Condensin controls mitotic chromosome stiffness and stability without forming a structurally contiguous scaffold

Abstract

During cell division, chromosomes must be folded into their compact mitotic form to ensure their segregation. This process is thought to be largely controlled by the action of condensin SMC protein complexes on chromatin fibers. However, how condensins organize metaphase chromosomes is not understood. We have combined micromanipulation of single human mitotic chromosomes, sub-nanonewton force measurement, siRNA interference of condensin subunit expression, and fluorescence microscopy, to analyze the role of condensin in large-scale chromosome organization. Condensin depletion leads to a dramatic (~10 fold) reduction in chromosome elastic stiffness relative to the native, non-depleted case. We also find that prolonged metaphase stalling of cells leads to overloading of chromosomes with condensin, with abnormally high chromosome stiffness. These results demonstrate that condensin is a main element controlling the stiffness of mitotic chromosomes. Isolated, slightly stretched chromosomes display a discontinuous condensing staining pattern, suggesting that condensins organize mitotic chromosomes by forming isolated compaction centers that do not form a continuous scaffold.

Cell-cycle and Age-Related Modulations of Mouse Chromosome Stiffness

“…In the past, mitotic chromosome stiffness has been measured and it was found that mitotic chromosomes had a chromatin network structure [9]. Several factors, including condensin, have been found to affect chromosome stiffness [46]. We aimed to identify whether cell stages and aging could influence chromosome stiffness.…”

“…Chromosomes stiffness has been extensively studied in various mitotic cells, revealing similarities and differences in chromosomes stiffness among different cell types [27][28][29].…”

The material properties of mitotic chromosomes

“…This chromosome axis, which also contains topoisomerase II, was originally proposed to form a rigid proteinaceous scaffold from which DNA loops are suspended [ 17 , 18 , 19 ]. Consistent with this “scaffolding” model, condensins impart structure and stiffness to mitotic chromosomes, allowing resistance to pulling forces exerted by the spindle in cells [ 20 , 21 , 22 , 23 , 24∗ , 25 ] or to micromanipulations on purified chromosomes [ 26 , 27 ].…”

“…In contrast to the mitotic bottlebrush structure formed by consecutive loops extruded by condensins, interphase loops formed by cohesin are short-lived, regulated by boundaries imposed by the protein CTCF, and interspersed with gaps [ 90 , 94∗ , 95∗ , 96 , 97 ]. This altered cross-link distribution leads to a less constrained chromatin state than that observed in mitosis, which may explain why interphase chromatin is more mobile than mitotic chromatin [ 26 , 27 , 98∗ , 99 , 100 , 101∗ ] ( Figure 4 ). The functional consequences of increased interphase chromatin mobility, and the interplay between interphase loop extrusion and the regulation of genome expression and maintenance, are now beginning to be understood.…”