A novel protein in a novel pathway maintains pluripotency in embryonic stem cells
June 6, 2003
By Cathy Holding
Discovering the transcription factors that define embryonic cell "stemness" is crucial for understanding of how cells become pluripotent and for assisting the search for more widely acceptable means of generating stem cells in vitro. The leukemia inhibitory factor (LIF) plays a major role in self-renewal of embryonic stem (ES) cells via the gp130/Stat3 signaling pathway, and the transcription factor Oct4 functions in the maintenance of pluripotency in blastocyst inner cell mass cells and in primordial germ cells. Overexpression of Oct4 drives ES cells into differentiation, suggesting that other factors may be involved in the maintenance of pluripotency and self-renewal in ES cells. In the May 30 Cell, two papers reveal the discovery of a single molecule that also functions in this capacity in the mouse. Each group cites a third group that has previously reported the same molecule as being expressed selectively in ES cells.
Kaoru Mitsui and colleagues at the Nara Institute of Science and Technology, used an in silico approach to find genes that have expression patterns identical to that of Oct4 (Cell, 113:631-642, May 30, 2003). Ian Chambers and colleagues at the Institute for Stem Cell Research, used an in vitro approach to identify which individual cDNA molecules from an ES cell library caused colonies to self-renew (Cell, 113:643-655, May 30, 2003).
Both teams used "supertransfection"—the viral-based strategy for DNA transfection that avoids epigenetic effects of chromosome integration—and identified the protein Nanog (from the Celtic "Tir nan Og" meaning "Land of the Ever Young"). Murine nanog cDNA is 2184bp long, comprising an open reading frame encoding 305 amino acids over four exons and a 3'-untranslated region containing a B2 (alu) repetitive element. The protein contains a homeodomain conserved in rats and humans, but no other conserved motifs. A sequence of tryptophan repeats, Trp-X-X-X, was also identified, although its function remains unclear.
Expression of Nanog was confined to preimplantation embryos (from the morula stage through to the epiblast) and primordial germ cells, as well as in various tumors and undifferentiated ES cells. Nanog conferred the ability for self-renewal on ES cells in a manner that was independent of, but augmented by, the presence of LIF, which was independent of Oct3/4 expression and which was resistant, but not refractory, to the effects of differentiation-promoting agents. Both teams put forward models proposing a cooperative effect between Oct4 and Nanog, suggesting that each functions in maintaining pluripotency by suppressing differentiation of specific pathways.
"Elevated expression of nanog is sufficient to sustain the cardinal attributes of ES cell identity," conclude Chambers et al.
Mitsui et al. concluded that their study revealed "two important properties
of the homeoprotein Nanog—the fundamental role in pluripotency of both
ICM and ES cells, and the ability to maintain ES cell self-renewal without
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