世界生命科學前沿動態(tài)周報（八十一）

（6.24-6.30/2012）
美寶國際集團:陶國新 

　　主要內容：小分子DNA適體促進神經髓鞘的再生和修復；食物纖維改變胃腸道菌群促進胃腸健康；肌營養(yǎng)不良疾病LGMD2D的自體細胞移植；干細胞和癌細胞中端粒酶的調節(jié)；源于人胚胎干細胞的胰腺祖細胞發(fā)育成有功能的胰島治療老鼠的糖尿病；剝奪葡萄糖激活代謝和信號的放大回路導致細胞死亡。

　　焦點動態(tài)：小分子DNA適體促進神經髓鞘的再生和修復。

1.  小分子DNA適體促進神經髓鞘的再生和修復

【動態(tài)】多發(fā)性硬化病是一種中樞神經系統(tǒng)炎癥，會局部破壞包裹神經軸突的絕緣的髓鞘，使人體越來越虛弱。全世界有20多萬多發(fā)性硬化病人，而缺乏預防病情進展和誘導修復的治療手段使該病很是棘手?？寡庄煼ㄖ辉陬A防復發(fā)方面有有限效果。以前所做的人血清樣本的篩查顯示天然IgM抗體結合少突細胞，在腦脊髓炎泰勒病毒慢性感染的易感老鼠模型和局部脫髓鞘作用的溶血卵磷脂模型中促進細胞信號傳導并對中樞神經系統(tǒng)再髓鞘化。這個有趣的結果表明能夠特異結合少突細胞或髓鞘組分的分子可能會促進多發(fā)性硬化病的治療性髓鞘再生。鑒于IgM抗體太大太復雜，有必要尋找更小的能夠在體內促進髓鞘再生的髓鞘特異性分子。美國科學家最近的研究顯示篩選出的特異結合老鼠髓鞘的一種40個核苷的單鏈DNA適體具有這種特性。該DNA適體在體外能與髓鞘的多個組分結合，腹腔注射的該適體能夠分布到中樞神經組織并促進泰勒病毒感染的老鼠受損的模型中樞神經系統(tǒng)髓鞘再生。相對于單克隆抗體，DNA適體更小，更穩(wěn)定，沒有免疫原性，有望成為新的治療多發(fā)性硬化的手段。

【點評】 該研究發(fā)現(xiàn)的小分子DNA能夠替代IgM抗體在老鼠模型中促進神經髓鞘的再生和修復，有可能成為治療人類多發(fā)性硬化病的選項，但是還需要在其他的動物模型上確認其能否成為人體臨床實驗的候選物。

【參考論文】   
PLoS ONE, 2012; 7 (6): e39595 DOI: 10.1371/journal.pone.0039595
Remyelination Induced by a DNA Aptamer in a Mouse Model of Multiple Sclerosis
Branislav Nastasijevic, Brent R. Wright, John Smestad, et al.
Multiple sclerosis (MS) is a debilitating inflammatory disease of the central nervous system (CNS) characterized by local destruction of the insulating myelin surrounding neuronal axons. With more than 200 million MS patients worldwide, the absence of treatments that prevent progression or induce repair poses a major challenge. Anti-inflammatory therapies have met with limited success only in preventing relapses. Previous screening of human serum samples revealed natural IgM antibodies that bind oligodendrocytes and promote both cell signaling and remyelination of CNS lesions in an MS model involving chronic infection of susceptible mice by Theiler’s encephalomyelitis virus and in the lysolecithin model of focal demyelination. This intriguing result raises the possibility that molecules with binding specificity for oligodendrocytes or myelin components may promote therapeutic remyelination in MS. Because of the size and complexity of IgM antibodies, it is of interest to identify smaller myelin-specific molecules with the ability to promote remyelination in vivo. Here we show that a 40-nucleotide single-stranded DNA aptamer selected for affinity to murine myelin shows this property. This aptamer binds multiple myelin components in vitro. Peritoneal injection of this aptamer results in distribution to CNS tissues and promotes remyelination of CNS lesions in mice infected by Theiler’s virus. Interestingly, the selected DNA aptamer contains guanosine-rich sequences predicted to induce folding involving guanosine quartet structures. Relative to monoclonal antibodies, DNA aptamers are small, stable, and non-immunogenic, suggesting new possibilities for MS treatment.

 2. 食物纖維改變胃腸道菌群促進胃腸健康

【動態(tài)】美國科學家的最新研究表明食物纖維能夠促進胃腸道不同類型有益菌群的繁殖，而這些有益菌群據(jù)信不止能夠促進胃腸道的健康，還能夠影響我們對多種疾病的敏感性，比如2型糖尿病、肥胖、炎癥性腸病、結腸癌以及免疫性疾病像類風濕性關節(jié)炎。這些有益菌群在腸道發(fā)酵纖維，制造短鏈脂肪酸和其他代謝產物，對宿主產生諸多健康方面的益處。如果能夠了解哪種食物纖維能夠最好的滋養(yǎng)這些促進健康的菌群，就能夠糾正飲食和腸道菌群的不均衡，支持和促進胃腸道健康。該研究表明食物纖維除了促進胃腸蠕動，對人體還有更多好處。他們進行了有安慰劑對照的雙盲試驗，每天給與受試人員不同種類的補充纖維（聚葡萄糖或可溶性谷物纖維），在第16-21天，檢查其大便樣品中菌群的“指紋圖譜”。兩種纖維都在門種屬水平上影響菌群的數(shù)量。補充可溶性谷物纖維的人，其腸道益生菌乳酸桿菌的數(shù)量增加。補充聚葡萄糖或可溶性谷物纖維的人都能增加腸道有益菌群。因此，聚葡萄糖或可溶性谷物纖維有潛力作為益生元使用促進腸道益生菌的繁殖。

【點評】 該研究表明食物纖維不僅僅是幫助人體促進胃腸蠕動和食物消化，而且是人體很重要的營養(yǎng)物質，間接通過滋養(yǎng)腸道益生菌促進人體健康。

【參考論文】   
Journal of Nutrition, 2012; 142 (7): 1259 DOI: 10.3945/jn.112.158766
454 Pyrosequencing Reveals a Shift in Fecal Microbiota of Healthy Adult Men Consuming Polydextrose or Soluble Corn Fiber
S. Hooda, B. M. V. Boler, M. C. R. Serao, et al.
The relative contribution of novel fibers such as polydextrose and soluble corn fiber (SCF) to the human gut microbiome and its association with host physiology has not been well studied. This study was conducted to test the impact of polydextrose and SCF on the composition of the human gut microbiota using 454 pyrosequencing and to identify associations among fecal microbiota and fermentative end-products. Healthy adult men (n = 20) with a mean dietary fiber (DF) intake of 14 g/d were enrolled in a randomized, double-blind, placebo-controlled crossover study. Participants consumed 3 treatment snack bars/d during each 21-d period that contained no supplemental fiber (NFC), polydextrose (PDX; 21 g/d), or SCF (21 g/d) for 21 d. There were no washout periods. Fecal samples were collected on d 16–21 of each period; DNA was extracted, followed by amplification of the V4-V6 region of the 16S rRNA gene using barcoded primers. PDX and SCF significantly affected the relative abundance of bacteria at the class, genus, and species level. The consumption of PDX and SCF led to greater fecal Clostridiaceae and Veillonellaceae and lower Eubacteriaceae compared with a NFC. The abundance of Faecalibacterium,Phascolarctobacterium, and Dialister was greater (P < 0.05) in response to PDX and SCF intake, whereas Lactobacillus was greater (P < 0.05) only after SCF intake.Faecalibacterium prausnitzii, well known for its antiinflammatory properties, was greater (P < 0.05) after fiber consumption. Principal component analysis clearly indicated a distinct clustering of individuals consuming supplemental fibers. Our data demonstrate a beneficial shift in the gut microbiome of adults consuming PDX and SCF, with potential application as prebiotics.

 3.  肌營養(yǎng)不良疾病LGMD2D的自體細胞移植

【動態(tài)】一項歐洲和日本科學家參與的最新研究顯示源于一種罕見的肌營養(yǎng)不良癥患者的干細胞被成功移植入有同樣癥狀的老鼠模型中。  Mesoangioblast（一種與血管有關的干細胞）干/祖細胞是源于表達堿性磷酸酶的肌肉中發(fā)現(xiàn)的周皮細胞亞群，已顯示出在肌營養(yǎng)不良癥的不同動物模型中改善癥狀的作用，并正在患有杜興肌營養(yǎng)不良患兒中進行臨床測試?；加邢嚓P肌營養(yǎng)不良疾病LGMD2D的發(fā)病原因是編碼α-肌糖 的基因發(fā)生突變，周皮細胞減少，不能產生足夠自體細胞治療的Mesoangioblast。因此，該研究從LGMD2D患者的成纖維細胞和成肌細胞重編程成iPSC，并建立一套程序從iPSC 轉化成Mesoangioblast樣細胞，通過在體外基因修正并擴增該源于iPSC的Mesoangioblast細胞，然后將其移植到沒有α-肌糖的免疫缺陷老鼠，它們產生了表達α-肌糖的肌纖維。而將源于老鼠iPSC的Mesoangioblast細胞移植到沒有α-肌糖的免疫缺陷老鼠改善了癥狀，恢復了缺失的祖細胞。

【點評】 該研究結果意味著移植基因修正后的源于LGMD2D患者iPSC的Mesoangioblast樣細胞可能對該種以及其他種類的肌營養(yǎng)不良的治療有用。

【參考論文】   
Sci Transl Med, 27 June 2012 DOI: 10.1126/scitranslmed.3003541
Transplantation of Genetically Corrected Human iPSC-Derived Progenitors in Mice with Limb-Girdle Muscular Dystrophy
Francesco Saverio Tedesco, Mattia F. M. Gerli, Laura Perani, et al.  
Mesoangioblasts are stem/progenitor cells derived from a subset of pericytes found in muscle that express alkaline phosphatase. They have been shown to ameliorate the disease phenotypes of different animal models of muscular dystrophy and are now undergoing clinical testing in children affected by Duchenne’s muscular dystrophy. Here, we show that patients with a related disease, limb-girdle muscular dystrophy 2D (LGMD2D), which is caused by mutations in the gene encoding α-sarcoglycan, have reduced numbers of this pericyte subset and thus produce too few mesoangioblasts for use in autologous cell therapy. Hence, we reprogrammed fibroblasts and myoblasts from LGMD2D patients to generate human induced pluripotent stem cells (iPSCs) and developed a protocol for the derivation of mesoangioblast-like cells from these iPSCs. The iPSC-derived mesoangioblasts were expanded and genetically corrected in vitro with a lentiviral vector carrying the gene encoding human α-sarcoglycan and a promoter that would ensure expression only in striated muscle. When these genetically corrected human iPSC-derived mesoangioblasts were transplanted into α-sarcoglycan–null immunodeficient mice, they generated muscle fibers that expressed α-sarcoglycan. Finally, transplantation of mouse iPSC-derived mesoangioblasts into α-sarcoglycan–null immunodeficient mice resulted in functional amelioration of the dystrophic phenotype and restoration of the depleted progenitors. These findings suggest that transplantation of genetically corrected mesoangioblast-like cells generated from iPSCs from LGMD2D patients may be useful for treating this type of muscular dystrophy and perhaps other forms of muscular dystrophy as well.

 4. 干細胞和癌細胞中端粒酶的調節(jié)

【動態(tài)】德國科學家最近發(fā)現(xiàn)了Wnt/β-聯(lián)蛋白信號途徑與端粒酶亞基 Tert表達之間的分子聯(lián)系。端粒酶的活性控制著端粒長度，在干細胞，衰老和癌癥中起關鍵作用。β-聯(lián)蛋白缺陷的老鼠胚胎干細胞端粒較短，相反地，表達有活性的β-聯(lián)蛋白的胚胎干細胞其端粒比較長。研究表明β-聯(lián)蛋白通過與多能性轉錄網絡的核心組分Klf4相互作用調節(jié)Tert表達，在老鼠腸癌模型和人體癌細胞中β-聯(lián)蛋白結合到Tert促進子，直接調節(jié)端粒酶基因。調節(jié)錯誤或β-聯(lián)蛋白突變會導致腫瘤發(fā)生。

【點評】 該研究關于β-聯(lián)蛋白調節(jié)Tert表達進而調節(jié)端粒酶活性和端粒長度的發(fā)現(xiàn)對于干細胞的研究以及抗癌研究可能產生重要影響。

【參考論文】   
Science, 2012; 336 (6088): 1549 DOI: 10.1126/science.1218370
Wnt/ -Catenin Signaling Regulates Telomerase in Stem Cells and Cancer Cells
K. Hoffmeyer, A. Raggioli, S. Rudloff, et al.  
Telomerase activity controls telomere length and plays a pivotal role in stem cells, aging, and cancer. Here, we report a molecular link between Wnt/β-catenin signaling and the expression of the telomerase subunit Tert. β-Catenin–deficient mouse embryonic stem (ES) cells have short telomeres; conversely, ES cell expressing an activated form of β-catenin (β-catΔEx3/+) have long telomeres. We show that β-catenin regulates Tert expression through the interaction with Klf4, a core component of the pluripotency transcriptional network. β-Catenin binds to the Tert promoter in a mouse intestinal tumor model and in human carcinoma cells. We uncover a previously unknown link between the stem cell and oncogenic potential whereby β-catenin regulates Tert expression, and thereby telomere length, which could be critical in human regenerative therapy and cancer.

 5. 源于人胚胎干細胞的胰腺祖細胞發(fā)育成有功能的胰島治療老鼠的糖尿病

【動態(tài)】加拿大科學家最近報道了一套方案，可以使商業(yè)化的人體胚胎干細胞體外分化成富集的PDX1+胰腺祖細胞群，進而在體內進一步發(fā)育成熟為胰腺分泌細胞。未成熟的胰腺前體細胞被移植到鏈脲霉素誘導的糖尿病免疫缺陷老鼠，而最初通過外源性胰島素控制高血糖癥狀，隨著移植細胞產生的胰島素隨時間的增加，糖尿病老鼠逐漸可以脫離外源性胰島素，人C-肽的分泌最終可以通過飲食和葡萄糖耐受調節(jié)。在免疫缺陷的大白鼠中也觀察到了類似的胰腺祖細胞的分化，體內源于人體胚胎干細胞的分泌細胞的成熟過程中呈現(xiàn)的基因和蛋白表達明顯類似于人體胚胎發(fā)育中的胰腺。

【點評】 該研究發(fā)現(xiàn)的結果支持利用分化的人體胚胎干細胞治療糖尿病的可能性，但是因為此類的動物試驗都是利用不大排斥細胞移植的免疫缺陷動物模型進行的，明顯不同于正常的生命，因此其試驗結果也只是提供某種可能性的參考，并不具備實際意義。

【參考論文】   
Diabetes, June 27, 2012 DOI:10.2337/db11-1711
Maturation of Human Embryonic Stem Cell–Derived Pancreatic Progenitors into Functional Islets Capable of Treating Pre-existing Diabetes in Mice
Alireza Rezania, Jennifer E. Bruin, Michael J. Riedel, et al.
Diabetes is a chronic debilitating disease that results from insufficient production of insulin from pancreatic β-cells. Islet cell replacement can effectively treat diabetes but is currently severely limited by the reliance upon cadaveric donor tissue. We have developed a protocol to efficiently differentiate commercially available human embryonic stem cells (hESCs) in vitro into a highly enriched PDX1+ pancreatic progenitor cell population that further develops in vivo to mature pancreatic endocrine cells. Immature pancreatic precursor cells were transplanted into immunodeficient mice with streptozotocin-induced diabetes, and glycemia was initially controlled with exogenous insulin. As graft-derived insulin levels increased over time, diabetic mice were weaned from exogenous insulin and human C-peptide secretion was eventually regulated by meal and glucose challenges. Similar differentiation of pancreatic precursor cells was observed after transplant in immunodeficient rats. Throughout the in vivo maturation period hESC-derived endocrine cells exhibited gene and protein expression profiles that were remarkably similar to the developing human fetal pancreas. Our findings support the feasibility of using differentiated hESCs as an alternative to cadaveric islets for treating patients with diabetes.

 6. 剝奪葡萄糖激活代謝和信號的放大回路導致細胞死亡

【動態(tài)】癌細胞變異的代謝讓細胞生存依賴于代謝底物的存在。美國科學家通過研究依賴葡萄糖生存的細胞死亡的信號機制闡明迅速剝奪葡萄糖供應導致超正常生理水平的磷酸化酪氨酸信號，甚至是在表達結構上活躍的酪氨酸激酶的細胞里。通過客觀的基于質譜的磷酸化蛋白譜，該研究發(fā)現(xiàn)剝奪葡萄糖激活啟動了與局部連接有關的獨特的磷酸化酪氨酸簽名，激活了涉及NADPH和線粒體生產活性氧，氧化抑制酪氨酸蛋白磷酸化酶，以及增加酪氨酸激酶信號的正反饋回路，最終導致活性氧介導的細胞死亡。

【點評】 該研究說明了癌細胞維持動態(tài)平衡過程中代謝和信號在系統(tǒng)水平上的交流。剝奪葡萄糖誘生活性氧，抑制磷酸化酶，激活酪氨酸激酶轉而產生更多活性氧，正反饋的回路不斷放大活性氧水平直至細胞進行活性氧介導的細胞死亡。

【參考論文】   
Molecular Systems Biology, 2012; 8 DOI: 10.1038/msb.2012.20
Glucose deprivation activates a metabolic and signaling amplification loop leading to cell death
Nicholas A Graham, Martik Tahmasian, Bitika Kohli, et al.    
The altered metabolism of cancer can render cells dependent on the availability of metabolic substrates for viability. Investigating the signaling mechanisms underlying cell death in cells dependent upon glucose for survival, we demonstrate that glucose withdrawal rapidly induces supra-physiological levels of phospho-tyrosine signaling, even in cells expressing constitutively active tyrosine kinases. Using unbiased mass spectrometry-based phospho-proteomics, we show that glucose withdrawal initiates a unique signature of phospho-tyrosine activation that is associated with focal adhesions. Building upon this observation, we demonstrate that glucose withdrawal activates a positive feedback loop involving generation of reactive oxygen species (ROS) by NADPH oxidase and mitochondria, inhibition of protein tyrosine phosphatases by oxidation, and increased tyrosine kinase signaling. In cells dependent on glucose for survival, glucose withdrawal-induced ROS generation and tyrosine kinase signaling synergize to amplify ROS levels, ultimately resulting in ROS-mediated cell death. Taken together, these findings illustrate the systems-level cross-talk between metabolism and signaling in the maintenance of cancer cell homeostasis.