NOR polyagglutination and Sta glycophorin in one family: relation of NOR polyagglutination to terminal alpha-galactose residues and abnormal glycolipids

Missing the sweet spot: one of the two N-glycans on human Gb3/CD77 synthase is expendable

N-glycosylation is a ubiquitous posttranslational modification that may influence folding, subcellular localization, secretion, solubility and oligomerization of proteins. In this study, we examined the effects of N-glycans on the activity of human Gb3/CD77 synthase, which catalyzes the synthesis of glycosphingolipids with terminal Galα1 → 4Gal (Gb3 and the P1 antigen) and Galα1 → 4GalNAc disaccharides (the NOR antigen). The human Gb3/CD77 synthase contains two occupied N-glycosylation sites at positions N121 and N203. Intriguingly, we found that while the N-glycan at N203 is essential for activity and correct subcellular localization, the N-glycan at N121 is dispensable and its absence did not reduce, but, surprisingly, even increased the activity of the enzyme. The fully N-glycosylated human Gb3/CD77 synthase and its glycoform missing the N121 glycan correctly localized in the Golgi, whereas a glycoform without the N203 site partially mislocalized in the endoplasmic reticulum. A double mutein missing both N-glycans was inactive and accumulated in the endoplasmic reticulum. Our results suggest that the decreased specific activity of human Gb3/CD77 synthase glycovariants results from their improper subcellular localization and, to a smaller degree, a decrease in enzyme solubility. Taken together, our findings show that the two N-glycans of human Gb3/CD77 synthase have opposing effects on its properties, revealing a dual nature of N-glycosylation and potentially a novel regulatory mechanism controlling the biological activity of proteins.

Single nucleotide polymorphisms in A4GALT spur extra products of the human Gb3/CD77 synthase and underlie the P1PK blood group system

Contrary to the mainstream blood group systems, P1PK continues to puzzle and generate controversies over its molecular background. The P1PK system comprises three glycosphingolipid antigens: P^k, P1 and NOR, all synthesised by a glycosyltransferase called Gb3/CD77 synthase. The P^k antigen is present in most individuals, whereas P1 frequency is lesser and varies regionally, thus underlying two common phenotypes: P₁, if the P1 antigen is present, and P₂, when P1 is absent. Null and NOR phenotypes are extremely rare. To date, several single nucleotide polymorphisms (SNPs) have been proposed to predict the P₁/P₂ status, but it has not been clear how important they are in general and in relation to each other, nor has it been clear how synthesis of NOR affects the P₁ phenotype. Here, we quantitatively analysed the phenotypes and A4GALT transcription in relation to the previously proposed SNPs in a sample of 109 individuals, and addressed potential P1 antigen level confounders, most notably the red cell membrane cholesterol content. While all the SNPs were associated with the P₁/P₂ blood type and rs5751348 was the most reliable, we found large differences in P1 level within groups defined by their genotype and substantial intercohort overlaps, which shows that the P1PK blood group system still eludes full understanding.

Evaluation of an amino acid residue critical for the specificity and activity of human Gb3/CD77 synthase

Human Gb3/CD77 synthase (α1,4-galactosyltransferase) is the only known glycosyltransferase that changes acceptor specificity because of a point mutation. The enzyme, encoded by A4GALT locus, is responsible for biosynthesis of Gal(α1-4)Gal moiety in Gb3 (CD77, P^k antigen) and P1 glycosphingolipids. We showed before that a single nucleotide substitution c.631C > G in the open reading frame of A4GALT, resulting in replacement of glutamine with glutamic acid at position 211 (substitution p. Q211E), broadens the enzyme acceptor specificity, so it can not only attach galactose to another galactose but also to N-acetylgalactosamine. The latter reaction leads to synthesis of NOR antigens, which are glycosphingolipids with terminal Gal(α1-4)GalNAc sequence, never before described in mammals. Because of the apparent importance of position 211 for enzyme activity, we stably transfected the 2102Ep cells with vectors encoding Gb3/CD77 synthase with glutamine substituted by aspartic acid or asparagine, and evaluated the cells by quantitative flow cytometry, high-performance thin-layer chromatography and real-time PCR. We found that cells transfected with vectors encoding Gb3/CD77 synthase with substitutions p. Q211D or p. Q211N did not express P^k, P1 and NOR antigens, suggesting complete loss of enzymatic activity. Thus, amino acid residue at position 211 of Gb3/CD77 synthase is critical for specificity and activity of the enzyme involved in formation of P^k, P1 and NOR antigens. Altogether, this approach affords a new insight into the mechanism of action of the human Gb3/CD77 synthase.

Human Gb3/CD77 synthase reveals specificity toward two or four different acceptors depending on amino acid at position 211, creating P(k), P1 and NOR blood group antigens

Human Gb3/CD77 synthase (α1,4-galactosyltransferase, P(k) synthase), encoded by A4GALT gene, is known for synthesis of Gal(α1-4)Gal moiety in globotriaosylceramide (Gb3Cer, CD77, P(k) blood group antigen), a glycosphingolipid of the globo series. Recently, it was shown that c.631C > G mutation in A4GALT, which causes p.Q211E substitution in the open reading frame of the enzyme, broadens the enzyme specificity, making it able also to synthesize Gal(α1-4)GalNAc moiety, which constitutes the defining terminal disaccharide of the NOR antigen (carried by two glycosphingolipids: NOR1 and NOR2). Terminal Gal(α1-4)Gal disaccharide is also present in another glycosphingolipid blood group antigen, called P1, which together with P(k) and NOR comprises the P1PK blood group system. Despite several attempts, it was never clearly shown that P1 antigen is synthesized by Gb3/CD77 synthase, leaving open an alternative hypothesis that there are two homologous α1,4-galactosyltransferases in humans. In this study, using recombinant Gb3/CD77 synthase produced in insect cells, we show that the consensus enzyme synthesizes both the P(k) and P1 antigens, while its p.Q211E variant additionally synthesizes the NOR antigen. This is the first direct biochemical evidence that Gb3/CD77 synthase is able to synthesize two different glycosphingolipid antigens: P(k) and P1, and when p.Q211E substitution is present, the NOR antigen is also synthesized.

Human genetic disorders of sphingolipid biosynthesis

Monogenic defects of sphingolipid biosynthesis have been recently identified in human patients. These enzyme deficiencies affect the synthesis of sphingolipid precursors, ceramides or complex glycosphingolipids. They are transmitted as autosomal recessive or dominant traits, and their resulting phenotypes often replicate the abnormalities seen in murine models deficient for the corresponding enzymes. In quite good agreement with the known critical roles of sphingolipids in cells from the nervous system and the epidermis, these genetic defects clinically manifest as neurological disorders, including paraplegia, epilepsy or peripheral neuropathies, or present with ichthyosis. The present review summarizes the genetic alterations, biochemical changes and clinical symptoms of this new group of inherited metabolic disorders. Hypotheses regarding the molecular pathophysiology and potential treatments of these diseases are also discussed.

A single point mutation in the gene encoding Gb3/CD77 synthase causes a rare inherited polyagglutination syndrome

Rare polyagglutinable NOR erythrocytes contain three unique globoside (Gb4Cer) derivatives, NOR1, NOR(int), and NOR2, in which Gal(α1-4), GalNAc(β1-3)Gal(α1-4), and Gal(α1-4)GalNAc(β1-3)Gal(α1-4), respectively, are linked to the terminal GalNAc residue of Gb4Cer. NOR1 and NOR2, which both terminate with a Gal(α1-4)GalNAc- sequence, react with anti-NOR antibodies commonly present in human sera. While searching for an enzyme responsible for the biosynthesis of Gal(α1-4)GalNAc, we identified a mutation in the A4GALT gene encoding Gb3/CD77 synthase (α1,4-galactosyltransferase). Fourteen NOR-positive donors were heterozygous for the C>G mutation at position 631 of the open reading frame of the A4GALT gene, whereas 495 NOR-negative donors were homozygous for C at this position. The enzyme encoded by the mutated gene contains glutamic acid instead of glutamine at position 211 (substitution Q211E). To determine whether this mutation could change the enzyme specificity, we transfected a teratocarcinoma cell line (2102Ep) with vectors encoding the consensus Gb3/CD77 synthase and Gb3/CD77 synthase with Glu at position 211. The cellular glycolipids produced by these cells were analyzed by flow cytometry, high-performance thin-layer chromatography, enzymatic degradation, and MALDI-TOF mass spectrometry. Cells transfected with either vector expressed the P1 blood group antigen, which was absent from untransfected cells. Cells transfected with the vector encoding the Gb3/CD77 synthase with Glu at position 211 expressed both P1 and NOR antigens. Collectively, these results suggest that the C631G mutation alters the acceptor specificity of Gb3/CD77 synthase, rendering it able to catalyze synthesis of the Gal(α1-4)Gal and Gal(α1-4)GalNAc moieties.

Lectins as tools in glycoconjugate research

Lectins are ubiquitous proteins of nonimmune origin, present in plants, microorganisms, animals and humans which specifically bind defined monosugars or oligosaccharide structures. Great progress has been made in recent years in understanding crucial roles played by lectins in many biological processes. Elucidation of carbohydrate specificity of human and animal lectins is of great importance for better understanding of these processes. Long before the role of carbohydrate-protein interactions had been explored, many lectins, mostly of plant origin, were identified, characterized and applied as useful tools in studying glycoconjugates. This review focuses on the specificity-based lectin classification and the methods of measuring lectin-carbohydrate interactions, which are used for determination of lectin specificity or for identification and characterization of glycoconjugates with lectins of known specificity. The most frequently used quantitative methods are shortly reviewed and the methods elaborated and used in our laboratories, based on biotinylated lectins, are described. These include the microtiter plate enzyme-linked lectinosorbent assay, lectinoblotting and lectin-glycosphingolipid interaction on thin-layer plates. Some chemical modifications of lectin ligands on the microtiter plates and blots (desialylation, Smith degradation, beta-elimination), which extend the applicability of these methods, are also described.

Structures of unique globoside elongation products present in erythrocytes with a rare NOR phenotype

Rare polyagglutinable erythrocytes of NOR phenotype were found to contain two unique glycosphingolipids (designated NOR1 and NOR2). These components (not detected in normal erythrocytes) were reactive with Griffonia simplicifolia isolectin IB4 (GSL-IB4) and commonly present human anti-NOR antibodies. The NOR1 component has been reported to be a globoside containing a single galactose residue linked alpha1,4 to the terminal N-acetylgalactosamine. Here, we report the structural studies on a second glycolipid, NOR2, and a third novel component migrating in high-performance thin-layer chromatography (HPTLC) between NOR1 and NOR2. The structures were determined by a combination of ion trap sequential mass spectrometry (MALDI-QIT-TOF) and step-wise treatment with glycosidases, followed by identification of products on HPTLC plates with lectins and mouse monoclonal anti-NOR antibody. The NOR2 component was found to be a disaccharide extension of NOR1 with the following structure: Galalpha1-4GalNAcbeta1-3Galalpha1-4GalNAcbeta1-3Galalpha1-4Galbeta1-4Glcbeta1-Cer. Treatment of NOR2 with alpha-galactosidase gave a glycolipid migrating between NOR1 and NOR2, which did not react with either GSL-IB4 or anti-NOR antibodies but did react with GalNAc-specific soybean agglutinin. This intermediate glycolipid (now designated NOR(int)) was identified as a relatively abundant component of a neutral glycolipid fraction from NOR erythrocytes, suggesting its presence as a precursor to NOR2. The structure of NOR(int) was also confirmed by sequential mass spectrometry studies. These results indicate that polyagglutination in NOR subjects is due to unique erythrocyte glycolipids that are synthesized by sequential addition of Galalpha1,4 and GalNAcbeta1,3 to globoside.

Presence of natural anti-Galα1-4GalNAcβ1-3Gal (anti-NOR) antibodies in animal sera

Rare polyagglutinable NOR erythrocytes contain unusual globoside extention products terminating with a Galalpha1-4GalNAcbeta1-3Gal- unit. This trisaccharide epitope is recognized by recently characterized antibodies naturally occurring in most human sera (Duk et al., Glycobiology, 15, 109, 2005). These antibodies represent two major types of fine specificity. All these antibodies are most strongly inhibited by Galalpha1-4GalNAcbeta1-3Gal (NOR-tri), and weakly by Galalpha1-4Gal. However, the type 1 antibodies are strongly inhibited by Galalpha1-4Galbeta1-3Gal-R and weakly by Galalpha1-4GalNAc, while the type 2 antibodies show the opposite reactivities with these two oligosaccharides. Similar antibodies have now been found in horse, rabbit and pig sera. The antibodies were purified from animal sera by affinity chromatography on Galalpha1-4GalNAcbeta1-3Gal-human serum albumin(HSA)-Sepharose 4B conjugate. The specificity of the antibodies was determined by binding to ELISA plates coated with several alpha-galactosylated oligosaccharide-polyacrylamide (PAA) or -HSA conjugates and by inhibition with synthetic oligosaccharides. The purified antibodies bound specifically to conjugates containing NOR-tri. The inhibition of binding showed that the animal sera also contain two types of anti-NOR antibodies: type 2 was found in the horse serum, and a mixture of both types was present in rabbit and pig serum. These results indicate that anti-NOR, a new and distinct kind of anti-alphaGal antibody, are present in animal sera and show similar specificties and diversity as their counterparts found in human sera.

Red blood cell antigens responsible for inherited types of polyagglutination

The three described types on inheritable polyagglutination are related to altered carbohydrate structures in glycoproteins or/and glycolipds on the erythrocyte surface. HEMPAS, a condition causing anemia and other pathological symptoms, is characterized by impaired biosynthesis of N-glycans, mostly those carried by band 3 and band 4.5 erythrocyte membrane proteins. Cad erythrocytes have abnormal glycophorin O-glycans, structurally related to the more common human Sd(a) and murine CT determinants, and accumulate an Sd(a)-like ganglioside. NOR erythrocytes express recently detected abnormal alpha-galactose-terminated glycosphingolipids, which strongly react with G. simplicifolia IB4 isolectin, but do not react with human anti-Galalpha1-3Gal antibodies.

Synthesis and inhibitory activity of a di- and a trisaccharide corresponding to an erythrocyte glycolipid responsible for the nor polyagglutination

The polyagglutinable erythrocytes NOR contain unusual neutral glycolipids reactive with anti-NOR antibodies. The disaccharide alpha-D-Galp-(1-->4)-D-GalpNAc and the trisaccharide alpha-D-Galp-(1-->4)-beta-D-GalpNAc-(1-->3)-D-Gal corresponding to the non-reducing end of a NOR glycolipid (NOR1) were chemically synthesized. The syntheses were based on a common (1-->4)-beta-D-GalNAc precursor, and utilized benzyl glycoside and benzyl ether functions for persistent blocking of hydroxyls. The alpha-D-Galp-(1-->4)-beta-D-GalpNAc structural element has been found only recently in Nature, and derivatives thereof have not been synthesized before. Both the synthesized oligosaccharides inhibited specifically human anti-NOR antibodies, the trisaccharide being 300 times more active than the disaccharide.

Structure of a neutral glycosphingolipid recognized by human antibodies in polyagglutinable erythrocytes from the rare NOR phenotype

NOR is a rare inheritable polyagglutination phenomenon that has been described in two families. Our recent studies on these erythrocytes showed they contained at least two unique neutral glycosphingolipids, and based on their reactivity with Griffonia simplicifolia IB4 (GSL-IB4) isolectin (Kusnierz-Alejska, G., Duk, M., Storry, J. R., Reid, M. E., Wiecek, B., Seyfried, H., and Lisowska, E. (1999) Transfusion 39, 32-38), both oligosaccharide chains terminated with an alpha-galactose residue. The reactivity with GSL-IB4 suggested that these oligosaccharide chains terminated with a Galalpha1-->3Gal- sequence and that anti-NOR agglutinins were common human anti-Galalpha1-->3Gal xenoantibodies. In this report we describe the structure of one NOR component (NOR1) that migrated on thin-layer chromatographic plates in the region of pentaglycosylceramides. Treatment of this sample with alpha-galactosidase and beta-N-acetylhexosaminidase was followed by high-performance thin-layer chromatography with product detection by lectins and the anti-Gb4 monoclonal antibody. The results suggested that NOR1 was an alpha-galactosylated Gb4Cer with a beta-N-acetylhexosaminidase-resistant GalNAc residue. Gas phase disassembly by ion trap mass spectrometry analysis showed the sequence to be Hex1-->4HexN1-->3Hex1-->4Hex1-->4Hex linked to a ceramide composed of C18 sphingosine and a C24 monounsaturated fatty acid. Together these data indicate NOR1 to be a novel Galalpha1-->4GalNAcbeta1-->3Galalpha1-->4Galbeta1-->4 Glc-Cer structure. Additionally it has been shown that NOR glycolipids are recognized by human antibodies that were distinct from the known anti-Galalpha1-->3Gal xenoantibodies.