A XEN-like State Bridges Somatic Cells to Pluripotency during Chemical Reprogramming

Abstract

Somatic cells can be reprogrammed into pluripotent stem cells (PSCs) by using pure chemicals, providing a different paradigm to study somatic reprogramming. However, the cell fate dynamics and molecular events that occur during the chemical reprogramming process remain unclear. We now show that the chemical reprogramming process requires the early formation of extra-embryonic endoderm (XEN)-like cells and a late transition from XEN-like cells to chemically-induced (Ci)PSCs, a unique route that fundamentally differs from the pathway of transcription factor-induced reprogramming. Moreover, precise manipulation of the cell fate transition in a step-wise manner through the XEN-like state allows us to identify small-molecule boosters and establish a robust chemical reprogramming system with a yield up to 1,000-fold greater than that of the previously reported protocol. These findings demonstrate that chemical reprogramming is a promising approach to manipulate cell fates.

The XEN of reprogramming

“…The study published by Zhao et al [5] now confirms that CiPSCs are derived via a XEN-like state, which allowed them to enhance the efficiency of the chemical-induced reprogramming by dissection of the cell fate transitions. They first observed that CiPSCs emerged from epitheloid colonies that expressed XEN markers and knocking these markers down hampered CiPSC formation.…”

“…Although the past decade has seen methodological improvements, generation of iPSCs still requires genetic manipulation that hinders their therapeutic application. The chemical cocktail developed by Zhao et al [5] of Deng's group circumvents this problem.…”

“…They first performed lineage tracing experiments via fibroblast specific protein 1 (Fsp1) to confirm that CiPSCs can be indeed generated from differentiated fibroblasts rather than a specific cell type amenable to pluripotency within the culture. Neural stem cells isolated from mouse brains by SSEA1 surface marker required another chemical, Ch55 (5), in addition to VC6TFE and decreased dosage of 616452 (6). They then tried to reprogram epithelial cells isolated from the small intestine based on the markers, EpCAM or VILLIN.…”

“…However, although the developmental fates of epiblast and XEN are divergent from each other in vivo , XEN cells have been derived from PSCs in vitro [12]. These observations combined with the observation of XEN-to-PSC transition by Zhao et al [5] indicate that cells can be more plastic in vitro, and the chemical cocktail of Deng's group might be hijacking such plasticity of XEN cells instead of reversing ontogeny. It is thus possible that the same principle can be applied to other embryonic sources, such as neural crest cells, to achieve pluripotency by hijacking their plasticity as an intermediate state to pluripotency in vitro.…”

“…Another limiting factor is the long duration. CiPSC formation from fibroblasts takes 40 days even with the optimized protocol of Zhao et al [5] …”