The Antagonist pGlu-βGlu-Pro-NH2 Binds to an Allosteric Site of the Thyrotropin-Releasing Hormone Receptor

After we identified pGlu-βGlu-Pro-NH2 as the first functional antagonist of the cholinergic central actions of the thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH2), we became interested in finding the receptor-associated mechanism responsible for this antagonism. By utilizing a human TRH receptor (hTRH-R) homology model, we first refined the active binding site within the transmembrane bundle of this receptor to enhance TRH’s binding affinity. However, this binding site did not accommodate the TRH antagonist. This prompted us to consider a potential allosteric binding site in the extracellular domain (ECD). Searches for ECD binding pockets prompted a remodeling of the extracellular loops and the N-terminus. We found that different trajectories of ECDs produced novel binding cavities that were then systematically probed with TRH, as well as its antagonist. This led us to establish not only a surface-recognition binding site for TRH, but also an allosteric site that exhibited a selective and high-affinity binding for pGlu-βGlu-Pro-NH2. The allosteric binding of this TRH antagonist is more robust than TRH’s binding to its own active site. The findings reported here may shed light on the mechanisms and the multimodal roles by which the ECD of a TRH receptor is involved in agonist and/or antagonist actions.

IgG anti-(Fab�)2 Antibodies in Early Pregnancy Sera of Women with Anti-thyroid Antibodies and Normal Outcome or Spontaneous Abortions

Anti-thyroid antibodies (atbs) (anti-TPO, anti-TG, anti-TRH receptor) are autoatbs that recognize specific antigens that belong to thyroid structures. Several mechanisms were proposed to explain the breaking of immunotolerance to thyroid antigens. Our previous studies showed that IgG-anti-F(ab�)2 atbs exert immunosuppresives effect in vitro and inverse correlate with autoatbs in certain autoimmune diseases. Relying on sera from pregnant women with and without anti-thyroid atbs, respectively with or without spontaneous pregnancy loss we intended to analyze the IgG-anti-F(ab)2 atbs titers in sera of these categories of patients. The lot of patients consists of 126 pregnant women out of which 47 had a normal course of pregnancy and 79 experienced spontaneous abortion. Anti-TPO, anti-TG, and IgG-anti-F(ab�)2 atbs titers were measured in sera of these women. Although a difference was found between IgG-anti-F(ab�)2 atbs titer in pregnant women with positive versus negative anti-thyroid atbs, this was not statistically significant. IgG-anti-F(ab)2 atbs titer is higher in women with a normal course of pregnancy compared to women spontaneous pregnancy loss. Differently from other autoimmune diseases, our data show that IgG-anti-F(ab�)2 atbs are not inversely correlated to anti-thyroid atbs titers. Higher IgG-anti-(Fab�)2 atbs titers were found in pregnant women with normal course of pregnancy compared with those with pregnancy loss.

60 YEARS OF NEUROENDOCRINOLOGY: TRH, the first hypophysiotropic releasing hormone isolated: control of the pituitary–thyroid axis

This review presents the findings that led to the discovery of TRH and the understanding of the central mechanisms that control hypothalamus–pituitary–thyroid axis (HPT) activity. The earliest studies on thyroid physiology are now dated a century ago when basal metabolic rate was associated with thyroid status. It took over 50 years to identify the key elements involved in the HPT axis. Thyroid hormones (TH: T4and T3) were characterized first, followed by the semi-purification of TSH whose later characterization paralleled that of TRH. Studies on the effects of TH became possible with the availability of synthetic hormones. DNA recombinant techniques permitted the identification of all the elements involved in the HPT axis, including their mode of regulation. Hypophysiotropic TRH neurons, which control the pituitary–thyroid axis, were identified among other hypothalamic neurons which express TRH. Three different deiodinases were recognized in various tissues, as well as their involvement in cell-specific modulation of T3concentration. The role of tanycytes in setting TRH levels due to the activity of deiodinase type 2 and the TRH-degrading ectoenzyme was unraveled. TH-feedback effects occur at different levels, including TRH and TSH synthesis and release, deiodinase activity, pituitary TRH-receptor and TRH degradation. The activity of TRH neurons is regulated by nutritional status through neurons of the arcuate nucleus, which sense metabolic signals such as circulating leptin levels.Trhexpression and the HPT axis are activated by energy demanding situations, such as cold and exercise, whereas it is inhibited by negative energy balance situations such as fasting, inflammation or chronic stress. New approaches are being used to understand the activity of TRHergic neurons within metabolic circuits.