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STUDY QUESTION 30Is SARS-CoV-2 receptor, angiotensin-converting enzyme 2 (ACE 2) expressed in the human 31 endometrium during the menstrual cycle, and does it participate in endometrial decidualization? 32 SUMMARY ANSWER 33 ACE2 protein is highly expressed in human endometrial stromal cells during the secretory phase 34 and is essential for human endometrial stromal cell decidualization. 35
WHAT IS KNOWN ALREADY 36ACE2 is expressed in numerous human tissues including the lungs, heart, intestine, kidneys and 37 placenta. ACE2 is also the receptor by which SARS-CoV-2 enters human cells. 38
STUDY DESIGN, SIZE, DURATION 39Proliferative (n = 9) and secretory (n = 6) phase endometrium biopsies from healthy reproductive-40 age women and primary human endometrial stromal cells from proliferative phase endometrium 41 were used in the study. 42
PARTICIPANTS/MATERIALS, SETTING, METHODS 43ACE2 expression and localization were examined by qRT-PCR, Western blot, and 44 immunofluorescence in both human endometrial samples and mouse uterine tissue. The effect of 45 ACE2 knockdown on morphological and molecular changes of human endometrial stromal cell 46 decidualization were assessed. Ovariectomized mice were treated with estrogen or progesterone 47 to determine the effects of these hormones on ACE2 expression. 48
MAIN RESULTS AND THE ROLE OF CHANCE 49In human tissue, ACE2 protein is expressed in both endometrial epithelial and stromal cells in the 50 proliferative phase of the menstrual cycle, and expression increases in stromal cells in the 51 secretory phase. The ACE2 mRNA (P ˂ 0.0001) and protein abundance increased during primary 52 human endometrial stromal cell (HESC) decidualization. HESCs transfected with ACE2-targeting 53 siRNA were less able to decidualize than controls, as evidenced by a lack of morphology change 54 and lower expression of the decidualization markers PRL and IGFBP1 (P ˂ 0.05). In mice during 55 pregnancy, ACE2 protein was expressed in uterine epithelial and stromal cells increased through 56 day six of pregnancy. Finally, progesterone induced expression of Ace2 mRNA in mouse uteri 57 more than vehicle or estrogen (P ˂ 0.05). 58
LIMITATIONS, REASONS FOR CAUTION 61Experiments assessing the function of ACE2 in human endometrial stromal cell decidualization 62 were in vitro. Whether SARS-CoV-2 can enter human endometrial stromal cells and affect 63 decidualization have not been assessed. 64
WIDER IMPLICATIONS OF THE FINDINGS 65Expression of ACE2 in the endometrium allow SARS-CoV-2 to enter endometrial epithelial and 66 stromal cells, which could impair in vivo decidualization, embryo implantation, and placentation. 67If so, women with COVID-19 may be at increased risk of early pregnancy loss. 68
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