Distinct but overlapping binding sites of agonist and antagonist at the relaxin family peptide 3 (RXFP3) receptor.

The relaxin-3 neuropeptide activates the relaxin family peptide 3 (RXFP3) receptor to modulate stress, appetite, and cognition. RXFP3 shows promise as a target for treating neurological disorders, but realization of its clinical potential requires development of smaller RXFP3-specific drugs that can penetrate the blood-brain barrier. Designing such drugs is challenging and requires structural knowledge of agonist- and antagonist-binding modes. Here, we used structure-activity data for relaxin-3 and a peptide RXFP3 antagonist termed R3 B1-22R to guide receptor mutagenesis and develop models of their binding modes. RXFP3 residues were alanine-substituted individually and in combination and tested in cell-based binding and functional assays to refine models of agonist and antagonist binding to active- and inactive-state homology models of RXFP3, respectively. These models suggested that both agonists and antagonists interact with RXFP3 via similar residues in their B-chain central helix. The models further suggested that the B-chain Trp27 inserts into the binding pocket of RXFP3 and interacts with Trp138 and Lys271, the latter through a salt bridge with the C-terminal carboxyl group of Trp27 in relaxin-3. R3 B1-22R, which does not contain Trp27, used a non-native Arg23 residue to form cation-π and salt-bridge interactions with Trp138 and Glu141 in RXFP3, explaining a key contribution of Arg23 to affinity. Overall, relaxin-3 and R3 B1-22R appear to share similar binding residues but may differ in binding modes, leading to active and inactive RXFP3 conformational states, respectively. These mechanistic insights may assist structure-based drug design of smaller relaxin-3 mimetics to manage neurological disorders.

Binding conformation and determinants of a single-chain peptide antagonist at the relaxin-3 receptor RXFP3.

The neuropeptide relaxin-3 and its receptor relaxin family peptide receptor-3 (RXFP3) play key roles in modulating behavior such as memory and learning, food intake, and reward seeking. A linear relaxin-3 antagonist (R3 B1-22R) based on a modified and truncated relaxin-3 B-chain was recently developed. R3 B1-22R is unstructured in solution; thus, the binding conformation and determinants of receptor binding are unclear. Here, we have designed, chemically synthesized, and pharmacologically characterized more than 60 analogues of R3 B1-22R to develop an extensive understanding of its structure-activity relationships. We show that the key driver for affinity is the nonnative C-terminal Arg23 Additional contributors to binding include amino acid residues that are important also for relaxin-3 binding, including Arg12, Ile15, and Ile19 Intriguingly, amino acid residues that are not exposed in native relaxin-3, including Phe14 and Ala17, also interact with RXFP3. We show that R3 B1-22R has a propensity to form a helical structure, and modifications that support a helical conformation are functionally well-tolerated, whereas helix breakers such as proline residues disrupt binding. These data suggest that the peptide adopts a helical conformation, like relaxin-3, upon binding to RXFP3, but that its smaller size allows it to penetrate deeper into the orthosteric binding site, creating more extensive contacts with the receptor.

Synthesis of relaxin-2 and insulin-like peptide 5 enabled by novel tethering and traceless chemical excision.

This report presents an entirely chemical, general strategy for the synthesis of relaxin-2 and insulin-like peptide 5. Historically, these two peptides have represented two of the more synthetically challenging members of the insulin superfamily. The key synthetic steps involve two sequential oxime ligations to covalently link the individual A-chain and B-chain, followed by disulfide bond formation under aqueous, redox conditions. This is followed by two chemical reactions that employ diketopiperazine cyclization-mediated cleavage and ester hydrolysis to liberate the connecting peptide and the heterodimeric product. This approach avoids the conventional iodine-mediated disulfide bond formation and enzyme-assisted proteolysis to generate biologically active two-chain peptides. This novel synthetic strategy is ideally suited for peptides such as relaxin and insulin-like peptide 5 as they possess methionine and tryptophan that are labile under strong oxidative conditions. Additionally, these peptides possess multiple arginine and lysine residues that preclude the use of trypsin-like enzymes to obtain biologically active hormones. This synthetic methodology is conceivably applicable to other two-chain peptides that contain multiple disulfide bonds. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Synthetic Route to Human Relaxin-2 via Iodine-Free Sequential Disulfide Bond Formation.

A new synthetic route to human relaxin-2 has been established through a sequential disulfide bond formation process in the absence of iodine. It is enabled by a combination of cysteine protection with penicillin G acylase-labile Phacm and a newly identified thiol activator bis(5-(2-methoxyethoxy)-2-pyrimidinyl disulfide. The long-standing challenges in relaxin B-chain assembly and its poor solubility have been solved by the insertion of two isoacyl dipeptide segments. The overall yield was 25% from the B chain and 5.8% from the B-chain starting resin.

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

H2 relaxin is a pleiotropic peptide hormone with clinical potential. Here we report on the reaction and use of hexafluorobenzene as an intramolecular disulfide replacement between Cys10 and Cys15 in the A-chain of H2 relaxin. Using flow-based Fmoc solid-phase peptide synthesis methodology we were able to obtain high-quality H2 relaxin fragments that were previously reported as challenging to synthesize. Subsequent native chemical ligation and oxidative folding enabled total synthesis of both wild type H2 relaxin and a C6F4 linked analog. Cell-based activity assays revealed modest activity for the C6F4 linked H2 relaxin analog, albeit 100-fold reduced relative to wild type. This work demonstrates how perfluoroarylation-cysteine SNAr chemistry may be a useful tool for the selective replacement of native disulfide bonds in proteins.

Chemically synthesized dicarba H2 relaxin analogues retain strong RXFP1 receptor activity but show an unexpected loss of in vitro serum stability.

Peptides and proteins are now acknowledged as viable alternatives to small molecules as potential therapeutic agents. A primary limitation to their more widespread acceptance is their generally short in vivo half-lives due to serum enzyme susceptibility and rapid renal clearance. Numerous chemical approaches to address this concern have been undertaken in recent years. The replacement of disulfide bonds with non-reducible elements has been demonstrated to be one effective means by eliminating the deleterious effect of serum reductases. In particular, substitution with dicarba bonds via ring closure metathesis has been increasingly applied to many bioactive cystine-rich peptides. We used this approach for the replacement of the A-chain intramolecular disulfide bond of human relaxin 2 (H2 relaxin), an insulin-like peptide that has important regulatory roles in cardiovascular and connective tissue homeostasis that has led to successful Phase IIIa clinical trials for the treatment of acute heart failure. Use of efficient solid phase synthesis of the two peptide chains was followed by on-resin ring closure metathesis and formation of the dicarba bond within the A-chain and then by off-resin combination with the B-chain via sequential directed inter-chain disulfide bond formation. After purification and comprehensive chemical characterization, the two isomeric synthetic H2 relaxin analogues were shown to retain near-equipotent RXFP1 receptor binding and activation propensity. Unexpectedly, the in vitro serum stability of the analogues was greatly reduced compared with the native peptide. Circular dichroism spectroscopy studies showed subtle differences in the secondary structures between dicarba analogues and H2 relaxin suggesting that, although the overall fold is retained, it may be destabilized which could account for rapid degradation of dicarba analogues in serum. Caution is therefore recommended when using ring closure metathesis as a general approach to enhance peptide stability.

Synthetic relaxins.

The relaxin subfamily of peptides within the human insulin superfamily consists of seven members including relaxin-2 and relaxin-3. The former is a pleiotropic hormone that is a vasodilator and cardiac stimulant in the cardiovascular system and an antifibrotic agent whereas the latter is primarily a neuropeptide involved in stress and metabolic control. Both possess the unique three-disulfide heterodimeric peptide structure of insulin. Consequently, the synthesis, both chemical and biological, of relaxin-2 and relaxin-3 has long represented a special challenge to further understanding their structural and functional relationships. This review highlights past and recent developments in the use of chemical and recombinant DNA methods of synthesis of these peptides and current resulting knowledge of their biology.

Chemical synthesis and orexigenic activity of rat/mouse relaxin-3.

The insulin-like peptide, relaxin-3 was first identified just a decade ago via a genomic database search and is now recognized to be a key neuropeptide with several roles including the regulation of arousal, stress responses and neuroendocrine homeostasis. It also has significant potential as a drug to treat stress and obesity. Its actions are mediated via its cognate G protein-coupled receptor, RXFP3, which is found in abundant numbers in the brain. However, much remains to be determined with respect to the mechanism of neurological action of this peptide. Consequently, the chemical synthesis of the rat and mouse (which share identical primary structures) two-chain, three disulfide peptide was undertaken and the resulting peptide subjected to detailed in vitro and in vivo assay. Use of efficient solid-phase synthesis methods provided the two regioselectively S-protected A- and B-chains which were readily combined via sequential disulfide bond formation. The synthetic rat/mouse relaxin-3 was obtained in high purity and good overall yield. It demonstrated potent orexigenic activity in rats in that central intracerebroventricular infusion led to significantly increased food intake and water drinking.

Site-specific conjugation of a lanthanide chelator and its effects on the chemical synthesis and receptor binding affinity of human relaxin-2 hormone.

Diethylenetriamine pentaacetic acid (DTPA) is a popular chelator agent for enabling the labeling of peptides for their use in structure-activity relationship study and biodistribution analysis. Solid phase peptide synthesis was employed to couple this commercially available chelator at the N-terminus of either the A-chain or B-chain of H2 relaxin. The coupling of the DTPA chelator at the N-terminus of the B-chain and subsequent loading of a lanthanide (europium) ion into the chelator led to a labeled peptide (Eu-DTPA-(B)-H2) in low yield and having very poor water solubility. On the other hand, coupling of the DTPA and loading of Eu at the N-terminus of the A-chain led to a water-soluble peptide (Eu-DTPA-(A)-H2) with a significantly improved final yield. The conjugation of the DTPA chelator at the N-terminus of the A-chain did not have any impact on the secondary structure of the peptide determined by circular dichroism spectroscopy (CD). On the other hand, it was not possible to determine the secondary structure of Eu-DTPA-(B)-H2 because of its insolubility in phosphate buffer. The B-chain labeled peptide Eu-DTPA-(B)-H2 required solubilization in DMSO prior to carrying out binding assays, and showed lower affinity for binding to H2 relaxin receptor, RXFP1, compared to the water-soluble A-chain labeled peptide Eu-DTPA-(A)-H2. The mono-Eu-DTPA labeled A-chain peptide, Eu-DTPA-(A)-H2, thus can be used as a valuable probe to study ligand-receptor interactions of therapeutically important H2 relaxin analogs. Our results show that it is critical to choose an approriate site for incorporating chelators such as DTPA. Otherwise, the bulky size of the chelator, depending on the site of incorporation, can affect yield, solubility, structure and pharmacological profile of the peptide.

The relaxin peptide family--structure, function and clinical applications.

The relaxin peptide family in humans consists of seven members, relaxin-1, -2 and -3 and insulin-like (INSL) peptides 3, 4, 5 and 6. It is an offshoot of the large insulin superfamily. Each member consists of two chains, commonly referred to as A and B, which are held together by two inter-chain disulfide bonds and another intra-chain disulfide bond present within the A chain. The cysteine residues present in each chain, together with the distinctive disulfide bonding pattern, are conserved across all members of the superfamily. The chemical synthesis of these complex peptides poses a significant challenge. In the past, random combination of the two synthetic S-reduced chains under oxidizing conditions was utilized to form the three disulfide bonds. Nowadays, with the aid of highly efficient solid phase peptide synthesis methodologies, in conjunction with selective S-thiol-protecting groups, combination of individual A- and B- chains by sequential chemical formation of each of the three disulfide bonds is now possible resulting in good yields of these peptides. The relaxin peptide family members bind to G-protein coupled receptors (GPCRs) which have been classified as relaxin family peptide (RXFP) receptors. The various unique receptor-ligand interactions are outlined in this review, together with the physiological roles of the relaxin peptide family members and lastly their past and present clinical applications.

An optimized chemical synthesis of human relaxin-2.

Human gene 2 relaxin (RLX) is a member of the insulin superfamily and is a multi-functional factor playing a vital role in pregnancy, aging, fibrosis, cardioprotection, vasodilation, inflammation, and angiogenesis. RLX is currently applied in clinical trials to cure among others acute heart failure, fibrosis, and preeclampsia. The synthesis of RLX by chemical methods is difficult because of the insolubility of its B-chain and the required laborious and low yielding site-directed combination of its A (RLXA) and B (RLXB) chains. We report here that oxidation of the Met(25) residue of RLXB improves its solubility, allowing its effective solid-phase synthesis and application in random interchain combination reactions with RLXA. Linear Met(O)(25)-RLX B-chain (RLXBO) reacts with a mixture of isomers of bicyclic A-chain (bcRLXA) giving exclusively the native interchain combination. Applying this method Met(O)(25)-RLX (RLXO) was obtained in 62% yield and was easily converted to RLX in 78% yield, by reduction with ammonium iodide.

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.

De novo design and synthesis of cyclic and linear peptides to mimic the binding cassette of human relaxin.

A synthetic cyclic regioselectively addressable functionalized template was used to prepare six analogs that presented the side chains of the key receptor-binding residues of each of H2 and H3 relaxins and insulin-like peptide 3. None showed any binding affinity for RXFP1, RXFP2, or RXFP3, indicating that the key residues were either incorrectly oriented or that additional residues are required for receptor binding.

Solid phase synthesis and structural analysis of novel A-chain dicarba analogs of human relaxin-3 (INSL7) that exhibit full biological activity.

Replacement of disulfide bonds with non-reducible isosteres can be a useful means of increasing the in vivo stability of a protein. We describe the replacement of the A-chain intramolecular disulfide bond of human relaxin-3 (H3 relaxin, INSL7), an insulin-like peptide that has potential applications in the treatment of stress and obesity, with the physiologically stable dicarba bond. Solid phase peptide synthesis was used to prepare an A-chain analogue in which the two cysteine residues that form the intramolecular bond were replaced with allylglycine. On-resin microwave-mediated ring closing metathesis was then employed to generate the dicarba bridge. Subsequent cleavage of the peptide from the solid support, purification of two isomers and their combination with the B-chain via two intermolecular disulfide bonds, then furnished two isomers of dicarba-H3 relaxin. These were characterized by CD spectroscopy, which suggested a structural similarity to the native peptide. Additional analysis by solution NMR spectroscopy also identified the likely cis/trans form of the analogs. Both peptides demonstrated binding affinities that were equivalent to native H3 relaxin on RXFP1 and RXFP3 expressing cells. However, although the cAMP activity of the analogs on RXFP3 expressing cells was similar to the native peptide, the potency on RXFP1 expressing cells was slightly lower. The data confirmed the use of a dicarba bond as a useful isosteric replacement of the disulfide bond.

Improved chemical synthesis and demonstration of the relaxin receptor binding affinity and biological activity of mouse relaxin.

The primary stored and circulating form of relaxin in humans, human gene-2 (H2) relaxin, has potent antifibrotic properties with rapidly occurring efficacy. However, when administered to experimental models of fibrosis, H2 relaxin can only be applied over short-term (2-4 week) periods, due to rodents mounting an antibody response to the exogenous human relaxin, resulting in delayed clearance and, hence, increased and variable circulating levels. To overcome this problem, the current study investigated the therapeutic potential of mouse relaxin over long-term exposure in vivo. Mouse relaxin is unique among the known relaxins in that it possesses an extra residue within the C-terminal region of its A-chain. To enable a detailed assessment of its receptor interaction and biological properties, it was chemically synthesized in good overall yield by the separate preparation of each of its A- and B-chains followed by regioselective formation of each of the intramolecular and two intermolecular disulfide bonds. Murine relaxin was shown to bind with high affinity to the human, mouse, and rat RXFP1 (primary relaxin) receptor but with a slightly lower affinity to that of H2 relaxin. When administered to relaxin-deficient mice (which undergo an age-dependent progression of organ fibrosis) over a 4 month treatment period, mouse relaxin was able to significantly inhibit the progression of collagen accumulation in several organs including the lung, kidney, testis, and skin (all p < 0.05 vs untreated group), consistent with the actions of H2 relaxin. These combined data demonstrate that mouse relaxin can effectively inhibit collagen deposition and accumulation (fibrosis) over long-term treatment periods.

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.

Synthesis, conformational studies and biological activity of N(alpha)-mono-biotinylated rat relaxin.

Biotin-avidin immobilization can be a useful tool in structure-function studies of hormone receptors. A crucial step is the preparation of a specifically biotinylated hormone that is able to bind to its receptor while leaving the biotin group free for interaction with avidin. The receptor for relaxin, an ovarian peptidic hormone produced during pregnancy, has not yet been isolated. We therefore undertook to prepare a specifically monobiotinylated rat relaxin for use in ligand-searching strategies. Rat relaxin is a convenient analogue because reliable bioassays exist, thus allowing assessment of the effect of N-biotinylation on bioactivity. To help improve the yield of the two-chain, three-disulfide bond rat relaxin, 2-hydroxy-4-methoxybenzyl (Hmb) backbone protection was used during the solid-phase assembly of the B-chain to help prevent any possible chain aggregation. As a final step, while the protected peptide was still on the resin, the biotin label was introduced at the N-terminus of the B-chain using standard coupling protocols. The chain combination with the A-chain was accomplished in reasonable yield. Secondary structural measurements demonstrated that the biotin caused the starting B-chain to adopt a more ordered conformation. The labelled synthetic relaxin exhibited similar circular dichroism spectra to native and synthetic single B-chain peptides. In addition, the biotinylated relaxin showed no significant difference in its chronotropic activity in the rat isolated heart assay compared with the native peptide. Biosensor studies showed that antibody recognition was retained upon attachment of the synthetic relaxin to the streptavidin-derivatized surface.

The chemical synthesis of rat relaxin and the unexpectedly high potency of the synthetic hormone in the mouse.

Rat relaxin, as isolated from ovaries, has been described in the literature as a low potency hormone in the mouse symphysis pubis assay. Searching for an explanation, a helix-breaking glycine residue in the B chain seemed to be the most auspicious perturbation. Rat relaxin was chemically synthesized and analyzed by reverse-phase high performance liquid chromatography, amino acid composition, mass spectrometry and circular dichroic spectroscopy. Analogs of rat relaxin were synthesized either with aspartic acid in place of the helix-breaking glycine residue in the receptor-binding region of the B chain or with Asp-Leu-Val instead of Gly-Tyr-Val at positions B14-B16. In receptor-binding assays [B14D, B15L, B16V]relaxin was a better ligand than rat relaxin, whereas the [B14D]relaxin was less potent. In the mouse symphysis pubis assay, both analogs were less potent than unmodified rat relaxin, but the [B14D, B15L, B16V]relaxin was better than [B14D]relaxin. In contrast to previous reports on native rat relaxin, the chemically synthesized rat relaxin proved to be as active as human and porcine relaxin with respect to the standard mouse assay system. Glycine, which is considered to be a perturbator in an alpha helix, is not only tolerated in the B14 position but is required for full biological potency.

Chemical synthesis of a Zwitterhormon, insulaxin, and of a relaxin-like bombyxin derivative.

The structural motif of insulin and relaxin is frequently seen in molecules of divergent functions and origins. The insect developmental factor bombyxin, the relaxin-like factor from Leydig cells, and the insulin-like factor 4 (INSL4) all are made of two disulfide-linked chains and have one disulfide bond within the A-chain. The polyclonal antibody R6, which was raised against porcine relaxin, reacts with a wide variety of naturally occurring relaxins from primates, marine and terrestrial mammals, and elasmobranchs but does not recognize insulin. The antibody binds mainly to the arginines that occur in the N, N+4 positions in the B-chains of all relaxins which also constitute the receptor-binding site. The receptor-binding haptens were incorporated by total synthesis into human despentapeptide insulin and bombyxin II, a developmental factor from the silk moth Bombyx mori. In the process the insect factor became a perfect antigen for the anti-relaxin antibody, whereas the human insulin was transformed into a bona fide relaxin. The conversion was affected by changing four critical residues so that the insulin activity was retained to the extent of 10% of the original level. This, to the best of our knowledge, is the first designer protein to incorporate two unrelated biological functions in one primary sequence, and we are therefore proposing that, analogous to zwitterion, the generic name "Zwitterhormon" (German spelling) be used for this type of construct.

Use of synthetic canine relaxin to develop a rapid homologous radioimmunoassay.

Canine relaxin (cRlx) was synthesized by a combination of solid-phase methods and sequential site-directed disulfide bond formation. Proof that the intended molecule had been synthesized was obtained by analytical HPLC of the intact and reduced molecule, by amino acid and sequence analysis, and by receptor binding and in vivo mouse interpubic ligament bioassays. Antisera to synthetic cRlx were raised in six male rabbits; these cross-reacted with relaxins of other species, but not with insulin, LH, FSH, hCG, or prolactin (PRL). Three of the antisera neutralized relaxin-induced interpubic ligament formation in estrogen-primed mice in vivo. A new homologous cRlx RIA was developed through the use of rabbit antiserum 79888, synthetic cRlx for standards and 125l-labeled trace, and a goat anti-rabbit lgG-polyethylene glycol precipitant. The new RIA can be completed in 26 h and has a sensitivity of 0.195-0.39 ng cRlx/tube. Intra- and interassay coefficients of variation were 3% and 12.5%. During pregnancy in bitches, serum cRlx rose to about 10 micrograms/ml. Immunoactive cRlx was also detected in serum, colostrum, and milk of lactating bitches, but not in large volumes (100-300 microliters) of serum of pseudopregnant or estrous bitches. Immunoreactive cRlx was also found in seminal plasma, but not in serum, of male dogs. The new homologous cRlx RIA is simple, rapid, sensitive, and specific, and will be used in future studies of canine relaxin physiology.