A Single Adaptive Mutation in Sodium Taurocholate Cotransporting Polypeptide Induced by Hepadnaviruses Determines Virus Species Specificity

Structure of antiviral drug bulevirtide bound to hepatitis B and D virus receptor protein NTCP

Cellular entry of the hepatitis B and D viruses (HBV/HDV) requires binding of the viral surface polypeptide preS1 to the hepatobiliary transporter Na+-taurocholate co-transporting polypeptide (NTCP). This interaction can be blocked by bulevirtide (BLV, formerly Myrcludex B), a preS1 derivative and approved drug for treating HDV infection. Here, to elucidate the basis of this inhibitory function, we determined a cryo-EM structure of BLV-bound human NTCP. BLV forms two domains, a plug lodged in the bile salt transport tunnel of NTCP and a string that covers the receptor's extracellular surface. The N-terminally attached myristoyl group of BLV interacts with the lipid-exposed surface of NTCP. Our structure reveals how BLV inhibits bile salt transport, rationalizes NTCP mutations that decrease the risk of HBV/HDV infection, and provides a basis for understanding the host specificity of HBV/HDV. Our results provide opportunities for structure-guided development of inhibitors that target HBV/HDV docking to NTCP.

Structural basis of hepatitis B virus receptor binding

Hepatitis B virus (HBV), a leading cause of developing hepatocellular carcinoma affecting more than 290 million people worldwide, is an enveloped DNA virus specifically infecting hepatocytes. Myristoylated preS1 domain of the HBV large surface protein binds to the host receptor sodium-taurocholate cotransporting polypeptide (NTCP), a hepatocellular bile acid transporter, to initiate viral entry. Here, we report the cryogenic-electron microscopy structure of the myristoylated preS1 (residues 2-48) peptide bound to human NTCP. The unexpectedly folded N-terminal half of the peptide embeds deeply into the outward-facing tunnel of NTCP, whereas the C-terminal half formed extensive contacts on the extracellular surface. Our findings reveal an unprecedented induced-fit mechanism for establishing high-affinity virus-host attachment and provide a blueprint for the rational design of anti-HBV drugs targeting virus entry.

Role of sodium taurocholate cotransporting polypeptide (NTCP) in HBV-induced hepatitis: Opportunities for developing novel therapeutics

Hepatitis B is an infectious disease caused by the HBV virus. It presents a significant challenge for treatment due to its chronic nature and the potential for developing severe complications, including hepatocirrhosis and hepatocellular carcinoma. These complications not only cause physical and psychological distress to patients but also impose substantial economic and social burdens on both individuals and society as a whole. The internalization of HBV relies on endocytosis and necessitates the involvement of various proteins, including heparin sulfate proteoglycans, epidermal growth factor receptors, and NTCP. Among these proteins, NTCP is pivotal in HBV internalization and is primarily located in the liver's basement membrane. As a transporter of bile acids, NTCP also serves as a receptor facilitating HBV entry into cells. Numerous molecules have been identified to thwart HBV infection by stifling NTCP activity, although only a handful exhibit low IC50 values. In this systematic review, our primary focus dwells on the structure and regulation of NTCP, as well as the mechanism involved in HBV internalization. We underscore recent drug breakthroughs that specifically target NTCP to combat HBV infection. By shedding light on these advances, this review contributes novel insights into developing effective anti-HBV medications.

Conserved use of the sodium/bile acid cotransporter (NTCP) as an entry receptor by hepatitis B virus and domestic cat hepadnavirus

The Orthohepadnavirus genus includes hepatitis B virus (HBV) that can cause chronic hepatitis and hepatocarcinoma in humans. Recently, a novel hepadnavirus in cats, domestic cat hepadnavirus (DCH), was identified that is genetically close to HBV. DCH infection is associated with chronic hepatitis in cats, suggesting a similarity with HBV pathogenesis and the potential to use DCH as a novel animal model for HBV research. HBV is shown to use the sodium/bile acid cotransporter (NTCP) as a major cell entry receptor, but the equivalent receptor for DCH remains unknown. Here we sought to identify the entry receptor for DCH. HBV- and DCH-derived preS1 peptides efficiently bound to both human and cat NTCPs, and residue 158 of NTCP proteins determined the species-specific binding of the DCH preS1 peptide. Myrcludex B, an HBV entry inhibitor, blocked the binding of the DCH preS1 peptide. Thus, DCH and HBV may share cell entry molecules, suggesting a possibility of inter-species transmission. Furthermore, our study suggests that DCH can be useful as a novel model for HBV research.

Targeted viral adaptation generates a simian-tropic hepatitis B virus that infects marmoset cells

Hepatitis B virus (HBV) only infects humans and chimpanzees, posing major challenges for modeling HBV infection and chronic viral hepatitis. The major barrier in establishing HBV infection in non-human primates lies at incompatibilities between HBV and simian orthologues of the HBV receptor, sodium taurocholate co-transporting polypeptide (NTCP). Through mutagenesis analysis and screening among NTCP orthologues from Old World monkeys, New World monkeys and prosimians, we determined key residues responsible for viral binding and internalization, respectively and identified marmosets as a suitable candidate for HBV infection. Primary marmoset hepatocytes and induced pluripotent stem cell-derived hepatocyte-like cells support HBV and more efficient woolly monkey HBV (WMHBV) infection. Adapted chimeric HBV genome harboring residues 1-48 of WMHBV preS1 generated here led to a more efficient infection than wild-type HBV in primary and stem cell derived marmoset hepatocytes. Collectively, our data demonstrate that minimal targeted simianization of HBV can break the species barrier in small NHPs, paving the path for an HBV primate model.

Tyrosine 146 of the Human Na+/Taurocholate Cotransporting Polypeptide (NTCP) Is Essential for Its Hepatitis B Virus (HBV) Receptor Function and HBV Entry into Hepatocytes

Na⁺/taurocholate cotransporting polypeptide (NTCP, gene symbol SLC10A1) is a hepatic bile acid uptake carrier participating in the enterohepatic circulation of bile acids. Apart from its transporter function, NTCP acts as the high-affinity liver-specific receptor for the hepatitis B virus (HBV), which attaches via its preS1-peptide domain of the large surface protein to NTCP, subsequently leading to endocytosis of the virus/NTCP-receptor complex. Although the process of NTCP-dependent HBV infection of hepatocytes has received much attention over the last decade, the precise molecular sites of the virus/NTCP interaction have not been fully identified. Inspection of the primary protein sequence of human NTCP revealed 139YIYSRGIY146 as a highly conserved tyrosine-rich motif. To study the role of Y139, Y141 and Y146 amino acids in NTCP biology, the aforementioned residues were substituted with alanine, phenylalanine or glutamate (mimicking phosphorylation) using site-directed mutagenesis. Similar to wt NTCP, the Y139A, Y141A, Y146A, Y141F, Y146F, and Y146E mutants were expressed at the plasma membrane of HEK293 cells and exhibited intact bile acid transport function. Y146A, Y146E, and Y146F demonstrated transport kinetics comparable to wild-type NTCP with K_m values of 57.3–112.4 µM and V_max values of 6683–7579 pmol/mg protein/min. Only Y141E was transport deficient, most likely due to an intracellular accumulation of the mutant protein. Most importantly, Y146A and Y146E mutation completely abrogated binding of the viral preS1-peptide to NTCP, while the Y146F mutant of NTCP showed some residual binding competence for preS1. Consequently, the NTCP mutants Y146A and Y146E, when expressed in HepG2 hepatoma cells, showed complete loss of susceptibility for in vitro HBV infection. In conclusion, tyrosine 146, and to some extent tyrosine 141, both belonging to the tyrosine-rich motif 139YIYSRGIY146 of human NTCP, are newly identified amino acid residues that play an essential role in the interaction of HBV with its receptor NTCP and, thus, in the process of virus entry into hepatocytes.

Structural insights into the HBV receptor and bile acid transporter NTCP

Around 250 million people are infected with hepatitis B virus (HBV) worldwide¹, and 15 million may also carry the satellite virus hepatitis D virus (HDV), which confers even greater risk of severe liver disease². The HBV receptor has been identified as sodium taurocholate co-transporting polypeptide (NTCP), which interacts directly with the first 48 amino acid residues of the N-myristoylated N-terminal preS1 domain of the viral large protein³. Despite the pressing need for therapeutic agents to counter HBV, the structure of NTCP remains unsolved. This 349-residue protein is closely related to human apical sodium-dependent bile acid transporter (ASBT), another member of the solute carrier family SLC10. Crystal structures have been reported of similar bile acid transporters from bacteria^4,5, and these models are believed to resemble closely both NTCP and ASBT. Here we have used cryo-electron microscopy to solve the structure of NTCP bound to an antibody, clearly showing that the transporter has no equivalent of the first transmembrane helix found in other SLC10 proteins, and that the N terminus is exposed on the extracellular face. Comparison of our structure with those of related proteins indicates a common mechanism of bile acid transport, but the NTCP structure displays an additional pocket formed by residues that are known to interact with preS1, presenting new opportunities for structure-based drug design.

Cryo-electron structures of the hepatitis B virus receptor NTCP show a distinct membrane topology compared with other SLC10 proteins, but a common bile acid transport mechanism that is shared with related mammalian and bacterial proteins.

Structure of the bile acid transporter and HBV receptor NTCP

Chronic infection with hepatitis B virus (HBV) affects more than 290 million people worldwide, is a major cause of cirrhosis and hepatocellular carcinoma, and results in an estimated 820,000 deaths annually^1,2. For HBV infection to be established, a molecular interaction is required between the large glycoproteins of the virus envelope (known as LHBs) and the host entry receptor sodium taurocholate co-transporting polypeptide (NTCP), a sodium-dependent bile acid transporter from the blood to hepatocytes³. However, the molecular basis for the virus-transporter interaction is poorly understood. Here we report the cryo-electron microscopy structures of human, bovine and rat NTCPs in the apo state, which reveal the presence of a tunnel across the membrane and a possible transport route for the substrate. Moreover, the cryo-electron microscopy structure of human NTCP in the presence of the myristoylated preS1 domain of LHBs, together with mutation and transport assays, suggest a binding mode in which preS1 and the substrate compete for the extracellular opening of the tunnel in NTCP. Our preS1 domain interaction analysis enables a mechanistic interpretation of naturally occurring HBV-insusceptible mutations in human NTCP. Together, our findings provide a structural framework for HBV recognition and a mechanistic understanding of sodium-dependent bile acid translocation by mammalian NTCPs.

Regulation of the HBV Entry Receptor NTCP and its Potential in Hepatitis B Treatment

Hepatitis B virus (HBV) is a globally prevalent human DNA virus responsible for more than 250 million cases of chronic liver infection, a condition that can lead to liver inflammation, cirrhosis, and hepatocellular carcinoma. Sodium taurocholate co-transporting polypeptide (NTCP), a transmembrane protein highly expressed in human hepatocytes and a mediator of bile acid transport, has been identified as the receptor responsible for the cellular entry of both HBV and its satellite, hepatitis delta virus (HDV). This has led to significant advances in our understanding of the HBV life cycle, especially the early steps of infection. HepG2-NTCP cells and human NTCP-expressing transgenic mice have been employed as the primary cell culture and animal models, respectively, for the study of HBV, and represent valuable approaches for investigating its basic biology and developing treatments for infection. However, the mechanisms involved in the regulation of NTCP transcription, translation, post-translational modification, and transport are still largely elusive. Improvements in our understanding of NTCP biology would likely facilitate the design of new therapeutic drugs for the prevention of the de novo infection of naïve hepatocytes. In this review, we provide critical findings regarding NTCP biology and discuss important questions that remain unanswered.

Fungal Secondary Metabolite Exophillic Acid Selectively Inhibits the Entry of Hepatitis B and D Viruses

Current anti-hepatitis B virus (HBV) drugs are suppressive but not curative for HBV infection, so there is considerable demand for the development of new anti-HBV agents. In this study, we found that fungus-derived exophillic acid inhibits HBV infection with a 50% maximal inhibitory concentration (IC50) of 1.1 µM and a 50% cytotoxic concentration (CC50) of >30 µM in primary human hepatocytes. Exophillic acid inhibited preS1-mediated viral attachment to cells but did not affect intracellular HBV replication. Exophillic acid appears to target the host cells to reduce their susceptibility to viral attachment rather than acting on the viral particles. We found that exophillic acid interacted with the HBV receptor, sodium taurocholate cotransporting polypeptide (NTCP). Exophillic acid impaired the uptake of bile acid, the original function of NTCP. Consistent with our hypothesis that it affects NTCP, exophillic acid inhibited infection with HBV and hepatitis D virus (HDV), but not that of hepatitis C virus. Moreover, exophillic acid showed a pan-genotypic anti-HBV effect. We thus identified the anti-HBV/HDV activity of exophillic acid and revealed its mode of action. Exophillic acid is expected to be a potential new lead compound for the development of antiviral agents.

Multitasking Na+/Taurocholate Cotransporting Polypeptide (NTCP) as a Drug Target for HBV Infection: From Protein Engineering to Drug Discovery

Hepatitis B virus (HBV) infections are among the major public health concerns worldwide with more than 250 million of chronically ill individuals. Many of them are additionally infected with the Hepatitis D virus, a satellite virus to HBV. Chronic infection frequently leads to serious liver diseases including cirrhosis and hepatocellular carcinoma, the most common type of liver cancer. Although current antiviral therapies can control HBV replication and slow down disease progress, there is an unmet medical need to identify therapies to cure this chronic infectious disease. Lately, a noteworthy progress in fighting against HBV has been made by identification of the high-affinity hepatic host receptor for HBV and HDV, namely Na⁺/taurocholate cotransporting polypeptide (NTCP, gene symbol SLC10A1). Next to its primary function as hepatic uptake transporter for bile acids, NTCP is essential for the cellular entry of HBV and HDV into hepatocytes. Due to this high-ranking discovery, NTCP has become a valuable target for drug development strategies for HBV/HDV-infected patients. In this review, we will focus on a newly predicted three-dimensional NTCP model that was generated using computational approaches and discuss its value in understanding the NTCP’s membrane topology, substrate and virus binding taking place in plasma membranes. We will review existing data on structural, functional, and biological consequences of amino acid residue changes and mutations that lead to loss of NTCP’s transport and virus receptor functions. Finally, we will discuss new directions for future investigations aiming at development of new NTCP-based HBV entry blockers that inhibit HBV tropism in human hepatocytes.

Na+-Taurocholate Co-Transporting Polypeptide (NTCP) in Livers, Function, Expression Regulation, and Potential in Hepatitis B Treatment

Chronic hepatitis B virus (HBV) infection has become one of the leading causes of liver cirrhosis and hepatocellular carcinoma globally. The discovery of sodium taurocholate co-transporting polypeptide (NTCP), a solute carrier, as a key receptor for HBV and hepatitis D virus (HDV) has opened new avenues for HBV treatment. Additionally, it has led researchers to generate hepatoma cell lines (including HepG2-NTCP and Huh-7-NTCP) susceptible to HBV infection in vitro, hence, paving the way to develop and efficiently screen new and novel anti-HBV drugs. This review summarizes the history, function and critical findings regarding NTCP as a viral receptor for HBV/HDV, and it also discusses recently developed drugs targeting NTCP.

Toll-Like Receptor Response to Hepatitis B Virus Infection and Potential of TLR Agonists as Immunomodulators for Treating Chronic Hepatitis B: An Overview

Chronic hepatitis B virus (HBV) infection remains a major global health problem. The immunopathology of the disease, especially the interplay between HBV and host innate immunity, is poorly understood. Moreover, inconsistent literature on HBV and host innate immunity has led to controversies. However, recently, there has been an increase in the number of studies that have highlighted the link between innate immune responses, including Toll-like receptors (TLRs), and chronic HBV infection. TLRs are the key sensing molecules that detect pathogen-associated molecular patterns and regulate the induction of pro- and anti-inflammatory cytokines, thereby shaping the adaptive immunity. The suppression of TLR response has been reported in patients with chronic hepatitis B (CHB), as well as in other models, including tree shrews, suggesting an association of TLR response in HBV chronicity. Additionally, TLR agonists have been reported to improve the host innate immune response against HBV infection, highlighting the potential of these agonists as immunomodulators for enhancing CHB treatment. In this study, we discuss the current understanding of host innate immune responses during HBV infection, particularly focusing on the TLR response and TLR agonists as immunomodulators.

Entry of hepatitis B virus: going beyond NTCP to the nucleus

Hepatitis B virus (HBV) infection remains a major cause of liver diseases and hepatocellular carcinoma. HBV infection begins by low-affinity attachment to hepatocytes and subsequent binding with a specific receptor sodium taurocholate cotransporting polypeptide (NTCP) on sinusoidal-basolateral side of liver parenchymal cells. Following internalization with an unclear mechanism, HBV undergoes uncoating, capsid disassembling and culminates in delivering its genome into the nucleus and forms the covalently closed circular (ccc) DNA. In this review, we briefly summarize the current understanding of HBV entry and discuss some unanswered questions along the entry pathway beyond NTCP binding into the nucleus.

Molecular regulation of the hepatic bile acid uptake transporter and HBV entry receptor NTCP

Transporters expressed by hepatocytes and enterocytes play a critical role in maintaining the enterohepatic circulation of bile acids. The sodium taurocholate cotransporting polypeptide (NTCP), exclusively expressed at the basolateral side of hepatocytes, mediates the uptake of conjugated bile acids. In conditions where bile flow is impaired (cholestasis), pharmacological inhibition of NTCP-mediated bile acid influx is suggested to reduce hepatocellular damage due to bile acid overload. Furthermore, NTCP has been shown to play an important role in hepatitis B virus (HBV) and hepatitis Delta virus (HDV) infection by functioning as receptor for viral entry into hepatocytes. This review provides a summary of current molecular insight into the regulation of NTCP expression at the plasma membrane, hepatic bile acid transport, and NTCP-mediated viral infection.

Animal Models for the Study of Hepatitis B Virus Pathobiology and Immunity: Past, Present, and Future

Hepatitis B virus (HBV) infection is a global public health problem that plagues approximately 240 million people. Chronic hepatitis B (CHB) often leads to liver inflammation and aberrant repair which results in diseases ranging from liver fibrosis, cirrhosis, to hepatocellular carcinoma. Despite its narrow species tropism, researchers have established various in vivo models for HBV or its related viruses which have provided a wealth of knowledge on viral lifecycle, pathogenesis, and immunity. Here we briefly revisit over five decades of endeavor in animal model development for HBV and summarize their advantages and limitations. We also suggest directions for further improvements that are crucial for elucidation of the viral immune-evasion strategies and for development of novel therapeutics for a functional cure.

Animal Models of Hepatitis B Virus Infection-Success, Challenges, and Future Directions

Chronic hepatitis B virus (HBV) infection affects more than 250 million people worldwide, which greatly increases the risk for terminal liver diseases, such as liver cirrhosis and hepatocellular carcinoma (HCC). Even though current approved antiviral therapies, including pegylated type I interferon (IFN) and nucleos(t)ide analogs, can effectively suppress viremia, HBV infection is rarely cured. Since HBV exhibits a narrow species tropism and robustly infects only humans and higher primates, progress in HBV research and preclinical testing of antiviral drugs has been hampered by the scarcity of suitable animal models. Fortunately, a series of surrogate animal models have been developed for the study of HBV. An increased understanding of the barriers towards interspecies transmission has aided in the development of human chimeric mice and has greatly paved the way for HBV research in vivo, and for evaluating potential therapies of chronic hepatitis B. In this review, we summarize the currently available animal models for research of HBV and HBV-related hepadnaviruses, and we discuss challenges and future directions for improvement.

Circadian control of hepatitis B virus replication

Chronic hepatitis B virus (HBV) infection is a major cause of liver disease and cancer worldwide for which there are no curative therapies. The major challenge in curing infection is eradicating or silencing the covalent closed circular DNA (cccDNA) form of the viral genome. The circadian factors BMAL1/CLOCK and REV-ERB are master regulators of the liver transcriptome and yet their role in HBV replication is unknown. We establish a circadian cycling liver cell-model and demonstrate that REV-ERB directly regulates NTCP-dependent hepatitis B and delta virus particle entry. Importantly, we show that pharmacological activation of REV-ERB inhibits HBV infection in vitro and in human liver chimeric mice. We uncover a role for BMAL1 to bind HBV genomes and increase viral promoter activity. Pharmacological inhibition of BMAL1 through REV-ERB ligands reduces pre-genomic RNA and de novo particle secretion. The presence of conserved E-box motifs among members of the Hepadnaviridae family highlight an evolutionarily conserved role for BMAL1 in regulating this family of small DNA viruses.

The NTCP p.Ser267Phe Variant Is Associated With a Faster Anti-HBV Effect on First-Line Nucleos(t)ide Analog Treatment

Sodium taurocholate cotransporting polypeptide (NTCP) acts as a cellular receptor for the hepatitis B virus infection of host hepatocytes. Previously, many studies confirmed that the NTCP p.Ser267Phe variant was a protective factor against HBV-related disease progression. We therefore designed this study to investigate whether the NTCP p.Ser267Phe variant exerts an additive anti-HBV effect in chronic hepatitis B (CHB) patients on mainstream NAs treatment. After propensity score matching (PSM), a total of 136 CHB patients were included, among whom 68 were heterozygous carriers and 68 were wild-type controls. Proportions of primary nonresponse, partial virological response, virological breakthrough and hepatitis B reactivation and the HBV DNA clearance rate at each time point were compared using the chi-square test. Kaplan-Meier analysis and matched t-tests were also performed to estimate the speed of viral clearance and serum HBV DNA reduction, respectively. The proportion of primary nonresponse was significantly lower in heterozygous carriers than in wild-type controls (p < 0.001), especially in patients using entecavir (p = 0.013). Specifically, heterozygous carriers achieved HBV DNA clearance faster than wild-type controls (log-rank p = 0.0198). HBV DNA levels were reduced more in heterozygous carriers after 12 weeks (p < 0.001) and 24 weeks (p = 0.006) of treatment, especially among patients using ETV. Here, our study demonstrated that heterozygous mutations in rs2296651 enhanced the antiviral response of first-line NAs and helped to explore the possibility of combining NAs and NTCP blockers for a better anti-HBV effect.

Host-Virus Arms Races Drive Elevated Adaptive Evolution in Viral Receptors

Viral receptors are the cell surface proteins that are hijacked by viruses to initialize their infections. Viral receptors are subject to two conflicting directional forces, namely, negative selection due to functional constraints and positive selection due to host-virus arms races. It remains largely obscure whether negative pleiotropy limits the rate of adaptation in viral receptors. Here, we perform evolutionary analyses of 96 viral receptor genes in primates and find that 41 out of 96 viral receptors experienced adaptive evolution. Many positively selected residues in viral receptors are located at the virus-receptor interfaces. Compared with control proteins, viral receptors exhibit significantly elevated rate of adaptation. Further analyses of genetic polymorphisms in human populations reveal signals of positive selection and balancing selection for 53 and 5 viral receptors, respectively. Moreover, we find that 49 viral receptors experienced different selection pressures in different human populations, indicating that viruses represent an important driver of local adaptation in humans. Our findings suggest that diverse viruses, many of which have not been known to infect nonhuman primates, have maintained antagonistic associations with primates for millions of years, and the host-virus conflicts drive accelerated adaptive evolution in viral receptors.IMPORTANCE Viruses hijack cellular proteins, termed viral receptors, to assist their entry into host cells. While viral receptors experience negative selection to maintain their normal functions, they also undergo positive selection due to an everlasting evolutionary arms race between viruses and hosts. A complete picture on how viral receptors evolve under two conflicting forces is still lacking. In this study, we systematically analyzed the evolution of 96 viral receptors in primates and human populations. We found around half of viral receptors underwent adaptive evolution and exhibit significantly elevated rates of adaptation compared to control genes in primates. We also found signals of past natural selection for 58 viral receptors in human populations. Interestingly, 49 viral receptors experienced different selection pressures in different human populations, indicating that viruses represent an important driver of local adaptation in humans. Our results suggest that host-virus arms races drive accelerated adaptive evolution in viral receptors.

Innovative HBV Animal Models Based on the Entry Receptor NTCP

Hepatitis B is a major global health problem, with an estimated 257 million chronically infected patients and almost 1 million deaths per year. The causative agent is hepatitis B virus (HBV), a small, enveloped, partially double-stranded DNA virus. HBV has a strict species specificity, naturally infecting only humans and chimpanzees. Sodium taurocholate co-transporting polypeptide (NTCP), a bile acid transporter expressed on hepatocytes, has been shown to be one of the key factors in HBV infection, playing a crucial role in the HBV entry process in vitro and in vivo. Variations in the amino acid sequence of NTCP can inhibit HBV infection and, therefore, contributes, in part, to the species barrier. This discovery has revolutionized the search for novel animal models of HBV. Indeed, it was recently shown that variations in the amino acid sequence of NTCP represent the sole species barrier for HBV infection in macaques. Here, we review what is known about HBV entry through the NTCP receptor and highlight how this knowledge has been harnessed to build new animal models for the study of HBV pathogenesis and curative therapies.

Diverse Effects of the NTCP p.Ser267Phe Variant on Disease Progression During Chronic HBV Infection and on HBV preS1 Variability

The sodium taurocholate co-transporting polypeptide (NTCP) acts as a cellular receptor for the hepatitis B virus (HBV) infection on host hepatocytes. We aim to investigate how the NTCP p.Ser267Phe variant affects HBV-related disease progression and analyze viral genomic variability under a host genetic background carrying the p.Ser267Phe variant. A total of 3187 chronic hepatitis B (CHB) patients were enrolled and genotyped for the p.Ser267Phe variant. The variant's association with disease progression was evaluated by logistic regression analysis. We also enrolled 83 treatment-naive CHB patients to analyze the variability of the HBV preS1 region. The frequency of the NTCP p.Ser267Phe variant was significantly lower in patients diagnosed with acute-on-chronic liver failure [OR (95% CI) = 0.33 (0.18-0.58), P = 1.34 × 10-4], cirrhosis [OR (95% CI) = 0.47 (0.31-0.72), P = 4.04 × 10-4], and hepatocellular carcinoma [OR (95% CI) = 0.54 (0.34-0.86), P = 9.83 × 10-3] as compared with CHB controls under the additive model after adjustment. Furthermore, the percentage of amino acid mutations in HBV preS1 region was significantly higher in the NTCP p.Ser267Phe heterozygote group than in the NTCP wild type homozygote group (P < 0.05). We herein demonstrate that the NTCP p.Ser267Phe variant is a protective factor reducing CHB patient risk for liver failure, cirrhosis, and hepatocellular carcinoma. A host genetic background carrying NTCP p.Ser267Phe exerts selective pressure on the virus, leading to more variability.

A Single Adaptive Mutation in Sodium Taurocholate Cotransporting Polypeptide Induced by Hepadnaviruses Determines Virus Species Specificity

Hepatitis B virus (HBV) and its hepadnavirus relatives infect a wide range of vertebrates, from fish to human. Hepadnaviruses and their hosts have a long history of acquiring adaptive mutations. However, there are no reports providing direct molecular evidence for such a coevolutionary "arms race" between hepadnaviruses and their hosts. Here, we present evidence suggesting that the adaptive evolution of the sodium taurocholate cotransporting polypeptide (NTCP), an HBV receptor, has been influenced by virus infection. Evolutionary analysis of the NTCP-encoding genes from 20 mammals showed that most NTCP residues are highly conserved among species, exhibiting evolution under negative selection (dN/dS ratio [ratio of nonsynonymous to synonymous evolutionary changes] of <1); this observation implies that the evolution of NTCP is restricted by maintaining its original protein function. However, 0.7% of NTCP amino acid residues exhibit rapid evolution under positive selection (dN/dS ratio of >1). Notably, a substitution at amino acid (aa) 158, a positively selected residue, converting the human NTCP to a monkey-type sequence abrogated the capacity to support HBV infection; conversely, a substitution at this residue converting the monkey Ntcp to the human sequence was sufficient to confer HBV susceptibility. Together, these observations suggested a close association of the aa 158 positive selection with the pressure by virus infection. Moreover, the aa 158 sequence determined attachment of the HBV envelope protein to the host cell, demonstrating the mechanism whereby HBV infection would create positive selection at this NTCP residue. In summary, we provide the first evidence in agreement with the function of hepadnavirus as a driver for inducing adaptive mutation in host receptor.IMPORTANCE HBV and its hepadnavirus relatives infect a wide range of vertebrates, with a long infectious history (hundreds of millions of years). Such a long history generally allows adaptive mutations in hosts to escape from infection while simultaneously allowing adaptive mutations in viruses to overcome host barriers. However, there is no published molecular evidence for such a coevolutionary arms race between hepadnaviruses and hosts. In the present study, we performed coevolutionary phylogenetic analysis between hepadnaviruses and the sodium taurocholate cotransporting polypeptide (NTCP), an HBV receptor, combined with virological experimental assays for investigating the biological significance of NTCP sequence variation. Our data provide the first molecular evidence supporting that HBV-related hepadnaviruses drive adaptive evolution in the NTCP sequence, including a mechanistic explanation of how NTCP mutations determine host viral susceptibility. Our novel insights enhance our understanding of how hepadnaviruses evolved with their hosts, permitting the acquisition of strong species specificity.