What is the cell surface receptor(s) for the different serotypes of botulinum neurotoxin?

Electrospray mass spectrometry of NeuAc oligomers associated with the C fragment of the tetanus toxin

The Clostridial neurotoxins, botulinum and tetanus, gain entry into motor neurons by binding to the sialic or N-acetylneuraminic acid (NeuAc) residues of gangliosides and specific protein receptors attached to the cell's surface. While the C-fragment of tetanus toxin (TetC) has been identified to be the ganglioside binding domain, remarkably little is known about how this domain discriminates between the structural features of different gangliosides. We have used electrospray ionization mass spectrometry (ESI-MS) to examine the formation of complexes between TetC and carbohydrates containing NeuAc groups to determine how NeuAc residues contribute to ganglioside binding. ESI-MS was used to obtain an estimate of the dissociation constants (KD values) for TetC binding to a number of related NeuAc-containing carbohydrates (sialyllactose and disialyllactose), as well as six (NeuAc)n oligomers (n = 1-6). KD values were found to range between approximately 10-35 microM. The strength of the interactions between the C fragment and (NeuAc)n are consistent with the topography of the targeting domain of tetanus toxin and the nature of its carbohydrate binding sites. These results suggest that the targeting domain of tetanus toxin contains two binding sites that can accommodate NeuAc (or a dimer) and that NeuAc may play an important role in ganglioside binding and molecular recognition, a process critical for normal cell function and one frequently exploited by toxins, bacteria, and viruses to facilitate their entrance into cells.

Genetic and immunological comparison of anti-botulinum type A antibodies from immune and non-immune human phage libraries

Understanding the antibody response in botulinum intoxication is important for vaccine design and passive prophylaxis. To investigate this activity, we have studied the immune response to BoNT/A (botulinum neurotoxin serotype A) binding domain (HC) at the molecular level using phage display. The scFv antibodies were isolated from V-gene repertoires prepared from (a) human volunteer immunized with pentavalent botulinum toxoid and (b) non-immune human peripheral blood lymphocytes and spleenocytes. A large panel of serotype specific phage expressing botulinum binding scFv could be selected from both libraries. Epitope mapping of immune scFv binders towards BoNT/A HC revealed surprisingly a limited number of scFv recognizing conformational epitopes that corresponded to two distinct groups, clusters I and II. Only scFv from cluster I exhibited neutralizing activity in the mouse hemidiaphragm assay. Anti- BoNT/A HC clones derived from a non-immune library could be conveniently grouped into clusters III-XI and appeared to share no overlapping epitopes with cluster I or II. In addition they showed no neutralization of toxin at biologically significant concentrations. We therefore suggest that a vaccine based on the pentavalent botulinum toxoid directs the humoral immune response to a limited number of immunodominant epitopes exposed on the binding domain HC.

Role of the disulfide cleavage induced molten globule state of type a botulinum neurotoxin in its endopeptidase activity

Botulinum neurotoxins are produced by anaerobic Clostridium botulinum in an inactive form. The endopeptidase activity of type A botulinum neurotoxin (BoNT/A) is triggered by reduction of its disulfide bond between its heavy chain and light chain. By using circular dichroism spectroscopy, we show that, upon reduction of BoNT/A and under physiological temperature (37 degrees C), the BoNT/A loses most of its native tertiary structure, while retaining most of its secondary structure. This type of structure is characterized as a molten globule type conformation, which was further confirmed for BoNT/A by the characteristic binding of 1-anilinonaphthalene-8-sulfonic acid. Under nonreducing conditions where the interchain disulfide bond is intact, the enzymatically inactive BoNT/A did not show a molten globule type of structure. A temperature profile of the structure and enzyme activity of BoNT/A revealed that, under reducing conditions, there was a strong correlation in the existence of the molten globule structure and optimum endopeptidase activity at about 37 degrees C.

Spectroscopic analysis of pH-induced changes in the molecular features of type A botulinum neurotoxin light chain

Clostridial botulinum neurotoxins (BoNTs) cause neuroparalysis by blocking neurotransmitter release at the neuromuscular junctions. While the toxin's heavy chain (HC) is involved in binding and internalization, the light chain (LC) acts as a unique Zn(2+)-endopeptidase against a target protein in the exocytotic docking/fusion machinery. During the translocation of the LC to the cytosol, it is exposed to the endosomal low pH. Low pH showed a dramatic change in the BoNT/A LC polypeptide folding as indicated by differential heat denaturation. Furthermore, binding of 1-anilinonaphthalenesulfonate (ANS) revealed exposure of hydrophobic domains of BoNT/A LC at low pH. Low-pH-induced structural (and by implication the endopeptidase activity) changes were completely reversible. Exposure of BoNT/A LC to low pH (4.7) did not, however, evoke the loss of Zn(2+) bound to its active site. Implications of these observations to the delivery of active BoNT/A LC to the nerve cell are discussed. We further analyzed the nature of low-pH-induced change in the polypeptide folding of BoNT/A LC by Trp fluorescence measurements. The Trp fluorescence peak was observed at 322 nm, and the two fluorescence lifetime components estimated at 2.1 ns (88%) and 0.6 ns (12%) did not change much at low pH. These observations suggested that the two Trp residues are buried and constrained in a hydrophobic environment, and it is likely that the core of the BoNT/A LC protein matrix does not participate in the low-pH-induced structural alteration. This conclusion was further supported by the near-UV circular dichroism spectra under two pH conditions.

The immunopathogenesis of Miller Fisher syndrome

Over the past decade, remarkable progress has been made in our understanding of the pathogenesis of Miller Fisher syndrome (MFS), a clinical variant of Guillain Barré syndrome (GBS). MFS comprises the clinical triad of ataxia, areflexia and ophthalmoplegia. It is associated with acute-phase IgG antibodies to GQ1b and GT1a gangliosides in over 90% of cases which are highly disease specific. Like GBS, MFS is a post-infectious syndrome following diverse infections, but particular attention has been paid to its association with Campylobacter jejuni enteritis. Serostrains of C. jejuni isolated from infected patients bear ganglioside-like epitopes in their lipopolysaccharide core oligosaccharides, which elicit humoral immune responses exhibiting molecular mimicry with GQ1b/GT1a gangliosides. These antibodies are believed to be the principal cause of the syndrome and physiological studies aimed at proving this have focused on the motor-nerve terminal as a potential site of pathogenic action. This review describes these findings and formulates a pathogenesis model based on our current state of knowledge.