Structure of the R3/I5 chimeric relaxin peptide, a selective GPCR135 and GPCR142 agonist

The human relaxin family comprises seven peptide hormones with various biological functions mediated through interactions with G-protein-coupled receptors. Interestingly, among the hitherto characterized receptors there is no absolute selectivity toward their primary ligand. The most striking example of this is the relaxin family ancestor, relaxin-3, which is an agonist for three of the four currently known relaxin receptors: GPCR135, GPCR142, and LGR7. Relaxin-3 and its endogenous receptor GPCR135 are both expressed predominantly in the brain and have been linked to regulation of stress and feeding. However, to fully understand the role of relaxin-3 in neurological signaling, the development of selective GPCR135 agonists and antagonists for in vivo studies is crucial. Recent reports have demonstrated that such selective ligands can be achieved by making chimeric peptides comprising the relaxin-3 B-chain combined with the INSL5 A-chain. To obtain structural insights into the consequences of combining A- and B-chains from different relaxins we have determined the NMR solution structure of a human relaxin-3/INSL5 chimeric peptide. The structure reveals that the INSL5 A-chain adopts a conformation similar to the relaxin-3 A-chain, and thus has the ability to structurally support a native-like conformation of the relaxin-3 B-chain. These findings suggest that the decrease in activity at the LGR7 receptor seen for this peptide is a result of the removal of a secondary LGR7 binding site present in the relaxin-3 A-chain, rather than conformational changes in the primary B-chain receptor binding site.     

The A-chain of human relaxin family peptides has distinct roles in the binding and activation of the different relaxin family peptide receptors

The relaxin peptides are a family of hormones that share a structural fold characterized by two chains, A and B, that are cross-braced by three disulfide bonds. Relaxins signal through two different classes of G-protein-coupled receptors (GPCRs), leucine-rich repeat-containing GPCRs LGR7 and LGR8 together with GPCR135 and GPCR142, now referred to as the relaxin family peptide (RXFP) receptors 1-4, respectively. Although key binding residues have been identified in the B-chain of the relaxin peptides, the role of the A-chain in their activity is currently unknown. A recent study showed that INSL3 can be truncated at the N terminus of its A-chain by up to 9 residues without affecting the binding affinity to its receptor RXFP2 while becoming a high affinity antagonist. This suggests that the N terminus of the INSL3 A-chain contains residues essential for RXFP2 activation. In this study, we have synthesized A-chain truncated human relaxin-2 and -3 (H2 and H3) relaxin peptides, characterized their structure by both CD and NMR spectroscopy, and tested their binding and cAMP activities on RXFP1, RXFP2, and RXFP3. In stark contrast to INSL3, A-chain-truncated H2 relaxin peptides lost RXFP1 and RXFP2 binding affinity and concurrently cAMP-stimulatory activity. H3 relaxin A-chain-truncated peptides displayed similar properties on RXFP1, highlighting a similar binding mechanism for H2 and H3 relaxin. In contrast, A-chain-truncated H3 relaxin peptides showed identical activity on RXFP3, highlighting that the B-chain is the sole determinant of the H3 relaxin-RXFP3 interaction. Our results provide new insights into the action of relaxins and demonstrate that the role of the A-chain for relaxin activity is both peptide- and receptor-dependent.     

Relaxin-3: improved synthesis strategy and demonstration of its high-affinity interaction with the relaxin receptor LGR7 both in vitro and in vivo

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

Human relaxin-2: historical perspectives and role in cancer biology

One of the most recognised and studied family of peptide hormones is the insulin superfamily. Within this family is the relaxin subfamily which comprises seven members: relaxin-1, -2 and -3 and insulin-like peptides 3, 4, 5 and 6. Besides exhibiting sequence similarities, each member exists as an active A-B heterodimer linked by three disulfide bonds. This mini-review is divided into three broad themes: an overview of all insulin superfamily members (including structural similarities); roles of each superfamily member and finally, a focus on the pleiotropic peptide hormone, human relaxin-2. In addition to promoting vasodilatory effects leading to evaluation in Phase III clinical trials for the treatment of acute heart failure, relaxin has recently been shown to be highly expressed by cancer cells, aiding in their proliferation, invasiveness and metastasis. These contrary effects of relaxin are discussed together with current efforts in the development of relaxin antagonists that may possess future therapeutic potential for the treatment of certain cancers.     

R3(BDelta23 27)R/I5 chimeric peptide, a selective antagonist for GPCR135 and GPCR142 over relaxin receptor LGR7: in vitro and in vivo characterization

Both relaxin-3 and its receptor (GPCR135) are expressed predominantly in brain regions known to play important roles in processing sensory signals. Recent studies have shown that relaxin-3 is involved in the regulation of stress and feeding behaviors. The mechanisms underlying the involvement of relaxin-3/GPCR135 in the regulation of stress, feeding, and other potential functions remain to be studied. Because relaxin-3 also activates the relaxin receptor (LGR7), which is also expressed in the brain, selective GPCR135 agonists and antagonists are crucial to the study of the physiological functions of relaxin-3 and GPCR135 in vivo. Previously, we reported the creation of a selective GPCR135 agonist (a chimeric relaxin-3/INSL5 peptide designated R3/I5). In this report, we describe the creation of a high affinity antagonist for GPCR135 and GPCR142 over LGR7. This GPCR135 antagonist, R3(BDelta23-27)R/I5, consists of the relaxin-3 B-chain with a replacement of Gly23 to Arg, a truncation at the C terminus (Gly24-Trp27 deleted), and the A-chain of INSL5. In vitro pharmacological studies showed that R3(BDelta23-27)R/I5 binds to human GPCR135 (IC50=0.67 nM) and GPCR142 (IC50=2.29 nM) with high affinity and is a potent functional GPCR135 antagonist (pA2=9.15) but is not a human LGR7 ligand. Furthermore, R3(BDelta23-27)R/I5 had a similar binding profile at the rat GPCR135 receptor (IC50=0.25 nM, pA2=9.6) and lacked affinity for the rat LGR7 receptor. When administered to rats intracerebroventricularly, R3(BDelta23-27)R/I5 blocked food intake induced by the GPCR135 selective agonist R3/I5. Thus, R3(BDelta23-27)R/I5 should prove a useful tool for the further delineation of the functions of the relaxin-3/GPCR135 system.     

Relaxin-3/insulin-like peptide 7, a neuropeptide involved in the stress response and food intake

Relaxin-3, also known as insulin-like peptide-7, is a newly-identified peptide of the insulin superfamily. All members of this superfamily have a similar structure, which consists of two subunits (A-chain and B-chain) linked by disulfide bonds. Relaxin-3 is so named because it has a motif that can interact with the relaxin receptor. By contrast to other relaxins, relaxin-3 is mainly expressed in the brain and testis. In rodent brain, anatomical studies have revealed its predominant expression in neurons of the nucleus incertus of the dorsal pons, and a few other regions of the brainstem. On the other hand, relaxin-3-expressing nerve fibers and the relaxin-3 receptors, RXFP3 and RXFP1, are widely distributed in the forebrain, with the hypothalamus being one of the most densely-innervated regions. Therefore, relaxin-3 is considered to exert various actions through its ligand-receptor system. This minireview describes the expression of relaxin-3 in the brain, as well as its functions in the hypothalamus, including the stress response and food intake.     

The highly conserved negatively charged Glu141 and Asp145 of the G-protein-coupled receptor RXFP3 interact with the highly conserved positively charged arginine residues of relaxin-3

Relaxin-3 is a newly identified insulin/relaxin superfamily peptide that plays a putative role in the regulation of food intake and stress response by activating its cognate G-protein-coupled receptor RXFP3. Relaxin-3 has three highly conserved arginine residues, B12Arg, B16Arg and B26Arg. We speculated that these positively charged arginines may interact with certain negatively charged residues of RXFP3. To test this hypothesis, we first replaced the negatively charged residues in the extracellular domain of RXFP3 with arginine, respectively. Receptor activation assays showed that arginine replacement of Glu141 or Asp145, especially Glu141, significantly decreased the sensitivity of RXFP3 to wild-type relaxin-3. In contrast, arginine replacement of other negatively charged extracellular residues had little effect. Thus, we deduced that Glu141 and Asp145, locating at the extracellular end of the second transmembrane domain, played a critical role in the interaction of RXFP3 with relaxin-3. To identify the ligand residues interacting with the negatively charged EXXXD motif of RXFP3, we replaced the three conserved arginines of relaxin-3 with negatively charged glutamate or aspartate, respectively. The mutant relaxin-3s retained the native structure, but their binding and activation potencies towards wild-type RXFP3 were decreased significantly. The compensatory effects of the mutant relaxin-3s towards mutant RXFP3s suggested two probable interaction pairs during ligand–receptor interaction: Glu141 of RXFP3 interacted with B26Arg of relaxin-3, meanwhile Asp145 of RXFP3 interacted with both B12Arg and B16Arg of relaxin-3. Based on these results, we proposed a relaxin-3/RXFP3 interaction model that shed new light on the interaction mechanism of the relaxin family peptides with their receptors.     

The Importance of Tryptophan B28 in H2 Relaxin for RXFP2 Binding and Activation

H2 relaxin (relaxin) is a member of the insulin–relaxin superfamily and exhibits several non-reproductive functions in addition to its well-known properties as a pregnancy hormone. Over the years, the therapeutic potential of relaxin has been examined for a number of conditions. It is currently in phase III clinical trials for the treatment of acute heart failure. The 53 amino acid peptide hormone consists of two polypeptide chains (A and B) which are cross-linked by two inter-chains and one intra-A chain disulfide bridge. Although its cognate receptor is relaxin family peptide receptor (RXFP) 1, relaxin is also able to cross-react with RXFP2, for which the native ligand is INSL3. The “RXXXRXXI” motif in the B-chain of H2 relaxin is responsible for primary binding to LRR of the RXFP1 receptor (Büllesbach and Schwabe, J Biol Chem 280:14051–14056, 2005). Previous RXFP2 receptor mutation and molecular modelling studies strongly suggest that, in addition to this motif, the Trp-B28 residue in the B-chain is responsible for H2–RXFP2 interaction. To confirm this finding, here we have mutated H2 relaxin in which Trp-B28 was replaced with alanine. The synthetic relaxin analogue was then tested on cells expressing either RXFP1 or 2 to determine the affinity and potency for the respective receptors. Our results confirm that Trp-B28 in the B-chain is crucial for binding and activating RXFP2, but not for RXFP1.     

Identification of relaxin-3/INSL7 as an endogenous ligand for the orphan G-protein-coupled receptor GPCR135

GPCR135, publicly known as somatostatin- and angiotensin-like peptide receptor, is expressed in the central nervous system and its cognate ligand(s) has not been identified. We have found that both rat and porcine brain extracts stimulated 35S-labeled guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) incorporation in cells over-expressing GPCR135. Multiple rounds of extraction, purification, followed by N-terminal sequence analysis of the ligand from porcine brain revealed that the ligand is a product of the recently identified gene, relaxin-3 (aka insulin-7 or INSL7). Recombinant human relaxin-3 potently stimulates GTPgammaS binding and inhibits cAMP accumulation in GPCR135 overexpressing cells with EC50 values of 0.25 and 0.35 nM, respectively. 125I-Relaxin-3 binds GPCR135 at high affinity with a Kd value of 0.31 nM. Relaxin-3 is the only member of the insulin/relaxin superfamily that can activate GPCR135. In situ hybridization showed that relaxin-3 mRNA is predominantly expressed in the dorsomedial ventral tegmental nucleus of the brainstem (aka nucleus incertus), as well as in discrete cells in the lateral periaqueductal gray and in the central gray nucleus. GPCR135 is expressed abundantly in the hypothalamus with discrete expression in the paraventricular nucleus of the hypothalamus and supraoptic nucleus, as well as in the cortex, septal nucleus, and preoptical area. Relaxin-3 has previously been shown to bind and activate the LGR7 relaxin receptor. However, we believe that neuroanatomical colocalization of GPCR135 and relaxin-3, coupled with a clear high affinity interaction, suggest that GPCR135 is the receptor for relaxin-3. The identification of relaxin-3 as the ligand for GPCR135 provides the framework for the discovery of a new brainstem/hypothalamus circuitry.     

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.     

Regioselective Disulfide Solid Phase Synthesis, Chemical Characterization and In Vitro Receptor Binding Activity of Equine Relaxin

In the equine industry, pregnancy loss during the third trimester constitutes a large percentage of fetal and neonatal mortality and represents a major financial loss and time investment for the breeder. Early identification of placental insufficiency would, in some cases, make it possible to sustain the pregnancy through medical intervention. Recent work suggests that relaxin is a valuable clinical tool for diagnosing placental insufficiency and monitoring treatment efficacy in mares. Relaxin is a polypeptide member of the insulin superfamily that consists of a two-chain structure and three disulfide bonds in a disposition identical to that of insulin. It is typically produced in the ovary during pregnancy and has primary roles in maintaining mammalian pregnancy and facilitating the delivery of the young via remodelling of the reproductive tract. The placenta is the primary source of relaxin in the mare during pregnancy. Its primary structure has been determined and shown to be the smallest of the known mammalian relaxins. It consists of a 20 residue A-chain and a 28-residue B-chain. To undertake detailed biophysical and biological characterization of the peptide, its chemical synthesis was undertaken using regioselective disulfide formation methods. The synthetic equine relaxin showed typical α-helical structure under physiological conditions. The peptide was found to bind to the relaxin receptor, LGR7, in vitro, and its binding affinity was found to be higher than that of the “gold standard”, porcine relaxin, and similar to that of the human relaxin-2 (H2 relaxin).     

Relaxin-3 stimulates the hypothalamic-pituitary-gonadal axis

The hypothalamus plays a key role in the regulation of both energy homeostasis and reproduction. Evidence suggests that relaxin-3, a recently discovered member of the insulin superfamily, is an orexigenic hypothalamic neuropeptide. Relaxin-3 is thought to act in the brain via the RXFP3 receptor, although the RXFP1 receptor may also play a role. Relaxin-3, RXFP3, and RXFP1 are present in the hypothalamic paraventricular nucleus, an area with a well-characterized role in the regulation of energy balance that also modulates reproductive function by providing inputs to hypothalamic gonadotropin-releasing hormone (GnRH) neurons. Other members of the relaxin family are known to play a role in the regulation of reproduction. However, the effects of relaxin-3 on reproductive function are unknown. We studied the role of relaxin-3 in the regulation of the hypothalamo-pituitary-gonadal (HPG) axis. Intracerebroventricular (5 nmol) and intraparaventricular (540-1,620 pmol) administration of human relaxin-3 (H3) in adult male Wistar rats significantly increased plasma luteinizing hormone (LH) 30 min postinjection. This effect was blocked by pretreatment with a peripheral GnRH antagonist. Central administration of human relaxin-2 showed no significant effect on plasma LH. H3 dose-dependently stimulated the release of GnRH from hypothalamic explants and GT(1)-7 cells, which express RXFP1 and RXFP3, but did not influence LH or follicle-stimulating hormone release from pituitary fragments in vitro. We have demonstrated a novel role for relaxin-3 in the stimulation of the HPG axis, putatively via hypothalamic GnRH neurons. Relaxin-3 may act as a central signal linking nutritional status and reproductive function.     

Solution structure and characterization of the LGR8 receptor binding surface of insulin-like peptide 3

Insulin-like peptide 3 (INSL3), a member of the relaxin peptide family, is produced in testicular Leydig cells and ovarian thecal cells. Gene knock-out experiments have identified a key biological role in initiating testes descent during fetal development. Additionally, INSL3 has an important function in mediating male and female germ cell function. These actions are elicited via its recently identified receptor, LGR8, a member of the leucine-rich repeat-containing G-protein-coupled receptor family. To identify the structural features that are responsible for the interaction of INSL3 with its receptor, its solution structure was determined by NMR spectroscopy together with in vitro assays of a series of B-chain alanine-substituted analogs. Synthetic human INSL3 was found to adopt a characteristic relaxin/insulin-like fold in solution but is a highly dynamic molecule. The four termini of this two-chain peptide are disordered, and additional conformational exchange is evident in the molecular core. Alanine-substituted analogs were used to identify the key residues of INSL3 that are responsible for the interaction with the ectodomain of LGR8. These include Arg(B16) and Val(B19), with His(B12) and Arg(B20) playing a secondary role, as evident from the synergistic effect on the activity in double and triple mutants involving these residues. Together, these amino acids combine with the previously identified critical residue, Trp(B27), to form the receptor binding surface. The current results provide clear direction for the design of novel specific agonists and antagonists of this receptor.     

Relaxin-3 and RXFP3 expression, and steroidogenic actions in the ovary of teleost fish

Relaxins are peptides similar in secondary structure to insulins. In teleost genomes, five or six relaxin genes have been identified. Two relaxins group closely with mammalian relaxin-3 on phylogenetic analysis and are named relaxin-3a and b. We refer to the remainder as relaxins c to f. Ovarian expression of relaxin-3a, d and f genes, and the relaxin-3 receptor gene Rxfp3, was studied in Danio rerio using RT-PCR. Immunohistochemistry was used to determine the distribution of relaxin-3 peptides and RXFP3 in the ovary of Fundulus heteroclitus (killifish). Thirdly, enzyme immunoassays and ovarian follicular culture were used to determine the effect of treatment with human recombinant relaxin-3 on the production of 17beta-estradiol and 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one in killifish ovarian follicles. Relaxin-3a, d, f, and Rxfp3 genes were expressed regardless of sex or reproductive condition. Relaxin-3 immunostaining was present in mid to late follicular stages within cortical alveoli of the oocyte cytopasm, whereas receptor staining was localized to follicular cells. Treatment with relaxin-3 enhanced 17beta-estradiol production in early and late maturing follicles, but did not have an effect in vitellogenic follicles. Relaxin-3 appeared to suppress the release of MIS production. This suggests that relaxin peptides may be involved with estradiol-dependant events in follicular development.     

Conformationally constrained single-chain peptide mimics of relaxin B-chain secondary structure.

Relaxin is a member of the insulin superfamily and has many biological actions including angiogenesis and collagen degradation. It is a 6 kDa peptide hormone consisting of two peptide chains (A and B) tethered by two disulphide bonds. Past structure-function relationship studies have shown the key receptor binding site of relaxin to be principally situated within the B-chain alpha-helix. Molecular dynamic simulations were performed to aid the design of conformationally constrained relaxin B-chain analogues that possess alpha-helical structure and relaxin-like activity. Restraints included disulphide bonds, both single and double, and lactam bonds. Each peptide was prepared by solid phase synthesis and, following purification, subjected to detailed conformational analysis by circular dichroism spectroscopy. Of 15 prepared relaxin B-chain mimetics, one was able to mimic the secondary structure of the native ligand as indicated by biomolecular recognition/interaction analysis using surface enhanced laser desorption ionization mass spectroscopy together with a relaxin antibody. However, none of the mimetics possess characteristic relaxin-like biological activity which strongly indicates that the pharmacophore comprises additional structural elements other than the relaxin B-chain alpha-helix. These findings will assist in the design and preparation of novel relaxin agonists and antagonists.     

Hydrocarbon stapled B chain analogues of relaxin-3 retain biological activity

Relaxin-3 or insulin-like peptide 7 (INSL7) is the most recently discovered relaxin/insulin-like family peptide. Mature relaxin-3 consists of an A chain and a B chain held by disulphide bonds. According to structure activity relationship studies, the relaxin-3 B chain is more important in binding and activating the receptor. RXFP3 (also known as Relaxin-3 receptor 1, GPCR 135, somatostatin- and angiotensin- like peptide receptor or SALPR) was identified as the cognate receptor for relaxin-3 by expression profiles and binding studies. Recent studies imply roles of this system in mediating stress and anxiety, feeding, metabolism and cognition. Stapling of peptides is a technique used to develop peptide drugs for otherwise undruggable targets. The main advantages of stapling include, increased activity due to reduced proteolysis, increased affinity to receptors and increased cell permeability. Stable agonists and antagonists of RXFP3 are crucial for understanding the physiological significance of this system. So far, agonists and antagonists of RXFP3 are peptides. In this study, for the first time, we have introduced stapling of the relaxin-3 B chain at 14th and 18th positions (14s18) and 18th and 22nd position (18s22). These stapled peptides showed greater helicity than the unstapled relaxin-3 B chain in circular dichroism analysis. Both stapled peptides bound RXFP3 and activated RXFP3 as observed in an inhibition of forskolin-induced cAMP assay and a ERK1/2 activation assay, although with different potencies. Therefore, we conclude that stapling of the relaxin3 B chain does not compromise its ability to activate RXFP3 and is a promising method for developing stable peptide agonists and antagonists of RXFP3 to aid relaxin-3/RXFP3 research.     

The relaxin peptide family and their novel G-protein coupled receptors

Relaxin-1 is a heterodimeric peptide hormone primarily produced by the pregnant corpus luteum and/or placenta and is involved in many essential physiological processes centered on its action as a potent extracellular matrix (ECM) remodeling agent. Insulin-like peptide 3 (INSL3), also known as relaxin-like factor, is predominantly expressed in the Leydig cells of the testes and is an important mediator of testicular descent. The relaxin-1 equivalent peptide in humans is actually the product of the human RLN2 gene, human 2 (H2) relaxin. Recently identified and thought to be the ancestral relaxin, relaxin-3 is specifically expressed in the nucleus incertus of the mouse and rat brain and is most likely an important neuropeptide. Each of the hormones above act on cell membrane G-protein coupled receptors (GPCRs). The relaxin-1 receptor is leucine-rich repeat-containing GPCR 7 (LGR7) whereas INSL3 acts on the closely related LGR8. These receptors have large extra-cellular domains containing multiple leucine-rich repeats (LRRs) and a unique LDL receptor-like cysteine-rich motif (LDLR-domain). Relaxin-3 will bind and activate LGR7 with 50-fold lower activity than H2 relaxin. Two relaxin-3 selective GPCRs; somatostatin and angiotensin like peptide receptor (SALPR) and GPCR 142 were recently identified, these type I GPCRs are unrelated to LGR7 and LGR8. The discovery and characterisation of these receptors is greatly aiding the quest to unravel the mechanics of these important hormones, however with three other family members, insulin-like peptides 4–6 (INSL4, INSL5 and INSL6) with unknown functions and unidentified receptors, there is still much to be learnt about this hormone family.     

Hypothalamic mapping of orexigenic action and Fos-like immunoreactivity following relaxin-3 administration in male Wistar rats

The insulin superfamily, characterized by common disulphide bonds, includes not only insulin but also insulin-like peptides such as relaxin-1 and relaxin-3. The actions of relaxin-3 are largely unknown, but recent work suggests a role in regulation of food intake. Relaxin-3 mRNA is highly expressed in the nucleus incertus, which has extensive projections to the hypothalamus, and relaxin immunoreactivity is present in several hypothalamic nuclei. In the rat, relaxin-3 binds and activates both relaxin family peptide receptor 1, which also binds relaxin-1, and a previously orphaned G protein-coupled receptor, RXFP3. These receptors are extensively expressed in the hypothalamus. The aims of these studies were twofold: 1) map the hypothalamic site(s) of the orexigenic action of relaxin-3 and 2) examine the site(s) of neuronal activation following central relaxin-3 administration. After microinjection into hypothalamic sites, human relaxin-3 (H3; 180 pmol) significantly stimulated 0- to 1-h food intake in the supraoptic nucleus (SON), arcuate nucleus (ARC), and the anterior preoptic area (APOA) [SON 0.4+/-0.2 (vehicle) vs. 2.9+/-0.5 g (H3), P<0.001; ARC 0.7+/-0.3 (vehicle) vs. 2.7+/-0.2 g (H3), P<0.05; and APOA 0.8+/-0.1 (vehicle) vs. 2.2+/-0.2 g (H3), P<0.05]. Cumulative food intake was significantly increased相似文献   

Site-specific DOTA/europium-labeling of recombinant human relaxin-3 for receptor-ligand interaction studies

Relaxin-3 (also known as INSL7) is a recently identified neuropeptide belonging to the insulin/relaxin superfamily. It has putative roles in the regulation of stress responses, food intake, and reproduction by activation of its cognate G-protein-coupled receptor RXFP3. It also binds and activates the relaxin family peptide receptors RXFP1 and RXFP4 in vitro. To obtain a europium-labeled relaxin-3 as tracer for studying the interaction of these receptors with various ligands, in the present work we propose a novel site-specific labeling strategy for the recombinant human relaxin-3 that has been previously prepared in our laboratory. First, the N-terminal 6 × His-tag of the single-chain relaxin-3 precursor was removed by Aeromonas aminopeptidase and all of the primary amines of the resultant peptide were reversibly blocked by citroconic anhydride. Second, the A-chain N-terminus of the blocked peptide was released by endoproteinase Asp-N cleavage that removed the linker peptide between the B- and A-chains. Third, an alkyne moiety was introduced to the newly released A-chain N-terminus by reaction with the highly active primary amine-specific N-hydroxysuccinimide ester. Fourth, after removal of the reversible blockage under mild acidic condition, europium-loaded DOTA with an azide moiety was introduced to the two-chain relaxin-3 carrying the alkyne moiety through click chemistry. Using this site-specific labeling strategy, homogeneous monoeuropium-labeled human relaxin-3 could be obtained with good overall yield. In contrast, conventional random labeling resulted in a complex mixture that was poorly resolved because human relaxin-3 has four primary amine moieties that all react with the modification reagent. Both saturation and competition binding assays demonstrated that the DOTA/Eu(3+)-labeled relaxin-3 retained high binding affinity for human RXFP3, RXFP4, and RXFP1 and was therefore a suitable non-radioactive and stable tracer to study the interaction of various natural or designed ligands with these receptors. Using this site-specific labeling strategy, other functional probes, such as fluorescent dyes, biotin, or nanoparticles could also be introduced to the A-chain N-terminal of the recombinant human relaxin-3. Additionally, we improved the time-resolved fluorescence assay for the DOTA-bound europium ion which paves the way for the use of DOTA as a lanthanide chelator for protein and peptide labeling in future studies.