Relaxin-like factor gene is highly expressed in the bovine ovary of the cycle and pregnancy: sequence and messenger ribonucleic acid analysis

The diverse influences of relaxin-like peptide family on tumor progression: Potential opportunities and emerging challenges

Relaxin-like peptide family exhibit differential expression patterns in various types of cancers and play a role in cancer development. This family participates in tumorigenic processes encompassing proliferation, migration, invasion, tumor microenvironment, immune microenvironment, and anti-cancer resistance, ultimately influencing patient prognosis. In this review, we explore the mechanisms underlying the interaction between the RLN-like peptide family and tumors and provide an overview of therapeutic approaches utilizing this interaction.

Age-related changes in circulating INSL3 concentrations and their associations with ovarian conditions in Japanese Black beef cattle

Information on circulating levels of insulin-like peptide 3 (INSL3) in female domesticated animals is limited, as their concentrations are significantly lower than in males. The objectives of the present study were to 1) develop a sandwich time-resolved fluorescence immunoassay (TRFIA) with higher detectability to measure blood INSL3 concentrations in female cattle, 2) determine INSL3 concentrations in female cattle among age groups and reproductive conditions, and 3) explore associations between INSL3 levels and ultrasonographic ovarian measurements. Blood was collected repeatedly from Japanese Black beef female calves (n = 12; 0－8 mo), heifers (n = 10; 10－26 mo), and cows (n = 20; 27－200 mo). Blood was taken from the cows (n = 13) at follicular, post-ovulatory, and luteal phases, and from cows with follicular cysts (n = 12). Ultrasonography of ovaries was conducted in the calves (n = 12) and the cows without ovarian diseases (n = 9). The ovarian area, as well as the number and diameters of antral follicles ≥ 2 mm, were determined in each ovary. The proposed method detected a difference in plasma INSL3 between calves (0.01 ng/mL) and heifers (0.18 ng/mL). However, the conventional assay showed similar levels for calves and heifers (1.82 vs 2.07 ng/mL). Plasma INSL3 and testosterone concentrations increased from calves to heifers (P < 0.0001), but only INSL3 rose from heifers to cows (P < 0.0001). INSL3 and testosterone concentrations did not change across the estrus cycle in cows, and the levels of both hormones in follicular cystic cows did not differ from those in the follicular phase. Ovarian area, maximal and average follicular diameters, and total volume of all follicles per animal were higher in cows than calves (P < 0.001). Plasma INSL3 concentrations correlated positively with the total volumes of all follicles in calves (P < 0.05) and cows (P < 0.05), whereas testosterone concentrations did not correlate with ovarian follicular measurements. In conclusion, plasma INSL3 concentrations measured by the proposed sandwich TRFIA showed a clear increase from female calves to cows in beef cattle. These results suggest that circulating levels of INSL3, but not of testosterone, are associated with the total volume of all antral follicles in both ovaries per animal in female cattle.

Insulin-like peptide 3 in domestic animals with normal and abnormal reproductive functions, in comparison to rodents and humans

Background

Insulin-like peptide 3 (INSL3) is a circulating hormone secreted from only testis and ovaries in mammals. Findings on INSL3 have been gathered from subjects with normal and abnormal reproductive statuses, especially rodents and humans. However, little to no review articles focusing on INSL3 in domestic animals exist.

Methods

The author reviewed the past and recent literature regarding the structure, expression, roles of INSL3 in the reproductive organs, and its circulation under normal and aberrant reproductive conditions in domestic animals in comparison with rodents and humans.

Main findings

As with humans and rodents, blood INSL3 concentrations rise around puberty in normal male domestic animals and are associated with testicular size. INSL3 levels are acutely upregulated by luteinizing hormone (LH), and the increase is smaller than that of testosterone in male ruminants, whereas the acute regulation of INSL3 by LH does not occur in human men. Dogs with cryptorchidism and bulls with abnormal semen have lowered INSL3 levels.

Conclusion

The findings regarding INSL3 secretions in male domestic animals with normal and aberrant reproductive functions illustrate similar or dissimilar points to humans and rodents. Data on blood INSL3 levels in normal and abnormal female domestic species are still limited and require further investigation.

Effects of colostrum management on transfer of passive immunity and the potential role of colostral bioactive components on neonatal calf development and metabolism

Neonatal dairy and beef calves are required to ingest adequate volumes of high-quality colostrum during their first hours of life to acquire transfer of passive immunity (TPI). As such, immunoglobulin G (IgG) has largely been the focus of colostrum research over recent decades. Yet, little is known about the additional bioactive compounds in colostrum that potentially influence newborn calf development and metabolism. The purpose of this narrative review is to synthesize research regarding the effects of colostrum management practices on TPI, as well as to address the potential role of additional colostral bioactive molecules, including oligosaccharides, fatty acids, insulin, and insulin-like growth factor-I, in promoting calf development and metabolism. Due to the importance of IgG in ensuring calf immunity and health, we review past research describing the process of colostrogenesis and dam factors influencing the concentrations of IgG in an effort to maximize TPI. We also address the transfer of additional bioactive compounds in colostrum and prepartum management and dam factors that influence their concentrations. Finally, we highlight key areas of future research for the scientific community to pursue to ultimately improve the health and welfare of neonatal dairy calves.

Evidence for existence of insulin-like factor 3 (INSL3) hormone-receptor system in the ovarian corpus luteum and extra-ovarian reproductive organs during pregnancy in goats

Insulin-like factor 3 (INSL3), initially described as a male hormone, is expressed in female reproductive organs during the estrous cycle and pregnancy but its function has not yet been established. This study explores the function of INSL3 in pregnant Saanen goats by characterizing the expression dynamics of INSL3 and its receptor, relaxin family peptide receptor 2 (RXFP2) and by demonstrating specific INSL3 binding in reproductive organs, using molecular and immunological approaches and ligand-receptor interaction assays. We demonstrate that the corpus luteum (CL) acts as both a source and target of INSL3 in pregnant goats, while extra-ovarian reproductive organs serve as additional INSL3 targets. The expression of INSL3 and RXFP2 in the CL reached maximum levels in middle pregnancy, followed by a decrease in late pregnancy; in contrast, RXFP2 expression levels in extra-ovarian reproductive organs were higher in the mammary glands but lower in the uterus, cervix and placenta and did not significantly change during pregnancy. The functional RXFP2 enabling INSL3 to bind was identified as an ~ 85 kDa protein in both the CL and mammary glands and localized in large and small luteal cells in the CL and in tubuloalveolar and ductal epithelial cells in the mammary glands. Additionally, INSL3 also bound to multiple cell types expressing RXFP2 in the uterus, cervix and placenta in a hormone-specific and saturable manner. These results provide evidence that an active intra- and extra-ovarian INSL3 hormone-receptor system operates during pregnancy in goats.

Physiology and evolution of the INSL3/RXFP2 hormone/receptor system in higher vertebrates

Although the insulin-like peptide hormone INSL3 and its cognate receptor RXFP2 (relaxin-family peptide receptor 2) have existed throughout chordate evolution, their physiological diversification appears to be linked closely with mammalian emergence and radiation. In contrast, they have been lost in birds and reptiles. Both hormone and receptor are expressed from autosomal genes which have maintained their synteny across vertebrate evolution. Whereas the INSL3 gene comprises only two exons closely linked to the JAK3 gene, RXFP2 is normally encoded by 18 exons. Both genes, however, are subject to alternative splicing to yield a variety of possibly inactive or antagonistic molecules. In mammals, the INSL3-RXFP2 dyad has maintained a probably primitive association with gametogenesis, seen also in fish, whereby INSL3 promotes the survival, growth and differentiation of male germ cells in the testis and follicle development in the ovary. In addition, however, the INSL3/RXFP2 system has adopted a typical 'neohormone' profile, essential for the promotion of internal fertilisation and viviparity; fetal INSL3 is essential for the first phase of testicular descent into a scrotum, and also appears to be associated with male phenotype, in particular horn and skeletal growth. Circulating INSL3 is produced exclusively by the mature testicular Leydig cells in male mammals and acts as a potent biomarker for testis development during fetal and pubertal development as well as in ageing. As such it can be used also to monitor seasonally breeding animals as well as to investigate environmental or lifestyle conditions affecting development. Nevertheless, most information about INSL3 and RXFP2 comes from a very limited selection of species; it will be especially useful to gain further information from a more diverse range of animals, especially those whose evolution has led them to express unusual reproductive phenotypes.

Putative adverse outcome pathways for female reproductive disorders to improve testing and regulation of chemicals

Modern living challenges female reproductive health. We are witnessing a rise in reproductive disorders and drop in birth rates across the world. The reasons for these manifestations are multifaceted and most likely include continuous exposure to an ever-increasing number of chemicals. The cause–effect relationships between chemical exposure and female reproductive disorders, however, have proven problematic to determine. This has made it difficult to assess the risks chemical exposures pose to a woman’s reproductive development and function. To address this challenge, this review uses the adverse outcome pathway (AOP) concept to summarize current knowledge about how chemical exposure can affect female reproductive health. We have a special focus on effects on the ovaries, since they are essential for lifelong reproductive health in women, being the source of both oocytes and several reproductive hormones, including sex steroids. The AOP framework is widely accepted as a new tool for toxicological safety assessment that enables better use of mechanistic knowledge for regulatory purposes. AOPs equip assessors and regulators with a pragmatic network of linear cause–effect relationships, enabling the use of a wider range of test method data in chemical risk assessment and regulation. Based on current knowledge, we propose ten putative AOPs relevant for female reproductive disorders that can be further elaborated and potentially be included in the AOPwiki. This effort is an important step towards better safeguarding the reproductive health of all girls and women.

Prepubertal nutrition alters Leydig cell functional capacity and timing of puberty

Leydig cell functional capacity reflects the numbers and differentiation status of the steroidogenic Leydig cells in the testes and becomes more or less fixed in early adulthood with the final establishment of the hypothalamo-pituitary-gonadal (HPG) axis after puberty. Factors influencing Leydig cell functional capacity and its role in puberty are poorly understood. Using a bovine model of dairy bulls fed four different nutritional regimes from 1 month to 12 months, and applying circulating Insulin-like peptide 3 (INSL3) as an accurate biomarker of Leydig cell functional capacity, showed that a high plane of nutrition in the first 6 months of life, but not later, significantly increased INSL3 in young adulthood. Moreover, INSL3 concentration at 4 months indicated a marked differential in early feeding regime and correlated well (negatively) with the timing of puberty, as reflected by the age in days for the first production of an ejaculate with >50 million sperm and >10% forward motility, as well as with testis size at 18 months. Reversing the diet at 6 months was unable to rectify the trend in either parameter, unlike for other parameters such as testosterone, body weight, and scrotal circumference. This study has shown that early prepubertal nutrition is a key factor in the development of Leydig cell functional capacity in early adulthood and appears to be a key driver in the dynamic progression of puberty.

Intergenerational response of steroidogenesis-related genes to maternal malnutrition

We sought to examine whether rat maternal food restriction (MFR) affects the expression of steroidogenesis-related genes Cyp19, Cyp17a1, Insl3 and Gdf-9 in the ovaries of offspring from the first (FRG1) and second (FRG2) generations at pre-pubertal age (week 4) and during adulthood (week 8). At week 4, MFR significantly increased the expression of RNAs for all analyzed genes in both FRG1 and FRG2 females, which may indicate that MFR affects the onset of the reproductive lifespan, by inducing early pubertal onset. At week 8, the Cyp19 gene was still upregulated in MRF-subjected animals (Cyp19: P=0.0049 and P=0.0508 in FRG1 and FRG2, respectively), but MFR induced a significant decrease in Cyp17 and Gdf-9 gene expression in the offspring of both FRG1 and FRG2 females when compared with the controls (Cyp17: P=0.0018 and P=0.0016, respectively; Gdf-9: P=0.0047 and P=0.0023, respectively). This suggests that females at week 8, which should normally be in their optimal reproductive capacity, experience premature ovarian aging. At week 4, the activation of Cyp19 and Cyp17 was higher in the FRG1 ovaries than in the FRG2 ovaries, whereas the extent of Insl3 and Gdf-9 activation was lower in the FRG1 ovaries. This may indicate that FRG2 females were more vulnerable to MFR than their mothers (FRG1) and grandmothers, which is consistent with the 'predictive adaptive response' hypothesis. Our findings reveal that MFR may induce intergenerational ovarian changes as an adaptive response to ensure reproductive success before death.

Theca Cell INSL3 and Steroids Together Orchestrate the Growing Bovine Antral Follicle

Insulin-like peptide 3 (INSL3) and its specific receptor RXFP2 are both expressed by theca interna cells of the growing antral follicle where they form an essential regulatory element in the production of the steroid precursor androstenedione. Using primary cultures of bovine theca cells from the mid follicular phase together with steroid agonists and antagonists we have examined how ovarian steroids modulate INSL3 expression. Transcript analysis shows that these cells express estrogen receptors α and β, androgen and progesterone receptors, besides the orphan nuclear receptors SF1 and nur77. Whereas, exogenous androgens have little or no effect, the androgen antagonist bicalutamide stimulates INSL3 production. In contrast, estrogen receptor agonists, as also progesterone, are stimulatory. Importantly, estrogen receptor signaling is convergent with the protein kinase A signaling pathway activated by LH, such that the estrogen receptor antagonist can inhibit the mild stimulatory effect of LH, and vice versa the PKA antagonist H89 blocks stimulation by estradiol. A significant finding is that the major steroid metabolite androstenedione appears to act predominantly as an estrogen and not an androgen in this system. Transfection of INSL3 gene promoter-reporter constructs together with various steroid receptor expression plasmids supports these findings and shows that steroid action uses non-classical pathways not requiring canonical steroid-responsive elements in the proximal promoter region. Together, the results indicate that increasing estrogens in the follicular phase stimulate a feedforward loop driving INSL3 signaling and thereby promoting steroidogenesis in the growing antral follicle until the LH surge which effectively switches off INSL3 expression.

Relaxin-Family Peptide Receptors 1 and 2 Are Fully Functional in the Bovine

In most mammals the peptide hormone relaxin is a key physiological component regulating early pregnancy and birth. However, synteny analysis shows that the gene encoding ovarian relaxin-2 is deleted in cows and sheep. While, these ruminants appear to exhibit a relaxin-like physiology, as in other mammals, until now a molecular understanding of this has been lacking. Cloning and expression analysis of the cognate bovine receptor for relaxin, RXFP1, as well as of the structurally related receptor, RXFP2, in female tissues, shows that these are expressed in a similar way to other mammals. RXFP1 transcripts are found in ovarian theca cells, endometrium, and myometrium, whereas RXFP2 transcripts are expressed in ovarian theca cells, oocytes, as well as in myometrium. Transfection of receptor-expressing gene constructs into HEK293 cells indicates that bovine RXFP1 has a greater EC50 at 10–50 nM for porcine or human relaxin, compared to human RXFP1. For bovine RXFP2, in contrast, the EC50 is <1 nM for its cognate ligand, bovine INSL3, but also 10–30 nM for porcine or human relaxin. Functional analysis shows that bovine myometrial cells are able to respond to exogenous relaxin and INSL3 with a significant increase in cAMP. Although expressing mRNA for both RXFP1 and RXFP2, bovine follicular theca cells only respond to INSL3 with a dose-dependent increase in cAMP. Altogether the results suggest that the cow is able to compensate for the missing hormone, and moreover imply that relaxin analogs could offer an important therapeutic option in treating female ruminant infertility.

Relaxin-like peptides in male reproduction - a human perspective

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

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Functional insulin-like factor 3 (INSL3) hormone-receptor system in the testes and spermatozoa of domestic ruminants and its potential as a predictor of sire fertility

Insulin-like factor 3 (INSL3) is essential for fetal testis descent, and has been implicated in the testicular and sperm functions in adult males; however, similar functions in domestic ruminants remain largely unknown. This study investigated the functional INSL3 hormone-receptor system in adult ruminant testes and spermatozoa, and explored its potential to diagnose the fertility of sires. Testes and spermatozoa were obtained from fertile bulls, rams and he-goats, whereas subfertile testes and spermatozoa were obtained only from bulls. As expected, INSL3 was visualized in Leydig cells, while we clearly demonstrated that the functional receptor, relaxin family peptide receptor 2 (RXFP2), enabling INSL3 to bind was identified in testicular germ cells and in the sperm equatorial segment of bulls, rams and he-goats. In comparison to fertile bulls, the percentage of INSL3- and RXFP2-expressing cells and their expression levels per cell were significantly reduced in the testes of subfertile bulls. In addition, the population of INSL3-binding spermatozoa was also significantly reduced in the semen of subfertile bulls. These results provide evidence for a functional INSL3 hormone-receptor system operating in ruminant testes and spermatozoa, and its potential to predict subfertility in sires.

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

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

Relaxin-related gene expression differs between anadromous and stream-resident stickleback (Gasterosteus aculeatus) following seawater transfer

Relaxin (RLN) is a hormone that was originally identified as a regulator of pregnancy and reproduction. However, recent mammalian studies have demonstrated that relaxins also have potent osmoregulatory actions. In mammals, six relaxin family peptides have been identified: RLN1/2, RLN3, insulin-like peptide (INSL) 3, INSL4, INSL5, and INSL6. Previous genome database searches have revealed that teleosts also possess multiple relaxin family genes. However, the functions of these relaxin family peptides in teleosts remain unclear. In order to gain insight into the osmoregulatory functions of teleost relaxins, we studied the relaxin family peptides in euryhaline three-spined sticklebacks (Gasterosteus aculeatus), which have diversified into a variety of ecotypes. Rln3a, rln3b, and rln transcripts were abundant in the stickleback brain, whereas insl5b transcript levels were highest in the intestine among tissues. Seawater challenge experiments showed that transcript levels of rln3a, rln3b, and rln in the brain changed significantly after seawater transfer. Particularly, rln3b showed different patterns of temporal changes between anadromous and stream-resident morphs. The transcript levels of relaxin family peptide receptors, rxfp1, rxfp2b, rxfp3-2a, and rxfp3-2b, did not exhibit substantial changes in the brain, although these were constantly higher in the anadromous morph than the stream-resident morph. These results suggest that stickleback relaxin systems are differentially regulated by salinity signals, at least at the transcriptional level, and anadromous and stream-resident morphs differ in relaxin signaling pathways. The differences in the expression of relaxin-related genes between these two morphs provide a foundation for further exploration of the osmoregulatory function of relaxins in teleosts.

The INSL3 gene is a direct target for the orphan nuclear receptor, COUP-TFII, in Leydig cells

Insulin-like 3 (INSL3), a hormone produced by Leydig cells, regulates testicular descent during foetal life and bone metabolism in adults. Despite its importance, little is known about the molecular mechanisms controlling INSL3 expression. Reduced Insl3 mRNA levels were reported in the testis of mice deficient for chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII), an orphan nuclear receptor known to play critical roles in cell differentiation and lineage determination in several tissues. Although COUP-TFII-deficient mice had Leydig cell dysfunction and impaired fertility, it remained unknown whether Insl3 expression was directly regulated by COUP-TFII. In this study, we observed a significant decrease in Insl3 mRNA levels in MA-10 Leydig cells depleted of COUP-TFII. Furthermore, a -1087 bp mouse Insl3 promoter was activated fourfold by COUP-TFII in MA-10 Leydig cells. Using 5' progressive deletions, the COUP-TFII-responsive element was located between -186 and -79 bp, a region containing previously uncharacterised direct repeat 0-like (DR0-like) and DR3 elements. The recruitment and direct binding of COUP-TFII to the DR0-like element were confirmed by chromatin immunoprecipitation and DNA precipitation assay respectively. Mutation of the DR0-like element, which prevented COUP-TFII binding, significantly decreased COUP-TFII-mediated activation of the -1087 bp Insl3 reporter in CV-1 fibroblast cells but not in MA-10 Leydig cells. Finally, we found that COUP-TFII cooperates with the nuclear receptor steroidogenic factor 1 (SF1) to further enhance Insl3 promoter activity. Our results identify Insl3 as a target for COUP-TFII in Leydig cells and revealed that COUP-TFII acts through protein-protein interactions with other DNA-bound transcription factors, including SF1, to activate Insl3 transcription in these cells.

Regulation of the reproductive cycle and early pregnancy by relaxin family peptides

The relaxin family of peptide hormones are structurally closely related to one another sharing a heterodimeric A-B structure, like that of insulin. They may also be active as unprocessed B-C-A pro-forms. Relaxin has been shown to pay a key role within the ovary, being involved in follicle growth, and ovulation. Relaxin is produced in large amounts also by the corpus luteum where it acts as an endocrine hormone positively affecting implantation, placentation and vascularization during the all-important first trimester phase of pregnancy establishment. Relaxin exerts its functions via the receptor RXFP1. Insulin-like peptide 3 (INSL3) in contrast acts through the related receptor RXFP2, and plays an essential role in the production of androgens within growing antral follicles. INSL3 is also produced in large amounts by the male fetus shortly after sex determination, where it controls the first transabdominal phase of testicular descent. However, this fetal INSL3 is also able to influence placental and maternal physiology, indicating associations with later preeclampsia and/or fetal growth restriction. Other members of this relaxin-like family of peptides, such as INSL4, INSL5 and INSL6 are less well studied, though all suggest modulatory roles in ovarian and/or placental function.

Insulin-like 3-induced rat preantral follicular growth is mediated by growth differentiation factor 9

The communication of somatic cells and oocytes by intrafollicular paracrine factors is essential for follicular growth in the ovary. Insulin-like 3 (INSL3) is a theca cell-secreted paracrine factor. Androgens and growth differentiation factor 9 (GDF9), an oocyte-derived growth factor, are essential for follicular development. Using a rat preantral follicle culture model, we examined in the present study the influence of INSL3 on preantral follicular growth and the molecular mechanisms involved. We have observed that the receptor for INSL3, relaxin/insulin-like family peptide receptor 2 (RXFP2), was exclusively expressed in oocytes. Recombinant INSL3 stimulated Gdf9 expression, preantral follicular growth, and testosterone synthesis in vitro. Inhibition of the cAMP/protein kinase A signaling pathway (with cAMP antagonist, 8-bromoadenosine 3',5'-cyclic monophosphorothioate, Rp-isomer) attenuated INSL3-induced Gdf9 expression and preantral follicular growth. Moreover, knocking down Gdf9 expression (with small interfering RNA) or inhibiting GDF9 signaling (with SB431542, an activin receptor-like kinase receptor 5 inhibitor, or specific inhibitor of mothers against decapentaplegic homolog 3) or androgen action (with flutamide, an androgen receptor antagonist) suppressed INSL3-induced preantral follicular growth. In addition, LH and DHT regulated the expression of Insl3 mRNA in preantral follicles. These observations suggest that INSL3 is a key theca cell-derived growth factor for preantral follicle and that its action is mediated by GDF9.

Ovarian expression of insulin-like peptide 3 (INSL3) and its receptor (RXFP2) during development of bovine antral follicles and corpora lutea and measurement of circulating INSL3 levels during synchronized estrous cycles

Insulin-like peptide 3 (INSL3), a major product of testicular Leydig cells, is also expressed by the ovary, but its functional role remains poorly understood. Here, we quantified expression of INSL3 and its receptor RXFP2 in theca interna cell (TIC) and granulosa cell compartments of developing bovine antral follicles and in corpora lutea (CL). INSL3 and RXFP2 mRNA levels were much higher in TIC than granulosa cell and increased progressively during follicle maturation with INSL3 peaking in large (11-18 mm) estrogen-active follicles and RXFP2 peaking in 9- to 10-mm follicles before declining in larger (11-18 mm) follicles. Expression of both INSL3 and RXFP2 in CL was much lower than in TIC. In situ hybridization and immunohistochemistry confirmed abundant expression of INSL3 mRNA and protein in TIC. These observations indicate follicular TIC rather than CL as the primary site of both INSL3 production and action, implying a predominantly autocrine/paracrine role in TIC. To corroborate the above findings, we showed that in vitro exposure of TIC to a luteinizing concentration of LH greatly attenuated expression of both INSL3 and its receptor while increasing progesterone secretion and expression of STAR and CYP11A1. Moreover, in vivo, a significant cyclic variation in plasma INSL3 was observed during synchronized estrous cycles. INSL3 and estradiol-17β followed a similar pattern, both increasing after luteolysis, before falling sharply after the LH surge. Thus, theca-derived INSL3, likely from the dominant preovulatory follicle, is detectable in peripheral blood of cattle, and expression is down-regulated during luteinization induced by the preovulatory LH surge. Collectively, these findings underscore the likely role of INSL3 as an important intrafollicular modulator of TIC function/steroidogenesis, while raising doubts about its potential contribution to CL function.

Relaxin family peptides and their receptors

There are seven relaxin family peptides that are all structurally related to insulin. Relaxin has many roles in female and male reproduction, as a neuropeptide in the central nervous system, as a vasodilator and cardiac stimulant in the cardiovascular system, and as an antifibrotic agent. Insulin-like peptide-3 (INSL3) has clearly defined specialist roles in male and female reproduction, relaxin-3 is primarily a neuropeptide involved in stress and metabolic control, and INSL5 is widely distributed particularly in the gastrointestinal tract. Although they are structurally related to insulin, the relaxin family peptides produce their physiological effects by activating a group of four G protein-coupled receptors (GPCRs), relaxin family peptide receptors 1-4 (RXFP1-4). Relaxin and INSL3 are the cognate ligands for RXFP1 and RXFP2, respectively, that are leucine-rich repeat containing GPCRs. RXFP1 activates a wide spectrum of signaling pathways to generate second messengers that include cAMP and nitric oxide, whereas RXFP2 activates a subset of these pathways. Relaxin-3 and INSL5 are the cognate ligands for RXFP3 and RXFP4 that are closely related to small peptide receptors that when activated inhibit cAMP production and activate MAP kinases. Although there are still many unanswered questions regarding the mode of action of relaxin family peptides, it is clear that they have important physiological roles that could be exploited for therapeutic benefit.

Possible role of insulin-like factor 3 in the bovine corpus luteum

Insulin-like factor 3 (INSL3) is a local regulator in mammalian gonads, but little is known of its function in bovine corpus luteum (CL). Here, we show that RXFP2 protein, the receptor of INSL3, was expressed throughout the estrous cycle and significantly high at the early luteal stage compared to the regressed luteal stage. INSL3 stimulated progesterone secretion, but not prostaglandin F2α and viability in cultured luteal cells. Together, these results suggest that INSL3 plays a luteotropic role as a local regulator in the bovine CL.

Insulin-like peptide 3 stimulates testosterone secretion in mouse Leydig cells via cAMP pathway

Testicular Leydig cells secrete insulin-like peptide 3 (INSL3) and express its receptor, RXFP2. However, the effects of INSL3 on endocrine function of Leydig cells are unknown. The present study examines the effects of INSL3 on mouse Leydig cells taking testosterone and cAMP secretions as endpoints. Leydig cells were isolated from testicular interstitial cells obtained from 8-week-old male mice. Cells were then plated in the presence or absence of mouse, human, canine or bovine INSL3 (0-100 ng/ml) for 18 h in multiwell-plates (96 wells) in different cell densities (2500, 5000, 10,000 or 20,000 cells per well). The effects of bovine INSL3 (100 ng/ml) on testosterone secretion by Leydig cells were examined in the presence or absence of, an adenylate cyclase inhibitor, SQ 22536 (1μM) or INSL3 antagonist (bovine and human; 100 ng/ml). Testosterone and cAMP in spent medium were measured by enzyme immunoassay. All INSL3 species tested significantly stimulated the testosterone secretion in Leydig cells, and the maximum stimulation was observed with 100 ng/ml bovine INSL3 at the lowest Leydig cell density (2500 cells per well). Moreover, bovine INSL3 (100 ng/ml) significantly stimulated the cAMP production from Leydig cells maximally at 1h, and remained significantly elevated even at 18 h. SQ 22536 and INSL3 antagonists (bovine and human) significantly reduced INSL3-stimulated testosterone secretion from Leydig cells. Taken together, stimulatory effects of INSL3 on testosterone secretion in Leydig cells are exerted via the activation of cAMP, suggesting a new autocrine function of INSL3 in males.

The structural determinants of insulin-like Peptide 3 activity

Insulin-like peptide 3 (INSL3) is a hormone and/or paracrine factor which is a member of the relaxin peptide family. It has key roles as a fertility regulator in both males and females. The receptor for INSL3 is the leucine rich repeat (LRR) containing G-protein coupled receptor 8 (LGR8) which is now known as relaxin family peptide receptor 2 (RXFP2). Receptor activation by INSL3 involves binding to the LRRs in the large ectodomain of RXFP2 by residues within the B-chain of INSL3 as well as an interaction with the transmembrane exoloops of the receptor. Although the binding to the LRRs is well characterized the features of the peptide and receptor involved in the exoloop interaction are currently unknown. This study was designed to determine the key INSL3 determinants for RXFP2 activation. A chimeric peptide approach was first utilized to demonstrate that the A-chain is critical for receptor activation. Replacement of the INSL3 A-chain with that from the related peptides INSL5 and INSL6 resulted in complete loss of activity despite only minor changes in binding affinity. Subsequent replacement of specific A-chain residues with those from the INSL5 peptide highlighted that the N-terminus of the A-chain of INSL3 is critical for its activity. Remarkably, replacement of the entire N-terminus with four or five alanine residues resulted in peptides with near native activity suggesting that specific residues are not necessary for activity. Additionally removal of two amino acids at the C-terminus of the A-chain and mutation of Lys-8 in the B-chain also resulted in minor decreases in peptide activity. Therefore we have demonstrated that the activity of the INSL3 peptide is driven predominantly by residues 5-9 in the A-chain, with minor additional contributions from the two C-terminal A-chain residues and Lys-8 in the B-chain. Using this new knowledge, we were able to produce a truncated INSL3 peptide structure which retained native activity, despite having 14 fewer residues than the parent peptide.

Uterine cysts in female mice deficient for caveolin-1 and insulin-like 3 receptor RXFP2

Gene mutations of insulin-like 3 (INSL3) peptide or its G protein-coupled receptor RXFP2 (relaxin family peptide receptor 2) lead to cryptorchidism. The role of INSL3 in adult females is less known, although INSL3 expression has been described in female reproductive organs. Caveolin-1 (CAV1), the main component of caveoli cell membrane invaginations, has been shown to play an important role in epithelial organization and stromal-epithelial interactions. We created a null allele of Cav1 mice by deleting its second exon through embryonic stem cell targeting. Immunohistochemical analysis demonstrated that CAV1 expression was primarily localized to endothelial blood vessel cells and the myometrium uterus, whereas the strongest expression of Rxfp2 was detected in the endometrial epithelium. By 12 months of age approximately 18% of Cav1-/- females developed single or multiple dilated endometrial cysts lined by a flattened, simple low epithelium. A deficiency for Rxfp2 on Cav1-deficient background led to more than a 2-fold increase in the incidence of uterine cysts (54-58%). Appearance of cysts led to a severe disorganization of uterine morphology. We have found that the cysts had an increased expression of β-catenin and estrogen receptor β in endometrial stromal and epithelial cells and increased epithelial proliferation. An analysis of simple dilated cysts in human patients for CAV1 expression did not show appreciable differences with control regardless of menstrual phase, suggesting an involvement of additional factors in human disease. The results of this study suggest a novel synergistic role of INSL3/RXFP2 and CAV1 in structural maintenance of the uterus.

INSL3 in the ruminant: a powerful indicator of gender- and genetic-specific feto-maternal dialogue

The hormone Insulin-like peptide 3 (INSL3) is a major secretory product of the Leydig cells from both fetal and adult testes. Consequently, it is a major gender-specific circulating hormone in the male fetus, where it is responsible for the first phase of testicular descent, and in the adult male. In most female mammals, circulating levels are very low, corresponding to only a small production of INSL3 by the mature ovaries. Female ruminants are exceptional in exhibiting high INSL3 gene expression by the thecal cells of antral follicles and by the corpora lutea. We have developed a specific and sensitive immunoassay to measure ruminant INSL3 and show that, corresponding to the high ovarian gene expression, non-pregnant adult female sheep and cows have up to four times the levels observed in other female mammals. Significantly, this level declines during mid-pregnancy in cows carrying a female fetus, in which INSL3 is undetectable. However, in cows carrying a male fetus, circulating maternal INSL3 becomes elevated further, presumably due to the transplacental transfer of fetal INSL3 into the maternal circulation. Within male fetal blood, INSL3 is high in mid-pregnancy (day 153) corresponding to the first transabdominal phase of testicular descent, and shows a marked dependence on paternal genetics, with pure bred or hybrid male fetuses of Bos taurus (Angus) paternal genome having 30% higher INSL3 levels than those of Bos indicus (Brahman) paternity. Thus INSL3 provides the first example of a gender-specific fetal hormone with the potential to influence both placental and maternal physiology.

Expression of insulin-like 3 (INSL3) and differential splicing of its receptor in the ovary of rhesus macaques

BACKGROUND

Although insulin-like 3 (INSL3) has been identified in the gonad of both sexes in many species, there are only limited reports on the distribution of INSL3 and its receptor, relaxin/insulin-like family peptide receptor 2 (RXFP2), in the primate ovary. Since the hormone-receptor pair is believed to play a role in female reproduction, investigating the transcription of INSL3/RXFP2 genes and the spatiotemporal expression of INSL3 in the nonhuman primate may shed light on the functional aspects of the system in humans.

METHODS

Database mining, molecular and immunological methods were applied.

RESULTS

One single INSL3 transcript and three novel splice variant transcripts of RXFP2 were identified in the ovary of rhesus macaques. While the full-length RXFP2 transcript is barely detectable in granulosa cells during the periovulatory period, INSL3 transcript and protein are highly abundant in theca cells surrounding antral follicles. Moreover, the INSL3 level in follicular fluid is 3-4 times higher than that in female serum which remains low throughout the menstrual cycle.

CONCLUSIONS

The presence of INSL3 and its receptor in the ovary implies a potential role of the ligand-receptor pair in female reproduction in nonhuman primates. However, the existence of multiple splice variants of RXFP2 indicates a very complex nature of the hormone-receptor system.

Relaxin family peptide systems and the central nervous system

Since its discovery in the 1920s, relaxin has enjoyed a reputation as a peptide hormone of pregnancy. However, relaxin and other relaxin family peptides are now associated with numerous non-reproductive physiologies and disease states. The new millennium bought with it the sequence of the human genome and subsequently new directions for relaxin research. In 2002, the ancestral relaxin gene RLN3 was identified from genome databases. The relaxin-3 peptide is highly expressed in a small region of the brain and in species from teleost to primates and has both conserved sequence and sites of expression. Combined with the discovery of the relaxin family peptide receptors, interest in the role of the relaxin family peptides in the central nervous system has been reignited. This review explores the relaxin family peptides that are expressed in or act upon the brain, the receptors that mediate their actions, and what is currently known of their functions.

Membrane receptors: structure and function of the relaxin family peptide receptors

The receptors for members of the relaxin peptide family have only recently been discovered and are G-protein-coupled receptors (GPCRs). Relaxin and insulin-like peptide 3 (INSL3) interact with the leucine-rich-repeat-containing GPCRs (LGRs) LGR7 and LGR8, respectively. These receptors show closest similarity to the glycoprotein hormone receptors and contain large ectodomains with 10 leucine-rich repeats (LRRs) but are unique members of the LGR family (class C) as they have an LDL class A (LDLa) module at their N-terminus. In contrast, relaxin-3 and INSL5 interact with another class of type I GPCRs which lack a large ectodomain, the peptide receptors GPCR135 and GPCR142, respectively. These receptors are now classified as relaxin family peptide (RXFP) receptors, RXFP1 (LGR7), RXFP2 (LGR8), RXFP3 (GPCR135) and RXFP4 (GPCR142). This review outlines the identification of the peptides and receptors, their expression profiles and physiological roles and the functional interactions of the peptides with their unique receptors.

The hinge region fragment of immunoglobulin G improves immunogenicity of recombinant gonadotrophin-releasing hormone conjugated to the T-helper epitope in designing peptide vaccines

In our previous study, the hinge fragment (225-232/225'-232') of human immunoglobulin G1 (IgG1) was used as a space peptide linker for synthesizing the GnRH3-hinge-MVP chimeric peptide, whereby three repeated gonadotrophin-releasing hormone (GnRH) units and a T-cell epitope from measles virus fusion protein (MVP) were amide-bond-linked at the N and C terminus, respectively, to the hinge peptide for producing anti-GnRH antibody responses. To investigate whether or not the hinge region fragment can improve the immunogenicity of GnRH, we further synthesized and purified GnRH3-hinge-MVP, GnRH3-hinge and GnRH3-MVP using recombinant DNA technology. Under high pH conditions, GnRH3-hinge-MVP was capable of forming double-chain structures. Immunization of male mice with the immunogens of GnRH3-hinge-MVP resulted in the generation of high-titre antibodies specific for GnRH. The synthetic GnRH3-hinge and GnRH3-MVP induced a lower titre of anti-GnRH antibody than GnRH3-hinge-MVP. This was followed by a decrease in serum testosterone levels, which resulted in a low level of expression of the relaxin-like factor gene in the testis. Our data suggest that peptide and T-cell epitopes oriented at the N-terminus or C-terminus of hinge peptides simplify the antigenic peptide conjugates and may be considered as potential synthetic immunogens.

Relaxin, the relaxin-like factor and their receptors

In 1926 Frederick Hisaw discovered a blood-borne factor in pregnant guinea pigs that would cause relaxation of the pubic symphysis in virgin females of the species. The relaxin-like factor gene (RLF), also known as insulin-like 3 (INSL3), was recovered from a library of testicular cDNA. The function of RLF as the mediator of testicular positioning in mice was discovered by gene deletion experiments. The report that deletion of a G-protein-coupled receptor in a mouse mutant caused cryptorchidism and that relaxin and RLF and their receptors were structurally and functionally similar may well have inspired Drs. Hsueh and Sherwood to put LGR7 and relaxin together and thus, after many agonizing years of uncertainty, the relaxin receptor had yielded its identity. LGR8 was recognized as the human version of the RLF receptor and together LGR7 and LGR8, with their respective ligands, opened to detailed investigation the large and important field of G-protein activated leucine-rich repeat receptors. In the process RLF and LGR8 have yielded some general information that might contribute to our knowledge of receptor/ligand interaction, in particular the enigmatic signal initiation process.

The evolution of the relaxin peptide family and their receptors

The relaxin peptide family in humans consists of relaxin-1, 2 and 3 and the insulin-like peptides (INSL)-3, 4, 5 and 6. The evolution of this family has been controversial; points of contention include the existence of an invertebrate relaxin and the absence of a ruminant relaxin. Over the past four years we have performed a comprehensive analysis of the relaxin peptide family using all available vertebrate and invertebrate genomes. Contrary to previous reports an invertebrate relaxin was not found; sequence similarity searches indicate the family emerged during early vertebrate evolution. Phylogenetic analyses revealed the presence ofpotential relaxin-3, relaxin and INSL5 homologs in fish; dating their emergence far earlier than previously believed. There are four known relaxin peptide family receptors; the relaxin and INSL3 receptors, the leucine rich repeat containing G protein-coupled receptors (LGR), LGR7 and LGR8 respectively; and the two relaxin-3 receptors, GPCR135 and GPCR142. Database searching identified several invertebrate ancestors of LGR7 and LGR8; the absence of an invertebrate relaxin suggests the presence of an unidentified invertebrate ligand for these receptors. No invertebrate ancestors of GPCR135 or GPCR142 were found. Based on the theory that interacting proteins co-evolve together, phylogenetic analyses of the relaxin peptide family receptors were performed to provide insight into interactions within the relaxin system. Co-evolution between INSL5 and GPCR142, as evidenced by the loss of both genes in the rat and dog and their similar expression profiles, predicted GPCR142 to be the endogeneous INSL5 receptor. This interaction has since been confirmed experimentally. The emergence and presence of multiple GPCR135 homologs in fish reflected similar findings for relaxin-3. It seems likely the ancestral relaxin system was relaxin-3 acting through GPCR135, before LGR7 was "acquired" as a relaxin receptor early in vertebrate development.

The expression level of frog relaxin mRNA (fRLX), in the testis of Rana esculenta, is influenced by testosterone

Frog relaxin (fRLX) belongs to the relaxin/insulin gene family present in the testis of Rana esculenta and is specifically expressed by Leydig cells. Since the expression of fRLX transcript changes during the reproductive cycle and is more abundant when circulating levels of androgens are relatively high, we investigated the effect(s) of testosterone and its antagonist (cyproterone acetate, CPA) on its expression pattern, in the testis of the frog Rana esculenta. Results from in vivoand in vitro experiments demonstrate that testosterone strongly induces a significant increase of fRLX mRNA expression in frog testes and, this effect is counteracted by CPA, supporting the existence of intratesticular (autocrine/paracrine) mechanisms of action. Interestingly, in both the control and testosterone-treated testes, fRLX mRNA expression was markedly decreased 24 h post-treatment, as compared to that measured at 2 h and 8 h post-treatment, suggesting that factor(s), other than testosterone, may act(s) in controlling its expression. In addition, RT-PCR analysis and in situ hybridization performed on frog testis injected with CPA for 15 days, on alternate days, showed a strong decrease of fRLX expression, suggesting that CPA counteracts the effect of testosterone on fRLX expression. Taken together our results strongly indicate that changes in the production, by the Leydig cells, of both testosterone and fRLX may represent a marker for the study of Leydig cell activity in the testis of the frog Rana esculenta.

Plasma estrone sulphate (E1S) and estradiol-17β (E2β) profiles during pregnancy and their relationship with the relaxation of sacrosciatic ligament, and prediction of calving time in Holstein–Friesian cattle

The objectives of this study were to investigate the plasma E(1)S and E(2)beta profiles during pregnancy and their relationship with the relaxation of sacrosciatic ligament in Holstein-Friesian cattle (n=37) and then to predict the calving time on the basis of E(1)S and E(2)beta profiles and relaxation of the ligament. Blood samples were collected at 4 weeks intervals from days 100 to 190, at 2 weeks intervals from days 190 to 250, every week from days 250 to 270 and thereafter every day from day 270 of gestation until the day after calving. The relaxation in the ligament was measured by using two scales as a distance at a schedule similar to blood sampling plus 5 days postpartum. One scale was kept firm exactly parallel to the ligament between the sacrum and the tuber ischii and other scale was erected perpendicularly to the first scale with the bottom just touching the ligament and the depth was measured in the second scale from the point where it touched the ligament to the point where it touched the first scale. Plasma samples were analyzed for E(1)S and E(2)beta by enzyme immunoassay. E(1)S concentration was low at day 100 (0.8+/-0.3 ng/ml), then increased progressively and drastically to reach the level of 28.4+/-3.6 ng/ml on the day before calving and declined significantly (p<0.05) at 9.5+/-3.1 ng/ml within 1 day postpartum. There was a gradual increase in concentration of E(2)beta from day 100 of gestation (0.1+/-0 ng/ml) until day 4 prepartum (0.6+/-0 ng/ml). Thereafter, it increased drastically and reached the peak level of 1.0+/-0.1 ng/ml (p<0.05) on the day before calving and declined markedly at 0.4+/-0.1 ng/ml within 1 day postpartum (p<0.05). Corresponding to E(1)S and E(2)beta concentrations, a gradual increase in the relaxation of the ligament was observed from day 100 of gestation (8+/-1mm) until day 2 prepartum (24+/-2mm). Thereafter, it showed a significant increase (p<0.05) within 1 day before calving (31+/-2mm) and almost no difference between day 1 prepartum and day 1 postpartum. A marked decrease (p<0.05) was observed thereafter until day 3 postpartum (10+/-2mm) and no significant change between days 3 and 4 as well as 4 and 5 postpartum. The increment of E(2)beta by >or=0.20 ng/ml from the preceding day concentration was 85.2% accurate for predicting calving within 24h in many of the cows (23 of 37) in the herd. The increment in ligament relaxation measurement by >or=5mm from the preceding day measurement was the most efficacious to predict calving within 24h with the highest accuracy (93.9%) in high proportions of cows (31 of 37) in the herd. In conclusion, plasma E(1)S and E(2)beta concentrations and relaxation of sacrosciatic ligament increased gradually as gestation advanced and reached the peak level on the day before calving. The relaxation in the ligament corresponded well to plasma E(2)beta concentrations. Prediction of calving was possible by E(2)beta profile and relaxation in the ligament but not by E(1)S profile. The increment in ligament measurement by >or=5mm from the preceding day measurement was the most useful and accurate in predicting calving within 24h. It is economical and easily applicable in the field condition.

International Union of Pharmacology LVII: recommendations for the nomenclature of receptors for relaxin family peptides

Although the hormone relaxin was discovered 80 years ago, only in the past 5 years have the receptors for relaxin and three other receptors that respond to related peptides been identified with all four receptors being G-protein-coupled receptors. In this review it is suggested that the receptors for relaxin (LGR7) and those for the related peptides insulin-like peptide 3 (LGR8), relaxin-3 (GPCR135), and insulin-like peptide 5 (LGPCR142) be named the relaxin family peptide receptors 1 through 4 (RXFP1-4). RXFP1 and RXFP2 are leucine-rich repeat-containing G-protein-coupled receptors with complex binding characteristics involving both the large ectodomain and the transmembrane loops. RXFP1 activates adenylate cyclase, protein kinase A, protein kinase C, phosphatidylinositol 3-kinase, and extracellular signaling regulated kinase (Erk1/2) and also interacts with nitric oxide signaling. RXFP2 activates adenylate cyclase in recombinant systems, but physiological responses are sensitive to pertussis toxin. RXFP3 and RXFP4 resemble more conventional peptide liganded receptors and both inhibit adenylate cyclase, and in addition RXFP3 activates Erk1/2 signaling. Physiological studies and examination of the phenotypes of transgenic mice have established that relaxin has roles as a reproductive hormone involved in uterine relaxation (some species), reproductive tissue growth, and collagen remodeling but also in the cardiovascular and renal systems and in the brain. The connective tissue remodeling properties of relaxin acting at RXFP1 receptors have potential for the development of agents effective for the treatment of cardiac and renal fibrosis, asthma, and scleroderma and for orthodontic remodelling. Agents acting at RXFP2 receptors may be useful for the treatment of cryptorchidism and infertility, whereas antagonists may be used as contraceptives. The brain distribution of RXFP3 receptors suggests that actions at these receptors have the potential for the development of antianxiety and antiobesity drugs.

Immunohistochemical Localization of Relaxin-Like Factor/Insulin-Like Peptide-3 in the Bovine Corpus Luteum

Relaxin-like factor/insulin-like peptide (INSL)-3 is highly expressed in the bovine corpus luteum throughout the estrous cycle and pregnancy. Demonstration of translation of the relaxin-like factor message was previously shown for the follicle but not the corpus luteum. In this study, relaxin-like factor mRNA was highly expressed in the corpus luteum on days 7 and 14 of pregnancy. Tissues were fixed in 10% neutral buffered formalin, and utilizing two different antibodies to relaxin-like factor, W3 rabbit anti-bovine and 2-8F mouse anti-bovine, relaxin-like factor was localized in fibroblast-like cells. Staining was also observed in the Leydig cells of bovine testicular tissue. No staining was observed in small and large steroidogenic luteal cells, indicating a nonsteroidogenic source of luteal relaxin-like factor. Definitive cell type identification is currently being determined via electron microscopy.

Relaxin Research in the Postgenomic Era

Because of the coevolution of ligands and their cognate receptors, analysis of human genomic sequences allows prediction of the pairing of these elements. Initially, we identified a group of five human leucine-rich repeat-containing G-protein-coupled receptor (LGR) genes homologous to LH, FSH, and TSH receptors. Based on common phenotypes of INSL3 null mice and transgenic mice with LGR8 gene deletion, we hypothesized that INSL3, relaxin, and related genes are likely ligands for the paralogous LGR7 and LGR8 genes. Matching the relaxin family peptides with these two orphan LGRs led to the finding that relaxin is capable of activating LGR7 and LGR8 through the Gs pathway. In addition, INSL3 and relaxin 3 were found to be specific ligands for LGR8 and LGR7, respectively. Based on the known production of INLS3 by testicular Leydig cells and ovarian theca cells, we demonstrated the expression of the INSL3 receptor LGR8 in oocytes in ovary and in male germ cells in the testis. Furthermore, we found that LH stimulates INSL3 transcripts in ovarian theca and testicular Leydig cells. INSL3, in turn, binds LGR8 expressed in germ cells to initiate the meiotic progression of arrested oocytes in preovulatory follicles in vitro and in vivo and to suppress male germ cell apoptosis in vivo. INSL3 interacts with germ cells to activate the inhibitory G protein, thus leading to decreases in cAMP production. Our data demonstrate the importance of the INSL3-LGR8 paracrine system in mediating gonadotropic actions in both ovary and testis.

Insulin-Like Factor 3: Where Are We Now?

Insulin-like factor 3 (INSL3), previously known as the relaxin-like factor (RLF), is a major peptide hormone secreted from the testicular Leydig cells of adult men and circulating in the blood at a concentration of approximately 1 ng/mL. Women also produce INSL3 in the theca interna cells of ovarian follicles, but circulating levels remain below 100 pg/mL. INSL3 is structurally related to relaxin and insulin, but unlike the latter, signals through a novel G-protein-coupled receptor, LGR8. Ablation of the gene for INSL3 leads primarily to cryptorchidism because of a defect in the first, transabdominal phase of testicular descent. In the adult knockout mouse, a mild phenotype is evident in the testis and ovary. We have developed a panel of antibodies specific for INSL3 from various species, which are suitable for immunohistochemistry and, more recently, for immunoassays. INSL3 is an important marker for the mature Leydig cell phenotype, where it appears to be expressed constitutively, once the mature differentiation state is achieved. It is also an indicator of differentiation status not only for Leydig cells but also for the theca interna cells of the ovary.

New members of the insulin family: regulators of metabolism, growth and now ... reproduction

Insulin, IGF, and relaxin are established members of the insulin protein superfamily. The application of the techniques of cellular, molecular, and computational biology has permitted the identification of new insulin-like ligands and their cognate receptors. Information regarding the biologic role is available for some of these newly identified ligand-receptor systems and indicates novel roles in diverse processes such as testicular descent, germ cell function, and cell migration.

Seasonal Expression of INSL3 and Lgr8/Insl3 Receptor Transcripts Indicates Variable Differentiation of Leydig Cells in the Roe Deer Testis

Roe deer are seasonal breeders and show cyclic variation in testicular volume and cellular differentiation within the tubular and interstitial testis compartment. We have employed the roe deer as a model to elucidate the expression of the postpubertal Leydig cell marker INSL3 during seasonal changes in Leydig cell differentiation. Roe deer testis and serum samples were collected bimonthly throughout the complete reproductive cycle. Peak levels of testicular Insl3 mRNA and INSL3 immunoprotein were detected well before the onset of rut in April and coincided with the highest percentage of INSL3-positive cell number/square millimeter of testicular interstitial area. During the winter (December, February), roe deer INSL3 was exclusively detected in a subpopulation of alpha-actin-negative, spindle-shaped peritubular cells. Concordant with the increase in INSL3 production in April and 1 mo after the reported LH peak, a sharp increase in serum testosterone concentrations was observed. High serum testosterone concentrations coincided with the presence of detectable 17alpha-hydroxylase, mRNA and protein, in Leydig cells. Upregulation of INSL3 production in spring appeared to reflect LH-dependent differentiation of Leydig cells. The considerable changes in percentage of INSL3 immunopositive cells within the numerically constant interstitial cell population indicated cyclic seasonal de- and redifferentiation of Leydig cells. A complex functional role of the INSL3/LGR8 ligand-receptor system in the roe deer testis was suggested by the detection of specific hybridization signals for roe deer Lgr8 transcripts in Sertoli cells of the roe deer testis.

Paracrine regulation of mammalian oocyte maturation and male germ cell survival

Mammalian oocytes are arrested at the prophase of meiosis before induction of maturation by the preovulatory luteinizing hormone (LH) surge. LH also promotes the survival of meiotic male germ cells in the testis. Because LH binds somatic cells, the mechanism underlying its regulation of germ cell function is unclear. We found that LH stimulates Leydig insulin-like 3 (INSL3) transcripts in ovarian theca and testicular Leydig cells. INSL3, in turn, binds a G protein-coupled receptor, LGR8 (leucine-rich repeat-containing G protein-coupled receptor 8), expressed in germ cells to activate the inhibitory G protein, thus leading to decreases in cAMP production. Treatment with INSL3 initiates meiotic progression of arrested oocytes in preovulatory follicles in vitro and in vivo and suppresses male germ cell apoptosis in vivo, thus demonstrating the importance of the INSL3-LGR8 paracrine system in mediating gonadotropin actions.

Isolation and expression analysis of the canine insulin-like factor 3 gene

The insulin-like factor 3 (INSL3 or relaxin-like factor) is a hormone produced mainly in gonadal tissues in males and females. Deletion of INSL3 or its receptor in male mice leads to the undescended testes, or cryptorchidism. Here we describe an isolation and analysis of full-length canine INSL3 gene. The INSL3 gene is composed of two exons within a small genomic region. Putative translation of the isolated cDNA yields 132 amino acid preproINSL3 that has the domain structure characteristic for the insulin-relaxin peptide superfamily with a well-conserved receptor-binding domain. Northern blot hybridization showed stronger expression of INSL3 in testis than in ovary. Reverse transcription-polymerase chain reaction analysis of the INSL3 expression revealed a minor splice variant of INSL3 potentially encoding 105 amino acids peptide. We established that the medium, conditioned with recombinant canine INSL3, produced from the full-length cDNA, but not from the minor splice variant, activated human GREAT/LGR8 receptor in vitro. In addition to the functional allele of INSL3, genomic DNA of one of the analyzed dogs contained an intronless nonexpressed pseudogene of INSL3. We isolated canine INSL3 promoter and showed that its activity was strongly mediated by steroidogenic factor-1 in vitro. Using site-specific mutagenesis, we identified a well-conserved steroidogenic factor-1 binding site within canine INSL3 promoter.

INSL3 ligand-receptor system in the equine testis

We employed molecular and immunological techniques to investigate the expression of INSL3, a member of the insulin-like superfamily, in prepubertal testis, postpubertal testes exhibiting normal and disturbed spermatogenesis, and cryptorchid testes of male horses. In addition, the partial cDNA coding sequences of the equine homologue of the human relaxin/INSL3-receptor Lgr8 were determined. Nonradioactive in-situ hybridization with a cRNA probe for equine Insl3 and immunohistochemistry with a specific rabbit INSL3 antiserum localized Insl3 transcripts and immunoreactive INSL3 ligand to Leydig cells in all types of testes investigated. Quantitative polymerase chain reaction analysis revealed a down-regulation of Insl3 and an up-regulation of the relaxin/INSL3-receptor expression in unilateral cryptorchid versus descended testes. Western blot analysis of protein extracts from adult normal and cryptorchid testes and prepubertal testes showed a single immunoreactive band at 14.5 kDa, which correlates with the predicted size of equine proINSL3. Densitometric analysis of Western blot data of adult normal testes revealed significantly stronger expression of immunoreactive proINSL3 as compared to extracts derived from cryptorchid or prepubertal testes. Thus, decreased expression of immunoreactive INSL3 in cryptorchid and prepubertal equine testis is transcriptionally regulated. The detection of transcripts for equine Lgr8 in the testis has identified the testis as a potential target of INSL3.

Structural requirements for the interaction of sheep insulin-like factor 3 with relaxin receptors in rat atria

Relaxin is a peptide with various reproductive and nonreproductive functions. The site for the peptide-receptor interaction contains two arginines (Arg) and an isoleucine (Ile) or valine (Val) residue in the B-chain with a configuration of -Arg-X-X-X-Arg-X-X-Ile/Val-X-. The sheep insulin-like peptide 3 (INSL3), a structural homologue of relaxin, also contains the n, n+4 arginines in the B-chain but they are displaced towards the carboxyl terminus by four residues (-X-X-X-X-Arg-X-X-Val-Arg-). Human INSL3 increases the activity of human relaxin in mouse bioassays. Here, we investigated whether sheep synthetic INSL3 affects the relaxin activity in rat atria. INSL3 lacked relaxin-like agonist activity but blocked the activity of relaxin and competed for relaxin binding sites at high concentrations. We also synthesized analogues of INSL3, with amino acid substitutions in the arginine-binding region. Analogues A, D and E, which have the arginines in positions identical to relaxin, showed weak relaxin-like agonist activity. These results suggest that other sites in the relaxin molecule are involved in high-affinity peptide-receptor interaction for the production of the relaxin biological responses.