Low molecular weight proteins in secondary lysosomes as activators of different sphingolipid hydrolases

Sphingolipids and lysosomal pathologies

Endocytosed (glyco)sphingolipids are degraded, together with other membrane lipids in a stepwise fashion by endolysosomal enzymes with the help of small lipid binding proteins, the sphingolipid activator proteins (SAPs), at the surface of intraluminal lysosomal vesicles. Inherited defects in a sphingolipid-degrading enzyme or SAP cause the accumulation of the corresponding lipid substrates, including cytotoxic lysosphingolipids, such as galactosylsphingosine and glucosylsphingosine, and lead to a sphingolipidosis. Analysis of patients with prosaposin deficiency revealed the accumulation of intra-endolysosmal vesicles and membrane structures (IM). Feeding of prosaposin reverses the storage, suggesting inner membrane structures as platforms of sphingolipid degradation. Water soluble enzymes can hardly attack sphingolipids embedded in the membrane of inner endolysosomal vesicles. The degradation of sphingolipids with few sugar residues therefore requires the help of the SAPs, and is strongly stimulated by anionic membrane lipids. IMs are rich in anionic bis(monoacylglycero)phosphate (BMP). This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Gangliosides and gangliosidoses: principles of molecular and metabolic pathogenesis

Gangliosides are the main glycolipids of neuronal plasma membranes. Their surface patterns are generated by coordinated processes, involving biosynthetic pathways of the secretory compartments, catabolic steps of the endolysosomal system, and intracellular trafficking. Inherited defects in ganglioside biosynthesis causing fatal neurodegenerative diseases have been described so far almost exclusively in mouse models, whereas inherited defects in ganglioside catabolism causing various clinical forms of GM1- and GM2-gangliosidoses have long been known. For digestion, gangliosides are endocytosed and reach intra-endosomal vesicles. At the level of late endosomes, they are depleted of membrane-stabilizing lipids like cholesterol and enriched with bis(monoacylglycero)phosphate (BMP). Lysosomal catabolism is catalyzed at acidic pH values by cationic sphingolipid activator proteins (SAPs), presenting lipids to their respective hydrolases, electrostatically attracted to the negatively charged surface of the luminal BMP-rich vesicles. Various inherited defects of ganglioside hydrolases, e.g., of β-galactosidase and β-hexosaminidases, and of GM2-activator protein, cause infantile (with tetraparesis, dementia, blindness) and different protracted clinical forms of GM1- and GM2-gangliosidoses. Mutations yielding proteins with small residual catabolic activities in the lysosome give rise to juvenile and adult clinical forms with a wide range of clinical symptomatology. Apart from patients' differences in their genetic background, clinical heterogeneity may be caused by rather diverse substrate specificities and functions of lysosomal hydrolases, multifunctional properties of SAPs, and the strong regulation of ganglioside catabolism by membrane lipids. Currently, there is no treatment available for neuronal ganglioside storage diseases. Therapeutic approaches in mouse models and patients with juvenile forms of gangliosidoses are discussed.

Characterization of the phospholipid requirement of a rat liver beta-glucosidase

The lipid requirement of membrane-bound rat liver beta-glucosidase was investigated using 4-methylumbelliferyl-beta-D-glucopyranoside as the substrate. The enzyme was solubilized and delipidated by sequential extraction of a crude lysosomal fraction from rat liver lysosomes with sodium cholate and ice-cold butan-1-ol. Neither saturated nor unsaturated phosphatidylcholine activated this enzyme. In contrast, acidic phospholipids like phosphatidylglycerol (PtdGro) and phosphatidylserine (PtdSer) were effective activators. For the PtdGro series, fatty acid composition was important, with the shorter chain or unsaturated fatty acid-containing PtdGro species being the best activators. Heat-stable factor (HSF) from Gaucher spleen by itself (1-2 micrograms) had no effect on enzyme activity. However, the same amount of HSF when combined with 10 micrograms of PtdSer markedly stimulated beta-glucosidase activity. In the presence of HSF, di-9-cis-octadecenoyl-PtdGro (1 microgram) or -PtdSer (5 micrograms) provided maximum protection of beta-glucosidase against heat (60 degrees C) inactivation. In the absence of phospholipids, HSF had no effect on the rate of inactivation of the enzyme by the suicide inhibitor conduritol B epoxide (t0.5, 12 +/- 0.5 min); the maximum rate of inactivation was achieved in the presence of a mixture of PtdGro (2.5-5 micrograms) and HSF (t0.5, 2.8 min). The combination of PtdSer (10 micrograms) and HSF (1.3 micrograms) lowered the Km for 4-methylumbelliferyl-beta-D-glucopyranoside from 24 to 2.7 mM. Inhibition of the enzyme by the glucocerebrosidase substrate analogues N-hexyl-O-glucosylsphingosine and glucosylsphingosine was influenced by the activator substances. The inclusion of PtdSer and HSF in the beta-glucosidase assay medium lowered the Ki of N-hexyl-O-glucosylsphingosine 20-fold. The same combination of activators decreased the I0.5 of the enzyme for glucosylsphingosine from 89.4 to 7.6 microM. A study of log (Vmax./Km) versus pH indicated that the PtdSer-HSF pair creates the active site of beta-glucosidase, making apparent three ionizable groups on the enzyme with pK values in the range 4.5-5.1.

A protein activator of galactosylceramide beta-galactosidase

A heat-stable protein was isolated from the spleen of a patient with Gaucher's disease. This protein will activate glucosylceramide beta-glucosidase activity (Ho, M.W. and O'Brien, J.S. (1971) Proc. Natl. Acad. Sci. U.S.A. 68, 2810-2813). When the specificity of this activator was tested using other enzymes and substrates, it was found to activate galactosylceramide beta-galactosidase activity and sphingomyelinase but not GM1 beta-galactosidase or sulfatide sulfatase. The ability to stimulate galactosylceramide beta-galactosidase was optimum at pH 4.6 in the presence of pure phosphatidylserine or other acidic lipids such as sulfatide and phosphatidylinositol. The partially purified activator protein could stimulate galactosylceramide beta-galactosidase activity in brain, liver, leukocytes and cultured fibroblasts. It was not able to stimulate the activity of this enzyme in tissue samples from patients with Krabbe's disease, demonstrating that it was acting on galactosylceramide beta-galactosidase and not GM1 beta-galactosidase. It was slowly denatured by treatment with Pronase, reaching 16% of starting levels after 24 h at 50 degrees C. Attempts to separate the abilities of this activator preparation to stimulate several lysosomal hydrolases by column chromatography were not successful.

Apolipoprotein C-III-1 activates lysosomal sphingomyelinase in vitro

Apolipoprotein (apo)C-III-1 from human very low density lipoprotein stimulates 14-fold the activity of lysosomal sphingomyelinase from human fibroblasts. At the sphingomyelin concentrations tested, maximal stimulation was obtained with 5 microM apoC-III-1 or apoC mixture. Apolipoproteins A-I, A-II, B, and C-I conferred little or no stimulation. Sphingomyelinase was stimulated 20-fold by lysophosphatidylcholine with an optimum concentration of 70 microM using 0.3 mM substrate. Sphingomyelinase activity was inhibited by concentrations of apoC-III-1 and lysophosphatidylcholine three- to fivefold above stimulatory levels. Triton X-100 activated sphingomyelinase 300-fold with a pH optimum of 5.0, while the pH optimum with the biological activators was 4.0. These results raise the possibility of an in vivo activity for the biological activators. The proteins that enter lysosomes as part of a lipoprotein complex may activate lysosomal enzymes that degrade the lipid components.

Mechanism of activation of glucocerebrosidase by co-beta-glucosidase (glucosidase activator protein)

The nature of the stimulatory action of the protein 'coglucosidase' on glucocerebrosidase was investigated with the use of highly purified cofactor from bovine spleen, radioactive glucosyl ceramide and methylumbelliferyl-beta-glucoside. A complex between glucosidase and either substrate could not be detected under equilibrium and non-equilibrium binding conditions. Complex formation between stimulating protein and the enzyme could be shown by the binding of the enzyme to an affinity column containing coglucosidase. This binding could be blocked by adding phosphatidylserine to the enzyme. The lipid also stimulated the enzyme. Additional evidence for binding of the enzyme to the two kinds of stimulators was the finding that they protected the enzyme against inactivation by N-ethylmaleimide and chloromercuriphenylsulfonate. A role for lipids in the stimulatory action of coglucosidase was shown by extracting lipids from the enzyme; this resulted in a loss of basal enzyme activity and of sensitivity to activation by the protein. Adding back to the lipids or phosphatidylserine increased the sensitivity of the delipidated enzyme to coglucosidase. Using the crude, unextracted enzyme we could show that low concentrations of phosphatidylserine augmented the effectiveness of coglucosidase but high concentrations of the lipid blocked the effect of the protein. It is proposed that lipids, particularly acidic ones, act on solubilized glucocerebrosidase to produce an enzyme conformation which allows binding and stimulation by coglucosidase. At higher lipid concentrations, the acidic lipids bind, in competition with coglucosidase, to the latter's binding site on the enzyme.

[Chance and discovery in research on the cause of infantile amaurotic idiocy]

Infantile amaurotic idiocy-the classical type known as "Tay-Sachs disease" -is the consequence of the accumulation of a ganglioside and a closely related derivation in the human brain. The accumulation of both substances is due to a genetically induced deficiency of their common catabolic enzyme system. K. Sandhoff discovered three enzymic variants of the disease, which, taken together, did not reveal any apparent causal relationship between enzymic defect and substrate accumulation. The role of chance and discovery in finding the three variants as well as in the elucidation of their causes is described.

[Lipid-protein interactions: mechanisms of enzymatic glycolipid catabolism and their genetic restrictive escapes]

Investigations of the genetic basis of ganglioside catabolism have led to the characterisation of two types of lipid-enzyme interaction: a) Breakdown of membrane-bound glycolipids as far as catalysed by membrane-bound enzymes is regulated by the membrane itself. b) The degradation of micelle-forming glycolipids by water-soluble lysosomal enzymes is facilitated by cofactors known as activator proteins. A genetic defect in an activator protein can be just as fatal as the lack of the enzyme itself.

Specificity of galactosylceramidase activation by phosphatidylserine

Bovine brain phosphatidylserine effectively activates human brain galactosylceramidase (Hanada, E. and Suzuki, K. (1979) Biochim. Biophys. Acta 575, 410-420). Its effect on the other beta-galactosidase (Gm1-ganglioside beta-galactosidase) in human tissues, genetically distinct from galactosylceramidase, was examined. When partially purified human brain beta-galactosidase preparations, pure with respect to each other, were used as the enzyme source and when lactosylceramide, a common glycosphingolipid substrate for both beta-galactosidases, was used as the substrate, phosphatidylserine activated only hydrolysis of lactosylceramide by galactosylceramidase but not by GM1-ganglioside beta-galactosidase. With either galactosylceramide or lactosylceramide as substrate, and with phosphatidylserine as the activator, diagnosis of globoid cell leukodystrophy was possible using whole homogenates of cultured fibroblasts. Since 80-90% of lactosylceramide-cleaving activity in normal fibroblasts is due to GM1-ganglioside beta-galactosidase and since fibroblasts of globoid cell leukodystrophy patients are genetically deficient in galactosylceramidase but normal in GM1-ganglioside beta-galactosidase, these rsults are also consistent with specific activation of galactosylceramidase by phosphatidylserine.

Somatic cell hybridisation studies showing different gene mutations in Niemann-Pick variants

Cultured skin fibroblasts from patients with different clinical types of Niemann-Pick disease were hybridized and sphingomyelinase activities were measured in the heterokaryon cell population. Both the natural substrate (3H-choline) sphingomyelin and the chromogenic analogue hexadecanoylamino-4-nitrophenylphosphorylcholine were used in the complementation analysis. In fusions between cells from type C Niemann-Pick disease with those from type A or B a clear restoration of sphingomyelinase activity occurred, whereas no complementation was found in other fusion combinations. The results indicate that at least two different genes are involved in the mutations leading to the different Niemann-Pick variants.

The specificity of human N-acetyl-beta-D-hexosaminidases towards glycosphincolipids is determined by an activator protein

It has been very difficult to correlate, on the basis of in vitro measurements of substrate specificities, the glycosphingolipid storage patterns observed in different variants of infantile GM2 gangliosidosis with the hexosaminidase (hex) isoenzyme deficiencies underlying these diseases. However, the in vitro enzyme assays included detergents, which greatly enhanced the enzymic degradation of lipids by breaking down the large lipid micelles that cannot otherwise be attacked by the hydrolases. In vivo, the role of detergent is taken over by water-soluble, low molecular weight proteins, so-called activators, which bind the lipid monomers, thus solubilizing them. It can be shown that the activator protein for the enzymic degradation of ganglioside GM2 has a very strong preference for hex A over hex B; it also acts on glycolipid GA2 and, to a lesser extent, on kidney globoside. This isoenzyme specificity is much less prominent or even reversed when detergents are used to solubilize the substrates. The substrate specificities of hex A and hex B measured in the presence of sufficient amounts of the activator protein most probably reflect the conditions occurring in vivo. They can explain the lipid storage patterns observed in different variants of infantile GM2 gangliosidosis, especially the accumulation of ganglioside GM2 in variant B (where hex B is still present) and the reduced storage of GA2 in the same variant as compared to variants O and AB. The physiological significance of the activator protein is demonstrated in variant AB in which the activator is deficient, resulting in an accumulation of glycolipids GM2 and GA2.

Activation of human brain galactosylceramidase by phosphatidylserine

Assays of sphingolipid hydrolases in vitro generally require bile salts or other detergents. A few 'activator proteins' have been reported that can partially replace the detergents in the assay mixture. We report here that phosphatidylserine from bovine brain is a relatively specific activator of human brain galactosylceramidase in the absence of sodium taurocholate (phosphatidylserine system). Activity similar to that obtained with the conventional assay system containing taurocholate and oleic acid (taurocholate system) could be obtained. Other lipids tested generally gave less than 10% of the taurocholate system activity, but sulfatide could activate human brain galactosylceramidase to 20--30% of the taurocholate system. The properties of the reaction in the phosphatidylserine system were examined with human brain whole homogenate, crude soluble post-concanavalin A preparations, and partially purified preparations as the enzyme source and compared with those obtained with the taurocholate system. The pH optimum shifted from 4.2 in the taurocholate system to 4.7 in the phosphatidylserine system. The phosphatidylserine system was superior in the linearity of the reaction with respect to the enzyme protein. Reasonably linear Lineweaver-Burk plots could be obtained. The Km values for the phosphatidylserine system were greater than those for the taurocholate system. The effect of phosphatidylserine was not additive to that of taurocholate. Additional phosphatidylserine to the taurocholate system was either without effect at lower concentrations or inhibitory at higher concentrations. The assays of galactosylceramidase with phosphatidylserine and without taurocholate do not necessarily provide pragmatic advantages but offer a potentially useful system with which to study the mechanism of in vivo degradation of the membrane-bound glycosphingolipid.

Biochemistry and genetics of gangliosidoses

The gangliosidoses comprise an-ever increasing number of biochemically and phenotypically variant diseases. In most of them an autosomal recessive inherited deficiency of a lysosomal hydrolase results in the fatal accumulation of glucolipids (predominantly in the nervous tissue) and of oligosaccharides. The structure, substrate specificity, immunological properties of and genetic studies on the relevant glycosidases, ganglioside GM1 beta-galactosidase and beta-hexosaminidase isoenzymes, are reviewed in this paper. Contrary to general expectation, only a poor correlation is observed between the severity of the disease and residual activity of the defective enzyme when measured with synthetic or natural substrates in the presence of detergents. For the understanding of variant diseases and for their pre- and postnatal diagnosis, the necessity of studying the substrate specificity of normal and mutated enzymes under conditions similar to the in vivo situation, e.g., with natural substrates in the presence of appropriate activator proteins, is stressed. The possibility that detergents may have adverse affects on the substrate specificity of the enzymes is discussed for the beta-hexosaminidases. The significance of activator proteins for the proper interaction of lipid substrates and water-soluble hydrolases is illustrated by the fatal glycolipid storage resulting from an activator protein deficiency in the AB variant of GM2-gangliosidosis. Recent somatic complementation studies have revealed the existence of a presumably post-translational modification factor necessary for the expression of ganglioside GM1 beta-galactosidase activity. This factor is deficient in a group of variants of GM1-glangliosidosis. Among the possible reasons for the variability of enzyme activity levels in heterozygotes and patients, allelic mutations, formation of hybrid enzymes, and the existence of patients as compound heterozygotes are discussed. All these may result in the production of mutant enzymes with an altered specificity for a variety of natural substrates.

Magnesium-dependent sphingomyelinase of infantile brain. Effect of detergents and a heat-stable factor

The properties of the Mg2+-dependent sphingomyelinase, whose pH optimum is between 7 and 8, were investigated using post-mortem infantile brain. The enzyme could be extracted with 0.2% Triton X-100 and remained soluble when centrifuged at 170,000 X g. Subsequent removal of the detergent with SM2-Biobeads resulted in resedimentation of the enzyme at 80,000 X g. A detergent was needed for assaying enzymatic activity; either Triton X-100 or bile salts could be used. With increasing concentrations of detergent, the rates of hydrolysis of sphinomyelin increased, reached an optimum and then decreased, suggesting inhibition of the enzyme. The concentrations of detergent which resulted in optimal reaction rates were directly related to the protein concentration of the enzymatic preparation. A heat-stable factor which counteracts inhibition by the above detergents is present in brain as well as several other tissues. A lipid extract of the enzymatic preparation, or several purified lipids could not mimic the effect of the heat-stable factor. The interrelationship between enzyme, detergent and the heat-stable factor was investigated.

Studies on the function of the activator of sulphatase A

The activator of sulphatase A is necessary for the enzymic degradation of sulphatides to cerebrosides at ionic concentrations in the physiological range (1). Activation is probably due to the reversible formation of a one-to-one complex between activator and sulphatides (1,2). Formation of this complex is partly inhibited by cerebrosides due to competitive binding (2), as well as by phospholipids (e.g. lecithin or phosphatidylserine). Inhibition of the complex formation between activator and sulphatides by cerebrosides and phosphatidyl-serine depends on the concentration of the lipids and is of the same order of magnitude as the inhibition (by these lipids) of the enzymic degradation of sulphatides in the presence of activator (1). Moreover the degradation rate of sulphatides increases with the concentration of activator-sulphatide complex in the reaction mixture (1) indicating that the activator-sulphatide complex is the substrate for the enzyme in the degradation of sulphatides by sulphatase A.

The activator of cerebroside sulphatase. Binding studies with enzyme and substrate demonstrating the detergent function of the activator protein

1. Sulphatase A (cerebroside sulphatase) (EC 3.1.6.1.) and a 12-fold excess of its physiological activator protein were chromatographed together on Sephadex G-75. The elution buffer was the same as that used in the enzymic degradation of sulphatides. The two proteins were eluted in different peaks indicating that no stable complex formed. 2. Activator protein was incubated with sulphatides under conditions used favouring the sulphatase activity. Incubation solutions were then examined by electrophoresis on a polyacrylamide gel gradient. An one-to-one complex between activator and sulphatides was observed. Half maximal binding occurred with 2.5 nmol of sulphatides together with 1 or 2 nmol of activator in 100 micronl. 3. Cerebrosides as the enzymic degradation products of sulphatides, bind also to the activator protein. A ratio of one-to-one could possibly be obtained at high cerebroside concentrations. The binding to cerebrosides is less specific than that to sulphatides. A 7-fold excess of cerebrosides was necessary for half maximal binding. 4. In a mixture of sulphatides and cerebrosides the formation of the complex with the activator protein is partly inhibited. The total amount of bound lipids changed as the composition of the lipid mixture was varied. In a one-to-one mixture of the two lipids 60% of the total bound lipids are sulphatides and 40% are cerebrosides.