The Experts below are selected from a list of 1617 Experts worldwide ranked by ideXlab platform
Edward D. Salmon - One of the best experts on this subject based on the ideXlab platform.
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Motile kinetochores and Polar ejection forces dictate chromosome position on the vertebrate mitotic spindle
Journal of Cell Biology, 1994Co-Authors: Conly L Rieder, Edward D. SalmonAbstract:We argue that hypotheses for how chromosomes achieve a metaphase alignment, that are based solely on a tug-of-war between poleward pulling forces produced along the length of opposing kinetochore fibers, are no longer tenable for vertebrates. Instead, kinetochores move themselves and their attached chromosomes, poleward and away from the pole, on the ends of relatively stationary but shortening/elongating kinetochore fiber Microtubules. Kinetochores are also "smart" in that they switch between persistent constant-velocity phases of poleward and away from the pole motion, both autonomously and in response to information within the spindle. Several molecular mechanisms may contribute to this directional instability including kinetochore-associated Microtubule motors and kinetochore Microtubule dynamic instability. The control of kinetochore directional instability, to allow for congression and anaphase, is likely mediated by a vectorial mechanism whose magnitude and orientation depend on the density and orientation or growth of Polar Microtubules. Polar Microtubule arrays have been shown to resist chromosome poleward motion and to push chromosomes away from the pole. These "Polar ejection forces" appear to play a key role in regulating kinetochore directional instability, and hence, positions achieved by chromosomes on the spindle.
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Kinetochore Directional Instability in Vertebrate Mitotic Cells
Biomechanics of Active Movement and Division of Cells, 1994Co-Authors: Robert V. Skibbens, Edward D. SalmonAbstract:Vertebrate mitotic spindles are formed from oppositely oriented Polar Microtubule (MT) arrays nucleated from centrosomes. MTs mostly grow and shorten by the addition and loss of tubulin subunits from the MT plus-ends distal from the pole. Chromosomes become attached to the spindle when kinetochores capture and stabilize the dynamically instable MT plus-ends (reviewed in Salmon, 1989).
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Poleward force at the kinetochore in metaphase depends on the number of kinetochore Microtubules.
Journal of Cell Biology, 1990Co-Authors: Thomas S. Hays, Edward D. SalmonAbstract:To examine the dependence of poleward force at a kinetochore on the number of kinetochore Microtubules (kMTs), we altered the normal balance in the number of Microtubules at opposing homologous kinetochores in meiosis I grasshopper spermatocytes at metaphase with a focused laser microbeam. Observations were made with light and electron microscopy. Irradiations that partially damaged one homologous kinetochore caused the bivalent chromosome to shift to a new equilibrium position closer to the pole to which the unirradiated kinetochore was tethered; the greater the dose of irradiation, the farther the chromosome moved. The number of kMTs on the irradiated kinetochore decreased with severity of irradiation, while the number of kMTs on the unirradiated kinetochore remained constant and independent of chromosome-to-pole distance. Assuming a balance of forces on the chromosome at congression equilibrium, our results demonstrate that the net poleward force on a chromosome depends on the number of kMTs and the distance from the pole. In contrast, the velocity of chromosome movement showed little dependence on the number of kMTs. Possible mechanisms which explain the relationship between the poleward force at a kinetochore, the number of kinetochore Microtubules, and the lengths of the kinetochore fibers at congression equilibrium include a "traction fiber model" in which poleward force producers are distributed along the length of the kinetochore fibers, or a "kinetochore motor-Polar ejection model" in which force producers located at or near the kinetochore pull the chromosomes poleward along the kMTs and against an ejection force that is produced by the Polar Microtubule array and increases in strength toward the pole.
Conly L Rieder - One of the best experts on this subject based on the ideXlab platform.
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Motile kinetochores and Polar ejection forces dictate chromosome position on the vertebrate mitotic spindle
Journal of Cell Biology, 1994Co-Authors: Conly L Rieder, Edward D. SalmonAbstract:We argue that hypotheses for how chromosomes achieve a metaphase alignment, that are based solely on a tug-of-war between poleward pulling forces produced along the length of opposing kinetochore fibers, are no longer tenable for vertebrates. Instead, kinetochores move themselves and their attached chromosomes, poleward and away from the pole, on the ends of relatively stationary but shortening/elongating kinetochore fiber Microtubules. Kinetochores are also "smart" in that they switch between persistent constant-velocity phases of poleward and away from the pole motion, both autonomously and in response to information within the spindle. Several molecular mechanisms may contribute to this directional instability including kinetochore-associated Microtubule motors and kinetochore Microtubule dynamic instability. The control of kinetochore directional instability, to allow for congression and anaphase, is likely mediated by a vectorial mechanism whose magnitude and orientation depend on the density and orientation or growth of Polar Microtubules. Polar Microtubule arrays have been shown to resist chromosome poleward motion and to push chromosomes away from the pole. These "Polar ejection forces" appear to play a key role in regulating kinetochore directional instability, and hence, positions achieved by chromosomes on the spindle.
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Kinetochores are transported poleward along a single astral Microtubule during chromosome attachment to the spindle in newt lung cells.
Journal of Cell Biology, 1990Co-Authors: Conly L Rieder, S P AlexanderAbstract:During mitosis in cultured newt pneumocytes, one or more chromosomes may become positioned well removed (greater than 50 microns) from the Polar regions during early prometaphase. As a result, these chromosomes are delayed for up to 5 h in forming an attachment to the spindle. The spatial separation of these chromosomes from the Polar Microtubule-nucleating centers provides a unique opportunity to study the initial stages of kinetochore fiber formation in living cells. Time-lapse Nomarski-differential interference contrast videomicroscopic observations reveal that late-attaching chromosomes always move, upon attachment, into a single Polar region (usually the one closest to the chromosome). During this attachment, the kinetochore region of the chromosome undergoes a variable number of transient poleward tugs that are followed, shortly thereafter, by rapid movement of the chromosome towards the pole. Anti-tubulin immunofluorescence and serial section EM reveal that the kinetochores and kinetochore regions of nonattached chromosomes lack associated Microtubules. By contrast, these methods reveal that the attachment and subsequent poleward movement of a chromosome correlates with the association of a single long Microtubule with one of the kinetochores of the chromosome. This Microtubule traverses the entire distance between the spindle pole and the kinetochore and often extends well past the kinetochore. From these results, we conclude that the initial attachment of a chromosome to the newt pneumocyte spindle results from an interaction between a single Polar-nucleated Microtubule and one of the kinetochores on the chromosome. Once this association is established, the kinetochore is rapidly transported poleward along the surface of the Microtubule by a mechanism that is not dependent on Microtubule depolymerization. Our results further demonstrate that the motors for prometaphase chromosome movement must be either on the surface of the kinetochore (i.e., within the corona but not the plate), distributed along the surface of the kinetochore Microtubules, or both.
Darryl L Kropf - One of the best experts on this subject based on the ideXlab platform.
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Localization and function of Kinesin-5-like proteins during assembly and maintenance of mitotic spindles in Silvetia compressa
BMC Research Notes, 2009Co-Authors: Nick T Peters, Anne Catherine Miller, Darryl L KropfAbstract:Background Kinesin-5 (Eg-5) motor proteins are essential for maintenance of spindle biPolarity in animals. The roles of Kinesin-5 proteins in other systems, such as Arabidopsis, Dictyostelium, and sea urchin are more varied. We are studying Kinesin-5-like proteins during early development in the brown alga Silvetia compressa . Previously, this motor was shown to be needed to assemble a biPolar spindle, similar to animals. This report builds on those findings by investigating the localization of the motor and probing its function in spindle maintenance. Findings Anti-Eg5 antibodies were used to investigate localization of Kinesin-5-like proteins in brown algal zygotes. In interphase zygotes, localization was predominantly within the nucleus. As zygotes entered mitosis, these motor proteins strongly associated with spindle poles and, to a lesser degree, with the Polar Microtubule arrays and the spindle midzone. In order to address whether Kinesin-5-like proteins are required to maintain spindle biPolarity, we applied monastrol to synchronized zygotes containing biPolar spindles. Monastrol is a cell-permeable chemical inhibitor of the Kinesin-5 class of molecular motors. We found that inhibition of motor function in pre-formed spindles induced the formation of multiPolar spindles and short biPolar spindles. Conclusion Based upon these localization and inhibitor studies, we conclude that Kinesin-5-like motors in brown algae are more similar to the motors of animals than those of plants or protists. However, Kinesin-5-like proteins in S. compressa serve novel roles in spindle formation and maintenance not observed in animals.
Ronald Hancock - One of the best experts on this subject based on the ideXlab platform.
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An antigen located in the kinetochore region in metaphase and on Polar Microtubule ends in the midbody region in anaphase, characterised using a monoclonal antibody
Chromosoma, 1990Co-Authors: Roumen Pankov, Margot Lemieux, Ronald HancockAbstract:We describe a new component of the kinetochore region of Chinese hamster ovary cells, which was characterised using a monoclonal antibody (mAb). This antigen was localised on the kinetochore regions of purified metaphase chromosomes, but in anaphase it was instead located on the Polar Microtubules in the midbody region, where they terminate in the stembody. It was not detectable in prophase or interphase cells by immunofluorescence, but was present in the interphase nucleus as shown by immunoblotting after SDS-polyacrylamide gel electrophoresis. The mAb recognised two polypeptides of M_r 140 000 and 155 000. The localisation of this antigen in metaphase on the kinetochore region, where the plus ends of the kinetochore Microtubules are temporarily stabilised when they attach, and later in the stembody and midbody where the plus ends of the Polar Microtubules are stabilised in anaphase and telophase, suggests that it could play a role in stabilising the plus ends of Microtubules and thus in the control of Microtubule dynamics during mitosis.
E D Salmon - One of the best experts on this subject based on the ideXlab platform.
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regional variation of Microtubule flux reveals Microtubule organization in the metaphase meiotic spindle
Journal of Cell Biology, 2008Co-Authors: Ge Yang, Lisa A Cameron, Paul S Maddox, E D Salmon, Gaudenz DanuserAbstract:Continuous poleward movement of tubulin is a hallmark of metaphase spindle dynamics in higher eukaryotic cells and is essential for stable spindle architecture and reliable chromosome segregation. We use quantitative fluorescent speckle microscopy to map with high resolution the spatial organization of Microtubule flux in Xenopus laevis egg extract meiotic spindles. We find that the flux velocity decreases near spindle poles by ∼20%. The regional variation is independent of functional kinetochores and centrosomes and is suppressed by inhibition of dynein/dynactin, kinesin-5, or both. Statistical analysis reveals that tubulin flows in two distinct velocity modes. We propose an association of these modes with two architecturally distinct yet spatially overlapping and dynamically cross-linked arrays of Microtubules: focused Polar Microtubule arrays of a uniform Polarity and slower flux velocities are interconnected by a dense barrel-like Microtubule array of antiparallel Polarities and faster flux velocities.
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Microtubule assembly and kinetochore directional instability in vertebrate monoPolar spindles: implications for the mechanism of chromosome congression.
Journal of cell science, 1994Co-Authors: L Cassimeris, C L Rieder, E D SalmonAbstract:We have proposed previously a kinetochore motor-Polar ejection model for chromosome congression to the metaphase plate where forces generated at the kinetochore are antagonized by away-from-the pole forces generated within each half-spindle on the chromosome arms. This model was based in large part on observations of the behavior of chromosomes on monoPolar spindles. In these cells chromosomes typically become attached to the pole by only one kinetochore fiber. These mono-oriented chromosomes move to positions away from the pole even though they are pulled poleward at their kinetochores. Their arms are also ejected away from the pole when severed from the centromere. Here we have characterized further the properties of monoPolar spindles in newt lung epithelial cells to determine the similarities between monoPolar and biPolar spindles. We found no significant differences between monoPolar and biPolar spindles over the parameters examined, which included: Microtubule dynamics as measured by fluorescence redistribution after photobleaching; the ability of Polar Microtubule arrays to push chromosome arms away from the pole; the dependence of chromosome position relative to the pole on Microtubule assembly; the number of kinetochore Microtubules per kinetochore; and the directional instability of kinetochore motion during chromosome oscillations poleward and away-from-the-pole. As in biPolar spindles, kinetochore directional instability is characterized by abrupt switching between constant velocity phases of poleward and away-from-the-pole motion. From these data we conclude that the mechanism(s) responsible for chromosome positioning in monoPolar spindles are fundamentally the same as those in biPolar spindles; only the geometry of the two spindle forms and the interplay between sister kinetochore directional instabilities are different. We also found no correlation in the kinetochore-to-pole distance with kinetochore Microtubule number in monoPolar spindles, but a strong qualitative correlation with Microtubule density. This finding indicates that oscillations of mono-oriented chromosomes in both monoPolar and biPolar spindles occur because chromosomes persist in poleward motion until they reach a density of Polar Microtubules sufficiently high to promote switching to away-from-the-pole motion. As the kinetochore and chromosome arms move away-from-the-pole, Microtubule density decreases and the kinetochore switches to poleward motion, pulling the chromosome arms back into regions of higher Microtubule density. The mechanism regulating kinetochore switching between poleward and away-from-the-pole motion is poorly understood, but may depend on tension at the kinetochore generated by pushing forces on the chromosome arms produced by the Polar Microtubule arrays.