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A team of scientists from the University of Queensland and King’s College London has discovered that the venom of the Australian Dendrochnid Trees contain previously unidentified neurotoxic peptides and that the 3D structure of these pain-inducing peptides is reminiscent of spider and cone snail venoms that target the same pain receptors, representing a remarkable case of convergent evolution between kingdoms of animal and plant poisons.
Nettles of the genus Dendrochnid produce powerful neurotoxins: (A) sign in a North Queensland National Park warning of biting trees to be careful; (YES) Dendrocnide excelsa petioles are covered with sharp hairs; (C) Scanning electron micrograph of the trichome structure in the leaf of Dendrocnide moroides; (DG) skin reaction resulting from an accidental sting with Dendrocnide moroides documented with an iPhone XR and NEC G120W2 thermal imaging camera, illustrating near-immediate local piloerection (arrowheads at D), the development of welts where sharp hairs penetrate the skin (arrows at E), as well as an erythema of the reflex long-term axonal (arrows in F and G) and associated local increase in skin temperature (degrees Celsius); (H) HPLC chromatogram of trichome extract of Dendrocnide excelsa; diamonds indicate deleterious responses elicited by intraplantar administration of individual fractions in vivo in C57BL6 / J mice, with a single late-eluting peak identified as the major pain-causing fraction. Image credit: Irina Vetter, Thomas Durek & Darren Brown, University of Queensland.
Australia is notoriously home to some of the most poisonous animals in the world, but while less well known, its poisonous flora is just as remarkable.
The giant tree that stingsDendrocnide excelsa) reigns supreme in size, with some specimens growing up to 35 m (115 ft) tall along the slopes and gullies of the tropical rainforests of eastern Australia. However, these members of the Urticaceae family are much more than large nettles.
Of the six species of the genus Dendrochnid native to the tropical and subtropical forests of eastern Australia, the giant biting tree and the mulberry-shaped biting tree (Dendrocnide moroides) are particularly notorious for producing painful stings, which can cause symptoms that last for days or weeks in extreme cases.
“Like other stinging plants, such as nettles, the giant tree is covered in needle-shaped appendages called trichomes that are about five millimeters long; trichomes look like fine hairs, but they actually act like hypodermic needles that inject toxins when they come into contact with the skin, ”said Dr Irina Vetter, a researcher at the Institute for Molecular Bioscience and the College of Pharmacy at the University of Queensland.
Small molecules have previously been tested in trichomes, such as histamine, acetylcholine, and formic acid, but injection of these did not cause the severe and long-lasting pain of the itchy tree, suggesting that an unidentified neurotoxin could be found.
“We were interested in finding out if there were neurotoxins that could explain these symptoms and why Gympie-Gympie can cause such long-lasting pain,” said Dr. Vetter.
The scientists found a completely new class of neurotoxin miniproteins that they named ‘gympietides’, after the indigenous name of the plant.
“Although they come from a plant, gympietides are similar to the toxins in spiders and cone snails in the way that they fold into their three-dimensional molecular structures and target the same pain receptors; this possibly makes the Gympie-Gympie tree a truly ‘poisonous’ plant, ”Dr. Vetter said.
“The prolonged pain of the burning tree can be explained by the permanent change of the gympietides in the sodium channels in the sensory neurons, not because the fine hairs stick to the skin.”
“By understanding how this toxin works, we hope to provide better treatment for those who have been bitten by the plant, to relieve or eliminate pain.”
“We can also potentially use gympietides as scaffolds for new pain relief therapies.”
The findings were published online in the journal. Scientific advances.
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Edward K. Dorado et al. 2020. Neurotoxic Peptides from Australian Giant Tree Venom. Scientific advances 6 (38): eabb8828; doi: 10.1126 / sciadv.abb8828
