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The Journal of Physiology Dec 2007The original 'lipid raft' hypothesis proposed that lipid-platforms/rafts form in the exoplasmic plasmalemmal leaflet by tight clustering of sphingolipids and... (Review)
Review
The original 'lipid raft' hypothesis proposed that lipid-platforms/rafts form in the exoplasmic plasmalemmal leaflet by tight clustering of sphingolipids and cholesterol. Their physical state, presumably similar to liquid-ordered phases in model membranes, would confer detergent resistance to rafts and enriched proteins therein. Based on this concept, detergent resistant membranes (DRMs) from solubilized cells were considered to reflect pre-existing 'lipid rafts' in live cells. To date, more than 200 proteins were found in DRMs including also members of the SNARE superfamily, which are small membrane proteins involved in intracellular fusion steps. Their raft association indicates that they are not uniformly distributed, and, indeed, microscopic studies revealed that SNAREs concentrate in submicrometre-sized, cholesterol-dependent clusters at which vesicles fuse. However, the idea that SNARE clusters are 'lipid rafts' was challenged, as they do not colocalize with raft markers, and SNAREs are excluded from liquid-ordered phases in model membranes. Independent from this disagreement, in recent years the solubilization criterion has been criticized for several reasons, calling for a more exact definition of rafts. At a recent consensus on a revised raft model, the term 'lipid rafts' was replaced by 'membrane rafts' that were defined as 'small (10-200 nm), heterogeneous, highly dynamic, sterol- and sphingolipid-enriched domains that compartmentalize cellular processes'. As a result, after dismissing the terms 'detergent resistant' and 'liquid-ordered', it now appears that SNARE clusters are bona fide 'membrane rafts'.
Topics: Animals; Cell Membrane; Cholesterol; Detergents; Exocytosis; Humans; Membrane Microdomains; SNARE Proteins
PubMed: 17478530
DOI: 10.1113/jphysiol.2007.134346 -
Biochimica Et Biophysica Acta.... Oct 2018One of the main questions in the membrane biology is the functional roles of membrane heterogeneity and molecular localization. Although segregation and local enrichment... (Review)
Review
One of the main questions in the membrane biology is the functional roles of membrane heterogeneity and molecular localization. Although segregation and local enrichment of protein/lipid components (rafts) have been extensively studied, the presence and functions of such membrane domains still remain elusive. Along with biochemical, cell observation, and simulation studies, model membranes are emerging as an important tool for understanding the biological membrane, providing quantitative information on the physicochemical properties of membrane proteins and lipids. Segregation of fluid lipid bilayer into liquid-ordered (Lo) and liquid-disordered (Ld) phases has been studied as a simplified model of raft in model membranes, including giant unilamellar vesicles (GUVs), giant plasma membrane vesicles (GPMVs), and supported lipid bilayers (SLB). Partition coefficients of membrane proteins between Lo and Ld phases were measured to gauze their affinities to lipid rafts (raftophilicity). One important development in model membrane is patterned SLB based on the microfabrication technology. Patterned Lo/Ld phases have been applied to study the partition and function of membrane-bound molecules. Quantitative information of individual molecular species attained by model membranes is critical for elucidating the molecular functions in the complex web of molecular interactions. The present review gives a short account of the model membranes developed for studying the lateral heterogeneity, especially focusing on patterned model membranes on solid substrates.
Topics: Biophysical Phenomena; Cell Membrane; Lipid Bilayers; Membrane Fluidity; Membrane Microdomains; Membrane Proteins; Membranes; Models, Biological; Spectrometry, Fluorescence; Unilamellar Liposomes
PubMed: 29550290
DOI: 10.1016/j.bbamem.2018.03.010 -
International Journal of Biological... Nov 2022Phenylketonuria (PKU) is a metabolic disorder connected to an excess of phenylalanine (Phe) in the blood and tissues, with neurological consequences. The disease's...
Phenylketonuria (PKU) is a metabolic disorder connected to an excess of phenylalanine (Phe) in the blood and tissues, with neurological consequences. The disease's molecular bases seem to be related to the accumulation of Phe at the cell membrane surface. Radiological outcomes in the brain demonstrate decreased water diffusivity in white matter, involving axon dysmyelination of not yet understood origin. We used a biophysical approach and model membranes to extend our knowledge of Phe-membrane interaction by clarifying Phe's propensity to affect membrane structure and dynamics based on lipid composition, with emphasis on modulating cholesterol and glycolipid components to mimic raft domains and myelin sheath membranes. Phe showed affinity for the investigated membrane mimics, mainly affecting the Phe-facing membrane leaflet. The surfaces of our neuronal membrane raft mimics were strong anchoring sites for Phe, showing rigidifying effects. From a therapeutic perspective, we further investigated the role of doxycycline, known to disturb Phe packing, unveiling its action as a competitor in Phe interactions with the membrane, suggesting its potential for treatment in the early stages of PKU. Our results suggest how Phe accumulation in extracellular fluids can impede normal growth of myelin sheaths by interfering with membrane slipping and by remodulating free water and myelin-associated water contents.
Topics: Humans; Phenylalanine; Glycolipids; Phenylketonurias; Brain; Water
PubMed: 36099998
DOI: 10.1016/j.ijbiomac.2022.09.062 -
Biochimica Et Biophysica Acta Aug 2013There is great diversity in the composition and structure of biological lipid membranes. We are beginning to appreciate the crucial role of lipids in many cellular... (Review)
Review
There is great diversity in the composition and structure of biological lipid membranes. We are beginning to appreciate the crucial role of lipids in many cellular processes, and characterize some of the lateral structures within membranes that could play a role in the activity of lipids. Simulations probe molecular level interactions between single molecules, which provide complementary information to experiments. Lipid membrane simulations have reached an exciting point, where the time and length scales of our simulations are approaching experimental resolutions and can be used to interpret experiments on increasingly complex model membranes. The focus of this review is on recent molecular simulations of domain formation in lipid bilayers. We highlight a number of recent examples where simulations are used in collaboration with experiments. We review recent simulation studies on lipid-lipid interactions related to domain formation and on lipid-protein interactions relevant for lipid raft function.
Topics: Animals; Computer Simulation; Humans; Lipid Bilayers; Membrane Lipids; Membrane Microdomains; Membrane Proteins
PubMed: 23500617
DOI: 10.1016/j.bbamem.2013.03.004 -
Biophysical Journal Jun 2021The lipid-raft hypothesis postulates that cell membranes possess some degree of lateral organization. The hypothesis has attracted much attention while remaining...
The lipid-raft hypothesis postulates that cell membranes possess some degree of lateral organization. The hypothesis has attracted much attention while remaining controversial, with an underlying mechanism that remains elusive. One idea that supports rafts relies on the membrane lying near a critical point. Although supported by experimental evidence, holding a many-component membrane at criticality requires a delicate tuning of all components-a daunting task. Here, we propose a coherent framework to reconcile critical behavior and lipid regulation. Using a lattice model, we show that lipid regulation of a complex membrane, i.e., allowing composition to fluctuate based on relative chemical potentials, can lead to critical behavior. The results are robust against specific values of the chemical potentials. Instead of a conventional transition point, criticality is observed over a large temperature range. This surprising behavior arises from finite-size effects, causing nonequivalent time and space averages. The instantaneous lipid distribution effectively develops a translational symmetry, which we relate to long-wavelength Goldstone modes. The framework is robust and reproduces important experimental trends; membrane-demixing temperature closely follows cell-growth temperature. It also ensures criticality of fixed-composition extracts, such as giant plasma membrane vesicles. Our clear picture provides a strong argument in favor of the critical-membrane hypothesis, without the need for specific sensing mechanisms.
Topics: Cell Membrane; Lipids; Membrane Microdomains; Membranes; Temperature
PubMed: 33961864
DOI: 10.1016/j.bpj.2021.03.043 -
Virology Nov 2006Membrane association is believed to be a prerequisite for the biological activity of the HIV-1 pathogenicity factor Nef. Attachment to cellular membranes as well as...
Membrane association is believed to be a prerequisite for the biological activity of the HIV-1 pathogenicity factor Nef. Attachment to cellular membranes as well as incorporation into detergent-insoluble microdomains (lipid rafts) require the N-terminal myristoylation of Nef. However, this modification is not sufficient for sustained membrane association and a specific raft-targeting signal for Nef has not yet been identified. Using live cell confocal microscopy and membrane fractionation analyses, we found that the N-terminal anchor domain (aa 1-61) is necessary and sufficient for efficient membrane binding of Nef from HIV-1(SF2). Within this domain, highly conserved lysine and arginine residues significantly contributed to Nef's membrane association and localization. Plasma membrane localization of Nef was also governed by an additional membrane-targeting motif between residues 40 and 61. Importantly, two lysines at positions 4 and 7 were not essential for the overall membrane association but critically contributed to Nef's incorporation into lipid raft domains. Cell surface receptor downmodulation was largely unaffected by mutations of all N-terminal basic residues, while the association of Nef with Pak2 kinase activity and its ability to augment virion infectivity correlated with its lysine-mediated raft incorporation. In contrast, all basic residues were required for efficient HIV-1 replication in primary human T lymphocytes but did not contribute to the incorporation of Nef into HIV-1 virions. Together, these results unravel that Nef's membrane association is governed by a complex pattern of signature motifs that differentially contribute to individual Nef activities. The identification of a critical raft targeting determinant and the functional characterization of a membrane-bound, non-raft-associated Nef variant indicate raft incorporation as a regulatory mechanism that determines the biological activity of distinct subpopulations of Nef in HIV-infected cells.
Topics: Amino Acid Motifs; Artificial Gene Fusion; Blotting, Western; Cell Fractionation; Cell Line; Cell Membrane; Cells, Cultured; Gene Products, nef; Green Fluorescent Proteins; HIV-1; Humans; Membrane Microdomains; Microscopy, Confocal; Microscopy, Fluorescence; Protein Binding; Protein Serine-Threonine Kinases; Protein Sorting Signals; Protein Structure, Tertiary; Virus Assembly; Virus Replication; nef Gene Products, Human Immunodeficiency Virus; p21-Activated Kinases
PubMed: 16916529
DOI: 10.1016/j.virol.2006.07.003 -
Frontiers in Molecular Biosciences 2022The lipid matrix of cellular membranes, directly and indirectly, regulates many vital functions of the cell. The diversity of lipids in membranes leads to the formation...
The lipid matrix of cellular membranes, directly and indirectly, regulates many vital functions of the cell. The diversity of lipids in membranes leads to the formation of ordered domains called rafts, which play a crucial role in signal transduction, protein sorting and other cellular processes. Rafts are believed to impact the development of different neurodegenerative diseases, such as Alzheimer's, Parkinson's, Huntington's ones, amyotrophic lateral sclerosis, some types of cancer, etc. These diseases correlate with the change in the membrane lipid composition resulting from an oxidative stress, age-related processes, dysfunction of proteins, and many others. In particular, a lot of studies report a significant rise in the level of lysolipids. Physicochemical properties of rafts are determined by membrane composition, in particular, by the content of lysolipids. Lysolipids may thus regulate raft-involving processes. However, the exact mechanism of such regulation is unknown. Although studying rafts still seems to be rather complicated, liquid-ordered domains are well observed in model systems. In the present study, we used atomic force microscopy (AFM) to examine how lysophospholipids influence the liquid-ordered domains in model ternary membranes. We demonstrated that even a small amount of lysolipids in a membrane significantly impacts domain size depending on the saturation of the lysolipid hydrocarbon tails and the amount of cholesterol. The mixture with the bigger relative fraction of cholesterol was more susceptible to the action of lysolipids. This data helped us to generalize our previous theoretical model of the domain size regulation by lipids with particular molecular shape expanding it to the case of lysolipids and dioleoylglycerol.
PubMed: 36275621
DOI: 10.3389/fmolb.2022.1021321 -
Biomolecules Jan 2024The purpose of this review is to succinctly examine the methodologies used in lipid raft research in the brain and to highlight the drawbacks of some investigative... (Review)
Review
The purpose of this review is to succinctly examine the methodologies used in lipid raft research in the brain and to highlight the drawbacks of some investigative approaches. Lipid rafts are biochemically and biophysically different from the bulk membrane. A specific lipid environment within membrane domains provides a harbor for distinct raftophilic proteins, all of which in concert create a specialized platform orchestrating various cellular processes. Studying lipid rafts has proved to be arduous due to their elusive nature, mobility, and constant dynamic reorganization to meet the cellular needs. Studying neuronal lipid rafts is particularly cumbersome due to the immensely complex regional molecular architecture of the central nervous system. Biochemical fractionation, performed with or without detergents, is still the most widely used method to isolate lipid rafts. However, the differences in solubilization when various detergents are used has exposed a dire need to find more reliable methods to study particular rafts. Biochemical methods need to be complemented with other approaches such as live-cell microscopy, imaging mass spectrometry, and the development of specific non-invasive fluorescent probes to obtain a more complete image of raft dynamics and to study the spatio-temporal expression of rafts in live cells.
Topics: Detergents; Membrane Microdomains; Brain
PubMed: 38397393
DOI: 10.3390/biom14020156 -
ACS Applied Materials & Interfaces Dec 2022Lipid rafts are highly ordered regions of the plasma membrane enriched in signaling proteins and lipids. Their biological potential is realized in exosomes, a subclass...
Lipid rafts are highly ordered regions of the plasma membrane enriched in signaling proteins and lipids. Their biological potential is realized in exosomes, a subclass of extracellular vesicles (EVs) that originate from the lipid raft domains. Previous studies have shown that EVs derived from human placental mesenchymal stromal cells (PMSCs) possess strong neuroprotective and angiogenic properties. However, clinical translation of EVs is challenged by very low, impure, and heterogeneous yields. Therefore, in this study, lipid rafts are validated as a functional biomaterial that can recapitulate the exosomal membrane and then be synthesized into biomimetic nanovesicles. Lipidomic and proteomic analyses show that lipid raft isolates retain functional lipids and proteins comparable to PMSC-EV membranes. PMSC-derived lipid raft nanovesicles (LRNVs) are then synthesized at high yields using a facile, extrusion-based methodology. Evaluation of biological properties reveals that LRNVs can promote neurogenesis and angiogenesis through modulation of lipid raft-dependent signaling pathways. A proof-of-concept methodology further shows that LRNVs could be loaded with proteins or other bioactive cargo for greater disease-specific functionalities, thus presenting a novel type of biomimetic nanovesicles that can be leveraged as targeted therapeutics for regenerative medicine.
Topics: Female; Humans; Pregnancy; Proteomics; Placenta; Membrane Microdomains; Cell Membrane; Proteins; Lipids
PubMed: 36448709
DOI: 10.1021/acsami.2c13868 -
FEBS Letters May 2010In this review we describe the history of the development of the raft concept for membrane sub-compartmentalization. From its early beginnings as a mechanism for apical... (Review)
Review
In this review we describe the history of the development of the raft concept for membrane sub-compartmentalization. From its early beginnings as a mechanism for apical sorting in epithelial cells the concept has evolved to a general principle for membrane organisation. After a shaky start with crude methodology based on detergent extraction the field has become increasingly sophisticated, employing a host of different methods that support the existence of dynamic raft domains in membranes. These are composed of fluctuating nanoscale assemblies of sphingolipid, cholesterol and proteins that can be stabilized to coalesce, forming platforms that function in membrane signalling and trafficking.
Topics: Animals; Biological Transport; Cell Compartmentation; Cell Polarity; Cholesterol; Cyclodextrins; Detergents; Fatty Acid-Binding Proteins; Humans; Lipid Metabolism; Membrane Microdomains; Models, Biological; Phase Transition
PubMed: 20036659
DOI: 10.1016/j.febslet.2009.12.043