Fusion accessibility of endocytic compartments along the recycling and lysosomal endocytic pathways in intact cells

Intracellular Pathway for the Generation of Functional MHC Class II Peptide Complexes in Immature Human Dendritic Cells

Binding of antigenic peptides to MHC class II (MHC-II) molecules occurs in the endocytic pathway. From previous studies in B lymphocytes, it is believed that most but not all of the newly synthesized MHC-II molecules are directly targeted from the trans-Golgi network to endosomal compartments. By using pulse-chase metabolic labeling followed by cell surface biotinylation, we show here that in contrast to an EBV-transformed B cell line and human monocytes, the majority of newly synthesized MHC-II molecules (at least 55 ± 13%) are first routed to the plasma membrane of dendritic cells derived from human monocytes. They reach the cell surface in association with the invariant chain (Ii), a polypeptide known to target MHC-II to the endosomal/lysosomal system. Following rapid internalization and degradation of Ii, these αβIi complexes are converted into αβ-peptide complexes as shown by their SDS stability. These SDS-stable dimers appear as soon as 15 to 30 min after internalization of the αβIi complexes. More than 80% of αβ dimers originating from internalized αβIi complexes are progressively delivered to the cell surface within the next 2 h. Depolymerization of microtubules, which delays the transport to late endosomal compartments, did not affect the kinetics of conversion of surface αβIi into SDS-stable and -unstable αβ dimers. Altogether, these data suggest that newly liberated class II αβ heterodimers may bind peptides in different compartments along the endocytic pathway in dendritic cells derived from human monocytes.

Slow degradation of aggregates of the Alzheimer's disease amyloid beta-protein by microglial cells

Microglia are immune system cells associated with senile plaques containing beta-amyloid (Abeta) in Alzheimer's disease. Although microglia are an integral part of senile plaques, their role in the development of Alzheimer's disease is not known. Because microglia are phagocytic cells, it has been suggested that microglia may function as plaque-attacking scavenger cells. Microglia bind and internalize microaggregates of Abeta that resemble those present in dense Alzheimer's disease plaques. In this study, we compared the degradation by microglia of Abeta microaggregates with the degradation of two other proteins, acetylated low density lipoprotein and alpha2-macroglobulin. We found that the majority of the internalized Abeta in microaggregates was undegraded 72 h after uptake, whereas 70-80% of internalized acetylated low density lipoprotein or alpha2-macroglobulin was degraded and released from cells in trichloroacetic acid-soluble form after 4 h. In the continued presence of fluorescent Abeta microaggregates for 4 days, microglia took up huge amounts of Abeta and became engorged with undigested material. These data suggest that microglia can slowly degrade limited amounts of Abeta plaque material, but the degradation mechanisms can be overwhelmed by larger amounts of Abeta.

Controlling receptor/ligand trafficking: effects of cellular and molecular properties on endosomal sorting

Receptor-mediated endocytosis is the process by which cells internalize ligands that have specifically interacted with cell surface receptors. Within intracellular endosomal compartments, receptor/ligand complexes can be targeted to lysosomes for degradation, recycled back to the plasma membrane, or sorted separately to these destinations. We have developed a mechanistic mathematical model that can account for the spectrum of experimentally observed endosomal sorting outcomes. The central hypothesis of this model is that receptors may be selectively retained by putative endosomal retention components and that this process may be modulated by receptor occupancy. This hypothesis is supported by the recent discovery of an endosomal retention component involved in targeting epidermal growth factor receptors to lysosomes. In this paper, we use the model to predict how changes in key cellular and molecular parameters alter sorting outcomes. This analysis provides guidance for rationally modulating the sorting process in a variety of biomedical applications, either by the manipulation of cellular parameters or the design of ligand properties.

Evidence for prolonged cell-surface contact of acetyl-LDL before entry into macrophages

Acetyl-LDL stimulates acyl-CoA:cholesterol acyltransferase (ACAT) much more effectively than LDL in mouse peritoneal macrophages. Previous work with another potent ACAT stimulator, beta-VLDL, suggested that atherogenic lipoproteins may use internalization pathways distinct from that of LDL. Brief incubation of fluorescently labeled acetyl-LDL and LDL followed by a short chase period without lipoproteins was used to compare endocytic pathways. LDL was delivered rapidly to perinuclear vesicles, corresponding to late endosomes and lysosomes. A substantial fraction (> 40%) of acetyl-LDL was initially retained in the cell periphery, while the rest was rapidly delivered to late endosomes that also contained LDL. Fluorescence of peripheral 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI)-acetyl-LDL could be quenched by TNBS, indicating accessibility of the peripheral acetyl-LDL to the extracellular space. Quantification of fluorescence intensities demonstrated that > 40% of the cell-associated DiI-acetyl-LDL but only about 10% of DiI-LDL fluorescence was quenchable by TNBS after a 3-minute chase. Fucoidin can efficiently displace DiI-acetyl-LDL bound to cells at 0 degree C. DiI-acetyl-LDL in the TNBS-quenchable peripheral compartments, however, was resistant to fucoidin. Electron microscopy of colloidal gold-acetyl-LDL showed that acetyl-LDL on the cell surface was often associated with microvilli or ruffles. After clearance from the surface, the peripheral acetyl-LDL was also delivered to the late endosomes and lysosomes. These results indicate that a substantial portion of acetyl-LDL enters macrophages through a pathway that initially differs from that of LDL. This pathway involves a prolonged retention of acetyl-LDL on the plasma membrane. This surface retention may affect ACAT activation in macrophages.

Bafilomycin A1 treatment retards transferrin receptor recycling more than bulk membrane recycling

Treatment of Chinese hamster ovary cells with the vacuolar proton pump inhibitor bafilomycin A1 causes a 2-fold retardation in the rate of recycling of transfected human transferrin receptors back to the cell surface as measured using biochemical assays (Johnson, L. S. , Dunn, K. W., Pytowski, B., and McGraw, T. E. (1993) Mol. Biol. Cell 4, 1251-1266). We have used quantitative fluorescence microscopy to determine which step(s) in the endocytic recycling pathway are affected. We show that removal of transferrin from sorting endosomes and accumulation in the peri-centriolar endocytic recycling compartment takes place normally in bafilomycin A1-treated cells. However, the rate constant for exit of transferrin receptors from recycling endosomes (ke) is reduced from 0.063 min-1 in untreated cells to 0.034 min-1 in the presence of bafilomycin A1. This retardation appears to be dependent on the presence of internalization motifs in the cytoplasmic domain since modified receptors lacking these oligopeptide motifs do not show as large a decrease in recycling rate in the presence of bafilomycin A1. Bulk membrane recycling (measured by efflux of an internalized fluorescent lipid analog, 6-[N-[7-nitrobenzo-2-oxa-1, 3-diazol-4-yl--amino-hexoyl- sphingosylphosphorylcholine) is slowed from an exit rate constant of 0.060 min-1 without drug to 0.046 min-1 in the presence of bafilomycin A1. We conclude that bafilomycin A1 slows bulk membrane flow, but it causes additional inhibition of receptor recycling in a manner that is dependent on a peptide motif on the cytoplasmic domain.

Microglial cells internalize aggregates of the Alzheimer's disease amyloid beta-protein via a scavenger receptor

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.

Dephosphorylation of the mannose-6-phosphate recognition marker is localized in later compartments of the endocytic route. Identification of purple acid phosphatase (uteroferrin) as the candidate phosphatase

The mannose-6-phosphate (Man6P) recognition marker in lysosomal proteins is known to be dephosphorylated after the delivery of lysosomal proteins to the endosome/lysosome compartment. The rate of Man6P recognition marker inactivation depends on the cell type and lysosomal protein. In the present study we show that in BHK 21 cells, which rapidly dephosphorylate lysosomal proteins, the recognition marker is stable in the endosomal compartment, to which lysosomal enzymes such as arylsulfatase A are delivered during endocytosis at 20 degrees C. Dephosphorylation depends on the transfer of internalized lysosomal enzymes from the 20 degrees C compartment to later compartments, most likely lysosomes. This transfer is sensitive to NH4C1 and nocodazole. In vitro experiments identified purple acid phosphatase (uteroferrin) as a candidate for the lysosomal phosphatase catalyzing in vivo the dephosphorylation of Man6P recognition marker.

Wortmannin alters the transferrin receptor endocytic pathway in vivo and in vitro

Treatment with the phosphatidylinositol 3-kinase inhibitor wortmannin promotes approximately 30% decrease in the steady-state number of cell-surface transferrin receptors. This effect is rapid and dose dependent, with maximal down-regulation elicited with 30 min of treatment and with an IC50 approximately 25 nM wortmannin. Wortmannin-treated cells display an increased endocytic rate constant for transferrin internalization and decreased exocytic rate constants for transferrin recycling. In addition to these effects in vivo, wortmannin is a potent inhibitor (IC50 approximately 15 nM) of a cell-free assay that detects the delivery of endocytosed probes into a common compartment. Inhibition of the in vitro assay involves the inactivation of a membrane-associated factor that can be recruited onto the surface of vesicles from the cytosol. Its effects on the cell-free assay suggest that wortmannin inhibits receptor sorting and/or vesicle budding required for delivery of endocytosed material to "mixing" endosomes. This idea is consistent with morphological changes induced by wortmannin, which include the formation of enlarged transferrin-containing structures and the disruption of the perinuclear endosomal compartment. However, the differential effects of wortmannin, specifically increased transferrin receptor internalization and inhibition of receptor recycling, implicate a role for phosphatidylinositol 3-kinase activity in multiple sorting events in the transferrin receptor's membrane traffic pathway.

Binding of nucleosomes to a cell surface receptor: redistribution and endocytosis in the presence of lupus antibodies

In the present study, we sought evidence for a surface nucleosome receptor in the fibroblastic cell line CV-1, and questioned whether anti-double-stranded (ds) DNA and/or anti-histone autoantibodies could recognize and influence the fate of cell surface-bound nucleosomes. 125I-labeled mononucleosomes were shown to bind to the cell layer in a specific, concentration-dependent and a saturable manner. Scatchard analysis revealed the presence of two binding sites: a high-affinity site with a Kd of approximately 7nM and a low-affinity site (Kd approximately 400 nM) with a high capacity of 9 x 10(7) sites. Visualization of bound mononucleosomes by fluorescence revealed staining on both the cell surface and the extracellular matrix (ECM). Purified mononucleosome-derived ds DNA (180-200 bp) was found to complete for binding of 125I-mononucleosomes on the low-affinity site, to stain exclusively the ECM in immunofluorescence, and to precipitate three specific proteins of 43, 180 and 240 kDa from 125-I-labeled cell lysates. Nucleosomes were found to precipitate not only the 180-kDa ds DNA-reactive component, but also a unique protein of 50 kDa, suggesting that this protein is a cell surface receptor for nucleosomes on these fibroblasts. Once bound on the cell surface, mononucleosomes were recognized and secondarily complexed by lupus anti-ds DNA or anti-histone antibodies (i.e. anti-nucleosome antibodies), thus forming immune complexes in situ. The presence of these complexing auto-antibodies was found dramatically to enhance the kinetics of mononucleosome internalization. Following the internalization of the nucleosome-anti-nucleosome complexes by immunofluorescence, we observed the formation of vesicles at the edge of the cells by 5-10 min which moved toward the perinuclear region by 20-30 min. By means of double-fluorescence labeling and proteolytic treatment, these fluorescent vesicles were shown to be in the cytoplasm, suggesting true endocytosis of nucleosome-anti-nucleosome immune complexes. As shown by confocal microscopy, at no stage of this endocytic process was there any indication that coated pits or coated vesicles participated. Co-distribution of the endocytic vesicles with regions rich in actin filaments and inhibition of endocytosis of nucleosome-anti-nucleosome complexes by disruption of the microfilament network with cytochalasin D suggest a mechanism mediated by the cytoskeleton. Taken together, our data provide evidence for the presence of a surface nucleosome receptor. We also show that anti-ds DNA and anti-histone antibodies can form nucleosome-anti-nucleosome immune complexes in situ at the cell surface, and thus dramatically enhance the kinetics of nucleosome endocytosis.

Actin filaments facilitate two steps of endocytosis

Recent reports have suggested a role for microtubules in the endocytic process and pointed out the role for actin filaments for the early steps of apical endocytosis in polarized epithelial cells. However, these studies do not address the respective contribution of these two types of filaments along the endocytic pathway. In addition, several studies failed to demonstrate the role for actin filaments in clathrin dependent endocytosis in non polarized cells. In the present study we have investigated the role for both filaments in the endocytosis of two ligands internalized via clathrin coated pits (transferrin and alpha 2-macroglobulin) in a mouse hepatoma cell line. By immunocytochemical analysis with confocal microscopy, and biochemical analysis using a temperature sensitive step at 18 degrees C, we have shown that actin filaments are involved in two steps of the degradative pathway, and that microtubules are required at a stage in between. Actin filaments increase first, the uptake of ligands and second, their delivery to the degradative compartment, whereas microtubules are required to maintain the distribution of the late endocytic compartment in its juxtanuclear position and facilitate the delivery of the ligands to the degradative compartment upstream of the actin filament requirement. Furthermore actin filaments facilitate the recycling of transferrin from the perinuclear region to the plasma membrane. Our data indicate for the first time the sequential involvement of actin filaments and microtubules along one intracellular membrane trafficking pathway.

VPS27 controls vacuolar and endocytic traffic through a prevacuolar compartment in Saccharomyces cerevisiae

Newly synthesized vacuolar hydrolases such as carboxypeptidase Y (CPY) are sorted from the secretory pathway in the late-Golgi compartment and reach the vacuole after a distinct set of membrane-trafficking steps. Endocytosed proteins are also delivered to the vacuole. It has been proposed that these pathways converge at a "prevacuolar" step before delivery to the vacuole. One group of genes has been described that appears to control both of these pathways. Cells carrying mutations in any one of the class E VPS (vacuolar protein sorting) genes accumulate vacuolar, Golgi, and endocytosed proteins in a novel compartment adjacent to the vacuole termed the "class E" compartment, which may represent an exaggerated version of the physiological prevacuolar compartment. We have characterized one of the class E VPS genes, VPS27, in detail to address this question. Using a temperature-sensitive allele of VPS27, we find that upon rapid inactivation of Vps27p function, the Golgi protein Vps10p (the CPY-sorting receptor) and endocytosed Ste3p rapidly accumulate in a class E compartment. Upon restoration of Vps27p function, the Vps10p that had accumulated in the class E compartment could return to the Golgi apparatus and restore correct sorting of CPY. Likewise, Ste3p that had accumulated in the class E compartment en route to the vacuole could progress to the vacuole upon restoration of Vps27p function indicating that the class E compartment can act as a functional intermediate. Because both recycling Golgi proteins and endocytosed proteins rapidly accumulate in a class E compartment upon inactivation of Vps27p, we propose that Vps27p controls membrane traffic through the prevacuolar/endosomal compartment in wild-type cells.

Imaging of endosome fusion in BHK fibroblasts based on a novel fluorimetric avidin-biotin binding assay

A fluorescence assay of in vivo endosome fusion was developed and applied to define the kinetics of endosome fusion in baby hamster kidney (BHK) fibroblasts. The assay is based on an approximately 10-fold enhancement of the green fluorescence of BODIPY-avidin upon biotin binding. The BODIPY-avidin fluorescence enhancement occurred in < 25 ms, was pH-independent, and involved a BODIPY-tryptophan interaction. For endocytosis in vivo, BHK fibroblasts were pulse-labeled with BODIPY-avidin together with a red (rhodamine) fluorescent fusion-independent chromophore (TMR). After specified chase times in a nonfluorescent medium, a second cohort of endosomes was pulse-labeled with biotin-conjugated albumin, dextran, or transferrin. Fusion of biotin-containing endosomes with avidin-containing endosomes was quantified by ratio imaging of BODIPY-to-TMR fluorescence in individual endosomes, using imaging methods developed for endosome pH studies. Analysis of BODIPY-to-TMR ratio distributions in avidin-labeled endosomes exposed to zero and maximum biotin indicated > 90% sensitivity for detection of endosome fusion. In avidin pulse (10 min) -chase-biotin albumin pulse (10 min) studies, both fused and unfused endosomes were identified; the fractions of avidin-labeled endosomes that fused with biotin-labeled endosomes were 0.48, 0.21, 0.16, and 0.07 for 0-, 5-, 10-, and 20-min chase times. Fitting of fusion data to a mathematical model of in vivo endosome fusion required the existence of an intermediate fusion compartment. Pulse-chase studies performed with biotin-transferrin to label the early/recycling endosomes indicated that after a 10-min chase, avidin-labeled endosomes reached a compartment that was inaccessible to biotin-transferrin. The assay was also applied to determine whether endosome fusion was influenced by temperature, pH (bafilomycin A1), second messengers (cAMP agonists, phorbol 12-myristate 13-acetate, staurosporine), and growth-related factors (platelet-derived growth factor, genistein). The results establish a sensitive fluorescence assay to quantify the fusion of vesicular compartments in living cells.

Membrane transport in the endocytic pathway

Despite controversies and debates, some fundamental properties of endosomes become apparent when comparing results from in vivo and in vitro strategies used to study endosomal membrane traffic. In addition, recent studies are starting to unravel the complex organization of early endosomes, in particular along the route followed by recycling receptors.

Oligomerized transferrin receptors are selectively retained by a lumenal sorting signal in a long-lived endocytic recycling compartment

Cross-linking of surface receptors results in altered receptor trafficking in the endocytic system. To better understand the cellular and molecular mechanisms by which receptor cross-linking affects the intracellular trafficking of both ligand and receptor, we studied the intracellular trafficking of the transferrin receptor (TfR) bound to multivalent-transferrin (Tf10) which was prepared by chemical cross-linking of transferrin (Tf). Tf10 was internalized about two times slower than Tf and was retained four times longer than Tf, without being degraded in CHO cells. The intracellular localization of Tf10 was investigated using fluorescence and electron microscopy. Tf10 was not delivered to the lysosomal pathway followed by low density lipoprotein but remained accessible to Tf in the pericentriolar endocytic recycling compartment for at least 60 min. The retained Tf10 was TfR-associated as demonstrated by a reduction in surface TfR number when cells were incubated with Tf10. The presence of Tf10 within the recycling compartment did not affect trafficking of subsequently endocytosed Tf. Retention of Tf10 within the recycling compartment did not require the cytoplasmic domain of the TfR since Tf10 exited cells with the same rate when bound to the wild-type TfR or a mutated receptor with only four amino acids in the cytoplasmic tail. Thus, cross-linking of surface receptors by a multivalent ligand acts as a lumenal retention signal within the recycling compartment. The data presented here show that the recycling compartment labeled by Tf10 is a long-lived organelle along the early endosome recycling pathway that remains fusion accessible to subsequently endocytosed Tf.

Maturation of early endosomes and vesicular traffic to lysosomes in relation to membrane recycling

The controversy whether endocytic processing occurs by organellar maturation or by vesicular traffic has not been resolved. It is also not clear whether maturation continues to the stage of lysosomes, to what extent it involves a decrease in organellar fusogenicity, and how it relates to membrane recycling. Maturation and vesicular traffic imply distinct kinetics for the intermingling of endocytic markers after sequential endocytic uptake. We have studied the kinetics of intermingling of fluid-phase markers (fluorescein-labelled dextran and horseradish peroxidase) and cell surface-derived membrane (labelled by galactosylation) in organelles at early and late stages of the endocytic pathway in macrophage-like P388D1 cells. Intermingling declined by sigmoid kinetics, indicating that endosomes matured within about 3 minutes to become non-fusogenic towards early endosomes. During maturation about 60% of internalized membrane was recycled with T1/2 approximately 2 minutes. Whereas matured endosomes were non-fusogenic towards early endosomes and towards each other, a second phase of intermingling was observed upon delivery to lysosomes. This intermingling occurred by a first-order process (T1/2 approximately 4 minutes), concurrent with recycling of the remaining 40% of internalized membrane marker. These kinetic observations suggest a model for endocytic processing which reconciles maturation of early endosomes with the known function of carrier vesicles: Endocytic carrier vesicles do not bud off from permanent early endosomes as proposed for vesicular traffic, but are derived, together with recycling vesicles, from the maturation of early endosomes which are consumed by this process; these carrier vesicles subsequently mediate delivery to lysosomes by vesicular traffic during which the remaining surface-derived membrane is recycled.

Evidence for nonvectorial, retrograde transferrin trafficking in the early endosomes of HEp2 cells

We have previously characterized the trafficking of transferrin (Tf) through HEp2 human carcinoma cells (Ghosh, R. N., D. L. Gelman, and F. R. Maxfield, 1994. J. Cell Sci. 107:2177-2189). Early endosomes in these cells are comprised of both sorting endosomes and recycling compartments, which are distinct separate compartments. Endocytosed Tf initially appears in punctate sorting endosomes that also contain recently endocytosed LDL. After short loading pulses, Tf rapidly sorts from LDL with first-order kinetics (t1/2 approximately 2.5 min), and it enters the recycling compartment before leaving the cell (t1/2 approximately 7 min). Here, we report a second, slower rate for Tf to leave sorting endosomes after HEp2 cells were labeled to steady state with fluorescein Tf instead of the brief pulse used previously. We determined this rate using digital image analysis to measure the Tf content of sorting endosomes that also contained LDL. With an 11-min chase, the Tf in sorting endosomes was 24% of steady-state value. This was in excess of the amount expected (5% of steady state) from the rate of Tf exit after short filling pulses. The excess could not be accounted for by reinternalization of recycled cell surface Tf, implying that either some Tf was retained in sorting endosomes, or that Tf was delivered back to the sorting endosomes from the recycling compartment. The former is unlikely since nearly all sorting endosomes contain detectable Tf after an 11-min chase, even though more than one third of the sorting endosomes were formed during the chase time. Furthermore, while observing living cells by confocal microscopy, we saw vesicle movements that appeared to be fluorescent Tf returning from recycling compartments to sorting endosomes. The slow rate of exit after steady-state labeling was similar to the Tf exit rate from the cell, suggesting an equilibration of Tf throughout the early endosomal system by this retrograde pathway. This retrograde traffic may be important for delivering molecules from the recycling compartment, which is a long-lived organelle, to sorting endosomes, which are transient.

Effects of apoprotein E on intracellular metabolism of model triglyceride-rich particles are distinct from effects on cell particle uptake

Apoprotein E (apoE) enhances uptake of triglyceride-rich lipoprotein particles (TGRP). We questioned whether apoE would also modulate intracellular metabolism of TGRP in addition to its effects on particle uptake. We prepared model TGRP with triolein and cholesteryl oleate (1:1, w/w) as the core lipids, emulsified by egg yolk phosphatidylcholine, and containing a non-degradable marker, [3H]cholesteryl hexadecyl ether. Particles were intermediate density lipoprotein-sized as determined by core lipid/phospholipid ratios (2.0-3.0/1) and gel filtration chromatography on Sepharose CL-2B. Emulsions were incubated with J774 macrophages for 5 min to 6 h at core lipid concentrations of 300-1200 micrograms/ml and 0-0.2 microgram recombinant apoE/mg core lipid. Particle uptake was determined by [3H]cholesteryl ether uptake and fluorescence microscopy in the absence and presence of apoE. Similar uptake of particles with and without apoE was achieved by utilizing a 4 times higher particle concentration in the absence of apoE. At equivalent levels of uptake, particles with apoE lead to one-half of the triglyceride mass accumulation and twice the triglyceride utilization as compared to particles without apoE. Further, apoE doubles cell cholesteryl ester hydrolysis and to a lesser extent (approximately 30%) increases cholesteryl ester resynthesis by acyl-CoA cholesterol acyltransferase. Particles, both with and without apoE, reach the lysosomal compartment as determined by colocalization with fluorescein-labeled alpha 2-macroglobulin. These results suggest that, in addition to its role in enhancing TGRP uptake, apoE has additional effects on modulating the cellular metabolism of both triglyceride and cholesteryl ester, after particle internalization.

The endocytic process in CHO cells, a toxic pathway of the polyene antibiotic amphotericin B

We describe the fate of the polyene antibiotic amphotericin B (AmB) after its interaction with Chinese hamster ovary (CHO) cells. The global uptake of AmB by these cells was measured at 37 degrees C after a 1-h incubation in the presence of 5% fetal bovine serum. It increased with the total concentration of drug and reached a plateau of approximately 1 nmol/mg of cell protein for an external concentration of 25 microM. The same experiment performed at 5 degrees C revealed a drastic decrease in uptake. The distribution of the drug among plasma membranes, endosomes, and lysosomes was then investigated after the separation of the postnuclear fractions by a Percoll gradient. After a 10-min incubation, AmB was found only in the plasma membrane fraction, regardless of the drug concentrations used (5 to 100 microM). After 60 min, at low drug concentrations (5 and 10 microM) AmB was found to be incorporated mainly in plasma and lysosomal fractions. At high concentrations (50 microM) AmB accumulated in endosomal fractions and plasma membranes. At intermediate concentrations (25 microM) AmB was distributed among the three fractions. When the same experiment was carried out at 5 degrees C, AmB was associated only with the plasma membrane even after 60 min, which was consistent with the absence of endocytotic process at low temperature. The effect of AmB on the endocytic process resulted in the increased uptake of sulforhodamine B, a fluid-phase marker of endocytosis, as well as by the accumulation of sulforhodamine in spots scattered in the cytoplasms of AmB-treated cells, in contrast to the accumulation around the nuclei observed in the control cells. These results are interpreted as indicating that AmB is internalized by the cells through endocytosis and that high concentrations of the drug block the fusion between endosomes and/or the fusion between endosomes and lysosomes.

Quantification of low density lipoprotein and transferrin endocytic sorting HEp2 cells using confocal microscopy

Numerous experiments on CHO cells have shown that endosomes are composed of separate vesicular and tubulovesicular compartments, such as the sorting endosome, the recycling compartment, and the late endosome. However, Hopkins et al. (Nature 346, 335–339, 1990) have reported that endosomes in HEp2 human carcinoma cells form an extensive tubular reticulum. To resolve their observations with previous results from CHO and other cells, we examined the sorting and intracellular transport of endocytosed macromolecules in HEp2 cells, using low density lipoprotein (LDL) and transferrin (Tf) to probe the lysosomally directed and recycling pathways, respectively. Fluorescent LDL and Tf were observed with laser scanning confocal microscopy to visualize simultaneously both probes' sorting and subsequent post-sorting behavior in HEp2 cells. Quantifying the 3-dimensional cellular distributions of fluorescent LDL and Tf, after a variety of pulsechase schemes, gave the ligands' trafficking rates. Initially, both ligands appear in the same punctate sorting endosomes, and fingers of Tf start extending from these sorting endosomes. Tf rapidly leaves dual-labeled sorting endosomes (t1/2 approximately 2.5 minutes) and enters a post-sorting recycling compartment from which it is recycled out of the cell (t1/2 approximately 7 minutes). We present both morphological and kinetic data supporting the existence of these two separate compartments along the recycling pathway in HEp2 cells. LDL remains in punctate sorting endosomes that eventually lose the ability to receive newly endocytosed LDL, and mature into late endosomes. The trafficking and sorting of Tf and LDL in HEp2 cells follow the same general scheme as in CHO cells, indicating that the tubular endosomes previously seen may be the tubular parts of the sorting endosomes and recycling compartments in these cells. We propose that the endosomes in the recycling pathway of HEp2 cells, as in CHO cells, are composed of short-lived sorting endosomes, accessible to both Tf and LDL, and long-lived post-sorting recycling compartments, which contain Tf and recycling receptors but not LDL.

The coated pit and macropinocytic pathways serve distinct endosome populations

Clathrin-coated vesicle endocytosis and macropinocytosis are distinct endocytic pathways demonstrable in several cell types including human epidermoid A431 cells (West, M.A., M.S. Bretscher, and C. Watts. 1989. J. Cell Biol. 109:2731-2739). Here we analyze the extent of mixing of macropinocytic endosome (macropinosome) content with that of conventional endosomes served by coated vesicle endocytosis. Using laser scanning confocal fluorescence microscopy we detected very little delivery of macropinosome content to either early or late endosomes-lysosomes as defined by labeling with transferrin or with LDL. Mixing of the contents of the macropinosomes and conventional endosomes was not induced by the addition of brefeldin A. Moreover, the morphology of macropinosomes was not grossly altered in the presence of brefeldin A, whilst in the same cells there were dramatic tubulation effects on conventional endosomes as reported by others. Although refractory to fusion with conventional endosomes, macropinosomes were nonetheless dynamic structures which sometimes exhibited vesiculo-tubular morphology in living cells and were capable of fusing with each other. We suggest that different endocytic mechanisms can give rise to distinct endosome populations.

The cytoplasmic domain of P-selectin contains a sorting determinant that mediates rapid degradation in lysosomes

P-selectin is a cell adhesion molecule required transiently on the surface of activated platelets and endothelial cells as a receptor for leukocytes. It is stored in secretory granules in platelets, endothelial cells, and transfected neuroendocrine cells and is rapidly delivered to the plasma membrane upon exocytosis of the secretory granules. It is then rapidly internalized in endothelial cells and transfected cells. We find that in transfected neuroendocrine PC12 cells, the fraction of P-selectin that is not targeted to secretory granules is rapidly degraded. In transfected CHO fibroblasts, which lack secretory granules, P-selectin was degraded with a half time of 2.3 h in plated cells, while low density lipoprotein receptor (LDL-R) had a half life of 9 h. In cells cultured in ammonium chloride to inhibit lysosomal proteinases, P-selectin was protected from degradation and rapidly accumulated in vesicles enriched in lgp-B, a resident lysosomal membrane protein. The cytoplasmic domain of P-selectin was sufficient to confer rapid turnover on LDL-R. Deletion of 10 amino acids from the cytoplasmic domain of P-selectin extended the half life to 9.5 h and abrogated rapid lysosomal targeting in the presence of ammonium chloride, implicating this sequence as a necessary element of a novel lysosomal targeting signal. The rate limiting step in degradation occurred after internalization from the cell surface, indicating that sorting of P-selectin away from efficiently recycled proteins occurs in endosomes. We propose that this sorting event represents a constitutive equivalent of receptor down regulation, and may function to regulate the expression of P-selectin at the surface of activated endothelial cells.

Beta-very low density lipoprotein is sequestered in surface-connected tubules in mouse peritoneal macrophages

beta-very low density lipoprotein (VLDL) is a large lipoprotein with multiple apoprotein E (apoE) molecules that bind to the LDL receptors on mouse macrophages. Even though they bind to the same receptor, the endocytic processing of beta-VLDL differs from low density lipoprotein (LDL). LDL is rapidly delivered to perinuclear lysosomes and degraded, but much of the beta-VLDL is retained in peripheral compartments for several minutes. We have investigated the properties of these peripheral compartments. Measurement of the pH was made using FITC-phosphatidylethanolamine incorporated into the beta-VLDL, and we found that the peripheral compartments were near neutral in pH. These peripheral, beta-VLDL containing compartments were poorly accessible to antibodies, but a low molecular weight fluorescence quencher (trypan blue) entered the compartments within a few seconds. Intermediate voltage EM of cells labeled with colloidal-gold-beta-VLDL revealed that the peripheral compartments are tubular, surface-connected invaginations. Kinetic studies with fluorescent beta-VLDL showed that the compartments become fully sealed with a half-time of 6 min, and the beta-VLDL is then delivered rapidly to perinuclear lysosomes. By monitoring fluorescence energy transfer between lipid analogs incorporated into the beta-VLDL, some processing of the lipoprotein in the peripheral tubular compartments is demonstrated. The novel mode of uptake of beta-VLDL may account for the high cholesterol ester accumulation induced by this lipoprotein.

The End2 mutation in CHO cells slows the exit of transferrin receptors from the recycling compartment but bulk membrane recycling is unaffected

We have characterized a new CHO cell line (12-4) derived from a parental line, TRVb-1, that expresses the human transferrin receptor. This mutant belongs to the end2 complementation group of endocytosis mutants. Like other end2 mutants, the endosomes in 12-4 cells show a partial acidification defect. These cells internalize LDL and transferrin at 70% of the rate of parental cells and externalize transferrin at 55% of the parental rate (Johnson, L. S., J. F. Presley, J. C. Park, and T. E. McGraw. J. Cell Physiol. 1993). In this report, we have used fluorescence microscopy to determine which step in receptor trafficking is affected in the mutants. Transferrin is sorted from LDL and is delivered to a peri-centriolar recycling compartment at rates similar to parental cells. However, the rate constant for exit of transferrin from the recycling compartment in mutant cells is 0.025 min-1 vs 0.062 min-1 in the parental line. We also measured the trafficking of a bulk membrane marker, 6-[N-[7-nitrobenzo-2-oxa-1,3-diazol-4-yl]-amino]hexanoyl- sphingosylphosphorylcholine (C6-NBD-SM) that labels the exofacial side of the plasma membrane. C6-NBD-SM enters the same recycling compartment as transferrin, and it exits the recycling compartment at a rate of 0.060-0.065 min-1 in both parental and 12-4 cells. We conclude that bulk membrane flow in the recycling pathway of 12-4 cells is normal, but exit of transferrin from the recycling compartment is slowed due to retention in this compartment. Thus, in the mutant cell line the recycling compartment carries out a sorting function, retaining transferrin over bulk membrane.

Sorting of membrane components from endosomes and subsequent recycling to the cell surface occurs by a bulk flow process

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 carrier, membrane lipids, to address this issue directly. We show that N-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-epsilon-aminohexanoyl]- sphingosylphosphorylcholine (C6-NBD-SM) in endocytosed as bulk membrane, and it transits the endocytic system kinetically and morphologically identically to fluorescently labeled transferrin 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.

Isolation of a temperature-sensitive variant Chinese hamster ovary cell line with a morphologically altered endocytic recycling compartment

We have enriched a mutagenized population of Chinese hamster ovary (CHO) cells for those defective in endocytosis by selection for survival to treatment with transferrin (Tf)-ricin and Tf-diphtheria toxin conjugates. Surviving cells were screened with a fluorescently labeled Tf uptake assay to identify cells with morphologically aberrant endocytic phenotypes. One of the cell lines identified, B104-5, has a striking temperature-induced alteration in the morphology of its endocytic receptor recycling compartment. In parental cells the tightly clustered endocytic recycling compartment is located near the Golgi complex. In the mutant cells, following incubation at 40 degrees C, this compartment appears fragmented and widely dispersed. Surprisingly, this alteration in the morphology of the recycling compartment has no effect on the kinetics of Tf internationalization and recycling. The wild-type endocytic compartment is closely aligned with the microtubule-organizing center and the Golgi apparatus, and like the Golgi, its clustered appearance is dependent upon intact microtubules. Although the disruption of the B104-5 receptor recycling compartment morphology can be phenocopied in wild-type cells by microtubule depolymerizing drugs, the microtubule cytoskeleton in B104-5 cells appears normal in immunofluorescent staining. B104-5 cells, unlike the parental cells, do not proliferate at 40 degrees C. The mutation in B104-5 cells is recessive, as fusion with wild-type cells results in a reversion of the B104-5 phenotype. The finding that the morphology of the recycling compartment in CHO cells can be altered without affecting recycling of endocytosed Tf is consistent with the variety of recycling compartment morphologies observed among different cell lines. An interpretation of this result is that the lesion in B104-5 cells is in a gene that is involved in determining the endocytic compartment morphologies observed in different cell lines.

Clathrin assembly protein AP-2 induces aggregation of membrane vesicles: a possible role for AP-2 in endosome formation

We have examined the in vitro behavior of clathrin-coated vesicles that have been stripped of their surface coats such that the majority of the clathrin is removed but substantial amounts of clathrin assembly proteins (AP) remain membrane-associated. Aggregation of these stripped coated vesicles (s-CV) is observed when they are placed under conditions that approximate the pH and ionic strength of the cell interior (pH 7.2, approximately 100 mM salt). This s-CV aggregation reaction is rapid (t1/2 < or = 0.5 min), independent of temperature within a range of 4-37 degrees C, and unaffected by ATP, guanosine-5'-O- (3-thiophosphate), and in particular EGTA, distinguishing it from Ca(2+)-dependent membrane aggregation reactions. The process is driven by the action of membrane-associated AP molecules since partial proteolysis results in a full loss of activity and since aggregation is abolished by pretreatment of the s-CVs with a monoclonal antibody that reacts with the alpha subunit of AP-2. However, vesicle aggregation is not inhibited by PPPi, indicating that the previously characterized polyphosphate-sensitive AP-2 self-association is not responsible for the reaction. The vesicle aggregation reaction can be reconstituted: liposomes of phospholipid composition approximating that found on the cytoplasmic surfaces of the plasma membrane and of coated vesicles (70% L-alpha-phosphatidylethanolamine (type I-A), 15% L-alpha-phosphatidyl-L- serine, and 15% L-alpha-phosphatidylinositol) aggregated after addition of AP-2, but not of AP-1, AP-3 (AP180), or pure clathrin triskelions. Aggregation of liposomes is abolished by limited proteolysis of AP-2 with trypsin. In addition, a highly purified AP-2 alpha preparation devoid of beta causes liposome aggregation, whereas pure beta subunit does not, consistent with results obtained in the s-CV assay which also indicate the involvement of the alpha subunit. Using a fluorescence energy transfer assay we show that AP-2 does not cause fusion of liposomes under physiological solution conditions. However, since the fusion of membranes necessarily requires the close opposition of the two participating bilayers, the AP-2-dependent vesicle aggregation events that we have identified may represent an initial step in the formation and fusion of endosomes that occur subsequent to endocytosis and clathrin uncoating in vivo.

Intracellular trafficking and the parasitophorous vacuole of Leishmania mexicana-infected macrophages

The continued success of Leishmania as an intramacrophage parasite is dependent on its ability to survive within an acidic intracellular compartment, resist degradation by lysosomal hydrolases, exploit the host cell as a source of nutrients, and avoid the macrophage's antigen-presenting capabilities. All these requirements are dependent on the properties of the parasitophorous vacuole in which Leishmania resides. This study shows that the vacuole possesses membrane proteins characteristic of a lysosome, and has MHC class II molecules. The trafficking of a variety of endocytic markers supports this finding. However, a temporal study up to 14 days post-infection indicates that, as it matures, the vacuole gains mannose 6-phosphate receptor, and becomes more accessible to endocytosed ligand, suggesting that the vacuole has functionally translocated from a lysosomal to late endosomal compartment. Endocytosed material was detected in the flagellar pocket and inside the amastigote, demonstrating parasite uptake of intra-vacuolar material. Careful analysis of amastigotes suggests that they avoid antigen presentation by their host cell by limiting the release of potential antigens. These findings significantly extend our understanding of the mechanisms employed by Leishmania to ensure its survival in the macrophage.

Delivery of ligands from sorting endosomes to late endosomes occurs by maturation of sorting endosomes

After endocytosis, lysosomally targeted ligands pass through a series of endosomal compartments. The endocytic apparatus that accomplishes this passage may be considered to take one of two forms: (a) a system in which lysosomally targeted ligands pass through preexisting, long-lived early sorting endosomes and are then selectively transported to long-lived late endosomes in carrier vesicles, or (b) a system in which lysosomally targeted ligands are delivered to early sorting endosomes which themselves mature into late endosomes. We have previously shown that sorting endosomes in CHO cells fuse with newly formed endocytic vesicles (Dunn, K. W., T. E. McGraw, and F. R. Maxfield. 1989. J. Cell Biol. 109:3303-3314) and that previously endocytosed ligands lose their accessibility to fusion with a half-time of approximately 8 min (Salzman, N. H., and F. R. Maxfield. 1989. J. Cell Biol. 109:2097-2104). Here we have studied the properties of individual endosomes by digital image analysis to distinguish between the two mechanisms for entry of ligands into late endosomes. We incubated TRVb-1 cells (derived from CHO cells) with diO-LDL followed, after a variable chase, by diI-LDL, and measured the diO content of diI-containing endosomes. As the chase period was lengthened, an increasing percentage of the endosomes containing diO-LDL from the initial incubation had no detectable diI-LDL from the second incubation, but those endosomes that contained both probes showed no decrease in the amount of diO-LDL per endosomes. These results indicate that (a) a pulse of fluorescent LDL is retained by individual sorting endosomes, and (b) with time sorting endosomes lose the ability to fuse with primary endocytic vesicles. These data are inconsistent with a preexisting compartment model which predicts that the concentration of ligand in sorting endosomes will decline during a chase interval, but that the ability of the stable sorting endosome to receive newly endocytosed ligands will remain high. These data are consistent with a maturation mechanism in which the sorting endosome retains and accumulates lysosomally directed ligands until it loses its ability to fuse with newly formed endocytic vesicles and matures into a late endosome. We also find that, as expected according to the maturation model, new sorting endosomes are increasingly labeled during the chase period indicating that new sorting endosomes are continuously formed to replace those that have matured into late endosomes.(ABSTRACT TRUNCATED AT 400 WORDS)

Processing of vasoactive intestinal peptide and transferrin in human cancerous colonic cells

Endocytosis of vasoactive intestinal peptide (VIP) and of transferrin (Tf) was comparatively studied in human cancerous colonic HT-29 cells. Cellular depletion in potassium inhibits the internalization of VIP (23%) and to a greater extent (42%) that of Tf. This indicates that clathrin-coated pits are also involved, at least in part, in VIP uptake. The distribution of 125I-Tf- or 125I-VIP-containing vesicles in sucrose gradients revealed low and high density vesicle subpopulations. The low density vesicle subpopulation represented a transient compartment from which incoming vesicles containing N-leucyl-beta naphthylamidase were recycled back to the membrane while those containing beta-hexosaminidase (HA) and ligand were mostly transferred into the high density compartment. Subsequent fusion of the latter with heavy vesicles was demonstrated by the shift of HA and ligand with vesicles that had been prelabeled with horseradish peroxidase (HRP). Simultaneous internalization of Tf-HRP and 125I-VIP showed that both the low and high density vesicle subpopulations comprised of two types of VIP-containing vesicle, as confirmed by the density shift reaction: two-thirds of VIP shifted with the Tf-HRP-containing vesicles to denser fractions and the remaining was found with unshifted vesicles. These findings indicate that the VIP-receptor complex processing in HT-29 cells follows two routes, the major route being common with Tf endocytosis.

The influence of particle size and multiple apoprotein E-receptor interactions on the endocytic targeting of beta-VLDL in mouse peritoneal macrophages

Low density lipoprotein (LDL) and beta-very low density lipoprotein (beta-VLDL) are internalized by the same receptor in mouse peritoneal macrophages and yet their endocytic patterns differ; beta-VLDL is targeted to both widely distributed and perinuclear vesicles, whereas LDL is targeted almost entirely to perinuclear lysosomes. This endocytic divergence may have important metabolic consequences since beta-VLDL is catabolized slower than LDL and is a more potent stimulator of acyl-CoA/cholesterol acyl transferase (ACAT) than LDL. The goal of this study was to explore the determinants of beta-VLDL responsible for its pattern of endocytic targeting. Fluorescence microscopy experiments revealed that large, intestinally derived, apoprotein (Apo) E-rich beta-VLDL was targeted mostly to widely distributed vesicles, whereas small, hepatically derived beta-VLDL was targeted more centrally (like LDL). Furthermore, the large beta-VLDL had a higher ACAT-stimulatory potential than the smaller beta-VLDL. The basis for these differences was not due to fundamental differences in the means of uptake; both large and small beta-VLDL were internalized by receptor-mediated endocytosis (i.e., not phagocytosis) involving the interaction of Apo E of the beta-VLDL with the macrophage LDL receptor. However, large beta-VLDL was much more resistant to acid-mediated release from LDL receptors than small beta-VLDL. Furthermore, partial neutralization of the multiple Apo Es on these particles by immunotitration resulted in a more perinuclear endocytic pattern, a lower ACAT-stimulatory potential, and an increased sensitivity to acid-mediated receptor release. These data are consistent with the hypothesis that the interaction of the multivalent Apo Es of large beta-VLDL with multiple macrophage LDL receptors leads to a diminished or retarded release of the beta-VLDL from its receptor in the acidic sorting endosome which, in turn, may lead to the widely distributed endocytic pattern of large beta-VLDL. These findings may represent a physiologically relevant example of a previously described laboratory phenomenon whereby receptor cross-linking by multivalent ligands leads to a change in receptor targeting.

An endogenous MDCK lysosomal membrane glycoprotein is targeted basolaterally before delivery to lysosomes

Using surface immunoprecipitation at 37 degrees C to "catch" the transient apical or basolateral appearance of an endogenous MDCK lysosomal membrane glycoprotein, the AC17 antigen, we demonstrate that the bulk of newly synthesized AC17 antigen is polarly targeted from the Golgi apparatus to the basolateral plasma membrane or early endosomes and is then transported to lysosomes via the endocytic pathway. The AC17 antigen exhibits very similar properties to members of the family of lysosomal-associated membrane glycoproteins (LAMPs). Parallel studies of an avian LAMP, LEP100, transfected into MDCK cells revealed colocalization of the two proteins to lysosomes, identical biosynthetic and degradation rates, and similar low levels of steady-state expression on both the apical (0.8%) and basolateral (2.1%) membranes. After treatment of the cells with chloroquine, newly synthesized AC17 antigen, while still initially targeted basolaterally, appears stably in both the apical and basolateral domains, consistent with the depletion of the AC17 antigen from lysosomes and its recycling in a nonpolar fashion to the cell surface.

Colocalization of synaptophysin with transferrin receptors: implications for synaptic vesicle biogenesis

We have reported previously that the synaptic vesicle (SV) protein synaptophysin, when expressed in fibroblastic CHO cells, accumulates in a population of recycling microvesicles. Based on preliminary immunofluorescence observations, we had suggested that synaptophysin is targeted to the preexisting population of microvesicles that recycle transferrin (Johnston, P. A., P. L. Cameron, H. Stukenbrok, R. Jahn, P. De Camilli, and T. C. Südhof. 1989. EMBO (Eur. Mol. Biol. Organ.) J. 8:2863-2872). In contrast to our results, another group reported that expression of synaptophysin in cells which normally do not express SV proteins results in the generation of a novel population of microvesicles (Leube, R. E., B. Wiedenmann, and W. W. Franke. 1989. Cell. 59:433-446). We report here a series of morphological and biochemical studies conclusively demonstrating that synaptophysin and transferrin receptors are indeed colocalized on the same vesicles in transfected CHO cells. These observations prompted us to investigate whether an overlap between the distribution of the two proteins also occurs in endocrine cell lines that endogenously express synaptophysin and other SV proteins. We have found that endocrine cell lines contain two pools of membranes positive for synaptophysin and other SV proteins. One of the two pools also contains transferrin receptors and migrates faster during velocity centrifugation. The other pool is devoid of transferrin receptors and corresponds to vesicles with the same sedimentation characteristics as SVs. These findings suggest that in transfected CHO cells and in endocrine cell lines, synaptophysin follows the same endocytic pathway as transferrin receptors but that in endocrine cells, at some point along this pathway, synaptophysin is sorted away from the recycling receptors into a specialized vesicle population. Finally, using immunofluorescent analyses, we found an overlap between the distribution of synaptophysin and transferrin receptors in the dendrites of hippocampal neurons in primary cultures before synapse formation. Axons were enriched in synaptophysin immunoreactivity but did not contain detectable levels of transferrin receptor immunoreactivity. These results suggest that SVs may have evolved from, as well as coexist with, a constitutively recycling vesicular organelle found in all cells.

Molecular and cellular mechanisms of receptor-mediated endocytosis

In general, receptors are involved in pathways of endocytosis, either constitutive or ligand induced. These receptors cluster in clathrin-coated pits, enter the cell via clathrin-coated vesicles, pass through an acidified endosome in which the receptors and ligands are sorted, and then either recycle to the cell surface, become stored intracellularly, or are degraded in lysosomes. The internalization pathways serve a variety of functions, such as nutrient uptake, removal of activated proteins, clearance of macromolecules, opportunistic entry of certain viruses and toxins, dissociation and degradation of ligand, and receptor-level regulation. Many receptors follow more than one intracellular pathway, depending on the cell type, receptor concentration, type of ligand, ligand valency, and ligand concentration. Although endocytosis is common to all nucleated eukaryotic cells, the factors that regulate these receptor-mediated endocytic pathways are not fully understood. Defective receptors that are not capable of undergoing normal endocytosis can lead to certain disease states, as in the case of familial hypercholesteremia (FH). This review has three objectives: (i) to describe the different routes that receptors and ligands follow after internaliation; (ii) to describe the potential mechanisms which regulate the initiation and subsequent sorting of receptors and ligands so they reach their final destination; and (iii) to describe the potential functions of receptor-mediated endocytosis.

Late endosomes derive from early endosomes by maturation

Endocytosed proteins destined for degradation in lysosomes are targeted mainly to early endosomes following uptake. Late endosomes are the major site for entry of newly synthesized lysosomal hydrolases via the cation-independent mannose 6-phosphate receptor into the degradative pathway. No consensus exists as to the mechanism of transport from early to late endosomes. We used asialoorosomucoid and transferrin to label selected parts of the degradative and receptor-recycling pathways, respectively, in the human hepatoma cell line HepG2. Intracellular mixing of sequentially endocytosed 125I- and HRP-labeled ligands was monitored by using 3,3'-diaminobenzidine-mediated density perturbation. The entire endocytic pathway of asialoorosomucoid, except for the lysosomes, remained fully accessible to subsequently endocytosed transferrin conjugated to HRP with unchanged kinetics. These results together with immunoelectron microscopic data support a model in which early endosomes gradually mature into late endosomes.

Recycling of epidermal growth factor-receptor complexes in A431 cells: identification of dual pathways

The intracellular sorting of EGF-receptor complexes (EGF-RC) has been studied in human epidermoid carcinoma A431 cells. Recycling of EGF was found to occur rapidly after internalization at 37 degrees C. The initial rate of EGF recycling was reduced at 18 degrees C. A significant pool of internalized EGF was incapable of recycling at 18 degrees C but began to recycle when cells were warmed to 37 degrees C. The relative rate of EGF outflow at 37 degrees C from cells exposed to an 18 degrees C temperature block was slower (t1/2 approximately 20 min) than the rate from cells not exposed to a temperature block (t1/2 approximately 5-7 min). These data suggest that there might be both short- and long-time cycles of EGF recycling in A431 cells. Examination of the intracellular EGF-RC dissociation and dynamics of short- and long-time recycling indicated that EGF recycled as EGF-RC. Moreover, EGF receptors that were covalently labeled with a photoactivatable derivative of 125I-EGF recycled via the long-time pathway at a rate similar to that of 125I-EGF. Since EGF-RC degradation was also blocked at 18 degrees C, we propose that sorting to the lysosomal and long-time recycling pathway may occur after a highly temperature-sensitive step, presumably in the late endosomes.

Isolation and characterization of the Fc receptor from the fetal yolk sac of the rat

The yolk sac of the fetal rat and the proximal small intestine of the neonatal rat selectively transport maternal IgG. IgG-Fc receptors are thought to mediate transport across the epithelium of both tissues. We used a mouse mAb (MC-39) against the 45-54-kD component of the Fc receptor of the neonatal intestine to find an antigenically related protein that might function as an Fc receptor in fetal yolk sac. In immunoblots of yolk sac, MC-39 recognized a protein band with apparent molecular mass of 54-58 kD. MC-39 bound to the endoderm of yolk sac in immunofluorescence studies. In immunogold-labeling experiments MC-39 was associated mainly with small vesicles in the apical cytoplasm and in the region near the basolateral membrane of endodermal cells. The MC-39 cross-reactive protein and beta 2-microglobulin, a component of the intestinal Fc receptor, were copurified from detergent-solubilized yolk sac by an affinity purification that selected for proteins which, like the intestinal receptor, bound to IgG at pH 6.0 and eluted at pH 8.0. In summary, the data suggest that we have isolated the Fc receptor of the yolk sac and that this receptor is structurally and functionally related to the Fc receptor of the neonatal intestine. An unexpected finding is that, unlike the intestinal receptor which binds maternal IgG on the apical cell surface, the yolk sac receptor appears to bind IgG only within apical compartments which we suggest represent the endosomal complex.

Internalization of vasopressin analogs in kidney and smooth muscle cells: evidence for receptor-mediated endocytosis in cells with V2 or V1 receptors

To determine whether receptor-mediated endocytosis occurs in vasopressin-responsive cells, we developed a model system using synthetic fluorescent-labeled vasopressin analogs and A10 (smooth muscle) and LLC-PK1 (kidney epithelial) cells in culture; these cell lines express V1 and V2 vasopressin cell surface receptor types, respectively. We used epifluorescence microscopy to examine the binding, internalization, and intracellular destination of [1-(2-mercapto)propionic acid,8-lysine-N6-carboxytetramethylrhodamine] vasopressin (R-MLVP) and [1-(2-mercapto)propionic acid,8-lysine-N6-carboxyfluorescein]vasopressin (F-MLVP) in these cells. The rhodamine-labeled fluorescent vasopressin analog, R-MLVP, initially bound in a diffuse manner at the cell surface of both A10 and LLC-PK1 cells and could be displaced by excess unlabeled [8-arginine]vasopressin. After incubation at 37 degrees C, bound ligand rapidly aggregated into small clusters or patches, which were internalized in a manner consistent with receptor-mediated endocytosis. Subsequent processing of internalized ligand-receptor complexes appeared to differ between A10 and LLC-PK1 cells. In the case of LLC-PK1 cells, ligand was delivered to a tightly focused lysosome compartment in the perinuclear region of the cell, and receptor molecules were replenished at the cell surface. The lysosomal location of ligand was supported by the quenching of fluorescence in the internalized vesicles when F-MLVP was used as a fluorescent tracer. In the case of A10 cells, ligand became localized to a vesicular compartment and reappearance of receptor at the cell surface was limited. Our data are consistent with the occurrence of receptor-mediated endocytosis of vasopressin in cells with V1 and V2 receptors.