Claudin-2, a component of the tight junction, forms a paracellular water channel.
Whether 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.
Rosenthal
Rita
R
Institute of Clinical Physiology, Charité, Campus Benjamin Franklin, Freie Universität and Humboldt-Universität, 12200 Berlin, Germany.
Milatz
Susanne
S
Krug
Susanne M
SM
Oelrich
Beibei
B
Schulzke
Jörg-Dieter
JD
Amasheh
Salah
S
Günzel
Dorothee
D
Fromm
Michael
M
D000818:Animals; D020346:Aquaporins / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D002463:Cell Membrane Permeability / Q000235:genetics; D057167:Claudins; D003001:Cloning, Molecular; D004285:Dogs; D004847:Epithelial Cells / Q000378:metabolism* / Q000473:pathology; D006801:Humans; D007687:Kidney Tubules, Proximal / Q000473:pathology; D008138:Loop of Henle / Q000473:pathology; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport; D015222:Sodium Channels / Q000235:genetics / Q000378:metabolism; D019108:Tight Junctions / Q000378:metabolism*; D019076:Transgenes / Q000235:genetics; D014867:Water / Q000378:metabolism*
D020346:Aquaporins; C491875:CLDN2 protein, human; D057167:Claudins; D008565:Membrane Proteins; D015222:Sodium Channels; C502009:claudin 10; D014867:Water