Comparative molecular topography of botulinum neurotoxins from Clostridium butyricum and Clostridium botulinum type E

Decoding Organelle Interactions: Unveiling Molecular Mechanisms and Disease Therapies

Organelles, substructures in the cytoplasm with specific morphological structures and functions, interact with each other via membrane fusion, membrane transport, and protein interactions, collectively termed organelle interaction. Organelle interaction is a complex biological process involving the interaction and regulation of several organelles, including the interaction between mitochondria-endoplasmic reticulum, endoplasmic reticulum-Golgi, mitochondria-lysosomes, and endoplasmic reticulum-peroxisomes. This interaction enables intracellular substance transport, metabolism, and signal transmission, and is closely related to the occurrence, development, and treatment of many diseases, such as cancer, neurodegenerative diseases, and metabolic diseases. Herein, the mechanisms and regulation of organelle interactions are reviewed, which are critical for understanding basic principles of cell biology and disease development mechanisms. The findings will help to facilitate the development of novel strategies for disease prevention, diagnosis, and treatment opportunities.

Unique ganglioside binding by botulinum neurotoxins C and D-SA

The botulinum neurotoxins (BoNTs) are the most potent protein toxins for humans. There are seven serotypes of BoNTs (A-G), based on a lack of cross-antiserum neutralization. The BoNT/C and BoNT/D serotypes include mosaic toxins that are organized as D-C and C-D toxins. One BoNT D-C mosaic toxin, BoNT/D-South Africa (BoNT/D-SA), was not fully neutralized by immunization with a vaccine composed of either prototype BoNT/C-Stockholm or BoNT/D-1873. Whereas several BoNT serotypes utilize dual receptors (gangliosides and proteins) to bind to and enter neurons, the basis for BoNT/C and BoNT/D entry into neurons is less well understood. Recent studies solved the crystal structures of the receptor-binding domains of BoNT/C, BoNT/D, and BoNT/D-SA. Comparative structural analysis showed that BoNT/C, BoNT/D and BoNT/D-SA lacked components of the ganglioside-binding pocket that exists within other BoNT serotypes. With the use of structure-based alignments, biochemical analyses, and cell-binding approaches, BoNT/C and BoNT/D-SA have been shown to possess a unique ganglioside-binding domain, the ganglioside-binding loop. Defining how BoNTs enter host cells provides insights towards understanding the evolution and extending the potential therapeutic and immunological values of the BoNT serotypes.

Quaternary structure of botulinum and tetanus neurotoxins as probed by chemical cross-linking and native gel electrophoresis

Botulinum and tetanus neurotoxins are water-soluble proteins (mol. wt 150,000) produced by Clostridium botulinum and Clostridium tetani, respectively. It is believed that these neurotoxins, once internalized via receptor-mediated endocytosis, form membrane channels in order to traverse the endosomal membrane and enter the cytoplasm of the nerve terminal. Investigation of the associative properties between neurotoxin molecules could yield an understanding of this channel formation. That is, an association between neurotoxin monomers could result in an oligomeric form of the neurotoxin necessary for assembly of a channel through the hydrophobic interior of the endosomal membrane, thereby allowing passage of the neurotoxin or its active fragment through the resulting pore. Based on the native gel electrophoresis and chemical cross-linking experiments, tetanus neurotoxin exists as a dimer and a trimer, type A botulinum neurotoxin exists as a dimer, trimer, and a larger species, type E botulinum neurotoxin exists as a monomer and dimer, and type B botulinum neurotoxin appears to exist as a dimer in aqueous solution. The results imply that quaternary structures of these neurotoxins may play an important role in their mode of action during neuronal poisoning.

Clostridium butyricum neurotoxin: partial amino acid sequence of major proteolytic fragments and intracellular activity in PC12 cells

The approximately 150 kDa single-chain neurotoxin produced by Clostridium butyricum, reported to be similar to C. botulinum neurotoxin serotype E (Giménez and Sugiyama, Infect. Immun. 54, 926-929, 1988), was probed with trypsin and endoproteinase Lys-C. The two proteases cleaved the butyricum neurotoxin between residues Arg 421-Lys 422 and Lys 418-Gly 419, respectively. Cleavage at this region, highly susceptible to proteolysis, generated approximately 50 kDa light and approximately 100 kDa heavy chains, whose identities were established by amino acid sequence determination. In the approximately 150 kDa dichain neurotoxin these two chains remained linked by an interchain disulfide bond. The endoproteinase Lys-C also cleaved the heavy chain between residues Lys 594-Ile 595, yielding a approximately 73 kDa fragment. A total of 77 amino acid residues was identified by Edman degradation of the major fragments generated by proteolysis. By analogy with other botulinum neurotoxin serotypes, the light chain contains the intracellular inhibitory domain and the heavy chain the receptor-binding and channel-forming domains. Butyricum neurotoxin, whether in single-chain or dichain form, inhibited approximately 70% of the Ca2+ stimulated [3H]norepinephrine release from permeabilized PC12 cells. Reduction with 5 mM dithiothreitol enhanced the intracellular inhibitory activity of dichain neurotoxin about 30-fold and increased the extent of inhibition to about 80%, while the activity of single-chain neurotoxin was slightly affected. Increase in the intracellular inhibitory activity of butyricum neurotoxin following mild treatment with trypsin (i.e. conversion of the single-chain protein to the dichain form) was virtually complete within 6 min.

Characterization of the neurotoxin isolated from a Clostridium baratii strain implicated in infant botulism

Botulism is widely known to result from ingestion of food containing botulinum neurotoxin produced in situ by certain strains of Clostridium botulinum. Infant botulism caused by C. botulinum, unlike the food-borne intoxication, is the toxicoinfectious form of botulism (S. S. Arnon, p. 331-345, in G. E. Lewis, ed., Biomedical Aspects of Botulism, 1981). The strain of Clostridium baratii implicated in infant botulism produced a neurotoxin that was neutralized with antiserum for botulinum neurotoxin serotype F (J. D. Hall, L. M. McCroskey, B. J. Pincomb, and C. L. Hatheway, J. Clin. Microbiol. 21:654-655, 1985). We developed a procedure to culture the toxigenic C. baratii (strain 6341) in dialysis bags and a simple purification scheme (precipitation of 900-ml culture supernatant with ammonium sulfate and two anion-exchange chromatographic steps at pH 5.5 and 8.0) that yielded up to 150 micrograms of purified neurotoxin. It is an approximately 140-kDa single-chain protein and has the following sequence of amino acid residues at the N terminus: Pro-Val-Asn-Ile-Asn-Asn-Phe-Asn-Tyr-Asn-Asp-Pro-Ile-Asn-Asn-Thr-Thr-Ile- Leu. Comparison of this amino acid sequence with those of the botulinum neurotoxin serotypes A, B, and E showed 40 to 50% identical residues in comparable positions. The specific toxicity of the neurotoxin, approximately 2 x 10(6) 50% lethal doses for mice per mg of protein injected, was not enhanced significantly by mild trypsinization, although the protease cleaved the neurotoxin within a disulfide loop that generated at least two primary fragments, approximately 47 and approximately 86 kDa, that remained linked by an interchain disulfide. These two fragments resembled the light and heavy chains of the well-characterized neurotoxin serotypes A, B, C, D, E, and F produced by C. botulinum.