Towards a quantitative representation of the cell signaling mechanisms of hallucinogens: Measurement and mathematical modeling of 5-HT1A and 5-HT2A receptor-mediated ERK1/2 activation

Through a multidisciplinary approach involving experimental and computational studies, we address quantitative aspects of signaling mechanisms triggered in the cell by the receptor targets of hallucinogenic drugs, the serotonin 5-HT2A receptors. To reveal the properties of the signaling pathways, and the way in which responses are elicited through these receptors alone and in combination with other serotonin receptors subtypes (the 5-HT1A), we developed a detailed mathematical model of receptor-mediated extracellular signal-regulated kinase 1 and 2 (ERK1 and 2, ERK1/2) activation in cells expressing the 5-HT1A and 5-HT2A subtypes individually, and together. In parallel, we measured experimentally the activation of ERK1/2 by the action of selective agonists on these receptors expressed in HEK 293 cells. We show that the 5-HT1AR agonist Xaliproden HCI elicits transient activation of ERK1/2 by phosphorylation, whereas 5-HT2AR activation by TCB-2 leads to greater, and more sustained responses. The 5-HT2A response dominates the MAPK signaling pathway when co-expressed with 5-HT1A, and diminution of the response by the 5-HT2A antagonist Ketanserin cannot be rescued by the 5-HT1AR agonist. Computational simulations of the activation pathway models produced qualitative results in good agreement with these experimental data, and parameter optimization makes this agreement quantitative. In silico simulation experiments using the models with the deletion of the positive regulators PKC in the 5-HT2AR pathway, or PLA2 in the combined 5-HT1AR/2AR models, showed that the basal level of active ERK1/2 is greatly decreased. Deletion of negative regulators of MKP and PP2A in 5-HT1AR and 5-HT2AR models was found to have even stronger effects. The model simulations under various conditions show that the extent of constitutive activity in a particular tissue and the specific drug efficacy properties determine the distinct dynamics of the 5-HT receptor-mediated ERK1/2 activation pathways. The mathematical models shown here to yield quantitative, function-related information about the signaling pathways activated by receptor binding of the drugs should enable ongoing efforts to establish a mechanistic understanding and an experimentally testable representation of hallucinogen-specific signaling in the cellular machinery.