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

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.