Number of Sialic Acid Residues in Ganglioside Headgroup Affects Interactions with Neighboring Lipids

Monolayers of binary mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and asialo-(GA1), disialo-(GD1b) and trisialo-(GT1b) gangliosides were used to determine the effect of ganglioside headgroup charge and geometry on its interactions with the neighboring zwitterionic lipid. Surface pressure versus molecular area isotherm measurements along with concurrent fluorescence microscopy of the monolayers at the air-water interface were complemented with atomic force microscopy imaging of monolayers deposited on solid substrates. Results were used to further develop a proposed geometric packing model that the complementary geometry of DPPC and monosialoganglioside GM1 headgroups affects their close molecular packing, inducing condensation of the layer at small mol % of ganglioside. For GA1, GD1b, and GT1b, a similar condensing effect, followed by a fluidizing effect is seen that varies with glycosphingolipid concentration, but results do not directly follow from geometric arguments because less DPPC is needed to condense ganglioside molecules with larger cross-sectional areas. The variations in critical packing mole ratios can be explained by global effects of headgroup charge and resultant dipole moments within the monolayer. Atomic force microscopy micrographs further support the model of ganglioside-induced DPPC condensation with condensed domains composed of a striped phase of condensed DPPC and DPPC/ganglioside geometrically packed complexes at low concentrations

Sphingomyelin and GM1 Influence Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Huntington disease (HD) is an inherited neurodegenerative disease caused by the expansion beyond a critical threshold of a polyglutamine (polyQ) tract near the N-terminus of the huntingtin (htt) protein. Expanded polyQ promotes the formation of a variety of oligomeric and fibrillar aggregates of htt that accumulate into the hallmark proteinaceous inclusion bodies associated with HD. htt is also highly associated with numerous cellular and subcellular membranes that contain a variety of lipids. As lipid homeostasis and metabolism abnormalities are observed in HD patients, we investigated how varying both the sphingomyelin (SM) and ganglioside (GM1) contents modifies the interactions between htt and lipid membranes. SM composition is altered in HD, and GM1 has been shown to have protective effects in animal models of HD. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy (AFM) were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added SM or GM1. The addition of either SM or GM1 decreased htt insertion into the lipid monolayers. However, TBLE vesicles with an increased SM content were more susceptible to htt-induced permeabilization, whereas GM1 had no effect on permeablization. Pure TBLE bilayers and TBLE bilayers enriched with GM1 developed regions of roughened, granular morphologies upon exposure to htt-exon1, but plateau-like domains with a smoother appearance formed in bilayers enriched with SM. Oligomeric aggregates were observed on all bilayer systems regardless of induced morphology. Collectively, these observations suggest that the lipid composition and its subsequent effects on membrane material properties strongly influence htt binding and aggregation on lipid membranes

Sphingomyelin and GM1 Influence Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Huntington disease (HD) is an inherited neurodegenerative disease caused by the expansion beyond a critical threshold of a polyglutamine (polyQ) tract near the N- terminus of the huntingtin (htt) protein. Expanded polyQ promotes the formation of a variety of oligomeric and fibrillar aggregates of htt that accumulate into the hallmark proteina- ceous inclusion bodies associated with HD. htt is also highly associated with numerous cellular and subcellular membranes that contain a variety of lipids. As lipid homeostasis and metabolism abnormalities are observed in HD patients, we investigated how varying both the sphingomyelin (SM) and ganglioside (GM1) contents modifies the interactions between htt and lipid membranes. SM composition is altered in HD, and GM1 has been shown to have protective effects in animal models of HD. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy (AFM) were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added SM or GM1. The addition of either SM or GM1 decreased htt insertion into the lipid monolayers. However, TBLE vesicles with an increased SM content were more susceptible to htt-induced permeabilization, whereas GM1 had no effect on permeablization. Pure TBLE bilayers and TBLE bilayers enriched with GM1 developed regions of roughened, granular morphologies upon exposure to htt-exon1, but plateau-like domains with a smoother appearance formed in bilayers enriched with SM. Oligomeric aggregates were observed on all bilayer systems regardless of induced morphology. Collectively, these observations suggest that the lipid composition and its subsequent effects on membrane material properties strongly influence htt binding and aggregation on lipid membranes

Triblock Copolymer as an Effective Membrane-Sealing Material

An intact cell membrane serves as a permeable barrier, regulating the influx and efflux of ions and small molecules. When the integrity of the membrane is compromised, its barrier function is also disrupted, threatening the survival of the cell. Triblock copolymer surfactants of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) have been shown to help seal structurally damaged membranes, arresting the leakage of intracellular materials. In order to understand how this particular family of triblock copolymers helps seal damaged membranes, model lipid monolayer and bilayer systems have been used to unravel the nature of the lipid/copolymer interaction. The copolymer surfactant is found to selectively insert into structurally compromised membranes, thus localizing its sealing effect on the damaged regions. The inserted polymer is “squeezed out” when the lipid packing density is increased, suggesting a mechanism for the cell to be rid of the polymer when the membrane integrity is restored

Lateral Phase Behavior of Human Skin Lipids

The human stratum corneum (SC) is the outermost layer of the epidermis, the first barrier against the environment. This largely impenetrable layer consists of anucleated, flattened, protein-rich cells called corneocytes suspended in an extracellular lipid matrix. This lipid matrix is composed of ceramides (Cers), cholesterol (Chol), and free fatty acids (FFAs). The SC lipids are unique from other biologically relevant lipids commonly found in cell membranes in that the Cers and FFAs have very long (\u3e20 carbons) fatty acid chains, nearly all of which are fully saturated. Lateral packing and domain formation within Langmuir monolayers of the individual Cer, Chol, and FFA components and their binary and ternary mixtures were investigating using surface pressure vs. molecular area isotherms and fluorescence microscopy to determine the role of each component in the material properties of the SC layer. Particular attention was paid to the effect of FFA length on membrane fluidity – FFA chains ranging from 16 to 30 carbons (24 carbons being the most common in the SC) were studied. A physiologically accurate mixture of several FFAs with different chain lengths (FFAmix) is shown to be more stable to compression-expansion cycling than any single FFA. Longer chain FFAs have a condensing effect in Cer monolayers, while Chol tends to disrupt the Cer-FFA organization. The trans double bond in the sphingosine backbone of certain SC Cers allows close lateral lipid packing, stiffening the layer. Additionally, there are significant differences between the canonical 1:1:1 Cer:Chol:FFA mixture and a more physiologically accurate 10:9:4 mixture with more representative Cers and FFAmix. Including a higher ratio of Cers and a lower one of FFAs in such mixtures condenses the monolayer despite FFAs being more incompressible than Cers alone, indicating that ternary SC lipid mixtures experience non-additive complex lipid interactions