Lipid rafts as platforms for sphingosine 1-phosphate metabolism and signalling

Highlights

• Association of sphingosine kinase 1 to lipid rafts is essential for its activity.

• Membrane localization of LPPs makes them essential to regulate extracellular S1P.

• ABC transporters not only export S1P, they are also regulated by S1P signalling.

• S1P₁ recruited to caveolin-rich domains is required for endothelial barrier function.

• S1P receptors can cross-talk actively cross-talk with other receptor systems.

Abstract

Spontaneous segregation of cholesterol and sphingolipids as a liquid-ordered phase leads to their clustering in selected membrane areas, the lipid rafts. These specialized membrane domains enriched in gangliosides, sphingomyelin, cholesterol and selected proteins involved in signal transduction, organize and determine the function of multiprotein complexes involved in several aspects of signal transduction, thus regulating cell homeostasis. Sphingosine 1-phosphate, an important biologically active mediator, is involved in several signal transduction processes regulating a plethora of cell functions and, not only several of its downstream effectors tend to localize in lipid rafts, some of the enzymes involved in its pathway, of receptors involved in its signalling and its transporters have been often found in these membrane microdomains. Considering this, in this review we address what is currently known regarding the relationship between sphingosine 1-phosphate metabolism and signalling and plasma membrane lipid rafts.

Introduction

Sphingolipids (SLs) are minor, yet essential, components of eukaryotic cell membranes, mainly residing in the outer layer of plasma membrane with their hydrophilic head group facing out toward the extracellular milieu [1,2]. Ceramide, the simplest sphingolipid, is the backbone of all complex sphingolipids which are characterized by the presence of neutral or charged groups linked to the hydroxylated group in position 1 of the sphingoid base [3], the most common base being sphingosine (2S,3R-D-erythro-2-amino-1,3-dihydroxy-octadec-4-ene), linked to a fatty acid through an amide bond. In addition to the C18 molecular species, which is the most abundant in complex sphingolipids in mammals, homologous lipids with a different length of the carbon chain or with a saturated chain, sphinganine or 4-hydroxy-sphinganine, have been identified as minor amount components in different cells [4]. The sphingolipid class is commonly defined by the head group of the molecule, a phosphate group in ceramide 1-phosphate, phosphocholine in sphingomyelin, monosaccharides in cerebrosides, and one or more sugar residues linked with a β-glycosidic bond in complex glycosphingolipids. The latter being the most structurally diverse class of complex sphingolipids, particularly enriched in the nervous system where they are not merely structural components of the membranes, but also play other essential roles, especially in signalling [5,6]. Of the simple SLs, ceramide, ceramide 1-phosphate, sphingosine and sphingosine 1-phosphate have been shown to be involved in several cellular events, like proliferation, motility, growth, differentiation, and apoptosis. Complex glycosphingolipids (GSLs) are involved in cell physiology by acting as antigens, mediators of cell adhesion, binding agents for growth factors, and as modulators of signal transduction [5]. Gangliosides, sialic acid-containing GSLs, have been shown to be involved in the development, differentiation, and function of nervous system in vertebrates [7]. Galactosylceramide (GalCer) and sulfatide, instead, are involved in the formation and maintenance of a myelin with a correct structure [8,9], and deeply affect the survival, proliferation and differentiation of oligodendrocytes [10,11]. Glycosphingolipids are not randomly distributed along the membrane surface; moreover, they are highly segregated, together with cholesterol, in lipid domains with specialized signalling functions, organizing and determining the function of multiprotein complexes involved in several aspects of signal transduction, thus regulating the homeostasis of the brain [12]. Within the cell, they are highly asymmetrically enriched in the external leaflet of plasma membranes, with the oligosaccharide chain protruding toward the extracellular space, where the sugar residues can engage cis and trans interactions with a wide variety of cell surface and extracellular molecules [13]. The local concentration of GSLs in the membrane affects these interactions. Direct lateral interactions (cis interactions) with plasma membrane proteins are strongly favoured within a sphingolipid-enriched membrane domain [14], whereas in the case of trans interactions, it has been shown that recognition of lipid-bound oligosaccharides by soluble ligands (for example antibodies or toxins) or by complementary carbohydrates and by carbohydrate binding proteins (such as selectins or lectins) belonging to the interfacing membrane of adjacent cells is strongly affected by their degree of segregation (or dispersion) [15].

Over the past few decades, the biochemical pathways of sphingolipids metabolism and the intracellular sites of synthesis and degradation, respectively in the endoplasmic reticulum/Golgi apparatus and lysosomes, have been extensively characterized [16,17]. Sphingolipids synthesis is set in motion by a sequence of three enzyme-catalyzed reactions that, at the cytosolic leaflet of the membranes of the endoplasmic reticulum (ER), lead to the formation of ceramide starting from l-Serine and palmitoyl-CoA [1]. The use of L-Alanine or glycine instead of l-Serine leads to the synthesis of 1-deoxysphingolipids lacking the C₁-OH group of canonical sphingolipids. The lack of this group renders 1-deoxySLs resistant to normal SL catabolism as the essential catabolic intermediate sphingosine-1-phosphate cannot be formed [18]. Ceramide acts as precursor of at least six different products, namely ceramide-1-phosphate, acyl-ceramide, sphingosine, sphingomyelin (SM), glucosylceramide (GlcCer), and galactosylceramide. In turn, degradation of SM and glycosphingolipids can yield ceramide. Sphingosine can be phosphorylated by two sphingosine kinases (SK1 and SK2) to produce sphingosine 1-phosphate, which, depending on where in the cell it is generated will have different fates. ER generated S1P, in fact, can be reconverted to sphingosine and then ceramide by the action of S1P phosphatases (SPP) and ceramide synthases [19]. At ER levels, S1P can also be irreversibly cleaved by S1P lyase to generate a fatty aldehyde and phosphoethanolamine [20]. S1P generated at plasma level, instead, tends to be exported from the cell more efficiently, through the action of different transporters, where it can then act as an extracellular mediator [21].

Ceramide and S1P are bioactive sphingolipids, whose levels are finely regulated, and these lipids, in turn, modulate cell growth and survival, regulating opposing signalling pathways. Increase of ceramide levels is associated with apoptosis and cell growth arrest [22], while S1P is required for optimal cell proliferation induced by growth factors [23] and suppresses ceramide-mediated apoptosis [24]. S1P has also been shown to play crucial roles in a plethora of processes such as cell migration, proliferation, differentiation, adhesion but also in stress response, inflammation and development for example during angiogenesis, cardiogenesis, limb development and neurogenesis (reviewed in [[25], [26], [27]]). Ceramide also plays critical roles in different biological processes including apoptosis, inflammation, autophagy, senescence, fatty acid oxidation and ER stress (reviewed in [28,29]). Interestingly, ceramide, also acts as a signal for the reorganization of sphingolipid-enriched plasma membrane signalling platforms [30].

These cholesterol- and sphingolipid-enriched plasma membrane signalling platforms, commonly known as lipid rafts, are involved in several biological processes. For example they play roles in cell adhesion and motility [[31], [32], [33]], endocytosis and trafficking [34], inflammatory response [35], cancer cell survival and invasion [36], neuroinflammation and pain response [37], hematopoietic stem cell retention in the bone marrow and their trafficking [38], and insulin resistance [39]. They are also involved in the pathogenesis of several neurodegenerative diseases [40].