3-Iodothyronamine Activates a Set of Membrane Proteins in Murine Hypothalamic Cell Lines

Julia Bräunig; Stefan Mergler; Sabine Jyrch; Carolin S Hoefig; Mark Rosowski; Jens Mittag; Heike Biebermann; Noushafarin Khajavi

doi:10.3389/fendo.2018.00523

3-Iodothyronamine Activates a Set of Membrane Proteins in Murine Hypothalamic Cell Lines

Front Endocrinol (Lausanne). 2018 Sep 11:9:523. doi: 10.3389/fendo.2018.00523. eCollection 2018.

Authors

Julia Bräunig^{1

2}, Stefan Mergler³, Sabine Jyrch^{1

2}, Carolin S Hoefig^{4

5}, Mark Rosowski⁶, Jens Mittag^{5

7}, Heike Biebermann^{1

2}, Noushafarin Khajavi^{1

2}

Affiliations

¹ Charité - Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany.
² Institute of Experimental Pediatric Endocrinology, Berlin, Germany.
³ Klinik für Augenheilkunde, Charité - Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany.
⁴ Institute of Experimental Endocrinology, Charité - Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany.
⁵ Department of Cell & Molecular Biology, Karolinska Instituet, Stockholm, Sweden.
⁶ Department Medical Biotechnology, Institute of Biotechnology, Technical University of Berlin, Berlin, Germany.
⁷ University of Lübeck - Center of Brain Behavior and Metabolism, Lübeck, Germany.

Abstract

3-Iodothyronamine (3-T₁AM) is an endogenous thyroid hormone metabolite. The profound pharmacological effects of 3-T₁AM on energy metabolism and thermal homeostasis have raised interest to elucidate its signaling properties in tissues that pertain to metabolic regulation and thermogenesis. Previous studies identified G protein-coupled receptors (GPCRs) and transient receptor potential channels (TRPs) as targets of 3-T₁AM in different cell types. These two superfamilies of membrane proteins are largely expressed in tissue which influences energy balance and metabolism. As the first indication that 3-T₁AM virtually modulates the function of the neurons in hypothalamus, we observed that intraperitoneal administration of 50 mg/kg bodyweight of 3-T₁AM significantly increased the c-FOS activation in the paraventricular nucleus (PVN) of C57BL/6 mice. To elucidate the underlying mechanism behind this 3-T₁AM-induced signalosome, we used three different murine hypothalamic cell lines, which are all known to express PVN markers, GT1-7, mHypoE-N39 (N39) and mHypoE-N41 (N41). Various aminergic GPCRs, which are the known targets of 3-T₁AM, as well as numerous members of TRP channel superfamily, are expressed in these cell lines. Effects of 3-T₁AM on activation of GPCRs were tested for the two major signaling pathways, the action of Gα_s/adenylyl cyclase and G_i/o. Here, we demonstrated that this thyroid hormone metabolite has no significant effect on G_i/o signaling and only a minor effect on the Gα_s/adenylyl cyclase pathway, despite the expression of known GPCR targets of 3-T₁AM. Next, to test for other potential mechanisms involved in 3-T₁AM-induced c-FOS activation in PVN, we evaluated the effect of 3-T₁AM on the intracellular Ca²⁺ concentration and whole-cell currents. The fluorescence-optic measurements showed a significant increase of intracellular Ca²⁺ concentration in the three cell lines in the presence of 10 μM 3-T₁AM. Furthermore, this thyroid hormone metabolite led to an increase of whole-cell currents in N41 cells. Interestingly, the TRPM8 selective inhibitor (10 μM AMTB) reduced the 3-T₁AM stimulatory effects on cytosolic Ca²⁺ and whole-cell currents. Our results suggest that the profound pharmacological effects of 3-T₁AM on selected brain nuclei of murine hypothalamus, which are known to be involved in energy metabolism and thermoregulation, might be partially attributable to TRP channel activation in hypothalamic cells.

Keywords: 3-T1AM; GPCR; TRP Channel; hypothalamus; signaling.