Sphingolipid hydrolases and activator proteins

Synthesis of Fluorescent Membrane-Spanning Lipids for Studies of Lipid Transfer and Membrane Fusion

For uncompromised in vitro assays for intermembrane lipid transfer and membrane fusion fluorescent membrane-spanning lipids have proved to be invaluable tools. These lipids in contrast to phosphoglycerolipids and sphingolipids are resistant to spontaneous as well as protein-mediated intermembrane transfer. Here I describe the synthesis of some homo-substituted fluorescent bipolar membrane-spanning lipids that bear a fluorescent tag either directly or via a phosphoethanolamine spacer to the lipid core. For the synthesis the lipid core of the bipolar membrane-spanning lipids, i.e., the tetraether lipid caldarchaeol, is prepared from cultures of the archaea Thermoplasma acidophilum.

Membrane lipids and their degradation compounds control GM2 catabolism at intralysosomal luminal vesicles

The catabolism of ganglioside GM2 is dependent on three gene products. Mutations in any of these genes result in a different type of GM2 gangliosidosis (Tay-Sachs disease, Sandhoff disease, and the B1 and AB variants of GM2 gangliosidosis), with GM2 as the major lysosomal storage compound. GM2 is also a secondary storage compound in lysosomal storage diseases such as Niemann-Pick disease types A-C, with primary storage of SM in type A and cholesterol in types B and C, respectively. The reconstitution of GM2 catabolism at liposomal surfaces carrying GM2 revealed that incorporating lipids into the GM2-carrying membrane such as cholesterol, SM, sphingosine, and sphinganine inhibits GM2 hydrolysis by β-hexosaminidase A assisted by GM2 activator protein, while anionic lipids, ceramide, fatty acids, lysophosphatidylcholine, and diacylglycerol stimulate GM2 catabolism. In contrast, the hydrolysis of the synthetic, water-soluble substrate 4-methylumbelliferyl-6-sulfo-2-acetamido-2-deoxy-β-d-glucopyranoside was neither significantly affected by membrane lipids such as ceramide or SM nor stimulated by anionic lipids such as bis(monoacylglycero)phosphate added as liposomes, detergent micelles, or lipid aggregates. Moreover, hydrolysis-inhibiting lipids also had an inhibiting effect on the solubilization and mobilization of membrane-bound lipids by the GM2 activator protein, while the stimulating lipids enhanced lipid mobilization.

Membrane-spanning lipids for an uncompromised monitoring of membrane fusion and intermembrane lipid transfer

A Förster resonance energy transfer-based fusion and transfer assay was developed to study, in model membranes, protein-mediated membrane fusion and intermembrane lipid transfer of fluorescent sphingolipid analogs. For this assay, it became necessary to apply labeled reporter molecules that are resistant to spontaneous as well as protein-mediated intermembrane transfer. The novelty of this assay is the use of nonextractable fluorescent membrane-spanning bipolar lipids. Starting from the tetraether lipid caldarchaeol, we synthesized fluorescent analogs with fluorophores at both polar ends. In addition, we synthesized radioactive glycosylated caldarchaeols. These labeled lipids were shown to stretch through bilayer membranes rather than to loop within a single lipid layer of liposomes. More important, the membrane-spanning lipids (MSLs) in contrast to phosphoglycerides proved to be nonextractable by proteins. We could show that the GM2 activator protein (GM2AP) is promiscuous with respect to glycero- and sphingolipid transfer. Saposin (Sap) B also transferred sphingolipids albeit with kinetics different from GM2AP. In addition, we could unambiguously show that the recombinant activator protein Sap C x His6 induced membrane fusion rather than intermembrane lipid transfer. These findings showed that these novel MSLs, in contrast with fluorescent phosphoglycerolipids, are well suited for an uncompromised monitoring of membrane fusion and intermembrane lipid transfer.

Sortilin and prosaposin localize to detergent-resistant membrane microdomains

Most soluble lysosomal hydrolases are sorted in the trans-Golgi network (TGN) and delivered to the lysosomes by the mannose 6-phosphate receptor (M6PR). However, the non-enzymic sphingolipid activator protein (SAP), prosaposin, as well as certain soluble lysosomal hydrolases, is sorted and trafficked to the lysosomes by sortilin. Based on previous results demonstrating that prosaposin requires sphingomyelin to be targeted to the lysosomes, we hypothesized that sortilin and its ligands are found in detergent-resistant membranes (DRMs). To test this hypothesis we have analyzed DRM fractions and demonstrated the presence of sortilin and its ligand, prosaposin. Our results showed that both the M6PR and its cargo, cathepsin B, were also present in DRMs. Cathepsin H has previously been demonstrated to interact with sortilin, while cathepsin D interacts with both sortilin and the M6PR. Both of these soluble lysosomal proteins were also found in DRM fractions. Using sortilin shRNA we have showed that prosaposin is localized to DRM fractions only in the presence of sortilin. These observations suggest that in addition to interacting with the same adaptor proteins, such as GGAs, AP-1 and retromer, both sortilin and the M6PR localize to similar membrane platforms, and that prosaposin must interact with sortilin to be recruited to DRMs.

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.

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.

The lysosomal trafficking of sphingolipid activator proteins (SAPs) is mediated by sortilin

Most soluble lysosomal proteins bind the mannose 6-phosphate receptor (M6P-R) to be sorted to the lysosomes. However, the lysosomes of I-cell disease (ICD) patients, a condition resulting from a mutation in the phosphotransferase that adds mannose 6-phosphate to hydrolases, have near normal levels of several lysosomal proteins, including the sphingolipid activator proteins (SAPs), GM2AP and prosaposin. We tested the hypothesis that SAPs are targeted to the lysosomal compartment via the sortilin receptor. To test this hypothesis, a dominant-negative construct of sortilin and a sortilin small interfering RNA (siRNA) were introduced into COS-7 cells. Our results showed that both the truncated sortilin and the sortilin siRNA block the traffic of GM2AP and prosaposin to the lysosomal compartment. This observation was confirmed by a co-immunoprecipitation, which demonstrated that GM2AP and prosaposin are interactive partners of sortilin. Furthermore, a dominant-negative mutant GGA prevented the trafficking of prosaposin and GM2AP to lysosomes. In conclusion, our results show that the trafficking of SAPs is dependent on sortilin, demonstrating a novel lysosomal trafficking.

Purification, cloning and characterization of a GPI inositol deacylase from Trypanosoma brucei

Inositol acylation is an obligatory step in glycosylphosphatidylinositol (GPI) biosynthesis whereas mature GPI anchors often lack this modification. The GPI anchors of Trypanosoma brucei variant surface glycoproteins (VSGs) undergo rounds of inositol acylation and deacylation during GPI biosynthesis and the deacylation reactions are inhibited by diisopropylfluorophosphate (DFP). Inositol deacylase was affinity labelled with [3H]DFP and purified. Peptide sequencing was used to clone GPIdeAc, which encodes a protein with significant sequence and hydropathy similarity to mammalian acyloxyacyl hydrolase, an enzyme that removes fatty acids from bacterial lipopolysaccharide. Both contain a signal sequence followed by a saposin domain and a GDSL-lipase domain. GPIdeAc(-/-) trypanosomes were viable in vitro and in animals. Affinity-purified HA-tagged GPIdeAc was shown to have inositol deacylase activity. However, total inositol deacylase activity was only reduced in GPIdeAc(-/-) trypanosomes and the VSG GPI anchor was indistinguishable from wild type. These results suggest that there is redundancy in T.brucei inositol deacylase activity and that there is another enzyme whose sequence is not recognizably related to GPIdeAc.

Update on genetic disorders affecting white matter

The classification of diseases affecting white matter has changed dramatically with the use of magnetic resonance imaging. Classical leukodystrophies, such as metachromatic leukodystrophy and Krabbe's disease, account for only a small number of inherited diseases that affect white matter. Magnetic resonance imaging has clarified genetic disorders that result in white matter changes or leukoencephalopathies. The term leukoencephalopathy is used to reflect the broader number of diseases that may cause as either primary or secondary changes in myelin development. This review attempts to categorize white matter disorders into classes such as lipid, myelin protein, organic acids, and defects in energy metabolism, in addition to other causes.