August 15, 2020

The Niche

Knoepfler lab stem cell blog

Review of Obokata STAP cells Nature papers

In two Nature papers (here and here) published today researchers report the astounding finding of reprogramming differentiated cells back to a pluripotent or even totipotent state simply by exposing the cells to extreme environmental stress, creating cells they called STAP cells. Update: see more thoughts on STAP stem cells here.

No genes. No proteins. No nuclear transfer. Just stressing the heck out of the cells, for example, by exposing them to acid. Nature writer David Cyranoski entitled his news piece on these papers “Acid bath offers easy path to stem cells”, which I thought was clever.

The authors report the creation of iPS like cells via sub-lethal stress and have named the cells stimulus-trigged acquisition of pluripotency (STAP) cells.

I agree with Cyranoski that these new papers on STAP cells are like to fuel a long-running debate. I also later in this post raise 6 key questions about this finding that should for now somewhat temper the over-exuberance that I’m hearing.

For now, the two papers are making a big splash. Stimulus-trigged fate conversion of somatic cells into pluripotency has first author Haruko Obokata and senior author Charles Vacanti. The second paper in the same issue of Nature and also with Obokata as first author has Teruhiko Wakayama as senior author: Bidirectional developmental potential in reprogrammed cells with acquired pluripotency.

What to make of these papers?

The second paper seems to be really just showing that the STAP cells are in fact not just pluripotent, but totipotent and can make extraembryonic tissues too. That seems surprising.

Obokata FIg. 1, STAP cells production process.
Obokata FIg. 1, STAP cells production process.

The first paper on STAP cells is really where we need to dig in deeper at this point I think.

The schematic above from the paper’s Fig. 1a shows the key protocol employed. By doing something akin to hitting the cells over the head with a sledgehammer of a pH 5.7 (physiological pH is more typically thought of as around 7.4), they report the blood cells of 1-week old mice turned on expression of an Oct-GFP reporter as they floated around in clusters in the media.

Obokata FIg. 2a, STAP cells
Obokata FIg. 2a, supposed colonies of STAP cells.

One counterintuitive thing is that the team reported that they could make this happen with T cells and also a wider population of CD45+ cells of the spleen, but not from more primitive hematopoietic progenitor cells, which offhand I would have imagined should be more amenable to reprogramming. Importantly the team provided pretty good evidence that the STAP cells arose from the differentiated blood cells themselves rather than potentially from rare pre-existing primitive stem cells in the cell populations.

The Oct4-GFP+ cells glow green and they also reportedly functionally possess pluripotency. The cells not only express pluripotency factor proteins including Oct4 and have pluripotency-associated surface markers (see Fig. 2 including panel a shown above), but also could reportedly differentiate into ectoderm, endoderm, and mesoderm by in vitro differentiation assays and teratoma assays. The pluripotency factor transcriptional activation is mechanistically tied to promoter demethylation.

The STAP cells formed colonies over time after they were dissociated. The colonies definitely were not as pretty as regular mouse ES or iPS cells however (see Figure 2f-i).

In Fig. 3 they go on to show that not just blood cells, but a variety of other cells can be made into STAP cells including brain, skin, muscle, fat, bone marrow, lung, and liver of 1-week old Oct4-GFP mice.

In Fig. 4 they show that STAP cells (from cag-gfp reporter mice) can contribute to the germ-line via chimeric mouse generation. In Fig. 5 they report that they can make ES cell-like cells from STAP cells. My only hesitation with this particular data is that again the STAP colonies don’t look particularly ES cell-like, but I wonder how much does that matter given the rather solid functional and molecular assays? What do you think?

The authors themselves point out that the actual mechanism of reprogramming remains unclear at this time:

A remaining question is whether cellular reprogramming is initiated specifically by the low-pH treatment or also by some other types of sublethal stress such as physical damage, plasma membrane perforation, osmotic pressure shock, growth-factor deprivation, heat shock or high Ca2+ exposure.

After a relatively quick read, no particular red flags jump out at me from the STAP cell paper. It just seems too good and too simple of a method to be true, but the data would suggest so far at least that this team is onto something really important.

But key open questions remain before anyone can really say just how important this is.

  • 1. Will it be reproducible by other labs?
  • 2. Will it work in human cells?
  • 3. Will it work in adult cells?
  • 4. What are the molecular mechanisms?
  • 5. Do these cells possess significant rates of mutations or epi-mutations, the latter being abnormalities in the epigenome?
  • 6. Are these cells tumorigenic (besides forming teratoma)?

In particular, if the answer to one or more of the first 3 questions is no, then the impact could be significantly muted.

Still the studies have both practical implications for potentially simple reprogramming of cells, and also suggests some fundamental concepts about cell and organismal biology that are intriguing including the idea that differentiated cells can be far more plastic than we all imagined. In addition it invokes the idea that when animals and hence their cells sustain injuries, a stem cell-like program may be induced.

Bottom line, I want to see what the future data addressing the 6 questions above tell us before getting too excited, but it’s definitely cool.

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