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

Pro-cathepsin D, Prosaposin, and Progranulin: Lysosomal Networks in Parkinsonism

Mutations in GBA1, the gene encoding the lysosomal hydrolase glucocerebrosidase (GCase), are a risk factor for parkinsonism. Pursuing the potential mechanisms underlying this risk in aging neurons, we propose a new network uniting three major lysosomal proteins: (i) cathepsin D (CTSD), which plays a major role in α-synuclein (SNCA) degradation and prosaposin (PSAP) cleavage; (ii) PSAP, essential for GCase activation and progranulin (PGRN) transport; and (iii) PGRN, impacting lysosomal biogenesis, PSAP trafficking, and CTSD maturation. We hypothesize that alterations to this network and associated receptors modify lysosomal function and subsequently impact both SNCA degradation and GCase activity. By exploring the interactions between this protein trio and each of their respective transporters and receptors, we may identify secondary risk factors that provide insight into the relationship between these lysosomal proteins, GCase, and SNCA, and reveal novel therapeutic targets.

Combination of acid β-glucosidase mutation and Saposin C deficiency in mice reveals Gba1 mutation dependent and tissue-specific disease phenotype

Gaucher disease is caused by mutations in GBA1 encoding acid β-glucosidase (GCase). Saposin C enhances GCase activity and protects GCase from intracellular proteolysis. Structure simulations indicated that the mutant GCases, N370S (0 S), V394L (4L) and D409V(9V)/H(9H), had altered function. To investigate the in vivo function of Gba1 mutants, mouse models were generated by backcrossing the above homozygous mutant GCase mice into Saposin C deficient (C*) mice. Without saposin C, the mutant GCase activities in the resultant mouse tissues were reduced by ~50% compared with those in the presence of Saposin C. In contrast to 9H and 4L mice that have normal histology and life span, the 9H;C* and 4L;C* mice had shorter life spans. 9H;C* mice developed significant visceral glucosylceramide (GC) and glucosylsphingosine (GS) accumulation (GC»GS) and storage macrophages, but lesser GC in the brain, compared to 4L;C* mice that presents with a severe neuronopathic phenotype and accumulated GC and GS primarily in the brain. Unlike 9V mice that developed normally for over a year, 9V;C* pups had a lethal skin defect as did 0S;C* mice resembled that of 0S mice. These variant Gaucher disease mouse models presented a mutation specific phenotype and underscored the in vivo role of Saposin C in the modulation of Gaucher disease.

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.

A saposin deficiency model in Drosophila: Lysosomal storage, progressive neurodegeneration and sensory physiological decline

Saposin deficiency is a childhood neurodegenerative lysosomal storage disorder (LSD) that can cause premature death within three months of life. Saposins are activator proteins that promote the function of lysosomal hydrolases that mediate the degradation of sphingolipids. There are four saposin proteins in humans, which are encoded by the prosaposin gene. Mutations causing an absence or impaired function of individual saposins or the whole prosaposin gene lead to distinct LSDs due to the storage of different classes of sphingolipids. The pathological events leading to neuronal dysfunction induced by lysosomal storage of sphingolipids are as yet poorly defined. We have generated and characterised a Drosophila model of saposin deficiency that shows striking similarities to the human diseases. Drosophila saposin-related (dSap-r) mutants show a reduced longevity, progressive neurodegeneration, lysosomal storage, dramatic swelling of neuronal soma, perturbations in sphingolipid catabolism, and sensory physiological deterioration. Our data suggests a genetic interaction with a calcium exchanger (Calx) pointing to a possible calcium homeostasis deficit in dSap-r mutants. Together these findings support the use of dSap-r mutants in advancing our understanding of the cellular pathology implicated in saposin deficiency and related LSDs.

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Drosophila model of PSD recapitulates neurodegenerative phenotype of human PSD.

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Preferential degeneration of sensory regions correlates with loss of sensory function.

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Sphingosine levels rise with age with an imbalance in sphingosine/ceramide ratios.

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Genetic interaction with the Na +/Ca + exchanger points to a calcium regulation deficit.

Membrane-spanning lipids for an uncompromised monitoring of membrane fusion and intermembrane lipid transfer

A Förster resonance energy transfer-based fusion and transfer assay was developed to study, in model membranes, protein-mediated membrane fusion and intermembrane lipid transfer of fluorescent sphingolipid analogs. For this assay, it became necessary to apply labeled reporter molecules that are resistant to spontaneous as well as protein-mediated intermembrane transfer. The novelty of this assay is the use of nonextractable fluorescent membrane-spanning bipolar lipids. Starting from the tetraether lipid caldarchaeol, we synthesized fluorescent analogs with fluorophores at both polar ends. In addition, we synthesized radioactive glycosylated caldarchaeols. These labeled lipids were shown to stretch through bilayer membranes rather than to loop within a single lipid layer of liposomes. More important, the membrane-spanning lipids (MSLs) in contrast to phosphoglycerides proved to be nonextractable by proteins. We could show that the GM2 activator protein (GM2AP) is promiscuous with respect to glycero- and sphingolipid transfer. Saposin (Sap) B also transferred sphingolipids albeit with kinetics different from GM2AP. In addition, we could unambiguously show that the recombinant activator protein Sap C x His6 induced membrane fusion rather than intermembrane lipid transfer. These findings showed that these novel MSLs, in contrast with fluorescent phosphoglycerolipids, are well suited for an uncompromised monitoring of membrane fusion and intermembrane lipid transfer.

The immunological functions of saposins

Saposins or sphingolipid activator proteins (SAPs) are small, nonenzymatic glycoproteins that are ubiquitously present in lysosomes. SAPs comprise the five molecules saposins A-D and the GM2 activator protein. Saposins are essential for sphingolipid degradation and membrane digestion. On the one hand, they bind the respective hydrolases required to catabolize sphingolipid molecules; on the other hand, saposins can interact with intralysosomal membrane structures to render lipids accessible to their degrading enzymes. Thus, saposins bridge the physicochemical gap between lipid substrate and hydrophilic hydrolases. Accordingly, defects in saposin function can lead to lysosomal lipid accumulation. In addition to their specific functions in sphingolipid metabolism, saposins have membrane-perturbing properties. At the low pH of lysosomes, saposins get protonated and exhibit a high binding affinity for anionic phospholipids. Based on their universal principle to interact with membrane bilayers, we present the immunological functions of saposins with regard to lipid antigen presentation to CD1-restricted T cells, processing of apoptotic bodies for antigen delivery and cross-priming, as well as their potential antimicrobial impact.

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.

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.

Molecular imaging of membrane interfaces reveals mode of beta-glucosidase activation by saposin C

Acid beta-glucosidase (GCase) is a soluble lysosomal enzyme responsible for the hydrolysis of glucose from glucosylceramide and requires activation by the small nonenzymatic protein saposin C (sapC) to gain access to the membrane-embedded glycosphingolipid substrate. We have used in situ atomic force microscopy (AFM) with simultaneous confocal and epifluorescence microscopies to investigate the interactions of GCase and sapC with lipid bilayers. GCase binds to sites on membranes transformed by sapC, and enzyme activity occurs at loci containing both GCase and sapC. Using FRET, we establish the presence of GCase/sapC and GCase/product contacts in the bilayer. These data support a mechanism in which sapC locally alters regions of bilayer for subsequent attack by the enzyme in stably bound protein complexes.

Structure of acid beta-glucosidase with pharmacological chaperone provides insight into Gaucher disease

Gaucher disease results from mutations in the lysosomal enzyme acid beta-glucosidase (GCase). Although enzyme replacement therapy has improved the health of some affected individuals, such as those with the prevalent N370S mutation, oral treatment with pharmacological chaperones may be therapeutic in a wider range of tissue compartments by restoring sufficient activity of endogenous mutant GCase. Here we demonstrate that isofagomine (IFG, 1) binds to the GCase active site, and both increases GCase activity in cell lysates and restores lysosomal trafficking in cells containing N370S mutant GCase. We also compare the crystal structures of IFG-bound GCase at low pH with those of glycerol-bound GCase at low pH and apo-GCase at neutral pH. Our data indicate that IFG induces active GCase, which is secured by interactions with Asn370. The design of small molecules that stabilize substrate-bound conformations of mutant proteins may be a general therapeutic strategy for diseases caused by protein misfolding and mistrafficking.

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.

A novel saposin-like protein of Entamoeba histolytica with membrane-fusogenic activity

Amoebapores, the pore-forming proteins of Entamoeba histolytica, are major pathogenicity factors of the parasite. Upon a comprehensive survey in the recently completed genome data sets for the protozoon, we identified in addition to the three amoebapore genes, 16 genes which are constitutively expressed and code for structurally similar proteins, all belonging to the family of saposin-like proteins. Here, we recombinantly expressed in bacteria a defined single entity of this expansive amoebic protein family, namely SAPLIP 3. The protein consists of the saposin-like domain only, comparable to amoebapores, and we characterized its interactions with membranes using different assays. In contrast to amoebapores, SAPLIP 3 neither forms pores in liposomes nor permeabilizes bacterial membranes. However, SAPLIP 3 induces leaky fusion of lipid vesicles as evidenced by fluorescence microscopic analysis and by using a fusion assay that monitors the dequenching of a lipophilic dye. The membrane-fusogenic activity of SAPLIP 3 which is dependent on the presence of negatively charged lipids and on acidic pH resembles in combination with the negative surface charge of the protein characteristics of human saposin C. Beside its function as a cofactor of sphingolipid hydrolysing enzymes, the human protein is considered to be involved in the reorganization of lysosomal compartments due to its fusogenic activity. We hypothesize that in the amoeba, SAPLIP 3 fulfils a similar function in the multifarious endo- and exocytotic transport processes.

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.

A short guided tour through functional and structural features of saposin-like proteins

SAPLIPs (saposin-like proteins) are a diverse family of lipid-interacting proteins that have various and only partly understood, but nevertheless essential, cellular functions. Their existence is conserved in phylogenetically most distant organisms, such as primitive protozoa and mammals. Owing to their remarkable sequence variability, a common mechanism for their actions is not known. Some shared principles beyond their diversity have become evident by analysis of known three-dimensional structures. Whereas lipid interaction is the basis for their functions, the special cellular tasks are often defined by interaction partners other than lipids. Based on recent findings, this review summarizes phylogenetic relations, function and structural features of the members of this family.

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.

Phospholipid membrane interactions of saposin C: in situ atomic force microscopic study

Saposin C (Sap C) is a small glycoprotein required for hydrolysis of glucosylceramidase in lysosomes. The full activity of glucosylceramidase requires the presence of both Sap C and acidic phospholipids. Interaction between Sap C and acidic phospholipid-containing membranes, a crucial step for enzyme activation, is not fully understood. In this study, the dynamic process of Sap C interaction with acidic phospholipid-containing membranes was investigated in aqueous buffer using atomic force microscopy. Sap C induced two types of membrane restructuring: formation of patch-like structural domains and the occurrence of membrane destabilization. The former caused thickness increase whereas the latter caused thickness reduction in the gel-phase membrane bilayer, possibly as a result of lipid loss or an interdigitating process. Patch-like domain formation was independent of acidic phospholipids, whereas membrane destabilization is dependent on the presence and concentration of acidic phospholipids. Sap C effects on membrane restructuring were further studied using synthetic peptides. Synthetic peptides corresponding to the amphipathic alpha-helical domains 1 (designated "H1 peptide") and 2 (H2 peptide) of Sap C were used. Our results indicated that H2 contributed to domain formation but not to membrane destabilization, whereas H1 induced neither type of membrane restructuring. However, H1 was able to mimic Sap C's destabilization effect in conjunction with H2, but only when H1 was present first and H2 was added afterwards. This study provides an approach to investigate the structure-function aspects of Sap C interaction with phospholipid membranes, with insights into the mechanism(s) of Sap C-membrane interaction.

Sphingolipid activator proteins: proteins with complex functions in lipid degradation and skin biogenesis

Sphingolipid activator proteins (SAPs or saposins) are essential cofactors for the lysosomal degradation of membrane-anchored sphingolipids. Four of the five known proteins of this class, SAPs A--D, derive from a single precursor protein and show high homology, whereas the fifth protein, GM2AP, is larger and displays a different secondary structure. Although the main function of all five proteins is assumed to lie in the activation of lipid degradation, their specificities and modes of action seem to differ considerably. It has recently been demonstrated that the action of the proteins is highly enhanced by the presence of acidic lipids in the target membranes. These results have some interesting implications for the topology of lysosomal degradation of lipids and may provide new insights into the function of these interesting proteins, which are ubiquitously expressed in the different tissues of the body. Recent studies indicated that the SAPs play an important role in the biogenesis of the epidermal water barrier, which has been demonstrated by the analysis of the skin phenotype displayed by SAP-knockout mice. The results obtained so far have led to some new insights into the formation of the epidermal water permeability barrier and may lead to a better understanding of this complex process.

Differential membrane interactions of saposins A and C: implications for the functional specificity

Saposins are small, heat-stable glycoprotein activators of lysosomal glycosphingolipid hydrolases that derive from a single precursor, prosaposin, by proteolytic cleavage. Three of these saposins (B, C, and D) share common structural features including a lack of tryptophan, a single glycosylation sequence, the presence of three conserved disulfide bonds, and a common multiamphipathic helical bundle motif. Saposin A contains an additional glycosylation site and a single tryptophan. The oligosaccharides on saposins are not required for in vitro activation functions. Saposins A and C were produced in Escherichia coli to contain single tryptophans at various locations to serve as intrinsic fluorescence reporters, i.e. as topological probes, for interaction with phospholipid membranes. Maximum emission shifts, aqueous and solid quenching, and resonance energy transfer were quantified by fluorescence spectroscopy. Amphipathic helices at the amino- and carboxyl termini of saposins A and C were shown to insert into the lipid bilayer to about five carbon bond lengths. In comparison, the middle region of saposins A or C were either embedded in the bilayer or solvent-exposed, respectively. Conformational changes of saposin C induced by phosphatidylserine interaction suggested the reorientation of functional helical domains. Differential interaction models are proposed for the membrane-bound saposins A and C. By site-directed mutagenesis of saposin A and C, their membrane topological structures were correlated with their activation effects on acid beta-glucosidase. These findings show that proper orientation of the middle segment of saposin C to the outside of the membrane surface is critical for its specific and multivalent interaction with acid beta-glucosidase. Such membrane interactions and orientations of the saposins determine the proximity of their activation and/or binding sites to lysosomal hydrolases or lipoid substrates.

Sphingolipid activator proteins are required for epidermal permeability barrier formation

The epidermal permeability barrier is maintained by extracellular lipid membranes within the interstices of the stratum corneum. Ceramides, the major components of these multilayered membranes, derive in large part from hydrolysis of glucosylceramides mediated by stratum corneum beta-glucocerebrosidase (beta-GlcCerase). Prosaposin (pSAP) is a large precursor protein that is proteolytically cleaved to form four distinct sphingolipid activator proteins, which stimulate enzymatic hydrolysis of sphingolipids, including glucosylceramide. Recently, pSAP has been eliminated in a mouse model using targeted deletion and homologous recombination. In addition to the extracutaneous findings noted previously, our present data indicate that pSAP deficiency in the epidermis has significant consequences including: 1) an accumulation of epidermal glucosylceramides together with below normal levels of ceramides; 2) alterations in lipids that are bound by ester linkages to proteins of the cornified cell envelope; 3) a thickened stratum lucidum with evidence of scaling; and 4) a striking abnormality in lamellar membrane maturation within the interstices of the stratum corneum. Together, these results demonstrate that the production of pSAP, and presumably mature sphingolipid activator protein generation, is required for normal epidermal barrier formation and function. Moreover, detection of significant amounts of covalently bound omega-OH-GlcCer in pSAP-deficient epidermis suggests that deglucosylation to omega-OH-Cer is not a requisite step prior to covalent attachment of lipid to cornified envelope proteins.

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.

Gaucher's disease: molecular, genetic and enzymological aspects

The molecular, genetic and enzymological abnormalities in Gaucher's disease have been delineated during the past decade. Although our understanding of the primary predisposition to the Gaucher's disease phenotypes has improved, the relationships remain poorly understood between the mutant alleles, the resultant enzyme variants, the saposin C (activator protein) locus and phenotypes. Of the more than 100-disease associated alleles, about 8 to 10 have significant frequencies in various ethnic and demographic groups. The N370S(1226G) allele is very frequent in Caucasian populations, but absent in Asian groups. In the Ashkenazi Jewish population, the N370S homozygosity predisposes to Gaucher's disease, but over 50% of such patients escape medical detection because of their mild to absent involvement, i.e. N370S may be a prediposing polymorphic variant. Clarification of genotype/phenotype relationships and the identification of modifier loci that impact on Gaucher's disease phenotypes remain a critical area for research. Greater understanding of these issues will facilitate genetic counselling and appropriate interventive therapy to prevent the morbid long-term manifestations of Gaucher's disease.

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.

Saposin C induces pH-dependent destabilization and fusion of phosphatidylserine-containing vesicles

We have previously shown that saposin C (Sap C), a glucosylceramidase activator protein, interacts with phosphatidylserine (PS) large unilamellar vesicles (LUV), promoting the glucosylceramidase binding to the bilayer [(1993) FEBS Lett. 336, 159-162]. In the present paper the consequences of the Sap C interaction on the lipid organization of the vesicles are reported. It was found that Sap C perturbs the PS bilayer as shown by the release of an encapsulated fluorescent dye. Three different procedures, resonance energy transfer, gel filtration and electron microscopy, indicated that the activator protein is also able to make PS liposomes fuse. The effects of Sap C on PS vesicles were observed at low but not at neutral pH. The lipid composition of the bilayer also affected the Sap C-induced destabilization; in fact, the presence of PS in mixed LUV was essential for significant leakage to occur. These results demonstrate for the first time that Sap C is a protein capable of destabilizing and fusing acidic phospholipid-containing membranes in a pH-dependent fashion.

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.

Intracellular trafficking of glycosphingolipids: role of sphingolipid activator proteins in the topology of endocytosis and lysosomal digestion

Glycosphingolipids (GSL) are components of the outer leaflet of the plasma membrane (PM) of vertebrate tissues. Our current knowledge of GSL metabolism and their intracellular traffic has been derived from metabolic studies but the exact mechanisms by which GSLs are transported from sites of synthesis (endoplasmic reticulum and Golgi) to the sites of residence (PM) and degradation (lysosomes) have not been clearly defined. It is now established that components of the PM reach the lysosomal compartment mainly by endocytic membrane flow. According to a new model, GSLs derived from the PM are thought to end up in intra-endosomal vesicles which could be delivered, by successive processes of membrane fission and fusion, along the endocytic pathway directly into the lumen of the lysosomes. Here the GSLs are degraded in a step-wise manner by exohydrolases. However, the catabolism of membrane-bound GSLs with short hydrophilic head groups needs the assistance of sphingolipid activator proteins (SAPs), which lift the GSLs from the plane of the membrane and present them for degradation to the lysosomal exohydrolases, which are usually water-soluble. The inherited deficiency of one of these enzymes or SAPs causes the lysosomal storage of their respective GSL substrates. In the case of the simultaneous deficiency of all 4 different SAPs the storage of all GSLs with short hydrophilic head groups occurs within multivesicular bodies and/or intra-lysosomal vesicles.

Mutations causing Gaucher disease

Glucocerebrosidase is a lysosomal enzyme responsible for hydrolysis of glucosylceramide to ceramide and glucose. Mutations disrupting the function of this enzyme cause autosomal recessive Gaucher disease. This disease is very heterogeneous. The clinical heterogeneity is due to a large number of mutations within the gene encoding glucocerebrosidase. To date 36 mutations have been described in Gaucher disease. In this part we present the mutations and review the more common ones. We also review the glucocerebrosidase natural activator, designated saposin C and mutations in its gene, associated with Gaucher disease.

Function of saposin C in the reconstitution of glucosylceramidase by phosphatidylserine liposomes

The function of saposin C (Sap C), a glucosylceramidase activator protein, in the enzyme stimulation by phosphatidylserine (PS) liposomes has been investigated. Using gel filtration experiments evidence was obtained for Sap C binding to PS large unilamellar vesicles (LUV) but not to glucosylceramidase. PS LUV, which by themselves are unable to tightly bind and stimulate the enzyme, acquire the capacity to also bind the enzyme after interaction with Sap C, making it express its full activity. Our results indicate that the primary step in the Sap C mode of action resides in its association with PS membranes; in turn, this association promotes the interaction between the membranes and glucosylceramidase.

Modulation of the serine base exchange enzyme activity of rat brain membranes by amphiphilic cations and amphiphilic anions

The biosynthesis of phosphatidylserine in mammalian tissues is catalyzed by the serine base exchange enzyme. The activity of this membrane-bound enzyme can be manipulated by amphiphiles. Amphiphilic cations, such as oleylamine, W-7, chlorpromazine, and didodecyldimethylamine, stimulate the serine base exchange activity. Amphiphilic anions, such as bis(2-ethylhexyl) hydrogen phosphate and cholesterol sulfate, inhibit the serine base exchange activity. These effects are more pronounced at pH 7.0 than at the pH optimum of 8.5 for this enzyme. Both the stimulators and the inhibitors alter the Vmax values without changing the Km value for serine, suggesting that their mechanism of action is related to interactions of the membrane-bound cosubstrate, phosphatidylethanolamine, with the membrane-bound enzyme. The optimal concentration of stimulator varies with the amount of membrane protein present; however, supraoptimal concentrations cause inhibitions. It is proposed that the amphiphilic cations enhance the interaction of the phosphorylethanolamine moiety of the membrane-bound cosubstrate with the enzyme and the amphiphilic anions interfere with such an interaction. Some of the pharmacological properties of these amphiphilic cations, employed clinically as antidepressants, may be mediated by modulation of the serine base exchange enzyme activity.

Additional biochemical findings in a patient and fetal sibling with a genetic defect in the sphingolipid activator protein (SAP) precursor, prosaposin. Evidence for a deficiency in SAP-1 and for a normal lysosomal neuraminidase

It has been shown that sphingolipid activator proteins (SAPs) 1 and 2 are encoded on the same gene along with two other putative activator proteins [Fürst, Machleidt & Sandhoff (1988) Biol. Chem. Hoppe-Seyler 369, 317-328 and O'Brien, Kretz, Dewji, Wenger, Esch & Fluharty (1988) Science 241, 1098-1101]. We have undertaken further biochemical investigations on a patient and fetal sibling, who were previously shown to have a unique sphingolipid storage disorder associated with an SAP-2 deficiency [Harzer, Paton, Poulos, Kustermann-Kuhn, Roggendorf, Grisar & Popp (1989) Eur. J. Pediatr. 149, 31-39]. The severity of their disorder suggested that other products of the SAP precursor or prosaposin gene may also be deficient. The turnover of cerebroside sulphate and globotriaosylceramide were investigated and were both impaired in fibroblasts from the patient and fetus. However, the activities of cerebroside sulphate sulphatase and globotriaosylceramide alpha-galactosidase in vitro were normal in cells from the fetus and patient respectively. In addition, there was an increase in cerebroside sulphate concentration in the kidney of the affected fetus. These results indicate that, in addition to the SAP-2 deficiency, there was a defect in SAP-1 function in this disorder. Additional increases in the concentration of monohexosyl- and dihexosyl-ceramide in the fetal kidney probably reflect the deficiency of SAP-2 in the case of monohexosylceramides, and the combined activator deficiency in the case of dihexosylceramides. Lactosylceramide-loading studies confirmed that there was a defect in the turnover of this lipid in fibroblasts from the affected patient and fetus but not from a patient with an isolated SAP-1 deficiency, or from patients with Krabbe disease, GM1 gangliosidosis or galactosialidosis. It has been suggested [Potier, Lamontagne, Michaud & Tranchemontagne (1990) Biochem. Biophys. Res. Commun. 173, 449-456] that the prosaposin gene also codes for lysosomal neuroaminidase. However, we found normal neuraminidase activity in fibroblasts from our patient, using assay conditions which are diagnostic for sialidosis patients. The role of prosaposin gene products in sphingolipid metabolism is discussed in view of our biochemical findings in this genetic disorder.

Activator proteins and topology of lysosomal sphingolipid catabolism

The lysosomal degradation of several sphingolipids by acid hydrolases is dependent on small non-enzymic cofactors, called sphingolipid activator proteins some of which have been identified as sphingolipid binding proteins. This review summarizes the information available on the structure, function, biosynthesis, gene organization and pathobiochemistry of the known sphingolipid activator proteins. It also offers models for their mode of action and for the topology of lysosomal digestion of glycolipids.

Saposin-C from bovine spleen; complete amino acid sequence and relation between the structure and its biological activity

Saposin-C, a small acidic glycoprotein that can activate glucosylceramide-beta-glucosidase, has been isolated from bovine spleen. The complete amino acid sequence of bovine saposin-C was determined by Edman degradation of the purified protein and its fragmented peptides. It contains 80 amino acids, one carbohydrate chain attached to a single asparagine residue and six cysteine residues in oxidized form. The sequence of bovine saposin-C is 76 and 65% identical with the sequences of saposin-C from human spleen and guinea pig liver, respectively. Hydropathy profiles of the sequence of saposin-C from three species were similar despite the significant residue substitutions. Bovine saposin-C had a stronger effect in stimulating bovine beta-glucosidase compared to human saposin-C. However, the effect of human saposin-C in stimulating human enzyme was stronger than that of bovine saposin-C. The region around residue 35, which is next to the extremely hydrophilic region, seems to be important to produce an interaction with the enzyme.

Secretion of sphingolipid hydrolase activator precursor, prosaposin

Sphingolipid hydrolases are activated by activator proteins or saposins. The precursor protein has been expected from the studies on the cDNA for saposins. Here we demonstrate that prosaposin occurs in various kinds of human secretory fluids such as cerebrospinal fluid, semen, milk, pancreatic juice, and bile. However, mature type saposins were not detected in these fluids. In human milk the amount of prosaposin changed during the lactating period; it became high in concentration within a few days after delivery, decreased during the transitional milk lactating stage, and then increased again toward the mature milk lactating stage. Prosaposin was released from human platelets in response to stimulation by thrombin, but mature saposins were not. From the time course of the release of prosaposin induced by thrombin and from the fact that weak platelet agonists, ADP, epinephrine, and collagen, did not cause the release of prosaposin, prosaposin secretion from platelets seemed to be from lysosome like granules. We postulate that some prosaposin works as a precursor for saposins in the lysosomes and the other serves as an extracellular protein with other specific roles.