Metabolomic Analysis of Key Metabolites and Regulatory Mechanisms in the Transition of Uterine Receptivity in Water Buffalo (Bubalus bubalis).

Background: While economically vital, buffalo exhibits low reproductive efficiency largely due to embryonic losses during implantation. Successful implantation requires precise embryo-maternal communication and metabolic/immune adaptations in the endometrium. We aimed to identify key serum metabolic signatures and associated peripheral immune responses that characterize the endometrial receptivity window during early pregnancy in water buffalo. Methods: Blood samples from pregnant (Preg, n = 12) and non-pregnant (Non-P, n = 10) buffaloes were collected on days 15, 18, and 21 post-artificial insemination (AI). We measured leukocyte counts and hormone levels and performed untargeted serum metabolomic profiling using LC-MS. Results: Pregnant buffaloes showed significantly reduced total white blood cell count, lymphocyte (LYM%), and neutrophil (NEU%) percentages, indicating immune remodeling at the beginning of pregnancy establishment. Metabolomic analysis identified 131 differentially expressed metabolites (DEMs) associated with pregnancy status at different stages. Enriched pathways included steroid hormone synthesis, retinol metabolism, starch/sucrose metabolism, and phenylalanine biosynthesis. Crucially, alterations in unsaturated fatty acids, retinol, and phenylalanine metabolism, along with monocyte (MON%)/LYM% ratios, were strongly linked to receptivity changes and successful implantation. Conclusions: Endometrial receptivity in buffalo during the embryonic implantation window was associated with changes in immune cells and metabolism in the blood, suggesting that immunometabolism may play an essential role in modulating endometrial receptivity during the implantation window. This study provides potential clues and a metabolic framework for understanding the underlying mechanisms of buffalo embryonic implantation.