Vacuolar ATPase inactivation blocks recycling to the trans-Golgi network from the plasma membrane

Squaramides enhance NLRP3 inflammasome activation by lowering intracellular potassium

The NLRP3 inflammasome is a component of the inflammatory response to infection and injury, orchestrating the maturation and release of the pro-inflammatory cytokines interleukin-1β (IL-1β), IL-18, and triggering pyroptotic cell death. Appropriate levels of NLRP3 activation are needed to avoid excessive tissue damage while ensuring host protection. Here we report a role for symmetrical diarylsquaramides as selective K+ efflux-dependent NLRP3 inflammasome enhancers. Treatment of macrophages with squaramides potentiated IL-1β secretion and ASC speck formation in response to K+ efflux-dependent NLRP3 inflammasome activators without affecting priming, endosome cargo trafficking, or activation of other inflammasomes. The squaramides lowered intracellular K+ concentration which enabled cells to respond to a below-threshold dose of the inflammasome activator nigericin. Taken together these data further highlight the role of ion flux in inflammasome activation and squaramides as an interesting platform for therapeutic development in conditions where enhanced NLRP3 activity could be beneficial.

Deep and fast label-free Dynamic Organellar Mapping

The Dynamic Organellar Maps (DOMs) approach combines cell fractionation and shotgun-proteomics for global profiling analysis of protein subcellular localization. Here, we enhance the performance of DOMs through data-independent acquisition (DIA) mass spectrometry. DIA-DOMs achieve twice the depth of our previous workflow in the same mass spectrometry runtime, and substantially improve profiling precision and reproducibility. We leverage this gain to establish flexible map formats scaling from high-throughput analyses to extra-deep coverage. Furthermore, we introduce DOM-ABC, a powerful and user-friendly open-source software tool for analyzing profiling data. We apply DIA-DOMs to capture subcellular localization changes in response to starvation and disruption of lysosomal pH in HeLa cells, which identifies a subset of Golgi proteins that cycle through endosomes. An imaging time-course reveals different cycling patterns and confirms the quantitative predictive power of our translocation analysis. DIA-DOMs offer a superior workflow for label-free spatial proteomics as a systematic phenotype discovery tool.

The NLRP3 inflammasome: activation and regulation

The NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome is a cytoplasmic supramolecular complex that is activated in response to cellular perturbations triggered by infection and sterile injury. Assembly of the NLRP3 inflammasome leads to activation of caspase-1, which induces the maturation and release of interleukin-1β (IL-1β) and IL-18, as well as cleavage of gasdermin D (GSDMD), which promotes a lytic form of cell death. Production of IL-1β via NLRP3 can contribute to the pathogenesis of inflammatory disease, whereas aberrant IL-1β secretion through inherited NLRP3 mutations causes autoinflammatory disorders. In this review, we discuss recent developments in the structure of the NLRP3 inflammasome, and the cellular processes and signaling events controlling its assembly and activation.

Disruptions in endocytic traffic contribute to the activation of the NLRP3 inflammasome

Inflammation driven by the NLRP3 inflammasome is coordinated through multiple signaling pathways and is regulated by subcellular organelles. Here, we tested the hypothesis that NLRP3 senses disrupted endosome trafficking to trigger inflammasome formation and inflammatory cytokine secretion. NLRP3-activating stimuli disrupted endosome trafficking and triggered localization of NLRP3 to vesicles positive for endolysosomal markers and for the inositol lipid PI4P. Chemical disruption of endosome trafficking sensitized macrophages to the NLRP3 activator imiquimod, driving enhanced inflammasome activation and cytokine secretion. Together, these data suggest that NLRP3 can sense disruptions in the trafficking of endosomal cargoes, which may explain in part the spatial activation of the NLRP3 inflammasome. These data highlight mechanisms that could be exploited in the therapeutic targeting of NLRP3.

S-palmitoylation determines TMEM55B-dependent positioning of lysosomes

The spatiotemporal cellular distribution of lysosomes depends on active transport mainly driven by microtubule motors such as kinesins and dynein. Different protein complexes attach these molecular motors to their vesicular cargo. TMEM55B (also known as PIP4P1), as an integral lysosomal membrane protein, is a component of such a complex that mediates the retrograde transport of lysosomes by establishing interactions with the cytosolic scaffold protein JIP4 (also known as SPAG9) and dynein-dynactin. Here, we show that TMEM55B and its paralog TMEM55A (PIP4P2) are S-palmitoylated proteins that are lipidated at multiple cysteine residues. Mutation of all cysteines in TMEM55B prevents S-palmitoylation and causes retention of the mutated protein in the Golgi. Consequently, non-palmitoylated TMEM55B is no longer able to modulate lysosomal positioning and the perinuclear clustering of lysosomes. Additional mutagenesis of the dileucine-based lysosomal sorting motif in non-palmitoylated TMEM55B leads to partial missorting to the plasma membrane instead of retention in the Golgi, implicating a direct effect of S-palmitoylation on the adaptor protein-dependent sorting of TMEM55B. Our data suggest a critical role for S-palmitoylation in the trafficking of TMEM55B and TMEM55B-dependent lysosomal positioning.

Respiratory RNA Viruses: How to Be Prepared for an Encounter with New Pandemic Virus Strains

The characteristics of the biology of influenza viruses and coronavirus that determine the implementation of the infectious process are presented. With provision for pathogenesis of infection possible effects of serine proteinase inhibitors, heparin, and inhibitors of heparan sulfate receptors in the prevention of cell contamination by viruses are examined. It has been determined that chelators of metals of variable valency and antioxidants should be used for the reduction of replicative activity of viruses and anti-inflammatory therapy. The possibility of a pH-dependent impairment of glycosylation of cellular and viral proteins was traced for chloroquine and its derivatives. The use of low-toxicity drugs as part of adjunct therapy increases the effectiveness of synthetic antiviral drugs and interferons and ensures the safety of baseline therapy.

The past, present and future of RNA respiratory viruses: influenza and coronaviruses

Influenza virus and coronaviruses continue to cause pandemics across the globe. We now have a greater understanding of their functions. Unfortunately, the number of drugs in our armory to defend us against them is inadequate. This may require us to think about what mechanisms to address. Here, we review the biological properties of these viruses, their genetic evolution and antiviral therapies that can be used or have been attempted. We will describe several classes of drugs such as serine protease inhibitors, heparin, heparan sulfate receptor inhibitors, chelating agents, immunomodulators and many others. We also briefly describe some of the drug repurposing efforts that have taken place in an effort to rapidly identify molecules to treat patients with COVID-19. While we put a heavy emphasis on the past and present efforts, we also provide some thoughts about what we need to do to prepare for respiratory viral threats in the future.

The NLRP3-inflammasome as a sensor of organelle dysfunction

Diverse pathogen- and damage-associated stresses drive inflammation via activation of the multimolecular NLRP3-inflammasome complex. How the effects of diverse stimuli are integrated by the cell to regulate NLRP3 has been the subject of intense research, and yet an accepted unifying hypothesis for the control of NLRP3 remains elusive. Here, we review the literature on the effects of NLRP3-activating stimuli on subcellular organelles and conclude that a shared feature of NLRP3-activating stresses is an organelle dysfunction. In particular, we propose that the endosome may be more important than previously recognized as a signal-integrating hub for NLRP3 activation in response to many stimuli and may also link to the dysfunction of other organelles. In addition, NLRP3-inflammasome-activating stimuli trigger diverse posttranslational modifications of NLRP3 that are important in controlling its activation. Future research should focus on how organelles respond to specific NLRP3-activating stimuli, and how this relates to posttranslational modifications, to delineate the organellar control of NLRP3.

Genome-scale identification of cellular pathways required for cell surface recognition

Interactions mediated by cell surface receptors initiate important instructive signaling cues but can be difficult to detect in biochemical assays because they are often highly transient and membrane-embedded receptors are difficult to solubilize in their native conformation. Here, we address these biochemical challenges by using a genome-scale, cell-based genetic screening approach using CRISPR gene knockout technology to identify cellular pathways required for specific cell surface recognition events. By using high-affinity monoclonal antibodies and low-affinity ligands, we determined the necessary screening parameters, including the importance of establishing binding contributions from the glycocalyx, that permitted the unequivocal identification of genes encoding directly interacting membrane-embedded receptors with high statistical confidence. Importantly, we show that this genome-wide screening approach additionally identified receptor-specific pathways that are required for functional display of receptors on the cell surface that included chaperones, enzymes that add post-translational modifications, trafficking proteins, and transcription factors. Finally, we demonstrate the utility of the approach by identifying IGF2R (insulin like growth factor 2 receptor) as a binding partner for the R2 subunit of GABA_B receptors. We show that this interaction is direct and is critically dependent on mannose-6-phosphate, providing a mechanism for the internalization and regulation of GABA_B receptor signaling. We conclude that this single approach can reveal both the molecular nature and the genetic pathways required for functional cell surface display of receptors recognized by antibodies, secreted proteins, and membrane-embedded ligands without the need to make any prior assumptions regarding their biochemical properties.

The position of lysosomes within the cell determines their luminal pH

Analysis of luminal lysosomal pH in combination with heterologous expression of lysosomal-associated proteins indicates that peripheral lysosomes are more alkaline than juxtanuclear ones and that depletion of Rab7 and its effector, RILP, are associated with and can account for the reduced acidification.

We examined the luminal pH of individual lysosomes using quantitative ratiometric fluorescence microscopy and report an unappreciated heterogeneity: peripheral lysosomes are less acidic than juxtanuclear ones despite their comparable buffering capacity. An increased passive (leak) permeability to protons, together with reduced vacuolar H⁺–adenosine triphosphatase (V-ATPase) activity, accounts for the reduced acidifying ability of peripheral lysosomes. The altered composition of peripheral lysosomes is due, at least in part, to more limited access to material exported by the biosynthetic pathway. The balance between Rab7 and Arl8b determines the subcellular localization of lysosomes; more peripheral lysosomes have reduced Rab7 density. This in turn results in decreased recruitment of Rab-interacting lysosomal protein (RILP), an effector that regulates the recruitment and stability of the V1G1 component of the lysosomal V-ATPase. Deliberate margination of lysosomes is associated with reduced acidification and impaired proteolytic activity. The heterogeneity in lysosomal pH may be an indication of a broader functional versatility.

Omeprazole Induces Apoptosis in Jurkat Cells

We report for the first time a potent apoptotic effect of omeprazole (OM). Apoptosis was induced in Jurkat cells in a time and concentration-dependent mode. Caspase 3 and PARP were rapidly cleaved in response to OM, but apoptosis was only partially inhibited by the caspase 3 inhibitor DEVD-CHO. OM also induced an early lysosomal destabilization which increased progressively and was correlated with a parallel increase in apoptotic cells. The cysteine protease inhibitor E64d gave strong protection against apoptosis thus proving the involvement of lysosomal enzymes in OM-induced apoptosis whereas, it did not impede the caspase 3 cleavage. Instead ZVAD-fmk, a general caspase inhibitor, also able to inhibit cathepsin activity, protected cells completely from OM-induced apoptosis. It therefore seems that both caspases and cysteine cathepsins are involved in the execution stage of OM-induced apoptosis.

VARP is recruited on to endosomes by direct interaction with retromer, where together they function in export to the cell surface

VARP is a Rab32/38 effector that also binds to the endosomal/lysosomal R-SNARE VAMP7. VARP binding regulates VAMP7 participation in SNARE complex formation and can therefore influence VAMP7-mediated membrane fusion events. Mutant versions of VARP that cannot bind Rab32:GTP, designed on the basis of the VARP ankyrin repeat/Rab32:GTP complex structure described here, unexpectedly retain endosomal localization, showing that VARP recruitment is not dependent on Rab32 binding. We show that recruitment of VARP to the endosomal membrane is mediated by its direct interaction with VPS29, a subunit of the retromer complex, which is involved in trafficking from endosomes to the TGN and the cell surface. Transport of GLUT1 from endosomes to the cell surface requires VARP, VPS29, and VAMP7 and depends on the direct interaction between VPS29 and VARP. Finally, we propose that endocytic cycling of VAMP7 depends on its interaction with VARP and, consequently, also on retromer.

Inhibitors of intravesicular acidification protect against Shiga toxin in a pH-independent manner

Shiga toxin inhibits protein synthesis after being transported from the cell surface to endosomes and retrogradely through the Golgi apparatus to the endoplasmic reticulum (ER) and into the cytosol. In this study, we have abolished proton gradients across internal membranes in different ways and investigated the effect on the various transport steps of Shiga toxin. Although inhibitors of the proton pump such as bafilomycin A1 and concanamycin A as well as some ionophores and chloroquine all protect against Shiga toxin, they mediate protection by inhibiting different transport steps. For instance, chloroquine protects the cells, although the toxin is transported to the ER. Importantly, our data indicate that proton pump activity is required for efficient endosome-to-Golgi transport of Shiga toxin, although acidification as such does not seem to be required.

Cholesterol depletion of hepatoma cells impairs hepatitis B virus envelopment by altering the topology of the large envelope protein

Previous reports have shown that cholesterol depletion of the membrane envelope of the hepatitis B virus (HBV) impairs viral infection of target cells. A potential function of this lipid in later steps of the viral life cycle remained controversial, with secretion of virions and subviral particles (SVP) being either inhibited or not affected, depending on the experimental approach employed to decrease the intracellular cholesterol level. This work addressed the role of host cell cholesterol on HBV replication, assembly, and secretion, using an alternative method to inhibition of the enzymes involved in the biosynthesis pathway. Growing HBV-producing cells with lipoprotein-depleted serum (LPDS) resulted in an important reduction of the amount of cholesterol within 24 h of treatment (about 40%). Cell exposure to chlorpromazine, an inhibitor of the clathrin-mediated pathway used by the low-density lipoprotein receptor for endocytosis, also impacted the cholesterol level; however, this level of inhibition was not achievable when the synthesis inhibitor lovastatin was used. HBV secretion was significantly inhibited in cholesterol-depleted cells (by ∼80%), while SVP release remained unaffected. The viral DNA genome accumulated in LPDS-treated cells in a time-dependent manner. Specific immunoprecipitation of nucleocapsids and mature virions revealed an increased amount of naked nucleocapsids, while synthesis of the envelope proteins occurred as normally. Following analysis of the large envelope protein conformation in purified microsomes, we concluded that cholesterol is important in maintaining the dual topology of this polypeptide, which is critical for viral envelopment.

The acidic environment of the Golgi is critical for glycosylation and transport

Proteins and glycolipids are modified by various modes of glycosylation in the endoplasmic reticulum (ER) and the Golgi apparatus. It is well known that the lumen of the Golgi is acidic and compromising acidification by chemical compounds causes impaired glycosylation and transport of proteins (Axelsson et al., 2001; Chapman and Munro, 1994; Palokangas et al., 1994; Presley et al., 1997; Puri et al., 2002; Reaves and Banting, 1994; Rivinoja et al., 2006; Tartakoff et al., 1978). The mechanisms by which glycosylation and transport are regulated by an acidic pH remain largely unknown. Recent findings that the impaired regulation of an acidic environment may be implicated in the pathology of several diseases emphasize the importance of pH regulation (Jentsch, 2007; Kasper et al., 2005; Kornak et al., 2001; Kornak et al., 2008; Piwon et al., 2000; Stobrawa et al., 2001; Teichgraber et al., 2008). We recently established a mutant cell line in which Golgi acidification was selectively impaired and the raised luminal Golgi pH caused impaired transport and glycosylation of proteins and altered Golgi morphology (Maeda et al., 2008). As alkalinizing compounds nonselectively affect all acidic organelles including lysosomes, endosomes, and the Golgi, the mutant cell is thought to be useful in analyzing how the acidic environment of the Golgi regulates glycosylation. In this chapter, we have introduced how we established mutant cells with impaired Golgi acidification and methods for measuring Golgi pH.

Expression and distribution of cystic fibrosis transmembrane conductance regulator in neurons of the human brain

The importance of the molecule cystic fibrosis transmembrane conductance regulator (CFTR) is reflected in the many physiological functions it regulates. It is known to be present in epithelial cells of the lungs, pancreas, sweat glands, gut, and other tissues, and gene mutations of CFTR cause cystic fibrosis (CF). We studied the expression and distribution of CFTR in the human brain with reverse transcriptase polymerase chain reaction, in situ hybridization, and immunohistochemistry. This study demonstrates widespread and abundant expression of CFTR in neurons of the human brain. Techniques of double labeling and evaluation of consecutive tissue sections localized CFTR protein and mRNA signals to the cytoplasm of neurons in all regions of the brain studied, but not to glial cells. The presence of CFTR in central neurons not only provides a possible explanation for the neural symptoms observed in CF patients, but also may lead to a better understanding of the functions of CFTR in the human brain.

GPHR is a novel anion channel critical for acidification and functions of the Golgi apparatus

The organelles within secretory and endocytotic pathways in mammalian cells have acidified lumens, and regulation of their acidic pH is critical for the trafficking, processing and glycosylation of cargo proteins and lipids, as well as the morphological integrity of the organelles. How organelle lumen acidification is regulated, and how luminal pH elevation disturbs these fundamental cellular processes, is largely unknown. Here, we describe a novel molecule involved in Golgi acidification. First, mutant cells defective in Golgi acidification were established that exhibited delayed protein transport, impaired glycosylation and Golgi disorganization. Using expression cloning, a novel Golgi-resident multi-transmembrane protein, named Golgi pH regulator (GPHR), was identified as being responsible for the mutant cells. After reconstitution in planar lipid bilayers, GPHR exhibited a voltage-dependent anion-channel activity that may function in counterion conductance. Thus, GPHR modulates Golgi functions through regulation of acidification.

Altered localisation of the copper efflux transporters ATP7A and ATP7B associated with cisplatin resistance in human ovarian carcinoma cells

Background

Copper homeostasis proteins ATP7A and ATP7B are assumed to be involved in the intracellular transport of cisplatin. The aim of the present study was to assess the relevance of sub cellular localisation of these transporters for acquired cisplatin resistance in vitro. For this purpose, localisation of ATP7A and ATP7B in A2780 human ovarian carcinoma cells and their cisplatin-resistant variant, A2780cis, was investigated.

Methods

Sub cellular localisation of ATP7A and ATP7B in sensitive and resistant cells was investigated using confocal fluorescence microscopy after immunohistochemical staining. Co-localisation experiments with a cisplatin analogue modified with a carboxyfluorescein-diacetate residue were performed. Cytotoxicity of the fluorescent cisplatin analogue in A2780 and A2780cis cells was determined using an MTT-based assay. The significance of differences was analysed using Student's t test or Mann-Whitney test as appropriate, p values of < 0.05 were considered significant.

Results

In the sensitive cells, both transporters are mainly localised in the trans-Golgi network, whereas they are sequestrated in more peripherally located vesicles in the resistant cells. Altered localisation of ATP7A and ATP7B in A2780cis cells is likely to be a consequence of major abnormalities in intracellular protein trafficking related to a reduced lysosomal compartment in this cell line. Changes in sub cellular localisation of ATP7A and ATP7B may facilitate sequestration of cisplatin in the vesicular structures of A2780cis cells, which may prevent drug binding to genomic DNA and thereby contribute to cisplatin resistance.

Conclusion

Our results indicate that alterations in sub cellular localisation of transport proteins may contribute to cisplatin resistance in vitro. Investigation of intracellular protein localisation in primary tumour cell cultures and tumour tissues may help to develop markers of clinically relevant cisplatin resistance. Detection of resistant tumours in patients may in turn enable individualization of the chemotherapy in the early stage of treatment.

Kalirin/Trio Rho guanine nucleotide exchange factors regulate a novel step in secretory granule maturation

The molecular mechanisms involved in the maturation of secretory granules, organelles that store hormones and neuropeptides, are poorly understood. As granule content proteins are processed, the composition of granule membranes changes, yielding constitutive-like secretion of immature content proteins and producing secretagogue-responsive mature granules. Constitutive-like secretion was not previously recognized as a process subject to regulation. We show that Kalirin and Trio, homologous Rho guanine nucleotide exchange factors (GEFs), which interact with a secretory granule resident protein, modulate cargo secretion from immature granules. Some of the Kalirin and Trio isoforms expressed in neuroendocrine cells colocalize with immature granules. Overexpression of their N-terminal GEF domain (GEF1) enhances secretion from immature granules, depleting cells of secretory cargo in the absence of secretagogue. This response requires GEF1 activity and is mimicked by Kalirin/Trio substrates Rac1 and RhoG. Accordingly, selective pharmacological inhibition of endogenous GEF1 activity decreases secretagogue-independent release of hormone precursors, accumulating product peptide in mature secretory granules. Kalirin/Trio modulation of cargo secretion from immature granules provides secretory cells with an extra layer of control over the sets of peptides released. Control of this step enhances the range of physiological responses that can be elicited, whereas lack of control could have pathological consequences.

The luminal domain participates in the endosomal trafficking of the cation-independent mannose 6-phosphate receptor

Although the role of the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor (CIMPR) has been well established in the receptor trafficking, that of the luminal domain is still controversial. We noticed that the peripheral distribution of GFP, fused to the transmembrane and cytoplasmic domains of CIMPR (G-CIMPR-tail), was distinct from that of endogenous CIMPR or of GFP fused to the full-length CIMPR (G-CIMPR-full). By live-cell imaging, trans-Golgi-network (TGN)-derived transport carriers containing G-CIMPR-full more frequently stopped and overlapped with transferrin-containing endosomes in the peripheral region than those containing G-CIMPR-tail. G-CIMPR-full was recycled back to the perinuclear TGN more slowly than that for G-CIMPR-tail, evidenced by fluorescence recovery after photobleaching analysis. Moreover, endogenous CIMPR and G-CIMPR-full, but not GFP-CIMPR-tail, drastically altered the characteristic distribution after treatment with chloroquine. A mutant receptor, G-CIMPR-full R/A, that cannot recognize the mannose 6-phosphate (M6P)-signal, behaved similarly to G-CIMPR-full, indicating that these differences are not attributable to the M6P-ligands binding situation. Interestingly, we also found that U18666A treatment was able to discriminate the M6P-ligand binding-dependent trafficking of CIMPR. Based on these findings, we propose that the CIMPR luminal domain is required for tight interaction with endocytic compartments, and retention by them, and that there are additional transport steps, in which the binding to M6P-ligands is involved.

GMx33 associates with the trans-Golgi matrix in a dynamic manner and sorts within tubules exiting the Golgi

The trans-Golgi matrix consists of a group of proteins dynamically associated with the trans-Golgi and thought to be involved in anterograde and retrograde Golgi traffic, as well as interactions with the cytoskeleton and maintenance of the Golgi structure. GMx33 is localized to the cytoplasmic face of the trans-Golgi and is also present in a large cytoplasmic pool. Here we demonstrate that GMx33 is dynamically associated with the trans-Golgi matrix, associating and dissociating with the Golgi in seconds. GMx33 can be locked onto the trans-Golgi matrix by GTPgammaS, indicating that its association is regulated in a GTP-dependent manner like several other Golgi matrix proteins. Using live-cell imaging we show that GMx33 exits the Golgi associated with tubules and within these tubules GMx33 segregates from transmembrane proteins followed by fragmentation of the tubules into smaller tubules and vesicles. Within vesicles produced by an in vitro budding reaction, GMx33 remains segregated in a matrixlike tail region that sometimes contains Golgin-245. This trans-matrix often links a few vesicles together. Together these data suggest that GMx33 is a member of the trans-Golgi matrix and offer clues regarding the role of the trans-Golgi matrix in sorting and exit from the Golgi.

ADP-ribosylation factor 1-independent protein sorting and export from the trans-Golgi network

Polarized epithelial cells efficiently sort newly synthesized apical and basolateral proteins into distinct transport carriers that emerge from the trans-Golgi network (TGN), and this sorting is recapitulated in nonpolarized cells. While the targeting signals of basolaterally destined proteins are generally cytoplasmically disposed, apical sorting signals are not typically accessible to the cytosol, and the transport machinery required for segregation and export of apical cargo remains largely unknown. Here we investigated the molecular requirements for TGN export of the apical marker influenza hemagglutinin (HA) in HeLa cells using an in vitro reconstitution assay. HA was released from the TGN in intact membrane-bound compartments, and export was dependent on addition of an ATP-regenerating system and exogenous cytosol. HA release was inhibited by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) as well as under conditions known to negatively regulate apical transport in vivo, including expression of the acid-activated proton channel influenza M2. Interestingly, release of HA was unaffected by depletion of ADP-ribosylation factor 1, a small GTPase that has been implicated in the recruitment of all known adaptors and coat proteins to the Golgi complex. Furthermore, regulation of HA release by GTPgammaS or M2 expression was unaffected by cytosolic depletion of ADP-ribosylation factor 1, suggesting that HA sorting remains functionally intact in the absence of the small GTPase. These data suggest that TGN sorting and export of influenza HA does not require classical adaptors involved in the formation of other classes of exocytic carriers and thus appears to proceed via a novel mechanism.

TMF is a golgin that binds Rab6 and influences Golgi morphology

BACKGROUND

Golgins are coiled-coil proteins associated with the Golgi apparatus, that are believed to be involved in the tethering of vesicles and the stacking of cisternae, as well as other functions such as cytoskeletal association. Many are peripheral membrane proteins recruited by GTPases. Several have been described in animal cells, and some in yeast, but the relationships between golgins from different species can be hard to define because although they share structural features, their sequences are not well conserved.

RESULTS

We show here that the yeast protein Sgm1, previously shown to be recruited to the Golgi by the GTPase Ypt6, binds to Ypt6:GTP via a conserved 100-residue coiled-coil motif that can be identified in a wide range of eukaryotes. The mammalian equivalent of Sgm1 is TMF/ARA160, a protein previously identified in various screens as a putative transcription or chromatin remodelling factor. We show that it is a Golgi protein, and that it binds to the three known isoforms of the Ypt6 homologue Rab6. Depletion of the protein by RNA interference in rat NRK cells results in a modest dispersal of Golgi membranes around the cell, suggesting a role for TMF in the movement or adherence of Golgi stacks.

CONCLUSION

We have identified TMF as an evolutionarily conserved golgin that binds Rab6 and contributes to Golgi organisation in animal cells.

Important role for the V-type H(+)-ATPase and the Golgi apparatus in the recycling of PTH/PTHrP receptor

Our previous studies demonstrated that a green fluorescent protein-tagged parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor stably expressed in LLCPK-1 cells undergoes agonist-dependent internalization into clathrin-coated pits. The subcellular localization of the internalized PTH/PTHrP receptor is not known. In the present study, we explored the intracellular pathways of the internalized PTH/PTHrP receptor. Using immunofluorescence and confocal microscopy, we show that the internalized receptors localize at a juxtanuclear compartment identified as the Golgi apparatus. The receptors do not colocalize with lysosomes. Furthermore, whereas the internalized receptors exhibit rapid recycling, treatment with proton pump inhibitors (bafilomycin-A1 and concanamycin A) or brefeldin A, Golgi disrupting agents, reduces PTH/PTHrP receptor recycling. Together, these data indicate an important role for the vacuolar-type hydrogen-ATPase and the Golgi apparatus in postendocytic PTH/PTHrP receptor recovery.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.

The p55 tumour necrosis factor receptor TNFR1 contains a trans-Golgi network localization signal in the C-terminal region of its cytoplasmic tail

It has been reported in some human cells that, in addition to a plasma membrane localization, members of the tumour necrosis factor receptor superfamily may be localized to the Golgi complex. We have shown by immunofluorescence and immunoelectron microscopy that the p55 tumour necrosis factor receptor, TNFR1, is principally localized to the trans-Golgi network in the human breast carcinoma cell line, MCF7. Chimaeras consisting of the extracellular and transmembrane domains of CD8 together with the cytoplasmic tail of TNFR1 were targeted to the trans-Golgi network in stably transfected rat fibroblastic cells. Deletions in the cytoplasmic tails of these chimaeras demonstrated the requirement for the C-terminal sequence of 23 amino acids for this targeting. The 23 amino acid sequence is mostly outside the death domain and contains both an acid patch and a dileucine motif. Interaction of this sequence with membrane traffic adaptor proteins may play an important role in controlling the responses of cells to tumour necrosis factor, since binding of signalling adaptor proteins has only been demonstrated for plasma membrane, and not Golgi-localized, TNFR1.

S100A13 and S100A6 exhibit distinct translocation pathways in endothelial cells

S100 proteins have attracted great interest in recent years because of their cell- and tissue-specific expression and association with various human pathologies. Most S100 proteins are small acidic proteins with calcium-binding domains — the EF hands. It is thought that this group of proteins carry out their cellular functions by interacting with specific target proteins, an interaction that is mainly dependent on exposure of hydrophobic patches, which result from calcium binding. S100A13, one of the most recently identified members of the S100 family, is expressed in various tissues. Interestingly,hydrophobic exposure was not observed upon calcium binding to S100A13 even though the dimeric form displays two high- and two low- affinity sites for calcium. Here, we followed the translocation of S100A13 in response to an increase in intracellular calcium levels, as protein translocation has been implicated in assembly of signaling complexes and signaling cascades, and several other S100 proteins are involved in such events. Translocation of S100A13 was observed in endothelial cells in response to angiotensin II, and the process was dependent on the classic Golgi-ER pathway. By contrast, S100A6 translocation was found to be distinct and dependent on actin-stress fibers. These experiments suggest that different S100 proteins utilize distinct translocation pathways, which might lead them to certain subcellular compartments in order to perform their physiological tasks in the same cellular environment.

Sonic hedgehog induces the segregation of patched and smoothened in endosomes

BACKGROUND

Sonic hedgehog (Shh) signal transduction involves the ligand binding Patched1 (Ptc1) protein and a signaling component, Smoothened (Smo). A select group of compounds inhibits both Shh signaling, regulated by Ptc1, and late endosomal lipid sorting, regulated by the Ptc-related Niemann-Pick C1 (NPC1) protein. This suggests that Ptc1 regulates Smo activity through a common late endosomal sorting pathway also utilized by NPC1. During signaling, Ptc accumulates in endosomal compartments, but it is unclear if Smo follows Ptc into the endocytic pathway.

RESULTS

We characterized the dynamic subcellular distributions of Ptc1, Smo, and activated Smo mutants individually and in combination. Ptc1 and Smo colocalize extensively in the absence of ligand and are internalized together after ligand binding, but Smo becomes segregated from Ptc1/Shh complexes destined for lysosomal degradation. In contrast, activated Smo mutants do not colocalize with nor are cotransported with Ptc1. Agents that block late endosomal transport and protein sorting inhibit the ligand-induced segregation of Ptc1 and Smo. We show that, like NPC1-regulated lipid sorting, Shh signal transduction is blocked by antibodies that specifically disrupt the internal membranes of late endosomes, which provide a platform for protein and lipid sorting.

CONCLUSIONS

These data support a model in which Ptc1 inhibits Smo only when in the same compartment. Ligand-induced segregation allows Smo to signal independently of Ptc1 after becoming sorted from Ptc1/Shh complexes in the late endocytic pathway.

Abnormal glycosylation and altered Golgi structure in colorectal cancer: dependence on intra-Golgi pH

Abnormal glycosylation of cellular glycoconjugates is a common phenotypic change in many human tumors. Here, we explore the possibility that an altered Golgi pH may also be responsible for these cancer-associated glycosylation abnormalities. We show that a mere dissipation of the acidic Golgi pH results both in increased expression of some cancer-associated carbohydrate antigens and in structural disorganization of the Golgi apparatus in otherwise normally glycosylating cells. pH dependence of these alterations was confirmed by showing that an acidification-defective breast cancer cell line (MCF-7) also displayed a fragmented Golgi apparatus, whereas the Golgi apparatus was structurally normal in its acidification-competent subline (MCF-7/AdrR). Acidification competence was also found to rescue normal glycosylation potential in MCF-7/AdrR cells. Finally, we show that abnormal glycosylation is also accompanied by similar structural disorganization and fragmentation of the Golgi apparatus in colorectal cancer cells in vitro and in vivo. These results suggest that an inappropriate Golgi pH may indeed be responsible for the abnormal Golgi structure and lowered glycosylation potential of the Golgi apparatus in malignant cells.

Role of adaptor complex AP-3 in targeting wild-type and mutated CD63 to lysosomes

CD63 is a lysosomal membrane protein that belongs to the tetraspanin family. Its carboxyterminal cytoplasmic tail sequence contains the lysosomal targeting motif GYEVM. Strong, tyrosine-dependent interaction of the wild-type carboxyterminal tail of CD63 with the AP-3 adaptor subunit mu 3 was observed using a yeast two-hybrid system. The strength of interaction of mutated tail sequences with mu 3 correlated with the degree of lysosomal localization of similarly mutated human CD63 molecules in stably transfected normal rat kidney cells. Mutated CD63 containing the cytosolic tail sequence GYEVI, which interacted strongly with mu 3 but not at all with mu 2 in the yeast two-hybrid system, localized to lysosomes in transfected normal rat kidney and NIH-3T3 cells. In contrast, it localized to the cell surface in transfected cells of pearl and mocha mice, which have genetic defects in genes encoding subunits of AP-3, but to lysosomes in functionally rescued mocha cells expressing the delta subunit of AP-3. Thus, AP-3 is absolutely required for the delivery of this mutated CD63 to lysosomes. Using this AP-3-dependent mutant of CD63, we have shown that AP-3 functions in membrane traffic from the trans-Golgi network to lysosomes via an intracellular route that appears to bypass early endosomes.

Insulin and isoproterenol have opposing roles in the maintenance of cytosol pH and optimal fusion of GLUT4 vesicles with the plasma membrane

Insulin treatment of rat adipocytes increases both cytoplasmic alkalinity and glucose transport activity. Both processes are blocked by the phosphatidylinositol 3-kinase inhibitor wortmannin. Isoproterenol pre-treatment reverses the alkalinizing effects of insulin and leads to attenuation of insulin-stimulated glucose transport activity and exposure of GLUT4 to photolabeling reagents at the cell surface. These effects of isoproterenol are mimicked by acid loading and are reversed by cell-alkalinizing conditions. However, neither isoproterenol nor acid loading alters the total level of GLUT4 at the plasma membrane as revealed by Western blotting of plasma membrane fractions or immunodetection of GLUT4 in plasma membrane lawns. GLUT4 is therefore occluded from participation in glucose transport catalysis by a pH-sensitive process. To examine the kinetics of trafficking that lead to these changes in cell surface GLUT4 occlusion, we have utilized a new biotinylated photolabel, GP15. This reagent has a 70-atom spacer between the biotin and the photolabeling diazirine group, and this allows quenching of the surface signal of biotinylated GLUT4 by extracellular avidin. The rates of GLUT4 internalization are only slightly altered by isoproterenol or acidification, mainly due to reduced recycling over long internalization times. By contrast, insulin stimulation of GLUT4 exocytosis is slowed by isoproterenol or acidification pre-treatments. Biphasic time courses are evident, with an initial burst of exposure at the cell surface followed by a slow phase. It is hypothesized that the burst kinetics are a consequence of a two-phase fusion reaction that is rapid in the presence of insulin but slowed by cytosol acidification.

Elevated endosomal pH in HeLa cells overexpressing mutant dynamin can affect infection by pH-sensitive viruses

Many viruses gain access to the cell via the endosomal route and require low endosomal pH for infectivity. The GTPase dynamin is essential for clathrin-dependent endocytosis, and in HeLa cells overexpressing the nonfunctional dynaminK44A mutant the formation of clathrin-coated vesicles is halted. HRV2, a human minor group rhinovirus, is internalized by members of the low-density lipoprotein receptor family in a clathrin-independent manner. The low endosomal pH then leads to conversion of the capsid to C-antigen, which is required for release (uncoating) and transfer of the viral RNA into the cytosol and de novo synthesis of infectious virus. We here demonstrate that overexpression of dynaminK44A reduces this antigenic conversion and results in diminished viral synthesis. In contrast, lysosomal degradation is unaffected. The kinetics of the formation of C-antigen in vitro and in vivo suggest that the pH in endosomes is elevated by about 0.4 units upon overexpression of dynaminK44A. As a consequence, HRV2 uncoating is diminished early after internalization but attains control levels upon prolonged internalization. Thus, overexpression of dynaminK44A, in addition to trafficking defects, results in an elevated endosomal pH and thereby affects virus infection and most likely endosomal sorting and processing.

Prolactin signalling to milk protein secretion but not to gene expression depends on the integrity of the Golgi region

Prolactin added to the incubation medium of lactating mammary epithelial cells is transported from the basal to the apical region of cells through the Golgi region and concomitantly stimulates arachidonic acid release and protein milk secretion. We report that when PRL is added after disorganisation of the Golgi apparatus by brefeldin A treatment, prolactin signalling to expression of genes for milk proteins and prolactin endocytosis are not affected. However, prolactin transport to the apical region of cells (transcytosis), as well as prolactin-induced arachidonic acid release and subsequent stimulation of the secretion of caseins, which are located in a post-Golgi compartment, are inhibited. This inhibition was not a consequence of damage to the secretory machinery, as under the same conditions, protein secretion could be stimulated by the addition of arachidonic acid to the incubation medium. Thus, it is possible to discriminate between prolactin-induced actions that are dependent (signalling to milk protein secretion) or independent (signalling to milk gene expression) on the integrity of the Golgi apparatus. These results suggest that these two biological actions may be transduced via distinct intracellular pathways, and support the hypothesis that prolactin signals may be emitted at various cellular sites.

Cholera toxin and Escherichia coli enterotoxin B-subunits inhibit macrophage-mediated antigen processing and presentation: evidence for antigen persistence in non-acidic recycling endosomal compartments

Cholera toxin (Ctx) and the closely related Escherichia coli heat-labile enterotoxin (Etx) not only act as mediators of diarrhoeal disease but also exert potent immunomodulatory properties on mammalian immune systems. The toxins normally exert their diarrhoeagenic effects by initiating receptor-mediated uptake into vesicles that enter a retrograde trafficking pathway, circumventing degradative compartments and targeting them to the trans-Golgi network (TGN) and endoplasmic reticulum. Here, we examine whether receptor-mediated binding and cellular entry by the toxin B-subunits also lead to concomitant changes in uptake and trafficking of exogenous antigens that could contribute to the potent immunomodulatory properties of these toxins. Treatment of the macrophage (J774.2) cell line with Etx B-subunit (EtxB) resulted in EtxB transport to the TGN and also led to the formation of large, translucent, non-acidic, EtxB-devoid vacuoles. When exogenous antigens were added, EtxB-treated cells were found to be proficient in both internalization of ovalbumin (OVA) and phagocytosis of bacterial particles. However, the internalized OVA, instead of trafficking along a lysosome-directed endocytic pathway via acidified endosomes, persisted in a non-acidic, light-density compartment that was distinct from the translucent vacuoles. The rerouted OVA did not co-localize with the endosomal markers rab5 or rab11, nor with EtxB, but was retained in a transferrin receptor-positive compartment. The failure of OVA to enter the late endosomal/lysosomal compartments correlated with a striking inhibition of OVA peptide processing and presentation to OVA-responsive CD4+ T-cells. CtxB also modulated OVA trafficking and inhibited antigen presentation. These findings demonstrate that the B-subunits of Ctx and Etx alter the progression of exogenous antigens along the endocytic processing pathway, and prevent or delay efficient epitope presentation and T-cell stimulation. The formation of such 'antigen depots' could contribute to the immunomodulatory properties of these bacterial virulence determinants.

Legionella pneumophila replication vacuoles mature into acidic, endocytic organelles

After ingestion by macrophages, Legionella pneumophila inhibits acidification and maturation of its phagosome. After a 6-10-h lag period, the bacteria replicate for 10-14 h until macrophage lysis releases dozens of progeny. To examine whether the growth phase of intracellular L. pneumophila determines the fate of its phagosome, interactions between the endosomal network and pathogen vacuoles were analyzed throughout the primary infection period. Surprisingly, as L. pneumophila replicated exponentially, a significant proportion of the vacuoles acquired lysosomal characteristics. By 18 h, 70% contained lysosomal-associated membrane protein 1 (LAMP-1) and 40% contained cathepsin D; 50% of the vacuoles could be labeled by endocytosis, and the pH of this population of vacuoles averaged 5.6. Moreover, L. pneumophila appeared to survive and replicate within lysosomal compartments: vacuoles harboring more than five bacteria also contained LAMP-1, inhibition of vacuole acidification and maturation by bafilomycin A1 inhibited bacterial replication, bacteria within endosomal vacuoles responded to a metabolic inducer by expressing a gfp reporter gene, and replicating bacteria obtained from macrophages, but not broth, were acid resistant. Understanding how L. pneumophila first evades and then exploits the endosomal pathway to replicate within macrophages may reveal the mechanisms governing phagosome maturation, a process also manipulated by Mycobacteria, Leishmania, and Coxiella.

Apical and basolateral endocytic pathways of MDCK cells meet in acidic common endosomes distinct from a nearly-neutral apical recycling endosome

Quantitative confocal microscopic analyses of living, polarized MDCK cells demonstrate different pH profiles for apical and basolateral endocytic pathways, despite a rapid and extensive intersection between the two. Three-dimensional characterizations of ligand trafficking demonstrate that the apical and basolateral endocytic pathways share early, acidic compartments distributed throughout the medial regions of the cell. Polar sorting for both pathways occurs in these common endosomes as IgA is sorted from transferrin to alkaline transcytotic vesicles. While transferrin is directly recycled from the common endosomes, IgA is transported to a downstream apical compartment that is nearly neutral in pH. By several criteria this compartment appears to be equivalent to the previously described apical recycling endosome. The functional significance of the abrupt increase in lumenal pH that accompanies IgA sorting is not clear, as disrupting endosome acidification has no effect on polar sorting. These studies provide the first detailed characterizations of endosome acidification in intact polarized cells and clarify the relationship between the apical and basolateral endocytic itineraries of polarized MDCK cells. The extensive mixing of apical and basolateral pathways underscores the importance of endocytic sorting in maintaining the polarity of the plasma membrane of MDCK cells.

Primaquine interferes with membrane recycling from endosomes to the plasma membrane through a direct interaction with endosomes which does not involve neutralisation of endosomal pH nor osmotic swelling of endosomes

The anti-malaria drug primaquine is a weak base which accumulates in endosomes in a protonated form and consequently neutralises the endosomal pH. Bafilomycin A1 prevents endosome acidification by inhibiting the vacuolar proton pump. Although both agents neutralise the endosomal pH, only primaquine has a strong inhibitory effect on recycling of endocytosed proteins to the plasma membrane (Van Weert et al. (1995), J. Cell Biol. 130, 821-834). This suggests that primaquine interferes with a parameter, other than endosomal pH, that is essential for membrane recycling. In the presence of 0.3 mM primaquine, endocytosed transferrin-receptors accumulated intracellularly, but not in the additional presence of bafilomycin A1. Thus, at relative low concentrations proton pump-driven accumulation of primaquine in endosomes was required to inhibit membrane recycling, suggesting that the target of primaquine is associated with endosomes. The inhibitory effect of 1 mM primaquine on transferrin receptor recycling was not reversed by the additional presence of bafilomycin A1, indicating that osmotic swelling of endosomes due to accumulation of protonated primaquine could also not explain its effect. To study endosome swelling morphologically, we introduce a novel technique for fluorescent labelling of endosomes involving HRP-catalysed biotinylation. In the presence of 0.2 mM primaquine endosomal vacuoles with diameters up to 2 microm were observed. Endosome swelling was not observed when in addition to primaquine also bafilomycin A1 was present, supporting the notion that vacuolar proton pump activity lowers the dose response for primaquine. Factors that are crucial for membrane recycling and may be affected by primaquine are discussed.

Recycling of AQP2 occurs through a temperature- and bafilomycin-sensitive trans-Golgi-associated compartment

The exo- and endocytotic pathway in which aquaporin-2 (AQP2) travels between the plasma membrane and intracellular vesicles is only partially characterized. It is known that the antidiuretic hormone vasopressin induces a translocation of AQP2 from an intracellular to a plasma membrane location, both in kidney collecting duct principal cells and in transfected epithelial cells. Here we provide evidence suggesting that while AQP2 shifts from an intracellular location to the cell surface in response to vasopressin, AQP2 also constitutively recycles through a similar pathway in transfected LLC-PK(1) cells even in the absence of hormonal stimulation. Incubating cells at 20 degrees C blocks AQP2 recycling in a perinuclear compartment, regardless of whether vasopressin is present. The H(+)-ATPase inhibitor bafilomycin A1 also blocks the recycling pathway of AQP2 in a perinuclear compartment adjacent to the Golgi in the presence and absence of vasopressin stimulation, indicating a role of vesicle acidification in both the constitutive and regulated recycling of AQP2. Colocalization of AQP2 with clathrin, but not with giantin, after both bafilomycin treatment and a 20 degrees C block suggests that the compartment in which recycling AQP2 is blocked may be the trans-Golgi, and not cis- and medial-Golgi cisternae.

Role of vacuolar H(+)-ATPase in interferon-induced inhibition of viral glycoprotein transport

We have shown previously that interferon-beta (IFN-beta) induces the alkalinization of trans-Golgi network (TGN) and inhibits the transport of G protein of vesicular stomatitis virus (VSV) in L(B) cells and gD protein of herpes simplex virus (HSV-1) in LMtk- cells transfected with gD cDNA. The vacuolar H(+)-ATPase (V-ATPase) is responsible for maintaining pH in TGN, and V-ATPase-mediated acidification is required for normal transport of proteins. To examine whether alkalinization caused by IFN is mediated through V-ATPase, the activity of V-ATPase was determined in IFN-treated cells by coupling ATP hydrolysis to NADH oxidation. Bafilomycin (Baf) was used as positive control, as it specifically inhibits V-ATPase. The activity of V-ATPase was reduced in IFN-treated or Baf-treated cells compared with untreated cells. Doses of IFN-beta or Baf that neither alter pHi nor inhibit the transport of viral glycoproteins concomitantly inhibited the transport of G and gD proteins in TGN, as demonstrated by indirect immunofluorescence studies, and raised the pH of TGN as demonstrated by a decrease in the uptake of DAMP. Further, the effect of Baf on IFN-induced antiviral activity against VSV was examined to correlate the biologic significance of these findings. Data showed that Baf significantly enhances (5-50-fold) the IFN-induced antiviral activity as demonstrated by viral titers from supernatants. These findings suggest that the inhibition of transport of G and gD proteins by IFN-beta, may be related to the inhibition of V-ATPase-mediated acidification of TGN.

MAL, an integral element of the apical sorting machinery, is an itinerant protein that cycles between the trans-Golgi network and the plasma membrane

The MAL proteolipid is a nonglycosylated integral membrane protein found in glycolipid-enriched membrane microdomains. In polarized epithelial Madin-Darby canine kidney cells, MAL is necessary for normal apical transport and accurate sorting of the influenza virus hemagglutinin. MAL is thus part of the integral machinery for glycolipid-enriched membrane-mediated apical transport. At steady state, MAL is predominantly located in perinuclear vesicles that probably arise from the trans-Golgi network (TGN). To act on membrane traffic and to prevent their accumulation in the target compartment, integral membrane elements of the protein-sorting machinery should be itinerant proteins that cycle between the donor and target compartments. To establish whether MAL is an itinerant protein, we engineered the last extracellular loop of MAL by insertion of sequences containing the FLAG epitope or with sequences containing residues that became O-glycosylated within the cells or that displayed biotinylatable groups. The ectopic expression of these modified MAL proteins allowed us to investigate the surface expression of MAL and its movement through different compartments after internalization with the use of a combination of assays, including surface biotinylation, surface binding of anti-FLAG antibodies, neuraminidase sensitivity, and drug treatments. Immunofluorescence and flow cytometric analyses indicated that, in addition to its Golgi localization, MAL was also expressed on the cell surface, from which it was rapidly internalized. This retrieval implies transport through the endosomal pathway and requires endosomal acidification, because it can be inhibited by drugs such as chloroquine, monensin, and NH(4)Cl. Resialylation experiments of surface MAL treated with neuraminidase indicated that approximately 30% of the internalized MAL molecules were delivered to the TGN, probably to start a new cycle of cargo transport. Together, these observations suggest that, as predicted for integral membrane members of the late protein transport machinery, MAL is an itinerant protein cycling between the TGN and the plasma membrane.

Stage-dependent redistribution of the V-ATPase during bovine implantation

The 16-kD subunit of the vacuolar H(+)-ATPase (V-ATPase), or ductin, is essential for the activity of this proton pump and has roles in intercellular communication and control of cell growth and differentiation. The V-ATPase is important for acidification-dependent degradation of tissue matrices through which some cell types move, and for pH regulation across some epithelial cell layers. Placentation involves intricate signaling, cell proliferation, and controlled invasion. We examined the distribution of three subunits of the V-ATPase in bovine trophoblast and endometrium at the time of implantation to determine the relationship of ductin expression to that of two other subunits, A (approximately 73 kD) and B (approximately 58 kD). Epithelial expression of all three subunits was observed, and in nonpregnant animals this expression was apical. As pregnancy proceeded, expression of all subunits became pericellular in luminal but not glandular epithelium, suggesting a redistribution of V-ATPase activity. The trophoblast expressed all three subunits during initial contact with the epithelium. In the stroma, ductin expression was reduced after implantation, and we discuss the possibility that ductin plays a role in the shifting communication between stromal and epithelial cells induced by embryo attachment. (J Histochem Cytochem 47:1247-1254, 1999)

Endosomal/lysosomal retention and degradation of major histocompatibility complex class I molecules is induced by myxoma virus

The highly immunosuppressive leporipoxvirus myxoma, previously was shown to promote the loss of cell surface class I major histocompatibility complex (MHC I) molecules. Here, we show that myxoma virus induces the loss of both cell surface and intracellular post-Golgi, beta(2)-microglobulin-associated MHC I. Myxoma-induced loss of these MHC I molecules is abrogated by vacuolar ATPase inhibitors, NH(4)Cl, and leupeptin. Furthermore, immunofluorescence microscopic studies reveal that in myxoma-infected cells, beta(2)-microglobulin-associated MHC I accumulates in Lamp-1(+) vesicular structures, suggesting that myxoma virus targets MHC I for degradation in late endosomes and/or lysosomes. These events are regulated by early gene product or products because they occur unabated in cells infected with myxoma virus in the presence of cytosine arabinoside, an inhibitor of DNA synthesis. Studies with baby green monkey kidney cells transfected with wild-type and tail-less forms of a mouse MHC I molecule, H-2L(d), indicate that the MHC I cytoplasmic tail is required for myxoma-induced localization in Lamp-1(+) organelles. Myxoma-induced endocytosis and degradation of MHC I may provide the virus with a means of dispensing with cell surface MHC I molecules that were loaded with peptides derived from viral proteins synthesized early in infection.

beta(1) integrin binds the 16-kDa subunit of vacuolar H(+)-ATPase at a site important for human papillomavirus E5 and platelet-derived growth factor signaling

Integrins mediate adhesive interactions between cells and the extracellular matrix, and play a role in cell migration, proliferation, differentiation, cytoskeletal organization, and signal transduction. We have identified an interaction between the beta(1) integrin and the 16-kDa subunit of vacuolar H(+)-ATPase (16K). This interaction was first isolated in a yeast two-hybrid screen and confirmed by coimmunoprecipitation and in in vitro binding assays using bacterially expressed proteins. Immunofluorescent studies performed in L6 myoblasts expressing both native and epitope-tagged 16K demonstrate co-localization with beta(1) integrin in focal adhesions. Deletion of the fourth of four transmembrane helices in 16K results in loss of interaction with beta(1) integrin in vitro and in the two-hybrid system, and less prominent staining in focal adhesions. This helix is also required for ligand-independent activation of platelet-derived growth factor-beta receptor signaling by the human papillomavirus E5 oncoprotein. Overexpression of 16K or expression of 16K lacking this helix alters the morphology of myoblasts and fibroblasts, suggesting that the interaction of 16K with integrins could be important for cell growth control. We also discuss the possible role 16K might play in integrin movement.

The steady state distribution of humTGN46 is not significantly altered in cells defective in clathrin-mediated endocytosis

It has been shown previously that whilst the rat type I integral membrane protein TGN38 (ratTGN38) is predominantly localised to the trans-Golgi network this protein does reach the cell surface from where it is internalised and delivered back to the trans-Golgi network. This protein thus provides a suitable tool for the investigation of trafficking pathways between the trans-Golgi network and the cell surface and back again. The human orthologue of ratTGN38, humTGN46, behaves in a similar fashion. These proteins are internalised from the cell surface via clathrin mediated endocytosis, a process which is dependent upon the GTPase activity of dynamin. We thus reasoned that humTGN46 would accumulate at the surface of cells rendered defective in clathrin mediated endocytosis by virtue of the fact that they express a GTPase defective mutant of dynamin I. It did not. In fact, expression of a dominant negative GTPase defective mutant of dynamin I had no detectable effect on the steady state distribution of humTGN46. One explanation for this observation is that humTGN46 does not travel directly to the cell surface from the trans-Golgi network. Further studies on cells expressing the dominant negative GTPase defective mutant of dynamin I indicate that the major recycling pathway for humTGN46 is in fact between the trans-Golgi network and the early endosome.

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.

Lumenal and transmembrane domains play a role in sorting type I membrane proteins on endocytic pathways

Previous studies have shown that when the cytosolic domains of the type I membrane proteins TGN38 and lysosomal glycoprotein 120 (lgp120) are added to a variety of reporter molecules, the resultant chimeric molecules are localized to the trans-Golgi network (TGN) and to lysosomes, respectively. In the present study we expressed chimeric constructs of rat TGN38 and rat lgp120 in HeLa cells. We found that targeting information in the cytosolic domain of TGN38 could be overridden by the presence of the lumenal and transmembrane domains of lgp120. In contrast, the presence of the transmembrane and cytosolic domains of TGN38 was sufficient to deliver the lumenal domain of lgp120 to the trans-Golgi network. On the basis of steady-state localization of the various chimeras and antibody uptake experiments, we propose that there is a hierarchy of targeting information in each molecule contributing to sorting within the endocytic pathway. The lumenal and cytosolic domains of lgp120 contribute to sorting and delivery to lysosomes, whereas the transmembrane and cytosolic domains of TGN38 contribute to sorting and delivery to the trans-Golgi network.

Endothelin-converting enzyme: ultrastructural localization and its recycling from the cell surface

The potent vasoconstrictor endothelin-1 (ET-1) is secreted constitutively by endothelial cells and has been implicated in the pathophysiology of several cardiovascular diseases. It is generated from its inactive intermediate, big ET-1, through the action of endothelin-converting enzyme (ECE). Using several complementary techniques, we have demonstrated that ECE is present at the cell surface and on intracellular vesicles and that it recycles from the cell surface in endothelial cells. This is the first ultrastructural localization of ECE in lung and the first time big ET-1 and ECE have been colocalized by immunogold in a vesicular population, 50 to 100 nm in diameter. In addition, by double immunogold staining of ultrathin cryosections, we have localized ECE together with angiotensin-converting enzyme on the luminal membrane of endothelial cells. With cell surface biotinylation of a transformed rat endothelial cell line and of human umbilical vein endothelial cells, we have confirmed the presence of ECE on the plasma membrane. After treatment of endothelial cells with chloroquine, ECE and trans-Golgi network 38 protein were shown by immunofluorescence staining to localize to the same intracellular compartment.

pH-dependent processing of secretogranin II by the endopeptidase PC2 in isolated immature secretory granules

We have previously characterized the processing of secretogranin II (SgII) in PC12 cells that were stably transfected with the endopeptidase PC2. Here we show that processing of SgII can be observed in isolated immature secretory granules (ISGs) derived from this cell line in a temperature- and ATP-dependent manner. The stimulatory effect of ATP on processing can be attributed to the activation of the vacuolar H(+)-ATPase and a concomitant decrease in intragranular pH. The immature secretory granule therefore provides an adequate environment for correct processing of SgII by PC2. The rate of SgII processing was strongly dependent on the intragranular pH, suggesting that processing of SgII can be used as a pH indicator for the granule interior. A standard curve was prepared using SgII processing in ISGs equilibrated at a range of pH values. The extent of processing in ISGs incubated in the presence of ATP at physiological pH was compared with the standard curve, and the intragranular pH was determined. From these observations, we propose an intragranular pH of 6.3 +/- 0.1 for ISGs in a physiological buffer in the presence of ATP. Hence, the pH of ISGs seems to be similar to the pH of the trans-Golgi network (TGN) and is clearly higher than the pH of mature secretory granules (pH 5.0-5.5). Interestingly, no processing of SgII could be observed in a membrane fraction that is highly enriched in TGN under conditions for which processing was readily obtained in isolated ISGs.

TGN38-green fluorescent protein hybrid proteins expressed in stably transfected eukaryotic cells provide a tool for the real-time, in vivo study of membrane traffic pathways and suggest a possible role for ratTGN38

The green fluorescent protein (GFP) of Aquorea victoria is fluorescent when expressed as a recombinant protein in eukaryotic cells and has been used as a convenient marker of gene expression in vivo. It has also been used as a marker of the intracellular targeting of recombinant fusion proteins (part GFP, part protein of interest) which have been transiently expressed in eukaryotic cells grown in tissue culture. Thus, the use of GFP has proved a useful tool to study intracellular events in real-time. However, some transiently transfected cells fail to express, or localise correctly, the GFP-tagged protein. Therefore the production of stable cell lines expressing GFP-tagged integral membrane proteins may be essential for long-term studies. The generation of stably transfected eukaryotic cells expressing an integral membrane protein with a known, but poorly characterised intracellular trafficking pathway, would provide useful reagents for future, more precise, analysis of that pathway. TGN38 is a type I integral membrane protein which cycles between the trans-Golgi network (TGN) and cell surface; at steady state it is localised to the TGN. As such, TGN38 is an ideal candidate for tagging with GFP. We have generated cDNA constructs encoding ratTGN38 tagged at either the N- or C terminus with GFP. Transiently transfected rat (NRK) cells expressed active fluorophore, but failed to show correct localisation of the fusion protein. In contrast, both constructs are appropriately localised in stably transfected NRK cells and both are fluorescent. Furthermore, the recombinant GFP-tagged proteins and the endogenous TGN38 molecules show identical responses to drugs and temperature blocks known to perturb intracellular morphology and membrane traffic pathways. In fact morphological changes to the TGN induced by brefeldin A were observed at earlier time points than had been described previously using immunofluorescence analysis of fixed cells, thus validating the use of in vivo, real-time analysis of GFP-tagged proteins. In addition, we show that (in contrast to the situation in COS cells) elevated expression of ratTGN38 in NRK cells does not lead to a fragmentation of the TGN; this has implications for the role which TGN38 is playing in the maintenance of the morphology of the TGN. The data we present demonstrate that: (i) it is possible to generate stable cell lines expressing integral membrane proteins tagged with GFP; (ii) the GFP tag remains fluorescent when expressed on either the cytosolic or the lumenal side of all membranes of the secretory pathway up to and including that of the TGN; (iii) the GFP tag does not interfere with the transport of TGN38 along the secretory pathway or its retention in the TGN; (iv) GFP remains fluorescent in cells which have been processed for immunofluorescence analysis (using either paraformaldehyde or methanol fixation); and (v) TGN38 plays a role in maintaining the morphology of the TGN. Thus, stably transfected cells expressing GFP-tagged integral membrane proteins can be used as effective tools for the real-time study of intracellular morphology and membrane traffic pathways in eukaryotic cells.