I.  The neural basis of host seeking in parasitic nematodes
Parasitic nematodes cause extensive disease and economic loss worldwide. Many parasitic nematodes actively search for hosts to infect using host-emitted sensory cues, but their host-seeking behaviors remain poorly understood. We are studying host seeking at the levels of genes, cells, circuits, and behaviors. We are interested in the behavioral responses of parasitic nematodes to host-emitted sensory cues, and the neural circuits and signaling pathways that underlie them. We use both the skin-penetrating human parasite Strongyloides stercoralis and entomopathogenic nematodes as model systems. Our research will provide insight into the neural basis of parasitic behaviors, and may enable the development of new strategies for combatting harmful nematode infections.

Host seeking by skin-penetrating nematodes
Skin-penetrating nematodes are gastrointestinal parasites that infect approximately 1 billion people worldwide. Most of our research on 

S. stercoralis

Photo by M. Castelletto.
skin-penetrating worms focuses on the human threadworm Strongyloides stercoralis, the only human-parasitic worm that is currently amenable to molecular genetic analysis. S. stercoralis is endemic to tropical and subtropical regions throughout the world, including the United States, and is estimated to infect 30-100 million people worldwide. Infection with skin-penetrating worms can cause chronic gastrointestinal distress as well as stunted growth and long-term cognitive impairment in children. S. stercoralis infection can also be fatal for immunosuppressed individuals. S. stercoralis infective larvae actively seek out human hosts, but little is known about their host-seeking behavior. 

We recently showed that S. stercoralis is highly motile relative to other parasitic worms, uses a cruising strategy to actively search for hosts, and is attracted to a diverse array of human skin and sweat odorants. A comparison of olfactory behavior in skin-penetrating nematodes, passively ingested nematodes, insect-parasitic nematodes, and C. elegans revealed that parasitic nematodes show species-specific olfactory preferences. In addition, worms with similar host specificity and infection mode respond similarly to odorants even when they are phylogenetically distant, suggesting an important role for olfaction in host finding and host selection (Castelletto et al., PLoS Pathogens 2014). We are now elucidating the neural circuitry that underlies odor-driven host seeking in skin-penetrating nematodes. By comparing olfactory neural circuit function in S. stercoralis and C. elegans, we hope to gain insight into how parasitic nervous systems have evolved to support parasite-specific behavioral repertoires. We are also investigating the responses of S. stercoralis to other host-emitted sensory cues.

Host seeking by entomopathogenic nematodes (EPNs)
Insect-parasitic nematodes in the genera Heterorhabditis and Steinernema are lethal parasites of insects.  The active host-seeking and

Entomopathogenic nematodes

Entomopathogenic nematodes emerging from a waxworm cadaver.
host-invasion behavior of EPNs resembles that of many human-parasitic nematodes, making them excellent models for human parasites.  EPNs are also of interest as biocontrol agents for insect pests and disease vectors. We have shown that different EPN species respond differently to the odor blends emitted by live hosts as well as individual host-derived odorants. EPNs use the general host cue carbon dioxide (CO2) as well as host-specific odorants for host location, but the relative importance of CO2 versus host-specific odorants varies for different parasite-host combinations and for different host-seeking behaviors. We also identified host-emitted odorants by GC-MS and found that many of them stimulate host seeking. These results demonstrated that EPNs have specialized olfactory systems that contribute to host selection (Dillman et al., Proc. Natl. Acad. Sci. 2012).

We recently found that the host-seeking behaviors of EPNs depend on the prior cultivation temperature of the infective larvae. Many odorants that are attractive for infective larvae cultivated at colder temperatures are repulsive for infective larvae cultivated at warmer temperatures, and vice versa. The skin-penetrating rat parasite Strongyloides ratti also shows temperature-dependent changes in its host-seeking behavior, demonstrating that context-dependent modulation of host seeking also occurs in mammalian-parasitic nematodes. Parasitic nematode infective larvae are long-lived, often surviving in the environment through multiple seasonal temperature changes. Temperature-dependent modulation of olfactory behavior may allow the infective larvae to optimize host seeking in response to changing environmental conditions, and may play a previously unrecognized role in shaping the interactions of parasitic nematodes with their hosts (Lee et al., BMC Biol. 2016).

II.  The neural basis of carbon dioxide response in C. elegans

Carbon dioxide avoidance by C. elegans‎

CO2 is emitted by all aerobic animals as a byproduct of cellular respiration.  CO2 is a critical host cue for many parasites, including many parasitic nematodes.  CO2 is also an important sensory cue for many free-living animals, as it can signal the presence of food, mates, predators, or pathogens.  We are investigating the neural basis of CO2 response in the free-living nematode C. elegans. We are particularly interested in using the CO2 response of C. elegans as a model system for understanding the context-dependent modulation of sensory neural circuits.

We have shown that C. elegans adults are repelled by CO2. For example, exposing the head of a forward-moving worm to CO2 causes the worm to reverse. We have identified a pair of sensory neurons, the BAG neurons, that are required for CO2-avoidance behavior.  We have also identified multiple signaling pathways that regulate CO2 avoidance (Hallem et al., Proc. Natl. Acad. Sci. 2011).  We are now investigating the interactions between these pathways, as well as their regulation and sites of action, to gain further insight into the mechanism of CO2 avoidance by C. elegans.  We are also investigating the neural circuits that mediate CO2 avoidance. 

In contrast to C. elegans adults, which are repelled by CO2, we have found that C. elegans dauers are attracted to CO2.  The dauer stage of C. elegans is a developmentally arrested third larval stage that is analogous to the infective stage of parasitic worms. CO2 attraction by dauers also requires the BAG sensory neurons (Hallem et al., 2011; Dillman et al., 2012). Thus, another area of research in the lab is how the CO2 microcircuit can give rise to both attractive and repulsive responses.

We have also shown that the CO2 response of C. elegans depends on ambient oxygen (O2) levels, and that the O2-dependent modulation of CO2 response requires the polymorphic neuropeptide Y receptor NPR-1. In npr-1 mutants and wild isolates with a low activity allele of npr-1, CO2 response is regulated by ambient O2 levels such that increases in ambient O2 inhibit CO2 avoidance. Thus, a CO2 stimulus can be either repulsive or neutral depending on ambient O2 levels. The regulation of CO2 response by O2 requires the O2-detecting URX neurons, which are active at high ambient O2. Activation of URX neurons at high O2 blocks CO2 response downstream of the calcium response of the BAG neurons, changing CO2 from repulsive to neutral. By contrast, in animals with a high activity allele of npr-1, NPR-1 acts in URX neurons to decrease their activity regardless of ambient O2 levels and thereby enables CO2 avoidance. Thus, the O2 and CO2 circuits interact to direct context-appropriate responses to fluctuating ambient gas levels (Carrillo et al., J. Neurosci. 2013). We are now further investigating how the O2 and CO2 circuits interact to regulate CO2 response.

Research in the Hallem lab is currently supported by the NIH, NSF, MacArthur Foundation, Burroughs-Wellcome Fund, and HHMI.