After endocytosis, most membrane proteins and lipids return to the cell surface, but some membrane components are delivered to late endosomes or the Golgi. We now understand that the pathways taken by internalized molecules that eventually recycle to the cell surface can be surprisingly complex and can involve a series of sorting events that occur in several organelles. The molecular basis for many of these sorting processes is only partly understood.

Mammalian cells take up extracellular material by a variety of different mechanisms that are collectively termed endocytosis. Endocytic mechanisms serve many important cellular functions including the uptake of extracellular nutrients, regulation of cell-surface receptor expression, maintenance of cell polarity, and antigen presentation. Endocytic pathways are also utilized by viruses, toxins, and symbiotic microorganisms to gain entry into cells. One of the best-characterized endocytic mechanisms is receptor-mediated endocytosis via clathrin-coated pits. This type of endocytosis constitutes the major emphasis of this review, with a brief discussion of other endocytic mechanisms and their comparison with the receptor-mediated pathway. This review describes and evaluates critically current understanding of the mechanisms of entry of plasma membrane components such as the receptor-ligand complexes and membrane lipids as well as the extracellular fluid into cells. The intracellular sorting and trafficking of these molecules upon internalization are also described. The roles of endocytosis in physiological and pathological processes are discussed. These include maintenance of cell polarization, antigen presentation, glucose transport, atherosclerosis, Alzheimer's disease, and the endocytosis of toxins and viruses.

Membrane lipids are essential for biological functions ranging from membrane trafficking to signal transduction. The composition of lipid membranes influences their organization and properties, so it is not surprising that disorders in lipid metabolism and transport have a role in human disease. Significant recent progress has enhanced our understanding of the molecular and cellular basis of lipid-associated disorders such as Tangier disease, Niemann-Pick disease type C and atherosclerosis. These insights have also led to improved understanding of normal physiology.

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Abstract. A central question in the endocytic process concerns the mechanism for sorting of recycling components (such as transferrin or low density lipoprotein receptors) from lysosomally directed components; membrane-associated molecules including receptors are generally directed towards the recycling pathway while the luminal content of sorting endosomes, consisting of the acid-released ligands, are lysosomally targeted. However, it is not known whether recycling membrane receptors follow bulk membrane flow or if these proteins are actively sorted from lysosomally directed material because of specific protein sequences and/or structural features. Using quantitative fluorescence microscopy we have determined the endocytic route and kinetics of traffic of the bulk carder, membrane lipids, to address this issue directly. We show that N- [N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-eaminohexanoyl] -sphingosylphosphorylcholine (Ca-NBD-SM) in endocytosed as bulk membrane, and it transits the endocytic system kinetically and morphologically identically to fluorescently labeled transferdn in a CHO cell line. With indistinguishable kinetics, the two labeled markers sort from lysosomally destined molecules in peripherally located sorting endosomes, accumulate in a peri-centriolar recycling compartment, and finally exit the cell. Other fluorescently labeled lipids, C6-NBD-phosphatidylcholine and galactosylceramide also traverse the same pathway. The constitutive nature of sorting of bulk membrane towards the recycling pathway and the lysosomal direction of fluid phase implies a geometric basis of sorting.I NTRACELLULAR trafficking of proteins during endocytosis has been extensively characterized (see Fig. 1). This process is mediated by organelles such as coated vesicles, sorting endosomes or early endosomes, and late endosomes (Goldstein et al., 1985;Maxfield and Yamashiro, 1991;van Deurs et al., 1989). A central question in the endocytic process concerns the mechanism for sorting of recycling components (e.g., the transferrin receptor [Tf-R] 1 or the low density lipoprotein receptor [LDL-R]) from lysosomally directed components (e.g., acid-released ligands such as low density lipoprotein ). This sorting is a rapid step (tu2 < 3 min), and as depicted in Fig. 1, takes place in acidic organelles called sorting endosomes (Yamashiro and Maxfield, 1987), having 1. Abbreviations used in this paper: ce2M, ce2-macroglobulin; C6-NBD-PC, C6-NBD-phosphatidylcholine; C6-NBD-SM, N-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-~-aminohexanoyl]-sphingosylphosphorylcholine; CCD, chargedcouped device; Cy3-ct2M, Cy3-1abeled ce2M; DiO-LDL, 3,3tdioctadecyl -oxacarbocyanine-labeled LDL; DiI-LDL, 3,3'-dioctadecylindocarbocyanine-labeled LDL; HF-12, Hepes-buffered Hams F-12 medium; LDL, low density lipoprotein; LDL-R, LDL-receptor; Rh-Tf, rhodamine-labeled Tf; "If, transferrin; Tf-R, "IT receptor; Tx, Texas-red; Tx-Tf, Texas-redlabeled Tf.Please address all correspondence to Dr. F.R. Maxfield, Department of Pathology, Rm 15-420, College of Physicians and S...

Microglia are immune system cells associated with Alzheimer's disease plaques containing beta-amyloid (A beta). Murine microglia internalize microaggregates of fluorescently labeled or radioiodinated A beta peptide 1-42. Uptake was confirmed using aggregates of unlabeled A beta detected by immunofluorescence. Uptake of A beta was reduced by coincubation with excess acetyl-low density lipoprotein (Ac-LDL) or other scavenger receptor (SR) ligands, and Dil-labeled Ac-LDL uptake by microglia was blocked by excess A beta. CHO cells transfected with class A or B SRs showed significantly enhanced uptake of A beta. These results show that microglia express SRs that may play a significant role in the clearance of A beta plaques. Binding to SRs could activate inflammation responses that contribute to the pathology of Alzheimer's disease.

Leukocytes enter sites of inflammation by squeezing through the borders between endothelial cells that line postcapillary venules at that site. This rapid process, called transendothelial migration (TEM) or diapedesis, is completed within 90 s after a leukocyte arrests on the endothelial surface. In this time, the leukocyte moves in ameboid fashion across the endothelial borders, which remain tightly apposed to it during transit. It is not known how the endothelial cell changes its borders rapidly and reversibly to accommodate the migrating leukocyte. Here we show that there is a membrane network just below the plasmalemma at the cell borders that is connected at intervals to the junctional surface. PECAM-1, an integral membrane protein with an essential role in TEM, is found in this compartment and constitutively recycles evenly along endothelial cell borders. During TEM, however, recycling PECAM is targeted to segments of the junction across which monocytes are in the act of migration. In addition, blockade of TEM with antibodies against PECAM specifically blocks the recruitment of this membrane to the zones of leukocyte migration, without affecting the constitutive membrane trafficking.