Brefeldin A causes a microtubule-mediated fusion of the trans-Golgi network and early endosomes

Actin and microtubule regulation ofTrans-Golgi network architecture, and copper-dependent protein transport to the cell surface

The Menkes disease ATPase (MNK) is a copper transporter that localizes to the mammalian trans-Golgi network (TGN) and shows substantial co-localization wih a ubiquitous TGN resident protein and marker, TGN46. We tested our hypothesis that these two TGN residents and integral membrane proteins are localized to biochemically distinct TGN sub-compartments using constitutively active mutant proteins and drugs that disrupt membrane traffic, lumenal pH and the cellular cytoskeleton. The pH-disrupting agent, monensin, causes MNK to be more diffusely distributed with partial separation of staining patterns for these two TGN residents. Expression of a constitutively active Rho-kinase (ROCK-KIN), which causes formation of juxta-nuclear astral actin arrays, also effects separation of MNK and TGN46 staining patterns. Treatment of ROCK-KIN expressing cells with latrunculin B, an actin-depolymerizing agent, causes complete overlap of MNK and TGN46 staining patterns with concomitant disappearance of polymerized actin. When microtubules are depolymerized in ROCK-KIN expressing cells by nocodazole, both MNK and TGN46 are found in puncate structures throughout the cell. However, a substantial proportion of MNK is still found in a juxta-nuclear location in contrast to TGN46. Actin distribution in these cells reveals that juxta-nuclear MNK is distinct to the astral actin clusters in ROCK-KIN expressing cells where the microtubules were depolymerized. The TGN to cell-surface transport of MNK requires both actin and microtubules networks, whilst the constitutive trafficking of proteins is independent of actin. Taken together, our findings indicate that at least two TGN sub-domains are regulated by separate cytoskeletal dynamics involving actin and tubulin.

Synaptic vesicle recycling at CNS snapses without AP-2

Synaptic vesicles (SVs) are composed of approximately 10 types of transmembrane proteins that must be recycled after exocytosis of neurotransmitter. The mechanisms for resorting these proteins into synaptic vesicles once incorporated into the plasma membrane after exocytosis are poorly understood. The adaptor complex AP-2 is the major clathrin-associated adaptor for cargo recognition at the plasma membrane. Here, we have investigated its role in synaptic vesicle endocytosis. shRNA-mediated knockdown of the AP-2 complex results in an approximately 96% reduction of this protein complex in primary neurons. We used simultaneous expression of shRNA and pHluorin-tagged vesicle components to show that the absence of AP-2 significantly slows but does not prevent the endocytosis of four of the major synaptic vesicle transmembrane proteins. We show that in the absence of AP-2, the AP-1 adaptor complex appears to functionally substitute for AP-2 but results in complex internalization kinetics that are now sensitive to the guanine-nucleotide exchange factor for ADP-ribosylation factor GTPase (ARF-GEF) inhibitor brefeldin-A (BFA). Simultaneous removal of both AP-2 and AP-1 prevents this compensatory substitution and results in slowed but functional endocytosis. These results demonstrate that in the absence of AP-2, SV proteins still become endocytosed, and synaptic vesicle recycling remains operational.

A novel small molecule regulator of guanine nucleotide exchange activity of the ADP-ribosylation factor and golgi membrane trafficking

An image-based phenotypic screen was developed to identify small molecule regulators of intracellular traffic. Using this screen we found that AG1478, a previously known inhibitor of epidermal growth factor receptor, had epidermal growth factor receptor-independent activity in inducing the disassembly of the Golgi in human cells. Similar to brefeldin A (BFA), a known disrupter of the Golgi, AG1478 inhibits the activity of small GTPase ADP-ribosylation factor. Unlike BFA, AG1478 exhibits low cytotoxicity and selectively targets the cis-Golgi without affecting endosomal compartment. We show that AG1478 inhibits GBF1, a large nucleotide exchange factor for the ADP-ribosylation factor, in a Sec7 domain-dependent manner and mimics the phenotype of a GBF1 mutant that has an inactive mutation. The treatment with AG1478 leads to the recruitment of GBF1 to the vesicular-tubular clusters adjacent to the endoplasmic reticulum exit sites, a step only transiently observed previously in the presence of BFA. We propose that the treatment with AG1478 delineates a membrane trafficking intermediate step that depends upon the Sec7 domain.

The secretion inhibitor Exo2 perturbs trafficking of Shiga toxin between endosomes and the trans-Golgi network

The small-molecule inhibitor Exo2 {4-hydroxy-3-methoxy-(5,6,7,8-tetrahydrol[1]benzothieno[2,3-d]pyrimidin-4-yl)hydraz-one benzaldehyde} has been reported to disrupt the Golgi apparatus completely and to stimulate Golgi–ER (endoplasmic reticulum) fusion in mammalian cells, akin to the well-characterized fungal toxin BFA (brefeldin A). It has also been reported that Exo2 does not affect the integrity of the TGN (trans-Golgi network), or the direct retrograde trafficking of the glycolipid-binding cholera toxin from the TGN to the ER lumen. We have examined the effects of BFA and Exo2, and found that both compounds are indistinguishable in their inhibition of anterograde transport and that both reagents significantly disrupt the morphology of the TGN in HeLa and in BS-C-1 cells. However, Exo2, unlike BFA, does not induce tubulation and merging of the TGN and endosomal compartments. Furthermore, and in contrast with its effects on cholera toxin, Exo2 significantly perturbs the delivery of Shiga toxin to the ER. Together, these results suggest that the likely target(s) of Exo2 operate at the level of the TGN, the Golgi and a subset of early endosomes, and thus Exo2 provides a more selective tool than BFA for examining membrane trafficking in mammalian cells.

alpha-Adducin mutations increase Na/K pump activity in renal cells by affecting constitutive endocytosis: implications for tubular Na reabsorption

Genetic variation in alpha-adducin cytoskeletal protein is implicated in the polymerization and bundling of actin and alteration of the Na/K pump, resulting in abnormal renal sodium transport and hypertension in Milan hypertensive rats and humans. To investigate the molecular involvement of alpha-adducin in controlling Na/K pump activity, wild-type or mutated rat and human alpha-adducin forms were, respectively, transfected into several renal cell lines. Through multiple experimental approaches (microscopy, enzymatic assays, coimmunoprecipitation), we showed that rat and human mutated forms increased Na/K pump activity and the number of pump units; moreover, both variants coimmunoprecipitate with Na/K pump. The increased Na/K pump activity was not due to changes in its basolateral localization, but to an alteration of Na/K pump residential time on the plasma membrane. Indeed, both rat and human mutated variants reduced constitutive Na/K pump endocytosis and similarly affected transferrin receptor trafficking and fluid-phase endocytosis. In fact, alpha-adducin was detected in clathrin-coated vesicles and coimmunoprecipitated with clathrin. These results indicate that adducin, besides its modulatory effects on actin cytoskeleton dynamics, might play a direct role in clathrin-dependent endocytosis. The constitutive reduction of the Na/K pump endocytic rate induced by mutated adducin variants may be relevant in Na-dependent hypertension.

A palmitoylation cycle dynamically regulates partitioning of the GABA-synthesizing enzyme GAD65 between ER-Golgi and post-Golgi membranes

GAD65, the smaller isoform of the enzyme glutamic acid decarboxylase, synthesizes GABA for fine-tuning of inhibitory neurotransmission. GAD65 is synthesized as a soluble hydrophilic protein but undergoes a hydrophobic post-translational modification and becomes anchored to the cytosolic face of Golgi membranes. A second hydrophobic modification, palmitoylation of Cys30 and Cys45 in GAD65, is not required for the initial membrane anchoring but is crucial for post-Golgi trafficking of the protein to presynaptic clusters. The mechanism by which palmitoylation directs targeting of GAD65 through and out of the Golgi complex is unknown. Here, we show that prior to palmitoylation, GAD65 anchors to both ER and Golgi membranes. Palmitoylation, however, clears GAD65 from the ER-Golgi, targets it to the trans-Golgi network and then to a post-Golgi vesicular pathway. FRAP analyses of trafficking of GAD65-GFP reveal a rapid and a slow pool of protein replenishing the Golgi complex. The rapid pool represents non-palmitoylated hydrophobic GAD65-GFP, which exchanges rapidly between the cytosol and ER/Golgi membranes. The slow pool represents palmitoylation-competent GAD65-GFP, which replenishes the Golgi complex via a non-vesicular pathway and at a rate consistent with a depalmitoylation step. We propose that a depalmitoylation-repalmitoylation cycle serves to cycle GAD65 between Golgi and post-Golgi membranes and dynamically control levels of enzyme directed to the synapse.

Brefeldin A action and recovery in Chlamydomonas are rapid and involve fusion and fission of Golgi cisternae

CHLAMYDOMONAS NOCTIGAMA has a non-motile Golgi apparatus consisting of several Golgi stacks adjacent to transitional ER. These domains are characterized by vesicle-budding profiles and the lack of ribosomes on the side of the ER proximal to the Golgi stacks. Immunogold labelling confirms the presence of COPI-proteins at the periphery of the Golgi stacks, and COPII-proteins at the ER-Golgi interface. After addition of BFA (10 microg/ml) a marked increase in the number of vesicular profiles lying between the ER and the Golgi stacks is seen. Serial sections of cells do not provide any evidence for the existence of tubular connections between the ER and the Golgi stacks, supporting the notion that COPI- but not COPII-vesicle production is affected by BFA. The fusion of COPII-vesicles at the CIS-Golgi apparatus apparently requires the presence of retrograde COPI-vesicles. After 15 min the cisternae of neighbouring Golgi stacks begin to fuse forming "mega-Golgis", which gradually curl before fragmenting into clusters of vesicles and tubules. These are surrounded by the transitional ER on which vesicle-budding profiles are still occasionally visible. Golgi remnants continue to survive for several hours and do not completely disappear. Washing out BFA leads to a very rapid reassembly of Golgi cisternae. At first, clusters of vesicles are seen adjacent to transitional ER, then "mini Golgis" are seen whose cisternae grow in length and number to produce "mega Golgis". These structures then divide by vertical fission to produce Golgi stacks of normal size and morphology roughly 60 min after drug wash-out.

Hepatitis B virus translocates across a trophoblastic barrier

Mother-infant transmission of hepatitis B virus (HBV) accounts for up to 30% of worldwide chronic infections. The mechanism and high-risk period of HBV transmission from mother to infant are unknown. Although largely prevented by neonatal vaccination, significant transmission continues to occur in high-risk populations. It is unclear whether HBV can traverse an intact epithelial barrier to infect a new host. Transplacental transmission of a number of viruses relies on transcytotic pathways across placental cells. We wished to determine whether infectious HBV can traverse a polarized trophoblast monolayer. We used a human placenta-derived cell line, BeWo, cultured on membranes as polarized monolayers, to model the maternal-fetal barrier. We assessed the effects of placental maturity and maternal immunoglobulin on viral transport. Intracellular viral trafficking pathways were investigated by confocal microscopy. Free HBV (and infectious duck hepatitis B virus) transcytosed across trophoblastic cells at a rate of 5% in 30 min. Viral transport occurred in microtubule-dependent endosomal vesicles. Additionally, confocal microscopy showed that the internalized virus traverses a monensin-sensitive endosomal compartment. Differentiation of the cytotrophoblasts to syncytiotrophoblasts resulted in a 25% reduction in viral transcytosis, suggesting that placental maturity may protect the fetus. Virus translocation was also reduced in the presence of HBV immunoglobulin. We show for the first time that transcytosis of infectious hepadnavirus can occur across a trophoblastic barrier early in gestation, with the risk of transmission being reduced by placental maturity and specific maternal antibody. This study suggests a mechanism by which mother-infant transmission may occur.

Dynamic response of prevacuolar compartments to brefeldin a in plant cells

Little is known about the dynamics and molecular components of plant prevacuolar compartments (PVCs) in the secretory pathway. Using transgenic tobacco (Nicotiana tabacum) Bright-Yellow-2 (BY-2) cells expressing membrane-anchored yellow fluorescent protein (YFP) reporters marking Golgi or PVCs, we have recently demonstrated that PVCs are mobile multivesicular bodies defined by vacuolar sorting receptor proteins. Here, we demonstrate that Golgi and PVCs have different sensitivity in response to brefeldin A (BFA) treatment in living tobacco BY-2 cells. BFA at low concentrations (5-10 microg mL(-1)) induced YFP-marked Golgi stacks to form both endoplasmic reticulum-Golgi hybrid structures and BFA-induced aggregates, but had little effect on YFP-marked PVCs in transgenic BY-2 cells at both confocal and immunogold electron microscopy levels. However, BFA at high concentrations (50-100 microg mL(-1)) caused both YFP-marked Golgi stacks and PVCs to form aggregates in a dose- and time-dependent manner. Normal Golgi or PVC signals can be recovered upon removal of BFA from the culture media. Confocal immunofluorescence and immunogold electron microscopy studies with specific organelle markers further demonstrate that the PVC aggregates are distinct, but physically associated, with Golgi aggregates in BFA-treated cells and that PVCs might lose their internal vesicle structures at high BFA concentration. In addition, vacuolar sorting receptor-marked PVCs in root-tip cells of tobacco, pea (Pisum sativum), mung bean (Vigna radiata), and Arabidopsis (Arabidopsis thaliana) upon BFA treatment are also induced to form similar aggregates. Thus, we have demonstrated that the effects of BFA are not limited to endoplasmic reticulum and Golgi, but extend to PVC in the endomembrane system, which might provide a quick tool for distinguishing Golgi from PVC for its identification and characterization, as well as a possible new tool in studying PVC-mediated protein traffic in plant cells.

Members of RTP and REEP gene families influence functional bitter taste receptor expression

Functional characterization of chemosensory receptors is usually achieved by heterologous expression in mammalian cell lines. However, many chemoreceptor genes, including bitter taste receptors (TAS2Rs), show only marginal cell surface expression. Usually, these problems are circumvented by using chimeric receptors consisting of "export tags" and the receptor sequence itself. It seems likely that chemoreceptor cells express factors for cell surface targeting of native receptor molecules in vivo. For TAS2Rs, however, such factors are still unknown. The present study investigates the influence of RTP and REEP proteins on the functional expression of human TAS2Rs in heterologous cells. We expressed hTAS2Rs in HEK 293T cells and observed dramatic differences in responsiveness to agonist stimulation. By immunocytochemistry we show accumulation of the bitter beta-glucopyranoside receptor hTAS2R16 in the Golgi compartment. Coexpression of RTP and REEP proteins changed the responses of some hTAS2Rs upon agonist stimulation, which is likely due to efficient cell surface localization as demonstrated by cell surface biotinylation experiments. The coimmunoprecipitation of hTAS2R16 and RTP3 or RTP4 suggests that the mechanism by which these cofactors influence hTAS2R16 function might involve direct protein-protein interaction. Finally, expression analyses demonstrate RTP and REEP gene expression in human circumvallate papillae and testis, both of which are sites of TAS2R gene expression.

The endocytic network in plants

Endocytosis and vesicle recycling via secretory endosomes are essential for many processes in multicellular organisms. Recently, higher plants have provided useful experimental model systems to study these processes. Endocytosis and secretory endosomes in plants play crucial roles in polar tip growth, a process in which secretory and endocytic pathways are integrated closely. Plant endocytosis and endosomes are important for auxin-mediated cell-cell communication, gravitropic responses, stomatal movements, cytokinesis and cell wall morphogenesis. There is also evidence that F-actin is essential for endocytosis and that plant-specific myosin VIII is an endocytic motor in plants. Last, recent results indicate that the trans Golgi network in plants should be considered an integral part of the endocytic network.

A unique lysophospholipid acyltransferase (LPAT) antagonist, CI-976, affects secretory and endocytic membrane trafficking pathways

Previous studies have shown that inhibition of a Golgi-complex-associated lysophospholipid acyltransferase (LPAT) activity by the drug CI-976 stimulates Golgi tubule formation and subsequent redistribution of resident Golgi proteins to the endoplasmic reticulum (ER). Here, we show that CI-976 stimulates tubule formation from all subcompartments of the Golgi complex, and often these tubules formed independently, i.e. individual tubules usually did not contain markers from different subcompartments. Whereas the cis, medial and trans Golgi membranes redistributed to the ER, the trans Golgi network (TGN) collapsed back to a compact juxtanuclear position similar to that seen with brefeldin A (BFA) treatment. Also similar to BFA, CI-976 induced the formation of endosome tubules, but unlike BFA, these tubules did not fuse with TGN tubules. Finally, CI-976 produced an apparently irreversible block in the endocytic recycling pathway of transferrin (Tf) and Tf receptors (TfRs) but had no direct effect on Tf uptake from the cell surface. Tf and TfRs accumulated in centrally located, Rab11-positive vesicles indicating that CI-976 inhibits export of cargo from the central endocytic recycling compartment. These results, together with previous studies, demonstrate that CI-976 inhibits multiple membrane trafficking steps, including ones found in the endocytic and secretory pathways, and imply a wider role for lysophospholipid acyltransferases in membrane trafficking.

Plasticity and rigidity in adaptor protein-2-mediated internalization of the TCR:CD3 complex

Many cell surface proteins are internalized via dileucine- or tyrosine-based motifs within their cytoplasmic domains by the heterotetrameric adaptor protein complex, AP-2. In this study we have examined how AP-2 mediates internalization of large cell surface receptors, such as the eight-chain TCR:CD3 complex. Although most receptors have a single signal that drives internalization, the TCR complex has two (D/E)xxxL(L/I) motifs and 20 YxxØ motifs. Using 293T cells, we show that AP-2 is completely dependent on both signals to mediate TCR internalization, because deletion of either completely blocks this process. Significant plasticity and redundancy were observed in the use of the YxxØ motifs, with a clear hierarchy in their use (CD3delta > CD3gamma >or= CD3zeta >> CD3epsilon). Remarkably, a single, membrane-distal YxxØ motif in CD3delta could mediate approximately 75% of receptor internalization, whereas its removal only reduced internalization by approximately 20%. In contrast, significant rigidity was observed in use of the (D/E)xxxL(L/I) motif in CD3gamma. This was due to an absolute requirement for the position of this signal in the context of the TCR complex and for a highly conserved lysine residue, K128, which is not present in CD3delta. These contrasting requirements suggest a general principle by which AP-2 may mediate the internalization of large, multichain complexes.

Cytosolic phospholipase A2-alpha mediates endothelial cell proliferation and is inactivated by association with the Golgi apparatus

Arachidonic acid and its metabolites are implicated in regulating endothelial cell proliferation. Cytosolic phospholipase A2-alpha (cPLA2alpha) is responsible for receptor-mediated arachidonic acid evolution. We tested the hypothesis that cPLA2alpha activity is linked to endothelial cell proliferation. The specific cPLA2alpha inhibitor, pyrrolidine-1, inhibited umbilical vein endothelial cell (HUVEC) proliferation in a dose-dependent manner. Exogenous arachidonic acid addition reversed this inhibitory effect. Inhibition of sPLA2 did not affect HUVEC proliferation. The levels of cPLA2alpha did not differ between subconfluent and confluent cultures of cells. However, using fluorescence microscopy we observed a novel, confluence-dependent redistribution of cPLA2alpha to the distal Golgi apparatus in HUVECs. Association of cPLA2alpha with the Golgi was linked to the proliferative status of HUVECs. When associated with the Golgi apparatus, cPLA2alpha activity was seen to be 87% inhibited. Relocation of cPLA2alpha to the cytoplasm and nucleus, and cPLA2alpha enzyme activity were required for cell cycle entry upon mechanical wounding of confluent monolayers. Thus, cPLA2alpha activity and function in controlling endothelial cell proliferation is regulated by reversible association with the Golgi apparatus.

Overexpression of Rab22a hampers the transport between endosomes and the Golgi apparatus

The transport and sorting of soluble and membrane-associated macromolecules arriving at endosomal compartments require a complex set of Rab proteins. Rab22a has been localized to the endocytic compartment; however, very little is known about the function of Rab22a and inconsistent results have been reported in studies performed in different cell lines. To characterize the function of Rab22a in endocytic transport, the wild-type protein (Rab22a WT), a hydrolysis-deficient mutant (Rab22a Q64L), and a mutant with reduced affinity for GTP (Rab22a S19N) were expressed in CHO cells. None of the three Rab22a constructs affected the transport of rhodamine-dextran to lysosomes, the digestion of internalized proteins, or the lysosomal localization of cathepsin D. In contrast with the mild effect of Rab22a on the endosome-lysosome route, cells expressing Rab22a WT and Rab22a Q64L presented a strong delay in the retrograde transport of cholera toxin from endosomes to the Golgi apparatus. Moreover, these cells accumulated the cation independent mannose 6-phosphate receptor in endosomes. These observations indicate that Rab22a can affect the trafficking from endosomes to the Golgi apparatus probably by promoting fusion among endosomes and impairing the proper segregation of membrane domains required for targeting to the trans-Golgi network (TGN).

Dynamics of Golgi matrix proteins after the blockage of ER to Golgi transport

When the ER to Golgi transport is blocked by a GTP-restricted mutant of Sar1p (H79G) in NRK-52E cells, most Golgi resident proteins are transported back into the ER. In contrast, the cis-Golgi matrix proteins GM130 and GRASP65 are retained in punctate cytoplasmic structures, namely Golgi remnants. Significant amounts of the medial-Golgi matrix proteins golgin-45, GRASP55 and giantin are retained in the Golgi remnants, but a fraction of these proteins relocates to the ER. Golgin-97, a candidate trans-Golgi network matrix protein, is retained in Golgi remnant-like structures, but mostly separated from GM130 and GRASP65. Interestingly, most Sec13p, a COPII component, congregates into larger cytoplasmic clusters soon after the microinjection of Sar1p(H79G), and these move to accumulate around the Golgi apparatus. Sec13p clusters remain associated with Golgi remnants after prolonged incubation. Electron microscopic analysis revealed that Golgi remnants are clusters of larger vesicles with smaller vesicles, many of which are coated. GM130 is mainly associated with larger vesicles and Sec13p with smaller coated vesicles. The Sec13p clusters disperse when p115 binding to the Golgi apparatus is inhibited. These results suggest that cis-Golgi matrix proteins resist retrograde transport flow and stay as true residents in Golgi remnants after the inhibition of ER to Golgi transport.

Atrophic macular degeneration mutations in ELOVL4 result in the intracellular misrouting of the protein

Elongation of very long chain fatty acids 4 (ELOVL4) is a novel member of the ELO family of genes that are involved in fatty acid metabolism. ELOVL4 encodes a putative transmembrane protein of 314 amino acids that carries a possible endoplasmic reticulum (ER) retention/retrieval signal (KXKXX) at the C-terminus. Two distinct mutations, a 5-bp deletion and a complex mutation from the same region in exon 6 of this gene, have been reported so far and are associated with autosomal dominant atrophic macular degeneration (adMD/STGD3). Both of these deletions could result in C-terminal truncation and loss of the ER retention signal in the mutant protein. We expressed the wild-type and mutant proteins in COS-7 and CHO cells to study the intracellular distribution of ELOVL4 and to identify possible implications of the above mutations in its localization. Immunofluorescence analysis of these proteins along with organelle marker antibodies revealed predominant ER localization for wild-type ELOVL4. Targeted deletion of the dilysine motif at the C-terminus of the protein resulted in the loss of ER localization. Immunoelectron microscopy and immunofluorescence analysis revealed a similar ER localization pattern for the protein in human photoreceptors. These data indicate that ELOVL4 is an ER-resident protein, which supports its suggested function in fatty acid elongation. We also demonstrate that the localization of both mutant proteins was dramatically changed from an ER to a Golgi distribution. Our observations suggest that the consequences of defective protein trafficking could underlie the molecular mechanism associated with degeneration of the macula in the patients with adMD/STGD3.

BIG2, a guanine nucleotide exchange factor for ADP-ribosylation factors: its localization to recycling endosomes and implication in the endosome integrity

Small GTPases of the ADP-ribosylation factor (ARF) family play a key role in membrane trafficking by regulating coated vesicle formation, and guanine nucleotide exchange is essential for the ARF function. Brefeldin A blocks the ARF-triggered coat assembly by inhibiting the guanine nucleotide exchange on ARFs and causes disintegration of the Golgi complex and tubulation of endosomal membranes. BIG2 is one of brefeldin A-inhibited guanine nucleotide exchange factors for the ARF GTPases and is associated mainly with the trans-Golgi network. In the present study, we have revealed that another population of BIG2 is associated with the recycling endosome and found that expression of a catalytically inactive BIG2 mutant, E738K, selectively induces membrane tubules from this compartment. We also have shown that BIG2 has an exchange activity toward class I ARFs (ARF1 and ARF3) in vivo and inactivation of either ARF exaggerates the BIG2(E738K)-induced tubulation of endosomal membranes. These observations together indicate that BIG2 is implicated in the structural integrity of the recycling endosome through activating class I ARFs.

Characterization of the trafficking pathway of cystic fibrosis transmembrane conductance regulator in baby hamster kidney cells

To examine the unknown trafficking pathway of the cystic fibrosis transmembrane conductance regulator (CFTR) from the endoplasmic reticulum (ER), we utilized baby hamster kidney cells stably expressing CFTR fused with green fluorescent protein. CFTR trafficking from the ER was visualized and analyzed by immunocytochemical analyses. Here we show that CFTR was exported from the ER to the cis-Golgi and early endosome, suggesting that CFTR transport in the early secretory pathway may utilize a non-conventional pathway. This CFTR trafficking pathway may be a target for pharmacological modulation that selectively stimulates CFTR transport.

Reproduction of Langerin/CD207 traffic and Birbeck granule formation in a human cell line model

Birbeck granules (BG) are organelles specific to Langerhans cells (LCs), which form where the C-type lectin Langerin accumulates. Their function remains obscure due to morphologic and dynamic alterations induced by maturation of isolated LC. In this study, we attempted to reconstitute Langerin traffic and BG formation in the endosomal pathway of a human melanoma cell line. In the selected Langerin-transfected cell line, M10-22E, Langerin is distributed between the early recycling endosomal compartment and the plasma membrane, as in LC. Whereas mainly concentrated in membranes related to the Rab11(+) endosomal recycling compartment at the steady state, Langerin also recycles in M10-22E cells and drives BG biogenesis in the endosomal recycling compartment. Interruption of endocytosis or recycling induces redistribution of intracellular Langerin with an associated alteration in BG location and morphology. We have, therefore, generated a stable, Langerin-transfected cell line in which Langerin traffic and distribution and BG morphology replicate that seen in freshly isolated LC. This practical model can now be used to further delineate the nature and function of BG.

Human neutrophil-derived CAP37 inhibits lipopolysaccharide-induced activation in murine peritoneal macrophages

Human cationic antimicrobial protein, CAP37, is released from neutrophil granules during infection. CAP37 attracts monocytes, binds Gram-negative endotoxin (lipopolysaccharide, LPS), is bactericidal for a range of Gram-negative bacteria, and reduces mortality in murine polymicrobial sepsis. Here, we report that recombinant CAP37 specifically targets murine peritoneal macrophages. Under steady-state conditions, the bulk of cell-associated CAP37 was localized at the plasma membrane. However, a fraction of CAP37 gained access to the endocytic system, but did not accumulate in recycling endosomes or in the trans-Golgi network (TGN). Instead, CAP37 was internalized by fluid phase endocytosis and accumulated in a prelysosomal compartment. Macrophages that were preexposed to CAP37 exhibited diminished LPS responsiveness, as determined by analysis of c-Jun phosphorylation. Further examination showed that pretreatment with CAP37 reduced the ability of macrophages to bind LPS. Taken together, these observations demonstrate that prolonged exposure to CAP37 desensitizes macrophages to LPS and suggest that this protein plays a novel anti-inflammatory role in polymicrobial sepsis.

Inhibition of membrane tubule formation and trafficking by isotetrandrine, an antagonist of G-protein-regulated phospholipase A2 enzymes

Previous studies have established a role for cytoplasmic phospholipase A(2) (PLA(2)) activity in tubule-mediated retrograde trafficking between the Golgi complex and the endoplasmic reticulum (ER). However, little else is known about how membrane tubule formation is regulated. This study demonstrates that isotetrandrine (ITD), a biscoclaurine alkaloid known to inhibit PLA(2) enzyme activation by heterotrimeric G-proteins, effectively prevented brefeldin A (BFA)-induced tubule formation from the Golgi complex and retrograde trafficking to the ER. In addition, ITD inhibited BFA-stimulated tubule formation from the trans-Golgi network and endosomes. ITD inhibition of the BFA response was potent (IC(50) approximately 10-20 microM) and rapid (complete inhibition with a 10-15-min preincubation). ITD also inhibited normal retrograde trafficking as revealed by the formation of nocodazole-induced Golgi mini-stacks at ER exit sites. Treatment of cells with ITD alone caused the normally interconnected Golgi ribbons to become fragmented and dilated, but cisternae were still stacked and located in a juxtanuclear position. These results suggest that a G-protein-binding PLA(2) enzyme plays a pivotal role in tubule mediated trafficking between the Golgi and the ER, the maintenance of the interconnected ribbons of Golgi stacks, and tubule formation from endosomes.

IL-4 selectively enhances FcgammaRIII expression and signaling on mouse mast cells

Fc receptors for IgG (FcgammaR) are widely expressed in the hematopoietic system and mediate a variety of inflammatory responses. There are two functional classes of FcgammaR, activation and inhibitory receptors. Since IgG immune complexes (IgG IC) bind each class with similar affinity, co-expression of these receptors leads to their co-ligation. Thus, expression levels of this antagonistic pair play a critical role in determining the cellular response. Murine mast cells co-express the activation receptor FcgammaRIII and the inhibitory receptor FcgammaRIIb and can be activated by IgG IC. Mast cell activation contributes to allergic and other inflammatory diseases-particularly those in which IgG IC may play important roles. Using mouse bone marrow-derived mast cells, we report that IL-4 selectively increases FcgammaRIII expression without altering FcgammaRIIb. This enhanced expression could be induced by Stat6 activation alone, and appeared to be mediated in part by increased FcgammaRIIIalpha protein synthesis without significant changes in transcription. The increase in FcgammaRIII expression was functionally significant, as it was matched by enhanced FcgammaR-mediated degranulation and cytokine production. Selective regulation of mast cell FcgammaR by interleukin-4 could alter inflammatory IgG responses and subsequently disease severity and progression.

TNF-α and IL-4 regulate expression of IL-13 receptor α2 on human fibroblasts

Two interleukin 13 receptors (IL-13Rs) have been identified as IL-13Ralpha1 and IL-13Ralpha2. IL-13Ralpha1 is composed of a heterodimer consisting of IL-13Ralpha1 and IL-4 receptor alpha (IL-4Ralpha) as a signaling subunit. In contrast, IL-13Ralpha2 is known as a decoy receptor for IL-13. In this study, we investigated the expression of IL-13Rs on human fibroblasts. IL-13Ralpha2 was significantly up-regulated after stimulation with tumor necrosis factor-alpha (TNF-alpha) and/or IL-4. In contrast, IL-13Ralpha1 was constitutively detectable and was not up-regulated. After the induction of IL-13alpha2 by IL-4, STAT6 phosphorylation through IL-13Ralpha1 by IL-13 was inhibited. We also detected large intracellular pools of IL-13Ralpha2 in fibroblasts quantitatively. Furthermore, mobilization of the IL-13Ralpha2 protein stores from the cytoplasm to the cell surface was prevented by an inhibitor of protein transport, brefeldin-A. These results indicate that TNF-alpha and IL-4 synergistically up-regulate the expression of IL-13Ralpha2 decoy receptor on human fibroblasts by inducing gene expression and mobilizing intracellular receptors, and thus may down-regulate the IL-13 signaling.

Mechanisms of TfR-mediated transcytosis and sorting in epithelial cells and applications toward drug delivery

Transferrin receptor has been an important protein for many of the advances made in understanding the intricacies of the intramolecular sorting pathways of endocytosed molecules. The unique internalization and recycling functions of transferrin receptor have also made it an attractive choice for drug targeting and delivery of large protein-based therapeutics and toxins. Recent advances in elucidating the role of the intracellular controllers of transferrin recycling and sorting, such as Rab proteins and their effectors, have led to enhancement of transferrin receptor as a drug delivery vehicle. This review focuses on the use of transferrin receptor as an agent for facilitating drug delivery and targeting, and the role that mechanisms of transferrin receptor sorting and transcytosis play in these events.

Arabidopsis sterol endocytosis involves actin-mediated trafficking via ARA6-positive early endosomes

BACKGROUND

In contrast to the intense attention devoted to research on intracellular sterol trafficking in animal cells, knowledge about sterol transport in plant cells remains limited, and virtually nothing is known about plant endocytic sterol trafficking. Similar to animals, biosynthetic sterol transport occurs from the endoplasmic reticulum (ER) via the Golgi apparatus to the plasma membrane. The vesicle trafficking inhibitor brefeldin A (BFA) has been suggested to disrupt biosynthetic sterol transport at the Golgi level.

RESULTS

Here, we report on early endocytic sterol trafficking in Arabidopsis root epidermal cells by introducing filipin as a tool for fluorescent sterol detection. Sterols can be internalized from the plasma membrane and localize to endosomes positive for the early endosomal Rab5 GTPase homolog ARA6 fused to green fluorescent protein (GFP) (ARA6-GFP). Early endocytic sterol transport is actin dependent and highly BFA sensitive. BFA causes coaccumulation of sterols, endocytic markers like ARA6-GFP, and PIN2, a polarly localized presumptive auxin transport protein, in early endosome agglomerations that can be distinguished from ER and Golgi. Sterol accumulation in such aggregates is enhanced in actin2 mutants, and the actin-depolymerizing drug cytochalasin D inhibits sterol redistribution from endosome aggregations.

CONCLUSIONS

Early endocytic sterol trafficking involves transport via ARA6-positive early endosomes that, in contrast to animal cells, is actin dependent. Our results reveal sterol-enriched early endosomes as targets for BFA interference in plants. Early endocytic sterol trafficking and recycling of polar PIN2 protein share a common pathway, suggesting a connection between plant endocytic sterol transport and polar sorting events.

Brefeldin A-regulated retrograde transport into the endoplasmic reticulum of internalised wheat germ agglutinin

The effects of the fungal metabolite brefeldin A (BFA) on the endocytic routes of internalised wheat germ agglutinin (WGA) were studied in human HepG2 hepatoma cells, drawing particular attention to the application times in relation to the membrane dynamics occurring at the trans Golgi face during endocytosis. As shown in previous studies, transport of internalised WGA into the Golgi apparatus can be classified in three stages being characterised by predominance of vesicular endosomes (stage I), formation of an extended endocytic trans Golgi network (stage II) and uptake of WGA into the stacked Golgi cisternae (stage III). BFA treatment of the cells led to rapid tubular-reticular transformations of the Golgi stacks. Retrograde transport and further destinations of internalised WGA depended on the time of BFA application. When BFA was administered during stages I or II, WGA was localised within the BFA-induced tubules and networks, but never was found within the endoplasmic reticulum. By contrast, BFA treatment during stage III led to a redistribution of internalised WGA into cisternae of the endoplasmic reticulum. These results show that BFA administered according to a precise time schedule can be used as a regulatory agent that allows to control retrograde traffic of internalised molecules into the endoplasmic reticulum.

Inhibition of a Golgi complex lysophospholipid acyltransferase induces membrane tubule formation and retrograde trafficking

Recent studies have suggested that formation of Golgi membrane tubules involves the generation of membrane-associated lysophospholipids by a cytoplasmic Ca2+-independent phospholipase A2 (PLA2). Herein, we provide additional support for this idea by showing that inhibition of lysophospholipid reacylation by a novel Golgi-associated lysophosphatidylcholine acyltransferase (LPAT) induces the rapid tubulation of Golgi membranes, leading in their retrograde movement to the endoplasmic reticulum. Inhibition of the Golgi LPAT was achieved by 2,2-dimethyl-N-(2,4,6-trimethoxyphenyl)dodecanamide (CI-976), a previously characterized antagonist of acyl-CoA cholesterol acyltransferase. The effect of CI-976 was similar to that of brefeldin A, except that the coatomer subunit beta-COP remained on Golgi-derived membrane tubules. CI-976 also enhanced the cytosol-dependent formation of tubules from Golgi complexes in vitro and increased the levels of lysophosphatidylcholine in Golgi membranes. Moreover, preincubation of cells with PLA2 antagonists inhibited the ability of CI-976 to induce tubules. These results suggest that Golgi membrane tubule formation can result from increasing the content of lysophospholipids in membranes, either by stimulation of a PLA2 or by inhibition of an LPAT. These two opposing enzyme activities may help to coordinately regulate Golgi membrane shape and tubule formation.

TCR dynamics in human mature T lymphocytes lacking CD3 gamma

The contribution of CD3gamma to the surface expression, internalization, and intracellular trafficking of the TCR/CD3 complex (TCR) has not been completely defined. However, CD3gamma is believed to be crucial for constitutive as well as for phorbol ester-induced internalization. We have explored TCR dynamics in resting and stimulated mature T lymphocytes derived from two unrelated human congenital CD3gamma-deficient (gamma(-)) individuals. In contrast to gamma(-) mutants of the human T cell line Jurkat, which were selected for their lack of membrane TCR and are therefore constitutively surface TCR negative, these natural gamma(-) T cells constitutively expressed surface TCR, mainly through biosynthesis of new chains other than CD3gamma. However, surface (but not intracellular) TCR expression in these cells was less than wild-type cells, and normal surface expression was clearly CD3gamma-dependent, as it was restored by retroviral transduction of CD3gamma. The reduced surface TCR expression was likely caused by an impaired assembly or membrane transport step during recycling, whereas constitutive internalization and degradation were apparently normal. Ab binding to the mutant TCR, but not phorbol ester treatment, caused its down-modulation from the cell surface, albeit at a slower rate than in normal controls. Kinetic confocal analysis indicated that early ligand-induced endocytosis was impaired. After its complete down-modulation, TCR re-expression was also delayed. The results suggest that CD3gamma contributes to, but is not absolutely required for, the regulation of TCR trafficking in resting and Ag-stimulated mature T lymphocytes. The results also indicate that TCR internalization is regulated differently in each case.

Association of cPLA2-alpha and COX-1 with the Golgi apparatus of A549 human lung epithelial cells

Cytosolic phospholipase A2-alpha (cPLA2-alpha) is an 85 kDa, Ca2+-sensitive enzyme involved in receptor-mediated prostaglandin synthesis. In airway epithelial cells, the release of prostaglandins is crucial in regulating the inflammatory response. Although prostaglandin release has been studied in various epithelial cell models, the subcellular location of cPLA2-alpha in these cells is unknown. Using high-resolution confocal microscopy of the human A549 lung epithelial cell line, we show that cPLA2-alpha relocates from the cytosol and nuclei to a juxtanuclear region following stimulation with the Ca2+ ionophore A23187. Double staining with rhodamine-conjugated wheat germ agglutinin confirmed this region to be the Golgi apparatus. Markers specific for Golgi subcompartments revealed that cPLA2-alpha is predominantly located at the trans-Golgi stack and the trans-Golgi network following elevation of cytosolic Ca2+. Furthermore, treatment of cells with the Golgi-disrupting agent brefeldin A caused a redistribution of cPLA2-alpha, confirming that cPLA2-alpha associates with Golgi-derived membranes. Finally, a specific co-localization of cPLA2-alpha with cyclooxygenase-1 but not cyclooxygenase-2 was evident at the Golgi apparatus. These results, combined with recent data on the role of PLA2 activity in maintaining Golgi structure and function, suggest that Golgi localization of cPLA2-alpha may be involved in membrane trafficking in epithelial cells.

The cell surface expression of SAP-binding receptor CD229 is regulated via its interaction with clathrin-associated adaptor complex 2 (AP-2)

CD229 (Ly9) is a cell surface receptor selectively expressed on T and B lymphocytes, and it belongs to the CD150 receptor family. Like other receptors of this family, CD229 interacts with SAP/SH2D1a protein, mutation of which is responsible for the fatal X-linked lymphoproliferative disease. Receptors of the CD150 family function as costimulatory molecules, regulating cytokine production and cytotoxicity. Thus, their signaling and regulation in lymphocytes may be critical to an understanding of the pathogenesis of the X-linked lymphoproliferative disease. Here we show that CD229 interacts with the mu(2) chain of the AP-2 adaptor complex that links transmembrane proteins to clathrin-coated pits. CD229 was the only member of the CD150 family associated with AP-2. We also show that the mu(2) chain interacts with the Y(470)EKL motif of CD229. The integrity of this site was necessary for CD229 internalization, but it was not involved in SAP recruitment. Moreover, CD229 binds to the AP-2 complex in T and B cell lines, and it is internalized rapidly from the cell surface on T cells after antibody ligation. In contrast, cross-linking of CD229 receptors with intact antibody inhibited CD229 internalization on B cells. However, when F(ab')(2) antibodies were used, CD229 internalization was similar on T and B cells, suggesting that Fcgamma receptors control CD229 cell surface expression. Furthermore, CD229 was regulated by T cell receptor and B cell receptor signaling because coligation with antibodies against anti-CD3 and anti-IgM increased the rate of CD229 endocytosis. These data suggest that CD229 cell surface expression on lymphocytes surface is strongly and differentially regulated within the CD150 family members.

YY1 is regulated by O-linked N-acetylglucosaminylation (O-glcNAcylation)

YY1 is a zinc finger DNA-binding transcription factor that influences expression of a wide variety of cellular and viral genes. YY1 is essential for the development of mammalian embryos. It regulates the expression of genes with important functions in DNA replication, protein synthesis, and cellular response to external stimuli during cell growth and differentiation. How YY1 accomplishes such a variety of functions is unknown. Here, we show that a subset of the nuclear YY1 appears to be O-GlcNAcylated regardless of the differentiation status of the cells. We found that glucose strongly stimulates O-linked N-acetylglucosaminylation (O-GlcNAcylation) on YY1. Glycosylated YY1 no longer binds the retinoblastoma protein (Rb). Upon dissociation from Rb, the glycosylated YY1 is free to bind DNA. The ability of the O-glycosylation on YY1 to disrupt the complex with Rb leads us to propose that O-glycosylation might have a profound effect on cell cycle transitions that regulate the YY1-Rb heterodimerization and promote the activity of YY1. Our observations provide strong evidence that YY1-regulated transcription is very likely connected to the pathway of glucose metabolism that culminates in the O-GlcNAcylation on YY1, changing its function in transcription.

A role for the cytoplasmic tail of the pre-T cell receptor (TCR) alpha chain in promoting constitutive internalization and degradation of the pre-TCR

Engagement of the alpha beta T cell receptor (TCR) by its ligand results in the down-modulation of TCR cell surface expression, which is thought to be a central event in T cell activation. On the other hand, pre-TCR signaling is a key process in alpha beta T cell development, which appears to proceed in a constitutive and ligand-independent manner. Here, comparative analyses on the dynamics of pre-TCR and TCR cell surface expression show that unligated pre-TCR complexes expressed on human pre-T cells behave as engaged TCR complexes, i.e. they are rapidly internalized and degraded in lysosomes and proteasomes but do not recycle back to the cell surface. Thus, pre-TCR down-regulation takes place constitutively without the need for extracellular ligation. By using TCR alpha/p Tau alpha chain chimeras, we demonstrate that prevention of recycling and induction of degradation are unique pre-TCR properties conferred by the cytoplasmic domain of the pT alpha chain. Finally, we show that pre-TCR internalization is a protein kinase C-independent process that involves the combination of src kinase-dependent and -independent pathways. These data suggest that constitutive pre-TCR down-modulation regulates pre-TCR surface expression levels and hence the extent of ligand-independent signaling through the pre-TCR.

Phospholipase A2 (PLA2) enzymes in membrane trafficking: mediators of membrane shape and function

Since the mid-1990s, there have been tremendous advances in our understanding of the roles that lipid-modifying enzymes play in various intracellular membrane trafficking events. Phospholipases represent the largest group of lipid-modifying enzymes and accordingly display a wide range of functions. The largest class of phospholipases are the phospholipase A(2) (PLA2) enzymes, and these have been most extensively studied for their roles in the generation lipid signaling molecules, e.g. arachidonic acid. In recent years, however, cytoplasmic PLA2 enzymes have also become increasingly associated with various intracellular trafficking events, such as the formation of membrane tubules from the Golgi complex and endosomes, and membrane fusion events in the secretory and endocytic pathways. Moreover, the ability of cytoplasmic PLA2 enzymes to directly affect the structure and function of membranes by altering membrane curvature suggests novel functional roles for these enzymes. This review will focus on the role of cytoplasmic PLA2 enzymes in intracellular membrane trafficking and the mechanisms by which they influence membrane structure and function.

Coordinated regulation and colocalization of alphav integrin and its activating enzyme proprotein convertase PC5 in vivo

Integrin alphav is involved in intracellular-extracellular signaling important for cytoskeleton alterations and control of cell movement. In vitro experiments indicate that the integrin alphav-subunit undergoes post-translational endoproteolytic cleavage. This type of activation requires the presence of suitable kexin/subtilisin-like proprotein convertases. In vitro experiments have demonstrated that, among several proprotein convertases, PC5A, and to a threefold lesser extent furin, can activate alphav integrin. The biological significance of these in vitro data would be further supported by a coexpression and coordinated regulation of the gene expression of alphav integrin and its activating enzyme PC5 in vivo. In the present study we investigated the regulation of alphav integrin and PC5 following balloon injury in vivo. Comparative immunocytochemistry revealed a coordinated regulation of alphav integrin and PC5 during vascular remodeling in rodents. Integrin alphav was found to be upregulated in PCNA-positive, proliferating vascular smooth muscle cells. Northern blots revealed no significant regulation of furin mRNA, whereas PC5A mRNA increased during vascular remodeling, suggesting that PC5 is the major convertase during neointima formation in vivo. Incubation of vascular smooth muscle cells with the Golgi-disturbing agent brefeldin A inhibited alphav integrin maturation, indicating that endoproteolytic cleavage occurs in the trans-Golgi network, were PC5 is localized. Thus, the present study further supports the concept that activation of alphav integrin occurs in the trans-Golgi network in vascular smooth muscle cells and involves PC5.

Cored tubules are present in human epidermal Langerhans cells

Cored tubules are ultrastructural organelles described to date only in murine cells belonging to the Langerhans cell family and located in the dermis and its draining lymph nodes. These organelles, the function of which is unknown, differ from Birbeck granules and are interestingly not found in murine epidermal Langerhans cells. In this work we demonstrate that cored tubules are present in freshly isolated human epidermal Langerhans cells. The tubules were found to be interconnected with structures known to belong to the early endosomal pathway and could be immunolabeled with gold-conjugated anti-CD1a and anti-Langerin monoclonal antibodies, but only at 37 degrees C. At this temperature such antibodies are able to progress from the early sorting endosomes to the early recycling endosomes, which in human Langerhans cells include the Birbeck granules. These findings strongly suggest that cored tubules form part of the early recycling compartment.

The Arabidopsis GNOM ARF-GEF mediates endosomal recycling, auxin transport, and auxin-dependent plant growth

Exchange factors for ARF GTPases (ARF-GEFs) regulate vesicle trafficking in a variety of organisms. The Arabidopsis protein GNOM is a brefeldin A (BFA) sensitive ARF-GEF that is required for the proper polar localization of PIN1, a candidate transporter of the plant hormone auxin. Mutations in GNOM lead to developmental defects that resemble those caused by interfering with auxin transport. Both PIN1 localization and auxin transport are also sensitive to BFA. In this paper, we show that GNOM localizes to endosomes and is required for their structural integrity. We engineered a BFA-resistant version of GNOM. In plants harboring this fully functional GNOM variant, PIN1 localization and auxin transport are no longer sensitive to BFA, while trafficking of other proteins is still affected by the drug. Our results demonstrate that GNOM is required for the recycling of auxin transport components and suggest that ARF-GEFs regulate specific endosomal trafficking pathways.

In vitro transport on cis and trans sides of the Golgi involves two distinct types of coatomer and ADP-ribosylation factor-independent transport intermediates

The cisternal maturation model proposes that secretory proteins transit the Golgi in cisternae that mature by the continuous retrograde transport of Golgi enzymes in vesicles. We have tested the hypothesis that de novo generation of transport intermediates containing medial, trans, and trans Golgi network (TGN) enzymes is reconstituted in vitro. Our analysis shows that the majority of transport is mediated by a steady state of transport intermediate production and consumption by Golgi cisternae, with only a minor contribution of pre-existing transport intermediates. Transport in the medial and trans regions of the stack involved intermediates containing Golgi enzymes, apparently moving in a retrograde direction. In contrast, transport between the trans Golgi and TGN was exclusively mediated by intermediates containing secretory protein, as expected for anterograde transport. These intermediates may be physiologically relevant, because only these two specific types of intermediates can be detected in cell homogenates. By analogy to the coatomer (COPI)-independent transport of Golgi enzymes to the endoplasmic reticulum, the steady-state production of intra-Golgi transport intermediates was not impaired by inhibition of COPI vesicle formation. These data suggest a model for COPI-independent intra-Golgi transport by cisternal maturation with a shift in mechanism to anterograde transport at the trans Golgi and TGN boundary.

NK cell CD94/NKG2A inhibitory receptors are internalized and recycle independently of inhibitory signaling processes

Human CD94/NKG2A is an inhibitory receptor that recognizes HLA-E and is expressed by NK cells and a subset of T cells. We have analyzed the cellular trafficking of the CD94/NKG2A receptor using the NKL cell line and peripheral blood NK cells. Flow cytometric, confocal microscopic, and biochemical analyses show that CD94/NKG2A continuously recycles in an active process that requires the cytoskeleton between the cell surface and intracellular compartments that are distinguishable from recycling compartments used by well-characterized receptors, such as transferrin receptor (CD71). CD94/NKG2A, an inhibitory receptor, traffics differently from the closely related CD94/NKG2C molecule, an activating receptor. Using transfection/expression analyses of wild-type and mutant CD94/NKG2A molecules in the HLA-E negative rat basophilic cell line RBL-2H3, we demonstrate that CD94/NKG2A internalization is independent of ligand cross-linking or the presence of functional immunoreceptor tyrosine-based inhibition motifs. Thus, the mechanisms that control cell surface homeostasis of CD94/NKG2A are independent of functional signaling.

Bromodeoxyuridine-labeled oligonucleotides as tools for oligonucleotide uptake studies

The mechanisms by which various oligonucleotides (ODNs) and their analogs enter cells are not fully understood. A common technique used in studies on cellular uptake of ODNs is their conjugation with fluorochromes. However, fluorescently labeled ODNs may vary from the parent compounds in charge and hydrophilicity, and they may interact differently with some components of cellular membranes. In this report, we present an alternative method based on the immunofluorescent detection of ODNs with incorporated 5-bromo-2'-deoxyuridine (BrdUrd). Localization of BrdUrd-modified ODNs has been achieved using FITC-labeled anti-BrdUrd antibodies. This technique allowed determination of the differences in cellular uptake of phosphodiester (PO) and phosphorothioate (PS) ODNs and their derivatives conjugated with cholesterol and menthol. The immunocytochemical method also has shown that the cellular uptake of some ODNs may be influenced by specific sequences that are responsible for the formation of higher-order structures.

Evidence for endocytosis of ROMK potassium channel via clathrin-coated vesicles

ROMK channels are present in the cortical collecting ducts of kidney and are responsible for K(+) secretion in this nephron segment. Recent studies suggest that endocytosis of ROMK channels is important for regulation of K(+) secretion in cortical collecting ducts. We investigated the molecular mechanisms for endocytosis of ROMK channels expressed in Xenopus laevis oocytes and cultured Madin-Darby canine kidney cells. When plasma membrane insertion of newly synthesized channel proteins was blocked by incubation with brefeldin A, ROMK currents decreased with a half-time of ~6 h. Coexpression with the Lys44-->Ala dominant-negative mutant dynamin, but not wild-type dynamin, reduced the rate of reduction of ROMK in the presence of brefeldin A. Mutation of Asn371 to Ile in the putative NPXY internalization motif of ROMK1 abolished the effect of the Lys44-->Ala dynamin mutant on endocytosis of the channel. Coimmunoprecipitation study and confocal fluorescent imaging revealed that ROMK channels associated with clathrin coat proteins in Madin-Darby canine kidney cells. These results provide compelling evidence for endocytosis of ROMK channels via clathrin-coated vesicles.

AP2 adaptor complex-dependent internalization of CD5: differential regulation in T and B cells

CD5 is a key regulator of Ag receptor-mediated activation, selection, and differentiation in both T and B cells. Accumulating evidence indicates that lymphocyte activation and selection are sensitive to variations in levels of CD5 on the cell surface. We now show that CD5 expression on the surface of B and T cells is regulated posttranslationally by direct interaction with the mu(2) subunit of the AP2 adaptor complex that links transmembrane proteins to clathrin-coated pits. CD5 is rapidly internalized from the cell surface in lymphoid cell lines, mature splenic T and B cells, and peritoneal CD5(+) B cells following monovalent or bivalent ligation of the receptor. We mapped the mu(2) subunit binding site on CD5 to Y(429) and determined that the integrity of this site was necessary for CD5 internalization. Cross-linking of the Ag receptor with intact Abs inhibited CD5 internalization in B cells, but had the opposite effect in T cells. However, if F(ab')(2) Abs were used to stimulate the Ag receptor in B cells, the effect on CD5 internalization was now similar to that observed in T cells, indicating that signals through the Ag receptor and FcR regulate CD5 endocytosis in B cells. This was confirmed using an FcgammaRIIB1-deficient B cell line. The ability to differentially alter posttranslational CD5 expression in T and B cells is likely to be key in regulation of Ag receptor signaling and generation of tolerance in T and B lymphocytes.

Overexpression of an ADP-ribosylation factor-guanine nucleotide exchange factor, BIG2, uncouples brefeldin A-induced adaptor protein-1 coat dissociation and membrane tubulation

BIG2 is a guanine nucleotide exchange factor (GEF) for the ADP-ribosylation factor (ARF) family of small GTPases, which regulate membrane association of COPI and adaptor protein (AP)-1 coat protein complexes. A fungal metabolite, brefeldin A (BFA), inhibits ARF-GEFs and leads to redistribution of coat proteins from membranes to the cytoplasm and membrane tubulation of the Golgi complex and the trans-Golgi network (TGN). To investigate the function of BIG2, we examined the effects of BIG2-overexpression on the BFA-induced redistribution of ARF, coat proteins, and organelle markers. The BIG2 overexpression blocked BFA-induced redistribution from membranes of ARF1 and the AP-1 complex but not that of the COPI complex. These observations indicate that BIG2 is implicated in membrane association of AP-1, but not that of COPI, through activating ARF. Furthermore, not only BIG2 but also ARF1 and AP-1 were found as queues of spherical swellings along the BFA-induced membrane tubules emanating from the TGN. These observations indicate that BFA-induced AP-1 dissociation from TGN membranes and tubulation of TGN membranes are not coupled events and suggest that a BFA target other than ARF-GEFs exists in the cell.

Toxoplasma gondii Rab5 enhances cholesterol acquisition from host cells

The role of endocytosis in nutrient uptake by Toxoplasma gondii is unknown. To explore this issue, we characterized an endosomal compartment by identifying a T. gondii Rab5 homologue, a molecular marker for early endosomes in eukaryotic cells. The deduced amino acid sequence of the T. gondii Rab5 gene encodes a protein of 240 amino acids, which we termed TgRab51. TgRab51 was epitope-tagged at the N-terminus, expressed in the parasite, and localized by immunofluorescence and immunoelectron microscopy to tubulovesicular structures anterior to the parasite nucleus and adjacent to, but distinct from the Golgi. By immunofluorescence analysis, TgRab51wt-HA staining partially overlapped with Golgi/TGN markers, but not with the T. gondii secretory organelles. A dominant positive mutant, TgRab51Q103L-HA, enhanced uptake of exogenous cholesterol analogues in intracellular parasites, augmented formation of lipid droplets and accelerated parasite growth. Brefeldin A disrupted the TgRab51 compartment, and altered the distribution of fluorescent exogenous cholesterol in cells expressing TgRab51Q103L-HA. These results suggest that TgRab51 facilitates sterol uptake, possibly through a Golgi-dependent pathway.

Birbeck granules are subdomains of endosomal recycling compartment in human epidermal Langerhans cells, which form where Langerin accumulates

Birbeck granules are unusual rod-shaped structures specific to epidermal Langerhans cells, whose origin and function remain undetermined. We investigated the intracellular location and fate of Langerin, a protein implicated in Birbeck granule biogenesis, in human epidermal Langerhans cells. In the steady state, Langerin is predominantly found in the endosomal recycling compartment and in Birbeck granules. Langerin internalizes by classical receptor-mediated endocytosis and the first Birbeck granules accessible to endocytosed Langerin are those connected to recycling endosomes in the pericentriolar area, where Langerin accumulates. Drug-induced inhibition of endocytosis results in the appearance of abundant open-ended Birbeck granule-like structures appended to the plasma membrane, whereas inhibition of recycling induces Birbeck granules to merge with a tubular endosomal network. In mature Langerhans cells, Langerin traffic is abolished and the loss of internal Langerin is associated with a concomitant depletion of Birbeck granules. Our results demonstrate an exchange of Langerin between early endosomal compartments and the plasma membrane, with dynamic retention in the endosomal recycling compartment. They show that Birbeck granules are not endocytotic structures, rather they are subdomains of the endosomal recycling compartment that form where Langerin accumulates. Finally, our results implicate ADP-ribosylation factor proteins in Langerin trafficking and the exchange between Birbeck granules and other endosomal membranes.

Molecular complexes that contain both c-Cbl and c-Src associate with Golgi membranes

Cbl is an adaptor protein that is phosphorylated and recruited to several receptor and non-receptor tyrosine kinases upon their activation. After binding to the activated receptor, Cbl plays a key role as a kinase inhibitor and as an E3 ubiquitin ligase, thereby contributing to receptor down-regulation and internalization. In addition, Cbl translocates to intracellular vesicular compartments following receptor activation. We report here that Cbl also associates with Golgi membranes. Confocal immunofluorescence staining of Cbl in a variety of unstimulated cells, including CHO cells, revealed a prominent perinuclear colocalization of Cbl and a Golgi marker. Both the prominent Cbl staining and the Golgi marker were dispersed by brefeldin A. Subcellular fractionation of CHO cells demonstrated that about 10% of Cbl is stably associated with membranes, and that Golgi-enriched membrane fractions produced by isopycnic density centrifugation and free-flow electrophoresis are also enriched in Cbl, relative to other membrane fractions. The membrane-bound Cbl was hyperphosphorylated and it co-immunoprecipitated with endogenous Src. By immunofluorescence, some Src colocalized with Cbl and Golgi markers, and Src, like Cbl, was present in the Golgi-enriched fraction prepared by sequential density centrifugation and free-flow electrophoresis. Transfection of an activated form of Src, but not wild-type Src, increased the amount of Src that co-immunoprecipitated with Cbl, and increased the intensity of Cbl staining on the Golgi. This result, together with the increased tyrosine phosphorylation of the membrane-associated Cbl, suggests that Golgi-associated Cbl could be part of a molecular complex that contains activated Src. The localization and interaction of Src and Cbl at the Golgi and the regulation of the interaction of Cbl with Golgi membrane suggest that this complex may contribute to the regulation of Golgi function.

Correction of cross-linker sensitivity of Fanconi anemia group F cells by CD33-mediated protein transfer

Studies have previously described the feasibility of receptor-mediated protein transfer in a cell culture model of Fanconi anemia (FA) group C. This study explores the versatility of this approach by using an antibody single-chain fusion protein to correct the phenotypic defect in FA group F cells. A 68.5-kd chimeric protein (His-M195FANCF) was expressed, consisting of a His tag, a single-chain antibody to the myeloid antigen CD33, and the FANCF protein, as well as a 43-kd His-FANCF fusion protein lacking the antibody motif, in Escherichia coli. The nickel-agarose-purified His-M195FANCF protein bound specifically to the surface of HeLa cells transfected with CD33 and internalized through vesicular structures. The fusion protein, but not CD33, sorted to the nucleus, consistent with the known nuclear localization of FANCF. No similar binding or internalization was observed with His-FANCF. Pretreatment of the transfected cells with chloroquine abolished nuclear accumulation, but there was little change with brefeldin A, indicating a minimal if any role for the Golgi apparatus in mediating transport from endosomes to the cytosol and the nucleus. The intracellular half-life of His-M195FANCF was approximately 160 minutes. Treatment of CD33-transfected FA group F lymphoblastoid cells with 0.1 mg/mL His-M195FANCF conferred resistance to mitomycin C. No similar protection was noted in CD33(-) parental cells or CD33(+) FA cells belonging to groups A and C. These results demonstrate that antibody-directed, receptor-mediated protein transfer is a versatile method for the delivery of biologically active proteins into hematopoietic cells.

Inhibition of transferrin recycling and endosome tubulation by phospholipase A2 antagonists

We report here that a broad spectrum of phospholipase A(2) (PLA(2)) antagonists produce a concentration-dependent, differential block in the endocytic recycling pathway of transferrin (Tf) and Tf receptors (TfRs) but have no acute affect on Tf uptake from the cell surface. At low concentrations of antagonists (approximately 1 microm), Tf and TfR accumulated in centrally located recycling endosomes, whereas at higher concentrations (approximately 10 microm), Tf-TfR accumulated in peripheral sorting endosomes. Several independent lines of evidence suggest that this inhibition of recycling may result from the inhibition of tubule formation. First, BFA-stimulated endosome tubule formation was similarly inhibited by PLA(2) antagonists. Second, endocytosed tracers were found in larger spherical endosomes in the presence of PLA(2) antagonists. And third, endosome tubule formation in a cell-free, cytosol-dependent reconstitution system was equally sensitive PLA(2) antagonists. These results are consistent with the conclusion that endosome membrane tubules are formed by the action of a cytoplasmic PLA(2) and that PLA(2)-dependent tubules are involved in intracellular recycling of Tf and TfR. When taken together with previous studies on the Golgi complex, these results also indicate that an intracellular PLA(2) activity provides a novel molecular mechanism for inducing tubule formation from multiple organelles.