Muscarinic Stimulation of α1E Ca Channels Is Selectively Blocked by the Effector Antagonist Function of RGS2 and Phospholipase C-β1

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Abstract
Neuronal α1E Ca channel subunits are widely expressed in mammalian brain, where they are thought to form R-type Ca channels. Recent studies have demonstrated that R-type channels contribute to neurosecretion and dendritic Ca influx, but little is known concerning their modulation. Here we show that α1E channels are strongly stimulated, and only weakly inhibited, through M1 muscarinic acetylcholine receptors. Both forms of channel modulation are mediated by pertussis toxin-insensitive G-proteins. Channel stimulation is blocked by regulator of G-protein signaling 2 (RGS2) or the C-terminal region of phospholipase C-β1 (PLCβ1ct), which have been previously shown to function as GTPase-activating proteins for Gαq. In contrast, RGS2 and PLCβ1ct do not block inhibition of α1E through M1 receptors. Inhibition is prevented, however, by the C-terminal region of β-adrenergic receptor kinase 1, which sequesters Gβγ dimers. Thus, stimulation of α1E is mediated by a pertussis toxin-insensitive Gα subunit (e.g., Gαq), whereas inhibition is mediated by Gβγ. The ability of RGS2 and PLCβ1ct to selectively block stimulation indicates these proteins functioned primarily as effector antagonists. In support of this interpretation, RGS2 prevented stimulation of α1E with non-hydrolyzable guanosine 5′-0-(3-thiotriphosphate). We also report strong muscarinic stimulation of rbE-II, a variant α1E Ca channel that is insensitive to voltage-dependent inhibition. Our results predict that Gαq-coupled receptors predominantly stimulate native R-type Ca channels. Receptor-mediated enhancement of R-type Ca currents may have important consequences for neurosecretion, dendritic excitability, gene expression, or other neuronal functions.