RPRFamide is a

neurotoxin Neurotoxins are toxins that are destructive to nervous tissue, nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insult (medical), insultsSpencer 2000 that can adversely affect function ...

belonging to the conorfamide family of

neuropeptides Neuropeptides are chemical messengers made up of small chains of amino acids that are synthesized and released by neurons. Neuropeptides typically bind to G protein-coupled receptors (GPCRs) to modulate neural activity and other tissues like the ...

, which can be found in the venom of cone snails.

Etymology and source

RPRFamide is a toxin from the carnivorous marine cone snail ''

Conus textile ''Conus textile'', the textile cone or the cloth of gold cone is a venomous species of sea snail, a marine gastropod mollusk in the family Conidae, the cone snails, cone shells or cones. Textile cone snails live mostly in the Indian Ocean, along ...

'', a predatory species that mainly lives in tropical waters. The venom of marine cone snails contains a diverse variety of toxins, which include conotoxins. RPRFamide belongs to the family of conotoxins, more specifically to the conorfamide family or RFamide family which are

peptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...

s that target neuronal

ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by Gating (electrophysiol ...

s in their prey.

Chemistry

The sequence for this toxin is identified as RPRF (R =

arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidinium, guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) a ...

, P =

proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the p ...

, and F =

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the chemical formula, formula . It can be viewed as a benzyl group substituent, substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of ...

). An amide group (-NH₂) is located at the terminal end (C-terminus). The presence of this group is paramount for its biological activity as it enhances its interaction with ion channels. The short length, the C-terminal Arg–Phe–NH₂ (RFa) motif, and the lack of cysteines clearly distinguishes these peptides from conotoxins and categorises them as cono-RFamides.

Target

Two main molecular targets have been discovered for RPRFamide. Firstly, it targets the acid sensing ion channel 3 (ASIC3), involved in the pain pathway. Secondly, it can target and inhibit

nicotinic acetylcholine receptor Nicotinic acetylcholine receptors, or nAChRs, are Receptor (biochemistry), receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the c ...

s (nAChRs), specifically the alpha-7 subtype.

Mode of action

The RPRFamide peptide modulates ASIC3, a proton-gated ion channel that is sensitive to acidic conditions and involved in pain perception. This channel is a proton-gated sodium channel involved in

nociception In physiology, nociception , also nocioception; ) is the Somatosensory system, sensory nervous system's process of encoding Noxious stimulus, noxious stimuli. It deals with a series of events and processes required for an organism to receive a pai ...

in response to acidic environments in a tissue, such as muscle fatigue. The toxin enhances ASIC3 currents, leading to increased pain signalling, particularly in response to acidic stimuli. This explains why RPRFamide can induce pain, particularly muscle pain, through the activation of ASIC3 channels. The peptide delays the desensitization of ASIC3 channels, keeping them open longer and allowing sustained ion flow, which increases sensitivity to pain stimuli and prolongs the nociceptive effect. Studies show that injecting cono-RFamide into mice muscle leads to increased acid induced pain. Additionally, studies showed that RPRFamide causes an increase in excitability of

dorsal root ganglion A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the do ...

(DRG) neurons. The RPRFamide also modulates nACh receptors by inhibiting them, specifically the alpha-7 and muscle-type nAChRs. These receptors are

ligand-gated ion channel Ligand-gated ion channels (LICs, LGIC), also commonly referred to as ionotropic receptors, are a group of transmembrane ion-channel proteins which open to allow ions such as sodium, Na+, potassium, K+, calcium, Ca2+, and/or chloride, Cl− to ...

s that mediate fast synaptic transmission in the nervous system and are involved in neuromuscular function. The toxin's inhibitory effect prevents the influx of ions that would normally result from

acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

binding, disrupting neurotransmission and impairing muscle contraction, depending on the receptor subtype.

Toxicity

The toxicity of RPRFamide has yet to be assessed in humans. However, available literature suggests that ASIC3 channels are expressed in muscle pain receptors, leading to extreme, long-lasting pain when injected into muscle tissue in mice, particularly when administered with an acidic solution.

Treatment

Currently, no available literature describes a method to counteract the neurotoxic activity of RPRFamide.

Therapeutic use

The therapeutic potential of RPRFamide has yet to be fully assessed. Some authors have discussed the neurotoxin's modulation of ASIC3 and nAChRs receptors, suggesting that further research could explore its role in pain modulation, including potential treatments for chronic pain.

References

{{reflist Ion channel toxins Neurotoxins Snail toxins Tetrapeptides Amides