Altered ligand dissociation rates in thyrotropin-releasing hormone receptors mutated in glutamine 105 of transmembrane helix III

Expression analysis of the thyrotropin-releasing hormone receptor (TRHR) in the immune system using agonist anti-TRHR monoclonal antibodies

Monoclonal anti-rat thyrotropin-releasing hormone (TRH) receptor (TRHR)-specific antibodies (mAb) were generated by immunization with synthetic peptides of rat TRHR partial amino acid sequences; one (TRHR01) was directed against a sequence (84-98) in the extracellular portion of the rat TRHR reported to be constant among different species, including man, and the second (TRHR02) recognizes the C-terminal region sequence 399-412. In lysates from GH4C1 cells, a clonal rat pituitary cell line, both mAb recognize the TRHR in Western blot analysis, and TRHR02 immunoprecipitates the TRHR. Incubation of GH4C1 cells with the mAb causes a fluorescence shift in fluorescence-activated cell sorting analysis. The cells were stained specifically by both mAb using immunocytochemical techniques. Furthermore, TRHR01 is agonistic in its ability to trigger Ca2+ flux, and desensitizes the TRH receptor. We tested for TRHR in several rat organs and found expression in lymphoid tissues. TRHR01 recognizes the human TRHR, and analysis of human peripheral blood lymphocyte and tonsil-derived leukocyte populations showed receptor expression in non-activated and phytohemagglutinin-activated T and B cells.

Modulation of human erg K+ channel gating by activation of a G protein-coupled receptor and protein kinase C

1. Modulation of the human ether-à-go-go-related gene (HERG) K+ channel was studied in two-electrode voltage-clamped Xenopus oocytes co-expressing the channel protein and the thyrotropin-releasing hormone (TRH) receptor. 2. Addition of TRH caused clear modifications of HERG channel gating kinetics. These variations consisted of an acceleration of deactivation, as shown by a faster decay of hyperpolarization-induced tail currents, and a slower time course of activation, measured using an envelope of tails protocol. The voltage dependence for activation was also shifted by nearly 20 mV in the depolarizing direction. Neither the inactivation nor the inactivation recovery rates were altered by TRH. 3. The alterations in activation gating parameters induced by TRH were demonstrated in a direct way by looking at the increased outward K+ currents elicited in extracellular solutions in which K+ was replaced by Cs+. 4. The effects of TRH were mimicked by direct pharmacological activation of protein kinase C (PKC) with beta-phorbol 12-myristate, 13-acetate (PMA). The TRH-induced effects were antagonized by GF109203X, a highly specific inhibitor of PKC that also abolished the PMA-dependent regulation of the channels. 5. It is concluded that a PKC-dependent pathway links G protein-coupled receptors that activate phospholipase C to modulation of HERG channel gating. This provides a mechanism for the physiological regulation of cardiac function by phospholipase C-activating receptors, and for modulation of adenohypophysial neurosecretion in response to TRH.

A G protein beta gamma dimer-mediated pathway contributes to mitogen-activated protein kinase activation by thyrotropin-releasing hormone receptors in transfected COS-7 cells

Activation of mitogen-activated protein kinase (MAPK) is induced by adding thyrotropin-releasing hormone (TRH) to COS-7 cells cotransfected with TRH receptors and an epitope-tagged MAPK. Long term treatment of the cells with pertussis toxin has no effect on TRH-induced MAPK activation. Incubation of the cells with the protein kinase C (PKC) inhibitor GF109203X causes an almost complete inhibition of MAPK activation by the PKC activator phorbol-12-myristate-13-acetate. In contrast, only approximately 50% of the TRH-induced MAPK activity is inhibited by GF109203X, indicating that activation of MAPK by TRH is only partially dependent on PKC. The inhibitory effect of GF109203X is additive with that of p21(N17ras), a dominant negative mutant of p21(ras) that exerts little effect on PKC-dependent MAPK activation by phorbol-12-myristate-13-acetate. The TRH-induced activation of MAPK also is inhibited partially by overexpression of transducin alpha subunits (alpha t), an agent known to sequester free G protein beta gamma dimers. However, the inhibitory potency of alpha t on TRH-induced activation is about half of that obtained in cells transfected with m2 muscarinic receptors, which activate MAPK exclusively through beta gamma dimers. The effect of alpha t is also additive with that of GF109203X but not with that of p21(N17ras). MAPK activation is not induced by the constitutively active form of G alpha q due to an inhibitory effect of its expression at a step downstream of that at which PKC-dependent and -independent routes to MAPK converge. Our results demonstrate that TRH receptors activate MAPK by a pathway only partially dependent on PKC activity. Furthermore, they indicate that beta gamma dimers of a pertussis and cholera toxin-insensitive G protein are involved in the PKC-independent fraction of the dual signaling route to MAPK initiated in the TRH receptor.