DEPHOSPHORYLATION OF EZRIN AS AN EARLY EVENT IN RENAL MICROVILLAR BREAKDOWN AND ANOXIC INJURY

Disruption of the renal proximal tubule (PT) brush border is a promine nt early event during ischemic injury to the kidney. The molecular bas is for this event is unknown, Within the brush border, ezrin may norma lly link the cytoskeleton to the cell plasma membrane, Anoxia causes e zrin to dissociate from the cytoskeleton and also causes many cell pro teins to become dephosphorylated in renal PTs, This study examines the hypothesis that ezrin dephosphorylation accompanies and may mediate t he anoxic disruption of the rabbit renal PT, During normoxia, 73 +/- 3 % of the cytoskeleton-associated (Triton-insoluble) ezrin was phosphor ylated, but 88 +/- 6% of dissociated (Triton-soluble) ezrin was dephos phorylated, Phosphorylation was on serine/threonine residues, since ez rin was not detectable by an antibody against phosphotyrosine. After 6 0 min of anoxia, phosphorylation of total intracellular ezrin signific antly decreased from 72 +/- 2% to 21 +/- 9%, and ezrin association wit h the cytoskeleton decreased from 91 +/- 2% to 58 +/- 2%. Calyculin A (1 mu M), the serine/threonine phosphatase inhibitor, inhibited the de phosphorylation of ezrin during anoxia by 57% and also blocked the dis sociation of ezrin from the cytoskeleton by 53%. Our results demonstra te that (i) the association of ezrin with the renal microvillar cytosk eleton is correlated with phosphorylation of ezrin serine/threonine re sidues and (ii) anoxia may cause disruption of the renal brush border by dephosphorylating ezrin and thereby dissociating the brush border m embrane from the cytoskeleton.