Receptor-operated activation of TRPC4 cation channels requires G proteins and phospholipase-Cδ1 (PLCδ1) activation by intracellular Ca Concurrent stimulation of G pathway accelerates G activation of TRPC4, which is not mimicked by increasing cytosolic Ca The kinetic effect of G was diminished by alkaline intracellular pH (pH ) and increased pH buffer capacity Acidic pH (6.75-6.25) together with the cytosolic Ca rise accelerated G -mediated TRPC4 activation Protons exert their facilitation effect through Ca -dependent activation of PLCδ1 The data suggest that G -PLCβ pathway facilitates G activation of TRPC4 through hydrolyzing PIP to produce the initial proton signal that triggers a self-propagating PLCδ1 activity supported by regenerative H and Ca . The findings provide novel mechanistic insights on receptor-operated TRPC4 activation by coincident G and G pathways and shed lights on how aberrant activation of TRPC4 may occur under pathological conditions to cause cell damage.
Transient Receptor Potential Canonical 4 (TRPC4) forms non-selective cation channels activated downstream from receptors that signal through G proteins. Our recent work suggests that TRPC4 channels are particularly coupled to pertussis toxin-sensitive G proteins, with a codependence on phospholipase-Cδ1 (PLCδ1). The G -mediated TRPC4 activation is dually dependent on and bimodally regulated by phosphatidylinositol 4,5-bisphosphate (PIP ), the substrate hydrolyzed by PLC, and intracellular Ca . As a byproduct of PLC-mediated PIP ydrolysis, protons have been shown to play an important role in the activation of Drosophila TRP channels. However, how intracellular pH affects mammalian TRPC channels remains obscure. Here, using patch-clamp recordings of HEK293 cells heterologously coexpressing mouse TRPC4β and G -coupled μ opioid receptor, we investigated the role of intracellular protons on G -mediated TRPC4 activation. We found that acidic cytosolic pH greatly accelerated the rate of TRPC4 activation without altering the maximal current density and this effect was dependent on intracellular Ca elevation. However, protons did not accelerate channel activation by directly acting upon TRPC4. We additionally demonstrated that protons exert their effect through sensitization of PLCδ1 to Ca , which in turn promotes PLCδ1 activity and further potentiates TRPC4 via a positive feedback mechanism. The mechanism elucidated here helps explain how G and G co-stimulation induces a faster activation of TRPC4 than G activation alone and highlights again the critical role of PLCδ1 in TRPC4 gating. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.