Reprogramming with Low Ph

RESEARCH HIGHLIGHTS

Nature Reviews Molecular Cell Biology | AOP, published online 12 February 2014; doi:10.1038/nrm3754

STEM CELLS in blastocyst injection assays, STAP cells contributed to both the embryonic and placental membranes. This Reprogramming with low pH was unexpected, as injected ES cells are usually found to only contrib- ute to the embryonic portion of Reprogramming of somatic cells to The Oct4–GFP+ cells expressed chimaeras, not the placental portion. pluripotency is generally achieved several pluripotency-related marker Intriguingly, STAP stem cells lost the temporary by perturbing nuclear functions, for genes (including Nanog) and marker capacity to differentiate into extra- exposure example, by nuclear transfer or by proteins, similarly to embryonic stem embryonic tissues. However, when introducing exogenous transcription (ES) cells. Moreover, the promoter primary STAP cells were cultured to low pH factors. Now, Obokata et al. show regions of Oct4 and Nanog were in a medium containing fibroblast can quickly that lineage-committed lymphocytes extensively demethylated in low- growth factor 4 (FGF4), they could reprogramme and other cells can be converted to pH-induced Oct4–GFP+ cells, which be converted into proliferative cells cells to a pluripotent state by altering only indicates that key pluripotency genes capable of contributing to placental the properties of their extracellular underwent substantial epigenetic tissues. It is important to note that pluripotency environment. They termed this reprogramming. such ‘FGF4‑induced stem cells’ differ unusual fate-conversion phenomenon When carrying out assays to from trophoblast stem cells (TSCs), ‘stimulus-triggered acquisition of test the differentiation capacity of the precursors of differentiated pluripotency’ (STAP). Oct4–GFP+ cells, the authors found placental cells, by also contributing to The authors used mouse CD45+ that cells that expressed GFP at embryonic tissues and by continuing leukocytes, isolated from postnatal high levels could give rise to three to express Oct4. (1‑week-old) spleens, carrying GFP germ-layer derivatives in vitro and, Together, these studies show under the control of the octamer- when injected into mice, they formed that temporary exposure to low pH binding protein 4 (Oct4) promoter. teratomas. Thus, committed somatic can quickly reprogramme cells to These cells normally do not express cells can be converted into a state pluripotency. Converted cells can dif- pluripotency-associated markers and, of pluripotency by external stresses, ferentiate into all somatic cell, germ consistently, did not express Oct4 such as low pH; the resulting cells cell and extra-embryonic lineages, when cultured in a medium contain- were referred to as ‘STAP cells’. depending on the culture medium ing leukaemia inhibitory factor (LIF) Blastocyst injection of STAP used for expansion. Importantly, as and B27, which is used for the main- cells led to chimaeric embryos with they are different from ES cells and tenance of pluripotent mouse cells. STAP cells contributing to all the TSCs and have a wider differentia- However, a 30 minute incubation in tissues examined. Furthermore, the tion potential, these cells constitute a low-pH medium (pH 5.4–5.8) led offspring of chimaeric mice included a distinct state of pluripotency. to the appearance of several clusters animals derived from STAP cells, The authors suggest that transient of cells expressing Oct4 (Oct4–GFP+ demonstrating their germline trans- exposure to strong stimuli might cells). After culture for a further mission, which is a strict criterion for ‘unlock’ somatic cells from lineage 7 days in medium with LIF pluripotency. However, when com- commitment, revealing a surpris- and B27, most of the cells died. pared to mouse ES cells, the authors ing degree of plasticity. The exact However, approximately one-third observed that STAP cells differed mechanisms underlying this unlock- of the surviving cells expressed in that they had limited capacity ing remain to be elucidated, and the Oct4 pluripotency marker for self-renewal in LIF-containing intriguing questions arise such as and lost expression of the CD45 medium. This limitation was for what purpose somatic cells may marker (Oct4–GFP+ CD45– cells), circumvented by addition of adreno- latently possess a self-driven ability which is indicative of dedifferentia- corticotropic hormone (ACTH) to for reprogramming. tion. Importantly, no cell divisions the medium, which converted STAP Kim Baumann were observed after the low-pH cells into proliferative cells. Such treatment, which suggests that in vitro expandable cells were named ORIGINAL RESEARCH PAPERS Obokata, H. et al. Oct4–GFP+ cells were gener- ‘STAP stem cells’. Stimulus-triggered fate conversion of somatic cells into pluripotency. Nature 505, ated de novo, rather than In a separate study, Obokata et al. 641–647(2014) | Obokata, H. et al. Bidirectional through selection followed further examined the developmental developmental potential in reprogrammed by amplification of potential of STAP cells. This was cells with acquired pluripotency. Nature 505, 676–680 (2014) NPG stress-resistant cells. prompted by the observation that,

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 15 | MARCH 2014