ZC3H14 phosphorylation is required for nuclear speckle assembly during zygotic splicing activation in mice

Abstract

Abstract Following zygotic genome activation (ZGA) in embryos, nuclear speckles, which are subnuclear domains where pre-mRNA undergoes splicing, are reassembled in a process called zygotic splicing activation (ZSA). ZSA is vital for early development, but its regulation is poorly understood. This study identifies the RNA-binding protein ZC3H14 as a key promoter of nuclear speckle formation during ZSA. Although viable, Zc3h14 knockout mice exhibit reduced female fertility. Most embryos from these females arrest during the 1-cell to 2-cell transition, and the remaining embryos arrest at the 2-cell stage, highlighting a critical role for maternally deposited ZC3H14. The protein localizes to nuclear speckles, and its phosphorylation by CDK12/13 regulates mRNA binding and initial speckles assembly. Blocking this phosphorylation disrupts proper splicing and leads to abnormal RNA accumulation. Consequently, phosphorylation-mimicking or phosphorylation-blocking ZC3H14 mutants expressed in zygotes positively and negatively affect development, respectively. Taken together, our results indicate that ZC3H14 phosphorylation by CDK12/13 maintains fertility in female mice by promoting nuclear speckle formation during ZSA.