Coding of two sphingolipid activator proteins (SAP-1 and SAP-2) by same genetic locus

Regulation of cellular and systemic sphingolipid homeostasis

One hundred and fifty years ago, Johann Thudichum described sphingolipids as unusual "Sphinx-like" lipids from the brain. Today, we know that thousands of sphingolipid molecules mediate many essential functions in embryonic development and normal physiology. In addition, sphingolipid metabolism and signalling pathways are dysregulated in a wide range of pathologies, and therapeutic agents that target sphingolipids are now used to treat several human diseases. However, our understanding of sphingolipid regulation at cellular and organismal levels and their functions in developmental, physiological and pathological settings is rudimentary. In this Review, we discuss recent advances in sphingolipid pathways in different organelles, how secreted sphingolipid mediators modulate physiology and disease, progress in sphingolipid-targeted therapeutic and diagnostic research, and the trans-cellular sphingolipid metabolic networks between microbiota and mammals. Advances in sphingolipid biology have led to a deeper understanding of mammalian physiology and may lead to progress in the management of many diseases.

Protein engineering using circular permutation - structure, function, stability, and applications

Protein engineering is important for creating novel variants from natural proteins, enabling a wide range of applications. Approaches such as rational design and directed evolution are routinely used to make new protein variants. Computational tools like de novo design can introduce new protein folds. Expanding the amino acid repertoire to include unnatural amino acids with non-canonical side chains in vitro by native chemical ligation and in vivo via codon expansion methods broadens sequence and structural possibilities. Circular permutation (CP) is an invaluable approach to redesigning a protein by rearranging the amino acid sequence, where the connectivity of the secondary structural elements is altered without changing the overall structure of the protein. Artificial CP proteins (CPs) are employed in various applications such as biocatalysis, sensing of small molecules by fluorescence, genome editing, ligand-binding protein switches, and optogenetic engineering. Many studies have shown that CP can lead to either reduced or enhanced stability or catalytic efficiency. The effects of CP on a protein's energy landscape cannot be predicted a priori. Thus, it is important to understand how CP can affect the thermodynamic and kinetic stability of a protein. In this review, we discuss the discovery and advancement of techniques to create protein CP, and existing reviews on CP. We delve into the plethora of biological applications for designed CP proteins. We subsequently discuss the experimental and computational reports on the effects of CP on the thermodynamic and kinetic stabilities of proteins of various topologies. An understanding of the various aspects of CP will allow the reader to design robust CP proteins for their specific purposes.

Expression patterns of prosaposin and its receptors, G protein-coupled receptor (GPR) 37 and GPR37L1, in the mouse olfactory organ

Prosaposin is a glycoprotein conserved widely in vertebrates, because it is a precursor for saposins that are required for normal lysosomal function and thus for autophagy, and acts as a neurotrophic factor. Most tetrapods possess two kinds of olfactory neuroepithelia, namely, the olfactory epithelium (OE) and the vomeronasal epithelium (VNE). This study examined the expression patterns of prosaposin and its candidate receptors, G protein-coupled receptor (GPR) 37 and GPR37L1, in mouse OE and VNE by immunofluorescence and in situ hybridization. Prosaposin immunoreactivity was observed in the olfactory receptor neurons, vomeronasal receptor neurons, Bowman's gland (BG), and Jacobson's gland (JG). Prosaposin expression was mainly observed in mature neurons. Prosaposin mRNA expression was observed not only in these cells but also in the apical region of the VNE. GPR37 and GPR37L1 immunoreactivities were found only in the BG and/or the JG. Prosaposin was suggested to secrete and facilitate the autophagic activities of the neurons and modulate the mucus secretion in mouse olfactory organ.

A saposin domain-containing protein of tongue sole Cynoglossus semilaevis: Antimicrobial activity and mechanism

Prosaposin is a precursor that can be processed into four different saposins, designated as A, B, C, and D, which have multiple functions in mammals, including neuroprotection and immune modulation. The immune function of saposin in teleost remains largely unknown. In the present study, a saposin (SAP) domain-containing protein was identified in half-smooth tongue sole Cynoglossus semilaevis and named CsSDP. CsSDP harbors one SAP A domain and two SAP B domains. When expressed in HEK293T cells, CsSDP was specifically localized in the lysosome. When overexpressed in Escherichia coli, CsSDP markedly inhibited bacterial growth, and the inhibitory effect depended on two specific regions in the SAP A and SAP B domains. Two polypeptides (P32 and P30) derived from the above SAP A and B domains could bind to and inhibit the growth of both Gram-negative and Gram-positive bacteria. The ultrastructural analysis revealed that P32 and P30 killed target bacteria by disrupting the bacterial cell wall and inducing substantial release of cytoplasmic contents. These results shed new lights on the immune function of saposin domain-containing protein in teleost.

The Biology of Lysosomes: From Order to Disorder

Since its discovery in 1955, the understanding of the lysosome has continuously increased. Once considered a mere waste removal system, the lysosome is now recognised as a highly crucial cellular component for signalling and energy metabolism. This notable evolution raises the need for a summarized review of the lysosome's biology. As such, throughout this article, we will be compiling the current knowledge regarding the lysosome's biogenesis and functions. The comprehension of this organelle's inner mechanisms is crucial to perceive how its impairment can give rise to lysosomal disease (LD). In this review, we highlight some examples of LD fine-tuned mechanisms that are already established, as well as others, which are still under investigation. Even though the understanding of the lysosome and its pathologies has expanded through the years, some of its intrinsic molecular aspects remain unknown. In order to illustrate the complexity of the lysosomal diseases we provide a few examples that have challenged the established single gene-single genetic disorder model. As such, we believe there is a strong need for further investigation of the exact abnormalities in the pathological pathways in lysosomal disease.

G protein-coupled receptor GPR37-like 1 regulates adult oligodendrocyte generation

Oligodendrocytes (OLs) continue to be generated from OL precursors (OPs) in the adult mammalian brain. Adult-born OLs are believed to contribute to neural plasticity, learning and memory through a process of "adaptive myelination," but how adult OL generation and adaptive myelination are regulated remains unclear. Here, we report that the glia-specific G protein-coupled receptor 37-like 1 (GPR37L1) is expressed in subsets of OPs and newly formed immature OLs in adult mouse brain. We found that OP proliferation and differentiation are inhibited in the corpus callosum of adult Gpr37l1 knockout mice, leading to a reduction in the number of adult-born OLs. Our data raise the possibility that GPR37L1 is mechanistically involved in adult OL generation and adaptive myelination, and suggest that GPR37L1 might be a useful functional marker of OPs that are committed to OL differentiation.

Prosaposin mediates inflammation in atherosclerosis

Macrophages play a central role in the pathogenesis of atherosclerosis. The inflammatory properties of these cells are dictated by their metabolism, of which the mechanistic target of rapamycin (mTOR) signaling pathway is a key regulator. Using myeloid cell-specific nanobiologics in apolipoprotein E-deficient (Apoe -/-) mice, we found that targeting the mTOR and ribosomal protein S6 kinase-1 (S6K1) signaling pathways rapidly diminished plaque macrophages' inflammatory activity. By investigating transcriptome modifications, we identified Psap, a gene encoding the lysosomal protein prosaposin, as closely related with mTOR signaling. Subsequent in vitro experiments revealed that Psap inhibition suppressed both glycolysis and oxidative phosphorylation. Transplantation of Psap -/- bone marrow to low-density lipoprotein receptor knockout (Ldlr -/-) mice led to a reduction in atherosclerosis development and plaque inflammation. Last, we confirmed the relationship between PSAP expression and inflammation in human carotid atherosclerotic plaques. Our findings provide mechanistic insights into the development of atherosclerosis and identify prosaposin as a potential therapeutic target.

Prosaposin, a neurotrophic factor, protects neurons against kainic acid-induced neurotoxicity

Prosaposin (PS) is the precursor of four sphingolipid activator proteins, saposin A-D. PS is both a precursor protein and a neuroprotective factor, and is up-regulated in response to excitotoxicity induced by kainic acid (KA), a glutamate analogue. Excess glutamate release induces neuropathological disorders such as ischemia and seizure. Our group's research revealed that PS immunoreactivity (IR) increased significantly in the hippocampal and cortical neurons on day 3 after KA injection, and high PS levels were maintained even after 3 weeks. The increase in PS, but not saposins, as detected by immunoblotting, suggests that the increase in PS-IR after KA injection was not caused by an increase in saposins acting as lysosomal enzymes after neuronal damage but, rather, by an increase in PS as a neurotrophic factor to improve neuronal survival. An 18-mer peptide (PS18) derived from the PS neurotrophic region significantly protected hippocampal neurons against KA-induced destruction. Furthermore, parvalbumin-positive GABAergic inhibitory interneurons and their axons exhibited intense PS expression. These results suggest that axonally transported PS protects damaged hippocampal pyramidal neurons from KA-induced neurotoxicity. Further in vitro studies that include the transfection of the PS gene will help with clarifying the mechanisms underlying the transport and secretion of PS.

Prosaposin and its receptors GRP37 and GPR37L1 show increased immunoreactivity in the facial nucleus following facial nerve transection

Neurotrophic factor prosaposin (PS) is a precursor for saposins A, B, C, and D, which are activators for specific sphingolipid hydrolases in lysosomes. Both saposins and PS are widely contained in various tissues. The brain, skeletal muscle, and heart cells predominantly contain unprocessed PS rather than saposins. PS and PS-derived peptides stimulate neuritogenesis and increase choline acetyltransferase activity in neuroblastoma cells and prevent programmed cell death in neurons. We previously detected increases in PS immunoactivity and its mRNA in the rat facial nucleus following facial nerve transection. PS mRNA expression increased not only in facial motoneurons, but also in microglia during facial nerve regeneration. In the present study, we examined the changes in immunoreactivity of the PS receptors GPR37 and GPR37L1 in the rat facial nucleus following facial nerve transection. Following facial nerve transection, many small Iba1- and glial fibrillary acidic protein (GFAP)-positive cells with strong GPR37L1 immunoreactivity, including microglia and astrocytes, were observed predominately on the operated side. These results indicate that GPR37 mainly works in neurons, whereas GPR37L1 is predominant in microglia or astrocytes, and suggest that increased PS in damaged neurons stimulates microglia or astrocytes via PS receptor GPR37L1 to produce neurotrophic factors for neuronal recovery.

Expression of the G protein-coupled receptor (GPR) 37 and GPR37L1 in the mouse digestive system

G protein-coupled receptor (GPR) 37 and GPR37L1 are known to modulate the dopaminergic neuron activity, and recently, they are identified as candidate prosaposin receptors. Intercellular prosaposin is proteolytically processed into four saposins, each of which acts as a sphingolipid hydrolase activator in the lysosome. In contrast, extracellular prosaposin exerts a trophic effect on neurons via GPR37 and GPR37L1. In this study, the expression patterns of GPR37 and GPR37L1 in the mouse digestive system were examined immunohistochemically. The islets of Langerhans of the pancreas showed intense immunoreactivity for GPR37 and GPR37L1. Weak immunoreactivity for GPR37 and GPR37L1 was found in the nerve plexuses of the esophagus and small and large intestines. Colocalization of GPR37 and tyrosine hydroxylase immunoreactivity was observed in the neuron of the nerve plexus of the large intestine. This study suggests the possibility that prosaposin affects the function of islet-secreting cells. Also, the expression of GPR37 and GPR37L1 in the nerve plexus suggests that prosaposin exerts a trophic effect not only in the central nervous system, but also in the enteric nervous system.

Hypoxia-ischemia alters distribution of lysosomal proteins in rat cortex and hippocampus

Neuronal excitotoxicity induced by glutamatergic receptor overstimulation contributes to brain damage. Recent studies have shown that lysosomal membrane permeabilization (LMP) is involved in ischemia-associated neuronal death. In this study we evaluated the effect of neonatal hypoxia-ischemia (HI), as a model of excitotoxicity, on the lysosomal integrity throughout the distribution of the lysosomal proteins cathepsin D and prosaposin. Rat pups (7 days old) of the Wistar Kyoto strain were submitted to HI and they were euthanized 4 days after treatment and the cerebral cortex (Cx) and hippocampus (HIP) were processed for immunohistochemistry or immunoblotting. Treatment induced an increase of gliosis and also a redistribution of both prosaposin and cathepsin D (as intermediate and mature forms), into the cytosol of the HIP and Cx. In addition, HI induced a decrease of LAMP-1 in the membranous fraction and the appearance of a reactive band to anti-LAMP-1 in the cytosolic fraction, suggesting a cleavage of this protein. From these results, we propose that the abnormal release of Cat D and PSAP to the cytosol is triggered as a result of LAMP-1 cleavage in HI animals, which leads to cell damage. This could be a common mechanism in pathological conditions that compromises neuronal survival and brain function.

Summary: Hypoxia ischemia (HI) induces an increase of gliosis and redistribution of prosaposin and cathepsin D into the cytosol of rat hippocampus. This could be triggered by LAMP-1 cleavage in HI.

Interneurons secrete prosaposin, a neurotrophic factor, to attenuate kainic acid-induced neurotoxicity

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PS increased mainly in the axons of PV positive interneurons after kainic acid (KA) injection.

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Electron microscopy revealed PS containing vesicles in PV positive axons.

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PS is secreted with secretogranin from synapses.

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The increased PS in the interneurons was due to increases in PS + 0, as in the choroid plexus.

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Interneurons produce and secrete intact PS around the hippocampal pyramidal neurons to protect them from KA neurotoxicity.

Prosaposin (PS) is a secretory neurotrophic factor, as well as a regulator of lysosomal enzymes. We previously reported the up-regulation of PS and the possibility of its axonal transport by GABAergic interneurons after exocitotoxicity induced by kainic acid (KA), a glutamate analog. In the present study, we performed double immunostaining with PS and three calcium binding protein markers: parvalbumin (PV), calbindin, and calretinin, for the subpopulation of GABAergic interneurons, and clarified that the increased PS around the hippocampal pyramidal neurons after KA injection existed mainly in the axons of PV positive interneurons. Electron microscopy revealed PS containing vesicles in the PV positive axon. Double immunostaining with PS and secretogranin or synapsin suggested that PS is secreted with secretogranin from synapses. Based on the results from in situ hybridization with two alternative splicing forms of PS mRNA, the increase of PS in the interneurons was due to the increase of PS + 0 (mRNA without 9-base insertion) as in the choroid plexus, but not PS + 9 (mRNA with 9-base insertion). These results were similar to those from the choroid plexus, which secretes an intact form PS + 0 to the cerebrospinal fluid. Neurons, especially PV positive GABAergic interneurons, produce and secrete the intact form of PS around hippocampal pyramidal neurons to protect them against KA neurotoxicity.

Lysosomal storage diseases

Lysosomes are cytoplasmic organelles that contain a variety of different hydrolases. A genetic deficiency in the enzymatic activity of one of these hydrolases will lead to the accumulation of the material meant for lysosomal degradation. Examples include glycogen in the case of Pompe disease, glycosaminoglycans in the case of the mucopolysaccharidoses, glycoproteins in the cases of the oligosaccharidoses, and sphingolipids in the cases of Niemann-Pick disease types A and B, Gaucher disease, Tay-Sachs disease, Krabbe disease, and metachromatic leukodystrophy. Sometimes, the lysosomal storage can be caused not by the enzymatic deficiency of one of the hydrolases, but by the deficiency of an activator protein, as occurs in the AB variant of GM2 gangliosidosis. Still other times, the accumulated lysosomal material results from failed egress of a small molecule as a consequence of a deficient transporter, as in cystinosis or Salla disease. In the last couple of decades, enzyme replacement therapy has become available for a number of lysosomal storage diseases. Examples include imiglucerase, taliglucerase and velaglucerase for Gaucher disease, laronidase for Hurler disease, idursulfase for Hunter disease, elosulfase for Morquio disease, galsulfase for Maroteaux-Lamy disease, alglucosidase alfa for Pompe disease, and agalsidase alfa and beta for Fabry disease. In addition, substrate reduction therapy has been approved for certain disorders, such as eliglustat for Gaucher disease. The advent of treatment options for some of these disorders has led to newborn screening pilot studies, and ultimately to the addition of Pompe disease and Hurler disease to the Recommended Uniform Screening Panel (RUSP) in 2015 and 2016, respectively.

Deciphering alternative splicing and nonsense-mediated decay modulate expression in primary lymphoid tissues of birds infected with avian pathogenic E. coli (APEC)

Background

Avian pathogenic E. coli (APEC) can lead to a loss in millions of dollars in poultry annually because of mortality and produce contamination. Studies have verified that many immune-related genes undergo changes in alternative splicing (AS), along with nonsense mediated decay (NMD), to regulate the immune system under different conditions. Therefore, the splicing profiles of primary lymphoid tissues with systemic APEC infection need to be comprehensively examined.

Results

Gene expression in RNAseq data were obtained for three different immune tissues (bone marrow, thymus, and bursa) from three phenotype birds (non-challenged, resistant, and susceptible birds) at two time points. Alternative 5′ splice sites and exon skipping/inclusion were identified as the major alternative splicing events in avian primary immune organs under systemic APEC infection. In this study, we detected hundreds of differentially-expressed-transcript-containing genes (DETs) between different phenotype birds at 5 days post-infection (dpi). DETs, PSAP and STT3A, with NMD have important functions under systemic APEC infection. DETs, CDC45, CDK1, RAG2, POLR1B, PSAP, and DNASE1L3, from the same transcription start sites (TSS) indicate that cell death, cell cycle, cellular function, and maintenance were predominant in host under systemic APEC.

Conclusions

With the use of RNAseq technology and bioinformatics tools, this study provides a portrait of the AS event and NMD in primary lymphoid tissues, which play critical roles in host homeostasis under systemic APEC infection. According to this study, AS plays a pivotal regulatory role in the immune response in chicken under systemic APEC infection via either NMD or alternative TSSs. This study elucidates the regulatory role of AS for the immune complex under systemic APEC infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-017-0488-4) contains supplementary material, which is available to authorized users.

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

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

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Drosophila model of PSD recapitulates neurodegenerative phenotype of human PSD.

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

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

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

Role of saposin C and D in auditory and vestibular function

OBJECTIVES/HYPOTHESIS

Saposins are small proteins derived from a precursor protein, prosaposin. Each of the four saposins (A-D) is necessary for the activity of lysosomal glycosphingolipid hydrolases. Individual saposin mutations lead to lysosomal storage diseases, some of which are associated with hearing loss. Here we evaluate the effects of the loss of saposins C and D on auditory and vestibular function in transgenic mice.

METHODS

Transgenic mice with either loss of saposin C function or a combined loss of saposin C + D function were studied. Light microscopy and immunofluorescence were used to evaluate histologic and morphologic changes in the auditory and vestibular organs. Acoustic brainstem response thresholds and distortion product otoacoustic emissions were used to study the auditory phenotype.

RESULTS

A null mutation of saposin C did not result in any identifiable histologic changes or loss of hearing through postnatal day 55. Combined losses of saposins C and D similarly did not result in any changes in organ of Corti histology or loss of hearing. However, inclusions within the vestibular end organs was noted, consistent with afferent and efferent neuronal sprouting, although to a much milder degree than seen in the previously studied prosaposin knockout mouse.

CONCLUSIONS

Loss of saposin C and D function, although causing mild phenotypic changes in the vestibular end organs, otherwise results in minimal functional impairment and no changes in the auditory system. It is more likely that the auditory and vestibular effects of the loss of prosaposin are mediated through the actions of saposin A and/or B.

LEVEL OF EVIDENCE

NA.

A prosaposin-derived Peptide alleviates kainic Acid-induced brain injury

Four sphingolipid activator proteins (i.e., saposins A–D) are synthesized from a single precursor protein, prosaposin (PS), which exerts exogenous neurotrophic effects in vivo and in vitro. Kainic acid (KA) injection in rodents is a good model in which to study neurotrophic factor elevation; PS and its mRNA are increased in neurons and the choroid plexus in this animal model. An 18-mer peptide (LSELIINNATEELLIKGL; PS18) derived from the PS neurotrophic region prevents neuronal damage after ischemia, and PS18 is a potent candidate molecule for use in alleviating ischemia-induced learning disabilities and neuronal loss. KA is a glutamate analog that stimulates excitatory neurotransmitter release and induces ischemia-like neuronal degeneration; it has been used to define mechanisms involved in neurodegeneration and neuroprotection. In the present study, we demonstrate that a subcutaneous injection of 0.2 and 2.0 mg/kg PS18 significantly improved behavioral deficits of Wistar rats (n = 6 per group), and enhanced the survival of hippocampal and cortical neurons against neurotoxicity induced by 12 mg/kg KA compared with control animals. PS18 significantly protected hippocampal synapses against KA-induced destruction. To evaluate the extent of PS18- and KA-induced effects in these hippocampal regions, we performed histological evaluations using semithin sections stained with toluidine blue, as well as ordinal sections stained with hematoxylin and eosin. We revealed a distinctive feature of KA-induced brain injury, which reportedly mimics ischemia, but affects a much wider area than ischemia-induced injury: KA induced neuronal degeneration not only in the CA1 region, where neurons degenerate following ischemia, but also in the CA2, CA3, and CA4 hippocampal regions.

Prosaposin overexpression following kainic acid-induced neurotoxicity

Because excessive glutamate release is believed to play a pivotal role in numerous neuropathological disorders, such as ischemia or seizure, we aimed to investigate whether intrinsic prosaposin (PS), a neuroprotective factor when supplied exogenously in vivo or in vitro, is up-regulated after the excitotoxicity induced by kainic acid (KA), a glutamate analog. In the present study, PS immunoreactivity and its mRNA expression in the hippocampal and cortical neurons showed significant increases on day 3 after KA injection, and high PS levels were maintained even after 3 weeks. The increase in PS, but not saposins, detected by immunoblot analysis suggests that the increase in PS-like immunoreactivity after KA injection was not due to an increase in saposins as lysosomal enzymes after neuronal damage, but rather to an increase in PS as a neurotrophic factor to improve neuronal survival. Furthermore, several neurons with slender nuclei inside/outside of the pyramidal layer showed more intense PS mRNA expression than other pyramidal neurons. Based on the results from double immunostaining using anti-PS and anti-GABA antibodies, these neurons were shown to be GABAergic interneurons in the extra- and intra-pyramidal layers. In the cerebral cortex, several large neurons in the V layer showed very intense PS mRNA expression 3 days after KA injection. The choroid plexus showed intense PS mRNA expression even in the normal rat, and the intensity increased significantly after KA injection. The present study indicates that inhibitory interneurons as well as stimulated hippocampal pyramidal and cortical neurons synthesize PS for neuronal survival, and the choroid plexus is highly activated to synthesize PS, which may prevent neurons from excitotoxic neuronal damage. To the best of our knowledge, this is the first study that demonstrates axonal transport and increased production of neurotrophic factor PS after KA injection.

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

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

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.

Differential expression of the alternatively spliced forms of prosaposin mRNAs in rat choroid plexus

Prosaposin has two distinct profiles. One is a precursor form that is processed into saposins thus promoting lysosomal sphingolipid hydrolase function, whereas the other is an intact form that is not processed into saposins but is abundant in certain tissues and secretory fluids, including the cerebrospinal fluid. In rats, alternative splicing in the prosaposin gene generates mRNAs with and without a 9-base insertion (Pro+9 and Pro+0 mRNAs, respectively). Pro+9 mRNA is reported to be preferentially expressed in tissues in which the intact form of prosaposin dominates, whereas Pro+0 mRNA is preferentially expressed in tissues in which the precursor dominates. The expression patterns of Pro+9 and Pro+0 mRNAs in the rat choroid plexus are examined in the present study. The specificities of 36-mer oligonucleotide probes used to detect the 9-base insertion by in situ hybridization were demonstrated by dot-blot hybridization. Next, these probes were used for in situ hybridization, which showed predominant expression of Pro+0 mRNA and weak expression of Pro+9 mRNA in the choroid plexus. These expression patterns were confirmed by reverse transcription plus the polymerase chain reaction with AlwI restriction enzyme treatment. Expression of the intact form of prosaposin in the choroid plexus was assessed by Western blotting and immunohistochemistry. Because the choroid plexus is responsible for the generation of cerebrospinal fluid containing the intact form of prosaposin, the present study raises the possibility that Pro+0 mRNA is related to the intact form in the choroid plexus and that the alternatively spliced forms of mRNAs do not simply correspond to the precursor and intact forms of prosaposin.

Tissue-specific effects of saposin A and saposin B on glycosphingolipid degradation in mutant mice

Individual saposin A (A-/-) and saposin B (B-/-)-deficient mice show unique phenotypes caused by insufficient degradation of myelin-related glycosphingolipids (GSLs): galactosylceramide and galactosylsphingosine and sulfatide, respectively. To gain insight into the interrelated functions of saposins A and B, combined saposin AB-deficient mice (AB-/-) were created by knock-in point mutations into the saposins A and B domains on the prosaposin locus. Saposin A and B proteins were undetectable in AB-/- mice, whereas prosaposin, saposin C and saposin D were expressed near wild-type (WT) levels. AB-/- mice developed neuromotor deterioration at >61 days and exhibited abnormal locomotor activity and enhanced tremor. AB-/- mice (~96 days) lived longer than A-/- mice (~85 days), but shorter than B-/- mice (~644 days). Storage materials were observed in Schwann cells and neuronal processes by electron microscopy. Accumulation of p62 and increased levels of LC3-II were detected in the brainstem suggesting altered autophagy. GSL analyses by (liquid chromatography) LC/MS identified substantial increases in lactosylceramide in AB-/- mouse livers. Sulfatide accumulated, but galactosylceramide remained at WT levels, in the AB-/- mouse brains and kidneys. Brain galactosylsphingosine in AB-/- mice was ~68% of that in A-/- mice. These findings indicate that combined saposins A and B deficiencies attenuated GalCer-β-galactosylceramidase and GM1-β-galactosidase functions in the degradation of lactosylceramide preferentially in the liver. Blocking sulfatide degradation from the saposin B deficiency diminished galactosylceramide accumulation in the brain and kidney and galctosylsphingosine in the brain. These analyses of AB-/- mice continue to delineate the tissue differential interactions of saposins in GSL metabolism.

Distribution of prosaposin in rat lymphatic tissues

Prosaposin (PSAP) is as a trophic factor and an activator protein for sphingolipid hydrolase in lysosomes. We generated a specific antibody to PSAP and examined the spatiotemporal distribution of PSAP-immunoreactive (PSAP-IR) cells in the lymphatic tissues of Wistar rats. Immunoblots of tissue homogenates separated electrophoretically showed a single band for PSAP in brain but two bands in spleen. PSAP-IR cells were distributed in both the red and white pulp of the spleen, in both the cortex and medulla of the thymus and in mesenteric lymph nodes. Many PSAP-IR cells were found in the dome portion of Peyer's patches and the number of PSAP-IR cells increased with the age of the rat. To identify the PSAP-IR cells, double- and triple-immunostainings were performed with antibodies against PSAP, CD68 and CD1d. The large number of double- and triple-positive cells suggested that antigen-presenting cells contained much PSAP in these lymphatic tissues. Intense expression of PSAP mRNA, examined by in situ hybridisation, was observed in the red pulp and corona of the spleen. In rats, the PSAP gene generates two alternative splicing forms of mRNA: Pro+9 containing a 9-base insertion and Pro+0 without the insertion. We examined the expression patterns of the alternative splicing forms of PSAP mRNA in the spleen. The presence of both types of mRNA (Pro+9 and Pro+0) indicated that the spleen contains various types of prosaposin-producing and/or secreting cells. These findings suggest diverse functions for PSAP in the immune system.

The role of saposin C in Gaucher disease

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

Gaucher disease paradigm: from ERAD to comorbidity

Mutations in the GBA gene, encoding the lysosomal acid beta-glucocerebrosidase (GCase), lead to deficient activity of the enzyme in the lysosomes, to glucosylceramide accumulation and to development of Gaucher disease (GD). More than 280 mutations in the GBA gene have been directly associated with GD. Mutant GCase variants present variable levels of endoplasmic reticulum (ER) retention, due to their inability to correctly fold, and undergo ER-associated degradation (ERAD) in the proteasomes. The degree of ER retention and proteasomal degradation is one of the factors that determine GD severity. In the present review, we discuss ERAD of mutant GCase variants and its possible consequences in GD patients and in carriers of GD mutations.

A method to determine the kinetics of multiple proteins in human infants with respiratory distress syndrome

We report a method to measure in vivo turnover of four proteins from sequential tracheal aspirates obtained from human newborn infants with respiratory distress syndrome using targeted proteomics. We detected enrichment for all targeted proteins approximately 3 h from the start of infusion of [5,5,5-(2)H(3)] leucine, secretion times that varied from 1.2 to 2.5 h, and half lives that ranged between 10 and 21 h. Complement factor B, a component of the alternative pathway of complement activation, had an approximately twofold-longer half-life than the other three proteins. In addition, the kinetics of mature and carboxy-terminal tryptic peptides from the same protein (surfactant protein B) were not statistically different (p = 0.49).

Structure of saposin A lipoprotein discs

The saposins are small, membrane-active proteins that exist in both soluble and lipid-bound states. Saposin A has roles in sphingolipid catabolism and transport and is required for the breakdown of galactosylceramide by β-galactosylceramidase. In the absence of lipid, saposin A adopts a closed monomeric apo conformation typical of this family. To study a lipid-bound state of this protein, we determined the crystal structure of saposin A in the presence of detergent to 1.9 Å resolution. The structure reveals two chains of saposin A in an open conformation encapsulating 40 internally bound detergent molecules organized in a highly ordered bilayer-like hydrophobic core. The complex provides a high-resolution view of a discoidal lipoprotein particle in which all of the internalized acyl chains are resolved. Saposin A lipoprotein discs exhibit limited selectivity with respect to the incorporated lipid, and can solubilize phospholipids, sphingolipids, and cholesterol into discrete, monodisperse particles with mass of approximately 27 kDa. These discs may be the smallest possible lipoprotein structures that are stabilized by lipid self-assembly.

Invertebrate models of lysosomal storage disease: what have we learned so far?

The lysosomal storage diseases (LSDs) collectively account for death in 1 in 8,000 children. Although some forms are treatable, they are essentially incurable and usually are lethal in the first decade of life. The most intractable forms of LSD are those with neuronal involvement. In an effort to identify the pathological signaling driving pathology in the LSDs, invertebrate models have been developed. In this review, we outline our current understanding of LSDs and recent findings using invertebrate models. We outline strategies and pitfalls for the development of such models. Available models of LSD in Drosophila and Caenorhabditis elegans are uncovering roles for LSD-related proteins with previously unknown function using both gain-of-function and loss-of-function strategies. These models of LSD in Drosophila and C. elegans have identified potential pathogenic signaling cascades that are proving critical to our understanding of these lethal diseases.

Lysosomal storage disorders: molecular basis and laboratory testing

Lysosomal storage disorders (LSDs) are a large group of more than 50 different inherited metabolic diseases which, in the great majority of cases, result from the defective function of specific lysosomal enzymes and, in cases, of non-enzymatic lysosomal proteins or non-lysosomal proteins involved in lysosomal biogenesis. The progressive lysosomal accumulation of undegraded metabolites results in generalised cell and tissue dysfunction, and, therefore, multi-systemic pathology. Storage may begin during early embryonic development, and the clinical presentation for LSDs can vary from an early and severe phenotype to late-onset mild disease. The diagnosis of most LSDs--after accurate clinical/paraclinical evaluation, including the analysis of some urinary metabolites--is based mainly on the detection of a specific enzymatic deficiency. In these cases, molecular genetic testing (MGT) can refine the enzymatic diagnosis. Once the genotype of an individual LSD patient has been ascertained, genetic counselling should include prediction of the possible phenotype and the identification of carriers in the family at risk. MGT is essential for the identification of genetic disorders resulting from non-enzymatic lysosomal protein defects and is complementary to biochemical genetic testing (BGT) in complex situations, such as in cases of enzymatic pseudodeficiencies. Prenatal diagnosis is performed on the most appropriate samples, which include fresh or cultured chorionic villus sampling or cultured amniotic fluid. The choice of the test--enzymatic and/or molecular--is based on the characteristics of the defect to be investigated. For prenatal MGT, the genotype of the family index case must be known. The availability of both tests, enzymatic and molecular, enormously increases the reliability of the entire prenatal diagnostic procedure. To conclude, BGT and MGT are mostly complementary for post- and prenatal diagnosis of LSDs. Whenever genotype/phenotype correlations are available, they can be helpful in predicting prognosis and in making decisions about therapy.

Prosaposin sorting is mediated by oligomerization

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

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.

Transcript variants, expression, and polymorphisms of the pig prosaposin gene

Prosaposin (PASP) is a sphingolipid hydrolysis protein that plays roles in both the nervous and reproduction systems. In this study, we cloned the pig PASP gene and studied its genomic organization, polymorphism, and expression pattern. Two PASP transcripts, TV1 (HQ245644) and TV2 (HQ245646), were identified in pig. TV1 was the complete transcript that encoded 527 amino acids, whereas TV2 was 9 bp shorter due to an exon 8 deletion. The pig PASP gene spanned over 34 kb in length on chromosome 14 (SSC14), and consisted of 15 exons and 14 introns. The pig PASP gene (TV1 and TV2) expressed predominantly in the cerebellum, lymphnode, pituitary, abdominal fat, hypothalamus, and cerebrum in both females and males. PASP TV1 expressed mainly in teh cerebrum, cerebellum, hypothalamus, pituitary, heart, subcutaneous fat, and foreleg muscle, while TV2 was expressed in the liver, spleen, lung, kidney, and lymphnode. In foreleg muscle, the predominant transcript was TV2 in males and TV1 in females. Some potential transcriptional elements were predicted in 5' flanking region (~3000 bp) of the PASP gene, and they were TATA boxes, RORE, Sp1, SRY, oct-1, Cdx A, and cap. Additionally, we identified 68 single-nucleotide polymorphisms and 9 indels in the pig PASP gene, and three single-nucleotide polymorphisms (C77932320T or L15F; C77928094T or P191L; A77917401G or K522R) were nonsynonymous substitutions. These results provide useful information for future functional investigations of the pig PASP gene.

The immunological functions of saposins

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

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

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

Lysosomal degradation of membrane lipids

The constitutive degradation of membrane components takes place in the acidic compartments of a cell, the endosomes and lysosomes. Sites of lipid degradation are intralysosomal membranes that are formed in endosomes, where the lipid composition is adjusted for degradation. Cholesterol is sorted out of the inner membranes, their content in bis(monoacylglycero)phosphate increases, and, most likely, sphingomyelin is degraded to ceramide. Together with endosomal and lysosomal lipid-binding proteins, the Niemann-Pick disease, type C2-protein, the GM2-activator, and the saposins sap-A, -B, -C, and -D, a suitable membrane lipid composition is required for degradation of complex lipids by hydrolytic enzymes.

Neurological deficits and glycosphingolipid accumulation in saposin B deficient mice

Saposin B derives from the multi-functional precursor, prosaposin, and functions as an activity enhancer for several glycosphingolipid (GSL) hydrolases. Mutations in saposin B present in humans with phenotypes resembling metachromatic leukodystrophy. To gain insight into saposin B's physiological functions, a specific deficiency was created in mice by a knock-in mutation of an essential cysteine in exon 7 of the prosaposin locus. No saposin B protein was detected in the homozygotes (B−/−) mice, whereas prosaposin, and saposins A, C and D were at normal levels. B−/− mice exhibited slowly progressive neuromotor deterioration and minor head tremor by 15 months. Excess hydroxy and non-hydroxy fatty acid sulfatide levels were present in brain and kidney. Alcian blue positive (sulfatide) storage cells were found in the brain, spinal cord and kidney. Ultrastructural analyses showed lamellar inclusion material in the kidney, sciatic nerve, brain and spinal cord tissues. Lactosylceramide (LacCer) and globotriaosylceramide (TriCer) were increased in various tissues of B−/− mice supporting the in vivo role of saposin B in the degradation of these lipids. CD68 positive microglial cells and activated GFAP positive astrocytes showed a proinflammatory response in the brains of B−/− mice. These findings delineate the roles of saposin B for the in vivo degradation of several GSLs and its primary function in maintenance of CNS function. B−/− provide a useful model for understanding the contributions of this saposin to GSL metabolism and homeostasis.

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.

Combined saposin C and D deficiencies in mice lead to a neuronopathic phenotype, glucosylceramide and α-hydroxy ceramide accumulation, and altered prosaposin trafficking

Saposins (A, B, C and D) are approximately 80 amino acid stimulators of glycosphingolipid (GSL) hydrolases that derive from a single precursor, prosaposin. In both humans and mice, prosaposin/saposin deficiencies lead to severe neurological deficits. The CD-/- mice with saposin C and D combined deficiencies were produced by introducing genomic point mutations into a critical cysteine in each of these saposins. These mice develop a severe neurological phenotype with ataxia, kyphotic posturing and hind limb paralysis. Relative to prosaposin null mice ( approximately 30 days), CD-/- mice had an extended life span ( approximately 56 days). Loss of Purkinje cells was evident after 6 weeks, and storage bodies were present in neurons of the spinal cord, brain and dorsal root ganglion. Electron microscopy showed well-myelinated fibers and axonal inclusions in the brain and sciatic nerve. Marked accumulations of glucosylceramides and alpha-hydroxy ceramides were present in brain and kidney. Minor storage of lactosylceramide (LacCer) was observed when compared with tissues from the prosaposin null mice, suggesting a compensation in LacCer degradation by saposin B for the saposin C deficiency. Skin fibroblasts and tissues from CD-/- mice showed an increase of intracellular prosaposin, impaired prosaposin secretion, deficiencies of saposins C and D and decreases in saposins A and B. In addition, the deficiency of saposin C in CD-/- mice resulted in cellular decreases of acid beta-glucosidase activity and protein. This CD null mouse model provides a tool to explore the in vivo functional interactions of saposins in GSL metabolism and lysosomal storage diseases, and prosaposin's physiological effects.

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.

Delivery of lysosomal enzymes for therapeutic use: glucocerebrosidase as an example

Enzyme therapies for lysosomal storage diseases have developed over the past decade into the standard-of-care for affected patients. Such therapy for Gaucher disease has been the prototype, using natural source or recombinant forms of human acid beta-glucosidase (GCase). In Gaucher disease, macrophages are the repository for the pathological lipid and the target for delivery of GCase. The macrophage mannose receptor provides a Trojan horse for intracellular delivery of intravenously administered GCase (man-GCase) with mannosyl-terminated oligosaccharide chains. Passage through several hostile compartments (e.g., plasma) leads to inefficient delivery of man-GCase to macrophage lysosomes. However, regular infusions of man-GCase re-establishes health in affected patients. Similar results are being obtained in several other lysosomal storage diseases. Evolving gene and chaperone approaches provide alternative treatment strategies.

Sphingolipid metabolism diseases

Human diseases caused by alterations in the metabolism of sphingolipids or glycosphingolipids are mainly disorders of the degradation of these compounds. The sphingolipidoses are a group of monogenic inherited diseases caused by defects in the system of lysosomal sphingolipid degradation, with subsequent accumulation of non-degradable storage material in one or more organs. Most sphingolipidoses are associated with high mortality. Both, the ratio of substrate influx into the lysosomes and the reduced degradative capacity can be addressed by therapeutic approaches. In addition to symptomatic treatments, the current strategies for restoration of the reduced substrate degradation within the lysosome are enzyme replacement therapy (ERT), cell-mediated therapy (CMT) including bone marrow transplantation (BMT) and cell-mediated "cross correction", gene therapy, and enzyme-enhancement therapy with chemical chaperones. The reduction of substrate influx into the lysosomes can be achieved by substrate reduction therapy. Patients suffering from the attenuated form (type 1) of Gaucher disease and from Fabry disease have been successfully treated with ERT.

Conditional expression of human acid β-glucosidase improves the visceral phenotype in a Gaucher disease mouse model

The reversibility and regression of histological and biochemical findings in a mouse model of Gaucher disease (4L/PS-NA) was evaluated using a liver-enriched activator protein promoter control of a tetracycline-controlled transcriptional activation-responsive human acid beta-glucosidase (hGCase) transgenic system. 4L/PS-NA has the acid beta-glucosidase (GCase) V394L/V394L (4L) point mutation combined with hypomorphic ( approximately 6% wild-type) expression of the mouse prosaposin transgene (PS-NA). The hGCase/4L/PS-NA had exclusive liver expression of hGCase controlled by doxycycline (DOX). In the absence of DOX, hGCase was secreted from liver at levels of approximately 120 microg/ml serum with only approximately 8% of full activity, following exposure to pH 7.4 in serum. The hGCase activity and protein were detected in cells of the liver (massive), lung, and spleen, but not the brain. The visceral tissue storage cells and glucosylceramide (GC) accumulation in hGCase/4L/PS-NA were decreased from that in 4L/PS-NA mice. Turning off hGCase expression with dietary DOX led to reaccumulation of storage cells and of GC in liver, lung, and spleen, and macrophage activation in those tissues. This study demonstrates that conditionally expressed hGCase supplemented the existing mutant mouse GCase to control visceral substrate accumulation in vivo.

Principles of lysosomal membrane digestion: stimulation of sphingolipid degradation by sphingolipid activator proteins and anionic lysosomal lipids

Sphingolipids and glycosphingolipids are membrane components of eukaryotic cell surfaces. Their constitutive degradation takes place on the surface of intra-endosomal and intra-lysosomal membrane structures. During endocytosis, these intra-lysosomal membranes are formed and prepared for digestion by a lipid-sorting process during which their cholesterol content decreases and the concentration of the negatively charged bis(monoacylglycero)phosphate (BMP)--erroneously also called lysobisphosphatidic acid (LBPA)--increases. Glycosphingolipid degradation requires the presence of water-soluble acid exohydrolases, sphingolipid activator proteins, and anionic phospholipids like BMP. The lysosomal degradation of sphingolipids with short hydrophilic head groups requires the presence of sphingolipid activator proteins (SAPs). These are the saposins (Saps) and the GM2 activator protein. Sphingolipid activator proteins are membrane-perturbing and lipid-binding proteins with different specificities for the bound lipid and the activated enzyme-catalyzed reaction. Their inherited deficiency leads to sphingolipid- and membrane-storage diseases. Sphingolipid activator proteins not only facilitate glycolipid digestion but also act as glycolipid transfer proteins facilitating the association of lipid antigens with immunoreceptors of the CD1 family.

Gaucher disease mouse models: point mutations at the acid beta-glucosidase locus combined with low-level prosaposin expression lead to disease variants

Gaucher disease is a common lysosomal storage disease caused by a defect of acid beta-glucosidase (GCase). The optimal in vitro hydrolase activity of GCase requires saposin C, an activator protein that derives from a precursor, prosaposin. To develop additional models of Gaucher disease and to test in vivo effects of saposin deficiencies, mice expressing low levels (4--45% of wild type) of prosaposin and saposins (PS-NA) were backcrossed into mice with specific point mutations (V394L/V394L or D409H/D409H) of GCase. The resultant mice were designated 4L/PS-NA and 9H/PS-NA, respectively. In contrast to PS-NA mice, the 4L/PS-NA and 9H/PS-NA mice displayed large numbers of engorged macrophages and nearly exclusive glucosylceramide (GC) accumulation in the liver, lung, spleen, thymus, and brain. Electron microscopy of the storage cells showed the characteristic tubular storage material of Gaucher cells. Compared with V394L/V394L mice, 4L/PS-NA mice that expressed 4--6% of wild-type prosaposin levels had approximately 25--75% decreases in GCase activity and protein in liver, spleen, and fibroblasts. These results imply that reduced saposin levels increased the instability of V394L or D409H GCases and that these additional decreases led to large accumulations of GC in all tissues. These models mimic a more severe Gaucher disease phenotype and could be useful for therapeutic intervention studies.

Conservation of expression and alternative splicing in the prosaposin gene

Prosaposin is the precursor of four lysosomal activator molecules known as saposins A, B, C and D. It is also secreted and was proposed to be a neurotrophic factor. The neurotrophic function was attributed to the amino terminus of saposin C. In man, mouse and rat prosaposin is transcribed to two major isoforms differing in the inclusion of 9 bps of exon 8 within the saposin B domain. In the present study, we show that there is evolutionary conservation of the prosaposin structure and alternative splicing in chick and zebrafish as well. Moreover, there is conservation in prosaposin expression as tested immunohistochemically in the mouse and chick developing brain. We developed a sensitive assay to quantitate the prosaposin alternatively spliced forms. Our results indicate that, in mouse brain, skeletal and cardiac muscle the exon 8-containing RNA is most abundant, while it is almost absent from visceral and smooth muscle-containing organs. We observed temporal and differential expression of the alternatively spliced prosaposin mRNAs in mouse and chick brain as well as during development. The elevation in the abundance of exon 8-containing prosaposin RNA during mouse and chick brain development may suggest a role for the exon 8-containing prosaposin form in this process.

Lysosomal storage disorders

Although the first description of a lysosomal storage disorder was that of Tay-Sachs disease in 1881, the lysosome was not discovered until 1955, by Christian De Duve. The first demonstration by Hers in 1963 of a link between an enzyme deficiency and a storage disorder (Pompe's disease) paved the way for a series of seminal discoveries about the intracellular biology of these enzymes and their substrates, culminating in the successful treatment of Gaucher's disease with beta-glucosidase in the early 1990s. It is now recognized that these disorders are not simply a consequence of pure storage, but result from perturbation of complex cell signalling mechanisms. These in turn give rise to secondary structural and biochemical changes, which have important implications for therapy. Significant challenges remain, particularly the treatment of central nervous system disease. It is hoped that recent advances in our understanding of lysosomal biology will enable successful therapies to be developed.

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.