Homo sapiens Raw sequence reads. Homo sapiens

A precise regulatory network of gene transcription is required for the cross-talk of nuclear and cytoplasm in the process of oocyte maturation. However, the mechanisms coordinating the maturation with environment-driven metabolic shift, a critical step in determining development potential of human in-vitro matured (IVM) oocytes, remains to be elucidated. To determine the mechanisms behind this difference, we compared the global transcriptional characteristics of vivo– and in vitro–matured oocytes using single-cell RNA-sequencing. Of the pathways affected by differentially expressed genes, about 30% belonged to substance and energy metabolism. ACAT1 and HADHA, which respectively encode enzymes controlling the supply of Acetyl-CoA and succinate, the substrates of the tricarboxylic acid (TCA) cycle, were silenced and therefore induced the failure of energy metabolism. Cytoplasmic levels of calcium, a key activator of these enzymes, were reduced because of abnormal expression of genes encoding transmembrane calcium carrier proteins. Although release of endogenous calcium from the ER and mitochondria met the requirement for maturation, excessive release resulted in cell apoptosis and mitochondrial dysfunction. Although the dysfunctional TCA cycle resulted in deficient NADH production, high nicotinamide nucleotide transhydrogenase expression suggested that a NADH shortage could be compensated for by NADPH dehydrogenation. Moreover, high NADP+ activated DPYD and thus enhanced the repair of DNA double-strand breaks to maintain chromosome euploidy. Thus in-vitro maturation of human oocytes is mediated through a cascade of competing and compensatory actions driven by enzyme-encoding genes.