Biology Reference
In-Depth Information
the role of LIN-28 in blocking expression of mature let-7 early in worm
development, high levels of Lin28 in mammalian stem cells also prevent
accumulation of let-7 miRNAs.
For manymiRNAs, including let-7, only one-half of the duplex that results
from Dicer processing accumulates as the mature miRNA, while the other
strand is presumably degraded. One factor that appears to influence miRNA
stability is the association with Argonaute proteins (
Kai and Pasquinelli, 2010
).
The ability to base pair with target sequences influences the accumulation of
mature miRNAs through a mechanism dubbed target-mediated miRNA
protection (TMMP;
Chatterjee
et al.
, 2011
). Thus, miRNA passenger strands
that lack target sites are released from Argonaute and subject to degradation.
The 5
0
!
3
0
exonucleases XRN-1 and XRN-2 degrade miRNAs that lose
association with Argonaute (
Fig. 1.7
;
Chatterjee and Groszhans, 2009;
Chatterjee
et al.
,2011
). Although the accumulation of let-7 miRNA can be
regulated by the availability of target site interactions in
C. elegans
,whether
TMMP functions in other organisms is yet to be determined.
While Argonaute and the AIN-1/-2 (GW182-related) proteins are
required for miRNA function, several cofactors have been found to modu-
late miRNA activity. The TRIM-NHL family of proteins, which contain
TRIM (tripartite-containing motif; RING, B-Box, coiled-coil) and NHL
domains (named after the first three proteins discovered to contain this
motif; NCL-1, Ht2A, and LIN-41), includes a broadly conserved class of
proteins involved in diverse biological pathways (
Slack and Ruvkun, 1998
).
Two members of this class, NHL-2 in
C. elegans
and TRIM32 in mice,
enhance the ability of let-7 miRNA to regulate target genes (
Hammell
et al.
,
2009b; Schwamborn
et al.
, 2009
). These proteins associate with Argonaute
complexes and stimulate the repressive activity of miRNAs on certain
targets through an unknown mechanism (
Fig. 1.7
). The small ribosome
subunit protein, RPS-14, also co-precipitates with ALG-1 but, instead,
seems to negatively regulate let-7 function in
C. elegans
(
Fig. 1.7
;
Chan
and Slack, 2009
). Thus, the effectiveness of let-7 in target regulation is
influenced not only by the level of the miRNA but also by the presence of
specific miRNA complex accessory proteins.
3.2. Identification of let-7 targets
The first miRNA targets were identified as genetic suppressors of miRNA
mutant phenotypes (
Lee
et al.
,1993;Moss
et al.
, 1997; Reinhart
et al.
,2000;
Slack
et al.
, 2000; Wightman
et al.
, 1993
). Since miRNAs negatively regulate
target gene expression, lf mutations that reduce miRNA target levels can
compensate for the absence of the miRNA. For example, the reiteration of
hypodermal seam cell divisions and rupturing vulva phenotypes displayed by
let-7
mutants are suppressed in worms that also contain mutations in
lin-41
,
a direct target of
let-7
regulation (
Reinhart
et al.
, 2000; Slack
et al.
, 2000
).
