Assessment of the Role of the Inositol 1,4,5-Trisphosphate Receptor in the Activation of Transient Receptor Potential Channels and Store-operated Ca 2+ Entry Channels

Abstract

The mechanism for coupling between Ca2+ stores and store-operated channels (SOCs) is an important but unresolved question. Although SOCs have not been molecularly identified, transient receptor potential (TRP) channels share a number of operational parameters with SOCs. The question of whether activation of SOCs and TRP channels is mediated by the inositol 1,4,5-trisphosphate receptor (InsP3R) was examined using the permeant InsP3R antagonist, 2-aminoethoxydiphenyl borate (2-APB) in both mammalian and invertebrate systems. In HEK293 cells stably transfected with human TRPC3 channels, the actions of 2-APB to block carbachol-induced InsP3R-mediated store release and carbachol-induced Sr2+ entry through TRPC3 channels were both reversed at high agonist levels, suggesting InsP3Rs mediate TRPC3 activation. However, electroretinogram recordings of the light-induced current in Drosophila revealed that the TRP channel-mediated responses in wild-type as well as trp and trpl mutant flies were all inhibited by 2-APB. This action of 2-APB is likely InsP3R-independent since InsP3Rs are dispensable for the light response. We used triple InsP3R knockout DT40 chicken B-cells to further assess the role of InsP3Rs in SOC activation. 45Ca2+ flux analysis revealed that although DT40 wild-type cells retained normal InsP3Rs mediating 2-APB-sensitive Ca2+ release, the DT40InsP3R-k/o cells were devoid of functional InsP3Rs. Using intact cells, all parameters of Ca 2+ store function and SOC activation were identical in DT40wt and DT40InsP3R-k/o cells. Moreover, in both cell lines SOC activation was completely blocked by 2-APB, and the kinetics of action of 2-APB on SOCs (time dependence and IC50) were identical. The results indicate that (a) the action of 2-APB on Ca2+ entry is not mediated by the InsP3R and (b) the effects of 2-APB provide evidence for an important similarity in the function of invertebrate TRP channels, mammalian TRP channels, and mammalian storeoperated channels.