Membrane Lipid Homeostasis

Quantification of lysosphingomyelin and lysosphingomyelin-509 for the screening of acid sphingomyelinase deficiency

BACKGROUND

Acid sphingomyelinase deficiency (ASMD) is a lysosomal disorder caused by deficiency of acid sphingomyelinase (ASM) leading to the accumulation of sphingomyelin (SM) in a variety of cell types. Lysosphingomyelin (LysoSM) is the de-acetylated form of SM and it has been shown as a biomarker for ASMD in tissues, plasma, and dried blood spots (DBS) and lysosphingomyelin-509 (LysoSM509) is the carboxylated analogue of LysoSM. High levels of Lysosphingomyelin 509 (LysoSM509) have also been shown in ASMD patients. In this study, we report the utility of the quantification of LysoSM and LysoSM509 in DBS of patients from Latin America with ASMD by ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS).

RESULTS

DBS samples from 14 ASMD patients were compared with 15 controls, and 44 general newborns. All patients had their diagnosis confirmed by the quantification of ASM and the measurement of the activity of chitotriosidase. All patients had significantly higher levels of lysoSM and lysoSM509 compared to controls and general newborns.

CONCLUSIONS

The quantification of lysosphingolipids in DBS is a valuable tool for the diagnosis of ASMD patients and lysoSM can be useful in the differential diagnosis with NPC. This method is also valuable in the ASMD newborn screening process.

Plasmalogens the neglected regulatory and scavenging lipid species

Plasmalogens are a class of phospholipids carrying a vinyl ether bond in sn-1 and an ester bond in sn-2 position of the glycerol backbone. Although they are widespread in all tissues and represent up to 18% of the total phospholipid mass in humans, their physiological function is still poorly understood. The aim of this review is to give an overview over the current knowledge in plasmalogen biology and pathology with an emphasis on neglected aspects of their involvement in neurological and metabolic diseases. Furthermore a better understanding of plasmalogen biology in health and disease could also lead to the development of better diagnostic and prognostic biomarkers for vascular and metabolic diseases such as obesity and diabetes mellitus, inflammation, neuro-degeneration and cancer.

A glance at the structural and functional diversity of membrane lipids

In the postgenomic era, spatially and temporally regulated molecular interactions as signals are beginning to take center stage in the understanding of fundamental biological events. For years, reductionism derived from the "fluid mosaic" model of the cell membrane has portrayed membrane lipids as rather passive molecules that, whereas separating biologically relevant aqueous phases, provided an environment so that membrane proteins could fulfill the specificity and selectivity required for proper cell signaling. Whereas these roles for membrane lipids still stand, the structural diversity of lipids and their complex arrangement in supramolecular assemblies have expanded such limited, although fundamental roles. Growing developments in the field of membrane lipids help to understand biological phenomena at the nanoscale domain, and reveal this heterogeneous group of organic compounds as a long underestimated group of key regulatory molecules. In this introductory chapter, brief overviews of the structural diversity of membrane lipids, the impact of different lipids on membrane properties, the vertical organization of lipids into rafts and caveolae, and the functional role of lipids as mediators of inter- and intracellular signals are provided. Any comprehensive review on membrane lipids, whether emphasizing structural or functional aspects, will require several volumes. The purpose of this chapter is to provide both introduction and rationale for the selection of topics that lie ahead in this book. For this reason, the list of references primarily includes reviews on particular issues dealing with membrane lipids wherein the reader can find further references.

The pathogenesis and treatment of acid sphingomyelinase-deficient Niemann-Pick disease

Patients with types A and B Niemann-Pick disease (NPD) have an inherited deficiency of acid sphingomyelinase (ASM) activity. The clinical spectrum of this disorder ranges from the infantile, neurological form that results in death by 3 years of age (type A NPD) to the non-neurological form (type B NPD) that is compatible with survival into adulthood. Intermediate cases also have been reported, and the disease is best thought of as a single entity with a spectrum of phenotypes. ASM deficiency is panethnic, but appears to be more frequent in individuals of Middle Eastern and North African descent. Current estimates of the disease incidence range from approximately 0.5 to 1 per 100,000 births. However, these approximations likely under estimate the true frequency of the disorder since they are based solely on cases referred to biochemical testing laboratories for enzymatic confirmation. The gene encoding ASM (SMPD1) has been studied extensively; it resides within an imprinted region on chromosome 11, and is preferentially expressed from the maternal chromosome. Over 100 SMPD1 mutations causing ASM-deficient NPD have been described, and some useful genotype-phenotype correlations have been made. Based on these findings, DNA-based carrier screening has been implemented in the Ashkenazi Jewish community. ASM 'knockout' mouse models also have been constructed and used to investigate disease pathogenesis and treatment. Based on these studies in the mouse model, an enzyme replacement therapy clinical trial has recently begun in adult patients with non-neurological ASM-deficient NPD.

Docosahexaenoic acid: a positive modulator of Akt signaling in neuronal survival

Phosphatidylinositol 3-kinase [PI (3)K]/Akt signaling is a critical pathway in cell survival. Here, we demonstrate a mechanism where membrane alteration by the n-3 fatty acid status affects Akt signaling, impacting neuronal survival. Docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid highly enriched in neuronal membranes, promotes neuronal survival by facilitating membrane translocation/activation of Akt through its capacity to increase phosphatidylserine (PS), the major acidic phospholipid in cell membranes. The activation of PI (3)K and phosphatidylsinositol triphosphate formation were not affected by DHA, indicating that membrane interaction of Akt is the event responsible for the DHA effect. Docosapentaenoic acid, which replaces DHA during n-3 fatty acid deficiency, was less effective in accumulating PS and translocating Akt and thus less effective in preventing apoptosis. Consistently, in vivo reduction of DHA by dietary depletion of n-3 fatty acids decreased hippocampal PS and increased neuronal susceptibility to apoptosis in cultures. This mechanism may contribute to neurological deficits associated with n-3 fatty acid deficiency and support protective effects of DHA in pathological models such as brain ischemia or Alzheimer's disease.