世界生命科學前沿動態(tài)周報（四十八）

【點評】
    這一研究在老鼠身上看到了后天營養(yǎng)狀況通過表觀遺傳修飾影響了個體胎兒的基因表達，改變了成年個體的代謝狀態(tài)。

【參考論文】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. 新化合物通過阻斷對高氧化壓力的適應來選擇性殺死癌細胞
【動態(tài)】
　　激活癌基因的突變和失活腫瘤抑制基因的突變所驅動的細胞惡變導致常常與升高的細胞壓力（如氧化、復制、代謝和蛋白毒性壓力、DNA損傷）有關的細胞失調。為了生存，癌細胞必須適應這種壓力，結果癌細胞可能變得依賴那些平常在正常細胞中不起如此關鍵作用的非癌基因。因此，在表型變換時針對這類非癌基因依賴性可能造成人為的致命相互作用和選擇性癌細胞死亡。利用基于細胞的小分子篩選和定量的蛋白組學方法客觀地識別出能夠選擇性殺死癌細胞而非正常細胞的小分子。不管p53是何狀況，來自胡椒的天然產物蓽茇酰胺在癌細胞和設計帶有癌基因型的普通細胞中都能增加活性氧自由基水平和細胞凋亡，但它對快分裂或者慢分裂的基本正常細胞作用很小。在異種移植腫瘤模型老鼠中觀察到蓽茇酰胺明顯的抗腫瘤作用，而對正常老鼠無明顯毒性。而且，蓽茇酰胺強力抑制老鼠自發(fā)形成的惡性乳腺腫瘤的生長和轉移。該研究結果表明一種小分子能選擇性地在有癌基因型的細胞中通過靶向在癌基因表達時產生的應對轉變引起的氧化壓力的非癌基因依賴性來誘導該細胞凋亡。

【點評】
　選擇性阻斷癌細胞對環(huán)境的適應從而讓環(huán)境因素引起癌細胞凋亡。該研究中的蓽茇酰胺對處于高氧化壓力下的癌細胞中的抗氧化酶在其濃度超出正常細胞中的水平時有抑制作用，從而選擇性地誘導癌細胞在氧化壓力下凋亡。這一點對研究美寶再生物質的抗癌作用機理有啟示。

【參考論文】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. 誘導多能干細胞的表觀遺傳記憶
【動態(tài)】  
    人誘導多能干細胞看似非常像人胚胎干細胞。以色列科學家用兩種遺傳譜系追蹤系統(tǒng)證明了從人胰島β細胞生成誘導多能干細胞系。這些重組的細胞獲得了多能干細胞的標記并分化成三胚層。然而，從β細胞而來的誘導多能干細胞(BiPSCs)在關鍵β細胞基因上維持開放的染色質結構，同時有一個獨特的DNA甲基化簽名以區(qū)別于其他多能干細胞。與胚胎干細胞和同基因的非β細胞來的iPSCs 相比，BiPSCs也顯示了更高的體內體外分化成生產胰島素的細胞的能力。其研究結果提示表觀遺傳記憶可能使BiPSCs更傾向于分化成生產胰島素的細胞。這些發(fā)現(xiàn)證明人誘導多能干細胞的表現(xiàn)型可能受其來源細胞的影響，也提示其受影響的分化潛力可能有利于細胞替代治療。

【點評】
    該研究的誘導多能干細胞能夠傾向性地分化成其來源細胞，看似技術進步，實則無奈之舉，因為人們無法駕馭誘導多能干細胞，不知道如何創(chuàng)造生命的條件使誘導多能干細胞能夠正常分化成任何需要的細胞。此外，誘導多能干細胞基因組的改變使其再也不是正常細胞，成為人造細胞。

【參考論文】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指導人纖維母細胞轉變成神經元
【動態(tài)】
    神經元的轉錄因子和進化上保守的信號通路被發(fā)現(xiàn)是有助于神經元的形成。然而，小RNA在神經新生中的指導作用還不清楚。美國科學家最近發(fā)現(xiàn)miR-9* 和miR-124指導SWI/SNF-like BAF 染色質重塑復合體組分的變化，該過程對神經元分化和功能很重要。在靠近神經元祖細胞有絲分裂出口時，miR-9* 和miR-124抑制神經元祖細胞BAF 染色質重塑復合體的BAF53a 亞基。在有絲分裂出口之后，BAF53a 被BAF53b取代，BAF45a被 BAF45b 和 BAF45c取代，后者合并成為神經元特異的BAF復合體為有絲分裂后的功能所必需。由于miR-9* 和miR-124也控制多個調節(jié)神經元分化和功能的基因，他們認為這些小RNA可能與神經元命運有關。他們的研究顯示人纖維母細胞中miR-9/9* 和miR-124的表達誘導其轉變成神經元，該過程受NEUROD2的促進。進一步加入神經元的轉錄因子ASCL1 和 MYT1L提高了轉化率和轉化成的神經元的成熟，這其中只表達這些轉錄因子不表達miR-9/9*-124 的話是沒有效果的。該研究暗示涉及miR-9/9*-124的遺傳線路對神經元命運決定有指導作用。

【點評】
    小RNA指導人纖維母細胞轉變成神經元，揭示了表觀調控對細胞分化的控制。表觀調控的研究將是比基因組研究更有實用價值的課題。

【參考論文】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. 多能神經干細胞生成大腦中的神經元和膠質細胞
【動態(tài)】
    神經元生成和膠質生成在成體哺乳動物大腦的分立的區(qū)域持續(xù)進行。一個尚未解決的基本問題是細胞生成是來自譜系限制的祖細胞還是來自成體大腦中自我更新的多能神經干細胞。美國科學家的最新研究采用遺傳標記策略在成年老鼠齒狀回中追蹤單個的、靜止的、表達巢蛋白的放射狀膠質樣前體細胞（RGL）?？寺》治霭l(fā)現(xiàn)多種激活RGL的方式，包括不對稱和對稱自我更新。體內的長期譜系追蹤顯示有明顯比例的克隆包含RGLs、神經元和星形膠質細胞，表明單個RGL能夠自我更新同時進行多譜系分化。此外，在RGL中有條件地去除Pten一開始促進其激活和對稱的自我更新，但最終導致星形膠質細胞的終極分化和成體海馬區(qū)RGL的耗竭。該研究鑒定RGL為自我更新的多能神經干細胞，提供了體內成體神經干細胞特性的新見解。

【點評】
    體內的克隆分析揭示自我更新和成體多能神經干細胞特性，對于神經新生過程有了進一步了解。