Mitotic catastrophe and endomitosis in tumour cells: an evolutionary key to a molecular solution

Following genotoxic insult, p53 mutated tumour cells undergo mitotic catastrophe. This is characterised by a switch from mitosis to the endocycle. The essential difference between mitosis and the endocycle is that in the latter, DNA synthesis is uncoupled from cell division, which leads to the formation of endopolyploid cells. Recent data suggests that a return from the endocycle into mitosis is also possible. Furthermore, our observations indicate that a particular type of endocycle known as endomitosis may be involved in this return. Here we review the role of endomitosis in the somatic reduction of polyploidy during development and its postulated role in the evolution of meiosis. Finally, we incorporate these evolutionary data to help interpret our most recent observations in the tumour cell system, which indicate a role for endomitosis and meiotic regulators, in particular p39mos in the segregation of genomes (somatic reduction) of these endopolyploid cells.     

CLASP Mediates Microtubule Repair by Restricting Lattice Damage and Regulating Tubulin Incorporation

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Heat induced liver cell proliferation in the livebearing fish Poeciliopsis

Synopsis Liver regeneration is induced by heat stress in the small viviparous fish, Poeciliopsis. Acute exposure to sublethal temperatures, one to two degrees below their killing temperature, damages tissue and initiates liver cell proliferation in P. lucida, P. monacha, and P. monacha-lucida hybrid clones, SYN-4 and SYN-5. Regeneration of liver cells began within 1–2 days following heat stress and proceeded over 5 days. Peak cell proliferation occurred 2–3 days after treatment in fish of all four genotypes. Cell proliferation was induced in the two all-female clones, SYN-4 and SYN-5, by exposure to 40.5° C for 60 minutes. This treatment imposed mortalities of 17.9% and 16.7%, respectively, whereas reduction of the temperature to 39.5°C and reduction of the time to 30 minutes resulted in no mortalities without significantly lowering the level of cell proliferation (p > 0.05). Liver cell proliferation induced by both heat treatments was significantly higher (p < 0.05) in the SYN-5 hybrids than in SYN-4. The induction of liver cell proliferation with sublethal temperature exposures is discussed as it may relate to chemical carcinogenesis in both feral and laboratory fish. Acute heat exposure may be used experimentally in fish as an independent stimulus for liver cell proliferation in carcinogenesis studies. In poikilothermic animals-heat exposure offers an alternative to surgical removal of approximately two-thirds of the liver, the method most frequently used in rodents to study the process of liver regeneration.     

DIAPH1 regulates chromosomal instability of cancer cells by controlling microtubule dynamics

Chromosomal instability (CIN) is a hallmark of cancer, resulting from misalignment and missegregation of chromosomes during meta- and anaphase, due to non-precise regulation of spindle-MT dynamics. Diaphanous Related Formin 1 (DIAPH1) is an actin nucleator and also binds microtubule (MT) with high affinity. In this study, we analyzed the role of DIAPH1 in regulation of spindle MT-dynamics and CIN in HT29 and HCT-116 colorectal cancer (CRC) cells. Our data show that down-regulation of DIAPH1 in these cell lines decreased spindle-MT speed by 50 % and the fraction of cells with misaligned and missegregated chromosomes was significantly increased. Furthermore, in HCT-116 DIAPH1 depleted cells deviation of chromosome number was elevated and the number of cells with micronuclei and cytosolic DNA was increased in both DIAPH1-knock down cell lines. In line with these results, database analysis revealed a significant correlation with low DIAPH1 mRNA expression and aneuploidy. Thus, DIAPH1 is substantially involved in the control of CIN in CRC cells. Since in vitro, DIAPH1 directly increased MT-polymerization, we assume that DIAPH1 controls CIN by regulating spindle-MT dynamics.     

Damaged DNA-binding Protein 1 (DDB1) Interacts with Cdh1 and Modulates the Function of APC/CCdh1

APC/C^Cdh1 plays a key role in mitotic exit and has essential targets in the G₁ phase; however, these mechanisms are poorly understood. In this report, we provide evidence that damaged DNA-binding protein 1 (DDB1) is capable of binding the WD40 domains of Cdh1, but not of Cdc20, through its BPA and BPC domains. Moreover, cells lacking DDB1 exhibit markedly elevated levels of the protein substrates of APC/C^Cdh1. Depletion of DDB1 in mitotic cells significantly delays mitotic exit, which demonstrates that the interaction between DDB1 and Cdh1 plays a critical role in regulating APC/C^Cdh1 activity. However, cells depleted of Cdh1 demonstrated no change in the UV-induced degradation of Cdt1, the main function of DDB1 as an E3 ligase. Strikingly, the APC/C^Cdh1 substrate levels are normal in cell knockdowns of Cul4A and Cul4B, which, along with DDB1, form an E3 ligase complex. This finding indicates that DDB1 modulates the function of APC/C^Cdh1 in a manner independent on the Cul4-DDB1 complex. Our results suggest that DDB1 may functionally regulate mitotic exit by modulating APC/C^Cdh1 activity. This study reveals that there may be cross-talk among DDB1, Cdh1, and Skp2 in the control of cell cycle division.     

Thyroid Hormone Receptor Interacting Protein 13 (TRIP13) AAA-ATPase Is a Novel Mitotic Checkpoint-silencing Protein

The mitotic checkpoint (or spindle assembly checkpoint) is a fail-safe mechanism to prevent chromosome missegregation by delaying anaphase onset in the presence of defective kinetochore-microtubule attachment. The target of the checkpoint is the E3 ubiquitin ligase anaphase-promoting complex/cyclosome. Once all chromosomes are properly attached and bioriented at the metaphase plate, the checkpoint needs to be silenced. Previously, we and others have reported that TRIP13 AAA-ATPase binds to the mitotic checkpoint-silencing protein p31^comet. Here we show that endogenous TRIP13 localizes to kinetochores. TRIP13 knockdown delays metaphase-to-anaphase transition. The delay is caused by prolonged presence of the effector for the checkpoint, the mitotic checkpoint complex, and its association and inhibition of the anaphase-promoting complex/cyclosome. These results suggest that TRIP13 is a novel mitotic checkpoint-silencing protein. The ATPase activity of TRIP13 is essential for its checkpoint function, and interference with TRIP13 abolished p31^comet-mediated mitotic checkpoint silencing. TRIP13 overexpression is a hallmark of cancer cells showing chromosomal instability, particularly in certain breast cancers with poor prognosis. We suggest that premature mitotic checkpoint silencing triggered by TRIP13 overexpression may promote cancer development.     

Polar organizers mark division axis prior to preprophase band formation in mitosis of the hepaticReboulia hemisphaerica (Bryophyta)

Summary Changes in the pattern of microtubules during the cell cycle of the hepaticReboulia hemisphaerica (Bryophyta) were studied by indirect immunofluorescence using conventional and confocal laser scanning microscopy (CLSM). The first indication that a cell is preparing for division is fusiform shaping of the nucleus accompanied by the appearance of well-defined polar organizers (POs) at the future spindle poles. Microtubules emanating from the POs ensheath the nucleus and eventually develop into the half-spindles of mitosis. Some of the microtubules from each PO pass tangential to the nucleus and interact in the region of the future mitotic equator. A preprophase band (PPB) forms in this region later in prophase and coexists with the prophase spindle. Thus, the plane of division appears to be determined by interaction of opposing arrays of microtubules emanating from POs. Prometaphase is marked by disappearance of the POs, loss of astral microtubules, and conversion of the fusiform spindle of prophase to a truncated, barrel-shaped spindle more typical of higher plants. Restoration of cortical microtubules in daughter cell occurs on the cell side distal to the new cell plate, but nucleation of microtubules is associated with the nuclear envelope and not with organized POs. At the next division POs appear at opposite poles of preprophase nuclei with no evidence of division and migration that is characteristic of cells with centriolar centrosomes. These data lend additional support for the view that mitosis in hepatics is transitional between green algae and higher plants.Abbreviations AMS axial microtubule system - CLSM confocal laser scanning microscopy - MTOC microtubule organizing center - PO polar organizer - PPB preprophase band of microtubules - QMS quadripolar microtubule system - TEM transmission electron microscopy     

Immunofluorescence and electron microscopic studies of microtubule organization during the cell cycle ofDictyota dichotoma (Phaeophyta,Dictyotales)

Summary Interphase cells ofDictyota dichotoma (Hudson) Lamour. lack cortical microtubules (Mts) but display an impressive network of cytoplasmic microtubules (c-Mts). These are focussed on two opposed perinuclear centriolar sites where centrin or a centrin-homologue is localized. Some of the Mts surround the nucleus, but the majority traverse the cytoplasm as bundles variously directed towards the plasmalemma. In apical cells, and to a lesser extent in the square or slightly elongated meristematic cells, Mts are more or less evenly arranged. In elongated cells they form thick bundles longitudinally traversing the cytoplasm; a pattern maintained in differentiated cells. In early prophase the non-perinuclear Mts disappear but by late prophase a bi-astral arrangement of short Mts is observed. They enter polar nuclear depressions and attach to differentiated regions of the nuclear envelope where polar gaps open. By metaphase the spindle Mts converge on the centrioles at the polar gaps. At anaphase, interzonal Mts are evident and the asters start to reassemble. After telophase disruption of the interzonal Mts, the daughter nuclei approach each other, but move apart again before cytokinesis. The latter movement keeps pace with the development of two interdigitating Mt systems, ensheathing both daughter nuclei. The partition membrane bisects this Mt cage. Between telophase and cytokinesis the centrosomes separate, finally occupying opposed perinuclear sites. New Mts arise at the new centrosomes, some terminating on the consolidating partition membrane. Our data show thatD. dichotoma vegetative cells display a prominent cytoplasmic Mt cytoskeleton, which undergoes continual, but definite, change in organization during the cell cycle.     

A new model for periphyton growth in running waters

A new model for biofilm growth is based on a logistic approach but accounts for detachment of the biofilm by the running water and the existence of a threshold point for growth. Above a critical current velocity, development of periphyton is not stable. A stability analysis shows the existence of a fold catastrophe in the model and the amplification of water velocity fluctuations by the algal biomass.