Identification of ovine proteins targeted by pregnancy-induced ISGylation and inference on function I: endometrium†

The establishment of pregnancy in ruminants is dependent on the production of interferon-tau by the conceptus which stimulates the production of interferon-stimulated gene 15 in maternal tissues. Interferon-stimulated gene 15 covalently binds to (ISGylation) and regulates intracellular proteins processes in a manner like ubiquitin. The aim of this study was to identify ISGylated proteins and infer their specific functions and biochemical pathways within endometrium. Endometrium was collected from pregnant ewes on Day 17 (P17, n = 4), synchronized non-inseminated ewes on Day 10 (EC10, n = 4) and Day 17 (EC17, n = 4) after estrus (Day 0). In addition, bovine endometrial cells were cultured with 10 ng/ml of recombinant bovine interferon-tau for 24 h. ISGylated proteins in protein extracts were immunoprecipitated with anti-bovine interferon-stimulated gene 15 antibody. The ISGylated proteins in eluates were identified using liquid chromatography-tandem mass spectrometry and confirmed by reciprocal immunoprecipitation of selected ISGylated proteins. Seventy-five ISGylated proteins were differentially expressed in the endometrium of EC10, EC17, and P17 ewes. Ingenuity pathway analysis indicated that interferon α/β signaling, interferon-stimulated gene 15 antiviral mechanism, Glycolysis I, and Gluconeogenesis I pathways were the most highly significant pathways induced in the pregnant endometrium. Enrichment of the ISGylated proteins STAT1, MX1, and enolase 1 was achieved by immunoprecipitation with specific antibodies and their conjugation to interferon-stimulated gene 15 was confirmed. The ISGylated endometrial proteins identified in pregnant ovine endometrium may function in interferon signaling, antiviral mechanisms, and glucose metabolism with implications for these pathways in the preparation of endometrium for successful implantation and maintenance of pregnancy.