ABSTRACT
Acid-sensing ion channels (ASICs) are proton-gated Na+ channels that are expressed throughout the nervous system. ASICs have been implicated in several neuronal disorders, like ischemic stroke, neuronal inflammation, and pathological pain. Several toxins from venomous animals have been identified that target ASICs with high specificity and potency. These toxins are extremely useful in providing protein pharmacophores and to characterize function and structure of ASICs. Marine cone snails contain a high diversity of toxins in their venom such as conotoxins, which are short polypeptides stabilized by disulfide bonds, and conopeptides, which have no or only one disulfide bond. Whereas conotoxins selectively target specific neuronal proteins, mainly ion channels, the targets of conopeptides are less well known. Here, we perform an in vitro screen of venoms from 18 cone snail species to identify toxins targeting ASICs. We identified a small conopeptide of only four amino acids from the venom of Conus textile that strongly potentiated currents of ASIC3, which has a specific role in the pain pathway. This peptide, RPRFamide, belongs to the subgroup of cono-RFamides. Electrophysiological characterization of isolated dorsal root ganglion (DRG) neurons revealed that RPRFamide increases their excitability. Moreover, injection of the peptide into the gastrocnemius muscle strongly enhanced acid-induced muscle pain in mice that was abolished by genetic inactivation of ASIC3. In summary, we identified a conopeptide that targets the nociceptor-specific ion channel ASIC3.
