ORP2 couples LDL-cholesterol transport to FAK activation by endosomal cholesterol/PI(4,5)P2 exchange.

Low-density lipoprotein (LDL)-cholesterol delivery from late endosomes to the plasma membrane regulates focal adhesion dynamics and cell migration, but the mechanisms controlling it are poorly characterized. Here, we employed auxin-inducible rapid degradation of oxysterol-binding protein-related protein 2 (ORP2/OSBPL2) to show that endogenous ORP2 mediates the transfer of LDL-derived cholesterol from late endosomes to focal adhesion kinase (FAK)-/integrin-positive recycling endosomes in human cells. In vitro, cholesterol enhances membrane association of FAK to PI(4,5)P2 -containing lipid bilayers. In cells, ORP2 stimulates FAK activation and PI(4,5)P2 generation in endomembranes, enhancing cell adhesion. Moreover, ORP2 increases PI(4,5)P2 in NPC1-containing late endosomes in a FAK-dependent manner, controlling their tubulovesicular trafficking. Together, these results provide evidence that ORP2 controls FAK activation and LDL-cholesterol plasma membrane delivery by promoting bidirectional cholesterol/PI(4,5)P2 exchange between late and recycling endosomes.

Neuraminidases 3 and 4 regulate neuronal function by catabolizing brain gangliosides.

Gangliosides (sialylated glycolipids) play an essential role in the CNS by regulating recognition and signaling in neurons. Metabolic blocks in processing and catabolism of gangliosides result in the development of severe neurologic disorders, including gangliosidoses manifesting with neurodegeneration and neuroinflammation. We demonstrate that 2 mammalian enzymes, neuraminidases 3 and 4, play important roles in catabolic processing of brain gangliosides by cleaving terminal sialic acid residues in their glycan chains. In neuraminidase 3 and 4 double-knockout mice, GM3 ganglioside is stored in microglia, vascular pericytes, and neurons, causing micro- and astrogliosis, neuroinflammation, accumulation of lipofuscin bodies, and memory loss, whereas their cortical and hippocampal neurons have lower rate of neuritogenesis in vitro Double-knockout mice also have reduced levels of GM1 ganglioside and myelin in neuronal axons. Furthermore, neuraminidase 3 deficiency drastically increased storage of GM2 in the brain tissues of an asymptomatic mouse model of Tay-Sachs disease, a severe human gangliosidosis, indicating that this enzyme is responsible for the metabolic bypass of ß-hexosaminidase A deficiency. Together, our results provide the first in vivo evidence that neuraminidases 3 and 4 have important roles in CNS function by catabolizing gangliosides and preventing their storage in lipofuscin bodies.-Pan, X., De Britto Pará De Aragão, C., Velasco-Martin, J. P., Priestman, D. A., Wu, H. Y., Takahashi, K., Yamaguchi, K., Sturiale, L., Garozzo, D., Platt, F. M., Lamarche-Vane, N., Morales, C. R., Miyagi, T., Pshezhetsky, A. V. Neuraminidases 3 and 4 regulate neuronal function by catabolizing brain gangliosides.

Sialidase NEU3 defines invasive potential of human glioblastoma cells by regulating calpain-mediated proteolysis of focal adhesion proteins.

BACKGROUND: Glioblastoma multiforme is one of the most malignant tumors of the human central nervous system characterized by high degree of invasiveness. Focusing on this invasive nature, we investigated whether ganglioside-specific sialidase NEU3 might be involved, because gangliosides are major components of brain tissues, and cell surface sialic acids, as target residues of sialidase catalysis, are thought to be closely related to cell invasion. METHODS: NEU3 mRNA levels of human glioblastoma specimens were evaluated by quantitative RT-PCR. Human glioblastoma cell lines, U251, A172, and T98G were used for cell invasion and migration by transwell and cell scratching assay. The molecules involved in the signaling cascade were investigated by western blot and immunofluorescent microscopy. RESULTS: NEU3 expression was down-regulated in human glioblastoma specimens. In the human glioblastoma cell lines, NEU3 overexpression reduced invasion and migration by promoting the assembly of focal adhesions through reduced calpain-dependent proteolysis, but NEU3 silencing resulted in accelerating cell invasion via disassembly of focal adhesions. In NEU3-silenced cells, elevation of calpain activity and GM3 accumulation were observed, as results of reduced sialidase hydrolysis, localization of calpain and GM3 at the cell lamellipodium being demonstrated by immunofluorescence microscopy. CONCLUSION: Sialidase NEU3 was found to exert a great influence on cell invasion in regulation of calpain activity and focal adhesion disassembly and consequent invasive potential of glioblastoma cells. GENERAL SIGNIFICANCE: This first demonstration of sialidase involvement in invasive potential of gliolastoma cells may point to NEU3 as an attractive treatment target of human gliomas.

Phosphatidic acid-mediated activation and translocation to the cell surface of sialidase NEU3, promoting signaling for cell migration.

The plasma membrane-associated sialidase NEU3 plays crucial roles in regulation of transmembrane signaling, and its aberrant up-regulation in various cancers contributes to malignancy. However, it remains uncertain how NEU3 is naturally activated and locates to plasma membranes, because of its Triton X-100 requirement for the sialidase activity in vitro and its often changing subcellular location. Among phospholipids examined, we demonstrate that phosphatidic acid (PA) elevates its sialidase activity 4 to 5 times at 50 µM in vitro at neutral pH and promotes translocation to the cell surface and cell migration through Ras-signaling in HeLa and COS-1 cells. NEU3 was found to interact selectively with PA as assessed by phospholipid array, liposome coprecipitation, and ELISA assays and to colocalize with phospholipase D (PLD) 1 in response to epidermal growth factor (EGF) or serum stimulation. Studies using tagged NEU3 fragments with point mutations identified PA- and calmodulin (CaM)-binding sites around the N terminus and confirmed its participation in translocation and catalytic activity. EGF induced PLD1 activation concomitantly with enhanced NEU3 translocation to the cell surface, as assessed by confocal microscopy. These results suggest that interactions of NEU3 with PA produced by PLD1 are important for regulation of transmembrane signaling, this aberrant acceleration probably promoting malignancy in cancers.

Total Synthesis of the Proposed Structure for Aromin and Its Structural Revision.

This paper describes the first total synthesis of the proposed structure for aromin, an annonaceous acetogenin possessing an unusual bis-THF ring system, and its 4S,7R-isomer. The key steps involve an oxidative cyclization of a couple of terminal-diene alcohols and an intermolecular metathesis of an alkenyl tetrahydrofuran with an enone carrying a tetrahydrofuranyl lactone. The spectral data of both samples did not match those of aromin. Re-examination of the NMR data using the CAST/CNMR Structure Elucidator and chemical derivations suggested that the real structure of aromin should be revised to be a tetrahydropyran acetogenin, montanacin D. Cytotoxicities in human solid tumor cell lines for synthetic samples were also evaluated.

Sialidase NEU3 contributes neoplastic potential on colon cancer cells as a key modulator of gangliosides by regulating Wnt signaling.

The plasma membrane-associated sialidase NEU3 is a key enzyme for ganglioside degradation. We previously demonstrated remarkable up-regulation of NEU3 in various human cancers, with augmented malignant properties. Here, we provide evidence of a close link between NEU3 expression and Wnt/ß-catenin signaling in colon cancer cells by analyzing tumorigenic potential and cancer stem-like characteristics. NEU3 silencing in HT-29 and HCT116 colon cancer cells resulted in significant decrease in clonogenicity on soft agar and in vivo tumor growth, along with down-regulation of stemness and Wnt-related genes. Analyses further revealed that NEU3 enhanced phosphorylation of the Wnt receptor LRP6 and consequently ß-catenin activation by accelerating complex formation with LRP6 and recruitment of GSK3ß and Axin, whereas its silencing exerted the opposite effects. NEU3 activity-null mutants failed to demonstrate the activation, indicating the requirement of ganglioside modulation by the sialidase for the effects. Under sphere-forming conditions, when stemness genes are up-regulated, endogenous NEU3 expression was found to be significantly increased, whereas NEU3 silencing suppressed sphere-formation and in vivo tumor incidence in NOD-SCID mice. Increased ability of clonogenicity on soft agar and sphere formation by Wnt stimulation was abrogated by NEU3 silencing. Furthermore, NEU3 was found to regulate phosphorylation of ERK and Akt via EGF receptor and Ras cascades, thought to be additionally required for tumor progression. The results indicate an essential contribution of NEU3 to tumorigenic potential through maintenance of stem-like characteristics of colon cancer cells by regulating Wnt signaling at the receptor level, in addition to tumor progression via Ras/MAPK signaling.

Upregulation of sialidase NEU3 in head and neck squamous cell carcinoma associated with lymph node metastasis.

Regional lymph node metastasis in head and neck squamous cell carcinoma (HNSCC) is a crucial event for its progression, associated with a high rate of mortality. Sialidase, a key enzyme for the regulation of cellular sialic acids through catalyzing the initial step of degradation of glycoproteins and glycolipids, has been implicated in cancer progression. To facilitate the development of novel treatments for HNSCC, we have investigated whether sialidase is involved in the progression of this cancer. We found plasma membrane-associated sialidase (NEU3) to be significantly upregulated in tumor compared to non-tumor tissues; particularly, an increase in its mRNA levels was significantly associated with lymph node metastasis. To understand the mechanisms, we analyzed the NEU3-mediated effects on the malignant phenotype using squamous carcinoma HSC-2 and SAS cells. NEU3 promoted cell motility and invasion, accompanied by the increased expression of MMP-9, whereas NEU3 silencing or the activity-null mutant did not. NEU3 enhanced phosphorylation of epidermal growth factor receptor (EGFR), and an EGFR inhibitor, AG1478, abrogated the NEU3-induced MMP9 augmentation. These findings identify NEU3 as a participant in HNSCC progression through the regulation of EGFR signaling and thus as a potential target for inhibiting EGFR-mediated tumor progression.

Increased sialidase activity in serum of cancer patients: Identification of sialidase and inhibitor activities in human serum.

Aberrant sialylation in glycoproteins and glycolipids is a characteristic feature of malignancy. Human sialidases, which catalyze the removal of sialic acid residues from glycoconjugates, have been implicated in cancer progression. They have been detected in a wide variety of human cells and tissues, but few studies have focused on their existence in human serum. Among the four types identified to date, we previously demonstrated that plasma membrane-associated ganglioside sialidase (NEU3) is markedly upregulated in various human cancers, including examples in the colon and prostate. Here, using a sensitive assay method, we found a significant increase of sialidase activity in the serum of patients with prostate cancer compared with that in healthy subjects having low activity, if any. Activity was apparent with gangliosides as substrates, but only to a very limited extent with 4-methylumbelliferyl sialic acid, a good synthetic substrate for sialidases other than human NEU3. The serum sialidase was also almost entirely immunoprecipitated with anti-NEU3 antibody, but not with antibodies for other sialidases. Interestingly, sera additionally contained inhibitory activity against the sialidase and also against recombinant human NEU3. The sialidase and inhibitor activities could be separated by exosome isolation and by hydrophobic column chromatography. The serum sialidase was assessed by a sandwich ELISA method using two anti-NEU3 antibodies. The results provide strong evidence that the serum sialidase is, in fact, NEU3, and this subtype may, therefore, be a potential utility for novel diagnosis of human cancers.

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.

N-acetylglucosamine-6-O sulfation on intestinal mucins prevents obesity and intestinal inflammation by regulating gut microbiota.

Intestinal mucins play an essential role in the defense against bacterial invasion and the maintenance of gut microbiota, which is instrumental in the regulation of host immune systems; hence, its dysregulation is a hallmark of metabolic disease and intestinal inflammation. However, the mechanism by which intestinal mucins control the gut microbiota as well as disease phenotypes remains nebulous. Herein, we report that N-acetylglucosamine (GlcNAc)-6-O sulfation of O-glycans on intestinal mucins performs a protective role against obesity and intestinal inflammation. Chst4-/- mice, lacking GlcNAc-6-O sulfation of the mucin O-glycans, showed significant weight gain and increased susceptibility to dextran sodium sulfate-induced colitis as well as colitis-associated cancer accompanied by significantly reduced immunoglobulin A (IgA) production caused by an impaired T follicular helper cell-mediated IgA response. Interestingly, the protective effects of GlcNAc-6-O sulfation against obesity and intestinal inflammation depend on the gut microbiota, evidenced by the modulation of the gut microbiota by cohousing or microbiota transplantation reversing disease phenotypes and IgA production. Collectively, our findings provide insight into the significance of host glycosylation, more specifically GlcNAc-6-O sulfation on intestinal mucins, in protecting against obesity and intestinal inflammation via regulation of the gut microbiota.

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.

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.

Identification of Crucial Amino Acid Residues for Antimicrobial Activity of Angiogenin 4 and Its Modulation of Gut Microbiota in Mice.

Angiogenin 4 bearing ribonuclease activity is an endogenous antimicrobial protein expressed in small and large intestine. However, the crucial amino acid residues responsible for the antibacterial activity of Ang4 and its impact on gut microbiota remain unknown. Here, we report the contribution of critical amino acid residues in the functional regions of Ang4 to its activity against Salmonella typhimurium LT2 and the effect of Ang4 on gut microbiota in mice. We found that Ang4 binds S. typhimurium LT2 through two consecutive basic amino acid residues, K58 and K59, in the cell-binding segment and disrupts the bacterial membrane integrity at the N-terminal α-helix containing residues K7 and K30, as evidenced by the specific mutations of cationic residues of Ang4. We also found that the RNase activity of Ang4 was not involved in its bactericidal activity, as shown by the H12 mutant, which lacks RNase activity. In vivo administration of Ang4 through the mouse rectum and subsequent bacterial 16S rRNA gene sequencing analyses demonstrated that administration of Ang4 not only increased beneficial bacteria such as Lactobacillus, Akkermansia, Dubosiella, Coriobacteriaceae UCG-002, and Adlercreutzia, but also decreased certain pathogenic bacteria, including Alistipes and Enterohabdus, indicating that Ang4 regulates the shape of gut microbiota composition. We conclude that Ang4 kills bacteria by disrupting bacterial membrane integrity through critical basic amino acid residues with different functionalities rather than overall electrostatic interactions and potentially maintains gut microflora in vivo under physiological and pathophysiological conditions.

[A randomized controlled study comparing clinical effects of naftopidil and tamsulosin on benign prostatic hyperplasia].

A prospective randomized controlled study was performed to compare the clinical effects of naftopidil and tamsulosin. Men complaining of lower urinary tract symptoms due to benign prostatic hyperplasia were randomized into two groups : one receiving 50mg naftopidil once daily (Naf group, n=36 patients), and the other receiving 0.2 mg tamsulosin once daily (Tam group, n=32 patients). In the Naf group at 12 weeks, 7 items of the International Prostate Symptom Score (IPSS), storage and voiding symptoms, total IPSS, quality of life (QOL) index (QOLI) and Qmax were improved significantly. In the Tam group at 12 weeks, 6 items of IPSS except urgency, storage and voiding symptoms, total IPSS, QOLI and Qmax were improved significantly. Improvement of residual urine volume (PVR) was insignificant in both groups. In intergroup comparison between the Naf and the Tam groups, variations of 7 items of IPSS, storage and voiding symptoms, total IPSS, QOLI, Qmax and PVR at 4 and 12 weeks after treatment were not statistically significant. There was almost no difference in clinical efficacy between Naf and Tam.

Differential regulation of repeated histone genes during the fission yeast cell cycle.

The histone genes are highly reiterated in a wide range of eukaryotic genomes. The fission yeast, Schizosaccharomyces pombe, has three pairs of histone H3-H4 genes: hht1+-hhf1+, hht2+-hhf2+ and hht3+-hhf3+. While the deduced amino acid sequences are identical, it remains unknown whether transcriptional regulation differs among the three pairs. Here, we report the transcriptional properties of each H3-H4 gene pair during the cell cycle. The levels of transcripts of hht1+-hhf1+ and hht3+-hhf3+ pairs and hhf2+ are increased at S-phase, while that of hht2+ remains constant throughout the cell cycle. We showed that the GATA-type transcription factor, Ams2, binds to the promoter regions of core histone genes in an AACCCT-box-dependent manner and is required for activation of S-phase-specific transcription. Furthermore, we found that Ams2-depletion stimulates feedback regulation of histone transcripts, mainly up-regulating the basal levels of hht2+-hhf2+ transcription, which are normally down-regulated by Hip1 and Slm9, homologs of the human histone chaperone, HIRA. These observations provide insight into the molecular mechanisms of differential regulation of transcripts from repeated histone genes in the fission yeast.

ORP2 interacts with phosphoinositides and controls the subcellular distribution of cholesterol.

ORP2 is a sterol-binding protein with documented functions in lipid and glucose metabolism, Akt signaling, steroidogenesis, cell adhesion, migration and proliferation. Here we investigate the interactions of ORP2 with phosphoinositides (PIPs) by surface plasmon resonance (SPR), its affinity for cholesterol with a pull-down assay, and its capacity to transfer sterol in vitro. Moreover, we determine the effects of wild-type (wt) ORP2 and a mutant with attenuated PIP binding, ORP2(mHHK), on the subcellular distribution of cholesterol, and analyze the interaction of ORP2 with the related cholesterol transporter ORP1L. ORP2 showed specific affinity for PI(4,5)P2, PI(3,4,5)P3 and PI(4)P, with suggestive Kd values in the µM range. Also binding of cholesterol by ORP2 was detectable, but a Kd could not be determined. Wt ORP2 was in HeLa cells mainly detected in the cytosol, ER, late endosomes, and occasionally on lipid droplets (LDs), while ORP2(mHHK) displayed an enhanced LD localization. Overexpression of wt ORP2 shifted the D4H cholesterol probe away from endosomes, while ORP2(mHHK) caused endosomal accumulation of the probe. Although ORP2 failed to transfer dehydroergosterol in an in vitro assay where OSBP is active, its knock-down resulted in the accumulation of cholesterol in late endocytic compartments, as detected by both D4H and filipin probes. Interestingly, ORP2 was shown to interact and partially co-localize on late endosomes with ORP1L, a cholesterol transporter/sensor at ER-late endosome junctions. Our data demonstrates that ORP2 binds several phosphoinositides, both PI(4)P and multiply phosphorylated species. ORP2 regulates the subcellular distribution of cholesterol dependent on its PIP-binding capacity. The interaction of ORP2 with ORP1L suggests a concerted action of the two ORPs.

Spindle checkpoint signaling requires the mis6 kinetochore subcomplex, which interacts with mad2 and mitotic spindles.

The spindle checkpoint coordinates cell cycle progression and chromosome segregation by inhibiting anaphase promoting complex/cyclosome until all kinetochores interact with the spindle properly. During early mitosis, the spindle checkpoint proteins, such as Mad2 and Bub1, accumulate at kinetochores that do not associate with the spindle. Here, we assess the requirement of various kinetochore components for the accumulation of Mad2 and Bub1 on the kinetochore in fission yeast and show that the necessity of the Mis6-complex and the Nuf2-complex is an evolutionarily conserved feature in the loading of Mad2 onto the kinetochore. Furthermore, we demonstrated that Nuf2 is required for maintaining the Mis6-complex on the kinetochore during mitosis. The Mis6-complex physically interacts with Mad2 under the condition that the Mad2-dependent checkpoint is activated. Ectopically expressed N-terminal fragments of Mis6 localize along the mitotic spindle, highlighting the potential binding ability of Mis6 not only to the centromeric chromatin but also to the spindle microtubules. We propose that the Mis6-complex, in collaboration with the Nuf2-complex, monitors the spindle-kinetochore attachment state and acts as a platform for Mad2 to accumulate at unattached kinetochores.

BAF53/Arp4 homolog Alp5 in fission yeast is required for histone H4 acetylation, kinetochore-spindle attachment, and gene silencing at centromere.

Nuclear actin-related proteins play vital roles in transcriptional regulation; however, their biological roles remain elusive. Here, we characterize Alp5, fission yeast homolog of Arp4/BAF53. The temperature-sensitive mutant alp5-1134 contains a single amino acid substitution in the conserved C-terminal domain (S402N) and displays mitotic phenotypes, including chromosome condensation and missegregation. Alp5 forms a complex with Mst1-HAT (histone acetyltransferase). Consistently, inhibition of histone deacetylases (HDACs), by either addition of a specific inhibitor or a mutation in HDAC-encoding clr6+ gene, rescues alp5-1134. Immunoblotting with specific antibodies against acetylated histones shows that Alp5 is required for histone H4 acetylation at lysines 5, 8, and 12, but not histone H3 lysines 9 or 14, and furthermore Clr6 plays an opposing role. Mitotic arrest is ascribable to activation of the Mad2/Bub1 spindle checkpoint, in which both proteins localize to the mitotic kinetochores in alp5-1134. Intriguingly, alp5-1134 displays transcriptional desilencing at the core centromere without altering the overall chromatin structure, which also is suppressed by a simultaneous mutation in clr6+. This result shows that Alp5 is essential for histone H4 acetylation, and its crucial role lies in the establishment of bipolar attachment of the kinetochore to the spindle and transcriptional silencing at the centromere.

Biological and Pathological Roles of Ganglioside Sialidases.

Sialidases are glycosidases responsible for the removal of α-glycosidically linked sialic acid residues from carbohydrate portions of glycoproteins and glycolipids, this process being the initial step in the degradation of such glycoconjugates. Sialic acids are considered to play important roles in various biological processes largely in two ways, one related to their hydrophilic and acidic properties exerting physicochemical effects on the glycoconjugates to which they are attached, and the other as recognition sites or in an opposing fashion as masking sites. The removal of sialic acids catalyzed by a sialidase, therefore greatly influences many biological processes through changing the conformation of glycoproteins and through recognition and masking of biological sites of functional molecules. Sialidases are found widely distributed in metazoan animals, from echinoderms to mammals, and are also present in viruses and other microorganisms, including fungi, protozoa, and bacteria even mostly lacking sialic acids. In mammals, there are four forms of sialidase (Neu1, Neu2, Neu3, and Neu4), differing in their major subcellular localization and enzymatic properties. They have been implicated in regulation of various cellular activities, such as cell differentiation, cell growth, and cell adhesion and motility, depending on their particular properties. In contrast, in microorganisms the enzymes appear to play roles limited to nutrition and pathogenesis. In this chapter, the focus is on mammalian sialidases preferentially hydrolyzing gangliosides, mostly Neu3 and Neu4, with an attempt to provide a brief overview of their physiological and pathological roles.