Regulation of Phosphoinositide Hydrolysis by GTP-Binding Proteins, Phorbol Esters and Botulinum Toxin Type D

Abstract

The muscarinic cholinergic receptor in 1321N1 human astrocytoma cells does not inhibit adenylate cyclase activity but regulates cyclic AMP levels by affecting a cyclic AMP phosphodiesterase (1). Because muscarinic activation of phosphodiesterase is calcium-dependent, Ken Harden and I postulated that muscarinic receptor stimulation in these cells caused phosphoinositide hydrolysis and calcium mobilization. Subsequently, in a series of joint and independent publications, we demonstrated that in 1321N1 cells muscarinic agonists stimulate the formation of inositol phosphates, including the active 1,4,5 isomer of InsP₃ (2–6) and mobilize calcium by release from intracellular stores (2,4,7). The muscarinic receptor in these cells had been shown to be regulated by guanine nucleotides (8), and the interaction of the receptor with a putative GTPbinding (G-) protein could be shown to correlate with the capacity of an agonist to stimulate inositol phosphate formation and ^{4 5}Ca⁺⁺ efflux (4). More direct evidence that a G-protein transduced muscarinic receptor binding into phospholipase C activation came from studies with cell free systems. In these washed membranes GTPγS caused marked stimulation of inositol phosphate production, a response that could be potentiated by the agonist carbachol but not effected by the agonist alone in the absence of added guanine nucleotide (9,10). The studies described in this paper deal primarily with our attempts to manipulate and characterize the G-protein that regulates phospholipase C.