世界生命科学前沿动态周报（五十七）

（9.12-9.18/2011）
美宝国际集团:陶国新 

　　主要内容：胰腺β细胞富含的Bace2酶调节β细胞的功能和数量；人体内多功能溶瘤细胞痘病毒靶向癌细胞的静脉输送；炎症激活蛋白维持血糖正常水平；Ccl2与LIF合作激活Stat3途径来维持ESCs/iPSCs的多能性；通过葡萄糖代谢控制胰腺β细胞再生；选择性剪接调控胚胎干细胞多能性。

　　焦点动态：选择性剪接调控胚胎干细胞多能性。

1.  胰腺β细胞富含的Bace2酶调节β细胞的功能和数量

【动态】
　　糖尿病研究中的圣杯之一就是发现刺激β细胞生长的生物分子和找到以此为靶点的药物。一个瑞士德国美国的国际研究团队与瑞士罗氏制药公司合作，不仅发现了一个调节β细胞生长的蛋白分子，也找到了一个刺激该蛋白的化合物。β细胞数量减少功能减弱是2型糖尿病的标志。该项研究通过siRNA筛选，在鼠科和人类β细胞中鉴定出Bace2酶是增殖前细胞膜蛋白Tmem27的选择性脱落酶。Bace2酶失效的老鼠和用新发现的Bace2酶抑制剂治疗的胰岛素抗性的老鼠都表现出β细胞数量扩增和提高的胰岛素水平导致的血糖稳态的控制改善。

【点评】
　　这些发现提示Bace2酶参与了控制维持β细胞并提供了合理策略通过抑制该酶来扩增有功能的胰腺β细胞数量。

【参考论文】
Cell Metabolism 2011, 14(3) pp. 365 – 377
Bace2 Is a β Cell-Enriched Protease that Regulates Pancreatic β Cell Function and Mass
Daria Esterházy, Ina Stützer, Haiyan Wang, et al.
Decreased β cell mass and function are hallmarks of type 2 diabetes. Here we identified, through a siRNA screen, beta site amyloid precursor protein cleaving enzyme 2 (Bace2) as the sheddase of the proproliferative plasma membrane protein Tmem27 in murine and human β cells. Mice with functionally inactive Bace2 and insulin-resistant mice treated with a newly identified Bace2 inhibitor both display augmented β cell mass and improved control of glucose homeostasis due to increased insulin levels. These results implicate Bace2 in the control of β cell maintenance and provide a rational strategy to inhibit this protease for the expansion of functional pancreatic β cell mass.

2.  人体内多功能溶瘤细胞痘病毒靶向癌细胞的静脉输送

【动态】
　　如果静脉注射后相对正常细胞在肿瘤组织中能选择性放大到很高的浓度，像蛋白和siRNAs等生物分子在癌症治疗中的效果和安全性会明显增加。可惜这在人体上还没实现。美国和加拿大的科学家假设进化为在哺乳动物血液中进行全身传播的一种痘病毒可以改造成选择性在癌组织中复制并用作静脉输送和肿瘤中转基因的表达的载体。JX-594是一种生物工程改造的溶瘤细胞痘病毒，能够在帮助激活表皮生长因子受体（EGFR）/ Ras途径的癌细胞中复制、表达转基因和扩增，随后细胞溶解产生抗癌免疫。研究报告了临床试验中JX-594在静脉注射后会以剂量相关的方式选择性感染肿瘤组织并在其中复制和表达转基因产物。临床上正常组织不被感染。这一平台技术开启了在人类转移性实体瘤中选择性表达高浓度的几种互补的治疗剂和成像剂的多功能产品的可能性。

【点评】
　　该研究为癌症的靶向治疗提供了很好的平台，也为开发更多更特异性的抗癌疗法和诊断提供了技术支持。

【参考论文】
Nature, 2011; 477 (7362): 99 DOI: 10.1038/nature10358
Intravenous delivery of a multi-mechanistic cancer-targeted oncolytic poxvirus in humans
Caroline J. Breitbach, James Burke, Derek Jonker, et al.
The efficacy and safety of biological molecules in cancer therapy, such as peptides and small interfering RNAs (siRNAs), could be markedly increased if high concentrations could be achieved and amplified selectively in tumour tissues versus normal tissues after intravenous administration. This has not been achievable so far in humans. We hypothesized that a poxvirus, which evolved for blood-borne systemic spread in mammals, could be engineered for cancer-selective replication and used as a vehicle for the intravenous delivery and expression of transgenes in tumours. JX-594 is an oncolytic poxvirus engineered for replication, transgene expression and amplification in cancer cells harbouring activation of the epidermal growth factor receptor (EGFR)/Ras pathway, followed by cell lysis and anticancer immunity. Here we show in a clinical trial that JX-594 selectively infects, replicates and expresses transgene products in cancer tissue after intravenous infusion, in a dose-related fashion. Normal tissues were not affected clinically. This platform technology opens up the possibility of multifunctional products that selectively express high concentrations of several complementary therapeutic and imaging molecules in metastatic solid tumours in humans.

3.  炎症激活蛋白维持血糖正常水平

【动态】
　　美国科学家最近报道了p38丝裂原活化蛋白激酶（MAPK）在苏氨酸Thr48 和 丝氨酸Ser61残基磷酸化叠接型X-box结合蛋白1（Xbp1s）并大大加强了老鼠中Xbp1s的核迁移，而任一残基突变为丙氨酸的话都会切实降低Xbp1s的核迁移和活性。还报道了同瘦老鼠相比，胖老鼠的肝脏中p38 MAPK活性显著降低。进而，他们报告了通过表达结构性活性MAP激酶激酶6（MKK6Glu）来激活p38 MAPK能够大大增强Xbp1s的核迁移，减少内质网压力，在严重肥胖老鼠和糖尿病鼠中建立正常血糖。因此，他们的研究结果定义了在苏氨酸Thr48 和 丝氨酸Ser61残基磷酸化Xbp1s在肥胖动物中维持稳定血糖所起的关键作用，也提示在肥胖老鼠肝脏中激活p38MAPK可能发展出新的治疗2型糖尿病的手段。

【点评】
　　肥胖导致的低度炎症增多被广泛认为促进了2型糖尿病的发生，而该研究表明炎症激活的两个蛋白实际上对于维持正常血糖水平很关键，说明低度炎症增多是身体防止肥胖引起糖尿病的抵御手段。

【参考论文】
Nature Medicine, 2011; DOI: 10.1038/nm.2449
p38 MAPK–mediated regulation of Xbp1s is crucial for glucose homeostasis
Jaemin Lee, Cheng Sun, Yingjiang Zhou, et al. 
Here we show that p38 mitogen-activated protein kinase (p38 MAPK) phosphorylates the spliced form of X-box binding protein 1 (Xbp1s) on its Thr48 and Ser61 residues and greatly enhances its nuclear migration in mice, whereas mutation of either residue to alanine substantially reduces its nuclear translocation and activity. We also show that p38 MAPK activity is markedly reduced in the livers of obese mice compared with lean mice. Further, we show that activation of p38 MAPK by expression of constitutively active MAP kinase kinase 6 (MKK6Glu) greatly enhances nuclear translocation of Xbp1s, reduces endoplasmic reticulum stress and establishes euglycemia in severely obese and diabetic mice. Hence, our results define a crucial role for phosphorylation on Thr48 and Ser61 of Xbp1s in the maintenance of glucose homeostasis in obesity, and they suggest that p38 MAPK activation in the livers of obese mice could lead to a new therapeutic approach to the treatment of type 2 diabetes.

4.  Ccl2与LIF合作激活Stat3途径来维持ESCs/iPSCs的多能性

【动态】
　　老鼠胚胎干细胞和诱导多能干细胞（ESCs/iPSCs）的多能性能够通过分泌白细胞抑制因子（LIF）的饲养细胞维持。日本科学家发现饲养细胞提供的低浓度的（25单位/毫升）LIF，在没有饲养细胞的条件下不足以维持ESCs/iPSCs的多能性。他们在有和没有饲养细胞的条件下对照培养老鼠诱导多能干细胞并分别测定整体转录谱以确定维持多能性所涉及的其他因子。结果找到17种表达明显不同的基因，包括7种在有饲养细胞的条件下培养的诱导多能干细胞中过高表达的趋化因子。这些趋化因子在iPSCs中的异位表达显示CC趋化因子配体2（Ccl2）诱导了多能性的关键转录因子基因Klf4, Nanog, Sox2 和 Tbx3。而且，添加重组的Ccl2蛋白急剧增加生长于低LIF无饲养细胞条件下的Nanog（绿色荧光蛋白）阳性 iPSCs 数量。 并进一步显示Ccl2促进多能性是通过激活Stat3途径和随后的Klf4上调来调节的。他们证明了Ccl2介导增加细胞多能性是独立于PI3-激酶和MAPK途径的，以及Tbx3可能是被Klf4上调的。总的来说，在无饲养细胞条件下Ccl2与LIF合作激活Stat3途径来维持ESCs/iPSCs的多能性。

【点评】
　　CCL2有在感染或发炎部位招募特定细胞的作用，而该研究第一次表明CCL2也能够帮助维持诱导多能干细胞的多能性，在另一个角度解释了炎症和细胞增殖分化时的相似性，正如在胃肠粘膜再生复原过程中观察到的那样。

【参考论文】
Stem Cells, 2011; 29 (8): 1196 DOI: 10.1002/stem.673
CC Chemokine Ligand 2 and Leukemia Inhibitory Factor Cooperatively Promote Pluripotency in Mouse Induced Pluripotent Cells
Yuki Hasegawa, Naoko Takahashi, Alistair R. R. Forrest, et al.
The pluripotency of mouse embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) can be maintained by feeder cells, which secrete leukemia inhibitory factor (LIF). We found that feeder cells provide a relatively low concentration (25 unit/ml) of LIF, which is insufficient to maintain the ESCs/iPSCs pluripotency in feeder free conditions. To identify additional factors involved in the maintenance of pluripotency, we carried out a global transcript expression profiling of mouse iPSCs cultured on feeder cells and in feeder-free (LIF-treated) conditions. This identified 17 significantly differentially expressed genes (adjusted p value <0.05) including seven chemokines overexpressed in iPSCs grown on feeder cells. Ectopic expression of these chemokines in iPSCs revealed that CC chemokine ligand 2 (Ccl2) induced the key transcription factor genes for pluripotency, Klf4, Nanog, Sox2, and Tbx3. Furthermore, addition of recombinant Ccl2 protein drastically increased the number of Nanog–green fluorescent protein–positive iPSCs grown in low-LIF feeder free conditions. We further revealed that pluripotency promotion by Ccl2 is mediated by activating the Stat3-pathway followed by Klf4 upregulation. We demonstrated that Ccl2-mediated increased pluripotency is independent of phosphoinositide 3-kinase and mitogen-activated protein kinase pathways and that Tbx3 may be upregulated by Klf4. Overall, Ccl2 cooperatively activates the Stat3-pathway with LIF in feeder-free conditions to maintain pluripotency for ESCs/iPSCs.

5.  通过葡萄糖代谢控制胰腺β细胞再生

【动态】
　　以色列科学家最近的研究表明老鼠中生产胰岛素的β细胞有惊人的再生能力， 提示理论上也能再生治疗人类糖尿病。压力条件下的生理性β细胞再生依赖幸存β细胞的加速增殖，但是触发和控制这一反应的因素还不清楚。利用胰岛移植实验，他们证明β细胞数量是整体控制的而非像组织损伤之类的局部因素控制。β细胞内葡萄糖代谢的长期变化而不是血糖水平本身是体内β细胞基础增殖和补偿性增殖的主要正向调节器。在细胞内，遗传操作和药理学处理显示葡萄糖通过葡萄糖激酶代谢，葡萄糖酵解的第一步，随之的KATP通道的关闭和膜去极化，来诱导β细胞复制。这些数据为通过代谢需求控制β细胞数量的自我平衡提供了分子机理。

【点评】
　　该研究说明身体的整体葡萄糖代谢水平是影响和控制胰腺β细胞增殖的主要因素，通过调节体内的代谢有可能启动和控制胰腺β细胞的生理性再生，对于治疗缺乏β细胞的I型糖尿病有潜在重要意义。

【参考论文】
Cell Metabolism, 2011; 13 (4): 440 DOI:10.1016/j.cmet.2011.02.012
Control of Pancreatic β Cell Regeneration by Glucose Metabolism
Shay Porat, Noa Weinberg-Corem, Sharona Tornovsky-Babaey,et al.
Recent studies revealed a surprising regenerative capacity of insulin-producing β cells in mice, suggesting that regenerative therapy for human diabetes could in principle be achieved. Physiologic β cell regeneration under stressed conditions relies on accelerated proliferation of surviving β cells, but the factors that trigger and control this response remain unclear. Using islet transplantation experiments, we show that β cell mass is controlled systemically rather than by local factors such as tissue damage. Chronic changes in β cell glucose metabolism, rather than blood glucose levels per se, are the main positive regulator of basal and compensatory β cell proliferation in vivo. Intracellularly, genetic and pharmacologic manipulations reveal that glucose induces β cell replication via metabolism by glucokinase, the first step of glycolysis, followed by closure of KATP channels and membrane depolarization. Our data provide a molecular mechanism for homeostatic control of β cell mass by metabolic demand.

6.  选择性剪接调控胚胎干细胞多能性

【动态】
　　选择性剪接是扩大蛋白多样性和调节基因表达的重要过程。加拿大科学家确定了一种进化上保守的胚胎干细胞特异性选择性剪接，这种剪接会改变叉头族转录因子（FOXP1）对DNA结合的倾向性。他们的研究表明胚胎干细胞特异性的同工型FOXP1刺激了细胞多能性所需的转录因子，包括OCT4, NANOG, NR5A2, 和 GDF3的基因表达，同时抑制了胚胎干细胞分化所需的基因。该型FOXP1也促进胚胎干细胞多能性的维持并和有助于有效的重组体细胞为诱导多能干细胞。这些结果揭示了一种选择性剪接通过控制关键的胚胎干细胞特异性转录程序在调节细胞多能性中所起的关键作用。

【点评】
　　该研究发现了一种控制胚胎干细胞多能性的开关。对于了解细胞多能性以及细胞命运的决定有重要意义。同时也为研究如何调控细胞多能性找到了一个生物靶点。

【参考论文】
Cell, 2011; DOI: 10.1016/j.cell.2011.08.023
An Alternative Splicing Switch Regulates Embryonic Stem Cell Pluripotency and Reprogramming
Mathieu Gabut, Payman Samavarchi-Tehrani, Xinchen Wang, et al.
Alternative splicing (AS) is a key process underlying the expansion of proteomic diversity and the regulation of gene expression. Here, we identify an evolutionarily conserved embryonic stem cell (ESC)-specific AS event that changes the DNA-binding preference of the forkhead family transcription factor FOXP1. We show that the ESC-specific isoform of FOXP1 stimulates the expression of transcription factor genes required for pluripotency, including OCT4, NANOG, NR5A2, and GDF3, while concomitantly repressing genes required for ESC differentiation. This isoform also promotes the maintenance of ESC pluripotency and contributes to efficient reprogramming of somatic cells into induced pluripotent stem cells. These results reveal a pivotal role for an AS event in the regulation of pluripotency through the control of critical ESC-specific transcriptional programs.