Primary hepatocytes of Tupaia belangeri as a potential model for hepatitis C virus infection

Tree Shrew as an Emerging Small Animal Model for Human Viral Infection: A Recent Overview

Viral infection is a global public health threat causing millions of deaths. A suitable small animal model is essential for viral pathogenesis and host response studies that could be used in antiviral and vaccine development. The tree shrew (Tupaia belangeri or Tupaia belangeri chinenesis), a squirrel-like non-primate small mammal in the Tupaiidae family, has been reported to be susceptible to important human viral pathogens, including hepatitis viruses (e.g., HBV, HCV), respiratory viruses (influenza viruses, SARS-CoV-2, human adenovirus B), arboviruses (Zika virus and dengue virus), and other viruses (e.g., herpes simplex virus, etc.). The pathogenesis of these viruses is not fully understood due to the lack of an economically feasible suitable small animal model mimicking natural infection of human diseases. The tree shrew model significantly contributes towards a better understanding of the infection and pathogenesis of these important human pathogens, highlighting its potential to be used as a viable viral infection model of human viruses. Therefore, in this review, we summarize updates regarding human viral infection in the tree shrew model, which highlights the potential of the tree shrew to be utilized for human viral infection and pathogenesis studies.

Tree Shrew Cells Transduced with Human CD4 and CCR5 Support Early Steps of HIV-1 Replication, but Viral Infectivity Is Restricted by APOBEC3

The host range of human immunodeficiency virus type 1 (HIV-1) is narrow. Therefore, using ordinary animal models to study HIV-1 replication, pathogenesis, and therapy is impractical. The lack of applicable animal models for HIV-1 research spurred our investigation on whether tree shrews (Tupaia belangeri chinensis), which are susceptible to many types of human viruses, can act as an animal model for HIV-1. Here, we report that tree shrew primary cells are refractory to wild-type HIV-1 but support the early replication steps of HIV-1 pseudotyped with the vesicular stomatitis virus glycoprotein envelope (VSV-G), which can bypass entry receptors. The exogenous expression of human CD4 renders the tree shrew cell line infectible to X4-tropic HIV-1IIIB, suggesting that tree shrew CXCR4 is a functional HIV-1 coreceptor. However, tree shrew cells did not produce infectious HIV-1 progeny virions, even with the human CD4 receptor. Subsequently, we identified tree shrew (ts) apolipoprotein B editing catalytic polypeptide 3 (tsAPOBEC3) proteins as active inhibitors of HIV-1 particle infectivity, with virus infectivity reduced 10- to 1,000-fold. Unlike human APOBEC3G, the tsA3Z2c-Z1b protein was not degraded by the HIV-1 viral infectivity factor (Vif) but markedly restricted HIV-1 replication through mutagenicity and reverse transcription inhibition. The pooled knockout of tsA3Z2c-Z1b partially restored the infectivity of the HIV-1 progeny. This work suggests that tsAPOBEC3 proteins serve as an additional barrier to the development of HIV-1 tree shrew models, even when virus entry is overcome by exogenous expression of human CD4. IMPORTANCE The development of animal models is critical for studying human diseases and their pathogenesis and for evaluating drug and vaccine efficacy. For improved AIDS research, the ideal animal model of HIV-1 infection should be a small laboratory mammal that closely mimics virus replication in humans. Tree shrews exhibit considerable potential as animal models for the study of human diseases and therapeutic responses. Here, we report that human CD4-expressing tree shrew cells support the early steps of HIV-1 replication and that tree shrew CXCR4 is a functional coreceptor of HIV-1. However, tree shrew cells harbor additional restrictions that lead to the production of HIV-1 virions with low infectivity. Thus, the tsAPOBEC3 proteins are partial barriers to developing tree shrews as an HIV-1 model. Our results provide insight into the genetic basis of HIV inhibition in tree shrews and build a foundation for the establishment of gene-edited tree shrew HIV-1-infected models.

Comparing the hippocampal miRNA expression profiles of wild and domesticated Chinese tree shrews (Tupaia belangeri chinensis)

Background

The domestication of tree shrews represents an important advance in the development of standardized laboratory animals. Little is known regarding the miRNA changes that accompany the transformation of wild tree shrews into domestic tree shrews.

Results

By performing miRNA-seq analysis on wild and domestic tree shrews, we identified 2410 miRNAs and 30 differentially expressed miRNAs in the hippocampus during tree shrew domestication. A KEGG analysis of the differentially expressed genes showed that the differentially expressed miRNAs were associated with ECM-receptor interaction, the phosphatidylinositol signaling system, protein digestion and absorption, inositol phosphate metabolism, lysine degradation, fatty acid degradation and focal adhesion. Most of these pathways could be classified under environmental information processing, organismal systems and metabolism. The miRNAs exclusively expressed in wild and tame tree shrews GO enriched in terms of divergent functions. The miRNA-mRNA networks suggested that novel-m1388-5p and novel-m0746-5p might play regulatory roles in domestication of tree shrews. Real–time RT-PCR analysis was employed to verify the presence of these miRNAs.

Conclusion

We identified a number of candidate miRNA-regulated domestication genes that may represent targets for selection during the domestication of tree shrews.

Establishment and transcriptomic features of an immortalized hepatic cell line of the Chinese tree shrew

BACKGROUND

The Chinese tree shrew (Tupaia belangeri chinesis) is a rising experimental animal and has been used for studying a variety of human diseases, such as metabolic and viral infectious diseases.

METHODS

In this study, we established an immortalized tree shrew hepatic cell line, ITH6.1, by introducing the simian virus 40 large T antigen gene into primary tree shrew hepatocytes (PTHs).

RESULTS

The ITH6.1 cell line had a stable cell morphology and proliferation activity. This cell line could be infected by enterovirus 71 (EV71), but not hepatitis C virus (HCV), although the known HCV entry factors, including CD81, SR-BI, CLDN1 and OCLN, were all expressed in the PTHs and ITH6.1 of different passages. Comparison of the transcriptomic features of the PTHs and different passages of the ITH6.1 cells revealed the dynamic gene expression profiles during the transformation. We found that the DNA replication- and cell cycle-related genes were upregulated, whereas the metabolic pathway-related genes were downregulated in early passages of immortalized hepatocytes compared to the PTHs. Furthermore, expression of hepatocytes function-related genes were repressed in ITH6.1 compared to that of PTHs.

CONCLUSION

We believe these cellular expression alterations might cause the resistance of the ITH6.1 cell to HCV infection. This tree shrew liver cell line may be a good resource for the field.

KEY POINTS

• A tree shrew hepatic cell line (ITH6.1) was established. • ITH6.1 cells could be infected by EV71, but not HCV. • ITH6.1 had an altered expression profiling compared to the primary hepatocytes.

Cytokine-Mediated Tissue Injury in Non-human Primate Models of Viral Infections

Viral infections trigger robust secretion of interferons and other antiviral cytokines by infected and bystander cells, which in turn can tune the immune response and may lead to viral clearance or immune suppression. However, aberrant or unrestricted cytokine responses can damage host tissues, leading to organ dysfunction, and even death. To understand the cytokine milieu and immune responses in infected host tissues, non-human primate (NHP) models have emerged as important tools. NHP have been used for decades to study human infections and have played significant roles in the development of vaccines, drug therapies and other immune treatment modalities, aided by an ability to control disease parameters, and unrestricted tissue access. In addition to the genetic and physiological similarities with humans, NHP have conserved immunologic properties with over 90% amino acid similarity for most cytokines. For example, human-like symptomology and acute respiratory syndrome is found in cynomolgus macaques infected with highly pathogenic avian influenza virus, antibody enhanced dengue disease is common in neotropical primates, and in NHP models of viral hepatitis cytokine-induced inflammation induces severe liver damage, fibrosis, and hepatocellular carcinoma recapitulates human disease. To regulate inflammation, anti-cytokine therapy studies in NHP are underway and will provide important insights for future human interventions. This review will provide a comprehensive outline of the cytokine-mediated exacerbation of disease and tissue damage in NHP models of viral infections and therapeutic strategies that can aid in prevention/treatment of the disease syndromes.

The characterization of auditory brainstem response (ABR) waveforms: A study in tree shrews (Tupaia belangeri)

To characterize the patterns of ABR waves in tree shrews, we must understand the hearing sensitivity and auditory function of healthy adult tree shrews. Fifteen tree shrews (30 ears) were stimulated with clicks and tone-pips at 11 different frequencies from 1 to 60 kHz. The ABR waves were recorded and analyzed. The ABR consisted of five to seven positive waves in the first 10 ms after a click stimulus, and the average hearing threshold of component III was 27.86 ± 3.78 dB SPL. Wave III was the largest and most clear. The ABR threshold was related to the tone-pip sitmulus by a “U” shaped curve. The sensitive frequency was approximately 8 kHz in tree shrews. The latencies systematically decreased with increasing stimulus frequencies. The ABR amplitudes of wave III increased as the sound pressure level increased. All of these results provide an empirical basis for future studies of hearing diseases in tree shrews.

CD81 Receptor Regions outside the Large Extracellular Loop Determine Hepatitis C Virus Entry into Hepatoma Cells

Hepatitis C virus (HCV) enters human hepatocytes using four essential entry factors, one of which is human CD81 (hCD81). The tetraspanin hCD81 contains a large extracellular loop (LEL), which interacts with the E2 glycoprotein of HCV. The role of the non-LEL regions of hCD81 (intracellular tails, four transmembrane domains, small extracellular loop and intracellular loop) is poorly understood. Here, we studied the contribution of these domains to HCV susceptibility of hepatoma cells by generating chimeras of related tetraspanins with the hCD81 LEL. Our results show that non-LEL regions in addition to the LEL determine susceptibility of cells to HCV. While closely related tetraspanins (X. tropicalis CD81 and D. rerio CD81) functionally complement hCD81 non-LEL regions, distantly related tetraspanins (C. elegans TSP9 amd D. melanogaster TSP96F) do not and tetraspanins with intermediate homology (hCD9) show an intermediate phenotype. Tetraspanin homology and susceptibility to HCV correlate positively. For some chimeras, infectivity correlates with surface expression. In contrast, the hCD9 chimera is fully surface expressed, binds HCV E2 glycoprotein but is impaired in HCV receptor function. We demonstrate that a cholesterol-coordinating glutamate residue in CD81, which hCD9 lacks, promotes HCV infection. This work highlights the hCD81 non-LEL regions as additional HCV susceptibility-determining factors.

Tree shrew (Tupaia belangeri) as a novel laboratory disease animal model

The tree shrew (Tupaia belangeri) is a promising laboratory animal that possesses a closer genetic relationship to primates than to rodents. In addition, advantages such as small size, easy breeding, and rapid reproduction make the tree shrew an ideal subject for the study of human disease. Numerous tree shrew disease models have been generated in biological and medical studies in recent years. Here we summarize current tree shrew disease models, including models of infectious diseases, cancers, depressive disorders, drug addiction, myopia, metabolic diseases, and immune-related diseases. With the success of tree shrew transgenic technology, this species will be increasingly used in biological and medical studies in the future.

Creating animal models, why not use the Chinese tree shrew (Tupaia belangeri chinensis)?

The Chinese tree shrew (Tupaia belangeri chinensis), a squirrel-like and rat-sized mammal, has a wide distribution in Southeast Asia, South and Southwest China and has many unique characteristics that make it suitable for use as an experimental animal. There have been many studies using the tree shrew (Tupaia belangeri) aimed at increasing our understanding of fundamental biological mechanisms and for the modeling of human diseases and therapeutic responses. The recent release of a publicly available annotated genome sequence of the Chinese tree shrew and its genome database (www.treeshrewdb.org) has offered a solid base from which it is possible to elucidate the basic biological properties and create animal models using this species. The extensive characterization of key factors and signaling pathways in the immune and nervous systems has shown that tree shrews possess both conserved and unique features relative to primates. Hitherto, the tree shrew has been successfully used to create animal models for myopia, depression, breast cancer, alcohol-induced or non-alcoholic fatty liver diseases, herpes simplex virus type 1 (HSV-1) and hepatitis C virus (HCV) infections, to name a few. The recent successful genetic manipulation of the tree shrew has opened a new avenue for the wider usage of this animal in biomedical research. In this opinion paper, I attempt to summarize the recent research advances that have used the Chinese tree shrew, with a focus on the new knowledge obtained by using the biological properties identified using the tree shrew genome, a proposal for the genome-based approach for creating animal models, and the genetic manipulation of the tree shrew. With more studies using this species and the application of cutting-edge gene editing techniques, the tree shrew will continue to be under the spot light as a viable animal model for investigating the basis of many different human diseases.

Tree shrew, a potential animal model for hepatitis C, supports the infection and replication of HCV in vitro and in vivo

The tree shrew (Tupaia belangeri chinensis), a small animal widely distributed in Southeast Asia and southwest China, has the potential to be developed as an animal model for hepatitis C. To determine the susceptibility of the tree shrew to hepatitis C virus (HCV) infection in vitro and in vivo, a well-established HCV, produced from the J6/JFH1-Huh7.5.1 culture system, was used to infect cultured primary tupaia hepatocytes (PTHs) and tree shrews. The in vitro results showed that HCV genomic RNA and HCV-specific nonstructural protein 5A (NS5A) could be detected in the PTH cell culture from days 3-15 post-infection, although the viral load was lower than that observed in Huh7.5.1 cell culture. The occurrence of five sense mutations [S391A, G397A, L402F and M405T in the hypervariable region 1 (HVR1) of envelope glycoprotein 2 and I2750M in NS5B] suggested that HCV undergoes genetic evolution during culture. Fourteen of the 30 experimental tree shrews (46.7 %) were found to be infected, although the HCV viremia was intermittent in vivo. A positive test for HCV RNA in liver tissue provided stronger evidence for HCV infection and replication in tree shrews. The results of an immunohistochemistry assay also demonstrated the presence of four HCV-specific proteins (Core, E2, NS3/4 and NS5A) in the hepatocytes of infected tree shrews. The pathological changes observed in the liver tissue of infected tree shrews could be considered to be representative symptoms of mild hepatitis. These results revealed that the tree shrew can be used as an animal model supporting the infection and replication of HCV in vitro and in vivo.

Bone Marrow Mesenchymal Stem-Cell Transplantation Promotes Functional Improvement Associated with CNTF-STAT3 Activation after Hemi-Sectioned Spinal Cord Injury in Tree Shrews

Hemi-sectioned spinal cord injury (hSCI) can lead to spastic paralysis on the injured side, as well as flaccid paralysis on the contralateral side, which can negatively affect a patient’s daily life. Stem-cell therapy may offer an effective treatment option for individuals with hSCI. To examine the role of bone marrow mesenchymal stem cells (BMSCs) transplantation on hSCI and explore related mechanisms in the tree shrews, here, we created a model of hSCI by inducing injury at the tenth thoracic vertebra (T10). Hoechst 33342-labeled BMSCs derived from adult tree shrews were isolated, cultured, and implanted into the spinal cord around the injury site at 9 days after injury. The isolated BMSCs were able to survive, proliferate and release a variety of neurotrophic factors (NTFs) both in vitro and in vivo. At 28 days after injury, compared with the sham group, the hSCI group displayed scar formation and dramatic elevations in the mean interleukin 1 beta (IL-1β) density and cell apoptosis level, whereas the expression of signal transducer and activator of transcription 3 (STAT3) and ciliary neurotrophic factor (CNTF) mRNA was reduced. Following BMSC transplantation, motoneurons extent of shrinkage were reduced and the animals’ Basso, Beattie, and Bresnahan (BBB) locomotion scale scores were significantly higher at 21 and 28 days after injury when compared with the injured group. Moreover, the hSCI-induced elevations in scar formation, IL-1β, and cell apoptosis were reduced by BMSC transplantation to levels that were close to those of the sham group. Corresponding elevations in the expression of STAT3 and CNTF mRNA were observed in the hSCI + BMSCs group, and the levels were not significantly different from those observed in the sham group. Together, our results support that grafted BMSCs can significantly improve locomotor function in tree shrews subjected to hSCI and that this improvement is associated with the upregulation of CNTF and STAT3 signaling.

The Mouse Lemur, a Genetic Model Organism for Primate Biology, Behavior, and Health

Systematic genetic studies of a handful of diverse organisms over the past 50 years have transformed our understanding of biology. However, many aspects of primate biology, behavior, and disease are absent or poorly modeled in any of the current genetic model organisms including mice. We surveyed the animal kingdom to find other animals with advantages similar to mice that might better exemplify primate biology, and identified mouse lemurs (Microcebus spp.) as the outstanding candidate. Mouse lemurs are prosimian primates, roughly half the genetic distance between mice and humans. They are the smallest, fastest developing, and among the most prolific and abundant primates in the world, distributed throughout the island of Madagascar, many in separate breeding populations due to habitat destruction. Their physiology, behavior, and phylogeny have been studied for decades in laboratory colonies in Europe and in field studies in Malagasy rainforests, and a high quality reference genome sequence has recently been completed. To initiate a classical genetic approach, we developed a deep phenotyping protocol and have screened hundreds of laboratory and wild mouse lemurs for interesting phenotypes and begun mapping the underlying mutations, in collaboration with leading mouse lemur biologists. We also seek to establish a mouse lemur gene "knockout" library by sequencing the genomes of thousands of mouse lemurs to identify null alleles in most genes from the large pool of natural genetic variants. As part of this effort, we have begun a citizen science project in which students across Madagascar explore the remarkable biology around their schools, including longitudinal studies of the local mouse lemurs. We hope this work spawns a new model organism and cultivates a deep genetic understanding of primate biology and health. We also hope it establishes a new and ethical method of genetics that bridges biological, behavioral, medical, and conservation disciplines, while providing an example of how hands-on science education can help transform developing countries.

Long-term propagation of tree shrew spermatogonial stem cells in culture and successful generation of transgenic offspring

Tree shrews have a close relationship to primates and have many advantages over rodents in biomedical research. However, the lack of gene manipulation methods has hindered the wider use of this animal. Spermatogonial stem cells (SSCs) have been successfully expanded in culture to permit sophisticated gene editing in the mouse and rat. Here, we describe a culture system for the long-term expansion of tree shrew SSCs without the loss of stem cell properties. In our study, thymus cell antigen 1 was used to enrich tree shrew SSCs. RNA-sequencing analysis revealed that the Wnt/β-catenin signaling pathway was active in undifferentiated SSCs, but was downregulated upon the initiation of SSC differentiation. Exposure of tree shrew primary SSCs to recombinant Wnt3a protein during the initial passages of culture enhanced the survival of SSCs. Use of tree shrew Sertoli cells, but not mouse embryonic fibroblasts, as feeder was found to be necessary for tree shrew SSC proliferation, leading to a robust cell expansion and long-term culture. The expanded tree shrew SSCs were transfected with enhanced green fluorescent protein (EGFP)-expressing lentiviral vectors. After transplantation into sterilized adult male tree shrew's testes, the EGFP-tagged SSCs were able to restore spermatogenesis and successfully generate transgenic offspring. Moreover, these SSCs were suitable for the CRISPR/Cas9-mediated gene modification. The development of a culture system to expand tree shrew SSCs in combination with a gene editing approach paves the way for precise genome manipulation using the tree shrew.

Hepatitis B virus molecular biology and pathogenesis

As obligate intracellular parasites, viruses need a host cell to provide a milieu favorable to viral replication. Consequently, viruses often adopt mechanisms to subvert host cellular signaling processes. While beneficial for the viral replication cycle, virus-induced deregulation of host cellular signaling processes can be detrimental to host cell physiology and can lead to virus-associated pathogenesis, including, for oncogenic viruses, cell transformation and cancer progression. Included among these oncogenic viruses is the hepatitis B virus (HBV). Despite the availability of an HBV vaccine, 350-500 million people worldwide are chronically infected with HBV, and a significant number of these chronically infected individuals will develop hepatocellular carcinoma (HCC). Epidemiological studies indicate that chronic infection with HBV is the leading risk factor for the development of HCC. Globally, HCC is the second highest cause of cancer-associated deaths, underscoring the need for understanding mechanisms that regulate HBV replication and the development of HBV-associated HCC. HBV is the prototype member of the Hepadnaviridae family; members of this family of viruses have a narrow host range and predominately infect hepatocytes in their respective hosts. The extremely small and compact hepadnaviral genome, the unique arrangement of open reading frames, and a replication strategy utilizing reverse transcription of an RNA intermediate to generate the DNA genome are distinguishing features of the Hepadnaviridae. In this review, we provide a comprehensive description of HBV biology, summarize the model systems used for studying HBV infections, and highlight potential mechanisms that link a chronic HBV-infection to the development of HCC. For example, the HBV X protein (HBx), a key regulatory HBV protein that is important for HBV replication, is thought to play a cofactor role in the development of HBV-induced HCC, and we highlight the functions of HBx that may contribute to the development of HBV-associated HCC.

Detection and analysis of tupaia hepatocytes via mAbs against tupaia serum albumin

On the basis of its close phylogenetic relationship with primates, the development of Tupaia belangeri as an infection animal model and drug metabolism model could provide a new option for preclinical studies, especially in hepatitis virus research. As a replacement for primary human hepatocytes (PHHs), primary tupaia hepatocytes (PTHs) have been widely used. Similar to human serum albumin, tupaia serum albumin (TSA) is the most common liver synthesis protein and is an important biomarker for PTHs and liver function. However, no detection or quantitative method for TSA has been reported. In this study, mouse monoclonal antibodies (mAbs) 4G5 and 9H3 against TSA were developed to recognize PTHs, and they did not show cross-reactivity with serum albumin from common experimental animals, such as the mouse, rat, cow, rabbit, goat, monkey, and chicken. The two mAbs also exhibited good performance in fluorescence activated cell sorting (FACS) analysis and immunofluorescence (IF) detection of PTHs. A chemiluminescent enzyme immune assay method using the two mAbs, with a linear range from 96.89 pg/ml to 49,609.38 pg/ml, was developed for the quantitative detection of TSA. The mAbs and the CLEIA method provide useful tools for research on TSA and PTHs.

Tree shrew database (TreeshrewDB): a genomic knowledge base for the Chinese tree shrew

The tree shrew (Tupaia belangeri) is a small mammal with a close relationship to primates and it has been proposed as an alternative experimental animal to primates in biomedical research. The recent release of a high-quality Chinese tree shrew genome enables more researchers to use this species as the model animal in their studies. With the aim to making the access to an extensively annotated genome database straightforward and easy, we have created the Tree shrew Database (TreeshrewDB). This is a web-based platform that integrates the currently available data from the tree shrew genome, including an updated gene set, with a systematic functional annotation and a mRNA expression pattern. In addition, to assist with automatic gene sequence analysis, we have integrated the common programs Blast, Muscle, GBrowse, GeneWise and codeml, into TreeshrewDB. We have also developed a pipeline for the analysis of positive selection. The user-friendly interface of TreeshrewDB, which is available at http://www.treeshrewdb.org, will undoubtedly help in many areas of biological research into the tree shrew.

New Methods in Tissue Engineering: Improved Models for Viral Infection

New insights in the study of virus and host biology in the context of viral infection are made possible by the development of model systems that faithfully recapitulate the in vivo viral life cycle. Standard tissue culture models lack critical emergent properties driven by cellular organization and in vivo-like function, whereas animal models suffer from limited susceptibility to relevant human viruses and make it difficult to perform detailed molecular manipulation and analysis. Tissue engineering techniques may enable virologists to create infection models that combine the facile manipulation and readouts of tissue culture with the virus-relevant complexity of animal models. Here, we review the state of the art in tissue engineering and describe how tissue engineering techniques may alleviate some common shortcomings of existing models of viral infection, with a particular emphasis on hepatotropic viruses. We then discuss possible future applications of tissue engineering to virology, including current challenges and potential solutions.

Drug target mining and analysis of the Chinese tree shrew for pharmacological testing

The discovery of new drugs requires the development of improved animal models for drug testing. The Chinese tree shrew is considered to be a realistic candidate model. To assess the potential of the Chinese tree shrew for pharmacological testing, we performed drug target prediction and analysis on genomic and transcriptomic scales. Using our pipeline, 3,482 proteins were predicted to be drug targets. Of these predicted targets, 446 and 1,049 proteins with the highest rank and total scores, respectively, included homologs of targets for cancer chemotherapy, depression, age-related decline and cardiovascular disease. Based on comparative analyses, more than half of drug target proteins identified from the tree shrew genome were shown to be higher similarity to human targets than in the mouse. Target validation also demonstrated that the constitutive expression of the proteinase-activated receptors of tree shrew platelets is similar to that of human platelets but differs from that of mouse platelets. We developed an effective pipeline and search strategy for drug target prediction and the evaluation of model-based target identification for drug testing. This work provides useful information for future studies of the Chinese tree shrew as a source of novel targets for drug discovery research.

How hepatitis C virus invades hepatocytes: The mystery of viral entry

Hepatitis C virus (HCV) infection is a global health problem, with an estimated 170 million people being chronically infected. HCV cell entry is a complex multi-step process, involving several cellular factors that trigger virus uptake into the hepatocytes. The high- density lipoprotein receptor scavenger receptor class B type I, tetraspanin CD81, tight junction protein claudin-1, and occludin are the main receptors that mediate the initial step of HCV infection. In addition, the virus uses cell receptor tyrosine kinases as entry regulators, such as epidermal growth factor receptor and ephrin receptor A2. This review summarizes the current understanding about how cell surface molecules are involved in HCV attachment, internalization, and membrane fusion, and how host cell kinases regulate virus entry. The advances of the potential antiviral agents targeting this process are introduced.

CD81 and hepatitis C virus (HCV) infection

Hepatitis C Virus (HCV) infection is a global public health problem affecting over 160 million individuals worldwide. Its symptoms include chronic hepatitis, liver cirrhosis and hepatocellular carcinoma. HCV is an enveloped RNA virus mainly targeting liver cells and for which the initiation of infection occurs through a complex multistep process involving a series of specific cellular entry factors. This process is likely mediated through the formation of a tightly orchestrated complex of HCV entry factors at the plasma membrane. Among HCV entry factors, the tetraspanin CD81 is one of the best characterized and it is undoubtedly a key player in the HCV lifecycle. In this review, we detail the current knowledge on the involvement of CD81 in the HCV lifecycle, as well as in the immune response to HCV infection.

Animal models for hepatitis C

Hepatitis C remains a global epidemic. Approximately 3 % of the world's population suffers from chronic hepatitis C, which is caused by hepatitis C virus (HCV)-a positive sense, single-stranded RNA virus of the Flaviviridae family. HCV has a high propensity for establishing a chronic infection. If untreated chronic HCV carriers can develop severe liver disease including fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Antiviral treatment is only partially effective, costly, and poorly tolerated. A prophylactic or therapeutic vaccine for HCV does not exist. Mechanistic studies of virus-host interactions, HCV immunity, and pathogenesis as well as the development of more effective therapies have been hampered by the lack of a suitable small animal model. Besides humans, chimpanzees are the only species that is naturally susceptible to HCV infection. While experimentation in these large primates has yielded valuable insights, ethical considerations, limited availability, genetic heterogeneity, and cost limit their utility. In search for more tractable small animal models, numerous experimental approaches have been taken to recapitulate parts of the viral life cycle and/or aspects of viral pathogenesis that will be discussed in this review. Exciting new models and improvements in established models hold promise to further elucidate our understanding of chronic HCV infection.

Hepatitis C virus infection and related liver disease: the quest for the best animal model

Hepatitis C virus (HCV) is a major cause of cirrhosis and hepatocellular carcinoma (HCC) making the virus the most common cause of liver failure and transplantation. HCV is estimated to chronically affect 130 million individuals and to lead to more than 350,000 deaths per year worldwide. A vaccine is currently not available. The recently developed direct acting antivirals (DAAs) have markedly increased the efficacy of the standard of care but are not efficient enough to completely cure all chronically infected patients and their toxicity limits their use in patients with advanced liver disease, co-morbidity or transplant recipients. Because of the host restriction, which is limited to humans and non-human primates, in vivo study of HCV infection has been hampered since its discovery more than 20 years ago. The chimpanzee remains the most physiological model to study the innate and adaptive immune responses, but its use is ethically difficult and is now very restricted and regulated. The development of a small animal model that allows robust HCV infection has been achieved using chimeric liver immunodeficient mice, which are therefore not suitable for studying the adaptive immune responses. Nevertheless, these models allowed to go deeply in the comprehension of virus-host interactions and to assess different therapeutic approaches. The immunocompetent mouse models that were recently established by genetic humanization have shown an interesting improvement concerning the study of the immune responses but are still limited by the absence of the complete robust life cycle of the virus. In this review, we will focus on the relevant available animal models of HCV infection and their usefulness for deciphering the HCV life cycle and virus-induced liver disease, as well as for the development and evaluation of new therapeutics. We will also discuss the perspectives on future immunocompetent mouse models and the hurdles to their development.

Genome of the Chinese tree shrew

Chinese tree shrews (Tupaia belangeri chinensis) possess many features valuable in animals used as experimental models in biomedical research. Currently, there are numerous attempts to employ tree shrews as models for a variety of human disorders: depression, myopia, hepatitis B and C virus infections, and hepatocellular carcinoma, to name a few. Here we present a publicly available annotated genome sequence for the Chinese tree shrew. Phylogenomic analysis of the tree shrew and other mammalians highly support its close affinity to primates. By characterizing key factors and signalling pathways in nervous and immune systems, we demonstrate that tree shrews possess both shared common and unique features, and provide a genetic basis for the use of this animal as a potential model for biomedical research.

The tree shrew provides a useful alternative model for the study of influenza H1N1 virus

Background

The influenza pandemics have resulted in significant morbidity and mortality worldwide. Animal models are useful in the study of influenza virus pathogenesis. Because of various limitations in current laboratory animal models, it is essential to develop new alternative animal models for influenza virus research aimed at understanding the viral and host factors that contribute to virus infection in human.

Method

We investigated the replicative efficiency of influenza H1N1 virus (classic strain (Influenza A/PR/8/34), seasonal influenza isolate (A/Guangzhou/GIRD/02/09) and swine-origin human influenza virus (A/Guangzhou/GIRD/07/09)) at Day1,2,4,6 and 9 p.i. using TCID₅₀ and qPCR assay in tree shrew model. Body temperature was monitored in the morning and evening for 3 days before infection and for 14 days. Seroconversion was detected by determining the neutralizing antibody titers against the challenge viruses in the pre- and exposure serum samples collected before infection and at 14 days p.i., respectively. Lungs and tracheas of tree shews were collected at day 14 post p.i. for histopathological analysis. Lectinhistochemistry analysis was conducted to identify the distribution of SAα2,3 Gal and SAα2,6 Gal receptors in the lung and trachea.

Results

The infected tree shrew displayed mild or moderate systemic and respiratory symptoms and pathological changes in respiratory tracts. The human H1N1 influenza virus may replicate in the upper respiratory tract of tree shrews. Analysis of the receptors distribution in the respiratory tract of tree shrews by lectinhistochemistry showed that sialic acid (SA)α2,6-Gal receptors were widely distributed in the trachea and nasal mucosa, whereas (SA)α2,3-Gal receptor was the main receptor in the lung tissue.

Conclusions

Based on these findings, tree shrew seemed to mimic well influenza virus infection in humans. We propose that tree shrews could be a useful alternative mammalian model to study pathogenesis of influenza H1N1 virus.

Animal models for the study of hepatitis C virus infection and replication

Hepatitis C virus (HCV) hepatitis, initially termed non-A, non-B hepatitis, has become one of the leading causes of cirrhosis and hepatocellular carcinoma worldwide. With the help of animal models, our understanding of the virus has grown substantially from the time of initial discovery. There is a paucity of available animal models for the study of HCV, mainly because of the selective susceptibility limited to humans and primates. Recent work has focused modification of animals to permit HCV entry, replication and transmission. In this review, we highlight the currently available models for the study of HCV including chimpanzees, tupaia, mouse and rat models. Discussion will include methods of model design as well as the advantages and disadvantages of each model. Particular focus is dedicated to knowledge of pathophysiologic mechanisms of HCV infection that have been elucidated through animal studies. Research within animal models is critically important to establish a complete understanding of HCV infection, which will ultimately form the basis for future treatments and prevention of disease.

Advancement in the development of models for hepatitis C research

Hepatitis C virus (HCV) is a pandemic disease affecting an estimated 180 million individuals worldwide and infecting each year another ~3-4 million people making HCV a global public health issue. HCV is the main cause for chronic hepatitis, cirrhosis, and hepatocellular carcinoma. In the United States, HCV-related chronic liver disease is a leading cause of liver transplantation. Despite significant improvements in antiviral drugs, only ~50% of treated patients with HCV have viral clearance after treatment. Showing unique species specificity, HCV has a narrow range of potential hosts infecting only chimpanzees and humans. For decades, the chimpanzee model has been the only and instrumental primate for studying HCV infection; however, availability, economic, and ethical issues make the chimpanzee an unsuitable animal model today. Thus, significant research has been devoted to explore different models that are suitable in studying the biology of the virus and application in the clinical research for developing efficient and tolerable treatments for patients. This review focuses on experimental models that have been developed to date and their findings related to HCV.

Tupaia CD81, SR-BI, claudin-1, and occludin support hepatitis C virus infection

Hepatitis C virus (HCV)-related research has been hampered by the lack of appropriate small-animal models. It has been reported that tree shrews, or tupaias (Tupaia belangeri), can be infected with serum-derived HCV. However, these reports do not firmly establish the tupaia as a reliable model of HCV infection. Human CD81, scavenger receptor class B type I (SR-BI), claudin 1 (CLDN1), and occludin (OCLN) are considered essential receptors or coreceptors for HCV cell entry. In the present study, the roles of these tupaia orthologs in HCV infection were assessed. Both CD81 and SR-BI of tupaia were found to be able to bind with HCV envelope protein 2 (E2). In comparison with human CD81, tupaia CD81 exhibited stronger binding activity with E2 and increased HCV pseudoparticle (HCVpp) cell entry 2-fold. The 293T cells transfected with tupaia CLDN1 became susceptible to HCVpp infection. Moreover, simultaneous transfection of the four tupaia factors into mouse NIH 3T3 cells made the cells susceptible to HCVpp infection. HCVpp of diverse genotypes were able to infect primary tupaia hepatocytes (PTHs), and this infection could be blocked by either anti-CD81 or anti-SR-BI. PTHs could be infected by cell culture-produced HCV (HCVcc) and did produce infectious progeny virus in culture supernatant. These findings indicate that PTHs possess all of the essential factors required for HCV entry and support the complete HCV infection cycle. This highlights both the mechanisms of susceptibility of tupaia to HCV infection and the possibility of using tupaia as a promising small-animal model in HCV study.

Potential treatment options and future research to increase hepatitis C virus treatment response rate

Hepatitis C virus (HCV) is a liver-tropic blood-borne pathogen that affects more than 170 million people worldwide. Although acute infections are usually asymptomatic, up to 90% of HCV infections persist with the possibility of long-term consequences such as liver fibrosis, cirrhosis, steatosis, insulin resistance, or hepatocellular carcinoma. As such, HCV-associated liver disease is a major public health concern. Although the currently available standard of care therapy of pegylated interferon α plus ribavirin successfully treats infection in a subset of patients, the development of more effective, less toxic HCV antivirals is a health care imperative. This review not only discusses the limitations of the current HCV standard of care but also evaluates upcoming HCV treatment options and how current research elucidating the viral life cycle is facilitating the development of HCV-specific therapeutics that promise to greatly improve treatment response rates both before and after liver transplantation.

Gene therapeutic approach for inhibiting hepatitis C virus replication using a recombinant protein that controls interferon expression

The hepatitis C virus (HCV) is a continuing threat to public health. The systemic administration of interferon alpha with ribavirin is the only currently approved treatment. However, this treatment is associated with a wide spectrum of systemic side effects that limits its effectiveness; thus, there is an urgent need for new treatment modalities. In this study, we describe a novel anti-HCV strategy employing a recombinant transcription factor that we have engineered in such a way that NS3/4a viral protease controls its intracellular localization, thereby restoring interferon secretion specifically in cells infected with HCV. Proof-of-concept experiments validated the strategy, showing that the recombinant transcription factor was triggered to stimulate interferon promoter by NS3/4A and remained inactive in cells without NS3/4a. Using an adenovirus-associated viral vector delivery system, we found that the recombinant transcription factor inhibited HCV replication effectively in vitro in cultured cells.

Hepatitis C virus experimental model systems and antiviral drug research

An estimated 130 million people worldwide are chronically infected with hepatitis C virus (HCV) making it a leading cause of liver disease worldwide. Because the currently available therapy of pegylated interferon-alpha and ribavirin is only effective in a subset of patients, the development of new HCV antivirals is a healthcare imperative. This review discusses the experimental models available for HCV antiviral drug research, recent advances in HCV antiviral drug development, as well as active research being pursued to facilitate development of new HCV-specific therapeutics.

Pathogenesis of hepatitis C virus infection in Tupaia belangeri

The lack of a small-animal model has hampered the analysis of hepatitis C virus (HCV) pathogenesis. The tupaia (Tupaia belangeri), a tree shrew, has shown susceptibility to HCV infection and has been considered a possible candidate for a small experimental model of HCV infection. However, a longitudinal analysis of HCV-infected tupaias has yet to be described. Here, we provide an analysis of HCV pathogenesis during the course of infection in tupaias over a 3-year period. The animals were inoculated with hepatitis C patient serum HCR6 or viral particles reconstituted from full-length cDNA. In either case, inoculation caused mild hepatitis and intermittent viremia during the acute phase of infection. Histological analysis of infected livers revealed that HCV caused chronic hepatitis that worsened in a time-dependent manner. Liver steatosis, cirrhotic nodules, and accompanying tumorigenesis were also detected. To examine whether infectious virus particles were produced in tupaia livers, naive animals were inoculated with sera from HCV-infected tupaias, which had been confirmed positive for HCV RNA. As a result, the recipient animals also displayed mild hepatitis and intermittent viremia. Quasispecies were also observed in the NS5A region, signaling phylogenic lineage from the original inoculating sequence. Taken together, these data suggest that the tupaia is a practical animal model for experimental studies of HCV infection.

Interaction of hepatitis C virus envelope glycoprotein E2 with the large extracellular loop of tupaia CD81

AIM: To further analyze the interaction of tupaia CD81 with hepatitis C virus (HCV) envelope protein E2.

METHODS: A tupaia CD81 large extracellular loop (CD81 LEL), which binds to HCV E2 protein, was cloned and expressed as a GST-fusion protein, and interaction of HCV E2 protein with a tupaia CD81 LEL was evaluated by enzyme-linked immunosorbent assay (EIA).

RESULTS: Although tupaia and human CD81 LEL differed in 6 amino acid changes, tupaia CD81 LEL was strongly recognized by anti-CD81 antibodies against human CD81 LEL conformation-dependent epitopes. Investigating LEL CD81-E2 interactions by EIA, we demonstrated that binding of tupaia CD81 LEL GST fusion protein to recombinant HCV E2 protein was markedly reduced compared to binding of human CD81 LEL GST fusion protein to recombinant HCV E2 protein.

CONCLUSION: These data suggest that the structural differences in-between the tupaia and human CD81 may alter the interaction of the large extracellular loop with HCV envelope glycoprotein E2. These findings may be important for the understanding of the mechanisms of binding and entry of HCV to PTHs.

Direct infection and replication of naturally occurring hepatitis C virus genotypes 1, 2, 3 and 4 in normal human hepatocyte cultures

BACKGROUND

Hepatitis C virus (HCV) infection afflicts about 170 million individuals worldwide. However, the HCV life cycle is only partially understood because it has not been possible to infect normal human hepatocytes in culture. The current Huh-7 systems use cloned, synthetic HCV RNA expressed in hepatocellular carcinoma cells to produce virions, but these cells cannot be infected with naturally occurring HCV obtained from infected patients.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we describe a human hepatocyte culture permissible to the direct infection with naturally occurring HCV genotypes 1, 2, 3 and 4 in the blood of HCV-infected patients. The culture system mimics the biology and kinetics of HCV infection in humans, and produces infectious virions that can infect naïve human hepatocytes.

CONCLUSIONS/SIGNIFICANCE

This culture system should complement the existing systems, and may facilitate the understanding of the HCV life cycle, its effects in the natural host cell, the hepatocyte, as well as the development of novel therapeutics and vaccines.

Scavenger receptor class B type I and hepatitis C virus infection of primary tupaia hepatocytes

Hepatitis C virus (HCV) is a major cause of chronic hepatitis worldwide. The study of early steps during HCV infection has been hampered by the lack of suitable in vitro or in vivo models. Primary Tupaia hepatocytes (PTH) have been shown to be susceptible to HCV infection in vitro and in vivo. Human scavenger receptor class B type I (SR-BI) represents an HCV receptor candidate mediating the cellular binding of E2 glycoprotein to HepG2 hepatoma cells. However, the function of SR-BI for viral infection of hepatocytes is unknown. In this study, we used PTH to assess the functional role of SR-BI as a putative HCV receptor. Sequence analysis of cloned tupaia SR-BI revealed a high homology between tupaia and human SR-BI. Transfection of CHO cells with human or tupaia SR-BI but not mouse SR-BI cDNA resulted in cellular E2 binding, suggesting that E2-binding domains between human and tupaia SR-BI are highly conserved. Preincubation of PTH with anti-SR-BI antibodies resulted in marked inhibition of E2 or HCV-like particle binding. However, anti-SR-BI antibodies were not able to block HCV infection of PTH. In conclusion, our results demonstrate that SR-BI represents an important cell surface molecule for the binding of the HCV envelope to hepatocytes and suggest that other or additional cell surface molecules are required for the initiation of HCV infection. Furthermore, the structural and functional similarities between human and tupaia SR-BI indicate that PTH represent a useful model system to characterize the molecular interaction of the HCV envelope and SR-BI on primary hepatocytes.

Inhibition of hepatitis C virus-like particle binding to target cells by antiviral antibodies in acute and chronic hepatitis C

Hepatitis C virus (HCV) is a leading cause of chronic viral hepatitis worldwide. The study of antibody-mediated virus neutralization has been hampered by the lack of an efficient and high-throughput cell culture system for the study of virus neutralization. The HCV structural proteins have been shown to assemble into noninfectious HCV-like particles (HCV-LPs). Similar to serum-derived virions, HCV-LPs bind and enter human hepatocytes and hepatoma cell lines. In this study, we developed an HCV-LP-based model system for a systematic functional analysis of antiviral antibodies from patients with acute or chronic hepatitis C. We demonstrate that cellular HCV-LP binding was specifically inhibited by antiviral antibodies from patients with acute or chronic hepatitis C in a dose-dependent manner. Using a library of homologous overlapping envelope peptides covering the entire HCV envelope, we identified an epitope in the N-terminal E2 region (SQKIQLVNTNGSWHI; amino acid positions 408 to 422) as one target of human antiviral antibodies inhibiting cellular particle binding. Using a large panel of serum samples from patients with acute and chronic hepatitis C, we demonstrated that the presence of antibodies with inhibition of binding activity was not associated with viral clearance. In conclusion, antibody-mediated inhibition of cellular HCV-LP binding represents a convenient system for the functional characterization of human anti-HCV antibodies, allowing the mapping of envelope neutralization epitopes targeted by naturally occurring antiviral antibodies.

Re-evaluation and in silico annotation of the Tupaia herpesvirus proteins

Herpesviruses represent an exceptionally suitable model to analyze evolutionary old pathogens, their competency to adapt to existing and changing molecular niches in host species, and the modulation of the gene content and function to comply with the requirements of life. The basis for numerous studies dealing with these questions are reliable statements about the gene content of herpesviral genomes and the functions of viral proteins. The recent determination of the coding strategy of the chimpanzee cytomegalovirus genome and the re-evaluation of the gene content of the human cytomegalovirus genome made it also necessary to restructure the putative transcription map of the Tupaia herpesvirus (THV) genome. Twenty-three THV-specific ORFs formerly predicted to be coding for viral proteins were deleted from the THV transcription map resulting in a gene layout that is now characterized by the presence of conserved genes in the genome center, that probably reflect the genome structure of common herpesviral ancestors, and species-specific genes at the termini. The conserved regions in the THV genome are characterized by high G + C contents between 60% and 80%, a high CpG dinucleotide frequency, and the presence of densely packed putative CpG islands. The genome termini seem to provide the requirements of large scale rearrangements and complements of the gene content to adapt to new environmental demands. With the help of the recently designed method of dictionary-driven, pattern-based protein annotation it was possible to assign putative functions to almost all potential THV proteins, e.g. 123 were found to be putative membrane or secreted proteins, putative signal domains were identified in 69, and 29 proteins were predicted to be glycosylated. The present study adds new aspects to the knowledge about the precise gene composition of herpesvirus genomes and viral protein functions that are of exceptional importance for studies dealing with the phylogeny, the evolution, vaccine vector development, virus-host interactions, pathogenesis and the determination of protein functions of herpesviruses.

Molecular anatomy of Tupaia (tree shrew) adenovirus genome; evolution of viral genes and viral phylogeny

Adenoviruses are globally spread and infect species in all five taxons of vertebrates. Outstanding attention is focused on adenoviruses because of their transformation potential, their possible usability as vectors in gene therapy and their applicability in studies dealing with, e.g. cell cycle control, DNA replication, transcription, splicing, virus-host interactions, apoptosis, and viral evolution. The accumulation of genetic data provides the basis for the increase of our knowledge about adenoviruses. The Tupaia adenovirus (TAV) infects members of the genus Tupaiidae that are frequently used as laboratory animals in behavior research dealing with questions about biological and molecular processes of stress in mammals, in neurobiological and physiological studies, and as model organisms for human hepatitis B and C virus infections. In the present study the TAV genome underwent an extensive analysis including determination of codon usage, CG depletion, gene content, gene arrangement, potential splice sites, and phylogeny. The TAV genome has a length of 33,501 bp with a G+C content of 49.96%. The genome termini show a strong CG depletion that could be due to methylation of these genome regions during the viral replication cycle. The analysis of the coding capacity of the complete TAV genome resulted in the identification of 109 open reading frames (ORFs), of which 38 were predicted to be real viral genes. TAV was classified within the genus Mastadenovirus characterized by typical gene content, arrangement, and homology values of 29 conserved ORFs. Phylogenetic trees show that TAV is part of a separate evolutionary lineage and no mastadenovirus species can be considered as the most related. In contrast to other mastadenoviruses a direct ancestor of TAV captured a DUT gene from its mammalian host, presumably controlling local dUTP levels during replication and enhance viral replication in non-dividing host tissues. Furthermore, TAV possesses a second DNA-binding protein gene, that is likely to play a role in the determination of the host range. In view of these data it is conceivable that TAV underwent evolutionary adaptations to its biological environment resulting in the formation of special genomic components that provided TAV with the ability to expand its host range during viral evolution.