Research

CONDENSIN IDC REGULATES EXPRESSION OF X-LINKED GENES

Dosage compensation is the process of equalizing X chromosome gene dosage between sexes. In C. elegans, XX hermaphrodites downregulate expression from both X chromosomes by half, leading to X-linked gene expression levels equal to XO males. To achieve this, condensin IDC binds along the length of both hermaphrodite X chromosomes (Fig 1).

Figure 1. The model of dosage compensation in C. elegans. Condensin IDC binds both hermaphrodite X chromosomes to downregulate gene expression two-fold and to equalize X-linked gene expression between the sexes.

Due to the similarity of condensin IDC to condensin complexes functioning in mitosis, the processes likely use similar mechanisms. In fact, condensin IDC activity does lead to compaction of the X chromosomes in hermaphrodites, and as a result these chromosomes are much more compact than the X in males (Lau et al 2014) (Fig 2).

Figure 2. Three-dimensional reconstruction of X chromosome paint fluorescence in situ hybridization signals (red) in nuclei (blue) of hermaphrodite and male C. elegans. The X chromosomes in hermaphrodites are more compact than in males.

Studies of chromatin modifications revealed further parallels to mitosis. We found that the dosage compensated X chromosomes of hermaphrodites are enriched for histone H4 lysine 20 monomethylation (H4K20me1) compared to autosomes or the X chromosome in males, but they are depleted of histone H4 lysine 16 acetylation (H4K16ac) (Wells et al, 2012, Custer et al 2014) (Fig 3). Interestingly, histones are similarly modified during mitosis: chromosomal levels of H4K20me1 increase, while levels of H4K16ac decrease. H4K20 methylation levels in C. elegans are modulated by the methyltransferases SET-1 and SET-4, and the demethylase DPY-21. Our current research is investigating how condensin IDC activity impacts these enzymes and how this regulation is coordinated with cell cycle progression.

Figure 3. H4K20me1 (green, top) is enriched and H16K16ac (green, below) is depleted on dosage compensated X chromosomes (red) of hermaphrodites.

Chromatin compaction and histone modifications are not the only mechanisms contributing gene repression on the X chromosomes. Our research uncovered an additional mechanism: tethering the X chromosomes to the nuclear lamina (Snyder et al, 2016). Tethering requires H3K9me3-decorated heterochromatin and the nuclear lamina protein CEC-4 which binds this heterochromatic mark. This tethering machinery cooperates with condensin IDC to relocate not just heterochromatic regions, but the entire X chromosomes, to the nuclear periphery, a repressive compartment within the nucleus (Fig 4).

Figure 4. X chromosomes in wild type hermaphrodites are located near the nuclear periphery, while in mutants of the tethering machinery the chromosomes are decondensed and located more internally.

CONDENSINS I AND II FUNCTION IN CHROMOSOME SEGREGATION AND CELL DIVISION.

We are also investigating the functions of condensins I and II during chromosome segregation, with special attention paid to the less-well characterized complex, condensin I. We looked at two types of cell division: mitosis and meiosis. Mitotic divisions produce daughter cells identical to the parent cell, for example during cell divisions which give rise to the multitude of cell types making up a whole organism. Meiosis is a specialized form of chromosome segregation which gives rise to germ cells, such as sperm and oocytes, containing only half the number of chromosomes compared to the parent cell.

Our early initial characterization of condensin I showed that it colocalizes with the chromosomal passenger kinase AIR-2, both on chromosomes and on the mitotic spindle (Collette et al 2011). AIR-2 coordinates many aspects of cell division, including kinetochore-microtubule attachments, chromosome orientation, anaphase spindle dynamics, and cytokinesis. Through a collaboration with Dr. Raymond Chan’s laboratory at the Department of Human Genetics at the University of Michigan, we investigated the role of AIR-2 and condensin I during abscission, the final stage of cytokinesis. Our results revealed that in the presence of a chromatin bridge (chromosome segregation error), condensin I activity is required to maintain the integrity of the cytokinetic furrow until the lagging chromosomal material can be cleared (Bembenek et al 2013 ) (Fig 5).

Figure 5. The image shows the first cell division in a C. elegans embryo with chromosome segregation defects. Chromosomes (blue) are not fully separated, and microtubules (red) are unable to pull them apart. Condensin I (green) is recruited to the chromatin bridges between separating chromosomes.

During specific stages of meiosis, condensin I colocalizes with a structurally-related complex, cohesin (Fig 6). Cohesin is responsible for holding duplicated copies of DNA together and then regulating their timely separation. The colocalization of cohesin and condensin suggested that condensin may play a role in regulating cohesin. Analysis of condensin I mutants in fact showed that cohesin levels in meiosis are reduced, leading to defects in cohesin function. We found that condensin regulates cohesin by antagonizing the anti-cohesion factor WAPL-1. In the absence of condensin, WAPL-1 reduces cohesin levels on chromosomes during meiotic prophase I, similar to the role it plays during mitotic cell divisions (Hernandez et al 2018). Deterioration of cohesin function is thought to be the major contributor to age-related infertility. Our data suggest that condensin function may contribute to promoting the health of oocytes during aging.

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Figure 6. 3D reconstruction of the meiotic metaphase I spindle. Condensin I (green) localizes between paired homologs (blue), in a region where cohesin must be removed for chromosomes to separate. The spindle is shown in red.

RESEARCH VIDEOS

Wild-type X Chromosome Video

Wild-type X Chromosomes

Live Imaging of AIR-2 during Meiosis

Meiosis1