Wdr5通过胚胎干细胞核转录网络介导自我更新和改编

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INTRODUCTION& W+ V0 [5 v$ F8 @
The maintenance of ES cell self-renewal requires a network of transcription factors, including Oct4, Sox2, Nanog, Esrrb, Tbx3, and Tcf3 (Chen et al., 2008; Ivanova et al., 2006; Kim et al., 2008; Tam et al., 2008). These factors participate in auto- and cross-regulatory interactions to increase their own expression and that of other self-renewal-associated genes while repressing genes that promote differentiation. Perturbation of these factors collapses the self-renewal circuitry and triggers specific or mixed lineage differentiation (Ivanova et al., 2006). In contrast to the numerous transcription factors, only a handful of chromatin regulators that are important for self-renewal have been characterized (Loh et al., 2007; Pasini et al., 2007; Schaniel et al., 2009).
' Y; M( }% b$ Y; s    ES cells harbor an open, transcriptionally permissive chromatin that allows for efficient epigenomic remodeling during lineage commitment (Efroni et al., 2008). However, factors regulating this "hyperdynamic" epigenetic configuration remain poorly under-stood. ES cells also contain "bivalent domains" where nucleosomes are marked  by  trimethylation  at  histone3-lysine27 (H3K27me3) and histone3-lysine4 (H3K4me3) (Bernstein et al., 2006).  The Polycomb  group  (PcG)  complex  mediates H3K27me3, correlated with gene repression (Boyer et al., 2006). In  contrast,  the  Trithorax  group  (trxG)  complex  mediates H3K4me3, generally correlated with gene activation (Ringrose and Paro, 2004). Although PcG has been extensively investigated in the maintenance of ES cell self-renewal, pluripotency, and somatic cell reprogramming, there exists little complementary information for trxG-associated members. This imbalance of knowledge represents a significant shortcoming in the under-standing of the roles played by trimethylated H3K4 and H3K27 in regulating the ES cell identity. Moreover, it remains to be shown whetherthe well-established transcriptional network can functionally interact with epigenetic regulators to maintain pluripotency and, more importantly, which factors mediate such interactions.
9 j1 ~. v# Z+ J  An unresolved question in chromatin biology is the manner by which generic histone modification complexes, like PcG and trxG, become targeted to specific genomic loci to direct specific gene regulatory functions (Schuettengruber et al., 2007). This is especially intriguing in the context of ES cells. For example, Chd1，a chromodomain-helicase-DNA-binding protein that is not specific to ES cells, was recently described to be essential for  pluripotency  and  reprogramming  (Gaspar-Maia  et  al., 2009). The factor(s) or mechanism(s) conferring such functional specificity to epigenetic regulators remains unknown. Moreover, it  is  unclear  how  ectopic  expression  of four transcription factors一Oct4, Sox2, KIf4, and c-Myc (OSKM)一can reprogram somatic cells to iPS cells with epigenomes that are largely indistinguishable from ES cells (Carvajal-Vergara et al., 2010; Tsai et al., 2010). This is especially pertinent to the re-establishment of the bivalent signature. Interestingly, although the OSKM-iPS methodology has been replaced by various combinations of factors or small molecules, Oct4 remains the sole factor that, until recently, could not be substituted/omitted (Heng et al., 2010).  Accordingly,  we  reasoned that the  resetting  of the somatic epigenome must be achieved through the activity of Oct4-interacting proteins and/or Oct4 target genes.
; C) Q9 K5 v5 `' u/ S" w4 Z    Protein complexes of the Set/MLL histone methyltransferase (HMT)family are mammalian homologs of trxG that function as conserved, multisubunit ensembles to catalyze the methylation of H3K4. The human MLL gene, which contains a SET domain, was first identified based on translocations that are commonly associated with the pathogenesis of multiple forms of hematological  malignancies  (Shilatifard,  2006).  Notably,  Set/MLL proteins  alone  are  catalytically  inactive  but  require  core subunits一WdrS, Ash21 and Rbbp5一that are related to components of the yeast Set1 complex (Dou et al., 2006). The Rbbp5 and Ash21 heterodimer directly participates in HMT activity of the M LL1 complex (Cao et al., 2010). Ash21 is required for mouse embryogenesis（Taylor et al., 2010) and proper X-inactivation (Pullirsch  et al.,   2010),  whereas  diminished  recruitment of Rbbp5 is found in patients with Wiskott-Aldrich syndrome (Stoller et al., 2010). Other trxG-associated cofactors such as Menin, Hcf1， and Cxxc1 have been implicated in processes like pancreaticβcell growth (Karnik et al., 2007), tumorigenesis (Lairmore and Chen, 2009), apoptosis（Tyagi and Herr, 2009), and euchromatin formation (Thomson et al., 2010). In particular, Wdr5 is a key component of trxG acting as a "presenter" of the H3K4 residue and is indispensible for Set/MLL complex assembly and effective HMT activity (Dou et al., 2006). It was shown that Wdr5 interacts with H3K4me2 and mediates transition to the trimethylated state (Wysocka et al., 2005). However, it was also shown that Wdr5 is unable to distinguish between different H3K4 methylation states (Couture et al., 2006). Although Wdr5 function  is  required  for  vertebrate  development  (Wysocka et al., 2005) and osteoblast differentiation (Zhu et al., 2008), its role in ES or iPS cells remains to be determined.
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INTRODUCTION+ n. y& z% T* l$ ]/ K
The maintenance of ES cell self-renewal requires a network of transcription factors, including Oct4, Sox2, Nanog, Esrrb, Tbx3, and Tcf3 (Chen et al., 2008; Ivanova et al., 2006; Kim et al., 2008; Tam et al., 2008). These factors participate in auto- and cross-regulatory interactions to increase their own expression and that of other self-renewal-associated genes while repressing genes that promote differentiation. Perturbation of these factors collapses the self-renewal circuitry and triggers specific or mixed lineage differentiation (Ivanova et al., 2006). In contrast to the numerous transcription factors, only a handful of chromatin regulators that are important for self-renewal have been characterized (Loh et al., 2007; Pasini et al., 2007; Schaniel et al., 2009)." T8 j  X- R' A" b+ O. y
    ES cells harbor an open, transcriptionally permissive chromatin that allows for efficient epigenomic remodeling during lineage commitment (Efroni et al., 2008). However, factors regulating this "hyperdynamic" epigenetic configuration remain poorly under-stood. ES cells also contain "bivalent domains" where nucleosomes are marked  by  trimethylation  at  histone3-lysine27 (H3K27me3) and histone3-lysine4 (H3K4me3) (Bernstein et al., 2006).  The Polycomb  group  (PcG)  complex  mediates H3K27me3, correlated with gene repression (Boyer et al., 2006). In  contrast,  the  Trithorax  group  (trxG)  complex  mediates H3K4me3, generally correlated with gene activation (Ringrose and Paro, 2004). Although PcG has been extensively investigated in the maintenance of ES cell self-renewal, pluripotency, and somatic cell reprogramming, there exists little complementary information for trxG-associated members. This imbalance of knowledge represents a significant shortcoming in the under-standing of the roles played by trimethylated H3K4 and H3K27 in regulating the ES cell identity. Moreover, it remains to be shown whetherthe well-established transcriptional network can functionally interact with epigenetic regulators to maintain pluripotency and, more importantly, which factors mediate such interactions.- ?( o" u2 T8 F& N* m
  An unresolved question in chromatin biology is the manner by which generic histone modification complexes, like PcG and trxG, become targeted to specific genomic loci to direct specific gene regulatory functions (Schuettengruber et al., 2007). This is especially intriguing in the context of ES cells. For example, Chd1，a chromodomain-helicase-DNA-binding protein that is not specific to ES cells, was recently described to be essential for  pluripotency  and  reprogramming  (Gaspar-Maia  et  al., 2009). The factor(s) or mechanism(s) conferring such functional specificity to epigenetic regulators remains unknown. Moreover, it  is  unclear  how  ectopic  expression  of four transcription factors一Oct4, Sox2, KIf4, and c-Myc (OSKM)一can reprogram somatic cells to iPS cells with epigenomes that are largely indistinguishable from ES cells (Carvajal-Vergara et al., 2010; Tsai et al., 2010). This is especially pertinent to the re-establishment of the bivalent signature. Interestingly, although the OSKM-iPS methodology has been replaced by various combinations of factors or small molecules, Oct4 remains the sole factor that, until recently, could not be substituted/omitted (Heng et al., 2010).  Accordingly,  we  reasoned that the  resetting  of the somatic epigenome must be achieved through the activity of Oct4-interacting proteins and/or Oct4 target genes.3 N" @/ C! A" y4 l, O. Z. O2 i9 U
    Protein complexes of the Set/MLL histone methyltransferase (HMT)family are mammalian homologs of trxG that function as conserved, multisubunit ensembles to catalyze the methylation of H3K4. The human MLL gene, which contains a SET domain, was first identified based on translocations that are commonly associated with the pathogenesis of multiple forms of hematological  malignancies  (Shilatifard,  2006).  Notably,  Set/MLL proteins  alone  are  catalytically  inactive  but  require  core subunits一WdrS, Ash21 and Rbbp5一that are related to components of the yeast Set1 complex (Dou et al., 2006). The Rbbp5 and Ash21 heterodimer directly participates in HMT activity of the M LL1 complex (Cao et al., 2010). Ash21 is required for mouse embryogenesis（Taylor et al., 2010) and proper X-inactivation (Pullirsch  et al.,   2010),  whereas  diminished  recruitment of Rbbp5 is found in patients with Wiskott-Aldrich syndrome (Stoller et al., 2010). Other trxG-associated cofactors such as Menin, Hcf1， and Cxxc1 have been implicated in processes like pancreaticβcell growth (Karnik et al., 2007), tumorigenesis (Lairmore and Chen, 2009), apoptosis（Tyagi and Herr, 2009), and euchromatin formation (Thomson et al., 2010). In particular, Wdr5 is a key component of trxG acting as a "presenter" of the H3K4 residue and is indispensible for Set/MLL complex assembly and effective HMT activity (Dou et al., 2006). It was shown that Wdr5 interacts with H3K4me2 and mediates transition to the trimethylated state (Wysocka et al., 2005). However, it was also shown that Wdr5 is unable to distinguish between different H3K4 methylation states (Couture et al., 2006). Although Wdr5 function  is  required  for  vertebrate  development  (Wysocka et al., 2005) and osteoblast differentiation (Zhu et al., 2008), its role in ES or iPS cells remains to be determined.; V$ Z1 W% Z- d! s

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引言$ Q( }& h7 \) u2 e
包含Oct4, Sox2, Nanog, Esrrb, Tbx3和 Tcf3的转录因子网是维持胚胎干细胞自我更新所必需的(Chen et al., 2008; Ivanova et al., 2006; Kim et al.,2008; Tam et al., 2008). 这些因子参与自调控和相互作用的交叉调控以增加其自身和其它自我更新相关基因的表达并同时抑制促分化基因. 干扰这些因子会破坏自我更新环并引起特定或多种系谱分化(Ivanova et al., 2006). 相对于繁多的转录因子而言，已被鉴定的自我更新中重要的染色质调节器仅占少数 (Loh et al., 2007; Pasini et al., 2007; Schaniel et al., 2009).- f) ^% y9 n. U( ^
    胚胎干细胞包含一个开放、可转录的染色质，使细胞系 commitment期间有高效的表观重建(Efroni et al., 2008).然而因子对这种"高动力性的" 表观遗传结构的调节作用研究者们仍知之甚少. 胚胎干细胞仍含有histone3-lysine27(H3K27me3)和histone3-lysine4 (H3K4me3)的三甲基标记的核小体的"二价染色体域" (Bernstein et al.,2006).多梳家族(PcG) 复合物调节H3K27me3与基因的抑制有关(Boyer et al., 2006).相反的, Trithorax家族(trxG)复合物调节H3K4me3普遍与活化基因有关(Ringrose and Paro, 2004). 虽然多梳家族在维持胚胎干细胞自我更新、多能性和体细胞重编程中的作用已被广泛研究，但Trithorax家族相关成员的信息却几乎没有. 这种知识的失调显示了参与调节胚胎干细胞特征的三甲基 H3K4和H3K27不足这一显著缺点.此外,这种已确定的转录网是否与表观调节子有相互作用从而保持细胞多能性仍有待进一步研究，并且更为重要的是哪些因子介导这种联系？* h& ~+ f& }+ ]
    染色质生物学中一个未解决的问题是哪些一般组织蛋白修饰复合物如PcG和 trxG如何作用于特定基因位点指导特定基因调节功能 (Schuettengruber et al., 2007).这是胚胎干细胞特有的引人之处. 例如近来研究发现一种非胚胎干细胞特有的 chromodomain-helicase-DNA-binding蛋白Chd1是多能性和重编程  所必需的(Gaspar-Maia  et  al.,2009).这具体的表观遗传调节功能的相关因子和机制还不清楚. 此外,能将体细胞重编程为拥有难与胚胎干细胞区别的epigenomes的多能干细胞的四种转录因子即Oct4, Sox2, KIf4, and c-Myc (OSKM)如何异位表达也还不清楚 (Carvajal-Vergara et al., 2010; Tsai et al., 2010). 这对二价signature的重建尤为重要. 有趣的是, 虽然OSKM-多能干细胞方法学已被不同因子的组合或小分子取代, 但到目前为止Oct4仍是独立的无法被替代或忽略的因子 (Heng et al.,2010). 因此我们提出一个调节Oct4-互动蛋白和/或Oct4目标基因的活性来重组体epigenome的模型.
. j; X1 d, r6 `6 z9 [   Set/MLL 组蛋白甲基 (HMT)家族的蛋白复合物是哺乳动物体内起保存作用的trxG的同族体, 多亚单元聚集催化H3K4的甲基化.多病因的恶性血液病常引起人MLL基因易位，该基因含有一个固定位点可被首先识别 (Shilatifard,  2006).  值得注意的是,  无酵母 Set1复合物成分相关核亚单元即WdrS, Ash21 和Rbbp5的单独的Set/MLL蛋白无催化活性(Dou et al., 2006). Rbbp5和Ash21 异质二聚体直接参与调节MLL1复合物的HMT活性 (Cao et al., 2010). Ash21是鼠胚胎形成（Taylor et al., 2010)和适当的X染色体失活必需的(Pullirsch et al., 2010), 因此发现Wiskott-Aldrich综合症病人体内bbp5的补充减少 (Stol- er et al., 2010). 其他trxG相关辅助因子如Menin, Hcf1和Cxxc1也出现在如胰腺β细胞生长(Karnik et al., 2007)、肿瘤形成(Lairmore and Chen, 2009)、细胞凋亡（Tyagi and Herr, 2009), 和常染色质形成 (Thomson et al., 2010)过程中. Wdr5是H3K4剩余的一个 "presenter" trxG的一个特别关键的成分，也是Set/MLL 复合物聚集和有效的HMT活性不可或缺的 (Dou et al., 2006). 研究表明Wdr5与H3K4me2相互作用并介导其向三甲基化状态转换 (Wysocka et al., 2005). 然而, 也有研究显示无法区别不同H3K4甲基化状态的Wdr5 (Couture et al., 2006). 尽管Wdr5是脊椎动物发育(Wysocka et al., 2005)和成骨分化(Zhu et al., 2008)所必需的,但其在胚胎干细胞或多能干细胞中的作用仍不清楚.
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" p5 |$ f; {2 x* h/ g, e0 @. }红色标记部分是我不知道如何翻译的部分，望前辈们指教 % k) G3 {7 p' h& c, n+ A$ d