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sequence of the zebrafish microtubule actin crosslinking factor 1 (macf1). macf1 is
a member of the highly conserved spectraplakin family of cytoskeletal linker pro-
teins that play diverse roles in polarized cells. Distinct from buc mutants, primary
oocytes of mgn mutants are capable of recruiting germ plasm RNA and mitochon-
dria to form a bona fide Bb (Gupta et al. 2010 ). However, eventually the Bb grows
abnormally large and persists into later-stage oocytes with the particularity that the
Bb fails to transit to the oocyte cortex. Accordingly, germ plasm mRNAs are not
transferred properly to the vegetal cortex, being trapped in the persistent and
enlarged Bb. In mgn mutants, the oocyte nucleus is mislocalized. Bb-localized
mRNAs and organelles are absent from the oocyte periphery, consistent with a role
of macf1 in nuclear anchoring and cortical localization. Both buc and mgn regulate
animal–vegetal polarity and are critical for delivery of mRNAs to the vegetal cortex.
buc promotes Bb assembly, whereas mgn is required for Bb translocation and dis-
persal via regulation of the microtubule cytoskeleton (Gupta et al. 2010 ).
8.2.3 Posttranscriptional Regulation in PGCs
Posttranscriptional regulation plays a critical role during germ cell development.
Several maternal mRNAs, such as vasa (Knaut et al. 2002 ), nanos1 (Koprunner et al.
2001 ), dnd (Weidinger et al. 2003 ), and Tudor domain-containing protein 7 (tdrd7)
(Mishima et al. 2006 ), are known to be localized to the germ plasm. However, during
early cleavage stages, these RNAs are detected outside the germ plasm as well and are
subsequently incorporated into somatic cells in addition to PGCs. vasa, nanos1, and
tdrd7 mRNAs are rapidly degraded in somatic cells but are stabilized in PGCs in a
process mediated by cis-acting elements in their 3′ untranslated region (UTR) (Wolke
et al. 2002 ). Somatic cell must eliminate germ plasm mRNAs in order to preserve their
somatic cell fates. Small noncoding RNAs (snRNA) have emerged as important play-
ers in early zebrafish development. One kind of snRNAs is micro RNA (miR) which
functions to negatively regulate target genes by binding to their 3′UTR, promoting
deadenylation, translational repression, and/or ultimately degradation of the transcript
(Bushati and Cohen 2007 ). In zebrafish, miRNA-430 is the principally expressed
miRNA during early embryogenesis. It is worth mentioning that currently there are no
maternal miRNAs reported in the zebrafish embryo. In fact, miR-430 is first expressed
at the mid-blastula transition (Schier and Giraldez 2006 ). Because of miRNAs are also
present in PGCs, inhibition of miRNA function plays a vital role in germ cell develop-
ment. For example, nanos1 is resistant to miRNA repression in the germ line, but not
in somatic cells. This resistance is granted by the maternally expressed binding protein
Dnd, which interacts with the nanos1 3′ UTR, blocking the binding of miR-430
(Kedde et al. 2007 ). This protection from miRNA repression is also found in other
germ plasm mRNAs, Tdrd7 and dazl, suggesting a general protecting mechanism in
germ lineage (Kedde et al. 2007 ; Takeda et al. 2009 ). Intriguingly, the fact that another
germ plasm RNA such as vasa is eliminated from the somatic cells in a miR-430 inde-
pendent mechanism, suggests the existence of additional modes of modulating
T. Aguero et al.