Rhodobacter capsulatus O1sA is a bifunctional enyzme active in both ornithine lipid and phosphatidic acid biosynthesis

The Rhodobacter capsulatus genome contains three genes (olsA [plsC138], plsC316, and plsC3498) that are annotated as lysophosphatidic acid (1-acyl-sn-glycerol-3-phosphate) acyltransferase (AGPAT). Of these genes, olsA was previously shown to be an O-acyltransferase in the second step of ornithine lipid biosynthesis, which is important for optimal steady-state levels of c-type cytochromes (S. Aygun-Sunar, S. Mandaci, H.-G. Koch, I. V. J. Murray, H. Goldfine, and F. Daldal. Mol. Microbiol. 61:418-435, 2006). The roles of the remaining plsC316 and plsC3498 genes remained unknown. In this work, these genes were cloned, and chromosomal insertion-deletion mutations inactivating them were obtained to define their function. Characterization of these mutants indicated that, unlike the Escherichia coli plsC, neither plsC316 nor plsC3498 was essential in R. capsulatus. In contrast, no plsC316 olsA double mutant could be isolated, indicating that an intact copy of either olsA or plsC316 was required for R. capsulatus growth under the conditions tested. Compared to OlsA null mutants, PlsC316 null mutants contained ornithine lipid and had no c-type cytochrome-related phenotype. However, they exhibited slight growth impairment and highly altered total fatty acid and phospholipid profiles. Heterologous expression in an E. coli plsC(Ts) mutant of either R. capsulatus plsC316 or olsA gene products supported growth at a nonpermissive temperature, exhibited AGPAT activity in vitro, and restored phosphatidic acid biosynthesis. The more vigorous AGPAT activity displayed by PlsC316 suggested that plsC316 encodes the main AGPAT required for glycerophospholipid synthesis in R. capsulatus, while olsA acts as an alternative AGPAT that is specific for ornithine lipid synthesis. This study therefore revealed for the first time that some OlsA enzymes, like the enzyme of R. capsulatus, are bifunctional and involved in both membrane ornithine lipid and glycerophospholipid biosynthesis

A Complex Extracellular Sphingomyelinase of Pseudomonas aeruginosa Inhibits Angiogenesis by Selective Cytotoxicity to Endothelial Cells

The hemolytic phospholipase C (PlcHR) expressed by Pseudomonas aeruginosa is the original member of a Phosphoesterase Superfamily, which includes phosphorylcholine-specific phospholipases C (PC-PLC) produced by frank and opportunistic pathogens. PlcHR, but not all its family members, is also a potent sphingomyelinase (SMase). Data presented herein indicate that picomolar (pM) concentrations of PlcHR are selectively lethal to endothelial cells (EC). An RGD motif of PlcHR contributes to this selectivity. Peptides containing an RGD motif (i.e., GRGDS), but not control peptides (i.e., GDGRS), block the effects of PlcHR on calcium signaling and cytotoxicity to EC. Moreover, RGD variants of PlcHR (e.g., RGE, KGD) are significantly reduced in their binding and toxicity, but retain the enzymatic activity of the wild type PlcHR. PlcHR also inhibits several EC-dependent in vitro assays (i.e., EC migration, EC invasion, and EC tubule formation), which represent key processes involved in angiogenesis (i.e., formation of new blood vessels from existing vasculature). Finally, the impact of PlcHR in an in vivo model of angiogenesis in transgenic zebrafish, and ones treated with an antisense morpholino to knock down a key blood cell regulator, were evaluated because in vitro assays cannot fully represent the complex processes of angiogenesis. As little as 2 ng/embryo of PlcHR was lethal to ∼50% of EGFP-labeled EC at 6 h after injection of embryos at 48 hpf (hours post-fertilization). An active site mutant of PlcHR (Thr178Ala) exhibited 120-fold reduced inhibitory activity in the EC invasion assay, and 20 ng/embryo elicited no detectable inhibitory activity in the zebrafish model. Taken together, these observations are pertinent to the distinctive vasculitis and poor wound healing associated with P. aeruginosa sepsis and suggest that the potent antiangiogenic properties of PlcHR are worthy of further investigation for the treatment of diseases where angiogenesis contributes pathological conditions (e.g., vascularization of tumors, diabetic retinopathy)

Activation of Cytosolic Phospholipase A\u3csub\u3e2\u3c/sub\u3eα in Resident Peritoneal Macrophages by Listeria Monocytogenes Involves Listeriolysin O and TLR2

Eicosanoid production by macrophages is an early response to microbial infection that promotes acute inflammation. The intracellular pathogen Listeria monocytogenes stimulates arachidonic acid release and eicosanoid production from resident mouse peritoneal macrophages through activation of group IVA cytosolic phospholipase A2 (cPLA2α). The ability of wild type L. monocytogenes (WTLM) to stimulate arachidonic acid release is partially dependent on the virulence factor listeriolysin O; however, WTLM and L. monocytogenes lacking listeriolysin O (ΔhlyLM) induce similar levels of cyclooxygenase 2. Arachidonic acid release requires activation of MAPKs by WTLM and ΔhlyLM. The attenuated release of arachidonic acid that is observed in TLR2-/- and MyD88-/- macrophages infected with WTLM and ΔhlyLM correlates with diminished MAPK activation. WTLM but not ΔhlyLM increases intracellular calcium, which is implicated in regulation of cPLA2α. Prostaglandin E2, prostaglandin I 2, and leukotriene C4 are produced by cPLA 2α+/+ but not cPLA2α-/- macrophages in response to WTLM and ΔhlyLM. Tumor necrosis factor (TNF)-α production is significantly lower in cPLA2α +/+ than in cPLA2α-/- macrophages infected with WTLM and ΔhlyLM. Treatment of infected cPLA 2α+/+ macrophages with the cyclooxygenase inhibitor indomethacin increases TNFα production to the level produced by cPLA 2α-/- macrophages implicating prostaglandins in TNFα down-regulation. Therefore activation of cPLA2α in macrophages may impact immune responses to L. monocytogenes