Biology Reference
In-Depth Information
found to be Oct4-positive consistent with the relatively low efficiency of
this Cre transgenic line (
Lomel´, Ramos-Mej´a, Gertsenstein, Lobe, &
Nagy, 2000
). Furthermore,
TNAP
-cre only becomes active around
E9.5-E10.5, making this an unsuitable model for investigating Oct4 func-
tion in PGCs at earlier stages. An earlier requirement for Oct4 during PGCs
specification has been reported (
Okamura et al., 2008
). Generation of chi-
meric mice from ES cells maintained by an
Oct4
transgene resulted in con-
stitutive repression of the transgene and therefore Oct4 downregulation.
This generated
Oct4
null epiblast cells, and these appeared to be capable
of expressing
Blimp1
at E7.0. However, no descendants of these cells were
found to be expressing Stella at E7.25 or contribute to the germline there-
after (
Okamura et al., 2008
). These results provide some indication that
Oct4 may be required to complete PGC specification. In contrast,
Nanog
null ES cells appear to be able to colonize the germline and survive until
E11.5 in chimeric animals (
Chambers et al., 2007
).
Nanog
null PGCs
expressing Oct4 and mouse Vasa homologue (Mvh) can be detected in
the gonads of chimeric animals at E11.5. However, these are lost by
E12.5. This suggests that Nanog is not required for specification or indeed,
the vast majority of PGC development, but may be required to complete the
epigenetic reprogramming occurring at E11 (see later). However, a subse-
quent report that utilized tamoxifen-induced, short-hairpin (sh) RNA
knockdown to deplete Nanog in PGCs reported an earlier phenotype
(
Yamaguchi et al., 2009
). Tamoxifen administration at E7.5 resulted in
Nanog depletion by E8.5 and extensive apoptosis at E9.5, both
in vivo
and
in vitro
. However, the authors report that tamoxifen administration at
E9.5 did not result in significant germ-cell depletion (
Yamaguchi et al.,
2009
). The discrepancy between the two papers is most likely due to the
differing experimental approaches. It is possible that
Nanog
null ES cells
undergo some compensatory changes to enable their survival in culture
and that this results in resistance to apoptosis in the context of PGCs. Alter-
natively, the knockdown approach may result in off-target effects or a more
general toxicity in PGCs.
In mouse PGCs, Oct4 appears to be the only pluripotency factor which is
expressed continuously during the transition of epiblast cells to specified
PGCs. However, in pig embryos both
Oct4
and
Nanog
exhibit a similar
pattern of expression, with the earliest PGCs, located at the posterior
epiblast, expressing both these factors (
Wolf et al., 2010
).
Oct4
expression
is maintained until at least E42 in the pig, similar data is not yet avail-
able for
Nanog
(
Hyldig, Croxall, Contreras, Thomsen, & Alberio, 2011
).
