Lipid Polarity Is Maintained in Absence of Tight Junctions

Junichi Ikenouchi,Kazuaki Umeda,Donna B Stolz,Kazutaka Ikeda,Mayu Suzuki,Masato Umeda,Ryo Taguchi,Satoshi B Sato,Tetsuyuki Kobayashi,Toshihide Kobayashi

doi:10.1074/jbc.m111.327064

Abstract

The role of tight junctions (TJs) in the establishment and maintenance of lipid polarity in epithelial cells has long been a subject of controversy. We have addressed this issue using lysenin, a toxin derived from earthworms, and an influenza virus labeled with a fluorescent lipid, octadecylrhodamine B (R18). When epithelial cells are stained with lysenin, lysenin selectively binds to their apical membranes. Using an artificial liposome, we demonstrated that lysenin recognizes the membrane domains where sphingomyelins are clustered. Interestingly, lysenin selectively stained the apical membranes of epithelial cells depleted of zonula occludens proteins (ZO-deficient cells), which completely lack TJs. Furthermore, the fluorescent lipid inserted into the apical membrane by fusion with the influenza virus did not diffuse to the lateral membrane in ZO-deficient epithelial cells. This study revealed that sphingomyelin-cluster formation occurs only in the apical membrane and that lipid polarity is maintained even in the absence of TJs.

Highlights

Tight junctions (TJs) are thought to prevent lipids from diffusing freely between the apical and basolateral membrane
As TJs are thought to function as the diffusion barrier against membrane proteins and lipids and to play essential roles in epithelial polarity by maintaining the asymmetric distribution of membrane proteins and lipids [5, 6], we examined the distribution of sphingomyelin in the ZO-1/ ZO-2/ZO-3-deficient epithelial cells (ZO-deficient cells), which lack TJs completely
We have shown that lysenin strongly binds to the membrane domain where sphingomyelin is clustered

Summary

Background

Tight junctions (TJs) are thought to prevent lipids from diffusing freely between the apical and basolateral membrane. The role of tight junctions (TJs) in the establishment and maintenance of lipid polarity in epithelial cells has long been a subject of controversy. We have addressed this issue using lysenin, a toxin derived from earthworms, and an influenza virus labeled with a fluorescent lipid, octadecylrhodamine B (R18). The ratio of sphingomyelin/phosphatidylcholine was reported to be higher in the apical membrane than the basolateral membrane, based on biochemical experiments using two types of envelope viruses [4] This raises an important question; How is such lipid polarity maintained within continuous membranes in epithelial cells? We clearly demonstrated that the apical polarization of sphingomyelin clusters occur in the absence of TJs and TJs are not the lateral diffusion barrier of lipids in epithelial cells

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION