Neurological deficits and glycosphingolipid accumulation in saposin B deficient mice

Experimental Analysis of Tear Fluid and Its Processing for the Diagnosis of Multiple Sclerosis

A pilot analysis of the tear fluid of patients with multiple sclerosis (MS) collected by glass microcapillary was performed using various experimental methods: liquid chromatography-mass spectrometry, Raman spectroscopy, infrared spectroscopy, and atomic-force microscopy. Infrared spectroscopy found no significant difference between the tear fluid of MS patients and the control spectra; all three significant peaks were located at around the same positions. Raman analysis showed differences between the spectra of the tear fluid of MS patients and the spectra of healthy subjects, which indicated a decrease in tryptophan and phenylalanine content and changes in the relative contributions of the secondary structures of the polypeptide chains of tear proteins. Atomic-force microscopy exhibited a surface fern-shaped dendrite morphology of the tear fluid of patients with MS, with less roughness on both oriented silicon (100) and glass substrates compared to the tear fluid of control subjects. The results of liquid chromatography-mass spectrometry showed downregulation of glycosphingolipid metabolism, sphingolipid metabolism, and lipid metabolism. Proteomic analysis identified upregulated proteins in the tear fluid of patients with MS such as cystatine, phospholipid transfer protein, transcobalamin-1, immunoglobulin lambda variable 1-47, lactoperoxidase, and ferroptosis suppressor protein 1; and downregulated proteins such as haptoglobin, prosaposin, cytoskeletal keratin type I pre-mRNA-processing factor 17, neutrophil gelatinase-associated lipocalin, and phospholipase A2. This study showed that the tear proteome in patients with MS is modified and can reflect inflammation. Tear fluid is not a commonly used biological material in clinico-biochemical laboratories. Experimental proteomics has the potential to become a promising contemporary tool for personalized medicine, and it might be applied in clinical practice by providing a detailed analysis of the tear-fluid proteomic profile of patients with MS.

Metachromatic Leukodystrophy: Diagnosis, Modeling, and Treatment Approaches

Metachromatic leukodystrophy is a lysosomal storage disease, which is characterized by damage of the myelin sheath that covers most of nerve fibers of the central and peripheral nervous systems. The disease occurs due to a deficiency of the lysosomal enzyme arylsulfatase A (ARSA) or its sphingolipid activator protein B (SapB) and it clinically manifests as progressive motor and cognitive deficiency. ARSA and SapB protein deficiency are caused by mutations in the ARSA and PSAP genes, respectively. The severity of clinical course in metachromatic leukodystrophy is determined by the residual ARSA activity, depending on the type of mutation. Currently, there is no effective treatment for this disease. Clinical cases of bone marrow or cord blood transplantation have been reported, however the therapeutic effectiveness of these methods remains insufficient to prevent aggravation of neurological disorders. Encouraging results have been obtained using gene therapy for delivering the wild-type ARSA gene using vectors based on various serotypes of adeno-associated viruses, as well as using mesenchymal stem cells and combined gene-cell therapy. This review discusses therapeutic strategies for the treatment of metachromatic leukodystrophy, as well as diagnostic methods and modeling of this pathology in animals to evaluate the effectiveness of new therapies.

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.

Comparing the Superficial Vasculature of the Central Nervous System in Six Laboratory Animals: A Hypothesis About the Role of the "Circle of Willis"

We provide images of the entire central nervous system vasculature, and compare the anatomical findings in six different laboratory animals. A detailed understanding of the specific anatomy for each is important in the design of experimental modeling and for understanding the specific function of each target organ. Six different types of animals, the Korean wild mouse, C57BL/6J mouse, F344 rat, mongolian gerbil, Syrian hamsters, and guinea pigs, were included. To stain the blood vessels in each of the animals, Alcian blue reagent was used to perfuse each species. The bifurcation and anastomotic patterns of the anterior cerebral arteries differed in each species. The vascular supply to the olfactory nerve was visualized as a single artery supplying both olfactory nerves, and arteries supplying the lateral portion of the olfactory nerves originating from the olfactory bulb area. The posterior communicating arteries of the six animals demonstrated unique morphologies. The shape of the hypophyseal portal system varied by species. Most animals used in this study had a hexagonal Circle of Willis, except for the Korean wild mouse. Using this approach, we successfully mapped the brain vascular system in six different species of animals. This information and the images created can guide other researchers as they design research studies and create experimental models for new surgical procedures and approaches. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc. Anat Rec, 302:2049-2061, 2019. © 2019 American Association for Anatomy.

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.

Developmental manganese neurotoxicity in rats: Cognitive deficits in allocentric and egocentric learning and memory

Manganese (Mn) is an essential element but neurotoxic at higher exposure levels. The effects of Mn overexposure (MnOE) on hippocampal and striatal-dependent learning and memory in rats were tested in combination with iron deficiency (FeD) and developmental stress that often co-occur with MnOE. Moderate FeD affects up to 15% of U.S. children and developmental stress is common in lower socio-economic areas where MnOE occurs. Pregnant Sprague-Dawley rats and their litters were housed in cages with or without (barren cage (BAR)) standard bedding from embryonic day (E)7 to postnatal day (P)28. Dams were fed a 90% FeD or iron sufficient (FeS) diet from E15-P28. Within each litter, separate offspring were treated with 100mg/kg Mn (MnOE) or vehicle (VEH) by gavage on alternate days from P4-28. Offspring were tested as adults in the Morris and Cincinnati water mazes. FeD and developmental stress interactively impaired spatial learning in the Morris water maze. Developmental stress and MnOE impaired learning and memory in both mazes. MnOE resulted in reduced CA1 hippocampal long-term potentiation (LTP) and increased levels of α-synuclein. Preweaning MnOE resulted in cognitive deficits on multiple domains of learning and memory accompanied by impaired LTP and α-synuclein changes, effects worsened by developmental stress.

Spiral ganglion degeneration and hearing loss as a consequence of satellite cell death in saposin B-deficient mice

Saposin B (Sap B) is an essential activator protein for arylsulfatase A in the hydrolysis of sulfatide, a lipid component of myelin. To study Sap B's role in hearing and balance, a Sap B-deficient (B(-/-)) mouse was evaluated. At both light and electron microscopy (EM) levels, inclusion body accumulation was seen in satellite cells surrounding spiral ganglion (SG) neurons from postnatal month 1 onward, progressing into large vacuoles preceding satellite cell degeneration, and followed by SG degeneration. EM also revealed reduced or absent myelin sheaths in SG neurons from postnatal month 8 onwards. Hearing loss was initially seen at postnatal month 6 and progressed thereafter for frequency-specific stimuli, whereas click responses became abnormal from postnatal month 13 onward. The progressive hearing loss correlated with the accumulation of inclusion bodies in the satellite cells and their subsequent degeneration. Outer hair cell numbers and efferent function measures (distortion product otoacoustic emissions and contralateral suppression) were normal in the B(-/-) mice throughout this period. Alcian blue staining of SGs demonstrated that these inclusion bodies corresponded to sulfatide accumulation. In contrast, changes in the vestibular system were much milder, but caused severe physiologic deficits. These results demonstrate that loss of Sap B function leads to progressive sulfatide accumulation in satellite cells surrounding the SG neurons, leading to satellite cell degeneration and subsequent SG degeneration with a resultant loss of hearing. Relative sparing of the efferent auditory and vestibular neurons suggests that alternate glycosphingolipid metabolic pathways predominate in these other systems.

A symptomatic Fabry disease mouse model generated by inducing globotriaosylceramide synthesis

Fabry disease is a lysosomal storage disorder in which neutral glycosphingolipids, predominantly Gb3 (globotriaosylceramide), accumulate due to deficient α-Gal A (α-galactosidase A) activity. The GLAko (α-Gal A-knockout) mouse has been used as a model for Fabry disease, but it does not have any symptomatic abnormalities. In the present study, we generated a symptomatic mouse model (G3Stg/GLAko) by cross-breeding GLAko mice with transgenic mice expressing human Gb3 synthase. G3Stg/GLAko mice had high Gb3 levels in major organs, and their serum Gb3 level at 5-25 weeks of age was 6-10-fold higher than that in GLAko mice of the same age. G3Stg/GLAko mice showed progressive renal impairment, with albuminuria at 3 weeks of age, decreased urine osmolality at 5 weeks, polyuria at 10 weeks and increased blood urea nitrogen at 15 weeks. The urine volume and urinary albumin concentration were significantly reduced in the G3Stg/GLAko mice when human recombinant α-Gal A was administered intravenously. These data suggest that Gb3 accumulation is a primary pathogenic factor in the symptomatic phenotype of G3Stg/GLAko mice, and that this mouse line is suitable for studying the pathogenesis of Fabry disease and for preclinical studies of candidate therapies.

Metabolic and cellular bases of sphingolipidoses

Lysosomes are cellular stomachs. They degrade macromolecules and release their components as nutrients into the cytosol. Digestion of sphingolipids and other membrane lipids occurs at luminal intraendosomal vesicles and IMs (intraendosomal membranes). Sphingolipid and membrane digestion needs catabolic hydrolases with the help of lipid-binding proteins [SAPs (sphingolipid activator proteins)] and anionic lipids such as BMP [bis(monoacylglycero)phosphate]. Inherited defects of hydrolases or SAPs or uptake of cationic amphiphilic drugs cause lipid accumulation, eventually leading to death, especially in inherited sphingolipid storage diseases. IMs are formed during endocytosis and their lipid composition is adjusted for degradation. Their cholesterol content, which stabilizes membranes, decreases and the level of negatively charged BMP, which stimulates sphingolipid degradation, increases. At the level of late endosomes, cholesterol is transported out of the luminal vesicles preferentially by cholesterol-binding proteins, NPC (Niemann-Pick type C)-2 and NPC-1. Their defects lead to an endolysosomal accumulation of cholesterol and sphingolipids in Niemann-Pick type C disease. BMP and ceramide stimulate NPC-2-mediated cholesterol transfer, whereas sphingomyelin inhibits it. Anionic membrane lipids also activate sphingomyelin degradation by ASM (acid sphingomyelinase), facilitating cholesterol export by NPC-2. ASM is a non-specific phospholipase C and degrades more than 23 phospholipids. SAPs are membrane-perturbing proteins which solubilize lipids, facilitating glycolipid digestion by presenting them to soluble catabolic enzymes at acidic pH. High BMP and low cholesterol levels favour lipid extraction and membrane disintegration by saposin A and B. The simultaneous inherited defect of saposins A-D causes a severe membrane and sphingolipid storage disease, also disrupting the water permeability barrier of the skin.

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.

A novel homozygous splicing mutation in PSAP gene causes metachromatic leukodystrophy in two Moroccan brothers

Prosaposin (PSAP) gene mutations, affecting saposin B (Sap-B) domain, cause a rare metachromatic leukodystrophy (MLD) variant in which arylsulfatase A (ARSA) activity is normal. To date, only 10 different PSAP mutations have been associated with a total of 18 unrelated MLD patients worldwide. In this study, we report for the first time a family with Moroccan origins in which the proband, presenting with a late-infantile onset of neurological involvement and a brain MRI with the typical tigroid MLD pattern, showed normal values of ARSA activity in the presence of an abnormal pattern of urinary sulfatides. In view of these findings, PSAP gene was analyzed, identifying the newly genomic homozygous c.909 + 1G > A mutation occurring within the invariant GT dinucleotide of the intron 8 donor splice site. Reverse transcriptase-polymerase chain reaction (RT-PCR), showing the direct junction of exon 7 to exon 9, confirmed the skipping of the entire exon 8 (p.Gln260_Lys303) which normally contains two cysteine residues (Cys271 and Cys265) involved in disulfide bridges. Our report provides further evidence that phenotypes of patients with Sap-B deficiency vary widely depending on age of onset, type, and severity of symptoms. Awareness of this rare MLD variant is crucial to prevent delayed diagnosis or misdiagnosis and to promptly provide an accurate genetic counseling, including prenatal diagnosis, to families.

Sphingolipids and lysosomal pathologies

Endocytosed (glyco)sphingolipids are degraded, together with other membrane lipids in a stepwise fashion by endolysosomal enzymes with the help of small lipid binding proteins, the sphingolipid activator proteins (SAPs), at the surface of intraluminal lysosomal vesicles. Inherited defects in a sphingolipid-degrading enzyme or SAP cause the accumulation of the corresponding lipid substrates, including cytotoxic lysosphingolipids, such as galactosylsphingosine and glucosylsphingosine, and lead to a sphingolipidosis. Analysis of patients with prosaposin deficiency revealed the accumulation of intra-endolysosmal vesicles and membrane structures (IM). Feeding of prosaposin reverses the storage, suggesting inner membrane structures as platforms of sphingolipid degradation. Water soluble enzymes can hardly attack sphingolipids embedded in the membrane of inner endolysosomal vesicles. The degradation of sphingolipids with few sugar residues therefore requires the help of the SAPs, and is strongly stimulated by anionic membrane lipids. IMs are rich in anionic bis(monoacylglycero)phosphate (BMP). This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Gangliosides and gangliosidoses: principles of molecular and metabolic pathogenesis

Gangliosides are the main glycolipids of neuronal plasma membranes. Their surface patterns are generated by coordinated processes, involving biosynthetic pathways of the secretory compartments, catabolic steps of the endolysosomal system, and intracellular trafficking. Inherited defects in ganglioside biosynthesis causing fatal neurodegenerative diseases have been described so far almost exclusively in mouse models, whereas inherited defects in ganglioside catabolism causing various clinical forms of GM1- and GM2-gangliosidoses have long been known. For digestion, gangliosides are endocytosed and reach intra-endosomal vesicles. At the level of late endosomes, they are depleted of membrane-stabilizing lipids like cholesterol and enriched with bis(monoacylglycero)phosphate (BMP). Lysosomal catabolism is catalyzed at acidic pH values by cationic sphingolipid activator proteins (SAPs), presenting lipids to their respective hydrolases, electrostatically attracted to the negatively charged surface of the luminal BMP-rich vesicles. Various inherited defects of ganglioside hydrolases, e.g., of β-galactosidase and β-hexosaminidases, and of GM2-activator protein, cause infantile (with tetraparesis, dementia, blindness) and different protracted clinical forms of GM1- and GM2-gangliosidoses. Mutations yielding proteins with small residual catabolic activities in the lysosome give rise to juvenile and adult clinical forms with a wide range of clinical symptomatology. Apart from patients' differences in their genetic background, clinical heterogeneity may be caused by rather diverse substrate specificities and functions of lysosomal hydrolases, multifunctional properties of SAPs, and the strong regulation of ganglioside catabolism by membrane lipids. Currently, there is no treatment available for neuronal ganglioside storage diseases. Therapeutic approaches in mouse models and patients with juvenile forms of gangliosidoses are discussed.

Inactivation of ceramide synthase 6 in mice results in an altered sphingolipid metabolism and behavioral abnormalities

The N-acyl chain length of ceramides is determined by the specificity of different ceramide synthases (CerS). The CerS family in mammals consists of six members with different substrate specificities and expression patterns. We have generated and characterized a mouse line harboring an enzymatically inactive ceramide synthase 6 (CerS6KO) gene and lacz reporter cDNA coding for β-galactosidase directed by the CerS6 promoter. These mice display a decrease in C16:0 containing sphingolipids. Relative to wild type tissues the amount of C16:0 containing sphingomyelin in kidney is ∼35%, whereas we find a reduction of C16:0 ceramide content in the small intestine to about 25%. The CerS6KO mice show behavioral abnormalities including a clasping abnormality of their hind limbs and a habituation deficit. LacZ reporter expression in the brain reveals CerS6 expression in hippocampus, cortex, and the Purkinje cell layer of the cerebellum. Using newly developed antibodies that specifically recognize the CerS6 protein we show that the endogenous CerS6 protein is N-glycosylated and expressed in several tissues of mice, mainly kidney, small and large intestine, and brain.

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.

The genetics of sphingolipid hydrolases and sphingolipid storage diseases

The relationship of sphingolipids with human disease first arose from the study of sphingolipid storage diseases over 50 years ago. Most of these disorders are due to inherited deficiencies of specific sphingolipid hydrolases, although a small number also result from defects in sphingolipid transport or activator proteins. Due to the primary protein deficiencies sphingolipids and other macromolecules accumulate in cells and tissues of affected patients, leading to a diverse presentation of clinical abnormalities. Over 25 sphingolipid storage diseases have been described to date. Most of the genes have been isolated, disease-causing mutations have been identified, the recombinant proteins have been produced and characterized, and animal models exist for most of the human diseases. Since most sphingolipid hydrolases are enriched within the endosomal/lysosomal system, macromolecules first accumulate within these compartments. However, these abnormalities rapidly spread to other compartments and cause a wide range of cellular dysfunction. This review focuses on the genetics of sphingolipid storage diseases and related hydrolytic enzymes with an emphasis on the relationship between genetic mutations and human disease.

Ganglioside biochemistry

Gangliosides are sialic acid-containing glycosphingolipids. They occur especially on the cellular surfaces of neuronal cells, where they form a complex pattern, but are also found in many other cell types. The paper provides a general overview on their structures, occurrence, and metabolism. Key functional, biochemical, and pathobiochemical aspects are summarized.

My journey into the world of sphingolipids and sphingolipidoses

Analysis of lipid storage in postmortem brains of patients with amaurotic idiocy led to the recognition of five lysosomal ganglioside storage diseases and identification of their inherited metabolic blocks. Purification of lysosomal acid sphingomyelinase and ceramidase and analysis of their gene structures were the prerequisites for the clarification of Niemann-Pick and Farber disease. For lipid catabolism, intraendosomal vesicles are formed during the endocytotic pathway. They are subjected to lipid sorting processes and were identified as luminal platforms for cellular lipid and membrane degradation. Lipid binding glycoproteins solubilize lipids from these cholesterol poor membranes and present them to water-soluble hydrolases for digestion. Biosynthesis and intracellular trafficking of lysosomal hydrolases (hexosaminidases, acid sphingomyelinase and ceramidase) and lipid binding and transfer proteins (GM2 activator, saposins) were analyzed to identify the molecular and metabolic basis of several sphingolipidoses. Studies on the biosynthesis of glycosphingolipids yielded the scheme of Combinatorial Ganglioside Biosynthesis involving promiscuous glycosyltransferases. Their defects in mutagenized mice impair brain development and function.

Chronic caloric restriction attenuates a loss of sulfatide content in PGC-1α-/- mouse cortex: a potential lipidomic role of PGC-1α in neurodegeneration

Peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α), a key regulator of energy metabolism and lipid homeostasis in multiple highly oxidative tissues, has been implicated in the metabolic derangements of diabetes and obesity. However, relatively less is known regarding its role in neurological functions. Using shotgun lipidomics, we investigated the lipidome of mouse cerebral cortex with generalized deficiency of PGC-1α (PGC-1α(-/-)) versus wild-type (WT) mice under standard diet and chronically calorically restricted conditions. Specific deficiency in sulfatide, a myelin-specific lipid class critically involved in maintaining neurological function, was uncovered in the cortex of PGC-1α(-/-) mice compared with WT mice at all ages examined. Chronic caloric restriction (CR) for 22 months essentially restored the sulfatide reduction in PGC-1α(-/-) mice compared with WT, but sulfatide reduction was not restored in PGC-1α(-/-) with CR for a short term (i.e., 3 months). Mechanistic studies uncovered and differentiated the biochemical mechanisms underpinning the two conditions of altered sulfatide homeostasis. The former is modulated through PGC-1α-MAL pathway, whereas the latter is under the control of LXR/RXR-apoE metabolism pathway. These results suggest a novel mechanistic role of PGC-1α in sulfatide homeostasis, provide new insights into the importance of PGC-1α in neurological functions, and indicate a potential therapeutic approach for treatment of deficient PGC-1α-induced alterations in sulfatide homeostasis.

Isofagomine in vivo effects in a neuronopathic Gaucher disease mouse

The pharmacological chaperone, isofagomine (IFG), enhances acid β-glucosidase (GCase) function by altering folding, trafficking, and activity in wild-type and Gaucher disease fibroblasts. The in vivo effects of IFG on GCase activity, its substrate levels, and phenotype were evaluated using a neuronopathic Gaucher disease mouse model, 4L;C* (V394L/V394L + saposin C-/-) that has CNS accumulation of glucosylceramide (GC) and glucosylsphingosine (GS) as well as progressive neurological deterioration. IFG administration to 4L;C* mice at 20 or 600 mg/kg/day resulted in life span extensions of 10 or 20 days, respectively, and increases in GCase activity and protein levels in the brain and visceral tissues. Cerebral cortical GC and GS levels showed no significant reductions with IFG treatment. Increases of GC or GS levels were detected in the visceral tissues of IFG treated (600 mg/kg/day) mice. The attenuations of brain proinflammatory responses in the treated mice were evidenced by reductions in astrogliosis and microglial cell activation, and decreased p38 phosphorylation and TNFα levels. Terminally, axonal degeneration was present in the brain and spinal cord from untreated and treated 4L;C* mice. These data demonstrate that IFG exerts in vivo effects by enhancing V394L GCase protein and activity levels, and in mediating suppression of proinflammation, which led to delayed onset of neurological disease and extension of the life span of 4L;C* mice. However, this was not correlated with a reduction in the accumulation of lipid substrates.

Accumulation and distribution of α-synuclein and ubiquitin in the CNS of Gaucher disease mouse models

Gaucher disease, a prevalent lysosomal storage disease, is caused by insufficient activity of acid β-glucosidase (GCase) and resultant glucosylceramide accumulation. Recently in Parkinson disease (PD) patients, heterozygous mutations in GCase have been associated with earlier onset and more progressive PD. To understand the pathogenic relationships between GCase variants and Parkinsonism, α-synuclein and ubiquitin distributions and levels in the brains of several mouse models containing GCase variants were evaluated by immunohistochemistry. Progressive α-synuclein and ubiquitin aggregate accumulations were observed in the cortex, hippocampus, basal ganglia, brainstem, and some cerebellar regions between 4 and 24 weeks in mice that were homozygous for GCase [D409H (9H) or V394L (4L)] variants and also had a prosaposin hypomorphic (PS-NA) transgene. In 4L/PS-NA and 9H/PS-NA mice, this was coincident with progressive neurological manifestations and brain glucosylceramide accumulation. Ultrastructural studies showed electron dense inclusion bodies in neurons and axons of 9H/PS-NA brains. α-synuclein aggregates were also observed in ventricular, brainstem, and cerebellar regions of older mice (>42-weeks) with the GCase variant (D409H/D409H) without overt neurological disease. In a chemically induced GCase deficiency, α-synuclein aggregates and glucosylceramide accumulation also occurred. These studies demonstrate a relationship between glucosylceramide accumulation and α-synuclein aggregates, and implicate glucosylceramide accumulation as risk factor for the α-synucleinopathies.

Myelin lesions associated with lysosomal and peroxisomal disorders

Abnormalities of myelin are common in lysosomal and peroxisomal disorders. Most display a primary loss of myelin in which the myelin sheath and/or oligodendrocytes are selectively targeted by diverse pathogenetic processes. The most severe and, hence, clinically relevant are heritable diseases predominantly of infants and children, the leukodystrophies: metachromatic, globoid cell (Krabbe disease) and adreno-leukodystrophy. Our still limited understanding of these diseases has derived from multiple sources: originally, neurological-neuropathologic-neurochemical correlative studies of the natural disease in humans or other mammals, which has been enhanced by more sophisticated and contemporary techniques of cell and molecular biology. Transgenic mouse models seem to be the most promising methodology, allowing the examination of the cellular role of lysosomes and peroxisomes for formation and maintenance of both myelin and axons, and providing initial platforms to evaluate therapies. Treatment options are woefully inadequate and in their nascent stages, but still inspire some hope for the future.

The role of UDP-Glc:glycoprotein glucosyltransferase 1 in the maturation of an obligate substrate prosaposin

A natural substrate for UGT1 is confirmed, revealing how the enzyme functions in the calnexin chaperone system as a quality control step in protein folding.

An endoplasmic reticulum (ER) quality control system assists in efficient folding and disposal of misfolded proteins. N-linked glycans are critical in these events because their composition dictates interactions with molecular chaperones. UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1) is a key quality control factor of the ER. It adds glucoses to N-linked glycans of nonglucosylated substrates that fail a quality control test, supporting additional rounds of chaperone binding and ER retention. How UGT1 functions in its native environment is poorly understood. The role of UGT1 in the maturation of glycoproteins at basal expression levels was analyzed. Prosaposin was identified as a prominent endogenous UGT1 substrate. A dramatic decrease in the secretion of prosaposin was observed in ugt1^−/− cells with prosaposin localized to large juxtanuclear aggresome-like inclusions, which is indicative of its misfolding and the essential role that UGT1 plays in its proper maturation. A model is proposed that explains how UGT1 may aid in the folding of sequential domain–containing proteins such as prosaposin.

Multi-system disorders of glycosphingolipid and ganglioside metabolism

Glycosphingolipids (GSLs) and gangliosides are a group of bioactive glycolipids that include cerebrosides, globosides, and gangliosides. These lipids play major roles in signal transduction, cell adhesion, modulating growth factor/hormone receptor, antigen recognition, and protein trafficking. Specific genetic defects in lysosomal hydrolases disrupt normal GSL and ganglioside metabolism leading to their excess accumulation in cellular compartments, particularly in the lysosome, i.e., lysosomal storage diseases (LSDs). The storage diseases of GSLs and gangliosides affect all organ systems, but the central nervous system (CNS) is primarily involved in many. Current treatments can attenuate the visceral disease, but the management of CNS involvement remains an unmet medical need. Early interventions that alter the CNS disease have shown promise in delaying neurologic involvement in several CNS LSDs. Consequently, effective treatment for such devastating inherited diseases requires an understanding of the early developmental and pathological mechanisms of GSL and ganglioside flux (synthesis and degradation) that underlie the CNS diseases. These are the focus of this review.

Structural aspects of therapeutic enzymes to treat metabolic disorders

Protein therapeutics represents a niche subset of pharmacological agents that is rapidly gaining importance in medicine. In addition to the exceptional specificity that is characteristic of protein therapeutics, several classes of proteins have also been effectively utilized for treatment of conditions that would otherwise lack effective pharmacotherapeutic options. A particularly striking class of protein therapeutics is exogenous enzymes administered for replacement therapy in patients afflicted with metabolic disorders. To date, at least 11 enzymes have either been approved for use, or are in clinical trials for the treatment of selected inherited metabolic disorders. With the recent advancement in structural biology, a significantly larger amount of structural information for several of these enzymes is now available. This article is an overview of the correlation between structural perturbations of these enzymes with the clinical presentation of the respective metabolic conditions, as well as a discussion of the relevant structural modification strategies engaged in improving these enzymes for replacement therapies.

Neuronopathic Gaucher disease in the mouse: viable combined selective saposin C deficiency and mutant glucocerebrosidase (V394L) mice with glucosylsphingosine and glucosylceramide accumulation and progressive neurological deficits

Gaucher disease is caused by defective acid β-glucosidase (GCase) function. Saposin C is a lysosomal protein needed for optimal GCase activity. To test the in vivo effects of saposin C on GCase, saposin C deficient mice (C−/−) were backcrossed to point mutated GCase (V394L/V394L) mice. The resultant mice (4L;C*) began to exhibit CNS abnormalities ∼30 days: first as hindlimb paresis, then progressive tremor and ataxia. Death occurred ∼48 days due to neurological deficits. Axonal degeneration was evident in brain stem, spinal cord and white matter of cerebellum accompanied by increasing infiltration of the brain stem, cortex and thalamus by CD68 positive microglial cells and activation of astrocytes. Electron microscopy showed inclusion bodies in neuronal processes and degenerating cells. Accumulation of p62 and Lamp2 were prominent in the brain suggesting the impairment of autophagosome/lysosome function. This phenotype was different from either V394L/V394L or C−/− alone. Relative to V394L/V394L mice, 4L;C* mice had diminished GCase protein and activity. Marked increases (20- to 30-fold) of glucosylsphingosine (GS) and moderate elevation (1.5- to 3-fold) of glucosylceramide (GC) were in 4L;C* brains. Visceral tissues had increases of GS and GC, but no storage cells were found. Neuronal cells in thick hippocampal slices from 4L;C* mice had significantly attenuated long-term potentiation, presumably resulting from substrate accumulation. The 4L;C* mouse mimics the CNS phenotype and biochemistry of some type 3 (neuronopathic) variants of Gaucher disease and is a unique model suitable for testing pharmacological chaperone and substrate reduction therapies, and investigating the mechanisms of neuronopathic Gaucher disease.

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.

Specific saposin C deficiency: CNS impairment and acid beta-glucosidase effects in the mouse

Saposins A, B, C and D are derived from a common precursor, prosaposin (psap). The few patients with saposin C deficiency develop a Gaucher disease-like central nervous system (CNS) phenotype attributed to diminished glucosylceramide (GC) cleavage activity by acid β-glucosidase (GCase). The in vivo effects of saposin C were examined by creating mice with selective absence of saposin C (C−/−) using a knock-in point mutation (cysteine-to-proline) in exon 11 of the psap gene. In C−/− mice, prosaposin and saposins A, B and D proteins were present at near wild-type levels, but the saposin C protein was absent. By 1 year, the C−/− mice exhibited weakness of the hind limbs and progressive ataxia. Decreased neuromotor activity and impaired hippocampal long-term potentiation were evident. Foamy storage cells were observed in dorsal root ganglion and there was progressive loss of cerebellar Purkinje cells and atrophy of cerebellar granule cells. Ultrastructural analyses revealed inclusions in axonal processes in the spinal cord, sciatic nerve and brain, but no excess of multivesicular bodies. Activated microglial cells and astrocytes were present in thalamus, brain stem, cerebellum and spinal cord, indicating regional pro-inflammatory responses. No storage cells were found in visceral organs of these mice. The absence of saposin C led to moderate increases in GC and lactosylceramide (LacCer) and their deacylated analogues. These results support the view that saposin C has multiple roles in glycosphingolipid (GSL) catabolism as well as a prominent function in CNS and axonal integrity independent of its role as an optimizer/stabilizer of GCase.

Sphingolipid activator protein B deficiency: report of 9 Saudi patients and review of the literature

Mutated PSAP gene resulting in sphingolipid activator protein B deficiency is known to cause metachromatic leukodystrophy variant in which arylsulfatase A is normal. Of 16 patients with metachromatic leukodystrophy that were evaluated in our center, 7 patients were diagnosed with arylsulfatase A-deficient metachromatic leukodystrophy, whereas 9 children from 4 unrelated Saudi families were found to have sphingolipid activator protein B deficiency. PSAP analysis found that the 4 families segregate the same homozygous mutation that was a g.722G>C transversion resulting in C241S change, which was previously reported in an Arab patient. Our work, which reports the largest series of patients with sphingolipid activator protein B deficiency, suggests that this variant is likely to be more common than arylsulfatase A-deficient metachromatic leukodystrophy in Arabs, a notion that has potential diagnostic and preventive implications.

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.