世界生命科学前沿动态周报（四十八）

【点评】
    这一研究在老鼠身上看到了后天营养状况通过表观遗传修饰影响了个体胎儿的基因表达，改变了成年个体的代谢状态。

【参考论文】FASEB Journal, June 13, 2011 fj.11-181792; published ahead of print
Perinatal Undernutrition Affects the Methylation and Expression of the Leptin Gene in Adults: Implication for the Understanding of Metabolic Syndrome
C Jousse, L Parry, S Lambert-Langlais, et al.
Transient environmental influences, such as perinatal nutritional stress, may induce deleterious metabolic symptoms that last for the entire life of individuals, implying that epigenetic modifications play an important role in this process. We have investigated, in mice, the consequences of maternal undernutrition during gestation and lactation on DNA methylation and expression of the leptin gene, which plays a major regulatory role in coordinating nutritional state with many aspects of mammalian biology. We show that animals born to mothers fed a low-protein-diet (F1-LPD group) have a lower body weight/adiposity and exhibit a higher food intake than animals born to mothers fed a control diet (F1-CD group). These modifications persisted throughout life and were associated with lower levels of leptin mRNA and protein in starved F1-LPD mice, emphasizing that maternal protein-undernutrition affects the balance between food intake and energy expenditure in adults. Moreover, this nutritional stress resulted in the removal of methyls at CpGs located in the promoter of leptin, causing a permanent specific modification in the dynamics of the expression of leptin, which exhibits a stronger induction in the F1-LPD than in F1-CD mice in response to a meal. This study is an example of a molecular rationale linking transient environmental influences to permanent phenotypic consequences.

3. 新化合物通过阻断对高氧化压力的适应来选择性杀死癌细胞
【动态】
　　激活癌基因的突变和失活肿瘤抑制基因的突变所驱动的细胞恶变导致常常与升高的细胞压力（如氧化、复制、代谢和蛋白毒性压力、DNA损伤）有关的细胞失调。为了生存，癌细胞必须适应这种压力，结果癌细胞可能变得依赖那些平常在正常细胞中不起如此关键作用的非癌基因。因此，在表型变换时针对这类非癌基因依赖性可能造成人为的致命相互作用和选择性癌细胞死亡。利用基于细胞的小分子筛选和定量的蛋白组学方法客观地识别出能够选择性杀死癌细胞而非正常细胞的小分子。不管p53是何状况，来自胡椒的天然产物荜茇酰胺在癌细胞和设计带有癌基因型的普通细胞中都能增加活性氧自由基水平和细胞凋亡，但它对快分裂或者慢分裂的基本正常细胞作用很小。在异种移植肿瘤模型老鼠中观察到荜茇酰胺明显的抗肿瘤作用，而对正常老鼠无明显毒性。而且，荜茇酰胺强力抑制老鼠自发形成的恶性乳腺肿瘤的生长和转移。该研究结果表明一种小分子能选择性地在有癌基因型的细胞中通过靶向在癌基因表达时产生的应对转变引起的氧化压力的非癌基因依赖性来诱导该细胞凋亡。

【点评】
　选择性阻断癌细胞对环境的适应从而让环境因素引起癌细胞凋亡。该研究中的荜茇酰胺对处于高氧化压力下的癌细胞中的抗氧化酶在其浓度超出正常细胞中的水平时有抑制作用，从而选择性地诱导癌细胞在氧化压力下凋亡。这一点对研究美宝再生物质的抗癌作用机理有启示。

【参考论文】Nature, 2011; 475 (7355): 231 DOI: 10.1038/nature10167
Selective killing of cancer cells by a small molecule targeting the stress response to ROS
Lakshmi Raj, Takao Ide, Aditi U. Gurkar, et al.
Malignant transformation, driven by gain-of-function mutations in oncogenes and loss-of-function mutations in tumour suppressor genes, results in cell deregulation that is frequently associated with enhanced cellular stress (for example, oxidative, replicative, metabolic and proteotoxic stress, and DNA damage). Adaptation to this stress phenotype is required for cancer cells to survive, and consequently cancer cells may become dependent upon non-oncogenes that do not ordinarily perform such a vital function in normal cells. Thus, targeting these non-oncogene dependencies in the context of a transformed genotype may result in a synthetic lethal interaction and the selective death of cancer cells. Here we used a cell-based small-molecule screening and quantitative proteomics approach that resulted in the unbiased identification of a small molecule that selectively kills cancer cells but not normal cells. Piperlongumine increases the level of reactive oxygen species (ROS) and apoptotic cell death in both cancer cells and normal cells engineered to have a cancer genotype, irrespective of p53 status, but it has little effect on either rapidly or slowly dividing primary normal cells. Significant antitumour effects are observed in piperlongumine-treated mouse xenograft tumour models, with no apparent toxicity in normal mice. Moreover, piperlongumine potently inhibits the growth of spontaneously formed malignant breast tumours and their associated metastases in mice. Our results demonstrate the ability of a small molecule to induce apoptosis selectively in cells that have a cancer genotype, by targeting a non-oncogene co-dependency acquired through the expression of the cancer genotype in response to transformation-induced oxidative stress.

4. 诱导多能干细胞的表观遗传记忆
【动态】  
    人诱导多能干细胞看似非常像人胚胎干细胞。以色列科学家用两种遗传谱系追踪系统证明了从人胰岛β细胞生成诱导多能干细胞系。这些重组的细胞获得了多能干细胞的标记并分化成三胚层。然而，从β细胞而来的诱导多能干细胞(BiPSCs)在关键β细胞基因上维持开放的染色质结构，同时有一个独特的DNA甲基化签名以区别于其他多能干细胞。与胚胎干细胞和同基因的非β细胞来的iPSCs 相比，BiPSCs也显示了更高的体内体外分化成生产胰岛素的细胞的能力。其研究结果提示表观遗传记忆可能使BiPSCs更倾向于分化成生产胰岛素的细胞。这些发现证明人诱导多能干细胞的表现型可能受其来源细胞的影响，也提示其受影响的分化潜力可能有利于细胞替代治疗。

【点评】
    该研究的诱导多能干细胞能够倾向性地分化成其来源细胞，看似技术进步，实则无奈之举，因为人们无法驾驭诱导多能干细胞，不知道如何创造生命的条件使诱导多能干细胞能够正常分化成任何需要的细胞。此外，诱导多能干细胞基因组的改变使其再也不是正常细胞，成为人造细胞。

【参考论文】Cell Stem Cell, 2011; 9 (1): 17 DOI:10.1016/j.stem.2011.06.007
Epigenetic Memory and Preferential Lineage-Specific Differentiation in Induced Pluripotent Stem Cells Derived from Human Pancreatic Islet Beta Cells
Ori Bar-Nur, Holger A. Russ, Shimon Efrat, Nissim Benvenisty.
Human induced pluripotent stem cells (HiPSCs) appear to be highly similar to human embryonic stem cells (HESCs). Using two genetic lineage-tracing systems, we demonstrate the generation of iPSC lines from human pancreatic islet beta cells. These reprogrammed cells acquired markers of pluripotent cells and differentiated into the three embryonic germ layers. However, the beta cell-derived iPSCs (BiPSCs) maintained open chromatin structure at key beta-cell genes, together with a unique DNA methylation signature that distinguishes them from other PSCs. BiPSCs also demonstrated an increased ability to differentiate into insulin-producing cells both in vitro and in vivo, compared with ESCs and isogenic non-beta iPSCs. Our results suggest that the epigenetic memory may predispose BiPSCs to differentiate more readily into insulin producing cells. These findings demonstrate that HiPSC phenotype may be influenced by their cells of origin, and suggest that their skewed differentiation potential may be advantageous for cell replacement therapy.

5. 小RNA指导人纤维母细胞转变成神经元
【动态】
    神经元的转录因子和进化上保守的信号通路被发现是有助于神经元的形成。然而，小RNA在神经新生中的指导作用还不清楚。美国科学家最近发现miR-9* 和miR-124指导SWI/SNF-like BAF 染色质重塑复合体组分的变化，该过程对神经元分化和功能很重要。在靠近神经元祖细胞有丝分裂出口时，miR-9* 和miR-124抑制神经元祖细胞BAF 染色质重塑复合体的BAF53a 亚基。在有丝分裂出口之后，BAF53a 被BAF53b取代，BAF45a被 BAF45b 和 BAF45c取代，后者合并成为神经元特异的BAF复合体为有丝分裂后的功能所必需。由于miR-9* 和miR-124也控制多个调节神经元分化和功能的基因，他们认为这些小RNA可能与神经元命运有关。他们的研究显示人纤维母细胞中miR-9/9* 和miR-124的表达诱导其转变成神经元，该过程受NEUROD2的促进。进一步加入神经元的转录因子ASCL1 和 MYT1L提高了转化率和转化成的神经元的成熟，这其中只表达这些转录因子不表达miR-9/9*-124 的话是没有效果的。该研究暗示涉及miR-9/9*-124的遗传线路对神经元命运决定有指导作用。

【点评】
    小RNA指导人纤维母细胞转变成神经元，揭示了表观调控对细胞分化的控制。表观调控的研究将是比基因组研究更有实用价值的课题。

【参考论文】Nature, 2011; DOI:10.1038/nature10323
MicroRNA-mediated conversion of human fibroblasts to neurons
Andrew S. Yoo, Alfred X. Sun, Li Li, et al.
Neurogenic transcription factors and evolutionarily conserved signalling pathways have been found to be instrumental in the formation of neurons. However, the instructive role of microRNAs (miRNAs) in neurogenesis remains unexplored. We recently discovered that miR-9* and miR-124 instruct compositional changes of SWI/SNF-like BAF chromatin-remodelling complexes, a process important for neuronal differentiation and function. Nearing mitotic exit of neural progenitors, miR-9* and miR-124 repress the BAF53a subunit of the neural-progenitor (np)BAF chromatin-remodelling complex. After mitotic exit, BAF53a is replaced by BAF53b, and BAF45a by BAF45b and BAF45c, which are then incorporated into neuron-specific (n)BAF complexes essential for post-mitotic functions. Because miR-9/9* and miR-124 also control multiple genes regulating neuronal differentiation and function, we proposed that these miRNAs might contribute to neuronal fates. Here we show that expression of miR-9/9* and miR-124 (miR-9/9*-124) in human fibroblasts induces their conversion into neurons, a process facilitated by NEUROD2. Further addition of neurogenic transcription factors ASCL1 and MYT1L enhances the rate of conversion and the maturation of the converted neurons, whereas expression of these transcription factors alone without miR-9/9*-124 was ineffective. These studies indicate that the genetic circuitry involving miR-9/9*-124 can have an instructive role in neural fate determination.

6. 多能神经干细胞生成大脑中的神经元和胶质细胞
【动态】
    神经元生成和胶质生成在成体哺乳动物大脑的分立的区域持续进行。一个尚未解决的基本问题是细胞生成是来自谱系限制的祖细胞还是来自成体大脑中自我更新的多能神经干细胞。美国科学家的最新研究采用遗传标记策略在成年老鼠齿状回中追踪单个的、静止的、表达巢蛋白的放射状胶质样前体细胞（RGL）。克隆分析发现多种激活RGL的方式，包括不对称和对称自我更新。体内的长期谱系追踪显示有明显比例的克隆包含RGLs、神经元和星形胶质细胞，表明单个RGL能够自我更新同时进行多谱系分化。此外，在RGL中有条件地去除Pten一开始促进其激活和对称的自我更新，但最终导致星形胶质细胞的终极分化和成体海马区RGL的耗竭。该研究鉴定RGL为自我更新的多能神经干细胞，提供了体内成体神经干细胞特性的新见解。

【点评】
    体内的克隆分析揭示自我更新和成体多能神经干细胞特性，对于神经新生过程有了进一步了解。