Degradation of membrane-bound ganglioside GM1. Stimulation by bis(monoacylglycero)phosphate and the activator proteins SAP-B and GM2-AP

According to our hypothesis (Fürst, W., and Sandhoff, K. (1992) Biochim. Biophys. Acta 1126, 1-16) glycosphingolipids of the plasma membrane are digested after endocytosis as components of intraendosomal and intralysosomal vesicles and membrane structures. The lysosomal degradation of glycosphingolipids with short oligosaccharide chains by acid exohydrolases requires small, non-enzymatic cofactors, called sphingolipid activator proteins (SAPs). A total of five activator proteins have been identified as follows: namely the saposins SAP-A, -B, -C, and -D, which are derived from the single chain SAP-precursor protein (prosaposin), and the GM2 activator protein. A deficiency of prosaposin results in the storage of ceramide and sphingolipids with short oligosaccharide head groups. The loss of the GM2 activator protein blocks the degradation of the ganglioside GM2. The enzymatic hydrolysis of the ganglioside GM1 is catalyzed by beta-galactosidase, a water-soluble acid exohydrolase. The lack of ganglioside GM1 accumulation in patients suffering from either prosaposin or GM2 activator protein deficiency has led to the hypothesis that SAPs are not needed for the hydrolysis of the ganglioside GM1 in vivo. In this study we demonstrate that an activator protein is required for the enzymatic degradation of membrane-bound ganglioside GM1 and that both SAP-B and the GM2 activator protein significantly enhance the degradation of the ganglioside GM1 by acid beta-galactosidase in a liposomal, detergent-free assay system. These findings offer a possible explanation for the observation that no storage of the ganglioside GM1 has been observed in patients with either isolated prosaposin or isolated GM2 activator deficiency. We also demonstrate that anionic phospholipids such as bis(monoacylglycero)phosphate and phosphatidylinositol, which specifically occur in inner membranes of endosomes and in lysosomes, are essential for the activator-stimulated hydrolysis of the ganglioside GM1. Assays utilizing surface plasmon resonance spectroscopy showed that bis(monoacylglycero)phosphate increases the binding of both beta-galactosidase and activator proteins to substrate-carrying membranes.     

Lysosomal degradation of membrane lipids

The constitutive degradation of membrane components takes place in the acidic compartments of a cell, the endosomes and lysosomes. Sites of lipid degradation are intralysosomal membranes that are formed in endosomes, where the lipid composition is adjusted for degradation. Cholesterol is sorted out of the inner membranes, their content in bis(monoacylglycero)phosphate increases, and, most likely, sphingomyelin is degraded to ceramide. Together with endosomal and lysosomal lipid-binding proteins, the Niemann-Pick disease, type C2-protein, the GM2-activator, and the saposins sap-A, -B, -C, and -D, a suitable membrane lipid composition is required for degradation of complex lipids by hydrolytic enzymes.     

Enrichment of NPC1-deficient cells with the lipid LBPA stimulates autophagy,improves lysosomal function,and reduces cholesterol storage

Niemann–Pick C (NPC) is an autosomal recessive disorder characterized by mutations in the NPC1 or NPC2 genes encoding endolysosomal lipid transport proteins, leading to cholesterol accumulation and autophagy dysfunction. We have previously shown that enrichment of NPC1-deficient cells with the anionic lipid lysobisphosphatidic acid (LBPA; also called bis(monoacylglycerol)phosphate) via treatment with its precursor phosphatidylglycerol (PG) results in a dramatic decrease in cholesterol storage. However, the mechanisms underlying this reduction are unknown. In the present study, we showed using biochemical and imaging approaches in both NPC1-deficient cellular models and an NPC1 mouse model that PG incubation/LBPA enrichment significantly improved the compromised autophagic flux associated with NPC1 disease, providing a route for NPC1-independent endolysosomal cholesterol mobilization. PG/LBPA enrichment specifically enhanced the late stages of autophagy, and effects were mediated by activation of the lysosomal enzyme acid sphingomyelinase. PG incubation also led to robust and specific increases in LBPA species with polyunsaturated acyl chains, potentially increasing the propensity for membrane fusion events, which are critical for late-stage autophagy progression. Finally, we demonstrated that PG/LBPA treatment efficiently cleared cholesterol and toxic protein aggregates in Purkinje neurons of the NPC1^I1061T mouse model. Collectively, these findings provide a mechanistic basis supporting cellular LBPA as a potential new target for therapeutic intervention in NPC disease.     

Lipid cosorting mediated by shiga toxin induced tubulation

To maintain cell membrane homeostasis, lipids must be dynamically redistributed during the formation of transport intermediates, but the mechanisms driving lipid sorting are not yet fully understood. Lowering sphingolipid concentration can reduce the bending energy of a membrane, and this effect could account for sphingolipid depletion along the retrograde pathway. However, sphingolipids and cholesterol are enriched along the anterograde pathway, implying that other lipid sorting mechanisms, such as protein-mediated sorting, can dominate. To characterize the influence of protein binding on the lipid composition of highly curved membranes, we studied the interactions of the B-subunit of Shiga toxin (STxB) with giant unilamellar vesicles containing its glycosphingolipid receptor [globotriaosylceramide (Gb3)]. STxB binding induced the formation of tubular membrane invaginations, and fluorescence microscopy images of these highly curved membranes were consistent with co-enrichment of Gb3 and sphingolipids. In agreement with theory, sorting was stronger for membrane compositions close to demixing. These results strongly support the hypothesis that proteins can indirectly mediate the sorting of lipids into highly curved transport intermediates via interactions between lipids and the membrane receptor of the protein.     

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.     

Physiological relevance of sphingolipid activator proteins in cultured human fibroblasts

The physiological degradation of several membrane-bound glycosphingolipids (GSLs) by water-soluble lysosomal exohydrolases requires the assistance of sphingolipid activator proteins (SAPs). Four of these SAPs are synthesized from a single precursor protein (prosaposin). Inherited deficiency of this precursor results in a rare disease in humans with an accumulation of ceramide (Cer) and glycolipids such as glucosylceramide and lactosylceramide (LacCer). In a previous study, we have shown that human SAP-D stimulates the lysosomal degradation of Cer in precursor deficient cells. In order to study the role of SAPs (or saposins) A-D in cellular GSL catabolism, we recently investigated the catabolism of exogenously added [(3)H]labeled ganglioside GM1, Forssman lipid, and endogenously [(14)C]labeled GSLs in SAP-precursor deficient human fibroblasts after the addition of recombinant SAP-A, -B, -C and -D. We found that activator protein deficient cells are still able to slowly degrade gangliosides GM1 and GM3, Forssman lipid and globotriaosylceramide to a significant extent, while LacCer catabolism critically depends on the presence of SAPs. The addition of either of the SAPs, SAP-A, SAP-B or SAP-C, resulted in an efficient hydrolysis of LacCer.     

The trafficking of prosaposin (SGP-1) and GM2AP to the lysosomes of TM4 Sertoli cells is mediated by sortilin and monomeric adaptor proteins

Prosaposin (SGP-1) and GM2 activator protein (GM2AP) are soluble sphingolipid activator proteins (SAPs) that are targeted to the lysosomal compartment of Sertoli cells to aid hydrolases in the breakdown of glycosphingolipids. To reach the lysosome, most soluble proteins must interact with the mannose 6-phosphate receptor (MPR). To be sorted from the Golgi, the MPR must bind to the Golgi associated, gamma-adaptin homologous, ARF binding proteins (GGAs), a group of monomeric adaptor proteins responsible for the recruitment of clathrin. It is well established, however, that the lysosomes of I-cell disease (ICD) patients have near normal levels of several lysosomal proteins, including prosaposin and GM2AP. ICD results from a mutation in the phosphotransferase that adds mannose 6-phosphate to hydrolases. Thus, prosaposin and GM2AP can traffic to lysosomes in a MPR independent manner. Previous work has demonstrated that an interaction with sphingomyelin in the Golgi membrane is necessary for the targeting of prosaposin by an unknown receptor. Using a TM4 Sertoli cell line, we tested the hypothesis that prosaposin and GM2AP are targeted to the lysosomal compartment via the sortilin receptor, which has been recently shown to have a GGA binding motif. Interestingly, dominant-negative GGAs, unable to bind clathrin to shuttle from the Golgi, prevented the trafficking of prosaposin and GM2AP to lysosomes. A dominant negative construct of sortilin lacking the GGA binding domain retained prosaposin and GM2AP in the Golgi. In conclusion, our results showed that the trafficking of prosaposin and GM2AP to the lysosome is dependent on sortilin.     

Niemann–Pick C2 (NPC2) and intracellular cholesterol trafficking

Cholesterol is an important precursor for numerous biologically active molecules, and it plays a major role in membrane structure and function. Cholesterol can be endogenously synthesized or exogenously taken up via the endocytic vesicle system and subsequently delivered to post-endo/lysosomal sites including the plasma membrane and the endoplasmic reticulum. Niemann–Pick C (NPC) disease results in the accumulation of exogenously-derived cholesterol, as well as other lipids, in late endosomes and lysosomes (LE/LY). Identification of the two genes that underlie NPC disease, NPC1 and NPC2, has focused attention on the mechanisms by which lipids, in particular cholesterol, are transported out of the LE/LY compartment. This review discusses the role of the NPC2 protein in cholesterol transport, and the potential for concerted action of NPC1 and NPC2 in regulating normal intracellular cholesterol homeostasis.     

Sphingolipid synthesis is involved in autophagy in Saccharomyces cerevisiae

In eukaryotes, autophagy is a conserved protein degradation system that degrades cytoplasmic components by encompassing them with double-membrane structures, called autophagosomes, and delivering them to the lytic compartments of vacuoles/lysosomes. Certain Atg proteins are known to be involved in autophagy, yet the identity and function of lipid molecules involved remain largely unknown. We investigated the involvement of sphingolipids in autophagy using Saccharomyces cerevisiae. Inhibiting synthesis of the simplest complex sphingolipid, inositol phosphorylceramide (IPC), resulted in reduced autophagic activities. Similar results were obtained using myriocin, an inhibitor of the first step in sphingolipid synthesis. Our results indicate that sphingolipids, especially IPC, are required for autophagy. Inhibition of sphingolipid synthesis had no effect on formation of Atg12-Atg5 or Atg8-phosphatidylethanolamine conjugates, on maturation of vacuolar proteases, or on formation of the pre-autophagosomal structure (PAS). These results suggest that sphingolipids are not involved in the cellular signaling that leads to formation of the PAS, but may be involved in the process of autophagosome formation.     

Sphingolipids as modulators of membrane proteins

The diversity of the transmembranome of higher eukaryotes is matched by an enormous diversity of sphingolipid classes and molecular species. The intrinsic properties of sphingolipids are not only suited for orchestrating lateral architectures of biological membranes, but their molecular distinctions also allow for the evolution of protein motifs specifically recognising and interacting with individual lipids. Although various reports suggest a role of sphingolipids in membrane protein function, only a few cases have determined the specificity of these interactions. In this review we discuss examples of specific protein–sphingolipid interactions for which a modulator-like dependency on sphingolipids was assigned to specific proteins. These novel functions of sphingolipids in specific protein–lipid assemblies contribute to the complexity of the sphingolipid classes and other molecular species observed in animal cells. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.     

Sphingolipid Metabolism and Interorganellar Transport: Localization of Sphingolipid Enzymes and Lipid Transfer Proteins

In recent decades, many sphingolipid enzymes, sphingolipid‐metabolism regulators and sphingolipid transfer proteins have been isolated and characterized. This review will provide an overview of the intracellular localization and topology of sphingolipid enzymes in mammalian cells to highlight the locations where respective sphingolipid species are produced. Interestingly, three sphingolipids that reside or are synthesized in cytosolic leaflets of membranes (ceramide, glucosylceramide and ceramide‐1‐phosphate) all have cytosolic lipid transfer proteins (LTPs). These LTPs consist of ceramide transfer protein (CERT), four‐phosphate adaptor protein 2 (FAPP2) and ceramide‐1‐phosphate transfer protein (CPTP), respectively. These LTPs execute functions that affect both the location and metabolism of the lipids they bind. Molecular details describing the mechanisms of regulation of LTPs continue to emerge and reveal a number of critical processes, including competing phosphorylation and dephosphorylation reactions and binding interactions with regulatory proteins and lipids that influence the transport, organelle distribution and metabolism of sphingolipids.      

Saposin B mobilizes lipids from cholesterol-poor and bis(monoacylglycero)phosphate-rich membranes at acidic pH. Unglycosylated patient variant saposin B lacks lipid-extraction capacity

Sphingolipid activator proteins (SAPs), GM2 activator protein (GM2AP) and saposins (Saps) A-D are small, enzymatically inactive glycoproteins of the lysosome. Despite of their sequence homology, these lipid-binding and -transfer proteins show different specificities and varying modes of action. Water-soluble SAPs facilitate the degradation of membrane-bound glycosphingolipids with short oligosaccharide chains by exohydrolases at the membrane-water interface. There is strong evidence that degradation of endocytosed components of the cell membrane takes place at intraendosomal and intralysosomal membranes. The inner membranes of the lysosome differ from the limiting membrane of the organelle in some typical ways: the inner vesicular membranes lack a protecting glycocalix, and they are almost free of cholesterol, but rich in bis(monoacylglycero)phosphate (BMP), the anionic marker lipid of lysosomes. In this study, we prepared glycosylated Sap-B free of other Saps by taking advantage of the Pichia pastoris expression system. We used immobilized liposomes as a model for intralysosomal vesicular membranes to probe their interaction with recombinantly expressed Sap-B. We monitored this interaction using SPR spectroscopy and an independent method based on the release of radioactively labelled lipids from liposomal membranes. We show that, after initial binding, Sap-B disturbs the membrane structure and mobilizes the lipids from it. Lipid mobilization is dependent on an acidic pH and the presence of anionic lipids, whereas cholesterol is able to stabilize the liposomes. We also show for the first time that glycosylation of Sap-B is essential to achieve its full lipid-extraction activity. Removal of the carbohydrate moiety of Sap-B reduces its membrane-destabilizing quality. An unglycosylated Sap-B variant, Asn215His, which causes a fatal sphingolipid storage disease, lost the ability to extract membrane lipids at acidic pH in the presence of BMP.     

Distribution and Functions of Sterols and Sphingolipids

Sterols and sphingolipids are considered mainly eukaryotic lipids even though both are present in some prokaryotes, with sphingolipids being more widespread than sterols. Both sterols and sphingolipids differ in their structural features in vertebrates, plants, and fungi. Interestingly, some invertebrates cannot synthesize sterols de novo and seem to have a reduced dependence on sterols. Sphingolipids and sterols are found in the plasma membrane, but we do not have a clear picture of their precise intracellular localization. Advances in lipidomics and subcellular fractionation should help to improve this situation. Genetic approaches have provided insights into the diversity of sterol and sphingolipid functions in eukaryotes providing evidence that these two lipid classes function together. Intermediates in sphingolipid biosynthesis and degradation are involved in signaling pathways, whereas sterol structures are converted to hormones. Both lipids have been implicated in regulating membrane trafficking.Typical examples of eukaryotic lipids, sterols, and sphingolipids can both be found in membranes from simple unicellular fungi and protists to multicellular animals and plants. Their versatile use as structural elements but also as signaling molecules has probably played an important role during the evolution of this large and diverse group of organisms. There are also many eukaryotes that have lost the ability to synthesize sterols de novo including nematodes, insects, and marine invertebrates, which have to take up sterols with their diet. Sterol biosynthesis has also been reported in a number of bacteria. Sphingolipids are more widely spread among prokaryotes than sterols and also show a greater variety of structures among the different eukaryotes.In this short review we will first give an overview about the diversity of sterol and sphingolipid structures and their distribution in nature. Then we will discuss their subcellular distribution. A brief technical section will add some information on the separation and detection of these lipid molecules. Subsequently, we will summarize different genetic approaches to study the functions of sterols and sphingolipids, and finally, we will discuss the functional and possible physical interactions of the two lipid classes within the cell. Far from being comprehensive, we will focus only on a few interesting aspects and try to give new view points, which are less frequently discussed.     

Npc1 deficiency in the C57BL/6J genetic background enhances Niemann-Pick disease type C spleen pathology

Niemann–Pick type C (NPC) disease is an autosomal recessive neurovisceral lipid storage disorder. The affected genes are NPC1 and NPC2. Mutations in either gene lead to intracellular cholesterol accumulation. There are three forms of the disease, which are categorized based on the onset and severity of the disease: the infantile form, in which the liver and spleen are severely affected, the juvenile form, in which the liver and brain are affected, and the adult form, which affects the brain. In mice, a spontaneous mutation in the Npc1 gene originated in the BALB/c inbred strain mimics the juvenile form of the disease. To study the influence of genetic background on the expression of NPC disease in mice, we transferred the Npc1 mutation from the BALB/c to C57BL/6J inbred background. We found that C57BL/6J-Npc1^−/− mice present with a much more aggressive form of the disease, including a shorter lifespan than BALB/c-Npc1^−/− mice. Surprisingly, there was no difference in the amount of cholesterol in the brains of Npc1^−/− mice of either mouse strain. However, Npc1^−/− mice with the C57BL/6J genetic background showed striking spleen damage with a marked buildup of cholesterol and phospholipids at an early age, which correlated with large foamy cell clusters. In addition, C57BL/6J Npc1^−/− mice presented red cell abnormalities and abundant ghost erythrocytes that correlated with a lower hemoglobin concentration. We also found abnormalities in white cells, such as cytoplasmic granulation and neutrophil hypersegmentation that included lymphopenia and atypias. In conclusion, Npc1 deficiency in the C57BL6/J background is associated with spleen, erythrocyte, and immune system abnormalities that lead to a reduced lifespan.     

Sphingolipid homeostasis in the web of metabolic routes

Sphingolipids play a key role in cells as structural components of membrane lipid bilayers and signaling molecules implicated in important physiological and pathological processes. Their metabolism is tightly regulated. Mechanisms controlling sphingolipid metabolism are far from being completely understood. However, they already reveal the integration of sphingolipids in the whole metabolic network as signaling devices that coordinate different metabolic pathways. A picture of sphingolipids integrated into metabolic networks might help to understand sphingolipid homeostasis. This review describes recent advances in the regulation of de novo sphingolipid synthesis with a focus on the bridges that exist with other metabolic pathways and the importance of this crosstalk in the control of sphingolipid homeostasis. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.     

From sheep to mice to cells: Tools for the study of the sphingolipidoses

The sphingolipidoses are a group of inherited lysosomal storage diseases in which sphingolipids accumulate due to the defective activity of one or other enzymes involved in their degradation. For most of the sphingolipidoses, little is known about the molecular mechanisms that lead to disease, which has negatively impacted attempts to develop therapies for these devastating human diseases. Use of both genetically-modified animals, ranging from mice to larger mammals, and of novel cell culture systems, is of utmost importance in delineating the molecular mechanisms that cause pathophysiology, and in providing tools that enable testing the efficacy of new therapies. In this review, we discuss eight sphingolipidoses, namely Gaucher disease, Fabry disease, metachromatic leukodystrophy, Krabbe disease, Niemann–Pick diseases A and B, Farber disease, GM1 gangliosidoses, and GM2 gangliosidoses, and describe the tools that are currently available for their study. This article is part of a Special Issue entitled Tools to study lipid functions.     

The Physical and Genetic Organization of A Viral Genom

Membrane proteins that bind and transport lipids face special challenges. Since lipids typically have low water solubility, both accessibility of the substrate to the protein and delivery to the desired destination are problematical. The amphipathic nature of membrane lipids, and their relatively large molecular size, also means that these proteins must possess substrate-binding sites of a different nature than those designed to handle small polar molecules. This review considers two integral proteins whose function is to bind and transfer membrane lipids within or across a membrane. The first protein, MsbA, is a putative lipid flippase that is a member of the ATP-binding cassette (ABC) superfamily. The protein is found in the inner (cytoplasmic) membrane (IM) of Gram-negative bacteria such as E. coli, where it is proposed to move lipid A from the inner to the outer membrane (OM) leaflet, an important step in the lipopolysaccharide biosynthetic pathway. Cholesterol is a major component of the plasma membrane in eukaryotic cells, where it regulates bilayer fluidity. The other lipid-binding protein discussed here, mammalian NPC1 (Niemann-Pick disease, Type C1), binds cholesterol inside late endosomes/lysosomes (LE/LY) and is involved in its transfer to the cytosol as part of a key intracellular sterol-trafficking pathway. Mutations in NPC1 lead to a devastating neurodegenerative condition, Niemann-Pick Type C disease, which is characterized by massive cholesterol accumulation in LE/LY. The accelerating pace of membrane protein structure determination over the past decade has allowed us a glimpse of how lipid binding and transfer by membrane proteins such as MsbA and NPC1 might be achieved.     

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.     

Segregation of sphingolipids and sterols during formation of secretory vesicles at the trans-Golgi network

The trans-Golgi network (TGN) is the major sorting station in the secretory pathway of all eukaryotic cells. How the TGN sorts proteins and lipids to generate the enrichment of sphingolipids and sterols at the plasma membrane is poorly understood. To address this fundamental question in membrane trafficking, we devised an immunoisolation procedure for specific recovery of post-Golgi secretory vesicles transporting a transmembrane raft protein from the TGN to the cell surface in the yeast Saccharomyces cerevisiae. Using a novel quantitative shotgun lipidomics approach, we could demonstrate that TGN sorting selectively enriched ergosterol and sphingolipid species in the immunoisolated secretory vesicles. This finding, for the first time, indicates that the TGN exhibits the capacity to sort membrane lipids. Furthermore, the observation that the immunoisolated vesicles exhibited a higher membrane order than the late Golgi membrane, as measured by C-Laurdan spectrophotometry, strongly suggests that lipid rafts play a role in the TGN-sorting machinery.     

Increased lipid droplet accumulation associated with a peripheral sensory neuropathy

Hereditary sensory neuropathy type 1 (HSN-1) is an autosomal dominant neurodegenerative disease caused by missense mutations in the SPTLC1 gene. The SPTLC1 protein is part of the SPT enzyme which is a ubiquitously expressed, critical and thus highly regulated endoplasmic reticulum bound membrane enzyme that maintains sphingolipid concentrations and thus contributes to lipid metabolism, signalling, and membrane structural functions. Lipid droplets are dynamic organelles containing sphingolipids and membrane bound proteins surrounding a core of neutral lipids, and thus mediate the intracellular transport of these specific molecules. Current literature suggests that there are increased numbers of lipid droplets and alterations of lipid metabolism in a variety of other autosomal dominant neurodegenerative diseases, including Alzheimer’s and Parkinson’s disease. This study establishes for the first time, a significant increase in the presence of lipid droplets in HSN-1 patient-derived lymphoblasts, indicating a potential connection between lipid droplets and the pathomechanism of HSN-1. However, the expression of adipophilin (ADFP), which has been implicated in the regulation of lipid metabolism, was not altered in lipid droplets from the HSN-1 patient-derived lymphoblasts. This appears to be the first report of increased lipid body accumulation in a peripheral neuropathy, suggesting a fundamental molecular linkage between a number of neurodegenerative diseases.