BANANASTOCK

Reprogramming of differenti-ated somatic cells to a pluripotent embryonic stem (ES) cell-like state can be achieved by expressing a combination of four transcrip-tion factors — octamer-binding protein 4 (OCT4; also known as POU5F1), SRY box-containing factor 2 (SOX2), krüppel-like fac-tor 4 (KLF4) and MYC. However, the efficiency of reprogramming by these transcription factors alone is low. Singhal et al. now show that chromatin-remodelling components of the BRG1-associated factor (BAF) complex promote reprogramming by achieving a euchromatic state and facilitating the binding of reprogram-ming factors onto pluripotency gene promoters.

The authors first set out to identify factors that mediate Oct4 activation in mouse embryonic fibroblasts (MEFs; the somatic cell source). MEFs treated with pluripotent stem cell whole-cell

extracts showed increased expression of Oct4 and stable activation of the endogenous Oct4 promoter. Further analysis showed that a nuclear extract from these whole-cell extracts was responsible for activating endogenous Oct4 expression. Mass spectrometry-based proteomics revealed that the reprogramming-competent component of this fraction comprised proteins from the BAF complex, which is known to facilitate transcrip-tional activity by remodelling the nucleosome.

Overexpression of the BAF com-plex components BRG1 and BAF155 (also known as SMARCC1) was shown to enhance reprogramming mediated by OCT4, SOX2, KLF4 and MYC. The promoters of pluripotency genes underwent faster demeth-ylation (which is associated with activation) when cells were treated with BRG1, BAF155 and the four transcription factors than when the four transcription factors acted alone. Furthermore, pluripotency-related genes in the resulting reprogrammed cells were expressed at levels compa-rable to those of ES cells.

So how do BRG1 and BAF55 promote reprogramming? When examining histone methylation pat-terns, the authors found that BRG1 and BAF55 increased histone 3 Lys4 trimethylation (H3K4me3) and H3K9 acetylation (both are activa-tion marks) on specific pluripotency genes and decreased the repressive mark H3K27me3 on one pluripo-tency gene. Thus, BRG1 and BAF55 seem to promote a euchromatic

chromatin state in the promoters of pluripotency genes, which in turn enhances OCT4 binding and there-fore transcription.

Next, the authors investigated the differentiation capacity of the cells reprogrammed with the four transcription factors, BRG1 and BAF55. Subcutaneous injection of these cells in nude mice resulted in the formation of teratomas with tissues of ectoderm, mesoderm and endoderm derivatives. Furthermore, when aggregated with eight-cell embryos in vitro, the reprogrammed cells contribute to the formation of the inner cell mass of developing blastocysts and, when transferred to mice, contribute to the germ line. Thus, reprogrammed cells have a differentiation and developmental capacity similar to that of ES cells.

This work shows that compo-nents of the BAF complex promote chromatin remodelling on specific reprogramming gene promoters, which facilitates the binding of repro-gramming factors, leading to efficient reprogramming into pluripotent cells. Increased understanding of reprogramming mechanisms should enable cells to be reprogrammed in a regulated and efficient manner.

Kim Baumann

ORIGINAL RESEARCH PAPER Singhal, N. et al. Chromatin-remodeling components of the BAF complex facilitate reprogramming. Cell 141, 943–955 (2010)FURTHER READING Egli, D., Birkhoff, G. & Eggan, K. Mediators of reprogramming: transcription factors and transitions through mitosis. Nature Rev. Mol. Cell Biol. 9, 505–516 (2008)

R E P R O G R A m m I N G

Remodelling for pluripotency

the BAF complex … facilitates the binding of reprogramming factors

R e s e a R c h h i g h l i g h t s

NATURE REvIEWS | Molecular cell Biology vOLUME 11 | AUGUST 2010

Nature Reviews Molecular Cell Biology | AOP, published online 14 July 2010; doi:10.1038/nrm2942

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