体细胞核移植胚胎核重编程的研究进展

尽管在多种哺乳动物种系中成功制备了体细胞克隆后代,但当前的克隆技术仍有许多亟待解决的问题。体细胞核移植胚胎大多存在许多发育异常,造成了妊娠早期高流产率和出生后高死亡率。有研究认为,克隆胚胎发育障碍的一个重要的原因是供体细胞的遗传重编程不完全。哺乳动物种系中,DNA甲基化是胚胎发育期转录调节的必需步骤,除了单拷贝基因序列外,在基因组很多的区域都可以观测到克隆胚胎的异常甲基化。此外,克隆胚胎的基因印迹也存在异常。     

动物核移植中核的重编程

动物体细胞核能被去核卵重新编程,获得发育的全能性.在重编程过程中,核仁结构发生变化,组蛋白被修饰.端粒酶基因能被重编程,从而恢复核移植后代的端粒长度.核移植后,克隆后代出现X染色体失活.核基因能被重编程,引起基因表达改变.     

The Nuclear Pore Complex and Nuclear Transport

Internal membrane bound structures sequester all genetic material in eukaryotic cells. The most prominent of these structures is the nucleus, which is bounded by a double membrane termed the nuclear envelope (NE). Though this NE separates the nucleoplasm and genetic material within the nucleus from the surrounding cytoplasm, it is studded throughout with portals called nuclear pore complexes (NPCs). The NPC is a highly selective, bidirectional transporter for a tremendous range of protein and ribonucleoprotein cargoes. All the while the NPC must prevent the passage of nonspecific macromolecules, yet allow the free diffusion of water, sugars, and ions. These many types of nuclear transport are regulated at multiple stages, and the NPC carries binding sites for many of the proteins that modulate and modify the cargoes as they pass across the NE. Assembly, maintenance, and repair of the NPC must somehow occur while maintaining the integrity of the NE. Finally, the NPC appears to be an anchor for localization of many nuclear processes, including gene activation and cell cycle regulation. All these requirements demonstrate the complex design of the NPC and the integral role it plays in key cellular processes.Taxonomically speaking, all life on earth falls into one of two fundamental groups, the prokaryotes and the eukaryotes. The prokaryotes, the first group to evolve, are single cell organisms bounded by a single membrane. About 1.5 billion years later, a series of evolutionary innovations led to the emergence of eukaryotes. Eukaryotes have multiple inner membrane structures that allow for compartmentalization within the cell, and therefore differentiation of the cell and regulation within it. Ultimately, the greater cellular complexity of eukaryotes allowed them to adopt a multicellular lifestyle, as seen in the plants, fungi and animals of today (reviewed in Field and Dacks 2009).Internal membrane bound structures sequester all genetic material in eukaryotic cells. The most prominent of these structures, which gives the eukaryotes their Greek-rooted name, is the nucleus—the central “kernel” (gr. “karyo-”) of the cell. The nucleus is bounded by a double membrane termed the nuclear envelope (NE), which separates the nucleoplasm and genetic material from the surrounding cytoplasm. However the genetic material in the nucleus is not totally isolated from the rest of the cell. Studded throughout the NE are portals called nuclear pore complexes (NPCs). The NPC is a highly selective, bidirectional transporter for a tremendous range of cargoes. Going into the nucleus, these cargoes include inner nuclear membrane proteins and all the proteins in the nucleoplasm. Going out are RNA-associated proteins that are assembled into ribosomal subunits or messenger ribonucleoproteins (mRNPs). Once transported, the NPC must ensure these cargos are retained in their respective nuclear and cytoplasmic compartments. All the while the NPC must prevent the passage of nonspecific macromolecules, yet allow the free diffusion of water, sugars, and ions. These many types of nuclear transport are regulated at multiple stages, providing a powerful extra level of cellular control that is not necessary in prokaryotes. Assembly, maintenance, and repair of the NPC must somehow occur while maintaining the integrity of the NE. Finally, the NPC appears to be an anchor for localization of many nuclear processes, including gene activation and cell cycle regulation (reviewed in Ahmed and Brickner 2007; Hetzer and Wente 2009). All these requirements demonstrate the complex design of the NPC and the integral role it plays in key cellular processes.     

核移植后的细胞核再程序化机制研究进展

目前动物克隆技术体系极待完善,其极低的成功率及克隆动物普遍存在的早衰、早天现象是阻碍研究深入进行的首要问题,其突破的关键在于对核移植后的细胞核再程序化机制的阐明。从移植核在结构上的重塑、移植核与受体卵细胞质所处的细胞周期及其相互作用、重构胚与两性胚在分子水平的变化等多方面研究表明：受体细胞质的环境对于细胞核的再程序化至关重要,处于有丝分裂各时期的细胞作为核供体一旦移植到卵母细胞后,移植核在卵质环境里将出现结构上的重塑和分子的再程序化;移植核与受体卵问细胞周期的相容性、重构胚的染色体倍性的正确与否,可能是决定重构胚发育率高低的重要因素;合子型基因激活是基因表达再程序化的关键事件之一;印记基因对于体细胞克隆动物移植核的再程序化过程可能起着非常独特的作用。     

Nuclear electrophysiology

Conclusions Patch-clamp, fluorescence microscopy and high-resolution EM have yielded new data which question current concepts of ion transport across the nuclear envelope. The current challenge is to prove that NICs play an important role in nuclear function either through their identity with NPCs or parts thereof. Electrophysiological designs must incorporate cell biology approaches as done for putative protein-conducting channels of the ER (Simon & Blobel, 1991, 1992).Preliminary studies (J.O. Bustamante et al., in preparation), illustrated in Fig. 1, confirm that, as is the case of NPCs, NICs cannot function in an extracellular environment deprived of cytosolic factors. Our current efforts aim at clarifying if the lysate factors required for macromolecular transport through NPCs (e.g., Adam et al., 199la,b) are those required for NIC open-shut gating. Monoclonal antibodies to identified NPC proteins should be helpful in furthering the identification of NICs with NPCs. Our observation of blockade of NIC activity with wheat germ agglutinin, discussed above, supports the idea that NPCs are the structural foundation for NICs. Should NICs be identified with NPCs or otherwise proven essential to nucleocytoplasmic transport, NIC response to cytoplasmic signals would suggest that they are relevant to mediating gene control by transduction and other cytosolic signals (Karin, 1991; Davis, 1992). NIC influence on intranuclear free ion concentrations is potentially important to controlling gene activation, repression, as well as the efficiency and fidelity of gene expression (e.g., Kroeger, 1963; Lezzi & Gilbert, 1970; Leake et al., 1972; Morgan & Curran, 1986; Li & Rokita, 1991; Lippard, 1993). As electrophysiological and cell/molecular biology approaches merge, the prospects improve for the field of nuclear electrophysiology.The author thanks (in alphabetical order) the intellectual contributions of Drs. Christopher W. Akey, Gregory S. Beckler (Promega), Louis J. DeFelice, Colin Dingwall, Alexander Fabiato, Julio M. Fernández, Larry Gerace, John A. Hanover, Bertil Hille, Stuart L. Jacobson, W. Jonathan Lederer, Andrejs Liepins, Gilbert N. Ling, Michele Mazzanti, Ernst Niggli, Sanford M. Simon, Walter Stühmer, and W. Gil Wier. Special thanks are tendered to Drs. Dingwall, Gerace, Hanover and Liepins for their observations on nuclear electrophysiology within the context of cell/molecular biology. Thanks are also extended to Drs. Lederer and Wier for discussions on fluorescence microscopy of Ca²⁺ transients. Dr. Niggli provided the preprint of his paper, with P. Lipp, confirming previous observations that cardiomyocyte nuclei behave as a barrier to intracellular Ca²⁺ waves. Drs. DeFelice and Mazzanti provided a draft of their review on the biophysics of the nuclear envelope. This work is supported by the American Heart Association, Maryland Affiliate. Institutional support and facilities have come through Drs. C. William Balke, Michael R. Gold, W. Gil Wier and W. Jonathan Lederer, to whom the author is deeply grateful. This work is dedicated to my parents for introducing me to scientific curiosity and for their constant incentive and support. A special dedication to my father who recently passed away.