Novel Aspects of the Physiology of Pregnancy in Domestic Ruminants

Mammals exhibit unique and highly variable mechanisms for the establishment and maintenance of pregnancy. Ruminants (e.g., sheep, cows, and goats) have novel mechanisms whereby the conceptus (embryo and its extra-embryonic membranes) signals for the establishment of pregnancy and exhibits unique metabolic pathways favoring conceptus development. Embryos of ruminants reach the spherical blastocyst stage at 5 to 10 mm in diameter and then elongate rapidly to elongated filamentous conceptuses of greater than 250 mm as they make contact with the uterine luminal epithelium (LE) for implantation. During conceptus elongation the trophectoderm cells secrete interferon tau (IFNT), a novel pregnancy recognition signal for ruminants to ensure maintenance of a functional corpus luteum (CL) to secrete progesterone (P4) required for pregnancy. P4 induces uterine epithelia cells to express the endogenous Jaagsiekte Retrovirus (enJSRV) that may transactivate toll-like receptors 7 and 8 in the conceptus trophectoderm to induce secretion of IFNT, a classical viral–antiviral mechanism. IFNT silences expression of receptors for estradiol (E2) and oxytocin (OXTR), which abrogates the mechanism whereby oxytocin from CL and posterior pituitary would otherwise induce large pulses of prostaglandin F_2α (PGF) by uterine epithelia to cause regression of the CL and its secretion of P4. IFNT has another novel role in silencing expression of not only ESR1 and OXTR, but all classical interferon-stimulated genes in the uterine LE and superficial glandular epithelium (sGE), but with P4 increasing expression of genes for transport of nutrients such as glucose and arginine into the uterine lumen to support conceptus development. Ruminant conceptuses convert glucose to fructose, a novel hexose sugar that cannot be transported back to the maternal circulation. Fructose is converted to fructose-1-PO4 for metabolism, not via the pathway for glycolysis but via the novel fructolysis pathway uninhibited by low pH, citrate, or ATP as is the case for glycolysis. Thus, fructose and its metabolites support the pentose cycle, hexosamine biosynthesis pathway, one-carbon metabolism, and the citric acid cycle for all cells of the conceptus. Arginine is another key nutrient transported into the uterine lumen by the uterine LE/sGE in response to P4 and IFNT. Arginine is metabolized to generate nitric oxide, polyamines, and creatine, essential for conceptus growth and development, while enhancing production of IFNT as a novel pregnancy recognition signal, and upregulating expression of genes in the uterine LE/sGE for transport of nutrients. Fructose is the major hexose sugar supporting major metabolic pathways required for conceptus growth and development in ruminants.