Oscillatory nuclear movement in fission yeast meiotic prophase is driven by astral microtubules, as revealed by continuous observation of chromosomes and microtubules in living cells

Homologous chromosome pairing: The linchpin of accurate segregation in meiosis

Meiosis is a specialized cell division that occurs in sexually reproducing organisms, generating haploid gametes containing half the chromosome number through two rounds of cell division. Homologous chromosomes pair and prepare for their proper segregation in subsequent divisions. How homologous chromosomes recognize each other and achieve pairing is an important question. Early studies showed that in most organisms, homologous pairing relies on homologous recombination. However, pairing mechanisms differ across species. Evidence indicates that chromosomes are dynamic and move during early meiotic stages, facilitating pairing. Recent studies in various model organisms suggest conserved mechanisms and key regulators of homologous chromosome pairing. This review summarizes these findings and compare similarities and differences in homologous chromosome pairing mechanisms across species.

The SUN-like protein TgSLP1 is essential for nuclear division in the apicomplexan parasite Toxoplasma gondii

Connections between the nucleus and the cytoskeleton are important for positioning and division of the nucleus. In most eukaryotes, the linker of nucleoskeleton and cytoskeleton (LINC) complex spans the outer and inner nuclear membranes and connects the nucleus to the cytoskeleton. In opisthokonts, it is composed of Klarsicht, ANC-1 and Syne homology (KASH) domain proteins and Sad1 and UNC-84 (SUN) domain proteins. Given that the nucleus is positioned at the posterior pole of Toxoplasma gondii, we speculated that apicomplexan parasites must have a similar mechanism that integrates the nucleus and the cytoskeleton. Here, we identified three UNC family proteins in the genome of the apicomplexan parasite T. gondii. Whereas the UNC-50 protein TgUNC1 localised to the Golgi and appeared to be not essential for the parasite, the SUN domain protein TgSLP2 showed a diffuse pattern throughout the parasite. The second SUN domain protein, TgSLP1, was expressed in a cell cycle-dependent manner and was localised close to the mitotic spindle and, more detailed, at the kinetochore. We demonstrate that conditional knockout of TgSLP1 leads to failure of nuclear division and loss of centrocone integrity.

The courtship choreography of homologous chromosomes: timing and mechanisms of DSB-independent pairing

Meiosis involves deep changes in the spatial organisation and interactions of chromosomes enabling the two primary functions of this process: increasing genetic diversity and reducing ploidy level. These two functions are ensured by crucial events such as homologous chromosomal pairing, synapsis, recombination and segregation. In most sexually reproducing eukaryotes, homologous chromosome pairing depends on a set of mechanisms, some of them associated with the repair of DNA double-strand breaks (DSBs) induced at the onset of prophase I, and others that operate before DSBs formation. In this article, we will review various strategies utilised by model organisms for DSB-independent pairing. Specifically, we will focus on mechanisms such as chromosome clustering, nuclear and chromosome movements, as well as the involvement of specific proteins, non-coding RNA, and DNA sequences.

Temporal Changes in Nuclear Envelope Permeability during Semi-Closed Mitosis in Dictyostelium Amoebae

The Amoebozoan Dictyostelium discoideum exhibits a semi-closed mitosis in which the nuclear membranes remain intact but become permeabilized to allow tubulin and spindle assembly factors to access the nuclear interior. Previous work indicated that this is accomplished at least by partial disassembly of nuclear pore complexes (NPCs). Further contributions by the insertion process of the duplicating, formerly cytosolic, centrosome into the nuclear envelope and nuclear envelope fenestrations forming around the central spindle during karyokinesis were discussed. We studied the behavior of several Dictyostelium nuclear envelope, centrosomal, and nuclear pore complex (NPC) components tagged with fluorescence markers together with a nuclear permeabilization marker (NLS-TdTomato) by live-cell imaging. We could show that permeabilization of the nuclear envelope during mitosis occurs in synchrony with centrosome insertion into the nuclear envelope and partial disassembly of nuclear pore complexes. Furthermore, centrosome duplication takes place after its insertion into the nuclear envelope and after initiation of permeabilization. Restoration of nuclear envelope integrity usually occurs long after re-assembly of NPCs and cytokinesis has taken place and is accompanied by a concentration of endosomal sorting complex required for transport (ESCRT) components at both sites of nuclear envelope fenestration (centrosome and central spindle).

The many faces of the bouquet centrosome MTOC in meiosis and germ cell development

Meiotic chromosomal pairing is facilitated by a conserved cytoskeletal organization. Telomeres associate with perinuclear microtubules via Sun/KASH complexes on the nuclear envelope (NE) and dynein. Telomere sliding on perinuclear microtubules contributes to chromosome homology searches and is essential for meiosis. Telomeres ultimately cluster on the NE, facing the centrosome, in a configuration called the chromosomal bouquet. Here, we discuss novel components and functions of the bouquet microtubule organizing center (MTOC) in meiosis, but also broadly in gamete development. The cellular mechanics of chromosome movements and the bouquet MTOC dynamics are striking. The newly identified zygotene cilium mechanically anchors the bouquet centrosome and completes the bouquet MTOC machinery in zebrafish and mice. We hypothesize that various centrosome anchoring strategies evolved in different species. Evidence suggests that the bouquet MTOC machinery is a cellular organizer, linking meiotic mechanisms with gamete development and morphogenesis. We highlight this cytoskeletal organization as a new platform for creating a holistic understanding of early gametogenesis, with direct implications to fertility and reproduction.

A force balance model for a cell size-dependent meiotic nuclear oscillation in fission yeast

Fission yeast undergoes premeiotic nuclear oscillation, which is dependent on microtubules and is driven by cytoplasmic dynein. Although the molecular mechanisms have been analyzed, how a robust oscillation is generated despite the dynamic behaviors of microtubules has yet to be elucidated. Here, we show that the oscillation exhibits cell length-dependent frequency and requires a balance between microtubule and viscous drag forces, as well as proper microtubule dynamics. Comparison of the oscillations observed in living cells with a simulation model based on microtubule dynamic instability reveals that the period of oscillation correlates with cell length. Genetic alterations that reduce cargo size suggest that the nuclear movement depends on viscous drag forces. Deletion of a gene encoding Kinesin-8 inhibits microtubule catastrophe at the cell cortex and results in perturbation of oscillation, indicating that nuclear movement also depends on microtubule dynamic instability. Our findings link numerical parameters from the simulation model with cellular functions required for generating the oscillation and provide a basis for understanding the physical properties of microtubule-dependent nuclear movements.

Katnip is needed to maintain microtubule function and lysosomal delivery to autophagosomes and phagosomes

The efficient delivery of lysosomes is essential for many cell functions, such as the degradation of unwanted intracellular components by autophagy and the killing and digestion of extracellular microbes within phagosomes. Using the amoeba Dictyostelium discoideum, we find that cells lacking Katnip (Katanin interacting protein) have a general defect in lysosomal delivery and although they make autophagosomes and phagosomes correctly, cells are then unable to digest them. Katnip is largely unstudied yet highly conserved across evolution. Previously studies found that Katnip mutations in animals cause defects in cilia structure. Here we show that Katnip plays a more general role in maintaining microtubule function. We find that loss of Katnip has no overall effect on microtubule dynamics or organization, but is important for the transport and degradation of endocytic cargos. Strikingly, Katnip mutants become highly sensitive to GFP-tubulin expression, which leads to microtubule tangles, defective anaphase extension, and slow cell growth. Our findings establish a general role for Katnip in regulating microtubule function, beyond the roles previously described in cilia. We speculate this is via a key function in microtubule repair, needed to maintain endosomal trafficking and lysosomal degradation.

How and Why Chromosomes Interact with the Cytoskeleton during Meiosis

During the early meiotic prophase, connections are established between chromosomes and cytoplasmic motors via a nuclear envelope bridge, known as a LINC (linker of nucleoskeleton and cytoskeleton) complex. These widely conserved links can promote both chromosome and nuclear motions. Studies in diverse organisms have illuminated the molecular architecture of these connections, but important questions remain regarding how they contribute to meiotic processes. Here, we summarize the current knowledge in the field, outline the challenges in studying these chromosome dynamics, and highlight distinctive features that have been characterized in major model systems.

Rec8 Cohesin-mediated Axis-loop chromatin architecture is required for meiotic recombination

During meiotic prophase, cohesin-dependent axial structures are formed in the synaptonemal complex (SC). However, the functional correlation between these structures and cohesion remains elusive. Here, we examined the formation of cohesin-dependent axial structures in the fission yeast Schizosaccharomyces pombe. This organism forms atypical SCs composed of linear elements (LinEs) resembling the lateral elements of SC but lacking the transverse filaments. Hi-C analysis using a highly synchronous population of meiotic S. pombe cells revealed that the axis-loop chromatin structure formed in meiotic prophase was dependent on the Rec8 cohesin complex. In contrast, the Rec8-mediated formation of the axis-loop structure occurred in cells lacking components of LinEs. To dissect the functions of Rec8, we identified a rec8-F204S mutant that lost the ability to assemble the axis-loop structure without losing cohesion of sister chromatids. This mutant showed defects in the formation of the axis-loop structure and LinE assembly and thus exhibited reduced meiotic recombination. Collectively, our results demonstrate that the Rec8-dependent axis-loop structure provides a structural platform essential for LinE assembly, facilitating meiotic recombination of homologous chromosomes, independently of its role in sister chromatid cohesion.

Stochastic model of T Cell repolarization during target elimination (II)

Cytotoxic T lymphocytes (T cells) and natural killer cells form a tight contact, the immunological synapse (IS), with target cells, where they release their lytic granules containing perforin/granzyme and cytokine-containing vesicles. During this process the cell repolarizes and moves the microtubule organizing center (MTOC) toward the IS. In the first part of our work we developed a computational model for the molecular-motor-driven motion of the microtubule cytoskeleton during T cell polarization and analyzed the effects of cortical-sliding and capture-shrinkage mechanisms. Here we use this model to analyze the dynamics of the MTOC repositioning in situations in which 1) the IS is in an arbitrary position with respect to the initial position of the MTOC and 2) the T cell has two IS at two arbitrary positions. In the case of one IS, we found that the initial position determines which mechanism is dominant and that the time of repositioning does not rise monotonously with the MTOC-IS distance. In the case of two IS, we observe several scenarios that have also been reported experimentally: the MTOC alternates stochastically (but with a well-defined average transition time) between the two IS; it wiggles in between the two IS without transiting to one of the two; or it is at some point pulled to one of the two IS and stays there. Our model allows one to predict which scenario emerges in dependency of the mechanisms in action and the number of dyneins present. We report that the presence of capture-shrinkage mechanism in at least one IS is necessary to assure the transitions in every cell configuration. Moreover, the frequency of transitions does not decrease with the distance between the two IS and is the highest when both mechanisms are present in both IS.

Rec8 Cohesin: A Structural Platform for Shaping the Meiotic Chromosomes

Meiosis is critically different from mitosis in that during meiosis, pairing and segregation of homologous chromosomes occur. During meiosis, the morphology of sister chromatids changes drastically, forming a prominent axial structure in the synaptonemal complex. The meiosis-specific cohesin complex plays a central role in the regulation of the processes required for recombination. In particular, the Rec8 subunit of the meiotic cohesin complex, which is conserved in a wide range of eukaryotes, has been analyzed for its function in modulating chromosomal architecture during the pairing and recombination of homologous chromosomes in meiosis. Here, we review the current understanding of Rec8 cohesin as a structural platform for meiotic chromosomes.

Pcp1/pericentrin controls the SPB number in fission yeast meiosis and ploidy homeostasis

During sexual reproduction, the zygote must inherit exactly one centrosome (spindle pole body [SPB] in yeasts) from the gametes, which then duplicates and assembles a bipolar spindle that supports the subsequent cell division. Here, we show that in the fission yeast Schizosaccharomyces pombe, the fusion of SPBs from the gametes is blocked in polyploid zygotes. As a result, the polyploid zygotes cannot proliferate mitotically and frequently form supernumerary SPBs during subsequent meiosis, which leads to multipolar nuclear divisions and the generation of extra spores. The blockage of SPB fusion is caused by persistent SPB localization of Pcp1, which, in normal diploid zygotic meiosis, exhibits a dynamic association with the SPB. Artificially induced constitutive localization of Pcp1 on the SPB is sufficient to cause blockage of SPB fusion and formation of extra spores in diploids. Thus, Pcp1-dependent SPB quantity control is crucial for sexual reproduction and ploidy homeostasis in fission yeast.

Tell the Difference Between Mitosis and Meiosis: Interplay Between Chromosomes, Cytoskeleton, and Cell Cycle Regulation

Meiosis is a specialized style of cell division conserved in eukaryotes, particularly designed for the production of gametes. A huge number of studies to date have demonstrated how chromosomes behave and how meiotic events are controlled. Yeast substantially contributed to the understanding of the molecular mechanisms of meiosis in the past decades. Recently, evidence began to accumulate to draw a perspective landscape showing that chromosomes and microtubules are mutually influenced: microtubules regulate chromosomes, whereas chromosomes also regulate microtubule behaviors. Here we focus on lessons from recent advancement in genetical and cytological studies of the fission yeast Schizosaccharomyces pombe, revealing how chromosomes, cytoskeleton, and cell cycle progression are organized and particularly how these are differentiated in mitosis and meiosis. These studies illuminate that meiosis is strategically designed to fulfill two missions: faithful segregation of genetic materials and production of genetic diversity in descendants through elaboration by meiosis-specific factors in collaboration with general factors.

Collective dynein transport of the nucleus by pulling on astral microtubules during Saccharomyces cerevisiae mitosis

Positioning the nucleus at the bud neck during Saccharomyces cerevisiae mitosis involves pulling forces of cytoplasmic dynein localized in the daughter cell. Although genetic analysis has revealed a complex network positioning the nucleus, quantification of the forces acting on the nucleus and the number of dyneins driving the process has remained difficult. To better understand the collective forces involved in nuclear positioning, we compare a model of dyneins-driven microtubule (MT) pulling, MT pushing, and cytoplasmic drag to experiments. During S. cerevisiae mitosis, MTs interacting with the cortex nucleated by the daughter spindle pole body (SPB) (SPB-D) are longer than the mother SPB (SPB-M), increasing further during spindle elongation in anaphase. Interphasic SPB mobility is effectively diffusive, while the mitotic mobility is directed. By optimizing a computational model of the mobility of the nucleus due to diffusion and MTs pushing at the cell membrane to experiment, we estimate the viscosity governing the drag force on nuclei during positioning. A force balance model of mitotic SPB mobility compared to experimental mobility suggests that even one or two dynein dimers are sufficient to move the nucleus in the bud neck. Using stochastic computer simulations of a budding cell, we find that punctate dynein localization can generate sufficient force to reel in the nucleus to the bud neck. Compared to uniform motor localization, puncta involve fewer motors suggesting a functional role for motor clustering. Stochastic simulations also suggest that a higher number of force generators than predicted by force balance may be required to ensure the robustness of spindle positioning.

A visual atlas of meiotic protein dynamics in living fission yeast

Meiosis is a carefully choreographed dynamic process that re-purposes proteins from somatic/vegetative cell division, as well as meiosis-specific factors, to carry out the differentiation and recombination pathway common to sexually reproducing eukaryotes. Studies of individual proteins from a variety of different experimental protocols can make it difficult to compare details between them. Using a consistent protocol in otherwise wild-type fission yeast cells, this report provides an atlas of dynamic protein behaviour of representative proteins at different stages during normal zygotic meiosis in fission yeast. This establishes common landmarks to facilitate comparison of different proteins and shows that initiation of S phase likely occurs prior to nuclear fusion/karyogamy.

Babo1, formerly Vop1 and Cop1/2, is no eyespot photoreceptor but a basal body protein illuminating cell division in Volvox carteri

In photosynthetic organisms many processes are light dependent and sensing of light requires light-sensitive proteins. The supposed eyespot photoreceptor protein Babo1 (formerly Vop1) has previously been classified as an opsin due to the capacity for binding retinal. Here, we analyze Babo1 and provide evidence that it is no opsin. Due to the localization at the basal bodies, the former Vop1 and Cop1/2 proteins were renamed V.c. Babo1 and C.r. Babo1. We reveal a large family of more than 60 Babo1-related proteins from a wide range of species. The detailed subcellular localization of fluorescence-tagged Babo1 shows that it accumulates at the basal apparatus. More precisely, it is located predominantly at the basal bodies and to a lesser extent at the four strands of rootlet microtubules. We trace Babo1 during basal body separation and cell division. Dynamic structural rearrangements of Babo1 particularly occur right before the first cell division. In four-celled embryos Babo1 was exclusively found at the oldest basal bodies of the embryo and on the corresponding d-roots. The unequal distribution of Babo1 in four-celled embryos could be an integral part of a geometrical system in early embryogenesis, which establishes the anterior-posterior polarity and influences the spatial arrangement of all embryonic structures and characteristics. Due to its retinal-binding capacity, Babo1 could also be responsible for the unequal distribution of retinoids, knowing that such concentration gradients of retinoids can be essential for the correct patterning during embryogenesis of more complex organisms. Thus, our findings push the Babo1 research in another direction.

Quantifying Tubulin Concentration and Microtubule Number Throughout the Fission Yeast Cell Cycle

The fission yeast Schizosaccharomyces pombe serves as a good genetic model organism for the molecular dissection of the microtubule (MT) cytoskeleton. However, analysis of the number and distribution of individual MTs throughout the cell cycle, particularly during mitosis, in living cells is still lacking, making quantitative modelling imprecise. We use quantitative fluorescent imaging and analysis to measure the changes in tubulin concentration and MT number and distribution throughout the cell cycle at a single MT resolution in living cells. In the wild-type cell, both mother and daughter spindle pole body (SPB) nucleate a maximum of 23 ± 6 MTs at the onset of mitosis, which decreases to a minimum of 4 ± 1 MTs at spindle break down. Interphase MT bundles, astral MT bundles, and the post anaphase array (PAA) microtubules are composed primarily of 1 ± 1 individual MT along their lengths. We measure the cellular concentration of αβ-tubulin subunits to be ~5 µM throughout the cell cycle, of which one-third is in polymer form during interphase and one-quarter is in polymer form during mitosis. This analysis provides a definitive characterization of αβ-tubulin concentration and MT number and distribution in fission yeast and establishes a foundation for future quantitative comparison of mutants defective in MTs.

The shape of pinned forced polymer loops

Loop geometry is a frequent encounter in synthetic and biological polymers. Here we provide an analytical theory to characterize the shapes of polymer loops subjected to an external force field. We show how to calculate the polymer density, gyration radius and its distribution. Interestingly, the distribution of the gyration radius shows a non-monotonic behavior as a function of the external force. Furthermore, we analyzed the gyration tensor of the polymer loop characterizing its overall shape. Two parameters called asphericity and the nature of asphericity derived from the gyration tensor, along with the gyration radius, can be used to quantify the shape of polymer loops in theory and experiments.

Collective effects of yeast cytoplasmic dynein based microtubule transport

Teams of cortically anchored dyneins pulling at microtubules (MTs) are known to be essential for aster, spindle and nuclear positioning during cell division and fertilization. While the single-molecule basis of dynein processivity is now better understood, the effect of increasing numbers of motors on transport is not clear. Here, we examine the collective transport properties of a Saccharomyces cerevisiae cytoplasmic dynein fragment, widely used as a minimal model, by a combination of quantitative MT gliding assays and stochastic simulations. We find both MT lengths and motor densities qualitatively affect the degree of randomness of MT transport. However, the directionality and velocity of MTs increase above a threshold number of motors (N) interacting with a filament. To better understand this behavior, we simulate a gliding assay based on a model of uniformly distributed immobilized motors transporting semi-flexible MTs. Each dynein dimer is modeled as an effective stochastic stepper with asymmetric force dependent detachment dynamics, based on single-molecule experiments. Simulations predict increasing numbers of motors (N) result in a threshold dependent transition in directionality and transport velocity and a monotonic decrease in effective diffusivity. Thus both experiment and theory show qualitative agreement in the emergence of coordination in transport above a threshold number of motor heads. We hypothesize that the phase-transition like property of this dynein could play a role in vivo during yeast mitosis, when this dynein localizes to the cortex and pulls astral MTs of increasing length, resulting in correct positioning and orientation of the nucleus at the bud-neck.

Measuring microtubule dynamics

Microtubules are key players in cellular self-organization, acting as structural scaffolds, cellular highways, force generators and signalling platforms. Microtubules are polar filaments that undergo dynamic instability, i.e. transition between phases of growth and shrinkage. This allows microtubules to explore the inner space of the cell, generate pushing and pulling forces and remodel themselves into arrays with different geometry and function such as the mitotic spindle. To do this, eukaryotic cells employ an arsenal of regulatory proteins to control microtubule dynamics spatially and temporally. Plants and microorganisms have developed secondary metabolites that perturb microtubule dynamics, many of which are in active use as cancer chemotherapeutics and anti-inflammatory drugs. Here, we summarize the methods used to visualize microtubules and to measure the parameters of dynamic instability to study both microtubule regulatory proteins and the action of small molecules interfering with microtubule assembly and/or disassembly.

Evolution of research on cellular motility over five decades

This short review traces how our knowledge of the molecular mechanisms of cellular movements originated and developed over the past 50 years. Work on actin-based and microtubule-based movements developed in different ways, but in both fields, the discovery of the key proteins drove progress. Starting from an inventory of zero molecules in 1960, both fields matured spectacularly, so we now know the atomic structures of the important proteins, understand the kinetics and thermodynamics of their interactions, have documented how the molecules behave in cells, and can test theories with molecularly explicit computer simulations of cellular processes.

Estimation of GFP-Nucleoporin Amount Based on Fluorescence Microscopy

Cellular structures and biomolecular complexes are not simply assemblies of proteins, but are organized with defined numbers of protein molecules in precise locations. Thus, evaluating the spatial localization and numbers of protein molecules is of fundamental importance in understanding cellular structures and functions. The amounts of proteins of interest have conventionally been determined by biochemical methods. However, biochemical measurements based on the population average have limitations: it is sometimes difficult to determine the amounts of insoluble proteins or low expression proteins localized in small portions of the cell. In contrast, microphotometric measurements using fluorescence microscopes enable us to detect the amounts of such proteins in situ in a particular subcellular region. Here, we describe a method to measure the amounts of fluorescently tagged proteins by fluorescence microscopy, and present an example of an application to nuclear pore proteins in the fission yeast Schizosaccharomyces pombe.

LINC complexes as regulators of meiosis

Meiosis is a key processes of sexual reproduction in eukaryotes. By combining two cell division cycles with a single round of DNA replication meiosis provides a mechanism to generate haploid gametes. Coincidentally, processes involved in ensuring appropriate segregation of homologous chromosomes also result in genetic recombination and shuffling of genes between each generation. During the first meiotic prophase, rapid telomere-led chromosome movements facilitate alignment and pairing of homologous chromosomes. Forces that produce these movements are generated by the cytoskeleton. Force transmission across the nuclear envelope is dependent upon LINC complexes. These structures consist of SUN and KASH domain proteins that span the two nuclear membranes. Together they represent a pair of links in a molecular chain that couples telomeres to the cytoskeleton. In addition to their force transducing role, LINC complexes also have essential functions ensuring the fidelity of recombination between homologous chromosomes. In this way, LINC complexes are now seen as playing an active and integral role in meiotic progression.

Module-based systematic construction of plasmids for episomal gene expression in fission yeast

The fission yeast Schizosaccharomyces pombe is a powerful model organism for cell biology and molecular biology, as genetic manipulation is easily achieved. Introduction of exogenous genes cloned in episomal plasmids into yeast cells can be done through well-established transformation methods. For expression of genes in S. pombe cells, the multi-copy plasmid pREP1 and its derivatives, including pREP41 and pREP81, have been widely used as vectors. Although recent advancement of technology brought a number of useful genetic elements such as new promoters, selection marker genes and fluorescent protein tags, introduction of those elements into conventional pREP1 requires a large commitment of both time and effort because cloning procedures need to be repeated until the final products are constructed. Here, we introduce materials and methods to construct many pREP1-type plasmids easily and systematically using the Golden Gate shuffling method, which enables one-step ligation of many DNA fragments into a plasmid. These materials and methods support creation of expression plasmids employing a variety of novel genetic elements, which will further facilitate genetic studies using S. pombe.

Meiotic Nuclear Oscillations Are Necessary to Avoid Excessive Chromosome Associations

Pairing of homologous chromosomes is a crucial step in meiosis, which in fission yeast depends on nuclear oscillations. However, how nuclear oscillations help pairing is unknown. Here, we show that homologous loci typically pair when the spindle pole body is at the cell pole and the nucleus is elongated, whereas they unpair when the spindle pole body is in the cell center and the nucleus is round. Inhibition of oscillations demonstrated that movement is required for initial pairing and that prolonged association of loci leads to mis-segregation. The double-strand break marker Rec25 accumulates in elongated nuclei, indicating that prolonged chromosome stretching triggers recombinatory pathways leading to mis-segregation. Mis-segregation is rescued by overexpression of the Holliday junction resolvase Mus81, suggesting that prolonged pairing results in irresolvable recombination intermediates. We conclude that nuclear oscillations exhibit a dual role, promoting initial pairing and restricting the time of chromosome associations to ensure proper segregation.

Distinct TERB1 Domains Regulate Different Protein Interactions in Meiotic Telomere Movement

Meiotic telomeres attach to the nuclear envelope (NE) and drive the chromosome movement required for the pairing of homologous chromosomes. The meiosis-specific telomere proteins TERB1, TERB2, and MAJIN are required to regulate these events, but their assembly processes are largely unknown. Here, we developed a germ-cell-specific knockout mouse of the canonical telomere-binding protein TRF1 and revealed an essential role for TRF1 in directing the assembly of TERB1-TERB2-MAJIN. Further, we identified a TERB2 binding (T2B) domain in TERB1 that is dispensable for the TRF1-TERB1 interaction but is essential for the subsequent TERB1-TERB2 interaction and therefore for telomere attachment to the NE. Meanwhile, cohesin recruitment at telomeres, which is required for efficient telomere movement, is mediated by the MYB-like domain of TERB1, but not by TERB2-MAJIN. Our results reveal distinct protein interactions through various domains of TERB1, which enable the sequential assembly of the meiotic telomere complex for their movements.

Moving and stopping: Regulation of chromosome movement to promote meiotic chromosome pairing and synapsis

Meiosis is a specialized cellular division occurring in organisms capable of sexual reproduction that leads to the formation of gametes containing half of the original chromosome number. During the earliest stage of meiosis, prophase I, pairing of homologous chromosomes is achieved in preparation for their proper distribution in the coming divisions. An important question is how do homologous chromosomes find each other and establish pairing interactions. Early studies demonstrated that chromosomes are dynamic in nature and move during this early stage of meiosis. More recently, there have been several studies across different models showing the conserved nature and importance of this chromosome movement, as well as the key components involved in chromosome movement. This review will cover these major findings and also introduce unexamined areas of regulation in meiotic prophase I chromosome movement.

Position matters: multiple functions of LINC-dependent chromosome positioning during meiosis

Chromosome positioning is crucial for multiple chromosomal events, including DNA replication, repair, and recombination. The linker of nucleoskeleton and cytoskeleton (LINC) complexes, which consist of conserved nuclear membrane proteins, were shown to control chromosome positioning and facilitate various biological processes by interacting with the cytoskeleton. However, the precise functions and regulation of LINC-dependent chromosome positioning are not fully understood. During meiosis, the LINC complexes induce clustering of telomeres, forming the bouquet chromosome arrangement, which promotes homologous chromosome pairing. In fission yeast, the bouquet forms through LINC-dependent clustering of telomeres at the spindle pole body (SPB, the centrosome equivalent in fungi) and detachment of centromeres from the SPB-localized LINC. It was recently found that, in fission yeast, the bouquet contributes to formation of the spindle and meiotic centromeres, in addition to homologous chromosome pairing, and that centromere detachment is linked to telomere clustering, which is crucial for proper spindle formation. Here, we summarize these findings and show that the bouquet chromosome arrangement also contributes to nuclear fusion during karyogamy. The available evidence suggests that these functions are universal among eukaryotes. The findings demonstrate that LINC-dependent chromosome positioning performs multiple functions and controls non-chromosomal as well as chromosomal events, and that the chromosome positioning is stringently regulated for its functions. Thus, chromosome positioning plays a much broader role and is more strictly regulated than previously thought.

Fission yeast myosin I facilitates PI(4,5)P2-mediated anchoring of cytoplasmic dynein to the cortex

Several key processes in the cell, such as vesicle transport and spindle positioning, are mediated by the motor protein cytoplasmic dynein, which produces force on the microtubule. For the functions that require movement of the centrosome and the associated nuclear material, dynein needs to have a stable attachment at the cell cortex. In fission yeast, Mcp5 is the anchor protein of dynein and is required for the oscillations of the horsetail nucleus during meiotic prophase. Although the role of Mcp5 in anchoring dynein to the cortex has been identified, it is unknown how Mcp5 associates with the membrane as well as the importance of the underlying attachment to the nuclear oscillations. Here, we set out to quantify Mcp5 organization and identify the binding partner of Mcp5 at the membrane. We used confocal and total internal reflection fluorescence microscopy to count the number of Mcp5 foci and the number of Mcp5 molecules in an individual focus. Further, we quantified the localization pattern of Mcp5 in fission yeast zygotes and show by perturbation of phosphatidylinositol 4-phosphate 5-kinase that Mcp5 binds to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂]. Remarkably, we discovered that the myosin I protein in fission yeast, Myo1, which is required for organization of sterol-rich domains in the cell membrane, facilitates the localization of Mcp5 and that of cytoplasmic dynein on the membrane. Finally, we demonstrate that Myo1-facilitated association of Mcp5 and dynein to the membrane determines the dynamics of nuclear oscillations and, in essence, dynein activity.

Identification and Analysis of MS5d: A Gene That Affects Double-Strand Break (DSB) Repair during Meiosis I in Brassica napus Microsporocytes

Here, we report the identification of the Brassica-specific gene MS5d, which is responsible for male sterility in Brassica napus. The MS5d gene is highly expressed in the microsporocyte and encodes a protein that localizes to the nucleus. Light microscopy analyses have demonstrated that the MS5d gene affects microsporocyte meiosis in the thermosensitive genic male sterility line TE5A. Sequence comparisons and genetic complementation revealed a C-to-T transition in MS5d, encoding a Leu-to-Phe (L281F) substitution and causing abnormal male meiosis in TE5A. These findings suggest arrested meiotic chromosome dynamics at pachytene. Furthermore, immunofluorescence analyses showed that double-strand break (DSB) formation and axial elements were normal but that DSB repair and spindle behavior were aberrant in TE5A meiocytes. Collectively, our results indicate that MS5d likely encodes a protein required for chromosomal DSB repair at early stages of meiosis in B. napus.

Spatial organization of the Schizosaccharomyces pombe genome within the nucleus

The fission yeast Schizosaccharomyces pombe is a useful experimental system for studying the organization of chromosomes within the cell nucleus. S. pombe has a small genome that is organized into three chromosomes. The small size of the genome and the small number of chromosomes are advantageous for cytological and genome-wide studies of chromosomes; however, the small size of the nucleus impedes microscopic observations owing to limits in spatial resolution during imaging. Recent advances in microscopy, such as super-resolution microscopy, have greatly expanded the use of S. pombe as a model organism in a wide range of studies. In addition, biochemical studies, such as chromatin immunoprecipitation and chromosome conformation capture, have provided complementary approaches. Here, we review the spatial organization of the S. pombe genome as determined by a combination of cytological and biochemical studies. Copyright © 2016 John Wiley & Sons, Ltd.

Mitotic Nuclear Envelope Breakdown and Spindle Nucleation Are Controlled by Interphase Contacts between Centromeres and the Nuclear Envelope

Faithful genome propagation requires coordination between nuclear envelope (NE) breakdown, spindle formation, and chromosomal events. The conserved linker of nucleoskeleton and cytoskeleton (LINC) complex connects fission yeast centromeres and the centrosome, across the NE, during interphase. During meiosis, LINC connects the centrosome with telomeres rather than centromeres. We previously showed that loss of telomere-LINC contacts compromises meiotic spindle formation. Here, we define the precise events regulated by telomere-LINC contacts and address the analogous possibility that centromeres regulate mitotic spindle formation. We develop conditionally inactivated LINC complexes in which the conserved SUN-domain protein Sad1 remains stable but severs interphase centromere-LINC contacts. Strikingly, the loss of such contacts abolishes spindle formation. We pinpoint the defect to a failure in the partial NE breakdown required for centrosome insertion into the NE, a step analogous to mammalian NE breakdown. Thus, interphase chromosome-LINC contacts constitute a cell-cycle control device linking nucleoplasmic and cytoplasmic events.

A cohesin-based structural platform supporting homologous chromosome pairing in meiosis

The pairing and recombination of homologous chromosomes during the meiotic prophase is necessary for the accurate segregation of chromosomes in meiosis. However, the mechanism by which homologous chromosomes achieve this pairing has remained an open question. Meiotic cohesins have been shown to affect chromatin compaction; however, the impact of meiotic cohesins on homologous pairing and the fine structures of cohesion-based chromatin remain to be determined. A recent report using live-cell imaging and super-resolution microscopy demonstrated that the lack of meiotic cohesins alters the chromosome axis structures and impairs the pairing of homologous chromosomes. These results suggest that meiotic cohesin-based chromosome axis structures are crucial for the pairing of homologous chromosomes.

The effect of magnesium on mitotic spindle formation in Schizosaccharomyces pombe

Magnesium (Mg2+), an essential ion for cells and biological systems, is involved in a variety of cellular processes, including the formation and breakdown of microtubules. The results of a previous investigation suggested that as cells grow the intracellular Mg2+ concentration falls, thereby stimulating formation of the mitotic spindle. In the present work, we used a Mg2+-deficient Schizosaccharomyces pombe strain GA2, in which two essential membrane Mg2+ transporter genes (homologs of ALR1 and ALR2 in Saccharomyces cerevisae) were deleted, and its parental strain Sp292, to examine the extent to which low Mg2+ concentrations can affect mitotic spindle formation. The two S. pombe strains were transformed with a plasmid carrying a GFP-α2-tubulin construct to fluorescently label microtubules. Using the free Mg2+-specific fluorescent probe mag-fura-2, we confirmed that intracellular free Mg2+ levels were lower in GA2 than in the parental strain. Defects in interphase microtubule organization, a lower percentage of mitotic spindle formation and a reduced mitotic index were also observed in the GA2 strain. Although there was interphase microtubule polymerization, the lower level of mitotic spindle formation in the Mg2+-deficient strain suggested a greater requirement for Mg2+ in this phenomenon than previously thought.

Meiotic chromosome mobility in fission yeast is resistant to environmental stress

The formation of healthy gametes requires pairing of homologous chromosomes (homologs) as a prerequisite for their correct segregation during meiosis. Initially, homolog alignment is promoted by meiotic chromosome movements feeding into intimate homolog pairing by homologous recombination and/or synaptonemal complex formation. Meiotic chromosome movements in the fission yeast, Schizosaccharomyces pombe, depend on astral microtubule dynamics that drag the nucleus through the zygote; known as horsetail movement. The response of microtubule-led meiotic chromosome movements to environmental stresses such as ionizing irradiation (IR) and associated reactive oxygen species (ROS) is not known. Here, we show that, in contrast to budding yeast, the horsetail movement is largely radiation-resistant, which is likely mediated by a potent antioxidant defense. IR exposure of sporulating S. pombe cells induced misrepair and irreparable DNA double strand breaks causing chromosome fragmentation, missegregation and gamete death. Comparing radiation outcome in fission and budding yeast, and studying meiosis with poisoned microtubules indicates that the increased gamete death after IR is innate to fission yeast. Inhibition of meiotic chromosome mobility in the face of IR failed to influence the course of DSB repair, indicating that paralysis of meiotic chromosome mobility in a genotoxic environment is not a universal response among species.

A few of our favorite things: Pairing, the bouquet, crossover interference and evolution of meiosis

Meiosis presents many important mysteries that await elucidation. Here we discuss two such aspects. First, we consider how the current meiotic program might have evolved. We emphasize the central feature of this program: how homologous chromosomes find one another ("pair") so as to create the connections required for their regular segregation at Meiosis I. Points of emphasis include the facts that: (i) the classical "bouquet stage" is not required for initial homolog contacts in the current evolved meiotic program; and (ii) diverse observations point to commonality between molecules that mediate meiotic inter-homolog interactions and molecules that are integral to centromeres and/or to microtubule organizing centers (a.k.a. spindle pole bodies or centrosomes). Second, we provide an overview of the classical phenomenon of crossover (CO) interference in an effort to bridge the gap between description on the one hand versus logic and mechanism on the other.

Pulled Polymer Loops as a Model for the Alignment of Meiotic Chromosomes

During recombination, the DNA of parents exchange their genetic information to give rise to a genetically unique offspring. For recombination to occur, homologous chromosomes need to find each other and align with high precision. Fission yeast solves this problem by folding chromosomes in loops and pulling them through the viscous nucleoplasm. We propose a theory of pulled polymer loops to quantify the effect of drag forces on the alignment of chromosomes. We introduce an external force field to the concept of a Brownian bridge and thus solve for the statistics of loop configurations in space.

Meiotic cohesin-based chromosome structure is essential for homologous chromosome pairing in Schizosaccharomyces pombe

Chromosome structure is dramatically altered upon entering meiosis to establish chromosomal architectures necessary for the successful progression of meiosis-specific events. An early meiotic event involves the replacement of the non-SMC mitotic cohesins with their meiotic equivalents in most part of the chromosome, forming an axis on meiotic chromosomes. We previously demonstrated that the meiotic cohesin complex is required for chromosome compaction during meiotic prophase in the fission yeast Schizosaccharomyces pombe. These studies revealed that chromosomes are elongated in the absence of the meiotic cohesin subunit Rec8 and shortened in the absence of the cohesin-associated protein Pds5. In this study, using super-resolution structured illumination microscopy, we found that Rec8 forms a linear axis on chromosomes, which is required for the organized axial structure of chromatin during meiotic prophase. In the absence of Pds5, the Rec8 axis is shortened whereas chromosomes are widened. In rec8 or pds5 mutants, the frequency of homologous chromosome pairing is reduced. Thus, Rec8 and Pds5 play an essential role in building a platform to support the chromosome architecture necessary for the spatial alignment of homologous chromosomes.

Microtubule-driven nuclear rotations promote meiotic chromosome dynamics

At the onset of meiosis, each chromosome needs to find its homologue and pair to ensure proper segregation. In Drosophila, pairing occurs during the mitotic cycles preceding meiosis. Here we show that germ cell nuclei undergo marked movements during this developmental window. We demonstrate that microtubules and Dynein are driving nuclear rotations and are required for centromere pairing and clustering. We further found that Klaroid (SUN) and Klarsicht (KASH) co-localize with centromeres at the nuclear envelope and are required for proper chromosome motions and pairing. We identified Mud (NuMA in vertebrates) as co-localizing with centromeres, Klarsicht and Klaroid. Mud is also required to maintain the integrity of the nuclear envelope and for the correct assembly of the synaptonemal complex. Our findings reveal a mechanism for chromosome pairing in Drosophila, and indicate that microtubules, centrosomes and associated proteins play a crucial role in the dynamic organization of chromosomes inside the nucleus.

Histone H4 acetylation required for chromatin decompaction during DNA replication

Faithful DNA replication is a prerequisite for cell proliferation. Several cytological studies have shown that chromosome structures alter in the S-phase of the cell cycle. However, the molecular mechanisms behind the alteration of chromosome structures associated with DNA replication have not been elucidated. Here, we investigated chromatin structures and acetylation of specific histone residues during DNA replication using the meiotic nucleus of the fission yeast Schizosaccharomyces pombe. The S. pombe meiotic nucleus provides a unique opportunity for measuring the levels of compaction of chromatin along the chromosome in a defined orientation. By direct measurement of chromatin compaction in living cells, we demonstrated that decompaction of chromatin occurs during meiotic DNA replication. This chromatin decompaction was suppressed by depletion of histone acetyltransferase Mst1 or by arginine substitution of specific lysine residues (K8 and K12) of histone H4. These results suggest that acetylation of histone H4 residues K8 and K12 plays a critical role in loosening chromatin structures during DNA replication.

Microtubule minus end motors kinesin-14 and dynein drive nuclear congression in parallel pathways

Long-term imaging via microfluidic chambers shows that two minus end–directed motors, dynein and Klp2, work in parallel at distinct subcellular structures to promote efficient nuclear congression.

Microtubules (MTs) and associated motors play a central role in nuclear migration, which is crucial for diverse biological functions including cell division, polarity, and sexual reproduction. In this paper, we report a dual mechanism underlying nuclear congression during fission yeast karyogamy upon mating of haploid cells. Using microfluidic chambers for long-term imaging, we captured the precise timing of nuclear congression and identified two minus end–directed motors operating in parallel in this process. Kinesin-14 Klp2 associated with MTs may cross-link and slide antiparallel MTs emanating from the two nuclei, whereas dynein accumulating at spindle pole bodies (SPBs) may pull MTs nucleated from the opposite SPB. Klp2-dependent nuclear congression proceeds at constant speed, whereas dynein accumulation results in an increase of nuclear velocity over time. Surprisingly, the light intermediate chain Dli1, but not dynactin, is required for this previously unknown function of dynein. We conclude that efficient nuclear congression depends on the cooperation of two minus end–directed motors.

Telomeres and centromeres have interchangeable roles in promoting meiotic spindle formation

Both centromere–centrosome and telomere–centrosome contacts can promote spindle formation during meiosis.

Telomeres and centromeres have traditionally been considered to perform distinct roles. During meiotic prophase, in a conserved chromosomal configuration called the bouquet, telomeres gather to the nuclear membrane (NM), often near centrosomes. We found previously that upon disruption of the fission yeast bouquet, centrosomes failed to insert into the NM at meiosis I and nucleate bipolar spindles. Hence, the trans-NM association of telomeres with centrosomes during prophase is crucial for efficient spindle formation. Nonetheless, in approximately half of bouquet-deficient meiocytes, spindles form properly. Here, we show that bouquet-deficient cells can successfully undergo meiosis using centromere–centrosome contact instead of telomere–centrosome contact to generate spindle formation. Accordingly, forced association between centromeres and centrosomes fully rescued the spindle defects incurred by bouquet disruption. Telomeres and centromeres both stimulate focal accumulation of the SUN domain protein Sad1 beneath the centrosome, suggesting a molecular underpinning for their shared spindle-generating ability. Our observations demonstrate an unanticipated level of interchangeability between the two most prominent chromosomal landmarks.

Mechanism and regulation of rapid telomere prophase movements in mouse meiotic chromosomes

Telomere-led rapid prophase movements (RPMs) in meiotic prophase have been observed in diverse eukaryote species. A shared feature of RPMs is that the force that drives the chromosomal movements is transmitted from the cytoskeleton, through the nuclear envelope, to the telomeres. Studies in mice suggested that dynein movement along microtubules is transmitted to telomeres through SUN1/KASH5 nuclear envelope bridges to generate RPMs. We monitored RPMs in mouse seminiferous tubules using 4D fluorescence imaging and quantitative motion analysis to characterize patterns of movement in the RPM process. We find that RPMs reflect a combination of nuclear rotation and individual chromosome movements. The telomeres move along microtubule tracks that are apparently continuous with the cytoskeletal network and exhibit characteristic arrangements at different stages of prophase. Quantitative measurements confirmed that SUN1/KASH5, microtubules, and dynein, but not actin, were necessary for RPMs and that defects in meiotic recombination and synapsis resulted in altered RPMs.

Dynactin and Num1 cooperate to establish the cortical anchoring of cytoplasmic dynein in S. pombe

Chromosome movement during meiosis is crucial for homologous pairing and meiotic recombination. During meiotic prophase in fission yeast, rapid nuclear migration is dependent on cytoplasmic dynein, which is anchored to the cell cortex and pulls microtubules, thereby driving nuclear migration. However, the precise mechanisms underlying dynein localization and activation remain unclear. Here, we identified three subunits of dynactin in fission yeast: Arp1, Mug5 and Jnm1 (also known as Mug1). These subunits transiently colocalized with dynein foci at the cell cortex and were essential for the cortical anchoring of dynein. Cortical factor Num1 (also known as Mcp5), which was also required for dynein anchoring, bound to dynein independently of dynactin. Whereas Num1 suppressed the sliding of dynein foci along the cortex, Arp1, Mug5 and Jnm1 were involved in the regulation of shrinkage and bundling of microtubules. From these data, we propose that dynein anchoring is established by cooperation of transient assembly of dynactin and function of Num1 at the cell cortex.

The dissection of meiotic chromosome movement in mice using an in vivo electroporation technique

During meiosis, the rapid movement of telomeres along the nuclear envelope (NE) facilitates pairing/synapsis of homologous chromosomes. In mammals, the mechanical properties of chromosome movement and the cytoskeletal structures responsible for it remain poorly understood. Here, applying an in vivo electroporation (EP) technique in live mouse testis, we achieved the quick visualization of telomere, chromosome axis and microtubule organizing center (MTOC) movements. For the first time, we defined prophase sub-stages of live spermatocytes morphologically according to ^GFP-TRF1 and ^GFP-SCP3 signals. We show that rapid telomere movement and subsequent nuclear rotation persist from leptotene/zygotene to pachytene, and then decline in diplotene stage concomitant with the liberation of SUN1 from telomeres. Further, during bouquet stage, telomeres are constrained near the MTOC, resulting in the transient suppression of telomere mobility and nuclear rotation. MTs are responsible for these movements by forming cable-like structures on the NE, and, probably, by facilitating the rail-tacking movements of telomeres on the MT cables. In contrast, actin regulates the oscillatory changes in nuclear shape. Our data provide the mechanical scheme for meiotic chromosome movement throughout prophase I in mammals.

Meiosis is a special type of cell division for gametogenesis, errors in which cause several genetic disorders such as infertility and Down syndrome. In meiotic prophase I, chromosomes are tethered to the nuclear envelope (NE) through telomeres, and move rapidly along the NE to get homologs aligned and juxtaposed. Following homologous recombination and synapsis, the bivalent chromosome structure is established, which promotes genetic varieties, and also ensures accurate chromosome segregation in following anaphase I. Although there have been extensive studies addressing meiotic chromosome dynamics in yeast and worms, the same in mammalian meiosis remains largely elusive. Here, we utilized an in vivo electroporation (EP) technique to visualize chromosome movement in live mouse spermatocytes. We, for the first time, define the meiotic sub-stages in live cells based on telomeres and chromosome axis morphologies, and reveal chromosome movements regulated in a stage-specific manner. Putting the live-observations together with our cytological observations in fixed cells, we propose that meiotic chromosome movements in mammals are mediated by the rail-tracking movement of telomeres along the MT cables surrounding the meiotic nucleus.

Meiotic nuclear movements in fission yeast are regulated by the transcription factor Mei4 downstream of a Cds1-dependent replication checkpoint pathway

In meiosis, the fission yeast nucleus displays an elongated morphology, moving back and forth within the cell; these nuclear movements continue for approximately 2 h before meiotic nuclear divisions. Meiotic DNA replication occurs in an early phase of the nuclear movements and is followed by meiotic prophase. Here we report that in mutants deficient in meiotic DNA replication, the duration of nuclear movements is strikingly prolonged to four to 5 h. We found that this prolongation was caused by the Cds1-dependent replication checkpoint, which represses expression of the mei4⁺ gene encoding a meiosis-specific transcription factor. In the absence of Mei4, nuclear movements persisted for more than 8 h. In contrast, overproduction of Mei4 accelerated termination of nuclear movements to approximately 30 min. These results show that Mei4 is involved in the termination of nuclear movements and that Mei4-mediated regulatory pathways link a DNA replication checkpoint to the termination of nuclear movements.

Chromosomes rein back the spindle pole body during horsetail movement in fission yeast meiosis

In meiosis, pairing and recombination of homologous chromosomes are crucial for the correct segregation of chromosomes, and substantial movements of chromosomes are required to achieve homolog pairing. During this process, it is known that telomeres cluster to form a bouquet arrangement of chromosomes. The fission yeast Schizosaccharomyces pombe provides a striking example of bouquet formation, after which the entire nucleus oscillates between the cell poles (these oscillations are generally called horsetail nuclear movements) while the telomeres remain clustered to the spindle pole body (SPB; a centrosome-equivalent structure in fungi) at the leading edge of the moving nucleus. S. pombe mutants defective in telomere clustering frequently form aberrant spindles, such as monopolar or nonpolar spindles, leading to missegregation of the chromosomes at the subsequent meiotic divisions. Here we demonstrate that such defects in meiotic spindle formation caused by loss of meiotic telomere clustering are rescued when nuclear movement is prevented. On the other hand, stopping nuclear movement does not rescue defects in telomere clustering, nor chromosome missgregation even in cells that have formed a bipolar spindle. These results suggest that movement of the SPB without attachment of telomeres leads to the formation of aberrant spindles, but that recovering bipolar spindles is not sufficient for rescue of chromosome missegregation in mutants lacking telomere clustering.

Gathering up meiotic telomeres: a novel function of the microtubule-organizing center

During meiosis, telomeres cluster and promote homologous chromosome pairing. Telomere clustering depends on conserved SUN and KASH domain nuclear membrane proteins, which form a complex called the linker of nucleoskeleton and cytoskeleton (LINC) and connect telomeres with the cytoskeleton. It has been thought that LINC-mediated cytoskeletal forces induce telomere clustering. However, how cytoskeletal forces induce telomere clustering is not fully understood. Recent study of fission yeast has shown that the LINC complex forms the microtubule-organizing center (MTOC) at the telomere, which has been designated as the "telocentrosome", and that microtubule motors gather telomeres via telocentrosome-nucleated microtubules. This MTOC-dependent telomere clustering might be conserved in other eukaryotes. Furthermore, the MTOC-dependent clustering mechanism appears to function in various other biological events. This review presents an overview of the current understanding of the mechanism of meiotic telomere clustering and discusses the universality of the MTOC-dependent clustering mechanism.

Nuclear motility in glioma cells reveals a cell-line dependent role of various cytoskeletal components

Nuclear migration is a general term for the movement of the nucleus towards a specific site in the cell. These movements are involved in a number of fundamental biological processes, such as fertilization, cell division, and embryonic development. Despite of its importance, the mechanism of nuclear migration is still poorly understood in mammalian cells. In order to shed light on the mechanical processes underlying nuclear movements, we adapted a micro-patterning based assay. C6 rat and U87 human glioma cells seeded on fibronectin patterns - thereby forced into a bipolar morphology - displayed oscillatory movements of the nucleus or the whole cell, respectively. We found that both the actomyosin system and microtubules are involved in the nuclear/cellular movements of both cell lines, but their contributions are cell-/migration-type specific. Dynein activity was necessary for nuclear migration of C6 cells but active myosin-II was dispensable. On the other hand, coupled nuclear and cellular movements of U87 cells were driven by actomyosin contraction. We explain these cell-line dependent effects by the intrinsic differences in the overall mechanical tension due to the various cytoskeletal elements inside the cell. Our observations showed that the movements of the nucleus and the centrosome are strongly correlated and display large variation, indicating a tight but flexible coupling between them. The data also indicate that the forces responsible for nuclear movements are not acting directly via the centrosome. Based on our observations, we propose a new model for nuclear oscillations in C6 cells in which dynein and microtubule dynamics are the main drivers of nuclear movements. This mechanism is similar to the meiotic nuclear oscillations of Schizosaccharomyces pombe and may be evolutionary conserved.