Central spindle microtubules are strongly coupled to chromosomes during both anaphase A and anaphase B

Abstract

18Spindle microtubules, whose dynamics vary over time and at different locations, 19 cooperatively drive chromosome segregation. Measurements of microtubule dynamics and 20 spindle ultrastructure can provide insight into the behaviors of microtubules, helping elucidate 21 the mechanism of chromosome segregation. Much work has focused on the dynamics and 22 organization of kinetochore microtubules, i.e. on the region between chromosomes and poles. In 23 comparison, microtubules in the central spindle region, between segregating chromosomes, have 24 been less thoroughly characterized. Here, we report measurements of the movement of central 25 spindle microtubules during chromosome segregation in human mitotic spindles, and 26Caenorhabditis elegans mitotic and female meiotic spindles. We found that these central spindle 27 microtubules slide apart at the same speed as chromosomes, even as chromosomes move towards 28 spindle poles. In these systems, damaging central spindle microtubules by laser ablation caused 29 an immediate and complete cessation of chromosome motion, suggesting a strong coupling 30 between central spindle microtubules and chromosomes. Electron tomographic reconstruction 31 revealed that the analyzed anaphase spindles all contain microtubules with both ends between 32 segregating chromosomes. Our results provide new dynamical, functional, and ultrastructural 33 characterizations of central spindle microtubules during chromosome segregation in diverse 34 spindles, and suggest that central spindle microtubules and chromosomes are strongly coupled in 35 anaphase. 36 102 2017; Redemann et al., 2018). These different structures make it unclear to what extent central 103 spindle microtubules are coupled to chromosomes in different spindles.104 6In this work, we investigated the relationship between central spindle microtubules and 105 chromosomes during anaphase in human mitotic spindles, and C. elegans mitotic and female 106 meiotic spindles. We used the same laser ablation and electron microscopy techniques on all of 107 these spindles to avoid the potential complications of interpretation that can arise when different 108 techniques are applied to different systems. We found that central microtubules slide apart at the 109 same speed as chromosomes, even when chromosomes move closer to poles. Damaging the 110 central spindle microtubules by laser ablation caused immediate and complete cessation of 111 chromosome motion, even when Anaphase A and Anaphase B occur simultaneously, suggesting 112 a strong coupling between central spindle microtubules and chromosomes. Our electron 113 tomographic reconstructions further reveal that these anaphase spindles all contain microtubules 114 with both ends between segregating sister chromosomes, and other central spindle microtubules 115 which terminate on either chromosomes or kinetochore fibers. Taken together, this work 116 suggests that central spindle microtubules are strongly coupled to chromosomes in anaphase 117 spindles. 118 7 Results 119 Central-sp...

Sumoylation regulates central spindle protein dynamics during chromosome segregation in oocytes

“…Previous reports have highlighted the importance of the central-spindle for chromosome segregation in oocytes Redemann et al, 2018;Yu et al, 2019). Remarkably, this central-spindle-based mechanism could be more widespread than anticipated, as it has been shown to exist also during mitosis in C. elegans and in human cells (Yu et al, 2019). Here we focused on the dynamic behaviour of key proteins and to what extent this is regulated by sumoylation.…”

“…Overall, we have shown that sumoylation regulates the dynamics of central-spindle proteins during female meiosis, namely BUB-1 and CLS-2. Previous reports have highlighted the importance of the central-spindle for chromosome segregation in oocytes Redemann et al, 2018;Yu et al, 2019). Remarkably, this central-spindle-based mechanism could be more widespread than anticipated, as it has been shown to exist also during mitosis in C. elegans and in human cells (Yu et al, 2019).…”

“…How is initial chromosome separation achieved during anaphase. Several lines of evidence point towards a two-step chromosome segregation mechanism operating during anaphase I in C. elegans oocytes: i) Two phases of chromosome segregation characterised by different segregation speeds were reported (McNally et al, 2016); ii) central-spindle CLS-2 localisation is at least partially BUB-1-independent ; iii) central-spindle ablation during mid-anaphase stops chromosomes segregation Yu et al, 2019); and iv) the area between segregating homologues is microtubule-free during early anaphase (Redemann et al, 2018), suggesting that the initial steps of segregation could be at least partially CLS-2-independent. Our results suggest that this early anaphase stage is subject to regulation by SUMO, and it will therefore be important to address which protein(s) are downstream of BUB-1 and SUMO.…”

“…During anaphase, while chromosomes are segregating, MTs populate the interchromosomal region creating the central-spindle. While many studies of MT-dependent chromosome movement focused on pulling forces generated by kinetochore MTs (kMTs) making end-on contacts with chromosomes (Cheeseman, 2014), there is also evidence for pushing forces that are exerted on the segregating chromosomes (Khodjakov et al, 2004;Nahaboo et al, 2015;Laband et al, 2017;Vukušić et al, 2017;Yu et al, 2019).…”

“…During C. elegans female meiosis, kinetochores disassemble in early anaphase I and appear to be dispensable for chromosome segregation (Dumont et al, 2010;Hattersley et al, 2016;McNally et al, 2016). In addition, tomographic reconstruction in electron microscopy of the C. elegans female meiotic spindle revealed that during anaphase I, central-spindle MTs transition from a lateral to an end-on orientation Redemann et al, 2018;Yu et al, 2019). Therefore, while the balance between central-spindle MTand kMT-driven forces may vary in different spindles, the former seems most important during female meiosis in C. elegans.…”