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The Replication cycle of Hepatitis B virus
Stephan Urban Andreas Schulze Maura Dandri Joerg Petersen
(1) Reversible and non-cell-type specific attachment to cellassociated
heparan sulfate proteoglycans.
(2) Specific and probably irreversible binding to an unknown
hepatocyte-specific preS1-receptor. This step presumably
requires activation of the virus resulting in exposure of
the myristoylated N-terminus of the L-protein [1].
(3) Two different entry pathways have been proposed: (3A)
endocytosis followed by release of nucleocapsids from
endocytic vesicles; (3B) fusion of the viral envelope at
the plasma membrane.
(4) Cytoplasmic release of the viral nucleocapsid containing
the relaxed circular partially double stranded DNA (rcDNA)
with its covalently linked polymerase.
(5) Transport of the nucleocapsid along microtubules. Accumulation
of the capsids at the nuclear envelope facilitates
interactions with adaptor proteins of the nuclear pore
complex.
(6) Possible trapping of the nucleocapsid in the nuclear basket
and release of rcDNA into the nucleoplasm. The mechanisms
determining the breakdown of the capsid and the
release of the viral DNA genome are unsolved [2].
(7) ‘‘Repair” of the incoming rcDNA: Completion of the plus
strand of the rcDNA by the viral polymerase. Removal of
the polymerase from the 50-end of the minus strand
DNA. Removal of a short RNA-primer used for the DNAplus
strand synthesis. Both processes are mediated by cellular
enzymes [3].
(8) cccDNA formation by covalent ligation of both DNA
strands (reviewed in [3]). The cccDNA molecule is organized
as a chromatin-like structure displaying the typical
beads-on-a string arrangement consisting of both histone
and non-histone proteins (minichromosome) [4]. The lack
of cccDNA in artificial host cells (e.g. hepatocytes of HBV
transgenic mice) suggests that host specific factors may
regulate cccDNA formation.
(9) Transcription. The cccDNA utilizes the cellular transcriptional
machinery to produce all viral RNAs necessary for
protein production and viral replication. Both host transcription
factors, such as CCAAT/enhancer-binding protein
(C/EBP) and hepatocyte nuclear factors (HNF) and viral
proteins (core, the regulatory X-protein) regulate this
process [4] and may modulate viral gene expression byinteracting with the viral promoters of the four major
overlapping open reading frames (ORFs): (I) the precore/
core gene, coding for the nucleocapsid protein and for
the non-structural, secreted, precore protein, the HBeAg;
(II) the polymerase gene coding for the reverse transcriptase,
RNase H and terminal protein domains; (III) the
L-,M-, and S-gene, coding for the three envelope proteins,
which are synthesized in frame from different promoters;
and (IV) the X gene, coding for the small regulatory X-protein.
A correlation between viremia levels and the acetylation
status of cccDNA-bound histones has been reported
[5], indicating that epigenetic mechanisms can regulate
the transcriptional activity of the cccDNA.
(10) All 4 major mRNAs utilize a single common polyadenylation
signal. Processing of viral RNAs, nuclear export as
well as stabilization of the viral RNAs appears to be exclusively
mediated by host factors (i.e. La RNA binding
protein).
(11) Translation of the pregenomic RNA (pgRNA) to the core
protein and the viral polymerase. The regulatory X-protein
and the three envelope proteins are translated from the
subgenomic RNAs.
(12) Complex formation of the pgRNA (via its epsilon stemloop
structure) with the core protein and the polymerase
and self-assembly of an RNA-containing nucleocapsid.
(13) Reverse transcription of the pgRNA followed by plusstrand
DNA-synthesis within the nucleocapsid. Maturation
of the RNA-containing nucleocapsids to DNA-containing
nucleocapsids within the cytoplasm.
(14) DNA-containing nucleocapsids can be either re-imported
into the nucleus to form additional cccDNA molecules
(14A) or can be enveloped for secretion (14B). The envelope
proteins are co-translationally inserted into the ER
membrane, where they bud into the ER lumen, and are
secreted by the cell, either as 22 nm subviral envelope particles
(SVPs) or as 42 nm infectious virions (Dane particles)
if they have enveloped the DNA-containing nucleocapsids
before budding. During synthesis of the L-protein, the
preS-domains remain cytoplasmically exposed and
become myristoylated. At some step after preS-mediated
nucleocapsid envelopment translocation across the membrane
occurs.
(15) Experiments performed using duck hepatitis B revealed
that the majority of cccDNA molecules in infected hepatocytes
comes from newly synthesized nucleocapsids. 1–50
cccDNA molecules appear to accumulate per cell, thoughdifferences in cccDNA dynamics and efficiency of cccDNA
accumulation may exist between HBV and the other hepadnaviruses.
Both viral and host factors controlling cccDNA
formation and pool size are yet poorly defined. A negativefeedback
mechanism suppressing cccDNA amplification
might involve the L-protein. As HBV polymerase inhibitors
do not directly affect the cccDNA, a decrease in cccDNA
levels is supposed to derive from the lack of sufficient recycling
of viral nucleocapsids to the nucleus, due to inhibition
of viral DNA-synthesis in the cytoplasm, and less
incoming viruses from the blood [6].
(16) Compared to virions spherical and filamentous SVPs are
secreted in a 103–106-fold excess into the blood of infected
individuals. SVPs lack a nucleocapsid and are therefore
non-infectious.
Therapeutic agents interfering with HBV life-cycle: (I) Acylated
preS1-peptides have been shown to bind the HBV-receptor
and block viral entry in vivo [7]; (II) Dihydroarylpyrimidines
interfere with nucleocapsid assembly and induce core protein
degradation [8]. (III) Polymerase inhibitors suppress reverse transcription
and synthesis of the DNA-plus strand. The preS1-
derived lipopeptides and the dihydroarylpyrimidines are presently
in preclinical development. Nucleos(t)ide analogues (Lamivudine,
Adefovir, Entecavir, Telbivudine, Tenofovir) and
interferon (IFN) a / PEG-IFN a are the only currently approved
therapeutic treatments. IFN a inhibits HBV both through immune
modulatory effects and directly by reducing steady-state levels of
HBV transcripts.
References
[1] Urban S. New insights into hepatitis B and hepatitis delta virus entry. Future
Virol 2008;3 (3):253–264.
[2] Kann M, Schmitz A, Rabe B. Intracellular transport of hepatitis B virus. World J
Gastroenterol 2007;13 (1):39–47.
[3] Nassal M. Hepatitis B viruses: reverse transcription a different way. Virus Res
2008;134 (1–2):235–249.
[4] Levrero M, Pollicino T, Petersen J, Belloni L, Raimondo G, Dandri M. Control of
cccDNA function in hepatitis B virus infection. J Hepatol 2009;51
(3):581–592.
[5] Pollicino T, Belloni L, Raffa G, Pediconi N, Squadrito G, Raimondo G, et al.
Hepatitis B virus replication is regulated by the acetylation status of hepatitis
B virus cccDNA-bound H3 and H4 histones. Gastroenterology 2006;130
(3):823–837.
[6] Zoulim F. New insight on hepatitis B virus persistence from the study of
intrahepatic viral cccDNA. J Hepatol 2005;42 (3):302–308.
[7] Petersen J, Dandri M, Mier W, Lutgehetmann M, Volz T, Von Weizsäcker F,
et al. Prevention of hepatitis B virus infection in vivo by entry inhibitors
derived from the large envelope protein. Nat Biotechnol 2008;26
(3):335–341.
[8] Deres K, Schröder CH, Paessens A, Goldmann S, Hacker HJ, Weber O, et al.
Inhibition of hepatitis B virus replication by drug-induced depletion of
nucleocapsids. Science 2003;299 (5608):893–896.
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楼主: StephenW
发表于 2011-5-13 08:48
