ExlA: A New Contributor to Pseudomonas aeruginosa Virulence

Abstract

ExlA (also called exolysin) is a recently discovered virulence factor secreted by a subset of Pseudomonas aeruginosa strains in which a type 3 secretion system is lacking. exlA-positive strains were identified worldwide in the clinic, causing several types of infectious diseases, and were detected in various locations in the environment. ExlA possesses pore-forming activity and is cytolytic for most human cell types. It belongs to a class of poorly characterized bacterial toxins, sharing a similar protein domain organization and a common secretion pathway. This review summarizes the recent findings regarding ExlA synthesis, its secretion pathway, and its toxic behavior for host cells.

Keywords: animal models of infection; bacterial virulence factors; cadherins; inflammasome; pore-forming toxins; two-partner secretion systems.

Figure 1

Genetics of the exlA strains and the synthesis of ExlA. (A) Phylogenetic tree of Pseudomonas aeruginosa strains created by genome comparison [derived from Kos et al. (2015)], showing the two genetic outlier clusters, PA7 and PA39. The PA7 strains and most PA39 strains possess the exlA gene, as opposed to classical strains. The lower panel shows the entire tree with uninterrupted lines. (B) Transcription of the exlB–exlA operon is enhanced by Vfr and negatively regulated by the ErfA transcription factor. The 172-kDa ExlA protein consists of a secretion signal peptide (SP), a two-partner secretion (TPS) domain that interacts with ExlB, filamentous hemagglutinin adhesin (FHA)-like repeats, and a C-terminal (C-ter) domain. (C) Both ExlA and ExlB are secreted into the periplasm via the sec pathway. ExlB is predicted to insert into the outer membrane. ExlA is exported into the extracellular milieu through the pore formed by ExlB. When intracellular concentrations of c-di-GMP are high, ExlA translocation stalls at midway with part of the toxin pointing outside. It is hypothesized that secretion resumes when ExlA interacts with a host receptor. (C) Created with BioRender.com.

Figure 2

Effect of ExlA pore formation in host cells. (A) Effect in epithelial and endothelial cells. Pore formation induces massive Ca2+ influx, which activates two processes. In the first process, Ca2+ binds to calmodulin (CaM), releasing calmodulin from pro-ADAM10, and activates two convertases, furin (Fur) and PC7, which excise the ADAM10 prodomain. Mature ADAM10 is then exported to the plasma membrane where it cleaves the cadherin ectodomain, leading to junction disruption. In the second phase, cells die from osmolysis owing to ion imbalance. (B) In macrophages, ExlA induces cell death by pyroptosis. K+ efflux through ExlA pores induces the assembly of the NLRP3 inflammasome, thereby activating caspase-1. Caspase-1 excises IL-1β and IL-18 prodomains and cleaves pro-gasdermin D. Mature gasdermin D oligomerizes and oligomers form a large pore in the plasma membrane, allowing the release of inflammatory cytokines. (A, B) Created with BioRender.com.