Biomedical Engineering Reference
In-Depth Information
2.1.2 Post-initiation Repression of Translation
Although most studies suggest that miRNA-mediated repression of translation
occurs at the translation initiation step, the presence of miRNP-mRNA in the
polysome fraction is not consistent with the inhibition of initiation model. It
was shown that lin-14 and lin-28 mRNAs (targets of lin-4 miRNA) remain
associated with polysome at different developmental stages (Olsen and
Ambros, 1999; Seggerson et al., 2002). These observations are now being
challenged, however, as the presence of these miRNP-mRNAs in the polysome
fraction might not be enough to suggest that these were, in fact, stalled com-
plexes and that translation had already been initiated.
2.2 Destabilization of mRNA (mRNA Deadenylation and Decay)
The levels of miRNA-regulated mRNAs have been shown to decrease in many
studies, and initial suggestions that levels of miRNA-regulated mRNAs remain
unchanged were only partially correct. Degradation of mRNA is triggered by
shortening of the poly(A) tail (deadenylation) and is followed by either 3
0
-5
0
exonuclease cleavage of mRNA or decapping and 5
0
-3
0
degradation by exonu-
clease (XRN1) (Parker and Song, 2004). A conserved interaction between
D. melanogaster GW182 and AGO1 protein has been proposed to be crucial
for marking miRNA-interacting mRNA for degradation (Behm-Ansmant et al.,
2006). The GW182 then recruits components of the deadenylation complex
CCR4-CAF1-NOT to the miRNA-regulated mRNAs. At this stage, if the dec-
apping enzymes DCP1 and 2 are missing, there is an accumulation of dead-
enylation mRNAs and a decrease in miRNA-mediated degradation of messages
(Behm-Ansmant et al., 2006; Eulalio et al., 2007b). It is unclear, however, if these
deadenylated mRNA further participate in translation inhibition mechanisms. It
is possible that the mechanisms of both translation inhibition and decay work
synergistically at this stage for effective miRNA-mediated regulation.
2.3 Sequestration of mRNA into Sub-cellular Locations
Sub-cellular locations such as P-bodies and stress granules have been known to
be accumulation sites for translationally repressed mRNA-protein complexes
(Eulalio et al., 2007a). There is evidence to suggest that mRNAs regulated by
miRNA-mediated translational repression accumulate in P-bodies. The inhibi-
tion of miRNA biogenesis or depletion of proteins involved in miRNA function
leads to loss of visible P-bodies in D. melanogaster. Studies from various
organisms suggest that the resident P-body protein, such as RCK/p54, 4E-T,
Pat1, and RAP55, may have an inhibitory effect on translation initiation
(Filipowicz et al., 2008). This sequestration of miRNA-repressed mRNAs in
P-bodies can also be partially attributed to the reversibility of miRNA action. It
has been shown in human hepatoma cells that after amino acid starvation or
