ExoY, an actin-activated nucleotidyl cyclase toxin from P. aeruginosa: A minireview

Pseudomonas aeruginosa aggregates elicit neutrophil swarming

Pseudomonas aeruginosa, a gram-negative multidrug-resistant (MDR) opportunist, belongs to the ESKAPE group of pathogens associated with the highest risk of mortality. Neutrophil swarming is a host defense strategy triggered by larger threats, where neutrophil swarms contain and clear damage/infection. Current ex vivo models designed to study neutrophil-pathogen interactions largely focus on individual neutrophil engagement with bacteria and fail to capture neutrophil swarming. Here, we report an ex vivo model that reproducibly elicits neutrophil swarming in response to bacterial aggregates. A rapid and robust swarming response follows engagement with pathogenic targets. Components of the type III secretion system (T3SS), a critical P. aeruginosa virulence determinant, are involved in swarm interaction. This ex vivo approach for studying neutrophil swarming in response to large pathogen targets constitutes a valuable tool for elucidating host-pathogen interaction mechanisms and for evaluating novel therapeutics to combat MDR infections.

cUMP elicits interendothelial gap formation during Pseudomonas aeruginosa infection

Pseudomonas aeruginosa utilizes a type 3 secretion system to intoxicate host cells with the nucleotidyl cyclase ExoY. After activation by its host cell cofactor, filamentous actin, ExoY produces purine and pyrimidine cyclic nucleotides, including cAMP, cGMP, and cUMP. ExoY-generated cyclic nucleotides promote interendothelial gap formation, impair motility, and arrest cell growth. The disruptive activities of cAMP and cGMP during the P. aeruginosa infection are established; however, little is known about the function of cUMP. Here, we tested the hypothesis that cUMP contributes to endothelial cell barrier disruption during P. aeruginosa infection. Using a membrane permeable cUMP analog, cUMP-AM, we revealed that during infection with catalytically inactive ExoY, cUMP promotes interendothelial gap formation in cultured pulmonary microvascular endothelial cells (PMVECs) and contributes to increased filtration coefficient in the isolated perfused lung. These findings indicate that cUMP contributes to endothelial permeability during P. aeruginosa lung infection.NEW & NOTEWORTHY During pneumonia, bacteria utilize a virulence arsenal to communicate with host cells. The Pseudomonas aeruginosa T3SS directly introduces virulence molecules into the host cell cytoplasm. These molecules are enzymes that trigger interkingdom communication. One of the exoenzymes is a nucleotidyl cyclase that produces noncanonical cyclic nucleotides like cUMP. Little is known about how cUMP acts in the cell. Here we found that cUMP instigates pulmonary edema during Pseudomonas aeruginosa infection of the lung.

Pharmacological potential of cyclic nucleotide signaling in immunity

Cyclic nucleotides are important signaling molecules that play many critical physiological roles including controlling cell fate and development, regulation of metabolic processes, and responding to changes in the environment. Cyclic nucleotides are also pivotal regulators in immune signaling, orchestrating intricate processes that maintain homeostasis and defend against pathogenic threats. This review provides a comprehensive examination of the pharmacological potential of cyclic nucleotide signaling pathways within the realm of immunity. Beginning with an overview of the fundamental roles of cAMP and cGMP as ubiquitous second messengers, this review delves into the complexities of their involvement in immune responses. Special attention is given to the challenges associated with modulating these signaling pathways for therapeutic purposes, emphasizing the necessity for achieving cell-type specificity to avert unintended consequences. A major focus of the review is on the recent paradigm-shifting discoveries regarding specialized cyclic nucleotide signals in the innate immune system, notably the cGAS-STING pathway. The significance of cyclic dinucleotides, exemplified by 2'3'-cGAMP, in controlling immune responses against pathogens and cancer, is explored. The evolutionarily conserved nature of cyclic dinucleotides as antiviral agents, spanning across diverse organisms, underscores their potential as targets for innovative immunotherapies. Findings from the last several years have revealed a striking diversity of novel bacterial cyclic nucleotide second messengers which are involved in antiviral responses. Knowledge of the existence and precise identity of these molecules coupled with accurate descriptions of their associated immune defense pathways will be essential to the future development of novel antibacterial therapeutic strategies. The insights presented herein may help researchers navigate the evolving landscape of immunopharmacology as it pertains to cyclic nucleotides and point toward new avenues or lines of thinking about development of therapeutics against the pathways they regulate.

Spread of multidrug-resistant Pseudomonas aeruginosa in animal-derived foods in Beijing, China

Pseudomonas aeruginosa is the most common bacterium occurred in nosocomial infections and is also an important indicator of food spoilage. The worldwide spread of multidrug resistant (MDR) P. aeruginosa is threatening public health. However, the prevalence and spread of MDR P. aeruginosa through the food chain is little referred under the One Health perspective. Here, we collected a total of 259 animal-derived foods (168 chicken and 91 pork) from 16 supermarkets and farmer's markets in six regions of Beijing, China. The prevalence of P. aeruginosa in chicken and pork was 42.1 %. The phenotypic antimicrobial susceptibility testing showed that 69.7 % of isolates were MDR, and isolates from Chaoyang district exhibited a higher resistance rate compared to that from Xicheng district (p < 0.05). P. aeruginosa isolates exhibited high levels of resistance against β-lactams (91.7 %), cephalosporins (29.4 %), and carbapenems (22.9 %). Interestingly, none of strains showed resistance to amikacin. Whole-genome sequencing showed that all isolates carried various kinds of antimicrobial resistance genes (ARGs) and virulence genes (VGs), especially for blaOXA genes and phz genes. Multilocus sequence typing (MLST) analysis indicated that ST111 (12.8 %) was the most predominant ST. Notably, the emergence of ST697 clones in food-borne P. aeruginosa was firstly reported. In addition, the toxin pyocyanin was detected in 79.8 % of P. aeruginosa strains. These findings help to decipher the prevalence and the strong toxigenic ability of MDR P. aeruginosa from animal-derived foods and highlight the effective supervision of animal-derived food hygiene should be strengthened to prevent the spread of ARGs in a One Health strategy.

Modulation of the immune response by the Pseudomonas aeruginosa type-III secretion system

Pseudomonas aeruginosa is an opportunistic pathogen that can cause critical cellular damage and subvert the immune response to promote its survival. Among the numerous virulence factors of P. aeruginosa, the type III secretion system (T3SS) is involved in host cell pathogenicity. Using a needle-like structure, T3SS detects eukaryotic cells and injects toxins directly into their cytosol, thus highlighting its ability to interfere with the host immune response. In this mini-review, we discuss how the T3SS and bacterial effectors secreted by this pathway not only activate the immune response but can also manipulate it to promote the establishment of P. aeruginosa infections.

Mechanism of actin-dependent activation of nucleotidyl cyclase toxins from bacterial human pathogens

Bacterial human pathogens secrete initially inactive nucleotidyl cyclases that become potent enzymes by binding to actin inside eukaryotic host cells. The underlying molecular mechanism of this activation is, however, unclear. Here, we report structures of ExoY from Pseudomonas aeruginosa and Vibrio vulnificus bound to their corresponding activators F-actin and profilin-G-actin. The structures reveal that in contrast to the apo-state, two flexible regions become ordered and interact strongly with actin. The specific stabilization of these regions results in an allosteric stabilization of the nucleotide binding pocket and thereby to an activation of the enzyme. Differences in the sequence and conformation of the actin-binding regions are responsible for the selective binding to either F- or G-actin. Other nucleotidyl cyclase toxins that bind to calmodulin rather than actin undergo a similar disordered-to-ordered transition during activation, suggesting that the allosteric activation-by-stabilization mechanism of ExoY is conserved in these enzymes, albeit the different activator.

The Role of Pseudomonas aeruginosa Virulence Factors in Cytoskeletal Dysregulation and Lung Barrier Dysfunction

Pseudomonas (P.) aeruginosa is an opportunistic pathogen that causes serious infections and hospital-acquired pneumonia in immunocompromised patients. P. aeruginosa accounts for up to 20% of all cases of hospital-acquired pneumonia, with an attributable mortality rate of ~30-40%. The poor clinical outcome of P. aeruginosa-induced pneumonia is ascribed to its ability to disrupt lung barrier integrity, leading to the development of lung edema and bacteremia. Airway epithelial and endothelial cells are important architecture blocks that protect the lung from invading pathogens. P. aeruginosa produces a number of virulence factors that can modulate barrier function, directly or indirectly, through exploiting cytoskeleton networks and intercellular junctional complexes in eukaryotic cells. This review summarizes the current knowledge on P. aeruginosa virulence factors, their effects on the regulation of the cytoskeletal network and associated components, and molecular mechanisms regulating barrier function in airway epithelial and endothelial cells. A better understanding of these processes will help to lay the foundation for new therapeutic approaches against P. aeruginosa-induced pneumonia.

Identification of Diverse Toxin Complex Clusters and an eCIS Variant in Serratia proteamaculans Pathovars of the New Zealand Grass Grub (Costelytra Giveni) and Manuka Beetle (Pyronota Spp.) Larvae

The grass grub endemic to New Zealand, Costelytra giveni (Coleoptera: Scarabaeidae), and the manuka beetle, Pyronota festiva and P. setosa (Coleoptera: Scarabaeidae), are prevalent pest species. Through assessment of bacterial strains isolated from diseased cadavers of these insect species, 19 insect-active Serratia proteamaculans variants and a single Serratia entomophila strain were isolated. When independently bioassayed, these isolates differed in host range, the rate of disease progression, and 12-day mortality rates, which ranged from 60 to 100% of the challenged larvae. A Pyronota spp.-derived S. proteamaculans isolate caused a transient disease phenotype in challenged C. giveni larvae, whereby larvae appeared diseased before recovering to a healthy state. Genome sequence analysis revealed that all but two of the sequenced isolates contained a variant of the S. entomophila amber-disease-associated plasmid, pADAP. Each isolate also encoded one of seven distinct members of the toxin complex (Tc) family of insect-active toxins, five of which are newly described, or a member of the extracellular contractile injection (eCIS) machine family, with a new AfpX variant designated SpF. Targeted mutagenesis of each of the predicted Tc- or eCIS-encoding regions abolished or attenuated pathogenicity. Host-range testing showed that several of the S. proteamaculans Tc-encoding isolates affected both Pyronota and C. giveni species, with other isolates specific for either Pyronota spp. or C. giveni. The isolation of several distinct host-specific pathotypes of Serratia spp. may reflect pathogen-host speciation. IMPORTANCE New pathotypes of the insect pathogen Serratia, each with differing virulence attributes and host specificity toward larvae of the New Zealand manuka beetle and grass grub, have been identified. All of the Serratia proteamaculans isolates contained one of seven different insect-active toxin clusters or one of three eCIS variants. The diversity of these Serratia-encoded virulence clusters, resulting in differences in larval disease progression and host specificity in endemic scarab larvae, suggests speciation of these pathogens with their insect hosts. The differing virulence properties of these Serratia species may affect their potential infectivity and distribution among the insect populations. Based on their differing geographic isolation and pathotypes, several of these Serratia isolates, including the manuka beetle-active isolates, are likely to be more effective biopesticides in specific environments or could be used in combination for greater effect.

High-Throughput Approaches for the Identification of Pseudomonas aeruginosa Antivirulents

Antimicrobial resistance is a serious medical threat, particularly given the decreasing rate of discovery of new treatments. Although attempts to find new treatments continue, it has become clear that merely discovering new antimicrobials, even if they are new classes, will be insufficient. It is essential that new strategies be aggressively pursued. Toward that end, the search for treatments that can mitigate bacterial virulence and tilt the balance of host-pathogen interactions in favor of the host has become increasingly popular. In this review, we will discuss recent progress in this field, with a special focus on synthetic small molecule antivirulents that have been identified from high-throughput screens and on treatments that are effective against the opportunistic human pathogen Pseudomonas aeruginosa.

Type 3 secretion system of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen, mainly affecting severe patients, such as those in intensive care units (ICUs). High levels of antibiotic resistance and a long battery of virulence factors characterise this pathogen. Among virulence factors, the T3SS (Type 3 Secretion Systems) are especially relevant, being one of the most important virulence factors in P. aeruginosa. T3SS are a complex "molecular syringe" able to inject different effectors in host cells, subverting cell machinery influencing immune responses, and increasing bacterial survival rates. While T3SS have been largely studied and the molecular structure and main effector functions have been established, a series of questions and further points remain to be clarified or established. The key role of T3SS in P. aeruginosa virulence has resulted in the search for T3SS-targeting molecules able to impair their functions and subsequently improve patient outcomes. This review aims to summarise the most relevant features of the P. aeruginosa T3SS.

The PopN Gate-keeper Complex Acts on the ATPase PscN to Regulate the T3SS Secretion Switch from Early to Middle Substrates in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic bacterium of which the main virulence factor is the Type III Secretion System. The ATPase of this machinery, PscN (SctN), is thought to be localized at the base of the secretion apparatus and to participate in the recognition, chaperone dissociation and unfolding of exported T3SS proteins. In this work, a protein-protein interaction ELISA revealed the interaction of PscN with a wide range of exported T3SS proteins including the needle, translocator, gate-keeper and effector. These interactions were further confirmed by Microscale Thermophoresis that also indicated a preferential interaction of PscN with secreted proteins or protein-chaperone complex rather than with chaperones alone, in line with the release of the chaperones in the bacterial cytoplasm after the dissociation from their exported proteins. Moreover, we suggest a new role of the gate-keeper complex and the ATPase in the regulation of early substrates recognition by the T3SS. This finding sheds a new light on the mechanism of secretion switching from early to middle substrates in P. aeruginosa.

Structure and function of adenylyl cyclases, key enzymes in cellular signaling

The adenylyl cyclases (ACs) catalyze the production of the ubiquitous second messenger, cAMP, which in turns acts on a number of effectors and thus regulates a plethora of cellular functions. As the key enzymes in the highly evolutionarily conserved cAMP pathway, the ACs control the physiology of the cells, tissues, organs and organisms in health and disease. A comprehensive understanding of the specific role of the ACs in these processes of life requires a deep mechanistic understanding of structure and mechanisms of action of these enzymes. Here we highlight the exciting recent reports on the biochemistry and structure and higher order organization of the ACs and their signaling complexes. These studies have provided the glimpses into the principles of the AC-mediated homeostatic control of cellular physiology.

Structural Biology and Molecular Modeling to Analyze the Entry of Bacterial Toxins and Virulence Factors into Host Cells

Understanding the functions and mechanisms of biological systems is an outstanding challenge. One way to overcome it is to combine together several approaches such as molecular modeling and experimental structural biology techniques. Indeed, the interplay between structural and dynamical properties of the system is crucial to unravel the function of molecular machinery’s. In this review, we focus on how molecular simulations along with structural information can aid in interpreting biological data. Here, we examine two different cases: (i) the endosomal translocation toxins (diphtheria, tetanus, botulinum toxins) and (ii) the activation of adenylyl cyclase inside the cytoplasm (edema factor, CyA, ExoY).

The Modes of Action of MARTX Toxin Effector Domains

Many Gram-negative bacterial pathogens directly deliver numerous effector proteins from the bacterium to the host cell, thereby altering the target cell physiology. The already well-characterized effector delivery systems are type III, type IV, and type VI secretion systems. Multifunctional autoprocessing repeats-in-toxin (MARTX) toxins are another effector delivery platform employed by some genera of Gram-negative bacteria. These single polypeptide exotoxins possess up to five effector domains in a modular fashion in their central regions. Upon binding to the host cell plasma membrane, MARTX toxins form a pore using amino- and carboxyl-terminal repeat-containing arms and translocate the effector domains into the cells. Consequently, MARTX toxins affect the integrity of the host cells and often induce cell death. Thus, they have been characterized as crucial virulence factors of certain human pathogens. This review covers how each of the MARTX toxin effector domains exhibits cytopathic and/or cytotoxic activities in cells, with their structural features revealed recently. In addition, future directions for the comprehensive understanding of MARTX toxin-mediated pathogenesis are discussed.

Differential regulation of actin-activated nucleotidyl cyclase virulence factors by filamentous and globular actin

Several bacterial pathogens produce nucleotidyl cyclase toxins to manipulate eukaryotic host cells. Inside host cells they are activated by endogenous cofactors to produce high levels of cyclic nucleotides (cNMPs). The ExoY toxin from Pseudomonas aeruginosa (PaExoY) and the ExoY-like module (VnExoY) found in the MARTX (Multifunctional-Autoprocessing Repeats-in-ToXin) toxin of Vibrio nigripulchritudo share modest sequence similarity (~38%) but were both recently shown to be activated by actin after their delivery to the eukaryotic host cell. Here, we further characterized the ExoY-like cyclase of V. nigripulchritudo. We show that, in contrast to PaExoY that requires polymerized actin (F-actin) for maximum activation, VnExoY is selectively activated by monomeric actin (G-actin). These two enzymes also display different nucleotide substrate and divalent cation specificities. In vitro in presence of the cation Mg2+, the F-actin activated PaExoY exhibits a promiscuous nucleotidyl cyclase activity with the substrate preference GTP>ATP≥UTP>CTP, while the G-actin activated VnExoY shows a strong preference for ATP as substrate, as it is the case for the well-known calmodulin-activated adenylate cyclase toxins from Bordetella pertussis or Bacillus anthracis. These results suggest that the actin-activated nucleotidyl cyclase virulence factors despite sharing a common activator may actually display a greater variability of biological effects in infected cells than initially anticipated.

The extreme C terminus of the Pseudomonas aeruginosa effector ExoY is crucial for binding to its eukaryotic activator, F-actin

Bacterial nucleotidyl cyclase toxins are potent virulence factors that upon entry into eukaryotic cells are stimulated by endogenous cofactors to catalyze the production of large amounts of 3'5'-cyclic nucleoside monophosphates. The activity of the effector ExoY from Pseudomonas aeruginosa is stimulated by the filamentous form of actin (F-actin). Utilizing yeast phenotype analysis, site-directed mutagenesis, functional biochemical assays, and confocal microscopy, we demonstrate that the last nine amino acids of the C terminus of ExoY are crucial for the interaction with F-actin and, consequently, for ExoY's enzymatic activity in vitro and toxicity in a yeast model. We observed that isolated C-terminal sequences of P. aeruginosa ExoY that had been fused to a carrier protein bind to F-actin and that synthetic peptides corresponding to the extreme ExoY C terminus inhibit ExoY enzymatic activity in vitro and compete with the full-length enzyme for F-actin binding. Interestingly, we noted that various P. aeruginosa isolates of the PA14 family, including highly virulent strains, harbor ExoY variants with a mutation altering the C terminus of this effector. We found that these naturally occurring ExoY variants display drastically reduced enzymatic activity and toxicity. Our findings shed light on the molecular basis of the ExoY-F-actin interaction, revealing that the extreme C terminus of ExoY is critical for binding to F-actin in target cells and that some P. aeruginosa isolates carry C-terminally mutated, low-activity ExoY variants.

Revealing how an adenylate cyclase toxin uses bait and switch tactics in its activation

Dissecting how bacterial pathogens escape immune destruction and cause respiratory infections in humans is a work in progress. One tactic employed by microbes is to use bacterial adenylate cyclase toxins (ACTs) to disarm immune cells and disrupt cellular signaling in host cells, which facilitates the infection process. Several clinically significant pathogens, such as Bacillus anthracis and Bordetella pertussis, have ACTs that are stimulated by an activator protein in human cells. Research has shown that these bacterial ACTs have evolved distinct ways of controlling their activities, but our understanding of how the B. pertussis ACT does this is limited. In a recent study, O’Brien and colleagues provide new and exciting evidence demonstrating that the regulation of B. pertussis ACT involves conformational switching between flexible and rigid states, which is triggered upon binding the host activator protein. This study increases our knowledge of how bacterial ACTs are unique enzymes, representing a potentially novel class of drug targets that may open new pathways to combat reemerging infectious diseases.