Fusion of sequentially internalized vesicles in alveolar macrophages

Rapid and facile quantitation of polyplex endocytic trafficking

Design of safe and effective synthetic nucleic acid delivery vectors such as polycation/DNA or polycation/siRNA complexes (polyplexes) will be facilitated by quantitative understanding of the mechanisms by which such materials escort cargo from the cell surface to the nucleus. In particular, the mechanisms of cellular internalization by various endocytosis pathways and subsequent endocytic vesicle trafficking have been shown to strongly affect nucleic acid delivery efficiency. Fluorescence microscopy and subcellular fractionation methods are commonly employed to follow intracellular trafficking of biomolecules and nanoparticulate delivery systems such as polyplexes. However, it is difficult to obtain quantitative data from microscopy and subcellular fractionation is experimentally difficult and low throughput. We have developed a method for quantifying the transport of polyplexes through important endocytic vesicles. The method is based on polymerization of 3,3'-diaminobenzidine by endocytosed horseradish peroxidase, causing an increase in the vesicle density, resistance to being solubilized by detergent and quenching of fluorophores within the vesicles, which makes them easy to separate and quantify. Using this method in HeLa cells, we have observed polyethylenimine/siRNA polyplexes initially appearing in early endosomes and rapidly moving to other compartments within 30min post-transfection. At the same time, we observed the kinetics of accumulation of the polyplexes in lysosomes at a similar rate. The results from the new method are consistent with similar measurements by confocal fluorescence microscopy and subcellular fractionation of endocytic vesicles on a Percoll gradient. The relative ease of this new method will aid investigation of gene delivery mechanisms by providing the means to rapidly quantify endocytic trafficking of polyplexes and other vectors.

Ferroportin-mediated mobilization of ferritin iron precedes ferritin degradation by the proteasome

Ferritin is a cytosolic molecule comprised of subunits that self-assemble into a nanocage capable of containing up to 4500 iron atoms. Iron stored within ferritin can be mobilized for use within cells or exported from cells. Expression of ferroportin (Fpn) results in export of cytosolic iron and ferritin degradation. Fpn-mediated iron loss from ferritin occurs in the cytosol and precedes ferritin degradation by the proteasome. Depletion of ferritin iron induces the monoubiquitination of ferritin subunits. Ubiquitination is not required for iron release but is required for disassembly of ferritin nanocages, which is followed by degradation of ferritin by the proteasome. Specific mammalian machinery is not required to extract iron from ferritin. Iron can be removed from ferritin when ferritin is expressed in Saccharomyces cerevisiae, which does not have endogenous ferritin. Expressed ferritin is monoubiquitinated and degraded by the proteasome. Exposure of ubiquitination defective mammalian cells to the iron chelator desferrioxamine leads to degradation of ferritin in the lysosome, which can be prevented by inhibitors of autophagy. Thus, ferritin degradation can occur through two different mechanisms.

Maturation models for endosome and lysosome biogenesis

The complexity and dynamic nature of the endocytic apparatus of mammalian cells have become increasingly clear over the past ten years. Structures collectively referred to as endosomes are at the crossroads of traffic with the plasma membrane and with the degradative pathway leading to lysosomes. They carry out the sorting and segregation of receptors and ligands, processes that are necessary for nutrient uptake and the maintenance of plasma membrane composition. This article addresses the question of whether endosomes are stable or transient compartments.

Hrs regulates early endosome fusion by inhibiting formation of an endosomal SNARE complex

Movement through the endocytic pathway occurs principally via a series of membrane fusion and fission reactions that allow sorting of molecules to be recycled from those to be degraded. Endosome fusion is dependent on SNARE proteins, although the nature of the proteins involved and their regulation has not been fully elucidated. We found that the endosome-associated hepatocyte responsive serum phosphoprotein (Hrs) inhibited the homotypic fusion of early endosomes. A region of Hrs predicted to form a coiled coil required for binding the Q-SNARE, SNAP-25, mimicked the inhibition of endosome fusion produced by full-length Hrs, and was sufficient for endosome binding. SNAP-25, syntaxin 13, and VAMP2 were bound from rat brain membranes to the Hrs coiled-coil domain. Syntaxin 13 inhibited early endosomal fusion and botulinum toxin/E inhibition of early endosomal fusion was reversed by addition of SNAP-25(150-206), confirming a role for syntaxin 13, and establishing a role for SNAP-25 in endosomal fusion. Hrs inhibited formation of the syntaxin 13-SNAP-25-VAMP2 complex by displacing VAMP2 from the complex. These data suggest that SNAP-25 is a receptor for Hrs on early endosomal membranes and that the binding of Hrs to SNAP-25 on endosomal membranes inhibits formation of a SNARE complex required for homotypic endosome fusion.

Fundamentals of pulmonary drug delivery

Aerosol administration of peptide-based drugs plays an important role in the treatment of pulmonary and systemic diseases and the unique cellular properties of airway epithelium offers a great potential to deliver new compounds. As the relative contributions from the large airways to the alveolar space are important to the local and systemic availability, the sites and mechanism of uptake and transport of different target compounds have to be characterized. Among the different respiratory cells, the ciliated epithelial cells of the larger and smaller airways and the type I and type II pneumocytes are the key players in pulmonary drug transport. With their diverse cellular characteristics, each of these cell types displays a unique uptake possibility. Next to the knowledge of these cellular aspects, the nature of aerosolized drugs, characteristics of delivery systems and the depositional and pulmonary clearance mechanisms display major targets to optimize pulmonary drug delivery. Based on the growing knowledge on pulmonary cell biology and pathophysiology due to modern methods of molecular biology, the future characterization of pulmonary drug transport pathways can lead to new strategies in aerosol drug therapy.

Receptor-mediated targeting of gene delivery vectors: insights from molecular mechanisms for improved vehicle design

One way to deliver transgenes to cells in a selective manner is to target the delivery vehicles, or vectors, to specific cell-surface receptors as a first step toward ultimate transport of the gene to the nucleus for expression. While selective delivery, although often to undesired cell types, occurs naturally for some viral vectors and can be achieved for nonviral vehicles, current understanding and control of the delivery mechanism is inadequate for many therapeutic applications. The complicated nature of receptor-mediated transgene uptake and transport requires improved analysis to more effectively evaluate delivery vehicles. As receptor-mediated pathways for gene delivery typically involve vector binding, internalization, subcellular trafficking, vesicular escape, nuclear translocation, and unpackaging for transcription, each of these processes offer mechanisms that can be exploited to enhance targeted gene delivery via properly designed vehicles. For the purpose of this review, current targeted gene delivery vehicles are divided into three approaches: viral, synthetic, and hybrid vectors. Each approach possesses advantages as well as disadvantages at the present time for in vitro and in vivo application, and provides particular challenges to overcome in order to gain significantly improved targeted delivery properties. Quantitative experiments and mathematical modeling of the gene delivery pathway will serve to provide insight into molecular mechanisms and rate-limiting steps for effective gene expression. Information on molecular mechanisms obtained by such methodologies can then be applied to specific vectors, whether viral, synthetic, or hybrid, allowing for the creation of targeted, effective, and safe gene therapeutics.

Syntaxin 7 and VAMP-7 are soluble N-ethylmaleimide-sensitive factor attachment protein receptors required for late endosome-lysosome and homotypic lysosome fusion in alveolar macrophages

Endocytosis in alveolar macrophages can be reversibly inhibited, permitting the isolation of endocytic vesicles at defined stages of maturation. Using an in vitro fusion assay, we determined that each isolated endosome population was capable of homotypic fusion. All vesicle populations were also capable of heterotypic fusion in a temporally specific manner; early endosomes, isolated 4 min after internalization, could fuse with endosomes isolated 8 min after internalization but not with 12-min endosomes or lysosomes. Lysosomes fuse with 12-min endosomes but not with earlier endosomes. Using homogenous populations of endosomes, we have identified Syntaxin 7 as a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) required for late endosome-lysosome and homotypic lysosome fusion in vitro. A bacterially expressed human Syntaxin 7 lacking the transmembrane domain inhibited homotypic late endosome and lysosome fusion as well as heterotypic late endosome-lysosome fusion. Affinity-purified antibodies directed against Syntaxin 7 also inhibited lysosome fusion in vitro but had no affect on homotypic early endosome fusion. Previous work suggested that human VAMP-7 (vesicle-associated membrane protein-7) was a SNARE required for late endosome-lysosome fusion. A bacterially expressed human VAMP-7 lacking the transmembrane domain inhibited both late endosome-lysosome fusion and homotypic lysosome fusion in vitro. These studies indicate that: 1) fusion along the endocytic pathway is a highly regulated process, and 2) two SNARE molecules, Syntaxin 7 and human VAMP-7, are involved in fusion of vesicles in the late endocytic pathway in alveolar macrophages.

The pathogenesis of foam cell formation: modified LDL stimulates uptake of co-incubated LDL via macropinocytosis

Previously, modified LDLs were shown to stimulate macropinocytosis in pigeon macrophages. Simultaneous intracellular trafficking of LDL and AcLDL, differentially labeled with colloidal gold, was done to determine whether uptake of LDL, which does not cause foam cell formation, was internalized via a separate route from AcLDL, which stimulates foam cell formation. AcLDL and LDL were followed at either low (12 microg/mL) concentrations near the saturation of high affinity binding sites or high (50 to 150 microg/mL) lipoprotein concentrations used to induce foam cell formation. The colloidal gold distribution and percentage of co-labeling as observed by transmission electron microscopy were determined for organelles involved with coated-pit endocytosis or macropinocytosis. LDL simultaneously incubated with AcLDL on macrophages at the low concentration was predominately internalized via coated-pit endocytosis. AcLDL was internalized via both coated-pit endocytosis and macropinocytosis at low concentration. At higher lipoprotein concentrations (50 to 150 microg/mL), AcLDL continued to be internalized via macropinocytosis. Interestingly, a significant portion of the co-incubated LDL, at high concentrations, also trafficked via macropinocytosis. LDL internalized by macropinosomes at high lipoprotein concentrations suggests that AcLDL-stimulated macropinocytosis might increase uptake of co-incubated lipoproteins. When (125)I-LDL was incubated with cold AcLDL, LDL degradation at 37 degrees C doubled, without a corresponding increase in cell association or total binding of LDL at 4 degrees C. These studies suggest that modified LDL-stimulated macropinocytosis is a mechanism for increased degradation of co-incubated LDL potentially leading to foam cell formation.

Construction of chimeric phagosomes that shelter Mycobacterium avium and Coxiella burnetii (phase II) in doubly infected mouse macrophages: an ultrastructural study

Dual infection of cells may divert pathogens to intracellular compartments different from those occupied in mono-infected cells. In the present studies, mouse bone marrow in vitro-derived macrophages were first infected with virulent Mycobacterium avium, which are normally singly lodged within tight phagosomes. These phagosomes do not mature; they undergo homotypic fusion with early endosomes and do not fuse with lysosomes. Seven days later, the cultures were superinfected with phase II (non-virulent) Coxiella burnetii, organisms sheltered in lysosome- (or prelysosome)-like, multi-occupancy phagosomes. The latter can attain large size and engage in efficient homo- and heterotypic fusion with other phagosomes. Cultures were fixed for transmission electron microscopy 6, 12, 24, and 48 h later. Other M. avium-infected cultures were superinfected with amastigotes of the trypanosomatid flagellate Leishmania amazonensis, which are also sheltered in lysosome- (or prelysosome)-like multi-occupancy vacuoles, and fixed at the same time periods. Chimeric phagosomes containing both M. avium and C. burnetii, were found already at 6 h and the proportion of M. avium that colocalized with C. burnetii in the same phagosomes reached over 90% after 48 h. In such phagosomes, both organisms were ultrastructurally well preserved. In contrast, colocalization of M. avium and L. amazonensis was rarely found. Speculative scenarios that could underlie the formation of chimeric phagosomes could involve delayed maturation of C. burnetii-containing phagosomes in presence of M. avium, which would allow for fusion of C. burnetii- and M. avium-containing phagosomes; the production, by C. burnetii, of molecules that upregulate the fusion of M. avium-containing phagosomes with those that contain C. burnetii; and the secretion of factors that could favour the survival of M. avium within chimeric vacuoles.

Transport of residual endocytosed products into terminal lysosomes occurs slowly in rat liver endothelial cells

Receptor-mediated endocytosis of circulating collagen is a major physiological scavenger function of the liver endothelial cell and an important catabolic event in the complete turnover of this abundant connective tissue protein. In the present study, transport of collagen through the endocytic pathway was investigated in cultured liver endothelial cells. Collagen conjugated to fluorescein isothiocyanate, to allow detection of the ligand by fluorescence and immunoelectron microscopy, was found sequentially in three different organelles that compose the basic degradative endocytic pathway of eukaryotic cells: early endosomes, late endosomes, and terminal lysosomes. Early endosomes were identified as vesicles positive for early endosome antigen 1 (EEA1). Late endosomes were distinguished as structures positive for the late endosomal/lysosomal marker rat lysosomal membrane glycoprotein 120, but negative for EEA1 and lysosomally targeted BSA-gold. Lysosomes were defined by their content of BSA-gold, injected 24 hours before isolation of cells. Coated pits and coated vesicles mediated an extremely rapid internalization. Shortly after internalization and during the first 20 minutes, ligand was found in early endosomes. From 20 minutes on, ligand started to appear in late endosomes (23%), and by 2 hours the transfer was largely complete (82.5%). Only 2.5% of ligand was transferred to the lysosomes after 2 hours, and this number slowly increased to 21% and 53% after 6 and 16 hours, respectively. We conclude that 1) EEA1 is a useful marker for tracing early events of endocytosis in liver endothelial cells; 2) in contrast to the rapid internalization, transit of internalized ligand through early sorting endosomes generally takes from 20 minutes to 2 hours; and 3) exit from the late endosomes is very slow, requiring several hours.

Homotypic lysosome fusion in macrophages: analysis using an in vitro assay

Lysosomes are dynamic structures capable of fusing with endosomes as well as other lysosomes. We examined the biochemical requirements for homotypic lysosome fusion in vitro using lysosomes obtained from rabbit alveolar macrophages or the cultured macrophage-like cell line, J774E. The in vitro assay measures the formation of a biotinylated HRP-avidin conjugate, in which biotinylated HRP and avidin were accumulated in lysosomes by receptor-mediated endocytosis. We determined that lysosome fusion in vitro was time- and temperature-dependent and required ATP and an N-ethylmaleimide (NEM)-sensitive factor from cytosol. The NEM-sensitive factor was NSF as purified recombinant NSF could completely replace cytosol in the fusion assay whereas a dominant-negative mutant NSF inhibited fusion. Fusion in vitro was extensive; up to 30% of purified macrophage lysosomes were capable of self-fusion. Addition of GTPgammas to the in vitro assay inhibited fusion in a concentration-dependent manner. Purified GDP-dissociation inhibitor inhibited homotypic lysosome fusion suggesting the involvement of rabs. Fusion was also inhibited by the heterotrimeric G protein activator mastoparan, but not by its inactive analogue Mas-17. Pertussis toxin, a Galphai activator, inhibited in vitro lysosome fusion whereas cholera toxin, a Galphas activator did not inhibit the fusion reaction. Addition of agents that either promoted or disrupted microtubule function had little effect on either the extent or rate of lysosome fusion. The high value of homotypic fusion was supported by in vivo experiments examining lysosome fusion in heterokaryons formed between cells containing fluorescently labeled lysosomes. In both macrophages and J774E cells, almost complete mixing of the lysosome labels was observed within 1-3 h of UV sendai-mediated cell fusion. These studies provide a model system for identifying the components required for lysosome fusion.

Simultaneous labeling of lipoprotein intracellular trafficking in pigeon monocyte-derived macrophages

Macrophage foam cell formation resulting from the accumulation of cholesterol and cholesterol esters derived from plasma lipoproteins is important for progression of atherosclerosis. Hypothetically, intracellular processing of lipoproteins that stimulate foam cell formation differs from processing of lipoproteins that do not. To test this, we examined simultaneous subcellular trafficking of lipoproteins in pigeon monocyte-derived macrophages. Pigeon beta-very-low-density lipoprotein (beta-VLDL), low-density lipoprotein (LDL), and acetylated low-density lipoprotein (Ac-LDL), differentially labeled with colloidal gold, were added in pairs to cells at 4 degrees C for 2 hours before uptake at 18 degrees C, 22 degrees C, or 37 degrees C for either 30 minutes or 2 hours. The colloidal gold distribution and percent co-labeling as observed by transmission electron microscopy were determined for organelles of the endocytic pathway. Incubations at 18 degrees C and 22 degrees C blocked lipoprotein trafficking to lysosomes. Incubation at 18 degrees C increased the percent distribution of lipoproteins in the endocytic pathway up to the early cisternal endosomes. Incubations at 22 degrees C resulted in a greater distribution of lipoproteins in the spherical late endosomes and late endosomal-prelysosomal tubular reticular compartment. The distribution in the endocytic pathway was a factor of time and temperature rather than lipoprotein type. The percentage of co-labeling of organelles for the three pairs of lipoproteins examined, Ac-LDL plus beta-VLDL, LDL plus beta-VLDL, and LDL plus Ac-LDL, was similar. Fewer noncoated and clathrin-coated pits and vesicles were co-labeled (average of 6%, maximum of 17%) than the rest of the endocytic pathway, early cisternal endosomes, spherical late endosomes, late endosomal-prelysosomal tubuloreticular compartment, and spherical lysosomes (average of 36%, maximum of 47%). The 36% of co-labeled later endocytic organelles contained an average of 58% of the labeled lipoproteins. This study suggests differential sorting does not occur for high-affinity uptake of lipoproteins.

Reconstitution of transcytosis in SLO-permeabilized MDCK cells: existence of an NSF-dependent fusion mechanism with the apical surface of MDCK cells

Recently, it was demonstrated that delivery from the trans-Golgi network (TGN) to the basolateral surface of Madin-Darby canine kidney (MDCK) cells required N-ethylmaleimide-sensitive factor (NSF)-alpha soluble NSF attachment protein (SNAP)-SNAP receptor (SNARE) complexes, while delivery from the TGN to the apical surface was independent of NSF-alpha SNAP-SNARE. To determine if all traffic to the apical surface of this cell line was NSF independent, we reconstituted the transcytosis of pre-internalized IgA to the apical surface and recycling to the basolateral surface. Transcytosis and the recycling of IgA required ATP and cytosol, and both were inhibited by treatment with N-ethylmaleimide. This inhibition was reversed by the addition of recombinant NSF. Botulinum neurotoxin serotype E, which is known to cleave the 25,000 Da synaptosomal associated protein, inhibited both transcytosis and recycling, although incompletely. We conclude that membrane traffic to a target membrane is not determined by utilizing a single molecular mechanism for fusion. Rather, a target membrane, e.g. the apical plasma membrane of MDCK cells, may use multiple molecular mechanisms to fuse with incoming vesicle.

"Synchronized" endocytosis and intracellular sorting in alveolar macrophages: the early sorting endosome is a transient organelle

Incubation of alveolar macrophages in hypoosmotic K(+)-containing buffers results in persistent cell swelling and an inability to undergo regulatory volume decrease. We demonstrate that cells incubated in hypo-K+ show an inhibition of endocytosis without any observed alteration in recycling. The inhibition of endocytosis affected all forms of membrane internalization, receptor and fluid phase. Both increased cell volume and the inhibition of endocytosis could be released upon return of cells to iso-Na+ buffers. The ability to synchronize the endocytic apparatus allowed us to examine hypotheses regarding the origin and maturation of endocytic vesicles. Incubation in hypo-K+ buffers had no effect on the delivery of ligands to degradative compartments or on the return of previously internalized receptors to the cell surface. Thus, membrane recycling and movement of internalized components to lysosomes occurred in the absence of continued membrane influx. We also demonstrate that fluorescent lipids, that had been incorporated into early endosomes, returned to the cell surface upon exposure of cells to hypo-K+ buffers. These results indicate that the early sorting endosome is a transient structure, whose existence depends upon continued membrane internalization. Our data supports the hypothesis that the transfer of material to lysosomes can best be explained by the continuous maturation of endosomes.

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.

The kinetic aspects of intracellular fluorescence labeling with TMA-DPH support the maturation model for endocytosis in L929 cells

TMA-DPH (1-(4-trimethylammonium)-6-phenyl-1,3,5-hexatriene), a hydrophobic fluorescent membrane probe, interacts with living cells by instantaneous incorporation into the plasma membrane, where it becomes fluorescent. It then follows the intracellular constitutive membrane traffic and acts as a bulk membrane marker of the endocytic pathway (Illinger, D., P. Poindron, P. Fonteneau, M. Modolell, and J. G. Kuhry. 1990. Biochim. Biophys. Acta. 1030:73-81; Illinger, D., P. Poindron, and J. G. Kuhry. 1991. Biol. Cell. 73:131-138). As such, TMA-DPH displays particular properties mainly due to partition between membranes and aqueous media. From these properties, original arguments can be inferred in favor of the maturation model for the endocytic pathway, against that of pre-existing compartments, in L929 cultured mouse fibroblasts. (a) TMA-DPH labeling is seen to progress from the cell periphery to perinuclear regions during endocytosis without any noticeable loss in fluorescence intensity; with a vesicle shuttle model this evolution would be accompanied by probe dilution with a decrease in the overall intracellular fluorescence intensity, and the labeling of the inner (late) compartments could in no way become more intense than that of the peripheral (early) ones. (b) From TMA-DPH fluorescence anisotropy assays, it is concluded that membrane fluidity is the same in the successive endocytic compartments as in the plasma membrane, which probably denotes a similar phospholipidic membrane composition, as might be expected in the maturation model. (c) TMA-DPH internalization and release kinetics are more easily described with the maturation model.

Receptor-mediated transcytosis of IgA in MDCK cells is via apical recycling endosomes

Classically, the polymeric immunoglobulin receptor and its ligand, IgA, are thought to be sorted from basolateral early endosomes into transcytotic vesicles that directly fuse with the apical plasma membrane. In contrast, we have found that in MDCK cells IgA is delivered from basolateral endosomes to apical endosomes and only then to the apical cell surface. When internalized from the basolateral surface of MDCK cells IgA is found to accumulate under the apical plasma membrane in a compartment that is accessible to two apically added membrane markers: anti-secretory component Fab fragments, and avidin internalized from the biotinylated apical pole of the cell. This accumulation occurs in the presence of apical trypsin, which prevents internalization of the ligand from the apical cell surface. Using a modification of the diaminobenzidine density-shift assay, we estimate that approximately 80% of basolaterally internalized IgA resides in the apical endosomal compartment. In addition, approximately 50% of basolaterally internalized transferrin, a basolateral recycling protein, has access to this apical endosomal compartment and is efficiently recycled back to the basolateral surface. Microtubules are required for the organization of the apical endosomal compartment and it is dispersed in nocodazole-treated cells. Moreover, this compartment is largely inaccessible to fluid-phase markers added to either pole of the cell, and therefore seems analogous to the recycling endosome described in nonpolarized cells. We propose a model in which transcytosis is not a specialized pathway that uses unique transcytotic vesicles, but rather combines portions of pathways used by non-transcytosing molecules.

Plasticity of early endosomes

We observed that the structural organization of early endosomes was significantly modified after cell surface biotinylation followed by incubation in the presence of low concentrations of avidin. Under these conditions early endosomes increased in size to form structures which extended over several micrometers and which had an intra-luminal content with a characteristic electron-dense appearance. The modified early endosomes were not formed when either avidin or biotinylation was omitted, suggesting that they resulted from the cross-linking of internalized biotinylated proteins by avidin. Accumulation of a fluid-phase tracer was increased after the avidin-biotin treatment (145% after 45 min). Both recycling and transport to the late endosomes still occurred, albeit to a somewhat lower extent than in control cells. Quantitative electron microscopy showed that the volume of the endosomal compartment was increased approximately 1.5-fold but that the surface area of the compartment decreased relative to its volume after avidin-biotin treatment. Finally, overexpression of a rab5 mutant, which is known to inhibit early endosome fusion in vitro, prevented the formation of these structures in vivo and caused early endosome fragmentation. Altogether, our data suggest that early endosomes exhibit a high plasticity in vivo. Cross-linking appears to interfere with this dynamic process but does not arrest membrane traffic to/from early endosomes.

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)

Insulin receptors internalize by a rapid, saturable pathway requiring receptor autophosphorylation and an intact juxtamembrane region

The effect of receptor occupancy on insulin receptor endocytosis was examined in CHO cells expressing normal human insulin receptors (CHO/IR), autophosphorylation- and internalization-deficient receptors (CHO/IRA1018), and receptors which undergo autophosphorylation but lack a sequence required for internalization (CHO/IR delta 960). The rate of [125I]insulin internalization in CHO/IR cells at 37 degrees C was rapid at physiological concentrations, but decreased markedly in the presence of increasing unlabeled insulin (ED50 = 1-3 nM insulin, or 75,000 occupied receptors/cell). In contrast, [125I]insulin internalization by CHO/IRA1018 and CHO/IR delta 960 cells was slow and was not inhibited by unlabeled insulin. At saturating insulin concentrations, the rate of internalization by wild-type and mutant receptors was similar. Moreover, depletion of intracellular potassium, which has been shown to disrupt coated pit formation, inhibited the rapid internalization of [125I]insulin at physiological insulin concentrations by CHO/IR cells, but had little or no effect on [125I]insulin uptake by CHO/IR delta 960 and CHO/IRA1018 cells or wild-type cells at high insulin concentrations. These data suggest that the insulin-stimulated entry of the insulin receptor into a rapid, coated pit-mediated internalization pathway is saturable and requires receptor autophosphorylation and an intact juxtamembrane region. Furthermore, CHO cells also contain a constitutive nonsaturable pathway which does not require receptor autophosphorylation or an intact juxtamembrane region; this second pathway is unaffected by depletion of intracellular potassium, and therefore may be independent of coated pits. Our data suggest that the ligand-stimulated internalization of the insulin receptor may require specific saturable interactions between the receptor and components of the endocytic system.

Identification of an endosome-specific antigen

We have used successive density gradient centrifugation with vesicles prepared from a human hepatoma Hep G2 post nuclear supernatant to obtain a highly enriched preparation of early endosomes. A monoclonal antibody (8E4) raised against this early endosome preparation recognizes a single polypeptide highly enriched in light vesicle membranes. The antigen has a molecular weight of 195 kDa by SDS-PAGE in the presence or absence of a reducing agent. Western blot analysis shows that the 8E4 antigen is detectable only in light vesicle membranes and not among heavy membranes, whole cytosol, or nuclear pellet proteins. The 8E4 antigen appears to be an integral membrane protein as it is precipitated by Triton X-114. The distribution of the 8E4 antigen in a Nycodenz density gradient fractionation of light vesicle membranes is identical to the distribution of 125I-ASOR-labeled early endosomes but distinct from the distribution of the plasma membrane enzyme, alkaline phosphodiesterase. In addition, incubation of cells with a horseradish peroxidase-transferrin conjugate followed by 3,3'-diaminobenzidine cytochemistry specifically quenches 8E4 antigen detection by protein dot blot analysis. These data strongly suggest that the 8E4 antigen is an integral membrane protein primarily located in endocytic vesicles.

Inhibition of late endosome-lysosome fusion: studies on the mechanism by which isotonic-K+ buffers alter intracellular ligand movement

Incubation of alveolar macrophages or hepatocytes in media in which Na+ is replaced by K+ ("isotonic-K buffer") inhibited the movement of internalized ligand from late endosomes to lysosomes (Ward et al.: Journal of Cell Biology 110:1013-1022, 1990). In this study we investigate the mechanism responsible for the isotonic-K+ block in movement of ligand from late endosomes to lysosomes. We observed that iso-K+ inhibition of endosome-lysosome fusion is not unique to alveolar macrophages or hepatocytes but can be seen in a variety of cell types including J774 and Hela cells. The inhibition in intracellular ligand movement was time dependent with the maximum change occurring after 60 minutes. Once established the inhibition resulted in a prolonged and apparently permanent decrease in vesicle movement. Cells were able to recover from the effects of iso-K+ buffers over a time course of 5-10 minutes when placed back in Na(+)-containing media. The effect of iso-K+ buffers was independent of intracellular pH changes and appeared to involve cell swelling. When cells were incubated in iso-K+ buffers under conditions in which cell volume changes were reduced, intracellular ligand movement approached normal levels. Such conditions included replacing Cl- with the less permeant anion gluconate, and by addition of sucrose to isotonic-K+ buffers. Analysis of the mechanism by which changes in cell volume could alter intracellular movement ruled out changes in cyclic nucleotides, Ca2+, or microtubules. These results suggest that changes in cell shape or volume can alter intracellular transport systems by novel routes.

The rate of internalization of different receptor-ligand complexes in alveolar macrophages is receptor-specific

To probe the mechanisms of endocytosis in alveolar macrophages, we examined the internalization rates of three different receptors. Initial rates of internalization for mannosylated BSA, diferric transferrin and alpha-macroglobulin-proteinase complexes were all different. Although the absolute rates of internalization varied depending on the cell preparation, transferrin was internalized at 10-20% and alpha-macroglobulin-proteinase complex at 40-60% of the rate of manosylated-BSA. Incubation of cells with transferrin did not affect the rate of internalization of mannosylated BSA or alpha-macroglobulin-proteinase complexes, and the rates of internalization were independent of receptor occupancy. These different internalization rates could not be ascribed to different rates of diacytosis. Altering the distribution of unoccupied surface receptors by either trypsin treatment of cells at 0 degree C or exposure to hyperosmotic solutions resulted in the absolute internalization rates being affected by the experimental condition, but the hierarchy in receptor internalization rates was maintained. The fact that a variety of conditions affect receptor internalization rates to the same degree implies the existence of co-ordinate regulation at a single rate-limiting step. Based on these results, we suggest that differences in internalization rate reflect the ability of ligand-receptor complexes to be captured by coated pits.