Biomedical Engineering Reference
In-Depth Information
phenotype. Mutant embryos develop through cleavage and compaction to form
blastocyst-like structures. Normal numbers of cells seem to be distributed to the
prospective ICM region. These ICM-like cells are viable, however, they are
unable to advance further to form primitive ectoderm and extra-embryonic
endoderm. On the other hand, trophoblast lineages appear normal. Only tro-
phoblast giant cells grow out from Oct4-deficient blastocysts in vitro. These
data indicate that OCT4 is a crucial factor for generation of hypoblast and
epiblast and maintenance of the pluripotent state during embryo development
(Nichols et al., 1998). No clear functions for Oct4 have been identified in adult
somatic stem cells (Lengner et al., 2007). Both hESCs and mESCs contain
abundant OCT4 protein in the nucleus. Expression of Oct4 declines upon
differentiation. Inactivation of Oct4 in embryo and ESCs causes spontaneous
differentiation to trophoblast lineage (Niwa et al., 2000). However, constitutive
Oct4 expression in mESCs is insufficient to maintain self-renewal without LIF
(Niwa et al., 2000). Overexpression of Oct4 yields the same phenotype as
STAT3 deficiency. This suggests that LIF does not regulate Oct4, and Oct4
does not regulate the LIF/STAT3 pathway. The Oct4 pathway appears to be a
parallel pathway for maintaining ESCs self-renewal. Many Oct4 target genes
also contain STAT-binding sites, suggesting that the two transcription factors
may cooperate in ESCs (Tanaka et al., 2002).
Sox2 is a member of the SOX (SRY-related HMG box) DNA-binding
protein family. POU and SOX proteins function together to regulate gene
expression both positively and negatively (Remenyi et al., 2004). Several reports
suggest cooperative activity between Oct4 and Sox2 on ESCs-specific enhan-
cers, such as those at the Utf1, Fgf4, Lefty1, and Nanog genes (Kuroda et al.,
2005; Nakatake et al., 2006; Nishimoto et al., 2001; Rodda et al., 2005; Tokuzawa
et al., 2003; Yuan et al., 1995). Furthermore, Oct-Sox enhancers are important
for the expression of Oct4 and Sox2 themselves, suggesting that these two
transcription factors are regulated by a positive-feedback loop (Chew et al.,
2005; Okumura-Nakanishi et al., 2005; Tomioka et al., 2002). Expression of
Sox2 in early embryos parallels that of Oct4, as it is expressed in the ICM, and
then in the early primitive ectoderm (epiblast), and germ cells (Avilion et al.,
2003). However, unlike Oct4, Sox2 is also expressed in neural stem cells
(Uwanogho et al., 1995; Zappone et al., 2000). Indeed, loss of Sox2 in the central
nervous system yields a phenotype that is independent of Oct4 (Avilion et al.,
2003; Miyagi et al., 2008). Both Sox2-deficient and Oct4-deficient embryos arrest
at similar stages (Avilion et al., 2003). Blastocyst-like structures are formed in
Sox2 mutants, but primitive ectoderm development is defective. The primary
defect lies in the epiblast, as illustrated by chimera rescue experiments, in which
wild-type ESCs were injected into Sox2-deficient blastocyst. In many 7.5 dpc
chimeras, the entire embryo is derived from the wild-type ESCs, revealing the
defect to be cell-autonomous to the epiblast. Consistent with this finding, no
outgrowth from blastocysts occurs in culture. ICM isolated from Sox2-deficient
embryos gives raise to trophoblast giant cells in culture. Silencing of Sox2 by
RNAi (RNA interference) in ESCs induces differentiation into multiple lineages,
