Identification of Potent and Long-Acting Single-Chain Peptide Mimetics of Human Relaxin-2 for Cardiovascular Diseases.

The insulin-like peptide human relaxin-2 was identified as a hormone that, among other biological functions, mediates the hemodynamic changes occurring during pregnancy. Recombinant relaxin-2 (serelaxin) has shown beneficial effects in acute heart failure, but its full therapeutic potential has been hampered by its short half-life and the need for intravenous administration limiting its use to intensive care units. In this study, we report the development of long-acting potent single-chain relaxin peptide mimetics. Modifications in the B-chain of relaxin, such as the introduction of specific mutations and the trimming of the sequence to an optimal size, resulted in potent, structurally simplified peptide agonists of the relaxin receptor Relaxin Family Peptide Receptor 1 (RXFP1) (e.g., 54). Introduction of suitable spacers and fatty acids led to the identification of single-chain lipidated peptide agonists of RXFP1, with sub-nanomolar activity, high subcutaneous bioavailability, extended half-lives, and in vivo efficacy (e.g., 64).

Engineered Relaxin as theranostic nanomedicine to diagnose and ameliorate liver cirrhosis.

Hepatic cirrhosis is a growing health problem with increasing mortality worldwide. So far, there is a lack of early diagnosis and no clinical therapy is approved for the treatment. In this study, we developed a novel theranostic nanomedicine by targeting relaxin (RLX) that is known to possess potent anti-fibrotic properties but simultaneously has poor pharmacokinetics and detrimental off-target effects. We conjugated RLX to PEGylated superparamagnetic iron-oxide nanoparticles (RLX-SPIONs) and examined hepatic stellate cells (HSCs) specific binding/uptake. Thereafter, we assessed the therapeutic efficacy of RLX-SPIONs on human HSCs in vitro and in vivo in CCl4-induced liver cirrhosis mouse model. RLX-SPIONs showed specific binding and uptake in TGFß-activated HSCs, and inhibited TGFß-induced HSCs differentiation, migration and contraction. In vivo, RLX-SPIONs strongly attenuated cirrhosis and showed enhanced contrast in MR imaging. Altogether, this study presents RLX-SPIONs as a novel theranostic nanomedicine that provides new opportunities for the diagnosis and treatment of liver cirrhosis.

Enzyme-linked immunosorbent assay of relaxin-like gonad-stimulating peptide in the starfish Patiria (Asterina) pectinifera.

A relaxin-like gonad-stimulating peptide (RGP) from starfish Patiria (Asterina) pectinifera is the first identified invertebrate gonadotropin for final gamete maturation. Recently, we succeeded in obtaining specific antibodies against P. pectinifera RGP (PpeRGP). In this study, the antibodies were used for the development of a specific and sensitive enzyme-linked immunosorbent assay (ELISA) for the measurement of PpeRGP. A biotin-conjugated peptide that binds to peroxidase-conjugated streptavidin is specifically detectable using 3,3',5,5'-tetramethylbenzidine (TMB)/hydrogen peroxide as a substrate; therefore, biotin-conjugated RGP (biotin-PpeRGP) was synthesized chemically. Similarly to PpeRGP, synthetic biotin-PpeRGP bound to the antibody against PpeRGP. In binding experiments with biotin-PpeRGP using wells coated with the antibody, a displacement curve was obtained using serial concentrations of PpeRGP. The ELISA system showed that PpeRGP could be measured in the range 0.01-10pmol per 50µl assay buffer. On the contrary, the B-chains of PpeRGP, Asterias amurensis RGP, Aphelasterias japonica RGP, and human relaxin showed minimal cross-reactivity in the ELISA, except that the A-chain of PpeRGP affected it slightly. These results strongly suggest that this ELISA system is highly specific and sensitive with respect to PpeRGP.

H3 Relaxin Protects Against Myocardial Injury in Experimental Diabetic Cardiomyopathy by Inhibiting Myocardial Apoptosis, Fibrosis and Inflammation.

BACKGROUND/AIMS: Apoptosis, fibrosis and NLRP3 inflammasome activation are involved in the development of diabetic cardiomyopathy (DCM). Human recombinant relaxin-3 (H3 relaxin) is a novel bioactive peptide that inhibits cardiac injury; however, whether H3 relaxin prevents cardiac injury in rats with DCM and the underlying mechanisms are unknown. METHODS: To investigate the effect of H3 relaxin on DCM, we performed a study using H3 relaxin treatment in male Sprague-Dawley (SD) rats with streptozotocin (STZ)-induced diabetes (DM). We measured apoptosis, fibrosis and NLRP3 inflammasome markers in the rat hearts four and eight weeks after the rats were injected with STZ (65 mg/kg) by western blot analysis. Subsequently, 2 or 6 weeks after the STZ treatment, the rats were treated with H3 relaxin [2 µg/kg/d (A group) or 0.2 µg/kg/d (B group)] for 2 weeks. Cardiac function was evaluated by echocardiography to determine the extent of myocardial injury in the DM rats. The protein levels of apoptosis, fibrosis and NLRP3 inflammasome markers were used to assess myocardial injury. In addition, we determined the plasma levels of IL-1ß and IL-18 using a Milliplex MAP Rat Cytokine/Chemokine Magnetic Bead Panel kit. RESULTS: The protein expression of cleaved caspase-8, caspase-9 and caspase-3 as well as fibrosis markers increased at 4 and 8 weeks in the STZ-induced diabetic hearts compared with the levels in the control group. Furthermore, the NLRP3 inflammasome was substantially activated in STZ-induced diabetic hearts, leading to increased IL-1ß and IL-18 levels. Compared with the DM group, the A group exhibited substantially better cardiac function. The protein levels of apoptosis markers were attenuated by H3 relaxin, indicating that H3 relaxin inhibited myocardial apoptosis in the hearts of diabetic rats. The protein expression of fibrosis markers was inhibited by H3 relaxin. Additionally, the protein expression and activation of the NLRP3 inflammasome were also effectively attenuated by H3 relaxin. CONCLUSIONS: This study is the first to demonstrate that H3 relaxin plays an anti-apoptotic, anti-fibrotic and anti-inflammatory role in DCM.

A validated UPLC-MS/MS method coupled with protein precipitation and ion exchange solid phase extraction for the quantitation of porcine relaxin B29 in dog plasma and its application to a pharmacokinetic study.

Porcine relaxin is a 6 kDa peptide hormone of pregnancy with important physiological and pharmacological effects. It contains a number of analogs of which porcine relaxin B29 is one of the most important. To support the development of porcine relaxin B29 as a new drug, we established an UPLC-MS/MS method for its quantitation in dog plasma. Sample preparation by protein precipitation and ion exchange solid phase extraction was followed by UPLC on an XBridge™ BEH300 C18 column at 40 °C in a run time of only 5.5 min. Detection was performed on a Qtrap 6500 mass spectrometer using ESI in the positive ion mode with MRM of the transitions at m/z 831.7 [M+7H]7+ â†’ 505.4 and m/z 1162.4 [M+5H]5+ â†’ 226 for pRLX B29 and internal standard (recombinant human insulin), respectively. The method was linear over the concentration range 30-2000 ng/mL with no matrix effects. Intra- and inter-day precisions were < 15% with accuracies in the range 98.8-100.6%. The method was successfully applied to a pharmacokinetic study in beagle dogs after administration of a 0.15 mg/kg intravenous dose. Graphical abstract Sample preparation and detection procedure.

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.

Relaxin-like peptides in male reproduction - a human perspective.

The relaxin family of peptide hormones and their cognate GPCRs are becoming physiologically well-characterized in the cardiovascular system and particularly in female reproductive processes. Much less is known about the physiology and pharmacology of these peptides in male reproduction, particularly as regards humans. H2-relaxin is involved in prostate function and growth, while insulin-like peptide 3 (INSL3) is a major product of the testicular Leydig cells and, in the adult, appears to modulate steroidogenesis and germ cell survival. In the fetus, INSL3 is a key hormone expressed shortly after sex determination and is responsible for the first transabdominal phase of testicular descent. Importantly, INSL3 is becoming a very useful constitutive biomarker reflecting both fetal and post-natal development. Nothing is known about roles for INSL4 in male reproduction and only very little about relaxin-3, which is mostly considered as a brain peptide, or INSL5. The former is expressed at very low levels in the testes, but has no known physiology there, whereas the INSL5 knockout mouse does exhibit a testicular phenotype with mild effects on spermatogenesis, probably due to a disruption of glucose homeostasis. INSL6 is a major product of male germ cells, although it is relatively unexplored with regard to its physiology or pharmacology, except that in mice disruption of the INSL6 gene leads to a disruption of spermatogenesis. Clinically, relaxin analogues may be useful in the control of prostate cancer, and both relaxin and INSL3 have been considered as sperm adjuvants for in vitro fertilization. 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.

A negatively charged transmembrane aspartate residue controls activation of the relaxin-3 receptor RXFP3.

Relaxin-3 is an insulin/relaxin superfamily neuropeptide involved in the regulation of food intake and stress response via activation of its cognate receptor RXFP3, an A-class G protein-coupled receptor (GPCR). In recent studies, a highly conserved ExxxD motif essential for binding of relaxin-3 has been identified at extracellular end of the second transmembrane domain (TMD2) of RXFP3. For most of the A-class GPCRs, a highly conserved negatively charged Asp residue (Asp(2.50) using Ballesteros-Weinstein numbering and Asp128 in human RXFP3) is present at the middle of TMD2. To elucidate function of the conserved transmembrane Asp128, in the present work we replaced it with other residues and the resultant RXFP3 mutants all retained quite high ligand-binding potency, but their activation and agonist-induced internalization were abolished or drastically decreased. Thus, the negatively charged transmembrane Asp128 controlled transduction of agonist-binding information from the extracellular region to the intracellular region through maintaining RXFP3 in a metastable state for efficient conformational change induced by binding of an agonist.

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.

International Union of Basic and Clinical Pharmacology. XCV. Recent advances in the understanding of the pharmacology and biological roles of relaxin family peptide receptors 1-4, the receptors for relaxin family peptides.

Relaxin, insulin-like peptide 3 (INSL3), relaxin-3, and INSL5 are the cognate ligands for the relaxin family peptide (RXFP) receptors 1-4, respectively. RXFP1 activates pleiotropic signaling pathways including the signalosome protein complex that facilitates high-sensitivity signaling; coupling to Gα(s), Gα(i), and Gα(o) proteins; interaction with glucocorticoid receptors; and the formation of hetero-oligomers with distinctive pharmacological properties. In addition to relaxin-related ligands, RXFP1 is activated by Clq-tumor necrosis factor-related protein 8 and by small-molecular-weight agonists, such as ML290 [2-isopropoxy-N-(2-(3-(trifluoromethylsulfonyl)phenylcarbamoyl)phenyl)benzamide], that act allosterically. RXFP2 activates only the Gα(s)- and Gα(o)-coupled pathways. Relaxin-3 is primarily a neuropeptide, and its cognate receptor RXFP3 is a target for the treatment of depression, anxiety, and autism. A variety of peptide agonists, antagonists, biased agonists, and an allosteric modulator target RXFP3. Both RXFP3 and the related RXFP4 couple to Gα(i)/Gα(o) proteins. INSL5 has the properties of an incretin; it is secreted from the gut and is orexigenic. The expression of RXFP4 in gut, adipose tissue, and ß-islets together with compromised glucose tolerance in INSL5 or RXFP4 knockout mice suggests a metabolic role. This review focuses on the many advances in our understanding of RXFP receptors in the last 5 years, their signal transduction mechanisms, the development of novel compounds that target RXFP1-4, the challenges facing the field, and current prospects for new therapeutics.

C-terminus of the B-chain of relaxin-3 is important for receptor activity.

Human relaxin-3 is a neuropeptide that is structurally similar to human insulin with two chains (A and B) connected by three disulfide bonds. It is expressed primarily in the brain and has modulatory roles in stress and anxiety, feeding and metabolism, and arousal and behavioural activation. Structure-activity relationship studies have shown that relaxin-3 interacts with its cognate receptor RXFP3 primarily through its B-chain and that its A-chain does not have any functional role. In this study, we have investigated the effect of modification of the B-chain C-terminus on the binding and activity of the peptide. We have chemically synthesised and characterized H3 relaxin as C-termini acid (both A and B chains having free C-termini; native form) and amide forms (both chains' C-termini were amidated). We have confirmed that the acid form of the peptide is more potent than its amide form at both RXFP3 and RXFP4 receptors. We further investigated the effects of amidation at the C-terminus of individual chains. We report here for the first time that amidation at the C-terminus of the B-chain of H3 relaxin leads to significant drop in the binding and activity of the peptide at RXFP3/RXFP4 receptors. However, modification of the A-chain C-terminus does not have any effect on the activity. We have confirmed using circular dichroism spectroscopy that there is no secondary structural change between the acid and amide form of the peptide, and it is likely that it is the local C-terminal carboxyl group orientation that is crucial for interacting with the receptors.

The plasma levels of relaxin-2 and relaxin-3 in patients with diabetes.

OBJECTIVES: Relaxin-2 has been found to alleviate fibrosis in experimental diabetic cardiomyopathy. In addition, the levels of serum relaxin-3 were increased and correlated with all the component traits of metabolic syndrome. We investigated the levels of plasma relaxin-2 or relaxin-3 and their relationship to component traits in patients with diabetes. DESIGN AND METHODS: We studied 33 newly diagnosed type 2 diabetes patients and 38 age-matched healthy subjects. Blood samples were taken at study entry, and relaxin-3, relaxin-2, fasting blood glucose, total cholesterol, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides, serum insulin and hemoglobin A1c (HbA1c) levels were measured. RESULTS: Relaxin-2 levels were significantly lower in patients with diabetes than in controls: the median plasma relaxin-2 concentration was 34.68 pg/mL (range, <29.00-50.81 pg/mL) in patients with diabetes and 45.80 pg/mL (range, <37.42-54.46 pg/mL) in controls (p=0.0150). However, no differences in relaxin-3 levels were observed between the diabetes group and controls (p=0.6550). The plasma levels of relaxin-2 or relaxin-3 were not correlated with systolic blood pressure (BP), diastolic BP, total cholesterol, LDL-C, HDL-C, triglyceride, fasting blood glucose, fasting insulin and HbA1c in patients with diabetes. Additionally, there was no correlation between the plasma concentrations of relaxin-2 and relaxin-3 in patients with diabetes (rs=0.225; p=0.208). CONCLUSIONS: We conclude that the plasma levels of relaxin-2 in diabetes patients were lower than in controls, however, there are no difference in plasma relaxin-3 concentrations between controls and patients with diabetes. Relaxin-2 or relaxin-3 levels are not related to component traits in patients with diabetes.

Release of relaxin-like gonad-stimulating substance from starfish radial nerves by lonomycin.

In starfish, the peptide hormone gonad-stimulating substance (GSS) secreted from nervous tissue stimulates oocyte maturation to induce 1-methyladenine (1-MeAde) production by ovarian follicle cells. Recently, GSS was purified from radial nerves of the starfish Asterina pectinifera and identified as a relaxin-like peptide. This study examines the mechanism of GSS secretion from radial nerves. When radial nerves isolated from A. pectinifera were incubated in artificial seawater containing ionomycin as a calcium ionophore, GSS release increased in a dose-dependent manner; 50% activity of GSS release was obtained with approximately 10 µM ionomycin. Another calcium ionophore, A23187, also stimulated GSS release from radial nerves. In contrast, membrane permeable cyclic AMP and cyclic GMP analogs failed to induce GSS release. These results suggest that GSS secretion is induced by intracellular Ca(2+) as a second messenger.

Increased feeding and body weight gain in rats after acute and chronic activation of RXFP3 by relaxin-3 and receptor-selective peptides: functional and therapeutic implications.

This paper provides a review of the effects of relaxin-3 and structurally related analogues on food intake and related behaviours, in relation to hypothalamic neural networks and chemical messengers known to control feeding, metabolism and body weight, including other neuropeptides and hormones. Soon after relaxin-3 was discovered, pharmacological studies identified the ability of the native peptide to stimulate feeding acutely in adult rats. Although interpretation of these data was confounded by ligand cross-reactivity at relaxin-family peptide (RXFP) receptors, studies with relaxin-3 analogues selective for the native relaxin-3 receptor, RXFP3, confirmed that acute and chronic activation of RXFP3 increased feeding and weight gain, and produced changes in plasma leptin and insulin. These studies also identified the hypothalamus as a locus of action. Studies are now required to identify RXFP3-positive neuron populations involved in the effects of relaxin-3/RXFP3 signalling on metabolic and neuroendocrine homeostasis, and to determine whether peptide-based, nonpeptide-based or gene-based RXFP3 treatments can alter food intake and body weight in animal models of obesity and eating disorders, as a reflection of the therapeutic potential of this newly identified transmitter system.

Design, synthesis, and characterization of a single-chain peptide antagonist for the relaxin-3 receptor RXFP3.

Relaxin-3 is a two-chain disulfide-rich peptide that is the ancestral member of the relaxin peptide family and, together with its G protein-coupled receptor RXFP3, is highly expressed in the brain. Strong evolutionary conservation of relaxin-3 suggests a critical biological function and recent studies have demonstrated modulation of sensory, neuroendocrine, metabolic, and cognitive systems. However, detailed studies of central relaxin-3-RXFP3 signaling have until now been severely hampered by the lack of a readily available high-affinity antagonist for RXFP3. Previous studies have utilized a complex two-chain chimeric relaxin peptide, R3(BΔ23-27)R/I5, in which a truncated relaxin-3 B-chain carrying an additional C-terminal Arg residue was combined with the insulin-like peptide 5 (INSL5) A-chain. In this study we demonstrate that, by replacing the native Cys in this truncated relaxin-3 B-chain with Ser, a single-chain linear peptide of 23 amino acids that retains high-affinity antagonism for RXFP3 can be achieved. In vivo studies demonstrate that this peptide, R3 B1-22R, antagonized relaxin-3/RXFP3 induced increases in feeding in rats after intracerebroventricular injection. Thus, R3 B1-22R represents an excellent tool for biological studies probing relaxin pharmacology and a lead molecule for the development of synthetically tractable, single-chain RXFP3 modulators for clinical use.

H3 relaxin demonstrates antifibrotic properties via the RXFP1 receptor.

Human gene 3 (H3) relaxin is the most recently discovered member of the relaxin peptide family and can potentially bind all of the defined relaxin family peptide receptors (RXFP1-4). While its effects as a neuromodulator are being increasingly studied through its primary receptor, RXFP3, its actions via other RXFPs are poorly understood. Hence, we specifically determined the antifibrotic effects and mechanisms of action of H3 relaxin via the RXFP1 receptor using primary rat ventricular fibroblasts in vitro, which naturally express RXFP1, but not RXFP3, and a mouse model of fibrotic cardiomyopathy in vivo. Transforming growth factor ß1 (TGF-ß1) administration to ventricular fibroblasts significantly increased Smad2 phosphorylation, myofibroblast differentiation, and collagen deposition (all p < 0.05 vs untreated controls), while having no marked effect on matrix metalloproteinase (MMP) 9, MMP-13, tissue inhibitor of metalloproteinase (TIMP) 1, or TIMP-2 expression over 72 h. H3 relaxin (at 100 and 250 ng/mL) almost completely abrogated the TGF-ß1-stimulated collagen deposition over 72 h, and its effects at 100 ng/mL were equivalent to that of the same dose of H2 relaxin. Furthermore, H3 relaxin (100 ng/mL) significantly inhibited TGF-ß1-stimulated cardiac myofibroblast differentiation and TIMP-1 and TIMP-2 expression to an equivalent extent as H2 relaxin (100 ng/mL), while also inhibiting Smad2 phosphorylation to approximately half the extent of H2 relaxin (all p < 0.05 vs TGF-ß1). Lower doses of H3 (50 ng/mL) and H2 (50 ng/mL) relaxin additively inhibited TGF-ß1-stimulated collagen deposition in vitro, while H3 relaxin was also found to reverse left ventricular collagen overexpression in the model of fibrotic cardiomyopathy in vivo. These combined findings demonstrate that H3 relaxin exerts antifibrotic actions via RXFP1 and may enhance the collagen-inhibitory effects of H2 relaxin.

A simple approach for the preparation of mature human relaxin-3.

Relaxin-3 (also known as INSL7) is the most recently identified member of the insulin-like family. It is predominantly expressed in the nucleus incertus of the brain and involved in the control of stress response, food intake, and reproduction. In the present work, we have established a simple approach for the preparation of the mature human relaxin-3 peptide. We first designed and recombinantly expressed a single-chain relaxin-3 precursor in E. coli cells. After purification by immobilized metal ion affinity chromatography, refolding in vitro through disulfide reshuffling, and digestion by endoproteinase Asp-N, mature human relaxin-3 was obtained in high yield and at low cost. Peptide mapping and circular dichroism spectroscopy studies suggested that the recombinant relaxin-3 adopted an insulin-like fold with the expected disulfide linkages. The recombinant mature relaxin-3 was fully active in both RXFP3 binding and activation assays. The activity of the single-chain precursor was very low, suggesting that a free C-terminus of the B-chain is necessary for receptor-binding and activation of relaxin-3. Our present work provides a highly efficient approach for the preparation of relaxin-3 as well as its analogues for functional and structural analyses.

The chemically synthesized human relaxin-2 analog, B-R13/17K H2, is an RXFP1 antagonist.

Relaxin is a pleiotropic hormone which exerts its biological functions through its G-protein coupled receptor, RXFP1. While relaxin is well known for its reproductive and antifibrotic roles, recent studies suggest that it is produced by cancer cells and acts on RXFP1 to induce growth and metastasis. Furthermore, more recently Silvertown et al. demonstrated that lentiviral production of a human gene-2 (H2) relaxin analog reduced the growth of prostate xenograft tumors. The authors proposed that the lentivirally produced peptide was an RXFP1 antagonist; however, the processed form of the peptide produced was not demonstrated. In this study, we have chemically synthesized the H2 relaxin analog, B-R13/17K H2 relaxin, and subjected it to detailed chemical characterization by HPLC, MALDI-TOF mass spectrometry, and amino acid analysis. The biological activity of the synthetic peptide was then tested in three different cell lines. It was found to bind with 500-fold lower affinity than H2 relaxin to RXFP1 receptors over-expressed in HEK-293T cells where it acted as a partial agonist. However, in cells which natively express the RXFP1 receptor, rat renal myofibroblasts and MCF-7 cancer cells, it acted as a full antagonist. Importantly, it was able to significantly inhibit cell invasion induced by H2 relaxin in MCF-7 cells consistent with the results of the lentiviral-driven expression in prostate cancer cells. The relaxin analog, B-R13/17K H2, can now be used as a tool to further understand RXFP1 function, and serve as a template for drug design for a therapeutic to treat prostate and other cancers.

Structure and activity in the relaxin family of peptides.

The availability of improved peptide synthesis procedures, convenient and sensitive assays for receptor binding and activation, together with advances in methods for structural characterization, has enabled the key structural features of the relaxin family of peptides responsible for biological activity to be defined. Not surprisingly, despite the similarities in primary amino acid sequences, different structural domains and residues are involved in the binding and activation at the four known relaxin family peptide receptors (RXFP1 to -4). Most of our knowledge on structure and function relates to the relaxin-RXFP1, insulin-like peptide 3 (INSL3)-RXFP2, and relaxin-3-RXFP3 systems, with information accumulating not only on the critical ligand structures but also the domains and residues on the receptor itself that are required for specificity and activation. These studies provide the framework for the design of small-molecule mimetics. While the B-chain cassette R-X-X-X-R-X-X-I, defined by Büllesbach and Schwabe, is essential for binding and activation of RXFP1, it is now recognized that the A chain, particularly the N-terminal domain, is also critical for receptor specificity. Studies of the various endogenous ligand-receptor pairs have led to the design of potent and specific agonists and antagonists. The relaxin-3 A chain-INSL5 B chain chimeric peptide and analogs with C-terminal truncations of the B chain, developed by Liu and colleagues at Johnson & Johnson, have provided selective agonist and antagonist peptides that are proving invaluable for in vivo studies of the relaxin-3-RXFP3 system.

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.