Small RNAs in Germ Cell Development - Current Topics in Developmental Biology - page 85

A C

piRNA pathways

miRNA pathway

B

endo-siRNA pathway

in mammals in Drosophila in Drosophila

RNA polymerase II dsRNA fragment Long ssRNA

m7 Cleavage

Gppp Dicer-2

Loqs

Drosha

pri-miRNA

DGCR8

Transposon mRNA
Piwi/Aub

AAAA

Dicer-2

Exportin5

Ago loading
complex

Dicer-2

Dicer

pre-miRNA R2D2

ping-pong
cycle

Piwi/Aub

TRBP

Ago3

miRNA/miRNA*
duplex

Dicer

Ago2

siRISC

Ago2

miRISC AGO Ago3

piRNA cluster transcript

Ago2

AAAA Transposon mRNA

AAAA

m7

G m7 G

Figure 4.2 (A) Schematic representation of miRNA biogenesis and function in mam-
mals. miRNAs are transcribed by RNA polymerase II, and Drosha and DGCR8 process
the stem loop-containing pri-miRNAs. After export into the cytoplasm by Exportin 5,
Dicer and the accessory protein TRBP/Loqs (mammals/fly, respectively) further pro-
cess the pre-miRNA into the miRNA/miRNA duplex. The guide strand is then loaded
onto the effector protein AGO forming the miRISC complex that is guided toward the
3 0 UTR of the target mRNAs, resulting in deadenylation or translational repression.
(B) endo-siRNA biogenesis and function in D. melanogaster. Precursor double-stranded
RNAs (dsRNAs) fragments are processed by Dicer-2 and the dsRNA-binding protein
Loqs. The processed RNA bound to the R2D2 and Dicer-2 proteins is loaded onto
Ago2 forming the siRISC that targets the slicer activity of Ago2 toward target mRNAs
and RNA of transposable elements. (C) piRNA biogenesis and function D. melanogaster.
The single-strand RNA which is the precursor for piRNAs is cleaved and the resulting
antisense piRNA is loaded onto Piwi/Aub protein and binds transposon RNA thereby
initiating the ping-pong cycle. Cleavage of the transposon RNA generates a small sense
RNA molecule that is loaded onto Ago3 protein and the RNA-protein complex then
binds the piRNA transcript. Cleavage by piRNA/Ago3 and interaction of the resulting
small RNA with PIWI/Aub protein completes the cycle, leading to an increase in the
number of piRNA molecules and inhibition of transposon function.

(3 0 UTR). Association of miRNAs with their target commonly leads to
deadenylation and subsequent degradation of the target and can also result in
translational repression ( Carrington and Ambros, 2003; Lai, 2002 ; reviewed
in Bartel, 2009; Kim et al ., 2009; Krol et al ., 2010 ).

2.2. miRNA function during PGC specification
As mentioned above, PGC specification in the invertebrates C. elegans ,
D. melanogaster and in some vertebrates like X. laevis and Danio rerio follows
the preformation strategy that depends on maternally provided factors.