Acid sphingomyelinase possesses a domain homologous to its activator proteins: saposins B and D

SMPD1 expression profile and mutation landscape help decipher genotype-phenotype association and precision diagnosis for acid sphingomyelinase deficiency

BACKGROUND

Acid sphingomyelinase deficiency (ASMD) disorder, also known as Niemann-Pick disease (NPD) is a rare genetic disease caused by mutations in SMPD1 gene, which encodes sphingomyelin phosphodiesterase (ASM). Except for liver and spleen enlargement and lung disease, two subtypes (Type A and B) of NDP have different onset times, survival times, ASM activities, and neurological abnormalities. To comprehensively explore NPD's genotype-phenotype association and pathophysiological characteristics, we collected 144 NPD cases with strict quality control through literature mining.

RESULTS

The difference in ASM activity can differentiate NPD type A from other subtypes, with the ratio of ASM activity to the reference values being lower in type A (threshold 0.045 (4.45%)). Severe variations, such as deletion and insertion, can cause complete loss of ASM function, leading to type A, whereas relatively mild missense mutations generally result in type B. Among reported mutations, the p.Arg3AlafsX76 mutation is highly prevalent in the Chinese population, and the p.R608del mutation is common in Mediterranean countries. The expression profiles of SMPD1 from GTEx and single-cell RNA sequencing data of multiple fetal tissues showed that high expressions of SMPD1 can be observed in the liver, spleen, and brain tissues of adults and hepatoblasts, hematopoietic stem cells, STC2_TLX1-positive cells, mesothelial cells of the spleen, vascular endothelial cells of the cerebellum and the cerebrum of fetuses, indicating that SMPD1 dysfunction is highly likely to have a significant effect on the function of those cell types during development and the clinicians need pay attention to these organs or tissues as well during diagnosis. In addition, we also predicted 21 new pathogenic mutations in the SMPD1 gene that potentially cause the NPD, signifying that more rare cases will be detected with those mutations in SMPD1. Finally, we also analysed the function of the NPD type A cells following the extracellular milieu.

CONCLUSIONS

Our study is the first to elucidate the effects of SMPD1 mutation on cell types and at the tissue level, which provides new insights into the genotype-phenotype association and can help in the precise diagnosis of NPD.

The Niemann-Pick type diseases – A synopsis of inborn errors in sphingolipid and cholesterol metabolism

Disturbances of lipid homeostasis in cells provoke human diseases. The elucidation of the underlying mechanisms and the development of efficient therapies represent formidable challenges for biomedical research. Exemplary cases are two rare, autosomal recessive, and ultimately fatal lysosomal diseases historically named "Niemann-Pick" honoring the physicians, whose pioneering observations led to their discovery. Acid sphingomyelinase deficiency (ASMD) and Niemann-Pick type C disease (NPCD) are caused by specific variants of the sphingomyelin phosphodiesterase 1 (SMPD1) and NPC intracellular cholesterol transporter 1 (NPC1) or NPC intracellular cholesterol transporter 2 (NPC2) genes that perturb homeostasis of two key membrane components, sphingomyelin and cholesterol, respectively. Patients with severe forms of these diseases present visceral and neurologic symptoms and succumb to premature death. This synopsis traces the tortuous discovery of the Niemann-Pick diseases, highlights important advances with respect to genetic culprits and cellular mechanisms, and exposes efforts to improve diagnosis and to explore new therapeutic approaches.

Acid Sphingomyelinase Deficiency: A Clinical and Immunological Perspective

Acid sphingomyelinase deficiency (ASMD) is a lysosomal storage disease caused by deficient activity of acid sphingomyelinase (ASM) enzyme, leading to the accumulation of varying degrees of sphingomyelin. Lipid storage leads to foam cell infiltration in tissues, and clinical features including hepatosplenomegaly, pulmonary insufficiency and in some cases central nervous system involvement. ASM enzyme replacement therapy is currently in clinical trial being the first treatment addressing the underlying pathology of the disease. Therefore, presently, it is critical to better comprehend ASMD to improve its diagnose and monitoring. Lung disease, including recurrent pulmonary infections, are common in ASMD patients. Along with lung disease, several immune system alterations have been described both in patients and in ASMD animal models, thus highlighting the role of ASM enzyme in the immune system. In this review, we summarized the pivotal roles of ASM in several immune system cells namely on macrophages, Natural Killer (NK) cells, NKT cells, B cells and T cells. In addition, an overview of diagnose, monitoring and treatment of ASMD is provided highlighting the new enzyme replacement therapy available.

Antiviral Activity of a Turbot (Scophthalmus maximus) NK-Lysin Peptide by Inhibition of Low-pH Virus-Induced Membrane Fusion

Global health is under attack by increasingly-frequent pandemics of viral origin. Antimicrobial peptides are a valuable tool to combat pathogenic microorganisms. Previous studies from our group have shown that the membrane-lytic region of turbot (Scophthalmus maximus) NK-lysine short peptide (Nkl_71–100) exerts an anti-protozoal activity, probably due to membrane rupture. In addition, NK-lysine protein is highly expressed in zebrafish in response to viral infections. In this work several biophysical methods, such as vesicle aggregation, leakage and fluorescence anisotropy, are employed to investigate the interaction of Nkl_71–100 with different glycerophospholipid vesicles. At acidic pH, Nkl_71–100 preferably interacts with phosphatidylserine (PS), disrupts PS membranes, and allows the content leakage from vesicles. Furthermore, Nkl_71–100 exerts strong antiviral activity against spring viremia of carp virus (SVCV) by inhibiting not only the binding of viral particles to host cells, but also the fusion of virus and cell membranes, which requires a low pH context. Such antiviral activity seems to be related to the important role that PS plays in these steps of the replication cycle of SVCV, a feature that is shared by other families of virus-comprising members with health and veterinary relevance. Consequently, Nkl_71–100 is shown as a promising broad-spectrum antiviral candidate.

Characterization of Sphingomyelin Phosphodiesterase Expression in Bumblebee (Bombus lantschouensis)

Sphingomyelin phosphodiesterase (SMPD) is a hydrolase that plays a major role in metabolic reactions involving sphingomyelin. Here, we describe an analysis of the cDNA sequence and gene structure of SMPD in bumblebee (Bombus lantschouensis). The expression of SMPD in different tissues and at different developmental stages and reproductive statuses was examined by real-time polymerase chain reaction (PCR). The results showed that the SMPD cDNA has a length of 2000 bp and contains an open reading frame (ORF) of 1,801 nucleotides that encodes a polypeptide of 599 amino acids. The full-length SMPD gene is 4228 bp and contains eight exons and seven introns. A comparative analysis revealed that the SMPD gene sequence in B. lantschouensis shares high sequence identity with those in other Bombus species. The SMPD gene is expressed broadly in various tissues and presents higher transcript levels in the ovary, midgut, and epidermis and thoracic tissues. Among the different developmental stages, the highest expression of SMPD was detected at the Pw pupal stage (pupae with an unpigmented body cuticle and white eyes), and the expression of this gene decreased from the Pp (pupae with pink eyes) to the Pdd (dark-eye pupae with a dark-pigmented cuticle) stages. In addition, SMPD expression was significantly upregulated after female egg laying. In conclusion, our results show that the bumblebee SMPD gene might play a key role at the Pw developmental stage and in female oviposition.

Structure of Human Acid Sphingomyelinase Reveals the Role of the Saposin Domain in Activating Substrate Hydrolysis

Acid sphingomyelinase (ASM) is a lysosomal phosphodiesterase that catalyzes the hydrolysis of sphingomyelin to produce ceramide and phosphocholine. While other lysosomal sphingolipid hydrolases require a saposin activator protein for full activity, the ASM polypeptide incorporates a built-in N-terminal saposin domain and does not require an external activator protein. Here, we report the crystal structure of human ASM and describe the organization of the three main regions of the enzyme: the N-terminal saposin domain, the proline-rich connector, and the catalytic domain. The saposin domain is tightly associated along an edge of the large, bowl-shaped catalytic domain and adopts an open form that exposes a hydrophobic concave surface approximately 30Å from the catalytic center. The calculated electrostatic potential of the enzyme is electropositive at the acidic pH of the lysosome, consistent with the strict requirement for the presence of acidic lipids in target membranes. Docking studies indicate that sphingomyelin binds with the ceramide-phosphate group positioned at the binuclear zinc center and molecular dynamic simulations indicate that the intrinsic flexibility of the saposin domain is important for monomer-dimer exchange and for membrane interactions. Overall, ASM uses a combination of electrostatic and hydrophobic interactions to cause local disruptions of target bilayers in order to bring the lipid headgroup to the catalytic center in a membrane-bound reaction.

The structure and catalytic mechanism of human sphingomyelin phosphodiesterase like 3a--an acid sphingomyelinase homologue with a novel nucleotide hydrolase activity

Human sphingomyelinase phosphodiesterase like 3a (SMPDL3a) is a secreted enzyme that shares a conserved catalytic domain with human acid sphingomyelinase (aSMase), the enzyme carrying mutations causative of Niemann-Pick disease. We have solved the structure of SMPDL3a revealing a calcineurin-like fold. A dimetal site, glycosylation pattern and a disulfide bond network are likely to be conserved also in human aSMase. We show that the binuclear site of SMPDL3a is occupied by two Zn(2+) ions and that excess Zn(2+) leads to inhibition of enzyme activity through binding to additional sites. As an extension of recent biochemical work we uncovered that SMPDL3a catalyses the hydrolysis of several modified nucleotides that include cytidine 5'-diphosphocholine, cytidine diphosphate ethanolamine and ADP-ribose, but not the aSMase substrate, sphingomyelin. We subsequently determined the structure of SMPDL3a in complex with the product 5'-cytidine monophosphate (CMP), a structure that is consistent with several distinct coordination modes of the substrate/product in the active site during the reaction cycle. Based on the structure of CMP complexes, we propose a phosphoryl transfer mechanism for SMPDL3a. Finally, a homology model of human aSMase was constructed to allow for the mapping of selected Niemann-Pick disease mutations on a three-dimensional framework to guide further characterization of their effects on aSMase function.

DATABASE

Structural data are available in the PDB database under the accession numbers 5EBB and 5EBE.

SK. Structural exploration of acid sphingomyelinase at different physiological pH through molecular dynamics and docking studies

Acid sphingomyelinase (ASM) hydrolysis the sphingomyelin at physiological pH 5.0 and subsequently leads to ceramide production.

SMPD1 Mutation Update: Database and Comprehensive Analysis of Published and Novel Variants

Niemann-Pick Types A and B (NPA/B) diseases are autosomal recessive lysosomal storage disorders caused by the deficient activity of acid sphingomyelinase (ASM) because of the mutations in the SMPD1 gene. Here, we provide a comprehensive updated review of already reported and newly identified SMPD1 variants. Among them, 185 have been found in NPA/B patients. Disease-causing variants are equally distributed along the SMPD1 gene; most of them are missense (65.4%) or frameshift (19%) mutations. The most frequently reported mutation worldwide is the p.R610del, clearly associated with an attenuated NP disease type B phenotype. The available information about the impact of 52 SMPD1 variants on ASM mRNA and/or enzymatic activity has been collected and whenever possible, phenotype/genotype correlations were established. In addition, we created a locus-specific database easily accessible at http://www.inpdr.org/genes that catalogs the 417 SMPD1 variants reported to date and provides data on their in silico predicted effects on ASM protein function or mRNA splicing. The information reviewed in this article, providing new insights into the genotype/phenotype correlation, is extremely valuable to facilitate diagnosis and genetic counseling of families affected by NPA/B.

Acid sphingomyelinase activity is regulated by membrane lipids and facilitates cholesterol transfer by NPC2

During endocytosis, membrane components move to intraluminal vesicles of the endolysosomal compartment for digestion. At the late endosomes, cholesterol is sorted out mainly by two sterol-binding proteins, Niemann-Pick protein type C (NPC)1 and NPC2. To study the NPC2-mediated intervesicular cholesterol transfer, we developed a liposomal assay system. (Abdul-Hammed, M., B. Breiden, M. A. Adebayo, J. O. Babalola, G. Schwarzmann, and K. Sandhoff. 2010. Role of endosomal membrane lipids and NPC2 in cholesterol transfer and membrane fusion. J. Lipid Res. 51: 1747-1760.) Anionic lipids stimulate cholesterol transfer between liposomes while SM inhibits it, even in the presence of anionic bis(monoacylglycero)phosphate (BMP). Preincubation of vesicles containing SM with acid sphingomyelinase (ASM) (SM phosphodiesterase, EC 3.1.4.12) results in hydrolysis of SM to ceramide (Cer), which enhances cholesterol transfer. Besides SM, ASM also cleaves liposomal phosphatidylcholine. Anionic phospholipids derived from the plasma membrane (phosphatidylglycerol and phosphatidic acid) stimulate SM and phosphatidylcholine hydrolysis by ASM more effectively than BMP, which is generated during endocytosis. ASM-mediated hydrolysis of liposomal SM was also stimulated by incorporation of diacylglycerol (DAG), Cer, and free fatty acids into the liposomal membranes. Conversely, phosphatidylcholine hydrolysis was inhibited by incorporation of cholesterol, Cer, DAG, monoacylglycerol, and fatty acids. Our data suggest that SM degradation by ASM is required for physiological secretion of cholesterol from the late endosomal compartment, and is a key regulator of endolysosomal lipid digestion.

Sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) is a novel nucleotide phosphodiesterase regulated by cholesterol in human macrophages

Cholesterol-loaded foam cell macrophages are prominent in atherosclerotic lesions and play complex roles in both inflammatory signaling and lipid metabolism, which are underpinned by large scale reprogramming of gene expression. We performed a microarray study of primary human macrophages that showed that transcription of the sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A) gene is up-regulated after cholesterol loading. SMPDL3A protein expression in and secretion from primary macrophages are stimulated by cholesterol loading, liver X receptor ligands, and cyclic AMP, and N-glycosylated SMPDL3A protein is detectable in circulating blood. We demonstrate for the first time that SMPDL3A is a functional phosphodiesterase with an acidic pH optimum. We provide evidence that SMPDL3A is not an acid sphingomyelinase but unexpectedly is active against nucleotide diphosphate and triphosphate substrates at acidic and neutral pH. SMPDL3A is a major source of nucleotide phosphodiesterase activity secreted by liver X receptor-stimulated human macrophages. Extracellular nucleotides such as ATP may activate pro-inflammatory responses in immune cells. Increased expression and secretion of SMPDL3A by cholesterol-loaded macrophage foam cells in lesions may decrease local concentrations of pro-inflammatory nucleotides and potentially represent a novel anti-inflammatory axis linking lipid metabolism with purinergic signaling in atherosclerosis.

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.

Roles and regulation of secretory and lysosomal acid sphingomyelinase

Acid sphingomyelinase occupies a prominent position in sphingolipid catabolism, catalyzing the hydrolysis of sphingomyelin to ceramide and phosphorylcholine. Enzymatic dysfunction of acid sphingomyelinase results in Niemann-Pick disease, a lysosomal storage disorder characterized at the cellular level by accumulation of sphingomyelin within the endo-lysosomal compartment. Over the past decade interest in the role of acid sphingomyelinase has moved beyond its "housekeeping" function in constitutive turnover of sphingomyelin in the lysosome to include study of regulated ceramide generation. Ceramide functions as a bioactive sphingolipid with pleiotropic signaling properties, and has been implicated in diverse cellular processes of physiologic and pathophysiologic importance. Though many cellular enzymes have the capacity to generate ceramide,there is growing appreciation that "all ceramides are not created equal." Ceramides likely exert distinct effects in different cellular/subcellular compartments by virtue of access to other sphingolipid enzymes (e.g.ceramidases), effector molecules (e.g. ceramide-activated protein phosphatases), and neighboring lipids and proteins (e.g. cholesterol, ion channels). One of the unique features of acid sphingomyelinase is that it has been implicated in the hydrolysis of sphingomyelin in three different settings--the endo-lysosomal compartment,the outer leaflet of the plasma membrane, and lipoproteins. How a single gene product has the capacity to function in these diverse settings, and the subsequent impact on downstream ceramide-mediated biology is the subject of this review.

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.

Saposin A Mobilizes Lipids from Low Cholesterol and High Bis(monoacylglycerol)phosphate-containing Membranes

Saposin A (Sap-A) is one of five known sphingolipid activator proteins required for the lysosomal degradation of sphingolipids and for the loading of lipid antigens onto antigen-presenting molecules of the CD1 type. Sap-A assists in the degradation of galactosylceramide by galactosylceramide-beta-galactosidase in vivo, which takes place at the surface of intraendosomal/intralysosomal vesicles. Sap-A is believed to mediate the interaction between the enzyme and its membrane-bound substrate. Its dysfunction causes a variant form of Krabbe disease. In the present study we prepared glycosylated Sap-A free of other Saps, taking advantage of the Pichia pastoris expression system. Using liposomes and surface plasmon resonance spectroscopy, we tested the binding and lipid mobilization capacity of Sap-A under different conditions. Along the endocytic pathway, the pH value decreases, and the lipid composition of intraendosomal and intralysosomal membranes changes drastically. In the inner membranes the cholesterol concentration decreases, and that of the anionic phospholipid bis(monoacylglycero)phosphate increases. Here, we show that Sap-A is able to bind to liposomes and to mobilize lipids out of them at acidic pH values below pH 4.7. Low cholesterol levels and increasing concentrations of bis(monoacylglycero)phosphate favor lipid extraction significantly. Galactosylceramide as a bilayer component is not essential for lipid mobilization by Sap-A, which requires intact disulfide bridges for activity. We also show for the first time that glycosylation of Sap-A is essential for its lipid extraction activity. Variant Sap-A proteins, which cause storage of galactosylceramide in humans (Krabbe disease, Spiegel, R., Bach, G., Sury, V., Mengistu, G., Meidan, B., Shalev, S., Shneor, Y., Mandel, H., and Zeigler, M. (2005) Mol. Genet. Metab. 84, 160-166) and in mutant mice (Matsuda, J., Vanier, M. T., Saito, Y., Tohyama, J., and Suzuki, K. (2001) Hum. Mol. Genet. 10, 1191-1199) are deficient in lipid extraction capacity.

The Lysosomal Trafficking of Acid Sphingomyelinase is Mediated by Sortilin and Mannose 6-phosphate Receptor

Acid sphingomyelinase (ASM), a member of the saposin-like protein (SAPLIP) family, is a lysosomal hydrolase that converts sphingomyelin to ceramide. Deficiency of ASM causes a variant form of Niemann-Pick disease. The mechanism of lysosomal targeting of ASM is poorly known. Previous studies suggest that ASM could use in part the mannose 6-phosphate receptor (M6P-Rc). Sortilin, a type I transmembrane glycoprotein that belongs to a novel family of receptor proteins, presents structural features of receptors involved in lysosomal targeting. In this study we examined the hypothesis that sortilin may be implicated in the trafficking of ASM to the lysosomes. Using a dominant-negative sortilin construct lacking the cytoplasmic tail, which is essential to recruit adaptor proteins and clathrin, we demonstrated that sortilin is also involved in the lysosomal targeting of ASM. Confocal microscopy revealed that truncated sortilin partially inhibited the lysosomal trafficking of ASM in COS-7 cells and abolished the lysosomal targeting of ASM in I-cells. Pulse-chase experiments corroborated that sortilin is involved in normal sorting of newly synthesized ASM. Furthermore, over-expression of truncated sortilin accelerated and enhanced the secretion of ASM from COS-7 cells and I-cells. Co-immunoprecipitation assays confirmed the interaction between sortilin and ASM. In conclusion, ASM uses sortilin as an alternative receptor to be targeted to the lysosomes.

A short guided tour through functional and structural features of saposin-like proteins

SAPLIPs (saposin-like proteins) are a diverse family of lipid-interacting proteins that have various and only partly understood, but nevertheless essential, cellular functions. Their existence is conserved in phylogenetically most distant organisms, such as primitive protozoa and mammals. Owing to their remarkable sequence variability, a common mechanism for their actions is not known. Some shared principles beyond their diversity have become evident by analysis of known three-dimensional structures. Whereas lipid interaction is the basis for their functions, the special cellular tasks are often defined by interaction partners other than lipids. Based on recent findings, this review summarizes phylogenetic relations, function and structural features of the members of this family.

A model of the acid sphingomyelinase phosphoesterase domain based on its remote structural homolog purple acid phosphatase

Sequence profile and fold recognition methods identified mammalian purple acid phosphatase (PAP), a member of a dimetal-containing phosphoesterase (DMP) family, as a remote homolog of human acid sphingomyelinase (ASM). A model of the phosphoesterase domain of ASM was built based on its predicted secondary structure and the metal-coordinating residues of PAP. Due to the low sequence identity between ASM and PAP (approximately 15%), the highest degree of confidence in the model resides in the metal-binding motifs. The ASM model predicts residues Asp 206, Asp 278, Asn 318, His 425, and His 457 to be dimetal coordinating. A putative orientation for the phosphorylcholine head group of the ASM substrate, sphingomyelin (SM), was made based on the predicted catalysis of the phosphorus-oxygen bond in the active site of ASM and on a structural comparison of the PAP-phosphate complex to the C-reactive protein-phosphorylcholine complex. These complexes revealed similar spatial interactions between the metal-coordinating residues, the metals, and the phosphate groups, suggesting a putative orientation for the head group in ASM consistent with the mechanism considerations. A conserved sequence motif in ASM, NX3CX3N, was identified (Asn 381 to Asn 389) and is predicted to interact with the choline amine moiety in SM. The resulting ASM model suggests that the enzyme uses an SN2-type catalytic mechanism to hydrolyze SM, similar to other DMPs. His 319 in ASM is predicted to protonate the ceramide-leaving group in the catalysis of SM. The putative functional roles of several ASM Niemann-Pick missense mutations, located in the predicted phosphoesterase domain, are discussed in context to the model.

Functional characterization of the postulated intramolecular sphingolipid activator protein domain of human acid sphingomyelinase

Degradation of membrane-bound sphingomyelin to phosphorylcholine and ceramide is catalyzed by the water-soluble lysosomal acid sphingomyelinase (A-SMase). The presence of sphingolipid activator proteins (Saps: saposins A-D; GM2 activator) is not essential to mediate this reaction at the water-lipid interface in vivo . A hypothesis based on amino acid sequence alignments suggests that the enzyme possesses an N-terminal saposin-homologous domain, which may facilitate the enzymatic reaction at the interface. We mutated one homologous and three conserved amino acid residues of this domain and studied the activity of the variant enzymes using different sphingomyelin degradation assays. A variant with an exchange of a conserved amino acid residue, Pro153Ala, still exhibited enzyme activity of approximately 52% of normal in a detergent-containing micellar assay, but only 13% of normal in a detergent-free liposomal assay system, which suggests that the Sap-homologous domain fulfills membrane-disturbing functions. Addition of saposin C to the liposomal assay mixtures increased the Pro153Ala variant sphingomyelinase activity to 46% of normal, indicating that the variant saposin-like domain can be substituted by the presence of the sphingolipid activator protein. On the other hand, the addition of saposin C did not result in complete restoration of the variant activity. Thus, the Sap-like domain may also have another role, e.g., to stabilize the fold of acid sphingomyelinase, which cannot be compensated by the presence of saposin C or a detergent. Such an essential second function of the saposin-like domain as an integral part of acid sphingomyelinase is confirmed by our observation that the Lys118Glu, Cys120Ser and Cys131Ser variants were almost completely devoid of activity in the detergent-containing micellar assay system as well as in the liposomal assay system in the presence of saposin C.

Acid and neutral sphingomyelinases: roles and mechanisms of regulation

Ceramide, an emerging bioactive lipid and second messenger, is mainly generated by hydrolysis of sphingomyelin through the action of sphingomyelinases. At least two sphingomyelinases, neutral and acid sphingomyelinases, are activated in response to many extracellular stimuli. Despite extensive studies, the precise cellular function of each of these sphingomyelinases in sphingomyelin turnover and in the regulation of ceramide-mediated responses is not well understood. Therefore, it is essential to elucidate the factors and mechanisms that control the activation of acid and neutral sphingomyelinases to understand their the roles in cell regulation. This review will focus on the molecular mechanisms that regulate these enzymes in vivo and in vitro, especially the roles of oxidants (glutathione, peroxide, nitric oxide), proteins (saposin, caveolin 1, caspases), and lipids (diacylglycerol, arachidonic acid, and ceramide).

Hypothetical proteins with putative enzyme activity in human amnion, lymphocyte, bronchial epithelial and kidney cell lines

A myriad of predicted proteins have been described based upon nucleic acid sequences but the existence of these structures has not been confirmed at the protein level. The aim of the study was therefore to show expression of hypothetical proteins in several cell lines and to provide the analytical basis for their identification and characterisation. We used two-dimensional gel electrophoresis with in-gel digestion of high protein spots and subsequent MALDI-TOF analysis of cell lysates from human amnion, lymphocyte, bronchial epithelial and kidney cell lines. A pI range from 3 to 10 was selected and second dimension was run using 9-16% gradient gels. A series of structures that have not been described before at the protein level were identified in several cell lines and were assigned to major enzyme systems including proteolysis (proteases, peptidases, ubiquitin), intermediary metabolism and oxidoreductases. We conclude that the proteomic approach used serves as a suitable tool to verify the existence of predicted/hypothetical proteins. The herein identified enzymes may contribute to several pathways/cascades in the human organism. Furthermore, analytical data given are of major relevance as pIs, a prerequisite to find proteins in a map, cannot be predicted from nucleic acid sequences.

Human acid sphingomyelinase

Human acid sphingomyelinase (haSMase, EC 3.1.4.12) catalyzes the lysosomal degradation of sphingomyelin to ceramide and phosphorylcholine. An inherited haSMase deficiency leads to Niemann-Pick disease, a severe sphingolipid storage disorder. The enzyme was purified and cloned over 10 years ago. Since then, only a few structural properties of haSMase have been elucidated. For understanding of its complex functions including its role in certain signaling and apoptosis events, complete structural information about the enzyme is necessary. Here, the identification of the disulfide bond pattern of haSMase is reported for the first time. Functional recombinant enzyme expressed in SF21 cells using the baculovirus expression system was purified and digested by trypsin. MALDI-MS analysis of the resulting peptides revealed the four disulfide bonds Cys120-Cys131, Cys385-Cys431, Cys584-Cys588 and Cys594-Cys607. Two additional disulfide bonds (Cys221-Cys226 and Cys227-Cys250) which were not directly accessible by tryptic cleavage, were identified by a combination of a method of partial reduction and MALDI-PSD analysis. In the sphingolipid activator protein (SAP)-homologous N-terminal domain of haSMase, one disulfide bond was assigned as Cys120-Cys131. The existence of two additional disulfide bridges in this region was proved, as was expected for the known disulfide bond pattern of SAP-type domains. These results support the hypothesis that haSMase possesses an intramolecular SAP-type activator domain as predicted by sequence comparison [Ponting, C.P. (1994) Protein Sci., 3, 359-361]. An additional analysis of haSMase isolated from human placenta shows that the recombinant and the native human protein possess an identical disulfide structure.

J3-crystallin of the jellyfish lens: similarity to saposins

J3-crystallin, one of the three major eye-lens proteins of the cubomedusan jellyfish (Tripedalia cystophora), shows similarity to vertebrate saposins, which are multifunctional proteins that bridge lysosomal hydrolases to lipids and activate enzyme activity. Sequence alignment of deduced J3-crystallin indicates two saposin-like motifs arranged in tandem, each containing six cysteines characteristic of this protein family. The J3-crystallin cDNA encodes a putative precursor analogous to vertebrate prosaposins. The J3-crystallin gene has seven exons, with exons 2-4 encoding the protein. Exon 3 encodes a circularly permutated saposin motif, called a swaposin, found in plant aspartic proteases. J3-crystallin RNA was found in the cubomedusan lens, statocyst, in bands radiating from the pigmented region of the ocellus, in the tentacle tip by in situ hybridization, and in the embryo and larva by reverse transcription-PCR. Our data suggest a crystallin role for the multifunctional saposin protein family in the jellyfish lens. This finding extends the gene sharing evolutionary strategy for lens crystallins to the cnidarians and indicates that the putative primordial saposin/swaposin J3-crystallin reflects both the chaperone and enzyme connections of the vertebrate crystallins.

Sphingolipid activator proteins: proteins with complex functions in lipid degradation and skin biogenesis

Sphingolipid activator proteins (SAPs or saposins) are essential cofactors for the lysosomal degradation of membrane-anchored sphingolipids. Four of the five known proteins of this class, SAPs A--D, derive from a single precursor protein and show high homology, whereas the fifth protein, GM2AP, is larger and displays a different secondary structure. Although the main function of all five proteins is assumed to lie in the activation of lipid degradation, their specificities and modes of action seem to differ considerably. It has recently been demonstrated that the action of the proteins is highly enhanced by the presence of acidic lipids in the target membranes. These results have some interesting implications for the topology of lysosomal degradation of lipids and may provide new insights into the function of these interesting proteins, which are ubiquitously expressed in the different tissues of the body. Recent studies indicated that the SAPs play an important role in the biogenesis of the epidermal water barrier, which has been demonstrated by the analysis of the skin phenotype displayed by SAP-knockout mice. The results obtained so far have led to some new insights into the formation of the epidermal water permeability barrier and may lead to a better understanding of this complex process.

Interfacial regulation of acid ceramidase activity. Stimulation of ceramide degradation by lysosomal lipids and sphingolipid activator proteins

The lysosomal degradation of ceramide is catalyzed by acid ceramidase and requires sphingolipid activator proteins (SAP) as cofactors in vivo. The aim of this study was to investigate how ceramide is hydrolyzed by acid ceramidase at the water-membrane interface in the presence of sphingolipid activator proteins in a liposomal assay system. The degradation of membrane-bound ceramide was significantly increased both in the absence and presence of SAP-D when anionic lysosomal phospholipids such as bis(monoacylglycero)phosphate, phosphatidylinositol, and dolichol phosphate were incorporated into substrate-bearing liposomes. Higher ceramide degradation rates were observed in vesicles with increased membrane curvature. Dilution assays indicated that acid ceramidase remained bound to the liposomal surface during catalysis. Not only SAP-D, but also SAP-C and SAP-A, were found to be stimulators of ceramide hydrolysis in the presence of anionic phospholipids. This finding was confirmed by cell culture studies, in which SAP-A, -C, and -D reduced the amount of ceramide storage observed in fibroblasts of a patient suffering from prosaposin deficiency. Strong protein-lipid interactions were observed for both SAP-D and acid ceramidase in surface plasmon resonance experiments. Maximum binding of SAP-D and acid ceramidase to lipid bilayers occurred at pH 4.0. Our results demonstrate that anionic, lysosomal lipids are required for efficient hydrolysis of ceramide by acid ceramidase.

Stimulation of acid sphingomyelinase activity by lysosomal lipids and sphingolipid activator proteins

Acid sphingomyelinase is a water-soluble, lysosomal glycoprotein that catalyzes the degradation of membrane-bound sphingomyelin into phosphorylcholine and ceramide. Sphingomyelin itself is an important component of the extracellular leaflet of various cellular membranes. The aim of the present investigation was to study sphingomyelin hydrolysis as a membrane-bound process. We analyzed the degradation of sphingomyelin by recombinant, highly purified acid sphingomyelinase in a detergent-free, liposomal assay system. In order to mimic the in vivo intralysosomal conditions as closely as possible a number of negatively charged, lysosomally occuring lipids including bis(monoacylglycero)phosphate and phosphatidylinositol were incorporated into substrate-carrying liposomes. Dolichol and its phosphate ester dolicholphosphate were also included in this study. Bis(monoacylglycero)phosphate and phosphatidylinositol were both effective stimulators of sphingomyelin hydrolysis. Dolichol and dolicholphosphate also significantly increased sphingomyelin hydrolysis. The influence of membrane curvature was investigated by incorporating the substrate into small (SUVs) and large unilamellar vesicles (LUVs) with varying mean diameter. Degradation rates were substantially higher in SUVs than in LUVs. Surface plasmon resonance experiments demonstrated that acid sphingomyelinase binds strongly to lipid bilayers. This interaction is significantly enhanced by anionic lipids such as bis(monoacylglycero)phosphate. Under detergent-free conditions only the sphingolipid activator protein SAP-C had a pronounced influence on sphingomyelin degradation in both neutral and negatively charged liposomes, catalyzed by highly purified acid sphingomyelinase, while SAP-A, -B and -D had no noticeable effect on sphingomyelin degradation.

Posttranslational regulation of acid sphingomyelinase in niemann-pick type C1 fibroblasts and free cholesterol-enriched chinese hamster ovary cells

Niemann-Pick type C disease is characterized by the accumulation of cholesterol and other lipids within the lysosomal compartment, a process that is often accompanied by a reduction in acid sphingomyelinase activity. These studies demonstrate that a CHO cell mutant (CT-60), which accumulates lysosomal cholesterol because of a defective NP-C1 protein, has approximately 5-10% of the acid sphingomyelinase activity of its parental cell line (25-RA) or wild type (CHO-K1) cells. The cholesterol-induced reduction in acid sphingomyelinase activity can be reproduced in CHO-K1 cells by incubation in the presence of low density lipoprotein (LDL) and progesterone, which impairs the normal egress of LDL-derived cholesterol from the lysosomal compartment. Kinetic analysis of sphingomyelin hydrolysis in cell extracts suggests that the CT60 cells have a reduced amount of functional acid sphingomyelinase as indicated by a 10-fold reduction in the apparent V(max). Western blot analysis using antibodies generated to synthetic peptides corresponding to segments within the carboxyl-terminal region of acid sphingomyelinase demonstrate that both the CT60 and the LDL/progesterone-treated CHO-K1 cells possess near normal levels of acid sphingomyelinase protein. Likewise, Niemann-Pick type C fibroblasts also displayed normal acid sphingomyelinase protein but negligible levels of acid sphingomyelinase activity. These data suggest that cholesterol-induced inhibition is a posttranslational event, perhaps involving cofactor mediated modulation of enzymatic activity or alterations in acid sphingomyelinase protein trafficking and maturation.

The amoebapore superfamily

Amoebapores, synthesized by human protozoan parasites, form ion channels in target cells and artificial lipid membranes. The major pathogenic effect of these proteins is due to their cytolytic capability which results in target cell death. They comprise a coherent family and are homologous to other proteins and protein domains found in eight families. These families include in addition to the amoebapores (1) the saposins, (2) the NK-lysins and granulysins, (3) the pulmonary surfactant proteins B, (4) the acid sphingomyelinases, (5) acyloxyacyl hydrolases and (6) the aspartic proteases. These amoebapore homologues have many properties in common including membrane binding and stability. We note for the first time that a new protein, countin, from the cellular slime mold, Dictyostelium discoideum, comprises the eighth family within this superfamily. All currently sequenced members of these eight families are identified, and the structural, functional and phylogenetic properties of these proteins are discussed.

Identification of a novel sequence involved in lysosomal sorting of the sphingolipid activator protein prosaposin

Prosaposin is synthesized as a 53-kDa protein, post-translationally modified to a 65-kDa form and further glycosylated to a 70-kDa secretory product. The 65-kDa protein is associated to Golgi membranes and is targeted to lysosomes, where four smaller nonenzymatic saposins implicated in the hydrolysis of sphingolipids are generated by its partial proteolysis. The targeting of the 65-kDa protein to lysosomes is not mediated by the mannose 6-phosphate receptor. The Golgi apparatus appears to accomplish the molecular sorting of the 65-kDa prosaposin by decoding a signal from its amino acid backbone. This investigation deals with the characterization of the sequence involved in this process by deleting the saposin functional domains A, B, C, and D and the highly conserved N and C termini of prosaposin. The truncated cDNAs were subcloned into expression vectors and transfected to COS-7 cells. The destination of the mutated proteins was assessed by immunocytochemistry. Deletion of the C terminus did not interfere with the secretion of prosaposin but abolished its transport to lysosomes. Deletion of saposins and the N-terminal domain did not affect the lysosomal or secretory routing of prosaposin. A chimeric construct of albumin and the C terminus of prosaposin was not directed to lysosomes. However, albumin connected to the C terminus and one or more functional domains of prosaposin reached lysosomes, indicating that the C terminus and at least one saposin domain are required for this process. In summary, we are reporting a novel sequence involved in the targeting of prosaposin to lysosomes.

Stimulation of lysosomal sphingomyelin degradation by sphingolipid activator proteins

Lysosomal breakdown of glycosphingolipids with short hydrophilic carbohydrate headgroups is achieved by the simultaneous action of specific hydrolases and sphingolipid activator proteins (SAPs). Activator proteins are considered to facilitate the enzyme/substrate interaction between water-soluble enzymes and membrane-bound substrates. Sphingomyelin, containing the small hydrophilic phosphorylcholine moiety, is hydrolysed by acid sphingomyelinase (acid SMase). Recent experimental data on the in vivo and in vitro role of activator proteins in sphingomyelin breakdown by acid SMase are reviewed. These data combined with the results using homogenous protein preparations as well as a liposomal assay system mimicking the physiological conditions suggest that lysosomal sphingomyelin degradation is not critically dependent on any of the known activator proteins. Moreover, evidence is provided that the assumed intramolecular activator domain of acid SMase and especially the presence of negatively charged lipids in the lysosomes are sufficient for sphingomyelin turnover.

Saposins and their interaction with lipids

The lysosomal degradation of several sphingolipids requires the presence of four small glycoproteins called saposins, generated by proteolytic processing of a common precursor, prosaposin. Saposins share several structural properties, including six similarly located cysteines forming three disulfide bridges with the same cysteine pairings. Recently it has been noted that also other proteins have the same polypeptide motif characterized by the similar location of six cysteines. These saposin-like (SAPLIP) proteins are surfactant protein B (SP-B), 'Entamoeba histolytica' pore-forming peptide, NK-lysin, acid sphingomyelinase and acyloxyacyl hydrolase. The structural homology and the conserved disulfide bridges suggest for all SAPLIPs a common fold, called 'saposin fold'. Up to now a precise fold, comprising five alpha-helices, has been established only for NK-lysin. Despite their similar structure each saposin promotes the degradation of specific sphingolipids in lysosomes, e.g. Sap B that of sulfatides and Sap C that of glucosylceramides. The different activities of the saposins must reside within the module of the alpha-helices and/or in additional specific regions of the molecule. It has been reported that saposins bind to lysosomal hydrolases and to several sphingolipids. Their structural and functional properties have been extensively reviewed and hypotheses regarding their molecular mechanisms of action have been proposed. Recent work of our group has evidenced a novel property of saposins: some of them undergo an acid-induced change in hydrophobicity that triggers their binding to phospholipid membranes. In this article we shortly review recent findings on the structure of saposins and on their interactions with lipids, with special attention to interactions with phospholipids. These findings offer a new approach for understanding the physiological role of saposins in lysosomes.

Expression of recombinant human acid sphingomyelinase in insect Sf21 cells: purification, processing and enzymatic characterization

Biochemical and structural studies on human acid sphingomyelinase (haSMase) depend on the access to homogeneous biologically active enzyme. Due to the low abundance of native haSMase (n-haSMase) in human tissue, conventional purification strategies are not suitable for the isolation of preparative amounts of the enzyme. We describe a novel approach to the functional expression and purification of haSMase employing the baculovirus expression vector system. Infection of Spodoptera frugiperda 21 cells with recombinant baculovirus encoding haSMase leads to the expression of a glycosylated 75 kDa precursor protein, which is subsequently processed to an enzymatically active secreted 72 kDa haSMase. Variations in N-glycosylation and proteolytic maturation account for the difference in molecular mass between mature recombinant (72 kDa) and human placental haSMase (75 kDa). N-terminal amino acid sequencing of recombinant haSMase (r-haSMase) reveals a 23-residue N-terminal extension compared to the placental enzyme. The apparent K(m) and Vmax values for sphingomyelin degradation by r-haSMase in a micellar assay system are 32 microM and 0.56 mmol h-1 mg-1, respectively. In conclusion, the established baculovirus expression vector system provides an efficient tool for the expression and functional characterization of haSMase.

Caenorhabditis elegans contains two distinct acid sphingomyelinases

Mounting evidence supports a role for acid sphingomyelinase (ASM) in cellular stress signaling. Only murine and human sphingomyelinases have been defined at the molecular level. These enzymes are the products of a conserved gene and at the amino acid level share 82% identity. In this study, we show that the nematode Caenorhabditis elegans possesses two ASMs, termed ASM-1 and ASM-2 encoded by two distinct genes, but lacks detectable neutral sphingomyelinase activity. The C. elegans ASMs are about 30% identical with each other and with the human and murine enzymes. The conserved regions include a saposin-like domain, proline-rich domain, and a putative signal peptide. In addition, 16 cysteines distributed throughout the molecules, and selected glycosylation sites, are conserved. The expression of these genes in C. elegans is regulated during development. Asm-1 is preferentially expressed in the embryo, whereas asm-2 is predominantly expressed in postembryonic stages. When transfected as Flag-tagged proteins into COS-7 cells, ASM-1 is found almost entirely in a secreted form whereas only 20% of ASM-2 is secreted. Only the secreted forms display enzymatic activity. Furthermore, ASM-2 requires addition of Zn2+ to be fully active, whereas ASM-1 is active in the absence of cation. C. elegans is the first organism to display two ASMs. This finding suggests the existence of an ASM gene family.

Functional characterization of the N-glycosylation sites of human acid sphingomyelinase by site-directed mutagenesis

Most soluble lysosomal enzymes require a mannose-6-phosphate recognition marker present on asparagine-linked oligosaccharides for proper targeting to lysosomes. We have determined the influence of the six potential N-linked oligosaccharide chains of human acid sphingomyelinase (ASM) on catalytic activity, targeting, and processing of the enzyme. Each N-glycosylation site was modified by site-directed mutagenesis and subsequently expressed in COS-1 cells. Evidence is presented that five of these sites are used. Elimination of the four N-terminal glycosylation sites does not disturb lysosomal targeting, processing, or enzymatic activity. However, removal of the two C-terminal N-glycosylation sites inhibits the formation of mature enzyme. Absence of glycosylation site five resulted in rapid cleavage of the primary translation product to an enzymatically inactive protein which accumulated inside the endoplasmic reticulum/Golgi, whereas deletion of glycosylation site six led to the formation of an inactive ASM precursor, also retained inside the endoplasmic reticulum/Golgi. Our results also provide evidence that the site of early proteolytic cleavage of newly synthesized ASM must be located between the second and third glycosylation sites.

An amphipathic helical motif common to tumourolytic polypeptide NK-lysin and pulmonary surfactant polypeptide SP-B

The tumour-lysing and antimicrobial polypeptide NK-lysin and the pulmonary surfactant-associated polypeptide SP-B exhibit 24% residue identities (49% similarities), including six half-cystine residues in the same disulphide bonding pattern, and similar far-UV circular dichroism spectra corresponding to 45-55% alpha-helix and 20-25% beta-sheet structures. From this, we conclude that the conformations of NK-lysin and SP-B are similar. In contrast, the functional properties of the two proteins are dissimilar: SP-B does not exhibit antibacterial activity and NK-lysin does not significantly effect phospholipid spreading at an air/water interface. Saposins, which solubilize lipids and activate lysosomal hydrolases, the pore-forming amoebapores, and parts of acid sphingomyelinase and acyloxyacylhydrolase, also share 18-27% sequence identities with NK-lysin (and SP-B), including the six conserved half-cystine residues. The inclusion of NK-lysin extends the family of saposin-like polypeptides, all members of which appear to interact with lipids. Strictly conserved structural features with implications for helix topology and lipid interactions are observed.