Breanna Walsh

An animal's response to chemosensory cues depends on the animal's prior experience, internal state, or life stage. However, the molecular mechanisms that regulate sensory valence ( i.e., whether a chemosensory cue is attractive or repulsive) remain poorly understood. We investigated the mechanisms that specify sensory valence using the responses of the free-living nematode Caenorhabditis elegans to carbon dioxide (CO ₂ ). C. elegans exhibits highly flexible responses to CO ₂ : well-fed animals are repelled by CO ₂ , while both starved animals and well-fed animals raised under high CO ₂ conditions are attracted to CO ₂ . Here, we show that CO ₂ attraction in animals raised at high CO ₂ requires a cGMP signaling pathway that involves the cGMP-dependent protein kinase EGL-4. This pathway does not regulate CO ₂ response in starved animals, indicating that the role of EGL-4 in mediating CO ₂ attraction depends on satiety state. Cultivation under high CO ₂ conditions leads to increased cGMP levels in the CO ₂ -detecting BAG neurons, consistent with a specific requirement for EGL-4 in high-CO ₂ -cultivated animals. We also show that EGL-4 regulates CO ₂ valence by altering neuropeptide expression in BAG. Our results indicate that sensory valence is established in a context-dependent manner at the level of the primary sensory neuron.

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.

The skin-penetrating gastrointestinal parasitic nematode Strongyloides stercoralis causes strongyloidiasis, which is a neglected tropical disease that is associated with severe chronic illness and fatalities. Unlike other human-infective nematodes, S. stercoralis cycles through a single free-living generation and thus serves as a genetically tractable model organism for understanding the mechanisms that enable parasitism. Techniques such as CRISPR/Cas9-mediated mutagenesis and transgenesis are now routinely performed in S. stercoralis by introducing exogenous DNA into free-living adults and then screening their F₁ progeny for transgenic or mutant larvae. However, transgenesis in S. stercoralis has been severely hindered by the inability to establish stable transgenic lines that can be propagated for multiple generations through a host; to date, studies of transgenic S. stercoralis have been limited to heterogeneous populations of transgenic F₁ larvae. Here, we develop an efficient pipeline for the generation of stable transgenic lines in S. stercoralis. We also show that this approach can be used to efficiently generate stable transgenic lines in the rat-infective nematode Strongyloides ratti. The ability to generate stable transgenic lines circumvents the limitations of working with heterogeneous F₁ populations, such as variable transgene expression and the inability to generate transgenics of all life stages. Our transgenesis approach will enable novel lines of inquiry into parasite biology, such as transgene-based comparisons between free-living and parasitic generations.