Uterine fluid extracellular vesicle miR-223-3p as endometrial receptivity biomarker and regulator in implantation.

Endometrial receptivity is a pivotal determinant of successful embryo implantation, yet its regulation by uterine fluid (UF) extracellular vesicles (EVs) remains poorly understood. This study aimed to identify key UF EV miRNAs during the endometrial receptive period and observe the role in implantation. UF EVs were isolated from the same in vitro fertilization/intracytoplasmic sperm injection patients at two time points: the prereceptive (LH + 3/progesterone [P] + 1) and receptive (LH + 7/P + 5). MiRNA sequencing and validation identified miR-223-3p as the key miRNA. Bioinformatic analysis predicted FOXO1 (Forkhead box protein O1) as a primary target, forming a hub genes complex with H3-3B (H3.3 histone B) and SIRT1 (silent information regulator 2 homolog 1). To investigate the functional role of UF EV miR-223-3p, EVs with reduced miR-223-3p were isolated from the culture medium of sh-miR-223-3p Ishikawa cells. Functional studies demonstrated that EVs internalization by trophoblast cells led to a significant reduction in miR-223-3p levels. This knockdown resulted in markedly impaired trophoblast proliferation, migration, invasion and adhesion. Concurrently, FOXO1 were significantly downregulated. We propose a novel model whereby receptive endometrium selectively packages miR-223-3p into UF EVs, which are then internalized by the embryo to modulate trophoblast function through the FOXO1/H3-3B/SIRT1 complex. UF EV miR-223-3p emerges as a critical regulator of endometrial receptivity and a promising noninvasive biomarker for assessing implantation competence.