＞PGT-A（Preimplantation genetic testing for aneuploidy）とは、体外受精によって得られた胚の染色体数を、移植する前に網羅的に調べる検査です。欧米では流産を防ぐ目的で既に実施されていますが、日本では日本産科婦人科学会が命の選別につながるとの観点から認めていませんでした。しかし、日本国内でのニーズの高まりを受け、PGT-Aが有用かどうか検証するため、2017年11月から2019年1月の間、日本産科婦人科学会主導による「PGT-Aの有用性に関する多施設共同研究のためのパイロット試験」が、国内の体外受精実施施設4施設において実施されました。
Fig. 5. Rewiring the TF network during the transition of ESCs to TSCs. (A) In ESCs, LIF signal stimulates the PI3K-Akt, Jak-Stat3 and Mek-Erk pathways that regulate the activities of different TFs of the ESC-specific TF network either positively (orange arrows) or negatively (blue bar). Fgf and Wnt signals integrate into the TF network in different ways. (B) In TSCs, Fgf stimulates the Mek-Erk pathway to activate TFs of the TSC-specific TF network. Sox2 and Esrrb are included in both TF networks but they are regulated by different signals from different enhancers. The red dashed rectangles depict mESC-specific enhancer usage, while the green dashed rectangles depict TSC-specific enhancer usage.
Rewiring of the interconnections among TFs in the network
In the mESC-specific TF network, Sox2 is believed to be regulated by mESC-specific enhancers that are bound by Oct3/4 and Sox2. A recent report has revealed that a distal enhancer located >100 kb downstream of Sox2 makes a greater contribution to regulating its expression in mESCs than the proximal OCT-SOX motifs (Zhou et al., 2014). This enhancer is a typical super-enhancer that is occupied by multiple TFs, such as Oct3/4, Sox2, Nanog, Smad1, Esrrb, Klf4, Nr5a2, Tfcp2l1 and Stat3, which are specific to mESCs. A different enhancer is therefore likely to be responsible for regulating the expression of Sox2 in TSCs, although this enhancer remains to be discovered. Changes in enhancer activity are often observed during differentiation (Long et al., 2016). In the case of Sox2, multiple enhancers are present in the regions upstream and downstream of this gene, each of which activates its expression in a distinct tissue-specific manner (Uchikawa et al., 2003; Okamoto et al., 2015). Such differential usage of enhancer elements allows the same TF to be incorporated into different networks (Fig. 5).
During the transition of mESCs to TSCs, the binding sites of Sox2 change dramatically (Adachi et al., 2013). The occupancy of most Sox2 binding sites in mESCs is lost in TSCs, and new TSC-specific binding sites come into use (Adachi et al., 2013). By comparing the consensus binding site sequences for Sox2 in mESCs and TSCs, the OCT-SOX motif was found to be the most enriched motif at mESC-specific binding sites, whereas the AP-2 and Sox motifs were enriched at TSC-specific binding sites (Adachi et al., 2013). Tfap2c (AP2γ) was identified as the AP-2 family member responsible for the recruitment of Sox2 to TSC-specific sites. In this case, switching binding partners could be a primary mechanism by which Sox2 binds to different target sites (Fig. 2). How a TF interacts with other TFs at a super-enhancer might also shape how it interacts with its target genes, as is the case for Sox2. Sox2 is recruited to mESC-specific super-enhancers, including one near the Sox2 gene. Other mESC-specific TFs are co-recruited to these super-enhancers, where they act synergistically to stabilize their binding to the super-enhancer, resulting in the activation of transcription. When the cells exit from the mESC state, most of these TFs become downregulated, and as a result Sox2 binding to these mESC-specific super-enhancers is lost. Concurrently, multiple TSC-specific TFs become active and cooperate with Sox2 to bind and activate TSC-specific super-enhancers (Figs 2 and 5).
TF network と、extracellular signals Programming the TF network In the developmental context, the TF network transitions from one state to another under the regulation of extracellular signals. Cell differentiation can occur in a short time period, and the direction of differentiation can be defined by the capacity of the original cell type. The number of extracellular signals required for differentiation can be limited, and the same signalling pathway can be used in different processes to provide different outcomes. An obvious example is found in the varied functions of the LIF signal.
Following the discovery of iPSCs, it was confirmed that the combinatorial overexpression of TFs could induce the reprogramming of multiple different cell types from one state to another, albeit with low efficiency (Morris, 2016). Forced reprogramming does not take place in a physiological context and requires very specific combinations of TFs to work, which might reflect the nature of the TF network in the starting cell. In the case of Sox2, its mode of function is defined by other TFs in the network. Co-expression of Oct3/4 allows Sox2 to bind mESC-specific target genes but it is not sufficient to activate the mESC-specific TF network. Klf4 is a minimal requirement to support the activation of the mESC-specific TF network, which might be due to sufficient activation of mESC-specific super-enhancers.
ES, TS細胞を移行させるTF機能のまとめ 細胞の一方向性の分化メカニズムの考察です。
Unidirectional transition of the TF network
The presence of shared TFs among networks might facilitate state transitions via modulation of the activity of a single TF. However, cell fate transitions in the developmental context occur in a unidirectional manner. Indeed, the transition of mESCs to TSCs occurs efficiently in response to the manipulation of a single TF and yet the reverse transition is inefficient, as with other reprogramming events in general (Wu et al., 2011) (Fig. 6). TSCs can be reprogrammed to the mESC state using the traditional reprogramming cocktail minus Sox2 (Wu et al., 2011), but the efficiency is ∼0.1% at best with four factors. Therefore, a mechanism that defines the direction of cell state transitions must exist between two linked TF networks. ・・・・ The relationship between two developmentally linked TF networks can be defined by several characteristics: (1) connections between the overlapping members; (2) the capacity of extracellular signals to activate key TFs; and (3) strong negative regulation of the original TF network by the network of the new cell state. Sequential differentiation events during development would be programmed according to these principles, while (1) and (3) could be important parameters in defining the capability of reprogramming.