Uptake of injected 125I-ricin by rat liver in vivo. Subcellular distribution and characterization of the internalized ligand

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.

Evaluation of ricinine, a ricin biomarker, from a non-lethal castor bean ingestion

A case is presented of the attempted suicide of a 58-year-old man using castor beans. The patient came to the emergency room complaining of nausea, vomiting and diarrhea for nine hours following the ingestion of six castor beans. Urine samples were taken throughout the hospital stay and submitted to the Centers for Disease Control and Prevention for analysis of ricinine, a castor bean component. The samples were found to be positive for ricinine, with a maximum concentration of 674 µg/g-creatinine excreted approximately 23 h post-exposure. Subsequent samples demonstrated lower ricinine concentrations, with the final sample taken at 62 h post-exposure at a concentration of 135 µg/g-creatinine of ricinine. The estimated urinary excretion half-life was approximately 15 h and the recovery of ricinine in the urine over the three days was estimated to be less than 10%. The patient fully recovered with supportive care and was discharged from the hospital six days after admission.

Different in vitro toxicity of ribosome-inactivating proteins (RIPs) on sensory neurons and Schwann cells

OBJECTIVE

To study the neurotoxicity induced by Ricinus communis agglutinin (RCA), ricin A chain (RTA), and trichosanthin (TCS) in vitro.

METHODS

Rat neurons and Schwann cells were cultured and real-time up-take of RIPs was traced. TUNEL, Annexin V and DAPI were employed to study the mechanism.

RESULTS

The purity of both primary neuronal and Schwann cell cultures attained 80-90%. In neuritis, transport of FITC-RCA was demonstrated, but RTA and TCS were not detected. RCA elicited the strongest TUNEL and annexin V signals in both cultures. RTA evoked a stronger apoptotic signal than TCS in neurons. In contrast, compared with TCS, RTA elicited an attenuated apoptotic reaction in Schwann cells. All internalized RIPs were concentrated in the cytoplasm of the cells and their nuclei were not stained by DAPI.

CONCLUSION

The toxicity of these RIPs on neurons is different from that on Schwann cells. Although they enter cells by different mechanisms they all induce apoptosis. These results may find application in in vivo neural lesioning studies and clinical therapy.

Proteolysis of Pseudomonas exotoxin A within hepatic endosomes by cathepsins B and D produces fragments displaying in vitro ADP-ribosylating and apoptotic effects

To assess Pseudomonas exotoxin A (ETA) compartmentalization, processing and cytotoxicity in vivo, we have studied the fate of internalized ETA with the use of the in vivo rodent liver model following toxin administration, cell-free hepatic endosomes, and pure in vitro protease assays. ETA taken up into rat liver in vivo was rapidly associated with plasma membranes (5-30 min), internalized within endosomes (15-60 min), and later translocated into the cytosolic compartment (30-90 min). Coincident with endocytosis of intact ETA, in vivo association of the catalytic ETA-A subunit and low molecular mass ETA-A fragments was observed in the endosomal apparatus. After an in vitro proteolytic assay with an endosomal lysate and pure proteases, the ETA-degrading activity was attributed to the luminal species of endosomal acidic cathepsins B and D, with the major cleavages generated in vitro occurring mainly within domain III of ETA-A. Cell-free endosomes preloaded in vivo with ETA intraluminally processed and extraluminally released intact ETA and ETA-A in vitro in a pH-dependent and ATP-dependent manner. Rat hepatic cells underwent in vivo intrinsic apoptosis at a late stage of ETA infection, as assessed by the mitochondrial release of cytochrome c, caspase-9 and caspase-3 activation, and DNA fragmentation. In an in vitro assay, intact ETA induced ADP-ribosylation of EF-2 and mitochondrial release of cytochrome c, with the former effect being efficiently increased by a cathepsin B/cathepsin D pretreatment. The data show a novel processing pathway for internalized ETA, involving cathepsins B and D, resulting in the production of ETA fragments that may participate in cytotoxicity and mitochondrial dysfunction.

Retrospective identification of ricin in animal tissues following administration by pulmonary and oral routes

A previously characterised amplified ELISA for ricin (sensitivity limit approximately 200 pgmL(-1)) has been employed to quantify ricin following a novel recovery method from selected tissues. Tissue samples from rats dosed by pulmonary instillation or orally with ricin were homogenised and treated with an elution buffer to extract ricin. This is the first time that ex vivo recovery of ricin post exposure following pulmonary or oral challenge has been achieved using clinically acceptable sampling methods, with promise in terms of diagnosis for the timely implementation of therapy. The toxin was detected and quantified using the ELISA in conjunction with pure ricin standards. Extracts from tissues sampled, including lung, blood, liver and spleen tested positive for ricin with maximum yield in lung associated fractions for pulmonary dosing and liver tissue for oral administration. This indicates the potential of lavage and blood sampling for timely diagnosis of ricin poisoning by pulmonary and oral routes, respectively. Time course analysis at 24 and 48 h also indicated the progression of ricin from surfaces of the lung into the lung tissue. Inter-subject variation was observed in the case of oral dosing, with data for ricin-treated and vehicle control tissues not statistically different in all samples. In addition the oral toxicity of the crude ricin administered was found to be higher than expected in the rat, based upon published information and an unpublished in house murine study.

Simultaneous existence of cinnamomin (a type II RIP) and small amount of its free A- and B-chain in mature seeds of camphor tree

Cinnamomin, a type II ribosome-inactivating protein (RIP), was isolated from the mature seeds of camphor tree (Cinnamomum camphora). In this paper, small amount of free A- and B-chain of cinnamomin were found to be present in the mature seed cell of C. camphora besides the intact cinnamomin. Our results demonstrated that camphorin, a type I RIP previously reported to coexist with cinnamomin in the seeds of C. camphora, actually was the A-chain of cinnamomin. The percentage of free A- and B-chain in the total cinnamomin was 2.6-2.8% in the seed extract. Of these free A- and B-chain approximate 80% already existed in the seed cell, only about 20% were produced during the purification operation. As the enzymatic activity to reduce disulfide bond of cinnamomin in the seed extract of C. camphora was detected, we proposed that the free A- and B-chain were derived from the enzymatic reduction of the interchain disulfide bond of cinnamomin. It was demonstrated that the endogenous type II RIPs of several plant species, such as Cinnamomum porrectum, Cinnamomum bodinieri and Ricinus communis, could be enzymatically reduced into the free A- and B-chain in their respective seed cells. The function of the free A-chain in the seed cell and the possibility that metabolic enzymes might be involved in the reduction of the interchain disulfide bond of type II RIPs in vivo are discussed.

Expression of functional ricin B chain using the baculovirus system

The ricin B chain (RTB) was expressed using a baculovirus expression system. The RTB coding sequence downstream of the preproricin signal sequence was inserted in the baculovirus transfer vector pM34T. After cotransfection of Spodoptera frugiperda Sf9 cells with linearized baculovirus DNA, recombinant viruses were selected, cloned and amplified. Upon infection of Sf9 cells with these recombinant baculoviruses, RTB production was revealed by immunoblotting. RTB expression using this system was optimum 72 h after infection of the cells at a multiplicity of infection of 3. RTB produced was glycosylated and had an apparent molecular mass of 34 kDa. Most of the signal sequence was removed, but the resulting recombinant RTB had a 13-residue N-terminus extension. Immunofluorescence analysis showed that this protein was located in the endoplasmic reticulum/Golgi region of the cell. RTB was not present at the plasma membrane. Secretion was enhanced by the addition of lactose to the cell-culture medium up to 50 mM. Purification was achieved from both cells and media using immobilized lactose and the lectin activity of RTB. Results obtained with the purified recombinant protein (more than 2 mg/l culture) were identical to those obtained with native RTB in all assays for biological activity; binding, internalization and reassociation with the ricin A chain to produce toxic ricin. Moreover, the RTB translocation capacity was not altered by the N-terminal peptide, showing that recombinant RTB could be used to deliver antigenic peptides to the cytosol for the induction of cell-mediated immunity.

Cytotoxic effects of ricin without an interchain disulfide bond: genetic modification and chemical crosslinking studies

Ricin is a toxic glycoprotein made of two polypeptide chains (A and B) linked by a disulfide bond. Ricin binds to cells by the B chain and is then internalized. The interchain disulfide bond is believed to be reduced in endosomes, and the A chain is then subsequently translocated to cytoplasm where it inactivates ribosomes. To understand the role of the disulfide bond in ricin toxicity, we prepared two types of ricin molecules. First, cysteine 259 of the A chain was mutated to an alanine residue. The mutant A chain was then reassociated with the native B chain to determine whether ricin is biologically active in the absence of an interchain disulfide bond. Reassociated mutant ricin showed a 40-fold reduction in biological activity. Binding studies using a hydrophobic fluorescence probe indicated that the associated complex was stable only at neutral pH and became highly unstable at a lower pH characteristic of the endosomal milieu. In the second construct, the interchain disulfide bond was replaced with a non-reducible bond by chemical derivatization. Interestingly, the non-reducible ricin molecule was equally cytotoxic as native ricin. These results show: (i) that the interchain disulfide bond is necessary to hold the A chain and the B chain together at endosomal pH, and (ii) that intact ricin may be transported to the cytoplasm where proteolysis or hydrolysis may occur to release the biologically active moiety.

Ricin-membrane interaction: membrane penetration depth by fluorescence quenching and resonance energy transfer

The entry of the plant toxin ricin and its A- and B-subunits in model membranes in the presence as well as absence of monosialoganglioside (GM1) has been studied. Dioleoylphosphatidylcholine and 5-, 10-, and 12-doxyl- or 9,10-dibromophosphatidylcholines serve as quenchers of intrinsic tryptophan fluorescence of the proteins. The parallax method of Chattopadhyay and London [(1987) Biochemistry 26, 39-45] has been employed to measure the average membrane penetration depth of tryptophans of ricin and its B-chain and the actual depth of the sole Trp 211 in the A-chain. The results indicate that both of the chains as well as intact ricin penetrate the membrane deeply and the C-terminal end of the A-chain is well inside the bilayer, especially at pH 4.5. An extrinsic probe N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine (I-AEDANS) has been attached to Cys 259 of the A-chain, and the kinetics of penetration has been followed by monitoring the increase in AEDANS fluorescence at 480 nm. The insertion follows first-order kinetics, and the rate constant is higher at a lower pH. The energy transfer distance analysis between Trp 211 and AEDANS points out that the conformation of the A-chain changes as it inserts into the membrane. CD studies indicate that the helicity of the proteins increases after penetration, which implies that some of the unordered structure in the native protein is converted to the ordered form during this process. Hydrophobic forces seem to be responsible for stabilizing a particular protein conformation inside the membrane.

Intracellular dynamics of ricin followed by fluorescence microscopy on living cells reveals a rapid accumulation of the dimeric toxin in the Golgi apparatus

The intracellular dynamics of fluorescent conjugates of the toxic lectin ricin was followed by video fluorescence microscopy on living CHO cells, demonstrating that the ricin heterodimer and its isolated B chain, after binding to the plasma membrane receptors, migrate to and accumulate in the Golgi apparatus following internalization. A ricin derivative labelled with fluorescein on the A chain and rhodamine on the B chain did not display significant splitting of the A-B heterodimer during translocation of the toxin to the Golgi; this novel finding provides support for the hypothesis that further processing of ricin takes place in this cellular compartment.

The variants of the protein toxins abrin and ricin. A useful guide to understanding the processing events in the toxin transport

Kinetic data on inhibition of protein synthesis in thymocyte by three abrins and ricin have been obtained. The intrinsic efficiencies of A chains of four toxins to inactivate ribosomes, as analyzed by ki-versus-concentration plots were abrin II, III > ricin > abrin I. The lag times were 90, 66, 75 and 105 min at a 0.0744 nM concentration of each of abrin I, II, III and ricin, respectively. To account for the observed differences in the dose-dependent lag time, functional and structural variables of toxins such as binding efficiency of B chains to receptors and low-pH-induced structural alterations have been analyzed. The association constants obtained by stopped flow studies showed that abrin-I (4.13 x 10(5) M-1 s-1) association with putative receptor (4-methylumbelliferyl-alpha-D-galactoside) is nearly two times more often than abrin III (2.6 x 10(5) M-1 s-1) at 20 degrees C. Equilibrium binding constants of abrin I and II to thymocyte at 37 degrees C were 2.26 x 10(7) M-1 and 2.8 x 107 M-1 respectively. pH-induced structural alterations as studied by a parallel enhancement in 8-anilino-L-naphthalene sulfonate fluorescence revealed a high degree of qualitative similarity. These results taken with a nearly identical concentration-independent lag time (minimum lag of 41-42 min) indicated that the binding efficiencies and internalization efficiencies of these toxins are the same and that the observed difference in the dose-dependent lag time is causally related to the proposed processing event. The rates of reduction of inter-subunit disulfide bond, an obligatory step in the intoxication process, have been measured and compared under a variety of conditions. Intersubunit disulfide reduction of abrin I is fourfold faster than that of abrin II at pH 7.2. The rate of disulfide reduction in abrin I could be decreased 11-fold by adding lactose, compared to that without lactose. The observed differences in the efficiencies of A chains, the dose-dependent lag period, the modulating effect of lactose on the rates of disulfide reduction and similarity in binding properties make the variants a valuable tool to probe the processing events in toxin transport in detail.

Endocytosis of ricin by rat liver cells in vivo and in vitro is mainly mediated by mannose receptors on sinusoidal endothelial cells

Upon intravenous injection into rats, the plant toxin ricin was rapidly cleared from the circulation by the liver. Among the different liver cell populations, most of the injected ricin associated with the sinusoidal endothelial cells (EC), whereas the liver parenchymal cells (PC) and Kupffer cells (KC) yielded minor contributions to the total liver uptake in vivo. Co-injection of mannan strongly inhibited ricin uptake by the EC, showing that it was mediated by mannose receptors. On the other hand, co-injection of lactose, which inhibits the galactose-specific association of ricin with cells, enhanced ricin uptake by the EC. The carbohydrate-dependency of the EC contribution to the uptake of ricin in vivo was reflected in the carbohydrate-dependency of the uptake in vivo by whole liver. In vitro, the EC also endocytosed ricin more efficiently than did the PC or KC. Whereas uptake in vitro in the EC was mainly mannose-specific, uptake in the two other cell types was mainly galactose-specific. Western blotting showed that the mannose receptors of liver non-parenchymal cells are identical with the mannose receptor previously isolated from alveolar macrophages. The mannose receptors are expressed at a higher level in EC than in KC. Ligand blotting showed that, in the presence of lactose, the mannose receptor is the only protein in the EC that binds ricin, and the binding is mannose-specific and Ca(2+)-dependent.