Ruminant placental lactogens: structure and biology

Abstract

Ruminant placental lactogens (PL) are members of the somatotropin, prolactin gene family that are synthesized by trophectodermal binucleate cells. The structure and biology of PL has been studied in the cow, sheep, and goat. Ruminant PL have greater structural identity to prolactin than somatotropin, although they bind to both lactogenic and somatogenic receptors. The molecular weights of ovine and caprine PL are approximately 23,000, whereas bovine PL is larger (31,000 to 34,000) due to glycosylation. Placental lactogen is secreted into both the fetal and maternal circulations. The concentration of PL in the fetus decreases with advancing gestation, whereas PL concentration peaks in the maternal circulation during the last third of pregnancy then reaches a plateau. Furthermore, the maternal concentration of PL is 100- to 1,000-fold higher in sheep and goats than in cows. The precise factors that modulate secretion of PL are unknown, although placental mass and nutrition seem to play a role. Ruminant PL have both lactogenic and somatogenic biological activities and may also have unique activities mediated through a specific receptor. There is circumstantial evidence to suggest that PL plays a role in stimulating mammogenesis. Placental lactogen secreted into the fetal compartment may also help regulate fetal growth. Direct experimental data indicate that PL can regulate maternal intermediary metabolism. Thus, it may act as a partitioning agent to regulate nutrient supply for fetal growth. The precise biological function of PL in ruminants, therefore, still needs to be defined.

Expression profile and protein levels of placental products as indirect measures of placental function in in vitro-derived bovine pregnancies

“…Although its function and secretory control are not completely understood, in vivo and in vitro studies have suggested PL to have multiple somatogenic (GH-like) and lactogenic (PRLlike) biological effects, depending on the species, such as placental angiogenesis, maternal and foetal intermediate metabolism, mammary gland development and function, ovarian and placental steroidogenesis, growth rate and luteal function (Patel et al 1996, Pedersen et al 1998, Gregoraszczuk et al 2000, Corbacho et al 2002, Gertler & Djiane 2002, Gootwine 2004. Bovine (b) PL is more structurally similar to PRL than GH and is secreted into both the maternal and foetal systems (Gootwine 2004), with a decrease in foetal concentrations during the course of gestation and a peak in maternal plasma during the last Online version via www.reproduction-online.org trimester of pregnancy (Byatt et al 1992a). It has been postulated that bPL recognizes homologous and heterologous somatogenic and lactogenic receptors, acting through components of the insulin-like growth factor system, which may modulate foetal growth (Byatt et al 1992a, Anthony et al 1995.…”

“…Despite the unknown function and control of PSPB secretion, many potential biological functions have been suggested based on in vivo and in vitro effects, including prostaglandin release by the endometrium, implantation, immunotolerance of the conceptus as a tissue alograft and uterine remodelling after parturition, among others (Del Vecchio et al 1990, Kiracofe et al 1993, Austin et al 1999, Tefera et al 2001. Both PAG-1 and PSPB diagnostic assays have been developed for early plasma diagnosis of pregnancy in cattle and other ruminants (Sasser et al 1986, Zoli et al 1991, Green et al 2000.The amounts of BC products in maternal circulation appear to be associated with placental mass, foetal number and neonatal birth weight, and have been postulated to be an index for conceptus viability and pregnancy normalcy in cattle (Byatt et al 1992a, Patel et al 1995, Vasquez et al 1995, Szenci et al 1998, Tefera et al 2001, Ravelich et al 2004. Associations between abnormal placental and foetal development after in vitro embryo manipulations have been suggested to play a key role in the occurrence of high birth weights.…”

“…Bovine (b) PL is more structurally similar to PRL than GH and is secreted into both the maternal and foetal systems (Gootwine 2004), with a decrease in foetal concentrations during the course of gestation and a peak in maternal plasma during the last Online version via www.reproduction-online.org trimester of pregnancy (Byatt et al 1992a). It has been postulated that bPL recognizes homologous and heterologous somatogenic and lactogenic receptors, acting through components of the insulin-like growth factor system, which may modulate foetal growth (Byatt et al 1992a, Anthony et al 1995. PSPB was first isolated from bovine placental tissues, as a product of the BCs of the trophoblastic ectoderm (Butler et al 1982).…”

Interactions of the bovine placental lactogen and prolactin receptor genes are associated with fertility traits in cattle

“…Bovine placental lactogen (bPL) shares about 50% and 24% amino acid sequence homology with PRL and GH, respectively (Anthony et al, 1995). However, no specific bPL receptor has been identified to date, yet it has been shown to be able to act through PRL receptor (PRLR) and GH receptor (GHR) (Byatt et al, 1992, Anthony et al, 1995. bPL has a wide range of biological -E-mail: hkhatib@wisc.edu 1743 activities in vivo, including mammogenesis, ovarian steroidogenesis and fetal growth (Byatt et al, 1992).…”

“…However, no specific bPL receptor has been identified to date, yet it has been shown to be able to act through PRL receptor (PRLR) and GH receptor (GHR) (Byatt et al, 1992, Anthony et al, 1995. bPL has a wide range of biological -E-mail: hkhatib@wisc.edu 1743 activities in vivo, including mammogenesis, ovarian steroidogenesis and fetal growth (Byatt et al, 1992). Nonetheless, the roles of bPL in fertility are less clear.…”