The stinging nettle (Dendrocnide excels) is one example of a large number of plants belonging to the nettle family, Urticaceae. While stinging nettle can be found almost anywhere, it is most common in Europe, North America, North Africa, and parts of Asia. Urticaceae family is notable for its species with a painful, occasionally lethal sting due to the presence of stinging trichomes, carrying a fluid rich in formic acid, histamine, acetylcholine, and serotonin. Physical contact with stinging nettle often results in a nonimmunologic-induced contact urticarial or irritant dermatitis. There is an immediate burning, stinging, pruritic sensation in the exposed area, followed within minutes by a blanching urticarial rash. The sensitivity of humans to stinging nettle contact appears to be variable, and many do not seek medical attention because of the short-lived duration of the symptoms (minutes to hours). Some individuals experience symptoms for greater than 12 hours.
The mechanism of action of toxicity following the penetration of human skin and injection of liquid from the base glands of the nettle hairs may be biochemical or mechanical. Based on animal experiments, some of the trichomes may remain in the skin after contact and cause direct mechanical irritation. The biochemical indicators of inflammation injected by the nettle hairs appear to include histamine, serotonin, acetylcholine and leukotrienes B4 and C4; however, it has been suggested that there may be additional substances in nettle fluid directly toxic to nerves or capable of secondary release of other mediators.
The toxins present in the
trichomes belong to the gympietide family of stinging nettle toxins. ExTxA, the
first identified member of the gympietide family, elicits spontaneous pain
behaviors and an axon reflex flare. The major effect of ExTxA is a striking
inhibition of NaV inactivation, leading to persistent currents that
likely contribute to enhanced excitability and spontaneous action potential
firing. Voltage-gated sodium (NaV) channels are critical
regulators of neuronal excitability and are targeted by many toxins that
directly interact with the pore-forming α subunit, typically via
extracellular loops of the voltage-sensing domains, or residues forming part of
the pore domain. Excelsatoxin A (ExTxA), a pain-causing knott in peptide from
the Australian stinging tree Dendrocnide excelsa, is the first
reported plant-derived NaV channel modulating peptide toxin.
Nav 1.7 is a sodium ion channel that in humans is encoded by the SCN9A gene and usually expressed at high levels in two types of neurons viz. the nociceptive neurons at dorsal root ganglion and trigeminal ganglion and sympathetic ganglion neurons, which are part of the autonomic nervous system. Nav 1.7 plays a critical role in the generation and conduction of action potentials and thus important for electrical signaling by most excitable cells. Nav 1.7 is present at the ending of pain-sensing nerves, the nociceptors, close to the region where the impulse is initiated. Stimulation of the nociceptor nerve endings produces "generator potentials", which are small changes in the voltage across the neuronal membranes. The Nav1.7 channel amplifies these membrane depolarizations, and when the membrane potential difference reaches a specific threshold, the neuron fires. The Nav1.7 channel produces a rapidly activating and inactivating current which is sensitive to the level of tetrodotoxin, a voltage-gated sodium-channel blocker. Nav1.7 activity consists of a slow transition of the channel into an inactive state when it is depolarized, even to a minor degree. This property allows these channels to remain available for activation with even small or slowly developing depolarizations. In sensory neurons, at least five of the nine mammalian isoforms NaV1.1-NaV1.9 are expressed, where they contribute to physiological and pathological neuronal excitability. NaV channels are composed of a Na+ ion conducting α subunit that can associate with one or more accessory β subunits, and respond to changes in transmembrane voltage with conformational rearrangements leading to opening of the channel pore.
There are numerous toxins directly interact with α subunit, either at the pore or via extracellular loops of the voltage-sensing domains, and belong to structurally diverse classes that inhibit or enhance channel function. Gympietides are one of the main causative agents eliciting spontaneous action potential discharge, an axon reflex flare and nocifensive behaviors in vivo. These remarkable peptides are characterized by a unique primary amino acid sequence and a tertiary structure closely resembling inhibitory cystine knot peptides typically found in animal venoms. Consistent with this high structural homology, synthetic gympietides inhibit NaV channel inactivation in dissociated dorsal root ganglion neurons, analogous to effects caused by NaV channel-targeting inhibitory cystine knot peptides from cone snail or spider venoms.
Earlier it was considered that Pn3a inhibits ExTxA-induced pain-like responses in mice, and supports the evolution of the gympietides as vertebrate-specific pain-causing defensive agents. But recently it was found that, toxins activity actually requires co-expression of TMEM233, a member of the dispanin family that highly expressed in sensory neurons, and that can associate with NaV1.7 to subtly modify inactivation properties. NaV channels are known to function as multi-subunit complexes in native neurons, where they associate not only with auxiliary β subunits, but also several other proteins that may affect post-translational modifications, channel expression, trafficking, and function. The β subunits are known to modulate interaction of various venom-derived peptides with the pore-forming NaV α subunit; for example, the kinetics of inhibition by μ-conotoxins is affected by the presence of β subunits, while co-expression of β2 or β4 prevents inhibition by the μO§-conotoxin GVIIJ. However, all previously studied NaV-targeting toxins appear to bind to α subunit, making the gympietides toxins that require an interacting protein to modulate NaV channel function. In addition, the gympietides are also dispanin ligands, raising the possibility of future development of selective ligands that may find use as tool compounds to interrogate the function of this protein family, or that may ultimately lead to therapeutic applications. Results revealed that, TMEM233 is essential for pharmacological activity of ExTxA at NaV channels, and that co-expression of TMEM233 modulates the gating properties of NaV1.7.
Sources
Kathleen, G. 2022. Establishing the monophyly and a first phylogeny for the nettle tree genus Dendrocnide (Urticaceae). A Master’s Dissertation.
Jami, S., Deuis, J.R., Klasfauseweh, T. et al. Pain-causing stinging nettle toxins target TMEM233 to modulate NaV1.7 function. Nat Commun 14, 2442 (2023). https://doi.org/10.1038/s41467-023-37963-2
Baumgardner DJ. Stinging nettle: the bad, the good, the unknown. J Patient Cent Res Rev. 2016;3:48-53. doi: 10.17294/2330-0698.1216