Direct interaction of GD3 ganglioside with mitochondria generates reactive oxygen species followed by mitochondrial permeability transition, cytochrome c release, and caspase activation.

Glycosphingolipids, including gangliosides, are emerging as signaling intermediates of extracellular stimuli. Because mitochondria play a key role in the orchestration of death signals, we assessed the interaction of GD3 ganglioside (GD3) with mitochondria and the subsequent cascade of events that culminate in cell death. In vitro studies with isolated mitochondria from rat liver demonstrate that GD3 elicited a burst of peroxide production within 15-30 min, which preceded the opening of the mitochondrial permeability transition, followed by cytochrome c (cyt c) release. These effects were mimicked by lactosylceramide and N-acetyl-sphingosine but not by sphinganine or sphingosine and were prevented by cyclosporin A and butylated hydroxytoluene (BHT). Reconstitution of mitochondria pre-exposed to GD3 with cytosol from rat liver in a cell-free system resulted in the proteolytic processing of procaspase 3 and subsequent caspase 3 activation. Intact hepatocytes or U937 cells selectively depleted of glutathione in mitochondria by 3-hydroxyl-4-pentenoate (HP) with the sparing of cytosol reduced glutathione (GSH) were sensitized to GD3, manifested as an apoptotic death. Inhibition of caspase 3 prevented the apoptotic phenotype of HP-treated cells caused by GD3 without affecting cell survival; in contrast, BHT fully protected HP-treated cells to GD3 treatment. Treatment of cells with tumor necrosis factor increased the level of GD3, whereas blockers of mitochondrial respiration at complex I and II protected sensitized cells to GD3 treatment. Thus, the effect of GD3 as a lipid death effector is determined by its interaction with mitochondria leading to oxidant-dependent caspase activation. Mitochondrial glutathione plays a key role in controlling cell survival through modulation of the oxidative stress induced by glycosphingolipids.     

Effects of disialoganglioside GD3 on the mitochondrial membrane potential

GD3 is an intracellular mediator of apoptotic signaling. Although GD3 is known to directly act on mitochondria, the dynamic responses of individual mitochondria to GD3 remain to be elucidated. In the current study, the membrane potential of single mitochondria is observed in the presence of GD3 or its analogues. Here, we report that (1) GD3 specifically induces gradual depolarizations of the inner membrane by a mechanism that differs from the permeability transition, and (2) the GD3-induced depolarizations are suppressed by cyclosporin A. These results suggest that GD3 depolarizes mitochondria by a mechanism distinct from but relevant to the permeability transition.     

Three dimensional structure of GD1b and GD1b-monolactone gangliosides in dimethylsulphoxide: a nuclear Overhauser effect investigation supported by molecular dynamics calculations.

A comparative study on the conformational features of the oligosaccharide moiety of GD1b and GD1b lactone gangliosides, in dimethylsulphoxide, has been carried out by nuclear Overhauser effect investigation; the experimental interresidue contacts have been used for restrained molecular mechanics and dynamics calculations. For GD1b, the tetrasaccharide beta-GalNAc-(1----4)-[alpha-Neu5Ac-(2 ----8)-alpha-Neu5Ac-(2----3)]-beta-Gal has a circular arrangement leaving a highly hydrophobic region with seven hydrogens pointing towards the center. At one side of this region the three electron rich groups GalNAc--NH, external Neu5Ac--OH4 and internal Neu5Ac--COO- are grouped together; at the other side five polar groups (four hydroxy groups and the external Neu5Ac carboxylate) define a large annular hydrophilic region. The external Neu5Ac is close to the external Gal residue, and the external Neu5Ac--COO- is within van der Waals contact with the inner Neu5Ac-OH9 group. The beta-Gal-(1----3)-beta-GalNAc glycosidic linkage shows a high degree of freedom. For GD1b-L, the trisaccharide beta-GalNAc-(1----4)-[alpha-Neu5Ac-(2----3)]-beta-Gal is disposed to forming rigid partially circular arrangement showing strong interresidue contacts between the inner Neu5Ac-H8 and both GalNAc-H1 and GalNAc-H5. The conformation of the lactone ring is the boat 9(A),2(B)B. The lactonization of the disialosyl residue induces a strong variation of the preexisting torsional glycosidic angles phi and psi, leaving the external Neu5Ac far from the external Gal. In both GD1b and GD1b lactone gangliosides, the conformation of the sialic acid side chain is the same as that of the free sialic acid in which the H7 is trans to H8 and gauche to H6, thus indicating that the presence of glycosidic and/or ester linkages does not affect the conformational properties of sialic acid. Both GD1b and GD1b lactone containing sialic acid carboxylate anion(s) or undissociated carboxyl group(s) show the same three dimensional structure, indicating that the presence of charges does not affect the intrinsic conformational features of gangliosides.     

GD3 ganglioside directly targets mitochondria in a bcl-2-controlled fashion.

Lipid and glycolipid diffusible mediators are involved in the intracellular progression and amplification of apoptotic signals. GD3 ganglioside is rapidly synthesized from accumulated ceramide after the clustering of death-inducing receptors and triggers apoptosis. Here we show that GD3 induces dissipation of DeltaPsim and swelling of isolated mitochondria, which results in the mitochondrial release of cytochrome c, apoptosis inducing factor, and caspase 9. Soluble factors released from GD3-treated mitochondria are sufficient to trigger DNA fragmentation in isolated nuclei. All these effects can be blocked by cyclosporin A, suggesting that GD3 is acting at the level of the permeability transition pore complex. We found that endogenous GD3 accumulates within mitochondria of cells undergoing apoptosis after ceramide exposure. Accordingly, suppression of GD3 synthase (ST8) expression in intact cells substantially prevents ceramide-induced DeltaPsim dissipation, indicating that endogenously synthesized GD3 induces mitochondrial changes in vivo. Finally, enforced expression of bcl-2 significantly prevents GD3-induced mitochondrial changes, caspase 9 activation, and apoptosis. These results show that mitochondria are a key destination for apoptogenic GD3 ganglioside along the lipid pathway to programmed cell death and indicate that relevant GD3 targets are under bcl-2 control.     

Molecular characterization of membrane type and ganglioside-specific sialidase (Neu3) expressed in E. coli

Endogenous expression of human membrane type ganglioside sialidase (Neu3) was examined in various cell lines including NB-1, U87MG, SK-MEL-2, SK-N-MC, HepG2, Hep3B, Jurkat, HL-60, K562, ECV304, Hela and MCF-7. Expression was detected in the neuroblastoma cell lines NB-1 and SK-N-MC, and also in erythroleukemia K562 cells, but not in any other cells. We isolated a Neu3 cDNA from K562 cells and expressed a His-tagged derivative in a bacterial expression system. The purified recombinant product of approximately 48 kDa had sialidase activity toward 4-methyl-umbelliferyl-alpha-D-N-acetylneuraminic acid (4MU-NeuAc). The optimal pH of the purified Neu3 protein for GD3 ganglioside was 4.5. The enzyme also efficiently hydrolyzed GD3, GD1a, GD1b and GM3 whereas sialyllactose, 4MU-NeuAc, GM1 and GM2 were poor substrates, and it had no activity against sialylated glycoproteins such as fetuin, transferrin and orosomucoid. We conclude that the sialidase activity of Neu3 is specific for gangliosides.     

Ganglioside GD3 sensitizes human hepatoma cells to cancer therapy

Ganglioside GD3 (GD3) has emerged as a modulator of cell death pathways due to its ability to interact with mitochondria and disable survival pathways. Because NF-kappaB activation contributes to cancer therapy resistance, this study was undertaken to test whether GD3 modulates the response of human hepatoblastoma HepG2 cells to radio- and chemotherapy. NF-kappaB was activated in HepG2 cells shortly after therapeutic doses of ionizing radiation or daunorubicin treatment that translated into up-regulation of kappaB-dependent genes. These effects were accompanied by minimal killing of HepG2 cells by either ionizing radiation or daunorubicin. However, GD3 pretreatment blocked the nuclear translocation of active kappaB members, without effect on Akt phosphorylation, induced by either treatment. The suppression of kappaB-dependent gene induction by GD3 was accompanied by enhanced apoptotic cell death caused by these therapies. Furthermore, the combination of GD3 plus ionizing radiation stimulated the formation of reactive species followed by the mitochondrial release of cytochrome c and Smac/Diablo and caspase 3 activation. Pretreatment with cyclosporin A before radiotherapy protected HepG2 cells from the therapeutic combination of GD3 plus ionizing radiation. These findings underscore a key role of mitochondria in the response of tumor cells to cancer therapy and highlight the potential relevance of GD3 to overcome resistance to cancer therapy by combining its dual action as a mitochondria-interacting and NF-kappaB-inactivating agent.     

Trafficking of ganglioside GD3 to mitochondria by tumor necrosis factor-alpha

The interaction of mitochondria with proapoptotic proteins activates apoptosis pathways. Previous findings have identified ganglioside GD3 (GD3) as an emerging apoptotic lipid intermediate that targets mitochondria in response to death signals. Using immunoelectron and laser scanning confocal microscopy, we characterize the trafficking of GD3 to mitochondria in response to tumor necrosis factor-alpha (TNF-alpha) in rat hepatocytes. In control hepatocytes, GD3 is present predominantly at the plasma membrane as well as in the endosomal/Golgi network, as verified by its colocalization with the asialoglycoprotein receptor. Following TNF-alpha exposure, GD3 undergoes a rapid cellular redistribution with a gradual loss from the plasma membrane before its colocalization with mitochondria. This process is mimicked by acidic sphingomyelinase and ionizing radiation but not by neutral sphingomyelinase or staurosporin. TNF-alpha stimulated the colocalization of GD3 with early and late endosomal markers, Rab 5 and Rab 7, whereas perturbation of plasma membrane cholesterol or actin cytoskeleton or inhibition of glucosylceramide synthase prevented the trafficking of GD3 to mitochondria. Finally, prevention of the TNF-alpha-stimulated neosynthesis of GD3, cyclosporin A, and latrunculin A or filipin protected sensitized hepatocytes from TNF-alpha-mediated cell death. Thus, the intracellular redistribution and mitochondrial targeting of GD3 during TNF-alpha signaling occurs through actin cytoskeleton vesicular trafficking and contributes to TNF-alpha-mediated hepatocellular cell death.     

Production and molecular characterization of clinical phase i anti-melanoma mouse IgG3 monoclonal antibody R24

R24 is a mouse IgG3 monoclonal antibody (mab) that reacts with the ganglioside GD3 expressed by cells of neuroectodermal origin. The anti-tumor activity of R24 has been demonstrated in initial phase I and pilot trials in patients suffering from metastatic melanoma. The purpose of this study was to investigate the biotechnological production and particularly the glycosylation of this clinically important antibody. Growth, metabolism, and IgG production of R24 secreting hybridoma cells were analyzed on 1 L bioreactor bench scale using repeated-batch mode. The amount of 57 mg of pure mab was obtained from 1.6 L crude supernatant by protein A chromatography. Western blot binding assays with sugar-specific lectins revealed glycosylation of the heavy chains, whereas no carbohydrates were detectable on the light chains. Because glycosylation is essential for antibody effector functions in vivo (such as complement fixation or binding to macrophage Fc receptors), mab R24 was subjected to both enzymatic deglycosylation using PNGase F and chemical deglycosylation by hydrazinolysis. Released glycans were structurally characterized by high pH anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD), matrix assisted laser desorption ionization time-of-flight (MALDI-TOF), and electrospray ionization quadrupole time-of-flight (ESI-QTOF) mass spectrometry. Six major biantennary chains of the complex glycosylation phenotype were found with variations in galactosylation and core fucosylation. The predominant N-linked structure, indicating the high degree of agalactosyl glycoforms, was the agalacto biantennary chain with a relative percentage of 57% (51% core-fucosylated, 6% nonfucosylated). The second most abundant oligosaccharide was the monogalacto biantennary chain amounting to 30% (26% core- and 4% nonfucosylated). The antibody contained 0.46 microg sialic acid per mg protein, which splits into 0.243 microg Neu5Gc and 0.217 microg Neu5Ac, corresponding to a Neu5Ac:Neu5Gc ratio of 1:1.06. Furthermore, the antigen specificity of R24 was determined by immunodetection of GD3 on thin-layer chromatograms, and real time GD3-antibody binding interactions were measured with an optical biosensor (BIAcore). From the structural data obtained in this study it is concluded that glycosylation of the antibody may be important in the clinical outcome of targeted anti-cancer immunotherapy.     

Apoptogenic ganglioside GD3 directly induces the mitochondrial permeability transition.

Early events in apoptotic cascades initiated by ceramides or by activation of the surface receptor CD95 (Fas/APO-1) include the formation of ganglioside GD3. GD3 appears to be both necessary and sufficient to propagate this lipid-mediated apoptotic pathway. Later events common to many apoptotic pathways include induction of the mitochondrial permeability transition (PT) and cytochrome c release, which in turn triggers downstream caspases and cell death. The links between GD3 formation and downstream stages of apoptosis are unknown. We report that ganglioside GD3 directly induces the PT in isolated rat liver mitochondria at 30-100 microM in the presence of exogenous substrate (succinate) and at approximately 3 microM in the absence of exogenous substrate. In contrast, other gangliosides tested (e.g. GM1) have only weak stimulatory effects in the presence of succinate and protect against PT induction in the absence of respiratory substrates. GD3-mediated induction of PT was antagonized by known PT inhibitors, namely cyclosporin A, ADP, trifluoperazine, and Mg(2+). GD3 induced PT even in the presence of submicromolar Ca(2+); GD3 is therefore the first biological PT inducer identified that does not require elevated Ca(2+). Exposure to GD3 also led to mitochondrial cytochrome c release. In contrast, C(2)-ceramide, which can initiate the lipid-mediated apoptotic cascade in susceptible cells, failed to either induce PT or release cytochrome c. These observations suggest that GD3 propagates apoptosis by inducing the PT and cytochrome c release. This model provides a mechanistic link between the earlier and later stages of CD95-induced/ceramide-mediated apoptosis.     

GD3-7-aldehyde is an apoptosis inducer and interacts with adenine nucleotide translocase

We prepared GD3-7-aldehyde (GD3-7) and determined its apoptotic potential. GD3-7 proved to be more efficient to induce pro-apoptotic mitochondrial alterations than GD3 when tested on mouse liver mitochondria. GD3-7-induced mitochondrial swelling and depolarization was blocked by cyclosporin A (CsA) supporting a critical role of the permeability transition pore complex (PTPC) during GD3-7-mediated apoptosis. In contrast to GD3, GD3-7 was able to induce channel formation in proteoliposomes containing adenine nucleotide translocase (ANT). This suggests that ANT is the molecular target of GD3-7. Using a specific antiserum, GD3-7 was detected in the lipid extract of the myeloid tumor cell line HL-60 after apoptosis induction, but not in living cells. Therefore, GD3-7 might be a novel mediator of PTPC-dependent apoptosis in cancer cells.     

Identification of the sialic acid structures recognized by minute virus of mice and the role of binding affinity in virulence adaptation

Sialic acid binding is required for infectious cell surface receptor recognition by parvovirus minute virus of mice (MVM). We have utilized a glycan array consisting of approximately 180 different carbohydrate structures to identify the specific sialosides recognized by the prototype (MVMp) and immunosuppressive (MVMi) strains of MVM plus three virulent mutants of MVMp, MVMp-I362S, MVMp-K368R, and MVMp-I362S/K368R. All of the MVM capsids specifically bound to three structures with a terminal sialic acid-linked alpha2-3 to a common Galbeta1-4GlcNAc motif: Neu5Acalpha2-3Galbeta1-4GlcNAcbeta1-4Galbeta1-4GlcNAc (3'SiaLN-LN), Neu5Acalpha2-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAc (3'SiaLN-LN-LN), and Neu5Acalpha2-3Galbeta1-4(Fucalpha1-3)-GlcNAcbeta1-3Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4(Fucalpha1-3)GlcNAc (sLe(x)-Le(x)-Le(x)). In addition, MVMi also recognized four multisialylated glycans with terminal alpha2-8 linkages: Neu5Acalpha2-8Neu5Acalpha2-8Neu5Acalpha ((Sia)(3)), Neu5Acalpha2-8Neu5Acalpha2-3Galbeta1-4Glc (GD3), Neu5Acalpha2-8Neu5Acalpha2-8Neu5Acalpha2-3Galbeta1-4Glc (GT3), and Neu5Acalpha2-8Neu5Acalpha2-3(GalNAcbeta1-4)Galbeta1-4Glc (GD2). Interestingly, the virulent MVMp-K368R mutant also recognized GT3. Analysis of the relative binding affinities using a surface plasmon resonance biospecific interaction (BIAcore) assay showed the wild-type MVMp and MVMi capsids binding with higher affinity to selected glycans compared with the virulent MVMp mutants. The reduced affinity of the virulent MVMp mutants are consistent with previous in vitro cell binding assays that had shown weaker binding to permissive cells compared with wild-type MVMp. This study identifies the sialic acid structures recognized by MVM. It also provides rationale for the tropism of MVM for malignant transformed cells that contain sLe(x) motifs and the neurotropism of MVMi, which is likely mediated via interactions with multisialylated glycans known to be tumor cell markers. Finally, the observations further implicate a decreased binding affinity for sialic acid in the in vivo adaptation of MVMp to a virulent phenotype.     

Specificity of human trans-sialidase as probed with gangliosides

It has been shown that human blood contains a soluble 67 kDa enzyme, belonging by its donor-acceptor properties to trans-sialidases. The enzyme is capable of both cleaving and synthesizing alpha2-3 and alpha2-6 sialosides [Atherosclerosis2001, 159, 103]. In this work the study of donor-acceptor specificity of the new enzyme was extended. It has been demonstrated in vitro that trans-sialidase possesses the ability of transferring Neu5Ac residue to acceptor (asialofetuin) both from alpha2-3- (GM1, GM3, GD1a), and alpha2-8-sialylated gangliosides (GD3 and GD1b, but not GT1b and GQ1b). Transfer of radiolabeled Neu5Ac from fetuin to glycosphingolipids demonstrated that Lac-Cer>mono- and disialogangliosides>GT1b>GQ1b were acceptors for this enzyme. Two methods were used to reveal whether alpha2-8 bond can be formed between Neu5Ac residues during trans-sialylation, that is immunochemical detection using monoclonal antibodies specific to alpha2-8 di- and oligosialic acids, and fluorometric C7/C9 analysis. Both methods demonstrated the formation of Neu5Acalpha2-8Neu5Ac termination by trans-sialidase, for example, in case of the use 3'SL as sialic acid donor and Neu5Ac-PAA or LDL as acceptor. Thus, human trans-sialidase in vitro displays wide substrate specificity: the enzyme is capable of digesting as well as synthesizing alpha2-3, alpha2-6, and alpha2-8 sialosides.     

Glycosphingolipids and mitochondria: Role in apoptosis and disease

Glycosphingolipids (GSLs) comprise a class of lipids with important structural and signaling functions. Synthesized from ceramide in the Golgi, they are subsequently distributed to different compartments, most predominantly in the plasma membrane where they integrate signaling platforms. A recently characterized trafficking of ganglioside GD3 (GD3), a GSLs with two sialic-acid residues, to mitochondria has revealed a novel function of this lipid as a death effector. In addition to the interaction of GD3 with mitochondria recruiting these organelles to apoptotic pathways, GD3 disables survival paths dependent on NF-B, thus favoring the balance towards cell death. The present review gathers the evidence documenting this emerging function of GSLs in cell death and their involvement in pathological states. Published in 2004..     

Glycosphingolipids and mitochondria: role in apoptosis and disease

Glycosphingolipids (GSLs) comprise a class of lipids with important structural and signaling functions. Synthesized from ceramide in the Golgi, they are subsequently distributed to different compartments, most predominantly in the plasma membrane where they integrate signaling platforms. A recently characterized trafficking of ganglioside GD3 (GD3), a GSLs with two sialic-acid residues, to mitochondria has revealed a novel function of this lipid as a death effector. In addition to the interaction of GD3 with mitochondria recruiting these organelles to apoptotic pathways, GD3 disables survival paths dependent on NF-kappaB, thus favoring the balance towards cell death. The present review gathers the evidence documenting this emerging function of GSLs in cell death and their involvement in pathological states.     

O-acetylation of GD3 prevents its apoptotic effect and promotes survival of lymphoblasts in childhood acute lymphoblastic leukaemia

We have previously demonstrated induction of O-acetylated sialoglycoproteins on lymphoblasts of childhood acute lymphoblastic leukaemia (ALL). These molecules promote survival of lymphoblasts by preventing apoptosis. Although O-acetylated sialoglycoproteins are over expressed, the status of O-acetylation of gangliosides and their role in lymphoblasts survival remains to be explored in ALL patients. Here, we have observed enhanced levels of 9-O-acetylated GD3 (9-O-AcGD3) in the lymphoblasts of patients and leukaemic cell line versus disialoganglioside GD3 in comparison to the normal cells. Localization of GD3 and 9-O-AcGD3 on mitochondria of patient's lymphoblasts has been demonstrated by immuno-electron microscopy. The exogenous administration of GD3-induced apoptosis in lymphoblasts as evident from the nuclear fragmentation and sub G0/G1 apoptotic peak. In contrast, 9-O-AcGD3 failed to induce such apoptosis. We further explored the mitochondria-dependent pathway triggered during GD3-induced apoptosis in lymphoblasts. GD3 caused a time-dependent depolarization of mitochondrial membrane potential, release of cytochrome c and 7.4- and 8-fold increased in caspase 9 and caspase 3 activity respectively. However, under identical conditions, an equimolar concentration of 9-O-AcGD3 failed to induce similar effects. Interestingly, 9-O-AcGD3 protected the lymphoblasts from GD3-induced apoptosis when administered in equimolar concentrations simultaneously. In situ de-O-acetylation of 9-O-AcGD3 with sodium salicylate restores the GD3-responsiveness to apoptotic signals. Although both GD3 and 9-O-acetyl GD3 localize to mitochondria, these two structurally related molecules may play different roles in ALL-disease biology. Taken together, our results suggest that O-acetylation of GD3, like that of O-acetylated sialoglycoproteins, might be a general strategy adopted by leukaemic blasts towards survival in ALL.     

Isolation and characterization of gangliosides from pig lymphocytes

Two major gangliosides from pig spleen lymphocytes, accounting for 57% of the total lipid-bound sialic acids, were isolated and purified to homogeneity by column chromatography on DEAE-Sephadex and silica gel. They were identified as GM3 (II3Neu5GcLacCer), and GD3 (II3(Neu5Gc)2LacCer), by thin-layer chromatography in comparison with standards and by analysis of the constituent sugars. The major fatty acids of these gangliosides were stearic acid and myristic acid, respectively. In addition to these gangliosides, GD2 and bands comigrating on thin-layer chromatography with authentic GM2, GM1, GD1a and GD1b were found. These compounds also occur in pig peripheral blood lymphocytes, where, however, GD3 represents about 70% of the total lipid-bound sialic acid.     

Bax, Bid and the permeabilization of the mitochondrial outer membrane in apoptosis

Mitochondria provide a key amplification step in the apoptotic pathway of many cells by releasing apoptogenic proteins into the cytosol. Recent studies have provided insights into how Bax and Bid may operate synergistically to recruit mitochondria into the pathway and how GD3 ganglioside, a metabolite of the sphingomyelin pathway, may also be used. In ischaemic disease, activation of the mitochondrial permeability transition pore may bypass the requirement for these factors.     

Involvement of the Galbeta1 - 3GalNAcbeta structure in the recognition of apoptotic bodies by THP-1 cells

A specific apoptotic glycosylation pattern may play an assistant or even a causative role in phagocytosis of apoptotic bodies. To elucidate the role of macrophages in lectin-mediated phagocytosis, an experimental system was used, where monocyte-derived THP-1 cells engulf the apoptotic bodies from the melanoma cell line MELJUSO. A flow cytometry assay was performed to reveal lectin expression and quantify the phagocytosis of apoptotic bodies. Taking into account that siglecs, a mannose receptor and galectins expressed on macrophages could be involved in engulfment of apoptotic bodies we studied their potential expression on THP-1 cells by means of polyacrylamide glycoconjugates. A strong binding of the cells to siglec ligands (3'SiaLac, 6'SiaLac, [Neu5Acalpha2-8]2) and galectin ligands (LacNAc, GalNAcbeta1 - 4GlcNAc, Galbeta1 - 3GalNAcbeta and asialoGM1) was observed. To reveal the corresponding targets on apoptotic bodies, the carbohydrate pattern of MELJUSO cells was analyzed. The apoptotic membrane was characterized by a high level of glycans terminated by galactose or sialic acid. To study lectin-mediated phagocytosis of apoptotic bodies by THP-1 cells, an inhibitory phagocytosis assay was performed. Binding of Galbeta1 - 3GalNAc- or LacNAc-specific reagents (lectins and antibodies) to apoptotic bodies abolished their engulfment by the THP-1 cells whereas blocking of Neu5Acalpha2 - 6 or Neu5Acalpha2 - 3 sites by the corresponding lectins was not effective. Furthermore, Galbeta1 - 3GalNAcbeta-PAA or asialoGM1-PAA binding to the THP-1 cells decreased phagocytosis, whereas two other potent THP-1-binding probes, LacNAc-PAA and GalNAcbeta1 - 4GlcNAc-PAA did not inhibit phagocytosis. Thus, Galbeta1 - 3GalNAcbeta-terminated chains represented on the apoptotic bodies but not the other tested galectin ligands appear to be a target for THP-1 cells.     

Nile Tilapia Neu3 sialidases: Molecular cloning,functional characterization and expression in Oreochromis niloticus

Mammalian Neu3 is a ganglioside specific sialidase. Gangliosides are involved in various physiological events such as cell growth, differentiation and diseases. Significance of Neu3 and gangliosides is still unclear in aquaculture fish species. To gain more insights of fish Neu3 sialidases, molecular cloning and characterization were carried out in tilapia (Oreochromis niloticus). A tilapia genome-wide search for orthologues of human NEU1, NEU2, NEU3 and NEU4 yielded eight putative tilapia sialidases, five of which were neu3-like and designated as neu3a, neu3b, neu3c, neu3d and neu3e. Among five neu3 genes, neu3a, neu3d and neu3e were amplified by PCR from adult fish brain cDNA with consensus sequences of 1227 bp, 1194 bp and 1155 bp, respectively. Multiple alignments showed conserved three Asp-boxes (SXDXGXTW), YRIP and VGPG motifs. The molecular weights for Neu3a, Neu3d and Neu3e were confirmed using immunoblotting analysis as 45.9 kDa, 44.4 kDa and 43.6 kDa, respectively. Lysate from neu3 genes transfected HEK293 cells showed sialidase activity in Neu3a towards ganglioside mix optimally at pH 4.6. Using pure gangliosides as substrates, highest sialidase activity for Neu3a was observed towards GD3 followed by GD1a and GM3, but not GM1. On the other hand, sialidase activities were not observed in Neu3d and Neu3e towards various sialoglycoconjugates. Indirect immunofluorescence showed that tilapia Neu3a and Neu3d are localized at the plasma membrane, while most Neu3e showed a cytosolic localization. RT-PCR analyses for neu3a showed significant expression in the brain, liver, and spleen tissues, while neu3d and neu3e showed different expression patterns. Based on these results, tilapia Neu3 exploration is an important step towards full understanding of a more comprehensive picture of Neu3 sub-family of proteins in fish.