2.1 Clinical sample collection
Uterine tissues were collected via pipe suction curettage (Wallace) from patients who underwent IVF-ET/ICSI treatment at the Reproductive Medical Center of Ruijin Hospital, School of Medicine, Shanghai Jiao Tong University between January 2016 and December 2020. All patients signed an informed consent form before they were recruited in the study. Patients with a history of tubal infertility who successfully achieved conception after the first embryo transfer were recruited as the control group (n = 38) and divided to three phases of the menstrual cycle: early proliferative phase (days 4–5, n = 10); late proliferative phase (days 11–13, n = 10); implantation phase (days 20–23, n = 18). Patients with no pregnancy after at least three embryo transfers including a total of ≥ 4 good-quality embryos were recruited as the RIF group (n = 18). The inclusion criteria for all participants included: 25–35 years old, regular ovulatory cycles with every 28–32 days, normal serum levels including follicle stimulating hormone (FSH < 10 mIU/mL), luteinizing hormone (LH < 10 mIU/mL) and estradiol (E2 < 50 mIU/mL) on day 3 of the menstrual cycle. The exclusion criteria for all patients included: presence of intrauterine pathology, hydrosalpinx, salpingitis, polycystic ovary syndrome, endometriosis, adenomyosis, chromosome abnormalities, or autoimmune disease, and any intrauterine device or contraceptive drug within the last 6 months.
For endometrial biopsy, specimens collected from the control group in the three phases were used to analyze differences in the HMGB1 phase-specific expression in the normal natural menstrual cycle (Supplementary Table 1). The implantation phase endometrium from 18 patients in the control group and 18 patients in the RIF group was collected on day LH + 7 to examine the expression of HMGB1 and HOXA10 (Supplementary Table 2). As we described previously [21], patients were monitored for follicle development by transvaginal ultrasound (Hitachi EUB-2000; Tokyo, Japan) until the diameter of the dominant follicle was ≥ 15 mm, and then serum LH and E2 levels were quantified daily. The day on which the LH level peaked (≥ 20 mIU/mL) was considered as day LH 0; the window/implantation phase was defined as day LH + 7. To isolate primary endometrial cells, the endometrium was collected from the control group (late proliferative phase, n = 12).
2.2 Microarray assay
Whole-genome gene expression profiling was conducted using uterine tissues from three patients with RIF and three tissues from the control group in the implantation phase as described previously [21]. Data were analyzed using the Gene Expression Omnibus database (GSE103465), and genes were considered as differentially expressed when they showed a fold-change ≥ 2 and adjusted P-value ≤ 0.05.
2.3 Real-time reverse transcription PCR (qRT-PCR)
Total RNA was extracted from cultured cells and endometrial biopsy tissues using the TaKaRa9767 kit in accordance with the manufacturer’s instructions (Takara, Shiga, Japan). Next, 1 µg RNA was reverse-transcribed into cDNA using Primescript Reverse transcriptase (Takara). The synthesized template cDNA, gene-specific primers (Supplementary Table 3), and fluorescent probes (Takara) were used for qRT-PCR. PCR was conducted on a QuantStudio™ 6 Flex Real-Time PCR System (Thermo Fisher Scientific, Waltham, MA, USA). The mRNA expression level was normalized to that of GAPDH. Relative mRNA expression levels were calculated using the 2−ΔΔCt method [22].
2.4 Western blotting analysis
Total protein was extracted from endometrial biopsy tissues and cultured cells using RIPA buffer (Thermo Fisher Scientific) supplemented with 1% protease inhibitor cocktail (Roche, Basel, Switzerland). After centrifuging the samples at 12,000 ×g for 30 min at 4°C, the protein-containing supernatants were collected and with 30 µg protein per sample separated by 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred onto nitrocellulose membranes. The membranes were blocked and then incubated with primary antibodies against HOXA10 (ab191470, Abcam, Cambridge, UK; 1:1,000), HMGB1 (ab79823, Abcam; 1:10,000) and GAPDH (5174, Cell Signaling Technology, Danvers, MA, USA; 1:500) for 12 h at 4°C. After washing the membranes four times with PBS, they were incubated with secondary antibodies. Specific protein bands were visualized using an Odyssey Infrared Imager (LI-COR, Lincoln, NE, USA).
2.5 Immunohistochemical staining
Immunohistochemical (IHC) staining was performed on 5-µm sections from 4% paraformaldehyde-fixed paraffin-embedded endometrial tissue samples. The slides were incubated with 5% bovine serum albumin (Amresco, Solon, OH, USA) (25°C, 30 min), and then incubated with anti-HMGB1 (dilution 1:400, ab79823, Abcam) and normal rabbit IgG antibodies overnight at 4°C. Images were acquired using an Olympus BX53M fluorescence microscope (Tokyo, Japan). The H-score was calculated to determine the expression of HMGB1 based on the percentage of cells stained at each intensity level, ranging from 3 to 0 (equaling to strong, moderate, weak, and negative staining, respectively).
2.6 Cell culture and transfection
Isolation of primary human endometrial stromal cells (HESCs) and primary human endometrial epithelial cells (HEECs) from endometrial samples was performed as described previously [20]. Briefly, fresh endometrial biopsy samples were rinsed twice with PBS, and then minced and digested with 1 mg/mL collagenase type IV (Thermo Fisher Scientific) at 37°C for 30–45 min. After enzymatic digestion, the mixture was passed through a 100-mm sieve followed by a 40-mm sieve, with HESCs collected by passing the mixture through a 40-µm sieve and HEECs collected on the opposite side of the 40-µm sieve. Ishikawa cells (human endometrial epithelial cell line) and JAR cells (human trophoblastic cell line) were obtained from the European Collection of Authenticated Cell Cultures (Salsibury, UK). The human endometrial stromal cell line (T-HESCs) were obtained from American Type Culture Collection (Manassas, VA, USA). All cells were cultured in Dulbecco’s modified Eagle medium (DMEM) (Thermo Fisher Scientific) supplemented with 10% fetal bovine serum at 37°C.
Negative control and HMGB1 over-expressing plasmids were purchased from GeneCopoeia (Rockville, MD, USA) and transfected into cells using transfection reagent (Roche) following the manufacturer’s instructions.
2.7 Proliferation assay
The capacity for cell proliferation was examined using Cell Counting Kit-8 (CCK-8; Dojindo, Kumamoto, Japan) in endometrial stromal cells (ESCs) including T-HESCs and HESCs. After 2 days of plasmid transfection, the cells were seeded into 96-well plates at 2000 cells/well and 10 µL of CCK-8 reagent was added to each well. After incubating at 37°C for 2 h, the optical densities were determined at 450 nm excitation using a microplate reader.
2.8 Cell migration assay
Ishikawa cells and HEECs were transfected with negative control and HMGB1 over-expressing plasmids and plated into 6-well plates. After transduction for 3 days when the cell confluence reached 80–90%, the cells were cultured in DMEM containing 0.1% fetal bovine serum for 24 h to achieve deprivation conditions. A vertical linear scratch was created in the Ishikawa monolayer cells using a 200-µL micropipette tip, with the floating cells removed by washing with sterile PBS. The cell migration distance was measured under a light microscope via Image J 1.46r (Nunes and Dias, NIH, Bethesda, MD, USA) at 0, 24, and 48 h after continuous culturing of the scratched cells in serum deprivation media at 37°C.
2.9 Embryo adhesion assay
The co-culture model of “embryo” adhesion was constructed as described by Jemma Evans et al. (2020) [23]. Briefly, HEECs were isolated from four late proliferative-phase endometrium samples from controls as described above. Ishikawa cells, possessing apical adhesiveness and similar characteristics of endometrial luminal and glandular epithelium, were commonly used to study endometrial receptivity and embryo adhesion [24–26]. Ishikawa cells and HEECs transfected with HMGB1 over-expressing or negative control plasmids were grown to confluence in a 48-well plate. Endometrial cells were then treated with a hormonal paradigm to mimic hormonal exposure throughout the menstrual cycle: cells were primed with 10− 8 M β-estradiol (E2758, Thermo Fisher Scientific, referred to as E) for 24 h, and then treated with combined E and 10− 7 progesterone (V900699, Thermo Fisher Scientific, referred to as P) for 24 h, followed by treatment with E/P and 10 IU human chorionic gonadotropin for 24 h. An adhesion assay was then performed. A total of 44 hatched blastocysts from C57BL/6 mice with a normal morphology was cocultured with confluent monolayers of acceptable HEECs for 6 h in DMEM/F12 complete medium. After washing the cells three times with PBS, images of the adherent blastocysts were captured for morphological examination with a TS2 microscope (Nikon, Tokyo, Japan). JAR cells (human trophoblastic cell line) labeled with PKH26 (PKH26GL-1KT, Abcam) for 10 min at 37°C were then added to fully confluent endometrial epithelial cells (EECs) including Ishikawa cells and HEECs monolayers for 1 h at 37°C, followed by vigorous agitation three times at 140 rpm for 5 min before refreshing the medium. The attached JAR cells were then imaged with a fluorescence microscope (Leica V3.8, Wetzlar, Germany).
2.10 In vitro decidualization experiment
HESCs and T-HESCs were used for decidualization experiments. After 3 days of plasmid transfection, HESCs and T-HESCs were cultured in serum-free DMEM containing estradiol (10 nM, E2758, Sigma, St. Louis, MO, USA), progesterone (1 µM V900699, Sigma), and 8-Br-cAMP (1 mM, ab141448, Abcam) for 3 and 8 days, respectively. Total RNA was extracted to analyze the expression of decidual markers: prolactin (PRL) and insulin-like growth factor-binding protein 1 (IGFBP1).
2.11 Statistical analysis
All experiments were independently performed at least three times. The results are shown as the mean ± standard error of the mean (SEM) and analyzed using SPSS software (version 23.0, SPSS, Inc., Chicago, IL, USA). Data were analyzed using Student’s t-test between two groups and one-way analysis of variance among more than two groups. Data were considered as statistically significant when P < 0.05.