Effect of gangliosides on binding, internalization and cytotoxic activity of ricin

Specificity of viscumin revised. As probed with a printed glycan array

Viscumin, a lectin used in anti-cancer therapy, was originally considered as βGal recognizing protein; later, an ability to bind 6'-sialyl N-acetyllactosamine (6'SLN) terminated gangliosides was found. Here we probed viscumin with a printed glycan array (PGA) containing a large number of mammalian sulfated glycans, and found a strong binding to glycans with 6-O-SuGal moiety as lactose, N-acetyllactosamine (LN), di-N-acetyllactosamine (LacdiNAc), and even 6-O-SuGalNAcα (but not SiaTn). Also, the ability to bind some of αGal terminated glycans, including Galα1-3Galβ1-4GlcNAc, was observed. Unexpectedly, only weak interaction was detected with parent neutral β-galactosides including LN-LN-LN and branched (LN)₂LN oligolactosamines; in the light of these data, one should not confidently classify viscumin as a β-galactoside-binding lectin. Carrying out PGA in the presence of neutral or sulfated/sialylated glycan, together with sequential elution from lactose-sepharose and consideration of the protein structure, lead to the conclusion that two glycan-binding sites of viscumin have different specificities, one of which prefers charged sulfated and sialylated moieties.

Treatments for Pulmonary Ricin Intoxication: Current Aspects and Future Prospects

Ricin, a plant-derived toxin originating from the seeds of Ricinus communis (castor beans), is one of the most lethal toxins known, particularly if inhaled. Ricin is considered a potential biological threat agent due to its high availability and ease of production. The clinical manifestation of pulmonary ricin intoxication in animal models is closely related to acute respiratory distress syndrome (ARDS), which involves pulmonary proinflammatory cytokine upregulation, massive neutrophil infiltration and severe edema. Currently, the only post-exposure measure that is effective against pulmonary ricinosis at clinically relevant time-points following intoxication in pre-clinical studies is passive immunization with anti-ricin neutralizing antibodies. The efficacy of this antitoxin treatment depends on antibody affinity and the time of treatment initiation within a limited therapeutic time window. Small-molecule compounds that interfere directly with the toxin or inhibit its intracellular trafficking may also be beneficial against ricinosis. Another approach relies on the co-administration of antitoxin antibodies with immunomodulatory drugs, thereby neutralizing the toxin while attenuating lung injury. Immunomodulators and other pharmacological-based treatment options should be tailored according to the particular pathogenesis pathways of pulmonary ricinosis. This review focuses on the current treatment options for pulmonary ricin intoxication using anti-ricin antibodies, disease-modifying countermeasures, anti-ricin small molecules and their various combinations.

Cytolethal Distending Toxin: A Unique Variation on the AB Toxin Paradigm

Some of the most potent toxins produced by plants and bacteria are members of a large family known as the AB toxins. AB toxins are generally characterized by a heterogenous complex consisting of two protein chains arranged in various monomeric or polymeric configurations. The newest class within this superfamily is the cytolethal distending toxin (Cdt). The Cdt is represented by a subfamily of toxins produced by a group of taxonomically distinct Gram negative bacteria. Members of this subfamily have a related AB-type chain or subunit configuration and properties distinctive to the AB paradigm. In this review, the unique structural and cytotoxic properties of the Cdt subfamily, target cell specificities, intoxication pathway, modes of action, and relationship to the AB toxin superfamily are compared and contrasted.

Endosomal ricin transport: involvement of Rab4- and Rab5-positive compartments

Transport of the ribosome-inactivating protein ricin through endosomes was studied in A431 cells expressing Rab5-, Rab4-, and Rab11-GFP. It was shown that Rab5- and Rab4-positive functional domains of early endosomes are involved in ricin transport. Ricin enters cells by both clathrin-dependent and clathrin-independent mechanisms. The main pool of internalized toxin accumulates in early endosomes and remains associated with them for a long time. In contrast to earlier observations, current observations indicate that the majority of ricin avoids transport to lysosomes. The low level of ricin association with Rab11 as well as with transferrin accumulated in the pericentriolar recycling compartment shows that the compartment is not responsible for keeping ricin away from degradation in lysosomes. Escape from degradation in lysosomes is assumed to result from the potentiality of ricin to form assemblies within compartments.

Membrane destabilization by ricin

Ricin is a promising candidate for the treatment of cancer because it can be selectively targeted to tumor cells via linkage to monoclonal antibodies. Biochemical evidence suggests that escape of ricin or its ribosome-inactivating subunit from an intracellular compartment is mediated by retrograde transport to the endoplasmic reticulum and subsequent direction into the ER-associated degradation pathway. Alternatively, lipase activity of ricin may facilitate leakage from endocytic vesicles. We have observed ricin-mediated release of macromolecular dyes from lipid vesicles that mimic the composition of endosomal membranes. Release of small molecules occurs to the same extent, suggesting an all-or-none mechanism due to bilayer destabilization. The level of accompanying membrane fusion depends on vesicle composition. Since it takes 24 h of incubation before the first traces of lysolipids are detectable by matrix-assisted laser desorption/ionization mass spectrometry, membrane destabilization is not due to the lipase activity of ricin.

Differences in endocytosis and intracellular sorting of ricin and viscumin in 3T3 cells

Ricin and viscumin are heterodimeric protein toxins. Their A-chain is enzymatically active and removes an adenine residue from the 28S rRNA, the B-chain has lectin activity and binds to terminal galactose residues of cell surface receptors. The toxins reveal a high degree of identity in their amino acid sequences. Nevertheless, uptake into 3T3 cells occurs via different receptors and endocytotic pathways. This has been revealed by enzyme linked based analysis of ricin competition with viscumin, and by fluorochrome-labeled toxins (viscumin-FITC, ricin-Alexa 568), which were added simultaneously or separately to living cells. Then the uptake was followed by confocal laser scanning microscopy. Ricin immediately is delivered to the tubular and vesicular structures of endosomes in the perinuclear area while viscumin becomes endocytosed into small vesicles preferentially in the cell periphery. After about 60 min both these toxins may be found in tubo-vesicular structures of endosomes where the sorting process can directly be observed. The fact that this sorting takes place is a strong argument for the assumption that the toxins are bound to membrane proteins, either to their original receptors or to other proteins inside the endosomal compartment exhibiting terminal galactose residues. The toxins are biologically fully active as has been proven by binding and by toxicity experiments, thus the differences in targeting do not arise from labeling.

Dehydration of model membranes induced by lectins from Ricinus communis and Viscum album

The effects of ribosome-inactivating proteins (RIPs) from Ricinus communis and from Viscum album on the water permeability, Pf, and the surface dielectric constant, epsilon, of model membranes were studied. Pf was calculated from microelectrode measurements of the ion concentration distribution in the immediate vicinity of a planar membrane, and epsilon was obtained from the fluorescence of dansyl phosphatidylethanolamine incorporated into unilamellar vesicles. Pf and epsilon of fully saturated phosphatidylcholine membranes were affected only in the presence of a lectin receptor (monosialoganglioside, GM1) in the bilayer. It is suggested that the membrane area occupied by clustered lectin-receptor complexes is markedly less permeable to water. Protein binding to the receptor was not a prelude for hydrophobic lipid-protein interactions when the membranes were formed from a mixture of natural phospholipids with a high content of unsaturated fatty acids. These membranes, characterized by a high initial water permeability, were found to interact with the RIPs unspecifically. From a decrease of both Pf and epsilon it was concluded that not only water partitioning but also protein adsorption correlates with looser packing of polyunsaturated lipids at the lipid-water interface.

Membrane fusion mediated by ricin and viscumin

The ribosome inactivating plant proteins (RIPs) ricin and viscumin but not Ricinus communis agglutinin are able induce vesicle-vesicle fusion. A model is suggested in which the toxicity of the RIPs is partially determined by their fusogenicity. Herein, fusion is hypothesized to allow the RIPs to leak across endocytic vesicles to approve their access to cytoplasmic ribosomes.

Immunotoxins containing A-chain of mistletoe lectin I are more active than immunotoxins with ricin A-chain

Conjugates of anti-CD25 monoclonal antibodies against cell surface IL-2 receptor with MLIA and RTA were prepared and investigated. Both of the immunotoxins had high specific cytotoxic activity on target cells. The IC50 value of the anti-CD25/MLIA immunotoxin was 15-fold greater than that of the anti-CD25/RTA. Previous studies of the anti-CD5 immunotoxins with MLIA and RTA showed that the anti-CD5/MLIA IT was 80-fold more active than anti-CD5/RTA IT [Tonevitsky et al. (1991) Int. J. Immunopharmacol. 13, 1037-1041]. The surface hydrophobicity of the MLI A-chain was 4-fold higher than that of the ricin A-chain as estimated by binding with ANS. In model experiments with small unilamellar DMPC liposomes, MLIA but not RTA increased the turbidity of liposome suspensions at pH 4.5. Our results indicate that the greater cytotoxic activity of the MLI A-chain immunotoxin probably provided a higher surface hydrophobicity of the protein and the ability to interact with phospholipid membranes.

Dynamic lipid heterogeneity and receptor events

Receptor occupation by specific ligands induces changes in the dynamic domain organization of surrounding lipids. Such changes were observed by measuring changes in the fluorescence parameters of fluorescent-labelled lipids incorporated into plasma membranes of intact cells, membrane vesicles or lipoprotein particles in response to specific binding of a broad range of biologically active agents, including drugs, prostaglandins, neuropeptides, antibodies and viruses. The high sensitivity of the fluorescence response allowed us to register changes in lipid heterogeneity induced in a multitude of discrete targets by transient weak binding of a single rapidly translocating molecule. To explain these observations a non-equilibrium model of ligand-receptor interaction based on low relaxation phenomena in heterogeneous lipid matrixes is proposed.