Mutations in the lipid A deacylase PagL which release the enzyme from its latency affect the ability of PagL to interact with lipopolysaccharide in Salmonella enterica serovar Typhimurium

LPS Remodeling Triggers Formation of Outer Membrane Vesicles in Salmonella

Outer membrane vesicles (OMV) are proposed to mediate multiple functions during pathogenesis and symbiosis. However, the mechanisms responsible for OMV formation remain poorly understood. It has been shown in eukaryotic membranes that lipids with an inverted-cone shape favor the formation of positive membrane curvatures. Based on these studies, we formulated the hypothesis that lipid A deacylation might impose shape modifications that result in the curvature of the outer membrane (OM) and subsequent OMV formation. We tested the effect of lipid A remodeling on OMV biogenesis employing Salmonella enterica serovar Typhimurium as a model organism. Expression of the lipid A deacylase PagL resulted in increased vesiculation, without inducing an envelope stress response. Mass spectrometry analysis revealed profound differences in the patterns of lipid A in OM and OMV, with accumulation of deacylated lipid A forms exclusively in OMV. OMV biogenesis by intracellular bacteria upon macrophage infection was drastically reduced in a pagL mutant strain. We propose a novel mechanism for OMV biogenesis requiring lipid A deacylation in the context of a multifactorial process that involves the orchestrated remodeling of the outer membrane.

IMPORTANCE

The role of lipid remodeling in vesiculation is well documented in eukaryotes. Similarly, bacteria produce membrane-derived vesicles; however, the molecular mechanisms underlying their production are yet to be determined. In this work, we investigated the role of outer membrane remodeling in OMV biogenesis in S Typhimurium. We showed that the expression of the lipid A deacylase PagL results in overvesiculation with deacylated lipid A accumulation exclusively in OMV. An S Typhimurium ΔpagL strain showed a significant reduction in intracellular OMV secretion relative to the wild-type strain. Our results suggest a novel mechanism for OMV biogenesis that involves outer membrane remodeling through lipid A modification. Understanding how OMV are produced by bacteria is important to advance our understanding of the host-pathogen interactions.

A proteomic approach of SigX function in Pseudomonas aeruginosa outer membrane composition

SigX is one of the 19 extracytoplasmic function sigma factors that have been predicted in the human opportunistic pathogen Pseudomonas aeruginosa genome. SigX is involved in the transcription of oprF, encoding the major outer membrane protein OprF, a pleiotropic porin that contributes to the maintaining of the wall structure, and is essential to P. aeruginosa virulence. This study aimed to get further insights into the functions of SigX. We performed here an outer membrane subproteome of a sigX mutant. Proteomic investigations revealed lower production of 8 porins among which 4 gated channels involved in iron or hem uptake, OprF, and the three substrate-specific proteins OprD, OprQ and OprE. On the other side, the glucose-specific porin OprB and the lipid A 3-O-deacylase that is involved in LPS modification were up-regulated. Our results indicate that SigX may be involved in the control and/or regulation of the outer membrane composition.

BIOLOGICAL SIGNIFICANCE

A proteomic approach was used herein to get further insights into SigX functions in P. aeruginosa. The data presented here suggest that SigX is involved in the outer membrane protein composition, and could be linked to a regulatory network involved in OM homeostasis.

Deciphering the acylation pattern of Yersinia enterocolitica lipid A

Pathogenic bacteria may modify their surface to evade the host innate immune response. Yersinia enterocolitica modulates its lipopolysaccharide (LPS) lipid A structure, and the key regulatory signal is temperature. At 21°C, lipid A is hexa-acylated and may be modified with aminoarabinose or palmitate. At 37°C, Y. enterocolitica expresses a tetra-acylated lipid A consistent with the 3′-O-deacylation of the molecule. In this work, by combining genetic and mass spectrometric analysis, we establish that Y. enterocolitica encodes a lipid A deacylase, LpxR, responsible for the lipid A structure observed at 37°C. Western blot analyses indicate that LpxR exhibits latency at 21°C, deacylation of lipid A is not observed despite the expression of LpxR in the membrane. Aminoarabinose-modified lipid A is involved in the latency. 3-D modelling, docking and site-directed mutagenesis experiments showed that LpxR D31 reduces the active site cavity volume so that aminoarabinose containing Kdo₂-lipid A cannot be accommodated and, therefore, not deacylated. Our data revealed that the expression of lpxR is negatively controlled by RovA and PhoPQ which are necessary for the lipid A modification with aminoarabinose. Next, we investigated the role of lipid A structural plasticity conferred by LpxR on the expression/function of Y. enterocolitica virulence factors. We present evidence that motility and invasion of eukaryotic cells were reduced in the lpxR mutant grown at 21°C. Mechanistically, our data revealed that the expressions of flhDC and rovA, regulators controlling the flagellar regulon and invasin respectively, were down-regulated in the mutant. In contrast, the levels of the virulence plasmid (pYV)-encoded virulence factors Yops and YadA were not affected in the lpxR mutant. Finally, we establish that the low inflammatory response associated to Y. enterocolitica infections is the sum of the anti-inflammatory action exerted by pYV-encoded YopP and the reduced activation of the LPS receptor by a LpxR-dependent deacylated LPS.

Lipopolysaccharide (LPS) is one of the major surface components of Gram-negative bacteria. The LPS contains a molecular pattern recognized by the innate immune system. Not surprisingly, the modification of the LPS pattern is a virulence strategy of several pathogens to evade the innate immune system. Yersinia enterocolitica causes food-borne infections in animals and humans (yersiniosis). Temperature regulates most, if not all, virulence factors of yersiniae including the structure of the LPS lipid A. At 21°C, lipid A is mainly hexa-acylated and may be modified with aminoarabinose or palmitate. In contrast, at 37°C, Y. enterocolitica expresses a unique tetra-acylated lipid A. In this work, we establish that Y. enterocolitica encodes a lipid A deacylase, LpxR, responsible for the lipid A structure expressed by the pathogen at 37°C, the host temperature. Our findings also revealed that the low inflammatory response associated to Y. enterocolitica infections is the sum of the anti-inflammatory action exerted by a Yersinia protein translocated into the cytosol of macrophages and the reduced activation of the LPS receptor complex due to the expression of a LpxR-dependent deacylated LPS.

Latency of the lipid A deacylase PagL is involved in producing a robust permeation barrier in the outer membrane of Salmonella enterica

Lipid A deacylase PagL, which detoxifies endotoxin, is latent in Salmonella enterica. This study determined the biological significance of this latency. PagL latency was beneficial for bacteria in producing a robust permeation barrier through lipid A modifications under host-mimetic conditions that induced the modification enzymes, including PagL.