Integrated ultrasensitive metabolomics and single-cell transcriptomics identify crucial regulators of sheep oocyte maturation and early embryo development in vitro

Developmental competence of oocytes matured in vitro is limited due to a lack of complete understanding of metabolism and metabolic gene expression during oocyte maturation and embryo development. Conventional metabolic analysis requires a large number of samples and is not efficiently applicable in oocytes and early embryos, thereby posing challenges in identifying key metabolites and regulating their in vitro culture system. To enhance the developmental competence of sheep oocytes, this study aimed to identify and supplement essential metabolites that were deficient in the culture systems. The metabolic characteristics of oocytes and embryos were determined using ultrasensitive metabolomics analysis on trace samples and single-cell RNA-seq. By conducting integrated analyses of metabolites in cells (oocytes and embryos) and their developmental microenvironment (follicular fluid, oviductal fluid, and in vitro culture systems), we identified key missing metabolites in the in vitro culture systems. In order to assess the impact of these key missing metabolites on oocyte development competence, we performed in vitro culture experiments. Furthermore, omics analyses were employed to elucidate the underlying mechanisms. Our findings demonstrated that betaine, carnitine and creatine were the key missing metabolites in vitro culture systems and supplementation of betaine and L-carnitine significantly improved the blastocyst formation rate (67.48% and 48.61%). Through in vitro culture experiments and omics analyses, we have discovered that L-carnitine had the potential to promote fatty acid oxidation, reduce lipid content and lipid peroxidation level, and regulate spindle morphological grade through fatty acid degradation pathway. Additionally, betaine may participate in methylation modification and osmotic pressure regulation, thereby potentially improving oocyte maturation and early embryo development in sheep. Together, these analyses identified key metabolites that promote ovine oocyte maturation and early embryo development, while also providing a new viewpoint to improve clinical applications such as oocyte maturation or embryo culture.