Imaging of influenza virus sialidase activity in living cells

Analysis of neuraminidase activity of human parainfluenza viruses using enzyme-linked lectin assay and BTP3-Neu5Ac assay

Human parainfluenza viruses (hPIVs) are causative agents of upper and lower respiratory tract infections and they have four serotypes. The virion surface displays hemagglutinin-neuraminidase (HN), having hemagglutinating (HA) and neuraminidase (NA) activities in a single molecule. The HA activity binds the virion to sialic acid on the viral receptor on host cells and the NA releases the progeny viruses from the cell surface. There are several methods for assaying viral NA activity, such as the thiobarbituric acid assay, 4-methylumbelliferyl-N-acetyl-α-d-neuraminic acid assay, NA-Star assay, and enzyme-linked lectin assay (ELLA). However, these are mainly used for influenza viruses and not for hPIVs. A fluorescent-based cytochemical NA assay using BTP3-Neu5Ac as the substrate was recently developed and used for orthomyxo- and paramyxoviruses, including types 1 and 3 hPIVs. In this study, we used the ELLA, and BTP-Neu5Ac assay for 14 field isolate strains of hPIVs including all four serotypes. The reaction in ELLA at pH 6.5 using peanut agglutinin (PNA) as a lectin was very low for all tested viruses except a type 3 virus strain with the maximum reaction at pH 6.5 and the acidic conditions did not enhance the reaction. ELLA with another lectin, Erythrina cristagalli agglutinin exhibited significant and stronger reactions than with PNA in some strains of types 1 and 3 viruses. The BTP3-Neu5Ac assay showed a fluorescent signal on cells infected with all the viruses except the hPIV1/Sendai/713/2018 strain in LLC-MK2 and/or MNT-1. The signal was detected in cell-free virus, as well, in all the viruses except the hPIV4a/Sendai/3935/2003 strain. The strength of the signal varied among viral strains but it was stronger in the reaction at pH 4.0 than pH 7.0 and strongest in type 2 hPIVs.

Novel sialidase inhibitors suppress mumps virus replication and infection

The prevalent human pathogen, mumps virus (MuV; orthorubulavirus parotitidis) causes various complications and serious sequelae, such as meningitis, encephalitis, deafness, and impaired fertility. Direct-acting antivirals (DAAs) targeting MuV which can prevent mumps and mumps-associated complications and sequelae are yet to be developed. Paramyxoviridae family members, such as MuV, possess viral surface hemagglutinin-neuraminidase (HN) protein with sialidase activity which facilitates efficient viral replication. Therefore, to develop DAAs targeting MuV we synthesized MuV sialidase inhibitors. It is proposed that the viral HN has a single functional site for N-acetylneuraminic acid (Neu5Ac) binding and sialidase activity. Further, the known MuV sialidase inhibitor is an analog of Neu5Ac-2,3-didehydro-2-deoxy-N-acetylneuraminic acid (DANA)-which lacks potency. DANA derivatives with higher MuV sialidase inhibitory potency are lacking. The MuV-HN-Neu5Ac binding site has a hydrophobic cavity adjacent to the C4 position of Neu5Ac. Exploiting this, here, we synthesized DANA derivatives with increasing hydrophobicity at its C4 position and created 3 novel sialidase inhibitors (Compounds 1, 2, and 3) with higher specificity for MuV-HN than DANA; they inhibited MuV replication step to greater extent than DANA. Furthermore, they also inhibited hemagglutination and the MuV infection step. The insight-that these 3 novel DANA derivatives possess linear hydrocarbon groups at the C4-hydroxyl group of DANA-could help develop highly potent sialidase inhibitors with high specificity for MuV sialidase, which may function as direct-acting MuV-specific antivirals.

Visualizing intracellular sialidase activity of influenza A virus neuraminidase using a fluorescence imaging probe

In influenza A virus-infected cells, newly synthesized viral neuraminidases (NAs) transiently localize at the host cell Golgi due to glycosylation, before their expression on the cell surface. It remains unproven whether Golgi-localized intracellular NAs exhibit sialidase activity. We have developed a sialidase imaging probe, [2-(benzothiazol-2-yl)-5-(non-1-yn-1-yl) phenyl]-α-D-N-acetylneuraminic acid (BTP9-Neu5Ac). This probe is designed to be cleaved by sialidase activity, resulting in the release of a hydrophobic fluorescent compound, 2-(benzothiazol-2-yl)-5-(non-1-yn-1-yl) phenol (BTP9). BTP9-Neu5Ac makes the location of sialidase activity visually detectable by the BTP9 fluorescence that results from the action of sialidase activity. In this study, we established a protocol to visualize the sialidase activity of intracellular NA at the Golgi of influenza A virus-infected cells using BTP9-Neu5Ac. Furthermore, we employed this fluorescence imaging protocol to elucidate the intracellular inhibition of laninamivir octanoate, an anti-influenza drug. At approximately 7 h after infection, newly synthesized viral NAs localized at the Golgi. Using our developed protocol, we successfully histochemically stained the sialidase activity of intracellular viral NAs localized at the Golgi. Importantly, we observed that laninamivir octanoate effectively inhibited the intracellular viral NA, in contrast to drugs like zanamivir or laninamivir. Our study establishes a visualization protocol for intracellular viral NA sialidase activity and visualizes the inhibitory effect of laninamivir octanoate on Golgi-localized intracellular viral NA in infected cells.

Evaluation of catalytic activity of human and animal origin viral neuraminidase: Current prospect

With the exception of plants, almost all living organisms synthesize neuraminidase/sialidase. It is a one among the crucial proteins that controls how virulent a microorganism is. An essential enzyme in orthomyxoviruses and paramyxoviruses that destroys receptors is neuraminidase. It plays a number of roles throughout the viral life cycle in addition to one that involves the release of progeny virus particles. This protein is an important target for therapeutic interventions and diagnostic assays. Neuraminidase inhibitors effectively prevent the spread of disease and viral infection. Sensitive, quick, and inexpensive high throughput assays are needed to screen for specific neuraminidase inhibitory chemicals. To characterize the neuraminidase catalytic activity, however, the traditional assays are still the most common in laboratories. This review gives a brief overview of these neuraminidase assays and recent, innovative developments, particularly those involving biosensors.

[Analysis and Control of Viral Infection Mechanisms by Glycobiology]

Glycans are present in all living organisms, including viruses, bacteria, and animals, and perform various biological functions. The author has been studying influenza viruses' glycan usage mechanisms, particularly the functional analysis of neuraminidase (NA), a viral sialidase. The authors recently focused on influenza virus NAs' high sialidase activity with the aim of using sialidase activity detection as a virus detection technology. Using the probe BTP3-Neu5Ac, allows fluorescent imaging of sialidase activity, we created a new technique for easy, rapid, and high sensitivity fluorescent imaging of virus-infected cells. The detection of viruses using BTP3-Neu5Ac does not require specific antibodies and can be performed by simply adding reagents. Furthermore, fluorescence imaging of sialidase as a virus detection technology has many advantages, including isolating viruses from fluorescently imaged infected cells. This detection technique is easy to use in basic research and hygiene testing, where viral culture is conducted and is expected to be widely used.

Enzymatic Substrates and Fluorescence Imaging of Influenza Virus Sialidase

Immunostaining with an antiviral antibody is usually performed to visualize virus-infected cells. In contrast, this study established an easy method for fluorescence (FL) imaging of cells infected with influenza A and B viruses and some paramyxoviruses without the need for cell fixation and an antiviral antibody. These viruses and the cells they have infected express the viral surface enzymes "neuraminidase" or "hemagglutinin-neuraminidase," which show sialidase activity. Sialidase activity is fluorescently visualized by using a sialidase fluorogenic probe developed in our previous study. The probe enables histochemical FL imaging of the virus-infected cells and applies to virus isolation and detection of an influenza virus resistant to sialidase inhibitors anti-influenza drugs.

Ultrasensitive chemiluminescent neuraminidase probe for rapid screening and identification of small-molecules with antiviral activity against influenza A virus in mammalian cells

Influenza A virus is the most virulent influenza subtype and is associated with large-scale global pandemics characterized by high levels of morbidity and mortality. Developing simple and sensitive molecular methods for detecting influenza viruses is critical. Neuraminidase, an exo-glycosidase displayed on the surface of influenza virions, is responsible for the release of the virions and their spread in the infected host. Here, we present a new phenoxy-dioxetane chemiluminescent probe (CLNA) that can directly detect neuraminidase activity. The probe exhibits an effective turn-on response upon reaction with neuraminidase and produces a strong emission signal at 515 nm with an extremely high signal-to-noise ratio. Comparison measurements of our new probe with previously reported analogous neuraminidase optical probes showed superior detection capability in terms of response time and sensitivity. Thus, as far as we know, our probe is the most sensitive neuraminidase probe known to date. The chemiluminescence turn-on response produced by our neuraminidase probe enables rapid screening for small molecules that inhibit viral replication through different mechanisms as validated directly in influenza A-infected mammalian cells using the known inhibitors oseltamivir and amantadine. We expect that our new chemiluminescent neuraminidase probe will prove useful for various applications requiring neuraminidase detection including drug discovery assays against various influenza virus strains in mammalian cells.

A new chemiluminescence neuraminidase probe enables rapid screening of small molecules that inhibit viral replication, directly in influenza A-infected mammalian cells.

The Function of Sialidase Revealed by Sialidase Activity Imaging Probe

Sialidase cleaves sialic acid residues from glycans such as glycoproteins and glycolipids. In the brain, desorption of the sialic acid by sialidase is essential for synaptic plasticity, learning and memory and synaptic transmission. BTP3-Neu5Ac has been developed for sensitive imaging of sialidase enzyme activity in mammalian tissues. Sialidase activity in the rat hippocampus detected with BTP3-Neu5Ac increases rapidly by neuronal depolarization. It is presumed that an increased sialidase activity in conjunction with neural excitation is involved in the formation of the neural circuit for memory. Since sialidase inhibits the exocytosis of the excitatory neurotransmitter glutamate, the increased sialidase activity by neural excitation might play a role in the negative feedback mechanism against the glutamate release. Mammalian tissues other than the brain have also been stained with BTP3-Neu5Ac. On the basis of information on the sialidase activity imaging in the pancreas, it was found that sialidase inhibitor can be used as an anti-diabetic drug that can avoid hypoglycemia, a serious side effect of insulin secretagogues. In this review, we discuss the role of sialidase in the brain as well as in the pancreas and skin, as revealed by using a sialidase activity imaging probe. We also present the detection of influenza virus with BTP3-Neu5Ac and modification of BTP3-Neu5Ac.

Sialidase Activity in Human Blood Serum Has a Distinct Seasonal Pattern: A Pilot Study

Desialylation—loss of terminal sialic acid residues from glycoconjugates catalyzed by sialidases—is involved in many human diseases and is considered a key molecular event of atherosclerosis onset. Desialylated low-density lipoproteins with atherogenic properties have been detected in human blood previously. However, there is currently no consensus on the origin of desialylation activity in the bloodstream. Here, we suggest viral intervention as a possible explanation. In order to address our hypothesis, we studied seasonal patterns of blood serum sialidase enzymatic activity and designed an approach to detect and quantify viral sialidase genetic presence. Increased sialidase activity in autumn-winter combined with detectable levels of influenza virus sialidase mRNA suggests exogenous viral sialidase as a viable component of desialylation in human blood, providing new insights on the molecular background of atherogenesis.

Fluorescent Materials With Aggregation-Induced Emission Characteristics for Array-Based Sensing Assay

Array-based sensing is a powerful tool for identifying analytes in complex environments with unknown interferences. In array-based sensing, the sensors, which transduce binding details to signal outputs, are of crucial importance for identifying analytes. Aggregation-induced emission luminogens (AIEgens) enjoy the advantages of easy synthesis and high sensitivity, which enable them to facilely form a sensor pool through structural modifications and sensitively reflect the subtle changes associated with binding events. All these features make AIEgens excellent candidates for array-based sensing, and attempts have been made by several research groups to explore their potentials in array-based sensing. In this review, we introduce the recent progresses of employing AIEgens as sensors in sensing assays and in building up sensor arrays for identification of varied biological analytes, including biomolecules and bacteria. Examples are selected to illustrate the working mechanism, probe design and selection, capability of the sensor array, and implications of these sensing methods.

Fluorogenic Probes for Accurate in Situ Imaging of Viral and Mammalian Sialidases

Sialidases are widely distributed in nature and are involved in many physiological and pathological processes. Sialidases are expressed and work in various tissues and organelles. Clarification of the localization of sialidases is very helpful as a way to understand their functions. We previously developed a novel fluorogenic probe for sialidases, BTP3-Neu5Ac, that visualized the localization of sialidase activity in live cells and tissues by precipitating the hydrophobic fluorescent compound; however, for the purpose of accurate fluorescence imaging of sialidase-expressing cells or the distribution of intracellular sialidase activity, BTP3-Neu5Ac was inadequate in imaging performance. We report the design and development of a sialidase imaging probe that improves the sensitivity and accuracy of in situ fluorescence imaging performance as well as increases the hydrophobicity by attaching linear unsaturated hydrocarbon chains into the hydrophobic fluorescent compound of BTP3-Neu5Ac. The newly developed probe showed low diffusivity and high brightness for fluorescence imaging, and it enabled sensitive and highly accurate imaging of viral sialidase in virus-infected cells and sialidase-expressing cells as well as mammalian sialidase in the rat brain. The probe also enabled the fluorescence imaging of intracellular viral sialidase in live-virus-infected cells. The newly developed probe is expected to be a useful tool that will contribute to the progress of research on sialidases in various fields such as research on viruses and brains.

A dual-functional molecular strategy for in situ suppressing and visualizing of neuraminidase in aqueous solution using iridium(iii) complexes

We have designed for the first time a dual-functional luminescent probe and inhibitor of neuraminidase (NA), a key influenza target. The lead iridium(iii) complex exhibited enhanced inhibition against NA compared to the FDA-approved antiviral drug, oseltamivir, and could detect NA even in the presence of an autofluorescent background.

Proximity Ligation-Based Fluorogenic Imaging Agents for Neuraminidases

Reagents to visualize and localize neuraminidase activity would be valuable probes to study the role of neuraminidases in normal cellular processes as well as during viral infections or cancer development. Herein, a new class of neuraminidase-imaging probes that function as proximity ligation reagents by releasing a highly reactive fluorophore that tags nearby cellular material is described. It is further demonstrated that it is possible to create an influenza virus-specific reagent, which can specifically detect influenza virus infections in mammalian cells. These reagents have potential use as specific histological probes independent of viral antigenicity and, therefore, offer some advantages over commonly used anti-neuraminidase antibodies.

Imaging of Sialidase Activity and Its Clinical Application

Sialidase releases sialic acid residues from the ends of sugar chains. The sialidases are involved in many physiological processes including cell differentiation and proliferation and immune function as well as pathophysiological conditions such as various human cancers and infections. Therefore visualization of sialidase activities with high sensitivity could provide valuable insights into these isozyme's activity. We developed novel fluorescent sialidase substrates, 2-benzothiazol-2-yl-phenol derivatives-based N-acetylneuraminic acid (Neu5Ac) (BTP-Neu5Ac) substrates, for highly sensitive and specific visualization of sialidase activity in living mammalian tissues and virus-infected cells. We found that BTP-Neu5Ac can visualize sialidase activities sensitively and selectively in rat tissues including brain slices. BTP-Neu5Ac can also clearly detect cancer cells implanted orthotopically in mouse colons and human colon cancers. In this review, I describe imaging of sialidase activity with BTP-Neu5Ac in animal tissues, detection of colon cancer, memory formation, detection of virus-infected cells, and application to drug-resistant influenza virus detection and separation.

An easy, rapid, and sensitive method for detection of drug-resistant influenza virus by using a sialidase fluorescent imaging probe, BTP3-Neu5Ac

Immunochromatographic kits and RT-PCR are widely used as diagnostic tools for influenza detection in clinical and hygiene fields. Immunochromatographic kits are useful for differential typing of influenza A and influenza B but cannot show if the detected virus strains have acquired drug resistance against neuraminidase inhibitors that target sialidase activity of viral neuraminidase. Although RT-PCR enables determination of drug-resistant mutants, its efficacy is limited to viruses carrying a known substitution in their neuraminidase genome sequence. In the present study, an easy, rapid and sensitive method for detection of drug-resistant influenza viruses regardless of major antigenic changes or genomic mutations was developed. By using the method in combination with virus-concentrated membranes in centrifugal filter units and a sialidase imaging probe, 2-(benzothiazol-2-yl)-4-bromophenyl-N-acetylneuraminic acid (BTP3-Neu5Ac), sialidase activity of influenza neuraminidase was visualized on membranes by the green fluorescence of produced hydrophobic BTP3 under UV irradiation with a handheld UV flashlight. Fluorescence images in the presence or absence of neuraminidase inhibitors clearly discriminated drug-resistant influenza viruses from drug-sensitive ones. The assay can be done within 15 min. The detection sensitivity was shown to be equal to or higher than the sensitivities of commercial immunochromatographic kits. The assay will be a powerful tool for screening and monitoring of emerging drug-resistant influenza viruses and would help clinicians decide effective antiviral treatment strategies when such mutants have become prevalent.

A light-up imaging protocol for neutral pH-enhanced fluorescence detection of lysosomal neuraminidase activity in living cells

A lysosome-accessing nanoprobe is designed for recognition of lysosomal neuraminidases (Lyso-Neus), which can cleave the 4-methylumbelliferone moieties of the substrate from the nanoprobe, and lead to the escape of the moieties from acidic lysosomes into the neutral cytosol assisted by cationic poly(ethyleneimine) to light up the pH-responsive fluorescence for visual detection and dynamic tracking of Lyso-Neu activity in living cells.

Senescence-associated sialidase revealed by an activatable fluorescence-on labeling probe

A fluorescence-quenched substrate of sialidase enables fluorescence-on live cell imaging of sialidases, revealing up-regulation of lysosome-associated sialidase in cell senescence.

Fluorescent Neuraminidase Assay Based on Supramolecular Dye Capture After Enzymatic Cleavage

A conceptually new type of enzymatic cleavage assay is reported that utilizes in situ supramolecular capture of the fluorescent product. A squaraine-derived substrate with large blocking groups at each end of its structure cannot be threaded by a tetralactam macrocycle until the blocking groups are removed by enzyme cleavage. A prototype design responds to viral neuraminidase, an indicator of influenza infection, and also measures susceptibility of the sample to neuraminidase inhibitor drugs. The substrate structure incorporates three key features: (a) a bis(4-amino-3-hydroxyphenyl)squaraine core with bright deep-red fluorescence and excellent photostability, (b) an N-methyl group at each end of the squaraine core that ensures fast macrocycle threading kinetics, and (c) sialic acid blocking groups that prevent macrocycle threading until they are removed by viral neuraminidase. The enzyme assay can be conducted in aqueous solution where dramatic colorimetric and fluorescence changes are easily observed by the naked eye. Alternatively, affinity capture beads coated with macrocycle can be used to immobilize the liberated squaraine and enable a range of heterogeneous analysis options. With further optimization, this new type of neuraminidase assay may be useful in a point of care clinic to rapidly diagnose influenza infection and also determine which of the approved antiviral inhibitor drugs is likely to be the most effective treatment for an individual patient. The assay design is generalizable and can be readily modified to monitor virtually any type of enzyme-catalyzed cleavage reaction.

[Multidimensional Analysis of Hippocampal Excitatory Neurotransmission and Development of Analytical Tools for Glycans]

Sialidase removes sialic acid residues from sialoglycoconjugates such as glycoproteins and glycolipids. Since sialic acid plays crucial roles in synaptic plasticity and memory in the hippocampus, the regulation of sialyl signaling by sialidase is also necessary for neural functions. However, since mammalian sialidase activity is remarkably weak, it has been difficult to detect sialidase activity in mammalian tissues. Determination of the distribution of sialidase activity in living mammalian tissues would provide much valuable information for understanding the roles of sialidase in physiological functions. Therefore, we synthesized a novel benzothiazolylphenol-based sialic acid derivative (BTP-Neu5Ac) as a fluorescent sialidase substrate. After cleavage of BTP-Neu5Ac, which is water soluble and shows little fluorescence, with sialidase, the water-insoluble fluorophore benzothiazolylphenol (BTP) released from BTP-Neu5Ac stains tissue and shows bright fluorescence. BTP-Neu5Ac can visualize sialidase activity in brain tissue with high levels of sensitivity and specificity. The sialidase expression level is markedly high in various human cancers such as colon, renal, prostate, and ovarian cancers. BTP-Neu5Ac can detect human colon cancers sensitively. Thus, BTP-Neu5Ac is useful not only for physiological research but also as a cancer probe. BTP-Neu5Ac is now being used in virology research. In this review, methods for histochemical imaging of sialidase activity and the role of sialidase in hippocampal memory are described based on the author's study of multidimensional analysis of hippocampal excitatory neurotransmission and development of analytical tools for glycans, which was awarded a prize by the Tokai branch of the Pharmaceutical Society of Japan.

High-Efficiency Capture of Drug Resistant-Influenza Virus by Live Imaging of Sialidase Activity

Influenza A and B viruses possess a neuraminidase protein that shows sialidase activity. Influenza virus-specific neuraminidase inhibitors (NAIs) are commonly used for clinical treatment of influenza. However, some influenza A and B viruses that are resistant to NAIs have emerged in nature. NAI-resistant viruses have been monitored in public hygiene surveys and the mechanism underlying the resistance has been studied. Here, we describe a new assay for selective detection and isolation of an NAI-resistant virus in a speedy and easy manner by live fluorescence imaging of viral sialidase activity, which we previously developed, in order to achieve high-efficiency capture of an NAI-resistant virus. An NAI-resistant virus maintains sialidase activity even at a concentration of NAI that leads to complete deactivation of the virus. Infected cells and focuses (infected cell populations) of an oseltamivir-resistant virus were selectively visualized by live fluorescence sialidase imaging in the presence of oseltamivir, resulting in high-efficiency isolation of the resistant viruses. The use of a combination of other NAIs (zanamivir, peramivir, and laninamivir) in the imaging showed that the oseltamivir-resistant virus isolated in 2008 was sensitive to zanamivir and laninamivir but resistant to peramivir. Fluorescence imaging in the presence of zanamivir also succeeded in selective live-cell visualization of cells that expressed zanamivir-resistant NA. Fluorescence imaging of NAI-resistant sialidase activity will be a powerful method for study of the NAI resistance mechanism, for public monitoring of NAI-resistant viruses, and for development of a new NAI that shows an effect on various NAI-resistant mutations.

Rapid Fluorescent Detection Assay for Human Parainfluenza Viruses

Human parainfluenza virus type 1 (hPIV1) does not form clear plaque by the conventional plaque formation assay because of slightly a cytopathic effects in many cell lines infected with hPIV1, thus making in virus titration, isolation and inhibitor evaluation difficult. We have succeeded in fluorescent histochemical visualization of sialidase activities of influenza A and B viruses, Newcastle disease virus and Sendai virus by using a novel fluorescent sialidase substrate, 2-(benzothiazol-2-yl)-4-bromophenyl 5-acetamido-3,5-dideoxy-α-D-glycero-D-galacto-2-nonulopyranosidonic acid (BTP3-Neu5Ac). In this study, we applied the BTP3-Neu5Ac assay for rapid detection of hPIV1 and hPIV type 3. The BTP3-Neu5Ac assay could histochemically visualize dot-blotted hPIVs on a membrane and hPIV-infected cells as local fluorescence under UV irradiation. We succeeded in distinct fluorescent visualization of hPIV1-infected cells in only 3 d using the BTP3-Neu5Ac assay. Due to there being no fixation, hPIV1 was isolated directly from fluorescent stained focus cells by the BTP3-Neu5Ac assay. Establishment of a sensitive, easy, and rapid fluorescent focus detection assay for hPIV, hPIV1 in particular will contribute greatly to progress in hPIV studies.

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.

Easy and Rapid Detection of Mumps Virus by Live Fluorescent Visualization of Virus-Infected Cells

Mumps viruses show diverse cytopathic effects (CPEs) of infected cells and viral plaque formation (no CPE or no plaque formation in some cases) depending on the viral strain, highlighting the difficulty in mumps laboratory studies. In our previous study, a new sialidase substrate, 2-(benzothiazol-2-yl)-4-bromophenyl 5-acetamido-3,5-dideoxy-α-D-glycero-D-galacto-2-nonulopyranosidonic acid (BTP3-Neu5Ac), was developed for visualization of sialidase activity. BTP3-Neu5Ac can easily and rapidly perform histochemical fluorescent visualization of influenza viruses and virus-infected cells without an antiviral antibody and cell fixation. In the present study, the potential utility of BTP3-Neu5Ac for rapid detection of mumps virus was demonstrated. BTP3-Neu5Ac could visualize dot-blotted mumps virus, virus-infected cells, and plaques (plaques should be called focuses due to staining of infected cells in this study), even if a CPE was not observed. Furthermore, virus cultivation was possible by direct pick-up from a fluorescent focus. In conventional methods, visible appearance of the CPE and focuses often requires more than 6 days after infection, but the new method with BTP3-Neu5Ac clearly visualized infected cells after 2 days and focuses after 4 days. The BTP3-Neu5Ac assay is a precise, easy, and rapid assay for confirmation and titration of mumps virus.

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.

Histochemical imaging of alkaline phosphatase using a novel fluorescent substrate

Histochemical visualization of phosphatase is exclusively required for Western immunoblotting and antigen-positive cell staining using an alkaline phosphatase (AP)-labeled secondary antibody. This detection has been performed by several reagents including 5-bromo-4-chloro-3-indolyl-phosphate (X-Phos), nitro blue tetrazolium (NBT), 3-(2'-spiroadamantane)-4-methoxy-4-(3″-phosphoryloxy)phenyl-1,2-dioxetane and 2-(5'-chloro-2'-phosphoryloxyphenyl)-6-chloro-4-[3H]-quinazolinone (ELF® 97 Phosphate). We previously reported that 2-(benzothiazol-2-yl)-4-bromophenol bonded with N-acetylneuraminic acid (BTP3-Neu5Ac), enabled fluorescent histochemical visualization of sialidase activity. 2-(Benzothiazol-2-yl)-4-bromophenol (BTP3), which is formed from BTP3-Neu5Ac by sialidase reaction, is a crystalline, insoluble and stable fluorogenic compound, deposited at the site of enzyme activity. We developed a BTP3 phosphate ester (BTP3-Phos) for the purpose of fluorescent histochemical visualization of phosphatase activity. BTP3-Phos emitted fluorescence in a manner dependent on the concentration of the AP-labeled antibody. BTP3-Phos also enabled fluorescent histochemical visualization of AP-blotted dots in a manner dependent on the concentration of the AP-labeled antibody. The detection sensitivity of BTP3-Phos was estimated to be greater than that of the conventional method using X-Phos and NBT. Influenza A virus-infected cells were fixed and reacted with anti-influenza A virus antibodies and incubated continuously with an AP-labeled secondary antibody. BTP3-Phos stained the infected cells with distinct green fluorescence. These results indicate that BTP3-Phos can enable fluorescent immunohistochemical staining analysis using an AP-labeled antibody. BTP3-Phos would be beneficial for histochemical staining of AP activity, and may be applicable for multi-color staining or a cell sorter.

[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.