Mechanisms for the establishment and maintenance of pregnancy: synergies from scientific collaborations

Abstract

Research on the functions of interferon tau (IFNT) led to the theory of pregnancy recognition signaling in ruminant species. But IFNT does much more as it induces expression of interferon regulatory factor 2 (IRF2) in uterine luminal (LE), superficial glandular (sGE), but not glandular (GE) epithelia. First, IRF2 silences transcription of the estrogen receptor alpha gene and, indirectly, transcription of the oxytocin receptor gene to abrogate development of the luteolytic mechanism to prevent regression of the corpus luteum and its production of progesterone for establishing and maintaining pregnancy. Second, IRF2 silences expression of classical interferon-stimulated genes in uterine LE and sGE; however, uterine LE and sGE respond to progesterone (P4) and IFNT to increase expression of genes for transport of nutrients into the uterine lumen such as amino acids and glucose. Other genes expressed by uterine LE and sGE encode for adhesion molecules such as galectin 15, cathepsins, and cystatins for tissue remodeling, and hypoxia-inducible factor relevant to angiogenesis and survival of blastocysts in a hypoxic environment. IFNT is also key to a servomechanism that allows uterine epithelia, particularly GE, to proliferate and to express genes in response to placental lactogen and placental growth hormone in sheep. The roles of secreted phosphoprotein 1 are also discussed regarding its role in implantation in sheep and pigs, as well as its stimulation of expression of mechanistic target of rapamycin mRNA and protein which is central to proliferation, migration, and gene expression in the trophectoderm cells.

Figure 1.

Schematic illustrating the current working hypothesis on IFNT signaling in endometrial epithelia and stroma of the ovine uterus. IFNT, produced in large amounts by the developing conceptus, binds to the Type I IFN receptor (IFNAR) present on cells of the ovine endometrium. In the cells of the endometrial luminal epithelium (LE) and superficial glands (sGE), IFNT induces IRF2 which prevents uterine LE and sGE from expressing classical ISGs such as STAT1, STAT2, and IRF9 (A). IRF2, a potent and stable repressor of transcription of genes in the nucleus, increases in abundance during early pregnancy only in uterine LE and sGE. The presence of IRF2 (interferon regulatory factor 2) inhibits binding of IRF1 to the promoter region of ISRE-containing target genes through direct competitive binding of IRF2 to ISRE and subsequent coactivator repulsion. IFNT can stimulate the transcription of a number of nonclassical IFNT-stimulated genes (ISG) as well as increase the activity of certain intracellular enzymes (prostaglandin synthase 2, PTGS2 and HSD11B1, hydroxysteroid 11-beta dehydrogenase). However, the noncanonical signaling pathways mediating these effects of IFNT in the LE/sGE are largely unknown. In cells of the stroma and middle to deep glands (B), IFNT-mediated association of the IFNAR subunits facilitates the cross-phosphorylation and activation of two Janus kinases, Tyk2 and JAK1, which in turn phosphorylate the receptor and creates a docking site for signal transducer and activator of transcription 2 (STAT2). STAT2 is then phosphorylated, thus creating a docking site for STAT1 that is then phosphorylated. STAT1 and STAT2 are then released from the receptor and can form two transcription factor complexes. Interferon-stimulated gene factor 3 (ISGF3), formed by association of the STAT1-2 heterodimer with IRF9 in the cytoplasm, translocates to the nucleus, and transactivates genes containing an ISRE, such as STAT1, STAT2, and IRF9. Gamma activation factor (GAF) is formed by binding of STAT1 homodimers, which translocates to the nucleus and transactivate genes containing a gamma activation sequence (GAS) element(s), such as IRF1. IRF1 can also bind and transactivate ISRE-containing genes. The simultaneous induction of STAT2 and IRF9 gene expression by IFNT appears to shift transcription factor formation from GAF towards predominantly ISGF3. Therefore, IFNT activation of the JAK-STAT signal transduction pathway allows for constant formation of ISGF3 and GAF transcription factor complexes and hyperactivation of ISG expression.

Figure 2.

Synthesis of polyamines from arginine in the mammalian conceptus. Arginine is hydrolyzed to ornithine plus urea by arginase I and arginase II in many cell types (possibly except for porcine placentae and neonatal small intestine). Synthesis of putrescine from ornithine is catalyzed by ornithine decarboxylase (a cytosolic enzyme) in all cell types. In placental mitochondria of sheep, arginine may be decarboxylated to form agmatine by arginine decarboxylase (ADC) and agmatine is then converted into putrescine by agmatinase. DCAM, decarboxylated 5-adenosylmethionine; α-KG, α-ketoglutarate; MTA, methylthioadenosine; SAMD, S-adenosyl-methionine decarboxylase; SAM, S-adenosylmethionine. Adapted from Wu et al. [213].

Figure 3.

Expression, regulation, and proposed functions of SPP1 produced by the uterine epithelia of pregnant pigs and sheep. Center to left: as porcine conceptuses (trophectoderm) elongate they secrete estrogens for pregnancy recognition. These estrogens also induce the synthesis and secretion of secreted phosphoprotein 1 (SPP1 or osteopontin [OPN]) from the uterine LE directly adjacent to the conceptus undergoing implantation. The implantation cascade is initiated when progesterone from CL downregulates Muc 1 on the surface of uterine LE. This exposes integrins on the uterine LE and conceptus trophectoderm cell surfaces for interaction with SPP1, and likely other ECM proteins, to mediate adhesion of conceptus trophectoderm to uterine LE for implantation. Center to right: as the lifespan of the CL is extended due to the actions of IFNT secretion from elongating ovine conceptuses they secrete progesterone. Progesterone then induces the synthesis and secretion of SPP1 from the uterine GE. The implantation cascade is initiated with downregulation of mucin 1 (Muc 1) (the regulatory mechanism remains to be identified) on the uterine LE surface to expose integrins on the LE and conceptus trophectoderm cell surfaces for interaction with SPP1 to mediate adhesion of conceptus trophectoderm to uterine LE for implantation.