Pharmacological Targeting of Pore-Forming Toxins as Adjunctive Therapy for Invasive Bacterial Infection

Abstract

For many of the most important human bacterial infections, invasive disease severity is fueled by the cell damaging and pro-inflammatory effects of secreted pore-forming toxins (PFTs). Isogenic PFT-knockout mutants, e.g., Staphylococcus aureus lacking α-toxin or Streptococcus pneumoniae deficient in pneumolysin, show attenuation in animal infection models. This knowledge has inspired multi-model investigations of strategies to neutralize PFTs or counteract their toxicity as a novel pharmacological approach to ameliorate disease pathogenesis in clinical disease. Promising examples of small molecule, antibody or nanotherapeutic drug candidates that directly bind and neutralize PFTs, block their oligomerization or membrane receptor interactions, plug establishment membrane pores, or boost host cell resiliency to withstand PFT action have emerged. The present review highlights these new concepts, with a special focus on β-PFTs produced by leading invasive human Gram-positive bacterial pathogens. Such anti-virulence therapies could be applied as an adjunctive therapy to antibiotic-sensitive and -resistant strains alike, and further could be free of deleterious effects that deplete the normal microflora.

Assessing the Extent of Structural and Dynamic Modulation of Membrane Lipids due to Pore Forming Toxins: Insights from Molecular Dynamics Simulations

“…72 With improved instrumentation and single molecule methods probing dynamics at length scales currently studied in molecular dynamics studies is a distinct possibility. An understanding of the lytic action of PFTs and the concomitant lipid reorganization could potentially help in developing novel treatment protocols which seek to disrupt protein binding and pore formation 73 , as well as understand neurodegenerative diseases such as Alzheimers which are caused by membrane-bound protein aggregates on the neuronal cell membranes, whose formation largely mimics self-assembly mechanisms followed by PFTs. 74…”

AnIn silicoAlgorithm for Identifying Amino Acids that Stabilize Oligomeric Membrane-Toxin Pores through Electrostatic Interactions

“…In fact, PFTs are the main virulence factors of pathogenic bacteria that cause most human diseases, such as Vibrio cholerae, Staphylococcus aureaus, Pseudomonas aeruginosa, Bacillus anthracis , and others. With the increase in the prevalence of human and animal infections propagated by antibiotic-resistant bacterial strains (Blair et al, 2015; Opal, 2016), an important research focus is on targeting bacterial virulence factors rather than the pathogens themselves (Gammon, 2014; Escajadillo & Nizet, 2018). Since PFTs form the single largest class of bacterial virulence factors (Los et al, 2013), PFTs of pathogenic bacteria are currently being targeted by many novel therapies, and therefore, an understanding of the molecular features and mechanisms of PFTs are crucial for such therapies to succeed.…”

“…To target PFTs directly, small molecules, peptides and liposome decoys that inhibit the binding of PFTs to host receptors, oligomerization of the pores on the host plasma membrane, or block already formed pores have been designed (recently reviewed in Escajadillo & Nizet, 2018) to protect against infection propagated by PFTs of Bacillus anthracis (anthrax) (Nestorovich & Bezrukov, 2014; Rai et al, 2006), Staphylococcus aureaus (pneumonia, gut and skin infections) (Ragle et al, 2010), Escherichia coli (gut and urinary tract infections) (Mandal et al, 2016), Vibrio cholerae (cholera) (Rai et al, 2006), Streptococcus pneumoniae (pneumonia) (Henry et al, 2015), and many other bacteria. While inhibitors that block fully formed pores may yield some therapeutic benefit, blocked pores can nonetheless cause other disruptive effects such as bending and deformation of the plasma membrane (Tzokov et al, 2006; Drücker et al, 2019) and alteration of the lateral organization of lipid domains (Yilmaz & Kobayashi, 2015) as well as lipid dynamics (Ponmalar et al, 2019).…”

“…Therefore, rather than blocking fully formed pores, a more effective strategy to neutralize PFTs would be to prevent pore formation, either by shielding the host plasma membrane from PFTs, or by blocking oligomerization and thus pore formation. This strategy underlies the design of therapeutics such as toxin-absorbing nanosponges, which act as a sink for PFTs and therefore protect the host cell membrane (Henry et al, 2015; Hu et al, 2013), small molecule inhibitors of PFT oligomerization (Escajadillo & Nizet, 2018), passive immunization with PFT-targeting antibodies (Foletti et al, 2013; Tkaczyk et al, 2016), PFT-receptor blockade on host cell membrane that prevents membrane-binding and conformational change (Rosch et al, 2010; Johnson et al, 2013), and even vaccination strategies with PFT epitopes as antigen that could promote anti-PFT adaptive-immune responses preventing the binding and oligomerization of PFTs (Adhikari et al, 2016; Wei et al, 2017).…”

“…Effective inhibitors that target PFT oligomerization and pore formation should bind to critical residues on membrane-bound PFT protomers and block protomer-protomer interfacial interactions, thus impairing self-assembly and the stability of these oligomeric and non-covalently bound protein complexes (Escajadillo & Nizet, 2018). However, identification of these critical residues and their interactions in the inter-protomer interface has been a challenge.…”