Fluorescence changes reveal kinetic steps of muscarinic receptor–mediated modulation of phosphoinositides and Kv7.2/7.3 K+ channels

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Abstract
G protein–coupled receptors initiate signaling cascades. M1 muscarinic receptor (M1R) activation couples through Gαq to stimulate phospholipase C (PLC), which cleaves phosphatidylinositol 4,5-bisphosphate (PIP2). Depletion of PIP2 closes PIP2-requiring Kv7.2/7.3 potassium channels (M current), thereby increasing neuronal excitability. This modulation of M current is relatively slow (6.4 s to reach within 1/e of the steady-state value). To identify the rate-limiting steps, we investigated the kinetics of each step using pairwise optical interactions likely to represent fluorescence resonance energy transfer for M1R activation, M1R/Gβ interaction, Gαq/Gβ separation, Gαq/PLC interaction, and PIP2 hydrolysis. Electrophysiology was used to monitor channel closure. Time constants for M1R activation (<100 ms) and M1R/Gβ interaction (200 ms) are both fast, suggesting that neither of them is rate limiting during muscarinic suppression of M current. Gαq/Gβ separation and Gαq/PLC interaction have intermediate 1/e times (2.9 and 1.7 s, respectively), and PIP2 hydrolysis (6.7 s) occurs on the timescale of M current suppression. Overexpression of PLC accelerates the rate of M current suppression threefold (to 2.0 s) to become nearly contemporaneous with Gαq/PLC interaction. Evidently, channel release of PIP2 and closure are rapid, and the availability of active PLC limits the rate of M current suppression.