Genetically altered mice with different sialyltransferase deficiencies show tissue-specific alterations in sialylation and sialic acid 9-O-acetylation

Removal of sialic acid involving Klotho causes cell-surface retention of TRPV5 channel via binding to galectin-1

Klotho is a mammalian senescence-suppression protein that has homology with glycosidases. The extracellular domain of Klotho is secreted into urine and blood and may function as a humoral factor. Klotho-deficient mice have accelerated aging and imbalance of ion homeostasis. Klotho treatment increases cell-surface abundance of the renal epithelial Ca(2+) channel TRPV5 by modifying its N-linked glycans. However, the precise sugar substrate and mechanism for regulation by Klotho is not known. Here, we report that the extracellular domain of Klotho activates plasma-membrane resident TRPV5 through removing terminal sialic acids from their glycan chains. Removal of sialic acids exposes underlying disaccharide galactose-N-acetylglucosamine, a ligand for a ubiquitous galactoside-binding lectin galectin-1. Binding to galectin-1 lattice at the extracellular surface leads to accumulation of functional TRPV5 on the plasma membrane. Knockdown of beta-galactoside alpha2,6-sialyltransferase (ST6Gal-1) by RNA interference, but not other sialyltransferases, in a human cell line prevents the regulation by Klotho. Moreover, the regulation by Klotho is absent in a hamster cell line that lacks endogenous ST6Gal-1, but is restored by forced expression of recombinant ST6Gal-1. Thus, Klotho participates in specific removal of alpha2,6-linked sialic acids and regulates cell surface retention of TRPV5 through this activity. This action of Klotho represents a novel mechanism for regulation of the activity of cell-surface glycoproteins and likely contributes to maintenance of calcium balance by Klotho.

Sialic acids in human health and disease

The surfaces of all vertebrate cells are decorated with a dense and complex array of sugar chains, which are mostly attached to proteins and lipids. Most soluble secreted proteins are also similarly decorated with such glycans. Sialic acids are a diverse family of sugar units with a nine-carbon backbone that are typically found attached to the outermost ends of these chains. Given their location and ubiquitous distribution, sialic acids can mediate or modulate a wide variety of physiological and pathological processes. This review considers some examples of their established and newly emerging roles in aspects of human physiology and disease.

Disruption of thymopoiesis in ST6Gal I-deficient mice

Thymocyte development is accompanied by sequential changes in cell surface glycosylation. For example, medullary thymocytes have increased levels of alpha2,3-linked sialic acid and a loss of asialo core 1 O-glycans as compared to cortical thymocytes. Some of these changes have been linked to fine tuning of the T cell receptor avidity. We analyzed ST6Gal I transcript abundance and levels of alpha2,6-linked sialic acid across thymocyte subsets. We found that ST6Gal I transcript levels increased following T cell receptor beta-selection suggesting that this sialyltransferase may influence the development of early thymocyte populations. Indeed, low levels of alpha2,6-linked sialic acid were found in the earliest T lineage cells, and then increased in T cell receptor beta-selected cells. To determine whether ST6Gal I influences T cell development, we analyzed ST6Gal I-deficient mice for disruptions in thymocyte populations. We found reduced thymic cellularity in the ST6Gal I-deficient mice starting in the early thymocyte compartments.

Disruption of GM2/GD2 synthase gene resulted in overt expression of 9-O-acetyl GD3 irrespective of Tis21

GM2/GD2 synthase gene knockout mice lack all complex gangliosides, which are abundantly expressed in the nervous systems of vertebrates. In turn, they have increased precursor structures GM3 and GD3, probably replacing the roles of the depleted complex gangliosides. In this study, we found that 9-O-acetyl GD3 is also highly expressed as one of the major glycosphingolipids accumulating in the nervous tissues of the mutant mice. The identity of the novel component was confirmed by neuraminidase treatment, thin layer chromatography-immunostaining, two-dimensional thin layer chromatography with base treatment, and mass spectrometry. All candidate factors reported to be possible inducer of 9-O- acetylation, such as bitamine D binding protein, acetyl CoA transporter, or O-acetyl ganglioside synthase were not up-regulated. Tis21 which had been reported to be a 9-O-acetylation inducer was partially down-regulated in the null mutants, suggesting that Tis21 is not involved in the induction of 9-O-acetyl-GD3 and that accumulated high amount of GD3 might be the main factor for the dramatic increase of 9-O-acetyl GD3. The ability to acetylate exogenously added GD3 in the normal mouse astrocytes was examined, showing that the wild-type brain might be able to synthesize very low levels of 9-O-acetyl GD3. Increased 9-O-acetyl GD3, in addition to GM3 and GD3, may play an important role in the compensation for deleted complex gangliosides in the mutant mice.

Endogenous galectin-1 enforces class I-restricted TCR functional fate decisions in thymocytes

During thymocyte development, the T-cell receptor (TCR) can discriminate major histocompatibility complex (MHC)/peptide ligands over a narrow range of affinities and translate subtle differences into functional fate decisions. How small differences in TCR input are translated into absolute differences in functional output is unclear. We examined the effects of galectin-1 ablation in the context of class-I-restricted thymocyte development. Galectin-1 expression opposed TCR partial agonist-driven positive selection, but promoted TCR agonist-driven negative selection of conventional CD8(+) T cells. Galectin-1 expression also promoted TCR agonist-driven CD8alphaalpha intestinal intraepithelial lymphocytes (IEL) development. Recombinant galectin-1 enhanced TCR binding to agonist/MHC complexes and promoted a negative-selection-signaling signature, reflected in intensified rapid and transient extracellular signal-regulated kinase (ERK) activation. In contrast, galectin-1 expression antagonized ERK activity in thymocytes undergoing positive selection. We propose that galectin-1 aids in discriminating TCR-directed fate decisions by promoting TCR binding to agonist/MHC complexes and enforcing agonist-driven signals, while opposing partial-agonist signals. In this way, galectin-1 widens the distinction between TCR-directed functional fate cues.

Cell surface glycosylation diversity of embryonic thymic tissues

In the thymus, glycosylation status of many cell surface molecules changes during the thymocyte maturation and selection processes. In this study, we evaluated the glycosylation changes and possible relationships with programmed cell death in the thymic tissues from mouse embryos at the days 14 (E14), 15 (E15), 16 (E16), 17 (E17) and 18 (E18) of embryonic development. In order to determine glycosylation changes we used three different plant lectins: peanut agglutinin (PNA), Maackia amurensis leucoagglutinin (MAL or MAAI) and Sambucus nigra agglutinin (SNA), which recognize core disaccharide galactose (1-3) N-acetylgalactosamine [Galbeta(1-->3)GalNAc], sialic acid linked (2-->3) to galactose [SAalpha(2-->3)Gal] and sialic acid linked to galactose [SAalpha(2-->6)Gal] structures, respectively. Our lectin histochemistry and lectin blotting studies indicated that glycosylation pattern was modified in thymocytes at the embryonic developmental stages analyzed. The immature cortical thymocytes were labeled by PNA, whereas medullary thymocytes were positive for MAL and SNA binding. Many medullary thymocytes exhibited alpha(2-->6)-linked sialic acid on their surface and this increased throughout the gestational stages. In the lectin blotting studies, different protein bands of various molecular weights were identified in thymocytes. Two of them were putatively identified as CD43 and CD45 glycoproteins. In addition, TUNEL (deoxynucleotdyltransferase-mediated dUDP nick end labeling) indicated that only PNA-positive cortical thymocytes were deleted in all embryonic stages. These results indicate that the glycosylation pattern was modified in thymocytes at all embryonic developmental stages, and these modifications can affect the T cell deletion, probably via the galectin-1 molecule in the embryonic thymus.

N-glycan structures and associated gene expression reflect the characteristic N-glycosylation pattern of human hematopoietic stem and progenitor cells

OBJECTIVE

Cell surface glycans contribute to the adhesion capacity of cells and are essential in cellular signal transduction. Yet, the glycosylation of hematopoietic stem and progenitor cells (HSPC), such as CD133+ cells, is poorly explored.

MATERIALS AND METHODS

N-glycan structures of cord blood-derived CD133+ and CD133- cells were analyzed with mass spectrometric profiling and exoglycosidase digestion, cell surface glycan epitopes with lectin binding assay, and expression of N-glycan biosynthesis-related genes with microarray analysis.

RESULTS

Over 10% difference was demonstrated in the N-glycan profiles of CD133+ and CD133- cells. Biantennary complex-type N-glycans were enriched in CD133+ cells. Of the genes regulating the synthesis of these structures, CD133+ cells overexpressed MGAT2 and underexpressed MGAT4. Moreover, the amount of high-mannose type N-glycans and terminal alpha2,3-sialylation was increased in CD133+ cells. Elevated alpha2,3-sialylation was supported by the overexpression of ST3GAL6.

CONCLUSION

Our work presents new information on the characters of HSPCs. The new knowledge of HSPC-specific N-glycosylation advances their identification and provides tools to promote HSPC homing and mobilization or targeting to specific tissues.

Effective replication of human influenza viruses in mice lacking a major alpha2,6 sialyltransferase

The hemagglutinins of influenza viruses isolated from humans typically prefer binding to sialic acid in an alpha2,6 linkage. Presumably, the virus uses the presence of these receptors on the respiratory tract to gain entrance into the host cell. The ST6Gal I sialyltransferase knock-out mouse lacks the main enzyme necessary for the attachment of alpha2,6 sialic acid to N-linked glycoproteins on the cell surface. Yet even in the absence of detectable alpha2,6 sialic acid in the mouse respiratory tract, human influenza viruses can still infect these mice and grow to similar titers in the lung and trachea as compared to wild-type animals. This work demonstrates that the presence of a major alpha2,6 sialic acid on N-linked glycoproteins is not essential for human influenza virus infection in mice.

Diversity in cell surface sialic acid presentations: implications for biology and disease

Sialic acids (Sias) are typically found as terminal monosaccharides attached to cell surface glycoconjugates. They play many important roles in many physiological and pathological processes, including microbe binding that leads to infections, regulation of the immune response, the progression and spread of human malignancies and in certain aspects of human evolution. This review will provide some examples of these diverse roles of Sias and briefly address immunohistochemical approaches to their detection.

Increased α2,6-sialylation of surface proteins on tolerogenic, immature dendritic cells and regulatory T cells

OBJECTIVE

Surface protein glycosylation of lymphocytes plays a key role in development, maturation, and immune regulation. Sialic acid most often is the terminal carbohydrate in these posttranslational modifications. Receptors for sialic acids are expressed on lymphocytes and can generate an inhibitory signal. This study compared the sialic acid expression pattern of tolerogenic cells and effector cells.

METHODS

Gene expression profiles of immature and mature monocyte-derived dendritic cells were compared using cDNA array technology. We analyzed the cell-surface protein sialylation of dendritic cells and different T cell subpopulations by flow cytometry using plant lectins.

RESULTS

Monocyte-derived dendritic cells showed a separation according to alpha2,6-linked sialic acid density. Tolerogenic, immature DC showed a higher alpha2,6-linked sialic acid, which was drastically downregulated after maturation of DC with proinflammatory cytokines. This differential expression of alpha2,6-linked sialic acid was reflected by transcriptional regulation of specific glycosyl transferases during DC maturation shown by cDNA array analysis. Furthermore, CD4(+) T cells significantly upregulated alpha2,6-linked sialic acid density, whereas alpha2,3-linked sialic acid density remained largely unchanged after stimulation. Isolated CD4(+)CD25(+) T cells showed a population with high density of alpha2,6-linked sialic acid and a population with low expression. The density of this particular carbohydrate was further increased during culture conditions expanding inhibitory T cells.

CONCLUSION

Surface proteins on tolerogenic, immature dendritic cells and regulatory T cells are highly alpha2,6-sialylated, suggesting a glycan motif of tolerogenic cells which might serve as ligand for inhibitory siglecs on the surface of effector cells.

Influenza virus receptor specificity and cell tropism in mouse and human airway epithelial cells

Recent human infections caused by the highly pathogenic avian influenza virus H5N1 strains emphasize an urgent need for assessment of factors that allow viral transmission, replication, and intra-airway spread. Important determinants for virus infection are epithelial cell receptors identified as glycans terminated by an alpha2,3-linked sialic acid (SA) that preferentially bind avian strains and glycans terminated by an alpha2,6-linked SA that bind human strains. The mouse is often used as a model for study of influenza viruses, including recent avian strains; however, the selectivity for infection of specific respiratory cell populations is not well described, and any relationship between receptors in the mouse and human lungs is incompletely understood. Here, using in vitro human and mouse airway epithelial cell models and in vivo mouse infection, we found that the alpha2,3-linked SA receptor was expressed in ciliated airway and type II alveolar epithelial cells and was targeted for cell-specific infection in both species. The alpha2,6-linked SA receptor was not expressed in the mouse, a factor that may contribute to the inability of some human strains to efficiently infect the mouse lung. In human airway epithelial cells, alpha2,6-linked SA was expressed and functional in both ciliated and goblet cells, providing expanded cellular tropism. Differences in receptor and cell-specific expression in these species suggest that differentiated human airway epithelial cell cultures may be superior for evaluation of some human strains, while the mouse can provide a model for studying avian strains that preferentially bind only the alpha2,3-linked SA receptor.

Altered granulopoietic profile and exaggerated acute neutrophilic inflammation in mice with targeted deficiency in the sialyltransferase ST6Gal I

Elevation of serum sialic acid and the ST6Gal-1 sialyltransferase is part of the hepatic system inflammatory response, but the contribution of ST6Gal-1 has remained unclear. Hepatic ST6Gal-1 elevation is mediated by P1, 1 of 6 promoters regulating the ST6Gal1 gene. We report that the P1-ablated mouse, Siat1DeltaP1, and a globally ST6Gal-1-deficient mouse had significantly increased peritoneal leukocytosis after intraperitoneal challenge with thioglycollate. Exaggerated peritonitis was accompanied by only a modest increase in neutrophil viability, and transferred bone marrow-derived neutrophils from Siat1DeltaP1 mice migrated to the peritonea of recipients with normal efficiency after thioglycollate challenge. Siat1DeltaP1 mice exhibited 3-fold greater neutrophilia by thioglycollate, greater pools of epinephrine-releasable marginated neutrophils, greater sensitivity to G-CSF, elevated bone marrow CFU-G and proliferative-stage myeloid cells, and a more robust recovery from cyclophosphamide-induced myelosuppression. Bone marrow leukocytes from Siat1DeltaP1 are indistinguishable from those of wild-type mice in alpha2,6-sialylation, as revealed by the Sambucus nigra lectin, and in the expression of total ST6Gal-1 mRNA. Together, our study demonstrated a role for ST6Gal-1, possibly from extramedullary sources (eg, produced in liver) in regulating inflammation, circulating neutrophil homeostasis, and replenishing granulocyte numbers.

α2,3/α2,6-Sialylation of N-glycans: non-synonymous signals with marked developmental regulation in bovine reproductive tracts

The glycan part endows cellular glycoconjugates with significant potential for biological recognition. N-Glycan branches often end with alpha2,3/alpha2,6-sialylation, posing the question whether and how placement of the sialic acid at 3 - or 6 -acceptor positions of galactose has cell biological relevance. As attractive model to study developmental regulation we monitored the expression of alpha2,3/alpha2,6-sialylated determinants in fetal and adult bovine testes and ovaries by lectin histochemistry. Distinct expression patterns were detected in both organ types. Oocyte staining, as a prominent example, was restricted to the presence of alpha2,6-sialylated glycans. Treatment with sialidase abolished binding and thus excluded sulfate esters as lectin targets. We added computer simulations to rationalize the observed evidence for non-random expression of the two closely related sialylgalactose isomers. Extensive molecular mechanics and molecular dynamics calculations reveal that the seemingly minor shift of the glycosidic bond from the alpha2,3 position to the alpha2,6 configuration causes significant shape and flexibility changes. They give each disaccharide its own characteristic meaning as signal in the sugar code.

Sialoadhesin-deficient mice exhibit subtle changes in B- and T-cell populations and reduced immunoglobulin M levels

Sialoadhesin (Sn, also called Siglec-1 or CD169) is a transmembrane receptor and the prototypic member of the Siglec family of sialic acid binding immunoglobulin-like lectins. It is expressed on specialized subsets of resident macrophages in hematopoietic and lymphoid tissues and on inflammatory macrophages. In order to investigate its function, we generated Sn-deficient mice and confirmed that these mice are true nulls by fluorescence-activated cell sorter analysis and immunohistochemistry. Mice deficient in Sn were viable and fertile and showed no developmental abnormalities. Analysis of cell populations revealed no differences in bone marrow, peritoneal cavity, and thymus, but there was a small increase in CD8 T cells and a decrease in B220-positive cells in spleens and lymph nodes of Sn-deficient mice. Furthermore, in spleen there was a slight decrease in follicular B cells with an increase in numbers of marginal zone B cells. B- and T-cell maturation as well as responses to stimulation with thioglycolate were only slightly affected by Sn deficiency. Immunoglobulin titers in Sn-deficient mice were significantly decreased for immunoglobulin M (IgM) but similar for IgG subclasses. These results suggest a role for sialoadhesin in regulating cells of the immune system rather than in influencing steady-state hematopoiesis.

Glycomics: an integrated systems approach to structure-function relationships of glycans

In comparison with genomics and proteomics, the advancement of glycomics has faced unique challenges in the pursuit of developing analytical and biochemical tools and biological readouts to investigate glycan structure-function relationships. Glycans are more diverse in terms of chemical structure and information density than are DNA and proteins. This diversity arises from glycans' complex nontemplate-based biosynthesis, which involves several enzymes and isoforms of these enzymes. Consequently, glycans are expressed as an 'ensemble' of structures that mediate function. Moreover, unlike protein-protein interactions, which can be generally viewed as 'digital' in regulating function, glycan-protein interactions impinge on biological functions in a more 'analog' fashion that can in turn 'fine-tune' a biological response. This fine-tuning by glycans is achieved through the graded affinity, avidity and multivalency of their interactions. Given the importance of glycomics, this review focuses on areas of technologies and the importance of developing a bioinformatics platform to integrate the diverse datasets generated using the different technologies to allow a systems approach to glycan structure-function relationships.

Loss of CD8 and TCR binding to Class I MHC ligands following T cell activation

The capacity of T cells to bind peptide/MHC ligands changes with T cell development and differentiation. Here we study changes in peptide/MHC multimer binding following T cell activation. Surprisingly, T cell activation caused a marked reduction in specific peptide/MHC Class I multimer binding, which was distinct from transient TCR down-regulation, and was especially dramatic for engagement with low-affinity peptide/MHC ligands. Direct CD8-Class I interactions were also profoundly and rapidly impaired following T cell stimulation, even though surface CD8alpha and CD8beta levels were unchanged after activation, suggesting that decreased CD8 co-receptor binding contributes to this effect. Finally, we show that enzymatic desialylation restores much of the multimer binding on activated T cells, suggesting that altered glycosylation may inhibit TCR/CD8 binding to peptide/MHC ligands. These radical changes in activated T cells' ability to perceive peptide/MHC ligands may contribute to selective outgrowth of clones with high affinity for the stimulatory ligand.

Thyroid sialyltransferase mRNA level and activity are increased in Graves' disease

Sialylation of cell components is an important immunomodulating mechanism affecting cell response to hormones and adhesion molecules. To study alterations in sialic acid metabolism in Graves' disease (GD) we measured the following parameters in various human thyroid tissues: lipid-bound sialic acid (LBSA) content, ganglioside profile, total sialyltransferase activity, and the two major sialyltransferase mRNAs for sialyltransferase-1 (ST6Gal I) and for sialyltransferase-4A (ST3Gal I). Fragments of toxic thyroid nodules (TN), nontoxic thyroid nodules (NN) and nontumorous tissue from patients with nodular goiter or thyroid cancer were used as a control (C). The LBSA content and sialyltransferase activity were the highest in the GD group (164 +/- 4.44 versus 120 +/- 2.00 nmoL/g, p = 0.005 and 1625 +/- 283.5 versus 324 +/- 54.2 cpm/mg of protein, p < 0.005 compared to control group C). Ganglioside profile in the GD group was similar to that in control tissues. Sialyltransferase- 1 mRNA and sialyltransferase-4A mRNA levels were significantly higher in the GD group than in the control group (12.52 +/- 6.90 versus 2.54 +/- 1.24 arbitrary units, p < 0.005 and 2,49 +/- 1.16 versus 1.23 +/- 0.46 arbitrary units, p < 0.05, respectively). There was a positive correlation between the increased sialyltransferase-1 mRNA level and the TSH-receptor antibody titer determined by the TRAK test. These results indicate that sialyltransferases expression and activity are increased in GD. Exact mechanism of this upregulation remains unknown, though one of possible explanations is the activation of the thyrotropin (TSH) receptor.

Regulation of sialic acid O-acetylation in human colon mucosa

The expression of O-acetylated sialic acids in human colonic mucins is developmentally regulated, and a reduction of O-acetylation has been found to be associated with the early stages of colorectal cancer. Despite this, however, little is known about the enzymatic process of sialic acid O-acetylation in human colonic mucosa. Recently, we have reported on a human colon sialate-7(9)-O-acetyltransferase capable of incorporating acetyl groups into sialic acids at the nucleotide-sugar level [Shen et al., Biol. Chem. 383 (2002), 307-317]. In this report, we show that the CMP-N-acetyl-neuraminic acid (CMP-Neu5Ac) and acetyl-CoA (AcCoA) transporters are critical components for the O-acetylation of CMP-Neu5Ac in Golgi lumen, with specific inhibition of either transporter leading to a reduction in the formation of CMP-5-N-acetyl-9-O-acetyl-neuraminic acid (CMP-Neu5,9Ac2). Moreover, the finding that 5-N-acetyl-9-O-acetyl-neuraminic acid (Neu5,9Ac2 could be transferred from neo-synthesised CMP-Neu5,9Ac2 to endogenous glycoproteins in the same Golgi vesicles, together with the observation that asialofetuin and asialo-human colon mucin are much better acceptors for Neu5,9Ac2 than asialo-bovine submandibular gland mucin, suggests that a sialyltransferase exists that preferentially utilises CMP-Neu5,9Ac2 as the donor substrate, transferring Neu5,9Ac2 to terminal Galbeta1,3(4)R- residues.

Expression of glycosphingolipids in lymph nodes of mice lacking TNF receptor 1: biochemical and flow cytometry analysis

The expression of gangliosides and neutral glycosphingolipids (GSLs) in the lymph nodes of mice lacking the gene for the tumour necrosis factor-alpha receptor p55 (TNFR1) has been investigated. GSL expression in the tissues of mice homozygous (TNFR1-/-) or heterozygous (TNFR1+/-) for the gene deletion was analysed by flow cytometry and high-performance thin-layer chromatography (HPTLC) followed by immunostaining with specific antibodies. HPTLC immunostaining revealed that lymph nodes from TNFR1-/- mice had reduced expression of ganglioside GM1b and GalNAc-GM1b, neolacto-series gangliosides, as well as the globo- (Gb3, Gb4 and Gb5) and ganglio-series (Gg3 and Gg4) neutral GSLs. Flow cytometry of freshly isolated lymph node cells showed no significant differences in GSL expression, except for the GalNAc-GM1b ganglioside, which was less abundant on T lymphocytes from TNFR1-/- lymph nodes. In TNFR1-/- mice, GalNAc-GM1b+/CD4+ T cells were twofold less abundant (3.8% vs 7.6% in the control mice), whereas GalNAc-GM1b+/CD8+ T cells were fourfold less abundant (5.0% vs 20.2% in the control mice). This study provides in vivo evidence that TNF signalling via the TNFR1 is important for the activation of GM1b-type ganglioside biosynthetic pathway in CD8 T lymphocytes, suggesting its possible role in the effector T lymphocyte function.

Human-specific regulation of alpha 2-6-linked sialic acids

Many microbial pathogens and toxins recognize animal cells via cell surface sialic acids (Sias) that are alpha 2-3- or alpha 2-8-linked to the underlying glycan chain. Human influenza A/B viruses are unusual in preferring alpha 2-6-linked Sias, undergoing a switch from alpha 2-3 linkage preference during adaptation from animals to humans. This correlates with the expression of alpha 2-6-linked Sias on ciliated human airway epithelial target cells and of alpha 2-3-linked Sias on secreted soluble airway mucins, which are unable to inhibit virus binding. Given several known differences in Sia biology between humans and apes, we asked whether this pattern of airway epithelial Sia linkages is also human-specific. Indeed, we show that since the last common ancestor with apes, humans underwent a concerted bidirectional switch in alpha 2-6-linked Sia expression between airway epithelial cell surfaces and secreted mucins. This can explain why the chimpanzee appears relatively resistant to experimental infection with human Influenza viruses. Other tissues showed additional examples of human-specific increases or decreases in alpha 2-6-linked Sia expression and only one example of a change specific to certain great apes. Furthermore, while human and great ape leukocytes both express alpha 2-6-linked Sias, only human erythrocytes have markedly up-regulated expression. These cell type-specific changes in alpha 2-6-Sia expression during human evolution represent another example of a human-specific change in Sia biology. Because the data set involves multiple great apes, we can also conclude that Sia linkage expression patterns can be conserved during millions of years of evolution within some vertebrate taxa while undergoing sudden major changes in other closely related ones.

Spatiotemporal expression patterns of sialoglycoconjugates during nephron morphogenesis and their regional and cell type-specific distribution in adult rat kidney

The expression of alpha2,6- and alpha2,3-linked sialic acids on N-glycans was studied in embryonic, postnatal, and adult rat kidney. Histochemistry and blotting using Polyporus squamosus and Sambucus nigra lectins for alpha2,6-linked sialic acids and the Maackia amurensis lectin for alpha2,3-linked sialic acids were performed and sialyltransferase activity was assayed. N-glycans with alpha2,6- and alpha2,3-linked sialic acid were differently expressed in the two embryonic anlagen and early stages of nephron. Metanephrogenic mesenchyme was positive for alpha2,3-linked sialic acid but not for the alpha2,6-linked one, which became detectable initially in the proximal part of S-shaped bodies. Collecting ducts were positive for alpha2,6-linked sialic acid, whereas alpha2,3-linked sialic acid was restricted to their ampullae. Although positive in embryonic kidney, S1 and S2 of proximal tubules became unreactive for alpha2,3-linked sialic acid in postnatal and adult kidneys. In adult kidney, intercalated but not principal cells of collecting ducts were reactive for alpha2,3-linked sialic acid. In contrast, alpha2,6-linked sialic acids were detected in all cells of adult kidney nephron. Blot analysis revealed a different but steady pattern of bands reactive for alpha2,6- and alpha2,3-linked sialic acid in embryonic, postnatal, and adult kidney. Activity of alpha2,6 and alpha2,3 sialyltransferases was highest in embryonic kidney and decreased over postnatal to adult kidney with the activity of alpha2,6 sialyltransferase always being three to fourfold that of alpha2,3 sialyltransferase. Thus, alpha2,6- and alpha2,3-linked sialic acids are differently expressed in embryonic anlagen and mesenchyme-derived early stages of nephron and show regional and cell type-specific differences in adult kidney.

Sweet 'n' sour: the impact of differential glycosylation on T cell responses

The fate and functional activity of T lymphocytes depend largely on the precise timing of gene expression and protein production. However, it is clear that post-translational modification of proteins affects their functional properties. Although modifications such as phosphorylation have been intensely studied by immunologists, less attention has been paid to the impact that changes in glycosylation have on protein function. However, there is considerable evidence that glycosylation plays a key role in immune regulation. We will focus here on examples in which differential glycosylation affects the development, survival or reactivity of T cells.