Inhibitory effects of bile acids and synthetic farnesoid X receptor agonists on rotavirus replication

Deoxycholic acid inhibits ASFV replication by inhibiting MAPK signaling pathway

African swine fever (ASF) is an acute, febrile, highly contagious infection of pigs caused by the African swine fever virus (ASFV). The purpose of this study is to understand the molecular mechanism of ASFV infection and evaluate the effect of DCA on MAPK pathway, so as to provide scientific basis for the development of new antiviral drugs. The transcriptome analysis found that ASFV infection up-regulated the IL-17 and MAPK signaling pathways to facilitate viral replication. Metabolome analysis showed that DCA levels were up-regulated after ASFV infection, and that exogenous DCA could inhibit activation of the MAPK pathway by ASFV infection and thus inhibit viral replication. Dual-luciferase reporter assays were used to screen the genes of ASFV and revealed that I73R could significantly up-regulate the transcription level of AP-1 transcription factor in the MAPK pathway. Confocal microscopy demonstrated that I73R could promote AP-1 entry into the nucleus, and that DCA could inhibit the I73R-mediated nuclear entry of AP-1, inhibiting MAPK pathway, and I73R interacts with AP-1. These results indicated that DCA can inhibit ASFV-mediated activation of the MAPK pathway, thus inhibiting ASFV replication. This study provides a theoretical basis for research on ASF pathogenesis and for antiviral drug development.

Epidemiology, pathogenesis, immune evasion mechanism and vaccine development of porcine Deltacoronavirus

Coronaviruses have been identified as pathogens of gastrointestinal and respiratory diseases in humans and various animal species. In recent years, the global spread of new coronaviruses has had profound influences for global public health and economies worldwide. As highly pathogenic zoonotic viruses, coronaviruses have become the focus of current research. Porcine Deltacoronavirus (PDCoV), an enterovirus belonging to the family of coronaviruses, has emerged on a global scale in the past decade and significantly influenced the swine industry. Moreover, PDCoV infects not only pigs but also other species, including humans, chickens and cattles, exhibiting a broad host tropism. This emphasizes the need for in-depth studies on coronaviruses to mitigate their potential threats. In this review, we provided a comprehensive summary of the current studies on PDCoV. We first reviewed the epidemiological investigations on the global prevalence and distribution of PDCoV. Then, we delved into the studies on the pathogenesis of PDCoV to understand the mechanisms how the virus impacts its hosts. Furthermore, we also presented some exploration studies on the immune evasion mechanisms of the virus to enhance the understanding of host-virus interactions. Despite current limitations in vaccine development for PDCoV, we highlighted the inhibitory effects observed with certain substances, which offers a potential direction for future research endeavors. In conclusion, this review summarized the scientific findings in epidemiology, pathogenesis, immune evasion mechanisms and vaccine development of PDCoV. The ongoing exploration of potential vaccine candidates and the insights gained from inhibitory substances have provided a solid foundation for future vaccine development to prevent and control diseases associated with PDCoV.

Impact of the Microbiota on Viral Infections

The mammalian gastrointestinal tract (GIT) hosts a diverse and highly active microbiota composed of bacteria, eukaryotes, archaea, and viruses. Studies of the GIT microbiota date back more than a century, although modern techniques, including mouse models, sequencing technology, and novel therapeutics in humans, have been foundational to our understanding of the roles of commensal microbes in health and disease. Here, we review the impacts of the GIT microbiota on viral infection, both within the GIT and systemically. GIT-associated microbes and their metabolites alter the course of viral infection through a variety of mechanisms, including direct interactions with virions, alteration of the GIT landscape, and extensive regulation of innate and adaptive immunity. Mechanistic understanding of the full breadth of interactions between the GIT microbiota and the host is still lacking in many ways but will be vital for the development of novel therapeutics for viral and nonviral diseases alike.

Advances in the development of antivirals for rotavirus infection

Rotavirus (RV) causes 200,000 deaths per year and imposes a serious burden to public health and livestock farming worldwide. Currently, rehydration (oral and intravenous) remains the main strategy for the treatment of rotavirus gastroenteritis (RVGE), and no specific drugs are available. This review discusses the viral replication cycle in detail and outlines possible therapeutic approaches including immunotherapy, probiotic-assisted therapy, anti-enteric secretory drugs, Chinese medicine, and natural compounds. We present the latest advances in the field of rotavirus antivirals and highlights the potential use of Chinese medicine and natural compounds as therapeutic agents. This review provides an important reference for rotavirus prevention and treatment.

A perspective study of the possible impact of obeticholic acid against SARS-CoV-2 infection

The causative agent of CoV disease 2019 is a new coronavirus CoV type 2, affecting the respiratory tract with severe manifestations (SARS-CoV-2). Covid-19 is mainly symptomless, with slight indications in about 85% of the affected cases. Many efforts were done to face this pandemic by testing different drugs and agents to make treatment protocols in different countries. However, the use of these proposed drugs is associated with the development of adverse events. Remarkably, the successive development of SARS-CoV-2 variants which could affect persons even they were vaccinated, prerequisite wide search to find efficient and safe agents to face SARS-CoV-2 infection. Obeticholic acid (OCA), which has anti-inflammatory effects, may efficiently treat Covid-19. Thus, the goal of this perspective study is to focus on the possible medicinal effectiveness in managing Covid-19. OCA is a powerful farnesoid X receptor (FXR) agonist possessing marked antiviral and anti-inflammatory effects. FXR is dysregulated in Covid-19 resulting in hyper-inflammation with concurrent occurrence of hypercytokinemia. Interestingly, OCA inhibits the reaction between this virus and angiotensin-converting enzyme type 2 (ACE2) receptors. FXR agonists control the expression of ACE2 and the inflammatory signaling pathways in this respiratory syndrome, which weakens the effects of Covid-19 disease and accompanied complications. Taken together, FXR agonists like OCA may reveal both direct and indirect impacts in the modulation of immune reaction in SARS-CoV-2 conditions. It is highly recommended to perform many investigations regarding different phases of the discovery of new drugs.

Metabolic syndrome and aberrant immune responses to viral infection and vaccination: Insights from small animal models

This review outlines the propensity for metabolic syndrome (MetS) to induce elevated disease severity, higher mortality rates post-infection, and poor vaccination outcomes for viral pathogens. MetS is a cluster of conditions including high blood glucose, an increase in circulating low-density lipoproteins and triglycerides, abdominal obesity, and elevated blood pressure which often overlap in their occurrence. MetS diagnoses are on the rise, as reported cases have increased by greater than 35% since 1988, resulting in one-third of United States adults currently diagnosed as MetS patients. In the aftermath of the 2009 H1N1 pandemic, a link between MetS and disease severity was established. Since then, numerous studies have been conducted to illuminate the impact of MetS on enhancing virally induced morbidity and dysregulation of the host immune response. These correlative studies have emphasized the need for elucidating the mechanisms by which these alterations occur, and animal studies conducted as early as the 1940s have linked the conditions associated with MetS with enhanced viral disease severity and poor vaccine outcomes. In this review, we provide an overview of the importance of considering overall metabolic health in terms of cholesterolemia, glycemia, triglyceridemia, insulin and other metabolic molecules, along with blood pressure levels and obesity when studying the impact of metabolism-related malignancies on immune function. We highlight the novel insights that small animal models have provided for MetS-associated immune dysfunction following viral infection. Such animal models of aberrant metabolism have paved the way for our current understanding of MetS and its impact on viral disease severity, dysregulated immune responses to viral pathogens, poor vaccination outcomes, and contributions to the emergence of viral variants.

Alterations in bile acids as metabolic signatures in the patients with human adenovirus type 7 infection

Objectives

The changes in metabolism by human adenovirus (HAdV) infection was unclear. The potential mechanism of HAdV-7 causing acute respiratory tract infection was explored.

Methods

Totally 35 patients with HAdV-7 infection, 32 asymptomatic cases with HAdV-7 and 14 healthy controls were enrolled from an outbreak of HAdV-7 in the army. The serum samples were analyzed by untargeted and targeted metabolomics. The effects of differential metabolites were verified on HAdV-7 replication in an A549 cell line.

Results

The untargeted metabolomics analysis revealed more significant changes in the classes of sphingolipids, polyketides, glycerolipids, fatty acyls, and carboxylic acids and their derivatives in the patients with HAdV-7 than in healthy controls. Two key metabolic pathways of secondary and primary bile acid biosynthesis were noted from pathway enrichment analysis. Targeted metabolomics analysis showed that the levels of unconjugated bile acids in the patients were significantly lower, while the levels of glyco- and tauro- conjugated bile acids in patients and asymptomatic cases were higher than those in the healthy controls. The profiles of cytokines and peripheral lymphocyte subsets obviously varied at different levels of bile acids, with significant differences after HAdV-7 infection. A cell verification test demonstrated that the replication of HAdV-7 significantly reduced when GCDCA and TCA were added.

Conclusion

Bile acids inhibited HAdV-7 replication in vitro. Alterations in bile acids was metabolic signatures of HAdV-7 infected subjects, and our results suggested bile acids might play protective roles against HAdV-7 infection.

Rotavirus-Induced Lipid Droplet Biogenesis Is Critical for Virus Replication

A variety of pathogens, including viruses, bacteria and parasites, target cellular lipid droplets for their replication. Rotaviruses (RVs) infect the villous epithelium of the small intestine and are a major cause of acute gastroenteritis in infants and young children worldwide. RVs induce and require lipid droplets for the formation of viroplasms, sites of virus genome replication, and nascent particle assembly. Here we review the role of lipid droplets in RV replication. Inhibitors of fatty acid synthesis or chemicals that interfere with lipid droplet homeostasis decrease the number and size of viroplasms and the yield of infectious virus. We used a genetically engineered RV, delayed in viroplasm assembly, to show an early interaction of RV nonstructural protein NSP2 and the lipid droplet-associated protein phospho-PLIN1. The interaction between NSP2 and phospho-PLIN1 suggests that we have identified part of the mechanism of RV-induced lipid droplet formation. These studies demonstrate that RV is an excellent model to dissect the cellular process of lipid droplet formation and to determine how RV induces and usurps lipid droplet biogenesis to form viroplasm/lipid droplets for virus replication.

A glucose-like metabolite deficient in diabetes inhibits cellular entry of SARS-CoV-2

The severity and mortality of COVID-19 are associated with pre-existing medical comorbidities such as diabetes mellitus. However, the underlying causes for increased susceptibility to viral infection in patients with diabetes is not fully understood. Here we identify several small-molecule metabolites from human blood with effective antiviral activity against SARS-CoV-2, one of which, 1,5-anhydro-D-glucitol (1,5-AG), is associated with diabetes mellitus. The serum 1,5-AG level is significantly lower in patients with diabetes. In vitro, the level of SARS-CoV-2 replication is higher in the presence of serum from patients with diabetes than from healthy individuals and this is counteracted by supplementation of 1,5-AG to the serum from patients. Diabetic (db/db) mice undergo SARS-CoV-2 infection accompanied by much higher viral loads and more severe respiratory tissue damage when compared to wild-type mice. Sustained supplementation of 1,5-AG in diabetic mice reduces SARS-CoV-2 loads and disease severity to similar levels in nondiabetic mice. Mechanistically, 1,5-AG directly binds the S2 subunit of the SARS-CoV-2 spike protein, thereby interrupting spike-mediated virus-host membrane fusion. Our results reveal a mechanism that contributes to COVID-19 pathogenesis in the diabetic population and suggest that 1,5-AG supplementation may be beneficial to diabetic patients against severe COVID-19.

Treading a HOSTile path: Mapping the dynamic landscape of host cell-rotavirus interactions to explore novel host-directed curative dimensions

Viruses are intracellular pathogens and are dependent on host cellular resources to carry out their cycles of perpetuation. Obtaining an integrative view of host-virus interaction is of utmost importance to understand the complex and dynamic interplay between viral components and host machineries. Besides its obvious scholarly significance, a comprehensive host-virus interaction profile also provides a platform where from host determinants of pro-viral and antiviral importance can be identified and further be subjected to therapeutic intervention. Therefore, adjunct to conventional methods of prophylactic vaccination and virus-directed antivirals, this host-targeted antiviral approach holds promising therapeutic potential. In this review, we present a comprehensive landscape of host cellular reprogramming in response to infection with rotavirus (RV) which causes profuse watery diarrhea in neonates and infants. In addition, an emphasis is given on how host determinants are either usurped or subverted by RV in course of infection and how therapeutic manipulation of specific host factors can effectively modulate the RV life cycle.

Roles of bile acids in enteric virus replication

Bile acids (BAs) are evolutionally conserved molecules synthesized in the liver from cholesterol to facilitating the absorption of fat-soluble nutrients. In the intestines, where enteric viruses replicate, BAs also act as signaling molecules that modulate various biological functions via activation of specific receptors and cell signaling pathways. To date, BAs present either pro-viral or anti-viral effects for the replication of enteric viruses in vivo and in vitro. In this review, we summarized current information on biosynthesis, transportation and metabolism of BAs and the role of BAs in replication of enteric caliciviruses, rotaviruses, and coronaviruses. We also discussed the application of BAs for cell culture adaptation of fastidious enteric caliciviruses and control of virus infection, which may provide novel insights into the development of antivirals and/or disinfectants for enteric viruses.

The influence of microbiota-derived metabolites on viral infections

Intestinal microbiota have profound effects on viral infections locally and systemically. While they can directly influence enteric virus infections, there is also an increasing appreciation for the role of microbiota-derived metabolites in regulating virus infections. Because metabolites diffuse across the intestinal epithelium and enter circulation, they can influence host response to pathogens at extraintestinal sites. In this review, we summarize the effects of three types of microbiota-derived metabolites on virus infections. While short-chain fatty acids serve to regulate the extent of inflammation associated with viral infections, the flavonoid desaminotyrosine and bile acids generally regulate interferon responses. A common theme that emerges is that microbiota-derived metabolites can have proviral and antiviral effects depending on the virus in question. Understanding the molecular mechanisms by which microbiota-derived metabolites impact viral infections and the highly conditional nature of these responses should pave the way to developing novel rational antivirals.

Bile acids LCA and CDCA inhibited porcine deltacoronavirus replication in vitro

Porcine deltacoronavirus (PDCoV) is an emerging enteric coronavirus that causes gastroenteritis in pigs and no vaccines or antiviral drugs are available. Bile acids are active factors in intestines and influence the replication of enteric viruses. Currently, the role of bile acids on PDCoV replication is unknown. In this study, we tested the effects of different types of bile acids on the replication of PDCoV in cell culture. We found that physiological concentrations of bile acids chenodeoxycholic acid (CDCA) and lithocholic acid (LCA) had antiviral activity against PDCoV in porcine kidney cell line (LLC-PK1) and porcine small intestinal epithelial cell line (IPEC-J2). In IPEC-J2 cells, CDCA and LCA inhibited PDCoV replication at post-entry stages by inducing the production of interferon (IFN)-λ3 and IFN-stimulated gene 15 (ISG15) via G protein-coupled receptor (GPCR). In summary, bile acids CDCA and LCA restricted PDCoV infection and LCA functioned through a GPCR-IFN-λ3-ISG15 signaling axis in IPEC-J2 cells. Our results may open new avenues for the development of antiviral drugs to treat PDCoV infection in pigs.

Next-Generation Probiotics and Their Metabolites in COVID-19

Since December 2019, a global pandemic has been observed, caused by the emergence of a new coronavirus, SARS CoV-2. The latter is responsible for the respiratory disease, COVID-19. The infection is also characterized by renal, hepatic, and gastrointestinal dysfunctions suggesting the spread of the virus to other organs. A dysregulated immune response was also reported. To date, there is no measure to treat or prevent SARS CoV-2 infection. Additionally, as gut microbiota composition is altered in patients with COVID-19, alternative therapies using probiotics can be considered to fight SARS CoV-2 infection. This review aims at summarizing the current knowledge about next-generation probiotics (NGPs) and their benefits in viral respiratory tract infections and in COVID-19. We describe these bacteria, highlighted by studies using metagenomic approaches. In addition, these bacteria generate metabolites such as butyrate, desaminotyrosine, and secondary bile acid, suggested to prevent viral respiratory infections. Gut microbial metabolites transported via the circulation to the lungs could inhibit viral replication or improve the immune response against viruses. The use of probiotics and/or their metabolites may target either the virus itself and/or the immunologic process. However, this review showed that more studies are needed to determine the benefits of probiotics and metabolite products in COVID-19.

Fecal Microbiota Transplantation during and Post-COVID-19 Pandemic

COVID-19 is a major pandemic facing the world today, which has implications on current microbiome-based treatments such as fecal microbiota transplantation (FMT) used for recurrent Clostridioides difficile infections. The bidirectional relationship between the inhabitants of our gut, the gut microbiota, and COVID-19 pathogenesis, as well as the underlying mechanism involved, must be elucidated in order to increase FMT safety and efficacy. In this perspective, we discuss the crucial cross-talk between the gut microbiota and the lungs, known as the gut-lung axis, during COVID-19 infection, as well as the putative effect of these microorganisms and their functional activity (i.e., short chain fatty acids and bile acids) on FMT treatment. In addition, we highlight the urgent need to investigate the possible impact of COVID-19 on FMT safety and efficacy, as well as instilling stringent screening protocols of donors and recipients during COVID-19 and post-COVID-19 pandemic to produce a cohesive and optimized FMT treatment plan across all centers and in all countries across the globe.

Microbiota-immune system interactions and enteric virus infection

Enteric viruses are important human pathogens that pose a significant global health problem. These viruses infect the gastrointestinal tract, which contains a community of microbes called the 'microbiota'. We and others have shown that intestinal microbiota are crucial for the replication, pathogenesis, and transmission of a variety of enteric viruses. However, the mechanisms underlying microbiota enhancement of enteric virus infection remain unclear. Interestingly, the host immune system is dependent on both the abundance and composition of the intestinal microbiota. Here we review several aspects of how microbiota influence the immune system and how this could potentially impact enteric virus infection.

The Complex Interactions Between Rotavirus and the Gut Microbiota

Human rotavirus (HRV) is the leading worldwide cause of acute diarrhea-related death in children under the age of five. RV infects the small intestine, an important site of colonization by the microbiota, and studies over the past decade have begun to reveal a complex set of interactions between RV and the gut microbiota. RV infection can temporarily alter the composition of the gut microbiota and probiotic administration alleviates some symptoms of infection in vivo, suggesting reciprocal effects between the virus and the gut microbiota. While development of effective RV vaccines has offered significant protection against RV-associated mortality, vaccine effectiveness in low-income countries has been limited, potentially due to regional differences in the gut microbiota. In this mini review, we briefly detail research findings to date related to HRV vaccine cohorts, studies of natural infection, explorations of RV-microbiota interactions in gnotobiotic pig models, and highlight various in vivo and in vitro models that could be used in future studies to better define how the microbiota may regulate RV infection and host antiviral immune responses.

The Intestinal Microbiome Restricts Alphavirus Infection and Dissemination through a Bile Acid-Type I IFN Signaling Axis

Chikungunya virus (CHIKV), an emerging alphavirus, has infected millions of people. However, the factors modulating disease outcome remain poorly understood. Here, we show in germ-free mice or in oral antibiotic-treated conventionally housed mice with depleted intestinal microbiomes that greater CHIKV infection and spread occurs within 1 day of virus inoculation. Alteration of the microbiome alters TLR7-MyD88 signaling in plasmacytoid dendritic cells (pDCs) and blunts systemic production of type I interferon (IFN). Consequently, circulating monocytes express fewer IFN-stimulated genes and become permissive for CHIKV infection. Reconstitution with a single bacterial species, Clostridium scindens, or its derived metabolite, the secondary bile acid deoxycholic acid, can restore pDC- and MyD88-dependent type I IFN responses to restrict systemic CHIKV infection and transmission back to vector mosquitoes. Thus, symbiotic intestinal bacteria modulate antiviral immunity and levels of circulating alphaviruses within hours of infection through a bile acid-pDC-IFN signaling axis, which affects viremia, dissemination, and potentially transmission.

The intestinal regionalization of acute norovirus infection is regulated by the microbiota via bile acid-mediated priming of type III interferon

Evidence has accumulated to demonstrate that the intestinal microbiota enhances mammalian enteric virus infections1. For example, we and others previously reported that commensal bacteria stimulate acute and persistent murine norovirus infections2-4. However, in apparent contradiction of these results, the virulence of murine norovirus infection was unaffected by antibiotic treatment. This prompted us to perform a detailed investigation of murine norovirus infection in microbially deplete mice, revealing a more complex picture in which commensal bacteria inhibit viral infection of the proximal small intestine while simultaneously stimulating the infection of distal regions of the gut. Thus, commensal bacteria can regulate viral regionalization along the intestinal tract. We further show that the mechanism underlying bacteria-dependent inhibition of norovirus infection in the proximal gut involves bile acid priming of type III interferon. Finally, the regional effects of the microbiota on norovirus infection may result from distinct regional expression profiles of key bile acid receptors that regulate the type III interferon response. Overall, these findings reveal that the biotransformation of host metabolites by the intestinal microbiota directly and regionally impacts infection by a pathogenic enteric virus.

Structural Basis for Human Norovirus Capsid Binding to Bile Acids

A recently developed human norovirus cell culture system revealed that the presence of bile enhanced or was an essential requirement for the growth of certain genotypes. Before this discovery, histo-blood group antigens (HBGAs) were the only well-studied cofactor known for human noroviruses, and there was evidence that several genotypes poorly bound HBGAs. Therefore, the purpose of this study was to investigate how human norovirus capsids interact with bile acids. We found that bile acids had low-micromolar affinities for GII.1, GII.10, and GII.19 capsids but did not bind GI.1, GII.3, GII.4, or GII.17. We showed that bile acid bound at a partially conserved pocket on the norovirus capsid-protruding (P) domain using X-ray crystallography. Amino acid sequence alignment and structural analysis delivered an explanation of selective bile acid binding. Intriguingly, we discovered that binding of the bile acid was the critical step to stabilize several P domain loops that optimally placed an essential amino acid side chain (Asp375) to bind HBGAs in an otherwise HBGA nonbinder (GII.1). Furthermore, bile acid enhanced HBGA binding for a known HBGA binder (GII.10). Altogether, these new data suggest that bile acid functions as a loop-stabilizing regulator and enhancer of HBGA binding for certain norovirus genotypes.IMPORTANCE Given that human norovirus virions likely interact with bile acid during a natural infection, our evidence that an HBGA nonbinder (GII.1) can be converted to an HBGA binder after bile acid binding is of major significance. Our data provide direct evidence that, like HBGAs, bile acid interaction on the capsid is an important cofactor for certain genotypes. However, more unanswered questions seem to arise from these new discoveries. For example, is there an association between the bile acid requirement and the prevalence of certain genotypes? That is, the GII.1 and GII.10 (bile acid binders) genotypes rarely caused outbreaks, whereas the GII.4 and GII.17 genotypes (bile acid nonbinders) were responsible for large epidemics. Therefore, it seems plausible that certain genotypes require bile acids, whereas others have modified their bile acid requirements on the capsid.

Chenodeoxycholic Acid from Bile Inhibits Influenza A Virus Replication via Blocking Nuclear Export of Viral Ribonucleoprotein Complexes

Influenza A virus (IAV) infection is still a major global threat for humans, especially for the risk groups: young children and the elderly. The currently licensed antiviral drugs target viral factors and are prone to viral resistance. In recent years, a few endogenous small molecules from host, such as estradiol and omega-3 polyunsaturated fatty acid (PUFA)-derived lipid mediator protection D1 (PD1), were demonstrated to be capable of inhibiting IAV infection. Chenodeoxycholic acid (CDCA), one of the main primary bile acids, is synthesized from cholesterol in the liver and classically functions in emulsification and absorption of dietary fats. Clinically, CDCA has been used in the treatment of patients with cholesterol gallstones for more than five decades. In this study, we showed that CDCA attenuated the replication of three subtypes of influenza A virus, including a highly pathogenic H5N1 strain, in A549 and MDCK cell cultures with IC₅₀ ranging from 5.5 to 11.5 μM. Mechanistically, CDCA effectively restrained the nuclear export of viral ribonucleoprotein (vRNP) complexes. In conclusion, as an endogenous physiological small molecule, CDCA can inhibit IAV replication in vitro, at least in part, by blocking vRNP nuclear export, and affords further studies for development as a potential antiviral agent against IAV infections.

Therapeutics and Immunoprophylaxis Against Noroviruses and Rotaviruses: The Past, Present, and Future

Background:

Noroviruses and rotaviruses are important viral etiologies of severe gastroenteritis. Noroviruses are the primary cause of nonbacterial diarrheal outbreaks in humans, whilst rotaviruses are a major cause of childhood diarrhea. Although both enteric pathogens substantially impact human health and economies, there are no approved drugs against noroviruses and rotaviruses so far. On the other hand, whilst the currently licensed rotavirus vaccines have been successfully implemented in over 100 countries, the most advanced norovirus vaccine has recently completed phase-I and II trials.

Methods:

We performed a structured search of bibliographic databases for peer-reviewed research litera-ture on advances in the fields of norovirus and rotavirus therapeutics and immunoprophylaxis.

Results:

Technological advances coupled with a proper understanding of viral morphology and replication over the past decade has facilitated pioneering research on therapeutics and immunoprophylaxis against noroviruses and rotaviruses, with promising outcomes in human clinical trials of some of the drugs and vaccines. This review focuses on the various developments in the fields of norovirus and rotavirus thera-peutics and immunoprophylaxis, such as potential antiviral drug molecules, passive immunotherapies (oral human immunoglobulins, egg yolk and bovine colostral antibodies, llama-derived nanobodies, and anti-bodies expressed in probiotics, plants, rice grains and insect larvae), immune system modulators, probiot-ics, phytochemicals and other biological substances such as bovine milk proteins, therapeutic nanoparti-cles, hydrogels and viscogens, conventional viral vaccines (live and inactivated whole virus vaccines), and genetically engineered viral vaccines (reassortant viral particles, virus-like particles (VLPs) and other sub-unit recombinant vaccines including multi-valent viral vaccines, edible plant vaccines, and encapsulated viral particles).

Conclusions:

This review provides important insights into the various approaches to therapeutics and im-munoprophylaxis against noroviruses and rotaviruses..

Metabolite-Sensing G Protein Coupled Receptor TGR5 Protects Host From Viral Infection Through Amplifying Type I Interferon Responses

The metabolite-sensing G protein–coupled receptors (GPCRs) bind to various metabolites and transmit signals that are important for proper immune and metabolic functions. However, the roles of metabolite-sensing GPCRs in viral infection are not well characterized. Here, we identified metabolite-sensing GPCR TGR5 as an interferon (IFN)-stimulated gene (ISG) which had increased expression following viral infection or IFN-β stimulation in a STAT1-dependent manner. Most importantly, overexpression of TGR5 or treatment with the modified bile acid INT-777 broadly protected host cells from vesicular stomatitis virus (VSV), newcastle disease virus (NDV) and herpes simplex virus type 1 (HSV-1) infection. Furthermore, VSV and HSV-1 replication was increased significantly in Tgr5-deficient macrophages and the VSV distribution in liver, spleen and lungs was increased in Tgr5-deficient mice during VSV infection. Accordingly, Tgr5-deficient mice were much more susceptible to VSV infection than wild-type mice. Mechanistically, TGR5 facilitates type I interferon (IFN-I) production through the AKT/IRF3-signaling pathway, which is crucial in promoting antiviral innate immunity. Taken together, our data reveal a positive feedback loop regulating IRF3 signaling and suggest a potential therapeutic role for metabolite-sensing GPCRs in controlling viral diseases.

Fexaramine as an entry blocker for feline caliciviruses

Feline calicivirus (FCV) is a small non-enveloped virus containing a single-stranded, positive-sense RNA genome of approximately 7.7 kb. FCV is a highly infectious pathogen of cats and typically causes moderate, self-limiting acute oral and upper respiratory tract diseases or chronic oral diseases. In addition, in recent years, virulent, systemic FCV (vs-FCV) strains causing severe systemic diseases with a high mortality rate of up to 67% have been reported in cats. Although FCV vaccines are commercially available, their efficacy is limited due to antigenic diversity of FCV strains and short duration of immunity. In this study, we identified fexaramine as a potent inhibitor of FCV including vs-FCV strains in cell culture and demonstrated that fexaramine is a entry blocker for FCV by using various experiments including time-of-addition studies, generation of resistant viruses in cell culture and the reverse genetics system. A fexaramine resistant FCV mutant has a single amino acid change in the P2 domain of VP1 (the major capsid), and the importance of this mutation for conferring resistance was confirmed using the reverse genetics system. A comparative analysis of viral resistance was also performed using a peptidyl inhibitor (NPI52) targeting FCV 3C-like protease. Finally, the effects of combination treatment of fexaramine and NPI52 against FCV replication and emergence of resistant viruses were investigated in cell culture.

Trypsin-independent porcine epidemic diarrhea virus US strain with altered virus entry mechanism

BACKGROUND

Porcine Epidemic Diarrhea Virus (PEDV) is a coronavirus that infects the intestinal tract and causes diarrhea and vomiting in older pigs or extreme dehydration and death that could reach 100% mortality in neonatal piglets. In the US, the first PEDV outbreaks occurred in 2013 and since then US PEDV strains have quickly spread throughout the US and worldwide, causing significant economic and public health concerns. Currently two conditionally approved vaccines exist in the US, but there is no live attenuated vaccine, which is considered the best option in controlling PEDV by inducing transferrable mucosal immunity to susceptible neonatal piglets. In this study, we passaged an US PEDV isolate under various conditions to generate three strains and characterized their growth and antigenicity in cell culture using various assays including Western blot analysis, serum neutralization assay, sequencing analysis and confocal microscopy. Finally, these strains were evaluated for pathogenicity in nursing piglets (1-4 days old).

RESULTS

One of the PEDV strains generated in this study (designated as PEDV 8aa) is able to replicate in cells without any protease and grows to a high titer of >8 log10 TCID50/ml in cell culture. Interestingly, replication of PEDV 8aa was severely reduced by trypsin and this correlated with the inhibition of virus attachment and entry into the cells. In neonatal nursing piglets, PEDV 8aa (passage number 70 or 105) was found to be fully attenuated with limited virus shedding.

CONCLUSIONS

These results suggest that applying selective pressure during viral passages can facilitate attainment of viral attenuation and that PEDV 8aa warrants further investigation as an attenuated vaccine.

Chenodeoxycholic acid requires activation of EGFR, EPAC, and Ca2+ to stimulate CFTR-dependent Cl- secretion in human colonic T84 cells

Bile acids are known to initiate intricate signaling events in a variety of tissues, primarily in the liver and gastrointestinal tract. Of the known bile acids, only the 7α-dihydroxy species, deoxycholic acid and chenodeoxycholic acid (CDCA), and their conjugates, activate processes that stimulate epithelial Cl^- secretion. We have previously published that CDCA acts in a rapid manner to stimulate colonic ion secretion via protein kinase A (PKA)-mediated activation of the dominant Cl^- channel, the cystic fibrosis transmembrane conductance regulator (CFTR) (Ao M, Sarathy J, Domingue J, Alrefai WA, and Rao MC. Am J Physiol Cell Physiol 305: C447-C456, 2013); however, PKA signaling did not account for the entire CDCA response. Here we show that in human colonic T84 cells, CDCA's induction of CFTR activity, measured as changes in short-circuit current (I_sc), is dependent on epidermal growth factor receptor (EGFR) activation and does not involve the bile acid receptors TGR5 or farnesoid X receptor. CDCA activation of Cl^- secretion does not require Src, mitogen-activated protein kinases, or phosphoinositide 3-kinase downstream of EGFR but does require an increase in cytosolic Ca²⁺ In addition to PKA signaling, we found that the CDCA response requires the novel involvement of the exchange protein directly activated by cAMP (EPAC). EPAC is a known hub for cAMP and Ca²⁺ cross talk. Downstream of EPAC, CDCA activates Rap2, and changes in free cytosolic Ca²⁺ were dependent on both EPAC and EGFR activation. This study establishes the complexity of CDCA signaling in the colonic epithelium and shows the contribution of EGFR, EPAC, and Ca²⁺ in CDCA-induced activation of CFTR-dependent Cl^- secretion.

Bile Acids Act as Soluble Host Restriction Factors Limiting Cytomegalovirus Replication in Hepatocytes

The liver constitutes a prime site of cytomegalovirus (CMV) replication and latency. Hepatocytes produce, secrete, and recycle a chemically diverse set of bile acids, with the result that interactions between bile acids and cytomegalovirus inevitably occur. Here we determined the impact of naturally occurring bile acids on mouse CMV (MCMV) replication. In primary mouse hepatocytes, physiological concentrations of taurochenodeoxycholic acid (TCDC), glycochenodeoxycholic acid, and to a lesser extent taurocholic acid significantly reduced MCMV-induced gene expression and diminished the generation of virus progeny, while several other bile acids did not exert antiviral effects. The anticytomegalovirus activity required active import of bile acids via the sodium-taurocholate-cotransporting polypeptide (NTCP) and was consistently observed in hepatocytes but not in fibroblasts. Under conditions in which alpha interferon (IFN-α) lacks antiviral activity, physiological TCDC concentrations were similarly effective as IFN-γ. A detailed investigation of distinct steps of the viral life cycle revealed that TCDC deregulates viral transcription and diminishes global translation in infected cells.

IMPORTANCE

Cytomegaloviruses are members of the Betaherpesvirinae subfamily. Primary infection leads to latency, from which cytomegaloviruses can reactivate under immunocompromised conditions and cause severe disease manifestations, including hepatitis. The present study describes an unanticipated antiviral activity of conjugated bile acids on MCMV replication in hepatocytes. Bile acids negatively influence viral transcription and exhibit a global effect on translation. Our data identify bile acids as site-specific soluble host restriction factors against MCMV, which may allow rational design of anticytomegalovirus drugs using bile acids as lead compounds.

Lipid droplets form complexes with viroplasms and are crucial for rotavirus replication

Recent evidence has demonstrated that a variety of pathogens target cellular lipid metabolism for their replication. Lipid droplets are a major contributor to lipid homeostasis and contain neutral fats but are also recognized as dynamic organelles involved in signal transduction, membrane trafficking and modulation of immune and inflammatory responses. Rotaviruses co-opt lipid droplets for their replication. Rotavirus viroplasms, sites of viral RNA replication and immature particle assembly, form complexes with cellular lipid droplets early in infection. Chemical compounds blocking fatty acid synthesis or interfering with lipid droplet homeostasis decrease viroplasm formation and the yield of infectious viral progeny. Lipid droplets are vital for the replication of rotaviruses as well as various members of the Flaviviridae family and several intracellular bacteria. Chemical compounds decreasing intracellular triglyceride content reduced rotavirus replication in an animal model and should be considered as potential therapeutic agents against disease caused by rotaviruses, flaviviruses and intracellular bacteria.

Rotavirus replication and the role of cellular lipid droplets: New therapeutic targets?

Rotaviruses (RVs) are a major cause of acute gastroenteritis in infants and young children worldwide. These viruses infect the villous epithelium of the small intestine. Part of their replication occurs in cytoplasmic inclusion bodies termed viroplasms. Viroplasms and the lipid droplets (LDs) of cellular organelles are known to interact both physically and functionally. Compounds interfering with the homoeostasis of LDs significantly decrease the production of infectious RV progeny. There is considerable scope for more detailed exploration of such compounds as potential antiviral agents for a disease for which at present no specific therapy exists.

Upregulation of bile acid receptor TGR5 and nNOS in gastric myenteric plexus is responsible for delayed gastric emptying after chronic high-fat feeding in rats

Chronic high-fat feeding is associated with functional dyspepsia and delayed gastric emptying. We hypothesize that high-fat feeding upregulates gastric neuronal nitric oxide synthase (nNOS) expression, resulting in delayed gastric emptying. We propose this is mediated by increased bile acid action on bile acid receptor 1 (TGR5) located on nNOS gastric neurons. To test this hypothesis, rats were fed regular chow or a high-fat diet for 2 wk. Rats fed the high-fat diet were subjected to concurrent feeding with oral cholestyramine or terminal ileum resection. TGR5 and nNOS expression in gastric tissue was measured by immunohistochemistry, PCR, and Western blot. Gastric motility was assessed by organ bath and solid-phase gastric emptying studies. The 2-wk high-fat diet caused a significant increase in neurons coexpressing nNOS and TGR5 in the gastric myenteric plexus and an increase in nNOS and TGR5 gene expression, 67 and 111%, respectively. Enhanced nonadrenergic, noncholinergic (NANC) relaxation, deoxycholic acid (DCA)-induced inhibition in fundic tissue, and a 26% delay in gastric emptying accompanied these changes. A 24-h incubation of whole-mount gastric fundus with DCA resulted in increased nNOS and TGR5 protein expression, 41 and 37%, respectively. Oral cholestyramine and terminal ileum resection restored the enhanced gastric relaxation, as well as the elevated nNOS and TGR5 expression evoked by high-fat feeding. Cholestyramine also prevented the delay in gastric emptying. We conclude that increased levels of circulatory bile acids induced by high-fat feeding upregulate nNOS and TGR5 expression in the gastric myenteric plexus, resulting in enhanced NANC relaxation and delayed gastric emptying.

The crucial role of bile acids in the entry of porcine enteric calicivirus

Replication of porcine enteric calicivirus (PEC) in LLC-PK cells is dependent on the presence of bile acids in the medium. However, the mechanism of bile acid-dependent PEC replication is unknown. Understanding of bile acid-mediated PEC replication may provide insight into cultivating related human noroviruses, currently uncultivable, which are the major cause of viral gastroenteritis outbreaks in humans. Our results demonstrated that while uptake of PEC into the endosomes does not require bile acids, the presence of bile acids is critical for viral escape from the endosomes into cell cytoplasm to initiate viral replication. We also demonstrated that bile acid transporters including the sodium-taurocholate co-transporting polypeptide and the apical sodium-dependent bile acid transporter are important in exerting the effects of bile acids in PEC replication in cells. In summary, our results suggest that bile acids play a critical role in virus entry for successful replication.

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Bile acids are essential for the replication of porcine enteric calicivirus (PEC).

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Bile acids are critical for virus escape from the endosomes for PEC replication.

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Bile acid transporters are important in bile acid-mediated PEC replication.

The role of cellular lipid droplets in rotavirus replication

Viroplasms, cytoplasmic inclusion bodies in rotavirus (RV)-infected cells in which viral RNA replication and early morphogenesis take place, were found to be associated with the cellular organelles lipid droplets (LDs). Compounds affecting LD homoeostasis, including agents causing lipolysis and others that inhibit fatty acid biosynthesis, decrease RV replication. Gradient ultracentrifugation of infected cell extracts shows LD components cosedimenting with viroplasms in low-density fractions. Disturbance of fatty acid biosynthesis decreases the production of both double-layered and triple-layered (infectious) RV particles. Future studies should explore the LD components important for RV replication, and the potential of chemical compounds interfering with lipid metabolism for treatment of RV disease.

Lipidome analysis of rotavirus-infected cells confirms the close interaction of lipid droplets with viroplasms

Rotaviruses (RVs) cause acute gastroenteritis in infants and young children, and are globally distributed. Within the infected host cell, RVs establish replication complexes in viroplasms (‘viral factories’) to which lipid droplet organelles are recruited. To further understand this recently discovered phenomenon, the lipidomes of RV-infected and uninfected MA104 cells were investigated. Cell lysates were subjected to equilibrium ultracentrifugation through iodixanol gradients. Fourteen different classes of lipids were differentiated by mass spectrometry. The concentrations of virtually all lipids were elevated in RV-infected cells. Fractions of low density (1.11–1.15 g ml⁻¹), in which peaks of the RV dsRNA genome and lipid droplet- and viroplasm-associated proteins were observed, contained increased amounts of lipids typically found concentrated in the cellular organelle lipid droplets, confirming the close interaction of lipid droplets with viroplasms. A decrease in the ratio of the amounts of surface to internal components of lipid droplets upon RV infection suggested that the lipid droplet–viroplasm complexes became enlarged.

Synthesis and anti-norovirus activity of pyranobenzopyrone compounds

During the last decade, noroviruses have gained media attention as the cause of large scale outbreaks of gastroenteritis on cruise ships, dormitories, nursing homes, etc. Although noroviruses do not multiply in food or water, they can cause large outbreaks because approximately 10-100 virions are sufficient to cause illness in a healthy adult. Recently, it was shown that the activity of acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT1) enzyme may be important in norovirus infection. In search of anti-noroviral agents based on the inhibition of ACAT1, we synthesized and evaluated the inhibitory activities of a class of pyranobenzopyrone molecules containing amino, pyridine, substituted quinolines, or 7,8-benzoquinoline nucleus. Three of the sixteen evaluated compounds possess ED(50) values in the low micrometer range. 2-Quinolylmethyl derivative 3A and 4-quinolylmethyl derivative 4A showed ED(50) values of 3.4 and 2.4 μM and TD(50) values of >200 and 96.4 μM, respectively. The identified active compounds are suitable for further modification for the development of anti-norovirus agents.

Novel triacsin C analogs as potential antivirals against rotavirus infections

Recently our group has demonstrated that cellular triglyceride (TG) levels play an important role in rotavirus replication. In this study, we further examined the roles of the key enzymes for TG synthesis (lipogenesis) in the replication of rotaviruses by using inhibitors of fatty acid synthase, long chain fatty acid acyl-CoA synthetase (ACSL), and diacylglycerol acyltransferase and acyl-CoA:cholesterol acyltransferase in association with lipid droplets of which TG is a major component. Triacsin C, a natural ACSL inhibitor from Streptomyces aureofaciens, was found to be highly effective against rotavirus replication. Thus, novel triacsin C analogs were synthesized and evaluated for their efficacies against the replication of rotaviruses in cells. Many of the analogs significantly reduced rotavirus replication, and one analog (1e) was highly effective at a nanomolar concentration range (ED(50) 0.1μM) with a high therapeutic index in cell culture. Our results suggest a crucial role of lipid metabolism in rotavirus replication, and triacsin C and/or its analogs as potential therapeutic options for rotavirus infections.