TRPA1: the species difference

provided a comprehensive review on the recent progress and remaining puzzles. Ample evidence suggests that the sensory function of TRPA1 is evolutionarily conserved. This notion is further supported by the fact that TRPA1 from several mammalian species (human, rat, and mouse) is activated by common ligands (a plethora of electrophylus) through a common mechanism (covalent modification). However, recent studies from us and others have revealed species-specific activation or blockade of TRPA1 by many ligands. Here, we wish to raise the awareness of the species difference issue, which has been largely overlooked thus far, but will have a profound impact on TRPA1 research and drug development. The first case of species difference between TRPA1 orthologs was demonstrated by Klionsky et al. (2007). They reported the identification of several small molecules that potently block human TRPA1 (Fig. 1 A). However , when tested against rat TRPA1, these compounds are either inactive (AMG2504 and AMG7160) or demonstrate reverse pharmacology and function as activa-tors of rat TRPA1 (AMG9090 and AMG5445) (Klionsky et al., 2007). From a high throughput screening, we identified dozens of structurally related, thioaminal-containing analogues that block human TRPA1 but activate rat TRPA1 (Chen et al., 2008) (e.g., CMP1-3; Fig. 1 A). Several lines of evidence indicate that these compounds interact with channel proteins through covalent modification. First, they are electrophilic compounds that are predicted to be reactive to nucleophilic cysteine and lysine residues. Second, when tested in La antigen-based ALARM NMR and ALARM MS studies, CMP1 modifies surface-exposed cysteines to form predicted adducts, confirming its chemical reactivity. Third, structural analogues with the reactive sulfur atom exhibit reactivity and effects on TRPA1, whereas close analogues without the reactive sulfur atom (e.g., CMP4 in Fig. 1 A) do not affect channel function. Finally, Cys-621 of human TRPA1 and Cys-622 of rat TRPA1, residues important for allyl isothiocya-nate modification, were found to be important for CMP1-mediated effects. Therefore, thioaminals covalently modify human and rat channels but produce opposite gating effects. In addition to thioaminals, several nonreactive compounds also demonstrate species-specific effects. Menthol activates mouse TRPA1 at low micromolar concentrations and blocks at higher concentration. However, menthol exhibits only an agonist effect on human TRPA1 across a range of concentrations (Xiao et al., 2008). Furthermore , caffeine was shown to activate mouse TRPA1 but suppresses human TRPA1 activity (Nagatomo and Kubo, 2008). From high throughput screening and medicinal chemistry, we identified human TRPA1 antagonists from …