Sulfatide Binds to Influenza B Virus and Enhances Viral Replication

Seasonal influenza epidemics caused by influenza A viruses (IAV) and influenza B viruses (IBV) pose a substantial public health burden. Despite the significant impact of IBV, its restricted host range and the absence of documented pandemics have resulted in limited research attention relative to IAV. Understanding the viral infection mechanisms of both IAV and IBV is crucial for controlling seasonal epidemics. Previously, we demonstrated that 3'-O-sulfated galactosylceramide sulfatide binds to IAV and enhances viral replication, a finding with potential therapeutic implications. However, the role sulfatide plays in other influenza virus infections, including those caused by IBV, remains unknown. Accordingly, in this paper, we investigate the function of sulfatide during IBV infection. We demonstrate that sulfatide binds to IBV hemagglutinin (HA), and that sulfatide overexpression significantly enhances IBV replication, whereas treatment with sulfatase or an anti-sulfatide antibody markedly suppressed IBV replication. Moreover, further tests involving the inhibition of sulfatide biosynthesis resulted in the suppression of viral replication with impaired nuclear export of viral ribonucleoproteins (vRNPs). These findings establish that sulfatide is a critical regulator of IBV replication, which parallels its role in IAV infection, and suggest that targeting sulfatide-virus interactions can lead to broad-spectrum therapeutic strategies against influenza virus.

VP1 of human and murine noroviruses recognizes glycolipid sulfatide via the P domain

Noroviruses are a prevalent cause of human viral gastroenteritis, yet the precise mechanisms underlying their infection cycle, particularly their interactions with and entry into cells, remain poorly understood. Human norovirus (HuNoV) primarily targets human small intestinal epithelial cells, within which 3-O-sulfogalactosylceramide (sulfatide) ranks among the most abundant glycosphingolipids (GSLs). While sulfatide involvement in the binding and infection mechanism of several viruses has been documented, its interaction with noroviruses remains underexplored. This study investigated whether noroviruses interact with sulfatide. We found that the recombinant viral capsid protein VP1 of HuNoV (genogroups I and II) and murine norovirus (genogroup V) exhibited robust binding to sulfatide compared with other tested GSLs using enzyme-linked immunosorbent assay, thin-layer chromatography binding assay and real-time quantitative reverse transcription polymerase chain reaction binding assay. VP1 also bound 3-O-sulfated lactosylceramide, which shares the 3-O-sulfated galactose moiety with sulfatide. However, both VP1 and its P domain, identified as the sulfatide-binding domain, exhibited limited binding to structural analogues of sulfatide and other sulfated compounds. These findings suggest a specific recognition of the 3-O-sulfated galactose moiety. Notably, we found that sulfatide is a novel binding target for norovirus particles. Overall, our findings reveal a previously unknown norovirus-sulfatide interaction, proposing sulfatide as a potential candidate for norovirus infection receptors.

Chronic brain damage in HIV-infected individuals under antiretroviral therapy is associated with viral reservoirs, sulfatide release, and compromised cell-to-cell communication

HIV infection has become a chronic and manageable disease due to the effective use of antiretroviral therapies (ART); however, several chronic aging-related comorbidities, including cognitive impairment, remain a major public health issue. However, these mechanisms are unknown. Here, we identified that glial and myeloid viral reservoirs are associated with local myelin damage and the release of several myelin components, including the lipid sulfatide. Soluble sulfatide compromised gap junctional communication and calcium wave coordination, essential for proper cognition. We propose that soluble sulfatide could be a potential biomarker and contributor to white matter compromise observed in HIV-infected individuals even in the current ART era.

Roles of conserved residues in the receptor binding sites of human parainfluenza virus type 3 HN protein

Human parainfluenza virus type 3 (hPIV-3) entry and intrahost spread through membrane fusion are initiated by two envelope glycoproteins, hemagglutinin-neuraminidase (HN) and fusion (F) protein. Binding of HN protein to the cellular receptor via its receptor-binding sites triggers conformational changes in the F protein leading to virus-cell fusion. However, little is known about the roles of individual amino acids that comprise the receptor-binding sites in the fusion process. Here, residues R192, D216, E409, R424, R502, Y530 and E549 located within the receptor-binding site Ⅰ, and residues N551 and H552 at the putative site Ⅱ were replaced by alanine with site-directed mutagenesis. All mutants except N551A displayed statistically lower hemadsorption activities ranging from 16.4% to 80.2% of the wild-type (wt) level. With standardization of the number of bound erythrocytes, similarly, other than N551A, all mutants showed reduced fusogenic activity at three successive stages: lipid mixing (hemifusion), content mixing (full fusion) and syncytium development. Kinetic measurements of the hemifusion process showed that the initial hemifusion extent for R192A, D216A, E409A, R424A, R502A, Y530A, E549A and H552A was decreased to 69.9%, 80.6%, 71.3%, 67.3%, 50.6%, 87.4%, 84.9% and 25.1%, respectively, relative to the wt, while the initial rate of hemifusion for the E409A, R424A, R502A and H552A mutants was reduced to 69.0%, 35.4%, 62.3%, 37.0%, respectively. In addition, four mutants with reduced initial hemifusion rates also showed decreased percentages of F protein cleavage from 43.4% to 56.3% of the wt. Taken together, Mutants R192A, D216A, E409A, R424A, R502A, Y530A, E549A and H552A may lead to damage on the fusion activity at initial stage of hemifusion, of which decreased extent and rate may be associated with impaired receptor binding activity resulting in the increased activation barrier of F protein and the cleavage of it, respectively.

Fenofibrate increases the amount of sulfatide which seems beneficial against Covid-19

Fenofibrate, which is a PPAR-alfpha agonist, increases the level of sulfatide. In this letter we hypothesize on the background of various findings that this is beneficial against COVID-19. Fenofibrate has been used for decades against hypercholesterolemia and has no serious side effects. Therefore, a trial giving fenofibrate to patients with corona virus infection is recommended.

Role of sulfatide in influenza A virus replication

Sulfatide is a 3-O-sulfated galactosylceramide that is abundantly expressed in the gastrointestinal tract, kidney, trachea, and particularly the central nervous system. Cellular sulfatide is mainly localized in the Golgi apparatus, cellular membrane, and lysosomes in cytosol. Since our earlier report showed that the influenza A virus specifically binds to sulfatide, we have investigated the roles of sulfatide in the influenza A virus lifecycle. The viral binding is independent of sialic acids, which function as virus receptors in virus attachment to the host cell surface. Sulfatide is recognized by the ectodomain of the viral envelope glycoprotein hemagglutinin (HA). Nascent HA is transported on the surface membrane of infected cells. The binding of HA with sulfatide on the cell surface induces apoptosis through potential loss of the mitochondrial membrane and nuclear translocation of apoptosis-inducing factor in mitochondria, where PB1-F2 peptide from the viral gene is accumulated. In the nucleus of infected cells, viral ribonucleoprotein (vRNP) complexes are formed from viral RNA genomes, viral nucleoprotein, and viral RNA polymerase subunits, and these complexes are selectively exported into cytosol through the nuclear membrane. The apoptosis significantly enhances the nuclear export of vRNP complexes, resulting in efficient formation of progeny viruses and facilitation of virus replication. At that time, activation of the Raf/mitogen-activated protein extracellular kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway through sulfatide is associated with virus replication. Our studies have demonstrated that sulfatide is not a viral receptor for virus infection, and that the binding of HA with sulfatide functions as an initiation switch for the formation of progeny viruses.

Functional analysis of glyco-molecules that bind with influenza virus

Influenza A virus (IAV) recognizes terminal sialic acid of sialoglyco-conjugates on host cells through the viral envelope glycoprotein hemagglutinin (HA), followed by initiation of entry into the cells. Molecular species of sialic acid are largely divided into two moieties: N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). A receptor for IAV infection generally means Neu5Ac. Almost all equine IAVs and some human, swine, and duck IAVs bind not only to Neu5Ac but also to Neu5Gc. In nonhuman animals, Neu5Gc has been detected in swine and equine tracheas and the duck colon, which are the main replication sites of mammalian and avian IAVs. Therefore, Neu5Gc in these sites has been suggested to be a functional receptor for IAV infection. Humans cannot synthesize Neu5Gc due to a genetic defect of the Neu5Gc-synthesizing enzyme. We evaluated the receptor function of Neu5Gc in IAV infection in human cells. Our results indicated that Neu5Gc expression on the surface of human cells is not a functional receptor for IAV infection and that it has a negative effect on infectivity of IAV possessing Neu5Gc binding ability. IAV also binds to non-sialo 3-O-sulfated galactosylceramide (sulfatide). Sulfatide has been suggested to be a functional receptor for IAV infection. However, we have shown that sulfatide is not a functional receptor for IAV infection and that the binding of HA with sulfatide enhances progeny virus production. It is expected that functions of these glyco-molecules can be used in prevention and development of new drugs against IAV.

Dysfunction of platelet-derived growth factor receptor α (PDGFRα) represses the production of oligodendrocytes from arylsulfatase A-deficient multipotential neural precursor cells

The membrane-bound receptor for platelet-derived growth factor A (PDGFRα) is crucial for controlling the production of oligodendrocytes (OLs) for myelination, but regulation of its activity during OL differentiation is largely unknown. We have examined the effect of increased sulfated content of galactosylceramides (sulfatides) on the regulation of PDGFRα in multipotential neural precursors (NPs) that are deficient in arylsulfatase A (ASA) activity. This enzyme is responsible for the lysosomal hydrolysis of sulfatides. We show that sulfatide accumulation significantly impacts the formation of OLs via deregulation of PDGFRα function. PDGFRα is less associated with detergent-resistant membranes in ASA-deficient cells and showed a significant decrease in AKT phosphorylation. Rescue experiments with ASA showed a normalization of the ratio of long versus short sulfatides, restored PDGFRα levels, corrected its localization to detergent-resistant membranes, increased AKT phosphorylation, and normalized the production of OLs in ASA-deficient NPs. Moreover, our studies identified a novel mechanism that regulates the secretion of PDGFRα in NPs, in glial cells, and in the brain cortex via exosomal shedding. Our study provides a first step in understanding the role of sulfatides in regulating PDGFRα levels in OLs and its impact in myelination.

A novel method for detection of Newcastle disease virus with a fluorescent sialidase substrate

Newcastle disease virus (NDV), belonging to the family Paramixoviridae, causes respiratory and neuronal symptoms in almost all birds. NDV has haemagglutinin-neuraminidase (HN) glycoprotein possessing sialidase activity. HN glycoprotein is highly expressed on the surface of NDV-infected cells, resulting in much higher sialidase activity in NDV-infected cells than in non-infected cells. It was reported that mouse and human cancer cells up-regulating sialidase expression were histochemically stained with a fluorescent sialidase substrate, 2-(benzothiazol-2-yl)-4-bromophenyl 5-acetamido-3,5-dideoxy-α-D-glycero-D-galacto-2-nonulopyranosidonic acid (BTP3-Neu5Ac), which deposits water-insoluble fluorescent compound BTP3 on locations of sialidase activity. By using the BTP3-Neu5Ac assay, we showed that NDV-infected cells and HN gene-expressing cells could be simply detected at room temperature after only 5min. Infection of the cells with the virus resulted in apparent green fluorescence, which disappeared with addition of a sialidase inhibitor. Cells that were stained in the BTP3-Neu5Ac assay were immunostained with an anti-NDV antibody. Moreover, BTP3-Neu5Ac staining was applied to a virus overlay binding assay with NDV particles. NDV-bound protein bands on guinea pig red blood cells were easily and rapidly detected by the BTP3-Neu5Ac assay after Western blotting. BTP3-Neu5Ac offers an easy and rapid protocol for fluorescent staining of NDV and virus-infected cells without antibodies.

[Function of glycochains in virus infection]

  Influenza A virus (IAV) has two envelope glycoproteins, hemagglutinin (HA) and neuraminidase (NA). HA binds to sialic acids at the terminals of glycochains on the host cell surface as virus receptors. NA shows sialidase activity, which cleaves sialic acids from the terminals of glycochains. A new subtype (antigenicities of HA and NA) of IAV for humans has pandemic potential. We investigated the functions of HA and NA in IAV replication and pandemic potential in terms of glycoscience. We found that the sialidase activity of pandemic IAV had low pH stability, which enhanced IAV replication. It is thought that the low pH stability contributed to the pandemics in 1968 and 2009. HA also binds to sulfatide not containing sialic acid, and we found that sulfatide enhanced IAV replication. Binding of HA to sulfatide on the host cell surface enhanced progeny IAV formation in infected cells through the induction of the nuclear export of viral ribonucleoproteins by apoptosis. Sialic acid species are divided into N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). The HAs of some human IAVs bind not only to Neu5Ac but also to Neu5Gc, which may facilitate the occurrence of a human IAV-based pandemic by genetic reassortment among IAV genomes in pig tracheas expressing Neu5Gc. We identified the amino acid residues of human IAV HA responsible for Neu5Gc binding and developed new techniques for the sensitive detection of IAV receptor specificities and infected cells. Our "glycovirology" research will provide new insights into the mechanisms of IAV replication and pandemic potential.