Detection of enteric pathogens by the nodosome

Detection of enteric pathogens by thenodosomeA Marijke Keestra and Andreas J Ba umler

Department of Medical Microbiology and Immunology School of Medicine University of California at Davis One Shields Avenue

Davis CA 95616 USA

Review

Nucleotide-binding oligomerization domain protein(NOD)1 and NOD2 participate in signaling pathways thatdetect pathogen-induced processes such as the pres-ence of peptidoglycan fragments in the host cell cytosolas danger signals Recent work suggests that peptido-glycan fragments activate NOD1 indirectly through ac-tivation of the small Rho GTPase Ras-related C3botulinum toxin substrate 1 (RAC1) Excessive activationof small Rho GTPases by virulence factors of entericpathogens also triggers the NOD1 signaling pathwayMany enteric pathogens use virulence factors that alterthe activation state of small Rho GTPases thereby ma-nipulating the host cell cytoskeleton of intestinal epithe-lial cells to promote bacterial attachment or entry Thesedata suggest that the NOD1 signaling pathway in intes-tinal epithelial cells provides an important sentinel func-tion for detecting lsquobreaking and enteringrsquo by entericpathogens

Distinguishing friend from foe in the gutThe human intestine is host to a large microbial commu-nity that provides benefit by conferring niche protectionand educating the immune system An excessive immuneresponse against this beneficial microbial community mustbe avoided to prevent chronic intestinal inflammationMechanisms that restrict immune responses against theresident microbial community include limiting the pres-ence at the luminal surface of innate immune receptorssuch as Toll-like receptors (TLRs) which recognize patho-gen-associated molecular patterns (PAMPs) effectivelydistinguishing microbes from self For example expressionof TLR5 which recognizes bacterial flagellin is restrictedto the basolateral surface of epithelial cells [1] In additionresident macrophages in the mucosal tissue lack expres-sion of the lipopolysaccharide (LPS) detector TLR4 and itsco-receptor CD14 thus making them poor inducers ofinflammatory cytokine production [2]

At the same time however the intestinal mucosa mustmaintain the ability to mount adequate antimicrobial

1471-4906$ ndash see front matter

Corresponding author Baumler AJ (ajbaumlerucdavisedu)Keywords Nod-like receptors patterns of pathogenesis pathogen detection type IIIsecretions system

responses when enteric pathogens enter the resident mi-crobial community To distinguish pathogens from com-mensal microbes the innate immune system detectspathogen-induced processes such as the presence of mi-crobial products in the cytosol of epithelial cells [3] Thesepathogen-induced processes are detected by nucleotide-binding oligomerization domain-like receptors (NLRs) agroup of innate immune receptors located in the host cellcytosol [4] Two of these NLRs NOD1 and NOD2 are majorcontributors to responses against bacterial pathogens inthe gastrointestinal tract This article aims to summarizerecent advances in our understanding of how NOD pro-teins enable the host to gauge the pathogenic potential ofintestinal microbes by detecting the activation state ofsmall Rho GTPases

NOD proteins in pathogen detection and maintenanceof gut immune homeostasisNOD1 is expressed constitutively in intestinal epithelialcells [5] where it helps to monitor the integrity of thecytosol and respond to the presence of enteric pathogens[6] NOD1 contributes to nuclear factor (NF)-kB activationelicited by several enteric bacterial pathogens includingShigella flexneri [7] enteroinvasive Escherichia coli [5]Campylobacter jejuni [8] Helicobacter pylori [910] andSalmonella enterica serotype Typhimurium (S Typhimur-ium) [1112] a process initiating proinflammatoryresponses Furthermore NOD1 along with a second pro-tein involved in monitoring the integrity of epithelialcytosol NOD2 is critical for the autophagic response toinvasive enteric pathogens [13]

Although NOD2 expression was initially thought to berestricted to monocytes [14] it is now clear that thisprotein is also expressed in Paneth cells of the intestinalepithelium [15] A loss-of-function mutation in the humanNOD2 gene is associated with an increased risk of devel-oping Crohnrsquos disease [16] and reduced ileal expression ofhuman defensin (HD)-5 and HD-6 two antimicrobial pep-tides produced by Paneth cells [17] HD-5 has bactericidalactivity [18] and HD-6 prevents invasion of enteric patho-gens through the formation of peptide nanonets whichresults from an ordered self-assembly of the defensin intofibrils [19] Paneth cells of mice produce related antimicro-bial peptides known as cryptins Interestingly expressionof cryptins is abrogated in NOD2-deficient mice [15] Theopportunistic pathogen Helicobacter hepaticus triggersgranulomatous inflammation of the ileum of NOD2-defi-cient mice and inflammatory responses can be reduced by

Trends in Immunology March 2014 Vol 35 No 3 123

Review Trends in Immunology March 2014 Vol 35 No 3

transgenic expression of a-defensin in Paneth cells [20]These data highlight the importance of NOD2 expressionin Paneth cells of the crypt epithelium for maintaininghomeostasis of the ileal mucosa

Collectively these studies highlight the importance ofNOD1 and NOD2 in the detection and control of pathogensat the epithelial surface To understand immune homeo-stasis it is important to be familiar with the signalsactivating this surveillance system and to know the iden-tities of host proteins participating in the NOD1NOD2signaling pathway

Formation of the nodosomeThe NOD1 and NOD2 signaling pathways can be triggeredby the presence of peptidoglycan fragments in the host cellcytosol which might be indicative of bacterial invasion orescape of bacteria from a pathogen-containing vacuoleDiaminopimelic-acid-type peptidoglycan fragments de-rived from Gram-negative bacteria such as g-D-gluta-myl-meso-diaminopimelic acid (iE-DAP) lead to theactivation of the NOD1 signaling pathway [2122] whereasmuramyl dipeptide (MurNAc-L-Ala-D-isoGln) ndash a peptido-glycan motif common to all bacteria ndash activates the NOD2signaling pathway [2324]

Upon stimulation with peptidoglycan NOD1 or NOD2form a protein complex termed the nodosome which con-tains receptor-interacting protein (RIP)2 [2526] Recruit-ment of RIP2 to the nodosome leads to activation of NF-kBand mitogen-activated protein (MAP) kinases [72728]thereby inducing proinflammatory and antimicrobialresponses [51529] Furthermore NOD1 and NOD2 inter-act with BID BH3 interacting-domain death agonist aBCL2 family protein thereby providing a link betweeninflammation and programmed cells death [30] Severalstudies have suggested that recruitment of NOD1 andNOD2 to the cell membrane is required for the formationof a functional nodosome For example NOD1 is recruited tothe membrane at the site of bacterial entry during S flexneriinfection where it colocalizes with NF-kB essential modula-tor (NEMO) a key component of the NF-kB signaling com-plex [31] Similarly recruitment of NOD2 to the membraneis required for responding to stimulation of intestinal epi-thelial cells with muramyl dipeptide [32] NOD2 promotesthe membrane recruitment of RIP2 [33] leading to ubiqui-tinylation of RIP2 and NEMO [3435] The nodosome con-tains the co-chaperone-like ubiquitin-ligase-associatedprotein SGT1 suppressor of G2 allele of SKP1 which isrequired for NF-kB activation [3637] presumably becauseit activates signaling components by assisting in the ubiqui-tinylation of NOD1 RIP2 andor NEMO Additional reg-ulators acting downstream of NOD2 in the signaling cascadehave been identified in genome-wide small interfering RNA(siRNA) screens [3839] However the functional mecha-nism leading to a membrane association of NOD1 and NOD2has long remained elusive

Recently a siRNA screen identified FRMPD2 FERMand PDZ domain containing 2 as a membrane-scaffoldingcomplex that supports a basolateral localization of NOD2in epithelial cells [38] Furthermore small Rho GTPaseswere identified as components of the NOD1 nodosomewhich provides a plausible explanation for its assembly

at the membrane [40] Small Rho GTPases are a family ofmembrane-bound signaling proteins that transition be-tween an inactive GDP-bound form and an active GTP-bound form The transition from an inactive to an activeform is catalyzed by guanosine nucleotide-exchange factors(GEFs) whereas GTPase-activating proteins (GAPs) accel-erate GTP hydrolysis thereby facilitating transition froman active to an inactive form of small Rho GTPases [41]The small Rho GTPases studied most extensively includeRAC1 cell division control protein 42 homolog (CDC42)and Ras homolog gene family member A (RHOA) Activeforms of the small Rho GTPases RAC1 CDC42 and RHOAcan activate NF-kB [42] Recent evidence suggests thatNF-kB activation mediated by small Rho GTPases requiresfunctional NOD1 RIP1 and RIP2 proteins [4043]

NOD2 colocalizes with the activated form of RAC1 inmembrane ruffles indicating that this Rho GTPase mightbe present in nodosomes [44] Furthermore NOD2 coloca-lizes with GEF-H1 a GEF for RHOA [45] Evidence for aninteraction of NOD1 with the activated forms of RAC1 andCDC42 comes from the observation that these proteins canbe coimmunoprecipitated [40] Similarly the small RhoGTPase RAC2 colocalizes with RIP2 in membrane rufflesand both proteins are present in a protein complex [43] Inplants the heat shock protein (HSP)90 chaperone bindsRAC1 in innate immune protein complexes [46] and thisinteraction is thought to function in the recruitment ofinnate immune receptors to the membrane-bound smallRho GTPase [47] Interestingly the nodosome of mammalscontains HSP90 [48] the presence of which is required forNF-kB activation mediated by NOD1 [40] and NOD2 [37]These observations raise the possibility that similar to itsfunction in plants HSP90 of mammals associates withNOD1 and NOD2 and functions in recruiting the resultingprotein complexes to the activated forms of membrane-bound small Rho GTPases [48] The membrane localizationof small Rho GTPases is essential for their ability toactivate the nodosome because a constitutively active formof RAC1 lacking its prenyl-group is unable to activateNOD1 [40] Thus the picture emerging from these studiesis that excessive activation of the membrane-bound pro-teins RAC1 CDC42 or RHOA leads to assembly of thenodosome at the plasma membrane a process that ulti-mately results in NF-kB activation The finding that acti-vation of RAC1 CDC42 and RHOA triggers the NOD1signaling pathway has important ramifications becausesmall Rho GTPases are well-known targets of bacterialvirulence factors [49] (Box 1)

Detection of bacterial virulence factors by thenodosomeVirulence factors activating small Rho GTPases are partic-ularly common among enteric pathogens (Figure 1) Forexample many type III secretion systems (T3SSs) of entericpathogens inject proteins termed effectors into host cellsthat activate small Rho GTPases The T3SS effectors IpgB1from S flexneri EspT from Citrobacter rodentium and SopEfrom S Typhimurium serve as GEFs that activate RAC1and CDC42 [50ndash52] The T3SS effectors Map from entero-pathogenic E coli and SopE2 from S Typhimurium serve asGEFs that activate CDC42 [5053] Furthermore EspM2

Box 1 Small Rho GTPases are common targets for bacterial

virulence factors

Over the past two decades manipulation of small Rho GTPases has

emerged as a conserved pattern in bacterial pathogenesis (reviewed

in [49]) For example the T3SS encoded by Salmonella pathogeni-

city island 1 triggers actin rearrangements and NF-kB activation in

host cells [6970] by injecting effector proteins into the cytosol that

activate Rac1 and CDC42 [52] In addition to the many effector

proteins and toxins from a variety of enteric pathogens which

activate small Rho GTPases (see Figure 1 in main text) there are

multiple examples of virulence factors that inactivate these

molecular switches For instance Clostridium difficile toxin A and

B glycosylate Rho GTPases thereby inhibiting their function

[101102] The T3SS effector proteins ExoS of Pseudomonas

aeruginosa YopE of Yersinia pseudotuberculosis and SptP of S

Typhimurium inhibit the function of small Rho GTPases by acting as

GAPs [103ndash106]

Small Rho GTPases mediate actin polymerization and NF-kB

activation through two distinct pathways [107] A model for the

pathway leading to NF-kB activation is shown in Figure 1 main text

Interestingly some virulence factors of enteric pathogens target

components of the signaling pathway that are located downstream

of small Rho GTPases For example the S Typhimurium T3SS

effector protein SspH2 Salmonella secreted protein H2 cooperates

with the NLR co-chaperone SGT1 to ubiquitinate NOD1 thereby

inducing IL-8 secretion [108] (see Figure 1 in main text) In case of

enteropathogenic Escherichia coli the T3SS effector proteins NleD

non-LEE-encoded effector D and NleC function in silencing this

signaling pathway by cleaving C-Jun N-terminal kinase (JNK) and

the NF-kB p65 subunit respectively [109110]

from enterohemorrhagic E coli and EspM2 from C roden-tium serve as GEFs for RHOA [54] The T3SS effectorsIpgB2 and OspB from S flexneri activate GEF-H1 whichin turn activates RHOA [5055] The S Typhimurium inosi-tol phosphatase SopB alters the inositol phosphate (IP)metabolism of the host cell thereby indirectly activatingCDC42 [5657] Recent studies have suggested that VAV2can activate CDC42 after it senses changes in the IP metab-olism [58] which suggest a possible mechanism for SopB-mediated CDC42 activation Cytotoxic necrotizing factor(CNF)1 ndash a toxin produced by uropathogenic E coli strainsndash constitutively activates RAC1 by inactivating its GTPaseactivity through deamidation of a glutamine residue [59ndash61] Finally Cam jejuni secretes Cia proteins through itsflagellar apparatus [62ndash65] and these proteins are ulti-mately translocated into the cytosol of host cells [66] wherethey contribute to activation of RAC1 [67] A second path-way by which Cam jejuni elicits RAC1 activation is CadF-mediated binding of fibronectin to the bacterial surfacewhich induces a b1-integrin-dependent signaling pathwayleading to the activation of two GEFs termed dedicator ofcytokinesis (DOCK)180 and T cell lymphoma invasion andmetastasis (TIAM)-1 [68] (Figure 1)

It is well established that activation of small RhoGTPases by bacterial virulence factors leads to inductionof MAP kinase signaling and nuclear translocation of NF-kB [526169ndash74] but evidence that the NOD1RIP2RIP1signaling pathway might be involved in this process hasemerged only recently The first indication that bacterialvirulence factors might activate the nodosome was theobservation that engagement of GEF-H1 by the S flexneriT3SS effectors IpgB2 and OspB led to NF-kB activationthat was NOD1 dependent [55] The S Typhimurium T3SSeffector SipA was subsequently shown to induce NF-kB

activation through a NOD1NOD2-dependent signalingpathway [12] Work with the E coli CNF-1 toxin estab-lished a clear role for RAC2 in inducing RIP1RIP2-depen-dent NF-kB activation [43] Finally activation of RAC1 bythe S Typhimurium T3SS effector SopE was shown to leadto assembly of the NOD1 nodosome at the host cell mem-brane thereby inducing NF-kB activation [40] The gua-nosine nucleotide-exchange factor activity of SopE isrequired for NF-kB activation [75] and for the recruitmentof NOD1 to membrane ruffles [40] The observation thatSopE can activate inflammasome-dependent cytokine mat-uration [7677] might be a consequence of nodosome acti-vation because NOD1 is known to enhance interleukin(IL)-1b secretion [78] IL-1b secretion requires two signalsThe first signal stimulates TLRs or NOD proteins therebydriving NF-kB-dependent expression of the IL1B genewhich leads to the production of pro-IL-1b The secondsignal stimulates NLRs to activate caspase-1 which inturn cleaves pro-IL-1b into its active form NOD1-depen-dent IL-1b secretion [78] is likely an indirect consequenceof NF-kB activation (signal 1) rather than resulting from adirect activation of caspase-1 (signal 2)

Not all effector proteins that activate small Rho GTPasesdo necessarily activate NF-kB For example SopE andSopE2 are both GEFs for RAC1 [53] However althoughan S Typhimurium strain translocating only SopE inducesproinflammatory responses in host cells [40] a strain trans-locating only SopE2 does not [73] Thus the signal detectedby NOD1 might be an excessive or prolonged activation ofsmall Rho GTPases which is observed for instance withconstitutively active forms of RAC1 CDC42 and RHOA[42] This model suggests that proinflammatory responsesare not observed when Rho GTPases are activated duringprocess involved in normal cell physiology

Collectively these data suggest that excessive activa-tion of small Rho GTPases by bacterial virulence factors orby the presence of peptidoglycan fragments in the cytosolare pathogen-induced processes that activate the NOD1signaling pathway Although the detection of peptidogly-can fragments in the host cell cytosol evidently representsan innate immune surveillance mechanism the situationis less clear-cut in case of bacterial virulence factors Onebenefit to the pathogen of activating small Rho GTPases isthat this process leads to cytoskeletal rearrangements(reviewed in [79]) promoting bacterial entry into host cells(reviewed in [80]) or the formation of pedestals on epithe-lial surfaces (reviewed in [81]) A byproduct of deployingvirulence factors to induce these actin rearrangementswould in turn be the induction of proinflammatoryresponses that are aimed at killing microbes [82] whichcould be viewed as the cost of doing business By this logicmonitoring the activation state of Rho GTPases could beconsidered a NOD1-dependent immune surveillance mech-anism that sets off an alarm when it detects breaking andentering by enteric pathogens This is reminiscent of effec-tor-triggered immune responses in plants which detectbacterial virulence factors either directly or through theireffects on host targets (reviewed in [8384])

An alternative but not mutually exclusive interpreta-tion of the data would be that enteric pathogens deployvirulence factors that activate the NOD1 pathway because

Intesnal lumenCadF

Integrinβ1

GEF-H1

EspM2 SopB Map IpgB1 Pepdo-glycan

PI3-kinase

SopE2 EspT

SopERAC1

DOCK180

TIAM-1

RHOA CDC42 RAC1

HSP90NOD1

RIP1RIP2

NEMOSGT1

IκB NF-κB

JNK ERK12

NF-κB AP-1

Epithelialcell Nucleus

Proinflammatorygene expression

GTP GTPCNF1

GDP GDP GDP

Nodosome

GTP GTP GTP

GDP GDPGDP

TRENDS in Immunology

Figure 1 Model for activation of the Nucleotide-binding oligomerization domain protein (NOD)1 nodosome in epithelial cells by diaminopimelic acid-type peptidoglycan or

bacterial virulence factors which requires excessive or prolonged activation of small Rho GTPases Peptidoglycan (blue box) or bacterial virulence factors (blue circles)

engage host proteins (red circles) to activate (arrows) the nodosome thereby inducing the expression of proinflammatory genes AP-1 activator protein-1 CDC42 cell

division control protein 42 homolog CNF1 cytotoxic necrotizing factor 1 DOCK dedicator of cytokinesis ERK extracellular signal-regulated kinase FAK focal adhesion

kinase Fn fibronectin GEF guanosine nucleotide-exchange factor HSP90 heat shock protein 90 IkB nuclear factor of kappa light polypeptide gene enhancer in B cells

inhibitor Ikk I kB kinase JNK C-Jun N-terminal kinase NEMO NF-kB essential modulator NF-kB nuclear factor-kB PI3 phosphoinositide 3 PIP phosphatidylinositol

phosphate RAC1 Ras-related C3 botulinum toxin substrate 1 RHOA Ras homolog gene family member A RIP receptor-interacting protein SGT1 suppressor of G-two

allele of Skp1 SspH2 Salmonella secreted protein H2 TIAM-1 T cell lymphoma invasion and metastasis-1

they benefit from the ensuing inflammatory host responseThis viewpoint is supported by recent data suggesting thatintestinal inflammation confers a fitness advantage uponS Typhimurium E coli and C rodentium during theircompetition with the obligate anaerobic microbial commu-nity that dominates the healthy gut [85ndash87] A bloom of STyphimurium and E coli in the inflamed gut is supportedby host-derived electron acceptors which are generated asbyproducts of the inflammatory host response [87ndash89]Interestingly the SopE protein which is present in onlya subset of S Typhimurium isolates alters host responsesby increasing expression of inducible NO synthase (iNOS)an enzyme that produces NO Conversion of NO to nitrateincreases growth of SopE-producing S Typhimuriumstrains in the intestinal lumen through nitrate respiration[88] Enhanced growth of S Typhimurium in the intestinallumen increases transmission of the pathogen [90] whichrepresents a potent selective force for the evolution ofenteric pathogens In other words the reason for deployingvirulence factors that trigger the NOD1 signaling pathwaymight be that the ensuing host response selectively feedsthe respective enteric pathogens

In summary manipulation of small Rho GTPases bybacterial virulence factors is a common theme in microbialpathogenesis (Box 1) and detection of these pathogen-induced processes by the innate immune system enablesthe host to distinguish pathogens from microbes with lowerpathogenic potential From an evolutionary point of view itis also interesting to note that detection of bacterial effec-tor proteins is a feature of the innate immune system thatis conserved from plants to mammals This observationraises the question whether signaling pathways involvedin detecting these patterns of pathogenesis are also con-served between the different phyla

Evolution of the NOD1 signaling pathwayNOD1 and NOD2 of mammals are thought to have evolvedindependently from plant NLRs suggesting that function-al similarities and the presence of similar protein domains(ie the presence of similar C-terminal leucine-richrepeats and similar central nucleotide-binding oligomeri-zation motifs) are features that developed independently indifferent lineages Furthermore the Drosophila melano-gaster genome does not encode homologs of the mammalianNOD1 or NOD2 proteins (reviewed in [9192]) Insteaddiaminopimelic acid-type peptidoglycan from Gram-nega-tive bacteria is detected in D melanogaster through theimmune deficiency (IMD) pathway which shares homologyto the tumor necrosis factor (TNF)-a signaling pathway inmammals [93] In the IMD pathway peptidoglycan isdetected by the transmembrane receptor peptidoglycan-recognition protein (PGRP)-LC [94] or the cytoplasmicreceptor PGRP-LE [95] Upon binding of peptidoglycanPGRP-LC and PGRP-LE activate IMD a death domainprotein with homology to RIP1 the RIP kinase associatedwith TNF-a-dependent NF-kB activation [96] In turnIMD activates the transcription factor Relish which is amember of the NF-kB family [97]

Interestingly recent studies raise the possibility thatthe IMD and NOD1 signaling pathways represent evolu-tionary conserved signaling cascades for the detection of

diaminopimelic acid-type peptidoglycan (reviewed in [98])First the E coli CNF-1 toxin constitutively activatesRAC2 in Drosophila cells thereby inducing the IMD path-way [43] and diaminopimelic acid-type peptidoglycan acti-vates the NOD1-signalling pathway in human cellsthrough a RAC1-dependent mechanism [40] Thus a re-quirement for small Rho GTPases is a feature sharedbetween the IMD and NOD1 pathways but absent fromthe TNF-a signaling pathway Second human RAC2induces NF-kB activation through RIP1 and RIP2 [43]These data suggest that the IMD homolog RIP1 is not onlyinvolved in TNF-a signaling but also participates indetecting the activation state of small Rho GTPases whichconfirms the similarities between the IMD and NOD1signaling pathways There are however some notable dif-ferences between the IMD and NOD1 pathways includingthe absence of a NOD1 homolog in insects and the fact thatmammalian PGRPs do not function in the NOD1 signalingpathway [99]

In summary recent evidence provides some support forthe idea that the signaling pathways detecting diamino-pimelic acid-type peptidoglycan in mammals and insectsshare a common ancestry (reviewed in [98]) This observa-tion along with the finding that the Drosophila Toll proteinand the homologous mammalian TLRs both function ascellular sensors of microbes highlights the conservation ofinnate immune surveillance mechanism within the animalkingdom

Concluding remarksThe picture emerging from these studies is that NODproteins provide epithelial cells with important sentinelfunctions for detecting pathogen-induced violations of theintegrity of the host cell cytoplasm Many enteric patho-gens produce virulence factors that manipulate the activityof small Rho GTPases to mediate attachment or entry or toinhibit phagocytosis (Figure 1 and Box 1) As a result thehost can detect the presence of pathogens by monitoringthe activation state of small Rho GTPases The down-stream signaling pathways enable the intestinal epitheli-um to mount responses that are appropriate to the threatincluding NF-kB activation activation of the autophagypathway and the release of antimicrobial peptides Theability of the epithelium to react appropriately is impor-tant for balancing host responses against microbial com-munities in the gastrointestinal tract When thesemechanisms for maintaining immune homeostasis aredisrupted by mutations that impair NOD2 signaling orthe autophagy pathway the host has an increasing risk ofdeveloping intestinal inflammation

Direct evidence for the binding of NOD1 to peptidogly-can is lacking therefore it has been speculated that thisprotein might function in signal transduction rather thanserving as an innate immune receptor for peptidoglycan[100] Interestingly NOD1-dependent NF-kB activationelicited by diaminopimelic acid-type peptidoglycanrequires the presence of active RAC1 [40] thereby raisingthe possibility that peptidoglycan is sensed in mammalsby an innate immune receptor acting upstream of thissmall Rho GTPase (Figure 1) which represents an excitingarea for future research

AcknowledgmentsWork in AJBrsquos laboratory is supported by Public Health Service GrantsAI044170 and AI076246 AMK is supported by the American HeartAssociation Grant 12SDG12220022

Appendix A Supplementary dataSupplementary data associated with this article can befound in the online version at httpdxdoiorg101016jit201310009

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55 Fukazawa A et al (2008) GEF-H1 mediated control of NOD1dependent NF-kappaB activation by Shigella effectors PLoSPathog 4 e1000228

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66 Neal-McKinney JM and Konkel ME (2012) The Campylobacterjejuni CiaC virulence protein is secreted from the flagellum anddelivered to the cytosol of host cells Front Cell Infect Microbiol2 31

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Detection of enteric pathogens by the nodosome

Distinguishing friend from foe in the gut

NOD proteins in pathogen detection and maintenance of gut immune homeostasis

Formation of the nodosome

Detection of bacterial virulence factors by the nodosome

Evolution of the NOD1 signaling pathway

Concluding remarks

Acknowledgments

Supplementary data

References