Ki-67 and CDK1 control the dynamic association of nuclear lipids with mitotic chromosomes.

Nuclear lipids play roles in regulatory processes, such as signaling, transcriptional regulation, and DNA repair. In this report, we demonstrate that nuclear lipids may contribute to Ki-67-regulated chromosome integrity during mitosis. In COS-7 cells, nuclear lipids are enriched at the perichromosomal layer and excluded from intrachromosomal regions during early mitosis but are then detected in intrachromosomal regions during late mitosis, as revealed by TT-ExM (expansion microscopy with trypsin digestion and tyramide signal amplification), an improved expansion microscopy technique that enables high-sensitivity and super-resolution imaging of proteins, lipids, and nuclear DNA. The nuclear nonhistone protein Ki-67 acts as a surfactant to form a repulsive molecular brush around fully condensed sister chromatids in early mitosis, preventing the diffusion or penetration of nuclear lipids into intrachromosomal regions. Ki-67 is phosphorylated during mitosis by cyclin-dependent kinase 1 (CDK1), the best-known master regulator of the cell cycle. Both Ki-67 knockdown and reduced Ki-67 phosphorylation by CDK1 inhibitors allow nuclear lipids to penetrate chromosomal regions. Thus, both Ki-67 protein level and phosphorylation status during mitosis appear to influence the perichromosomal distribution of nuclear lipids. Ki-67 knockdown and CDK1 inhibition also lead to uneven chromosome disjunction between daughter cells, highlighting the critical role of this regulatory mechanism in ensuring accurate chromosome segregation. Given that Ki-67 has been proposed to promote chromosome individualization and establish chromosome-cytoplasmic compartmentalization during open mitosis in vertebrates, our results reveal that nuclear lipid enrichment at the perichromosomal layer enhances the ability of Ki-67 to form a protective perichromosomal barrier (chromosome envelope), which is critical for correct chromosome segregation and maintenance of genome integrity during mitosis.