Ethics statement
All animal procedures in this study were performed in accordance with the guidelines of the Committee for Experimental Animals at Zen-noh Embryo Transfer Center (Hokkaido, Japan), with the approval of the Institutional Animal Care and Use Committee of Zen-noh Embryo Transfer Center (Approval number: ZET20190628). All animals used were raised and kept at this center throughout the course of this experimentation. This study was carried out in compliance with the ARRIVE guidelines.
Collection of bovine blood samples
Estrous synchronization, superovulation, and embryo transfer (ET) processes were performed as previously described 33. Recipient heifers (14~16 months old, n = 20) were given a single injection of 0.75 mg cloprostenol to synchronize their estrous cycles and the heifers exhibited behavioral estrus 40-48 h after the injection (n = 17, day 0 = day of estrus). For ET processes, day 7 embryos were collected from super-ovulated and artificially inseminated (AI) Japanese black cows (3~5 years old, n = 5). Two blastocysts each derived from the superovulation/AI procedure were then transferred non-surgically into the uterine horn of Holstein heifers, ipsilateral to the corpus luteum, on day 7 of the estrous cycle. Blood samples were collected on pregnant days 17, 20, and 22. In addition, blood samples from a group of heifers (14~16 months old, n = 3), which went through the same estrous synchronization without ET procedure, were collected on days 0, 7, 14, and 17 of the estrous cycle. The diagnosis of pregnancy or non-pregnancy (NP), indicative of embryonic loss, was done by real-time B-mode ultrasonography (Convex scanner HS-1500, Honda electronics Co. Ltd., Toyohashi, Japan) on days 30 of gestation, from which blood samples were divided into pregnant or NP group. After sorting of blood samples, albumin was removed from serum samples by ProMax Albumin Removal Kit (Polysciences, Warrington, PA, USA).
iTRAQ analysis
A global analysis of proteins using iTRAQ analysis was performed as described previously 12. Briefly, serum samples from ET-heifers on day 17, 20 or 22 and those without ET were resuspended in 30 µl iTRAQ lysis buffer (50 mM TAEB, 0.1 % SDS). Total protein (100 µg) was subjected to trypsin digestion and then reacted with appropriate iTRAQ reagent according to the manufacturer’s instructions. Sample fractionation was performed with an Agilent 3100 OFFGEL Fractionator (Agilent Technologies, Santa Clara, CA, USA). Furthermore, mass spectrometry analysis was performed with a Thermo Scientific LTQ Orbitrap XL mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Mascot software was used to simultaneously identify and quantify proteins.
Western blot analysis
Serum samples were separated through SDS-PAGE and were then transferred onto polyvinylidene difluoride (PVDF) membranes (Bio-Rad, Hercules, CA, USA). After blocking with Block Ace reagent (DS Pharma Biomedical, Osaka, Japan), membranes were incubated with goat polyclonal anti-SNX5 (1:2000, ab5983, abcam, Tokyo, Japan), rabbit polyclonal anti-SRFBP1 (1:2000, ab109598, abcam), rabbit polyclonal anti-FAS (1:2000, ab22759, abcam), rabbit polyclonal anti-DDB1 (1:2000, ab97522, abcam), rabbit monoclonal anti-CPSF6 (1:2000, ab175237, abcam), rabbit polyclonal anti-FUT8 (1:500, ab115925, abcam), mouse monoclonal anti-ACAC (1:2000, ab205883, abcam), rabbit polyclonal anti-SNX5 (1:2000, SAB2102260, Sigma-Aldrich, Tokyo, Japan), rabbit polyclonal anti-FUT8 (1:2000, LS-C145608, Life Span BioSciences, Seattle, WA, USA), or rabbit polyclonal anti-ISG15 (1:2000, Origene Technologies, Rockville MD, USA) antibody. Immunoreactive bands were detected using enhanced chemiluminescence (EMD Millipore, Temecula, CA, USA) after incubation with horseradish peroxidase labeled anti-mouse, rabbit, or goat IgG (1:5000, Vector Laboratories, Burlingame, CA, USA). Signals were detected using C-DiGit Blot Scanner (LI-COR) and then band density was assessed with Image Studio DiGit software (version 5.2) 34.
Metabolome analysis
A serum metabolomics analysis was performed using GC/MS as described previously 35 with some modifications. In brief, a sample of 50 μl of serum was mixed with 5 μl of 1 mg/ml 2-isopropylmalic acid (Sigma-Aldrich) in distilled water as an internal standard, and 250 μl of methanol–chloroform–water (2.5:1:1) mixture. Then samples were lyophilized, and added with 40 μl of 20 mg/ml methoxyamine hydrochloride (Sigma-Aldrich), dissolved in pyridine for oximation. After mixing, the samples were shaken for 90 min at 30°C. Next 20 μl of N-methyl N-trimethylsilyl-trifluoroacetamide (GL Science, Tokyo, Japan) was added for trimethylsilylation, and the mixture was incubated at 37°C for 45 min. The sample was subjected to GC/MS (GCMS QP2010-Ultra; Shimadzu, Kyoto, Japan). The Shimadzu Smart Metabolites Database (Shimadzu) was used to identify metabolites. Samples were normalized by a pooled sample from control group. A metabolic pathway analysis was performed using MetaboAnalyst 36. Metabolites that significantly diffed between two groups were subjected to an enrichment analysis (http://www.metaboanalyst.ca/faces/upload/EnrichUploadView.xhtml).
Statistical analysis
All experimental data represent the results obtained from three or more independent experiments each with triplicate assays. Data were expressed as the mean ± SEM. A P-value < 0.05 was considered statistically significant.