INTRODUCTION

Successful embryo implantation requires the coordinated progression of blastocyst development and synchronized adaptation of the uterine endometrium. The endometrium plays a central role in this process, and inadequate uterine preparation can result in implantation failure [1–3]. Throughout the estrous cycle, the endometrium undergoes dynamic structural and functional modifications to support embryo implantation [1–3]. These changes are tightly regulated by ovarian steroid hormones, cytokines, and growth factors, which collectively influence endometrial differentiation, pregnancy recognition signaling, uterine receptivity, and embryo–maternal interactions [1, 4, 5]. Embryo implantation is a highly coordinated process involving reciprocal communication between the blastocyst and the uterine epithelium [1, 3]. In rodents, trophoblast giant cells play a critical role in decidual invasion and placental formation [1, 6, 7], while cytoskeletal remodeling is essential for trophoblast differentiation and tissue reorganization during implantation [8–10].

In recent years, proteomic approaches have increasingly enabled the identification of dynamic protein-level changes associated with endometrial receptivity and implantation success [11, 12]. Advances in mass spectrometry-based proteomics, together with multi-omics integration, have enhanced our understanding of protein expression dynamics during the implantation window [11, 12]. Previous transcriptomic and multi-omics studies have provided important insights into uterine receptivity, primarily at the whole-tissue level, capturing combined signals from epithelial, stromal, and immune compartments [3, 11, 12]. However, cell type–specific proteomic characterization of the endometrial epithelium remains limited. Given that the luminal epithelium represents the primary interface for embryo–maternal interaction, targeted analysis of epithelial cells is essential to better define localized molecular changes associated with implantation. Whole-tissue analyses inherently reflect composite signals from epithelial, stromal, and immune compartments, which may obscure cell type–specific molecular changes critical for implantation [11, 12]. In contrast, the luminal epithelium serves as the first point of contact between the blastocyst and the maternal environment, mediating attachment, luminal closure, and early embryo–maternal signaling [3, 13]. Therefore, epithelial-specific proteomic analysis provides a more precise framework to resolve localized molecular adaptations at the implantation interface that may not be captured by whole-tissue approaches. Moreover, emerging evidence highlights redox regulation and antioxidant defense as key modulators of uterine receptivity [14–16]. Progesterone-mediated signaling and prostaglandin pathways also play crucial roles in regulating endometrial remodeling and implantation competence [1, 4, 17].

Despite these advances, significant knowledge gaps remain regarding the molecular mechanisms that govern epithelial adaptation during the establishment of uterine receptivity. Most previous investigations have focused on transcriptomic profiling or whole-endometrial tissue analyses, which do not adequately distinguish epithelial-specific responses from those occurring in stromal, vascular, or immune cell populations [3, 11, 12]. Consequently, the protein-level regulatory networks operating specifically within endometrial epithelial cells during the peri-implantation period remain insufficiently characterized. In particular, the coordinated involvement of redox homeostasis, endoplasmic reticulum stress responses, protein folding pathways, and cytoskeletal remodeling in facilitating epithelial receptivity has not been comprehensively explored. Because proteins represent the primary functional mediators of cellular activity, epithelial-specific proteomic profiling may provide critical insights that cannot be inferred solely from transcriptomic datasets. Addressing this gap is essential for improving our understanding of the molecular events that support embryo attachment and implantation and may contribute to the identification of biomarkers or regulatory pathways associated with reproductive success.

Therefore, this study aimed to characterize epithelial-specific proteomic changes in mouse endometrial cells during early pregnancy and to identify candidate regulators associated with uterine receptivity. By combining two-dimensional gel electrophoresis-based proteomic analysis with protein identification and molecular validation, this study sought to establish a focused proteomic resource for the receptive endometrial epithelium. Particular emphasis was placed on identifying proteins and pathways associated with redox regulation, endoplasmic reticulum (ER) stress adaptation, and epithelial remodeling during the transition from the pre-receptive to receptive uterine state.