Somatic Cells Turned Pluripotent by Acid Treatment – Major Breakthrough, Or Too Good To Be True?
The potential use of induced pluripotent stem cells in medicine has the power to completely revolutionize the way physicians treat human diseases. Imagine the possibility of treating diseased or damaged tissue by taking a patient’s own cells from an unaffected tissue, and turning them into healthy cells to replace the bad ones. This idea has been around for years, but has always seemed too far away to consider in reality because it was so difficult to reliably manipulate the cells to induce pluripotency and re-differentiation into the new tissue. The hope of using stem cells for therapeutic purposes has gained renewed attention recently due to the exciting finding by Haruko Obokata, Charles A. Vacanti, and colleagues that differentiated somatic cells can be induced into pluripotency by simply exposing them to extreme external stimuli, such as a low pH buffer.
The researchers started by using fluorescence-activated cell sorting (FACS) to isolate differentiated lymphocytes from mice containing Oct4, a stem cell marker, fused to the green fluorescent protein (GFP). The purified cells would glow green when converted into a pluripotent state, but not when differentiated because Oct4-GFP would not be expressed. Incubation of the differentiated lymphocytes in a citric acid buffer (pH ~5.5) for about 30 minutes induced significant Oct4-GFP expression, suggesting that these GFP+ cells were reverted into pluripotency. Similar results were also seen with other extreme external stimuli, including mechanical stress, heat shock, high calcium treatment, and membrane disruption. A week after the low pH treatment, the GFP+ cells also expressed several other pluripotency markers in addition to Oct4, such as Nanog, E-cadherin, and SSEA1. The mechanism for this transition to pluripotency is likely epigenetic in nature because the authors observed that promoters for the genes encoding Oct4 and Nanog were demethylated in the GFP+ cells relative to the differentiated somatic cells.
In a very important experiment in this study, the authors were able to show that the GFP+ cells with pluripotent properties could be re-differentiated into different cell types, including ectodermal, mesodermal, and endodermal cell lineages. This assay convinced the scientists that they actually created bonafide pluripotent cells from somatic cells, and they therefore referred to them as STAP cells, which stands for Stimulus-Triggered Acquisition of Pluripotency. They rounded out this study by performing additional experiments to further characterize the STAP cells by showing that they are generally non-proliferative, they can be derived from multiple types of tissue sources, and that STAP cells could give rise to chimeric mice when injected into blastocysts. GFP+ STAP cells were detected in all tissues investigated in these chimeric mice, and they could produce GFP+ offspring. These results indicate that the STAP cells have the ability to differentiate into all types of somatic cell lineages, and that they can even develop into germline cells. Furthermore, the authors went on to show that under certain media conditions, STAP cells could be maintained and replicated in culture, and these proliferative cells were called STAP stem cells. Similar to the case for the STAP cells, STAP stem cells were also pluripotent, and they could be differentiated into multiple different kinds of cells in culture, including beating heart muscles. STAP stem cells could also contribute to forming chimeric mice when injected into blastocysts, and mice derived from blastocysts containing STAP stem cells fully developed to become healthy adults and were competent for producing offspring.
In this report, the authors described a very easy and seemingly robust method to de-differentiate somatic mouse cells, giving hope to the possibility that cellular identity might also be controlled in human cells for therapeutic purposes in the future. This complex discovery from a simple protocol might leave many scientists thinking “Why didn’t I think of that?!?” However, there has recently been some controversy surrounding this report, and several labs have had difficulty reproducing the findings. What do you think? Is this a groundbreaking discovery, or does it just seem too good to be true?
Obokata H, Wakayama T, Sasai Y, Kojima K, Vacanti MP, Niwa H, Yamato M, & Vacanti CA (2014). Stimulus-triggered fate conversion of somatic cells into pluripotency. Nature, 505 (7485), 641-7 PMID: 24476887