Deciphering protein function during mitosis in PtK cells using RNAi

MCAK Inhibitors Induce Aneuploidy in Triple-Negative Breast Cancer Models

Standard of care for triple-negative breast cancer (TNBC) involves the use of microtubule poisons such as paclitaxel, which are proposed to work by inducing lethal levels of aneuploidy in tumor cells. While these drugs are initially effective in treating cancer, dose-limiting peripheral neuropathies are common. Unfortunately, patients often relapse with drug-resistant tumors. Identifying agents against targets that limit aneuploidy may be a valuable approach for therapeutic development. One potential target is the microtubule depolymerizing kinesin, MCAK, which limits aneuploidy by regulating microtubule dynamics during mitosis. Using publicly available datasets, we found that MCAK is upregulated in triple-negative breast cancer and is associated with poorer prognoses. Knockdown of MCAK in tumor-derived cell lines caused a two- to five-fold reduction in the IC50 for paclitaxel, without affecting normal cells. Using FRET and image-based assays, we screened compounds from the ChemBridge 50 k library and discovered three putative MCAK inhibitors. These compounds reproduced the aneuploidy-inducing phenotype of MCAK loss, reduced clonogenic survival of TNBC cells regardless of taxane-resistance, and the most potent of the three, C4, sensitized TNBC cells to paclitaxel. Collectively, our work shows promise that MCAK may serve as both a biomarker of prognosis and as a therapeutic target.

MCAK Inhibitors Induce Aneuploidy in Triple Negative Breast Cancer Models

Standard of care for triple negative breast cancer (TNBC) involves the use of microtubule poisons like paclitaxel, which are proposed to work by inducing lethal levels of aneuploidy in tumor cells. While these drugs are initially effective in treating cancer, dose-limiting peripheral neuropathies are common. Unfortunately, patients often relapse with drug resistant tumors. Identifying agents against targets that limit aneuploidy may be a valuable approach for therapeutic development. One potential target is the microtubule depolymerizing kinesin, MCAK, which limits aneuploidy by regulating microtubule dynamics during mitosis. Using publicly available datasets, we found that MCAK is upregulated in triple negative breast cancer and is associated with poorer prognoses. Knockdown of MCAK in tumor-derived cell lines caused a two- to five-fold reduction in the IC ₅₀ for paclitaxel, without affecting normal cells. Using FRET and image-based assays, we screened compounds from the ChemBridge 50k library and discovered three putative MCAK inhibitors. These compounds reproduced the aneuploidy-inducing phenotype of MCAK loss, reduced clonogenic survival of TNBC cells regardless of taxane-resistance, and the most potent of the three, C4, sensitized TNBC cells to paclitaxel. Collectively, our work shows promise that MCAK may serve as both a biomarker of prognosis and as a therapeutic target.

Simple Summary

Triple negative breast cancer (TNBC) is the most lethal breast cancer subtype with few treatment options available. Standard of care for TNBC involves the use of taxanes, which are initially effective, but dose limiting toxicities are common, and patients often relapse with resistant tumors. Specific drugs that produce taxane-like effects may be able to improve patient quality of life and prognosis. In this study we identify three novel inhibitors of the Kinesin-13 MCAK. MCAK inhibition induces aneuploidy; similar to cells treated with taxanes. We demonstrate that MCAK is upregulated in TNBC and is associated with poorer prognoses. These MCAK inhibitors reduce the clonogenic survival of TNBC cells, and the most potent of the three inhibitors, C4, sensitizes TNBC cells to taxanes, similar to the effects of MCAK knockdown. This work will expand the field of precision medicine to include aneuploidy-inducing drugs that have the potential to improve patient outcomes.

Role of kinesin superfamily in gastrointestinal cancer

Kinesins constitute a protein superfamily that belongs to motor proteins. Kinesins move along microtubules to exert their functions. They play a crucial role in intracellular transportation, mitosis, cell formation, and cell function. Kinesin are not only responsible for the transport of various membrane organelles, protein complexes, mRNA and so on to ensure the basic activity of cells, but also can regulate intracellular molecular signal pathways. Numerous studies have shown that kinesins are closely associated with the development of a variety of human diseases, especially the formation and development of gastrointestinal tumors. This article reviews the role of kinesins in gastrointestinal cancer.

KIF-2C expression is correlated with poor prognosis of operable esophageal squamous cell carcinoma male patients

To determine the prognostic significance of Kinesin family member 2C (KIF-2C) expression in patients with operable esophageal squamous cell carcinoma (ESCC), we conducted an immunohistochemical analysis of KIF-2C expression in 415 surgically resected primary tumor tissues and 40 adjacent non-cancerous tissues from patients with operable ESCC. The median duration of postoperative follow-up was 76.0 months. Higher KIF-2C expression was associated with significantly increased risks of higher pathologic tumor (pT) status (P=0.038) and poorer tumor differentiation (P=0.022). For the entire cohort, KIF-2C expression was not an independent factor significantly associated with overall survival (OS) (P=0.097) or disease-free survival (DFS) (P=0.152). In female patients, KIF-2C expression had no effect on OS (P=0.880) and DFS (P=0.864). However, OS (hazard ratio (HR)=1.480, P=0.013) and DFS (HR=1.418, P=0.024) were worse for male patients with high KIF-2C expression compared with male patients with low KIF-2C expression. Moreover, the OS and DFS of male patients with high KIF-2C expression were also significantly shorter compared with female patients with low KIF-2C expression (P=0.022, P=0.029) and female patients with high KIF-2C expression (P=0.014, P=0.018). Based on these findings, KIF-2C expression in tumor tissues promises to serve as an independent prognostic marker for male, but not female, patients with operable ESCC. Prognosis was worse for male patients with high KIF-2C expression compared with patients with the same pathologic tumor-node-metastasis (pTNM) stage.

Spatial regulation of astral microtubule dynamics by Kif18B in PtK cells

The spatial and temporal control of microtubule dynamics is fundamentally important for proper spindle assembly and chromosome segregation. This is achieved, in part, by the multitude of proteins that bind to and regulate spindle microtubules, including kinesin superfamily members, which act as microtubule-destabilizing enzymes. These fall into two general classes: the kinesin-13 proteins, which directly depolymerize microtubules, and the kinesin-8 proteins, which are plus end-directed motors that either destabilize microtubules or cap the microtubule plus ends. Here we analyze the contribution of a PtK kinesin-8 protein, Kif18B, in the control of mitotic microtubule dynamics. Knockdown of Kif18B causes defects in spindle microtubule organization and a dramatic increase in astral microtubules. Kif18B-knockdown cells had defects in chromosome alignment, but there were no defects in chromosome segregation. The long astral microtubules that occur in the absence of Kif18B are limited in length by the cell cortex. Using EB1 tracking, we show that Kif18B activity is spatially controlled, as loss of Kif18B has the most dramatic effect on the lifetimes of astral microtubules that extend toward the cell cortex. Together our studies provide new insight into how diverse kinesins contribute to spatial microtubule organization in the spindle.

Coupling changes in cell shape to chromosome segregation

Animal cells undergo dramatic changes in shape, mechanics and polarity as they progress through the different stages of cell division. These changes begin at mitotic entry, with cell-substrate adhesion remodelling, assembly of a cortical actomyosin network and osmotic swelling, which together enable cells to adopt a near spherical form even when growing in a crowded tissue environment. These shape changes, which probably aid spindle assembly and positioning, are then reversed at mitotic exit to restore the interphase cell morphology. Here, we discuss the dynamics, regulation and function of these processes, and how cell shape changes and sister chromatid segregation are coupled to ensure that the daughter cells generated through division receive their fair inheritance.

Using Photoactivatable GFP to Study Microtubule Dynamics and Chromosome Segregation

Mitosis is a highly dynamic process during which the genetic material is equally distributed between two daughter cells. During mitosis, the sister chromatids of replicated chromosomes interact with dynamic microtubules and such interactions lead to stereotypical chromosome movements that eventually result in chromosome segregation and successful cell division. Approaches that allow quantification of microtubule dynamics and chromosome movements are of utmost importance for a mechanistic understanding of mitosis. In this chapter, we describe methods based on activation of photoactivatable green fluorescent protein (PA-GFP) that can be used for quantitative studies of microtubule dynamics and chromosome segregation.

Role of the Number of Microtubules in Chromosome Segregation during Cell Division

Faithful segregation of genetic material during cell division requires alignment of chromosomes between two spindle poles and attachment of their kinetochores to each of the poles. Failure of these complex dynamical processes leads to chromosomal instability (CIN), a characteristic feature of several diseases including cancer. While a multitude of biological factors regulating chromosome congression and bi-orientation have been identified, it is still unclear how they are integrated so that coherent chromosome motion emerges from a large collection of random and deterministic processes. Here we address this issue by a three dimensional computational model of motor-driven chromosome congression and bi-orientation during mitosis. Our model reveals that successful cell division requires control of the total number of microtubules: if this number is too small bi-orientation fails, while if it is too large not all the chromosomes are able to congress. The optimal number of microtubules predicted by our model compares well with early observations in mammalian cell spindles. Our results shed new light on the origin of several pathological conditions related to chromosomal instability.

Femtosecond near-infrared laser microirradiation reveals a crucial role for PARP signaling on factor assemblies at DNA damage sites

Laser microirradiation is a powerful tool for real-time single-cell analysis of the DNA damage response (DDR). It is often found, however, that factor recruitment or modification profiles vary depending on the laser system employed. This is likely due to an incomplete understanding of how laser conditions/dosages affect the amounts and types of damage and the DDR. We compared different irradiation conditions using a femtosecond near-infrared laser and found distinct damage site recruitment thresholds for 53BP1 and TRF2 correlating with the dose-dependent increase of strand breaks and damage complexity. Low input-power microirradiation that induces relatively simple strand breaks led to robust recruitment of 53BP1 but not TRF2. In contrast, increased strand breaks with complex damage including crosslinking and base damage generated by high input-power microirradiation resulted in TRF2 recruitment to damage sites with no 53BP1 clustering. We found that poly(ADP-ribose) polymerase (PARP) activation distinguishes between the two damage states and that PARP activation is essential for rapid TRF2 recruitment while suppressing 53BP1 accumulation at damage sites. Thus, our results reveal that careful titration of laser irradiation conditions allows induction of varying amounts and complexities of DNA damage that are gauged by differential PARP activation regulating protein assembly at the damage site.

Draft De Novo Transcriptome of the Rat Kangaroo Potorous tridactylus as a Tool for Cell Biology

The rat kangaroo (long-nosed potoroo, Potorous tridactylus) is a marsupial native to Australia. Cultured rat kangaroo kidney epithelial cells (PtK) are commonly used to study cell biological processes. These mammalian cells are large, adherent, and flat, and contain large and few chromosomes—and are thus ideal for imaging intra-cellular dynamics such as those of mitosis. Despite this, neither the rat kangaroo genome nor transcriptome have been sequenced, creating a challenge for probing the molecular basis of these cellular dynamics. Here, we present the sequencing, assembly and annotation of the draft rat kangaroo de novo transcriptome. We sequenced 679 million reads that mapped to 347,323 Trinity transcripts and 20,079 Unigenes. We present statistics emerging from transcriptome-wide analyses, and analyses suggesting that the transcriptome covers full-length sequences of most genes, many with multiple isoforms. We also validate our findings with a proof-of-concept gene knockdown experiment. We expect that this high quality transcriptome will make rat kangaroo cells a more tractable system for linking molecular-scale function and cellular-scale dynamics.

Functional analysis of phosphorylation of the mitotic centromere-associated kinesin by Aurora B kinase in human tumor cells

Mitotic centromere-associated kinesin (MCAK) is the best characterized member of the kinesin-13 family and plays important roles in microtubule dynamics during mitosis. Its activity and subcellular localization is tightly regulated by an orchestra of mitotic kinases, such as Aurora B. It is well known that serine 196 of MCAK is the major phosphorylation site of Aurora B in Xenopus leavis extracts and that this phosphorylation regulates its catalytic activity and subcellular localization. In the current study, we have addressed the conserved phosphorylation site serine 192 in human MCAK to characterize its function in more depth in human cancer cells. Our data confirm that S192 is the major phosphorylation site of Aurora B in human MCAK and that this phosphorylation has crucial roles in regulating its catalytic activity and localization at the kinetochore/centromere region in mitosis. Interfering with this phosphorylation leads to a delayed progression through prometa- and metaphase associated with mitotic defects in chromosome alignment and segregation. We show further that MCAK is involved in directional migration and invasion of tumor cells, and interestingly, interference with the S192 phosphorylation affects this capability of MCAK. These data provide the first molecular explanation for clinical observation, where an overexpression of MCAK was associated with lymphatic invasion and lymph node metastasis in gastric and colorectal cancer patients.

The activity regulation of the mitotic centromere-associated kinesin by Polo-like kinase 1

The mitotic centromere-associated kinesin (MCAK), a potent microtubule depolymerase, is involved in regulating microtubule dynamics. The activity and subcellular localization of MCAK are tightly regulated by key mitotic kinases, such as Polo-like kinase 1 (Plk1) by phosphorylating multiple residues in MCAK. Since Plk1 phosphorylates very often different residues of substrates at different stages, we have dissected individual phosphorylation of MCAK by Plk1 and characterized its function in more depth. We have recently shown that S621 in MCAK is the major phosphorylation site of Plk1, which is responsible for regulating MCAK's degradation by promoting the association of MCAK with APC/CCdc20. In the present study, we have addressed another two residues phosphorylated by Plk1, namely S632/S633 in the C-terminus of MCAK. Our data suggest that Plk1 phosphorylates S632/S633 and regulates its catalytic activity in mitosis. This phosphorylation is required for proper spindle assembly during early phases of mitosis. The subsequent dephosphorylation of S632/S633 might be necessary to timely align the chromosomes onto the metaphase plate. Therefore, our studies suggest new mechanisms by which Plk1 regulates MCAK: the degradation of MCAK is controlled by Plk1 phosphorylation on S621, whereas its activity is modulated by Plk1 phosphorylation on S632/S633 in mitosis.

Imaging and physically probing kinetochores in live dividing cells

The kinetochore mediates chromosome segregation at cell division. It is the macromolecular machine that links chromosomes to spindle microtubules, and is made of more than 100 protein species in mammalian cells. Molecular tools are presently revealing the biochemical interactions and regulatory mechanisms that ensure proper kinetochore function. Here, we discuss two approaches for imaging and physically probing kinetochores despite mitotic cell rounding and rapid kinetochore dynamics. First, we describe how mild spindle compression can improve kinetochore imaging and how stronger compression can mechanically perturb the spindle and kinetochores. Second, we describe how simultaneously imaging two-colored kinetochore reporter probes at subpixel resolution can report on kinetochore structural dynamics under cellular forces. We hope that the experimental details we provide here will make these two approaches broadly accessible and help move forward our understanding of kinetochore function--and make these approaches adaptable to the study of other cellular structures.

DNA damage to a single chromosome end delays anaphase onset

Chromosome ends contain nucleoprotein structures known as telomeres. Damage to chromosome ends during interphase elicits a DNA damage response (DDR) resulting in cell cycle arrest. However, little is known regarding the signaling from damaged chromosome ends (designated here as "TIPs") during mitosis. In the present study, we investigated the consequences of DNA damage induced at a single TIP in mitosis. We used laser microirradiation to damage mitotic TIPs or chromosome arms (non-TIPs) in PtK2 kidney epithelial cells. We found that damage to a single TIP, but not a non-TIP, delays anaphase onset. This TIP-specific checkpoint response is accompanied by differential recruitment of DDR proteins. Although phosphorylation of H2AX and the recruitment of several repair factors, such as Ku70-Ku80, occur in a comparable manner at both TIP and non-TIP damage sites, DDR factors such as ataxia telangiectasia mutated (ATM), MDC1, WRN, and FANCD2 are specifically recruited to TIPs but not to non-TIPs. In addition, Nbs1, BRCA1, and ubiquitin accumulate at damaged TIPs more rapidly than at damaged non-TIPs. ATR and 53BP1 are not detected at either TIPs or non-TIPs in mitosis. The observed delay in anaphase onset is dependent on the activity of DDR kinases ATM and Chk1, and the spindle assembly checkpoint kinase Mps1. Cells damaged at a single TIP or non-TIP eventually exit mitosis with unrepaired lesions. Damaged TIPs are segregated into micronuclei at a significantly higher frequency than damaged non-TIPs. Together, these findings reveal a mitosis-specific DDR uniquely associated with chromosome ends.

Polo-like kinase 1 regulates the stability of the mitotic centromere-associated kinesin in mitosis

Proper bi-orientation of chromosomes is critical for the accurate segregation of chromosomes in mitosis. A key regulator of this process is MCAK, the mitotic centromere-associated kinesin. During mitosis the activity and localization of MCAK are regulated by mitotic key kinases including Plk1 and Aurora B. We show here that S621 in the MCAK's C-terminal domain is the major phosphorylation site for Plk1. This phosphorylation regulates MCAK's stability and facilitates its recognition by the ubiquitin/proteasome dependent APC/C(Cdc20) pathway leading to its D-box dependent degradation in mitosis. While phosphorylation of S621 does not directly affect its microtubule depolymerising activity, loss of Plk1 phosphorylation on S621 indirectly enhances its depolymerization activity in vivo by stabilizing MCAK, leading to an increased level of protein. Interfering with phosphorylation at S621 causes spindle formation defects and chromosome misalignments. Therefore, this study suggests a new mechanism by which Plk1 regulates MCAK: by regulating its degradation and hence controlling its turnover in mitosis.

Targeting telomere-containing chromosome ends with a near-infrared femtosecond laser to study the activation of the DNA damage response and DNA damage repair pathways

Telomeres are at the ends of chromosomes. Previous evidence suggests that laser-induced deoxyribose nucleic acid (DNA) breaks at chromosome ends during anaphase results in delayed cytokinesis. A possible explanation for this delay is that the DNA damage response (DDR) mechanism has been activated. We describe a live imaging method to study the effects of DDR activation following focal point near-infrared femtosecond laser microirradiation either at a single chromosome end or at a chromosome arm in mitotic anaphase cells. Laser microirradiation is used in combination with dual fluorescent labeling to monitor the co-localization of double-strand break marker γH2AX along with the DDR factors in PtK2 (Potorous tridactylus) cells. Laser-induced DNA breaks in chromosome ends as well as in chromosome arms results in recruitment of the following: poly(ADP-ribose) polymerase 1, checkpoint sensors (p-Chk1, p-Chk2), DNA repair protein Ku70/Ku80, and proliferating cell nuclear antigen. However, phosphorylated p53 at serine 15 is detected only at chromosome ends and not at chromosome arms. Full activation of DDR on damaged chromosome ends may explain previously published results that showed the delay of cytokinesis.

Mitotic centromere-associated kinase (MCAK/Kif2C) regulates cellular senescence in human primary cells through a p53-dependent pathway

Mitotic centromere-associated kinase (MCAK/Kif2C) plays a critical role in chromosome movement and segregation with ATP-dependent microtubule depolymerase activity. However, its role in cellular senescence remains unclear. MCAK/Kif2C expression decreased in human primary cells under replicative and premature senescence. MCAK/Kif2C down-regulation in young cells induced premature senescence. MCAK/Kif2C overexpression in old cells partially reversed cell senescence. Senescence phenotypes by MCAK/Kif2C knockdown were observed in p16-knockdown cells, but not in p53-knockdown cells. These results suggest that MCAK/Kif2C plays an important role in the regulation of cellular senescence through a p53-dependent pathway and might contribute to tissue/organism aging and protection of cellular transformation.

Biophysics of mitosis

Mitosis is the process by which eukaryotic cells organize and segregate their chromosomes in preparation for cell division. It is accomplished by a cellular machine composed largely of microtubules (MTs) and their associated proteins. This article reviews literature on mitosis from a biophysical point of view, drawing attention to the assembly and motility processes required to do this complex job with precision. Work from both the recent and the older literature is integrated into a description of relevant biological events and the experiments that probe their mechanisms. Theoretical work on specific subprocesses is also reviewed. Our goal is to provide a document that will expose biophysicists to the fascination of this quite amazing process and provide them with a good background from which they can pursue their own research interests in the subject.

Using cultured mammalian cells to study mitosis

Diverse cell types have been used to study various aspects of mitosis. Early investigators focused primarily on cells that were suited to morphological studies. More recently, experimental systems have been developed to study both morphology and the molecular basis of chromosome motion and cell-cycle regulation. This article briefly reviews cell types that have been used to study mitosis in live cells. It then discusses cell lines that have been used to examine mitosis in cultured mammalian cells and summarizes the methods that are used to culture and study these cells.

Kip3, the yeast kinesin-8, is required for clustering of kinetochores at metaphase

In Saccharomyces cerevisiae, chromosome congression clusters kinetochores on either side of the spindle equator at metaphase. Many organisms require one or more kinesin-8 molecular motors to achieve chromosome alignment. The yeast kinesin-8, Kip3, has been well studied in vitro but a role in chromosome congression has not been reported. We investigated Kip3's role in this process using semi-automated, quantitative fluorescence microscopy and time-lapse imaging and found that Kip3 is required for congression. Deletion of KIP3 increases inter-kinetochore distances and increases the variability in the position of sister kinetochores along the spindle axis during metaphase. Kip3 does not regulate spindle length and is not required for kinetochore-microtubule attachment. Instead, Kip3 clusters kinetochores on the metaphase spindle by tightly regulating kinetochore microtubule lengths.

Kinesins and protein kinases: key players in the regulation of microtubule dynamics and organization

Microtubule dynamics is controlled and amplified in vivo by complex sets of regulators. Among these regulatory proteins, molecular motors from the kinesin superfamily are taking an increasing importance. Here we review how microtubule disassembly or assembly into interphase microtubules, mitotic spindle or cilia may involve kinesins and how protein kinases may participate in these kinesin-dependent regulations.

Long astral microtubules uncouple mitotic spindles from the cytokinetic furrow

Depletion of MCAK, which lengthens astral microtubules, induces oscillations of the mitotic spindle, and displaces the plane of cell division.

Astral microtubules (MTs) are known to be important for cleavage furrow induction and spindle positioning, and loss of astral MTs has been reported to increase cortical contractility. To investigate the effect of excess astral MT activity, we depleted the MT depolymerizer mitotic centromere-associated kinesin (MCAK) from HeLa cells to produce ultra-long, astral MTs during mitosis. MCAK depletion promoted dramatic spindle rocking in early anaphase, wherein the entire mitotic spindle oscillated along the spindle axis from one proto-daughter cell to the other, driven by oscillations of cortical nonmuscle myosin II. The effect was phenocopied by taxol treatment. Live imaging revealed that cortical actin partially vacates the polar cortex in favor of the equatorial cortex during anaphase. We propose that this renders the polar actin cortex vulnerable to rupture during normal contractile activity and that long astral MTs enlarge the blebs. Excessively large blebs displace mitotic spindle position by cytoplasmic flow, triggering the oscillations as the blebs resolve.

How kinesin motor proteins drive mitotic spindle function: Lessons from molecular assays

Kinesins are enzymes that use the energy of ATP to perform mechanical work. There are approximately 14 families of kinesins within the kinesin superfamily. Family classification is derived primarily from alignments of the sequences of the core motor domain. For this reason, the enzymatic behavior and motility of each motor generally reflects its family. At the cellular level, kinesin motors perform a variety of functions during cell division and within the mitotic spindle to ensure that chromosomes are segregated with the highest fidelity possible. The cellular functions of these motors are intimately related to their mechanical and enzymatic properties at the single molecule level. For this reason, motility studies designed to evaluate the activity of purified molecular motors are a requirement in order to understand, mechanistically, how these motors make the mitotic spindle work and what can cause the spindle to fail. This review will focus on a selection of illustrative kinesins, which have been studied at the molecular level in order to inform our understanding of their function in cells. In addition, the review will endeavor to point out some kinesins that have been studied extensively but which still lack sufficient molecular underpinnings to fully predict their contribution to spindle function.

Functional and spatial regulation of mitotic centromere-associated kinesin by cyclin-dependent kinase 1

Mitotic centromere-associated kinesin (MCAK) plays an essential role in spindle formation and in correction of improper microtubule-kinetochore attachments. The localization and activity of MCAK at the centromere/kinetochore are controlled by Aurora B kinase. However, MCAK is also abundant in the cytosol and at centrosomes during mitosis, and its regulatory mechanism at these sites is unknown. We show here that cyclin-dependent kinase 1 (Cdk1) phosphorylates T537 in the core domain of MCAK and attenuates its microtubule-destabilizing activity in vitro and in vivo. Phosphorylation of MCAK by Cdk1 promotes the release of MCAK from centrosomes and is required for proper spindle formation. Interfering with the regulation of MCAK by Cdk1 causes dramatic defects in spindle formation and in chromosome positioning. This is the first study demonstrating that Cdk1 regulates the localization and activity of MCAK in mitosis by directly phosphorylating the catalytic core domain of MCAK.

Kinesin-13s in mitosis: Key players in the spatial and temporal organization of spindle microtubules

Dynamic microtubules are essential for the process of mitosis. Thus, elucidating when, where, and how microtubule dynamics are regulated is key to understanding this process. One important class of proteins that directly regulates microtubule dynamics is the Kinesin-13 family. Kinesin-13 proteins induce depolymerization uniquely from both ends of the microtubule. This activity coincides with their cellular localization and with their ability to regulate microtubule dynamics to control spindle assembly and kinetochore-microtubule attachments. In this review, we highlight recent findings that dissect the important actions of Kinesin-13 family members and summarize important studies on the regulation of their activity by phosphorylation and by protein-protein interactions.

Kinetochore-microtubule dynamics and attachment stability

Mitosis is the process by which a cell divides its genetic material equally into two daughter cells. Successful division requires that the two identical sister chromatids of a mitotic chromosome attach to the plus-ends of spindle microtubules (MTs) via their kinetochores, which are large protein structures built on centromeric DNA. Attachments between kinetochores and MTs must be persistent so that forces can be generated for chromosome movements, but at the same time they must be compliant, because attached MT plus-ends continuously polymerize and depolymerize to provide force for chromosome congression to the spindle equator. Both the attachment stability of kinetochore-MTs and the degree of dynamic instability exhibited by kinetochore-MTs must be precisely controlled to avoid errors in chromosome segregation. This chapter provides an overview of techniques used in cultured mammalian cells that measure stability and polymerization/depolymerization dynamics of kinetochore-MTs during mitosis.

Induction of asymmetrical cell division to analyze spindle-dependent organelle partitioning using correlative microscopy techniques

This protocol describes an assay for the induction of asymmetrical cell division where the entire spindle is segregated into only one of the daughter cells. The procedure consists of four stages: (i) generation of asymmetrical monoasters by arresting cells in early mitosis with a kinesin Eg5 inhibitor; (ii) induction of cell division by microinjection of recombinant Mad1 protein or by the addition of a Cdk1 inhibitor; (iii) monitoring the division process by phase-contrast time-lapse microscopy; and (iv) processing for correlative immunofluorescence or correlative electron microscopy. This approach can be applied to determine the requirement for the mitotic spindle in organelle partitioning as well as to investigate the role of the monopolar spindle in cytokinesis. Moreover, the generated nucleus-lacking cytoplast provides an ideal environment to test the feasibility and activity of biological processes in the absence of genomic influence. The protocol takes 2-4 d to complete.

Compression regulates mitotic spindle length by a mechanochemical switch at the poles

BACKGROUND

Although the molecules involved in mitosis are becoming better characterized, we still lack an understanding of the emergent mechanical properties of the mitotic spindle. For example, we cannot explain how spindle length is determined. To gain insight into how forces are generated and responded to in the spindle, we developed a method to apply controlled mechanical compression to metaphase mitotic spindles in living mammalian cells while monitoring microtubules and kinetochores by fluorescence microscopy.

RESULTS

Compression caused reversible spindle widening and lengthening to a new steady state. Widening was a passive mechanical response, and lengthening was an active mechanochemical process requiring microtubule polymerization but not kinesin-5 activity. Spindle morphology during lengthening and drug perturbations suggested that kinetochore fibers are pushed outward by pole-directed forces generated within the spindle. Lengthening of kinetochore fibers occurred by inhibition of microtubule depolymerization at poles, with no change in sliding velocity, interkinetochore stretching, or kinetochore dynamics.

CONCLUSIONS

We propose that spindle length is controlled by a mechanochemical switch at the poles that regulates the depolymerization rate of kinetochore fibers in response to compression and discuss models for how this switch is controlled. Poleward force appears to be exerted along kinetochore fibers by some mechanism other than kinesin-5 activity, and we speculate that it may arise from polymerization pressure from growing plus ends of interpolar microtubules whose minus ends are anchored in the fiber. These insights provide a framework for conceptualizing mechanical integration within the spindle.

Kinetochore-microtubule attachment relies on the disordered N-terminal tail domain of Hec1

Accurate chromosome segregation is dependent upon stable attachment of kinetochores to spindle microtubules during mitosis. A long-standing question is how kinetochores maintain stable attachment to the plus ends of dynamic microtubules that are continually growing and shortening. The Ndc80 complex is essential for persistent end-on kinetochore-microtubule attachment in cells [1, 2], but how the Ndc80 complex forms functional microtubule-binding sites remains unknown. We show that the 80 amino acid N-terminal unstructured "tail" of Hec1 is required for generating stable kinetochore-microtubule attachments. PtK1 cells depleted of endogenous Hec1 and rescued with Hec1-GFP fusion proteins deleted of the entire N terminus or the disordered N-terminal 80 amino acid tail domain fail to generate stable kinetochore-microtubule attachments. Mutation of nine amino acids within the Hec1 tail to reduce its positive charge also abolishes stable attachment. Furthermore, the mitotic checkpoint remains functional after deletion of the N-terminal 80 amino acid tail, but not after deletion of the N-terminal 207 amino acid region containing both the tail domain and a calponin homology (CH) domain. These results demonstrate that kinetochore-microtubule binding is dependent on electrostatic interactions mediated through the disordered N-terminal 80 amino acid tail domain and mitotic-checkpoint function is dependent on the CH domain of Hec1.

MCAK and paclitaxel have differential effects on spindle microtubule organization and dynamics

Within the mitotic spindle, there are multiple populations of microtubules with different turnover dynamics, but how these different dynamics are maintained is not fully understood. MCAK is a member of the kinesin-13 family of microtubule-destabilizing enzymes that is required for proper establishment and maintenance of the spindle. Using quantitative immunofluorescence and fluorescence recovery after photobleaching, we compared the differences in spindle organization caused by global suppression of microtubule dynamics, by treating cells with low levels of paclitaxel, versus specific perturbation of spindle microtubule subsets by MCAK inhibition. Paclitaxel treatment caused a disruption in spindle microtubule organization marked by a significant increase in microtubules near the poles and a reduction in K-fiber fluorescence intensity. This was correlated with a faster t(1/2) of both spindle and K-fiber microtubules. In contrast, MCAK inhibition caused a dramatic reorganization of spindle microtubules with a significant increase in astral microtubules and reduction in K-fiber fluorescence intensity, which correlated with a slower t(1/2) of K-fibers but no change in the t(1/2) of spindle microtubules. Our data support the model that MCAK perturbs spindle organization by acting preferentially on a subset of microtubules, and they support the overall hypothesis that microtubule dynamics is differentially regulated in the spindle.

Protein inhibition by microinjection and RNA-mediated interference in tissue culture cells: complementary approaches to study protein function

A major goal in cell biology is to understand the molecular mechanisms of the biological process under study, which requires functional information about the roles of individual proteins in the cell. For many non-genetic model organisms researchers have relied on the use of inhibitory reagents, such as antibodies that can be microinjected into cells. More recently, the advent of RNA-mediated interference (RNAi) has allowed scientists to knockdown individual proteins and to examine the consequences of the knockdown. In this chapter we present a comparison between microinjection of inhibitory reagents and RNAi for the analysis of protein function in mammalian tissue culture cells, providing both a description of the techniques as well as a discussion of the benefits and drawbacks of each approach. In addition, we present a strategy to employ RNAi for organisms without a sequenced genome. While the focus of our research is on the organization of the mitotic spindle during cell division and thus the examples utilized are from that system, the approaches described here should be readily applicable to multiple experimental models.

Microtubule binding and disruption and induction of premature senescence by disorazole C(1)

Disorazoles comprise a family of 29 macrocyclic polyketides isolated from the fermentation broth of the myxobacterium Sorangium cellulosum. The major fermentation product, disorazole A(1), was found previously to irreversibly bind to tubulin and to have potent cytotoxic activity against tumor cells, possibly because of its highly electrophilic epoxide moiety. To test this hypothesis, we synthesized the epoxide-free disorazole C(1) and found it retained potent antiproliferative activity against tumor cells, causing prominent G(2)/M phase arrest and inhibition of in vitro tubulin polymerization. Furthermore, disorazole C(1) produced disorganized microtubules at interphase, misaligned chromosomes during mitosis, apoptosis, and premature senescence in the surviving cell populations. Using a tubulin polymerization assay, we found disorazole C(1) inhibited purified bovine tubulin polymerization, with an IC(50) of 11.8 +/- 0.4 microM, and inhibited [3H]vinblastine binding noncompetitively, with a K(i) of 4.5 +/- 0.6 microM. We also found noncompetitive inhibition of [3H]dolastatin 10 binding by disorazole C(1), with a K(i) of 10.6 +/- 1.5 microM, indicating that disorazole C(1) bound tubulin uniquely among known antimitotic agents. Disorazole C(1) could be a valuable chemical probe for studying the process of mitotic spindle disruption and its relationship to premature senescence.

Cell cycle dependent degradation of MCAK: evidence against a role in anaphase chromosome movement

MCAK, a kinesin related motor protein with microtubule depolymerizing activity, is known to play an important role in spindle assembly and correcting errors in mitotic chromosome alignment. Experiments to determine how cellular levels of the protein are regulated demonstrate that MCAK accumulates during cell cycle progression, reaches a maximum at G(2)/M phase, and is rapidly degraded by the proteasome during mitosis. Immunofluorescence microscopy further indicates that MCAK largely disappears from kinetochores and spindle poles at the metaphase to anaphase transition. A phosphorylated form of MCAK appears during mitosis and seems to be preferentially degraded, but degradation does not appear to depend on Aurora B, a kinase reported to be involved in regulating the error correcting activity of the protein. These studies indicate that MCAK activity is limited during the latter stages of mitosis by protein degradation, and argue against a role for the protein in anaphase chromosome movement.

Work flow for multiplexing siRNA assays by solid-phase reverse transfection in multiwell plates

Solid-phase reverse transfection on cell microarrays is a high-throughput method for the parallel transfection of mammalian cells. However, the cells transfected in this way have been restricted so far to microscopy-based analyses. Analysis methods such as reverse transcriptase-polymerase chain reaction (RT-PCR) and access to higher cell numbers for statistical reasons in microscopy-based assays are not possible with solid-phase reverse transfection on cell microarrays. We have developed a quick and reliable protocol for automated solid-phase reverse transfection of human cells with siRNAs in multiwell plates complementing solid-phase reverse transfection on cell microarrays. The method retains all advantages of solid-phase reverse transfection such as long-term storage capacity after fabrication, reduced cytotoxicity, and reduced cost per screen compared with liquid-phase transfection in multiwell plates. The protocol has been tested for the RNAi-mediated knockdown of several genes in different cell lines including U20S, RPE1, A549, and HeLa cells. We show that even 3 months after production of the "ready to transfect" multiwell plates, there is no reduction in their transfection efficiency as assessed by RT-PCR and nuclear phenotyping by fluorescence microscopy. We conclude that solid-phase reverse transfection in multiwell plates is a cost-efficient and flexible tool for multiplexing cellular assays.

What generates flux of tubulin in kinetochore microtubules?

We discuss models for production of tubulin flux in kinetochore microtubules. Current models concentrate solely on microtubules and their associated motors and enzymes. For example, in some models the driving force for flux is enzymes at the poles and the kinetochores; in others the driving force is motor molecules that are associated with a stationary spindle matrix. We present a different viewpoint, that microtubules are propelled poleward by forces arising from the spindle matrix, that the forces on the microtubules "activate" polymerising and depolymerising enzymes at kinetochores and poles, that matrix forces utilise actin, myosin, and microtubule motors, and that the matrix itself may not necessarily be static.

MCAK facilitates chromosome movement by promoting kinetochore microtubule turnover

Mitotic centromere-associated kinesin (MCAK)/Kif2C is the most potent microtubule (MT)-destabilizing enzyme identified thus far. However, MCAK's function at the centromere has remained mechanistically elusive because of interference from cytoplasmic MCAK's global regulation of MT dynamics. In this study, we present MCAK chimeras and mutants designed to target centromere-associated MCAK for mechanistic analysis. Live imaging reveals that depletion of centromere-associated MCAK considerably decreases the directional coordination between sister kinetochores. Sister centromere directional antagonism results in decreased movement speed and increased tension. Sister centromeres appear unable to detach from kinetochore MTs efficiently in response to directional switching cues during oscillatory movement. These effects are reversed by anchoring ectopic MCAK to the centromere. We propose that MCAK increases the turnover of kinetochore MTs at all centromeres to coordinate directional switching between sister centromeres and facilitate smooth translocation. This may contribute to error correction during chromosome segregation either directly via slow MT turnover or indirectly by mechanical release of MTs during facilitated movement.

Aurora B phosphorylates multiple sites on mitotic centromere-associated kinesin to spatially and temporally regulate its function

Chromosome congression and segregation require the proper attachment of microtubules to the two sister kinetochores. Disruption of either Aurora B kinase or the Kinesin-13 mitotic centromere-associated kinesin (MCAK) increases chromosome misalignment and missegregation due to improper kinetochore-microtubule attachments. MCAK localization and activity are regulated by Aurora B, but how Aurora B phosphorylation of MCAK affects spindle assembly is unclear. Here, we show that the binding of MCAK to chromosome arms is also regulated by Aurora B and that Aurora B-dependent chromosome arm and centromere localization is regulated by distinct two-site phosphoregulatory mechanisms. MCAK association with chromosome arms is promoted by phosphorylation of T95 on MCAK, whereas phosphorylation of S196 on MCAK promotes dissociation from the arms. Although targeting of MCAK to centromeres requires phosphorylation of S110 on MCAK, dephosphorylation of T95 on MCAK increases the binding of MCAK to centromeres. Our study reveals a new role for Aurora B, which is to prevent excess MCAK binding to chromatin to facilitate chromatin-nucleated spindle assembly. Our study also shows that the interplay between multiple phosphorylation sites of MCAK may be critical to temporally and spatially control MCAK function.

The kinesin related motor protein, Eg5, is essential for maintenance of pre-implantation embryogenesis

Eg5 is a plus end directed kinesin related motor protein (KRP) previously shown to be involved in the assembly and maintenance of the mitotic spindle. KRPs are molecular motors capable of generating forces upon microtubules (MTs) in dividing cells and driving structural rearrangements necessary in the developing spindle. In vitro experiments demonstrate that loss of Eg5 results in cell cycle arrest and defective centrosome separation resulting in the development of monopolar spindles. Here we describe mice with a genetrap insertion in Eg5. Heterozygous mutant mice appear phenotypically normal. In contrast, embryos homozygous for the Eg5 null allele recovered at embryonic days 2.5-3.5 display signs of a proliferation defect as reduced cell numbers and failure of compaction and progression to the blastocyst stage was observed. These data, in conjunction with previous in vitro data, suggest that loss of Eg5 results in abnormal spindle structure, cell cycle arrest and thereby reduced cell proliferation of early cleavage pre-implantation embryos. These observations further support the conclusion that Eg5 is essential for cell division early in mouse development, and that maternal contribution may sustain the embryo through the maternal to zygotic transition at which point supplies of functional Eg5 are exhausted, preventing further cell cleavage.

The interplay of the N- and C-terminal domains of MCAK control microtubule depolymerization activity and spindle assembly

Spindle assembly and accurate chromosome segregation require the proper regulation of microtubule dynamics. MCAK, a Kinesin-13, catalytically depolymerizes microtubules, regulates physiological microtubule dynamics, and is the major catastrophe factor in egg extracts. Purified GFP-tagged MCAK domain mutants were assayed to address how the different MCAK domains contribute to in vitro microtubule depolymerization activity and physiological spindle assembly activity in egg extracts. Our biochemical results demonstrate that both the neck and the C-terminal domain are necessary for robust in vitro microtubule depolymerization activity. In particular, the neck is essential for microtubule end binding, and the C-terminal domain is essential for tight microtubule binding in the presence of excess tubulin heterodimer. Our physiological results illustrate that the N-terminal domain is essential for regulating microtubule dynamics, stimulating spindle bipolarity, and kinetochore targeting; whereas the C-terminal domain is necessary for robust microtubule depolymerization activity, limiting spindle bipolarity, and enhancing kinetochore targeting. Unexpectedly, robust MCAK microtubule (MT) depolymerization activity is not needed for sperm-induced spindle assembly. However, high activity is necessary for proper physiological MT dynamics as assayed by Ran-induced aster assembly. We propose that MCAK activity is spatially controlled by an interplay between the N- and C-terminal domains during spindle assembly.