Claudin-2, a component of the tight junction, forms a paracellular water channel

Abstract

SummaryWhether or not significant amounts of water pass the tight junction (TJ) of leaky epithelia is still unresolved, because it is difficult to separate transcellular water flux from TJ-controlled paracellular water flux. Using an approach without differentiating technically between the transcellular and paracellular route, we measured transepithelial water flux with and without selective molecular perturbation of the TJ to unequivocally attribute changes to the paracellular pathway. To this end, MDCK C7 cells were stably transfected with either claudin-2 or claudin-10b, two paracellular cation-channel-forming TJ proteins that are not endogenously expressed in this cell line. Claudin-2 is typical of leaky, water-transporting epithelia, such as the kidney proximal tubule, whereas claudin-10b is present in numerous epithelia, including water-impermeable segments of the loop of Henle. Neither transfection altered the expression of endogenous claudins or aquaporins. Water flux was induced by an osmotic gradient, a Na + gradient or both. Under all conditions, water flux in claudin-2-transfected cells was elevated compared with vector controls, indicating claudin-2-mediated paracellular water permeability. Na + -driven water transport in the absence of an osmotic gradient indicates a single-file mechanism. By contrast, claudin-10b transfection did not alter water flux. We conclude that claudin-2, but not claudin-10b, forms a paracellular water channel and thus mediates paracellular water transport in leaky epithelia.

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

“…The finding that cldn10b-based TJs are highly permeable to Na + and, to a lesser extent, are permeable to Mg 2+ is in good agreement with previous reports that cldn10b forms a paracellular channel for small cations with a preference for monovalent over divalent cations (7,8,11). Furthermore, it fits well with the observation that the absence of cldn10b from the TAL led to decreased paracellular Na + permeability in mice (12).…”

“…This spatial separation is important for renal ion homeostasis and its discovery improves our understanding of paracellular transport organization. (8,11). Mice lacking cldn10 in the TAL display reduced paracellular Na + selectivity and develop hypermagnesemia, hypocalciuria, and nephrocalcinosis (12), i.e., renal retention of divalent cations.…”

Epidermal tight junctions in health and disease

“…29 However, it is not clear whether ochratoxin A is specific for cldn-4, or whether it may also affect expression/localization of other barrier related proteins. It is puzzling that reduced expression of cldn-1 in keratinocytes did not show an effect on water barrier as measured by a modified Ussing chamber, 30 neither in human primary keratinocytes (siRNA mediated K D ; 83 C/-8% knockdown) nor in mouse keratinocytes isolated from Cldn-1 homozygous knock-out (KO) mice. 24 This was highly surprising, because Cldn-1 KO mice die within the first day of birth due to high transepidermal water loss.…”

ILDR1 is important for paracellular water transport and urine concentration mechanism

“…Rosenthal et al previously have demonstrated that a putative paracellular water channel-claudin-2-mediates transepithelial water permeation in MDCK-II cells (3). We found that endogenous ILDR1 expression in MDCK-II cells was below the detection limit by immunoblotting or immunofluorescent labeling (Fig.…”

“…Jorge Fischbarg noticed that the corneal endothelium retained over 60% of water permeability even when AQP1, the only aquaporin expressed by these cells, was knocked out by genetic deletion (2). Rosenthal et al have demonstrated that, in cultured renal epithelium expressing the claudin-2 protein, transepithelial water permeability was significantly higher than in cells without its expression (3). Using an optical microscopic approach, Spring and coworkers have directly measured the velocity of water flow across the tight junction and have ruled out the existence of any significant transjunctional water flow, at least for the bicellular TJ (bTJ) (4).…”