Effect of ZO Protein Knockdown on Leak Pathway Pore Size in MDCK Renal Epithelial Cell Populations
Authors: Monaco Ashley , Axis Josephine , Ovryn Ben , Amsler Kurt 
Published: April 2020

The tight junction is the primary barrier to the paracellular movement of water and solutes across epithelial cells (paracellular permeability). Paracellular permeability is divided into two pathways: the Pore Pathway (small solute size, high capacity, charge‐selective) and the Leak Pathway (large solute size, low capacity, charge‐nonselective). Much has been learned about the molecular basis, properties, and regulation of the Pore Pathway. The Leak Pathway is much less well understood. Two basic questions about the Leak Pathway that remain controversial are: 1) does the Leak Pathway have a size limit; and 2) if so, how is the Leak Pathway size limit affected by manipulating specific tight junction proteins. We have initiated an investigation of these two questions by measuring the paracellular permeability of a size range of fluorescein dextrans (4 kDa to 70 kDa) and calculating pore size using the mathematical model, the Renkin sieving equation. In this mathematical model, calculation of pore size is independent of total flux rate, which is also affected by the pore volume fraction and pore length. We first used this approach to examine Leak Pathway permeability in the wild type MDCK renal epithelial cell line. The paracellular movement of all fluorescein dextrans across MDCK cell monolayers was linear with time. The apparent permeability (Papp) decreased with increasing solute Stokes radius. Using the Renkin sieving equation, the Leak Pathway pore size in wild type MDCK cell monolayers was calculated to be ~423 Angstroms. We then examined the effect of knockdown of either of the tight junction‐associated cytoplasmic proteins, ZO‐1 or ZO‐2, on pore size. The paracellular flux rates for all fluorescein dextrans were higher across ZO‐1 knockdown MDCK (ZO‐1 KD) cell populations than across either wild type MDCK cell populations or ZO‐2 knockdown MDCK (ZO‐2 KD) cell populations, which were similar. The paracellular flux of each fluorescein dextran was linear with time across populations of either ZO‐1 KD cells or ZO‐2 KD cells. Papp decreased with increasing Stokes radius for both knockdown cell lines. The calculated pore size for ZO‐2 KD cell populations was similar to that calculated for wild type MDCK cell populations. In contrast, the pore size for ZO‐1 KD cell populations was substantially smaller (~171 Angstroms) than was calculated for wild type MDCK cell populations despite the fact that the total flux rate for all fluorescein dextrans was significantly higher across ZO‐1 KD cell populations. These results indicate that knockdown of ZO‐1 in MDCK cells decreases the Leak Pathway pore size, whereas, knockdown of ZO‐2 does not alter Leak Pathway pore size. In addition, since the flux rate across ZO‐1 KD cell monolayers is higher than across monolayers of the other two cell lines, these results suggest ZO‐1 KD cell populations exhibit either a greater pore volume fraction and/or a shorter pore length than do either wild type MDCK cell populations or ZO‐2 KD cell populations.

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