Mitotic chromosomes are mechanically connected by chromatin-based interchromosome linkers

Abstract

The spatial organization of chromosomes is crucial for gene regulation and genome stability. Metaphase chromosomes appear physically discrete from one another in conventional chromosome preparations. However, interchromosomal connections, or "linkers", have been observed among mitotic chromosomes, and may play important roles in coordinated chromosome movements and genome stability. We use micropipette-based isolation and manipulation to analyze interchromosome linkers between mammalian mitotic chromosomes. Pulling an isolated chromosome reveals that thin linkers connect them; CREST staining indicates their locations to be at centromeres. The linkers display linear elasticity and have a length-doubling force of approximately 300 pN, comparable to the length-doubling force of an entire metaphase chromosome. Enzymatic treatments demonstrate that the linkers are disrupted by DNase, but not by RNase, protease, or microtubule polymerization (spindle fiber) inhibitors, indicating that linker connectivity is based on DNA. Immunofluorescence experiments indicate that histones and topo II are on the linkers, with topo II organized into discrete foci spaced by approximately 0.4 Mbp. After prolonged metaphase arrest using spindle inhibitors isolated genomes do not display interchromosome linkers. Finally, experiments on intact cells show that interchromosome linkers containing CENP-B can be observed between mitotic chromosomes, indicating they are not an artifact of genome isolation.