The World Health Organization (WHO) announced on May 5, 2023, that COVID-19 was no more a global public health emergency. This announcement was a major turning point in the global effort to combat the pandemic. In the 'post-epidemic era', we should pay long-term attention to the impact of the new coronavirus on human long-term quality of life. Since 2020, there has been a brief decline in population worldwide, a phenomenon that was alleviated in 2023. In addition to the economic and social factors brought by the epidemic, we also noted the potential impact of the epidemic on reproductive health.
ASRM and ESHRE recommended that fertility services be stopped during the early stages of the pandemic, with the exception of urgent cases [14, 15]. Further research has revealed a correlation between SARS-CoV-2 infection and some aspects of human fertility.
Molecular studies indicated that the ovarian cortex, medulla, oocytes, endometrium, and the membranes of trophectoderm, hypoblast, and epiblast cells in blastocysts have all been demonstrated to co-express ACE2 and TMPRSS2 in females [5, 6, 16]. SARS-CoV-2 infection has been associated with ovarian dysfunction, disruptions in the follicular microenvironment, changes in menstrual volume and cycle length, reproductive outcomes, and potential interference with embryo implantation and pregnancy [17–19].
Clinical assessments of the outcomes of assisted reproductive technology revealed that women who had frozen embryo transfer (ET) within 60 days of infection had reduced pregnancy rates than those who delayed ET [20]. However, other studies have shown that a history of mild or asymptomatic SARS-CoV-2 infection in females does not appear to have a negative impact on laboratory and clinical outcomes in fresh and frozen ET cycles [21, 22]. This suggests that current research on COVID-19's effects on reproduction might only represent the tip of the iceberg, necessitating extensive basic research and clinical observation. To find out if SARS-CoV-2 affects the endometrium during implantation or the embryo in its early stages of development, perhaps influencing implantation or raising the incidence of miscarriages, more research is required.
Using total RNA next-generation sequencing, Professor Lucía de Miguel-Gomez explains for the first time how systemic COVID-19 modifies global gene expression in the cyanobacteria of symptomatic women [9]. The purpose of this endeavor is to look at how COVID-19 affects essential elements of a healthy menstrual cycle and associated reproductive consequences. Professor I. Henarejos-Castillo's MSE gene co-expression network model provides insights into how transcriptome alterations caused by COVID-19, inferred from nasopharyngeal tissue, may impair important endometrial processes [10]. This model is instrumental in shedding light on the influence of COVID-19 on endometrial processes. These two articles reveal potential mechanisms by which COVID-19 may affect endometrial functions, however the patients’ age, menstrual cycle parameters, fertility status and biopsy collection timing of the selected endometrial specimens and throat swabs in both studies varied greatly. Using the endometrium [9] and throat swabs [10] of COVID-19-positive women as our reference, we overlapped the differentially expressed genes to investigate the relatively stable expressed molecule and lessen the discrepancy from the unparallel background of patients.
Without distinguishing expression trends, 50 overlapping molecules were identified; however, only 10 showed consistent trends, while 40 showed opposite trends. This strongly indicates the tissue specificity of COVID-19 infection and the impact of potential differences such as age and menstrual timing on the results. It also indirectly highlights the limitations of relying solely on bioinformatics analysis or using throat swab differential gene databases for such analyses. Upon analyzing these 10 molecules, we identified EPSTI1 and SUGT1 as two prioritized molecules.
In mixed cultures of human breast cancer cells and fibroblasts, the interferon-responsive gene EPSTI1 was first discovered [23]. It has been implicated in various biological processes, including tumor cell metastasis, epithelial-mesenchymal transition, chronic inflammation, tissue reconstruction, embryonic development, and the regulation of cell apoptosis [24, 25]. Capdevila-Busquets et al. reported that EPSTI1 can modulate the extrinsic apoptotic pathway in both estrogen receptor-positive and triple-negative breast cancer cell lines [24]. Methylation of EPSTI1 may play a role in SARS-CoV-2 infection processes and influence inflammatory and immune responses by regulating EPSTI1 expression [26]. Previous studies have reported that the expression of EPSTI1 was significantly increased in BAL cells, PBMCs, leukocytes, and nasopharyngeal tissue of COVID-19 patients [27, 28]. In our study, women with COVID-19 infection had considerably higher levels of EPSTI1 in their endometrial cells.
The cytokines, such as IFN-γ, IL-1β, IL-6, IL-12, IL-18, IL-33 and TGF-β, are associated with different clinical features of COVID-19. A long-lasting cytokine signature consisting of elevated levels of interleukin IL-1β could potentially underlie many of the clinical symptoms of post Covid-19 infection [13]. EPSTI1 is an interferon (IFN)-responsive gene and is highly expressed in exposed to virus infection as well as lipopolysaccharide-induced inflammatory [29]. Here, we further defined that IL-1β, LPS and INF-γ could upregulate EPSTI expression in Ishikawa cells and T-hESCs cells, suggesting that elevated EPSTI1 expression in the endometrium of patients after covid-19 infection may be associated with an inflammatory storm.
There have been few studies on the function of EPSTI1 in the endometrium. EPSTI1 levels is lower in endometrial tissues of pregnant mares compared to tissues from non-pregnant mares [30], suggesting a potential link to pregnancy. In our in vitro decidualization experiment, EPSTI1 inhibited the expression of dPRL, IGFBP-1 in T-hESCs. We also observed that EPSTI1, known as a molecule promotes epithelial-mesenchyma transition (EMT), was down-regulated in T-hESCs during decidualization in vitro. The ability of the mature endometrium to undergo decidualization and cyclic regeneration is essential for successful human reproduction. During the implantation phase, ovarian hormones regulate the mesenchyma-epithelial transition (MET) process, which is consistent with changes in stromal cellular morphology that arise with decidualization [31]. Therefore, it is reasonable to presume that EPSTI1 restrain decidualization by disturbing the MET process of endometrial stromal cells during the peri-implantation window.
By stimulating leucine-rich repeat-containing and nucleotide-binding domain proteins, SUGT1 participates in the innate immune response. When heat shock protein 90 was discovered, SUGT1 was shown to be a co-chaperone. According to reports [32–34], SUGT1 is connected to both HIV-1 infection and colorectal cancer. According to Ata et al., individuals experiencing recurrent pregnancy loss due to endometriosis may benefit from targeting endometrial SUGT1 as a possible therapeutic target [35]. Within one month of the COVID-19 infection, we found in this study that there was a elevation of SUGT1 in the endometrial cells of the patients. SUGT was also induced by IL-1β and LPS in Ishikawa cells and T-hESCs. In vitro, SUGT1 suppressed Bewo spheroids adhesion and decidualization while remaining unresponsive to the stimuli of progesterone or estrogen. We speculated that SUGT1 may participate in the dysregulation of endometrial receptivity though inflammatory pathway. More experiments will conducted to explore the concrete mechanism.
Although the peak of the COVID-19 epidemic has passed, research on COVID-19 should not be confined to this period. Our research demonstrates that following COVID-19 infection, the endometrium of the peri-implantation window experiences protein-level alterations that impact endometrial receptivity. It is noteworthy that we collected our endometrial samples between 10 and 30 days after the COVID-19 diagnosis, which better captures the relatively long-term consequences of the infection on the endometrium.
The immune response is a critical factor in the evolution of COVID-19. There is a growing understanding that the immune system's reaction to COVID-19, rather than the virus itself, is frequently the cause of the morbidity linked to it. With a growing population of recovering patients, it became clear that in 32–87% of patients (including those with mild acute disease), health impairments persist beyond the acute phase of infection [12]. The start and maintenance in a healthy pregnancy depend on the endometrium's dynamic balance between pro-and anti-inflammatory mediators. SARS-CoV-2 may disrupt the immunological milieu in the uterus, which might impact embryo implantation. The results of this investigation may lead to a reexamination of the connection between COVID-19 and the endometrium and generate theories regarding the emergence of protracted COVID and other post-COVID-19 related adverse effects.
The study faces limitations. The clinical sample size is limited. However, we verify potential molecular expression based on analysis from database of high quality and large scale. Our study led to hypotheses concerning the infection of COVID-19 influence the endometrium by regulating key molecules, which could potentially affect endometrial function and implantation. It is now imperative to further investigate this relationship to deepen our understanding.