Phenol-soluble modulins and staphylococcal infection

Abstract

Staphylococcus aureus is an important human pathogen and a leading cause of death worldwide. Phenol-soluble modulins (PSMs) have recently emerged as a novel toxin family defining the virulence potential of highly aggressive S. aureus isolates. PSMs have multiple roles in staphylococcal pathogenesis, causing lysis of red and white blood cells, stimulating inflammatory responses and contributing to biofilm development and the dissemination of biofilm-associated infections. Moreover, the pronounced capacity of PSMs to kill human neutrophils after phagocytosis might explain failures in the development of anti-staphylococcal vaccines. Here, we discuss recent progress made in our understanding of the biochemical and genetic properties of PSMs and their role in S. aureus pathogenesis, and suggest potential avenues to target PSMs for the development of anti-staphylococcal drugs.

Figure 1. PSM genes, amino acid sequences, and structure

(A,B) psm genes and amino acid sequences in S. epidermidis and S. aureus. Gene annotations are according to S. epidermidis strain RP62A and S. aureus strain USA300 FPR3757. All PSMs are secreted with an N-terminal N-formyl methionine (fM). Several α-type psm genes are not annotated in staphylococcal genomes owing to their short length. Note that the psmα and psmδ genes of S. epidermidis are located at a position in the genome corresponding to that of the S. aureus psmα operon, suggesting a common ancestor of these genes. The S. epidermidis psmβ operon contains a gene, psmβ3, whose gene product could not be detected in culture filtrates of S. epidermidis strains. Some S. epidermidis strains, such as RP62A, may contain two identical copies of the psmβ1 gene, resulting in higher PSMβ1 production than in strains that contain only one copy. The δ-toxin (sometimes called PSMγ), highly similar between S. epidermidis and S. aureus, is encoded by the gene “hld” (for “hemolysin delta”), located within RNAIII in the Agr system. (C) Location of the psm-mec gene in SCCmec elements. The psm-mec gene is found in SCCmec elements of types II, III, and VIII, in the J2 region next to the class A mec gene complex (with the core genes of the SCCmec element in the order IS431-mecA-mecR-mecI), which is characteristic for these SCCmec types. Type III is shown here as example. (D) α-helical wheel presentation of PSMα3, showing the extreme amphipathy that is characteristic of PSMs, with hydrophobic and hydrophilic amino acids found on opposite sides of the α-helix. (A–C), numbers behind amino acid sequences show peptide length and charge (in parentheses).

Figure 2. PSM regulation

PSMs are regulated tightly by the Agr system, a quorum-sensing (QS) system that produces and senses the presence of a post-translationally modified pheromone called AIP (auto-inducing peptide). This QS circuit is shown at the top; the necessary components are encoded by the agrACDB operon. The response regulator AgrA activates transcription of the agrACDB operon in an auto-feedback loop, but also that of RNAIII, the effector molecule in charge of changing transcription levels of most genes of the Agr operon. The psmα and psmβ genes are the only known and confirmed exceptions, as they are directly regulated by AgrA, indicating an early evolutionary link of QS and PSM production, and suggesting that RNAIII-dependent gene regulation was added later in evolution by formation of the RNAIII-encoding genetic information around the δ-toxin gene, hld. The psm-mec gene is also under Agr control, but it is not known whether this occurs by direct binding of AgrA. Modified from ref. , with permission.

Figure 3. PSM export

PSMs are secreted by the Pmt (Phenol-soluble modulin transporter) four-component ABC transporter. Presence of the Pmt transporter is crucial for PSM-mediated phenotypes, such as cytolysis, inflammation, and biofilm structuring, and immunity to PSMs of self and non-self. In analogy to other transporters that export membrane-active drugs, the substrate is likely bound from within the membrane, using the same mechanism for PSMs originating from the cytosol or the surrounding fluid. Modified from ref. , with permission.

Figure 4. Overview over PSM activities

In a likely receptor-independent fashion and in the micromolar range, PSMs cause biofilm structuring and detachment, spreading on surfaces, and cytolysis. Some PSMs may also be antimicrobial, in particular towards streptococci. Cytolytic activity is found exclusively in α-type PSMs. Most likely, due to the receptor-independent nature of cytolytic activity, many cell types are subject to destruction by PSMs. Erythrocytes and neutrophils are shown as examples. Lysis of neutrophils by α-type PSMs may occur after phagocytosis, making PSMs a particularly valuable weapon against elimination by innate host defense. PSMs also affect the adaptive immune system by inducing a tolerogenic phenotype in dendritic cells (DCs) and inhibiting Th1 differentiation in T cells. In the nanomolar range, all PSMs activate the FPR2 receptor, leading to neutrophil activation, chemotaxis, and cytokine release.