Eun-Mi Ha

Innate immune homeostasis by the homeobox gene caudal and commensal-gut mutualism in Drosophila.

Although commensalism with gut microbiota exists in all metazoans, the host factors that maintain this homeostatic relationship remain largely unknown. We show that the intestinal homeobox gene Caudal regulates the commensal-gut mutualism by repressing nuclear factor kappa B–dependent antimicrobial peptide genes. Inhibition of Caudal expression in flies via RNA interference led to overexpression of antimicrobial peptides, which in turn altered the commensal population within the intestine. In particular, the dominance of one gut microbe, Gluconobacter sp. strain EW707, eventually led to gut cell apoptosis and host mortality. However, restoration of a healthy microbiota community and normal host survival in the Caudal-RNAi flies was achieved by reintroduction of the Caudal gene. These results reveal that a specific genetic deficiency within a host can profoundly influence the gut commensal microbial community and host physiology.

Regulation of DUOX by the Gαq-Phospholipase Cβ-Ca2+ Pathway in Drosophila Gut Immunity

All metazoan guts are in constant contact with diverse food-borne microorganisms. The signaling mechanisms by which the host regulates gut-microbe interactions, however, are not yet clear. Here, we show that phospholipase C-beta (PLCbeta) signaling modulates dual oxidase (DUOX) activity to produce microbicidal reactive oxygen species (ROS) essential for normal host survival. Gut-microbe contact rapidly activates PLCbeta through Galphaq, which in turn mobilizes intracellular Ca(2+) through inositol 1,4,5-trisphosphate generation for DUOX-dependent ROS production. PLCbeta mutant flies had a short life span due to the uncontrolled propagation of an essential nutritional microbe, Saccharomyces cerevisiae, in the gut. Gut-specific reintroduction of the PLCbeta restored efficient DUOX-dependent microbe-eliminating capacity and normal host survival. These results demonstrate that the Galphaq-PLCbeta-Ca(2+)-DUOX-ROS signaling pathway acts as a bona fide first line of defense that enables gut epithelia to dynamically control yeast during the Drosophila life cycle.

An essential complementary role of NF‐κB pathway to microbicidal oxidants in Drosophila gut immunity

In the Drosophila gut, reactive oxygen species (ROS)‐dependent immunity is critical to host survival. This is in contrast to the NF‐κB pathway whose physiological function in the microbe‐laden epithelia has yet to be convincingly demonstrated despite playing a critical role during systemic infections. We used a novel in vivo approach to reveal the physiological role of gut NF‐κB/antimicrobial peptide (AMP) system, which has been ‘masked’ in the presence of the dominant intestinal ROS‐dependent immunity. When fed with ROS‐resistant microbes, NF‐κB pathway mutant flies, but not wild‐type flies, become highly susceptible to gut infection. This high lethality can be significantly reduced by either re‐introducing Relish expression to Relish mutants or by constitutively expressing a single AMP to the NF‐κB pathway mutants in the intestine. These results imply that the local ‘NF‐κB/AMP’ system acts as an essential ‘fail‐safe’ system, complementary to the ROS‐dependent gut immunity, during gut infection with ROS‐resistant pathogens. This system provides the Drosophila gut immunity the versatility necessary to manage sporadic invasion of virulent pathogens that somehow counteract or evade the ROS‐dependent immunity.

Innate immunity and gut-microbe mutualism in Drosophila.

Metazoan guts face a wide variety of microorganisms upon exposure to the environment, including beneficial symbionts, non-symbionts, food-borne microbes and life-threatening pathogens. Recent evidence has shown that the innate immunity of gut epithelia, such as anti-microbial peptide- and reactive oxygen species-based immune systems, actively participate in gut-microbe homeostasis by shaping the commensal community while efficiently eliminating unwanted bacteria. Therefore, elucidation of the regulatory mechanism by which gut innate immunity occurs at the molecular level will provide a novel perspective of gut-microbe mutualisms as well as of gut diseases caused by alterations in the innate immunity.