Animal Toxins Providing Insights into TRPV1 Activation Mechanism

Abstract

Beyond providing evolutionary advantages, venoms offer unique research tools, as they were developed to target functionally important proteins and pathways. As a key pain receptor in the nociceptive pathway, transient receptor potential vanilloid 1 (TRPV1) of the TRP superfamily has been shown to be a target for several toxins, as a way of producing pain to deter predators. Importantly, TRPV1 is involved in thermoregulation, inflammation, and acute nociception. As such, toxins provide tools to understand TRPV1 activation and modulation, a critical step in advancing pain research and the development of novel analgesics. Indeed, the phytotoxin capsaicin, which is the spicy chemical in chili peppers, was invaluable in the original cloning and characterization of TRPV1. The unique properties of each subsequently characterized toxin have continued to advance our understanding of functional, structural, and biophysical characteristics of TRPV1. By building on previous reviews, this work aims to provide a comprehensive summary of the advancements made in TRPV1 research in recent years by employing animal toxins, in particular DkTx, RhTx, BmP01, Echis coloratus toxins, APHCs and HCRG21. We examine each toxin's functional aspects, behavioral effects, and structural features, all of which have contributed to our current knowledge of TRPV1. We additionally discuss the key features of TRPV1's outer pore domain, which proves to be the target of the currently discussed toxins.

Keywords: TRPV1; centipede toxin; nociception; outer pore domain; pain; scorpion toxin; sea anemone; snake toxin; spider toxin; venom.

Figure 1

Pore-forming domains in the TRPV1 tetramer. (A) Top view of closed tetrameric TRPV1 channel showing transmembrane helices and emphasizing the intertwined subunits arranged around a central pore. Each subunit is color-coded individually. PDB ID: 5IRZ. (B) Color coded outer pore domain and pore-forming structures from a top-down view of the tetrameric channel. S5 (green) links with the pore helix (pink) via its linker (black). The pore helix connects to S6 (gold) via an outer pore linker (blue), which also harbors the upper selectivity filter Gly 643 (red). A lower selectivity filter (red) appears further down S6 at Ile 679 (red). S1–S4 are represented in grey. Note that the pore turret (23 AA) situated between S5 and the pore helix is omitted in this structure. PDB ID: 5IRZ. (C) Side view of a single TRPV1 subunit color coded as described in B. PDB ID: 5IRZ.

Figure 2

The amphipathic nature of the DkTx structure allows it to protrude the membrane bilayer. Individually visualized knots of DkTx, with the linker (cyan) between the two knots appearing on K1 (A) Hydrophobic residues are labeled in red and polar residues in blue. This amphipathic nature presumably enables DkTx to successfully protrude into the lipid environment of the cell membrane. Key amino acids indicated in TRPV1 binding according to computational scan studies have been labeled (). Disulfide bridges are labeled in yellow. K1, PDB ID2N9Z. (B) K2 knot of DkTx labeled as described in A. Amino acids in K2 are numbered according to the molecule in its entirety.PDB ID: 2NAJ.

Figure 3

The polar RhTx binds TRPV1 through its charged surface. Structure of RhTx indicating the polarity of the molecule. Charged residues D20, K21, Q22 and E27 (red) have been indicated in TRPV1 binding, along with the polar residue R15. Two cysteine bridges are highlighted in yellow. PDB ID: 2MVA.

Figure 4

BmP01’s K23 forms an electrostatic interaction with TRPV1 outer pore region. Structure of BmP01 indicating the typical ICK motif formed by three disulfide bonds (indicated in yellow). Red indicates key amino acid K23, which interacts with E649 of TRPV1, an important proton-binding site in TRPV1 channel activation. PDB ID: 1WM7.

Figure 5

Significant amino acids for toxin-induced modulation of TRPV1 activity. (A) The outer pore region is collectively colored in gold, and S1–S4 are labeled in fuchsia. Intracellular structures are blue. PDB ID: 5IRZ. (B) Key amino acids indicated in channel activation by each individual toxin are labeled according to the toxin: DkTx, Red; RhTx, Blue; BmP01, Purple; APHC, Green. Whole subunits are lightly colored to visualize the interface between adjacent subunits. PDB ID: 5IRZ.