Animal development is a unidirectional process that steers undifferentiated cells of the early embryo into specialized cell types such as brain, muscle or skin cells. Once differentiated, there is almost no way back. A notable exception is the de-differentiation of mature into immature cell types during injured tissue regeneration. It is unknown if the reversal of cellular specialization proceeds along the same path used by natural development - in reverse direction - or along distinct paths. The Betschinger group has now uncovered a bi-directional and reversible differentiation mechanism, shedding light on how developmental unidirectionality is safeguarded.
Although humans have some tissue regeneration capacities, for example to repair skin or cartilage after injury, those are very limited compared to other animals, such as salamanders, that can rebuild entire limbs upon amputation. It has long been thought that humans, like most mammals, are poor regenerators because the development and maturation of adult cell types is irreversible, thus preventing de-differentiation into cell types that are able to reform tissues. However, this view radically changed when Shinya Yamanaka (awarded with the Nobel Prize in Physiology or Medicine 2012 for that discovery) demonstrated that adult mammalian cells can be experimentally reprogrammed into pluripotent stem cells, capable of differentiating into any adult cell type. Therefore, poor regeneration is not due to an inability of our cells to de-differentiate, but because this process is actively restricted.
Molecular mechanisms that limit de-differentiation have already been identified, but a fundamental question remains: To what extent is de-differentiation a reversion of natural development? To what extent do de-differentiation and differentiation employ common mechanisms?
Intrigued by that question, Daniela Mayer, a PhD student in the Betschinger group, set out to identify common regulators of differentiation and de-differentiation. Using interconversion of unspecialized pluripotent embryonic stem cells (ESCs) and more specialized epiblast stem cells (EpiSCs) - ESCs can differentiate into EpiSCs and EpiSCs can reprogram into ESCs - she performed a large-scale comparative loss-of-function approach to systematically identify genes that both drive ESC differentiation and inhibit EpiSC de-differentiation.
While Mayer identified many genes acting in one or the other direction, only one - the transcription factor Zfp281 - acted bi-directionally. ESC differentiation and EpiSC de-differentiation therefore proceed along vastly distinct trajectories but go through a common, Zfp281-dependent, bottleneck. Mechanistically, Mayer further showed that Zfp281 is invariantly bound to DNA but controls cell type-specific gene expression and, consequentially, cell type conversion by interacting with accessory factors in the differentiated cell state.
“We were surprised to identify only one bi-directionally acting factor,” says Joerg Betschinger. “This suggests that Zfp281 has a prominent role in safeguarding the directionality of an early developmental mechanism and it will be exciting to see if this or related mechanisms also restrict adult tissue regeneration.”
Daniela Mayer, Michael B Stadler, Melanie Rittirsch, Daniel Hess, Ilya Lukonin, Maria Winzi, Austin Smith, Frank Buchholz, Joerg Betschinger (2019) Zfp281 orchestrates interconversion of pluripotent states by engaging Ehmt1 and Zic2 . EMBO J. e102591