Role of Lipid rafts in Signal Transduction
1. What are lipid rafts
According to the Singer–Nicholson fluid mosaic concept proposed in 1972, the lipid bilayer acts as a neutral two-dimensional solvent, having minimal impact on membrane protein function. More recently there is growing evidence that lipids exist in different phases: gel, liquid-ordered and liquid-disordered states, in order of increasing fluidity. In the gel state lipids are semi-frozen, in contracts to the other end where the liquid-disordered state, the whole lipid bilayer is fluid, as proposed by the Singer–Nicholson model. In the liquid ordered phase, phospholipids with saturated hydrocarbon chains pack tightly with cholesterol but still maintain their mobility in the plane of the membrane. Despite a detailed biophysical studies of model membranes, it has been difficult to demonstrate that lipids exist in these different phases in the complex environment of the cell. Lipid rafts can be considered as microdomains in plasma membranes with higher composition of cholesterol and sphingolipids than the surrounding lipid bilayer. The fatty acid chain of the lipid bilayer is also more saturated and this makes the lipid rafts more rigid in structure. Researchers related lipid rafts in model membranes to the immiscibility of ordered (Lo phase) and disordered (Ld or L? phase) liquid phases (1). It has been claimed that because of their resistance to detergents like Triton X100 at low temperatures, lipid rafts can be extracted from the rest of the lipid membrane(2). Due to this property, lipid rafts are also called detergent-insoluble glycolipid-enrichedcomplexes (GEMs). Lipid rafts have the property of including as well as excluding proteins in different extents. Proteins with affinity to raft include glycosylphosphatidylinositol (GPI)-anchored proteins (3, 4) doubly acylated proteins, such as Src-family kinases (SKFs) or the ?-subunits of heterotrimeric G proteins (5), cholesterol-linked and palmitoylated proteins such as Hedgehog (6), and transmembrane proteins, particularly palmitoylated ones (3). GPI-anchored proteins or proteins that carry hydrophobic modifications probably partition into rafts owing to preferential packing of their saturated membrane anchors. How transmembrane proteins are included in to lipid rafts is not yet clear but , mutational analysis has shown that amino acids in the transmembrane domains near the exoplasmic leaflet are critical(7). Despite significant efforts in developing methodology and techniques for lipid raft research, there are still some shortcomings in precise determination of their size, structure, and composition which were recently critically discussed by Simons et al. (8)
2. Rafts in Signal Transduction
At cellular level, signal or stimulus is a converted to chemical information in a process called Signal transduction. Transduction can be simple where only receptor molecules are involved or more complex in which it involves the coupling of ligand-receptor interactions that lead to many intracellular events. These include phosphorylation by tyrosine kinases and/or serine/threonine kinases. For cells to respond efficiently environmental changes, the specificity and fidelity of signal transduction are very important. This can be in part achieved by the differential localization of proteins that participate in signaling pathways. In the plasma membrane, lipid rafts compartmentalize membrane functions. Lipid rafts have both positive and negative effects on cellular signaling. Specifically, rafts may act as platforms to localize components of a signaling pathway in close proximity, or may facilitate crosstalk between signaling pathways by keeping components of both pathways near each other. However, lipid rafts may also act as sequestering regions in the membrane to prevent association of pathway components, leading to downregulation of signaling events. The identification of lipid rafts has changed the way we look the regulation of signal transduction in cell membranes. Lipid rafts are involved in immune response, as well as in cell proliferation and survival and other not yet elucidated signal pathways.
2.1. Cell Adhesion Signaling in Lipid Rafts
Evidence are accumulating on the important role Lipid rafts play in migration and adhesion of cancer cells. It is known that the transmembrane proteins known as Integrins facilitate the anchorage of cells to components of the extracellular matrix (ECM) or bind to ligands on other cells to support cell-cell adhesion. Recent evidence suggests that the micro organization of lipids in the plasma membrane can affect integrin-mediated cellular functions (9). Integrin-mediated cell adhesion to the ECM is regarded as one of the primary stages of SFKs’ function. SFKs are activated in lipid rafts, and lipid-raft-specific inhibition of SFKs abrogates adhesion of breast cancer cells (10). Future studies elucidating the mechanism underlying the lipid raft-mediated regulation of cancer cell adhesion and migration will provide new insights into the mechanism of cancer invasion and metastasis and also provide a wealth of new targets for cancer prevention and therapy for clinical medicine.
2.2. Lipid rafts in Apoptotic signaling
More recently a large body of evidence accumulated in recent years has shown that lipid raft platforms can play a critical role in Death receptor (DR) ?mediated apoptotic signaling. Recruitment and aggregation of DRs, such as Fas/CD95, and downstream signaling molecules in lipid?raft platforms generate membrane domains, where protein–protein interactions and signal transmission are greatly favored. As Drs do not have enzymatic activity, and therefore the protein –protien interaction to transmit apoptotic signal is very important. This concentration of DRs and downstream apoptotic signaling molecules in raft clusters or platforms has led to the concept of CASMERs, which represent raft?based supramolecular entities, where DRs, together with downstream apoptotic signaling molecules, are recruited in raft platforms (11, 12) hence facilitating protein–protein interactions and the transmission of apoptotic signals. Here the formation of CASMER marks the induction of DR?mediated apoptosis in hematologic cancer cells. This recruitment of Fas/CD95 and downstream signaling molecules in rafts in hematologic cancer cells, forming a CASMER, favors first, the formation of the DISC apoptotic complex in membrane rafts and then, their putative connection with other subcellular organelles. Of particular interest in cell?death response is the putative connection between plasma membrane rafts and mitochondria, as unveiled by the use of the raft?targeted drug edelfosine (13). On these grounds, raft?like domains have been identified in the membranes of different subcellular organelles, such as mitochondria and endoplasmic reticulum, where their role remains largely elusive (14). These data, together with recent evidence showing distinct organelle interactions, particularly between endoplasmic reticulum and mitochondria (13), and that rafts or raft constituents can traffic to other subcellular organelles (15), suggest the existence of a raft?mediated dynamic network of communication between plasma membrane and different subcellular organelles that could modulate cell death. A detailed cellular and molecular characterization of CASMERS with respect to formation, clustering and compartmentalization will help to elucidate how Drs regulate cell fate decisions. The relevant function of lipid rafts in Fas/CD95?mediated apoptosis, together with the fact that the recruitment of Fas/CD95 and additional DRs can be modulated pharmacologically, highlights the role of lipid rafts as a target in a number of diseases.
2.3. Lipid rafts in T and B cell receptor signaling
An emerging area of cellular process where lipid raft play a role in signal transduction is the Immune reception. As a common principle of these processes, ligand induced receptor engagement triggers the recruitment of proximal machinery to the cytoplasmic tail of the receptor to initiate a cascade of tyrosine phosphorylation events that cause the induction of cytoskeletal rearrangements, the mobilization of Ca2+ stores and the activation of select transcription factors (16). These processes are highly dynamic and require a delicate adjustment in order to elicit strong signal and induce the desired downstream effector functions and, at the same time, prevent or delay the triggering of apoptotic programs due to cell hyper activation. This requires compartmentalization of proteins in a coordinated manner, the raft concept provides an attractive model for the regulation of such signaling cascades. Extensive studies of signal transduction initiated by the T cell and B cell receptor (TCR and BCR, respectively) revealed that receptor triggering, initiated in the context of complex cell-cell contacts referred to as the immunological synapse (IS) (17), indeed leads to the recruitment of the respective receptor and its downstream machinery to raft-like micro domains from where signaling initiates (18). Simultaneously, negative regulators of signaling are often excluded from these domains. These steps also involve cytoskeleton-mediated clustering of individual raft domains to enforce and sustain signal output. However, also in IS experimental systems, the existence and functional significance of raft domains is debated. While earlier experiments suggested an enrichment of lipid rafts at the IS in various cell systems (19), a fluorescence resonance energy transfer (FRET)-based analysis failed to detect the recruitment of glycophosphatidylinositol GPI-linked raft reporter proteins at the IS (20). In contrast, the use of Lauran membrane dye, which allows direct visualization of plasma membrane order, revealed the accumulation of ordered lipid domains at the sites of T lymphocyte activation (21). These findings may be reconciled at least in parts by reports demonstrating that GPI-linked proteins are not preferentially incorporated into signal transduction competent clusters (22). It appears that instead of raft recruitment, protein-protein interaction drives the assembly of signaling competent clusters. This emerging concept is in line with the role of nanoclusters in plasma membrane compartmentalization for optimal TCR/BCR signal transduction. But to what extent theses clusters fulfil all criteria of the original raft hypothesis still remains to be elucidated. However, the detection of clusters as raft-like domains by classical approaches such as flotation in cold detergent, suggests extensive but indeterminate fraction of results obtained with these methods reflects the activity of such nanoclusters.