Staphylococcus aureus Lpl Lipoproteins Delay G2/M Phase Transition in HeLa Cells

Targeting host-specific metabolic pathways-opportunities and challenges for anti-infective therapy

Microorganisms can takeover critical metabolic pathways in host cells to fuel their replication. This interaction provides an opportunity to target host metabolic pathways, in addition to the pathogen-specific ones, in the development of antimicrobials. Host-directed therapy (HDT) is an emerging strategy of anti-infective therapy, which targets host cell metabolism utilized by facultative and obligate intracellular pathogens for entry, replication, egress or persistence of infected host cells. This review provides an overview of the host lipid metabolism and links it to the challenges in the development of HDTs for viral and bacterial infections, where pathogens are using important for the host lipid enzymes, or producing their own analogous of lecithin-cholesterol acyltransferase (LCAT) and lipoprotein lipase (LPL) thus interfering with the human host's lipid metabolism.

Staphylococcus aureus lipoproteins in infectious diseases

Infections with the Gram-positive bacterial pathogen Staphylococcus aureus remain a major challenge for the healthcare system and demand new treatment options. The increasing antibiotic resistance of S. aureus poses additional challenges, consequently inflicting a huge strain in the society due to enormous healthcare costs. S. aureus expresses multiple molecules, including bacterial lipoproteins (Lpps), which play a role not only in immune response but also in disease pathogenesis. S. aureus Lpps, the predominant ligands of TLR2, are important for bacterial survival as they maintain the metabolic activity of the bacteria. Moreover, Lpps possess many diverse properties that are of vital importance for the bacteria. They also contribute to host cell invasion but so far their role in different staphylococcal infections has not been fully defined. In this review, we summarize the current knowledge about S. aureus Lpps and their distinct roles in various infectious disease animal models, such as septic arthritis, sepsis, and skin and soft tissue infections. The molecular and cellular response of the host to S. aureus Lpp exposure is also a primary focus.

Skin Cancer-Associated S. aureus Strains Can Induce DNA Damage in Human Keratinocytes by Downregulating DNA Repair and Promoting Oxidative Stress

Actinic keratosis (AK) is a premalignant lesion, common on severely photodamaged skin, that can progress over time to cutaneous squamous cell carcinoma (SCC). A high bacterial load of Staphylococcus aureus is associated with AK and SCC, but it is unknown whether this has a direct impact on skin cancer development. To determine whether S. aureus can have cancer-promoting effects on skin cells, we performed RNA sequencing and shotgun proteomics on primary human keratinocytes after challenge with sterile culture supernatant ('secretome') from four S. aureus clinical strains isolated from AK and SCC. Secretomes of two of the S. aureus strains induced keratinocytes to overexpress biomarkers associated with skin carcinogenesis and upregulated the expression of enzymes linked to reduced skin barrier function. Further, these strains induced oxidative stress markers and all secretomes downregulated DNA repair mechanisms. Subsequent experiments on an expanded set of lesion-associated S. aureus strains confirmed that exposure to their secretomes led to increased oxidative stress and DNA damage in primary human keratinocytes. A significant correlation between the concentration of S. aureus phenol soluble modulin toxins in secretome and the secretome-induced level of oxidative stress and genotoxicity in keratinocytes was observed. Taken together, these data demonstrate that secreted compounds from lesion-associated clinical isolates of S. aureus can have cancer-promoting effects in keratinocytes that may be relevant to skin oncogenesis.

Bacterial lipoproteins in sepsis

Bacterial lipoproteins are membrane proteins derived from both gram-negative and gram-positive bacteria. They seem to have diverse functions not only on bacterial growth, but also play an important role in host's virulence. Bacterial lipoproteins exert their action on host immune cells via TLR2/1 or TLR2/6. Therefore, bacterial lipoproteins also need to be considered while addressing bacterial pathogenicity besides classical bacterial endotoxin like LPS and other microbial associated molecular patterns such as LTA, and peptidoglycans. In this mini-review, we provide an overview of general bacterial lipoprotein biosynthesis and the need to understand the lipoprotein-mediated pathogenicity in diseases like sepsis.

Staphylococcus aureus Internalization in Osteoblast Cells: Mechanisms, Interactions and Biochemical Processes. What Did We Learn from Experimental Models?

Bacterial internalization is a strategy that non-intracellular microorganisms use to escape the host immune system and survive inside the human body. Among bacterial species, Staphylococcus aureus showed the ability to interact with and infect osteoblasts, causing osteomyelitis as well as bone and joint infection, while also becoming increasingly resistant to antibiotic therapy and a reservoir of bacteria that can make the infection difficult to cure. Despite being a serious issue in orthopedic surgery, little is known about the mechanisms that allow bacteria to enter and survive inside the osteoblasts, due to the lack of consistent experimental models. In this review, we describe the current knowledge about S. aureus internalization mechanisms and various aspects of the interaction between bacteria and osteoblasts (e.g., best experimental conditions, bacteria-induced damages and immune system response), focusing on studies performed using the MG-63 osteoblastic cell line, the best traditional (2D) model for the study of this phenomenon to date. At the same time, as it has been widely demonstrated that 2D culture systems are not completely indicative of the dynamic environment in vivo, and more recent 3D models—representative of bone infection—have also been investigated.

Lipoproteins in Gram-Positive Bacteria: Abundance, Function, Fitness

When one thinks of the Gram+ cell wall, the peptidoglycan (PG) scaffold in particular comes to mind. However, the cell wall also consists of many other components, for example those that are covalently linked to the PG: the wall teichoic acid and the cell wall proteins tethered by the sortase. In addition, there are completely different molecules that are anchored in the cytoplasmic membrane and span the cell wall. These are lipoteichoic acids and bacterial lipoproteins (Lpp). The latter are in the focus of this review. Lpp are present in almost all bacteria. They fulfill a wealth of different tasks. They represent the window to the outside world by recognizing nutrients and incorporating them into the bacterial cell via special transport systems. Furthermore, they perform very diverse and special tasks such as acting as chaperonin, as cyclomodulin, contributing to invasion of host cells or uptake of plasmids via conjugation. All these functions are taken over by the protein part. Nevertheless, the lipid part of the Lpp plays an as important role as the protein part. It is the released lipoproteins and derived lipopeptides that massively modulate our immune system and ultimately play an important role in immune tolerance or non-tolerance. All these varied activities of the Lpp are considered in this review article.

Involvement of caspase-1 in inflammasomes activation and bacterial clearance in S. aureus-infected osteoblast-like MG-63 cells

Staphylococcus aureus, a versatile Gram-positive bacterium, is the main cause of bone and joint infections (BJI), which are prone to recurrence. The inflammasome is an immune signaling platform that assembles after pathogen recognition. It activates proteases, most notably caspase-1 that proteolytically matures and promotes the secretion of mature IL-1β and IL-18. The role of inflammasomes and caspase-1 in the secretion of mature IL-1β and in the defence of S. aureus-infected osteoblasts has not yet been fully investigated. We show here that S. aureus-infected osteoblast-like MG-63 but not caspase-1 knock-out CASP1 -/- MG-63 cells, which were generated using CRISPR-Cas9 technology, activate the inflammasome as monitored by the release of mature IL-1β. The effect was strain-dependent. The use of S. aureus deletion and complemented phenole soluble modulins (PSMs) mutants demonstrated a key role of PSMs in inflammasomes-related IL-1β production. Furthermore, we found that the lack of caspase-1 in CASP1 -/- MG-63 cells impairs their defense functions, as bacterial clearance was drastically decreased in CASP1 -/- MG-63 compared to wild-type cells. Our results demonstrate that osteoblast-like MG-63 cells play an important role in the immune response against S. aureus infection through inflammasomes activation and establish a crucial role of caspase-1 in bacterial clearance.

"Gene accordions" cause genotypic and phenotypic heterogeneity in clonal populations of Staphylococcus aureus

Gene tandem amplifications are thought to drive bacterial evolution, but they are transient in the absence of selection, making their investigation challenging. Here, we analyze genomic sequences of Staphylococcus aureus USA300 isolates from the same geographical area to identify variations in gene copy number, which we confirm by long-read sequencing. We find several hotspots of variation, including the csa1 cluster encoding lipoproteins known to be immunogenic. We also show that the csa1 locus expands and contracts during bacterial growth in vitro and during systemic infection of mice, and recombination creates rapid heterogeneity in initially clonal cultures. Furthermore, csa1 copy number variants differ in their immunostimulatory capacity, revealing a mechanism by which gene copy number variation can modulate the host immune response.

Gene tandem amplifications can drive bacterial evolution. Here, Belikova et al. identify copy number variations of lipoprotein-encoding genes in Staphylococcus aureus clinical isolates, and show that the loci expand and contract during bacterial growth in vitro and in mice, leading to changes in immunostimulatory capacity.

Staphylococcus aureus Lpl protein triggers human host cell invasion via activation of Hsp90 receptor

Staphylococcus aureus is a facultative intracellular pathogen. Recently, it has been shown that the protein part of the lipoprotein-like lipoproteins (Lpls), encoded by the lpl cluster comprising of 10 lpls paralogue genes, increases pathogenicity, delays the G2/M phase transition, and also triggers host cell invasion. Here, we show that a recombinant Lpl1 protein without the lipid moiety binds directly to the isoforms of the human heat shock proteins Hsp90α and Hsp90ß. Synthetic peptides covering the Lpl1 sequence caused a twofold to fivefold increase of S. aureus invasion in HaCaT cells. Antibodies against Hsp90 decrease S. aureus invasion in HaCaT cells and in primary human keratinocytes. Additionally, inhibition of ATPase function of Hsp90 or silencing Hsp90α expression by siRNA also decreased the S. aureus invasion in HaCaT cells. Although the Hsp90ß is constitutively expressed, the Hsp90α isoform is heat-inducible and appears to play a major role in Lpl1 interaction. Pre-incubation of HaCaT cells at 39°C increased both the Hsp90α expression and S. aureus invasion. Lpl1-Hsp90 interaction induces F-actin formation, thus, triggering an endocytosis-like internalisation. Here, we uncovered a new host cell invasion principle on the basis of Lpl-Hsp90 interaction.

Staphylococcus aureus induces DNA damage in host cell

Staphylococcus aureus causes serious medical problems in human and animals. Here we show that S. aureus can compromise host genomic integrity as indicated by bacteria-induced histone H2AX phosphorylation, a marker of DNA double strand breaks (DSBs), in human cervix cancer HeLa and osteoblast-like MG-63 cells. This DNA damage is mediated by alpha phenol-soluble modulins (PSMα_1–4), while a specific class of lipoproteins (Lpls), encoded on a pathogenicity island in S. aureus, dampens the H2AX phosphorylation thus counteracting the DNA damage. This DNA damage is mediated by reactive oxygen species (ROS), which promotes oxidation of guanine forming 7,8-dihydro-8-oxoguanine (8-oxoG). DNA damage is followed by the induction of DNA repair that involves the ATM kinase-signaling pathway. An examination of S. aureus strains, isolated from the same patient during acute initial and recurrent bone and joint infections (BJI), showed that recurrent strains produce lower amounts of Lpls, induce stronger DNA-damage and prompt the G2/M transition delay to a greater extent that suggest an involvement of these mechanisms in adaptive processes of bacteria during chronicization. Our findings redefine our understanding of mechanisms of S. aureus-host interaction and suggest that the balance between the levels of PSMα and Lpls expression impacts the persistence of the infection.

Artificial Selection for Pathogenicity Mutations in Staphylococcus aureus Identifies Novel Factors Relevant to Chronic Infection

Adaptation of Staphylococcus aureus to host microenvironments during chronic infection involves spontaneous mutations, yet changes underlying adaptive phenotypes remain incompletely explored. Here, we employed artificial selection and whole-genome sequencing to better characterize spontaneous chromosomal mutations that alter two pathogenicity phenotypes relevant to chronic infection in S. aureus: intracellular invasiveness and intracellular cytotoxicity. We identified 23 genes whose alteration coincided with enhanced virulence, 11 that were previously known and 12 (52%) that had no previously described role in S. aureus pathogenicity. Using precision genome editing, transposon mutants, and gene complementation, we empirically assessed the contributions of individual genes to the two virulence phenotypes. We functionally validated 14 of 21 genes tested as measurably influencing invasion and/or cytotoxicity, including 8 newly implicated by this study. We identified inactivating mutations (murA, ndhC, and a hypothetical membrane protein) and gain-of-function mutations (aroE Thr182Ile, yhcF Thr74Ile, and Asp486Glu in a hypothetical peptidase) in previously unrecognized S. aureus virulence genes that enhance pathogenesis when introduced into a clean genetic background, as well as a novel activating mutation in the known virulence regulator gene saeS (Ala106Thr). Investigation of potentially epistatic interactions identified a tufA mutation (Ala271Val) that enhances virulence only in the context of purine operon repressor gene (purR) inactivation. This project reveals a functionally diverse range of genes affected by gain- or loss-of-function mutations that contribute to S. aureus adaptive virulence phenotypes. More generally, the work establishes artificial selection as a means to determine the genetic mechanisms underlying complex bacterial phenotypes relevant to adaptation during infection.

The hidden lipoproteome of Staphylococcus aureus

Lipoproteins are attached to the outer leaflet of the membrane by a di- or tri-acylglyceryl moiety and are thus positioned in the membrane-cell wall interface. Consequently, lipoproteins are involved in many surface associated functions, including cell wall synthesis, electron transport, uptake of nutrients, surface stress response, signal transduction, and they represent a reservoir of bacterial virulence factors. Inspection of 123 annotated Staphylococcus aureus genome sequences in the public domain revealed that this organism devotes about 2-3% of its coding capacity to lipoproteins, corresponding to about 70 lipoproteins per genome. 60 of these lipoproteins were identified in 95% of the genomes analyzed, which thus constitute the core lipoproteome of S. aureus. 30% of the conserved staphylococcal lipoproteins are substrate-binding proteins of ABC transporters with roles in nutrient transport. With a few exceptions, much less is known about the function of the remaining lipoproteins, representing a large gap in our knowledge of this functionally important group of proteins. Here, we summarize current knowledge, and integrate information from genetic context analysis, expression and regulatory data, domain architecture, sequence and structural information, and phylogenetic distribution to provide potential starting points for experimental evaluation of the biological function of the poorly or uncharacterized lipoproteome of S. aureus.

Toll-Like Receptor 2 and Lipoprotein-Like Lipoproteins Enhance Staphylococcus aureus Invasion in Epithelial Cells

Staphylococcus aureus contains a certain subclass of lipoproteins, the so-called lipoprotein-like lipoproteins (Lpl's), that not only represent Toll-like receptor 2 (TLR2) ligands but are also involved in host cell invasion. Here we addressed the question of which factors contribute to Lpl-mediated invasion of epithelial cells and keratinocytes. For this purpose, we compared the invasiveness of USA300 and its Δlpl mutant under different conditions. In the presence of the matrix proteins IgG, fibrinogen (Fg), and fibronectin (Fn), and of fetal bovine serum (FBS), the invasion ratio was increased in both strains, and always more in USA300 than in its Δlpl mutant. Interestingly, when we compared the invasion of HEK-0 and HEK-TLR2 cells, the cells expressing TLR2 showed a 9-times-higher invasion frequency. When HEK-TLR2 cells were additionally stimulated with a synthetic lipopeptide, Pam₃CSK₄ (P3C), the invasion frequency was further increased. A potential reason for the positive effect of TLR2 on invasion could be that TLR2 activation by P3C also activates F-actin formation. Here we show that S. aureus invasion depends on a number of factors, on the host side as well as on the bacterial side.

Staphylococcal (phospho)lipases promote biofilm formation and host cell invasion

Most Staphylococcus aureus strains secrete two lipases SAL1 and SAL2 encoded by gehA and gehB. These two lipases differ with respect to their substrate specificity. Staphylococcus hyicus secretes another lipase, SHL, which is in contrast to S. aureus lipases Ca²⁺-dependent and has a broad-spectrum lipase and phospholipase activity. The aim of this study was to investigate the role of staphylococcal (phospho) lipases in virulence. For this we constructed a gehA-gehB double deletion mutant in S. aureus USA300 and expressed SHL in agr-positive (accessory gene regulator) and agr-negative S. aureus strains. The lipases themselves have no hemolytic or cytotoxic activity. However, in agr-negative strains SHL-expression caused an upregulation of hemolytic activity. We further show that SHL-expression significantly enhanced biofilm formation probably due to an increase of extracellular DNA release. SHL-expression also increased host cell invasion 4-6-fold. Trioleate, a main triacylglycerol component of mammalian skin, induced lipase production. Finally, in the mouse sepsis and skin colonization models the lipase producing and mutant strain showed no significant difference compared to the WT strain. In conclusion, we show that staphylococcal lipases promote biofilm formation and host cell invasion and thereby contribute to S. aureus virulence.

Aspartate tightens the anchoring of staphylococcal lipoproteins to the cytoplasmic membrane

In gram-negative bacteria, the ABC transporter LolCDE complex translocates outer membrane-specific lipoproteins (Lpp) from the inner membrane to the outer membrane. Lpp possessing aspartate (Asp) at position +2 are not translocated because it functions as a LolCDE avoidance signal. In gram-positive bacteria, lacking an outer membrane and the Lol system, Lpp are only anchored at the outer leaflet of the cytoplasmic membrane. However, the release of Lpp particularly in pathogenic or commensal species is crucial for immune modulation. Here, we provide evidence that in Staphylococcus aureus Asp at position +2 plays a role in withholding Lpp to the cytoplasmic membrane. Screening of published exoproteomic data of S. aureus revealed that Lpp mainly with Gly or Ser at position +2 were found in exoproteome, but there was no Lpp with Asp+2. The occurrence of Lpp with Asp+2 is infrequent in gram-positive bacteria. In S. aureus USA300 only seven of the 67 Lpp possess Asp+2; among them five Lpp represented Lpl lipoproteins involved in host cell invasion. Our study demonstrated that replacing the Asp+2 present in Lpl8 with a Ser enhances its release into the supernatant. However, there is no different release of Asp+2 and Ser+2 in mprF mutant that lacks the positive charge of lysyl-phosphatidylglycerol (Lys-PG). Moreover, substitution of Ser+2 by Asp in SitC (MntC) did not lead to a decreased release indicating that in staphylococci positions +3 and +4 might also be important for a tighter anchoring of Lpp. Here, we show that Asp in position +2 and adjacent amino acids contribute in tightening the anchoring of Lpp by interaction of the negative charged Asp with the positive charged Lys-PG.