Partial synchronization of cell division in root meristem induced by 5-aminouracil

5-aminouracil induces a partial synchronization of mitoses in barley, onion and garlic root tips. The highest degree of synchronization has been achieved in garlic where the mitotic index reached the value of about 36%, while in onion and barley the values equalled about 20%. The concentration causing the maximal synchronization in barley (400–750 ppm) was many times higher than in garlic (62.5 ppm) and onion (100 ppm). The occurrence of micronuclei was evaluated in garlic, under the conditions when synchronization was maximal. It was increased nearly tenfold as compared with the control.     

The influence of 5-aminouracil on the mitotic index of barley root meristems

Isolated barley embryos were cultivated in the complete liquid medium for 24 h and then treated with 200 ppm or 750 ppm 5-AU for 2 h, 8 h or 24 h. During the 24 h treatment, MI decreased from 5–12% to 2–5%. After putting the embryos into the fresh medium without 5-AU, the original decrease of MI was followed by slight increase which was manifested only in part of the roots of some experimental variants. The time of the occurrence of MI maxima varied with the duration of treatment and concentration of 5-AU. After treatment with 750 ppm, the increase of MI occurred later but it was more pronounced than after cessation of the treatment with 200 ppm. In the former case, frequencies exceeding the controls appeared 12 h–16 h after treatment and they were manifested especially after treatments of shorter durations (2 h and 8 h). In the latter case, the increase of MI occurred mainly 6 h after cessation of the 24 h treatment. A slight chromosome breaking ability of this drug was found.     

Enhancing the mitotic index of blastomeres by thymidine synchronization

Mouse blastomeres were synchronized by exposure of the embryo to thymidine and colchicine. The mitosis index increased from 9% metaphases (colchicine synchronization) to 18% (thymidine + colchicine synchronization) per embryo. The in vitro development of embryos was not affected by treatment. This method appears to approach a level where successful sexing of embryos becomes possible.     

Mechanism of mitotic arrest induced by dolastatin 15 involves loss of tension across kinetochore pairs

Dolastatin 15 (DL15) is a potent, tubulin-targeted, vinca-site binding, anticancer agent that induces mitotic arrest and inhibit cell proliferation in a variety of cell types. Several analogs of DL15, including LU 103793 and tasidotin, have been progressed to clinical trials for different types of cancer. DL15 has been known to interfere with cellular microtubules and purified tubulin in vitro. However, the molecular mechanism with which the peptide arrests cells in mitosis is poorly understood. This study reports a possible antimitotic mechanism of action of DL15. DL15 inhibited HeLa cell proliferation in a concentration-dependent manner with a half-maximal inhibitory concentration (IC₅₀) of 2.8 ± 0.3 nM, induced mitotic arrest, disrupted cellular microtubules near its IC₅₀ for cell proliferation, and inhibited the re-polymerization of cellular microtubules. By staining the centrosomes of DL15-treated cells with anti-γ tubulin antibodies, the study found a significant reduction in interpolar distances in mitotic HeLa cells, indicating a disruption in the normal assembly dynamics of the microtubules. The study further found that DL15 induced a loss of tension across the kinetochore pairs as indicated by a reduction in interkinetochore distance. In response to this loss of tension, the tension-sensing checkpoint protein BuBR1 accumulated at the kinetochores, promoting mitotic arrest. In vitro, DL15 promoted formation of curved and fragmented polymers of microtubule proteins and inhibited tubulin decay in a manner similar to vinca-site binding agents such as phomopsin A. Together, the data indicate that the mitotic arrest induced by DL15 involves a loss of tension across the kinetochore pairs due to disruption of normal assembly dynamics of microtubules.     

Hydroxyurea-induced mitotic synchronization in Allium sativum root meristems

The synchronized divisions following a treatment with hydroxyurea (HU) — an inhibitor of DNA synthesis — were studied in root meristems of Allium sativum using two methods: autoradiography of median sections and morphological labeling with a cytokinesis inhibitor. It is shown that the second wave of mitoses is heterogeneous: it is composed mostly of cells which have been synchronized in the S phase by the HU treatment, of cells coming from the quiescent center stimulated to enter DNA synthesis and of cells which were not blocked by the 23 h HU treatment (slow cycling cells). It is also shown that the cell cycle following the first synchronized division is considerably shortened by the synchronization procedure.Abbreviations QC quiescent center - HU hydroxyurea - MHQD methyl-3 hydroxy-6 quinazoline dione 2–4     

Increases in nuclear volume and cell size in meristematic cells arrested by 5-aminouracil

Summary Lateral roots ofVicia faba were treated with a solution of 5-aminouracil (3.93 × 10^–3 M). They were either treated for 6 hours and allowed to recover for up to 10 hours, or were treated continuously for up to 24 hours. Mitotic index decreased as the duration of treatment increased,e.g., it was < 0.5 after 6 hours treatment and 4 hours recovery and 0.23 after 12 hours continuous treatment. During this period of low mitotic activity nuclei and cells increased in size: mean nuclear volume, for example, was 1505±651 m³ 8 hours after the end of a 6 hours treatment. In roots treated continuously, nuclear volume increased from 559±204 m³ at 0 hour to 1272±636 m³ at 12 hours. In the first 3 hours it was the larger nuclei that grew,i.e., nuclei that would have proceeded into mitosis if they had not been blocked by 5-AU. But between 3 and 12 hours of continuous exposure to 5-AU all nuclei increased in volume. Cells, on the other hand, showed no response during the first 6 hours of treatment; their areas did not increase till 6–12 hours had elapsed. It appears that in cells blocked by 5-AU growth continues for about 12 hours. Initially, nuclei grow disproportionately large, suggesting that synthesis of nuclear components is favoured at the expense of cytoplasmic constituents, at least during the first 6 hours of treatment; there is an internal imbalance between nuclear and cell growth and a temporary change in the nuclear cytoplasmic ratio. When cells recover from the 5-AU block and enter mitosis their prophase nuclei are also much larger than those of untreated cells. The response to 5-AU is discussed in terms of internal restrictions on cell growth, due to the presence of cell walls, and the heterogeneity in nuclear volumes.     

Enhanced mitotic activity induced by irradiation is abolished by PGI2 pretreatment

It is well known that irradiation may induce pronounced vascular lesions. Experimental studies revealed that irradiation induces an increased mitotic activity. As PGI2 has been claimed to be an antilesional agent, we wondered whether a pretreatment with PGI2 might abolish some of the effects induced by irradiation. 2 Groups of 24 rabbits were studied. 8 Rabbits each were irradiated with either 5 or 10 Gy on an abdominal aortic segment; 8 animals were sham treated. In each of the 3 groups half of the animals (n = 4) received PGI2 and half the buffer vehicle only. It is demonstrated that PGI2 is able to depress the enhanced mitotic activity induced by irradiation. In comparison to the controls, vascular thromboxane formation is decreased, the temporary increase in PGI2-formation by the vessel wall is less pronounced, whereas the conversion of exogenous arachidonic acid is unchanged. It is hypothetized that stable PGI2-analogues given during irradiation may probably prevent at least in part radiation-induced vascular changes and finally radiation-induced vasculopathy; this claim has to be proven in human.     

Acceleration of mitosis induced by mitotic stimulators of Physarum polycephalum

Extracts of the myxomycete Physarum polycephalum exhibit an accelerating effect on nuclear division which fluctuates during the synchronous nuclear division cycle. Extracts from late G2 phase plasmodia can advance mitosis in recipient test plasmodia by up to 30% of the length of the control cycle. The advancing capacity of extracts is heat- and ammonium sulphate-precipitable, non-dialysable and destroyed by pronase, suggesting that the active substance is a protein. The advance of mitosis is in strong correlation with the applied dose of stimulatory material.     

High sensitivity of DNA replication to 5-aminouracil in the middle of the S period

Summary Cell distribution in different compartments of the cell cycle (G₁, early, middle and late S, G₂ and mitosis) has been studied during treatment with 0.5 mM 5-aminouracil and recovery inAllium cepa L. root meristems by cytophotometric and autoradiographic methods. At optimum conditions for obtaining mitotic synchronization, 5-aminouracil gives rise to cell accumulation in the S period, preferentially in its middle zone where the relative DNA content is 2.8 ± 0.1 C. After a 14-hour treatment 33% of the proliferative population is accumulated in this particular region.During recovery, a drastic reduction of the S phase and a clear increase of the mitotic frequency are the most important events observed. Apparently, the removal of the drug frees the blockage and the accumulated cells complete their interphase making up the mitotic wave.     

5-methyl-dCTP deaminase induced by bacteriophage XP-12.

Bacteriophage XP-12, whose DNA contains 34 mol% 5-methylcytosine, induces the synthesis of a unique enzyme, 5-methyl-dCTP deaminase. The substrate for this enzyme, 5-methyl-dCTP, is produced by reactions catalyzed in part by other phage-induced enzymes, and the product of the reaction is dTTP. The deaminase therefore provides a novel pathway for biosynthesis of thymine residues for phage XP-12 DNA. Evidence is presented that this pathway is used for dTTP synthesis in XP-12-infected Xanthomonas oryzae.     

Morphological characteristics and the nature of gaps (achromatic lesions) induced by 4-aminouracil in chromosomes of Vicia faba

The cytological analysis of gaps induced by 4-aminouracil in Vicia faba shows that they are characterized by the presence of 1–2 very fine parallel chains, probably a chromatid skeleton mentioned by other authors. These achromatic lesions of the chromosomes are probably due to the loss or despiralization of both DNA and chromosomal proteins.     

低氧暴露诱导肌萎缩发生的机制探讨

高海拔地区常伴随着诸多不利环境因素,使初上高原者处于应激状态,对机体代谢系统产生一系列不良影响,其中之一就是诱发骨骼肌萎缩。肌萎缩是一种肌肉功能减退的反应,表现为肌纤维横截面积减小,肌纤维类型转变和肌肉力量、耐力下降。高原环境造成的肌萎缩与低氧环境密切相关。高原训练是竞技体育中一种提高氧运输能力、心脏供血能力及最大摄氧量的有效训练方式,但此过程中造成的骨骼肌质量丢失会影响运动员力量和耐力的发挥,不利于运动表现;同时,随着高原旅游和低氧减肥的兴起,世居平原大众进入高原后发生的肌量丢失和肌力下降也会影响其健康状态。低氧暴露所诱发的肌萎缩程度由低氧浓度和暴露时长所决定,是一个多器官、多组织参与的整体调控骨骼肌蛋白代谢失衡的过程,且在不同类型的肌纤维中表现不同。但目前关于低氧暴露导致肌萎缩的机制还不完全清楚。因此,本文将就该问题相关研究进展进行综述,以期进一步阐明低氧诱导肌萎缩的生物学机制,从而利用低氧刺激更好地提高运动成绩和服务大众健康。