Receptor-mediated endocytosis in cultured fibroblasts: cryptic coated pits and the formation of receptosomes

Enhanced cellular uptake of maleimide-modified liposomes via thiol-mediated transport

With a small amount of maleimide modification on the liposome surface, enhanced cellular uptake of liposomes and drug-delivery efficiency can be obtained both in vitro and in vivo. Herein, we describe the mechanisms underlying this enhanced cellular uptake. Suppression of the cellular uptake of maleimide-modified liposomes (M-GGLG, composed of 1,5-dihexadecyl N,N-diglutamyl-lysyl-L-glutamate [GGLG]/cholesterol/poly(ethylene glycol) - 1,2-distearoyl-sn-glycero-3-phosphoethanolamine [PEG₅₀₀₀-DSPE]/maleimide [M]-PEG₅₀₀₀-Glu2C18 at a molar ratio of 5:5:0.03:0.03) caused by temperature block and addition of serum was alleviated compared with that of liposomes without maleimide modification (GGLG liposomes, composed of GGLG/cholesterol/PEG₅₀₀₀-DSPE/PEG₅₀₀₀-Glu2C₁₈ at a molar ratio of 5:5:0.03:0.03). When 0.01 nM N-ethylmaleimide was used to pre-block cellular thiols, the cellular uptake of M-GGLG liposomes was decreased to approximately 70% in HeLa, HCC1954, MDA-MB-468, and COS-7 cell lines. Moreover, inhibition of a thiol-related reductase such as protein disulfide isomerase resulted in a 15%-45% inhibition of the cellular uptake of M-GGLG liposomes, whereas GGLG liposomes were not influenced. Further, single and mixed inhibitors of clathrin-mediated endocytosis, caveolae-mediated endocytosis, and macropinocytosis did not efficiently inhibit the cellular uptake of M-GGLG liposomes. Using confocal microscopy, we verified that M-GGLG liposomes were localized partially in lysosomes after inhibition of the mentioned conventional endocytic pathways. Therefore, it was hypothesized that the mechanisms underlying the enhanced cellular uptake of liposomes by maleimide modification was thiol-mediated membrane trafficking, including endocytosis and energy-independent transport.

FCH domain only-2 organizes clathrin-coated structures and interacts with Disabled-2 for low-density lipoprotein receptor endocytosis

Clathrin-mediated endocytosis regulates the internalization of many nutrient and signaling receptors. Clathrin and endocytic accessory proteins are recruited to receptors by specific adaptors. The adaptor Disabled-2 (Dab2) recruits its cargoes, including the low-density lipoprotein receptor (LDLR), and mediates endocytosis, even when the major adaptor protein AP2 is depleted. We hypothesized that the accessory proteins normally recruited by AP2 may be recruited by Dab2 if AP2 is absent. We identified one such accessory protein, the F-BAR protein FCH domain only-2 (FCHO2), as a major Dab2-interacting protein. The μ-homology domain (μHD) of FCHO2 binds directly to DPF sequences in Dab2 that also bind AP2. Disrupting the Dab2-FCHO2 interaction inhibited Dab2-mediated LDLR endocytosis in AP2-depleted cells. Depleting FCHO2 reduced the number but increased the size of clathrin structures on the adherent surface of HeLa cells and inhibited LDLR and transferrin receptor clustering. However, LDLR was internalized efficiently by FCHO2-deficient cells when additional time was provided for LDLR to enter the enlarged structures before budding, suggesting that later steps of endocytosis are normal under these conditions. These results indicate FCHO2 regulates the size of clathrin structures, and its interaction with Dab2 is needed for LDLR endocytosis under conditions of low AP2.

Coupling between clathrin-dependent endocytic budding and F-BAR-dependent tubulation in a cell-free system

Cell-free reconstitution of membrane traffic reactions and the morphological characterization of membrane intermediates that accumulate under these conditions have helped to elucidate the physical and molecular mechanisms involved in membrane transport. To gain a better understanding of endocytosis, we have reconstituted vesicle budding and fission from isolated plasma membrane sheets and imaged these events. Electron and fluorescence microscopy, including subdiffraction-limit imaging by stochastic optical reconstruction microscopy (STORM), revealed F-BAR (FBP17) domain coated tubules nucleated by clathrin-coated buds when fission was blocked by GTPgammaS. Triggering fission by replacing GTPgammaS with GTP led not only to separation of clathrin-coated buds, but also to vesicle formation by fragmentation of the tubules. These results suggest a functional link between FBP17-dependent membrane tubulation and clathrin-dependent budding. They also show that clathrin spatially directs plasma membrane invaginations that lead to the generation of endocytic vesicles larger than those enclosed by the coat.

The oocyte as an endocytic cell

Oocytes of Xenopus laevis grow primarily by sequestering vitellogenin (VTG) selectively from the maternal bloodstream. Morphological observations have demonstrated that an endocytic system is responsible for VTG uptake. Binding studies indicate the presence of 2-28 X 10(10) surface VTG receptors per oocyte. These are continuously internalized into endosomes whether or not they are occupied by VTG, and other macromolecules may become trapped in the process. VTG-containing endosomes give rise to dense transitional yolk bodies; these fuse with yolk platelets only after the cleavage of vitellogenin. In the absence of VTG, endosomes appear to fuse directly with yolk platelets. From these observations it is postulated that receptor occupancy can act as a transmembrane signal which directs the postendocytic compartmentation of proteins. Yolk platelet proteins do not undergo subsequent turnover, whereas adventitiously incorporated protein is gradually lost from the oocyte by a dual mechanism which may involve both lysosomal proteolysis and secretion from the oocyte as a consequence of membrane recycling. Although these observations may not apply to all growing oocytes, the X. laevis oocyte nevertheless appears to be a particularly attractive experimental system for studies of endocytic compartmentation and membrane receptor recycling.

Dynamics of Endocytic Vesicle Creation

Clathrin-mediated endocytosis is the main path for receptor internalization in metazoans and is essential for controlling cell integrity and signaling. It is driven by a large array of protein and lipid interactions that have been deciphered mainly by biochemical and genetic means. To place these interactions into context, and ultimately build a fully operative model of endocytosis at the molecular level, it is necessary to know the kinetic details of the role of each protein in this process. In this review, we describe the recent efforts made, by using live cell imaging, to define clear steps in the formation of endocytic vesicles and to observe the recruitment of key proteins during membrane invagination, the scission of a newly formed vesicle, and its movement away from the plasma membrane.

Coupling between clathrin-coated-pit invagination, cortactin recruitment, and membrane scission observed in live cells

During clathrin-mediated endocytosis, membrane scission marks the isolation of a cargo-laden clathrin-coated pit (CCP) from the cell exterior. Here we used live-cell imaging of a pH-sensitive cargo to visualize the formation of clathrin-coated vesicles (CCVs) at single CCPs with a time resolution of seconds. We show that CCPs are highly dynamic and can produce multiple vesicles in succession. Using alternating evanescent field and epifluorescence illumination, we show that CCP invagination and scission are tightly coupled, with scission coinciding with maximal displacement of CCPs from the plasma membrane and with peak recruitment of cortactin-DsRed, a dynamin and F-actin binding protein. Finally, perturbing actin polymerization with latrunculin-B drastically reduces the efficiency of membrane scission and affects many aspects of CCP dynamics. We propose that CCP invagination, actin polymerization, and CCV formation are highly coordinated for efficient endocytosis.

Tegumental surface modulation in Schistosoma mansoni primary sporocysts in response to ligand binding

The clearance of host molecules from the surface of a parasite constitutes a potential immune evasive strategy. The possibility that certain ligands, when bound to the tegument of Schistosoma mansoni primary sporocysts, could induce such a modulating effect was investigated. Live, in vitro cultured primary sporocysts were first treated with either snail host Biomphalaria glabrata plasma, an anti-sporocyst monoclonal antibody (MoAb III-1), or concanavalin A (con A). The capacity of these primary ligands to produce a modulating effect alone, or when subsequently crosslinked by secondary or tertiary ligands, was measured using quantitative fluorescence microscopy. Snail plasma alone, or plasma crosslinked at the sporocyst surface with a mouse anti-plasma MoAb had little or no modulating effect. However, a tertiary level of ligand crosslinking with an anti-mouse IgG antibody produced an average 1.8-fold decrease in surface fluorescence within 1 h post-labelling. The anti-sporocyst MoAb III-1 also required secondary antibody reactivity to induce an average 1.5-fold decrease in MoAb III-1 recognized epitopes. Sporocysts labelled with con A crosslinked by secondary and tertiary ligands showed inconsistent modulation, with a 1.5-fold decrease in fluorescence in one out of three replicates. Overall, however, analysis of combined data revealed no significant effect of tertiary ligand level crosslinkage on modulation of con A-tegumental receptor complexes. In contrast, con A binding alone to tegumental determinants induced a small, but significant, reduction in surface con A complexes. Modulation of ligand-receptor complexes on the sporocyst tegumental membrane appears to be an energy-requiring event, since clearance of surface complexes was inhibited in the presence of sodium azide and/or sodium iodoacetate, or when larvae were incubated at 4 degrees C. It is concluded that alterations in sporocyst tegumental surface components may be triggered by specific (but as yet undefined) signals. Sporocysts are capable of exhibiting different responses depending on the nature of the binding signal and reactive tegumental receptor, and the degree of ligand crosslinkage.

ATP is required for receptor-mediated endocytosis in intact cells

We have demonstrated a requirement for cellular ATP in the receptor-mediated endocytosis of transferrin. This has been accomplished using a novel assay for endocytosis based on acquisition of resistance to the membrane impermeable reducing agent, glutathione (GSH). Diferric-transferrin was conjugated to biotin via a cleavable disulfide bond and iodinated. Internalization of 125I-biotin-S-S-transferrin (125I-BSST) was quantitated by adsorption to avidin-Sepharose after treatment of cells with GSH. Receptor-mediated endocytosis of 125I-BSST was severely inhibited in ATP-depleted cells. Similar results were obtained when ATP was depleted by incubation of cells either under a N2-atmosphere or in the presence of NaN3 and NaF. The latter treatment, alone, also resulted in a loss of surface transferrin receptors which could not be correlated to reductions in cellular ATP. In contrast to the acquisition of GSH resistance, the apparent internalization of 125I-BSST as assessed by inaccessibility to antitransferrin antibodies reached control levels in ATP-depleted cells. Our biochemical and morphological data suggested that, although ATP is required for receptor-mediated endocytosis, in ATP-depleted cells ligands can become efficiently sequestered into deeply invaginated pits that are inaccessible to large probes such as antibodies, but remain accessible to small molecules such as GSH.

Decreased density distribution of mesenteric and diaphragmatic microvascular anionic charges during murine abdominal sepsis

Pulmonary edema of sepsis is a consequence of increased transmural conductance for water and proteins at the level of lung microvessels induced by vasoactive endogenous mediators, liberated after activation of complement by bacterial endotoxins. Intermittent opening of interendothelial junctions at the level of post-capillary venules has been implicated as being the pathway for the leaking plasma proteins and water. Microvascular basement membranes and endothelial cell surfaces have fixed anionic charges (AS) which prevent the escape of plasma proteins from the circulation as well as the adhesion of blood cells to the luminal endothelium. The density distribution of these AS was substantially reduced in visceral and systemic microvessels during murine abdominal sepsis. This observation suggest that MOF secondary to sepsis is the consequence of a severe and generalized alteration of the microvascular electronegative charge, induced by liberation of inflammatory mediators.

Study on the origin of apical tubules in ileal absorptive cells of suckling rats using concanavalin-A as a membrane-bound tracer

The ileal absorptive cells of suckling rats exhibit high levels of endocytic activity being engaged in nonselective uptake of macromolecules from the intestinal lumen. The apical cytoplasm usually contains an extensive network of small, membrane-limited tubules (apical tubules: AT), in addition to newly formed endocytic vesicles and large endocytic vacuoles. To determine whether the AT are directly involved in the endocytic process by carrying the tracer into the cell, we have analysed movements of the apical cell membrane of the ileal absorptive cells by using a membrane-bound tracer (horseradish peroxidase-labelled concanavalin-A: Con-A HRP). The ileal absorptive cells were exposed in vitro to Con-A HRP for 10 min at 4 degrees C, incubated for different times in Con-A free medium at 37 degrees C, and prepared for electron microscopy. After 1 min incubation at 37 degrees C, invaginations of the apical cell membrane, including coated pits, and endocytic vesicles were labelled with HRP-reaction product, whereas the AT and large endocytic vacuoles were negative. After 2.5 min, almost all the large endocytic vacuoles were labelled with reaction product, which was seen in their vacuolar lumen and along the luminal surface of their limiting membrane. A few AT with reaction product were seen in the apical cytoplasm; they were in frequent connection with the reaction-positive large endocytic vacuoles. With increasing incubation time, the number of the labelled AT increased. Thus, after 15 min at 37 degrees C, the apical cytoplasm was fully occupied by the reaction-positive AT. The ends of these AT were often continuous with small spherical coated vesicles. No reaction product was detected in the Golgi complex at any time after incubation. These observations indicate that the AT located in the apical cytoplasm probably originate by budding off from the large endocytic vacuoles, rather than being involved in the process of endocytosis.

Ultrastructural localization of steroid sulphatase in cultured human fibroblasts by immunocytochemistry: a comparative study with lysosomal enzymes and the mannose 6-phosphate receptor

Immunocytochemistry was used to study the subcellular localization of steroid sulphatase in cultured human fibroblasts. Ultra-thin cryosections were incubated with antibodies raised against steroid sulphatase purified from human placenta and immune complexes were visualized with gold probes as electron dense markers. Steroid sulphatase was found in rough endoplasmic reticulum, Golgi cisternae and in the trans-Golgi reticulum, where it co-distributes with lysosomal enzymes and the mannose 6-phosphate receptor. The enzyme was not detected in lysosomes. Steroid sulphatase was also found at the plasma membrane and in the endocytic pathway (i.e. coated pits, endosomes and multivesicular endosomes). These may be the sites where sulphated oestrogen precursors are hydrolysed. Also here, it co-localizes with lysosomal enzymes and the mannose 6-phosphate receptor. It is concluded that microsomal steroid sulphatase and lysosomal enzymes share several cellular compartments.

Insulin and alpha 2-macroglobulin-methylamine undergo endocytosis by different mechanisms in rat adipocytes: II. Comparison of intracellular events

A previous ultrastructural study showed that gold-labeled insulin (Au-I) and the non-hormonal ligand gold-labeled alpha-2-macroglobulin-methylamine (Au-alpha 2MGMA) underwent endocytosis by dissimilar cell surface structures on rat adipocytes. The present ultrastructural study compared the intracellular routes taken by these two ligands in adipocytes. Intracellular Au-alpha 2MGMA was initially found within apparent coated vesicles but Au-I was not, consistent with the previous demonstration that Au-alpha 2MGMA underwent endocytosis by coated pits whereas Au-I was internalized by uncoated micropinocytotic invaginations. Early in the endocytic pathway, the two ligands were segregated within separate small vesicles and tubulovesicles. Au-alpha 2MGMA was concentrated in a small number of these structures whereas Au-I was sparsely distributed among a relatively large number. Subsequently, the two endocytic pathways converged as the ligands intermingled within pale multivesicular bodies and lysosome-like structures. Au-I was less efficiently transferred to lysosomes than Au-alpha 2MGMA since a greater proportion of intracellular Au-I remained associated with small vesicles and tubulovesicles. This study indicates that early intracellular events in the endocytic pathways of insulin and alpha 2MGMA are distinct. These findings are discussed in light of the fundamentally dissimilar biological roles of these two molecules and the possible involvement of the endocytic pathway in the insulin signaling mechanism.

Chemoattractant-regulated mobilization of a novel intracellular compartment in human neutrophils

A novel mobilizable intracellular compartment was identified in human neutrophils by latent alkaline phosphatase activity. This compartment is mobilized to the plasma membrane much more readily than any identified granule subset and has kinetics of up-regulation in the membrane similar to those reported for a variety of receptor proteins. Triton X-100 permeabilization of both intact human neutrophils and subcellular fractions obtained by density-gradient centrifugation revealed that 70 percent of the alkaline phosphatase is located in an intracellular compartment distinct from primary, secondary, and gelatinase granules and from the plasma membrane. This compartment fully translocates to the plasma membrane after stimulation with nanomolar concentrations of the chemotactic peptide N-formylmethionylleucylphenylalanine.

Early interactions between animal viruses and the host cell: relevance to viral vaccines

Viral recognition of specific receptors in the host cell plasma membrane is the first step in virus infection. Attachment is followed by a redistribution or capping of virus particles on the cell surface which may play a role in the uptake process. Certain viruses penetrate the plasma membrane directly but many, both enveloped and non-enveloped viruses, are endocytosed at coated pits and subsequently pass into endosomes. The low pH environment of the endosome facilitates passage of the viral genome into the cytoplasm. For some viruses the mechanism of membrane penetration is now known to be linked to a pH-mediated conformational change in external virion proteins. As a consequence of infection there are alterations in the permeability of the plasma membrane which may contribute to cellular damage. Recent advances in the understanding of these processes are reviewed and their relevance to the development of new strategies for vaccines emphasised.

Serial section analysis of clathrin-coated pits in rat liver sinusoidal endothelial cells

Electron microscopy and serial sections were used to examine the shape of clathrin-coated pits in sinusoidal endothelial cells of rat livers. Livers were perfused at 4 degrees C with either concanavalin A-horseradish peroxidase (conA-HRP), or HRP alone, followed by warm-up to 37 degrees C and fixation with glutaraldehyde. Alternatively, the livers were perfused with HRP at 37 degrees C, followed by fixation. All tissue was preserved using a membrane contrast enhancement technique (R-OTO) consisting of sequential osmium-ferrocyanide, thiocarbohydrazide, and osmium-ferrocyanide treatment. Peroxidase reaction product was used to identify structures participating in endocytosis. One hundred and ninety-three clathrin-coated structures were examined. Sixty-six were from livers perfused with conA-HRP at 4 degrees C, 63 were from livers perfused with only HRP at 4 degrees C, and 64 were from livers perfused with HRP at 37 degrees C. These coated structures were morphologically classified into three categories: (a) flat pits; (b) cup-shaped pits; (c) pits with a narrow neck. No isolated coated vesicles were found. In cells perfused at 4 degrees C followed by warming to 37 degrees C, the percentage of coated pits found connected to the cell surface by narrow necks was 31%, using conA-HRP, and 27% using HRP alone. In cells perfused continuously at 37 degrees C, the percentage of coated pits with narrow neck connections was 21% using HRP alone. These results suggest that the formation of coated pits connected to the surface by narrow necks is not an artifact of cell type, of experimental protocol or of incubation with a lectin.

Blocked coated pits in AtT20 cells result from endocytosis of budding retrovirions

AtT20 cells support the replication of two endogenous retroviruses, a murine leukemia virus and a mouse mammary tumor virus. On glass or plastic substrates, AtT20 cells grow in clumps. In this situation, retroviruses budding from the plasma membrane of one cell can, on rare occasions, be invested by coated pits in the plasma membranes of contiguous cells. These pits can invaginate to depths of 2,000-4,000 A within the cytoplasm drawing with them the viral buds which remain connected to their parental cells by tubular stalks, some of which are only 225 +/- 15 A in diameter. These stalks run down the straight necks of the pits from the buds to the parental cell surfaces. Several lines of evidence indicate that these unique structures are blocked such that neither endocytosis nor budding can go to completion, and that they persist for several hours. The properties of these blocked coated pits are relevant to models of both endocytosis and viral budding. First, they indicate that the invagination of a coated pit is not absolutely dependent on its pinching off to form a coated vesicle, but that uncoating appears to be dependent upon the generation of a free vesicle. Secondly, they suggest that the final stages in the maturation of a retroviral core into a mature nucleoid are dependent on the detachment of the bud from its parental cell and that the driving force of budding is the association of viral transmembrane proteins with viral core proteins. An explanation is offered to account for the formation of these structures despite the phenomenon of viral interference.

Study on membrane recycling in the rat visceral yolk-sac endoderm using concanavalin-A conjugates

The internalization and intracellular movements of apical-cell-membrane material were investigated in the endodermal cells of cultured visceral yolk-sacs of rats (whole-embryo culture; explanted at 10.5 days of gestation and cultured for 24 h) using horseradish peroxidase- and ferritin-labelled concanavalin A (Con-A HRP, Con-A Fer). When visceral yolk-sac endoderm was exposed to Con-A HRP or Con-A Fer for 5 min at 4 degrees C, the apical cell membranes containing a well-developed fuzzy coat were heavily labelled, whereas apical vacuoles, lysosomes and apical canaliculi were not. Incubation of Con-A-labelled endoderm for 5-60 min at 20 degrees and 37 degrees C in Con-A-free serum resulted in a temperature-dependent internalization of membrane-bound lectin into coated vesicles, apical vacuoles and lysosomes, and the apical cell membranes were cleared of the heavy labelling. With increasing incubation time, the number of labelled vacuolar structures and the intensity of their labelling decreased gradually, whereas the number of labelled apical canaliculi increased. Thus, after 30 and 60 min at 37 degrees C, most of the apical canaliculi contained high concentrations of the markers. It was possible to observe labelled apical canaliculi that were in continuity with labelled apical vacuoles and lysosomes as well as with the apical cell membrane. These findings in rat endodermal cells indicate that constituents of the apical cell membrane are internalized in apical vacuoles and lysosomes, and are then brought back to the apical cell membrane by the apical canaliculi, which concentrate and store this membrane material.

Phagocytotic and iron-storing capacities of stromal cells in the rat endometrium. A histochemical and ultrastructural study

Cytoplasmic pigment inclusions of rat endometrial stromal cells were studied by histology, histochemistry, fluorescence microscopy, electron microscopy and X-ray microprobe analysis. It is shown that a number of endometrial perivascular stromal cells contain numerous free cytoplasmic ferritin particles as well as hemosiderin vacuoles. The larger pigment inclusions reveal also positive PAS- and Schmorl reactions indicating that they contain polysaccharide and lipofuscin material, respectively. These pigment-storing stromal cells also display acid phosphatase activity; they avidly phagocytose instillated latex particles. No pigment-storing cells occur within the surface or glandular epithelium, either in the basal endometrium or in the myometrium. It is demonstrated that the endometrial iron-storing cells function as iron depots; they take part in the phagocytosis and endocytosis of extracellular tissue components and therefore can be named phagocytes. Our data show that "fibroblastoid" endometrial stromal cells may differentiate into endometrial resident phagocytes, which ensure interstitial proteolysis and hence facilitate the drainage of extracellular fluid into the venous blood capillaries.

Fluid-phase and adsorptive endocytosis in ciliated epithelial cells of the rat ductuli efferentes

Ciliated cells of the ductuli efferentes show at their apex a discrete endocytic apparatus composed of small vesicles connected to or subjacent to the apical plasma membrane, small apical membranous tubules, and pale multivesicular bodies. Deeper in the cytoplasm, there are acid phosphatase-positive denser, multivesicular bodies and secondary lysosomes showing an electron-dense cortex and a crystalline, paler stained core. Cationic ferritin and concanavalin A-ferritin used to demonstrate adsorptive endocytosis, when injected into the rete testis, rapidly reached the lumen of the ductuli efferentes. At 1 min after injection, these tracers were seen bound to the apical plasma membrane of ciliated cells and within small endocytic vesicles and by 5 min in narrow apical tubules. At 15 and 30 min after injection, the tracers appeared in pale multivesicular bodies while at 1 hr they were found within dense multivesicular bodies. At 2 hr and longer time intervals these tracers accumulated within secondary lysosomes. Native ferritin, concanavalin A-ferritin in the presence of alpha-methyl-D-mannoside, and horseradish peroxidase or albumin-colloidal gold complexes were used to analyze fluid-phase endocytosis. At various intervals following their injection into the rete testis, these tracers presented a distribution identical in all respects to that described for cationic ferritin and concanavalin A-ferritin. In the present work, none of the above tracers were transported to the abluminal aspect of the ciliated cells. These cells, like the nonciliated epithelial cells of the ductuli efferentes are thus involved in adsorptive as well as in fluid-phase endocytosis.

Evidence for an association between calmodulin and membrane patches containing gonadotropin-releasing hormone--receptor complexes in cultured gonadotropes

participation of calmodulin, clathrin, and actin in receptor mediated endocytosis of gonadotropin-releasing hormone (GnRH) was studied in an in vitro system of dispersed pituitary cells with a triple staining procedure. Cells were incubated in D-Lys6-Pro9-Des10-GnRH-biotin and stained with avidin-peroxidase-diaminobenzidine. Calmodulin, clathrin, and actin as well as luteinizing hormone were identified by indirect immunofluorescence with FITC- and rhodamine-labeled second antibody. The results indicate a close spatial association of calmodulin, but not of clathrin and actin, with GnRH-containing plasma membrane patches.

Cell biology of the asialoglycoprotein receptor system: a model of receptor-mediated endocytosis

Substantial information about the ASGP-R has accumulated in the 10 years following the initial studies of this receptor by Ashwell and Morell. Many of its biochemical properties, its structure, and its orientation within the plasma membrane are now known. The pathways of ASGP ligand and receptor, with the CURL organelle being a central component, are summarized in Fig. 18. The major pathway of the ligand through the cell, beginning with binding at the cell surface and ending with degradation in lysosomes, has been investigated in detail. Recently, alternate routes of the ligand such as the ligand recycling pathway have been observed. With regard to the itinerary of the receptor, there is now biochemical, kinetic, and morphological evidence to support receptor recycling. The new concept of CURL as an important intracellular organelle has originated from studies of ASGP-R recycling. Its importance in the dissociation and segregation of ligand and receptor as well as in receptor recycling is now evident. In addition, there has been a concurrent investigation of other receptor systems that participate in receptor-mediated endocytosis, providing parallels and contrasts to the ASGP-R of hepatocytes. Many critical issues still exist in the cell biology of the ASGP-R. What are the structural requirements of the receptor for ligand binding and subsequent endocytosis of the receptor-ligand complex? Very little is known about the interactions between the receptor and the lipid bilayer in which it resides. How does the receptor move laterally in the plasma membrane? Are there proteins or glycolipids closely associated with the ASGP-R and, if so, what is their function? What is the mechanism that causes receptor clustering into coated pits? Although the existence of a pathway for ligand recycling has been demonstrated, there are still many issues to be addressed. What signals a particular ligand molecule for recycling? Is it a stochastic process? What is the function of this route of ligand movement? How are the various ligand pathways coordinated and regulated? In addition, there are many unanswered questions regarding the receptor pathway. How does CURL mediate the sorting of ASGP-R from ligand? How are receptors with different destinations (e.g., ASGP-R and IgA receptor) sorted in CURL? What is the mechanism of ASGP-R degradation and how is it regulated? Finally, how does the Golgi function in the ASGP system and what is the relationship between the Golgi and CURL? Future investigation of these issues will require further observations with existing techniques as well as new approaches.(ABSTRACT TRUNCATED AT 400 WORDS)

Endocytic uptake, transport, and catabolism of proteins by epithelial cells

Adsorptive and/or receptor-mediated endocytosis of proteins is a universal cell property, which is highly expressed in epithelial cells. Some absorbed proteins are transported intact across cells and in this manner subserve specialized functions such as the transference of immunity from mother to child. Mainly, however, absorbed proteins are transported to lysosomes, where they undergo complete hydrolysis to amino acids. This process is essential for the homeostasis of circulating proteins. This brief review considers the intracellular pathways taken by endocytosed proteins and the quantitative aspects and determinants of protein uptake and catabolism by epithelial cells. The topics to be briefly discussed are initial internalization sites, transport organelles (endosomes), and lysosomal and nonlysosomal pathways of transport; intracellular sorting of internalized proteins, membranes, and receptors; kinetics and selectivity of renal cell uptake of low-molecular-weight proteins and proteohormones; receptor-mediated endocytosis of larger proteins (e.g., glycoproteins) by hepatocytes; and lysosomal catabolism of absorbed proteins and its dependence on protein load and endosomal-lysosomal pH and function. The perspectives of the field and some of the outstanding unsolved problems are briefly discussed.

Receptor-mediated endocytosis of glucagon in isolated mouse hepatocytes

The binding of glucagon to the cell surface and the pathway of intracellular transport of the hormone in isolated mouse hepatocytes were studied by autoradiography, colloidal gold-labeled glucagon (Au-glucagon), and biochemical methods. In cells incubated with 1251-glucagon at 4 degrees C, the label was mainly localized to the plasma membrane even after 60 min of incubation. At 20 degrees C, the labeled ligand was internalized by the cells and the amount of internalized ligand increased with time of incubation. At 37 degrees C, the ligand was rapidly internalized and found to be associated with coated or uncoated vesicles. Au-glucagon experiments revealed clearly the process of internalization of glucagon. Au-glucagon bound to the plasma membrane was transported to coated regions and then internalized into vesicles via coated pits. Biochemical results supported these findings from autoradiography and Au-glucagon experiments. Thus, glucagon is internalized by hepatocytes via receptor-mediated endocytosis.

Endosomes and Golgi vesicles in adsorptive and fluid phase endocytosis

We studied with morphometric methods the endocytosis by pheochromocytoma cells of a conjugate of wheat germ agglutinin with ferritin (WGA-Ft) and of horseradish peroxidase (HRP). Quantitative studies indicated that WGA-Ft was cleared slowly from cell surfaces and that it was not recycled to the surface. Cells labeled with WGA-Ft for 15 min at room temperature were washed and incubated in medium containing HRP for 15 or 30 min at 37 degrees C. The greatest proportion of labeled vesicles and tubules contained only WGA-Ft (83.4% at 15 min and 85.3% at 30 min). A very small fraction of labeled vesicles and tubules contained only HRP (0.2% at 15 min and 0.9% at 30 min). Vesicles and tubules at the Golgi apparatus were labeled almost exclusively with WGA-Ft (97% at 15 min and 30 min); the rest had both labels. Most labeled lysosomes contained both labels (80.1% at 15 min and 80.8% at 30 min). Of the remainder more contained WGA-Ft alone (20% at 15 min and 10.9% at 30 min), then HRP alone (none at 15 min and 8.2% at 30 min). In contrast to the various and varying patterns of labeling with WGA-Ft and HRP of the other organelles studied, the vast majority of endosomes contained both markers (94.1% at 15 min and 100% at 30 min); the rest contained WGA-Ft only. These results demonstrate that endosomes are recipients of both fluid phase and adsorptive endocytosis markers; these findings are consistent with the hypothesis that endosomes mediate the sorting out and subsequent intracellular traffic of membrane bound and fluid phase markers. Cisterns of the Golgi apparatus did not contain WGA-Ft; in sharp contrast, when WGA-HRP was used, the cisterns of the Golgi apparatus consistently contained HRP.

Diffusion-limited forward rate constants in two dimensions. Application to the trapping of cell surface receptors by coated pits

A variety of receptors are known to aggregate in specialized cell surface structures called coated pits, prior to being internalized when the coated pits close off. At 37 degrees C on human fibroblasts, as well as on other cell types, a recycling process maintains a constant number of coated pits on the cell surface. In this paper, we explore implications for receptor aggregation and internalization of the two types of recycling models that have been proposed for the maintenance of the coated pit concentration. In one model, coated pits alternate between accessible and inaccessible states at fixed locations on the cell surface, while in the other model, coated pits recycle to random locations on the cell surface. We consider receptors that are randomly inserted in the membrane, move by pure diffusion with diffusion coefficient D, and are instantly and irreversibly trapped when they reach a coated pit boundary (the diffusion limit). For such receptors, we calculate for each of the two models: the mean time tau to reach a coated pit, the forward rate constant k+ for the interaction of a receptor with a coated pit, and the fraction phi of receptors aggregated in coated pits. We show that for the parameters that characterize coated pits on human fibroblasts, the way in which coated pits return to the surface has a negligible effect on the values of tau, k+, and phi for mobile receptors, D greater than or equal to 1.0 X 10(-11) cm2/s, but has a substantial effect for "immobile" receptors, D much less than 1 X 10(-11) cm2/s. We present numerical examples to show that it may be possible to distinguish between these models if one can monitor slowly diffusing receptors (D less than 1 X 10(-11) cm2/s) on cells whose coated pits have relatively short lifetimes (less than or equal to 1 min). Finally, we show that for the low-density lipoprotein (LDL) receptor on human fibroblasts (D = 4.5 X 10(-11) cm2/s), the predicted and observed values of K+ and phi are in close agreement. Therefore, even for slowly diffusing LDL receptor, unaided diffusion as the transport mechanism of receptors to coated pits is consistent with measured rates of LDL internalization.

Endocytosis in nonciliated epithelial cells of the ductuli efferentes in the rat

The nonciliated cells lining the ductuli efferentes presented three distinct cytoplasmic regions. The apical region contained, in addition to cisternae of endoplasmic reticulum and mitochondria, two distinct membranous elements. The tubulovesicular system consisted of dilated tubules connected to the apical plasma membrane and subjacent distended vesicular profiles. The apical tubules, not connected to the cell surface, consisted of numerous densely stained tubules of small size which contain a compact, finely granulated material. The supranuclear region, in addition to a Golgi apparatus and ER cisternae, contained dilated vacuoles, pale and dense multivesicular bodies, as well as numerous dense granules identified cytochemically as lysosomes. The basal region contained the nucleus and many lipid droplets. The endocytic activity of these cells was investigated using cationic ferritin (CF) and concanavalin-A-ferritin (Con-A-ferritin) as markers of adsorptive endocytosis; and native ferritin (NF), concanavalin-A-ferritin in the presence of alpha-methyl mannoside, and horseradish peroxidase or albumin bound to colloidal gold for demonstrating fluid-phase endocytosis. These tracers were injected separately into the rete testis, and animals were sacrificed at various time intervals after injection. At 1 min, CF or Con-A-ferritin were seen bound to the apical plasma membrane, to the membrane of microvilli, and to the membrane delimiting elements of the tubulovesicular system. Between 2 and 5 min, these tracers accumulated in the densely stained apical tubules and at 15 min in the dilated vacuoles. Between 30 min and 1 hr, the tracers appeared in multivesicular bodies of progressively increasing density, whereas at 2 hr and later time intervals, many dense lysosomal elements became labeled. The tracers for fluid-phase endocytosis showed a distribution similar to that for CF or Con-A-ferritin except that they did not bind to the apical plasma membrane, microvilli, or membrane delimiting the tubulovesicular system. At no time interval were any of the tracers observed in the abluminal spaces. Thus, the nonciliated epithelial cells of the ductuli efferentes are actively involved in fluid-phase and adsorptive endocytosis, both of which result in the sequestration of endocytosed material within the lysosomal apparatus of the cell.

Initial steps in receptor-mediated endocytosis. The influence of temperature on the shape and distribution of plasma membrane clathrin-coated pits in cultured mammalian cells

We have examined the shape and distribution of clathrin-coated pits in Swiss 3T3 cells at 4 or 37 degrees C using electron microscopy with serial sections and immunofluorescence light microscopy. Both groups were fixed in glutaraldehyde and preserved further using a membrane contrast enhancement technique consisting of sequential osmium-ferrocyanide, thiocarbohydrazide and osmium-ferrocyanide treatment in situ. Concanavalin A-horseradish peroxidase (conA-HRP) was used to identify these structures participating in endocytosis. Two hundred twenty-two clathrin-coated structures were analysed; 126 from cells fixed at 4 degrees C, and 96 from cells fixed after a 3 min warm-up to 37 degrees C. All coated structures labeled with conA-HRP had demonstrable connections to the plasma membrane. These coated structures were morphologically classified into three categories: (a) flat pits; (b) curved pits; and (c) pits with narrow-neck connections to the plasma membrane. At 37 degrees C, 27% of coated pits had narrow neck connections to the plasma membrane whereas at 4 degrees C only 1% had such connections. Receptosomes (endosomes) labeled with conA-HRP were found only after incubation at 37 degrees C, indicating that active endocytosis was occurring in cells at 37 degrees C, but not at 4 degrees C. Immunofluorescence with anti-clathrin antibody was used to quantitate the number of clathrin-coated pits in Swiss 3T3 cells incubated at 4 and 37 degrees C prior to fixation. No difference was detected. There were 426 +/- 122 pits per cell at 37 degrees C and 441 +/- 106 at 4 degrees C. These results support the hypothesis that formation of a narrow neck connected a coated pit to the cell surface is an early step in the mechanism of receptor-mediated endocytosis.

Endocytosis in epithelial cells lining the rete testis of the rat

The endocytic activity of the low cuboidal cells lining the rete testis was analyzed by electron microscopy following injection of various tracers into the lumen of these anastomotic channels. At 1 and 5 minutes after injection, cationic ferritin (CF) and concanavalin A-ferritin (Con A) were seen bound to the apical plasma membrane and to the membrane of subjacent vesicles or invaginations connected to this apical membrane. At 30 and 60 minutes, these tracers were found in intracytoplasmic vesicles and in vesicles connected to the lateral or basal plasma membrane as well as in the lateral intercellular space and in the lamina lucida of basal lamina. At 30 minutes, CF and Con A also appeared in the matrix of pale multivesicular bodies while at 1 hour dense multivesicular bodies were labeled. At 2 hours and later time intervals, the tracers accumulated in dense granules identified as lysosomes. Native ferritin (NF), concanavalin A-ferritin in presence of alpha-methyl-D-mannoside, and horseradish peroxidase or albumin bound to colloidal gold were all to be incorporated by the lysosomal system of these epithelial cells, as just described for CF and Con A, but these various tracers were not bound to the apical plasma membrane or to the membrane of cytoplasmic vesicles, nor were they found in the intercellular spaces or the lamina lucida at the base of the cells. Thus, the epithelial cells of the rete testis do not appear to be only involved in the uptake of substances from the lumen and their disposal by the lysosomal system, but also appear to contribute to the transport of certain macromolecules from the lumen to the laterobasal surfaces of the cells. These cells may thus play a role in determining the composition of the rete testis fluid.

Coated vesicle morphology and sub-populations at the neuromuscular junction

Serial sections of frog cutaneous pectoris neuromuscular junctions were examined to determine if isolated coated vesicles in one section are connected to infoldings of the presynaptic membrane in adjacent sections or are truly pinched off from the plasmalemma. Twenty percent of the coated vesicles examined serially were isolated from plasmalemma. In addition, two populations of coated vesicles were observed: those the size of synaptic vesicles and a smaller population (8% of total) of larger diameter (100 nm).

Isolation and characterization of monopinocytotic vesicles containing polyomavirus from the cytoplasm of infected mouse kidney cells

Monopinocytotic vesicles containing polyomavirus were isolated from the cytoplasm of mouse kidney cells infected with polyomavirus using sucrose density gradients. Nonenclosed, membrane-associated virions released by the action of neuraminidase separated from vesicle-enclosed virions in the sucrose gradient. Marker enzyme assays indicated the derivation of the vesicle membrane from the plasma membrane of the cell. The 125I-labeled virus enclosed in the vesicle sedimented more slowly in the gradient and was not observed unless infection and endocytosis had occurred. Detergent treatment of virion-containing vesicles caused the release of polyomavirus with sedimentation properties similar to those of purified polyoma virions. In addition, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of virion proteins from vesicles containing virions demonstrated patterns of proteins similar to those of purified intact virions. Electron microscopy confirmed the presence of single intact virions inside vesicles. The study of these monopinocytotic virion-containing vesicles represents a further step in elucidating the early events of polyomavirus infection.

Membrane retrieval in the guinea-pig neurohypophysis. Isolation and characterization of secretory vesicles and coated microvesicles after radiolabel incorporation in vivo

We have developed small-scale methods for the isolation and biochemical characterization of subcellular fractions from single guinea-pig posterior-pituitary glands. Secretory vesicles and coated microvesicles produced in this way were of similar purity to those isolated from large amounts of tissue by conventional ultracentrifugation. [35S]Cysteine injected into the hypothalamus was found in the soluble contents of secretory vesicles isolated from the neural lobes 24 h later. High-pressure liquid-chromatographic analysis revealed that the radiolabel was incorporated into the expected neurosecretory products (oxytocin, vasopressin and neurophysin) and also into a biosynthetic intermediate in the vasopressin system. The membranes of secretory vesicles were labelled with [3H]choline 24 h after its hypothalamic injection. Little or no [3H]choline could be demonstrated in coated microvesicles at this time, although these structures were labelled 5 days after injection. Stimulating hormone secretion by chronic dehydration produced a significant fall in [3H]choline content of the secretory-vesicle membranes without any transfer of label into coated microvesicles, suggesting that coated microvesicles are not involved in membrane retrieval in the neurohypophysis.

Transglutaminase and receptor-mediated endocytosis in macrophages and cultured fibroblasts

The function of intracellular transglutaminases remains to be clarified. In fibroblasts the links between the activity of this enzyme and receptor-mediated endocytosis are complex and open to interpretation. However, the issue cannot be firmly laid to rest until the structural specificity of the alkylamine inhibitors of endocytosis is explained. In macrophages, there is substantial evidence that the enzyme plays some role in receptor-mediated phagocytosis, but what this role is and how it might relate to endocytosis in other types of cells is at present an unresolved issue.

Use of colloidal gold and ruthenium red in stereo high-voltage electron microscopic study of Con A-binding sites in mouse macrophages

Concanavalin A (Con A)-binding sites were labeled with colloidal gold (CG), stained with ruthenium red, and observed under a high-voltage electron microscope. Mouse peritoneal macrophages were labeled by the indirect Con A/CG labeling method at 0 degree C. After washing, some of the cells were incubated in phosphate-buffered saline (PBS) at 37 degrees C. The specimens were then stained with ruthenium red, to enhance the contrast of the cell surface, and embedded in Epon. Sections (0.3 approximately 3 micron thick) were cut and examined by high-voltage electron microscopy at accelerating voltages of 200 approximately 1,000 kV. Staining with ruthenium red provided a strong contrast of the cell surface and the invaginating tubules beneath it against the cytoplasm; in thick sections, both of them were clearly seen by stereomicroscopy. CG particles which represented Con A-binding sites were also sufficiently electron dense to be recognized by high-voltage electron microscopy of thick sections. The two- and three-dimensional distribution of CG particles on the ruthenium-red-positive cell surface was clearly visualized. At 0 degree C, Con A-binding sites were randomly distributed on the cell surface. The redistribution and endocytosis of Con A-binding sites were seen at 37 degrees C. The three-dimensional organization of membrane invagination, which represented the process of endocytosis, was clearly seen by stereomicroscopy. The combination of CG labeling and ruthenium red staining is a useful method for high-voltage electron microscopic analysis of the two- and three-dimensional distribution of CG-labeled ligands on the cell surface in thick sections.

Receptor-mediated binding and uptake of GnRH agonist and antagonist by pituitary cells

The intracellular pathway of an enzyme resistant GnRH agonist (D-Lys6-GnRH) conjugated to ferritin or to colloidal gold was followed in cultured pituitary cells. After an initial uniform distribution over the cell surface of gonadotropes, the electrondense marker was internalized, either individually or in small groups. Some, but not all marker was associated with invaginated membrane specializations showing a proteineous coat at their cytoplasmic site. After longer incubation times, the marker appeared in the lysosomal compartment and the Golgi apparatus, where it could be found in the vesicular as well as cisternal portion. In addition, the receptor-mediated endocytosis of the GnRH antagonist D-p-Glu1-D-Phe2-D-Trp3-D-Lys6-GnRH was studied by light and electron microscopic autoradiography after 30 and 60 min of incubation to ensure uptake. At both time points, in in vitro as well as in vivo studies, silver grains were localized over cytoplasmic organelles of castration cells, including dilated endoplasmic reticulum, lysosomes, and clear vesicles. No consistent association with cell nuclei, mitochondria, or secretory vesicles could be observed. The results suggest that both agonist and antagonist are binding selectively to the plasma membrane of gonadotropes and subsequently are taken up via receptor-mediated endocytosis for degradation or possible action on synthetic processes.

Endocytosis and exocytosis: current concepts of vesicle traffic in animal cells

Animal cells have specific pathways to transport macromolecules from their surrounding environment to their interior, and from internal compartments to the cell surface or other intracellular locations. Many of these movements appear to be receptor-dependent processes in which specific membrane receptors bind macromolecules, segregate them into discrete membrane-limited compartments, and move the molecules to new locations. Such processes include the clustering and internalization of receptor-bound ligands at the cell surface in clathrin-coated pits, the formation of endocytic vesicles (receptosomes) from coated pits, the movement of receptosomes by saltatory motion to the Golgi system, the concentration of materials in the coated pits of the Golgi system that are destined for delivery to lysosomes, and the directed traffic of materials destined for exocytosis out of the Golgi to the cell surface. This review describes some of the experiments which have led to our current understanding of the various organelles involved in this traffic and some of the biochemical mechanisms involved.

Reappearance of beta-adrenergic receptors after isoproterenol treatment in intact C6-cells

The reappearance of beta-adrenergic receptors in C6-glioma cells after desensitization with isoproterenol was studied using the antagonist [3H]CGP-12177. Reappearance had the following properties: (a) it occurred in intact cells only, (b) it was temperature dependent, (c) it required an Na+/H+ gradient, low intracellular Ca2+ activity, and (d) it required ATP, and (e) intact lysosomes. The results suggest endocytosis and recycling of the beta-adrenergic receptor after agonist treatment.

Biochemical aspects of conceptus--endometrial interactions

Mammalian conceptuses must provide a chemical signal to the maternal system to insure maintenance of corpus luteum (CL) function and of progesterone production and continuation of uterine endometrial secretory activity. These events insure that the developing conceptus is provided with appropriate nutrients, regulatory enzymes and endocrine state to allow successful establishment and maintenance of pregnancy. Pig blastocysts begin to produce estrogens by Day 11 of pregnancy, which prevents secretion of the uterine luteolytic factor (PGF2 alpha) in an endocrine direction, but allows secretion in an exocrine direction, i.e., into the uterine lumen. Therefore, CL are "protected." Blastocyst estrogens also trigger secretion of a group of proteins, including uteroferrin, an iron transport protein, and a family of protease inhibitors whose biosynthesis within the uterine glandular epithelium is under the control of progesterone. Estrogen also appears to promote accumulation of glucose and fructose within the uterine lumen. A complex in vivo "culture medium" is thereby established to promote conceptus development. Pig blastocysts do not undergo invasive implantation within the uterine lumen although they produce the protease, plasminogen activator. Invasion may be prevented by endometrial secretion of progesterone-induced protease inhibitors which are produced in large amounts. In addition to estrogens of conceptus origin, calcium and prostaglandins PGF2 alpha and E2 may affect the uterine vasculature, water and electrolyte transport, capillary permeability, conceptus steroid production, and related events during pregnancy. The blastocysts of the large domestic animals also secrete proteins which include a large glycoprotein (Mr approximately 600,000) and a small acidic protein (Mr approximately 17,000). The latter has been purified from sheep and named ovine trophoblast protein I. These proteins may play unique roles in early pregnancy with respect to establishment and maintenance of pregnancy in the ewe, sow, mare, and cow.

Formation of receptosomes from plasma membrane coated pits during endocytosis: analysis by serial sections with improved membrane labeling and preservation techniques

The early events in the entry of ligands into cultured fibroblasts through plasma membrane clathrin-coated pits were examined by using serial sections and electron microscopy. Swiss 3T3 cells were labeled at 4 degrees C with concanavalin A-horseradish peroxidase and warmed to 37 degrees C for 1 or 2 min to label organelles participating in endocytosis. Cells were fixed in glutaraldehyde and further preserved with a membrane contrast-enhancement technique that utilized sequential osmium/ferrocyanide, thiocarbohydrazide, and osmium/ferrocyanide treatment in situ. This technique provides exceptional preservation and enhanced contrast of membranous structures. Serial sections taken both parallel to and perpendicular to the substratum were examined by electron microscopy, and 263 clathrin-coated structures containing horseradish peroxidase were evaluated for their connection to the plasma membrane. All of these coated structures were found to be connected to the plasma membrane either directly or via necks approximately equal to 170 A in width and up to 7,000 A in length. These necks were often surrounded by a dense filamentous network. These results strongly suggest that isolated coated vesicles do not form during the endocytosis process in Swiss 3T3 cells.

Isolation and characterization of a highly enriched preparation of receptosomes (endosomes) from a human cell line

Receptor-mediated endocytosis proceeds by transfer of receptor-ligand complexes from clathrin-coated pits at the cell surface to uncoated endocytic vesicles termed receptosomes (or endosomes). These vesicles have now been purified more than 37-fold based on their content of newly internalized epidermal growth factor. 125I-labeled EGF was bound to human KB carcinoma cells at 4 degrees C, and then the cells were warmed to 37 degrees C for 8 min and disrupted. The purification scheme involved density gradient centrifugation on colloidal silica and sucrose and gel filtration on Sephacryl S-1000. Relative to homogenate, receptosomes are enriched 4.3-fold in their cholesterol content and depleted in enzyme markers for plasma membranes (2- to 3-fold) and lysosomes (9-fold). Receptosomes have a polypeptide composition that is different from plasma membrane, lysosome, and other homogenate fractions. They are enriched in transferrin receptors (30-fold) and in unidentified Mr 70,000-75,000 glycoprotein(s); they contain phosphomannosyl receptors. They do not contain detectable amounts of clathrin.

Filipin-cholesterol complexes form in uncoated vesicle membrane derived from coated vesicles during receptor-mediated endocytosis of low density lipoprotein

Filipin has been widely used as an electron microscopic probe to detect 3-beta-hydroxysterols, principally cholesterol, in cellular membranes. When it complexes with sterol, it forms globular deposits that disrupt the planar organization of the membrane. Previous studies have shown that coated pits and coated vesicles, specialized membranes involved in receptor-mediated endocytosis, do not appear to bind filipin. This has led to the suggestion that these membranes are low in cholesterol compared with the remainder of the plasma membrane. Since coated endocytic vesicles become uncoated vesicles during the transport of internalized ligands to the lysosome, we have carried out studies to determine whether or not the membranes that surround these transport vesicles are unable to bind filipin and therefore, are also low in cholesterol. Cells were incubated with ferritin-conjugated ligands that bind to low density lipoprotein (LDL) receptors in coated pits. After allowing internalization of the conjugates, we fixed the cells in either the presence or absence of filipin. This permitted us to identify all of the vesicles involved in the transport of LDL to the lysosome and to determine whether the membranes of these vesicles were able to bind filipin. We found that, coordinate with the dissociation of the clathrin coat from the endocytic vesicles, the membranes became sensitive to the formation of filipin-sterol complexes. Furthermore, all of the uncoated endocytic vesicle membranes, as well as the lysosomal membranes, bound filipin. This suggests either that coated membrane contains normal cholesterol levels, which is not easily detected with filipin, or that cholesterol rapidly moves into endocytic vesicles after the clathrin coat dissociates from the membrane.