Biology Reference
In-Depth Information
5.3. Signaling pathways
The signal transduction pathways downstream of growth factors have been
less thoroughly investigated in PGCs. The Nakano laboratory has demon-
strated that conditional deletion of
Pten
in PGCs leads to testicular teratomas
in mice and also increases the efficiency of EG cell derivation (
Kimura
et al., 2003
). Deletion of
Pten
in ES cells leads to an increase in pho-
sphatidylinositol (3,4,5)-triphosphate (PIP3) and activation of the down-
stream Akt signaling pathway (
Sun et al., 1999
). In keeping with this,
Akt is hyperphosphorylated in
Pten
-deleted PGCs and teratomas (
Kimura
et al., 2003
). PGCs in
Pten
-deleted mice display hyperproliferation,
increased survival, and tumor formation (
Kimura et al., 2003
). Over-
expression of Akt has been demonstrated to improve the survival of PGCs
in culture (
de Miguel, Cheng, Holland, Federspiel, & Donovan, 2002
), and
forced Akt activation in cultured PGCs leads to an increased EG cell deri-
vation efficiency, phenocopying
Pten
-deletion (
Kimura et al., 2008
). This
constitutively active Akt construct also obviates the need for bFGF during
derivation. The authors also demonstrated that
p53
deletion similarly
improves EG cell derivation efficiency in the presence or absence of bFGF
(
Kimura et al., 2008
). Taken together, these findings indicate that the PI3K/
Akt pathway is a critical regulator of PGC proliferation and viability, and
provide compelling evidence that activation of this axis is an important step
to enable establishment of EG cells. This suggests a model of the signal trans-
duction pathway downstream of bFGF during EG cell derivation (
Fig. 5.4
).
In addition, it has been reported that bFGF is required for only the first 24 h
during derivation (
Durcova-Hills et al., 2006
), as is the forced activation of
Akt (
Kimura et al., 2008
). If bFGF is omitted during this time, EG cells can-
not be derived even if bFGF is subsequently added back (
Durcova-Hills
et al., 2006
). This may indicate that Akt activation by bFGF provides an early
stimulus to PGCs and acts during a critical time window. However, whether
the bFGF/PI3K/Akt axis acts purely to increase proliferation of PGCs or in
some way directly triggers reprogramming is unclear.
5.4. EG cells
Upon blastocyst injection, EG cells contribute to chimeric mice and exhibit
germline transmission, confirming their pluripotency (
Labosky, Barlow, &
Hogan, 1994; Stewart, Gadi, & Bhatt, 1994
). In contrast, PGCs do not con-
tribute to chimeras following blastocyst injection (
Durcova-Hills et al.,
2006
; Papaioannou & Gardner, personal communication;
Rossant, 1993
)
