Recently, it was reported in Kaiser Health News (https://khn.org/news/search-for-a-snakebite-drug-might-lead-to-a-covid-treatment-too/) and the AVMA Animal Health SmartBrief that a new small molecule inhibitor of phospholipase A2, Varespladib, (https://www.nature.com/articles/s41598-019-53755-5.pdf) of snake venom for treatment of snake bite might treat COVID-19 lung disease as well because another PLA2 enzyme in human tissue is in the inflammatory cascade. The Brooks Counterproliferation Team made neutralizing aptamers against AB binary toxins: Botox, Shiga toxin, phospholipase A2 of South American rattlesnake venom, and anthrax toxins; United States Patent US 9,273,345 B2, Mar.1,2016. The snake antivenin aptamers were going to be pursued for this snake (Crotalus durissus terrificus) as well as other significantly dangerous snakes in collaboration with Administración Nacional De Laboratorios E Institutos De Salud (National Administration of Laboratories and Institutes of Health (ANLIS) ) but because of the closure of Brooks in 2011, this never happened.
Many bacterial toxins are AB toxins, that is, they contain a trans-cell membrane cell transporting component B and an affector or toxic effect generator A. Oddly, this motif is seen in the venoms of snakes. The most famous of the AB toxins is produced by the anaerobic bacterium Clostridium botulinum. It is the number one biotoxin on the Select Agent, potential biowarfare/ bioterrorism, list with an estimated human median lethal dose (LD-50) of 1.3–2.1 ng/ kg by injection or 10–13 ng/kg by inhalation. Other species of this genera also produce AB toxins which cause neurological toxicity, tetanus (tetanus toxin produced in wounds by Clostridium tetani), or necrotizing effects (iota toxin of Clostridium perfringens), gangrene. Botulinum toxin is composed of a heavy chain (equivalent to the B chain) and a light chain (equivalent to the A chain) linked together by a single disulfide bond (as many of the snake venoms and related polypeptide hormones such as insulin and nerve growth factor are linked by more than one disulfide linkage). The toxin is produced by the bacteria as an inactive pre-toxin (approximately 150k) which is activated by proteolytic cleavage into the 100kD heavy chain and the 50kD light chain. Botulinum neurotoxin exists as 7 different serotypes, A, B, C, D, E, F and G. All these various serotypes inhibit acetylcholine release from nerve endings, but by targeting different intracellular neuronal protein components of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) acetylcholine transport complex. However, their potencies vary substantially, with some being more toxic and more prevalent clinically in some host species than others. Some are more likely to be associated with human disease like Type A (Western North America) and B (Eastern North America) and others with animal disease, in waterfowl Type C and Type E sporadically among fish-eating birds, such as common loons (Gavia immer) and gulls. Even though botulinum toxicity is usually considered only a toxicity, it can be produced in infants by the consumption of honey contaminated with C. botulinum spores which grow into vegetative forms and produce toxin in vivo, in situ. Occasionally this is seen in adults. An even rarer form is wound botulism related to C. botulinum which grows and produces Type A and Type B toxins, but in at least one case, Type E, in place. Another prominent AB toxin which has seen much press is Shiga (from Shigella dysenteriae shiga toxigenic group of Escherichia coli (STEC), which includes serotypes O157:H7, O104:H4, and other enterohemorrhagic E. coli (EHEC)), which produce related Shiga-like toxin, which is composed of Shiga-like toxin 1 and 2 (SLT-1 and 2 or Stx-1 and 2), with Stx-1 differing from Stx by only 1 amino acid and Stx-2 sharing 56% of its sequences with Stx-1. Shiga toxin inhibits protein synthesis by a mechanism similar to ricin toxin produced by the castor bean, Ricinus communis. After entering a cell, the affector portion of the toxin acts as a N-glycosidase, cleaving a specific adenine from the 28S RNA of the 60S subunit of the eukaryotic ribosome, stopping protein synthesis. Besides causing the gastroenteritis of O157 infection, it also is neurotoxic. The glycosphingolipid, Gb3, is the receptor for Shiga and Shiga-like toxins. It is present in great amounts on renal epithelial cells leading to renal toxicity. Gb3-type receptors are also found in central nervous system neurons and endothelium, which may explain the toxin’s neurotoxicity. Stx-2 increases the expression of its receptor Gb3 and causes neuronal dysfunctions through this positive feedback mechanism. Our group of researchers at AFRL, Brooks City-Base, discovered that curcumin, if given before exposure to the toxin, decreases the Gb3 on target cells. We also developed small synthetic oligonucleotides which bind to the toxin and prevent its action in cell culture. Therefore, Shiga toxicity is a toxigenic disease which is infectious, but not only for the human host. Its genes are encoded on a latent, temperate lambdoid prophage in Escherichia coli. The phage regulatory network is a significant contributor to toxin production and release by this pathogenic E. coli and allows the phage to be released from lysogenic E. coli to co-opt other normally non-hemorrhagic enteric E. coli. This opens another variant from Koch’s Postulates, the conversion of resident microbes which are non-pathogenic into pathogenic ones, violating the first hypothesis by having all hosts carrying potentially pathogenic microbes. This mechanism of toxin genes being conveyed to a susceptible co-opted microbial host does not end with E. coli. It exists in one of the most common bacteria which we vaccinate against in childhood with the classic DPT shot, the toxin of the diphtheria bacterium, Corynebacterium diphtheria. Diphtheria toxin was discovered in 1890 by Emil Adolf von Behring. In 1951, it was discovered that the toxin gene was not encoded on the bacterial chromosome, but by a latent temperate phage infecting all toxigenic lysogenic strains of the diphtheria bacteria. The toxin inhibits protein synthesis as does Shiga and other bacterial toxins and some antibiotics. It does this by acting as the enzyme NAD-ADP-ribosyltransferase (EC 2.4.2.36). It catalyzes the transfer of nicotinamide adenine dinucleotide to the eukaryotic cell elongation factor-2 (eEF2), inactivating this protein so that it cannot participate in the protein synthesizing function of the ribosomes. It ADP-ribosylates the unusual amino acid diphthamide in eEF2. The toxin, like Shiga toxin and snake venoms, is an AB toxin. It is structurally a single polypeptide chain of 535 amino acids composed of two subunits linked by disulfide bridges. Cancer drugs were eventually developed using the toxin, Denileukin Diftitox, which uses diphtheria toxin as an anti-neoplastic pharmaceutical, and Resimmune ™ which is an immunotoxin for cutaneous T cell lymphoma. The latter uses diphtheria toxin (truncated by the cell binding domain) coupled to anti-CD3 antibody. Cholera, which Robert Koch investigated and used the observation of its carrier state in people to disregard his first postulate, is caused by the bacterium Vibrio cholerae, which produces an AB toxin similar to diphtheria toxin and is also co-opted by a bacteriophage which conveys the toxin genes. The cholera toxin is an oligomeric protein made up of six subunits, a single copy of the A (enzymatic “affector” subunit) and five copies of the B subunit (receptor binding), AB5. Subunit B binds and delivers subunit A to the cell where it activates the G protein which activates adenylate cyclase. The five B subunits have a mass of 11 kDa each and form a five-membered ring. The A subunit is 28 kDa and has two functional substructures, the A1 portion of the chain: a globular ADP-G protein-ribosylase and the A2 chain, an extended alpha helix which sits in the center of the B subunit ring. The toxin is similar in structure and mechanism to the heat-labile enterotoxin of Escherichia coli. The subunit A enzyme activates, with different specificity, but essentially the same catalytic activity as subunit A of diphtheria toxin. All these toxins are potential targets for therapeutic neutralizing aptamers which can be selected from a large library of DNA sequences and amplified for clinical use https://patentimages.storage.googleapis.com/4d/9f/d0/ce71a48231f7c5/US9273345.pdf.
Global Biosurveillance 