HP1α is a chromatin crosslinker that controls nuclear and mitotic chromosome mechanics

Abstract

Chromatin, which consists of DNA and associated proteins, contains genetic information and is a mechanical component of the nucleus. Heterochromatic histone methylation controls nucleus and chromosome stiffness, but the contribution of heterochromatin protein HP1α (CBX5) is unknown. We used a novel HP1α auxin-inducible degron human cell line to rapidly degrade HP1α. Degradation did not alter transcription, local chromatin compaction, or histone methylation, but did decrease chromatin stiffness. Single-nucleus micromanipulation reveals that HP1α is essential to chromatin-based mechanics and maintains nuclear morphology, separate from histone methylation. Further experiments with dimerization-deficient HP1αI165E indicate that chromatin crosslinking via HP1α dimerization is critical, while polymer simulations demonstrate the importance of chromatin-chromatin crosslinkers in mechanics. In mitotic chromosomes, HP1α similarly bolsters stiffness while aiding in mitotic alignment and faithful segregation. HP1α is therefore a critical chromatin-crosslinking protein that provides mechanical strength to chromosomes and the nucleus throughout the cell cycle and supports cellular functions.

Heterochromatin flexibility contributes to chromosome segregation in the cell nucleus

“…Given the complexity of the in vivo nuclear environment, the e↵ect that we demonstrate here could enhance the heterochromatin-rich domain formations in the nuclear interior in vivo in conjunction with liquid-liquid phase separation [25,27,28,64]. Chromatin with heterochromatin markers chemically favors interactions with HP1 [26,59], and this could e↵ectively induce an attraction between heterochromatin units.…”

“…The copyright holder for this preprint this version posted December 2, 2020. ; https://doi.org/10.1101/2020.12.01.403832 doi: bioRxiv preprint 2 Understanding behavior of chromatin copolymer in vivo is further complicated by the very di↵erent chemistry of its euchromatin and heterochromatin sections. For instance, recent experiments have shown that heterochromatin and the HP1 protein undergo a liquid-liquid phase separation, while euchromatin does not [25][26][27][28]. Indeed, for polymers, chemical a nity di↵erences between the involving species (heterochromatin and HP1 in this case) can lead to microphase separation instead of homogeneous mixtures, in which domains with various sizes and morphologies can be observed [29,30].…”

Regulation of Nuclear Mechanics and the Impact on DNA Damage

“…This crosslinking effect of HP1 is thought to stabilise compacted chromatin states and to be essential in heterochromatin phase separation [ 84 , 85 ] through the formation of membrane-less condensates. A recent report by Strom et al also showed that HP1 chromatin crosslinking capabilities are important for nuclear shape maintenance, and its degradation leads to decreased chromatin stiffness and nuclear rigidity [ 86 ].…”

Reflections on the organization and the physical state of chromatin in eukaryotic cells

“…Biomechanical studies measuring force transduction within the nucleus under different conditions indicate a viscoelastic, hydrogel-like behavior of chromatin (Shimamoto et al 2017). The degree of nuclear rigidity appears to be highly dependent on the status of histone acetylation (Shimamoto et al 2017), histone methylation (Nava et al 2020), the presence of CBX5 (Strom et al 2020), and the nuclear concentration of bivalent ions (Shimamoto et al 2017). Some cell types can undergo drastic reversible deformations of the nucleus, with a striking example provided by the nuclei of granulocytes squeezing through gaps between endothelial cells of capillary walls (Labernadie and Trepat 2018).…”