Computer Simulations Suggest a Key Role of Membranous Nanodomains in Biliary Lipid Secretion

The bile fluid contains various lipids that are secreted at the canalicular membrane of hepatocytes. As the secretion mechanism is still a matter of debate and a direct experimental observation of the secretion process is not possible so far, we used a mathematical model to simulate the extraction of the major bile lipids cholesterol, phosphatidylcholine and sphingomyelin from the outer leaflet of the canalicular membrane. Lipid diffusion was modeled as random movement on a triangular lattice governed by next-neighbor interaction energies. Phase separation in liquid-ordered and liquid-disordered domains was modeled by assigning two alternative ordering states to each lipid species and minimization of next-neighbor ordering energies. Parameterization of the model was performed such that experimentally determined diffusion rates and phases in ternary lipid mixtures of model membranes were correctly recapitulated. The model describes the spontaneous formation of nanodomains in the external leaflet of the canalicular membrane in a time window between 0.1 ms to 10 ms at varying lipid proportions. The extraction of lipid patches from the bile salt soluble nanodomain into the bile reproduced observed biliary phospholipid compositions for a physiologi-cal membrane composition. Comparing the outcome of model simulations with available experi-mental observations clearly favors the extraction of tiny membrane patches composed of about 100–400 lipids as the likely mechanism of biliary lipid secretion.Author Summary: Formation of the bile is one of the central functions of the liver. The bile fluid aids in the digestion of edible fats and removal of drugs and toxins from the body. The bile fluid is mainly composed of bile salts (BS), phosphatidylcholine (PC) and cholesterol (CH) in a fairly fixed proportion that prevents liver impairment by gallstone formation or cholestasis. During bile formation, BS are actively pumped out of the hepatocyte into the extracellular space where they extract PC and CH from the canalicular membrane. This extraction process bears the risk for the canalicular membrane to be destructed. Hence, only a certain fraction of the membrane should be accessible to the solubilizing activity of BS. We have developed a mathematical model that describes the temporal formation of CH-enriched ordered and PC-enriched disordered nanodomains in the canalicular membrane. Model simulations reveal that the disordered nanodomains exhibit a composition of PC and CH similar to that also found in the bile. From this finding and the good concordance of model simulations with experimental data we conclude that PC and CH are mainly secreted into the bile from the disordered nanodomain. Our work adds a new layer of physiological importance to the spontaneous formation of lipid domains in biological membranes.