Small RNAs in Germ Cell Development - Current Topics in Developmental Biology - page 84

During gametogenesis, the male germ cells undergo mitotic arrest and resume
division postnatally as spermatogonia cells. In contrast, the germ cells in the
female continue to divide, arrest in the diplotene stage of meiosis I, and
resume meiotic division postnatally during the ovarian folliculogenesis
( Fig. 4.1 C(b); reviewed in Edson et al ., 2009 ).

1.3. Small RNAs in germ cell development
The first evidence for small noncoding RNAs (sRNAs) function in devel-
opment was the discovery that introduction of double-stranded RNA
(dsRNA) fragments triggered effective inhibition of RNA function, namely
RNA interference (RNAi) in C. elegans ( Fire et al ., 1998 ) following the
discovery of genes encoding for small RNAs ( Lee et al ., 1993; Wightman
et al ., 1993 ). Prominent species of small RNAs are the miRNAs, the endo-
siRNAs, both playing more common roles in all tissues, and the piRNAs
whose function is mostly required in the germline ( Ambros et al ., 2003;
Aravin et al ., 2007a; Lagos-Quintana et al ., 2001; Lau et al ., 2001; Lee and
Ambros, 2001; Reinhart and Bartel, 2002 ).
Below, we discuss the function these three small RNA families play in
germline development by regulating gene expression and transposon activity.

2. miRNAs in Germ Cell Development

2.1. miRNA biogenesis and function
miRNAs comprise a large family of 21-25-nucleotide (nt)-long noncoding
RNAs that function by regulating gene expression posttranscriptionally. The
biogenesis of miRNAs was recently reviewed in great detail by Winter et al .
(2009) and Krol et al . (2010) . Briefly, miRNAs are processed from precursor
molecules (pri-miRNAs) transcribed by RNA polymerase II ( Fig. 4.2 A). The
pri-miRNAs fold into a characteristic hairpin structure that is processed by
the RNase III enzyme Drosha and the accessory protein DGCR8 into the

70-nt-long pre-miRNA ( Lund et al ., 2004 ). Following export into the
cytoplasm mediated by Exportin 5, the pre-miRNA is further processed into
the 21-25-nt-long miRNA/miRNA* duplex by another RNase III, Dicer,
supported by the RNA-binding protein TRBP ( Carmell and Hannon, 2004;
Hutvagner et al ., 2001; Yi et al ., 2003 ). To allow targeting of specific
mRNAs, the guiding strand of the now mature miRNA is loaded onto an
Argonaut (AGO) protein, one of the main components in the miRNA-
induced silencing complex, miRISC. The miRNA incorporated into the
miRISC will guide the complex to the specific target mRNAs.
miRNAs interact with their target mRNAs by recognizing short
sequence signatures often located within their 3 0 untranslated region