Interaction between Epithelial Sodium Channel γ-Subunit and Claudin-8 Modulates Paracellular Sodium Permeability in Renal Collecting Duct
Authors: Sassi Ali , Wang Yubao , Chassot Alexandra , Komarynets Olga , Roth Isabelle , Olivier Valérie , Crambert Gilles , Dizin Eva , Boscardin Emilie , Hummler Edith , Feraille Eric 
Published: April 2020

Significance Statement
In the renal collecting duct, fluid and solute reabsorption are the result of both transcellular and paracellular transport. Although the mechanisms of transcellular transport have been extensively studied, less is known regarding the regulation of the paracellular pathway. The authors investigated the physiologic role and regulation of the transmembrane protein claudin-8 in cultured mouse cortical collecting duct cell models and in knockout mice lacking kidney tubule–specific expression of the epithelial sodium channel γ-subunit, discovering an interaction between the γ-subunit and claudin-8. This interaction modulates paracellular permeability to sodium and may play an important role in preventing the backflow of reabsorbed solutes and water to the tubular lumen, as well as in coupling paracellular and transcellular sodium transport.

Background
Water and solute transport across epithelia can occur via the transcellular or paracellular pathways. Tight junctions play a key role in mediating paracellular ion reabsorption in the kidney. In the renal collecting duct, which is a typical absorptive tight epithelium, coordination between transcellular sodium reabsorption and paracellular permeability may prevent the backflow of reabsorbed sodium to the tubular lumen along a steep electrochemical gradient.

Methods
To investigate whether transcellular sodium transport controls tight-junction composition and paracellular permeability via modulating expression of the transmembrane protein claudin-8, we used cultured mouse cortical collecting duct cells to see how overexpression or silencing of epithelial sodium channel (ENaC) subunits and claudin-8 affect paracellular permeability. We also used conditional kidney tubule–specific knockout mice lacking ENaC subunits to assess the ENaC’s effect on claudin-8 expression.

Results
Overexpression or silencing of the ENaC γ-subunit was associated with parallel and specific changes in claudin-8 abundance. Increased claudin-8 abundance was associated with a reduction in paracellular permeability to sodium, whereas decreased claudin-8 abundance was associated with the opposite effect. Claudin-8 overexpression and silencing reproduced these functional effects on paracellular ion permeability. Conditional kidney tubule–specific ENaC γ-subunit knockout mice displayed decreased claudin-8 expression, confirming the cell culture experiments' findings. Importantly, ENaC β-subunit or α-subunit silencing or kidney tubule–specific β-ENaC or α-ENaC knockout mice did not alter claudin-8 abundance.

Conclusions
Our data reveal the specific coupling between ENaC γ-subunit and claudin-8 expression. This coupling may play an important role in preventing the backflow of reabsorbed solutes and water to the tubular lumen, as well as in coupling paracellular and transcellular sodium permeability.