Epigenetic reprogramming in mammalian nuclear transfer

Somatic cloning has been succeeded in some species, but the cloning efficiency is very low, which limits the application of the technique in many areas of research and biotechnology. The cloning of mammals by somatic cell nuclear transfer (NT) requires epigenetic reprogramming of the differentiated state of donor cell to a totipotent, embryonic ground state. Accumulating evidence indicates that incomplete or inappropriate epigenetic reprogramming of donor nuclei is likely to be the primary cause of failures in nuclear transfer. This review summarizes the roles of various epigenetic mechanisms, including DNA methylation, histone acetylation, imprinting, X-chromosome inactivation, telomere maintenance and expressions of development-related genes on somatic nuclear transfer.     

加入 WTO后中国粮食生产对策

历经１５年艰苦谈判 ,中国终于在２００１年加入世界贸易组织 ,这势必对中国经济和农业发展产生深远影响。以小麦为例 ,由于中国一直重视产量而忽视品质 ,面筋度超过３０ %的优质硬麦仅占小麦总量的２０ % ,而进口小麦不仅价格低 ,而且又具有面筋度较高的优质特性。根据加入ＷＴＯ协议 ,中国将在较短过渡期内以较低关税允许进口较多的小麦、玉米和稻米等粮食品种。根据乌拉圭回合《农业协议》规定 ,加入ＷＴＯ后就必须扩大国内市场对外开放程度、降低关税税率、逐步取消关税壁垒和粮食出口补贴 ,这给中国粮食生产带来重大影响。受冲击影响最…     

Retrovirus transfer of a bacterial gene into mouse haematopoietic progenitor cells

The haematopoietic system is made up of a hierarchy of cells with different developmental, functional and proliferative capacities. Although cellular diversity appears to arise from the commitment and maturation of stem cells, the molecular basis for this differentiation process is unknown. The introduction of cloned DNA sequences into haematopoietic progenitor cells would provide a novel approach for studying this differentiating in vivo system. One laboratory has reported DNA-mediated transfer of genes into mouse bone marrow cells. However, retroviruses offer a number of advantages over DNA-mediated gene transfer procedures, including high efficiency infection of a wide range of cell types in vitro and in vivo, stable and low copy integration into the host chromosome, and a defined integrated provirus structure. For these reasons recombinant DNA techniques have been utilized to construct high efficiency retrovirus vectors expressing foreign genes. We demonstrate here, using such a retrovirus vector, the transfer of a dominant selectable drug-resistance gene into defined classes of mouse haematopoietic progenitor cells. These observations should facilitate the development of molecular genetic approaches to fundamental and clinical problems in haematopoiesis.     

Chromatin dynamics during epigenetic reprogramming in the mouse germ line

A unique feature of the germ cell lineage is the generation of totipotency. A critical event in this context is DNA demethylation and the erasure of parental imprints in mouse primordial germ cells (PGCs) on embryonic day 11.5 (E11.5) after they enter into the developing gonads. Little is yet known about the mechanism involved, except that it is apparently an active process. We have examined the associated changes in the chromatin to gain further insights into this reprogramming event. Here we show that the chromatin changes occur in two steps. The first changes in nascent PGCs at E8.5 establish a distinctive chromatin signature that is reminiscent of pluripotency. Next, when PGCs are residing in the gonads, major changes occur in nuclear architecture accompanied by an extensive erasure of several histone modifications and exchange of histone variants. Furthermore, the histone chaperones HIRA and NAP-1 (NAP111), which are implicated in histone exchange, accumulate in PGC nuclei undergoing reprogramming. We therefore suggest that the mechanism of histone replacement is critical for these chromatin rearrangements to occur. The marked chromatin changes are intimately linked with genome-wide DNA demethylation. On the basis of the timing of the observed events, we propose that if DNA demethylation entails a DNA repair-based mechanism, the evident histone replacement would represent a repair-induced response event rather than being a prerequisite.     

In vivo reprogramming of murine cardiac fibroblasts into induced cardiomyocytes

The reprogramming of adult cells into pluripotent cells or directly into alternative adult cell types holds great promise for regenerative medicine. We reported previously that cardiac fibroblasts,which represent 50%of the cells in the mammalian heart, can be directly reprogrammed to adult cardiomyocyte-like cells in vitro by the addition of Gata4, Mef2c and Tbx5 (GMT). Here we use genetic lineage tracing to show that resident non-myocytes in the murine heart can be reprogrammed into cardiomyocyte-like cells in vivo by local delivery of GMT after coronary ligation. Induced cardiomyocytes became binucleate, assembled sarcomeres and had cardiomyocyte-like gene expression. Analysis of single cells revealed ventricular cardiomyocyte-like action potentials, beating upon electrical stimulation, and evidence of electrical coupling. In vivo delivery of GMT decreased infarct size and modestly attenuated cardiac dysfunction up to 3 months after coronary ligation. Delivery of the pro-angiogenic and fibroblast-activating peptide, thymosin b4, along with GMT, resulted in further improvements in scar area and cardiac function. These findings demonstrate that cardiac fibroblasts can be reprogrammed into cardiomyocyte-like cells in their native environment for potential regenerative purposes.     

Functional genetic analysis of mouse chromosome 11

Now that the mouse and human genome sequences are complete, biologists need systematic approaches to determine the function of each gene. A powerful way to discover gene function is to determine the consequence of mutations in living organisms. Large-scale production of mouse mutations with the point mutagen N-ethyl-N-nitrosourea (ENU) is a key strategy for analysing the human genome because mouse mutants will reveal functions unique to mammals, and many may model human diseases. To examine genes conserved between human and mouse, we performed a recessive ENU mutagenesis screen that uses a balancer chromosome, inversion chromosome 11 (refs 4, 5). Initially identified in the fruitfly, balancer chromosomes are valuable genetic tools that allow the easy isolation of mutations on selected chromosomes. Here we show the isolation of 230 new recessive mouse mutations, 88 of which are on chromosome 11. This genetic strategy efficiently generates and maps mutations on a single chromosome, even as mutations throughout the genome are discovered. The mutations reveal new defects in haematopoiesis, craniofacial and cardiovascular development, and fertility.     

Chfr defines a mitotic stress checkpoint that delays entry into metaphase

Chemicals that target microtubules induce mitotic stress by affecting several processes that occur during mitosis. These processes include separation of the centrosomes in prophase, alignment of the chromosomes on the spindle in metaphase and sister-chromatid separation in anaphase. Many human cancers are sensitive to mitotic stress. This sensitivity is being exploited for therapy and implies checkpoint defects. The known mitotic checkpoint genes, which prevent entry into anaphase when the chromosomes are not properly aligned on the mitotic spindle, are, however, rarely inactivated in human cancer. Here we describe the chfr gene, which is inactivated owing to lack of expression or by mutation in four out of eight human cancer cell lines examined. Normal primary cells and tumour cell lines that express wild-type chfr exhibited delayed entry into metaphase when centrosome separation was inhibited by mitotic stress. In contrast, the tumour cell lines that had lost chfr function entered metaphase without delay. Ectopic expression of wild-type chfr restored the cell cycle delay and increased the ability of the cells to survive mitotic stress. Thus, chfr defines a checkpoint that delays entry into metaphase in response to mitotic stress.     

入世后我国农业科技发展战略调整与政府职能定位

加入ＷＴＯ后 ,农业是我国承受压力最大的行业 ,但也带来新的发展机遇。实现由传统农业向现代农业转变 ,根本出路在于创新。现行的农业宏观管理体制和农业科技发展基本不能适应新形势的要求。为了趋利避害、迎接挑战 ,必须调整农业科技发展的战略和方向 ,以观念创新推动体制创新、体制创新支撑科技创新 ,才能从根本上增强我国农业发展能力 ,提高农产品国际竞争力。一、入世后我国农业科技发展的机遇与挑战１．入世后我国农业面临的形势和任务加入ＷＴＯ后 ,我国农业发展面临市场竞争加剧、人均自然资源短缺、生态环境恶化、投入与补贴手段有…     

Mediation of meiotic and early mitotic chromosome segregation in Drosophila by a protein related to kinesin.

Contrary to the traditional view that microtubules pull chromosomes polewards during the anaphase stage of meiotic and mitotic cell divisions, new evidence suggests that the chromosome movements are driven by a motor located at the kinetochore. The process of chromosome segregation involves proper arrangement of kinetochores for spindle attachment, followed by spindle attachment and chromosome movement. Mechanisms in Drosophila for chromosome segregation in meiosis differ in males and females, implying the action of different gene products in the two sexes. A product encoded at the claret locus in Drosophila is required for normal chromosome segregation in meiosis in females and in early mitotic divisions of the embryo. Here we show that the predicted amino-acid sequence of this product is related to the heavy chain of kinesin. The conserved region corresponds to the kinesin motor domain and includes the ATP-binding site and a region that can bind microtubules. A second region contains a leucine repeat motif which may mediate protein-subunit interactions necessary for attachment of chromosomes to the spindle. The mutant phenotype of chromosome nondisjunction and loss, and its similarity to the kinesin ATP-binding domain, suggest that the product encoded at claret not only stabilizes chromosome attachments to the spindle, but may also be a motor that drives chromosome segregation in female meiosis.     

加入WTO与中国农业产业化发展对策

加入ＷＴＯ ,标志着中国的改革开放进入“高级阶段”或进入“高速公路” ,国际环境和中国本身需要加大改革力度。这就是说 ,中国的开放要按照有关协议加大步伐 ,要最大限度地走向开放 ;而最大限度的开放 ,要求或者是促使改革不得不加大深度和广度。那么 ,中国农村细小分散、以农户为单元的农业生产如何和世界发达的工厂化 生产的现代化农业企业在农产品市场开放的条件下展开竞争呢？这里 ,有许多问题需要研究 ,尤其是要研究需要采取的应对措施。一、中国的农业产业化发展在中国 ,从１９９３年提出农业产业化到现在已近１０年 ,农业产业化已…