Measurement of Microtubule Half-Life and Poleward Flux in the Mitotic Spindle by Photoactivation of Fluorescent Tubulin

The cell cycle oscillator and spindle length set the speed of chromosome separation in Drosophila embryos

Anaphase is tightly controlled in space and time to ensure proper separation of chromosomes. The mitotic spindle, the self-organized microtubule structure driving chromosome segregation, scales in size with the available cytoplasm. Yet, the relationship between spindle size and chromosome movement remains poorly understood. Here, we address how the movement of chromosomes changes during the cleavage divisions of the Drosophila blastoderm. We show that the speed of chromosome separation gradually decreases during the 4 nuclear divisions of the blastoderm. This reduction in speed is accompanied by a similar reduction in the length of the spindle, thus ensuring that these two quantities are tightly linked. Using a combination of genetic and quantitative imaging approaches, we find that two processes contribute to controlling the speed at which chromosomes move at mitotic exit: the activity of molecular motors important for microtubule depolymerization and the cell cycle oscillator. Specifically, we found that the levels of Klp10A, Klp67A, and Klp59C, three kinesin-like proteins important for microtubule depolymerization, contribute to setting the speed of chromosome separation. This observation is supported by quantification of microtubule dynamics indicating that poleward flux rate scales with the length of the spindle. Perturbations of the cell cycle oscillator using heterozygous mutants of mitotic kinases and phosphatases revealed that the duration of anaphase increases during the blastoderm cycles and is the major regulator of chromosome velocity. Thus, our work suggests a potential link between the biochemical rate of mitotic exit and the forces exerted by the spindle. Collectively, we propose that the cell cycle oscillator and spindle length set the speed of chromosome separation in anaphase.

Modification of the neck-linker of KIF18A alters Microtubule subpopulation preference

Kinesins support many diverse cellular processes, including facilitating cell division through mechanical regulation of the mitotic spindle. However, how kinesin activity is controlled to facilitate this process is not well understood. Interestingly, posttranslational modifications have been identified within the enzymatic region of all 45 mammalian kinesins, but the significance of these modifications has gone largely unexplored. Given the critical role of the enzymatic region in facilitating nucleotide and microtubule binding, it may serve as a primary site for kinesin regulation. Consistent with this idea, a phosphomimetic mutation at S357 in the neck-linker of KIF18A alters the localization of KIF18A within the spindle from kinetochore microtubules to nonkinetochore microtubules at the periphery of the spindle. Changes in localization of KIF18A-S357D are accompanied by defects in mitotic spindle positioning and the ability to promote mitotic progression. This altered localization pattern is mimicked by a shortened neck-linker mutant, suggesting that KIF18A-S357D may cause the motor to adopt a shortened neck-linker-like state that decreases KIF18A accumulation at the plus-ends of kinetochore microtubules. These findings demonstrate that posttranslational modifications in the enzymatic region of kinesins could be important for biasing their localization to particular microtubule subpopulations.

Modification of the Neck Linker of KIF18A Alters Microtubule Subpopulation Preference

Kinesins support many diverse cellular processes, including facilitating cell division through mechanical regulation of the mitotic spindle. However, how kinesin activity is controlled to facilitate this process is not well understood. Interestingly, post-translational modifications have been identified within the enzymatic region of all 45 mammalian kinesins, but the significance of these modifications has gone largely unexplored. Given the critical role of the enzymatic region in facilitating nucleotide and microtubule binding, it may serve as a primary site for kinesin regulation. Consistent with this idea, a phosphomimetic mutation at S357 in the neck-linker of KIF18A alters the localization of KIF18A within the spindle from kinetochore microtubules to peripheral microtubules. Changes in localization of KIF18A-S357D are accompanied by defects in mitotic spindle positioning and the ability to promote mitotic progression. This altered localization pattern is mimicked by a shortened neck-linker mutant, suggesting that KIF18A-S357D may cause the motor to adopt a shortened neck-linker like state that prevents KIF18A from accumulating at the plus-ends of kinetochore microtubules. These findings demonstrate that post-translational modifications in the enzymatic region of kinesins could be important for biasing their localization to particular microtubule subpopulations.

α-Tubulin detyrosination links the suppression of MCAK activity with taxol cytotoxicity

α/β-Tubulin posttranslational modifications (PTMs) generate microtubule diversity, but whether they account for cancer cell resistance to microtubule-targeting drugs remains unknown. Here, we performed a pilot dissection of the "cancer tubulin code" using the NCI-60 cancer cell panel. We found that acetylated, detyrosinated, and ∆2-α-tubulin that typically accumulate on stable microtubules were uncoupled in many cancer cells. Acetylated α-tubulin did not affect microtubule dynamics, whereas its levels correlated with, but were not required for, taxol-induced cytotoxicity. In contrast, experimental increase of α-tubulin detyrosination, and/or depletion of the detyrosination-sensitive microtubule-depolymerizing enzyme MCAK, enhanced taxol-induced cytotoxicity by promoting cell death in mitosis and the subsequent interphase, without causing a cumulative effect. Interestingly, only increased detyrosinated α-tubulin aggravated taxol-induced spindle multipolarity. Overall, we identified high α-tubulin acetylation as a potential biomarker for cancer cell response to taxol and uncovered a mechanistic link between α-tubulin detyrosination and the suppression of MCAK activity in taxol-induced cytotoxicity, likely by promoting chromosome missegregation, regardless of spindle defects.

Chromosome segregation fidelity requires microtubule polyglutamylation by the cancer downregulated enzyme TTLL11

Regulation of microtubule (MT) dynamics is key for mitotic spindle assembly and faithful chromosome segregation. Here we show that polyglutamylation, a still understudied post-translational modification of spindle MTs, is essential to define their dynamics within the range required for error-free chromosome segregation. We identify TTLL11 as an enzyme driving MT polyglutamylation in mitosis and show that reducing TTLL11 levels in human cells or zebrafish embryos compromises chromosome segregation fidelity and impairs early embryonic development. Our data reveal a mechanism to ensure genome stability in normal cells that is compromised in cancer cells that systematically downregulate TTLL11. Our data suggest a direct link between MT dynamics regulation, MT polyglutamylation and two salient features of tumour cells, aneuploidy and chromosome instability (CIN).

Augmin-dependent microtubule self-organization drives kinetochore fiber maturation in mammals

Chromosome segregation in mammals relies on the maturation of a thick bundle of kinetochore-attached microtubules known as k-fiber. How k-fibers mature from initial kinetochore microtubule attachments remains a fundamental question. By combining molecular perturbations and phenotypic analyses in Indian muntjac fibroblasts containing the lowest known diploid chromosome number in mammals (2N = 6) and distinctively large kinetochores, with fixed/live-cell super-resolution coherent-hybrid stimulated emission depletion (CH-STED) nanoscopy and laser microsurgery, we demonstrate a key role for augmin in kinetochore microtubule self-organization and maturation, regardless of pioneer centrosomal microtubules. In doing so, augmin promotes kinetochore and interpolar microtubule turnover and poleward flux. Tracking of microtubule growth events within individual k-fibers reveals a wide angular dispersion, consistent with augmin-mediated branched microtubule nucleation. Augmin depletion reduces the frequency of kinetochore microtubule growth events and hampers efficient repair after acute k-fiber injury by laser microsurgery. Together, these findings underscore the contribution of augmin-mediated microtubule amplification for k-fiber self-organization and maturation in mammals.

An anaphase surveillance mechanism prevents micronuclei formation from frequent chromosome segregation errors

Micronuclei are a hallmark of cancer and several other human disorders. Recently, micronuclei were implicated in chromothripsis, a series of massive genomic rearrangements that may drive tumor evolution and progression. Here, we show that Aurora B kinase mediates a surveillance mechanism that integrates error correction during anaphase with spatial control of nuclear envelope reassembly to prevent micronuclei formation. Using high-resolution live-cell imaging of human cancer and non-cancer cells, we uncover that anaphase lagging chromosomes are more frequent than previously anticipated, yet they rarely form micronuclei. Micronuclei formation from anaphase lagging chromosomes is prevented by a midzone-based Aurora B phosphorylation gradient that stabilizes kinetochore-microtubule attachments and assists spindle forces required for anaphase error correction while delaying nuclear envelope reassembly on lagging chromosomes, independently of microtubule density. We propose that a midzone-based Aurora B phosphorylation gradient actively monitors and corrects frequent chromosome segregation errors to prevent micronuclei formation during human cell division.

•

Anaphase lagging chromosomes are frequent but rarely form micronuclei

•

A midzone Aurora B activity gradient prevents micronuclei from segregation errors

•

Midzone Aurora B assists spindle forces at the kinetochores to correct errors

•

Aurora B spatially regulates nuclear envelope reformation on lagging chromosomes

CLASP2 binding to curved microtubule tips promotes flux and stabilizes kinetochore attachments

Girão et al. use structure-guided functional mutants of CLASP2 to show that recognition of growing microtubule plus-ends through EB–protein interaction and the ability to associate with curved microtubule protofilaments through TOG2 and TOG3 domains promote growth and stabilization of kinetochore–microtubules required for poleward flux.

CLASPs are conserved microtubule plus-end–tracking proteins that suppress microtubule catastrophes and independently localize to kinetochores during mitosis. Thus, CLASPs are ideally positioned to regulate kinetochore–microtubule dynamics required for chromosome segregation fidelity, but the underlying mechanism remains unknown. Here, we found that human CLASP2 exists predominantly as a monomer in solution, but it can self-associate through its C-terminal kinetochore-binding domain. Kinetochore localization was independent of self-association, and driving monomeric CLASP2 to kinetochores fully rescued normal kinetochore–microtubule dynamics, while partially sustaining mitosis. CLASP2 kinetochore localization, recognition of growing microtubule plus-ends through EB–protein interaction, and the ability to associate with curved microtubule protofilaments through TOG2 and TOG3 domains independently sustained normal spindle length, timely spindle assembly checkpoint satisfaction, chromosome congression, and faithful segregation. Measurements of kinetochore–microtubule half-life and poleward flux revealed that CLASP2 regulates kinetochore–microtubule dynamics by integrating distinctive microtubule-binding properties at the kinetochore–microtubule interface. We propose that kinetochore CLASP2 suppresses microtubule depolymerization and detachment by binding to curved protofilaments at microtubule plus-ends.