Introduction
Slx-2 was discovered in 1977 by Konowalchuki, Speirs, and Stavric in filtrate cultures of Escherichia coli. Due to its ability to affect African Green monkey kidney cells (Vero lineage), it it was first known as verotoxin. The protein was later found to be analogous to A5B-type Shiga toxin produced by Shigella dysenteriae, and was thus was also given the name Shiga-like toxin. Due to its role in hemolytic-uremic syndrome, a disease characterized by hemolytic anemia, kidney failure, and thrombocytopenia, this protein has been a topic of immunological research in recent years. Unlike their true-Shiga relatives, verotoxins are not heat-stable nor heat-labile, although their method of action remains relatively similar in regarding their role as an N-glycosidase.
Mechanism of Action
Once ingested, enterohemorrhagic Escherichia coli (EHEC), such as serotype O157:H7, release verotoxin into the intestinal tract. The verotoxin B subunits bind to receptors on the intestinal, renal cell, or neuron membrane known as glycolipid globotriaosylceramides. This glycolipid, also known as Gb3, is reported as the receptor for both Slx-1 and Slx-2. The analogous stx-2’s binding to Gb3 is heavily dependent on the presence of residues, such as phosphatidylcholine or glycolipid galactosylceramide, upon ceramide portion of Gb3, as well as the lipid environment (i.e. the presence of cholesterol). Both verotoxins can also bind Gb4, but Stx-2 cannot bind. Although it is speculative, it is suggested as possible that cholesterol may “form hydrogen bonds with the ceramide on glycolipids, leading to conformational changes that make cells more susceptible…”. This is the case for HIV, but has not been shown for verotoxin to date.
No work has yet been done on specific Slx-2 binding to Gb3, but it can be assumed due to the homology present between the two, as well as their shared receptor, that the mechanisms are roughly the same. The binding of the B subunit to G3b occurs at one to three receptor sites on the subunits, the third of which is has a lower affinity than the first two. As the B subunits bind with high affinity, the cluster of verotoxin expands laterally across the membrane, causing it to invaginate with or without the assistance of cellular tubules such as actin or dynamin. Such invagination give way to the formation of vesicles which transport the toxins into the cell. Once there, the A subunit is cleaved by the action of furin to form A1 and A2. A2 keeps A1 bound to the B pentamer, while A1 carries out the endohydrolysis of the N-glycosidic bond at a specific adenosine on the 28S rRNA unit of a 60S ribosome. This halts protein synthesis, killing the cell. With Stx, the dead cells release cytokines and chemokines, which play a role in platelet activation. Stx also inactivates ADAMTS13, which is involved in cleaving a protein that is involved in blood clotting, thus increasing the level of platelet activation further, eventually leading to the formation of microthrombi.
Disease
Due to the high density of G3b receptors in renal and intestinal cells, especially in children, verotoxin can cause hemolytic-uremic syndrome in infected people. Infection usually begins via the ingestion of food or water contaminated by the feces of deer, cattle, or other animals without G3b receptors (i.e. EHEC/verotoxin harboring organisms). Once inside the body, the toxins will bind to and inhibit protein synthesis in those Gb3-dense tissues, killing them. Damaging the glomeruli causes kidney failure, while damaging the intestinal wall causes bloody diarrhea (since the hemorrhaging intestinal wall attempts to repair itself continuously). As mentioned in the Mode of Action, these proteins also can cause microthrombus formation, which destroy red blood cells in small blood vessels. Thus, the hemolytic anemia and thrombocytopenia is caused by the formation of microthrombi, and the uremia is caused by the damage done to glomeruli.
Neural tissue and the lungs also have high densities of G3b in their cell membranes, which makes them susceptible to this toxin. In extreme cases, mice have experiences limb paralysis from lethal doses of Stx-2, and damage to lung tissue can exasperate anemia.
Treatments
The use of antibiotics to treat E. coli O157:H7 related diseases is extremely controversial, because there is evidence that certain antibiotics such as polymyxin B, trimethoprime / sulphamethoxazole, ciprofloxacin, cefixime, and tetracycline can increase verotoxin production, worsening the issue and increasing mortality rates. However, some antibotics such as quinolones and fosfomycin, may prevent the development of haemolytic uraemic syndrome or thrombotic thrombocytopenic purpura. The issue, like any case dealing with antibiotics, is ensuring that the bacteria do not become resistant to the few plausible treatment options that we seems to have.
Relevance
Shiga and shiga-like proteins are primarily relevant in an epidemiological sense—they cause disease, primarily in regions where agriculture is abundant and sanitary practices may not be as much so.
Structural highlights
Verotoxin-1 exists as hetero-hexamer (~74300 Da), made up of an A and B subunit. The A-subunit, a monomer, is ~32000 Da, while each of the five chains of the B-subunit pentamer is ~8000-8500 Da. Thus, the protein stoichiometry is A5B. The A By itself, the B subunit has cyclic C5 symmetry. It is the A subunit that causes disease via catalytically inactivating 60S ribosomal subunit by cleaving into subunits A1 and A2 via furin, A1 being the catalytic unit and A2 being the unit binding A1 to the β pentamer. The role of the B subunit is to bind to receptors on target cells, most notably globotriaosylceramide (Gb3) in humans. There are three Gb3 binding sites within the B subunit, sites 1 and 2 having a higher affinity than site 3. Thus, the A subunit is the disease-causing subunit. The A1 subunit contains ten alpha-helices and thirteen beta-strands while the A2 subunit contain two alpha-helices and two beta-strands. The A2 subunit is also entirely hydrophilic while the A1 subunit has regions of hydrophobicity, likely due to its function as a catalytic inhibitor of the 60S subunit. The B subunit is largely hydrophilic with one region of hydrophobicity, which are alpha-to-beta transitions on the outer side of the pentamer.
Evolution
Subunits A and B are encoded for by (stxA2 + stx2A + L0103) and (stxB2 + stx2B + L0104) respectively. It has been suggested by some that, due to the more appropriate phylogenetic placement of Shigella as a subgenus of Escherichia rather than as its own genus, it is possible that Stx could have been transduced into E. coli by a toxin-converting lambdoid bacteriophage, such as H-19B or 933W. This would explain the high conservation of amino acid sequences in functionally relevant (and otherwise structural) portions of the subunits.
