The underlying mechanisms of chromosome separation synchrony and kinetics

Abstract

Accurate segregation of sister chromatids in anaphase is a crucial step of cell division. Its success ensures the proper passage of genetic information to progeny cells, whereas its failure can result in aneuploid or inviable descendant cells. Synchronous and sudden separation of sister chromatids is ubiquitously observed among cell types, but the regulation underlying this synchrony remains poorly understood. Following DNA replication, sister chromatids are held together by the cohesin complex. The protease separase cleaves cohesin at anaphase, which allows microtubules to pull sister chromatids to opposite spindle poles. Separase activity is inhibited by its binding partner securin and is released upon Anaphase-Promoting Complex/Cyclosome (APC/C)-dependent securin degradation at anaphase onset. Although this sequence of events, which form the core mechanism of chromosome segregation, is well understood, the mechanisms behind the dynamic regulation of this process are not fully known. The abrupt and synchronous onset of sister chromatid separation suggests switch-like control of separase activation, while chromosome-specific differences in separation timing suggest contribution of chromosome-specific features. Determining how the regulatory components of anaphase influence the synchrony and order of chromosome segregation is key to understanding how eukaryotic cells achieve proper division and faithful genome inheritance. Using live-cell imaging of centromere-labeled chromosomes in Schizosaccharomyces pombe, we show a slight preference for the order in which chromosomes separate (chromosome I before II, and II before III), but a large amount of variability between cells. By perturbing microtubule dynamics, securin degradation, separase activity, and cohesin amount and cleavage, we found that cohesin loading influences chromosome separation order, impaired microtubule function disrupts typical separation speeds, and the dynamics of securin degradation and separase activity determine separation synchrony. Through collaborative stochastic and deterministic modeling, we demonstrate that synchronous chromosome separation can be explained without previously proposed positive feedback regulation of separase. Deterministic modeling shows that switch-like separase activation can be achieved without feedback, while stochastic modeling shows that the temporal dynamics of separase activation are sufficient to explain the separation synchrony and order observed in both wild-type cells and cells with perturbations to the core segregation machinery. Together, our findings show that cohesin distribution and separase activation kinetics are sufficient to explain the observed synchrony and order of chromosome segregation, offering insight into the coordinated and rapid onset of anaphase.