Reprogramming cells into neurons: At least two roads do lead to Rome

Two paths can diverge… and still lead to the same outcome. In their latest publication, researchers from the Elling lab at IMBA show that two closely related transcription factors kickstart distinct developmental paths to reprogram mouse embryonic stem cells into a convergent neuronal fate. The findings were published on September 2 in Nature Communications.

During development, a single cell differentiates into a complex embryo, built out of different cell and tissue types. To achieve this differentiation, transcription factors interact and compete to establish cell identity – in intricate paths that remain to be deciphered. Now, IMBA group leader Ulrich Elling and PhD student Gintautas Vainorius together with a team of IMBA and international researchers examined how two closely related transcription factors, Ascl1 and Ngn2, turn mouse embryonic stem cells into neurons. The researchers show that the paths initiated by these two “family members” diverge drastically, but both transition cells between two identical states, and eventually give rise to neurons.

The two transcription factors Ascl1 and Ngn2 seem like two siblings with opposing characters. Ascl1 and Ngn2 are structurally similar and convert different cell types into functioning neurons – so efficiently that they are used to reprogram cells into neurons in vitro. In their new study, Elling and Vainorius disentangle how the different transcription factors lead to the same outcome, the specification of a functioning neuron. 

These paths are not straightforward, the researchers found. They show that Ascl1 works by swiftly and completely shutting down the stem cell program in the cells and arresting the cell cycle. Thus, Ascl1 wipes the cell's initial identity and then installs neuronal cell fate. Surprisingly, Ascl1 additionally induces a competing trophoblast lineage by mimicking Ascl2, a trophoblast specific bHLH transcription factor.  In contrast, Ngn2 does not fully eliminate the stem cell program but keeps some stem cell genes turned on, overlaying pluripotency and neuronal stem cell networks. This allows the cells to transition through an intermediate stem cell-like state before they become neurons.

Taken together, the researchers show that Ascl1 and Ngn2, although related structurally, induce distinct genetic networks and downregulate the pluripotency network differently. This leads to the formation of different alternative cell lineages and different genetic dependencies that govern the transitions between cell lineages. This highlights how closely related transcription factors can take unique approaches to push cells from one identity to another, while eventually reaching the same outcome.