Pathways to Pluripotency: How Germ Cells Make Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 14

PGCs to a pluripotent state. Studies of EG derivation from PGCs demonstrate that
down-regulation of Prdm1 may indeed be an important first step in the formation of
pluripotent cells. When PGCs are isolated from the embryo and placed into culture
they express both Oct4 and Prdm1. After exposure of PGCs to FGF, Prdm1 is rapidly
down-regulated in some of the PGCs within 24 h (Fig. 1.3 ). Later in the culture period,
Prmt5, a protein that acts in a complex with Prdm1, translocates from the nucleus to
the cytoplasm. While the role of Prmt5 in the regulation of pluripotency is unclear,
some of the targets of Prdm1 are known and include Myc and Klf4, two of the factors
required for the reprogramming on fibroblasts to iPSCs. Examination of Myc and
Klf4 expression during EG formation reveals that both genes are up-regulated
following exposure of PGCs to FGF. Thus one key role of Prdm1 in response to FGF
might be to cause up-regulation of two of the key genes required for cellular repro-
gramming. Interestingly, these studies also suggest that up-regulation of Myc could
also be brought about by activation of the signal transducer and transcriptional activa-
tor-3 (STAT-3), which is a direct target of the LIF signaling pathway. Taken together
these data suggest a key series of events must occur in order to convert PGCs to EG
cells. Down-regulation of Prdm1 must occur in order to relieve repression that main-
tains the germ cell fate. Together with activation of STAT3 via the LIF signaling
pathway this leads to up-regulation of a set of genes, including Myc and Klf4, required
for establishment of the pluripotent stem cell state (Durcova-Hills et al. 2008 ). One of
the other genes involved in the specification of the germline is Prdm14 (Yamaji et al.
2008 ). Interestingly, PGCs isolated from Prdm14 −/− embryos seem unable to form EG
cells (Yamaji et al. 2008 ). Thus, unlike Prdm1, whose down-regulation may be
required for EG formation, loss of Prdm14 seems to inhibit the formation of these
pluripotent stem cells. Although both proteins have been proposed to have repressive
activities, clarification of their precise function will likely shed light on these results.
One proposed role of Prdm14 in normal PGC development is to up-regulate Sox2 in
nascent PGCs (Yamaji et al. 2008 ). Therefore, the inability of Prdm14 −/− PGCs to be
able to give rise to EG cells may be due to the fact that, unlike normal PGCs, they may
have low levels of Sox2 and therefore may be resistant to reprogramming.
It seems likely that conversion of PGCs to the pluripotent state might also
require the down-regulation of many other genes involved in germ cell develop-
ment. Some of these genes have been identified by differential screening of PGC
and pluripotent stem cell-derived cDNA libraries and include genes such as CREB-
binding protein (CBP), a transcriptional co-repressor/histone acetyltransferase,
which has been found to play an important role in PGC development (Elliott et al.
2007 ). One of the key questions is how FGF signaling leads to PGC conversion via
down-regulation of Prdm1 and activation or repression of other genes. An impor-
tant clue comes from the proposed mode of action of FGFs, which can act to
modify chromatin and allow access of transcription factors to promoter regions.
Presumably this action leads to transcriptional and epigenetic changes that cause
PGCs to convert into pluripotent EG cells. The finding that TSA can replace FGF2
in the cocktail of growth factors used to convert PGCs to EG cells also suggests that
chromatin modification plays a key role in converting PGCs to EG cells because of the
ability of TSA to act as a HDAC inhibitor (Durcova-Hills et al. 2008 ). Indeed these