Cotranslational assembly of membrane protein/nanoparticles in cell-free systems

Nanoparticles composed of amphiphilic scaffold proteins and small lipid bilayers are valuable tools for reconstitution and subsequent functional and structural characterization of membrane proteins. In combination with cell-free protein production systems, nanoparticles can be used to cotranslationally and translocon independently insert membrane proteins into tailored lipid environments. This strategy enables rapid generation of protein/nanoparticle complexes by avoiding detergent contact of nascent membrane proteins. Frequently in use are nanoparticles assembled with engineered derivatives of either the membrane scaffold protein (MSP) or the Saposin A (SapA) scaffold. Furthermore, several strategies for the formation of membrane protein/nanoparticle complexes in cell-free reactions exist. However, it is unknown how these strategies affect functional folding, oligomeric assembly and membrane insertion efficiency of cell-free synthesized membrane proteins. We systematically studied membrane protein insertion efficiency and sample quality of cell-free synthesized proteorhodopsin (PR) which was cotranslationally inserted in MSP and SapA based nanoparticles. Three possible PR/nanoparticle formation strategies were analyzed: (i) PR integration into supplied preassembled nanoparticles, (ii) coassembly of nanoparticles from supplied scaffold proteins and lipids upon PR expression, and (iii) coexpression of scaffold proteins together with PR in presence of supplied lipids. Yield, homogeneity as well as the formation of higher PR oligomeric complexes from samples generated by the three strategies were analyzed. Conditions found optimal for PR were applied for the synthesis of a G-protein coupled receptor. The study gives a comprehensive guideline for the rapid synthesis of membrane protein/nanoparticle samples by different processes and identifies key parameters to modulate sample yield and quality.

Two short antimicrobial peptides derived from prosaposin-like proteins in the starry flounder (Platichthys stellatus)

Prosaposin (PSAP) is a precursor of saposin (SAP), which is present in lysosomal and secreted proteins. PSAP is a member of the SAP-like protein families, which comprise multifunctional proteins. In particular, their antimicrobial activity has been reported. We identified PSAP-like (PsPSAPL) sequences from starry flounder and analysed their expression and antimicrobial activity based on cDNA and amino acid sequences. PsPSAPL showed conservation of three saposin B type domains at high levels, and PsPSAPL mRNA was relatively abundantly distributed in the brain and gills of healthy starry founders. PsPSAPL mRNA showed significant expression changes in response to viral haemorrhagic septicaemia virus and Streptococcus parauberis. Synthetic peptides (PsPSAPL-1 and -2), prepared based on amino acid sequences, were used to confirm as well as analyse the antimicrobial activity against bacteria and parasites. Consequently, PsPSAPL-1 and -2 were found to significantly inhibit the growth of various bacteria and kill the Miamiensis avidus. In addition, bacterial biofilm formation was significantly inhibited. Safety was also confirmed by analysing cell haemolysis. These results indicate the immunological function of PsPSAP and the potential antimicrobial activity of the AMPs PsPSAPL-1 and -2.

Recognition of Potential COVID-19 Drug Treatments through the Study of Existing Protein-Drug and Protein-Protein Structures: An Analysis of Kinetically Active Residues

We report the results of our in silico study of approved drugs as potential treatments for COVID-19. The study is based on the analysis of normal modes of proteins. The drugs studied include chloroquine, ivermectin, remdesivir, sofosbuvir, boceprevir, and α-difluoromethylornithine (DMFO). We applied the tools we developed and standard tools used in the structural biology community. Our results indicate that small molecules selectively bind to stable, kinetically active residues and residues adjoining them on the surface of proteins and inside protein pockets, and that some prefer hydrophobic sites over other active sites. Our approach is not restricted to viruses and can facilitate rational drug design, as well as improve our understanding of molecular interactions, in general.

Progranulin mutations result in impaired processing of prosaposin and reduced glucocerebrosidase activity

Frontotemporal dementia (FTD) is a common neurogenerative disorder characterized by progressive degeneration in the frontal and temporal lobes. Heterozygous mutations in the gene encoding progranulin (PGRN) are a common genetic cause of FTD. Recently, PGRN has emerged as an important regulator of lysosomal function. Here, we examine the impact of PGRN mutations on the processing of full-length prosaposin to individual saposins, which are critical regulators of lysosomal sphingolipid metabolism. Using FTD-PGRN patient-derived cortical neurons differentiated from induced pluripotent stem cells, as well as post-mortem tissue from patients with FTLD-PGRN, we show that PGRN haploinsufficiency results in impaired processing of prosaposin to saposin C, a critical activator of the lysosomal enzyme glucocerebrosidase (GCase). Additionally, we found that PGRN mutant neurons had reduced lysosomal GCase activity, lipid accumulation and increased insoluble α-synuclein relative to isogenic controls. Importantly, reduced GCase activity in PGRN mutant neurons is rescued by treatment with saposin C. Together, these findings suggest that reduced GCase activity due to impaired processing of prosaposin may contribute to pathogenesis of FTD resulting from PGRN mutations.

A Comparative Study of Human Saposins

Saposins are small proteins implicated in trafficking and loading of lipids onto Cluster of Differentiation 1 (CD1) receptor proteins that in turn present lipid antigens to T cells and a variety of T-cell receptors, thus playing a crucial role in innate and adaptive immune responses in humans. Despite their low sequence identity, the four types of human saposins share a similar folding pattern consisting of four helices linked by three conserved disulfide bridges. However, their lipid-binding abilities as well as their activities in extracting, transporting and loading onto CD1 molecules a variety of sphingo- and phospholipids in biological membranes display two striking characteristics: a strong pH-dependence and a structural change between a compact, closed conformation and an open conformation. In this work, we present a comparative computational study of structural, electrostatic, and dynamic features of human saposins based upon their available experimental structures. By means of structural alignments, surface analyses, calculation of pH-dependent protonation states, Poisson-Boltzmann electrostatic potentials, and molecular dynamics simulations at three pH values representative of biological media where saposins fulfill their function, our results shed light into their intrinsic features. The similarities and differences in this class of proteins depend on tiny variations of local structural details that allow saposins to be key players in triggering responses in the human immune system.

An Adaptable Phospholipid Membrane Mimetic System for Solution NMR Studies of Membrane Proteins

Based on the saposin-A (SapA) scaffold protein, we demonstrate the suitability of a size-adaptable phospholipid membrane-mimetic system for solution NMR studies of membrane proteins (MPs) under close-to-native conditions. The Salipro nanoparticle size can be tuned over a wide pH range by adjusting the saposin-to-lipid stoichiometry, enabling maintenance of sufficiently high amounts of phospholipid in the Salipro nanoparticle to mimic a realistic membrane environment while controlling the overall size to enable solution NMR for a range of MPs. Three representative MPs, including one G-protein-coupled receptor, were successfully incorporated into SapA-dimyristoylphosphatidylcholine nanoparticles and studied by solution NMR spectroscopy.

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

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

In silico insights into protein-protein interactions and folding dynamics of the saposin-like domain of Solanum tuberosum aspartic protease

The plant-specific insert is an approximately 100-residue domain found exclusively within the C-terminal lobe of some plant aspartic proteases. Structurally, this domain is a member of the saposin-like protein family, and is involved in plant pathogen defense as well as vacuolar targeting of the parent protease molecule. Similar to other members of the saposin-like protein family, most notably saposins A and C, the recently resolved crystal structure of potato (Solanum tuberosum) plant-specific insert has been shown to exist in a substrate-bound open conformation in which the plant-specific insert oligomerizes to form homodimers. In addition to the open structure, a closed conformation also exists having the classic saposin fold of the saposin-like protein family as observed in the crystal structure of barley (Hordeum vulgare L.) plant-specific insert. In the present study, the mechanisms of tertiary and quaternary conformation changes of potato plant-specific insert were investigated in silico as a function of pH. Umbrella sampling and determination of the free energy change of dissociation of the plant-specific insert homodimer revealed that increasing the pH of the system to near physiological levels reduced the free energy barrier to dissociation. Furthermore, principal component analysis was used to characterize conformational changes at both acidic and neutral pH. The results indicated that the plant-specific insert may adopt a tertiary structure similar to the characteristic saposin fold and suggest a potential new structural motif among saposin-like proteins. To our knowledge, this acidified PSI structure presents the first example of an alternative saposin-fold motif for any member of the large and diverse SAPLIP family.

Saposin C protects glucocerebrosidase against α-synuclein inhibition

Mutations in GBA1, the gene for glucocerebrosidase (GCase), are genetic risk factors for Parkinson disease (PD). α-Synuclein (α-Syn), a protein implicated in PD, interacts with GCase and efficiently inhibits enzyme activity. GCase deficiency causes the lysosomal storage disorder Gaucher disease (GD). We show that saposin C (Sap C), a protein vital for GCase activity in vivo, protects GCase against α-syn inhibition. Using nuclear magnetic resonance spectroscopy, site-specific fluorescence, and Förster energy transfer probes, Sap C was observed to displace α-syn from GCase in solution and on lipid vesicles. Our results suggest that Sap C might play a crucial role in GD-related PD.

Targeting and cytotoxicity of SapC-DOPS nanovesicles in pancreatic cancer

Only a small number of promising drugs target pancreatic cancer, which is the fourth leading cause of cancer deaths with a 5-year survival of less than 5%. Our goal is to develop a new biotherapeutic agent in which a lysosomal protein (saposin C, SapC) and a phospholipid (dioleoylphosphatidylserine, DOPS) are assembled into nanovesicles (SapC-DOPS) for treating pancreatic cancer. A distinguishing feature of SapC-DOPS nanovesicles is their high affinity for phosphatidylserine (PS) rich microdomains, which are abnormally exposed on the membrane surface of human pancreatic tumor cells. To evaluate the role of external cell PS, in vitro assays were used to correlate PS exposure and the cytotoxic effect of SapC-DOPS in human tumor and nontumorigenic pancreatic cells. Next, pancreatic tumor xenografts (orthotopic and subcutaneous models) were used for tumor targeting and therapeutic efficacy studies with systemic SapC-DOPS treatment. We observed that the nanovesicles selectively killed human pancreatic cancer cells in vitro by inducing apoptotic death, whereas untransformed cells remained unaffected. This in vitro cytotoxic effect correlated to the surface exposure level of PS on the tumor cells. Using xenografts, animals treated with SapC-DOPS showed clear survival benefits and their tumors shrank or disappeared. Furthermore, using a double-tracking method in live mice, we showed that the nanovesicles were specifically targeted to orthotopically-implanted, bioluminescent pancreatic tumors. These data suggest that the acidic phospholipid PS is a biomarker for pancreatic cancer that can be effectively targeted for therapy utilizing cancer-selective SapC-DOPS nanovesicles. This study provides convincing evidence in support of developing a new therapeutic approach to pancreatic cancer.

The role of saposin C in Gaucher disease

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

Saposin C coupled lipid nanovesicles specifically target arthritic mouse joints for optical imaging of disease severity

Rheumatoid arthritis is a chronic inflammatory disease affecting approximately 1% of the population and is characterized by cartilage and bone destruction ultimately leading to loss of joint function. Early detection and intervention of disease provides the best hope for successful treatment and preservation of joint mobility and function. Reliable and non-invasive techniques that accurately measure arthritic disease onset and progression are lacking. We recently developed a novel agent, SapC-DOPS, which is composed of the membrane-associated lysosomal protein saposin C (SapC) incorporated into 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) lipid nanovesicles. SapC-DOPS has a high fusogenic affinity for phosphatidylserine-enriched microdomains on surfaces of target cell membranes. Incorporation of a far-red fluorophore, CellVue Maroon (CVM), into the nanovesicles allows for in vivo non-invasive visualization of the agent in targeted tissue. Given that phosphatidylserine is present only on the inner leaflet of healthy plasma membranes but is “flipped” to the outer leaflet upon cell damage, we hypothesized that SapC-DOPS would target tissue damage associated with inflammatory arthritis due to local surface-exposure of phosphatidylserine. Optical imaging with SapC-DOPS-CVM in two distinct models of arthritis, serum-transfer arthritis (e.g., K/BxN) and collagen-induced arthritis (CIA) revealed robust SapC-DOPS-CVM specific localization to arthritic paws and joints in live animals. Importantly, intensity of localized fluorescent signal correlated with macroscopic arthritic disease severity and increased with disease progression. Flow cytometry of cells extracted from arthritic joints demonstrated that SapC-DOPS-CVM localized to an average of 7–8% of total joint cells and primarily to CD11b+Gr-1+ cells. Results from the current studies strongly support the application of SapC-DOPS-CVM for advanced clinical and research applications including: detecting early arthritis onset, assessing disease progression real-time in live subjects, and providing novel information regarding cell types that may mediate arthritis progression within joints.

Identification of prosaposin as a novel interaction partner for Rhox5

Prosaposin (Psap) has multiple cellular functions. It is involved in the development of the reproductive system, nervous system, and prostate cancer as well as in the regulation of sphingolipid catabolism by activating several lysosomal hydrolases involved in the metabolism of various sphingolipids. In this research, it was found to be a novel interaction partner for Rhox5 using yeast two-hybrid screening. The interaction between Rhox5 and the full-length prosapsoin (the transcript without exon 8) as well as the C-terminal domain of prosaposin, was further confirmed in both yeast two hybrid analysis and in vitro assay. It suggested that the C-terminal domain of prosaposin may be critical for the Rhox5-prosaposin interaction. Given the important roles played by both Rhox5 and prosaposin in maintaining the differentiation of male reproductive organs, spermatogenesis, and fertilization, the interaction between Rhox5 and prosaposin might regulate the development of male reproductive organs dynamically.

An evolutionary and structure-based docking model for glucocerebrosidase-saposin C and glucocerebrosidase-substrate interactions - relevance for Gaucher disease

Gaucher disease, the most prevalent lysosomal storage disorder, is principally caused by malfunction of the lysosomal enzyme glucocerebrosidase (GBA), a 497-amino acid membrane glycoprotein that catalyzes the hydrolysis of glucosylceramide to ceramide and glucose in the presence of an essential 84-residue activator peptide named saposin C (SapC). Knowledge of the GBA structure, a typical (beta/alpha)(8) TIM barrel, explains the effect of few mutations, directly affecting or located near the catalytic site. To identify new regions crucial for proper GBA functionality, we analyzed the interactions of the enzyme with a second (substrate) and a third (cofactor) partner. We build 3D docking models of the GBA-SapC and the GBA-ceramide interactions, by means of methodologies that integrate both evolutive and structural information. The GBA-SapC docking model confirm the implication of three spatially closed regions of the GBA surface (TIM barrel-helix 6 and helix 7, and the Ig-like domain) in binding the SapC molecule. This model provides new basis to understand the pathogenicity of several mutations, such as the prevalent Leu444Pro, and the additive effect of Glu326Lys in the double mutant Glu326Lys-Leu444Pro. Overall, 39 positions in which amino acid changes are known to cause Gaucher disease were localized in the GBA regions identified in this work. Our model is discussed in relation to the phenotype (pathogenic effect) of these mutations, as well as to the enzymatic activity of the recombinant proteins when available. Both data fully correlates with the proposed model, which will provide a new tool to better understand Gaucher disease and to design new therapy strategies.

Investigating the interaction of saposin C with POPS and POPC phospholipids: a solid-state NMR spectroscopic study

The interaction of Saposin C (Sap C) with negatively charged phospholipids such as phosphatidylserine (PS) is essential for its biological function. In this study, Sap C (initially protonated in a weak acid) was inserted into multilamellar vesicles (MLVs) consisting of either 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-L-serine] (negatively charged, POPS) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (neutrally charged, POPC). The MLVs were then investigated using solid-state NMR spectroscopy under neutral pH (7.0) conditions. The (2)H and (31)P solid-state NMR spectroscopic data of Sap C-POPS and Sap C-POPC MLVs (prepared under the same conditions) were compared using the (2)H order parameter profiles of the POPC-d(31) or POPS-d(31) acyl chains as well as the (31)P chemical shift anisotropy width and (31)P T(1) relaxation times of the phospholipids headgroups. All those solid-state NMR spectroscopic approaches indicate that protonated Sap C disturbs the POPS bilayers and not the POPC lipid bilayers. These observations suggest for the first time that protonated Sap C inserts into PS bilayers and forms a stable complex with the lipids even after resuspension under neutral buffer conditions. Additionally, (31)P solid-state NMR spectroscopic studies of mechanically oriented phospholipids on glass plates were conducted and perturbation effect of Sap C on both POPS and POPC bilayers was compared. Unlike POPC bilayers, the data indicates that protonated Sap C (initially protonated in a weak acid) was unable to produce well-oriented POPS bilayers on glass plates at neutral pH. Conversely, unprotonated Sap C (initially dissolved in a neutral buffer) did not interact significantly with POPS phospholipids allowing them to produce well-oriented bilayers at neutral pH.

Fasciola hepatica: identification of CD4+ T-helper epitopes from the 11.5 kDa saposin-like protein SAP-2 using synthetic peptides

Fasciola hepatica saposin-like protein (FhSAP-2) is a novel antigen expressed at an early stage of infection and has been shown to induce in rabbits a significant protection to infection with F. hepatica. There are no studies to identify the immunologically relevant regions of FhSAP-2. In this work the amino acid sequence of FhSAP-2 was analyzed to identify potential T-cell epitopes. A predictive algorithm identified four possible sites. Experimental determination of the T-cell epitopes was achieved using a panel of overlapping peptides spanning the entire sequence of FhSAP-2, which was evaluated for their ability to induce lymphoproliferative responses of spleen cells from 8 immunized BALB/c (H-2d) mice. Five different epitopes were identified. There was minimal agreement between theoretical and experimental approaches. It was found that peptides containing amino acid residues AVTFA and IDIDLCDICT as part of their structure induce high levels of IL-2 and IFNgammain vitro and was classified as Th1 epitopes. Peptides that contain the residues ADQTV, CIEFVQQEVD and YIIDHVDQHN induced significant amount of IL-4 and IL-2 were considered as containers of Th0 epitopes. Identification of prominent T-cell epitopes from FhSAP-2 offers the possibility of understanding how the CD4+ T-cell response is involved in protection against fasciolosis and how it is implicated in susceptibility to infection.

Saposin B is the dominant saposin that facilitates lipid binding to human CD1d molecules

CD1d molecules bind lipid antigens in the endocytic pathway, and access to the pathway is important for the development of CD1d-restricted natural killer T (NKT) cells. Saposins, derived from a common precursor, prosaposin, are small, heat-stable lysosomal glycoproteins required for lysosomal degradation of sphingolipids. Expression of prosaposin is required for efficient lipid binding and recognition of human CD1d molecules by NKT cells. Despite high sequence homology among the four saposins, they have different specificities for lipid substrates and different mechanisms of action. To determine the saposins involved in promoting lipid binding to CD1d, we expressed prosaposin deletion mutants lacking individual saposins in prosaposin-negative, CD1d-positive cells. No individual saposin proved to be absolutely essential, but the absence of saposin B resulted in the lowest recognition of alpha-galactosylceramide by NKT cells. When recombinant exogenous saposins were added to the prosaposin-negative cells, saposin B was the most efficient in restoring CD1d recognition. Saposin B was also the most efficient in mediating alpha-galactosylceramide binding to recombinant plate-bound CD1d and facilitating NKT cell activation. Saposin B could also mediate lipid binding to soluble CD1d molecules in a T cell-independent assay. The optimal pH for saposin B-mediated lipid binding to CD1d, pH 6, is higher than that of lysosomes, suggesting that saposin B may facilitate lipid binding to CD1d molecules throughout the endocytic pathway.

J-Spectroscopy in the presence of residual dipolar couplings: determination of one-bond coupling constants and scalable resolution

The access to weak alignment media has fuelled the development of methods for efficiently and accurately measuring residual dipolar couplings (RDCs) in NMR-spectroscopy. Among the wealth of approaches for determining one-bond scalar and RDC constants only J-modulated and J-evolved techniques retain maximum resolution in the presence of differential relaxation. In this article, a number of J-evolved experiments are examined with respect to the achievable minimum linewidth in the J-dimension, using the peptide PA4 and the 80-amino-acid-protein Saposin C as model systems. With the JE-N-BIRDd,X-HSQC experiment, the average full-width at half height could be reduced to approximately 5 Hz for the protein, which allows the additional resolution of otherwise unresolved peaks by the active (J+D)-coupling. Since RDCs generally can be scaled by the choice of alignment medium and alignment strength, the technique introduced here provides an effective resort in cases when chemical shift differences alone are insufficient for discriminating signals. In favorable cases even secondary structure elements can be distinguished.

Spontaneously forming ellipsoidal phospholipid unilamellar vesicles and their interactions with helical domains of saposin C

We have observed a bimodal distribution of ellipsoidal unilamellar vesicles (ULVs) in a phospholipid mixture composed of dioleoyl phosphatidylserine (DOPS) and dipalmitoyl and dihexanoyl phosphatidylcholine, DPPC and DHPC, respectively. Dynamic light scattering and transmission electron microscopy data indicate a bimodal size distribution of these nanoparticles with hydrodynamic radii of approximately 200 and >500 nm, while small-angle neutron scattering data were fit using a model of coexisting monodisperse morphologies, namely, oblate and triaxial ellipsoidal vesicles. Unlike DOPS ULV formed by sonication, which can fuse days after being formed, these ULVs are stable over a period of 12 months at 4 degrees C. We also report on the structure of these ULVs associated with the two helical peptide domains (H1 and H2) of a glucosylprotein, namely, Saposin C, to gain some insight into protein-membrane interactions.

Saposin-like proteins from the intestine of the blood-feeding hookworm, Ancylostoma caninum

Hookworms feed on blood, utilizing haemoglobin for nutrition, growth and reproduction. The haemoglobin digestion cascade has been partially elucidated, but the process immediately preceding this event, haemolysis, has received considerably less attention. We have cloned and expressed Ancylostoma caninum mRNAs encoding 2 proteins belonging to the saposin-like protein (SAPLIP) family, termed Ac-slp-1 and Ac-slp-2. The open reading frames of SLP-1 and SLP-2 were used to identify expressed sequence tags encoding SAPLIPs from the 4 major clades of animal parasitic nematodes. Both Ac-slp-1 and slp-2 mRNAs were shown to be expressed in all life stages assessed, with slp-1 predominantly being expressed in third-stage larvae (L3) before and after activation with dog serum. Recombinant SLP-1 and SLP-2 were expressed in insect cells and used to raise specific antisera in mice. These antisera were used as probes in fluorescence microscopy to localize the anatomic expression sites of both proteins to small, punctate organelles or vesicles within the intestinal cells of adult worms; weak staining was detected on the microvillar brush border of the intestine. Using transmission electron microscopy, both proteins were localized to similar vesicles in the intestinal cells of the L3. Recombinant proteins contained C-terminal purification tags that potentially precluded dimerization and possibly interfered with the subsequent detection of haemolytic activity. Their expression in the gut of the L3 and adult stages suggests a role for these hookworm SAPLIPs in the lysis of host cells during tissue migration and/or feeding.

Direct visualization of saposin remodelling of lipid bilayers

Saposins A, B, C and D are soluble, non-enzymatic proteins that interact with lysosomal membranes to activate the breakdown and transfer of glycosphingolipids. The mechanisms of hydrolase activation and lipid transfer by saposins remain unknown. We have used in situ atomic force microscopy (AFM) with simultaneous confocal fluorescence microscopy to investigate the interactions of saposins with lipid membranes. AFM images of the effect of saposins A, B and C on supported lipid bilayers showed a time and concentration-dependent nucleated spread of membrane transformation. Saposin B produced deep gaps that ultimately filled with granular material, while saposins A and C lead to localized areas of membrane that were reduced in height by approximately 1.5 nm. Fluorescence-labeled saposin C co-localized with the transformed areas of the bilayer, indicating stable binding to the membrane. Fluorescence resonance energy transfer confirmed a direct interaction between saposin C and lipid. Under certain conditions of membrane lipid composition and saposin concentration, extensive bilayer lipid removal was observed. We propose a multi-step mechanism that integrates the structural features and amphipathic properties of the saposin proteins.

Mapping of B-cell epitopes on a novel 11.5-kilodalton Fasciola hepatica-Schistosoma mansoni cross-reactive antigen belonging to a member of the F. hepatica saposin-like protein family

FhSAP-2 is a novel member of the Fasciola hepatica saposin-like protein family that induces protection in rabbits against a challenge infection. We investigated the presence of lineal B-cell epitopes within this protein using a set of overlapping synthetic peptides. Peptides were tested in enzyme-linked immunosorbent assays against sera from rabbits infected with F. hepatica. Two dominant epitopes were identified, which were also highly reactive with sera from mice infected with Schistosoma mansoni. These peptides may be suitable to incorporate into a polyepitope-based vaccine formulation against F. hepatica.

Crystal structures of saposins A and C

Saposins A and C are sphingolipid activator proteins required for the lysosomal breakdown of galactosylceramide and glucosylceramide, respectively. The saposins interact with lipids, leading to an enhanced accessibility of the lipid headgroups to their cognate hydrolases. We have determined the crystal structures of human saposins A and C to 2.0 Angstroms and 2.4 Angstroms, respectively, and both reveal the compact, monomeric saposin fold. We confirmed that these two proteins were monomeric in solution at pH 7.0 by analytical centrifugation. However, at pH 4.8, in the presence of the detergent C(8)E(5), saposin A assembled into dimers, while saposin C formed trimers. Saposin B was dimeric under all conditions tested. The self-association of the saposins is likely to be relevant to how these small proteins interact with lipids, membranes, and hydrolase enzymes.

Characterization of human saposins by NMR spectroscopy

Saposins are lipid-binding and membrane-perturbing glycoproteins of the mammalian lysosomes involved in sphingolipid and membrane digestion. Although the four human saposins (Saps), A-D, are sequence-related, they are responsible for the activation of different steps in the cascade of lysosomal glycosphingolipid degradation. Saposin activity is maximal under acidic conditions, and the pH dependence of lipid and membrane binding has been assigned to conformational variability. We have employed solution NMR spectroscopy to all four (15)N-labeled human saposins at both neutral and acidic pH. Using backbone NOEs and residual dipolar couplings, the "saposin fold" comprising five alpha-helices was confirmed for Sap-A, Sap-C, and Sap-D. Structural variations within these proteins are in the order of variations between the known structures of Sap-C and NK-lysin. In contrast, Sap-B yielded spectra of very poor quality, presumably due to conformational heterogeneity and molecular association. Sap-D exists in a slow dynamic equilibrium of two conformational states with yet unknown function. At pH 4.0, where all saposins are highly unstable, Sap-C undergoes a transition to a specific dimeric state, which is likely to resemble the structure recently found in both Sap-C in a detergent environment and crystals of Sap-B.

A novel saposin-like protein of Entamoeba histolytica with membrane-fusogenic activity

Amoebapores, the pore-forming proteins of Entamoeba histolytica, are major pathogenicity factors of the parasite. Upon a comprehensive survey in the recently completed genome data sets for the protozoon, we identified in addition to the three amoebapore genes, 16 genes which are constitutively expressed and code for structurally similar proteins, all belonging to the family of saposin-like proteins. Here, we recombinantly expressed in bacteria a defined single entity of this expansive amoebic protein family, namely SAPLIP 3. The protein consists of the saposin-like domain only, comparable to amoebapores, and we characterized its interactions with membranes using different assays. In contrast to amoebapores, SAPLIP 3 neither forms pores in liposomes nor permeabilizes bacterial membranes. However, SAPLIP 3 induces leaky fusion of lipid vesicles as evidenced by fluorescence microscopic analysis and by using a fusion assay that monitors the dequenching of a lipophilic dye. The membrane-fusogenic activity of SAPLIP 3 which is dependent on the presence of negatively charged lipids and on acidic pH resembles in combination with the negative surface charge of the protein characteristics of human saposin C. Beside its function as a cofactor of sphingolipid hydrolysing enzymes, the human protein is considered to be involved in the reorganization of lysosomal compartments due to its fusogenic activity. We hypothesize that in the amoeba, SAPLIP 3 fulfils a similar function in the multifarious endo- and exocytotic transport processes.

The saposin-like proteins 1, 2, and 3 of Fasciola gigantica

The SAP genes of Fasciola encode proteins belonging to the saposin-like protein family. The saposin signature, a compact domain of mainly alpha-helical character, contains six conserved cysteine residues and has been implicated in membrane-binding, pore formation, and subsequent cell lysis in several family members. Recombinant SAP-2 of F. hepatica has been shown to induce lysis of human erythrocytes and peripheral blood mononuclear cells. This suggests that the SAPs are involved in the nutrition of Fasciola as the released content of lysed host cells is available for further enzymatic processing and uptake by the parasite. In the present study a new SAP-3 cDNA was obtained in an immunoscreen of an adult stage F. gigantica cDNA library with an antiserum against the parasite's excretion/secretion antigens. SAP-1 and SAP-2 cDNAs were isolated from F. gigantica cDNA libraries using oligonucleotide primers specific to the SAP-1 and SAP-2 DNA sequences from F. hepatica. Transcripts of the three SAPs are present from the metacercarial to the adult stage and are located to the gut epithelium. In immatures SAP-1 RNA is the predominant product whereas in adults SAP-2 and -3 are the more abundant products. Polyclonal anti-SAP-1 and SAP-2 antisera confirmed the tissue-specificity and revealed the subcellular localization of SAPs in large granules concentrated in the apical part of the gut epithelial cells of the parasite. Interestingly, evolutionary conservation of the Fasciola SAP sequences among other trematodes is low at 20-30% sequence identity comparable to the Entamoeba amoebapore sequences.

Crystallization and preliminary characterization of three different crystal forms of human saposin C heterologously expressed in Pichia pastoris

The amphiphilic saposin proteins (A, B, C and D) act at the lipid-water interface in lysosomes, mediating the hydrolysis of membrane building blocks by water-soluble exohydrolases. Human saposin C activates glucocerebrosidase and beta-galactosylceramidase. The protein has been expressed in Pichia pastoris, purified and crystallized in three different crystal forms, diffracting to a maximum resolution of 2.5 A. Hexagonal crystals grew from 2-propanol-containing solution and contain a single molecule in the asymmetric unit according to the Matthews coefficient. Orthorhombic and tetragonal crystals were both obtained with pentaerythritol ethoxylate and are predicted to contain two molecules in the asymmetric unit. Attempts to determine the respective crystal structures by molecular replacement using either the known NMR structure of human saposin C or a related crystal structure as search models have so far failed. The failure of the molecular-replacement method is attributed to conformational changes of the protein, which are known to be required for its biological activity. Crystal structures of human saposin C therefore might be the key to mapping out the conformational trajectory of saposin-like proteins.

Role of lysine residues in membrane anchoring of saposin C

Molecular dynamics (MD) simulations of the N-terminal region of saposin C, containing amino acid residues 4-20 (saposin C4-20), were performed over 2.5 ns in 1,2-dioleoyl-sn-glycero-3-phosphoserine (DOPS) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) monolayers. The simulations revealed several strong specific interactions of lysine 13 (Lys13) and lysine 17 (Lys17) in saposin C4-20 with the anionic phospholipids, which are required for membrane anchoring of the peptide. Membrane anchoring of saposin C4-20 facilitates saposin C-induced liposomal membrane fusion. Substitutions of Lys13 or Lys17 with alanine or glutamic acid led to a substantial loss of saposin C's fusogenicity. However, arginine replacement of Lys13 or Lys17 caused a partial loss of saposin C's fusogenic activity. The membrane anchoring of saposin C was altered in the presence of 0.4 M sodium chloride. Differential salt effects on Lys-mutant saposin Cs were observed using Trp fluorescence analysis. Low salt concentration had a more significant impact on Lys-mutant saposin C with a negatively charged amino acid residue replacement than those mutants with a positively charged or neutral residue replacement. These results indicate that positively charged amino acids at positions 13 and 17 are required for the fusogenic function of saposin C. In addition, the side-chain structure of lysine is crucial to the precise membrane anchoring which is necessary for the total fusion activity of saposin C. The MD simulations and vesicle size measurements of lysine-mutant saposins confirm the importance of the two lysine residues in saposin C4-20 for saposin C-induced fusion of negatively charged phospholipid membranes.

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.

Crystal structure and functional studies reveal that PAS factor from Vibrio vulnificus is a novel member of the saposin-fold family

PAS factor is a novel putative bacterial secretion factor thought to induce secretion of periplasmic proteins. We solved the crystal structure of PAS factor from Vibrio vulnificus at 1.8A resolution and found it to be comprised of five alpha helices that form an antiparallel bundle with an up-and-down topology, and to adopt the saposin-fold characteristic of a family of proteins that bind to membranes and lipids. PAS factor lacks the disulfide bridge characteristic of mammalian saposin-fold proteins; in fact, it shows no sequence homology with mammalian proteins. Nevertheless, the molecular architectures are similar, and the shared propensity for membrane interaction suggests strongly that PAS factor is another member of the saposin-fold family. Analysis of the CD spectra showed that PAS factor binds to membranes directly, while measurement of calcein dye leakage showed that PAS factor interacts strongly with liposomes composed of anionic phospholipids, making them leaky, but binds very weakly with liposomes composed of zwitterionic phospholipids. Moreover, by analyzing tryptophan fluorescence emission from four single-tryptophan mutants (V10W, T22W, F35W, and L70W), we identified the putative phospholipid-binding site of PAS factor. The resultant membrane destabilization likely mediates secretion of periplasmic proteins required for the in vivo survival and pathogenesis of V.vulnificus.

The N370S (Asn370-->Ser) mutation affects the capacity of glucosylceramidase to interact with anionic phospholipid-containing membranes and saposin C

The properties of the endolysosomal enzyme GCase (glucosylceramidase), carrying the most prevalent mutation observed in Gaucher patients, namely substitution of an asparagine residue with a serine at amino acid position 370 [N370S (Asn370-->Ser) GCase], were investigated in the present study. We previously demonstrated that Sap (saposin) C, the physiological GCase activator, promotes the association of GCase with anionic phospholipid-containing membranes, reconstituting in this way the enzyme activity. In the present study, we show that, in the presence of Sap C and membranes containing high levels of anionic phospholipids, both normal and N370S GCases are able to associate with the lipid surface and to express their activity. Conversely, when the amount of anionic phospholipids in the membrane is reduced (approximately 20% of total lipids), Sap C is still able to promote binding and activation of the normal enzyme, but not of N370S GCase. The altered interaction of the mutated enzyme with anionic phospholipid-containing membranes and Sap C was further demonstrated in Gaucher fibroblasts by confocal microscopy, which revealed poor co-localization of N370S GCase with Sap C and lysobisphosphatidic acid, the most abundant anionic phospholipid in endolysosomes. Moreover, we found that N370S Gaucher fibroblasts accumulate endolysosomal free cholesterol, a lipid that might further interfere with the interaction of the enzyme with Sap C and lysobisphosphatidic acid-containing membranes. In summary, our results show that the N370S mutation primarily affects the interaction of GCase with its physiological activators, namely Sap C and anionic phospholipid-containing membranes. We thus propose that the poor contact between N370S GCase and its activators may be responsible for the low activity of the mutant enzyme in vivo.

Structural changes in a binary mixed phospholipid bilayer of DOPG and DOPS upon saposin C interaction at acidic pH utilizing 31P and 2H solid-state NMR spectroscopy

Saposin C (Sap C) is known to stimulate the catalytic activity of the lysosomal enzyme glucosylceramidase (GCase) that facilitates the hydrolysis of glucosylceramide to ceramide and glucose. Both Sap C and acidic phospholipids are required for full activity of GCase. In order to better understand this interaction, mixed bilayer samples prepared from dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylserine (DOPS) (5:3 ratio) and Sap C were investigated using (2)H and (31)P solid-state NMR spectroscopy at temperatures ranging from 25 to 50 degrees C at pH 4.7. The Sap C concentrations used to carry out these experiments were 0 mol%, 1 mol% and 3 mol% with respect to the phospholipids. The molecular order parameters (S(CD)) were calculated from the dePaked (2)H solid-state NMR spectra of Distearoyl-d70-phosphatidylglycerol (DSPG-d70) incorporated with DOPG and DOPS binary mixed bilayers. The S(CD) profiles indicate that the addition of Sap C to the negatively charged phospholipids is concentration dependent. S(CD) profiles of 1 mol% of the Sap C protein show only a very slight decrease in the acyl chain order. However, the S(CD) profiles of the 3 mol% of Sap C protein indicate that the interaction is predominantly increasing the disorder in the first half of the acyl chain near the head group (C1-C8) indicating that the amino and the carboxyl termini of Sap C are not inserting deep into the DOPG and DOPS mixed bilayers. The (31)P solid-state NMR spectra show that the chemical shift anisotropy (CSA) for both phospholipids decrease and the spectral broadening increases upon addition of Sap C to the mixed bilayers. The data indicate that Sap C interacts similarly with the head groups of both acidic phospholipids and that Sap C has no preference to DOPS over DOPG. Moreover, our solid-state NMR spectroscopic data agree with the structural model previously proposed in the literature [X. Qi, G.A. Grabowski, Differential membrane interactions of saposins A and C. Implication for the functional specificity, J. Biol. Chem. 276 (2001) 27010-27017] [1].

Direct AFM observation of saposin C-induced membrane domains in lipid bilayers: from simple to complex lipid mixtures

Saposin C (Sap C) is a small glycoprotein required by glucosylceramidase (GCase) for hydrolysis of glucosylceramide to ceramide and glucose in lysosomes. The molecular mechanism underlying Sap C stimulation of the enzyme activation is not fully understood. Here, atomic force microscopy (AFM) has been used to study Sap C-membrane interactions under physiological conditions. First, to establish how Sap C-membrane interactions affect membrane structure, lipid bilayers containing zwitterionic and anionic phospholipids were used. It was observed that Sap C induced two types of membrane restructuring effects, i.e., the formation of patch-like domains and membrane destabilization. Bilayers underwent extensive structural reorganization. To validate the biological importance of the membrane restructuring effects, interaction of Sap C with lipid bilayers composed of cholesterol, sphingomyelin, and zwitterionic and anionic phospholipids were studied. Although similar membrane restructuring effects were observed, Sap C-membrane interactions, in this case, were remarkably modulated and their effects were restricted to a limited area. As a result, nanometer-sized domains were formed. The establishment of a model membrane system will allow us to further study the dynamics, structure and mechanism of the Sap C-associated membrane domains and to examine the important role that these domains may play in enzyme activation.

Molecular Characterization of a Saposin-Like Protein Family Member Isolated from Bovine Lymphocytes

Human and porcine T lymphocytes and natural killer (NK) cells produce antibacterial proteins that belong to the saposin-like family of proteins (SAPLIP). The objective of this study was to determine if a bovine homolog of SAPLIP exists in lymphocytes that exhibit antibacterial activity. Following stimulation with IL-2, bactericidal activity against Staphylococcus aureus was detected to some extent in most major subpopulations of T lymphocytes including CD4+, CD8+, CD3+, and WC1+ gammadelta T lymphocytes. However, the majority of antibacterial activity was observed in the CD2+CD3- lymphocytes, which are similar phenotypically to NK cells. A partial sequence of a bovine SAPLIP was generated using low specificity primers designed from regions of homology between other SAPLIP including porcine NK-lysin and human granulysin. Enhanced expression of the bovine lysin gene was detected in mRNA isolated from IL-2-stimulated CD2+CD3- lymphocytes. The partial cDNA sequence was then used to make gene specific primers for a rapid amplification of cDNA ends (RACE) procedure that provided repeatable 5' and 3' cDNA ends. By examining overlapping regions from the RACE procedure, full-length sequence information was obtained for the bovine lysin homologue. Conceptual translation of the cDNA demonstrated conserved similarities to known SAPLIP members. Further characterization of the bovine lysin may facilitate its use in protecting dairy cattle against bacterial infections.

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.

Trichinella spiralis secretes a homologue of prosaposin

Infective larvae and adult stage Trichinella spiralis secrete a protein homologous to prosaposin, the precursor of sphingolipid activator proteins (saposins) A-D originally defined in vertebrates. The protein contains four saposin domains, with the six cysteine residues which form the three intramolecular disulphide bonds in close register in each case. It differs substantially from vertebrate prosaposins in the N-terminal prodomain, the region separating saposins A and B, and completely lacks the C-terminal domain which has been demonstrated to be essential for lysosomal targetting in these organisms. The protein is secreted in unprocessed form with an estimated mass of 56 kDa, and contains a single N-linked glycan which is bound by the monoclonal antibody NIM-M1, characteristic of the TSL-1 antigens which are capped by tyvelose (3,6-dideoxy-D-arabinohexose). Immuno-electron microscopy localised the protein to membrane-bound vesicles and more complex multi-lamellar organelles in diverse tissues including the hypodermis, intestine and stichosomes, although it was absent from the dense-core secretory granules typical of the latter. Possible functions of a secreted prosaposin are discussed.

Solution structure of human saposin C in a detergent environment

Saposin C is a lysosomal, membrane-binding protein that acts as an activator for the hydrolysis of glucosylceramide by the enzyme glucocerebrosidase. We used high-resolution NMR to determine the three-dimensional solution structure of saposin C in the presence of the detergent sodium dodecyl sulfate (SDS). This structure provides the first representation of membrane bound saposin C at the atomic level. In the presence of SDS, the protein adopts an open conformation with an exposed hydrophobic pocket. In contrast, the previously reported NMR structure of saposin C in the absence of SDS is compact and contains a hydrophobic core that is not exposed to the solvent. NMR data indicate that the SDS molecules interact with the hydrophobic pocket. The structure of saposin C in the presence of SDS is very similar to a monomer in the saposin B homodimer structure. Their comparison reveals possible similarity in the type of protein/lipid interaction as well as structural components differentiating their quaternary structures and functional specificity.

Amplification and overexpression of prosaposin in prostate cancer

We identified prosaposin (PSAP) as a secreted protein expressed in androgen-independent (AI) prostate cancer cells by cloning/sequencing, after probing a PC-3 cDNA library expressed in the lambdaTriplEx phagemid expression vector with a polyclonal rabbit antibody generated against pooled human seminal plasma. PSAP is a neurotrophic molecule; its deficiency or inactivation has proved to be lethal in man and mice, and in mice, it leads to abnormal development and atrophy of the prostate gland, despite normal testosterone levels. We used Southern hybridization, quantitative real-time polymerase chain reaction, and/or single nucleotide polymorphism (SNP) array analysis, and we now report the genomic amplification of PSAP in the metastatic AI prostate cancer cell lines, PC-3, DU-145, MDA-PCa 2b, M-12, and NCI-H660. In addition, by using SNP arrays and a set of 25 punch biopsy samples of human prostate cancer xenografts (LAPC3, LuCaP 23.1, 35, 49, 58, 73, 77, 81, 86.2, 92.1, 93, 96, 105, and 115), lymph nodes, and visceral-organ metastases, we detected amplification of the PSAP locus (10q22.1) in LuCaP 58 and 96 xenografts and two lymph node metastases. In addition, AI metastatic prostate cancer cell lines C4-2B and IA8-ARCaP over-expressed PSAP mRNA without evidence of genomic amplification. Taken together with prior data that demonstrated the growth-, migration-, and invasion-promoting activities, the activation of multiple signal transduction pathways, and the antiapoptotic effect of PSAP (or one of its active domains, saposin C) in prostate cancer cells, our current observation of PSAP amplification or overexpression in prostate cancer suggests, for the first time, a role for this molecule in the process of carcinogenesis or cancer progression in the prostate.

An alpha-subunit loop structure is required for GM2 activator protein binding by beta-hexosaminidase A

The alpha- and/or beta-subunits of human beta-hexosaminidase A (alphabeta) and B (betabeta) are approximately 60% identical. In vivo only beta-hexosaminidase A can utilize GM2 ganglioside as a substrate, but requires the GM2 activator protein to bind GM2 ganglioside and then interact with the enzyme, placing the terminal GalNAc residue in the active site of the alpha-subunit. A model for this interaction suggests that two loop structures, present only in the alpha-subunit, may be critical to this binding. Three amino acids in one of these loops are not encoded in the HEXB gene, while four from the other are removed posttranslationally from the pro-beta-subunit. Natural substrate assays with forms of hexosaminidase A containing mutant alpha-subunits demonstrate that only the site that is removed from the beta-subunit during its maturation is critical for the interaction. Our data suggest an unexpected biological role for such proteolytic processing events.

Mutation in saposin D domain of sphingolipid activator protein gene causes urinary system defects and cerebellar Purkinje cell degeneration with accumulation of hydroxy fatty acid-containing ceramide in mouse

The sphingolipid activator proteins (saposins A, B, C and D) are small homologous glycoproteins that are encoded by a single gene in tandem within a large precursor protein (prosaposin) and are required for in vivo degradation of some sphingolipids with relatively short carbohydrate chains. Human patients with prosaposin or specific saposin B or C deficiency are known, and prosaposin- and saposin A-deficient mouse lines have been generated. Experimental evidence suggests that saposin D may be a lysosomal acid ceramidase activator. However, no specific saposin D deficiency state is known in any mammalian species. We have generated a specific saposin D(-/-) mouse by introducing a mutation (C509S) into the saposin D domain of the mouse prosaposin gene. Saposin D(-/-) mice developed progressive polyuria at around 2 months and ataxia at around 4 months. Pathologically, the kidney of saposin D(-/-) mice showed renal tubular degeneration and eventual hydronephrosis. In the nervous system, progressive and selective loss of the cerebellar Purkinje cells in a striped pattern was conspicuous, and almost all Purkinje cells disappeared by 12 months. Biochemically, ceramides, particularly those containing hydroxy fatty acids accumulated in the kidney and the brain, most prominently in the cerebellum. These results not only indicate the role of saposin D in in vivo ceramide metabolism, but also suggest possible cytotoxicity of ceramide underlying the cerebellar Purkinje cell and renal tubular cell degeneration.