Departments of 1 Medicine, 2 Cell Biology, and 4 Surgery, Yale University School of Medicine, New Haven, Connecticut 06520; and 3 Brown University, Providence, Rhode Island 02912
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ABSTRACT |
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Epithelia separate tissue
spaces by regulating the passage of ions, solutes, and water through
both the transcellular and paracellular pathways. Paracellular
permeability is defined by intercellular tight junctions, which vary
widely among tissues with respect to solute flux, electrical
resistance, and ionic charge selectivity. To test the hypothesis that
members of the claudin family of tight junction proteins create charge
selectivity, we assessed the effect of reversing the charge of selected
extracellular amino acids in two claudins using site-directed
mutagenesis. Claudins were expressed in cultured Madin-Darby canine
kidney cell monolayers under an inducible promoter, and clones were
compared with and without induction for transmonolayer electrical
resistance and dilution potentials. Expression and localization of
claudins were determined by immunoblotting, immunofluorescence
microscopy, and freeze-fracture electron microscopy. We observed that
substituting a negative for a positive charge at position 65 in the
first extracellular domain of claudin-4 increased paracellular
Na+ permeability. Conversely, substituting positive for
negative charges at three positions in the first extracellular domain
of claudin-15, singly and in combination, reversed paracellular charge selectivity from a preference for Na+ to Cl.
These results support a model where claudins create charge-selective channels in the paracellular space.
tight junction; claudin-4; claudin-15; dilution potential; intercellular junction
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INTRODUCTION |
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THE BODY SPACES of multicellular animals are lined by continuous sheets of epithelial cells. Movement of solutes, ions, and water across these barriers occurs through both the transcellular pathway, via a large number of transmembrane channels and carriers, and the paracellular pathway, via intercellular tight junctions (TJs). Together these complementary pathways establish the selective and regulated barriers required for absorption and secretion. Our understanding of transcellular transport mechanisms is quite advanced, whereas the molecular basis for the fundamental property of charge selectivity in the paracellular pathway remains less well defined.
The barrier properties of TJs vary widely among tissues in both magnitude, typically quantified as electrical resistance, and charge selectivity. In so-called "tight" epithelia, where transcellular mechanisms generate steep electrochemical gradients, the paracellular permeability is small and the issue of charge selectivity is of minor importance. However, in "leaky" epithelia, like the small intestine and proximal tubules of the kidney, the paracellular pathway is a major component of overall transport and variations in charge selectivity have a significant impact on the composition of transported fluids (17, 23).
TJs are positioned as continuous circumferential cell-cell contacts at the border of the apical and lateral cell membranes. In freeze-fracture electron micrographs, the intercellular contacts correspond to rows, or fibrils, of transmembrane proteins that make extracellular adhesive contacts with rows from adjacent cells to seal the intercellular space. Claudins, small (~23 kDa) tetraspan proteins with two extracellular domains, are likely the critical structural proteins in the fibrils (5, 7).
Circumstantial evidence supports the idea that claudins create the variable properties of the TJs. The 20 claudin genes in mammals show developmental and tissue-specific expression patterns (18, 21).
Experimentally increasing the levels of different claudins in cultured
epithelial models has been reported to either increase or decrease
electrical resistance (6, 10, 12). This was interpreted to
result from increasing or decreasing cell-to-cell adhesion. However,
our recent studies suggest an alternative mechanism. We demonstrated
that expression of claudin-4 in cultured Madin-Darby canine kidney
(MDCK) cell monolayers increased transmonolayer electrical resistance
by selectively decreasing the paracellular permeability for
Na+ (PNa), whereas
PCl and the flux of a noncharged solute were
unchanged (22). This finding implies that claudin-4
altered charge discrimination without altering the diffusion of
nonelectrolyte solutes. However, these studies did not prove that
claudin-4 was directly responsible. This led us, in the present study,
to test whether claudins might behave like paracellular channels and
directly determine ion permeability through the electrostatic charges
on their extracellular amino acid residues. The extracellular
domains of claudins contain regions of highly conserved residues and
intervening positions where the charge can be positive, negative, or
neutral. In this study we have focused on variable residues in the
larger first extracellular domain (see Fig. 1, A and
B).
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MATERIALS AND METHODS |
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Plasmid constructs and cell lines. Claudin-4K65D (K65D) was generated using the Stratagene (La Jolla, CA) Quik-Change site-directed mutagenesis kit. Full-length human claudin-15 was amplified by PCR with the use of human kidney cDNA (Quick-clone cDNA; Clontech Laboratories, Palo Alto, CA) as a template and primers 53988 (5'-CCCACCATGTCGATGGCTG-3') and 53989 (5'-CAGAGCTGCTACACGTAGGC-3'). The amplified product was cloned into the TOPO-TA vector (InVitrogen, San Diego, CA) and subcloned into the pTRE vector (Clontech Laboratories). Claudin-15 mutations were made using the Quik-Change site-directed mutagenesis kit (Stratagene). All wild-type and mutant claudin constructs were verified by DNA sequencing in both directions. Clonal cell lines of MDCK II Tet-Off cells (Clontech Laboratories) were derived by standard transfection and selection techniques; regulated expression of the transgene products was accomplished by varying doxycycline levels in the culture media as previously described (22).
Immunoblots and immunofluorescence. Anti-human claudin-4 mouse monoclonal antibody (Zymed, South San Francisco, CA) was used for immunoblots at a dilution of 1:4,000. Immunofluorescence for claudin-4 and K65D was performed using established protocols (22). Anti-human claudin-15 mouse monoclonal antibody (Zymed) was used for immunoblots at a dilution of 1:2,000 and for immunofluorescence on 1.0% paraformaldehyde-fixed cells at a 1:100 dilution. Immunoblots and immunofluorescence for ZO-1, occludin, and claudin-2 were performed using established protocols (22). Autofluorograms were scanned (Afga Duoscan T1200; Agfa-Gevaert, Mortsel, Belgium), and the integrated optical densities of constant areas at a specific molecular mass range were determined using Gel-Pro Analyzer (Media Cybernetics, Silver Spring, MD). Local area background-integrated optical density was subtracted from each band.
Freeze-fracture electron microscopy. Freeze-fracture electron microscopy was carried out using established protocols (13).
Electrophysiology. Electrophysiological characterization of MDCK II monolayers was carried out according to published methods (22). Stable MDCK II Tet-Off cell lines (Clontech Laboratories) transfected with wild-type or mutant claudins were grown on Snapwell filters (Costar; Corning Life Sciences, Acton, MA) for 4 days, induced without doxycycline or noninduced with doxycycline (50 ng/ml). Protein expression is maximal by day 2. Transmonolayer resistance was measured by using a modified Ussing chamber with a microcomputer-controlled voltage-current clamp (Harvard Apparatus, Holliston, MA) with buffer A (120 mM NaCl, 10 mM HEPES, pH 7.4, 5 mM KCl, 10 mM NaHCO3, 1.2 mM CaCl2, and 1 mM MgSO4) in the apical and basolateral chambers. Transmonolayer dilution potentials were measured upon dilution of the apical chamber (buffer B: 60 mM NaCl, 120 mM mannitol, 10 mM HEPES, pH 7.4, 5 mM KCl, 10 mM NaHCO3, 1.2 mM CaCl2, and 1 mM MgSO4) relative to the basolateral chamber (buffer A) (22). Dilution potentials were immediately stable and repeatedly measured (every 6 s) for at least 30 s after buffer A had been replaced with buffer B in the apical chamber. Voltage and current electrodes consisted of a Ag-AgCl wire in 3 M KCl saturated with AgCl housed in a glass barrel with a microporous ceramic tip (Harvard Apparatus). Liquid junction potentials were calculated by using the Henderson diffusion equation for univalent ions (11) under dilution potential conditions and were factored into the reported dilution potentials.
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RESULTS |
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A charge-reversing mutation of claudin-4, K65D, eliminates its ability to discriminate against Na+. Claudin-4 contains a single basic residue, Lys65, in a charge-variable region of the first extracellular domain (Fig. 1, A and B). To test whether electrostatic repulsion from Lys65 of claudin-4 discriminates against the paracellular movement of Na+ (22), we reversed the charge at this position by replacement with aspartic acid (K65D) using site-specific mutagenesis (Fig. 1A). The K65D mutant was expressed in stably transfected clones of MDCK cells under a tightly regulated doxycycline-repressible promoter (1). Properties of monolayers induced to express claudin-4 or the K65D mutant were compared with noninduced controls of the same clone; at least three clones were examined for each example.
The cellular localization of the K65D mutant protein, like both endogenous and experimentally induced wild-type claudin-4, was predominantly at apicolateral cell membranes (Fig. 1C). The immunolocalization of claudins was compared with that of the cytoplasmic TJ protein ZO-1. In all cases the focused TJ location of ZO-1 was unaffected by forced expression of the claudins (Fig. 1C). Likewise, induction of either wild-type claudin-4 or the K65D mutant did not affect the levels of several other TJ proteins (Fig. 1D), suggesting that changes in permeability can be interpreted as a specific effect of the claudin transgene product. We next determined whether either claudin-4 or the K65D mutant altered paracellular charge selectivity by comparing transepithelial dilution potentials (22) with and without induction of protein expression. In the low-resistance monolayers used for our studies, the majority of ionic conductance is paracellular and the transmonolayer electrical potential immediately following imposition of a NaCl gradient reflects the relative permeabilities of Na+ vs. ClClaudin-15 creates Na+-selective
paracellular channels.
To test the generality of the hypothesis that charged residues on the
extracellular domains of claudins establish paracellular charge
selectivity, we performed charge-reversing mutations in the opposite
direction, from acidic to basic residues, and on a different claudin.
Claudin-15 was selected for study because it has three acidic residues
at positions that are typically variable among the claudin family
members. Furthermore, their combined reversal creates a first
extracellular domain with only positive charges (Fig.
2A). We reasoned that if the
electrostatic channel model were correct, this extreme case might
reverse the junction selectivity from a preference for Na+
to a preference for Cl ions.
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Charge-reversing mutations on the first extracellular domain of
claudin-15 create Cl-selective
paracellular channels.
To determine whether all three acidic residues in the first
extracellular domain of claudin-15 influence paracellular charge selectivity, we replaced them with basic residues, singly and in
combination (Fig. 2A). Immunoblot analysis demonstrated that expression of all mutant proteins could be tightly regulated and that,
except for occludin, their induction did not alter levels of other TJ
proteins (Fig. 2B). Freeze-fracture electron microscopy performed on a subset of the mutants revealed an increase in TJ fibril
complexity (Fig. 2D) in a pattern that was indistinguishable from that of cells expressing exogenous wild-type claudin-15. Immunofluorescence microscopy of all mutants revealed localization similar to that seen with wild-type claudin-15 (Fig. 2C).
Dilution potentials revealed that reversing the charge at the first
acidic position, m1 (E46K), had no effect on paracellular charge
selectivity. In contrast, mutation of either position m2 (D55R) or m3
(E64K) resulted in a significant decrease in the ratio of
PNa to PCl (Fig.
2E). Introducing a positive charge at position m2 actually reversed the overall permeability ratio to favor Cl
ions.
We conclude that some, but not all, charged positions in claudin-15
influence paracellular charge selectivity.
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DISCUSSION |
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The results presented here provide the first conclusive evidence that claudins directly regulate charge selectivity of the paracellular pathway. Their extracellular sequences share a core of conserved residues and presumably a similar tertiary structure. We speculate that the extracellular adhesive contacts between claudins bring them together in a manner that creates aqueous channels lined by the variably charged positions. This charged-channel model is consistent with the observation that TJs in all epithelia have a relatively similar size discrimination for uncharged solutes but have variable charge selectivity and overall electrical resistance (17). A similar model was proposed almost four decades ago based on elegant physiological experiments (25), but the physical basis has remained unknown.
The paracellular pathway has been characterized as having large aqueous spaces and unique charge selectivities. Unlike transmembrane ion channels, the paracellular pathway shows selectivity for net charge rather than specific ions (3, 14). Parallels to the paracellular pathway can be found in the properties of gap junctions, where connexons provide large aqueous cell-to-cell channels and show moderate charge and solute selectivity (16). Similar to our work on claudins, an extracellular domain of connexins with charged residues has been characterized to influence gap junction charge selectivity (20).
We chose to study the larger first extracellular domain of claudins because it has greater numbers and variability of charges than the second domain. Furthermore, consistent with a role for charge in the first extracellular domain influencing charge selectivity, indirect published evidence suggests that claudin-16, with 10 negative residues on its first extracellular domain, forms paracellular channels selective for divalent cations (19). Studies are currently in progress to characterize any contribution of the second extracellular domain in paracellular charge selectivity.
We speculate that paracellular ionic selectivities of different epithelia are determined by varying combinations and levels of different claudins. It is already known that claudins show unique expression patterns and that some TJs contain several different claudins (4, 8, 15, 18, 19). Physiological or pathological alterations in claudins are expected to have a significant impact on epithelial barrier properties. These could be manifest as changes in transport, antigen and pathogen entry, or, conceivably, tissue morphogenesis. Indeed, two recent reports of human diseases resulting from mutations in claudin-14 and 16 and the phenotype of a mouse knockout of claudin-11 (9, 19, 24) can be rationalized as defects in paracellular ionic charge selectivity. Understanding and manipulating the molecular basis of selectivity should also have therapeutic applications for enhancing the level and location of drug delivery.
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ACKNOWLEDGEMENTS |
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We express thanks to Emile Boulpaep, Lukas Landmann, and members of our laboratory, Laura Mitic, Alan Fanning, and Zenta Walther.
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FOOTNOTES |
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These studies were supported by National Institute of Diabetes and Digestive and Kidney Diseases Grants DK-45134, DK-34989, and DK-38979. O. R. Colegio is an investigator of the National Institutes of Health Medical Scientist Training Program.
Address for reprint requests and other correspondence: O. R. Colegio, Yale Univ., PO Box 208019, New Haven, CT 06520-8019 (E-mail: oscar.colegio{at}yale.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpcell.00038.2002
Received 24 January 2002; accepted in final form 12 March 2002.
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