Saposins (sap) A and C activate the degradation of galactosylsphingosine

Selective prosaposin expression in Langerhans islets of the mouse pancreas

The islets of Langerhans are clusters of endocrine cells surrounded by exocrine acinar cells in the pancreas. Prosaposin is a housekeeping protein required for normal lysosomal function, but its expression level is significantly different among tissues. Prosaposin also exists in various body fluids including serum. Intracellularly, prosaposin activates lysosomes and may support autophagy, and extracellularly, prosaposin promotes survival of neurons via G protein-coupled receptors. In this study, prosaposin and its mRNA expression were examined in endocrine cells of the islets as well as in exocrine acinar cells in the pancreas of mice by in situ hybridization and immunostaining. High expression levels of prosaposin were found in Alpha, Beta and Delta cells in the islets, whereas prosaposin mRNA expression was faint or negative and prosaposin immunoreactivity was negative in exocrine acinar cells. The high expression levels of prosaposin in endocrine cells may indicate that prosaposin plays a crucial role in crinophagy, which is a characteristic autophagy in peptide-secreting endocrine cells, and/or that prosaposin is secreted from pancreatic islets. Since prosaposin has been reported in serum, this study suggests a new possible function of the Langerhans islets.

Hepatomegaly and Splenomegaly: An Approach to the Diagnosis of Lysosomal Storage Diseases

Clinical findings of hepatomegaly and splenomegaly, the abnormal enlargement of the liver and spleen, respectively, should prompt a broad differential diagnosis that includes metabolic, congestive, neoplastic, infectious, toxic, and inflammatory conditions. Among the metabolic diseases, lysosomal storage diseases (LSDs) are a group of rare and ultrarare conditions with a collective incidence of 1 in 5000 live births. LSDs are caused by genetic variants affecting the lysosomal enzymes, transporters, or integral membrane proteins. As a result, abnormal metabolites accumulate in the organelle, leading to dysfunction. Therapeutic advances, including early diagnosis and disease-targeted management, have improved the life expectancy and quality of life of people affected by certain LSDs. To access these new interventions, LSDs must be considered in patients presenting with hepatomegaly and splenomegaly throughout the lifespan. This review article navigates the diagnostic approach for individuals with hepatosplenomegaly particularly focusing on LSDs. We provide hints in the history, physical exam, laboratories, and imaging that may identify LSDs. Additionally, we discuss molecular testing, arguably the preferred confirmatory test (over biopsy), accompanied by enzymatic testing when feasible.

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.

Expression of prosaposin and its G protein-coupled receptor (GPR) 37 in mouse cochlear and vestibular nuclei

Prosaposin is a precursor of lysosomal hydrolases activator proteins, saposins, and also acts as a secretory protein that is not processed into saposins. Prosaposin elicits neurotrophic function via G protein-coupled receptor (GPR) 37, and prosaposin deficiency causes abnormal vestibuloauditory end-organ development. In this study, immunohistochemistry was used to examine prosaposin and GPR37 expression patterns in the mouse cochlear and vestibular nuclei. Prosaposin immunoreactivity was observed in neurons and glial cells in both nuclei. GPR37 immunoreactivity was observed in only some neurons, and its immunoreactivity in the vestibular nucleus was weaker than that in the cochlear nucleus. This study suggests a possibility that prosaposin deficiency affects not only the end-organs but also the first center of the vestibuloauditory system.

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.

The lipid transfer protein Saposin B does not directly bind CD1d for lipid antigen loading

Background: Lipid antigens are presented on the surface of cells by the CD1 family of glycoproteins, which have structural and functional similarity to MHC class I molecules. The hydrophobic lipid antigens are embedded in membranes and inaccessible to the lumenal lipid-binding domain of CD1 molecules. Therefore, CD1 molecules require lipid transfer proteins for lipid loading and editing. CD1d is loaded with lipids in late endocytic compartments, and lipid transfer proteins of the saposin family have been shown to play a crucial role in this process. However, the mechanism by which saposins facilitate lipid binding to CD1 molecules is not known and is thought to involve transient interactions between protein components to ensure CD1-lipid complexes can be efficiently trafficked to the plasma membrane for antigen presentation. Of the four saposin proteins, the importance of Saposin B (SapB) for loading of CD1d is the most well-characterised. However, a direct interaction between CD1d and SapB has yet to be described. Methods: In order to determine how SapB might load lipids onto CD1d, we used purified, recombinant CD1d and SapB and carried out a series of highly sensitive binding assays to monitor direct interactions. We performed equilibrium binding analysis, chemical cross-linking and co-crystallisation experiments, under a range of different conditions. Results: We could not demonstrate a direct interaction between SapB and CD1d using any of these binding assays. Conclusions: This work strongly indicates that the role of SapB in lipid loading does not involve direct binding to CD1d. We discuss the implication of this for our understanding of lipid loading of CD1d and propose several factors that may influence this process.

The lipid transfer protein Saposin B does not directly bind CD1d for lipid antigen loading

Background: Lipid antigens are presented on the surface of cells by the CD1 family of glycoproteins, which have structural and functional similarity to MHC class I molecules. The hydrophobic lipid antigens are embedded in membranes and inaccessible to the lumenal lipid-binding domain of CD1 molecules. Therefore, CD1 molecules require lipid transfer proteins for lipid loading and editing. CD1d is loaded with lipids in late endocytic compartments, and lipid transfer proteins of the saposin family have been shown to play a crucial role in this process. However, the mechanism by which saposins facilitate lipid binding to CD1 molecules is not known and is thought to involve transient interactions between protein components to ensure CD1-lipid complexes can be efficiently trafficked to the plasma membrane for antigen presentation. Of the four saposin proteins, the importance of Saposin B (SapB) for loading of CD1d is the most well-characterised. However, a direct interaction between CD1d and SapB has yet to be described. Methods: In order to determine how SapB might load lipids onto CD1d, we used purified, recombinant CD1d and SapB and carried out a series of highly sensitive binding assays to monitor direct interactions. We performed equilibrium binding analysis, chemical cross-linking and co-crystallisation experiments, under a range of different conditions. Results: We could not demonstrate a direct interaction between SapB and CD1d using any of these binding assays. Conclusions: This work establishes comprehensively that the role of SapB in lipid loading does not involve direct binding to CD1d. We discuss the implication of this for our understanding of lipid loading of CD1d and propose several factors that may influence this process.

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

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

Can psychosine and galactocerebrosidase activity predict early-infantile Krabbe's disease presymptomatically?

Krabbe's disease (KD) is a fatal neurodegenerative disorder, with the early-infantile form (EIKD) defined by onset of symptoms before age 6 months. Early and highly accurate identification of EIKD is required to maximize benefits of hematopoietic stem cell transplantation treatment. This study investigates the potential for accurate prediction of EIKD based on a novel newborn screening (NBS) tool developed from two biomarkers, galactocerebrosidase (GALC) enzyme activity and galactosylsphingosine concentration (psychosine [PSY]). Normative information about PSY and GALC, derived from distinct samples of normal newborns, was used to develop the novel diagnostic tool. Bivariate normal limits (BVNL) were constructed, assuming a multivariate normal distribution of natural logarithms of GALC and PSY of normal newborns. The (lnGALC, lnPSY) points for newborns in various "abnormal groups," including one group of infants who subsequently suffered EIKD, were plotted on a graph of BVNL. The points for all EIKD patients fell outside of BVNL (100% sensitivity). In a simulation study to compare the false-positive rate of existing univariate methods of diagnosis with our new BVNL-based method, we generated 100 million normal newborn data points. All fell within BVNL (i.e., zero false positives), whereas 5,682 false positives were observed when applying a two-tiered univariate method of the type suggested in the literature. These results suggest that (lnGALC, lnPSY) BVNLs will allow highly accurate prediction of EIKD, whereas two-tiered univariate approaches will not. Redevelopment of the BVNL based on GALCs and PSYs measured on a common large sample of normal newborns is required for NBS use. © 2016 Wiley Periodicals, Inc.

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.

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.

Mycobacterium tuberculosis and the intimate discourse of a chronic infection

Mycobacterium tuberculosis is an extremely successful pathogen that demonstrates the capacity to modulate its host both at the cellular and tissue levels. At the cellular level, the bacterium enters its host macrophage and arrests phagosome maturation, thus avoiding many of the microbicidal responses associated with this phagocyte. Nonetheless, the intracellular environment places certain demands on the pathogen, which, in response, senses the environmental shifts and upregulates specific metabolic programs to allow access to nutrients, minimize the consequences of stress, and sustain infection. Despite its intracellular niche, Mycobacterium tuberculosis demonstrates a marked capacity to modulate the tissues surrounding infected cells through the release of potent, bioactive cell wall constituents. These cell wall lipids are released from the host cell by an exocytic process and induce physiological changes in neighboring phagocytes, which drives formation of a granuloma. This tissue response leads to the generation and accumulation of caseous debris and the progression of the human tuberculosis granuloma. Completion of the life cycle of tuberculosis requires damaging the host to release infectious bacteria into the airways to spread the infection. This damage reflects the pathogen's ability to subvert the host's innate and acquired immune responses to its own nefarious ends.

Caseation of human tuberculosis granulomas correlates with elevated host lipid metabolism

The progression of human tuberculosis (TB) to active disease and transmission involves the development of a caseous granuloma that cavitates and releases infectious Mycobacterium tuberculosis bacilli. In the current study, we exploited genome-wide microarray analysis to determine that genes for lipid sequestration and metabolism were highly expressed in caseous TB granulomas. Immunohistological analysis of these granulomas confirmed the disproportionate abundance of the proteins involved in lipid metabolism in cells surrounding the caseum; namely, adipophilin, acyl-CoA synthetase long-chain family member 1 and saposin C. Biochemical analysis of the lipid species within the caseum identified cholesterol, cholesteryl esters, triacylglycerols and lactosylceramide, which implicated low-density lipoprotein-derived lipids as the most likely source. M. tuberculosis infection in vitro induced lipid droplet formation in murine and human macrophages. Furthermore, the M. tuberculosis cell wall lipid, trehalose dimycolate, induced a strong granulomatous response in mice, which was accompanied by foam cell formation. These results provide molecular and biochemical evidence that the development of the human TB granuloma to caseation correlates with pathogen-mediated dysregulation of host lipid metabolism.

A mutation in the saposin C domain of the sphingolipid activator protein (Prosaposin) gene causes neurodegenerative disease in mice

Saposins A, B, C, and D are small amphiphatic glycoproteins that are encoded in tandem within a precursor protein (prosaposin, PSAP), and are required for in vivo degradation of sphingolipids. Humans with saposin C deficiency exhibit the clinical presentation of Gaucher-like disease. We generated two types of saposin C mutant mice, one carrying a homozygous missense mutation (C384S) in the saposin C domain of prosaposin (Sap-C(-/-)) and the other carrying the compound heterozygous mutation with a second null Psap allele (Psap(-/C384S)). During early life stages, both Sap-C(-/-) and Psap(-/C384S) mice grew normally; however, they developed progressive motor and behavioral deficits after 3 months of age and the majority of affected mice could scarcely move by about 15 months. They showed no signs of hepatosplenomegaly throughout their lives. No accumulation of glucosylceramide and glucosylsphingosine was detected in the brain or liver of both Sap-C(-/-) and Psap(-/C384S) mice. Neuropathological analyses revealed patterned loss of cerebellar Purkinje cells, widespread axonal spheroids filled with membrane-derived concentric or lamellar electron-dense bodies, and lipofuscin-like deposition in the neurons. Soap-bubble-like inclusion bodies were detected in the trigeminal ganglion cells and the vascular endothelial cells. Compound heterozygous Psap(-/C384S) mice showed qualitatively identical but faster progression of the neurological phenotypes than Sap-C(-/-) mice. These results suggest the in vivo role of saposin C in axonal membrane homeostasis, the disruption of which leads to neurodegeneration in lysosomal storage disease.

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.

A mutation in the saposin A coding region of the prosaposin gene in an infant presenting as Krabbe disease: first report of saposin A deficiency in humans

A six-month-old infant girl presenting with progressive encephalopathy and abnormal myelination in the cerebral white matter was originally diagnosed as suffering from Krabbe disease. The diagnosis was based on a deficiency of galactocerebrosidase activity found in leukocytes isolated from whole blood. When cultured skin fibroblasts did not show a similar enzyme deficiency and sulphatide (stearoyl-1-14C) uptake indicated an abnormal storage of galactosylceramide, a deficiency of an activator was implied. A three base pair deletion was found in the saposin A coding sequence of the prosaposin gene leading to the deletion of a conserved valine at amino acid number 11 of the saposin A protein. This deletion in saposin A is proposed as the cause for the abnormal galactosylceramide metabolism in this infant. This is the first report of a saposin A mutation in humans leading to pathological consequences.

Delayed clinical and pathological signs in twitcher (globoid cell leukodystrophy) mice on a C57BL/6 x CAST/Ei background

Modifier genes may account for the phenotypic variability observed in the late-onset forms of globoid cell leukodystrophy (GCL) in humans. In order to begin a search for modifier genes, the effect of genetic background on the clinical and pathological manifestations of GCL was investigated in twitcher mice. Twitcher mice on a C57BL/6 x CAST/Ei background had an increased life span (61.4 +/- 2.5 vs 37.0 +/- 0.6 days), a delayed onset of tremor (24 vs 21 days), and a delayed decline in walking ability compared to C57BL/6 twitcher mice. Pathologically, C57BL/6 x CAST/Ei twitcher mice had fewer lectin-positive globoid cells, less gliosis, and a greater preservation of myelin compared to C57BL/6 twitcher mice under moribund conditions. Similar concentrations of psychosine, the toxic species that accumulates in GCL, were measured by tandem mass spectrometry between moribund C57BL/6 twitcher mice (286.5 pmol/mg protein), 40-day C57BL/6 x CAST/Ei twitcher mice (276.5 pmol/mg), and moribund C57BL/6 x CAST/Ei twitcher mice (247.0 pmol/mg), suggesting that the milder phenotype in CAST/Ei x C57BL/6 twitcher mice did not correlate with less psychosine. In summary, the introduction of modifier genes from the wild, inbred CAST/Ei strain had a phenotypic effect resulting in a significantly slower disease course.

Establishment and characterization of spontaneously immortalized Schwann cells from murine model of globoid cell leukodystrophy (twitcher)

The twitcher mouse is a murine model of human globoid cell leukodystrophy (GLD; Krabbe disease) caused by a genetic defect in the activity of galactosylceramidase (GALC). An accumulation of cytotoxic metabolite, galactosylsphingosine (psychosine), in myelin forming cells (oligodendrocytes and Schwann cells) of the twitcher mouse as well as patients with GLD has been suggested to cause dysfunction of these cells and subsequent demyelination in the central and peripheral nervous system. To investigate further the cellular pathomechanism of GLD, we established spontaneously immortalized Schwann cell lines from the twitcher mouse. Long-term cultures of Schwann cells derived from dorsal root ganglia and consecutive peripheral nerves of 3-week-old twitcher mice were maintained for 6 months, and spontaneously developed colonies were expanded further and characterized. One of the cell lines, designated TwS1, showed distinct Schwann cell phenotypes, was passaged twice a week and maintained for over 10 months without phenotypic alterations. The TwS1 cells had a nonsense mutation in the GALC genome, and showed markedly reduced GALC activity and elevated psychosine levels. Ultrastructurally, varieties of cytoplasmic inclusions were demonstrated in TwS1 cells. When TwS1 cells were infected with a retrovirus vector encoding GALC, GALC activity was markedly increased and psychosine levels were significantly decreased. These immortalized Schwann cells can be useful in studies on the nervous system lesions in GLD.