Cytosolic sialidase Neu2 upregulation during PC12 cells differentiation

Neuraminidases-Key Players in the Inflammatory Response after Pathophysiological Cardiac Stress and Potential New Therapeutic Targets in Cardiac Disease

Glycoproteins and glycolipids on the cell surfaces of vertebrates and higher invertebrates contain α-keto acid sugars called sialic acids, terminally attached to their glycan structures. The actual level of sialylation, regulated through enzymatic removal of the latter ones by NEU enzymes, highly affects protein-protein, cell-matrix and cell-cell interactions. Thus, their regulatory features affect a large number of different cell types, including those of the immune system. Research regarding NEUs within heart and vessels provides new insights of their involvement in the development of cardiovascular pathologies and identifies mechanisms on how inhibiting NEU enzymes can have a beneficial effect on cardiac remodelling and on a number of different cardiac diseases including CMs and atherosclerosis. In this regard, a multitude of clinical studies demonstrated the potential of N-acetylneuraminic acid (Neu5Ac) to serve as a biomarker following cardiac diseases. Anti-influenza drugs i.e., zanamivir and oseltamivir are viral NEU inhibitors, thus, they block the enzymatic activity of NEUs. When considering the improvement in cardiac function in several different cardiac disease animal models, which results from NEU reduction, the inhibition of NEU enzymes provides a new potential therapeutic treatment strategy to treat cardiac inflammatory pathologies, and thus, administrate cardioprotection.

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.

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.

Establishment and characterization of Neu1-knockout zebrafish and its abnormal clinical phenotypes

Mammalian sialidase Neu1 is involved in various physiological functions, including cell adhesion, differentiation, cancer metastasis, and diabetes through lysosomal catabolism and desialylation of glycoproteins at the plasma membrane. Various animal models have been established to further explore the functions of vertebrate Neu1. The present study focused on zebrafish (Danio rerio) belonging to Cypriniformes as an experimental animal model with neu1 gene deficiency. The results revealed that the zebrafish Neu1 desialyzed both α2-3 and α2-6 sialic acid linkages from oligosaccharides and glycoproteins at pH 4.5, and it is highly conserved with other fish species and mammalian Neu1. Furthermore, Neu1-knockout zebrafish (Neu1-KO) was established through CRISPR/Cas9 genome editing. Neu1-KO fish exhibited slight abnormal embryogenesis with the accumulation of pleural effusion; however, no embryonic lethality was observed. Although Neu1-KO fish were able to be maintained as homozygous, they showed smaller body length and weight than the wild-type (WT) fish, and muscle atrophy and curvature of the vertebra were observed in adult Neu1-KO fish (8 months). The expression patterns of myod and myog transcription factors regulating muscle differentiation varied between Neu1-KO and WT fish embryo. Expression of lysosomal-related genes, including ctsa, lamp1a, and tfeb were up-regulated in adult Neu1-KO muscle as compared with WT. Furthermore, the expression pattern of genes involved in bone remodeling (runx2a, runx2b, and mmp9) was decreased in Neu1-KO fish. These phenotypes were quite similar to those of Neu1-KO mice and human sialidosis patients, indicating the effectiveness of the established Neu1-KO zebrafish for the study of vertebrate Neu1 sialidase.

Desialylation of Sonic-Hedgehog by Neu2 Inhibits Its Association with Patched1 Reducing Stemness-Like Properties in Pancreatic Cancer Sphere-forming Cells

Cancer stem cells (CSCs) are crucial regulators of tumor recurrence/progression. The maintenance of CSCs is dependent on aberrant activation of various pathways, including Hedgehog. Prevalent sialylations contribute to aggressiveness in CSCs. Here, we have addressed the role of sialylation in regulating stemness-like properties of pancreatic cancer sphere-forming cells (PCS) through modulation of the Hedgehog (Hh) pathway. The status of CD133/CD44/surface-sialylation was checked by flow cytometry and effects of Neu2 overexpression in PCS were compared using qPCR, immunoblotting, co-immunoprecipitation and also by colony-formation assays. The work was also validated in a xenograft model after Neu2 overexpression. Neu2 and Shh status in patient tissues were examined by immunohistochemistry. PCS showed higher Hh-pathway activity and sialylation with reduced cytosolic-sialidase (Neu2). Neu2 overexpression caused desialylation of Shh, thereby reducing Shh-Patched1 binding thus causing decreased Hh-pathway activity with lower expression of Snail/Slug/CyclinD1 leading to reduction of stemness-like properties. Neu2-overexpression also induced apoptosis in PCS. Additionally, Neu2-overexpressed PCS demonstrated lower mTORC2 formation and inhibitory-phosphorylation of Gsk3β, reflecting a close relationship with reduced Hh pathway. Moreover, both Neu2 and Rictor (a major component of mTORC2) co-transfection reduced stem cell markers and Hh-pathway activity in PCS. Neu2-overexpressed tumors showed reduction in tumor mass with downregulation of stem cell markers/Shh/mTOR and upregulation of Bax/Caspase8/Caspase3. Thus, we established that reduced sialylation by Neu2 overexpression leads to decreased stemness-like properties by desialylation of Shh, which impaired its association with Patched1 thereby inhibiting the Hh pathway. All these may be responsible for enhanced apoptosis in Neu2-overexpressed PCS.

Association of cytosolic sialidase Neu2 with plasma membrane enhances Fas-mediated apoptosis by impairing PI3K-Akt/mTOR-mediated pathway in pancreatic cancer cells

Modulation of sialylation by sialyltransferases and sialidases plays essential role in carcinogenesis. There are few reports on sialyltransferase, however, the contribution of cytosolic sialidase (Neu2) remains unexplored in pancreatic ductal adenocarcinoma (PDAC). We observed lower expression of Neu2 in different PDAC cells, patient tissues, and a significant strong association with clinicopathological characteristics. Neu2 overexpression guided drug-resistant MIAPaCa2 and AsPC1 cells toward apoptosis as evidenced by decreased Bcl2/Bax ratio, activation of caspase-3/caspase-6/caspase-8, PARP reduction, reduced CDK2/CDK4/CDK6, and cyclin-B1/cyclin-E with unaffected caspase-9. Neu2-overexpressed cells exhibited higher expression of Fas/CD95-death receptor, FasL, FADD, and Bid cleavage confirming extrinsic pathway-mediated apoptosis. α2,6-linked sialylation of Fas helps cancer cells to survive, which is a substrate for Neu2. Therefore, their removal should enhance Fas-mediated apoptosis. Neu2-overexpressed cells indeed showed increased enzyme activity even on membrane. Interestingly, this membrane-bound Neu2 exhibited enhanced association with Fas causing its desialylation and activation as corroborated by decreased association of Fas with α2,6-sialic acid-binding lectin. Additionally, enhanced cytosolic Neu2 inhibited the expression of several growth factor-mediated signaling molecules involved in PI3K/Akt–mTOR pathway probably through desialylation which in turn also causes Fas activation. Furthermore, Neu2-overexpressed cells exhibited reduced cell migration, invasion with decreased VEGF, VEGFR, and MMP9 levels. To the best of our knowledge, this is the first report of cytosolic Neu2 on membrane, its association with Fas, enhanced desialylation, activation, and Fas-mediated apoptosis. Taken together, our study ascertains a novel concept by which the function of Fas/CD95 could be modulated indicating a critical role of upstream Neu2 as a promising target for inducing apoptosis in pancreatic cancer.

Exploring the link between ceramide and ionizing radiation

The aim of radiotherapy is to eradicate cancer cells with ionizing radiation; tumor cell death following irradiation can be induced by several signaling pathways, most of which are triggered as a consequence of DNA damage, the primary and major relevant cell response to radiation. Several lines of evidence demonstrated that ceramide, a crucial sensor and/or effector of different signalling pathways promoting cell cycle arrest, death and differentiation, is directly involved in the molecular mechanisms underlying cellular response to irradiation. Most of the studies strongly support a direct relationship between ceramide accumulation and radiation-induced cell death, mainly apoptosis; for this reason, defining the contribution of the multiple metabolic pathways leading to ceramide formation and the causes of its dysregulated metabolism represent the main goal in order to elucidate the ceramide-mediated signaling in radiotherapy. In this review, we summarize the current knowledge concerning the different routes leading to ceramide accumulation in radiation-induced cell response with particular regard to the role of the enzymes involved in both ceramide neogenesis and catabolism. Emphasis is placed on sphingolipid breakdown as mechanism of ceramide generation activated following cell irradiation; the functional relevance of this pathway, and the role of glycosphingolipid glycohydrolases as direct targets of ionizing radiation are also discussed. These new findings add a further attractive point of investigation to better define the complex interplay between sphingolipid metabolism and radiation therapy.

Molecular cloning and biochemical characterization of medaka (Oryzias latipes) lysosomal neu4 sialidase

Glycoconjugates are known to be involved in many physiological events in vertebrates. Sialidase is one of the glycosidases, which removes sialic acid from glycoconjugates. In mammals, the properties and physiological functions of sialidases have been investigated, while there is little understanding of fish sialidase. Here, to investigate the significance of fish neu4 sialidase, neu4 gene was cloned from medaka brain mRNA and identified. Sialidase-specific motifs (GPG, YRVP and Asp-Box) were well conserved in the medaka neu4 polypeptide. Optimal pH of medaka neu4 sialidase was 4.6, but its activity was sustained even at neutral and weak alkaline pH. The neu4 considerably cleaved sialic acid from 4-methylumbelliferyl-N-acetyl-α-D-neuraminic acid and sialyllactose, but not from ganglioside and fetuin, which are good substrates for human NEU4. neu4 activity was mostly detected in mitochondria/lysosome fraction after biochemical fractionation, and indirect immunofluorescence assays revealed neu4 localization in lysosome in neu4 overexpressed cells. Next, developmental change in medaka neu4 and other sialidase mRNA levels were estimated by real-time PCR. Each sialidases showed different expression patterns in embryonic development: neu4 was up-regulated at late developmental stage in embryo, and neu3a mRNA level was quite high in 0.5 dpf. On the other hand, neu3b expression was drastically increased after hatching, suggesting that each sialidase may play a different role in embryonic development.

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.

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.

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.

An investigation of intracellular glycosylation activities in CHO cells: effects of nucleotide sugar precursor feeding

Controlling glycosylation of recombinant proteins produced by CHO cells is highly desired as it can be directed towards maintaining or increasing product quality. To further our understanding of the different factors influencing glycosylation, a glycosylation sub-array of 79 genes and a capillary electrophoresis method which simultaneously analyzes 12 nucleotides and 7 nucleotide sugars; were used to generate intracellular N-glycosylation profiles. Specifically, the effects of nucleotide sugar precursor feeding on intracellular glycosylation activities were analyzed in CHO cells producing recombinant human interferon-gamma (IFN-gamma). Galactose (+/-uridine), glucosamine (+/-uridine), and N-acetylmannosamine (ManNAc) (+/-cytidine) feeding resulted in 12%, 28%, and 32% increase in IFN-gamma sialylation as compared to the untreated control cultures. This could be directly attributed to increases in nucleotide sugar substrates, UDP-Hex ( approximately 20-fold), UDP-HexNAc (6- to 15-fold) and CMP-sialic acid (30- to 120-fold), respectively. Up-regulation of B4gal and St3gal could also have enhanced glycan addition onto the proteins, leading to more complete glycosylation (sialylation). Combined feeding of glucosamine + uridine and ManNAc + cytidine increased UDP-HexNAc and CMP-sialic acid by another two- to fourfold as compared to feeding sugar precursors alone. However, it did not lead to a synergistic increase in IFN-gamma sialylation. Other factors such as glycosyltransferase or glycan substrate levels could have become limiting. In addition, uridine feeding increased the levels of uridine- and cytidine-activated nucleotide sugars simultaneously, which could imply that uridine is one of the limiting substrates for nucleotide sugar synthesis in the study. Hence, the characterization of intracellular glycosylation activities has increased our understanding of how nucleotide sugar precursor feeding influence glycosylation of recombinant proteins produced in CHO cells. It has also led to the optimization of more effective strategies for manipulating glycan quality.

Enhancing glycoprotein sialylation by targeted gene silencing in mammalian cells

Recombinant glycoproteins produced by mammalian cells represent an important category of therapeutic pharmaceuticals used in human health care. Of the numerous sugars moieties found in glycoproteins, the terminal sialic acid is considered particularly important. Sialic acid has been found to influence the solubility, thermal stability, resistance to protease attack, antigenicity, and specific activity of various glycoproteins. In mammalian cells, it is often desirable to maximize the final sialic acid content of a glycoprotein to ensure its quality and consistency as an effective pharmaceutical. In this study, CHO cells overexpressing recombinant human interferon gamma (hIFNgamma) were treated using short interfering RNA (siRNA) and short-hairpin RNA (shRNA) to reduce expression of two newly identified sialidase genes, Neu1 and Neu3. By knocking down expression of Neu3 we achieved a 98% reduction in sialidase function in CHO cells. The recombinant hIFNgamma was examined for sialic acid content that was found to be increased 33% and 26% respectively with samples from cell stationary phase and death phase as compared to control. Here, we demonstrate an effective targeted gene silencing strategy to enhance protein sialylation using RNA interference (RNAi) technology.

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.

Glycobiology in the cytosol: the bitter side of a sweet world

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of "glycobiology in the cytosol" which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.

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.

Dependence of neurotrophic factor activation of Trk tyrosine kinase receptors on cellular sialidase

A direct link between receptor glycosylation and activation following natural ligand interaction has not been observed. Here, we discover a membrane sialidase-controlling mechanism that depends on ligand binding to its receptor to induce enzyme activity which targets and desialylates the receptor and, consequently, causes the induction of receptor dimerization and activation. We also identify a specific sialyl alpha-2,3-linked beta-galactosyl sugar residue of TrkA tyrosine kinase receptor, which is rapidly targeted and hydrolyzed by the sialidase. Trk-expressing cells and primary cortical neurons following stimulation with specific neurotrophic growth factors express a vigorous membrane sialidase activity. Neuraminidase inhibitors, Tamiflu, BCX1812, and BCX1827, block sialidase activity induced by nerve growth factor (NGF) in TrkA-PC12 cells and by brain-derived neurotrophic factor (BDNF) in primary cortical neurons. In contrast, the neuraminidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid, specific for plasma membrane ganglioside Neu3 and Neu2 sialidases has no inhibitory effect on NGF-induced pTrkA. The GM1 ganglioside specific cholera toxin subunit B applied to TrkA-PC12 cells has no inhibitory effect on NGF-induced sialidase activity. Neurite outgrowths induced by NGF-treated TrkA-PC12 and BDNF-treated PC12(nnr5) stably transfected with TrkB receptors (TrkB-nnr5) cells are significantly inhibited by Tamiflu. Our results establish a novel mode of regulation of receptor activation by its natural ligand and define a new function for cellular sialidases.