TRPA1 is a non-selective cation channel that is mainly expressed in primary sensory neurons. TRPA1 functions as a nociceptor to be activated by various noxious compounds and physiological stresses such as mechanical and temperature stimuli. The pharmacological properties of TRPA1 have been compared in a wide variety of species, and species diversity among them has been reported. Zinc is a TRPA1 activator and intracellular histidine residues of TRPA1 are involved in the sensitivity to intracellular zinc. However, the mechanism of TRPA1 activation by extracellular zinc is unclear. The purpose of this study is to elucidate the molecular mechanism of TRPA1 activation by extracellular zinc to use human and chicken TRPA1.
Functional analysis of TRPA1 was performed using Ca²⁺ and Zn²⁺ indicators and a heterologous expression system with HEK293 cells. HEK293 cells expressing human TRPA1 (hTRPA1), chicken TRPA1 (chTRPA1), and their mutants were loaded with fura-2 and FluoZin-3.
Although hTRPA1 was activated by extracellular application of zinc, chTRPA1 was not even in the presence of essential Zn²⁺-sensitive histidine residues. When mustard oil was applied to chTRPA1-expressing cells to open the pore of chTRPA1, the addition of extracellular zinc induced a large increase in intracellular Ca²⁺ concentration. These results suggest that the channel state of chTRPA1 may limit zinc-permeability compared to hTRPA1.