Plat safeguards aged mouse oocytes against programmed cell death through activating the Erk1/2 pathway

Female reproductive aging is characterized by the decline in oocytes quality and developmental potentials, but the underlying mechanisms remain insufficiently investigated. In this study, we analyze the transcriptomes of individual oocytes from young and aged mice, identifying a distinct group of aged oocytes, which display morphological abnormalities, such as fragmentation and darkened cytoplasm. These defective aged oocytes show a global shrink in gene expression, with yet a notable expression increase in mitochondrial RNA. Further gene expression analysis indicates that these defective oocytes exhibit typical features of programmed cell death (PCD), and functional assays demonstrate that those oocytes are specifically subject to both apoptosis and ferroptosis. Blocking the PCD pathways largely reverses the defective morphology. Interestingly, the expression of tissue-type plasminogen activator (tPA, or Plat) decreases with age, and even becomes undetectable in defective oocytes. Loss of Plat expression in oocytes downregulates Erk1/2 pathway activity, closely resembling the effects observed in Erk1/2 deficiency. Combining with tPA-interactome assay, it is suggested that Plat functions as an upstream regulator of Erk1/2 pathway activity, likely through Erk1/2 phosphorylation. Supplementing exogenous tPA in oocyte cultures reduces defect rate and therefore improves oocyte quality. Collectively, our findings highlight Plat as a pivotal factor in protecting aged mouse oocytes from PCD, with potential implications for developing better clinical strategies to enhance oocyte quality in aging females.