Received for publication April 26, 2005.
Revised September 15, 2005.
Accepted for publication October 18, 2005.

Altered Expression of G_q/11 Protein Shapes mGlu1 and mGlu5 Receptor-mediated Single Cell Inositol 1,4,5-trisphosphate and Ca²⁺ Signaling

Abstract

The metabotropic glutamate (mGlu) receptors, mGlu1 and mGlu5, mediate distinct inositol 1,4,5-trisphosphate (IP₃) and Ca²⁺ signaling patterns, governed in part by differential mechanisms of feedback regulation following activation. Single cell imaging has previously shown that mGlu1 receptors initiate sustained elevations in IP₃ and Ca²⁺, which are sensitive to agonist concentration. In contrast, mGlu5 receptors are subject to cyclical PKC-dependent uncoupling and consequently mediate co-incident IP₃ and Ca²⁺ oscillations that are largely agonist concentration-independent. Here, we have investigated the contribution of G_q/11 protein expression levels in shaping mGlu1/5 receptor-mediated IP₃ and Ca²⁺ signals, using RNA interference (RNAi). RNAi-mediated knockdown of G_q/11 almost abolished the single-cell increase in IP₃ caused by mGlu1 and mGlu5 receptor activation. For the mGlu1 receptor, this unmasked base-line Ca²⁺ oscillations that persisted even at maximal agonist concentrations. mGlu5 receptor-activated Ca²⁺ oscillations were still observed, but were only initiated at high agonist concentrations. Recombinant over-expression of G_q enhanced IP₃ signals following mGlu1 and mGlu5 receptor activation. Interestingly, although mGlu5 receptor-mediated IP₃ and Ca²⁺ oscillations in control cells were largely insensitive to agonist concentration, increasing G_q expression converted these oscillatory signatures to sustained plateau responses in a high proportion of cells. In addition to modulating temporal Ca²⁺ signals, up- or down-regulation of G_q/11 expression alters the threshold for the concentration of glutamate at which a measurable Ca²⁺ signal could be detected. These experiments indicate that altering G_q/11 expression levels differentially affects spatio-temporal aspects of IP₃ and Ca²⁺ signaling mediated by the mGlu1 and mGlu5 receptors.

Key words: Metabotropic glutamate, Gq/11 family, Phospholipase C's, Calcium (G Protein Coupled Signals), Ca imaging

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