Targeting of hepatitis C virus core protein to mitochondria through a novel C-terminal localization motif

Processing of human cytomegalovirus UL37 mutant glycoproteins in the endoplasmic reticulum lumen prior to mitochondrial importation

The human cytomegalovirus (HCMV) UL37 glycoprotein (gpUL37) is internally cleaved and its products divergently traffic to mitochondria or are retained in the secretory pathway. To define the requirements for gpUL37 cleavage, residues -1 and -3 of the consensus endoplasmic reticulum (ER) signal peptidase I site within exon 3 (UL37x3) were replaced by bulky tyrosines (gpUL37 cleavage site mutant I). Internal cleavage of this UL37x3 mutant was inhibited, verifying usage of the consensus site at amino acids (aa) 193/194. The full-length mitochondrial species of gpUL37 cleavage site mutant I was N glycosylated and endoglycosidase H sensitive, indicating that ER translocation and processing took place prior to its mitochondrial importation. Moreover, these results suggest that internal cleavage of gpUL37 is not necessary for its N glycosylation. Partial deletion or disruption of the UL37 hydrophobic core immediately upstream of the cleavage site resulted in decreased protein abundance, suggesting that the UL37x3 hydrophobic alpha-helix contributes to either correct folding or stability of gpUL37. Insertion of the UL37x3 hydrophobic core and cleavage site into pUL37(M), a splice variant of gpUL37 which lacks these sequences and is neither proteolytically cleaved nor N glycosylated, resulted in its internal cleavage and N glycosylation. Its NH(2)-terminal fragment, pUL37(M-NH2), was detected more abundantly in mitochondria, while its N-glycosylated C-terminal fragment, gpUL37(M-COOH), was detected predominantly in the ER in a manner analogous to that of gpUL37 cleavage products. These results indicate that UL37x3 aa 178 to 205 are prerequisite for gpUL37 internal cleavage and alter UL37 protein topology allowing N glycosylation of its C-terminal sequences. In contrast, the NH(2)-terminal UL37x1 hydrophobic leader, present in pUL37x1, pUL37(M), and gpUL37, is not cleaved from mature UL37 protein, retaining a membrane anchor for UL37 isoforms during trafficking. Taken together, these results suggest that HCMV gpUL37 undergoes sequential trafficking, during which it is ER translocated, processed, and then mitochondrially imported.

Interaction of the smooth endoplasmic reticulum and mitochondria

The ER (endoplasmic reticulum) is composed of multiple domains including the nuclear envelope, ribosome-studded rough ER and the SER (smooth ER). The SER can also be functionally segregated into domains that regulate ER-Golgi traffic (transitional ER), ERAD (ER-associated degradation), sterol and lipid biosynthesis and calcium sequestration. The last two, as well as apoptosis, are critically regulated by the close association of the SER with mitochondria. Studies with AMFR (autocrine motility factor receptor) have defined an SER domain whose integrity and mitochondrial association can be modulated by ilimaquinone as well as by free cytosolic calcium levels in the normal physiological range. AMFR is an E3 ubiquitin ligase that targets its ligand directly to the SER via a caveolae/raft-dependent pathway. In the present review, we will address the relationship between the calcium-dependent morphology and mitochondrial association of the SER and its various functional roles in the cell.

Non-structural protein 4A of Hepatitis C virus accumulates on mitochondria and renders the cells prone to undergoing mitochondria-mediated apoptosis

Non-structural protein 4A (NS4A) of Hepatitis C virus (HCV) functions as a cofactor for NS3 by forming a complex with it to augment its enzymic activities. NS4A also forms a complex with other HCV proteins, such as NS4B/NS5A, to facilitate the formation of the viral RNA replication complex on the endoplasmic reticulum (ER) membrane. In addition to its essential role in HCV replication, NS4A is thought to be involved in viral pathogenesis by affecting cellular functions. In this study, it was demonstrated that NS4A was localized not only on the ER, but also on mitochondria when expressed either alone or together with NS3 in the form of the NS3/4A polyprotein and in the context of HCV RNA replication in Huh7 cells harbouring an HCV RNA replicon. Moreover, NS4A expression altered the intracellular distribution of mitochondria significantly and caused mitochondrial damage, as evidenced by the collapsed mitochondrial transmembrane potential and release of cytochrome c into the cytoplasm, which led ultimately to induction of apoptosis through activation of caspase-3, but not caspase-8. Consistently, Huh7 cells expressing NS3/4A and those harbouring an HCV RNA replicon were shown to be more prone to undergoing actinomycin D-induced, mitochondria-mediated apoptosis, compared with the control Huh7 cells. Taken together, these results suggest the possibility that HCV exerts cytopathic effect (CPE) on the infected cells under certain conditions and that NS4A is responsible, at least in part, for the conditional CPE in HCV-infected cells.

Human herpesvirus 1 protein US3 induces an inhibition of mitochondrial electron transport

Previous studies have identified virus proteins that traffic to mitochondria and may affect mitochondrial function. Here, it is reported that Human herpesvirus 1 (HHV-1, herpes simplex virus 1) and influenza virus reduced mitochondrial respiration, whilst Measles virus, cytomegalovirus, coxsackievirus B4 and Feline calicivirus did not. The inhibition of total cellular respiration was caused by a block in the mitochondrial electron-transport chain. This effect occurred during β-phase protein synthesis and the inhibition of mitochondrial respiration could be reproduced by ectopic expression of the β-phase protein US3. An HHV-1 mutant lacking this protein failed to inhibit oxygen consumption in infected cells relative to controls. It was concluded that US3 was mediating the suppression of mitochondrial respiration following HHV-1 infection. The integrity of the electron-transport chain in HHV-1-infected cells was analysed further and the site of the block in electron transport was located between complexes II and III, a site previously shown to be affected by Poliovirus.

HCV proteins increase expression of heme oxygenase-1 (HO-1) and decrease expression of Bach1 in human hepatoma cells

BACKGROUND/AIMS

Hepatitis C infection induces hepatic oxidative stress. Heme oxygenase (HO), the rate-controlling enzyme of heme catabolism, plays a key role as a protector against oxidative, and other stresses. Other recent work has implicated Bach1, a heme binding protein that represses gene expression, in the regulation of HO-1 gene expression.

METHODS

We investigated the effects of HCV polyprotein expression on expression of HO-1 and Bach1 genes in human hepatoma cells (Huh-7 cells).

RESULTS

HO-1 was up-regulated in the cell line expressing HCV proteins from core up to the aminoterminal domain of NS3. Addition of increasing concentrations of N-acetylcysteine (NAC) led to down-regulation of HO-1 in cells expressing HCV proteins. In contrast, Bach1 was significantly down-regulated in these cells. Sodium arsenite, a strong inducer of oxidative stress and HO-1, reduced Bach1 expression in wild type Huh-7 cells, and NAC partially abrogated this decrease.

CONCLUSIONS

Huh-7 cells expressing HCV proteins show significant up-regulation of the HO-1 gene, and reciprocal down-regulation of the Bach1 gene. Exogenous oxidative stressors and anti-oxidants can modulate expression of these genes. These and other results suggest a key role of down-regulation of Bach1 and up-regulation of HO-1 in diminishing cytotoxic effects of HCV proteins in human hepatocytes.

Calpain 10: a mitochondrial calpain and its role in calcium-induced mitochondrial dysfunction

Calpains, Ca(2+)-activated cysteine proteases, are cytosolic enzymes implicated in numerous cellular functions and pathologies. We identified a mitochondrial Ca(2+)-inducible protease that hydrolyzed a calpain substrate (SLLVY-AMC) and was inhibited by active site-directed calpain inhibitors as calpain 10, an atypical calpain lacking domain IV. Immunoblot analysis and activity assays revealed calpain 10 in the mitochondrial outer membrane, intermembrane space, inner membrane, and matrix fractions. Mitochondrial staining was observed when COOH-terminal green fluorescent protein-tagged calpain 10 was overexpressed in NIH-3T3 cells and the mitochondrial targeting sequence was localized to the NH(2)-terminal 15 amino acids. Overexpression of mitochondrial calpain 10 resulted in mitochondrial swelling and autophagy that was blocked by the mitochondrial permeability transition (MPT) inhibitor cyclosporine A. With the use of isolated mitochondria, Ca(2+)-induced MPT was partially decreased by calpain inhibitors. More importantly, Ca(2+)-induced inhibition of Complex I of the electron transport chain was blocked by calpain inhibitors and two Complex I proteins were identified as targets of mitochondrial calpain 10, NDUFV2, and ND6. In conclusion, calpain 10 is the first reported mitochondrially targeted calpain and is a mediator of mitochondrial dysfunction through the cleavage of Complex I subunits and activation of MPT.

Structural determinants that target the hepatitis C virus core protein to lipid droplets

Hepatitis C virus core protein is targeted to lipid droplets, which serve as intracellular storage organelles, by its C-terminal domain, termed D2. From circular dichroism and nuclear magnetic resonance analyses, we demonstrate that the major structural elements within D2 consist of two amphipathic alpha-helices (Helix I and Helix II) separated by a hydrophobic loop. Both helices require a hydrophobic environment for folding, indicating that lipid interactions contribute to their structural integrity. Mutational studies revealed that a combination of Helix I, the hydrophobic loop, and Helix II is essential for efficient lipid droplet association and pointed to an in-plane membrane interaction of the two helices at the phospholipid layer interface. Aside from lipid droplet association, membrane interaction of D2 is necessary for folding and stability of core following maturation at the endoplasmic reticulum membrane by signal peptide peptidase. These studies identify critical determinants within a targeting domain that enable trafficking and attachment of a viral protein to lipid droplets. They also serve as a unique model for elucidating the specificity of protein-lipid interactions between two membrane-bound organelles.

Hepatitis C infection and alcohol use: A dangerous mix for the liver and antiviral immunity

This article presents the proceedings of a symposium presented at the meeting of the Research Society on Alcoholism, held in Santa Barbara, California, in June 2005. The organizers and chairs were Sam Zakhari and Gyongyi Szabo. The presentations included (1) Mitochondrial Abnormalities Induced by Hepatitis C -Alcohol Interaction by Steven Weinman; (2) Effects of Acute and Chronic Ethanol on Innate Antiviral Signaling Pathways, Hepatitis C Replication, and Human Liver Cell Transcription by Stephen Polyak; (3) Ethanol Alters Dendritic Cell Function In Vivo and Impairs the Subsequent Cellular Immune Responses to Hepatitis C Proteins by Costica Aloman; and (4) Pathogenic Interactions Between Hepatitis C Virus and Alcohol Use in Humans: Dendritic Cells as Common Targets by Gyongyi Szabo. This symposium summarizes the state of knowledge of cellular and molecular pathways by which alcohol and HCV have pathogenic interactions resulting in depression of the immune response and liver damage in chronic HCV infection.

Apolipoprotein E4: a causative factor and therapeutic target in neuropathology, including Alzheimer's disease

The premise of this review is that apolipoprotein (apo) E4 is much more than a contributing factor to neurodegeneration. ApoE has critical functions in redistributing lipids among CNS cells for normal lipid homeostasis, repairing injured neurons, maintaining synapto-dendritic connections, and scavenging toxins. In multiple pathways affecting neuropathology, including Alzheimer's disease, apoE acts directly or in concert with age, head injury, oxidative stress, ischemia, inflammation, and excess amyloid beta peptide production to cause neurological disorders, accelerating progression, altering prognosis, or lowering age of onset. We envision that unique structural features of apoE4 are responsible for apoE4-associated neuropathology. Although the structures of apoE2, apoE3, and apoE4 are in dynamic equilibrium, apoE4, which is detrimental in a variety of neurological disorders, is more likely to assume a pathological conformation. Importantly, apoE4 displays domain interaction (an interaction between the N- and C-terminal domains of the protein that results in a compact structure) and molten globule formation (the formation of stable, reactive intermediates with potentially pathological activities). In response to CNS stress or injury, neurons can synthesize apoE. ApoE4 uniquely undergoes neuron-specific proteolysis, resulting in bioactive toxic fragments that enter the cytosol, alter the cytoskeleton, disrupt mitochondrial energy balance, and cause cell death. Our findings suggest potential therapeutic strategies, including the use of "structure correctors" to convert apoE4 to an "apoE3-like" molecule, protease inhibitors to prevent the generation of toxic apoE4 fragments, and "mitochondrial protectors" to prevent cellular energy disruption.

L’apoptose hépatique

Apoptosis or programmed cell death occurs in the liver as in other organs. In the normal state it is not a frequent mode of hepatic cell destruction. Morphological and biochemical characteristics of liver cell apoptosis do not differ from what is observed in other cells. The Fas receptor pathway, a frequent hepatic apoptotic pathway among various others, involves intra-cellular signals amplified by mitochondria. Although hepatic apoptosis may occur by following several others pathways, Fas, which is abundantly expressed in the plasma membrane of hepatocytes, is very often involved in hepatocyte demise during B or C viral hepatitis irrespective of their clinical form, alcoholic hepatitis, cholestasis due to accumulation of hepatic biliary salts, or certain types of drug-induced hepatitis. Fas is also probably responsible for the death of biliary cells in primary biliary cirrhosis. In contrast one of the causes of resistance to apoptosis of hepatic cancerous cells could be related to an alteration of the Fas receptor. This is why much experimental work is presently performed to achieve inhibition of the Fas receptor either at the mRNA level or at the level of Fas-inductible proteolytic enzymes called caspases. One perspective is a specific treatment of apoptosis as an adjuvant treatment of liver diseases.

Signal peptide cleavage and internal targeting signals direct the hepatitis C virus p7 protein to distinct intracellular membranes

The hepatitis C virus (HCV) p7 protein forms an amantadine-sensitive ion channel required for viral replication in chimpanzees, though its precise role in the life cycle of HCV is unknown. In an attempt to gain some insights into p7 function, we examined the intracellular localization of p7 using epitope tags and an anti-p7 peptide antibody, antibody 1055. Immunofluorescence labeling of p7 at its C terminus revealed an endoplasmic reticulum (ER) localization independent of the presence of its signal peptide, whereas labeling the N terminus gave a mitochondrial-type distribution in brightly labeled cells. Both of these patterns could be visualized within individual cells, suggestive of separate pools of p7 where the N and C termini differed in accessibility to antibody. These patterns were disrupted by preventing signal peptide cleavage. Subcellular fractionation revealed that p7 was enriched in a heavy membrane fraction associated with mitochondria as well as normal ER-derived microsomes. The complex regulation of the intracellular distribution of p7 suggests that p7 plays multiple roles in the HCV life cycle either intracellularly or as a virion component.

Hepatitis C virus core protein inhibits deoxycholic acid-mediated apoptosis despite generating mitochondrial reactive oxygen species

BACKGROUND

Hepatitis C virus (HCV) core protein is known to cause oxidative stress and alter apoptosis pathways. However, the apoptosis results are inconsistent, and the real significance of oxidative stress is not well known. The aim of this study was twofold. First, we wanted to confirm whether core-induced oxidative stress was really significant enough to cause DNA damage, and whether it induced cellular antioxidant responses. Second, we wanted to evaluate whether this core-induced oxidative stress and the antioxidant response to it was responsible for apoptosis changes.

METHODS

HCV core protein was expressed under control of the Tet-Off promoter in Huh-7 cells and HeLa cells. We chose to use deoxycholic acid (DCA) as a model because it is known to produce both reactive oxygen species (ROS) and apoptosis.

RESULTS

Core expression uniformly increased ROS and 8-hydroxy-2'-deoxyguanosine (8-OHdG) under basal and DCA-stimulated conditions. Core protein expression also increased manganese superoxide dismutase levels. Core protein inhibited DCA-mediated mitochondrial membrane depolarization and DCA-mediated activation of caspase-9 and caspase-3, despite the increase in ROS by DCA. Core protein inhibited DCA-mediated apoptosis by increasing Bcl-x(L) protein and decreasing Bax protein, without affecting the proportion of Bax between mitochondria and cytosol, resulting in suppression of cytochrome c release from mitochondria into cytoplasm.

CONCLUSIONS

HCV core protein induces oxidative DNA damage, whereas it inhibits apoptosis that is accompanied by enhancement of ROS production. Thus, oxidative stress and apoptosis modulation by core protein are independent of each other.

Subcellular localization of hepatitis C virus structural proteins in a cell culture system that efficiently replicates the virus

Due to the recent development of a cell culture model, hepatitis C virus (HCV) can be efficiently propagated in cell culture. This allowed us to reinvestigate the subcellular localization of HCV structural proteins in the context of an infectious cycle. In agreement with previous reports, confocal immunofluorescence analysis of the subcellular localization of HCV structural proteins indicated that, in infected cells, the glycoprotein heterodimer is retained in the endoplasmic reticulum. However, in contrast to other studies, the glycoprotein heterodimer did not accumulate in other intracellular compartments or at the plasma membrane. As previously reported, an association between the capsid protein and lipid droplets was also observed. In addition, a fraction of labeling was consistent with the capsid protein being localized in a membranous compartment that is associated with the lipid droplets. However, in contrast to previous reports, the capsid protein was not found in the nucleus or in association with mitochondria or other well-defined intracellular compartments. Surprisingly, no colocalization was observed between the glycoprotein heterodimer and the capsid protein in infected cells. Electron microscopy analyses allowed us to identify a membrane alteration similar to the previously reported "membranous web." However, no virus-like particles were found in this type of structure. In addition, dense elements compatible with the size and shape of a viral particle were seldom observed in infected cells. In conclusion, the cell culture system for HCV allowed us for the first time to characterize the subcellular localization of HCV structural proteins in the context an infectious cycle.

Signal peptide peptidase promotes the formation of hepatitis C virus non-enveloped particles and is captured on the viral membrane during assembly

The maturation of the core protein (C) of Hepatitis C virus (HCV) is controlled by the signal peptidase (sp) and signal peptide peptidase (spp) of the host. To date, it remains unknown whether spp cleavage influences viral infectivity and/or the assembly process. Here, evidence is provided that cleavage by spp is not required for assembly of nucleocapsid-like particles (NLPs) in yeast (Pichia pastoris). The immature NLPs (not processed by spp) show a density of 1.11 g ml(-1) on sucrose gradients and a diameter of 50 nm. Co-expression of human spp (hspp) with C generates the 21 kDa mature form of the protein and promotes the accumulation of non-enveloped particles. The amount of non-enveloped particles accumulating in the cell was correlated directly with the expression level of hspp. Furthermore, immunocapture studies showed that hspp was embedded in the membranes of enveloped particles. These results suggest that maturation of the C protein can occur after formation of the enveloped particles and that the abundance of hspp influences the types of particle accumulating in the cells.

Antigen presentation and the ubiquitin-proteasome system in host-pathogen interactions

Relatively small genomes and high replication rates allow viruses and bacteria to accumulate mutations. This continuously presents the host immune system with new challenges. On the other side of the trenches, an increasingly well-adjusted host immune response, shaped by coevolutionary history, makes a pathogen's life a rather complicated endeavor. It is, therefore, no surprise that pathogens either escape detection or modulate the host immune response, often by redirecting normal cellular pathways to their advantage. For the purpose of this chapter, we focus mainly on the manipulation of the class I and class II major histocompatibility complex (MHC) antigen presentation pathways and the ubiquitin (Ub)-proteasome system by both viral and bacterial pathogens. First, we describe the general features of antigen presentation pathways and the Ub-proteasome system and then address how they are manipulated by pathogens. We discuss the many human cytomegalovirus (HCMV)-encoded immunomodulatory genes that interfere with antigen presentation (immunoevasins) and focus on the HCMV immunoevasins US2 and US11, which induce the degradation of class I MHC heavy chains by the proteasome by catalyzing their export from the endoplasmic reticulum (ER)-membrane into the cytosol, a process termed ER dislocation. US2- and US11-mediated subversion of ER dislocation ensures proteasomal degradation of class I MHC molecules and presumably allows HCMV to avoid recognition by cytotoxic T cells, whilst providing insight into general aspects of ER-associated degradation (ERAD) which is used by eukaryotic cells to purge their ER of defective proteins. We discuss the similarities and differences between the distinct pathways co-opted by US2 and US11 for dislocation and degradation of human class I MHC molecules and also a putatively distinct pathway utilized by the murine herpes virus (MHV)-68 mK3 immunoevasin for ER dislocation of murine class I MHC. We speculate on the implications of the three pathogen-exploited dislocation pathways to cellular ER quality control. Moreover, we discuss the ubiquitin (Ub)-proteasome system and its position at the core of antigen presentation as proteolysis and intracellular trafficking rely heavily on Ub-dependent processes. We add a few examples of manipulation of the Ub-proteasome system by pathogens in the context of the immune system and such diverse aspects of the host-pathogen relationship as virus budding, bacterial chromosome integration, and programmed cell death, to name a few. Finally, we speculate on newly found pathogen-encoded deubiquitinating enzymes (DUBs) and their putative roles in modulation of host-pathogen interactions.

Hepatitis C virus protease NS3/4A cleaves mitochondrial antiviral signaling protein off the mitochondria to evade innate immunity

Hepatitis C virus (HCV) is a global epidemic manifested mainly by chronic infection. One strategy that HCV employs to establish chronic infection is to use the viral Ser protease NS3/4A to cleave some unknown cellular targets involved in innate immunity. Here we show that the target of NS3/4A is the mitochondrial antiviral signaling protein, MAVS, that activates NF-kappaB and IFN regulatory factor 3 to induce type-I interferons. NS3/4A cleaves MAVS at Cys-508, resulting in the dislocation of the N-terminal fragment of MAVS from the mitochondria. Remarkably, a point mutation of MAVS at Cys-508 renders MAVS resistant to cleavage by NS3/4A, thus maintaining the ability of MAVS to induce interferons in HCV replicon cells. NS3/4A binds to and colocalizes with MAVS in the mitochondrial membrane, and it can cleave MAVS directly in vitro. These results provide an example of host-pathogen interaction in which the virus evades innate immunity by dislodging a pivotal antiviral protein from the mitochondria and suggest that blocking the cleavage of MAVS by NS3/4A may be applied to the prevention and treatment of HCV.

Identification and characterization of MAVS, a mitochondrial antiviral signaling protein that activates NF-kappaB and IRF 3

Viral infection triggers host innate immune responses through activation of the transcription factors NF-kappaB and IRF 3, which coordinately regulate the expression of type-I interferons such as interferon-beta (IFN-beta). Herein, we report the identification of a novel protein termed MAVS (mitochondrial antiviral signaling), which mediates the activation of NF-kappaB and IRF 3 in response to viral infection. Silencing of MAVS expression through RNA interference abolishes the activation of NF-kappaB and IRF 3 by viruses, thereby permitting viral replication. Conversely, overexpression of MAVS induces the expression of IFN-beta through activation of NF-kappaB and IRF 3, thus boosting antiviral immunity. Epistasis experiments show that MAVS is required for the phosphorylation of IRF 3 and IkappaB and functions downstream of RIG-I, an intracellular receptor for viral RNA. MAVS contains an N-terminal CARD-like domain and a C-terminal transmembrane domain, both of which are essential for MAVS signaling. The transmembrane domain targets MAVS to the mitochondria, implicating a new role of mitochondria in innate immunity.

Hepatitis C virus core variants isolated from liver tumor but not from adjacent non-tumor tissue interact with Smad3 and inhibit the TGF-beta pathway

Hepatitis C virus (HCV) is a major risk factor for human hepatocellular carcinoma (HCC) but the mechanisms underlying HCV-induced carcinogenesis are still poorly understood. We have hypothesized that viral variants, selected during long-term infection, might contribute to cellular transformation. To address this issue, we have investigated the effect of natural HCV core variants isolated from liver tumors (T), or their non-tumor (NT) counterparts, on the tumor growth factor-beta (TGF-beta) pathway, a major regulator of cellular proliferation, differentiation and apoptosis. We have found a significant reduction in TGF-beta reporter gene activity with the expression of core sequences isolated from liver tumors. In contrast, moderate or no effects were observed with non-tumor mutants or a core reference sequence. The molecular mechanisms have been characterized and involved the inhibition, by tumor-derived cores, of the DNA-binding activity of the Smad3/4 transcription factors complex. This inhibition occurs through a direct interaction between the central domain (amino acids 59-126) of tumor-derived core and the MH1 DNA-binding domain of Smad3, thus preventing its binding to DNA. We have therefore identified a new cell-signaling pathway targeted by HCV core and inhibited by tumor-derived core sequences. These results suggest that during chronic infection, there is selection of viral variants that may promote cell transformation by providing, to clonally expanding cells, resistance to TGF-beta antiproliferative effects.

Differential display analysis of gene expression in brains from hepatitis C-infected patients

OBJECTIVES

Hepatitis C virus (HCV) infection is often associated with cognitive dysfunction, fatigue and depression. The current study was undertaken to determine whether HCV infection affects gene expression in brain tissue.

DESIGN

We analysed the gene expression pattern in brain tissue in a group of HCV-infected patients compared with HCV-negative controls.

METHODS

Brain tissue samples were obtained at autopsy from three HCV-positive patients and three HCV-negative control patients. The analysis of gene expression was conducted using differential display and reverse Northern hybridization. Only those genes that were up or downregulated more than 1.8 times were considered to be differentially expressed.

RESULTS

Altogether, 29 differentially expressed genes were identified by differential display and subsequently confirmed by reverse Northern hybridization. A prominent finding was the downregulation of mitochondrial oxidative phosphorylation genes in HCV-infected patients. The impairment of brain oxidative/energy metabolism has previously been suggested to be the proximate cause of many disorders that impair mentation. Another finding was the downregulation of some ribosomal protein genes and several genes involved in transcription regulation, perhaps reflecting reduced metabolic activities.

CONCLUSION

Our findings suggest for the first time that there may be a biological basis for the neuropsychiatric symptoms and cognitive impairment associated with HCV infection.

Hepatitis C virus core protein is a dimeric alpha-helical protein exhibiting membrane protein features

The building block of hepatitis C virus (HCV) nucleocapsid, the core protein, together with viral RNA, is composed of different domains involved in RNA binding and homo-oligomerization. The HCV core protein 1-169 (C(HCV)169) and its N-terminal region from positions 1 to 117 (C(HCV)117) were expressed in Escherichia coli and purified to homogeneity suitable for biochemical and biophysical characterizations. The overall conformation and the oligomeric properties of the resulting proteins C(HCV)169 and C(HCV)117 were investigated by using analytical centrifugation, circular dichroism, intrinsic fluorescence measurements, and limited proteolysis. Altogether, our results show that core protein (C(HCV)169) behaves as a membranous protein and forms heterogeneous soluble micelle-like aggregates of high molecular weight in the absence of detergent. In contrast, it behaves, in the presence of mild detergent, as a soluble, well-folded, noncovalent dimer. Similar to findings observed for core proteins of HCV-related flaviviruses, the HCV core protein is essentially composed of alpha-helices (50%). In contrast, C(HCV)117 is soluble and monodispersed in the absence of detergent but is unfolded. It appears that the folding of the highly basic domain from positions 2 to 117 (2-117 domain) depends on the presence of the 117-169 hydrophobic domain, which contains the structural determinants ensuring the binding of core with cellular membranes. Finally, our findings provide valuable information for further investigations on isolated core protein, as well as for attempts to reconstitute nucleocapsid particles in vitro.

Hepatitis C virus core protein inhibits mitochondrial electron transport and increases reactive oxygen species (ROS) production

Hepatitis C infection causes a state of chronic oxidative stress, which may contribute to fibrosis and carcinogenesis in the liver. Previous studies have shown that expression of the HCV core protein in hepatoma cells depolarized mitochondria and increased reactive oxygen species (ROS) production, but the mechanisms of these effects are unknown. In this study we examined the properties of liver mitochondria from transgenic mice expressing HCV core protein, and from normal liver mitochondria incubated with recombinant core protein. Liver mitochondria from transgenic mice expressing the HCV proteins core, E1 and E2 demonstrated oxidation of the glutathione pool and a decrease in NADPH content. In addition, there was reduced activity of electron transport complex I, and increased ROS production from complex I substrates. There were no abnormalities observed in complex II or complex III function. Incubation of control mitochondria in vitro with recombinant core protein also caused glutathione oxidation, selective complex I inhibition, and increased ROS production. Proteinase K digestion of either transgenic mitochondria or control mitochondria incubated with core protein showed that core protein associates strongly with mitochondria, remains associated with the outer membrane, and is not taken up across the outer membrane. Core protein also increased Ca(2+) uptake into isolated mitochondria. These results suggest that interaction of core protein with mitochondria and subsequent oxidation of the glutathione pool and complex I inhibition may be an important cause of the oxidative stress seen in chronic hepatitis C.

Molecular basis for mitochondrial localization of viral particles during beet necrotic yellow vein virus infection

During infection, Beet necrotic yellow vein virus (BNYVV) particles localize transiently to the cytosolic surfaces of mitochondria. To understand the molecular basis and significance of this localization, we analyzed the targeting and membrane insertion properties of the viral proteins. ORF1 of BNYVV RNA-2 encodes the 21-kDa major coat protein, while ORF2 codes for a 75-kDa minor coat protein (P75) by readthrough of the ORF1 stop codon. Bioinformatic analysis highlighted a putative mitochondrial targeting sequence (MTS) as well as a major (TM1) and two minor (TM3 and TM4) transmembrane regions in the N-terminal part of the P75 readthrough domain. Deletion and gain-of-function analyses based on the localization of green fluorescent protein (GFP) fusions showed that the MTS was able to direct a reporter protein to mitochondria but that the protein was not persistently anchored to the organelles. GFP fused either to MTS and TM1 or to MTS and TM3-TM4 efficiently and specifically associated with mitochondria in vivo. The actual role of the individual domains in the interaction with the mitochondria seemed to be determined by the folding of P75. Anchoring assays to the outer membranes of isolated mitochondria, together with in vivo data, suggest that the TM3-TM4 domain is the membrane anchor in the context of full-length P75. All of the domains involved in mitochondrial targeting and anchoring were also indispensable for encapsidation, suggesting that the assembly of BNYVV particles occurs on mitochondria. Further data show that virions are subsequently released from mitochondria and accumulate in the cytosol.

Proapoptotic and pronecrosis effect of different truncated hepatitis C virus core proteins

OBJECTIVE

To study the roles of different truncated hepatitis C virus (HCV) core proteins (CORE) in the pathogenesis of HCV persistent infection and hepatocellular carcinoma (HCC) and to assess intracellular localization in transiently transfected cells.

METHODS

Seven truncated GFP (green fluorescent protein)-CORE fusion protein expression plasmids were constructed, which contained HCV CORE sequences derived from tumor tissues (BT) and non-tumor tissues (BNT) from one patient infected with HCV. Amino acid (aa) lengths were BT: 1-172 aa, 1-126 aa, 1-58 aa, 59-126 aa, 127-172 aa; BNT: 1-172 aa and C191: 1-172 aa respectively. Subcellular localization of CORE-GFP was analyzed by con-focal laser scanning microscope. Apoptosis and necrosis were quantified by flow cytometry.

RESULTS

Different truncated CORE-GFP localized mainly in the cytoplasm, but nuclear staining was also observed. HCV CORE could induce apoptosis and necrosis, and different truncated COREs could induce cell apoptosis and necrosis at different levels. Among the same length 1-172 aa of BT, BNT and C191, the cell apoptosis and necrosis percentage of BT is highest, and C191 is the lowest (BT>BNT>C191). To the different fragment COREs of BT, N-terminal of CORE induced apoptosis and necrosis higher, compared with that of C-terminal (1-172 aa>1-126 aa>1-58 aa>127-172 aa>59-126 aa).

CONCLUSION

These results suggest HCV CORE could induce apoptosis and necrosis of cells, which might play an important role in the pathogenesis of HCV persistent infection and HCC and the different CORE domains of different HCV quasi-species might have some difference in their pathogenesis.

Mitochondrial dysfunction in hepatitis C

Chronic hepatitis C induces a state of hepatic oxidative stress that is more pronounced than that present in many other inflammatory liver diseases. This review summarizes recent information that the hepatitis C virus (HCV) core protein plays an important role in this phenomenon. Core protein localizes to mitochondria, particularly at the points of contact between mitochondrial outer membrane and endoplasmic reticulum. Its expression causes inhibition of electron transport at complex I, increased complex I reactive oxygen species (ROS) production, decreased mitochondrial glutathione, and increased mitochondrial permeability transition in response to exogenous oxidants and tumor necrosis factor-alpha. Possible mechanisms of the core protein effects include direct interaction with electron carriers and indirect effects mediated by changes in mitochondrial calcium. These results suggest that antioxidant approaches may prove beneficial for patients with chronic hepatitis C.

Hepatitis C virus core protein, cytochrome P450 2E1, and alcohol produce combined mitochondrial injury and cytotoxicity in hepatoma cells

BACKGROUND & AIMS

Alcohol consumption exacerbates liver injury in chronic hepatitis C, and enhanced mitochondrial oxidative stress is one possible mechanism. The aim of this study was to determine whether hepatitis C virus core protein and alcohol-inducible cytochrome P450 2E1 contribute to reactive oxygen species production and cytotoxicity in human hepatoma cells.

METHODS

Huh-7 cells expressing core protein, cytochrome P450 2E1, or both were exposed to 0.1 mmol/L tertiary butyl hydroperoxide, tumor necrosis factor alpha, and/or 25 mmol/L ethanol. Cytotoxicity, reactive oxygen species production, glutathione content, and mitochondrial membrane potential were measured.

RESULTS

Expression of core/cytochrome P450 2E1 synergistically enhanced cell death induced by either tertiary butyl hydroperoxide or tumor necrosis factor alpha. After tertiary butyl hydroperoxide treatment, total reactive oxygen species production was increased more than 3-fold compared with cells that did not express core and cytochrome P450 2E1. Mitochondrial depolarization and reduced glutathione depletion occurred as well, and cell death was prevented by inhibition of mitochondrial permeability transition or caspase activity. Confocal microscopy showed that the mitochondria themselves were the origin of the reactive oxygen species. In the absence of core/cytochrome P450 2E1 expression, mitochondrial changes and cell death did not occur. Ethanol treatment further decreased mitochondrial reduced glutathione content and exacerbated mitochondrial reactive oxygen species production, depolarization, and cell death. All these effects were prevented by the antioxidant N -acetylcysteine.

CONCLUSIONS

Mitochondrial reactive oxygen species production is induced by hepatitis C virus core and cytochrome P450 2E1, resulting in a reduction of mitochondrial antioxidant capacity and sensitivity to oxidants and tumor necrosis factor alpha. Alcohol further depletes mitochondrial reduced glutathione, which exacerbates depolarization and cell death. Sensitization of mitochondria to oxidative insults is thus a potential mechanism for alcohol-related exacerbation of liver injury in chronic hepatitis C.