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Accurate chromosome segregation is vital for cell and organismal viability. The mitotic spindle, a bipolar macromolecular machine composed largely of dynamic microtubules, is responsible for chromosome segregation during each cell replication cycle. Prior to anaphase, a bipolar metaphase spindle must be formed in which each pair of chromatids is attached to microtubules from opposite spindle poles. In this bipolar configuration pulling forces from the dynamic microtubules can generate tension across the sister kinetochores. The tension status acts as a signal that can destabilize aberrant kinetochore-microtubule attachments and reinforces correct, bipolar connections. Historically it has been challenging to isolate the specific role of tension in mitotic processes due to the interdependency of attachment and tension status at kinetochores. Recent technical and experimental advances have revealed new insights into how tension functions during mitosis. Here we summarize the evidence that tension serves as a biophysical signal that unifies multiple aspects of kinetochore and centromere function to ensure accurate chromosome segregation.
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CellDynaMo–stochastic reaction-diffusion-dynamics model: Application to search-and-capture process of mitotic spindle assembly
We introduce a Stochastic Reaction-Diffusion-Dynamics Model (SRDDM) for simulations of cellular mechanochemical processes with high spatial and temporal resolution. The SRDDM is mapped into the CellDynaMo package, which couples the spatially inhomogeneous reaction-diffusion master equation to account for biochemical reactions and molecular transport within the Langevin Dynamics (LD) framework to describe dynamic mechanical processes. This computational infrastructure allows the simulation of hours of molecular machine dynamics in reasonable wall-clock time. We apply SRDDM to test performance of the Search-and-Capture of mitotic spindle assembly by simulating, in three spatial dimensions, dynamic instability of elastic microtubules anchored in two centrosomes, movement and deformations of geometrically realistic centromeres with flexible kinetochores and chromosome arms. Furthermore, the SRDDM describes the mechanics and kinetics of Ndc80 linkers mediating transient attachments of microtubules to the chromosomal kinetochores. The rates of these attachments and detachments depend upon phosphorylation states of the Ndc80 linkers, which are regulated in the model by explicitly accounting for the reactions of Aurora A and B kinase enzymes undergoing restricted diffusion. We find that there is an optimal rate of microtubule-kinetochore detachments which maximizes the accuracy of the chromosome connections, that adding chromosome arms to kinetochores improve the accuracy by slowing down chromosome movements, that Aurora A and kinetochore deformations have a small positive effect on the attachment accuracy, and that thermal fluctuations of the microtubules increase the rates of kinetochore capture and also improve the accuracy of spindle assembly.
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- PAR ID:
- 10419983
- Editor(s):
- Meier-Schellersheim, Martin
- Date Published:
- Journal Name:
- PLOS Computational Biology
- Volume:
- 18
- Issue:
- 6
- ISSN:
- 1553-7358
- Page Range / eLocation ID:
- e1010165
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The accurate segregation of chromosomes is essential for the survival of organisms and cells. Mistakes can lead to aneuploidy, tumorigenesis and congenital birth defects. The spindle assembly checkpoint ensures that chromosomes properly align on the spindle, with sister chromatids attached to microtubules from opposite poles. Here, we review how tension is used to identify and selectively destabilize incorrect attachments, and thus serves as a trigger of the spindle assembly checkpoint to ensure fidelity in chromosome segregation. Tension is generated on properly attached chromosomes as sister chromatids are pulled in opposing directions but resisted by centromeric cohesin. We discuss the role of the Aurora B kinase in tension-sensing and explore the current models for translating mechanical force into Aurora B-mediated biochemical signals that regulate correction of chromosome attachments to the spindle.more » « less
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Edelstein-Keshet, Leah (Ed.)The search-and-capture model of spindle assembly has been a guiding principle for understanding prometaphase for decades. The computational model presented allows one to address two questions: how rapidly the microtubule–kinetochore connections are made, and how accurate these connections are. In most previous numerical simulations, the model geometry was drastically simplified. Using the CellDynaMo computational platform, we previously introduced a geometrically and mechanically realistic 3D model of the prometaphase mitotic spindle, and used it to evaluate thermal noise and microtubule kinetics effects on the capture of a single chromosome. Here, we systematically investigate how geometry and mechanics affect a spindle assembly’s speed and accuracy, including nuanced distinctions between merotelic, mero-amphitelic, and mero-syntelic chromosomes. We find that softening of the centromere spring improves accuracy for short chromosome arms, but accuracy disappears for long chromosome arms. Initial proximity of chromosomes to one spindle pole makes assembly accuracy worse, while initial chromosome orientation matters less. Chromokinesins, added onto flexible chromosome arms, allow modeling of the polar ejection force, improving a spindle assembly’s accuracy for a single chromosome. However, spindle space crowding by multiple chromosomes worsens assembly accuracy. Our simulations suggest that the complex microtubule network of the early spindle is key to rapid and accurate assembly.more » « less
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Abstract Macromolecular assembly depends on tightly regulated pairwise binding interactions that are selectively favored at assembly sites while being disfavored in the soluble phase. This selective control can arise due to molecular density-enhanced binding, as recently found for the kinetochore scaffold protein CENP-T. When clustered, CENP-T recruits markedly more Ndc80 complexes than its monomeric counterpart, but the underlying molecular basis remains elusive. Here, we use quantitativein vitroassays to reveal two distinct mechanisms driving this behavior. First, Ndc80 binding to CENP-T is a two-step process: initially, Ndc80 molecules rapidly associate and dissociate from disordered N-terminal binding sites on CENP-T. Over time, these sites undergo maturation, resulting in stronger Ndc80 retention. Second, we find that this maturation transition is regulated by a kinetic barrier that is sensitive to the molecular environment. In the soluble phase, binding site maturation is slow, but within CENP-T clusters, this process is markedly accelerated. Notably, the two Ndc80 binding sites in human CENP-T exhibit distinct maturation rates and environmental sensitivities, which correlate with their different amino-acid content and predicted binding conformations. This clustering-induced maturation is evident in dividing human cells, suggesting a distinct regulatory entry point for controlling kinetochore assembly. We propose that the tunable acceleration of binding site maturation by molecular crowding may represent a general mechanism for promoting the formation of macromolecular structures. Significance StatementA distinctive mechanism of protein-protein interaction underpins the assembly of kinetochores, which is critical for human cell division. During mitosis, the Ndc80 complex must bind tightly to the unstructured N-terminus of its receptor, CENP-T, which is densely clustered at kinetochores. Using single-moleculein vitroassays, we show that Ndc80 binding is mediated by an initially unstable yet tunable interface. The high molecular density of CENP-T at the kinetochores accelerates the maturation of this binding interface, favoring the formation of stable complexes within the kinetochore structure, rather than in the soluble phase. This environment-driven modulation of binding site maturation may represent a key regulatory mechanism for ensuring strong and specific interactions during the assembly of macromolecular complexes such as kinetochores.more » « less
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Bloom, Kerry (Ed.)During mitosis, kinetochore–microtubule attachments are monitored by a molecular surveillance system known as the spindle assembly checkpoint. The prevailing model posits that dynein evicts checkpoint proteins (e.g., Mad1, Mad2) from stably attached kinetochores by transporting them away from kinetochores, thus contributing to checkpoint silencing. However, the mechanism by which dynein performs this function, and its precise role in checkpoint silencing remain unresolved. Here, we find that dynein’s role in checkpoint silencing is restricted to evicting checkpoint effectors from the fibrous corona, and not the outer kinetochore. Dynein evicts these molecules from the corona in a manner that does not require stable, end-on microtubule attachments. Thus, by disassembling the corona through indiscriminate microtubule encounters, dynein primes the checkpoint signaling apparatus so it can respond to stable end-on microtubule attachments and permit cells to progress through mitosis. Accordingly, we find that dynein function in checkpoint silencing becomes largely dispensable in cells in which checkpoint effectors are excluded from the corona.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1371/journal.pcbi.1010165
