Hepatic secretion of phospholipid vesicles in the mouse critically depends on mdr2 or MDR3 P-glycoprotein expression. Visualization by electron microscopy.

Abstract

Hepatocellular secretion of bile salts into the biliary space induces phospholipid and cholesterol secretion, but the mechanism for integrated lipid secretion is poorly understood. Knockout mice unable to make the canalicular membrane mdr2 P-glycoprotein exhibit normal rates of bile salt secretion, yet are virtually incapable of secreting biliary phospholipid and cholesterol. As the mdr2 P-glycoprotein is thought to mediate transmembrane movement of phospholipid molecules, this mouse model was used to examine the mechanism for biliary phospholipid secretion. In wild-type mdr2 ( ϩ / ϩ ) mice, ultrarapid cryofixation of livers in situ revealed abundant unilamellar lipid vesicles within bile canalicular lumina. Although 74% of vesicles were adherent to the external aspect of the canalicular plasma membrane, bilayer exocytosis was not observed. Vesicle numbers in mdr2 ( ϩ / Ϫ ) and ( Ϫ / Ϫ ) mice were 55 and 12% of wild-type levels, respectively. In a strain of mdr2 ( Ϫ / Ϫ ) mice which had been "rescued" by heterozygous genomic insertion of the MDR3 gene, the human homologue of the murine mdr2 gene, vesicle numbers returned to 95% of wild-type levels. Our findings indicate that biliary phospholipid is secreted as vesicles by a process largely dependent on the action of the murine mdr2 P-glycoprotein or human MDR3 P-glycoprotein. We conclude that mdr2-mediated phospholipid translocation from the internal to external hemileaflet of the canalicular membrane permits exovesiculation of the external hemileaflet, a vesiculation process promoted by the detergent environment of the bile canalicular lumen.

Atp8b1 deficiency in mice reduces resistance of the canalicular membrane to hydrophobic bile salts and impairs bile salt transport

“…44 The vesicular structures observed in canaliculi of mutant livers are likely formed as a consequence of the elevated biliary cholesterol levels. These structures must not be confused with the vesicles that have been described by Crawford and coworkers 45 to support the vesiculation theory, a proposed mechanism for biliary phospholipid excretion. In contrast to the vesicles observed in our study, they described unilamellar vesicles, most of which were adherent to the canalicular membrane.…”

Tauroursodeoxycholic acid inserts the bile salt export pump into canalicular membranes of cholestatic rat liver

“…A relevant loss of Bsep into the bile canalicular lumen during administration of TLCA appears unlikely to explain the decrease of Bsep density in canalicular membranes. Although hydrophobic bile salts have been shown to induce external hemileaflet exocytosis of phospholipid vesicles, 38,39 the mechanisms for protein retention within the canalicular membrane during biliary phospholipid secretion appear to be very effective. 40 Thus, Bsep retrieval into the pericanalicular zone (see Figures 1 and 2) appears to be the predominant mechanism leading to Bsep deprivation of apical hepatocyte membranes exposed to TLCA.…”

Impaired response of biliary lipid secretion to a lithogenic diet in phosphatidylcholine transfer protein-deficient mice

“…Evidence has been presented that Abcb4 (Mdr2), which rate limits biliary phosphatidylcholine secretion at the canalicular membrane, may function as a specific flippase for phosphatidylcholines, thereby providing a selective mechanism for phosphatidylcholines (36). Because of the high rates of Abcb4-mediated biliary phosphatidylcholine secretion (37,38), hepatocytes would be expected to possess efficient mechanisms to replenish the canalicular membrane as phosphatidylcholine molecules are removed during bile formation. Earlier observations that microtubule-disrupting agents could inhibit biliary secretion of cholesterol and phospholipid (39)(40)(41) suggested that transport of vesicles along microtubules could provide a resupply route (reviewed in 37).…”