Biology Reference
In-Depth Information
(
Ruggiero
et al
., 2009
;
Trabucchi
et al
., 2009
). The pre-miRNA is then
relocated to the cytoplasm via Exportin 5 where it is processed further by
Dicer. Interestingly, it has been recently shown that certain miRNAs, such
as the erythrocyte regulator miR-451, are processed independently of Dicer
although via an Ago2-dependent pathway (
Cheloufi
et al
., 2010
). Addi-
tional forms of posttranscriptional modification, and thus control, have also
been reported for certain miRNAs such as let-7, where Tutase mediates
uridylation of pre-let-7 in a Lin28-dependent manner (
Hagan
et al
., 2009
).
This, in turn, causes a block to subsequent let-7 processing. The sequence of
the miRNA itself can be subject to variations that include alterations in
length at the 3
0
end (
Wu
et al
., 2007
), as well as Adar-dependent deamina-
tion that can impact processing and/or “seed” targeting (
Yang
et al
., 2006
).
Once a miRNA duplex is created, it is loaded into the RISC complex.
The miRNA duplex is unwound and a guide strand is used to direct RISC
to cognate 3
0
UTRs. This is followed by reduced transcript stability through
uncapping and deadenylation, and/or a block in translation, ultimately
resulting in lower amounts of protein expression (
Filipowicz
et al
., 2008
).
3.2. MicroRNA turnover
The relative proportions of miRNAs and their targets needed for adequate
repression of gene expression are just beginning to be carefully assessed with
the use of next-generation sequencing approaches. Like all chemical reac-
tions, the stoichiometry must be within an optimal range for a miRNA to
have a meaningful impact on its targets. It has been shown that the most
highly expressed miRNAs make up the bulk of those loaded into the RISC
complex (
Landthaler
et al
., 2008
). However, because miRNAs can work in
concert to enhance target repression (as mentioned above), it is likely that
miRNAs expressed at lower levels can work together to repress common
targets. Although such details are still being studied, it is clear that the
abundance of a mature miRNA is an important determinant of its func-
tionality. Thus, mechanisms that control both its initial production and the
turnover of its mature form will have a significant impact on miRNA
biology.
While miRNA biogenesis has been well studied, active mechanisms
governing miRNA turnover are just beginning to be understood. Perhaps,
the best examples of active miRNA turnover have come from experiments
using nonmammalian model organisms. Studies in plants and nematodes
have identified the exonucleases Sdn1 and Xrn2, respectively, as mediators
of miRNA degradation (
Chatterjee and Grosshans, 2009
;
Ramachandran
and Chen, 2008
). However, recent studies using human cells have found
that the exosome 3
0
-5
0
exoribonuclease complex is also involved in
miRNA decay (
Bail
et al
., 2010
). Further investigation into the mechanisms
regulating miRNA turnover remains an important direction for the field.
