Cellular Membrane Trafficking Machineries Used by the Hepatitis Viruses

Autophagy-related protein 5 (ATG5) interacts with bone marrow stromal cell antigen 2 (BST2) to stimulate HBV replication through antagonizing the antiviral activity of BST2

Hepatitis B virus (HBV) infection is a major global health burden with 820 000 deaths per year. In our previous study, we found that the knockdown of autophagy-related protein 5 (ATG5) significantly upregulated the interferon-stimulated genes (ISGs) expression to exert the anti-HCV effect. However, the regulation of ATG5 on HBV replication and its underlying mechanism remains unclear. In this study, we screened the altered expression of type I interferon (IFN-I) pathway genes using RT² Profiler™ PCR array following ATG5 knock-down and we found the bone marrow stromal cell antigen 2 (BST2) expression was significantly increased. We then verified the upregulation of BST2 by ATG5 knockdown using RT-qPCR and found that the knockdown of ATG5 activated the Janus kinase/signal transducer and activator of transcription (JAK-STAT) signaling pathway. ATG5 knockdown or BST2 overexpression decreased Hepatitis B core Antigen (HBcAg) protein, HBV DNA levels in cells and supernatants of HepAD38 and HBV-infected NTCP-HepG2. Knockdown of BST2 abrogated the anti-HBV effect of ATG5 knockdown. Furthermore, we found that ATG5 interacted with BST2, and further formed a ternary complex together with HBV-X (HBx). In conclusion, our finding indicates that ATG5 promotes HBV replication through decreasing BST2 expression and interacting with it directly to antagonize its antiviral function.

The programed death-1/programed death ligand-1 axis and its potential as a therapeutic target for virus-associated tumours

As an important and serious condition impacting human health, the diagnosis, and treatment of tumours is clinically vital because tumour cell immune escape sustains tumour development. Programed death ligand-1 (PD-L1) on tumour cell surfaces binds to the programed death-1 (PD-1), inhibits T cell activation, and induces apoptosis, and incapacitates cells. This allows tumour cells to evade recognition and clearance by the immune system, thereby permitting tumour occurrence, and development and poor prognosis outcomes in patients with tumours. Currently, anti-PD-1/PD-L1 immunotherapy has become pivotal in tumour treatment. Pathogens, especially viruses, are important factors which induce many tumours. In this article, we examine associations between Epstein-Barr virus, human papilloma virus, hepatitis B virus, hepatitis C virus, and human immunodeficiency virus type 1-related tumours and PD-1/PD-L1 axis.

Switching to tenofovir alafenamide versus continued therapy in chronic hepatitis B patients who were treated with entecavir: A prospective, multicenter, randomized controlled study

BACKGROUNDS

Entecavir (ETV) and tenofovir alafenamide fumarate (TAF) have been used widely to treat patients with chronic hepatitis B virus (HBV) infection, but it is still unclear how best to use these drugs. Although some studies compared the efficacies of treatment switch from ETV to TAF, there has been no randomized study.

METHODS

We performed a prospective multicenter randomized controlled study in which subjects were enrolled from April 2018 to June 2019 and observed for 2 years until March 2021 to clarify the efficacy and safety of switching from ETV to TAF.

RESULTS

Thirty-three patients were enrolled and randomized into 2 groups, and a total of 30 patients were evaluated; a TAF-switching group (n = 16) and an ETV-continuing group (n = 14). The mean age of the 30 patients was 61 years old and 18 patients (60%) were male. The serum HBV DNA in all patients were below detection limit. The mean change in hepatitis B surface antigen (HBsAg) levels after 2 years was not significantly different between the TAF and ETV groups (-0.08 vs -0.20 log IU/mL, P = .07). Comparing the group with a HBsAg decline (≤ -0.1 log IU/mL) and a group without a HBsAg decline in an overall analysis, the prior ETV duration was significantly shorter in the HBsAg-declined group (49 vs 92 months, P = .03). Although the eGFR levels tended to decrease in the TAF group compared to ETV (-6.15 vs -2.26 mL/min/1.73 m2, P = .09), no significant differences were observed in patients with baseline eGFR < 60 (-2.49 vs 0.40 mL/min/1.73 m2, P = .25).

CONCLUSION

The efficacy and safety were comparable in the TAF-switching group and the ETV-continuing group. Because the present study was conducted in limited patients, a larger study will be required.

Non-Achievement of Alanine Aminotransferase Normalization Associated with the Risk of Hepatocellular Carcinoma during Nucleos(t)ide Analogue Therapies: A Multicenter Retrospective Study

Patients with a chronic hepatitis B virus (HBV) infection who are treated with nucleos(t)ide analogues (NAs) are still at risk for hepatocellular carcinoma (HCC), and it has been clinically questioned whether patients with a high risk of HCC can be identified efficiently. We aimed to clarify the risk factors associated with the development of HCC during NA therapies. A total of 611 chronically HBV-infected patients without a history of HCC, who were treated with NAs for more than 6 months (median 72 months), from 2000 to 2021, were included from 16 hospitals in the Tohoku district in Japan. Incidences of HCC occurrence were analyzed with clinical factors, including on-treatment responses. Alanine aminotransferase (ALT) normalization, based on the criteria of three guidelines, was analyzed with other parameters, including the age−male−ALBI−platelets (aMAP) risk score. During the observation period, 48 patients developed HCC, and the cumulative HCC incidence was 10.6% at 10 years. Non-achievement of ALT normalization at 1 year of therapy was mostly associated with HCC development when ALT ≤ 30 U/L was used as the cut-off (cumulative incidence, 19.9% vs. 5.3% at 10 years, p < 0.001). The effectiveness of the aMAP risk score at the start of treatment was validated in this cohort. A combination of an aMAP risk score ≥ 50 and non-achievement of ALT normalization could stratify the risk of HCC significantly, and notably, there was no HCC development in 103 patients without these 2 factors. In conclusion, non-achievement of ALT normalization (≤30 U/L) at 1 year might be useful in predicting HCC during NA therapies and, in combination with the aMAP risk score, could stratify the risk more precisely.

Functional role of miR‑155 in physiological and pathological processes of liver injury (Review)

There are several types of liver injury, including alcohol‑induced liver injury, drug‑induced liver injury, infectious liver injury, cirrhosis, liver ischemia/reperfusion injury and liver failure. In recent years, accumulated data have demonstrated that microRNAs (miRNAs/miRs) may be involved in the occurrence and development of a variety of systemic diseases, such as immune diseases, tumors and nervous system diseases. miR‑155 is a key miRNA, which has been studied extensively and has been shown to target different genes. In the present review, the potential effects and mechanisms of miR‑155 on the physiological and pathological processes of liver injury were reviewed from the perspective of cell stress, inflammation and activation of fibrosis. In addition, the potential benefits of miR‑155 as a therapeutic target and predictor of liver injury were summarized.

Ubiquitination-Dependent Regulation of Small GTPases in Membrane Trafficking: From Cell Biology to Human Diseases

Membrane trafficking is critical for cellular homeostasis, which is mainly carried out by small GTPases, a class of proteins functioning in vesicle budding, transport, tethering and fusion processes. The accurate and organized membrane trafficking relies on the proper regulation of small GTPases, which involves the conversion between GTP- and GDP-bound small GTPases mediated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Emerging evidence indicates that post-translational modifications (PTMs) of small GTPases, especially ubiquitination, play an important role in the spatio-temporal regulation of small GTPases, and the dysregulation of small GTPase ubiquitination can result in multiple human diseases. In this review, we introduce small GTPases-mediated membrane trafficking pathways and the biological processes of ubiquitination-dependent regulation of small GTPases, including the regulation of small GTPase stability, activity and localization. We then discuss the dysregulation of small GTPase ubiquitination and the associated human membrane trafficking-related diseases, focusing on the neurological diseases and infections. An in-depth understanding of the molecular mechanisms by which ubiquitination regulates small GTPases can provide novel insights into the membrane trafficking process, which knowledge is valuable for the development of more effective and specific therapeutics for membrane trafficking-related human diseases.

The Exosome-Associated Tetraspanin CD63 Contributes to the Efficient Assembly and Infectivity of the Hepatitis B Virus

Currently, the hepatocellular trafficking pathways that are used by the hepatitis B virus (HBV) during viral infection and shedding are poorly defined. It is known that the HBV uses late endosomal and multivesicular body (MVB) compartments for assembly and release. The intraluminal vesicles (ILVs) generated within MVBs have also been implicated in the late synthesis stages of a variety of pathogenic viruses. We recently observed that the HBV within infected hepatocytes appears to associate with the tetraspanin protein CD63, known to be a prominent and essential component of ILVs. Immunofluorescence microscopy of HBV-expressing cells showed that CD63 colocalized with HBV proteins (large hepatitis B surface antigens [LHBs] and hepatitis B core) and labeled an exceptionally large number of secreted extracellular vesicles of uniform size. Small interfering RNA (siRNA)-mediated depletion of CD63 induced a substantial accumulation of intracellular LHBs protein but did not alter the levels of either intracellular or extracellular HBV DNA, nor pregenomic RNA. Consistent with these findings, we found that markedly less LHBs protein was associated with the released HBV particles from CD63 siRNA-treated cells. Importantly, the HBV viral particles that were shed from CD63-depleted cells were substantially less infective than those collected from control cells with normal CD63 levels. Conclusion: These findings implicate the tetraspanin protein CD63 as a marker and an important component in the formation and release of infectious HBV particles.

CCDC88A/GIV promotes HBV replication and progeny secretion via enhancing endosomal trafficking and blocking autophagic degradation

Hepatitis B virus (HBV) particles are thought to be secreted from hepatocytes through multivesicular bodies (MVBs); however, the cellular trafficking mechanisms prior to this process remain elusive. It has been reported that CCDC88A/GIV expression, which is involved in multiple aspects of vesicular trafficking, changes dynamically at different phases of chronic HBV infection. In this study, we focused on the role of CCDC88A/GIV in HBV replication. In the liver tissues of chronically HBV-infected patients, HBV infection significantly enhanced CCDC88A/GIV expression, and increased endoplasmic reticulum (ER) stress and autophagosome formation without changing endosome formation. Additionally, colocalization of SHBsAg with early endosomes (~30.2%) far exceeded that with autophagosomes (~3.2%). In hepatoma cells, CCDC88A/GIV and its downstream proteins, DNM2 (dynamin 2; a CCDC88A/GIV effector), CLTC and RAB5A significantly enhanced HBV replication and endosome formation but inhibited autophagosome formation. Blocking endocytosis disrupted HBsAg trafficking to endosomes and caused its accumulation in the ER lumen, which triggered ER stress to initiate the unfolded protein response (UPR). Therefore, HBsAg trafficking into autophagosomes was increased, and the lysosomal activity and maturation, which was inhibited by HBV infection, were restored. Meanwhile, core particles were prevented from entering MVBs. CCDC88A/GIV and its other effector, GNAI3, decreased autophagic flux by enhancing the insulin-induced AKT-MTOR pathway, thereby inhibiting HBV antigens autophagic degradation. In conclusion, CCDC88A/GIV enhanced HBV replication by increasing endosomal trafficking and reducing autophagic degradation of HBV antigens, suggesting that CCDC88A/GIV-mediated endosomal trafficking plays an important role in HBV replication and progeny secretion.Abbreviations: ACTB: actin beta; AO: acridine orange; ATF6: activating transcription factor 6; CCDC88A/GIV: coiled-coil domain containing 88A; CLTC: clathrin heavy chain; CQ: chloroquine; DAPI: 4ʹ,6-diamidino-2-phenylindole; DNM2: dynamin 2; ER: endoplasmic reticulum; ERN1: endoplasmic reticulum to nucleus signaling 1; EIF2A: eukaryotic translation initiation factor 2A; FBS: fetal bovine serum; GNAI3: G protein subunit alpha i3; HBV: hepatitis B virus; HBV RIs: HBV replication intermediates; HBcAg: HBV core protein; HBsAg: HBV surface antigen; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MVBs: multivesicular bodies; MTOR: mechanistic target of rapamycin kinase; PDI: protein disulfide isomerase; PHH: primary human hepatocyte; pSM2: a HBV replication-competent plasmid; HSPA5/BIP: heat shock protein family A (Hsp70) member 5; SQSTM1/p62: sequestosome 1; siRNA: small interfering RNA; SEM: standard error of the mean; UPR: unfolded protein response

Hepatitis B Virus Reactivation with Discontinuation of Nucleoside Analogue in Patients Who Received Allogeneic Hematopoietic Stem Cell Transplantation

Reactivation of hepatitis B virus (HBV) is known to occur frequently after hematopoietic stem cell transplantation (HSCT). The reactivation can be prevented by nucleos(t)ide analogue (NA), but it is unclear how long NA should be continued. Here, we report 3 cases of HBV reactivation with discontinuation of NA following the discontinuation of immunosuppressive therapies after HSCT. Three male patients aged 34, 59, and 54 years received allogeneic HSCT (allo-HSCT) for chronic myeloid leukemia, mixed phenotype acute leukemia, and myelodysplastic syndrome, respectively. Before HSCT, 2 patients were positive for hepatitis B surface antigen (HBsAg) and 1 patient was negative for HBsAg and positive for antibodies to hepatitis B core antigen. NA (lamivudine or entecavir) was started at the same time as HSCT and stopped after the discontinuation of immunosuppressive therapies. In all patients, the serum HBV DNA levels were increased after the discontinuation of NAs. Two of the three patients developed severe hepatitis with high levels of HBV DNA (7.5 and 7.4 log IU/mL, respectively). A patient without hepatitis was re-administered NA soon after the HBV DNA started to increase (3.3 log IU/mL). Interestingly, the 2 patients who developed hepatitis cleared HBsAg promptly after the recovery from hepatitis and they could stop NAs without the reversion of HBsAg. It was speculated that transplanted immune cells, which were naïve for HBV, react strongly with HBV antigens that were increased after the NA discontinuation. The discontinuation of NA after allo-HSCT is not recommended generally because strong hepatitis might be induced even after several years.

A narrative review of the role of necroptosis in liver disease: a double-edged sword

Acute and chronic liver injuries lead to hepatocyte death and turnover. When injuries become chronic, continuous cell death and transformation lead to chronic inflammation, fibrosis, cirrhosis, and eventually carcinoma. A therapeutic strategy of great significance for liver disease is to control hepatocyte death in acute and chronic injuries. This strategy prevents hepatocytes from causing liver failure and inhibits both secondary inflammation and fibrosis. Both apoptosis and necrosis have been proven to occur in the liver, but the role of necroptosis in liver diseases is controversial. Necroptosis, which has features of necrosis and apoptosis, is a regulatory process that occurs in some cell types when caspases are inhibited. The signaling pathway of necroptosis is characterized by the activation of receptor-interacting proteins kinase (RIPK) and mixed lineage kinase domain-like (MLKL). Necroptosis is associated with a variety of inflammatory diseases and has been the focus of research in recent years. The incidence of necroptosis in liver tissues has been studied recently in several liver injury models, but the results of the studies are not consistent. The purpose of this review is to summarize the published data on the involvement of necroptosis in liver injury, focusing on the controversies, issues remaining to be discussed, and potential therapeutic applications in this area.

Switching to tenofovir disoproxil fumarate in entecavir-treated chronic hepatitis B patients: A pilot randomized controlled study

Although hepatitis B surface antigen (HBsAg) removal is considered the goal of chronic hepatitis B treatment, it can rarely be achieved with nucleos(t)ide analogues (NAs). It has been reported that tenofovir disoproxil fumarate (TDF) is superior in reducing HBsAg compared with entecavir (ETV) in treatment-naïve patients; however, the effect of TDF in patients who have received NAs is still unclear. The aim of the present study was to evaluate the efficacy of switching from ETV to TDF in patients who were already receiving ETV. A pilot randomized controlled study for 2 years in patients who had been treated with ETV for >1 year and did not exhibit drug resistance was performed (Clinical trial registration: UMIN000021948, UMIN-CTR, May 1, 2016). A total of 20 patients were enrolled and 19 patients were randomized into 2 groups, a TDF-switching group (n=12) or an ETV-continuing group (n=7). The mean change in HBsAg levels after 2 years was greater in the TDF group compared with the ETV group, but the difference was not significant (-0.25 vs. -0.06 log IU/ml). In the TDF group, hepatitis B e antigen (HBeAg)-positive patients at baseline showed significantly greater changes in HBsAg (-0.63 vs. -0.03 log IU/ml; P=0.030). In contrast, no difference between HBeAg-positive and HBeAg-negative patients was observed in the ETV group. No significant differences of estimated glomerular filtration rate and inorganic phosphorus changes were observed among the TDF and ETV groups. In conclusion, a significant HBsAg decrease was not achieved after switching from ETV to TDF in the overall analysis, but HBeAg-positive patients showed a larger HBsAg decrease after switching treatment.

Recent advances targeting C-C chemokine receptor type 2 for liver diseases in monocyte/macrophage

Liver plays a critical role in metabolism, nutrient storage and detoxification. Emergency signals or appropriate immune response leads to pathological inflammation and breaks the steady state when liver dysfunction appears, which makes body more susceptible to chronic liver infection, autoimmune diseases and tumour. Compelling proof has illustrated the non-redundant importance of C-C chemokine receptor type 2 (CCR2), one of G-protein-coupled receptors, in different diseases. Selectively expressed on the surface of cells, CCR2 is involved in various signalling pathways and regulates the migration of cells. Especially, a peculiar role of CCR2 has been identified within decades in the onset and progression of hepatic diseases, which led to particular focusing on CCR2 as a new therapeutic and diagnostic target for non-alcoholic fatty liver disease and hepatocellular carcinoma. In this review, we discuss the effect of CCR2 in monocytes/macrophages on liver diseases. The application and translation of the decades of discoveries into therapies promise novel approaches in the treatment of liver disease.

Synchronised infection identifies early rate-limiting steps in the hepatitis B virus life cycle

Hepatitis B virus (HBV) is an enveloped DNA virus that contains a partially double-stranded relaxed circular (rc) DNA. Upon infection, rcDNA is delivered to the nucleus where it is repaired to covalently closed circular (ccc) DNA that serves as the transcription template for all viral RNAs. Our understanding of HBV particle entry dynamics and host pathways regulating intracellular virus trafficking and cccDNA formation is limited. The discovery of sodium taurocholate co-transporting peptide (NTCP) as the primary receptor allows studies on these early steps in viral life cycle. We employed a synchronised infection protocol to quantify HBV entry kinetics. HBV attachment to cells at 4°C is independent of NTCP, however, subsequent particle uptake is NTCP-dependent and reaches saturation at 12 h post-infection. HBV uptake is clathrin- and dynamin dependent with actin and tubulin playing a role in the first 6 h of infection. Cellular fractionation studies demonstrate HBV DNA in the nucleus within 6 h of infection and cccDNA was first detected at 24 h post-infection. Our studies show the majority (83%) of cell bound particles enter HepG2-NTCP cells, however, only a minority (<1%) of intracellular rcDNA was converted to cccDNA, highlighting this as a rate-limiting in establishing infection in vitro. This knowledge highlights the deficiencies in our in vitro cell culture systems and will inform the design and evaluation of physiologically relevant models that support efficient HBV replication.

Effect of viral DNA methylation on expression of hepatitis B virus proteins depends on the virus genotype

The frequency of HBV genomic methylation in the liver was reported to vary among patients, but the detailed mechanism is still unknown. In this study, the effects of HBV genome methylation on HBV replication were investigated in vitro. A total of 6 plasmids containing 1.24-fold the HBV genome of different genotypes (subgenotypes A1, A2, B1, and C2) were purified after in vitro methylation with CpG methyltransferase (M.SssI) and transfected into HepG2 cells. In genotype B and C strains, methylation markedly decreased the amount of hepatitis B e antigen (HBeAg) in the culture supernatant. A reduction of hepatitis B surface antigen (HBsAg) was found in some HBV strains but the reduction was smaller than that of HBeAg. There was no significant difference in particle-associated HBV DNA in the culture supernatant. These findings suggest that HBV genomic methylation might be involved in the HBeAg decline in genotype B and C, in part, and that the reduction of HBsAg was less than that of HBeAg. In conclusion, this study showed that the effect of HBV genomic methylation differs among HBV genotypes, suggesting a potential explanation for the different clinical outcomes of genotypes A, B, and C.

Cellular endo-lysosomal dysfunction in the pathogenesis of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD), an increasingly devastating human disorder, is characterized by intrahepatic fat accumulation. Although important progress has been made in understanding NAFLD, the fundamental mechanisms involved in the pathogenesis of NAFLD have not been fully explained. The endo-lysosomal trafficking network is central to lipid metabolism, protein degradation and signal transduction, which are involved in a variety of diseases. In recent years, many genes and pathways in the endo-lysosomal trafficking network and involved in lysosomal biogenesis have been associated with the development and progression of NAFLD. Mutations of these genes and impaired signalling lead to dysfunction in multiple steps of the endo-lysosomal network (endocytic trafficking, membrane fusion and lysosomal degradation), resulting in the accumulation of pathogenic proteins. In this review, we will focus on how alterations in these genes and pathways affect endo-lysosomal trafficking as well as the pathophysiology of NAFLD.

Comparison of hepatitis B virus genotypes B and C among chronically hepatitis B virus-infected patients who received nucleos(t)ide analogs: A multicenter retrospective study

AIM

Hepatitis B virus genotype B (HBV/B) has been reported to have less risk of liver cirrhosis and hepatocellular carcinoma (HCC), but long-term observation has rarely been reported. We aimed to clarify the characteristics of HBV/B in nucleos(t)ide analog-treated patients in an area where HBV/B is more prevalent than in other areas of Japan.

METHODS

A total of 498 chronically HBV-infected patients treated with nucleos(t)ide analog (lamivudine, entecavir, tenofovir disoproxil fumarate, or tenofovir alafenamide fumarate) for >6 months (mean 70.6 months) were included from nine hospitals in northeast Japan. The frequencies of hepatitis B surface antigen loss and HCC occurrence were analyzed.

RESULTS

Among 427 patients whose genotype could be determined, 34.0% and 64.4% were infected with HBV/B and genotype C (HBV/C), respectively. The age of patients with HBV/B was significantly older than those with HBV/C (57.7 vs. 48.1). The cumulative rate of hepatitis B surface antigen loss was significantly higher in HBV/B than in HBV/C (3.6% vs. 0.7% at 10 years). Among 480 patients without HCC history, HCC occurrence was found in 40 patients (13.4% at 10 years). There was no cumulative rate difference of HCC occurrence among the genotypes, but after propensity score matching for age/sex, it was significantly lower in HBV/B than in HBV/C (5.3% vs. 18.5% at 10 years).

CONCLUSIONS

Although a lower rate of HCC occurrence in HBV/B was shown by an age/sex-matched analysis than that in HBV/C, patients with HBV/B were significantly older and had a comparative risk of HCC occurrence in nucleos(t)ide analog-treated patients.

Small Interfering RNA Screening for the Small GTPase Rab Proteins Identifies Rab5B as a Major Regulator of Hepatitis B Virus Production

Viruses are considered to use vesicular trafficking in infected cells, but the details of assembly/release pathways of hepatitis B virus (HBV) are still unknown. To identify key regulators of HBV production, we performed short interfering RNA (siRNA) screening for Rab proteins, which are considered to act as molecular switches in vesicular trafficking using HepG2.2.15 cells. Among 62 Rab proteins, the suppression of Rab5B most significantly increased HBV DNA in the culture supernatant. Surprisingly, 5 days after the transfection of Rab5B siRNA, HBV DNA in the supernatant was increased more than 30-fold, reflecting the increase of infectious HBV particles. Northern blotting showed that transcription of 2.4/2.1-kb mRNA coding envelope proteins containing large hepatitis B surface protein (LHBs) was increased. Analysis of hepatocyte nuclear factors (HNFs) showed that transcription of HNF4α, which is known to enhance 2.4-kb mRNA transcription, was regulated by Rab5B. Also, it was revealed that LHBs had accumulated in the endoplasmic reticulum (ER) after Rab5B depletion but not in the multivesicular body (MVB), which is thought to be an organelle utilized for HBV envelope formation. Therefore, it was considered that Rab5B is required for the transport of LHBs from the ER to MVB. Immunofluorescent microscopy showed that HBs proteins, including LHBs, colocalized with HBc in the ER of Rab5B-depleted cells, suggesting that HBV envelopment occurs not only in the MVB but also in the ER. In conclusion, Rab5B is a key regulator of HBV production and could be a target of antiviral therapy.IMPORTANCE HBV infection is a worldwide health problem, but the mechanisms of how HBV utilizes cellular machinery for its life cycle are poorly understood. In particular, it has been unclear how the viral components and virions are transported among the organelles. The HBV budding site has been reported to be the ER or MVB, but it has not been clearly determined. In this study, siRNA-based screening of Rab proteins using HBV-expressing cells showed that Rab5B, one of the Rab5 isoforms, has important roles in late steps of the HBV life cycle. Although Rab5 is known to work on early endosomes, this study showed that Rab5B plays a role in the transport of LHBs between the ER and MVB. Furthermore, it affects the transcription of LHBs. This is the first report on the mechanisms of HBV envelope protein transport among the organelles, and the results provide important insights into the therapeutic control of HBV infection.

The cell biology of the hepatocyte: A membrane trafficking machine

The liver performs numerous vital functions, including the detoxification of blood before access to the brain while simultaneously secreting and internalizing scores of proteins and lipids to maintain appropriate blood chemistry. Furthermore, the liver also synthesizes and secretes bile to enable the digestion of food. These diverse attributes are all performed by hepatocytes, the parenchymal cells of the liver. As predicted, these cells possess a remarkably well-developed and complex membrane trafficking machinery that is dedicated to moving specific cargos to their correct cellular locations. Importantly, while most epithelial cells secrete nascent proteins directionally toward a single lumen, the hepatocyte secretes both proteins and bile concomitantly at its basolateral and apical domains, respectively. In this Beyond the Cell review, we will detail these central features of the hepatocyte and highlight how membrane transport processes play a key role in healthy liver function and how they are affected by disease.