Pathways to Pluripotency: How Germ Cells Make Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 5

1.2 Early Germ Cell Development

In mice up until about 6.25 days post coitus (dpc) there are no cells that can be dis-
tinguished as germ cells. At that time inductive signals, including bone morphoge-
netic proteins (BMPs), create a population of cells, termed primordial germ cells
(PGCs), that begin to express what we know as germ cell markers (Saitou et al. 2003 ;
Surani 2007 ) (Fig. 1.1 ). These markers include tissue non-specific alkaline phos-
phatase (TNAP or AlpL) and the C-Kit receptor tyrosine kinase (De 2000 ), both
expressed on the cell surface of PGCs. While TNAP is either not required or redun-
dant in germ cell development, the C-Kit receptor has a critical role in germline
development (see below). An early event in the specification of the germline is the
expression of Prdm1 (also known as Blimp1), a zinc finger containing, DNA-binding,
transcriptional repressor. During early development in the pre-gastrulation embryo
Prdm1 is expressed in a subpopulation of cells in the visceral endoderm overlying the
proximal epiblast (Fig. 1.1 ). In the next stages of development, it is expressed in the
anterior visceral endoderm and in the nascent PGCs. Prdm1 then remains expressed
in developing PGCs up until the time at which they enter the developing gonads
(Vincent et al. 2005 ; Ohinata et al. 2005 ). Evidence for the role of Prdm1 in PGC
development comes from studies in mice lacking the gene. Loss of Prdm1 leads to
failure of PGCs to form properly at the very earliest stages of germline development
(Vincent et al. 2005 ; Ohinata et al. 2005 ). A few PGC-like cells form in Prdm1-
deficient embryos but they fail to proliferate, migrate, or show the consistent down-
regulation of homeobox genes observed during normal germ cell development.
Importantly, these studies demonstrate that Prdm1 is required for formation of the
germ cell lineage. It has been suggested that Prdm1 acts on differentiating pluripotent
cells of the epiblast to suppress the somatic cell differentiation pathways including
expression of Hox genes (which would be counterproductive to their effective dif-
ferentiation into the germline) and to prevent newly formed PGCs from slipping back
towards a pluripotent state (Ancelin et al. 2006 ; Kurimoto et al. 2008 ). Prdm1 is a
member of a larger superfamily of proteins that contain an N-terminal PR/SET
domain linked to C-terminal C2H2 zinc fingers (reviewed in Bikoff et al. 2009 ; John
and Garrett-Sinha 2009 ). These proteins are known to mediate nuclear import and
DNA binding. In other cell types Prdm1 blocks transcription at promoters of target
genes known to regulate cell cycle progression such as Myc. In addition, Prdm1 is
known to silence transcription factors in order to maintain the identity of those cells
(reviewed in Bikoff et al. 2009 ; John and Garrett-Sinha 2009 ).
Interestingly, a second member of this family of factors, Prdm14, also plays a key
role in PGC specification and development (Yamaji et al. 2008 ). Using a transgenic
reporter line in which a fluorescent reporter is expressed from the Prdm14 upstream
elements and using whole-mount immunofluorescence it has been shown that
Prdm14 is co-expressed with Prdm1 in the few cells fated to become PGCs at the
posterior of the early embryo (Fig. 1.1 ). Loss of Prdm14 in mice results both in loss
of PGCs and in adult animals that are sterile. In Prdm14 mutant embryos it was
found that PGC specification is defective from the earliest stages of the development
of the lineage. Further studies on the role of Prdm14 in germ cell specification sug-