Gloria Bartolo

Parasitic nematodes infect over a billion people worldwide and cause some of the most prevalent neglected tropical diseases ^1-5 . Many of these parasites are skin penetrating and have both extra-host life stages that inhabit host feces and surrounding soil, and intra-host life stages that inhabit host niches such as skin, vasculature, and intestine ^2,6-8 . Across life stages, these parasites encounter oxygen (O ₂ ) levels that range from ~21% at the soil-air interface to near-anaerobic levels in the host intestine ^9-12 . However, whether parasitic nematodes detect and respond to changes in O ₂ levels was unknown. Here, we examine O ₂ sensation in skin-penetrating parasitic nematodes and find that they show robust responses to changes in O ₂ levels. Moreover, their O ₂ -evoked behaviors differ from those of the free-living nematode Caenorhabditis elegans . We then investigate the molecular and neural mechanisms of O ₂ sensing in Strongyloides stercoralis , a genetically tractable human-infective nematode, and find that parasite-specific behavioral responses to O ₂ arise in part from evolutionary changes in their soluble guanylate cyclase repertoire. Finally, we find that neuronal O ₂ sensing regulates intra-host development in S. stercoralis . Our results demonstrate that skin-penetrating nematodes exhibit neuronally mediated O ₂ responses that are critical for multiple steps of their parasitic life cycle.

Skin-penetrating nematodes are one of the most prevalent causes of disease worldwide. The World Health Organization has targeted these parasites for elimination by 2030, but the lack of preventative measures is a major obstacle to this goal. Infective larvae enter hosts through skin and blocking skin penetration could prevent infection. However, in order to prevent worm ingress via the skin, an understanding of the behavioral and neural mechanisms that drive skin penetration is required. Here, we describe the skin-penetration behavior of the human-infective threadworm Strongyloides stercoralis. We show that S. stercoralis engages in repeated cycles of pushing, puncturing, and crawling on the skin surface before penetrating. Pharmacological inhibition of dopamine signaling inhibits these behaviors in S. stercoralis and the human hookworm Ancylostoma ceylanicum, suggesting a critical role for dopamine signaling in driving skin penetration across distantly related nematodes. CRISPR-mediated disruption of dopamine biosynthesis and chemogenetic silencing of dopaminergic neurons also inhibit skin penetration. Finally, inactivation of the TRPN channel TRP-4, which is expressed in the dopaminergic neurons, blocks skin penetration. Our results suggest that drugs targeting TRP-4 and other nematode-specific components of the dopaminergic pathway could be developed into topical prophylactics that block skin penetration, thereby preventing infections.

Anthrax is a major zoonotic disease of wildlife, and in places like West Africa, it can be caused by Bacillus anthracis in arid nonsylvatic savannahs, and by B. cereus biovar anthracis (Bcbva) in sylvatic rainforests. Bcbva-caused anthrax has been implicated in as much as 38% of mortality in rainforest ecosystems, where insects can enhance the transmission of anthrax-causing bacteria. While anthrax is well-characterized in mammals, its transmission by insects points to an unidentified anthrax-resistance mechanism in its vectors. In mammals, a secreted anthrax toxin component, 83 kDa Protective Antigen (PA₈₃), binds to cell-surface receptors and is cleaved by furin into an evolutionary-conserved PA₂₀ and a pore-forming PA₆₃ subunits. We show that PA₂₀ increases the resistance of Drosophila flies and Culex mosquitoes to bacterial challenges, without directly affecting the bacterial growth. We further show that the PA₈₃ loop known to be cleaved by furin to release PA₂₀ from PA₆₃ is, in part, responsible for the PA₂₀-mediated protection. We found that PA₂₀ binds directly to the Toll activating peptidoglycan-recognition protein-SA (PGRP-SA) and that the Toll/NF-κB pathway is necessary for the PA₂₀-mediated protection of infected flies. This effect of PA₂₀ on innate immunity may also exist in mammals: we show that PA₂₀ binds to human PGRP-SA ortholog. Moreover, the constitutive activity of Imd/NF-κB pathway in MAPKK Dsor1 mutant flies is sufficient to confer the protection from bacterial infections in a manner that is independent of PA₂₀ treatment. Lastly, Clostridium septicum alpha toxin protects flies from anthrax-causing bacteria, showing that other pathogens may help insects resist anthrax. The mechanism of anthrax resistance in insects has direct implications on insect-mediated anthrax transmission for wildlife management, and with potential for applications, such as reducing the sensitivity of pollinating insects to bacterial pathogens. Gram-positive Bacillus anthracis and B. cereus biovar anthracis cause anthrax in mammals by secreting anthrax toxin. Recent studies report the devastating ecological effects anthrax has on the wildlife in areas like tropical rainforests, where in the past 26 years, anthrax had been implicated in 38% of wildlife mortality, and where insects can act as the vectors for anthrax transmission. The anthrax-resistance mechanism in its insect vectors has not been identified. We discovered that one of anthrax toxin components reduces the sensitivity of insects to anthrax-causing bacilli and other bacteria, and that insect immunity Toll/NF-κB and Imd/NF-κB pathways play a role in this toxin-mediated protection. The toxin-mediated protection of insects would benefit the anthrax-causing bacteria by extending the lifespan of vectors and enhancing the pathogen transmission. Anthrax toxin component activation of NF-κB has also been found to exists in humans. Moreover, the activation of the innate NF-κB-dependent immunity by this anthrax toxin component has potential implications in protecting pollinating insects from bacterial pathogens.