The kinesin KIF4 mediates HBV/HDV entry through the regulation of surface NTCP localization and can be targeted by RXR agonists in vitro

Hepatitis B and D virus entry

Hepatitis B virus (HBV) entry is the initial step of viral infection, leading to the formation of covalently closed circular DNA, which is a molecular reservoir of viral persistence and a key obstacle for HBV cure. The restricted entry of HBV into specific cell types determines the nature of HBV, which has a narrow host range in tissues and species. Hepatitis D virus (HDV) shares viral surface antigens with HBV and thus follows a similar entry mechanism at its early stages. In late 2012, sodium taurocholate cotransporting polypeptide was discovered as an HBV and HDV entry receptor. Since then, the mechanisms of HBV and HDV entry have been extensively analysed. These analyses have expanded our understanding of HBV and HDV host tropism and have provided new strategies for the development of antiviral agents. Notably, the structures of sodium taurocholate cotransporting polypeptide and its interaction with the 2-48 amino acid region of viral preS1 have been recently solved. These findings will stimulate further entry studies. In this Review, we summarize current understanding of HBV and HDV entry and future perspectives.

Classical swine fever virus recruits ALIX and ESCRT-III to facilitate viral budding

Classical swine fever virus (CSFV) incurs substantial economic losses in the global swine industry due to its persistent emergence and re-emergence across various countries. However, the precise mechanisms governing CSFV budding remain inadequately understood. Our study elucidates that the endosomal sorting complex required for transport (ESCRT)-associated protein ALIX, in conjunction with ESCRT-III, plays a pivotal role in orchestrating CSFV budding. Genomic sequence analysis identified a critical interaction between the YPXnL late domain on the E2 protein and ALIX. Through immunoprecipitation and structural domain deletion assays, we demonstrated that the ALIX Bro1 domain specifically recognized viral particles by binding to the YPXnL motif. Immunoelectron and transmission electron microscopy further confirmed that, upon infection, ALIX accumulated at the periphery of subcellular organelles, including COPII vesicles, endosomes, and the Golgi apparatus, thereby facilitating CSFV budding. Our findings also revealed that ESCRT-III subunits CHMP2B, CHMP4B, CHMP7, and VPS4A interacted with ALIX to expedite CSFV budding. Notably, Rab8 activated by Kif4A contributed to the release of CSFV particles by interacting with ALIX and directing ALIX-containing vesicles along microtubules toward the cytosol. Our study demonstrates that ALIX specifically recognizes E2 and orchestrates the recruitment of ESCRT-III and Rab8 to facilitate the vesicular budding of CSFV particles from the Golgi apparatus to the cytosol. Ultimately, virus-laden vesicles propelled by Kif4A are transported along microtubules to the plasma membrane for release. Our findings offer the first comprehensive elucidation of the CSFV budding process and contribute to the identification of antiviral targets, thereby advancing the development of antiviral therapeutics.IMPORTANCEThe endosomal sorting complex required for transport (ESCRT) machinery plays a pivotal role in the sorting of membrane proteins in eukaryotic cells and regulating various stages of infection for numerous viruses. Previous studies have underscored the indispensable role of ESCRT in the cellular entry and replication of classical swine fever virus (CSFV). However, the precise mechanisms by which ESCRT recognizes CSFV particles and initiates viral vesicle budding have remained elusive. This study reveals that the Bro1 domain of ALIX initiates viral budding proximal to the Golgi apparatus by specifically recognizing the YPXnL late domain on the CSFV E2 protein. Mechanistically, ALIX and ESCRT-III facilitate Rab8-regulated endosomal transport of CSFV particles from the Golgi apparatus to the plasma membrane. Subsequently, virions are propelled by the kinesin Kif4A along microtubules for egress into the extracellular space. In summary, these findings significantly advance our understanding of CSFV pathogenesis and offer valuable insights into the vesicular transport and budding mechanisms of CSFV particles.

Novel mechanistic insights - A brand new Era for anti-HBV drugs

Hepatitis B Virus (HBV) remains a critical global health issue, with substantial morbidity and mortality. Current therapies, including interferons and nucleoside analogs, often fail to achieve complete cure or functional eradication. This review explores recent advances in anti-HBV agents, focusing on their innovative mechanisms of action. HBV entry inhibitors target the sodium taurocholate cotransporting polypeptide (NTCP) receptor, impeding viral entry, while nucleus translocation inhibitors disrupt key viral life cycle steps, preventing replication. Capsid assembly modulators inhibit covalently closed circular DNA (cccDNA) formation, aiming to eradicate the persistent viral reservoir. Transcription inhibitors targeting cccDNA and integrated DNA offer significant potential to suppress HBV replication. Immunomodulatory agents are highlighted for their ability to enhance host immune responses, facil-itating better control and possible eradication of HBV. These novel approaches represent significant advancements in HBV therapy, providing new strategies to overcome current treatment limitations. The development of cccDNA reducers is particularly critical, as they directly target the persistent viral reservoir, offering a promising pathway towards achieving a functional cure or complete viral eradication. Continued research in this area is essential to advance the effectiveness of anti-HBV therapies.

The Culprit Behind HBV-Infected Hepatocytes: NTCP

Hepatitis B virus (HBV) is a globally prevalent human DNA virus responsible for over 250 million cases of chronic liver infections, leading to conditions such as liver inflammation, cirrhosis and hepatocellular carcinoma (HCC). Sodium taurocholate co-transporting polypeptide (NTCP) is a transmembrane protein highly expressed in human hepatocytes and functions as a bile acid (BA) transporter. NTCP has been identified as the receptor that HBV and its satellite virus, hepatitis delta virus (HDV), use to enter hepatocytes. HBV entry into hepatocytes is tightly regulated by various signaling pathways, and NTCP plays an important role as the initial stage of HBV infection. NTCP acts as an initiation signal, causing metabolic changes in hepatocytes and facilitating the entry of HBV into hepatocytes. Thus, a comprehensive understanding of NTCP's role is crucial. In this review, we will examine the regulatory mechanisms governing HBV pre-S1 binding to liver membrane NTCP, the role of NTCP in HBV internalization, and the transcriptional and translational regulation of NTCP expression. Additionally, we will discuss clinical drugs targeting NTCP, including combination therapies involving NTCP inhibitors, and consider the safety of NTCP as a therapeutic target.

Identification of Host Proteins Involved in Hepatitis B Virus Genome Packaging

A critical part of the hepatitis B virus (HBV) life cycle is the packaging of the pregenomic RNA (pgRNA) into nucleocapsids. While this process is known to involve several viral elements, much less is known about the identities and roles of host proteins in this process. To better understand the role of host proteins, we isolated pgRNA and characterized its protein interactome in cells expressing either packaging-competent or packaging-incompetent HBV genomes. We identified over 250 host proteins preferentially associated with pgRNA from the packaging-competent version of the virus. These included proteins already known to support capsid formation, enhance viral gene expression, catalyze nucleocapsid dephosphorylation, and bind to the viral genome, demonstrating the ability of the approach to effectively reveal functionally significant host-virus interactors. Three of these host proteins, AURKA, YTHDF2, and ATR, were selected for follow-up analysis. RNA immunoprecipitation qPCR (RIP-qPCR) confirmed pgRNA-protein association in cells, and siRNA knockdown of the proteins showed decreased encapsidation efficiency. This study provides a template for the use of comparative RNA-protein interactome analysis in conjunction with virus engineering to reveal functionally significant host-virus interactions.

Targeting NTCP for liver disease treatment: A promising strategy

The sodium taurocholate co-transporting polypeptide (NTCP), a bile acids transporter, has been identified as a new therapeutic target for the treatment of liver disease. This paper thoroughly investigates the function of NTCP for regulating bile acid regulation, its correlation with hepatitis B and D infections, and its association with various liver diseases. Additionally, in this review we examine recent breakthroughs in creating NTCP inhibitors and their prospective applications in liver disease treatment. While this review emphasizes the promising potential of targeting NTCP, it concurrently underscores the need for broader and more detailed research to fully understand the long-term implications and potential side effects associated with NTCP inhibition.

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.

FREEDA: An automated computational pipeline guides experimental testing of protein innovation

Cell biologists typically focus on conserved regions of a protein, overlooking innovations that can shape its function over evolutionary time. Computational analyses can reveal potential innovations by detecting statistical signatures of positive selection that lead to rapid accumulation of beneficial mutations. However, these approaches are not easily accessible to non-specialists, limiting their use in cell biology. Here, we present an automated computational pipeline FREEDA that provides a simple graphical user interface requiring only a gene name; integrates widely used molecular evolution tools to detect positive selection in rodents, primates, carnivores, birds, and flies; and maps results onto protein structures predicted by AlphaFold. Applying FREEDA to >100 centromere proteins, we find statistical evidence of positive selection within loops and turns of ancient domains, suggesting innovation of essential functions. As a proof-of-principle experiment, we show innovation in centromere binding of mouse CENP-O. Overall, we provide an accessible computational tool to guide cell biology research and apply it to experimentally demonstrate functional innovation.

Interactions of Na+/taurocholate cotransporting polypeptide with host cellular proteins upon hepatitis B and D virus infection: novel potential targets for antiviral therapy

Na⁺/taurocholate cotransporting polypeptide (NTCP) is a member of the solute carrier (SLC) family 10 transporters (gene symbol SLC10A1) and is responsible for the sodium-dependent uptake of bile salts across the basolateral membrane of hepatocytes. In addition to its primary transporter function, NTCP is the high-affinity hepatic receptor for hepatitis B (HBV) and hepatitis D (HDV) viruses and, therefore, is a prerequisite for HBV/HDV virus entry into hepatocytes. The inhibition of HBV/HDV binding to NTCP and internalization of the virus/NTCP receptor complex has become a major concept in the development of new antiviral drugs called HBV/HDV entry inhibitors. Hence, NTCP has emerged as a promising target for therapeutic interventions against HBV/HDV infections in the last decade. In this review, recent findings on protein-protein interactions (PPIs) between NTCP and cofactors relevant for entry of the virus/NTCP receptor complex are summarized. In addition, strategies aiming to block PPIs with NTCP to dampen virus tropism and HBV/HDV infection rates are discussed. Finally, this article suggests novel directions for future investigations evaluating the functional contribution of NTCP-mediated PPIs in the development and progression of HBV/HDV infection and subsequent chronic liver disorders.

FREEDA: an automated computational pipeline guides experimental testing of protein innovation by detecting positive selection

Cell biologists typically focus on conserved regions of a protein, overlooking innovations that can shape its function over evolutionary time. Computational analyses can reveal potential innovations by detecting statistical signatures of positive selection that leads to rapid accumulation of beneficial mutations. However, these approaches are not easily accessible to non-specialists, limiting their use in cell biology. Here, we present an automated computational pipeline FREEDA (Finder of Rapidly Evolving Exons in De novo Assemblies) that provides a simple graphical user interface requiring only a gene name, integrates widely used molecular evolution tools to detect positive selection, and maps results onto protein structures predicted by AlphaFold. Applying FREEDA to >100 mouse centromere proteins, we find evidence of positive selection in intrinsically disordered regions of ancient domains, suggesting innovation of essential functions. As a proof-of-principle experiment, we show innovation in centromere binding of CENP-O. Overall, we provide an accessible computational tool to guide cell biology research and apply it to experimentally demonstrate functional innovation.