Glucosylceramidase mass and subcellular localization are modulated by cholesterol in Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is characterized by the accumulation of cholesterol and sphingolipids in the late endosomal/lysosomal compartment. The mechanism by which the concentration of sphingolipids such as glucosylceramide is increased in this disease is poorly understood. We have found that, in NPC fibroblasts, the cholesterol storage affects the stability of glucosylceramidase (GCase), decreasing its mass and activity; a reduction of cholesterol raises the level of GCase to nearly normal values. GCase is activated and stabilized by saposin C (Sap C) and anionic phospholipids. Here we show by immunofluorescence microscopy that in normal fibroblasts, GCase, Sap C, and lysobisphosphatidic acid (LBPA), the most abundant anionic phospholipid in the endolysosomal system, reside in the same intracellular vesicular structures. In contrast, the colocalization of GCase, Sap C, and LBPA is markedly impaired in NPC fibroblasts but can be re-established by cholesterol depletion. These data show for the first time that the level of cholesterol modulates the interaction of GCase with its protein and lipid activators, namely Sap C and LBPA, regulating the GCase activity and stability.     

Structural and membrane-binding properties of saposin D.

Saposin D is generated together with three similar proteins, saposins A, B and C, from a common precursor, called prosaposin, in acidic organelles such as late endosomes and lysosomes. Although saposin D has been reported to stimulate the enzymatic hydrolysis of sphingomyelin and ceramide, its physiological role has not yet been clearly established. In the present study we examined structural and membrane-binding properties of saposin D. At acidic pH, saposin D showed a great affinity for phospholipid membranes containing an anionic phospholipid such as phosphatidylserine or phosphatidic acid. The binding of saposin D caused destabilization of the lipid surface and, conversely, the association with the membrane markedly affected the fluorescence properties of saposin D. The presence of phosphatidylserine-containing vesicles greatly enhanced the intrinsic tyrosine fluorescence of saposin D, which contains tyrosines but not tryptophan residues. The structural properties of saposin D were investigated in detail using advanced MS analysis. It was found that the main form of saposin D consists of 80 amino acid residues and that the six cysteine residues are linked in the following order: Cys5-Cys78, Cys8-Cys72 and Cys36-Cys47. The disulfide pattern of saposin D is identical with that previously established for two other saposins, B and C, which also exhibit a strong affinity for lipids. The common disulfide structure probably has an important role in the interaction of these proteins with membranes. The analysis of the sugar moiety of saposin D revealed that the single N-glycosylation site present in the molecule is mainly modified by high-mannose-type structures varying from two to six hexose residues. Deglycosylation had no effect on the interaction of saposin D with phospholipid membranes, indicating that the glycosylation site is not related to the lipid-binding site. The association of saposin D with membranes was highly dependent on the composition of the bilayer. Neither ceramide nor sphingomyelin, sphingolipids whose hydrolysis is favoured by saposin D, promoted its binding, while the presence of an acidic phospholipid such as phosphatidylserine or phosphatidic acid greatly favoured the interaction of saposin D with vesicles at low pH. These results suggest that, in the acidic organelles where saposins are localized, anionic phospholipids may be determinants of the saposin D topology and, conversely, saposin D may affect the lipid organization of anionic phospholipid-containing membranes.     

Direct visualization of saposin remodelling of lipid bilayers

Saposins A, B, C and D are soluble, non-enzymatic proteins that interact with lysosomal membranes to activate the breakdown and transfer of glycosphingolipids. The mechanisms of hydrolase activation and lipid transfer by saposins remain unknown. We have used in situ atomic force microscopy (AFM) with simultaneous confocal fluorescence microscopy to investigate the interactions of saposins with lipid membranes. AFM images of the effect of saposins A, B and C on supported lipid bilayers showed a time and concentration-dependent nucleated spread of membrane transformation. Saposin B produced deep gaps that ultimately filled with granular material, while saposins A and C lead to localized areas of membrane that were reduced in height by approximately 1.5 nm. Fluorescence-labeled saposin C co-localized with the transformed areas of the bilayer, indicating stable binding to the membrane. Fluorescence resonance energy transfer confirmed a direct interaction between saposin C and lipid. Under certain conditions of membrane lipid composition and saposin concentration, extensive bilayer lipid removal was observed. We propose a multi-step mechanism that integrates the structural features and amphipathic properties of the saposin proteins.     

Phospholipid vesicle fusion induced by saposin C

Saposin C is a small Trp-free, multifunctional glycoprotein that enhances the hydrolytic activity of acid beta-glucosidase in lysosomes. Saposin C's functions have been shown to include neuritogenic/neuroprotection effects and membrane fusion induction. Here, the mechanism and kinetics of saposin C's fusogenic activity were evaluated by fluorescence spectroscopic methods including dequenching, fluorescence resonance energy transfer, and stopped-flow analyses. Trp or dansyl groups were introduced as fluorescence reporters into selected sites of saposin C to serve as topological probes for protein-protein and protein-membrane interactions. Saposin C induction of liposomal vesicle enlargement was dependent upon anionic phospholipids and acidic pH. The initial fusion burst was completed in the timeframe of a few seconds to minutes and was dependent upon the unsaturated anionic phospholipid content. Two events were associated with saposin C-membrane interaction: membrane insertion of the saposin C terminal helices and reorientation of its central helical region. The latter conformational change likely exposed a binding site for saposins anchored on vesicles. Addition of selected saposin C peptides prior to intact saposin C in reaction mixtures abolished the liposomal fusion. These results indicated that saposin-membrane and saposin-saposin interactions are needed for the fusion process.     

Conformational and amino acid residue requirements for the saposin C neuritogenic effect.

Prosaposin is the precursor of four activator proteins, termed saposins A, B, C, and D, that are required for much of glycosphingolipid hydrolysis. The intact precursor also has neurite outgrowth activity ex vivo and in vivo that is localized to amino acid residues 22-31 of saposin C. Across species, this saposin C region has a high degree of identity and similarity with amino acids in the analogous region of saposin A. Wild-type and mutant saposins C and A from human and mouse were expressed in E. coli. Pure proteins, synthetic peptide analogues, conformation-specific antibodies, and CD spectroscopy were used to evaluate the basis of the ex vivo neuritogenic effect. Wild-type saposin A had no neuritogenic activity whereas reduced and alkylated saposin A did. Introduction of the conserved saposin A Tyr 30 (Y30) into saposin C at the analogous position 31, a conserved Ala(A)/Gly(G)31, diminished neuritogenic activity by 50-60%. Nondenatured saposin A with an introduced A30 acquired substantial neuritogenic activity. Polyclonal antibodies directed against the NH2-terminus of saposin C cross-reacted well with reduced and alkylated saposins C and A, wild-type saposin C, and saposin A [Y30A], poorly with saposin C [A31Y], and not at all with wild-type saposin A. CD spectra of wild-type and mutant saposins C and A, the corresponding neuritogenic region of saposin C, and the analogous region of saposin A showed that more "saposin C-like" molecules had neuritogenic properties. Those with more "saposin A-like" spectra did not. These studies show that the neuritogenic activity of saposin C requires specific placement of amino acids, and that Y30 of saposin A significantly alters local conformation in this critical region and suppresses neuritogenic activity.     

Saposin C is required for normal resistance of acid beta-glucosidase to proteolytic degradation

Saposins (A, B, C, and D) are small sphingolipid activator proteins that are derived by proteolytic processing of a common precursor, prosaposin. In the lysosomal sphingolipid degradation pathway, acid beta-glucosidase (GCase) requires saposin C for optimal in vitro and in vivo hydrolysis of glucocerebroside. The deficiency of prosaposin/saposins (PS-/-) in humans and mice leads to a decrease of GCase activity in selected tissues. Concordant decreases (>50%) of GCase protein and in vitro activity were detected in extracts of cultured fibroblasts and hepatocytes from PS-/- mice and human prosaposin-deficient fibroblasts. GCase RNA in the PS-/- cells was at wild-type levels. Compared with that in wild-type cells (t(1/2) >24 h), the GCase protein in the PS-/- cells had a faster disappearance rate (t(1/2) approximately 1 h in mouse and approximately 8 h in human) as determined by metabolic labeling and immunoprecipitation with anti-GCase antibodies. Treatment of PS-/- cells with leupeptin, an inhibitor of cysteine proteases, led to significant increases (approximately 2-fold) in GCase protein and in vitro activity. Loading saposin C to human PS-/- fibroblasts resulted in an enhancement of GCase protein and in vitro activity. Saposin D loading had no effect. These data indicate that saposin C is required for GCase resistance to proteolytic degradation in the cell. Thus, diminished in vivo GCase activity would be greater than expected only from the lack of GCase activation by saposin C. These results indicate a new property for saposin C, an anti-proteolytic protective function toward GCase.     

Structural requirements for lysosomal targeting of the prosaposin precursor protein

Although the Man-6-P-independent lysosomal sorting of prosaposin, a precursor of four saposins (A, B, C, and D) is not understood, a protein/lipid interaction is considered. Immunocytochemical analysis revealed that each single saposin linked to the C-terminus of prosaposin and to secretory albumin, drives the chimeric protein to lysosomes in COS-7 cells. Quantitative image analysis demonstrated that saposins are targeted with different efficiency (P<0.05) and in a less smooth manner than the precursor. Despite a very close homology, the charge distribution at the surface of 3D comparative models between saposins appeared different. Western blotting monitored prosaposin in cells also as a di- or trimeric form, whereas the chimeric saposins as monomeric. This implies that each amphipathic saposin-like motif may be a part of the overall structural requirements for binding of the precursor to the membrane lipids of transport vesicle. The crystal structure of saposin B demonstrating two dimeric units for lipid binding supports current findings.     

Crystal structures of saposins A and C

Saposins A and C are sphingolipid activator proteins required for the lysosomal breakdown of galactosylceramide and glucosylceramide, respectively. The saposins interact with lipids, leading to an enhanced accessibility of the lipid headgroups to their cognate hydrolases. We have determined the crystal structures of human saposins A and C to 2.0 Angstroms and 2.4 Angstroms, respectively, and both reveal the compact, monomeric saposin fold. We confirmed that these two proteins were monomeric in solution at pH 7.0 by analytical centrifugation. However, at pH 4.8, in the presence of the detergent C(8)E(5), saposin A assembled into dimers, while saposin C formed trimers. Saposin B was dimeric under all conditions tested. The self-association of the saposins is likely to be relevant to how these small proteins interact with lipids, membranes, and hydrolase enzymes.     

Crystal structures of human saposins C andD: implications for lipid recognition and membrane interactions

Human saposins are essential proteins required for degradation of sphingolipids and lipid antigen presentation. Despite the conserved structural organization of saposins, their distinct modes of interaction with biological membranes are not fully understood. We describe two crystal structures of human saposin C in an "open" configuration with unusual domain swapped homodimers. This form of SapC dimer supports the "clip-on" model for SapC-induced vesicle fusion. In addition, we present the crystal structure of SapD in two crystal forms. They reveal the monomer-monomer interface for the SapD dimer, which was confirmed in solution by analytical ultracentrifugation. The crystal structure of SapD suggests that side chains of Lys10 and Arg17 are involved in initial association with the preferred anionic biological membranes by forming salt bridges with sulfate or phosphate lipid headgroups.     

Determination of saposin proteins (sphingolipid activator proteins) in human tissues

Saposins are small glycoproteins which are required for sphingolipid hydrolysis by lysosomal hydrolases. Each saposin (A, B, C, and D) stimulates a different enzymatic activity. A new simple HPLC method to determine the levels of saposins A, C, and D in tissue was developed. Tissues were homogenized in 20 vol of water, boiled, and centrifuged. The supernatant was lyophilized and redissolved in 5 ml of water. A 1.5-ml sample of the solution was applied to a reverse-phase HPLC column (C4 column) and eluted with an acetonitrile gradient. Most contaminants eluted from the column prior to the saposins, which were eluted later as a cluster of peaks. This cluster was collected and then analyzed by another HPLC system equipped with an AX-300 anion-exchange column using a NaCl gradient. Saposins D, A, and C eluted from the AX-300 column separately and in that order. Quantitation of the saposins was made by measuring the sizes of each peak. Standard curves made from pure saposins showed that quantification was linear over a range from 1 to 5 micrograms. Saposin B was measured by its stimulation activity on pure human liver GM1 ganglioside beta-galactosidase. Stimulation was linear up to 80 micrograms of saposin B. Application of this method to analysis of human tissues for their saposin content is presented.     

Saposin D solubilizes anionic phospholipid-containing membranes.

Saposin (Sap) D is a late endosomal/lysosomal small protein, generated together with three other similar proteins, Sap A, B, and C, from the common precursor, prosaposin. Although the functions of saposins such as Sap B and C are well known (Sap B promotes the hydrolysis of sulfatides and Sap C that of glucosylceramide), neither the physiological function nor the mechanism of action of Sap D are yet fully understood. We previously found that a dramatic increase of Sap D superficial hydrophobicity, occurring at the low pH values characteristic of the late endosomal/lysosomal environment, triggers the interaction of the saposin with anionic phospholipid-containing vesicles. We have presently found that, upon lipid binding, Sap D solubilizes the membranes, as shown by the clearance of the vesicles turbidity. The results of gel filtration, density gradient centrifugation, and negative staining electron microscopy demonstrate that this effect is due to the transformation of large vesicles to smaller particles. The solubilizing effect of Sap D is highly dependent on pH, the lipid/saposin ratio, and the presence of anionic phospholipids; small variations in each of these conditions markedly influences the activity of Sap D. The present study documents the interaction of Sap D with membranes as a complex process. Anionic phospholipids attract Sap D from the medium; when the concentration of the saposin on the lipid surface reaches a critical value, the membrane breaks down into recombinant small particles enriched in anionic phospholipids. Our results suggest that the role played by Sap D is more general than promoting sphingolipid degradation, e.g. the saposin might also be a key mediator of the solubilization of intralysosomal/late endosomal anionic phospholipid-containing membranes.     

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.     

Palmitoyl protein thioesterase 1 (PPT1) deficiency causes endocytic defects connected to abnormal saposin processing

Infantile neuronal ceroid lipofuscinosis (INCL) is a severe neurodegenerative disorder of the childhood caused by mutations in the gene encoding palmitoyl protein thioesterase 1 (PPT1). PPT1 localizes to late endosomes/lysosomes of non-neuronal cells and in neurons also to presynaptic areas. PPT1-deficiency causes massive death of cortical neurons and most tissues show an accumulation of saposins A and D. We have here studied endocytic pathways, saposin localization and processing in PPT1-deficient fibroblasts to elucidate the cellular defects resulting in accumulation of specific saposins. We show that PPT1-deficiency causes a defect in fluid-phase and receptor-mediated endocytosis, whereas marker uptake and recycling endocytosis remain intact. Furthermore, we show that saposins A and D are more abundant and relocalized in PPT-deficient fibroblasts and mouse primary neurons. Metabolic labeling and immunoprecipitation analyses revealed hypersecretion and abnormal processing of prosaposin, implying that the accumulation of saposins may result from endocytic defects. We show for the first time a connection between saposin storage and a defect in the endocytic pathway of INCL cells. These data provide new insights into the metabolism of PPT1-deficient cells and offer a basis for further studies on cellular processes causing neuronal death in INCL and other neurodegenerative diseases.     

Saposins (Sphingolipid Activator Proteins) in the Twitcher Mutant Mouse

The twitcher mutant mouse, the animal model of Krabbe disease (human globoid cell leukodystrophy), is characterized by apparent deficiency of galactosylceramide beta-galactosidase activity. Saposin A and C, the heat-stable small sphingolipid activator glycoproteins, stimulate the activity of galactosylceramide beta-galactosidase as well as glucosylceramide beta-glucoside. The role of these saposins in the twitcher mutation was investigated. Boiled supernatant fractions, which contained saposins, were prepared from homogenates of twitcher brain, liver, kidney, and spleen. These preparations showed an almost identical effect on the activity of purified glucosylceramide beta-glucosidase (measured by hydrolysis of 4-methylumbelliferyl-beta-glucoside) with similar preparations from control tissues. The effect on the activity of galactosylceramide beta-galactosidase as well as 4-methylumbelliferyl-beta-glucoside beta-glucosidase in the twitcher brain and liver homogenates by authentic saposin A and C was similar to that in control tissues. These results suggest that the twitcher mutation does not affect the concentrations of saposin A or C or their interaction with galactosylceramide beta-galactosidase.     

Mesotrypsin and Caspase-14 Participate in Prosaposin Processing: POTENTIAL RELEVANCE TO EPIDERMAL PERMEABILITY BARRIER FORMATION

A proteomics-based search for molecules interacting with caspase-14 identified prosaposin and epidermal mesotrypsin as candidates. Prosaposin is a precursor of four sphingolipid activator proteins (saposins A–D) that are essential for lysosomal hydrolysis of sphingolipids. Thus, we hypothesized that caspase-14 and mesotrypsin participate in processing of prosaposin. Because we identified a saposin A sequence as an interactor with these proteases, we prepared a specific antibody to saposin A and focused on saposin A-related physiological reactions. We found that mesotrypsin generated saposins A–D from prosaposin, and mature caspase-14 contributed to this process by activating mesotrypsinogen to mesotrypsin. Knockdown of these proteases markedly down-regulated saposin A synthesis in skin equivalent models. Saposin A was localized in granular cells, whereas prosaposin was present in the upper layer of human epidermis. The proximity ligation assay confirmed interaction between prosaposin, caspase-14, and mesotrypsin in the granular layer. Oil Red staining showed that the lipid envelope was significantly reduced in the cornified layer of skin from saposin A-deficient mice. Ultrastructural studies revealed severely disorganized cornified layer structure in both prosaposin- and saposin A-deficient mice. Overall, our results indicate that epidermal mesotrypsin and caspase-14 work cooperatively in prosaposin processing. We propose that they thereby contribute to permeability barrier formation in vivo.     

Immunolocalization of the saposin-like insert of plant aspartic proteinases exhibiting saposin C activity. Expression in young flower tissues and in barley seeds

The plant-specific insert (PSI) of cypro11 gene-encoding cyprosin, an aspartic proteinase from Cynara cardunculus , has been cloned by polymerase chain reaction (PCR) into a bacterial expression vector. A rearranged form of this PSI in which the N- and C-terminal sequences were permutated to make it more similar to the structural arrangement observed in saposins was also cloned and expressed in the same system. The biological activities of the two purified recombinant proteins were compared to those of human saposins B and C. The proteins showed similar activity to saposin C, i.e. capacity to activate human glucosylceramidase. At a concentration of 5 µ M , wild-type PSI, saposin C, and rearranged PSI activated human glucosylceramidase two-, three-, and five-fold, respectively. The K_m for 4-methylumbelliferyl β-glucopyranoside was around 7 µ M in the presence of any of the three activators (5 µ M ). The neurotropic activity using NS20Y cells and lipid-binding properties of the plant recombinant proteins were tested. The two plant proteins showed lipid-binding properties similar to those of saposins but did not have any effect on neurite outgrowth. Immunolocalization of PSI showed its expression in protective tissues in flower meristem – protodermis, in C. cardunculus and embryonic root cap and coleorhiza in mature barley grains – as well as husk, pericarp, and the aleurone layer. Possible biological functions suggested for the plant homologue to saposins besides the general activation of enzymes involved in lipid metabolism would be involvement in plant defence.     

Saposins: structure, function, distribution, and molecular genetics.

Saposins A, B, C, and D are small heat-stable glycoproteins derived from a common precursor protein, prosaposin. These mature saposins, as well as prosaposin, activate several lysosomal hydrolases involved in the metabolism of various sphingolipids. All four saposins are structurally similar to one another including placement of six cysteines, a glycosylation site, and conserved prolines in identical positions. In spite of the structural similarities, the specificity and mode of activation of sphingolipid hydrolases differs among individual saposins. Saposins appear to be lysosomal proteins, exerting their action upon lysosomal hydrolases. Prosaposin is a 70 kDa glycoprotein containing four domains, one for each saposin, placed in tandem. Prosaposin is proteolytically processed to saposins A, B, C and D, apparently within lysosomes. However, prosaposin also exists as an integral membrane protein not destined for lysosomal entry and exists uncleaved in many biological fluids such as seminal plasma, human milk, and cerebrospinal fluid, where it appears to have a different function. The physiological significance of saposins is underlined by their accumulation in tissues of lysosomal storage disease patients and the occurrence of sphingolipidosis due to mutations in the prosaposin gene. This review presents an overview of the occurrence, structure and function of these saposin proteins.     

Induction of autophagy promotes fusion of multivesicular bodies with autophagic vacuoles in k562 cells

Morphological and biochemical studies have shown that autophagosomes fuse with endosomes forming the so-called amphisomes, a prelysosomal hybrid organelle. In the present report, we have analyzed this process in K562 cells, an erythroleukemic cell line that generates multivesicular bodies (MVBs) and releases the internal vesicles known as exosomes into the extracellular medium. We have previously shown that in K562 cells, Rab11 decorates MVBs. Therefore, to study at the molecular level the interaction of MVBs with the autophagic pathway, we have examined by confocal microscopy the fate of MVBs in cells overexpressing green fluorescent protein (GFP)-Rab11 and the autophagosomal protein red fluorescent protein-light chain 3 (LC3). Autophagy inducers such as starvation or rapamycin caused an enlargement of the vacuoles decorated with GFP-Rab11 and a remarkable colocalization with LC3. This convergence was abrogated by a Rab11 dominant negative mutant, indicating that a functional Rab11 is involved in the interaction between MVBs and the autophagic pathway. Interestingly, we presented evidence that autophagy induction caused calcium accumulation in autophagic compartments. Furthermore, the convergence between the endosomal and the autophagic pathways was attenuated by the Ca2+ chelator acetoxymethyl ester (AM) of the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), indicating that fusion of MVBs with the autophagosome compartment is a calcium-dependent event. In addition, autophagy induction or overexpression of LC3 inhibited exosome release, suggesting that under conditions that stimulates autophagy, MVBs are directed to the autophagic pathway with consequent inhibition in exosome release.     

The primary structure of mouse saposin.

The primary structure of mouse sphingolipid activator protein (saposin) was determined by cDNA sequencing. The amino acid sequence predicted by the cDNA sequence revealed that mouse saposin was highly homologous to human saposin and also to rat sertoli cell glycoprotein. Mouse saposin also has four functional domains, which are structurally similar to each other, and each domain has cysteines, prolines, and a potential glycosylation site at an almost identical position. An amino acid comparison between human and mouse saposins revealed that the similarity was approximately 70%, and human saposin lacks thirty-one amino acids between domains C and D. Heterogeneities of mRNA were found in both the coding and noncoding regions.