Pathogenic hantaviruses bind plexin-semaphorin-integrin domains present at the apex of inactive, bent alphavbeta3 integrin conformers

Replication kinetics of pathogenic Eurasian orthohantaviruses in human mesangial cells

BACKGROUND

Eurasian pathogenic orthohantaviruses cause hemorrhagic fever with renal syndrome (HFRS) characterized by acute kidney injury (AKI). The virulence of orthohantaviruses varies enormously and direct infection of different renal cell types contribute to pathogenesis. Glomerular mesangial cells play an essential role in the interplay between kidney cells and proper kidney function. Therefore, we analyzed the replication competence of different orthohantavirus species in primary mesangial cells and a mesangial cell line.

METHODS

We tested the suitability of the mesangial cell line CIHGM-1 (conditionally immortalized human glomerular mesangial cells) as cell culture model for orthohantavirus kidney infection by comparison with primary human renal mesangial cells (HRMCs). We analyzed infection with high pathogenic Hantaan virus (HTNV), moderate pathogenic Puumala virus (PUUV) and non-/low-pathogenic Tula virus (TULV).

RESULTS

Effective viral spread was observed for PUUV only, whereas infection with HTNV and TULV was abortive. However, in contrast to TULV, HTNV exhibits an initially high infection rate and declines afterwards. This replication pattern was observed in HRMCs and CIHGM-1 cells. Viability or adhesion was neither impaired for PUUV-infected CIHGM-1 nor HRMCs. A loss of migration capacity was observed in PUUV-infected CIHGM-1 cells, but not in HRMCs.

CONCLUSIONS

The identification of differences in the replication competence of pathogenic orthohantavirus strains in renal mesangial cells is of special interest and may provide useful insights in the virus-specific mechanisms of orthohantavirus induced AKI. The use of CIHGM-1 cells will facilitate the research in a relevant cell culture system.

Hantavirus: an overview and advancements in therapeutic approaches for infection

Hantaviruses are a significant and emerging global public health threat, impacting more than 200,000 individuals worldwide each year. The single-stranded RNA viruses belong to the Hantaviridae family and are responsible for causing two acute febrile diseases in humans: Hantavirus pulmonary syndrome (HPS) and hemorrhagic fever with renal syndrome (HFRS). Currently, there are no licensed treatments or vaccines available globally for HTNV infection. Various candidate drugs have shown efficacy in increasing survival rates during the early stages of HTNV infection. Some of these drugs include lactoferrin, ribavirin, ETAR, favipiravir and vandetanib. Immunotherapy utilizing neutralizing antibodies (NAbs) generated from Hantavirus convalescent patients show efficacy against HTNV. Monoclonal antibodies such as MIB22 and JL16 have demonstrated effectiveness in protecting against HTNV infection. The development of vaccines and antivirals, used independently and/or in combination, is critical for elucidating hantaviral infections and the impact on public health. RNA interference (RNAi) arised as an emerging antiviral therapy, is a highly specific degrades RNA, with post-transcriptional mechanism using eukaryotic cells platform. That has demonstrated efficacy against a wide range of viruses, both in vitro and in vivo. Recent antiviral methods involve using small interfering RNA (siRNA) and other, immune-based therapies to target specific gene segments (S, M, or L) of the Hantavirus. This therapeutic approach enhances viral RNA clearance through the RNA interference process in Vero E6 cells or human lung microvascular endothelial cells. However, the use of siRNAs faces challenges due to their low biological stability and limited in vivo targeting ability. Despite their successful inhibition of Hantavirus replication in host cells, their antiviral efficacy may be hindered. In the current review, we focus on advances in therapeutic strategies, as antiviral medications, immune-based therapies and vaccine candidates aimed at enhancing the body's ability to control the progression of Hantavirus infections, with the potential to reduce the risk of severe disease.

Differences in the Susceptibility of Human Tubular Epithelial Cells for Infection with Orthohantaviruses

Diseases induced by infection with pathogenic orthohantaviruses are characterized by a pronounced organ-specific manifestation. Pathogenic Eurasian orthohantaviruses cause hemorrhagic fever with renal syndrome (HFRS) with often massive proteinuria. Therefore, the use of a relevant kidney cell culture would be favorable to analyze the underlying cellular mechanisms of orthohantavirus-induced acute kidney injury (AKI). We tested different human tubular epithelial cell lines for their suitability as an in vitro infection model. Permissiveness and replication kinetics of highly pathogenic Hantaan virus (HTNV) and non-/low-pathogenic Tula virus (TULV) were analyzed in tubular epithelial cell lines and compared to human primary tubular epithelial cells. Ana-lysis of the cell line HK-2 revealed the same results for viral replication, morphological and functional effects as observed for HTNV in primary cells. In contrast, the cell lines RPTEC/TERT1 and TH1 demonstrated only poor infection rates after inoculation with HTNV and are unusable as an infection model. While pathogenic HNTV infects primary tubular and HK-2 cells, non-/low-pathogenic TULV infects neither primary tubular cells nor the cell line HK-2. Our results show that permissiveness of renal cells varies between orthohantaviruses with differences in pathogenicity and that HK-2 cells demonstrate a suitable in vitro model to study viral tropism and pathogenesis of orthohantavirus-induced AKI.

Therapeutic potential for P2Y2 receptor antagonism

G protein-coupled receptors are the target of more than 30% of all FDA-approved drug therapies. Though the purinergic P2 receptors have been an attractive target for therapeutic intervention with successes such as the P2Y₁₂ receptor antagonist, clopidogrel, P2Y₂ receptor (P2Y₂R) antagonism remains relatively unexplored as a therapeutic strategy. Due to a lack of selective antagonists to modify P2Y₂R activity, studies using primarily genetic manipulation have revealed roles for P2Y₂R in a multitude of diseases. These include inflammatory and autoimmune diseases, fibrotic diseases, renal diseases, cancer, and pathogenic infections. With the advent of AR-C118925, a selective and potent P2Y₂R antagonist that became commercially available only a few years ago, new opportunities exist to gain a more robust understanding of P2Y₂R function and assess therapeutic effects of P2Y₂R antagonism. This review discusses the characteristics of P2Y₂R that make it unique among P2 receptors, namely its involvement in five distinct signaling pathways including canonical Gα_q protein signaling. We also discuss the effects of other P2Y₂R antagonists and the pivotal development of AR-C118925. The remainder of this review concerns the mounting evidence implicating P2Y₂Rs in disease pathogenesis, focusing on those studies that have evaluated AR-C118925 in pre-clinical disease models.

Structural basis for non-canonical integrin engagement by Bordetella adenylate cyclase toxin

Integrins are ubiquitous cell-surface heterodimers that are exploited by pathogens and toxins, including leukotoxins that target β₂ integrins on phagocytes. The Bordetella adenylate cyclase toxin (ACT) uses the α_Mβ₂ integrin as a receptor, but the structural basis for integrin binding and neutralization by antibodies is poorly understood. Here, we use cryoelectron microscopy to determine a 2.7 Å resolution structure of an ACT fragment bound to α_Mβ₂. This structure reveals that ACT interacts with the headpiece and calf-2 of the α_M subunit in a non-canonical manner specific to bent, inactive α_Mβ₂. Neutralizing antibody epitopes map to ACT residues involved in α_M binding, providing the basis for antibody-mediated attachment inhibition. Furthermore, binding to α_Mβ₂ positions the essential ACT acylation sites, which are conserved among toxins exported by type I secretion systems, at the cell membrane. These findings reveal a structural mechanism for integrin-mediated attachment and explain antibody-mediated neutralization of ACT intoxication.

Expression Profile of Human Renal Mesangial Cells Is Altered by Infection with Pathogenic Puumala Orthohantavirus

Acute kidney injury (AKI) with proteinuria is a hallmark of infections with Eurasian orthohantaviruses. Different kidney cells are identified as target cells of hantaviruses. Mesangial cells may play a central role in the pathogenesis of AKI by regulation of inflammatory mediators and signaling cascades. Therefore, we examined the characteristics of hantavirus infection on human renal mesangial cells (HRMCs). Receptor expression and infection with pathogenic Puumala virus (PUUV) and low-pathogenic Tula virus (TULV) were explored. To analyze changes in protein expression in infected mesangial cells, we performed a proteome profiler assay analyzing 38 markers of kidney damage. We compared the proteome profile of in vitro-infected HRMCs with the profile detected in urine samples of 11 patients with acute hantavirus infection. We observed effective productive infection of HRMCs with pathogenic PUUV, but only poor abortive infection for low-pathogenic TULV. PUUV infection resulted in the deregulation of proteases, adhesion proteins, and cytokines associated with renal damage. The urinary proteome profile of hantavirus patients demonstrated also massive changes, which in part correspond to the alterations observed in the in vitro infection of HRMCs. The direct infection of mesangial cells may induce a local environment of signal mediators that contributes to AKI in hantavirus infection.

OUP accepted manuscript

Hantavirus-induced diseases are emerging zoonoses with endemic appearances and frequent outbreaks in different parts of the world. In humans, hantaviral pathology is characterized by the disruption of the endothelial cell barrier followed by increased capillary permeability, thrombocytopenia due to platelet activation/depletion and an overactive immune response. Genetic vulnerability due to certain human leukocyte antigen haplotypes is associated with disease severity. Typically, two different hantavirus-caused clinical syndromes have been reported: hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). The primarily affected vascular beds differ in these two entities: renal medullary capillaries in HFRS caused by Old World hantaviruses and pulmonary capillaries in HCPS caused by New World hantaviruses. Disease severity in HFRS ranges from mild, e.g. Puumala virus-associated nephropathia epidemica, to moderate, e.g. Hantaan or Dobrava virus infections. HCPS leads to a severe acute respiratory distress syndrome with high mortality rates. Due to novel insights into organ tropism, hantavirus-associated pathophysiology and overlapping clinical features, HFRS and HCPS are believed to be interconnected syndromes frequently involving the kidneys. As there are no specific antiviral treatments or vaccines approved in Europe or the USA, only preventive measures and public awareness may minimize the risk of hantavirus infection. Treatment remains primarily supportive and, depending on disease severity, more invasive measures (e.g., renal replacement therapy, mechanical ventilation and extracorporeal membrane oxygenation) are needed.

αVβ3 Integrin Expression Is Essential for Replication of Mosquito and Tick-Borne Flaviviruses in Murine Fibroblast Cells

The Flavivirus genus includes a number of important viruses that are pathogenic to humans and animals and are responsible for outbreaks across the globe. Integrins, a family of heterodimeric transmembrane molecules expressed in all nucleated cells mediate critical functions of cell physiology and cell cycle. Integrins were previously postulated to be involved in flavivirus entry and to modulate flavivirus replication efficiency. In the present study, mouse embryonic fibroblasts (MEF), lacking the expression of αVβ3 integrin (MEF-αVβ3^−/−), were infected with four different flaviviruses, namely yellow fever virus (YFV), West Nile virus (WNV), Usutu virus (USUV) and Langat virus (LGTV). The effects of the αVβ3 integrin absence in double-knockout MEF-αVβ3^−/− on flavivirus binding, internalization and replication were compared to the respective wild-type cells. Binding to the cell surface for all four flaviviruses was not affected by the ablation of αVβ3 integrin, whereas internalization of USUV and WNV was slightly affected by the loss of αVβ3 integrin expression. Most interestingly, the deletion of αVβ3 integrin strongly impaired replication of all flaviviruses with a reduction of up to 99% on virus yields and a strong reduction on flavivirus anti-genome RNA synthesis. In conclusion, our results demonstrate that αVβ3 integrin expression in flavivirus-susceptible cell lines enhances the flavivirus replication.

Binding of the Andes Virus Nucleocapsid Protein to RhoGDI Induces the Release and Activation of the Permeability Factor RhoA

Andes virus (ANDV) nonlytically infects pulmonary microvascular endothelial cells (PMECs), causing acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). In HPS patients, virtually every PMEC is infected; however, the mechanism by which ANDV induces vascular permeability and edema remains to be resolved. The ANDV nucleocapsid (N) protein activates the GTPase RhoA in primary human PMECs, causing VE-cadherin internalization from adherens junctions and PMEC permeability. We found that ANDV N protein failed to bind RhoA but coprecipitates RhoGDI (Rho GDP dissociation inhibitor), the primary RhoA repressor that normally sequesters RhoA in an inactive state. ANDV N protein selectively binds the RhoGDI C terminus (residues 69 to 204) but fails to form ternary complexes with RhoA or inhibit RhoA binding to the RhoGDI N terminus (residues 1 to 69). However, we found that ANDV N protein uniquely inhibits RhoA binding to an S34D phosphomimetic RhoGDI mutant. Hypoxia and vascular endothelial growth factor (VEGF) increase RhoA-induced PMEC permeability by directing protein kinase Cα (PKCα) phosphorylation of S34 on RhoGDI. Collectively, ANDV N protein alone activates RhoA by sequestering and reducing RhoGDI available to suppress RhoA. In response to hypoxia and VEGF-activated PKCα, ANDV N protein additionally directs the release of RhoA from S34-phosphorylated RhoGDI, synergistically activating RhoA and PMEC permeability. These findings reveal a fundamental edemagenic mechanism that permits ANDV to amplify PMEC permeability in hypoxic HPS patients. Our results rationalize therapeutically targeting PKCα and opposing protein kinase A (PKA) pathways that control RhoGDI phosphorylation as a means of resolving ANDV-induced capillary permeability, edema, and HPS. IMPORTANCE HPS-causing hantaviruses infect pulmonary endothelial cells (ECs), causing vascular leakage, pulmonary edema, and a 35% fatal acute respiratory distress syndrome (ARDS). Hantaviruses do not lyse or disrupt the endothelium but dysregulate normal EC barrier functions and increase hypoxia-directed permeability. Our findings reveal a novel underlying mechanism of EC permeability resulting from ANDV N protein binding to RhoGDI, a regulatory protein that normally maintains edemagenic RhoA in an inactive state and inhibits EC permeability. ANDV N sequesters RhoGDI and enhances the release of RhoA from S34-phosphorylated RhoGDI. These findings indicate that ANDV N induces the release of RhoA from PKC-phosphorylated RhoGDI, synergistically enhancing hypoxia-directed RhoA activation and PMEC permeability. Our data suggest inhibiting PKC and activating PKA phosphorylation of RhoGDI as mechanisms of inhibiting ANDV-directed EC permeability and therapeutically restricting edema in HPS patients. These findings may be broadly applicable to other causes of ARDS.

Genetic depletion studies inform receptor usage by virulent hantaviruses in human endothelial cells

Hantaviruses are RNA viruses with known epidemic threat and potential for emergence. Several rodent-borne hantaviruses cause zoonoses accompanied by severe illness and death. However, assessments of zoonotic risk and the development of countermeasures are challenged by our limited knowledge of the molecular mechanisms of hantavirus infection, including the identities of cell entry receptors and their roles in influencing viral host range and virulence. Despite the long-standing presumption that β3/β1-containing integrins are the major hantavirus entry receptors, rigorous genetic loss-of-function evidence supporting their requirement, and that of decay-accelerating factor (DAF), is lacking. Here, we used CRISPR/Cas9 engineering to knockout candidate hantavirus receptors, singly and in combination, in a human endothelial cell line that recapitulates the properties of primary microvascular endothelial cells, the major targets of viral infection in humans. The loss of β3 integrin, β1 integrin, and/or DAF had little or no effect on entry by a large panel of hantaviruses. By contrast, loss of protocadherin-1, a recently identified entry receptor for some hantaviruses, substantially reduced hantavirus entry and infection. We conclude that major host molecules necessary for endothelial cell entry by PCDH1-independent hantaviruses remain to be discovered.

Mutational analysis of structural proteins of SARS-CoV-2

SARS-CoV-2 transmissibility is higher than that of other human coronaviruses; therefore, it poses a threat to the populated communities. We investigated mutations among envelope (E), membrane (M), and spike (S) proteins from different isolates of SARS-CoV-2 and plausible signaling influenced by mutated virus in a host. We procured updated protein sequences from the NCBI virus database. Mutations were analyzed in the retrieved sequences of the viral proteins through multiple sequence alignment. Additionally, the data was subjected to ScanPROSITE to analyse if the mutations generated a relevant sequence for host signaling. Unique mutations in E, M, and S proteins resulted in modification sites like PKC phosphorylation and N-myristoylation sites. Based on structural analysis, our study revealed that the D614G mutation in the S protein diminished the interaction with T859 and K854 of adjacent chains. Moreover, the S protein of SARS-CoV-2 consists of an Arg-Gly-Asp (RGD) tripeptide sequence, which could potentially interact with various members of integrin family receptors. RGD sequence in S protein might aid in the initial virus attachment. We speculated crucial host pathways which the mutated isolates of SARS-CoV-2 may alter like PKC, Src, and integrin mediated signaling pathways. PKC signaling is known to influence the caveosome/raft pathway which is critical for virus entry. Additionally, the myristoylated proteins might activate NF-κB, a master molecule of inflammation. Thus the mutations may contribute to the disease pathogenesis and distinct lung pathophysiological changes. Further the frequently occurring mutations in the protein can be studied for possible therapeutic interventions.

Endothelial Cells in Emerging Viral Infections

There are several reasons to consider the role of endothelial cells in COVID-19 and other emerging viral infections. First, severe cases of COVID-19 show a common breakdown of central vascular functions. Second, SARS-CoV-2 replicates in endothelial cells. Third, prior deterioration of vascular function exacerbates disease, as the most common comorbidities of COVID-19 (obesity, hypertension, and diabetes) are all associated with endothelial dysfunction. Importantly, SARS-CoV-2's ability to infect endothelium is shared by many emerging viruses, including henipaviruses, hantavirus, and highly pathogenic avian influenza virus, all specifically targeting endothelial cells. The ability to infect endothelium appears to support generalised dissemination of infection and facilitate the access to certain tissues. The disturbed vascular function observed in severe COVID-19 is also a prominent feature of many other life-threatening viral diseases, underscoring the need to understand how viruses modulate endothelial function. We here review the role of vascular endothelial cells in emerging viral infections, starting with a summary of endothelial cells as key mediators and regulators of vascular and immune responses in health and infection. Next, we discuss endotheliotropism as a possible virulence factor and detail features that regulate viruses' ability to attach to and enter endothelial cells. We move on to review how endothelial cells detect invading viruses and respond to infection, with particular focus on pathways that may influence vascular function and the host immune system. Finally, we discuss how endothelial cell function can be dysregulated in viral disease, either by viral components or as bystander victims of overshooting or detrimental inflammatory and immune responses. Many aspects of how viruses interact with the endothelium remain poorly understood. Considering the diversity of such mechanisms among different emerging viruses allows us to highlight common features that may be of general validity and point out important challenges.

T-cell immunoglobulin and mucin (TIM) contributes to the infection of human airway epithelial cells by pseudotype viruses containing Hantaan virus glycoproteins

Hantaviruses are rodent-borne hemorrhagic fever viruses leading to serious diseases. Viral attachment and entry represent the first steps in virus transmission and are promising targets for antiviral therapeutic intervention. Here we investigated receptor use in human airway epithelium of the Old and New World hantaviruses Hantaan virus (HTNV) and Andes virus (ANDV). Using a biocontained recombinant vesicular stomatitis virus pseudotype platform, we provide first evidence for a role of the cellular phosphatidylserine (PS) receptors of the T-cell immunoglobulin and mucin (TIM) protein family in HTNV and ANDV infection. In line with previous studies, HTNV, but not ANDV, was able to use glycosaminoglycan heparan sulfate and α_vβ₃ integrin as co-receptors. In sum, our studies demonstrate for the first time that hantaviruses make use of apoptotic mimicry for infection of human airway epithelium, which may explain why these viruses can easily break the species barrier.

Immunocytochemical and Ultrastructural Evidence Supporting That Andes Hantavirus (ANDV) Is Transmitted Person-to-Person Through the Respiratory and/or Salivary Pathways

In South America Andes hantavirus (ANDV) is hosted by the rodent Oligoryzomys longicaudatus (also known as pygmy rice rat). In humans, ANDV causes Hantavirus Pulmonary Syndrome (HPS), with a fatality rate of about 40%. Epidemiologic and molecular evidence has shown that ANDV can be transmitted from person to person. Sin Nombre hantavirus, occurring in North America, and ANDV are genetically related, and both cause HPS with similar clinical evolution and mortality rate. However, only ANDV is transmitted from person to person. A recent hantavirus outbreak in a small village in Southern Argentine, with 29 HPS cases and 11 deaths has brought to mind that person-to-person transmission continues to be a public health emergency. The present investigation was aimed to understand how does ANDV actually spread between persons. Tissue samples of lung and salivary glands from infected Oligoryzomys longicaudatus and lethal cases of human HPS were investigated by bright field immunocytochemistry, multichannel immunofluorescence, and transmission electron microscopy. The findings are consistent with ANDV infection and replication in the lung alveolar epithelium and macrophages, and in the secretory cells of the submandibular salivary glands. In the lung of infected Oligoryzomys longicaudatus and human cases HPS, the bulk of immunoreactive hantavirus antigens was localized in epithelial cells of the alveolar walls and macrophages. The ultrastructural study supports that in the lung of HPS patients the virus replicates in the alveolar epithelial cells with virus particles being discharged into the alveolar lumen. Virus-like particles were seen within vacuoles of the lung macrophages. Considering that these macrophages can reach the conductive segments of the airways, their expectoration becomes a deadly bullet for ANDV transmission. In the submandibular glands of infected rodents and HPS cases, ANDV antigens were in capillary endothelium, the secretory cells and filling the lumen of the excretory pathway. It is proposed that in patients with HPS caused by ANDV the alveolar epithelium and macrophages would be the gate for the airway spreading of the virus, while the salivary glands are a target for virus replication and an exit pathway through saliva.

Hantavirus entry: Perspectives and recent advances

Hantaviruses are important zoonotic pathogens of public health importance that are found on all continents except Antarctica and are associated with hemorrhagic fever with renal syndrome (HFRS) in the Old World and hantavirus pulmonary syndrome (HPS) in the New World. Despite the significant disease burden they cause, no FDA-approved specific therapeutics or vaccines exist against these lethal viruses. The lack of available interventions is largely due to an incomplete understanding of hantavirus pathogenesis and molecular mechanisms of virus replication, including cellular entry. Hantavirus Gn/Gc glycoproteins are the only viral proteins exposed on the surface of virions and are necessary and sufficient to orchestrate virus attachment and entry. In vitro studies have implicated integrins (β_1-3), DAF/CD55, and gC1qR as candidate receptors that mediate viral attachment for both Old World and New World hantaviruses. Recently, protocadherin-1 (PCDH1) was demonstrated as a requirement for cellular attachment and entry of New World hantaviruses in vitro and lethal HPS in vivo, making it the first clade-specific host factor to be identified. Attachment of hantavirus particles to cellular receptors induces their internalization by clathrin-mediated, dynamin-independent, or macropinocytosis-like mechanisms, followed by particle trafficking to an endosomal compartment where the fusion of viral and endosomal membranes can occur. Following membrane fusion, which requires cholesterol and acid pH, viral nucleocapsids escape into the cytoplasm and launch genome replication. In this review, we discuss the current mechanistic understanding of hantavirus entry, highlight gaps in our existing knowledge, and suggest areas for future inquiry.

Early Host Interactions That Drive the Dysregulated Response in Sepsis

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. While many individual cells and systems in the body are involved in driving the excessive and sometimes sustained host response, pathogen engagement with endothelial cells and platelets early in sepsis progression, are believed to be key. Significant progress has been made in establishing key molecular interactions between platelets and pathogens and endothelial cells and pathogens. This review will explore the growing number of compensatory connections between bacteria and viruses with platelets and endothelial cells and how a better understanding of these interactions are informing the field of potential novel ways to treat the dysregulated host response during sepsis.

Unique Interferon Pathway Regulation by the Andes Virus Nucleocapsid Protein Is Conferred by Phosphorylation of Serine 386

Andes virus (ANDV) causes hantavirus pulmonary syndrome (HPS) and is the only hantavirus shown to spread person to person and cause a highly lethal HPS-like disease in Syrian hamsters. The unique ability of ANDV N protein to inhibit beta interferon (IFNβ) induction may contribute to its virulence and spread. Here we analyzed IFNβ regulation by ANDV N protein substituted with divergent residues from the nearly identical Maporal virus (MAPV) N protein. We found that MAPV N fails to inhibit IFNβ signaling and that replacing ANDV residues 252 to 296 with a hypervariable domain (HVD) from MAPV N prevents IFNβ regulation. In addition, changing ANDV residue S386 to the histidine present in MAPV N or the alanine present in other hantaviruses prevented ANDV N from regulating IFNβ induction. In contrast, replacing serine with phosphoserine-mimetic aspartic acid (S386D) in ANDV N robustly inhibited interferon regulatory factor 3 (IRF3) phosphorylation and IFNβ induction. Additionally, the MAPV N protein gained the ability to inhibit IRF3 phosphorylation and IFNβ induction when ANDV HVD and H386D replaced MAPV residues. Mass spectroscopy analysis of N protein from ANDV-infected cells revealed that S386 is phosphorylated, newly classifying ANDV N as a phosphoprotein and phosphorylated S386 as a unique determinant of IFN regulation. In this context, the finding that the ANDV HVD is required for IFN regulation by S386 but dispensable for IFN regulation by D386 suggests a role for HVD in kinase recruitment and S386 phosphorylation. These findings delineate elements within the ANDV N protein that can be targeted to attenuate ANDV and suggest targeting cellular kinases as potential ANDV therapeutics.IMPORTANCE ANDV contains virulence determinants that uniquely permit it to spread person to person and cause highly lethal HPS in immunocompetent hamsters. We discovered that ANDV S386 and an ANDV-specific hypervariable domain permit ANDV N to inhibit IFN induction and that IFN regulation is directed by phosphomimetic S386D substitutions in ANDV N. In addition, MAPV N proteins containing D386 and ANDV HVD gained the ability to inhibit IFN induction. Validating these findings, mass spectroscopy analysis revealed that S386 of ANDV N protein is uniquely phosphorylated during ANDV infection. Collectively, these findings reveal new paradigms for ANDV N protein as a phosphoprotein and IFN pathway regulator and suggest new mechanisms for hantavirus regulation of cellular kinases and signaling pathways. Our findings define novel IFN-regulating virulence determinants of ANDV, identify residues that can be modified to attenuate ANDV for vaccine development, and suggest the potential for kinase inhibitors to therapeutically restrict ANDV replication.

Analysis of the integrin β3 receptor for pathogenic orthohantaviruses in rodent host species

Host reservoir specificity of pathogens is complex and may depend on receptor variability. For pathogenic orthohantaviruses, integrin β₃ had been previously identified as entry receptor and the presence of aspartic acid residue at position 39 (D39) in human integrin β₃ was described to be a prerequisite for infection of primate cells with Hantaan virus (HTNV). However, the role of integrin β₃ in orthohantavirus infection of host animals is not completely understood. Therefore, we analyzed the nucleotide sequence of the integrin β₃ gene of Myodes glareolus and Apodemus agrarius, the hosts of Puumala virus (PUUV) and HTNV, respectively. Sequence analysis in tissue samples demonstrated that the amino acid residue D39 is not present in integrin β₃ of these natural orthohantavirus hosts. Furthermore, we analyzed the transcription and protein expression levels of integrin β₃ in the renal cell line BVK168 generated from the PUUV host, bank vole. Transcription level of integrin β₃ was 100-fold lower in BVK168 cells than in Vero E6 cells and integrin β₃ expression was not detectable in BVK168 cells. However, despite the absence of amino acid residue D39 and no detectable integrin β₃ expression, BVK168 cells are susceptible to infection with both PUUV and HTNV. These results indicate that the mechanism of orthohantaviral entry in rodent species does not correspond to the requirements that were described for the entry in primate cells in vitro.

Macropinocytosis contributes to hantavirus entry into human airway epithelial cells

Hantaviruses are emerging rodent-borne negative-strand RNA viruses associated with severe human diseases. Zoonotic transmission occurs via aerosols of contaminated rodent excreta and cells of the human respiratory epithelium represent likely early targets. Here we investigated cellular factors involved in entry of the pathogenic Old and New World hantaviruses Hantaan virus (HTNV) and Andes virus (ANDV) into human respiratory epithelial cells. Screening of a kinase inhibitor library using a biocontained recombinant vesicular stomatitis virus pseudotype platform revealed differential requirement for host kinases for HTNV and ANDV entry and provided first hints for an involvement of macropinocytosis. Examination of a selected panel of well-defined inhibitors of endocytosis confirmed that both HTNV and ANDV enter human respiratory epithelial cells via a pathway that critically depends on sodium proton exchangers and actin, hallmarks of macropinocytosis. However, HTNV and ANDV differed in their individual requirements for regulatory factors of macropinocytosis, indicating virus-specific differences.

A Single-Nucleotide Polymorphism of αVβ₃ Integrin Is Associated with the Andes Virus Infection Susceptibility

The AndesOrthohantavirus (ANDV), which causes the hantavirus cardiopulmonary syndrome, enters cells via integrins, and a change from leucine to proline at residue 33 in the PSI domain (L33P), impairs ANDV recognition. We assessed the association between this human polymorphism and ANDV infection. We defined susceptible and protective genotypes as “TT” (coding leucine) and “CC” (coding proline), respectively. TT was present at a rate of 89.2% (66/74) among the first cohort of ANDV cases and at 60% (63/105) among exposed close-household contacts, who remained uninfected (p < 0.05). The protective genotype (CC) was absent in all 85 ANDV cases, in both cohorts, and was present at 11.4% of the exposed close-household contacts who remained uninfected. Logistic regression modeling for risk of infection had an OR of 6.2–12.6 (p < 0.05) in the presence of TT and well-known ANDV risk activities. Moreover, an OR of 7.3 was obtained when the TT condition was analyzed for two groups exposed to the same environmental risk. Host genetic background was found to have an important role in ANDV infection susceptibility, in the studied population.

Structure of an extended β3 integrin

Cells use adhesion receptor integrins to communicate with their surroundings. Integrin activation and cellular signaling are coupled with change from bent to extended conformation. β3 integrins, including αIIbβ3, which is essential for the function of platelets in hemostasis and thrombosis, and αVβ3, which plays multiple roles in diverse cell types, have been prototypes in understanding integrin structure and function. Despite extensive structural studies, a high-resolution integrin structure in an extended conformation remains to be determined. The human β3 Leu33Pro polymorphism, located at the PSI domain, defines human platelet-specific alloantigens 1a and 1b (HPA-1a/b), immune response to which is a cause of posttransfusion purpura and fetal/neonatal alloimmune thrombocytopenia. Leu33Pro substitution has also been suggested to be a risk factor for thrombosis. Here we report the crystal structure of the β3 headpiece in either Leu33 or Pro33 form, both of which reveal intermediate and fully extended conformations coexisting in 1 crystal. These were used to build high-resolution structures of full-length β3 integrin in the intermediate and fully extended states, agreeing well with the corresponding conformations observed by electron microscopy. Our structures reveal how β3 integrin becomes extended at its β-knee region and how the flexibility of β-leg domains is determined. In addition, our structures reveal conformational changes of the PSI and I-EGF1 domains upon β3 extension, which may affect the binding of conformation-dependent anti-HPA-1a alloantibodies. Our structural and functional data show that Leu33Pro substitution does not directly alter the conformation or ligand binding of β3 integrin.

Upregulation of P2Y2R, Active uPA, and PAI-1 Are Essential Components of Hantavirus Cardiopulmonary Syndrome

Sin Nombre virus (SNV) causes hantavirus cardiopulmonary pulmonary syndrome (HCPS) with the loss of pulmonary vascular endothelial integrity, and pulmonary edema without causing cytopathic effects on the vascular endothelium. HCPS is associated primarily with a dysregulated immune response. We previously found occult signs of hemostatic imbalance in the form of a sharp >30-100 fold increase in the expression of plasminogen activator inhibitor type 1 (PAI-1), in serial blood plasma draws of terminal stage-patients. However, the mechanism of the increase in PAI-1 remains unclear. PAI-1 is a primary inhibitor of fibrinolysis caused by tissue plasminogen activator (tPA) and urokinase plasminogen activator plasma (uPA). Here, we investigate factors that contribute to PAI-1 upregulation during HCPS. Using zymography, we found evidence of PAI-1-refractory uPA activity and no tPA activity in plasma samples drawn from HCPS patients. The sole prevalence of uPA activity suggested that severe inflammation drove PAI-1 activity. We have recently reported that the P2Y₂ receptor (P2Y₂R) mediates SNV infectivity by interacting in cis with β₃ integrins, which activates the latter during infection. P2Y₂R is a known effector for several biological processes relevant to HCPS pathogenesis, such as upregulation of tissue factor (TF), a primary initiator of the coagulation cascade, stimulating vascular permeability and leukocyte homing to sites of infection. As P2Y₂R is prone to upregulation under conditions of inflammation, we compared the expression level of P2Y₂R in formalin fixed tissues of HCPS decedents using a TaqMan assay and immunohistochemistry. Our TaqMan results show that the expression of P2Y₂R is upregulated significantly in HCPS cases compared to non- HCPS controls (P < 0.001). Immunohistochemistry showed that lung macrophages were the primary reservoir of high and coincident localization of P2Y₂R, uPA, PAI-1, and TF antigens. We also observed increased staining for SNV antigens in the same tissue segments where P2Y₂R expression was upregulated. Conversely, sections of low P2Y₂R expression showed weak manifestations of macrophages, SNV, PAI-1, and TF. Coincident localization of P2Y₂R and PAI-1 on macrophage deposits suggests an inflammation-dependent mechanism of increasing pro-coagulant activity in HCPS in the absence of tissue injury.

A High-Throughput Flow Cytometry Screen Identifies Molecules That Inhibit Hantavirus Cell Entry

Hantaviruses cause hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS), which infects more than 200,000 people worldwide. Sin Nombre virus (SNV) and Andes virus (ANDV) cause the most severe form of HCPS, with case fatality ratios of 30%-40%. There are no specific therapies or vaccines for SNV. Using high-throughput flow cytometry, we screened the Prestwick Chemical Library for small-molecule inhibitors of the binding interaction between UV-inactivated and fluorescently labeled SNV^R18 particles, and decay-accelerating factor (DAF) expressed on Tanoue B cells. Eight confirmed hit compounds from the primary screen were investigated further in secondary screens that included infection inhibition, cytotoxicity, and probe interference. Antimycin emerged as a bona fide hit compound that inhibited cellular infection of the major HCPS (SNV)- and HCPS (Hantaan)-causing viruses. Confirming our assay's ability to detect active compounds, orthogonal testing of the hit compound showed that antimycin binds directly to the virus particle and blocks recapitulation of physiologic integrin activation caused by SNV binding to the integrin PSI domain.

Transcriptomics Sequencing Provides Insights into Understanding the Mechanism of Grass Carp Reovirus Infection

Grass carp is an important aquaculture fish species in China that is affected by severe diseases, especially haemorrhagic disease caused by grass carp reovirus (GCRV). However, the mechanisms of GCRV invasion and infection remain to be elucidated. In the present study, Ctenopharyngodon idellus kidney (CIK) cells were infected with GCRV, harvested at 0, 8, 24, and 72 h post infection, respectively, and then subjected to transcriptomics sequencing. Each sample yielded more than 6 Gb of clean data and 40 million clean reads. To better understand GCRV infection, the process was divided into three phases: the early (0-8 h post infection), middle (8-24 h post infection), and late (24-72 h) stages of infection. A total of 76 (35 up-regulated, 41 down-regulated), 553 (463 up-regulated, 90 down-regulated), and 284 (150 up-regulated, 134 down-regulated) differently expressed genes (DEGs) were identified during the early, middle, and late stages of infection, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed that DEGs were mainly involved in carbohydrate biosynthesis, transport, and endocytosis in the early stage, phagocytosis and lysosome pathways were mainly enriched in the middle stage, and programmed cell death, apoptosis, and inflammation were largely associated with the late stage. These results suggest GCRV infection is a gradual process involving adsorption on the cell surface, followed by endocytosis into cells, transport by lysosomes, and eventually resulted in cell necrosis and/or apoptosis. Our findings provide insight into the mechanisms of grass carp reovirus infection.

Low-affinity binding in cis to P2Y2R mediates force-dependent integrin activation during hantavirus infection

Atomic force microscopy is used to establish that low-affinity integrins bind in cis to P2Y₂R. Integrin activation is initiated by a membrane-normal switchblade motion triggered by integrin priming after the virus binds to the integrin PSI domain. Tensile force between the P2Y₂R and unbending integrin stimulates outside-in signaling.

Pathogenic hantaviruses bind to the plexin-semaphorin-integrin (PSI) domain of inactive, β₃ integrins. Previous studies have implicated a cognate cis interaction between the bent conformation β₅/β₃ integrins and an arginine-glycine-aspartic acid (RGD) sequence in the first extracellular loop of P2Y₂R. With single-molecule atomic force microscopy, we show a specific interaction between an atomic force microscopy tip decorated with recombinant α_IIbβ₃ integrins and (RGD)P2Y₂R expressed on cell membranes. Mutation of the RGD sequence to RGE in the P2Y₂R removes this interaction. Binding of inactivated and fluorescently labeled Sin Nombre virus (SNV) to the integrin PSI domain stimulates higher affinity for (RGD)P2Y₂R on cells, as measured by an increase in the unbinding force. In CHO cells, stably expressing α_IIbβ₃ integrins, virus engagement at the integrin PSI domain, recapitulates physiologic activation of the integrin as indicated by staining with the activation-specific mAB PAC1. The data also show that blocking of the Gα₁₃ protein from binding to the cytoplasmic domain of the β₃ integrin prevents outside-in signaling and infection. We propose that the cis interaction with P2Y₂R provides allosteric resistance to the membrane-normal motion associated with the switchblade model of integrin activation, where the development of tensile force yields physiological integrin activation.

Endocytic Pathways Used by Andes Virus to Enter Primary Human Lung Endothelial Cells

Andes virus (ANDV) is the major cause of hantavirus pulmonary syndrome (HPS) in South America. Despite a high fatality rate (up to 40%), no vaccines or antiviral therapies are approved to treat ANDV infection. To understand the role of endocytic pathways in ANDV infection, we used 3 complementary approaches to identify cellular factors required for ANDV entry into human lung microvascular endothelial cells. We screened an siRNA library targeting 140 genes involved in membrane trafficking, and identified 55 genes required for ANDV infection. These genes control the major endocytic pathways, endosomal transport, cell signaling, and cytoskeleton rearrangement. We then used infectious ANDV and retroviral pseudovirions to further characterize the possible involvement of 9 of these genes in the early steps of ANDV entry. In addition, we used markers of cellular endocytosis along with chemical inhibitors of known endocytic pathways to show that ANDV uses multiple routes of entry to infect target cells. These entry mechanisms are mainly clathrin-, dynamin-, and cholesterol-dependent, but can also occur via a clathrin-independent manner.

The Andes Virus Nucleocapsid Protein Directs Basal Endothelial Cell Permeability by Activating RhoA

Andes virus (ANDV) predominantly infects microvascular endothelial cells (MECs) and nonlytically causes an acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). In HPS patients, virtually every pulmonary MEC is infected, MECs are enlarged, and infection results in vascular leakage and highly lethal pulmonary edema. We observed that MECs infected with the ANDV hantavirus or expressing the ANDV nucleocapsid (N) protein showed increased size and permeability by activating the Rheb and RhoA GTPases. Expression of ANDV N in MECs increased cell size by preventing tuberous sclerosis complex (TSC) repression of Rheb-mTOR-pS6K. N selectively bound the TSC2 N terminus (1 to 1403) within a complex containing TSC2/TSC1/TBC1D7, and endogenous TSC2 reciprocally coprecipitated N protein from ANDV-infected MECs. TSCs normally restrict RhoA-induced MEC permeability, and we found that ANDV infection or N protein expression constitutively activated RhoA. This suggests that the ANDV N protein alone is sufficient to activate signaling pathways that control MEC size and permeability. Further, RhoA small interfering RNA, dominant-negative RhoA(N19), and the RhoA/Rho kinase inhibitors fasudil and Y27632 dramatically reduced the permeability of ANDV-infected MECs by 80 to 90%. Fasudil also reduced the bradykinin-directed permeability of ANDV and Hantaan virus-infected MECs to control levels. These findings demonstrate that ANDV activation of RhoA causes MEC permeability and reveal a potential edemagenic mechanism for ANDV to constitutively inhibit the basal barrier integrity of infected MECs. The central importance of RhoA activation in MEC permeability further suggests therapeutically targeting RhoA, TSCs, and Rac1 as potential means of resolving capillary leakage during hantavirus infections.

HPS is hallmarked by acute pulmonary edema, hypoxia, respiratory distress, and the ubiquitous infection of pulmonary MECs that occurs without disrupting the endothelium. Mechanisms of MEC permeability and targets for resolving lethal pulmonary edema during HPS remain enigmatic. Our findings suggest a novel underlying mechanism of MEC dysfunction resulting from ANDV activation of the Rheb and RhoA GTPases that, respectively, control MEC size and permeability. Our studies show that inhibition of RhoA blocks ANDV-directed permeability and implicate RhoA as a potential therapeutic target for restoring capillary barrier function to the ANDV-infected endothelium. Since RhoA activation forms a downstream nexus for factors that cause capillary leakage, blocking RhoA activation is liable to restore basal capillary integrity and prevent edema amplified by tissue hypoxia and respiratory distress. Targeting the endothelium has the potential to resolve disease during symptomatic stages, when replication inhibitors lack efficacy, and to be broadly applicable to other hemorrhagic and edematous viral diseases.

Early Bunyavirus-Host Cell Interactions

The Bunyaviridae is the largest family of RNA viruses, with over 350 members worldwide. Several of these viruses cause severe diseases in livestock and humans. With an increasing number and frequency of outbreaks, bunyaviruses represent a growing threat to public health and agricultural productivity globally. Yet, the receptors, cellular factors and endocytic pathways used by these emerging pathogens to infect cells remain largely uncharacterized. The focus of this review is on the early steps of bunyavirus infection, from virus binding to penetration from endosomes. We address current knowledge and advances for members from each genus in the Bunyaviridae family regarding virus receptors, uptake, intracellular trafficking and fusion.

Human Parechovirus 1 Infection Occurs via αVβ1 Integrin

Human parechovirus 1 (HPeV-1) (family Picornaviridae) is a global cause of pediatric respiratory and CNS infections for which there is no treatment. Although biochemical and in vitro studies have suggested that HPeV-1 binds to αVβ1, αVβ3 and αVβ6 integrin receptor(s), the actual cellular receptors required for infectious entry of HPeV-1 remain unknown. In this paper we analyzed the expression profiles of αVβ1, αVβ3, αVβ6 and α5β1 in susceptible cell lines (A549, HeLa and SW480) to identify which integrin receptors support HPeV-1 internalization and/or replication cycle. We demonstrate by antibody blocking assay, immunofluorescence microscopy and RT-qPCR that HPeV-1 internalizes and replicates in cell lines that express αVβ1 integrin but not αVβ3 or αVβ6 integrins. To further study the role of β1 integrin, we used a mouse cell line, GE11-KO, which is deficient in β1 expression, and its derivate GE11-β1 in which human integrin β1 subunit is overexpressed. HPeV-1 (Harris strain) and three clinical HPeV-1 isolates did not internalize into GE11-KO whereas GE11-β1 supported the internalization process. An integrin β1-activating antibody, TS2/16, enhanced HPeV-1 infectivity, but infection occurred in the absence of visible receptor clustering. HPeV-1 also co-localized with β1 integrin on the cell surface, and HPeV-1 and β1 integrin co-endocytosed into the cells. In conclusion, our results demonstrate that in some cell lines the cellular entry of HPeV-1 is primarily mediated by the active form of αVβ1 integrin without visible receptor clustering.

A Molecular-Level Account of the Antigenic Hantaviral Surface

Hantaviruses, a geographically diverse group of zoonotic pathogens, initiate cell infection through the concerted action of Gn and Gc viral surface glycoproteins. Here, we describe the high-resolution crystal structure of the antigenic ectodomain of Gn from Puumala hantavirus (PUUV), a causative agent of hemorrhagic fever with renal syndrome. Fitting of PUUV Gn into an electron cryomicroscopy reconstruction of intact Gn-Gc spike complexes from the closely related but non-pathogenic Tula hantavirus localized Gn tetramers to the membrane-distal surface of the virion. The accuracy of the fitting was corroborated by epitope mapping and genetic analysis of available PUUV sequences. Interestingly, Gn exhibits greater non-synonymous sequence diversity than the less accessible Gc, supporting a role of the host humoral immune response in exerting selective pressure on the virus surface. The fold of PUUV Gn is likely to be widely conserved across hantaviruses.

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We describe the high-resolution crystal structure of a hantaviral Gn ectodomain

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Electron cryotomography analysis reveals the ultrastructure of Gn-Gc assembly

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X-ray fitting and mapping analysis reveals the antigenic hantavirus surface

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The Gn fold is likely to be widely conserved across this group of viruses

Beyond RGD: virus interactions with integrins

Viruses successfully infect host cells by initially binding to the surfaces of the cells, followed by an intricate entry process. As multifunctional heterodimeric cell-surface receptor molecules, integrins have been shown to usefully serve as entry receptors for a plethora of viruses. However, the exact role(s) of integrins in viral pathogen internalization has yet to be elaborately described. Notably, several viruses harbor integrin-recognition motifs displayed on viral envelope/capsid-associated proteins. The most common of these motifs is the minimal peptide sequence for binding integrins, RGD (Arg-Gly-Asp), which is known for its role in virus infection via its ability to interact with over half of the more than 20 known integrins. Not all virus-integrin interactions are RGD-dependent, however. Non-RGD-binding integrins have also been shown to effectively promote virus entry and infection as well. Such virus-integrin binding is shown to facilitate adhesion, cytoskeleton rearrangement, integrin activation, and increased intracellular signaling. Also, we have attempted to discuss the role of carbohydrate moieties in virus interactions with receptor-like host cell surface integrins that drive the process of internalization. As much as possible, this article examines the published literature regarding the role of integrins in terms of virus infection and virus-encoded glycosylated proteins that mediate interactions with integrins, and it explores the idea of targeting these receptors as a therapeutic treatment option.

Hantavirus-induced pathogenesis in mice with a humanized immune system

Hantaviruses are emerging zoonotic pathogens that can cause severe disease in humans. Clinical observations suggest that human immune components contribute to hantavirus-induced pathology. To address this issue we generated mice with a humanized immune system. Hantavirus infection of these animals resulted in systemic infection associated with weight loss, decreased activity, ruffled fur and inflammatory infiltrates of lung tissue. Intriguingly, after infection, humanized mice harbouring human leukocyte antigen (HLA) class I-restricted human CD8+ T cells started to lose weight earlier (day 10) than HLA class I-negative humanized mice (day 15). Moreover, in these mice the number of human platelets dropped by 77 % whereas the number of murine platelets did not change, illustrating how differences between rodent and human haemato-lymphoid systems may contribute to disease development. To our knowledge this is the first description of a humanized mouse model of hantavirus infection, and our results indicate a role for human immune cells in hantaviral pathogenesis.

Endothelial cell dysfunction in viral hemorrhage and edema

The endothelium maintains a vascular barrier by controlling platelet and immune cell interactions, capillary tone and interendothelial cell (EC) adherence. Here we suggest common elements in play during viral infection of the endothelium that alter normal EC functions and contribute to lethal hemorrhagic or edematous diseases. In viral reservoir hosts, infection of capillaries and lymphatic vessels may direct immunotolerance without disease, but in the absence of these cognate interactions they direct the delayed onset of human disease characterized by thrombocytopenia and vascular leakage in a severe endothelial dysfunction syndrome. Here we present insight into EC controls of hemostasis, immune response and capillary permeability that are altered by viral infection of the endothelium.

Virus interactions with endothelial cell receptors: implications for viral pathogenesis

The endothelial lining of the vasculature performs a vital role in maintaining fluid barrier functions despite balancing nutrient and fluid content of tissues, repairing localized damage, coordinating responses of a plethora of factors, immune cells and platelets through a multitude of endothelial cell surface receptors. Viruses that nonlytically cause lethal hemorrhagic or edematous diseases engage receptors on vascular and lymphatic endothelial cells, altering normal cellular responses that control capillary leakage and fluid clearance functions with lethal consequences. Recent studies indicate that receptors directing dengue virus and hantavirus infection of the endothelium contribute to the dysregulation of normal endothelial cell signaling responses that control capillary permeability and immune responses that contribute to pathogenesis. Here we present recent studies of virally altered endothelial functions that provide new insight into targeting barrier functions of the endothelium as a potential therapeutic approach.

Immunogenetic factors affecting susceptibility of humans and rodents to hantaviruses and the clinical course of hantaviral disease in humans

We reviewed the associations of immunity-related genes with susceptibility of humans and rodents to hantaviruses, and with severity of hantaviral diseases in humans. Several class I and class II HLA haplotypes were linked with severe or benign hantavirus infections, and these haplotypes varied among localities and hantaviruses. The polymorphism of other immunity-related genes including the C4A gene and a high-producing genotype of TNF gene associated with severe PUUV infection. Additional genes that may contribute to disease or to PUUV infection severity include non-carriage of the interleukin-1 receptor antagonist (IL-1RA) allele 2 and IL-1β (-511) allele 2, polymorphisms of plasminogen activator inhibitor (PAI-1) and platelet GP1a. In addition, immunogenetic studies have been conducted to identify mechanisms that could be linked with the persistence/clearance of hantaviruses in reservoirs. Persistence was associated during experimental infections with an upregulation of anti-inflammatory responses. Using natural rodent population samples, polymorphisms and/or expression levels of several genes have been analyzed. These genes were selected based on the literature of rodent or human/hantavirus interactions (some Mhc class II genes, Tnf promoter, and genes encoding the proteins TLR4, TLR7, Mx2 and β3 integrin). The comparison of genetic differentiation estimated between bank vole populations sampled over Europe, at neutral and candidate genes, has allowed to evidence signatures of selection for Tnf, Mx2 and the Drb Mhc class II genes. Altogether, these results corroborated the hypothesis of an evolution of tolerance strategies in rodents. We finally discuss the importance of these results from the medical and epidemiological perspectives.

The integrin adhesome: from genes and proteins to human disease

The adhesive interactions of cells with their environment through the integrin family of transmembrane receptors have key roles in regulating multiple aspects of cellular physiology, including cell proliferation, viability, differentiation and migration. Consequently, failure to establish functional cell adhesions, and thus the assembly of associated cytoplasmic scaffolding and signalling networks, can have severe pathological effects. The roles of specific constituents of integrin-mediated adhesions, which are collectively known as the 'integrin adhesome', in diverse pathological states are becoming clear. Indeed, the prominence of mutations in specific adhesome molecules in various human diseases is now appreciated, and experimental as well as in silico approaches provide insights into the molecular mechanisms underlying these pathological conditions.

Hantavirus Gn and Gc envelope glycoproteins: key structural units for virus cell entry and virus assembly

In recent years, ultrastructural studies of viral surface spikes from three different genera within the Bunyaviridae family have revealed a remarkable diversity in their spike organization. Despite this structural heterogeneity, in every case the spikes seem to be composed of heterodimers formed by Gn and Gc envelope glycoproteins. In this review, current knowledge of the Gn and Gc structures and their functions in virus cell entry and exit is summarized. During virus cell entry, the role of Gn and Gc in receptor binding has not yet been determined. Nevertheless, biochemical studies suggest that the subsequent virus-membrane fusion activity is accomplished by Gc. Further, a class II fusion protein conformation has been predicted for Gc of hantaviruses, and novel crystallographic data confirmed such a fold for the Rift Valley fever virus (RVFV) Gc protein. During virus cell exit, the assembly of different viral components seems to be established by interaction of Gn and Gc cytoplasmic tails (CT) with internal viral ribonucleocapsids. Moreover, recent findings show that hantavirus glycoproteins accomplish important roles during virus budding since they self-assemble into virus-like particles. Collectively, these novel insights provide essential information for gaining a more detailed understanding of Gn and Gc functions in the early and late steps of the hantavirus infection cycle.

An innate immunity-regulating virulence determinant is uniquely encoded by the Andes virus nucleocapsid protein

Andes virus (ANDV) is the only hantavirus known to spread from person to person and shown to cause highly lethal hantavirus pulmonary syndrome (HPS) in patients and Syrian hamsters. Hantaviruses replicate in human endothelial cells and accomplish this by restricting the early induction of beta interferon (IFN-β)- and IFN-stimulated genes (ISGs). Our studies reveal that the ANDV nucleocapsid (N) protein uniquely inhibits IFN signaling responses directed by cytoplasmic double-stranded RNA (dsRNA) sensors RIG-I and MDA5. In contrast, N proteins from Sin Nombre, New York-1, and Prospect Hill hantaviruses had no effect on RIG-I/MDA5-directed transcriptional responses from IFN-β-, IFN-stimulated response element (ISRE)-, or κB-containing promoters. Ablating a potential S-segment nonstructural open reading frame (ORF) (NSs) within the ANDV plasmid expressing N protein failed to alter IFN regulation by ANDV N protein. Further analysis demonstrated that expressing the ANDV N protein inhibited downstream IFN pathway activation directed by MAVS, TBK1, and IκB kinase ε (IKKε) but failed to inhibit transcriptional responses directed by constitutive expression of active interferon regulatory factor IRF3-5D or after stimulation by alpha interferon (IFN-α) or tumor necrosis factor alpha (TNF-α). Consistent with IFN pathway-specific regulation, the ANDV N protein inhibited TBK1-directed IRF3 phosphorylation (phosphorylation of serine 396 [pS396]) and TBK1 autophosphorylation (pS172). Collectively, these findings indicate that the ANDV N inhibits IFN signaling responses by interfering with TBK1 activation, upstream of IRF3 phosphorylation and NF-κB activation. Moreover, our findings reveal that ANDV uniquely carries a gene encoding a virulence determinant within its N protein that is capable of restricting ISG and IFN-β induction and provide a rationale for the novel pathogenesis and spread of ANDV.

Andes virus (ANDV) is distinguished from other hantaviruses by its unique ability to spread from person to person and cause lethal hantavirus pulmonary syndrome (HPS)-like disease in Syrian hamsters. However, virulence determinants that distinguish ANDV from other pathogenic hantaviruses have yet to be defined. Here we reveal that ANDV uniquely contains a virulence determinant within its nucleocapsid (N) protein that potently inhibits innate cellular signaling pathways. This novel function of the N protein provides a new mechanism for hantaviruses to regulate interferon (IFN) and IFN-stimulated gene (ISG) induction that is likely to contribute to the enhanced ability of ANDV to replicate, spread, and cause disease. These findings differentiate ANDV from other HPS-causing hantaviruses and provide a potential target for viral attenuation that needs to be considered in vaccine development.

The major cellular sterol regulatory pathway is required for Andes virus infection

The Bunyaviridae comprise a large family of RNA viruses with worldwide distribution and includes the pathogenic New World hantavirus, Andes virus (ANDV). Host factors needed for hantavirus entry remain largely enigmatic and therapeutics are unavailable. To identify cellular requirements for ANDV infection, we performed two parallel genetic screens. Analysis of a large library of insertionally mutagenized human haploid cells and a siRNA genomic screen converged on components (SREBP-2, SCAP, S1P and S2P) of the sterol regulatory pathway as critically important for infection by ANDV. The significance of this pathway was confirmed using functionally deficient cells, TALEN-mediated gene disruption, RNA interference and pharmacologic inhibition. Disruption of sterol regulatory complex function impaired ANDV internalization without affecting virus binding. Pharmacologic manipulation of cholesterol levels demonstrated that ANDV entry is sensitive to changes in cellular cholesterol and raises the possibility that clinically approved regulators of sterol synthesis may prove useful for combating ANDV infection.

As obligate, intracellular parasites viruses are dependent upon the host cell for numerous factors and processes. However, for many important viruses few of the required host factors have been identified. Hantaviruses are rodent-borne viruses that are associated with severe human disease. Transmission to humans occurs sporadically with a recent notable example in Yosemite National park. In the present study, we utilized two independent genetic strategies to discover cellular factors needed for replication of the highly pathogenic hantavirus Andes virus. We found that four genes, encoding components of a complex involved in regulation of cholesterol synthesis and uptake, were critical for Andes virus infection. Drugs that inhibit an enzyme in this complex or that reduce cellular cholesterol levels effectively blocked Andes virus infection, suggesting new ways for combating this pathogenic virus.

Hantavirus interferon regulation and virulence determinants

Hantaviruses predominantly replicate in primary human endothelial cells and cause 2 diseases characterized by altered barrier functions of vascular endothelium. Most hantaviruses restrict the early induction of interferon-β (IFNβ) and interferon stimulated genes (ISGs) within human endothelial cells to permit their successful replication. PHV fails to regulate IFN induction within human endothelial cells which self-limits PHV replication and its potential as a human pathogen. These findings, and the altered regulation of endothelial cell barrier functions by pathogenic hantaviruses, suggest that virulence is determined by the ability of hantaviruses to alter key signaling pathways within human endothelial cells. Our findings indicate that the Gn protein from ANDV, but not PHV, inhibits TBK1 directed ISRE, kB and IFNβ induction through virulence determinants in the Gn cytoplasmic tail (GnT) that inhibit TBK1 directed IRF3 phosphorylation. Further studies indicate that in response to hypoxia induced VEGF, ANDV infection enhances the permeability and adherens junction internalization of microvascular and lymphatic endothelial cells. These hypoxia/VEGF directed responses are rapamycin sensitive and directed by mTOR signaling pathways. These results demonstrate the presence of at least two hantavirus virulence determinants that act on endothelial cell signaling pathways: one that regulates antiviral IFN signaling responses, and a second that enhances normal hypoxia-VEGF-mTOR signaling pathways to facilitate endothelial cell permeability. These findings suggest signaling pathways as potential targets for therapeutic regulation of vascular deficits that contribute to hantavirus diseases and viral protein targets for attenuating pathogenic hantaviruses.

Old World hantaviruses: aspects of pathogenesis and clinical course of acute renal failure

Hantavirus-associated diseases represent emerging infections that are ranked in the highest priority group of communicable diseases for surveillance and epidemiological research. In the last years, several novel hantavirus species were described and the number of host reservoir species harboring hantaviruses is also increasing. Reports of cases with severe or atypical clinical courses become also more frequent. These facts raise more and more questions concerning host reservoir specificity, pathogenicity and molecular mechanism of pathogenesis. Hantavirus disease is characterized by vascular leakage due to increased capillary permeability. The infection manifests often in the lung (hantaviral cardiopulmonary syndrome; HCPS) or in the kidney (hemorrhagic fever with renal syndrome, HFRS). The underlying mechanisms of both syndromes are probably similar despite the difference in organ tropism. Characterization of hantaviral replication cycle and of patient-specific determinants will help to identify factors responsible for the clinical symptoms and course.

Vascular endothelial growth factor levels in dobrava/belgrade virus infections

The levels of vascular endothelial growth factor-A (VEGF) were estimated in 102 serum samples from 63 hospitalized Greek patients with hemorrhagic fever with renal syndrome (HFRS) caused by Dobrava/Belgrade virus. Significantly higher VEGF levels were seen in the severe when compared with non-severe cases (mean values 851.96 pg/mL and 326.75 pg/mL, respectively; p = 0.003), while a significant difference was observed among groups based on the day after the onset of illness. In both severe and non-severe cases, VEGF peaked in the second week of illness; however, elevation of VEGF in the severe cases started later and remained high until convalescence, suggesting that the role of VEGF was associated with repair of vascular damage rather than with increased permeability.

Hypoxia induces permeability and giant cell responses of Andes virus-infected pulmonary endothelial cells by activating the mTOR-S6K signaling pathway

Andes virus (ANDV) is a South American hantavirus that causes a highly lethal hantavirus pulmonary syndrome (HPS) characterized by hypoxia, thrombocytopenia, and vascular leakage leading to acute pulmonary edema. ANDV infects human pulmonary microvascular and lymphatic endothelial cells (MECs and LECs, respectively) and nonlytically enhances the permeability of interendothelial cell adherence junctions in response to vascular endothelial growth factor (VEGF). Recent findings also indicate that ANDV causes the formation of giant endothelial cells. Here, we demonstrate that hypoxic conditions alone enhance permeability and giant cell responses of ANDV-infected MECs and LECs through activation of the mTOR signaling pathway. In contrast to infection of cells with nonpathogenic Tula virus (TULV), we observed that exposure of ANDV-infected MECs and LECs to hypoxic conditions resulted in a 3- to 6-fold increase in monolayer permeability and the formation of giant cells 3× to 5× normal size. ANDV infection in combination with hypoxic conditions resulted in the enhancement of hypoxia-inducible factor 1α (HIF1α)-directed VEGF A, angiopoietin 4, and EGLN3 transcriptional responses. Constitutive mTOR signaling induces the formation of giant cells via phosphorylation of S6K, and mTOR regulates hypoxia and VEGF A-induced cellular responses. We found that S6K was hyperphosphorylated in ANDV-infected, hypoxia-treated MECs and LECs and that rapamycin treatment for 1 h inhibited mTOR signaling responses and blocked permeability and giant cell formation in ANDV-infected monolayers. These findings indicate that ANDV infection and hypoxic conditions enhance mTOR signaling responses, resulting in enhanced endothelial cell permeability and suggest a role for rapamycin in therapeutically stabilizing the endothelium of microvascular and lymphatic vessels during ANDV infection.

Role of vascular and lymphatic endothelial cells in hantavirus pulmonary syndrome suggests targeted therapeutic approaches

BACKGROUND

Hantaviruses in the Americas cause a highly lethal acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). Hantaviruses nonlytically infect microvascular and lymphatic endothelial cells and cause dramatic changes in barrier functions without disrupting the endothelium. Hantaviruses cause changes in the function of infected endothelial cells that normally regulate fluid barrier functions. The endothelium of arteries, veins, and lymphatic vessels are unique and central to the function of vast pulmonary capillary beds that regulate pulmonary fluid accumulation.

RESULTS

We have found that HPS-causing hantaviruses alter vascular barrier functions of microvascular and lymphatic endothelial cells by altering receptor and signaling pathway responses that serve to permit fluid tissue influx and clear tissue edema. Infection of the endothelium provides several mechanisms for hantaviruses to cause acute pulmonary edema, as well as potential therapeutic targets for reducing the severity of HPS disease.

CONCLUSIONS

Here we discuss interactions of HPS-causing hantaviruses with the endothelium, roles for unique lymphatic endothelial responses in HPS, and therapeutic targeting of the endothelium as a means of reducing the severity of HPS disease.

Slit2-Robo4 receptor responses inhibit ANDV directed permeability of human lung microvascular endothelial cells

Hantaviruses nonlytically infect human endothelial cells (ECs) and cause edematous and hemorrhagic diseases. Andes virus (ANDV) causes hantavirus pulmonary syndrome (HPS), and Hantaan virus (HTNV) causes hemorrhagic fever with renal syndrome (HFRS). Hantaviruses enhance vascular endothelial growth factor directed EC permeability resulting in the disassembly of inter-endothelial cell adherens junctions (AJs). Recent studies demonstrate that Slit2 binding to Robo1/Robo4 receptors on ECs has opposing effects on AJ disassembly and vascular fluid barrier functions. Here we demonstrate that Slit2 inhibits ANDV and HTNV induced permeability and AJ disassembly of pulmonary microvascular ECs (PMECs) by interactions with Robo4. In contrast, Slit2 had no effect on the permeability of ANDV infected human umbilical vein ECs (HUVECs). Analysis of Robo1/Robo4 expression determined that PMECs express Robo4, but not Robo1, while HUVECs expressed both Robo4 and Robo1 receptors. SiRNA knockdown of Robo4 in PMECs prevented Slit2 inhibition of ANDV induced permeability demonstrating that Robo4 receptors determine PMEC responsiveness to Slit2. Collectively, this data demonstrates a selective role for Slit2/Robo4 responses within PMECs that inhibits ANDV induced permeability and AJ disassembly. These findings suggest Slit2s utility as a potential HPS therapeutic that stabilizes the pulmonary endothelium and antagonizes ANDV induced pulmonary edema.

Amino Acid Sequences Mediating Vascular Cell Adhesion Molecule 1 Binding to Integrin Alpha 4: Homologous DSP Sequence Found for JC Polyoma VP1 Coat Protein

The JC polyoma viral coat protein VP1 was analyzed for amino acid sequences homologies to the IDSP sequence which mediates binding of VLA-4 (integrin alpha 4) to vascular cell adhesion molecule 1. Although the full sequence was not found, a DSP sequence was located near the critical arginine residue linked to infectivity of the virus and binding to sialic acid containing molecules such as integrins (3). For the JC polyoma virus, a DSP sequence was found at residues 70, 71 and 72 with homology also noted for the mouse polyoma virus and SV40 virus. Three dimensional modeling of the VP1 molecule suggests that the DSP loop has an accessible site for interaction from the external side of the assembled viral capsid pentamer.

Host cell factors in filovirus entry: novel players, new insights

Filoviruses cause severe hemorrhagic fever in humans with high case-fatality rates. The cellular factors exploited by filoviruses for their spread constitute potential targets for intervention, but are incompletely defined. The viral glycoprotein (GP) mediates filovirus entry into host cells. Recent studies revealed important insights into the host cell molecules engaged by GP for cellular entry. The binding of GP to cellular lectins was found to concentrate virions onto susceptible cells and might contribute to the early and sustained infection of macrophages and dendritic cells, important viral targets. Tyrosine kinase receptors were shown to promote macropinocytic uptake of filoviruses into a subset of susceptible cells without binding to GP, while interactions between GP and human T cell Ig mucin 1 (TIM-1) might contribute to filovirus infection of mucosal epithelial cells. Moreover, GP engagement of the cholesterol transporter Niemann-Pick C1 was demonstrated to be essential for GP-mediated fusion of the viral envelope with a host cell membrane. Finally, mutagenic and structural analyses defined GP domains which interact with these host cell factors. Here, we will review the recent progress in elucidating the molecular interactions underlying filovirus entry and discuss their implications for our understanding of the viral cell tropism.

Elevated VEGF Levels in Pulmonary Edema Fluid and PBMCs from Patients with Acute Hantavirus Pulmonary Syndrome

Hantavirus pulmonary syndrome is characterized by vascular permeability, hypoxia, and acute pulmonary edema. Vascular endothelial growth factor (VEGF) is induced by hypoxia, potently induces vascular permeability, and is associated with high-altitude-induced pulmonary edema. Hantaviruses alter the normal regulation of β3 integrins that restrict VEGF-directed permeability and hantavirus infected endothelial cells are hyperresponsive to the permeabilizing effects of VEGF. However, the role of VEGF in acute pulmonary edema observed in HPS patients remains unclear. Here we retrospectively evaluate VEGF levels in pulmonary edema fluid (PEF), plasma, sera, and PBMCs from 31 HPS patients. VEGF was elevated in HPS patients PEF compared to controls with the highest levels observed in PEF samples from a fatal HPS case. VEGF levels were highest in PBMC samples during the first five days of hospitalization and diminished during recovery. Significantly increased PEF and PBMC VEGF levels are consistent with acute pulmonary edema observed in HPS patients and HPS disease severity. We observed substantially lower VEGF levels in a severe HPS disease survivor after extracorporeal membrane oxygenation. These findings suggest the importance of patients' VEGF levels during HPS, support the involvement of VEGF responses in HPS pathogenesis, and suggest targeting VEGF responses as a potential therapeutic approach.

The Role of the Endothelium in HPS Pathogenesis and Potential Therapeutic Approaches

American hantaviruses cause a highly lethal acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). Hantaviruses nonlytically infect endothelial cells and cause dramatic changes in barrier functions of the endothelium without disrupting the endothelium. Instead hantaviruses cause changes in the function of infected endothelial cells that normally regulate fluid barrier functions of capillaries. The endothelium of arteries, veins, and lymphatic vessels is unique and central to the function of vast pulmonary capillary beds, which regulate pulmonary fluid accumulation. The endothelium maintains vascular barrier functions through a complex series of redundant receptors and signaling pathways that serve to both permit fluid and immune cell efflux into tissues and restrict tissue edema. Infection of the endothelium provides several mechanisms for hantaviruses to alter capillary permeability but also defines potential therapeutic targets for regulating acute pulmonary edema and HPS disease. Here we discuss interactions of HPS causing hantaviruses with the endothelium, potential endothelial cell-directed permeability mechanisms, and therapeutic targeting of the endothelium as a means of reducing the severity of HPS disease.

Andes virus infection of lymphatic endothelial cells causes giant cell and enhanced permeability responses that are rapamycin and vascular endothelial growth factor C sensitive

Hantaviruses primarily infect endothelial cells (ECs) and nonlytically cause vascular changes that result in hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). Acute pulmonary edema during HPS may be caused by capillary leakage and failure of lymphatic vessels to clear fluids. Uniquely regulated lymphatic ECs (LECs) control fluid clearance, although roles for lymphatics in hantavirus disease remain undetermined. Here we report that hantaviruses productively infect LECs and that LEC infection by HPS causing Andes virus (ANDV) and HFRS causing Hantaan virus (HTNV) are inhibited by α(v)β(3) integrin antibodies. Although α(v)β(3) integrins regulate permeabilizing responses directed by vascular endothelial growth factor receptor 2 (VEGFR2), we found that only ANDV-infected LECs were hyperpermeabilized by the addition of VEGF-A. However, VEGF-C activation of LEC-specific VEGFR3 receptors blocked ANDV- and VEGF-A-induced LEC permeability. In addition, ∼75% of ANDV-infected LECs became viable mononuclear giant cells, >4 times larger than normal, in response to VEGF-A. Giant cells are associated with constitutive mammalian target of rapamycin (mTOR) activation, and we found that both giant LECs and LEC permeability were sensitive to rapamycin, an mTOR inhibitor, and VEGF-C addition. These findings indicate that ANDV uniquely alters VEGFR2-mTOR signaling responses of LECs, resulting in giant cell and LEC permeability responses. This suggests that ANDV infection alters normal LEC and lymphatic vessel functions which may contribute to edematous fluid accumulation during HPS. Moreover, the ability of VEGF-C and rapamycin to normalize LEC responses suggests a potential therapeutic approach for reducing pulmonary edema and the severity of HPS following ANDV infection.