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Interfering with outer membrane biogenesis to fight
Gram-negative bacterial pathogens
Raffaele Ieva
To cite this version:
Raffaele Ieva. Interfering with outer membrane biogenesis to fight Gram-negative bacterial pathogens. Virulence, Taylor & Francis, 2017, 8, pp.1049 - 1052. �10.1080/21505594.2017.1296617�. �hal-03039249�
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Virulence
ISSN: 2150-5594 (Print) 2150-5608 (Online) Journal homepage: https://www.tandfonline.com/loi/kvir20
Interfering with outer membrane biogenesis to
fight Gram-negative bacterial pathogens
Raffaele Ieva
To cite this article: Raffaele Ieva (2017) Interfering with outer membrane biogenesis to fight Gram-negative bacterial pathogens, Virulence, 8:7, 1049-1052, DOI: 10.1080/21505594.2017.1296617 To link to this article: https://doi.org/10.1080/21505594.2017.1296617
© 2017 Taylor & Francis
Accepted author version posted online: 17 Feb 2017.
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EDITORIAL
Interfering with outer membrane biogenesis to fight Gram-negative
bacterial pathogens
Raffaele Ieva
Laboratoire de Microbiologie et de Genetique Moleculaires (LMGM), Centre de Biologie Integrative (CBI), Universite de Toulouse, CNRS, UPS, Toulouse, France
ARTICLE HISTORYReceived 10 February 2017; Accepted 10 February 2017
KEYWORDSantivirulence drugs; bacterial envelope; BAM; OMP; outer membrane;Yersinia enterocolitica
The multilayer envelope of Gram-negative bacteria con-sists of an inner and an outer membrane (OM) separated by the periplasmic space and a thin cell wall. The exter-nal leaflet of the OM is mainly composed of lipopolysac-charide (LPS). Thanks to the highly compacted saturated acyl chains of its lipidic moiety, and the covalently linked core oligosaccharide and O-antigen polysaccharide, the LPS forms an effective hydrophobic and hydrophilic bar-rier that protects these microorganisms from external insults, including several antibiotics.1 The OM further harbours an arsenal of protein assemblies that promote interaction with the colonized environment, biofilm for-mation, as well as adhesion to and invasion of host tis-sues.2 Thus accurate biogenesis of the OM is critical for bacterial survival. Pathogens with defects in OM func-tionality are likely to succumb in the competition with bacteria of the surrounding microbiota or may be attenu-ated in their ability to infect the host. Recently, due to difficulties in combating infectious diseases with tradi-tional antibiotics, much effort has been put into develop-ing novel antivirulence drugs that can specifically interfere with mechanisms of pathogenesis.3,4,5 As these
medicaments do not compromise essential functions of bacterial cells, but rather impair the overall fitness of pathogenic strains in the context of the infection process, they are likely to generate a low selective pressure for suppressor mutations. As part of this effort, a great deal of research work is currently focused on deciphering the intricate network of cellular processes that ensure ef fi-cient OM biogenesis in bacteria and its impact on dis-eases. In this issue of Virulence, Weirich et al. shed light on how loss of activity of non-essential proteins involved in OM biogenesis influences the pathogenic potential of
Yersinia enterocolitica,6 an enteropathogen that can be
transmitted from farm animals to humans.7
Two main classes of proteins populate the OM: 1) Lipoproteins, which associate to the membrane via a lipi-dated amino(N)-terminal cysteine residue; 2) Integral OM proteins (OMPs) that span the lipid bilayer by adopting b-barrel structures of variable size (8–26 b-strands) and may additionally carry soluble domains that ultimately localize in the periplasm or on the cell surface.8Both lipoproteins and OMPs are synthesized in the cytosol, transported across the inner membrane and shuttled through the periplasm to their site of assembly at the OM. Due to the amphipathic character of their membrane spanning domains, OMPs are aggregation-prone and their transport across the periplasm is aided by chaperones.9 Some OMPs are preferentially cha-peroned by SurA, while other OMPs can be transported via a pathway assisted by the chaperone Skp in coopera-tion with the protease DegP.10,11In all cases, precursors are delivered to theb-barrel assembly machinery (BAM) for assembly into the OM. BAM is a pentamer in Escheri-chia coli and possibly in other Enterobacteriaceae,12,13,14
which include several pathogens (strains of E. coli, Sal-monella enterica, Shigella flexneri, Klebsiella pneumonia, Yersinia species). The central and essential component BamA spans through the OM via a carboxy(C)-terminal b-barrel domain and interacts via an N-terminal, peri-plasm-disposed domain with partner subunits. BamA associates with the lipoproteins BamBCDE, of which only BamD is essential.13,14 Numerous lines of evidence indicate that the folding and assembly of distinct OMPs are differentially impacted by the lack of a periplasmic chaperone or a non-essential BAM lipoprotein.
CONTACT Raffaele Ieva raffaele.ieva@ibcg.biotoul.fr Laboratoire de Microbiologie et de Genetique Moleculaires (LMGM), Universite de Toulouse, CNRS,
31062 Toulouse, France.
Comment on: Weirich J, et al. Identifying components required for OMP biogenesis as novel targets for antiinfective drugs. Virulence 2017; 1-20; PMID:28118090; https://doi.org/10.1080/21505594.2016.1278333
© 2017 Taylor & Francis
VIRULENCE
2017, VOL. 8, NO. 7, 1049–1052
However, it still remains rather elusive how periplasmic chaperones or BAM lipoproteins contribute to the assembly of different OMPs, despite numerous genetic, biochemical and structural studies.15 Of note, while BamA is widely conserved across different phyla of bac-teria, the set of bacterial lipoproteins or periplasmic fac-tors that stably associate with BamA may differ.14 It is tempting to speculate that the heterogeneity of the BAM complex in different families of bacteria reflects specific requirements in OMP biogenesis and niches coloniza-tion, and that these differences could potentially be exploited in antivirulence therapies to selectively target different microorganisms.
Weirich et al. describe a multi-approach analysis of the consequences produced by the loss of non-essential factors involved in OMP biogenesis in Y. enterocoli-tica.6 The main determinants that contribute to its
pathogenesis are the LPS, the type III secretion system and its effector proteins, and the OMPs Ail (attach-ment and invasion locus), YadA (Yersinia adhesin A) and Inv (invasin).16 The pathogenic potential of Y. enterocolitica strains can be estimated in a mouse infection model, providing the means to assess the contribution of different cellular factors to the infec-tion process. Weirich et al. focus their analysis on the loss of SurA, Skp, DegP, BamB, BamC and BamE. Theyfind that losses of BamB or SurA and, to a lower extent, Skp enhance the cell susceptibility to concen-trations of antibiotics and detergents that are normally tolerated by wild type cells.6 The observed chemical susceptibility of these mutant cells underlines, together with previousfindings, the potential of pharmacologi-cal inactivation of BamB and SurA to enhance the potency of traditional antibiotics.6,17,18,19,20,21 Impor-tantly, by combining semi-quantitative proteomics and measurements of mRNA levels, Weirich at al. further investigate how losses of BamB, SurA and Skp influ-ence the steady-state levels of proteins of the OM. Accumulation of OMP assembly intermediates in the periplasm induces the envelope stress-activated sE response,22which causes upregulation of OMP biogen-esis factors and downregulation of mRNA levels encoding abundant OMPs.23Variations in the levels of
proteins encoded by genes controlled by sE are diffi-cult to interpret as these variations may result from additive effects owing to regulation of mRNA levels and proteostasis. The collected data set published by Weirich et al. is of great interest as it contributes to reveal the regulatory and proteostatic pathways that govern OM biogenesis in Y. enterocolitica. Further-more, comparisons of the data set published here and results obtained using similar approaches on E. coli may help to understand differential strategies adopted
by diverse, albeit evolutionarily related, pathogens in responding to envelope stress. In E. coli for instance, transcripts encoding BAM subunits are activated upon loss of SurA11 via the sE response.23 In Y. enterocoli-tica cells lacking SurA or BamB, the levels of tran-scripts encoding BAM subunits are unaffected, while the amounts of transcripts encoding other OMPs (OmpF and LamB) are reduced, which is most proba-bly due tosE activation.6 The hypothesis that Yersinia species have a reduced sE regulon23 may explain the
observed differential regulation of BAM transcripts. One key finding reported by Weirich et al. concerns the differential effects of lack of SurA or BamB on dis-tinct OMPs, among which autotransporters, a subclass of the type V secretion system. Autotransporters are vir-ulence factors consisting generally of a C-terminal b-bar-rel domain (b-domain) that promotes translocation across the OM of a covalently linked N-terminal passen-ger domain.24,25,26Inv is an inverse autotransporter with a N-terminal b-domain. YadA is a trimeric autotrans-porter. Theb-domains of both Inv and YadA ultimately fold into 12-strand b-barrels, however in the case of YadA 3 identical monomers each contribute 4b-strands to thefinal structure. Weirich et al. find that the levels of Inv detected on the cell surface are reduced in surA¡and bamB¡cells. The levels of the corresponding transcript
are unaffected in surA¡cells,6indicating that biogenesis of Inv strongly depends on SurA. This chaperon has been shown to specifically interact with the passenger domain of a classical autotransporter.27 SurA was also suggested to extensively interact with the passenger domain of an inverse autotransporter.28 At least in the case of classical autotransporters, completion of b-domain assembly into the OM is licensed upon secre-tion of the passenger domain.27,29,30Thus further experi-ments will be necessary to establish whether, in the case of Y. enterocolitica Inv, SurA is mainly required to main-tain the passenger domain in a secretion competent con-formation or to chaperone its b-domain across the periplasm. On the other hand, the data presented by Weirich et al. clearly show that biogenesis of YadA does not depend on SurA nor BamB. The reason for this dif-ferential dependence may relate to the low number of b-strands of the YadA protein monomer.6 This is an
interesting hypothesis in line with the observation that TolC, another trimeric OMP (each TolC monomer con-tributes 4b-strands to a 12-strand b-barrel) is indepen-dent of SurA and BamB.11,31As suggested by the authors and elsewhere, it is conceivable that the small size of the amphipathicb-domain determines a low dependence on the biogenesis factors SurA and BamB.6,32,33In addition, the proposed cooperation of multiple BAM complexes in folding and assembling trimeric b-barrel structures32
might generate a protein rich environment in which YadA can efficiently assemble even in the absence of SurA or BamB.6
Most convincingly, Weirich et al. show that loss of SurA or BamB (or to a lower extent Skp) attenuates the virulence of Y. enterocolitica in both orogastric and sys-temic mouse infection models. Several factors are likely to contribute to these phenotypes at different steps of the infection process. Besides Inv, also the iron uptake recep-tor (FyuA) is reduced in all mutants tested in mouse infection.6 Furthermore, the authors show that the LPS of bamB¡cells lacks O-antigen,6a feature that might sig-nificantly contribute to its serum susceptibility.34,35 The
authors further comment that although the overall bio-genesis on YadA is not affected,6 minor changes in pas-senger domain properties, due to the altered landscape of OM components, may influence its ability to bind complement and promote serum resistance. In sum-mary, Weirich et al. present a multifaceted analysis to identify targets of the OM biogenesis machineries for antivirulence drugs that can attenuate Y. enterocolitica and, potentially, other enterobacterial pathogens. Similar approaches should be extended to mutant strains lacking other components involved in OMP biogenesis, maintenance and in envelope stress response pathways.23,36,37,38,39,40
Disclosure of potential conflicts of interest
No potential conflicts of interest were disclosed.
Acknowledgments
The author thanks Drs. Peter Redder and Harris Bernstein for discussion.
Funding
This work was supported by the ATIP-Avenir program (CNRS).
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