一种能够维持胚胎干细胞多能性和自我更新能力的蛋白REST

来自德克萨斯M.D Anderson肿瘤中心的科学家在自然杂质上发表文章称，某种小RNA分子具有阻止胚胎干细胞自我更新和使干细胞定向分化为特定细胞类型的能力，一种名为REST的蛋白能够通过阻断该小RNA分子的表达从而维持胚胎干细胞多能性和自我更新能力的蛋白。

研究人员发现RE1沉默转录因子（REST）在胚胎干细胞中扮演着双重角色，文章的主要作者来自M.D Anderson肿瘤遗传学中心的Sadhan Majumder教授(博士)表示“REST具有维持胚胎干细胞的自我更新能力，也就是细胞克隆更多自己的能力，同时REST保持了胚胎干细胞的多能性，即胚胎干细胞分化为体内任何种类细胞的能力。

3月23日先于印刷版的自然杂志网络版刊登了M.D Anderson肿瘤中心关于REST在成神经管细胞瘤（一种极具攻击性的儿科脑瘤）中的作用的研究。

胚胎干细胞本质上就像空白名单。其具有独特的发育成功能完全相同的未分化细胞的能力，同时，如果需要，他们又能分化成具有特定功能的不同类型的细胞。在实验中，科学家们已经成功地诱导胚胎干细胞分化为心肌细胞和分泌胰岛素的胰腺细胞。希望有一天，人们可以利用胚胎干细胞来重塑或者代替人体凋亡的细胞，而且有可能用来治疗许多疾病。

“胚胎干细胞在药用方面具有非常大的潜力”Majumder说，“关键的是掌握可以获得许多具有自我更新能力的胚胎干细胞以及使其分化为各种类型细胞的方法。”REST可以在维持胚胎干细胞稳定供应以及保持它们分化能力方面起到关键作用。

抑制小分子RNA-21

针对小鼠胚胎干细胞的研究发现，REST能够抑制一种名为microRNA-21或者miR-21的特定小RNA分子。MicroRNA分子是小片段RNA分子，其可以通过结合基因的信使RNA来控制基因的表达。

研究小组发现：MiR-21具有抑制胚胎干细胞自我更新的能力并且与某些和胚胎干细胞自我更新能力相关的如：Oct4,Nanog,Sox2以及c-Myc 等关键调节子表达的缺失有关。REST可以通过抑制miR-21来阻止上述事件的发生，从而维持胚胎干细胞的自我更新能力和多潜能性。

研究人员通过对人工培养的处于自我更新（增殖）期和分化期的小鼠胚胎干细胞进行的一系列实验发现了REST和miR-21的功能。他们发现当细胞处于增殖期时REST的表达明显增高。REST的缺失导致干细胞增殖能力的下降并开始分化--即使当胚胎干细胞处于非常有利于增殖的条件下时，情况也一样。在处于分化期的胚胎干细胞中加入REST可以保持他们自我更新的能力。

这些实验同时揭示：REST结合在一系列以与胚胎干细胞自我更新能力相关基因为靶子的小RNA分子的基因染色质上。REST控制了11种小RNA分子的转录。

REST与小儿脑瘤有关
先前的实验室结果表明用来维持干细胞自我更新能力和多潜能性的REST的用量可能会导致成神经管细胞瘤，一种侵略性很强的小儿脑瘤。成神经管细胞瘤被认为是由小脑外部粒层的未分化的神经干细胞演化而来的。

M.D Anderson中心的Majumder研究小组在早先的研究中发现，大概一半左右的成神经管细胞瘤都过表达REST，而这种现象在大多数细胞中并不存在。“我们认为REST是导致这种小儿脑瘤的关键因素”Majumder表示，“而且它的主要功能就是维持一部分特定的脑干细胞或者说祖细胞的“干性 ”（stemness）状态。

研究人员猜想，通过维持神经干细胞的“干性”状态，REST阻止神经干细胞分化成普通的细胞和特殊类型的细胞，反而导致肿瘤的形成。M.D Anderson的科学家们现在正在研究是否小RNA分子于成神经管细胞瘤也存在某种关系。

对REST功能的了解有助于对成神经管细胞瘤和胚胎干细胞生物学的研究。“就像阻断REST的功能对成神经管细胞瘤有潜在的治疗效果一样，阻断REST的功能使胚胎干细胞分化对再生性药物的研究来说也是潜在的关键一步。”Majumder说。

刊登在自然杂志上研究报告获得了国立卫生研究院和肿瘤遗传学Dodie Hawn协会的资助。

和 Majumder一起的作者还包括，作者Sanjay K. Singh和Mohamedi N.Kagalwala（来自于M.D Anderson中心的肿瘤遗传学部和胚胎干细胞与发育生物学中心）；Jan Parker-Thornburg（生物化学与分子生物学部）和Henry Adams（肿瘤遗传学部）。另外，M.D Anderson中心的神经肿瘤部、脑瘤中心、胚胎干细胞和发育生物学部以及休斯敦的得克萨斯大学研究生院生物医药学科的基因与发育项目也为 Majumder的该项研究作出了贡献。

Protein Protects Embryonic Stem Cells' Versatility And Self-renewal

ScienceDaily (Mar. 23, 2008) — A protein known as REST blocks the ex[x]pression of a microRNA that prevents embryonic stem cells from reproducing themselves and causes them to differentiate into specific cell types, scientists at The University of Texas M. D. Anderson Cancer Center report in the journal Nature.

Researchers show RE1-silencing transc[x]ription factor (REST) plays a dual role in embryonic stem cells, said senior author Sadhan Majumder, Ph.D., professor in M. D. Anderson's Department of Cancer Genetics. "It maintains self-renewal, or the cell's ability to make more and more cells of its own type, and it maintains pluripotency, meaning that the cells have the potential to become any type of cell in the body."

The paper posted online March 23 in advance of publication grew from M. D. Anderson research on the protein's role in medulloblastoma -- an exceptionally aggressive pediatric brain cancer.

Embryonic stem cells are essentially blank slates. They have the unique ability to develop from identical, unspecialized cells and then differentiate into distinct types of cells with special functions. In the laboratory, scientists have been able to induce embryonic stem cells to develop into heart muscle cells or insulin-producing cells of the pancreas. The hope is that embryonic stem cells might one day be used to restore or replace failing cells in the human body and perhaps treat a wide range of diseases.

"Embryonic stem cells have a very high potential in medicine," Majumder said. "The critical thing is to learn the mechanisms that could be used to generate a lot of self-renewing embryonic stem cells and be able to differentiate them into various cell types." REST could play a key role in maintaining a steady supply of these cells and in preserving their differentiation capability.

Suppressing MicroRNA-21

In studies using mouse embryonic stem cells, the researchers found that REST disarms a specific microRNA called microRNA-21 or miR-21. MicroRNAs are tiny pieces of RNA that control gene ex[x]pression by binding to the gene's messenger RNA.

The team found that MiR-21 suppresses embryonic stem cell self-renewal and is associated with a corresponding loss of ex[x]pression of critical self-renewal regulators, such as Oct4, Nanog, Sox2 and c-Myc. REST counters this by suppressing miR-21 to preserve the cells' self-renewal and pluripotency.

The researchers discovered the roles of REST and miR-21 in a series of experiments using cultured mouse embryonic stem cells in either a self-renewal state or a differentiating state. They found that REST ex[x]pression was significantly higher in the self-renewal state. Withdrawing REST reduced the stem cells' ability to reproduce themselves and started differentiation -- even when the cells were grown under conditions conducive to self-renewal. Adding REST to differentiating cells maintained their self-renewal.

These experiments also revealed that REST is bound to the gene chromatin of a set of microRNAs with the potential to target self-renewal genes. REST controls transc[x]ription of 11 microRNAs.

REST Implicated in Pediatric Brain Cancer

Previous laboratory research suggests that the qualities that make REST beneficial in stem cell production and pluripotency may contribute to the development of medulloblastoma, an aggressive type of children's brain tumor. Medulloblastomas are believed to develop from undifferentiated neural stem cells in the external granule la[x]yer of the cerebellum.

In earlier research, Majumder's group at M. D. Anderson discovered that about half of these tumors overexpress REST, which is not found in most neural cells. "We found that REST is a critical factor in this group of children's brain tumors," Majumder said, "and that its major function is to keep a group of specific brain stem cells, or progenitor cells, in a state of stemness."

The researchers hypothesize that by maintaining the neural stem cells' 'stemness,' REST prevents their differentiation into normal and distinct types of cells, leading instead to tumor formation. The M. D. Anderson scientists are now exploring whether microRNAs might also play a role in medulloblastomas.

Understanding REST function has applications in both medulloblastoma and embryonic stem cell biology. "Just as blocking REST function has therapeutic potential in medulloblastoma, blocking REST function to allow for differentiation of embryonic stem cells is a potentially critical step in regenerative medicine," Majumder said.

The research reported in Nature was supported by grants from the National Institutes of Health and the Dodie Hawn Fellowship in Cancer Genetics.

Co-authors with Majumder are first author Sanjay K. Singh and Mohamedi N. Kagalwala, both from M. D. Anderson's Department of Cancer Genetics and the Center for Stem Cell and Developmental Biology; Jan Parker-Thornburg from the Department of Biochemistry and Molecular Biology; and Henry Adams from the Department of Cancer Genetics. Majumder is also affiliated with M. D. Anderson's Department of Neuro-Oncology, The Brain Tumor Center, and the Center for Stem Cell and Developmental Biology, as well as the Program in Genes and Development at The University of Texas Graduate School of Biomedical Sciences at Houston