Structural and functional characterization of cold-active sialidase isolated from Antarctic fungus Penicillium griseofulvum P29

The fungal strain, Penicillium griseofulvum P29, isolated from a soil sample taken from Terra Nova Bay, Antarctica, was found to be a good producer of sialidase (P29). The present study was focused on the purification and structural characterization of the enzyme. P29 enzyme was purified using a Q-Sepharose column and fast performance liquid chromatography separation on a Mono Q column. The determined molecular mass of the purified enzyme of 40 kDa by SDS-PAGE and 39924.40 Da by matrix desorption/ionization mass spectrometry (MALDI-TOF/MS) analysis correlated well with the calculated mass (39903.75 kDa) from the amino acid sequence of the enzyme. P29 sialidase shows a temperature optimum of 37 °C and low-temperature stability, confirming its cold-active nature. The enzyme is more active towards α(2 → 3) sialyl linkages than those containing α(2 → 6) linkages. Based on the determined amino acid sequence and 3D structural modeling, a 3D model of P29 sialidase was presented, and the properties of the enzyme were explained. The conformational stability of the enzyme was followed by fluorescence spectroscopy, and the new enzyme was found to be conformationally stable in the neutral pH range of pH 6 to pH 9. In addition, the enzyme was more stable in an alkaline environment than in an acidic environment. The purified cold-active enzyme is the only sialidase produced and characterized from Antarctic fungi to date.

Bacterial Enzyme Assay for Mucin Glycan Degradation

Bacterial sialidase and sulfoglycosidase may act on the acidic modifications of mucin O-glycans, producing sialic acid and 6-sulfated N-acetylglucosamine, respectively. Assays for these enzymes, using mucin as a substrate, are enabled by the detection and/or quantification of the free monosaccharides that are released by these enzymes. This chapter describes enzyme reactions with mucin, detection by thin-layer chromatography of sialic acid, and quantification of 6-sulfated N-acetylglucosamine by liquid chromatography-tandem mass spectrometry.

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.

Lysosomal sialidase NEU1, its intracellular properties, deficiency, and use as a therapeutic agent

Neuraminidase 1 (NEU1) is a lysosomal sialidase that cleaves terminal α-linked sialic acid residues from sialylglycans. NEU1 is biosynthesized in the rough endoplasmic reticulum (RER) lumen as an N-glycosylated protein to associate with its protective protein/cathepsin A (CTSA) and then form a lysosomal multienzyme complex (LMC) also containing β-galactosidase 1 (GLB1). Unlike other mammalian sialidases, including NEU2 to NEU4, NEU1 transport to lysosomes requires association of NEU1 with CTSA, binding of the CTSA carrying terminal mannose 6-phosphate (M6P)-type N-glycan with M6P receptor (M6PR), and intralysosomal NEU1 activation at acidic pH. In contrast, overexpression of the single NEU1 gene in mammalian cells causes intracellular NEU1 protein crystallization in the RER due to self-aggregation when intracellular CTSA is reduced to a relatively low level. Sialidosis (SiD) and galactosialidosis (GS) are autosomal recessive lysosomal storage diseases caused by the gene mutations of NEU1 and CTSA, respectively. These incurable diseases associate with the NEU1 deficiency, excessive accumulation of sialylglycans in neurovisceral organs, and systemic manifestations. We established a novel GS model mouse carrying homozygotic Ctsa IVS6 + 1 g/a mutation causing partial exon 6 skipping with simultaneous deficiency of Ctsa and Neu1. Symptoms developed in the GS mice like those in juvenile/adult GS patients, such as myoclonic seizures, suppressed behavior, gargoyle-like face, edema, proctoptosis due to Neu1 deficiency, and sialylglycan accumulation associated with neurovisceral inflammation. We developed a modified NEU1 (modNEU1), which does not form protein crystals but is transported to lysosomes by co-expressed CTSA. In vivo gene therapy for GS and SiD utilizing a single adeno-associated virus (AAV) carrying modNEU1 and CTSA genes under dual promoter control will be created.

β-Sitosterol could serve as a dual inhibitor of Trypanosoma congolense sialidase and phospholipase A2: in vitro kinetic analyses and molecular dynamic simulations

The involvement of Trypanosoma congolense sialidase alongside phospholipase A2 has been widely accepted as the major contributing factor to anemia during African animal trypanosomiasis. The enzymes aid the parasite in scavenging sialic acid and fatty acids necessary for survival in the infected host, but there are no specific drug candidates against the two enzymes. This study investigated the inhibitory effects of β-sitosterol on the partially purified T. congolense sialidase and phospholipase A2. Purification of the enzymes using DEAE cellulose column led to fractions with highest specific activities of 8016.41 and 39.26 µmol/min/mg for sialidase and phospholipase A2, respectively. Inhibition kinetics studies showed that β-sitosterol is non-competitive and an uncompetitive inhibitor of sialidase and phospholipase A2 with inhibition binding constants of 0.368 and 0.549 µM, respectively. Molecular docking of the compound revealed binding energies of - 8.0 and - 8.6 kcal/mol against the sialidase and phospholipase A2, respectively. Furthermore, 100 ns molecular dynamics simulation using GROMACS revealed stable interaction of β-sitosterol with both enzymes. Hydrogen bond interactions between the ligand and Glu284 and Leu102 residues of the sialidase and phospholipase A2, respectively, were found to be the major stabilizing forces. In conclusion, β-sitosterol could serve as a dual inhibitor of T. congolense sialidase and phospholipase A2; hence, the compound could be exploited further in the search for newer trypanocides.

Sialylation of cell surface glycoconjugates modulates cytosolic galectin-mediated responses upon organelle damage : Minireview

Sialylation is an important terminal modification of glycoconjugates that mediate diverse functions in physiology and disease. In this review we focus on how altered cell surface sialylation status is sensed by cytosolic galectins when the integrity of intracellular vesicles or organelles is compromised to expose luminal glycans to the cytosolic milieu, and how this impacts galectin-mediated cellular responses. In addition, we discuss the roles of mammalian sialidases on the cell surface, in the organelle lumen and cytosol, and raise the possibility that intracellular glycan processing may be critical in controlling various galectin-mediated responses when cells encounter stress.

Siglec receptors as new immune checkpoints in cancer

Cancer immunotherapy in the form of immune checkpoint inhibitors and cellular therapies has improved the treatment and prognosis of many patients. Nevertheless, most cancers are still resistant to currently approved cancer immunotherapies. New approaches and rational combinations are needed to overcome these resistances. There is emerging evidence that Siglec receptors could be regarded as new immune checkpoints and targets for cancer immunotherapy. In this review, we summarize the experimental evidence supporting Siglec receptors as new immune checkpoints in cancer and discuss their mechanisms of action, as well as current efforts to target Siglec receptors and their interactions with sialoglycan Siglec-ligands.

Ureaplasma parvum infection induces inflammatory changes in vaginal epithelial cells independent of sialidase

BACKGROUND

Ureaplasma, a genus of the order Mycoplasmatales and commonly grouped with Mycoplasma as genital mycoplasma is one of the most common microbes isolated from women with infection/inflammation-associated preterm labor (PTL). Mycoplasma spp. produce sialidase that cleaves sialic acid from glycans of vaginal mucous membranes and facilitates adherence and invasion of the epithelium by pathobionts, and dysregulated immune response. However, whether Ureaplasma species can induce the production of sialidase is yet to be demonstrated. We examined U. parvum-infected vaginal epithelial cells (VECs) for the production of sialidase and pro-inflammatory cytokines.

METHODS

Immortalized VECs were cultured in appropriate media and treated with U. parvum in a concentration of 1 × 10⁵ DNA copies/ml. After 24 h of treatment, cells and media were harvested. To confirm infection and cell uptake, immunocytochemistry for multi-banded antigen (MBA) was performed. Pro-inflammatory cytokine production and protein analysis for sialidase confirmed pro-labor pathways.

RESULTS

Infection of VECs was confirmed by the presence of intracellular MBA. Western blot analysis showed no significant increase in sialidase expression from U. parvum-treated VECs compared to uninfected cells. However, U. parvum infection induced 2-3-fold increased production of GM-CSF (p = 0.03), IL-6 (p = 0.01), and IL-8 (p = 0.01) in VECs compared to controls.

CONCLUSION

U. parvum infection of VECs induced inflammatory imbalance associated with vaginal dysbiosis but did not alter sialidase expression at the cellular level. These data suggest that U. parvum's pathogenic effect could be propagated by locally produced pro-inflammatory cytokines and, unlike other genital mycoplasmas, may be independent of sialidase.

The genome and phylogenetic analyses of tit siadenoviruses reveal both a novel avian host and viral species

In both a Eurasian blue tit (Cyanistes caeruleus) and a great tit (Parus major), found dead in North Rhine-Westphalia, Germany, intranuclear inclusion bodies were observed in the kidneys during the histologic examination. Siadenoviruses were detected in both samples, and the nucleotide sequence of the partial DNA polymerase, obtained from the blue tit, was almost identical with that of great tit adenovirus type 1, reported from Hungary previously. The sequence, derived from the German great tit sample was more similar to great tit adenovirus 2, yet divergent enough to forecast the possible establishment of a novel viral type and species. Therefore, the complete genome was subjected to next generation sequencing. The annotation revealed a typical siadenoviral genome layout, and phylogenetic analyses proved the distinctness of the novel virus type: great tit adenovirus 3. We propose the establishment of a new species (Siadenovirus carbocapituli) within the genus Siadenovirus to contain great tit adenovirus types 2 and 3. As both of the newly-detected viruses originated from histologically confirmed cases, and several siadenoviruses have been associated with avian nephritis earlier, we assume that the renal pathology might have been also of adenoviral origin. Additionally, we performed structural studies on two virus-coded proteins. The viral sialidase and the fiber knob were modeled using the AlphaFold2 program. According to the results of the sialic acid docking studies, the fiber trimer of the new great tit adenovirus 3 binds this acid with good affinity. As sialic acid is expressed in the kidney, it can be hypothesized that it is used during the receptor binding and entry of the virus. Strong binding of sialic acid was also predictable for the viral sialidase albeit its enzymatic activity remains disputable since, within its catalytic site, an asparagine residue was revealed instead of the conserved aspartic acid.

The sialidase NEU3 promotes pulmonary fibrosis in mice

BACKGROUND

Sialic acid is often the distal sugar on glycoconjugates, and sialidases are enzymes that remove this sugar. In fibrotic lesions in human and mouse lungs, there is extensive desialylation of glycoconjugates, and upregulation of sialidases including the extracellular sialidase NEU3. In the bleomycin model of pulmonary fibrosis, mice lacking NEU3 (Neu3^-/-) showed strongly attenuated bleomycin-induced weight loss, lung damage, inflammation, and fibrosis. This indicates that NEU3 is necessary for the full spectrum of bleomycin-induced pulmonary fibrosis.

METHODS

To determine if NEU3 is sufficient to induce pulmonary fibrosis, recombinant murine NEU3 and a mutated inactive recombinant murine NEU3 protein were produced. Mice were given recombinant NEU3 proteins by oropharyngeal aspiration, either alone or 10 days after bleomycin challenge. Over the course of 21 days, mice were assessed for weight change, and after euthanasia, bronchoalveolar lavage fluid cells and lung tissue were assessed for inflammation and fibrosis.

RESULTS

Aspiration of recombinant murine NEU3 caused inflammation and fibrosis in the lungs, while inactive NEU3 caused inflammation but not fibrosis. Mice were also treated with recombinant murine NEU3 starting 10 days after bleomycin. In male but not female mice, recombinant murine NEU3 increased inflammation and fibrosis. Inactive NEU3 did not enhance bleomycin-induced lung fibrosis.

CONCLUSION

These results suggest that NEU3 is sufficient to induce fibrosis in the lungs, that aspiration of NEU3 has a greater effect on male mice, and that this effect is mediated by NEU3's enzymic activity.

Neu1 deficiency increases the susceptibility of zebrafish to Edwardsiella piscicida infection via lysosomal dysfunction

Neu1 is a lysosomal glycosidase that catalyzes the removal of sialic acids from glycoconjugates. Although Neu1 sialidase is highly conserved among vertebrates, the role of fish Neu1 is not fully understood because of its unique aquatic living situation. Compared to land animals, fish have a higher chance of bacterial infection, and to understand the role of fish Neu1, the susceptibility of Neu1 knockout zebrafish (Neu1-KO) was evaluated using Edwardsiella piscicida, a fish pathogen. Neu1-KO larvae showed high susceptibility to E. piscicida, despite the activation of macrophages, and presented increased lysosomal signals induced by the accumulation of Sia α2-3 linked oligosaccharides. The accumulation coincided with the signal of the macrophage marker, suggesting that the dysfunction of lysosomes in macrophages would result in a high susceptibility of Neu1-KO to E. piscicida. Chloroquine, an inhibitor of lysosomal degradation, induced high mortality of wild type zebrafish with E. piscicida infection accompanied by increased lysosomal accumulation, similar to Neu1-KO zebrafish. This study revealed that Neu1 sialidase plays a crucial role in the lysosomal degradation of macrophages with a bacterial infection.

Review sialidase NEU3 and its pathological significance

Sialidases (EC 3.2.1.18, also called neuraminidases) catalyze the removal of α-glycosidically linked sialic acid residues from glycoproteins and glycolipids; this is the initial step in the degradation of these glycoconjugates. Sialidases of mammalian origin have been implicated in not only lysosomal catabolism but also the modulation of functional molecules involved in many biological processes. To date, four types of mammalian sialidases have been cloned and designated as Neu1, Neu2, Neu3 and Neu4. These sialidases differ in their subcellular localization and enzymatic properties, as well as their chromosomal localization, and they are expressed in a tissue-specific manner. Among the sialidases, the plasma membrane-associated sialidase Neu3 appears to play particular roles in controlling transmembrane signaling through the modulation of gangliosides, and its aberrant expression is closely related to various pathogeneses, including that of cancer. Interestingly, the human orthologue NEU3 acts in two ways, catalytic hydrolysis of gangliosides and protein interactions with other signaling molecules. Aberrant NEU3 expression can induce various pathological conditions. This review briefly summarizes recent studies, focusing on the involvement of NEU3 in various pathological phenomena.

Involvement of N-acetylneuraminate cytidylyltransferase in Edwardsiella piscicida pathogenicity

Edwardsiella piscicida is a gram-negative bacterium that causes Edwardsiellosis in cultured fish. Edwardsiellosis is accompanied by symptoms such as skin lesions, hemorrhage, and necrosis in fish organs, which leads to significant economic losses in the aquaculture industry. Recently, we found that bacterial sialoglycoconjugates may be involved in the infectivity of E. piscicida. The more infectious strains of E. piscicida contain more sialic acid in the bacterial body, and the mRNA level of putative CMP-Neu5Ac synthase (css) is upregulated compared to that in the non-pathogenic strain. However, this putative css gene is yet to be cloned, and the involvement of CSS in E. piscicida pathogenicity remains unclear. Here, we cloned and transferred the css gene from E. piscicida into the FPC498 strain. CSS promoted infection in cultured cells originating from different fish species, and enhanced the mortality of E. piscicida-infected zebrafish larvae. CSS enhanced cell attachment and motility in E. piscicida, which differs from the decreased bacterial growth observed with the sialic acid-supplemented M9 medium. Both fractions (chloroform-methanol)-soluble and -insoluble fraction) prepared from E. piscicida pellet exhibited the increment of sialo-conjugates induced by CSS. Further, lectin blotting revealed the increment of Sia α2-3- and α2-6-, but not α2-8-, -linked glycoprotein in CSS-overexpressing E. piscicida. Overall, these findings indicate the physiological significance of CSS and the role of sialylation in E. piscicida pathogenicity.

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.

Characterization of Human Parainfluenza Virus Receptor Using Terminal Sialic Acid Linkage-Modified Cells

Human parainfluenza virus type 1 (hPIV1) and type 3 (hPIV3) are respiratory pathogen viruses that bind to terminal sialic acids of glycoconjugates on the cell surface hemagglutinin-neuraminidase glycoprotein. Sialic acid residues are linked to the galactose residue primarily by α2,3 or α2,6 linkages on the terminal of glycoprotein or glycolipids. One of the major determinants of pathogenicity or tissue tropism is virus binding or infection specificity for each sialyl linkage. Sialic linkage-modified human blood cells or mammalian cells that mainly have α2,3- or α2,6-linked sialic acid residues on the surface can be prepared by treatment with linkage-specific sialidases or sialyltransferases. These linkage-modified cells can be used in hemagglutination assays to estimate virus particles' binding specificity, hemadsorption assays to estimate virus glycoproteins' binding specificity, and virus infectivity assays. These methods contribute to identifying the specificity of sialic acid lineage recognition of the hPIV or other sialic acid-binding viruses.

Assays for Determining the Sialidase Activity of Influenza Viruses and Monitoring Influenza Virus Susceptibility to Neuraminidase Inhibitors

Three types of assays--colorimetric, fluorescent, and chemiluminescent--are used to determine the sialidase (neuraminidase: NA) activity of influenza viruses. The fluorescent assay is cost-effective and applicable for many laboratories and is, therefore, commonly used for global monitoring of the NA inhibitor susceptibility of influenza viruses. Here, I describe, in detail, protocols for the fluorescence-based NA activity assay and the NA inhibition assay, which are used to determine the NA activity and NA inhibitor susceptibility, respectively, of influenza viruses.

Sialidase neu4 deficiency is associated with neuroinflammation in mice

Sialidases catalyze the removal of sialic acid residues from glycoproteins, oligosaccharides, and sialylated glycolipids. Sialidase Neu4 is in the lysosome and has broad substrate specificity. Previously generated Neu4-/- mice were viable, fertile and lacked gross morphological abnormalities, but displayed a marked vacuolization and lysosomal storage in lung and spleen cells. In addition, we showed that there is an increased level of GD1a ganglioside and a markedly decreased level of GM1 ganglioside in the brain of Neu4-/- mice. In this study, we further explored whether sialidase Neu4 deficiency causes neuroinflammation. We demostrated that elevated level of GD1a and GT1b is associated with an increased level of LAMP1-positive lysosomal vesicles and Tunel-positive neurons correlated with alterations in the expression of cytokines and chemokines in adult Neu4-/- mice. Astrogliosis and microgliosis were also significantly enhanced in the hippocampus, and cerebellum. These changes in brain immunity were accompanied by motor impairment in these mice. Our results indicate that sialidase Neu4 is a novel mediator of an inflammatory response in the mouse brain due to the altered catabolism of gangliosides.

Neuraminidase 1 and its Inhibitors from Chinese Herbal Medicines: An Emerging Role for Cardiovascular Diseases

Neuraminidase, also known as sialidase, is ubiquitous in animals and microorganisms. It is predominantly distributed in the cell membrane, cytoplasmic vesicles, and lysosomes. Neuraminidase generally recognizes the sialic acid glycosidic bonds at the ends of glycoproteins or glycolipids and enzymatically removes sialic acid. There are four types of neuraminidases, named as Neu1, Neu2, Neu3, and Neu4. Among them, Neu1 is the most abundant in mammals. Recent studies have revealed the involvement of Neu1 in several diseases, including cardiovascular diseases, diabetes, cancers, and neurological disorders. In this review, we center the attention to the role of Neu1 in cardiovascular diseases, including atherosclerosis, ischemic myocardial injury, cerebrovascular disease, congenital heart disease, and pulmonary embolism. We also summarize inhibitors from Chinese herbal medicines (CHMs) in inhibiting virus neuraminidase or human Neu1. Many Chinese herbs and Chinese herb preparations, such as Lonicerae Japonicae Flos, Scutellariae Radix, Yupingfeng San, and Huanglian Jiedu Decoction, have neuraminidase inhibitory activity. We hope to highlight the emerging role of Neu1 in humans and potentially titillate interest for further studies in this area.

The involvement of Nile tilapia (Oreochromis niloticus) Neu4 sialidase in neural differentiation during early ontogenesis

Neurogenesis is an important process for the formation of the central nervous system during ontogenesis. Mammalian sialidases are involved in neurogenesis through desialylation of sialo-glycoconjugates. However, the significance of fish sialidases, unlike that of mammals, in neurogenesis has not been investigated. The present study focuses on Nile tilapia (Oreochromis niloticus) because of its unique profiles of sialidases related to enzymatic properties, subcellular localization, and tissue-specific gene expression. First, the fish were cultured under aphotic condition, which is known to cause the delayed development of the retina and brain in various fish. Next, we investigate the effect of aphotic condition on the levels of tilapia sialidases. Our results revealed that the tilapia showed a decrease in the number of ganglion cell in the retina. The expression level of neu4 mRNA is up-regulated in the eyes from tilapia reared in Dark accompanied by the increase of retinal differentiation markers. These results indicated that tilapia Neu4 is involved in retinal development in Nile tilapia. Furthermore, we tried to clarify the function of tilapia Neu4 in the neuronal cells using two neuroblast cell lines (SH-SY5Y and Neuro2a cell lines). Tilapia Neu4 decreased sialic acid level of both nuclear glycoproteins as well as glycolipids. Moreover, tilapia Neu4 accelerated neurite formation in both two neural cell lines and, increased the acetylcholinesterase activity, but it did not affect cell proliferation. Collectively, these results suggest that Neu4 accelerates neurite differentiation during ontogenesis in tilapia.

Structurally homologous sialidases exhibit a commonality in reactivity: Glycoside hydrolase-catalyzed hydrolysis of Kdn-thioglycosides

Aspergillus fumigatus is one of the main causative agents of invasive aspergillosis, an often-lethal fungal disease that affects immunocompromised individuals. A. fumigatus produces a sialidase that cleaves the nine-carbon carbohydrate Kdn from glycoconjugates. This enzyme plays a critical role in A. fumigatus pathogenicity, and is thus a target for the development of new therapeutics. In order to understand the reactivity of this Kdnase, and to develop a sensitive and selective assay for its catalytic activity we determined whether, like its close structural homolog the excreted sialidase produced by Micromonospora viridifaciens, this enzyme can efficiently hydrolyze thioglycoside substrates. We synthesized a panel of seven aryl 2-thio-d-glycero-α-d-galacto-non-2-ulopyranosonides and measured the activity of the A. fumigatus Kdnase towards these substrates. Four of these substrates were hydrolyzed by the A. fumigatus enzyme, although M. viridifaciens sialidase-catalyzed the hydrolysis of these Kdn thioglycosides with higher catalytic efficiencies (k_cat/K_m). We also tested an enzyme that was evolved from MvNA to improve its activity against Kdn glycosides (Glycobiology 2020, 30, 325). All three enzymes catalyzed the hydrolysis of the four most reactive Kdn thioglycosides and their second-order rate constants (k_cat/K_m) display a concave downwards Brønsted plot. The kinetic data, for each enzyme, is consistent with a change in rate-limiting step from CS bond cleavage for thioglycosides in which the pK_a of the corresponding aryl thiol is >3.6, to a non-chemical step, which is likely a conformational change, that occurs prior to CS bond cleavage for the 2,3,4,5,6-pentafluorothiophenyl glycoside.

The potential antiviral effect of major royal jelly protein2 and its isoform X1 against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2): Insight on their sialidase activity and molecular docking

•

Major royal jelly protein 2 and its isoform X1 are two predicted inhibitors of SARS-CoV-2.

•

Major royal jelly protein 2 and its isoform X1 have sialidase activity.

•

Major royal jelly protein 2 and its isoform X1 might inhibit SARS-CoV-2 entry and replication.

•

Major royal jelly protein 2 and its isoform X1 might inhibit SARS-CoV-2-induced hypoxia.

Severe acute respiratory syndrome-coronavirus (SARS-CoV)-2 is a newly emerging type of CoV. We evaluated the predicted anti-SARS-CoV-2 effect of major royal jelly protein (MRJP)2 and MRJP2 isoform X1, which recently showed high efficacy against other enveloped RNA-viruses (HCV and HIV). Some in-silico analyses have been performed to predict the impact of these proteins on viral entry, replication, and complications. These proteins have shown a high potency in sialic acid hydrolysis from the lung cells (WI-38) surface. Docking analysis showed that these proteins have a high binding affinity to viral receptor-binding sites in the receptor-binding domain, causing attachment prevention. Moreover, MRJPs can exert an inhibitory influence, via different mechanisms, for SARS-CoV-2 non-structural proteins (main and papain proteases, RNA replicase, RNA-dependent RNA polymerase, and methyltransferase). Also, they can bind to hemoglobin-binding sites on viral-nsps and prevent their hemoglobin attack. Thus, MRJP2 and MRJP2 X1 can be a promising therapy for SARS-CoV-2 infection.

Ex vivo assays to detect complement activation in complementopathies

In complement-driven thrombotic microangiopathies, failure to regulate complement activation leads to end-organ damage. The modified Ham (mHam) test measures complement-mediated killing of a nucleated cell in vitro but lacks a confirmatory assay and reliable positive controls. We demonstrate that C5b-9 accumulation on the surface of TF1 PIGAnull cells correlates with cell killing in the mHam. We also show that Sialidase treatment of cells or addition of Shiga toxin 1 to human serum serve as a more reliable positive control for the mHam than cobra venom factor or lipopolysaccharide. Simultaneously performing the mHam and measuring C5b-9 accumulation either in GVB⁺⁺ or GVB⁰ MgEGTA buffer with the addition of complement pathway specific inhibitors (anti-C5 antibody or a factor D inhibitor, ACH-145951) can be used to localize defects in complement regulation. As more targeted complement inhibitors become available, these assays may aid in the selection of personalized treatments for patients with complement-mediated diseases.

Identification of novel fish sialidase genes responsible for KDN-cleaving activity

2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) is a minor component of sialic acids detected in vertebrates, such as human cancer cells, rat liver, and fish tissues. Although the enzyme activity of KDN-cleaving sialidase (KDN-sialidase) has been detected in rainbow trout, the gene responsible for its expression has not been identified in vertebrates. We evaluated sialidases in human and various fish for their KDN-cleaving activity using an artificial substrate, methylumbelliferyl-KDN (MU-KDN). Four of the human sialidases tested (NEU1, NEU2, NEU3, and NEU4) did not hydrolyze MU-KDN. Although most fish Neu1s showed negligible KDN-sialidase activity, two Neu1b sialidases from Oreochromis niloticus and Astyanax mexicanus, a paralog of Neu1, exhibited a potent KDN-sialidase activity. Further, O. niloticus and Oryzias latipes Neu3a exhibited a drastically high KDN-sialidase activity, while Danio rerio Neu3.1 showed moderate activities and other Neu3 proteins exhibited little activity. All the Neu4 sialidases tested in fish cleaved KDN and Neu5Ac from MU-KDN and MU-Neu5Ac, respectively, with equivalent potential. To our knowledge, this is the first report to identify KDN-sialidase genes in vertebrates and we believe that KDN-sialidase activity could be conserved among fish Neu4s.

Human neuraminidases have reduced activity towards modified sialic acids on glycoproteins

Multiple levels of diversity in sialic acid presentation can influence the substrate activity of sialosides for glycoside hydrolases. Few reports have investigated the specificity of human neuraminidase (hNEU) activity towards modified sialic acid residues that can occur on glycoproteins. Previously, we evaluated hNEU activity towards 9-O-acetylated sialic acid in glycolipid substrates and found that hNEU generally discriminated against 9-O-acetylated sialic acid over Neu5Ac. Here, we have investigated the substrate specificity of hNEU enzymes for a glycoprotein substrate (bovine submaxillary mucin) containing 9-O-acetylated and Neu5Gc residues. Using this model substrate, we observe a general trend for hNEU tolerance of Neu5Ac > Neu5Gc ≫ Neu5,9Ac₂, consistent with our previous results with glycolipid substrates. These results expand our understanding of hNEU enzyme specificity and suggest that naturally occurring modifications of sialic acids can play a role in regulating hNEU activity.

The therapeutic potential of phytol towards Trypanosoma congolense infection and the inhibitory effects against trypanosomal sialidase

The search for novel therapeutic candidates against animal trypanosomiasis is an ongoing scientific endevour because of the negative impacts of the disease to the African livestock industry. In this study, the in vivo therapeutic potentials of phytol toward Trypanosoma congolense infection and the inhibitory effects on trypanosomal sialidase were investigated. Rats were infected with T. congolense and administered daily oral treatment of 50 and 100 mg/kg BW of phytol. Within the first 10 days of the treatment, no antitrypanosomal activity was recorded but a moderate trypanostatic activity was observed from day 17-day 21 pi. However, at 100 mg/kg BW, phytol demonstrated a significant (p < 0.05) ameliorative potentials toward T. congolense-induced host-associated pathological damages such as anaemia, hepatic and renal damages; and the data was comparable to diminazine aceturate. Moreover, the T. congolense caused a significant (p < 0.05) increase in free serum sialic acid level which was significantly (p < 0.05) prevented in the presence of phytol (100 mg/kg BW). In an in vitro analysis, phytol inhibited partially purified T. congolense sialidase using an uncompetitive inhibition pattern with inhibition binding constant of 261.24 μmol/mL. Subsequently, molecular docking revealed that the compound binds to homology modelled trypanosomal sialidase with a binding free energy of -6.7 kcal/mol which was mediated via a single hydrogen bond while Trp324 and Pro274 were the critical binding residues. We concluded that phytol has moderate trypanostatic activity but with a great potential in mitigating the host-associated cellular damages while the anaemia amelioration was mediated, in part, through the inhibition of sialidase.

Production and characterization of a monoclonal antibody against the sialidase of Gardnerella vaginalis using a synthetic peptide in a MAP8 format

Abstract

Bacterial vaginosis is one of the most frequent vaginal infections. Its main etiological agent is Gardnerella vaginalis, which produces several virulence factors involved in vaginal infection and colonization, in particular, sialidase (SLD), a potential clinical biomarker that participates in immune response modulation and mucus degradation. The main objective of this work was the production and evaluation of a monoclonal antibody against G. vaginalis sialidase and its validation in immunoassays. For immunization of mice, a synthetic multiantigenic peptide was used, and hybridomas were generated. After fusion, hybridomas were evaluated for antibody production and cloned by limited dilution. One clone producing IgG1 was selected and characterized by indirect ELISA, dot blot, and Western blot, and we also tested clinical isolates and HeLa cells infected with G. vaginalis. The results showed that the anti-SLD antibody recognized a single protein of ~90 kDa that correlated with the estimated molecular weight of SLD. In addition, anti-SLD antibody recognized SLD from complete bacteria and from culture supernatants of infected Hela cells. In conclusion, our results showed that the anti-SLD antibody recognized SLD from different sources and could be considered a new tool for the diagnosis of bacterial vaginosis.

Key Points

• Anti-sialidase mAb was generated using a synthetic peptide

• The mAb recognizes synthetic peptide and intact protein from multiple sources

• The antibody was characterized by several immunological methods

Sialidase NEU1 suppresses progression of human bladder cancer cells by inhibiting fibronectin-integrin α5β1 interaction and Akt signaling pathway

Background

Sialic acids are widely distributed in animal tissues, and aberrantly expressed in a variety of cancer types. High expression of sialic acid contributes to tumor aggressiveness by promoting cell proliferation, migration, angiogenesis, and metastasis. Sialidases are responsible for removal of sialic acids from glycoproteins and glycolipids.

Methods

N-glycomics of bladder cancer cells were detected by MALDI-TOF mass spectrometry. Sialic acid modification in bladder cancer tissue was determined by lectin blot. The down-regulation of NEU1 in bladder cancer cells was determined by high resolution liquid chromatography mass spectrometry (HR LC-MS). The effects of sialidase NEU1 expression on proliferation and apoptosis of human bladder cancer cells were examined by western blot, RT-PCR, confocal imaging and flow cytometry. Moreover, the function of sialic acids on fibronectin-integrin α5β1 interaction were assayed by immunoprecipitation and ELISA. The importance of NEU1 in tumor formation in vivo was performed using BALB/c-nu mice. Expression of NEU1 in primary human bladder cancer tissue samples was estimated using bladder cancer tissue microarray.

Results

(1) Downregulation of NEU1 was primarily responsible for aberrant expression of sialic acids in bladder cancer cells. (2) Decreased NEU1 expression was correlated with bladder cancer progression. (3) NEU1 overexpression enhanced apoptosis and reduced proliferation of bladder cancer cells. (4) NEU1 disrupted FN-integrin α5β1 interaction and deactivated the Akt signaling pathway. (5) NEU1 significantly suppressed in vivo tumor formation in BALB/c-nu mice.

Conclusions

Our data showed that NEU1 inhibited cancer cell proliferation, induced apoptosis, and suppressed tumor formation both in vitro and in vivo, by disrupting interaction of FN and integrin β1 and inhibiting the Akt signaling pathway. Our observations indicate that NEU1 is an important modulator of the malignant properties of bladder cancer cells, and is a potential therapeutic target for prognosis and treatment of bladder cancer.

Identification and characterization of a novel, versatile sialidase from a Sphingobacterium that can hydrolyze the glycosides of any sialic acid species at neutral pH

Bacterial sialidases are widely used to remove sialic acid (Sia) residues from glycans. Most of them cleave the glycosides of N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) under acidic pHs; however, currently available bacterial sialidases had no activity to the glycosides of deaminoneuraminic acid (Kdn). In this study, we found a novel sialidase from Sphingobacterium sp. strain HMA12 that could cleave any of the glycosides of Neu5Ac, Neu5Gc, and Kdn. It also had a broad linkage specificity, i.e., α2,3-, α2,6-, α2,8-, and α2,9-linkages, and the optimal pH at neutral ranges, pH 6.5-7.0. These properties are particularly important when sialidases are applied for in vivo digestion of the cell surface sialosides under physiological conditions. Interestingly, 2,3-didehydro-2-deoxy-N-acetylneuraminic acid (Neu5Ac2en), which is a transition state analog-based inhibitor, competitively inhibited the enzyme-catalyzed reaction for Kdn as well as for Neu5Ac, suggesting that the active site is common to the Neu5Ac and Kdn residues. Taken together, this sialidase is versatile and useful for the in vivo research on sialo-glycoconjugates.

Characterization of Porphyromonas gingivalis sialidase and disruption of its role in host-pathogen interactions

Key to onset and progression of periodontitis is a complex relationship between oral bacteria and the host. The organisms most associated with severe periodontitis are the periodontal pathogens of the red complex: Tannerella forsythia, Treponema denticola and Porphyromonas gingivalis. These organisms express sialidases, which cleave sialic acid from host glycoproteins, and contribute to disease through various mechanisms. Here, we expressed and purified recombinant P. gingivalis sialidase SiaPG (PG_0352) and characterized its activity on a number of substrates, including host sialoglycoproteins and highlighting the inability to cleave diacetylated sialic acids - a phenomenon overcome by the NanS sialate-esterase from T. forsythia. Indeed SiaPG required NanS to maximize sialic acid harvesting from heavily O-acetylated substrates such as bovine salivary mucin, hinting at the possibility of interspecies cooperation in sialic acid release from host sources by these members of the oral microbiota. Activity of SiaPG and P. gingivalis was inhibited using the commercially available chemotherapeutic zanamivir, indicating its potential as a virulence inhibitor, which also inhibited sialic acid release from mucin, and was capable of inhibiting biofilm formation of P. gingivalis on oral glycoprotein sources. Zanamivir also inhibited attachment and invasion of oral epithelial cells by P. gingivalis and other periodontal pathogens, both in monospecies but also in multispecies infection experiments, indicating potential to suppress host-pathogen interactions of a mixed microbial community. This study broadens our understanding of the multifarious roles of bacterial sialidases in virulence, and indicates that their inhibition with chemotherapeutics could be a promising strategy for periodontitis therapy.

Investigation of substrate specificity of sialidases with membrane mimetic glycoconjugates

Sialidases or neuraminidases play important roles in various physiological and pathological processes by cleaving terminal sialic acids (Sias) (desialylation) from the glycans of both glycoproteins and glycolipids. To understand the biological significance of desialylation by sialidases, it is important to investigate enzyme specificity with native substrate in biological membrane of cells. Herein, we report a membrane-mimicking system with liposome ganglioside conjugates containing different lipids for evaluating substrate specificity of sialidase and the lipid effect on the enzyme activity. Briefly, liposomes of phosphatidylcholine (PC) and cholesterol with ganglioside (GM3 or GM1) along with different percentage of phosphatidylserine (PS) or phosphatidylethanolamine (PE) were prepared and characterized. Their desialylation profiles with Arthrobacter ureafaciens (bacterial) sialidase and H1N1 (influenza viral) sialidase were quantified by HPLC method. A diversity of substrate preference was found for both bacterial and viral sialidase to the liposome ganglioside conjugate platform. The apparent K_m and V_max were dependent on the type of lipid. These results indicate that the intrinsic characteristics of the membrane-like system affect the sialidase specificity and activity. This biomimetic substrate provides a better tool for unravelling the substrate specificity and the biological function of sialidases and for screening of functional sialidase inhibitors as well.

A novel sialidase-activatable fluorescence probe with improved stability for the sensitive detection of sialidase

Sialidases catalyse the hydrolysis of terminal sialic acid residues of various glycoconjugates and visualising sialidase activity is important for understanding its function in the biological and pathological context. Upon developing a novel fluorescence probe for sialidase with improved fluorescence characteristics based on our previously reported fluorophore, HMRef, an inherent instability of sialic acid conjugates was found to both reduce selectivity and sensitivity. We aimed at increasing the stability of the probes by incorporating a self-immolative spacer with a higher pK_a between the sialic acid residue and HMRef to develop HMRef-S-Neu5Ac, which shows superior stability allowing for the specific detection of sialidase.

Knockout of sialidase and pro-apoptotic genes in Chinese hamster ovary cells enables the production of recombinant human erythropoietin in fed-batch cultures

Sialic acid, a terminal monosaccharide present in N-glycans, plays an important role in determining both the in vivo half-life and the therapeutic efficacy of recombinant glycoproteins. Low sialylation levels of recombinant human erythropoietin (rhEPO) in recombinant Chinese hamster ovary (rCHO) cell cultures are considered a major obstacle to the production of rhEPO in fed-batch mode. This is mainly due to the accumulation of extracellular sialidases released from the cells. To overcome this hurdle, three sialidase genes (Neu1, 2, and 3) were initially knocked-out using the CRISPR/Cas9-mediated large deletion method in the rhEPO-producing rCHO cell line. Unlike wild type cells, sialidase knockout (KO) clones maintained the sialic acid content and proportion of tetra-sialylated rhEPO throughout fed-batch cultures without exhibiting a detrimental effect with respect to cell growth and rhEPO production. Additional KO of two pro-apoptotic genes, BAK and BAX, in sialidase KO clones (5X KO clones) further improved rhEPO production without any detrimental effect on sialylation. On day 10 in fed-batch cultures, the 5X KO clones had 1.4-times higher rhEPO concentration and 3.0-times higher sialic acid content than wild type cells. Furthermore, the proportion of tetra-sialylated rhEPO on day 10 in fed-batch cultures was 42.2-44.3% for 5X KO clones while it was only 2.2% for wild type cells. Taken together, KO of sialidase and pro-apoptotic genes in rCHO cells is a useful tool for producing heavily sialylated glycoproteins such as rhEPO in fed-batch mode.

Diagnostics and Therapy of Human Diseases - Focus on Sialidases

Sialic acid residues that make part of the cell surface repertoire of carbohydrate residues are implicated in various physiological processes and human pathologies. Sialidases, or neuraminidases, are the enzymes that are able to cleave and release the sialic acid residues, while trans-sialidases can transfer the residues from donor to acceptor molecules. They are important for processing the surface glycolipids and glycoproteins. Therapeutic potential of pharmacological sialidases inhibition is currently actively studied. Knowledge and expertise gained from genetic defects leading to human sialidase deficiency can be used for designing such drugs. In this review, we discuss the current progress in studying sialidases and their inhibitors and the relevance of these studies to developing novel therapeutic approaches. In vitro studies suggest that some sialidase inhibitors might be useful therapeutics for treating sialidosis, cancer, infections, immune diseases, atherosclerosis and other pathologies. Consequently, there is a field for further research and development. A thorough investigation of human sialidases is therefore crucial to human health.

Phenotypic diversity in an emerging mycoplasmal disease

The avian pathogen Mycoplasma gallisepticum (MG) is a known pathogen of poultry, and newly emerged pathogen of house finches wherein it is associated with lethal conjunctivitis. Factors present in MG that are known to mediate virulence include cytadherence, sialidase activity, peroxide production, and biofilm formation. We have quantitatively assessed these factors for MG isolates from house finches from a temporal and geographic distribution across the continental United States that show differing capacity for virulence in vivo. Statistically significant (P < 0.05) differences were observed across strains for sialidase activity, cytadherence, and hydrogen peroxide production. Sialidase activity increased over time in geographically static populations, but did not correlate with time overall. All strains were able to bind α-2,6-linked sialic acid. No strains were found to bind α-2,3-linked sialic acid. All strains produced biofilms in vitro; however, no significant differences were observed in the density of biofilms across strains. Quantitative variance in virulence-associated traits is consistent with within-host evolutionary adaptation in response to a change in ecological niche by a parasitic pathogen.

A glycal-based photoaffinity probe that enriches sialic acid binding proteins

To identify sialic acid binding proteins from complex proteomes, three photocrosslinking affinity-based probes were constructed using Neu5Ac (5 and 6) and Neu5Ac2en (7) scaffolds. Kinetic inhibition assays and Western blotting revealed the Neu5Ac2en-based 7 to be an effective probe for the labeling of a purified gut microbial sialidase (BDI_2946) and a purified human sialic acid binding protein (hCD33). Additionally, LC-MS/MS affinity-based protein profiling verified the ability of 7 to enrich a low-abundance sialic acid binding protein (complement factor H) from human serum thus validating the utility of this probe in a complex context.

Sialic acid metabolism as a potential therapeutic target of atherosclerosis

Sialic acid (Sia), the acylated derivative of the nine-carbon sugar neuraminic acid, is a terminal component of the oligosaccharide chains of many glycoproteins and glycolipids. In light of its important biological and pathological functions, the relationship between Sia and coronary artery disease (CAD) has been drawing great attentions recently. Large-scale epidemiological surveys have uncovered a positive correlation between plasma total Sia and CAD risk. Further research demonstrated that N-Acetyl-Neuraminic Acid, acting as a signaling molecule, triggered myocardial injury via activation of Rho/ROCK-JNK/ERK signaling pathway both in vitro and in vivo. Moreover, there were some evidences showing that the aberrant sialylation of low-density lipoprotein, low-density lipoprotein receptor and blood cells was involved in the pathological process of atherosclerosis. Significantly, the Sia regulates immune response by binding to sialic acid-binding immunoglobulin-like lectin (Siglecs). The Sia-Siglecs axis is involved in the immune inflammation of atherosclerosis. The generation of Sia and sialylation of glycoconjugate both depend on many enzymes, such as sialidase, sialyltransferase and trans-sialidase. Abnormal activation or level of these enzymes associated with atherosclerosis, and inhibitors of them might be new CAD treatments. In this review, we focus on summarizing current understanding of Sia metabolism and of its relevance to atherosclerosis.

A Glycomic Approach Towards Identification of Signature Molecules in CD34(+) Haematopoietic Stem Cells from Umbilical Cord Blood

Umbilical cord blood (UCB) is a powerful storehouse for normal CD34⁺ haematopoietic stem cells (HSCs), often used for allogeneic bone marrow (BM) transplantation in malignant and non-malignant diseases. The glycomic especially the sialoglycomic aspect of these HSCs has been unravelled in this study. Cell surface expression of the glycans with the related enzymatic activities has been compared with the BM of childhood acute lymphoblastic leukaemia, a common BM-associated malignancy. An enhanced cell surface expression of α2,3-linked sialic acid, P- and E-selectins, and intercellular adhesion molecule along with reduced expression of L-selectin distinguishes CD34⁺ HSCs of UCB from leukaemic samples. More importantly, high expression of O-acetylated sialoglycoproteins, a hallmark of lymphoblasts, is drastically reduced in the CD34⁺ HSCs of UCB and is substantiated by the low activity of sialylate-O-acetyltransferase and high sialidase activity. In contrast, a significant variation is evident in the expression of sialic acid, α2,6-linked sialic acids, and the sialyltransferase activity. Taken together, these studies indicate a few signature molecules, forming a unique glycomic template, which may be a potential indicator, reassuring the normal profile of these stem cells, to be used for future transplantation.

A substrate tagging and two-step enzymatic reaction strategy for large-scale synthesis of 2,7-anhydro-sialic acid

A sialyltransferase acceptor tagging and two-step enzymatic reaction strategy has been developed for multigram-scale chemoenzymatic synthesis of 2,7-anhydro-N-acetylneuraminic acid (2,7-anhydro-Neu5Ac), a compound that can serve as a sole carbon source for the growth of Ruminococcus gnavus, a common human gut commensal. Different approaches of introducing hydrophobic UV-active tags to lactose as well-suited sialyltransferase acceptors have been explored and a simple two-step high-yield chemical synthetic procedure has been identified. The UV-active hydrophobic tag facilitates monitoring reaction progress and allows facile product purification by C18-cartridges. A two-step enzyme-catalyzed reaction procedure has been established to combine with C18 cartridge-based purification process for high-yield production of the desired product in multigram scales with the recycled use of chromophore-tagged lactoside starting material and sialoside intermediate. This study demonstrated an environmentally friendly highly-efficient synthetic and purification strategy for the production of 2,7-anhydro-Neu5Ac to explore its potential functions.

Trypanostatic activity of geranylacetone: Mitigation of Trypanosoma congolense-associated pathological pertubations and insight into the mechanism of anaemia amelioration using in vitro and in silico models

Trypanosoma congolense is an important pathogen that wreaks havoc in the livestock industry of the African continent. This study evaluated the in vivo antitrypanosomal activity of geranylacetone and its ameliorative effect on the disease-induced anaemia and organ damages as well as its inhibitory effects against trypanosomal sialidase using in vitro and in silico techniques. Geranylacetone was used to treat T. congolense infected rats, at a dose of 50 and 100 mg/kg BW, for 14 days where it was found to reduce the parasite burden in the infected animals. Moreover, 100 mg/kg BW of geranylacetone significantly (p < 0.05) ameliorated the anaemia, hepatic and renal damages caused by the parasite. This is in addition to the alleviation of the parasite-induced hepatosplenomegaly and upsurge in free serum sialic acid levels in the infected animals which were associated with the observed anaemia amelioration by the compound. Consequently, bloodstream T. congolense sialidase was partially purified on DEAE cellulose column and inhibition kinetic studies revealed that the enzyme was inhibited by geranylacetone via an uncompetitive inhibition pattern. In silico analysis using molecular docking with Autodock Vina indicated that geranylacetone binds to trypanosomal sialidase with a minimum free binding energy of -5.8 kcal/mol which was mediated by 26 different kinds of non-covalent interactions excluding hydrogen bond whilst Asp163 and Phe421 had the highest number of the interactions. The data suggests that geranylacetone has trypanostatic activity and could protect animals against the T. congolense-induced anaemia through the inhibition of sialidase and/or the protection of the parasite-induced hepatosplenomegaly.

In silico analysis as a strategy to identify candidate epitopes with human IgG reactivity to study Porphyromonas gingivalis virulence factors

Porphyromonas gingivalis (Pg) is one of the main pathogens in chronic periodontitis (CP). Studies on the immunogenicity of its virulence factors may contribute to understanding the host response to infection. The present study aimed to use in silico analysis as a tool to identify epitopes from Lys-gingipain (Kgp) and neuraminidase virulence factors of the Pg ATCC 33277 strain. Protein sequences were obtained from the NCBI Protein Database and they were scanned for amino acid patterns indicative of MHC II binding using the MHC-II Binding Predictions tool from the Immune Epitope Database (IEDB). Peptides from different regions of the proteins were chemically synthesized and tested by the indirect ELISA method to verify IgG immunoreactivity in serum of subjects with CP and without periodontitis (WP). T cell epitope prediction resulted in 16 peptide sequences from Kgp and 18 peptide sequences from neuraminidase. All tested Kgp peptides exhibited IgG immunoreactivity whereas tested neuraminidase peptides presented low IgG immunoreactivity. Thus, the IgG reactivity to Kgp protein could be reaffirmed and the low IgG reactivity to Pg neuraminidase could be suggested. The novel peptide epitopes from Pg were useful to evaluate its immunoreactivity based on the IgG-mediated host response. In silico analysis was useful for preselecting epitopes for immune response studies in CP.

Desialylation in physiological and pathological processes: New target for diagnostic and therapeutic development

Desialylation is a pivotal part of sialic acid metabolism, which initiates the catabolism of glycans by removing the terminal sialic acid residues on glycans, thereby modulating the structure and functions of glycans, glycoproteins, or glycolipids. The functions of sialic acids have been well recognized, whereas the function of desialylation process is underappreciated or largely ignored. However, accumulating evidence demonstrates that desialylation plays an important role in a variety of physiological and pathological processes. This chapter summarizes the current knowledge pertaining to desialylation in a variety of physiological and pathological processes, with a focus on the underlying molecular mechanisms. The potential of targeting desialylation process for diagnostic and therapeutic development is also discussed.

Sialidase Attenuates Epidermal Growth Factor Response and Abolishes Antiproliferative Effects of Erlotinib in A549Alveolar Epithelial Cells

Erlotinib is a widely used, reversible tyrosine kinase inhibitor (TKI), targeting pro-proliferative signaling of epidermal growth factor receptor (EGFR). The drug is approved for the first-line treatment of patients with metastatic non-small cell lung cancer with EGFR mutations. Extracellular glycans can affect EGFR expression, dimerization, phosphorylation, and EGF binding. In this study we investigated the effects of EGF and erlotinib on the cell cycle of naive and sialidase (alpha-neuraminidase)-pretreated human A549 alveolar epithelial cells. A549 cells were labeled with propidium iodide, and fractions of cells in different phases of cycle were quantified by flow cytometry. We found that neither did desialilation nor EGF, as well as erlotinib treatment, increase the number of damaged cells (subG0/G1 cell fraction), while erlotinib did significantly increase the number of G0/G1 cells and decrease S + G2/M cell fractions. In naive cells, EGF increased proliferating cell numbers by more than 40%, and this effect was blocked by erlotinib. In desialylated cells, however, proliferation was significantly decreased by about 29%, and EGF and erlotinib did not exert significant effects. We conclude that changes in alveolar epithelial cell membrane glycosylation may affect function of growth-promoting receptors and erlotinib effectiveness.

cholerae sialidase

Sialidases or neuraminidases are enzymes that catalyze the cleavage of terminal sialic acids from oligosaccharides and glycoconjugates. They play important roles in bacterial and viral infection and have been attractive targets for drug development. Structure-based drug design has led to potent inhibitors against neuraminidases of influenza A viruses that have been used successfully as approved therapeutics. However, selective and effective inhibitors against bacterial and human sialidases are still being actively pursued. Guided by crystal structural analysis, several derivatives of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en or DANA) were designed and synthesized as triazole-linked transition state analogs. Inhibition studies revealed that glycopeptide analog E-(TriazoleNeu5Ac2en)-AKE and compound (TriazoleNeu5Ac2en)-A were selective inhibitors against Vibrio cholerae sialidase, while glycopeptide analog (TriazoleNeu5Ac2en)-AdE selectively inhibited Vibrio cholerae and A. ureafaciens sialidases.

Sialidase activity in human pathologies

Sialic acid residues are frequently located at the terminal positions of glycoconjugate chains of cellular glycocalyx. Sialidases, or neuraminidases, catalyse removal of these residues thereby modulating various normal and pathological cellular activities. Recent studies have revealed the involvement of sialidases in a wide range of human disorders, including neurodegenerative disorders, cancers, infectious diseases and cardiovascular diseases. The accumulating data make sialidases an interesting potential therapeutic target. Modulating the activity of these enzymes may have beneficial effects in several pathologies. Four types of mammalian sialidases have been described: NEU1, NEU2, NEU3 and NEU4. They are encoded by different genes and characterized by different subcellular localization. In this review, we will summarize the current knowledge on the roles of different sialidases in pathological conditions.

Bifunctional properties and characterization of a novel sialidase with esterase activity from Bifidobacterium bifidum

Sialidases catalyze the removal of terminal sialic acid from various complex carbohydrates. In the gastrointestinal tract, sialic acid is commonly found in the sugar chain of mucin, and many enteric commensals use mucin as a nutrient source. We previously identified two different sialidase genes in Bifidobacterium bifidum, and one was cloned and expressed as an extracellular protein designated as exo-α-sialidase SiaBb2. The other exo-α-sialidase gene (siabb1) from the same bifidobacterium encodes an extracellular protein (SiaBb1) consisting of 1795 amino acids with a molecular mass of 189 kDa. SiaBb1 possesses a catalytic domain that classifies this enzyme as a glycoside hydrolase family 33 member. SiaBb1 preferentially hydrolyzes α2,3-linked sialic acid over α2,6-linked sialic acid from sialoglycan, which is the same as SiaBb2. However, SiaBb1 has an SGNH hydrolase domain with sialate-O-acetylesterase activity and an N-terminal signal sequence and C-terminal transmembrane region. SiaBb1 is the first bifunctional sialidase identified with esterase activity. Abbreviations: GalNAc: N-acetyl-D-galactosamine; Fuc: L-fucose; Gal: D-galactose.

Sialidases: Therapeutic and Antiatherogenic Potential

This review focuses on the biological role and clinical relevance of relatively poor studied enzymes known as sialidases. We describe structure and function of sialic acid, in particular as a component of gangliosides and plasma lipoproteins. Several types of sialidases are known in mammals, of which trans-sialidase is of special interest, since it is capable of removing sialic acid from low density lipoprotein (LDL) particles and transferring it to different acceptors in blood plasma. Desialylation of LDL, in turn, endows it a capacity to accumulate in the smooth muscle cells of human aortic intima, and therefore is important for atherogenesis. Moreover, sialidases appear to be involved in a variety of pathological processes, including viral infections and cancer, which makes these enzymes an attractive therapeutic target.

Inhibition of sialidase activity and cellular invasion by the bacterial vaginosis pathogen Gardnerella vaginalis

Bacterial vaginosis is a genital tract infection, thought to be caused by transformation of a lactobacillus-rich flora to a dysbiotic microbiota enriched in mixed anaerobes. The most prominent of these is Gardnerella vaginalis (GV), an anaerobic pathogen that produces sialidase enzyme to cleave terminal sialic acid residues from human glycans. Notably, high sialidase activity is associated with preterm birth and low birthweight. We explored the potential of the sialidase inhibitor Zanamavir against GV whole cell sialidase activity using methyl–umbelliferyl neuraminic acid (MU-NANA) cleavage assays, with Zanamavir causing a 30% reduction in whole cell GV sialidase activity (p < 0.05). Furthermore, cellular invasion assays using HeLa cervical epithelial cells, infected with GV, demonstrated that Zanamivir elicited a 50% reduction in cell association and invasion (p < 0.05). Our data thus highlight that pharmacological sialidase inhibitors are able to modify BV-associated sialidase activity and influence host–pathogen interactions and may represent novel therapeutic adjuncts.

Characterization of a recombinant Bacteroides fragilis sialidase expressed in Escherichia coli

The human gut commensal Bacteroides fragilis produces sialidases that remove a terminal sialic acid from host-derived polysaccharides. Sialidase is considered to be involved in B. fragilis infection pathology. A native B. fragilis sialidase has been purified and characterized, and was shown to be post-translationally modified by glycosylation. However, the biochemical properties of recombinant B. fragilis sialidase expressed in a heterologous host remain uncharacterized. In this study, we examined the enzymatic properties of the 60-kDa sialidase NanH1 of B. fragilis YCH46, which was prepared as a recombinant protein (rNanH1) in Escherichia coli. In E. coli rNanH1 was expressed as inclusion bodies, which were separated from soluble proteins to allow solubilization of insoluble rNanH1 in a buffer containing 8 M urea and renaturation in refolding buffer containing 100 mM CaCl₂ and 50 mM L-arginine. The specific activity of renatured rNanH1 measured using 4-methylumberiferyl-α-D-N-acetyl neuraminic acid as a substrate was 6.16 μmol/min/mg. The optimal pH of rNanH1 ranged from 5.0 to 5.5. The specific activity of rNanH1 was enhanced in the presence of calcium ions. rNanH1 preferentially hydrolyzed the sialyl α2,8 linkage and cleaved sialic acids from mucin and serum proteins (e.g., fetuin and transferrin) but not from α1-acid glycoprotein, which is similar to the previously observed biochemical properties for a native sialidase purified from B. fragilis SBT3182. The results and methods described in this study will be useful for preparing and characterizing recombinant proteins for other B. fragilis sialidase isoenzymes.

Lactobacillus crispatus represses vaginolysin expression by BV associated Gardnerella vaginalis and reduces cell cytotoxicity

Using a chemically-defined medium simulating genital tract secretions, we have shown that pre-adhering Lactobacillus crispatus to Hela epithelial cells reduced cytotoxicity caused by Gardnerella vaginalis. This effect was associated to the expression of vaginolysin and was specific to L. crispatus interference, as other vaginal facultative anaerobes had no protective effect.

Synthesis and anti-influenza virus evaluation of triterpene-sialic acid conjugates

We are interested in new non-natural glycosides with sialic acid conjugates and their biological activities. We report the synthesis of eleven non-natural occurring glycosides, which are triterpene (glycyrrhetinic acid and its derivatives)-sialic acid conjugates, and their inhibitory activities against influenza virus sialidases and influenza virus multiplication in MDCK host cells. Deoxoglycyrrhetol-sialic acid conjugates (6d and 6e) and oleanolic acid-sialic acid conjugates (7d and 7e) showed strong inhibitory activities against three subtypes of influenza virus sialidases. These four compounds (6d, 6e, 7d and 7e) showed clear inhibition to influenza virus multiplication but not to MDCK host cell survival.