Hydrolysis of Tay-Sachs Ganglioside by β-Hexosaminidase A of Human Liver and Urine

Expression of the GM2 activator protein in mouse testis

The GM2-activator protein (GM2-AP), revealed by Li et al. in 1973 in human liver, was initially identified as a protein cofactor that stimulated β-hexosaminidase A to hydrolyze N-acetylgalactosamine from GM2 ganglioside. This cofactor was found to be missing in human variant AB Tay-Sachs disease. Over the years, the GM2-AP has also been shown to be involved in kidney vesicular transport, lipid presentation by CD1 molecule to T-cells, and interaction of human sperm with zona pellucida. Since the expression of the GM2-AP via mRNA detection in mouse tissues was found to be the highest in testis, we became interested in the localization of the GM2-AP at cellular level in mouse testis during spermatogenesis. Using immunohistochemical analysis and electron microscopy, we found that the GM2-AP was predominantly localized in the basal cytoplasm and the attenuated processes of Sertoli cells. The stained structure appeared to be lysosomes. The most interesting finding was the association of the GM2-AP with the acrosomal apparatus in early spermatids. A modest to intense staining was observed in some acrosomal granules and acrosomal caps. The GM2-AP seemed to disappear from acrosomal caps in the later stage of spermatids, in which the nucleus became elongated and condensed. These results suggest that the GM2-AP may be involved in the normal functions of Sertoli cells and play important roles during the development of acrosomal caps in the early spermatids.

Effect of structural modifications of ganglioside GM2 on intra-molecular carbohydrate-to-carbohydrate interaction and enzymatic susceptibility

The effect of inter-molecular carbohydrate-to-carbohydrate interaction on basic cell biological processes has been well documented and appreciated. In contrast, very little is known about the intra-molecular carbohydrate-to-carbohydrate interaction. The presence of an interaction between the GalNAc and the Neu5Ac in GM2 detected by NMR spectroscopy represents a well-defined intra-molecular carbohydrate-to-carbohydrate interaction. This intriguing interaction is responsible for the GM2-epitope, GalNAcbeta1-->4(Neu5Acalpha2-->3)Gal-, to exhibit a rigid and compact conformation. We hypothesized that this compact conformation may be the cause for both the GalNAc and the Neu5Ac in GM2 to be refractory to enzymatic hydrolysis and the GM2 activator protein is able to interact with the compact trisaccharide GM2-epitope, rendering the GalNAc and the Neu5Ac accessible to beta-hexosaminidase A and sialidase. We have used a series of structurally modified GM2 to study the effect of modifications of sugar chains on the conformation and enzymatic susceptibility of this ganglioside. Our hypothesis was borne out by the fact that when the GalNAcbeta1-->4Gal linkage in GM2 was converted to the GalNAcbeta1-->6Gal, both the GalNAc and the Neu5Ac became susceptible to beta-hexosaminidase A and sialidase, respectively, without GM2 activator protein. We hope our work will engender interest in identifying other intra-molecular carbohydrate-to-carbohydrate interactions in glycoconjugates.

Synthesis and enzymatic susceptibility of a series of novel GM2 analogs

A series of GM2 analogs in which GM2 epitope was coupled to a variety of glycosyl lipids were designed and synthesized to investigate the mechanism of enzymatic hydrolysis of GM2 ganglioside. The coupling of N-Troc-protected sialic acid and p-methoxyphenyl galactoside acceptor gave the crystalline disaccharide, which was further coupled with galactosamine donor to give the desired GM2 epitope trisaccharide. After conversion into the corresponding glycosyl donor, the trisaccharide was coupled with galactose, glucose and artificial ceramide (B30) to give the final compounds. The result on hydrolysis of GM2 analogs indicates that GM2 activator protein requires one spacer sugar between GM2 epitope and the lipid moiety to assist the hydrolysis of the terminal GalNAc residue.

Identification and tissue-specific expression of amphioxus GM2 activator protein gene from amphioxus Branchiostoma belcheri

An amphioxus cDNA, AmphiGM2AP, encoding GM2 activator protein was isolated from the gut cDNA library of Branchiostoma belcheri. It is 907 bp long, and its longest open reading frame codes for a precursor protein consisting of 242 amino acid residues with a signal peptide of 14 amino acids. The deduced amino acid sequence includes a conserved domain typical of GM2APs between residues 53 and 224, a single N-linked glycosylation site at position 65 and 8 conserved cysteines. Phylogenetic analysis showed that amphiGM2AP forms a club together with invertebrate GM2APs, indicating that AmphiGM2AP is evolutionarily closely related to invertebrate GM2APs rather than vertebrate ones. Both Northern blotting and in situ hybridization histochemistry analyses revealed a tissue-specific expression pattern of AmphiGM2AP in adult amphioxus with the strongest expression in the digestive system, which is in contrast to the widespread expression pattern of human, mouse and sheep GM2AP genes. It is suggested that AmphiGM2AP is possibly involved in the take-in of digested food components like lipid molecules.

Chemical and Chemoenzymatic Synthesis of S-Linked Ganglioside Analogues and Their Protein Conjugates for Use as Immunogens

Analogues of the tumor-associated gangliosides GM(3) and GM(2) containing terminal S-linked neuraminic acid residues and an amino terminated, truncated ceramide homologue have been synthesized and conjugated to a protein. The synthesis involved coupling of a S-linked sialyl alpha(2-->3) galactose disaccharide with a glucosyl sphingosine analogue, followed by elaboration and deprotection to give amino-terminated glycosyl ceramide 1. Glycosyltransferase-catalyzed extension of the trisaccharide 1 provided access to the modified GM(2) tetrasaccharide 2 or sulphur-containing GD(3) analogue 30. Owing to their potentially enhanced resistance to endogenous exo-glycoside hydrolases and their inherent non-self character, carbohydrate antigens containing non-reducing terminal thioglycosidic linkages may be more immunogenic than O-linked antigens and may stimulate the production of antibodies capable of recognizing naturally occurring oligosaccharides. Our initial results suggest that in fact these antigens are viable immunogens and furthermore, that immune sera cross reacts with O-gangliosides in the context of a heterologous glycoprotein conjugate.

Interaction of GM2 activator protein with glycosphingolipids

GM2 activator protein is a protein cofactor that stimulates the hydrolysis of the GalNAc and the NeuAc in GM2 by beta-hexosaminidase A and sialidase, respectively. To understand the mechanism of action of GM2 activator, the interaction of this protein with GM2 and/or beta-hexosaminidase A has been the subject of interest since the purified GM2 activator became available. Numerous techniques including ultracentrifugation, isoelectric a focusing, polyacrylamide gel electrophoresis, gel filtration, thin layer chromatogram overlay, and fluorescence dequenching assay have been used to investigate the binding and the affinity of GM2 activator to various glycosphingolipids. It has been generally accepted that GM2 activator must have a very weak binding with the enzyme, because they can be easily separated from each other by gel filtration. Therefore, the interaction of GM2 and GM2 activator has been the focus for most of he study. Although preferential association of GM2 activator with GM2 was detected by some methods, GM2 activator was found also to bind other glycosphingolipids. Isolation of the specific complex that consists of only GM2 activator and GM2 from incubation mixture containing the activator protein and mixed glycosphingolipids has not been successfully carried out. Ultracentrifugation and gel-filtration are the mildest methods for the isolation of the complexes. However, these methods do not separate the complexes formed by specific interaction from that formed by non-specific association. In fluorescence dequenching assay, the attempt to isolate the complex of R18 lipid probe with GM2 activator was also not successful. Since GM2 activator and glycosphingolipids contain hydrophobic domains in their molecules, the non-specific hydrophobic interactions between the two components can greatly interfere with the isolation of true functional complexes. Among the reported methods, thin layer chromatography overlay and the assay based on the inhibition of fluorescence dequenching by various glycosphingolipids are more informative than the others on the binding between GM2 activator and the carbohydrate head groups of glycosphingolipids.

Effect of GM2 activator protein on the enzymatic hydrolysis of phospholipids and sphingomyelin

GM2 activator protein (GM2AP) is a specific protein cofactor that stimulates the enzymatic hydrolysis of the GalNAc from GM2, a sialic acid containing glycosphingolipid, both in vitro and in lysosomes. While phospholipids together with glycosphingolipids are important membrane constituents, little is known about the possible effect of GM2AP on the hydrolysis of phospholipids. Several recent reports suggest that GM2AP might have functions other than stimulating the conversion of GM2 into GM3 by beta-hexosaminidase A, such as inhibiting the activity of platelet activating factor and enhancing the degradation of phosphatidylcholine by phospholipase D (PLD). We therefore examined the effect of GM2AP on the in vitro hydrolyses of a number of phospholipids and sphingomyelin by microbial (Streptomyces chromofuscus) and plant (cabbage) PLD. GM2AP, at the concentration as low as 1.08 microM (1 microg/50 microl) was found to inhibit about 70% of the hydrolyses of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol by PLD, whereas the same concentration of GM2AP only inhibited about 20-25% of the hydrolysis of sphingomyelin by sphingomyelinase and had no effect on the hydrolysis of sphingosylphosphorylcholine by PLD. Thus, GM2AP exerts strong and broad inhibitory effects on the hydrolysis of phospholipids carried out by plant and microbial PLDs. High ammonium sulfate concentration (1.6 M or 21.1%) masks this inhibitory effect, possibly due to the alteration of the ionic property of GM2AP.

Catabolism of asialo-GM2 in man and mouse. Specificity of human/mouse chimeric GM2 activator proteins

Tay-Sachs disease is an inborn lysosomal disease characterized by excessive cerebral accumulation of GM2. The catabolism of GM2 to GM3 in man requires beta-hexosaminidase A (HexA) and a protein cofactor, the GM2 activator. Thus, Tay-Sachs disease can be caused by the deficiency of either HexA or the GM2 activator. The same cofactor found in mouse shares 74.1% amino acid identity (67% nucleotide identity) with the human counterpart. Between the two activators, the mouse GM2 activator can effectively stimulate the hydrolysis of both GM2 and asialo-GM2 (GA2) by HexA and, to a lesser extent, also stimulate HexB to hydrolyze GA2, whereas the human activator is ineffective in stimulating the hydrolysis of GA2 (Yuziuk, J. A., Bertoni, C., Beccari, T., Orlacchio, A., Wu, Y.-Y., Li, S.-C., and Li, Y.-T. (1998) J. Biol. Chem. 273, 66-72). To understand the role of these two activators in stimulating the hydrolyses of GM2 and GA2, we have constructed human/mouse chimeric GM2 activators and studied their specificities. We have identified a narrow region (Asn(106)-Tyr(114)) in the mouse cDNA sequence that might be responsible for stimulating the hydrolysis of GA2. Replacement of the corresponding site in the human sequence with the specific mouse sequence converted the ineffective human activator into an effective chimeric protein for stimulating the hydrolysis of GA2. This chimeric activator protein, like the mouse protein, is also able to stimulate the hydrolysis of GA2 by HexB. The mouse model of human type B Tay-Sachs disease recently engineered by the targeted disruption of the Hexa gene showed less severe clinical manifestation than found in human patients. This has been considered to be the result of the catabolism of GM2 via converting it to GA2 and further hydrolysis of GA2 to lactosylceramide by HexB with the assistance of mouse GM2 activator protein. The chimeric activator protein that bears the characteristics of the mouse GM2 activator may therefore be able to induce an alternative catabolic pathway for GM2 in human type B Tay-Sachs patients.

Tay-Sachs disease carrier screening: a model for prevention of genetic disease

Tay-Sachs disease (TSD) is an autosomal-recessive, progressive, and ultimately fatal neurodegenerative disorder. Within the last 30 years, the discovery of the enzymatic basis of the disease, namely deficiency of the enzyme hexosaminidase A, made possible both enzymatic diagnosis of TSD and heterozygote identification. In the last decade, the cloning of the HEXA gene and the identification of more than 80 associated TSD-causing mutations has permitted molecular diagnosis in many instances. TSD was the first genetic condition for which community-based screening for carrier detection was implemented. As such, the TSD experience can be viewed as a prototypic effort for public education, carrier testing, and reproductive counseling for avoiding fatal childhood disease. More importantly, the outcome of TSD screening over the last 28 years offers convincing evidence that such an effort can dramatically reduce incidence of the disease.

Structural basis for the resistance of Tay-Sachs ganglioside GM2 to enzymatic degradation

To understand the reason why, in the absence of GM2 activator protein, the GalNAc and the NeuAc in GM2 (GalNAcbeta1-->4(NeuAcalpha2-->3)Galbeta1-->4Glcbet a1-1'Cer) are refractory to beta-hexosaminidase A and sialidase, respectively, we have recently synthesized a linkage analogue of GM2 named 6'GM2 (GalNAcbeta1-->6(NeuAcalpha2-->3)Galbeta1-->4Glcbet a1-1'Cer). While GM2 has GalNAcbeta1-->4Gal linkage, 6'-GM2 has GalNAcbeta1-->6Gal linkage (Ishida, H., Ito, Y., Tanahashi, E., Li, Y.-T., Kiso, M., and Hasegawa, A. (1997) Carbohydr. Res. 302, 223-227). We have studied the enzymatic susceptibilities of GM2 and 6'GM2, as well as that of the oligosaccharides derived from GM2, asialo-GM2 (GalNAcbeta1-->4Galbeta1--> 4Glcbeta1-1'Cer) and 6'GM2. In addition, the conformational properties of both GM2 and 6'GM2 were analyzed using NMR spectroscopy and molecular mechanics computation. In sharp contrast to GM2, the GalNAc and the Neu5Ac of 6'GM2 were readily hydrolyzed by beta-hexosaminidase A and sialidase, respectively, without GM2 activator. Among the oligosaccharides derived from GM2, asialo-GM2, and 6'GM2, only the oligosaccharide from GM2 was resistant to beta-hexosaminidase A. Conformational analyses revealed that while GM2 has a compact and rigid oligosaccharide head group, 6'GM2 has an open spatial arrangement of the sugar units, with the GalNAc and the Neu5Ac freely accessible to external interactions. These results strongly indicate that the resistance of GM2 to enzymatic hydrolysis is because of the specific rigid conformation of the GM2 oligosaccharide.

The GM2 activator protein, its roles as a co-factor in GM2 hydrolysis and as a general glycolipid transport protein

Although there is only one documented function carried out by the GM2 activator protein in the lysosome, new information suggests that other less obvious roles may also be played by this protein in vivo. This information includes data demonstrating that the GM2 activator is a secretory, as well as a lysosomal protein, and that cells possess a carbohydrate-independent mechanism to re-capture the activator, with or without bound lipid, from the extracellular fluid. Additionally the GM2 activator has been shown to bind, solubilize and transport a broad spectrum of lipid molecules, such as glycolipids, gangliosides and at least one phosphoacylglycerol, between liposomes. At pH 7 the GM2 activator's rate of lipid transport is reduced by only 50% from its maximum rate which is achieved at approx. pH 5, suggesting that the GM2 activator may serve as a general intra- and/or inter-cellular lipid transport protein in vivo. Since the late 1970s the lysosomal form of the GM2 activator has been known to act as a substrate-specific co-factor for the hydrolysis of GM2 ganglioside by beta-hexosaminidase A. Gangliosides are a class of negatively charged glycolipids particularly abundant in neuronal cells which have been linked to numerous in vivo functions, such as memory formation and signal transduction events. Deficiency of the GM2 activator protein results in the storage of GM2 ganglioside and severe neurological disease, the AB-variant form of GM2 gangliosidosis, usually culminating in death before the age of 4 years. The exact mode-of-action of the GM2 activator in its role as a co-factor, and its specificity for various glycolipids are currently matters of debate in the literature.

Specificity of mouse GM2 activator protein and beta-N-acetylhexosaminidases A and B. Similarities and differences with their human counterparts in the catabolism of GM2

Tay-Sachs disease, an inborn lysosomal disease featuring a buildup of GM2 in the brain, is caused by a deficiency of beta-hexosaminidase A (Hex A) or GM2 activator. Of the two human lysosomal Hex isozymes, only Hex A, not Hex B, cleaves GM2 in the presence of GM2 activator. In contrast, mouse Hex B has been reported to be more active than Hex A in cleaving GM2 (Burg, J., Banerjee, A., Conzelmann, E., and Sandhoff, K. (1983) Hoppe Seyler's Z. Physiol. Chem. 364, 821-829). In two independent studies, mice with the targeted disruption of the Hexa gene did not display the severe buildup of brain GM2 or the concomitant abnormal behavioral manifestations seen in human Tay-Sachs patients. The results of these two studies were suggested to be attributed to the reported GM2 degrading activity of mouse Hex B. To clarify the specificity of mouse Hex A and Hex B and to better understand the observed results of the mouse model of Tay-Sachs disease, we have purified mouse liver Hex A and Hex B and also prepared the recombinant mouse GM2 activator. Contrary to the findings of Burg et al., we found that the specificities of mouse Hex A and Hex B toward the catabolism of GM2 were not different from the corresponding human Hex isozymes. Mouse Hex A, but not Hex B, hydrolyzes GM2 in the presence of GM2 activator, whereas GM2 is refractory to mouse Hex B with or without GM2 activator. Importantly, we found that, in contrast to human GM2 activator, mouse GM2 activator could effectively stimulate the hydrolysis of GA2 by mouse Hex A and to a much lesser extent also by Hex B. These results provide clear evidence on the existence of an alternative pathway for GM2 catabolism in mice by converting GM2 to GA2 and subsequently to lactosylceramide. They also provide the explanation for the lack of excessive GM2 accumulation in the Hexa gene-disrupted mice.

Interaction of GM2 activator protein with glycosphingolipids

GM2 activator protein is a protein cofactor that has been shown to stimulate the enzymatic hydrolysis of both GalNAc and NeuAc from GM2 (Wu, Y. Y., Lockyer, J. M., Sugiyama, E., Pavlova, N.V., Li, Y.-T., and Li, S.-C. (1994) J. Biol. Chem. 269, 16276-16283). To understand the mechanism by which GM2 activator stimulates the hydrolysis of GM2, we examined the interaction of this activator protein with GM2 as well as with other glycosphingolipids by TLC overlay and Sephacryl S-200 gel filtration. The TLC overlay analysis unveiled the binding specificity of GM2 activator, which was not previously revealed. Under the conditions optimal for the activator protein to stimulate the hydrolysis of GM2 by beta-hexosaminidase A, GM2 activator was found to bind avidly to acidic glycosphingolipids, including gangliosides and sulfated glycosphingolipids, but not to neutral glycosphingolipids. The gangliosides devoid of sialic acids, such as asialo-GM1 and asialo-GM2, and the GM2 derivatives whose carboxyl function in the NeuAc had been modified by methyl esterification or reduction, were only very weakly bound to GM2 activator. These results indicate that the negatively charged sugar residue or sulfate group in gangliosides is one of the important sites recognized by GM2 activator. For comparison, we also studied in parallel the complex formation between glycosphingolipids and saposin B, a separate activator protein with broad specificity to stimulate the hydrolysis of various glycosphingolipids. We found that saposin B bound to neutral glycosphingolipids and gangliosides equally well, and there was an exceptionally strong binding to sulfatide. In contrast to previous reports, we found that GM2 activator formed complexes with GM2 and other gangliosides in different proportions depending on the ratio between the activator protein and the ganglioside in the incubation mixture prior to gel filtration. We were not able to detect the specific binding of GM2 activator to GM2 when GM2 was mixed with GM1 or GM3. Thus, the specificity or the mode of action of GM2 activator cannot be simply explained by its interaction with glycosphingolipids based on complex formation. The binding of GM2 activator to a wide variety of negatively charged glycosphingolipids may indicate that this activator protein has functions other than assisting the enzymatic hydrolysis of GM2.

Characterization of an alternatively spliced GM2 activator protein, GM2A protein. An activator protein which stimulates the enzymatic hydrolysis of N-acetylneuraminic acid, but not N-acetylgalactosamine, from GM2

GM2 activator protein is a protein cofactor which stimulates the enzymatic hydrolysis of both GalNAc and NeuAc from GM2. We have previously isolated two cDNA clones, GM2 activator cDNA and GM2A cDNA, for human GM2 activator protein (Nagarajan, S., Chen, H.-C., Li, S.-C., Li, Y.-T., and Lockyer, J. M. (1992) Biochem. J. 282, 807-813). GM2A mRNA is an RNA alternative splicing product that contains exons 1, 2, 3, and intron 3 of the genomic DNA sequence of GM2 activator protein (Klima, H., Tanaka, A., Schnabel, D., Nakano, T., Schröder, M., Suzuki, K., and Sandhoff, K. (1991) FEBS Lett. 289, 260-264). GM2A cDNA encodes a protein (GM2A protein) containing 1-109 of the 160 amino acids of human GM2 activator protein, plus a tripeptide (VST) encoded by intron 3 at the COOH terminus. Thus, GM2A protein can be regarded as a form (truncated version) of GM2 activator protein. We have expressed GM2A cDNA in Escherichia coli using pT7-7 as the vector. The recombinant GM2A protein was purified to an electrophoretically homogeneous form and was found to stimulate the hydrolysis of NeuAc from GM2 by clostridial sialidase, but not the hydrolysis of GalNAc from GM2 by beta-hexosaminidase A. Like GM2 activator protein, GM2A protein also specifically recognized the terminal GM2 epitope in GalNAc-GD1a and stimulated the hydrolysis of only the external NeuAc from this ganglioside by clostridial sialidase. These results enabled us to discern the enzymatic hydrolyses of GalNAc and NeuAc from the GM2 epitope and established that the NeuAc recognition domain of GM2 activator protein is located within amino acids 1-109. The presence of GM2A mRNA in human tissues and the selective stimulation of NeuAc hydrolysis by GM2A protein indicate that this activator protein may be involved in the catabolism of GM2 through the asialo-GM2 pathway.

Isolation and structural characterization of N-acetyl- and N-glycolylneuraminic-acid-containing GalNAc-GD1a isomers, IV4GalNAcIV3Neu5AcII3Neu5GcGgOse4Cer and IV4GalNAcIV3Neu5GcII3Neu5AcGgOse4Cer, from bovine brain

A ganglioside preparation containing two structurally related minor gangliosides (Gg 1 + 2) was isolated from bovine brain ganglioside mixture and characterized. Treatment of 50 g ganglioside mixture with Clostridium perfrigens sialidase, followed by chromatography on DEAE-Sepharose and silica gel columns, yielded 20 mg Gg 1 + 2. By chemical analyses, 1H- and 13C-NMR spectroscopy, enzymic hydrolyses using human beta-hexosaminidase A and clostridial sialidase, and TLC overlay with the conjugated cholera toxin B subunit, the two novel gangliosides Gg 1 and Gg 2 were identified to be: Gg 1, GalNAc-GD1a(Neu5Ac/Neu5Gc), beta-GalNAc-(1-4)-[alpha-Neu5Ac-(2-3)]-beta- Gal-(1-3)-beta-GalNAc-(1-4)-[alpha-Neu5Gc-(2-3)]-beta-Gal-(1-4)-be ta- Glc-(1-1)-Cer; Gg 2, GalNAc-GD1a(Neu5Gc/Neu5Ac), beta-GalNAc-(1-4)-[alpha-Neu5Gc-(2-3)]- beta-Gal-(1-3)-beta-GalNAc-(1-4)-[alpha-Neu5Ac-(2-3)]-beta-Gal-(1- 4)-beta- Glc-(1-1)-Cer. These two gangliosides contain the identical pentasaccharide backbone except that the substitution of the two sialic acids, Neu5Ac and Neu5Gc, are in the reversed position of the external and the internal Gal residues. Our analyses showed that the content of Gg 1 and Gg 2 were approximately 0.12% and 0.08%, respectively, of the total brain ganglioside mixture.

Neuronal expression of a minor monosialosyl ganglioside GM1b in rat brain: immunochemical characterization using a specific monoclonal antibody

Monoclonal antibodies (mAbs) against GM1b ganglioside were raised by immunizing NZB/n mice with the antigen purified from bovine brains, and the details of binding specificity of the mAbs were characterized. Anti-GM1b mAb, termed NA-6, reacted specifically with GM1b (NeuAc) and GM1b(NeuGc). NA-6 antibody did not react with other structurally related gangliosides, indicating that the antibody recognizes NeuAc or NeuGc alpha 2-3Gal beta 1-4GalNAc beta 1-4Gal structure. Using NA-6 antibody, GM1b ganglioside in developing rat brain was investigated by TLC/enzyme-immunostaining and detected first on gestational day 16. The specific content of brain GM1b increased until postnatal day 10, and then gradually decreased in later stage of development. Immunohistochemically GM1b was found in proximal dendrites and cell bodies of neurons in extensive regions of adult rat brain. The immunoreactivity tended to be confined in patch-like clusters on cell membranes, as typically found in the hippocampus and neocortex. The GM1b synthase activity, when assayed in the cerebellar microsome proteins, was significantly reduced in lurcher mutant mouse that is devoid of both cerebellar granule and Purkinje cells. These findings demonstrate that GM1b ganglioside exists in neurons and is actively synthesized during the development in rat brain.

GM2 ganglioside activator occurs in multiple forms

The protein which activates the hydrolysis of GM2 ganglioside by hexosaminidase A was purified from human kidney. The GM2 activator had a molecular mass of 28 kDa by gel filtration and was resolved into three major bands using polyacrylamide gel electrophoresis in the presence of SDS with molecular masses of 23, 22 and 21 kDa. These three bands corresponded respectively to strongly binding, weakly binding and non-binding fractions of GM2 activator chromatographed through concanavalin A-Sepharose, indicating that GM2 activator exists in multiple glycosylated forms.

Tay-Sachs disease screening and diagnosis: evolving technologies

Tay-Sachs disease (TSD) is an autosomal recessive, progressive, and fatal neurodegenerative disorder. Within the last 25 years, the discovery of the enzymatic basis of the disease, the deficiency of the enzyme hexosaminidase A, has made possible both enzymatic diagnosis of TSD and heterozygote identification. TSD is the first genetic condition for which a community-based heterozygote screening program was attempted with the intention of reducing the incidence of a genetic disease. In this article we review the clinical, biochemical, and molecular features of TSD as well as the development of laboratory technology that has been deployed in community genetic screening programs. We describe the assay procedures used and some of the limitations in their accuracy. We consider the impact of DNA-based technology on the process of identification of individuals carrying mutant genes associated with TSD and we discuss the social context within which genetic screening occurs.

A NeuGc-containing trisialoganglioside of bovine brain

A N-glycolyneuraminic acid containing trisialoganglioside was isolated from bovine brains ganglioside mixture using Q-Sepharose. Its chemical structure was characterized as IV3NeuAc, II3NeuAc-NeuGc, Gg4Cer by gas-liquid chromatography, a permethylation study, sialidase degradation, TLC/enzyme-immunostaining, fast atom bombardment-mass spectrometry, fluorometric HPLC and proton nuclear magnetic resonance spectroscopy. This was unique in the mixed sialic acid constituents. (formula; see text) This accounted for 0.78% of the gangliosides. The ceramide structure was almost identical with those of major bovine brain ganglioside, as mainly composed of 18:0 fatty acid (90.9%) and d20:0 sphingosine base.

Biochemistry and genetics of Tay-Sachs disease

Tay-Sachs disease is one of the few neurodegenerative diseases of known causes. It results from mutations of the HEXA gene encoding the alpha subunit of beta-hexosaminidase, producing a destructive ganglioside accumulation in lysosomes, principally in neurons. With the determination of the protein sequence of the alpha and beta subunits, deduced from cDNA sequences, the complex pathway of subcellular and lysosomal processing of the enzyme has been determined. More recently, detailed knowledge of the gene structure has allowed the determination of specific mutations causing Tay-Sachs disease. The high incidence of the disease in Ashkenazi Jews is attributed predominantly to three mutations present in high frequency, while in non-Jews some two dozen mutations have been identified thus far. The cataloguing of mutations has important implications for carrier screening and prenatal diagnosis for Tay-Sachs disease.

Properties of acid ceramidase from human spleen

We have characterised ceramidase activity in extracts of human spleen from control subjects and from patients with Gaucher disease. In Triton X-100 extracts of control spleens, a broad pH optimum of pH 3.5-5.0 was found; no ceramidase activity was detectable at neutral or alkaline pH. About 45-60% of acid ceramidase could be extracted from spleen without detergents, but for complete extraction, Triton X-100 was required. For the radiolabelled substrate oleoylsphingosine, a Km of 0.22 +/- 0.09 mM and a Vmax of 57 +/- 11 nmol/h per mg protein was calculated in spleen from a control subject. Flat-bed isoelectric focussing in the presence of Triton X-100 revealed a pI of 6.0-7.0 for acid ceramidase; similar values were found for sphingomyelinase and glucerebrosidase. HPLC-gel filtration indicated that in the presence of Triton X-100, acid ceramidase has an Mr of about 100 kDa. In the absence of detergents, the enzyme forms high-molecular-weight aggregates. Similar aggregation behaviour was observed for sphingomyelinase, while the elution of beta-hexosaminidase was not affected by detergents. The elution profile of glucocerebrosidase was only slightly altered by Triton X-100. There was no difference in the properties of acid ceramidase present in spleen from control subjects and from patients with type I Gaucher disease.

Identity of the activator proteins for the enzymatic hydrolysis of sulfatide, ganglioside GM1, and globotriaosylceramide

The activator protein for the enzymatic hydrolysis of sulfatide, ganglioside GM1, and globotriaosylceramide was purified from human kidney, brain, and urine. As far as they could be assayed, these three activities cochromatographed during all steps, indicating that they are due to the same protein. This result was corroborated by immunochemical comparison of individually purified activator preparations. In contrast, the activator for ganglioside GM2 hydrolysis could clearly be separated from the other activities. Kinetic data were determined for the interaction of the sulfatide activator with the different glycolipids and hydrolases.

Alteration of beta-hexosaminidase activity and isoenzymes in human leukemic cells

beta-Hexosaminidase (EC 3.2.1.20; Hex) activity and isoenzyme characteristics were analyzed in human normal and leukemic leukocytes. Unseparated CLL and CML cells had a specific activity that was lower, whereas ALL and AML blasts had a higher specific activity than normal lymphocytes and granulocytes. CLL B-cells had a lower specific activity compared with that in normal non-T-lymphocytes; CLL T-cells and normal T-cells had similar activity. Isoenzyme separation was performed by chromatofocusing on PBE-94 coupled with an automated enzyme assay. When using a single linear pH elution gradient, normal leukocytes and all leukemia cells contained two forms of isoenzyme (B and A). When a double pH elution gradient was performed, an extra distinct form of Hex (I) was recorded. Hex I was present in small amounts in normal granulocytes and PHA-stimulated normal lymphocytes; isoenzyme I was found in high amounts in all leukemias tested. The activity ratios I/B and I/A, as well as the I isoenzyme profile, may facilitate differentiation between normal and leukemic cells and between lymphoblastic and myeloblastic leukemias.

Glycolipid-binding proteins

Proteins which bind glycolipids with high specificity are tentatively divided into two groups. One group consists of activator proteins involved in the catabolism of glycolipids by acid lysosomal hydrolases. Two activator proteins, GM2-activator and sphingolipid activator protein-1, are critically appraised on their glycolipid-binding properties and on their activity to facilitate the transfer of glycolipids. These proteins are glycoproteins localized in the lysosomes. Their molecular weights are in a range of 21 000-27 000, and isoelectric points are 4-5. Glycolipid transfer protein (GLTP) is included in the other group. GLTP purified from pig brain has a molecular weight of about 20 000 and an isoelectric point of 8.3. GLTP facilitates the transfer of various glycosphingolipids and glyceroglycolipids between membranes. The protein does not facilitate the transfer of phospholipids or cholesterol. GLTP binds galactosylceramide. The galactosylceramide-GLTP complex participates in the transfer reaction as the intermediate. Each protein in both groups binds glycolipids with a characteristic specificity to the sugar moiety. A stoichiometry of 1 mol of lipid per mol of protein has been found in all three proteins. Proteins in both groups seem to have a hydrophobic region on their surface, since all three proteins have been efficiently purified by hydrophobic chromatography.

Motor neuron disease and adult hexosaminidase A deficiency in two families: evidence for multisystem degeneration

We studied three patients from two unrelated families with adult hexosaminidase A deficiency. A 30-year-old, non-Jewish proband in the first family had juvenile amyotrophic lateral sclerosis that evolved to mild dementia, ataxia, and axonal (neuronal) motor-sensory peripheral neuropathy. A 36-year-old Jewish proband in the second family had "pure" spinal muscular atrophy. One supposedly healthy brother of the first proband was found to have borderline IQ, mild spasticity, and ataxia but no evidence of motor neuron disease. Marked cerebellar atrophy was detected by head scans in all three patients. In both probands electromyograms were characterized by prominent, complex repetitive discharges in many muscles. Hexosaminidase A activities against the artificial substrate were similar to those reported in infantile Tay-Sachs disease; however, the hexosaminidase A level against GM2 substrates was higher than that found in infantile Tay-Sachs disease. The hexosaminidase A levels of the parents were in the heterozygous range. Motor neuron disease in our patients and in those previously described appears to be part of a multisystem degeneration of the nervous system.

Mapping of the gene coding for the human GM2 activator protein to chromosome 5

The gene coding for the GM2 activator protein has been mapped to human chromosome 5, using an enzyme-linked immunoadsorbent assay (ELISA) to identify the human protein in human-mouse somatic cell hybrids.

Molecular forms of beta-N-acetylhexosaminidase in Epstein-Barr virus-transformed lymphoid cell lines from normal subjects and patients with Tay-Sachs disease

In whole leukocytes and in lymphocytes from normal subjects, the percentage activity of heat-stable beta-N-acetylhexosaminidase (30 +/- 5% and 45 +/- 5%, respectively) was higher than in the transformed lymphoid cell line (19 +/- 3%). In Tay-Sachs transformed cells as well as non-transformed beta-N-acetylhexosaminidase was almost completely heat-stable (95 - 98%). In the transformed cells from normal subjects, the beta-N-acetylhexosaminidase B (Hex B) activity (5% of total) was significantly lower than in blood lymphocytes (average 25 - 30% of total activity), whereas Hex A and Hex I were similar in the either cell type. Blood lymphocytes and lymphoid cell lines established from a Tay-Sachs patient lacked heat-labile Hex A and expressed high heat-stable Hex I and Hex B activities (3-6-fold). After neuraminidase treatment, Hex A peak sharpened while Hex I peaks switched to higher pI than normal Hex I, in the region of Hex B. PreHex A/S pI was not affected. Hydrolytic properties using the both substrates (4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-glucopyranoside and 4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-galactopyranoside) of each molecular form were similar in transformed and non-transformed cells. Data derived from the use of a mixture of substrates were consistent with the model which proposes a common active site for either substrate in the case of preHex A, Hex B and Hex I, but not for Hex A. Thus Epstein-Barr virus-transformed lymphoid cell lines represent an accurate model system for studies on Tay-Sachs disease.

Properties of hexosaminidases in cell-free extracts of rainbow trout livers and effects of thyroid hormones

1. The kinetic and physical properties of hexosaminidase (EC 3.2.1.30) activity were studied in liver extracts of juvenile rainbow trout. 2. Differential centrifugation studies failed to unequivocally demonstrate the sub-cellular localization of hexosaminidase, but indicated that the hexosaminidase activity was associated with the mitochondrial and lysosomal fractions. 3. Cell-free, Triton X-100 liver extracts indicated a pH optimum and activation energies comparable to those of mammals, but for other properties trout hepatic hexosaminidase differed markedly from its mammalian counterpart. 4. In contrast to the situation in rats, trout hexosaminidase was uninfluenced by a wide range of T4 and T3 doses administered by either injection or immersion routes. 5. T4 administration increased plasma T4 with minor change in plasma T3, indicating inhibition of T4 to T3 conversion by high T4 levels.