I’ve written about human stem cells from all sorts of sources, from human embryos circa 1997 (“Embryonic Stem Cells Debut Amid Little Media Attention,” The Scientist, too ancient for a link), to old people’s teeth. Dominating the field in recent years have been the human induced pluripotent stem (hiPS or reprogrammed) cells that Shinya Yamanaka introduced in 2007. Add four key genes to fibroblasts, and the cells go back in developmental time, regaining pluripotency: the ability to yield cell types descending from any of the three layers that make up the early embryo. (The genes encode transcription factors, which are proteins that turn on other genes. These four are called Yamanaka factors. RNA or proteins are used directly, too.)
Although the terminology for all these stem cells can be confusing, the meaning of “pluripotency” is thankfully clear: a cell that can divide, giving rise to many types of specialized cells. And by definition, a stem cell renews itself, keeping things going.
A New Type of Stem Cell
Pascale Guillot, PhD, from Imperial College London and her colleagues describe another source of human pluripotent stem cells today in the journal Molecular Therapy: human amniotic fluid stem cells exposed to valproic acid, or AFSC_VPA. Surely they need a catchier name -- this one’s even harder to remember than hES or hiPS – so I’ll call them simply “new”. The cells lie in the human life cycle between the embryos and old people’s teeth: floating in the fluid that cushions a fetus.
The amniotic fluid in the study came from abortion material collected at 8 to 12 weeks of gestation. Unaltered, the cells aren’t pluripotent, but exposure to valproic acid does the trick. It’s a seizure medication and mood stabilizer that controls gene expression, a histone deacetylase inhibitor). The cells are grown on a protein-rich gel and also exposed to the medium used to culture hES cells.
Valproic acid is cheap, FDA approved, and although it causes neural tube defects in an embryo, is used on the amniotic stem cells outside the body. “I think this could be an exciting new player in the pluripotent stem cell field, and certainly has the potential of making a big impact on regenerative medicine – provided the somewhat controversial way of obtaining these cells can be overcome,” says Florian Siebzehnrubl, PhD, a researcher in the department of neurosurgery at the University of Florida in Gainesville. Collecting the cells at amniocentesis, a few weeks later, would solve that problem.
Valproic acid as a route to pluripotency offers advantages other than availability and price. It’s safer than injecting genes, RNA, or proteins, which is inefficient and can induce tumors. And applying the drug to isolated amniotic fluid cells doesn’t entail culturing stem cells from embryos, as is the case for hES cells. The new cells change shape and form compact colonies – something’s clearly happening.
Unaltered amniotic fluid stem cells are an intermediate of sorts between hES cells and adult and probably iPS cells, in terms of developmental potential. They seem poised at a critical precipice in development when they can still give rise to a few cell types, but no longer have the full spectrum of possibilities that comes with pluripotency. “It’s easier to derive pluripotent stem cells from these cells than from adult cells, in which crucial pluripotency genes are totally switched off,” explains Dr. Guillot. In amniotic fluid stem cells, the Yamanaka factors are scant, but not absent as they are in the adult cells. “Valproic acid is a small molecule epigenetic modifier that relaunches transcription” she adds, restoring some developmental potential to the amniotic cells.
Researchers already knew that hiPS cells are not the same as hES cells, transcriptionally speaking. And the new research supports the idea that stem cells that may look alike on the outside can behave differently. Figuring out the distinctions will be critical to using pluripotent stem cells clinically.
Intriguing Origins and an Amazing Journey
The new stem cells share 82% of their transcriptome – the set of active genes – with hES cells. A Venn diagram is a good way to sort out what the different types of stem cells share – and how they differ (see figure).
As expected, the new stem cells share expression of some genes with the amniotic stem cells from which they’re derived, but they also share expression of 273 genes with hES cells that aren’t expressed in the non-valproic acid amniotic stem cells. That is, the drug does something. Zeroing in on the functions of the 273 genes, particularly four that sculpt the tadpole-shaped sperm from blobby precursors, provided clues to the normal role of the stem cells in amniotic fluid: they may be primordial germ cells (PGCs) -– future sperm and eggs.
“It’s possible that PGCs retain their naïve status, because while floating in the amniotic fluid they do not have any contact with the extracellular matrix and other cells, which normally drive stem cells to differentiation,” explains Dr. Guillot. The researchers looked at cells from male pregnancies, because the fetal cells are easy to distinguish from maternal cells by the presence of the Y chromosome.
Like amniotic fluid stem cells, PGCs aren’t new to stem cell science. In fact, PGCs were the source of the embryonic germ (EG) cells that John Gearhart’s group, then at Johns Hopkins, described in 1997 and I wrote about. But Gearhart’s EG cells, as well as hES cells, come from the inner cell mass, the smear of cells within a hollow ball of cells (the blastocyst) that becomes the embryo. Instead, PGCs, and the new stem cells that resemble them, arise from the surrounding “extra-embryonic” portion of the blastocyst.
Whether a stem cell hails from the “embryo proper” or the extra-embryonic cells makes a big difference, to some people, when it comes to the ethics of using embryos, although sampling from either as early as the blastocyst stage destroys the whole. But the PGCs are especially confounding, ethically, because they originate outside the embryo, then join it, then may leave it. The wandering cells are caught up in the embryo’s initial folds into three layers, then, following signals sent out by nearby cells, migrate to a structure called the genital ridge, which becomes the testes. The PGCs arrive by the 7th week.
And during this developmental journey, a few PGCs may lose their way, winding up afloat in the amniotic fluid, once again outside the actual embryo as it nears the 8-week-mark that defines the fetal stage.
Maximal Potential, Minimal Risk
The researchers speculate that the AFSC_VPAs will find clinical uses because they can give rise to structures from all three embryo layers. So far in experiments the cells yield neural tube and squamous epithelium (ectoderm), connective tissues including blood (mesoderm), and gut and lung linings (endoderm).
“One of the hurdles to overcome in regenerative medicine, either for cell-based therapies or for tissue engineering, is the limited capacity of the cells to differentiate into non-mesodermal lineages. Here, we can use these cells not only for the treatment of hematopoietic (blood) diseases, but also for pathologies affecting tissues of endodermal or ectodermal origin,” explains Dr. Guillot.
But rather than saving amniotic stem cells doused with valproic acid for individuals, the researchers envision a limited number of cell lines stored in banks from which many people can make withdrawals. The precedent lies in a simulation for hES cells. Researchers at
Cambridge University calculated that 150 hES cell lines could provide cell therapies for about a third of the human population, based on HLA types (the cell surface molecules used in tissue matching). The new stem cells might be used in a similar way, and also for drug testing and studying “diseases-in-a-dish,” Dr. Guillot says.
The elephant in the room, of course, is the embryo question. “The ethical advantage of amniotic fluid stem cells over hES cells is that, technically, no embryo gets destroyed when they are obtained. The derivation of hES cells requires a blastocyst, which after hES isolation cannot implant and give rise to an embryo anymore. The possibility of isolating pluripotent cells without affecting the embryo is very appealing. This is a bit of a double-edged sword, though, since the authors had to use amniotic fluid from terminations to get to the stem cells,” says Dr. Siebzehnrubl. But the cells can be isolated from amniocentesis material, according to a news release from the journal.
My fascination with the Molecular Therapy paper is more philosophical: this new source of stem cells represents an organism within an organism within an organism, like Russian nesting dolls. Within pregnant women, fetuses float in fluid that contains cells that are seeds of the next, the third, generation.
Although cautioning that further experiments are necessary to confirm that the new cells are functionally equal to PGCs, Dr. Guillot agrees. Use of the AFSC_VPA stem cells “would allow us to capitalize on the past to repair the future.”
It’s a new way of looking at the potential of stem cells.