Investigating the regulation of cohesin dynamics during meiotic prophase in C. elegans

Abstract

The physical linking of sister chromatids after S-phase is known as sister chromatid cohesion (SCC) and is largely provided by the cohesin complex. The coordinated loss of SCC at anaphase onset is essential for correct chromosome segregation, a process mediated by proteolysis of the kleisin subunit of cohesin. In addition, during mitotic prophase of most organisms a large portion of cohesin is removed from chromosomes by a non-proteolytic pathway that depends on the conserved protein WAPL. Cohesin is also known to reload and SCC is re-established in response to DSBs after mitotic S-phase. During meiosis, SCC is similarly established during S-phase, but is then released in two steps during the sequential meiotic divisions. Also, unlike mitosis, multiple cohesin complexes with divergent functions exist in meiosis. Whether cohesin is dynamically associated with chromosomes during meiotic prophase and how this may be regulated was not known. Thus, the key aims of this project were to determine if WAPL mediates cohesin removal during meiotic prophase, and to find out if meiotic cohesin complexes display turnover on prophase chromosomes of the nematode C. elegans. Alternative meiosis-specific kleisins (REC-8, COH-3, and COH-4) define the different complexes present during worm meiosis. I show here that WAPL-1 limits the association of COH-3/4 complexes with meiotic prophase chromosomes, which severely limits their cohesive function. REC-8 complexes on the other hand are not affected much by WAPL-1. I show that loss of WAPL-1 affects the structure of axial elements and disrupts chromosome segregation and DSB repair. I also demonstrate by FRAP live imaging that there is significant turnover of cohesin on meiotic prophase chromosomes. Dynamic turnover of COH-3 is much greater that REC-8, as predicted by the different sensitivity to WAPL-1 of REC-8 and COH-3 cohesin complexes. These findings demonstrate that cohesin is actively removed and reloaded during meiotic prophase. Dysregulation of these processes could be relevant for human fertility, since SCC exhaustion over time is thought to contribute to the decline in fertility with increased maternal age.