The gene encoding the precursor protein to the hormone oxytocin and its associated neurophysin has been isolated from a rat genomic library, and its sequence has been determined. The small gene (approximately equal to 850 base pairs) predicts a mRNA of approximately equal to 500 bases [without the poly(A) tail]. The exon-intron organization is similar to that of the vasopressin gene, with two splice sites in the protein-coding region. The first exon (A) comprises the 5' noncoding promoter region, a putative signal peptide, the nonapeptide hormone oxytocin, and the NH2-terminal, variable region of neurophysin. The second exon (B) encodes the central, conserved region of neurophysin, and the third exon (C) encodes the remaining COOH terminus of neurophysin, with an additional arginine residue at its end, presumably cleaved off during post-translational processing. A stretch of 143 nucleotides within exon B, except for a single base change, is entirely homologous to the equivalent part of the rat vasopressin gene, offering support for a gene conversion event having recently affected the two genes.

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.

Insulin-like factor 3 (INSL3) is a small peptide hormone made and secreted uniquely by mature Leydig cells in the testes of all mammals. Importantly, this expression and secretion appears to be constitutive and therefore reflects the differentiation status and number of the Leydig cells present, differing thereby from testosterone, which is acutely and homeostatically regulated by the hormones of the hypothalamic-pituitary-gonadal axis. As a consequence, the measurement of INSL3 either as mRNA in the testis or as secreted peptide circulating in the blood provides an excellent assessment of Leydig cell differentiation, for example, during fetal development, puberty, or aging or following exposure to endocrine-disrupting agents. Whereas INSL3 is proving increasingly useful as a biomarker for testis status, less is known about its functions, particularly in the adult male. Current evidence points to autocrine, paracrine, and endocrine roles, acting through the G-protein-coupled receptor called RXFP2, although more research is required to characterize these functions in detail.

Insulin-like factor 3 (INSL3) serum levels were measured in 135 andrologically well-characterized normal men and 85 patients with testicular disorders to investigate how the hormone, which is a major secretory product of human Leydig cells, is related to testosterone (T), LH, and semen quality. INSL3 was measured by using a newly developed fluorescence immunoassay.Median (2.5–97.5 percentiles) INSL3 serum levels were as follows: normal men (n = 135), 0.99 (0.55–1.73) ng/ml; infertile men (n = 23), 1.11 (0.60–2.07) ng/ml; anorchid men (n = 21), nondetectable (ND); patients with 47, XXY, Klinefelter syndrome (n = 21), 0.12 (ND–0.78) ng/ml; men with hypogonadotropic hypogonadism and T substitution (n = 11), ND; and men with hypogonadotropic hypogonadism and human chorionic gonadotropin (hCG) treatment (n = 5), 0.36 (0.13–0.73) ng/ml. Before testicular biopsy, two infertile men had blood samples drawn directly from vena spermatica. Here, the serum INSL3 levels were 15-fold higher than in serum from peripheral blood samples (13.84 and 14.00 ng/ml, respectively). In two unilaterally orchiectomized former testis cancer patients, who underwent hCG stimulation test, INSL3 serum levels were unchanged 72 and 96 h after hCG stimulation. In conclusion, we provide a normal range for INSL3 serum levels in adult men and show that the majority, if not all, circulating INSL3 derives from the testes. Furthermore, our data strongly indicate that INSL3 secretion is dependent on the differentiating effect of LH on Leydig cells but independent of the steroidogenic LH-mediated action. Thus, even though T and INSL3 are both dependent on LH, these two Leydig cell hormones are regulated differently.

Human epididymal protein 6 (HE6; also known as GPR64) is an orphan member of the LNB-7TM (B₂) subfamily of G-protein-coupled receptors. Family members are characterized by the dual presence of a secretin-like (type II) seven-transmembrane (7TM) domain and a long cell adhesion-like extracellular domain. HE6 is specifically expressed within the efferent ductules and the initial segment of the epididymis, ductal systems involved in spermatozoon maturation. Here, we report that targeted deletion of the 7TM domain of the murine HE6 gene results in male infertility. Mutant mice reveal a dysregulation of fluid reabsorbtion within the efferent ductules, leading to a backup of fluid accumulation in the testis and a subsequent stasis of spermatozoa within the efferent ducts. The fertility phenotype of HE6 knockout mice identifies this receptor as a potential nonsteroidal, nontesticular target for future male contraceptives and identifies an in vivo function for a member of this unusual gene family.

The possession of a scrotum to contain the male gonads is a characteristic feature of almost all mammals, and appears to have evolved to allow the testes and epididymis to be exposed to a temperature a few degrees below that of core body temperature. Analysis of cryptorchid patients, and those with varicocele suggest that mild scrotal warming can be detrimental to sperm production, partly by effects on the stem cell population, and partly by effects on later stages of spermatogenesis and sperm maturation. Recent studies on the effects of clothing and lifestyle emphasize that these can also lead to chronically elevated scrotal temperatures. In particular, the wearing of nappies by infants is a cause for concern in this regard. Together all of the evidence indirectly supports the view that lifestyle factors in addition to other genetic and environmental influences could be contributing to the secular trend in declining male reproductive parameters. The challenge will be to provide relevant and targeted experimental results to support or refute the currently circumstantial evidence.