Harnessing the Membrane Translocation Properties of AB Toxins for Therapeutic Applications

Abstract

Over the last few decades, proteins and peptides have become increasingly more common as FDA-approved drugs, despite their inefficient delivery due to their inability to cross the plasma membrane. In this context, bacterial two-component systems, termed AB toxins, use various protein-based membrane translocation mechanisms to deliver toxins into cells, and these mechanisms could provide new insights into the development of bio-based drug delivery systems. These toxins have great potential as therapies both because of their intrinsic properties as well as the modular characteristics of both subunits, which make them highly amenable to conjugation with various drug classes. This review focuses on the therapeutical approaches involving the internalization mechanisms of three representative AB toxins: botulinum toxin type A, anthrax toxin, and cholera toxin. We showcase several specific examples of the use of these toxins to develop new therapeutic strategies for numerous diseases and explain what makes these toxins promising tools in the development of drugs and drug delivery systems.

Clostridium botulinum C3 Toxin for Selective Delivery of Cargo into Dendritic Cells and Macrophages

“…However, the cell-type selectivity of C3bot towards monocytic cells can be exploited for pharmacological purposes in a second way, i.e., for targeted delivery of therapeutic (macro-) molecules into these target cells and their controlled release into the cytosol. Various bacterial AB-type protein toxins have been used as drug delivery systems because of their unique mode of action, i.e., endocytic uptake and endosomal release of the therapeutic cargo molecules to reach cytosolic drug targets [ 46 , 47 , 48 ]. Since C3bot by nature enters monocyte-derived cells, a non-toxic variant of C3bot should represent an ideal molecule for targeted drug delivery into macrophages and DCs.…”

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

“…As modular protein toxins are increasingly employed as potential cytosolic delivery platforms for therapeutics ( 8 , 37 , 38 , 39 ), it is imperative that we continue to investigate the intricate regulation of their cytosolic cargo-delivery mechanisms to optimize their therapeutic potential. We anticipate results from these studies will inform our understanding of the pH-dependent translocation mechanism of CNF toxins and will have broader implications for the development of BTIDD platforms.…”

“…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.…”

Targeting the Inside of Cells with Biologicals: Toxin Routes in a Therapeutic Context

“…an active catalytic subunit (A) and five identical binding subunits (B) A subunit: 32 kDa B subunit: 7.7 kDa RIP, inhibits protein synthesis Several types of bacteria: Shigella dysenteriae , Shiga-like toxin-producing bacteria (STEC) [ 19 , 67 , 68 ] Cholera toxin Ganglioside GM1, e.g. on the plasma membrane of intestinal epithelial cells AB5 structure, A subunit: 27 kDa, B subunit: 11.5 kDa Increase in cAMP due to persistently active adenylate cyclase in the cytosol followed by chloride secretion in intestinal cells Bacterium Vibriae cholerae [ 8 , 69 ] Pertussis toxin Sialic acid galactose moiety on glycoproteins or glycolipids 117 kDa S1 subunit (A component): 28 kDa Increase in cAMP due to persistently active adenylate cyclase in the cytosol, derailed cell signalling as a consequence Bordetella pertussis [ 12 , 70 – 72 ] Pseudomonas exotoxin A Alpha 2-macroglobulin-receptor 28 kDa domain that contains the binding domain 37 kDa domain that contains the enzymatically active domain ADP ribosylation of the eukaryotic elongation factor eEF2, as a consequence: inhibition of protein synthesis Pseudomonas aeruginosa [ 15 , 73 , 74 ] Cytolethal distending toxins For glycans and cholesterol, an influence on binding has been discussed, glycolipid deficiencies were found to sensitise host cells towards the toxins Subunit A: ~ 23–30 kDa Subunit B: ~ 28–32 kDa Subunit C: ~ 19–20 kDa Cell-cycle arrest Various Gram-negative bacteria [ 75 , 76 ] ADP adeno...…”