Biomedical Engineering Reference
In-Depth Information
Nanog fail to develop due to the absence of primitive ectoderm (Chambers
et al., 2003; Mitsui et al., 2003). A clear epiblast and extra-embryonic ectoderm
at 5.5 dpc do not form. Nanog-deficient blastocysts are indistinguishable from
wild-type embryos. The ICM fails to proliferate, and yet differentiates to
parietal-endoderm-like cells. These data suggest that Nanog expression is indis-
pensable for the maintenance of primitive ectoderm in the embryos and pre-
vents differentiation into extra-embryonic endoderm. Consistent with these
findings, Nanog represses Gata6 to prevent ICM differentiation to hypoblast
(Mitsui et al., 2003; Singh et al., 2007). mESCs cannot be derived from Nanog-
null blastocysts (Mitsui et al., 2003).
In contrast with these in vivo results in Nanog-deficient embryos, Nanog-
null ESCs can be generated under specific circumstances. mESCs with a con-
ditionally modified Nanog locus were manipulated to form Nanog-null ESCs.
Surprisingly, ESCs lacking both Nanog alleles may remain undifferentiated, yet
are prone to differentiate (Chambers et al., 2007). Nanog-deficient mESCs
express stem cell markers (Oct4, Sox2). Thus, Nanog is required for the estab-
lishment of ESCs, but dispensable for their maintenance (Chambers et al., 2007;
Mitsui et al., 2003). Nanog expression is mosaic within ESCs population and
appears to undergo oscillary circuits. This dynamic expression pattern is com-
patible with the existence of Nanog-deficient ESCs (Singh et al., 2007). Like
Nanog-deficient ESCs, the Nanog negative population is susceptible to differ-
entiation signals (Chambers et al., 2007). In contrast, overexpression of Nanog
sustains mESCs pluripotency in the absence of LIF and or STAT3 activation.
Thus, the Nanog and STAT3 pathways are independent (Chambers et al.,
2003). However, ChIP (Chromatin immunoprecipitation) analysis suggests
that Nanog may be a direct downstream effector of the LIF/STAT3 pathway
in maintaining pluripotency (Suzuki et al., 2006). Further work is needed to
confirm the direct link between Nanog and the LIF/STAT3 pathway. Recent
reports suggest that NANOG is a direct target of TGFb/activin-mediated
SMAD signaling in hESCs (Xu et al., 2008).
The three ''core'' factors are integrated with LIF/BMP/Wnt pathways,
cooperate with each other to prevent differentiation, and maintain pluripotency
by a delicate balance. These ''core'' factors do not act in isolation, but rather
within a network comprised of many other transcription factors that are also
critical to the properties of ESCs. We review recent elucidation of a wider
transcription network in ESCs.
4 Pluripotency Genomics and Proteomics
In large part, RNA expression marks important differences between two cell
types at the molecular level. Several high-throughput technologies are now
available to characterize all transcripts expressed in a given cell population.
These include sequencing-based approaches, such as EST (expressed sequence
