Deletion of the Isg15 gene results in up-regulation of decidual cell survival genes and down-regulation of adhesion genes: implication for regulation by IL-1beta

The diverse repertoire of ISG15: more intricate than initially thought

ISG15, the product of interferon (IFN)-stimulated gene 15, is the first identified ubiquitin-like protein (UBL), which plays multifaceted roles not only as a free intracellular or extracellular molecule but also as a post-translational modifier in the process of ISG15 conjugation (ISGylation). ISG15 has only been identified in vertebrates, indicating that the functions of ISG15 and its conjugation are restricted to higher eukaryotes and have evolved with IFN signaling. Despite the highlighted complexity of ISG15 and ISGylation, it has been suggested that ISG15 and ISGylation profoundly impact a variety of cellular processes, including protein translation, autophagy, exosome secretion, cytokine secretion, cytoskeleton dynamics, DNA damage response, telomere shortening, and immune modulation, which emphasizes the necessity of reassessing ISG15 and ISGylation. However, the underlying mechanisms and molecular consequences of ISG15 and ISGylation remain poorly defined, largely due to a lack of knowledge on the ISG15 target repertoire. In this review, we provide a comprehensive overview of the mechanistic understanding and molecular consequences of ISG15 and ISGylation. We also highlight new insights into the roles of ISG15 and ISGylation not only in physiology but also in the pathogenesis of various human diseases, especially in cancer, which could contribute to therapeutic intervention in human diseases.

Influence of conceptus presence and preovulatory estradiol exposure on uterine gene transcripts and proteins around maternal recognition of pregnancy in beef cattle

The uterine environment must provide sufficient endocrine conditions and nutrients for pregnancy maintenance and conceptus survival. The objective of this study was to determine the effects of preovulatory estradiol and conceptus presence on uterine transcripts and uterine luminal fluid (ULF) proteins. Beef cows/heifers were synchronized and artificially inseminated (d 0). Uteri were flushed (d 16); conceptuses and endometrial biopsies were collected. Total cellular RNA was extracted from endometrium for RNA sequencing and RT-PCR validation. There were two independent ULF pools made for each of the following groups: highE2/conceptus, highE2/noconceptus, lowE2/conceptus, and lowE2/noconceptus that were analyzed using the 2D LC-MS/MS based iTRAQ method. There were 64 differentially expressed genes (DEGs) and 77 differentially expressed proteins (DEPs) in common among the highE2/conceptus vs highE2/noconceptus and lowE2/conceptus vs lowE2/noconceptus groups. In summary, the interaction between preovulatory estradiol and the conceptus induces the expression of genes, proteins, and pathways necessary for pregnancy.

Relative abundance and localization of interferon-stimulated gene 15 mRNA transcript in intra- and extra-uterine tissues during the early stages of pregnancy in sheep

The aim of this study was to investigate relative abundance and localization of ISG15 mRNA transcript in intra-uterine (trophoblast, endometrium) and extra-uterine (hypothalamus, anterior pituitary, corpus luteum) tissues before and during the period of conceptus implantation. Multiparous ewes (n = 16) were randomly allotted into four groups: pregnant or estrous cyclic on days of 12 and 16 (n = 4 per group) following estrus. Relative abundances of ISG15 mRNA transcript were determined in the endometrium, corpus luteum, hypothalamus, and anterior-pituitary using real time quantitative PCR. Localization of ISG15 mRNA transcript was evaluated using in situ hybridization. The presence of ISG15 mRNA transcript was only visualized in intra-uterine tissues including the endometrium and trophoblast on day 12 of pregnancy. The ISG15 mRNA transcript was detected in all tissues evaluated on day 16 of pregnancy. The abundance of ISG15 mRNA transcript was greater in the endometrium on day 12 of pregnancy than at other days when evaluations occurred while in all other tissues except the hypothalamus there were large abundances of ISG15 mRNA on day 16 of pregnancy. It is concluded that the ISG15 mRNA transcript is only present in intra-uterine tissues before conceptus implantation. The ISG15 mRNA transcript, however, is present in extra-uterine tissues of ewes during implantation probably due to an increased amount of interferon-tau in blood circulation that is produced by the developing embryo. Results also indicate, for the first time, that pregnancy is associated with an intra-hypothalamus and anterior pituitary increased abundance of ISG15 mRNA transcript in ewes.

Extracellular Vesicles Mediated Early Embryo-Maternal Interactions

Embryo-maternal crosstalk is an important event that involves many biological processes, which must occur perfectly for pregnancy success. This complex communication starts from the zygote stage within the oviduct and continues in the uterus up to the end of pregnancy. Small extracellular vesicles (EVs) are part of this communication and carry bioactive molecules such as proteins, lipids, mRNA, and miRNA. Small EVs are present in the oviductal and uterine fluid and have important functions during fertilization and early embryonic development. Embryonic cells are able to uptake oviductal and endometrium-derived small EVs. Conversely, embryo-derived EVs might modulate oviductal and uterine function. In this review, our aim is to demonstrate the role of extracellular vesicles modulating embryo-maternal interactions during early pregnancy.

Upregulation of Enzymes involved in ISGylation and Ubiquitination in patients with hepatocellular carcinoma

Background: ISGylation is the conjugation of ISG15 with target proteins. ISGylation occurs through an enzymatic cascade, which is similar to that of ubiquitination. Through ISGylation, ISG15 can bind to proteins involved in cell proliferation and differentiation, thus promoting genesis and progression of malignancies. The present study aims to investigate expression of genes involved in ISGylation and ubiquitination in patients with hepatocellular carcinoma and to correlate gene expression with clinical laboratory parameters of these patients. Methods: mRNA expression of genes encoding enzymes involved in the ISGylation process (EFP, HERC5, UBA1, UBC and USP18) was evaluated by quantitative real-time PCR in 38 pairs of tumour and adjacent non-tumour tissues from patients with hepatocellular carcinoma and correlated with distinct clinical laboratory parameters. Results: Relative mRNA expression of EFP, HERC5, UBA1 and USP18 was significantly higher in tumour tissues compared to adjacent non-tumour tissues (P=0.006; 0.012; 0.02 and 0.039, respectively). The correlation pattern of mRNA expression between genes in the tumours differed from the pattern in adjacent non-tumour tissues. Relative expression of EFP, HERC5 and UBA1 in adjacent non-tumour tissues was positively associated with direct bilirubin levels (Spearman's rho=0.31, 0.33 and 0.45; P=0.06, 0.05 and 0.01, respectively) and relative expression of USP18 in adjacent non-tumour tissues correlated negatively with ALT levels (Spearman's rho= -0.33, P=0.03). Conclusions: EFP, HERC5, UBA1, and USP18 genes are upregulated in tumour tissues of patients with HCC and, thus, may be associated with the pathogenesis of hepatocellular carcinoma.

Molecular characterization and expression profile of interferon-stimulated gene 15 (ISG15) in the endometrium of goat (Capra hircus)

Interferon-stimulated gene 15 (ISG15), a ubiquitin-like protein, is responsible for uterine receptivity, implantation and conceptus development in different ruminant species, but in goat (Capra hircus) its role is yet to be explicated. In the present study, the ISG15 gene was cloned, characterized and its temporal expression profile was examined in the endometrium of caprine (cp). A fragment of cpISG15 gene, 1033 bp in length, was amplified, cloned and sequenced from genomic DNA covering the coding region. Sequence analysis of cpISG15 gene revealed that it was comprised of two exons of 59 bp and 496 bp encoding a peptide of 157 amino acids. Complementary DNA (cDNA) and deduced amino acid sequences of cpISG15 exhibited 99 and 98, 93 and 88, 94 and 89, 76 and 66, and 73 and 62% identity with that of sheep, cattle, buffalo, human and mice, respectively. Further, relative expression of cpISG15 mRNA and protein was determined by quantitative real-time PCR (qPCR) and Western blot, respectively, in the endometrium of pregnant and cyclic does. Both cpISG15 mRNA and protein were expressed maximally (P < 0.05) in the endometrium during early stage of pregnancy (16-24 d) as compared to cyclic does, but no significant difference was observed in cpISG15 mRNA and protein expression in the endometrium between the later stage of pregnancy (25-40 d) and cyclic does. It is concluded that cpISG15 is almost similar in structure and probably in function also to other species as it has been found significantly upregulated during early pregnancy.

Proteomic profiling of human decidual immune proteins during Toxoplasma gondii infection

A Toxoplasma gondii infection during pregnancy can result in spontaneous abortion, preterm labor, or congenital fetal defects. The decidual immune system plays a critical role in regulating the immune micro-environment and in the induction of immune tolerance. To better understand the factors that mediate the decidual immune response associated with the T. gondii infection, a large-scale study employing TMT proteomics was conducted to characterize the differential decidual immune proteomes from infected and uninfected human decidual immune cells samples. The decidual immune cells from 105 human voluntary abortion tissues were purified, and of the 5510 unique proteins identified, 181 proteins were found to be differentially abundant (>1.2-fold cutoff, p < 0.05) in the T. gondii-infected decidual immune cells. 11 proteins of 181 differentially expressed proteins associated with trophoblast invasion, placental development, intrauterine fetal growth, and immune tolerance were verified using a quantitative real-time polymerase chain reaction and western blotting. This systematic analysis for the proteomics of decidual immune cells identified a broad range of immune factors in human decidual immune cells, shedding a new insight into the decidual immune molecular mechanism for abnormal pregnancy outcomes associated with T. gondii infection.

Roles of interferon-stimulated gene 15 protein in bovine embryo development

Interferon (IFN)-stimulated gene 15 (ISG15) is one of several proteins induced by conceptus-derived Type I or II IFNs in the uterus, and is implicated as an important factor in determining uterine receptivity to embryos in ruminants. But little is known about the role the ISG15 gene or gene product plays during embryo development. In the present study, both the expression profile and function of ISG15 were investigated in early bovine embryos in vitro. ISG15 mRNA was detectable in Day 0, 2, 6 and 8 bovine embryos, but IFN-τ (IFNT) mRNA only appeared from Day 6. This means that embryonic expression of ISG15 on Days 0 and 2 was not induced by embryonic IFNT. However, ISG15 mRNA expression paralleled the expression of IFNT mRNA in Day 6 and 8 embryos. ISG15–lentivirus interference plasmid (ISG15i) was injected into 2-cell embryos to knockdown ISG15 expression. This resulted in decreases in the proportion of hatching blastocysts, the diameter of blastocysts and cell number per diameter of blastocysts compared with control embryos. In addition, ISG15i inhibited IFNT, Ets2 (E26 oncogene homolog 2) mRNA and connexion 43 protein expression in Day 8 blastocysts, whereas exogenous IFNT treatment (100 ng mL–1, from Day 4 to Day 8) improved ISG15 mRNA and connexion 43 protein expression. In conclusion, it appears that ISG15 is involved in early bovine embryo development and that it regulates IFNT expression in the blastocyst.

Differential expression of ISG 15 mRNA in peripheral blood mononuclear cells of nulliparous and multiparous pregnant versus non-pregnant Bos indicus cattle

Embryonic mortality is found to be the main source of reproductive wastage in domestic ruminants. Many genes are involved in the growth and development of the embryo, and the interferon-stimulated gene 15 (ISG 15) is one of the major gene stimulated by interferon tau, the maternal recognition of pregnancy signal in ruminants. In this study, both genomic and cDNA sequences of ISG 15 from Bos indicus (Deoni breed) were amplified and characterized. The genomic sequence of Deoni ISG 15 exhibited 99% identity with Bos taurus and 97% identity with that of Bos mutus and Bubalus bubalis. Moreover qRT-PCR analysis revealed constitutive expression of the ISG 15 mRNA in peripheral blood mononuclear cells of Deoni heifers and multiparous cows during early pregnancy. Fourteen Deoni heifers and fifteen multiparous Deoni cows were synchronized for timed AI by CIDR-Ovsynch protocol, and six animals were kept as cyclic control in each group. Blood samples were collected on days 7, 14, 16, 18, 21, 30 and 45 from the day of AI. Pregnancy was confirmed by plasma progesterone level through ELISA. A significantly higher expression of ISG 15 mRNA was found on day 16 (p < .05) and day 18 (p < .05) of pregnancy in nulliparous heifers. Although in multiparous Deoni cows ISG 15 expression was greater in pregnant cows, difference was statistically non-significant. The result of this study indicates that ISG 15 gene expression is upregulated during 16-18 days of pregnancy and could be used as an early pregnancy marker in dairy cows especially in heifers.

Possible role of interferon tau on the bovine corpus luteum and neutrophils during the early pregnancy

When pregnancy is established, interferon tau (IFNT), a well-known pregnancy recognition signal in ruminants, is secreted by embryonic trophoblast cells and acts within the uterus to prepare for pregnancy. IFNT acts as an endocrine factor on the corpus luteum (CL) to induce refractory ability against the luteolytic action of PGF2α. Hypothesising that IFNT may influence not only the uterine environment but also the CL in cows via local or peripheral circulation, we investigated qualitative changes in the CL of pregnant cows during the maternal recognition period (day 16) and the CL of non-pregnant cows. The CL of pregnant animals had a higher number of neutrophils, and the expression of interleukin 8 (IL8) mRNA and its protein was higher as well as compared with the CL of non-pregnant animals. Although IFNT did not affect progesterone (P4) secretion and neutrophil migration directly, it stimulated IL8 mRNA expression on luteal cells (LCs), influencing the neutrophils, resulting in the increased migration of IFNT-activated neutrophils. Moreover, both IFNT-activated neutrophils and IL8 increased P4 secretion from LCs in vitro. Our novel finding was the increase in neutrophils and IL8 within the CL of pregnant cows, suggesting the involvement of IFNT function within the CL toward establishment of pregnancy in cows. The present results suggest that IFNT upregulates neutrophil numbers and function via IL8 on LCs in the CL of early pregnant cows and that both neutrophils and IL8, stimulated by IFNT, are associated with an increase in P4 concentrations during the maternal recognition period in cows.

Comparative analysis between endometrial proteomes of pregnant and non-pregnant ewes during the peri-implantation period

BACKGROUND

Early pregnancy failure has a profound impact on both human reproductive health and animal production. 2/3 pregnancy failures occur during the peri-implantation period; however, the underlying mechanism(s) remains unclear. Well-organized modification of the endometrium to a receptive state is critical to establish pregnancy. Aberrant endometrial modification during implantation is thought to be largely responsible for early pregnancy loss.

RESULT

In this study, using well-managed recipient ewes that received embryo transfer as model, we compared the endometrial proteome between pregnant and non-pregnant ewes during implantation period. After embryo transfer, recipients were assigned as pregnant or non-pregnant ewes according to the presence or absence of an elongated conceptus at Day 17 of pregnancy. By comparing the endometrial proteomic profiles between pregnant and non-pregnant ewes, we identified 94 and 257 differentially expressed proteins (DEPs) in the endometrial caruncular and intercaruncular areas, respectively. Functional analysis showed that the DEPs were mainly associated with immune response, nutrient transport and utilization, as well as proteasome-mediated proteolysis.

CONCLUSION

These analysis imply that dysfunction of these biological processes or pathways of DEP in the endometrium is highly associated with early pregnancy loss. In addition, many proteins that are essential for the establishment of pregnancy showed dysregulation in the endometrium of non-pregnant ewes. These proteins, as potential candidates, may contribute to early pregnancy loss.

Embryo mortality in Isg15-/- mice is exacerbated by environmental stress

The interferon-stimulated gene 15 (Isg15) encodes a ubiquitin-like protein that is induced in the endometrium by pregnancy in mice, humans, and ruminants. Because ISG15 is a component of the innate immune system, we hypothesized that development of the embryo, fetus, and postnatal pup may be impaired in mice lacking Isg15 (Isg15(-/-)) and that this development would be further impaired in response to environmental insults such as hypoxia. The number of implantation sites, resorption sites, dead embryos, and the changes in overall gross morphology of the uterus were evaluated in Isg15(-/-) mice on Days 7.5 and 12.5 postcoitum (dpc). Postnatal development also was monitored from birth to 12 wk of age. On 7.5 dpc, the number of implantation sites and serum progesterone concentrations were similar. However, embryo mortality increased (P < 0.05) in Isg15(-/-) dams by 12.5 dpc, resulting in smaller litter sizes (4.26 ± 0.21 embryos; n = 83 litters) compared to Isg15(+/+) females (7.78 ± 0.29 pups; n = 47 litters). Embryo mortality in Isg15(-/-) mice was further exacerbated to 70% when dams were stressed through housing under hypoxic conditions (PB = 445 mmHg; 6.5-12.5 dpc). Transmission electron microscopy revealed lesions in antimesometrial decidua as well as trophoblast cells adjacent to decidual cells on 7.5 dpc. ISG15 was localized to mesometrial decidua on 7.5 dpc. By 12.5 dpc, ISG15 was intensely localized to the labyrinth of the placenta. By 7.5 dpc, uterine natural killer cell migration into the mesometrial pole was diminished by 65% and was less prevalent in Isg15(-/-) compared to Isg15(+/+) deciduum. Postnatal growth rate of offspring that survived to birth from Isg15(-/-) and Isg15(+/+) dams was not different. Embryo mortality occurs in pregnant Isg15(-/-) mice, is exacerbated by environmental insults like maternal hypoxia, and might result from impaired early decidualization, vascular development, and formation of the labyrinth.

Conceptus elongation in ruminants: roles of progesterone, prostaglandin, interferon tau and cortisol

The majority of pregnancy loss in ruminants occurs during the first three weeks after conception, particularly during the period of conceptus elongation that occurs prior to pregnancy recognition and implantation. This review integrates established and new information on the biological role of ovarian progesterone (P4), prostaglandins (PGs), interferon tau (IFNT) and cortisol in endometrial function and conceptus elongation. Progesterone is secreted by the ovarian corpus luteum (CL) and is the unequivocal hormone of pregnancy. Prostaglandins (PGs) and cortisol are produced by both the epithelial cells of the endometrium and the trophectoderm of the elongating conceptus. In contrast, IFNT is produced solely by the conceptus trophectoderm and is the maternal recognition of pregnancy signal that inhibits production of luteolytic pulses of PGF_2α by the endometrium to maintain the CL and thus production of P4. Available results in sheep support the idea that the individual, interactive, and coordinated actions of P4, PGs, IFNT and cortisol regulate conceptus elongation and implantation by controlling expression of genes in the endometrium and/or trophectoderm. An increased knowledge of conceptus-endometrial interactions during early pregnancy in ruminants is necessary to understand and elucidate the causes of infertility and recurrent early pregnancy loss and provide new strategies to improve fertility and thus reproductive efficiency.

Interferon stimulated gene 15 expression at the human embryo-maternal interface

PURPOSE

In early pregnancy the dialogue between maternal endometrium and embryo is a key process in establishing a receptive decidua and placental network. Decidual ISG15 induction is thought to promote pregnancy maintenance and development. ISG15 is involved in RNA splicing, cytoskeletal organization, stress response and further intracellular processes.

METHODS

ISG15 expression was examined immunohistologically in paraffin-embedded human placental and decidual tissue samples of all pregnancy trimesters on adjacent sections (first trimester n = 5, second n = 5, third n = 3). Samples were processed using a protocol applying a rabbit polyclonal ISG15 antibody. A mouse monoclonal cytokeratin seven antibody was utilized to identify the different placental departments and decidual glands. Staining results and anatomical features were evaluated blindly with strict rating criteria.

RESULTS

ISG15 expression was identified in first and second trimester tissue samples. ISG15 localized especially to the extravillous cytotrophoblasts in the maternal wall and in maternal blood vessel. Expression was detected in cytotrophoblast progenitor cells in the placental villi and the cell column with a maximum in the first trimester. The syncytial layer stained positive in first and second trimester samples. Third trimester samples showed no expression of ISG15 at all.

CONCLUSIONS

ISG15 abundance in the human placenta is an interesting finding, with implications for placental development, fetal growth and potential defense mechanism against infections. The maximal expression of ISG15 in the first and second trimester of pregnancy suggests that ISG function is needed when placental and embryo development is enormous and embryo susceptibility to external influences is high.

ISGylation: a conserved pathway in mammalian pregnancy

Successful pregnancy includes remodeling and differentiation of the endometrium in response to sex steroid hormones, development of maternal immunotolerance to the implanting embryo, and modification of the local uterine environment by the embryo to suit its own needs. The major signal released by the ruminant conceptus during establishment of pregnancy is interferon-tau (IFNT) that stimulates the expression of many genes in the endometrium and ovary. One of these genes is called interferon stimulated gene 15 (ISG15), which encodes a ubiquitin homolog with a C-terminal Gly that becomes covalently attached to Lys residues on targeted proteins through an ATP-dependent multi-step enzymatic reaction called ISGylation. The conceptus-derived induction of endometrial ISGs also occurs in mouse and human deciduas and placenta, in response to pregnancy presumably through action of cytokines such as interleukins and type I IFN. Described herein is evidence to support the concept that ISGylation is a maternal response to the developing conceptus, implantation and placentation that is conserved across mammalian pregnancy. Although the precise role for ISG15 remains elusive during pregnancy, it is clear that up-regulation in response to pregnancy may impart a pre-emptive defense to infection or other environmental insults, and protection of the conceptus against inflammatory insults across species.

Conceptus-derived prostaglandins regulate gene expression in the endometrium prior to pregnancy recognition in ruminants

In cattle, the blastocyst hatches from the zona pellucida on days 8-9 and then forms a conceptus that grows and elongates into an ovoid and then filamentous shape between days 9 and 16. The growing conceptus synthesizes and secretes prostaglandins (PGs) and interferon τ (IFNT). Our hypothesis was that the ovoid conceptus exerts a local effect on the endometrium prior to maternal recognition of pregnancy on day 16 in cattle. In study one, synchronized cyclic heifers received no blastocysts or 20 in vitro-produced blastocysts on day 7 and their uteri were collected on day 13. IFNT was not detected by RIA in the uterine flushing samples of pregnant heifers containing multiple ovoid conceptuses; however, total PG levels were higher in the uterine lumen of pregnant heifers than in that of cyclic heifers. Microarray analysis revealed that the expression of 44 genes was increased in the endometria of day 13 pregnant heifers when compared with that in the endometria of cyclic heifers, and many of these genes were classical Type I IFN-stimulated genes (ISGs). In studies two and three, the effects of infusing PGs at the levels produced by the elongating day 14 conceptus into the uterine lumen of cyclic ewes on ISG expression in the endometrium were determined. Results indicated that the infusion of PGs increased the abundance of several ISGs in the endometrium. These studies support the hypothesis that the day 13 conceptus secretes PGs that act locally in a paracrine manner to alter gene expression in the endometrium prior to pregnancy recognition in cattle.

Interaction of the conceptus and endometrium to establish pregnancy in mammals: role of interleukin 1β

Implantation and the establishment of pregnancy in mammals involves an intricate interplay of hormones, cytokines, growth factors, proteins, lipids, ions and the extracellular matrix between the uterine epithelium, stroma, immune cells and the conceptus trophectoderm. The divergent nature of implantation in the mouse, human and pig provides not only an interesting contrast in the establishment of pregnancy and early embryonic development but also intriguing similarities with regard to early endometrial-conceptus signaling. An interesting pro-inflammatory cytokine expressed in a number of mammalian species during the period of implantation is interleukin-1β (IL1B). The presence of IL1B might be involved with immunotolerance at the maternal-placental interface and has been proposed as one of the mediators in placental viviparity. The production of IL1B and other proinflammatory cytokines might play a role in establishing pregnancy through modulation of the nuclear factor kappa-B (NFKB) system in a number of species. A model for the regulation of cellular progesterone receptor expression and NFKB activation for endometrial receptivity and conceptus attachment is continuing to evolve and is discussed in the present review.

The expression of interferon-stimulated gene 15 in equine endometrium

Establishment of pregnancy is critically dependent upon a precisely orchestrated embryo-maternal interaction leading to a receptive uterine environment. The up-regulation of the interferon-stimulated protein 15 kDa (ISG15) during pregnancy has been described in various species and has been hypothesized to be part of the molecular repertoire that makes the uterus receptive to conceptus development. In the current study, the expression of ISG15 and enzymes involved in ISG15ylation was examined at the mRNA and protein level in equine endometrium at Day 14 of the luteal phase and at Day 14 and 50 of pregnancy. ISG15 mRNA showed a 2.63-fold higher expression at Day 14 of pregnancy when compared to Day 14 of the cycle, while mRNA abundance at Day 50 of pregnancy was unchanged compared to Day 14 of the cycle. Upon Western blot analysis using anti-ISG15 antibody, several higher molecular weight bands could be observed, representing proteins conjugated to ISG15. No free ISG15 could be detected. The pattern of ISG15 reactive proteins differed from those observed in non-uterine samples. Upon immunohistochemistry, ISG15 reactive proteins located primarily to luminal and glandular epithelial cells, while stromal cells showed weaker staining. In conclusion, the expression of ISG15-conjugated proteins in equine endometrium did not differ between cyclic and pregnant 14 days after ovulation and Day 50 of pregnancy. It is hypothesized that the unique subset of ISG15ylated proteins expressed in endometrial tissue contributes to normal cellular function and that, unlike other species, the modification of ISG15-conjugated proteins is not an active contributor to conceptus-maternal interaction in the mare.