两篇顶级文章：β细胞再生由成体非祖细胞维持

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两篇顶级文章：β细胞再生由成体非祖细胞维持




   最近，哈佛大学Doug Melton率领的研究小组和宾州大学医学院Jake Kushner率领的研究小组各自独立发现，胰岛素分泌β细胞的数量是由成体非祖细胞维持的，这一发现推翻了一直流行的“β细胞再生依赖于特定祖细胞”的假说。Melton研究成果刊登于6月份PLoS Biology杂志，Kushner的研究成果刊登于5月份Developmental Cell 杂志。

  数据很有说服力，但不能排除一小部分祖细胞辅助β细胞再生的可能性。纽约州立大学Gladys Teitelman说寻找参与再生的稀有细胞很难。 伦敦玛丽皇后大学医学与牙科学院Malcolm Alison 说：“因此我们应该放弃研究胰腺中的干细胞吗？我不这么认为。”

  研究人员已经提出了几种β细胞再生资源，包括胰腺中的干细胞，骨髓或者胰腺管，以及能够横向分化（trans-differentiate）的外分泌胰腺细胞。Melton 2004年公布的数字显示β细胞起源于之前存在的β细胞。Kushner说：“尽管如此，这种理论仍尚存争议。”

  两支研究小组利用不同的技术获得相似的结论。Kushner小组依靠连续胸苷标记，动物在不同周期吸收两种分裂细胞（dividing-cell）标记。研究人员设计起源于祖细胞，将两种标记整合在一起的细胞。细胞经历多重分裂循环，那些只合并一种标记——只分裂一次的细胞——被认为是自我更新。

  Kushner 说：“我们惊奇地发现没有专门的祖细胞。”几乎没有含有两种标记的β细胞。相反，β细胞只是在特定时间经历细胞分裂，因为即便研究人员操作两种标记，β细胞也只是含有一种标记，证明这些细胞只是在这段造作时间内分裂了一次。

  Melton小组跟踪细胞分裂时荧光标记物的衰退，以及重组阶段绿色荧光蛋白（GFP）的结合情况。两种分析证实之前存在的β细胞是新β细胞的来源，没有成体干细胞。荧光标记在β细胞中衰退均匀，GFP标记提示细胞在G0或G1期重组。

  Melton认为胰腺中仍然存在祖细胞。“这项实验并不能排除它。”在肝脏中，胆管来源的干细胞在发生损伤后被激活，由此某些研究人员推测胰腺管中也有严阵以待的干细胞。Kushner说这项发现对治疗糖尿病有提示意思，可以发展补充肌体β细胞的技术。但Melton说，研究人员仍有很长一段路要走，尽管研究结果提示了β细胞复制的方式，但没有找到调节β细胞分裂的物质， “如果我们想更新它们，我们应该知道控制复制速度的物质。”

相关英文文献：

PLoS Biology, Received: January 26, 2007; Accepted: April 16, 2007; Published: May 29, 2007

All β Cells Contribute Equally to Islet Growth and Maintenance
Kristen Brennand1,2,3, Danwei Huangfu1,2,3, Doug Melton1,2,3*

1 Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts, United States of America2 Harvard Stem Cell Institute, Harvard University, Cambridge, Massachusetts, United States of America, 3 Howard Hughes Medical Institute, Harvard University, Cambridge, Massachusetts, United States of America

In healthy adult mice, the β cell population is not maintained by stem cells but instead by the replication of differentiated β cells. It is not known, however, whether all β cells contribute equally to growth and maintenance, as it may be that some cells replicate while others do not. Understanding precisely which cells are responsible for β cell replication will inform attempts to expand β cells in vitro, a potential source for cell replacement therapy to treat diabetes. Two experiments were performed to address this issue. First, the level of fluorescence generated by a pulse of histone 2B–green fluorescent protein (H2BGFP) expression was followed over time to determine how this marker is diluted with cell division; a uniform loss of label across the entire β cell population was observed. Second, clonal analysis of dividing β cells was completed; all clones were of comparable size. These results support the conclusion that the β cell pool is homogeneous with respect to replicative capacity and suggest that all β cells are candidates for in vitro expansion. Given similar observations in the hepatocyte population, we speculate that for tissues lacking an adult stem cell, they are replenished equally by replication of all differentiated cells.

Abbreviations: CAGG, a constitutive promoter containing the CMV enhancer and the chicken β-actin promoter; FACS, fluorescence-activated cell sorter; H2BGFP, histone 2B–green fluorescent protein; LRC, label-retaining cell; MADM, mosaic analysis with double markers; mEF, mouse embryonic fibroblast; mES cell, mouse embryonic stem cell; RIP, rat insulin promoter; rtTA, reverse tetracycline transactivator; tetO, tetracycline-inducible promoter; tTA, tetracycline transactivator.
* To whom correspondence should be addressed. E-mail: dmelton@mcb.harvard.edu

  Brennand K. et al.,\"All