Insights into the micromechanical properties of the metaphase spindle

Centromere drive may propel the evolution of chromosome and genome size in plants

BACKGROUND

Genome size is influenced by natural selection and genetic drift acting on variations from polyploidy and repetitive DNA sequences. We hypothesized that centromere drive, where centromeres compete for inclusion in the functional gamete during meiosis, may also affect genome and chromosome size. This competition occurs in asymmetric meiosis, where only one of the four meiotic products becomes a gamete. If centromere drive influences chromosome size evolution, it may also impact post-polyploid diploidization, where a polyploid genome is restructured to function more like a diploid through chromosomal rearrangements, including fusions. We tested if plant lineages with asymmetric meiosis exhibit faster chromosome size evolution compared to those with only symmetric meiosis, which lack centromere drive as all four meiotic products become gametes. We also examined if positive selection on centromeric histone H3 (CENH3), a protein that can suppress centromere drive, is more frequent in these asymmetric lineages.

METHODS

We analyzed plant groups with different meiotic modes: asymmetric in gymnosperms and angiosperms, and symmetric in bryophytes, lycophytes, and ferns. We selected species based on available CENH3 gene sequences and chromosome size data. Using Ornstein-Uhlenbeck evolutionary models and phylogenetic regressions, we assessed the rates of chromosome size evolution and the frequency of positive selection on CENH3 in these clades.

RESULTS

Our analyses showed that clades with asymmetric meiosis have a higher frequency of positive selection on CENH3 and increased rates of chromosome size evolution compared to symmetric clades.

CONCLUSIONS

Our findings support the hypothesis that centromere drive accelerates chromosome and genome size evolution, potenatially also influencing the process of post-polyploid diploidization. We propose a model which in a single famework helps explain the stability of chromosome size in symmetric lineages (bryophytes, lycophytes, and ferns) and its variability in asymmetric lineages (gymnosperms and angiosperms), providing a foundation for future research in plant genome evolution.

The oncogene cyclin D1 promotes bipolar spindle integrity under compressive force

The mitotic spindle is the bipolar, microtubule-based structure that segregates chromosomes at each cell division. Aberrant spindles are frequently observed in cancer cells, but how oncogenic transformation affects spindle mechanics and function, particularly in the mechanical context of solid tumors, remains poorly understood. Here, we constitutively overexpress the oncogene cyclin D1 in human MCF10A cells to probe its effects on spindle architecture and response to compressive force. We find that cyclin D1 overexpression increases the incidence of spindles with extra poles, centrioles, and chromosomes. However, it also protects spindle poles from fracturing under compressive force, a deleterious outcome linked to multipolar cell divisions. Our findings suggest that cyclin D1 overexpression may adapt cells to increased compressive stress, possibly contributing to its prevalence in cancers such as breast cancer by allowing continued proliferation in mechanically challenging environments.

PIGB maintains nuclear lamina organization in skeletal muscle of Drosophila

The nuclear lamina (NL) plays various roles and participates in nuclear integrity, chromatin organization, and transcriptional regulation. Lamin proteins, the main components of the NL, form a homogeneous meshwork structure under the nuclear envelope. Lamins are essential, but it is unknown whether their homogeneous distribution is important for nuclear function. Here, we found that PIGB, an enzyme involved in glycosylphosphatidylinositol (GPI) synthesis, is responsible for the homogeneous lamin meshwork in Drosophila. Loss of PIGB resulted in heterogeneous distributions of B-type lamin and lamin-binding proteins in larval muscles. These phenotypes were rescued by expression of PIGB lacking GPI synthesis activity. The PIGB mutant exhibited changes in lamina-associated domains that are large heterochromatic genomic regions in the NL, reduction of nuclear stiffness, and deformation of muscle fibers. These results suggest that PIGB maintains the homogeneous meshwork of the NL, which may be essential for chromatin distribution and nuclear mechanical properties.

Biomolecular Condensates: Structure, Functions, Methods of Research

The term "biomolecular condensates" is used to describe membraneless compartments in eukaryotic cells, accumulating proteins and nucleic acids. Biomolecular condensates are formed as a result of liquid-liquid phase separation (LLPS). Often, they demonstrate properties of liquid-like droplets or gel-like aggregates; however, some of them may appear to have a more complex structure and high-order organization. Membraneless microcompartments are involved in diverse processes both in cytoplasm and in nucleus, among them ribosome biogenesis, regulation of gene expression, cell signaling, and stress response. Condensates properties and structure could be highly dynamic and are affected by various internal and external factors, e.g., concentration and interactions of components, solution temperature, pH, osmolarity, etc. In this review, we discuss variety of biomolecular condensates and their functions in live cells, describe their structure variants, highlight domain and primary sequence organization of the constituent proteins and nucleic acids. Finally, we describe current advances in methods that characterize structure, properties, morphology, and dynamics of biomolecular condensates in vitro and in vivo.

Mechanisms underlying spindle assembly and robustness

The microtubule-based spindle orchestrates chromosome segregation during cell division. Following more than a century of study, many components and pathways contributing to spindle assembly have been described, but how the spindle robustly assembles remains incompletely understood. This process involves the self-organization of a large number of molecular parts - up to hundreds of thousands in vertebrate cells - whose local interactions give rise to a cellular-scale structure with emergent architecture, mechanics and function. In this Review, we discuss key concepts in our understanding of spindle assembly, focusing on recent advances and the new approaches that enabled them. We describe the pathways that generate the microtubule framework of the spindle by driving microtubule nucleation in a spatially controlled fashion and present recent insights regarding the organization of individual microtubules into structural modules. Finally, we discuss the emergent properties of the spindle that enable robust chromosome segregation.

The oncogene cyclin D1 promotes bipolar spindle integrity under compressive force

The mitotic spindle is the bipolar, microtubule-based structure that segregates chromosomes at each cell division. Aberrant spindles are frequently observed in cancer cells, but how oncogenic transformation affects spindle mechanics and function, particularly in the mechanical context of solid tumors, remains poorly understood. Here, we constitutively overexpress the oncogene cyclin D1 in human MCF10A cells to probe its effects on spindle architecture and response to compressive force. We find that cyclin D1 overexpression increases the incidence of spindles with extra poles, centrioles, and chromosomes. However, it also protects spindle poles from fracturing under compressive force, a deleterious outcome linked to multipolar cell divisions. Our findings suggest that cyclin D1 overexpression may adapt cells to increased compressive stress, contributing to its prevalence in cancers such as breast cancer by allowing continued proliferation in mechanically challenging environments.

Engineering metaphase spindles: Construction site and building blocks

In an active, crowded cytoplasm, eukaryotic cells construct metaphase spindles from conserved building blocks to segregate chromosomes. Yet, spindles execute their function in a stunning variety of cell shapes and sizes across orders of magnitude. Thus, the current challenge is to understand how unique mesoscale spindle characteristics emerge from the interaction of molecular collectives. Key components of these collectives are tubulin dimers, which polymerise into microtubules. Despite all conservation, tubulin is a genetically and biochemically complex protein family, and we only begin to uncover how tubulin diversity affects microtubule dynamics and thus spindle assembly. Moreover, it is increasingly appreciated that spindles are dynamically intertwined with the cytoplasm that itself exhibits cell-type specific emergent properties with yet mostly unexplored consequences for spindle construction. Therefore, on our way toward a quantitative picture of spindle function, we need to understand molecular behaviour of the building blocks and connect it to the entire cellular context.

Modeling and mechanical perturbations reveal how spatially regulated anchorage gives rise to spatially distinct mechanics across the mammalian spindle

During cell division, the spindle generates force to move chromosomes. In mammals, microtubule bundles called kinetochore-fibers (k-fibers) attach to and segregate chromosomes. To do so, k-fibers must be robustly anchored to the dynamic spindle. We previously developed microneedle manipulation to mechanically challenge k-fiber anchorage, and observed spatially distinct response features revealing the presence of heterogeneous anchorage (Suresh et al., 2020). How anchorage is precisely spatially regulated, and what forces are necessary and sufficient to recapitulate the k-fiber's response to force remain unclear. Here, we develop a coarse-grained k-fiber model and combine with manipulation experiments to infer underlying anchorage using shape analysis. By systematically testing different anchorage schemes, we find that forces solely at k-fiber ends are sufficient to recapitulate unmanipulated k-fiber shapes, but not manipulated ones for which lateral anchorage over a 3 μm length scale near chromosomes is also essential. Such anchorage robustly preserves k-fiber orientation near chromosomes while allowing pivoting around poles. Anchorage over a shorter length scale cannot robustly restrict pivoting near chromosomes, while anchorage throughout the spindle obstructs pivoting at poles. Together, this work reveals how spatially regulated anchorage gives rise to spatially distinct mechanics in the mammalian spindle, which we propose are key for function.

Morphological growth dynamics, mechanical stability, and active microtubule mechanics underlying spindle self-organization

The spindle is a dynamic intracellular structure self-organized from microtubules and microtubule-associated proteins. The spindle’s bipolar morphology is essential for the faithful segregation of chromosomes during cell division, and it is robustly maintained by multifaceted mechanisms. However, abnormally shaped spindles, such as multipolar spindles, can stochastically arise in a cell population and cause chromosome segregation errors. The physical basis of how microtubules fail in bipolarization and occasionally favor nonbipolar assembly is poorly understood. Here, using live fluorescence imaging and quantitative shape analysis in Xenopus egg extracts, we find that spindles of varied shape morphologies emerge through nonrandom, bistable self-organization paths, one leading to a bipolar and the other leading to a multipolar phenotype. The bistability defines the spindle’s unique morphological growth dynamics linked to each shape phenotype and can be promoted by a locally distorted microtubule flow that arises within premature structures. We also find that bipolar and multipolar spindles are stable at the steady-state in bulk but can infrequently switch between the two phenotypes. Our microneedle-based physical manipulation further demonstrates that a transient force perturbation applied near the assembled pole can trigger the phenotypic switching, revealing the mechanical plasticity of the spindle. Together with molecular perturbation of kinesin-5 and augmin, our data propose the physical and molecular bases underlying the emergence of spindle-shape variation, which influences chromosome segregation fidelity during cell division.

Coupling of microtubule bundles isolates them from local disruptions to set the structural stability of the anaphase spindle

Chromosome segregation requires load-bearing interactions across kinetochore fibers and antiparallel microtubule bundles, which constitute the spindle midzone. Mechanical properties of kinetochore fibers have been characterized during metaphase, when the mitotic spindle achieves steady state. However, it has been difficult to probe the mechanics of the spindle midzone that elongates during anaphase. Here, we combine superresolution expansion and electron microscopies, lattice light-sheet imaging, and laser microsurgery to examine how midzone organization sets its mechanics. We find that individual midzone bundles extend out to multiple positions across chromosomes and form multiple apparent microtubule-based connections with each other. Across the spindle's short axis, these microtubule bundles exhibit restricted, submicrometer-amplitude motions, which are weakly correlated on <10s timescales. Severing individual midzone bundles near their center does not substantially affect positions of neighboring bundles, nor the overall structural stability of the midzone. In contrast, severing multiple midzone bundles or individual bundles at their chromosome-proximal ends significantly displaces neighboring microtubule bundles. Together, these data suggest a model wherein multiple midzone connections both reinforce its structure and mechanically isolate individual bundles from local perturbations. This feature sets the robust midzone architecture to accommodate disruptions, including those which result from lagging chromosomes, and achieve stereotypic outputs, such as proper chromosome separation.

Kinetochore size scales with chromosome size in bimodal karyotypes of Agavoideae

BACKGROUND AND AIMS

In eukaryotes, the total kinetochore size (defined as a chromosomal region containing CENH3-positive nucleosomes) per nucleus strongly correlates with genome size, a relationship that has been hypothesized to stem from general intracellular scaling principles. However, if larger chromosomes within a karyotype required larger kinetochores to move properly, it could also be derived from the mechanics of cell division.

METHODS

We selected seven species of the plant subfamily Agavoideae whose karyotypes are characterized by the presence of small and very large chromosomes. We visualized the kinetochore regions and chromosomes by immunolabelling with an anti-CENH3 antibody and DAPI (6'-diamidino-2-phenylindole) staining. We then employed 2D widefield and 3D super-resolution microscopy to measure chromosome and kinetochore areas and volumes, respectively. To assess the scaling relationship of kinetochore size to chromosome size inside a karyotype, we log-transformed the data and analysed them with linear mixed models which allowed us to control for the inherent hierarchical structure of the dataset (metaphases within slides and species).

KEY RESULTS

We found a positive intra-karyotype relationship between kinetochore and chromosome size. The slope of the regression line of the observed relationship (0.277 for areas, 0.247 for volumes) was very close to the theoretical slope of 0.25 for chromosome width based on the expected physics of chromosome passage through the cytoplasm during cell division. We obtained similar results by reanalysing available data from human and maize.

CONCLUSIONS

Our findings suggest that the total kinetochore size to genome size scaling observed across eukaryotes may also originate from the mechanics of cell division. Moreover, the potential causal link between kinetochore and chromosome size indicates that evolutionary mechanisms capable of leading kinetochore size changes to fixation, such as centromere drive, could promote the size evolution of entire chromosomes and genomes.

Contribution of cytoplasm viscoelastic properties to mitotic spindle positioning

The regulation of mitotic spindle positioning is a key process for tissue architecture, embryo development, and stem cells. To date, most models have assumed that spindles are positioned by forces exerted by polar cytoskeleton networks, like microtubule asters or actomyosin bundles. Here, using in situ magnetic tweezers to apply calibrated forces and torques to mitotic spindles in live dividing sea urchin cells, we found that the viscoelastic properties of the cytoplasm medium in which spindles are embedded can hold spindles in place and move them back if their original position is perturbed. These viscoelastic forces are large and may significantly participate in the force balance that position and orient mitotic spindles in many cell types.

Cells are filled with macromolecules and polymer networks that set scale-dependent viscous and elastic properties to the cytoplasm. Although the role of these parameters in molecular diffusion, reaction kinetics, and cellular biochemistry is being increasingly recognized, their contributions to the motion and positioning of larger organelles, such as mitotic spindles for cell division, remain unknown. Here, using magnetic tweezers to displace and rotate mitotic spindles in living embryos, we uncovered that the cytoplasm can impart viscoelastic reactive forces that move spindles, or passive objects with similar size, back to their original positions. These forces are independent of cytoskeletal force generators yet reach hundreds of piconewtons and scale with cytoplasm crowding. Spindle motion shears and fluidizes the cytoplasm, dissipating elastic energy and limiting spindle recoils with functional implications for asymmetric and oriented divisions. These findings suggest that bulk cytoplasm material properties may constitute important control elements for the regulation of division positioning and cellular organization.

Opposing motors provide mechanical and functional robustness in the human spindle

At each cell division, the spindle self-organizes from microtubules and motors. In human spindles, the motors dynein and Eg5 generate contractile and extensile stress, respectively. Inhibiting dynein or its targeting factor NuMA leads to unfocused, turbulent spindles, and inhibiting Eg5 leads to monopoles; yet, bipolar spindles form when both are inhibited together. What, then, are the roles of these opposing motors? Here, we generate NuMA/dynein- and Eg5-doubly inhibited spindles that not only attain a typical metaphase shape and size but also undergo anaphase. However, these spindles have reduced microtubule dynamics and are mechanically fragile, fracturing under force. Furthermore, they exhibit lagging chromosomes and a dramatic left-handed twist at anaphase. Thus, although these opposing motors are not required for spindle shape, they are essential to its mechanical and functional robustness. This work suggests a design principle whereby opposing active stresses provide robustness to force-generating cellular structures.

The Xenopus spindle is as dense as the surrounding cytoplasm

The mitotic spindle is a self-organizing molecular machine, where hundreds of different molecules continuously interact to maintain a dynamic steady state. While our understanding of key molecular players in spindle assembly is significant, it is still largely unknown how the spindle's material properties emerge from molecular interactions. Here, we use correlative fluorescence imaging and label-free three-dimensional optical diffraction tomography (ODT) to measure the Xenopus spindle's mass density distribution. While the spindle has been commonly referred to as a denser phase of the cytoplasm, we find that it has the same density as its surrounding, which makes it neutrally buoyant. Molecular perturbations suggest that spindle mass density can be modulated by tuning microtubule nucleation and dynamics. Together, ODT provides direct, unbiased, and quantitative information of the spindle's emergent physical properties-essential to advance predictive frameworks of spindle assembly and function.

Scaling Laws for Mitotic Chromosomes

During mitosis in higher eukaryotes, each chromosome condenses into a pair of rod-shaped chromatids. This process is co-regulated by the activity of several gene families, and the underlying biophysics remains poorly understood. To better understand the factors regulating chromosome condensation, we compiled a database of mitotic chromosome size and DNA content from the tables and figures of >200 published papers. A comparison across vertebrate species shows that chromosome width, length and volume scale with DNA content to the powers ∼1/4, ∼1/2, and ∼1, respectively. Angiosperms (flowering plants) show a similar length scaling, so this result is not specific to vertebrates. Chromosome shape and size thus satisfy two conditions: (1) DNA content per unit volume is approximately constant and (2) the cross-sectional area increases proportionately with chromosome length. Since viscous drag forces during chromosome movement are expected to scale with length, we hypothesize that the cross-section increase is necessary to limit the occurrence of large chromosome elongations that could slow or stall mitosis. Lastly, we note that individual vertebrate karyotypes typically exhibit a wider range of chromosome lengths as compared with angiosperms.

The microRNA-210-Stathmin1 Axis Decreases Cell Stiffness to Facilitate the Invasiveness of Colorectal Cancer Stem Cells

Simple Summary

Metastasis of tumor cells is the leading cause of death in cancer patients. Concurrent therapy with surgical removal of primary and metastatic lesions is the main approach for cancer therapy. Currently, therapeutic resistant properties of cancer stem cells (CSCs) are known to drive malignant cancer progression, including metastasis. Our study aimed to identify molecular tools dedicated to the detection and treatment of CSCs. We confirmed that microRNA-210-3p (miR-210) was upregulated in colorectal stem-like cancer cells, which targeted stathmin1 (STMN1), to decrease cell elasticity for increasing mobility. We envision that strategies for softening cellular elasticity will reduce the onset of CSC-orientated metastasis.

Abstract

Cell migration is critical for regional dissemination and distal metastasis of cancer cells, which remain the major causes of poor prognosis and death in patients with colorectal cancer (CRC). Although cytoskeletal dynamics and cellular deformability contribute to the migration of cancer cells and metastasis, the mechanisms governing the migratory ability of cancer stem cells (CSCs), a nongenetic source of tumor heterogeneity, are unclear. Here, we expanded colorectal CSCs (CRCSCs) as colonospheres and showed that CRCSCs exhibited higher cell motility in transwell migration assays and 3D invasion assays and greater deformability in particle tracking microrheology than did their parental CRC cells. Mechanistically, in CRCSCs, microRNA-210-3p (miR-210) targeted stathmin1 (STMN1), which is known for inducing microtubule destabilization, to decrease cell elasticity in order to facilitate cell motility without affecting the epithelial–mesenchymal transition (EMT) status. Clinically, the miR-210-STMN1 axis was activated in CRC patients with liver metastasis and correlated with a worse clinical outcome. This study elucidates a miRNA-oriented mechanism regulating the deformability of CRCSCs beyond the EMT process.

Principles and Applications of Single Particle Tracking in Cell Research

It is a tough challenge for many decades to decipher the complex relationships between cell behaviors and cellular physical properties. Single particle tracking (SPT) with high spatial and temporal resolution has been applied extensively in cell research to understand physicochemical properties of cells and their bio-functions by tracking endogenous or exogenous probes. This review describes the fundamental principles of SPT as well as its applications in intracellular mechanics, membrane dynamics, organelles distribution, and processes of internalization and transport. Finally, challenges and future directions of SPT are also discussed.

Mechanical Mechanisms of Chromosome Segregation

Chromosome segregation—the partitioning of genetic material into two daughter cells—is one of the most crucial processes in cell division. In all Eukaryotes, chromosome segregation is driven by the spindle, a microtubule-based, self-organizing subcellular structure. Extensive research performed over the past 150 years has identified numerous commonalities and contrasts between spindles in different systems. In this review, we use simple coarse-grained models to organize and integrate previous studies of chromosome segregation. We discuss sites of force generation in spindles and fundamental mechanical principles that any understanding of chromosome segregation must be based upon. We argue that conserved sites of force generation may interact differently in different spindles, leading to distinct mechanical mechanisms of chromosome segregation. We suggest experiments to determine which mechanical mechanism is operative in a particular spindle under study. Finally, we propose that combining biophysical experiments, coarse-grained theories, and evolutionary genetics will be a productive approach to enhance our understanding of chromosome segregation in the future.

Microtubule poleward flux in human cells is driven by the coordinated action of four kinesins

Mitotic spindle microtubules (MTs) undergo continuous poleward flux, whose driving force and function in humans remain unclear. Here, we combined loss-of-function screenings with analysis of MT-dynamics in human cells to investigate the molecular mechanisms underlying MT-flux. We report that kinesin-7/CENP-E at kinetochores (KTs) is the predominant driver of MT-flux in early prometaphase, while kinesin-4/KIF4A on chromosome arms facilitates MT-flux during late prometaphase and metaphase. Both these activities work in coordination with kinesin-5/EG5 and kinesin-12/KIF15, and our data suggest that the MT-flux driving force is transmitted from non-KT-MTs to KT-MTs by the MT couplers HSET and NuMA. Additionally, we found that the MT-flux rate correlates with spindle length, and this correlation depends on the establishment of stable end-on KT-MT attachments. Strikingly, we find that MT-flux is required to regulate spindle length by counteracting kinesin 13/MCAK-dependent MT-depolymerization. Thus, our study unveils the long-sought mechanism of MT-flux in human cells as relying on the coordinated action of four kinesins to compensate for MT-depolymerization and regulate spindle length.

Mechanics of the spindle apparatus

During mitosis microtubules self-organize to form a bipolar mitotic spindle structure, which positions the sister chromatids on the spindle mid-plane and separates them afterwards. Previous studies have identified many spindle associated proteins. Yet, we do not fully understand how these nanoscopic proteins lead to force generation through interactions of individual microtubules, motor proteins and chromosomes, and how a large number of these local interactions ultimately determine the structure and mechanics of the spindle in micron scale. Here we review the current understanding and open questions related to the structure and mechanics of the mitotic spindle. We then discuss how a combination of electron microscopy and computational modeling can be used to tackle some of these open questions.

Microneedle manipulation of the mammalian spindle reveals specialized, short-lived reinforcement near chromosomes

The spindle generates force to segregate chromosomes at cell division. In mammalian cells, kinetochore-fibers connect chromosomes to the spindle. The dynamic spindle anchors kinetochore-fibers in space and time to move chromosomes. Yet, how it does so remains poorly understood as we lack tools to directly challenge this anchorage. Here, we adapt microneedle manipulation to exert local forces on the spindle with spatiotemporal control. Pulling on kinetochore-fibers reveals the preservation of local architecture in the spindle-center over seconds. Sister, but not neighbor, kinetochore-fibers remain tightly coupled, restricting chromosome stretching. Further, pulled kinetochore-fibers pivot around poles but not chromosomes, retaining their orientation within 3 μm of chromosomes. This local reinforcement has a 20 s lifetime, and requires the microtubule crosslinker PRC1. Together, these observations indicate short-lived, specialized reinforcement in the spindle center. This could help protect chromosome attachments from transient forces while allowing spindle remodeling, and chromosome movements, over longer timescales.

Meiotic Spindle Has a Soft Spot

The spindle relies on forces exerted by microtubules and motor proteins to align and segregate chromosomes. In this issue of Developmental Cell, Takagi et al. (2019) show that meiotic spindle microtubules respond differently to forces at different spindle locations, depending on microtubule organization and motor proteins that crosslink them.

Light-inducible activation of cell cycle progression in Xenopus egg extracts under microfluidic confinement

Cell-free Xenopus egg extract is a widely used and biochemically tractable model system that allows recapitulation and elucidation of fundamental cellular processes. Recently, the introduction of microfluidic extract manipulation has enabled compartmentalization of bulk extract and a newfound ability to study organelles on length scales that recapitulate key features of cellular morphology. While the microfluidic confinement of extracts has produced a compelling platform for the in vitro study of cell processes at physiologically-relevant length scales, it also imposes experimental limitations by restricting dynamic control over extract properties. Here, we introduce photodegradable polyethylene glycol (PEG) hydrogels as a vehicle to passively and selectively manipulate extract composition through the release of proteins encapsulated within the hydrogel matrix. Photopatterned PEG hydrogels, passive to both extract and encapsulated proteins, serve as protein depots within microfluidic channels, which are subsequently flooded with extract. Illumination by ultraviolet light (UV) degrades the hydrogel structures and releases encapsulated protein. We show that an engineered fluorescent protein with a nuclear localization signal (GST-GFP-NLS) retains its ability to localize within nearby nuclei following UV-induced release from hydrogel structures. When diffusion is considered, the kinetics of nuclear accumulation are similar to those in experiments utilizing conventional, bulk fluid handling. Similarly, the release of recombinant cyclin B Δ90, a mutant form of the master cell cycle regulator cyclin B which lacks the canonical destruction box, was able to induce the expected cell cycle transition from interphase to mitosis. This transition was confirmed by the observation of nuclear envelope breakdown (NEBD), a phenomenological hallmark of mitosis, and the induction of mitosis-specific biochemical markers. This approach to extract manipulation presents a versatile and customizable route to regulating the spatial and temporal dynamics of cellular events in microfluidically confined cell-free extracts.

Mitsui-7, heat-treated, and nitrogen-doped multi-walled carbon nanotubes elicit genotoxicity in human lung epithelial cells

Background

The unique physicochemical properties of multi-walled carbon nanotubes (MWCNT) have led to many industrial applications. Due to their low density and small size, MWCNT are easily aerosolized in the workplace making respiratory exposures likely in workers. The International Agency for Research on Cancer designated the pristine Mitsui-7 MWCNT (MWCNT-7) as a Group 2B carcinogen, but there was insufficient data to classify all other MWCNT. Previously, MWCNT exposed to high temperature (MWCNT-HT) or synthesized with nitrogen (MWCNT-ND) have been found to elicit attenuated toxicity; however, their genotoxic and carcinogenic potential are not known. Our aim was to measure the genotoxicity of MWCNT-7 compared to these two physicochemically-altered MWCNTs in human lung epithelial cells (BEAS-2B & SAEC).

Results

Dose-dependent partitioning of individual nanotubes in the cell nuclei was observed for each MWCNT material and was greatest for MWCNT-7. Exposure to each MWCNT led to significantly increased mitotic aberrations with multi- and monopolar spindle morphologies and fragmented centrosomes. Quantitative analysis of the spindle pole demonstrated significantly increased centrosome fragmentation from 0.024–2.4 μg/mL of each MWCNT. Significant aneuploidy was measured in a dose-response from each MWCNT-7, HT, and ND; the highest dose of 24 μg/mL produced 67, 61, and 55%, respectively. Chromosome analysis demonstrated significantly increased centromere fragmentation and translocations from each MWCNT at each dose. Following 24 h of exposure to MWCNT-7, ND and/or HT in BEAS-2B a significant arrest in the G1/S phase in the cell cycle occurred, whereas the MWCNT-ND also induced a G2 arrest. Primary SAEC exposed for 24 h to each MWCNT elicited a significantly greater arrest in the G1 and G2 phases. However, SAEC arrested in the G1/S phase after 72 h of exposure. Lastly, a significant increase in clonal growth was observed one month after exposure to 0.024 μg/mL MWCNT-HT & ND.

Conclusions

Although MWCNT-HT & ND cause a lower incidence of genotoxicity, all three MWCNTs cause the same type of mitotic and chromosomal disruptions. Chromosomal fragmentation and translocations have not been observed with other nanomaterials. Because in vitro genotoxicity is correlated with in vivo genotoxic response, these studies in primary human lung cells may predict the genotoxic potency in exposed human populations.

Electronic supplementary material

The online version of this article (10.1186/s12989-019-0318-0) contains supplementary material, which is available to authorized users.

Cell Division: NuMA Bears the Load in the Spindle

The mitotic spindle bears the load of chromosomes during mitosis, but how this load is distributed across the spindle is unclear. A new study shows that load distribution in the spindle is confined and requires the microtubule cross-linking protein NuMA.

Self-organization of spindle-like microtubule structures

Microtubule self-organization is an essential physical process underlying several essential cellular functions, including cell division. In cell division, the dominant arrangement is the mitotic spindle, a football-shaped microtubule-based machine responsible for separating the chromosomes. We are interested in the underlying fundamental principles behind the self-organization of the spindle shape. Prior biological works have hypothesized that motor proteins control the proper formation of the spindle. Many of these motor proteins are also microtubule-crosslinkers, so it is unclear if the critical aspect is the motor activity or the crosslinking. In this study, we seek to address this question by examining the self-organization of microtubules using crosslinkers alone. We use a minimal system composed of tubulin, an antiparallel microtubule-crosslinking protein, and a crowding agent to explore the phase space of organizations as a function of tubulin and crosslinker concentration. We find that the concentration of the antiparallel crosslinker, MAP65, has a significant effect on the organization and resulted in spindle-like arrangements at relatively low concentration without the need for motor activity. Surprisingly, the length of the microtubules only moderately affects the equilibrium phase. We characterize both the shape and dynamics of these spindle-like organizations. We find that they are birefringent homogeneous tactoids. The microtubules have slow mobility, but the crosslinkers have fast mobility within the tactoids. These structures represent a first step in the recapitulation of self-organized spindles of microtubules that can be used as initial structures for further biophysical and active matter studies relevant to the biological process of cell division.

The Physics of the Metaphase Spindle

The assembly of the mitotic spindle and the subsequent segregation of sister chromatids are based on the self-organized action of microtubule filaments, motor proteins, and other microtubule-associated proteins, which constitute the fundamental force-generating elements in the system. Many of the components in the spindle have been identified, but until recently it remained unclear how their collective behaviors resulted in such a robust bipolar structure. Here, we review the current understanding of the physics of the metaphase spindle that is only now starting to emerge.

The Spindle: Integrating Architecture and Mechanics across Scales

The spindle segregates chromosomes at cell division, and its task is a mechanical one. While we have a nearly complete list of spindle components, how their molecular-scale mechanics give rise to cellular-scale spindle architecture, mechanics, and function is not yet clear. Recent in vitro and in vivo measurements bring new levels of molecular and physical control and shed light on this question. Highlighting recent findings and open questions, we introduce the molecular force generators of the spindle, and discuss how they organize microtubules into diverse architectural modules and give rise to the emergent mechanics of the mammalian spindle. Throughout, we emphasize the breadth of space and time scales at play, and the feedback between spindle architecture, dynamics, and mechanics that drives robust function.

Chromatin as a nuclear spring

The nucleus in eukaryotic cells is the site for genomic functions such as RNA transcription, DNA replication, and DNA repair/recombination. However, the nucleus is subjected to various mechanical forces associated with diverse cellular activities, including contraction, migration, and adhesion. Although it has long been assumed that the lamina structure, underlying filamentous mesh-work of the nuclear envelope, plays an important role in resisting mechanical forces, the involvement of compact chromatin in mechanical resistance has also recently been suggested. However, it is still unclear how chromatin functions to cope with the stresses. To address this issue, we studied the mechanical responses of human cell nuclei by combining a force measurement microscopy setup with controlled biochemical manipulation of chromatin. We found that nuclei with condensed chromatin possess significant elastic rigidity, whereas the nuclei with a decondensed chromatin are considerably soft. Further analyses revealed that the linker DNA and nucleosome-nucleosome interactions via histone tails in the chromatin act together to generate a spring-like restoring force that resists nuclear deformation. The elastic restoring force is likely to be generated by condensed chromatin domains, consisting of interdigitated or "melted" 10-nm nucleosome fibers. Together with other recent studies, it is suggested that chromatin functions not only as a "memory device" to store, replicate, and express the genetic information for various cellular functions but also as a "nuclear spring" to resist and respond to mechanical forces.

Microfluidic Encapsulation of Demembranated Sperm Nuclei in Xenopus Egg Extracts

The cell-free nature of Xenopus egg extract makes it a uniquely tractable experimental model system. The extract, effectively unconfined cytoplasm, allows the direct and relatively straight-forward addition of purified proteins and other reagents, a characteristic that renders the system amenable to many biochemical and cell biological manipulations. Accessibility to the system also facilitates the direct physical manipulation and probing of biological structures, in turn enabling mechanical properties of intracellular assemblies and organelles, such as the mitotic spindle and nucleus, to be measured. Recently, multiphase microfluidics have been combined with Xenopus egg extracts to encapsulate discrete cytoplasmic volumes. Described here is a protocol detailing the use of multiphase microfluidic devices to encapsulate sperm nuclei within extract droplets of defined size and shape. This protocol can also be applied more generally to encapsulation of microbeads and other particles.

Analyzing the micromechanics of the cell division apparatus

Cell division involves mechanical processes, such as chromosome transport and centrosome separation. Quantitative micromanipulation-based approaches have been central to dissecting the forces driving these processes. We highlight two biophysical assays that can be employed for such analyses. First, an in vitro "mini-spindle" assay is described that can be used to examine the collective mechanics of mitotic motor proteins cross-linking two microtubules. In the spindle, motor proteins (e.g., kinesin-5, kinesin-14, and dynein) can localize to overlapping microtubules that slide relative to each other, work as an ensemble, and equilibrate between cytoplasm and the microtubules. The "mini-spindle" assay can recapitulate these features and allows measurements of forces generated between adjacent microtubules and their dependence on filament orientation, sliding speed, overlap length, and motor protein density. Second, we describe a force-calibrated microneedle-based "whole-spindle" micromechanics assay. Microneedle-based micromanipulation can be a useful technique to examine cellular scale mechanics, but its use has been restricted by the difficulty in getting probes to penetrate the plasma membrane without disrupting cell physiology. As detailed here, the use of cell-free extracts prepared from metaphase-arrested Xenopus eggs can address this limitation. These micromanipulation studies also benefit from the use of frozen stocks of Xenopus egg extract. Together, these approaches can be used to decipher how micromechanics and biochemical activities ensure successful cell division.

P190RhoGAP prevents mitotic spindle fragmentation and is required to activate Aurora A kinase at acentriolar poles

Assembly of the mitotic spindle is essential for proper chromosome segregation during mitosis. Maintenance of spindle poles requires precise regulation of kinesin- and dynein-generated forces, and improper regulation of these forces disrupts pole integrity leading to pole fragmentation. The formation and function of the mitotic spindle are regulated by many proteins, including Aurora A kinase and the motor proteins Kif2a and Eg5. Here, we characterize a surprising role for the RhoA GTPase-activating protein, p190RhoGAP, in regulating the mitotic spindle. We show that cells depleted of p190RhoGAP arrest for long periods in mitosis during which cells go through multiple transitions between having bipolar and multipolar spindles. Most of the p190RhoGAP-depleted cells finally achieve a stable bipolar attachment and proceed through anaphase. The multipolar spindle phenotype can be rescued by low doses of an Eg5 inhibitor. Moreover, we show that p190RhoGAP-depleted multipolar cells localize Aurora A to all the poles, but the kinase is only activated at the two centriolar poles. Overall, our data identify an unappreciated connection between p190RhoGAP and the proteins that control spindle poles including Aurora A kinase and Eg5 that is required to prevent or correct spindle pole fragmentation.

Radial stiffness characteristics of the overlap regions of sarcomeres in isolated skeletal myofibrils in pre-force generating state

We have studied the stiffness of myofilament lattice in sarcomeres in the pre-force generating state, which was realized by a relaxing reagent, BDM (butane dione monoxime). First, the radial stiffness for the overlap regions of sarcomeres of isolated single myofibrils was estimated from the resulting decreases in diameter by osmotic pressure applied with the addition of Dextran. Then, the radial stiffness was also estimated from force-distance curve measurements with AFM technology. The radial stiffness for the overlap regions thus obtained was composed of a soft and a rigid component. The soft component visco-elastically changed in a characteristic fashion depending on the physiological conditions of myofibrils, suggesting that it comes from cross-bridge structures. BDM treatments significantly affected the soft radial component of contracting myofibrils depending on the approach velocity of cantilever: It was nearly equal to that in the contracting state at high approach velocity, whereas as low as that in the relaxing state at low approach velocity. However, comparable BDM treatments greatly suppressed the force production and the axial stiffness in contracting glycerinated muscle fibers and also the sliding velocity of actin filaments in the in vitro motility assay. Considering that BDM shifts the cross-bridge population from force generating to pre-force generating states in contracting muscle, the obtained results strongly suggest that cross-bridges in the pre-force generating state are visco-elastically attached to the thin filaments in such a binding manner that the axial stiffness is low but the radial stiffness significantly high similar to that in force generating state.

High-quality frozen extracts of Xenopus laevis eggs reveal size-dependent control of metaphase spindle micromechanics

Cell-free extracts from unfertilized Xenopus laevis eggs offer the opportunity for a variety of biochemical and biophysical assays for analyzing essential cell cycle events such as metaphase spindle assembly. However, the extracts often exhibit substantial variation in quality and have low storage stability, factors that hamper their experimental utility. Here we report a simple two-step method for preparing frozen egg extracts that retain spindle assembly activity levels similar to those of freshly prepared extracts. Extract degradation associated with the freeze-thaw process can be substantially reduced by using centrifugal filter-based dehydration and slow sample cooling. Large amounts of frozen extract stocks from single-batch preparations allowed us to collect extensive data in micromanipulation experiments, which are often low-throughput, and thus enabled the clarification of correlations between metaphase spindle size and stiffness. Our method provides an assay platform with minimized biological variability and improves the accessibility of egg extracts for research.

Mapping Load-Bearing in the Mammalian Spindle Reveals Local Kinetochore Fiber Anchorage that Provides Mechanical Isolation and Redundancy

Active forces generated at kinetochores move chromosomes, and the dynamic spindle must robustly anchor kinetochore fibers (k-fibers) to bear this load. The mammalian spindle bears the load of chromosome movement far from poles, but we do not know where and how-physically and molecularly-this load distributes across the spindle. In part, this is because probing spindle mechanics in live cells is difficult. Yet answering this question is key to understanding how the spindle generates and responds to force and performs its diverse mechanical functions. Here, we map load-bearing across the mammalian spindle in space-time and dissect local anchorage mechanics and mechanism. To do so, we laser-ablate single k-fibers at different spindle locations and in different molecular backgrounds and quantify the immediate relaxation of chromosomes, k-fibers, and microtubule speckles. We find that load redistribution is locally confined in all directions: along the first 3-4 μm from kinetochores, scaling with k-fiber length, and laterally within ∼2 μm of k-fiber sides, without detectable load sharing between neighboring k-fibers. A phenomenological model suggests that dense, transient crosslinks to the spindle along k-fibers bear the load of chromosome movement but that these connections do not limit the timescale of spindle reorganization. The microtubule crosslinker NuMA is needed for the local load-bearing observed, whereas Eg5 and PRC1 are not detectably required, suggesting specialization in mechanical function. Together, the data and model suggest that NuMA-mediated crosslinks locally bear load, providing mechanical isolation and redundancy while allowing spindle fluidity. These features are well suited to support robust chromosome segregation.

Multiciliated cell basal bodies align in stereotypical patterns coordinated by the apical cytoskeleton

Herawati et al. developed a long-term and high-resolution live imaging system for cultured mouse tracheal multiciliated cells. Using both experimental and theoretical studies, they reveal the developmental principle of ciliary basal body alignment directed by apical cytoskeletons.

Multiciliated cells (MCCs) promote fluid flow through coordinated ciliary beating, which requires properly organized basal bodies (BBs). Airway MCCs have large numbers of BBs, which are uniformly oriented and, as we show here, align linearly. The mechanism for BB alignment is unexplored. To study this mechanism, we developed a long-term and high-resolution live-imaging system and used it to observe green fluorescent protein–centrin2–labeled BBs in cultured mouse tracheal MCCs. During MCC differentiation, the BB array adopted four stereotypical patterns, from a clustering “floret” pattern to the linear “alignment.” This alignment process was correlated with BB orientations, revealed by double immunostaining for BBs and their asymmetrically associated basal feet (BF). The BB alignment was disrupted by disturbing apical microtubules with nocodazole and by a BF-depleting Odf2 mutation. We constructed a theoretical model, which indicated that the apical cytoskeleton, acting like a viscoelastic fluid, provides a self-organizing mechanism in tracheal MCCs to align BBs linearly for mucociliary transport.

The mechanics of microtubule networks in cell division

Forth and Kapoor review the mechanical properties of the spindle microtubule network during cell division.

The primary goal of a dividing somatic cell is to accurately and equally segregate its genome into two new daughter cells. In eukaryotes, this process is performed by a self-organized structure called the mitotic spindle. It has long been appreciated that mechanical forces must be applied to chromosomes. At the same time, the network of microtubules in the spindle must be able to apply and sustain large forces to maintain spindle integrity. Here we consider recent efforts to measure forces generated within microtubule networks by ensembles of key proteins. New findings, such as length-dependent force generation, protein clustering by asymmetric friction, and entropic expansion forces will help advance models of force generation needed for spindle function and maintaining integrity.

Nucleosome-nucleosome interactions via histone tails and linker DNA regulate nuclear rigidity

A force-calibrated microneedle setup and controlled biochemical perturbation reveal that chromatin acts as a spring-like mechanical module that controls the rigidity of cell nuclei. The underlying molecular mechanism involves linker DNA and internucleosomal interaction via histone tails.

Cells, as well as the nuclei inside them, experience significant mechanical stress in diverse biological processes, including contraction, migration, and adhesion. The structural stability of nuclei must therefore be maintained in order to protect genome integrity. Despite extensive knowledge on nuclear architecture and components, however, the underlying physical and molecular mechanisms remain largely unknown. We address this by subjecting isolated human cell nuclei to microneedle-based quantitative micromanipulation with a series of biochemical perturbations of the chromatin. We find that the mechanical rigidity of nuclei depends on the continuity of the nucleosomal fiber and interactions between nucleosomes. Disrupting these chromatin features by varying cation concentration, acetylating histone tails, or digesting linker DNA results in loss of nuclear rigidity. In contrast, the levels of key chromatin assembly factors, including cohesin, condensin II, and CTCF, and a major nuclear envelope protein, lamin, are unaffected. Together with in situ evidence using living cells and a simple mechanical model, our findings reveal a chromatin-based regulation of the nuclear mechanical response and provide insight into the significance of local and global chromatin structures, such as those associated with interdigitated or melted nucleosomal fibers.

Mechanical properties of spindle poles are symmetrically balanced

The metaphase spindle is organized for accurate chromosome segregation. One of the fundamental features of the spindle across the species is its symmetrical shape; the spindle consists of two polar arrays of microtubules at both ends. Although it has been suggested that the formation of the bipolar shape requires force balance coordination by molecular motors, i.e., kinesins and dyneins, quantitative analysis for the pole mechanics has not been conducted. Here, we demonstrate that it is not only the shape but also the stiffness and microtubule density of the pairs of pole regions are symmetrically balanced in single spindles self-assembled in Xenopus egg extracts. We found that the inhibition of dynein functions dramatically reduced the stiffness and microtubule density in the pole region. By contrast, the inhibition of one of the kinesins, Eg5, which is the antagonistic motor protein of dynein, increased the value of these parameters. Moreover, the inhibition of both dynein and Eg5 recovered these parameter values to those of non-treated spindle poles. We also found that, when one pole structure was held widened with the use of two glass microneedles, the opposite pole structure spontaneously widened, resulting in the formation of the barrel-like shaped spindle. The values of stiffness and microtubule density in the manipulated pole region decreased, following the spontaneous decrement of those in the paired unmanipulated pole region. These results suggest that the spindle possesses a mechanism to dynamically maintain its symmetry in mechanical properties.

Endoplasmic-reticulum-mediated microtubule alignment governs cytoplasmic streaming

Cytoplasmic streaming refers to a collective movement of cytoplasm observed in many cell types. The mechanism of meiotic cytoplasmic streaming (MeiCS) in Caenorhabditis elegans zygotes is puzzling as the direction of the flow is not predefined by cell polarity and occasionally reverses. Here, we demonstrate that the endoplasmic reticulum (ER) network structure is required for the collective flow. Using a combination of RNAi, microscopy and image processing of C. elegans zygotes, we devise a theoretical model, which reproduces and predicts the emergence and reversal of the flow. We propose a positive-feedback mechanism, where a local flow generated along a microtubule is transmitted to neighbouring regions through the ER. This, in turn, aligns microtubules over a broader area to self-organize the collective flow. The proposed model could be applicable to various cytoplasmic streaming phenomena in the absence of predefined polarity. The increased mobility of cortical granules by MeiCS correlates with the efficient exocytosis of the granules to protect the zygotes from osmotic and mechanical stresses.

Fluidization and Active Thinning by Molecular Kinetics in Active Gels

We derive the constitutive equations of an active polar gel from a model for the dynamics of elastic molecules that link polar elements. Molecular binding kinetics induces the fluidization of the material, giving rise to Maxwell viscoelasticity and, provided that detailed balance is broken, to the generation of active stresses. We give explicit expressions for the transport coefficients of active gels in terms of molecular properties, including nonlinear contributions on the departure from equilibrium. In particular, when activity favors linker unbinding, we predict a decrease of viscosity with activity-active thinning-of kinetic origin, which could explain some experimental results on the cell cortex. By bridging the molecular and hydrodynamic scales, our results could help understand the interplay between molecular perturbations and the mechanics of cells and tissues.

Mechanisms of Chromosome Congression during Mitosis

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called "direct congression" pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call "peripheral congression", is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called "tubulin code" might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

Metaphase Spindle Assembly

A microtubule-based bipolar spindle is required for error-free chromosome segregation during cell division. In this review I discuss the molecular mechanisms required for the assembly of this dynamic micrometer-scale structure in animal cells.

A force-generating machinery maintains the spindle at the cell center during mitosis

The position and orientation of the mitotic spindle is precisely regulated to ensure the accurate partition of the cytoplasm between daughter cells and the correct localization of the daughters within growing tissue. Using magnetic tweezers to perturb the position of the spindle in intact cells, we discovered a force-generating machinery that maintains the spindle at the cell center during metaphase and anaphase in one- and two-cell Caenorhabditis elegans embryos. The forces increase with the number of microtubules and are larger in smaller cells. The machinery is rigid enough to suppress thermal fluctuations to ensure precise localization of the mitotic spindle, yet compliant enough to allow molecular force generators to fine-tune the position of the mitotic spindle to facilitate asymmetric division.

Laser microsurgery reveals conserved viscoelastic behavior of the kinetochore

Accurate chromosome segregation depends on proper kinetochore-microtubule attachment. Upon microtubule interaction, kinetochores are subjected to forces generated by the microtubules. In this work, we used laser ablation to sever microtubules attached to a merotelic kinetochore, which is laterally stretched by opposing pulling forces exerted by microtubules, and inferred the mechanical response of the kinetochore from its length change. In both mammalian PtK1 cells and in the fission yeast Schizosaccharomyces pombe, kinetochores shortened after microtubule severing. Interestingly, the inner kinetochore-centromere relaxed faster than the outer kinetochore. Whereas in fission yeast all kinetochores relaxed to a similar length, in PtK1 cells the more stretched kinetochores remained more stretched. Simple models suggest that these differences arise because the mechanical structure of the mammalian kinetochore is more complex. Our study establishes merotelic kinetochores as an experimental model for studying the mechanical response of the kinetochore in live cells and reveals a viscoelastic behavior of the kinetochore that is conserved in yeast and mammalian cells.