Biology Reference
In-Depth Information
despite the lack of direct evidence that it increases PGC proliferation rather
than survival.
The plant extract Forskolin (FK) acts as a potent mitogen for PGCs
(
de Felici et al., 1993; Koshimizu, Watanabe, & Nakatsuji, 1995
). FK has
been shown to activate protein kinase A (PKA) but has pleiotropic effects
on cells (
Laurenza, Sutkowski, & Seamon, 1989; Lippincott-Schwartz
et al., 1991
). However, FK treatment increases cAMP in PGCs (
Dolci,
Pesce, & de Felici, 1993
), and blockade of PKA eliminates the effect of
FK on mitotic activity (
de Felici et al., 1993
). Furthermore, other factors that
raise cAMP also increase PGC proliferation (
Pesce, Canipari, Ferri, Siracusa,
& de Felici, 1996
). Thus, there is strong evidence that FK acts through PKA/
cAMP and that this pathway is potentially an important regulator of PGC
proliferation. However, the discovery of growth factors that act through
PKA/cAMP
in vivo
have not been forthcoming (
de Felici & Pesce,
1994
). Retinoic acid (RA) is an equally potent PGC mitogen and can act
synergistically with LIF to further increase proliferation (
Koshimizu et al.,
1995
). Both FK and RA can replace bFGF in EG cell derivation protocols
and, in fact, lead to an increase in derivation efficiency (
Koshimizu et al.,
1996
). This provides indirect evidence that, like FK and RA, bFGF acts
as a mitogen in the context of EG cell derivation. Furthermore, although
LIF and SF can increase the mitotic activity of PGCs, proliferation may
be a limiting factor during conversion of PGCs to EG cells.
Other factors have also been reported to influence PGC culture or EG
cell derivation. LIF can be substituted by Oncostatin-M and IL-6 (in com-
bination with soluble IL-6 receptor), both of which activate the gp130
receptor (
Koshimizu et al., 1996
). Similarly, other FGFs can substitute for
bFGF to stimulate EG cell derivation. However, while FGF5 seems to have
similar activity to bFGF on E8.5 PGCs, FGF9 and FGF10 are less efficient at
generating EG cell colonies (
Durcova-Hills, Adams, Barton, Surani, &
Mclaren, 2006
). These findings are not adequately accounted for by the dif-
ferent receptor affinities of the FGF molecules. However, it is noteworthy
that mutants in FGFR2-IIIB have reduced numbers of PGCs at E11.5 and
that addition of FGF7, a ligand for this receptor, causes a marginal increase in
PGC numbers in embryo slice cultures (
Takeuchi et al., 2005
).
TGF
b
(
Godin & Wylie, 1991
) and Activin (
Richards, Enders, &
Resnick, 1999
) have been shown to have a negative effect on PGC prolif-
eration, while TNF
a
(
Kawase, Yamamoto, Hashimoto, & Nakatsuji, 1994
)
and IL-4 (
Cooke, Heasman, & Wylie, 1996
) have a positive effect. How-
ever, no subsequent work has built on these observations, and the role of
