Occurrence of prosaposin as a neuronal surface membrane component

Pan-cancer analysis of PSAP identifies its expression and clinical relevance in gastric cancer

Prosaposin (PSAP) plays a critical role in sphingolipid and cancer metabolism. Reports have shown that PSAP was involved in proliferation, tumorigenesis, and metastasis. However, the expression pattern of PSAP and its prognostic roles in gastric cancer remain elusive. PSAP expression pattern and its prognostic roles in gastric cancer (GC) were explored using data from the TCGA and Kaplan-Meier Plotter. Immunohistochemical staining of GC tissues was performed to validate the prognostic role of PSAP. TISIDB was used to analyze its correlation with immunomodulators. PSAP-associated genes, PDCD1, TGFB1, and CSF1R were used to build a risk model to evaluate immunotherapy outcomes of patients with stomach adenocarcinoma (STAD). Results showed that PSAP was highly expressed in GC. High PSAP expression in GC patients also significantly indicated a poor prognosis. The results of immunohistochemical staining showed that PSAP was an independent prognostic factor in GC patients. Based on three PSAP-associated genes, a risk model that could predict the prognosis and immunotherapy outcome of STAD was bulit. PSAP was an independent prognostic factor in GC. Our results have identified three prognosis-related genes which were useful to evaluate immunotherapy outcomes of STAD patients.

Temporal changes in prosaposin expression in the rat dentate gyrus after birth

Neurogenesis in the hippocampal dentate gyrus occurs constitutively throughout postnatal life. Adult neurogenesis includes a multistep process that ends with the formation of a postmitotic and functionally integrated new neuron. During adult neurogenesis, various markers are expressed, including GFAP, nestin, Pax6, polysialic acid-neural cell adhesion molecule (PSA-NCAM), neuronal nuclei (NeuN), doublecortin, TUC-4, Tuj-1, and calretinin. Prosaposin is the precursor of saposins A–D; it is found in various organs and can be excreted. Strong prosaposin expression has been demonstrated in the developing brain including the hippocampus, and its neurotrophic activity has been proposed. This study investigated changes in prosaposin in the dentate gyrus of young and adult rats using double immunohistochemistry with antibodies to prosaposin, PSA-NCAM, and NeuN. Prosaposin immunoreactivity was intense in the dentate gyrus at postnatal day 3 (P3) and P7, but decreased gradually after P14. In the dentate gyrus at P28, immature PSA-NCAM-positive neurons localized exclusively in the subgranular zone were prosaposin-negative, whereas mature Neu-N-positive neurons were positive for prosaposin. Furthermore, these prosaposin-negative immature neurons were saposin B-positive, suggesting that the neurons take up and degrade prosaposin. In situ hybridization assays showed that prosaposin in the adult dentate gyrus is dominantly the Pro+9 type, a secreted type of prosaposin. These results imply that prosaposin secreted from mature neurons stimulates proliferation and maturation of immature neurons in the dentate gyrus.

Prosaposin, a regulator of estrogen receptor alpha, promotes breast cancer growth

Prosaposin, a secreted protein, is a well-known pleiotropic growth factor. Although a previous report has indicated that prosaposin is overexpressed in breast cancer cell lines, the role of prosaposin in the development of breast cancer remains to be identified. Here, we first revealed that prosaposin upregulated estrogen receptor alpha expression, nuclear translocation and transcriptional activity by western blot, immunofluorescence assay and dual luciferase reporter gene assay, respectively. Furthermore, we demonstrated prosaposin upregulated estrogen receptor alpha expression through MAPK-signaling pathway using MAPK inhibitor. Proliferation assay and tumor xenograft experiments in nude mice (n = 6 per group) further confirmed prosaposin could promote breast cancer growth significantly in vitro and in vivo. These findings suggested that prosaposin might enhance estrogen receptor alpha-mediated signaling axis and play a role in breast cancer development and progression.

Chronological changes in prosaposin in the developing rat brain

Prosaposin is the precursor protein of four glycoproteins, saposins A, B, C, and D, which activate sphingolipid hydrolases in lysosomes. Besides this role, intact prosaposin is also known as a potent neurotrophic factor that prevents neuronal cell death and stimulates neurite outgrowth in in vivo and in vitro experiments. In the present study, we examined chronological changes in prosaposin immunoreactivity in the rat brain using immunofluorescence staining and Diaminobenzidine (DAB) immunohistochemistry. In the hippocampal regions CA1, CA3, and dentate gyrus, the strongest staining of prosaposin was observed on postnatal day 1. The prosaposin immunoreactivity then decreased gradually until postnatal day 28. But in the cerebral cortex, prosaposin staining intensity increased from postnatal day 1 to 14, then decreased until postnatal day 28. The prosaposin immunoreactivity co-localized with the lysosomal granules labeled by an anti-Cathepsin D antibody, indicating that prosaposin mainly localized in the lysosomes of the neurons. We also examined the chronological changes in prosaposin mRNA and its two alternatively spliced variants using in situ hybridization. We found that both the mRNA forms, especially the one without a nine-base insertion, increased significantly from embryonic day 15 to postnatal day 7, then decreased gradually until postnatal day 28. Abundant prosaposin expression in the perinatal stages indicates a potential role of prosaposin in the early development of the rat brain.

Proteomic analysis of synaptosomal protein expression reveals that cerebral ischemia alters lysosomal Psap processing

Cerebral ischemia (CI) induces dramatic changes in synaptic structure and function that precedes delayed post-ischemic neuronal death. Here, a proteomic analysis was used to identify the effects of focal CI on synaptosomal protein levels. Contralateral and ipsilateral synaptosomes, prepared from adult mice subjected to 60 min middle cerebral artery occlusion, were isolated following 3, 6 and 20 h of reperfusion. Synaptosomal protein samples (n=3) were labeled using the cleavable ICAT system prior to analysis with nanoLC-MS/MS. Each sample was analyzed by LC-MS to identify differential expressions using InDEPT software and differentially expressed peptides were identified by targeted LC-MS/MS. A total of 62 differentially expressed proteins were identified and Gene Ontology classification (cellular component) indicated that the majority of the proteins were located in the mitochondria and other components consistent with synaptic localization. The observed alterations in synaptic protein levels poorly correlated with gene expression, indicating the involvement of post-transcriptional regulatory mechanisms in determining post-ischemic synaptic protein content. Additionally, immunohistochemistry analysis of prosaposin (Psap) and saposin C (SapC) indicates that CI disrupts Psap processing and glycosphingolipid metabolism. These results demonstrate that the synapse is adversely affected by CI and may play a role in mediating post-ischemic neuronal viability.

An association analysis of murine anxiety genes in humans implicates novel candidate genes for anxiety disorders

BACKGROUND

Human anxiety disorders are complex diseases with largely unknown etiology. We have taken a cross-species approach to identify genes that regulate anxiety-like behavior with inbred mouse strains that differ in their innate anxiety levels as a model. We previously identified 17 genes with expression levels that correlate with anxiety behavior across the studied strains. In the present study, we tested their 13 known human homologues as candidate genes for human anxiety disorders with a genetic association study.

METHODS

We describe an anxiety disorder study sample derived from a Finnish population-based cohort and consisting of 321 patients and 653 carefully matched control subjects, all interviewed to obtain DSM-IV diagnoses. We genotyped altogether 208 single nucleotide polymorphisms (SNPs) (all non-synonymous SNPs, SNPs that alter potential microRNA binding sites, and gap-filling SNPs selected on the basis of HapMap information) from the investigated anxiety candidate genes.

RESULTS

Specific alleles and haplotypes of six of the examined genes revealed some evidence for association (p < or = .01). The most significant evidence for association with different anxiety disorder subtypes were: p = .0009 with ALAD (delta-aminolevulinate dehydratase) in social phobia, p = .009 with DYNLL2 (dynein light chain 2) in generalized anxiety disorder, and p = .004 with PSAP (prosaposin) in panic disorder.

CONCLUSIONS

Our findings suggest that variants in these genes might predispose to specific human anxiety disorders. These results illustrate the potential utility of cross-species approaches in identification of candidate genes for psychiatric disorders.

Molecular mechanism for neuro-protective effect of prosaposin against oxidative stress: its regulation of dimeric transcription factor formation

Prosaposin triggers G-protein-coupled receptor (GPCR)-mediated protein kinase B (Akt)/extracellular signal-regulated kinase (ERK) phosphorylation cascades to exert its neurotrophic and myelinotrophic activity capable of preventing neural cell death and promoting neural proliferation and glial differentiation. In the present study, we investigated the down-stream neurotrophic signaling mechanism of prosaposin by which rat pheochromocytoma (PC-12) cells are protected from cell death induced by oxidative stress. When PC-12 cells were exposed to H2O2, the cells underwent abrupt shrinkage followed by apoptosis. Prosaposin treatment at as low as 1 nM protected PC-12 cells from cell death by the oxidative stress with the activation of an ERK phosphorylation cascade. Simultaneously, prosaposin blocked the oxidative stress induced-Akt phosphorylation that acts on the down-stream of caspase-3 activation. A MEK inhibitor, PD98059, or a phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, abolished the survival effect of prosaposin on the oxidative stress-induced cell death. Furthermore, prosaposin blocked the oxidative stress-induced phosphorylations of c-Jun N-terminal kinase (JNK) and p38 stress-activated protein kinase. We further investigated the effect of prosaposin treatment on the phosphorylation of activating protein-1 (AP-1) complex components, c-Jun and activating transcription factor (ATF)-3. Western blot analysis demonstrated that prosaposin treatment at 100 ng/ml decreased the levels of c-Jun and ATF-3 induced by H2O2 stimulation. Our results suggest that prosaposin aids survival of PC-12 cells from oxidative stress not only by reducing the phosphorylation levels of JNK and p38, but also by regulating the c-Jun/AP-1 pathway.

Expression patterns in alternative splicing forms of prosaposin mRNA in the rat facial nerve nucleus after facial nerve transection

Prosaposin acts as a neurotrophic factor, in addition to its role as the precursor protein for saposins A, B, C, and D, which are activators for specific sphingolipid hydrolases in lysosomes. In rats, the prosaposin gene generates two alternative splicing forms of mRNA: Pro+9 containing a 9-base insertion and Pro+0 without. The expression of these mRNAs changes after brain injury. We examined the expression patterns of the alternative splicing forms of prosaposin mRNA in the rat facial nerve nucleus for 52 days following facial nerve transection. Pro+0 mRNA increased within 3 days of transection, peaked after 5-10 days, and remained significantly elevated for 21 days. In contrast, the expression of Pro+9 mRNA was constant throughout the regenerative period. Prosaposin mRNA expression increased not only in facial motoneurons, but also in microglia during facial nerve regeneration. Our findings indicate that the saposin B domain of prosaposin, which is the domain affected by alternative splicing, plays an important role in both neurons and microglia during neuroregeneration.

Distribution of prosaposin in the rat nervous system

Prosaposin is the precursor of four sphingolipid activator proteins (saposins A, B, C, and D) for lysosomal hydrolases and is abundant in the nervous system and muscle. In addition to its role as a precursor of saposins in lysosomes, intact prosaposin has neurotrophic effects in vivo or in vitro when supplied exogenously. We examined the distribution of prosaposin in the central and peripheral nervous systems and its intracellular distribution. Using a monospecific antisaposin D antibody that crossreacts with prosaposin but not with saposins A, B, or C, immunoblot experiments showed that both the central and peripheral nervous systems express unprocessed prosaposin and little saposin D. Using the antisaposin D antibodies, we demonstrated that prosaposin is abundant in almost all neurons of both the central and peripheral nervous systems, including autonomic nerves, as well as motor and sensory nerves. Immunoelectron microscopy using double staining with antisaposin D and anticathepsin D antibodies showed strong prosaposin immunoreactivity mainly in the lysosomal granules in the neurons in both the central and peripheral nervous systems. The expression of prosaposin mRNA, examined using in situ hybridization, was observed in these same neurons. Our results suggest that prosaposin is synthesized ubiquitously in neurons of both the central and peripheral nervous systems.

Neurolysosomal pathology in human prosaposin deficiency suggests essential neurotrophic function of prosaposin

A neuropathologic study of three cases of prosaposin (pSap) deficiency (ages at death 27, 89 and 119 days), carried out in the standard autopsy tissues, revealed a neurolysosomal pathology different from that in the non-neuronal cells. Non-neuronal storage is represented by massive lysosomal accumulation of glycosphingolipids (glucosyl-, galactosyl-, lactosyl-, globotriaosylceramides, sulphatide, and ceramide). The lysosomes in the central and peripheral neurons were distended by pleomorphic non-lipid aggregates lacking specific staining and autofluorescence. Lipid storage was borderline in case 1, and at a low level in the other cases. Neurolysosomal storage was associated with massive ubiquitination, which was absent in the non-neuronal cells and which did not display any immunohistochemical aggresomal properties. Confocal microscopy and cross-correlation function analyses revealed a positive correlation between the ubiquitin signal and the late endosomal/lysosomal markers. We suppose that the neuropathology most probably reflects excessive influx of non-lipid material (either in bulk or as individual molecules) into the neurolysosomes. The cortical neurons appeared to be uniquely vulnerable to pSap deficiency. Whereas in case 1 they populated the cortex, in cases 2 and 3 they had been replaced by dense populations of both phagocytic microglia and astrocytes. We suggest that this massive neuronal loss reflects a cortical neuronal survival crisis precipitated by the lack of pSap. The results of our study may extend the knowledge of the neurotrophic function of pSap, which should be considered essential for the survival and maintenance of human cortical neurons.

Progressive deafness and altered cochlear innervation in knock-out mice lacking prosaposin

After a yeast two-hybrid screen identified prosaposin as a potential interacting protein with the nicotinic acetylcholine receptor (nAChR) subunit alpha10, studies were performed to characterize prosaposin in the normal rodent inner ear. Prosaposin demonstrates diffuse organ of Corti expression at birth, with gradual localization to the inner hair cells (IHCs) and its supporting cells, inner pillar cells, and synaptic region of the outer hair cells (OHCs) and Deiters' cells (DCs) by postnatal day 21 (P21). Microdissected OHC and DC quantitative reverse transcriptase-PCR and immunohistology localizes prosaposin mRNA to DCs and OHCs, and protein predominantly to the apex of the DCs. Subsequent studies in a prosaposin knock-out (KO) (-/-) mouse showed intact but slightly reduced hearing through P19, but deafness by P25 and reduced distortion product otoacoustic emissions from P15 onward. Beginning at P12, the prosaposin KO mice showed histologic organ of Corti changes including cellular hypertrophy in the region of the IHC and greater epithelial ridge, a loss of OHCs from cochlear apex, and vacuolization of OHCs. Immunofluorescence revealed exuberant overgrowth of auditory afferent neurites in the region of the IHCs and proliferation of auditory efferent neurites in the region of the tunnel of Corti. IHC recordings from these KO mice showed normal I-V curves and responses to applied acetylcholine. Together, these results suggest that prosaposin helps maintain normal innervation patterns to the organ of Corti. Furthermore, prosaposin's overlapping developmental expression pattern and binding capacity toward the nAChR alpha10 suggest that alpha10 may also play a role in this function.

Shedding and uptake of gangliosides and glycosylphosphatidylinositol-anchored proteins

Gangliosides and glycosylphosphatidylinositol (GPI)-anchored proteins have very different biosynthetic origin, but they have one thing in common: they are both comprised of a relatively large hydrophilic moiety tethered to a membrane by a relatively small lipid tail. Both gangliosides and GPI-anchored proteins can be actively shed from the membrane of one cell and taken up by other cells by insertion of their lipid anchors into the cell membrane. The process of shedding and uptake of gangliosides and GPI-anchored proteins has been independently discovered in several disciplines during the last few decades, but these discoveries were largely ignored by people working in other areas of science. By bringing together results from these, sometimes very distant disciplines, in this review, we give an overview of current knowledge about shedding and uptake of gangliosides and GPI-anchored proteins. Tumor cells and some pathogens apparently misuse this process for their own advantage, but its real physiological functions remain to be discovered.

The exon 8-containing prosaposin gene splice variant is dispensable for mouse development, lysosomal function, and secretion

Prosaposin is a multifunctional protein with diverse functions. Intracellularly, prosaposin is a precursor of four sphingolipid activator proteins, saposins A to D, which are required for hydrolysis of sphingolipids by several lysosomal exohydrolases. Secreted prosaposin has been implicated as a neurotrophic, myelinotrophic, and myotrophic factor as well as a spermatogenic factor. It has also been implicated in fertilization. The human and the mouse prosaposin gene has a 9-bp exon (exon 8) that is alternatively spliced, resulting in an isoform with three extra amino acids, Gln-Asp-Gln, within the saposin B domain. Alternative splicing in the prosaposin gene is conserved from fish to humans, tissue specific, and regulated in the brain during development and nerve regeneration-degeneration processes. To elucidate the physiological role of alternative splicing, we have generated a mouse lacking exon 8 by homologous recombination. The exon 8 prosaposin mutant mice are healthy and fertile with no obvious phenotype. No changes were detected in prosaposin secretion or in accumulation and metabolism of gangliosides, sulfatides, neutral glycosphingolipids, neutral phospholipids, other neutral lipids, and ceramide. These data strongly indicate that the prosaposin variant containing the exon 8-encoded three amino acids is dispensable for normal mouse development and fertility as well as for prosaposin secretion and its lysosomal function, at least in the presence of the prosaposin variant missing the exon 8-encoded three amino acids.

Changes in expression of prosaposin in the rat facial nerve nucleus after facial nerve transection

Prosaposin is the precursor of saposins A, B, C and D, which are activators of sphingolipid hydrolases. In addition, unprocessed prosaposin functions as a neurotrophic factor in the central and peripheral nervous systems by acting to prevent neuronal apoptosis, to elongate neurites and to facilitate myelination. In this study, the expression pattern of prosaposin in the facial nerve nucleus after facial nerve transection was examined by immunohistochemistry and in situ hybridization. Prosaposin immunoreactivity in the neurons on the operated side facial nerve nucleus showed a biphasic pattern: it was significantly increased on day 3 after transection, decreased dramatically on day 7, started to increase gradually on day 14 and reached another peak on day 21 after transection. Significant increases in the levels of prosaposin mRNA were identified in the neurons on the operated side, suggesting that prosaposin was synthesized vigorously by the neurons themselves in the case of facial nerve transection. The diverse changes in prosaposin immunoreactivity during the process of facial nerve regeneration may reflect the diverse neurotrophic activities of prosaposin in facial motoneurons.

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.

Hippocampal prosaposin changes during stress: a glucocorticoid-independent event

Several studies indicate that stress can produce remarkable effects on neurotrophic factors. In this regard, hippocampus is the most interesting structure of the brain because of its broad involvement in behavioral and neuroendocrine phenomena. In the present study, we investigated the effect of stress on hippocampal prosaposin, which is known to act as a neurotrophic and neuroprotective factor. Rats subjected to restraint stress (120 min) had a significant and transient reduction of hippocampal, but not hypothalamic, prosaposin full-length protein. Indeed, when this stressful stimulus was applied daily for 3 days, no differences were detected in comparison with naive rats. To investigate the role of glucocorticoids in the stress-induced decrease in hippocampal prosaposin, adrenalectomized and corticosterone-treated rats were studied. The results indicate that adrenalectomized rats behave as intact animals. This finding indicates that the absence of endogenous corticosterone does not prevent a decrease in hippocampal prosaposin. When an increase of corticosterone was achieved through exogenous administration, hippocampal prosaposin concentrations were unchanged in comparison with vehicle-injected (sesame oil) rats. These results led to the conclusion that stress, not via an increase of glucocorticoid hormone, transiently reduces hippocampal prosaposin levels. This phenomenon is followed by rapid recovery of the neurotrophin level, even when the stress stimulus persists.

Regulation of gene expression in response to brain injury: enhanced expression and alternative splicing of rat prosaposin (SGP-1) mRNA in injured brain

Prosaposin, the precursor of saposins or saps, is an injury-repair protein that acts on both neurons and glia. Previous studies identified the prosaposin gene as one of differentially expressed genes following nerve injury. In the present study, we investigated expression of prosaposin mRNA in injured brain utilizing rat models of focal cerebral ischemia and cortical stab wound in order to explore the significance of prosaposin in nerve injury. In ischemic brain, the level of prosaposin mRNA was elevated greater than 400% over controls within 5 days after ischemic insults. Importantly, this induction was accompanied by a 9-base splicing consistent with the alternative Exon-8 splicing of human prosaposin mRNA. In normal brain, two prosaposin mRNA species with and without the 9-base insertion were expressed at a ratio of 85:15; however, this equilibrium reverted to 5:95 following ischemic injury. Similar inductions were observed in stab wound brains. Immunohistochemical staining and in situ hybridization demonstrated an enhanced signal distribution of prosaposin mRNA and injury-induced prosaposin protein around the lesion. The data suggest the expression and processing of prosaposin mRNA may be crucially regulated not only for cerebral homeostasis but also during nerve regenerative and degenerative processes.

Prosaptide exacerbates ischemia-induced behavioral deficits in vivo; an effect that does not involve mitogen-activated protein kinase activation

Prosaposin is a 517 amino acid membrane component and secreted protein(5,7,9) that is proteolytically cleaved to generate the four small glycoproteins; saposins A, B, C and D.(9,13,19) Prosaposin's ability to promote neurite outgrowth(31) and to protect neurons from programmed cell death(28) in vitro, as well as to rescue neurons from ischemia and other damage in vivo(11,12,15,25) implied that prosaposin was neurotrophic/neuroprotectant.(1,7,24,31) The neurotrophic sequence of prosaposin was isolated to smaller peptide fragments termed prosaptides(15,31) within the amino terminal portion of saposin C.(1,6,8,10,17,20,21,28) The proposed use of synthetic prosaptides as peripherally administered neuroprotective and/or neurotrophic therapeutic agents has stemmed from their ability to cross the blood-brain barrier,(27) as well as their reported neurotrophic activity in vitro.(15,23,31) Few studies, however, have attempted to characterize these peptides, presumably due to their reported instability following peripheral administration.(27) With the recent design of a stable 11-mer retro-inverso prosaptide,(15,31) it has become feasible to investigate the pharmacological effects of a stable version of these peptides in the validated rabbit spinal cord ischemia model that has been used extensively in the development of therapeutics to treat ischemic stroke.(4,14,16,18) Our results show not only that prosaptide was not neurotrophic/neuroprotectant in vivo, but rather it worsened ischemia-induced behavioral deficits.

Prosaposin treatment induces PC12 entry in the S phase of the cell cycle and prevents apoptosis: activation of ERKs and sphingosine kinase

We report that prosaposin treatment induced extracellular signal-regulated kinases (ERKs) and sphingosine kinase activity, increased DNA synthesis, and prevented cell apoptosis. Prosaposin treatment induced pheochromocytoma cells (PC12) to enter the S phase of the cell cycle; this effect was inhibited by the MEK inhibitor PD98059, indicating that prosaposin-induced ERK phosphorylation is required for stimulation of DNA synthesis. The prosaposin effect was also inhibited by pertussis toxin, indicating that the prosaposin receptor is a G-protein-coupled receptor. Prosaposin rescued PC12 cells from apoptosis induced by staurosporine or ceramide. Sphingosine kinase activity was increased by prosaposin treatment. We propose that this effect is a mechanism underlying the proliferative and anti-apoptotic functions of prosaposin. Prosaposin appears to be a key regulatory factor in the ceramide-S-1-P rheostat, which regulates cell fate.

Prosaposin is immunolocalized to muscle and prosaptides promote myoblast fusion and attenuate loss of muscle mass after nerve injury

Prosaposin is the precursor of the saposins and has both neurotrophic and myelinotrophic activity in vitro and in vivo. Using an antibody specific for the holoprotein, an immunocytochemical survey demonstrated intense staining of adult rat skeletal, cardiac, and smooth muscle cells. Prosaposin immunoreactivity in muscle appears dependent on innervation, as denervated adult rat skeletal muscles showed decreased immunostaining that returned to normal levels after reinnervation. TX14(A), a peptide derived from the neurotrophic sequence of prosaposin, attenuated the decline in muscle mass loss following nerve injury induced by a constricting ligature. In vitro, both L6 myoblasts and primary chick-embryo myoblasts showed similar prosaposin immunopositivity, mainly in myotubes. TX14(A) induced a threefold increase in L6 myoblast fusion during early stages of differentiation without affecting cell proliferation. The fusion process was decreased in vitro in a dose-dependent fashion by addition of a neutralizing anti-prosaposin antibody. These data suggest that, in addition to neurotrophic and myelinotrophic activities, prosaposin has myotrophic properties.

Secretion of prosaposin, a multifunctional protein, by breast cancer cells

Western blotting and immunodetection with three antibodies were used to probe conditioned media of breast cancer cells (MDA231, MDA435, MCF-7) for prosaposin, a lysosomal protein that occurs in milk. It was readily detected in media from these cells, and from that of an sv40-transformed mammary epithelial cell, HBL100, but not from medium of human neural tumor cells (SK-N-MC). In cultures of MCF-7 cells, the prosaposin pattern of secretion over time closely resembled that of procathepsin D, another lysosomal protein occurring in milk. Supplementing medium with 17beta-estradiol (0. 1-100 nM) dose dependently increased secretion of both proteins after 48 h without changes in cell viability. The influence of 17beta-estradiol on secretion could play a role in the trophic activity of prosaposin in cellular differentiation and cell death protection. In concert with other lysosomal proteins in the tumor environment, such as procathepsin D, prosaposin may be a factor in eliminating barriers to tumor metastasis by facilitating hydrolysis of membrane glycolipids. The number of milk proteins known to be secreted by breast cancer cells is growing. There is evidence that at least some of these may be secreted in an endocrine manner in the normal, non-lactating breast.

Sphingolipid activator proteins are required for epidermal permeability barrier formation

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

Prosaposin receptor: evidence for a G-protein-associated receptor

Prosaposin, the precursor of sphingolipid activator protein (saposins A-D), has been identified as a neurotrophic factor capable of inducing neural differentiation and preventing cell death. The putative prosaposin receptor was partially purified from baboon brain membranes by affinity chromatography using a saposin C-column. The purified preparation gave a single major protein band with an apparent molecular weight of 54 kDa on SDS-PAGE. Affinity cross-linking of 11 kDa 125I-saposin C demonstrated the presence of a 66 kDa product, indicative of an apparent molecular weight of 55 kDa for the receptor. A GTP gamma S-binding assay using cell membranes from SHSY5Y neural cells demonstrated agonist stimulated binding of [35S]-GTP gamma S upon treatment with prosaptide TX14(A) a peptide from the neurotrophic region; maximal binding was obtained at 2 nM. TX14(A) stimulated binding was abolished by prior treatment of SHSY5Y cells with pertussis toxin and by a scrambled and an all D-amino acid-derivative of the 14-mer. A 14-mer mutant prosaptide (6N-->6D) competed with TX14(A) with a Ki of 0.7 nM. Immunoblot analysis using an antibody against the G0 alpha subunit demonstrated that the purified receptor preparation contained a 40 kDa reactive band consistent with association of G0 alpha and the receptor. These findings indicate that the signaling induced by prosaposin and TX14(A) is generated by binding to a G0-protein associated receptor.

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.

Prosaposin, a neurotrophic factor: presence and properties in milk

The presence of prosaposin, the precursor of the sphingolipid activator proteins (saposins A, B, C, and D), was investigated in bovine milk. The milk proteins were resolved by SDS-PAGE, blotted onto nitrocellulose sheets, and immunostained. Each of three appropriate antibodies defined a band from milk that matched in mobility the reference prosaposin from human milk at a relative molecular mass of 66,000. Evidence of mature saposins was not found. Prosaposin was detected in milk of other species chimpanzee, rhesus, goat, and rat) and was consistently observed in samples of retail milk and from individual cows. Prosaposin was not associated with particulate matter (fat globules, casein micelles, membrane fragments, and somatic cells) in either human or bovine milk. Rather, prosaposin was located exclusively in the milk serum (whey), existing in monomeric form, as revealed by nondenaturing PAGE. A commercial whey protein concentrate (75% protein) appeared to retain milk prosaposin quantitatively. Properties that were useful in the isolation of prosaposin from milk were its binding to concanavalin A, retention by anion-exchange cellulose, and resistance to precipitation by heating. The possibility that bovine milk prosaposin nutritionally benefits the humans who consume it is enhanced by the fact that only part of its saposin C segment is required for neurotrophic activity.

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.