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Lipid rafts are nanoscopic assemblies of sphingolipids, cholesterol, and specific membrane proteins that contribute to lateral heterogeneity in eukaryotic membranes. Separation of artificial membranes into liquid-ordered (Lo) and liquid-disordered phases is regarded as a common model for this compartmentalization. However, tight lipid packing in Lo phases seems to conflict with efficient partitioning of raft-associated transmembrane (TM) proteins. To assess membrane order as a component of raft organization, we performed fluorescence spectroscopy and microscopy with the membrane probes Laurdan and C-laurdan. First, we assessed lipid packing in model membranes of various compositions and found cholesterol and acyl chain dependence of membrane order. Then we probed cell membranes by using two novel systems that exhibit inducible phase separation: giant plasma membrane vesicles [Baumgart et al. (2007) Proc Natl Acad Sci USA 104:3165-3170] and plasma membrane spheres. Notably, only the latter support selective inclusion of raft TM proteins with the ganglioside GM1 into one phase. We measured comparable small differences in order between the separated phases of both biomembranes. Lateral packing in the ordered phase of giant plasma membrane vesicles resembled the Lo domain of model membranes, whereas the GM1 phase in plasma membrane spheres exhibited considerably lower order, consistent with different partitioning of lipid and TM protein markers. Thus, lipid-mediated coalescence of the GM1 raft domain seems to be distinct from the formation of a Lo phase, suggesting additional interactions between proteins and lipids to be effective.

The plasma membrane (PM) of cells is a dynamic structure whose morphology and composition is in constant flux. PM morphologic changes are particularly relevant for the assembly and disassembly of signaling platforms involving surface-bound signaling proteins, as well as for many other mechanochemical processes that occur at the PM surface. Surface-bound membrane proteins (SBMP) require efficient association with the PM for their function, which is often achieved by the coordinated interactions of intrinsically disordered regions (IDRs) and globular domains with membrane lipids. This review focuses on the role of IDR-containing SBMPs in remodeling the composition and curvature of the PM. The ability of IDR-bearing SBMPs to remodel the Gaussian and mean curvature energies of the PM is intimately linked to their ability to sort subsets of phospholipids into nanoclusters. We therefore discuss how IDRs of many SBMPs encode lipid-binding specificity or facilitate cluster formation, both of which increase their membrane remodeling capacity, and how SBMP oligomers alter membrane shape by monolayer surface area expansion and molecular crowding.

We have recently developed a minimal system for generating long tubular nanostructures that resemble tubes observed in vivo with biological membranes. Here, we studied membrane tube pulling in ternary mixtures of sphingomyelin, phosphatidylcholine and cholesterol. Two salient results emerged: the lipid composition is significantly different in the tubes and in the vesicles; tube fission is observed when phase separation is generated in the tubes. This shows that lipid sorting may depend critically on both membrane curvature and phase separation. Phase separation also appears to be important for membrane fission in tubes pulled out of giant liposomes or purified Golgi membranes.

The plasma membrane is a highly compartmentalized, dynamic material and this organization is essential for a wide variety of cellular processes. Nanoscale domains allow proteins to organize for cell signaling, endo- and exocytosis, and other essential processes. Even in the absence of proteins, lipids have the ability to organize into domains as a result of a variety of chemical and physical interactions. One feature of membranes that affects lipid domain formation is membrane curvature. To directly test the role of curvature in lipid sorting, we measured the accumulation of two similar lipids, 1,2-Dihexadecanoyl-sn-glycero-3-phosphoethanolamine (DHPE) and hexadecanoic acid (HDA), using a supported lipid bilayer that was assembled over a nanopatterned surface to obtain regions of membrane curvature. Both lipids studied contain 16 carbon, saturated tails and a head group tag for fluorescence microscopy measurements. The accumulation of lipids at curvatures ranging from 28 nm to 55 nm radii was measured and fluorescein labeled DHPE accumulated more than fluorescein labeled HDA at regions of membrane curvature. We then tested whether single biotinylated DHPE molecules sense curvature using single particle tracking methods. Similar to groups of fluorescein labeled DHPE accumulating at curvature, the dynamics of single molecules of biotinylated DHPE was also affected by membrane curvature and highly confined motion was observed.

Cytoplasmic coat proteins are required for cargo selection and budding of tubulovesicular transport intermediates that shuttle between intracellular compartments. To better understand the physical parameters governing coat assembly and coat-induced membrane deformation, we have reconstituted the Arf1-dependent assembly of the COPI coat on giant unilamellar vesicles by using fluorescently labeled Arf1 and coatomer. Membrane recruitment of Arf1-GTP occurs exclusively on disordered lipid domains and does not induce optically visible membrane deformation. In the presence of Arf1-GTP, coatomer self-assembles into weakly curved coats on membranes under high tension, while it induces extensive membrane deformation at low membrane tension. These deformations appear to have a composition different from the parental membrane because they are protected from phase transition. These findings suggest that the COPI coat is adapted to liquid disordered membrane domains where it could promote lipid sorting and that its mechanical effects can be tuned by membrane tension.

Transport of biologically active molecules across tight epithelial barriers is a major challenge preventing therapeutic peptides from oral drug delivery. Here, we identify a set of synthetic glycosphingolipids that harness the endogenous process of intracellular lipid-sorting to enable mucosal absorption of the incretin hormone GLP-1. Peptide cargoes covalently fused to glycosphingolipids with ceramide domains containing C6:0 or smaller fatty acids were transported with 20-100-fold greater efficiency across epithelial barriers in vitro and in vivo. This was explained by structure-function of the ceramide domain in intracellular sorting and by the affinity of the glycosphingolipid species for insertion into and retention in cell membranes. In mice, GLP-1 fused to short-chain glycosphingolipids was rapidly and systemically absorbed after gastric gavage to affect glucose tolerance with serum bioavailability comparable to intraperitoneal injection of GLP-1 alone. This is unprecedented for mucosal absorption of therapeutic peptides, and defines a technology with many other clinical applications. To work properly, drugs need to be absorbed efficiently into the body. Medications that are injected directly into the bloodstream are often quickly transported to the organs or tissues they target. But injections are not always convenient, and many patients would instead prefer to swallow a pill or tablet. If a drug is swallowed, however, it must first be absorbed through the gut before it can enter the bloodstream. The lining of the gut consists of tightly linked layers of cells that readily take up small molecules, such as water and simple nutrients, but exclude almost all larger ones. Since several important types of drugs are large or poorly absorbed molecules, such as proteins, finding methods to help them cross the gut barrier is a major part of drug development. Originally from bacteria, cholera toxin is an example of a large, naturally occurring protein that does cross the gut lining. To do this, the toxin specifically attaches onto GM1, a type of lipid molecule that is found on the outer surface of gut cells, and hijacks the system that moves this lipid within cells. Previous studies identified several key features of GM1’s structure that enable this movement; and, in 2014, researchers tested GM1 as a ‘carrier’ to help the gut to absorb large therapeutic molecules. This approach was successful in cells grown in the laboratory, but not when the drugs were fed to animals. To overcome this issue, Garcia-Castillo, Chinnapen et al. – who include some of the researchers involved in the earlier studies – set out to further boost GM1’s ability to transport drugs across the gut lining. First several hybrid molecules were made, consisting of different structures of GM1 (the ‘carrier’) fused to a reporter peptide (the ‘cargo’). Laboratory experiments with human intestinal cells and dog kidney cells, both of which form tightly-linked layers much like the actual lining of the gut, revealed specific structural variations of the GM1-derived carrier that transported the cargo across the cell barrier more efficiently. Garcia-Castillo, Chinnapen et al. went on to test the efficiency of these carriers further by switching the reporter cargo to a therapeutic hormone called GLP-1. This hormone is used to treat people with type II diabetes but is currently given via an injection. The same structural variants of GM1 that enhanced delivery of the reporter cargo also worked for the larger GLP-1 hormone. Garcia-Castillo, Chinnapen et al. then fed the GM1-GLP-1 fusions to mice, and measured the amount of GLP-1 hormone absorbed into the blood. Crucially, the mice fed GM1-GLP-1 molecules absorbed the drug just as well as mice injected with the GLP-1 that is normally given to diabetes patients. Together these findings represent a major contribution to the pharmaceutical toolbox. They may also ultimately lead to more drugs that can be given as a patient-friendly pill or tablet, readily cross the gut barrier and achieve widespread drug delivery around the body.

The small intestinal brush border is a specialized cell membrane that needs to withstand the solubilizing effect of bile salts during assimilation of dietary nutrients and to achieve detergent resistance; it is highly enriched in glycolipids organized in lipid raft microdomains. In the present work, the fluorescent lipophilic probes FM 1–43 ( N -(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide), FM 4–64 ( N -(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino) phenyl)hexatrienyl)pyridinium dibromide), TMA-DPH (1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene p -toluenesulfonate), and CellMask Orange plasma membrane stain were used to study endocytosis from the enterocyte brush border of organ-cultured porcine mucosal explants. All the dyes readily incorporated into the brush border but were not detectably endocytosed by 5 min, indicating a slow uptake compared with other cell types. At later time points, FM 1–43 clearly appeared in distinct punctae in the terminal web region, previously shown to represent early endosomes (TWEEs). In contrast, the other dyes were relatively “endocytosis resistant” to varying degrees for periods up to 2 h, indicating an active sorting of lipids in the brush border prior to internalization. For some of the dyes, a diphenylhexatriene motif in the lipophilic tail seemed to confer the relative endocytosis resistance. Lipid sorting by selective endocytosis therefore may be a process in the enterocytes aimed to generate and maintain a unique lipid composition in the brush border.

Uric acid crystals, the causative agent of gout, have recently gained widespread attention due to their role as a natural endogenous adjuvant. Uric acid crystals, first sensed extracellularly by membrane lipid alterations, are internalized and subsequently activate the NLRP3 inflammasome. Currently, various aspects of this particular novel pathway are poorly defined. This short review will focus on some recent discoveries regarding this simple crystalline structure and address areas requiring further investigation. The fact that uric acid crystals activate innate host defense mechanisms, triggering robust inflammation and immune activation, may lead to engineering potent adjuvants for future vaccines. Furthermore, the elucidation of uric acid's mechanism of inflammation may lay the foundation for other solid inflammatory structures such as silica, asbestos, and alum.

Stabilized lipid rafts in membrane transport