Transcript Review Viral and Cellular MicroRNAs as Determinants of

November 24th 2008
Eva Gottwein and Bryan R. Cullen
Cell Host & Microbe, June 2008
Viral and Cellular MicroRNAs as
Determinants of Viral Pathogenesis and
Immunity
Role of miRNAs in interaction between viruses and their hosts
Ines Hahn, Department of Pulmonary Medicine, Laboratory for Experimental Lung
Hannover Medical School
What are microRNAs ?
•
microRNAs (miRNAs) are small, single-stranded, non-coding RNA
molecules
•
about 19-23 nucleotides in length
•
encoded in the genomes of plants and animals
•
regulate the expression of genes by binding to the 3´ untranslated
regions (3'-UTR) of specific mRNAs
•
mammalian genomes are predicted to encode 200-500 miRNAs
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miRNA formation
and processing
1.
miRNAs are initially transcribed as
part of a long RNA molecule:
Primary-miRNA (500-3000nt)
2.
In the nucleus, the dsRNA-specific
ribonuclease Drosha processes
primary-miRNAs into 70-100nt
hairpin RNAs: pre-miRNAs
3.
Drosha acts in conjunction with
RNA-binding protein DGCR8
www.ambion.com
Pir-miRNA: Primary microRNA
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4.
pre-miRNAs are transported to the
cytoplasm via an exportin 5-dependent
mechanism
5.
In the cytoplasm, the pre-miRNAs are
digested by a second double strandspecific ribonuclease: Dicer
6.
Dicer processing results in a sequencespecific, single-stranded, mature miRNA
molecule
7.
The single-stranded, 19-23nt
mature miRNA is bound by a
complex that is similar to the
RNA-induced silencing complex
(RISC) that participates in RNA
interference
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8.
RISCs consist minimally of the
miRNA and one of four Argonaute
(Ago) proteins
9.
In animals, the RISC-bound miRNA
binds mRNAs through a sequence
that at least partially matches the
3`-UTR of target mRNA
10.
Seed region: 2-7 nt from the
miRNA 5`end
11.
The bound mRNA remains
untranslated, resulting in reduced
expression of the corresponding
gene
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Function of miRNA as regulators of viral and/or
host cell gene expression
•
miRNA are attractive candidates as virally encoded regulators of viral
and/or host cell gene expression due to:
– their small size
– lack of immunogenicity
– remarkable functional flexibility
•
miRNAs may regulate viral replication and pathogenesis:
– viruses may use cellular miRNA for their replication
– viral miRNAs may directly regulate viral and/or host cell gene expression to
benefit the virus
– expression of celluar miRNAs may be induced/inhibited to benefit the virus
•
On the other hand: cellular miRNA can be disadvantageous to the
virus!
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Viral miRNA expression in human pathogenic
herpesviruses
•
Viral miRNAs exclusively derive from dsDNA viruses mainly of the
herpesvirus family e.g.:
–
–
–
–
Herpes simplexvirus type 1 (HSV)
Human and murine cytomegalovirus (hCMV, mCMV)
Karposi Sarcoma Herpesvirus (KSHV)
Human Epstein-Barr Virus (EBV)
•
Some herpesviruses express multiple viral miRNAs e.g. hCMV, mCMV,
EBV, KSHV
•
Herpesvirus miRNAs, like cellular miRNAs, are processed from POL II
transcripts
•
Herpesviral miRNAs derive from non-coding regions, open reading
frames of protein-coding mRNAs, intronic regions
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Viral miRNA expression during latency and
productive infection
γ-herpesvirus
e.g. KSHV, EBV
α- and β-herpesviruses
e.g. HSV-1, hCMV, mCMV
expression of miRNA primarily
associated with viral latency
≠
miRNAs expressed during
productive virus replication
clustered miRNA
≠
miRNAs are dispersed
throughout the viral genome
in closely related viruses miRNAs ≠
are arranged in the same
genomic location
encoded in mostly different
locations
miRNA sequences are poorly
conserved, even closely related
viruses exhibit no evident
sequence homology
miRNAs sequences are poorly
conserved, even closely related
viruses exhibit no evident
sequence homology
=
☺The exception prooves the rule.
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Viral miRNAs may directly regulate viral gene expression
- perfect complementarity to the 3´UTR of the mRNA -
• Virus-derived miRNAs may
target viral transcripts for
degradation with RISC
RISC
Figure: 1A
BART: BamA rightward transcripts
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Viral miRNAs may directly regulate viral gene expression
- imperfect complementarity to the 3´UTR of the mRNA -
• Viral miRNAs may inhibit
translation of viral transcripts
carrying imperfect matches
to the miRNA
RISC
Figure: 1B
IE: Immediate Early
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Viral miRNAs may directly reduce host cell gene
expression
•
•
virus-derived miRNAs may inhibit host mRNAs or
function as orthologs of cellular miRNA, thereby inhibiting host cell
mRNA targets for host cell miRNA
Figure: 1C
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Expression of celluar miRNAs may be restructured to
benefit the virus replication
•
Cellular miRNAs directly or indirectly affect virus replication
Figure: 2
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Celluar miRNAs may function as antiviral miRNA to
inhibit virus replication
•
Several viruses are accessible to inhibition by experimentally introduced
siRNAs, making it likely that they are also accessible to inhibition by
miRNA-RISC
•
There is no evidence for antiviral miRNAs so far
•
Reported interactions between viruses and cellular miRNAs resulting in
reduced viral replication might be random
•
Viruses should be able to escape such inhibitory interactions by mutation
and viral escape from experimentally introduced siRNA has been reported
for a range of viruses
•
Celluar miRNAs may inhibit virus replication by repressing cellular factors
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Conclusion
•
•
miRNA are produced by both viruses and their hosts
can either benefit the virus or the host
•
Viral miRNAs can
– regulate viral gene repression and replication
– affect cellular gene expression
•
Cellular miRNAs can alter viral life cycle (e.g. miR-155 / HCV)
Thanks for the attention!
Department of Pulmonary Medicine, Laboratory for Experimental Lung
Hannover Medical School