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In-Depth Information
X. laevis.
The identification of additional zebrafish germ plasm components,
many of which are RNA-binding proteins, or encode for such proteins
(
Hashimoto
et al
., 2004, 2006; K¨prunner
et al
., 2001; Kosaka
et al
., 2007;
Maegawa
et al
., 1999; Olsen
et al
., 1997; Weidinger
et al
., 2003; Yoon
et al
.,
1997
), suggested that similar to its role in other organisms, the germ plasm
plays a role in controllingRNA stability, localization, and function (reviewed
in
Ikenishi, 1998
). Whereas many zebrafish germ plasm components such as
dazl
,
nanos
, and
vasa
are conserved between zebrafish and other organisms
whose germ cells are specified by germ plasm, some appear to be vertebrate
specific, notably theRNA-binding proteinDead end (Dnd) (
Hay
et al
., 1988;
Houston
et al
., 1998; Komiya
et al
., 1994; K
¨
prunner
et al
., 2001; Lehmann
and Nusslein-Volhard, 1991; Maegawa
et al
., 1999; Weidinger
et al
., 2003;
Yoon
et al
., 1997; Zhou and King, 1996
).
Following their specification, the PGCs proliferate and migrate toward
the region where the gonad develops, where they associate with somatic
cells (
Fig. 4.1
B(b and c)) and differentiate into gametes
(Richardson and
Lehmann, 2010)
.
1.2.3. Germline development in the mouse
Mouse PGCs are specified around embryonic day E5.5-E6.0 in response to
signals encoded by members of the transforming growth factor
b
(TGF
b
)
superfamily and the family of bone morphogenetic proteins (BMPs) originat-
ing from cells of the extraembryonic ectoderm and the visceral endoderm
cells (
Fig. 4.1
C(a);
Lawson
et al
., 1999; Ying and Zhao, 2001; Ying
et al
.,
2001
). These signals establish “competence” of the cells to assume the fate of
PGC precursors as marked by BMP-induced expression of IFITM3 (inter-
feron-induced transmembrane protein3, also referred to as “Fragilis”) (
de
Sousa Lopes
et al
., 2007; Saitou
et al
., 2002
). At E6.25, some of the responding
cells start expressing PRDM1 (PRDI-BF1-RIZ domain-containing 1;
BLIMP1) and the closely related PRDM14, becoming the first cells in the
mammalian embryo to be committed to a specific fate. These BMP-depen-
dent events ultimately lead to suppression of somatic gene expression (e.g.,
Hox
genes) and concomitant induction of germline characteristic gene
expression and
de novo
expression of pluripotency genes such as
Sox2
,
Oct4
,
and
Nanog
(
Kurimoto
et al
., 2008; Ohinata
et al
., 2005; Saitou
et al
., 2002;
Vincent
et al
., 2005; Yamaji
et al
., 2008
). Similar to many other organisms,
following their specification, the specified PGCs migrate toward the region
where the gonad develops
(Richardson and Lehmann, 2010)
. Several gene
products have been shown to be required for the survival of mouse PGC
during their migration. Relevant for this review, the function of RNA-
binding proteins that are associated with the germ plasm of different organ-
isms such as Nanos and Dead end homolog 1 (DND1, Ter) is essential at this
stage (
Tsuda
et al
., 2003; Youngren
et al
., 2005
). The PGCs then loose their
migratory behavior and eventually differentiate into gametes, sperm, and egg.
