Biomedical Engineering Reference
In-Depth Information
including trophoblast (Greber et al., 2007b; Ivanova et al., 2006). This result is
consistent with a role of Sox2 inmaintaining pluripotency, similar to that of Oct4.
SOX2 protein shuttles between the cytoplasm and nucleus during early embry-
ogenesis (Li et al., 2007b). SOX2 protein is detected in the cytoplasm of growing
oocytes. In contrast to OCT4 protein, maternal SOX2 actively enters the nucleus
by the 2-cell stage. However, SOX2 is exclusively cytoplasmic in trophoblast
at the blastocyst stage. Unknown mechanism for active protein export out of
nucleus in trophoblast has not yet been elucidated (Avilion et al., 2003). These
events occur independent of zygotic Sox2 transcription, which begins in the late
morula, as maternal SOX2 protein in the cytoplasm of trophoblast is distributed
identically in wild-type and Sox2 null blastocysts (Avilion et al., 2003). Recently,
a nuclear translocation signal has been identified within the HMG box of several
SOX proteins. SOX10 requires active nucleo-cytoplasmic shuttling for transacti-
vation of target genes in vitro (Rehberg et al., 2002), whereas sex reversal can be
induced in cultured XX gonads using an inhibitor that results in nuclear seques-
tration of SOX9 (Babaie et al., 2007; Gasca et al., 2002). SOX2 has two distinct
nuclear translocation signals. The Dmu-mSox2 mutant gene has mutations in
these signals and fails to remain in the nucleus, but yet is competent to interact
with wild-type SOX2 (Li et al., 2007b). Dmu-mSox2 is unable to cooperate with
OCT4 at Oct-Sox target promoters in ESCs. Since Dmu-mSox2 can still interact
with wild-type SOX2, it inhibits the activity of wild-type SOX2 in a dominant-
negative fashion, and subsequently suppresses the activity of downstream genes,
such as Oct4 and Nanog. Overexpression of Dmu-mSox2 in ESCs triggers
progressive doublings of cell ploidy (
<
8 N), accompanying trophoblast differ-
entiation. These results resemble the knockout of Sox2. In toto, these data
indicate that SOX2 maintains stem cell pluripotency by shuttling between the
nucleus and cytoplasm and in cooperation with OCT4 prevents trophoblast
differentiation and polyploid formation in ESCs. Surprisingly, overexpression
of Oct4 restores self-renewal in Sox2 null cells (Masui et al., 2007). Sox2-null-
Oct4-rescued cells seem normal and LIF-dependent for proliferation. Stem cell
markers (Fgf4, Nanog, Utf,1 and Foxd3) are expressed at relatively high levels
in the rescued cells. These observations suggest that Sox2 may be dispensable
at a subset of Oct-Sox enhancers, or other Sox proteins, such as Sox15, may
compensate (Maruyama et al., 2005) with overexpressed Oct4.
The third ''core'' factor, Nanog, a member of NK homeodomain transcrip-
tion factor family, was first identified by Chambers et al. and Mitui et al. using
different strategies (Chambers et al., 2003; Mitsui et al., 2003). The name
''Nanog'' derives from ''Tı´ rnanO
´
g'', the mythological Celtic land of the ''ever
young''. Nanog mRNA is first detected in the interior cells of the compacted
morula and then restricted to the ICM. In later blastocysts, Nanog expression is
further restricted and excluded from the primitive ectoderm. Nanog is expressed
in germ cells during embryogenesis, but downregulated thereafter. Adult tissues
do not express Nanog. Upon differentiation of pluripotent cell lines, such as
ESCs, EGCs, and ECCs, Nanog expression is progressively extinguished.
Unlike Oct4 and Sox2, Nanog is not a maternal factor. Mouse embryos lacking
