BASOLATERAL BUT NOT APICAL APPLICATION OF PROTEASE RESULTS IN A RAPIDRISE OF TRANSEPITHELIAL ELECTRICAL-RESISTANCE AND FORMATION OF ABERRANT TIGHT JUNCTION STRANDS IN MDCK CELLS

In the presence of Ca2+ application of trypsin to the basolateral surf ace of confluent MDCK cell monolayers with formed tight junctions (TJ) , induces the formation of basolaterally oriented aberrant TJ strands, Induction of aberrant TJ strands is accompanied by an increase in tra nsepithelial electrical resistance (TER), up to 90%, which upon additi on of trypsin inhibitor is maintained for up to 1 h. Thereafter TER re turns slowly to baseline values. Under similar conditions, application of trypsin to the apical surface has little or no effect on either TE R or the number of aberrant TJ strands, Confocal microscopy of monolay ers, immunostained for ZO-1, revealed that this TJ associated cytoplas mic protein, extended below the TJ along the basolateral surface follo wing brief exposure to trypsin. Removing Ca2+ after treatment of the m onolayer with basolaterally applied trypsin resulted, after 20 min, in the increased partitioning of TJ particles onto the E fracture face, of both normal and aberrant TJ strands. Like the TJ strands themselves , therefore, aberrant strands may be linked to cytoskeletal elements. Aberrant TJ strands do not form when monolayers, maintained in low Ca2 + medium, are exposed to trypsin, suggesting that under these conditio ns TJ precursors, and/or trypsin-sensitive proteins regulating TJ stra nd assembly are sequestered in a vesicular compartment that is inacces sible to exogenous trypsin. Prolonged exposure of the apical surface o f an established, polarized epithelium with intact TJ to trypsin, had Little effect on TJ integrity and did not induce aberrant strands.