Lipid composition of hepatitis B virus surface antigen particles and the particle-producing human hepatoma cell lines

Serum Metabolome Signatures Characterizing Co-Infection of Plasmodium falciparum and HBV in Pregnant Women

Plasmodium falciparum (P. falciparum) and hepatitis B virus (HBV) co-infection is on the rise among pregnant women in northern Ghana. Mono-infection with either of these two pathogens results in unique metabolic alterations. Thus, we aimed to explicate the effects of this co-infection on the metabolome signatures of pregnant women, which would indicate the impacted metabolic pathways and provide useful prognostic or diagnostic markers. Using an MS/MS-based targeted metabolomic approach, we determined the serum metabolome in pregnant women with P. falciparum mono-infection, HBV mono-infection, P. falciparum, and HBV co-infection and in uninfected (control) women. We observed significantly decreased sphingolipid concentrations in subjects with P. falciparum mono-infection, whereas amino acids and phospholipids were decreased in subjects with HBV mono-infection. Co-infections were found to be characterized distinctively by reduced concentrations of phospholipids and hexoses (mostly glucose) as well as altered pathways that contribute to redox homeostasis. Overall, PC ae C40:1 was found to be a good discriminatory metabolite for the co-infection group. PC ae C40:1 can further be explored for use in the diagnosis and treatment of malaria and chronic hepatitis B co-morbidity as well as to distinguish co-infections from cases of mono-infections.

Improved pharmacokinetics of tenofovir ester prodrugs strengthened the inhibition of HBV replication and the rebalance of hepatocellular metabolism in preclinical models

Tenofovir (TFV) ester prodrugs, a class of nucleotide analogs (NAs), are the first-line clinical anti-hepatitis B virus (HBV) drugs with potent antiviral efficacy, low resistance rate and high safety. In this work, three marketed TFV ester drugs, tenofovir disoproxil fumarate (TDF), tenofovir alafenamide fumarate (TAF) and tenofovir amibufenamide fumarate (TMF), were used as probes to investigate the relationships among prodrug structures, pharmacokinetic characteristics, metabolic activations, pharmacological responses and to reveal the key factors of TFV ester prodrug design. The results indicated that TMF and TAF exhibited significantly stronger inhibition of HBV DNA replication than did TDF in HBV-positive HepG2.2.15 cells. The anti-HBV activity of TMF was slightly stronger than TAF after 9 days of treatment (EC₅₀ 7.29 ± 0.71 nM vs. 12.17 ± 0.56 nM). Similar results were observed in the HBV decline period post drug administration to the HBV transgenic mouse model, although these three TFV prodrugs finally achieved the same anti-HBV effect after 42 days treatments. Furthermore, TFV ester prodrugs showed a correcting effect on disordered host hepatic biochemical metabolism, including TCA cycle, glycolysis, pentose phosphate pathway, purine/pyrimidine metabolism, amino acid metabolism, ketone body metabolism and phospholipid metabolism. The callback effects of the three TFV ester prodrugs were ranked as TMF > TAF > TDF. These advantages of TMF were believed to be attributed to its greater bioavailability in preclinical animals (SD rats, C57BL/6 mice and beagle dogs) and better target loading, especially in terms of the higher hepatic level of the pharmacologically active metabolite TFV-DP, which was tightly related to anti-HBV efficacy. Further analysis indicated that stability in intestinal fluid determined the actual amount of TFV prodrug at the absorption site, and hepatic/intestinal stability determined the maintenance amount of prodrug in circulation, both of which influenced the oral bioavailability of TFV prodrugs. In conclusion, our research revealed that improved pharmacokinetics of TFV ester prodrugs (especially intestinal stability) strengthened the inhibition of HBV replication and the rebalance of hepatocellular metabolism, which provides new insights and a basis for the design, modification and evaluation of new TFV prodrugs in the future.

Cryo-EM structures of human hepatitis B and woodchuck hepatitis virus small spherical subviral particles

The loss of detectable hepatitis B surface antigen (HBsAg) is considered a functional cure in chronic hepatitis B. Naturally, HBsAg can be incorporated into the virion envelope or assembled into subviral particles (SVPs) with lipid from host cells. Until now, there has been no detailed structure of HBsAg, and the published SVP structures are controversial. Here, we report the first subnanometer-resolution structures of spherical SVP from hepatitis B virus (HBV) and the related woodchuck hepatitis virus (WHV) determined by cryo-electron microscopy in combination with AlphaFold2 prediction. Both structures showed unique rhombicuboctahedral symmetry with 24 protruding spikes comprising dimer of small HBsAg with four helical domains. The lipid moiety in the SVP is organized in a noncanonical lipid patch instead of a lipid bilayer, which can accommodate the exposed hydrophobic surface and modulate particle stability. Together, these findings advance our knowledge of viral membrane organization and the structures of HBV and WHV spherical SVPs.

Targeting lipid biosynthesis pathways for hepatitis B virus cure

Chronic hepatitis B virus (HBV) infection is characterized by the presence of high circulating levels of non-infectious lipoprotein-like HBV surface antigen (HBsAg) particles thought to contribute to chronic immune dysfunction in patients. Lipid and metabolomic analysis of humanized livers from immunodeficient chimeric mice (uPA/SCID) revealed that HBV infection dysregulates several lipid metabolic pathways. Small molecule inhibitors of lipid biosynthetic pathway enzymes acetyl-CoA carboxylase (ACC), fatty acid synthase, and subtilisin kexin isozyme-1/site-1 protease in HBV-infected HepG2-NTCP cells demonstrated potent and selective reduction of extracellular HBsAg. However, a liver-targeted ACC inhibitor did not show antiviral activity in HBV-infected liver chimeric mice, despite evidence of on-target engagement. Our study suggests that while HBsAg production may be dependent on hepatic de novo lipogenesis in vitro, this may be overcome by extrahepatic sources (such as lipolysis or diet) in vivo. Thus, a combination of agents targeting more than one lipid metabolic pathway may be necessary to reduce HBsAg levels in patients with chronic HBV infection.

HBsAg, Subviral Particles, and Their Clearance in Establishing a Functional Cure of Chronic Hepatitis B Virus Infection

In diverse viral infections, the production of excess viral particles containing only viral glycoproteins (subviral particles or SVP) is commonly observed and is a commonly evolved mechanism for immune evasion. In hepatitis B virus (HBV) infection, spherical particles contain the hepatitis B surface antigen, outnumber infectious virus 10 000-100 000 to 1, and have diverse inhibitory effects on the innate and adaptive immune response, playing a major role in the chronic nature of HBV infection. The current goal of therapies in development for HBV infection is a clinical outcome called functional cure, which signals a persistent and effective immune control of the infection. Although removal of spherical SVP (and the HBsAg they carry) is an important milestone in achieving functional cure, this outcome is rarely achieved with current therapies due to distinct mechanisms for assembly, secretion, and persistence of SVP, which are poorly targeted by direct acting antivirals or immunotherapies. In this Review, the current understanding of the distinct mechanisms involved in the production and persistence of spherical SVP in chronic HBV infection and their immunoinhibitory activity will be reviewed as well as current therapies in development with the goal of clearing spherical SVP and achieving functional cure.

Coronavirus-Induced Host Cubic Membranes and Lipid-Related Antiviral Therapies: A Focus on Bioactive Plasmalogens

Coronaviruses have lipid envelopes required for their activity. The fact that coronavirus infection provokes the formation of cubic membranes (CM) (denoted also as convoluted membranes) in host cells has not been rationalized in the development of antiviral therapies yet. In this context, the role of bioactive plasmalogens (vinyl ether glycerophospholipids) is not completely understood. These lipid species display a propensity for non-lamellar phase formation, facilitating membrane fusion, and modulate the activity of membrane-bound proteins such as enzymes and receptors. At the organism level, plasmalogen deficiency is associated with cardiometabolic disorders including obesity and type 2 diabetes in humans. A straight link is perceived with the susceptibility of such patients to SARS-CoV-2 (severe acute respiratory syndrome-coronavirus-2) infection, the severity of illness, and the related difficulty in treatment. Based on correlations between the coronavirus-induced modifications of lipid metabolism in host cells, plasmalogen deficiency in the lung surfactant of COVID-19 patients, and the alterations of lipid membrane structural organization and composition including the induction of CM, we emphasize the key role of plasmalogens in the coronavirus (SARS-CoV-2, SARS-CoV, or MERS-CoV) entry and replication in host cells. Considering that plasmalogen-enriched lung surfactant formulations may improve the respiratory process in severe infected individuals, plasmalogens can be suggested as an anti-viral prophylactic, a lipid biomarker in SARS-CoV and SARS-CoV-2 infections, and a potential anti-viral therapeutic component of lung surfactant development for COVID-19 patients.

Biochemical phenotyping of multiple myeloma patients at diagnosis reveals a disorder of mitochondrial complexes I and II and a Hartnup-like disturbance as underlying conditions, also influencing different stages of the disease

The aim of this study was to identify novel plasma metabolic signatures with possible relevance during multiple myeloma (MM) development and progression. A biochemical quantitative phenotyping platform based on targeted electrospray ionization tandem mass spectrometry technology was used to aid in the identification of any eventual perturbed biochemical pathway in peripheral blood plasma from 36 MM patients and 73 healthy controls. Our results showed that MM cases present an increase in short and medium/long-chain species of acylcarnitines resembling Multiple AcylCoA Dehydrogenase Deficiency (MADD), particularly, associated with MM advanced International Staging System (ISS). Lipids profile showed lower concentrations of phosphatidylcholine (PC), lysophosphatidylcholine (LPC) and sphingomyelins (SM) in the MM patients and its respective ISS groups. MM cases were accompanied by a drop in the concentration of essential amino acids, especially tryptophan, with a significant inverse correlation between the progressive drop in tryptophan with the elevation of β2-microglobulin, with the increase in systemic methylation levels (Symmetric Arginine Dimethylation, SDMA) and with the accumulation of esterified carnitines in relation to free carnitine (AcylC/C0). Serotonin was significantly elevated in cases of MM, without a clear association with ISS. Kynurenine/tryptophan ratio demonstrates that the activity of dioxigenases is even higher in the cases classified as ISS 3. In conclusion, our study showed that MM patients at diagnosis showed metabolic disorders resembling both mitochondrial complexes I and II and Hartnup-like disturbances as underlying conditions, also influencing different stages of the disease.

A Genome-wide CRISPR Screen Identifies ZCCHC14 as a Host Factor Required for Hepatitis B Surface Antigen Production

A hallmark of chronic hepatitis B (CHB) virus infection is the presence of high circulating levels of non-infectious small lipid HBV surface antigen (HBsAg) vesicles. Although rare, sustained HBsAg loss is the idealized endpoint of any CHB therapy. A small molecule, RG7834, has been previously reported to inhibit HBsAg expression by targeting terminal nucleotidyltransferase proteins 4A and 4B (TENT4A and TENT4B). In this study, we describe a genome-wide CRISPR screen to identify other potential host factors required for HBsAg expression and to gain further insights into the mechanism of RG7834. We report more than 60 genes involved in regulating HBsAg and identify additional factors involved in RG7834 activity, including a zinc finger CCHC-type containing 14 (ZCCHC14) protein. We show that ZCCHC14, together with TENT4A/B, stabilizes HBsAg expression through HBV RNA tailing, providing a potential new therapeutic target to achieve functional cure in CHB patients.

Construction of atomic models of full hepatitis B vaccine particles at different stages of maturation

Hepatitis B, one of the world's most common liver infections, is caused by the Hepatitis B Virus (HBV). Via the infected cells, this virus generates non pathogen particles with similar surface structures as those found in the full virus. These particles are used in a recombinant form (HBsAg) to produce efficient vaccines. The atomic structure of the HBsAg particles is currently unsolved, and the only existing structural data for the full particle were obtained by electronic microscopy with a maximum resolution of 12 Å. As many vaccines, HBsAg is a complex bio-system. This complexity results from numerous sources of heterogeneity, and traditional bio-immuno-chemistry analytic tools are often limited in their ability to fully describe the molecular surface or the particle. For the Hepatitis B vaccine particle (HBsAg), no atomic data are available so far. In this study, we used the principal well-known elements of HBsAg structure to reconstitute and model the full HBsAg particle assembly at a molecular level (protein assembly, particle formation and maturation). Full HBsAg particle atomic models were built based on an exhaustive experimental data review, amino acid sequence analysis, iterative threading modeling, and molecular dynamic approaches.

Hepatitis B Virus (HBV) Subviral Particles as Protective Vaccines and Vaccine Platforms

Hepatitis B remains one of the major global health problems more than 40 years after the identification of human hepatitis B virus (HBV) as the causative agent. A critical turning point in combating this virus was the development of a preventative vaccine composed of the HBV surface (envelope) protein (HBsAg) to reduce the risk of new infections. The isolation of HBsAg sub-viral particles (SVPs) from the blood of asymptomatic HBV carriers as antigens for the first-generation vaccines, followed by the development of recombinant HBsAg SVPs produced in yeast as the antigenic components of the second-generation vaccines, represent landmark advancements in biotechnology and medicine. The ability of the HBsAg SVPs to accept and present foreign antigenic sequences provides the basis of a chimeric particulate delivery platform, and resulted in the development of a vaccine against malaria (RTS,S/AS01, Mosquirix^TM), and various preclinical vaccine candidates to overcome infectious diseases for which there are no effective vaccines. Biomedical modifications of the HBsAg subunits allowed the identification of strategies to enhance the HBsAg SVP immunogenicity to build potent vaccines for preventative and possibly therapeutic applications. The review provides an overview of the formation and assembly of the HBsAg SVPs and highlights the utilization of the particles in key effective vaccines.

Stimulated phospholipid synthesis is key for hepatitis B virus replications

Chronic hepatitis B Virus (HBV) infection has high morbidity, high pathogenicity and unclear pathogenesis. To elucidate the relationship between HBV replication and host phospholipid metabolites, we measured 10 classes of phospholipids in serum of HBV infected patients and cells using ultra performance liquid chromatograph-triple quadruple mass spectrometry. We found that the levels of phosphatidylcholine (PC), phosphatidylethanolamine, and lyso-phosphatidic acid were increased in HBsAg (+) serum of infected patients compared with HBsAg (-), while phosphatidylserine, phosphatidylglycerol, phosphatidylinositol, and sphingomyelin were decreased, which were confirmed in an HBV infected HepG2.2.15 cell line. We further evaluated the enzyme levels of PC pathways and found that PCYT1A and LPP1 for PC synthesis were up-regulated after HBV infection. Moreover, HBV replication was inhibited when PCYT1A and LPP1 were inhibited. These results indicated that the PC synthesis in HBV infected host are regulated by PCYT1A and LPP1, which suggests that PCYT1A, LPP1 could be new potential targets for HBV treatment.

Lipidomic Analysis Reveals Serum Alteration of Plasmalogens in Patients Infected With ZIKA Virus

Zika virus (ZIKV) is an arthropod-borne virus (arbovirus) in the Flavivirus genus of the Flaviviridae family. Since the large outbreaks in French Polynesia in 2013–2014 and in Brazil in 2015, ZIKV has been considered a new public health threat. Similar to other related flavivirus, ZIKV is associated with mild and self-limiting symptoms such as rash, pruritus, prostration, headache, arthralgia, myalgia, conjunctivitis, lower back pain and, when present, a short-term low grade fever. In addition, ZIKV has been implicated in neurological complications such as neonatal microcephaly and Guillain–Barré syndrome in adults. Herein, serum lipidomic analysis was used to identify possible alterations in lipid metabolism triggered by ZIKV infection. Patients who presented virus-like symptoms such as fever, arthralgia, headache, exanthema, myalgia and pruritus were selected as the control group. Our study reveals increased levels of several phosphatidylethanolamine (PE) lipid species in the serum of ZIKV patients, the majority of them plasmenyl-phosphatidylethanolamine (pPE) (or plasmalogens) linked to polyunsaturated fatty acids. Constituting up to 20% of total phospholipids in humans, plasmalogens linked to polyunsaturated fatty acids are particularly enriched in neural membranes of the brain. The biosynthesis of plasmalogens requires functional peroxisomes, which are important sites for viral replication, including ZIKV. Thus, increased levels of plasmalogens in serum of ZIKV infected subjects suggest a link between ZIKV life cycle and peroxisomes. Our data provide important insights into specific host cellular lipids that are likely associated with ZIKV replication and may serve as platform for antiviral strategy against ZIKV.

Cryo-EM structure of native spherical subviral particles isolated from HBV carriers

Hepatitis B virus (HBV) contains 3 types of particles, i.e., 22-nm-diameter spherical and tubular subviral particles (SVPs) and 44-nm-diameter Dane particles. The SVPs are non-infectious and present strong immunogenicity, while Dane particles are infectious. In this study, we isolated spherical SVPs from HBV carriers' sera and determined their 3D structure at the resolution of ∼30 Å by cryo-electron microscopy (cryo-EM) single-particle reconstruction. Our cryo-EM structure suggests that the native HBV spherical SVP is irregularly organized, where spike-like features are arranged in a crystalline-like pattern on the surface. Strikingly, the hepatitis B surface antigen (HBsAg) in the native spherical SVPs folds as protrusions on the surface, as those on the native tubular SVPs and Dane particles, but is largely different from that in the recombinant octahedral SVPs. These results suggest a universal folding shape of HBsAg on the native HBV viral and subviral particles.

Fatty acid translocase promoted hepatitis B virus replication by upregulating the levels of hepatic cytosolic calcium

Hepatitis B virus (HBV) is designated a "metabolovirus" due to the intimate connection between the virus and host metabolism. The nutrition state of the host plays a relevant role in the severity of HBV infection. Metabolic syndrome (MS) is prone to increasing HBV DNA loads and accelerating the progression of liver disease in patients with chronic hepatitis B (CHB). Cluster of differentiation 36 (CD36), also named fatty acid translocase, is known to facilitate long-chain fatty acid uptake and contribute to the development of MS. We recently found that CD36 overexpression enhanced HBV replication. In this study, we further explored the mechanism by which CD36 overexpression promotes HBV replication. Our data showed that CD36 overexpression increased HBV replication, and CD36 knockdown inhibited HBV replication. RNA sequencing found some of the differentially expressed genes were involved in calcium ion homeostasis. CD36 overexpression elevated the cytosolic calcium level, and CD36 knockdown decreased the cytosolic calcium level. Calcium chelator BAPTA-AM could override the HBV replication increased by CD36 overexpression, and the calcium activator thapsigargin could improve the HBV replication reduced by CD36 knockdown. We further found that CD36 overexpression activated Src kinase, which plays an important role in the regulation of the store-operated Ca²⁺ channel. An inhibitor of Src kinase (SU6656) significantly reduced the CD36-induced HBV replication. We identified a novel link between CD36 and HBV replication, which is associated with cytosolic calcium and the Src kinase pathway. CD36 may represent a potential therapeutic target for the treatment of CHB patients with MS.

Serum lipid alterations identified in chronic hepatitis B, hepatitis B virus-associated cirrhosis and carcinoma patients

The incidences of chronic hepatitis B (CHB), Hepatitis B virus (HBV)-associated cirrhosis and HBV-associated carcinoma are high and increasing. This study was designed to evaluate serum lipid metabolite changes that are associated with the progression from CHB to HBV-associated cirrhosis and ultimately to HBV-associated HCC. A targeted metabolomic assay was performed in fasting sera from 136 CHB patients, 104 HBV-associated cirrhosis, and 95 HBV-associated HCC using ultra-performance liquid chromatography triple quadrupole mass spectrometry. A total of 140 metabolites were identified. Clear separations between each two groups were obtained using the partial least squares discriminate analysis of 9 lipid metabolites. Progressively lower levels of long-chain lysophosphatidylcholines (lysoPC a C18:2, lysoPC a C20:3, lysoPC a C20:4) were observed from CHB to cirrhosis to carcinoma; lower levels of lysoPC a C20:4 were found in patients with higher model for end-stage liver disease in the same disease group; and lysoPC a C20:3 levels were lower in Child-Pugh Class C than in Class A and Class B in HBV-associated cirrhosis and HBV-associated HCC groups. The octadecadienyl carnitine level was higher in HBV-associated cirrhosis group than in other two groups. Serum levels of selected long-chain lysoPCs are promising markers for the progression of HBV-associated liver diseases.

Metabolic characterization of the natural progression of chronic hepatitis B

Background

Worldwide, over 350 million people are chronically infected with the hepatitis B virus (HBV) and are at increased risk of developing progressive liver diseases. The confinement of HBV replication to the liver, which also acts as the central hub for metabolic and nutritional regulation, emphasizes the interlinked nature of host metabolism and the disease. Still, the metabolic processes operational during the distinct clinical phases of a chronic HBV infection—immune tolerant, immune active, inactive carrier, and HBeAg-negative hepatitis phases—remains unexplored.

Methods

To investigate this, we conducted a targeted metabolomics approach on serum to determine the metabolic progression over the clinical phases of chronic HBV infection, using patient samples grouped based on their HBV DNA, alanine aminotransferase, and HBeAg serum levels.

Results

Our data illustrate the strength of metabolomics to provide insight into the metabolic dysregulation experienced during chronic HBV. The immune tolerant phase is characterized by the speculated viral hijacking of the glycerol-3-phosphate–NADH shuttle, explaining the reduced glycerophospholipid and increased plasmalogen species, indicating a strong link to HBV replication. The persisting impairment of the choline glycerophospholipids, even during the inactive carrier phase with minimal HBV activity, alludes to possible metabolic imprinting effects. The progression of chronic HBV is associated with increased concentrations of very long chain triglycerides together with citrulline and ornithine, reflective of a dysregulated urea cycle peaking in the HBV envelope antigen-negative phase.

Conclusions

The work presented here will aid in future studies to (i) validate and understand the implication of these metabolic changes using a thorough systems biology approach, (ii) monitor and predict disease severity, as well as (iii) determine the therapeutic value of the glycerol-3-phosphate–NADH shuttle.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-016-0318-8) contains supplementary material, which is available to authorized users.

The metabolic responses to hepatitis B virus infection shed new light on pathogenesis and targets for treatment

Chronic infection caused by the hepatitis B virus (HBV), is strongly associated with hepatitis, fatty liver and hepatocellular carcinoma. To investigate the underlying mechanisms, we characterize the metabolic features of host cells infected with the virus using systems biological approach. The results show that HBV replication induces systematic metabolic alterations in host cells. HBV infection up-regulates the biosynthesis of hexosamine and phosphatidylcholine by activating glutamine-fructose-6-phosphate amidotransferase 1 (GFAT1) and choline kinase alpha (CHKA) respectively, which were reported for the first time for HBV infection. Importantly suppressing hexosamine biosynthesis and phosphatidylcholine biosynthesis can inhibit HBV replication and expression. In addition, HBV induces oxidative stress and stimulates central carbon metabolism and nucleotide synthesis. Our results also indicate that HBV associated hepatocellular carcinoma could be attributed to GFAT1 activated hexosamine biosynthesis and CHKA activated phosphatidylcholine biosynthesis. This study provides further insights into the pathogenesis of HBV-induced diseases, and sheds new light on drug target for treating HBV infection.

Host factors involved in hepatitis B virus maturation, assembly, and egress

Hepatitis B virus (HBV) is a major cause of liver disease. Due to the tiny size of its genome, HBV depends on the critical interplay between viral and host factors for the generation of new viral particles from infected cells. Recent work has illuminated a multiplicity of spatially and temporally coordinated virus-host interactions that accompany HBV particle genesis. These interactions include the requirement of cellular chaperones for the maturation of the three viral envelope proteins, the cellular factors involved in dynamic modification, maturation, and intracellular trafficking of the nucleocapsids, and the host components of the multivesicular body (MVB) pathway enabling virion budding at intracellular compartments. Beside infectious virions, HBV produces at least two other types of particles, subviral empty envelope particles and subviral naked capsid particles, likely as a result of the engagement of different host factors by the viral structural proteins. Accordingly, HBV exploits distinct cellular pathways to release its particle types. Here, I review recent progress in these areas of the cell biology of HBV genesis.

Hepatitis B virus-induced lipid alterations contribute to natural killer T cell-dependent protective immunity

In most adult humans, hepatitis B is a self-limiting disease leading to life-long protective immunity, which is the consequence of a robust adaptive immune response occurring weeks after hepatitis B virus (HBV) infection. Notably, HBV-specific T cells can be detected shortly after infection, but the mechanisms underlying this early immune priming and its consequences for subsequent control of viral replication are poorly understood. Using primary human and mouse hepatocytes and mouse models of transgenic and adenoviral HBV expression, we show that HBV-expressing hepatocytes produce endoplasmic reticulum (ER)-associated endogenous antigenic lipids including lysophospholipids that are generated by HBV-induced secretory phospholipases and that lead to activation of natural killer T (NKT) cells. The absence of NKT cells or CD1d or a defect in ER-associated transfer of lipids onto CD1d results in diminished HBV-specific T and B cell responses and delayed viral control in mice. NKT cells may therefore contribute to control of HBV infection through sensing of HBV-induced modified self-lipids.

The structure of HBsAg particles is not modified upon their adsorption on aluminium hydroxide gel

Current Hepatitis B vaccines are based on recombinant Hepatitis B surface antigen (HBsAg) virus-like particles adsorbed on aluminium (Al) gel. These particles exhibit a lipoprotein-like structure with about 70 protein S molecules in association with various types of lipids. To determine whether the adsorption on Al gel affects HBsAg structure, we investigated the effect of adsorption and mild desorption processes on the protein and lipid parts of the particles, using various techniques. Electron microscopy showed that the size and morphology of native and desorbed HBsAg particles were comparable. Moreover, infrared and Raman spectroscopy revealed that the secondary structure of the S proteins was not affected by the adsorption/desorption process. Affinity measurements with Surface Plasmon Resonance showed no difference between native and desorbed HBsAg for HBsAg-specific RF-1 monoclonal antibody. Steady-state and time-resolved fluorescence data of the intrinsic fluorescence of the S proteins further indicated that the adsorption/desorption of HBsAg particles on Al gel did not modify the environment of the most emitting Trp residues, confirming that the conformation of the S proteins remains intact. Moreover, using environment-sensitive 3-hydroxyflavone probes, no significant changes of the lipid core and lipid membrane surface of the HBsAg particles were observed during the adsorption/desorption process. Finally, the ratio between lipids and proteins in the particles was found to be similar before and after the adsorption/desorption process. Taken together, our data show that adsorption on Al gel does not affect the structure of the HBsAg particles.

Biochemical and morphological properties of hepatitis C virus particles and determination of their lipidome

A hallmark of hepatitis C virus (HCV) particles is their association with host cell lipids, most notably lipoprotein components. It is thought that this property accounts for the low density of virus particles and their large heterogeneity. However, the composition of infectious virions and their biochemical and morphological properties are largely unknown. We developed a system in which the envelope glycoprotein E2 was N-terminally tagged with a FLAG epitope. This virus, designated Jc1E2(FLAG), produced infectivity titers to wild type levels and allowed affinity purification of virus particles that were analyzed for their protein and lipid composition. By using mass spectrometry, we found the lipid composition of Jc1E2(FLAG) particles to resemble the one very low- and low density-lipoprotein with cholesteryl esters accounting for almost half of the total HCV lipids. Thus, HCV particles possess a unique lipid composition that is very distinct from all other viruses analyzed so far and from the human liver cells in which HCV was produced. By electron microscopy (EM), we found purified Jc1E2(FLAG) particles to be heterogeneous, mostly spherical structures, with an average diameter of about 73 nm. Importantly, the majority of E2-containing particles also contained apoE on their surface as assessed by immuno-EM. Taken together, we describe a rapid and efficient system for the production of large quantities of affinity-purified HCV allowing a comprehensive analysis of the infectious virion, including the determination of its lipid composition.

New insights into hepatitis B and hepatitis delta virus entry

Notwithstanding the medical importance of the HBV infection, our understanding of how this pathogen enters hepatocytes is incomplete. This reflects a long-lasting dependence of in vitro infection studies solely on primary human hepatocytes, which are difficult to obtain and maintain in a susceptible state. The establishment of a polarizable HBV-susceptible human hepatoma cell line (HepaRG) and the utilization of Tupaia belangeri hepatocytes (PTHs) resolved this issue. Since then, important insight into viral and cellular determinants participating in HBV binding and infection have been achieved. We now know that the large viral surface protein (L) plays a pivotal role in HBV entry. It mediates diverse functions, commencing binding of virions to heparan sulfate proteoglycans at the hepatocytes surface as a prerequisite for entry. Subsequently, (a) highly specific event(s) involving the myristoylated N-terminal preS1 subdomain of L, as well as the cytosolic and antigenic loops of the S-domain, initiates a series of less well understood steps, resulting in a pH independent, reduction-sensitive fusion of the viral membrane with a cellular membrane. One of these steps is highly sensitive to synthetic N-acylated preS1 lipopeptides and can be blocked in vitro and in vivo at picomolar concentrations. This opens novel therapeutic options addressing virus entry. Future approaches aiming at the elucidation of HBV hepatotropism, the identification of (a) specific receptor(s), the clarification of the endocytic entry pathway and imaging of fluorescently-labeled virions will allow us to decipher more precisely the HBV entry pathway in the near future. Furthermore, clinical efficacy studies with HBV–preS-derived lipopeptides will tell us whether entry inhibition is a passable way to defend acute and chronic HBV and hepatitis delta virus infections.

Assembly and budding of a hepatitis B virus is mediated by a novel type of intracellular vesicles

Formation of enveloped viruses involves assembly and budding at cellular membranes. In this study, we elucidated the morphogenesis of hepadnaviruses on the ultrastructural and biochemical level using duck hepatitis B virus (DHBV) as a model system. Formation of virus progeny initiates at the endoplasmic reticulum (ER) and is conserved both in vitro and in vivo. The morphogenesis proceeds via membrane-surrounded vesicles containing both virions and subviral particles, indicating a common morphogenetic pathway. The virus particle-containing vesicles (VCVs) are generated and maintained by reorganization of endomembranes accompanied by a striking disorganization of the rough ER (rER). VCVs are novel organelles with unique identity and properties of ER, intermediate compartment, endosomes, and multivesicular bodies. VCVs are dynamic structures whose size and shape are regulated by both membrane fusion and fission.

CONCLUSION

Our data indicate a strong reorganization of endomembranes during DHBV infection, resulting in the biogenesis of novel organelles serving as multifunctional platforms for assembly and budding of virus progeny.

Damage to the gills, skin and other tissues by lysenin and the coelomic fluid of the earthworm Eisenia foetida in two teleosts, Tanichthys albonubes and Oreochromis mossambicus

Lysenin is a 33-kDa protein found in the coelomic fluid (CF) of the earthworm Eisenia foetida. Purified lysenin binds specifically to sphingomyelin (SM). In the present studies, we found that the white cloud mountain minnow Tanichthys albonubes and the Mozambique tilapia Oreochromis mossambicus died in solutions of lysenin (at concentrations above 2.5 microg/ml) and CF (0.6%, v/v) within 2 h. The gills of both species of fish were damaged similarly by lysenin and by CF. Most gill lamellae became irregularly bent or curled, with swelling of the epithelial cells of the lamellae. Red blood cells in the lamellar vascular sinuses, in the central venous sinuses, and in the blood vessels of the entire body became swollen and lysed, choking the sinuses. Epithelial cells in the skin were also damaged. When fish of both species were treated with lysenin or CF that had been incubated with SM-liposomes, they did not die. Their behavior remained normal and there was no damage to any cells or tissues. These findings suggest that SM might be involved in the lethal effects of lysenin and CF. It is likely that purified lysenin and lysenin in CF bound to SM in the cell membranes of the tissues mentioned above, damaging the cells. The presence of SM in the gills and skin was confirmed, supporting this hypothesis. The damage to gills and hemolysis might have resulted in lethal respiratory problems. Damage to the skin might disturb the exchange of ions through the skin, hastening death. Damage by lysenin and CF to epithelial cells of the cornea and the wall of the oral cavity was also recognized, but there was no such damage to the intestine.

A novel membrane protein, Ros3p, is required for phospholipid translocation across the plasma membrane in Saccharomyces cerevisiae

Ro09-0198 (Ro) is a tetracyclic peptide antibiotic that binds specifically to phosphatidylethanolamine (PE) and causes cytolysis. To investigate the molecular basis of transbilayer movement of PE in biological membranes, we have isolated a series of budding yeast mutants that are hypersensitive to the Ro peptide. One of the most sensitive mutants, designated ros3 (Ro-sensitive 3), showed no significant change in the cellular phospholipid composition or in the sensitivity to amphotericin B, a sterol-binding polyene macrolide antibiotic. These results suggest that the mutation of ros3 affects the PE organization on the plasma membrane, rather than PE synthesis or overall organization of the membrane structures. By functional complementation screening, we identified the gene ROS3 affected in the mutant, and we showed that the hypersensitive phenotype was caused by the defective expression of the ROS3 gene product, Ros3p, an evolutionarily conserved protein with two putative transmembrane domains. Disruption of the ROS3 gene resulted in a marked decrease in the internalization of fluorescence-labeled analogs of PE and phosphatidylcholine, whereas the uptake of fluorescence-labeled phosphatidylserine and endocytic markers was not affected. Neither expression levels nor activities of ATP-binding cassette transporters of the ros3Delta cells differed from those of wild type cells, suggesting that Ros3p is not related to the multidrug resistance activities. Immunochemical analyses of the structure and subcellular localization showed that Ros3p was a glycosylated membrane protein localized in both the plasma membrane and the endoplasmic reticulum, and that a part of Ros3p was associated with the detergent-insoluble glycolipid-enriched complexes. These results indicate that Ros3p is a membrane glycoprotein that plays an important role in the phospholipid translocation across the plasma membrane.

LPS-binding protein and CD14-dependent attachment of hepatitis B surface antigen to monocytes is determined by the phospholipid moiety of the particles

It was observed recently that recombinant yeast-derived hepatitis B surface antigen (rHBsAg) particles, which contain the S protein only, bind almost exclusively to monocytes. It is shown here that binding requires the presence of the LPS receptor CD14. Furthermore, evidence is presented that a domain on CD14 that is identical to or largely overlaps with the LPS-binding pocket is instrumental for the attachment of rHBsAg. Additionally, it is shown that the heat-labile LPS-binding protein (LBP) catalyses the binding of rHBsAg to the cells. Remarkably, natural plasma-derived HBsAg (pHBsAg) does not have this property. pHBsAg devoid of its lipids and reconstituted with phosphatidylserine or phosphatidylglycerol acquires the characteristic of yeast-derived HBsAg. Clearly, the interaction of rHBsAg with the cell membrane is determined by the presence of charged phospholipids that are absent in pHBsAg. Although a lipid-receptor interaction is suggested, antibody-inhibition experiments suggest a possible involvement of the C-terminal region of the S protein in the interaction with monocytes. The possible implications of these observations for hepatitis B virus (HBV) infection and HBV vaccine efficiency are discussed.

Comparison between hepatitis B surface antigen (HBsAg) particles derived from mammalian cells (CHO) and yeast cells (Hansenula polymorpha): composition, structure and immunogenicity

The composition, structure and immunogenicity of hepatitis B surface antigen (HBsAg) particles derived from Chinese hamster ovary (CHO) cells and from cells of the yeast Hansenula polymorpha were compared. The particles were similar in size distribution (mean 20-33 nm), in shape (spherical), in gross composition (protein to lipid weight ratio of 60:40), and in types of lipids (phospholipids > > sterols = sterol esters = triacylglycerols). Differences related to genetic engineering and type of host cells were found in peptide and lipid compositions. CHO-HBsAg has three peptides: S, M and L, each in two forms of glycosylation, while the Hansenula-HBsAg has only the nonglycosylated S peptide. The electrical surface potential at the lipid/water interface of HBsAg derived from Hansenula is more negative than that of HBsAg derived from CHO, which was close to neutrality. Although the numbers of cysteine residues (all in the S peptides) are identical (14), 11 of them are free thiols in the CHO-HBsAg, compared with three to four in the Hansenula-HBsAg. The fact that 85% of the phospholipids are hydrolyzed by phospholipase C and that all the aminophospholipids react with trinitrobenzenesulfate suggests that the particles derived from both cell types are either leaky vesicles or have a lipoprotein-like structure. Subcutaneous injection into mice of fluorescein-isothiocyanate-labeled HBsAg particles from both sources resulted in their accumulation in the marginal sinus of lymph nodes. The humoral responses to subcutaneous injection into mice of CHO- and Hansenula-HBsAg were similar: however, the cytotoxic T lymphocyte response to CHO-HBsAg was lower.

Ether lipids in biomembranes

Plasmalogens (1-O-1'-alkenyl-2-acylglycerophospholipids) and to a lesser extent the 1-O-alkyl analogs are ubiquitous and in some cases major constituents of mammalian cellular membranes and of anaerobic bacteria. In archaebacteria polar lipids of the cell envelope are either diphytanylglycerolipids or bipolar macrocyclic tetraether lipids capable of forming covalently linked 'bilayers'. Information on the possible role of ether lipids as membrane constituents has been obtained from studies on the biophysical properties of model membranes consisting of these lipids. In addition, effects of modified ether lipid content on properties of biological membranes have been investigated using microorganisms or mammalian cells which carry genetic defects in ether lipid biosynthesis. Differential utilization of ether glycerophospholipids by specific phospholipases might play a role in the generation of lipid mediators that are involved in signal transduction. A possible function of plasmalogens as antioxidants has been demonstrated with cultured cells and might play a role in serum lipoproteins. Synthetic ether lipid analogs exert cytostatic effects, most likely by interfering with membrane structure and by specific interaction with components of signal transmission pathways, such as phospholipase C and protein kinase C.

Reconstitution of hepatitis B surface antigen proteins into phospholipid vesicles

Hepatitis B surface antigen (HBsAg), devoid of 75% of its total lipids has been reconstituted with several phospholipids by the detergent dialysis method, using the non-ionic detergent beta-D-octyl glucoside. Upon reconstitution with both neutral and acidic phospholipids, HBsAg particles had the same morphology and, as indicated by trypsin hydrolysis, the topology of the surface proteins was maintained. However, only negatively charged phospholipids were able to completely revert the conformational changes which had been induced by removal of the lipids. The helical content, as indicated by CD techniques, and the antigenic activity, as measured by binding to polyclonal antibodies, of HBsAg reconstituted with acidic phospholipids were practically identical to those of the native antigen. Cholesterol had no effect on the antigenic activity recovered by reconstitution with any of the phospholipids.