Chemical synthesis and biological activity of rat INSL3

Development of an LC-MS/MS assay for quantification of intact INSL3 in rat plasma

Background: Relatively large disulfide-linked polypeptides can serve as signaling molecules for a diverse array of biological processes and may be studied in animal models to investigate their function in vivo. The aim of this work was to develop an LC-MS/MS assay to measure a model peptide, INSL3, in rat plasma. Results: A dual enrichment strategy incorporating both protein precipitation and solid phase extraction was utilized to isolate INSL3 from rat plasma, followed by targeted LC-MS/MS detection. The method was able to measure full-length INSL3 (6.1 kDa) down to 0.2 ng/ml with acceptable accuracy and precision. Conclusion: The final assay was applied to support an exploratory pharmacokinetic study to evaluate steady-state concentrations of dosed INSL3 in rat plasma.

Leydig cells contribute to the inhibition of spermatogonial differentiation after irradiation of the rat

Irradiation with 6 Gy produces a complete block of spermatogonial differentiation in LBNF1 rats that would be permanent without treatment. Subsequent suppression of gonadotropins and testosterone (T) restores differentiation to the spermatocyte stage; however, this process requires 6 weeks. We evaluated the role of Leydig cells (LCs) in maintenance of the block in spermatogonial differentiation after exposure to radiation by specifically eliminating functional LCs with ethane dimethane sulfonate (EDS). EDS (but not another alkylating agent), given at 10 weeks after irradiation, induced spermatogonial differentiation in 24% of seminiferous tubules 2 weeks later. However, differentiation became blocked again at 4 weeks as LCs recovered. When EDS was followed by treatment with GnRH antagonist and flutamide, sustained spermatogonial differentiation was induced in >70% of tubules within 2 weeks. When EDS was followed by GnRH antagonist plus exogenous T, which also inhibits LC recovery but restores follicle stimulating hormone (FSH) levels, the spermatogonial differentiation was again rapid but transient. These results confirm that the factors that block spermatogonial differentiation are indirectly regulated by T, and probably FSH, and that adult and possibly immature LCs contribute to the production of such inhibitory factors. We tested whether insulin-like 3 (INSL3), a LC-produced protein whose expression correlated with the block in spermatogonial differentiation, was indeed responsible for the block by injecting synthetic INSL3 into the testes and knocking down its expression in vivo with siRNA. Neither treatment had any effect on spermatogonial differentiation. The Leydig cell products that contribute to the inhibition of spermatogonial differentiation in irradiated rats remain to be elucidated.

The active form of goat insulin-like peptide 3 (INSL3) is a single-chain structure comprising three domains B-C-A, constitutively expressed and secreted by testicular Leydig cells

Relaxin-like factor (RLF), also called insulin-like peptide 3 (INSL3), is a member of the insulin/relaxin gene family and is produced by testicular Leydig cells. While the understanding of its effects is growing, very little is known about the structural and functional properties of native INSL3. Here, we demonstrate that native INSL3 isolated from goat testes is a single-chain structure with full biological activity, and is constitutively expressed and secreted by Leydig cells. Using a series of chromatography steps, native INSL3 was highly purified as a single 12-kDa peak as revealed by SDS-PAGE. MS/MS analysis provided 81% sequence coverage and revealed a distinct single-chain structure consisting of the B-, C-, and A-domains deduced previously from the INSL3 cDNA sequence. Moreover, the N-terminal peptide was six amino acid residues longer than predicted. Native INSL3 exhibited full bioactivity in HEK-293 cells expressing the receptor for INSL3. Immunoelectron microscopy and Western blot analysis revealed that INSL3 was secreted by Leydig cells through the constitutive pathway into blood and body fluids. We conclude, therefore, that goat INSL3 is constitutively secreted from Leydig cells as a B-C-A single-chain structure with full biological activity.

Relaxin-like factor (RLF)/insulin-like peptide 3 (INSL3) is secreted from testicular Leydig cells as a monomeric protein comprising three domains B-C-A with full biological activity in boars

RLF (relaxin-like factor), also known as INSL3 (insulin-like peptide 3), is a novel member of the relaxin/insulin gene family that is expressed in testicular Leydig cells. Despite the implicated role of RLF/INSL3 in testis development, its native conformation remains unknown. In the present paper we demonstrate for the first time that boar testicular RLF/INSL3 is isolated as a monomeric structure with full biological activity. Using a series of chromatography steps, the native RLF/INSL3 was highly purified as a single peak in reverse-phase HPLC. MS/MS (tandem MS) analysis of the trypsinized sample provided 66% sequence coverage and revealed a distinct monomeric structure consisting of the B-, C- and A-domains deduced previously from the RLF/INSL3 cDNA. Moreover, the N-terminal peptide was four amino acid residues longer than predicted previously. MS analysis of the intact molecule and PMF (peptide mass fingerprinting) analysis at 100% sequence coverage confirmed this structure and indicated the existence of three site-specific disulfide bonds. RLF/INSL3 retained full bioactivity in HEK (human embryonic kidney)-293 cells expressing RXFP2 (relaxin/insulin-like family peptide receptor 2), the receptor for RLF/INSL3. Furthermore, RLF/INSL3 was found to be secreted from Leydig cells into testicular venous blood. Collectively, these results indicate that boar RLF/INSL3 is secreted from testicular Leydig cells as a B-C-A monomeric structure with full biological activity.

Solid phase synthesis and structural analysis of novel A-chain dicarba analogs of human relaxin-3 (INSL7) that exhibit full biological activity

Replacement of disulfide bonds with non-reducible isosteres can be a useful means of increasing the in vivo stability of a protein. We describe the replacement of the A-chain intramolecular disulfide bond of human relaxin-3 (H3 relaxin, INSL7), an insulin-like peptide that has potential applications in the treatment of stress and obesity, with the physiologically stable dicarba bond. Solid phase peptide synthesis was used to prepare an A-chain analogue in which the two cysteine residues that form the intramolecular bond were replaced with allylglycine. On-resin microwave-mediated ring closing metathesis was then employed to generate the dicarba bridge. Subsequent cleavage of the peptide from the solid support, purification of two isomers and their combination with the B-chain via two intermolecular disulfide bonds, then furnished two isomers of dicarba-H3 relaxin. These were characterized by CD spectroscopy, which suggested a structural similarity to the native peptide. Additional analysis by solution NMR spectroscopy also identified the likely cis/trans form of the analogs. Both peptides demonstrated binding affinities that were equivalent to native H3 relaxin on RXFP1 and RXFP3 expressing cells. However, although the cAMP activity of the analogs on RXFP3 expressing cells was similar to the native peptide, the potency on RXFP1 expressing cells was slightly lower. The data confirmed the use of a dicarba bond as a useful isosteric replacement of the disulfide bond.

Synthesis, conformation, and activity of human insulin-like peptide 5 (INSL5)

Insulin-like peptide 5 (INSL5) was first identified through searches of the expressed sequence tags (EST) databases. Primary sequence analysis showed it to be a prepropeptide that was predicted to be processed in vivo to yield a two-chain sequence (A and B) that contained the insulin-like disulfide cross-links. The high affinity interaction between INSL5 and the receptor RXFP4 (GPCR142) coupled with their apparent coevolution and partially overlapping tissue expression patterns strongly suggest that INSL5 is an endogenous ligand for RXFP4. Given that the primary function of the INSL5–RXFP4 pair remains unknown, an effective means of producing sufficient quantities of this peptide and its analogues is needed to systematically investigate its structural and biological properties. A combination of solid-phase peptide synthesis methods together with regioselective disulfide bond formation were used to obtain INSL5. Both chains were unusually resistant to standard synthesis protocols and required highly optimized conditions for their acquisition. In particular, the use of a strong tertiary amidine, DBU, as N^α-deprotection base was required for the successful assembly of the B chain; this highlights the need to consider incomplete deprotection rather than acylation as a cause of failed synthesis. Following sequential disulfide bond formation and chain combination, the resulting synthetic INSL5, which was obtained in good overall yield, was shown to possess a similar secondary structure to human relaxin-3 (H3 relaxin). The peptide was able to inhibit cAMP activity in SK-N-MC cells that expressed the human RXFP4 receptor with a similar activity to H3 relaxin. In contrast, it had no activity on the human RXFP3 receptor. Synthetic INSL5 demonstrates equivalent activity to the recombinant-derived peptide, and will be an important tool for the determination of its biological function.

Role of hormones, genes, and environment in human cryptorchidism

Cryptorchidism is the most frequent congenital birth defect in male children (2-4% in full-term male births), and it has the potential to impact the health of the human male. In fact, although it is often considered a mild malformation, it represents the best-characterized risk factor for reduced fertility and testicular cancer. Furthermore, some reports have highlighted a significant increase in the prevalence of cryptorchidism over the last few decades. Etiology of cryptorchidism remains for the most part unknown, and cryptorchidism itself might be considered a complex disease. Major regulators of testicular descent from intraabdominal location into the bottom of the scrotum are the Leydig-cell-derived hormones testosterone and insulin-like factor 3. Research on possible genetic causes of cryptorchidism has increased recently. Abundant animal evidence supports a genetic cause, whereas the genetic contribution to human cryptorchidism is being elucidated only recently. Mutations in the gene for insulin-like factor 3 and its receptor and in the androgen receptor gene have been recognized as causes of cryptorchidism in some cases, but some chromosomal alterations, above all the Klinefelter syndrome, are also frequently involved. Environmental factors acting as endocrine disruptors of testicular descent might also contribute to the etiology of cryptorchidism and its increased incidence in recent years. Furthermore, polymorphisms in different genes have recently been investigated as contributing risk factors for cryptorchidism, alone or by influencing susceptibility to endocrine disruptors. Obviously, the interaction of environmental and genetic factors is fundamental, and many aspects have been clarified only recently.

Hormonal and genetic control of testicular descent

Cryptorchidism has the potential to affect the health of the human male. Although it is often considered a mild malformation, it represents the best-characterized risk factor for reduced fertility and testicular cancer. The aetiology of cryptorchidism remains, for the most part, unknown and cryptorchidism itself might be considered a complex disease. This reflects the intricate mechanisms regulating testicular development and descent from intra-abdominal location into the bottom of the scrotum, involving different anatomical and hormonal factors. Major actors of testicular descent are the Leydig cell-derived hormones testosterone and insulin-like factor 3, even if other factors may play a role. Although considerable evidence exists in animals to support a genetic cause, the genetic contribution to human cryptorchidism is only recently being elucidated. Environmental factors might also contribute to the aetiology of cryptorchidism and its increased incidence in recent years. Mutations in the gene for insulin-like factor 3 and its receptor and in the androgen receptor gene explain a minority of cases of cryptorchidism, but research on genetic polymorphisms that may also influence susceptibility to endocrine disruptors is shedding light on this field.

Solution Structure and Characterization of the LGR8 Receptor Binding Surface of Insulin-like Peptide 3

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

The Mode of Interaction of the Relaxin-like Factor (RLF) with the Leucine-rich Repeat G Protein-activated Receptor 8

The relaxin-like factor (RLF, also named INSL3) is a critical component in the chain of events that lead to the normal positioning of the gonads in the male fetus. RLF and relaxin share features of the secondary structure to the extent that relaxin cross-reacts with the LGR8, the RLF receptor. Although both hormones interact with their receptors essentially via the B chain, the sharply defined binding cassette of relaxin is not present in RLF. Structure and function analysis of RLF derivatives with single amino acid replacements revealed that the most important binding residues are tryptophan B27, followed by arginine B16 and valine B19. Single alanine replacements for each individual position resulted in a relative receptor affinity of 4.0% (B16), 6.1% (B19), and 0.5% (B27). Tryptophan B27 is located on an extended structure, and arginine B16 and valine B19 are positioned on the exposed surface of the B chain helix. The 3 residues could be brought together to form a contiguous binding area if the C-terminal end of the B chain were free to fold back against the central portion of the B chain helix. Such a movement depends critically on the flexibility of the C-terminal end, which is controlled by positions B23-25. In as much as these major binding residues seem hardly sufficient to explain the strong binding of RLF to LGR8 we searched for and found an extended region where little contributions by individual residues added up to a strong receptor affinity. This mode of interaction could drive the binding energy sufficiently high to account for the picomolar binding constant of RLF and its receptor.

Paracrine and endocrine roles of insulin-like factor 3

Insulin-like factor 3 (INSL3) is expressed in Leydig cells of the testis and theca cells of the ovary. This peptide affects testicular descent by acting on gubernaculum via its specific receptor leucine-rich repeat-containing G protein-coupled receptor 8 (LGR8). From initial animal data showing the cryptorchid phenotype of Insl3/Lgr8 mutants, an extensive search for mutations in INSL3 and LGR8 genes was undertaken in human patients with cryptorchidism, and a frequency of mutation of 4-5% has been detected. However, definitive proofs of a causative role for some of these mutations are still lacking. More recent data suggest additional paracrine (in the testis and ovary) and endocrine actions of INSL3 in adults. INSL3 circulates at high concentrations in serum of adult males and its production is dependent on the differentiation effect of LH. Therefore, INSL3 is increasingly used as a specific marker of Leydig cell differentiation and function.

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.

INSL3/LGR8 role in testicular descent and cryptorchidism

Cryptorchidism, generally referred to a failure of testicular descent into the scrotum, is the most frequent (up to 3-4% at birth) congenital anomaly in newborn boys. Cryptorchidism is closely associated with impaired fertility, and represents an established risk factor for testicular cancer. Like other genital defects, cryptorchidism is believed to be caused by either endocrine or genetic abnormalities, or both. Recent elucidation of the molecular mechanism of the rodent testicular descent, and, in particular, the critical role of Insl3 (insulin-like 3) and its receptor Great/Lgr8 encouraged the search for naturally occurring mutations in the human homologues of these genes in the affected patient population. Genetic analysis revealed several functionally deleterious mutations in both INSL3 and GREAT/LGR8 genes. However, although some of mutations were found only in cryptorchid patients, it remains to be verified whether there is a causative link between the presence of mutations in INSL3 or GREAT/LGR8 and the undescended testis phenotype in men. The data and analysis of published studies indicate that mutations in these two genes might account for only a small portion of all cases of this disease in the human population.

Testicular descent and cryptorchidism: the state of the art in 2004

The understanding of testicular descent has changed much in the 20 years since the authors' laboratory began studying the mechanism. The process is now known to occur in 2 steps with different anatomy and hormonal regulation but with many still unresolved controversies. Recent advances include the recognition of acquired cryptorchidism of critical early postnatal germ cell development and the recommendation for surgery at 6 months of age. The authors still await long-term outcome studies.

Synthesis, conformation, receptor binding and biological activities of monobiotinylated human insulin-like peptide 3

Biotin-avidin immobilization has been routinely used as a tool to study peptide-receptor and peptide-antibody interactions. Biotinylated peptides can also be employed to localize cells that express the peptides' receptor, and to analyse ligand-receptor binding. Insulin-like peptide 3 (INSL3) is a peptide hormone which contains A- and B-chains connected by two disulphide bonds and plays a role in testicular descent during sexual development. In order to study the interaction of INSL3 with its receptor LGR8, a G protein-coupled receptor, we chemically synthesized Nalpha-mono-biotinylated human INSL3 (B-hINSL3) and compared it structurally and biologically with hINSL3. Both peptides exhibited similar, but high, receptor binding affinities on human foetal kidney fibroblast 293T cells transfected human LGR8 based on a competition radioreceptor assay with 33P-labelled relaxin H2 (B33). The modified B-hINSL3 showed full biological activity as determined by the stimulation of gubernacular cell proliferation. The labelled B-hINSL3 contains a higher alpha-helix content, and this increased helical structure is accompanied by an increase in ability to stimulate cAMP accumulation in 293T cells expressing LGR8. Our results suggest that the N-terminal region of the A-chain is not involved in the interaction of INSL3 with its receptor. However, the introduction of biotin onto the N-terminus of the A-chain promoted conformational stability which, in turn, permitted better receptor activation.

Insulin-like 3 signalling in testicular descent

Undescended testis is one of the most common congenital defects in the newborn boys and the common cause of cryptorchidism. If left untreated, this condition is strongly associated with infertility and drastically increased risk of testicular cancer in adulthood. Testis position in developing males is defined by sexual dimorphic differentiation of two gonadal ligaments, gubernaculum and cranial suspensory ligament. Recent transgenic mouse studies identified testicular hormone insulin-like 3 (INSL3), and its receptor, GREAT/LGR8, as the critical regulators of the gubernacular differentiation. Mutation analysis of the two genes in patients with undescended testis revealed functionally deleterious mutations, which may be responsible for the abnormal phenotype in some of the patients.

H3 relaxin is a specific ligand for LGR7 and activates the receptor by interacting with both the ectodomain and the exoloop 2

Leucine-rich repeat-containing, G protein-coupled receptors (LGRs) represent a unique subgroup of G protein-coupled receptors with a large ectodomain. Recent studies demonstrated that relaxin activates two orphan LGRs, LGR7 and LGR8, whereas INSL3/Leydig insulin-like peptide specifically activates LGR8. Human relaxin 3 (H3 relaxin) was recently discovered as a novel ligand for relaxin receptors. Here, we demonstrate that H3 relaxin activates LGR7 but not LGR8. Taking advantage of the overlapping specificity of these three ligands for the two related LGRs, chimeric receptors were generated to elucidate the mechanism of ligand activation of LGR7. Chimeric receptor LGR7/8 with the ectodomain from LGR7 but the transmembrane region from LGR8 maintains responsiveness to relaxin but was less responsive to H3 relaxin based on ligand stimulation of cAMP production. The decreased ligand signaling was accompanied by decreases in the ability of H3 relaxin to compete for (33)P-relaxin binding to the chimeric receptor. However, replacement of the exoloop 2, but not exoloop 1 or 3, of LGR7 to the chimeric LGR7/8 restored ligand binding and receptor-mediated cAMP production. These results suggested that activation of LGR7 by H3 relaxin involves specific binding of the ligand to both the ectodomain and the exoloop 2, thus providing a model with which to understand the molecular basis of ligand signaling for this unique subgroup of G protein-coupled receptors.

Relaxin-like bioactivity of ovine Insulin 3 (INSL3) analogues

Relaxin is an insulin-like peptide consisting of two separate chains (A and B) joined by two inter- and one intrachain disulfide bonds. Binding to its receptor requires an Arg-X-X-X-Arg-X-X-Ile motif in the B-chain. A related member of the insulin superfamily, INSL3, has a tertiary structure that is predicted to be similar to relaxin. It also possesses an Arg-X-X-X-Arg motif within its B-chain, although this is displaced by four amino acids towards the C-terminus from the corresponding position within relaxin. We have previously shown that synthetic INSL3 itself does not display relaxin-like activity although analogue (Analogue A) with an introduced arginine residue in the B-chain giving it an Arg cassette in the exact relaxin position does possess weak activity. In order to identify further the structural features that impart relaxin function, solid phase peptide synthesis was used to prepare three additional analogues for bioassay. Each of these contained point substitutions within the arginine cassette. Analogue D contained the full human relaxin binding cassette, Analogue G consisted of the native INSL3 sequence containing an Arg to Ala substitution, and Analogue E was a further modification of Analogue A, with the same substitution. Each analogue was fully chemically characterized by a number of criteria. Detailed circular dichroism spectroscopy analyses showed that the changes caused little alteration of secondary structure and, hence, overall conformation. However, each analogue displayed only weak relaxin-like activity. These results indicate that while the arginine cassette is vital for relaxin-like activity, there are additional, as yet unidentified structural requirements for relaxin binding.

Reproductive biology of the relaxin-like factor (RLF/INSL3)

The relaxin-like factor (RLF), which is the product of the insulin-like factor 3 (INSL3) gene, is a new circulating peptide hormone of the relaxin-insulin family. In male mammals, it is a major secretory product of the testicular Leydig cells, where it appears to be expressed constitutively but in a differentiation-dependent manner. In the adult testis, RLF expression is a good marker for fully differentiated adult-type Leydig cells, but it is only weakly expressed in prepubertal immature Leydig cells or in Leydig cells that have become hypertrophic or transformed. It is also an important product of the fetal Leydig cell population, where it has been demonstrated using knockout mice to be responsible for the second phase of testicular descent acting on the gubernaculum. INSL3 knockout mice are cryptorchid, and in estrogen-induced cryptorchidism, RLF levels in the testis are significantly reduced. RLF is also made in female tissues, particularly in the follicular theca cells of small antral follicles and in the corpus luteum of the cycle and pregnancy. The ruminant ovary has a very high level of RLF expression, and analysis of primary cultures of ovarian theca-lutein cells indicated that, as in the testis, expression is probably constitutive but differentiation dependent. Female INSL3 knockout mice have altered estrous cycles, where RLF may be involved in follicle selection, an idea strongly supported by observations on bovine secondary follicles. Recently, a novel 7-transmembrane domain receptor (LGR8 or Great) has been tentatively identified as the RLF receptor, and its deletion in mice leads also to cryptorchidism.

INSL3/Leydig insulin-like peptide activates the LGR8 receptor important in testis descent

Several orphan G protein-coupled receptors homologous to gonadotropin and thyrotropin receptors have recently been identified and named as LGR4-8. INSL3, also known as Leydig insulin-like peptide or relaxin-like factor, is a relaxin family member expressed in testis Leydig cells and ovarian theca and luteal cells. Male mice mutant for INSL3 exhibit cryptorchidism or defects in testis descent due to abnormal gubernaculum development whereas overexpression of INSL3 induces ovary descent in transgenic females. Because transgenic mice missing the LGR8 gene are also cryptorchid, INSL3 was tested as the ligand for LGR8. Here, we show that treatment with INSL3 stimulated cAMP production in cells expressing recombinant LGR8 but not LGR7. In addition, interactions between INSL3 and LGR8 were demonstrated following ligand receptor cross-linking. Northern blot analysis indicated that the LGR8 transcripts are expressed in gubernaculum whereas treatment of cultured gubernacular cells with INSL3 stimulated cAMP production and thymidine incorporation. The present study identified the ligand for an orphan G protein-coupled receptor based on common phenotypes of ligand and receptor null mice. Demonstration of INSL3 as the ligand for LGR8 facilitates understanding of the mechanism of testis descent and allows studies on the role of INSL3 in gonadal and other physiological processes.

Purification and characterization of relaxin from the tammar wallaby (Macropus eugenii): bioactivity and expression in the corpus luteum

The objective of this study was to isolate and purify prorelaxin or mature relaxin from the tammar wallaby corpus luteum (CL), determine their structure and bioactivity, and test the hypothesis that enzymatic cleavage of prorelaxin occurs in late gestation. Tammar relaxin peptides were extracted from pooled corpora lutea of late pregnant tammars using a combination of HPLC methods, and they were identified using Western blotting with a human (H2) relaxin antisera and matrix-assisted laser desorption ionization time of flight mass spectrometry. Although no prorelaxin was identified, multiple 6-kDa peptides were detected, which corresponded to the predicted mature tammar relaxin amino acid sequence, with an A chain of 24 amino acids, and different B chain lengths of 28, 29, 30, and 32 amino acids. Tammar relaxin bound with high affinity to rat cortical relaxin receptors and stimulated cAMP production in the human monocytic cell line, THP-1, which expresses the relaxin receptor. Analysis of individual CL indicated that equivalent amounts of mature relaxin peptides were present throughout gestation and also in unmated tammars at equivalent stages of the luteal phase in the nonpregnant cycle. Immunoreactive relaxin was localized specifically to the luteal cells of the CL and the intensity of immunostaining did not vary between gestational stages. These data show that the CL of both pregnant and unmated tammar wallabies produces mature relaxin and suggests that relaxin expression in this species is not influenced by the conceptus. Moreover, the presence of mature relaxin throughout gestation implies that prohormone cleavage is not limited to the later stages of pregnancy