Activation of rat liver adenylate cyclase by cholera toxin requires toxin internalization and processing in endosomes

ADP-ribosylation systems in bacteria and viruses

ADP-ribosylation is an ancient posttranslational modification present in all kingdoms of life. The system likely originated in bacteria where it functions in inter- and intra-species conflict, stress response and pathogenicity. It was repeatedly adopted via lateral transfer by eukaryotes, including humans, where it has a pivotal role in epigenetics, DNA-damage repair, apoptosis, and other crucial pathways including the immune response to pathogenic bacteria and viruses. In other words, the same ammunition used by pathogens is adapted by eukaryotes to fight back. While we know quite a lot about the eukaryotic system, expanding rather patchy knowledge on bacterial and viral ADP-ribosylation would give us not only a better understanding of the system as a whole but a fighting advantage in this constant arms race. By writing this review we hope to put into focus the available information and give a perspective on how this system works and can be exploited in the search for therapeutic targets in the future. The relevance of the subject is especially highlighted by the current situation of being amid the world pandemic caused by a virus harbouring and dependent on a representative of such a system.

Insights into binding of cholera toxin to GM1 containing membrane

Interactions of cholera toxin (CT) with membrane are associated with the massive secretory diarrhea seen in Asiatic cholera. Ganglioside GM1 has been shown to be responsible for the binding of the B subunit of cholera toxin (CT-B), which then helps CT to pass through the membrane, but the exact mechanism remains to be explored. In this work, we have carried out atomistic scale molecular dynamics simulation to investigate the structural changes of CT upon membrane binding and alteration in membrane structure and dynamics. Starting from the initial structure where the five units of B subunit bind with five GM1, only three of five units remain bound and the whole CT is tilted such that the three binding units are deeper in the membrane. The lipids that are in contact with those units of the CT-B behave differently from the rest of the lipids. Altogether, our results demonstrate the atomistic interaction of CT with GM1 containing lipid membrane and provide a probable mechanism of the early stage alteration of lipid structure and dynamics, which can make a passage for penetration of CT on membrane surface.

Dispensability and dynamics of caveolin-1 during liver regeneration and in isolated hepatic cells

Caveolae participate in several cellular processes such as vesicular transport, cholesterol homeostasis, regulation of signal transduction, integrin signaling, and cell growth. The expression and functional role of caveolin (Cav), the most abundant protein of caveolae, has been reported in liver and in different hepatocyte cell lines, in human cirrhotic liver, and in hepatocellular carcinomas. The role of Cav-1 in liver regeneration after partial hepatectomy (PH) has been investigated as a model of liver proliferation in vivo. Our results show that Cav-1 increases in liver after PH with a redistribution of the protein from the caveola-enriched domain to the noncaveolar fraction. Moreover, the Cav-1 located in the noncaveolar fraction is phosphorylated in tyrosine 14, even though the Cav-1 gene is dispensable for liver regeneration after PH, as deduced from data obtained with commercially available animals lacking this gene. In addition to this, the proinflammatory stimulation of hepatocytes induces Cav-1 translocation to a noncaveolar fraction and tyrosine 14 phosphorylation mainly through the activation of tyrosine kinases such as Src.

CONCLUSION

These results support a dynamic role for Cav-1 in liver proliferation both in vivo after PH and in vitro in cultured hepatic cell lines, but with minimal implications for the liver regeneration process.

Role of receptor-mediated endocytosis, endosomal acidification and cathepsin D in cholera toxin cytotoxicity

Using the in situ liver model system, we have recently shown that, after cholera toxin binding to hepatic cells, cholera toxin accumulates in a low-density endosomal compartment, and then undergoes endosomal proteolysis by the aspartic acid protease cathepsin-D [Merlen C, Fayol-Messaoudi D, Fabrega S, El Hage T, Servin A, Authier F (2005) FEBS J272, 4385-4397]. Here, we have used a subcellular fractionation approach to address the in vivo compartmentalization and cytotoxic action of cholera toxin in rat liver parenchyma. Following administration of a saturating dose of cholera toxin to rats, rapid endocytosis of both cholera toxin subunits was observed, coincident with massive internalization of both the 45 kDa and 47 kDa Gsalpha proteins. These events coincided with the endosomal recruitment of ADP-ribosylation factor proteins, especially ADP-ribosylation factor-6, with a time course identical to that of toxin and the A subunit of the stimulatory G protein (Gsalpha) translocation. After an initial lag phase of 30 min, these constituents were linked to NAD-dependent ADP-ribosylation of endogenous Gsalpha, with maximum accumulation observed at 30-60 min postinjection. Assessment of the subsequent postendosomal fate of internalized Gsalpha revealed sustained endolysosomal transfer of the two Gsalpha isoforms. Concomitantly, cholera toxin increased in vivo endosome acidification rates driven by the ATP-dependent H(+)-ATPase pump and in vitro vacuolar acidification in hepatoma HepG2 cells. The vacuolar H(+)-ATPase inhibitor bafilomycin and the cathepsin D inhibitor pepstatin A partially inhibited, both in vivo and in vitro, the cAMP response to cholera toxin. This cathepsin D-dependent action of cholera toxin under the control of endosomal acidity was confirmed using cellular systems in which modification of the expression levels of cathepsin D, either by transfection of the cathepsin D gene or small interfering RNA, was followed by parallel changes in the cytotoxic response to cholera toxin. Thus, in hepatic cells, a unique endocytic pathway was revealed following cholera toxin administration, with regulation specificity most probably occurring at the locus of the endosome and implicating endosomal proteases, such as cathepsin D, as well as organelle acidification.

Glucagon-mediated internalization of serine-phosphorylated glucagon receptor and Gsalpha in rat liver

To assess glucagon receptor compartmentalization and signal transduction in liver parenchyma, we have studied the functional relationship between glucagon receptor endocytosis, phosphorylation and coupling to the adenylate cyclase system. Following administration of a saturating dose of glucagon to rats, a rapid internalization of glucagon receptor was observed coincident with its serine phosphorylation both at the plasma membrane and within endosomes. Co-incident with glucagon receptor endocytosis, a massive internalization of both the 45- and 47-kDa Gsalpha proteins was also observed. In contrast, no change in the subcellular distribution of adenylate cyclase or beta-arrestin 1 and 2 was observed. In response to des-His(1)-[Glu(9)]glucagon amide, a glucagon receptor antagonist, the extent and rate of glucagon receptor endocytosis and Gsalpha shift were markedly reduced compared with wild-type glucagon. However, while the glucagon analog exhibited a wild-type affinity for endosomal acidic glucagonase activity and was processed at low pH with similar kinetics and rates, its proteolysis at neutral pH was 3-fold lower. In response to tetraiodoglucagon, a glucagon receptor agonist of enhanced biological potency, glucagon receptor endocytosis and Gsalpha shift were of higher magnitude and of longer duration, and a marked and prolonged activation of adenylate cyclase both at the plasma membrane and in endosomes was observed. The subsequent post-endosomal fate of internalized Gsalpha was evaluated in a cell-free rat liver endosome-lysosome fusion system following glucagon injection. A sustained endo-lysosomal transfer of the two 45- and 47-kDa Gsalpha isoforms was observed. Therefore, these results reveal that within hepatic target cells and consequent to glucagon-mediated internalization of the serine-phosphorylated glucagon receptor and the Gsalpha protein, extended signal transduction may occur in vivo at the locus of the endo-lysosomal apparatus.

Mouse polyomavirus enters early endosomes, requires their acidic pH for productive infection, and meets transferrin cargo in Rab11-positive endosomes

Mouse polyomavirus (PyV) virions enter cells by internalization into smooth monopinocytic vesicles, which fuse under the cell membrane with larger endosomes. Caveolin-1 was detected on monopinocytic vesicles carrying PyV particles in mouse fibroblasts and epithelial cells (33). Here, we show that PyV can be efficiently internalized by Jurkat cells, which do not express caveolin-1 and lack caveolae, and that overexpression of a caveolin-1 dominant-negative mutant in mouse epithelial cells does not prevent their productive infection. Strong colocalization of VP1 with early endosome antigen 1 (EEA1) and of EEA1 with caveolin-1 in mouse fibroblasts and epithelial cells suggests that the monopinocytic vesicles carrying the virus (and vesicles containing caveolin-1) fuse with EEA1-positive early endosomes. In contrast to SV40, PyV infection is dependent on the acidic pH of endosomes. Bafilomycin A1 abolished PyV infection, and an increase in endosomal pH by NH4Cl markedly reduced its efficiency when drugs were applied during virion transport towards the cell nucleus. The block of acidification resulted in the retention of a fraction of virions in early endosomes. To monitor further trafficking of PyV, we used fluorescent resonance energy transfer (FRET) to determine mutual localization of PyV VP1 with transferrin and Rab11 GTPase at a 2- to 10-nm resolution. Positive FRET between PyV VP1 and transferrin cargo and between PyV VP1 and Rab11 suggests that during later times postinfection (1.5 to 3 h), the virus meets up with transferrin in the Rab11-positive recycling endosome. These results point to a convergence of the virus and the cargo internalized by different pathways in common transitional compartments.

Proteolytic activation of internalized cholera toxin within hepatic endosomes by cathepsin D

We have defined the in vivo and in vitro metabolic fate of internalized cholera toxin (CT) in the endosomal apparatus of rat liver. In vivo, CT was internalized and accumulated in endosomes where it underwent degradation in a pH-dependent manner. In vitro proteolysis of CT using an endosomal lysate required an acidic pH and was sensitive to pepstatin A, an inhibitor of aspartic acid proteases. By nondenaturating immunoprecipitation, the acidic CT-degrading activity was attributed to the luminal form of endosomal cathepsin D. The rate of toxin hydrolysis using an endosomal lysate or pure cathepsin D was found to be high for native CT and free CT-B subunit, and low for free CT-A subunit. On the basis of IC(50) values, competition studies revealed that CT-A and CT-B subunits share a common binding site on the cathepsin D enzyme, with native CT and free CT-B subunit displaying the highest affinity for the protease. By immunofluorescence, partial colocalization of internalized CT with cathepsin D was confirmed at early times of endocytosis in both hepatoma HepG2 and intestinal Caco-2 cells. Hydrolysates of CT generated at low pH by bovine cathepsin D displayed ADP-ribosyltransferase activity towards exogenous Gsalpha protein suggesting that CT cytotoxicity, at least in part, may be related to proteolytic events within endocytic vesicles. Together, these data identify the endocytic apparatus as a critical subcellular site for the accumulation and proteolytic degradation of endocytosed CT, and define endosomal cathepsin D an enzyme potentially responsible for CT cytotoxic activation.

Molecular pathogenesis, epidemiology, and clinical manifestations of respiratory infections due to Bordetella pertussis and other Bordetella subspecies

Bordetella respiratory infections are common in people (B. pertussis) and in animals (B. bronchiseptica). During the last two decades, much has been learned about the virulence determinants, pathogenesis, and immunity of Bordetella. Clinically, the full spectrum of disease due to B. pertussis infection is now understood, and infections in adolescents and adults are recognized as the reservoir for cyclic outbreaks of disease. DTaP vaccines, which are less reactogenic than DTP vaccines, are now in general use in many developed countries, and it is expected that the expansion of their use to adolescents and adults will have a significant impact on reducing pertussis and perhaps decrease the circulation of B. pertussis. Future studies should seek to determine the cause of the unique cough which is associated with Bordetella respiratory infections. It is also hoped that data gathered from molecular Bordetella research will lead to a new generation of DTaP vaccines which provide greater efficacy than is provided by today's vaccines.

Caveolin-stabilized membrane domains as multifunctional transport and sorting devices in endocytic membrane traffic

Endocytosis comprises several routes of internalization. An outstanding question is whether the caveolar and endosomal pathways intersect. Following transport of the caveolar protein Caveolin-1 and two cargo complexes, Simian Virus 40 and Cholera toxin, in live cells, we uncovered a Rab5-dependent pathway in which caveolar vesicles are targeted to early endosomes and form distinct and stable membrane domains. In endosomes, the low pH selectively allowed the toxin to diffuse out of the caveolar domains into the surrounding membrane, while the virus remained trapped. Thus, we conclude that, unlike cyclic assembly and disassembly of coat proteins in vesicular transport, oligomeric complexes of caveolin-1 confer permanent structural stability to caveolar vesicles that transiently interact with endosomes to form subdomains and release cargo selectively by compartment-specific cues.

Toxin entry: how reversible is the secretory pathway?

A number of proteins produced by plants and bacteria are extremely toxic to eukaryotic cells. Their potency arises from their ability to catalyse the modification of crucial cellular components. Only a few toxin molecules are required to kill a cell, but to do so they must first reach the cytosol. How such proteins are translocated across the target cell membrane is poorly understood, but we argue here that some toxins may travel the secretory pathway in reverse, passing all the way from the cell surface to the endoplasmic reticulum (ER) before entering the cytosol.

The role of bacterial and non-bacterial toxins in the induction of changes in membrane transport: implications for diarrhea

Bacterial toxins induce changes in membrane transport which underlie the loss of electrolyte homeostasis associated with diarrhea. Bacterial- and their secreted toxin-types which have been linked with diarrhea include: (a) Vibrio cholerae (cholera toxin, E1 Tor hemolysin and accessory cholera enterotoxin); (b) Escherichia coli (heat stable enterotoxin, heat-labile enterotoxin and colicins); (c) Shigella dysenteriae (shiga-toxin); (d) Clostridium perfringens (C. perfringens enterotoxin, alpha-toxin, beta-toxin and theta-toxin); (e) Clostridium difficile (toxins A and B); (f) Staphylococcus aureus (alpha-haemolysin); (g) Bacillus cereus (cytotoxin K and haemolysin BL); and (h) Aeromonas hydrophila (aerolysin, heat labile cytotoxins and heat stable cytotoxins). The mechanisms of toxin-induced diarrhea include: (a) direct effects on ion transport in intestinal epithelial cells, i.e. direct toxin interaction with intrinsic ion channels in the membrane and (b) indirect interaction with ion transport in intestinal epithelial cells mediated by toxin binding to a membrane receptor. These effects consequently cause the release of second messengers, e.g. the release of adenosine 3',5'-cyclic monophosphate/guanosine 3',5'-monophosphate, IP(3), Ca2+ and/or changes in second messengers that are the result of toxin-formed Ca2+ and K+ permeable channels, which increase Ca2+ flux and augment changes in Ca2+ homeostasis and cause depolarisation of the membrane potential. Consequently, many voltage-dependent ion transport systems, e.g. voltage-dependent Ca2+ influx, are affected. The toxin-formed ion channels may act as a pathway for loss of fluid and electrolytes. Although most of the diarrhea-causing toxins have been reported to act via cation and anion channel formation, the properties of these channels have not been well studied, and the available biophysical properties that are needed for the characterization of these channels are inadequate.

Biochemical analysis of a caveolae-enriched plasma membrane fraction from rat liver

We isolated and characterized a subcellular fraction derived from the blood-sinusoidal plasma membrane of hepatocytes enriched in caveolin and containing several of the molecular components described to be present in caveolae isolated from other cell types. A morphological study by electron microscopy revealed that it was composed of caveolae-attached membrane profiles. Immunoelectron microscopy of isolated fraction showed the specific labeling of internal caveolae membranes with anti-caveolin antibody. Finally, one- and two-dimensional electrophoresis and Western blotting were used for the biochemical analysis of this new rat liver plasma membrane fraction. From the biochemical and the morphological characterization, we conclude that the caveolae-enriched plasma membrane fraction is a plasma membrane fraction, which originates from specialized regions of the sinusoidal plasma membrane, enriched in caveolae.

Membrane traffic and the cellular uptake of cholera toxin

In nature, cholera toxin (CT) and the structurally related E. coli heat labile toxin type I (LTI) must breech the epithelial barrier of the intestine to cause the massive diarrhea seen in cholera. This requires endocytosis of toxin-receptor complexes into the apical endosome, retrograde transport into Golgi cisternae or endoplasmic reticulum (ER), and finally transport of toxin across the cell to its site of action on the basolateral membrane. Targeting into this pathway depends on toxin binding ganglioside GM1 and association with caveolae-like membrane domains. Thus to cause disease, both CT and LTI co-opt the molecular machinery used by the host cell to sort, move, and organize their cellular membranes and substituent components.

The "early-sorting" endocytic compartment of rat hepatocytes is involved in the intracellular pathway of caveolin-1 (VIP-21)

The sinusoidal plasma membrane of the hepatocyte is organized into functional and structural microdomains whose origin, maintenance, and functioning are closely related with the endocytic compartment. Three different subcellular fractions, from rat liver, containing caveolin-1, the structural protein of caveolae, were morphologically and biochemically characterized. A caveolae-enriched plasma membrane fraction (CEF), contains large membrane structures surrounding attached internal plasmalemmal vesicles; the receptor-recycling compartment (RRC), contains tubules and vesicles with similar morphology to the internal vesicles observed by electron microscopy in CEF; and finally, caveolin-1 was also detected in early-sorting endosomes (CURL, compartment of uncoupling receptors and ligands). In this study, we show that following an intravenous administration of retinol-binding protein (RBP), there was a redistribution of caveolin-1 from the plasma membrane (CEF) to intracellular endocytic compartments (RRC and early-sorting endosomes). Thus, these results indicate that, in the hepatocyte, caveolae are dynamic structures actively interacting with the endocytic compartment.

Cholera and pertussis toxins increase acidification of endocytic vesicles without altering ion conductances

Acidification of endocytic vesicles, driven by the vacuolar H+ pump, is affected by parallel ion transporters. Because adenosine 3',5'-cyclic monophosphate (cAMP) and heterotrimeric G proteins may alter ion transporters, I tested whether cholera and pertussis toxins affected acidification of rat liver endosomes. Fluorescein-labeled dextran-loaded "10-min" endosomes from cholera toxin-treated rats exhibited ATP-dependent rates of acidification in the presence and absence of Cl- or K+ that were approximately 60-120% (P < 0.05) faster than rates from control endosomes. This increase was greater for "older" "20-min" endosomes and less for 'early" "2-min" endosomes. Ion transport functions of 10-min and 20-min toxin-exposed endosomes were similar to those of 2-min control endosomes. Cholera toxin also increased ATP-dependent steady-state intravesicular H+ concentration by 38-218% (P < 0.05). Pertussis toxin increased endosome acidification rates by 20-54% (P < 0.05). Both toxins increased liver cAMP content, and endosomes prepared from perfused livers exposed to 0.75 mM dibutyryl cAMP exhibited similar increases in acidification rates. These studies indicate that both cholera and pertussis toxins markedly alter the function of rat liver endosomes. The mechanism is unlikely to reflect major changes in vesicle ion transporters but rather may indicate either an increase in the number of H+ pumps per endosome and/or changes in fusion, remodeling, and maturation of early endocytic vesicles in response to cAMP.

Intracellular transport and processing of protein toxins produced by enteric bacteria

Bacterial toxins are associated with disease in humans and animals. Toxins can either be preformed in food or produced by bacteria in the intestine. There are two types of toxins: heat-labile protein toxins and heat stabile toxins. Heat labile toxins are produced by Bacillus cereus, Clostridium perfringens, Escherichia coli, and Vibrio cholerae, and heat-stabile enterotoxins consisting of relatively few amino acids are produced by Escherichia coli and acts by activation of guanylate cyclase. Similarly, heat-stabile entero-toxins are also produced by Staphylococcus aureus, a common cause of food poisoning in the United States, and Yersenia enterocolitica. Protein toxins produced by enteric bacteria can intoxicate intestinal cells and can also be taken up from the gut and reach other cells in the body. For example the Shiga-like toxins (vero-toxins) can intoxicate endothelial cells in the kidney and cause kidney failure. Intracellular transport and processing of a few of the protein toxins produced by enteric bacteria, namely Clostridium difficile toxin A and B, cholera toxin and the related heat-labile toxin produced by Escherichia coli, and Shiga toxin and Shiga-like toxins are presented.

Endosomal proteolysis of internalized proteins

Endosomal proteases have been implicated in the degradation of internalized regulatory peptides involved in the control of metabolic pathways and in the processing of intracellular antigens for cytolytic immune responses. Processing in the endocytic vesicles is regulated by changes in endosomal acidity due to the presence of an ATP-dependent proton pump which modulates protease activity, protein unfolding and receptor-ligand interactions. A limited number of proteases appear to reside in endosomes which do not contain the full complement of active proteases capable of completely degrading all internalized polypeptides. Retention of some acid hydrolases in endosomes is apparently related to their association with undefined endosomal membrane receptors. The limited number of proteases and the pH gradient from neutral to acidic (pH 7 to 5) within endosomes make possible a selective and controlled processing environment in comparison to lysosomes. The full set of endo- and exopeptidases that break down proteins to amino acids are active later in the pathway in lysosomes.

Crystal structure of a new heat-labile enterotoxin, LT-IIb

BACKGROUND

Cholera toxin from Vibrio cholerae and the type I heat-labile enterotoxins (LT-Is) from Escherichia coli are oligomeric proteins with AB5 structures. The type II heat-labile enterotoxins (LT-IIs) from E. coli are structurally similar to, but antigenically distinct from, the type I enterotoxins. The A subunits of type I and type II enterotoxins are homologous and activate adenylate cyclase by ADP-ribosylation of a G protein subunit, G8 alpha. However, the B subunits of type I and type II enterotoxins differ dramatically in amino acid sequence and ganglioside-binding specificity. The structure of LT-IIb was determined both as a prototype for other LT-IIs and to provide additional insights into structure/function relationships among members of the heat-labile enterotoxin family and the superfamily of ADP-ribosylating protein toxins.

RESULTS

The 2.25 A crystal structure of the LT-IIb holotoxin has been determined. The structure reveals striking similarities with LT-I in both the catalytic A subunit and the ganglioside-binding B subunits. The latter form a pentamer which has a central pore with a diameter of 10-18 A. Despite their similarities, the relative orientation between the A polypeptide and the B pentamer differs by 24 degrees in LT-I and LT-IIb. A common hydrophobic ring was observed at the A-B5 interface which may be important in the cholera toxin family for assembly of the AB5 heterohexamer. A cluster of arginine residues at the surface of the A subunit of LT-I and cholera toxin, possibly involved in assembly, is also present in LT-IIb. The ganglioside receptor binding sites are localized, as suggested by mutagenesis, and are in a position roughly similar to the sites where LT-I binds its receptor.

CONCLUSIONS

The structure of LT-IIb provides insight into the sequence diversity and structural similarity of the AB5 toxin family. New knowledge has been gained regarding the assembly of AB5 toxins and their active-site architecture.

Developmental differences in the expression of the cholera toxin sensitive subunit (Gs alpha) of adenylate cyclase in the rat small intestine

BACKGROUND

The stimulatory guanosine triphosphate (GTP) binding protein alpha subunit (Gs alpha) of adenylate cyclase is the target protein for cholera toxin.

AIMS/METHODS

The expression of this signal transducer was analysed in the small intestine of developing rats by RNA transfer (northern blot) analysis by immunoblotting, and by ADP-ribosylation of membrane proteins.

RESULTS

Intestinal Gs alpha mRNA (about 1.9 kb) was increased in the neonate compared with the adult rat. Two isoforms of Gs alpha proteins, a 45,000 and a 52,000 form, were expressed in the small intestinal epithelial cell and both were ADP-ribosylated by cholera toxin. A significant increase in the larger isoform (52,000) and in its ribosylation was noted in the 2 week old suckling compared with post-weaned older animals. The protein content or ribosylation of the smaller form (45,000) did not significantly change with age.

CONCLUSION

These data show that a developmental decline of intestinal Gs alpha expression seems to be, in part, regulated at the mRNA level. An increased Gs alpha expression in the immature intestine may help to explain a previously reported, dose dependent increased adenylate cyclase response and an increase in fluid secretion to cholera toxin in neonates compared with adults.

Pertussis toxin-catalyzed ADP-ribosylation of Gi-2 and Gi-3 in CHO cells is modulated by inhibitors of intracellular trafficking

In previous studies, an in vitro ADP-ribosylation assay was developed to quantitatively evaluate the in vivo ADP-ribosylation of eukaryotic target proteins in intact Chinese hamster ovary (CHO) cells by pertussis toxin (PT). Immunoblot analysis identified the two PT-sensitive target proteins in CHO cells as Gi-2 and Gi-3. In this in vitro ADP-ribosylation assay, the ability of PT and ADP-ribosylate Gi-2 and Gi-3 intact CHO cells was not inhibited by cytochalasin D but was inhibited by chloroquine, monensin, and nocodazole. These data implicated the involvement of a cytochalasin D-independent endocytic mechanism, a pH-sensitive step, and microtubules in the ADP-ribosylation of Gi-2 and Gi-3 by PT in intact CHO cells. Preincubation of CHO cells with cycloheximide, at concentrations that reduced protein synthesis by > 95%, did not inhibit the ability of PT to ADP-ribosylate Gi-2 and Gi-3. Control experiments showed that these agents did not affect either the ability of PT to directly ADP-ribosylate the heterotrimeric G protein, Gt, or the binding of PT to CHO cells, except that monensin slightly inhibited the binding of PT to CHO cells. These results are consistent with a model in which PT is internalized by receptor-mediated endocytosis, probably via a cytochalasin D-independent pathway, which involves intracellular trafficking through late endosomes and the Golgi apparatus. An alternative model predicts the presence of a eukaryotic factor that traffics within cells via this pathway and is required by PT to ADP-ribosylate Gi proteins.

Intracellular signal transduction: The role of endosomes

Polypeptide hormones, growth factors, and other biologically significant molecules are specifically internalized by target cells. Exposure of cells to these ligands results in the formation of ligand-receptor complexes on the cell surface and subsequent internalization of these complexes into the endosomal apparatus (endosomes, or ENs). The study of ENs has identified several important functions for this unique cellular organelle. These include the dissociation of ligand from receptor and receptor recycling to the cell surface and the degradation of some internalized ligands, as well as the delivery of others to lysosomes. More recently, it has become apparent that ENs fulfill another critical role, that of signal transduction. In this article, we review the evidence substantiating this role for ENs and propose three models by which ENs participate in cell signaling.

Cationic amphiphilic drugs inhibit the internalization of cholera toxin to the Golgi apparatus and the subsequent elevation of cyclic AMP

Cholera toxin (CT) consists of a pentameric B subunit which binds with high affinity to ganglioside GM1, and an A subunit which stimulates adenylate cyclase, resulting in the elevation of cAMP. We now examine the effect of cationic amphiphilic drugs (CADs) on the internalization of rhodamine (Rh)-CT in cultured hippocampal neurons. CADs have recently been shown to inhibit receptor recycling by disrupting the assembly-disassembly of clathrin at the plasma membrane and on endosomes (Wang, L.-H., Rothberg, K. G., and Anderson, R. G. W. (1993) J. Cell Biol. 123, 1107-1117). Rh-CT was internalized by an energy- and temperature-dependent (presumably vesicular) mechanism to the Golgi apparatus. Internalization to the Golgi apparatus was completely but reversibly blocked by CADs, and the ability of CT to stimulate the elevation of cAMP was significantly reduced. In control cells, cAMP levels were elevated 2.3-fold after 20 min of incubation with CT, but in CAD-treated cells cAMP levels were only elevated 1.3-fold. The effect of CADs on CT internalization was not due to a direct effect of CADs on the Golgi apparatus. Our data demonstrate that CADs inhibit vesicular transport of CT to the Golgi apparatus and imply that the sorting of CT to the Golgi apparatus occurs in the same endosomal compartment involved in sorting recycling receptors to the plasma membrane, since both pathways are inhibited by CADs.

Role of a potential endoplasmic reticulum retention sequence (RDEL) and the Golgi complex in the cytotonic activity of Escherichia coli heat-labile enterotoxin

Recent experimental evidence indicates that Escherichia coli heat-labile enterotoxin and the closely related cholera toxin gain access to intracellular target substrates through a brefeldin A-sensitive pathway that may involve retrograde transport through the Golgi-endoplasmic reticulum network. The A subunits of both toxins possess a carboxy-terminal tetrapeptide sequence (KDEL in cholera toxin and RDEL in the heat-labile enterotoxins) that is known to mediate the retention of eukaryotic proteins in the endoplasmic reticulum. To investigate the potential role of the RDEL sequence in the toxic activity of the heat-labile enterotoxin we constructed mutant analogues of the toxin containing single substitutions (RDGL and RDEV) or a reversed sequence (LEDR). The single substitutions had little effect on Chinese hamster ovary cell elongation or the ability to stimulate cAMP accumulation in Caco-2 cells. Reversal of the sequence reduced the ability of the toxin to increase cAMP levels in Caco-2 cells by approximately 60% and decreased the ability to elicit elongation of Chinese hamster ovary cells. The effects of the heat-labile enterotoxin were not diminished in a mutant Chinese hamster ovary cell line (V.24.1) that belongs to the End4 complementation group and possesses a temperature-sensitive block in secretion that correlates directly with the disappearance of the Golgi stacks. Collectively, these findings suggest that the brefeldin A-sensitive process involved in intoxication by the heat-labile enterotoxin does not involve RDEL-dependent retrograde transport of the A subunit through the Golgi-endoplasmic reticulum complex. The results are more consistent with a model of internalization involving translocation of the A subunit from an endosomal or a trans-Golgi network compartment.

Fluorescence analysis of the interaction between ganglioside GM1-containing phospholipid vesicles and the B subunit of cholera toxin

Binding of cholera toxin B protomer (CT-B) to a pyrene-labeled analogue of its ganglioside GM1 receptor (pyrene-GM1) in the absence and presence of phosphatidylcholine vesicles was monitored using steady-state fluorescence spectroscopy. CT-B association with pyrene-GM1 micelles induces changes in the fluorescence properties of this ganglioside analogue that are consistent with its conversion from an excimer to a monomer form. Incubation of pyrene-GM1 with preformed vesicles of phosphatidylcholine (PC) results in complete conversion of pyrene-GM1 to its monomer form, however, unlike with CT-B binding, incorporation of pyrene-GM1 into PC vesicles occurs with a concomitant loss of fluorescence quenching by the small polar quenching agent acrylamide. Subsequent binding of CT-B to the PC-GM1 composite vesicles causes no further change in the pyrene fluorescence emission spectrum but does appear to increase acrylamide accessibility. These data lead to the conclusion that cholera toxin binding to a cell membrane alters membrane packing at the site of attachment. Furthermore, this phenomenon appears to be influenced by environmental conditions such as pH. A pH of about 4.0 or less causes acrylamide quenching to decrease to approximately the levels observed in the absence of CT-B. These results may be useful in describing the dynamics of the interaction between cholera toxin and target cell membranes. Moreover, these data could provide clues to the mechanism by which the toxic portion of CT is able to enter the cytoplasm of target cells.

The family of bacterial ADP-ribosylating exotoxins

Pathogenic bacteria utilize a variety of virulence factors that contribute to the clinical manifestation of their pathogenesis. Bacterial ADP-ribosylating exotoxins (bAREs) represent one family of virulence factors that exert their toxic effects by transferring the ADP-ribose moiety of NAD onto specific eucaryotic target proteins. The observations that some bAREs ADP-ribosylate eucaryotic proteins that regulate signal transduction, like the heterotrimeric GTP-binding proteins and the low-molecular-weight GTP-binding proteins, has extended interest in bAREs beyond the bacteriology laboratory. Molecular studies have shown that bAREs possess little primary amino acid homology and have diverse quaternary structure-function organization. Underlying this apparent diversity, biochemical and crystallographic studies have shown that several bAREs have conserved active-site structures and possess a conserved glutamic acid within their active sites.

The effect of GTP on the aluminum fluoride- and forskolin-activated adenylyl cyclase from human embryonic kidney 293 cells

GTP has been shown to inhibit AlF4(-)-stimulated, and to activate forskolin-stimulated adenylyl cyclase activity in the presence of Mg2+ in cell membranes from human embryonic kidney 293 cells. The maximal inhibitory response of AlF4(-)-stimulated adenylyl cyclase activity by GTP was not dependent on the concentration of Mg2+, but was so in the case of forskolin-activated activity at all forskolin concentrations assayed. Mn2+ ions stimulated AlF4(-)- or forskolin-activated adenylyl cyclase activity to a greater extent than Mg2+. The inhibition of AlF4(-)-stimulated cyclase by GTP was still observed with Mn2+, but the activation of forskolin-stimulated cyclase by GTP was not. When assayed together, Mn2+ and Mg2+ showed non-additive behaviours with respect to the amount of cyclic AMP formed after AlF4(-)-stimulation of adenylyl cyclase. The temperature dependence of the activation of adenylyl cyclase by forskolin, AlF4- or under basal conditions was observed to be somehow different in the presence of Mn2+ than in the presence of Mg2+ ions. Cholera toxin treatment produced a markedly increased cyclase activity, specially when assayed with AlF4-. In the case of forskolin-activated adenylyl cyclase, UTP and CTP were unable to reproduce the cyclase activation detected with GTP. However, in the case of AlF4(-)-stimulated adenylyl cyclase, UTP was as good as GTP at inhibiting cyclase activity, and CTP virtually eliminated the activation of the cyclase with AlF4-.

Interaction of a cholera toxin derivative containing a reduced number of receptor binding sites with intact cells in culture

Hybrid CTB (hCTB), having only one or two functional binding sites, has been constructed from two chemically inactivated derivatives of CTB. One inactive derivative consisted of CTB formylated in the lone Trp-88 of each beta-chain (fCTB), whereas the other inactive derivative consisted of CTB specifically succinylated in three amino groups located in or near the receptor binding site (sssCTB). hCTB, fCTB and sssCTB were able to reassociate with CTA and form the corresponding holotoxins hCT, fCT and sssCT as measured by gel filtration chromatography. In contrast to fCT and sssCT, hCT could increase the cAMP content of intact Vero cells in a time- and dose-dependent way: concentrations as low as a few nanograms of hCT per milliliter caused a significant increase in the intracellular cAMP level. The maximal cAMP level induced by hCT (1 microgram/ml) was, however, more than 2-fold lower than that elicited by its native counterpart. At saturating ligand concentrations and at 37 degrees C, the lag periods and rates of CT and hCT induced cAMP accumulation were essentially the same. Treatment of Vero and HeLa cells with GM1 did not affect their difference in response to CT and hCT. When Vero cells treated with hCT were incubated for longer periods of time, a further slow accumulation of cAMP occurred until after about 20 h cAMP levels of cells exposed to CT or hCT were essentially the same. In contrast to Vero and HeLa cells, human skin fibroblasts exhibited an almost identical response to CT as well as to hCT. Acidotropic agents such as chloroquine and monensin affected the CT and hCT induced increase in cAMP content of Vero cells, fibroblasts and GM1 treated Hela cells in a similar way. The results are consistent with the view that CT receptor recognition domains are shared between adjacent beta-chains, that pentavalent binding appears not to be essential for cytotoxicity and that in the cell types studied intracellular processing of CT, hCT is involved.

Adhesion and internalization of E. coli strains expressing various pathogenicity determinants

The adhesion of Escherichia coli to host epithelial cells is the very first step of urinary tract infections followed by the internalization of some bacteria into these cells. These steps are influenced by several surface antigens or products of the pathogen, e.g. fimbriae or adhesins, K antigen, and hemolysin. The bacterial adherence and the internalization of several mutants of an E. coli O18:K5 strain differing in the expression of K5 antigen, hemolysin, and fimbriae were measured using a permanent line of porcine tubuloepithelial cells (LLC-PK1). Strains with K5 antigen were reduced in adherence and internalization in comparison to the K-negative strains. The expression of hemolysin by these strains lead to an increase of adherence and internalization. The internalization of bacteria is influenced mainly by their adherence to the epithelial cells. Thus, the internalization of attached bacteria is rather a kind of endocytosis than an invasion of the bacteria. To confirm this thesis, we investigated the influence of cytoskeletal inhibitors (cytochalasine B, cytochalasine D, colchicine, and chloroquine) on bacterial adherence and internalization. The cytoskeletal inhibitors lead to a significant inhibition of internalization of the bacteria tested. The receptor-mediated endocytosis of bacteria by tubuloepithelial cells may be of importance in the pathogenesis of recurrent urinary tract infection.

Involvement of the Golgi region in the intracellular trafficking of cholera toxin

The intracellular pathway following receptor-mediated endocytosis of cholera toxin was studied using brefeldin A (BFA), which inhibited protein secretion and induced dramatic morphological changes in the Golgi region. In both mouse Y1 adrenal cells and CHO cells, BFA at 1 micrograms/ml caused a 80-90% inhibition of the cholera toxin (CT)-induced elevation of intracellular cAMP. The inhibition of the cytotoxicity of CT by BFA was also observed in a rounding assay of Y1 adrenal cells. The inhibition of CT cytotoxicity by BFA was dose dependent, with the ID50 value similar to the LD50 of BFA in Y1 adrenal cells. Binding and internalization of [125I]-labeled cholera toxin in Y1 adrenal cells was not affected by BFA. Unlike the BFA-sensitive cell lines such as Y1 adrenal and CHO cells, BFA at 1 micrograms/ml did not inhibit the cytotoxicity of CT in PtK1 cells, of which the Golgi structure was BFA-resistant. These results strongly suggest that a BFA-sensitive Golgi is required for the protection of CT cytotoxicity by BFA. In contrast, elevation of the intracellular cAMP by forskolin, which acts directly on the plasma membrane adenylate cyclase, was not affected by BFA. These observations indicate that the intoxication of target cells by CT requires an intact Golgi region for its intracellular trafficking and/or processing. In this respect, CT shares a common intracellular pathway with ricin, Pseudomonas toxin, and modeccin, even though their structures and modes of action are very different.

Intoxication of cultured cells by cholera toxin: evidence for different pathways when bound to ganglioside GM1 or neoganglioproteins

We previously reported that when the oligosaccharide of ganglioside GM1 is covalently attached to cell surface proteins of GM1-deficient rat glioma C6 cells, the cells bind large amounts of cholera toxin (CT) but their cAMP response to CT is not enhanced [Pacuszka, T., & Fishman, P. H. (1990) J. Biol. Chem. 265, 7673-7668]. We now report that when such cells were exposed to CT in the presence of chloroquine, an acidotropic agent, they accumulated cAMP. This raised the possibility that CT bound to cell surface "neoganglioproteins" may be entering the cells through a different pathway from that of CT-bound GM1. To further explore this phenomenon, we covalently attached GM1 oligosaccharide to human transferrin (Tf). The modified protein (GM1OS-Tf) bound with high affinity to Tf receptors on HeLa cells and increased the binding of CT to the cells. The bound CT, however, was unable to activate adenylyl cyclase as measured by cyclic AMP accumulation. By contrast, treatment of HeLa cells with GM1 increased both CT binding and stimulation of cyclic AMP accumulation. Control cells and cells treated with either GM1 or GM1OS-Tf were exposed to CT in the presence of chloroquine. Whereas chloroquine had little or no effect on the response of control or GM1-treated cells to CT, it made the cells treated with GM1OS-Tf responsive to the toxin. Our results indicate that CT bound to its natural receptor GM1 enters the cells through a pathway different from that of toxin bound to neoganglioproteins.

Lactose binding to heat-labile enterotoxin revealed by X-ray crystallography

Recognition of the oligosaccharide portion of ganglioside GM1 in membranes of target cells by the heat-labile enterotoxin from Escherichia coli is the crucial first step in its pathogenesis, as it is for the closely related cholera toxin. These toxins have five B subunits, which are essential for GM1 binding, and a single A subunit, which needs to be nicked by proteolysis and reduced, yielding an A1-'enzyme' and an A2-'linker' peptide. A1 is translocated across the membrane of intestinal epithelial cells, possibly after endocytosis, upon which it ADP-ribosylates the G protein Gs alpha. The mechanism of binding and translocation of these toxins has been extensively investigated, but how the protein is orientated on binding is still not clear. Knowing the precise arrangement of the ganglioside binding sites of the toxins will be useful for designing drugs against the diarrhoeal diseases caused by organisms secreting these toxins and in the development of oral vaccines against them. We present here the three-dimensional structure of the E. coli heat-labile enterotoxin complexed with lactose. This reveals the location of the binding site of the terminal galactose of GM1, which is consistent with toxin binding to the target cell with its A1 fragment pointing away from the membrane. A small helix is identified at the carboxy terminus of A2 which emerges through the central pore of the B subunits and probably comes into contact with the membrane upon binding, whereas the A1 subunit is flexible with respect to the B pentamer.