SERUM OPENS TIGHT JUNCTIONS AND REDUCES ZO-1 PROTEIN IN RETINAL EPITHELIAL-CELLS

We have shown previously that serum inhibits tight junction formation in a retinal epithelial cell culture model for the blood-brain barrier , We have now examined in detail the effects of serum on the tight jun ctions. Our data show that serum induces a breakdown in tight junction function as indicated by decreased transepithelial electrical resista nce and increased permeability. Rat serum had effects similar to those of bovine serum, indicating that the activity is species-independent. The effect is concentration-dependent, reversible, and specific for t he apical surface, suggesting the involvement of a specific receptor-l igand interaction. Differences in the time course, response magnitude, and structural manifestations between the serum-induced breakdown and that induced by switching the cultures to a low-calcium medium sugges t fundamental differences in their mechanisms, The calcium switch resu lts in an immediate and complete junctional breakdown with cell retrac tion and perinuclear translocation of both actin and the tight junctio n protein zonula occludens-1. The serum-induced breakdown occurs slowl y, is incomplete, and is manifested structurally by decreases in zonul a occludens-1. protein, whereas actin organization is unchanged. Thus, serum induces a specific breakdown in retinal epithelial cell tight j unctions that may be mediated by effects on the expression of zonula o ccludens-1.