We are studying a family of G-protein coupled receptors (referred to as the secretin-VIP family, family B, or type II) that is structurally distinct from the majority of G-protein coupled receptors (rhodopsin- or beta-adrenergic-like receptors). Peptide effects on G-protein coupled receptors are often relatively long lasting and may have a modulatory role. These subtle effects may be relevant to the basis or treatment of mental illness. Recently we have focussed on the parathyroid hormone 2 (PTH2) receptor, which we identified in a screen for novel central nervous system (CNS) polypeptide receptors. Our initial studies of the human PTH2 receptor showed that it was activated by parathyroid hormone (PTH). Mapping the PTH2 receptor's cellular distribution revealed expression within CNS areas not normally exposed to circulating peptides, such as PTH. Further, we found that PTH poorly activated the rat PTH2 receptor. Using selective activation of the PTH2 receptor as an assay we purified a previously unknown 39 amino acid peptide from bovine hypothalamus, which we named tuberoinfundibular peptide of 39 residues (TIP39).This year we examined the receptor selectivity of TIP39. It activates PTH2 receptors from human, rat and zebrafish and does not activate PTH1 receptors. Both rat and zebrafish PTH2 receptors are much more effectively activated by TIP39 than PTH, supporting the suggestion that a TIP39 homologue is their natural ligand. We found that a zebrafish PTH3 receptor is activated by both PTH and PTHrP, suggesting that it is unlikely to be the homologue of a PTHrP-selective mammalian receptor that is involved in hypothalamic function. We completed a detailed survey of the brain distribution of the PTH2 receptor, providing a basis for studies of the receptors involvement in different functional pathways. We studied the interaction of TIP39 and several amino terminal truncated TIP39 peptides with PTH2, PTH1 and PTH2/PTH1 receptor chimeras, to increase understanding of the molecular basis of the ligand's interaction with the receptor, and to contribute to antagonist development. We found that most of the binding energy can be attributed to TIP39 interaction with receptor juxtamembrane domains, as opposed to interaction with receptor amino-terminal sequences, which is more significant for PTH. We found that removing 6 amino terminal residues from TIP39 creates a PTH2 receptor antagonist of modest potency. However, it removed a structural barrier for TIP39 interaction with the PTH1 receptor and created a potent PTH1 receptor antagonist. This peptide may be useful in evaluating PTH1 receptor antagonism as a therapeutic strategy in hypercalcemia of malignancy. Ongoing efforts include evaluating the effects of TIP39 on pituitary hormone release, nociception, and a more general functional mapping of its effects. Identification of the rodent and human forms of the peptide, and characterization of these genes, as well as development of selective antibodies, and an antagonist, are immediate goals important for elucidation of the biological role of TIP39.
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