Small RNAs in Germ Cell Development - Current Topics in Developmental Biology - page 86

These factors include RNA molecules and proteins that together constitute
the germ plasm. Cells inheriting the germ plasm are determined to assume
the germ cell fate.
The first indications for a possible role of miRNAs in germline specifi-
cation were observations in D. melanogaster demonstrating that components
of the miRNA machinery like Dicer-1 and dFMRP (fragile X mental
retardation protein) could influence the expression of Oskar, Vasa, and
Nanos, which are crucial pole plasm components ( Megosh et al ., 2006 ).
Further analysis of the defects in pole cell formation upon loss of the
miRNA machinery revealed that the miRNAs themselves, namely miR-6
and miR-9, are key regulators of this process ( Leaman et al ., 2005 ).
As in Drosophila , the germ plasm of zebrafish is enriched at specific
locations of the developing embryo ( Braat et al ., 1999; Olsen et al ., 1997;
Yoon et al ., 1997 ). Importantly, whereas germ plasm and mRNAs that
are part of it (e.g., nanos , dead end , and tdrd7 ) are highly concentrated in
certain locations in the early embryo, a significant amount of the material is
actually not localized and is found in other cells as well. This point is
exemplified by the nanos mRNA, whose translation is strongly inhibited
in cells destined to give rise to somatic cells ( Blaser et al ., 2005; K¨prunner
et al ., 2001 ). As development progresses, these mRNA molecules are
preferentially cleared from the soma, establishing PGC specific expression
of the transcripts.
Two important studies deciphered the mechanisms underlying the
somatic silencing and degradation of such germline-enriched mRNAs,
demonstrating the role of miRNA 430 (miR-430) in controlling nanos
and tdrd7 function ( Giraldez et al ., 2006; Mishima et al ., 2006 ). miRNAs
of the miR-430 cluster are highly abundant in early zebrafish embryos and
are key regulators of mRNA turnover during maternal-to-zygotic transi-
tion. The role of miRNAs in regulating maternal mRNA turnover was
demonstrated in embryos lacking Dicer function (MZ dicer mutant embryos)
that exhibit grossly abnormal embryonic development due to misexpression
of a large set of mRNAs ( Giraldez et al ., 2005, 2006 ). Further, mRNAs that
are normally silenced and degraded in somatic cells, while being expressed
in germ cells, retain their somatic expression in MZ dicer mutant embryos.
Whereas miRNAs are responsible for the somatic silencing of “germ cell”
mRNAs, they are also present in the germ cells ( Mishima et al ., 2006 ), but
their function is counteracted in these cells. One factor known to inhibit
miRNA activity in zebrafish germ cells is the Dead end protein. Dead end
binds poly-U stretches within the 3 0 UTR of the target mRNA (e.g., nanos ,
tdrd7 , and hub ) to inhibit miRNA function by a yet unknown mechanism
( Kedde et al ., 2007; Mickoleit et al ., 2011; Slanchev et al ., 2009 ). In addition,
the DAZ-like protein (Dazl) was found to increase translation efficiency of
mRNAs by promoting poly(A) tail elongation in the germ line and can
thereby counteract deadenylation mediated by miR-430 ( Takeda et al .,