Received: 6-Oct-15 Accepted: 26-Apr-16 Published online: 13-Jun-16
受付 2015/10/6 受諾 2016/4/26 オンライン公開 2016/6/13
In January 2014, it was reported that strong external stimuli, such as a transient low-pH stressor, was capable of inducing the reprogramming of mammalian somatic cells, resulting in the generation of pluripotent cells. This cellular reprograming event was designated ‘stimulus-triggered acquisition of pluripotency’ (STAP) by the authors of these reports. However, after multiple instances of scientific misconduct in the handling and presentation of the data were brought to light, both reports were retracted. To investigate the actual scientific significance of the purported STAP phenomenon, we sought to repeat the original experiments based on the methods presented in the retracted manuscripts and other relevant information. As a result, we have concluded that the STAP phenomenon as described in the original studies is not reproducible.
Cellular reprograming is a biological event in which a differentiated metazoan cell is induced to revert to a state functionally resembling that of cells at earlier developmental stages,. Full reprograming of somatic cells results in the acquisition of the ability to give rise to an entire organism, or totipotency; this can be achieved by somatic cell nuclear transfer. Pluripotency in contrast is the ability of a cell to differentiate into all somatic cell lineages. It has been shown that the artificial expression of pluripotency-associated transcription factors results in reprogramming of somatic cells to a state of pluripotency, such cells are referred to as as induced pluripotent stem (iPS) cells.
(英文) Mouse pluripotent stem cells share common features. Authentic pluripotent stem cells are embryonic stem (ES) cells derived from pre-implantation embryos,. Under optimized culture conditions, these maintain self-renewal by giving rise to pluripotent daughter cells via cell division. Leukemia inhibitory factor (LIF) is a well-known factor sufficient to maintain the pluripotency of mouse pluripotent stem cells in vitro. Such cells express a unique set of genes associated with pluripotency, such as a transcription factor Oct3/4, and contribute to embryo development when transferred into pre-implantation embryos, resulting in the formation of germline chimeras. These properties are shared by iPS cells derived from somatic cells. Therefore, acquisition of pluripotency by somatic cells via reprograming is typically assessed based on such criteria.
(英文) In 2014, Obokata et al. reported that sublethal external stimuli, such as exposure to a transient low-pH stressor, reprogrammed mammalian somatic cells, resulting in the generation of pluripotent cells,. In these reports, this cellular reprograming event was designated ‘stimulus-triggered acquisition of pluripotency’ (STAP). The reports also described how the primary pluripotent cells, STAP cells, were able to give rise to two types of pluripotent stem cells in a culture-condition-dependent manner. However, both reports were subsequently retracted due to multiple intsances<→instances> of scientific misconduct,. To investigate the scientific significance of the STAP phenomenon, we repeated the reported experiments based on the methods presented in the retracted manuscripts and other relevant information subsequently obtained. We examined the expression of pluripotency-associated genes in cell aggregates obtained in cultures of somatic cells treated with transient low-pH, and the ability of such cell aggregates to contribute to chimeric embryos after injection into pre-implantation embryos. The results of this reevaluation indicate that the previously reported STAP phenomenon is not reproducible.
2014年、Obokata et alは一過性の低pH刺激への曝露などの亜致死的外的刺激が、哺乳類の体細胞を再プログラムし、多能性細胞の発生をもたらすことを報告した,。これらの報告では、この細胞再プログラミング事象を「刺激惹起性多能性獲得」（STAP）と命名した。報告書はまた、如何にして一次多能性細胞であるSTAP細胞が培養条件に依存した2種類の多能性幹細胞を生じさせることができるかを記載した。しかし、両方の報告は、その後、科学的な誤処置の複数の事例のために取り下げられた,。 STAP現象の科学的意義を調べるために、取り下げられた論文に示された方法とその後得られた関連情報に基づいた記載実験を繰り返した。我々は一時的な低pHで処理された体細胞の培養物中で得られた細胞凝集体における多能性関連遺伝子の発現および着床前胚への注入後にそのような細胞凝集体がキメラ胚に寄与する能力を調べた。この再評価の結果は、以前に報告されたSTAP現象には再現性がないことを示している。
In the original report, transient low-pH stress induced by addition of hydrochloric acid (HCl) caused massive cell death of dissociated somatic cells around 1–2 days after treatment (Fig. 1d of Obokata et al. 2014); the surviving cells formed aggregates (Fig. 1b of Obokata et al.). In the present study, we examined the effect of HCl treatment on dissociated cells derived from spleen, liver and heart of 4–9-day-old mice. The amount of the diluted HCl solution used to achieve optimized low-pH condition was set at 10 µl based on the titration assay (Fig. 1a). This amount resulted in massive cell death routinely although it gave higher pH than 5.7 that was indicated in the previous manuscript (Figure S1a<→Extended Data Figure 1a> of Obokata et al.), and 12 µl often resulted in complete cell death. However, although massive cell death was observed at two days after treatment, aggregate formation was rarely observed in any cell type (Fig. 1b). Occasional formation of aggregates was also observed in the culture of non-treated cells, suggesting that low-pH treatment does not enhance the formation of cell aggregates.
元の報告では、塩酸（HCl）の添加によって誘発された一過性低pHストレスは、処理後約1～2日で解離した体細胞の大量の細胞死を引き起こした（Obokataら、20141の図1d）。生存細胞は凝集体を形成した(Fig. 1d of Obokata et al. 2014);生き残った細胞は凝集塊を形成した。本研究では、4～9日齢のマウスの脾臓、肝臓および心臓由来の解離細胞に対する塩酸処理の効果を調べた。最適化された低pH条件を達成するために使用される希塩酸溶液の量は、滴定実験に基づいて10μlに設定した (Fig. 1a)。この量は、以前の論文に示された5.7よりも高いpHを与えたが(Figure S1a<→Extended Data Figure 1a> of Obokata et al.)、大量の細胞死を恒常的にもたらし、12μlはしばしば完全な細胞死をもたらした。しかし、処理後2日目に大量の細胞死が観察されたが、いずれの細胞タイプにおいても凝集体形成が稀にしか観察されなかった（図1b）。非処理細胞の培養でも時折凝集体の形成が観察され、低pH処理が細胞凝集体の形成を増強しないことを示唆している。
(図注) Figure 1: Optimization of the condition for low-pH treatment. Figure 1 (a) Titration of HCl and ATP to achieve optimal low-pH condition of cell suspension. Indicated volumes of the diluted HCl or ATP solution was added to 500 μl of HBSS containing 7 × 10 liver cells and pH was measured. (b) Frequency of formation of cell aggregates from the cells prepared from various tissues after low-pH treatment. The numbers of the total experimental trials and the trials with formation of cell aggregates at each combination of cell types and low-pH stressors are indicated.
(英文) Next we examined the effect of adenosine triphosphate (ATP) as a transient low-pH stressor based on personal communication with the authors of the original study. The amount of the diluted ATP solution to achieve optimized low-pH (~5.7) was adjusted(Fig. 1a) and experiments were repeated several times. Massive cell death was again observed at two days after treatment (Fig. 2a); however, we found that liver cells reproducibly gave rise to cell aggregates morphologically similar to those shown in the previous report, whereas spleen and heart cells only occasionally formed similar cell aggregates (Fig. 1b). The efficiency of aggregate formation was clearly higher for ATP-treated cells than for HCl-treated or non-treated cells, especially in the case of liver cells.
(図注) Figure 2: Formation of cell aggregates from low-pH treated cells. Figure 2
(a) Time course of the cultures of liver, heart and spleen cells treated with ATP. The cells were prepared from 5-days old of C57BL6 mice carrying CAG-GFP. Scale bar = 100 μm. (b) Cell aggregates derived from liver cells treated with ATP with the culture for 7 days. Liver cells were prepared from 4-days old of C57BL6/129 F1 mice. Scale bar = 100 μm. (c) Frequency of formation of cell aggregates from liver cells with different genetic backgrounds. B6; C57BL6, F1; C57BL6/129 or 129/C57BL6. The numbers of the total experimental trials and the trials with formation of cell aggregates at each combination of cell types and genetic backgrounds are indicated.
(英文) In the case of liver cells, when 5 × 10 cells were seeded in a well of 12 well plate, 20–30 aggregates were observed after seven days on average (Fig. 2b). Addition of fibroblast growth factor (Fgf)-2, based on personal communication with the authors of the 2014 reports, slightly enhanced cell agregate formation. Since the culture medium contains leukemia inhibitory factor (LIF), which shows differential action on ES cells derived from different genetic backgrounds, we suspected that the genetic background might affect aggregate formation. However, as shown in Fig. 2c, although it was known that the 129 background confers a dominant effect in obligating the LIF signal input to maintain pluripotency, there was no difference between C57BL6 and C57BL6 × 129 F1 (either C57BL6/129 or 129/C57BL6) in the observed frequency of aggregate formation in the present study.
(英文) Induced cell aggregates show poor induction of pluripotency-associated markers
To test the induction of pluripotency markers in cell aggregates obtained from ATP-treated liver cells, we assessed the expression of pluripotency-associated genes. Oct3/4 is a well-defined marker of pluripotent stem cells. Using a primer pair to detect Oct3/4 transcript from the Pou5f1 allele, but not pseudo-genes, we did not find a detectable level (above 0.1% of the expression level in mouse ES cells, relative to the expression levels of Gapdh) of the transcript by quantitative polymerase chain reaction (Q-PCR) using a total RNA sample prepared from all cells in the culture (Fig. 3a), indicating that extremely few or no cells expressing Oct3/4 were present. Interestingly, expression of Gfp from the Oct3/4-GFP transgene (GOF18)「14 」was detected in liver cells cultured for seven days irrespective of ATP treatment, suggesting leaky expression of this transgene.
(図注) Figure 3: Q-PCR analysis for the expression of pluripotency markers in induced cell aggregates. Figure 3
(a) Q-PCR analysis of the low-pH treated liver cells cultured for 7 days. Liver cells were prepared from 7-day old GOF mice and treated with either ATP or HCl, or without stressor. RNA samples were prepared from all cells in the wells at day 7 of culture and the relative expression levels of Gfp (derived from GOF Tg) and Oct3/4 (derived from the endogenous Pou5f1 allele) to Gapdh were indicated with standard deviation. The expression levels in control ES cells carrying CAG-GFP Tg were set at 1.0. (b) Q-PCR analysis of the single cell aggregates derived from the ATP-treated or non-treated liver cells cultured for seven days. The liver cells were prepared from 4-days old of C57BL6/129 mice and the single cell aggregates were separately treated for quantification of gene expression. The relative expression levels of pluripotency-associated genes to Gnb2l1 were indicated with standard deviation. The expression levels in 10 control ES cells were set at 1.0. (c) Frequency of cell aggregates showing the levels of Oct3/4 expression comparable to ES cells. The relative expression levels of Oct3/4 in single cell aggregates derived from liver cells were measured as b and the frequency of the cell aggregates with the levels of Oct3/4 expression over 0.001 of relative expression to ES cells is indicated.
(英文) We next performed qPCR on individual cell aggregates isolated from culture. Aggregates were selected and RNA samples were prepared separately. These RNAs were reverse-transcribed and qPCR was performed. We found that some aggregates expressed a comparable amount—more than 10% of the expression level in ES cells—of pluripotency-associated genes, including Oct3/4. Since the cell aggregates consist of ~10 cells, such expression level indicated possible existence of the cell(s) expressing pluripotency-associated genes at the equivalent level to that in ES cells. Klf4 expression was detected in all samples, which may reflect its expression in liver cells, and thus serves as a positive control in this assay. Of cell aggregates derived from liver cells treated with ATP, 19% expressed the amount of Oct3/4 comparable to ES cells (Fig. 3c). These data suggest that some proportion of cells in the aggregates express pluripotency-associated genes at comparable levels to those of ES cells.
(英文) To examine the proportion of the cells expressing Oct3/4 in the aggregates, we next applied immuno-staining using a specific antibody against Oct3/4 we raised and assessed previously. Cell aggregates derived from low-PH treated liver cells were fixed, stained by anti-Oct3/4 antibody, and observed using confocal microscopy. We stained morula-stage mouse embryos as positive controls. By comparison with these positive controls, we found that some of the cell aggregates contained cells expressing Oct3/4 at comparable levels (Fig. 4a). In the case of cell aggregates derived from liver cells treated by ATP, 20% of cell aggregates contained Oct3/4-positive cells (Fig. 4b), which is consistent to the proportion of cell aggregates expressing the amounts of Oct3/4 comparable to ES cells detected by QPCR (Fig. 3c). In contrast, cell aggregates derived from liver cells treated by HCl included Oct3/4-positive cells at a frequency comparable to that of non-treated cells. The presence of Oct3/4-positive cells in the cell aggregates found only 1 in 14 cases in cultures of non-treated liver cells suggests that such cells are derived from Oct3/4-positive cells present in the liver cell population, that in vitro culture may itself be a source of the stress to the cells, or that the immuno-staining technique may produce some non-specific signal. In cell aggregates derived from ATP-treated liver cells, the Oct3/4-positive cells were typically positioned in the center of the cell aggregates and exhibit large nuclei, and were surrounded by Oct3/4-negative cells with small nuclei at the peripheries of the cell aggregates (Fig. 4c).
(図注) Figure 4: Immuno-staining of cell aggregates derived from low-pH treated liver cells. Figure 4
(a) Immunostaining of morula-stage embryos and cell aggregates for Oct3/4 and Nanog. Both samples were treated in parallel and confocal microscopic images were captured with the same exposure time. The embryo is wild-type C57BL6 whereas the cell aggregates were derived from 8-days old of Alb-cre/Rosa-GFP Tg mice. (b) Frequency of cell aggregates carrying Oct3/4-positive cells. The numbers of the immune-stained cell aggregates derived from liver cells and that of carrying Oct3/4-positive cells are indicated for each stressor treatment. (c) Immuno-staining image of cell aggregate for Oct3/4 derived from liver cells prepared from 4-days old C57BL6/129 mice.
(英文) Oct3/4-GFP transgene expression not detected in low-pH treated cells
In the original report, the authors used transgenic reporter gene expression as a marker of pluripotency. This reporter consisted of the transcriptional regulatory element of Oct3/4 and the fluorescent marker GFP, designated GOF, which is silent in somatic cells and activated in pluripotent cells. When we used the same transgenic mouse line as a source of dissociated cells, we found that they began to acquire strong auto-fluorescence in culture after ATP treatment. On observation with fluorescent microscopy, most aggregates showed both green and red fluorescence, a sign of auto-fluorescence (Fig. 5a) although this may also include the fluorescent signal from the GOF transgene, since we detected Gfp mRNA by qPCR in these cells after culture in vitro for seven days (Fig. 3a).
(図注) Figure 5: アnalyses of fluorescent signals from GOF transgene. Figure 5
(a) Fluorescent microscopic アnalysis of cell aggregates derived from GOF Tg mice. The cell aggregates (top) were derived from liver cells of 6-days old of GOF Tg mice. Fluorescent images with the filter sets for detection of GFP and RFP signals are shown. Images of ES cells carrying CAG-GFP (middle) captured with the same conditions are shown as a control to confirm no leaky signal of GFP in RFP channel. Images of wild-type ES cells (bottom) are shown as a control to confirm spesific signal of GFP in GFP channel. (b) FACS アnalysis of the low-pH treated spleen cells derived from GOF Tg mice. The spleen cells were isolated from 7-day-old GOF Tg mice and prepared with Lympholyte followed by treatment with the indicated stressors. After the culture for seven days, the cells were dissociated, stained with anti-E-cadherin with PE and anti-CD45 with APC, and アnalyzed by FACS. Wild-type ES cells were used as a positive control for E-cadherin staining and a negative control for CD45-staining as well as GFP fluorescence.
(英文) Specific detection of GFP fluorescence by fluorescence-activated cell sorting (FACS) was also performed. In spleen cells collected using Lympholyte, CD45-positive/E-cadherin-negative blood cells were enriched. The reprogramming of such cells to a state of pluripotency can be monitored by their conversion to CD45-negative/E-cadherin-positive cells and acquisition of GFP expression from the GOF transgene. However, although we again observed increased auto-fluorescence and some reduction of CD45 expression, neither a specific signal of GFP fluorescence nor an increase of E-cadherin expression was observed in the low-pH treated cells (Fig. 5b). Given these findings, we suggest that that there was no evident sign of reprogramming in low-pH treated spleen cells based on the expression of the GOF transgene.
(英文) Induced cell aggregates do not contribute to chimeric embryos after injection into pre-implantation embryos
In the original report, the authors showed that the cell aggregates obtained by the culture of low-pH treated cells contribute to chimeras when the cell aggregates were chosen by their morphologies under the microscopic observation, manually dissected and injected into blastocysts (Fig. 4a of Obokata et al.). The frequency of obtaining chimeric mice from injected blastocysts reached 24% (64 chimeric mice from 264 injected blastocysts; Figure S7b<→Extended Data Figure 7b> of Obokata et al.). We prepared cell aggregates from the liver cells dissociated from the livers of transgenic mice carrying CAG-EGFP or selected cell aggregates expressing GFP prepared from the liver cells derived from Alb-cre: Rosa-GFP double transgenic mice, (Fig. 6, top) and repeated injections of these into morula and blastocysts eight times. However, we found no chimeric embryos carrying GFP-positive cells among 117 embryos derived from 244 injected embryos based on observation by fluorescent microscopy (Fig. 6, bottom), although the embryo manipulation technique of the animal facility should be excellent as the huge number of knockout mice were routinely generated by the injection of ES cells into blastosysts (*ttp://www2.clst.riken.jp/arg/mutant_mice_generated_in_CDB.html and ⁑ttp://www2.clst.riken.jp/arg/publications.html). These data strongly suggest that the acquisition of pluripotency occurs in cell aggregates derived from low-pH treated liver cells only extremely rarely, if at all.
元の報告では、著者らは、低pH処理細胞の培養によって得られた細胞凝集塊が、顕微鏡観察下で形態によって選択され、手動で解剖され、胚盤胞に注入されると、キメラに寄与することを示した(Fig. 4a of Obokata et al.)。注入された胚盤胞からキメラマウスを得る頻度は24％に達した(264個の注入された胚盤胞から64のキメラマウス; Figure S7b<→Extended Data Figure 7b> of Obokata et al.)。我々は、CAG-EGFP を持つトランスジェニックマウスの肝細胞から分離した肝細胞からの細胞凝集体を用意し、もしくはAlb-cre：Rosa-GFP二重挿入トランスジェニックマウス, 由来肝細胞から用意されたGFP発現細胞凝集体を選択し、これらを桑実胚および胚盤胞へ8回注入を繰り返した。しかしながら我々は、蛍光顕微鏡による観察に基づいて、244個の注入された胚由来の117個の胚の中で、GFP陽性細胞を保持するキメラ胚を一つも見いださなかった（図6、下）。同じ動物施設の胚操作技法がES細胞を胚盤胞に注入することにより、ノックアウトマウスを日常的に見事に作成しているにも関わらずである（*ttp://www2.clst.riken.jp/arg/mutant_mice_generated_in_CDB.htmlおよび*ttp://www2.clst.riken.jp/arg/publications.html）。 これらのデータは、多分化能の獲得が低pH処理肝細胞に由来する細胞凝集体でまったく起きないか、ごくまれにしか起こらないことを強く示唆している。
(図注) Figure 6: Chimera assay of cell aggregates. Figure 6
Cell aggregates derived from liver cells prepared from 8-day-old Alb-cre:Rosa-GFP Tg (upper panels) were injected into morula-stage embryos. The manipulated embryos were transferred into the uterus and the embryos were recovered at E10.5. The dissected embryos were observed under fluorescent microscopy for the contribution of GFP-positive cells.
(英文) Induced cell aggregates do not give rise to stem cell lines
The original reports stated that two different types of stem cell lines could be established from cell aggregates obtained by the culture of low-pH treated cells: ES-like ‘STAP stem cells’ (Fig. 5 of Obokata et al.1) and trophoblast stem (TS)-like ‘FGF-induced (FI) stem cells’ (Fig. 2 of Obokata et al.2). To reevaluate these claims, we transferred cell aggregates derived from liver cells from various genetic backgrounds into the culture conditions for derivation of either ES-like or TS-like stem cells. In the case of the culture for ES-like stem cells in serum-free culture containing knockout serum replacement (KSR), adrenocorticotropic hormone (ACTH) and LIF19, most of the cell aggregates died without outgrowth, which may attributable to the absence of serum, while a small number aggregates gave rise to colonies containing small cells with large nuclei, resembling the morphology of embryonic stem cells. However, most of these cells ceased proliferation at day 7 and gradually regressed. Marked proliferation after day 7 was observed in only three of 492 cell aggregates and none of these gave rise to cell lines (Fig. 7a). In the case of the culture for TS-like stem cells containing FGF-4 and heparin20, many clumps showed outgrowth of fibroblastic cells, which may be due to the presence of FGF2 in the medium. Few of these (22 of 391 cell aggregates) gave rise to colonies of small stem cell-like cells, and one could be passaged three times (Fig. 7b). However, all ultimately regressed without giving rise to cell lines. These data showed that we are unable to derive stem cell lines from aggregates derived from low-pH treated liver cells.
元の報告は、低pH処理細胞の培養によって得られた細胞凝集体から2つの異なるタイプの幹細胞株を樹立することができたと述べている。即ちES細胞様STAP幹細胞（Obokata et al.1の図5）および栄養膜（TS）様の「FGF誘導（FI）幹細胞」（Obokataらの図2）である。これらの主張を再評価するために、ES様またはTS様幹細胞の誘導のための培養条件に、様々な遺伝的背景の肝細胞由来の細胞凝集体を移した。ノックアウト血清代替物（KSR）、副腎皮質刺激ホルモン（ACTH）およびLIF19を含む無血清培養物におけるES様幹細胞の培養の場合、細胞凝集物の大部分は、おそらく血清の不在に起因して増殖することなく死滅した。一方、少数の凝集物は、ES細胞の形態に似た大きな核を有する小さな細胞を含むコロニーを形成した。しかし、これらの細胞のほとんどは、7日目に増殖を停止し、徐々に退行した。 7日後の顕著な増殖は、492個の細胞凝集体のうちのわずか3つで観察され、いずれも細胞株を生じなかった（図7a）。 FGF-4およびヘパリン20を含有するTS様幹細胞の培養の場合、多くの塊が線維芽細胞の増殖を示したが、これは培地中にFGF2が存在するためである可能性がある。これらのうちの少数（391個の細胞集合体のうち22個）が小幹細胞様細胞のコロニーを生じ、3回継代することができた（図7b）。しかしながら、最終的に細胞株を生じさせることなくすべて退行した。これらのデータは、低pH処理肝細胞由来の凝集塊から幹細胞株を得ることができないことを示している。
(図注) Figure 7: Culture of cell aggregates in vitro. Figure 7
(a) The outgrowth culture of cell aggregate derived from liver cells. Liver cells were prepared from 7-days old of C57BL6 CAG-GFP Tg, treated with ATP and cultured for six days. Single cell aggregates were isolated and cultured on MEF feeder cells with medium containing KSR, ACTH and LIF adapted to the culture of ES cells. The cells continue<→continued> to grow for 15 days but did not give secondary colony after passage. (b) Outgrowth culture of cell aggregates derived from liver cells. Liver cells were prepared from 4-day-old C57BL6/129 mice, treated with ATP, and cultured for six days. The cell aggregates were isolated and cultured on MEF feeder cells with medium containing FGF4 and heparin adapted to the culture of TS cells. The cells continued to grow for 11 days, but did not give rise to secondary colonies after passage.
In the present study, we investigated the properties of cell aggregates obtained by culture of liver cells transiently treated with low-pH stimulus, which was performed by the group directed by the author. We initially followed the protocol described in the original paper with the detail description in protocol exchange where HCl was applied to achieve low-pH condition. However, we merely obtained the cell aggregates expressing the pluripotency makers as described in this report even when it was combined with the culture in medium containing Fgf2, which was not described in the original protocol but subsequently suggested by the authors. However, when we used ATP instead of HCl, also based on a suggestion by the authors, a few cells in a subset of cell aggregates expressed the pluripotency marker Oct3/4 at levels comparable to those in ES cells that were reproducibly detected by QPCR (Fig. 3c) and immunostaining (Fig. 4b). However, the frequency was very low; 5 × 10 liver cells yielded only ~30 cell aggregates, in which about 20% of the cell aggregates contained 1–2 Oct3/4 positive cells, indicating a frequency per seeded liver cell of 0.0012–0.0024%. Moreover, the pluripotency of such cells was not confirmed by chimera formation assay, and they did not give rise to any stem cell lines. We thus conclude that such cell aggregates do not fulfill the definition for STAP cells proposed in the original studies.
(英文) Moreover, since the frequency of Oct3/4-positive cells in the cell aggregates was quite low, it was impossible to determine whether they were selected from the original population or induced in culture, again highlighting the lack of evidence supporting the existence of the reported STAP phenomenon. An independent examination was made on chimeric potency of STAP-like cell aggregates that were generated by Haruko Obokata. Among 1154 embryos injected with the aggregates, 671 developed beyond E8.5; however, none of the aggregates made significant contribution to any tissues, the details of which was reported in a Biorixiv website (bioRxiv doi: ⁑ttp://dx.doi.org/10.1101/028472). These data are consistent to the recent report by De Los Angeles et al..
加えて、細胞凝集体中のOct3 / 4陽性細胞の頻度は非常に低かったので、それらが元の集団から選択されたか、培養で誘導されたかを決定することは不可能であり、再度、報告されたSTAP現象の存在を保証する証拠の欠如が注目された。小保方晴子自身によって作製されたSTAP様細胞凝集体のキメラ形成能力について独立した検証がなされた。凝集体を注入された1154胚のうち671がE8.5日を超えて発生した。しかし、どの凝集体もいかなる組織にも有意な寄与をしなかった。その詳細はBiorixivのウェブサイト（bioRxiv doi：*ttp://dx.doi.org/10.1101/028472）で報告されている。これらのデータは、De Los Angelesらによる最近の報告書と一致している。
(英文) Materials and Methods
C57BL/6NJcl (CLEA Japan) and 129X1/SvJJmsSlc (Japan SLC) mice were purchased from suppliers. C57BL/6-Tg(CAG-EGFP)C14-Y01-FM131Osb transgenic mouse (CAG-GFP Tg) line was provided by Research Institute for Microbial Disease, Osaka University. C57BL/6J-Tg(GOFGFP)11Imeg transgenic mouse (GOF-Tg) line was obtained from RIKEN Bio-Resource Center (RBRC00771). B6.Cg-Tg(Alb-cre)21Mgn/J transgenic mouse (Alb-cre Tg) line was supplied by Jackson Laboratory. R26R-H2B-EGFP transgenic mouse (Rosa-GFP) line was generated by Laboratory for Animal Resources and Genetic Engineering (LARGE), RIKEN CDB. All animal experiments were carried out in accordance with our Guidelines for the Care and Use of Laboratory animals and were approved by the Institutional Committee for Laboratory Animal Experimentation (RIKEN Kobe Institute).
4–9-day-old mice were euthanized using carbon dioxide and then sterilized with 70% ethanol. For the isolation of spleen cells, excised spleen was minced with scissors and the tissue fragments were dissociated in phosphate buffered serine (PBS) by pipetting.The cell suspension was strained through a cell strainer followed by the collection of cells by centrifugation at 1,000 rpm for 5 min. The collected cells were re-suspended in 5 ml of Dulbecco’s Modified Eagle medium (DMEM; Life Technologies) and added to the same volume of Lympholyte® (Cedarlane), and then centrifuged at 1,000 g<→rpm > for 20 min. The lymphocyte layer was isolated and washed with PBS to obtain single cell suspension.For the isolation of liver cells, excised liver was minced with scissors and the tissue fragments were dissociated by incubation in Type I collagenase (Worthington Biochemical) solution (0.5 mg/ml in Hanks Balanced Salt Solution (HBSS, no calcium, no magnesium; Life Technologies)). Next, the cell suspension was strained through a cell strainer followed by the collection of cells by centrifugation at 1,000 rpm for 5 min. For the isolation of heart cells, the excised heart was minced with scissors and the tissue fragments were dissociated by incubation in Type II collagenase (Worthington Biochemical) solution (0.5 mg/ml in HBSS).The cell suspension was strained through a cell strainer followed by the collection of cells by centrifugation at 1,000 rpm for 5 min.
4から9日齢のマウスを二酸化炭素を用いて安楽死させ、次いで70％エタノールで滅菌した。脾臓細胞の単離のために、摘出した脾臓をはさみで細かく刻み、組織断片をピペットでリン酸緩衝セリン（PBS）中で解離させた。細胞懸濁液を細胞濾過器に通し、続いて1,000rpmで5分間遠心分離することによって細胞を回収した。回収した細胞を5mlのダルベッコ改変イーグル培地（DMEM; Life Technologies）に再懸濁し、同量の Lympholyte® (Cedarlane)に加え、次いで1,000rpmで20分間遠心分離した。リンパ球層を単離し、PBSで洗浄して単細胞懸濁液を得た。肝臓細胞の単離のために、切除した肝臓をハサミで細かく刻み、I型コラゲナーゼ（Worthington Biochemical）溶液(0.5 mg/ml in Hanks Balanced Salt Solution (HBSS, no calcium, no magnesium; Life Technologies))の中で組織断片を保温維持によって単離した。次に、細胞懸濁液を細胞濾過器に通し、続いて1000rpmで5分間の遠心分離によって細胞を回収した。心臓細胞の単離のために、切除した心臓をハサミで細かく刻み、II型コラゲナーゼ（Worthington Biochemical）溶液（HBSS中0.5mg / ml）中での保温維持により組織断片を解離させた。細胞懸濁液を細胞濾過器に通し、続いて1,000rpmで5分間遠心分離することによって細胞を回収した。
(英文) Low-pH treatment and culture of cell aggregates
Diluted HCl solution was prepared with 10 μl of 35% HCl (Nakarai) in 590 μl HBSS. Diluted ATP solution was prepared with ATP (Sigma) in distilled water at 200 mM. Titration of pH with various amount of diluted HCl or ATP was performed with 500 μl of HBSS containing 7 × 10 liver cells. As a routine method, 10 μl of either diluted HCl or ATP solution was added into 500 μl of cell suspension containing 5 × 10 cells in HBSS followed by incubation for 25 min at 37 °C, and then centrifuged at 1,000 rpm at room temperature for 5 min. After the supernatant was removed, precipitated cells were re-suspended and plated onto either adhesive or non-adhesive plates at cell density of 1–5 × 10 cells per well in 1 ml of the culture medium. The culture medium consists of DMEM/HamF12 (Life Technologies) supplemented with 1,000 U/ml of mouse LIF (home-made) and 2% of B27® Supplement (Life Technologies). Optionally, recombinant human Fgf2 (Wako) was added at final concentration of 10 ng/ml.
To quantify the levels of mRNA transcripts, total RNA was prepared by TRIzol® (Life Technologies). cDNA were synthesized from 1 μg of total RNA using SuperScript® III (Life Technologies), and quantified by real-time PCR using a CFX384 system (BioRad). All samples were tested in triplicate, and the mean relative amounts of each transcript were calculated by normalization to an endogenous control Gapdh.
Each cell aggregate was washed with PBS and transferred in 2 μl of PBS into 8 μl RealTime ready Cell Lysis Buffer (Roche) supplied with NP-40, RNAsin and RNase inhibitor. Then 3 μl of cell lysis solution was mixed with 1.5 μl of DNaseI solution (0.2 U/μl) to degradate genomic DNA followed by addition of 1.5 μl of 8 mM EDTA solution to stop the reaction. For reverse transcription of RNA, 3 μl of pre-mixture of SuperScript® VILO reverse transcriptase (Life Technologies) was added into 6 μl of DNaseI-treated cell lysate and incubated at 42 °C for 1 hour. The reverse-transcribed product was pre-amplified with Plutinum multiplex PCR master mix using pooled primer mixture using the reaction cycle (95 °C for 30 sec; 60 °C for 90 sec; 72 °C for 60 sec) for 14 cycles. The mixture was treated with Exonuclease I to remove the primers for pre-amplification, and quantitative PCR was performed with the primer pairs specific for each gene using Quantitest SYBR Green PCR mix (Qiagen) in BioRad CFX384 Real-Time System (Bio-Rad). All samples were tested in triplicate, and the mean relative amounts of each transcript were calculated by normalization to an endogenous control Gapdh or Gnb2l1.
mRNA転写物のレベルを定量するために、 TRIzol® （Life Technologies）により全RNAを調製した。 SuperScript® III (Life Technologies)を用いて1μgの全RNAからcDNAを合成し、CFX384システム（BioRad）を用いたリアルタイムPCRによって定量した。 全ての試料を三重に試験し、各転写産物の平均相対量を、内在性コントロールGapdhに対する標準化によって計算した。
Cells were fixed by 4% paraformaldehyde in PBS for 30 min at 4 °C and then permeabilized by 0.1% Triton X-100 in PBS for 15 minutes at room temperature (RT). After brief washing with PBS followed by blocking with PBS containing 2% FCS, the cells were incubated with the following primary antibodies: anti-Oct3/4 rabbit antiserum15 and anti-Nanog rat monoclonal antibody (R&D) for overnight at 4 °C.After washing with PBS, the cells were incubated with Alexa Fluor 488- or 633-conjugated donkey antibodies (Invitrogen) were used in a proper combination of species specificity as indicated in Figure legends.Fluorescent images were captured with an IX51 microscope with DP70 digital camera (Olympus) or a Leica SP8 confocal microscope (Leica).
For flow cytometric analyses, cell aggregates were harvested, washed by PBS, and incubated with TrypLETM Select (Life Technologies) for 5 min. After dilution with culture medium, aggregates were dissociated into single cells by gentle pipetting. Cells adhered to the culture substrate were also harvested following a standard method. These cells were mixed and collected as pellets by a centrifugation. For quantification of GFP-positive population, dissociated cells were re-suspended in 500 μl HBSS containing 1 μl of DRAQ7, a cell-nonpermeable DNA dye (for the detection of dead cells; Cell Signaling). When combined with a staining for CD45 antigen, cell pellets were suspended with 50 μl HBSS containing 10 μl of APC-conjugated rat anti-CD45 antibody (BD Pharmingen), and incubated for 30 min on ice. For co-staining with CD45/E-cadherin antibodies, cell pellets were suspended in culture medium, and then incubated for 30 min in CO2 incubator. The cells were harvested and suspended with 50 μl HBSS containing 5 μl biotin-labeled rat anti-E-cadherin antibody (ECCD2). After incubation for 30 min on ice, the stained cells were once washed by HBSS, re-suspended with 50 μl HBSS containing 1 μl PE-conjugated streptavidin (Life Technologies) and 10 μl APC-conjugated anti-CD45 antibody, and further incubated for 30 min on ice. These stained cells were once washed by HBSS and suspended with 500 μl HBSS.
After the cell suspension was passed through a filter mesh, the cells were analyzed using a FACSAria IIIu cell sorter (Becton Dickinson).
Cell aggregates obtained by the ATP treatment of liver cells were cut into smaller pieces using a laser (XYClone, Nikko Hansen & Co., Ltd) or shaped glass capillaries, and single piece was then microinjected into each 8-cell or blastocyst stage embryo from ICR mice (Charles River Laboratories Japan, Inc.). Injected embryos were transferred to the uterus of 2.5 dpc pseudopregnant ICR females (Charles River Laboratories Japan, Inc.) on or the next day of injection.
肝細胞のATP処理により得られた細胞凝集体を、レーザー（XYClone、Nikko Hansen＆Co.、Ltd）または成形ガラス毛細管を用いてより小さい断片に切断し、次いで単片をそれぞれICRマウス（Charles River Laboratories Japan、Inc.)の8細胞期胚または胚盤胞期胚に注射注入した。 挿入胚は移植当日もしくは翌日、交尾後<訳注:不妊オスを使う>2.5日のメスの偽妊娠ICRマウス(Charles River Laboratories Japan、Inc.）の子宮に移入された。
(英文) Culture for derivation of stem cells
The culture medium for derivation of ES-like stem cells consists of Glasgow-modified eagles medium (GMEM, Sigma), 15% KnockOut Serum Replacement® (KSR, Life Technologies), 1 × non-essential amino acids (NEAA, Nakarai), 1 × Sodium Pyruvate (Nakarai), 10－4 M 2-mercaptoethanol (Nakarai), 1,000 U/ml of LIF and 10 μM ACTH (Kurabo on consignment). We confirmed the medium is optimal for the culture of conventional ES cells. The culture medium for derivation of TS-like stem cells consists of GMEM, 20% FCS, 1 × NEAA, 1 × Sodium Pyruvate, 10－4 M 2-mercaptoethanol, 25 ng/ml of recombinant mouse Fgf4 (Wako) and 1 μg/ml of heparin (Wako).We confirmed the medium is optimal for the culture of conventional TS cells. To derive stem cells, cell aggregates were isolated under a microscope and transferred into a well of 96-well plate with 100 μl of the culture medium and 1,000 feeder cells. Feeder cells were prepared by treatment of mouse embryonic fibroblasts prepared from day 14 C57BL6 embryos with Mitomycin C (Wako) for 3 hours.
ES様幹細胞の誘導のための培地は、グラスゴー改変イーグル培地（GMEM、Sigma）、15％KnockOut血清代替物®（KSR、Life Technologies）、1×非必須アミノ酸（NEAA、Nakarai） 1×ピルビン酸ナトリウム（Nakarai）、10-4M 2-メルカプトエタノール（Nakarai）、1,000U / ml LIFおよび10μMACTH（クラボウは委託）で構成されている。我々は、培地が従来のES細胞の培養に最適であることを確認した。TS様幹細胞の誘導のための培養培地は、GMEM、20％FCS、1×NEAA、1×ピルビン酸ナトリウム、10 -4 M 2-メルカプトエタノール、25ng / mlの組換えマウスFgf4（Wako）および1μg / mlのヘパリン（Wako）で構成されている。我々は、培地が従来のTS細胞の培養に最適であることを確認した。幹細胞を誘導するために、細胞凝集体を顕微鏡下で単離し、100μlの培養培地および1,000個のフィーダー細胞を有する96ウェルプレートのウェルに移した。フィーダー細胞は、14日目のC57BL6胚から得られ、マイトマイシンC（Wako）で3時間調整されたマウス胚線維芽細胞の処理によって準備された。
How to cite this article: Niwa, H. Investigation of the cellular reprogramming phenomenon referred to as stimulus-triggered acquisition of pluripotency (STAP). Sci. Rep. 6, 28003; doi: 10.1038/srep28003 (2016).
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We would like to thank the assistance of the members of the Scientific Validity Examination Team, Dr. Hiroshi Kiyonari and Mr. Kenichi Inoue for chimera production and animal breeding, and Laboratory of Animal Resources and Genetic Engineering for animal housing. We also thank Mr. Douglas Sipp for critical discussion of this report. This examination was supported by the grant for Scientific Validity Examination by RIKEN President.