The density of small tight junction pores varies among cell types and is increased by expression of claudin-2

Abstract

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.

Deletion of claudin-10 ( Cldn10 ) in the thick ascending limb impairs paracellular sodium permeability and leads to hypermagnesemia and nephrocalcinosis

“…Currently, we can only speculate on the molecular and biophysical implications of this finding. Recently, the presence of two pores of high and low capacity in the TAL has been suggested (25,26). The function of the pore impaired by deletion of Cldn10 would then mediate paracellular flux of Na + , whereas the pore for Ca 2+ and Mg 2+ would be dependent on the presence of other claudins.…”

The Blood–Brain Barrier

“…Most of what is known about TJs is from work on ECs, which have identified that these cellular adhesions are formed on the apical part of the lateral membrane by homotypic and heterotypic interactions of transmembrane molecules that are linked to the cytoskeleton through interactions with cytoplasmic adaptors. The strength of the junctions varies greatly depending on the tissue in which they are found, and work in cell culture suggests that they have a size-selective permeability to uncharged molecules of up to 4 nm, and then low permeability to larger molecules (Van Itallie and Anderson 2006;Van Itallie et al 2008). This suggests that the TJs form a 4-nm pore and that larger molecules would pass through discontinuities in the junctions.…”

Tricellulin Forms a Barrier to Macromolecules in Tricellular Tight Junctions without Affecting Ion Permeability

“…The paracellular barrier to uncharged solutes in MDCK II cells has been modeled as two distinct and independent pathways (Watson et al, 2001;Van Itallie et al, 2008), highcapacity "small pores" with a radius of 0.4 nm and a sizeindependent pathway allowing the flux of larger solutes. The permeability to small solutes was suggested to be proportional to the pore number and the profile of TJ proteins expressed.…”