Partner telomeres during anaphase in crane-fly spermatocytes are connected by an elastic tether that exerts a backward force and resists poleward motion

Centromere pairing enables correct segregation of meiotic chromosomes

Proper chromosome segregation in meiosis I relies on the formation of connections between homologous chromosomes. Crossovers between homologs provide a connection that allows them to attach correctly to the meiosis I spindle. Tension is transmitted across the crossover when the partners attach to microtubules from opposing poles of the spindle. Tension stabilizes microtubule attachments that will pull the partners toward opposite poles at anaphase. Paradoxically, in many organisms, non-crossover partners segregate correctly. The mechanism by which non-crossover partners become bioriented on the meiotic spindle is unknown. Both crossover and non-crossover partners pair their centromeres early in meiosis (prophase). In budding yeast, centromere pairing is correlated with subsequent correct segregation of the partners. The mechanism by which centromere pairing, in prophase, promotes later correct attachment of the partners to the metaphase spindle is unknown. We used live cell imaging to track the biorientation process of non-crossover chromosomes. We find that centromere pairing allows the establishment of connections between the partners that allows their later interdependent attachment to the meiotic spindle using tension-sensing biorientation machinery. Because all chromosome pairs experience centromere pairing, our findings suggest that crossover chromosomes also utilize this mechanism to achieve maximal segregation fidelity.

Structural evidence for elastic tethers connecting separating chromosomes in crane-fly spermatocytes

Different types of anaphase bridges are reported to form between segregating chromosomes during cell division. Previous studies using laser microsurgery suggested that elastic tethers connect the telomeres of separating anaphase chromosomes in many animal meiotic and mitotic cells. However, structural evidence is lacking for their existence. In this study, by correlating live imaging with electron tomography, we examined whether visible structures connect separating telomeres in meiosis I of crane-fly primary spermatocytes. We found structures extending between separating telomeres in all stages of anaphase. The structures consist of two components: one is darkly stained, looking somewhat like chromatin, whereas the other is more lightly stained, appearing filamentous. Although in early anaphase both structures extend between telomeres, in later anaphase, the darker structure extends shorter distances from the telomeres but the lighter structure still extends between the separating telomeres. From these observations, we deduced that these structures represent the "tethers" inferred from the laser-cutting experiments. Because elastic tethers have been detected in a variety of animal cells, they probably are present during anaphase in all animal cells.

Connections between sister and non-sister telomeres of segregating chromatids maintain euploidy

The complete separation of sister chromatids during anaphase is a fundamental requirement for successful mitosis. Therefore, divisions with either persistent DNA-based connections or lagging chromosome fragments threaten aneuploidy if unresolved. Here, we demonstrate the existence of an anaphase mechanism in normally dividing cells in which pervasive connections between telomeres of segregating chromosomes aid in rescuing lagging chromosome fragments. We observe that in a large proportion of Drosophila melanogaster neuronal stem cell divisions, early anaphase sister and non-sister chromatids remain connected by thin telomeric DNA threads. Normally, these threads are resolved in mid-to-late anaphase via a spatial mechanism. However, we find that the presence of a nearby unrepaired DNA break recruits histones, BubR1 kinase, Polo kinase, Aurora B kinase, and BAF to the telomeric thread of the broken chromosome, stabilizing it. Stabilized connections then aid lagging chromosome rescue. These results suggest a model in which pervasive anaphase telomere-telomere connections that are normally resolved quickly can instead be stabilized to retain wayward chromosome fragments. Thus, the liability of persistent anaphase inter-chromosomal connections in normal divisions may be offset by their ability to maintain euploidy in the face of chromosome damage and genome loss.

Optical tweezers across scales in cell biology

Optical tweezers (OT) provide a noninvasive approach for delivering minute physical forces to targeted objects. Controlling such forces in living cells or in vitro preparations allows for the measurement and manipulation of numerous processes relevant to the form and function of cells. As such, OT have made important contributions to our understanding of the structures of proteins and nucleic acids, the interactions that occur between microscopic structures within cells, the choreography of complex processes such as mitosis, and the ways in which cells interact with each other. In this review, we highlight recent contributions made to the field of cell biology using OT and provide basic descriptions of the physics, the methods, and the equipment that made these studies possible.

Structural Chromosome Instability: Types, Origins, Consequences, and Therapeutic Opportunities

Chromosomal instability (CIN) refers to an increased rate of acquisition of numerical and structural changes in chromosomes and is considered an enabling characteristic of tumors. Given its role as a facilitator of genomic changes, CIN is increasingly being considered as a possible therapeutic target, raising the question of which variables may convert CIN into an ally instead of an enemy during cancer treatment. This review discusses the origins of structural chromosome abnormalities and the cellular mechanisms that prevent and resolve them, as well as how different CIN phenotypes relate to each other. We discuss the possible fates of cells containing structural CIN, focusing on how a few cell duplication cycles suffice to induce profound CIN-mediated genome alterations. Because such alterations can promote tumor adaptation to treatment, we discuss currently proposed strategies to either avoid CIN or enhance CIN to a level that is no longer compatible with cell survival.

Blocking Protein Phosphatase 1 [PP1] Prevents Loss of Tether Elasticity in Anaphase Crane-Fly Spermatocytes

In normal anaphase cells, telomeres of each separating chromosome pair are connected to each other by tethers. Tethers are elastic at the start of anaphase: arm fragments cut from anaphase chromosomes in early anaphase move across the equator to the oppositely-moving chromosome, telomere moving toward telomere. Tethers become inelastic later in anaphase as the tethers become longer: arm fragments no longer move to their partners. When early anaphase cells are treated with Calyculin A (CalA), an inhibitor of protein phosphatases 1 (PP1) and 2A (PP2A), at the end of anaphase chromosomes move backward from the poles, with telomeres moving toward partner telomeres. Experiments described herein show that in cells treated with CalA, backwards movements are stopped in a variety of ways, by cutting the tethers of backwards moving chromosomes, by severing arms of backwards moving chromosomes, by severing arms before the chromosomes reach the poles, and by cutting the telomere toward which a chromosome is moving backwards. Measurements of arm-fragment velocities show that CalA prevents tethers from becoming inelastic as they lengthen. Since treatment with CalA causes tethers to remain elastic throughout anaphase and since inhibitors of PP2A do not cause the backwards movements, PP1 activity during anaphase causes the tethers to become inelastic.

Physiological relevance of post-translational regulation of the spindle assembly checkpoint protein BubR1

BubR1 is an essential component of the spindle assembly checkpoint (SAC) during mitosis where it functions to prevent anaphase onset to ensure proper chromosome alignment and kinetochore-microtubule attachment. Loss or mutation of BubR1 results in aneuploidy that precedes various potential pathologies, including cancer and mosaic variegated aneuploidy (MVA). BubR1 is also progressively downregulated with age and has been shown to be directly involved in the aging process through suppression of cellular senescence. Post-translational modifications, including but not limited to phosphorylation, acetylation, and ubiquitination, play a critical role in the temporal and spatial regulation of BubR1 function. In this review, we discuss the currently characterized post-translational modifications to BubR1, the enzymes involved, and the biological consequences to BubR1 functionality and implications in diseases associated with BubR1. Understanding the molecular mechanisms promoting these modifications and their roles in regulating BubR1 is important for our current understanding and future studies of BubR1 in maintaining genomic integrity as well as in aging and cancer.

The role of phosphorylation in the elasticity of the tethers that connect telomeres of separating anaphase chromosomes

Elastic tethers, connecting telomeres of all separating anaphase chromosome pairs, lose elasticity when they lengthen during anaphase. Treatment with phosphatase inhibitor CalyculinA causes anaphase chromosomes to move backwards after they reach the poles, suggesting that dephosphorylation causes loss of tether elasticity. We added 50nM CalyculinA to living anaphase crane-fly spermatocytes with different length tethers. When tethers were short, almost all partner chromosomes moved backwards after nearing the poles. When tethers were longer, fewer chromosomes moved backwards. With yet longer tethers none moved backward. This is consistent with tether elasticity being lost by dephosphorylation. 50nM CalyculinA blocks both PP1 and PP2A. To distinguish between PP1 and PP2A we treated cells with short tethers with 50nM okadaic acid which blocks solely PP2A, or with 1µM okadaic acid which blocks both PP1 and PP2A. Only 1µM okadaic acid caused chromosomes to move backward. Thus, tether elasticity is lost because of dephosphorylation by PP1.

Mechanisms driving acentric chromosome transmission

The kinetochore-microtubule association is a core, conserved event that drives chromosome transmission during mitosis. Failure to establish this association on even a single chromosome results in aneuploidy leading to cell death or the development of cancer. However, although many chromosomes lacking centromeres, termed acentrics, fail to segregate, studies in a number of systems reveal robust alternative mechanisms that can drive segregation and successful poleward transport of acentrics. In contrast to the canonical mechanism that relies on end-on microtubule attachments to kinetochores, mechanisms of acentric transmission largely fall into three categories: direct attachments to other chromosomes, kinetochore-independent lateral attachments to microtubules, and long-range tether-based attachments. Here, we review these "non-canonical" methods of acentric chromosome transmission. Just as the discovery and exploration of cell cycle checkpoints provided insight into both the origins of cancer and new therapies, identifying mechanisms and structures specifically involved in acentric segregation may have a significant impact on basic and applied cancer research.

The pericentromeric heterochromatin of homologous chromosomes remains associated after centromere pairing dissolves in mouse spermatocyte meiosis

In meiosis, crossovers between homologous chromosomes link them together. This enables them to attach to microtubules of the meiotic spindle as a unit, such that the homologs will be pulled away from one another at anaphase I. Homologous pairs can sometimes fail to become linked by crossovers. In some organisms, these non-exchange partners are still able to segregate properly. In several organisms, associations between the centromeres of non-exchange partners occur in meiotic prophase. These associations have been proposed to promote segregation in meiosis I. But it is unclear how centromere pairing could promote subsequent proper segregation. Here we report that meiotic centromere pairing of chromosomes in mouse spermatocytes allows the formation of an association between chromosome pairs. We find that heterochromatin regions of homologous centromeres remain associated even after centromere-pairing dissolves. Our results suggest the model that, in mouse spermatocytes, heterochromatin maintains the association of homologous centromeres in the absence crossing-over.

Anaphase Chromosomes in Crane-Fly Spermatocytes Treated With Taxol (Paclitaxel) Accelerate When Their Kinetochore Microtubules Are Cut: Evidence for Spindle Matrix Involvement With Spindle Forces

Various experiments have indicated that anaphase chromosomes continue to move after their kinetochore microtubules are severed. The chromosomes move poleward at an accelerated rate after the microtubules are cut but they slow down 1-3 min later and move poleward at near the original speed. There are two published interpretations of chromosome movements with severed kinetochore microtubules. One interpretation is that dynein relocates to the severed microtubule ends and propels them poleward by pushing against non-kinetochore microtubules. The other interpretation is that components of a putative "spindle matrix" normally push kinetochore microtubules poleward and continue to do so after the microtubules are severed from the pole. In this study we distinguish between these interpretations by treating cells with taxol. Taxol eliminates microtubule dynamics, alters spindle microtubule arrangements, and inhibits dynein motor activity in vivo. If the dynein interpretation is correct, taxol should interfere with chromosome movements after kinetochore microtubules are severed because it alters the arrangements of spindle microtubules and because it blocks dynein activity. If the "spindle matrix" interpretation is correct, on the other hand, taxol should not interfere with the accelerated movements. Our results support the spindle matrix interpretation: anaphase chromosomes in taxol-treated crane-fly spermatocytes accelerated after their kinetochore microtubules were severed.

Chromosomes selectively detach at one pole and quickly move towards the opposite pole when kinetochore microtubules are depolymerized in Mesostoma ehrenbergii spermatocytes

In a typical cell division, chromosomes align at the metaphase plate before anaphase commences. This is not the case in Mesostoma spermatocytes. Throughout prometaphase, the three bivalents persistently oscillate towards and away from either pole, at average speeds of 5-6 μm/min, without ever aligning at a metaphase plate. In our experiments, nocodazole (NOC) was added to prometaphase spermatocytes to depolymerize the microtubules. Traditional theories state that microtubules are the producers of force in the spindle, either by tubulin depolymerizing at the kinetochore (PacMan) or at the pole (Flux). Accordingly, if microtubules are quickly depolymerized, the chromosomes should arrest at the metaphase plate and not move. However, in 57/59 cells, at least one chromosome moved to a pole after NOC treatment, and in 52 of these cells, all three bivalents moved to the same pole. Thus, the movements are not random to one pole or other. After treatment with NOC, chromosome movement followed a consistent pattern. Bivalents stretched out towards both poles, paused, detached at one pole, and then the detached kinetochores quickly moved towards the other pole, reaching initial speeds up to more than 200 μm/min, much greater than anything previously recorded in this cell. As the NOC concentration increased, the average speeds increased and the microtubules disappeared faster. As the kinetochores approached the pole, they slowed down and eventually stopped. Similar results were obtained with colcemid treatment. Confocal immunofluorescence microscopy confirms that microtubules are not associated with moving chromosomes. Thus, these rapid chromosome movements may be due to non-microtubule spindle components such as actin-myosin or the spindle matrix.

A review of "tethers": elastic connections between separating partner chromosomes in anaphase

Recent work has demonstrated the existence of elastic connections, or tethers, between the telomeres of separating partner chromosomes in anaphase. These tethers oppose the poleward spindle forces in anaphase. Functional evidence for tethers has been found in a wide range of animal taxa, suggesting that they might be present in all dividing cells. An examination of the literature on cell division from the nineteenth century to the present reveals that connections between separating partner chromosomes in anaphase have been described in some of the earliest observations of cell division. Here, we review what is currently known about connections between separating partner chromosomes in anaphase, and we speculate on possible functions of tethers, and on what they are made of and how one might determine their composition.

Elastic tethers between separating anaphase chromosomes in crane-fly spermatocytes coordinate chromosome movements to the two poles

Separating anaphase chromosomes in crane-fly spermatocytes are connected by elastic tethers, as originally described by LaFountain et al. (2002): telomere-containing arm fragments severed from the arms move backwards to the partner telomeres. We have tested whether the tethers coordinate the movements of separating partner chromosomes. In other cell types anaphase chromosomes move faster, temporarily, when their kinetochore microtubules are severed. However, in crane-fly spermatocytes the chromosomes move at their usual speed when their kinetochore microtubules are severed. To test whether the absence of increased velocity is because tethers link the separating chromosomes and coordinate their movements, we cut tethers with a laser microbeam and then cut the kinetochore microtubules. After this procedure, the associated chromosome sped up, as in other cells. These results indicate that the movements of partner anaphase chromosomes in crane-fly spermatocytes are coordinated by elastic tethers connecting the two chromosomes and confirm that chromosomes speed up in anaphase when their kinetochore microtubules are severed. © 2016 Wiley Periodicals, Inc.

Mitotic tethers connect sister chromosomes and transmit "cross-polar" force during anaphase A of mitosis in PtK2 cells

Originally described in crane-fly spermatocytes, tethers physically link and transmit force between the ends of separating chromosomes. Optical tweezers and laser scissors were used to sever the tether between chromosomes, create chromosome fragments attached to the tether which move toward the opposite pole, and to trap the tethered fragments. Laser microsurgery in the intracellular space between separating telomeres reduced chromosome strain in half of tested chromosome pairs. When the telomere-containing region was severed from the rest of the chromosome body, the resultant fragment either traveled towards the proper pole (poleward), towards the sister pole (cross-polar), or movement ceased. Fragment travel towards the sister pole varied in distance and always ceased following a cut between telomeres, indicating the tether is responsible for transferring a cross-polar force to the fragment. Optical trapping of cross-polar traveling fragments places an upper boundary on the tethering force of ~1.5 pN.

Holocentric plant meiosis: first sisters, then homologues

Meiosis is a crucial process of sexual reproduction by forming haploid gametes from diploid precursor cells. It involves 2 subsequent divisions (meiosis I and meiosis II) after one initial round of DNA replication. Homologous monocentric chromosomes are separated during the first and sister chromatids during the second meiotic division. The faithful segregation of monocentric chromosomes is realized by mono-orientation of fused sister kinetochores at metaphase I and by bi-orientation of sister kinetochores at metaphase II. Conventionally this depends on a 2-step loss of cohesion, along chromosome arms during meiosis I and at sister centromeres during meiosis II.

Centromere pairing--tethering partner chromosomes in meiosis I

In meiosis, homologous chromosomes face the obstacle of finding, holding onto and segregating away from their partner chromosome. There is increasing evidence, in a diverse range of organisms, that centromere–centromere interactions that occur in late prophase are an important mechanism in ensuring segregation fidelity. Centromere pairing appears to initiate when homologous chromosomes synapse in meiotic prophase. Structural proteins of the synaptonemal complex have been shown to help mediate centromere pairing, but how the structure that maintains centromere pairing differs from the structure of the synaptonemal complex along the chromosomal arms remains unknown. When the synaptonemal complex proteins disassemble from the chromosome arms in late prophase, some of these synaptonemal complex components persist at the centromeres. In yeast and Drosophila these centromere-pairing behaviors promote the proper segregation of chromosome partners that have failed to become linked by chiasmata. Recent studies of mouse spermatocytes have described centromere pairing behaviors that are similar in several respects to what has been described in the fly and yeast systems. In humans, chromosomes that fail to experience crossovers in meiosis are error-prone and are a major source of aneuploidy. The finding that centromere pairing is a conserved phenomenon raises the possibility that it may play a role in promoting the segregation fidelity of non-exchange chromosome pairs in humans.

Chromosome interaction over a distance in meiosis

The challenge of cell division is to distribute partner chromosomes (pairs of homologues, pairs of sex chromosomes or pairs of sister chromatids) correctly, one into each daughter cell. In the 'standard' meiosis, this problem is solved by linking partners together via a chiasma and/or sister chromatid cohesion, and then separating the linked partners from one another in anaphase; thus, the partners are kept track of, and correctly distributed. Many organisms, however, properly separate chromosomes in the absence of any obvious physical connection, and movements of unconnected partner chromosomes are coordinated at a distance. Meiotic distance interactions happen in many different ways and in different types of organisms. In this review, we discuss several different known types of distance segregation and propose possible explanations for non-random segregation of distance-segregating chromosomes.

Franklin FC, Heckmann S. Atypical centromeres in plants-what they can tell us

The centromere, visible as the primary constriction of condensed metaphase chromosomes, is a defined chromosomal locus essential for genome stability. It mediates transient assembly of a multi-protein complex, the kinetochore, which enables interaction with spindle fibers and thus faithful segregation of the genetic information during nuclear divisions. Centromeric DNA varies in extent and sequence composition among organisms, but a common feature of almost all active eukaryotic centromeres is the presence of the centromeric histone H3 variant cenH3 (a.k.a. CENP-A). These typical centromere features apply to most studied species. However, a number of species display "atypical" centromeres, such as holocentromeres (centromere extension along almost the entire chromatid length) or neocentromeres (ectopic centromere activity). In this review, we provide an overview of different atypical centromere types found in plants including holocentromeres, de novo formed centromeres and terminal neocentromeres as well as di-, tri- and metapolycentromeres (more than one centromere per chromosomes). We discuss their specific and common features and compare them to centromere types found in other eukaryotic species. We also highlight new insights into centromere biology gained in plants with atypical centromeres such as distinct mechanisms to define a holocentromere, specific adaptations in species with holocentromeres during meiosis or various scenarios leading to neocentromere formation.

Normal segregation of a foreign-species chromosome during Drosophila female meiosis despite extensive heterochromatin divergence

The abundance and composition of heterochromatin changes rapidly between species and contributes to hybrid incompatibility and reproductive isolation. Heterochromatin differences may also destabilize chromosome segregation and cause meiotic drive, the non-Mendelian segregation of homologous chromosomes. Here we use a range of genetic and cytological assays to examine the meiotic properties of a Drosophila simulans chromosome 4 (sim-IV) introgressed into D. melanogaster. These two species differ by ∼12-13% at synonymous sites and several genes essential for chromosome segregation have experienced recurrent adaptive evolution since their divergence. Furthermore, their chromosome 4s are visibly different due to heterochromatin divergence, including in the AATAT pericentromeric satellite DNA. We find a visible imbalance in the positioning of the two chromosome 4s in sim-IV/mel-IV heterozygote and also replicate this finding with a D. melanogaster 4 containing a heterochromatic deletion. These results demonstrate that heterochromatin abundance can have a visible effect on chromosome positioning during meiosis. Despite this effect, however, we find that sim-IV segregates normally in both diplo and triplo 4 D. melanogaster females and does not experience elevated nondisjunction. We conclude that segregation abnormalities and a high level of meiotic drive are not inevitable byproducts of extensive heterochromatin divergence. Animal chromosomes typically contain large amounts of noncoding repetitive DNA that nevertheless varies widely between species. This variation may potentially induce non-Mendelian transmission of chromosomes. We have examined the meiotic properties and transmission of a highly diverged chromosome 4 from a foreign species within the fruitfly Drosophila melanogaster. This chromosome has substantially less of a simple sequence repeat than does D. melanogaster 4, and we find that this difference results in altered positioning when chromosomes align during meiosis. Yet this foreign chromosome segregates at normal frequencies, demonstrating that chromosome segregation can be robust to major differences in repetitive DNA abundance.

Topoisomerase II is required for the proper separation of heterochromatic regions during Drosophila melanogaster female meiosis

Heterochromatic homology ensures the segregation of achiasmate chromosomes during meiosis I in Drosophila melanogaster females, perhaps as a consequence of the heterochromatic threads that connect achiasmate homologs during prometaphase I. Here, we ask how these threads, and other possible heterochromatic entanglements, are resolved prior to anaphase I. We show that the knockdown of Topoisomerase II (Top2) by RNAi in the later stages of meiosis results in a specific defect in the separation of heterochromatic regions after spindle assembly. In Top2 RNAi-expressing oocytes, heterochromatic regions of both achiasmate and chiasmate chromosomes often failed to separate during prometaphase I and metaphase I. Heterochromatic regions were stretched into long, abnormal projections with centromeres localizing near the tips of the projections in some oocytes. Despite these anomalies, we observed bipolar spindles in most Top2 RNAi-expressing oocytes, although the obligately achiasmate 4^th chromosomes exhibited a near complete failure to move toward the spindle poles during prometaphase I. Both achiasmate and chiasmate chromosomes displayed defects in biorientation. Given that euchromatic regions separate much earlier in prophase, no defects were expected or observed in the ability of euchromatic regions to separate during late prophase upon knockdown of Top2 at mid-prophase. Finally, embryos from Top2 RNAi-expressing females frequently failed to initiate mitotic divisions. These data suggest both that Topoisomerase II is involved in the resolution of heterochromatic DNA entanglements during meiosis I and that these entanglements must be resolved in order to complete meiosis.

Proper chromosome segregation during egg and sperm development is crucial to prevent birth defects and miscarriage. During chromosome replication, DNA entanglements are created that must be resolved before chromosomes can fully separate. In the oocytes of the fruit fly Drosophila melanogaster, DNA entanglements persist between heterochromatic regions of the chromosomes until after spindle assembly and may facilitate the proper segregation of chromosomes during meiosis. Topoisomerase II enzymes can resolve DNA entanglements by cutting and untwisting tangled DNA. Decreasing Topoisomerase II (Top2) levels in the ovaries of fruit flies led to sterility. RNAi knockdown of the Top2 gene in oocytes resulted in chromosomes that failed to fully separate their heterochromatic regions during meiosis I and caused oocytes to arrest in meiosis I. These studies demonstrate that the Top2 enzyme is required for releasing DNA entanglements between homologous chromosomes before the onset of chromosome segregation during Drosophila female meiosis.

Alternative meiotic chromatid segregation in the holocentric plant Luzula elegans

Holocentric chromosomes occur in a number of independent eukaryotic lineages. They form holokinetic kinetochores along the entire poleward chromatid surfaces, and owing to this alternative chromosome structure, species with holocentric chromosomes cannot use the two-step loss of cohesion during meiosis typical for monocentric chromosomes. Here we show that the plant Luzula elegans maintains a holocentric chromosome architecture and behaviour throughout meiosis, and in contrast to monopolar sister centromere orientation, the unfused holokinetic sister centromeres behave as two distinct functional units during meiosis I, resulting in sister chromatid separation. Homologous non-sister chromatids remain terminally linked after metaphase I, by satellite DNA-enriched chromatin threads, until metaphase II. They then separate at anaphase II. Thus, an inverted sequence of meiotic sister chromatid segregation occurs. This alternative meiotic process is most likely one possible adaptation to handle a holocentric chromosome architecture and behaviour during meiosis.

Couples, pairs, and clusters: mechanisms and implications of centromere associations in meiosis

Observations of a wide range of organisms show that the centromeres form associations of pairs or small groups at different stages of meiotic prophase. Little is known about the functions or mechanisms of these associations, but in many cases, synaptonemal complex elements seem to play a fundamental role. Two main associations are observed: homology-independent associations very early in the meiotic program-sometimes referred to as centromere coupling-and a later association of homologous centromeres, referred to as centromere pairing or tethering. The later centromere pairing initiates during synaptonemal complex assembly, then persists after the dissolution of the synaptonemal complex. While the function of the homology-independent centromere coupling remains a mystery, centromere pairing appears to have a direct impact on the chromosome segregation fidelity of achiasmatic chromosomes. Recent work in yeast, Drosophila, and mice suggest that centromere pairing is a previously unappreciated, general meiotic feature that may promote meiotic segregation fidelity of the exchange and non-exchange chromosomes.

Distance segregation of sex chromosomes in crane-fly spermatocytes studied using laser microbeam irradiations

Univalent sex chromosomes in crane-fly spermatocytes have kinetochore spindle fibres to each spindle pole (amphitelic orientation) from metaphase throughout anaphase. The univalents segregate in anaphase only after the autosomes approach the poles. As each univalent moves in anaphase, one spindle fibre shortens and the other spindle fibre elongates. To test whether the directionality of force production is fixed at anaphase, that is, whether one spindle fibre can only elongate and the other only shorten, we cut univalents in half with a laser microbeam, to create two chromatids. In both sex-chromosome metaphase and sex-chromosome anaphase, the two chromatids that were formed moved to opposite poles (to the poles to which their fibre was attached) at speeds about the same as autosomes, much faster than the usual speeds of univalent movements. Since the chromatids moved to the pole to which they were attached, independent of the direction to which the univalent as a whole was moving, the spindle fibre that normally elongates in anaphase still is able to shorten and produce force towards the pole when allowed (or caused) to do so.

Pac-man motility of kinetochores unleashed by laser microsurgery

Experiments reveal pac-man motility in kinetochores of X-Y chromosomes, even though their normal behavior is dominated by traction fiber mechanics. A laser microbeam is used to release kinetochores in anaphase from tension. There is a poleward motion of released kinetochores twice as fast as normal and faster than tubulin flux.

We report on experiments directly in living cells that reveal the regulation of kinetochore function by tension. X and Y sex chromosomes in crane fly (Nephrotoma suturalis) spermatocytes exhibit an atypical segregation mechanism in which each univalent maintains K-fibers to both poles. During anaphase, each maintains a leading fiber (which shortens) to one pole and a trailing fiber (which elongates) to the other. We used this intriguing behavior to study the motile states that X-Y kinetochores are able to support during anaphase. We used a laser microbeam to either sever a univalent along the plane of sister chromatid cohesion or knock out one of a univalent's two kinetochores to release one or both from the resistive influence of its sister's K-fiber. Released kinetochores with attached chromosome arms moved poleward at rates at least two times faster than normal. Furthermore, fluorescent speckle microscopy revealed that detached kinetochores converted their functional state from reverse pac-man to pac-man motility as a consequence of their release from mechanical tension. We conclude that kinetochores can exhibit pac-man motility, even though their normal behavior is dominated by traction fiber mechanics. Unleashing of kinetochore motility through loss of resistive force is further evidence for the emerging model that kinetochores are subject to tension-sensitive regulation.

Functional states of kinetochores revealed by laser microsurgery and fluorescent speckle microscopy

The impact of mechanical forces on kinetochore motility was investigated using laser microsurgery and fluorescent speckle microscopy on kinetochores and associated microtubules during anaphase in crane fly spermatocytes. Kinetochores detached from their chromosomes moved at twice their normal speed, entering a motile state identified as “park.”

The impact of mechanical forces on kinetochore motility was investigated using laser microsurgery to detach kinetochores with associated chromatin (K fragment) from meiotic chromosomes in spermatocytes from the crane fly Nephrotoma suturalis. In spermatocytes, elastic tethers connect telomeres of homologues during anaphase A of meiosis I, thus preventing complete disjunction until mid- to late anaphase A. K fragments liberated from tethered arms moved at twice the normal velocity toward their connected poles. To assess functional states of detached and control kinetochores, we loaded cells with fluorescently labeled tubulin for fluorescent speckle microscopy on kinetochore microtubules. Control kinetochores added fluorescent speckles at the kinetochore during anaphase A, whereas kinetochores of K fragments generally did not. In cases in which speckles reappeared in K-fragment K fibers, speckles and K fragments moved poleward at similar velocities. Thus detached kinetochores convert from their normal polymerization (reverse pac-man) state to a different state, in which polymerization is not evident. We suggest that the converted state is “park,” in which kinetochores are anchored to plus ends of kinetochore microtubules that shorten exclusively at their polar ends.

BubR1- and Polo-coated DNA tethers facilitate poleward segregation of acentric chromatids

The mechanisms that safeguard cells against chromosomal instability (CIN) are of great interest, as CIN contributes to tumorigenesis. To gain insight into these mechanisms, we studied the behavior of cells entering mitosis with damaged chromosomes. We used the endonuclease I-CreI to generate acentric chromosomes in Drosophila larvae. While I-CreI expression produces acentric chromosomes in the majority of neuronal stem cells, remarkably, it has no effect on adult survival. Our live studies reveal that acentric chromatids segregate efficiently to opposite poles. The acentric chromatid poleward movement is mediated through DNA tethers decorated with BubR1, Polo, INCENP, and Aurora-B. Reduced BubR1 or Polo function results in abnormal segregation of acentric chromatids, a decrease in acentric chromosome tethering, and a great reduction in adult survival. We propose that BubR1 and Polo facilitate the accurate segregation of acentric chromatids by maintaining the integrity of the tethers that connect acentric chromosomes to their centric partners.

Duct tape for broken chromosomes

A cell undergoing mitosis is presented with a potentially catastrophic situation when a DNA double-strand break creates a chromosome fragment that lacks connection to a centromere. Royou et al. (2010) now reveal that this cellular crisis is averted in fruit fly neuroblasts by thin chromatin tethers that hold on to the ends of the broken chromosomes.

Heterochromatic threads connect oscillating chromosomes during prometaphase I in Drosophila oocytes

In Drosophila oocytes achiasmate homologs are faithfully segregated to opposite poles at meiosis I via a process referred to as achiasmate homologous segregation. We observed that achiasmate homologs display dynamic movements on the meiotic spindle during mid-prometaphase. An analysis of living prometaphase oocytes revealed both the rejoining of achiasmate X chromosomes initially located on opposite half-spindles and the separation toward opposite poles of two X chromosomes that were initially located on the same half spindle. When the two achiasmate X chromosomes were positioned on opposite halves of the spindle their kinetochores appeared to display proper co-orientation. However, when both Xs were located on the same half spindle their kinetochores appeared to be oriented in the same direction. Thus, the prometaphase movement of achiasmate chromosomes is a congression-like process in which the two homologs undergo both separation and rejoining events that result in the either loss or establishment of proper kinetochore co-orientation. During this period of dynamic chromosome movement, the achiasmate homologs were connected by heterochromatic threads that can span large distances relative to the length of the developing spindle. Additionally, the passenger complex proteins Incenp and Aurora B appeared to localize to these heterochromatic threads. We propose that these threads assist in the rejoining of homologs and the congression of the migrating achiasmate homologs back to the main chromosomal mass prior to metaphase arrest.

Proper chromosome segregation is essential during the production of eggs and sperm. Chromosome missegregation during meiosis results in the lethality of the offspring or in children carrying extra copies of a given chromosome (for example, Down syndrome). Recombination results in homologous chromosomes becoming physically interlocked in a manner that is normally sufficient to ensure proper segregation. Chromosomes that fail to undergo recombination require additional mechanisms to ensure their proper segregation. In Drosophila melanogaster oocytes we show that chromosomes that fail to recombine undergo dynamic movements on the meiotic spindle prior to their proper segregation. Although previous studies had shown that non-recombinant chromosomes move to opposite sides of the developing meiotic spindle, we show that these chromosomes can cross the spindle and re-associate with their homologs to attempt reorientation. Additionally, we observed threads connecting separated non-recombinant chromosomes that contained heterochromatic DNA and passenger complex proteins. These threads could assist the non-recombinant chromosomes in locating their homologs during their dynamic movements on the spindle. These chromosome movements and the heterochromatic threads are likely part of the mechanism ensuring proper segregation of nonexchange chromosomes.

Titin in insect spermatocyte spindle fibers associates with microtubules, actin, myosin and the matrix proteins skeletor, megator and chromator

Titin, the giant elastic protein found in muscles, is present in spindles of crane-fly and locust spermatocytes as determined by immunofluorescence staining using three antibodies, each raised against a different, spatially separated fragment of Drosophila titin (D-titin). All three antibodies stained the Z-lines and other regions in insect myofibrils. In western blots of insect muscle extract the antibodies reacted with high molecular mass proteins, ranging between rat nebulin (600-900 kDa) and rat titin (3000-4000 kDa). Mass spectrometry of the high molecular mass band from the Coomassie-Blue-stained gel of insect muscle proteins indicates that the protein the antibodies bind to is titin. The pattern of staining in insect spermatocytes was slightly different in the two species, but in general all three anti-D-titin antibodies stained the same components: the chromosomes, prophase and telophase nuclear membranes, the spindle in general, along kinetochore and non-kinetochore microtubules, along apparent connections between partner half-bivalents during anaphase, and various cytoplasmic components, including the contractile ring. That the same cellular components are stained in close proximity by the three different antibodies, each against a different region of D-titin, is strong evidence that the three antibodies identify a titin-like protein in insect spindles, which we identified by mass spectrometry analysis as being titin. The spindle matrix proteins skeletor, megator and chromator are present in many of the same structures, in positions very close to (or the same as) D-titin. Myosin and actin also are present in spindles in close proximity to D-titin. The varying spatial arrangements of these proteins during the course of division suggest that they interact to form a spindle matrix with elastic properties provided by a titin-like protein.

Laser microsurgery in the GFP era: a cell biologist's perspective

Modern biology is based largely on a reductionistic "dissection" approach-most cell biologists try to determine how complex biological systems work by removing their individual parts and studying the effects of this removal on the system. A variety of enzymatic and mechanical methods have been developed to dissect large cell assemblies like tissues and organs. Further, individual proteins can be inactivated or removed within a cell by genetic manipulations (e.g., RNAi or gene knockouts). However, there is a growing demand for tools that allow intracellular manipulations at the level of individual organelles. Laser microsurgery is ideally suited for this purpose and the popularity of this approach is on the rise among cell biologists. In this chapter, we review some of the applications for laser microsurgery at the subcellular level and describe practical requirements for laser microsurgery instrumentation demanded in the field. We also outline a relatively inexpensive but versatile laser microsurgery workstation that is being used in our laboratory. Our major thesis is that the limitations of the technology are no longer at the level of the laser, microscope, or software, but instead only in defining creative questions and in visualizing the target to be destroyed.

Calyculin A, an enhancer of myosin, speeds up anaphase chromosome movement

Actin and myosin inhibitors often blocked anaphase movements in insect spermatocytes in previous experiments. Here we treat cells with an enhancer of myosin, Calyculin A, which inhibits myosin-light-chain phosphatase from dephosphorylating myosin; myosin thus is hyperactivated. Calyculin A causes anaphase crane-fly spermatocyte chromosomes to accelerate poleward; after they reach the poles they often move back toward the equator. When added during metaphase, chromosomes at anaphase move faster than normal. Calyculin A causes prometaphase chromosomes to move rapidly up and back along the spindle axis, and to rotate. Immunofluorescence staining with an antibody against phosphorylated myosin regulatory light chain (p-squash) indicated increased phosphorylation of cleavage furrow myosin compared to control cells, indicating that calyculin A indeed increased myosin phosphorylation. To test whether the Calyculin A effects are due to myosin phosphatase or to type 2 phosphatases, we treated cells with okadaic acid, which inhibits protein phosphatase 2A at concentrations similar to Calyculin A but requires much higher concentrations to inhibit myosin phosphatase. Okadaic acid had no effect on chromosome movement. Backward movements did not require myosin or actin since they were not affected by 2,3-butanedione monoxime or LatruculinB. Calyculin A affects the distribution and organization of spindle microtubules, spindle actin, cortical actin and putative spindle matrix proteins skeletor and titin, as visualized using immunofluorescence. We discuss how accelerated and backwards movements might arise.

At the interface between signaling and executing anaphase--Cdc14 and the FEAR network

Anaphase is the stage of the cell cycle when the duplicated genome is separated to opposite poles of the cell. The irreversible nature of this event confers a unique burden on the cell and it is therefore not surprising that the regulation of this cell cycle stage is complex. In budding yeast, a signaling network known as the Cdc fourteen early anaphase release (FEAR) network and its effector, the protein phosphatase Cdc14, play a key role in the coordination of the multiple events that occur during anaphase, such as partitioning of the DNA, regulation of spindle stability, activation of microtubule forces, and initiation of mitotic exit. These functions of the FEAR network contribute to genomic stability by coordinating the completion of anaphase and the execution of mitotic exit.

Direct visualization of microtubule flux during metaphase and anaphase in crane-fly spermatocytes

Microtubule flux in spindles of insect spermatocytes, long-used models for studies on chromosome behavior during meiosis, was revealed after iontophoretic microinjection of rhodamine-conjugated (rh)-tubulin and fluorescent speckle microscopy. In time-lapse movies of crane-fly spermtocytes, fluorescent speckles generated when rh-tubulin incorporated at microtubule plus ends moved poleward through each half-spindle and then were lost from microtubule minus ends at the spindle poles. The average poleward velocity of approximately 0.7 microm/min for speckles within kinetochore microtubules at metaphase increased during anaphase to approximately 0.9 microm/min. Segregating half-bivalents had an average poleward velocity of approximately 0.5 microm/min, about half that of speckles within shortening kinetochore fibers. When injected during anaphase, rhtubulin was incorporated at kinetochores, and kinetochore fiber fluorescence spread poleward as anaphase progressed. The results show that tubulin subunits are added to the plus end of kinetochore microtubules and are removed from their minus ends at the poles, all while attached chromosomes move poleward during anaphase A. The results cannot be explained by a Pac-man model, in which 1) kinetochore-based, minus end-directed motors generate poleward forces for anaphase A and 2) kinetochore microtubules shorten at their plus ends. Rather, in these cells, kinetochore fiber shortening during anaphase A occurs exclusively at the minus ends of kinetochore microtubules.

Kinetochore fibre dynamics outside the context of the spindle during anaphase

Chromosomes move polewards as kinetochore fibres shorten during anaphase. Fibre dynamics and force production have been studied extensively, but little is known about these processes in the absence of the spindle matrix. Here we show that laser-microbeam-severed kinetochore fibres in the cytoplasm of grasshopper spermatocytes maintain a constant length while turning over in a polarized manner. Tubulin incorporates at or near the kinetochore and translocates towards severed ends without shortening the fibre. Consequently, the chromosome cannot move polewards unless the severed fibre reattaches to the pole through microtubules. A potential seclusion artefact has been ruled out, as fibres severed inside spindles behave identically despite being surrounded by the spindle matrix. Our data suggest that kinetochore microtubules constantly treadmill during anaphase in insect cells. Treadmilling is an intrinsic property of microtubules in the kinetochore fibre, independent of the context and attachment of the spindle. The machinery that depolymerizes minus ends of kinetochore microtubules is functional in a non-spindle context. Attachment to the pole, however, is required to cause net kinetochore fibre shortening to generate polewards forces during anaphase.

Telomere attachment, meiotic chromosome condensation, pairing, and bouquet stage duration are modified in spermatocytes lacking axial elements

During the extended prophase to the meiosis I division, chromosomes assemble axial elements (AE) along replicated sister chromatids whose ends attach to the inner nuclear membrane (NM) via a specialized conical thickening. Here, we show at the EM level that in Sycp3(-/-) spermatocyte chromosomes lack the AE and the conical end thickening, but still they attach their telomeres to the inner NM with an electron-dense plate that contains T(2)AG(3) repeats. Immunofluorescence detected telomere proteins, SCP2, and the meiosis-specific cohesin STAG3 at the Sycp3(-/-) telomere. Bouquet stage spermatocytes were approximately threefold enriched, and the number of telomere but not centromere signals was reduced to the haploid in advanced Sycp3(-/-) spermatocytes, which indicates a special mode of homolog pairing at the mammalian telomere. Fluorescence in situ hybridization with mouse chromosome 8- and 12-specific subsatellite probes uncovered reduced levels of regional homolog pairing, whereas painting of chromosomes 13 revealed partial or complete juxtapositioning of homologs; however, condensation of Sycp3(-/-) bivalents was defective. Electron microscopic analysis of AE-deficient spermatocytes revealed that transverse filaments formed short structures reminiscent of the synaptonemal complex central region, which likely mediate stable homolog pairing. It appears that the AE is required for chromosome condensation, rapid exit from the bouquet stage, and fine-tuning of homolog pairing.

Structure of kinetochore fibres in crane-fly spermatocytes after irradiation with an ultraviolet microbeam: Neither microtubules nor actin filaments remain in the irradiated region

We studied chromosome movement after kinetochore microtubules were severed. Severing a kinetochore fibre in living crane-fly spermatocytes with an ultraviolet microbeam creates a kinetochore stub, a birefringent remnant of the spindle fibre connected to the kinetochore and extending only to the edge of the irradiated region. After the irradiation, anaphase chromosomes either move poleward led by their stubs or temporarily stop moving. We examined actin and/or microtubules in irradiated cells by means of confocal fluorescence microscopy or serial-section reconstructions from electron microscopy. For each cell thus examined, chromosome movement had been recorded continuously until the moment of fixation. Kinetochore microtubules were completely severed by the ultraviolet microbeam in cells in which chromosomes continued to move poleward after the irradiation: none were seen in the irradiated regions. Similarly, actin filaments normally present in kinetochore fibres were severed by the ultraviolet microbeam irradiations: the irradiated regions contained no actin filaments and only local spots of non-filamentous actin. There was no difference in irradiated regions when the associated chromosomes continued to move versus when they stopped moving. Thus, one cannot explain motion with severed kinetochore microtubules in terms of either microtubules or actin-filaments bridging the irradiated region. The data seem to negate current models for anaphase chromosome movement and support a model in which poleward chromosome movement results from forces generated within the spindle matrix that propel kinetochore fibres or kinetochore stubs poleward.