Separation of endocytic vesicles in Nycodenz gradients

Subcellular fractionation of tissue culture cells

Cell fractionation techniques include some of the most important and widely used analytical tools in cell and molecular biology, and are essential for the development of cell-free assays that reconstitute complicated cellular processes. In addition to simple gradient systems, this unit discusses the immuno-purification of organelles, in particular endosomes. As antigens, purification can be achieved using endogenous or ectopically expressed proteins, provided that appropriate antibodies are available. Alternatively, tagged proteins can be used, when combined with anti-tag antibodies. Now that sequencing of the genomes of several organisms has been completed, biochemical strategies, and in particular fractionation and in vitro transport assays, are more necessary than ever to study the numerous protein and protein complexes that are being discovered.

Subcellular fractionation of tissue culture cells

Development of cell fractionation techniques over the last few decades has provided the means to analyze the composition and properties of purified cellular elements. In particular, subcellular fractionation is essential for the development of cell-free assays that reconstitute complicated cellular processes. These assays have provided new and important tools to understand the molecular mechanisms of complex cellular functions, permitting these functions to be studied in the test tube as a series of biochemical reactions. The protocols in this unit describe fractionation of tissue culture cells to immunoisolate early and late endosomes under conditions where these compartments retain their capacity to support membrane transport in vitro.

Endocytosed ricin and asialoorosomucoid follow different intracellular pathways in hepatocytes

Earlier studies have suggested that fluid phase endocytosis in rat hepatocytes takes place via a clathrin-independent mechanism [1,2]. This observation suggests that a relatively large amount of plasma membrane outside coated pits may be involved in hepatic endocytosis. Ricin, which binds to galactose residues on glycoproteins and glycolipids, has, in this report, been used as a general marker for the plasma membrane of hepatocytes. The endocytosis of ricin was compared with that of asialoorosomucoid (AOM) which is taken up exclusively via clathrin-coated pits. Hypertonic medium has been shown to inhibit uptake via coated pits more effectively than clathrin-independent uptake [3-5]. It was found, in this study, that the addition of 100 mM sucrose to the incubation medium inhibited the uptake of 125I-tyramine-cellobiose-asialoorosomucoid (125I-TC-AOM) more extensively than that of 125I-tyramine-cellobiose-ricin (125I-TC-ricin), compatible with the notion that the two probes are internalised via different mechanisms. Subcellular fractionation experiments indicated that 125I-TC-ricin entered a denser endocytic organelle than that receiving 125I-TC-AOM. To determine whether the separation of the two probes was due to a different transport kinetics (i.e. that 125I-TC-ricin is transported more rapidly to a later, denser compartment than 125I-TC-AOM) the cells were incubated at 18 degreesC to allow a slower internalisation/transport of the labelled probes. The results obtained showed, again, that the early endosomes containing 125I-TC-ricin were significantly denser than those containing 125I-TC-AOM. We also employed the horseradish peroxidase (HRP)-diaminobenzidine (DAB) density shift technique of Courtoy et al. [6] to determine whether 125I-TC-ricin and 125I-TC-AOM were in separate endosomes early after their uptake. The results showed that early endosomes containing 125I-TC-AOM were density shifted whereas those containing 125I-TC-ricin were unaffected by the density shift procedure. The use of probes labelled with 125I-TC allowed us to identify compartments involved in the degradation of 125I-TC-AOM and 125I-TC-ricin, by measuring acid soluble radioactivities in the gradient fractions. It was found that 125I-TC-ricin was degraded mainly in endosomes, whereas 125I-TC-AOM, as expected, was degraded mainly in lysosomes.

CD5-mediated specific delivery of DNA to T lymphocytes: compartmentalization augmented by adenovirus

Specific DNA delivery has been achieved via interactions between an asialoorosomucoid-polylysine conjugate and the asialoglycoprotein receptor. We have now extended this technology to another cell type. In order to achieve DNA delivery uniquely to T cells, we have employed an antibody-polylysine conjugate which binds and is internalized via CD5. Binding analyses of the T101 monoclonal antibody to Jurkat cells and freshly isolated human peripheral T lymphocytes were performed and Scatchard plots revealed Kd values of 1.4 and 1.2 pM, respectively. To introduce DNA into the T cell, a complex of T101-polylysine and the luciferase plasmid was formed (T101-PL-DNA). 125I-labeled antibody alone or T101-PL-DNA complexes were both shown to internalize. Subcellular fractionation indicated that the complex remained in the endosomal compartment of the cell for up to 90 min. However, with the addition of adenovirus particles, there was a decrease of labeled complex in the endosomal fraction over time suggesting it was no longer 'tethered' to the endosome vesicle. In vitro transfections confirmed this result showing the addition of adenovirus particles during incubation resulted in increased expression of the luciferase protein. Without adenovirus, there was limited expression of the transduced gene. These data revealed that T101 can deliver DNA via an antibody-PL conjugate. The addition of adenovirus allowed the DNA to escape the endosome enabling expression of the reporter gene.

The effect of temperature on intracellular transport of asialo‐glycoproteins in rainbow trout liver

The intracellular pathway of a endocytosed glycoprotein has been studied in rainbow trout (Oncorhynchus mykiss) liver by means of differential and isopycnic gradient centrifugation. When 125I‐tyramine‐cellobiose‐labelled asialoorosomucoid was injected intravenously, the protein was removed rapidly from the blood by hepatic endocytosis. The tracer was localized initially in a small, slowly sedimenting organelle (endosome) and transferred to the denser lysosomes where degradation took place. This process took several hours at the acclimated temperature of 10 ° C. The transport could be accelerated markedly or retarded by transfer of fish to higher or lower temperatures respectively. The intracellular transport steps were more susceptible to changing temperature than the internalization process. In particular, the degradation was temperature sensitive. These results may have implications for the effects of changing temperature on immune function in fishes. The impairment of transport upon a rapid temperature decrease, will reduce the rate of antigen processing and presentation in these animals.

A two-dimensional electrophoretic analysis of the proteins and glycoproteins of liver plasma membrane domains and endosomes. Implications for endocytosis and transcytosis

1. Polypeptides of liver plasma membrane fractions enriched in three surface domains of hepatocytes, blood-sinusoidal, lateral and bile canalicular, were analysed by isoelectric focusing (IEF) and non-equilibrium pH gel electrophoresis (NEPHGE) across a wide pH range, followed by SDS/PAGE. The overall Coomassie Blue-stained polypeptide patterns in the fractions were different. lateral plasma membrane fractions contained a characteristically higher number of polypeptides focusing at the basic pH range, whereas few basic polypeptides were present in sinusoidal plasma membrane fractions. The glycoproteins in these plasma membrane fractions stained by a lectin overlay technique with radio-iodinated concanavalin A, wheat-germ agglutinin and a slug lectin, were also different. 2. The polypeptides and glycoproteins of 'early' and 'late' endosome fractions were also compared by two-dimensional electrophoresis. Their composition was shown by Coomassie Blue staining, lectin overlay staining and in membranes metabolically labelled with [35S]methionine to be generally similar. The glycoproteins of sinusoidal plasma membranes and early and late endosomes were generally similar, but major differences in polypeptides of molecular mass 20-50 kDa, pI 7.5-8.5, in plasma membranes and endosomes were demonstrated, with a specific population of basic (pI 8-9) low-molecular-mass polypeptides being present at highest levels in 'late' endosomal fractions (shown by Coomassie Blue staining). 3. Analysis of the distribution of three specific membrane glycoproteins identified by using immunoblotting techniques showed that the asialoglycoprotein and the divalent-cation-sensitive mannose 6-phosphate receptors were present in sinusoidal plasma membrane and in early and late endocytic fractions: they were not detected in canalicular plasma membrane fractions. In contrast, 5'-nucleotidase was detected in all fractions examined. The role of the endocytic compartment in regulating trafficking pathways between the plasma membrane domains of the hepatocyte is discussed.

Intracellular transport of formaldehyde-treated serum albumin in liver endothelial cells after uptake via scavenger receptors

Endocytosis of formaldehyde-treated serum albumin (FSA) mediated by the scavenger receptor was studied in rat liver endothelial cells. Suspended cells had about 8000 receptors/cell, whereas cultured cells had about 19,000 receptors/cell. Kd was 10(-8) M in both systems. Cell-surface scavenger receptors were found exclusively in coated pits by electron microscopy, by using ligand labelled with colloidal gold. Cell-surface-bound FSA could be released by decreasing the pH to 6.0; it was therefore possible to assess the rate of internalization of surface-bound ligand. This rate was very high: t1/2 for internalization of ligand prebound at 4 degrees C was 24 s. The endocytic rate constant at 37 degrees C, Ke, measured as described by Wiley & Cunningham [(1982) J. Biol. Chem. 257, 4222-4229], was 2.44 min-1, corresponding to t1/2 = 12 s. Uptake of FSA at 37 degrees C after destruction of one cell-surface pool of receptors by Pronase was decreased to 60%. This finding is compatible with a relatively large intracellular pool of receptors. The intracellular handling of 125I-tyramine-cellobiose-labelled FSA (125I-TC-FSA) was studied by subcellular fractionation in sucrose gradients, Nycodenz gradients or by differential centrifugation. The density distributions of degraded and undegraded 125I-TC-FSA after fractionation of isolated non-parenchymal cells and whole liver were similar, when studied in Nycodenz and sucrose gradients, suggesting that the subcellular distribution of the ligand was not influenced by the huge excess of non-endothelial material in a whole liver homogenate. Fractionation in sucrose gradients showed that the ligand was sequentially associated with organelles banding at 1.14, 1.17 and 1.21 g/ml. At 9-12 min after intravenous injection the ligand was in a degradative compartment, as indicated by the accumulation of acid-soluble radioactivity at 1.21 g/ml. A rapid transfer of ligand to the lysosomes was also indicated by the finding that a substantial proportion of the ligand could be degraded by incubating mitochondrial fractions prepared 12 min after intravenous injection of the ligand. The results indicate that FSA is very rapidly internalized and transferred through an endosomal compartment to the lysosomes. The endosomes are gradually converted into lysosomes between 9 and 12 min after injection of FSA. The rate-limiting step in the intracellular handling of 125I-TC-FSA is the degradation in the lysosomes.

Investigation of endosomal compartments involved in endocytosis and transcytosis of polymeric immunoglobulin A by subcellular fractionation of perfused isolated rat liver

1. A gamma camera was used to monitor continuously the uptake of radiolabelled polymeric immunoglobulin A (pIgA) into the rat body after intravenous injection. Uptake into liver was fast but, since the peak of liver labelling occurred only after 9-15 min, it was not sufficiently rapid to constitute a pulse dose. A perfused, isolated rat liver system was therefore established which could be given a single pass dose of pIgA; a variety of tests showed such livers remained viable for at least 3 h and could be subsequently fractionated on Ficoll and Nycodenz gradients with normal distributions of marker enzymes. 2. Subcellular fractionation at different times after a single pass dose of pIgA showed that whilst pIgA appeared sequentially in sinusoidal plasma membrane, light endosomes, dense endosomes, very dense endosomes and lysosomes as in vivo, the predominance of pIgA in the light endosome compartment disappeared much earlier than after injection in vivo of pIgA, presumably because this compartment was not being continuously loaded over the first 10-15 min. The time course of appearance of label in bile was unchanged. A large excess of unlabelled asialofetuin did not change these patterns, indicating that the asialoglycoprotein receptor was not involved. 3. Low doses of the microtubule agent colchicine reduced the proportion of pIgA reaching the bile, but subcellular fractionation of treated liver showed that distribution of label amongst liver fractions was little changed, although the overall liver pIgA content had increased. This would suggest that pIgA did not remain in the common compartment which could have supplied bile or lysosomes but rather flowed out of it as rapidly as in untreated liver but towards those compartments supplying the lysosomes. 4. Experiments with nocodazole, which reversibly disrupts microtubules, showed that very little of the pIgA taken into an inhibited liver appeared in the bile after nocodazole was removed 30 min later, even though a second dose of pIgA, given after nocodazole removal, appeared in bile with a normal time course. The first dose of pIgA must therefore have passed beyond the compartments competent to supply the bile before nocodazole was removed. Such compartments were undamaged since the second dose of pIgA appeared in bile normally. We therefore conclude that the bulk of pIgA must be supplied to the bile from light or dense endosomes rather than from very dense endosomes and lysosomes.

The preparative isolation of endosome fractions: a review

The endosome is an intracellular, acidic, membrane-bound, subcellular compartment to which endocytosed ligands, receptors and plasma membrane proteins are conveyed before sorting and delivery to destinations elsewhere in the cell. The preparative isolation of elements of this compartment has been achieved successfully using various appropriate combinations of density gradient ultracentrifugation, electrophoretic, gel filtration and immunoaffinity techniques. These methods for isolating endosome fractions are reviewed together with the difficulties of establishing markers for such fractions. The isolation of an endosome fraction from the pathway of polymeric IgA transcytosis in rat liver is discussed to exemplify successful isolation procedures and appropriate subcellular markers.

Intracellular transport and degradation of chylomicron remnants in rat liver cells after in vivo endocytosis

The intracellular transport and degradation of in vivo endocytosed chylomicron remnants labelled with 125I in the protein moiety was studied in rat liver cells by means of subcellular fractionation in Nycodenz and sucrose density gradients. Initially, the radioactivity was located in low-density endosomes and was sequentially transferred to light and dense lysosomes. Data from gel filtration of the light and dense lysosomal fractions showed radioactive material with a molecular weight of about 1000-2000, representing short peptide fragments or amino acids which remain attached to iodinated tyramine cellobiose. In addition, undegraded apoproteins accumulated in both types of lysosome. Our data suggest that endocytosed chylomicron remnant apoproteins are first located in low-density endosomes and are sequentially transferred to light and dense lysosomes. Furthermore, the degradation process starts in the light lysosomes.

Rapid subcellular fractionation of the rat liver endocytic compartments involved in transcytosis of polymeric immunoglobulin A and endocytosis of asialofetuin

The distributions of two endocytosed radiolabelled ligands (polymeric immunoglobulin A and asialofetuin) in rat liver endocytic compartments were investigated by using rapid subcellular fractionation of post-mitochondrial supernatants on vertical density gradients of Ficoll or Nycodenz. Two endocytic compartments were identified, both ligands being initially associated with a light endocytic-vesicle fraction on Ficoll gradients, asialofetuin then accumulating in denser endosomes before transfer to lysosomes for degradation.

Intracellular transport of asialoglycoproteins in rat hepatocytes. Evidence for two subpopulations of lysosomes

The intracellular transport and degradation of asialoorosomucoid (AOM) in isolated rat hepatocytes was studied by means of subcellular fractionation in Nycodenz gradients. The asialoglycoprotein was labelled by covalent attachment of a radioiodinated tyramine-cellobiose adduct ( [125I]TC) which leads to labelled degradation products being trapped intracellularly and thus serving as markers for the degradative organelles. The ligand was initially (1 min) in a slowly sedimenting (small) vesicle and subsequently in larger endosomes. Acid-soluble, radioactive degradation products were first found in a relatively light lysosome whose distribution coincided in the gradient with that of the larger endosome. Later (30 min) degradation products were found in denser lysosomes which banded in the same region of the gradient as the lysosomal enzyme, beta-acetylglucosaminidase. Colchicine, monensin and leupeptin all inhibited degradation of [125I]tyramine-cellobiose asialoorosomucoid ( [125I]TC-AOM) and reduced the formation of degradation products in both the light and the dense lysosomes. In presence of monensin and colchicine no undegraded ligand was seen in the dense lysosome, suggesting that uptake in these vesicles was inhibited. Leupeptin allowed accumulation of undegraded ligand in the dense lysosome. Therefore, transfer from light to dense lysosomes is not dependent on degradation as such. In the presence of monensin two peaks of undegraded ligand were found in the gradients. It seems possible that in the monensin-sensitive endosomes, dissociation of the ligand-receptor complex is inhibited, allowing ligand to recycle with the receptors in small vesicles.

Subfractionation of hepatic endosomes in Nycodenz gradients and by free-flow electrophoresis. Separation of ligand-transporting and receptor-enriched membranes

The complexity of rat liver endosome fractions containing internalized radioiodinated asialotransferrin, asialo-(alkaline phosphatase), insulin and prolactin was investigated by using free-flow electrophoresis and isopycnic centrifugation in Nycodenz gradients. Two subfractions were separated by free-flow electrophoresis. Both subfractions contained receptors for asialoglycoprotein and insulin. Glycosyltransferase activities were associated with the more electronegative vesicles, whereas 5'-nucleotidase and alkaline phosphodiesterase activities were associated with the less electronegative vesicles. Three subfractions were separated on Nycodenz gradients. Two subfractions, previously shown to become acidified in vitro, contained the ligands. At short intervals after uptake (1-2 min), ligands were mainly in subfraction DN-2 (density 1.115 g/cm3), but movement into subfraction DN-1 (density 1.090 g/cm3) had occurred 10-15 min after internalization. Low amounts of glycosyltransferase activities were associated with subfraction DN-2, and 5'-nucleotidase and alkaline phosphodiesterase activities were mainly located in subfraction DN-1. The binding sites for asialoglycoproteins and insulin were distributed towards the higher density range in the Nycodenz gradients, thus indicating a segregation of receptor-enriched vesicles and those vesicles containing the various ligands 10-15 min after internalization. Electron microscopy of the subfractions separated on Nycodenz gradients indicated that whereas the ligand-transporting fractions consisted mainly of empty vesicles (average diameter 100-150 nm), the receptor-enriched component was more granular and smaller (average diameter 70-95 nm). The properties of the endosome subfraction are used to assign their origin to the regions of the endocytic compartment where ligand-receptor dissociation and separation occur.