CONTROL OF OLIGODENDROCYTE DEVELOPMENT BY OLIG2 AND CHROMATIN REMODELLING COMPLEXES

令人惊讶的是，组成我们身体的数千种不同类型的细胞--例如血细胞、肌肉细胞、神经细胞--都是从同一个受精卵细胞发育而来的。因此，所有类型的细胞都包含相同的DNA，但每种类型都包含一组特定细胞类型独有的特殊蛋白质，而这些蛋白质是在所有细胞中发现的一组核心“看家”蛋白质之上的。“特定于细胞类型”的蛋白质赋予细胞特定的身份--例如，它是血细胞或神经细胞的定义。例如，血红蛋白(仅存在于红细胞中)、角蛋白(仅存在于皮肤细胞中)、胰岛素(仅存在于胰腺细胞中)等。尽管所有细胞都含有相同的DNA--相同的基因集合，但这些特征蛋白如何只在一种或几种细胞类型中表达？如果我们能够理解一些基因关闭而另一些基因高度表达的机制，我们可能会学习如何将一种细胞类型转化为另一种细胞类型--例如，治愈特定类型的细胞受损或被摧毁的疾病。这类疾病的例子有1型糖尿病，即制造胰岛素的胰腺细胞被免疫系统破坏，或者运动神经元病，控制肌肉运动的脊髓神经元因不明原因死亡。如果我们能从人体或培养皿中的健康细胞中制造出替代细胞，这将是非常有帮助的。细胞中有许多蛋白质的功能纯粹是激活或抑制其他蛋白质编码基因，通过将特定的DNA序列结合到这些基因旁边。这种DNA结合蛋白被称为“转录因子”，因为它们控制着一个给定的基因是否被“转录”成组装相应蛋白质的指令。例如，转录因子OLIG2仅存在于中枢神经系统的细胞中，称为“少突胶质细胞”。这些细胞产生“髓鞘”，即围绕“轴突”的绝缘膜螺旋状包裹，“轴突”是神经细胞的细长延伸，将电脉冲从大脑的一个部位输送到另一个部位。这极大地提高了信息在大脑中的传播速度。如果没有髓鞘，我们真的不能快速思考，或者根本不能！此外，当髓鞘受损时，就像在脱髓鞘疾病多发性硬化症期间一样，神经功能可能会受到严重损害。因此，我们想了解OLIG2是如何在少突胶质细胞中独特地激活髓鞘形成基因的。染色体中的DNA(“染色质”)通常被紧密缠绕成一种形式，使其编码信息不可访问。像OLIG2这样的转录因子本身不能解开DNA--它们需要与许多其他蛋白质相互作用，形成大型的“染色质重塑复合体”，共同将基因从紧密折叠的“封闭”状态中释放出来。这些复合体有几种类型(如INO80和ISWI复合体)，其各自的功能尚不清楚。我们最近发现，OLIG2与这两个复合体和其他复合体密切相关，这引发了这样一个问题：为什么需要不同的复合体，它们各自有什么作用？我们的假设是，不同的染色质重塑复合体在少突胶质细胞发育的不同阶段发挥作用，激活不同的基因集，从而允许从早期胚胎干细胞到完全成熟的髓鞘形成的少突胶质细胞的发展。我们的项目将通过识别与INO80和ISWI复合体相关的基因来测试这一想法，确定OLIG2是否以及如何将INO80和ISWI复合体导向这些基因，以及在少突胶质细胞发育过程中是否有不同的基因集参与其中。我们的实验将有助于阐明适用于所有细胞类型的转录调控的一般机制，以及帮助我们了解哺乳动物大脑的详细工作原理。

Amazingly, the thousands of different cell types that make up our body - blood cells, muscle cells, nerve cells, for example - all develop from the same single cell, the fertilized egg. Hence, all cell types contain the same DNA, yet each contains a set of specialized proteins that are unique to that particular cell type, on top of a core set of "housekeeping" proteins found in all cells. The "cell-type-specific" proteins are what give a cell its particular identity - what defines it as a blood cell or nerve cell, for example. Examples are haemoglobin (present or "expressed" only in red blood cells), keratin (only in skin cells), insulin (only in pancreatic cells) and so on. How are these characteristic proteins expressed in only one or a few cell types despite the fact that all cells contain the same DNA - the same collection of genes? If we could understand the mechanisms that keep some genes shut down and others highly expressed, we might learn how to convert one cell type into another - for example, to cure diseases in which a particular type of cell is damaged or destroyed. Examples of such diseases are type-one diabetes, in which the pancreatic cells that make insulin are destroyed by the immune system, or motor neuron disease, in which spinal neurons that control muscle movement die for unknown reasons. If we could manufacture replacement cells from healthy cells in the body, or in a dish, this could be extremely helpful.There are many proteins in cells whose function is purely to activate or repress other protein-coding genes, by binding specific DNA sequences next to those genes. Such DNA-binding proteins are called "transcription factors" because they control whether a given gene is "transcribed" into the instructions for assembling the corresponding protein. For example, the transcription factor OLIG2 is present uniquely in cells in the central nervous system called "oligodendrocytes". These cells make "myelin", spiral wraps of insulating membrane around "axons", the long thin extensions of nerve cells that carry electrical impulses from one part of the brain to another. This greatly increases the speed at which information travels around the brain. Without myelin, we literally would not be able to think quickly, or at all! Moreover, when myelin is damaged, as it is during the demyelinating disease multiple sclerosis, nervous function can be seriously compromised. We therefore want to understand how OLIG2 can activate myelin-forming genes uniquely in oligodendrocytes.DNA in chromosomes ("chromatin") is normally tightly wound into a form that makes its encoded information inaccessible. Transcription factors like OLIG2 cannot by themselves unravel the DNA - they need to interact with many other proteins to form large "chromatin remodelling complexes" that can together release a gene from its tightly folded, "closed" state. These complexes are of several types (e.g. INO80 and ISWI complexes) whose individual functions are poorly understood. We recently found that OLIG2 is associated tightly with both of these complexes and others, raising the question of why different complexes are needed, and what do they individually do?Our hypothesis is that the different chromatin remodelling complexes come into play at different stages of oligodendrocyte development to activate different sets of genes that allow progression along the path from early embryonic stem cell to fully mature, myelin-forming oligodendrocyte. Our project will test this idea by identifying the genes associated with the INO80 and ISWI complexes, determining whether and how OLIG2 directs the INO80 and ISWI complexes to those genes and whether different sets of genes are engaged as oligodendrocytes develop. Our experiments will help to illuminate general mechanisms of transcriptional regulation, applicable to all cell types, as well as helping us understand the detailed workings of the mammalian brain.