Solution structure, aggregation behavior, and flexibility of human relaxin-2

Cumulative phylogenetic, sequence and structural analysis of Insulin superfamily proteins provide unique structure-function insights

The insulin superfamily proteins (ISPs), in particular, insulin, IGFs and relaxin proteins are key modulators of animal physiology. They are known to have evolved from the same ancestral gene and have diverged into proteins with varied sequences and distinct functions, but maintain a similar structural architecture stabilized by highly conserved disulphide bridges. The recent surge of sequence data and the structures of these proteins prompted a need for a comprehensive analysis, which connects the evolution of these sequences (427 sequences) in the light of available functional and structural information including representative complex structures of ISPs with their cognate receptors. This study reveals (a) unusually high sequence conservation of IGFs (>90 % conservation in 184 sequences) and provides a possible structure-based rationale for such high sequence conservation; (b) provides an updated definition of the receptor-binding signature motif of the functionally diverse relaxin family members (c) provides a probable non-canonical C-peptide cleavage site in a few insulin sequences. The high conservation of IGFs appears to represent a classic case of resistance to sequence diversity exerted by physiologically important interactions with multiple partners. We also propose a probable mechanism for C-peptide cleavage in a few distinct insulin sequences and redefine the receptor-binding signature motif of the relaxin family. Lastly, we provide a basis for minimally modified insulin mutants with potential therapeutic application, inspired by concomitant changes observed in other insulin superfamily protein members supported by molecular dynamics simulation.

Identification of Novel Series of Potent and Selective Relaxin Family Peptide Receptor 1 (RXFP1) Agonists

Relaxin H2 is a clinically relevant peptide agonist for relaxin family peptide receptor 1 (RXFP1), but a combination of this hormone's short plasma half-life and the need for injectable delivery limits its therapeutic potential. We sought to overcome these limitations through the development of a potent small molecule (SM) RXFP1 agonist. Although two large SM HTS campaigns failed in identifying suitable hit series, we uncovered novel chemical space starting from the only known SM RXFP1 agonist series, represented by ML290. Following a design-make-test-analyze strategy based on improving early dose to man ranking, we discovered compound 42 (AZ7976), a highly selective RXFP1 agonist with sub-nanomolar potency. We used AZ7976, its 10 000-fold less potent enantiomer 43 and recombinant relaxin H2 to evaluate in vivo pharmacology and demonstrate that AZ7976-mediated heart rate increase in rats was a result of RXFP1 agonism. As a result, AZ7976 was selected as lead for continued optimization.

Synthesis of A11Cys-B11Cys Disulfide Surrogates of H2 Relaxin through an Intermolecular Native Chemical Ligation-Assisted Diaminodiacid Strategy

We report an intermolecular native chemical ligation-assisted diaminodiacid strategy for the flexible construction of A11Cys-B11Cys disulfide surrogates of H2 relaxin. The practicality of this strategy was evidenced by the synthesis of four new H2 relaxin analogs, among which H2-2a-B28Ile is found to exhibit improved potency, selectivity, and stability compared with native H2 relaxin.

Electrochemical biosensors for monitoring of selected pregnancy hormones during the first trimester: A systematic review

The hormones human chorionic gonadotropin, progesterone, estrogen and four of its metabolites (estradiol, estrone, estriol, estetrol), as well as relaxin play an essential role in the development of the fetus during the first trimester. Imbalances in these hormones during the first trimester have been directly linked to miscarriages. However, frequent monitoring of the hormones is limited by the current conventional centralized analytical tools that do not allow a rapid response time. Electrochemical sensing is considered an ideal tool to detect hormones owing to its advantages such as quick response, user-friendliness, low economic costs, and possibility of use in point-of-care settings. Electrochemical detection of pregnancy hormones is an emerging field that has been demonstrated primarily at research level. Thus, it is timely with a comprehensive overview of the characteristics of the reported detection techniques. This is the first extensive review focusing on the advances related to electrochemical detection of hormones linked to the first trimester of pregnancy. Additionally, this review offers insights into the main challenges that must be addressed imminently to ensure progress from research to clinical applications.

Development of a long-acting relaxin analogue, LY3540378, for treatment of chronic heart failure

BACKGROUND AND PURPOSE

Chronic heart failure, a progressive disease with limited treatment options currently available, especially in heart failure with preserved ejection fraction (HFpEF), represents an unmet medical need as well as an economic burden. The development of a novel therapeutic to slow or reverse disease progression would be highly impactful to patients and society. Relaxin-2 (relaxin) is a human hormone regulating cardiovascular, renal, and pulmonary adaptations during pregnancy. A short-acting recombinant relaxin, Serelaxin, demonstrated short-term heart failure symptom relief and biomarker improvement in acute heart failure trials. Here, we present the development of a long-acting relaxin analogue to be tested in the treatment of chronic heart failure.

EXPERIMENTAL APPROACH

LY3540378 is a long-acting protein therapeutic composed of a human relaxin analogue and a serum albumin-binding VHH domain.

KEY RESULTS

LY3540378 is a potent agonist of the relaxin family peptide receptor 1 (RXFP1) and maintains selectivity against RXFP2/3/4 comparable to native relaxin. The half-life of LY3540378 in preclinical species is extended through high affinity binding of the albumin-binding VHH domain to serum albumin. When tested in a single dose administration, LY3540378 elicited relaxin-mediated pharmacodynamic responses, such as reduced serum osmolality and increased renal blood flow in rats. In an isoproterenol-induced cardiac hypertrophy mouse model, treatment with LY3540378 significantly reduced cardiac hypertrophy and improved isovolumetric relaxation time. In a monkey cardiovascular safety study, there were no adverse observations from administration of LY3540378.

CONCLUSION AND IMPLICATIONS

LY3540378 demonstrated to be a suitable clinical development candidate, and is progressing in clinical trials.

Single-Shot Solid-Phase Synthesis of Full-Length H2 Relaxin Disulfide Surrogates

Chemical synthesis of insulin superfamily proteins (ISPs) has recently been widely studied to develop next-generation drugs. Separate synthesis of multiple peptide fragments and tedious chain-to-chain folding are usually encountered in these studies, limiting accessibility to ISP derivatives. Here we report the finding that insulin superfamily proteins (e.g. H2 relaxin, insulin itself, and H3 relaxin) incorporating a pre-made diaminodiacid bridge at A-B chain terminal disulfide can be easily and rapidly synthesized by a single-shot automated solid-phase synthesis and expedient one-step folding. Our new H2 relaxin analogues exhibit almost identical structures and activities when compared to their natural counterparts. This new synthetic strategy will expediate production of new ISP analogues for pharmaceutical studies.

Liposomal Entrapment or Chemical Modification of Relaxin2 for Prolongation of Its Stability and Biological Activity

Relaxin (RLX) is a protein that is structurally similar to insulin and has interesting biological activities. As with all proteins, preservation of RLX’s structural integrity/biological functionality is problematic. Herein, we investigated two methods for increasing the duration of relaxin-2’s (RLX2) biological activity: synthesis of a palmitoyl RLX2 conjugate (P-RLX2) with the use of a Palmitoyl-l-Glu-OtBu peptide modifier, and encapsulation into liposomes of P-RLX2, RLX2, and its oxidized form (O-RLX2). For liposomal encapsulation thin-film hydration and DRV methods were applied, and different lipid compositions were tested for optimized protein loading. RLX2 and O-RLX2 were quantified by HPLC. The capability of the peptides/conjugate to stimulate transfected cells to produce cyclic adenosine monophosphate (cAMP) was used as a measure of their biological activity. The stability and bioactivity of free and liposomal RLX2 types were monitored for a 30 d period, in buffer (in some cases) and bovine serum (80%) at 37 °C. The results showed that liposome encapsulation substantially increased the RLX2 integrity in buffer; PEGylated liposomes demonstrated a higher protection. Liposome encapsulation also increased the stability of RLX2 and O-RLX2 in serum. Considering the peptide’s biological activity, cAMP production of RLX2 was higher than that of the oxidized form and the P-RLX2 conjugate (which demonstrated a similar activity to O-RLX2 when measured in buffer, but lower when measured in the presence of serum proteins), while liposome encapsulation resulted in a slight decrease of bioactivity initially, but prolonged the peptide bioactivity during incubation in serum. It was concluded that liposome encapsulation of RLX2 and synthetic modification to P-RLX2 can both prolong RLX2 peptide in vitro stability; however, the applied chemical conjugation results in a significant loss of bioactivity (cAMP production), whereas the effect of liposome entrapment on RLX2 activity was significantly lower.

The Structural Basis of Peptide Binding at Class A G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) represent the largest membrane protein family and a significant target class for therapeutics. Receptors from GPCRs’ largest class, class A, influence virtually every aspect of human physiology. About 45% of the members of this family endogenously bind flexible peptides or peptides segments within larger protein ligands. While many of these peptides have been structurally characterized in their solution state, the few studies of peptides in their receptor-bound state suggest that these peptides interact with a shared set of residues and undergo significant conformational changes. For the purpose of understanding binding dynamics and the development of peptidomimetic drug compounds, further studies should investigate the peptide ligands that are complexed to their cognate receptor.

Structural Insights into the Unique Modes of Relaxin-Binding and Tethered-Agonist Mediated Activation of RXFP1 and RXFP2

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

Relaxin and the Cardiovascular System: from Basic Science to Clinical Practice

The peptide hormone relaxin was originally linked to reproductive physiology, where it is believed to mediate systemic and renal hemodynamic adjustments to pregnancy. Recently, its broad range of effects in the cardiovascular system has been the focus of intensive research regarding its implications under pathological conditions and potential therapeutic potential. An understanding of the multitude of cardioprotective actions prompted the study of serelaxin, recombinant human relaxin-2, for the treatment of acute heart failure. Despite early promising results from phase II studies, recently revealed RELAX-AHF-2 outcomes were rather disappointing and the treatment for acute heart failure remains an unmet medical need. This article reviews the physiologic actions of relaxin on the cardiovascular system and its relevance in the pathophysiology of cardiovascular disease. We summarize the most updated clinical data and discuss future directions of serelaxin for the treatment of acute heart failure. This should encourage additional work to determine how can relaxin's beneficial effects be exploited for the treatment of cardiovascular disease.

Effects of C-Terminal B-Chain Modifications in a Relaxin 3 Agonist Analogue

The receptor for the neuropeptide relaxin 3, relaxin family peptide 3 (RXFP3) receptor, is an attractive pharmacological target for the control of eating, addictive, and psychiatric behaviors. Several structure-activity relationship studies on both human relaxin 3 (containing 3 disulfide bonds) and its analogue A2 (two disulfide bonds) suggest that the C-terminal carboxylic acid of the tryptophan residue in the B-chain is important for RXFP3 activity. In this study, we have added amide, alcohol, carbamate, and ester functionalities to the C-terminus of A2 and compared their structures and functions. As expected, the C-terminal amide form of A2 showed lower binding affinity for RXFP3 while ester and alcohol substitutions also demonstrated lower affinity. However, while these analogues showed slightly lower binding affinity, there was no significant difference in activation of RXFP3 compared to A2 bearing a C-terminal carboxylic acid, suggesting the binding pocket is able to accommodate additional atoms.

Engineering of chimeric peptides as antagonists for the G protein-coupled receptor, RXFP4

Insulin-like peptide 5 (INSL5) is a very important pharma target for treating human conditions such as anorexia and diabetes. However, INSL5 with two chains and three disulfide bridges is an extremely difficult peptide to assemble by chemical or recombinant means. In a recent study, we were able to engineer a simplified INSL5 analogue 13 which is a relaxin family peptide receptor 4 (RXFP4)-specific agonist. To date, however, no RXFP4-specific antagonist (peptide or small molecule) has been reported in the literature. The focus of this study was to utilize the non-specific RXFP3/RXFP4 antagonist ΔR3/I5 as a template to rationally design an RXFP4 specific antagonist. Unexpectedly, we demonstrated that ΔR3/I5 exhibited partial agonism at RXFP4 when expressed in CHO cells which is associated with only partial antagonism of INSL5 analogue activation. In an attempt to improve RXFP4 specificity and antagonist activity we designed and chemically synthesized a series of analogues of ΔR3/I5. While all the chimeric analogues still demonstrated partial agonism at RXFP4, one peptide (Analogue 17) exhibited significantly improved RXFP4 specificity. Importantly, analogue 17 has a simplified structure which is more amenable to chemical synthesis. Therefore, analogue 17 is an ideal template for further development into a specific high affinity RXFP4 antagonist which will be an important tool to probe the physiological role of RXFP4/INSL5 axis.

Single chain peptide agonists of relaxin receptors

There are seven human relaxin family peptides that have two chains (A and B) and three disulfide bonds. The target receptors for four of these peptides are known as relaxin family peptide receptors, RXFP1-RXFP4. Detailed structure-activity relationship (SAR) studies of relaxin family peptides have been reported over the years and have led to the design of new analogs with agonistic and antagonistic properties. This review briefly summarizes the SAR of human relaxin 2 (H2 relaxin) and human relaxin 3 (H3 relaxin) leading to the design and development of single-B-chain only agonists, B7-33 and peptide 5. The physiological functions of these new peptides agonists in cellular and animal models are also described.

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.

Distinct activation modes of the Relaxin Family Peptide Receptor 2 in response to insulin-like peptide 3 and relaxin

Relaxin family peptide receptor 2 (RXFP2) is a GPCR known for its role in reproductive function. It is structurally related to the human relaxin receptor RXFP1 and can be activated by human gene-2 (H2) relaxin as well as its cognate ligand insulin-like peptide 3 (INSL3). Both receptors possess an N-terminal low-density lipoprotein type a (LDLa) module that is necessary for activation and is joined to a leucine-rich repeat domain by a linker. This linker has been shown to be important for H2 relaxin binding and activation of RXFP1 and herein we investigate the role of the equivalent region of RXFP2. We demonstrate that the linker’s highly-conserved N-terminal region is essential for activation of RXFP2 in response to both ligands. In contrast, the linker is necessary for H2 relaxin, but not INSL3, binding. Our results highlight the distinct mechanism by which INSL3 activates RXFP2 whereby ligand binding mediates reorientation of the LDLa module by the linker region to activate the RXFP2 transmembrane domains in conjunction with the INSL3 A-chain. In contrast, relaxin activation of RXFP2 involves a more RXFP1-like mechanism involving binding to the LDLa-linker, reorientation of the LDLa module and activation of the transmembrane domains by the LDLa alone.

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-2 Expression in Oral Squamous Cell Carcinoma

Background

When advanced, oral squamous cell carcinoma (OSCC) may involve adjacent non-epithelial structures, and the prognosis is worse for bone invasion. Human relaxin-2 is a peptide hormone that has recently been associated with cancer. It can induce human osteoclast differentiation and activation, suggesting a role in tumor-driven osteolysis. This study was a preliminary assessment of the prognostic role of relaxin-2 in surgical specimens of OSCC tissue and adjacent but uninvolved mandibular/maxillary bone.

Methods

Relaxin-2 immunohistochemical expression and reaction intensity were assessed in tumor and uninvolved adjacent mandibular/maxillary bone specimens from 23 operated OSCC patients.

Results

All OSCC specimens were positive for relaxin-2. The intensity of its reaction in OSCC correlated significantly with the pattern of the tumor's invasion front (p = 0.02), being higher with the infiltrative pattern. Mean relaxin-2 immunohistochemical expression was higher in patients whose OSCC recurred after treatment than those experiencing no recurrence (81.3% ± 22.6% vs. 59.5% ± 29.7%, respectively). A significant direct association emerged between relaxin-2 expression in OSCC specimens and recurrence rate (p = 0.049).

Conclusions

Relaxin-2 expression in OSCC should be further investigated as a potentially useful marker for identifying patients at higher risk of recurrence, who might benefit from closer follow-up and more aggressive adjuvant therapy. In other oncological settings, increasing evidence of relaxin being produced by cancer cells is prompting efforts to synthesize human relaxin-2 analogs capable of acting as antagonists and limiting tumor growth.

Relaxin Inhibits High Glucose-Induced Matrix Accumulation in Human Mesangial Cells by Interfering with TGF-β1 Production and Mesangial Cells Phenotypic Transition

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease (ESRD). DN is characterized by glomerular extracellular matrix accumulation, mesangial expansion, basement membrane thickening, and renal interstitial fibrosis. To date, mounting evidence has shown that H2 relaxin possesses powerful antifibrosis properties; however, the mechanisms of H2 relaxin on diabetic nephropathy remain unknown. Here, we aimed to explore whether H2 relaxin can reduce production of extracellular matrix (ECM) secreted by human mesangial cells (HMC). HMC were exposed to 5.5 mM glucose (NG) or 30 mM glucose (HG) with or without H2 relaxin. Fibronectin (FN) and collagen type IV levels in the culture supernatants were examined by solid-phase enzyme-linked immunoadsorbent assay (ELISA). Western blot was used to detect the expression of α-smooth muscle actin (α-SMA) protein. Quantitative polymerase chain reaction (qPCR) method was employed to analyze transforming growth factor (TGF)-β1 mRNA expression. Compared with the normal glucose group, the levels of fibronectin and collagen type were markedly increased after being cultured in high glucose medium. Compared with the high glucose group, remarkable decreases of fibronectin, collagen type IV, α-smooth muscle actin, and TGF-β1 mRNA expression were observed in the H2 relaxin-treated group. The mechanism by which H2 relaxin reduced high glucose-induced overproduction of ECM may be associated with inhibition of TGF-β1 mRNA expression and mesangial cells' phenotypic transition. H2 relaxin is a potentially effective modality for the treatment of DN.

Identification of hydrophobic interactions between relaxin-3 and its receptor RXFP3: implication for a conformational change in the B-chain C-terminus during receptor binding

Relaxin-3 is an insulin/relaxin superfamily neuropeptide implicated in the regulation of food intake and stress response via activation of the G protein-coupled receptor RXFP3. Their electrostatic interactions have been recently identified, and involves three positively charged B-chain residues (B12Arg, B16Arg, and B26Arg) of relaxin-3 and two negatively charged residues (Glu141 and Asp145) in a highly conserved ExxxD motif at the extracellular end of the second transmembrane domain of RXFP3. To investigate their hydrophobic interactions, in the present work we deleted the highly conserved B-chain C-terminal B27Trp residue of relaxin-3, and mutated four highly conserved aromatic residues (Phe137, Trp138, Phe146, and Trp148) around the ExxxD motif of RXFP3. The resultant [∆B27W]relaxin-3 exhibited approximately tenfold lower binding potency and ~1000-fold lower activation potency towards wild-type RXFP3, confirming its importance for relaxin-3 function. Although the RXFP3 mutants could be normally trafficked to cell membrane, they had quite different activities. [F137A]RXFP3 could normally distinguish wild-type relaxin-3 and [∆B27W]relaxin-3 in binding and activation assays, whereas [W138A]RXFP3 lost most of this capability, suggesting that the Trp138 residue of RXFP3 forms hydrophobic interactions with the B27Trp residue of relaxin-3. The hydrophobic Trp138 residue and the formerly identified negatively charged Glu141 and Asp145 residues in the highly conserved WxxExxxD motif may thus form a functional surface that is important for interaction with relaxin-3. We hypothesize that the relaxin-3 B-chain C-terminus changes from the original folding-back conformation to an extended conformation during binding with RXFP3, to allow its B27Trp and B26Arg residues to interact with the Trp138 and Glu141 residues of RXFP3, respectively.

The complex binding mode of the peptide hormone H2 relaxin to its receptor RXFP1

H2 relaxin activates the relaxin family peptide receptor-1 (RXFP1), a class A G-protein coupled receptor, by a poorly understood mechanism. The ectodomain of RXFP1 comprises an N-terminal LDLa module, essential for activation, tethered to a leucine-rich repeat (LRR) domain by a 32-residue linker. H2 relaxin is hypothesized to bind with high affinity to the LRR domain enabling the LDLa module to bind and activate the transmembrane domain of RXFP1. Here we define a relaxin-binding site on the LDLa-LRR linker, essential for the high affinity of H2 relaxin for the ectodomain of RXFP1, and show that residues within the LDLa-LRR linker are critical for receptor activation. We propose H2 relaxin binds and stabilizes a helical conformation of the LDLa-LRR linker that positions residues of both the linker and the LDLa module to bind the transmembrane domain and activate RXFP1.

The mechanism by which relaxin activates the relaxin family peptide receptor-1 is poorly understood. Here, Sethi et al. identify a relaxin binding site in an extracellular linker between the LDLa and LRR domains and propose that relaxin binding stabilizes a helical conformation that leads to receptor activation.

A single-chain derivative of the relaxin hormone is a functionally selective agonist of the G protein-coupled receptor, RXFP1

A single-chain derivative of the relaxin hormone ameliorates fibrosis without side-effects.

Human gene-2 relaxin (H2 relaxin) is a pleiotropic hormone with powerful vasodilatory and anti-fibrotic properties which has led to its clinical evaluation and provisional FDA approval as a treatment for acute heart failure. The diverse effects of H2 relaxin are mediated via its cognate G protein coupled-receptor (GPCR), Relaxin Family Peptide Receptor (RXFP1), leading to stimulation of a combination of cell signalling pathways that includes cyclic adenosine monophosphate (cAMP) and extracellular-signal-regulated kinases (ERK)1/2. However, its complex two-chain (A and B), disulfide-rich insulin-like structure is a limitation to its facile preparation, availability and affordability. Furthermore, its strong activation of cAMP signaling is likely responsible for reported detrimental tumor-promoting actions that may preclude long-term use of this drug for treating human disease. Here we report the design and synthesis of a H2 relaxin B-chain-only analogue, B7-33, which was shown to bind to RXFP1 and preferentially activate the pERK pathway over cAMP in cells that endogenously expressed RXFP1. Thus, B7-33 represents the first functionally selective agonist of the complex GPCR, RXFP1. Importantly, this small peptide agonist prevented or reversed organ fibrosis and dysfunction in three pre-clinical rodent models of heart or lung disease with similar potency to H2 relaxin. The molecular mechanism behind the strong anti-fibrotic actions of B7-33 involved its activation of RXFP1-angiotensin II type 2 receptor heterodimers that induced selective downstream signaling of pERK1/2 and the collagen-degrading enzyme, matrix metalloproteinase (MMP)-2. Furthermore, in contrast to H2 relaxin, B7-33 did not promote prostate tumor growth in vivo. Our results represent the first known example of the minimisation of a two-chain cyclic insulin-like peptide to a single-chain linear peptide that retains potent beneficial agonistic effects.

Engineering of a Novel Simplified Human Insulin-Like Peptide 5 Agonist

Insulin-like peptide 5 (INSL5) has recently been discovered as only the second orexigenic gut hormone after ghrelin. As we have previously reported, INSL5 is extremely difficult to assemble and oxidize into its two-chain three-disulfide structure. The focus of this study was to generate structure-activity relationships (SARs) of INSL5 and use it to develop a potent and simpler INSL5 mimetic with RXFP4 agonist activity. A series of human and mouse INSL5 (hINSL5/mINSL5) analogues were designed and chemically synthesized, resulting in a chimeric INSL5 analogue exhibiting more than 10-fold higher potency (0.35 nM) at human RXFP4 compared with native hINSL5 (4.57 nM). The SAR study also identified a key residue (K(A15)) in the A-chain of mINSL5 that contributes to improved RXFP4 affinity and potency of mINSL5 compared with hINSL5. This knowledge ultimately led us to engineer a minimized hINSL5 mimetic agonist that retains native hINSL5-like RXFP4 affinity and potency at human RXFP4. This minimized analogue was synthesized in 17.5-fold higher yield and in less time compared with hINSL5.

The C-terminus of the B-chain of human insulin-like peptide 5 is critical for cognate RXFP4 receptor activity

Insulin-like peptide 5 (INSL5) is an orexigenic peptide hormone belonging to the relaxin family of peptides. It is expressed primarily in the L-cells of the colon and has a postulated key role in regulating food intake. Its G protein-coupled receptor, RXFP4, is a potential drug target for treating obesity and anorexia. We studied the effect of modification of the C-terminus of the A and B-chains of human INSL5 on RXFP4 binding and activation. Three variants of human INSL5 were prepared using solid phase peptide synthesis and subsequent sequential regioselective disulfide bond formation. The peptides were synthesized as C-terminal acids (both A- and B-chains with free C-termini, i.e., the native form), amides (both chains as the C-terminal amide) and one analog with the C-terminus of its A-chain as the amide and the C-terminus of the B-chain as the acid. The results showed that C-terminus of the B-chain is more important than that of the A-chain for RXFP4 binding and activity. Amidation of the A-chain C-terminus does not have any effect on the INSL5 activity. The difference in RXFP4 binding and activation between the three peptides is believed to be due to electrostatic interaction of the free carboxylate of INSL5 with a positively charged residue (s), either situated within the INSL5 molecule itself or in the receptor extracellular loops.

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.