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Sphingolipids, Membrane Rafts and Caveolae

1. Introduction

It is impossible to understand the functions of sphingolipids without some understanding of their distinctive physical location within membranes. Sphingolipids are located only in the outer (exoplasmic) leaflet of the plasma membrane bilayer, while certain glycerophospholipids, including phosphatidylinositol, phosphatidylserine and phosphatidylethanolamine, occur only in the inner (cytoplasmic) leaflet under normal circumstances. Cholesterol is believed to occur in roughly equal proportions in both leaflets. Further, sphingomyelin and other sphingolipids together with cholesterol are located in an intimate association in specific sub-domains or 'rafts' (or related invaginated structures termed 'caveolae') of membranes. These are laterally segregated regions that form as a result of selective affinities between sphingolipids and membrane proteins, which act to compartmentalize and provide a platform for the latter and thereby separate different biochemical functions. Several distinct types of protein are associated with rafts, including those with essential membrane functions (glycosylphosphatidylinositol-linked proteins, caveolins, flotillins), signalling proteins (e.g. Src family kinases), G protein-coupled receptors, and others that are lipid linked (palmitoylated, myristoylated, hedgehog).

Raft domains are present in both the inner and outer leaflets of asymmetric cell membranes, and they are believed to be coupled across leaflets. It has become evident that such rafts do not represent a single monolithic structure, but a heterogeneous collection of domains differing in their protein and lipid compositions as well as in their stability with respect to time, i.e. length scales of tens of nanometers and time scales of milliseconds. Many aspects of raft structure are uncertain and controversial, because of the technical difficulties involved in their study in membranes of living cells as opposed to model systems. Indeed, it has been argued by some that all of the evidence for the existence of rafts is indirect, and that alternative explanations may be possible. On the other hand, recent evidence obtained by using lipid-specific toxin fragments supports the concept of raft domains.

2. Raft Structure and Organization

The packing of the saturated acyl chains of sphingolipids with cholesterol is thermodynamically favoured over that with unsaturated acyl chains, and cholesterol is essential to the process of raft formation. If one of either the sphingolipid or cholesterol is depleted by any means, the other follows and vice versa. Indeed, there is evidence that in animal tissues sphingomyelin regulates the capacity of membranes to absorb cholesterol and thereby controls its flux between the plasma membrane and other regulatory pathways in the endoplasmic reticulum. One key to the phenomenon is that sphingomyelin forms extensive intramolecular hydrogen bonds from the 3-hydroxyl group of the long-chain base to the phosphate oxygens of the head group and also bonds intermolecularly via the amine moiety of the long-chain base.

A formal definition of rafts has been proposed (Pike, L.J. J. Lipid Res., 47, 1597-1598 (2006); DOI), i.e.

"Membrane rafts are small (10-200 nm), heterogeneous, highly dynamic, sterol- and sphingolipid-enriched domains that compartmentalize cellular processes. Small rafts can sometimes be stabilized to form larger platforms through protein-protein and protein-lipid interactions."

  - or more simply as transient, relatively ordered membrane domains, the formation of which is driven by lipid-lipid and lipid-protein interactions. Earlier definitions have tended to depend on the methods used experimentally to produce raft preparations and especially those depending on their resistance to non-ionic detergents, i.e. their insolubility in cold 1% Triton X-100, for example. They have then been described as detergent-resistant membranes or 'DRM'. This has resulted in much confusion and controversy in the literature. While DRM certainly contain raft material, it is now considered that they should not be treated as rafts per se, as more robust methods of studying raft formation are available.

Others do not like the term 'raft' and prefer the concept of 'membrane nanodomains' as "functional assemblies of lipids and proteins, when determined by biophysical or microscopic techniques". However, I will continue to the concept of 'rafts' here until an alternative consensus has clearly emerged.

Up to 50% of the plasma membrane in animal tissues may consist of rafts, and the apical membrane of epithelial cells especially may behave like a large raft. As sphingolipids containing long, largely saturated acyl chains, they pack more tightly together, thus giving sphingolipids much higher melting temperatures than membrane glycerophospholipids. This tight acyl chain packing is essential for raft lipid organization, since the differential packing facility of sphingolipids and cholesterol in comparison with glycerophospholipids is believed to lead to a phase separation in the membrane that gives rise to sphingolipid-rich regions ('liquid-ordered' phase) surrounded by glycerophospholipid-rich domains ('liquid-disordered' phase). Such ordering is responsible for the resistance to attack by detergents. As these rafts are relatively small (approximately 50 nm diameter and containing roughly 3000 sphingomyelin molecules) and mobile, they are not easy to study by microscopic methods; they are thicker than normal membranes (46 versus 40 angstroms). Sphingolipids tend to have more free hydroxyl groups, both in the long-chain bases and fatty acid components than glycerolipids, and these enter into hydrogen bonding and contribute to the stability of rafts, and the presence of very-long-chain fatty acid components (e.g. C26) is believed to be essential. Cholesterol interacts particularly strongly with sphingomyelin, and much less with glycosphingolipids.

cartoonIn addition to lipid-lipid interaction, raft formation is driven and stabilized by lipid-protein and protein-protein interactions, and membranes should be regarded as lipid-protein composites rather than a solution of protein in a lipid solvent. Indeed, membrane proteins are essential for raft formation, and the reggies/flotillins, which are sometimes described as molecular scaffolding, are especially important in that they promote the assembly of glycerophosphoinositol(GPI)-anchored proteins into rafts. These then recruit other proteins and facilitate interactions among them. However, many other proteins are present including tyrosine kinases, phosphatases and other signalling proteins. Palmitoylation or myristoylation can target proteins to lipid rafts (see our web page on proteolipids), and for example flotillin is anchored in the lipid bilayer via N-terminal myristoyl and palmitoyl groups; it also binds to actin and the cytoskeleton and it is now evident that these are involved in compartmentalization of the membrane and raft formation. Other proteins with trans-membrane segments are targeted to rafts by specific amino acid sequences that provide much of the important biological properties of rafts, and are also required to maintain their stability. By means of this interplay of lipid-based raft units with protein-mediated assembly of specific protein complexes, functional domains are generated with high biological activity in the plasma membrane. From comparisons of membrane solubility in different detergents, it appears that there may exist subsets of membrane raft domains, which differ in their molecular compositions. In particular, they may contain distinct ganglioside species as well as the glycosyl phosphatidylinositol anchors. On the other hand, others have expressed doubt that GPI-anchored proteins occur in raft domains in plasma membranes of live cells.

Rafts also contain some glycerophospholipids and these often consist of relatively simple molecular species with one saturated and one monounsaturated acyl chain. In addition, both ethanolamine plasmalogens, especially those containing arachidonic acid, and phosphatidylserine are enriched in rafts as compared to the plasma membrane as a whole. While a great deal of attention has been focused on rafts in the outer leaflet of the plasma membrane, relatively little is known of the structural organization and properties of the corresponding inner leaflet or how the two layers interact. However, it is believed that raft domains form on the inner leaflet also and there is presumed to be interaction between the two.

Other lipids can apparently form microdomains in membranes that are raft-like in their properties. One such is phosphatidylglucoside, in which the diacylglycerol component contains two saturated fatty acids, so is comparable to a ceramide in some respects. Similarly, phosphatidylinositol 4,5-bisphosphate is believed to interact with cationic residues of a large array of proteins together with cholesterol to form localized membrane domains.

Plants, yeasts and bacteria: Specific micro-domains or rafts that are enriched in the characteristic plant sterols, sterol glucosides and sphingolipids, such as glucosylceramide and especially the glycosylinositol phosphoceramides, have been detected in the plasma membrane and Golgi apparatus of plant cells. They are especially important in the plasmodesmata, the narrow passages through the cell walls of adjacent cells that allows communication between them, where the membranes are enriched in sterols and sphingolipids containing predominantly very-long-chain fatty acids. These microdomains contain distinct sets of proteins, including those anchored by glycosyl phosphatidylinositol or glycosylinositol phosphoceramides. As in animal cells, such rafts may assist in positioning proteins in specific regions of the cell where they can function in development, membrane trafficking and signalling. The proteins flotillins and plant-specific remorins are seen as markers for distinct types of nanodomain.

In the plasma membrane of the yeast Saccharomyces cerevisiae, raft-like structures have been identified that do not contain ergosterol but are composed primarily of sphingolipids, possibly inositol phosphorylceramide, and contain glycosylphosphatidylinositol-anchored proteins. However, micro-domains enriched in both ergosterol and sphingolipids have been found in the plasma membrane of other yeast species.

While bacteria in general lack much of the cellular compartmentation present in eukaryotes, as well as both the sphingolipids and sterols characteristic of rafts, they arrange various signal transduction, protein secretion and transport processes in 'functional membrane microdomains'. These operate in much the same way as rafts in eukaryotes, and their formation is believed to involve interaction of farnesol and/or farnesol-derived polyisoprenoid lipids with flotillin-like proteins. Proteins associated with signal transduction, membrane trafficking and the regulation of metabolism are held in close proximity, and so increase the possibilities for interaction and the efficiency of related cell processes. Indeed, membrane organization may be of special importance in unicellular organisms, as this is the boundary between the organism and its environment and is vital for many cellular processes, including cell division and signal transduction.

3. Caveolae

Those subdomains in the plasma membrane related to rafts and termed ‘caveolae’ are flask-shaped invaginations 50 to 100 nm in diameter in membranes that provide a cholesterol- and sphingolipid-rich environment and are found abundantly in vascular endothelial cells, adipocytes, smooth muscle cells and fibroblasts; for example, they can amount to 50% of the surface area of adipocytes. They are stabilized by particular membrane-spanning scaffolding proteins, the caveolins, a family of three palmitoylated hairpin-like proteins that bind strongly to cholesterol of which caveolin-1 is the main structural component, i.e. a 24 kDa integral membrane protein with three palmitoylation sites at the C-terminal domain, which bind it to the inner leaflet of the plasma membrane. Caveolin-2 is required for lung functions while caveolin-3 is a specific to muscle. The caveolin proteins are inserted into the endoplasmic reticulum co-translationally, and after self-association to form oligomers of 12 or 14 monomers in the Golgi, they are delivered to the plasma membrane via a secretory pathway requiring the ganglioside GM1 and glycosylphosphatidylinositol-linked proteins. They can then assemble with more caveolin oligomers to form caveolae with the aid of further structural proteins termed 'cavins' (~50 kDa), of which four occur in vertebrates. These have been shown to bind to each other in vitro, and it is suggested that they bind with the caveolins in a large caveolar coat complex in vivo to shape the caveolae. For example, cavin-1 stabilizes caveolin-1 indirectly at the plasma membrane with the aid of adaptor proteins. Cavins also have the ability to bind to phosphatidylserine. In addition to the caveolins, caveolae are known to contain some specific signalling proteins, including Ras proteins, G proteins and growth factor receptors, not present in other raft microdomains. The presence of caveolins is the defining property of caveolae, and it is believed that there may be different classes of caveolae with different compositions, physical forms and metabolic functions.

Scottish thistleOther than a negligible content of glycosyl phosphatidylinositol, the lipid composition of caveolae is similar to that of rafts in general in that they contain appreciable amounts of sphingolipids and cholesterol. However, the gangliosides GM1 and to some extent GM3 appear to be concentrated in caveolae, while phosphatidylserine is the main anionic lipid and is known to play crucial roles in their formation and stability. One hypothesis that cannot as yet be tested is that the neck region of caveolae with a strong positive curvature may be highly enriched in glycosphingolipids, which may be largely absent from their lumen. For example, there is evidence that the caveolins interact directly with glycosphingolipids especially, and that the latter may have regulatory effects in the signalling functions of caveolae. It has been argued that caveolae best meet the accepted definition of a raft and should be used as a provisional standard against which other proposed raft structures should be judged.

A general picture has emerged of caveolin-1 and sphingolipids forming the scaffold for an integrated network of interactions, which contribute to the regulation of cellular functions via their influence upon receptor molecules in the plasma membrane. Caveolae appear to have a role in controlling the level of free cholesterol in cells, and thence may affect signalling processes. They are especially abundant in endothelial cells, adipocytes, smooth muscle cells and fibroblasts, but have not been found in lymphocytes or mature neurons. In adipocytes, they may regulate the flux of fatty acids across the plasma membrane. Also in adipocytes, insulin is the main hormone that affects metabolism, and the receptor at the plasma membrane is located in caveolae with possible implications for diabetes, obesity and other metabolic disorders. It appears that caveolae may function as an important route by which nutrients, such as folate, glucose and fatty acids, are able to cross the plasma membrane. Although the mechanism is still obscure, one further suggestion is that caveolae may act as transport vesicles, which bud from the plasma membrane in response to specific cues. They may have a non-signalling role in the repair of membrane damage in response to physical stress, perhaps by acting as a reservoir of components that can flatten out under mechanical stress.

There is some evidence that changes in caveolae create a catabolic microenvironment in tumours that supports oxidative mitochondrial metabolism in cancer cells and so contributes to the poor survival rates for cancer patients.

4. Raft Function

It should be recognized that lipid rafts in general are dynamic structures, which can be formed or undergo compositional changes during signalling events and are short-lived (milliseconds or less). There may also be some form of cross-talk between different raft populations, which can coalesce during activity. Thus in resting cells, sphingolipids may exist in small and highly dynamic domains, which on stimulation can stabilize and grow. In the process, they initiate biochemical reactions by promoting interactions between proteins. Raft domains may also aid membrane trafficking, for example by facilitating the transfer of specific proteins, such as the glycosyl phosphatidylinositol-anchored proteins, from the endoplasmic reticulum or Golgi to the plasma membrane. As discussed above, they may have a role in cholesterol homeostasis.

Lipid rafts are believed to modulate signalling events in a number of different ways according to the composition of the specific subpopulations. Thus, the location of signalling molecules within one such micro-domain might in itself be a control mechanism, as a protein activated by phosphorylation within the raft might be prevented from interacting with an inactivating phosphatase in another region of the membrane, for example. Alternatively, interactions with specific raft lipids or with the distinctive biophysical environment could change the conformation of a resident protein and thence its activity. By concentrating all of the components of particular signalling pathways within one domain, lipid rafts could promote signalling in response to stimuli, while movement of signalling molecules in and out of the raft could control whether cells are able to respond to stimuli. Similarly, communication between different signalling pathways could be simplified if the relevant molecules were concentrated in the same lipid raft. In contrast, rafts might also regulate signals in a negative manner by sequestering signalling molecules in an inactive state. The physical properties of rafts may be key factors in these interactions. Thus, in response to receptor activation or other stimuli, sphingolipid compositions in rafts may be altered with effects on membrane architecture or morphology producing further downstream events.

Many of the signal transduction functions of rafts are related to cell adhesion, migration, survival and proliferation, but they can also support receptor-mediated apoptotic signalling. In effect, membrane rafts may act as supporting structures that enable segregation of pro- from anti-apoptotic molecules. As many oncogenic proteins are located in raft­like domains and mitogenic signalling stems from various cell surface receptors, it is believed that rafts may be involved in cancer development and progression and that rafts may be a novel target in the treatment of cancer. Similarly, the transition of macrophages into foam cells may be raft-dependent with the potential to affect cardiovascular disease, while in contrast ion channels in caveolae are essential for normal cardiac functions. A deficiency of flotillin in cells compromises many lipid raft-associated cell processes and is associated with Parkinson's and Alzheimer's diseases, for example.

cartoonRaft formation is believed to be particularly important to the activity of T cells, i.e. lymphocytes derived from the thymus gland that are intimately involved in antibody production. Thus, lipid rafts are a key element of membrane organization that appear to be crucial for the initiation of T cell signalling by enabling efficient interaction between antigens and receptors, and they provide a platform for the interaction of Toll-like receptors, which are critically involved in inflammatory responses, with their ligands in cells. Similarly, lipid rafts have an essential function in enabling tumor necrosis factor alpha (TNFα), one of the cytokines involved in systemic inflammation, to bind to its specific receptor; the activity of sphingomyelin synthase 2 is an important factor that modulates this activity. Also, it is apparent that the biological functions of neurotransmitters and their receptors, in particular acetylcholine and serotonin receptors, are highly dependent upon sphingolipids and cholesterol in raft domains. Among other events, the lipids interact with receptors to alter their conformation ("chaperone-like" effect), thus regulating neurotransmitter binding and signal transducing functions .

There is evidence that certain pathogens activate the acid sphingomyelinase that releases ceramide in membrane rafts transforming them into larger units, which can mediate the internalization of bacteria, viruses and parasites into host cells, to initiate programmed cell death (apoptosis) and release signalling molecules. They may also assist in the budding of viruses from infected cells. In effect, rafts and caveolae re-organize the receptor and intracellular signalling molecules in the cell membrane and enable the interaction of pathogens with cells. The lipids of viruses are derived from the host membranes, and for example, it has recently been demonstrated that the lipids of the HIV virus are enriched in sphingolipids that appear to be derived very specifically from rafts. Further, these lipids are essential for the infectivity of the virus. In contrast, rafts can assist cells to defeat infection by activation of transcription factors and the release of cytokines.

5. Non-Raft Regions of Membranes

A corollary of the existence of rafts, which are rich in cholesterol and with low levels of polyunsaturated fatty acids, is that micro-domains must also exist that are depleted in cholesterol and enriched in polyunsaturated fatty acids. Indeed, the rigid structure of cholesterol and the highly flexible chains of docosahexaenoic acid, for example, are incompatible and promote the lateral segregation of membranes into rafts. Micro-domains that are enriched in polyunsaturated lipids and cholesterol-poor are technically even less easy to study than rafts, but these may also contain distinctive proteins and have important biological functions. There are also suggestions that the interface between rafts and non-raft regions may attract a specific range of proteins.

Suggested Reading

Lipid listings Credits/disclaimer Updated: September 18th, 2017 Author: William W. Christie LipidWeb icon