Physical Determinants of Bipolar Mitotic Spindle Assembly and Stability in Fission Yeast

Abstract

Mitotic spindles use an elegant bipolar architecture to segregate duplicated chromosomes with high fidelity. Bipolar spindles form from a monopolar initial condition; this is the most fundamental construction problem that the spindle must solve. Microtubules, motors, and cross-linkers are important for bipolarity, but the mechanisms necessary and sufficient for spindle assembly remain unknown. We describe a physical model that exhibits de novo bipolar spindle formation. We began with physical properties of fission-yeast spindle pole body size and microtubule number, kinesin-5 motors, kinesin-14 motors, and passive cross-linkers. Our model results agree quantitatively with our experiments in fission yeast, thereby establishing a minimal system with which to interrogate collective selfassembly. By varying the features of our model, we identify a set of functions essential for the generation and stability of spindle bipolarity. When kinesin-5 motors are present, their bidirectionality is essential, but spindles can form in the presence of passive cross-linkers alone. We also identify characteristic failed states of spindle assemblythe persistent monopole, X spindle, separated asters, and short spindle, which are avoided by the creation and maintenance of antiparallel microtubule overlaps. Our model can guide the identification of new, multifaceted strategies to induce mitotic catastrophes; these would constitute novel strategies for cancer chemotherapy.

Distribution of Eg5 and TPX2 in mitosis: Insight from CRISPR tagged cells

“…A variety of experimental approaches have provided insight into the proteins that comprise various parts of the spindle and the spindle as a whole (Kagiali, Senturk, Kucuk, Queshi, & Ozlu, 2017). This information is critical for building realistic models of mitosis and gaining insight into mechanism from the stoichiometry of various spindle components (Blackwell et al, 2017;). This information is critical for building realistic models of mitosis and gaining insight into mechanism from the stoichiometry of various spindle components (Blackwell et al, 2017;).…”

“…Obtaining a parts list is a key step to understanding mitosis; however, it is also important to know the distribution of the various components and their relative concentrations (Wu & Pollard, 2005). This information is critical for building realistic models of mitosis and gaining insight into mechanism from the stoichiometry of various spindle components (Blackwell et al, 2017;).…”

Pivot-and-bond model explains microtubule bundle formation

“…1c, Supplementary Movie 2 in ref. [30]) and the increased Mal3 signal in the spindle after bundling ( Fig. 1d).…”

“…Studies in fission yeast have shown that passive (thermal) pivoting motion of MTs around the spindle pole body accelerates kinetochore capture (26)(27)(28), together with dynamic instability of MTs (29). MT rotational motion about a pivot at the SPB was also included in the model for spindle formation (30) and in vitro studies (31). However, observation of the dynamics of bundle formation in vivo and a corresponding physical model are required to understand the formation and stability of MT bundles.…”

“…By introducing the pivot and bond model we gain a deeper understanding of the mesoscopic properties of the bundling process, such as bundle stability and average bundling time, as well as predict their dependence on biological parameters such as MT length and cross-linker concentration. This approach is complementary to more exhaustive and detailed methods such as large-scale numerical simulations (for example(30,38)). Along with parallel MT bundles, mitotic spindles also contain bundles of anti-parallel MTs, which are made of MTs extending from the opposite spindle poles.…”