The Niemann-Pick type C (NPC) disease is a lysosomal lipid storage disorder caused by mutations in one of the two genes NPC1 or NPC2 encoding proteins of the late endosome/lysosome compartment (LE/LY). Defect in these proteins alters vesicular transport and leads to abnormal accumulation of cholesterol (Chol) in LE/LY. There are some lines of evidence suggesting that annexin A6 (AnxA6) participates in vesicular transport of Chol and may interact with membrane domains enriched in Chol and bind Chol. In this work we characterized the membrane microdomains resistant to Triton X-100, i.e., detergent-resistant membranes (DRMs) isolated from NPC patient-derived fibroblasts and from control cells. NPC cells contain a significantly higher amount of DRMs than the control cells that is consistent with the defect in Chol turnover in NPC cells. We also studied the mechanism of AnxA6 involvement in the NPC-induced changes in the membrane organization and showed that in the presence of calcium some AnxA6 molecules associate with the DRMs. This suggests that AnxA6 may play a role in the membrane lateral organization, contributing thus to the etiology of NPC disease. We then focused on the interaction of AnxA6-1 with Chol-rich membranes and on the involvement of its flexible region and VAAEIL sequence in these interactions. For this purpose, kinetics of the interfacial adsorption of human recombinant AnxA6 to Langmuir monolayers containing phosphatidylcholine, Chol and/or cholesteryl acetate were measured. Our data suggest that AnxA6 exhibits the highest affinity to Chol-containing monolayers and that the hydroxyl group of Chol plays a pivotal role in the AnxA6-lipid interactions in vitro.

La maladie de Niemann-Pick de type C (NPC) est une lipidose lysosomale complexe due à une mutation d’un des gènes NPC1 ou NPC2, qui codent pour ces protéines localisées dans les compartiments endo-lysosomaux (LE/LY). Leur absence altère le trafic intracellulaire et induit l’accumulation du cholestérol (Chol) dans les LE/LY. De plus, l’AnxA6 semble participer au transport vésiculaire du Chol en interagissant avec les microdomaines membranaires enrichis en Chol, ou avec le Chol lui-même. Dans ce travail, nous avons isolé des microdomaines membranaires résistant au Triton X-100 (également appelés DRMs pour detergent resistant membranes) à partir de lignée cellulaire NPC L1 ou de cellules saines. Les fibroblastes NPC contiennent plus de DRMs que les fibroblastes sains. Ceci semble être corrélé aux problèmes de transport du Chol dans les cellules NPC. Nous avons aussi montré qu’en présence de calcium, une partie de l’AnxA6 est associé aux DRMs, suggérant que l’AnxA6 participe à l’organisation de la membrane et par ce bias à l’étiologie de la maladie de NPC. Nous avons alors analysé les interactions de l’AnxA6-1 avec les microdomaines riches en Chol ainsi que l’implication de sa région flexible et de la séquence VAAEIL dans ces interactions. Leurs interactions avec des monocouches de Langmuir constituées de phosphatidylcholine, Chol et/ou d’acétate de cholestéryle. Nos résultats montrent que l’AnxA6 a la plus grande affinité pour les monocouches contenant du Chol ainsi que l’implication du groupement hydroxyle du Chol lors de ces interactions. / The Niemann-Pick type C (NPC) disease is a lysosomal lipid storage disorder caused by mutations in one of the two genes NPC1 or NPC2 encoding proteins of the late endosome/lysosome compartment (LE/LY). Defect in these proteins alters vesicular transport and leads to abnormal accumulation of cholesterol (Chol) in LE/LY. There are some lines of evidence suggesting that annexin A6 (AnxA6) participates in vesicular transport of Chol and may interact with membrane domains enriched in Chol and bind Chol. In this work we characterized the membrane microdomains resistant to Triton X-100, i.e., detergent-resistant membranes (DRMs) isolated from NPC patient-derived fibroblasts and from control cells. NPC cells contain a significantly higher amount of DRMs than the control cells that is consistent with the defect in Chol turnover in NPC cells. We also studied the mechanism of AnxA6 involvement in the NPC-induced changes in the membrane organization and showed that in the presence of calcium some AnxA6 molecules associate with the DRMs. This suggests that AnxA6 may play a role in the membrane lateral organization, contributing thus to the etiology of NPC disease. We then focused on the interaction of AnxA6-1 with Chol-rich membranes and on the involvement of its flexible region and VAAEIL sequence in these interactions. For this purpose, kinetics of the interfacial adsorption of human recombinant AnxA6 to Langmuir monolayers containing phosphatidylcholine, Chol and/or cholesteryl acetate were measured. Our data suggest that AnxA6 exhibits the highest affinity to Chol-containing monolayers and that the hydroxyl group of Chol plays a pivotal role in the AnxA6-lipid interactions in vitro.

The physical properties of glycerophospholipids (GPLs) are not only determined by the head group (HG), but also by their fatty acid (FA) chains, which affect their distribution and function within membranes in the cell. Understanding the microheterogenity of lipid membranes on a molecular level requires qualitative and quantitative characterization of individual lipids and identification of their FA moieties. The aim of my study was to introduce the new technology of multiple precursor ion scanning (MPIS) on a QSTAR Pulsar time-of-flight mass spectrometer (QqTOF) to analyze lipids. Detailed information on fatty acid composition of individual GPL molecules could be obtained in parallel with conventional profiling of lipid classes, and this could be done by direct analysis of total lipid extracts. This method was termed Fatty Acid Scanning (FAS) and Head Group Scanning HGS, respectively. In this way the molecular GPL composition of total lipid extracts could be charted in a single analysis accurately and rapidly at a low picomole concentration level. Furthermore, combining FAS and HGS together with ion trap MS3 analysis allowed complete charting of the molecular composition of PCs, including quantification of their positional isomers, thus providing a detailed and comprehensive characterization of molecular composition of the pool of PCs. Development of the Lipid Profiler software allowed full automation and rapid processing of complex data, including identification and quantification of molecular GPLs. This approach was evaluated by preliminary applications. First, the molecular composition of PCs of total lipid extracts of MDCK cells and of human red blood cells (RBC) could accurately be charted. Significant presence of positional isomers was observed increasing the total number of individual PC species close to one hundred. Secondly, the molecular PC and SM species distribution in detergent resistant membranes (DRMs) prepared by Triton X-100 DRMs were analyzed and were found to be enriched in distinct GPLs. The distribution in PCs and SMs of Triton X-100 DRMs of RBC were compared with those of the DRMs of MDCK cells. Finally, combining the use of a 96 well plate and a robotic system demonstrated that these analyses can be automated and analyzed with high throughput. This system we termed Shotgun Lipidomics. Taken together, this mass spectrometric methodology provides rapid and detailed insight into the distribution of the molecular GPLs of membranes and membrane sub-fractions.

The physical properties of glycerophospholipids (GPLs) are not only determined by the head group (HG), but also by their fatty acid (FA) chains, which affect their distribution and function within membranes in the cell. Understanding the microheterogenity of lipid membranes on a molecular level requires qualitative and quantitative characterization of individual lipids and identification of their FA moieties. The aim of my study was to introduce the new technology of multiple precursor ion scanning (MPIS) on a QSTAR Pulsar time-of-flight mass spectrometer (QqTOF) to analyze lipids. Detailed information on fatty acid composition of individual GPL molecules could be obtained in parallel with conventional profiling of lipid classes, and this could be done by direct analysis of total lipid extracts. This method was termed Fatty Acid Scanning (FAS) and Head Group Scanning HGS, respectively. In this way the molecular GPL composition of total lipid extracts could be charted in a single analysis accurately and rapidly at a low picomole concentration level. Furthermore, combining FAS and HGS together with ion trap MS3 analysis allowed complete charting of the molecular composition of PCs, including quantification of their positional isomers, thus providing a detailed and comprehensive characterization of molecular composition of the pool of PCs. Development of the Lipid Profiler software allowed full automation and rapid processing of complex data, including identification and quantification of molecular GPLs. This approach was evaluated by preliminary applications. First, the molecular composition of PCs of total lipid extracts of MDCK cells and of human red blood cells (RBC) could accurately be charted. Significant presence of positional isomers was observed increasing the total number of individual PC species close to one hundred. Secondly, the molecular PC and SM species distribution in detergent resistant membranes (DRMs) prepared by Triton X-100 DRMs were analyzed and were found to be enriched in distinct GPLs. The distribution in PCs and SMs of Triton X-100 DRMs of RBC were compared with those of the DRMs of MDCK cells. Finally, combining the use of a 96 well plate and a robotic system demonstrated that these analyses can be automated and analyzed with high throughput. This system we termed Shotgun Lipidomics. Taken together, this mass spectrometric methodology provides rapid and detailed insight into the distribution of the molecular GPLs of membranes and membrane sub-fractions.