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First published online 15 January 2008
doi: 10.1242/jcs.021485
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Research Article |
1 Department of Medicine, University of North Carolina, Chapel Hill, NC 27599, USA
2 Cell and Molecular Physiology, University of North Carolina, Chapel Hill, NC 27599, USA
3 School of Pharmacy, University of North Carolina, Chapel Hill, NC 27599, USA
4 Department of Molecular Biomedical Sciences, North Carolina State University, Raleigh, NC 27695, USA
5 Department of Dermatology, Yale University School of Medicine, New Haven, CT 06510, USA
* Author for correspondence (e-mail: vitallie{at}med.unc.edu)
Accepted 4 November 2007
Epithelial tight junctions contain size- and charge-selective pores that control the paracellular movement of charged and noncharged solutes. Claudins influence the charge selectivity and electrical resistance of junctions, but there is no direct evidence describing pore composition or whether pore size or density differs among cell types. To characterize paracellular pores independent of influences from charge selectivity, we profiled the `apparent permeabilities' (Papp) of a continuous series of noncharged polyethylene glycols (PEGs) across monolayers of five different epithelial cell lines and porcine ileum. We also characterized Papp of high and low electrical resistance MDCK cell monolayers expressing heterologous claudins. Papp profiling confirms that the paracellular barrier to noncharged solutes can be modeled as two distinct pathways: high-capacity small pores and a size-independent pathway allowing flux of larger solutes. All cell lines and ileum share a pore aperture of radius 4 Å. Using Papp of a PEG of radius 3.5 Å to report the relative pore number provides the novel insight that pore density along the junction varies among cell types and is not necessarily related to electrical resistance. Expression of claudin-2 results in a selective increase in pore number but not size and has no effect on the permeability of PEGs that are larger than the pores; however, neither knockdown of claudin-2 nor overexpression of several other claudins altered either the number of small pores or their size. We speculate that permeability of all small solutes is proportional to pore number but that small electrolytes are subject to further selectivity by the profile of claudins expressed, explaining the dissociation between the Papp for noncharged solutes and electrical resistance. Although claudins are likely to be components of the small pores, other factors might regulate pore number.
Key words: Claudin, Tight junctions, Paracellular permeability
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