Prolactin receptors in the mammary gland, corpus luteum and other tissues of the tammar wallaby, Macropus eugenii

Placental expression of pituitary hormones is an ancestral feature of therian mammals

BACKGROUND

The placenta is essential for supplying nutrients and gases to the developing mammalian young before birth. While all mammals have a functional placenta, only in therian mammals (marsupials and eutherians) does the placenta closely appose or invade the uterine endometrium. The eutherian placenta secretes hormones that are structurally and functionally similar to pituitary growth hormone (GH), prolactin (PRL) and luteinizing hormone (LH). Marsupial and eutherian mammals diverged from a common ancestor approximately 125 to 148 million years ago and developed distinct reproductive strategies. As in eutherians, marsupials rely on a short-lived but functional placenta for embryogenesis.

RESULTS

We characterized pituitary GH, GH-R, IGF-2, PRL and LHβ in a macropodid marsupial, the tammar wallaby, Macropus eugenii. These genes were expressed in the tammar placenta during the last third of gestation when most fetal growth occurs and active organogenesis is initiated. The mRNA of key growth genes GH, GH-R, IGF-2 and PRL were expressed during late pregnancy. We found significant up-regulation of GH, GH-R and IGF-2 after the start of the rapid growth phase of organogenesis which suggests that the placental growth hormones regulate the rapid phase of fetal growth.

CONCLUSIONS

This is the first demonstration of the existence of pituitary hormones in the marsupial placenta. Placental expression of these pituitary hormones has clearly been conserved in marsupials as in eutherian mammals, suggesting an ancestral origin of the evolution of placental expression and a critical function of these hormones in growth and development of all therian mammals.

Prolactin acts on the hypothalamic-pituitary axis to modulate follicle-stimulating hormone gene expression in the female brushtail possum (Trichosurus vulpecula)

Brushtail possums exhibit a distinct preovulatory pattern of prolactin (Prl) secretion suggesting that Prl is involved in normal reproductive function. In some mammals, Prl is essential for corpus luteum (CL) function and/or modulation of steroidal effects on hypothalamic-pituitary activity. The aim of this study was to test the effects of biologically active recombinant possum Prl (recPosPrl) on both pituitary gland and CL function in possums. To confirm biological activity, administration of recPosPrl-N2C1 (10 μg) resulted in an 18-fold stimulation (P<0.05) of progesterone (P(4)) production by possum granulosa cells in vitro. Based on these findings, minipumps containing either recPosPrl-N2C1 (n=10) or saline (n=8) were inserted into lactating female possums. The expression levels of pituitary-derived PRL, LHB, FSHB and GNRHR and CL-derived LHR mRNA were quantified. Following a resumption of reproductive activity, no differences in ovulation incidence or plasma Prl concentrations were observed. Plasma Prl levels were less variable (P<0.001) in Prl-treated possums, confirming a self-regulatory role for Prl in this species. There was a marked down-regulation (P<0.001) of FSHB mRNA at the mid-luteal stage in Prl-treated possums, whereas mean PRL, LHB, GNRHR and LHR mRNA expression levels were not different between experimental groups. Plasma P(4) concentrations were not different (P=0.05) in Prl-treated possums, although tended to be higher in the peri-ovulatory and early-luteal phase. We conclude in the brushtail possum that Prl is self-regulated via a short-feedback loop common to all mammals studied and is able to modulate FSHB expression probably at the level of the hypothalamus and/or pituitary gland.

Progesterone and reproduction in marsupials: a review

Progesterone (P4) profiles throughout pregnancy and the oestrous cycle are reviewed in a wide range of marsupial species, representing 12 Families, and focus on the corpus luteum (CL) and its functioning, compared with its eutherian counterpart. Physiologically, P4 subtends the same fundamental processes supporting gestation in marsupials as it does in eutherian mammals, from its role in stimulating the secretory endometrium, effecting nutritional transfer across the placenta and establishing lactogenesis. Before the formation of the CL, however, secretion of P4 is widespread throughout many Families and the dual roles of P4 in the induction of sexual behaviour and ovulation are exposed. In Dasyuridae, raised levels of P4 are linked with the induction of sexual receptivity and are also present around the time of mating in Burramyidae, Petauridae and Tarsipedidae, but their function is unknown. Only in Didelphidae has research established that the pheromonally-induced levels of pro-oestrous P4 trigger ovulation. This is principally the role of oestradiol in the eutherian and may be an important difference between the marsupial and the eutherian. The deposition of the shell coat around the early marsupial embryo is also a function of P4, but perhaps the most striking difference is seen in the time taken to form the CL. This is not always immediate and the maximum secretion of P4 from the granulosa cells may not occur until some 2 weeks after ovulation. The slower development of the CL in some species is linked with delays in the development of the embryo during its unattached phase and results in relatively long gestation periods. A common feature of these, in monovular species, is a short pulse of P4 from the newly-luteinised CL, which is all that is needed for the subsequent development of the embryo to term. Maternal recognition of pregnancy occurs soon after the formation of the blastocyst, with embryo-induced changes in ovarian production of P4 and the uterine endometrium. The embryo, similar to the eutherian, determines the length of the gestation period and initiates its own birth, but in direct contrast, the embryo of some marsupial species shortens the life-span of the CL. The evidence points to a different strategy; one of a reduction, rather than an expansion of the potential ovarian and placental support available during pregnancy. The marsupial mode of reproduction, where all species produce highly altricial young, receiving complex and extensive maternal care, has facilitated the adaptive radiation of this group and avoided the need for precociality.

Society for Reproductive Biology Founders' Lecture 2006 - life in the pouch: womb with a view

Marsupials give birth to an undeveloped altricial young after a relatively short gestation period, but have a long and sophisticated lactation with the young usually developing in a pouch. Their viviparous mode of reproduction trades placentation for lactation, exchanging the umbilical cord for the teat. The special adaptations that marsupials have developed provide us with unique insights into the evolution of all mammalian reproduction. Marsupials hold many mammalian reproductive 'records', for example they have the shortest known gestation but the longest embryonic diapause, the smallest neonate but the longest sperm. They have contributed to our knowledge of many mammalian reproductive events including embryonic diapause and development, birth behaviour, sex determination, sexual differentiation, lactation and seasonal breeding. Because marsupials have been genetically isolated from eutherian mammals for over 125 million years, sequencing of the genome of two marsupial species has made comparative genomic biology an exciting and important new area of investigation. This review will show how the study of marsupials has widened our understanding of mammalian reproduction and development, highlighting some mechanisms that are so fundamental that they are shared by all today's marsupial and eutherian mammals.

Prolactin in the brushtail possum (Trichosurus vulpecula): development of homologous radioimmunoassay using recombinant possum prolactin

We report the production of recombinant possum prolactin (posPrl), and its use in the development and validation of a highly specific homologous radioimmunoassay for the measurement of prolactin (Prl) in brushtail possums. This enabled the subsequent investigation of some basic mechanisms involved in the regulation of Prl secretion in this species. Recombinant posPrl spanning the entire coding region was expressed in Escherichia coli, resulting in a 199 amino acid protein with a molecular weight approximately 23 kDa. The potency of posPrl was 45.3 +/- 4.8% that of ovine Prl in a radioreceptor assay using possum mammary gland receptors and induced a 3.4 +/- 0.8-fold increase in progesterone secretion in primary possum granulosa cells. Antiserum (G27) was raised against recombinant posPrl and was highly specific for possum Prl (approximately 30% binding at 1:60,000 final dilution), and exhibited negligible cross-reactivity (<0.0001%) with possum growth hormone. Serial dilutions of pituitary gland extracts, and plasma samples from male and female possums gave parallel inhibition curves to recombinant posPrl standards in the assay. Biological validation of the RIA included treating possums with drugs known to alter Prl secretion in other mammals. In seasonally anoestrous female possums, administration of 20 microg thyrotropin-releasing hormone (TRH) resulted in a 15-fold increase (P < 0.01) in plasma Prl concentrations. In mid-late lactating female possums, a bolus of cabergoline (dopamine agonist; 75 microg) reduced (P < 0.05) plasma Prl levels to baseline for 24 h, while repeated administration (6 x 75 microg at 12 h intervals) suppressed (P < 0.01) plasma Prl concentrations until 24h after the last injection. Prolonged inhibition of Prl levels subsequently caused marked (P < 0.01) attenuation in rate of bodyweight increase of pouch young. The amplitude of the Prl surge in response to a bolus of TRH (15 microg) was 5-fold lower in cabergoline-treated, compared to control mid-late lactating possums. In conclusion, we report the development and validation of a robust and sensitive RIA for measuring Prl concentrations in the plasma of brushtail possums.

The corpus luteum and interstitial tissue in a marsupial, the brushtail possum (Trichosurus vulpecula)

The Australian brushtail possum (Trichosurus vulpecula) is a nocturnal, arboreal marsupial. It has become a pest of significant ecological and economic importance in New Zealand, and thus a renewed interest in understanding the reproductive biology of this species has been generated. The corpus luteum (CL) in possums is a largely autonomous gland in that it does not rely on pituitary hormones to function and is not responsive to luteolytic agents for its demise. Its importance in regulating the oestrous cycle and pregnancy has been established; however, little is known regarding the mechanisms involved in its function. Interstitial tissue (IT) is a prominent feature found throughout the ovarian stroma, yet little is known regarding the origin or function of these cells. Based on histological examinations, our data support the hypothesis that interstitial tissue arises from a unique cell type called medullary cords during early ovarian development. Using possum-specific probes for proteins involved in steroidogenesis, receptors for pituitary hormones and members of the TGF-beta superfamily we have initiated studies investigating the expression of genes that may be important in the function and regulation of the CL and interstitial tissue. Results show that both tissues are steroidogenic and that both express receptors for prolactin and luteinising hormone (LH). Collectively these findings suggest that prolactin and LH may be important in the regulation of steroidogenesis in the CL and interstitial tissue in possums.

Diapause

Embryonic diapause, or delayed implantation as it is sometimes known, is said to occur when the conceptus enters a state of suspended animation at the blastocyst stage of development. Blastocysts may either cease cell division so that their size and cell numbers remain constant, or undergo a period of very slow growth with minimal cell division and expansion. Diapause has independently evolved on many occasions. There are almost 100 mammals in seven different mammalian orders that undergo diapause. In some groups, such as rodents, kangaroos, and mustelids, it is widespread, whereas others such as the Artiodactyla have only a single representative (the roe deer). In each family the characteristics of diapause differ, and the specific controls vary widely from lactational to seasonal, from estrogen to progesterone, or from photoperiod to nutritional. Prolactin is a key hormone controlling the endocrine milieu of diapause in many species, but paradoxically it may act either to stimulate or inhibit growth and activity of the corpus luteum. Whatever the species-specific mechanisms, the ecological result of diapause is one of synchronization: It effectively lengthens the active gestation period, which allows mating to occur and young to be born at times of the year optimal for that species.

The prolactin receptor from the brushtail possum (Trichosurus vulpecula): cDNA cloning, expression and functional analysis

A full length, prolactin receptor cDNA clone has been isolated from the brushtail possum (Trichosurus vulpecula). This clone encodes a 625 amino acid protein which shares 60-70 and 54% sequence identity with prolactin receptor (long form) sequences from mammalian and avian species, respectively. Sequence similarity was highest in the extra-cellular, hormone-binding domain and in specific regions of the intracellular domain which regulates prolactin receptor signalling in cells. Prolactin receptor mRNA was detected in a wide range of possum tissues and in the mammary gland the PRL-R gene was differentially expressed during lactation with peak mRNA levels being detected during the first 6 days of lactation and after day 115 throughout late lactation. This pattern of PRL-R mRNA expression in the mammary gland is similar to that observed for circulating prolactin in the lactating possum. In CHO cells transiently transfected with the possum prolactin receptor, expression of a beta-lactoglobulin promoter/reporter gene construct was increased 3-fold by adding prolactin. The possum prolactin receptor is therefore capable of binding ovine prolactin and activating the Jak2/Stat5 signalling cascade. This provides evidence for the highly conserved nature of the prolactin signalling pathway in mammalian evolution.

Mesotocin receptors during pregnancy, parturition and lactation in the tammar wallaby

Mestocin receptor concentrations in membrane preparations from reproductive tissues of the tammar Macropus eugenii throughout gestation and lactation were assessed using [3H]-oxytocin as the ligand. There was a single binding site which bound both mesotocin and oxytocin with high and similar affinities. Mesotocin receptor concentrations in the myometrium were low (708 +/- 199 fmol mg-1 protein) in early and middle gestation but increased significantly on day 23 of pregnancy of the 26-day gestation period to 1921 +/- 552 fmol mg-1 protein. Myometrial receptors reached a peak of 2483 +/- 575 fmol mg-1 protein on days 25 and 26 of gestation, but returned to basal levels about an hour after birth. Receptor concentrations in the contralateral non-gravid uterus were much lower (605 +/- 75 fmol mg-1) and did not significantly increase throughout the period of gestation but dropped one day before birth. Mesotocin receptors were undetectable in the endometrium, the yolk sac placenta and the lateral, median and anterior vagina of all animals tested. In the lactating mammary gland after birth mesotocin receptors were initially high (588 +/- 38 fmol mg-1) but decreased after 200 days and by late lactation were 224 +/- 55 fmol mg-1 protein on day 240, close to the time of weaning. Mesotocin receptors in the ipsilateral non-lactating gland were also high in early lactation (430 +/- 153 fmol mg-1) and declined in late lactation (62 +/- 20 fmol mg-1). The changing concentrations of mesotocin receptors in pregnancy and lactation demonstrate that they are specifically regulated in tammar reproductive tissues. The increase in mesotocin receptors in gravid, but not in the non-gravid myometrium three days before birth may make the uterus responsive to the surge of mesotocin at birth. Since this rise is unilateral and only occurs in the gravid myometrium it must be due to local effects from the ipsilateral ovary or the feto-placental unit. Likewise, the down-regulation of mesotocin receptors in the contralateral, non-gravid myometrium may be due to its proximity to the developing follicle. The changing concentrations in the lactating and the adjacent, non-lactating mammary gland also reflect a differential regulation of mesotocin receptors, probably mediated via the sucking stimulus. Thus, local influences appear to be of primary importance in the regulation of mesotocin receptors during reproduction in this marsupial.

Diapause, pregnancy, and parturition in Australian marsupials

Marsupial pregnancy is characterised by a long lactation and a relatively short gestation. Marsupials have, in effect, exchanged the umbilical cord for the teat. However, gestation can be extended for up to 11 months by the imposition of a period of developmental arrest known as embryonic diapause. Diapause may be under either lactational or seasonal control, and in the kangaroos and wallabies these effects are mediated by prolactin and melatonin, respectively. At the other end of gestation, namely parturition, it appears that marsupials are fairly typical mammals and require all the same physiological and behavioural cues essential for the delivery of a viable young. Parturition depends on a synchronised cascade of hormonal events triggered by the fetus itself. Prostaglandin and prolactin concentrations pulse around the time of birth and progesterone falls. Successful parturition also depends on the adoption of the appropriate behaviour and birth posture by the mother. Despite the fact that the entire period of gestation is accomplished in such a short time, the neonate has perfectly adapted its growth and development to influence its mother's physiology to induce the change from nurturing the young in its uterus via a placenta, to a precise synchronisation of the birth process resulting in completion of its growth within the pouch sustained by a milk tailor-made for each developmental stage.

Characterization of renal prolactin-binding sites of two amphibians (Ambystoma tigrinum and Rana catesbeiana) and a reptile (Pseudemys scripta elegans)

The specific binding of 125I-ovine prolactin (oPRL) to renal membrane preparations from Pseudemys scripta elegans, Ambystoma tigrinum, and Rana catesbeiana was characterized. All three membranes showed specific oPRL binding that was dependent upon time, temperature, pH, and membrane concentration. Scatchard analysis of the binding of 125I-oPRL revealed high-affinity sites with KA values of 2.1 X 10(10), 3.6 X 10(10), and 1.25 X 10(10) M-1 for kidney membranes of Pseudemys, Ambystoma, and Rana, respectively. In addition, there was a low-affinity site on the Ambystoma membranes. The binding capacities ranged from 31 to 70 fmol/mg of membrane protein. The hormonal specificity of these membranes was studied by competing increasing amounts of oPRL, human growth hormone (hGH) derived from recombinant DNA techniques, bovine (b) GH, and human placental lactogen (hPL) with 125I-oPRL. The oPRL standard and hGH were the most potent competitors in all three assays, although hGH was not as potent as the oPRL. Human PL was moderately active in the turtle kidney assay, weakly active in the Ambystoma radioreceptor assay, and inactive in the bullfrog assay. Bovine GH had low potency on the turtle membranes and was inactive in the amphibian assays. The results of these studies indicate that the characteristics of renal PRL receptors of Pseudemys, Ambystoma, and Rana are similar to those of lactogenic receptors throughout the vertebrates. In addition, these data provide evidence for the first time of renal PRL receptors in a reptile.

Seasonal patterns of circulating progesterone and prolactin and response to bromocriptine in the female tammar Macropus eugenii

Concentrations of prolactin and progesterone in the plasma of female tammars (Macropus eugenii) were measured during lactational quiescence and seasonal quiescence and during the period of natural resumption of reproduction after the summer solstice in December. Prolactin concentrations were consistently low (less than 40 ng/ml) during the period of declining day length and consistently elevated (greater than 40 ng/ml) during the period of increasing day length. Basal levels of progesterone were lowest (118.9 +/- 9.1 pg/ml) at the winter solstice and highest (244.1 +/- 21.0 pg/ml) at the summer solstice. Treatment with bromocriptine (CB154) did not depress prolactin levels at either time of the year but during February to June a single injection of 5 mg/kg body wt induced development of the quiescent corpus luteum (CL) and the diapausing embryo was reactivated. In February and March the level of progesterone increased in association with the developing CL, but after treatment at the winter solstice in June plasma progesterone did not increase although pregnancy was successfully completed. From September through November only 1 of 50 females responded to bromocriptine. None responded to bromocriptine given at the summer solstice in December, but all these animals spontaneously reactivated 2 weeks later. The difference in response of female tammars to bromocriptine treatment in the two halves of the year suggests that different endocrine controls operate in lactational and seasonal quiescence and that the latter is more complex.

Prolactin binding in rat Langerhans islets

Membrane preparations of collagenase-dispersed Langerhans islets of female Wistar rats exhibit specific binding sites for 125I-labelled ovine prolactin (125I-oPrl). Almost negligible binding was detected in islets of male animals. The binding is a saturable and time-temperature dependent process, equilibrium being reached after 16 h incubation at 0 degrees C. The bound oPrl is not displaceable by hFSH, hLH, bGH or hGH. In contrast with other cell fractions, the 12,000 g pellet accounts for more than 80% of the specific binding of 125I-oPrl. Scatchard plots of data obtained in saturation studies indicate a single class of binding sites with Ka = 0.21 x 10(10)M-1. Protein and phospholipid moieties are essential for the receptor activity, since after trypsin or phospholipase C digestions marked loss of binding was verified. In islets of streptozotocin diabetic rats a marked reduction in the number of binding sites was observed. These findings may suggest that some of the actions of prolactin on endocrine pancreas could be explained by its specific interaction with islet cell membranes.