世界生命科学前沿动态周报（八十一）

（6.24-6.30/2012）
美宝国际集团:陶国新 

　　主要内容：小分子DNA适体促进神经髓鞘的再生和修复；食物纤维改变胃肠道菌群促进胃肠健康；肌营养不良疾病LGMD2D的自体细胞移植；干细胞和癌细胞中端粒酶的调节；源于人胚胎干细胞的胰腺祖细胞发育成有功能的胰岛治疗老鼠的糖尿病；剥夺葡萄糖激活代谢和信号的放大回路导致细胞死亡。

　　焦点动态：小分子DNA适体促进神经髓鞘的再生和修复。

1.  小分子DNA适体促进神经髓鞘的再生和修复

【动态】多发性硬化病是一种中枢神经系统炎症，会局部破坏包裹神经轴突的绝缘的髓鞘，使人体越来越虚弱。全世界有20多万多发性硬化病人，而缺乏预防病情进展和诱导修复的治疗手段使该病很是棘手。抗炎疗法只在预防复发方面有有限效果。以前所做的人血清样本的筛查显示天然IgM抗体结合少突细胞，在脑脊髓炎泰勒病毒慢性感染的易感老鼠模型和局部脱髓鞘作用的溶血卵磷脂模型中促进细胞信号传导并对中枢神经系统再髓鞘化。这个有趣的结果表明能够特异结合少突细胞或髓鞘组分的分子可能会促进多发性硬化病的治疗性髓鞘再生。鉴于IgM抗体太大太复杂，有必要寻找更小的能够在体内促进髓鞘再生的髓鞘特异性分子。美国科学家最近的研究显示筛选出的特异结合老鼠髓鞘的一种40个核苷的单链DNA适体具有这种特性。该DNA适体在体外能与髓鞘的多个组分结合，腹腔注射的该适体能够分布到中枢神经组织并促进泰勒病毒感染的老鼠受损的模型中枢神经系统髓鞘再生。相对于单克隆抗体，DNA适体更小，更稳定，没有免疫原性，有望成为新的治疗多发性硬化的手段。

【点评】 该研究发现的小分子DNA能够替代IgM抗体在老鼠模型中促进神经髓鞘的再生和修复，有可能成为治疗人类多发性硬化病的选项，但是还需要在其他的动物模型上确认其能否成为人体临床实验的候选物。

【参考论文】   
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. 食物纤维改变胃肠道菌群促进胃肠健康

【动态】美国科学家的最新研究表明食物纤维能够促进胃肠道不同类型有益菌群的繁殖，而这些有益菌群据信不止能够促进胃肠道的健康，还能够影响我们对多种疾病的敏感性，比如2型糖尿病、肥胖、炎症性肠病、结肠癌以及免疫性疾病像类风湿性关节炎。这些有益菌群在肠道发酵纤维，制造短链脂肪酸和其他代谢产物，对宿主产生诸多健康方面的益处。如果能够了解哪种食物纤维能够最好的滋养这些促进健康的菌群，就能够纠正饮食和肠道菌群的不均衡，支持和促进胃肠道健康。该研究表明食物纤维除了促进胃肠蠕动，对人体还有更多好处。他们进行了有安慰剂对照的双盲试验，每天给与受试人员不同种类的补充纤维（聚葡萄糖或可溶性谷物纤维），在第16-21天，检查其大便样品中菌群的“指纹图谱”。两种纤维都在门种属水平上影响菌群的数量。补充可溶性谷物纤维的人，其肠道益生菌乳酸杆菌的数量增加。补充聚葡萄糖或可溶性谷物纤维的人都能增加肠道有益菌群。因此，聚葡萄糖或可溶性谷物纤维有潜力作为益生元使用促进肠道益生菌的繁殖。

【点评】 该研究表明食物纤维不仅仅是帮助人体促进胃肠蠕动和食物消化，而且是人体很重要的营养物质，间接通过滋养肠道益生菌促进人体健康。

【参考论文】   
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.  肌营养不良疾病LGMD2D的自体细胞移植

【动态】一项欧洲和日本科学家参与的最新研究显示源于一种罕见的肌营养不良症患者的干细胞被成功移植入有同样症状的老鼠模型中。  Mesoangioblast（一种与血管有关的干细胞）干/祖细胞是源于表达碱性磷酸酶的肌肉中发现的周皮细胞亚群，已显示出在肌营养不良症的不同动物模型中改善症状的作用，并正在患有杜兴肌营养不良患儿中进行临床测试。患有相关肌营养不良疾病LGMD2D的发病原因是编码α-肌糖 的基因发生突变，周皮细胞减少，不能产生足够自体细胞治疗的Mesoangioblast。因此，该研究从LGMD2D患者的成纤维细胞和成肌细胞重编程成iPSC，并建立一套程序从iPSC 转化成Mesoangioblast样细胞，通过在体外基因修正并扩增该源于iPSC的Mesoangioblast细胞，然后将其移植到没有α-肌糖的免疫缺陷老鼠，它们产生了表达α-肌糖的肌纤维。而将源于老鼠iPSC的Mesoangioblast细胞移植到没有α-肌糖的免疫缺陷老鼠改善了症状，恢复了缺失的祖细胞。

【点评】 该研究结果意味着移植基因修正后的源于LGMD2D患者iPSC的Mesoangioblast样细胞可能对该种以及其他种类的肌营养不良的治疗有用。

【参考论文】   
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. 干细胞和癌细胞中端粒酶的调节

【动态】德国科学家最近发现了Wnt/β-联蛋白信号途径与端粒酶亚基 Tert表达之间的分子联系。端粒酶的活性控制着端粒长度，在干细胞，衰老和癌症中起关键作用。β-联蛋白缺陷的老鼠胚胎干细胞端粒较短，相反地，表达有活性的β-联蛋白的胚胎干细胞其端粒比较长。研究表明β-联蛋白通过与多能性转录网络的核心组分Klf4相互作用调节Tert表达，在老鼠肠癌模型和人体癌细胞中β-联蛋白结合到Tert促进子，直接调节端粒酶基因。调节错误或β-联蛋白突变会导致肿瘤发生。

【点评】 该研究关于β-联蛋白调节Tert表达进而调节端粒酶活性和端粒长度的发现对于干细胞的研究以及抗癌研究可能产生重要影响。

【参考论文】   
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. 源于人胚胎干细胞的胰腺祖细胞发育成有功能的胰岛治疗老鼠的糖尿病

【动态】加拿大科学家最近报道了一套方案，可以使商业化的人体胚胎干细胞体外分化成富集的PDX1+胰腺祖细胞群，进而在体内进一步发育成熟为胰腺分泌细胞。未成熟的胰腺前体细胞被移植到链脲霉素诱导的糖尿病免疫缺陷老鼠，而最初通过外源性胰岛素控制高血糖症状，随着移植细胞产生的胰岛素随时间的增加，糖尿病老鼠逐渐可以脱离外源性胰岛素，人C-肽的分泌最终可以通过饮食和葡萄糖耐受调节。在免疫缺陷的大白鼠中也观察到了类似的胰腺祖细胞的分化，体内源于人体胚胎干细胞的分泌细胞的成熟过程中呈现的基因和蛋白表达明显类似于人体胚胎发育中的胰腺。

【点评】 该研究发现的结果支持利用分化的人体胚胎干细胞治疗糖尿病的可能性，但是因为此类的动物试验都是利用不大排斥细胞移植的免疫缺陷动物模型进行的，明显不同于正常的生命，因此其试验结果也只是提供某种可能性的参考，并不具备实际意义。

【参考论文】   
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. 剥夺葡萄糖激活代谢和信号的放大回路导致细胞死亡

【动态】癌细胞变异的代谢让细胞生存依赖于代谢底物的存在。美国科学家通过研究依赖葡萄糖生存的细胞死亡的信号机制阐明迅速剥夺葡萄糖供应导致超正常生理水平的磷酸化酪氨酸信号，甚至是在表达结构上活跃的酪氨酸激酶的细胞里。通过客观的基于质谱的磷酸化蛋白谱，该研究发现剥夺葡萄糖激活启动了与局部连接有关的独特的磷酸化酪氨酸签名，激活了涉及NADPH和线粒体生产活性氧，氧化抑制酪氨酸蛋白磷酸化酶，以及增加酪氨酸激酶信号的正反馈回路，最终导致活性氧介导的细胞死亡。

【点评】 该研究说明了癌细胞维持动态平衡过程中代谢和信号在系统水平上的交流。剥夺葡萄糖诱生活性氧，抑制磷酸化酶，激活酪氨酸激酶转而产生更多活性氧，正反馈的回路不断放大活性氧水平直至细胞进行活性氧介导的细胞死亡。

【参考论文】   
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.