Chemosensory mechanisms of host-seeking and host-infection behaviors in skin-penetrating parasitic nematodes
- Author(s): Gang, Spencer S
- Advisor(s): Hallem, Elissa A
- et al.
Skin-penetrating parasitic nematodes infect approximately 1 billion people and are endemic in many developing regions worldwide. Nematode infections can lead to severe intestinal distress, anemia, stunted growth and cognitive impairment in children, and in some cases death. Skin penetrating nematodes live in the environment as developmentally arrested third stage infective larvae (iL3s) that actively search for mammals to infect, a process called host seeking. Despite the pervasive nature of these infections, very little is known about cellular and molecular mechanism that promote host-seeking behavior. In this dissertation I explore the behavioral strategies, and underlying chemosensory mechanisms, that allow skin penetrating iL3s to detect hosts in the environment. I also characterize how iL3s use sensory mechanisms to resume their development upon entering the host. We focused on the human parasitic threadworm Strongyloides stercoralis as a model for understanding chemosensory-driven behaviors in skin-penetrating nematodes. First, we investigated the host-seeking behaviors of S. stercoralis iL3s and compared its behavior to that of other parasitic nematodes. S. stercoralis iL3s are especially active in the environment, are stimulated by elevated temperatures approximating that of the human body, and also navigate toward volatile odorants emitted from human skin and sweat. The odorant response profile of S. stercoralis iL3s was distinct from that of other parasitic nematodes that infect other mammals, indicating that olfaction may play a critical role in host specificity. We observed that the host seeking behaviors of S. stercoralis iL3s are dramatically different from those of passively ingested parasitic nematode species. Thus, parasitic nematodes with different infection modes may identify and navigate to hosts in distinct ways, and in turn may require distinct environmental control measures. To address the molecular mechanisms of host seeking behavior, we leveraged the unique genetic tractability of S. stercoralis to develop CRISPR-Cas9-mediated targeted mutagenesis in parasitic nematodes. As a proof-of-concept for CRISPR-Cas9 feasibility in S. stercoralis we targeted the twitchin gene Ss-unc-22 and generated iL3s with severe motility defects, the first mutant phenotype resulting from targeted mutagenesis observed in any parasitic nematode species. Our CRISPR-Cas9 technique was then applied to disrupt S. stercoralis genes with roles in chemosensation. We targeted the Ss tax-4 gene, a subunit of a cyclic nucleotide-gated ion channel, since its ortholog in the model free-living nematode Caenorhabditis elegans is known to be required for sensory-driven behaviors. Ss-tax-4 iL3s were deficient in thermosensory driven host seeking and could not positively thermotaxis in a temperature gradient, a robust behavior observed in wild-type iL3s. Ss-tax-4 iL3s were also unable to navigate towards host-emitted 3-methyl-1-butanol (isoamyl alcohol), indicating that olfactory driven host-seeking behaviors were also disrupted in the absence of functional chemosensory pathways. Next, we asked if chemosensory mechanisms were also necessary for iL3s to successfully establish an infection in the host. To address this question, we subjected S. stercoralis iL3s to host-like conditions in vitro and assessed iL3 activation, an initial developmental step inside the host where iL3s resume feeding behavior. iL3s consistently activated in the presence of 37C and 5% CO2 environments, but removal of either stimulus eliminated activation. Ss-tax-4 iL3 were unable to activate in the presence of heat and CO2, indicating that sensory function is also critical for in-host development. Our results suggest that two novel avenues for reducing the disease burden caused by skin penetrating nematodes may be to interfere with chemosensory-driven host seeking in the environment, or to inhibit sensory-driven development inside the host. Our results also lay the foundation for investigating the sensory neural circuits underlying the parasite-specific behaviors of an important group of helminths.