New G protein coupled receptor targets for 3-iodotyronamine

So far most in vivo effects of 3-iodothyronamine (3-T1AM) are thought to be mediated by binding at the trace amine-associated receptor 1 (TAAR1). Inconsistently, 3-T1AM effects still persist in Taar1 knockout mice, suggesting additional receptor targets. It has previously been reported that 3-T1AM also binds to the alpha-2A-adrenergic receptor (ADRA2A) modulating insulin secretion. We speculated that this effect is mediated by interaction with ADRA2A or with a ADRA2A/TAAR1 complex. To unravel the signaling properties of ADRA2A in interplay with 3-T1AM we first investigated on a potential general signaling modification by the label-free EPIC-technology and then specified changes in signaling by cAMP inhibition and MAPK (ERK1/2) assays. We found that 3-T1AM induces Gi/o activation at ADRA2A. In contrast to nor-epinephrine (NorEpi), 3-T1AM did not activate the MAPK pathway but reduced the NorEpi-induced MAPK activation. Interestingly, co-expression of TAAR1 and ADRA2A as it occurs e.g. in pancreatic β-cells, abolished NorEpi-induced Gi/o signaling at ADRA2A. In addition, in ADRA2A/TAAR1 hetero-oligomers application of NorEpi results in uncoupling of Gi/o signaling pathway, not affecting MAPK activation. RNA sequencing analyses showed significant co-expression of ADRA2A and TAAR1 in mouse pancreatic islets but chronical 3-T1AM application in mice over a period of six days had no significant effects on glucose homeostasis. Our study provides important insights into the 3-T1AM-mediated signaling spectrum. It revealed a direct influence of 3-T1AM on ADRA2A function activating Gi/o pathway and 3-T1AM-induced uncoupling of NorEpi-mediated ADRA2A signaling in ADRA2A/TAAR1 hetero-oligomers. Supported by the Priority Program SPP1629 Thyroid Trans Act, BI893/5 – 1