Routing of internalized ricin and ricin conjugates to the Golgi complex

Separation of ricin A- and B-chains after dithiothreitol reduction

After complete cleavage of ricin interchain disulfide bridge by 0.05 M dithiothreitol in nondenaturing conditions at 37 degrees C during 1 h 30 min, A- and B-chains were separated on a lactosaminyl-aminoethyl Biogel P-150 column at 4 degrees C, in the presence of 0.01 M dithiothreitol and 0.5 M MgCl2. A- and B-chains have been characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunology. Their specific activities have been tested by protein synthesis inhibition in a cell-free assay (rabbit reticulocyte lysate) and on whole cells (Zajdela hepatoma cells) and by hemagglutination. From these tests, the apparent cross contamination of the chains was about 0.1%.

Estimation of the amount of internalized ricin that reaches the trans-Golgi network

We have used a protocol for internalization of ricin, a ligand binding to plasma membrane glycoproteins and glycolipids with terminal galactosyl residues, and infection with the vesicular stomatitis virus ts 045 mutant in BHK-21 cells to determine whether internalized plasma membrane molecules tagged by ricin reach distinct compartments of the biosynthetic-exocytic pathway. At 39.5 degrees C newly synthesized G protein of ts 045 was largely prevented from leaving the endoplasmic reticulum. At the same temperature ricin was endocytosed and reached, in addition to endosomes and lysosomes, elements of the Golgi complex. When the temperature was lowered to 19.5 degrees C, no more ricin was delivered to the Golgi complex, but now G protein accumulated in the Golgi stacks and the trans-Golgi network (TGN). Double-labeling immunogold cytochemistry on ultracryosections was used to detect G protein and ricin simultaneously. These data, combined with stereological and biochemical methods, showed that approximately 5% of the total amount of ricin within the cells, corresponding to 6-8 X 10(4) molecules per cell, colocalized with G protein in the Golgi complex after 60 min at 39.5 degrees C. Of this amount approximately 70-80% was present in the TGN. Since most of the ricin molecules remain bound to their binding sites at the low pH prevailing in compartments of the endocytic pathway, the results indicate that a fraction of the internalized plasma membrane molecules with terminal galactose are not recycled directly from endosomes or delivered to lysosomes, but are routed to the Golgi complex. Also, the results presented here, in combination with other recent studies on ricin internalization, suggest that translocation of the toxic ricin A-chain to the cytosol occurs in the TGN.

Inhibition of endocytosis from coated pits by acidification of the cytosol

Binding and endocytosis of the ligands transferrin, epidermal growth factor (EGF), and ricin were measured in a number of different cell lines after treatment of cells with compounds that react with SH-groups and under conditions where the cytosolic pH was lowered. N-ethylmalemide and diamide irreversibly inhibited endocytosis of all ligands tested, whereas low pH in the cytosol strongly inhibited endocytosis of transferrin and EGF. Data obtained by electron microscopy indicated that the formation of coated vesicles from coated pits is inhibited in acidified cells. Entry of ricin was much less affected, and ricin endocytosed under these conditions was able to intoxicate the cells. At low pH in the cytosol there was a calcium-dependent increase in the number of transferrin receptors at the cell surface. The increase was even larger in the presence of the calcium ionophore A23187, whereas it was completely blocked by the calmodulin antagonists trifluoperazine and W7. The results show that endocytosis from coated pits can be inhibited in a reversible way by acidification of the cytosol and they suggest that a second pathway of endocytosis exists, possibly involving formation of vesicles from uncoated areas of the membrane.

Ligands internalized through coated or noncoated invaginations follow a common intracellular pathway

Cholera toxin (CT) represents a class of ligands that binds preferentially to noncoated pits on the cell surface. In the present study, we have investigated the mechanism of endocytosis of this class of ligand and compared it to the classic coated pit mechanism. When either CT coupled to colloidal gold particles (CT-gold) or 125I-labeled CT were incubated with 3T3 L1 fibroblasts at 4 degrees C, both ligands bound in a preferential fashion to small noncoated pits on the cell surface. CT-gold surface-labeled cells were then incubated at 22 degrees C. The labeled ligand progressively moved into noncoated vesicles and a tubulovesicular compartment composed of a network of tubules and vesicles closely associated with multivesicular bodies but distinct from the Golgi complexes. The ligand next passed into multivesicular bodies. By contrast, alpha 2-macroglobulin (alpha 2m)-gold initially localized preferentially to coated pits and subsequently to coated vesicles and tubulovesicular structures before associating with multivesicular bodies. To directly compare the intracellular pathway followed by CT-gold to that followed by alpha 2m-gold, CT-gold (7 nm) was coincubated with alpha 2m-gold (15 nm). By 10 min of incubation at 22 degrees C, up to 66% of tubulovesicular units contained both ligands when analyzed in serial sections. Subsequently, both ligands were colocalized in multivesicular bodies. We conclude that CT-gold endocytosed via noncoated vesicles and alpha 2m-gold endocytosed through coated vesicles subsequently associate with the same tubulovesicular units, multivesicular bodies, and lysosomes.

IgE-immunotoxins. II. IgE-ricin A-chain

In order to develop a reagent capable of killing cells with high-affinity IgE Fc receptors, such as mast cells and basophils, ricin A-chain (the toxic portion of ricin) was conjugated to rat IgE myeloma protein, IR 162, via derivatization of the IgE by n-succinimidyl-3-(2-pyridyldithio)propionate (SPDP) thus creating an IgE-immunotoxin. Monensin (10(-7)-10(-8)M), a carboxylic ionophore, facilitated IgE-ricin A-chain (3 X 10(-7)M) toxicity in a dose-related fashion ith significant reductions in [3H]leucine incorporation compared to cells exposed only to monensin. This enhanced toxicity could be inhibited by the addition of both anti-ricin A-chain or anti-IgE, suggesting that different routes of intracellular processing may play a role in determining the toxicity of the IgE-ricin A-chain conjugate. Ricin B-chain (5 X 10(-7) and 5 X 10(-8)M) added to free ricin A-chain (10(-6)-10(-8)M) reproducibly facilitated toxicity, and this toxicity could be inhibited (30-90%) by lactose (50 mM). Ricin B-chain also facilitated IgE-ricin A-chain (2.75 X 10(-8)M) toxicity; however, this toxicity was not affected by lactose. The data suggest that ricin B-chain potentiates the cytosolic access of internalized IgE-immunotoxin and that the binding and internalization of the toxin was mediated via the IgE Fc receptor. A second type of IgE-ricin A-chain conjugate was synthesized whereby both IgE and ricin A-chain were derivatized with SPDP. RBL cells were killed in a dose-dependent manner by this IgE-ricin A-chain conjugate (2.5 X 10(-6)-2.5 X 10(-9)M) without requiring the addition of monensin or ricin B-chain. These data indicate that the intracellular route and processing of internalized immunotoxin is critical to eliciting toxicity.

Quantitative ultrastructural, autoradiographic evidence for the magnitude and early involvement of the Golgi apparatus complex in the endocytosis of wheat germ agglutinin by cultured neuroblastoma

Several ligands undergo endocytosis into the Golgi apparatus. We have examined with a quantitative ultrastructural, autoradiographic method the sequential endocytosis of tritiated wheat germ agglutinin (3H-WGA) by cultured murine neuroblastoma cells. Cells were incubated with 3H-WGA for 1 hour at 4 degrees C, washed, and incubated in complete medium without ligand at 37 degrees C for 5, 15, 30, and 120 minutes. At 5 minutes, the optimized sources/micron 2 of neuroblastoma cell area, which represented the grain density of each compartment, were as follows: smooth vesicles and tubules, 1.03 +/- 0.88; Golgi-associated vesicles, i.e., clusters of vesicles within a 1 micron radius of the Golgi cisterns, 1.03 +/- 0.31; Golgi cisterns, less than 0.01; and lysosomes, 0.26 +/- 0.16. At 15 minutes grain densities were: smooth vesicles and tubules, 0.9 +/- 0.34; Golgi-associated vesicles, 1.41 +/- 0.28; Golgi cisterns, 0.73 +/- 0.41; and lysosomes, 0.1 +/- 0.09. At 30 minutes grain densities were: smooth vesicles and tubules, 0.46 +/- 0.46; Golgi-associated vesicles, 1.78 +/- 0.34; Golgi cisterns, 0.89 +/- 0.78; and lysosomes, 0.39 +/- 0.14. At 2 hours, smooth vesicles, tubules, and Golgi cisterns were not labeled, Golgi-associated vesicles were still labeled (0.71 +/- 0.1), and lysosomes were heavily labeled (2.17 +/- 0.22). These results are consistent with the hypotheses that either the Golgi complex (cisterns and associated vesicles) is an early and intermediate step of the endocytosis of 3H-WGA into lysosomes or that it constitutes part of a separate and quantitatively significant pathway of endocytosis of this ligand.

Acidification of the cytosol inhibits endocytosis from coated pits

Acidification of the cytosol of a number of different cell lines strongly reduced the endocytic uptake of transferrin and epidermal growth factor. The number of transferrin binding sites at the cell surface was increased in acidified cells. Electron microscopic studies showed that the number of coated pits at the cell surface was not reduced in cells with acidified cytosol. Experiments with transferrin-horseradish peroxidase conjugates and a monoclonal anti-transferrin receptor antibody demonstrated that transferrin receptors were present in approximately 75% of the coated pits both in control cells and in cells with acidified cytosol. The data therefore indicate that the reason for the reduced endocytic uptake of transferrin at internal pH less than 6.5 is an inhibition of the pinching off of coated vesicles. In contrast, acidification of the cytosol had only little effect on the uptake of ricin and the fluid phase marker lucifer yellow. Ricin endocytosed by cells with acidified cytosol exhibited full toxic effect on the cells. Although the pathway of this uptake in acidified cells remains uncertain, some coated pits may still be involved. However, the data are also consistent with the possibility that an alternative endocytic pathway involving smooth (uncoated) pits exists.

Delivery of internalized ricin from endosomes to cisternal Golgi elements is a discontinuous, temperature-sensitive process

Galactose-terminating membrane glycoproteins and glycolipids on two established human breast carcinoma cell lines were tagged at 4 degrees C with a ricin-horseradish peroxidase conjugate (Ri-HRP). The cells were then incubated for various periods of time at 37 or 18 degrees C. After fixation and diaminobenzidine cytochemistry, the compartments reached by Ri-HRP were studied by analyzing thin serial sections. In both cell types a highly pleomorphic endosomal system comprising vacuolar elements as well as smaller, sometimes branched, tubular elements (tubular endosomes) was revealed at both 37 and 18 degrees C. At 37 degrees C Ri-HRP was consistently observed in flattened cisterns of the Golgi region in 30-40% of the Golgi complexes examined after 30-60 min of incubation. However, no Ri-HRP reached such Golgi elements at 18 degrees C, even after incubation for 180 min. Moreover, at 18 degrees C the ability of ricin to inhibit protein synthesis was virtually abolished, whereas the effect of diphtheria toxin was reduced much less. Following incubation with a monovalent transferrin-HRP conjugate or with unconjugated HRP, no labeling of cisternal Golgi elements was detected. These data indicate that delivery of galactose-terminating membrane molecules from endosomes to the Golgi complex is a discontinuous, temperature-sensitive process and that this process may be required for optimal ricin A-chain translocation.

Receptor-mediated endocytosis of apamin by liver cells

The binding and uptake of the bee venom toxin apamin, by guinea-pig and rat liver were studied. Guinea-pig liver plasma membranes contain inhibitable, high-affinity binding sites for [125I]monoiodoapamin: Kd = 12.6 +/- 0.8 pM (SE); Bmax = 4.2 +/- 0.2 fmol/mg protein. No binding sites for [125I]monoiodoapamin on rat liver plasma membranes were detected in agreement with the absence of a physiological response to the toxin by rat hepatocytes. [125I]Monoiodoapamin, injected into the portal vein of guinea-pigs, was recovered in an undegraded form in a liver endosome fraction. The uptake of [125I]monoiodoapamin by rat livers was less than 4% of that taken up by guinea-pig livers and there was little evidence of radiolabelled toxin appearing in isolated rat endocytic vesicles. Inhibitable, high-affinity binding sites for [125I]monoiodoapamin were also identified on isolated guinea-pig liver endosomal membranes; Kd = 10.6 +/- 3.3 pM; Bmax = 2.5 +/- 0.6 fmol/mg protein. No inhibitable apamin binding sites were detected on rat endosomal membranes. Plasma membranes and endosomal membranes isolated from guinea-pig liver showed a similar spectrum of polypeptides to that previously reported for plasma membranes and endosomal membranes isolated from rat liver. The enzymatic composition of guinea-pig endosomes was also similar to that previously reported for rat endosomes. The results indicate that apamin was internalised by receptor-mediated endocytosis by guinea-pig liver cells in an analogous manner to that already shown for a variety of endogenous ligands.

Membrane specializations and extracellular material associated with host astrocytes in peripheral neural transplants

The work of Aguayo and colleagues [Aguayo, David and Bray (1981) J. Exp. Biol. 95, 231-240] demonstrates that grafts of peripheral neural tissue are able to induce regenerative elongation of cut axons in the adult central nervous system. Elucidation of the mechanism of this response requires an understanding of the cellular interactions induced by these types of transplant. In previous studies [Zhou, Lawrence, Morris and Raisman (1986) Neuroscience 17, 815-827; Zhou, Lindsay, Lawrence and Raisman (1986) Neuroscience 17, 803-813] we have transplanted decapsulated adult superior cervical sympathetic ganglia or nodose ganglia into either the septal nuclei or the choroid fissure of adult syngeneic rat hosts. We found that host astrocytes invade the transplants along Schwann cell fascicles and around blood vessels. This raises the questions of what form the migrating astrocytes take, what routes they follow, and what is their fate. In the present study we have taken advantage of the fact that at longer survivals astrocytes accumulate as "paravascular cuffs", and we show that they have several specialized ultrastructural features, such as plasmalemmal caveolae, desmosomes, hemidesmosomes and accumulations of extracellular material. The specific stimuli inducing (or enhancing) these astrocytic specializations and their significance in relation to the wider morphogenetic events induced by peripheral neural transplants remain to be elucidated. However, the observations are further evidence of the remarkable mobility and plasticity of central astrocytes in transplantation situations, and in particular emphasize the involvement of the cell surface and its relationship to extracellular matrix.

A new diploid nontumorigenic human breast epithelial cell line isolated and propagated in chemically defined medium

A new, nontumorigenic human breast epithelial cell line, HMT-3522, has been established from fibrocystic breast tissue. Cells were explanted and propagated in chemically defined medium including insulin, transferrin, epidermal growth factor, hydrocortisone, estradiol, prolactin, and Na-selenite. The epithelial nature of the cell line was established by immunocytochemical detection of cytokeratins. Moreover, electronmicroscopy revealed monolayers of polarized cells connected by desmosomes and provided with apical microvilli. Milk fat globule membrane antigen, specific for the apical membrane domain of normal, luminal breast epithelial cells, was expressed only in confluent cultures where some cells overlaid others, indicating "stem cell"-like properties. After 25 to 30 passages, the cells are diploid with a few marker chromosomes and loss of chromosomes in the D-group. The cells are nontumorigenic in athymic mice; they lack estrogen receptors, and estradiol does not stimulate growth. The HMT-3522 cell line may represent a useful model for the study of breast cell differentiation and carcinogenesis in vitro.

The Golgi apparatus in the acinar cells of the developing embryonic pancreas: II. Localization of lectin-binding sites

The reaction patterns of the Golgi apparatus following staining with the lectins concanavalin A (ConA), Ricinus communis I agglutinin (RCA I), and Helix pomatia lectin (HPA) were studied in the pancreas acinar cells of rat embryos in the course of cell differentiation from day 13 through day 20 of gestation. The binding reactions were localized by means of pre-embedment incubation of 10-microns-thick cryosections of pancreas tissue, prefixed in a mixture of 4% formaldehyde/0.5% glutaraldehyde, using horseradish peroxidase for electron microscope visualization. ConA, which preferentially binds to alpha-D-mannosyl residues, consistently stained the cisternae of the cis Golgi side. The majority of the stacks also showed ConA staining of medial cisternae. The reaction of the trans side was variable; in each stage of development, the cisternae of the trans Golgi side either were devoid of labeling or appeared intensely stained. The reactions obtained with RCA I, which recognizes terminal beta-D-galactosyl residues, changed in the course of cell differentiation; in the protodifferentiated and early differentiated states, the system of "rigid lamellae," located at the trans side of the Golgi stacks, was intensely labeled, but became unreactive after production of secretion granules had started, the reaction then being restricted to the stacked saccules. In regard to the Golgi stacks in each of the developmental stages, RCA I binding sites either were confined to the trans cisternae, or, in addition, were found distributed across elements of the medial and cis compartments.(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative electron microscopic study of the intracellular localization of wheat germ agglutinin in retinal neurons

Previous work has established that, following endocytosis, wheat germ agglutinin, like a number of other plasma membrane bound ligands, is transported to the Golgi apparatus-complex. Previous studies that provided qualitative information about the intracellular distribution of internalized wheat germ agglutinin used techniques that precluded any quantitative conclusions about the relative magnitude of the labeling of endosomes, lysosomes, and the Golgi apparatus-complex. Using quantitative ultrastructural autoradiography, this study compares the time course and relative magnitude of labeling of various intracellular compartments to the labeling in the Golgi area. Fifteen minutes after intraocular injection, wheat germ agglutinin is confined to the inner surface of the retina and the immediate subsurface neuropil with little labeling of the retinal ganglion cell perikarya. Thirty minutes after injection, the plasma membrane (6.97 +/- 1.17), endosomes (10.27 +/- 3.98), smooth vesicles and tubules (1.94 +/- 1.66), and lysosomes (2.42 +/- 1.21) of the retinal ganglion cells are labeled, while the Golgi apparatus-complex is not labeled (0.29 +/- 0.25). (Figures in parentheses are the calculated relative radioactive source density +/- the standard deviation.) The relative labeling density of the plasma membrane and endosomes decreases somewhat during the next 90 minutes (plasma membrane, 4.76 +/- 0.67; endosomes, 7.23 +/- 2.02), while the labeling density of smooth vesicles and tubules and of lysosomes rises (smooth vesicles and tubules, 5.56 +/- 0.94; lysosomes, 7.76 +/- 1.56). The Golgi apparatus-complex, which is unlabeled at 30 minutes, is weakly labeled at 2 hours (1.26 +/- 0.28). The fact that the lysosomes are already labeled at 30 minutes while the Golgi apparatus-complex is unlabeled at that time indicates that the transport of wheat germ agglutinin to the Golgi apparatus-complex is a relatively late phenomenon, and suggests that the bulk of the wheat germ agglutinin destined for lysosomes does not pass through the Golgi apparatus-complex.

Effect of physical environment on the conformation of ricin. Influence of low pH

The molecular properties of ricin (the toxic lectin from Ricinus communis seeds, RCA II or RCA 60) were evaluated by analytical ultracentrifugation, viscosimetry, c.d., fluorescence and equilibrium dialysis. Measurements of sedimentation (S0(20,W) = 4.60 S) and viscosity (eta = 2.96 X 10(-2) dl/g) indicated that, at neutral pH, the ricin molecule is very compact. Various transitions were explored, and a pH-triggered change in the ricin conformation was observed between pH 7 and 4. In this range, the sedimentation coefficient, far-u.v. c.d. and fluorescence altered simultaneously without unfolding. Below pH 7 the change in the ricin conformation was accompanied by a decrease in the affinity of ricin for galactosides, and at pH 4.0 by an alteration in its binding capacity. These effects of low pH are discussed in relation to the physical conditions encountered by ricin molecules during their entry into living cells.