Endocytosis

Human granulocytic ehrlichiosis agent and Ehrlichia chaffeensis reside in different cytoplasmic compartments in HL-60 cells

The human granulocytic ehrlichiosis (HGE) agent resides and multiplies exclusively in cytoplasmic vacuoles of granulocytes. Double immunofluorescence labeling was used to characterize the nature of the HGE agent replicative inclusions and to compare them with inclusions containing the human monocytic ehrlichia, Ehrlichia chaffeensis, in HL-60 cells. Although both Ehrlichia spp. can coinfect HL-60 cells, they resided in separate inclusions. Inclusions of both Ehrlichia spp. were not labeled with either anti-lysosome-associated membrane protein 1 or anti-CD63. Accumulation of myeloperoxidase-positive granules were seen around HGE agent inclusions but not around E. chaffeensis inclusions. 3-(2, 4-Dinitroanilino)-3'-amino-N-methyldipropylamine and acridine orange were not localized to either inclusion type. Vacuolar-type H+-ATPase was not colocalized with HGE agent inclusions but was weakly colocalized with E. chaffeensis inclusions. E. chaffeensis inclusions were labeled with the transferrin receptor, early endosomal antigen 1, and rab5, but HGE agent inclusions were not. Some HGE agent and E. chaffeensis inclusions colocalized with major histocompatibility complex class I and II antigens. These two inclusions were not labeled for annexins I, II, IV, and VI; alpha-adaptin; clathrin heavy chain; or beta-coatomer protein. Vesicle-associated membrane protein 2 colocalized to both inclusions. The cation-independent mannose 6-phosphate receptor was not colocalized with either inclusion type. Endogenously synthesized sphingomyelin, from C6-NBD-ceramide, was not incorporated into either inclusion type. Brefeldin A did not affect the growth of either Ehrlichia sp. in HL-60 cells. These results suggest that the HGE agent resides in inclusions which are neither early nor late endosomes and does not fuse with lysosomes or Golgi-derived vesicles, while E. chaffeensis resides in an early endosomal compartment which accumulates the transferrin receptor.

Role of signal transduction in internalization of the G protein-coupled receptor for parathyroid hormone (PTH) and PTH-related protein

For G protein-coupled receptors, limited information is available on the role of agonist binding or of the second-messenger products of receptor signaling on receptor endocytosis. We explored this problem using the opossum PTH/PTH-related protein (PTHrP) receptor, a prototypical Class II G protein-coupled receptor, as a model. In one approach, we evaluated the endocytic properties of mutated forms of the opossum PTH/PTHrP receptor that we had previously shown to be impaired in their ability to initiate agonist-induced signaling when expressed in COS-7 cells. A point mutation in the third cytoplasmic loop (K382A) that severely impairs PTH/PTHrP receptor signaling significantly reduced internalization, whereas two mutant receptors that displayed only partial defects in signaling were internalized normally. To explore more directly the role of second-messenger pathways, we used a cleavable biotinylation method to assess endocytosis of the wild-type receptor stably expressed in human embryonic kidney (HEK) 293 cells. A low rate of constitutive internalization was detected (<5% over a 30-min incubation at 37 C); the rate of receptor internalization was enhanced about 10-fold by the receptor agonists PTH(1-34) or PTHrP(1-34), whereas the receptor antagonist PTH(7-34) had no effect. Forskolin treatment produced a minimal increase in constitutive receptor endocytosis, and the protein kinase (PK)-A inhibitor H-89 failed to block agonist-stimulated endocytosis. Similarly, activation of PK-C, by treatment with phorbol 12-myristate 13-acetate, elicited only a minimal increase in constitutive receptor endocytosis; and blockade of the PK-C pathway, by treatment with a bisindolylmaleimide, failed to inhibit agonist-induced receptor endocytosis. Immunofluorescence confocal microscopic studies of PTH/PTHrP receptor internalization confirmed the results using receptor biotinylation. These findings suggest that: 1) agonist binding is required for the efficient endocytosis of the PTH/PTHrP receptor; 2) receptor activation (agonist-induced receptor conformational change) and/or coupling to G proteins plays a critical role in receptor internalization; and 3) activation of PK-A and PK-C is neither necessary nor sufficient for agonist-stimulated receptor internalization.

A Critical Tyrosine Residue in the Cytoplasmic Tail Is Important for CD1d Internalization But Not for Its Basolateral Sorting in MDCK Cells

The CD1 family of polypeptides is divided into two groups, the CD1b and CD1d group. Both groups are involved in stimulation of T cell response. Molecules of the CD1b group can present Ag derived from bacterial cell walls to T cells; the process of Ag acquisition is thought to take place in endosomes. Little is known about Ag presentation by CD1d. We therefore studied the intracellular trafficking of human CD1d in Madin-Darby canine kidney (MDCK) and COS cells. CD1d was found in endosomal compartments after its internalization from the plasma membrane. It is therefore possible that CD1d acquires its yet unidentified exogenous ligand in the same compartments as the MHC class II and CD1b molecules. CD1d contains a tyrosine-based sorting signal in its cytoplasmic tail that is necessary for internalization. Furthermore, the cytoplasmic tail of CD1d also contains a signal for basolateral sorting that is, however, different from the internalization signal.

Genetic analysis of hook, a gene required for endocytic trafficking in drosophila

The Drosophila hook gene encodes a novel component of the endocytic compartment. Previously identified hook alleles, which still expressed truncated Hook proteins, affected the accumulation of internalized transmembrane ligands into multivesicular bodies (MVBs). To determine the hook null phenotype, we isolated nine new hook alleles on the basis of their characteristic hooked-bristle phenotype. At least one of these alleles, hk11, is a complete loss-of-function allele. Flies carrying the hk11 allele are viable and fertile but neither transmembrane ligands nor soluble ligands accumulate in MVBs. This effect on endocytosed ligands can be mimicked by the expression of Hook proteins truncated for the N- and C-terminal domains flanking the central coiled-coil region. The importance of all three domains for Hook function was confirmed by their conservation between two Drosophila and two human Hook proteins.

Mutational analysis demonstrates that ClC-4 and ClC-5 directly mediate plasma membrane currents

ClC-4 and ClC-5, together with ClC-3, form a distinct branch of the CLC chloride channel family. Although ClC-5 was shown to be mainly expressed in endocytotic vesicles, expression of ClC-5 in Xenopus oocytes elicited chloride currents. We now show that ClC-4 also gives rise to strongly outwardly rectifying anion currents when expressed in oocytes. They closely resemble ClC-5 currents with which they share a NO3- > Cl- > Br- > I- conductance sequence that differs from that reported for the highly homologous ClC-3. Both ClC-4 and ClC-5 currents are reduced by lowering extracellular pH. We could measure similar currents after expressing either channel in HEK293 cells. To demonstrate that these currents are directly mediated by the channel proteins, we introduced several point mutations that change channel characteristics. In ClC-5, several point mutations alter the kinetics of activation but leave macroscopic rectification and ion selectivity unchanged. A mutation (N565K) equivalent to a mutation reported to have profound effects on ClC-3 does not have similar effects on ClC-5. Moreover, a mutation at the end of D2 (S168T in ClC-5) changes ion selectivity, and a mutation at the end of D3 (E211A in ClC-5 and E224A in ClC-4) changes voltage dependence and ion selectivity. This shows that ClC-4 and ClC-5 can directly mediate plasma membrane currents.

Clathrin-Coated Pit-Associated Proteins Are Required for Alveolar Macrophage Phagocytosis

During phagocytosis, phagocytic receptors and membrane material must be inserted in the pseudopod membrane as it extends over the phagocytic target. This may require a clathrin-mediated recycling mechanism similar to that postulated for leading edge formation during cell migration. To investigate this possibility, liposomes were used to deliver to intact rat alveolar macrophages (AMs): 1) Abs to clathrin, clathrin adaptor AP-2, and hsc70, and 2) amantadine. Phagocytosis was assayed by fluorometric and colorimetric techniques. Liposome-delivered Abs to clathrin and AP-2 inhibited AM phagocytosis of zymosan-coated, fluorescent liposomes from 16.3 ± 0.3 to 5.8 ± 0.3, and 10.1 ± 0.9 to 4.8 ± 0.2 liposomes/cell (p &lt; 0.01). Similarly, liposome-delivered Ab to clathrin also inhibited AM phagocytosis of IgG-opsonized RBCs from 11.7 ± 1.7 to 3.8 ± 0.7 RBCs/cell (p &lt; 0.01). Amantadine, which blocks the budding of clathrin-coated vesicles, inhibited phagocytosis from 13.8 ± 0.8 to 5.7 ± 0.6 (p &lt; 0.01). Ab blockade of hsc70, which catalyzes clathrin turnover, also inhibited phagocytosis from 9.1 ± 0.5 to 4.3 ± 0.2 (p &lt; 0.01). These findings suggest that clathrin-mediated receptor/membrane recycling is required for phagocytosis.

Regulation of angiotensin II type 1 (AT1) receptor function

The type 1 angiotensin receptor (AT1) mediates the important biological actions of the peptide hormone, angiotensin II (AngII), by activating an array of intracellular signaling pathways. The unique temporal arrangement and duration of AngII-stimulated signals suggests a hierarchy of post-AT1 receptor binding events that permits activation of selective effector pathways. Moreover, it predicts that the coupling of AT1 receptors is tightly regulated, allowing cells to differentiate acute responses from those requiring longer periods of stimulation. Recent studies have concentrated on delineating the molecular processes involved in modulating AT1 receptor activity. In addition to AT1 receptor modification (phosphorylation), trafficking (internalization and degradation) and interaction with regulatory intracellular proteins, other processes may include receptor dimerization, cross-regulation by other receptor systems, and receptor isomerization between activated and non-activated forms. This review focuses on recent advances in this area of research, highlighting directions for future investigation.

Lipid-based systems for the intracellular delivery of genetic drugs

Currently available delivery systems for genetic drugs have limited utility for systemic applications. Cationic liposome/plasmid DNA or oligonucleotide complexes are rapidly cleared from circulation, and the highest levels of activity are observed in 'first pass' organs, such as the lungs, spleen and liver. Engineered viruses can generate an immune response, which compromises transfection resulting from subsequent injections and lack target specificity. A carrier, which can accumulate at sites of diseases such as infections, inflammations and tumours, has to be a small, neutral and highly serum-stable particle, which is not readily recognized by the fixed and free macrophages of the reticuloendothelial system (RES). This review summarizes lipid-based technologies for the delivery of nucleic acid-based drugs and introduces a new class of carrier systems, which solve, at least in part, the conflicting demands of circulation longevity and intracellular delivery. Plasmid DNA and oligonucleotides are entrapped into lipid particles that contain small amounts of a positively charged lipid and are stabilized by the presence of a polythylene glycol (PEG) coating. These carriers protect nucleic acid-based drugs from degradation by nucleases, are on average 70 nm in diameter, achieve long circulation lifetimes and are capable of transfecting cells.

Molecular aspects of the endocytic pathway

Observation of the flow of material along the endocytic pathway has lead to the description of the basic architecture of the pathway and provided insight into the relationship between compartments. Significant advances have been made in the study of endocytic transport steps at the molecular level, of which studies of cargo selection, vesicle budding and membrane fusion events comprise the major part. Progress in this area has been driven by two approaches, yeast genetics and in vitro or cell-free assays, which reconstitute particular transport steps and allow biochemical manipulation. The complex protein machineries that control vesicle budding and fusion are significantly conserved between the secretory and endocytic pathways such that proteins that regulate particular steps are often part of a larger family of proteins which exercise a conserved function at other locations within the cell. Well characterized examples include vesicle coat proteins, rabs (small GTPases) and soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (SNAP) receptors (SNAREs). Intracompartmental pH, lipid composition and cytoskeletal organization have also been identified as important determinants of the orderly flow of material within the endocytic pathway.

Effect of bafilomycin A1 and nocodazole on endocytic transport in HeLa cells: implications for viral uncoating and infection

Bafilomycin A1 (baf), a specific inhibitor of vacuolar proton ATPases, is commonly employed to demonstrate the requirement of low endosomal pH for viral uncoating. However, in certain cell types baf also affects the transport of endocytosed material from early to late endocytic compartments. To characterize the endocytic route in HeLa cells that are frequently used to study early events in viral infection, we used 35S-labeled human rhinovirus serotype 2 (HRV2) together with various fluid-phase markers. These virions are taken up via receptor-mediated endocytosis and undergo a conformational change to C-antigenic particles at a pH of <5.6, resulting in release of the genomic RNA and ultimately in infection (E. Prchla, E. Kuechler, D. Blaas, and R. Fuchs, J. Virol. 68:3713-3723, 1994). As revealed by fluorescence microscopy and subcellular fractionation of microsomes by free-flow electrophoresis (FFE), baf arrests the transport of all markers in early endosomes. In contrast, the microtubule-disrupting agent nocodazole was found to inhibit transport by accumulating marker in endosomal carrier vesicles (ECV), a compartment intermediate between early and late endosomes. Accordingly, lysosomal degradation of HRV2 was suppressed, whereas its conformational change and infectivity remained unaffected by this drug. Analysis of the subcellular distribution of HRV2 and fluid-phase markers in the presence of nocodazole by FFE revealed no difference from the control incubation in the absence of nocodazole. ECV and late endosomes thus have identical electrophoretic mobilities, and intraluminal pHs of <5.6 and allow uncoating of HRV2. As bafilomycin not only dissipates the low endosomal pH but also blocks transport from early to late endosomes in HeLa cells, its inhibitory effect on viral infection could in part also be attributed to trapping of virus in early endosomes which might lack components essential for uncoating. Consequently, inhibition of viral uncoating by bafilomycin cannot be taken to indicate a low pH requirement only.

Identification of phosphorylation sites in the G protein-coupled receptor for parathyroid hormone. Receptor phosphorylation is not required for agonist-induced internalization

In some G protein-coupled receptors (GPCRs), agonist-dependent phosphorylation by specific GPCR kinases (GRKs) is an important mediator of receptor desensitization and endocytosis. Phosphorylation and the subsequent events that it triggers, such as arrestin binding, have been suggested to be regulatory mechanisms for a wide variety of GPCRs. In the present study, we investigated whether agonist-induced phosphorylation of the PTH receptor, a class II GPCR, also regulates receptor internalization. Upon agonist stimulation, the PTH receptor was exclusively phosphorylated on serine residues. Phosphoamino acid analysis of a number of receptor mutants in which individual serine residues had been replaced by threonine identified serine residues in positions 485, 486, and 489 of the cytoplasmic tail as sites of phosphorylation after agonist treatment. When serine residues at positions 483, 485, 486, 489, 495, and 498 were simultaneously replaced by alanine residues, the PTH receptor was no longer phosphorylated either basally or in response to PTH. The substitution of these serine residues by alanine affected neither the number of receptors expressed on the cell surface nor the ability of the receptor to signal via Gs. Overexpression of GRK2, but not GRK3, enhanced PTH-stimulated receptor phosphorylation, and this phosphorylation was abolished by alanine mutagenesis of residues 483, 485, 486, 489, 495, and 498. Thus, phosphorylation of the PTH receptor by the endogenous kinase in HEK-293 cells occurs on the same residues targeted by overexpressed GRK2. Strikingly, the rate and extent of PTH-stimulated internalization of mutated PTH receptors lacking phosphorylation sites were identical to that observed for the wild-type PTH receptor. Moreover, overexpressed GRK2, while enhancing the phosphorylation of the wild-type PTH receptor, had no affect on the rate or extent of receptor internalization in response to PTH. Thus, the agonist-occupied PTH receptor is phosphorylated by a kinase similar or identical to GRK2 in HEK-293 cells, but this phosphorylation is not requisite for efficient receptor endocytosis.

Retrograde transport from the pre-Golgi intermediate compartment and the Golgi complex is affected by the vacuolar H+-ATPase inhibitor bafilomycin A1

The effect of the vacuolar H+-ATPase inhibitor bafilomycin A1 (Baf A1) on the localization of pre-Golgi intermediate compartment (IC) and Golgi marker proteins was used to study the role of acidification in the function of early secretory compartments. Baf A1 inhibited both brefeldin A- and nocodazole-induced retrograde transport of Golgi proteins to the endoplasmic reticulum (ER), whereas anterograde ER-to-Golgi transport remained largely unaffected. Furthermore, p58/ERGIC-53, which normally cycles between the ER, IC, and cis-Golgi, was arrested in pre-Golgi tubules and vacuoles, and the number of p58-positive approximately 80-nm Golgi (coatomer protein I) vesicles was reduced, suggesting that the drug inhibits the retrieval of the protein from post-ER compartments. In parallel, redistribution of beta-coatomer protein from the Golgi to peripheral pre-Golgi structures took place. The small GTPase rab1p was detected in short pre-Golgi tubules in control cells and was efficiently recruited to the tubules accumulating in the presence of Baf A1. In contrast, these tubules showed no enrichment of newly synthesized, anterogradely transported proteins, indicating that they participate in retrograde transport. These results suggest that the pre-Golgi structures contain an active H+-ATPase that regulates retrograde transport at the ER-Golgi boundary. Interestingly, although Baf A1 had distinct effects on peripheral pre-Golgi structures, only more central, p58-containing elements accumulated detectable amounts of 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine (DAMP), a marker for acidic compartments, raising the possibility that the lumenal pH of the pre-Golgi structures gradually changes in parallel with their translocation to the Golgi region.

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.

Bacterial Lipopolysaccharide Can Enter Monocytes Via Two CD14-Dependent Pathways

Host recognition and disposal of LPS, an important Gram-negative bacterial signal molecule, may involve intracellular processes. We have therefore analyzed the initial pathways by which LPS, a natural ligand of glycosylphosphatidylinositol (GPI)-anchored CD14 (CD14-GPI), enters CD14-expressing THP-1 cells and normal human monocytes. Exposure of the cells to hypertonic medium obliterated coated pits and blocked 125I-labeled transferrin internalization, but failed to inhibit CD14-mediated internalization of [3H]LPS monomers or aggregates. Immunogold electron microscope analysis found that CD14-bound LPS moved principally into noncoated structures (mostly tubular invaginations, intracellular tubules, and vacuoles), whereas relatively little moved into coated pits and vesicles. When studied using two-color laser confocal microscopy, internalized Texas Red-LPS and BODIPY-transferrin were found in different locations and failed to overlap completely even after extended incubation. In contrast, in THP-1 cells that expressed CD14 fused to the transmembrane and cytosolic domains of the low-density lipoprotein receptor, a much larger fraction of the cell-associated LPS moved into coated pits and colocalized with intracellular transferrin. These results suggest that CD14 (GPI)-dependent internalization of LPS occurs predominantly via noncoated plasma membrane invaginations that direct LPS into vesicles that are distinct from transferrin-containing early endosomes. A smaller fraction of the LPS enters via coated pits. Aggregation, which greatly increases LPS internalization, accelerates its entry into the nonclathrin-mediated pathway.

Cholesterol distribution in living cells: fluorescence imaging using dehydroergosterol as a fluorescent cholesterol analog

Cholesterol is an important constituent of most mammalian cell membranes and its concentration in various cellular membranes is tightly regulated. Although there is much information about cholesterol distribution and trafficking in cells, it is primarily derived from indirect measurements, and the results obtained using different approaches are often conflicting. A cholesterol analog that faithfully mimics the properties of cholesterol and can be followed in living cells would thus be very useful. In this study, we report the fluorescence imaging of such an analog, dehydroergosterol (DHE), in living cells. DHE differs from cholesterol in having three additional double bonds and an extra methyl group. In model systems, DHE closely mimics the behavior of native cholesterol. Using triple-labeling studies, we show that DHE colocalizes extensively with endocytosed transferrin, an endocytic recycling compartment marker, and with a marker for the trans-Golgi network, Tac-TGN38. This distribution of DHE is qualitatively similar to that observed when cells are labeled with the fluorescent cholesterol-binding polyene antibiotic, filipin, although there are differences in apparent proportions of DHE and filipin that are localized at the plasma membrane. Another cholesterol derivative, 25-NBD-cholesterol, has a structure that is compromised by the presence of a bulky NBD group and does not distribute to the same organelles as DHE or filipin. In addition, we show in this manuscript that kinetic processes can be followed in living cells by monitoring recovery of DHE fluorescence in a photobleached region over time. Our observations provide evidence for the presence of a large intracellular cholesterol pool in the endocytic recycling compartment and the trans-Golgi network that might play important roles in the trafficking of lipids, lipid-anchored proteins, and transmembrane proteins that preferentially partition into cholesterol-enriched membrane domains. In addition, this intracellular cholesterol pool might be involved in the maintenance of cellular cholesterol homeostasis.

Continuity between wound macrophage and fibroblast phenotype: analysis of wound fibroblast phagocytosis

Analysis of phagocytic activity in wound fibroblasts was chosen as a means to assess the possible continuity between macrophage and fibroblast phenotypes. Fibroblast phagocytosis of uncoated, IgG-coated, or collagen-coated fluorescent beads was analyzed by flow cytometry in vivo and in vitro. Phagocytosis of fluorescent beads by procollagen I-positive cells (fibroblasts) was evaluated in vivo by injecting beads into subcutaneously implanted sponge wounds in anesthetized Fisher rats. Phagocytic activity of a purified population of wound fibroblasts was measured in vitro and correlated with oxidation state using hydroethidium. In the wound environment, 50-60% of the cells that engulfed uncoated, IgG-coated, or collagen-coated beads were procollagen I-positive cells (i.e., fibroblasts). Procollagen I-positive cells engulfed uncoated and IgG-coated beads in preference to collagen-coated beads in vivo. Cultured wound fibroblasts engulfed uncoated, IgG-coated, and collagen-coated particles. The majority of fibroblasts that engulfed beads were in an elevated oxidation state. We conclude that substantial fibroblast phagocytosis occurs in the wound, but scavenger receptor-mediated fibroblast phagocytosis is different from that of macrophages. Additional markers will be helpful in defining the macrophage fibroblast continuum.

Analysis of GLUT4 distribution in whole skeletal muscle fibers: identification of distinct storage compartments that are recruited by insulin and muscle contractions

The effects of insulin stimulation and muscle contractions on the subcellular distribution of GLUT4 in skeletal muscle have been studied on a preparation of single whole fibers from the rat soleus. The fibers were labeled for GLUT4 by a preembedding technique and observed as whole mounts by immunofluorescence microscopy, or after sectioning, by immunogold electron microscopy. The advantage of this preparation for cells of the size of muscle fibers is that it provides global views of the staining from one end of a fiber to the other and from one side to the other through the core of the fiber. In addition, the labeling efficiency is much higher than can be obtained with ultracryosections. In nonstimulated fibers, GLUT4 is excluded from the plasma membrane and T tubules. It is distributed throughout the muscle fibers with approximately 23% associated with large structures including multivesicular endosomes located in the TGN region, and 77% with small tubulovesicular structures. The two stimuli cause translocation of GLUT4 to both plasma membrane and T tubules. Quantitation of the immunogold electron microscopy shows that the effects of insulin and contraction are additive and that each stimulus recruits GLUT4 from both large and small depots. Immunofluorescence double labeling for GLUT4 and transferrin receptor (TfR) shows that the small depots can be further subdivided into TfR-positive and TfR-negative elements. Interestingly, we observe that colocalization of TfR and GLUT4 is increased by insulin and decreased by contractions. These results, supported by subcellular fractionation experiments, suggest that TfR-positive depots are only recruited by contractions. We do not find evidence for stimulation-induced unmasking of resident surface membrane GLUT4 transporters or for dilation of the T tubule system (Wang, W., P.A. Hansen, B.A. Marshall, J.O. Holloszy, and M. Mueckler. 1996. J. Cell Biol. 135:415-430).

An endocytosed TGN38 chimeric protein is delivered to the TGN after trafficking through the endocytic recycling compartment in CHO cells

To examine TGN38 trafficking from the cell surface to the TGN, CHO cells were stably transfected with a chimeric transmembrane protein, TacTGN38. We used fluorescent and 125I-labeled anti-Tac IgG and Fab fragments to follow TacTGN38's postendocytic trafficking. At steady-state, anti-Tac was mainly in the TGN, but shortly after endocytosis it was predominantly in early endosomes. 11% of cellular TacTGN38 is on the plasma membrane. Kinetic analysis of trafficking of antibodies bound to TacTGN38 showed that after short endocytic pulses, 80% of internalized anti-Tac returned to the cell surface (t1/2 = 9 min), and the remainder trafficked to the TGN. When longer filling pulses and chases were used to load anti-Tac into the TGN, it returned to the cell surface with a t1/2 of 46 min. Quantitative confocal microscopy analysis also showed that fluorescent anti-Tac fills the TGN with a 46-min t1/2. Using the measured rate constants in a simple kinetic model, we predict that 82% of TacTGN38 is in the TGN, and 7% is in endosomes. TacTGN38 leaves the TGN slowly, which accounts for its steady-state distribution despite the inefficient targeting from the cell surface to the TGN.

Cholesterol-dependent retention of GPI-anchored proteins in endosomes

Several cell surface eukaryotic proteins have a glycosylphosphatidylinositol (GPI) modification at the Cterminal end that serves as their sole means of membrane anchoring. Using fluorescently labeled ligands and digital fluorescence microscopy, we show that contrary to the potocytosis model, GPI-anchored proteins are internalized into endosomes that contain markers for both receptor-mediated uptake (e.g. transferrin) and fluid phase endocytosis (e.g. dextrans). This was confirmed by immunogold electron microscopy and the observation that a fluorescent folate derivative bound to the GPI-anchored folate receptor is internalized into the same compartment as co-internalized horseradish peroxidase-transferrin; the folate fluorescence was quenched when cells subsequently were incubated with diaminobenzidine and H2O2. Most of the GPI-anchored proteins are recycled back to the plasma membrane but at a rate that is at least 3-fold slower than C6-NBD-sphingomyelin or recycling receptors. This endocytic retention is regulated by the level of cholesterol in cell membranes; GPI-anchored proteins are recycled back to the cell surface at the same rate as recycling transferrin receptors and C6-NBD-sphingomyelin in cholesterol-depleted cells. Cholesterol-dependent endocytic sorting of GPI-anchored proteins is consistent with the involvement of specialized lipid domains or 'rafts' in endocytic sorting. These results provide an alternative explanation for GPI-requiring functions of some GPI-anchored proteins.

Stimulated release of fluorescently labeled IgE fragments that efficiently accumulate in secretory granules after endocytosis in RBL-2H3 mast cells

Sensitization of RBL-2H3 mast cells with monomeric fluorescein-5-isothiocyanate (FITC)-labeled immunoglobulin E (IgE) results in slow but highly efficient accumulation of labeled IgE fragments in a pool of acidic peripheral vesicles that are visible by fluorescence microscopy after raising endosomal pH with ammonium chloride. Stimulation of cells containing these FITC-IgE fragments by aggregation of high affinity receptors for IgE (FcepsilonRI) or by Ca2+ ionophore and phorbol 12-myristate 13-acetate results in release of FITC fluorescence from the cells, which can be monitored continuously with a spectrofluorometer. The fluorescence release process corresponds to cellular degranulation: it is prevented under conditions that prevent stimulated beta-hexosaminidase release, and these two processes exhibit the same antigen dose-dependence and kinetics. Pulse-chase labeling reveals that aggregation of FITC-IgE bound to FcepsilonRI at the cell surface causes internalization and delivery to the regulated secretory vesicles with a high efficiency similar to monomeric IgE-FcepsilonRI, but more rapidly. Binding of Cy3-modified IgE to FcepsilonRI results in labeling of the same secretory vesicles as in FITC-IgE-sensitized cells, and these Cy3-labeled vesicles can be observed by fluorescence microscopy without neutralization of intracellular compartments. Simultaneous three-photon microscopy of serotonin fluorescence and two-photon microscopy of Cy3 fluorescence reveals that these Cy3-labeled vesicles coincide with serotonin-labeled secretory granules. After stimulation of the cells via aggregation of IgE-FcepsilonRI or addition of Ca2+ ionophore and phorbol 12-myristate 13-acetate, depletion of the Cy3 label from the intracellular vesicles is observed with confocal microscopy. These results provide strong evidence for the lysosomal nature of secretory granules in these cells. In addition, they provide the basis for a direct, real-time method for monitoring single cell degranulation.

Aspects of the structure and assembly of desmosomes

Desmosomes are found principally in epithelial cells and consist of disc-like plaques, the extracellular face of which is paired with that of a neighbouring cell. There is increasing evidence that desmosomes are adhesive structures, and that two types of desmosomal glycoproteins, the desmogleins (Dsg) and desmocollins (Dsc) both Ca(2+)-binding cadherin-like molecules, perform this role in adhesion through interaction of their extracellular domains. A number of isoforms of Dsg and Dsc are present in specific tissues. The cytoplasmic side of the plaque is attached to intermediate filaments through desmoplakin, a major plaque protein. Also associated with desmosomes are plakoglobin and beta-catenin, suggesting that the adhesive function of desmosomes might be mediated by signal transduction. Formation of desmosomes can be studied by growing epithelial cells in low-Ca2+ medium (LCM, < 0.1 mM), where desmosomal proteins are either synthesized but not assembled, or form partially assembled but unstable half-desmosomes. Addition of Ca2+ (to about 2mM) initiates cell contact and, in the case of half-desmosomes, leads to stabilization by incorporation into membranes and formation of typical paired structures. In cases where such pre-assembled structures are not formed, recruitment of desmosomal proteins appears to occur by vesicular transport of desmocollins and desmogleins to the cell surface, where association is made with plakoglobin and later, with desmoplakin. Although much remains to be learned of the assembly process, specific interacting domains of the molecular components are being recognized. Desmosome assembly is part of a coordinated pattern of junction formation which accompanies the establishment of cell polarity, resulting in differentiation of apical and basolateral cell surfaces. Desmosomes are now being regarded, not as static and inert structures, but as membrane specializations linked to systems involved in cell-cell communication as well as adhesion.

Host resistance to intracellular infection: mutation of natural resistance-associated macrophage protein 1 (Nramp1) impairs phagosomal acidification

The mechanisms underlying the survival of intracellular parasites such as mycobacteria in host macrophages remain poorly understood. In mice, mutations at the Nramp1 gene (for natural resistance-associated macrophage protein), cause susceptibility to mycobacterial infections. Nramp1 encodes an integral membrane protein that is recruited to the phagosome membrane in infected macrophages. In this study, we used microfluorescence ratio imaging of macrophages from wild-type and Nramp1 mutant mice to analyze the effect of loss of Nramp1 function on the properties of phagosomes containing inert particles or live mycobacteria. The pH of phagosomes containing live Mycobacterium bovis was significantly more acidic in Nramp1- expressing macrophages than in mutant cells (pH 5.5 +/- 0.06 versus pH 6.6 +/- 0.05, respectively; P <0.005). The enhanced acidification could not be accounted for by differences in proton consumption during dismutation of superoxide, phagosomal buffering power, counterion conductance, or in the rate of proton "leak", as these were found to be comparable in wild-type and Nramp1-deficient macrophages. Rather, after ingestion of live mycobacteria, Nramp1-expressing cells exhibited increased concanamycin-sensitive H+ pumping across the phagosomal membrane. This was associated with an enhanced ability of phagosomes to fuse with vacuolar-type ATPase-containing late endosomes and/or lysosomes. This effect was restricted to live M. bovis and was not seen in phagosomes containing dead M. bovis or latex beads. These data support the notion that Nramp1 affects intracellular mycobacterial replication by modulating phagosomal pH, suggesting that Nramp1 plays a central role in this process.

Mutation of individual serine residues in the C-terminal tail of the lutropin/choriogonadotropin receptor reveal distinct structural requirements for agonist-induced uncoupling and agonist-induced internalization

We have previously mapped the agonist-induced phosphorylation of the rat lutropin/choriogonadotropin receptor (rLHR) to a locus of four serines (Ser635, Ser639, Ser649, and Ser652) located in the C-terminal tail. The removal or mutation of this locus delays the time course of agonist-induced uncoupling of the rLHR from its effector system without affecting the overall magnitude of uncoupling, and it retards the endocytosis of the agonist-receptor complex. We have now prepared and analyzed four new rLHR mutants in which each of these serines were individually mutated to alanines. The data presented show that each mutation reduces agonist-promoted rLHR phosphorylation by 20-40%. Mutation of Ser635 or Ser639 delayed the time course of agonist-induced uncoupling to about the same extent as the simultaneous mutation of all four serines. Mutation of Ser635 or Ser639 also retarded agonist-induced internalization, but the magnitude of this decrease was less than that induced by the simultaneous mutation of all four serines. Mutation of Ser649 had no effect on agonist-induced uncoupling but retarded agonist-induced internalization to the same extent as the simultaneous mutation of all four serines. Mutation of Ser652 has little or no effect on either of these two parameters. Co-transfection studies with dominant-negative arrestins and dominant-negative dynamin reveal that, despite differences in their rates of internalization, rLHR-wild-type, rLHR-S639A, and rLHR-S649A are internalized by an arrestin- and dynamin-dependent pathway. These data show that the structural requirements needed for the agonist-induced uncoupling and internalization of the rLHR are distinct.

The cellular trafficking and zinc dependence of secretory and lysosomal sphingomyelinase, two products of the acid sphingomyelinase gene

The acid sphingomyelinase (ASM) gene, which has been implicated in ceramide-mediated cell signaling and atherogenesis, gives rise to both lysosomal SMase (L-SMase), which is reportedly cation-independent, and secretory SMase (S-SMase), which is fully or partially dependent on Zn2+ for enzymatic activity. Herein we present evidence for a model to explain how a single mRNA gives rise to two forms of SMase with different cellular trafficking and apparent differences in Zn2+ dependence. First, we show that both S-SMase and L-SMase, which contain several highly conserved zinc-binding motifs, are directly activated by zinc. In addition, SMase assayed from a lysosome-rich fraction of Chinese hamster ovary cells was found to be partially zinc-dependent, suggesting that intact lysosomes from these cells contain subsaturating levels of Zn2+. Analysis of Asn-linked oligosaccharides and of N-terminal amino acid sequence indicated that S-SMase arises by trafficking through the Golgi secretory pathway, not by cellular release of L-SMase during trafficking to lysosomes or after delivery to lysosomes. Most importantly, when Zn2+-dependent S-SMase was incubated with SMase-negative cells, the enzyme was internalized, trafficked to lysosomes, and became zinc-independent. We conclude that L-SMase is exposed to cellular Zn2+ during trafficking to lysosomes, in lysosomes, and/or during cell homogenization. In contrast, the pathway targeting S-SMase to secretion appears to be relatively sequestered from cellular pools of Zn2+; thus S-SMase requires exogeneous Zn2+ for full activity. This model provides important information for understanding the enzymology and regulation of L- and S-SMase and for exploring possible roles of ASM gene products in cell signaling and atherogenesis.

Size of IgG-opsonized particles determines macrophage response during internalization

It is generally assumed that particles > 1 micron elicit a phagocytic response. To determine whether this is the case, we examined the uptake and transport of IgG-opsonized polystyrene beads of defined size, ranging from 0.2 to 3 microns, by mouse bone marrow-derived macrophages. The kinetics of opsonized bead internalization were comparable for each of the different beads examined. We used rhodamine phalloidin to examine particle-induced assembly of F-actin phagocytic cups by fluorescence microscopy. Phagocytic cup formation was size dependent in a nonlinear fashion. Less than 30% of 0.2- to 0.75-micron particles and greater than 80% of 2- and 3-micron particles were associated with F-actin. Cells treated with 0.25 micron cytochalasin D showed decreased phagocytic cup formation and a linear decrease in bead uptake as a function of particle surface area. In contrast, potassium depletion, which preferentially inhibits clathrin-mediated endocytosis, was more effective at inhibiting the uptake of smaller beads. Thus, with increasing particle size, IgG-opsonized particle uptake became less clathrin dependent and more actin dependent. The kinetics of ligand delivery to lysosomes was measured using an immunoprecipitation assay based on the intermixing of internalized anti-dinitrophenol (DNP) IgG with DNP-derivitized beta-glucuronidase (DNP-beta-glu) incorporated into lysosomes. Soluble mannosylated anti-DNP IgG was delivered to lysosomes after an 8-min lag period. The kinetics of anti-DNP IgG-opsonized beads showed a size-dependent response, where beads sized 0.2, 0.5, and 0.75 micron showed a lag period prior to delivery to lysosomes. In contrast, beads 1.0 micron or larger showed no lag in delivery to lysosomes. Since beads that had no lag in delivery to lysosomes also showed high levels of phagocytic cup formation, this suggests that phagocytic cups may be important in the rapid delivery of internalized particles to lysosomes.

Inhibition of clathrin-mediated endocytosis selectively attenuates specific insulin receptor signal transduction pathways

To examine the role of clathrin-dependent insulin receptor internalization in insulin-stimulated signal transduction events, we expressed a dominant-interfering mutant of dynamin (K44A/dynamin) by using a recombinant adenovirus in the H4IIE hepatoma and 3T3L1 adipocyte cell lines. Expression of K44A/dynamin inhibited endocytosis of the insulin receptor as determined by both cell surface radioligand binding and trypsin protection analysis. The inhibition of the insulin receptor endocytosis had no effect on either the extent of insulin receptor autophosphorylation or insulin receptor substrate 1 (IRS1) tyrosine phosphorylation. In contrast, expression of K44A/dynamin partially inhibited insulin-stimulated Shc tyrosine phosphorylation and activation of the mitogen-activated protein kinases ERK1 and -2. Although there was an approximately 50% decrease in the insulin-stimulated activation of the phosphatidylinositol 3-kinase associated with IRS1, insulin-stimulated Akt kinase phosphorylation and activation were unaffected. The expression of K44A/dynamin increased the basal rate of amino acid transport, which was additive with the effect of insulin but had no effect on the basal or insulin-stimulated DNA synthesis. In 3T3L1 adipocytes, expression of K44A/dynamin increased the basal rate of glucose uptake, glycogen synthesis, and lipogenesis without any significant effect on insulin stimulation. Together, these data demonstrate that the acute actions of insulin are largely independent of insulin receptor endocytosis and are initiated by activation of the plasma membrane-localized insulin receptor.

Role of envelope protein gE endocytosis in the pseudorabies virus life cycle

Several groups have reported that certain herpesvirus envelope proteins do not remain on the surface of cells that express them but rather are internalized by endocytosis in a recycling process. The biological function of membrane protein endocytosis in the virus life cycle remains a matter of speculation and debate. In this report, we demonstrate that some, but not all, membrane proteins encoded by the alphaherpesvirus pseudorabies virus (PRV) are internalized after reaching the plasma membrane. Glycoproteins gE and gB are internalized from the plasma membrane of cells, while gI and gC are not internalized efficiently. We show for gE that the cytoplasmic domain of the protein is required for endocytosis. While the gI protein is incapable of endocytosis on its own, it can be internalized when complexed with gE. We demonstrate that endocytosis of the gE-gI complex and gB occurs early after infection of tissue culture cells but that this process stops completely after 6 h of infection, a time that correlates with significant shutoff of host protein synthesis. We also show that gE protein internalized at 4 h postinfection is not present in virions formed at a later time. We discuss the differences in PRV gE and gI endocytosis compared to that of the varicella-zoster virus homologs and the possible roles of glycoprotein endocytosis in the virus life cycle.

Recent advances in intestinal macromolecular drug delivery via receptor-mediated transport pathways

Receptor-mediated transport mechanisms provide a pathway for the trafficking of extracellular macromolecules into (endocytosis) and across (transcytosis) the cell. This comprises the binding of a ligand to a specific cell-surface receptor, clustering of the ligand-receptor complexes in endocytotic vesicles and vesicular sorting. This review focuses on recent advances in cellular and molecular biology pertaining to receptor-mediated endocytosis. A concise overview is presented of current and potential future applications of targeting to RME mechanisms to improve oral macromolecular drug delivery.

Hydrolysis of GTP on rab11 is required for the direct delivery of transferrin from the pericentriolar recycling compartment to the cell surface but not from sorting endosomes

Rab11 is a small GTP-binding protein that in cultured mammalian cells has been shown to be concentrated in the pericentriolar endosomal recycling compartment and to play a key role in passage of the recycling transferrin receptor through that compartment [Ullrich, O., Reinsch, S., Urbé, S., Zerial, M. & Parton, R. G. (1996) J. Cell Biol. 135, 913-924]. To obtain insights into the site(s) of action of rab11 within the recycling pathway, we have now compared the effects on recycling at 37 degreesC of overexpression of wild-type rab11 and various mutant forms of this protein in cells that had been loaded with transferrin at either 37 degreesC or 16 degreesC. We show that incubation at 16 degreesC blocks passage of endocytosed transferrin into the recycling compartment and that, whereas the rab11 dominant negative mutant form (S25N) inhibits transferrin recycling after interiorization at either temperature, the wild-type rab11 and constitutively active mutant (Q70L) have no inhibitory effect on the recycling of molecules that were interiorized at 16 degreesC. This differential inhibitory effect shows that two distinct pathways for recycling are followed by the bulk of the transferrin molecules interiorized at the two different temperatures. The incapacity of the constitutively active form of rab11 (Q70L) to inhibit recycling of molecules interiorized at 16 degreesC is consistent with their recycling taking place directly from sorting endosomes, in a process that does not require hydrolysis of GTP on rab11. The fact that the dominant negative (S25N) form of rab11 inhibits recycling of molecules interiorized at both temperatures indicates that activation of rab11 by GTP is required for exit of transferrin from sorting endosomes, regardless of whether this exit is toward the recycling compartment or directly to the plasma membrane.

Identification of a cytoplasmic motif in the erythropoietin receptor required for receptor internalization

Erythropoietin (EPO) promotes the viability, proliferation and differentiation of mammalian erythroid progenitor cells via its specific cell surface receptor. The EPO receptor (EPO-R) is a member of the cytokine receptor superfamily and is comprised of one identified subunit which homodimerizes upon ligand binding. To study the role of the intracellular domain of the EPO-R in the endocytosis of EPO, we compared the rate and extent of 125I-EPO endocytosis by wild type (wt) EPO-R and five cytoplasmically truncated EPO-Rs: 1-251 EPO-R, 1-257 EPO-R, 1-267 EPO-R, 1-276 EPO-R and 1-306 EPO-R which contain 4, 10, 20, 29 or 59 amino acids of the cytoplasmic domain, respectively. We also studied an EPO-R mutant (PB) which lacks amino acid residues 281-300 of the cytoplasmic domain. The experiments were conducted in COS 7 cells transfected with the EPO-R cDNAs and in Ba/F3 cells stably expressing the wt EPO-R, 1-251 or 1-257 EPO-R. Cells expressing wt EPO-R, PB EPO-R (delta281-300), 1-276 EPO-R or 1-306 EPO-R internalized approximately 50% of 125I-EPO bound to the cell surface, while cells expressing 1-251, 1-257 or 1-267 EPO-R internalized only 25% of the bound 125I-EPO. The steady-state expression levels of these latter receptors on the cell surface were typically 2-5-fold higher than wt EPO-R. Our data indicate that amino acid residues 267-276 (FEGLFTTHK) of the EPO-R cytoplasmic domain may have a role in receptor internalization. Metabolic labeling experiments suggest that in transiently transfected COS 7 cells most of the wt EPO-R and 1-257 EPO-Rs do not exit the ER and may be degraded there. The half-life of both receptors was essentially similar and was in the range of 1 h. In Ba/F3 cells the mature Golgi processed 1-257 EPO-R was more stable than the corresponding form of the wt EPO-R, possibly contributing to its higher cell surface expression.

Adenovirus internalization and infection require dynamin

The cell receptors that facilitate adenovirus internalization into cells have been identified; however, the infectious pathway of virus entry has not been established. Adenovirus entry and infection were examined in HeLa cells lacking or overexpressing mutant dynamin, a protein that specifically regulates clathrin-mediated endocytosis. Expression of mutant dynamin significantly reduced adenovirus internalization and gene delivery, indicating a functional requirement for this molecule. These findings are consistent with virus entry via the clathrin-coated pit pathway.

Sphingosine-1-phosphate as a ligand for the G protein-coupled receptor EDG-1

The sphingolipid metabolite sphingosine-1-phosphate (SPP) has been implicated as a second messenger in cell proliferation and survival. However, many of its biological effects are due to binding to unidentified receptors on the cell surface. SPP activated the heterotrimeric guanine nucleotide binding protein (G protein)-coupled orphan receptor EDG-1, originally cloned as Endothelial Differentiation Gene-1. EDG-1 bound SPP with high affinity (dissociation constant = 8.1 nM) and high specificity. Overexpression of EDG-1 induced exaggerated cell-cell aggregation, enhanced expression of cadherins, and formation of well-developed adherens junctions in a manner dependent on SPP and the small guanine nucleotide binding protein Rho.

Fluorescent virions: dynamic tracking of the pathway of adenoviral gene transfer vectors in living cells

The pathogenic agent, adenovirus (Ad), has taken on a new role as a vector for gene transfer in both laboratory and clinical settings. To help understand the intracellular pathways and fate of Ad gene transfer vectors, we covalently conjugated fluorophores to E1-, E3- Ad vectors and used quantitative fluorescence microscopy to assess essential steps of Ad vector gene transfer to the A549 human epithelial lung cell line including binding, internalization, escape from endosomes, translocation to the nucleus, dissociation of capsids and gene expression. The data demonstrate that Ad internalizes with a t1/2 2.5 min, breaks out of endosomes early, likely prior to endosome-endosome fusion, exhibits sustained, intracellular velocities averaging 0.58 microm/sec, and translocates to the nucleus with >80% of internalized fluorophore demonstrating nuclear localization within 60 min of infection. Interestingly, 24 hr after infection, half of the initially internalized fluorescence was detected but lacked nuclear localization, suggesting that the capsid is released from the nucleus and is likely degraded. Fluorescent labeling of virions provides a novel quantitative, morphological strategy to characterize the interaction of gene transfer vectors with the intracellular environment.

Sphingomyelinase treatment induces ATP-independent endocytosis

ATP hydrolysis has been regarded as a general requirement for internalization processes in mammalian cells. We found, however, that treatment of ATP-depleted macrophages and fibroblasts with exogenous sphingomyelinase (SMase) rapidly induces formation of numerous vesicles that pinch off from the plasma membrane; the process is complete within 10 min after adding SMase. By electron microscopy, the SMase-induced vesicles are approximately 400 nm in diameter and lack discernible coats. 15-30% of plasma membrane is internalized by SMase treatment, and there is no detectable enrichment of either clathrin or caveolin in these vesicles. When ATP is restored to the cells, the SMase-induced vesicles are able to deliver fluid-phase markers to late endosomes/lysosomes and return recycling receptors, such as transferrin receptors, back to the plasma membrane. We speculate that hydrolysis of sphingomyelin on the plasma membrane causes inward curvature and subsequent fusion to form sealed vesicles. Many cell types express a SMase that can be secreted or delivered to endosomes and lysosomes. The hydrolysis of sphingomyelin by these enzymes is activated by several signaling pathways, and this may lead to formation of vesicles by the process described here.