Human acid beta-glucosidase. Use of inhibitory and activating monoclonal antibodies to investigate the enzyme's catalytic mechanism and saposin A and C binding sites

Clinical Sphingolipids Pathway in Parkinson’s Disease: From GCase to Integrated-Biomarker Discovery

Alterations in the sphingolipid metabolism of Parkinson’s Disease (PD) could be a potential diagnostic feature. Only around 10–15% of PD cases can be diagnosed through genetic alterations, while the remaining population, idiopathic PD (iPD), manifest without validated and specific biomarkers either before or after motor symptoms appear. Therefore, clinical diagnosis is reliant on the skills of the clinician, which can lead to misdiagnosis. IPD cases present with a spectrum of non-specific symptoms (e.g., constipation and loss of the sense of smell) that can occur up to 20 years before motor function loss (prodromal stage) and formal clinical diagnosis. Prodromal alterations in metabolites and proteins from the pathways underlying these symptoms could act as biomarkers if they could be differentiated from the broad values seen in a healthy age-matched control population. Additionally, these shifts in metabolites could be integrated with other emerging biomarkers/diagnostic tests to give a PD-specific signature. Here we provide an up-to-date review of the diagnostic value of the alterations in sphingolipids pathway in PD by focusing on the changes in definitive PD (postmortem confirmed brain data) and their representation in “probable PD” cerebrospinal fluid (CSF) and blood. We conclude that the trend of holistic changes in the sphingolipid pathway in the PD brain seems partly consistent in CSF and blood, and could be one of the most promising pathways in differentiating PD cases from healthy controls, with the potential to improve early-stage iPD diagnosis and distinguish iPD from other Parkinsonism when combined with other pathological markers.

Glucosylceramide and galactosylceramide, small glycosphingolipids with significant impact on health and disease

Numerous clinical observations and exploitation of cellular and animal models indicate that glucosylceramide (GlcCer) and galactosylceramide (GalCer) are involved in many physiological and pathological phenomena. In many cases, the biological importance of these monohexosylcermides has been shown indirectly as the result of studies on enzymes involved in their synthesis and degradation. Under physiological conditions, GalCer plays a key role in the maintenance of proper structure and stability of myelin and differentiation of oligodendrocytes. On the other hand, GlcCer is necessary for the proper functions of epidermis. Such an important lysosomal storage disease as Gaucher disease (GD) and a neurodegenerative disorder as Parkinson’s disease are characterized by mutations in the GBA1 gene, decreased activity of lysosomal GBA1 glucosylceramidase and accumulation of GlcCer. In contrast, another lysosomal disease, Krabbe disease, is associated with mutations in the GALC gene, resulting in deficiency or decreased activity of lysosomal galactosylceramidase and accumulation of GalCer and galactosylsphingosine. Little is known about the role of both monohexosylceramides in tumor progression; however, numerous studies indicate that GlcCer and GalCer play important roles in the development of multidrug-resistance by cancer cells. It was shown that GlcCer is able to provoke immune reaction and acts as a self-antigen in GD. On the other hand, GalCer was recognized as an important cellular receptor for HIV-1. Altogether, these two molecules are excellent examples of how slight differences in chemical composition and molecular conformation contribute to profound differences in their physicochemical properties and biological functions.

Mechanism of glucocerebrosidase activation and dysfunction in Gaucher disease unraveled by molecular dynamics and deep learning

The lysosomal enzyme glucocerebrosidase-1 (GCase) catalyzes the cleavage of a major glycolipid glucosylceramide into glucose and ceramide. The absence of fully functional GCase leads to the accumulation of its lipid substrates in lysosomes, causing Gaucher disease, an autosomal recessive disorder that displays profound genotype-phenotype nonconcordance. More than 250 disease-causing mutations in GBA1, the gene encoding GCase, have been discovered, although only one of these, N370S, causes 70% of disease. Here, we have used a knowledge-based docking protocol that considers experimental data of protein-protein binding to generate a complex between GCase and its known facilitator protein saposin C (SAPC). Multiscale molecular-dynamics simulations were used to study lipid self-assembly, membrane insertion, and the dynamics of the interactions between different components of the complex. Deep learning was applied to propose a model that explains the mechanism of GCase activation, which requires SAPC. Notably, we find that conformational changes in the loops at the entrance of the substrate-binding site are stabilized by direct interactions with SAPC and that the loss of such interactions induced by N370S and another common mutation, L444P, result in destabilization of the complex and reduced GCase activation. Our findings provide an atomistic-level explanation for GCase activation and the precise mechanism through which N370S and L444P cause Gaucher disease.

The LIMP-2/SCARB2 binding motif on acid β-glucosidase: basic and applied implications for Gaucher disease and associated neurodegenerative diseases

The acid β-glucosidase (glucocerbrosidase (GCase)) binding sequence to LIMP-2 (lysosomal integral membrane protein 2), the receptor for intracellular GCase trafficking to the lysosome, has been identified. Heterologous expression of deletion constructs, the available GCase crystal structures, and binding and co-localization of identified peptides or mutant GCases were used to identify and characterize a highly conserved 11-amino acid sequence, DSPIIVDITKD, within human GCase. The binding to LIMP-2 is not dependent upon a single amino acid, but the interactions of GCase with LIMP-2 are heavily influenced by Asp(399) and the di-isoleucines, Ile(402) and Ile(403). A single alanine substitution at any of these decreases GCase binding to LIMP-2 and alters its pH-dependent binding as well as diminishing the trafficking of GCase to the lysosome and significantly increasing GCase secretion. Enterovirus 71 also binds to LIMP-2 (also known as SCARB2) on the external surface of the plasma membrane. However, the LIMP-2/SCARB2 binding sequences for enterovirus 71 and GCase are not similar, indicating that LIMP-2/SCARB2 may have multiple or overlapping binding sites with differing specificities. These findings have therapeutic implications for the production of GCase and the distribution of this enzyme that is delivered to various organs.

Influence of the variation of the alkyl chain length of N-alkyl-β-D-glycosylamine derivatives on antifungal properties

Twelve new glucosidic and galactosidic derivatives of N-alkylaminosugars with different alkylamines from 6 to 18 carbons were synthesized and characterized by (1)H and (13)C NMR. Their antifungal activity against the food fungal pathogen Aspergillus niger was evaluated using the radial growth assay. The influence of the variation of the alkyl chain length of N-alkylaminosugars on the mycelium growth was then discussed. Inhibition by the different alkylamines is shown as a biostatic effect rather than a biocidal effect. It was observed that alkylamines keep their antifungal properties after a thermal treatment compatible with food packaging and processing.

Insights into Krabbe disease from structures of galactocerebrosidase

Krabbe disease is a devastating neurodegenerative disease characterized by widespread demyelination that is caused by defects in the enzyme galactocerebrosidase (GALC). Disease-causing mutations have been identified throughout the GALC gene. However, a molecular understanding of the effect of these mutations has been hampered by the lack of structural data for this enzyme. Here we present the crystal structures of GALC and the GALC-product complex, revealing a novel domain architecture with a previously uncharacterized lectin domain not observed in other hydrolases. All three domains of GALC contribute residues to the substrate-binding pocket, and disease-causing mutations are widely distributed throughout the protein. Our structures provide an essential insight into the diverse effects of pathogenic mutations on GALC function in human Krabbe variants and a compelling explanation for the severity of many mutations associated with fatal infantile disease. The localization of disease-associated mutations in the structure of GALC will facilitate identification of those patients that would be responsive to pharmacological chaperone therapies. Furthermore, our structure provides the atomic framework for the design of such drugs.

Lysosomal degradation of membrane lipids

The constitutive degradation of membrane components takes place in the acidic compartments of a cell, the endosomes and lysosomes. Sites of lipid degradation are intralysosomal membranes that are formed in endosomes, where the lipid composition is adjusted for degradation. Cholesterol is sorted out of the inner membranes, their content in bis(monoacylglycero)phosphate increases, and, most likely, sphingomyelin is degraded to ceramide. Together with endosomal and lysosomal lipid-binding proteins, the Niemann-Pick disease, type C2-protein, the GM2-activator, and the saposins sap-A, -B, -C, and -D, a suitable membrane lipid composition is required for degradation of complex lipids by hydrolytic enzymes.

Involvement of acid beta-glucosidase 1 in the salvage pathway of ceramide formation

Activation of protein kinase C (PKC) promotes the salvage pathway of ceramide formation, and acid sphingomyelinase has been implicated, in part, in providing substrate for this pathway (Zeidan, Y. H., and Hannun, Y. A. (2007) J. Biol. Chem. 282, 11549-11561). In the present study, we examined whether acid beta-glucosidase 1 (GBA1), which hydrolyzes glucosylceramide to form lysosomal ceramide, was involved in PKC-regulated formation of ceramide from recycled sphingosine. Glucosylceramide levels declined after treatment of MCF-7 cells with a potent PKC activator, phorbol 12-myristate 13-acetate (PMA). Silencing GBA1 by small interfering RNAs significantly attenuated acid glucocerebrosidase activity and decreased PMA-induced formation of ceramide by 50%. Silencing GBA1 blocked PMA-induced degradation of glucosylceramide and generation of sphingosine, the source for ceramide biosynthesis. Reciprocally, forced expression of GBA1 increased ceramide levels. These observations indicate that GBA1 activation can generate the source (sphingosine) for PMA-induced formation of ceramide through the salvage pathway. Next, the role of PKCdelta, a direct effector of PMA, in the formation of ceramide was determined. By attenuating expression of PKCdelta, cells failed to trigger PMA-induced alterations in levels of ceramide, sphingomyelin, and glucosylceramide. Thus, PKCdelta activation is suggested to stimulate the degradation of both sphingomyelin and glucosylceramide leading to the salvage pathway of ceramide formation. Collectively, GBA1 is identified as a novel source of regulated formation of ceramide, and PKCdelta is an upstream regulator of this pathway.

Principles of lysosomal membrane degradation: Cellular topology and biochemistry of lysosomal lipid degradation

Cellular membranes enter the lysosomal compartment by endocytosis, phagocytosis, or autophagy. Within the lysosomal compartment, membrane components of complex structure are degraded into their building blocks. These are able to leave the lysosome and can then be utilized for the resynthesis of complex molecules or can be further degraded. Constitutive degradation of membranes occurs on the surface of intra-endosomal and intra-lysosomal membrane structures. Many integral membrane proteins are sorted to the inner membranes of endosomes and lysosome after ubiquitinylation. In the lysosome, proteins are degraded by proteolytic enzymes, the cathepsins. Phospholipids originating from lipoproteins or cellular membranes are degraded by phospholipases. Water-soluble glycosidases sequentially cleave off the terminal carbohydrate residues of glycoproteins, glycosaminoglycans, and glycosphingolipids. For glycosphingolipids with short oligosaccharide chains, the additional presence of membrane-active lysosomal lipid-binding proteins is required. The presence of lipid-binding proteins overcomes the phase problem of water soluble enzymes and lipid substrates by transferring the substrate to the degrading enzyme or by solubilizing the internal membranes. The lipid composition of intra-lysosomal vesicles differs from that of the plasma membrane. To allow at least glycosphingolipid degradation by hydrolases and activator proteins, the cholesterol content of these intraorganellar membranes decreases during endocytosis and the concentration of bis(monoacylglycero)phosphate, a stimulator of sphingolipid degradation, increases. A considerable part of our current knowledge about mechanism and biochemistry of lysosomal lipid degradation is derived from a class of human diseases, the sphingolipidoses, which are caused by inherited defects within sphingolipid and glycosphingolipid catabolism.

Sphingolipid metabolism diseases

Human diseases caused by alterations in the metabolism of sphingolipids or glycosphingolipids are mainly disorders of the degradation of these compounds. The sphingolipidoses are a group of monogenic inherited diseases caused by defects in the system of lysosomal sphingolipid degradation, with subsequent accumulation of non-degradable storage material in one or more organs. Most sphingolipidoses are associated with high mortality. Both, the ratio of substrate influx into the lysosomes and the reduced degradative capacity can be addressed by therapeutic approaches. In addition to symptomatic treatments, the current strategies for restoration of the reduced substrate degradation within the lysosome are enzyme replacement therapy (ERT), cell-mediated therapy (CMT) including bone marrow transplantation (BMT) and cell-mediated "cross correction", gene therapy, and enzyme-enhancement therapy with chemical chaperones. The reduction of substrate influx into the lysosomes can be achieved by substrate reduction therapy. Patients suffering from the attenuated form (type 1) of Gaucher disease and from Fabry disease have been successfully treated with ERT.

Saposin A Mobilizes Lipids from Low Cholesterol and High Bis(monoacylglycerol)phosphate-containing Membranes

Saposin A (Sap-A) is one of five known sphingolipid activator proteins required for the lysosomal degradation of sphingolipids and for the loading of lipid antigens onto antigen-presenting molecules of the CD1 type. Sap-A assists in the degradation of galactosylceramide by galactosylceramide-beta-galactosidase in vivo, which takes place at the surface of intraendosomal/intralysosomal vesicles. Sap-A is believed to mediate the interaction between the enzyme and its membrane-bound substrate. Its dysfunction causes a variant form of Krabbe disease. In the present study we prepared glycosylated Sap-A free of other Saps, taking advantage of the Pichia pastoris expression system. Using liposomes and surface plasmon resonance spectroscopy, we tested the binding and lipid mobilization capacity of Sap-A under different conditions. Along the endocytic pathway, the pH value decreases, and the lipid composition of intraendosomal and intralysosomal membranes changes drastically. In the inner membranes the cholesterol concentration decreases, and that of the anionic phospholipid bis(monoacylglycero)phosphate increases. Here, we show that Sap-A is able to bind to liposomes and to mobilize lipids out of them at acidic pH values below pH 4.7. Low cholesterol levels and increasing concentrations of bis(monoacylglycero)phosphate favor lipid extraction significantly. Galactosylceramide as a bilayer component is not essential for lipid mobilization by Sap-A, which requires intact disulfide bridges for activity. We also show for the first time that glycosylation of Sap-A is essential for its lipid extraction activity. Variant Sap-A proteins, which cause storage of galactosylceramide in humans (Krabbe disease, Spiegel, R., Bach, G., Sury, V., Mengistu, G., Meidan, B., Shalev, S., Shneor, Y., Mandel, H., and Zeigler, M. (2005) Mol. Genet. Metab. 84, 160-166) and in mutant mice (Matsuda, J., Vanier, M. T., Saito, Y., Tohyama, J., and Suzuki, K. (2001) Hum. Mol. Genet. 10, 1191-1199) are deficient in lipid extraction capacity.

Principles of lysosomal membrane digestion: stimulation of sphingolipid degradation by sphingolipid activator proteins and anionic lysosomal lipids

Sphingolipids and glycosphingolipids are membrane components of eukaryotic cell surfaces. Their constitutive degradation takes place on the surface of intra-endosomal and intra-lysosomal membrane structures. During endocytosis, these intra-lysosomal membranes are formed and prepared for digestion by a lipid-sorting process during which their cholesterol content decreases and the concentration of the negatively charged bis(monoacylglycero)phosphate (BMP)--erroneously also called lysobisphosphatidic acid (LBPA)--increases. Glycosphingolipid degradation requires the presence of water-soluble acid exohydrolases, sphingolipid activator proteins, and anionic phospholipids like BMP. The lysosomal degradation of sphingolipids with short hydrophilic head groups requires the presence of sphingolipid activator proteins (SAPs). These are the saposins (Saps) and the GM2 activator protein. Sphingolipid activator proteins are membrane-perturbing and lipid-binding proteins with different specificities for the bound lipid and the activated enzyme-catalyzed reaction. Their inherited deficiency leads to sphingolipid- and membrane-storage diseases. Sphingolipid activator proteins not only facilitate glycolipid digestion but also act as glycolipid transfer proteins facilitating the association of lipid antigens with immunoreceptors of the CD1 family.

Analysis of recombinant human saposin A expressed by Pichia pastoris

Saposins (SAPs) are small glycoproteins required for activation of sphingolipid hydrolysis by lysosomal enzymes. Four SAPs, SAP-A, -B, -C, and -D, are proteolytically cleaved from a single gene product termed prosaposin. The mature coding sequence of human SAP-A tagged with 6-histidine was expressed in Pichia pastoris and the recombinant protein was purified from the culture supernatant by simple purification steps with an immobilized metal ion affinity column, a Concanavalin A column, and reversed-phase HPLC. Secreted SAP-A contained both glycosylated and nonglycosylated forms. Both forms of SAP-A activated galactocerebroside and 4-methylumbelliferyl beta-d-glucoside hydrolysis by galactocerebrosidase and glucocerebrosidase. SAP-A expressed in P. pastoris should be useful for further structural and functional analysis of this protein.

Biosynthesis and degradation of mammalian glycosphingolipids

Glycolipids are a large and heterogeneous family of sphingolipids that form complex patterns on eukaryotic cell surfaces. This molecular diversity is generated by only a few enzymes and is a paradigm of naturally occurring combinatorial synthesis. We report on the biosynthetic principles leading to this large molecular diversity and focus on sialic acid-containing glycolipids of the ganglio-series. These glycolipids are particularly concentrated in the plasma membrane of neuronal cells. Their de novo synthesis starts with the formation of the membrane anchor, ceramide, at the endoplasmic reticulum (ER) and is continued by glycosyltransferases of the Golgi complex. Recent findings from genetically engineered mice are discussed. The constitutive degradation of glycosphingolipids (GSLs) occurs in the acidic compartments, the endosomes and the lysosomes. Here, water-soluble glycosidases sequentially cleave off the terminal carbohydrate residues from glycolipids. For glycolipid substrates with short oligosaccharide chains, the additional presence of membrane-active sphingolipid activator proteins (SAPs) is required. A considerable part of our current knowledge about glycolipid degradation is derived from a class of human diseases, the sphingolipidoses, which are caused by inherited defects within this pathway. A new post-translational modification is the attachment of glycolipids to proteins of the human skin.

Saposins (sap) A and C activate the degradation of galactosylsphingosine

As previously shown for [(3)H-galactosyl]ceramide, the breakdown of [(3)H-galactosyl]sphingosine was reduced in prosaposin-deficient skin fibroblast homogenates. Galactosylsphingosine hydrolysis was also deficient in cell homogenates from Krabbe's disease (beta-galactocerebrosidase-deficient) patients, but not acid beta-galactosidase-deficient patients. Moreover, hydrolysis of galactosylsphingosine in the prosaposin-deficient cell homogenates could be partially restored by adding pure saposin A or C, thereby identifying these saposins as essential facilitators of galactosylsphingosine hydrolysis. By contrast, saposins B and D had little effect on galactosylsphingosine hydrolysis in the prosaposin-deficient cells. The reduced galactosylsphingosine turnover in prosaposin-deficiency suggests that there could be a pathogenetic cerebral accumulation of galactosylsphingosine in this disorder.

Further studies on the reconstitution of glucosylceramidase activity by Sap C and anionic phospholipids

The reconstitution of the activity of the lysosomal enzyme glucosylceramidase requires anionic phospholipids and, at least, a protein factor, saposin C (Sap C). We have previously proposed a mechanism for the glucosylceramidase activation [Vaccaro et al. (1993) FEBS Lett. 336, 159-162] which implies that Sap C promotes the association of the enzyme with anionic phospholipid-containing membranes, thus favoring the contact between the enzyme and its lipid substrate, glucosylceramide. We have further investigated the properties of Sap C using a fluorescent hydrophobic probe such as 4, 4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (bis-ANS). The binding between bis-ANS and Sap C was pH-dependent, indicating that protonation leads to increased exposure of hydrophobic surfaces of Sap C. The interaction of Sap C with membranes, triggered by the development of hydrophobic properties at low pH values, was affected by the content of anionic phospholipids, such as phosphatidylserine or phosphatidylinositol, suggesting that anionic phospholipids have the potential to modulate the insertion of Sap C in the hydrophobic environment of lysosomal membranes. We previously showed that Sap C and anionic phospholipids are both required for the binding of glucosylceramidase to large vesicles. We have presently observed that Sap C is able to promote the association of glucosylceramidase with the lipid surface only when anionic phospholipids exceed a concentration of 5-10%. This level can be reached by summing lower amounts of individual anionic phospholipids, since they have additive effects. The present data extend and refine our model of the mechanism of glucosylceramidase activation and stress the key role of pH, Sap C and anionic phospholipids in promoting the interaction of the enzyme with membranes.

A non-glycosylated and functionally deficient mutant (N215H) of the sphingolipid activator protein B (SAP-B) in a novel case of metachromatic leukodystrophy (MLD)

The lysosomal degradation of sphingolipids with short oligosaccharide chains depends on small glycosylated non-enzymatic sphingolipid activator proteins (SAPs, saposins). Four of the five known SAPs, SAP-A, -B, -C and -D, are derived by proteolytic processing from a common precursor protein (SAP-precursor) that is encoded by a gene on chromosome 10 consisting of 15 exons and 14 introns. SAP-B is a non-specific glycolipid binding protein that stimulates in vitro the hydrolysis of about 20 glycolipids by different enzymes. In vivo SAP-B stimulates in particular the degradation of sulphatides by arylsulphatase A. So far, four different point mutations have been identified on the SAP-B domain of the SAP-precursor gene. The mutations result in a loss of mature SAP-B, causing the lysosomal accumulation of sulphatides and other sphingolipids, resulting in variant forms of metachromatic leukodystrophy (MLD). Here we report on a patient with SAP-B deficiency that is caused by a new homoallelic point mutation that has been identified by mRNA and DNA analysis. A 643A > C transversion results in the exchange of asparagine 215 to histidine and eliminates the single glycosylation site of SAP-B. Metabolic labelling experiments showed that the mutation had no effect on the intracellular transport of the encoded precursor to the acidic compartments and its maturation in the patient's cells. All four SAPs (SAP-A to SAP-D) were detectable by immunochemical methods. SAP-B in the patient's cells was found to be slightly less stable than the protein in normal cells and corresponded in size to the deglycosylated form of the wild-type SAP-B. Feeding studies with non-glycosylated SAP-precursor, generating non-glycosylated SAP-B, showed that the loss of the carbohydrate chain reduced the intracellular activity of the protein significantly. The additional structural change of the patient's SAP-B, caused by the change of amino acid 215 from asparagine to histidine, presumably resulted in an almost completely inactive protein.

Lysosomal degradation on vesicular membrane surfaces. Enhanced glucosylceramide degradation by lysosomal anionic lipids and activators

According to a recent hypothesis (Sandhoff, K., and Kolter, T. (1996) Trends Cell Biol. 6, 98-103), glycolipids, which originate from the plasma membrane, are exposed to lysosomal degradation on the surface of intralysosomal vesicles. Taking the interaction of membrane-bound lipid substrates and lysosomal hydrolases as an experimental model, we studied the degradation of glucosylceramides with different acyl chain lengths by purified glucocerebrosidase in a detergent-free liposomal assay system. Our investigation focused on the stimulating effect induced by lysosomal components such as sphingolipid activator protein C (SAP-C or saposin C), anionic lysosomal lipids, bis(monoacylglycero)phosphate, and dolichol phosphate, as well as degradation products of lysosomal lipids, e.g. dolichols and free fatty acids. The size of the substrate-containing liposomal vesicles was varied in the study. Enzymatic hydrolysis of glucosylceramide carried by liposomes made of phosphatidylcholine and cholesterol was rather slow and only weakly accelerated by the addition of SAP-C. However, the incorporation of anionic lipids such as bis(monoacylglycero)phosphate, dolichol phosphate, and phosphatidylinositol into the substrate carrying liposomes stimulated glucosylceramide hydrolysis up to 30-fold. Dolichol was less effective. SAP-C activated glucosylceramide hydrolysis under a variety of experimental conditions and was especially effective for the increase of enzyme activity when anionic lipids were inserted into the liposomes. Glucosylceramides with short acyl chains were found to be degraded much faster than the natural substrates. Dilution experiments indicated that the added enzyme molecules associate at least partially with the membranes and act there. Surface plasmon resonance experiments demonstrated binding of SAP-C at concentrations up to 1 microM to liposomes. At higher concentrations (2.5 microM SAP-C), liposomal lipids were released from the liposome coated chip. A model for lysosomal glucosylceramide hydrolysis is discussed.

Saposins (sap) A and C activate the degradation of galactosylceramide in living cells

In loading tests using galactosylceramide which had been labelled with tritium in the ceramide moiety, living skin fibroblast lines derived from the original prosaposin-deficient patients had a markedly reduced capacity to degrade galactosylceramide. The hydrolysis of galactosylceramide could be partially restored in these cells, up to about half the normal rate, by adding pure saposin A, pure saposin C, or a mixture of these saposins to the culture medium. By contrast, saposins B and D had little effect on galactosylceramide hydrolysis in the prosaposin-deficient cells. Cells from beta-galactocerebrosidase-deficient (Krabbe) patients had a relatively high residual galactosylceramide degradation, which was similar to the rate observed for prosaposin-deficient cells in the presence of saposin A or C. An SV40-transformed fibroblast line from the original saposin C-deficient patient, where saposin A is not affected, showed normal degradation of galactosylceramide. The findings support the hypothesis, which was deduced originally from in vitro experiments, that saposins A and C are the in vivo activators of galactosylceramide degradation. Although the results with saposin C-deficient fibroblasts suggest that the presence of only saposin A allows galactosylceramide breakdown to proceed at a normal rate in fibroblasts, it remains to be determined whether saposins A and C can substitute for each other with respect to their effects on galactosylceramide metabolism in the whole organism.

Effect of saposins A and C on the enzymatic hydrolysis of liposomal glucosylceramide

The degradation of glucosylceramide in lysosomes is accomplished by glucosylceramidase with the assistance of, at least, another protein, saposin C (Sap C), which is generated from a large precursor together with three other similar proteins, saposins A, B, and D. In the present study, we have examined the effects of saposins on the enzymatic hydrolysis of glucosylceramide inserted in large and small phospholipid liposomes. The glucosylceramide contained in large unilamellar vesicles (LUV) was degraded by glucosylceramidase at a rate 7-8-fold lower than glucosylceramide inserted in small unilamellar vesicles (SUV). The separate addition of either Sap A or Sap C to the LUV system partially stimulated the sphingolipid degradation while saposins B and D had no effect. In the presence of both Sap A and Sap C, the rate of sphingolipid degradation was higher than the sum of the rates with the two saposins individually, indicating synergism in their actions. The stimulatory effect of the two saposins depended on the incorporation of an acidic phospholipid such as phosphatidylserine (PS) into LUV. The characteristics of glucosylceramidase activation by Sap C were different from those of Sap A. Sap C increased the rate of hydrolysis of both the artificial water soluble substrate, 4-methylumbelliferyl-beta-D-glucopyranoside, and the lipid substrate, glucosylceramide, while Sap A only stimulated degradation of the sphingolipid. Also the binding properties of Saps A and C were markedly different. At acidic pH values, Sap C bound to PS-containing LUV and promoted the association of glucosylceramidase with the membrane. In contrast, Sap A had poor affinity for the membrane even in the presence of glucosylceramide; moreover, Sap A did not potentiate the capacity of Sap C to mediate glucosylceramidase binding. In conclusion, our results show that both Sap A and Sap C are required for maximal hydrolysis of glucosylceramide inserted in PS-containing LUV, that their effects are synergistic, and that their mode of action is different. Sap C is responsible for the membrane binding of glucosylceramidase, while Sap A stimulation is possibly related to its effect on the conformation of the enzyme. It can be envisaged that Sap A in conjunction with Sap C might have a physiological role in glucosylceramide degradation.

Subsite structure of the beta-glucosidase from Aspergillus niger, evaluated by steady-state kinetics with cello-oligosaccharides as substrates

The beta-glucosidase from a commercially available preparation from Aspergillus niger was highly purified. The Michaelis constant Km and the molar activity K0 for cello-oligosaccharide substrates Gn (n = 2-6) were obtained by steady-state kinetic analysis on the beta-glucosidase-catalyzed hydrolysis at 25 degrees C and pH 5.0. Stoichiometric production of Gn-1 by the beta-glucosidase reaction for Gn was confirmed by HPLC techniques. Based on Km and K0 for Gn, subsite affinities (Ai, i = 1-6) were estimated as follows (kcal/mol): A1 = 1.3, A2 = 5.2, A3 = 0.65, A4 = -0.10, A5 = -0.65, and A6 = -0.26, of which A1-A3 are much higher than those of the beta-glucosidase of Candida wickerhamii. The subsite structure is quite similar to that of the alpha-glucosidase of A. niger, whereas the dependence of k0 on n is highly characteristic for beta-glucosidase, and decreases with n, suggesting some interaction between the particular subsites.

Functional organization of saposin C. Definition of the neurotrophic and acid beta-glucosidase activation regions

Saposin C is an essential co-factor for the hydrolysis of glucosylceramide by acid beta-glucosidase in mammals. In addition, prosaposin promotes neurite outgrowth in vitro via sequences in saposin C. The regional organization of these neurotrophic and activation properties of saposin C was elucidated using recombinant or chemically synthesized saposin Cs from various regions of the molecule. Unreduced and reduced proteins were analyzed by electrospray-mass spectrometry to establish the complement of disulfide bonds in selected saposin Cs. Using saposin B as a unreactive backbone, chimeric saposins containing various length segments of saposin B and C localized the neurotrophic and acid beta-glucosidase activation properties to the carboxyl- and NH2-terminal 50% of saposin C, respectively. The peptide spanning residues 22-31 had neurotrophic effects. Molecular modeling and site-directed mutagenesis localized the activation properties of saposin C to the region spanning residues 47-62. Secondary structure was needed for retention of this property. Single substitutions of R and S at the conserved cysteines at 47 or 78 diminished but did not obliterate the activation properties. These results indicate the segregation of neurotrophic and activation properties of saposin C to two different faces of the molecule and suggest a topographic sequestration of the activation region of prosaposin for protection of the cell from adverse hydrolytic activity of acid beta-glucosidase.

Turnover and distribution of intravenously administered mannose-terminated human acid beta-glucosidase in murine and human tissues

Gaucher disease type 1, the most prevalent lysosomal storage disease, is caused by the defective activity of the lysosomal enzyme, acid beta-glucosidase, or glucocerebrosidase. Infusion of purified acid beta-glucosidase containing alpha-mannosyl-terminated oligosaccharides (alglucerase) is efficacious in reversing hematologic, hepatic, splenic, and bony disease manifestations. The murine tissue distribution and turnover of bolus injections of alglucerase was evaluated by enzymatic activity, quantitative cross-reacting immunologic material analyses, and immunofluorescence studies. Enzyme activity measurements detected distribution to liver, spleen, thymus, kidney, and bone marrow mononuclear cells, but not to lungs and brain. In kidney and thymus, the enzyme was transiently present. In liver and spleen, enzyme activity peaked at about 20 min postinjection followed by a biphasic decrease with t1/2 approximately 40-60 min and approximately 12-14 h. In bone marrow maximal enzyme activity was at 40-60 min with a disappearance t1/2 approximately 60 min. Quantitative cross-reacting immunologic material studies of liver and spleen showed delivery of enzyme with decreased catalytic rate constants whose degradation included denaturation and proteolytic components. By immunofluorescence the human enzyme was distributed primarily to reticuloendothelial cells of the liver and spleen. In autopsy material from a Gaucher disease type 2 patient treated with enzyme, immunohistochemical and activity studies showed distributions similar to those in mice. These studies indicate a complex delivery and intracellular degradation of acid beta-glucosidase with lower intrinsic activity than the administered therapeutic agent.

Identification of the binding and activating sites of the sphingolipid activator protein, saposin C, with glucocerebrosidase

Saposin C is a sphingolipid activator protein of 8.5 kDa that activates lysosomal glucocerebrosidase. Previously, we synthesized and characterized a synthetic full-length human saposin C protein that displays 85% of the activity of the native saposin C. In this study we use shorter synthetic peptides derived from the saposin C sequence to map binding and activation sites. By determining the activity and kinetic constant (Kact) values of these peptides, we have identified two functional domains, each comprising a binding site adjacent to or partially overlapping with an activation site. Domains 1 and 2 are located within amino acid positions 6-34 and 41-60, respectively. The activation sites span residues 27-34 and 41-49, whereas binding sites encompass residues 6-27 and 45-60. Peptides containing the sequences of either domain displayed 90% of the activity of the full-length synthetic saposin C. Domain 2, however, bound to glucocerebrosidase by at least an order of magnitude more strongly than domain 1. Binding sites within these domains contain sequences that are excellent candidates for forming amphipathic helical structures. Competition assays demonstrated that the binding of one domain to glucocerebrosidase prevents binding of the other domain, and that saposin A and saposin C bind to the same sites on glucocerebrosidase. A model predicting a saposin C:glucocerebrosidase complex with a stoichiometry of 4:2, respectively, is presented.

Functional human saposins expressed in Escherichia coli. Evidence for binding and activation properties of saposins C with acid beta-glucosidase

Small (80-amino acid) glycoproteins or saposins are important for the in vivo function of several lysosomal hydrolases. Four saposins, A, B, C, and D, are encoded by a single locus termed prosaposin. Saposins C and A are thought to function in vivo as activators of acid beta-glucosidase. The physiologic role of saposin C has been confirmed, whereas that of saposin A role has not. To investigate the effects of saposins C and A on acid beta-glucosidase activity, the coding sequence for the individual saposins was expressed in Escherichia coli and the recombinant proteins purified to homogeneity. Recombinant and natural saposins A and C activated acid beta-glucosidase similarly only in micromolar amounts. Saposin C had specific activation of acid beta-glucosidase activity at < 200 nM. A second phase of activation was achieved at > 1 microM. In comparison, saposin A consistently activated acid beta-glucosidase only at > 1 microM. Two mutant saposins C (Cys382-->Phe and Cys382--Gly) were created and shown to compete with saposin C for a site on acid beta-glucosidase. The mutant saposins did not activate the enzyme. Recombinant saposin A (< 200 nM) competed with saposin C for a site on the enzyme but without activating effects. These studies show that saposin A is not an in vitro activator of acid beta-glucosidase at physiologic concentrations, although binding occurs without activating acid beta-glucosidase. The studies with mutant saposins C indicate that the binding and activation effects of saposins C are distinct events. These results indicate that the saposin C-induced conformational change in the enzyme occurs via highly specific, probably multivalent, interactions between acid beta-glucosidase and saposin C.

Prosaposin deficiency: further characterization of the sphingolipid activator protein-deficient sibs. Multiple glycolipid elevations (including lactosylceramidosis), partial enzyme deficiencies and ultrastructure of the skin in this generalized sphingolipid storage disease

Sphingolipid activator protein (SAP) deficiency, previously described in two sibs and shown to be caused by the absence of the common saposin precursor (prosaposin), was further characterized by biochemical lipid and enzyme studies and by ultrastructural analysis. The 20-week-old fetal sib had increased concentrations of neutral glycolipids, including mono-, di-, tri- and tetrahexosylceramide, in liver, kidney and cultured skin fibroblasts compared with the controls. Glucosylceramide and lactosylceramide were particularly elevated. The kidney of the affected fetus showed additional increases in the concentration of sulphatide, galactosylceramide and digalactosylceramide. Free ceramide was stored in the liver and kidney, and GM3 and GM2 gangliosides were elevated in the liver, but not the brain, of the fetus. Phospholipids, however, were normal in the affected fetus. In the liver biopsy of the propositus, who later died at 16 weeks of age, only a few lipids could be studied. Glucosylceramide, dihexosylceramide and ceramide were elevated in agreement with our previous study. Enzyme studies were undertaken using detergent-free liposomal substrate preparations and fibroblast extracts. The sibs' beta-glucocerebrosidase and beta-galactocerebrosidase activities were clearly reduced, but their sphingomyelinase activities were normal. The normal activity of the latter enzyme and the almost normal tissue concentration of sphingomyelin in prosaposin deficiency suggest that the prosaposin-derived SAPs are not required for sphingomyelinase activity in vivo. In keeping with the biochemical findings, skin biopsies from the sibs showed massive lysosomal storage with a vesicular and membranous ultrastructure. The function of SAPs in sphingolipid degradation and the role of SAPs for enzyme activity in vitro are discussed. In addition, the similarity in neutral glycolipid accumulations in Niemann-Pick disease type C and in prosaposin deficiency are noted. The phenotype of the prosaposin deficient sibs resembled acute neuronopathic (type 2) Gaucher disease more than Farber disease in several aspects, but their genotype was unique.

Gaucher disease: a heterogeneous clinical complex for which effective enzyme replacement has come of age

Gaucher disease, the most common form of lysosomal storage disease, is the result of autosomal recessive inheritance of a lysosomal enzyme glucocerebrosidase deficiency, which produces defective hydrolysis of glucosylceramide that accumulates in reticuloendothelial (tissue macrophage) cells. The current review focuses on Type 1 (the nonneuronopathic) or adult Gaucher disease and defines the clinical manifestations (splenomegaly, hepatomegaly, bony lesions, and clinical metabolic dysfunction) in relationship to the known enzymatic defect. The clinical diversity and variability in symptoms and signs, the age at onset of the clinical manifestations and their rate of progression, and the heterogeneity of the organs involved are reviewed. Extensive delineation of the nature of the enzyme defect and the recent molecular characterization of the enzyme mutants has not provided an explantation for the remarkable clinical phenotypic heterogeneity. Enzyme assays now provide an excellent method for diagnosis. Effective enzyme replacement therapy emphasizes the value of early diagnosis and has altered the costs and potential risks of older therapeutic indications for splenectomy or cytokine therapy. Enzyme efficacy raises questions about the specific indications for replacement treatment and the most desirable rate and duration of enzyme delivery.

Structure and evolution of the human prosaposin chromosomal gene

The gene for prosaposin was characterized by sequence analysis of chromosomal DNA to gain insight into the evolution of this locus that encodes four highly conserved sphingolipid activator proteins or saposins. The 13 exons ranged in size from 57 to 1200 bp, while the introns were from 91 to 3812 bp in length. The regions encoding saposins A, B, and D each had three exons, while that for saposin C had only two. This sequence included the regions that encode the carboxy terminus of the signal peptide, the four mature prosaposin proteins, and the 3' untranslated region. Primer extension studies indicated that over 99% of the coding sequence was contained in these 19,985 bp. Use of PCR and reverse PCR techniques indicated that the most 5' coding approximately 140 bp contained large introns and at least two small exons. Analyses of the intronic positions in the saposin regions indicated that this gene evolved from an ancestral gene by two duplication events and at least one gene rearrangement involving a double crossover after introns had been inserted into the gene.