In a recent study published in the journal PLoS ONE, researchers in Spain performed a systematic review of pneumolysin (PLY) toxin as a molecular target to develop novel therapeutics against Streptococcus pneumoniae.
Pneumolysin as a target for new therapies against pneumococcal infections: A systematic review. Image Credit: Kateryna Kon / Shutterstock
Background
Streptococcus pneumoniae is the primary causative organism of community-acquired pneumonia (CAP) among children and adults and also causes septicemia, meningitis, otitis media, and sinusitis, despite the availability of pneumococcal vaccines. PLY is a key virulence factor produced by pneumococcal organisms and binds with cholesterol among eukaryotic cells, and forms pores resulting in cellular destruction by activating pathways of apoptosis, pyroptosis, and necroptosis, releasing membrane-derived exosomes and microvesicles.
PLY pores cause substantial mitochondrial Ca2+ influx, triggering changes in mitochondrial morphology and mitochondrial deoxyribonucleic acid (mtDNA) release via microvesicles. During inflammation, the toxin activates multiple pathways such as mitogen-activated protein kinase (MAPK), the nuclear factor kappa-B (NF-κB), and NOD-like receptor pyrin domain-containing 3 (NLRP3) inflammasomes, resulting in the elevated expression of chemokines and cytokines including interleukins (IL)-1,6,8, and tumor necrosis factor (TNF). The emergence of non-pneumococcal vaccine serotypes post-PCV introduction, coupled with antimicrobial resistance, warrants the development of more effective therapeutic options for Streptococcus pneumoniae infections.
About the study
In the present study, researchers investigated molecules with direct or indirect PLY-inhibitory activity and explored the mechanisms of PLY inhibition
Databases such as Scopus, MEDLINE, and Web of Science were searched for records assessing PLY as a molecular target for developing anti-pneumococcal agents, published between 1 January 2000 and 25 November 2021. In addition, references to included articles were reviewed. Two independent researchers screened all records, and disagreements were resolved by another researcher. The team excluded records published in non-English languages, including case reports, patents, book chapters, reviews, and notes.
In addition, studies presenting PLY toxins as vaccines, and those elucidating pneumococcal infection pathogenesis, were excluded. The quality of included studies and bias risks were determined using the office of health assessment & translation (OHAT) tools. Data extracted from the included studies included the study authors, type of publication, abstract, title, article source titles, digital object identifier (DOI), date of publication, tests performed on laboratory animals and cell lines, the tested dosage of the molecules, chemical abstracts service (CAS) number, delivery route, infection type, and the causative S. pneumoniae strain. In addition, knowledge networks of the anti-PLY molecules were evaluated.
Results
Initially, 366 relevant records were obtained, of which 203, 161, and two were retrieved from the Web of Science, MEDLINE, and Scopus, respectively. After duplicate removal, titles and abstracts of 192 records were screened, and after applying the eligibility criteria, only 41 records assessing the PLY-inhibitory activity of various molecules were considered for the final analysis. The molecules could be categorized under three groups as follows: (i) molecules exerting direct effects by blocking or binding with PLY, (ii) molecules with indirect effects by preventing its deleterious effects on cells of the host, and (iii) molecules with unknown mechanisms of action.
Several molecules have been considered toxin blockers; however, only a few of them, including peptides, antibiotics, statins, and sterols, have the potential to be implemented for clinical use. Contrastingly, studies demonstrating effectiveness in animal-based studies with adequate reliability were limited for other biological molecules. The majority of studies had a good confidence level, and the overall risk of bias in the included studies was low or probably low, but heterogeneity was high.
The types of anti-PLY molecules included (i) plant-derived compounds (Tannins, polyphenols, quinones, flavonoids, terpenoids, and phenethylamines); (ii) sterols (β-sitosterol and cholesterol); (iii) statins (pravastatin and simvastatin), omega-3 fatty acids [such as docosahexaenoic acid (DHA)]; (iv) purine-6-ones (PDPs); (v) thioethers (montelukast and CysLT1); (vi) antibiotics (clarithromycin and ceftriaxone); (vii) peptides (TIP and DM3); (viii) cations (Mg2+, Zn2+, and Ca2+); and (viii) antibodies.
Direct PLY inhibitors of PLY oligomerization include acacetin, morin, apigenin, epigallocatechin gallate, shikonin, juglone, quercetin, hederagenin, and dryocrassin ABBA. Amentoflavone interacts with PLY at Arg359, Ser254, and Glu277 sites to weaken PLY oligomerization, and verbascoside inhibits PLY oligomerization by binding to the cleft between the third and fourth PLY domains. Other direct inhibitor molecules are ephedrine hydrochloride, betulin, oleanolic acid, β-sitosterol, docosahexaenoic acid, DM3 peptide, mannose receptor peptides (MRC-1), and anti-PLY antibodies.
Indirect PLY inhibitors include simvastatin, 9-(6-phenyl-2-oxohex-3-yl)-2-(3,4-dimethoxybenzyl)-purin-6-one, montelukast, TNF-derived tonoplast intrinsic protein (TIP peptide), GHRH agonist JI-34 peptide, vasculotide, zVAD-fmk, Z-VAL-ALA-OH, and Ca2+. PLY inhibitors with unknown mechanisms include aloe-emodin, clarithromycin, ceftriaxone, Mg2+, and Zn2+. Mechanisms of indirect PLY inhibition include reducing cholesterol content and inhibiting platelet-activating factor receptor (PAFr), phosphodiesterase 2 (PDE2), cysteinyl leukotriene receptor 1 (CysLT1), TIP peptide, protein kinase C-alpha, and arginase activities. Other mechanisms include restoring sodium uptake and blunting myosin light chain (MLC) phosphorylation and PLY-coding gene deletions.
Overall, the study findings showed that several anti-PLY molecules lowered pneumococcal infection-associated inflammation, some of which are already in use for other purposes and can, therefore, be considered safe for use against pneumococcus. The patients may benefit from a second line of therapy in the early stages of infection, preventing invasive-type pneumococcal infections and acute respiratory distress syndrome (ARDS).
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