Download Significant water absorption goes paracellular in kidney proximal

Am J Physiol Renal Physiol 306: F51–F52, 2014;
doi:10.1152/ajprenal.00545.2013.
Editorial Focus
Significant water absorption goes paracellular in kidney proximal tubules
Rita Rosenthal and Michael Fromm
Institute of Clinical Physiology, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, Berlin, Germany
Submitted 11 October 2013; accepted in final form 16 October 2013
Address for reprint requests and other correspondence: M. Fromm, Institute of
Clinical Physiology, Charité-Universitätsmedizin Berlin, Campus Benjamin
Franklin, Hindenburgdamm 30, 12203 Berlin, Germany (e-mail: michael.
[email protected]).
http://www.ajprenal.org
They showed that in mice deficient of AQP-1, which is
highly expressed in both apical and basolateral membranes of
proximal tubule cells and, thus, represents the main pathway
for transcellular water movement, fluid reabsorption is reduced
by 20%. The absence of AQP-1 causes a transepithelial osmotic difference, which induces fluid reabsorption across
AQP1-independent transcellular pathways. This was observed
both in the absence and presence of claudin-2. In mice deficient
of both AQP-1 and claudin-2, a reduction of fluid flow by 26%
was found, which was not significantly different from values
measured in singly deficient animals (9). This was an unexpected finding of the study, which has not be sufficiently
explained yet. A possible interpretation could be the assumption that AQP-1- and claudin-2-mediated water fluxes are
interdependent, as it has been shown for paracellular permeability and AQP-5 function in salivary glands (5).
These findings are consistent with results obtained by Muto
et al. (7), who found a decreased net transepithelial reabsorption of Na, Cl, and water in isolated, perfused S2 segments of
proximal tubules of claudin-2-deficient mice. Aquaporin expression was not investigated in that study. Proximal tubule net
water reabsorption in Cldn2⫺/⫺ mice was decreased by about
30% compared with that in Cldn2⫹/⫹ mice.
A possible alternative explanation for the role of claudin-2 in
kidney tubular water flux has to be discussed, namely, that
claudin-2 mediates Na⫹ reabsorption through the paracellular
pathway, but water follows only along the transcellular pathway, mediated, e.g., by AQP-1. However, this possibility was
excluded in the study on MDCK C7 cells, as it demonstrated
that in claudin-2-expressing cell layers, water flux was induced
not only by an osmotic gradient, but also by a NaCl gradient in
the absence of an osmotic gradient. As a second piece of evidence,
net water flux induced by an osmotic gradient caused a net Na⫹
transport in claudin-2-expressing cells, indicating that water and
Na⫹ were transported through the same channel (8).
For claudin-2-based paracellular channels, the characteristics of the pore have been determined. A negatively charged
site within the pore formed by aspartate-65 causes the cation
selectivity by electrostatic interaction. The pore radius was
estimated to be 3.25 Å, which appears large enough for the
passage of both partially dehydrated cations and water molecules (diameter 2.8 Å) (10).
Among other claudins exhibiting undisputed cation or anion
permeability (4), neither overexpression of claudin-10b nor of
claudin-17 altered water permeability of MDCK C7 cell layers
(6, 8). Thus, claudin-2 so far is the only claudin for which
water permeability has been demonstrated. The present study
by Schnermann et al. (9) has convincingly proven and quantified that concept for kidney proximal tubules.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
1931-857X/14 Copyright © 2014 the American Physiological Society
F51
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.4 on August 3, 2017
IT HAD BEEN A FUNDAMENTAL dispute for several decades,
whether water passes epithelia such as kidney proximal tubule
or small intestine along the transcellular, paracellular, or both
pathways.
By discovery of the transmembranal aquaporin water channels (AQP), the molecular basis of the transcellular pathway
was uncovered (1), but the molecular basis of the tight junctional pathway was not resolved at that time. Transcellular and
paracellular water transport was, therefore, measured by methods that attempted to technically discriminate the two pathways. This turned out to be difficult, and as a result, the
contribution of the paracellular pathway was estimated in the
maximal range between 0 and 100%, depending on the method
used for analysis (3).
In recent years, several tight junction proteins were identified to form paracellular channels with selectivity for small
cations (claudin-2, claudin-10b, and claudin-15) or anions
(claudin-10a and claudin-17) (4). In consequence, new approaches were developed to measure transepithelial water
transport before and after molecular perturbation of the tight
junction by overexpression or downregulation of specific junctional proteins. In this way, the tight junctional contribution to
transepithelial water transport could be analyzed.
Using an overexpression study in the epithelial cell line
MDCK-C7, Rosenthal et al. (8) identified claudin-2 to form a
paracellular water channel. Claudin-2 is highly expressed in
the proximal tubule of the kidney and is known as a paracellular cation channel (2). Thus, claudin-2 forms channels for
both, small cations and water. Whereas this in vitro study
employed claudin-2 overexpression, others were performed on
claudin-2-deficient mice.
In a very evident study published in the American Journal of
Physiology—Renal Physiology, Schnermann et al. (9) measured proximal fluid reabsorption in wild-type mice and mice
deficient for claudin-2, AQP-1, and both. In claudin-2-deficient
mice, fluid reabsorption was reduced by 23% compared to
wild-type mice. This result corroborates the concept of a
claudin-2-based paracellular water permeability and convincingly demonstrates a paracellular contribution to proximal
tubule fluid reabsorption of 25%. Schnermann et al. (9) performed a comprehensive analysis not only of proximal fluid
reabsorption, but also on kidney and single-nephron filtration
rate on a large number of mice. Furthermore, in a detailed
expression analysis of wild-type mice and mice deficient for
claudin-2, AQP1, and both, the authors demonstrated that the
findings are due to the deletion of the specific protein and not
to further alterations in gene expression.
Editorial Focus
F52
AUTHOR CONTRIBUTIONS
Author contributions: R.R. and M.F. drafted manuscript; R.R. and M.F.
edited and revised manuscript; R.R. and M.F. approved final version of
manuscript.
6.
7.
REFERENCES
8.
9.
10.
AJP-Renal Physiol • doi:10.1152/ajprenal.00545.2013 • www.ajprenal.org
Downloaded from http://ajprenal.physiology.org/ by 10.220.33.4 on August 3, 2017
1. Agre P, Preston GM, Smith BL, Jung JS, Raina S, Moon C, Guggino
WB, Nielsen S. Aquaporin CHIP: the archetypal molecular water channel.
Am J Physiol Renal Fluid Electrolyte Physiol 265: F463–F476, 1993.
2. Amasheh S, Meiri N, Gitter AH, Schöneberg T, Mankertz J, Schulzke
JD, Fromm M. Claudin-2 expression induces cation-selective channels in
tight junctions of epithelial cells. J Cell Sci 115: 4969 –4976, 2002.
3. Fischbarg J. Fluid transport across leaky epithelia: central role of the tight
junction and supporting role of aquaporins. Physiol Rev 90: 1271–1290,
2010.
4. Günzel D, Yu AS. Claudins and the modulation of tight junction permeability. Physiol Rev 93: 525–569, 2013.
5. Kawedia JD, Nieman ML, Boivin GP, Melvin JE, Kikuchi K, Hand
AR, Lorenz JN, Menon AG. Interaction between transcellular and
paracellular water transport pathways through Aquaporin 5 and the tight
junction complex. Proc Natl Acad Sci USA 104: 3621–3626, 2007.
Krug SM, Günzel D, Conrad MP, Rosenthal R, Fromm A, Amasheh
S, Schulzke JD, Fromm M. Claudin-17 forms tight junction channels
with distinct anion selectivity. Cell Mol Life Sci 69: 2765–2778, 2012.
Muto S, Hata M, Taniguchi J, Tsuruoka S, Moriwaki K, Saitou M,
Furuse K, Sasaki H, Fujimura A, Imai M, Kusano E, Tsukita S,
Furuse M. Claudin-2-deficient mice are defective in the leaky and
cation-selective paracellular permeability properties of renal proximal
tubules. Proc Natl Acad Sci USA 107: 8011–8016, 2010.
Rosenthal R, Milatz S, Krug SM, Oelrich B, Schulzke JD, Amasheh S,
Günzel D, Fromm M. Claudin-2, a component of the tight junction, forms
a paracellular water channel. J Cell Sci 123: 1913–1921, 2010.
Schnermann J, Huang Y, Mizel D. Fluid reabsorption in proximal
convoluted tubules of mice with gene deletions of claudin-2 and/or
aquaporin. Am J Physiol Renal Physiol 305: F1352–F1364, 2013.
Yu ASL, Cheng MH, Angelow S, Günzel D, Kanzawa SA, Schneeberger EE, Fromm M, Coalson RD. Molecular basis for cation selectivity in claudin-2-based paracellular pores: Identification of an electrostatic interaction site. J Gen Physiol 133: 111–127, 2009.