European Research Council (ERC) Project

The driving forces of cell division

At the onset of division the cell forms a spindle, a micro-machine made of microtubules, which divide the chromosomes by pulling on kinetochores, protein complexes on the chromosome. The central question in the field is how accurate chromosome segregation results from the interactions between kinetochores, microtubules and the associated proteins.

According to the current paradigm, the forces on kinetochores are produced by k-fibers, bundles of microtubules extending between the spindle pole and the kinetochore. The proposed project is built upon a hypothesis that a new class of microtubules, which we term bridging microtubules, bridge sister kinetochores.

We are taking an interdisciplinary approach by combining laser microsurgery with genetic perturbations, quantitative measurements of the responses and comparison with theoretical models. Understanding the role of bridging microtubules in force generation and chromosome movements will not only shed light on the mechanism of chromosome segregation, but may also increase the potential of mitotic anticancer strategies, as the spindle is a major target for chemotherapy.


Research area: Cellular and Developmental Biology
Researcher: Prof Iva Tolić
Host Institution: Ruđer Bošković Institute, Zagreb
Project: A new class of microtubules in the spindle exerting forces on kinetochores (NewSpindleForce)
ERC call:  Consolidator Grant 2014
ERC funding: € 2.15 million for five years




Research papers

Microtubule Sliding within the Bridging Fiber Pushes Kinetochore Fibers Apart to Segregate Chromosomes.
Kruno Vukušić, Renata Buđa, Agneza Bosilj, Ana Milas, Nenad Pavin, Iva M. Tolić. Dev Cell, 43(1): 11-23 (2017).

During cell division, mitotic spindle microtubules segregate chromosomes by exerting forces on kinetochores. What forces drive chromosome segregation in anaphase remains a central question. The current model for anaphase in human cells includes shortening of kinetochore fibers and separation of spindle poles. Both processes require kinetochores to be linked with the poles. Here we show, by combining laser ablation, photoactivation, and theoretical modeling, that kinetochores can separate without any attachment to one spindle pole. This separation requires the bridging fiber, a microtubule bundle that connects sister kinetochore fibers. Bridging fiber microtubules in intact spindles slide apart with kinetochore fibers, indicating strong crosslinks between them. We conclude that sliding of microtubules within the bridging fibers drives pole separation and pushes kinetochore fibers poleward by the friction of passive crosslinks between these fibers. Thus, sliding within the bridging fiber works together with the shortening of kinetochore fibers to segregate chromosomes.


PRC1-labeled microtubule bundles and kinetochore pairs show one-to-one association in metaphase.
Bruno Polak, Patrik Risteski, Sonja Lesjak, Iva M. Tolić. EMBO Rep, 18(2): 217–230 (2017).

In the mitotic spindle, kinetochore microtubules form k‐fibers, whereas overlap or interpolar microtubules form antiparallel arrays containing the cross‐linker protein regulator of cytokinesis 1 (PRC1). We have recently shown that an overlap bundle, termed bridging fiber, links outermost sister k‐fibers. However, the relationship between overlap bundles and k‐fibers throughout the spindle remained unknown. Here, we show that in a metaphase spindle more than 90% of overlap bundles act as a bridge between sister k‐fibers. We found that the number of PRC1‐GFP‐labeled bundles per spindle is nearly the same as the number of kinetochore pairs. Live‐cell imaging revealed that kinetochore movement in the equatorial plane of the spindle is highly correlated with the movement of the coupled PRC1‐GFP‐labeled fiber, whereas the correlation with other fibers decreases with increasing distance. Analysis of endogenous PRC1 localization confirmed the results obtained with PRC1‐GFP. PRC1 knockdown reduced the bridging fiber thickness and interkinetochore distance throughout the spindle, suggesting a function of PRC1 in bridging microtubule organization and force balance in the metaphase spindle.

Relaxation of interkinetochore tension after severing of a k-fiber depends on the length of the k-fiber stub.
Ana Milas, Iva M. Tolić. Matters Select, doi: 10.19185/matters.201603000025 (2016).

During mitosis, tension forces acting on kinetochores are required for passage through the spindle assembly checkpoint and for chromatid segregation. It is generally thought that the interkinetochore tension is generated by molecular events occurring at the ends of k-fibers, bundles of microtubules bound to kinetochores. However, recent work has shown that bridging microtubules, which interact laterally with sister k-fibers and act as a bridge between sister kinetochores, contribute to interkinetochore tension. We set out to test the role of the bridging fibers in the origin of interkinetochore tension during metaphase in human U2OS and rat-kangaroo PtK1 cells. We severed k-fibers at different distances from the kinetochore and measured the change in the interkinetochore distance, which we use as a readout of the change in interkinetochore tension. We found that severing of a k-fiber closer to the kinetochore results in a larger relaxation of the interkinetochore tension than severing far from the kinetochore. These findings imply that a long k-fiber stub, which remains connected to the bridging fiber, withstands the interkinetochore tension and prevents relaxation of the interkinetochore distance, whereas a short stub does not. Thus, our results are consistent with a role of bridging microtubules in the origin of interkinetochore tension.

Overlap microtubules link sister k-fibres and balance the forces on bi-oriented kinetochores.
Janko Kajtez, Anastasia Solomatina, Maja Novak, Bruno Polak, Kruno Vukušić, Jonas Rüdiger, Gheorghe Cojoc, Ana Milas, Ivana Šumanovac Šestak, Patrik Risteski, Federica Tavano, Anna H. Klemm, Emanuele Roscioli, Julie Welburn, Daniela Cimini, Matko Glunčić, Nenad Pavin, Iva M. Tolić. Nat Commun, 7: 10298 (2016).

During metaphase, forces on kinetochores are exerted by k-fibres, bundles of microtubules that end at the kinetochore. Interestingly, non-kinetochore microtubules have been observed between sister kinetochores, but their function is unknown. Here we show by laser-cutting of a k-fibre in HeLa and PtK1 cells that a bundle of non-kinetochore microtubules, which we term ‘bridging fibre’, bridges sister k-fibres and balances the interkinetochore tension. We found PRC1 and EB3 in the bridging fibre, suggesting that it consists of antiparallel dynamic microtubules. By using a theoretical model that includes a bridging fibre, we show that the forces at the pole and at the kinetochore depend on the bridging fibre thickness. Moreover, our theory and experiments show larger relaxation of the interkinetochore distance for cuts closer to kinetochores. We conclude that the bridging fibre, by linking sister k-fibres, withstands the tension between sister kinetochores and enables the spindle to obtain a curved shape.

Reviews, commentaries and methods

Optogenetic reversible knocksideways, laser ablation and photoactivation on the mitotic spindle in human cells.
Ana Milas, Mihaela Jagrić, Jelena Martinčić, Iva M. Tolić. Methods Cell Biol, in press.

At the onset of mitosis, cells assemble the mitotic spindle, a dynamic micromachine made of microtubules and associated proteins. Although most of these proteins have been identified, it is still unknown how their collective behavior drives spindle formation and function. Over the last decade, RNA interference has been the main tool for revealing the role of spindle proteins. However, the effects of this method are evident only after a longer time period, leading to difficulties in the interpretation of phenotypes. Optogenetics is a novel technology that enables fast, reversible, and precise control of protein activity by utilization of light. In this chapter, we present an optogenetic knocksideways method for rapid and reversible translocation of proteins from the mitotic spindle to mitochondria using blue light. Furthermore, we discuss other optical approaches, such as laser ablation of microtubule bundles in the spindle and creation of reference marks on the bundles by photoactivation of photoactivatable GFP. Finally, we show how different optical perturbations can be combined in order to acquire deeper understanding of the mechanics of mitosis.

Mitotic spindle: kinetochore fibers hold on tight to interpolar bundles.
Iva M. Tolić. Eur Biophys J, doi: 10.1007/s00249-017-1244-4 (2017).

Spindle microtubules can be divided into three major classes: kinetochore microtubules, which form k-fibers ending at the kinetochore; interpolar microtubules, which extend from the opposite sides of the spindle and interact in the middle; and astral microtubules, which extend towards the cell cortex. Recent work in human cells has shown a close relationship between interpolar and kinetochore microtubules, where interpolar bundles are attached laterally to kinetochore fibers almost all along their length, acting as a bridge between sister k-fibers. Most of the interpolar bundles are attached to a pair of sister kinetochore fibers and vice versa. Thus, the spindle is made of modules consisting of a pair of sister kinetochore fibers and a bundle of interpolar microtubules that connects them. These interpolar bundles, termed bridging fibers, balance the forces acting at kinetochores and support the rounded shape of the spindle during metaphase. This review discusses the structure, function, and formation of kinetochore fibers and interpolar bundles, with an emphasis on how they interact. Their connections have an impact on the force balance in the spindle and on chromosome movement during mitosis because the forces in interpolar bundles are transmitted to kinetochore fibers and hence to kinetochores through these connections.

Dissection and characterization of microtubule bundles in the mitotic spindle using femtosecond laser ablation.
Renata Buđa, Kruno Vukušić, Iva M. Tolić. Methods Cell Biol, 139: 81–101 (2017).

The mitotic spindle is a highly organized and dynamic structure required for segregation of the genetic material into two daughter cells. Although most of the individual players involved in building the spindle have been characterized in vitro, a general understanding of how all of the spindle players act together in vivo is still missing. Hence, in recent years, experiments have focused on introducing mechanical perturbations of the spindle on a micron scale, thereby providing insight into its function and organization, as well as into forces acting in the spindle. Among different types of mechanical perturbations, optical ones are more flexible, less invasive, and more precise than other approaches. In this chapter, we describe a detailed protocol for cutting the microtubule bundles in human cells using a near-infrared femtosecond laser. This type of laser microsurgery provides the ability to precisely sever a single microtubule bundle while preserving spindle integrity and dynamics. Furthermore, we describe quantitative measurements obtained from the response of a severed microtubule bundle to laser ablation, which reveal the structure and function of individual parts of the spindle, such as the bridging fiber connecting sister k-fibers. Finally, the method described here can be easily combined with other quantitative techniques to address the complexity of the spindle.

TrendsTalk. Iva Tolić: Movements inside Cells and across Countries.
Iva M. Tolić. Trends Biochem Sci, 41(10): 815–816 (2016).

Cell division is the most fundamental process, without which there would be no life, but we still do not really understand how it works. Microtubules are crucial in this process because they attach to the chromosomes and pull them apart. The current paradigm says that only those microtubules that end at the chromosome are able to pull the chromosomes apart. In my lab, we recently found out that other microtubules, which we called ‘bridging microtubules’, also exert forces on chromosomes in metaphase. The name comes from the observation that these microtubules act like a bridge between sister kinetochores. This finding stimulated us to explore the function of bridging microtubules. We have shown that they balance the tension between sister kinetochores, and we are now exploring the role of bridging microtubules in the process of chromosome capture by spindle microtubules and in chromosome segregation.


Mitotic Spindle Assembly: Building the Bridge between Sister K-Fibers.
Juraj Simunić, Iva M. Tolić. Trends Biochem Sci, 41(10): 824–833 (2016).

The mitotic spindle performs the task of physically dividing the genetic material between the newly formed daughter cells. To achieve this, bundles of microtubules and associated proteins orchestrate forces that spatially organize and then separate the chromosomes. In the classic view of the spindle, the kinetochore microtubules (k-fibers) are tensed and, thus, straight, whereas interpolar bundles are curved and do not interact with k-fibers close to the spindle equator. The updated view of the spindle depicts k-fibers as curved and interacting with newly identified interpolar bundles, called bridging fibers, along their length. In this Opinion, we propose and discuss scenarios for the origin of this structure in the context of known spindle assembly mechanisms.

Self-Organization and Forces in the Mitotic Spindle.
Nenad Pavin, Iva M. Tolić. Annu Rev Biophys, 45: 279–298 (2016).

At the onset of division, the cell forms a spindle, a precise self-constructed micromachine composed of microtubules and the associated proteins, which divides the chromosomes between the two nascent daughter cells. The spindle arises from self-organization of microtubules and chromosomes, whose different types of motion help them explore the space and eventually approach and interact with each other. Once the interactions between the chromosomes and the microtubules have been established, the chromosomes are moved to the equatorial plane of the spindle and ultimately toward the opposite spindle poles. These transport processes rely on directed forces that are precisely regulated in space and time. In this review, we discuss how microtubule dynamics and their rotational movement drive spindle self-organization, as well as how the forces acting in the spindle are generated, balanced, and regulated.

Bridging the gap between sister kinetochores.
Iva M. Tolić, Nenad Pavin. Cell Cycle, 15(9): 1169-1170 (2016).

The results of our experiments led us to a mathematically exact yet simple model. In this model, microtubule fibers in the spindle are elastic rods, which bend under compressive force. Their curvature tells us about the forces that bend the k-fibers. At the same time, this model explains how the tension on kinetochores is balanced: Compression in the bridging fiber balances the tension at the kinetochore and the compression at the spindle pole. Thus, in our model the k-fiber is under tension close to the kinetochore and under compression close to the pole. The wooden model with 3 rods provides an intuitive understanding of this force balance.





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