Evidence for mitochondrial localization of a novel human sialidase (NEU4)

A proline-rich loop mediates specific functions of human sialidase NEU4 in SK-N-BE neuronal differentiation

Human sialidase NEU4 long (N4L) is a membrane-associated enzyme that has been shown to be localized in the outer mitochondrial membrane. A role in different cellular processes has been suggested for this enzyme, such as apoptosis, neuronal differentiation and tumorigenesis. However, the molecular bases for these roles, not found in any of the other highly similar human sialidases, are not understood. We have found that a proline-rich sequence of 81 amino acids, unique to NEU4 sequence, contains potential Akt and Erk1 kinase motifs. Molecular modeling, based on the experimentally determined three-dimensional structure of cytosolic human NEU2, showed that the proline-rich sequence is accommodated in a loop, thus preserving the typical beta-barrel structure of sialidases. In order to investigate the role of this loop in neuronal differentiation, we obtained SK-N-BE neuroblastoma cells stably overexpressing either human wild-type N4L or a deletion mutant lacking the proline-rich loop. Our results demonstrate that the proline-rich region can also enhance cell proliferation and retinoic acid (RA)-induced neuronal differentiation and it is also involved in NEU4 interaction with Akt, as well as in substrate recognition, modifying directly or through the interaction with other protein(s) the enzyme specificity toward sialylated glycoprotein(s). On the whole, our results suggest that N4L could be a downstream component of the PI3K/Akt signaling pathway required for RA-induced differentiation of neuroblastoma SK-N-BE cells.

Positive regulation of insulin signaling by neuraminidase 1

Neuraminidases (sialidases) catalyze the removal of sialic acid residues from sialylated glycoconjugates. We now report that mammalian neuraminidase 1 (Neu1), in addition to its catabolic function in lysosomes, is transported to the cell surface where it is involved in the regulation of insulin signaling. Insulin binding to its receptor rapidly induces interaction of the receptor with Neu1, which hydrolyzes sialic acid residues in the glycan chains of the receptor and, consequently, induces its activation. Cells from sialidosis patients with a genetic deficiency of Neu1 show impairment of insulin-induced phosphorylation of downstream protein kinase AKT, and treatment of these cells with purified Neu1 restores signaling. Genetically modified mice with ∼10% of the normal Neu1 activity exposed to a high-fat diet develop hyperglycemia and insulin resistance twice as fast as their wild-type counterparts. Together, these studies identify Neu1 as a novel component of the signaling pathways of energy metabolism and glucose uptake.

Identification of selective inhibitors for human neuraminidase isoenzymes using C4,C7-modified 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA) analogues

In the past two decades, human neuraminidases (human sialidases, hNEUs) have been found to be involved in numerous pathways in biology. The development of selective and potent inhibitors of these enzymes will provide critical tools for glycobiology, help to avoid undesired side effects of antivirals, and may reveal new small-molecule therapeutic targets for human cancers. However, because of the high active site homology of the hNEU isoenzymes, little progress in the design and synthesis of selective inhibitors has been realized. Guided by our previous studies of human NEU3 inhibitors, we designed a series of C4,C7-modified analogues of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA) and tested them against the full panel of hNEU isoenzymes (NEU1, NEU2, NEU3, NEU4). We identified inhibitors with up to 38-fold selectivity for NEU3 and 12-fold selectivity for NEU2 over all other isoenzymes. We also identified compounds that targeted NEU2 and NEU3 with similar potency.

Erythrophagocytosis of desialylated red blood cells is responsible for anaemia during Trypanosoma vivax infection

Trypanosomal infection-induced anaemia is a devastating scourge for cattle in widespread regions. Although Trypanosoma vivax is considered as one of the most important parasites regarding economic impact in Africa and South America, very few in-depth studies have been conducted due to the difficulty of manipulating this parasite. Several hypotheses were proposed to explain trypanosome induced-anaemia but mechanisms have not yet been elucidated. Here, we characterized a multigenic family of trans-sialidases in T. vivax, some of which are released into the host serum during infection. These enzymes are able to trigger erythrophagocytosis by desialylating the major surface erythrocytes sialoglycoproteins, the glycophorins. Using an ex vivo assay to quantify erythrophagocytosis throughout infection, we showed that erythrocyte desialylation alone results in significant levels of anaemia during the acute phase of the disease. Characterization of virulence factors such as the trans-sialidases is vital to develop a control strategy against the disease or parasite.

Cell-permeable probe for identification and imaging of sialidases

Alkyne-hinged 3-fluorosialyl fluoride (DFSA) containing an alkyne group was shown to be a mechanism-based target-specific irreversible inhibitor of sialidases. The ester-protected analog DFSA (PDFSA) is a membrane-permeable precursor of DFSA designed to be used in living cells, and it was shown to form covalent adducts with virus, bacteria, and human sialidases. The fluorosialyl-enzyme adduct can be ligated with an azide-annexed biotin via click reaction and detected by the streptavidin-specific reporting signals. Liquid chromatography-mass spectrometry/mass spectrometry analysis on the tryptic peptide fragments indicates that the 3-fluorosialyl moiety modifies tyrosine residues of the sialidases. DFSA was used to demonstrate influenza infection and the diagnosis of the viral susceptibility to the anti-influenza drug oseltamivir acid, whereas PDFSA was used for in situ imaging of the changes of sialidase activity in live cells.

Human cytosolic sialidase NEU2-low general tissue expression but involvement in PC-3 prostate cancer cell survival

Human cytosolic sialidase (NEU2) has been identified and characterized using a NEU2 cDNA constructed from a genomic library of human skeletal muscle. However, the tissue distribution of NEU2 mRNA and the physiological functions of the enzyme remain unclear. In the present study, unlike other human sialidases, NEU2 expression as assessed by quantitative real-time PCR was found to be extremely low or undetectable in many human tissues and cells, with notable exceptions like the placenta and testis. The gene forms obtained by PCR with cDNAs synthesized from poly (A)(+) RNA of human brain and colon were verified to encode cytosolic sialidase with appropriate activity, regardless of the brain gene feature of SNPs. Among a series of human cancer cell lines examined, only prostate cancer PC-3 cells exhibited relatively high expression and NEU2-silencing with an siRNA resulted in decreased cell survival and motility. To gain insights into the significance of the high levels, transcription factors in the promoter region of the NEU2 gene were surveyed for involvement. PC-3 cells were characterized by high expression of Runx2 and Sp3, and their silencing reduced NEU2, suggesting regulatory roles.

The therapeutic potential of pharmacological chaperones and proteosomal inhibitors, Celastrol and MG132 in the treatment of sialidosis

Sialidosis is an autosomal recessive disorder caused by a dysfunctional Sialidase enzyme. Categorised into two phenotypes, Sialidosis type I and II, Sialidosis is a highly heterogeneous disorder with varying ages of onset and pathologies. Currently, there is no viable therapy for the treatment of Sialidosis patients. At the molecular level, cells from Sialidosis patients with compound heterozygous mutations show improper enzyme folding, loss of Sialidase enzyme activity and subsequent accumulation of sialylconjugates within lysosomes. One promising treatment option is the use of small pharmacological molecules to increase the enzymatic activities of mutant proteins. In this study, we examined the efficacy of the immuno-suppressant (Celastrol) as well as a proteosomal inhibitor (MG132) to rescue mutant enzymes with altered conformation. Our results reveal that MG132 enhances enzyme activity and its localisation in cells expressing defective Sialidase. We also found that MG132 reduces accumulation of ganglioside products, GT1b, GD3, and GM3 in pre-loaded Sialidosis cells. Alternatively, Celastrol appears to reduce Sialidase expression and activity revealing a potentially novel effect of Celastrol on Sialidase. Interestingly, the combination of Celastrol and MG132 appears to amplify the beneficial impact of MG132 on both the endogenous and recombinant expression of defective Sialidase. This study explores a novel biological criteria to assess the efficacy of small molecules through accumulation analysis and points to a potential therapeutic strategy for the treatment of Sialidosis.

Reduced susceptibility to colitis-associated colon carcinogenesis in mice lacking plasma membrane-associated sialidase

Sialic acids are acidic monosaccharides that bind to the sugar chains of glycoconjugates and change their conformation, intermolecular interactions, and/or half-life. Thus, sialidases are believed to modulate the function of sialoglycoconjugates by desialylation. We previously reported that the membrane-associated mammalian sialidase NEU3, which preferentially acts on gangliosides, is involved in cell differentiation, motility, and tumorigenesis. The NEU3 gene expression is aberrantly elevated in several human cancers, including colon, renal, prostate, and ovarian cancers. The small interfering RNA-mediated knock-down of NEU3 in cancer cell lines, but not in normal cell-derived primary cultures, downregulates EGFR signaling and induces apoptosis. Here, to investigate the physiological role of NEU3 in tumorigenesis, we established Neu3-deficient mice and then subjected them to carcinogen-induced tumorigenesis, using a sporadic and a colitis-associated colon cancer models. The Neu3-deficient mice showed no conspicuous accumulation of gangliosides in the brain or colon mucosa, or overt abnormalities in their growth, development, behavior, or fertility. In dimethylhydrazine-induced colon carcinogenesis, there were no differences in the incidence or growth of tumors between the Neu3-deficient and wild-type mice. On the other hand, the Neu3-deficient mice were less susceptible than wild-type mice to the colitis-associated colon carcinogenesis induced by azoxymethane and dextran sodium sulfate. These results suggest that NEU3 plays an important role in inflammation-dependent tumor development.

Sialidase significance for cancer progression

Aberrant glycosylation is a characteristic feature of cancer cells. In particular, altered sialylation is closely associated with malignant properties, including invasiveness and metastatic potential. To elucidate the molecular mechanisms underlying the aberrancy, our studies have focused on mammalian sialidase, which catalyzes the removal of sialic acid residues from glycoproteins and glycolipids. The four types of mammalian sialidase identified to date show altered expression and behave in different manners during carcinogenesis. The present review briefly summarizes results on altered expression of sialidases and their possible roles in cancer progression. These enzymes are indeed factors defining cancer malignancy and thus potential targets for cancer diagnosis and therapy.

NEU4L sialidase overexpression promotes β-catenin signaling in neuroblastoma cells, enhancing stem-like malignant cell growth

Neuroblastoma (NB) is a frequently lethal tumor that occurs in childhood and originates from embryonic neural crest cells. The malignant and aggressive phenotype of NB is strictly related to the deregulation of pivotal pathways governing the proliferation/differentiation status of neural crest precursor cells, such as MYCN, Delta/Notch and Wnt/β-catenin (CTNNB1) signaling. In this article, we demonstrate that sialidase NEU4 long (NEU4L) influences the differentiation/proliferation behavior of NB SK-N-BE cells by determining hyperactivation of the Wnt/β-catenin signaling pathway. NEU4L overexpression in SK-N-BE cells induced significant increases in active, nonphosphorylated β-catenin content, β-catenin/TCF transcriptional activity and β-catenin gene target expression including MYCN, MYC, CCND2 (cyclin D2) and CDC25A. In turn, these molecular features strongly modified the behavior of NEU4L SK-N-BE overexpressing cells, promoting the following: (1) an enhanced proliferation rate, mainly due to a faster transition from G1 to S phase in the cell cycle; (2) a more undifferentiated cell phenotype, which was similar to stem-like NB cells and possibly mediated by an increase of the expression of the pluripotency genes, MYC, NANOG, OCT-4, CD133 and NES (nestin); (3) the failure of NB cell differentiation after serum withdrawal. The molecular link between NEU4L and Wnt/β-catenin signaling appeared to rely most likely on the capability of the enzyme to modify the sialylation level of cell glycoproteins. These findings could provide a new candidate for therapeutic treatment.

Sialidase NEU4 hydrolyzes polysialic acids of neural cell adhesion molecules and negatively regulates neurite formation by hippocampal neurons

Modulation of levels of polysialic acid (polySia), a sialic acid polymer, predominantly associated with the neural cell adhesion molecule (NCAM), influences neural functions, including synaptic plasticity, neurite growth, and cell migration. Biosynthesis of polySia depends on two polysialyltransferases ST8SiaII and ST8SiaIV in vertebrate. However, the enzyme involved in degradation of polySia in its physiological turnover remains uncertain. In the present study, we identified and characterized a murine sialidase NEU4 that catalytically degrades polySia. Murine NEU4, dominantly expressed in the brain, was found to efficiently hydrolyze oligoSia and polySia chains as substrates in sialidase in vitro assays, and also NCAM-Fc chimera as well as endogenous NCAM in tissue homogenates of postnatal mouse brain as assessed by immunoblotting with anti-polySia antibodies. Degradation of polySia by NEU4 was also evident in neuroblastoma Neuro2a cells that were co-transfected with Neu4 and ST8SiaIV genes. Furthermore, in mouse embryonic hippocampal primary neurons, the endogenously expressed NEU4 was found to decrease during the neuronal differentiation. Interestingly, GFP- or FLAG-tagged NEU4 was partially co-localized with polySia in neurites and significantly suppressed their outgrowth, whereas silencing of NEU4 showed the acceleration together with an increase in polySia expression. These results suggest that NEU4 is involved in regulation of neuronal function by polySia degradation in mammals.

Mammalian sialidases: physiological and pathological roles in cellular functions

Sialic acids are terminal acidic monosaccharides, which influence the chemical and biological features of glycoconjugates. Their removal catalyzed by a sialidase modulates various biological processes through change in conformation and creation or loss of binding sites of functional molecules. Sialidases exist widely in vertebrates and also in a variety of microorganisms. Recent research on mammalian sialidases has provided evidence for great importance of these enzymes in various cellular functions, including lysosomal catabolism, whereas microbial sialidases appear to play roles limited to nutrition and pathogenesis. Four types of mammalian sialidases have been identified and characterized to date, designated as NEU1, NEU2, NEU3 and NEU4. They are encoded by different genes and differ in major subcellular localization and enzymatic properties including substrate specificity, and each has been found to play a unique role depending on its particular properties. This review is an attempt to concisely summarize current knowledge concerning mammalian sialidases, with a special focus on their properties and physiological and pathological roles in cellular functions.

Structure and Function of Mammalian Sialidases

The removal of sialic acids, catalyzed by sialidase, is the initial step in degradation of oligosaccharides, glycoproteins, and glycolipids. The catalytic reaction may greatly influence biological processes through changing the conformation of glycoproteins and create or mask binding sites of functional molecules. Recent progress in sialidase research has clarified that mammalian sialidases indeed contribute to the regulation of various cellular functions as well as lysosomal catabolism, unlike the sialidases of microbial origin that probably play roles limited to nutrition and pathogenesis. However, the mammalian enzymes contain consensus sequences in the six-blade β-propeller structural organization typical of microbial sialidases, despite the low degree of similarity to the amino acid sequences of the microbial enzymes. The present review briefly summarizes structural and functional features of mammalian sialidases.

Where catabolism meets signalling: neuraminidase 1 as a modulator of cell receptors

Terminal sialic acid residues are found in abundance in glycan chains of glycoproteins and glycolipids on the surface of all live cells forming an outer layer of the cell originally known as glycocalyx. Their presence affects the molecular properties and structure of glycoconjugates, modifying their function and interactions with other molecules. Consequently, the sialylation state of glycoproteins and glycolipids has been recognized as a critical factor modulating molecular recognitions inside the cell, between the cells, between the cells and the extracellular matrix, and between the cells and certain exogenous pathogens. Sialyltransferases that attach sialic acid residues to the glycan chains in the process of their initial synthesis were thought to be mainly responsible for the creation and maintenance of a temporal and spatial diversity of sialylated moieties. However, the growing evidence also suggests that in mammalian cells, at least equally important roles belong to sialidases/neuraminidases, which are located on the cell surface and in intracellular compartments, and may either initiate the catabolism of sialoglycoconjugates or just cleave their sialic acid residues, and thereby contribute to temporal changes in their structure and functions. The current review summarizes emerging data demonstrating that neuraminidase 1 (NEU1), well known for its lysosomal catabolic function, can be also targeted to the cell surface and assume the previously unrecognized role as a structural and functional modulator of cellular receptors.

Neu1 sialidase and matrix metalloproteinase-9 cross-talk is essential for Toll-like receptor activation and cellular signaling

The signaling pathways of mammalian Toll-like receptors (TLRs) are well characterized, but the precise mechanism(s) by which TLRs are activated upon ligand binding remains poorly defined. Recently, we reported a novel membrane sialidase-controlling mechanism that depends on ligand binding to its TLR to induce mammalian neuraminidase-1 (Neu1) activity, to influence receptor desialylation, and subsequently to induce TLR receptor activation and the production of nitric oxide and proinflammatory cytokines in dendritic and macrophage cells. The α-2,3-sialyl residue of TLR was identified as the specific target for hydrolysis by Neu1. Here, we report a membrane signaling paradigm initiated by endotoxin lipopolysaccharide (LPS) binding to TLR4 to potentiate G protein-coupled receptor (GPCR) signaling via membrane Gα(i) subunit proteins and matrix metalloproteinase-9 (MMP9) activation to induce Neu1. Central to this process is that a Neu1-MMP9 complex is bound to TLR4 on the cell surface of naive macrophage cells. Specific inhibition of MMP9 and GPCR Gα(i)-signaling proteins blocks LPS-induced Neu1 activity and NFκB activation. Silencing MMP9 mRNA using lentivirus MMP9 shRNA transduction or siRNA transfection of macrophage cells and MMP9 knock-out primary macrophage cells significantly reduced Neu1 activity and NFκB activation associated with LPS-treated cells. These findings uncover a molecular organizational signaling platform of a novel Neu1 and MMP9 cross-talk in alliance with TLR4 on the cell surface that is essential for ligand activation of TLRs and subsequent cellular signaling.

Regulation of sialyl Lewis antigen expression in colon cancer cells by sialidase NEU4

Sialyl Lewis antigens, sialyl Lewis a and sialyl Lewis x, are utilized as tumor markers, and their increase in cancer is associated with tumor progression by enhancement of cancer cell adhesion to endothelial E-selectin. However, regulation mechanisms are not fully understood. We previously demonstrated that NEU4 is the only sialidase efficiently acting on mucins and it is down-regulated in colon cancer. To elucidate the significance of NEU4 down-regulation, we investigated sialyl Lewis antigens as endogenous substrates for the sialidase. NEU4 was found to hydrolyze the antigens in vitro and decrease cell surface levels much more effectively than other sialidases. Western blot, thin layer chromatography, and metabolic inhibition studies of desialylation products revealed NEU4 to preferentially catalyze sialyl Lewis antigens expressed on O-glycans. Cell adhesion to and motility and growth on E-selectin were significantly reduced by NEU4. E-selectin stimulation of colon cancer cells enhanced cell motility through activation of the p38/Hsp27/actin reorganization pathway, whereas NEU4 attenuated the signaling. On immunocytochemical analysis, some NEU4 molecules were localized at cell surfaces. Under hypoxia conditions whereby the antigens were increased concomitantly with several sialyl- and fucosyltransferases, NEU4 expression was markedly decreased. These results suggest that NEU4 plays an important role in control of sialyl Lewis antigen expression and its impairment in colon cancer.

Identifying selective inhibitors against the human cytosolic sialidase NEU2 by substrate specificity studies

Aberrant expression of human sialidases has been shown to associate with various pathological conditions. Despite the effort in the sialidase inhibitor design, less attention has been paid to designing specific inhibitors against human sialidases and characterizing the substrate specificity of different sialidases regarding diverse terminal sialic acid forms and sialyl linkages. This is mainly due to the lack of sialoside probes and efficient screening methods, as well as limited access to human sialidases. A low cellular expression level of the human sialidase NEU2 hampers its functional and inhibitory studies. Here we report the successful cloning and expression of the human sialidase NEU2 in E. coli. About 11 mg of soluble active NEU2 was routinely obtained from 1 L of E. coli cell culture. Substrate specificity studies of the recombinant human NEU2 using twenty p-nitrophenol (pNP)-tagged α2-3- or α2-6-linked sialyl galactosides containing different terminal sialic acid forms including common N-acetylneuraminic acid (Neu5Ac), non-human N-glycolylneuraminic acid (Neu5Gc), 2-keto-3-deoxy-D-glycero-D-galacto-nonulosonic acid (Kdn), or their C5-derivatives in a microtiter plate-based high-throughput colorimetric assay identified a unique structural feature specifically recognized by the human NEU2 but not two bacterial sialidases. The results obtained from substrate specificity studies were used to guide the design of a sialidase inhibitor that was selective against human NEU2. The selectivity of the inhibitor was revealed by the comparison of sialidase crystal structures and inhibitor docking studies.

Antibody-mediated sialidase activity in blood serum of patients with multiple myeloma

Cell surface sialylation is known to be tightly connected with tumorigenicity, invasiveness, metastatic potential, clearance of aged cells, while the sialylation of IgG molecules determines their anti-inflammatory properties. Four sialidases - hydrolytic enzymes responsible for cleavage of sialic residues - were described in different cellular compartments. However, sialidases activity in body fluids, and specifically in blood serum, remains poorly studied. Here, we characterize first known IgG antibodies possessing sialidase-like activity in blood serum of multiple myeloma (MM) patients. Ig fractions were precipitated with ammonium sulfate (50% of saturation) from blood serum of 12 healthy donors and 14 MM patients, and screened for the presence of sialidase activity by using 4-MUNA (2'-(4-methylumbelliferyl)-α-D-N-acetylneuraminic acid) as substrate. High level of sialidase activity was detected in the MM patients, but not in healthy donors. Subsequent antibody purification by protein-G affinity chromatography and HPLC size exclusion chromatography at acidic conditions demonstrated that sialidase activity was attributable to IgG molecules. Sialidase activity was also specific for (Fab)(2) fragment of IgG and blocked by sialidase inhibitor DANA. Sialidase activity of IgG molecule was also confirmed by in gel assay for cleavage of sialidase substrate. Kinetic parameters of the catalysis reaction were described by Michaelis-Menten equation with K(m)  = 44.4-108 µM and k(cat) = 2.7-23.1 min(-1). The action of IgG possessing sialidase-like activity towards human red blood cells resulted in a subsequent increase in their agglutination by the peanut agglutinin, that confirms their desialylation by the studied IgG. This is the first demonstration of the intrinsic sialidase activity of IgG isolated from blood serum of MM patients.

Mice doubly-deficient in lysosomal hexosaminidase A and neuraminidase 4 show epileptic crises and rapid neuronal loss

Tay-Sachs disease is a severe lysosomal disorder caused by mutations in the HexA gene coding for the α-subunit of lysosomal β-hexosaminidase A, which converts G_M2 to G_M3 ganglioside. Hexa^−/− mice, depleted of β-hexosaminidase A, remain asymptomatic to 1 year of age, because they catabolise G_M2 ganglioside via a lysosomal sialidase into glycolipid G_A2, which is further processed by β-hexosaminidase B to lactosyl-ceramide, thereby bypassing the β-hexosaminidase A defect. Since this bypass is not effective in humans, infantile Tay-Sachs disease is fatal in the first years of life. Previously, we identified a novel ganglioside metabolizing sialidase, Neu4, abundantly expressed in mouse brain neurons. Now we demonstrate that mice with targeted disruption of both Neu4 and Hexa genes (Neu4^−/−;Hexa^−/−) show epileptic seizures with 40% penetrance correlating with polyspike discharges on the cortical electrodes of the electroencephalogram. Single knockout Hexa^−/− or Neu4^−/− siblings do not show such symptoms. Further, double-knockout but not single-knockout mice have multiple degenerating neurons in the cortex and hippocampus and multiple layers of cortical neurons accumulating G_M2 ganglioside. Together, our data suggest that the Neu4 block exacerbates the disease in Hexa^−/− mice, indicating that Neu4 is a modifier gene in the mouse model of Tay-Sachs disease, reducing the disease severity through the metabolic bypass. However, while disease severity in the double mutant is increased, it is not profound suggesting that Neu4 is not the only sialidase contributing to the metabolic bypass in Hexa^−/− mice.

Tay-Sachs disease is the second most common lysosomal storage disorder, especially frequent in Ashkenazi Jews and French Canadians. The disorder is caused by mutations in the gene coding for lysosomal β-hexosaminidase A (HexA), resulting in accumulation of G_M2 ganglioside in neurons followed by progressive neurologic degeneration, fatal in early childhood. However mice, depleted of HexA, remain asymptomatic to at least 1 year of age, owing to the ability of these mice to catabolise stored G_M2 ganglioside via a lysosomal neuraminidase into glycolipid G_A2 further processed by β-hexosaminidase B, thereby completely bypassing the HexA defect. Our current study provides an explanation why the disease is severe in humans but not in mice. We showed that mice depleted of both HexA and ganglioside neuraminidase 4 (Neu4) show epileptic seizures similar to that often observed in Tay-Sachs patients. Single HexA or Neu4 knockout mice do not show such symptoms. Further, double-knockout but not single-knockout mice have multiple degenerating cortical and hippocampal neurons and multiple layers of cortical neurons accumulating G_M2 ganglioside. Our data suggest that the Neu4 depletion exacerbates the disease in HexA knockout mice, supporting the view that Neu4 is one of the modifier genes in the mouse model of Tay-Sachs disease.

LPS-induced cytokine production in human dendritic cells is regulated by sialidase activity

Removal of sialic acid from glycoconjugates on the surface of monocytes enhances their response to bacterial LPS. We tested the hypothesis that endogenous sialidase activity creates a permissive state for LPS-induced cytokine production in human monocyte-derived DCs. Of the four genetically distinct sialidases (Neu1-4), Neu1, Neu3, and Neu4 are expressed in human monocytes, but only Neu1 and Neu3 are up-regulated as cells differentiate into DCs. Neu1 and Neu3 are present on the surface of monocytes and DCs and are also present intracellularly. DCs contain a greater amount of sialic acid than monocytes, but the amount of sialic acid/mg total protein declines during differentiation to DCs. This relative hyposialylation of cells does not occur in mature DCs grown in the presence of zanamivir, a pharmacologic inhibitor of Neu3 but not Neu1, or DANA, an inhibitor of Neu1 and Neu3. Inhibition of sialidase activity during differentiation to DCs causes no detectable change in cell viability or expression of DC surface markers. Differentiation of monocytes into DCs in the presence of zanamivir results in reduced LPS- induced expression of IL-6, IL-12p40, and TNF-α by mature DCs, demonstrating a role for Neu3 in cytokine production. A role for Neu3 is supported by inhibition of cytokine production by DANA in DCs from Neu1⁻/⁻ and WT mice. We conclude that sialidase-mediated change in sialic acid content of specific cell surface glycoconjugates in DCs regulates LPS-induced cytokine production, thereby contributing to development of adaptive immune responses.

Dependence of pathogen molecule-induced toll-like receptor activation and cell function on Neu1 sialidase

The signaling pathways of mammalian Toll-like receptors (TLR) are well characterized, but the initial molecular mechanisms activated following ligand interactions with the receptors remain poorly defined. Here, we show a membrane controlling mechanism that is initiated by ligand binding to TLR-2, -3 and-4 to induce Neu1 sialidase activity within minutes in live primary bone marrow (BM) macrophage cells and macrophage and dendritic cell lines. Central to this process is that Neu1 and not Neu2,-3 and-4 forms a complex with TLR-2,-3 and-4 on the cell surface of naïve macrophage cells. Neuraminidase inhibitors BCX1827, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (DANA), zanamivir and oseltamivir carboxylate have a limited significant inhibition of the LPS-induced sialidase activity in live BMC-2 macrophage cells but Tamiflu (oseltamivir phosphate) completely blocks this activity. Tamiflu inhibits LPS-induced sialidase activity in live BMC-2 cells with an IC(50) of 1.2 microM compared to an IC(50) of 1015 microM for its hydrolytic metabolite oseltamivir carboxylate. Tamiflu blockage of LPS-induced Neu1 sialidase activity is not affected in BMC-2 cells pretreated with anticarboxylesterase agent clopidogrel. Endotoxin LPS binding to TLR4 induces Neu1 with subsequent activation of NFkappaB and the production of nitric oxide and pro-inflammatory IL-6 and TNFalpha cytokines in primary and macrophage cell lines. Hypomorphic cathepsin A mice with a secondary Neu1 deficiency respond poorly to LPS-induced pro-inflammatory cytokines compared to the wild-type or hypomorphic cathepsin A with normal Neu1 mice. Our findings establish an unprecedented mechanism for pathogen molecule-induced TLR activation and cell function, which is critically dependent on Neu1 sialidase activity associated with TLR ligand treated live primary macrophage cells and macrophage and dendritic cell lines.

Thymoquinone from nutraceutical black cumin oil activates Neu4 sialidase in live macrophage, dendritic, and normal and type I sialidosis human fibroblast cells via GPCR Galphai proteins and matrix metalloproteinase-9

Anti-inflammatory activities of thymoquinone (TQ) have been demonstrated in in vitro and in vivo studies. However, the precise mechanism(s) of TQ in these anti-inflammatory activities is not well understood. Using a newly developed assay to detect sialidase activity in live macrophage cells (Glycoconj J doi: 10.1007/s10719-009-9239-8 ), here we show that TQ has no inhibitory effect on endotoxin lipopolysaccharide (LPS) induced sialidase activity in live BMC-2 macrophage cells. In contrast, the parent black seed oil (BSO) and another constituent of BSO para-cymene (p-CY) completely block LPS induced sialidase activity. All of these compounds had no effect on cell viability. On the other hand, TQ induces a vigorous sialidase activity in live BMC-2 macrophage cells in a dose dependent manner as well in live DC-2.4 dendritic cells, HEK-TLR4/MD2, HEK293, SP1 mammary adenocarcinoma cells, human WT and 1140F01 and WG0544 type I sialidosis fibroblast cells. Tamiflu (oseltamivir phosphate) inhibits TQ-induced sialidase activity in live BMC-2 cells with an IC(50) of 0.0194 microM compared to an IC(50) of 19.1 microM for neuraminidase inhibitor DANA (2-deoxy-2,3-dehydro-N-acetylneuraminic acid). Anti-Neu1, -2 and -3 antibodies have no inhibition of TQ-induced sialidase activity in live BMC-2 and human THP-1 macrophage cells but anti-Neu4 antibodies completely block this activity. There is a vigorous sialidase activity associated with TQ treated live primary bone marrow (BM) macrophage cells derived from WT and hypomorphic cathepsin A mice with a secondary Neu1 deficiency (NeuI KD), but not from Neu4 knockout (Neu4 KO) mice. Pertussis toxin (PTX), a specific inhibitor of Galphai proteins of G-protein coupled receptor (GPCR) and the broad range inhibitors of matrix metalloproteinase (MMP) galardin and piperazine applied to live BMC-2, THP-1 and primary BM macrophage cells completely block TQ-induced sialidase activity. These same inhibitory effects are not observed with the GM1 ganglioside specific cholera toxin subunit B (CTXB) as well as with CTX, tyrosine kinase inhibitor K252a, and the broad range GPCR inhibitor suramin. The specific inhibitor of MMP-9, anti-MMP-9 antibody and anti-Neu4 antibody, but not the specific inhibitor of MMP-3 completely block TQ-induced sialidase activity in live THP-1 cells, which express Neu4 and MMP-9 on the cell surface. Neu4 sialidase activity in cell lysates from TQ-treated live THP-1 cells desialylates natural gangliosides and mucin substrates. RT-PCR and western blot analyses reveal no correlation between mRNA and protein values for Neu3 and Neu4 in human monocytic THP-1 cells, suggesting for the first time a varied post-transcriptional mechanism for these two mammalian sialidases independent of TQ activation. Our findings establish an unprecedented activation of Neu4 sialidase on the cell surface by thymoquinone, which is derived from the nutraceutical black cumin oil. The potentiation of GPCR-signaling by TQ via membrane targeting of Galphai subunit proteins and matrix metalloproteinase-9 activation may be involved in the activation process of Neu4 sialidase on the cell surface.

Desialylation of insulin receptors and IGF-1 receptors by neuraminidase-1 controls the net proliferative response of L6 myoblasts to insulin

We recently established that the subunit of cell surface-residing elastin receptor, neuraminidase-1 (Neu1), can desialylate adjacent insulin-like growth factor 1 receptors (IGF-1R) of arterial smooth muscle cells, thereby quenching their proliferative response to insulin-like growth factor II. In this study, we explored whether Neu1 would also desialylate the insulin receptors (IR), as well as the IGF-1R on rat skeletal L6 myoblasts, and whether desialylation of IR and IGF-1R would affect a net proliferative effect of insulin. First, we found that physiological (0.5-1 nM) and high therapeutic (10 nM) insulin concentrations induced a modest increase in proliferation rate of cultured L6 myoblasts. While IR kinase inhibitor could abolish the mitogenic effect of these insulin concentrations, the observed more pronounced proliferative response to supraphysiological concentration (100 nM) of insulin could be eliminated only by specific inhibition of IGF-1R. Then, we found that treatment of L6 cells with mouse-derived Neu1 or with Clostridium perfringens neuraminidase caused desialylation of IR, which coincided with a significant increase of their proliferative response to lower (0.5-10 nM) concentrations of insulin. In contrast, experimental desialylation of IGF-1R coincided with elimination of the heightened proliferative response of L6 myoblasts to 100 nM insulin. Importantly, we also found that inhibition of endogenous Neu1 abolished the increase in proliferation of L6 cells induced by 1 and 10 nM of insulin, but amplified the proliferative effect of 100 nM insulin. We therefore conclude that desialylation of both IR and IGF-1R by Neu1 controls the net proliferative response of skeletal myoblasts to insulin.

Regulation of intracellular signaling by extracellular glycan remodeling

The plasma membrane of eukaryotic cells is coated with carbohydrates. By virtue of their extracellular position and recognizable chemical features, cell surface glycans mediate many receptor-ligand interactions. Recently, mammalian extracellular hydrolytic enzymes have been shown to modify the structure of cell surface glycans and consequently alter their binding properties. These cell surface glycan remodeling events can cause rapid changes in critical signal transduction phenomena. This Review highlights recent studies on the roles of eukaryotic extracellular sialidases, sulfatases, and a deacetylase in regulation of intracellular signaling. We also describe possible therapies that target extracellular glycan remodeling processes and discuss the potential for new discoveries in this area.

Heterodimerization of the sialidase NEU1 with the chaperone protective protein/cathepsin A prevents its premature oligomerization

Lysosomal neuraminidase-1 (NEU1) forms a multienzyme complex with beta-galactosidase and protective protein/cathepsin A (PPCA). Because of its association with PPCA, which acts as a molecular chaperone, NEU1 is transported to the lysosomal compartment, catalytically activated, and stabilized. However, the mode(s) of association between these two proteins both en route to the lysosome and in the multienzyme complex has remained elusive. Here, we have analyzed the hydrodynamic properties of PPCA, NEU1, and a complex of the two proteins and identified multiple binding sites on both proteins. One of these sites on NEU1 that is involved in binding to PPCA can also bind to other NEU1 molecules, albeit with lower affinity. Therefore, in the absence of PPCA, as in the lysosomal storage disease galactosialidosis, NEU1 self-associates into chain-like oligomers. Binding of PPCA can reverse self-association of NEU1 by causing the disassembly of NEU1-oligomers and the formation of a PPCA-NEU1 heterodimeric complex. The identification of binding sites between the two proteins allowed us to create innovative structural models of the NEU1 oligomer and the PPCA-NEU1 heterodimeric complex. The proposed mechanism of interaction between NEU1 and its accessory protein PPCA provides a rationale for the secondary deficiency of NEU1 in galactosialidosis.

Human sialidase NEU4 long and short are extrinsic proteins bound to outer mitochondrial membrane and the endoplasmic reticulum, respectively

Sialidases are widely distributed glycohydrolytic enzymes removing sialic acid residues from glycoconjugates. In mammals, several sialidases with different subcellular localizations and biochemical features have been described. NEU4, the most recently identified member of the human sialidase family, is found in two forms, NEU4 long and NEU4 short, differing in the presence of a 12-amino-acid sequence at the N-terminus. Contradictory data are present in the literature about the subcellular distribution of these enzymes, their membrane anchoring mechanism being still unclear. In this work, we investigate the human NEU4 long and NEU4 short membrane anchoring mechanism and their subcellular localization. Protein extraction with Triton X-114 and sodium carbonate and cross-linking experiments demonstrate that both forms of NEU4 are extrinsic membrane proteins, anchored via protein-protein interactions. Moreover, through confocal microscopy and subcellular fractionation, we show that the long form localizes in mitochondria, while the short form is also associated with the endoplasmic reticulum. Finally, mitochondria subfractionation experiments suggest that NEU4 long is bound to the outer mitochondrial membrane.

Specific expression of Neu2 type B in mouse thymus and the existence of a membrane-bound form in COS cells

Neu2 mRNA from the mouse thymus, as we have reported [K. Kotani, A. Kuroiwa, T. Saito, Y. Matsuda, T. Koda, S. Kijimoto-Ochiai, Cloning, chromosomal mapping, and characteristic 5'-UTR sequence of murine cytosolic sialidase, Biochem. Biophys. Res. Commun. 286 (2001) 250-258], has a novel sequence at the 5' terminus that shows the ability to encode 6 extra amino acids in the N-terminus than that of the muscle. In this paper, we analyzed the cDNA and EST database and found the five types of alternative splicing of Neu2 mRNA: A, B, C, D and N. We studied the expression of these types in the immune tissues and found that the thymus expressed only type B. We constructed 2 types of plasmid that encode long (B) or short (C) form of Neu2 protein, and transfected them into COS7 cells to study them under the same conditions. We found that 30-40% of the both forms of Neu2 activity was located in the crude membrane-fraction, and hydrolyzed ganglioside effectively, while both soluble fraction showed particular behavior with substrate specificity. Microscopic study by active staining with X-NANA showed that they located not only in the cytoplasm but also in areas surrounding the nucleus and in the peripheral ruffled spot.

Developmental change of sialidase neu4 expression in murine brain and its involvement in the regulation of neuronal cell differentiation

Sialidase Neu4 is reported to be dominantly expressed in the mouse brain, but its functional significance is not fully understood. We previously demonstrated that sialidase Neu3, also rich in mouse brain, is up-regulated during neuronal differentiation with involvement in acceleration of neurite formation. To elucidate physiological functions of Neu4, as well as Neu3, we determined expression during mouse brain development by quantitative RT-PCR. Expression was relatively low in the embryonic stage and then rapidly increased at 3-14 days after birth, whereas Neu3 demonstrated high levels in the embryonic stage and down-regulation after birth. Murine Neu4 was found to possess two isoforms differing in expression levels, developmental pattern, and enzymatic character. Distinct from the human isoforms, the murine forms, to a different extent, both catalyzed the removal of sialic acid from gangliosides as well as glycoproteins, and one isoform seemed to act on polysialylated NCAM efficiently, despite the low activity toward ordinary substrates. In situ hybridization demonstrated Neu4 mRNA to be present mainly in the hippocampus in which NCAM is rich and decreases after birth. During retinoic acid-induced differentiation, Neu4 expression was down-regulated in Neuro2a cells. Overexpression of Neu4 resulted in suppression of neurite formation, and its knockdown showed the acceleration. Thin layer chromatography of the glycolipids from Neu4-transfected cells showed ganglioside compositions to be only slightly affected, although lectin blot analysis revealed increased binding to Ricinus communis agglutinin (RCA) lectin of a approximately 95-kDa glycoprotein, which decreased with cell differentiation. These results suggest that mouse Neu4 plays an important regulatory role in neurite formation, possibly through desialylation of glycoproteins.

Assessment of neuropsychiatric adverse events in influenza patients treated with oseltamivir: a comprehensive review

After reports from Japan of neuropsychiatric adverse events (NPAEs) in children taking oseltamivir phosphate (hereafter referred to as oseltamivir [Tamiflu; F. Hoffmann-La Roche Ltd, Basel, Switzerland]) during and after the 2004--5 influenza season, Roche explored possible reasons for the increase in reporting rate and presented regular updates to the US FDA and other regulatory authorities. This review summarizes the results of a comprehensive assessment of the company's own preclinical and clinical studies, post-marketing spontaneous adverse event reporting, epidemiological investigations utilizing health claims and medical records databases and an extensive review of the literature, with the aim of answering the following questions: (i) what the types and rates of neuropsychiatric abnormalities reported in patients with influenza are, and whether these differ in patients who have received oseltamivir compared with those who have not; (ii) what levels of oseltamivir and its active metabolite, oseltamivir carboxylate are achieved in the CNS; (iii) whether oseltamivir and oseltamivir carboxylate have pharmacological activity in the CNS; and (iv) whether there are genetic differences between Japanese and Caucasian patients that result in different levels of oseltamivir and/or oseltamivir carboxylate in the CNS, differences in their metabolism or differences in their pharmacological activity in the CNS. In total, 3051 spontaneous reports of NPAEs were received by Roche, involving 2466 patients who received oseltamivir between 1999 and 15 September 2007; 2772 (90.9%) events originated from Japan, 190 (6.2%) from the US and 89 (2.9%) from other countries. During this period, oseltamivir was prescribed to around 48 million people worldwide. Crude NPAE reporting rates (per 1,000,000 prescriptions) in children (aged < or =16 years) and adults, respectively, were 99 and 28 events in Japan and 19 and 8 in the US. NPAEs were more commonly reported in children (2218 events in 1808 children aged < or =16 years vs 833 in 658 adults) and generally occurred within 48 hours of the onset of influenza illness and initiation of treatment. After categorizing the reported events according to International Classification of Diseases (9th edition) codes, abnormal behaviour (1160 events, 38.0%) and delusions/perceptual disturbances (661 events, 21.7%) were the largest categories of events, and delirium or delirium-like events (as defined by the American Psychiatric Association) were very common in most categories. No difference in NPAE reporting rates between oseltamivir and placebo was found in phase III treatment studies (0.5% vs 0.6%). Analyses of US healthcare claims databases showed the risk of NPAEs in oseltamivir-treated patients (n = 159,386) was no higher than those not receiving antivirals (n = 159,386). Analysis of medical records in the UK General Practice Research Database showed that the adjusted relative risk of NPAEs in influenza patients was significantly higher (1.75-fold) than in the general population. Based on literature reports, NPAEs in Japanese and Taiwanese children with influenza have occurred before the initiation of oseltamivir treatment; events were also similar to those occurring after the initiation of oseltamivir therapy. No clinically relevant differences in plasma pharmacokinetics of oseltamivir and its active metabolite oseltamivir carboxylate were noted between Japanese and Caucasian adults or children. Penetration into the CNS of both oseltamivir and oseltamivir carboxylate was low in Japanese and Caucasian adults (cerebrospinal fluid/plasma maximum concentration and area under the plasma concentration-time curve ratios of approximately 0.03), and the capacity for converting oseltamivir to oseltamivir carboxylate in rat and human brains was low. In animal autoradiography and pharmacokinetic studies, brain : plasma radioactivity ratios were generally 20% or lower. Animal studies showed no specific CNS/behavioural effects after administration of doses corresponding to > or =100 times the clinical dose. Oseltamivir or oseltamivir carboxylate did not interact with human neuraminidases or with 155 known molecular targets in radioligand binding and functional assays. A review of the information published to date on functional variations of genes relevant to oseltamivir pharmacokinetics and pharmacodynamics and simulated gene knock-out scenarios did not identify any plausible genetic explanations for the observed NPAEs. The available data do not suggest that the incidence of NPAEs in influenza patients receiving oseltamivir is higher than in those who do not, and no mechanism by which oseltamivir or oseltamivir carboxylate could cause or worsen such events could be identified.

Contribution of sialidase NEU1 to suppression of metastasis of human colon cancer cells through desialylation of integrin beta4

We previously found an inverse relationship between sialidase Neu1 expression and metastatic potential of murine cancer cells. To elucidate the mechanism underlying the cellular events, the human sialidase gene NEU1 was overexpressed or silenced in colon cancer HT-29 cells. When NEU1-overexpressing cells were injected transsplenically into mice, in vivo liver metastasis was significantly reduced. NEU1 suppressed cell migration, invasion and adhesion in vitro, whereas the silencing resulted in the opposite. One of the major molecular changes by NEU1 was decreased sialylation of integrin beta4, assessed by PNA- and MAL-II-lectin blotting of immunoprecipitates with anti-integrin beta4 antibody. The desialylation was accompanied by decreased phosphorylation of the integrin followed by attenuation of focal adhesion kinase and Erk1/2 pathway. Moreover, NEU1 caused downregulation of matrix metalloproteinase-7, overexpression of which is associated with cancer metastasis. Treatment of the cells with GalNAc-alpha-O-benzyl, an inhibitor of O-glycosylation, showed increased PNA-positive integrin beta4 with its decreased phosphorylation, indicating that sialic acid removal from the integrin O-glycans results in the decreased phosphorylation. Biotinylation and immunofluorescence staining exhibited some NEU1 molecules to be at the cell surface accessible to the integrin. These results suggest that NEU1 is important in regulation of integrin beta4-mediated signaling, leading to suppression of metastasis.

A strategy for constructing C-sialosides based on Ireland-Claisen rearrangement and its application for synthesis of CF2-linked ganglioside GM4 analog

Sialidase-resistant ganglioside analogs having similar biological activities to natural gangliosides are expected to be important probes for clarifying the biological functions of gangliosides. Focusing on difluoromethylene-linked (CF2-linked) and methylene-linked (CH2-linked) α(2,3)sialylgalactose as a core structure of sialidase-resistant ganglioside mimics, we have developed novel, stereocontrolled, and efficient methodologies to synthesize C-sialosides based on Ireland-Claisen rearrangement. These methods were employed to synthesize CF2-linked GM4. The CF2-linked GM4 inhibited human sialidases NEU2 and NEU4 with IC50 values of 754 and 930 μM, respectively, and strongly inhibited human lymphocyte proliferation in the same concentration range as natural GM4.

Human sialidase as a cancer marker

Altered sialylation of cell surface glycoproteins and glycolipids is closely related to the malignant phenotype of cancer cells, including the metastatic potential and invasiveness. Many cancer-related antigens in clinical use contain sialic acids at the terminal position of sugar chains in the molecules. To elucidate the molecular mechanism, we focused our investigation on sialidase, which catalyzes the removal of sialic acid residues from the glycoconjugates. Four types of human sialidases identified to date behave in different manners during carcinogenesis. One of the sialidases, found in the lysosomes, showed downregulation in cancers, promoting anchorage-independent growth, and metastatic ability, while another, found in the plasma membrane, showed marked upregulation, causing apoptosis suppression. It was found that estimation of the mRNA levels of sialidases by real-time PCR allowed discrimination of cancerous from noncancerous tissues and even determination of the pathological stage in some cancers. Immunohistochemistry of cancer tissues using the antibody against the plasma membrane sialidase was useful for clinical diagnosis. This paper briefly summarizes our findings of the altered sialidase expression in cancers and the possibility of their clinical application as cancer markers. Human sialidases are indeed related to malignancy and may be potential targets for cancer diagnosis and therapy.

Neuraminidase-1, a subunit of the cell surface elastin receptor, desialylates and functionally inactivates adjacent receptors interacting with the mitogenic growth factors PDGF-BB and IGF-2

We recently established that the elastin-binding protein, which is identical to the spliced variant of beta-galactosidase, forms a cell surface-targeted complex with two proteins considered "classic lysosomal enzymes": protective protein/cathepsin A and neuraminidase-1 (Neu1). We also found that cell surface-residing Neu1 can desialylate neighboring microfibrillar glycoproteins and facilitate the deposition of insoluble elastin, which contributes to the maintenance of cellular quiescence. Here we provide evidence that cell surface-residing Neu1 contributes to a novel mechanism that limits cellular proliferation by desialylating cell membrane-residing sialoglycoproteins that directly propagate mitogenic signals. We demonstrated that treatment of cultured human aortic smooth muscle cells (SMCs) with either a sialidase inhibitor or an antibody that blocks Neu1 activity induced significant up-regulation in SMC proliferation in response to fetal bovine serum. Conversely, treatment with Clostridium perfringens neuraminidase (which is highly homologous to Neu1) decreased SMC proliferation, even in cultures that did not deposit elastin. Further, we found that pretreatment of aortic SMCs with exogenous neuraminidase abolished their mitogenic responses to recombinant platelet-derived growth factor (PDGF)-BB and insulin-like growth factor (IGF)-2 and that sialidosis fibroblasts (which are exclusively deficient in Neu1) were more responsive to PDGF-BB and IGF-2 compared with normal fibroblasts. Furthermore, we provide direct evidence that neuraminidase caused the desialylation of both PDGF and IGF-1 receptors and diminished the intracellular signals induced by the mitogenic ligands PDGF-BB and IGF-2.

Limited inhibitory effects of oseltamivir and zanamivir on human sialidases

Oseltamivir (Tamiflu) and zanamivir (Relenza), two extensively used clinically effective anti-influenza drugs, are viral sialidase (also known as neuraminidase) inhibitors that prevent the release of progeny virions and thereby limit the spread of infection. Recently mortalities and neuropsychiatric events have been reported with the use of oseltamivir, especially in pediatric cases in Japan, suggesting that these drugs might also inhibit endogenous enzymes involved in sialic acid metabolism, including sialidase, sialyltransferase, and CMP-synthase, in addition to their inhibitory effects on the viral sialidase. The possible inhibition could account for some of the rare side effects of oseltamivir. However, there has been little direct evidence in regard to the sensitivities of animal sialidases to these drugs. Here, we examined whether these inhibitors might indeed affect the activities of human sialidases, which differ in primary structures and enzyme properties but possess tertiary structures similar to those of the viral enzymes. Using recombinant enzymes corresponding to the four human sialidases identified so far, we found that oseltamivir carboxylate scarcely affected the activities of any of the sialidases, even at 1 mM, while zanamivir significantly inhibited the human sialidases NEU3 and NEU2 in the micromolar range (K(i), 3.7 +/- 0.48 and 12.9 +/- 0.07 microM, respectively), providing a contrast to the low nanomolar concentrations at which these drugs block the activity of the viral sialidases.

Neuraminidase-1 is required for the normal assembly of elastic fibers

The assembly of elastic fibers in tissues that undergo repeated cycles of extension and recoil, such as the lungs and blood vessels, is dependent on the proper interaction and alignment of tropoelastin with a microfibrillar scaffold. Here, we describe in vivo histopathological effects of neuraminidase-1 (Neu1) deficiency on elastin assembly in the lungs and aorta of mice. These mice exhibited a tight-skin phenotype very similar to the Tsk mouse. Normal septation of Neu1-null mice did not occur in neonatal mice, resulting in enlarged alveoli that were maintained in adults. The abnormal development of elastic fibers was remarkable under electron microscopy and confirmed by the overlapping distribution of elastin, fibrillin-1, fibrillin-2, and fibulin-5 (Fib-5) by the light microscopy immunostainings. Fib-5 fibers appeared diffuse and unorganized around the alveolar walls and the apex of developing secondary septal crests. Fibrillin-2 deposition was also abnormal in neonatal and adult lungs. Dispersion of myofibroblasts appeared abnormal in developing lungs of Neu1-null mice, with a random distribution of myofibroblast around the alveolar walls, rather than concentrating at sites of elastin synthesis. The elastic lamellae in the aorta of the Neu1-null mice were thinner and separated by hypertrophic smooth muscle cells that were surrounded by an excess of the sialic acid-containing moieties. The concentration of elastin, as measure by desmosine levels, was significantly reduced in the aorta of Neu1-null mice. Message levels for tropoelastin and Fib-5 were normal, suggesting the elastic fiber defects in Neu1-null mice result from impaired extracellular assembly.

Plasma membrane-associated sialidase as a crucial regulator of transmembrane signalling

Mammalian sialidases, glycosidases responsible for the removal of sialic acids from glycoproteins and glycolipids, has been implicated to participate in many biological processes as well as in lysosomal catabolism. Among those forms identified to date, plasma membrane-associated sialidase, Neu3, is a key enzyme in degradation of gangliosides, for which it exhibits a special substrate preference. This sialidase has been shown to control transmembrane signalling for many cellular processes, including cell differentiation, cell growth and apoptosis, and human orthologue NEU3 is markedly up-regulated in various cancers. It is known to suppress apoptosis in cancer cells. Furthermore, its overexpression causes impaired glucose tolerance and hyper-insulinaemia together with overproduction of insulin in enlarged islets in the transgenic mice. The present review primarily summarizes our recent results, focusing on Neu3 as a regulator of transmembrane signalling.

Accumulation of free complex-type N-glycans in MKN7 and MKN45 stomach cancer cells

During the N-glycosylation reaction, it has been shown that ‘free’ N-glycans are generated either from lipid-linked oligosaccharides or from misfolded glycoproteins. In both cases, occurrence of high mannose-type free glycans is well-documented, and the molecular mechanism for their catabolism in the cytosol has been studied. On the other hand, little, if anything, is known with regard to the accumulation of more processed, complex-type free oligosaccharides in the cytosol of mammalian cells. During the course of comprehensive analysis of N-glycans in cancer cell membrane fractions [Naka et al. (2006) J. Proteome Res. 5, 88–97], we found that a significant amount of unusual, complex-type free N-glycans were accumulated in the stomach cancer-derived cell lines, MKN7 and MKN45. The most abundant and characteristic glycan found in these cells was determined to be NeuAcα2-6Galβ1-4GlcNAcβ1-2Manα1-3Manβ1-4GlcNAc. Biochemical analyses indicated that those glycans found were cytosolic glycans derived from lysosomes due to low integrity of the lysosomal membrane. Since the accumulation of these free N-glycans was specific to only two cell lines among the various cancer cell lines examined, these cytosolic N-glycans may serve as a specific biomarker for diagnosis of specific tumours. A cytosolic sialidase, Neu2, was shown to be involved in the degradation of these sialoglycans, indicating that the cytosol of mammalian cells might be equipped for metabolism of complex-type glycans.

Low expression of Neu2 sialidase in the thymus of SM/J mice-existence of neuraminidase positive cells "Neu-medullocyte" in the murine thymus

We have already reported that the homogenate of the A/J mouse thymus shows a high sialidase activity at the neutral pH region and that in both soluble and membrane fractions optimal pH was 6.5-7 (Kijimoto-Ochiai et al., Glycoconj. J., 20:375-384, 2004). In the present study, we investigated the level of sialidase activities in the thymus of the SM/J mouse, a mouse strain that we know to have a Neu1(a) allele that reveals a low level of sialidase activity in the liver. We found that while in the A/J thymus the soluble sialidase activity at pH 6.5 was high, the SM/J thymus lacked all such activity. A QTL analysis of SMXA recombinant inbred strains showed that soluble sialidase activity correlated well with the D1Mit8/9 marker on chromosome 1. The murine whole DNA-sequence data and the results of our FISH analysis (Kotani et al., Biochem. Biophys. Res. Comm., 286:250-258, 2001) showed that this location is consistent with the position of Neu2 gene. We confirmed that it is hard to detect the Neu2 enzyme of the SM/J mouse thymus by an anti-Neu2 antibody using a Western blot analysis. We also found that while the mRNA expression of Neu2 was quite normal in the SM/J mouse liver, it was very low in the SM/J mouse thymus. We therefore conclude that the lack of soluble sialidase activity in the SM/J mouse thymus is due to the thymus-specific low expression level of the Neu2 gene. We have previously shown that the sialidase positive cell which contains the Mac-1 and immunoglobulin, and which is located sparsely in the corticomedullar region or medullary region of the A/J mouse thymus (Kijimoto-Ochiai et al., Glycoconj. J., 20:375-384, 2004). We showed now in this paper that the detection of this cell in the SM/J mouse thymus at pH 7.0 was difficult. We propose, therefore, to name the cell "Neu-medullocyte".