Luteal peptides and their genes as important markers of ovarian differentiation

Secreted peptide hormones and components of the steroidogenic machinery are molecules that are expressed usually in high amounts and in a time- and cell-specific fashion within the cells that give rise to the bovine corpus luteum. They thus serve as useful markers for the events occurring within the nuclei of these cells that result in differentiation and the expression of the specific luteal phenotype. We have studied the bovine genes of three such luteal products: oxytocin, the new relaxin-like factor (RLF), and the steroidogenic acute regulatory protein (StAR). The oxytocin gene is expressed in the granulosal cells of the preovulatory follicle and in the large luteal cells of the immediately resulting early corpus luteum. The RLF gene is a major thecal cell product in antral and atretic follicles. It is also transcribed in luteal cells, but only in the mid- to late ovarian cycle and in pregnancy, following a temporal pattern of expression very similar to that of relaxin in pigs. The StAR gene appears to be upregulated only in the mid- to late ovarian cycle, several days after the increase in steroidogenic enzymes associated with luteinization and progesterone production. All three genes make use of the transcription factor SF-1 (Ad4BP) and, although they all respond to LH activation of adenylate cyclase, none utilize CRE-linked systems. Specific transcriptional activation must involve other factors to encode the information for the widely diverse temporal and cellular patterns of gene expression for these three genes.

Expression and regulation of relaxin-like factor gene transcripts in the bovine ovary: differentiation-dependent expression in theca cell cultures

The relaxin-like factor (RLF) was recently discovered as a new member of the insulin-insulin-like growth factor-relaxin family of growth factors and hormones predominantly in the Leydig cells of the testis. In cattle, in contrast to other species, the RLF gene is also expressed to a high level in the ovary, where its expression pattern in the corpus luteum of the late cycle and pregnancy is similar to that of relaxin in the pig. The RLF gene was also transcribed to a high level in the theca cells of estrogen-rich, large antral follicles. Long-term primary cultures of bovine theca cells showed that expression was insulin dependent. After an initial decline in specific mRNA concentrations, there was a switch to a transcript with a longer poly(A) tail at about Day 6 of culture, which continued to increase to very high levels by Day 15 of culture. Addition of fetal calf serum to cultures caused a rapid and irreversible down-regulation of the RLF gene. Also, LH caused a decline in specific gene expression in long-term primary theca cell cultures. As in the Leydig cells of the testis, the pattern of RLF gene expression appears to reflect the differentiation state of the ovarian theca-luteal cell lineage, and should prove useful for mapping the fate of these cells under differing stimulation regimes.

Bovine endometrial epithelial cells as a model system to study oxytocin receptor regulation

Endometrial epithelial cell cultures were established from bovine uterine tissue collected during the oestrous cycle from commercially slaughtered animals. These cells were shown to express moderately high levels of oxytocin receptors (OTR) (up to 30000 per cell) after about one week in culture. These receptors have been characterized at the molecular, pharmacological and functional level and shown to be identical to those expressed in the bovine endometrium in vivo. Preliminary experiments to investigate the regulation of the OTR and its gene using this system, have shown that expression is to a large degree constitutive, the receptors being spontaneously upregulated during culture. Sex steroids at concentrations close to or above the serum limits observed in vivo appeared to have no effect, although the cells were shown to express mRNA for the specific steroid receptors throughout culture. Only the blastocyst product, interferon-tau, showed a significant effect, downregulating both OTR and their gene transcripts in the cultured endometrial epithelial cells. Although more extensive studies are necessary, these results support the view that the OTR gene is controlled in part at least by a combination of constitutive and inhibitory elements.

Oxytocin and male reproductive function

In the male mammal, the small peptide hormone oxytocin is produced in similar quantities within the hypothalamo-pituitary magnocellular system as in the female, yet for the male little is known about the physiology associated with this hormone. The present review summarizes what is known about the function of oxytocin in the male mammal and tries to take account of both central and systemic effects, and those linked with a local production of oxytocin within the male reproductive organs. In several species a pulse of systemic oxytocin, presumably of hypothalamic origin, appears to be associated with ejaculation. The systemic hormone could act peripherally stimulating smooth muscle cells of the male reproductive tract, but could also reflect central effects in the brain modulating sexual behaviour. In addition to systemic oxytocin, the peptide is also made locally within the testis, and possibly also the epididymis and prostate. In the former tissue it appears to have an autocrine/paracrine role modulating steroid metabolism, but may in addition be involved in contractility of the seminiferous tubules. However, the latter function may involve the mediacy of Sertoli cells which under some circumstances can also exhibit the components of a local oxytocin system. In the prostate of the rat and the dog oxytocin is linked again to steroid metabolism and may also act as a growth regulator. Finally, oxytocin in seminal fluid is discussed and its possible role in respect to the fate of the semen following ejaculation.

The role of N-terminal glycosylation in the human oxytocin receptor

The human oxytocin receptor includes three N-glycosylation sites in its extracellular N-terminal domain. We have established permanent cell-lines in which the gene for the human oxytocin receptor (OTR) has been introduced into HeLa cells. These cells differ by the disruption of one or more of the N-terminal N-glycosylation sites by site-directed mutagenesis of the transfected OTR constructs. The binding capacity of each transfectant, calculated per mg membrane protein, was 5-17 times higher than that of human term myometrium. The pharmacological characteristics of the transfected wild-type OTR are very similar to those of native myometrial OTR. The mutation of N-glycosylation sites (Asn-X-Ser/Thr), namely OTR-D8N15N26 (Asn8-->Asp8), -N8D15N26(Asn15-->Asp15), -N8D15D26(Asn15-->Asp15, Asn26-->Asp26) and -D8N15D26 (Asn8-->Asp8, Asn26-->Asp26) appear to affect neither their dissociation constant (Kd), nor the affinities for various oxytocin related ligands. As a high level of cell surface binding was retained for each clone, receptor trafficking appears to be normal. This suggests that the full glycosylation of OTR observed in vivo is not essential for its activity. These results indicate also that these cell lines may prove very useful for pharmacological screening of oxytocin related products.

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

Relaxin-like factor (RLF) is a new member of the insulin/insulin-like growth factor family of hormones and growth factors, which appears to be predominantly expressed in the Leydig cells of the testis. An analysis of male and female bovine tissues indicated that in the cow the RLF gene is also highly expressed in the female, mainly in the follicular theca interna and in the corpus luteum, with a pattern of gene expression very similar to that for the related peptide relaxin in other species. Sequence analysis of bovine testicular and luteal RLF and cDNA shows that the same gene is expressed in both male and female gonads. Because the bovine RLF sequence, like those from pigs, humans, and mice, retains, the putative receptor binding motif described for relaxin, it seems plausible that RLF might functionally substitute for relaxin in the cow, in which the latter peptide appears not to be significantly expressed.

Structure and expression of the bovine oxytocin receptor gene

The gene for the bovine oxytocin receptor has been sequenced using a combination of clones derived from a bovine endometrial cDNA library from estrus and a bovine genomic DNA library, with confirmation of structure using reverse transcription PCR programmed by term myometrial RNA. The receptor belongs to the seven transmembrane domain family and predicts a protein of 391 amino acids. A comparison of the genomic sequence with the cDNA structure, as well as reverse transcription polymerase chain reaction (RT-PCR) analysis, shows there are two introns, one in the 5'noncoding region that appears to be differentially spliced in the bovine uterus and a conserved intron within the open reading frame between the regions encoding the transmembrane domains VI and VII. Northern blot analysis indicated three major transcripts in myometrium and endometrium in vivo at approximately 6.5 kb, 3.5 kb, and 2.0 kb. In situ hybridization analysis of uterine tissue at term showed highest mRNA concentrations in the endometrial epithelium, particularly in the deep glands, a pattern confirmed also at the immunohistochemical level by monoclonal antibodies raised against a human amino-terminal peptide. Further confirmation of the identity of the receptor was obtained by transient transfection of a reconstituted receptor construct into COS-7 cells. The expressed receptor was shown to have identical pharmacological properties in respect to various oxytocin analogs to the natural bovine endometrial receptor.