Transcription at the centromere: Opportunity and danger for the maintenance of epigenetic identity

如何从单个细胞中制造出有机体是生物学中最令人惊叹的过程之一。从一个受精卵开始，这个细胞将开始多次分裂，形成多细胞动物、植物或真菌。一旦细胞完全生长，细胞分裂就不会停止，而是在有机体的整个生命周期中持续下去。这对于确保在经历细胞自然死亡或与环境相互作用时保持身体的形状和大小是必不可少的。重要的是，在生物体宏观层面上重要的东西也在细胞层面上发挥作用。对一个细胞来说，发展并保持其身份是一项基本任务。细胞身份编码在基因组中，基因组以染色体的形式出现。染色体是由染色质的长纤维组成的，染色质是由DNA和称为组蛋白的蛋白质包装在一起的。四种不同的组蛋白H2A、H2B、H3和H4形成了一个更大的盘状复合体，称为核小体，DNA被包裹在核小体周围，使染色质纤维看起来像串在一起的珠子。在细胞周期中，染色体在母细胞中复制，并在细胞分裂时传递给两个子细胞。虽然大多数遗传信息是在DNA序列中编码的，但人们普遍认为染色质携带的信息独立于潜在的DNA序列而传递。这种形式的遗传被称为“表观遗传”，可以决定某些基因是否表达，或定义染色体特定区域的特性。着丝粒是这些特殊区域的一部分。在显微镜下，它们在X形染色体上可见收缩，对于细胞分裂过程中染色体的分离是必不可少的。着丝粒的特性是通过组蛋白H3的同源物，即着丝粒特异的组蛋白CENP-A(果蝇中的dCENP-A)的存在而在表观遗传学上确定的。着丝粒染色质由CENP-A和典型的组蛋白H3核小体点阵组成。在S阶段，H3在DNA复制过程中被补充，而在果蝇和人类中，CENP-A的加载是在有丝分裂晚期到G1期以复制不依赖的方式发生的。这一过程需要去除所谓的占位符H3-核小体，它在上一个S阶段已经定位在着丝粒DNA序列上。近年来，我们和其他实验室提出的证据表明，一种称为转录的特定细胞过程可能允许染色质重塑发生，并允许CENP-A交换H3。需要直接DNA接触的细胞过程，如DNA复制或转录，会诱导大规模的染色质重塑事件，从而允许DNA和RNA聚合酶的进展。转录在基因组中的主要作用是转录基因并产生编码蛋白质、转移或核糖体RNA的RNA转录本。由于着丝粒上没有基因，我们假设转录被用来重塑染色质和破坏核小体的稳定，足以驱逐H3占位符。这将构成另一个有趣的例子，大自然如何通过改变现有工具箱的用途来解决复杂的问题。尽管这是一个有趣的假设，但这个过程仍然没有被理解，并提出了许多问题：着丝粒转录如何区分应该保留的CENP-A核小体和应该删除的占位符H3-核小体？这一过程是如何监管的？在寻找CENP-A的结合伙伴的过程中，我们先前发现了Spt6，一种参与转录过程中循环组蛋白的蛋白质。在这里，我们建议研究人和果蝇Spt6与CENP-A和H3组蛋白结合的分子细节，以及着丝粒上的转录如何影响CENP-A的载量。通过研究这一基本的进化保守过程，着丝粒可以作为一种范式来理解表观遗传保持细胞特性的潜在机制。

How to make an organism out of a single cell is one of the most amazing processes in biology. Starting with a single fertilized egg, this cell will start to divide many times to give rise to a multicellular animal, plant or fungus. The cell divisions will not stop once fully grown but continue through the lifetime of the organism. This is essential to ensure that body shape and size are maintained, while experiencing natural cell death or caused by interacting with its environment.Importantly, what is important at the macroscopic level of an organism also plays a role at the cellular level. Developing but also maintaining its identity is an essential task for a cell. Cellular identity is encoded in the genome which comes in the shape of chromosomes. Chromosomes are formed by long fibers of chromatin, made of DNA that is packaged together with proteins called histones. Four different histones H2A, H2B, H3 and H4 form a larger disk-shaped complex called a nucleosome around which the DNA is wrapped giving the chromatin fiber the appearance of beads on a string. During the cell cycle, chromosomes are duplicated in a mother cell and passed on to two daughter cells as cells divide. While most genetic information is encoded in the DNA sequence, it is widely appreciated that chromatin carries information that is passed on independently of the underlying DNA sequence. This form of inheritance is called "epigenetic" and can determine whether certain genes are expressed or define the identity of specific regions of the chromosome. Centromeres are among those specialised regions. They are visible as constrictions in X-shaped chromosomes under the microscope and essential for the separation of chromosomes during cell division.Centromere identity is determined epigenetically through the presence of a homolog of histone H3, the centromere-specific histone CENP-A (dCENP-A in Drosophila). Centromeric chromatin is composed of interspersed arrays of CENP-A and canonical histone H3 nucleosomes. While H3 is replenished during DNA replication in S-phase, loading of CENP-A in Drosophila and humans takes place in a replication-independent manner from late mitosis to G1. This process requires the removal of so-called placeholder H3-nucleosomes, which have been positioned on centromeric DNA-sequences during the previous S-phase. In recent years, we and other labs have presented evidence that one particular cellular process called transcription could allow for this chromatin remodeling to take place and allow the exchange of H3 by CENP-A.Cellular processes that require direct DNA contact like DNA replication or transcription induce large-scale chromatin remodeling events to allow the progression of DNA- and RNA- polymerases. The major role of transcription in the genome is to transcribe genes and produce RNA transcripts encoding proteins, transfer or ribosomal RNA. As there are no genes at the centromere, we hypothesise that transcription is instead used to remodel chromatin and destabilise nucleosomes enough to evict H3-placeholders. This would constitute another fascinating example, how nature solves complex problems by repurposing existing toolkits.Although an intriguing hypothesis, this process is still not understood and raises many questions: How does centromeric transcription distinguish CENP-A nucleosome that should be retained from placeholder H3-nucleosomes that should be removed? How is this process regulated? In a search for binding partners of CENP-A, we previously identified Spt6, a protein involved in recycling histones during transcription. Here, we propose to investigate the molecular details of human and Drosophila Spt6 binding to CENP-A and H3 histones and how transcription at the centromere contributes to CENP-A loading. By studying this essential evolutionary conserved process, centromeres can be used as a paradigm to understand the underlying mechanism of epigenetic inheritance to preserve the identity of the cell.