The creation of pluripotent stem-cell lines from early embryos has enabled the indefinite capture in vitro of one of nature’s most fascinating but fleeting faces. Yet, not all embryonic stem cells (ESCs) are alike. Mouse ESCs are free from differentiation bias and epigenetic restriction, whereas human ESCs have already made some fundamental developmental decisions. But can you tell the ‘naïve’ from the ‘primed’ stem cells, and why, when it comes to stem cells, is ignorance bliss?
Naïve versus primed pluripotent stem cells
To understand the differences between naïve and primed pluripotency, we must take a journey with the early mouse embryo. A few days after fertilization, the embryo is a free-floating ball of cells called the blastocyst. Within the blastocyst, the inner cell mass contains the epiblast, which produces all cells of the adult organism. ESCs are derived from the epiblast of the pre-implantation blastocyst. They have very low levels of marks involved in developmental decisions, including DNA methylation and H3K27me3, and female cells express both X chromosomes. Inject ESCs into another blastocyst, and they will contribute to the developing organism to produce chimeric animals.
Two days later, the blastocyst has undergone implantation and several morphological changes. Pluripotent cells can still be derived from the epiblast at this stage and are called epiblast stem cells (EpiSCs). Although they can still differentiate into all three germ layers, these cells express high levels of de novo DNA methyltransferases and have started to re-methylate their genome. They down-regulate key pluripotency genes but up-regulate differentiation markers. Female cells express only one X chromosome. Inject EpiSCs into a pre-implantation blastocyst and they die.
Human embryonic stem cells aren’t so naïve
Despite their blastocyst origin, human ESCs exist in a primed pluripotent state reminiscent of mouse EpiSCs. They share many properties with EpiSCs and their self-renewal in culture relies on the same signaling pathways. Why human ESCs fast-forward in developmental time when removed from their natural environment is currently unknown. Nonetheless, several attempts have been made to isolate the elusive naïve state. In mice, naïve and primed cells are interconvertible, which formed the basis for a publication in 2010 showing that the forced expression of pluripotency factors in primed cells generates naïve-like human ESCs. Nonetheless, these cells remained unstable and the search continued.
Capturing and maintaining naïve-like human embryonic stem cells
Four teams (Chan et al., Gafni et al., Ware et al., Theunissen et al.) have since isolated naïve-like cells without transgenes by culturing primed cells in complicated cocktails of cytokines and/or chemical inhibitors. Although these human naïve-like cells closely resemble mouse ESCs, it is still unclear whether a stable transition to naïve pluripotency has occurred in these cells.
Takashima et al. transiently expressed KLF2 and NANOG in primed cells to ‘rewire’ their transcriptional circuity. The expression profile of these rewired human ESCs was similar to that of mouse ESCs but distinct from that of naïve-like cells produced by altering culture conditions alone. Remarkably, despite this long quest to isolate naïve cells, a recent study showed that a small population of naïve-like ESCs is present in conventional cultures of human ESCs. These important developments bring us closer to understanding the biology of naïve cells and capturing this state indefinitely.
Ignorance is bliss – advantages of naïve over primed stem cells
So why all the fuss? Naïve cells have several advantages over primed cells. For starters, homologous recombination is more efficient in naïve cells, probably because their chromatin is more open. Thus, naïve cells could be used in future stem cell therapies for the targeted correction of genetic defects in patient-derived iPSCs. Moreover, naïve cells are uniquely capable of producing chimeras. Gafni et al. used naïve-like cells derived from iPSCs to create mouse–human chimeric embryos, showing that human naïve cells are developmentally compatible with other species. Although this may sound a little Frankensteinish, in the right hands this approach could generate human tissues for organ transplantation or ‘humanized’ animal models in which to study human diseases.
So primed cells forget what you have learnt, it’s of no use to us anyway.