An intermediate cell state of chemical reprogramming can be induced to convert directly between cell types.
A few years ago, Hongkui Deng and colleagues at Peking University in Beijing discovered a way to avoid supplying transcription factors by using chemicals to reprogram mouse somatic cells into pluripotent stem cells. As cells underwent chemical reprogramming, they passed through an intermediate state that resembles extra-embryonic endoderm (XEN). New work led by Deng and Zhen Chai at Peking University reveals that the XEN-like state is a unique multipotent condition from which cells can embark on diverse specification paths.
The original search for a chemical reprogramming cocktail was motivated by a wish to establish whether the Yamanaka factors used in traditional reprogramming were necessary, and to circumvent technical and safety concerns around transcription factor expression. “Compared to transgenic methods, small molecules are cell permeable, cost-effective, easy to synthesize, preserve and standardize, and their effects can be reversible,” Deng and first author Xiang Li write in a joint e-mail. Chemical reprogramming presented the opportunity to study an alternative route to pluripotency.
In their latest work, the researchers determined that fibroblast-specific gene expression is downregulated in XEN-like cells induced from fibroblasts. “We found that the induced XEN-like cells expressed master genes governing cell fate choices toward three germ layers and cell lineages, suggesting broader lineage plasticity,” write Deng and Li. By applying a version of a medium used to specify neurons from pluripotent stem cells, they found that XEN-like cells readily assume a neuronal fate.
The partially reprogrammed cells have some attractive properties. Their numbers can be expanded dramatically without losing key features. “XEN-like cells do not compromise their neuronal induction efficiency and retain genetic integrity and genome stability after long-term expansion,” state Deng and Li. Neurons induced from XEN-like cells closely resemble primary neurons with respect to their gene expression, functional properties and ability to engraft in the brains of adult mice upon transplantation. Despite their proliferative potential, they do not generate tumors in animals.
The conveniences of chemical programming can be somewhat offset, however, by the narrow concentration windows and specific application durations needed for small molecules to be effective. To achieve the best results, the researchers recommend closely following the details of their protocols.
In addition to generating neurons, which are cells of ectodermal origin, the researchers converted XEN-like cells to hepatocyte-like cells, which are of endodermal origin. The developmental plasticity of the XEN-like state is still something of a mystery. During development, extra-embryonic endoderm is an inductive tissue that does not contribute cells to the embryo. Gene expression in XEN-like cells clearly differs from expression in the pluripotent state induced by the Yamanaka transcription factors. But XEN-like cells do express at least two pluripotency factors found in fully chemically reprogrammed cells. The researchers are actively looking into the mechanisms behind XEN-like plasticity. It will be interesting to understand the epigenetic state of these cells, in addition to their other properties.
The teams are working to generate other functional cell types and are interested in determining whether cells of mesodermal origin, such as cardiomyocytes and blood cells, can be differentiated from XEN-like cells. They are also pursuing the question of whether an analogous cell type can be induced in human cells. Deng and Li predict that “chemical reprogramming will become a very easy, simple, and popular approach.” An expandable multipotent state that circumvents full pluripotency may have some practical advantages and shed light on the nature of pluripotency in cultured cells.
Li, X. et al. Direct reprogramming of fibroblasts via a chemically induced XEN-like state. Cell Stem Cell 21, 264–273.e7 (2017).