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of having both negative and positive regulators on the same mRNA. After Pumilio
is phosphorylated, cyclin B1 is translated, perhaps through Xdazl promoting its
polyadenylation (Ota et al. 2011 ). Xdazl protein is detected in PGCs continuously
from blastula stages until germ cells are within the gonads at stage 52 (Houston and
King 2000a; Kataoka et al. 2006 ). The persistent expression of Xdazl protein in the
germ line suggests that it may regulate the translation of RNAs responsible for
many PGC functions including migration, meiosis, and proliferation. Interestingly,
mouse PGCs lacking dazl fail to transcriptionally activate the Oct4 pluripotency
program, a hallmark of the germ line (Haston et al. 2009 ). Thus, there may be a role
for Xdazl in preserving the potential for totipotency in Xenopus PGCs.
Another level of translational control in the early embryo is mediated by microR-
NAs. Whereas germ line RNAs must be cleared from the soma to prevent abnormal
development, they must also be protected in the germ plasm. Overexpression of nanos
RNA in early staged embryos results in incomplete neural tube closure, most likely
because it represses required VegT activity (Luo et al. 2011 ). Evidence supports a role
for RNA binding proteins Dnd and Xdazl in protecting other germ line RNAs includ-
ing their own RNAs, from degradation. xdazl, dnd, xpat, nanos and deadsouth, appear
to be protected from miR-18 degradation by a combination of Dnd and ElrB1 binding
the target region in the 3′ UTR (Koebernick et al. 2010 ). These studies need to be
extended beyond using reporter assays to specific loss-of- function analyses of miR-
- Xdazl protein may also protect from miR-427 mediated degradation in the germ
line if its role has been conserved with its predicted zebrafish ortholog miR-430. Dazl
relieves miR-430-translation repression of tdrd7 mRNA by inducing its polyadenyl-
ation in zebrafish (Takeda et al. 2009 ). Caution is advised however in drawing too
many parallels between Xenopus and zebrafish miR-functions (Koebernick et al.
2010 ). Exactly when miR-18 is transcribed in development is not clear, but in the case
of miR-427 and other miRNAs in general, expression is dynamic during develop-
ment, reaching significant levels only at the MBT and predominantly in the animal
hemisphere (Lund et al. 2009 ; Harding et al. 2014 ). Thus a level of protection from
miR-mediated degradation in the germ line is accomplished simply by keeping miR
transcripts very low within the vegetal hemisphere before the MBT.
Another important component of germ plasm that plays a role in protection is a
member of a subclass of Argonaute proteins called Piwi proteins. Piwi proteins interact
with piRNAs, and both are very abundant as well as essential for germ cell develop-
ment and function in mouse and zebrafish (Ketting 2011 ; Houwing et al. 2008 ). piR-
NAs are enriched in transposon-derived sequences and appear to silence transposons
in the germ line by either cleaving transcripts or facilitating chromatin methylation of
transposon sequences (Armisen et al. 2009 ; reviewed in Siomi et al. 2011 ). Xiwi, one
of two Piwi proteins found in Xenopus oocytes, coimmunoprecipitated with nanos
RNA in large RNP particles from egg extracts (Lau et al. 2009 ; Minshall et al. 2007 ).
Because Xiwi1 was found to have putative nuclear localization and export signals, it
has been proposed that it accompanies nanos RNA, along with Sm proteins, during
nuclear transport of nuage into the ooplasm as germ plasm is formed (Bilinski et al.
2004 ; Lau et al. 2009 ). Clearly, much remains to be done before a mechanistic picture
is completed of how germ plasm components specify and protect the germ line.
T. Aguero et al.