Saposin D: a sphingomyelinase activator

Prosaposin maintains lipid homeostasis in dopamine neurons and counteracts experimental parkinsonism in rodents

Prosaposin (PSAP) modulates glycosphingolipid metabolism and variants have been linked to Parkinson's disease (PD). Here, we find altered PSAP levels in the plasma, CSF and post-mortem brain of PD patients. Altered plasma and CSF PSAP levels correlate with PD-related motor impairments. Dopaminergic PSAP-deficient (cPSAPDAT) mice display hypolocomotion and depression/anxiety-like symptoms with mildly impaired dopaminergic neurotransmission, while serotonergic PSAP-deficient (cPSAPSERT) mice behave normally. Spatial lipidomics revealed an accumulation of highly unsaturated and shortened lipids and reduction of sphingolipids throughout the brains of cPSAPDAT mice. The overexpression of α-synuclein via AAV lead to more severe dopaminergic degeneration and higher p-Ser129 α-synuclein levels in cPSAPDAT mice compared to WT mice. Overexpression of PSAP via AAV and encapsulated cell biodelivery protected against 6-OHDA and α-synuclein toxicity in wild-type rodents. Thus, these findings suggest PSAP may maintain dopaminergic lipid homeostasis, which is dysregulated in PD, and counteract experimental parkinsonism.

Acid Sphingomyelinase Deficiency: A Clinical and Immunological Perspective

Acid sphingomyelinase deficiency (ASMD) is a lysosomal storage disease caused by deficient activity of acid sphingomyelinase (ASM) enzyme, leading to the accumulation of varying degrees of sphingomyelin. Lipid storage leads to foam cell infiltration in tissues, and clinical features including hepatosplenomegaly, pulmonary insufficiency and in some cases central nervous system involvement. ASM enzyme replacement therapy is currently in clinical trial being the first treatment addressing the underlying pathology of the disease. Therefore, presently, it is critical to better comprehend ASMD to improve its diagnose and monitoring. Lung disease, including recurrent pulmonary infections, are common in ASMD patients. Along with lung disease, several immune system alterations have been described both in patients and in ASMD animal models, thus highlighting the role of ASM enzyme in the immune system. In this review, we summarized the pivotal roles of ASM in several immune system cells namely on macrophages, Natural Killer (NK) cells, NKT cells, B cells and T cells. In addition, an overview of diagnose, monitoring and treatment of ASMD is provided highlighting the new enzyme replacement therapy available.

Acid Sphingomyelinase, a Lysosomal and Secretory Phospholipase C, Is Key for Cellular Phospholipid Catabolism

Here, we present the main features of human acid sphingomyelinase (ASM), its biosynthesis, processing and intracellular trafficking, its structure, its broad substrate specificity, and the proposed mode of action at the surface of the phospholipid substrate carrying intraendolysosomal luminal vesicles. In addition, we discuss the complex regulation of its phospholipid cleaving activity by membrane lipids and lipid-binding proteins. The majority of the literature implies that ASM hydrolyses solely sphingomyelin to generate ceramide and ignores its ability to degrade further substrates. Indeed, more than twenty different phospholipids are cleaved by ASM in vitro, including some minor but functionally important phospholipids such as the growth factor ceramide-1-phosphate and the unique lysosomal lysolipid bis(monoacylglycero)phosphate. The inherited ASM deficiency, Niemann-Pick disease type A and B, impairs mainly, but not only, cellular sphingomyelin catabolism, causing a progressive sphingomyelin accumulation, which furthermore triggers a secondary accumulation of lipids (cholesterol, glucosylceramide, GM2) by inhibiting their turnover in late endosomes and lysosomes. However, ASM appears to be involved in a variety of major cellular functions with a regulatory significance for an increasing number of metabolic disorders. The biochemical characteristics of ASM, their potential effect on cellular lipid turnover, as well as a potential impact on physiological processes will be discussed.

Sphingolipid lysosomal storage diseases: from bench to bedside

Johann Ludwig Wilhelm Thudicum described sphingolipids (SLs) in the late nineteenth century, but it was only in the past fifty years that SL research surged in importance and applicability. Currently, sphingolipids and their metabolism are hotly debated topics in various biochemical fields. Similar to other macromolecular reactions, SL metabolism has important implications in health and disease in most cells. A plethora of SL-related genetic ailments has been described. Defects in SL catabolism can cause the accumulation of SLs, leading to many types of lysosomal storage diseases (LSDs) collectively called sphingolipidoses. These diseases mainly impact the neuronal and immune systems, but other systems can be affected as well. This review aims to present a comprehensive, up-to-date picture of the rapidly growing field of sphingolipid LSDs, their etiology, pathology, and potential therapeutic strategies. We first describe LSDs biochemically and briefly discuss their catabolism, followed by general aspects of the major diseases such as Gaucher, Krabbe, Fabry, and Farber among others. We conclude with an overview of the available and potential future therapies for many of the diseases. We strive to present the most important and recent findings from basic research and clinical applications, and to provide a valuable source for understanding these disorders.

Prosaposin in the rat oviductal epithelial cells

Prosaposin (PSAP) has two forms: a precursor and a secreted form. The secreted form has neurotrophic, myelinotrophic, and myotrophic properties. The precursor form is a precursor protein of saposins A-D. Although the distribution of PSAP in male reproductive organs is well known, its distribution in female reproductive organs, especially in the oviduct, is unclear. Immunoblots and immunohistochemistry of oviducts showed that oviductal tissues contain PSAP proteins, and a significant increase in PSAP was observed in the estrus-metestrus phase compared to the diestrus-proestrus phase in the ampulla. To identify PSAP trafficking in cells, double-immunostaining was performed with antibodies against PSAP in combination with sortilin, mannose 6 phosphate receptor (M6PR), or low-density lipoprotein receptor-related protein 1 (LRP1). PSAP and sortilin double-positive reactions were observed near the nuclei, as well as in the apical portion of microvillous epithelial cells, whereas these reactions were only observed near the nuclei of ciliated epithelial cells. PSAP and M6PR double-positive reactions were observed near the nuclei of microvillous and ciliated epithelial cells. PSAP and M6PR double-positive reactions were also observed in the apical portion of microvillous epithelial cells. PSAP and LRP1 double-positive reactions were observed in the plasma membrane and apical portion of both microvillous and ciliated epithelial cells. Immunoelectron staining revealed PSAP immunoreactive small vesicles with exocytotic features at the apical portion of microvillous epithelial cells. These findings suggest that PSAP is present in the oviductal epithelium and has a pivotal role during pregnancy in providing an optimal environment for gametes and/or sperm in the ampulla.

Fenretinide binding to the lysosomal protein saposin D alters ceramide solubilization and hydrolysis

Fenretinide is a synthetic retinoid pharmaceutical linked to ceramide build-up in vivo. Saposin D is an intralysosomal protein necessary for ceramide binding/degradation. We show, via electronic absorption spectroscopy, fluorescence spectroscopy, and ceramide hydrolysis assays, that fenretinide is bound by saposin D {K a = (1.45 ± 0.49) × 105 M-1}, and affects ceramide solubilization/degradation.

Exploring Extracellular Vesicles Biogenesis in Hypothalamic Cells through a Heavy Isotope Pulse/Trace Proteomic Approach

Studies have shown that the process of extracellular vesicles (EVs) secretion and lysosome status are linked. When the lysosome is under stress, the cells would secrete more EVs to maintain cellular homeostasis. However, the process that governs lysosomal activity and EVs secretion remains poorly defined and we postulated that certain proteins essential for EVs biogenesis are constantly synthesized and preferentially sorted to the EVs rather than the lysosome. A pulsed stable isotope labelling of amino acids in cell culture (pSILAC) based quantitative proteomics methodology was employed to study the preferential localization of the newly synthesized proteins into the EVs over lysosome in mHypoA 2/28 hypothalamic cell line. Through proteomic analysis, we found numerous newly synthesized lysosomal enzymes—such as the cathepsin proteins—that preferentially localize into the EVs over the lysosome. Chemical inhibition against cathepsin D promoted EVs secretion and a change in the EVs protein composition and therefore indicates its involvement in EVs biogenesis. In conclusion, we applied a heavy isotope pulse/trace proteomic approach to study EVs biogenesis in hypothalamic cells. The results demonstrated the regulation of EVs secretion by the cathepsin proteins that may serve as a potential therapeutic target for a range of neurological disorder associated with energy homeostasis.

Progranulin mutations result in impaired processing of prosaposin and reduced glucocerebrosidase activity

Frontotemporal dementia (FTD) is a common neurogenerative disorder characterized by progressive degeneration in the frontal and temporal lobes. Heterozygous mutations in the gene encoding progranulin (PGRN) are a common genetic cause of FTD. Recently, PGRN has emerged as an important regulator of lysosomal function. Here, we examine the impact of PGRN mutations on the processing of full-length prosaposin to individual saposins, which are critical regulators of lysosomal sphingolipid metabolism. Using FTD-PGRN patient-derived cortical neurons differentiated from induced pluripotent stem cells, as well as post-mortem tissue from patients with FTLD-PGRN, we show that PGRN haploinsufficiency results in impaired processing of prosaposin to saposin C, a critical activator of the lysosomal enzyme glucocerebrosidase (GCase). Additionally, we found that PGRN mutant neurons had reduced lysosomal GCase activity, lipid accumulation and increased insoluble α-synuclein relative to isogenic controls. Importantly, reduced GCase activity in PGRN mutant neurons is rescued by treatment with saposin C. Together, these findings suggest that reduced GCase activity due to impaired processing of prosaposin may contribute to pathogenesis of FTD resulting from PGRN mutations.

Progranulin-mediated deficiency of cathepsin D results in FTD and NCL-like phenotypes in neurons derived from FTD patients

Frontotemporal dementia (FTD) encompasses a group of neurodegenerative disorders characterized by cognitive and behavioral impairments. Heterozygous mutations in progranulin (PGRN) cause familial FTD and result in decreased PGRN expression, while homozygous mutations result in complete loss of PGRN expression and lead to the neurodegenerative lysosomal storage disorder neuronal ceroid lipofuscinosis (NCL). However, how dose-dependent PGRN mutations contribute to these two different diseases is not well understood. Using iPSC-derived human cortical neurons from FTD patients harboring PGRN mutations, we demonstrate that PGRN mutant neurons exhibit decreased nuclear TDP-43 and increased insoluble TDP-43, as well as enlarged electron-dense vesicles, lipofuscin accumulation, fingerprint-like profiles and granular osmiophilic deposits, suggesting that both FTD and NCL-like pathology are present in PGRN patient neurons as compared to isogenic controls. PGRN mutant neurons also show impaired lysosomal proteolysis and decreased activity of the lysosomal enzyme cathepsin D. Furthermore, we find that PGRN interacts with cathepsin D, and that PGRN increases the activity of cathepsin D but not cathepsins B or L. Finally, we show that granulin E, a cleavage product of PGRN, is sufficient to increase cathepsin D activity. This functional relationship between PGRN and cathepsin D provides a possible explanation for overlapping NCL-like pathology observed in patients with mutations in PGRN or CTSD, the gene encoding cathepsin D. Together, our work identifies PGRN as an activator of lysosomal cathepsin D activity, and suggests that decreased cathepsin D activity due to loss of PGRN contributes to both FTD and NCL pathology in a dose-dependent manner.

The mechanism of glycosphingolipid degradation revealed by a GALC-SapA complex structure

Sphingolipids are essential components of cellular membranes and defects in their synthesis or degradation cause severe human diseases. The efficient degradation of sphingolipids in the lysosome requires lipid-binding saposin proteins and hydrolytic enzymes. The glycosphingolipid galactocerebroside is the primary lipid component of the myelin sheath and is degraded by the hydrolase β-galactocerebrosidase (GALC). This enzyme requires the saposin SapA for lipid processing and defects in either of these proteins causes a severe neurodegenerative disorder, Krabbe disease. Here we present the structure of a glycosphingolipid-processing complex, revealing how SapA and GALC form a heterotetramer with an open channel connecting the enzyme active site to the SapA hydrophobic cavity. This structure defines how a soluble hydrolase can cleave the polar glycosyl headgroups of these essential lipids from their hydrophobic ceramide tails. Furthermore, the molecular details of this interaction provide an illustration for how specificity of saposin binding to hydrolases is encoded.

The protective role of prosaposin and its receptors in the nervous system

Prosaposin (also known as SGP-1) is an intriguing multifunctional protein that plays roles both intracellularly, as a regulator of lysosomal enzyme function, and extracellularly, as a secreted factor with neuroprotective and glioprotective effects. Following secretion, prosaposin can undergo endocytosis via an interaction with the low-density lipoprotein-related receptor 1 (LRP1). The ability of secreted prosaposin to promote protective effects in the nervous system is known to involve activation of G proteins, and the orphan G protein-coupled receptors GPR37 and GPR37L1 have recently been shown to mediate signaling induced by both prosaposin and a fragment of prosaposin known as prosaptide. In this review, we describe recent advances in our understanding of prosaposin, its receptors and their importance in the nervous system.

The role of saposin C in Gaucher disease

Saposin C is one of four homologous proteins derived from sequential cleavage of the saposin precursor protein, prosaposin. It is an essential activator for glucocerebrosidase, the enzyme deficient in Gaucher disease. Gaucher disease is a rare autosomal recessive lysosomal storage disorder caused by mutations in the GBA gene that exhibits vast phenotypic heterogeneity, despite its designation as a "simple" Mendelian disorder. The observed phenotypic variability has led to a search for disease modifiers that can alter the Gaucher phenotype. The PSAP gene encoding saposin C is a prime candidate modifier for Gaucher disease. In humans, saposin C deficiency due to mutations in PSAP results in a Gaucher-like phenotype, despite normal in vitro glucocerebrosidase activity. Saposin C deficiency has also been shown to modify phenotype in one mouse model of Gaucher disease. The role of saposin C as an activator required for normal glucocerebrosidase function, and the consequences of saposin C deficiency are described, and are being explored as potential modifying factors in patients with Gaucher disease.

Prosaposin sorting is mediated by oligomerization

The compartmental nature of eukaryotic cells requires sophisticated mechanisms of protein sorting. Prosaposin, the precursor of four sphingolipid activator proteins, is transported from the trans-Golgi network (TGN) to lysosomes as a partially glycosylated (65 kDa) protein with high-mannose/hybrid oligosaccharides. Prosaposin is also found in the extracellular space where it is secreted as a fully glycosylated (70 kDa) protein composed of complex glycans. Although the trafficking of prosaposin to lysosomes is known to be mediated by sortilin, the mechanism of secretion of this protein is still unknown. In this study, we report that prosaposin may covalently aggregate into oligomers. Our results demonstrate that while prosaposin oligomers are secreted into the extracellular space, monomeric prosaposin remains inside the cell bound to sortilin. We also found that deletion of the C-terminus of prosaposin, previously shown to block its lysosomal transport, did not abolish its oligomerization and secretion. On the other hand, elimination of the N-terminus and of each saposin domain inhibited its oligomerization and resulted in its retention as a fully glycosylated protein. In conclusion, we are reporting for the first time that oligomerization of prosaposin is crucial for its entry into the secretory pathway.

A stretch of 17 amino acids in the prosaposin C terminus is critical for its binding to sortilin and targeting to lysosomes

Prosaposin, the precursor of four lysosomal cofactors required for the hydrolysis of sphingolipids, is transported to the lysosomes via the alternative receptor, sortilin. In this study, we identified a specific domain of 17 amino acids within the C terminus of prosaposin involved in binding to this sorting receptor. We generated six prosaposin deletion constructs and examined the effect of truncation by coimmunoprecipitation and confocal microscopy. The experiments revealed that the first half of the prosaposin C terminus (aa 524-540), containing a saposin-like motif, was required and necessary to bind sortilin and to transport it to the lysosomes. Based on this result, we introduced twelve site-directed point mutations within the first half of the C terminus. Although the interaction of prosaposin with sortilin was pH dependent, the mutation of hydrophilic amino acids that usually modulate pH-dependent protein interactions did not affect the binding of prosaposin to sortilin. Conversely, a tryptophan (W530) and two cysteines (C528 and C536) were essential for its interaction with sortilin and for its transport to the lysosomes. In conclusion, our investigation demonstrates that a saposin-like motif within the first half of the prosaposin C terminus contains the sortilin recognition site.

Prosaposin inhibits tumor metastasis via paracrine and endocrine stimulation of stromal p53 and Tsp-1

Metastatic tumors can prepare a distant site for colonization via the secretion of factors that act in a systemic manner. We hypothesized that non- or weakly metastatic human tumor cells may act in an opposite fashion by creating a microenvironment in distant tissues that is refractory to colonization. By comparing cell lines with different metastatic potential, we have identified a tumor-secreted inhibitor of metastasis, prosaposin (Psap), which functions in a paracrine and endocrine fashion by stimulating the expression of thrombospondin-1 (Tsp-1) in fibroblasts present in both primary tumors and distant organs, doing so in a p53-dependent manner. Introduction of Psap in highly metastatic cells significantly reduced the occurrence of metastases, whereas inhibition of Psap production by tumor cells was associated with increased metastatic frequency. In human prostate cancer, decreased Psap expression was significantly associated with metastatic tumors. Our findings suggest that prosaposin, or other agents that stimulate p53 activity in the tumor stroma, may be an effective therapy by inhibition of the metastatic process.

Sortilin and prosaposin localize to detergent-resistant membrane microdomains

Most soluble lysosomal hydrolases are sorted in the trans-Golgi network (TGN) and delivered to the lysosomes by the mannose 6-phosphate receptor (M6PR). However, the non-enzymic sphingolipid activator protein (SAP), prosaposin, as well as certain soluble lysosomal hydrolases, is sorted and trafficked to the lysosomes by sortilin. Based on previous results demonstrating that prosaposin requires sphingomyelin to be targeted to the lysosomes, we hypothesized that sortilin and its ligands are found in detergent-resistant membranes (DRMs). To test this hypothesis we have analyzed DRM fractions and demonstrated the presence of sortilin and its ligand, prosaposin. Our results showed that both the M6PR and its cargo, cathepsin B, were also present in DRMs. Cathepsin H has previously been demonstrated to interact with sortilin, while cathepsin D interacts with both sortilin and the M6PR. Both of these soluble lysosomal proteins were also found in DRM fractions. Using sortilin shRNA we have showed that prosaposin is localized to DRM fractions only in the presence of sortilin. These observations suggest that in addition to interacting with the same adaptor proteins, such as GGAs, AP-1 and retromer, both sortilin and the M6PR localize to similar membrane platforms, and that prosaposin must interact with sortilin to be recruited to DRMs.

Cholesterol in negatively charged lipid bilayers modulates the effect of the antimicrobial protein granulysin

The release of granulysin, a 9-kDa cationic protein, from lysosomal granules of cytotoxic T lymphocytes and natural killer cells plays an important role in host defense against microbial pathogens. Granulysin is endocytosed by the infected target cell via lipid rafts and kills subsequently intracellular bacteria. The mechanism by which granulysin binds to eukaryotic and prokaryotic cells but lyses only the latter is not well understood. We have studied the effect of granulysin on large unilamellar vesicles (LUVs) and supported bilayers with prokaryotic and eukaryotic lipid mixtures or model membranes with various lipid compositions and charges. Binding of granulysin to bilayers with negative charges, as typically found in bacteria and lipid rafts of eukaryotic cells, was shown by immunoblotting. Fluorescence release assays using LUV revealed an increase in permeability of prokaryotic, negatively charged and lipid raft-like bilayers devoid of cholesterol. Changes in permeability of these bilayers could be correlated to defects of various sizes penetrating supported bilayers as shown by atomic force microscopy. Based on these results, we conclude that granulysin causes defects in negatively charged cholesterol-free membranes, a membrane composition typically found in bacteria. In contrast, granulysin is able to bind to lipid rafts in eukaryotic cell membranes, where it is taken up by the endocytotic pathway, leaving the cell intact.

Epidermal sphingolipids: Metabolism, function, and roles in skin disorders

Mammalian epidermis produces and delivers large quantities of glucosylceramide and sphingomyelin precursors to stratum corneum extracellular domains, where they are hydrolyzed to corresponding ceramide species. This cycle of lipid precursor formation and subsequent hydrolysis represents a mechanism that protects the epidermis against potentially harmful effects of ceramide accumulation within nucleated cell layers. Prominent skin disorders, such as psoriasis and atopic dermatitis, have diminished epidermal ceramide levels, reflecting altered sphingolipid metabolism, that may contribute to disease severity/progression. Enzymatic processes in the hydrolysis of glucosylceramide and sphingomyelin, and the roles of sphingolipids in skin diseases, are the focus of this review.

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.

Screening and identification of genes trans-regulated by a novel HBeAg binding protein E-18 with microarray assay

AIM: To investigate the biological functions of a novel hepatitis B virus e antigen (HBeAg) binding protein E-18, and to use cDNA microarray technique to screen genes regulated by E-18.

METHODS: A novel gene E-18 coding for HBeAg was screened and identified by using yeast two-hybrid system 3 and co-immunoprecipitation technique. The E-18 coding DNA fragment was amplified by reverse transcription polymerase chain reaction (RT-PCR) technique from HepG2 cell. The expressive vector of pcDNA3.1-E-18 was constructed by routine molecular biological methods. The HepG2 cells were transfected with pcDNA3.1(-) and pcDNA3.1-E-18, respectively by using lipofectamine. The total RNA was isolated and reverse transcribed. The cDNA of each sample were subjected to microarray screening with 1 152 cDNA probes and analyzed by bioinformatics.

RESULTS: E-18 cDNA sequence was obtained and identified by yeast two-hybrid screening and bioinformatics analysis. The expressive vector was constructed and confirmed by DNA sequencing analysis and restriction enzyme digestion. High quality mRNA and cDNA of transfected HepG2 cells had been prepared and successful microarray screening conducted. From the scanning results, there were 52 differential expression genes, of which 36 genes were down-regulated, and 16 genes were up-regulated.

CONCLUSION: Microarray technique is successfully used to screen the genes trans-regulated by E-18. The expression of E-18 protein affects the expression spectrum of HepG2 cell.

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.

The lysosomal transport of prosaposin requires the conditional interaction of its highly conserved d domain with sphingomyelin

Lysosomal prosaposin (65 kDa) is a nonenzymic protein that is transported to the lysosomes in a mannose 6-phosphate-independent manner. Selective deletion of the functional domains of prosaposin indicates that the D domain and the carboxyl-terminal region are necessary for its transport to the lysosomes. Inhibitors of sphingolipid biosynthesis, such as fumonisin B(1) (FB(1)) and tricyclodecan-9-yl xanthate potassium salt (D609), also interfere with the trafficking of prosaposin to lysosomes. In this study, we examine sphingomyelin as a direct candidate for the trafficking of prosaposin. Chinese hamster ovary and COS-7 cells overexpressing prosaposin or an albumin/prosaposin construct were incubated with these inhibitors, treated with sphingolipids, and then immunostained. Sphingomyelin restored the immunostaining in lysosomes in both FB(1)- and D609-treated cells and ceramide reestablished the immunostaining in FB(1)-treated cells only. D-Threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), which inhibits glycosphingolipids, had no effect on the immunostaining pattern. To determine whether sphingomyelin has the same effect on the transport of endogenous prosaposin, testicular explants were treated with FB(1) and D609. Sphingomyelin restored prosaposin immunogold labeling in the lysosomes of FB(1)- and D609-treated Sertoli cells, whereas ceramide restored the label in FB(1) treatment only. Albumin linked to the D and COOH-terminal domains of prosaposin was used as a dominant negative competitor. The construct blocked the targeting of prosaposin and induced accumulation of membrane in the lysosomes, demonstrating that the construct uses the same transport pathway as endogenous prosaposin. In conclusion, our results showed that sphingomyelin, the D domain, and its adjacent COOH-terminal region play a crucial role in the transport of prosaposin to lysosomes. Although the precise nature of this lipid-protein interaction is not well established, it is proposed that sphingomyelin microdomains (lipid rafts) are part of a mechanism ensuring correct intercellular trafficking of prosaposin.

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.

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.

The level of prosaposin is decreased in the skin of patients with psoriasis vulgaris

Ceramides are the most abundant lipids constituting the intercellular matrix of the skin stratum corneum and their critical role in skin homeostasis has been extensively documented. Their concentration in the skin highly depends on the rate of availability of the enzymes involved in ceramide generation. The aim of this study was to investigate whether the concentration of prosaposin was altered in the skin of patients with psoriasis vulgaris. Prosaposin, the precursor of saposins (sphingolipid activator proteins), was measured in lesional and nonlesional skin of psoriatic patients and in normal skin from surgical patients, both at the mRNA and at the protein level. Densitometric analysis of reverse transcriptase-polymerase chain reaction bands separated by gel-electrophoresis showed a progressive decrease of prosaposin mRNA expression in nonlesional and lesional psoriatic skin, being substantially decreased in lesional psoriatic skin compared with normal control skin. Immunohistochemical analysis showed a significant decrease of prosaposin level in the stratum corneum of psoriatic lesional skin (both in active-type and in chronic-type plaque) compared with nonlesional and with normal skin (p < 0.01), and in psoriatic nonlesional skin compared with normal control (p < 0.05). Immunolocalization of sphingomyelinase in lesional and nonlesional psoriatic skin showed a decrease in the level of this enzyme in the stratum corneum of psoriatic lesional, compared with nonlesional skin. These results support the concept that disturbance of epidermal barrier function caused by derangement in ceramide generation can be crucial for the development of psoriatic skin diseases.

The amoebapore superfamily

Amoebapores, synthesized by human protozoan parasites, form ion channels in target cells and artificial lipid membranes. The major pathogenic effect of these proteins is due to their cytolytic capability which results in target cell death. They comprise a coherent family and are homologous to other proteins and protein domains found in eight families. These families include in addition to the amoebapores (1) the saposins, (2) the NK-lysins and granulysins, (3) the pulmonary surfactant proteins B, (4) the acid sphingomyelinases, (5) acyloxyacyl hydrolases and (6) the aspartic proteases. These amoebapore homologues have many properties in common including membrane binding and stability. We note for the first time that a new protein, countin, from the cellular slime mold, Dictyostelium discoideum, comprises the eighth family within this superfamily. All currently sequenced members of these eight families are identified, and the structural, functional and phylogenetic properties of these proteins are discussed.

Identification of a novel sequence involved in lysosomal sorting of the sphingolipid activator protein prosaposin

Prosaposin is synthesized as a 53-kDa protein, post-translationally modified to a 65-kDa form and further glycosylated to a 70-kDa secretory product. The 65-kDa protein is associated to Golgi membranes and is targeted to lysosomes, where four smaller nonenzymatic saposins implicated in the hydrolysis of sphingolipids are generated by its partial proteolysis. The targeting of the 65-kDa protein to lysosomes is not mediated by the mannose 6-phosphate receptor. The Golgi apparatus appears to accomplish the molecular sorting of the 65-kDa prosaposin by decoding a signal from its amino acid backbone. This investigation deals with the characterization of the sequence involved in this process by deleting the saposin functional domains A, B, C, and D and the highly conserved N and C termini of prosaposin. The truncated cDNAs were subcloned into expression vectors and transfected to COS-7 cells. The destination of the mutated proteins was assessed by immunocytochemistry. Deletion of the C terminus did not interfere with the secretion of prosaposin but abolished its transport to lysosomes. Deletion of saposins and the N-terminal domain did not affect the lysosomal or secretory routing of prosaposin. A chimeric construct of albumin and the C terminus of prosaposin was not directed to lysosomes. However, albumin connected to the C terminus and one or more functional domains of prosaposin reached lysosomes, indicating that the C terminus and at least one saposin domain are required for this process. In summary, we are reporting a novel sequence involved in the targeting of prosaposin to lysosomes.

Role of prosaposin in the male reproductive system: effect of prosaposin inactivation on the testis, epididymis, prostate, and seminal vesicles

SGP-1/prosaposin can be secreted or targeted to the lysosomes where it is processed into smaller saposins (A, B, C, and D) required for the hydrolysis of glycosphingolipids. The deficiency of saposins B and C results in variant forms of metachromatic leukodystrophy and Gaucher's disease, respectively, which are characterized by lysosomal storage of undegraded glycosphingolipids. In the nervous system, prosaposin presents trophic activity. A mouse model was recently developed by creating a null allele in embryonic stem cells through gene targeting to investigate the phenotypic diversity of prosaposin mutations and the involvement of this protein in lysosomal storage diseases, and for the development of therapeutic approaches. Mice homozygous mutants die at the age of 35-40 days and neurological disorders contribute to the early demise of the mutant mice. The male reproductive organs in homozygous mutants show several abnormalities, such as a decrease in testis size with reduced spermiogenesis and an involution of the prostate, seminal vesicles, and epididymis. In these animals, the blood levels of testosterone remain normal. In the prostate of homozygous mutants, only the basal epithelial cells appear to be present, while the secretory cells are absent. These findings suggest that prosaposin may be involved in the development and maintenance of the male reproductive organs, as well as, in cellular differentiation.

Sphingomyelin-degrading pathways in human cells role in cell signalling

The ubiquitous sphingophospholipid sphingomyelin (SM) can be hydrolysed in human cells to ceramide by different sphingomyelinases (SMases). These enzymes exert a dual role, enabling not only the turnover of membrane SM and the degradation of exogenous (lipoprotein) SM, but also the signal-induced generation of the lipid second messenger ceramide. This review focuses on the function(s) of the different SMases in living cells. While both lysosomal and non-lysosomal pathways that ensure SM hydrolysis in intact cells can be distinguished, the precise contribution of each of these SM-cleaving enzymes to the production of ceramide as a signalling molecule remains to be clarified.

Stimulation of lysosomal sphingomyelin degradation by sphingolipid activator proteins

Lysosomal breakdown of glycosphingolipids with short hydrophilic carbohydrate headgroups is achieved by the simultaneous action of specific hydrolases and sphingolipid activator proteins (SAPs). Activator proteins are considered to facilitate the enzyme/substrate interaction between water-soluble enzymes and membrane-bound substrates. Sphingomyelin, containing the small hydrophilic phosphorylcholine moiety, is hydrolysed by acid sphingomyelinase (acid SMase). Recent experimental data on the in vivo and in vitro role of activator proteins in sphingomyelin breakdown by acid SMase are reviewed. These data combined with the results using homogenous protein preparations as well as a liposomal assay system mimicking the physiological conditions suggest that lysosomal sphingomyelin degradation is not critically dependent on any of the known activator proteins. Moreover, evidence is provided that the assumed intramolecular activator domain of acid SMase and especially the presence of negatively charged lipids in the lysosomes are sufficient for sphingomyelin turnover.

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.

Cloning, expression and map assignment of chicken prosaposin

Prosaposin is the precursor of four small glycoproteins, saposins A-D, that activate lysosomal sphingolipid hydrolysis. A full-length cDNA encoding prosaposin from chicken brain was isolated by PCR. The deduced amino acid sequence predicted that, similarly to human and other mammalian species studied, chicken prosaposin contains 518 residues, including four domains that correspond to saposins A-D. There was 59% identity and 76% similarity of human and chicken prosaposin amino acid sequences. The basic three-dimensional structures of these saposins is predicted to be similar on the basis of the conservation of six cysteine residues and an N-glycosylation site. Identity of amino acid sequences was higher among saposins A, B and D than in saposin C. The predicted amino acid sequence of saposin B matched exactly that of purified chicken saposin B protein. The chicken prosaposin gene was mapped to a single locus, PSAP, in chicken linkage group E11C10 and is closely linked to the ACTA2 locus. This confirms the homology between chicken and human prosaposins and defines a new conserved segment with human chromosome 10q21-q24.

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.

Structural analysis of the mouse prosaposin (SGP-1) gene reveals the presence of an exon that is alternatively spliced in transcribed mRNAs

SGP-1/prosaposin can be secreted or targeted to the lysosomes where it is processed into smaller saposins A, B, C, and D required for the hydrolysis of glycosphingolipids. The deficiency of saposins B and C results in variant forms of metachromatic leukodystrophy and Gaucher's disease, respectively, which are characterized by lysosomal storage of undegraded glycosphingolipids. A required step to correct these genetic defects, or to understand the targeting mechanism of SGP-1 to the lysosomes, or to the extracellular space as well as its interaction with specific glycosphingolipids, is the analysis of the gene encoding this protein. Thus our investigation dealt with the molecular cloning of the mouse SGP-1 gene. Sequence analysis revealed that the mouse SGP-1 gene consists of 15 exons ranging from nine base pairs to 298 base pairs and 14 introns, which ranged from 89 base pairs to >8 kb in length. Our data show that saposin A is encoded by the exons 3, 4, and 5, saposin B by exons 6, 7, 8, and 9, saposin C by exons 10 and 11, and saposin D by exons 12, 13, and 14. The translation start codon is located within exon 1, and the translation stop codon is located within exon 15. The exon/intron boundaries were in accordance to the AG/GT consensus sequences. Our data also revealed that the SGP-1 gene has an exon consisting of the nine base pairs (CAG GAT CAG) encoding the three amino acids of saposin B, which may be alternatively spliced in the SGP-1 mRNA. The presence of the different forms of alternatively spliced mRNAs in various tissues was analyzed by RT-PCR. This approach demonstrated that prosaposin mRNAs of brain, heart, and muscle contain the nine base pairs of exon 8, whereas the transcripts from testis, lung, pancreas, spleen, and kidney do not contain this exon 8. Sequence comparison between the human and mouse prosaposin showed that exon 11 of mouse SGP-1 consists of 279 base pairs, whereas the human prosaposin gene consists of 187 base pairs. The extra 93 base pairs encode 31 amino acids corresponding to a proline-rich region located between saposin C and saposin D in the mouse prosaposin molecule. Finally, the availability of these genomic clones provides a starting point for further studies on the genetic role of specific sequences on the structure and function of SGP-1/prosaposin and its derived saposin proteins. In conclusion, we cloned and sequenced the mouse prosaposin (SGP-1) gene. The structural analysis of this gene revealed the presence of an exon that is alternatively spliced in transcribed mRNAs in a tissue-specific manner.

Decreased level of prosaposin in atopic skin

In the skin of atopic dermatitis patients, the amount of ceramides in the stratum corneum is decreased. Although the cause of this decrease may be due to the higher activity of acylase, a decrease in the activity of sphingolipid activator proteins may also be the cause. A polyclonal antibody to saposin D, elicited by immunizing rabbits with the synthetic polypeptide from cDNA of saposin D, cross-reacted with a single 65-kDa epidermal protein of pI 5.6 in a 2-dimensional immunoblot study, suggesting that it was prosaposin, the precursor protein of saposin D, from its molecular weight and demonstrating its immunohistochemical localization in the innermost cell layers of the stratum corneum of the skin. The antigenic material was also observed in the epithelium of the esophagus, pneumocytes of the lungs, hepatocytes, and glandular cells of the stomach. Immunoelectron microscopy showed the antigenic material in the cytoplasm of the granular cells and the intercellular spaces, either between the stratum granulosum and the stratum corneum or on the stratum corneum cell envelope. By ELISA, the amount of the 65-kDa protein in the inner surface skin of the upper arm of atopic dermatitis patients (nonlesional skin) [4.1 +/- 2.0 microg per 7 mm2 (mean +/- SD), n = 10] was found to be significantly decreased (p < 0.05) to 66% of that in the normal control (6.2 +/- 1.5 microg per 7 mm2, n = 10). Therefore, the suppression of prosaposin synthesis may be related to the abnormal stratum corneum formation in atopic skin through lower activation of glucosylcerebrosidase or sphingomyelinase.

Model SV40-transformed fibroblast lines for metabolic studies of human prosaposin and acid ceramidase deficiencies

Skin fibroblasts from patients with Farber disease (acid ceramidase deficiency) and from two siblings of the only known family affected with prosaposin deficiency were transformed by transfection with a plasmid carrying the SV40 large T antigen. The prosaposin-deficient transformed cell lines conserved their original metabolic defects, and in particular they were free of detectable immunoreactivity when using anti-saposin B and anti-saposin C antisera. Ultrastructurally, the cells contained heterogeneous lysosomal storage products. As found for their parental cell lines, the SV40-transformed fibroblasts exhibited deficient in vitro activities of lysosomal ceramidase and beta-galactosylceramidase, but a normal activity of acid sphingomyelinase. As observed for SV40-transformed fibroblasts from Farber disease, degradation of radioactive glucosylceramide or low density lipoprotein-associated radiolabelled sphingomyelin by the prosaposin-deficient cells in situ showed a clear impairment in the turnover of lysosomal ceramide. Ceramide storage in prosaposin-deficient cells was also demonstrated by ceramide mass determination. In contrast to acid ceramidase deficient cells, both the accumulation of ceramide and the reduced in vitro activity of acid ceramidase in cells from prosaposin deficiency could be corrected by addition of purified saposin D. The data confirm that prosaposin is required for lysosomal ceramide degradation, but not for sphingomyelin turnover. The SV40-transformed fibroblasts will be useful for pathophysiological studies on human prosaposin deficiency.

Lysosomal proteolysis of prosaposin, the precursor of saposins (sphingolipid activator proteins): its mechanism and inhibition by ganglioside

Saposins A, B, C, and D, which are required for the enzymatic hydrolysis of sphingolipids by specific lysosomal hydrolases, are produced by proteolytic processing of their common precursor protein, prosaposin. Our previous observation suggested that lysosomal cathepsin D may be involved in the proteolysis of prosaposin. Herein we report the involvement of cathepsin D in the proteolytic processing of prosaposin. An antibody against human placental cathepsin D blocked the proteolytic activity toward prosaposin in a human testicular lysosomal protease mixture (glycoprotein fraction). On immunoblot analysis using a monoclonal antibody against human saposin C, cathepsin D showed a similar proteolytic pattern as that of a human testicular glycoprotein fraction and hydrolyzed prosaposin into products of 48 and 29 kDa. The Km and Vmax values were 0.9 microM and 167 nmol/h/mg, respectively. N-Terminal sequence analysis indicated that the 48-kDa band was a mixture of two trisaposins, including domains for saposins A, B, and C and saposins B, C, and D, respectively. A similar study also showed that the 29-kDa band contained two disaposins, including domains for saposins A and B and saposins C and D, respectively. By longer treatment with cathepsin D, disaposins were further processed into mature saposin A and small fragments (14.5-17.5 kDa) containing individual saposins and portions of interdomain sequences. These small fragments were no longer processed by cathepsin D, but trimmed to fragments having similar molecular sizes (10.5-11.5 kDa) to those of mature saposins by a rat lysosome preparation. These findings indicated that cathepsin D is involved in the maturation of saposins but that, in addition to cathepsin D, other proteases appear to be involved in the maturation of saposin B, C, and D in lysosomes. Gangliosides, which specifically form complexes with prosaposin and saposins, inhibit proteolysis of prosaposin by cathepsin D. This finding indicates that prosaposin may be protected from lysosomal proteolysis by forming a complex with gangliosides in vivo.

Proteolytic processing patterns of prosaposin in insect and mammalian cells

Prosaposin is a multifunctional protein encoded at a single locus in humans and mice. The precursor contains, in tandem, four glycoprotein activators or saposins, termed A, B, C, and D, that are essential for specific glycosphingolipid hydrolase activities. Prosaposin appears to be a potent neurotrophic factor. To explore the proteolytic processing from prosaposin to mature activator proteins, metabolic labeling was done with human prosaposin expressed in insect cells, human fibroblasts, neuronal stem cells (NT2) and retinoic acid-differentiated NT2 neurons. In all cell types, the major processing pathway was through a tetrasaposin, A-B-C-D, from which saposin A was then removed. In mammalian cells monosaposins were derived from the trisaposin B-C-D by cleavage to the disaposins, B-C and C-D, that were processed to monosaposins. In insect cells the major end products were the disaposins, with A-B and C-D derived from the tetrasaposin, A-B-C-D, or with B-C and C-D derived from the trisaposin, B-C-D. In insect and mammalian cells, the nonsignal NH2-terminal peptide preceding saposin A (termed Nter) was usually removed prior to saposin A cleavage. In NT2-derived differentiated neurons, precursor tetrasaposins containing A-B-C-D were secreted with and without Nter. Immunofluorescence studies using prosaposin-specific antisera showed large steady state amounts of uncleaved prosaposin in Purkinje cells, cortical neurons, and other specific cell types in adult mice. These studies indicate that prosaposin processing is highly regulated at a proteolytic level to produce prosaposin, tetrasaposins, or mature monosaposins in specific mammalian cells.

Structural analysis of saposin C and B. Complete localization of disulfide bridges

Saposins A, B, C, and D are a group of homologous glycoproteins derived from a single precursor, prosaposin, and apparently involved in the stimulation of the enzymatic degradation of sphingolipids in lysosomes. All saposins have six cysteine residues at similar positions. In the present study we have investigated the disulfide structure of saposins B and C using advanced mass spectrometric procedures. Electrospray analysis showed that deglycosylated saposins B and C are mainly present as 79- and 80-residue monomeric polypeptides, respectively. Fast atom bombardment mass analysis of peptide mixtures obtained by a combination of chemical and enzymatic cleavages demonstrated that the pairings of the three disulfide bridges present in each saposin are Cys4-Cys77, Cys7-Cys71, Cys36-Cys47 for saposin B and Cys5-Cys78, Cys8-Cys72, Cys36-Cys47 for saposin C. We have recently shown that saposin C interacts with phosphatidylserine-containing vesicles inducing destabilization of the lipid surface (Vaccaro, A. M., Tatti, M., Ciaffoni, F., Salvioli, R., Serafino, A., and Barca, A. (1994) FEBS Lett. 349, 181-186); this perturbation promotes the binding of the lysosomal enzyme glucosylceramidase to the vesicles and the reconstitution of its activity. It was presently found that the effects of saposin C on phosphatidylserine liposomes and on glucosylceramidase activity are markedly reduced when the three disulfide bonds are irreversibly disrupted. These results stress the importance of the disulfide structure for the functional properties of the saposin.

Role of sulfated glycoprotein-1 (SGP-1) in the disposal of residual bodies by Sertoli cells of the rat

Sulfated glycoprotein-1 (SGP-1) is a polypeptide secreted by Sertoli cells in the rat. Sequence analysis revealed a 76% sequence similarity with human prosaposin produced by various cell types. Human prosaposin is a 70 kDa protein which is cleaved in the lysosomes into four 10-15 kDa polypeptides termed saposins A, B, C, and D. The function of lysosomal saposins is to either solubilize certain membrane glycolipids or to form complexes with lysosomal enzymes and/or their glycolipid substrate to facilitate their hydrolysis. The present investigation dealt with the delivery of SGP-1 into the phagosomes of Sertoli cells; these phagosomes contain the residual bodies which detach from the late spermatids at the time of spermiation. Immunogold labeling with anti-SGP-1 antibody was found over Sertoli cell lysosomes, but was absent from phagosomes formed after phagocytosis of spermatid residual bodies in the apical Sertoli cell cytoplasm in stages VIII and early IX of the cycle of the seminiferous epithelium. The phagosomes found later in the basal Sertoli cell cytoplasm in stages IX and X of the cycle became labeled with the antibody as the components of the residual bodies rapidly underwent lysis and disappeared from the Sertoli cells. Sertoli cell lysosomes isolated by cell fractionation (estimated purity of 80%) were found to contain a 65 kDa form of SGP-1 or prosaposin, as well as the 15 kDa polypeptides or saposins. Thus, it appears that this unique lysosomal form of SGP-1 reached the Sertoli cell phagosomes and that their derived polypeptides, the saposins, must play a role in the hydrolysis of membrane glycolipids found in phagocytosed residual bodies.

Expression of SGP-1 mRNA in preimplantation mouse embryos

In a search for genes expressed in preimplantation mouse embryos that are important for the earliest steps in differentiation, we identified an abundant mRNA that codes for a sulfated glycoprotein, SGP-1. The amount of this RNA rises approximately 100-fold during preimplantation development to a level approximately equal to that of beta-actin mRNA in blastocysts, although the level of these transcripts per cell remains fairly constant during these stages at approximately 2,000-4,000 copies. An antisense RNA that is complementary to approximately the last one-third of the message and contains an open reading frame of 455 nt was found in blastocysts at a 2-3-fold higher level than the mRNA. In situ hybridization with sense and antisense riboprobes showed that both strands are distributed throughout the embryo. The abundance of the SGP-1 mRNA indicates that the encoded protein may play an important role in the development of embryos, and the excess of antisense RNA raises the possibility of an unusual mechanism of regulating its expression.

Hormonal regulation of sulfated glycoprotein-1 synthesis by nonciliated cells of the efferent ducts of adult rats

The objective of this study was to define the factors regulating the endogenous production of sulfated glycoprotein-1 (SGP-1) in nonciliated cells of the efferent ducts. To this end we examined five different groups of animals undergoing the following experimental procedures: (1) hypophysectomized animals at 7, 14, and 28 days, (2) 7-day hypophysectomized rats receiving testosterone implants given at various time intervals thereafter, (3) castration at various time intervals up to 7 days, (4) 7-day castrated rats receiving testosterone implants at various time intervals thereafter, and (5) castrated rats given testosterone implants immediately after castration and sacrificed at different time intervals thereafter. Efferent ducts were fixed by perfusion with 4% paraformaldehyde and 0.5% glutaraldehyde in phosphate buffer for quantitative immunocytochemical analysis at the level of the electron microscope. For each experimental condition and their controls, the number of gold particles/micron2 within the endosomal and lysosomal compartments was calculated taking into account the changes in both the volume of the cell and organelles being quantified and expressed as labeling content. The results revealed that hypophysectomy (up to 4 weeks) caused a marked significant decrease in the SGP-1 labeling content of the endosomal and lysosomal compartments. The labeling content of the lysosomal compartment of efferent ducts from rats castrated for up to 1 week did not change significantly. However, there was a significant decrease in the labeling content of endosomes. This decrease is due to SGP-1, which is secreted by Sertoli cells, not being available for uptake in the efferent ducts. These results suggested that testosterone is not required for maintaining the high labeling content of SGP-1 within lysosomes of nonciliated cells, but that a pituitary factor appears to be needed. The administration of testosterone at different intervals to 7-day castrated animals resulted in a significant decrease of lysosomal SGP-1, suggesting that testosterone under these experimental conditions inhibits the production of a pituitary factor that maintains the high labeling content of SGP-1 within lysosomes of the nonciliated cells. Testosterone administered to 7-day hypophysectomized animals over a 24-hr period had no effect on the labeling content of SGP-1 within lysosomes. However, the administration of testosterone to animals immediately following castration showed no differences in the labeling content of SGP-1 within compared to controls.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural and immunocytochemical study of skin fibroblasts from normal and sialidosis patients

The objectives of this study were to analyze morphologically, morphometrically and immunocytochemically the lysosomal compartment of normal fibroblasts and of fibroblasts with neuraminidase deficiency. The immunocytochemical analyses consisted of quantifying the distribution of saposins and beta-galactosidase in the lysosomes of these cells to test the hypothesis that neuraminidase deficiency is associated with an impairment in the transport of these proteins to the lysosomal compartment. To test this idea, cultured skin fibroblasts of patients with or without sialidosis were prepared for electron microscopy and probed with antibodies against lysosomal beta-galactosidase and lysosomal saposins. The lysosomes of the affected cells had an abnormal accumulation of incompletely digested membranes which was associated with a significant lowering in the density of antigenic sites per lysosome. However, due to a significant increase in the number of lysosomes per affected cell, the total number of antigenic sites in control and neuraminidase deficient cells was similar. This presumably compensatory effect indicates that although the rate of production of beta-galactosidase and saposins remains unchanged, the transport of these molecules to the lysosomes is somehow affected. Our data also indicate that in the fibroblasts, lysosomes require a normal concentration of the three enzymes to maintain neuraminidase activity and sphingolipid degradation.