Micromanipulation of prophase I chromosomes from mouse spermatocytes reveals high stiffness and gel-like chromatin organization

Abstract

Meiosis produces four haploid cells after two successive divisions in sexually reproducing organisms. A critical event during meiosis is construction of the synaptonemal complex (SC), a large, protein-based bridge that physically links homologous chromosomes. The SC facilitates meiotic recombination, chromosome compaction, and the eventual separation of homologous chromosomes at metaphase I. We present experiments directly measuring physical properties of captured mammalian meiotic prophase I chromosomes. Mouse meiotic chromosomes are about ten-fold stiffer than somatic mitotic chromosomes, even for genetic mutants lacking SYCP1, the central element of the SC. Meiotic chromosomes dissolve when treated with nucleases, but only weaken when treated with proteases, suggesting that the SC is not rigidly connected, and that meiotic prophase I chromosomes are a gel meshwork of chromatin, similar to mitotic chromosomes. These results are consistent with a liquid- or liquid-crystal SC, but with SC-chromatin stiff enough to mechanically drive crossover interference.

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

“…However, the chromosome stiffness at different cell cycles have not been completely studied. In this project, we measured the chromosome stiffness of MEFs (Mouse Embryonic Fibroblasts) and found that the average Young's Modulus of MEF chromosomes was 340 ± 80 Pa (Figure 2B), which is consistent with our previously published data that MEF chromosomes have a modulus of 370 ± 70 Pa [13]. Next, we isolated chromosomes from mouse MI oocytes and measured their stiffness (Figure 2A).…”

“…We previously demonstrated that the central elements of the synaptonemal complex do not contribute to chromosome stiffness, and so we shifted our focus to the role meiotic cohesins during the meiosis I stage [13]. Cohesins can still load onto chromosomes even when an intact synaptonemal complex is not formed [31].…”

“…Hold pipette, force pipette, and stiff pipette were prepared to grab chromosomes. They were forged by using a micropipette puller (Sutter P-97) and were cut into suitable sizes [13].…”

A Hypothesis: Linking Phase Separation to Meiotic Sex Chromosome Inactivation and Sex-Body Formation

“…While the factors involved in MSCI are well established, the mechanisms that physically separate sex chromosomes from autosomes have been an open question in the field. Wang et al (2019) demonstrated there are more long-distance interactions within the X chromosome compared to autosomes, which may indicate X chromosomes are softer than stiff autosomes (Biggs et al, 2020). Unlike the rod-shaped autosomes, the territory of the X and Y chromosomes gradually becomes globe-shaped during sex-body formation.…”

Centrosome dysfunction associated with somatic expression of the synaptonemal complex protein TEX12

“…This complexity is reflected in a dedicated meiotic structure called the synaptonemal complex (SC). This supramolecular protein structure binds together homologous chromosome pairs and is essential for the maturation of inter-homologue crossover recombination events, and thereby fertility 2 – 6 . Consequently, meiotic proteins responsible for this process are highly specialised and otherwise silenced in somatic cells.…”