Whitehead Institute, Cambridge, MA 02142, USA.
Proc Natl Acad Sci U S A. 2013 Jun 4;110(23):9368-73. doi: 10.1073/pnas.1304889110. Epub 2013 Apr 23.
Throughout the plant and animal kingdoms specific cell types become polyploid, increasing their DNA content to attain a large cell size. In mammals, megakaryocytes (MKs) become polyploid before fragmenting into platelets. The mammalian trophoblast giant cells (TGCs) exploit their size to form a barrier between the maternal and embryonic tissues. The mechanism of polyploidization has been investigated extensively in Drosophila, in which a modified cell cycle--the endocycle, consisting solely of alternating S and gap phases--produces polyploid tissues. During S phase in the Drosophila endocycle, heterochromatin and specific euchromatic regions are underreplicated and reduced in copy number. Here we investigate the properties of polyploidization in murine MKs and TGCs. We induced differentiation of primary MKs and directly microdissected TGCs from embryonic day 9.5 implantation sites. The copy number across the genome was analyzed by array-based comparative genome hybridization. In striking contrast to Drosophila, the genome was uniformly and integrally duplicated in both MKs and TGCs. This was true even for heterochromatic regions analyzed by quantitative PCR. Underreplication of specific regions in polyploid cells is proposed to be due to a slower S phase, resulting from low expression of S-phase genes, causing failure to duplicate late replicating genomic intervals. We defined the transcriptome of TGCs and found robust expression of S-phase genes. Similarly, S-phase gene expression is not repressed in MKs, providing an explanation for the distinct endoreplication parameters compared with Drosophila. Consistent with TGCs endocycling rather than undergoing endomitosis, they have low expression of M-phase genes.
在植物和动物王国中，特定的细胞类型会变成多倍体，增加其 DNA 含量以获得较大的细胞大小。在哺乳动物中，巨核细胞 (MKs) 在分裂成血小板之前变成多倍体。哺乳动物滋养层巨细胞 (TGCs) 利用其大小在母体和胚胎组织之间形成屏障。多倍体形成的机制在果蝇中得到了广泛研究，在果蝇中，一种改良的细胞周期——仅由交替的 S 和间隙期组成的内周期——产生多倍体组织。在果蝇内周期的 S 期，异染色质和特定的常染色质区域复制不足，拷贝数减少。在这里，我们研究了小鼠 MKs 和 TGCs 中的多倍体形成特性。我们诱导原代 MKs 分化，并直接从小鼠胚胎 9.5 天植入部位分离 TGCs。通过基于阵列的比较基因组杂交分析基因组的拷贝数。与果蝇形成鲜明对比的是，MKs 和 TGCs 中的基因组都被均匀地和完整地复制。这甚至适用于通过定量 PCR 分析的异染色质区域。多倍体细胞中特定区域的复制不足被认为是由于 S 期较慢，这是由于 S 期基因表达水平低，导致晚期复制基因组间隔无法复制。我们定义了 TGCs 的转录组，发现 S 期基因表达旺盛。同样，MKs 中的 S 期基因表达没有被抑制，这为与果蝇相比，内复制参数明显不同提供了一个解释。与 TGCs 内周期而不是经历有丝分裂一致，它们的 M 期基因表达水平较低。
