Contribution of the tricellular tight junction to paracellular permeability in leaky and tight epithelia

Abstract

The tricellular tight junction (tTJ) is a potential weak point of the paracellular barrier. For solving the proportional contribution of the tTJ, ion conductances and macromolecule permeabilities were analyzed in cell lines of different leakiness. MDCK II, Caco-2, and HT-29/B6 cells were subjected to two-path impedance spectroscopy and morphological analyses in order to calculate the contribution of the tTJ to paracellular and total ion conductivity. The contribution to macromolecule permeability was evaluated by tricellulin overexpression or knockdown. Tricellulin-dependent macromolecule passage was comparably regulated in leaky and tight epithelia, but relative and absolute ion permeabilities of the tTJs were different. Assuming a minimal (50 pS) and maximal (146 pS) conductivity per single tTJ, the possible range of contribution of the tTJ to paracellular ion conductance amounted to only 0.3-1.1% in the leaky cell line MDCK II, but 3-25% in the moderately tight cell line Caco-2, and not less than 29% in the tight cell line HT-29/B6. In these cells, this resulted in a contribution to total epithelial conductance of 9-32%. In conclusion, in leaky epithelia the bicellular TJ accounts for nearly the entire paracellular ion conductance, whereas in tight epithelia the low bicellular TJ conductance has large impact on the tTJ.

Morphological bases for intestinal paracellular absorption in bats and rodents

“…But, the vast majority of intestinal cells are enterocytes (e.g., >90% in laboratory rats; (Cheng & Leblond, )), which we assume is also the case in bats, and so we expect that our conclusions based on the relative differences using our simplifying assumptions should hold up. TJ permeability also can differ between bi‐ and tri‐cellular junctions (Krug, ), but we are not aware of any information on these in rodents versus bats and for now we assume no major differences due to that.…”

The Leak Pathway: A Pathway in Flux

“…Moderate tricellulin overexpression decreased the paracellular permeability of both 4 kDa fluorescein-dextran and 10 kDa fluorescein-dextran but not of 20 kDa fluorescein-dextran or horseradish peroxidase [62]. Knockdown of tricellulin protein content in HT29 intestinal epithelial cells, which express higher endogenous tricellulin levels, increased the paracellular permeability of both 4 kDa fluorescein-dextran and 10 kDa fluorescein-dextran [112]. These results suggest that the tricellular junction can form a paracellular channel/pore for molecules up to between 10 kDa (23 Å) and 20 kDa (33 Å).…”

Crystal structure of the tricellulin C‐terminal coiled‐coil domain reveals a unique mode of dimerization

“…To substantiate the above finding of tricellular localization, we next performed transepithelial electrical resistance (TER) and ion permeability measurements, as in the leaky-cell model MDCK II these parameters are only altered upon bicellular expression of tricellulin. [15][16][17] Neither TER, Na + and Clpermeability, nor ion-charge selectivity is significantly affected upon overexpression of wild-type or mutant tricellulin (Figs. 5B, S5A, and S5B), supporting mainly tTJ localization.…”

“…The tTJ is considered to be a potential weak point of the overall TJ and is specifically sealed against passage of macromolecules by tricellulin. The tricellulin‐dependent contribution of the tTJ to ion permeability depends on the leakiness of the epithelium . If strongly overexpressed, tricellulin is also involved in reducing paracellular permeability at bicellular TJs between two adjacent cells …”

“…The tricellulin-dependent contribution of the tTJ to ion permeability depends on the leakiness of the epithelium. 15 If strongly overexpressed, tricellulin is also involved in reducing paracellular permeability at bicellular TJs between two adjacent cells. 16,17 In the cell, tricellulin occurs as a homomer or a heteromer in complex with either occludin or MarvelD3.…”