Nanog promotes transfer of pluripotency after cell fusion

Through cell fusion, embryonic stem (ES) cells can erase the developmental programming of differentiated cell nuclei and impose pluripotency. Molecules that mediate this conversion should be identifiable in ES cells. One candidate is the variant homeodomain protein Nanog, which has the capacity to entrain undifferentiated ES cell propagation. Here we report that in fusions between ES cells and neural stem (NS) cells, increased levels of Nanog stimulate pluripotent gene activation from the somatic cell genome and enable an up to 200-fold increase in the recovery of hybrid colonies, all of which show ES cell characteristics. Nanog also improves hybrid yield when thymocytes or fibroblasts are fused to ES cells; however, fewer colonies are obtained than from ES x NS cell fusions, consistent with a hierarchical susceptibility to reprogramming among somatic cell types. Notably, for NS x ES cell fusions elevated Nanog enables primary hybrids to develop into ES cell colonies with identical frequency to homotypic ES x ES fusion products. This means that in hybrids, increased Nanog is sufficient for the NS cell epigenome to be reset completely to a state of pluripotency. We conclude that Nanog can orchestrate ES cell machinery to instate pluripotency with an efficiency of up to 100% depending on the differentiation status of the somatic cell.     

Control of ground-state pluripotency by allelic regulation of Nanog

Pluripotency is established through genome-wide reprogramming during mammalian pre-implantation development, resulting in the formation of the naive epiblast. Reprogramming involves both the resetting of epigenetic marks and the activation of pluripotent-cell-specific genes such as Nanog and Oct4 (also known as Pou5f1). The tight regulation of these genes is crucial for reprogramming, but the mechanisms that regulate their expression in vivo have not been uncovered. Here we show that Nanog--but not Oct4--is monoallelically expressed in early pre-implantation embryos. Nanog then undergoes a progressive switch to biallelic expression during the transition towards ground-state pluripotency in the naive epiblast of the late blastocyst. Embryonic stem (ES) cells grown in leukaemia inhibitory factor (LIF) and serum express Nanog mainly monoallelically and show asynchronous replication of the Nanog locus, a feature of monoallelically expressed genes, but ES cells activate both alleles when cultured under 2i conditions, which mimic the pluripotent ground state in vitro. Live-cell imaging with reporter ES cells confirmed the allelic expression of Nanog and revealed allelic switching. The allelic expression of Nanog is regulated through the fibroblast growth factor-extracellular signal-regulated kinase signalling pathway, and it is accompanied by chromatin changes at the proximal promoter but occurs independently of DNA methylation. Nanog-heterozygous blastocysts have fewer inner-cell-mass derivatives and delayed primitive endoderm formation, indicating a role for the biallelic expression of Nanog in the timely maturation of the inner cell mass into a fully reprogrammed pluripotent epiblast. We suggest that the tight regulation of Nanog dose at the chromosome level is necessary for the acquisition of ground-state pluripotency during development. Our data highlight an unexpected role for allelic expression in controlling the dose of pluripotency factors in vivo, adding an extra level to the regulation of reprogramming.     

Generation of germline-competent induced pluripotent stem cells

We have previously shown that pluripotent stem cells can be induced from mouse fibroblasts by retroviral introduction of Oct3/4 (also called Pou5f1), Sox2, c-Myc and Klf4, and subsequent selection for Fbx15 (also called Fbxo15) expression. These induced pluripotent stem (iPS) cells (hereafter called Fbx15 iPS cells) are similar to embryonic stem (ES) cells in morphology, proliferation and teratoma formation; however, they are different with regards to gene expression and DNA methylation patterns, and fail to produce adult chimaeras. Here we show that selection for Nanog expression results in germline-competent iPS cells with increased ES-cell-like gene expression and DNA methylation patterns compared with Fbx15 iPS cells. The four transgenes (Oct3/4, Sox2, c-myc and Klf4) were strongly silenced in Nanog iPS cells. We obtained adult chimaeras from seven Nanog iPS cell clones, with one clone being transmitted through the germ line to the next generation. Approximately 20% of the offspring developed tumours attributable to reactivation of the c-myc transgene. Thus, iPS cells competent for germline chimaeras can be obtained from fibroblasts, but retroviral introduction of c-Myc should be avoided for clinical application.     

Forced expression of theOct-4 gene influences differentiation of embryonic stem cells

By transfecting an Oct-4 expression plasmid into embryonic stem cells (ES cells), the ES-O cell line was constructed, which sustained the expression of Oct-4 gene when induced by retinoic acid. Forced expression of Oct-4 gene could not sustain the stem property of ES-O cells without the differentiation inhibiting factor LIF, but if LIF exists, forced expression of Oct-4 gene could enhance the ability to sustain the undifferentiation state and inhibit cell differentiation induced by retinoic acid. It was indicated that Oct-4 must cooperate with LIF to sustain the undifferentiation state of ES cells. During the cell differentiation, ES-O cells tend to differentiate into neural cells, suggesting that forced expression of Oct-4 gene may be in relation with the differentiation of neuroderm.     

增强型绿色荧光蛋白基因真核表达载体的构建

构建增强型绿色荧光蛋白(enhanced green fluorescent protein，EGFP)基因的真核表达载体pCDNA3．1( )-EGFP，转染至培养的Hela细胞中成功表达，并发出绿色荧光，证明EGFP一种良好的报告基因和筛选标记．     

Intrinsic transition of embryonic stem-cell differentiation into neural progenitors

The neural fate is generally considered to be the intrinsic direction of embryonic stem (ES) cell differentiation. However, little is known about the intracellular mechanism that leads undifferentiated cells to adopt the neural fate in the absence of extrinsic inductive signals. Here we show that the zinc-finger nuclear protein Zfp521 is essential and sufficient for driving the intrinsic neural differentiation of mouse ES cells. In the absence of the neural differentiation inhibitor BMP4, strong Zfp521 expression is intrinsically induced in differentiating ES cells. Forced expression of Zfp521 enables the neural conversion of ES cells even in the presence of BMP4. Conversely, in differentiation culture, Zfp521-depleted ES cells do not undergo neural conversion but tend to halt at the epiblast state. Zfp521 directly activates early neural genes by working with the co-activator p300. Thus, the transition of ES cell differentiation from the epiblast state into neuroectodermal progenitors specifically depends on the cell-intrinsic expression and activator function of Zfp521.     

人生长激素基因转化的小鼠胚胎干细胞特性的研究

报道了携带人生长激素基因（ｈＧＨ）的逆转录病毒载体ｐＩＮＳ－ＧＨ导入小鼠胚胎干细胞（ＥＳ细胞）ＣＣＥ后，虽然用放射免疫法未检测到ｈＧＨ基因的表达，但是，Ｓｏｕｔｈｅｒｎ杂交的结果表明，ｈＧＨ基因的确已经整合到细胞基因组中。对转化的ＥＳ细胞克隆进行了体内外分化能力及嵌合能力的检验，结果表明，经过一系列体外操作的ＥＳ细胞，仍具有分化成多种细胞类型的能力。转化的ＥＳ细胞通过显微注射注入囊胚后，能参与受体     

IGF and FGF cooperatively establish the regulatory stem cell niche of pluripotent human cells in vitro

Distinctive properties of stem cells are not autonomously achieved, and recent evidence points to a level of external control from the microenvironment. Here, we demonstrate that self-renewal and pluripotent properties of human embryonic stem (ES) cells depend on a dynamic interplay between human ES cells and autologously derived human ES cell fibroblast-like cells (hdFs). Human ES cells and hdFs are uniquely defined by insulin-like growth factor (IGF)- and fibroblast growth factor (FGF)-dependence. IGF 1 receptor (IGF1R) expression was exclusive to the human ES cells, whereas FGF receptor 1 (FGFR1) expression was restricted to surrounding hdFs. Blocking the IGF-II/IGF1R pathway reduced survival and clonogenicity of human ES cells, whereas inhibition of the FGF pathway indirectly caused differentiation. IGF-II is expressed by hdFs in response to FGF, and alone was sufficient in maintaining human ES cell cultures. Our study demonstrates a direct role of the IGF-II/IGF1R axis on human ES cell physiology and establishes that hdFs produced by human ES cells themselves define the stem cell niche of pluripotent human stem cells.     

新型呼肠病毒S1基因与宿主细胞相互作用的初步研究

摘要 目的 筛选出新型呼肠病毒S1基因和宿主相互作用的关键区段,为后续的宿主受体蛋白筛选工作提供可靠的基础。 方法 将编码全长S1基因,以及N端和C端阅读框分别克隆入真核表达载体pIRSE2-EGFP,转染敏感细胞株鼠成纤维细胞（L929）和人宫颈癌细胞（Hela）后,通过荧光报告基因EGFP的表达分析,筛选出S1基因与宿主细胞相互作用时起决定性影响的区段。 结果 同等量的3种真核表达质粒在单独转染L929时,pGSN质粒转染后荧光表达量最大,pGSC质粒转染后荧光表达量最小。 不同比例混合的pGS与pGSN进行共转染时,pGSN在转染质粒中的比例越高,荧光表达量也越高。 在Hela细胞中的转染结果与L929相同。 结论 新型呼肠病毒R4株的S1基因N端阅读框编码区（62-406bp）在S1基因转染时起关键作用。     

利用CRIS-PITCh系统构建原位表达CEP290-EGFP的细胞系

初级纤毛是一种存在于脊椎动物大多数细胞表面的细胞器,主要参与胚胎发育和传导胞外信号。在纤毛生长的过程中,过渡区关键蛋白CEP290主要定位在中心体上,而CEP290表达或定位异常影响初级纤毛的结构和功能。本实验基于一种新型基因敲入技术——CRIS-PITCh系统,通过微同源介导末端连接方法,以实现将绿色荧光蛋白EGFP编码序列插入CEP290基因中,最终构建融合表达CEP290-EGFP蛋白的人视网膜色素上皮细胞系。因此,我们可以通过荧光成像直接观察内源CEP290的动态定位,为今后进一步研究CEP290的功能提供更好的工具和方法。     

The molecular mechanism of embryonic stem cell pluripotency maintenance

In vitro cultured embryonic stem （ES） cells are derived from the inner cell mass （ICM） of pre-implantation embryos, and are capable of giving rise to all cell and tissue types of the three germ layers upon being injected back into blastocysts. These ceils are therefore said to possess pluripotency that can be maintained infinitely in culture under optimal conditions. Such pluripotency maintenance is believed to be due to the symmetrical cleavage of the cells in an undifferentiated state. The pluripotency of ES cells is the basis for their various practical and potential applications. ES cells can be used as donor cells to generate knockout or transgenic animals, as in vitro models of mammalian development, and as cell resources for cell therapy in regenerative medicine. The further success in these applications, particularly in the last two, is dependent on the establishment of a culture system with components in the medium clearly defined and the subsequent procedures for controlled differentiation of the cells into specific lineages. In turn, elucidating the molecular mechanism for pluripotency maintenance of ES cells is the prerequisite. This paper summarizes the recent progresses in this area, focusing mainly on the LIF/STAT3, BMPs/Smads, canonical Wnt, TGFβ/activin/nodal, PI3K and FGF signaling pathways and the genes such as oct4, nanog that are crucial in ES cell pluripotency maintenance. The regulatory systems of pluripotency maintenance in both mouse and human ES cells are also discussed. We believe that the cross-talkings between these signaling pathways, as well as the regulatory system underlying pluripotency maintenance will be the main focus in the area of ES cell researches in the future.     

Hotspots of aberrant epigenomic reprogramming in human induced pluripotent stem cells

Induced pluripotent stem cells (iPSCs) offer immense potential for regenerative medicine and studies of disease and development. Somatic cell reprogramming involves epigenomic reconfiguration, conferring iPSCs with characteristics similar to embryonic stem (ES) cells. However, it remains unknown how complete the reestablishment of ES-cell-like DNA methylation patterns is throughout the genome. Here we report the first whole-genome profiles of DNA methylation at single-base resolution in five human iPSC lines, along with methylomes of ES cells, somatic cells, and differentiated iPSCs and ES cells. iPSCs show significant reprogramming variability, including somatic memory and aberrant reprogramming of DNA methylation. iPSCs share megabase-scale differentially methylated regions proximal to centromeres and telomeres that display incomplete reprogramming of non-CG methylation, and differences in CG methylation and histone modifications. Lastly, differentiation of iPSCs into trophoblast cells revealed that errors in reprogramming CG methylation are transmitted at a high frequency, providing an iPSC reprogramming signature that is maintained after differentiation.