Proapoptotic effects of P. aeruginosa involve inhibition of surfactant phosphatidylcholine synthesis

Maternal and neonatal risk factors for neonatal respiratory distress syndrome in term neonates in Cyprus: a prospective case-control study

Background

Neonatal respiratory distress syndrome (NRDS) is strongly associated with premature birth, but it can also affect term neonates. Unlike the extent of research in preterm neonates, risk factors associated with incidence and severity of NRDS in term neonates are not well studied. In this study, we examined the association of maternal and neonatal risk factors with the incidence and severity of NRDS in term neonates admitted to Neonatal Intensive Care Unit (NICU) in Cyprus.

Methods

In a prospective, case-control design we recruited term neonates with NRDS and non-NRDS admitted to the NICU of Archbishop Makarios III hospital, the only neonatal tertiary centre in Cyprus, between April 2017–October 2018. Clinical data were obtained from patients’ files. We used univariate and multivariate logistic and linear regression models to analyse binary and continuous outcomes respectively.

Results

During the 18-month study period, 134 term neonates admitted to NICU were recruited, 55 (41%) with NRDS diagnosis and 79 with non-NRDS as controls. In multivariate adjusted analysis, male gender (OR: 4.35, 95% CI: 1.03–18.39, p = 0.045) and elective caesarean section (OR: 11.92, 95% CI: 1.80–78.95, p = 0.01) were identified as independent predictors of NRDS. Among neonates with NRDS, early-onset infection tended to be associated with increased administration of surfactant (β:0.75, 95% CI: − 0.02-1.52, p = 0.055). Incidence of pulmonary hypertension or systemic hypotension were associated with longer duration of parenteral nutrition (pulmonary hypertension: 11Vs 5 days, p < 0.001, systemic hypotension: 7 Vs 4 days, p = 0.01) and higher rate of blood transfusion (pulmonary hypertension: 100% Vs 67%, p = 0.045, systemic hypotension: 85% Vs 55%, p = 0.013).

Conclusions

This study highlights the role of elective caesarean section and male gender as independent risk factors for NRDS in term neonates. Certain therapeutic interventions are associated with complications during the course of disease. These findings can inform the development of evidence-based recommendations for improved perinatal care.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13052-021-01086-5.

Targeting the Type Three Secretion System in Pseudomonas aeruginosa

The injectisome type three secretion system (T3SS) is a major virulence factor in Pseudomonas aeruginosa. This bacterium is responsible for severe infections in immunosuppressed or cystic fibrosis patients and has become resistant to many antibiotics. Inhibitors of T3SS may therefore constitute an innovative therapeutic target. After a brief description of the T3SS and its regulation, this review presents strategies to inhibit T3SS-mediated toxicity and describes the main families of existing inhibitors. Over the past few years, 12 classes of small-molecule inhibitors and two types of antibody have been discovered and evaluated in vitro for their capacity to inhibit T3SS expression or function, and to protect host cells from T3SS-mediated cytotoxicity. While only one small molecule has been tested in vivo, a bifunctional antibody targeting both the translocation apparatus of the T3SS and a surface polysaccharide is currently in Phase II clinical trials.

The Role of Surfactant in Lung Disease and Host Defense against Pulmonary Infections

Pulmonary surfactant is essential for life as it lines the alveoli to lower surface tension, thereby preventing atelectasis during breathing. Surfactant is enriched with a relatively unique phospholipid, termed dipalmitoylphosphatidylcholine, and four surfactant-associated proteins, SP-A, SP-B, SP-C, and SP-D. The hydrophobic proteins, SP-B and SP-C, together with dipalmitoylphosphatidylcholine, confer surface tension-lowering properties to the material. The more hydrophilic surfactant components, SP-A and SP-D, participate in pulmonary host defense and modify immune responses. Specifically, SP-A and SP-D bind and partake in the clearance of a variety of bacterial, fungal, and viral pathogens and can dampen antigen-induced immune function of effector cells. Emerging data also show immunosuppressive actions of some surfactant-associated lipids, such as phosphatidylglycerol. Conversely, microbial pathogens in preclinical models impair surfactant synthesis and secretion, and microbial proteinases degrade surfactant-associated proteins. Deficiencies of surfactant components are classically observed in the neonatal respiratory distress syndrome, where surfactant replacement therapies have been the mainstay of treatment. However, functional or compositional deficiencies of surfactant are also observed in a variety of acute and chronic lung disorders. Increased surfactant is seen in pulmonary alveolar proteinosis, a disorder characterized by a functional deficiency of the granulocyte-macrophage colony-stimulating factor receptor or development of granulocyte-macrophage colony-stimulating factor antibodies. Genetic polymorphisms of some surfactant proteins such as SP-C are linked to interstitial pulmonary fibrosis. Here, we briefly review the composition, antimicrobial properties, and relevance of pulmonary surfactant to lung disorders and present its therapeutic implications.

CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.

Association between Pseudomonas aeruginosa type III secretion, antibiotic resistance, and clinical outcome: a review

Pseudomonas aeruginosa uses a complex type III secretion system to inject the toxins ExoS, ExoT, ExoU, and ExoY into the cytosol of target eukaryotic cells. This system is regulated by the exoenzyme S regulon and includes the transcriptional activator ExsA. Of the four toxins, ExoU is characterized as the major virulence factor responsible for alveolar epithelial injury in patients with P. aeruginosa pneumonia. Virulent strains of P. aeruginosa possess the exoU gene, whereas non-virulent strains lack this particular gene. The mechanism of virulence for the exoU⁺ genotype relies on the presence of a pathogenic gene cluster (PAPI-2) encoding exoU and its chaperone, spcU. The ExoU toxin has a patatin-like phospholipase domain in its N-terminal, exhibits phospholipase A₂ activity, and requires a eukaryotic cell factor for activation. The C-terminal of ExoU has a ubiquitinylation mechanism of activation. This probably induces a structural change in enzymatic active sites required for phospholipase A₂ activity. In P. aeruginosa clinical isolates, the exoU⁺ genotype correlates with a fluoroquinolone resistance phenotype. Additionally, poor clinical outcomes have been observed in patients with pneumonia caused by exoU⁺-fluoroquinolone-resistant isolates. Therefore, the potential exists to improve clinical outcomes in patients with P. aeruginosa pneumonia by identifying virulent and antimicrobial drug-resistant strains through exoU genotyping or ExoU protein phenotyping or both.

Risk of developing pneumonia is enhanced by the combined traits of fluoroquinolone resistance and type III secretion virulence in respiratory isolates of Pseudomonas aeruginosa

OBJECTIVES

To determine the differential association of host characteristics, antimicrobial resistance, and type III secretion system virulence of Pseudomonas aeruginosa isolates with respiratory syndromes in hospitalized adult patients.

DESIGN

Retrospective, cohort study.

SETTING

Community teaching hospital.

PATIENTS

Two hundred eighteen consecutive adult patients with respiratory culture positive for P. aeruginosa between January 2005 to January 2010.

INTERVENTIONS

Medical charts were reviewed to obtain demographic, laboratory, radiographic, and clinical information. Isolates were assayed by polymerase chain reaction for genes encoding the type III secretion system effectors (ExoU, ExoS, and PcrV) and for strain relatedness using randomly amplified polymorphic DNA analysis. Levofloxacin susceptibility was determined by broth microdilution. Patients were grouped by colonization, bronchitis, or pneumonia and were compared for differential risk of developing the clinical syndrome with respect to host and microbial characteristics.

MEASUREMENTS AND MAIN RESULTS

Half of the study cohort (54%, 117 of 218) had pneumonia, 32% (70 of 218) had bronchitis, and 14% (31 of 218) had colonization; in-hospital mortality was 35%, 11%, and 0%, respectively. Host factors strongly associated with pneumonia development were residence in long-term care facility, healthcare-associated acquisition of P. aeruginosa, higher Acute Physiology and Chronic Health Evaluation II score, presence of enteral feeding tube, mechanical ventilation, and recent history of pneumonia. Fluoroquinolone-resistant (57% vs 34%, 16%; p < 0.0001) and multidrug-resistant (36% vs 26%, 7%; p = 0.0045) strains were more likely to cause pneumonia than bronchitis or colonization, respectively. Analysis of host and microbial factors in a multivariate regression model yielded the combined traits of fluoroquinolone resistance and gene encoding the type III secretion system ExoU effector in P. aeruginosa as the single most significant predictor of pneumonia development.

CONCLUSIONS

These results suggest that fluoroquinolone-resistant phenotype in a type III secretion system exoU strain background contributes toward the pathogenesis of P. aeruginosa in pneumonia.

A mechanism for suppression of the CDP-choline pathway during apoptosis

Inhibition of the CDP-choline pathway during apoptosis restricts the availability of phosphatidylcholine (PtdCho) for assembly of membranes and synthesis of signaling factors. The N-terminal nuclear localization signal (NLS) in CTP:phosphocholine cytidylyltransferase (CCT)α is removed during apoptosis but the caspase(s) involved and the contribution to suppression of the CDP-choline pathway is unresolved. In this study we utilized siRNA silencing of caspases in HEK293 cells and caspase 3-deficient MCF7 cells to show that caspase 3 is required for CCTα proteolysis and release from the nucleus during apoptosis. CCTα-Δ28 (a caspase-cleaved mimic) expressed in CCTα-deficient Chinese hamster ovary cells was cytosolic and had increased in vitro activity. However, [³H]choline labeling experiments in camptothecin-treated MCF7 cells and MCF7 cells expressing caspase 3 (MCF7-C3) revealed a global suppression of the CDP-choline pathway that was consistent with inhibition of a step prior to CCTα. In camptothecin-treated MCF7 and MCF7-C3 cells, choline kinase activity was unaffected; however, choline transport into cells was reduced by 30 and 60%, respectively. We conclude that caspase 3-mediated removal of the CCTα NLS contributes minimally to the inhibition of PtdCho synthesis during DNA damage-induced apoptosis. Rather, the CDP-choline pathway is inhibited by caspase 3-independent and -dependent suppression of choline transport into cells.

The role of phosphatidylcholine and choline metabolites to cell proliferation and survival

The reorganization of metabolic pathways in cancer facilitates the flux of carbon and reducing equivalents into anabolic pathways at the expense of oxidative phosphorylation. This provides rapidly dividing cells with the necessary precursors for membrane, protein and nucleic acid synthesis. A fundamental metabolic perturbation in cancer is the enhanced synthesis of fatty acids by channeling glucose and/or glutamine into cytosolic acetyl-CoA and upregulation of key biosynthetic genes. This lipogenic phenotype also extends to the production of complex lipids involved in membrane synthesis and lipid-based signaling. Cancer cells display sensitivity to ablation of fatty acid synthesis possibly as a result of diminished capacity to synthesize complex lipids involved in signaling or growth pathways. Evidence has accrued that phosphatidylcholine, the major phospholipid component of eukaryotic membranes, as well as choline metabolites derived from its synthesis and catabolism, contribute to both proliferative growth and programmed cell death. This review will detail our current understanding of how coordinated changes in substrate availability, gene expression and enzyme activity lead to altered phosphatidylcholine synthesis in cancer, and how these changes contribute directly or indirectly to malignant growth. Conversely, apoptosis targets key steps in phosphatidylcholine synthesis and degradation that are linked to disruption of cell cycle regulation, reinforcing the central role that phosphatidylcholine and its metabolites in determining cell fate.

Impaired pulmonary defense against Pseudomonas aeruginosa in VEGF gene inactivated mouse lung

Repeated bacterial and viral infections are known to contribute to worsening lung function in several respiratory diseases, including asthma, cystic fibrosis, and chronic obstructive pulmonary disease (COPD). Previous studies have reported alveolar wall cell apoptosis and parenchymal damage in adult pulmonary VEGF gene ablated mice. We hypothesized that VEGF expressed by type II cells is also necessary to provide an effective host defense against bacteria in part by maintaining surfactant homeostasis. Therefore, Pseudomonas aeruginosa (PAO1) levels were evaluated in mice following lung-targeted VEGF gene inactivation, and alterations in VEGF-dependent type II cell function were evaluated by measuring surfactant homeostasis in mouse lungs and isolated type II cells. In VEGF-deficient lungs increased PAO1 levels and pro-inflammatory cytokines, TNFα and IL-6, were detected 24 h after bacterial instillation compared to control lungs. In vivo lung-targeted VEGF gene deletion (57% decrease in total pulmonary VEGF) did not alter alveolar surfactant or tissue disaturated phosphatidylcholine (DSPC) levels. However, sphingomyelin content, choline phosphate cytidylyltransferase (CCT) mRNA, and SP-D expression were decreased. In isolated type II cells an 80% reduction of VEGF protein resulted in decreases in total phospholipids (PL), DSPC, DSPC synthesis, surfactant associated proteins (SP)-B and -D, and the lipid transporters, ABCA1 and Rab3D. TPA-induced DSPC secretion and apoptosis were elevated in VEGF-deficient type II cells. These results suggest a potential protective role for type II cell-expressed VEGF against bacterial initiated infection.

Regulation of lung surfactant phospholipid synthesis and metabolism

The alveolar type II epithelial (ATII) cell is highly specialised for the synthesis and storage, in intracellular lamellar bodies, of phospholipid destined for secretion as pulmonary surfactant into the alveolus. Regulation of the enzymology of surfactant phospholipid synthesis and metabolism has been extensively characterised at both molecular and functional levels, but understanding of surfactant phospholipid metabolism in vivo in either healthy or, especially, diseased lungs is still relatively poorly understood. This review will integrate recent advances in the enzymology of surfactant phospholipid metabolism with metabolic studies in vivo in both experimental animals and human subjects. It will highlight developments in the application of stable isotope-labelled precursor substrates and mass spectrometry to probe lung phospholipid metabolism in terms of individual molecular lipid species and identify areas where a more comprehensive metabolic model would have considerable potential for direct application to disease states.

Surfactant phospholipid metabolism

Pulmonary surfactant is essential for life and is composed of a complex lipoprotein-like mixture that lines the inner surface of the lung to prevent alveolar collapse at the end of expiration. The molecular composition of surfactant depends on highly integrated and regulated processes involving its biosynthesis, remodeling, degradation, and intracellular trafficking. Despite its multicomponent composition, the study of surfactant phospholipid metabolism has focused on two predominant components, disaturated phosphatidylcholine that confers surface-tension lowering activities, and phosphatidylglycerol, recently implicated in innate immune defense. Future studies providing a better understanding of the molecular control and physiological relevance of minor surfactant lipid components are needed. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Surfactant and its role in the pathobiology of pulmonary infection

Pulmonary surfactant is a complex surface-active substance comprised of key phospholipids and proteins that has many essential functions. Surfactant's unique composition is integrally related to its surface-active properties, its critical role in host defense, and emerging immunomodulatory activities ascribed to surfactant lipids. Together these effector functions provide for lung stability and protection from a barrage of potentially virulent infectious pathogens.

Calmodulin antagonizes a calcium-activated SCF ubiquitin E3 ligase subunit, FBXL2, to regulate surfactant homeostasis

Calmodulin is a universal calcium-sensing protein that has pleiotropic effects. Here we show that calmodulin inhibits a new SCF (Skp1-Cullin-F-box) E3 ligase component, FBXL2. During Pseudomonas aeruginosa infection, SCF (FBXL2) targets the key enzyme, CCTα, for its monoubiquitination and degradation, thereby reducing synthesis of the indispensable membrane and surfactant component, phosphatidylcholine. P. aeruginosa triggers calcium influx and calcium-dependent activation of FBXL2 within the Golgi complex, where it engages CCTα. FBXL2 through its C terminus binds to the CCTα IQ motif. FBXL2 knockdown increases CCTα levels and phospholipid synthesis. The molecular interaction of FBXL2 with CCTα is opposed by calmodulin, which traffics to the Golgi complex, binds FBXL2 (residues 80 to 90) via its C terminus, and vies with the ligase for occupancy within the IQ motif. These observations were recapitulated in murine models of P. aeruginosa-induced surfactant deficiency, where calmodulin gene transfer reduced FBXL2 actions by stabilizing CCTα and lessening the severity of inflammatory lung injury. The results provide a unique model of calcium-regulated intermolecular competition between an E3 ligase subunit and an antagonist that is critically relevant to pneumonia and lipid homeostasis.

The type III secretion system of Pseudomonas aeruginosa: infection by injection

The Gram-negative bacterium Pseudomonas aeruginosa uses a complex type III secretion apparatus to inject effector proteins into host cells. The configuration of this secretion machinery, the activities of the proteins that are injected by it and the consequences of this process for infection are now being elucidated. This Review summarizes our current knowledge of P. aeruginosa type III secretion, including the secretion and translocation machinery, the regulation of this machinery, and the associated chaperones and effector proteins. The features of this interesting secretion system have important implications for the pathogenesis of P. aeruginosa infections and for other type III secretion systems.

Role of Pseudomonas aeruginosa type III effectors in disease

Pseudomonas aeruginosa uses a type III secretion system (T3SS) to directly inject four known effectors into host cells. ExoU is a potent cytotoxin with phospholipase A2 activity that causes rapid necrotic death in many cell types. The biological function of ExoY, an adenylate cyclase, remains incompletely defined. ExoS and ExoT are closely related bifunctional proteins with N-terminal GTPase activating protein (GAP) activity toward Rho family proteins and C-terminal ADP ribosylase (ADPRT) activity toward distinct and non-overlapping set of targets. While almost no strain encodes or secretes all four effectors, the commonly found combinations of ExoU/ExoT or ExoS/ExoT provides redundant and failsafe mechanisms to cause mucosal barrier injury, inhibit many arms of the innate immune response, and prevent wound repair.

15-deoxy-Delta12,14-prostaglandin J2 impairs phosphatidylcholine synthesis and induces nuclear accumulation of thiol-modified cytidylyltransferase

Synthesis of phosphatidylcholine, the major phospholipid of animal cell membranes, requires the key enzyme cytidylyltransferase (CCTalpha). Cysteine sulfhydryls within CCTalpha are needed for full catalytic activity. Here we show that prostaglandin 15-deoxy-Delta12,14-PGJ2 (15d-PGJ2) inactivates CCTalpha by inducing generation of reactive oxidant species and the appearance of a cross-linked CCTalpha dimer in cells. N-Acetyl-l-cysteine reduced oxidative stress, prevented CCTalpha cross-linking, and restored CCT function in 15d-PGJ2-treated cells. 15d-PGJ2 modified critical cysteine residues within CCTalpha as determined by mutagenesis studies and by incorporation of biotin-15d-PGJ2 into CCTalpha. These effects of 15d-PGJ2 were associated with CCTalpha accumulation within the nucleus. The data indicate that bioactive prostanoids significantly impair membrane phospholipid production by promoting cysteine cross-bridging within CCTalpha.

Calmodulin binds and stabilizes the regulatory enzyme, CTP: phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCTalpha) is a proteolytically sensitive enzyme essential for production of phosphatidylcholine, the major phospholipid of animal cell membranes. The molecular signals that govern CCTalpha protein stability are unknown. An NH(2)-terminal PEST sequence within CCTalpha did not serve as a degradation signal for the proteinase, calpain. Calmodulin (CaM) stabilized CCTalpha from calpain proteolysis. Adenoviral gene transfer of CaM in cells protected CCTalpha, whereas CaM small interfering RNA accentuated CCTalpha degradation by calpains. CaM bound CCTalpha as revealed by fluorescence resonance energy transfer and two-hybrid analysis. Mapping and site-directed mutagenesis of CCTalpha uncovered a motif (LQERVDKVK) harboring a vital recognition site, Gln(243), whereby CaM directly binds to the enzyme. Mutagenesis of CCTalpha Gln(243) not only resulted in loss of CaM binding but also led to complete calpain resistance in vitro and in vivo. Thus, calpains and CaM both access CCTalpha using a structurally similar molecular signature that profoundly affects CCTalpha levels. These data suggest that CaM, by antagonizing calpain, serves as a novel binding partner for CCTalpha that stabilizes the enzyme under proinflammatory stress.

Chronic Pseudomonas aeruginosa infection reduces surfactant levels by inhibiting its biosynthesis

Chronic Pseudomonas aeruginosa infection, as occurs in cystic fibrosis, is associated with decreased surfactant phospholipid levels. To investigate mechanisms, we measured synthesis of dipalmitoylphosphatidylcholine (DPPC), the major surfactant phospholipid. Mice received an agarose bead slurry alone, or were infected with beads containing a clinical mucoid isolate of P. aeruginosa. Bacterial infection after 3 days resulted in a approximately 50% reduction in surfactant DPPC content versus control. These changes in surfactant were associated with co-ordinate reductions in mRNAs and immunoreactive levels for CTP: phosphocholine cytidylyltransferase (CCTalpha), the rate-regulatory enzyme required for DPPC synthesis. P. aeruginosa infection of murine lung epithelia decreased CCTalpha gene transcription without altering mRNA stability and by a mechanism other than release of a soluble extracellular inhibitor. Promoter deletional analysis revealed that P. aeruginosa activates a negative response element from -1019 to -799 bp of the CCTalpha proximal 5'-flanking region. Exposure of cells to a P. aeruginosa mutant strain producing alginate reduced CCTalpha promoter activity, whereas these effects were not observed in strains defective in alginate synthesis. Murine type II cells isolated from P. aeruginosa-infected CCTalpha promoter-beta-galactosidase transgenic mice exhibited significantly reduced CCT and beta-galactosidase enzyme activities versus control. Thus, a mucoid P. aeruginosa strain reduces mRNA synthesis of a key biosynthetic enzyme thereby decreasing levels of surfactant.