Evidence for direct binding of Clostridium botulinum type E derivative toxin and its fragments to gangliosides and free fatty acids

Variations in the Botulinum Neurotoxin Binding Domain and the Potential for Novel Therapeutics

Botulinum neurotoxins (BoNTs) are categorised into immunologically distinct serotypes BoNT/A to /G). Each serotype can also be further divided into subtypes based on differences in amino acid sequence. BoNTs are ~150 kDa proteins comprised of three major functional domains: an N-terminal zinc metalloprotease light chain (LC), a translocation domain (H_N), and a binding domain (H_C). The H_C is responsible for targeting the BoNT to the neuronal cell membrane, and each serotype has evolved to bind via different mechanisms to different target receptors. Most structural characterisations to date have focussed on the first identified subtype within each serotype (e.g., BoNT/A1). Subtype differences within BoNT serotypes can affect intoxication, displaying different botulism symptoms in vivo, and less emphasis has been placed on investigating these variants. This review outlines the receptors for each BoNT serotype and describes the basis for the highly specific targeting of neuronal cell membranes. Understanding receptor binding is of vital importance, not only for the generation of novel therapeutics but also for understanding how best to protect from intoxication.

Two Feet on the Membrane: Uptake of Clostridial Neurotoxins

The extraordinary potency of botulinum neurotoxins (BoNT) and tetanus neurotoxin (TeNT) is mediated by their high neurospecificity, targeting peripheral cholinergic motoneurons leading to flaccid and spastic paralysis, respectively, and successive respiratory failure. Complex polysialo gangliosides accumulate BoNT and TeNT on the plasma membrane. The ganglioside binding in BoNT/A, B, E, F, G, and TeNT occurs via a conserved ganglioside-binding pocket within the most carboxyl-terminal 25 kDa domain H_CC, whereas BoNT/C, DC, and D display here two different ganglioside binding sites. This enrichment step facilitates subsequent binding of BoNT/A, B, DC, D, E, F, and G to the intraluminal domains of the synaptic vesicle glycoprotein 2 (SV2) isoforms A-C and synaptotagmin-I/-II, respectively. Whereas an induced α-helical 20-mer Syt peptide binds via side chain interactions to the tip of the H_CC domain of BoNT/B, DC and G, the preexisting, quadrilateral β-sheet helix of SV2C-LD4 binds the clinically most relevant serotype BoNT/A mainly through backbone-backbone interactions at the interface of H_CC and H_CN. In addition, the conserved, complex N559-glycan branch of SV2C establishes extensive interactions with BoNT/A resulting in delayed dissociation providing BoNT/A more time for endocytosis into synaptic vesicles. An analogous interaction occurs between SV2A/B and BoNT/E. Altogether, the nature of BoNT-SV2 recognition clearly differs from BoNT-Syt. Subsequently, the synaptic vesicle is recycled and the bound neurotoxin is endocytosed. Acidification of the vesicle lumen triggers membrane insertion of the translocation domain, pore formation, and finally translocation of the enzymatically active light chain into the neuronal cytosol to halt release of neurotransmitters.

Pharyngeal-cervical-brachial variant of Guillain-Barre syndrome

The pharyngeal-cervical-brachial (PCB) variant of Guillain-Barré syndrome is defined by rapidly progressive oropharyngeal and cervicobrachial weakness associated with areflexia in the upper limbs. Serial nerve conduction studies suggest that PCB represents a localised subtype of Guillain-Barré syndrome characterised by axonal rather than demyelinating neuropathy. Many neurologists are unfamiliar with PCB, which is often misdiagnosed as brainstem stroke, myasthenia gravis or botulism. The presence of additional ophthalmoplegia and ataxia indicates overlap with Fisher syndrome. Half of patients with PCB carry IgG anti-GT1a antibodies which often cross-react with GQ1b, whereas most patients with Fisher syndrome carry IgG anti-GQ1b antibodies which always cross-react with GT1a. Significant overlap between the clinical and serological profiles of these patients supports the view that PCB and Fisher syndrome form a continuous spectrum. In this review, we highlight the clinical features of PCB and outline new diagnostic criteria.

Botulinum neurotoxins B and E translocate at different rates and exhibit divergent responses to GT1b and low pH

Botulinum neurotoxins (BoNTs, serotypes A–G) are the most deadly substances known. Here, we investigated how BoNT/E, a serotype that causes human botulism, translocates into the cytosol of neurons. Analogous to BoNT/B, BoNT/E required binding of the coreceptor, GT1b, to undergo significant secondary structural changes and transform into a hydrophobic protein at low pH. These data indicate that both serotypes act as coincidence detectors for both GT1b and low pH, to undergo translocation. However, BoNT/E translocated much more rapidly than BoNT/B. Also, BoNT/E required only GT1b, and not low pH, to oligomerize, whereas BoNT/B required both. In further contrast to the case of BoNT/B, low pH alone altered the secondary structure of BoNT/E to some degree and resulted in its premature inactivation. Hence, comparison of two BoNT serotypes revealed that these agents exhibit both convergent and divergent responses to receptor interactions, and pH, in the translocation pathway.

Receptor mimicry as novel therapeutic treatment for biothreat agents

The specter of intentional release of pathogenic microbes and their toxins is a real threat. This article reviews the literature on adhesins of biothreat agents, their interactions with oligosaccharides and the potential for anti-adhesion compounds as an alternative to conventional therapeutics. The minimal binding structure of ricin has been well characterised and offers the best candidate for successful anti-adhesion therapy based on the Galβ1-4GlcNAc structure. The botulinum toxin serotypes A-F bind to a low number of gangliosides (GT1b, GQ1b, GD1a and GD1b) hence it should be possible to determine the minimal structure for binding. The minimal disaccharide sequence of GalNAcβ1-4Gal found in the gangliosides asialo-GM1 and asialo-GM2 is required for adhesion for many respiratory pathogens. Although a number of adhesins have been identified in bacterial biothreat agents such as Yersinia pestis, Bacillus anthracis, Francisella tularensis, Brucella species and Burkholderia pseudomallei, specific information regarding their in vivo expression during pneumonic infection is lacking. Limited oligosaccharide inhibition studies indicate the potential of GalNAcβ1-4Gal, GalNAcβ-3Gal and the hydrophobic compound, para-nitrophenol as starting points for the rational design of generic anti-adhesion compounds. A cocktail of multivalent oligosaccharides based on the minimal binding structures of identified adhesins would offer the best candidates for anti-adhesion therapy.

Botulinum neurotoxin D uses synaptic vesicle protein SV2 and gangliosides as receptors

Botulinum neurotoxins (BoNTs) include seven bacterial toxins (BoNT/A-G) that target presynaptic terminals and act as proteases cleaving proteins required for synaptic vesicle exocytosis. Here we identified synaptic vesicle protein SV2 as the protein receptor for BoNT/D. BoNT/D enters cultured hippocampal neurons via synaptic vesicle recycling and can bind SV2 in brain detergent extracts. BoNT/D failed to bind and enter neurons lacking SV2, which can be rescued by expressing one of the three SV2 isoforms (SV2A/B/C). Localization of SV2 on plasma membranes mediated BoNT/D binding in both neurons and HEK293 cells. Furthermore, chimeric receptors containing the binding sites for BoNT/A and E, two other BoNTs that use SV2 as receptors, failed to mediate the entry of BoNT/D suggesting that BoNT/D binds SV2 via a mechanism distinct from BoNT/A and E. Finally, we demonstrated that gangliosides are essential for the binding and entry of BoNT/D into neurons and for its toxicity in vivo, supporting a double-receptor model for this toxin.

BoNTs are a family of seven bacterial toxins (BoNT/A-G). Among the seven BoNTs, whether BoNT/D uses the same entry pathways and similar receptor-binding strategies as other BoNTs is not known. Previous studies have suggested that BoNT/D does not need a protein receptor nor ganglioside co-receptor, in contrast to all other BoNTs. Here we demonstrate that BoNT/D uses synaptic vesicle protein SV2 as its protein receptor and gangliosides as co-receptor, thus supporting the “double-receptor” model as a central theme for this class of toxins. Furthermore, we found that BoNT/D utilizes a SV2 binding mechanism distinct from BoNT/A and BoNT/E, two other BoNTs that use SV2 as receptors. This indicates that different BoNTs can develop their distinct mechanisms to target a common receptor protein.

Domain organization in Clostridium botulinum neurotoxin type E is unique: its implication in faster translocation

Clostridium botulinum produces seven antigenically distinct neurotoxins [C. botulinum neurotoxins (BoNTs) A-G] sharing a significant sequence homology. Based on sequence and functional similarity, it was believed that their three-dimensional structures will also be similar. Indeed, the crystal structures of BoNTs A and B exhibit similar fold and domain association where the translocation domain is flanked on either side by binding and catalytic domains. Here, we report the crystal structure of BoNT E holotoxin and show that the domain association is different and unique, although the individual domains are similar to those of BoNTs A and B. In BoNT E, both the binding domain and the catalytic domain are on the same side of the translocation domain, and all three have mutual interfaces. This unique association may have an effect on the rate of translocation, with the molecule strategically positioned in the vesicle for quick entry into cytosol. Botulism, the disease caused by BoNT E, sets in faster than any other serotype because of its speedy internalization and translocation, and the present structure offers a credible explanation. We propose that the translocation domain in other BoNTs follows a two-step process to attain translocation-competent conformation as in BoNT E. We also suggest that this translocation-competent conformation in BoNT E is a probable reason for its faster toxic rate compared to BoNT A. However, this needs further experimental elucidation.

Glycosylated SV2A and SV2B mediate the entry of botulinum neurotoxin E into neurons

Botulinum neurotoxin E (BoNT/E) can cause paralysis in humans and animals by blocking neurotransmitter release from presynaptic nerve terminals. How this toxin targets and enters neurons is not known. Here we identified two isoforms of the synaptic vesicle protein SV2, SV2A and SV2B, as the protein receptors for BoNT/E. BoNT/E failed to enter neurons cultured from SV2A/B knockout mice; entry was restored by expressing SV2A or SV2B, but not SV2C. Mice lacking SV2B displayed reduced sensitivity to BoNT/E. The fourth luminal domain of SV2A or SV2B alone, expressed in chimeric receptors by replacing the extracellular domain of the low-density lipoprotein receptor, can restore the binding and entry of BoNT/E into neurons lacking SV2A/B. Furthermore, we found disruption of a N-glycosylation site (N573Q) within the fourth luminal domain of SV2A rendered the mutant unable to mediate the entry of BoNT/E and also reduced the entry of BoNT/A. Finally, we demonstrate that BoNT/E failed to bind and enter ganglioside-deficient neurons; entry was rescued by loading exogenous gangliosides into neuronal membranes. Together, the data reported here demonstrate that glycosylated SV2A and SV2B act in conjunction with gangliosides to mediate the entry of BoNT/E into neurons.

Identification of the receptor-binding sites in the carboxyl-terminal half of the heavy chain of botulinum neurotoxin types C and D

Botulinum neurotoxin (BoNT) binds to presynaptic neuronal cells and blocks neurotransmitter release. The carboxyl-terminal half of the heavy chain (H(C)) of the neurotoxin recognizes its specific receptor on the plasma membrane. We have previously demonstrated that BoNT/C binds to gangliosides GD1b and GT1b under physiological conditions, while BoNT/D interacts with phosphatidylethanolamine (PE). Here we report that the recognition sites for gangliosides and PE are present in the carboxyl-terminal domain of H(C). Chimeric mutants and site-directed mutants of BoNT/C-H(C) and BoNT/D-H(C) were generated and their binding activities evaluated. The chimeric H(C) that consisted of the amino-terminal half of BoNT/D-H(C) and the carboxyl-terminal half of BoNT/C-H(C) possessed activity similar to the authentic BoNT/C-H(C), suggesting that the carboxyl-terminal region of H(C) is involved in the receptor recognition of BoNT/C. Moreover, analysis using site-directed mutants indicated that the peptide motif W(1257)Ycdots, three dots, centeredG(1270)cdots, three dots, centeredH(1282) plays an important role in the interaction between BoNT/C and gangliosides. In contrast, we revealed that two lysine residues of BoNT/D-H(C) are involved in the formation of the critical binding site for receptor binding.

Binding of Clostridium botulinum type C and D neurotoxins to ganglioside and phospholipid. Novel insights into the receptor for clostridial neurotoxins

Clostridium botulinum neurotoxins (BoNTs) act on nerve endings to block acetylcholine release. Their potency is due to their enzymatic activity and selective high affinity binding to neurons. Although there are many pieces of data available on the receptor for BoNT, little attempt has been made to characterize the receptors for BoNT/C and BoNT/D. For this purpose, we prepared the recombinant carboxyl-terminal domain of the heavy chain (H(C)) and then examined its binding capability to rat brain synaptosomes treated with enzymes and heating. Synaptosomes treated with proteinase K or heating retained binding capability to both H(C)/C and H(C)/D, suggesting that a proteinaceous substance does not constitute the receptor component. We next performed a thin layer chromatography overlay assay of H(C) with a lipid extract of synaptosomes. Under physiological or higher ionic strengths, H(C)/C bound to gangliosides GD1b and GT1b. These data are in accord with results showing that neuraminidase and endoglycoceramidase treatment decreased H(C)/C binding to synaptosomes. On the other hand, H(C)/D interacted with phosphatidylethanolamine but not with any ganglioside. Using cerebellar granule cells obtained from GM3 synthase knock-out mice, we found that BoNT/C did not elicit a toxic effect but that BoNT/D still inhibited glutamate release to the same extent as in granule cells from wild type mice. These observations suggested that BoNT/C recognized GD1b and GT1b as functional receptors, whereas BoNT/D induced toxicity in a ganglioside-independent manner, possibly through binding to phosphatidylethanolamine. Our results provide novel insights into the receptor for clostridial neurotoxin.

Glycosphingolipids-sweets for botulinum neurotoxin

A number of viruses, bacteria, and bacterial toxins can only act on cells that express the appropriate glycosphingolipids (GSLs) on the outer surface of their plasma membranes. An example of this dependency is provided by botulinum neurotoxin (BoNT) which is synthesized by Clostridium botulinum and inhibits neurotransmission at the neuromuscular junction by catalyzing hydrolysis of a SNARE protein, thereby inducing a flaccid paralysis. Haemagglutinin components of progenitor forms of BoNT mediate its adherence to glycosphingolipids (GSLs) on intestinal epithelial cells while the cellular activity of most isolated serotypes requires the presence of certain gangliosides, especially those of the Gg1b family. This review discusses available information about the identity and the roles of GSLs in the activity of BoNT. Observations that serotypes A-F of BoNT require gangliosides for optimum activity (serotype G apparently does not), permits the hypothesis that it should be possible to develop an antagonist of this interaction thereby inhibiting/reducing its effect.

Anti-GT1a IgG in Guillain-Barré syndrome

OBJECTIVE

To investigate the presence of serum anti-GT1a IgG in Guillain-Barré syndrome (GBS) and its relation to clinical manifestations.

BACKGROUND

Several patients with GBS and bulbar palsy have been reported to have serum anti-GT1a IgG. Most, however, also have anti-GQ1b IgG. A previous study failed to detect GT1a in human cranial nerves, but GQ1b was abundant in human ocular motor nerves. Whether anti-GT1a IgG itself determines the clinical manifestations is not yet clear.

METHODS

The association of clinical manifestations with the presence of anti-GT1a IgG and with its cross reactivity was investigated. An immunochemical study was performed to determine whether GT1a is present in human cranial nerves.

RESULTS

Anti-GT1a and anti-GQ1b IgG were positive in 10% and 9% respectively of 220 consecutive patients with GBS. Patients with anti-GT1a IgG often had cranial nerve palsy (ophthalmoparesis, 57%; facial palsy, 57%; bulbar palsy, 70%), and 39% needed artificial ventilation. These features were also seen in patients with anti-GQ1b IgG. There was no significant difference between the two groups with respect to the frequency of clinical findings. An enzyme-linked immunosorbent assay showed that anti-GT1a IgG cross reacted with GQ1b in 75% of the patients, GD1a in 30%, GM1 in 20%, and GD1b in 20%. All five patients who carried anti-GT1a IgG that did not cross react with GQ1b had bulbar palsy, neck weakness, absence of sensory disturbance, and positive Campylobacter jejuni serology. Thin-layer chromatography with immunostaining showed that GT1a is present in human oculomotor and lower cranial nerves.

CONCLUSIONS

These findings provide further evidence that anti-GT1a IgG itself can determine clinical manifestations. The distinctive clinical features of patients with anti-GT1a IgG without anti-GQ1b activity distinguish a specific subgroup within GBS.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

Is IgG anti-GT1a antibody associated with pharyngeal-cervical-brachial weakness or oropharyngeal palsy in Guillain-Barré syndrome?

The pharyngeal-cervical-brachial variant (PCB) of Guillain-Barré syndrome (GBS) has clinical features similar to those of botulism and diphtheria. Mizoguchi et al. (1994) [Mizoguchi, K., Hase, A., Obi, T., Matsuoka, H., Takatsu, M., Nishimura, Y., Irie, F., Seyama, Y., Hirabayashi, Y., 1994. Two species of antiganglioside antibodies in a patient with a pharyngeal-cervical-brachial variant of Guillain-Barré syndrome. J. Neurol. Neurosurg. Psychiatry 57, 1121-1123] reported a patient with PCB-like symptoms who had serum IgG anti-GT1a antibodies which did not cross-react with GQ1b. We assumed that PCB is associated with anti-GT1a antibodies that do not have reactivity to GQ1b and made a serological study of a PCB patient. We searched for PCB patients prospectively and found one with PCB. This patient had IgG anti-GT1a antibodies which were not absorbed with GQ1b in an absorption study, whereas IgG anti-GT1a antibodies from Fisher's syndrome patients were. The frequency of positive IgG anti-GT1a antibody did not differ in patients with and without bulbar palsy. Our findings indicate that IgG anti-GT1a antibodies which do not cross-react with GQ1b are specifically detectable in PCB and can be used as a diagnostic marker of PCB.

Structure and function of tetanus and botulinum neurotoxins

Tetanus and botulinum neurotoxins are produced by Clostridia and cause the neuroparalytic syndromes of tetanus and botulism. Tetanus neurotoxin acts mainly at the CNS synapse, while the seven botulinum neurotoxins act peripherally. Clostridial neurotoxins share a similar mechanism of cell intoxication: they block the release of neurotransmitters. They are composed of two disulfide-linked polypeptide chains. The larger subunit is responsible for neurospecific binding and cell penetration. Reduction releases the smaller chain in the neuronal cytosol, where it displays its zinc-endopeptidase activity specific for protein components of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxins B, D, F and G recognize specifically VAMP/ synaptobrevin. This integral protein of the synaptic vesicle membrane is cleaved at single peptide bonds, which differ for each neurotoxin. Botulinum A, and E neurotoxins recognize and cleave specifically SNAP-25, a protein of the presynaptic membrane, at two different sites within the carboxyl-terminus. Botulinum neurotoxin type C cleaves syntaxin, another protein of the nerve plasmalemma. These results indicate that VAMP, SNAP-25 and syntaxin play a central role in neuroexocytosis. These three proteins are conserved from yeast to humans and are essential in a variety of docking and fusion events in every cell. Tetanus and botulinum neurotoxins form a new group of zinc-endopeptidases with characteristic sequence, mode of zinc coordination, mechanism of activation and target recognition. They will be of great value in the unravelling of the mechanisms of exocytosis and endocytosis, as they are in the clinical treatment of dystonias.

Involvement of phospholipids in the intoxication mechanism of botulinum neurotoxin

Phospholipids were examined for their potential to interact with botulinum neurotoxin by an in vivo toxin-inactivation assay and a direct binding assay on a thin layer plate. Type E neurotoxin was inactivated by negatively charged phospholipids, phosphatidylserine (PS) and phosphatidylinositol (PI). The toxicity of the neurotoxin was not affected by phosphatidylcholine (PC) without an electric charge or phosphatidylethanolamine (PE) with a positive electric charge. The neurotoxin bound directly to PS and PI but not to PC or PE. These results suggest that the negatively charged phospholipids in the cell membranes are involved in the intoxication mechanism of botulinum neurotoxin. The phospholipids PS and PI were tested for their potential to interact within three domains [L, H-1, and H-2] which compose the neurotoxin. All three domains bound to PS; whereas, PI specifically accepted the binding of the H-1 domain relative to the penetration of the neurotoxin into the lipid membrane. In this paper, we discuss the interaction between the neurotoxin and the lipid membrane in the intoxication mechanism.

Gangliosides and bacterial toxins in Guillain-Barré syndrome

Autoimmune factors are strongly favoured as mediating Guillain-Barré syndrome (GBS); however, the precise mechanisms by which this occurs remain unknown. Microbial infections in a susceptible host resulting in an idiosyncratic immune response which cross-reacts with nerve constituents still remains the most plausible working hypothesis on which much current research is based. Considerable recent evidence indicates that this humoral immune response is at least in part directed to gangliosides. Interestingly, many bacterial toxins, including botulinum and tetanus neurotoxins, also bind to gangliosides and induce diseases with some similarities to GBS. This article discusses the evidence in favour of a pathogenic role for anti-ganglioside antibodies in GBS in the context of our knowledge of the biology of gangliosides and the factors that determine their immunogenicity.

Purification and characterization of the ganglioside-binding fragment of Clostridium botulinum type E neurotoxin

A way of fragmentation of Clostridium botulinum neurotoxin was carried out to elucidate the structure-function relationship of neurotoxin. The hitherto only plausible fragment was isolated from the trypsin-treated heavy chain of botulinum type E neurotoxin. In the presence of 4 M urea, one protein peak emerged from QAE-Sephadex column loaded with the heavy chain mildly treated with trypsin by elution with 0.1 M sodium chloride. Although many protein bands were detected in SDS-PAGE of the treated heavy chain, the eluted protein migrated in a single band to the position of 41,000 Da. The recovery of the 41,000-Da fragment was 28.6%, but with a 2 M urea-containing buffer as eluant, the recovery was less than 12%. The 41,000-Da fragment bound to gangliosides GD1a, GT1b, and GQ1b, to which neurotoxin and the heavy chain bound. The 41,000-Da fragment partially interfered with the binding of 125I-labeled neurotoxin to mouse brain synaptosomes. We have proposed a three-fragment structure (L.H-1.H-2) for botulinum type E neurotoxin. The characters of the 41,000-Da fragment described in this paper seem to substantiated our proposal that type E neurotoxin consists of three fragments, L.H-1.H-2, and that the ganglioside-binding fragment is H-2.

Binding of pulmonary surfactant protein A to galactosylceramide and asialo-GM2

The binding of pulmonary surfactant protein A (SP-A) to glycolipids was examined in the present study. The direct binding of SP-A on a thin-layer chromatogram was visualized using 125I-SP-A as a probe. 125I-SP-A bound to galactosylceramide and asialo-GM2, but failed to exhibit significant binding to GM1, GM2, asialo-GM1, sulfatide, and Forssman antigen. The study of 125I-SP-A binding to glycolipids coated onto microtiter wells also revealed that SP-A bound to galactosylceramide and asialo-GM2. SP-A bound to galactosylceramides with non-hydroxy or hydroxy fatty acids, but showed no binding to either glucosylceramide or galactosylsphingosine. Excess native SP-A competed with 125I-SP-A for the binding to asialo-GM2 and galactosylceramide. Specific antibody to rat SP-A inhibited 125I-SP-A binding to glycolipids. In spite of chelation of Ca2+ with EDTA or EGTA, SP-A retained a significant binding to glycolipids. Inclusion of excess monosaccharides in the binding buffer reduced the glycolipid binding of SP-A, but failed to achieve complete abolishment. The oligosaccharide isolated from asialo-GM2 is also effective at reducing 125I-SP-A binding to the solid-phase asialo-GM2. From these data, we conclude that SP-A binds to galactosylceramide and asialo-GM2, and that both saccharide and ceramide moieties in the glycolipid molecule are important for the binding of SP-A to glycolipids.

Binding of botulinum and tetanus neurotoxins to ganglioside GT1b and derivatives thereof

The ability of fragments derived from botulinum neurotoxin (BTx) serotype A to bind to GT1b-coated plastic wells was investigated and compared with the binding characteristics of the parent approximately 150-kDa protein. Although the approximately 50-kDa light chain of BTxA had a marginal binding capacity, the predominant adherence to GT1b-coated wells was exhibited by the approximately 50-kDa carboxy-terminal half of the approximately 100-kDa heavy chain of BTxA; the amino-terminal half of the heavy chain lacked the ability to bind. Binding to GT1b by BTxA and its fragments was compared with that of tetanus neurotoxin (TTx) and the carboxy-terminal half of its heavy chain. Binding of BTxA and the C-terminal half of the heavy chain was optimal in buffers of low ionic strength (mu less than or equal to 0.04 and 0.06, respectively), whereas the heavy chain bound GT1b best at mu greater than or equal to 0.10. TTx and the approximately 50-kDa C-terminal half of its approximately 100-kDa heavy chain bound GT1b at ionic strengths similar to those of BTxA. Comparison of the binding of BTx serotypes A, B, and E to GT1b (using conditions that were found to be optimal for binding by BTxA) indicated differences in the interaction of the three serotypes with GT1b. Compared with BTxA, adherence to GT1b by serotypes B and E was reduced by approximately 60 and approximately 90%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristic binding of human plasma apolipoprotein B to gangliotetraosylceramide and gangliotriaosylceramide

The binding of human plasma low-density lipoproteins (LDL), freshly prepared by discontinuous ultracentrifugation, to several neutral and acidic glycosphingolipids was examined by TLC immunostaining with the anti [apolipoprotein B (apoB)] antibody. ApoB was found to bind characteristically to the asialogangliosides, gangliotetraosylceramide (Gg4Cer) and gangliotriaosylceramide (Gg3Cer), the former being a more potent receptor than the latter, indicating that the sequences Gal beta 1-3GalNAc beta 1-4Gal and GalNAc beta 1-4Gal are involved in the binding of apoB. A weak positive reaction with fucosylgangliotetraosylceramide (IV2Fuc-Gg4Cer), which has the same internal recognition sequences, was also observed (the binding ability was only 1/7 of that in the case of Gg4Cer). No binding to other neutral glycosphingolipids, or glycosphingolipid sulfates (I3-SO3-GalCer) and gangliosides, was detected, and therefore substitution of the receptor glycolipid with sialic acid was thought to inhibit the binding. The results indicate that, along with the binding of apoB to the LDL-binding domain of the receptor glycoprotein, interaction with some carbohydrate chains in the receptor, or with glycolipids coexisting on the plasma membrane, may be important for the binding of apoB to cells.

Effect of pH on the interaction of botulinum neurotoxins A, B and E with liposomes

The interaction of botulinum neurotoxin serotypes A, B and E with membranes of different lipid compositions was examined by photolabelling with two photoreactive phosphatidylcholine analogues that monitor the polar region and the hydrophobic core of the lipid bilayer. At neutral pH the neurotoxins interacted both with the polar head groups and with fatty acid chains of phospholipids. At acidic pHs the neurotoxins underwent structural changes characterized by a more extensive interaction with lipids. Both the heavy and light chain subunits of the neurotoxins were involved in the process. The change in the nature and extent of toxin-lipid interaction occurred in the pH range 4-6 and was not influenced by the presence of polysialogangliosides. The present data are in agreement with the idea that botulinum neurotoxins enter into nerve cells from a low pH intracellular compartment.

Interaction of botulinum and tetanus toxins with the lipid bilayer surface

The interaction of botulinum neurotoxins serotypes A, B and E (from Clostridium botulinum) and of tetanus neurotoxin (from Clostridium tetani) with the surface of liposomes made of different lipid compositions was studied by photolabelling with a radioiodinated photoactive phosphatidylethanolamine analogue [125I-dipalmitoyl (3,4-azidosalicylamido)phosphatidylethanolamine]. When the vesicles were made of negatively charged lipids (asolectin), each of these neurotoxic proteins was radioiodinated, thus providing evidence for their attachment to the membrane surface. The presence of gangliosides on liposome membranes enhanced fixation of the neurotoxic proteins to the lipid vesicle surface. Both the heavy and light chains of the clostridial neurotoxins were involved in the attachment to the lipid bilayer surface. Each of the toxins tested here attached poorly to liposomes made of zwitterionic lipids (egg phosphatidylcholine), even when polysialogangliosides were present. The data suggest that the binding of botulinum and tetanus neurotoxins to their target neuronal cells involves negatively charged lipids and polysialogangliosides on the cell membrane.

Antigenic structure of Clostridium botulinum type B neurotoxin and its interaction with gangliosides, cerebroside, and free fatty acids

A fragment distinct from the heavy and light chains was obtained by treatment of Clostridium botulinum type B neurotoxin with chymotrypsin. Enzyme-linked immunosorbent assay and immunoblotting analysis with monoclonal antibodies showed that the fragment consisted of the light chain and part of the heavy chain (H-1 fragment) linked together by a disulfide bond. Monoclonal antibodies reacting to the heavy chain but not to the fragment were thought to recognize the epitopes on the remaining portion (H-2 fragment) of the heavy chain, being easily digested by chymotrypsin. Thus, the antigenic structure of type B neurotoxin resembles those of type A and E neurotoxins. The chymotrypsin-induced fragment bound to cerebroside and free fatty acids but not to gangliosides. The manner of binding of type B neurotoxin to gangliosides and free fatty acids was different from those of type A and E neurotoxins. Such differences in the reactivities to lipids may be related to the finding that each neurotoxin binds to a type-specific site on the neural membrane.