Therapeutic Uses of Bacterial Subunit Toxins

Abstract

The B subunit pentamer verotoxin (VT aka Shiga toxin-Stx) binding to its cellular glycosphingolipid (GSL) receptor, globotriaosyl ceramide (Gb3) mediates internalization and the subsequent receptor mediated retrograde intracellular traffic of the AB5 subunit holotoxin to the endoplasmic reticulum. Subunit separation and cytosolic A subunit transit via the ER retrotranslocon as a misfolded protein mimic, then inhibits protein synthesis to kill cells, which can cause hemolytic uremic syndrome clinically. This represents one of the most studied systems of prokaryotic hijacking of eukaryotic biology. Similarly, the interaction of cholera AB5 toxin with its GSL receptor, GM1 ganglioside, is the key component of the gastrointestinal pathogenesis of cholera and follows the same retrograde transport pathway for A subunit cytosol access. Although both VT and CT are the cause of major pathology worldwide, the toxin–receptor interaction is itself being manipulated to generate new approaches to control, rather than cause, disease. This arena comprises two areas: anti neoplasia, and protein misfolding diseases. CT/CTB subunit immunomodulatory function and anti-cancer toxin immunoconjugates will not be considered here. In the verotoxin case, it is clear that Gb3 (and VT targeting) is upregulated in many human cancers and that there is a relationship between GSL expression and cancer drug resistance. While both verotoxin and cholera toxin similarly hijack the intracellular ERAD quality control system of nascent protein folding, the more widespread cell expression of GM1 makes cholera the toxin of choice as the means to more widely utilise ERAD targeting to ameliorate genetic diseases of protein misfolding. Gb3 is primarily expressed in human renal tissue. Glomerular endothelial cells are the primary VT target but Gb3 is expressed in other endothelial beds, notably brain endothelial cells which can mediate the encephalopathy primarily associated with VT2-producing E. coli infection. The Gb3 levels can be regulated by cytokines released during EHEC infection, which complicate pathogenesis. Significantly Gb3 is upregulated in the neovasculature of many tumours, irrespective of tumour Gb3 status. Gb3 is markedly increased in pancreatic, ovarian, breast, testicular, renal, astrocytic, gastric, colorectal, cervical, sarcoma and meningeal cancer relative to the normal tissue. VT has been shown to be effective in mouse xenograft models of renal, astrocytoma, ovarian, colorectal, meningioma, and breast cancer. These studies are herein reviewed. Both CT and VT (and several other bacterial toxins) access the cell cytosol via cell surface ->ER transport. Once in the ER they interface with the protein folding homeostatic quality control pathway of the cell -ERAD, (ER associated degradation), which ensures that only correctly folded nascent proteins are allowed to progress to their cellular destinations. Misfolded proteins are translocated through the ER membrane and degraded by cytosolic proteosome. VT and CT A subunits have a C terminal misfolded protein mimic sequence to hijack this transporter to enter the cytosol. This interface between exogenous toxin and genetically encoded endogenous mutant misfolded proteins, provides a new therapeutic basis for the treatment of such genetic diseases, e.g., Cystic fibrosis, Gaucher disease, Krabbe disease, Fabry disease, Tay-Sachs disease and many more. Studies showing the efficacy of this approach in animal models of such diseases are presented.

Enterohemorrhagic Escherichia coli and a Fresh View on Shiga Toxin-Binding Glycosphingolipids of Primary Human Kidney and Colon Epithelial Cells and Their Toxin Susceptibility

“…Stx is a classic representative of the AB 5 toxins composed of a single A subunit (StxA, 32 kDa), where "A" stands for "activity", and five identical B subunits (StxB, 7.7 kDa each), where "B" stands for "binding" [4,20,164] as shown in Figure 1A,B. The catalytic A subunit of the ribosome-inactivating protein exerts interruption of the protein biosynthesis at the ribosomes and the pentameric B subunit binds to globo-series GSLs on the target cell surface [70,[165][166][167]. Stxs are grouped in the two types Stx1 and Stx2, which are further subdivided into three Stx1 and (at least until now) nine Stx2 subtypes (for correct nomenclature refer to Scheutz and collaborators) [50].…”

Botulinum Neurotoxins (BoNTs) and Their Biological, Pharmacological, and Toxicological Issues: A Scoping Review

“…It is noteworthy that he was also the first physician to propose the therapeutic use of the botulinum toxin. In 1897, after an epidemic of botulism in the city of Elezel (Belgium) due to the consumption of ham of homemade pork, the microbiologist Emile van Ermengem, discovered the bacterium and concluded that the disease was caused by the anaerobic growth of the microorganism and that the endospores produced a toxin [11]. In the 1920s, scientists were able to isolate one of the seven toxins produced by Clostridium, such as the toxin BoNT-A, which is now the active form of the commercial drug Botox™.…”

“…In the 1920s, scientists were able to isolate one of the seven toxins produced by Clostridium, such as the toxin BoNT-A, which is now the active form of the commercial drug Botox™. During the Second World War, the research on the manufacture of biological weapons that could infect and affect the nervous system had a strong impulse, so two military laboratories, one in the United States and one in England, studied the effects of toxin B [11,12].…”

“…Later, the same drug was recalled Botox, and was licensed by the American FDA for wrinkles. Botox has already caused a small revolution in cosmetic plastic surgery in an easy, simple, and safe way [11,14]. In England, in 1993, Khalaf Bushara and David Park were the first to use BTX-A for nonmuscular problems demonstrating that injections of toxins inhibit sweating and can therefore be useful in the treatment of hyperhidrosis [15].…”

Insertion-trigger residues differentially modulate endosomal escape by cytotoxic necrotizing factor toxins

“…After binding and cellular uptake, these toxins transport their toxic cargo to the cytosol through multiple trafficking pathways, the most common of which involve retrograde transport through the endoplasmic reticulum or endocytic trafficking from early to late endosomes followed by pH-dependent endosomal escape. Already, a number of modular toxins have been exploited for their ability to deliver heterologous cargo molecules to the cytosol, including fluorescent proteins ( 1 ), epitope tags ( 2 ), nanobodies ( 3 , 4 ), various recombinant enzymes ( 5 , 6 , 7 , 8 , 9 , 10 ), and nucleic-acid-binding proteins ( 11 , 12 , 13 ). Bacterial toxin-inspired drug delivery (BTIDD) platforms, such as those described for the cytotoxic necrotizing factor (CNF) toxins ( 14 ) that assemble from modular components, could be expanded to noncognate therapeutic cargos if the determinants for efficient cytosolic delivery of the biologic cargo were more fully understood.…”