Estimation of the amount of internalized ricin that reaches the trans-Golgi network

Targeting the Inside of Cells with Biologicals: Toxin Routes in a Therapeutic Context

Numerous toxins translocate to the cytosol in order to fulfil their function. This demonstrates the existence of routes for proteins from the extracellular space to the cytosol. Understanding these routes is relevant to multiple aspects related to therapeutic applications. These include the development of anti-toxin treatments, the potential use of toxins as shuttles for delivering macromolecular cargo to the cytosol or the use of drugs based on toxins. Compared with other strategies for delivery, such as chemicals as carriers for macromolecular delivery or physical methods like electroporation, toxin routes present paths into the cell that potentially cause less damage and can be specifically targeted. The efficiency of delivery via toxin routes is limited. However, low-delivery efficiencies can be entirely sufficient, if delivered cargoes possess an amplification effect or if very few molecules are sufficient for inducing the desired effects. This is known for example from RNA-based vaccines that have been developed during the coronavirus disease 2019 pandemic as well as for other approved RNA-based drugs, which elicited the desired effect despite their typically low delivery efficiencies. The different mechanisms by which toxins enter cells may have implications for their technological utility. We review the mechanistic principles of the translocation pathway of toxins from the extracellular space to the cytosol, the delivery efficiencies, and therapeutic strategies or applications that exploit toxin routes for intracellular delivery.

In Vitro Methods to Study the Golgi Apparatus Role in Proteoglycan and Glycosaminoglycan Synthesis

Subcellular fractionation is an introductory step in a variety of experimental approaches designed to study intracellular components, like membranes and organelle systems. Subcellular fractions enriched in membranes of the Golgi apparatus of mammalian cells have been isolated to address localization and activity of proteins, including enzymes, to study intracellular membrane transport mechanisms, and to reconstitute in vitro cellular processes associated with the Golgi apparatus. Here, I describe methods to purify Golgi membranes by subcellular fractionation, to assay nucleotide sulfate (PAPS) uptake into Golgi vesicles, and to measure sulfate incorporation into in vitro synthesized glycosaminoglycans.

Endocytosis of the CdtA subunit from the Haemophilus ducreyi cytolethal distending toxin

Many Gram-negative pathogens produce a cytolethal distending toxin (CDT) with two cell-binding subunits (CdtA + CdtC) and a catalytic CdtB subunit. After adhesion to the plasma membrane of a target cell, CDT moves by retrograde transport to endoplasmic reticulum. CdtB then enters the nucleus where it generates DNA breaks that lead to cell cycle arrest and apoptosis or senescence. CdtA anchors the CDT holotoxin to the plasma membrane and is thought to remain on the cell surface after endocytosis of the CdtB/CdtC heterodimer. Here, we re-examined the potential endocytosis and intracellular transport of CdtA from the Haemophilus ducreyi CDT. We recorded the endocytosis of holotoxin-associated CdtA with a cell-based enzyme-linked immunoabsorbent assay (CELISA) and visualised its presence in the early endosomes by confocal microscopy 10 min after CDT binding to the cell surface. Western blot analysis documented the rapid degradation of internalised CdtA. Most of internalised CdtB and CdtC were degraded as well. The rapid rate of CDT internalisation and turnover, which could explain why CdtA endocytosis was not detected in previous studies, suggests only a minor pool of cell-associated CdtB reaches the nucleus. Our work demonstrates that CDT is internalised as an intact holotoxin and identifies the endosomes as the site of CdtA dissociation from CdtB/CdtC. TAKE AWAYS: During the endocytosis of CDT, CdtA is thought to remain at the cell surface. A cell-based ELISA documented the rapid endocytosis of CdtA. CdtA was visualised in the early endosomes by confocal microscopy. Intracellular CdtA was rapidly degraded, along with most of CdtB and CdtC.

Structural Analysis of Toxin-Neutralizing, Single-Domain Antibodies that Bridge Ricin's A-B Subunit Interface

Ricin toxin kills mammalian cells with notorious efficiency. The toxin's B subunit (RTB) is a Gal/GalNAc-specific lectin that attaches to cell surfaces and promotes retrograde transport of ricin's A subunit (RTA) to the trans Golgi network (TGN) and endoplasmic reticulum (ER). RTA is liberated from RTB in the ER and translocated into the cell cytoplasm, where it functions as a ribosome-inactivating protein. While antibodies against ricin's individual subunits have been reported, we now describe seven alpaca-derived, single-domain antibodies (V_HHs) that span the RTA-RTB interface, including four Tier 1 V_HHs with IC₅₀ values <1 nM. Crystal structures of each V_HH bound to native ricin holotoxin revealed three different binding modes, based on contact with RTA's F-G loop (mode 1), RTB's subdomain 2γ (mode 2) or both (mode 3). V_HHs in modes 2 and 3 were highly effective at blocking ricin attachment to HeLa cells and immobilized asialofetuin, due to framework residues (FR3) that occupied the 2γ Gal/GalNAc-binding pocket and mimic ligand. The four Tier 1 V_HHs also interfered with intracellular functions of RTB, as they neutralized ricin in a post-attachment cytotoxicity assay (e.g., the toxin was bound to cell surfaces before antibody addition) and reduced the efficiency of toxin transport to the TGN. We conclude that the RTA-RTB interface is a target of potent toxin-neutralizing antibodies that interfere with both extracellular and intracellular events in ricin's cytotoxic pathway.

Sensitivity of Kupffer cells and liver sinusoidal endothelial cells to ricin toxin and ricin toxin-Ab complexes

Ricin toxin is a plant-derived, ribosome-inactivating protein that is rapidly cleared from circulation by Kupffer cells (KCs) and liver sinusoidal endothelial cells (LSECs)-with fatal consequences. Rather than being inactivated, ricin evades normal degradative pathways and kills both KCs and LSECs with remarkable efficiency. Uptake of ricin by these 2 specialized cell types in the liver occurs by 2 parallel routes: a "lactose-sensitive" pathway mediated by ricin's galactose/N-acetylgalactosamine-specific lectin subunit (RTB), and a "mannose-sensitive" pathway mediated by the mannose receptor (MR; CD206) or other C-type lectins capable of recognizing the mannose-side chains displayed on ricin's A (RTA) and B subunits. In this report, we investigated the capacity of a collection of ricin-specific mouse MAb and camelid single-domain (VH H) antibodies to protect KCs and LSECs from ricin-induced killing. In the case of KCs, individual MAbs against RTA or RTB afforded near complete protection against ricin in ex vivo and in vivo challenge studies. In contrast, individual MAbs or VH Hs afforded little (<40%) or even no protection to LSECs against ricin-induced death. Complete protection of LSECs was only achieved with MAb or VH H cocktails, with the most effective mixtures targeting RTA and RTB simultaneously. Although the exact mechanisms of protection of LSECs remain unknown, evidence indicates that the Ab cocktails exert their effects on the mannose-sensitive uptake pathway without the need for Fcγ receptor involvement. In addition to advancing our understanding of how toxins and small immune complexes are processed by KCs and LSECs, our study has important implications for the development of Ab-based therapies designed to prevent or treat ricin exposure should the toxin be weaponized.

Intracellular Transport and Cytotoxicity of the Protein Toxin Ricin

Ricin can be isolated from the seeds of the castor bean plant (Ricinus communis). It belongs to the ribosome-inactivating protein (RIP) family of toxins classified as a bio-threat agent due to its high toxicity, stability and availability. Ricin is a typical A-B toxin consisting of a single enzymatic A subunit (RTA) and a binding B subunit (RTB) joined by a single disulfide bond. RTA possesses an RNA N-glycosidase activity; it cleaves ribosomal RNA leading to the inhibition of protein synthesis. However, the mechanism of ricin-mediated cell death is quite complex, as a growing number of studies demonstrate that the inhibition of protein synthesis is not always correlated with long term ricin toxicity. To exert its cytotoxic effect, ricin A-chain has to be transported to the cytosol of the host cell. This translocation is preceded by endocytic uptake of the toxin and retrograde traffic through the trans-Golgi network (TGN) and the endoplasmic reticulum (ER). In this article, we describe intracellular trafficking of ricin with particular emphasis on host cell factors that facilitate this transport and contribute to ricin cytotoxicity in mammalian and yeast cells. The current understanding of the mechanisms of ricin-mediated cell death is discussed as well. We also comment on recent reports presenting medical applications for ricin and progress associated with the development of vaccines against this toxin.

How Ricin Damages the Ribosome

Ricin belongs to the group of ribosome-inactivating proteins (RIPs), i.e., toxins that have evolved to provide particular species with an advantage over other competitors in nature. Ricin possesses RNA N-glycosidase activity enabling the toxin to eliminate a single adenine base from the sarcin-ricin RNA loop (SRL), which is a highly conserved structure present on the large ribosomal subunit in all species from the three domains of life. The SRL belongs to the GTPase associated center (GAC), i.e., a ribosomal element involved in conferring unidirectional trajectory for the translational apparatus at the expense of GTP hydrolysis by translational GTPases (trGTPases). The SRL represents a critical element in the GAC, being the main triggering factor of GTP hydrolysis by trGTPases. Enzymatic removal of a single adenine base at the tip of SRL by ricin blocks GTP hydrolysis and, at the same time, impedes functioning of the translational machinery. Here, we discuss the consequences of SRL depurination by ricin for ribosomal performance, with emphasis on the mechanistic model overview of the SRL modus operandi.

Construction and characterization of the recombinant immunotoxin RTA-4D5-KDEL targeting HER2/neu-positive cancer cells and locating the endoplasmic reticulum

The specific targeting of immunotoxins enables their wide application in cancer therapy. The A-chain of the ricin protein (RTA) is an N-glycosidase that catalyzes the removal of adenine from the 28S rRNA, preventing protein translation and leading to cell death. Ricin is highly toxic but can only exert its toxic effects from within the cytoplasm. In this study, we linked the anti-HER2 single-chain variable fragment 4D5 scFv and the endoplasmic reticulum-targeting peptide KDEL to the C-terminal of the RTA to construct immunotoxin RTA-4D5-KDEL. In vitro experiments showed that the anticancer effect of RTA-4D5-KDEL towards ovarian cancer cells SKOV-3 increased 440-fold and 28-fold relative to RTA and RTA-4D5, respectively. RTA-4D5-KDEL had a strong inhibitory effect on HER2-overexpressing SKOV-3 cells and caused little damage to normal HEK-293 cells and H460 lung cancer cells. Immunofluorescence experiments showed that the immunotoxin RTA-4D5 could specifically bind to SKOV-3 cells, but not to normal cells HEK-293. The immunotoxin RTA-4D5-KDEL could rapidly localize the recombinant protein to the endoplasmic reticulum. These results suggest that the recombinant immunotoxin RTA-4D5-KDEL has a strong inhibitory effect on ovarian cancer cells that overexpress HER2 but little harm to the normal cells.

A vital sugar code for ricin toxicity

Ricin is one of the most feared bioweapons in the world due to its extreme toxicity and easy access. Since no antidote exists, it is of paramount importance to identify the pathways underlying ricin toxicity. Here, we demonstrate that the Golgi GDP-fucose transporter Slc35c1 and fucosyltransferase Fut9 are key regulators of ricin toxicity. Genetic and pharmacological inhibition of fucosylation renders diverse cell types resistant to ricin via deregulated intracellular trafficking. Importantly, cells from a patient with SLC35C1 deficiency are also resistant to ricin. Mechanistically, we confirm that reduced fucosylation leads to increased sialylation of Lewis X structures and thus masking of ricin-binding sites. Inactivation of the sialyltransferase responsible for modifications of Lewis X (St3Gal4) increases the sensitivity of cells to ricin, whereas its overexpression renders cells more resistant to the toxin. Thus, we have provided unprecedented insights into an evolutionary conserved modular sugar code that can be manipulated to control ricin toxicity.

Modelling toxin effects on protein biosynthesis in eukaryotic cells

We present a rather generic model for toxin (ricin) inhibition of protein biosynthesis in eukaryotic cells. We also study reduction of the ricin toxic effects with application of antibodies against the RTB subunit of ricin molecules. Both species initially are delivered extracellularly. The model accounts for the pinocytotic and receptor-mediated toxin endocytosis and the intact toxin exocytotic removal out of the cell. The model also includes the lysosomal toxin destruction, the intact toxin motion to the endoplasmic reticulum (ER) for separation of its molecules into the RTA and RTB subunits, and the RTA chain translocation into the cytosol. In the cytosol, one portion of the RTA undergoes degradation via the ERAD. The other its portion can inactivate ribosomes at a large rate. The model is based on a system of deterministic ODEs. The influence of the kinetic parameters on the protein concentration and antibody protection factor is studied in detail.

Yeast Reporter Assay to Identify Cellular Components of Ricin Toxin A Chain Trafficking

RTA, the catalytic A-subunit of the ribosome inactivating A/B toxin ricin, inhibits eukaryotic protein biosynthesis by depurination of 28S rRNA. Although cell surface binding of ricin holotoxin is mainly mediated through its B-subunit (RTB), sole application of RTA is also toxic, albeit to a significantly lower extent, suggesting alternative pathways for toxin uptake and transport. Since ricin toxin trafficking in mammalian cells is still not fully understood, we developed a GFP-based reporter assay in yeast that allows rapid identification of cellular components required for RTA uptake and subsequent transport through a target cell. We hereby show that Ypt6p, Sft2p and GARP-complex components play an important role in RTA transport, while neither the retromer complex nor COPIB vesicles are part of the transport machinery. Analyses of yeast knock-out mutants with chromosomal deletion in genes whose products regulate ADP-ribosylation factor GTPases (Arf-GTPases) and/or retrograde Golgi-to-ER (endoplasmic reticulum) transport identified Sso1p, Snc1p, Rer1p, Sec22p, Erv46p, Gea1p and Glo3p as novel components in RTA transport, suggesting the developed reporter assay as a powerful tool to dissect the multistep processes of host cell intoxication in yeast.

Applications in Stimulated Emission Depletion Microscopy: Localization of a Protein Toxin in the Endoplasmic Reticulum Following Retrograde Transport

Retrograde transport is a process in which proteins are trafficked from the plasma membrane and endosomes to biosynthetic and secretory organelles, namely the Golgi apparatus and endoplasmic reticulum (ER). A number of plant and bacterial toxins, including cholera toxin and ricin toxin, exploit retrograde transport to gain entry into host cells, although the specifics of this process have remained difficult to probe by laser scanning confocal microscopy (LSCM). Here we demonstrate the use of super-resolution and live-cell imaging [stimulated emission depletion (STED)] to visualize exogenously applied ricin toxin within the ER. The improved resolution obtained by STED, as compared with LSCM (0.09 versus 0.19 μm), provides a more accurate determination of the amount of ricin that had trafficked to the ER.

Ribosome-Inactivating Proteins from Plants: A Historical Overview

This review provides a historical overview of the research on plant ribosome-inactivating proteins (RIPs), starting from the first studies at the end of eighteenth century involving the purification of abrin and ricin, as well as the immunological experiments of Paul Erlich. Interest in these plant toxins was revived in 1970 by the observation of their anticancer activity, which has given rise to a large amount of research contributing to the development of various scientific fields. Biochemistry analyses succeeded in identifying the enzymatic activity of RIPs and allowed for a better understanding of the ribosomal machinery. Studies on RIP/cell interactions were able to detail the endocytosis and intracellular routing of ricin, thus increasing our knowledge of how cells handle exogenous proteins. The identification of new RIPs and the finding that most RIPs are single-chain polypeptides, together with their genetic sequencing, has aided in the development of new phylogenetic theories. Overall, the biological properties of these proteins, including their abortifacient, anticancer, antiviral and neurotoxic activities, suggest that RIPs could be utilized in agriculture and in many biomedical fields, including clinical drug development.

Thermal Unfolding of the Pertussis Toxin S1 Subunit Facilitates Toxin Translocation to the Cytosol by the Mechanism of Endoplasmic Reticulum-Associated Degradation

Pertussis toxin (PT) moves from the host cell surface to the endoplasmic reticulum (ER) by retrograde vesicular transport. The catalytic PTS1 subunit dissociates from the rest of the toxin in the ER and then shifts to a disordered conformation which may trigger its export to the cytosol through the quality control mechanism of ER-associated degradation (ERAD). Functional roles for toxin instability and ERAD in PTS1 translocation have not been established. We addressed these issues with the use of a surface plasmon resonance system to quantify the cytosolic pool of PTS1 from intoxicated cells. Only 3% of surface-associated PTS1 reached the host cytosol after 3 h of toxin exposure. This represented, on average, 38,000 molecules of cytosolic PTS1 per cell. Cells treated with a proteasome inhibitor contained larger quantities of cytosolic PTS1. Stabilization of the dissociated PTS1 subunit with chemical chaperones inhibited toxin export to the cytosol and blocked PT intoxication. ERAD-defective cell lines likewise exhibited reduced quantities of cytosolic PTS1 and PT resistance. These observations identify the unfolding of dissociated PTS1 as a trigger for its ERAD-mediated translocation to the cytosol.

Modeling neutralization of Shiga 2 toxin by A-and B-subunit-specific human monoclonal antibodies

A mathematical model for Shiga 2 toxin neutralization by A-and B-subunit-specific human monoclonal antibodies initially delivered in the extracellular domain is presented, taking into account toxin and antibodies interaction in the extracellular domain, diffusion of toxin, antibodies, and their reaction products toward the cell, the receptor-mediated toxin and complex composed of toxin and antibody to A-subunit internalization from the extracellular into the intracellular medium and excretion of this complex back to the extracellular environment via recycling endosomal carriers. The retrograde transport of the intact toxin to the endoplasmic reticulum and its anterograde movement back to the vicinity of the plasma membrane with its subsequent exocytotic removal to the extracellular space via the secretory vesicle pathway is also taken into account. The model is composed of a set of coupled PDEs. A mathematical model based on a system of ODEs for Shiga 2 toxin neutralization by antibodies in the absence of cell is also studied. Both PDE and ODE systems are solved numerically. Numerical results are illustrated by figures and discussed.

Modeling of toxin-antibody interaction and toxin transport toward the endoplasmic reticulum

A model for toxin-antibody interaction and toxin trafficking towards the endoplasmic-reticulum is presented. Antibody and toxin (ricin) initially are delivered outside the cell. The model involves: the pinocytotic (cellular drinking) and receptor-mediated toxin internalization modes from the extracellular into the intracellular domain, its exocytotic excretion from the cytosol back to the extracellular medium, the intact toxin retrograde transport to the endoplasmic reticulum, the anterograde toxin movement outward from the cell across the plasma membrane, the lysosomal toxin degradation, and the toxin clearance (removal from the system) flux. The model consists of a set of coupled PDEs. Using an averaging procedure, the model is reduced to a system of coupled ODEs. Both PDEs and ODEs systems are solved numerically. Numerical results are illustrated by figures and discussed.

Toxin effect on protein biosynthesis in eukaryotic cells: a simple kinetic model

A model for toxin inhibition of protein synthesis inside eukaryotic cells is presented. Mitigation of this effect by introduction of an antibody is also studied. Antibody and toxin (ricin) initially are delivered outside the cell. The model describes toxin internalization from the extracellular into the intracellular domain, its transport to the endoplasmic reticulum (ER) and the cleavage inside the ER into the RTA and RTB chains, the release of RTA into the cytosol, inactivation (depurination) of ribosomes, and the effect on translation. The model consists of a set of ODEs which are solved numerically. Numerical results are illustrated by figures and discussed.

Ricin trafficking in cells

The heterodimeric plant toxin ricin binds exposed galactosyls at the cell surface of target mammalian cells, and, following endocytosis, is transported in vesicular carriers to the endoplasmic reticulum (ER). Subsequently, the cell-binding B chain (RTB) and the catalytic A chain (RTA) are separated reductively, RTA embeds in the ER membrane and then retrotranslocates (or dislocates) across this membrane. The protein conducting channels used by RTA are usually regarded as part of the ER-associated protein degradation system (ERAD) that removes misfolded proteins from the ER for destruction by the cytosolic proteasomes. However, unlike ERAD substrates, cytosolic RTA avoids destruction and folds into a catalytic conformation that inactivates its target ribosomes. Protein synthesis ceases, and subsequently the cells die apoptotically. This raises questions about how this protein avoids the pathways that are normally sanctioned for ER-dislocating substrates. In this review we focus on the molecular events that occur with non-tagged ricin and its isolated subunits at the ER–cytosol interface. This focus reveals that intra-membrane interactions of RTA may control its fate, an area that warrants further investigation.

Role of Fc in antibody-mediated protection from ricin toxin

We have studied the role of the antibody (Ab) Fc region in mediating protection from ricin toxicity. We compared the in vitro and in vivo effects of intact Ig and of Fab fragments derived from two different neutralizing Ab preparations, one monoclonal, the other polyclonal. Consistent results were obtained from each, showing little difference between Ig and Fab in terms of antigen binding and in vitro neutralization, but with relatively large differences in protection of animals. We also studied whether importing Ab into the cell by Fc receptors enhanced the intracellular neutralization of ricin toxin. We found that the imported Ab was found in the ER and Golgi, a compartment traversed by ricin, as it traffics through the cell, but intracellular Ab did not contribute to the neutralization of ricin. These results indicate that the Fc region of antibody is important for in vivo protection, although the mechanism of enhanced protection by intact Ig does not appear to operate at the single cell level. When using xenogeneic antibodies, the diminished immunogenicity of Fab/F(ab’)₂ preparations should be balanced against possible loss of protective efficacy.

Antibody-mediated inhibition of ricin toxin retrograde transport

Ricin is a member of the ubiquitous family of plant and bacterial AB toxins that gain entry into the cytosol of host cells through receptor-mediated endocytosis and retrograde traffic through the trans-Golgi network (TGN) and endoplasmic reticulum (ER). While a few ricin toxin-specific neutralizing monoclonal antibodies (MAbs) have been identified, the mechanisms by which these antibodies prevent toxin-induced cell death are largely unknown. Using immunofluorescence confocal microscopy and a TGN-specific sulfation assay, we demonstrate that 24B11, a MAb against ricin’s binding subunit (RTB), associates with ricin in solution or when prebound to cell surfaces and then markedly enhances toxin uptake into host cells. Following endocytosis, however, toxin-antibody complexes failed to reach the TGN; instead, they were shunted to Rab7-positive late endosomes and LAMP-1-positive lysosomes. Monovalent 24B11 Fab fragments also interfered with toxin retrograde transport, indicating that neither cross-linking of membrane glycoproteins/glycolipids nor the recently identified intracellular Fc receptor is required to derail ricin en route to the TGN. Identification of the mechanism(s) by which antibodies like 24B11 neutralize ricin will advance our fundamental understanding of protein trafficking in mammalian cells and may lead to the discovery of new classes of toxin inhibitors and therapeutics for biodefense and emerging infectious diseases.

Ricin is the prototypic member of the AB family of medically important plant and bacterial toxins that includes cholera and Shiga toxins. Ricin is also a category B biothreat agent. Despite ongoing efforts to develop vaccines and antibody-based therapeutics against ricin, very little is known about the mechanisms by which antibodies neutralize this toxin. In general, it is thought that antibodies simply prevent toxins from attaching to cell surface receptors or promote their clearance through Fc receptor (FcR)-mediated uptake. In this report, however, we describe a neutralizing monoclonal antibody (MAb) against ricin’s binding subunit (RTB) that not only associates with ricin after the toxin has bound to the cell’s surface but actually enhances toxin uptake into host cells. Following endocytosis, the antibody-toxin complexes are then routed for degradation. The results of this study are important because they reveal a previously unappreciated role for B-subunit-specific antibodies in intracellular neutralization of ricin toxin.

Toxin instability and its role in toxin translocation from the endoplasmic reticulum to the cytosol

AB toxins enter a host cell by receptor-mediated endocytosis. The catalytic A chain then crosses the endosome or endoplasmic reticulum (ER) membrane to reach its cytosolic target. Dissociation of the A chain from the cell-binding B chain occurs before or during translocation to the cytosol, and only the A chain enters the cytosol. In some cases, AB subunit dissociation is facilitated by the unique physiology and function of the ER. The A chains of these ER-translocating toxins are stable within the architecture of the AB holotoxin, but toxin disassembly results in spontaneous or assisted unfolding of the isolated A chain. This unfolding event places the A chain in a translocation-competent conformation that promotes its export to the cytosol through the quality control mechanism of ER-associated degradation. A lack of lysine residues for ubiquitin conjugation protects the exported A chain from degradation by the ubiquitin-proteasome system, and an interaction with host factors allows the cytosolic toxin to regain a folded, active state. The intrinsic instability of the toxin A chain thus influences multiple steps of the intoxication process. This review will focus on the host-toxin interactions involved with A chain unfolding in the ER and A chain refolding in the cytosol.

Inter-Golgi transport mediated by COPI-containing vesicles carrying small cargoes

A core prediction of the vesicular transport model is that COPI vesicles are responsible for trafficking anterograde cargoes forward. In this study, we test this prediction by examining the properties and requirements of inter-Golgi transport within fused cells, which requires mobile carriers in order for exchange of constituents to occur. We report that both small soluble and membrane-bound secretory cargo and exogenous Golgi resident glycosyl-transferases are exchanged between separated Golgi. Large soluble aggregates, which traverse individual stacks, do not transfer between Golgi, implying that small cargoes (which can fit in a typical transport vesicle) are transported by a different mechanism. Super-resolution microscopy reveals that the carriers of both anterograde and retrograde cargoes are the size of COPI vesicles, contain coatomer, and functionally require ARF1 and coatomer for transport. The data suggest that COPI vesicles traffic both small secretory cargo and steady-state Golgi resident enzymes among stacked cisternae that are stationary.

DOI:http://dx.doi.org/10.7554/eLife.01296.001

All eukaryotic cells contain an organelle called the Golgi apparatus, which consists of a series of four to six flattened structures called cisternae. Proteins that are intended for secretion from the cell, or proteins that go on to become part of the cell membrane, must pass through the Golgi, where they undergo modifications that ensure they are targeted to the correct place.

There are two main models for how proteins are transported from the entry side of the Golgi, known as the cis face, to the exit side (trans face), through a process known as anterograde transport. One possibility is that the cargo protein matures within a single cisterna, which gradually moves from the cis to the trans face without the protein ever leaving it. Alternatively, the cisternae may remain fixed in position, while individual proteins are carried between them by specialized transport vesicles called COPI vesicles.

Now, Pellett et al. have used modern molecular biology techniques to revisit this question, more than 25 years after members of the same group first obtained evidence suggesting the involvement of COPI vesicles. To do this, they labelled the proteins that reside within the Golgi of one cell green, and those within the Golgi of another cell, red. They then fused the two cells together, and traced the movement of labelled proteins between the two organelles.

Proteins that are known to undergo anterograde transport were also transported between the two Golgi, whereas large protein aggregates were not. Super-resolution microscopy revealed that the transported proteins were carried in vesicles the size of COPI vesicles and surrounded by a coat protein that resembles COPI. Moreover, transport involved the adaptor protein ARF, which helps to load cargo into COPI vesicles.

By providing evidence that Golgi resident proteins and proteins that normally undergo anterograde transport can be carried by COPI vesicles between two physically separate Golgi, Pellett et al. increase the weight of evidence that COPI vesicles may also be responsible for both retrograde and anterograde transport within the Golgi itself.

DOI:http://dx.doi.org/10.7554/eLife.01296.002

Ricin crosses polarized human intestinal cells and intestines of ricin-gavaged mice without evident damage and then disseminates to mouse kidneys

Ricin is a potent toxin found in the beans of Ricinus communis and is often lethal for animals and humans when aerosolized or injected and causes significant morbidity and occasional death when ingested. Ricin has been proposed as a bioweapon because of its lethal properties, environmental stability, and accessibility. In oral intoxication, the process by which the toxin transits across intestinal mucosa is not completely understood. To address this question, we assessed the impact of ricin on the gastrointestinal tract and organs of mice after dissemination of toxin from the gut. We first showed that ricin adhered in a specific pattern to human small bowel intestinal sections, the site within the mouse gut in which a variable degree of damage has been reported by others. We then monitored the movement of ricin across polarized human HCT-8 intestinal monolayers grown in transwell inserts and in HCT-8 cell organoids. We observed that, in both systems, ricin trafficked through the cells without apparent damage until 24 hours post intoxication. We delivered a lethal dose of purified fluorescently-labeled ricin to mice by oral gavage and followed transit of the toxin from the gastrointestinal tracts to the internal organs by in vivo imaging of whole animals over time and ex vivo imaging of organs at various time points. In addition, we harvested organs from unlabeled ricin-gavaged mice and assessed them for the presence of ricin and for histological damage. Finally, we compared serum chemistry values from buffer-treated versus ricin-intoxicated animals. We conclude that ricin transverses human intestinal cells and mouse intestinal cells in situ prior to any indication of enterocyte damage and that ricin rapidly reaches the kidneys of intoxicated mice. We also propose that mice intoxicated orally with ricin likely die from distributive shock.

Neutralizing monoclonal antibodies against ricin's enzymatic subunit interfere with protein disulfide isomerase-mediated reduction of ricin holotoxin in vitro

The penultimate event in the intoxication of mammalian cells by ricin toxin is the reduction, in the endoplasmic reticulum (ER), of the intermolecular disulfide bond that links ricin's enzymatic (RTA) and binding (RTB) subunits. In this report we adapted an in vitro protein disulfide isomerase (PDI)-mediated reduction assay to test the hypothesis that the RTA-specific neutralizing monoclonal antibody (mAb) IB2 interferes with the liberation of RTA from RTB. IB2 recognizes an epitope located near the interface between RTA and RTB and, like a number of other RTA-specific neutralizing mAbs, is proposed to neutralize ricin intracellularly. In this study, we found that IB2 virtually eliminated the reduction of ricin holotoxin into RTA and RTB in vitro. Surprisingly, three other neutralizing mAbs (GD12, R70 and SyH7) that bind epitopes at considerable distance from ricin's disulfide bond were as effective (or nearly as effective) as IB2 in interfering with PDI-mediated liberation of RTA from RTB. By contrast, two non-neutralizing RTA-specific mAbs, FGA12 and SB1, did not affect PDI-mediated reduction of ricin. These data reveal a possible mechanism by which RTA-specific antibodies may neutralize ricin intracellularly, provided they are capable of trafficking in association with ricin from the cell surface to the ER.

Stapled Golgi cisternae remain in place as cargo passes through the stack

We have designed a membrane ‘staple’, which consists of membrane-anchored repeats of the trans-aggregating FM domain that face the lumen of the secretory pathway. In the presence of the disaggregating drug these proteins transit the secretory pathway. When the drug is removed these proteins form electron-dense plaques which we term staples. Unexpectedly, when initially positioned within the cis-Golgi, staples remained at the cis face of the Golgi even after many hours. By contrast, soluble FM-aggregates transited the Golgi. Staples and soluble aggregates placed in cis-Golgi cisternae therefore have different fates. Whereas the membrane staples are located in the flattened, stacked central regions of the cisternae, the soluble aggregates are in the dilated rims. This suggests that while the cisternae are static on the time scale of protein traffic, the dilated rims are mobile and progress in the cis → trans direction via a mechanism that we term ‘Rim Progression’.

DOI:http://dx.doi.org/10.7554/eLife.00558.001

Most plant and animal cells contain an organelle known as the Golgi apparatus, which consists of a series of four to six stacked cisternae. Almost all the proteins that are secreted from the cell, or targeted to its plasma membrane, transit through the Golgi. This process takes roughly 5–20 min.

Although transport of proteins through the Golgi was first observed more than 50 years ago, it is still unclear exactly how this process occurs. One possibility is that proteins to be packaged move through the cisternae enclosed in vesicles, as if on a conveyor belt. Alternatively, the proteins themselves may remain stationary while the Golgi cisternae move over them.

Now, Lavieu et al. provide evidence that the Golgi shows both mobile and static behaviour depending on the type and size of the cargo being processed. To distinguish between these two mechanisms, they created a new type of protein cargo—which they called a ‘staple’—that became fixed to the walls on each side of the cisternae and could not, therefore, move freely through the Golgi. They compared the processing of this protein to that of a more typical soluble protein cargo, which could move freely through the Golgi stack.

Surprisingly, the Golgi processed these two types of cargo in very different ways. The staples remained embedded in the walls in the center of the cisternae, whereas the conventional soluble cargo was transported past the staples and collected at the edges of the cisternae, which are known as rims. These are wider than the center of the cisternae, and the staples are too narrow to span them. Lavieu et al. suggest that the Golgi cisternae can be divided into two functionally distinct domains: the centers of cisternae, which remain stationary, and the edges or rims, which can move.

In addition to increasing our understanding of how proteins are prepared for transport inside cells, this new mechanism reconciles seemingly conflicting data by revealing that the Golgi can be both mobile and static.

DOI:http://dx.doi.org/10.7554/eLife.00558.002

Antibody to ricin a chain hinders intracellular routing of toxin and protects cells even after toxin has been internalized

Background

Mechanisms of antibody-mediated neutralization are of much interest. For plant and bacterial A-B toxins, A chain mediates toxicity and B chain binds target cells. It is generally accepted and taught that antibody (Ab) neutralizes by preventing toxin binding to cells. Yet for some toxins, ricin included, anti-A chain Abs afford greater protection than anti-B. The mechanism(s) whereby Abs to the A chain neutralize toxins are not understood.

Methodology/Principal Findings

We use quantitative confocal imaging, neutralization assays, and other techniques to study how anti-A chain Abs function to protect cells. Without Ab, ricin enters cells and penetrates to the endoplasmic reticulum within 15 min. Within 45–60 min, ricin entering and being expelled from cells reaches equilibrium. These results are consistent with previous observations, and support the validity of our novel methodology. The addition of neutralizing Ab causes ricin accumulation at the cell surface, delays internalization, and postpones retrograde transport of ricin. Ab binds ricin for >6hr as they traffic together through the cell. Ab protects cells even when administered hours after exposure.

Conclusions/Key Findings

We demonstrate the dynamic nature of the interaction between the host cell and toxin, and how Ab can alter the balance in favor of the cell. Ab blocks ricin’s entry into cells, hinders its intracellular routing, and can protect even after ricin is present in the target organelle, providing evidence that the major site of neutralization is intracellular. These data add toxins to the list of pathogenic agents that can be neutralized intracellularly and explain the in vivo efficacy of delayed administration of anti-toxin Abs. The results encourage the use of post-exposure passive Ab therapy, and show the importance of the A chain as a target of Abs.

Detection of toxin translocation into the host cytosol by surface plasmon resonance

AB toxins consist of an enzymatic A subunit and a cell-binding B subunit(1). These toxins are secreted into the extracellular milieu, but they act upon targets within the eukaryotic cytosol. Some AB toxins travel by vesicle carriers from the cell surface to the endoplasmic reticulum (ER) before entering the cytosol(2-4). In the ER, the catalytic A chain dissociates from the rest of the toxin and moves through a protein-conducting channel to reach its cytosolic target(5). The translocated, cytosolic A chain is difficult to detect because toxin trafficking to the ER is an extremely inefficient process: most internalized toxin is routed to the lysosomes for degradation, so only a small fraction of surface-bound toxin reaches the Golgi apparatus and ER(6-12). To monitor toxin translocation from the ER to the cytosol in cultured cells, we combined a subcellular fractionation protocol with the highly sensitive detection method of surface plasmon resonance (SPR)(13-15). The plasma membrane of toxin-treated cells is selectively permeabilized with digitonin, allowing collection of a cytosolic fraction which is subsequently perfused over an SPR sensor coated with an anti-toxin A chain antibody. The antibody-coated sensor can capture and detect pg/mL quantities of cytosolic toxin. With this protocol, it is possible to follow the kinetics of toxin entry into the cytosol and to characterize inhibitory effects on the translocation event. The concentration of cytosolic toxin can also be calculated from a standard curve generated with known quantities of A chain standards that have been perfused over the sensor. Our method represents a rapid, sensitive, and quantitative detection system that does not require radiolabeling or other modifications to the target toxin.

Immunity to ricin: fundamental insights into toxin-antibody interactions

Ricin toxin is an extraordinarily potent inducer of cell death and inflammation. Ricin is also a potent provocateur of the humoral immune system, eliciting a mixture of neutralizing, non-neutralizing and even toxin-enhancing antibodies. The characterization of dozens of monoclonal antibodies (mAbs) against the toxin's enzymatic (RTA) and binding (RTB) subunits has begun to reveal fundamental insights into the underlying mechanisms by which antibodies neutralize (or fail to neutralize) ricin in systemic and mucosal compartments. This information has had immediate applications in the design, development and evaluation of ricin subunit vaccines and immunotherapeutics.

Ricinus communis intoxications in human and veterinary medicine-a summary of real cases

Accidental and intended Ricinus communis intoxications in humans and animals have been known for centuries but the causative agent remained elusive until 1888 when Stillmark attributed the toxicity to the lectin ricin. Ricinus communis is grown worldwide on an industrial scale for the production of castor oil. As by-product in castor oil production ricin is mass produced above 1 million tons per year. On the basis of its availability, toxicity, ease of preparation and the current lack of medical countermeasures, ricin has gained attention as potential biological warfare agent. The seeds also contain the less toxic, but highly homologous Ricinus communis agglutinin and the alkaloid ricinine, and especially the latter can be used to track intoxications. After oil extraction and detoxification, the defatted press cake is used as organic fertilizer and as low-value feed. In this context there have been sporadic reports from different countries describing animal intoxications after uptake of obviously insufficiently detoxified fertilizer. Observations in Germany over several years, however, have led us to speculate that the detoxification process is not always performed thoroughly and controlled, calling for international regulations which clearly state a ricin threshold in fertilizer. In this review we summarize knowledge on intended and unintended poisoning with ricin or castor seeds both in humans and animals, with a particular emphasis on intoxications due to improperly detoxified castor bean meal and forensic analysis.

Role of the mannose receptor (CD206) in innate immunity to ricin toxin

The entry of ricin toxin into macrophages and certain other cell types in the spleen and liver results in toxin-induced inflammation, tissue damage and organ failure. It has been proposed that uptake of ricin into macrophages is facilitated by the mannose receptor (MR; CD206), a C-type lectin known to recognize the oligosaccharide side chains on ricin’s A (RTA) and B (RTB) subunits. In this study, we confirmed that the MR does indeed promote ricin binding, uptake and killing of monocytes in vitro. To assess the role of MR in the pathogenesis of ricin in vivo, MR knockout (MR^−/−) mice were challenged with the equivalent of 2.5× or 5× LD₅₀ of ricin by intraperitoneal injection. We found that MR^−/− mice were significantly more susceptible to toxin-induced death than their age-matched, wild-type control counterparts. These data are consistent with a role for the MR in scavenging and degradation of ricin, not facilitating its uptake and toxicity in vivo.

Passive and active vaccination strategies to prevent ricin poisoning

Ricin toxin (RT) is derived from castor beans, produced by the plant Ricinus communis. RT and its toxic A chain (RTA) have been used therapeutically to arm ligands that target disease-causing cells. In most cases these ligands are cell-binding monoclonal antibodies (MAbs). These ligand-toxin conjugates or immunotoxins (ITs) have shown success in clinical trials [1]. Ricin is also of concern in biodefense and has been classified by the CDC as a Class B biothreat. Virtually all reports of RT poisoning have been due to ingestion of castor beans, since they grow abundantly throughout the world and are readily available. RT is easily purified and stable, and is not difficult to weaponize. RT must be considered during any "white powder" incident and there have been documented cases of its use in espionage [2,3]. The clinical syndrome resulting from ricin intoxication is dependent upon the route of exposure. Countermeasures to prevent ricin poisoning are being developed and their use will depend upon whether military or civilian populations are at risk of exposure. In this review we will discuss ricin toxin, its cellular mode of action, the clinical syndromes that occur following exposure and the development of pre- and post-exposure approaches to prevent of intoxication.

Adapting yeast as model to study ricin toxin a uptake and trafficking

The plant A/B toxin ricin represents a heterodimeric glycoprotein belonging to the family of ribosome inactivating proteins, RIPs. Its toxicity towards eukaryotic cells results from the depurination of 28S rRNA due to the N-glycosidic activity of ricin toxin A chain, RTA. Since the extention of RTA by a mammalian-specific endoplasmic reticulum (ER) retention signal (KDEL) significantly increases RTA in vivo toxicity against mammalian cells, we here analyzed the phenotypic effect of RTA carrying the yeast-specific ER retention motif HDEL. Interestingly, such a toxin (RTA^HDEL) showed a similar cytotoxic effect on yeast as a corresponding RTA^KDEL variant on HeLa cells. Furthermore, we established a powerful yeast bioassay for RTA in vivo uptake and trafficking which is based on the measurement of dissolved oxygen in toxin-treated spheroplast cultures of S. cerevisiae. We show that yeast spheroplasts are highly sensitive against external applied RTA and further demonstrate that its toxicity is greatly enhanced by replacing the C-terminal KDEL motif by HDEL. Based on the RTA resistant phenotype seen in yeast knock-out mutants defective in early steps of endocytosis (∆end3) and/or in RTA depurination activity on 28S rRNA (∆rpl12B) we feel that the yeast-based bioassay described in this study is a powerful tool to dissect intracellular A/B toxin transport from the plasma membrane through the endosomal compartment to the ER.

Ricin trafficking in plant and mammalian cells

Ricin is a heterodimeric plant protein that is potently toxic to mammalian and many other eukaryotic cells. It is synthesized and stored in the endosperm cells of maturing Ricinus communis seeds (castor beans). The ricin family has two major members, both, lectins, collectively known as Ricinus communis agglutinin ll (ricin) and Ricinus communis agglutinin l (RCA). These proteins are stored in vacuoles within the endosperm cells of mature Ricinus seeds and they are rapidly broken down by hydrolysis during the early stages of post-germinative growth. Both ricin and RCA traffic within the plant cell from their site of synthesis to the storage vacuoles, and when they intoxicate mammalian cells they traffic from outside the cell to their site of action. In this review we will consider both of these trafficking routes.

Small-molecule inhibitors of ricin and Shiga toxins

This review summarizes the successes and continuing challenges associated with the identification of small-molecule inhibitors of ricin and Shiga toxins, members of the RNA N-glycosidase family of toxins that irreversibly inactivate eukaryotic ribosomes through the depurination of a conserved adenosine residue within the sarcin-ricin loop (SRL) of 28S rRNA. Virtual screening of chemical libraries has led to the identification of at least three broad classes of small molecules that bind in or near the toxin's active sites and thereby interfere with RNA N-glycosidase activity. Rational design is being used to improve the specific activity and solubility of a number of these compounds. High-throughput cell-based assays have also led to the identification of small molecules that partially, or in some cases, completely protect cells from ricin- and Shiga-toxin-induced death. A number of these recently identified compounds act on cellular proteins associated with intracellular trafficking or pro-inflammatory/cell death pathways, and one was reported to be sufficient to protect mice in a ricin challenge model.

Darwin's foundation for investigating self-incompatibility and the progress toward a physiological model for S-RNase-based SI

Charles Darwin made extensive observations of the pollination biology of a wide variety of plants. He carefully documented the consequences of self-pollination and described species that were self-sterile but that could easily be crossed with other plants of the same species. He believed that compatibility was controlled by the 'mutual action' of pollen and pistil contents. A genetic model for self-sterility was developed in the early 1900 s based on studies of the compatibility relationships among, what are now referred to as, self-incompatible (SI) Nicotiana species. Today, it is believed that SI in these species is controlled by an interaction between S-RNases produced in the pistil and F-box proteins expressed in pollen and, moreover, that this S-RNase-based SI system is shared by a great diversity of other plant species. Current research is aimed at understanding how the mutual actions of these S-gene products function in the physiological context of pollen tube growth.

Cytosolic chaperones influence the fate of a toxin dislocated from the endoplasmic reticulum

The plant cytotoxin ricin enters target mammalian cells by receptor-mediated endocytosis and undergoes retrograde transport to the endoplasmic reticulum (ER). Here, its catalytic A chain (RTA) is reductively separated from the cell-binding B chain, and free RTA enters the cytosol where it inactivates ribosomes. Cytosolic entry requires unfolding of RTA and dislocation across the ER membrane such that it arrives in the cytosol in a vulnerable, nonnative conformation. Clearly, for such a dislocated toxin to become active, it must avoid degradation and fold to a catalytic conformation. Here, we show that, in vitro, Hsc70 prevents aggregation of heat-treated RTA, and that RTA catalytic activity is recovered after chaperone treatment. A combination of pharmacological inhibition and cochaperone expression reveals that, in vivo, cytosolic RTA is scrutinized sequentially by the Hsc70 and Hsp90 cytosolic chaperone machineries, and that its eventual fate is determined by the balance of activities of cochaperones that regulate Hsc70 and Hsp90 functions. Cytotoxic activity follows Hsc70-mediated escape of RTA from an otherwise destructive pathway facilitated by Hsp90. We demonstrate a role for cytosolic chaperones, proteins typically associated with folding nascent proteins, assembling multimolecular protein complexes and degrading cytosolic and stalled, cotranslocational clients, in a toxin triage, in which both toxin folding and degradation are initiated from chaperone-bound states.

Effect of surface charge and density of distearylphosphatidylethanolamine-mPEG-2000 (DSPE-mPEG-2000) on the cytotoxicity of liposome-entrapped ricin: Effect of lysosomotropic agents

Ricin was encapsulated in various liposomes having neutral, negatively and positively charged and different density of DSPE-mPEG-2000 on the surface and cytotoxicity of ricin entrapped in these different charged liposomal formulations was studied in CHO pro(-) cells and compared with free ricin with a view to develop an optimum delivery system for ricin in vivo. It was observed that the cytotoxicity of ricin entrapped in various charged liposomes was significantly dependent on the charge on the surface of liposomes. The maximum cytotoxicity of ricin was observed when it was delivered through negatively charged liposomes. Monensin enhances the cytotoxicity of ricin entrapped in various charged liposomes and the extent of enhancement of the cytotoxicity is significantly dependent on the charge on the surface of liposomes. Maximum potentiation (213.14-fold) of cytotoxicity of ricin was observed when it was delivered through positively charged liposomes followed by negatively charged (83.36-fold) and neutral (71.30-fold) liposomes, respectively. Studies on the kinetics of inhibition of protein synthesis by ricin entrapped in various charged liposomes revealed that lag period of inhibition of protein synthesis is significantly lengthened following delivery through various charged liposomes. However, in the presence of monensin, the lag period was reduced. There is a marginal variation in the cytotoxicity of ricin entrapped in various charged liposomes after incorporation of 5mol% of DSPE-mPEG-2000 on the surface. However, there is a significant variation in the enhancing potency of monensin on the cytotoxicity of ricin entrapped in various charged liposomes in CHO pro(-) cells following incorporation of 5mol% DSPE-mPEG-2000 on the surface. Studies on the effect of variation of density of DSPE-mPEG-2000 on the surface of various charged liposomes on the enhancement of cytotoxicity of entrapped ricin by monensin in CHO pro(-) cells showed that the enhancing potency of monensin on the cytotoxicity of ricin entrapped in various charged liposomes is significantly dependent on the density of DSPE-mPEG-2000 on their surface. It was also observed that the efficacies of monensin on the enhancement of cytotoxicity of ricin entrapped in various charged PEG-liposomes in CHO pro(-) cells was highly related to their amount of cell-association. The present study has clearly shown that by suitable alteration of liposomal lipid composition, charge and density of hydrophilicity it would be possible to direct liposomal ricin to specific cells for their selective elimination in combination with monensin.

Pet, a non-AB toxin, is transported and translocated into epithelial cells by a retrograde trafficking pathway

The plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli is a 104-kDa autotransporter protein that exhibits proteolytic activity against the actin-binding protein alpha-fodrin. Intracellular cleavage of epithelial fodrin by Pet disrupts the actin cytoskeleton, causing both cytotoxic and enterotoxic effects. Intoxication requires the serine protease activity of Pet and toxin endocytosis from clathrin-coated pits. The additional events in the intracellular trafficking of Pet are largely uncharacterized. Here, we determined by confocal microscopy that internalized Pet is transferred from the early endosomes to the Golgi apparatus and then travels to the endoplasmic reticulum (ER). Pet associates with the Sec61p translocon before it moves into the cytosol as an intact, 104-kDa protein. This translocation process contrasts with the export of other ER-translocating toxins, in which only the catalytic A subunit of the AB toxin enters the cytosol. However, like intoxication with these AB toxins, Pet intoxication was inhibited in a subset of mutant CHO cell lines with aberrant activity in the ER-associated degradation pathway of ER-to-cytosol translocation. This is the first report which documents the cell surface-to-ER and ER-to-cytosol trafficking of a bacterial non-AB toxin.

Depletion of sphingolipids facilitates endosome to Golgi transport of ricin

It has been previously demonstrated that depletion of cholesterol inhibits endosome to Golgi transport. Whether this inhibition is due to disruption of sphingolipid- and cholesterol-containing lipid rafts that are selected for Golgi transport or whether there is a physical requirement of cholesterol for either membrane deformations, facilitating formation of transport vesicles, or for recruitment of cytosolic constituents is not obvious. To investigate this in more detail, we have studied endosome to Golgi transport of ricin in sphingolipid-deficient cells using either a mutant cell line that does not express serine palmitoyltransferase, the first enzyme in sphingolipid biosynthesis, or a specific inhibitor, myriocin, of the same enzyme. Depletion of sphingolipids gave an increased sensitivity to ricin, and this increased sensitivity was inhibited by addition of sphingolipids. Importantly, endosome to Golgi transport of ricin, measured as sulfation of a modified ricin molecule, was increased in sphingolipid-deficient cells. No effect was seen on other pathways taken by ricin. Interestingly, cholesterol depletion inhibited endosome to Golgi transport even in cells with reduced levels of sphingolipids, suggesting that cholesterol as such is required for formation of transport vesicles. Our results indicate that the presence of sphingolipids actually limits and may function to control endosome to Golgi transport of ricin.

New views of S-RNase-based self-incompatibility

S-RNase-based self-incompatibility (SI) is the most widespread form of genetically controlled mate selection in plants. S-RNase controls pollination specificity in the pistil, while the newly discovered SLF/SFB controls pollination specificity in the pollen. A widely discussed model suggests that compatibility is explained by ubiquitylation and degradation of nonself-S-RNase and that, conversely, incompatibility is caused by failure to degrade self-S-RNase. This model is consistent with the long-standing view that S-RNase inhibition is central to SI. Recent results show, however, that S-RNase is compartmentalized in pollen tubes and, significantly, that compatibility might not require SLF/SFB. S-RNase compartmentalization and dislocation into the pollen tube cytoplasm might be similar to the trafficking of other cytotoxins such as ricin.

Enhancement of the cytotoxicity of liposomal ricin by the carboxylic ionophore monensin and the lysosomotropic amine NH4Cl in Chinese hamster ovary cells

Ricin was encapsulated in negatively charged liposomes and its effect on the cytotoxicity was compared with native ricin in Chinese hamster ovarian (CHO) cells. The cytotoxicity of ricin, as measured by a marker protein synthesis (incorporation of 3H-leucine), was reduced markedly (300-fold) following encapsulation in liposomes. Lactose, a potent inhibitor of ricin cytotoxicity, had no effect on the binding, internalization, and cytotoxicity of liposomal ricin, indicating that liposomal ricin enter into mammalian cells by an alternative route, bypassing galactose-mediated endocytic pathway. Both monensin (a carboxylic ionophore) and NH4Cl (a lysosomotropic amine) markedly enhances the cytotoxicity of liposomal ricin, indicating endocytotic uptake of liposomal ricin. The degree of potentiation of the cytotoxicity of liposomal ricin by both monensin and NH4Cl was significantly higher (441- and 51-fold) as compared to native ricin (62.5- and 12.5-fold). The extent of exocytosis of free ricin was found to be much higher as compared to liposomal ricin; on the other hand, the extent of degradation of free and liposomal ricin was identical. Consequently, the intracellular level of liposomal ricin was increased to 3.5-fold. This higher level of intracellular liposomal ricin may allow more efficient ricin A-chain release into the cytosol under the influence of NH4Cl and monensin. Monensin-induced potentiation of liposomal ricin was prevented by brefeldin A, indicating that in the presence of monensin, the liposomal ricin was efficiently routed through the Golgi apparatus en route to the cytosol. Thus, liposomal ricin in combination with monensin may have potential application for selective elimination of malignant cells.

Transport of Ricin from Endosomes to the Golgi Apparatus is Regulated by Rab6A and Rab6A′

Ricin is transported from early endosomes and/or the recycling compartment to the trans-Golgi network (TGN) and subsequently to the endoplasmic recticulum (ER) before it enters the cytosol and intoxicates cells. We have investigated the role of the Rab6 isoforms in retrograde transport of ricin using both oligo- and vector-based RNAi assays. Ricin transport to the TGN was inhibited by the depletion of Rab6A when the Rab6A messenger RNA (mRNA) levels were reduced by more than 40% and less than 75%. However, when Rab6A mRNA was reduced by more than 75% and Rab6A' mRNA was simultaneously up-regulated, the inhibition of ricin sulfation was abolished, indicating that the up-regulation of Rab6A' may compensate for the loss of Rab6A function. In addition, we found that a near complete depletion of Rab6A' gave approximately 40% reduction in ricin sulfation. The up-regulation of Rab6A mRNA levels did not seem to compensate for the loss of Rab6A' function. The depletion of both Rab6A and Rab6A' gave a stronger inhibition of ricin sulfation than what was observed knocking down the two isoforms separately. In conclusion, both Rab6A and Rab6A' seem to be involved in the transport of ricin from endosomes to the Golgi apparatus.

Retrograde transport pathways utilised by viruses and protein toxins

A model has been presented for retrograde transport of certain toxins and viruses from the cell surface to the ER that suggests an obligatory interaction with a glycolipid receptor at the cell surface. Here we review studies on the ER trafficking cholera toxin, Shiga and Shiga-like toxins, Pseudomonas exotoxin A and ricin, and compare the retrograde routes followed by these protein toxins to those of the ER trafficking SV40 and polyoma viruses. We conclude that there is in fact no obligatory requirement for a glycolipid receptor, nor even with a protein receptor in a lipid-rich environment. Emerging data suggests instead that there is no common pathway utilised for retrograde transport by all of these pathogens, the choice of route being determined by the particular receptor utilised.