Dissociation of Regulated Trafficking of TRPC3 Channels to the Plasma Membrane from Their Activation

Regulated translocation of canonical transient receptor potential (TRPC) proteins to the plasma membrane has been proposed as a mechanism of their activation. By using total internal reflection fluorescence microscopy (TIRFM), we monitored green fluorescent protein-labeled TRPC3 (TRPC3-GFP) movement to the plasma membrane in HEK293 cells stably expressing this fusion protein. We observed no increase in TRPC3-GFP TIRFM in response to the muscarinic receptor agonist methacholine or the synthetic diacylglycerol, 1-oleoyl-2-acetyl-sn-glycerol, despite activation of TRPC3 by these agents. We did, however, observe a TIRFM response to epidermal growth factor (EGF). This TIRFM response to EGFwas accompanied by increased Ba entry and TRPC3 currents. However, 1-oleoyl-2-acetyl-sn-glycerol-induced TRPC3 activity was not increased. TIRFM also increased in response to Gd , a competitive inhibitor of TRPC3 channels. This may be indicative of constitutive trafficking of TRPC3, with Gd acting to “trap” cycling TRPC3 molecules in the plasma membrane. Consistent with this interpretation,TRPC3-expressing cells exhibited large variance in membrane capacitance, and this variancewas decreased by bothGd and EGF. These results indicate the following: (i) trafficking of TRPC3 may play a role in regulating the concentration of channels in the plasmamembrane but is not involved in activation through the phospholipase C pathway; (ii) TRPC3 undergoes constitutive cyclical trafficking in the plasmamembrane, and themechanismbywhich growth factors increase the number of plasma membrane channels may involve stabilizing them in the plasmamembrane.