Centrosome and kinetochore movement during mitosis.

During the past year important progress has been made in refining our understanding of how chromosomes become equally distributed to daughter cells during mitosis. Unlike the situation in diatoms and yeast, it now appears that spindle pole (centrosome) separation during spindle formation and anaphase B is mediated in vertebrates primarily by an astral pulling, and not a pushing, mechanism. Kinetochore motility is directionally unstable, which has important consequences for how chromosomes move to the equator of the forming spindle. Finally, the observation that sister chromatid disjunction occurs even in the presence of high levels of maturation promoting factor reveals that the series of biochemical events responsible for this phenomenon is not an obligatory part of the pathway by which the cell exits mitosis.

Crocidolite asbestos fibers undergo size-dependent microtubule-mediated transport after endocytosis in vertebrate lung epithelial cells.

The large respiratory epithelial cells within primary cultures of newt (Taricha granulosa) lung are uniquely suited for high resolution video-enhanced light-microscopic studies. We show here that these cells incorporate crocidolite asbestos fibers within 18 h by endocytosis. Once inside the cell, fibers less than 5 microns in length are seen by video light microscopy to undergo saltatory transport at a maximum velocity of 1.18 microns/s. By contrast, fibers over 5 microns long rarely exhibit saltatory motion. Over time, all of the fibers become preferentially located near the nucleus. This perinuclear accumulation is largely inhibited by disassembling the cytoplasmic microtubules with nocodazole. Same cell correlative light and electron microscopy reveal that fibers exhibiting saltatory behavior are enclosed within a membrane. From these observations we conclude that, upon incorporation into epithelial cells, asbestos fibers undergo size-dependent active transport along cytoplasmic microtubules. Our data are the first to link the dimension-dependent transforming ability of asbestos fibers to a basic cellular function, i.e., the microtubule-dependent transport of cellular components.

Behavior of crocidolite asbestos during mitosis in living vertebrate lung epithelial cells.

Asbestos has been described as a physical carcinogen in that long thin fibers are generally more carcinogenic than shorter thicker ones. It has been hypothesized that long thin fibers disrupt chromosome behavior during mitosis, causing chromosome abnormalities which lead to cell transformation and neoplastic progression. Using high-resolution time lapse video-enhanced light microscopy and the uniquely suited lung epithelial cells of the newt Taricha granulosa, we have characterized for the first time the behavior of crocidolite asbestos fibers, and their interactions with chromosomes, during mitosis in living cells. We found that the keratin cage surrounding the mitotic spindle inhibited fiber migration, resulting in spindles with few fibers. As in interphase, fibers displayed microtubule-mediated saltatory movements. Fiber position was only slightly affected by the ejection forces of the spindle asters. Physical interactions between crocidolite fibers and chromosomes occurred randomly within the spindle and along its edge. Crocidolite fibers showed no affinity toward chromatin and most encounters ended with the fiber passively yielding to the chromosome. In a few encounters along the spindle edge the chromosome yielded to the fiber, which remained stationary as if anchored to the keratin cage. We suggest that fibers thin enough to be caught in the keratin cage and long enough to protrude into the spindle are those fibers with the ability to snag or block moving chromosomes.

Increased mitochondrial cytochrome c levels and mitochondrial hyperpolarization precede camptothecin-induced apoptosis in Jurkat cells.

Mitochondria play a central role in apoptosis through release of cytochrome c and activation of caspases. In the present study, we showed that, in Jurkat human T cells, camptothecin-induced apoptosis is preceded by (i) an increase in cytochrome c and subunit IV of cytochrome c oxidase (COX IV) levels in mitochondria; and (ii) an elevation of the mitochondrial membrane potential (Delta(Psi)m). These events are followed by cytochrome c release into the cytosol, cytochrome c and COX IV depletion from mitochondria, externalization of phosphatidylserine (PS), disruption of Delta(Psi)m, caspase activation, poly(ADP-ribose)polymerase cleavage and DNA fragmentation. The pan-caspase inhibitor z-VAD.fmk blocked camptothecin-induced PS externalization, disruption of Delta(Psi)m and DNA fragmentation, suggesting that these events are mediated by caspase activation. In contrast, z-VAD did not prevent cytochrome c release, despite preventing cytochrome c and COX IV depletion from mitochondria. Together, these data suggest that mitochondrial cytochrome c and COX IV enrichment are early events preceding the onset of apoptosis and that cytochrome c release is upstream of caspase activation and loss of Delta(Psi)m. Furthermore, prevention by z-VAD of cytochrome c and COX IV depletion in mitochondria suggests the possibility that a caspase-like activity in mitochondria is involved in the proteolytic depletion of respiratory chain proteins. Activation of this activity may play an important role in drug-induced apoptosis.

Stable versus unstable orientations of sex chromosomes in two grasshopper species.

The structural basis of orientation stability was investigated. The stable unipolar orientation of the Melanoplus sanguinipes X-chromosome univalent is unique in that it is stable without tension created by forces towards opposite poles; tension is thought to be the principle component in stabilizing kinetochore orientations to a pole. Stable orientation of the X chromosome in Melanoplus sanguinipes was compared with unstable X orientation in Melanoplus differentialis. Ten cells (five of each species) were studied, firstly in living cultures where chromosome behavior was followed, then by serial-section electron microscopy where the structural basis for chromosome behavior was examined. Microtubules other than kinetochore microtubules were observed impinging on the X chromosomes. One end of these microtubules was buried in chromatin, while the other ran toward a pole. The X chromosomes of M. sanguinipes had more of these microtubules than did M. differentialis X chromosomes. It is suggested that M. sanguinipes X chromosomes are less condensed than M. differentialis X chromosomes and so allow more microtubules to penetrate the chromosome. The extra microtubules impinging on the M. sanguinipes X chromosome probably prevent reorientation by inhibiting the turning of the chromosome towards the opposite pole, i.e., more force is needed to turn a kinetochore towards the opposite pole than can be generated and attempts at reorientation fail. This may be analogous to the effect that tension has on the orientation stability of bivalents.

Chromosome rearrangement patterns of an SD chromosome (SDKona-2) in Drosophila melanogaster caused by hybrid dysgenesis.

The types and frequencies of spontaneous chromosome rearrangements caused by hybrid dysgenesis were studied in a second chromosome autosome of Drosophila melanogaster. This second chromosome, being an SD chromosome, had two important advantages over other autosomes for this study: (i) it had the two inversions characteristic of a standard SD-72 chromosome type, which distinguished it from its homolog in polytene chromosome spreads, and (ii) because of the meiotic drive associated with the segregation distorter system, it was preferentially transmitted to the next generation. The chromosome mutation frequency of this chromosome (given the name SDKona-2) was 8.3 and 11.7% in the F2 and F3 generations, respectively. The types of new chromosome rearrangements observed in the first four generations included paracentric inversions, pericentric inversions, duplications, deletions, reciprocal translocations (involving the third chromosome), and transpositions. Small paracentric inversions were the most common type of new rearrangement. Later, over 35 generations, some of these new rearrangements changed, either by becoming more complex or by being replaced with yet another new chromosome rearrangement. Duplications were unstable and were replaced by paracentric inversions whose breakpoints were on either side of the duplication. Transpositions arose both from a single multibreak event and from a series of two-break events.

Bivalent behavior in Drosophila melanogaster males containing the In(1)sc4Lsc8RX chromosome.

The sex chromosome bivalent was examined in Drosophila melanogaster males possessing the In(1)sc4Lsc8R X chromosome. Three-dimensional reconstructions from electron micrographs of serially cut thin sections were made. A large proportion of the kinetochores of In(1)sc4Lsc8R/Y bivalents did not face opposite poles during metaphase I and anaphase I. This suggests that In(1)sc4Lsc8R/Y bivalents may have difficulty achieving bipolar stability. Delay in achieving bipolar stability could contribute to the nondisjunctional behavior found in In(1)sc4Lsc8R/Y males.

Chromosome mal-orientation and reorientation during mitosis.

We argue that mal-orientation of mitotic chromosomes is not as rare as once believed. However, unlike bivalents during meiosis I, the reorientation of a mal-oriented mitotic chromosome has yet to be observed. This appears to be due, in part, to the difficulty in differentiating mal-oriented chromosomes from mono-oriented ones which are common during spindle formation in living mitotic cells. We assume that mitotic cells possess mechanisms for correcting chromosome mal-orientations that are similar to those operating during meiosis. However, unlike meiosis, where reorientation appears to be triggered when tension on a K-fiber is relieved or reduced, other factors related to the close proximity of sister kinetochores may also induce reorientation in mal-oriented mitotic chromosomes. We favor a model in which the reorientation of a mitotic kinetochore depends on, and is initiated by, the kinetochore capturing MTs from the pole to which it is reorienting.

Meiosis in Drosophila males. I. The question of separate conjunctive mechanisms for the XY and autosomal bivalents.

The conjunctive mechanism of the XY bivalent is believed to differ from that of the autosomal bivalents in the achiasmate Drosophila melanogaster male. It has been proposed that hypothetical cohesive elements, termed collochores, hold the X and Y chromosomes together at or near their nucleolar organizing regions (NORs) and that collochores are not exhibited by autosomal bivalents. In electron micrographs, unique fibrillar material is observed between the X and Y chromosomes at the synaptic site. Recently, the 240 bp nontranscribed spacer associated with rRNA genes at the NOR has been implicated as the essential DNA sequence for XY pairing. To test whether this DNA sequence is always associated with XY pairing and to determine its relationship to the unique fibrillar material, we studied the XY bivalent in Drosophila simulans. The D. simulans Y chromosome has few, if any, rRNA genes, but does have a large block (3,000 kb or 12,500 copies) of the nontranscribed spacer repeat located at the distal end of its long arm. This is in contrast to the D. melanogaster Y, which has the repeat located among rRNA genes on its short arm. Using light and electron microscopy, we show that the X does indeed pair with the distal end of the long arm of the D. simulans Y. However, no fibrillar material is evident in serial thin sections of the D. simulans XY bivalent, suggesting that this material (in D. melanogaster) may be remnants of the NOR rather than a morphological manifestation of the hypothetical collochores.(ABSTRACT TRUNCATED AT 250 WORDS)

Tension, microtubule rearrangements, and the proper distribution of chromosomes in mitosis.

The basis for stable versus unstable kinetochore orientation was investigated by a correlated living-cell/ultrastructural study of grasshopper spermatocytes. Mal-oriented bivalents having both kinetochores oriented to one spindle pole were induced by micromanipulation. Such mal-orientations are stable while the bivalent is subject to tension applied by micromanipulation but unstable after tension is released. Unstable bivalents always reorient with movement of one kinetochore toward the opposite pole. Microtubules associated with stably oriented bivalents, whether they are mal-oriented or in normal bipolar orientation, are arranged in orderly parallel bundles running from each kinetochore toward the pole. Similar orderly kinetochore microtubule arrangements characterized mal-oriented bivalents fixed just after release of tension. A significantly different microtubule arrangement is found only some time after tension release, when kinetochore movement is evident. The microtubules of a reorienting kinetochore always include a small number of microtubules running toward the pole toward which the kinetochore was moving at the time of fixation. All other microtubules associated with such a moving kinetochore appear to have lost their anchorage to the original pole and to be dragged passively as the kinetochore proceeds to the other pole. Thus, the stable anchorage of kinetochore microtubules to the spindle is associated with tension force and unstable anchorage with the absence of tension. The effect of tension is readily explained if force production and anchorage are both produced by mitotic motors, which link microtubules to the spindle as they generate tension forces.

Meiosis in Drosophila melanogaster. IV. The conjunctive mechanism of the XY bivalent.

Chromosome pairing during meiosis I in D. melanogaster males was investigated ultrastructurally by examining complete bivalents in electron micrographs of serial thin sections. The XY bivalent is characterized by the presence of the unique material located between the two half-bivalents at the site of synapsis. The material has a fibrillar appearance and is less electron dense than the surrounding chromatin. XY bivalents in XYY males and XY bivalents containing the X chromosome, In (1) sc4LSC8R, where the pairing sites of the X chromosome are inverted and partially deleted also possess this material. The material is not associated with autosomal bivalents and may represent a morphological manifestation of the hypothetical cohesive elements (collochores) which are thought to function in conjunction of the X and Y chromosomes (Cooper, 1964).

Meiosis in Drosophila melanogaster. I. Chromosome identification and kinetochore microtubule numbers during the first and second meiotic divisions in males.

Individual bivalents or chromosomes have been identified in Drosophila melanogaster spermatocytes at metaphase I, anaphase I, metaphase II and anaphase II in electron micrographs of serial sections. Identification was based on a combination of chromosome volume analysis, bivalent topology, and kinetochore position. - Kinetochore microtubule numbers have been obtained for the identified chromosomes at all four meiotic stages. Average numbers in D. melanogaster are relatively low compared to reported numbers of other higher eukaryotes. There is no differences in kinetochore microtubule numbers within a stage despite a large (approximately tenfold) difference in chromosome volume between the largest and the smallest chromosome. A comparison between the two meiotic metaphases (metaphase I and metaphase II) reveals that metaphase I kinetochores possess twice as many microtubules as metaphase II kinetochores. - Other microtubules in addition to those that end on or penetrate the kinetochore are found in the vicinity of the kinetochore. These microtubules penetrate the chromosome rather than the kinetochore proper and are more numerous at metaphase I than at the other division stages.

Meiosis in Drosophila melanogaster. V. Univalent behavior in In(1)sc4Lsc8R/BsY males.

Univalent behavior during meiosis has been examined in Drosophila melanogaster males possessing the In(1)sc4Lsc8R X chromosome using light microscopy and serial section electron microscopy. Males from two stocks, displaying high (0.40) and low (0.14) frequencies of sex chromosome nondisjunction, have been investigated. The results demonstrate that (i) sex chromosomes are more intimately paired during prometaphase I in males from the low nondisjunction stock than in males from the high nondisjunction stock, and (ii) the univalents are distributed to the poles in an unbiased manner during meiosis rather than by directed segregation of both univalents to the same pole as previously determined for other In(1)sc4Lsc8R/Y males.

Studies on the ejection properties of asters: astral microtubule turnover influences the oscillatory behavior and positioning of mono-oriented chromosomes.

The position of a mono-oriented chromosome changes as it oscillates to and from the pole to which it is attached. Such oscillatory behavior reveals that the net force on a mono-oriented chromosome is constantly changing. Fluctuations may occur in both the polewardly directed force acting at the kinetochore and the opposing outwardly directed force associated with the aster. We have examined the ejection properties of the aster--as well as the oscillatory behavior and positioning of mono-oriented chromosomes--in relation to astral microtubule turnover. We treated cells containing monopolar spindles with drugs that affect microtubule turnover, either by promoting the depletion of dynamically unstable astral microtubules (nocodazole and colcemid) or by augmenting their numbers and stability (taxol). Both types of drugs stopped the oscillatory behavior of mono-oriented chromosomes within seconds. The final position of the chromosomes depended on how microtubule turnover was affected. In the case of nocodazole and colcemid, non-kinetochore astral microtubules were depleted first and the kinetochore-to-pole distance shortened. In these cells chromosome fragments generated by laser microsurgery were no longer expelled from the center of the aster. By contrast, with taxol the number of non-kinetochore microtubules increased and the astral ejection force became stronger as shown by the finding that the chromosomes moved away from the pole to the periphery of the monaster. Moreover, arms severed from chromosomes at the periphery of the taxol monaster failed to move further away from the aster's center. From these observations we conclude that the oscillatory movements and changing position of a mono-oriented chromosome relative to the pole are mediated by changes in the number of astral microtubules. The dynamic instability of astral microtubules that leads to a rapid turnover may contribute to the astral ejection force by allowing the continual growth of microtubules out from the aster. Growing astral microtubules may exert a pushing force that their rigidity maintains until their depolymerization.

IAP insertion in the murine LamB3 gene results in junctional epidermolysis bullosa.

The laminin-5 molecule functions in the attachment of various epithelia to basement membranes. Mutations in the laminin-5-coding genes have been associated with Herlitz junctional epidermolysis bullosa (HJEB), a severe and often lethal blistering disease of humans. Here we report the characterization of a spontaneous mouse mutant with an autosomal recessive blistering disease. These mice exhibit sub-epithelial blisters of the skin and mucosal surfaces and abnormal hemidesmosomes lacking sub-basal dense plates. By linkage analysis the genetic defect was localized to a 2-cM region on distal Chromosome (Chr) 1 where a laminin-5 subunit gene, LamB3, was previously localized. LamB3 mRNA and laminin-5 protein were undetectable by Northern blot analysis and immunohistochemical methods, respectively. DNA sequence analysis indicated that the LamB3 genetic defect resulted from disruption of the coding sequence by insertion of an intracisternal-A particle (IAP) at an exon/intron junction. These findings suggest a role for laminin-5 in hemidesmosome formation and indicate that the LamB3(IAP) mutant is a useful mouse model for HJEB.

Long crocidolite asbestos fibers cause polyploidy by sterically blocking cytokinesis.

Asbestos has been described as a physical carcinogen in that its carcinogenic effects appear to be related primarily to fiber dimensions. It has been hypothesized that long asbestos fibers may interfere with chromosome distribution during cell division, causing genomic changes that lead to cell transformation and neoplastic progression. Using high-resolution time-lapse light microscopy and serial-section electron microscopy, we have followed individual crocidolite asbestos fibers through the later stages of cell division in LLC-MK2 epithelial cells, and have detailed for the first time their effect on cytokinesis. We found that long fibers (15-55 microgram), trapped by the cleavage furrow, sterically blocked cytokinesis, sometimes resulting in the formation of a binucleated cell. The ends of blocking fibers were usually found within invaginations of the newly formed nuclei. Nuclear envelope-fiber attachment was evident when a chromatin strand ran with the fiber into the intercellular bridge. Such strands may break, causing chromosome structural rearrangements. Our data are the first to show that individual crocidolite fibers can cause genomic changes by sterically blocking cytokinesis and that fiber length and affinity for the nuclear envelope are important factors. Such genomic changes may be among the initial events leading to asbestos-induced cancers.