Maternal TDP43 orchestrates nuclear speckle assembly and zygotic splicing activation during oocyte-to-embryo transition in mice

Zygotic splicing activation (ZSA) is a crucial process of mRNA post-transcriptional regulation during maternal-to-zygotic transition, ensuring normal embryonic development. However, the key factors and mechanisms underlying ZSA regulation remain unclear. We observed that the nuclear speckle (NS), a key splicing region, was newly established at the 2-cell stage in mice and was consistent with the ZSA period. Moreover, NS and TAR DNA-binding protein-43 (TDP43) always exhibited a partially adjacent and mutually exclusive localization relationship. TDP43 performed its function through liquid-liquid phase separation. The condensation-deficient state of TDP43 was the active form involved in NS regulation in 2-cell embryos. Additionally, TDP43 could also interact with both transcribed RNAs associated with NS and directly with NS proteins. Maternal TDP43 deficiency led to the failure of NS assembly in 2-cell embryos, resulting in the inability to skip exons during transcript splicing. In contrast, ectopic expression of TDP43 in zygotes led to abnormal enlargement of the NS, resulting in excessive skipping of transcript exons. Both bidirectional ZSA disorders led to 2-cell arrest during early embryogenesis. ZSA defects caused by TDP43 deficiency also impaired cell totipotency-pluripotency conversion. In this study, we identified an NS upstream regulatory factor, TDP43, which helps maintain the balance of ZSA, providing a new perspective on the post-transcriptional regulation of early embryos.