Tuma, Pamela L., and Ann L. Hubbard. Transcytosis: Crossing Cellular Barriers. Physiol Rev 83: 871–932, 2003; 10.1152/physrev.00001.2003.—Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

ABCG5 (G5) and ABCG8 (G8) are ATP-binding cassette (ABC) transporters that limit intestinal absorption and promote biliary excretion of neutral sterols. Mutations in either ABCG5 or ABCG8 result in an identical clinical phenotype, suggesting that these two half-transporters function as heterodimers. Expression of both G5 and G8 is required for either protein to be transported to the plasma membrane of cultured cells. In this paper we used immunofluorescence microscopy to confirm, in vivo, that G5 is localized to the apical membranes of mouse enterocytes and hepatocytes. Other ABC halftransporters function as homodimers or as heterodimers with other subfamily members. To determine whether G5 or G8 complex with other ABCG half-transporters, we co-expressed G1, G2, and G4 with either G5 or G8 in cultured cells. G1, G2, and G4 co-immunoprecipitated with G5, and G4 co-immunoprecipitated with G8, but the putative dimers were retained in the endoplasmic reticulum (ER). Adenovirus-mediated expression of either G5 or G8 in the liver of G5G8 null mice resulted in ER retention of the expressed proteins and no increase in biliary cholesterol. In contrast, co-expression of G5 and G8 resulted in transit of the proteins out of the ER and a 10-fold increase in biliary cholesterol concentration. Finally, adenoviral expression of G2 in the presence or absence of G5 or G8 failed to promote sterol excretion into bile. These experiments indicate that G5 and G8 function as obligate heterodimers to promote sterol excretion into bile. ABCG5 (G5)1 and ABCG8 (G8) are members of the large family of ATP-binding cassette (ABC) transporters that facilitate translocation of a wide variety of substrates across cellular membranes (1). Eukaryotic ABC transporters consist of two modules, a magnesium-dependent ATPase catalytic domain and a transmembrane domain containing 6 -12 membranespanning segments (2). ABC transporters are divided into halftransporters, which contain one ATPase domain and one membrane-spanning domain, and full-transporters, which contain paired modules in tandem (2). G5 and G8 are both half-transporters belonging to the G subfamily of ABC proteins. Like the other three members of the human G subfamily of ABC transporters (ABCG1, ABCG2, and ABCG4), the ATPase catalytic domains of G5 and G8 are located N-terminal to the transmembrane domain.Mutations in either ABCG5 or ABCG8 cause sitosterolemia, an autosomal recessive disorder characterized by the accumulation of both plant-derived (primarily sitosterol) and animalderived (cholesterol) sterols in plasma and tissues (3-5). In mice, G5 and G8 limit the absorption of dietary sterols and promote the excretion of cholesterol into bile (6, 7). In sitosterolemia there is a generalized increase in the absorption of dietary neutral sterols and a defect in the efflux of these sterols into bile (8 -10). These changes in sterol trafficking result in deposition of neutral sterols in skin as xanthomas and in coronary arteries, resulting in premature atherosclerosis.G5 and G8 are both glycop...

The liver performs numerous vital functions, including the detoxification of blood before access to the brain while simultaneously secreting and internalizing scores of proteins and lipids to maintain appropriate blood chemistry. Furthermore, the liver also synthesizes and secretes bile to enable the digestion of food. These diverse attributes are all performed by hepatocytes, the parenchymal cells of the liver. As predicted, these cells possess a remarkably well-developed and complex membrane trafficking machinery that is dedicated to moving specific cargos to their correct cellular locations. Importantly, while most epithelial cells secrete nascent proteins directionally toward a single lumen, the hepatocyte secretes both proteins and bile concomitantly at its basolateral and apical domains, respectively. In this Beyond the Cell review, we will detail these central features of the hepatocyte and highlight how membrane transport processes play a key role in healthy liver function and how they are affected by disease.