Does “Chromosome Envelope” Exist in Addition to the Cell Membrane and Nuclear Envelope?

Dr. Hsueh, Yi-Ping and Wang, Ting-Fang - December, 2024

During eukaryotic mitosis, sister chromatids first undergo condensation and individuation before being equally segregated into the two daughter cells. This process varies across species. In most fungi (e.g., budding yeast), mitosis occurs in a “closed” manner, where the nuclear envelope (NE) remains intact throughout. In this scenario, the NE, cytoplasm, and cell membrane divide simultaneously, preventing mixing of cytoplasmic and chromosomal components.

In contrast, animal and plant cells undergo “open mitosis,” where the NE ruptures and disassembles before sister chromatid separation. After cell division, the NE reforms around the daughter nuclei. A critical question in open mitosis is how chromosomal components are protected from cytoplasmic exposure during the transient absence of the NE—a longstanding challenge in cell biology.

Dr. Hsiao-Tang Hu, using the trypsin-tyramide expansion microscopy (TT-ExM) method developed by Dr. Bi-Chang Chen and Mr. Ueh-Ting Wang, demonstrated for the first time that “chromosome envelopes (CE)” exist in mammalian cells. This structure, composed of nuclear non-histone proteins such as Ki-67, is regulated by phosphorylation through cyclin-dependent kinases (CDKs). Ki-67 facilitates the accumulation of phosphatidylserine (PS) and other lipids on the outer surfaces of mitotic chromosomes. The CE acts as a barrier, preventing the diffusion or penetration of cytoplasmic components into the interiors of mitotic chromosomes, offering a novel mechanism to maintain chromosomal integrity during open mitosis.

"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, an improved expansion microscopy technique that enables high-sensitivity, super-resolution imaging of proteins, lipids, and nuclear DNA. The nuclear non-histone 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 Ki-67’s ability to form a protective perichromosomal barrier (chromosome envelope), which is critical for correct chromosome segregation and maintenance of genome integrity during mitosis.