Microbiota promote secretory cell determination in the intestinal epithelium by modulating host Notch signaling.

Resident microbes promote many aspects of host development, although the mechanisms by which microbiota influence host tissues remain unclear. We showed previously that the microbiota is required for allocation of appropriate numbers of secretory cells in the zebrafish intestinal epithelium. Because Notch signaling is crucial for secretory fate determination, we conducted epistasis experiments to establish whether the microbiota modulates host Notch signaling. We also investigated whether innate immune signaling transduces microbiota cues via the Myd88 adaptor protein. We provide the first evidence that microbiota-induced, Myd88-dependent signaling inhibits host Notch signaling in the intestinal epithelium, thereby promoting secretory cell fate determination. These results connect microbiota activity via innate immune signaling to the Notch pathway, which also plays crucial roles in intestinal homeostasis throughout life and when impaired can result in chronic inflammation and cancer.

Structural and functional features of a developmentally regulated lipopolysaccharide-binding protein.

UNLABELLED: Mammalian lipopolysaccharide (LPS) binding proteins (LBPs) occur mainly in extracellular fluids and promote LPS delivery to specific host cell receptors. The function of LBPs has been studied principally in the context of host defense; the possible role of LBPs in nonpathogenic host-microbe interactions has not been well characterized. Using the Euprymna scolopes-Vibrio fischeri model, we analyzed the structure and function of an LBP family protein, E. scolopes LBP1 (EsLBP1), and provide evidence for its role in triggering a symbiont-induced host developmental program. Previous studies showed that, during initial host colonization, the LPS of V. fischeri synergizes with peptidoglycan (PGN) monomer to induce morphogenesis of epithelial tissues of the host animal. Computationally modeled EsLBP1 shares some but not all structural features of mammalian LBPs that are thought important for LPS binding. Similar to human LBP, recombinant EsLBP1 expressed in insect cells bound V. fischeri LPS and Neisseria meningitidis lipooligosaccharide (LOS) with nanomolar or greater affinity but bound Francisella tularensis LPS only weakly and did not bind PGN monomer. Unlike human LBP, EsLBP1 did not bind N. meningitidis LOS:CD14 complexes. The eslbp1 transcript was upregulated ~22-fold by V. fischeri at 24 h postinoculation. Surprisingly, this upregulation was not induced by exposure to LPS but, rather, to the PGN monomer alone. Hybridization chain reaction-fluorescent in situ hybridization (HCR-FISH) and immunocytochemistry (ICC) localized eslbp1 transcript and protein in crypt epithelia, where V. fischeri induces morphogenesis. The data presented here provide a window into the evolution of LBPs and the scope of their roles in animal symbioses. IMPORTANCE: Mammalian lipopolysaccharide (LPS)-binding protein (LBP) is implicated in conveying LPS to host cells and potentiating its signaling activity. In certain disease states, such as obesity, the overproduction of this protein has been a reliable biomarker of chronic inflammation. Here, we describe a symbiosis-induced invertebrate LBP whose tertiary structure and LPS-binding characteristics are similar to those of mammalian LBPs; however, the primary structure of this distantly related squid protein (EsLBP1) differs in key residues previously believed to be essential for LPS binding, suggesting that an alternative strategy exists. Surprisingly, symbiotic expression of eslbp1 is induced by peptidoglycan derivatives, not LPS, a pattern converse to that of RegIIIγ, an important mammalian immunity protein that binds peptidoglycan but whose gene expression is induced by LPS. Finally, EsLBP1 occurs along the apical surfaces of all the host's epithelia, suggesting that it was recruited from a general defensive role to one that mediates specific interactions with its symbiont.

Investigating bacterial-animal symbioses with light sheet microscopy.

Microbial colonization of the digestive tract is a crucial event in vertebrate development, required for maturation of host immunity and establishment of normal digestive physiology. Advances in genomic, proteomic, and metabolomic technologies are providing a more detailed picture of the constituents of the intestinal habitat, but these approaches lack the spatial and temporal resolution needed to characterize the assembly and dynamics of microbial communities in this complex environment. We report the use of light sheet microscopy to provide high-resolution imaging of bacterial colonization of the intestine of Danio rerio, the zebrafish. The method allows us to characterize bacterial population dynamics across the entire organ and the behaviors of individual bacterial and host cells throughout the colonization process. The large four-dimensional data sets generated by these imaging approaches require new strategies for image analysis. When integrated with other "omics" data sets, information about the spatial and temporal dynamics of microbial cells within the vertebrate intestine will provide new mechanistic insights into how microbial communities assemble and function within hosts.

LBP/BPI proteins and their relatives: conservation over evolution and roles in mutualism.

LBP [LPS (lipopolysaccharide)-binding protein] and BPI (bactericidal/permeability-increasing protein) are components of the immune system that have been principally studied in mammals for their involvement in defence against bacterial pathogens. These proteins share a basic architecture and residues involved in LPS binding. Putative orthologues, i.e. proteins encoded by similar genes that diverged from a common ancestor, have been found in a number of non-mammalian vertebrate species and several non-vertebrates. Similar to other aspects of immunity, such as the activity of Toll-like receptors and NOD (nucleotide-binding oligomerization domain) proteins, analysis of the conservation of LBPs and BPIs in the invertebrates promises to provide insight into features essential to the form and function of these molecules. This review considers state-of-the-art knowledge in the diversity of the LBP/BPI proteins across the eukaryotes and also considers their role in mutualistic symbioses. Recent studies of the LBPs and BPIs in an invertebrate model of beneficial associations, the Hawaiian bobtail squid Euprymna scolopes' alliance with the marine luminous bacterium Vibrio fischeri, are discussed as an example of the use of non-vertebrate models for the study of LBPs and BPIs.

Taming the symbiont for coexistence: a host PGRP neutralizes a bacterial symbiont toxin.

In horizontally transmitted mutualisms between marine animals and their bacterial partners, the host environment promotes the initial colonization by specific symbionts that it harvests from the surrounding bacterioplankton. Subsequently, the host must develop long-term tolerance to immunogenic bacterial molecules, such as peptidoglycan and lipopolysaccaride derivatives. We describe the characterization of the activity of a host peptidoglycan recognition protein (EsPGRP2) during establishment of the symbiosis between the squid Euprymna scolopes and its luminous bacterial symbiont Vibrio fischeri. Using confocal immunocytochemistry, we localized EsPGRP2 to all epithelial surfaces of the animal, and determined that it is exported in association with mucus shedding. Most notably, EsPGRP2 was released by the crypt epithelia into the extracellular spaces housing the symbionts. This translocation occurred only after the symbionts had triggered host morphogenesis, a process that is induced by exposure to the peptidoglycan monomer tracheal cytotoxin (TCT), a bacterial 'toxin' that is constitutively exported by V. fischeri. Enzymatic analyses demonstrated that, like many described PGRPs, EsPGRP2 has a TCT-degrading amidase activity. The timing of EsPGRP2 export into the crypts provides evidence that the host does not export this protein until after TCT induces morphogenesis, and thereafter EsPGRP2 is constantly present in the crypts ameliorating the effects of V. fischeri TCT.

Peptidoglycan induces loss of a nuclear peptidoglycan recognition protein during host tissue development in a beneficial animal-bacterial symbiosis.

Peptidoglycan recognition proteins (PGRPs) are mediators of innate immunity and recently have been implicated in developmental regulation. To explore the interplay between these two roles, we characterized a PGRP in the host squid Euprymna scolopes (EsPGRP1) during colonization by the mutualistic bacterium Vibrio fischeri. Previous research on the squid-vibrio symbiosis had shown that, upon colonization of deep epithelium-lined crypts of the host light organ, symbiont-derived peptidoglycan monomers induce apoptosis-mediated regression of remote epithelial fields involved in the inoculation process. In this study, immunofluorescence microscopy revealed that EsPGRP1 localizes to the nuclei of epithelial cells, and symbiont colonization induces the loss of EsPGRP1 from apoptotic nuclei. The loss of nuclear EsPGRP1 occurred prior to DNA cleavage and breakdown of the nuclear membrane, but followed chromatin condensation, suggesting that it occurs during late-stage apoptosis. Experiments with purified peptidoglycan monomers and with V. fischeri mutants defective in peptidoglycan-monomer release provided evidence that these molecules trigger nuclear loss of EsPGRP1 and apoptosis. The demonstration of a nuclear PGRP is unprecedented, and the dynamics of EsPGRP1 during apoptosis provide a striking example of a connection between microbial recognition and developmental responses in the establishment of symbiosis.

Effects of colonization, luminescence, and autoinducer on host transcription during development of the squid-vibrio association.

The light-organ symbiosis between the squid Euprymna scolopes and the luminous bacterium Vibrio fischeri offers the opportunity to decipher the hour-by-hour events that occur during the natural colonization of an animal's epithelial surface by its microbial partners. To determine the genetic basis of these events, a glass-slide microarray was used to characterize the light-organ transcriptome of juvenile squid in response to the initiation of symbiosis. Patterns of gene expression were compared between animals not exposed to the symbiont, exposed to the wild-type symbiont, or exposed to a mutant symbiont defective in either of two key characters of this association: bacterial luminescence or autoinducer (AI) production. Hundreds of genes were differentially regulated as a result of symbiosis initiation, and a hierarchy existed in the magnitude of the host's response to three symbiont features: bacterial presence > luminescence > AI production. Putative host receptors for bacterial surface molecules known to induce squid development are up-regulated by symbiont light production, suggesting that bioluminescence plays a key role in preparing the host for bacteria-induced development. Further, because the transcriptional response of tissues exposed to AI in the natural context (i.e., with the symbionts) differed from that to AI alone, the presence of the bacteria potentiates the role of quorum signals in symbiosis. Comparison of these microarray data with those from other symbioses, such as germ-free/conventionalized mice and zebrafish, revealed a set of shared genes that may represent a core set of ancient host responses conserved throughout animal evolution.

An annotated cDNA library of juvenile Euprymna scolopes with and without colonization by the symbiont Vibrio fischeri.

BACKGROUND: Biologists are becoming increasingly aware that the interaction of animals, including humans, with their coevolved bacterial partners is essential for health. This growing awareness has been a driving force for the development of models for the study of beneficial animal-bacterial interactions. In the squid-vibrio model, symbiotic Vibrio fischeri induce dramatic developmental changes in the light organ of host Euprymna scolopes over the first hours to days of their partnership. We report here the creation of a juvenile light-organ specific EST database. RESULTS: We generated eleven cDNA libraries from the light organ of E. scolopes at developmentally significant time points with and without colonization by V. fischeri. Single pass 3' sequencing efforts generated 42,564 expressed sequence tags (ESTs) of which 35,421 passed our quality criteria and were then clustered via the UIcluster program into 13,962 nonredundant sequences. The cDNA clones representing these nonredundant sequences were sequenced from the 5' end of the vector and 58% of these resulting sequences overlapped significantly with the associated 3' sequence to generate 8,067 contigs with an average sequence length of 1,065 bp. All sequences were annotated with BLASTX (E-value < -03) and Gene Ontology (GO). CONCLUSION: Both the number of ESTs generated from each library and GO categorizations are reflective of the activity state of the light organ during these early stages of symbiosis. Future analyses of the sequences identified in these libraries promise to provide valuable information not only about pathways involved in colonization and early development of the squid light organ, but also about pathways conserved in response to bacterial colonization across the animal kingdom.

Identifying components of the NF-kappaB pathway in the beneficial Euprymna scolopes-Vibrio fischeri light organ symbiosis.

The Toll/NF-kappaB pathway is a common, evolutionarily conserved innate immune pathway that modulates the responses of animal cells to microbe-associated molecular patterns (MAMPs). Because MAMPs have been implicated as critical elements in the signaling of symbiont-induced development, an expressed sequence tag library from the juvenile light organ of Euprymna scolopes was used to identify members of the Toll/NF-kappaB pathway. Full-length transcripts were identified by using 5' and 3' RACE PCR. Seven transcripts critical for MAMP-induced triggering of the Toll/NF-kappaB phosphorylation cascade have been identified, including receptors, signal transducers, and a transcription factor. Further investigations should elucidate the role of the Toll/NF-kappaB pathway in the initiation of the beneficial symbiosis between E. scolopes and Vibrio fischeri.