The role of the endometrium in endocrine regulation of the animal oestrous cycle

A century of research on the uteroovarian pathway for uterine-induced luteolysis in mammals

Year 2023 is the 100-year anniversary of the discovery in guinea pigs that the lifespan of the corpus luteum (CL) is controlled by the uterus. The CL is the gatekeeper between two fundamental reproductive events - the estrous cycle and pregnancy. Uteroluteal research for the initial 33 years was productive but limited to laboratory species until the inclusion of farm animals in 1956. In the early 1960s, it was found that uterine luteolysin in sows travels unilaterally from a uterine horn to the adjacent CL which likely accounted for the heyday of uteroluteal research in the 1960s-70s. The luteolytic properties of PGF2α were demonstrated in rats in 1969. In 1971, (1) surgical separation of the lengthwise adherence between the uteroovarian vein and ovarian artery interfered with luteolysis in ewes, (2) species with primarily unilateral vs systemic uterine-induced luteolysis have a strong vs an absent or weak unilateral venoarterial transfer pathway, and (3) vascular infusions identified PGF2α as a uterine luteolysin. Vascular and PGF2α studies were beginning to merge. In 1973, a venoarterial pathway was firmly demonstrated in ewes and later in heifers by surgical anastomosis of a uterine vein or ovarian artery from a uterine-intact side to the corresponding vessel on the unilaterally hysterectomized side. More recent studies described how prostaglandins likely transfer through the walls of uterine and ovarian vessels using concentration gradients in sows and a prostaglandins transporter system in cows.

[Initiation and endocrine control of parturition in domestic mammals - Part 1]

Endocrine regulation of parturition is based on an intense exchange of signals between the fetus, placenta and mother. Apart from sheep, our knowledge of the endocrine control of parturition is still very incomplete. However, current observations suggest significant differences between the species. For the maintenance of pregnancy, progesterone (P4) is the crucial superordinate regulatory factor, although in some species, such as the horse, functions of P4 are at least partially fulfilled by other progestogens. In general, prepartum P4 withdrawal is considered a prerequisite for the onset of physiological birth. In species with exclusive (dog) or predominant (e. g., cattle, goat, pig) luteal P4 at the end of gestation, luteolysis is the crucial event. In sheep, where P4 is of placental origin prior to parturition, the prepartum P4 decline is due to a switch in placental steroid metabolism. The mechanism of prepartum progestogen withdrawal in the mare is still largely unclear. In sheep, initiation of parturition proceeds from maturation of the fetal hypothalamic-pituitary-adrenal (HPA) axis, which leads to a steep prepartum rise in fetal cortisol concentrations stimulating the collapse of placental P4 production. In cattle, fetal cortisol probably triggers luteolysis via stimulation of placental prostaglandin secretion. In several other domestic mammalian species, there is also evidence that the initiation of parturition proceeds from maturation of the fetal HPA axis. However, the functional relationships between fetal cortisol and prepartum P4 withdrawal are largely unknown in nonruminant species.

Prostacyclin Synthesis and Prostacyclin Receptor Expression in the Porcine Myometrium: Prostacyclin Potential to Regulate Fatty Acid Transporters, Cytokines and Contractility-Related Factors

Simple Summary

Prostacyclin (prostaglandin I2; PGI2) is an important modulator of vascular functions and is involved in various reproductive processes. PGI2 was also described as a modulator of uterine contractility in several species, including the pig. However, its synthesis and role in the myometrium of the porcine uterus are still not fully described. The objective of this study was to evaluate profiles of PGI2 synthesis and PGI2 receptor expression in the myometrium of gilts throughout the estrous cycle and during early pregnancy and to investigate the in vitro effect of PGI2 on the mRNA expression of factors engaged in smooth muscle contraction, nutrient transport, prostaglandin synthesis and action, and inflammatory response. The obtained results showed that the synthesis of PGI2 changes in the myometrium of pigs during both the estrous cycle and early pregnancy, resulting in much greater concentrations of PGI2 in cyclic than in pregnant gilts. Moreover, PGI2 stimulated the expression of fatty acid transporters and contractility-related calponin 1 and caldesmon 1, whereas it decreased cytokine expression. This study indicates that PGI2 may participate in the regulation of myometrial functions modulating the availability of factors involved in smooth muscle activity and inflammatory reaction in the uterus of pigs.

Abstract

Although prostacyclin (PGI2) has been well described as a regulator of smooth muscle activity, limited data are available concerning its role in the myometrium of pigs. The present research aimed to examine profiles of PGI2 synthase (PTGIS) and PGI2 receptor (PTGIR) expression and 6-keto PGF1α (a PGI2 metabolite) concentrations in the myometrium of gilts throughout the estrous cycle and during early pregnancy using qPCR, Western blot, and/or ELISA methods. Furthermore, myometrial explants were exposed to iloprost (a stable PGI2 analog) to investigate the effect of PGI2 on the mRNA expression of factors engaged in smooth muscle contraction, nutrient transport, prostaglandin synthesis and action, and inflammatory response. PTGIS mRNA expression was greater in cyclic than in pregnant gilts on days 11–12 after estrus and was accompanied by greater concentrations of 6-keto PGF1α detected in cyclic than in pregnant animals on days 11–20. Iloprost stimulated fatty acid transporters and contractility-related calponin 1 and caldesmon 1 mRNA expression and decreased interleukin 1β and tumor necrosis factor transcript abundance. The obtained results indicate a physiologically relevant role of PGI2 during the estrous cycle in the porcine myometrium with its importance for regulating the expression of contractility-, nutrient transport- and inflammatory response-related factors.

Difference in expression between AQP1 and AQP5 in porcine endometrium and myometrium in response to steroid hormones, oxytocin, arachidonic acid, forskolin and cAMP during the mid-luteal phase of the estrous cycle and luteolysis

BACKGROUND

Recently, we demonstrated in vitro that AQP1 and AQP5 in the porcine uterus are regulated by steroid hormones (P4, E2), arachidonic acid (AA), forskolin (FSK) and cAMP during the estrous cycle. However, the potential of the porcine separated uterine tissues, the endometrium and myometrium, to express these AQPs remains unknown. Thus, in this study, the responses of AQP1 and AQP5 to P4, E2 oxytocin (OT), AA, FSK and cAMP in the porcine endometrium and myometrium were examined during the mid-luteal phase of the estrous cycle and luteolysis.

METHODS

Real-time PCR and western blot analysis.

RESULTS

Progesterone up-regulated the expression of AQP1/AQP5 mRNAs and proteins in the endometrium and myometrium, especially during luteolysis. Similarly, E2 also stimulated the expression of both AQPs, but only in the endometrium. AA led to the upregulation of AQP1/AQP5 in the endometrium during luteolysis. In turn, OT increased the expression of AQP1/AQP5 mRNAs and proteins in the myometrium during mid-luteal phase. Moreover, a stimulatory effect of forskolin and cAMP on the expression of AQP1/AQP5 mRNAs and proteins in the endometrium and myometrium dominated during luteolysis, but during the mid-luteal phase their influence on the expression of these AQPs was differentiated depending on the type of tissue and the incubation duration.

CONCLUSIONS

These results seem to indicate that uterine tissues; endometrium and myometrium, exhibit their own AQP expression profiles in response to examined factors. Moreover, the responses of AQP1/AQP5 at mRNA and protein levels to the studied factors in the endometrium and myometrium are more pronounced during luteolysis. This suggests that the above effects of the studied factors are connected with morphological and physiological changes taking place in the pig uterus during the estrous cycle.

Progesterone, estradiol, arachidonic acid, oxytocin, forskolin and cAMP influence on aquaporin 1 and 5 expression in porcine uterine explants during the mid-luteal phase of the estrous cycle and luteolysis: an in vitro study

BACKGROUND

The cell membrane water channel protein, aquaporins (AQPs), regulate cellular water transport and cell volume and play a key role in water homeostasis. Recently, AQPs are considered as important players in the field of reproduction. In previous studies, we have established the presence of AQP1 and 5 in porcine uterus. Their expression at protein level altered in distinct tissues of the female reproductive system depending on the phase of the estrous cycle. However, the regulation of aquaporin genes and proteins expression has not been examined in porcine uterine tissue. Therefore, we have designed an in vitro experiment to explain whether steroid hormones, progesterone (P4) and estradiol (E2), and other factors: oxytocine (OT), arachidonic acid (AA; substrate for prostaglandins synthesis) as well as forskolin (FSK; adenylate cyclase activator) and cAMP (second messenger, cyclic adenosine monophosphate) may impact AQPs expression.

METHODS

Uterine tissues were collected on Days 10-12 and 14-16 of the estrous cycle representing the mid-luteal phase and luteolysis. Real-time PCR and Western blot analysis were performed to examine the expression of porcine AQP1 and AQP5. Their expression in the uterine explants was also evaluated by immunohistochemistry.

RESULTS

The results indicated that uterine expression of AQP1 and AQP5 potentially remains under control of steroid hormones and AA-derived compounds (e.g. prostaglandins). P4, E2, AA, FSK and cAMP cause translocation of AQP5 from apical to the basolateral plasma membrane of the epithelial cells, which might affect the transcellular water movement (through epithelial cells) between uterine lumen and blood vessels. The AC/cAMP pathway is involved in the intracellular signals transduction connected with the regulation of AQPs expression in the pig uterus.

CONCLUSIONS

This study documented specific patterns of AQP1 and AQP5 expression in response to P4, E2, AA, FSK and cAMP, thereby providing new indirect evidence of their role in maintaining the local fluid balance within the uterus during the mid-luteal phase of the estrous cycle and luteolysis in pigs.

VEGFR-3 neutralization inhibits ovarian lymphangiogenesis, follicle maturation, and murine pregnancy

Lymphatic vessels surround follicles within the ovary, but their roles in folliculogenesis and pregnancy, as well as the necessity of lymphangiogenesis in follicle maturation and health, are undefined. We used systemic delivery of mF4-31C1, a specific antagonist vascular endothelial growth factor receptor 3 (VEGFR-3) antibody to block lymphangiogenesis in mice. VEGFR-3 neutralization for 2 weeks before mating blocked ovarian lymphangiogenesis at all stages of follicle maturation, most notably around corpora lutea, without significantly affecting follicular blood angiogenesis. The numbers of oocytes ovulated, fertilized, and implanted in the uterus were normal in these mice; however, pregnancies were unsuccessful because of retarded fetal growth and miscarriage. Fewer patent secondary follicles were isolated from treated ovaries, and isolated blastocysts exhibited reduced cell densities. Embryos from VEGFR-3-neutralized dams developed normally when transferred to untreated surrogates. Conversely, normal embryos transferred into mF4-31C1-treated dams led to the same fetal deficiencies observed with in situ gestation. Although no significant changes were measured in uterine blood or lymphatic vascular densities, VEGFR-3 neutralization reduced serum and ovarian estradiol concentrations during gestation. VEGFR-3-mediated lymphangiogenesis thus appears to modulate the folliculogenic microenvironment and may be necessary for maintenance of hormone levels during pregnancy; both of these are novel roles for the lymphatic vasculature.

Distribution and quantitative changes in amounts of aquaporin 1, 5 and 9 in the pig uterus during the estrous cycle and early pregnancy

BACKGROUND

Aquaporins (AQPs) are a family of membrane channel proteins that facilitate bulk water transport. To date, 11 isoforms of AQPs have been reported to be expressed in the female and male reproductive systems. The purpose of our study was to determine the localization and quantitative changes in the expression of AQP1, 5 and 9 within the pig uterus during different stages of the estrous cycle and early pregnancy.

METHODS

Immunoperoxidase and semi-quantitative immunoblotting techniques were used to examine the distribution and changes in amounts of AQP1, AQP5 and AQP9 in uteral cells of pigs at the early (Days 2-4), middle (10-12), late (14-16) stage of the luteal phase and late (18-20) stage of the follicular phase of the estrous cycle as well as on Days 14-16 and 30-32 of gestation (the onset and the end of implantation process).

RESULTS

The results demonstrated that AQP1, 5, and 9 were clearly detected in all studied stages of the estrous cycle and pregnancy. AQP1 was localized within uterine blood vessels. In cyclic gilts, endometrial and myometrial expression of AQP1 protein did not change significantly but increased during gestation. AQP5 was localized in smooth muscle cells and uterine epithelial cells. Endometrial expression of AQP5 protein did not change significantly between Days 2-4 and 10-12 of the estrous cycle but increased on Days 14-16 and 18-20 as well as during early pregnancy. Myometrial expression of AQP5 did not differ significantly during the estrous cycle but increased in the pregnancy. The anti-AQP9 antibody labeled uterine epithelial cells of uterus. Endometrial expression of AQP9 did not change significantly between Days 2-4 and 10-12 of the estrous cycle but increased on Days 14-16 and 18-20 as well as during early pregnancy.

CONCLUSIONS

The results suggest that a functional and distinctive collaboration exists among diverse AQPs in water handling during the different uterine phases in the estrous cycle and early pregnancy.

Secretion of estradiol-17beta by porcine endometrium and myometrium during early pregnancy and luteolysis

Past studies of the source of estrogens secreted during maternal recognition of pregnancy in pigs have focused on embryonic rather than uterine origin of these steroids. The present study documents: (1) the expression of the gene CYP 17, encoding cytochrome P450 17alpha-hydroxylase/C(17-20) lyase and (2) the synthesis and secretion of estradiol-17 beta (E(2)) in endometrial and myometrial tissues in gilts. The expression of CYP 17 gene was shown in porcine endometrium and myometrium. Basal endometrial secretion of E(2) was higher in pregnant gilts than in cyclic gilts (days 14-16). The myometrium secreted more E(2) during the expected time of luteolysis compared to early pregnancy. Basal secretion of E(2) during pregnancy was higher from the endometrium than from the myometrium. Conversely, during luteolysis E(2) secretion was higher from the myometrium and lower from the endometrium. In pregnant and cyclic gilts (days 14-16), progesterone (P(4), 10(-5)M) in vitro significantly increased E(2) secretion regardless of reproductive status. Oxytocin (OT, 10(-7)M) had no influence on E(2) secretion and did not change the stimulatory effect of P(4) in both tissues examined. In conclusions: (1) the CYP 17 gene transcript is present in porcine endometrium and myometrium; (2) porcine endometrium and myometrium release E(2) in vitro; (3) the endometrium releases more E(2) than the myometrium during early pregnancy; (4) the myometrium releases E(2) mainly during luteolysis; (5) the endometrium and myometrium can increase E(2) release in vitro if substrate (P(4)) is provided during early pregnancy and luteolysis. These data suggest active estrogen production by the myometrium and endometrium as an alternative source for this signal for recognition of pregnancy in the pig.