Tight junction has a crucial role in regulating paracellular transports (as a 
barrier) and in separating apical from basolateral compartments to maintain cell 
polarity (as a fence). Tight junction can be disrupted by various stimuli, 
including oxidative stress, pathogens and proinflammatory cytokines. However, 
association of defective tight junction with kidney stone pathogenesis remains 
unknown. We therefore examined whether calcium oxalate monohydrate (COM) 
crystals, which are the major crystalline composition in kidney stones, have any 
effects on expression and function of tight junction of polarized renal tubular 
epithelial cells. Western blot analysis revealed marked decrease in levels of 
occludin and zonula occludens-1 (ZO-1) in COM-treated polarized Madin-Darby 
canine kidney (MDCK) cells. Immunofluorescence staining revealed not only the 
decline of these tight junction proteins but also their redistribution and 
dissociation in COM-treated cells. Additionally, transepithelial resistance was 
significantly decreased, indicating impaired tight junction barrier and 
increased paracellular permeability in COM-treated cells. Subcellular 
fractionation followed by western blot analysis of Na(+)/K(+)-ATPase-α1 revealed 
that this basolateral membrane marker was also detectable in apical membrane 
fraction of COM-treated cells, but not in apical membrane fraction of control 
cells. Immunofluorescence study confirmed the translocation of 
Na(+)/K(+)-ATPase-α1 (from basolateral to apical membranes) in COM-treated 
cells, indicating impaired fence function of the tight junction. Moreover, 
dihydrorhodamine assay using flow cytometry revealed the significantly higher 
level of hydrogen peroxide in the COM-treated cells. These data provide the 
first evidence to demonstrate decreased expression and defective barrier and 
fence functions of the tight junction of renal tubular epithelial cells exposed 
to COM crystals that may be fundamental for subsequent renal tubulointerstitial 
injury, which in turn enhances the stone pathogenesis.