High level of sialate-O-acetyltransferase activity in lymphoblasts of childhood acute lymphoblastic leukaemia (ALL): enzyme characterization and correlation with disease status

Influenza D virus utilizes both 9-O-acetylated N-acetylneuraminic and 9-O-acetylated N-glycolylneuraminic acids as functional entry receptors

Influenza D virus (IDV) utilizes bovines as a primary reservoir with periodical spillover to other hosts. We have previously demonstrated that IDV binds both 9-O-acetylated N-acetylneuraminic acid (Neu5,9Ac2) and 9-O-acetylated N-glycolylneuraminic acid (Neu5Gc9Ac). Bovines produce both Neu5,9Ac2 and Neu5Gc9Ac, while humans are genetically unable to synthesize Neu5Gc9Ac. 9-O-Acetylation of sialic acids is catalyzed by CASD1 via a covalent acetyl-enzyme intermediate. To characterize the role of Neu5,9Ac2 and Neu5Gc9Ac in IDV infection and determine which form of 9-O-acetylated sialic acids drives IDV entry, we took advantage of a CASD1 knockout (KO) MDCK cell line and carried out feeding experiments using synthetic 9-O-acetyl sialic acids in combination with the single-round and multi-round IDV infection assays. The data from our studies show that (i) CASD1 KO cells are resistant to IDV infection and lack of IDV binding to the cell surface is responsible for the failure of IDV replication; (ii) feeding CASD1 KO cells with Neu5,9Ac2 or Neu5Gc9Ac resulted in a dose-dependent rescue of IDV infectivity; and (iii) diverse IDVs replicated robustly in CASD1 KO cells fed with either Neu5,9Ac2 or Neu5Gc9Ac at a level similar to that in wild-type cells with a functional CASD1. These data demonstrate that IDV can utilize Neu5,9Ac2- or non-human Neu5Gc9Ac-containing glycan receptor for infection. Our findings provide evidence that IDV has acquired the ability to infect and transmit among agricultural animals that are enriched in Neu5Gc9Ac, in addition to posing a zoonotic risk to humans expressing only Neu5,9Ac2.IMPORTANCEInfluenza D virus (IDV) has emerged as a multiple-species-infecting pathogen with bovines as a primary reservoir. Little is known about the functional receptor that drives IDV entry and promotes its cross-species spillover potential among different hosts. Here, we demonstrated that IDV binds exclusively to 9-O-acetylated N-acetylneuraminic acid (Neu5,9Ac2) and non-human 9-O-acetylated N-glycolylneuraminic acid (Neu5Gc9Ac) and utilizes both for entry and infection. This ability in effective engagement of both 9-O-acetylated sialic acids as functional receptors for infection provides an evolutionary advantage to IDV for expanding its host range. This finding also indicates that IDV has the potential to emerge in humans because Neu5,9Ac2 is ubiquitously expressed in human tissues, including lung. Thus, results of our study highlight a need for continued surveillance of IDV in humans, as well as for further investigation of its biology and cross-species transmission mechanism.

9-O Acetylated Gangliosides in Health and Disease

Glycosphingolipids comprise a lipid class characterized by the presence of sugar moieties attached to a ceramide backbone. The role of glycosphingolipids in pathophysiology has gained relevance in recent years in parallel with the development of analytical technologies. Within this vast family of molecules, gangliosides modified by acetylation represent a minority. Described for the first time in the 1980s, their relation to pathologies has resulted in increased interest in their function in normal and diseased cells. This review presents the state of the art on 9-O acetylated gangliosides and their link to cellular disorders.

Sialic acid O-acetylation: From biosynthesis to roles in health and disease

Sialic acids are nine-carbon sugars that frequently cap glycans at the cell surface in cells of vertebrates as well as cells of certain types of invertebrates and bacteria. The nine-carbon backbone of sialic acids can undergo extensive enzymatic modification in nature and O-acetylation at the C-4/7/8/9 position in particular is widely observed. In recent years, the detection and analysis of O-acetylated sialic acids have advanced, and sialic acid-specific O-acetyltransferases (SOATs) and O-acetylesterases (SIAEs) that add and remove O-acetyl groups, respectively, have been identified and characterized in mammalian cells, invertebrates, bacteria, and viruses. These advances now allow us to draw a more complete picture of the biosynthetic pathway of the diverse O-acetylated sialic acids to drive the generation of genetically and biochemically engineered model cell lines and organisms with altered expression of O-acetylated sialic acids for dissection of their roles in glycoprotein stability, development, and immune recognition, as well as discovery of novel functions. Furthermore, a growing number of studies associate sialic acid O-acetylation with cancer, autoimmunity, and infection, providing rationale for the development of selective probes and inhibitors of SOATs and SIAEs. Here, we discuss the current insights into the biosynthesis and biological functions of O-acetylated sialic acids and review the evidence linking this modification to disease. Furthermore, we discuss emerging strategies for the design, synthesis, and potential application of unnatural O-acetylated sialic acids and inhibitors of SOATs and SIAEs that may enable therapeutic targeting of this versatile sialic acid modification.

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.

Expression of 9-O- and 7,9-O-Acetyl Modified Sialic Acid in Cells and Their Effects on Influenza Viruses

Sialic acids (Sia) are widely displayed on the surfaces of cells and tissues. Sia come in a variety of chemically modified forms, including those with acetyl modifications at the C-7, C-8, and C-9 positions. Here, we analyzed the distribution and amounts of these acetyl modifications in different human and canine cells. Since Sia or their variant forms are receptors for influenza A, B, C, and D viruses, we examined the effects of these modifications on virus infections. We confirmed that 9-O-acetyl and 7,9-O-acetyl modified Sia are widely but variably expressed across cell lines from both humans and canines. Although they were expressed on the cell surfaces of canine MDCK cell lines, they were located primarily within the Golgi compartment of human HEK-293 and A549 cells. The O-acetyl modified Sia were expressed at low levels of 1 to 2% of total Sia in these cell lines. We knocked out and overexpressed the sialate O-acetyltransferase gene (CasD1) and knocked out the sialate O-acetylesterase gene (SIAE) using CRISPR/Cas9 editing. Knocking out CasD1 removed 7,9-O- and 9-O-acetyl Sia expression, confirming previous reports. However, overexpression of CasD1 and knockout of SIAE gave only modest increases in 9-O-acetyl levels in cells and no change in 7,9-O-acetyl levels, indicating that there are complex regulations of these modifications. These modifications were essential for influenza C and D infection but had no obvious effect on influenza A and B infection.IMPORTANCE Sialic acids are key glycans that are involved in many different normal cellular functions, as well as being receptors for many pathogens. However, Sia come in diverse chemically modified forms. Here, we examined and manipulated the expression of 7,9-O- and 9-O-acetyl modified Sia on cells commonly used in influenza virus and other research by engineering the enzymes that produce or remove the acetyl groups.

Modulation of TLR4 Sialylation Mediated by a Sialidase Neu1 and Impairment of Its Signaling in Leishmania donovani Infected Macrophages

Altered sialylation is generally maintained by a fine balance between sialidases and sialyltransferases, which plays an essential role during disease pathogenesis. TLR4 is a membrane-bound highly sialylated glycoprotein predominantly having α2,3-linked sialic acids. It is one of the most important client molecules in the anti-leishmanial innate immune arm. Here, we initiated a comprehensive study on the modulation of TLR4 sialylation in Leishmania donovani (L. d)-infected macrophages by a mammalian sialidase/neuraminidase-1 (Neu1) having substrate specificity toward α2,3-linked sialic acids. We observed reduced membrane-associated Neu1 with its decreased enzyme activity in infected macrophages. Moreover, we demonstrated reduced association of Neu1 with TLR4 leading to enhanced sialylation of TLR4 in these infected cells. Conversely, Neu1 over expression exhibited enhanced association of TLR4 with Neu1 leading to reduced sialylation which possibly linked to increased association of TLR4 with its downstream adaptor protein, MyD88. This, in turn, activated downstream MAP kinase signaling pathway, with enhanced nuclear translocation of NFκB that resulted in increased genetic and protein levels expression of Th1 cytokines and effector molecule nitric oxide secretion which ultimately leads to reduced parasite burden in macrophages. This was further validated by Neu1 silencing in infected macrophages which reversed such a situation. Such events strongly confirm the importance of Neu1 in modulation of TLR4 sialylation during parasite infection resulting in impairment of innate immune response. Furthermore, decreased membrane-bound Neu1 in infected macrophages could be attributed to its reduced tyrosine-phosphorylation as well as diminished association with cathepsin A. Both these phenomenon possibly play significant roles in inhibiting translocation of the sialidase from cytosol to membrane. Taken together, our study first time demonstrated impaired translocation of cytosolic Neu1 to the membrane of L. donovani-infected macrophages due to impaired phosphorylation of this enzyme. This novel finding establishes a link between enhanced α2,3-linked sialic acids on TLR4 and reduced membrane-bound Neu1 which plays a significant role for inhibiting downstream signaling to establish successful infection in the host cells.

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

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

Mahanine drives pancreatic adenocarcinoma cells into endoplasmic reticular stress-mediated apoptosis through modulating sialylation process and Ca2+-signaling

Endoplasmic reticulum (ER) stress results from protein unfolding/misfolding during cellular maturation, which requires a coordinated action of several chaperones and enzymes and Ca²⁺ signalling. ER-stress possibly has a positive effect on survival of pancreatic cancer cell. Therefore, detailed insights into this complex signaling network are urgently needed. Here, we systematically analyzed the impact of ER stress-mediated unfolded protein response (UPR) and Ca²⁺-signaling cross-talk for the survival of pancreatic adenocarcinoma (PDAC) cells. We observed enhanced ER activity and initiation of UPR signaling induced by a carbazole alkaloid (mahanine). This event triggers a time-dependent increase of intracellular Ca²⁺ leakage from ER and subsequently Ca²⁺ signaling induced by enhanced reactive oxygen species (ROS) produced by this pro-oxidant agent. In addition, we observed an altered glycosylation, in particular with regard to reduced linkage-specific sialic acids possibly due to decreased sialyltransferase activity. Changes in sialylation entailed enhanced expression of the ganglioside GD3 in the treated cells. GD3, an inducer of apoptosis, inhibited pancreatic xenograft tumor. Taken together, our study describes a molecular scenario how PDAC cells are driven into apoptosis by mahanine by UPR-driven ER stress-associated and ROS-mediated calcium signaling and possibly defective sialylation.

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.

9-O-acetylated sialic acids differentiating normal haematopoietic precursors from leukemic stem cells with high aldehyde dehydrogenase activity in children with acute lymphoblastic leukaemia

Childhood acute lymphoblastic leukaemia (ALL) originates from mutations in haematopoietic progenitor cells (HPCs). For high-risk patients, treated with intensified post-remission chemotherapy, haematopoietic stem cell (HSC) transplantation is considered. Autologous HSC transplantation needs improvisation till date. Previous studies established enhanced disease-associated expression of 9-O-acetylated sialoglycoproteins (Neu5,9Ac2-GPs) on lymphoblasts of these patients at diagnosis, followed by its decrease with clinical remission and reappearance with relapse. Based on this differential expression of Neu5,9Ac2-GPs, identification of a normal HPC population was targeted from patients at diagnosis. This study identifies two distinct haematopoietic progenitor populations from bone marrow of diagnostic ALL patients, exploring the differential expression of Neu5,9Ac2-GPs with stem cell (CD34, CD90, CD117, CD133), haematopoietic (CD45), lineage-commitment (CD38) antigens and cytosolic aldehyde dehydrogenase (ALDH). Normal haematopoietic progenitor cells (ALDH(+)SSC(lo)CD45(hi)Neu5,9Ac2 -GPs(lo)CD34(+)CD38(-)CD90(+)CD117(+)CD133(+)) differentiated into morphologically different, lineage-specific colonies, being crucial for autologous HSC transplantation while leukemic stem cells (ALDH(+)SSC(lo)CD45(lo)Neu5,9Ac2 -GPs(hi)CD34(+)CD38(+)CD90(-)CD117(-)CD133(-)) lacking this ability can be potential targets for minimal residual disease detection and drug-targeted immunotherapy.

Disialoganglioside GD3-synthase over expression inhibits survival and angiogenesis of pancreatic cancer cells through cell cycle arrest at S-phase and disruption of integrin-β1-mediated anchorage

Gangliosides play important roles in the development, differentiation and proliferation of mammalian cells. They bind to other cell membrane components through their terminal sialic acids. Different gangliosides influence cellular functions based on the positions and linkages of sialic acids. Expression of gangliosides mainly depends on the status of sialic acid-modulatory enzymes, such as different types of sialyltransferases and sialidases. One such sialyltransferase, disialoganglioside GD3 synthase, is specifically responsible for the production of GD3. Pancreatic ductal adenocarcinoma, making up more than 90% of pancreatic cancers, is a fatal malignancy with poor prognosis. Despite higher sialylation status, the disialoganglioside GD3 level is very low in this cancer. However, the exact status and function of this disialoganglioside is still unknown. Here, we intended to study the intracellular mechanism of disialoganglioside GD3-induced apoptosis and its correlation with the adhesion and angiogenic pathways in pancreatic cancer. We demonstrated that disialoganglioside GD3 synthase-transfected cells showed enhanced apoptosis and it caused the arrest of these cells in the S-phase of the cell cycle. Integrins, a family of transmembrane proteins play important role in cell-cell recognition, invasion, adhesion and migration. disialoganglioside GD3 co-localised with integrin-β1 and thereby inhibited it's downstream signalling in transfected cells. Transfected cells exhibited inhibition of cell adhesion with extracellular matrix proteins. Enhanced GD3 expression down regulated angiogenesis-regulatory proteins and inhibited epidermal growth factor/vascular endothelial growth factor-driven angiogenic cell growth in these cells. Taken together, our study provides support for the GD3-induced cell cycle arrest, disruption of integrin-β1-mediated anchorage, inhibition of angiogenesis and thereby induced apoptosis in pancreatic cancer cells.

Identification and analysis of o-acetylated sialoglycoproteins

5-N-acetylneuraminic acid, commonly known as sialic acid (Sia), constitutes a family of N- and O-substituted 9-carbon monosaccharides. Frequent modification of O-acetylations at positions C-7, C-8, or C-9 of Sias generates a family of O-acetylated sialic acid (O-AcSia) and plays crucial roles in many cellular events like cell-cell adhesion, proliferation, migration, etc. Therefore, identification and analysis of O-acetylated sialoglycoproteins (O-AcSGPs) are important. In this chapter, we describe several approaches for successful identification of O-AcSGPs. We broadly divide them into two categories, i.e., invasive and noninvasive methods. Several O-AcSias-binding probes are used for this purpose. Detailed methodologies for step-by-step identification using these probes have been discussed. We have also included a few invasive analytical methods for identification and quantitation of O-AcSias. Several indirect methods are also elaborated for such purpose, in which O-acetyl group from sialic acids is initially removed followed by detection of Sias by several approaches. For molecular identification, we have described methods for affinity purification of O-AcSGPs using an O-AcSias-binding lectin as an affinity matrix followed by sequencing using matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF-TOF) mass spectroscopy (MS). In spite of special attention, loss of O-acetyl groups due to its sensitivity towards alkaline pH and high temperature along with migration of labile O-acetyl groups from C7-C8-C9 during sample preparation is difficult to avoid. Therefore there is always a risk for underestimation of O-AcSias.

Sialic acid metabolism and sialyltransferases: natural functions and applications

Sialic acids are a family of negatively charged monosaccharides which are commonly presented as the terminal residues in glycans of the glycoconjugates on eukaryotic cell surface or as components of capsular polysaccharides or lipooligosaccharides of some pathogenic bacteria. Due to their important biological and pathological functions, the biosynthesis, activation, transfer, breaking down, and recycle of sialic acids are attracting increasing attention. The understanding of the sialic acid metabolism in eukaryotes and bacteria leads to the development of metabolic engineering approaches for elucidating the important functions of sialic acid in mammalian systems and for large-scale production of sialosides using engineered bacterial cells. As the key enzymes in biosynthesis of sialylated structures, sialyltransferases have been continuously identified from various sources and characterized. Protein crystal structures of seven sialyltransferases have been reported. Wild-type sialyltransferases and their mutants have been applied with or without other sialoside biosynthetic enzymes for producing complex sialic acid-containing oligosaccharides and glycoconjugates. This mini-review focuses on current understanding and applications of sialic acid metabolism and sialyltransferases.

Functions and Biosynthesis of O-Acetylated Sialic Acids

Sialic acids have a pivotal functional impact in many biological interactions such as virus attachment, cellular adhesion, regulation of proliferation, and apoptosis. A common modification of sialic acids is O-acetylation. O-Acetylated sialic acids occur in bacteria and parasites and are also receptor determinants for a number of viruses. Moreover, they have important functions in embryogenesis, development, and immunological processes. O-Acetylated sialic acids represent cancer markers, as shown for acute lymphoblastic leukemia, and they are known to play significant roles in the regulation of ganglioside-mediated apoptosis. Expression of O-acetylated sialoglycans is regulated by sialic acid-specific O-acetyltransferases and O-acetylesterases. Recent developments in the identification of the enigmatic sialic acid-specific O-acetyltransferase are discussed.

The human Cas1 protein: a sialic acid-specific O-acetyltransferase?

Sialic acids are important sugars at the reducing end of glycoproteins and glycolipids. They are among many other functions involved in cell-cell interactions, host-pathogen recognition and the regulation of serum half-life of glycoproteins. An important modification of sialic acids is O-acetylation, which can alter or mask the biological properties of the parent sialic acid molecule. The nature of mammalian sialate-O-acetyltransferases (EC 2.3.1.45) involved in their biosynthesis is still unknown. We have identified the human CasD1 (capsule structure1 domain containing 1) gene as a candidate to encode the elusive enzyme. The human CasD1 gene encodes a protein with a serine-glycine-asparagine-histidine hydrolase domain and a hydrophobic transmembrane domain. Expression of the Cas1 protein tagged with enhanced green fluorescent protein in mammalian and insect cells directed the protein to the medial and trans-cisternae of the Golgi. Overexpression of the Cas1 protein in combination with α-N-acetyl-neuraminide α-2,8-sialyltransferase 1 (GD3 synthase) resulted in an up to 40% increased biosynthesis of 7-O-acetylated ganglioside GD3. By quantitative real-time polymerase chain reaction, we found up to 5-fold increase in CasD1 mRNA in tumor cells overexpressing O-Ac-GD3. CasD1-specific small interfering RNA reduced O-acetylation in tumor cells. These results suggest that the human Cas1 protein is directly involved in O-acetylation of α2-8-linked sialic acids.

Differentially regulated expression of 9-O-acetyl GD3 (CD60b) and 7-O-acetyl-GD3 (CD60c) during differentiation and maturation of human T and B lymphocytes

GD3 (CD60a) and its 9-O-acetylated variant (CD60b) are intracellular regulators of apoptosis in T lymphocytes. Surface expressed 9-O-acetyl- and 7-O-acetyl-GD3 (CD60b and CD60c) may have a functional impact on activated T and B cells. In order to investigate the balance between surface and intracellular expression and synthesis and degradation of these glycosphingolipids in human lymphocytes of various differentiation stages, we analyzed (i) expression of GD3 molecules on native T and B cells and thymocytes by flow cytometry and (ii) activity and regulation of possible key enzymes for CD60a,b,c synthesis and degradation at the transcriptional level. Both, surface and cytoplasmic expression of CD60a and CD60c was highest in tonsillar T cells. In thymocytes, CD60c outweighs the other CD60 variants and was mainly found in the cytoplasm. All lymphocyte preparations contained sialate O-acetyltransferase activity producing 7-O-acetyl-GD3. Sialidase activity was highest in peripheral blood lymphocytes followed by thymocytes and tonsillar T and B cells. Transcription of GD3 synthase (ST8SiaI), the key enzyme for GD3 synthesis, was highest in tonsillar T cells, whereas transcriptional levels of sialidase NEU3 and O-acetylesterase H-Lse were lowest in activated T cells. This balance between enzymes of sialic acid metabolism may explain the strong overall staining intensity for all GD3 forms in T cells. Both CASD1, presumably encoding a sialic acid-specific O-acetyltransferase, and H-Lse showed highest transcription in peripheral B lymphocytes corresponding to the low expression of CD60b and c in these cells. Our data point to regulatory functions of these anabolic and catabolic key enzymes for the expression of GD3 and its O-acetylated variants in lymphocytes at a given differentiation stage.

Disease-associated glycosylated molecular variants of human C-reactive protein activate complement-mediated hemolysis of erythrocytes in tuberculosis and Indian visceral leishmaniasis

Human C-reactive protein (CRP), as a mediator of innate immunity, removed damaged cells by activating the classical complement pathway. Previous studies have successfully demonstrated that CRPs are differentially induced as glycosylated molecular variants in certain pathological conditions. Affinity-purified CRPs from two most prevalent diseases in India viz. tuberculosis (TB) and visceral leishmaniasis (VL) have differential glycosylation in their sugar composition and linkages. As anemia is a common manifestation in TB and VL, we assessed the contributory role of glycosylated CRPs to influence hemolysis via CRP-complement-pathway as compared to healthy control subjects. Accordingly, the specific binding of glycosylated CRPs with erythrocytes was established by flow-cytometry and ELISA. Significantly, deglycosylated CRPs showed a 7-8-fold reduced binding with erythrocytes confirming the role of glycosylated moieties. Scatchard analysis revealed striking differences in the apparent binding constants (10(4)-10(5) M(-1)) and number of binding sites (10(6)-10(7)sites/erythrocyte) for CRP on patients' erythrocytes as compared to normal. Western blotting along with immunoprecipitation analysis revealed the presence of distinct molecular determinants on TB and VL erythrocytes specific to disease-associated CRP. Increased fragility, hydrophobicity and decreased rigidity of diseased-erythrocytes upon binding with glycosylated CRP suggested membrane damage. Finally, the erythrocyte-CRP binding was shown to activate the CRP-complement-cascade causing hemolysis, even at physiological concentration of CRP (10 microg/ml). Thus, it may be postulated that CRP have a protective role towards the clearance of damaged-erythrocytes in these two diseases.

Down regulation of membrane-bound Neu3 constitutes a new potential marker for childhood acute lymphoblastic leukemia and induces apoptosis suppression of neoplastic cells

Membrane-linked sialidase Neu3 is a key enzyme for the extralysosomal catabolism of gangliosides. In this respect, it regulates pivotal cell surface events, including trans-membrane signaling, and plays an essential role in carcinogenesis. In this report, we demonstrated that acute lymphoblastic leukemia (ALL), lymphoblasts (primary cells from patients and cell lines) are characterized by a marked down-regulation of Neu3 in terms of both gene expression (-30 to 40%) and enzymatic activity toward ganglioside GD1a (-25.6 to 30.6%), when compared with cells from healthy controls. Induced overexpression of Neu3 in the ALL-cell line, MOLT-4, led to a significant increase of ceramide (+66%) and to a parallel decrease of lactosylceramide (-55%). These events strongly guided lymphoblasts to apoptosis, as we assessed by the decrease in Bcl2/Bax ratio, the accumulation of Neu3 transfected cells in the sub G0-G1 phase of the cell cycle, the enhanced annexin-V positivity, the higher cleavage of procaspase-3. Therefore, the reduced expression of Neu3 in ALL could help lymphoblasts to survive, maintaining the cellular content of ceramide below a critical level. Interestingly, we found that Neu3 activity varied in relation to disease progression, increasing in clinical remission after chemotherapy, and decreasing again in patients that relapsed. In addition, a negative correlation was observed between Neu3 expression and the percentage of the ALL marker 9-OAcGD3 positive cells. Consequently, Neu3 could represent a new potent biomarker in childhood ALL, to assess the efficacy of therapeutic protocols and to rapidly identify an eventual relapse.

Sialic acids acquired by Pseudomonas aeruginosa are involved in reduced complement deposition and siglec mediated host-cell recognition

The opportunism of Pseudomonas aeruginosa (PA) in immunocompromised hosts prompted us to explore the potential role of sialic acids (Sia) in this phenomenon. Culture of PA in the presence of exogenous Sia resulted in linkage-specific incorporation of Sia which was associated with decreased complement deposition on the bacteria. Sia acquired by PA mediated enhanced binding of bacteria to recombinant-CHO cells expressing human siglec-7 or siglec-9, as well as to human NK-cells and monocytes naturally expressing these siglecs. Therefore, Sia may be acquired by PA in the host and contribute to bacterial pathogenicity and host-cell interactions via reduction of complement deposition and siglec-dependent recognition.

Human beta-galactoside alpha-2,3-sialyltransferase (ST3Gal III) attenuated Taxol-induced apoptosis in ovarian cancer cells by downregulating caspase-8 activity

Taxol triggers apoptosis in a variety of cancer cells, but it also upregulates cytoprotective proteins and/or pathways that compromise its therapeutic efficacy. In this report, we found that Taxol treatment resulted in caspase-8-dependent apoptosis in SKOV3 human ovarian cancer cells. Moreover, Taxol-induced apoptosis was associated with caspase-3 activation. Interestingly, Taxol treatment upregulated alpha-2,3-sialyltransferase (ST3Gal III) expression and forced expression of ST3Gal III attenuated Taxol-induced apoptosis. Furthermore, ST3Gal III overexpression inhibited Taxol-triggered caspase-8 activation, indicating that ST3Gal III upregulation produces cellular resistance to Taxol and hence reduces the efficacy of Taxol therapy.

Elevated mRNA level of hST6Gal I and hST3Gal V positively correlates with the high risk of pediatric acute leukemia

Altered sialylation occurs in essentially all types of human and experimental cancers. Although, aberrant sialylation is believed to mainly due to altered sialyltransferase (ST) level, so far, expression pattern of different STs in acute lymphoblastic leukemia has never been investigated. Accordingly, the aim of our study was to monitor the changes in mRNA expression of ST6Gal I, ST3Gal V and ST8Sia I in patients by real-time PCR, which may provide prognostic information useful in defining appropriate therapeutic options. Our data demonstrated that ST6Gal I and ST3Gal V mRNA were up-regulated in lymphoblasts whereas its presence was negligible in non-malignant donors. In contrast, ST8SiaI was downregulated in patients. The extents of linkage-specific sialylation of glycoconjugates were found to be associated with disease establishment. Additionally, ST6Gal I and ST3Gal V were positively correlated with the high risk of the disease (P=0.0032 and 0.0016). This differential ST level can be used as biomarker with the molecular method of quantitative PCR and may be useful to discriminate normal and cancer patients.

O-acetylated sialic acids: multifaceted role in childhood acute lymphoblastic leukaemia

Childhood acute lymphoblastic leukaemia (ALL), a malignant transformation of the lymphoblasts, is highly responsive to chemotherapy. However, due to certain inadequacy in detection of minimal residual disease (MRD), relapse is a common phenomenon. To address this question, the present review deals with the induction of an unique O-acetyl derivative of sialic acid on a few disease-associated glycoproteins and glycolipids at the onset of childhood ALL, a finding of our group in the last decade. This information has been successfully utilized for diagnosis and prognosis of the disease. Existing literature is included for comparison. Additionally, cell surface overexpression of 9-O-acetylated sialoglycoproteins and antibodies against them present in patients' sera aid the survival of the malignant lymphoblasts and suggest a multifaceted role played by these molecules. Taken together, monitoring these molecules helps not only in unravelling the biology of this paediatric malignancy but also in personalizing the treatment strategies for the betterment of the patient population.

9-O-acetylated sialoglycoproteins are important immunomodulators in Indian visceral leishmaniasis

Overexpression of disease-associated 9-O-acetylated sialoglycoproteins (9-O-AcSGPs) on peripheral blood mononuclear cells (PBMC) of visceral leishmaniasis (VL) patients (PBMC(VL)) compared to their levels of expression in healthy individuals has been demonstrated using a lectin, achatinin-H, with specificity toward 9-O-acetylated sialic acid derivatives alpha2-6 linkage with subterminal N-acetylgalactosamine (9-O-AcSAalpha2-6GalNAc). The decreased presence of disease-associated 9-O-AcSGPs on different immune cells of parasitologically cured individuals after successful treatment relative to the levels in patients with active VL prior to treatment was demonstrated. However, their contributory role as immunomodulatory determinants on PBMC(VL) remained unexplored. Accordingly, 9-O-AcSGPs on PBMC(VL) were sensitized with achatinin-H, leading to their enhanced proliferation compared to that observed with different known mitogens or parasite antigen. This lymphoproliferative response was characterized by evaluation of the TH1/TH2 response by intracellular staining and enzyme-linked immunosorbent assay for secreted cytokines, and the results were corroborated by their genetic expression. Sensitized PBMC(VL) evidenced a mixed TH1/TH2 cellular response with a predominance of the TH1 response, indicating the ability of 9-O-AcSGPs to modulate the host cell toward a favorable response. Interestingly, the humoral and cellular responses showed a good correlation. Further, high levels of anti-9-O-AcSGP antibodies with an order of distribution of immunoglobulin M (IgM) > IgG1 = IgG3 > IgG4 > IgG2 > IgE could be explained by a mixed TH1/TH2 response. A good correlation of enhanced 9-O-AcSGPs with both the cell-mediated (r = 0.98) and humoral (r = 0.99) response was observed. In summary, it may be concluded that sensitization of 9-O-AcSGPs on PBMC(VL) may provide a basis for the modulation of the host's immune response by their controlled expression, leading to a beneficial immune response and influencing the disease pathology.