Structural insights into the modes of relaxin-binding and tethered-agonist activation of RXFP1 and RXFP2

Our poor understanding of the mechanism by which the peptide-hormone H2 relaxin activates its G protein coupled receptor, RXFP1 and the related receptor RXFP2, has hindered progress in its therapeutic development. Both receptors possess large ectodomains, which bind H2 relaxin, and contain an N-terminal LDLa module that is essential for receptor signalling and postulated to be a tethered agonist. Here, we show that a conserved motif (GDxxGWxxxF), C-terminal to the LDLa, is critical for receptor activity. Importantly, this motif adopts different structures in RXFP1 and RXFP2, suggesting distinct activation mechanisms. For RXFP1, the motif is flexible, weakly associates with the LDLa, and requires H2 relaxin binding to stabilize an active conformation. Conversely, the GDxxGWxxxF motif in RXFP2 is more closely associated with the LDLa, forming an essential binding interface for H2 relaxin. These differences in the activation mechanism will aid drug development targeting these receptors.

Relaxin peptides reduce cellular damage in cultured brain slices exposed to transient oxygen–glucose deprivation: an effect mediated by nitric oxide

The effect of treatment with human relaxins on cell death was studied in oxygen- and glucose-deprived brain slices. In addition, involvement of nitric oxide and the relaxin receptor, RXFP3, was studied. Brain slices ( n = 12–18/group) were cultured under standard conditions for two weeks and then exposed to: ( i) an oxygenated balanced salt solution, ( ii) a deoxygenated, glucose-free balanced salt solution (OGD media), or ( iii) OGD media containing 10−7 mol/L H2 relaxin, 10−7 mol/L H2 relaxin with 50 μmol/L L-NIL, 10−7 mol/L H3 relaxin, or 10−7 mol/L H3 relaxin with 50 μmol/L L-NIL. Cell death was assessed using propidium iodide fluorescence. In a separate experiment, 10−5 mol/L R3 B1-22R (an antagonist of RXFP3) was added to both H2 and H3 relaxin treatments. H2 and H3 relaxin treatment reduced cell damage or death in OGD slices and L-NIL partially attenuated the effect of H3 relaxin. Antagonism of RXFP3 blocked the effect of H3 but not H2 relaxin. These data increase our understanding of the role of relaxin ligands and their receptors in protecting tissues throughout the body from ischemia and reperfusion injury.

NEW PROSPECTS IN PHARMACOLOGICAL TREATMENT OF ACUTE DECOMPENSATED HEART FAILURE. EXPERIENCE WITH SERELAXIN. A CLINICAL CASE

The article tells about new prospects in the treatment of acute heart failure with Serelaxin which is a recombinant molecule identical to the human peptide hormone - H2 relaxin. The author also shares personal experience on the use of the drug. Clinical trials of Serelaxin are reviewed. A clinical case of a prolonged, 48-hour intravenous infusion of Serelaxin to a patient with acute decompensated heart failure and results of the treatment are described. The hemodynamic parameters, safety profile and clinical efficacy in this patient during treatment with Serelaxin are evaluated.

Abstract P640: A Single-Chain Derivative of the Realaxin Hormone (B7-33) Protects Cytotrophoblasts from Hyperglycemia-Induced Preeclampsia Phenotype and Induces the Survival Pathway

Background: Relaxin is a peptide hormone that allows vasodilation and plays an important role in the process of parturition. The literature suggests potential therapeutic role of H2 relaxin in preeclampsia (PreE), however, there is a controversy on hypotensive action of the peptide. Due to the complex insulin-like structure of relaxin (A- and B- chains, 53 amino acids, 3 disulfide bonds), a novel H2 relaxin B-chain-only peptide variant B7-33 (27 amino acids without any disulfide bonds) has recently been developed. This single-chain peptide displayed equivalent efficacy to the natural H2 relaxin in several functional assays both in vitro and in vivo . Importantly, B7-33 was shown to have H2 relaxin-like RXFP1 specific effects, particularly in endogenously expressing RXFP1 cells, thus we hypothesized that B7-33 could be an alternative and cost-effective treatment option for PreE compared with H2 relaxin. Methods: Human CTBs were treated with 100, 150, 200, 300, or 400 mg/dL glucose for 48h and were co-treated with B7-33 (25 nM) with glucose exposure, while some cells were treated with 5, 10, 25 and 50 nM B7-33 alone. Levels of vascular endothelial growth factor (VEGF), placental growth factor (PlGF), soluble fms-like tyrosine kinase-1 (sFlt-1), and soluble endoglin (sEng) were measured in culture media using ELISA kits. Cell lysates were utilized to evaluate the mTOR, pAKT and total AKT expression by western blotting. Statistical comparisons were performed using analysis of variance with Duncan’s post hoc test. Results: Secretion of sFlt-1 and sEng were increased while VEGF and PIGF were decreased in CTBs treated with ≥150 mg/dl of glucose (*p < 0.05 for each). B7-33 co-treatment significantly rescued CTBs from hyperglycemia-induced anti-angiogenic profile (p < 0.05 for each). There is no effect of B7-33 on sFLT-1, sEng and PlGF; however, it increases expression of VEGF, while CTBs were treated only with B7-33. B7-33 also causes increased mTOR and pAKT expression in CTBs without any change in total AKT. Conclusions: B7-33 mitigates the hyperglycemia-induced dysfunction of CTBs by attenuating anti-angiogenic phenotype similar to that seen in PreE. This study supports the importance of continuing research of B7-33 in preE prevention.