Lipid raft

Lipid raft

Lipid raft organisation, region (1) is standard lipid bilayer, while region (2) is a lipid raft.

The plasma membranes of cells contain combinations of signaling molecules, influencing membrane fluidity and membrane protein trafficking, and regulating neurotransmission and receptor trafficking.[3][4] Lipid rafts are more ordered and tightly packed than the surrounding bilayer, but float freely in the membrane bilayer.[5] Although more common in plasma membrane, lipid rafts have also been reported in other parts of the cell, such as Golgi and lysosomes.


  • Properties of lipid rafts 1
  • History 2
  • Common types of lipid rafts 3
  • Lipid raft and signal transduction 4
    • Immunoglobulin E signaling 4.1
    • T-cell antigen receptor signaling 4.2
    • B-cell antigen receptor signaling 4.3
  • Lipid rafts as platforms for virus entry 5
    • Nonenveloped virus 5.1
    • Enveloped virus 5.2
  • Visualization of lipid rafts 6
  • Controversy about lipid rafts 7
  • See also 8
  • References 9
  • External links 10

Properties of lipid rafts

Space-filling models of sphingomyelin (a) and cholesterol (b).

One key difference between lipid rafts and the plasma membranes from which they are derived is lipid composition. Research has shown that lipid rafts generally contain 3 to 5-fold the amount of cholesterol found in the surrounding bilayer.[6] Also, lipid rafts are enriched in sphingolipids such as sphingomyelin, which is typically elevated by 50% compared to the plasma membrane. To offset the elevated sphingolipid levels, phosphatidylcholine levels are decreased which results in similar choline-containing lipid levels between the rafts and the surrounding plasma membrane. Cholesterol interacts preferentially, although not exclusively, with sphingolipids due to their structure and the saturation of the hydrocarbon chains. Although not all of the phospholipids within the raft are fully saturated, the hydrophobic chains of the lipids contained in the rafts are more saturated and tightly packed than the surrounding bilayer.[4] Cholesterol is the dynamic "glue" that holds the raft together.[3] Due to the rigid nature of the sterol group, cholesterol partitions preferentially into the lipid rafts where acyl chains of the lipids tend to be more rigid and in a less fluid state.[4] One important property of membrane lipids is their amphipathic character. Amphipathic lipids have a polar, hydrophilic head group and a non-polar, hydrophobic region.[7] The figure to the right shows the inverted cone-like shape of sphingomyelin and the cone-like shape of cholesterol based on the area of space occupied by the hydrophobic and hydrophilic regions. It should be noted that cholesterol has the ability to pack in between the lipids in rafts, serving as a molecular spacer and filling any voids between associated sphingolipids.[7]

Rietveld & Simons related lipid rafts in model membranes to the immiscibility of ordered (Lo phase) and disordered (Ld or Lα phase) liquid phases.[8] The cause of this immiscibility is uncertain, but the immiscibility is thought to minimize the free energy between the two phases. Studies have shown there is a difference in thickness of the lipid rafts and the surrounding membrane which results in hydrophobic mismatch at the boundary between the two phases. This phase height mismatch has been shown to increase line tension which may lead to the formation of larger and more circular raft platforms to minimize the energetic cost of maintaining the rafts as a separate phase. Other spontaneous events, such as curvature of the membrane and fusing of small rafts into larger rafts, can also minimize line tension.[4]

By one early definition of lipid rafts, lipid rafts differ from the rest of the plasma membrane. In fact, researchers have hypothesized that the lipid rafts can be extracted from a plasma membrane. The extraction would take advantage of lipid raft resistance to non-ionic detergents, such as Triton X-100 or Brij-98 at low temperatures (e.g., 4 °C). When such a detergent is added to cells, the fluid membrane will dissolve while the lipid rafts may remain intact and could be extracted.

Because of their composition and detergent resistance, lipid rafts are also called detergent-insoluble glycolipid-enriched complexes (GEMs) or DIGs[9] or Detergent Resistant Membranes (DRMs). However the validity of the detergent resistance methodology of membranes has recently been called into question due to ambiguities in the lipids and proteins recovered and the observation that they can also cause solid areas to form where there were none previously.[10]


Until 1982, it was widely accepted that

External links

  1. ^ a b Thomas, Sunil; Preda-Pais, Anca; Casares, Sofia; Brumeanu, Teodor-D (2004). "Analysis of lipid rafts in T cells". Molecular Immunology 41 (4): 399–409.  
  2. ^ Thomas, Sunil; Kumar S., Rajeev; Brumeanu, Teodor−D. (2004). "Role of lipid rafts in T cells". Archivum Immunologiae et Therapiae Experimentalis 52 (4): 215–24.  
  3. ^ a b c Korade, Zeljka; Kenworthy, Anne K. (2008). "Lipid rafts, cholesterol, and the brain". Neuropharmacology 55 (8): 1265–73.  
  4. ^ a b c d e f g h i Pike, L. J. (2008). "The challenge of lipid rafts". The Journal of Lipid Research 50: S323.  
  5. ^ Simons, Kai; Ehehalt, Robert (2002). "Cholesterol, lipid rafts, and disease". Journal of Clinical Investigation 110 (5): 597–603.  
  6. ^ Laura, Anchisi; Sandra Dessi; Alessandra Pani; Antonella Mandas (25 November 2012). "Cholesterol homeostasis: a key to prevent or slow down neurodegeneration". Frontiers 3.  
  7. ^ a b Fantini, Jacques; Garmy, Nicolas; Mahfoud, Radhia; Yahi, Nouara (2004). "Lipid rafts: Structure, function and role in HIV, Alzheimer's and prion diseases". Expert Reviews in Molecular Medicine 4 (27): 1–22.  
  8. ^ Rietveld, Anton; Simons, Kai (1998). "The differential miscibility of lipids as the basis for the formation of functional membrane rafts". Biochimica et Biophysica Acta (BBA) - Reviews on Biomembranes 1376 (3): 467–79.  
  9. ^ Fivaz, Marc; Abrami, Laurence; Van Der Goot, F.Gisou (1999). "Landing on lipid rafts". Trends in Cell Biology 9 (6): 212–3.  
  10. ^ Heerklotz, H. (2002). "Triton Promotes Domain Formation in Lipid Raft Mixtures". Biophysical Journal 83 (5): 2693–701.  
  11. ^ Singer, S. J.; Nicolson, Garth L. (1972). "The Fluid Mosaic Model of the Structure of Cell Membranes". Science 175 (4023): 720–31.  
  12. ^ Stier, A.; Sackmann, E. (1973). "Spin labels as enzyme substrates Heterogeneous lipid distribution in liver microsomal membranes". Biochimica et Biophysica Acta (BBA) - Biomembranes 311 (3): 400–8.  
  13. ^ Karnovsky, Morris J.; Kleinfeld, Alan M.; Hoover, Richard L.; Klausner, Richard D. (1982). "The concept of lipid domains in membranes". The Journal of Cell Biology 94 (1): 1–6.  
  14. ^ Israelachvili, J. N.; Marčelja, S.; Horn, R. G. (2009). "Physical principles of membrane organization". Quarterly Reviews of Biophysics 13 (2): 121–200.  
  15. ^ Estep, T. N.; Mountcastle, D. B.; Barenholz, Y.; Biltonen, R. L.; Thompson, T. E. (1979). "Thermal behavior of synthetic sphingomyelin-cholesterol dispersions". Biochemistry 18 (10): 2112–7.  
  16. ^ Goodsaid-Zalduondo, F.; Rintoul, D. A.; Carlson, J. C.; Hansel, W. (1982). "Luteolysis-Induced Changes in Phase Composition and Fluidity of Bovine Luteal Cell Membranes". Proceedings of the National Academy of Sciences of the United States of America 79 (14): 4332–6.  
  17. ^ Simons, Kai; Van Meer, Gerrit (1988). "Lipid sorting in epithelial cells". Biochemistry 27 (17): 6197–202.  
  18. ^ Simons, Kai; Ikonen, Elina (1997). "Functional rafts in cell membranes". Nature 387 (6633): 569–72.  
  19. ^ a b c d Allen, John A.; Halverson-Tamboli, Robyn A.; Rasenick, Mark M. (2006). "Lipid raft microdomains and neurotransmitter signalling". Nature Reviews Neuroscience 8 (2): 128–40.  
  20. ^ King, Michael W (February 10, 2013). "Mechanisms of Signal Transduction". 
  21. ^ a b Janes, Peter W.; Ley, Steven C.; Magee, Anthony I.; Kabouridis, Panagiotis S. (2000). "The role of lipid rafts in T cell antigen receptor (TCR) signalling". Seminars in Immunology 12 (1): 23–34.  
  22. ^ Schmitz, Gerd; Grandl, Margot (2008). "Update on lipid membrane microdomains". Current Opinion in Clinical Nutrition and Metabolic Care 11 (2): 106–12.  
  23. ^ Simons, Kai; Toomre, Derek (2000). "Lipid rafts and signal transduction". Nature Reviews Molecular Cell Biology 1 (1): 31–9.  
  24. ^ a b c Field, Kenneth A.; Holowka, David; Baird, Barbara (1995). "FcɛRI-Mediated Recruitment of p53/56lyn to Detergent- Resistant Membrane Domains Accompanies Cellular Signaling". Proceedings of the National Academy of Sciences of the United States of America 92 (20): 9201–5.  
  25. ^ a b Sheets, Erin D; Holowka, David; Baird, Barbara (1999). "Membrane organization in immunoglobulin E receptor signaling". Current Opinion in Chemical Biology 3 (1): 95–9.  
  26. ^ a b Baird, Barbara; Sheets, Erin D; Holowka, David (1999). "How does the plasma membrane participate in cellular signaling by receptors for immunoglobulin E?". Biophysical Chemistry 82 (2–3): 109–19.  
  27. ^ Stauffer, Thomas P.; Meyer, Tobias (1997). "Compartmentalized IgE Receptor-mediated Signal Transduction in Living Cells". The Journal of Cell Biology 139 (6): 1447–54.  
  28. ^ Holowka, David; Sheets, Erin D.; Baird, Barbara (2000). "Interactions between FcεRI and lipid raft components are regulated by the actin cytoskeleton". Journal of Cell Science 113 (6): 1009–19.  
  29. ^ Sheets, E. D.; Holowka, D; Baird, B (1999). "Critical Role for Cholesterol in Lyn-mediated Tyrosine Phosphorylation of Fcepsilon RI and Their Association with Detergent-resistant Membranes". The Journal of Cell Biology 145 (4): 877–87.  
  30. ^ Goitsuka, R.; Kanazashi, H; Sasanuma, H; Fujimura, Y; Hidaka, Y; Tatsuno, A; Ra, C; Hayashi, K; Kitamura, D (2000). "A BASH/SLP-76-related adaptor protein MIST/Clnk involved in IgE receptor-mediated mast cell degranulation". International Immunology 12 (4): 573–80.  
  31. ^ Langlet, Claire; Bernard, Anne-Marie; Drevot, Philippe; He, Hai-Tao (2000). "Membrane rafts and signaling by the multichain immune recognition receptors". Current Opinion in Immunology 12 (3): 250–5.  
  32. ^ Zhang, W; Trible, RP; Samelson, LE (1998). "LAT PalmitoylationIts Essential Role in Membrane Microdomain Targeting and Tyrosine Phosphorylation during T Cell Activation". Immunity 9 (2): 239–46.  
  33. ^ Brdička, Tomáš; Černý, Jan; Hořejší, Václav (1998). "T Cell Receptor Signalling Results in Rapid Tyrosine Phosphorylation of the Linker Protein LAT Present in Detergent-Resistant Membrane Microdomains". Biochemical and Biophysical Research Communications 248 (2): 356–60.  
  34. ^ Cooper, Jonathan A.; Cary, Leslie A. (2000). "Molecular switches in lipid rafts". Nature 404 (6781): 945.  
  35. ^ a b Gupta, Neetu; Defranco, Anthony L. (2007). "Lipid rafts and B cell signaling". Seminars in Cell & Developmental Biology 18 (5): 616–26.  
  36. ^ a b c d Chazal, Nathalie; Gerlier, Denis (2003). "Virus Entry, Assembly, Budding, and Membrane Rafts". Microbiology and Molecular Biology Reviews 67 (2): 226–37, table of contents.  
  37. ^ a b c d e f g h Pietiäinen, Vilja M.; Marjomäki, Varpu; Heino, Jyrki; Hyypiä, Timo (2005). "Viral entry, lipid rafts and caveosomes". Annals of Medicine 37 (6): 394–403.  
  38. ^ Rajendran, Lawrence; Simons, Kai (2005). "Lipid rafts and membrane dynamics". Journal of Cell Science 118 (6): 1099–102.  
  39. ^ Rawat, Satinder S.; Viard, Mathias; Gallo, Stephen A.; Rein, Alan; Blumenthal, Robert; Puri, Anu (2003). "Modulation of entry of enveloped viruses by cholesterol and sphingolipids (Review)". Molecular Membrane Biology 20 (3): 243–54.  
  40. ^ Campbell, S.M; Crowe, S.M; Mak, J (2001). "Lipid rafts and HIV-1: From viral entry to assembly of progeny virions". Journal of Clinical Virology 22 (3): 217–27.  
  41. ^ Alving, Carl R.; Beck, Zoltan; Karasavva, Nicos; Matyas, Gary R.; Rao, Mangala (2006). "HIV-1, lipid rafts, and antibodies to liposomes: Implications for anti-viral-neutralizing antibodies (Review)". Molecular Membrane Biology 23 (6): 453–65.  
  42. ^ Jacobson, Ken; Mouritsen, Ole G.; Anderson, Richard G. W. (2007). "Lipid rafts: At a crossroad between cell biology and physics". Nature Cell Biology 9 (1): 7–14.  
  43. ^ Sharma, Pranav; Varma, Rajat; Sarasij, R.C; Ira; Gousset, Karine; Krishnamoorthy, G; Rao, Madan; Mayor, Satyajit (2004). "Nanoscale Organization of Multiple GPI-Anchored Proteins in Living Cell Membranes". Cell 116 (4): 577–89.  
  44. ^ Ritchie, Ken; Shan, Xiao-Yuan; Kondo, Junko; Iwasawa, Kokoro; Fujiwara, Takahiro; Kusumi, Akihiro (2005). "Detection of Non-Brownian Diffusion in the Cell Membrane in Single Molecule Tracking". Biophysical Journal 88 (3): 2266–77.  
  45. ^ Eggeling, Christian; Ringemann, Christian; Medda, Rebecca; Schwarzmann, Günter; Sandhoff, Konrad; Polyakova, Svetlana; Belov, Vladimir N.; Hein, Birka; et al. (2009). "Direct observation of the nanoscale dynamics of membrane lipids in a living cell". Nature 457 (7233): 1159–62.  
  46. ^ Thomas, S.; Kumar, R.S.; Casares, S.; Brumeanu, T.-D. (2003). "Sensitive detection of GM1 lipid rafts and TCR partitioning in the T cell membrane". Journal of Immunological Methods 275 (1–2): 161–8.  
  47. ^ Thomas, Sunil; Kumar, Rajeev; Preda-Pais, Anca; Casares, Sofia; Brumeanu, Teodor-D. (2003). Signalosome from the Lipid Rafts by a Soluble, Dimeric Peptide-MHC Class II Chimera1"lck"A Model for Antigen-Specific T-Cell Anergy: Displacement of CD4-p56. The Journal of Immunology 170 (12): 5981–92.  
  48. ^ Munro, Sean (2003). "Lipid rafts: elusive or illusive?". Cell 115 (4): 377–88.  
  49. ^ Barenholz, Yechezkel (2004). "Sphingomyelin and Cholesterol: From Membrane Biophysics and Rafts to Potential Medical Applications". In Quinn, Peter J. Membrane Dynamics and Domains. Subcellular Biochemistry 37. pp. 167–215.  
  50. ^ Pike, L. J.; Miller, JM (1998). "Cholesterol Depletion Delocalizes Phosphatidylinositol Bisphosphate and Inhibits Hormone-stimulated Phosphatidylinositol Turnover". Journal of Biological Chemistry 273 (35): 22298–304.  
  51. ^ Caroni, P. (2001). "NEW EMBO MEMBERS' REVIEW: Actin cytoskeleton regulation through modulation of PI(4,5)P2 rafts". The EMBO Journal 20 (16): 4332–6.  
  52. ^ Kwik, Jeanne; Boyle, Sarah; Fooksman, David; Margolis, Leonid; Sheetz, Michael P.; Edidin, Michael (2003). "Membrane cholesterol, lateral mobility, and the phosphatidylinositol 4,5-bisphosphate-dependent organization of cell actin". Proceedings of the National Academy of Sciences 100 (24): 13964–9.  
  53. ^ Edidin, Michael (2003). "THE STATE OF LIPID RAFTS: From Model Membranes to Cells". Annual Review of Biophysics and Biomolecular Structure 32: 257–83.  


See also

A second argument questions the effectiveness of the experimental design when disrupting lipid rafts. Pike and Miller discuss potential pitfalls of using cholesterol depletion to determine lipid raft function.[50] They noted that most researchers were using acute methods of cholesterol depletion, which disrupt the rafts, but also disrupt another lipid known as PI(4,5)P2. PI(4,5)P2 plays a large role in regulating the cell’s cytoskeleton,[51] and disrupting PI(4,5)P2 causes many of the same results as this type of cholesterol depletion, including lateral diffusion of the proteins in the membrane.[52] Because the methods disrupt both rafts and PI(4,5)P2, Kwik et al. concluded that loss of a particular cellular function after cholesterol depletion cannot necessarily be attributed solely to lipid raft disruption, as other processes independent of rafts may also be affected. Finally, while lipid rafts are believed to be connected in some way to proteins, Edidin argues that proteins attract the lipids in the raft by interactions of proteins with the acyl chains on the lipids, and not the other way around.[53]

A first rebuttal to this point suggests that the Lo phase of the rafts is more tightly packed due to the intermolecular hydrogen bonding exhibited between sphingolipids and cholesterol that is not seen elsewhere.[49]

  • Fourth, the entire membrane may exist in the Lo phase.
  • Third, the time scale of lipid raft existence is unknown. If lipid rafts exist, they may only occur on a time scale that is irrelevant to biological processes.
  • Second, there is no consensus on lipid raft size, which has been reported anywhere between 1 and 1,000 nanometres.
  • First, a line tension should exist between the Lα and Lo phases. This line has been seen in model membranes, but has not been readily observed in cell systems.

Arguments against the existence of lipid rafts include the following:

The role of rafts in cellular signaling, trafficking, and structure has yet to be determined despite many experiments involving several different methods, and their very existence is controversial despite all the above.[48]

Controversy about lipid rafts

Other techniques used in the analysis of lipid rafts include ELISA, western blotting, and FACS.[46][47][1]

Also used are atomic force microscopy (AFM), Scanning Ion Conductance Microscopy (SICM), dual polarisation interferometry, Nuclear Magnetic Resonance (NMR) although fluorescence microscopy remains the dominant technique. In the future it is hoped that super-resolution microscopy such as Stimulated Emission Depletion (STED)[45] or various forms of structured illumination microscopy may overcome the problems imposed by the diffraction limit.

Other optical techniques are also used: Fluorescence Correlation and Cross-Correlation Spectroscopy (FCS/FCCS) can be used to gain information of fluorophore mobility in the membrane, Fluorescence Resonance Energy Transfer (FRET) can detect when fluorophores are in close proximity and optical tweezer techniques can give information on membrane viscosity.[19]

Sharma and colleagues used combination of high resolution imaging and mathematical modeling to provide the view that raft proteins are organized into high density nanoclusters with radii ranging over 5–20 nm. Using measurements of fluorescence resonance energy transfer between the same probes (homo-FRET or fluorescence anisotropy), Sharma and colleagues reported that a fraction (20–40%) of GPI-anchored proteins are organized into high density clusters of 4–5 nm radius, each consisting of a few molecules and different GPI-anchored proteins.[43] To combat the problems of small size and dynamic nature, single particle and molecule tracking using cooled, sensitive CCD cameras and total internal reflection (TIRF) microscopy is coming to prominence. This allows information of the diffusivity of particles in the membrane to be extracted as well as revealing membrane corrals, barriers and sites of confinement.[44]

Manipulation of cholesterol is one of the most widely used techniques for studying lipid rafts. Sequestration (using filipin, nystatin or amphotericin), depletion and removal (using methyl-B-cyclodextrin) and inhibition of cholesterol synthesis (using HMG-CoA reductase inhibitors) are ways cholesterol are manipulated in lipid raft studies. These studies allow for the observations of effects on neurotransmitter signaling upon reduction of cholesterol levels.[19]

One of the primary reasons for the controversy over lipid rafts has stemmed from the challenges of studying lipid rafts in living cells, which are not in thermodynamic equilibrium.[19] Lipid rafts are small microdomains ranging from 10–200 nm in size.[4] Due to their size being below the classical diffraction limit of a light microscope, lipid rafts have proved difficult to visualize directly. Currently synthetic membranes are studied; however, there are many drawbacks to using these membranes. First, synthetic membranes have a lower concentration of proteins compared to biomembranes. Also, it is difficult to model membrane-cytoskeletal interactions which are present in biomembranes. Other pitfalls include lack of natural asymmetry and inability to study the membranes in non-equilibrium conditions.[4][42] Despite this, fluorescence microscopy is used extensively in the field. For example, fluorophores conjugated to cholera-toxin B-subunit, which binds to the raft constituent ganglioside GM1 is used extensively. Also used are lipophilic membrane dyes which either partition between rafts and the bulk membrane, or change their fluorescent properties in response to membrane phase. Laurdan is one of the prime examples of such a dye. Rafts may also be labeled by genetic expression of fluorescent fusion proteins such as Lck-GFP.

Visualization of lipid rafts

Human Immunodeficiency virus (HIV), as a sexually-transmitted animal virus, must first penetrate a barrier of epithelial cells, who don’t express CD4 and chemokine receptors, to establish a productive infection. An alternative receptor for HIV-1 envelope glycoprotein on epithelial cells is glycosphingolipid galactosyl-ceramide (GalCer), which enriches at lipid raft.[40][41]

Influenza viruses bind to cellular receptor siaclic acid, which links to glycoconjugate on the cell surface, to initiate endocytosis. After transported into late endosomes, low-pH-dependent conformation change of HA induces fusion, and viral ribonucleoprotein complexes (RNP) are released by proton influx of viral ion channel M2 protein that requires binding with cholesterol. Semliki Forest virus (SFV) and Sindbis virus (SIN) require cholesterol and sphingolipids in target membrane lipid rafts for envelope glycoprotein-mediated membrane fusion and entry.[39] Human T-lymphotropic virus Type I (HTLV-1) enters cell via glucose transporter 1 (GLUT-1); Ebola virus and Marburg virus uses folate receptor-α (FRα), which is a GPI-anchored protein, as cellular receptor; Hepatitis B virus recognizes human complement receptor type 2 (CR2, or known as CD21); Human herpesvirus 6 (HHV-6) binds to human CD46 on host cell surface; all these viral receptors are located in lipid rafts or would be relocated into lipid rafts after infection.

Enveloped virus

There are also viruses that use the non-caveolar raft-mediated endocytosis, such as Echovirus 11 (EV11, picornavirus). However, detailed mechanisms still need to be further characterized.[37]

EV1 uses α2β1-integrin as cellular receptor.[36] Multiple integrin heterodimers can bind to the adjacent sites of the virus capsid.[37] Similar to SV40, attachment and binding with cells triggers clustering and relocation of integrin molecules from lipid rafts to the caveolae-like structures.[37] Depletion of cholesterol in lipid rafts inhibits EV1 infection.[36]

SV40 utilizes two different receptors to bind onto cell surface: ganglioside GM1 located in lipid rafts and major histocompatibility (MHC) class I molecule.[36][37] Binding of SV40 with MHC class I molecules triggers receptor clustering and redistribution. SV40 may recruit more caveolae from the cytoplasm or even new caveolae formed at the site of entry.[37] A cascade of virus-induced signaling events triggered by attachment results in caveolae-mediated endocytosis in about 20 min.[37] In some cell types the virus can enter the caveosomes directly from lipid rafts in non-coated vesicles.[37][38]

The best studied models of lipid rafts-related nonenveloped viral entry are simian virus 40 (SV40, Papovaviridae) and echovirus type 1 (EV1, Picornaviridae).[36][37]

Nonenveloped virus

Viruses, as obligate intracellular parasites, have to involve specific interaction of virus and cellular receptor expressed at the plasma membrane in order to enter cells. Accumulated evidence supports that viruses enter cells via penetration of specific membrane microdomains, including lipid rafts.

Lipid rafts as platforms for virus entry

The process of B cell antigen receptor signalling is similar to Immunoglobulin E signalling and T-cell antigen receptor signalling. It is commonly believed that other than BCR, lipid rafts play an important role in many of the cell surface events involved in B cell activation. Their functions include signaling by BCR, modulation of that signaling by co-receptors, signaling by CD40, endocytosis of antigen bound to the BCR and its routing to late endosomes to facilitate loading of antigen-derived peptides onto class II MHC molecules, routing of those peptide/MHC-II complexes to the cell surface, and their participation in antigen presentation to T cells.[35]

B cell antigen receptor (BCR) is a complex between a membrane bound Ig (mIg) molecule and a disulfide-linked Igα- Igβ heterodimer of two polypetides.[35] Igα and Igβ each contains an amino acid motif, called ITAM, whose sequence is D/ExxYxxL/Ix7YxxL/I.

B-cell antigen receptor signaling

T cell antigen receptor (TCR) is a molecule found on the surface of T lymphocytes (T cells). It is composed of αβ-heterodimers, CD3 (γδε) complex and ξ-homodimer. The α- and β- subunits contains extracellular binding sites for peptides that are presented by the major histocompatibility complex (MHC) class I and class II proteins on the surface of antigen presenting cells (APCs). The CD3 and ξ- subunits contain cytoplasmic ITAM motifs. During the signaling process, MHCs binding to TCRs brings two or more receptors together. This crosslinking, similar to IgE signaling, then recruit doubly acylated non-receptor Src-like tyrosine kinases to phosphorylate ITAM tyrosine residues. In addition to recruiting Lyn, TCR signaling also recruits Fyn.[21][31] Following this procedure, ZAP-70 (which is also different with IgE signalling) binds to phosphorylated ITAMs, which leads to its own activation and LAT activation. LAT activation is the source of signal amplification. Another difference between IgE and T cell antigen receptor signalling is that Lck activation by TCR could result in more severe raft clustering[32][33] thus more signal amplification. One possible mechanism of down-regulating this signaling involves the binding of cytosolic kinase Csk to the raft associated protein CBP. Csk may then suppress the Src-family kinases through phosphorylation.[34]

T-cell antigen receptor signalling process
componments for T-cell antigen receptor signalling

T-cell antigen receptor signaling

Immunoglobulin E (IgE) signaling is the first convincingly demonstrated lipid rafts involving signaling process.[24][25][26] Evidences for this fact includes decreased solubility of Fc-epsilon receptors (FcεR) in Triton X-100 from steady state to crosslinking state,[24] formation of patches large enough to be visualized by fluorescence microscopy from gangliosides and GPI-anchored proteins,[27][28] abolition of IgE signaling by surface cholesterol depletion with methyl-β-cyclodextrin[29] and so on. This signaling pathway can be described as follows: IgE first binds to Fc-epsilon receptors (FcεR) residing in the plasma membrane of mast cells and basophils through its Fc segment. FcεR is a tetramer consist of one α, one β and two γ chains.[26] It is monomeric and binds one IgE molecule. The α chain binds IgE and the other three chains contain immune receptor tyrosine-based activation motifs (ITAM). Then oligomeric antigens bind to receptor-bound IgE to crosslink two or more of these receptors. This crosslinking then recruits doubly acylated non-receptor Src-like tyrosine kinase Lyn to phosphorylate ITAMs. After that, Syk family tyrosine kinases bind these phosphotyrosine residues of ITAMs to initiate the signaling cascade.[24][25] Syk can, in turn, activate other proteins such as LAT. Through crosslinking LAT can recruit other proteins into the raft and further amplify the signal.[30]

IgE signalling process
Components for IgE signalling

Immunoglobulin E signaling

One reasonable way to consider lipid rafts is that small rafts can form concentrating platforms after ligand binding activation for individual receptors.[22] If receptor activation takes place in a lipid raft, the signalling complex is protected from non-raft enzymes such as membrane phosphatases. Overall, raft binding recruits proteins to a new micro-environment so that the phosphorylation state can be modified by local kinases and phosphatases to give downstream signalling.[23] Lipid rafts have been found by researchers to be involved in many signal transduction processes, such as Immunoglobulin E signalling, T cell antigen receptor signalling, B cell antigen receptor signalling, EGF receptor signalling, insulin receptor signalling and so on. In order to illustrate these principles, detailed examples of signalling pathways that involve lipid rafts are described below.

Signal transduction at the cellular level refers to any process by which a cell converts one kind of signal or stimulus into another. The movement of signal or stimulus can be simple, like that associated with receptor molecules. More complex signal transduction involves the coupling of ligand-receptor interactions to many intracellular events. These events include phosphorylations by tyrosine kinases and/or serine/threonine kinases.[20] The specificity and fidelity of signal transduction are essential for cells to respond efficiently to changes in their environment. This is achieved in part by the differential localization of proteins that participate in signalling pathways. In the plasma membrane, one approach of compartmentalization utilizes lipid rafts.[21]

Lipid raft and signal transduction

[19] Two types of lipid rafts have been proposed: planar lipid rafts (also referred to as non-caveolar, or glycolipid, rafts) and caveolae. Planar rafts are defined as being continuous with the plane of the plasma membrane (not invaginated) and by their lack of distinguishing morphological features. Caveolae, on the other hand, are flask shaped invaginations of the plasma membrane that contain

Common types of lipid rafts

  • What are the effects of membrane protein levels?
  • What is the physiological function of lipid rafts?
  • What effect does flux of membrane lipids have on raft formation?
  • What effect do diet and drugs have on lipid rafts?
  • What effect do proteins located at raft boundaries have on lipid rafts?[4]

Other questions yet to be answered include:

At the 2006 Keystone Symposium of Lipid Rafts and Cell Function, lipid rafts were defined as "small (10-200nm), 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 interactions" In recent years, lipid raft studies have tried to address many of the key issues that cause controversy in this field, including the size and lifetime of rafts. [18] to the plasma membrane. The idea was more formally developed in 1997 by Simons and Ikonen.trans Golgi network Subsequently, they called these microdomains, lipid "rafts". The original concept of rafts was used as an explanation for the transport of cholesterol from the [17], present in cell membranes.sphingolipids, and glycolipids and Gerrit van Meer from the University of Utrecht, Netherlands refocused interest on these membrane microdomains, enriched with lipids and cholesterol, Germany (EMBL) in European Molecular Biology Laboratory at the Kai Simons Later, [16] These microdomains (‘rafts’) were shown to exist also in cell membranes.[15]. They form because of the segregation of these lipids into a separate phase, demonstrated by Biltonen and Thompson and their coworkers.sphingolipids One type of microdomain is constituted by cholesterol and [4] In 1974, the effects of temperature on membrane behavior had led to the proposal of "clusters of lipids" in membranes and by 1975, data suggested that these clusters could be "quasicrystalline" regions within the more freely dispersed liquid crystalline lipid molecule. In 1978, X-Ray diffraction studies led to further development of the "cluster" idea defining the microdomains as "lipids in a more ordered state". Karnovsky and co-workers formalized the concept of lipid domains in membranes in 1982. Karnovsky's studies showed heterogeneity in the lifetime decay of 1,6-diphenyl-1,3,5-hexatriene, which indicated that there were multiple phases in the lipid environment of the membrane.[14]