Figure 1. Characterization of the histamine-triggered transgene switch.

(a) Illustration of the histamine sensor device (HSD). Following constitutive expression by the human elongation factor 1 alpha promoter (P_hEF1α), the histamine receptor H2 (HRH2) localizes to the plasma membrane. The G protein-coupled receptor HRH2 senses extracellular histamine and triggers G_s-mediated activation of adenylate cyclase (AC), which converts ATP to cyclic AMP (cAMP). cAMP binds the regulatory subunits of protein kinase A (PKA), whose catalytic subunits translocate into the nucleus where they phosphorylate the cAMP-responsive binding protein 1 (CREB1). Subsequently, CREB1 binds and activates a synthetic promoter (P_CRE), which drives the expression of intracellular (Citrine) or secreted (SEAP) reporter proteins. (b) Real-time monitoring of histamine-triggered cellular fluorescence of HSD_Citrine-engineered cell populations in a microplate reader. The plot shows the mean of three experiments. (c) Histamine sensitivity of transgenic HSD_SEAP32 cells. The data points were fitted using a sigmoidal dose-response curve. The blue translucent background represents the physiological range of histamine concentrations found in human blood. (d) Effect of histamine on the SEAP expression of HEK-293 cells with pcDNA3.1 and either pSEAP2-control (P_SV40-SEAP-pA) or pCK53 (P_CRE-SEAP-pA). (e) Profiling of the histamine-dependent intracellular cAMP surge in HSD_SEAP32 cells. (f) SEAP expression kinetics of HSD_SEAP32 cells exposed to different physiological histamine concentrations. (g) Impact of different histamine doses and induction time on HSD_SEAP32-mediated SEAP expression. (h) Influence of specific HRH1 (cetirizine) and HRH2 (cimetidine) antagonists on HSD_SEAP32-mediated SEAP expression. The data represent the means±s.d. (n≥3 experiments).