Total synthesis of human relaxin and human relaxin derivatives by solid-phase peptide synthesis and site-directed chain combination

Engineering and Characterization of a Long-Half-Life Relaxin Receptor RXFP1 Agonist

Relaxin-2 is a peptide hormone with important roles in human cardiovascular and reproductive biology. Its ability to activate cellular responses such as vasodilation, angiogenesis, and anti-inflammatory and antifibrotic effects has led to significant interest in using relaxin-2 as a therapeutic for heart failure and several fibrotic conditions. However, recombinant relaxin-2 has a very short serum half-life, limiting its clinical applications. Here, we present protein engineering efforts targeting the relaxin-2 hormone in order to increase its serum half-life while maintaining its ability to activate the G protein-coupled receptor RXFP1. To achieve this, we optimized a fusion between relaxin-2 and an antibody Fc fragment, generating a version of the hormone with a circulating half-life of around 3 to 5 days in mice while retaining potent agonist activity at the RXFP1 receptor both in vitro and in vivo.

Seleno-relaxin analogues: effect of internal and external diselenide bonds on the foldability and a fibrosis-related factor of endometriotic stromal cells

Human relaxin-2 (H2 relaxin) is a peptide hormone of about 6 kDa, first identified as a reproductive hormone involved in vasoregulation during pregnancy. It has recently attracted strong interest because of its diverse functions, including anti-inflammatory, anti-fibrotic, and vasodilatory, and has been suggested as a potential peptide-based drug candidate for a variety of diseases. Mature H2 relaxin is constituted by the A- and B-chains stabilized by two interchain disulfide (SS) bridges and one intrachain SS linkage. In this study, seleno-relaxins, SeRlx-α and SeRlx-β, which are [C11UA,C11UB] and [C10UA,C15UA] variants of H2 relaxin, respectively, were synthesized via a one-pot oxidative chain assembly (folding) from the component A- and B-chains. The substitution of SS bonds in a protein with their analogue, diselenide (SeSe) bonds, has been shown to alter the physical, chemical, and physiological properties of the protein. The surface SeSe bond (U11A-U11B) enhanced the yield of chain assembly while the internal SeSe bond (U10A-U15A) improved the reaction rate of the folding, indicating that these bridges play a major role in controlling the thermodynamics and kinetics, respectively, of the folding mechanism. Furthermore, SeRlx-α and SeRlx-β effectively reduced the expression of a tissue fibrosis-related factor in human endometriotic stromal cells. Thus, the findings of this study indicate that the S-to-Se substitution strategy not only enhances the foldability of relaxin, but also provides new guidance for the development of novel relaxin formulations for endometriosis treatment.

Cysteine protecting groups: applications in peptide and protein science

Protecting group chemistry for the cysteine thiol group has enabled a vast array of peptide and protein chemistry over the last several decades. Increasingly sophisticated strategies for the protection, and subsequent deprotection, of cysteine have been developed, facilitating synthesis of complex disulfide-rich peptides, semisynthesis of proteins, and peptide/protein labelling in vitro and in vivo. In this review, we analyse and discuss the 60+ individual protecting groups reported for cysteine, highlighting their applications in peptide synthesis and protein science.

Stepwise Construction of Disulfides in Peptides

The disulfide bond plays an important role in biological systems. It defines global conformation, and ultimately the biological activity and stability of the peptide or protein. It is frequently present, singly or multiply, in biologically important peptide hormones and toxins. Numerous disulfide-containing peptides have been approved by the regulatory agencies as marketed drugs. Chemical synthesis is one of the prerequisite tools needed to gain deep insights into the structure-function relationships of these biomolecules. Along with the development of solid-phase peptide synthesis, a number of methods of disulfide construction have been established. This minireview will focus on the regiospecific, stepwise construction of multiple disulfides used in the chemical synthesis of peptides. We intend for this article to serve a reference for peptide chemists conducting complex peptide syntheses and also hope to stimulate the future development of disulfide methodologies.

Relaxin 2/RXFP1 Signaling Induces Cell Invasion via the β-Catenin Pathway in Endometrial Cancer

Relaxin is known to play an important role in animal pregnancies, including those of humans. It is suggested that relaxin induces aggressive cell growth and invasiveness in several types of cancer, including endometrial cancer. However, the mechanisms of relaxin remain largely unclear. In this study, we examined the effects of relaxin 2 (RLN2), the major circulating relaxin in humans, on human endometrial carcinoma cell lines. RLN2 treatment induced invasion in HEC-1B and Ishikawa cells. RLN2-induced cell invasion was significantly decreased by transfection of relaxin receptor 1 (RXFP1) siRNAs. The β-catenin inhibitor, XAV939, also significantly inhibited the RLN2-induced cell invasions. Both a decrease of cadherin expression and an increase of β-catenin phosphorylation were observed in response to the RLN2 treatment in HEC-1B and Ishikawa cells. We then examined RLN2 and RXFP1 expression in 80 human endometrioid endometrial carcinoma tissues. RLN2 immunoreactivity was detected in the human endometrial carcinoma cells and had a correlative tendency with histological grade and RXFP1. These results suggest that adherens junctions in cancer cells are weakened by the breakdown of the cadherin/catenin complex, which is induced by β-catenin phosphorylation via RLN2/RXFP1 signaling.

Challenges in the design of insulin and relaxin/insulin-like peptide mimetics

Peptidomimetics are designed to overcome the poor pharmacokinetics and pharmacodynamics associated with the native peptide or protein on which they are based. The design of peptidomimetics starts from developing structure-activity relationships of the native ligand-target pair that identify the key residues that are responsible for the biological effect of the native peptide or protein. Then minimization of the structure and introduction of constraints are applied to create the core active site that can interact with the target with high affinity and selectivity. Developing peptidomimetics is not trivial and often challenging, particularly when peptides' interaction mechanism with their target is complex. This review will discuss the challenges of developing peptidomimetics of therapeutically important insulin superfamily peptides, particularly those which have two chains (A and B) and three disulfide bonds and whose receptors are known, namely insulin, H2 relaxin, H3 relaxin, INSL3 and INSL5.

Novel Methods for the Chemical Synthesis of Insulin Superfamily Peptides and of Analogues Containing Disulfide Isosteres

The insulin superfamily of peptides is ubiquitous within vertebrates and invertebrates and is characterized by the presence of a set of three disulfide bonds in a unique disposition. With the exception of insulin-like growth factors I and II, which are single chain peptides, the remaining 8 members of the human insulin superfamily are two-chain peptides containing one intramolecular and two intermolecular disulfide bridges. These structural features have long made the chemical synthesis of the peptides a considerable challenge, in particular, including their correct disulfide bond pairing and formation. However, they have also afforded the opportunity to develop modern solid phase synthesis methods for the preparation of such peptides that incorporate novel or improved chemical methods for the controlled introduction of both disulfide bonds and their surrogates, both during and after peptide chain assembly. In turn, this has enabled a detailed probing of the structure and function relationship of this small but complex superfamily of peptides. After initially using and subsequently identifying significant limitations of the approach of simultaneous random chain combination and oxidative folding, our laboratory undertook to develop robust chemical synthesis strategies in concert with orthogonal cysteine S-protecting groups and corresponding regioselective disulfide bond formation. These have included the separate synthesis of each of the two chains or of the two chains linked by an artificial C-peptide that is removed following postoxidative folding. These, in turn, have enabled an increased ease of acquisition in a good yield of not only members of human insulin superfamily but other insulin-like peptides. Importantly, these successful methods have enabled, for the first time, a detailed analysis of the role that the disulfide bonds play in the structure and function of such peptides. This was achieved by selective removal of the disulfide bonds or by the judicious insertion of disulfide isosteres that possess structurally subtle variations in bond length, hydrophobicity, and angle. These include lactam, dicarba, and cystathionine, each of which has required modifications to the peptide synthesis protocols for their successful placement within the peptides. Together, these synthesis improvements and the novel chemical developments of cysteine/cystine analogues have greatly aided in the development of novel insulin-like peptide (INSL) analogues, principally with intra-A-chain disulfide isosteres, possessing not only improved functional properties such as increased receptor selectivity but also, with one important and unexpected exception, greater in vivo half-lives due to stability against disulfide reductases. Such analogues greatly will aid further biochemical and pharmacological analyses to delineate the structure-function relationships of INSLs and also future potential drug development.

Synthetic Advances in Insulin-like Peptides Enable Novel Bioactivity

Insulin is a miraculous hormone that has served a seminal role in the treatment of insulin-dependent diabetes for nearly a century. Insulin resides within in a superfamily of structurally related peptides that are distinguished by three invariant disulfide bonds that anchor the three-dimensional conformation of the hormone. The additional family members include the insulin-like growth factors (IGF) and the relaxin-related set of peptides that includes the so-called insulin-like peptides. Advances in peptide chemistry and rDNA-based synthesis have enabled the preparation of multiple insulin analogues. The translation of these methods from insulin to related peptides has presented unique challenges that pertain to differing biophysical properties and unique amino acid compositions. This Account presents a historical context for the advances in the chemical synthesis of insulin and the related peptides, with division into two general categories where disulfide bond formation is facilitated by native conformational folding or alternatively orthogonal chemical reactivity. The inherent differences in biophysical properties of insulin-like peptides, and in particular within synthetic intermediates, have constituted a central limitation to achieving high yield synthesis of properly folded peptides. Various synthetic approaches have been advanced in the past decade to successfully address this challenge. The use of chemical ligation and metastable amide bond surrogates are two of the more important synthetic advances in the preparation of high quality synthetic precursors to high potency peptides. The discovery and application of biomimetic connecting peptides simplifies proper disulfide formation and the subsequent traceless removal by chemical methods dramatically simplifies the total synthesis of virtually any two-chain insulin-like peptide. We report the application of these higher synthetic yield methodologies to the preparation of insulin-like peptides in support of exploratory in vivo studies requiring a large quantity of peptide. Tangentially, we demonstrate the use of these methods to study the relative importance of the IGF-1 connecting peptide to its biological activity. We report the translation of these finding in search of an insulin analog that might be comparably enhanced by a suitable connecting peptide for interaction with the insulin receptor, as occurs with IGF-1 and its receptor. The results identify a unique receptor site in the IGF-1 receptor from which this enhancement derives. The selective substitution of this specific IGF-1 receptor sequence into the homologous site in the insulin receptor generated a chimeric receptor that was equally capable of signaling with insulin or IGF-1. This novel receptor proved to enhance the potency of lower affinity insulin ligands when they were supplemented with the IGF-1 connecting peptide that similarly enhanced IGF-1 activity at its receptor. The chimeric insulin receptor demonstrated no further enhancement of potency for native insulin when it was similarly prepared as a single-chain analogue with a native IGF-1 connecting peptide. These results suggest a more highly evolved insulin receptor structure where the requirement for an additional structural element to achieve high potency interaction as demonstrated for IGF-1 is no longer required.

Relaxin family peptides: structure-activity relationship studies

The human relaxin peptide family consists of seven cystine-rich peptides, four of which are known to signal through relaxin family peptide receptors, RXFP1-4. As these peptides play a vital role physiologically and in various diseases, they are of considerable importance for drug discovery and development. Detailed structure-activity relationship (SAR) studies towards understanding the role of important residues in each of these peptides have been reported over the years and utilized for the design of antagonists and minimized agonist variants. This review summarizes the current knowledge of the SAR of human relaxin 2 (H2 relaxin), human relaxin 3 (H3 relaxin), human insulin-like peptide 3 (INSL3) and human insulin-like peptide 5 (INSL5).

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Serelaxin for the treatment of heart failure

Outcomes for patients with acute heart failure remain suboptimal and treatments principally target improvement of symptoms. To date there has been no therapy approved for acute heart failure shown to improve mortality or readmission risk post-discharge. Serelaxin, a recombinant form of the naturally occurring polypeptide hormone relaxin, has demonstrated promise in preclinical and early clinical trials as a potentially novel therapy for acute heart failure. It is postulated through its anti-fibrotic and vasodilatory effects that this agent can improve outcomes in both the short and long term in these patients. Randomized clinical data has suggested that the medication is safe and well tolerated. However, definitive outcomes data is currently being assessed in a large multi-center trial.

Chemical synthesis of peptides within the insulin superfamily

The synthesis of insulin has inspired fundamental advances in the art of peptide science while simultaneously revealing the structure-function relationship of this centrally important metabolic hormone. This review highlights milestones in the chemical synthesis of insulin that can be divided into two separate approaches: (i) disulfide bond formation driven by protein folding and (ii) chemical reactivity-directed sequential disulfide bond formation. Common to the two approaches are the persistent challenges presented by the hydrophobic nature of the individual A-chain and B-chain and the need for selective disulfide formation under mildly oxidative conditions. The extension and elaboration of these synthetic approaches have been ongoing within the broader insulin superfamily. These structurally similar peptides include the insulin-like growth factors and also the related peptides such as relaxin that signal through G-protein-coupled receptors. After a half-century of advances in insulin chemistry, we have reached a point where synthesis is no longer limiting structural and biological investigation within this family of peptide hormones. The future will increasingly focus on the refinement of structure to meet medicinal purposes that have long been pursued, such as the development of a glucose-sensitive insulin. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

A perfluoroaromatic abiotic analog of H2 relaxin enabled by rapid flow-based peptide synthesis

We report on the rapid-flow based synthesis and functional characterization of a H2 relaxin analog that takes advantage of perfluoroarylation-cysteine SNAr chemistry for a disulfide replacement strategy.

Vimocin and vidapin, cyclic KTS peptides, are dual antagonists of α1β1/α2β1 integrins with antiangiogenic activity

Obtustatin and viperistatin, members of the disintegrin protein family, served as lead compounds for the synthesis of linear and cyclic peptides containing the KTS binding motif. The most active linear peptide, a viperistatin analog, indicated the importance of Cys(19) and Cys(29), as well as the presence of Arg at position 24 for their biologic activity, and was used as the basic sequence for the synthesis of cyclic peptides. Vimocin (compound 6) and vidapin (compound 10) showed a high potency (IC50 = 0.17 nM) and intermediate efficacy (20 and 40%) in inhibition of adhesion of α1/α2 integrin overexpressor cells to respective collagens. Vimocin was more active in inhibition of the wound healing (53%) and corneal micropocket (17%) vascularization, whereas vidapin was more potent in inhibition of migration in the Matrigel tube formation assay (90%). Both compounds similarly inhibited proliferation (50-90%) of endothelial cells, and angiogenesis induced by vascular endothelial growth factor (80%) and glioma (55%) in the chorioallantoic membrane assay. These peptides were not toxic to endothelial cell cultures and caused no acute toxicity upon intravenous injection in mice, and were stable for 10-30 hours in human serum. The in vitro and in vivo potency of the peptides are consistent with conformational ensembles and "bioactive" space shared by obtustatin and viperistatin. These findings suggest that vimocin and vidapin can serve as dual α1β1/α2β1 integrin antagonists in antiangiogenesis and cancer therapy.

Relaxin for preventing preterm birth

BACKGROUND

Preterm birth is a leading cause of perinatal morbidity and mortality. Early animal and clinical studies have provided some evidence to support an inhibitory effect of relaxin on preterm birth for women in preterm labour.

OBJECTIVES

To assess the effects of relaxin for women in preterm labour on preterm birth and associated maternal and neonatal/infant health outcomes.

SEARCH METHODS

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (30 June 2013), and the reference lists of relevant papers.

SELECTION CRITERIA

Randomised and quasi-randomised controlled trials assessing the effects of relaxin compared with no treatment, a placebo, or an alternative tocolytic, for preventing preterm birth for women in preterm labour. Primary review outcomes included birth within 28 hours of treatment, birth within seven days of treatment, perinatal mortality, and a serious neonatal adverse outcome composite.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed study eligibility, extracted data and assessed the risk of bias of included studies.

MAIN RESULTS

We included three quasi-randomised controlled trials, with a total of 149 women and their babies. All three trials were at a high risk of bias. When comparing women receiving relaxin with those who did not receive relaxin, there was a significant reduction in birth within seven days of treatment in one trial of 30 women (risk ratio (RR) 0.50, 95% confidence interval (CI) 0.29 to 0.87), yet no significant difference was seen for perinatal mortality in this trial (RR 0.83, 95% CI 0.32 to 2.15). The second and third included trials did not report on any of the primary outcomes pre-specified in the review, including birth within 48 hours of treatment, birth within seven days of treatment, perinatal mortality, and serious neonatal adverse outcomes.One trial found a significant increase in pregnancy prolongation for women receiving relaxin (RR 8.00, 95% CI 1.14 to 56.33; 30 women). None of the three included trials found significant differences in the outcomes of fetal death, neonatal death, birthweight or preterm birth, and no trial reported on longer-term outcomes for the babies.

AUTHORS' CONCLUSIONS

There is limited randomised controlled trial evidence available on the effect of relaxin during pregnancy for preventing preterm birth for women in preterm labour. Evidence from one quasi-randomised trial suggested a reduction in birth within seven days of treatment for women receiving relaxin, compared with women in a control group, however this trial was at a high risk of bias and included only 30 women. There is thus insufficient evidence to support or refute the use of relaxin in women in preterm labour for preventing preterm birth.

Sequential formation of regioselective disulfide bonds in synthetic peptides with multiple disulfide bonds

Numerous methods have been developed for the formation of disulfide bonds in recombinant DNA-derived and chemically synthesized peptides and proteins, but only a few have found widespread acceptance. The choice of method(s) for formation of disulfide in synthetic peptides and proteins needs to be tailored for each individual polypeptide in such a way so that the reaction conditions are selective, efficient, and safe and give the maximum yield. Here we describe the sequential formation of three disulfide bonds regioselectively which has been optimized for the synthesis of two-chained, heterodimeric polypeptide members of the insulin-relaxin superfamily.

Handling a tricycle: orthogonal versus random oxidation of the tricyclic inhibitor cystine knotted peptide gurmarin

Gurmarin is a 35 amino acid peptide with three disulfide bridges in an inhibitor cystine knot. It is found in the plant Gymnema sylvestre, and has been identified as a sweet taste inhibitor in rodents. In this article we provide an efficient route for the synthesis of gurmarin by a controlled random oxidation strategy. We compared two oxidation procedures to form the three disulfide bridges. In the first, based on random oxidation, reduced gurmarin was synthesized using trityl for cysteine protection, and oxidized for 48 h in a Tris-HCl buffer containing cystamine and reduced glutathione to facilitate disulfide scrambling. The second was based on step-wise deprotection followed by oxidation in which the cysteine pairs are orthogonally protected with tert-Butylthio, trityl and acetamidomethyl. To verify that the native gurmarin oxidation product was obtained, thermolysin cleavage was used. Cleavage of random oxidized gurmarin showed two possible disulfide combinations; the native and a non-native gurmarin disulfide isomer. The non-native isomer was therefore synthesized using the orthogonal deprotection-oxidation strategy and the native and the non-native gurmarin isomers were analyzed using UPLC. It was found that the random oxidation procedure leads to native gurmarin in high yield. Thus, the synthetic route was simple and significantly more efficient than previously reported syntheses of gurmarin and other cysteine rich peptides. Importantly, native gurmarin was obtained by random oxidation, which was confirmed by a synthetic approach for the first time.

Replacement of disulfides by amide bonds in the relaxin-like factor (RLF/INSL3) reveals a role for the A11-B10 link in transmembrane signaling

The relaxin-like factor (RLF) also named insulin-like 3 (INSL3) consists of two polypeptide chains linked by two interchain and one intrachain disulfide bond. RLF binds to its receptor (LGR8 also named RXFP2) through the B chain and initiates transmembrane communication by activating the adenylate cyclase through the N-terminal region of both chains. Cystine A11-B10 occupies a unique position on the molecular surface just outside the binding region and between the two signaling ports. We have synthesized an RLF analogue in which the disulfide A11-B10 was replaced by a peptide bond and found that cAMP production ceased while receptor binding was not affected. In contrast, replacing the disulfide A24-B22 by a peptide bond reduced potency proportional to the binding affinity and lowered efficacy to 65%, while replacing disulfide A10-A15 by a peptide bond reduced binding affinity to 32% and lowered potency to 7% but maintained 100% efficacy. The exceptional properties of the derivative bearing an A11-B10 isopeptide cross-link suggests that the disulfide has a special role in signal transduction. We propose that disulfide A11-B10 serves as an insulator between the two ports, whereas the amide functionality disturbs the signal transmission complex likely due to changes in polarity. The clear separation between receptor binding and signal activation sites within this small protein permits one to study how the relaxin-like factor initiates the signal on the receptor that induces intracellular cAMP production.

Chimeric relaxin peptides highlight the role of the A-chain in the function of H2 relaxin

Human gene-2 (H2) relaxin is a member of the insulin-relaxin peptide superfamily. Because of the potential clinical applications of H2 relaxin, there is a need for novel analogs that have improved biological activity and receptor specificity. In this respect, we have chemically assembled chimeric peptides consisting of the B-chain of H2 relaxin in combination with A-chains from other insulin/relaxin family members. The peptides were prepared using solid phase peptide synthesis together with regioselective disulfide bond formation and characterized by RP-HPLC, MALDI-TOF MS and amino acid analysis. Their in vitro activity was assessed in RXFP1 or RXFP2 expressing cells. Replacement of the H2 relaxin A-chain resulted in parallel losses of binding affinity and activity on RXFP1. Not surprisingly H1A-H2B demonstrated the highest activity as the H1 A-chain shares high homology with H2 relaxin whereas INSLA-H2B, which shows low homology, had very poor activity. Importantly A-chain replacements had a dramatic effect on RXFP2 activity similar to previous results demonstrating different modes of activation of A-chain variants on RXFP1 and RXFP2. H3A-H2B is particularly interesting as it displays moderate activity at RXFP1 but poor activity at RXFP2 indicating that it may be a template for specific RXFP1 agonist development. Our study confirms that the activity of H2 relaxin at both RXFP1 and RXFP2 relies on interactions with both the B- and A-chains, and also provide new biochemical insights into the mechanism of relaxin action that the A-chain needs to be in native or near-native form for strong RXFP1 or RXFP2 agonist activity.

The chemical synthesis of relaxin and related peptides

Successful methods for the chemical assembly of insulin-like peptides allow the detailed study of their structure and function relationships. However, the two-chain, three-disulfide bond structure of this family of peptides, which includes relaxin, has long represented a significant challenge with respect to their chemical synthesis. Early efforts involved the random combination of the two synthetic S-reduced chains under oxidizing conditions to spontaneously form the three disulfide bonds. Such an approach, while generally effective for native sequences, is critically dependent upon the presence of intact secondary structures within the individual chains which guide the subsequent folding and oxidation pathway. This limitation prevents the use of this approach for the preparation of analogs in which these secondary elements are either absent or modified. Nowadays, the use of highly efficient solid-phase peptide synthesis methodologies together with selective S-thiol-protecting groups allows the acquisition of individual chains that can be combined by effective sequential chemically directed formation of each of the three disulfide bonds. These approaches have allowed the high-yield assembly of an array of insulin-like peptides which, in turn, has provided considerable and valuable structural and biological information.

Relaxin, the relaxin-like factor and their receptors

In 1926 Frederick Hisaw discovered a blood-borne factor in pregnant guinea pigs that would cause relaxation of the pubic symphysis in virgin females of the species. The relaxin-like factor gene (RLF), also known as insulin-like 3 (INSL3), was recovered from a library of testicular cDNA. The function of RLF as the mediator of testicular positioning in mice was discovered by gene deletion experiments. The report that deletion of a G-protein-coupled receptor in a mouse mutant caused cryptorchidism and that relaxin and RLF and their receptors were structurally and functionally similar may well have inspired Drs. Hsueh and Sherwood to put LGR7 and relaxin together and thus, after many agonizing years of uncertainty, the relaxin receptor had yielded its identity. LGR8 was recognized as the human version of the RLF receptor and together LGR7 and LGR8, with their respective ligands, opened to detailed investigation the large and important field of G-protein activated leucine-rich repeat receptors. In the process RLF and LGR8 have yielded some general information that might contribute to our knowledge of receptor/ligand interaction, in particular the enigmatic signal initiation process.

RXFP1-inactive relaxin activates human glucocorticoid receptor: further investigations into the relaxin-GR pathway

The relaxin peptide family regulates diverse biological functions (central nervous processes, reproduction, cardiovascular and kidney function, and connective tissue composition) through different G protein-coupled receptors. We reported earlier that human relaxin-2 and porcine relaxin additionally interact with the human glucocorticoid receptor (GR) in an agonistic manner. Here we investigated whether the membrane receptor RXFP1 is critically involved in this pathway. We used chemically modified porcine relaxin which was biologically inactive at RXFP1. Native porcine relaxin, but not the modified peptide affected RXFP1-dependent and GR-independent readouts: ERK-1/2 and Akt phosphorylation as well as up-regulation of Akt and endothelin type-B receptor. In contrast, relaxin and modified relaxin inhibited endotoxin-stimulated secretion of TNF-alpha and IL-6 by human macrophages, an effect sensitive to the glucocorticoid receptor antagonists RU-486 and D-06. Both relaxins caused Ser(211) phosphorylation of GR, a biomarker of agonist-related receptor activation. Relaxin-induced accumulation of Ser(211)-phosphorylated GR was found in the cytoplasm and nucleus of HeLa cells, in endothelial cells, and in transfected HT-29 cells. In AP-1-luciferase assays, relaxin and modified relaxin inhibited endotoxin-induced activation of AP-1, a transcription factor essentially involved in endotoxin signaling. This study suggests that the inability of relaxin to interact with its membrane receptor does not interfere with its ability to activate GR.

Synthesis, conformation, and activity of human insulin-like peptide 5 (INSL5)

Insulin-like peptide 5 (INSL5) was first identified through searches of the expressed sequence tags (EST) databases. Primary sequence analysis showed it to be a prepropeptide that was predicted to be processed in vivo to yield a two-chain sequence (A and B) that contained the insulin-like disulfide cross-links. The high affinity interaction between INSL5 and the receptor RXFP4 (GPCR142) coupled with their apparent coevolution and partially overlapping tissue expression patterns strongly suggest that INSL5 is an endogenous ligand for RXFP4. Given that the primary function of the INSL5–RXFP4 pair remains unknown, an effective means of producing sufficient quantities of this peptide and its analogues is needed to systematically investigate its structural and biological properties. A combination of solid-phase peptide synthesis methods together with regioselective disulfide bond formation were used to obtain INSL5. Both chains were unusually resistant to standard synthesis protocols and required highly optimized conditions for their acquisition. In particular, the use of a strong tertiary amidine, DBU, as N^α-deprotection base was required for the successful assembly of the B chain; this highlights the need to consider incomplete deprotection rather than acylation as a cause of failed synthesis. Following sequential disulfide bond formation and chain combination, the resulting synthetic INSL5, which was obtained in good overall yield, was shown to possess a similar secondary structure to human relaxin-3 (H3 relaxin). The peptide was able to inhibit cAMP activity in SK-N-MC cells that expressed the human RXFP4 receptor with a similar activity to H3 relaxin. In contrast, it had no activity on the human RXFP3 receptor. Synthetic INSL5 demonstrates equivalent activity to the recombinant-derived peptide, and will be an important tool for the determination of its biological function.

Regulation of receptor signaling by relaxin A chain motifs: derivation of pan-specific and LGR7-specific human relaxin analogs

Relaxin peptides are important hormones for the regulation of reproductive tissue remodeling and the renal cardiovascular system during pregnancy. Recent studies demonstrated that two of the seven human relaxin family peptides, relaxin H2 (RLN2) and INSL3, signal exclusively through leucine-rich repeat-containing G protein-coupled receptors, LGR7 and LGR8. Although it was well characterized that an RXXXRXXI motif at the RLN2 B chain confers receptor activation activity, it is not clear what roles RLN2 A chain plays in receptor interaction. Analyses of relaxin family genes on syntenic regions of model tetrapods showed that the A chain of RLN2 orthologs exhibited a greater sequence divergence as compared with the receptor-binding domain-containing B chain, foreshadowing a potential role in receptor interactions; hence, defining receptor selectivity in this fast evolving peptide hormone. To test our hypothesis that select residues in the human RLN2 A chain play key roles in receptor interaction, we studied mutant peptides with residue substitution(s) in the A chain. Here, we showed that alanine substitution at the A16 and A17 positions enhances LGR8-activation activity of RLN2, whereas mutation at the A22-23 region (RLN2A22-23) ablates LGR8, but not LGR7, activation activity. In addition, we demonstrated that the functional characteristics of the RLN2A22-23 mutant are mainly attributed to modifications at the PheA23 position. Taken together, our studies indicated that ThrA16, LysA17, and PheA23 constitute part of the receptor-binding interface of human RLN2, and that modification of these residues has led to the generation of novel human RLN2 analogs that would allow selective activation of human LGR7, but not LGR8, in vivo.

Origin of INSL3-mediated testicular descent in therian mammals

Testicular descent is a unique physiological adaptation found in therian mammals allowing optimal spermatogenesis below core body temperature. Recent studies show that INSL3, produced by Leydig cells, and its receptor LGR8 (RXFP2) are essential for mediating the transabdominal phase of testicular descent during early development. However, the origin and genetic basis for this physiological adaptation is not clear. Using syntenic mapping and the functional characterization of contemporary and resurrected relaxin family hormones, we show that derivation of INSL3-mediated testicular descent involved the duplication of an ancestral RLN3-like gene that encodes an indiscriminate ligand for LGR7 (RXFP1) and LGR8. This event was followed by acquisition of the LGR7-selective characteristics by a daughter gene (RLN3) prior to the evolution of the common ancestor of monotremes, marsupials, and placentals. A subsequent mutation of the other daughter gene (INSL3) occurred before the emergence of therian mammals, which then led to the derivation of the reciprocal LGR8-specific characteristics of INSL3. The stepwise evolution of these independent signaling pathways through gene duplication and subsequent divergence is consistent with Darwinian theory of selection and adaptation, and the temporal proximity suggests an association between these genetic events and the concurrent evolution of testicular descent in ancestral therian mammals.

Relaxin—a pleiotropic hormone and its emerging role for experimental and clinical therapeutics

The insulin-related peptide hormone relaxin (Rlx) is known as pregnancy hormone for decades. In the 1980s, researchers began to recognize the highly intriguing fact that Rlx plays a role in a multitude of physiological processes far beyond pregnancy and reproduction. So, Rlx's contribution to the regulation of vasotonus, plasma osmolality, angiogenesis, collagen turnover, and renal function has been established. In addition, the peptide has been demonstrated to represent a mediator of cardiovascular pathology. The ongoing efforts to identify Rlx receptors eventually precipitated the discovery of the G protein-coupled receptors (GPCR) LGR7 and LGR8 as membrane receptors for human Rlx-2 in 2002. This review will summarize the current state of insight into this rapidly evolving field, which has further been expanded by the discovery of GPCR135 and GPCR142 as receptors for Rlx-3. In addition, Rlx has also been shown to activate the human glucocorticoid receptor (GR). There is evidence from Rlx and Rlx receptor knockouts suggesting that LGR7 is the only relevant receptor for mouse Rlx-1 (corresponding to human Rlx-2) in vivo and that insulin-like peptide (INSL)-3 represents the physiological ligand for LGR8. Regarding Rlx signal transduction, the cyclic adenosine monophosphate (cAMP) and nitric oxide (NO) pathways will be characterized as major cascades. Investigation of downstream signaling remains an important field for future research. Finally, the current state of therapeutical strategies using Rlx in animal models as well as in humans is summarized.

The Mode of Interaction of the Relaxin-like Factor (RLF) with the Leucine-rich Repeat G Protein-activated Receptor 8

The relaxin-like factor (RLF, also named INSL3) is a critical component in the chain of events that lead to the normal positioning of the gonads in the male fetus. RLF and relaxin share features of the secondary structure to the extent that relaxin cross-reacts with the LGR8, the RLF receptor. Although both hormones interact with their receptors essentially via the B chain, the sharply defined binding cassette of relaxin is not present in RLF. Structure and function analysis of RLF derivatives with single amino acid replacements revealed that the most important binding residues are tryptophan B27, followed by arginine B16 and valine B19. Single alanine replacements for each individual position resulted in a relative receptor affinity of 4.0% (B16), 6.1% (B19), and 0.5% (B27). Tryptophan B27 is located on an extended structure, and arginine B16 and valine B19 are positioned on the exposed surface of the B chain helix. The 3 residues could be brought together to form a contiguous binding area if the C-terminal end of the B chain were free to fold back against the central portion of the B chain helix. Such a movement depends critically on the flexibility of the C-terminal end, which is controlled by positions B23-25. In as much as these major binding residues seem hardly sufficient to explain the strong binding of RLF to LGR8 we searched for and found an extended region where little contributions by individual residues added up to a strong receptor affinity. This mode of interaction could drive the binding energy sufficiently high to account for the picomolar binding constant of RLF and its receptor.

International Union of Pharmacology LVII: recommendations for the nomenclature of receptors for relaxin family peptides

Although the hormone relaxin was discovered 80 years ago, only in the past 5 years have the receptors for relaxin and three other receptors that respond to related peptides been identified with all four receptors being G-protein-coupled receptors. In this review it is suggested that the receptors for relaxin (LGR7) and those for the related peptides insulin-like peptide 3 (LGR8), relaxin-3 (GPCR135), and insulin-like peptide 5 (LGPCR142) be named the relaxin family peptide receptors 1 through 4 (RXFP1-4). RXFP1 and RXFP2 are leucine-rich repeat-containing G-protein-coupled receptors with complex binding characteristics involving both the large ectodomain and the transmembrane loops. RXFP1 activates adenylate cyclase, protein kinase A, protein kinase C, phosphatidylinositol 3-kinase, and extracellular signaling regulated kinase (Erk1/2) and also interacts with nitric oxide signaling. RXFP2 activates adenylate cyclase in recombinant systems, but physiological responses are sensitive to pertussis toxin. RXFP3 and RXFP4 resemble more conventional peptide liganded receptors and both inhibit adenylate cyclase, and in addition RXFP3 activates Erk1/2 signaling. Physiological studies and examination of the phenotypes of transgenic mice have established that relaxin has roles as a reproductive hormone involved in uterine relaxation (some species), reproductive tissue growth, and collagen remodeling but also in the cardiovascular and renal systems and in the brain. The connective tissue remodeling properties of relaxin acting at RXFP1 receptors have potential for the development of agents effective for the treatment of cardiac and renal fibrosis, asthma, and scleroderma and for orthodontic remodelling. Agents acting at RXFP2 receptors may be useful for the treatment of cryptorchidism and infertility, whereas antagonists may be used as contraceptives. The brain distribution of RXFP3 receptors suggests that actions at these receptors have the potential for the development of antianxiety and antiobesity drugs.

Relaxin-3: Improved Synthesis Strategy and Demonstration of Its High-Affinity Interaction with the Relaxin Receptor LGR7 BothIn VitroandIn Vivo†

Relaxin-3 is a member of the human relaxin peptide family, the gene for which, RLN3, is predominantly expressed in the brain. Mapping studies in the rodent indicate a highly developed network of RLN3, RLN1, and relaxin receptor-expressing cells in the brain, suggesting that relaxin peptides have important functional roles in the central nervous system. A regioselective disulfide-bond synthesis protocol was developed and used for the chemical synthesis of human (H3) relaxin-3. The selectively S-protected A and B chains were combined by stepwise formation of each of the three insulin-like disulfides via aeration, thioloysis, and iodolysis. Judicious positioning of the three sets of S-protecting groups was crucial for acquisition of synthetic H3 relaxin in a good overall yield. The activity of the peptide was tested against relaxin family peptide receptors. Although the highest activity was demonstrated on the human relaxin-3 receptor (GPCR135), the peptide also showed high activity on relaxin receptors (LGR7) from various species and variable activity on the INSL3 receptor (LGR8). Recombinant mouse prorelaxin-3 demonstrated similar activity to H3 relaxin, suggesting that the presence of the C peptide did not influence the conformation of the active site. H3 relaxin was also able to activate native LGR7 receptors. It stimulated increased MMP-2 expression in LGR7-expressing rat ventricular fibroblasts in a dose-dependent manner and, following infusion into the lateral ventricle of the brain, stimulated water drinking in rats, activating LGR7 receptors located in the subfornical organ. Thus, H3 relaxin is able to interact with the relaxin receptor LGR7 both in vitro and in vivo.

Multiple binding sites revealed by interaction of relaxin family peptides with native and chimeric relaxin family peptide receptors 1 and 2 (LGR7 and LGR8)

Relaxin family peptide 1 (RXFP1) receptor (LGR7) and RXFP2 receptor (LGR8) were recently identified as the receptor targets for H2 relaxin and insulin-like peptide 3 (INSL3), respectively. In this study, we define the pharmacology of these two receptors by using a number of receptor chimeras and relaxin family peptides. We have identified two binding sites on these receptors: one primary, high-affinity site within the ectodomain and a secondary, lower affinity site within the transmembrane region. The primary site was found to dictate receptor binding characteristics, although the lower affinity site also exerts some influence and modulates ligand affinity for the primary site in a manner dependent upon the peptide in question. Not all relaxin peptides were able to bind to the RXFP2 receptor, indicating that the relaxin-RXFP2 receptor interaction is species-specific. INSL3 was found to exhibit characteristics of a partial agonist at the RXFP2 and chimeric RXFP1/2 receptors, with low maximal cAMP responses but high potency in coupling to this pathway. cAMP accumulation studies also revealed that the binding sites couple to cAMP signaling pathways with differing efficiency: the high-affinity site signals with high efficiency, whereas the lower affinity site signals with little to no efficiency. Comparisons between RXFP1, RXFP2, the chimeric receptors, and the truncated receptors revealed that the interaction between receptor sites is critical for optimal ligand binding and signal transduction and that the ectodomain is essential for signaling. Evidence obtained in this study supports a two-stage binding model of receptor activation: binding to the primary site allows a conformational change and interaction with the low-affinity transmembrane site.

Human relaxin gene 3 (H3) and the equivalent mouse relaxin (M3) gene. Novel members of the relaxin peptide family

We have identified a novel human relaxin gene, designated H3 relaxin, and an equivalent relaxin gene in the mouse from the Celera Genomics data base. Both genes encode a putative prohormone sequence incorporating the classic two-chain, three cysteine-bonded structure of the relaxin/insulin family and, importantly, contain the RXXXRXX(I/V) motif in the B-chain that is essential for relaxin receptor binding. A peptide derived from the likely proteolytic processing of the H3 relaxin prohormone sequence was synthesized and found to possess relaxin activity in bioassays utilizing the human monocytic cell line, THP-1, that expresses the relaxin receptor. The expression of this novel relaxin gene was studied in mouse tissues using RT-PCR, where transcripts were identified with a pattern of expression distinct from that of the previously characterized mouse relaxin. The highest levels of expression were found in the brain, whereas significant expression was also observed in the spleen, thymus, lung, and ovary. Northern blotting demonstrated an approximately 1.2-kb transcript present in mouse brain poly(A) RNA but not in other tissues. These data, together with the localization of transcripts in the pars ventromedialis of the dorsal tegmental nucleus of C57BLK6J mouse brain by in situ hybridization histochemistry, suggest a new role for relaxin in neuropeptide signaling processes. Together, these studies describe a third member of the human relaxin family and its equivalent in the mouse.

The primary structure and the disulfide links of the bovine relaxin-like factor (RLF)

The relaxin-like factor (RLF), produced by the Leydig cells, is an essential link in the chain of events leading to the proper positioning of the testes during fetal development. The primary structure of RLF, as reported in the literature, is based solely upon cDNA sequences with chain lengths determined according to deduced processing sites and with relaxin-like cross-links. Biochemical characterization of bovine testicular RLF shows clearly that the endogenous hormone does consist of a 26 residue A chain and two forms of B chain, one containing 40 residues, the other 45. In addition, both B chains are 9 residues longer at the C terminus than the cDNA-deduced chain, and about 20% of the B chains have an additional 5 residue extension at the N terminus. Sequence analysis in combination with mass spectrometry and tryptic peptide mapping showed unambiguously that RLF is larger than previously assumed and that it has the relaxin-type disulfide bond distribution that makes it a bona fide member of the relaxin family of hormones.

Synthesis and conformational analysis of the insulin-like 4 gene product

Insulin-like 4 (INSL-4) is a protein expressed in the early placenta. Its primary structure is insulin-like with reference to the distribution of cysteine residues and the single chain pro-form. Insulin-like 4 was generated by solid-phase peptide synthesis of the two chains followed by the sequential synthesis of the three disulfide bonds. Two disulfide isomers were produced, one with an insulin-like disulfide bonding pattern and the other with a reversed chain orientation. The CD spectra of the two disulfide isomers were indistinguishable without any features produced by periodic structures. In addition, the hydrodynamic properties of the two isomers were identical which implied a very open structure of the disulfide-bonded two-chain molecules. It appears that insulin-likeness cannot be defined solely on the basis of the primary structure of cDNA.

The relaxin receptor-binding site geometry suggests a novel gripping mode of interaction

Relaxin has a unique, clearly identifiable, mixed function receptor-binding region comprising amino acid residues that evolve sequentially from the central portion of the B chain alpha-helix. Two arginine residues in positions B13 and B17 that project like forefinger and middle finger from the helix provide the electrostatic element opposed by the hydrophobic (thumb) element isoleucine (B20), offset from the arginines by about 40 degrees. The binding intensity of relaxin to its receptor decreases by 3 orders of magnitude if alanine is substituted for the newly discovered binding component isoleucine in position B20. The arginine residues cannot be replaced by other positive charges, nor can the guanidinium group be presented on a longer or shorter hydrocarbon chain. In contrast, the hydrophobic interaction is incremental in nature, and the contribution to the total binding energy is roughly proportional to the number of hydrocarbon units in the side chain. It appears that a hydrophobic surface exists on the receptor that offers optimal van der Waals' interaction with beta-branched hydrophobic amino acids. The binding energy increases roughly 10-fold with each methylene group whereby beta-branching is more effective per surface unit than chain elongation. Aromatic side chains appear to demarcate the extent of the binding region in so far as residues larger than phenylalanine decrease receptor binding. The exceptional clarity of binding site geometry in relaxin makes for an excellent opportunity to design peptido-mimetics.

The receptor binding conformation of bombyxin is induced by alanine(B15)

Bombyxin is an insect hormone with an insulin-like structure which affects the reduction of stored carbohydrates in the silkworm Bombyx mori. The receptor binding surface of bombyxin includes a trough on the interface between the B chain helix and the N-terminal A chain helix. Alanine(B15) is located on the edge of this feature, whereas the bottom is formed by hydrophobic core residues Ile(A2) and Leu(B14). Replacement of alanine(B15) with bulkier residues produces a negative steric effect on bombyxin receptor binding; alpha-aminobutyric acid reduced the affinity to 6.5%, valine to 1.1%, norvaline to 0.88%, and leucine to 0.05%. CD spectra of these analogues were indistinguishable from each other and identical to that of bombyxin. Changing the backbone structure by replacing alanine with glycine and alpha-aminoisobutyric acid resulted in analogues with activities of 3.7 and 1.4%, respectively, but also a disturbed structure as determined by CD spectroscopy. Replacement of other residues on the periphery of the trough, i.e., arginines at positions B12 and B16, also reduced the level of receptor binding but to a lesser extent than the replacement of alanine(B15). The level of receptor binding for citrulline(B12) bombyxin was 17% and for citrulline(B16) bombyxin was 45%. When it is considered that glycine(A1) is located on the edge of the same trough but across from Ala(B15) and is required for maintenance of the overall structure of bombyxin, it is proposed that the bombyxin receptor binding site forms a contiguous hydrophobic area consisting of residues Ile(A2), Leu(B14), and Ala(B15).

Specific, high affinity relaxin-like factor receptors

The relaxin-like factor (RLF) is a circulating hormone that binds to specific membrane-bound uterine receptors in the mouse. Mono-iodinated RLF tracers were produced and characterized specifically to study the properties of the RLF receptor. The tracers bound to the RLF receptor in uterine crude membrane preparations with high affinity (73 nM for (125)I-Tyr(A9) RLF and 90 nM for (125)I-Tyr(A26) RLF) as determined by Scatchard analysis. The specificity of binding was confirmed by chemical cross-linking experiments. Binding of (125)I-Tyr(A9) RLF to the putative receptor was inhibited in the presence of a 640-fold excess of unlabeled human RLF but not by the same excess of human relaxin. SDS-gel electrophoresis of the RLF-receptor complex revealed a molecular mass of >200 kDa, which remained unchanged upon reduction. The size and the lack of subunit structure of the receptor is similar to the features reported for the relaxin receptor. In this regard both, the RLF and the relaxin receptor are different from the insulin- and the insulin-like growth factor-type 1 receptors. This observation supports the relaxin-likeness of this new factor not only toward potential target tissues but also as regards receptor features.