世界生命科學前沿動態(tài)周報（五十七）

（9.12-9.18/2011）
美寶國際集團:陶國新 

　　主要內容：胰腺β細胞富含的Bace2酶調節(jié)β細胞的功能和數量；人體內多功能溶瘤細胞痘病毒靶向癌細胞的靜脈輸送；炎癥激活蛋白維持血糖正常水平；Ccl2與LIF合作激活Stat3途徑來維持ESCs/iPSCs的多能性；通過葡萄糖代謝控制胰腺β細胞再生；選擇性剪接調控胚胎干細胞多能性。

　　焦點動態(tài)：選擇性剪接調控胚胎干細胞多能性。

1.  胰腺β細胞富含的Bace2酶調節(jié)β細胞的功能和數量

【動態(tài)】
　　糖尿病研究中的圣杯之一就是發(fā)現(xiàn)刺激β細胞生長的生物分子和找到以此為靶點的藥物。一個瑞士德國美國的國際研究團隊與瑞士羅氏制藥公司合作，不僅發(fā)現(xiàn)了一個調節(jié)β細胞生長的蛋白分子，也找到了一個刺激該蛋白的化合物。β細胞數量減少功能減弱是2型糖尿病的標志。該項研究通過siRNA篩選，在鼠科和人類β細胞中鑒定出Bace2酶是增殖前細胞膜蛋白Tmem27的選擇性脫落酶。Bace2酶失效的老鼠和用新發(fā)現(xiàn)的Bace2酶抑制劑治療的胰島素抗性的老鼠都表現(xiàn)出β細胞數量擴增和提高的胰島素水平導致的血糖穩(wěn)態(tài)的控制改善。

【點評】
　　這些發(fā)現(xiàn)提示Bace2酶參與了控制維持β細胞并提供了合理策略通過抑制該酶來擴增有功能的胰腺β細胞數量。

【參考論文】
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.  人體內多功能溶瘤細胞痘病毒靶向癌細胞的靜脈輸送

【動態(tài)】
　　如果靜脈注射后相對正常細胞在腫瘤組織中能選擇性放大到很高的濃度，像蛋白和siRNAs等生物分子在癌癥治療中的效果和安全性會明顯增加?？上н@在人體上還沒實現(xiàn)。美國和加拿大的科學家假設進化為在哺乳動物血液中進行全身傳播的一種痘病毒可以改造成選擇性在癌組織中復制并用作靜脈輸送和腫瘤中轉基因的表達的載體。JX-594是一種生物工程改造的溶瘤細胞痘病毒，能夠在幫助激活表皮生長因子受體（EGFR）/ Ras途徑的癌細胞中復制、表達轉基因和擴增，隨后細胞溶解產生抗癌免疫。研究報告了臨床試驗中JX-594在靜脈注射后會以劑量相關的方式選擇性感染腫瘤組織并在其中復制和表達轉基因產物。臨床上正常組織不被感染。這一平臺技術開啟了在人類轉移性實體瘤中選擇性表達高濃度的幾種互補的治療劑和成像劑的多功能產品的可能性。

【點評】
　　該研究為癌癥的靶向治療提供了很好的平臺，也為開發(fā)更多更特異性的抗癌療法和診斷提供了技術支持。

【參考論文】
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.  炎癥激活蛋白維持血糖正常水平

【動態(tài)】
　　美國科學家最近報道了p38絲裂原活化蛋白激酶（MAPK）在蘇氨酸Thr48 和 絲氨酸Ser61殘基磷酸化疊接型X-box結合蛋白1（Xbp1s）并大大加強了老鼠中Xbp1s的核遷移，而任一殘基突變?yōu)楸彼岬脑挾紩袑嵔档蚗bp1s的核遷移和活性。還報道了同瘦老鼠相比，胖老鼠的肝臟中p38 MAPK活性顯著降低。進而，他們報告了通過表達結構性活性MAP激酶激酶6（MKK6Glu）來激活p38 MAPK能夠大大增強Xbp1s的核遷移，減少內質網壓力，在嚴重肥胖老鼠和糖尿病鼠中建立正常血糖。因此，他們的研究結果定義了在蘇氨酸Thr48 和 絲氨酸Ser61殘基磷酸化Xbp1s在肥胖動物中維持穩(wěn)定血糖所起的關鍵作用，也提示在肥胖老鼠肝臟中激活p38MAPK可能發(fā)展出新的治療2型糖尿病的手段。

【點評】
　　肥胖導致的低度炎癥增多被廣泛認為促進了2型糖尿病的發(fā)生，而該研究表明炎癥激活的兩個蛋白實際上對于維持正常血糖水平很關鍵，說明低度炎癥增多是身體防止肥胖引起糖尿病的抵御手段。

【參考論文】
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與LIF合作激活Stat3途徑來維持ESCs/iPSCs的多能性

【動態(tài)】
　　老鼠胚胎干細胞和誘導多能干細胞（ESCs/iPSCs）的多能性能夠通過分泌白細胞抑制因子（LIF）的飼養(yǎng)細胞維持。日本科學家發(fā)現(xiàn)飼養(yǎng)細胞提供的低濃度的（25單位/毫升）LIF，在沒有飼養(yǎng)細胞的條件下不足以維持ESCs/iPSCs的多能性。他們在有和沒有飼養(yǎng)細胞的條件下對照培養(yǎng)老鼠誘導多能干細胞并分別測定整體轉錄譜以確定維持多能性所涉及的其他因子。結果找到17種表達明顯不同的基因，包括7種在有飼養(yǎng)細胞的條件下培養(yǎng)的誘導多能干細胞中過高表達的趨化因子。這些趨化因子在iPSCs中的異位表達顯示CC趨化因子配體2（Ccl2）誘導了多能性的關鍵轉錄因子基因Klf4, Nanog, Sox2 和 Tbx3。而且，添加重組的Ccl2蛋白急劇增加生長于低LIF無飼養(yǎng)細胞條件下的Nanog（綠色熒光蛋白）陽性 iPSCs 數量。 并進一步顯示Ccl2促進多能性是通過激活Stat3途徑和隨后的Klf4上調來調節(jié)的。他們證明了Ccl2介導增加細胞多能性是獨立于PI3-激酶和MAPK途徑的，以及Tbx3可能是被Klf4上調的?？偟膩碚f，在無飼養(yǎng)細胞條件下Ccl2與LIF合作激活Stat3途徑來維持ESCs/iPSCs的多能性。

【點評】
　　CCL2有在感染或發(fā)炎部位招募特定細胞的作用，而該研究第一次表明CCL2也能夠幫助維持誘導多能干細胞的多能性，在另一個角度解釋了炎癥和細胞增殖分化時的相似性，正如在胃腸粘膜再生復原過程中觀察到的那樣。

【參考論文】
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.  通過葡萄糖代謝控制胰腺β細胞再生

【動態(tài)】
　　以色列科學家最近的研究表明老鼠中生產胰島素的β細胞有驚人的再生能力， 提示理論上也能再生治療人類糖尿病。壓力條件下的生理性β細胞再生依賴幸存β細胞的加速增殖，但是觸發(fā)和控制這一反應的因素還不清楚。利用胰島移植實驗，他們證明β細胞數量是整體控制的而非像組織損傷之類的局部因素控制。β細胞內葡萄糖代謝的長期變化而不是血糖水平本身是體內β細胞基礎增殖和補償性增殖的主要正向調節(jié)器。在細胞內，遺傳操作和藥理學處理顯示葡萄糖通過葡萄糖激酶代謝，葡萄糖酵解的第一步，隨之的KATP通道的關閉和膜去極化，來誘導β細胞復制。這些數據為通過代謝需求控制β細胞數量的自我平衡提供了分子機理。

【點評】
　　該研究說明身體的整體葡萄糖代謝水平是影響和控制胰腺β細胞增殖的主要因素，通過調節(jié)體內的代謝有可能啟動和控制胰腺β細胞的生理性再生，對于治療缺乏β細胞的I型糖尿病有潛在重要意義。

【參考論文】
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.  選擇性剪接調控胚胎干細胞多能性

【動態(tài)】
　　選擇性剪接是擴大蛋白多樣性和調節(jié)基因表達的重要過程。加拿大科學家確定了一種進化上保守的胚胎干細胞特異性選擇性剪接，這種剪接會改變叉頭族轉錄因子（FOXP1）對DNA結合的傾向性。他們的研究表明胚胎干細胞特異性的同工型FOXP1刺激了細胞多能性所需的轉錄因子，包括OCT4, NANOG, NR5A2, 和 GDF3的基因表達，同時抑制了胚胎干細胞分化所需的基因。該型FOXP1也促進胚胎干細胞多能性的維持并和有助于有效的重組體細胞為誘導多能干細胞。這些結果揭示了一種選擇性剪接通過控制關鍵的胚胎干細胞特異性轉錄程序在調節(jié)細胞多能性中所起的關鍵作用。

【點評】
　　該研究發(fā)現(xiàn)了一種控制胚胎干細胞多能性的開關。對于了解細胞多能性以及細胞命運的決定有重要意義。同時也為研究如何調控細胞多能性找到了一個生物靶點。

【參考論文】
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.