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MicroRNAs: key participants in gene regulatory networks
Commentary
Xi-Song Ke, Chang-Mei Liu, De-Pei Liu and Chih-Chuan Liang
microRNAs (miRNAs) are a newly identified and surprisingly large
class of endogenous tiny regulatory RNAs. They exhibit various
expressional patterns and are highly conserved across species.
Recently, several regulatory targets of miRNAs have been
predicted. Functional analysis of the potential targets indicated
that miRNAs may be involved in a wide range of pivotally
biological events. The nature of miRNAs and their intersection
with small interfering RNAs endow them with many regulatory
advantages over proteins and make them a potent and novel
means to regulate gene expression at almost all levels.
Here we argue that miRNAs are key participants in gene
regulatory network.
from the purely informational medium to a variety of
structural, informational and even catalytic molecules in
the cell. Many other ncRNAs that function as regulators
have also been discovered [3], but their number and
importance seem marginal.
1367-5931/$ – see front matter
ß 2003 Elsevier Science Ltd. All rights reserved.
Recently, a remarkably large number of tiny ncRNA
genes have been identified and named microRNAs
(miRNAs) [4–6,7,8–10,11,12,13]. The first miRNAs
to be discovered were the heterochronic regulatory RNAs
Lin-4 and Let-7 [14,15]. miRNAs have been evolutionarily conserved over a wide spread of species and exhibit
diversity in expression profiles, suggesting that they
occupy a wide variety of regulatory functions and exert
profound effects on cell growth and development [4–6].
Recent work showed that miRNAs can regulate gene
expression at many levels, offering a novel and promising
gene regulatory mechanism and strongly supporting the
idea that RNA can perform the same regulatory roles as
proteins. Understanding this RNA-based regulation will
help us to finally understand the complexity of the
genome and the gene regulatory network.
DOI 10.1016/S1367-5931(03)00075-9
Identification
Addresses
National Laboratory of Medical Molecular Biology, Institute of Basic
Medical Sciences, Chinese Academy of Medical Science & Peking Union
Medical College, Beijing, 100005, China
e-mail: [email protected]
Current Opinion in Chemical Biology 2003, 7:516–523
Abbreviations
B-CLL
B-cell chronic lymphocytic leukemia
CAF
CARPEL Factory
miRNAs microRNAs
miRNP
micro-ribonucleoprotein
ncRNAs non-coding RNAs
PPD
PAZ/PIWI domain
RdRP
RNA-dependent RNA polymerase
RISC
RNA-induced silencing complex
RNAi
RNA silencing
siRNA
small interfering RNA
UTR
untranslated region
Introduction
Four decades ago, the Central Dogma was formulated and
simplified as ‘DNA makes RNA, and RNA makes protein’. As a result, RNAs have been looked as simple
molecules that merely convert genetic information into
protein. However, it has been estimated that although
most of the genome is transcribed, almost 97% of the
genome does not encode proteins in higher eukaryotes
[1]; it is difficult to believe that these vast transcripts have
no function. Now, putative non-coding RNAs (ncRNAs)
have been discovered that function directly as RNA
rather than encoding proteins [2]. ncRNAs seem to be
particularly suited for roles that require highly specific
nucleic-acid recognition. The view of RNA has changed
Current Opinion in Chemical Biology 2003, 7:516–523
miRNAs are 22 nt RNAs that arise from one arm of
longer endogenous hairpin transcripts [16]. Hundreds of
miRNA genes have been identified in many species,
mostly in intergenic regions. Several miRNAs that correspond to putative genes have also been identified
[5,10,13]. Some miRNAs have multiple loci in the genome
[9,11]; occasionally, several miRNA genes are arranged in
tandem clusters [4,7,12]. Despite the success of recent
efforts, the fact that many miRNAs have been isolated just
once suggests the search for new miRNAs is far from
complete [9,10,11]. An analysis of chromosomes 21 and
22 found that ten times more sequences of the genome
were transcribed than predicted. The phylogenic conservation of such transcripts indicates that they do have a
function [17], and also hints that many genes operate
‘below our radar’. The identified miRNAs probably represent only the tip of the iceberg, and the number of
miRNAs might turn out to be very large.
Evidence has been presented that mature miRNAs are
cleaved from their precursors by the RNaseIII family
(Dicer) [18]. Taking together all identified miRNAs, the
characteristics of miRNAs can be summarized as following:
1. They are single-stranded RNAs of about 22 nt.
2. They are cleaved from one arm of a longer endogenous
double-stranded hairpin precursor by Dicer.
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MicroRNAs: key participants in gene regulatory networks Ke et al. 517
3. They should match precisely the genomic regions that
can potentially encode precursor RNAs in the form of
double-stranded hairpins.
4. miRNAs and their predicted precursor secondary
structures are phylogenetically conserved.
5. miRNAs and their potential precursors can be confirmed by northern blots.
6. When Dicer is mutated or knocked-out, miRNA precursors accumulate.
Figure 1
5′–P
Hairpin
precursor
3′–OH
5′–P
3′–OH
Biosynthesis
Much evidence has supported that Dicer and Argonaute
are crucial participants in miRNA biosynthesis and function. Dicer and Argonaute animal mutants exhibit reduction in mature miRNA accumulation, essential reiteration
of cell division and delay of the switching to a later stage
developmental programme. In mutant Arabidopsis, disruption of embryo development, delay in flowering time
and over-proliferation of floral meristems occurs. These
lines of evidence suggest both Dicer and Argonaute are
indispensable for the maturation and activity of miRNAs
[9,19]. Argonaute has been shown to interact with
miRNAs genetically and biochemically, and the translation initiation factor eIF2C can cleave substrates that are
homologous to its constituent miRNAs [7,19–21,22].
Mutations in genes required for miRNA biosynthesis lead
to genetic developmental defects; these dramatic consequences are derived, at least in part, from the role of
generating miRNAs [22].
Although the details are still obscure, the outline of the
miRNA biosynthesis pathway is beginning to emerge. In
the earliest experiments, miRNAs were generated from
their precursors by Dicer in animals or its Arabidopsis
homologue CARPEL Factory (CAF) [9,11,18]. As several miRNA pairs were cloned from both sides of the same
precursors [4,5,7,9], these miRNAs could base pair with
each other and potentially form duplexes with 2 nt
overhangs at both 30 ends, a feature of the Dicer products
[23]. We propose that miRNAs are initially cleaved in
duplex form rather than from one arm of their precursors.
Secondly, the duplex miRNAs (pre-miRNAs) are transferred to pre-micro-ribonucleoprotein (pre-miRNP), a
multi-protein–RNA complex whose constituents include
Argonaute and RNA helicase [7,12,19] and that differs
from miRNP only in that it contains double-stranded
miRNA [7]. Thirdly, cofactors unwind the pre-miRNAs
into single strands, thus transforming pre-miRNP into
miRNP. Argonaute proteins belong to the PAZ/PIWI
domain (PPD) family, and PAZ is also a component of
Dicer and thought to mediate Argonaute–Dicer interaction [24,25]. Finally, miRNAs recognize targets whilst
sequestered within miRNP. Hence, miRNPs may play an
important role in the maturation and delivery of their
miRNAs to RNA targets [12], and different effectors
could integrate into and lead miRNAs to diverse functional pathways (Figure 1).
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Dicer
OH–3′
5′–P
3′–OH
P–5′
Pre-miRNA
Co-factors
Pre-miRNP
Degradation
miRNP
Diverse
effectors
Functions
Current Opinion in Chemical Biology
Model for the biosynthesis and function pathway of miRNAs. miRNAs
are cleaved by Dicer from the hairpin precursor in the form of duplex,
initially with 2 or 3 nt overhangs in the 30 ends, and are termed premiRNAs. Cofactors join the pre-miRNP and unwind the pre-miRNAs into
single-stranded miRNAs, and pre-miRNP is transformed to miRNP.
miRNAs can recognize regulatory targets while part of the miRNP
complex. Different effectors lead miRNAs into diverse pathways. The
structure of pre-miRNAs is consistent with the observation that 22 nt
RNA duplexes with 2 or 3 nt overhangs at the 30 ends are beneficial for
reconstitution of the protein complex and might be required for highaffinity binding of the short RNA duplex to the protein components [23].
Functions
Detailed functions
Growing evidence suggests that miRNAs play crucial
roles in eukaryotic gene regulation. The first miRNAs
genes to be discovered, lin-4 and let-7, are heterochronic
switching genes that are essential for the normal temporal
control of diverse developmental events in Caenorhabditis
elegans. Both lin-4 and let-7 base-pair incompletely to
Current Opinion in Chemical Biology 2003, 7:516–523
518 Commentary
repeated elements in the 30 untranslated regions (UTRs)
of other heterochronic genes, and regulate the translation
directly and negatively by antisense RNA–RNA interaction [14,15]. Mutations in lin-4 or let-7 causes temporal
transformations in cell fates with omission or reiteration of
stage-specific events [14,15]. Studies of lin-4 indicated
that the stability, polyadenylation level and polysome
association of target mRNAs were not affected [26,27].
This regulatory model seems flexible and efficient, for the
translational repression can be easily deleted just by
releasing the miRNAs, rather than destruction and synthesis of new mRNAs. The other miRNAs were thought to
interact with target mRNAs by limited complementary
and suppressed translation as well [4–6].
Many studies have shown, however, that the acting model
of lin-4 and let-7 might be just one type of functional
mechanism mediated by miRNAs. A large subset of
Drosophila miRNAs were shown to be precisely complementary to the K box, Brd box and GY box motifs in the
30 -UTR, and the two former motifs were previously
demonstrated to significantly affect both transcript stability and translational efficiency [28]. Given a perfectly
complementary target RNA, let-7 could direct target RNA
degradation rather than inhibit translation [22]. In addition, Arabidopsis miR39 /miR171 was perfectly complementary to internal regions of some mRNAs, and resulted
in specific cleavage of targets within the region of complementarity [29]. This suggests that only the degree of
complementarity between miRNAs and their RNA targets determines their functions [22].
Predicted functions
Predicting potential targets is the major challenge in
exploring miRNA functions. By identifying sequences
with near complementarity, several targets have been
predicted. Strikingly, most of the potential or putative
targets are transcriptional factors that are crucial in cell
growth and development. Both the basic-helix–loop–
helix (bHLH) repressors and Bearded family are predicted targets of many miRNAs, and miRNAs could be
the key to explain the plethoric developmental phenotypes [28]. The bHLH repressors belong to a superfamily of DNA-binding transcription factors that regulate
numerous biological processes, including cell fate decisions in both invertebrates and vertebrates [30]. Bearded
family members are modulators of the Notch signaling
pathway in multiple cell fate decisions during adult
sensory organ development [31]. Scarecrow-like family
are targets of miR39/miR171 and regulate an asymmetric
cell division that is essential for generating the radial
organization of the Arabidopsis root [32]. miRNAs in Arabidopsis are almost complementary to many sequences.
Some of these potential targets are transcriptional factors,
including squamousa-promoter-binding-protein-like proteins and CCAAT-binding-factor-HAP2-like proteins,
many of which are involved in developmental patterning
Current Opinion in Chemical Biology 2003, 7:516–523
or cell differentiation, and the complementary sites were
conserved among flowing plants [9,33]. Predicted targets
for miR165 encoded HD-Zip transcription factors. The
potential complementary sites might explain the ectopic
expression previously described for mutations in these
genes. It seemed that complementarity to miR165 was
required for confining these mRNA’s accumulation to the
proper cell types [33]. Targeting of developmental transcription factors suggested that many plant miRNAs function during cellular differentiation to clear key regulatory
transcripts from daughter cell lineages [33]. Interestingly,
Argonaute1, which was required both for axillary shoot
meristem formation, leaf development in Arabidopsis and
the biosynthesis and functions of miRNAs, was a predicted
target of miR168, suggesting a negative feedback mechanism to control expression of Argonaute and activities of
miRNAs [33]. The high percentage of predicted miRNA
targets acting as developmental regulators and the conservation of target sites suggested that miRNAs are
involved in a wide range of organism development and
behaviour and cell fate decisions [33].
Analysis of the genomic location of miRNAs indicates
that they play important roles in human development and
disease. For examples, mir10 was located and conserved
in the Hox gene cluster in multiple species. Considering
the spatial and temporal colinearity of Hox gene expression and the positional conservation of mir-10, it is conceivable that miR-10 is important for regulating
developmental events [13]. mir15 and mir16 are clustered
and located within the intron of LEU2, which lies within
the deleted minimal region of the B-cell chronic lymphocytic leukemia (B-CLL) tumor suppressor locus, and
both genes are deleted or down-regulated in the majority
of CLL cases [13,34]. It is possible that miRNA levels
are crucial in maintaining normal B cell differentiation.
B-CLL suppressor genes have yet to be identified and
mir-15 and mir-16 are strong candidates [13].
Proteins related to fragile X syndrome have been associated with protein complexes that contain miRNAs. The
possibility that these proteins use an RNA-related
mechanism for target recognition suggests that defects
in miRNA-related machinery may cause human disease
[35,36]. In addition, the locus of mir-175 is a candidate
region for Waisman syndrome and X-linked mental retardation [12]. Such findings present further evidence for
the involvement of miRNAs in human diseases, and the
significance of this growing gene family.
SiRNA-related functions
In addition to miRNAs, there are other 22-nt ncRNAs
named small interfering RNAs (siRNAs) [23]. siRNAs are
derived from exogenous dsRNA molecules including
very long hairpins or bimolecular duplexes. Numerous
siRNAs accumulate from both strands of the dsRNA after
the action of Dicer [16]. siRNAs function in the form of
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MicroRNAs: key participants in gene regulatory networks Ke et al. 519
RNA-induced silencing complex (RISC) to mediate the
destruction of homologous RNA targets during RNA
silencing (RNAi), which was thought to be the ‘immune
system’ of the genome against the proliferation of invading molecules [37]. It was demonstrated that perfect basepairing between siRNAs and target mRNAs is required
for mRNA cleavage [38]. Furthermore, siRNAs might
modulate gene expression through additional mechanisms including guiding cytosine methylation, histone
methylation and epigenetic modifications of homologous
DNA sequences [39–41]. siRNAs and RNAi-related proteins including Dicer, Argonaute and RNA-dependent
RNA polymerase (RdRP) appear to be central components of the chromatin-silencing machinery. siRNAs are
required for the proper regulation of chromosome architecture during mitosis and meiosis in fission yeast [42,43].
Provost’s work suggested an evolutionary conserved role
of Dicer in chromosome function [44]. The Argonaute
family members are important in post-transcriptional and
transcriptional gene silencing and programmed DNA
elimination [41]. Importantly, siRNAs were able to signal
intercellularly and trigger silencing systemically throughout the plants [45].
Except for the different origins of their precursors,
miRNAs and siRNAs cannot be distinguished from their
biochemical compositions, biosynthesis and functions.
The generation and activities of both require Dicer and
Argonaute [18,21,46]. In fact, there are several tantalizing
similarities between miRNP and RISC, including similar
sizes and both containing RNA helicase and the PPD
proteins. It is therefore tempting to speculate that miRNP
and RISC are the same RNP with multiple functions [47].
In addition, let-7 and miR39 could naturally enter the RNAi
pathway and cleave target mRNAs [22,29]. The nearperfect complementarity between plant miRNAs and their
targets suggests that they act similarly to siRNAs and direct
target RNA cleavage [33]. Interestingly, experiments
showed that siRNAs can function as miRNAs to repress
expression of a target mRNA with partially complementary
binding sites in its 30 -UTR [48]. Furthermore, siRNAs
could guide target RNA cleavage in RNAi in the form of
single strands such as miRNAs [49]. On the other hand,
Drosophila Argonaute coimmunoprecipitated with both
miRNAs and siRNAs [35]. All these lines of evidence
indicate that the two pathways intersect, and may even
be the same. In some sense, miRNP/RISC could be considered as a flexible platform into which different regulatory modulators could be incorporated. The core complex
would be required for receiving small RNAs from Dicer
and guiding them to recognize their homologous targets.
Different effectors could join in the core. When nucleases
are incorporated, they direct the degradation of targets,
and translational repressors lead to translational inhibition.
The inclusion of chromatin remodeling factors, on the
other hand, mediates transcriptional silencing. Other adaptations might also exist [50].
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Deduced functions
Considering their genomic location, the employment of
tiny RNAs to specific recognition and the connection with
siRNAs, it is reasonable to deduce the almost infinite
potential of miRNAs in regulating gene expression at
nearly all levels. As some miRNAs are located less than
1 kb from either 50 start or 30 end of putative genes that
contain many cis-acting elements, even in introns, exons
or the complementary spanned splice sites of known
protein coding genes [5,13,33], it is imaginable that
miRNAs have regulatory activities ranging from gene
transcription as trans-acting factors, translational initiation, pre-mRNAs splicing, RNA transport, mRNAs stability or localization as the base-pairing site diversion. But
it is not clear how to control these RNAs to function as
miRNAs or traditional RNAs. Some miRNAs come from
the same precursors [4,5,7,9], indicating that interaction
may take place between miRNAs. In addition, there are
miRNAs that are related in sequence to other ncRNAs,
such as rRNAs, tRNAs or snRNAs [10], arguing strongly
for a more general function of mRNAs in post-transcriptional regulation. Furthermore, as some miRNAs correspond to the sense strands of mRNAs [5], or are
complementary to potential promoter sequences, and
CAF has putative nuclear locational signals, it is conceivable that miRNAs might guide the modification of functional or structural chromatin DNA sequences and regulate
gene expression at the DNA level, such as replication,
transcription or epigenetic control of nuclear genomes [9].
Interestingly, miRNAs may also target proteins; a good
example was a regulatory RNA CsrB, which competed with
another mRNA to bind a global regulatory protein CsrA,
and antagonized its activity [51].
Advantages as regulators
As tiny RNA regulators, miRNAs seem to be highly
adaptable for participators in gene regulating processes.
By their very natures, miRNAs can do the same as
traditional protein regulators and even provide obvious
advantages over the later in many repects. Firstly,
Watson–Crick base pairing between miRNAs and target
RNAs or DNAs allows an exquisitely specific recognition,
a job that RNA is more suited to than proteins. Secondly,
miRNAs can be generated more rapidly when required,
for a miRNA precursor is much smaller than a typical
protein-coding gene, and without being delayed by translation and post-translation modification in synthesizing.
Rapid expression and the ability to act directly as RNA
may facilitate the precise temporal regulation, especially
in developmental transitions via miRNAs. Thirdly, it is
evident that production and degradation of miRNAs are
both more cost-efficient than a specific RNA- or DNAbinding protein domain. Fourthly, there are an enormous
number of different theoretical miRNAs, and each has
many potential targets with different base-pairing modes.
The diverse degree and details of base pairing provide a
wealth of possibilities and flexibilities in gene regulation.
Current Opinion in Chemical Biology 2003, 7:516–523
520 Commentary
Fifthly, evolution of a specific complementary miRNA
could be achieved more simply, just in a single step by a
partial duplication of a fragment of the target gene into an
appropriate context for expression of new genes [2].
Sixthly, many processes in gene regulation can be
obtained simply by steric interference of target DNAs,
RNAs or proteins, which could be carried out only via
miRNAs. Finally, given that they share the same mechanism with siRNAs to be amplified by RdRP activity
[52,53], miRNAs may scale-up greatly both in the number
and effect. Taking all this together, we can see that
miRNAs are more optimal materials than proteins in
many ways, and may be the simplest molecules for the
desired gene regulation.
Figure 2
Chromatin
1
Gene
2
2
3
3
Pre-mRNA
4
5
4
5
Although the details of miRNA-mediated gene regulatory networks are not clear yet, the tiny size and prevalence of miRNAs hint that they might collaborate with
proteins to control gene expression, and miRNA-based
mechanisms might be widespread in higher eukaryotes.
When processes require simply alteration of steric structure of the target molecules to indirectly control the
binding of other regulatory factors or blocking of active
sites, even tagging on targets to recruit other regulators,
miRNAs can carry out such roles alone. In cases requiring
more sophisticated functions, a small number of shared
proteins could be incorporated into a common protein
complex containing miRNAs, with the miRNAs acting as
guiders in specific sequence recognition, and the incorporated proteins performing specific functions. In this
sense, miRNAs could potentially regulate in trans any
biological process involving RNA–RNA, RNA–DNA or
RNA–protein interactions. In fact, the relatively short
length seems insufficient to induce complex structural
signals for stability or intracellular transport. In addition
to antisense sequence elements, therefore, it is likely
that many different miRNAs utilize a common protein
complex for intracellular trafficking and protection from
degradation [54]. It is possible that miRNAs could be
Current Opinion in Chemical Biology 2003, 7:516–523
mRNA
5,6
7,8
Precursor
4,5
6,7
Key participants in gene regulatory network
From the above discussion, we can see that miRNAs have
various expression patterns including stage-specific and/or
tissue-specific and uniform during development and are
involved in a broad range of key process in cell division,
and cellular and organism physiology; vast numbers of
miRNAs exist; their predicted targets and genomic locations are diverse and interesting; and, most importantly,
they have the potential to interact with RNA, DNA and
protein molecules to regulate gene expression in a diverse
array of developmental or physiological events at almost
all levels, including chromatin remodeling, gene transcription, RNA processing and location, translation efficiency,
RNA transport, RNA stability, even proteins activity. It is
logically concluded that miRNAs are key participants
even at the second center in a gene regulatory network
(Figure 2).
1
4
5,6
7,8
miRNA
Protein
9
1. Chromatin remodeling
2. Gene methylation
3. Transcription
4. RNA process
5. RNA stability
5
7
10
6. RNA transport
7. RNA location
8. Translation
9. Protein activity
10. miRNA stability
Current Opinion in Chemical Biology
miRNAs are key participants even at the second center in gene
regulatory networks. miRNAs potently interact with RNA, DNA and even
proteins and regulate gene expression at nearly all steps. Importantly, by
targeting Argonaute proteins, a negative feedback mechanism for the
biosynthesis and activity of miRNAs may exist. Purple arrows indicate
activities of proteins and green arrows indicate activities of miRNA.
trafficked systemically to distant organs like siRNAs and
contribute to long-distance regulation in response to
environmental signals. Demonstration that miRNAs traffic systemically and selectively in plants would significantly enrich the emerging paradigm and lay an RNA
signaling ‘superhighway’ to facilitate communication
and coordination of vital information for plant growth,
development and response to the environment [55].
The discovery of the phylogenic distribution of miRNAs
implies the ancient origin of the miRNA pathway, which
arose early in eukaryotic evolution and might have
played roles during the origins and evolution of both
plant and animal multi-cellular life [11]. However, considering the versatility of miRNAs and their advantages
over peptides for some mechanisms, it is likely that
miRNAs aren’t simply the remnants of ancient, preDNA and pre-protein control systems [56], but play
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MicroRNAs: key participants in gene regulatory networks Ke et al. 521
key roles in higher eukaryotes even now. Although
proteins are the known fundamental effectors of cellular
function, the basis of eukaryotic complexity and phenotypic variation might lie primarily in a control architecture that is composed of a highly parallel system of transacting RNAs. These RNAs relay information required for
the coordination and modulation of gene expression [57].
miRNAs might act as ancient but vigorous participants
and a second center of gene expression, and enable the
integration and completion of the traditional network.
These results indicate that Dicer mRNA was subject
to negative feedback regulation through the activity of
a miRNA.
Acknowledgements
This work is supported by the Chinese High-Technology (863) Programme
2001AA217171 and NSFC/RGC 3991061991.
References and recommended reading
Papers of particular interest, published within the annual period of
review, have been highlighted as:
of special interest
of outstanding interest
Conclusions and future challenges
We should be excited by the exploration of the novel
regulatory paradigm of miRNAs; it opens a new vista
and adds another dimension to the diversity and complexity of gene-regulatory networks. miRNAs also offer possible explanations for some puzzles in deeply studied but
unclear regulatory pathways and complex genetic phenomena such as imprinting and RNA interference that involve
RNA signaling. Furthermore, miRNAs might help to
partly decipher the mystery that higher organisms have
striking complexity in phenotypes yet a relatively limited
number of protein coding genes in their genomes [57]. The
field has advanced at an astonishing rate over the past few
years; however, the key roles of miRNAs are just beginning
to be glimpsed and we are far from fully appreciating the
mechanism of miRNA-mediated gene regulatory networks. In our opinion, the primary challenges for understanding miRNA-mediated networks are to verify the
predicted targets of miRNAs by genetic or biochemical
experiments, elucidate the exact mechanism of miRNAs
working on one another, and identify the pathways that
control the expression and modulate the activities of
miRNAs to respond environmental or cellular signals.
Update
Recent work has explored more evidence for miRNA
function. mir-14 has been shown to function as a cell
death suppressor in Drosophila [58]. Loss of mir-14
enhanced Reaper-dependent cell death, whereas overexpression suppressed cell death. Another miRNA gene
bantam simultaneously stimulated cell proliferation and
prevented apoptosis, and a pro-apoptotic gene, hid, as a
target for regulation by bantam miRNA provided an
explanation for bantam’s anti-apoptotic activity [59].
Conversely, a heterochronic gene, lin-57/hbl-1, which
ensures that postembryonic developmental events are
appropriately timed in C. elegans, is a probable target of
let-7 and lin-4 regulation through its 30 UTR [60,61].
Another example of a negative feedback mechanism
involved in control of activities of miRNAs has been
detected. The levels of Dicer RNAs were elevated in
miRNA-defective mutant plants, a sequence near the
middle of Dicer mRNA was the predicted target for
miR162, and Dicer-derived RNA with the properties of
a mir162-guided cleavage product was identified [62].
www.current-opinion.com
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