Distribution of saposin proteins (sphingolipid activator proteins) in lysosomal storage and other diseases

Degradation of hexosylceramides is required for timely corpse clearance via formation of cargo-containing phagolysosomal vesicles

Efficient degradation of phagocytic cargo in lysosomes is crucial to maintain cellular homeostasis and defending cells against pathogens. However, the mechanisms underlying the degradation and recycling of macromolecular cargo within the phagolysosome remain incompletely understood. We previously reported that the phagolysosome containing the corpse of the polar body in C. elegans tubulates into small vesicles to facilitate corpse clearance, a process that requires cargo protein degradation and amino acid export. Here we show that degradation of hexosylceramides by the prosaposin ortholog SPP-10 and glucosylceramidases is required for timely corpse clearance. We observed accumulation of membranous structures inside endolysosomes of spp-10-deficient worms, which are likely caused by increased hexosylceramide species. spp-10 deficiency also caused alteration of additional sphingolipid subclasses, like dihydroceramides, 2-OH-ceramides, and dihydrosphingomyelins. While corpse engulfment, initial breakdown of corpse membrane inside the phagolysosome and lumen acidification proceeded normally in spp-10-deficient worms, formation of the cargo-containing vesicles from the corpse phagolysosome was reduced, resulting in delayed cargo degradation and phagolysosome resolution. Thus, by combining ultrastructural studies and sphingolipidomic analysis with observing single phagolysosomes over time, we identified a role of prosaposin/SPP-10 in maintaining phagolysosomal structure, which promotes efficient resolution of phagocytic cargos.

Two short antimicrobial peptides derived from prosaposin-like proteins in the starry flounder (Platichthys stellatus)

Prosaposin (PSAP) is a precursor of saposin (SAP), which is present in lysosomal and secreted proteins. PSAP is a member of the SAP-like protein families, which comprise multifunctional proteins. In particular, their antimicrobial activity has been reported. We identified PSAP-like (PsPSAPL) sequences from starry flounder and analysed their expression and antimicrobial activity based on cDNA and amino acid sequences. PsPSAPL showed conservation of three saposin B type domains at high levels, and PsPSAPL mRNA was relatively abundantly distributed in the brain and gills of healthy starry founders. PsPSAPL mRNA showed significant expression changes in response to viral haemorrhagic septicaemia virus and Streptococcus parauberis. Synthetic peptides (PsPSAPL-1 and -2), prepared based on amino acid sequences, were used to confirm as well as analyse the antimicrobial activity against bacteria and parasites. Consequently, PsPSAPL-1 and -2 were found to significantly inhibit the growth of various bacteria and kill the Miamiensis avidus. In addition, bacterial biofilm formation was significantly inhibited. Safety was also confirmed by analysing cell haemolysis. These results indicate the immunological function of PsPSAP and the potential antimicrobial activity of the AMPs PsPSAPL-1 and -2.

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.

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.

Analysis of differential gene expression profiles in Caenorhabditis elegans knockouts for the v-SNARE master protein 1

At chemical synapses, neurons communicate information to other cells by secreting neurotransmitters or neuropeptides into the synaptic cleft, which then bind to receptors on the target cell. Preliminary work performed in our laboratory has shown that mutant nematodes lacking a protein called VSM-1 have increased synaptic density compared with the wild type. Consequently, we hypothesized that genes expressed in vsm-1 mutants mediate enhanced synaptogenesis. To identify these genes of interest, we utilized microarray technology and quantitative PCR. To this end, first we isolated the total RNA from young-adult wild-type and vsm-1 mutant Caenorhabditis elegans. Next, we synthesized cDNA from reverse transcription of the isolated RNA. Hybridization of the cDNA to a microarray was performed to facilitate gene expression profiling. Finally, fluorescently labeled microarrays were analyzed, and the identities of induced and repressed genes were uncovered in the open-source software Magic Tool. Analyses of microarray experiments performed using three independent biological samples per strain and three technical replicas and dye swaps showed induction of genes coding for major sperm proteins and repression of SPP-2 in vsm-1 mutants. Microarray results were also validated and quantified by using quantitative PCR.

Saposin C protects glucocerebrosidase against α-synuclein inhibition

Mutations in GBA1, the gene for glucocerebrosidase (GCase), are genetic risk factors for Parkinson disease (PD). α-Synuclein (α-Syn), a protein implicated in PD, interacts with GCase and efficiently inhibits enzyme activity. GCase deficiency causes the lysosomal storage disorder Gaucher disease (GD). We show that saposin C (Sap C), a protein vital for GCase activity in vivo, protects GCase against α-syn inhibition. Using nuclear magnetic resonance spectroscopy, site-specific fluorescence, and Förster energy transfer probes, Sap C was observed to displace α-syn from GCase in solution and on lipid vesicles. Our results suggest that Sap C might play a crucial role in GD-related PD.

Targeting and cytotoxicity of SapC-DOPS nanovesicles in pancreatic cancer

Only a small number of promising drugs target pancreatic cancer, which is the fourth leading cause of cancer deaths with a 5-year survival of less than 5%. Our goal is to develop a new biotherapeutic agent in which a lysosomal protein (saposin C, SapC) and a phospholipid (dioleoylphosphatidylserine, DOPS) are assembled into nanovesicles (SapC-DOPS) for treating pancreatic cancer. A distinguishing feature of SapC-DOPS nanovesicles is their high affinity for phosphatidylserine (PS) rich microdomains, which are abnormally exposed on the membrane surface of human pancreatic tumor cells. To evaluate the role of external cell PS, in vitro assays were used to correlate PS exposure and the cytotoxic effect of SapC-DOPS in human tumor and nontumorigenic pancreatic cells. Next, pancreatic tumor xenografts (orthotopic and subcutaneous models) were used for tumor targeting and therapeutic efficacy studies with systemic SapC-DOPS treatment. We observed that the nanovesicles selectively killed human pancreatic cancer cells in vitro by inducing apoptotic death, whereas untransformed cells remained unaffected. This in vitro cytotoxic effect correlated to the surface exposure level of PS on the tumor cells. Using xenografts, animals treated with SapC-DOPS showed clear survival benefits and their tumors shrank or disappeared. Furthermore, using a double-tracking method in live mice, we showed that the nanovesicles were specifically targeted to orthotopically-implanted, bioluminescent pancreatic tumors. These data suggest that the acidic phospholipid PS is a biomarker for pancreatic cancer that can be effectively targeted for therapy utilizing cancer-selective SapC-DOPS nanovesicles. This study provides convincing evidence in support of developing a new therapeutic approach to pancreatic cancer.

The identification of new biomarkers for identifying and monitoring kidney disease and their translation into a rapid mass spectrometry-based test: evidence of presymptomatic kidney disease in pediatric Fabry and type-I diabetic patients

Using label-free quantative proteomics, we have identified 2 potential protein biomarkers that indicate presymptomatic kidney disease in the urine of pediatric patients with type-I diabetes and Fabry disease (n = 20). Prosaposin and GM2 activator protein (GM2AP) were observed to be elevated in the urine of these patient groups compared to age- and sex-matched controls. These findings were validated by development of a rapid MRM-based tandem mass spectrometry test. Prosaposin was observed to be both significantly elevated in the urine of patients with Fabry disease compared to controls (p = 0.02) and reduced after 12 months enzyme replacement therapy (ERT, p = 0.01). Similarly, GM2AP concentrations were observed to be significantly higher compared to controls in the diabetic group (p = 0.049) and the pretreatment Fabry group (p = 0.003). In addition, this observed to be reduced significantly in the Fabry group following 12 months of ERT (p = 0.01). The process of detection of the biomarkers, development into a test and implications for monitoring patients and treatment are discussed.

Proteomic analysis of mouse models of Niemann-Pick C disease reveals alterations in the steady-state levels of lysosomal proteins within the brain

Niemann-Pick C disease (NPC) is a neurodegenerative lysosomal disorder characterized by storage of cholesterol and other lipids caused by defects in NPC1, a transmembrane protein involved in cholesterol export from the lysosome, or NPC2, an intralysosomal cholesterol transport protein. Alterations in lysosomal activities have been implicated in NPC pathogenesis therefore the aim of this study was to conduct a proteomic analysis of lysosomal proteins in mice deficient in either NPC1 or NPC2 to identify secondary changes that might be associated with disease. Lysosomal proteins containing the specific mannose 6-phosphate modification were purified from wild-type and Npc1(-/-) and Npc2(-/-) mutant mouse brains at different stages of disease progression and identified by bottom-up LC-MS/MS and quantified by spectral counting. Levels of a number of lysosomal proteins involved in lipid catabolism including prosaposin and the two subunits of β-hexosaminidase were increased in both forms of NPC, possibly representing a compensatory cellular response to the accumulation of glycosphingolipids. Several other lysosomal proteins were significantly altered, including proteases and glycosidases. Changes in lysosomal protein levels corresponded with similar alterations in activities and transcript levels. Understanding the rationale for such changes may provide insights into the pathophysiology of NPC.

Distribution of prosaposin mRNA in the central nervous system of the pigeon (Columba livia)

Bioassay and immunohistochemical studies have detected the presence of prosaposin in the central nervous system (CNS) of mammals. Here, first time, we have determined the partial cDNA sequence of pigeon prosaposin and mapped the distribution of its mRNA in the pigeon CNS. The predicted amino acid sequence of pigeon prosaposin showed 93 and 60% identity to chicken and human prosaposin, respectively. In situ hybridization, autoradiograms showed that the prosaposin mRNA expression was found in the olfactory bulb, prepiriform cortex, Wulst, mesopallium, nidopallium, hippocampal formation, thalamus, tuberis nucleus, pre-tectal nucleus, nucleus mesencephalicus lateralis, pars dorsalis, nucleus isthmi, pars parvocellularis and magnocellularis, Edinger-Westphal nucleus, optic tectum, cerebellar cortex and nuclei, vestibular nuclei and gray matter of the spinal cord. These results suggest that the cDNA sequence of pigeon prosaposin is comparable to other vertebrates, and the general distribution pattern of prosaposin mRNA resembles those are found in mammals.

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.

Cytotoxicity and Selectivity in Skin Cancer by SapC-DOPS Nanovesicles

Squamous cell carcinoma (SCC) and melanoma are malignant human cancers of the skin with an annual mortality that exceed 10,000 cases every year in the USA alone. In this study, the lysosomal protein saposin C (SapC) and the phospholipid dioloylphosphatidylserine (DOPS) were assembled into cancer-selective nanovesicles (SapC-DOPS) and successfully tested using several in vitro and in vivo skin cancer models. Using MTT assay that measures the percentage of cell death, SapC-DOPS cytotoxic effect on three skin tumor cell lines (squamous cell carcinoma, SK-MEL-28, and MeWo) was compared to two normal nontumorigenic skin cells lines, normal immortalized keratinocyte (NIK) and human fibroblast cell (HFC). We observed that the nanovesicles selectively killed the skin cancer cells by inducing apoptotic cell death whereas untransformed skin cancer cells remained unaffected. Using subcutaneous skin tumor xenografts, animals treated with SapC-DOPS by subcutaneous injection showed a 79.4 % tumor reduced compared to the control after 4 days of treatment. We observed that the nanovesicles killed skin cancer cells by inducing apoptotic cell death compared to the control as revealed by TUNEL staining of xenograft tumor sections.

Cancer-selective targeting and cytotoxicity by liposomal-coupled lysosomal saposin C protein

PURPOSE

Saposin C is a multifunctional protein known to activate lysosomal enzymes and induce membrane fusion in an acidic environment. Excessive accumulation of lipid-coupled saposin C in lysosomes is cytotoxic. Because neoplasms generate an acidic microenvironment, caused by leakage of lysosomal enzymes and hypoxia, we hypothesized that saposin C may be an effective anticancer agent. We investigated the antitumor efficacy and systemic biodistribution of nanovesicles comprised of saposin C coupled with dioleoylphosphatidylserine in preclinical cancer models.

EXPERIMENTAL DESIGN

Neuroblastoma, malignant peripheral nerve sheath tumor and, breast cancer cells were treated with saposin C-dioleoylphosphatidylserine nanovesicles and assessed for cell viability, ceramide elevation, caspase activation, and apoptosis. Fluorescently labeled saposin C-dioleoylphosphatidylserine was i.v. injected to determine in vivo tumor-targeting specificity. Antitumor activity and toxicity profile of saposin C-dioleoylphosphatidylserine were evaluated in xenograft models.

RESULTS

Saposin C-dioleoylphosphatidylserine nanovesicles, with a mean diameter of approximately 190 nm, showed specific tumor-targeting activity shown through in vivo imaging. Following i.v. administration, saposin C-dioleoylphosphatidylserine nanovesicles preferentially accumulated in tumor vessels and cells in tumor-bearing mice. Saposin C-dioleoylphosphatidylserine induced apoptosis in multiple cancer cell types while sparing normal cells and tissues. The mechanism of saposin C-dioleoylphosphatidylserine induction of apoptosis was determined to be in part through elevation of intracellular ceramides, followed by caspase activation. In in vivo models, saposin C-dioleoylphosphatidylserine nanovesicles significantly inhibited growth of preclinical xenografts of neuroblastoma and malignant peripheral nerve sheath tumor. I.v. dosing of saposin C-dioleoylphosphatidylserine showed no toxic effects in nontumor tissues.

CONCLUSIONS

Saposin C-dioleoylphosphatidylserine nanovesicles offer promise as a novel, nontoxic, cancer-targeted, antitumor agent for treating a broad range of cancers.

Regional expression of prosaposin in the wild-type and saposin D-deficient mouse brain detected by an anti-mouse prosaposin-specific antibody

Prosaposin is a precursor of saposins A, B, C, and D. Saposins are indispensable for lysosomal hydrolysis of sphingolipids. The notion that prosaposin itself is likely involved in brain development led us to generate an anti-mouse prosaposin-specific antibody that do not cross-react with any of the processed saposins. We have used it to study expression of prosaposin in the brain of wild-type (WT) and saposin D knockout mice (Sap-D(-/-)). Immunoblot studies indicated that prosaposin, already abundant in the brain of WT, was dramatically increased in Sap-D(-/-). By immunohistochemistry, the brain of WT was rich in prosaposin in hippocampal CA3 pyramidal neurons, tufted cells and mitral cells in olfactory bulb, and cerebellar Purkinje cells. In Sap-D(-/-), immunoreactivity of prosaposin was increased in these neurons, most notably in the CA3 pyramidal neurons which contained prosaposin immuno-positive inclusion bodies in the endoplasmic reticulum. Further characterization of these prosaposin-rich neurons may provide new insights into the physiological functions of prosaposin in the nervous system.

Replacement of alpha-galactosidase A in Fabry disease: effect on fibroblast cultures compared with biopsied tissues of treated patients

The function and intracellular delivery of enzyme therapeutics for Fabry disease were studied in cultured fibroblasts and in the biopsied tissues of two male patients to show diversity of affected cells in response to treatment. In the mutant fibroblasts cultures, the final cellular level of endocytosed recombinant alpha-galactosidases A (agalsidases, Fabrazyme, and Replagal) exceeded, by several fold, the amount in control fibroblasts and led to efficient direct intra-lysosomal hydrolysis of ((3)H)Gb3Cer. In contrast, in the samples from the heart and some other tissues biopsied after several months of enzyme replacement therapy (ERT) with Fabrazyme, only the endothelial cells were free of storage. Persistent Gb3Cer storage was found in cardiocytes (accompanied by increase of lipopigment), smooth muscle cells, fibroblasts, sweat glands, and skeletal muscle. Immunohistochemistry of cardiocytes demonstrated, for the first time, the presence of a considerable amount of the active enzyme in intimate contact with the storage compartment. Factors responsible for the limited ERT effectiveness are discussed, namely post-mitotic status of storage cells preventing their replacement by enzyme supplied precursors, modification of the lysosomal system by longstanding storage, and possible relative lack of Sap B. These observations support the strategy of early treatment for prevention of lysosomal storage.

Neuronal ceroid lipofuscinosis: clinical and morphologic findings in nine affected Polish Owczarek Nizinny (PON) dogs

OBJECTIVE

The aim of this study was to characterize the clinical and morphologic features of neuronal ceroid lipofuscinosis (NCL) in the Polish Owczarek Nizinny (PON) breed of dog.

ANIMALS

Nine Swedish PON dogs of both sexes were included in the study.

PROCEDURE

All dogs underwent a detailed clinical evaluation, with emphasis on ophthalmic exams. Histopathology and electron microscopy were performed on the eyes, brain and various internal organs. Immunohistochemical staining for detection of sphingolipid activator proteins (SAPs) and mitochondrial ATP synthase (SCMAS) was performed on the eyes and brain.

RESULTS

The dogs showed behavioral abnormalities, motor disturbances and visual impairment or blindness. Pupillary responses were abnormal while fundus changes varied from normal to severe retinal atrophy. Electroretinography (ERG) showed variable changes, from slight alterations in the process of dark adaptation to severely reduced or nonrecordable ERG a- and b-wave amplitudes. Histopathology revealed intracytoplasmic storage bodies within neurons of the brain and in retinal cells, especially the retinal pigment epithelium (RPE). Round to oval granular type of inclusion bodies, known as granular osmiophilic dense deposits (GRODS), were found in neuronal cells in the brain and in the retina. Immunohistochemistry identified the storage material in the brain and retina as consisting of SAPs.

CONCLUSION

The presently described NCL disease in PON dogs shows similarities to previously recorded cases in the Miniature Schnauzer. The closest human equivalent to this disease is infantile NCL (CLN1), in which the major stored proteins are SAPs and the ultrastructure of the inclusion bodies of neuronal cells is granular.

Agalsidase alfa therapy for Fabry disease

Fabry disease is a lysosomal storage disorder that results in neuropathic pain, progressive renal dysfunction, cardiomyopathy and stroke in affected individuals. The disease is caused by mutations in the GLA gene coding for α galactosidase A. The resulting deficiency of this enzyme causes accumulation of neutral glycosphingolipids in various tissues. Recombinant human agalsidase alfa has been developed to treat patients with Fabry disease. Preliminary data on this form of enzyme replacement therapy suggest that it improves pain, stabilizes renal function and improves cardiac hypertrophy in some patients. More data are needed on the ability of this therapy to prevent cardiac events, stroke and death.

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.

[Discovery of immature thymocyte proliferation factor]

An athymic mouse-derived immature T-cell clone, N-9F, was not maintained by interleukin-2 alone but required another soluble factor, contained in concanavalin A-stimulated rat splenocyte culture supernatant, namely T cell growth factor (TCGF), for its proliferation. An N-9F-proliferation factor (NPF) was isolated in a pure form from TCGF. N-9F cells and immature thymocytes proliferated in the presence of N-9F at 10(-12)-10(-9)M in a dose-dependent manner, but adult thymocytes were not stimulated by NPF. NPF increased DNA synthesis of N-9F. NPF increased CD4 and CD8 double negative, single positive and double positive thymocytes in fetal thymus organ culture. A hamster anti-NPF antiserum possessing the capacity to neutralize N-9F proliferation activity of NPF neutralized the increasing effect of NPF on immature thymocytes. All effects of NPF was inhibited by mAb QR6.6 to recognize a 100 kDa surface molecule of N-9F. The amino-terminal 20 amino acid sequence of NPF was identified and identical to that of rat saposin A. The apparent molecular weight of NPF, 16000, was comparable to that of saposin A. A Hitrap-mouse recombinant His-tag-saposin A antibody column bound NPF, pulled down the NPF activity in TCGF, and the antibody recognized a 16kDa molecule in western-blotting of TCGF. Thus, NPF in TCGF was a saposin A-like protein possessing the capacity for growth and differentiation of immature thymocytes. The physiological significance of NPF in the growth and differentiation of immature thymocytes was discussed in view of the characteristic distributions of NPF and the molecule recognized by its mAb QR6.6 in fetal thymi.

Inactivation of sortilin (a novel lysosomal sorting receptor) by dominant negative competition and RNA interference

To assess the role of sortilin in the sorting and trafficking of sphingolipid activator proteins (SAPs) the function of sortilin was abolished by a dominant-negative mutant and by the use of RNAi. Mutant sortilin lacking the carboxyl-terminal region that contains the sorting signal abolished the trafficking of SAPs to the lysosomes. Both sortilin and SAPs were retained in the Golgi apparatus. The use of chemically synthesized siRNA effectively blocked the trafficking of SAPs to the lysosomes as well. Additionally, we created a stable COS-7 cell line transfected with the pSilencer 3.1 H1 neo vector containing a selected siRNA template oligonucleotide (small hairpin interference RNA) where the levels of sortilin were greatly suppressed. The elimination of sortilin by this method will permit to determine whether or not sortilin is involved in a general mechanism of lysosomal sorting that involves the trafficking of various soluble lysosomal proteins other than SAPs.

Prosaposin: a new player in cell death prevention of U937 monocytic cells

We report that prosaposin binds to U937 and is active as a protective factor on tumor necrosis factor alpha (TNFalpha)-induced cell death. The prosaposin-derived saposin C binds to U937 cells in a concentration-dependent manner, suggesting that prosaposin behaves similarly. Prosaposin binding induces U937 cell death prevention, reducing both necrosis and apoptosis. This effect was inhibited by mitogen-activated protein ERK kinase (MEK) and sphingosine kinase (SK) inhibitors, indicating that prosaposin prevents cell apoptosis by activation of extracellular signal-regulated kinases (ERKs) and sphingosine kinase. Prosaposin led to rapid ERK phosphorylation in U937 cells as detected by anti-phospho-p44/42 mitogen-activated protein (MAP) kinase and anti-phosphotyrosine reactivity on ERK immunoprecipitates. It was partially prevented by apo B-100 and pertussis toxin (PT), suggesting that both lipoprotein receptor-related protein (LRP) receptor and Go-coupled receptor may play a role in the prosaposin-triggered pathway. Moreover, sphingosine kinase activity was increased by prosaposin treatment as demonstrated by the enhanced intracellular formation of sphingosine-1-phosphate (S-1-P). The observation that the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin prevented the prosaposin effect on cell apoptosis suggests that sphingosine kinase exerts its anti-apoptotic activity by the PI3K-Akt pathway. Thus, cell apoptosis prevention by prosaposin occurs through ERK phosphorylation and sphingosine kinase. The biological effect triggered by prosaposin might be extended to primary cells because it triggers Erk phosphorylation in peripheral blood mononuclear cells (PBMCs). This is the first evidence of a biological effect consequent to a signal transduction pathway triggered by prosaposin in cells of non-neurological origin.

Isolation of proliferation factor of immature T-cell clone in concanavalin A-stimulated splenocyte culture supernatant

An athymic mouse-derived immature T-cell clone, N-9F, was not maintained by interleukin-2 alone but required another soluble factor, contained in concanavalin A-stimulated rat splenocyte culture supernatant, namely T cell growth factor (TCGF), for its proliferation. An N-9F-proliferation factor (NPF) was isolated in a pure form from TCGF. N-9F cells and immature thymocytes proliferated in the presence of NPF at 10-11-10-8 g/ml in a dose-dependent manner, but adult thymocytes were not stimulated by NPF. NPF increased DNA synthesis of N-9F. NPF increased CD4 and CD8 double negative thymocytes and CD8 single positive thymocytes in fetal thymus organ culture. A hamster anti-NPF antiserum possessing the capacity to neutralize N-9F proliferation activity of NPF decreased double negative thymocytes. The amino-terminal amino acid sequence of NPF was identified to be Ser-Leu-Pro-Cys-Asp-Ile-Cys-Lys-Thr-Val-Val-Thr-Glu-Ala-Cys-Asn-Leu-Leu-Lys-Asp- and was identical to that of rat saposin A. The apparent molecular weight of NPF, 16000, was comparable to that of saposin A. A rabbit anti-mouse recombinant His-tag (mrH)-saposin A antibody recognized a 16000 MW molecule in TCGF. A Hitrap-saposin A antibody column bound NPF and pulled down the NPF activity in TCGF. Thus, NPF in TCGF was a saposin A-like protein possessing the capacity for growth and differentiation of immature thymocytes.

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.

Are there useful biochemical markers of disease activity in lysosomal storage diseases?

The primary biochemical consequence of a defect in a gene encoding a functional component of the lysosomal system is disruption of the catabolism or processing of macromolecules in the lumen of the lysosome. Transport of the resulting digestion products through the lysosomal membrane may also be affected. This leads to the accumulation of specific metabolites within the lysosomes of affected cells. The nature of these storage products depends upon the functional protein affected and the cell type. The accumulation of storage products is progressive and leads to hypertrophy of the lysosomal system, the hallmark of lysosomal storage diseases (LSDs). Subsequent cell necrosis or, possibly, exocytosis results in the appearance in body fluids of the storage products and components of the lysosomes at much higher concentrations than seen in normal unaffected individuals. Measurement of these increased levels of metabolites and proteins provides disease-specific and generic biochemical markers for LSDs. Secondary changes in metabolism and cellular function may also produce characteristic changes in the levels of metabolites or proteins, which can also be used as markers of the disease process. Although the rate of appearance of these biochemical markers in an individual will depend upon the underlying mutation in the gene and on other genetic and environmental factors, it provides a good indicator of the progression of the disease. As the novel forms of treatment being developed may reverse the hypertrophy of the lysosomal system, biochemical markers could also be used to monitor the reversal of pathology and the efficacy of treatment.

Quantification of mRNAs encoding proteins of the glycosphingolipid catabolism in mouse models of GM2 gangliosidoses and sphingolipid activator protein precursor (prosaposin) deficiency

We have investigated the mRNA amounts of six lysosomal proteins (beta-hexosaminidase alpha- and beta-subunit, sphingolipid activator protein precursor, GM2 activator protein, lysosomal sialidase, beta-glucocerebrosidase) involved in the degradation of glycosphingolipids. We analyzed extracts from brain tissues of mouse models for lysosomal storage diseases, i.e., the GM2 gangliosidoses and the deficiency of the sphingolipid activator protein precursor (prosaposin). The mRNA levels were quantified by real-time reverse transcription-polymerase chain reaction. Although storage of the respective lysosomal proteins has been reported in human and mice, no increase of their mRNA amounts could be detected here. Our results indicate that there is no transcriptional upregulation of lysosomal proteins in the examined neuronal storage disorders.

Methionine oxidation within the cerebroside-sulfate activator protein (CSAct or Saposin B)

The cerebroside-sulfate activator protein (CSAct or Saposin B) is a small water-soluble glycoprotein that plays an essential role in the metabolism of certain glycosphingolipids, especially sulfatide. Deficiency of CSAct in humans leads to sulfatide accumulation and neurodegenerative disease. CSAct activity can be measured in vitro by assay of its ability to activate sulfatide-sulfate hydrolysis by arylsulfatase A. CSAct has seven methionine residues and a mass of 8,845 Da when deglycosylated. Mildly oxidized, deglycosylated CSAct (+16 Da), separated from nonoxidized CSAct by reversed-phase high-performance liquid chromatography (RP-HPLC), showed significant modulation of the in vitro activity. Because oxidation partially protected against CNBr cleavage and could largely be reversed by treatment with dithiothreitol, it was concluded that the major modification was conversion of a single methionine to its sulfoxide. High-resolution RP-HPLC separated mildly oxidized CSAct into seven or more different components with shorter retention times than nonoxidized CSAct. Mass spectrometry showed these components to have identical mass (+16 Da). The shorter retention times are consistent with increased polarity accompanying oxidation of surface-exposed methionyl side chains, in general accordance with the existing molecular model. A mass-spectrometric CNBr mapping protocol allowed identification of five of the seven possible methionine-sulfoxide CSAct oxoforms. The most dramatic suppression of activity occurred upon oxidation of Met61 (26% of control) with other residues in the Q60MMMHMQ66 motif falling in the 30-50% activity range. Under conditions of oxidative stress, accumulation of minimally oxidized CSAct protein in vivo could perturb metabolism of sulfatide and other glycosphingolipids. This, in turn, could contribute to the onset and progression of neurodegenerative disease, especially in situations where the catabolism of these materials is marginal.

Analysis and classification of 304 mutant alleles in patients with type 1 and type 3 Gaucher disease

Gaucher disease results from the inherited deficiency of the enzyme glucocerebrosidase (EC 3.2.1.45). Although >100 mutations in the gene for human glucocerebrosidase have been described, most genotype-phenotype studies have focused upon screening for a few common mutations. In this study, we used several approaches-including direct sequencing, Southern blotting, long-template PCR, restriction digestions, and the amplification refraction mutation system (ARMS)-to genotype 128 patients with type 1 Gaucher disease (64 of Ashkenazi Jewish ancestry and 64 of non-Jewish extraction) and 24 patients with type 3 Gaucher disease. More than 97% of the mutant alleles were identified. Fourteen novel mutations (A90T, N117D, T134I, Y135X, R170C, W184R, A190T, Y304X, A341T, D399Y, c.153-154insTACAGC, c.203-204insC, c.222-224delTAC, and c.1122-1123insTG) and many rare mutations were detected. Recombinant alleles were found in 19% of the patients. Although 93% of the mutant alleles in our Ashkenazi Jewish type 1 patients were N370S, c.84-85insG, IVS2+1G-->A or L444P, these four mutations accounted for only 49% of mutant alleles in the non-Jewish type 1 patients. Genotype-phenotype correlations were attempted. Homozygosity or heterozygosity for N370S resulted in type 1 Gaucher disease, whereas homozygosity for L444P was associated with type 3. Genotype L444P/recombinant allele resulted in type 2 Gaucher disease, and homozygosity for a recombinant allele was associated with perinatal lethal disease. The phenotypic consequences of other mutations, particularly R463C, were more inconsistent. Our results demonstrate a high rate of mutation detection, a large number of novel and rare mutations, and an accurate assessment of the prevalence of recombinant alleles. Although some genotype-phenotype correlations do exist, other genetic and environmental factors must also contribute to the phenotypes encountered, and we caution against relying solely upon genotype for prognostic or therapeutic judgements.

Hydrogen-deuterium exchange signature of porcine cerebroside sulfate activator protein

Hydrogen-deuterium exchange can be a sensitive indicator of protein structural integrity. Comparisons were made between cerebroside sulfate activator protein (CSAct) in the native state and after treatment with guanidine hydrochloride plus dithiothreitol. Native protein has three internal disulfide bonds and treated protein has no internal disulfide bonds. The comparisons were made using hydrogen-deuterium exchange measured by electrospray ionization mass spectrometry, percentage alpha-helical content measured by circular dichroism and biological activity measured by the ability to support arylsulfatase A-catalyzed sulfate hydrolysis from cerebroside sulfate. In acidic solvent native protein has 59 exchange refractory protons and treated protein has 20 exchange refractory protons (44 and 14% of the exchangeable proton populations, respectively). In native protein the size of the exchange refractory proton population is sensitive to changes in pH, temperature and the presence of a ligand. It is uninfluenced by the presence or absence of glycosyl groups attached to Asn21. Helical content is virtually identical in native and treated protein. Biological activity is significantly reduced but not obliterated in treated protein. The hydrogen-deuterium exchange profile appears to be a sensitive signature of the correctly folded protein, and reflects a dimension of the protein structure that is not apparent in circular dichroic spectra or in the ability of the protein to support arylsulfatase A-catalyzed sulfate hydrolysis from sulfatide. The hydrogen-deuterium exchange profile will be a valuable criterion for characterizing mutant forms of CSAct produced by recombinant and synthetic paradigms and also the native and mutant forms of related proteins.

Glucocerebrosidase gene mutations in patients with type 2 Gaucher disease

Gaucher disease, the most common lysosomal storage disorder, results from the inherited deficiency of the enzyme glucocerebrosidase. Three clinical types are recognized: type 1, non-neuronopathic; type 2, acute neuronopathic; and type 3, subacute neuronopathic. Type 2 Gaucher disease, the rarest type, is progressive and fatal. We have performed molecular analyses of a cohort of 31 patients with type 2 Gaucher disease. The cases studied included fetuses presenting prenatally with hydrops fetalis, infants with the collodion baby phenotype, and infants diagnosed after several months of life. All 62 mutant glucocerebrosidase (GBA) alleles were identified. Thirty-three different mutant alleles were found, including point mutations, splice junction mutations, deletions, fusion alleles and recombinant alleles. Eleven novel mutations were identified in these patients: R131L, H255Q, R285H, S196P, H311R, c.330delA, V398F, F259L, c.533delC, Y304C and A190E. Mutation L444P was found on 25 patient alleles. Southern blots and direct sequencing demonstrated that mutation L444P occurred alone on 9 alleles, with E326K on one allele and as part of a recombinant allele on 15 alleles. There were no homozygotes for point mutation L444P. The recombinant alleles that included L444P resulted from either reciprocal recombination or gene conversion with the nearby glucocerebrosidase pseudogene, and seven different sites of recombination were identified. Homozygosity for a recombinant allele was associated with early lethality. We have also summarized the literature describing mutations associated with type 2 disease, and list 50 different mutations. This report constitutes the most comprehensive molecular study to date of type 2 Gaucher disease, and it demonstrates that there is significant phenotypic and genotypic heterogeneity among patients with type 2 Gaucher disease. Hum Mutat 15:181-188, 2000. Published 2000 Wiley-Liss, Inc.

Preparation of the cerebroside sulfate activator (CSAct or saposin B) from human urine

The purification of cerebroside sulfate activator (CSAct) or saposin B from pooled human urine is described. Urinary proteins are concentrated by ammonium sulfate precipitation. A suspension of the precipitate is heat-treated and the heat-stable proteins are fractionated through a series of chromatographic steps. An initial concanavalin A column retains little of the CSAct activity, but is important for subsequent purification. Passing the Con A effluent directly onto an octyl Sepharose column removes the protein of interest which is recovered by affinity elution with octyl glucoside. Subsequent ion-exchange and gel filtration chromatographies yield a protein of 80-90% purity, although it is sometimes necessary to repeat one or more steps. A small amount of CSAct can sometimes be recovered from the initial Con A Sepharose column by methyl mannoside elution and purified by a parallel chromatographic protocol. Mass spectral analysis suggests that the final material is a mixture of two major and several minor glycoforms of a 79 amino acid protein with the structure predicted from the human prosaposin cDNA by truncation of both N- and C-terminal regions. Sugar analysis revealed the presence of glucosamine, mannose, and fucose, consistent with the major isoforms bearing a five-sugar Man(2)GluNac(2)Fuc or a single GluNac substituent. The human urinary material is similar to the previously characterized pig kidney protein in most respects, but varies in some details.

Intracellular transport of acid beta-glucosidase and lysosome-associated membrane proteins is affected in Gaucher's disease (G202R mutation)

Gaucher's disease (GD) is caused by an inherited deficiency of acid beta-glucosidase with storage of glucosylceramides in the lysosomes of macrophages. This study identifies a G202R mutation in the acid beta-glucosidase gene in an infant with severe neuronopathic (type 2) GD and only slightly reduced acid beta-glucosidase activity. Western blot analysis, pulse chase experiments, and the thin frozen section immunogold method were used to analyse the implications of this mutation on the pathogenesis, clinical heterogeneity and diagnostic evaluation of GD. The results show that acid beta-glucosidase persists in the patient's fibroblasts as a mannose-rich polypeptide in the endoplasmic reticulum and is not transported to the lysosomes. By contrast, high expression of the lysosome-associated membrane proteins LAMP-1 and LAMP-2, saposin C, and cathepsin D was observed in the patient's lysosomes. Immunogold labelling of the integral membrane proteins LAMP-1 and LAMP-2 increases significantly at the cell surface of Kupffer cells and fibroblasts as well as at the apical membrane of hepatocytes. In addition, LAMP-1 and LAMP-2 associate with the bilayer of stored glucosylceramide. It is concluded that defective intracellular transport of mutant acid beta-glucosidase from the endoplasmic reticulum to lysosomes leads to a more severe clinical phenotype than the residual enzyme activity may indicate. Furthermore, the detection of LAMP in the tubular bundles of undigested glucosylceramides, as well as their increased concentration at the surfaces of the affected cells, suggests that these proteins play a role in the storage or removal of substrate in GD. Intracellular targeting of acid beta-glucosidase and LAMP contributes to the broad phenotypic heterogeneity of GD.

Biosynthesis, processing, and targeting of sphingolipid activator protein (SAP )precursor in cultured human fibroblasts. Mannose 6-phosphate receptor-independent endocytosis of SAP precursor

Sphingolipid activator proteins (SAPs) are essential cofactors for the lysosomal degradation of glycosphingolipids with short oligosaccharide chains by acidic exohydrolases. SAP-A, -B, -C, and -D derive from proteolysis of a 73-kDa glycoprotein, the SAP precursor. In the present publication, we studied the intracellular transport and the endocytosis of SAP precursor in human skin fibroblasts. Our data indicate that SAP precursor bears phosphate residues on noncomplex carbohydrate chains linked to the SAP-C and the SAP-D domain and sulfate residues on complex carbohydrate chains located within the SAP-A, -C, and possibly the SAP-D domain. Treatment of fibroblasts with either bafilomycin A1 or 3-methyladenine indicates that proteolytic cleavage of SAP precursor begins as early as in the late endosomes. To determine whether targeting of SAP precursor depends on mannose 6-phosphate residues, we analyzed the processing of SAP precursor in I-cell disease fibroblasts. In these cells nearly normal amounts of newly synthesized SAP-C were found, although secretion of SAP precursor was enhanced 2-3-fold. Moreover, SAP-C could be localized to lysosomal structures by indirect immunofluorescence in normal and in I-cell disease fibroblasts. Mannose 6-phosphate was not found to interfere significantly with endocytosis of SAP precursor. Normal fibroblasts internalized SAP precursor secreted from I-cells nearly as efficiently as the protein secreted from normal cells. To our surprise, deglycosylated SAP precursor was taken up by mannose 6-phosphate receptor double knock out mouse fibroblasts more efficiently than the glycosylated protein. We propose that intracellular targeting of SAP precursor to lysosomes is only partially dependent on mannose 6-phosphate residues, whereas its endocytosis occurs in a carbohydrate-independent manner.

Sphingolipid activator proteins in a human hereditary renal disease with deposition of disialogangliosides

Congenital nephrotic syndrome of the Finnish type is a recessively inherited renal disease with glomerular deposits of the disialoganglioside O-acetyl-GD3. Sphingolipid activator proteins (saposins) stimulate the degradation of glycosphingolipids by lysosomal enzymes, and defects in saposins cause accumulation of substrate lipids in the affected tissues in lysosomal storage disease. Here we report a study of the role of saposins in the accumulation of O-acetyl-GD3 in kidneys of congenital nephrotic syndrome patients. At the mRNA level, the expression of saposin precursor in diseased kidneys appeared normal, and the nucleotide sequence analysis of cDNA clones did not reveal abnormalities in the prosaposin gene. Immunohistologically, saposins were localized mainly to the epithelial cells of the distal renal tubules or to the parietal epithelial cells of glomeruli. In the nephrotic syndrome kidneys, the staining pattern was highly granular and appeared mostly in the apical part of the epithelial lining, unlike the control kidneys. These results show that a major site of ganglioside metabolism is located in the distal nephron. Furthermore, these results suggest that saposins are not directly involved in the metabolism of the terminal sialic acids of disialogangliosides in the nephrotic syndrome kidneys.

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.

Prosaposin and prosaptide, a peptide from prosaposin, induce an increase in ganglioside content on NS20Y neuroblastoma cells

Prosaposin has been recently identified as a neurotrophic factor eliciting differentiation in neuronal cultured cells (NS20Y). In this paper we investigate whether prosaposin and its active peptide (prosaptide) may modify the ganglioside pattern in neuroblastoma cells. The analysis by high performance thin layer chromatography did not reveal qualitative changes in the ganglioside pattern of NS20Y cells incubated in the presence of prosaposin, compared to control cells, but it did reveal an increase of the content of all three major resorcinol positive bands (GM3, GM2, GD1a). Cytofluorimetric and immunofluorescence microscopic analysis revealed that the increase of the ganglioside content was at the plasma membrane level. These findings suggest that the neurotrophic activity of prosaposin on NS20Y neuroblastoma cells might be mediated in part by the increase of cell surface gangliosides.

MRI evaluation of the brain in infantile neuronal ceroid-lipofuscinosis. Part 1: Postmortem MRI with histopathologic correlation

The purpose of this study was to correlate postmortem magnetic resonance imaging (MRI) with histopathologic findings in brains of a series of autopsied patients with infantile neuronal ceroid-lipofuscinosis, a recessively inherited progressive encephalopathy. Eight formalin-fixed brains (age range at death, 7 to 13 years) were examined with MRI. One patient had also undergone brain MRI 2 years before death. Histopathologic analyses were made from standard areas selected on the basis of the MRI scans. Postmortem MRI findings did not differ significantly from the findings in the patient who was also examined during life. Typical findings were extreme cerebral atrophy and hypointensity of the gray-matter structures in relation to the white matter on T2-weighted images, a pattern the reverse of normal. Characteristic histologic findings were almost complete loss of cortical neurons and secondary loss of axons and myelin sheaths in the white matter. The drastically altered relative intensities of the gray- and white-matter structures on the MRI scans reflected replacement of the neurons with hypertrophic astrocytes and/or macrophages filled with storage material.

Sphingolipid activator proteins (SAPs) are stored together with glycosphingolipids in the infantile neuronal ceroid-lipofuscinosis (INCL)

The storage material isolated from the brains of patients with infantile neuronal ceroid-lipofuscinosis (INCL) contains, on average, 43% protein and 35% lipids on a dry weight basis. Recently we identified the major storage proteins as sphingolipid activator proteins (SAPs) A and D by direct sequencing. In the present study we used monospecific anti-sap-B-, anti-sap-C, and anti-sap-D-antisera in immunohistochemical and Western analyses to show that sap-D is, indeed, an integral component of the storage bodies. In contrast, no (or little) immunoreactivity for sap-B or sap-C was detected in the INCL storage granules. This observation is of interest for an understanding of the pathogenesis because the four SAPs are produced from a single precursor protein by proteolytic cleavage. Furthermore, we analysed the stored lipids on high performance thin layer chromatography combined with different staining techniques. In this preliminary analysis we found two glycosphingolipids, yet to be identified, to be common for all INCL patients.

Sphingolipid activator proteins in the neuronal ceroid-lipofuscinoses: an immunological study

The molecular defects underlying neuronal ceroid-lipofuscinoses (NCL) are still unknown. However, more data exist on the composition of the hydrophobic storage material characteristic of NCL. Accumulation of subunit c of the mitochondrial ATP synthase has been shown in most forms of human NCL with the exception of the infantile NCL (INCL) for which we have recently demonstrated storage of sphingolipid activator proteins (SAP). In the present study we raised an antiserum against storage cytosomes purified from INCL brain. Using the anti-INCL antiserum and monospecific SAP antisera, we studied storage material isolated from the brains of patients affected with NCL by Western analysis, and found a 12-kDa protein showing a SAP-like immunoreactivity not only in INCL, but also in all the childhood forms of NCL. Furthermore, using the anti-sap-D antiserum for immunohistochemistry, we observed strong immunoreactivity of the storage cytosomes in all major forms of NCL, and also in tissues of non-neuroectodermal origin. From these data we conclude that the presence of SAP within the storage bodies is a phenomenon common to all major forms of human NCL.

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.

Developmental and tissue-specific expression of prosaposin mRNA in murine tissues

Prosaposin is a multifunctional locus in humans and mice that encodes in tandem and in the same reading frame four glycoprotein activators, or saposins, of lysosomal hydrolases. These ubiquitously expressed glycoproteins and the precursor, prosaposin, have been proposed to function in glycosphingolipid catabolic pathways and glycolipid transport. To characterize the temporal and spatial expression of the prosaposin locus, prenatal and postnatal mouse tissues were screened by in situ hybridization with a mouse antisense riboprobe for prosaposin. Prenatally, prosaposin mRNA was expressed differentially in the placenta and prominently in the decidua basalis and capsularis where expression was gestational age dependent. No other region of high-level expression was detectable in the prenatal mouse. In comparison, high-level differential expression of prosaposin was clearly evident postnatally in a variety of organs, including secretory epithelial cells of the choroid plexus, ependymal lining, upper trachea, esophagus, cortical tubules of the kidney, sertoli cells of the testes and epididymis. Discrete localization of prosaposin mRNA expression was also found in neurons of the cerebral cortex, cerebellar cortex, and lateral columns of the spinal cord as well as in hepatocytes of the mature liver. Very high levels of expression were found in specialized tissues including the Harderian glands and macrophages of lymph nodes, lungs, splenic tissue, and thymus. These studies indicate that the expression of the prosaposin locus, a presumed "housekeeping" gene, is under tissue- and cell-specific differential control. The spatial organization of expression suggests a role for this locus in the expression of glycosphingolipid-storage diseases.

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

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

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

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

Storage of saposins A and D in infantile neuronal ceroid-lipofuscinosis

We have isolated storage cytosomes from brain tissue of patients with infantile neuronal ceroid-lipofuscinosis. The purified storage bodies were subjected to compositional analysis which revealed a high content of proteins, accounting for 43% of dry weight. Saposins A and D, also known as sphingolipid activator proteins (SAPs), were shown to constitute a major portion of the accumulated protein using gel electrophoresis and sequence analysis. This is the first time that saposins have been found to be stored in any form of neuronal ceroid-lipofuscinosis.

Synthesis and characterization of a bioactive 82-residue sphingolipid activator protein, saposin C

The sphingolipid activator protein, saposin C (also termed SAP 2), was chemically synthesized, purified, and characterized. The fully protected 82-residue protein was synthesized by automated solid-phase methods, with multiple recoupling steps resulting in a high average coupling efficiency of 98.8%. The overall yield was estimated to be approx 40%. Deprotection and cleavage of the peptide from the resin was followed by folding in the absence of chaotropic agents at pH 8.5. The protein was purified by reversed-phase high pressure liquid chromatography (HPLC) and its purity determined by capillary electrophoresis and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The composition of the synthetic saposin C was determined by amino acid analysis. Its sequence was verified by Edman sequence analysis of overlapping peptide fragments generated by chymotryptic and Staphylococcus aureus V8 digestions. The sequence at the C-terminus was determined by digestion with carboxypeptidase P, followed by phenylthiohydantoin (PTH) derivitization and HPLC analysis of the released amino acid residues. Deglycosylated native saposin C appeared as a lower molecular-weight species than synthetic saposin C on SDS-PAGE. This has been explained by amino acid and C-terminal analysis showing native saposin C to be two amino acids shorter at the C terminus than a deduced sequence (from cDNA) previously published. Synthetic saposin C displayed 85% of full biological activity as determined by its ability to stimulate glucocerebrosidase activity in vitro: Synthetic and native saposin C increased glucocerebrosidase catalyzed hydrolysis of 4-methylumbelliferyl beta-D-glucoside by factors of 6.0 and 7.1, respectively. Furthermore, synthetic and native saposin C share similar K(act) values (0.5 and 1.5 microM respectively) indicating that they bind to glucocerebrosidase with similar affinities.

Distribution of prosaposin-like immunoreactivity in rat brain

Prosaposin is the precursor for saposins A, B, C, and D, which are small lysosomal proteins required for the hydrolysis of sphingolipids by specific lysosomal hydrolases. With a monospecific anti-saposin C antibody, which cross-reacts with prosaposin but not with saposin A, B, or D, the present immunoblot experiments showed that the rat brain expresses an unprocessed approximately 72 kDa protein (possibly prosaposin) and little saposin C. Regional analysis demonstrated that prosaposin is abundant in the brainstem, hypothalamus, cerebellum, striatum, and hippocampus, and less abundant in the cerebral cortex. Consistent with this finding, prosaposin-like immunoreactive neurons and fibers as revealed by immunohistochemistry were observed frequently in subcortical regions. The medial septum, diagonal bands, basal nucleus of Meynert, ventral striatum, medial habenular nucleus, and motor nuclei of cranial nerve had significant numbers of immunoreactive neurons. There were also nerve fibers with prosaposin-like immunoreactivity in several projection fields of the above nuclei. Other brain areas that contained prosaposin-like immunoreactive neurons and/or processes were: several brain nuclei (nucleus caudate putamen, globus pallidus, substantia nigra, red nucleus) constituting the so-called extrapyramidal system, reticular thalamic nucleus, entopeduncular nucleus, mammillary nuclei, auditory relay nuclei, cerebellum, sensory cranial nerve nuclei, and the reticular formation. The distribution pattern of prosaposin is apparently different from that of other neuroactive substances so far examined, and thus prosaposin may be involved in novel central events.

Structural study of the oligosaccharide moieties of sphingolipid activator proteins, saposins A, C and D obtained from the spleen of a Gaucher patient

We have determined and compared the structures of the oligosaccharide moieties of saposin A, C and D purified from the spleen of a patient with Gaucher disease. These saposins, together with saposin B, are small glycoproteins, derived from separate domains of a single precursor, prosaposin, and are required for the lysosomal hydrolysis of various sphingolipids. The characteristic features of the oligosaccharide moieties of saposin A are (a) the predominance of a fucosylated trimannosyl core structure and (b) the occurrence of several different oligomannose-type and N-acetyllactosamine-type oligosaccharides. Saposin C contains (a) a predominance of oligomannose-type oligosaccharides and monoantennary oligosaccharides and (b) the presence of four different oligosaccharides having bisecting N-acetylglucosamine residues (found only in this saposin). Saposin D is distinguished by the occurrence of oligomannose-type oligosaccharides, which comprise nearly 90% of its total oligosaccharides. The possible reasons for the unique glycosylation of each saposin is discussed.