Hamdane Nourdine, Tremblay Michel G, Dillinger Stefan, Stefanovsky Victor Y, Németh Attila, Moss Tom
Laboratory of Growth and Development, St-Patrick Research Group in Basic Oncology, Cancer Division of the Quebec University Hospital Research Centre, Canada; Department of Molecular Biology, Medical Biochemistry and Pathology, Faculty of Medicine, Laval University, Edifice St Patrick, 9 rue McMahon, Québec, QC G1R 3S3, Canada.
Department of Biochemistry III, Biochemistry Center Regensburg, University of Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany.
Gene. 2017 May 15;612:5-11. doi: 10.1016/j.gene.2016.09.013. Epub 2016 Sep 8.
The nucleolus is the site of ribosome biogenesis and forms around the actively transcribed ribosomal RNA (rRNA) genes. However, the nucleolus is also implicated in cell cycle regulation, tumour suppression and chromosome segregation and nucleolar disfunction is linked to a wide range of human diseases. Interestingly, the nucleolus is also required for genome reprogramming and the establishment of heterochromatin in the mammalian embryo. Mammalian oocytes contain a subnuclear structure that is believed to be the precursor of the functional nucleolus, the Nucleolar Precursor Body (NPB). But the NPB is also required for the organisation of the zygotic heterochromatin and the establishment of pluripotency. We found that disruption of the mouse Upstream Binding Factor (UBF (UBTF)) gene caused disassembly of somatic nucleoli and the accumulation of the key rRNA gene transcription factors into dense subnuclear foci resembling NPBs. Here we show that UBF deletion causes the rRNA genes to collapse onto their centromere-proximal chromosomal sites spatially distinct from NPB-like structures, and that these structures contain rRNA gene transcription factors but not all nucleolar proteins. We further find that embryonic NPBs and their surrounding heterochromatin are both disrupted in UBF-null mouse embryos. These embryos also display subnuclear foci containing the rRNA gene transcription factors and arrest development before completing the forth cleavage division. The data suggest that the rRNA gene transcription factors have an intrinsic ability to interact and form a discrete nuclear compartment even in the absence of any rRNA gene activity and that the formation or maintenance of the zygotic NPB and surrounding heterochromatin requires UBF.
核仁是核糖体生物发生的场所，围绕着活跃转录的核糖体RNA（rRNA）基因形成。然而，核仁也参与细胞周期调控、肿瘤抑制和染色体分离，核仁功能障碍与多种人类疾病有关。有趣的是，核仁对于哺乳动物胚胎中的基因组重编程和异染色质的建立也是必需的。哺乳动物卵母细胞含有一种亚核结构，被认为是功能性核仁的前体，即核仁前体小体（NPB）。但NPB对于合子异染色质的组织和多能性的建立也是必需的。我们发现，小鼠上游结合因子（UBF（UBTF））基因的破坏导致体细胞核糖体解体，关键rRNA基因转录因子聚集形成类似于NPB的致密亚核灶。在这里我们表明，UBF缺失导致rRNA基因在空间上与其着丝粒近端染色体位点塌陷，这些位点与NPB样结构不同，并且这些结构含有rRNA基因转录因子，但并非所有核仁蛋白。我们进一步发现，UBF基因敲除的小鼠胚胎中，胚胎NPB及其周围的异染色质均被破坏。这些胚胎还表现出含有rRNA基因转录因子的亚核灶，并在完成第四次卵裂之前发育停滞。数据表明，即使在没有任何rRNA基因活性的情况下，rRNA基因转录因子也具有相互作用并形成离散核区室的内在能力，并且合子NPB及其周围异染色质的形成或维持需要UBF。
