Zebrafish as a new model for herpes simplex virus type 1 infection

Application of the zebrafish model in human viral research

Viruses are a leading cause of infectious diseases. Well-developed animal models are valuable for understanding the immune responses to viral infections and the pathogenesis of viral diseases. Zebrafish is a commonly used small vertebrate model organism with strong reproductive ability, a short life cycle, and rapid embryonic development. Moreover, zebrafish and human genomes are highly similar; they have approximately 70 % homology in protein-coding genes, and 84 % of genes associated with human diseases have zebrafish counterparts. Recent years, different groups have developed zebrafish models for human viral infections and diseases, offering new insights into the molecular mechanisms of human viral pathogenesis as well as the development of antiviral strategies. The zebrafish model has become a simple and effective model system for understanding host-virus interaction. This review provides a comprehensive summary of the use of zebrafish models in human viral research, particularly in SARS-CoV-2.

A novel insight on SARS-CoV-2 S-derived fragments in the control of the host immunity

Despite all efforts to combat the pandemic of COVID-19, we are still living with high numbers of infected persons, an overburdened health care system, and the lack of an effective and definitive treatment. Understanding the pathophysiology of the disease is crucial for the development of new technologies and therapies for the best clinical management of patients. Since the manipulation of the whole virus requires a structure with an adequate level of biosafety, the development of alternative technologies, such as the synthesis of peptides from viral proteins, is a possible solution to circumvent this problem. In addition, the use and validation of animal models is of extreme importance to screen new drugs and to compress the organism's response to the disease. Peptides derived from recombinant S protein from SARS-CoV-2 were synthesized and validated by in silico, in vitro and in vivo methodologies. Macrophages and neutrophils were challenged with the peptides and the production of inflammatory mediators and activation profile were evaluated. These peptides were also inoculated into the swim bladder of transgenic zebrafish larvae at 6 days post fertilization (dpf) to mimic the inflammatory process triggered by the virus, which was evaluated by confocal microscopy. In addition, toxicity and oxidative stress assays were also developed. In silico and molecular dynamics assays revealed that the peptides bind to the ACE2 receptor stably and interact with receptors and adhesion molecules, such as MHC and TCR, from humans and zebrafish. Macrophages stimulated with one of the peptides showed increased production of NO, TNF-α and CXCL2. Inoculation of the peptides in zebrafish larvae triggered an inflammatory process marked by macrophage recruitment and increased mortality, as well as histopathological changes, similarly to what is observed in individuals with COVID-19. The use of peptides is a valuable alternative for the study of host immune response in the context of COVID-19. The use of zebrafish as an animal model also proved to be appropriate and effective in evaluating the inflammatory process, comparable to humans.

Zebrafish-based platform for emerging bio-contaminants and virus inactivation research

Emerging bio-contaminants such as viruses have affected health and environment settings of every country. Viruses are the minuscule entities resulting in severe contagious diseases like SARS, MERS, Ebola, and avian influenza. Recent epidemic like the SARS-CoV-2, the virus has undergone mutations strengthen them and allowing to escape from the remedies. Comprehensive knowledge of viruses is essential for the development of targeted therapeutic and vaccination treatments. Animal models mimicking human biology like non-human primates, rats, mice, and rabbits offer competitive advantage to assess risk of viral infections, chemical toxins, nanoparticles, and microbes. However, their economic maintenance has always been an issue. Furthermore, the redundancy of experimental results due to aforementioned aspects is also in examine. Hence, exploration for the alternative animal models is crucial for risk assessments. The current review examines zebrafish traits and explores the possibilities to monitor emerging bio-contaminants. Additionally, a comprehensive picture of the bio contaminant and virus particle invasion and abatement mechanisms in zebrafish and human cells is presented. Moreover, a zebrafish model to investigate the emerging viruses such as coronaviridae and poxviridae has been suggested.

Susceptibility and Permissivity of Zebrafish (Danio rerio) Larvae to Cypriniviruses

The zebrafish (Danio rerio) represents an increasingly important model organism in virology. We evaluated its utility in the study of economically important viruses from the genus Cyprinivirus (anguillid herpesvirus 1, cyprinid herpesvirus 2 and cyprinid herpesvirus 3 (CyHV-3)). This revealed that zebrafish larvae were not susceptible to these viruses after immersion in contaminated water, but that infections could be established using artificial infection models in vitro (zebrafish cell lines) and in vivo (microinjection of larvae). However, infections were transient, with rapid viral clearance associated with apoptosis-like death of infected cells. Transcriptomic analysis of CyHV-3-infected larvae revealed upregulation of interferon-stimulated genes, in particular those encoding nucleic acid sensors, mediators of programmed cell death and related genes. It was notable that uncharacterized non-coding RNA genes and retrotransposons were also among those most upregulated. CRISPR/Cas9 knockout of the zebrafish gene encoding protein kinase R (PKR) and a related gene encoding a protein kinase containing Z-DNA binding domains (PKZ) had no impact on CyHV-3 clearance in larvae. Our study strongly supports the importance of innate immunity-virus interactions in the adaptation of cypriniviruses to their natural hosts. It also highlights the potential of the CyHV-3-zebrafish model, versus the CyHV-3-carp model, for study of these interactions.

An Overview towards Zebrafish Larvae as a Model for Ocular Diseases

Despite the obvious morphological differences in the visual system, zebrafish share a similar architecture and components of the same embryonic origin as humans. The zebrafish retina has the same layered structure and cell types with similar metabolic and phototransduction support as humans, and is functional 72 h after fertilization, allowing tests of visual function to be performed. The zebrafish genomic database supports genetic mapping studies as well as gene editing, both of which are useful in the ophthalmological field. It is possible to model ocular disorders in zebrafish, as well as inherited retinal diseases or congenital or acquired malformations. Several approaches allow the evaluation of local pathological processes derived from systemic disorders, such as chemical exposure to produce retinal hypoxia or glucose exposure to produce hyperglycemia, mimicking retinopathy of prematurity or diabetic retinopathy, respectively. The pathogenesis of ocular infections, autoimmune diseases, or aging can also be assessed in zebrafish larvae, and the preserved cellular and molecular immune mechanisms can be assessed. Finally, the zebrafish model for the study of the pathologies of the visual system complements certain deficiencies in experimental models of mammals since the regeneration of the zebrafish retina is a valuable tool for the study of degenerative processes and the discovery of new drugs and therapies.

The knowns and many unknowns of CNS immunity in teleost fish

Many disease agents infect the central nervous system (CNS) of teleost fish causing severe losses for the fish farming sector. Yet, neurotropic fish pathogens remain poorly documented and immune responses in the teleost CNS essentially unknown. Previously thought to be devoid of an immune system, the mammalian CNS is now recognized to be protected from infection by diverse immune cells that mostly reside in the meningeal lymphatic system. Here we review the current body of work pertaining immune responses in the teleost CNS to infection. We identify important knowledge gaps with regards to CNS immunity in fish and make recommendations for rigorous experimentation and reporting in manuscripts so that fish immunologists can advance this burgeoning field.

Effect of novel therapeutic medicine swertiamarin from Enicostema axillare in zebrafish infected with Salmonella typhi

Herbal treatments have been practiced by humans over centuries and therefore possess time-proven safety. However, it is crucial to evaluate the toxic effects of herbal medicine to confirm their safety, particularly when developing therapeutic drugs. Use of laboratory animals such as mice, rat, and rabbits was considered as gold standard in herbal toxicity assessments. However, in the last few decades, the ethical consideration of using higher vertebrates for toxicity testing has become more controversial. As a possible alternative model involving lower vertebrates such as zebra fish were introduced. Hence in the present study, swertiamain compound isolated from E. axillare was assessed for it antimicrobial activity in zebra fish larvae againt S. typhi. The cumulative mortality rate and bacterial localization in zebra fish larvae were studied. Biochemical markers assays were performed to find the preventive role of the compound during the typhoid infection. The results showed that zebra fish can be successfully used as a model to study typhoid infection and the anti-bacterial compound swertiamarin used in this study clears the bacterial load and pathogenic symptoms to a great extent.

Using zebrafish to understand reciprocal interactions between the nervous and immune systems and the microbial world

Animals rely heavily on their nervous and immune systems to perceive and survive within their environment. Despite the traditional view of the brain as an immunologically privileged organ, these two systems interact with major consequences. Furthermore, microorganisms within their environment are major sources of stimuli and can establish relationships with animal hosts that range from pathogenic to mutualistic. Research from a variety of human and experimental animal systems are revealing that reciprocal interactions between microbiota and the nervous and immune systems contribute significantly to normal development, homeostasis, and disease. The zebrafish has emerged as an outstanding model within which to interrogate these interactions due to facile genetic and microbial manipulation and optical transparency facilitating in vivo imaging. This review summarizes recent studies that have used the zebrafish for analysis of bidirectional control between the immune and nervous systems, the nervous system and the microbiota, and the microbiota and immune system in zebrafish during development that promotes homeostasis between these systems. We also describe how the zebrafish have contributed to our understanding of the interconnections between these systems during infection in fish and how perturbations may result in pathology.

Intranasal delivery of SARS-CoV-2 spike protein is sufficient to cause olfactory damage, inflammation and olfactory dysfunction in zebrafish

Anosmia, loss of smell, is a prevalent symptom of SARS-CoV-2 infection. Anosmia may be explained by several mechanisms driven by infection of non-neuronal cells and damage in the nasal epithelium rather than direct infection of olfactory sensory neurons (OSNs). Previously, we showed that viral proteins are sufficient to cause neuroimmune responses in the teleost olfactory organ (OO). We hypothesize that SARS-CoV-2 spike (S) protein is sufficient to cause olfactory damage and olfactory dysfunction. Using an adult zebrafish model, we report that intranasally delivered SARS-CoV-2 S RBD mostly binds to the non-sensory epithelium of the olfactory organ and causes severe olfactory histopathology characterized by loss of cilia, hemorrhages and edema. Electrophysiological recordings reveal impaired olfactory function to both food and bile odorants in animals treated intranasally with SARS-CoV-2 S RBD. However, no loss of behavioral preference for food was detected in SARS-CoV-2 S RBD treated fish. Single cell RNA-Seq of the adult zebrafish olfactory organ indicated widespread loss of olfactory receptor expression and inflammatory responses in sustentacular, endothelial, and myeloid cell clusters along with reduced numbers of T_regs. Combined, our results demonstrate that intranasal SARS-CoV-2 S RBD is sufficient to cause structural and functional damage to the zebrafish olfactory system. These findings may have implications for intranasally delivered vaccines against SARS-CoV-2.

Recapitulation of Retinal Damage in Zebrafish Larvae Infected with Zika Virus

Zebrafish are increasingly being utilized as a model to investigate infectious diseases and to advance the understanding of pathogen–host interactions. Here, we take advantage of the zebrafish to recapitulate congenital ZIKV infection and, for the first time, demonstrate that it can be used to model infection and reinfection and monitor anti-viral and inflammatory immune responses, as well as brain growth and eye abnormalities during embryonic development. By injecting a Brazilian strain of ZIKV into the yolk sac of one-cell stage embryos, we confirmed that, after 72 h, ZIKV successfully infected larvae, and the physical condition of the virus-infected hosts included gross morphological changes in surviving embryos (84%), with a reduction in larval head size and retinal damage characterized by increased thickness of the lens and inner nuclear layer. Changes in locomotor activity and the inability to perceive visual stimuli are a result of changes in retinal morphology caused by ZIKV. Furthermore, we demonstrated the ability of ZIKV to replicate in zebrafish larvae and infect new healthy larvae, impairing their visual and neurological functions. These data reinforce the deleterious activity of ZIKV in the brain and visual structures and establish the zebrafish as a model to study the molecular mechanisms involved in the pathology of the virus.

Exploring Zebrafish Larvae as a COVID-19 Model: Probable Abortive SARS-CoV-2 Replication in the Swim Bladder

Animal models are essential to understanding COVID-19 pathophysiology and for preclinical assessment of drugs and other therapeutic or prophylactic interventions. We explored the small, cheap, and transparent zebrafish larva as a potential host for SARS-CoV-2. Bath exposure, as well as microinjection in the coelom, pericardium, brain ventricle, or bloodstream, resulted in a rapid decrease of SARS-CoV-2 RNA in wild-type larvae. However, when the virus was inoculated in the swim bladder, viral RNA stabilized after 24 h. By immunohistochemistry, epithelial cells containing SARS-CoV-2 nucleoprotein were observed in the swim bladder wall. Our data suggest an abortive infection of the swim bladder. In some animals, several variants of concern were also tested with no evidence of increased infectivity in our model. Low infectivity of SARS-CoV-2 in zebrafish larvae was not due to the host type I interferon response, as comparable viral loads were detected in type I interferon-deficient animals. A mosaic overexpression of human ACE2 was not sufficient to increase SARS-CoV-2 infectivity in zebrafish embryos or in fish cells in vitro. In conclusion, wild-type zebrafish larvae appear mostly non-permissive to SARS-CoV-2, except in the swim bladder, an aerial organ sharing similarities with the mammalian lung.

Antimicrobial Compounds from Microorganisms

Antimicrobial resistance is an exigent public health concern owing to the emergence of novel strains of human resistant pathogens and the concurrent rise in multi-drug resistance. An influx of new antimicrobials is urgently required to improve the treatment outcomes of infectious diseases and save lives. Plant metabolites and bioactive compounds from chemical synthesis have found their efficacy to be dwindling, despite some of them being developed as drugs and used to treat human infections for several decades. Microorganisms are considered untapped reservoirs for promising biomolecules with varying structural and functional antimicrobial activity. The advent of cost-effective and convenient model organisms, state-of-the-art molecular biology, omics technology, and machine learning has enhanced the bioprospecting of novel antimicrobial drugs and the identification of new drug targets. This review summarizes antimicrobial compounds isolated from microorganisms and reports on the modern tools and strategies for exploiting promising antimicrobial drug candidates. The investigation identified a plethora of novel compounds from microbial sources with excellent antimicrobial activity against disease-causing human pathogens. Researchers could maximize the use of novel model systems and advanced biomolecular and computational tools in exploiting lead antimicrobials, consequently ameliorating antimicrobial resistance.

Exploring Zebrafish Larvae as a COVID-19 Model: Probable Abortive SARS-CoV-2 Replication in the Swim Bladder

Animal models are essential to understanding COVID-19 pathophysiology and for preclinical assessment of drugs and other therapeutic or prophylactic interventions. We explored the small, cheap, and transparent zebrafish larva as a potential host for SARS-CoV-2. Bath exposure, as well as microinjection in the coelom, pericardium, brain ventricle, or bloodstream, resulted in a rapid decrease of SARS-CoV-2 RNA in wild-type larvae. However, when the virus was inoculated in the swim bladder, viral RNA stabilized after 24 h. By immunohistochemistry, epithelial cells containing SARS-CoV-2 nucleoprotein were observed in the swim bladder wall. Our data suggest an abortive infection of the swim bladder. In some animals, several variants of concern were also tested with no evidence of increased infectivity in our model. Low infectivity of SARS-CoV-2 in zebrafish larvae was not due to the host type I interferon response, as comparable viral loads were detected in type I interferon-deficient animals. A mosaic overexpression of human ACE2 was not sufficient to increase SARS-CoV-2 infectivity in zebrafish embryos or in fish cells in vitro. In conclusion, wild-type zebrafish larvae appear mostly non-permissive to SARS-CoV-2, except in the swim bladder, an aerial organ sharing similarities with the mammalian lung.

Tilapia Lake Virus-Induced Neuroinflammation in Zebrafish: Microglia Activation and Sickness Behavior

In mammals, the relationship between the immune system and behavior is widely studied. In fish, however, the knowledge concerning the brain immune response and behavioral changes during brain viral infection is very limited. To further investigate this subject, we used the model of tilapia lake virus (TiLV) infection of zebrafish (Danio rerio), which was previously developed in our laboratory. We demonstrated that TiLV persists in the brain of adult zebrafish for at least 90 days, even when the virus is not detectable in other peripheral organs. The virions were found in the whole brain. During TiLV infection, zebrafish displayed a clear sickness behavior: decreased locomotor activity, reduced food intake, and primarily localizes near the bottom zone of aquaria. Moreover, during swimming, individual fish exhibited also unusual spiral movement patterns. Gene expression study revealed that TiLV induces in the brain of adult fish strong antiviral and inflammatory response and upregulates expression of genes encoding microglia/macrophage markers. Finally, using zebrafish larvae, we showed that TiLV infection induces histopathological abnormalities in the brain and causes activation of the microglia which is manifested by changes in cell shape from a resting ramified state in mock-infected to a highly ameboid active state in TiLV-infected larvae. This is the first study presenting a comprehensive analysis of the brain immune response associated with microglia activation and subsequent sickness behavior during systemic viral infection in zebrafish.

Zebrafish Models for the Safety and Therapeutic Testing of Nanoparticles with a Focus on Macrophages

New nanoparticles and biomaterials are increasingly being used in biomedical research for drug delivery, diagnostic applications, or vaccines, and they are also present in numerous commercial products, in the environment and workplaces. Thus, the evaluation of the safety and possible therapeutic application of these nanomaterials has become of foremost importance for the proper progress of nanotechnology. Due to economical and ethical issues, in vitro and in vivo methods are encouraged for the testing of new compounds and/or nanoparticles, however in vivo models are still needed. In this scenario, zebrafish (Danio rerio) has demonstrated potential for toxicological and pharmacological screenings. Zebrafish presents an innate immune system, from early developmental stages, with conserved macrophage phenotypes and functions with respect to humans. This fact, combined with the transparency of zebrafish, the availability of models with fluorescently labelled macrophages, as well as a broad variety of disease models offers great possibilities for the testing of new nanoparticles. Thus, with a particular focus on macrophage-nanoparticle interaction in vivo, here, we review the studies using zebrafish for toxicological and biodistribution testing of nanoparticles, and also the possibilities for their preclinical evaluation in various diseases, including cancer and autoimmune, neuroinflammatory, and infectious diseases.

A Confocal Microscopic Study of Gene Transfer into the Mesencephalic Tegmentum of Juvenile Chum Salmon, Oncorhynchus keta, Using Mouse Adeno-Associated Viral Vectors

To date, data on the presence of adenoviral receptors in fish are very limited. In the present work, we used mouse recombinant adeno-associated viral vectors (rAAV) with a calcium indicator of the latest generation GCaMP6m that are usually applied for the dorsal hippocampus of mice but were not previously used for gene delivery into fish brain. The aim of our work was to study the feasibility of transduction of rAAV in the mouse hippocampus into brain cells of juvenile chum salmon and subsequent determination of the phenotype of rAAV-labeled cells by confocal laser scanning microscopy (CLSM). Delivery of the gene in vivo was carried out by intracranial injection of a GCaMP6m-GFP-containing vector directly into the mesencephalic tegmentum region of juvenile (one-year-old) chum salmon, Oncorhynchus keta. AAV incorporation into brain cells of the juvenile chum salmon was assessed at 1 week after a single injection of the vector. AAV expression in various areas of the thalamus, pretectum, posterior-tuberal region, postcommissural region, medial and lateral regions of the tegmentum, and mesencephalic reticular formation of juvenile O. keta was evaluated using CLSM followed by immunohistochemical analysis of the localization of the neuron-specific calcium binding protein HuCD in combination with nuclear staining with DAPI. The results of the analysis showed partial colocalization of cells expressing GCaMP6m-GFP with red fluorescent HuCD protein. Thus, cells of the thalamus, posterior tuberal region, mesencephalic tegmentum, cells of the accessory visual system, mesencephalic reticular formation, hypothalamus, and postcommissural region of the mesencephalon of juvenile chum salmon expressing GCaMP6m-GFP were attributed to the neuron-specific line of chum salmon brain cells, which indicates the ability of hippocampal mammal rAAV to integrate into neurons of the central nervous system of fish with subsequent expression of viral proteins, which obviously indicates the neuronal expression of a mammalian adenoviral receptor homolog by juvenile chum salmon neurons.

Modeling Virus-Induced Inflammation in Zebrafish: A Balance Between Infection Control and Excessive Inflammation

The inflammatory response to viral infection in humans is a dynamic process with complex cell interactions that are governed by the immune system and influenced by both host and viral factors. Due to this complexity, the relative contributions of the virus and host factors are best studied in vivo using animal models. In this review, we describe how the zebrafish (Danio rerio) has been used as a powerful model to study host-virus interactions and inflammation by combining robust forward and reverse genetic tools with in vivo imaging of transparent embryos and larvae. The innate immune system has an essential role in the initial inflammatory response to viral infection. Focused studies of the innate immune response to viral infection are possible using the zebrafish model as there is a 4-6 week timeframe during development where they have a functional innate immune system dominated by neutrophils and macrophages. During this timeframe, zebrafish lack a functional adaptive immune system, so it is possible to study the innate immune response in isolation. Sequencing of the zebrafish genome has revealed significant genetic conservation with the human genome, and multiple studies have revealed both functional conservation of genes, including those critical to host cell infection and host cell inflammatory response. In addition to studying several fish viruses, zebrafish infection models have been developed for several human viruses, including influenza A, noroviruses, chikungunya, Zika, dengue, herpes simplex virus type 1, Sindbis, and hepatitis C virus. The development of these diverse viral infection models, coupled with the inherent strengths of the zebrafish model, particularly as it relates to our understanding of macrophage and neutrophil biology, offers opportunities for far more intensive studies aimed at understanding conserved host responses to viral infection. In this context, we review aspects relating to the evolution of innate immunity, including the evolution of viral pattern recognition receptors, interferons and interferon receptors, and non-coding RNAs.

Infection of zebrafish larvae with human norovirus and evaluation of the in vivo efficacy of small-molecule inhibitors

We have recently established that human norovirus (HuNoV) replicates efficiently in zebrafish larvae after inoculation of a clinical sample into the yolk, providing a simple and robust in vivo system in which to study HuNoV. In this Protocol Extension, we present a detailed description of virus inoculation by microinjection, subsequent daily monitoring and harvesting of larvae, followed by viral RNA quantification. This protocol can be used to study viral replication of genogroup (G)I and GII HuNoVs in vivo within 3-4 d. Additionally, we describe how to evaluate the in vivo antiviral effect and toxicity of small molecules using HuNoV-infected zebrafish larvae, in multi-well plates and without the need for specific formulations. This constitutes a great advantage for drug discovery efforts, as no specific antivirals or vaccines currently exist to treat or prevent norovirus gastroenteritis.

Fucoidan But Not 2'-Fucosyllactose Inhibits Human Norovirus Replication in Zebrafish Larvae

Human noroviruses (hNoVs) cause heavy disease burden worldwide and there is no clinically approved vaccination or antiviral hitherto. In this study, with the use of a zebrafish larva in vivo platform, we investigated the anti-hNoV potentials of fucoidan (from brown algae Fucus vesiculosus) and 2'-Fucosyllactose (2'-FL). As a result, although both fucoidan and 2'-FL were able to block hNoV GII.4 virus-like particle (VLPs) from binding to type A saliva as expected, only fucoidan, but not 2'-FL, was able to inhibit the replication of hNoV GII.P16-GII.4 in zebrafish larvae, indicating the possible needs of higher molecular weights for fucosylated carbohydrates to exert anti-hNoV effect.

Withanone from Withania somnifera Attenuates SARS-CoV-2 RBD and Host ACE2 Interactions to Rescue Spike Protein Induced Pathologies in Humanized Zebrafish Model

Purpose

SARS-CoV-2 engages human ACE2 through its spike (S) protein receptor binding domain (RBD) to enter the host cell. Recent computational studies have reported that withanone and withaferin A, phytochemicals found in Withania somnifera, target viral main protease (M^Pro) and host transmembrane TMPRSS2, and glucose related protein 78 (GRP78), respectively, implicating their potential as viral entry inhibitors. Absence of specific treatment against SARS-CoV-2 infection has encouraged exploration of phytochemicals as potential antivirals.

Aim

This study aimed at in silico exploration, along with in vitro and in vivo validation of antiviral efficacy of the phytochemical withanone.

Methods

Through molecular docking, molecular dynamic (MD) simulation and electrostatic energy calculation the plausible biochemical interactions between withanone and the ACE2-RBD complex were investigated. These in silico observations were biochemically validated by ELISA-based assays. Withanone-enriched extract from W. somnifera was tested for its ability to ameliorate clinically relevant pathological features, modelled in humanized zebrafish through SARS-CoV-2 recombinant spike (S) protein induction.

Results

Withanone bound efficiently at the interacting interface of the ACE2-RBD complex and destabilized it energetically. The electrostatic component of binding free energies of the complex was significantly decreased. The two intrachain salt bridge interactions (K31-E35) and the interchain long-range ion-pair (K31-E484), at the ACE2-RBD interface were completely abolished by withanone, in the 50 ns simulation. In vitro binding assay experimentally validated that withanone efficiently inhibited (IC₅₀=0.33 ng/mL) the interaction between ACE2 and RBD, in a dose-dependent manner. A withanone-enriched extract, without any co-extracted withaferin A, was prepared from W. somnifera leaves. This enriched extract was found to be efficient in ameliorating human-like pathological responses induced in humanized zebrafish by SARS-CoV-2 recombinant spike (S) protein.

Conclusion

In conclusion, this study provided experimental validation for computational insight into the potential of withanone as a potent inhibitor of SARS-CoV-2 coronavirus entry into the host cells.

Blood Vessel Imaging at Pre-Larval Stages of Zebrafish Embryonic Development

The zebrafish (Danio rerio) is an increasingly popular animal model biological system. In cardiovascular research, it has been used to model specific cardiac phenomena as well as to identify novel therapies for human cardiovascular disease. While the zebrafish cardiovascular system functioning is well examined at larval stages, the mechanisms by which vessel activity is initiated remain a subject of intense investigation. In this research, we report on an in vivo stain-free blood vessel imaging technique at pre-larval stages of zebrafish embryonic development. We have developed the algorithm for the enhancement, alignment and spatiotemporal analysis of bright-field microscopy images of zebrafish embryos. It enables the detection, mapping and quantitative characterization of cardiac activity across the whole specimen. To validate the proposed approach, we have analyzed multiple data cubes, calculated vessel images and evaluated blood flow velocity and heart rate dynamics in the absence of any anesthesia. This non-invasive technique may shed light on the mechanism of vessel activity initiation and stabilization as well as the cardiovascular system’s susceptibility to environmental stressors at early developmental stages.

Tracking Mechanisms of Viral Dissemination In Vivo

Dissemination and replication of viruses into hosts is a multistep process where viral particles infect, navigate, and indoctrinate various cell types. Viruses can reach tissues that are distant from their infection site by subverting subcellular mechanisms in ways that are, sometimes, disruptive. Modeling these steps, at appropriate resolution and within animal models, is cumbersome. Yet, mimicking these strategies in vitro fails to recapitulate the complexity of the cellular ecosystem. Here, we will discuss relevant in vivo platforms to dissect the cellular and molecular programs governing viral dissemination and briefly discuss organoid and ex vivo alternatives. We will focus on the zebrafish model and will describe how it provides a transparent window to unravel new cellular mechanisms of viral dissemination in vivo.

The zebrafish model allows in vivo investigations of virus-induced molecular processes at subcellular resolution.

Viruses have evolved multiple strategies for disseminating over long distance, including by indoctrinating host cell types with high migration potential.

Organoids derived from stem cells emerge as powerful alternatives to unravel new molecular mechanisms of viral dissemination.

Zebrafish as an alternative animal model in human and animal vaccination research

Much of medical research relies on animal models to deepen knowledge of the causes of animal and human diseases, as well as to enable the development of innovative therapies. Despite rodents being the most widely used research model worldwide, in recent decades, the use of the zebrafish (Danio rerio) model has exponentially been adopted among the scientific community. This is because such a small tropical freshwater teleost fish has crucial genetic, anatomical and physiological homology with mammals. Therefore, zebrafish constitutes an excellent experimental model for behavioral, genetic and toxicological studies which unravels the mechanism of various human diseases. Furthermore, it serves well to test new therapeutic agents, such as the safety of new vaccines. The aim of this review was to provide a systematic literature review on the most recent studies carried out on the topic. It presents numerous advantages of this type of animal model in tests of efficacy and safety of both animal and human vaccines, thus highlighting gains in time and cost reduction of research and analyzes.

The Zebrafish Xenograft Platform-A Novel Tool for Modeling KSHV-Associated Diseases

Kaposi’s sarcoma associated-herpesvirus (KSHV, also known as human herpesvirus-8) is a gammaherpesvirus that establishes life-long infection in human B lymphocytes. KSHV infection is typically asymptomatic, but immunosuppression can predispose KSHV-infected individuals to primary effusion lymphoma (PEL); a malignancy driven by aberrant proliferation of latently infected B lymphocytes, and supported by pro-inflammatory cytokines and angiogenic factors produced by cells that succumb to lytic viral replication. Here, we report the development of the first in vivo model for a virally induced lymphoma in zebrafish, whereby KSHV-infected PEL tumor cells engraft and proliferate in the yolk sac of zebrafish larvae. Using a PEL cell line engineered to produce the viral lytic switch protein RTA in the presence of doxycycline, we demonstrate drug-inducible reactivation from KSHV latency in vivo, which enabled real-time observation and evaluation of latent and lytic phases of KSHV infection. In addition, we developed a sensitive droplet digital PCR method to monitor latent and lytic viral gene expression and host cell gene expression in xenografts. The zebrafish yolk sac is not well vascularized, and by using fluorogenic assays, we confirmed that this site provides a hypoxic environment that may mimic the microenvironment of some human tumors. We found that PEL cell proliferation in xenografts was dependent on the host hypoxia-dependent translation initiation factor, eukaryotic initiation factor 4E2 (eIF4E2). This demonstrates that the zebrafish yolk sac is a functionally hypoxic environment, and xenografted cells must switch to dedicated hypoxic gene expression machinery to survive and proliferate. The establishment of the PEL xenograft model enables future studies that exploit the innate advantages of the zebrafish as a model for genetic and pharmacologic screens.

Alternative Experimental Models for Studying Influenza Proteins, Host-Virus Interactions and Anti-Influenza Drugs

Ninety years after the discovery of the virus causing the influenza disease, this malady remains one of the biggest public health threats to mankind. Currently available drugs and vaccines only partially reduce deaths and hospitalizations. Some of the reasons for this disturbing situation stem from the sophistication of the viral machinery, but another reason is the lack of a complete understanding of the molecular and physiological basis of viral infections and host-pathogen interactions. Even the functions of the influenza proteins, their mechanisms of action and interaction with host proteins have not been fully revealed. These questions have traditionally been studied in mammalian animal models, mainly ferrets and mice (as well as pigs and non-human primates) and in cell lines. Although obviously relevant as models to humans, these experimental systems are very complex and are not conveniently accessible to various genetic, molecular and biochemical approaches. The fact that influenza remains an unsolved problem, in combination with the limitations of the conventional experimental models, motivated increasing attempts to use the power of other models, such as low eukaryotes, including invertebrate, and primary cell cultures. In this review, we summarized the efforts to study influenza in yeast, Drosophila, zebrafish and primary human tissue cultures and the major contributions these studies have made toward a better understanding of the disease. We feel that these models are still under-utilized and we highlight the unique potential each model has for better comprehending virus-host interactions and viral protein function.

A robust human norovirus replication model in zebrafish larvae

Human noroviruses (HuNoVs) are the most common cause of foodborne illness, with a societal cost of $60 billion and 219,000 deaths/year. The lack of robust small animal models has significantly hindered the understanding of norovirus biology and the development of effective therapeutics. Here we report that HuNoV GI and GII replicate to high titers in zebrafish (Danio rerio) larvae; replication peaks at day 2 post infection and is detectable for at least 6 days. The virus (HuNoV GII.4) could be passaged from larva to larva two consecutive times. HuNoV is detected in cells of the hematopoietic lineage and the intestine, supporting the notion of a dual tropism. Antiviral treatment reduces HuNoV replication by >2 log10, showing that this model is suited for antiviral studies. Zebrafish larvae constitute a simple and robust replication model that will largely facilitate studies of HuNoV biology and the development of antiviral strategies.

A Highly Divergent Picornavirus Infecting the Gut Epithelia of Zebrafish (Danio rerio) in Research Institutions Worldwide

Zebrafish have been extensively used as a model system for research in vertebrate development and pathogen-host interactions. We describe the complete genome of a novel picornavirus identified during a viral metagenomics analysis of zebrafish gut tissue. The closest relatives of this virus showed identity of <20% in their P1 capsids and <36% in their RdRp qualifying zebrafish picornavirus-1 (ZfPV-1) as member of a novel genus with a proposed name of Cyprivirus. Reverse transcription (RT)-PCR testing of zebrafish from North America, Europe, and Asia showed ZfPV-1 to be globally distributed, being detected in 23 of 41 (56%) institutions tested. In situ hybridization of whole zebrafish showed viral RNA was restricted to a subset of enterocytes and cells in the subjacent lamina propria of the intestine and the intestinal mucosa. This naturally occurring and apparently asymptomatic infection (in wild-type zebrafish lineage AB) provides a natural infection system to study picornavirus-host interactions in an advanced vertebrate model organism. Whether ZfPV-1 infection affects any immunological, developmental, or other biological processes in wild-type or mutant zebrafish lineages remains to be determined.

Evolutionary divergence of the vertebrate TNFAIP8 gene family: Applying the spotted gar orthology bridge to understand ohnolog loss in teleosts

Comparative functional genomic studies require the proper identification of gene orthologs to properly exploit animal biomedical research models. To identify gene orthologs, comprehensive, conserved gene synteny analyses are necessary to unwind gene histories that are convoluted by two rounds of early vertebrate genome duplication, and in the case of the teleosts, a third round, the teleost genome duplication (TGD). Recently, the genome of the spotted gar, a holostean outgroup to the teleosts that did not undergo this third genome duplication, was sequenced and applied as an orthology bridge to facilitate the identification of teleost orthologs to human genes and to enhance the power of teleosts as biomedical models. In this study, we apply the spotted gar orthology bridge to help describe the gene history of the vertebrate TNFAIP8 family. Members of the TNFAIP8 gene family have been linked to regulation of immune function and homeostasis and the development of multiple cancer types. Through a conserved gene synteny analysis, we identified zebrafish orthologs to human TNFAIP8L1 and TNFAIP8L3 genes and two co-orthologs to human TNFAIP8L2, but failed to identify an ortholog to human TNFAIP8. Through the application of the orthology bridge, we determined that teleost orthologs to human TNFAIP8 genes were likely lost in a genome inversion event after their divergence from their common ancestor with spotted gar. These findings demonstrate the value of this enhanced approach to gene history analysis and support the development of teleost models to study complex questions related to an array of biomedical issues, including immunity and cancer.

Zebrafish as a Model for the Study of Host-Virus Interactions

Zebrafish (Danio rerio) has become an increasingly important model for in vivo and in vitro studies on host-pathogen interaction, offering scientists with optical accessibility and genetic tractability, and a vertebrate-type immunity that can be separated into innate and adaptive ones. Although it is shown in previous studies that few species of viruses can naturally infect zebrafish, the spring viraemia of carp virus (SVCV), a rhabdovirus that causes contagious acute hemorrhagic viraemia in a variety of cyprinid fishes, can infect zebrafish by both injection and static immersion methods in laboratory conditions. In addition, SVCV can infect zebrafish fibroblast cell line (ZF4 cells), together with the Epithelioma papulosum cyprini (EPC) cell line (EPC cells), a common cell line used widely in fish disease research. The infection and propagation of SVCV in zebrafish and especially in these cell lines can be employed conveniently in laboratory for functional assays of zebrafish genes. The zebrafish, ZF4 and EPC cell, and SVCV can serve as a simple and efficient model system in understanding host-virus interactions. In the present chapter, we provide detailed protocols for the host-virus interaction analysis based on zebrafish embryos, ZF4/EPC cells, and SVCV, including infection methods of zebrafish embryos and cell lines, analyses of immune responses by quantitative PCR (qPCR) and RNA sequencing (RNA-Seq), antiviral assays based on ZF4 and EPC cells, and the analysis of host-virus interaction using luciferase assays. These protocols should provide efficient and typical means to address host-virus interactions in a more general biological sense.

Modeling Infectious Diseases in the Context of a Developing Immune System

Zebrafish has been used for over a decade to study the mechanisms of a wide variety of inflammatory disorders and infections, with models ranging from bacterial, viral, to fungal pathogens. Zebrafish has been especially relevant to study the differentiation, specialization, and polarization of the two main innate immune cell types, the macrophages and the neutrophils. The optical accessibility and the early appearance of myeloid cells that can be tracked with fluorescent labels in zebrafish embryos and the ability to use genetics to selectively ablate or expand immune cell populations have permitted studying the interaction between infection, development, and metabolism. Additionally, zebrafish embryos are readily colonized by a commensal flora, which facilitated studies that emphasize the requirement for immune training by the natural microbiota to properly respond to pathogens. The remarkable conservation of core mechanisms required for the recognition of microbial and danger signals and for the activation of the immune defenses illustrates the high potential of the zebrafish model for biomedical research. This review will highlight recent insight that the developing zebrafish has contributed to our understanding of host responses to invading microbes and the involvement of the microbiome in several physiological processes. These studies are providing a mechanistic basis for developing novel therapeutic approaches to control infectious diseases.

Infectious disease models in zebrafish

In recent years, the zebrafish (Danio rerio) has developed as an important alternative to mammalian models for the study of hostpathogen interactions. Because they lack a functional adaptive immune response during the first 4-6weeks of development, zebrafish rely upon innate immune responses to protect against injuries and infections. During this early period of development, it is possible to isolate and study mechanisms of infection and inflammation arising from the innate immune response without the complications presented by the adaptive immune response. Zebrafish possess several inherent characteristics that make them an attractive option to study hostpathogen interactions, including extensive sequence and functional conservation with the human genome, optical clarity in larvae that facilitates the high-resolution visualization of host cell-microbe interactions, a fully sequenced and annotated genome, robust forward and reverse genetic tools and techniques (e.g., CRISPR-Cas9 and TALENs), and amenability to chemical studies and screens. Here, we describe methods for studying hostpathogen interactions both through systemic infections and through localized infections that allow analysis of host cell response, migration patterns, and behavior. Each of the methods described can be modified for use in downstream applications that include ecotoxicant studies and chemical screens.

The Animal Model Determines the Results of Aeromonas Virulence Factors

The selection of an experimental animal model is of great importance in the study of bacterial virulence factors. Here, a bath infection of zebrafish larvae is proposed as an alternative model to study the virulence factors of Aeromonas hydrophila. Intraperitoneal infections in mice and trout were compared with bath infections in zebrafish larvae using specific mutants. The great advantage of this model is that bath immersion mimics the natural route of infection, and injury to the tail also provides a natural portal of entry for the bacteria. The implication of T3SS in the virulence of A. hydrophila was analyzed using the AH-1::aopB mutant. This mutant was less virulent than the wild-type strain when inoculated into zebrafish larvae, as described in other vertebrates. However, the zebrafish model exhibited slight differences in mortality kinetics only observed using invertebrate models. Infections using the mutant AH-1ΔvapA lacking the gene coding for the surface S-layer suggested that this protein was not totally necessary to the bacteria once it was inside the host, but it contributed to the inflammatory response. Only when healthy zebrafish larvae were infected did the mutant produce less mortality than the wild-type. Variations between models were evidenced using the AH-1ΔrmlB, which lacks the O-antigen lipopolysaccharide (LPS), and the AH-1ΔwahD, which lacks the O-antigen LPS and part of the LPS outer-core. Both mutants showed decreased mortality in all of the animal models, but the differences between them were only observed in injured zebrafish larvae, suggesting that residues from the LPS outer core must be important for virulence. The greatest differences were observed using the AH-1ΔFlaB-J (lacking polar flagella and unable to swim) and the AH-1::motX (non-motile but producing flagella). They were as pathogenic as the wild-type strain when injected into mice and trout, but no mortalities were registered in zebrafish larvae. This study demonstrates that zebrafish larvae can be used as a host model to assess the virulence factors of A. hydrophila. This model revealed more differences in pathogenicity than the in vitro models and enabled the detection of slight variations in pathogenesis not observed using intraperitoneal injections of mice or fish.

Liposome-Mediated Herpes Simplex Virus Uptake Is Glycoprotein-D Receptor-Independent but Requires Heparan Sulfate

Cationic liposomes are widely used to facilitate introduction of genetic material into target cells during transfection. This study describes a non-receptor mediated herpes simplex virus type-1 (HSV-1) entry into the Chinese hamster ovary (CHO-K1) cells that naturally lack glycoprotein D (gD)-receptors using a commercially available cationic liposome: lipofectamine. Presence of cell surface heparan sulfate (HS) increased the levels of viral entry indicating a potential role of HS in this mode of entry. Loss of viral entry in the presence of actin de-polymerizing or lysosomotropic agents suggests that this mode of entry results in the endocytosis of the lipofectamine-virus mixture. Enhancement of HSV-1 entry by liposomes was also demonstrated in vivo using a zebrafish embryo model that showed stronger infection in the eyes and other tissues. Our study provides novel insights into gD receptor independent viral entry pathways and can guide new strategies to enhance the delivery of viral gene therapy vectors or oncolytic viruses.

[Looking through zebrafish to study host-pathogen interactions]

The zebrafish offers many advantages that motivated and validated its use to study the virulence of numerous human pathogens, including viruses, bacteria and fungi. Its immune system is homologous to the one of mammals. The optical transparency of zebrafish embryos allows non-invasive and real-time monitoring of the infection processes through the use of imaging techniques. The zebrafish is therefore a useful and powerful model to study host-pathogen interactions at a cellular level. It may be used to describe pathophysiological events and subversion mechanisms that are specific to each pathogen. In addition to increasing our understanding of the host immune defense, this model is of high potential for medical application, being particularly amenable to high-throughput screening for the discovery of new anti-infective molecules.

Conservation of the STING-Mediated Cytosolic DNA Sensing Pathway in Zebrafish

Zebrafish (Danio rerio) is a unique potential model animal for dissecting innate immune signaling. Here we demonstrate that herpes simplex virus 1 (HSV-1) could infect zebrafish at its different developmental stages and trigger the expression of type I interferons (IFNs) as well as interferon-stimulated genes (ISGs) in zebrafish larvae. Silencing of zSTING, but not zMAVS, markedly attenuates the DNA virus-induced antiviral responses. Notably, a conserved serine residue (S373) is essential for the action of zSTING. Unexpectedly, zebrafish cyclic GMP-AMP synthase (cGAS) is dispensable for the STING signaling, whereas zDHX9 and zDDX41 are potential sensors for HSV-1 infection in vivo. Taken together, this proof-of-concept study establishes the zebrafish larva as a feasible model for investigating the cytosolic DNA sensing mechanism, shedding light on the conservation of the STING antiviral signaling pathway.

IMPORTANCE

The zebrafish larva provides technical advantages for understanding host-pathogen interactions. In this study, we established the zebrafish larva as a useful model for studying HSV-1 infection. HSV-1 infection triggers strong type I interferon production, which depends on STING expression. In addition, STING-mediated antiviral signaling is conserved in zebrafish. Interestingly, zDHX9 and zDDX41 are indispensable for detecting HSV-1, while cGAS is dispensable. This proof-of-concept study indicates that the zebrafish represents an amenable model for the investigation of cytosolic DNA sensing mechanisms.

Diversity of heparan sulfate and HSV entry: basic understanding and treatment strategies

A modified form of heparan sulfate (HS) known as 3-O-sulfated heparan sulfate (3-OS HS) generates fusion receptor for herpes simplex virus (HSV) entry and spread. Primary cultures of corneal fibroblasts derived from human eye donors have shown the clinical significance of this receptor during HSV corneal infection. 3-OS HS- is a product of a rare enzymatic modification at C3 position of glucosamine residue which is catalyzed by 3-O-sulfotransferases (3-OSTs) enzymes. From humans to zebrafish, the 3-OST enzymes are highly conserved and widely expressed in cells and tissues. There are multiple forms of 3-OSTs each producing unique subset of sulfated HS making it chemically diverse and heterogeneous. HSV infection of cells or zebrafish can be used as a unique tool to understand the structural-functional activities of HS and 3-OS HS and likewise, the infection can be used as a functional assay to screen phage display libraries for identifying HS and 3-OS HS binding peptides or small molecule inhibitors. Using this approach over 200 unique 12-mer HS and 3-OS HS recognizing peptides were isolated and characterized against HSV corneal infection where 3-OS HS is known to be a key receptor. In this review we discuss emerging role of 3-OS HS based therapeutic strategies in preventing viral infection and tissue damage.

Influenza A virus infection in zebrafish recapitulates mammalian infection and sensitivity to anti-influenza drug treatment

Seasonal influenza virus infections cause annual epidemics and sporadic pandemics. These present a global health concern, resulting in substantial morbidity, mortality and economic burdens. Prevention and treatment of influenza illness is difficult due to the high mutation rate of the virus, the emergence of new virus strains and increasing antiviral resistance. Animal models of influenza infection are crucial to our gaining a better understanding of the pathogenesis of and host response to influenza infection, and for screening antiviral compounds. However, the current animal models used for influenza research are not amenable to visualization of host-pathogen interactions or high-throughput drug screening. The zebrafish is widely recognized as a valuable model system for infectious disease research and therapeutic drug testing. Here, we describe a zebrafish model for human influenza A virus (IAV) infection and show that zebrafish embryos are susceptible to challenge with both influenza A strains APR8 and X-31 (Aichi). Influenza-infected zebrafish show an increase in viral burden and mortality over time. The expression of innate antiviral genes, the gross pathology and the histopathology in infected zebrafish recapitulate clinical symptoms of influenza infections in humans. This is the first time that zebrafish embryos have been infected with a fluorescent IAV in order to visualize infection in a live vertebrate host, revealing a pattern of vascular endothelial infection. Treatment of infected zebrafish with a known anti-influenza compound, Zanamivir, reduced mortality and the expression of a fluorescent viral gene product, demonstrating the validity of this model to screen for potential antiviral drugs. The zebrafish model system has provided invaluable insights into host-pathogen interactions for a range of infectious diseases. Here, we demonstrate a novel use of this species for IAV research. This model has great potential to advance our understanding of influenza infection and the associated host innate immune response.

Studying the immune response to human viral infections using zebrafish

Humans and viruses have a long co-evolutionary history. Viral illnesses have and will continue to shape human history: from smallpox, to influenza, to HIV, and beyond. Animal models of human viral illnesses are needed in order to generate safe and effective antiviral medicines, adjuvant therapies, and vaccines. These animal models must support the replication of human viruses, recapitulate aspects of human viral illnesses, and respond with conserved immune signaling cascades. The zebrafish is perhaps the simplest, most commonly used laboratory model organism in which innate and/or adaptive immunity can be studied. Herein, we will discuss the current zebrafish models of human viral illnesses and the insights they have provided. We will highlight advantages of early life stage zebrafish and the importance of innate immunity in human viral illnesses. We will also discuss viral characteristics to consider before infecting zebrafish with human viruses as well as predict other human viruses that may be able to infect zebrafish.

Comprehensive analysis of herpes simplex virus 1 (HSV-1) entry mediated by zebrafish 3-O-Sulfotransferase isoforms: implications for the development of a zebrafish model of HSV-1 infection

Binding of herpes simplex virus 1 (HSV-1) envelope glycoprotein D (gD) to the receptor 3-O-sulfated heparan sulfate (3-OS HS) mediates viral entry. 3-O-Sulfation of HS is catalyzed by the 3-O-sulfotransferase (3-OST) enzyme. Multiple isoforms of 3-OST are differentially expressed in tissues of zebrafish (ZF) embryos. Here, we performed a comprehensive analysis of the role of ZF 3-OST isoforms (3-OST-1, 3-OST-5, 3-OST-6, and 3-OST-7) in HSV-1 entry. We found that a group of 3-OST gene family isoforms (3-OST-2, -3, -4, and -6) with conserved catalytic and substrate-binding residues of the enzyme mediates HSV-1 entry and spread, while the other group (3-OST-1, -5, and -7) lacks these properties. These results demonstrate that HSV-1 entry can be recapitulated by certain ZF 3-OST enzymes, a significant step toward the establishment of a ZF model of HSV-1 infection and tissue-specific tropism.

Cellular visualization of macrophage pyroptosis and interleukin-1β release in a viral hemorrhagic infection in zebrafish larvae

Hemorrhagic viral diseases are distributed worldwide with important pathogens, such as dengue virus or hantaviruses. The lack of adequate in vivo infection models has limited the research on viral pathogenesis and the current understanding of the underlying infection mechanisms. Although hemorrhages have been associated with the infection of endothelial cells, other cellular types could be the main targets for hemorrhagic viruses. Our objective was to take advantage of the use of zebrafish larvae in the study of viral hemorrhagic diseases, focusing on the interaction between viruses and host cells. Cellular processes, such as transendothelial migration of leukocytes, virus-induced pyroptosis of macrophages. and interleukin-1β (Il-1β) release, could be observed in individual cells, providing a deeper knowledge of the immune mechanisms implicated in the disease. Furthermore, the application of these techniques to other pathogens will improve the current knowledge of host-pathogen interactions and increase the potential for the discovery of new therapeutic targets. Importance: Pathogenic mechanisms of hemorrhagic viruses are diverse, and most of the research regarding interactions between viruses and host cells has been performed in cell lines that might not be major targets during natural infections. Thus, viral pathogenesis research has been limited because of the lack of adequate in vivo infection models. The understanding of the relative pathogenic roles of the viral agent and the host response to the infection is crucial. This will be facilitated by the establishment of in vivo infection models using organisms such as zebrafish, which allows the study of the diseases in the context of a complete individual. The use of this animal model with other pathogens could improve the current knowledge on host-pathogen interactions and increase the potential for the discovery of new therapeutic targets against diverse viral diseases.

Fast gene transfer into the adult zebrafish brain by herpes simplex virus 1 (HSV-1) and electroporation: methods and optogenetic applications

The zebrafish has various advantages as a model organism to analyze the structure and function of neural circuits but efficient viruses or other tools for fast gene transfer are lacking. We show that transgenes can be introduced directly into the adult zebrafish brain by herpes simplex type I viruses (HSV-1) or electroporation. We developed a new procedure to target electroporation to defined brain areas and identified promoters that produced strong long-term expression. The fast workflow of electroporation was exploited to express multiple channelrhodopsin-2 variants and genetically encoded calcium indicators in telencephalic neurons for measurements of neuronal activity and synaptic connectivity. The results demonstrate that HSV-1 and targeted electroporation are efficient tools for gene delivery into the zebrafish brain, similar to adeno-associated viruses and lentiviruses in other species. These methods fill an important gap in the spectrum of molecular tools for zebrafish and are likely to have a wide range of applications.

Zebrafish: modeling for herpes simplex virus infections

For many years, zebrafish have been the prototypical model for studies in developmental biology. In recent years, zebrafish has emerged as a powerful model system to study infectious diseases, including viral infections. Experiments conducted with herpes simplex virus type-1 in adult zebrafish or in embryo models are encouraging as they establish proof of concept with viral-host tropism and possible screening of antiviral compounds. In addition, the presence of human homologs of viral entry receptors in zebrafish such as 3-O sulfated heparan sulfate, nectins, and tumor necrosis factor receptor superfamily member 14-like receptor bring strong rationale for virologists to test their in vivo significance in viral entry in a zebrafish model and compare the structure-function basis of virus zebrafish receptor interaction for viral entry. On the other end, a zebrafish model is already being used for studying inflammation and angiogenesis, with or without genetic manipulations, and therefore can be exploited to study viral infection-associated pathologies. The major advantage with zebrafish is low cost, easy breeding and maintenance, rapid lifecycle, and a transparent nature, which allows visualizing dissemination of fluorescently labeled virus infection in real time either at a localized region or the whole body. Further, the availability of multiple transgenic lines that express fluorescently tagged immune cells for in vivo imaging of virus infected animals is extremely attractive. In addition, a fully developed immune system and potential for receptor-specific knockouts further advocate the use of zebrafish as a new tool to study viral infections. In this review, we focus on expanding the potential of zebrafish model system in understanding human infectious diseases and future benefits.

Viral diseases in zebrafish: what is known and unknown

Naturally occurring viral infections have the potential to introduce confounding variability that leads to invalid and misinterpreted data. Whereas the viral diseases of research rodents are well characterized and closely monitored, no naturally occurring viral infections have been characterized for the laboratory zebrafish (Danio rerio), an increasingly important biomedical research model. Despite the ignorance about naturally occurring zebrafish viruses, zebrafish models are rapidly expanding in areas of biomedical research where the confounding effects of unknown infectious agents present a serious concern. In addition, many zebrafish research colonies remain linked to the ornamental (pet) zebrafish trade, which can contribute to the introduction of new pathogens into research colonies, whereas mice used for research are purpose bred, with no introduction of new mice from the pet industry. Identification, characterization, and monitoring of naturally occurring viruses in zebrafish are crucial to the improvement of zebrafish health, the reduction of unwanted variability, and the continued development of the zebrafish as a model organism. This article addresses the importance of identifying and characterizing the viral diseases of zebrafish as the scope of zebrafish models expands into new research areas and also briefly addresses zebrafish susceptibility to experimental viral infection and the utility of the zebrafish as an infection and immunology model.

Evaluation of the pathogenesis and treatment of Mycobacterium marinum infection in zebrafish

Mycobacterium marinum-infected zebrafish are used to study tuberculosis pathogenesis, as well as for antitubercular drug discovery. The small size of zebrafish larvae coupled with their optical transparency allows for rapid analysis of bacterial burdens and host survival in response to genetic and pharmacological manipulations of both mycobacteria and host. Automated fluorescence microscopy and automated plate fluorimetry (APF) are coupled with facile husbandry to facilitate large-scale, repeated analysis of individual infected fish. Both methods allow for in vivo screening of chemical libraries, requiring only 0.1 μmol of drug per fish to assess efficacy; they also permit a more detailed evaluation of the individual stages of tuberculosis pathogenesis. Here we describe a 16-h protocol spanning 22 d, in which zebrafish larvae are infected via the two primary injection sites, the hindbrain ventricle and caudal vein; this is followed by the high-throughput evaluation of pathogenesis and antimicrobial efficacy.

Role of Filopodia in HSV-1 Entry into Zebrafish 3-O-Sulfotransferase-3-Expressing Cells

Background:

Heparan sulfate proteoglycans (HSPGs) modified by zebrafish (ZF) encoded glucosaminyl 3-O sulfotransferase-3 (3-OST-3) generate a receptor for herpes simplex virus type-1 (HSV-1) entry and spread. In order to elucidate the mechanism by which HSV-1 enters into ZF-3-OST-3 cells, we investigated the mode of viral entry.

Results:

Under high resolution scanning electron microscopy (SEM), actin cytoskeleton changes were observed by a dramatic increase in the number of filopodia formed during early interactions of HSV-1 with the target cells. While the increase in number was common among all the infected cells, the highest numbers of filopodia was observed in cells expressing the 3-OST-3 modified form of heparan sulfate (HS) encoded either by human or ZF. The levels of viral infection and filopodia induction were reduced with the actin polymerization inhibitors, Cytochalasin-D and Lantriculin B, suggesting an important role for actin reorganization during ZF-3-OST-3 mediated HSV-1 entry. Supporting an interesting possibility of filopodia usage during HSV-1 spread, pre-treatment of cytochalasin D in ZF-3-OST-3 cells drastically reduced virus glycoprotein induced cell fusion.

Conclusions:

Taken together, our results provide new evidence on the involvement of filopodia during HSV-1 infection of ZF-3-OST-3 cells and confirm a role for modified heparan sulfate in cytoskeleton rearrangement during HSV-1 entry.

Zebrafish encoded 3-O-sulfotransferase-2 generated heparan sulfate serves as a receptor during HSV-1 entry and spread

Previously we reported the role of zebrafish (ZF) encoded glucosaminyl 3-O-sulfotransferase-3 (3-OST-3) isoform in assisting herpes simplex virus type-1 (HSV-1) entry and spread by generating an entry receptor to HSV-1 envelope glycoprotein D (gD). However, the ability of ZF encoded 3-OST-2 isoform to participate in HSV-1 entry has not been determined although it is predominantly expressed in ZF brain, a prime target for HSV-1 to infect and establish lifelong latency. Here we report the expression cloning of ZF encoded 3-OST-2 isoform and demonstrate HSV-1 entry into resistant Chinese hamster ovary (CHO-K1) cells expressing the clone. Additional significance of ZF encoded 3-OST-2 receptor was demonstrated using medically important isolates of HSV-1. In addition, interference to HSV-1 entry was observed upon co-expression of HSV-1 gD and ZF 3-OST-2. Similarly HSV-1 entry was significantly inhibited by the pre-treatment of cells with enzyme HS lyases (heparinase II/III). Finally, ZF-3-OST-2 expressing CHO-K1 was able to fuse with HSV-1 glycoprotein expressing cells suggesting their role in HSV-1 spread. Taken together our result demonstrates a role for ZF 3-OST-2 in HSV-1 pathogenesis.

The zebrafish as a model for paediatric diseases

The rapid increase in information about genes and their associations with human diseases has highlighted the need for model organisms suitable for genetic manipulation and drug testing. The zebrafish is a valuable vertebrate animal model that offers many advantages, including the relative ease of husbandry and genetic manipulation and the capacity for high-throughput screens. In this review, we describe the zebrafish as a model for paediatric diseases, with particular emphasis on haematopoietic and infectious diseases.

CONCLUSION

The zebrafish has become an established vertebrate model in which to elucidate the molecular mechanisms of various human diseases.