干细胞疗法的一条新途径（ZT）
利用新的方法，可更快、更有效的从成体皮肤细胞创建干细胞与肌细胞，并且这一方法也可能更加安全。该技术可制造诱导多功能干细胞（iPSCs）。iPSCs 有与原始胚胎干细胞相同的转化为体内任何组织的能力，但并不涉及胚胎的应用。更进一步的是，该技术不需要改造皮肤的DNA，从而回避了本技术用于人类时必然会遇到的重大障碍之一。
开发这一新方法的研究小组组长、美国麻省波士顿儿童医院的Derrick Rossi说： “我们的技术不改变宿主细胞的DNA，因此完全消除了与基于DNA的载体（即病毒载体——译者）相关的风险，（这些风险之所以出现是因为）DNA载体可与细胞的DNA整合并可能导致癌症。”
其他干细胞研究者对这一进展极为称许，称之为（医学上的）突破。麻省伍斯特市高级细胞技术公司首席科学官员Robert Lanza说， 该技术是个“规则改变者”。 他说：“如果可重复，这一技术将解决该领域内的一些最重要的问题。”
基因开关
要创建iPSCs，需要对用以创建iPSCs的皮肤细胞重新编程，使之暴露于超量的KLF4、c-MYC、OCT4 与 SOX2.四种关键的基因开关中，从而诱导皮肤细胞进入胚胎状态。
日本先驱者Shinya Yamanaka于2006年首次制出了iPSCs， 从那时起，制造iPSCs的主要方式就是用病毒来感染皮肤细胞。该病毒移植四种使控制开关进入皮肤细胞DNA的基因。但是因为该病毒是随机地把基因插入皮肤细胞DNA，所以存在意外触发致癌基因的危险，从而使这种“传统”方法风险太高，无法用于临床。
Rossi及其同事通过向细胞内的液体添加这四种基因的信使RNA（mRNA）拷贝来回避这一难题。信使RNA拷贝可在细胞内直接变成四种开关，期间皮肤细胞的DNA则保持不变。
在四种成体皮肤细胞类型的实验中，Rossi 及其研究小组显示了：通过每天注入四种基因开关的mRNA拷贝，他们能够在两周时间内创建iPSCs。这是利用病毒完成创建所需时间的一半。甚至更佳，因为比起利用病毒来，新的方法使原始细胞变成iPSCs的数量提高到四十倍。
临床潜力
在创建iPSCs后，Rossi及其研究小组成员显示：他们能够运用类似技术将其转化为肌细胞。通常的手段是：通过将其暴露于生长因子与营养素的混合物中来达到目的；Rossi及其同事则制作了MYOD（MYOD是将干细胞转化为肌细胞的基因）的一份mRNA拷贝，然后将拷贝注入iPSCs。在数天内，干细胞已经转化为肌细胞。
Rossi指出：“将iPSCs转化为具有临床价值的细胞的能力可能是我们的研究工作中的最重要的部分。它打开了支配细胞命运的大门！”
他的方法的其它意外收获是：将一种细胞类型转化为另一种类型时，其过程有严密的可控性。所注入的mRNA数精确地决定了细胞内所制造的基因开关数。并且，除非该mRNA每天都添加注入，否则细胞就会全面停止制造开关活动。
Rossi对此加以评论：“这意味着我们对开关有着临时控制权与剂量控制权。” 通过调整mRNA配方，有可能随意创建不同类型的组织。
绝密行动
虽然这一方法建立于一个非常简单的想法上，Rossi及其同事却整整花了一年时间才得以克服当初看来根本无法逾越的障碍。这个障碍就是皮肤细胞会把注入的mRNA视为“异己分子”从而“格杀勿论”， 这个清洗过程主要是通过产生保护性化学干扰素来达到。
Rossi给RNA套上了伪装，这样RNA可以不被细胞的防卫系统所觉察，从而成功地化解了这个难题。（说得具体点就是）把RNA的天然积木方块包装起来，搞成人工品模样，这样mRNA就可以在免疫响应面前得以瞒天过海，之后RNA就可以“大显神通”了 —— 被用来制造蛋白。
Rossi强调，迄今为止本工作只处于原理验证阶段（proof of principle）。他指出，在第一个病人得到治疗之前，有很多进一步的工作要做，也需要其他科学家的验证。
参考文献：Highly Efficient Reprogramming to Pluripotency and Directed Differentiation of Human Cells with Synthetic Modified mRNA Cell Stem Cell, DOI: 10.1016/j.stem.2010.08.012 New route to stem cell therapy opens up
Stem cells and muscle cells have been created from adult skin cells using a new method that is both quicker and more efficient than its predecessor. It is also likely to be safer.
The technique makes induced pluripotent stem cells (iPSCs), which have the same capacity as primordial embryonic stem cells to turn into every tissue of the body, but do not involve the use of embryos. What's more, it doesn't involve tinkering with the skin cells' DNA, and so gets round one of the main obstacles to using this technique in people.
"Our technology does not alter the DNA of the host cells, so completely eliminates the risks associated with DNA-based vectors which can integrate into the cell's DNA and possibly lead to cancer," says Derrick Rossi of the Children's Hospital Boston in Massachusetts, head of the team that developed the new method.
Other stem cell researchers have hailed this is a breakthrough. Robert Lanza, chief scientific officer at Advanced Cell Technology in Worcester, Massachusetts, calls it a game-changer. "If repeatable, it would solve some of the most important problems in the field," he says.
Gene switches
To create iPSCs, the skin cells from which they are made need to be reprogrammed to an embryonic state by exposing them to extra amounts of four critical gene switches, called KLF4, c-MYC, OCT4 and SOX2.
Since iPSCs were first made in 2006 by Japanese pioneer Shinya Yamanaka, the main way of doing this has been to infect skin cells with a virus which implants the four genes that make the control switches into the DNA of the skin cell. But because the virus inserts the genes randomly into the skin cell's DNA, there is a risk of accidentally activating cancer-causing genes, making the method too risky for clinical use.
Rossi and his colleagues got round this problem by adding messenger RNA (mRNA) copies of the four genes to the fluid within cells, where they are made directly into the four switches. The DNA of the skin cell is unchanged.
In experiments with four types of adult skin cell, Rossi and his team showed that by infusing mRNA copies of the four gene switches daily, they could create iPSCs within two weeks – half the time it takes using viruses. Even better, 40 times as many of the original cells become iPSCs as when using viruses.
Clinical potential
Once they had created iPSCs, Rossi and his team showed they could use a similar technique to turn them into muscle cells. Normally, this is done by exposing them to a cocktail of growth factors and nutrients, but Rossi and his colleagues instead made an mRNA copy of MYOD, a gene that turns stem cells into muscle cells, and infused this into the iPSCs. Within days, the stem cells had turned into muscle cells.
"The ability to turn the iPSCs into clinically valuable cells is probably the most important part of our work," says Rossi. "It opens a big door for directing cell fate."
The other bonus of his approach is that the process of converting one cell type to another can be tightly controlled. The amount of mRNA infused dictates precisely the amount of gene switch made inside the cell. And unless the mRNA is topped up each day, cells stop making the switches altogether.
"That means we have temporal and dose control over the switches," says Rossi. By tinkering with mRNA recipes, it may be possible to create different types of tissue at will.
Covert operation
Although the approach is based on a very simple idea, Rossi and his colleagues had to spend a year overcoming what at first seemed an insurmountable obstacle. The problem was that the skin cells identified the injected mRNA as "foreign" and destroyed it, mainly by producing the protective chemical interferon.
Rossi cracked the problem by disguising the RNA so that it went unnoticed by the cell's defences. By swapping natural building blocks of the RNA for artificial ones, they were able to mask the mRNA from the cell's immune responses, and so allow proteins to be made from the introduced RNA.
Rossi stresses that the work so far is simply a proof of principle. Much further work, and validation by other scientists, will be needed before the first patients can be treated, he points out.
Journal reference: Cell Stem Cell, DOI: 10.1016/j.stem.2010.08.012 学习@@ 学习
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