Elucidation of the serotonin 5-HT(2A) receptor-coupled phospholipase A(2) signaling pathway
Abstract
The purpose of the present study was to employ pharmacological and biochemical tools to explore the ability of the serotonin2A (5-HTZA) receptor to couple to multiple signaling cascades, namely the phospholipase C (PLC) and phospholipase A2 (PLA2) pathways, and particularly to elucidate the mechanism by which the 5-HTZA receptor activates PLA 2. Initial studies were conducted to verify that PLA2 activation was receptor-mediated and not subsequent to PLC activation. The data suggest that the 5HT2A receptor is able to activate both signaling pathways independently because several specific inhibitors of signaling molecules along each cascade were employed with no evidence that they could modulate signaling in the other pathway. Next, the ability of structurally distinct ligands to activate both the PLC and PLA2 signaling pathways was examined. In support of the hypothesis of agonist-directed trafficking, many 5-HT 2A receptor agonists displayed differential activation of the PLC and PLA2 signaling pathways. Furthermore, many of these agonists showed a difference in potency between PLC and PLA2 pathway activation, which is consistent with receptor-inactivation studies that suggested the existence of pathway-specific receptor reserve. Having verified that 5-HT2A receptor-mediated PLA activation was independent of PLC activation, studies were then conducted to elucidate a potential signal transduction cascade responsible for linking 5-HT 2A receptor activation to AA liberation. By employing various selective inhibitors, toxins, and antagonistic peptide constructs, the 5HT2A receptor was demonstrated to couple to PLA2 activation through a complex signaling mechanism involving both Gαi/o-associated Gβγ-mediated ERK1,2 activation and Gα12/13-coupled Rho-mediated p38 activation. Thus, these results suggest that the 5-HT 2A receptor can couple to multiple G proteins, namely PLC-coupled Gα q, PLA2-coupled Gαi/o, and PLA2-coupled Gα12/13, to stimulate PLA2-AA release and PLC-IP accumulation through largely independent signaling pathways. Overall, these studies demonstrate not only the ability, but also a potential mechanism by which the 5-HT2A receptor can couple to multiple signaling cascades. The data presented herein support the findings of other laboratories, which together suggest GPCR signaling to be more complex that thought previously. An understanding of the signal transduction pathways that are activated by a specific drug will provide insight into the physiological response mediated by its receptor.
Degree
Ph.D.
Advisors
Nichols, Purdue University.
Subject Area
Pharmacology|Neurology|Molecular biology
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