The C. elegans CHP1 homolog, pbo-1, functions in innate immunity by regulating the pH of the intestinal lumen.

Saida Benomar,Blake R Peterson,Aaron M Bender,Patrick Lansdon,Brian D Ackley,Josephine R Chandler,David S Schneider

doi:10.1371/journal.ppat.1008134

Abstract

Caenorhabditis elegans are soil-dwelling nematodes and models for understanding innate immunity and infection. Previously, we developed a novel fluorescent dye (KR35) that accumulates in the intestine of C. elegans and reports a dynamic wave in intestinal pH associated with the defecation motor program. Here, we use KR35 to show that mutations in the Ca2+-binding protein, PBO-1, abrogate the pH wave, causing the anterior intestine to be constantly acidic. Surprisingly, pbo-1 mutants were also more susceptible to infection by several bacterial pathogens. We could suppress pathogen susceptibility in pbo-1 mutants by treating the animals with pH-buffering bicarbonate, suggesting the pathogen susceptibility is a function of the acidity of the intestinal pH. Furthermore, we use KR35 to show that upon infection by pathogens, the intestinal pH becomes neutral in a wild type, but less so in pbo-1 mutants. C. elegans is known to increase production of reactive oxygen species (ROS), such as H2O2, in response to pathogens, which is an important component of pathogen defense. We show that pbo-1 mutants exhibited decreased H2O2 in response to pathogens, which could also be partially restored in pbo-1 animals treated with bicarbonate. Ultimately, our results support a model whereby PBO-1 functions during infection to facilitate pH changes in the intestine that are protective to the host.

Highlights

As increased antibiotic resistance is being observed in clinical settings, bacterial infections are becoming a crisis-level global health burden
We show that intestinal pH directly affects production of reactive oxygen species (e.g. H2O2) important for pathogen defense
KR35 is activated by acid in a range that is physiologically relevant to the C. elegans intestine, which maintains a pH gradient from anterior to posterior of ~5.5–3.5 [19]

Summary

Introduction

As increased antibiotic resistance is being observed in clinical settings, bacterial infections are becoming a crisis-level global health burden. Host barriers to infection can be physical (e.g. an exoskeleton or epidermal layer), chemical (e.g. shed-able mucosa that adhere to pathogens) or genetic (e.g. innate and/or adaptive immunity). One effective mechanism of host protection is the production of reactive oxygen species (ROS), e.g. superoxide (O2–) which can be converted to hydrogen peroxide (H2O2) [1]. In the vertebrate innate immune response, the enzymes NADPH oxidase and superoxide dismutase (SOD) convert O2 to O2and H2O2 [5, 6]. Because ROS can be self-destructive, it is no surprise that the production of ROS is tightly regulated in response to pathogens [7]. The identities of infection-dependent signals and regulatory responses that lead to ROS production remain incomplete

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Discussion

Conclusion