Dynamics of receptor-operated Ca(2+) currents through TRPC channels controlled via the PI(4,5)P2-PLC signaling pathway

Abstract

Transient receptor potential canonical (TRPC) channels are Ca(2+)-permeable, nonselective cation channels that carry receptor-operated Ca(2+) currents (ROCs) triggered by receptor-induced, phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Within the vasculature, TRPC channel ROCs contribute to smooth muscle cell depolarization, vasoconstriction, and vascular remodeling. However, TRPC channel ROCs exhibit a variable response to receptor-stimulation, and the regulatory mechanisms governing TRPC channel activity remain obscure. The variability of ROCs may be explained by their complex regulation by PI(4,5)P2 and its metabolites, which differentially affect TRPC channel activity. To resolve the complex regulation of ROCs, the use of voltage-sensing phosphoinositide phosphatases and model simulation have helped to reveal the time-dependent contribution of PI(4,5)P2 and the possible role of PI(4,5)P2 in the regulation of ROCs. These approaches may provide unprecedented insight into the dynamics of PI(4,5)P2 regulation of TRPC channels and the fundamental mechanisms underlying transmembrane ion flow. Within that context, we summarize the regulation of TRPC channels and their coupling to receptor-mediated signaling, as well as the application of voltage-sensing phosphoinositide phosphatases to this research. We also discuss the controversial bidirectional effects of PI(4,5)P2 using a model simulation that could explain the complicated effects of PI(4,5)P2 on different ROCs.

Keywords: Ca2+ signaling; PIP2; TRPC channels; receptor-operated calcium current; smooth muscle; voltage-sensing phosphatase.

FIGURE 1

Transient receptor potential canonical (TRPC) channels receptor-operated Ca²⁺ currents (ROCs). (A) Schematic representation of ROCs. Binding of an agonist to a G_q-protein-coupled receptor leads to phospholipase C (PLC) activation. The activated PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] to produce of diacylglycerol (DAG) and IP₃. DAG and the reduction of PI(4,5)P₂ levels directly contribute to TRPC channels activation, while IP₃ triggers Ca²⁺ release from intracellular stores. (B) ROCs through TRPC7 channels. TRPC7 currents were induced using carbachol, a muscarinic receptor agonist (gray line, 100 μM). Left and right panels respectively display currents observed with low and high levels of muscarinic receptor expression (data from Itsuki et al., 2014). The vertical and horizontal gray scale bars indicate 200 pA and 15 s, respectively.

FIGURE 2

The inhibition upon the voltage-sensing phosphatases (VSPs) activation. (A) TRPC6 currents induced by a DAG analog (OAG) are transiently inhibited by Dr-VSP activation (Imai et al., 2012). (B) Inhibitory effects of VSPs on TRPC6 and KCNQ2/3 channels. Data for TRPC6 channels are from Imai et al. (2012). Parenthesizes indicate values for KCNQ2/3 channels (Falkenburger et al., 2010). *Unpublished data from experiments in which equal amounts of plasmid harboring cDNA encoding Ci-, Dr-, or Gg-VSP, and TRPC6 were co-transfected into HEK293 cells. Currents were evoked using 50 μM OAG.