A structural view of ligand-dependent activation in thermoTRP channels

Abstract

Transient Receptor Potential (TRP) proteins are a large family of ion channels, grouped into seven sub-families. Although great advances have been made regarding the activation and modulation of TRP channel activity, detailed molecular mechanisms governing TRP channel gating are still needed. Sensitive to electric, chemical, mechanical, and thermal cues, TRP channels are tightly associated with the detection and integration of sensory input, emerging as a model to study the polymodal activation of ion channel proteins. Among TRP channels, the temperature-activated kind constitute a subgroup by itself, formed by Vanilloid receptors 1-4, Melastatin receptors 2, 4, 5, and 8, TRPC5, and TRPA1. Some of the so-called "thermoTRP" channels participate in the detection of noxious stimuli making them an interesting pharmacological target for the treatment of pain. However, the poor specificity of the compounds available in the market represents an important obstacle to overcome. Understanding the molecular mechanics underlying ligand-dependent modulation of TRP channels may help with the rational design of novel synthetic analgesics. The present review focuses on the structural basis of ligand-dependent activation of TRPV1 and TRPM8 channels. Special attention is drawn to the dissection of ligand-binding sites within TRPV1, PIP2-dependent modulation of TRP channels, and the structure of natural and synthetic ligands.

Keywords: PIP2; TRP channels; TRPM8; TRPV1; capsaicin; menthol; structure.

Figure 1

Structural features of the capsaicin receptor. (A) Conserved structural domains: Ankyrin repeat domain—Olive. Pre-TM1 helix—Salmon. TM1-TM4 domain—Pale pink. TM4-TM5 linker—Cyan. Selectivity filter—Green. Gate—Fuchsia. TM5-TM6 domain—Yellow. TRP domain—Orange. (B) Residues involved in ligand-binding and/or modulation of channel activity: Colors represent residues location. TM1-TM4 domain—Fuchsia. Selectivity filter and pore helix—Green. TM5-TM6 domain—Yellow. Intracellular loops—Orange. Extracellular loops—Red. (C) Putative ligand-binding sites: Vanilloids—Red. Fatty acids and lipids—Green. PIP₂—Cyan. Cysteine residues—Yellow. TRPV1 structure (PDB ID 3J5P) was visualized and colored using PyMOL Molecular Graphics System.

Figure 2

PIP₂ mediates intra- and inter-subunit contacts. (A) Identified PIP₂ binding pocket in the structure of TRPV1. (B) Residues proposed as mediators of PIP₂ interaction to the channel. (C) K710 is located at the distal end of the TD helix and interacts with Q498 located at the bottom of TM2. (D) Predicted/potential cation-π interactions connecting the TRP box of the TD helix with the TM4-TM5 helix and N-terminal region. In order to get a better placement of the lateral chains of the available structure, the TRPV1 channel (PDB ID 3J5P) was embedded in lipids (POPC), and placed in a periodic box containing water and ions (140 mM KCl). After 5 ns of all-atom MD simulations using the NAMD/CHARMM32 force-field, the channel was visualized using VMD.

Figure 3

The structure of natural and endovaniloids. (A) Natural compounds with agonist activity on TRPV1 receptors. Figure indicates the affinity for specific ³[H]-resiniferatoxin binding sites in rat spinal cord (Szallasi and Blumberg, 1999). (B) Structure of endovanilloids identified as activators of TRPV1 receptors.

Figure 4

The structure of natural thermoTRP ligands. (A) Leucettamol A and B identified with TRP channel activity (Chianese et al., 2012). (B) Cinnamaldehyde, TRPA1 activator (Macpherson et al., 2007). (C) Structure of monoterpenoids and related compounds evaluated on TRPM8, TRPV3, and TRPA1 receptors (Vogt-Eisele et al., ; Takaishi et al, 2012). (D) Structure of prostaglandins evaluated in TRPA1 receptors (Taylor-Clark et al., 2008). (E) Activity profile of human TRPA1 and mouse TRPM8 channels after addition of thymol (Lee et al., 2008) and menthol (Sherkheli et al., 2010), respectively.

Figure 5

Vanilloids and methylamines as thermoTRP channel ligands. (A) TRPV1 agonist activity. (B) TRPV1 antagonist activity. (C) TRP channel antagonists.

Figure 6

Fatty acids and lipids as thermoTRP channel ligands. (A) Polyunsaturated fatty acids (PUFAs) evaluated on TRPV1 and TRPA1 receptors (Matta et al., ; Motter and Ahern, 2012). (B) 5-nitro-2-(3-phenylpropylamino) benzoic acid derivatives evaluated on TRPA1 receptors (Liu et al., 2010).