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[
International Worm Meeting,
2011]
Many parasitic nematodes rely on olfactory cues to locate their hosts, yet little is known about how olfactory preferences contribute to the specificity of host-parasite interactions. We are investigating this question using the insect-parasitic nematodes Heterorhabditis bacteriophora and Steinernema carpocapsae; the mammalian-parasitic nematodes Nippostrongylus brasiliensis and Strongyloides ratti; and the free-living nematode C. elegans as a comparative model. We examined the chemotaxis behavior of parasitic infective juveniles (IJs) and C. elegans dauers in response to a large and diverse panel of ecologically-relevant odorants, including host odorants that we identified by gas chromatography-mass spectrometry of live hosts. We found that the insect-parasitic nematodes have different odor response profiles despite their overlapping host ranges. However, the odor response profiles of the insect-parasitic nematodes are more similar to each other than to C. elegans despite their phylogenetic distance, likely reflecting a key role for olfaction in their convergently evolved parasitic lifestyles. We also examined the responses of insect-parasitic nematodes to host versus non-host insects, and found that these parasites are capable of discriminating between different insect species using olfactory cues. We are now extending these studies to mammalian-parasitic nematodes to gain insight into the niche-specific olfactory adaptations of different parasitic species as well as the evolution of olfactory behavior. We are also examining the relative contributions of the universal cue carbon dioxide (CO2) versus host-specific odorants in mediating host-seeking behavior. Our results indicate that parasites rely on both CO2 and host-specific odorants for host location, but the relative importance of CO2 versus host-specific odorants varies for different parasites and different hosts. Finally, we are investigating the neural basis of host-seeking behavior by identifying the sensory neurons and downstream neural circuits that are required for successful host location.
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[
International Worm Meeting,
2009]
Parasitic nematodes are a major health concern worldwide, and current strategies for preventing or eliminating nematode infections are insufficient. One possible control strategy is to interfere with the ability of nematodes to locate hosts. Carbon dioxide is an important host-seeking cue for many parasitic nematodes, yet little is known about the mechanism of CO2 response in nematodes. We found that C. elegans displays acute CO2 avoidance: exposure to CO2 results in the cessation of forward movement and the initiation of backward movement. We found that multiple signaling molecules affect CO2 avoidance, including TAX-2/TAX-4, RGS-3, TAX-6, and NPR-1. Nutritional status also modulates CO2 responsiveness via the insulin and TGF-b pathways. Acute CO2 avoidance is mediated primarily by the BAG neurons, and TAX-2/TAX-4 are required in the BAG neurons for CO2 response. Additional ciliated sensory neurons also contribute to CO2 response. We are now extending this analysis to other nematodes. We have found that the BAG neurons are required for CO2 avoidance in the necromenic worm Pristionchus pacificus, and for CO2 attraction in the insect-parasitic nematode Heterorhabditis bacteriophora. We are now investigating the mechanism by which analogous neurons mediate attraction in parasitic nematodes and repulsion in free-living nematodes. We are also investigating how CO2 response changes depending on the life stage of the worm. Finally, we are examining behavioral responses to other potential host-seeking odorants in H. bacteriophora and a different insect-parasitic nematode, Steinernema carpocapsae. We have found that the two species show very different odor response spectra, raising the possibility that some of the tested odorants contribute to host specificity.
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[
International Worm Meeting,
2007]
Nematode parasitism is a worldwide health concern: over a quarter of the world''s population is infected with nematode parasites, and over a hundred species of nematodes are parasites of humans. Despite the extensive morbidity and mortality caused by infection with nematode parasites, the biological mechanisms of host-parasite interactions are poorly understood. This is largely due to the lack of genetically tractable model systems for parasitism. We have demonstrated that the insect parasitic nematode Heterorhabditis bacteriophora, its bacterial symbiont Photorhabdus luminescens, and the fruit fly Drosophila melanogaster constitute a tripartite model for nematode parasitism and parasitic infection. We find that infective juveniles (IJs) of H. bacteriophora, which contain P. luminescens in their gut, can infect and kill D. melanogaster larvae. We show that infection activates a humoral immune response in D. melanogaster that results in the temporally dynamic expression of a subset of antimicrobial peptide (AMP) genes, and that this immune response is induced specifically by P. luminescens. We also investigated the cellular and molecular mechanisms underlying IJ recovery, the developmental process that occurs in parasitic nematodes upon host invasion. We find that the chemosensory neurons and signaling pathways that control dauer recovery in C. elegans also control IJ recovery in H. bacteriophora. In particular, IJ recovery is mediated by the ASJ neurons, a cGMP signaling pathway, and muscarinic acetylcholine receptors. Our results suggest conservation of these developmental processes across free-living and parasitic nematodes.
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[
International Worm Meeting,
2021]
Skin-penetrating parasitic nematodes, such as the threadworm Strongyloides stercoralis, infect approximately 600 million people worldwide. During the infective life stage, Strongyloides infective larvae (iL3s) engage in behaviors that increase the likelihood of contact with a host. Upon contact, iL3s penetrate the skin of the host and develop into parasitic adults that colonize the gastrointestinal tract. How Strongyloides iL3s sense mechanical cues and the extent to which mechanosensation enables both location and infection of a host are not yet understood. To characterize the role of mechanosensation in location of a host, we first studied the response of the parasitic nematode S. ratti to mechanical cues such as vibration and touch. In preliminary experiments, S. ratti iL3s moved toward applied 50 Hz vibrations. In contrast, S. ratti free-living adults moved away from vibrations of the same frequency. Additionally, S. ratti iL3s reacted less frequently to gentle touch along the body and harsh touch at the nose relative to free-living adults. To examine how mechanosensation enables infection of a host, we developed an assay to study skin penetration in vivo. We observed that iL3s penetrate into 1% low-melt agarose media, whereas free-living adults do not. We are now characterizing the behaviors of iL3s that are penetrating rat skin. We have also identified S. ratti homologs of several C. elegans genes that encode mechanoreceptors. We are now generating reporter constructs to examine the expression patterns of these genes, and to identify and label putative mechanosensory neurons in S. ratti. To study how the neurons sense and respond to mechanical stimuli, we will use a combination of calcium imaging and neuronal silencing. Additionally, we will disrupt candidate genes using CRISPR/Cas9-mediated targeted mutagenesis, and then compare the behaviors of mutant and wild-type iL3s to identify genes and signaling pathways required for host-seeking and host-invasion behaviors. Together, these experiments will provide insight into the neural and molecular mechanisms that drive mechanosensory behaviors in skin-penetrating nematodes.
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[
International Worm Meeting,
2013]
Olfaction is a critical sensory modality that allows an organism to process and respond to the surrounding chemical environment. Despite the importance of olfaction for organismal survival, how olfactory behavior is affected by environmental conditions is poorly understood. We are using Steinernema carpocapsae, a widely distributed entomopathogenic nematode (EPN) that uses olfactory cues to locate insect hosts, to investigate the context-dependent modulation of olfactory behavior. Using chemotaxis assays, we found that temperature has a strong influence on the olfactory behavior of the infective juvenile (IJ) stage of these parasites. In particular, the cultivation temperature of IJs has a dramatic effect on their olfactory responses, and inducing temperature changes alters their olfactory preferences. We then extended the analysis of temperature's influence on olfactory behavior to other EPN species and found that the extent to which temperature affects olfactory preferences varies widely among EPN species with different climate and geographical distributions. We are now investigating the neural mechanisms that mediate temperature-dependent changes in EPN olfactory behavior. We are also investigating whether these changes reflect differences in host preference or host-seeking strategy at different temperatures. Our work with the developmentally-arrested IJs suggests environmental temperature is sufficient to induce neural plasticity and may pave the way for enhancing the efficacy of EPNs as biocontrol agents.
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[
International Worm Meeting,
2015]
Entomopathogenic nematodes (EPNs) are lethal parasites of insects that are used as biocontrol agents for insect pests. Although EPNs are commercially available as safer alternatives to chemical insecticides, their efficacy in the field is often inconsistent for reasons that remain elusive. Olfactory behavior of parasites is thought to be crucial for the identification of and movement toward preferred hosts, but how it is affected by environmental conditions is poorly understood. We find that the infective juveniles (IJs) of various EPN species, including Steinernema carpocapsae, a widely distributed EPN with a broad host range, show olfactory plasticity as a function of temperature and age. Odorants that elicit attractive responses for IJs at lower temperatures elicit repulsive responses at higher temperatures and vice versa. In addition, we find that EPNs shift their host-seeking strategy depending on cultivation temperature: S. carpocapsae IJs reared at lower temperatures appear to actively cruise more than IJs reared at higher temperatures. Many EPNs are capable of infecting many different insect hosts, and are exposed to seasonal variations in temperature. Temperature-dependent modulation of host-seeking behavior may enable these generalists to target seasonally appropriate hosts. Furthermore, we find that the roots of stressed plants, which may signal the presence of potential insect hosts, elicit different responses from IJs depending on IJ cultivation temperature. We are now comparing the influence of temperature on olfactory behavior in specialist EPN species with narrower host ranges, and also in mammalian-parasitic nematodes such as the skin-penetrating rat parasite Strongyloides ratti. We are also investigating the neural basis of olfactory plasticity in EPNs. In summary, our results demonstrate that EPNs exhibit dramatic temperature-dependent behavioral changes, which may allow optimization of their host-seeking behavior. A better understanding of olfactory plasticity in EPNs may ultimately pave the way for enhancing their efficacy as biocontrol agents.
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[
International Worm Meeting,
2013]
Soil-dwelling nematodes use environmental sensory cues to locate resources and potential mates. Many parasitic nematodes also use these cues to locate appropriate hosts. The human parasite Strongyloides stercoralis and the rat parasites Strongyloides ratti and Nippostrongylus brasiliensis are skin-penetrating nematodes that spend a portion of their life cycle in the soil. We are interested in how these worms use host-derived odorants to search for hosts. We examined the responses of S. stercoralis, S. ratti, and N. brasiliensis infective juveniles (IJs) to carbon dioxide (CO2), an important host cue for entomopathogenic nematodes (EPNs) as well as many hematophagous insects. We found that mammalian-parasitic IJs are not attracted to CO2, suggesting they rely instead on host-specific cues. We then examined the responses of these IJs to a large panel of human odorants, and found that many of these odorants attract parasitic IJs. Moreover, many of the odorants that elicited the strongest responses from S. stercoralis have also been shown to attract mosquitoes, suggesting that human-parasitic nematodes and mosquitoes use similar olfactory cues. These results raised the possibility that insect repellents might be effective against parasitic nematodes. However, we found that some insect repellents do not repel mammalian-parasitic nematodes, and in fact S. stercoralis and S. ratti are attracted to the insect repellent DEET. A comparison of the odor response profiles of the mammalian-parasitic IJs to those of EPN IJs and C. elegans dauers revealed that olfactory preferences reflect host range rather than phylogeny, suggesting an important role for olfaction in the evolution of host specificity among parasitic nematodes. Finally, we found that S. ratti IJs respond differently to odorants than S. ratti non-infective larvae and adults, indicating that the olfactory preferences of at least some parasitic nematodes are stage-specific. We are now expanding these studies to additional species of parasitic nematodes, including some that are passively ingested and some that have more diverse host ranges. We are also investigating the neural basis of odor-driven host-seeking behaviors in mammalian-parasitic nematodes.
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[
International Worm Meeting,
2009]
The BAG neurons are ciliated sensory neurons that recently have been shown to mediate avoidance behavior to carbon dioxide (CO2) and to inhibit egg-laying behavior. To test whether the BAG neurons are CO2 sensors, we generated transgenic animals expressing the high-affinity genetically encoded calcium sensor cameleon YC3.60 in the BAG neurons. We observed increases in BAG neuron calcium in response to application of as little as 0.1% CO2 with half maximal increases evoked by application of 0.9% CO2. The BAG neurons require the TAX-2 / TAX-4 cyclic nucleotide-gated cation channel to respond to environmental CO2. The BAG neurons of
rgs-3 mutants, which lack a negative regulator of heterotrimeric G proteins, are defective for CO2-evoked calcium responses, indicating that the BAG neurons are negatively regulated by a G protein signaling pathway. Using in vivo calcium imaging, we are seeking neurons that function downstream of the BAG sensory neurons in neuronal circuits that mediate CO2 avoidance and the regulation of egg laying.
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[
International Worm Meeting,
2021]
Skin-penetrating gastrointestinal parasitic nematodes are a major source of "diseases of disadvantage," infecting approximately one billion people. Their life cycle includes an infective third-larval (iL3) stage that searches for hosts in a poorly understood process involving host-emitted sensory cues, including heat. The iL3s of the human-parasitic threadworm Strongyloides stercoralis display robust attraction to mammalian body temperatures (37°C). What are the molecular and neural mechanisms that underlie this parasite-specific positive thermotaxis behavior? Previously, we used CRISPR/Cas9-mediated mutagenesis to demonstrate that positive thermotaxis by S. stercoralis iL3s is dependent on the S. stercoralis homolog of the C. elegans
tax-4 gene. The S. stercoralis
tax-4 gene is expressed in multiple head neurons in iL3s. Now, in order to identify the primary sensory neurons responsible for positive thermotaxis in iL3s, we again leveraged the genetic similarities between S. stercoralis and C. elegans. In C. elegans, TAX-4+ AFD sensory neurons provide the primary thermosensory drive for thermotaxis navigation. We genetically identified the S. stercoralis AFD neurons via a Strongyloides homolog of the C. elegans AFD-specific gene
gcy-23, and found that inducible silencing of these neurons suppresses positive thermotaxis in iL3s. Thus, the thermosensory role of AFD is conserved between S. stercoralis and C. elegans despite species-specific differences in thermal preference and behavior. Using genetically encoded calcium sensors, we found that thermosensory responses in the S. stercoralis AFD neurons are distinct from those in the C. elegans AFD neurons. In response to thermal stimuli that mimic those experienced by iL3 during positive thermotaxis, the S. stercoralis AFD neurons exhibit a warming-triggered hyperpolarization near ambient temperature followed by near-linear positive encoding of temperatures up to human body temperature. This result is the first direct evidence of differential sensory encoding by homologous neurons in parasitic and free-living nematodes. Finally, we identified three S. stercoralis AFD-specific thermosensitive receptor guanylate cyclases that each display an expanded responsiveness to mammalian body temperatures relative to C. elegans homologs. Thus, altered thermal encoding in primary thermosensory neurons likely contributes to parasite-specific behaviors. Together, our results provide insight into the molecular mechanisms and neural circuits that allow skin-penetrating nematodes to target hosts.
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[
International Worm Meeting,
2021]
Although similar behaviors across species are often established by different neural mechanisms, it is largely assumed that within a species, similar behaviors are established by the same neural mechanism. As a result, the possibility of divergent neural pathways shaping similar behaviors within a species has been largely unexplored. Here, we challenge this assumption by elucidating the neural basis of carbon dioxide (CO2) response in C. elegans across life stages. We previously showed that well-fed adults are repelled by CO2, while starved adults are attracted to CO2; this behavioral shift is achieved through modulation of the response properties of two interneurons in the CO2 microcircuit, RIG and AIY. Like starved adults, developmentally arrested dauer larvae are attracted to CO2 despite significant differences in their internal physiology. Using a combination of in vivo calcium imaging and behavioral analyses, we show that dauers utilize a different circuit mechanism to establish the same behavioral state as that of starved adults. Notably, the AIB and AVE interneurons promote CO2 attraction in dauers but not adults, whereas RIG promotes opposite responses to CO2 across the two life stages. Moreover, AIY promotes CO2 attraction in starved adults but does not modulate CO2 response in dauers. Our results also highlight the role of dauer-specific changes in electrical synaptic connectivity in shaping interneuron activity in the CO2 microcircuit. In addition, we show that distinct sets of neuropeptides modulate CO2 attraction in dauers vs. adults, and identify a novel role for the insulin signaling pathway in mediating CO2 attraction specifically in dauers. Together, our results demonstrate that distinct intra-species circuit mechanisms can establish similar behavioral states across life stages, highlighting an unexpected complexity to chemosensory processing.