+ transport

“…As recently reported, claudin expression differs along the corticomedullary axis of the TAL (10). Whereas TJ expression of cldn16 and cldn19 is confined to the cortex and the outer stripe of outer medulla (OSOM), cldn10b is expressed within TJs along the entire TAL from the cortex to the inner stripe of outer medulla (ISOM).…”

“…Whereas cldn3, cldn10, cldn16, and cldn19 were clearly expressed, cldn11 could not be detected. The anti-cldn10 antibody cannot discriminate between isoforms a and b, but PCR showed that only cldn10b is present in the TAL (10,12). Thus, a positive cldn10 staining in the TAL is considered to indicate the presence of the isoform cldn10b.…”

“…Rather, the TAL reacted to the Ca 2+ load by increased expression and TJ localization of cldn14 that was virtually absent under control conditions (10,(20)(21)(22). Cldn14 was able to interact with cldn16 in a yeast two-hybrid (Y2H) assay, and it is assumed that this interaction alters the cldn16/cldn19-based cation permeability (20).…”

“…Cldn14 is virtually absent from the TAL of mice under control conditions but is up-regulated in mice with a high dietary Ca 2+ intake (10,(20)(21)(22). This increase is accompanied by decreased Ca 2+ and Mg 2+ permeability in the TAL.…”

“…Cldn10 exists in several splice variants that differ in their ion selectivities (7,8). We and others have shown that the TAL expresses the isoform cldn10b (7,9,10) that forms a paracellular channel for small cations but not for water…”