Requirement of extracellular Ca 2+ binding to specific amino acids for heat-evoked activation of TRPA1.

KEY POINTS: We found that extracellular Ca2+ , but not other divalent cations (Mg2+ and Ba2+ ) or intracellular Ca2+ , is involved in heat-evoked activation of green anole (ga) TRPA1. Heat-evoked activation of chicken (ch) and rat snake (rs) TRPA1 does not depend solely on extracellular Ca2+ . Neutralization of acidic amino acids on the outer surface of TRPA1 by extracellular Ca2+ is important for heat-evoked large activation of gaTRPA1, chTRPA1 and rsTRPA1.

ABSTRACT: Transient receptor potential ankyrin 1 (TRPA1) is a homotetrameric non-selective cation-permeable channel that has six transmembrane domains and cytoplasmic N- and C-termini. The N-terminus is characterized by an unusually large number of ankyrin repeats. Although the 3-dimensional structure of human TRPA1 has been determined, and TRPA1 channels from insects to birds are known to be activated by heat stimulus, the mechanism for temperature-dependent TRPA1 activation is unclear. We previously reported that extracellular Ca2+ , but not intracellular Ca2+ , plays an important role in heat-evoked TRPA1 activation in green anole lizards (gaTRPA1). Here we focus on extracellular Ca2+ -dependent heat sensitivity of gaTRPA1 by comparing gaTRPA1 with heat-activated TRPA1 channels from rat snake (rsTRPA1) and chicken (chTRPA1). In the absence of extracellular Ca2+ , rsTRPA1 and chTRPA1 are activated by heat and generate small inward currents. A comparison of extracellular amino acids in TRPA1 identified three negatively charged amino acid residues (glutamate and aspartate) near the outer pore vestibule that are involved in heat-evoked TRPA1 activation in the presence of extracellular Ca2+ . These results suggest that neutralization of acidic amino acids by extracellular Ca2+ is important for heat-evoked activation of gaTRPA1, chTRPA1, and rsTRPA1, which could clarify mechanisms of heat-evoked channel activation.