The crucial role of CTCF in mitotic progression during early development of sea urchin

CCCTC-binding factor (CTCF), an insulator protein with 11 zinc fingers, is enriched at the boundaries of topologically associated domains (TADs) in eukaryotic genomes. In this study, we isolated and analyzed the cDNAs encoding HpCTCF, the CTCF homolog in the sea urchin Hemicentrotus pulcherrimus, to investigate its expression patterns and functions during the early development of sea urchin. HpCTCF contains nine zinc fingers corresponding to fingers 2-10 of the vertebrate CTCF. Expression pattern analysis revealed that HpCTCF mRNA was detected at all developmental stages and in the entire embryo. Upon expressing the HpCTCF-GFP fusion protein in early embryos, we observed its uniform distribution within interphase nuclei. However, during mitosis, it disappeared from the chromosomes and subsequently reassembled on the chromosome during telophase. Moreover, the morpholino-mediated knockdown of HpCTCF resulted in mitotic arrest during the morula to blastula stage. Most of the arrested chromosomes were not phospholylated at serine 10 of histone H3, indicating that mitosis was arrested at the telophase by HpCTCF depletion. Furthermore, impaired sister chromatid segregation was observed using time-lapse imaging of HpCTCF-knockdown embryos. Thus, HpCTCF is essential for mitotic progression during the early development of sea urchins, especially during the telophase-to-interphase transition. However, the normal development of pluteus larvae in CRISPR-mediated HpCTCF-knockout embryos suggests that disruption of zygotic HpCTCF expression has little effect on embryonic and larval development.