Reversible pinocytosis of horseradish peroxidase in lymphoid cells

Kinetics of antigen cross-presentation assessed experimentally and by a model of the complete endomembrane system

Dendritic cells (DCs) have a specialized endomembrane system capable of presenting exogenous antigens in the context of MHC class I (MHC-I) molecules. This process, named cross-presentation, is crucial to activate CD8+ T lymphocytes and initiate cytotoxic immune responses. In this report, we present an Agent-Based Model in combination with Ordinary Differential Equations with enough complexity to reproduce cross-presentation. The model embraces the secretory and endocytic pathways, in connection with the plasma membrane, the endoplasmic reticulum, and the cytosol. Key molecules required for cross-presentation were included as cargoes. In the simulations, the kinetics of MHC-I uptake and recycling, and cross-presentation accurately reproduced experimental values. The model proved to be a suitable tool to elaborate hypotheses and design experiments. In particular, the model predictions and the experimental results obtained indicate that the rate-limiting step in cross-presentation of soluble ovalbumin is MHC-I loading after proteasomal processing of the antigenic protein.

Dynamics of endocytosis and exocytosis of poly(D,L-lactide-co-glycolide) nanoparticles in vascular smooth muscle cells

PURPOSE

The purpose of this work was to characterize the process of endocytosis, exocytosis, and intracellular retention of poly (D,L-lactide-co-glycolide) nanoparticles in vitro using human arterial vascular smooth muscle cells (VSMCs).

METHODS

Nanoparticles containing bovine serum albumin (BSA) as a model protein and 6-coumarin as a fluorescent marker were formulated by a double emulsion-solvent evaporation technique. The endocytosis and exocytosis of nanoparticles in VSMCs were studied using confocal microscopy and their intracellular uptake and retention were determined quantitatively using high-performance liquid chromatography.

RESULTS

Cellular uptake of nanoparticles (mean particle size 97 +/- 3 nm) was a concentration-, time-, and energy-dependent endocytic process. Confocal microscopy demonstrated that nanoparticles were internalized rapidly, with nanoparticles seen inside the cells as early as within 1 min after incubation. The nanoparticle uptake increased with incubation time in the presence of nanoparticles in the medium; however, once the extracellular nanoparticle concentration gradient was removed, exocytosis of nanoparticles occurred with about 65% of the internalized fraction undergoing exocytosis in 30 min. Exocytosis of nanoparticles was slower than the exocytosis of a fluid phase marker, Lucifer yellow. Furthermore, the exocytosis of nanoparticles was reduced after the treatment of cells with the combination of sodium azide and deoxyglucose, suggesting that exocytosis of nanopartides is an energy-dependent process. The nanoparticle retention increased with increasing nanoparticle dose in the medium but the effect was relatively less significant with the increase in incubation time. Interestingly, the exocytosis of nanoparticles was almost completely inhibited when the medium was depleted of serum. Further studies suggest that the addition of BSA in the serum free medium with or without platelet derived growth factor (PDGF) induced exocytosis of nanoparticles. The above result suggests that the protein in the medium is either adsorbed onto nanoparticles and/or carried along with nanoparticles inside the cells, which probably interacts with the exocytic pathway and leads to greater exocytosis of nanoparticles.

CONCLUSIONS

The study demonstrated that endocytosis and exocytosis of nanoparticles are dynamic and energy-dependent processes. Better understanding of the mechanisms of endocytosis and exocytosis, studies determining the effect of nanoparticle formulation and composition that may affect both the processes, and characterization of intracellular distribution of nanoparticles with surface modifications would be useful in exploring nanoparticles for intracellular delivery of therapeutic agents.

Direct pathway from early/recycling endosomes to the Golgi apparatus revealed through the study of shiga toxin B-fragment transport

Shiga toxin and other toxins of this family can escape the endocytic pathway and reach the Golgi apparatus. To synchronize endosome to Golgi transport, Shiga toxin B-fragment was internalized into HeLa cells at low temperatures. Under these conditions, the protein partitioned away from markers destined for the late endocytic pathway and colocalized extensively with cointernalized transferrin. Upon subsequent incubation at 37 degreesC, ultrastructural studies on cryosections failed to detect B-fragment-specific label in multivesicular or multilamellar late endosomes, suggesting that the protein bypassed the late endocytic pathway on its way to the Golgi apparatus. This hypothesis was further supported by the rapid kinetics of B-fragment transport, as determined by quantitative confocal microscopy on living cells and by B-fragment sulfation analysis, and by the observation that actin- depolymerizing and pH-neutralizing drugs that modulate vesicular transport in the late endocytic pathway had no effect on B-fragment accumulation in the Golgi apparatus. B-fragment sorting at the level of early/recycling endosomes seemed to involve vesicular coats, since brefeldin A treatment led to B-fragment accumulation in transferrin receptor-containing membrane tubules, and since B-fragment colocalized with adaptor protein type 1 clathrin coat components on early/recycling endosomes. Thus, we hypothesize that Shiga toxin B-fragment is transported directly from early/recycling endosomes to the Golgi apparatus. This pathway may also be used by cellular proteins, as deduced from our finding that TGN38 colocalized with the B-fragment on its transport from the plasma membrane to the TGN.

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.

Trafficking of interleukin 2 and transferrin in endosomal fractions of T lymphocytes

The T lymphocyte growth factor interleukin 2 binds to surface high-affinity receptors and is rapidly internalized and degraded in acidic organelles. The alpha and beta chains of high-affinity interleukin 2 receptors are internalized together with interleukin 2. To identify the intracellular pathway followed by interleukin 2, we have compared the subcellular distribution of interleukin 2, transferrin and a fluid-phase marker, horseradish peroxidase, in the human T cell line IARC 301.5. Transferrin was used as a marker of early and recycling endosomes, and horseradish peroxidase to probe for the whole endocytic pathway. Fractionation of intracellular organelles on a discontinuous sucrose gradient showed that internalized interleukin 2 is initially mostly found in compartments with similar densities to transferrin, e.g. early and recycling endosomes. The kinetics of entry and exit of interleukin 2 from such organelles was much slower than that of transferrin. Later on, interleukin 2 is predominantly found in dense lysosome-containing fractions. Very little, if any, interleukin 2 was found in fractions corresponding to late endosomes containing horseradish peroxidase. These results suggest that, after endocytosis, interleukin 2 enters early or recycling endosomes before it reaches dense lysosomes.

Antigen processing and presentation: how can a foreign antigen be recognized in a sea of self proteins?

A mathematical model describing the time-dependent events of antigen processing and presentation is utilized to quantitatively analyze the importance of newly synthesized Ia molecules as well as Ia molecules internalized from the cell surface in the formation of Ia-antigen complexes, the T cell receptor ligand. It has recently been shown that antigen presenting cells are not selective for the proteins they process and present. Therefore, we also investigate the ability of macrophages and B cells to process and present antigen in the presence of competing proteins often present in the extracellular environment. A set of criteria is formulated based upon experimental data to determine the validity of two model variations. We draw two major conclusions from our simulations. First, we determine that macrophages and B cells can present between 1-3 Ia-antigen complexes micron-2 for antigen concentrations in the range of 4-7 microM while in the presence of approximately 0-10 microM competing proteins or peptides. Second, we find it likely that antigen presenting cells, both B cells and macrophages, need to internalize Ia molecules from the cell surface in order for a sufficient number of Ia-antigen complexes to be presented. Binding of antigen to newly synthesized Ia alone does not, given experimentally reported values for Ia synthesis, allow sufficient Ia-antigen complex formation.

A multicompartmental model of fluid-phase endocytosis in rabbit liver parenchymal cells

Fluid-phase endocytosis was studied in isolated rabbit liver parenchymal cells by using 125I-poly(vinylpyrrolidone) (PVP) as a marker. First, uptake of 125I-PVP by cells was determined. Also, cells were loaded with 125I-PVP for 20, 60 and 120 min, and release of marker was monitored for 120-220 min. Then we used the Simulation, Analysis and Modeling (SAAM) computer program and the technique of model-based compartmental analysis to develop a mechanistic model for fluid-phase endocytosis in these cells. To fit all data simultaneously, a model with three cellular compartments and one extracellular compartment was required. The three kinetically distinct cellular compartments are interpreted to represent (1) early endosomes, (2) a prelysosomal compartment equivalent to the compartment for uncoupling of receptor and ligand (CURL) and/or multivesicular bodies (MVB), and (3) lysosomes. The model predicts that approx. 80% of the internalized 125I-PVP was recycled to the medium from the early-endosome compartment. The apparent first-order rate constant for this recycling was 0.094 min-1, thus indicating that an average 125I-PVP molecule is recycled in 11 min. The model also predicts that recycling to the medium occurs from all three intracellular compartments. From the prelysosomal compartment, 40% of the 125I-PVP molecules are predicted to recycle to the medium and 60% are transferred to the lysosomal compartment. The average time for recycling from the prelysosomal compartment to the medium was estimated to be 66 min. For 125I-PVP in the lysosomal compartment, 0.3%/min was transferred back to the medium. These results, and the model developed to interpret the data, predict that there is extensive recycling of material endocytosed by fluid-phase endocytosis to the extracellular environment in rabbit liver parenchymal cells.

Reassessment of fluid-phase endocytosis and diacytosis in monolayer cultures of human fibroblasts

We have investigated the kinetics of fluid-phase endocytosis and diacytosis in confluent monolayers of human fibroblasts by comparing the behavior of three markers that have been previously used to study this process: [14C]sucrose, 125I-labeled polyvinylpyrrolidone ([125I]PVP), and Lucifer Yellow. Three distinct kinetic compartments were observed with all markers. The first was relatively large (10-60 fl/cell), reached steady state within 15 min at 37 degrees C, and was rapidly lost from monolayers after removing the markers at 37 degrees C but not at 0 degree C. These properties indicate that this compartment is the same as that previously proposed to be the major intracellular compartment involved in diacytosis. However, this compartment is probably extracellular fluid trapped between cells since it is rapidly lost into the medium when the cells are either scraped or enzymatically removed from the culture dishes at 0 degree C. In addition, it very slowly undergoes both filling and emptying at 0 degree C. However, we did observe a second, much smaller, kinetic compartment (approximately 2 fl/cell) undergoing rapid diacytosis that does seem to be intracellular. A third compartment that we observed accumulates markers at a linear rate (10-20 fl cell-1 hr-1) and is not lost from cells even after incubation periods greater than 6 hr. The markers [14C]sucrose and [125I]PVP displayed very similar behavior with respect to all three compartments and yielded nearly linear long-term uptake rates, thus indicating that there is little if any absorbed component in their uptake. However, Lucifer Yellow displayed significantly higher incorporation rates and its uptake rate was strongly nonlinear, indicating its uptake in fibroblasts is predominantly adsorptive. Our observations indicate that the rate of fluid-phase endocytosis in fibroblasts is significantly less than previously reported and that any compartment involved in diacytosis is very small and turns over very rapidly. Significantly, we estimate that the constitutive internalization of clathrin-coated pits is sufficient to account for the majority of fluid-phase endocytosis and thus represents a major mechanism of membrane retrieval in these cells.

Quantification of endocytosis-derived membrane traffic

The main data covered by this article have been summarized in Table I. A fairly uniform picture is obtained for endocytosis-derived membrane transfer and compartmentation. This may be due to the limited amount of information and the resulting low resolution. Data on mainly three cell types are presented: macrophages, fibroblasts and amoebae. The data vary as much for one cell type as between different cells. Therefore, no possible differences related to cell function emerge. More detailed data, for more cell types, may change the picture. The values for cell surface area, although significantly different in absolute terms (column S in Table I), are rather similar when related to cell diameter, all being about 3-fold in excess of the surface area of the smooth sphere of comparable volume (column xi in Table I). The rate of plasma membrane internalization for macrophages and amoebae both professional phagocytes, is about 2 cell surface area equivalents per h or more. This may be somewhat higher than for fibroblasts (column PM/h in Table I). The average residence time for membrane on the cell surface, therefore, is about 30 min. A most interesting finding seems to be the rather uniform values obtained for the average size (volume weighted) of primary pinosomes, being about 0.3 micron in diameter (column phi-Internalization in Table I). Due to their rapid increase in size as a result of fusion (cf. Fig. 2), it has not been feasible to directly measure the size of primary pinosomes by morphometric means. The values in Table I, give no information on the size distributions of primary pinosomes and on whether these consist of one or more size classes. The steady-state average diameter of pinosomes is noticeably larger than that of primary pinosomes (column phi-pinosomes in Table I; cf. Table II for Acanthamoebae). The corresponding decrease in surface-to-volume ratio can make about 50% of pinosomal membrane available for recycling directly from this membrane compartment. Membrane recycling from the pinosomal compartment occurs after an average residence time of about 3 min for macrophages and 4-6 min for fibroblasts (column tau-pinosomes in Table I). The relative pool size of intracellular membranes participating in shuttling to and from the cell surface is significantly different for animal cells and amoebae (column rho in Table I). For macrophages, fibroblasts, CHO cells, and mast cells, this intracellular membrane pool amounts to about 10-20% the plasma membrane area, compared to 150-200% in the case of amoebae.(ABSTRACT TRUNCATED AT 400 WORDS)

Endocytosis of cationized ferritin in human peripheral blood by resting T-lymphocytes

We have examined the binding and internalization of cationized ferritin in T-lymphocytes of human peripheral blood, as a model for resting cells. After 30 min of incubation only 8% of endocytotic vesicles contain cationized ferritin. T-cells internalize the equivalent of their entire surface area in approximately 54 h, a longer time than is required by non-resting cells such as PHA-stimulated human lymphocytes. These tracer experiments suggest that the endocytosis of cationized ferritin by T-lymphocytes follows a lysosome pathway similar to that described for other cell types.

Assembled and unassembled pools of clathrin: a quantitative study using an enzyme immunoassay

Using polyclonal antibodies raised against clathrin, we have developed an enzyme-linked immunoassay that can specifically measure the quantity of clathrin in crude cell extracts. We found that the quantity (weight percent of total protein) of clathrin was similar in cell types that exhibit large differences in their levels of endocytosis and exocytosis (lymphoid cells, 0.11%; liver cells, 0.07%, fibroblasts, 0.18%; myeloma cells, 0.16%). However, the quantity of clathrin was found to be significantly higher in brain cortex (0.75%). Cellular clathrin was separated by high-speed centrifugation into two fractions: an unassembled form present in high-speed supernatants and an assembled form (clathrin coats) present in the pellets. We show that the fraction of clathrin in the unassembled state varies considerably depending on the cell type studied (14% in brain cortex to 70% in lymphocytes). Our data support the view that the amount of clathrin (relative to total cell protein) in eucaryotic cells is not related to the extent of receptor-mediated endocytosis and intracellular membrane traffic. However, the fraction of assembled clathrin seems to be higher in endocytically and/or exocytically active cells.