Fig. 1: A comparative analysis of 9.5 day-old wild-type (left) and chimeric (right) embryos, which incorporate green fluorescent protein (GFP)-expressing OSE cells. Under normal illumination (top) the two appear identical, but analysis of GFP fluorescence (bottom) reveals that virtually every tissue of the chimera is in part derived from these iPSCs.
Reproduced from Ref. 2 © 2009 Macmillan Publishers
Amid intense interest in the clinical potential of embryonic stem cells (ESCs), the recent discovery that mature cells can be coaxed into becoming ‘stem-like’ promises to revolutionize the field.
Rather than obtaining cells directly from embryos, scientists can instead work with skin cells collected from patients. These cells are subsequently reprogrammed to express genes that cause them to de-differentiate, yielding ‘induced pluripotent stem cells’ (iPSCs). Virtually indistinguishable from ESCs, iPSCs can theoretically be instructed to develop into any mature cell type—with the added benefit of potentially enabling derivation of patient-specific tissues or even organs for transplant purposes.
The activity of four mouse genes—Oct4, Sox2, c-Myc and Klf4—is sufficient for iPSC generation, but evidence suggests that other genes could be substituted into this cohort. In an exploration of the contribution of the transcription factor Klf4 to maintaining ESC pluripotency, Huck-Hui Ng of A*STAR’s Genome Institute of Singapore and colleagues found that several closely related factors can effectively take its place1.
“As a follow-up project, we asked which other transcription factors can replace Klf4 in reprogramming,” says Ng. Using a screen to look for such factors, they identified the gene encoding estrogen-related receptor β (Esrrb), which has been implicated previously as a pluripotency factor. When Ng and colleagues used a retroviral vector to introduce the Oct4, Sox2, c-Myc and Esrrb genes into mouse fibroblasts, they confirmed that Esrrb can successfully replace Klf4 for the production of iPSCs—albeit at a somewhat lower efficiency2.
They noted that Esrrb failed to substitute for Oct4 or Sox2, but c-Myc was found to be entirely dispensable. Further, cells transduced with the three-gene set of Oct4, Sox2 and Esrrb (OSE cells) were similar to conventionally prepared iPSCs in terms of morphology and gene expression profile. Typically, truly pluripotent cells can be transplanted into early stage embryos to produce adult mice that are chimeric, containing a mixture of tissues derived from both donor and recipient cells, and the OSE cells passed this test as well (Fig. 1). “This shows that certain classical reprogramming factors can be substituted by different transcription factors,” says Ng.
Relatively little is known about Esrrb, but the investigators have gained some useful initial insights into its function. For example, Esrrb directly binds to and activates a number of ESC-specific genes, and was shown to form a complex with Oct4 and another key pluripotency factor, NANOG. “We are interested in further examining Esrrb’s role in the reprogramming of human somatic cells,” says Ng.
The A*STAR-affiliated authors in this highlight are from the Genome Institute of Singapore.
