Sushmita Mukherjee

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.

To understand the mechanisms for endocytic sorting of lipids, we investigated the trafficking of three lipid-mimetic dialkylindocarbocyanine (DiI) derivatives, DiIC16(3) (1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), DiIC12(3) (1,1'- didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), and FAST DiI (1,1'-dilinoleyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate), in CHO cells by quantitative fluorescence microscopy. All three DiIs have the same head group, but differ in their alkyl tail length or unsaturation; these differences are expected to affect their distribution in membrane domains of varying fluidity or curvature. All three DiIs initially enter sorting endosomes containing endocytosed transferrin. DiIC16(3), with two long 16-carbon saturated tails is then delivered to late endosomes, whereas FAST DiI, with two cis double bonds in each tail, and DiIC12(3), with saturated but shorter (12-carbon) tails, are mainly found in the endocytic recycling compartment. We also find that DiOC16(3) (3,3'- dihexadecyloxacarbocyanine perchlorate) and FAST DiO (3, 3'-dilinoleyloxacarbocyanine perchlorate) behave similarly to their DiI counterparts. Furthermore, whereas a phosphatidylcholine analogue with a BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) fluorophore attached at the end of a 5-carbon acyl chain is delivered efficiently to the endocytic recycling compartment, a significant fraction of another derivative with BODIPY attached to a 12-carbon acyl chain entered late endosomes. Our results thus suggest that endocytic organelles can sort membrane components efficiently based on their preference for association with domains of varying characteristics.

Considerable evidence shows that lateral inhomogeneities in lipid composition and physical properties exist in biological membranes. These membrane lipid domains are proposed to play important roles in processes such as signal transduction and membrane traffic. However, there is not at present an adequate description of the nature of these lipid domains in terms of their size, abundance, composition, or dynamics. We discuss the current analyses of the properties and function of membrane domains in cells and compare their properties with chemically simpler model membrane systems that can be understood in greater detail.