ZO-1 MAINTAINS ITS SPATIAL-DISTRIBUTION BUT DISSOCIATES FROM JUNCTIONAL FIBRILS DURING TIGHT JUNCTION REGULATION

Tight junctions restrict diffusion of hydrophilic solutes through the paracellular pathways of columnar epithelia. It is now apparent that t he barrier function of tight junctions is physiologically regulated. C urrent models of the tight junction envisage junctional subunits consi sting of extracellular ''kisses'' between plasma membranes of adjacent cells, intramembrane components represented by freeze-fracture fibril s, and cytoplasmic elements such as the tight junction-specific protei n ZO-1 and elements of the cytoskeleton. Insights into functional rela tionships between these various components of tight junctions should b e provided by mapping component interrelationships in states of altere d junctional permeability. Here we define the spatial distribution of ZO-1 during a state of physiological regulation of intestinal absorpti ve cell tight junctions. Enhanced permeation of absorptive cell juncti ons in response to activation of apical membrane Na+-solute cotranspor ters does not lead to redistribution of the ZO-1 pool, as judged from quantitative ultrastructural immunolocalization studies employing two different ZO-1 antibodies. Surprisingly, ZO-1, which normally localize s under junctional kisses/fibrils, focally persists at sites where jun ctional kisses/fibrils are cleared. These findings suggest that 1) spa tial redistribution of ZO-1 does not contribute to physiological regul ation of junctions elicited by activation of Na+-solute cotransport an d 2) ZO-1 and junctional fibrils may spatially dissociate during such regulated states.