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

Maternal Fed Zinc-Deficient Diet: Effects on Relaxin Family Peptides and Oxidant System in the Testis and Liver Tissue of Male Offspring

Today, the studies are limited on roles of insulin-like peptide 3 (INSL3), insulin-like peptide 7 (INSL7), and relaxin family peptide receptor 1 (RXFP1) which are synthesized by the testis. It is aimed to investigate the levels of the sex hormone as testosterone and the family of insulin-like proteins (relaxin family peptides), which are important in the puberty transition, in the testicular and liver tissues of male offspring born to female rats fed a zinc-deficient diet during the pregnancy, and in the changes in lipid peroxidation markers. The study was performed on 40 male offspring. In Group I: Control group, both male offspring and mothers were fed with standard rat chow. In Group II: Zinc deficient diet, both male offspring and mothers were fed a zinc-deficient diet (2.8 mg/kg zinc). In Group III: Normal diet, male offspring fed standard rat chow for 45 days (66th day) after being separated from their mothers with a maternal zinc-deficient diet. In Group IV: Zinc-supplemented diet, offspring fed with zinc supplemented (5 mg/kg/day intraperitoneal zinc sulfate, i.p.) in addition to standard rat chow after being separated from their mothers with maternal zinc deficiency until the termination of the study (66th day). Our study suggests that zinc-supplemented diets play an important role in the changes in INSL3, INSL7, RXFP1, and testosterone levels during spermatogenesis. INSL7, INSL3, and RXFP1 levels were higher in zinc-supplemented group than the zinc-deficient diet group. Liver levels of INSL3, INSL7, and MDA were significantly different in zinc-deficiency diet group than zinc-supplemented group.

Targeting the relaxin-3/RXFP3 system: a patent review for the last two decades

INTRODUCTION

The neuropeptide relaxin-3/RXFP3 system belongs to the relaxin/insulin superfamily and is involved in many important physiological processes, such as stress responses, appetite control, and motivation for reward. Although relaxin-3 is the endogenous agonist for RXFP3, it can also bind to and activate RXFP1 and RXFP4. Consequently, research has been focused on the development of RXFP3-specific peptides and small-molecule ligands to validate the relaxin-3/RXFP3 system as a novel drug target.

AREAS COVERED

This review provides an overview of patents on the relaxin-3/RXFP3 system covering ligand development and pharmacological studies since 2003. Related patents and literature reports were obtained from established sources including SciFinder, Google Patents, and Espacenet for patents and SciFinder, PubMed, and Google Scholar for literature reports.

EXPERT OPINION

There has been an increasing amount of patent activities around relaxin-3/RXFP3, highlighting the importance of this novel neuropeptide system for drug discovery. The development of relaxin-3 derived peptides and small-molecule modulators, as well as behavioral studies in rodents, have shown that the relaxin-3/RXFP3 system is a promising drug target for treating various metabolic and neuropsychiatric diseases including obesity, anxiety, and alcohol addiction.

Relaxin ligand/receptor systems in the developing teleost fish brain: Conserved features with mammals and a platform to address neuropeptide system functions

The relaxins (RLNs) are a group of peptide hormone/neuromodulators that can regulate a wide range of physiological processes ranging from reproduction to brain function. All the family members have originated from a RLN3-like ancestor via different rounds of whole genome and gene specific duplications during vertebrate evolution. In mammals, including human, the divergence of the different family members and the emergence of new members led to the acquisition of specific functions for the various relaxin family peptide and associated receptor genes. In particular, in mammals, it was shown, that the role of RLN3 is correlated to the modulation of arousal, stress responses, emotion, social recognition, and other brain functions, positioning this gene/peptide as a potential therapeutic target for neuropsychiatric disorders. This review highlights the evolutionary conservation of relaxin family peptide and receptor gene expression and their associated brain neural circuits. In the zebrafish, the expression pattern of the different relaxin family members has specific features that are conserved in higher species, including a likely similar functional role for the ancestral RLN3-like gene. The use of different model organisms, particularly the zebrafish, to explore the diversification and conservation of relaxin family ligands and receptor systems, provides a relatively high-throughput platform to identify their specific conserved or differential neuromodulatory roles in higher species including human.

The Relaxin-3 Receptor, RXFP3, Is a Modulator of Aging-Related Disease

During the aging process our body becomes less well equipped to deal with cellular stress, resulting in an increase in unrepaired damage. This causes varying degrees of impaired functionality and an increased risk of mortality. One of the most effective anti-aging strategies involves interventions that combine simultaneous glucometabolic support with augmented DNA damage protection/repair. Thus, it seems prudent to develop therapeutic strategies that target this combinatorial approach. Studies have shown that the ADP-ribosylation factor (ARF) GTPase activating protein GIT2 (GIT2) acts as a keystone protein in the aging process. GIT2 can control both DNA repair and glucose metabolism. Through in vivo co-regulation analyses it was found that GIT2 forms a close coexpression-based relationship with the relaxin-3 receptor (RXFP3). Cellular RXFP3 expression is directly affected by DNA damage and oxidative stress. Overexpression or stimulation of this receptor, by its endogenous ligand relaxin 3 (RLN3), can regulate the DNA damage response and repair processes. Interestingly, RLN3 is an insulin-like peptide and has been shown to control multiple disease processes linked to aging mechanisms, e.g., anxiety, depression, memory dysfunction, appetite, and anti-apoptotic mechanisms. Here we discuss the molecular mechanisms underlying the various roles of RXFP3/RLN3 signaling in aging and age-related disorders.

Effects of chronic silencing of relaxin-3 production in nucleus incertus neurons on food intake, body weight, anxiety-like behaviour and limbic brain activity in female rats

Eating disorders are frequently triggered by stress and are more prevalent in women than men. First signs often appear during early adolescence, but the biological basis for the sex-specific differences is unknown. Central administration of native relaxin-3 (RLN3) peptide or chimeric/truncated analogues produces differential effects on food intake and HPA axis activity in adult male and female rats, but the precise role of endogenous RLN3 signalling in metabolic and neuroendocrine control is unclear. Therefore, we examined the effects of microRNA-induced depletion (knock-down) of RLN3 mRNA/(peptide) production in neurons of the brainstem nucleus incertus (NI) in female rats on a range of physiological, behavioural and neurochemical indices, including food intake, body weight, anxiety, plasma corticosterone, mRNA levels of key neuropeptides in the paraventricular nucleus of hypothalamus (PVN) and limbic neural activity patterns (reflected by c-fos mRNA). Validated depletion of RLN3 in NI neurons of female rats (n = 8) produced a small, sustained (~ 2%) decrease in body weight, an imbalance in food intake and an increase in anxiety-like behaviour in the large open field, but not in the elevated plus-maze or light/dark box. Furthermore, NI RLN3 depletion disrupted corticosterone regulation, increased oxytocin and arginine-vasopressin, but not corticotropin-releasing factor, mRNA, in PVN, and decreased basal levels of c-fos mRNA in parvocellular and magnocellular PVN, bed nucleus of stria terminalis and the lateral hypothalamic area, brain regions involved in stress and feeding. These findings support a role for NI RLN3 neurons in fine-tuning stress and neuroendocrine responses and food intake regulation in female rats.

Chronic activation of the relaxin-3 receptor on GABA neurons in rat ventral hippocampus promotes anxiety and social avoidance

Anxiety disorders are highly prevalent in modern society and better treatments are required. Key brain areas and signaling systems underlying anxiety include prefrontal cortex, hippocampus, and amygdala, and monoaminergic and peptidergic systems, respectively. Hindbrain GABAergic projection neurons that express the peptide, relaxin-3, broadly innervate the forebrain, particularly the septum and hippocampus, and relaxin-3 acts via a G_i/o -protein-coupled receptor known as the relaxin-family peptide 3 receptor (RXFP3). Thus, relaxin-3/RXFP3 signaling is implicated in modulation of arousal, motivation, mood, memory, and anxiety. Ventral hippocampus (vHip) is central to affective and cognitive processing and displays a high density of relaxin-3-positive nerve fibers and RXFP3 binding sites, but the identity of target neurons and associated effects on behavior are unknown. Therefore, in adult, male rats, we assessed the neurochemical nature of hippocampal RXFP3 mRNA-expressing neurons and anxiety-like and social behavior following chronic RXFP3 activation in vHip by viral vector expression of an RXFP3-selective agonist peptide, R3/I5. RXFP3 mRNA detected by fluorescent in situ hybridization was topographically distributed across the hippocampus in somatostatin- and parvalbumin-mRNA expressing GABA neurons. Chronic RXFP3 activation in vHip increased anxiety-like behavior in the light-dark box and elevated-plus maze, but not the large open-field test, and reduced social interaction with a conspecific stranger. Our data reveal disruptive effects of persistent RXFP3 signaling on hippocampal GABA networks important in anxiety; and identify a potential therapeutic target for anxiety disorders that warrants further investigation in relevant preclinical models.

The impact of apelin and relaxin plasma levels in masked hypertension and white coat hypertension

Masked hypertension (HTN) and white coat hypertension represent two reverse forms of clinical HTN with questionable prognostic significance. Recent evidence supports that low apelin and relaxin plasma levels contribute to vascular damage accelerating atherogenesis and predisposing to HTN and cardiovascular (CV) events. The aim of this study was to compare apelin and relaxin plasma levels between patients with masked hypertension (MH) and those with white coat HTN (WCH). Overall, 130 patients not receiving antihypertensive therapy were studied. All patients underwent 24-hour ambulatory BP monitoring (ABPM) and office BP measurements. Plasma apelin and relaxin levels were measured by ELISA method. According to BP recordings, 24 subjects had MH (group A) and 32 had WCH (group B). Apelin (200 ± 111 pg/mL vs 305 ± 127 pg/mL, P < 0.01) and relaxin (35.2 ± 6.7 pg/mL vs 46.8 ± 23.6 pg/mL, P < 0.01) plasma levels were significantly lower in patients with MH compared to those with WCH, respectively. In conclusion, our findings showed that patients with MH had significantly lower apelin and relaxin levels compared to those with WCH. This observation implies an additional prognostic role for adipokines supporting the concept that MH is closer to essential HTN whereas WCH is a more benign condition.

Altered relaxin family receptors RXFP1 and RXFP3 in the neocortex of depressed Alzheimer's disease patients

RATIONALE

The G-protein-coupled relaxin family receptors RXFP1 and RXFP3 are widely expressed in the cortex and are involved in stress responses and memory and emotional processing. However, the identification of these receptors in human cortex and their status in Alzheimer's disease (AD), which is characterized by both cognitive impairments and neuropsychiatric behaviours, have not been reported.

OBJECTIVES

In this study, we characterized RXFP receptors for immunoblotting and measured RXFP1 and RXFP3 immunoreactivities in the postmortem neocortex of AD patients longitudinally assessed for depressive symptoms.

METHODS

RXFP1 and RXFP3 antibodies were characterized by immunoblotting with lysates from transfected HEK cells and preadsorption with RXFP3 peptides. Also, postmortem neocortical tissues from behaviourally assessed AD and age-matched controls were processed for immunoblotting with RXFP1 and RXFP3 antibodies.

RESULTS

Compared to controls, putative RXFP1 immunoreactivity was reduced in parietal cortex of non-depressed AD patients but unchanged in depressed patients. Furthermore, putative RXFP3 immunoreactivity was increased only in depressed AD patients. RXFP1 levels in the parietal cortex also correlated with severity of depression symptoms. In contrast, RXFP1 and RXFP3 levels did not correlate with dementia severity or β-amyloid burden.

CONCLUSION

Alterations of RXFP1 and RXFP3 may be neurochemical markers of depression in AD, and relaxin family receptors warrant further preclinical investigations as possible therapeutic targets for neuropsychiatric symptoms in dementia.

Increased alcohol consumption in relaxin-3 deficient male mice

Relaxin-3 is a neuropeptide expressed in the brainstem, and predominantly localized in the gray matter of the midline dorsal pons termed the nucleus incertus. Relaxin-3-expressing neurons densely project axons rostrally to various forebrain regions including the septum, hippocampus, and lateral hypothalamus. Several relaxin-3 functions have been reported including food intake, stress responses, neuroendocrine function, emotion, and spatial memory. In addition, recently relaxin-3 and its receptor, RXFP3, were shown to regulate alcohol intake using an RXFP3 antagonist and RXFP3 gene knockout mice. In the present study, we investigated alcohol consumption in relaxin-3 knockout mice, and found that male but not female mice significantly drank more alcohol than wild-type mice in the two-bottle choice test. However, after chronic alcohol vapor exposure, wild-type and mutant mice did not show this difference in alcohol intake, although both genotypes exhibited increased alcohol consumption compared with non-alcohol-exposed control mice. There was no genotype difference in sucrose or quinine preference. These results suggest that the relaxin-3 neuronal system modestly affects alcohol preference and consumption.

Evidence of D2 receptor expression in the nucleus incertus of the rat

The nucleus incertus (NI), located in the caudal brainstem, mainly consists of GABAergic neurons with widespread projections across the brain. It is the chief source of relaxin-3 in the mammalian brain and densely expresses corticotropin-releasing factor type 1 (CRF1) receptors. Several other neurotransmitters, peptides and receptors are reportedly expressed in the NI. In the present investigation, we show the expression of dopamine type-2 (D2) receptors in the NI by reverse transcriptase-polymerase chain reaction (RT-PCR), western blotting (WB) and immunofluorescence (IF). RT-PCR did not show expression of D3 receptors. D2 receptor short isoform (D2S)-like, relaxin-3, CRF1/2 receptor and NeuN immunoreactivity were co-expressed in the cells of the NI. Behavioural effects of D2 receptor activation by intra-NI infusion of quinpirole (a D2/D3 agonist) were evaluated. Hypolocomotion was observed in home cage monitoring system (LABORAS) and novel environment-induced suppression of feeding behavioural paradigms. Thus the D2 receptors expressed in the NI are likely to play a role in locomotion. Based on its strong bidirectional connections to the median raphe and interpeduncular nuclei, the NI was predicted to play a role in modulating behavioural activity and the present results lend support to this hypothesis. This is the first evidence of expression of a catecholamine receptor, D2-like immunoreactivity, in the NI.

Expression pattern of zebrafish rxfp2 homologue genes during embryonic development

RXFP2 is one of the 4 receptors for relaxin insulin-like peptides, in particular it binds with high affinity the INSL3 peptide. INSL3/RXFP2 pair is essential for testicular descent during placental mammalian development. The evolutionary history of this ligand/receptor pair has received much attention, since its function in vertebrate species lacking testicular descent, such as the fishes, remains elusive. Herein, we analyzed the expression pattern of three rxfp2 homologue genes in zebrafish embryonic development. For all the three rxfp2 genes (rxfp2a, rxfp2b, and rxfp2-like) we showed the presence of maternally derived transcripts. Later in the development, rxfp2a is only expressed at larval stage, whereas rxfp2b is expressed in all the analyzed stage with highest level in the larvae. The rxfp2-like gene is expressed in all the analyzed stage with a transcript level that increased starting at early pharyngula stage. The spatial localization analysis of rxfp2-like gene showed that it is expressed in many cell clusters in the developing brain. In addition, other rxfp2-like-expressing cells were identified in the retina and oral epithelium. This analysis provides new insights to elucidate the evolution of rxfp2 genes in vertebrate lineage and lays the foundations to study their role in vertebrate embryonic development.

Ascending control of arousal and motivation: role of nucleus incertus and its peptide neuromodulators in behavioural responses to stress

Arousal is a process that involves the activation of ascending neural pathways originating in the rostral pons that project to the forebrain through the midbrain reticular formation to promote the activation of key cortical, thalamic, hypothalamic and limbic centres. Established modulators of arousal include the cholinergic, serotonergic, noradrenergic and dopaminergic networks originating in the pons and midbrain. Recent data indicate that a population of largely GABAergic projection neurones located in the nucleus incertus (NI) are also involved in arousal and motivational processes. The NI has prominent efferent connections with distinct hypothalamic, amygdalar and thalamic nuclei, in addition to dense projections to key brain regions associated with the generation and pacing of hippocampal activity. The NI receives strong inputs from the prefrontal cortex, lateral habenula and the interpeduncular and median raphe nuclei, suggesting it is highly integrated in circuits regulating higher cognitive behaviours (hippocampal theta rhythm) and emotion. Anatomical and functional studies have revealed that the NI is a rich source of multiple peptide neuromodulators, including relaxin-3, and may mediate extra-hypothalamic effects of the stress hormone corticotrophin-releasing factor, as well as other key modulators such as orexins and oxytocin. This review provides an overview of earlier studies and highlights more recent research that implicates this neural network in the integration of arousal and motivated behaviours and has begun to identify the associated mechanisms. Future research that should help to better clarify the connectivity and function of the NI in major experimental species and humans is also discussed.

Sex-specific effects of relaxin-3 on food intake and brain expression of corticotropin-releasing factor in rats

This study compared the effects of relaxin-3 (RLN3) on food intake, plasma corticosterone, and the expression of corticotropin-releasing factor (CRF) in male and female rats. RLN3 was injected into the lateral ventricle at 25, 200, and 800 pmol concentrations. RLN3 at 25 pmol increased food intake (grams) at 30 and 60 minutes after injection in female but not male rats. Female rats also showed higher increase in relative to body weight (BW) food intake (mg/g BW) for all RLN3 concentrations at 30 minutes and for 800 pmol of RLN3 at 60 minutes. Moreover, RLN3 at 800 pmol significantly increased 24-hour BW gain in female but not male rats. At 60 minutes after administration, 800 pmol of RLN3 produced a significant increase in plasma corticosterone and in the expression of CRF and c-fos mRNAs in the parvocellular paraventricular hypothalamic nucleus (PVN) in male but not female rats. The levels of c-fos mRNA in the magnocellular PVN were increased by RLN3 but did not differ between the sexes. Conversely, expression of CRF mRNA in the medial preoptic area was increased in female rats but was not sensitive to 800 pmol of RLN3. In the bed nucleus of the stria terminalis, 800 pmol of RLN3 significantly increased CRF mRNA expression in female but not male rats. Therefore, female rats showed more sensitivity and stronger food intake increase in response to RLN3. The differential effects of RLN3 on CRF expression in the PVN and bed nucleus of the stria terminalis may contribute to the sex-specific difference in the behavioral response.

Expression analysis of five zebrafish RXFP3 homologues reveals evolutionary conservation of gene expression pattern

Relaxin peptides exert different functions in reproduction and neuroendocrine processes via interaction with two evolutionarily unrelated groups of receptors: RXFP1 and RXFP2 on one hand, RXFP3 and RXFP4 on the other hand. Evolution of receptor genes after splitting of tetrapods and teleost lineage led to a different retention rate between mammals and fish, with the latter having more gene copies compared to the former. In order to improve our knowledge on the evolution of the relaxin ligands/receptors system and have insights on their function in early stages of life, in the present paper we analyzed the expression pattern of five zebrafish RXFP3 homologue genes during embryonic development. In our analysis, we show that only two of the five genes are expressed during embryogenesis and that their transcripts are present in all the developmental stages. Spatial localization analysis of these transcripts revealed that the gene expression is restricted in specific territories starting from early pharyngula stage. Both genes are expressed in the brain but in different cell clusters and in extra-neural territories, one gene in the interrenal gland and the other in the pancreas. These two genes share expression territories with the homologue mammalian counterpart, highlighting a general conservation of gene expression regulatory processes and their putative function during evolution that are established early in vertebrate embryogenesis.

Sex- and age-specific differences in relaxin family peptide receptor expression within the hippocampus and amygdala in rats

Relaxin is an essential pregnancy-related hormone with broad peripheral effects mediated by activation of relaxin-like family peptide 1 receptors (RXFP1). More recent studies suggest an additional role for relaxin as a neuropeptide, with RXFP1 receptors expressed in numerous brain regions. Neurons in an area of the brainstem known as the nucleus incertus (NI) produce relaxin 3 (RLN3), the most recently identified neuropeptide in the relaxin family. RLN3 has been shown to activate both RXFP1 and relaxin-like family peptide receptor 3 (RXFP3) receptor subtypes. Studies suggest wide-ranging neuromodulatory effects of both RXFP1 and RXFP3 activation, although to date the majority of studies have been conducted in young males. In the current study, we examined potential sex- and age-related changes in RLN3 gene expression in the NI as well as RXFP1 and RXFP3 gene expression in the dorsal hippocampus (HI), ventral hippocampus (vHI) and amygdala (AMYG) using young adult (9-12weeks) and middle-aged (9-12months) male and female rats. In addition, regional changes in RXFP1 and RXFP3 protein expression were examined in the CA1, CA2/CA3 and dentate gyrus (DG) as well as within basolateral (BLA), central (CeA), and medial (MeA) amygdaloid nuclei. In the NI, RLN3 showed an age-related decrease in males. In the HI, only the RXFP3 receptor showed an age-related change in gene expression, however, both receptor subtypes showed age-related changes in protein expression that were region specific. Additionally, while gene and protein expression of both receptors increased with age in AMYG, these effects were both region- and sex-specific. Finally, overall males displayed a greater number of cells that express the RXFP3 protein in all of the amygdaloid nuclei examined. Cognitive and emotional processes regulated by activity within the HI and AMYG are modulated by both sex and age. The vast majority of studies exploring the influence of sex on age-related changes in the HI and AMYG have focused on sex hormones, with few studies examining the role of neuropeptides. The current findings suggest that changes in relaxin family peptides may contribute to the significant sex differences observed in these brain regions as a function of aging.

Relaxin-3 receptor (Rxfp3) gene knockout mice display reduced running wheel activity: implications for role of relaxin-3/RXFP3 signalling in sustained arousal

Anatomical and pharmacological evidence suggests the neuropeptide, relaxin-3, is the preferred endogenous ligand for the relaxin family peptide-3 receptor (RXFP3) and suggests a number of putative stress- and arousal-related roles for RXFP3 signalling. However, in vitro and in vivo evidence demonstrates exogenous relaxin-3 can activate other relaxin peptide family receptors, and the role of relaxin-3/RXFP3 signalling in specific brain circuits and associated behaviours in mice is not well described. In this study, we characterised the behaviour of cohorts of male and female Rxfp3 gene knockout (KO) mice (C57/B6J(RXFP3TM1/DGen)), relative to wild-type (WT) littermates to determine if this receptor KO strain has a similar phenotype to its ligand KO equivalent. Rxfp3 KO mice displayed similar performance to WT littermates in several acute behavioural paradigms designed to gauge motor coordination (rotarod test), spatial memory (Y-maze), depressive-like behaviour (repeat forced-swim test) and sensorimotor gating (prepulse inhibition of acoustic startle). Notably however, male and female Rxfp3 KO mice displayed robust and consistent (dark phase) hypoactivity on voluntary home-cage running wheels (∼20-60% less activity/h), and a small but significant decrease in anxiety-like behavioural traits in the elevated plus maze and light/dark box paradigms. Importantly, this phenotype is near identical to that observed in two independent lines of relaxin-3 KO mice, suggesting these phenotypes are due to the elimination of ligand or receptor and RXFP3-linked signalling. Furthermore, this behavioural characterisation of Rxfp3 KO mice identifies them as a useful experimental model for studying RXFP3-linked signalling and assessing the selectivity and/or potential off-target actions of RXFP3 agonists and antagonists, which could lead to an improved understanding of dysfunctional arousal in mental health disorders, including depression, anxiety, insomnia and neurodegenerative diseases.

Regulation of expression of relaxin-3 and its receptor RXFP3 in the brain of diet-induced obese rats

An animal model closely related to human obesity is diet-induced obesity in Sprague-Dawley rats. These rats placed on a high-energy (HE) diet show wide distribution in body weight gain with a subset of animals developing diet-induced obesity (DIO) and the remaining animals showing a diet-resistant (DR) phenotype. Once obesity is established, DIO rats strongly defend their increased body weight against caloric restriction. There is evidence that neuropeptide relaxin-3 is involved in food intake regulation, but the levels of expression of relaxin-3 and its receptor have not been yet demonstrated in the DIO model. The present study investigated the brain expression of relaxin-3 and its cognate receptor RXFP3 in DIO and DR rats maintained on an HE diet since weaning. Expression of relaxin-3 and RXFP3 mRNAs was assessed by in situ hybridization in ad libitum, food-deprived (12 h) and refed (1 h) feeding states. The levels of expression of relaxin-3 in the medial portion of the nucleus incertus (NI) were higher in the DIO rats compared to the DR rats in the ad libitum-fed state. Food deprivation increased the levels of expression of relaxin-3 in the medial NI in DR but not DIO rats. The stronger expression of relaxin-3 in the ad libitum-fed state in the DIO rats was accompanied by low expression of the RXFP3 receptor in the paraventricular hypothalamic nucleus (PVN), supraoptic nucleus, central amygdala (CeA), NI, and nucleus of the solitary tract (NTS). Refeeding increased expression of RXFP3 in the paraventricular thalamic nucleus, parvocellular PVN, CeA, NI, and NTS in the DIO rats. These results provide evidence that DIO rats show a constitutive increase in relaxin-3 expression in the medial NI and that refeeding after food deprivation may enhance the orexigenic effects of relaxin-3 in DIO rats by rapid upregulation of the expression of RXFP3 in the specific brain regions involved in food intake regulation.

Relaxin-3/RXFP3 networks: an emerging target for the treatment of depression and other neuropsychiatric diseases?

Animal and clinical studies of gene-environment interactions have helped elucidate the mechanisms involved in the pathophysiology of several mental illnesses including anxiety, depression, and schizophrenia; and have led to the discovery of improved treatments. The study of neuropeptides and their receptors is a parallel frontier of neuropsychopharmacology research and has revealed the involvement of several peptide systems in mental illnesses and identified novel targets for their treatment. Relaxin-3 is a newly discovered neuropeptide that binds, and activates the G-protein coupled receptor, RXFP3. Existing anatomical and functional evidence suggests relaxin-3 is an arousal transmitter which is highly responsive to environmental stimuli, particularly neurogenic stressors, and in turn modulates behavioral responses to these stressors and alters key neural processes, including hippocampal theta rhythm and associated learning and memory. Here, we review published experimental data on relaxin-3/RXFP3 systems in rodents, and attempt to highlight aspects that are relevant and/or potentially translatable to the etiology and treatment of major depression and anxiety. Evidence pertinent to autism spectrum and metabolism/eating disorders, or related psychiatric conditions, is also discussed. We also nominate some key experimental studies required to better establish the therapeutic potential of this intriguing neuromodulatory signaling system, including an examination of the impact of RXFP3 agonists and antagonists on the overall activity of distinct or common neural substrates and circuitry that are identified as dysfunctional in these debilitating brain diseases.

Apelin and relaxin plasma levels in young healthy offspring of patients with essential hypertension

Epidemiologic studies have shown that healthy offspring of hypertensive patients exhibit many features of the metabolic syndrome, such as hyperinsulinemia, insulin resistance, and lipid disorders, while hypoapelinemia and hyporelaxinemia may contribute to vascular damage that accelerates atherogenesis. The aim of this study was to determine apelin and relaxin plasma levels in the healthy offspring of hypertensive patients and to compare the findings with those of healthy offspring of healthy parents, matched for age, sex, and body mass index (BMI). Forty-six (24 men and 22 women) healthy offspring of hypertensive patients, mean age 18±3 years and BMI 22.4±1.4 kg/m(2) (group A), and 50 healthy offspring of healthy parents (28 men and 22 women), mean age 18±3.2 years and BMI 22.6±1.7 kg/m(2) (group B), were studied. The apelin and relaxin plasma levels (enzyme-linked immunosorbent assay method) were determined in the study population. The two groups were matched for age, sex, and BMI. Plasma apelin levels (6±3 vs 105 pg/mL, P<.001) and relaxin plasma levels (20±7 vs 29±8 pg/mL, P<.001) were significantly lower in group A compared with group B, respectively. Our findings suggest that healthy offspring of healthy parents have significantly lower plasma apelin and relaxin levels. This group of individuals needs closer follow-up and further examination.

Synthesis of fluorescent analogs of relaxin family peptides and their preliminary in vitro and in vivo characterization

Relaxin, a heterodimeric polypeptide hormone, is a key regulator of collagen metabolism and multiple vascular control pathways in humans and rodents. Its actions are mediated via its cognate G-protein-coupled receptor, RXFP1 although it also "pharmacologically" activates RXFP2, the receptor for the related, insulin-like peptide 3 (INSL3), which has specific actions on reproduction and bone metabolism. Therefore, experimental tools to facilitate insights into the distinct biological actions of relaxin and INSL3 are required, particularly for studies of tissues containing both RXFP1 and RXFP2. Here, we chemically functionalized human (H2) relaxin, the RXFP1-selective relaxin analog H2:A(4-24)(F23A), and INSL3 to accommodate a fluorophore without marked reduction in binding or activation propensity. Chemical synthesis of the two chains for each peptide was followed by sequential regioselective formation of their three disulfide bonds. Click chemistry conjugation of Cy5.5 at the B-chain N-terminus, with conservation of the disulfide bonds, yielded analogs displaying appropriate selective binding affinity and ability to activate RXFP1 and/or RXFP2 in vitro. The in vivo biological activity of Cy5.5-H2 relaxin and Cy5.5-H2:A(4-24)(F23A) was confirmed in mice, as acute intracerebroventricular (icv) infusion of these peptides (but not Cy5.5-INSL3) stimulated water drinking, an established behavioral response elicited by central RXFP1 activation. The central distribution of Cy5.5-conjugated peptides was examined in mice killed 30 min after infusion, revealing higher fluorescence within brain tissue near-adjacent to the cerebral ventricle walls relative to deeper brain areas. Production of fluorophore-conjugated relaxin family peptides will facilitate future pharmacological studies to probe the function of H2 relaxin/RXFP1 and INSL3/RXFP2 signaling in vivo while tracking their distribution following central or peripheral administration.

Relaxin: a missing link in the pathomechanisms of Systemic Lupus Erythematosus?

Systemic Lupus Erythematosus (SLE) is an autoimmune rheumatic disease which predominantly affects women of reproductive age. Despite significant progress in recent years to elucidate many potential mechanisms involved in the generation of autoimmunity the factors behind the high incidence among women, the relapsing-remitting clinical course and pregnancy-related complications in SLE remain unclear. In this review, we hypothesize a potential role for uterine endometrium through its production of relaxin, a peptide hormone, as a "missing-link" to explain this female predominance, variable clinical course and obstetric complications operating in SLE.

Developmental expression pattern of two zebrafish rxfp3 paralogue genes

In mammals, the RXFP3 is the cognate receptor of the relaxin-3 peptide (RLN3). In teleosts, many different orthologue genes for RXFP3 are present. In particular, two paralogue genes, rxfp3-2a and rxfp3-2b, likely encode the receptors for the Rln3a peptide. The transcription of these two rxfp3 genes is differentially regulated early during zebrafish embryogenesis. Indeed, reverse transcription-polymerase chain reaction analyses show that the rxfp3-2b transcript is always present during embryo development, while the rxfp3-2a transcript is detectable only at larval stage. By in situ hybridization experiments on embryos and larvae, the rxfp3-2b transcript was revealed in the brain and in the retinal ganglion cell layer and thymus. Particularly in the brain, many territories are involved in the rxfp3-2b expression, among them the optic tectum, thalamus, preoptic area, different nerve nuclei, habenula and pineal gland. The RXFP3 spatiotemporal expression pattern appears to be conserved between Danio rerio and mammals, as also previously showed for the corresponding ligand, the RLN3. Interestingly, the brain areas expressing the rxfp3-2b receptor gene are involved in the visual system, emotional behaviors and circadian rhythm and could be functionally related to the neurotransmitter Rln3a-expressing territories.

Masked hypertension and atherogenesis: the impact of apelin and relaxin plasma levels

Recent evidence demonstrates that masked hypertension (MH) is a significant predictor of cardiovascular disease, while apelin and relaxin are two novel factors with a significant role in vascular regulation. Apelin is an adipokine that elicits endothelium-dependent vasorelaxation and reduces arterial blood pressure, while relaxin is a protein hormone that induces the production of nitric oxide and vascular endothelial growth factor and inhibits endothelin and angiotensin II. This study aimed to investigate whether apelin and relaxin plasma levels are affected in patients with MH and compare the findings with those of healthy normotensives. One hundred-thirty (60 men, 70 women) healthy patients with a mean age of 45±12 years who had clinic blood pressure <140/90 mmHg were studied. The whole study population underwent 24-hour ambulatory blood pressure monitoring (ABPM). According to the ABPM recordings, 24 individuals (8 men, 16 women) had MH and the remaining 106 patients (52 men, 54 women) had normal ABPM recordings. Apelin and relaxin plasma levels were determined in both groups (enzyme-linked immunosorbent assay method). The apelin (220±121 vs 315±147 pg/mL, P=.001) and relaxin (35.2±6.7 vs 56.8±13.6 pg/mL, P<.001) plasma levels were significantly lower in the masked hypertensive group compared with normotensive controls. Our findings suggest that patients with masked hypertension have significantly lower apelin and relaxin levels. This observation may have prognostic significance for future cardiovascular events in patients with MH and needs further investigation.

Sex differences in the effects of chronic stress and food restriction on body weight gain and brain expression of CRF and relaxin-3 in rats

This study investigated sex-specific effects of repeated stress and food restriction on food intake, body weight, corticosterone plasma levels and expression of corticotropin-releasing factor (CRF) in the hypothalamus and relaxin-3 in the nucleus incertus (NI). The CRF and relaxin-3 expression is affected by stress, and these neuropeptides produce opposite effects on feeding (anorexigenic and orexigenic, respectively), but sex-specific regulation of CRF and relaxin-3 by chronic stress is not fully understood. Male and female rats were fed ad libitum chow (AC) or ad libitum chow and intermittent palatable liquid Ensure without food restriction (ACE), or combined with repeated food restriction (60% chow, 2 days per week; RCE). Half of the rats were submitted to 1-h restraint stress once a week. In total, seven weekly cycles were applied. The body weight of the RCE stressed male rats significantly decreased, whereas the body weight of the RCE stressed female rats significantly increased compared with the respective control groups. The stressed female RCE rats considerably overate chow during recovery from stress and food restriction. The RCE female rats showed elevated plasma corticosterone levels and low expression of CRF mRNA in the paraventricular hypothalamic nucleus but not in the medial preoptic area. The NI expression of relaxin-3 mRNA was significantly higher in the stressed RCE female rats compared with other groups. An increase in the expression of orexigenic relaxin-3 and misbalanced hypothalamic-pituitary-adrenal axis activity may contribute to the overeating and increased body weight seen in chronically stressed and repeatedly food-restricted female rats.

Relaxin: new pathophysiological aspects and pharmacological perspectives for an old protein

Human relaxin-2 (hereafter simply defined as "relaxin") is a 6-kDa peptidic hormone best known for the physiological role played during pregnancy in the growth and differentiation of the reproductive tract and in the renal and systemic hemodynamic changes. This factor can also be involved in the pathophysiology of arterial hypertension and heart failure, in the molecular pathways of fibrosis and cancer, and in angiogenesis and bone remodeling. It belongs to the relaxin peptide family, whose members comprehensively exert numerous effects through interaction with different types of receptors, classified as relaxin family peptide (RXFP) receptors (RXFP1, RXFP2, RXFP3, RXFP4). Research looks toward the in-depth examination and complete understanding of relaxin in its various pleiotropic actions. The intent is to evaluate the likelihood of employing this substance for therapeutic purposes, for instance in diseases where a deficit could be part of the underlying pathophysiological mechanisms, also avoiding any adverse effect. Relaxin is already being considered as a promising drug, especially in acute heart failure. A careful study of the different RXFPs and their receptors and the comprehension of all biological activities of these hormones will probably provide new drugs with a potential wide range of therapeutic applications in the near future.

Silencing relaxin-3 in nucleus incertus of adult rodents: a viral vector-based approach to investigate neuropeptide function

Relaxin-3, the most recently identified member of the relaxin peptide family, is produced by GABAergic projection neurons in the nucleus incertus (NI), in the pontine periventricular gray. Previous studies suggest relaxin-3 is a modulator of stress responses, metabolism, arousal and behavioural activation. Knockout mice and peptide infusions in vivo have significantly contributed to understanding the function of this conserved neuropeptide. Yet, a definitive role remains elusive due to discrepancies between models and a propensity to investigate pharmacological effects over endogenous function. To investigate the endogenous function of relaxin-3, we generated a recombinant adeno-associated viral (rAAV) vector expressing microRNA against relaxin-3 and validated its use to knock down relaxin-3 in adult rats. Bilateral stereotaxic infusion of rAAV1/2 EmGFP miR499 into the NI resulted in significant reductions in relaxin-3 expression as demonstrated by ablation of relaxin-3-like immunoreactivity at 3, 6 and 9 weeks and by qRT-PCR at 12 weeks. Neuronal health was unaffected as transduced neurons in all groups retained expression of NeuN and stained for Nissl bodies. Importantly, qRT-PCR confirmed that relaxin-3 receptor expression levels were not altered to compensate for reduced relaxin-3. Behavioural experiments confirmed no detrimental effects on general health or well-being and therefore several behavioural modalities previously associated with relaxin-3 function were investigated. The validation of this viral vector-based model provides a valuable alternative to existing in vivo approaches and promotes a shift towards more physiologically relevant investigations of endogenous neuropeptide function.

Galanin-like peptide and the regulation of feeding behavior and energy metabolism

The hypothalamic neuropeptides modulate physiological activity via G protein-coupled receptors (GPCRs). Galanin-like peptide (GALP) is a 60 amino acid neuropeptide that was originally isolated from porcine hypothalamus using a binding assay for galanin receptors, which belong to the GPCR family. GALP is mainly produced in neurons in the hypothalamic arcuate nucleus. GALP-containing neurons form neuronal networks with several other types of peptide-containing neurons and then regulate feeding behavior and energy metabolism. In rats, the central injection of GALP produces a dichotomous action that involves transient hyperphasia followed by hypophasia and a reduction in body weight, whereas, in mice, it has only one action that reduces both food intake and body weight. In the present minireview, we discuss current evidence regarding the function of GALP, particularly in relation to feeding and energy metabolism. We also examine the effects of GALP activity on food intake, body weight and locomotor activity after intranasal infusion, a clinically viable mode of delivery. We conclude that GALP may be of therapeutic value for obesity and life-style-related diseases in the near future.