一種能夠維持胚胎干細胞多能性和自我更新能力的蛋白REST

來自德克薩斯M.D Anderson腫瘤中心的科學家在自然雜質上發(fā)表文章稱，某種小RNA分子具有阻止胚胎干細胞自我更新和使干細胞定向分化為特定細胞類型的能力，一種名為REST的蛋白能夠通過阻斷該小RNA分子的表達從而維持胚胎干細胞多能性和自我更新能力的蛋白。

研究人員發(fā)現(xiàn)RE1沉默轉錄因子（REST）在胚胎干細胞中扮演著雙重角色，文章的主要作者來自M.D Anderson腫瘤遺傳學中心的Sadhan Majumder教授(博士)表示“REST具有維持胚胎干細胞的自我更新能力，也就是細胞克隆更多自己的能力，同時REST保持了胚胎干細胞的多能性，即胚胎干細胞分化為體內任何種類細胞的能力。

3月23日先于印刷版的自然雜志網(wǎng)絡版刊登了M.D Anderson腫瘤中心關于REST在成神經(jīng)管細胞瘤（一種極具攻擊性的兒科腦瘤）中的作用的研究。

胚胎干細胞本質上就像空白名單。其具有獨特的發(fā)育成功能完全相同的未分化細胞的能力，同時，如果需要，他們又能分化成具有特定功能的不同類型的細胞。在實驗中，科學家們已經(jīng)成功地誘導胚胎干細胞分化為心肌細胞和分泌胰島素的胰腺細胞。希望有一天，人們可以利用胚胎干細胞來重塑或者代替人體凋亡的細胞，而且有可能用來治療許多疾病。

“胚胎干細胞在藥用方面具有非常大的潛力”Majumder說，“關鍵的是掌握可以獲得許多具有自我更新能力的胚胎干細胞以及使其分化為各種類型細胞的方法。”REST可以在維持胚胎干細胞穩(wěn)定供應以及保持它們分化能力方面起到關鍵作用。

抑制小分子RNA-21

針對小鼠胚胎干細胞的研究發(fā)現(xiàn)，REST能夠抑制一種名為microRNA-21或者miR-21的特定小RNA分子。MicroRNA分子是小片段RNA分子，其可以通過結合基因的信使RNA來控制基因的表達。

研究小組發(fā)現(xiàn)：MiR-21具有抑制胚胎干細胞自我更新的能力并且與某些和胚胎干細胞自我更新能力相關的如：Oct4,Nanog,Sox2以及c-Myc 等關鍵調節(jié)子表達的缺失有關。REST可以通過抑制miR-21來阻止上述事件的發(fā)生，從而維持胚胎干細胞的自我更新能力和多潛能性。

研究人員通過對人工培養(yǎng)的處于自我更新（增殖）期和分化期的小鼠胚胎干細胞進行的一系列實驗發(fā)現(xiàn)了REST和miR-21的功能。他們發(fā)現(xiàn)當細胞處于增殖期時REST的表達明顯增高。REST的缺失導致干細胞增殖能力的下降并開始分化--即使當胚胎干細胞處于非常有利于增殖的條件下時，情況也一樣。在處于分化期的胚胎干細胞中加入REST可以保持他們自我更新的能力。

這些實驗同時揭示：REST結合在一系列以與胚胎干細胞自我更新能力相關基因為靶子的小RNA分子的基因染色質上。REST控制了11種小RNA分子的轉錄。

REST與小兒腦瘤有關
先前的實驗室結果表明用來維持干細胞自我更新能力和多潛能性的REST的用量可能會導致成神經(jīng)管細胞瘤，一種侵略性很強的小兒腦瘤。成神經(jīng)管細胞瘤被認為是由小腦外部粒層的未分化的神經(jīng)干細胞演化而來的。

M.D Anderson中心的Majumder研究小組在早先的研究中發(fā)現(xiàn)，大概一半左右的成神經(jīng)管細胞瘤都過表達REST，而這種現(xiàn)象在大多數(shù)細胞中并不存在。“我們認為REST是導致這種小兒腦瘤的關鍵因素”Majumder表示，“而且它的主要功能就是維持一部分特定的腦干細胞或者說祖細胞的“干性 ”（stemness）狀態(tài)。

研究人員猜想，通過維持神經(jīng)干細胞的“干性”狀態(tài)，REST阻止神經(jīng)干細胞分化成普通的細胞和特殊類型的細胞，反而導致腫瘤的形成。M.D Anderson的科學家們現(xiàn)在正在研究是否小RNA分子于成神經(jīng)管細胞瘤也存在某種關系。

對REST功能的了解有助于對成神經(jīng)管細胞瘤和胚胎干細胞生物學的研究。“就像阻斷REST的功能對成神經(jīng)管細胞瘤有潛在的治療效果一樣，阻斷REST的功能使胚胎干細胞分化對再生性藥物的研究來說也是潛在的關鍵一步。”Majumder說。

刊登在自然雜志上研究報告獲得了國立衛(wèi)生研究院和腫瘤遺傳學Dodie Hawn協(xié)會的資助。

和 Majumder一起的作者還包括，作者Sanjay K. Singh和Mohamedi N.Kagalwala（來自于M.D Anderson中心的腫瘤遺傳學部和胚胎干細胞與發(fā)育生物學中心）；Jan Parker-Thornburg（生物化學與分子生物學部）和Henry Adams（腫瘤遺傳學部）。另外，M.D Anderson中心的神經(jīng)腫瘤部、腦瘤中心、胚胎干細胞和發(fā)育生物學部以及休斯敦的得克薩斯大學研究生院生物醫(yī)藥學科的基因與發(fā)育項目也為 Majumder的該項研究作出了貢獻。

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