Structure, function, and evolution of nettle caterpillar venom toxins

Insects mostly use their venom to subdue prey, or for both subduing prey and defense from predators, but some rare species such as larval lepidopterans (caterpillars) use venom solely for deterring predators. Of the 133 lepidopteran families, nine (Erebidae, Eucliedae, Lasiocampidae, Limacodidae, Megalopygidae, Notodontidae, Nymphalidae, Saturniidae and Zygaenidae) can cause serious pathophysiological conditions in humans and animals. Although they present a health hazard, caterpillars are under-represented in venom research and little is known about the structure and function of their toxins.

Limacodidae is a family of more than 1,500 species, over half of which, known as stinging nettles, have venomous larvae (1). Envenomation by limacodids mainly causes intense pain, itch, and erythema, and occasionally numbness, weakness, nausea, and dizziness. Using a combination of imaging technologies, transcriptomics, proteomics and functional assays, we recently provided a holistic portrait of the venom system of one species, Doratifera vulnerans, which produces a complex peptide-rich venom, contrary to the common belief that defensive venoms have simple composition (2). Three of the most abundant families of venom peptides are: (i) disulfide-rich knottins similar to the inhibitor cystine knot (ICK) peptides that dominate spider venoms; (ii) linear, cationic cecropin-like peptides that cause pain when injected into mammals, and also kill bacteria, insects and parasites; and (iii) homologues of adipokinetic hormone/corazonin-related neuropeptide (ACPs) (2).

Using proteomics and transcriptomics, I am currently examining the venom of additional limacodid species, including the saddleback caterpillar Acharia stimulea native to eastern North America, and an undescribed species collected in Townsville, Australia. Surprisingly, I found that the North American species possesses venom that is very similar to that of the Australian D. vulnerans whereas venom of the undescribed Australian species is radically different, hinting at complex evolution of venom composition within Limacodidae. I am also expanding knowledge of the evolutionary trajectory of individual toxins by synthesising and characterising peptides intermediate in structure between the ancestral cecropin of non-venomous Lepidoptera and D. vulnerans cecropin-like venom toxins. Finally, I am producing a library of limacodid venom peptides using solid-phase peptide synthesis and recombinant expression that I will test on mammalian ion channels including those involved in pain. This research will provide novel insights into multiple aspects of limacodid venoms.

References:

1. Epstein M, ed. (1996) Revision and phylogeny of the Limacodid-group families, with evolutionary studies on slug caterpillars (Lepidoptera:Zygaenoidea), Smithsonian Institution Press, Washington DC.
2. Walker AA, Robinson SD, Paluzzi JV, Merritt DJ, Nixon SA, Schroeder CI, et al. (2021) Production, composition, and mode of action of the painful defensive venom produced by a limacodid caterpillar, Doratifera vulnerans. Proc Natl Acad Sci USA. 118, e2023815118.