Unraveling UVB effects: Catalase activity and molecular alterations in the stratum corneum

AIM

Ultraviolet B (UVB) radiation can compromise the functionality of the skin barrier through various mechanisms. We hypothesize that UVB induce photochemical alterations in the components of the outermost layer of the skin, known as the stratum corneum (SC), and modulate its antioxidative defense mechanisms. Catalase is a well-known antioxidative enzyme found in the SC where it acts to scavenge reactive oxygen species. However, a detailed characterization of acute UVB exposure on the activity of native catalase in the SC is lacking. Moreover, the effects of UVB irradiation on the molecular dynamics and organization of the SC keratin and lipid components remain unclear. Thus, the aim of this work is to characterize consequences of UVB exposure on the structural and antioxidative properties of catalase, as well as on the molecular and global properties of the SC matrix surrounding the enzyme.

EXPERIMENTS

The effect of UVB irradiation on the catalase function is investigated by chronoamperometry with a skin covered oxygen electrode, which probes the activity of native catalase in the SC matrix. Circular dichroism is used to explore changes of the catalase secondary structure, and gel electrophoresis is used to detect fragmentation of the enzyme following the UVB exposure. UVB induced alterations of the SC molecular dynamics and structural features of the SC barrier, as well as its water sorption behavior, are investigated by a complementary set of techniques, including natural abundance 13C polarization transfer solid-state NMR, wide-angle X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, and dynamic vapor sorption microbalance.

FINDINGS

The findings show that UVB exposure impairs the antioxidative function of catalase by deactivating both native catalase in the SC matrix and lyophilized catalase. However, UVB radiation does not alter the secondary structure of the catalase nor induce any observable enzyme fragmentation, which otherwise could explain deactivation of its function. NMR measurements on SC samples show a subtle increase in the molecular mobility of the terminal segments of the SC lipids, accompanied by a decrease in the mobility of lipid chain trans-gauche conformers after high doses of UVB exposure. At the same time, the NMR data suggest increased rigidity of the polypeptide backbone of the keratin filaments, while the molecular mobility of amino acid residues in random coil domains of keratin remain unaffected by UVB irradiation. The FTIR data show a consistent decrease in absorbance associated with lipid bond vibrations, relative to the main protein bands. Collectively, the NMR and FTIR data suggest a small modification in the composition of fluid and solid phases of the SC lipid and protein components after UVB exposure, unrelated to the hydration capacity of the SC tissue. To conclude, UVB deactivation of catalase is anticipated to elevate oxidative stress of the SC, which, when coupled with subtle changes in the molecular characteristics of the SC, may compromise the overall skin health and elevate the likelihood of developing skin disorders.

Interplay of α-synuclein with Lipid Membranes: Cooperative Adsorption, Membrane Remodeling and Coaggregation

α-Synuclein is a small neuronal protein enriched at presynaptic termini. It is hypothesized to play a role in neurotransmitter release and synaptic vesicle cycling, while the formation of α-synuclein amyloid fibrils is associated with several neurodegenerative diseases, most notably Parkinson's Disease. The molecular mechanisms of both the physiological and pathological functions of α-synuclein remain to be fully understood, but in both cases, interactions with membranes play an important role. In this Perspective, we discuss several aspects of α-synuclein interactions with lipid membranes including cooperative adsorption, membrane remodeling and α-synuclein amyloid fibril formation in the presence of lipid membranes. We highlight the coupling between the different phenomena and their interplay in the context of physiological and pathological functions of α-synuclein.

Structural characterisation of α-synuclein-membrane interactions and the resulting aggregation using small angle scattering

The presence of amyloid fibrils is a hallmark of several neurodegenerative diseases. Some amyloidogenic proteins, such as α-synuclein and amyloid β, interact with lipids, and this interaction can strongly favour the formation of amyloid fibrils. In particular the primary nucleation step, i.e. the de novo formation of amyloid fibrils, has been shown to be accelerated by lipids. However, the exact mechanism of this acceleration is still mostly unclear. Here we use a range of scattering methods, such as dynamic light scattering (DLS) and small angle X-ray and neutron scattering (SAXS and SANS) to obtain structural information on the binding of α-synuclein to model membranes formed from negatively charged lipids and their co-assembly into amyloid fibrils. We find that the model membranes take an active role in the reaction. The binding of α synuclein to the model membranes immediately induces a major structural change in the lipid assembly, which leads to a break-up into small and mostly disc- or rod-like lipid-protein particles. This transition can be reversed by temperature changes or proteolytic protein removal. Incubation of the small lipid-α-synuclein particles for several hours, however, leads to amyloid fibril formation, whereby the lipids are incorporated into the amyloid fibrils.

A label-free high-throughput protein solubility assay and its application to Aβ40

A major hallmark of Alzheimer's disease is the accumulation of aggregated amyloid β peptide (Aβ) in the brain. Here we develop a solubility assay for proteins and measure the solubility of Aβ40. In brief, the method utilizes 96-well filter plates to separate monomeric Aβ from aggregated Aβ, and the small species are quantified with the amine reactive dye o-phthalaldehyde (OPA). This procedure ensures that solubility is measured for unlabeled species, and makes the assay high-throughput and inexpensive. We demonstrate that the filter plates successfully separate fibrils from monomer, with negligible monomer adsorption, and that OPA can quantify Aβ peptides in a concentration range from 40 nM to 20 μM. We also show that adding a methionine residue to the N-terminus of Aβ1-40 decreases the solubility by <3-fold. The method will facilitate further solubility studies, and contribute to the understanding of the thermodynamics of amyloid fibril formation.

Extracellular Vesicles Slow Down Aβ(1-42) Aggregation by Interfering with the Amyloid Fibril Elongation Step

Formation of amyloid-β (Aβ) fibrils is a central pathogenic feature of Alzheimer's disease. Cell-secreted extracellular vesicles (EVs) have been suggested as disease modulators, although their exact roles and relations to Aβ pathology remain unclear. We combined kinetics assays and biophysical analyses to explore how small (<220 nm) EVs from neuronal and non-neuronal human cell lines affected the aggregation of the disease-associated Aβ variant Aβ(1-42) into amyloid fibrils. Using thioflavin-T monitored kinetics and seeding assays, we found that EVs reduced Aβ(1-42) aggregation by inhibiting fibril elongation. Morphological analyses revealed this to result in the formation of short fibril fragments with increased thicknesses and less apparent twists. We suggest that EVs may have protective roles by reducing Aβ(1-42) amyloid loads, but also note that the formation of small amyloid fragments could be problematic from a neurotoxicity perspective. EVs may therefore have double-edged roles in the regulation of Aβ pathology in Alzheimer's disease.

The density of anionic lipids modulates the adsorption of α-Synuclein onto lipid membranes

α-Synuclein is an intrinsically disordered presynaptic protein associated with Parkinson's disease. The physiological role of α-Synuclein is not fully understood, but the protein is known to interact with lipid membranes. We here study how membrane charge affects the adsorption of α-Synuclein to (i) supported lipid bilayers and (ii) small unilamellar vesicles with varying amounts of anionic lipids. The results showed that α-Synuclein adsorbs onto membranes containing ≥5% anionic phosphatidylserine (DOPS) lipids, but not to membranes containing ≤1% DOPS. The density of adsorbed α-Synuclein increased steadily with the DOPS content up to 20% DOPS, after which it leveled off. The vesicles were saturated with α-Synuclein at a 3-5 times higher protein density compared to the supported bilayers, which suggests that a more deformable membrane binds more α-Synuclein. Altogether, the results show that both membrane charge density and flexibility influence the association of α-Synuclein to lipid membranes.

Ganglioside Micelles Affect Amyloid β Aggregation by Coassembly

Amyloid β peptide (Aβ) is the crucial protein component of extracellular plaques in Alzheimer's disease. The plaques also contain gangliosides lipids, which are abundant in membranes of neuronal cells and in cell-derived vesicles and exosomes. When present at concentrations above its critical micelle concentration (cmc), gangliosides can occur as mixed micelles. Here, we study the coassembly of the ganglioside GM1 and the Aβ peptides Aβ40 and 42 by means of microfluidic diffusional sizing, confocal microscopy, and cryogenic transmission electron microscopy. We also study the effects of lipid-peptide interactions on the amyloid aggregation process by fluorescence spectroscopy. Our results reveal coassembly of GM1 lipids with both Aβ monomers and Aβ fibrils. The results of the nonseeded kinetics experiments show that Aβ40 aggregation is delayed with increasing GM1 concentration, while that of Aβ42 is accelerated. In seeded aggregation reactions, the addition of GM1 leads to a retardation of the aggregation process of both peptides. Thus, while the effect on nucleation differs between the two peptides, GM1 may inhibit the elongation of both types of fibrils. These results shed light on glycolipid-peptide interactions that may play an important role in Alzheimer's pathology.

Retardation of Aβ42 fibril formation by apolipoprotein A-I and recombinant HDL particles

The double nucleation mechanism of amyloid β (Aβ) peptide aggregation is retained from buffer to cerebrospinal fluid (CSF) but with reduced rate of all microscopic processes. Here, we used a bottom-up approach to identify retarding factors in CSF. We investigated the Aβ42 fibril formation as a function of time in the absence and presence of apolipoprotein A-I (ApoA-I), recombinant high-density lipoprotein (rHDL) particles, or lipid vesicles. A retardation was observed in the presence of ApoA-I or rHDL particles, most pronounced with ApoA-I, but not with lipid vesicles. Global kinetic analysis implies that rHDL interferes with secondary nucleation. The effect of ApoA-I could best be described as an interference with secondary and to a smaller extent primary nucleation. Using surface plasmon resonance and microfluidics diffusional sizing analyses, we find that both rHDL and ApoA-I interact with Aβ42 fibrils but not Aβ42 monomer, thus the effect on kinetics seems to involve interference with the catalytic surface for secondary nucleation. The Aβ42 fibrils were imaged using cryogenic-electron microscopy and found to be longer when formed in the presence of ApoA-I or rHDL, compared to formation in buffer. A retarding effect, as observed in CSF, could be replicated using a simpler system, from key components present in CSF but purified from a CSF-free host. However, the effect of CSF is stronger implying the presence of additional retarding factors.

Wrapping anisotropic microgel particles in lipid membranes: Effects of particle shape and membrane rigidity

Cellular engulfment and uptake of macromolecular assemblies or nanoparticles via endocytosis can be associated to both healthy and disease-related biological processes as well as delivery of drug nanoparticles and potential nanotoxicity of pollutants. Depending on the physical and chemical properties of the system, the adsorbed particles may remain at the membrane surface, become wrapped by the membrane, or translocate across the membrane through an endocytosis-like process. In this paper, we address the question of how the wrapping of colloidal particles by lipid membranes can be controlled by the shape of the particles, the particle-membrane adhesion energy, the membrane phase behavior, and the membrane-bending rigidity. We use a model system composed of soft core-shell microgel particles with spherical and ellipsoidal shapes, together with phospholipid membranes with varying composition. Confocal microscopy data clearly demonstrate how tuning of these basic properties of particles and membranes can be used to direct wrapping and membrane deformation and the organization of the particles at the membrane. The deep-wrapped states are more favorable for ellipsoidal than for spherical microgel particles of similar volume. Theoretical calculations for fixed adhesion strength predict the opposite behavior-wrapping becomes more difficult with increasing aspect ratio. The comparison with the experiments implies that the microgel adhesion strength must increase with increasing particle stretching. Considering the versatility offered by microgels systems to be synthesized with different shapes, functionalizations, and mechanical properties, the present findings further inspire future studies involving nanoparticle-membrane interactions relevant for the design of novel biomaterials and therapeutic applications.

Molecular Mobility in Keratin-Rich Materials Monitored by Nuclear Magnetic Resonance: A Tool for the Evaluation of Structure-Giving Properties

Keratins are structural proteins that are abundant in human skin, nails, and hair, where they provide mechanical strength. In the present study, we investigate the molecular mobilities and structures of three keratin-rich materials with clearly different mechanical properties: nails, stratum corneum (upper layer of epidermis), and keratinocytes (from lower layer of epidermis). We use solid-state NMR on natural-abundance ¹³C to characterize small changes in molecular dynamics in these biological materials with close to atomistic resolution. One strong advantage of this method is that it detects small fractions of mobile components in a molecularly complex material while it simultaneously gives information on the rigid components in the very same sample. The molecular mobility can be linked to mechanical material properties in different conditions, including hydration or exposure to osmolytes or organic solvents. Importantly, the study revealed that the response to both hydration and addition of urea is clearly different for the nail keratin compared to the stratum corneum keratin. The comparative examination of these materials may provide a better understanding of skin diseases originating from keratin malfunction and contributes to the design and development of new materials.

Solubility of Foreign Molecules in Stratum Corneum Brick and Mortar Structure

The barrier function of the skin is mainly assured by its outermost layer, stratum corneum (SC). One key aspect in predicting dermal drug delivery and in safety assessment of skin exposure to chemicals is the need to determine the amount of chemical that is taken up into the SC. We here present a strategy that allows for direct measures of the amount of various solid chemicals that can be dissolved in the SC in any environmental relative humidity (RH). A main advantage of the presented method is that it distinguishes between molecules that are dissolved within the SC and molecules that are not dissolved but might be present at, for example, the skin surface. In addition, the method allows for studies of uptake of hydrophobic chemicals without the need to use organic solvents. The strategy relies on the differences in the molecular properties of the added molecules in the dissolved and the excess states, employing detection methods that act as a dynamic filter to spot only one of the fractions, either the dissolved molecules or the excess solid molecules. By measuring the solubility in SC and delipidized SC at the same RHs, the same method can be used to estimate the distribution of the added chemical between the extracellular lipids and corneocytes at different hydration conditions. The solubility in porcine SC is shown to vary with hydration, which has implications for the molecular uptake and transport across the skin. The findings highlight the importance of assessing the chemical uptake at hydration conditions relevant to the specific applications. The methodology presented in this study can also be generalized to study the solubility and partitioning of chemicals in other heterogeneous materials with complex composition and structure.

Ganglioside GM3 stimulates lipid-protein co-assembly in α-synuclein amyloid formation

Parkinson's disease is characterized by the aggregation of the presynaptic protein α-synuclein (αSyn), and its co-assembly with lipids and other cellular matter in the brain. Here we investigated lipid-protein co-assembly in a system composed of αSyn and model membranes containing the glycolipid ganglioside GM3. We quantified the uptake of lipids into the co-assembled aggregates and investigated how lipid molecular dynamics is altered by being present in the co-assemblies using solution ¹H- and solid-state ¹³C NMR spectroscopy. Aggregate morphology was studied using cryo-TEM. The overall lipid uptake in the co-assembled aggregates was found to increase with the molar ratio of GM3 in the vesicles. The lipids present in the co-assembled aggregates have reduced acyl chain and headgroup dynamics compared to the protein-free bilayer system. These findings may improve our understanding of how different types of lipids can influence the composition of αSyn aggregates, which may have consequences for amyloid formation in vivo.

Thermal fluctuations and osmotic stability of lipid vesicles

Biological membranes constantly change their shape in response to external stimuli, and understanding the remodeling and stability of vesicles in heterogeneous environments is therefore of fundamental importance for a range of cellular processes. One crucial question is how vesicles respond to external osmotic stresses, imposed by differences in solute concentrations between the vesicle interior and exterior. Previous analyses of the membrane bending energy have predicted that micron-sized giant unilamellar vesicles (GUVs) should become globally deformed already for nanomolar concentration differences, in contrast to experimental findings that find deformations at much higher osmotic stresses. In this article, we analyze the mechanical stability of a spherical vesicle exposed to an external osmotic pressure in a statistical-mechanical model, including the effect of thermally excited membrane bending modes. We find that the inclusion of thermal fluctuations of the vesicle shape changes renders the vesicle deformation continuous, in contrast to the abrupt transition in the athermal picture. Crucially, however, the predicted critical pressure associated with global vesicle deformation remains the same as when thermal fluctuations are neglected, approximately six orders of magnitude smaller than the typical collapse pressure recently observed experimentally for GUVs. We conclude by discussing possible sources of this persisting dissonance between theory and experiments.

Single-vesicle intensity and colocalization fluorescence microscopy to study lipid vesicle fusion, fission, and lipid exchange

Interactions of lipid vesicles play important roles in a large variety of functions and dysfunctions in the human body. Vital for several biochemical functions is the interaction between monomeric proteins and lipid membranes, and the induced phenomena such as fusion between vesicles and cell membranes, lipid exchange between the membranes, or vesicle fission. Identification of single events and their frequency of occurrence would provide valuable information about protein-lipid interactions in both healthy and degenerative pathways. In this work, we present a single-vesicle intensity and colocalization fluorescence microscopy assay with a custom-written MATLAB analysis program. The assay can be used to study lipid exchange as well as vesicle fusion and fission between two vesicle populations labeled with different fluorescent dyes. Vesicles from the two populations are first mixed and docked to a glass surface. The sample is then simultaneously imaged using two separate wavelength channels monitoring intensity changes and colocalization of vesicles from the two populations. The monomeric pre-synaptic protein α-synuclein (α-syn) and small unilamellar vesicles consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dioleoyl-sn-glycero-3-phospho-L-serine, (DOPS), and monosialotetrahexosylganglioside (GM1) were used as a model system to evaluate the method. From our analysis, neither α-syn induced fusion nor lipid exchange was observed for vesicles consisting of DOPC:DOPS (7:3). However, including 10% GM1 in the vesicles resulted in a 91% increase of the number of vesicles within 10 min, combined with a 57% decrease in the average fluorescence intensity per vesicle, indicating that approximately half of the vesicles underwent fission. The method facilitates the study of lipid vesicle fusion, fission, and lipid exchange under controlled conditions. It also allows these events to be studied for systems with more complex composition including exosomes and lipid-based drug carriers, to enable a better understanding of their physicochemical properties.

Molecular dynamics simulations and solid-state nuclear magnetic resonance spectroscopy measurements of C-H bond order parameters and effective correlation times in a POPC-GM3 bilayer

Glycolipids such as gangliosides affect the properties of lipid membranes and in extension the interactions between membranes and other biomolecules like proteins. To better understand how the properties of individual lipid molecules can contribute to shape the functional aspects of a membrane, the spatial restriction and dynamics of C-H bond segments can be measured using nuclear magnetic resonance (NMR) spectroscopy. We combine solid-state NMR spectroscopy with all-atom molecular dynamics (MD) simulations to investigate how ganglioside GM3 affects the bilayer structure and dynamics of C-H bond segments. These two methods yield reorientational correlation functions, molecular profiles of C-H bond order parameters |S_CH| and effective correlation times τ_e, which we compare for lipids in POPC bilayers with and without 30 mol% GM3. Our results revealed that all C-H segments of POPC reorient slower in the presence of GM3 and that the defining features of the GM3-POPC bilayer lie in the GM3 headgroup; it gives the bilayer an extended headgroup layer with high order (|S_CH| up to 0.3-0.4) and slow dynamics (τ_e up to 100 ns), a character that may be mechanistically important in ganglioside interactions with other biomolecules.

α-Synuclein Interaction with Lipid Bilayer Discs

α-Synuclein (aSyn) is a 140 residue long protein present in presynaptic termini of nerve cells. The protein is associated with Parkinson's disease, in which case it has been found to self-assemble into long amyloid fibrils forming intracellular inclusions that are also rich in lipids. Furthermore, its synaptic function is proposed to involve interaction with lipid membranes, and hence, it is of interest to understand aSyn-lipid membrane interactions in detail. In this paper we report on the interaction of aSyn with model membranes in the form of lipid bilayer discs. Using a combination of cryogenic transmission electron microscopy and small-angle neutron scattering, we show that circular discs undergo a significant shape transition after the adsorption of aSyn. When aSyn self-assembles into fibrils, aSyn molecules desorb from the bilayer discs, allowing them to recover to their original shape. Interestingly, the desorption process has an all-or-none character, resulting in a binary coexistence of circular bilayer discs with no adsorbed aSyn and deformed bilayer discs having a maximum amount of adsorbed protein. The observed coexistence is consistent with the recent finding of cooperative aSyn adsorption to anionic lipid bilayers.

The effects of glycols on molecular mobility, structure, and permeability in stratum corneum

The skin provides an attractive alternative to the conventional drug administration routes. Still, it comes with challenges as the upper layer of the skin, the stratum corneum (SC), provides an efficient barrier against permeation of most compounds. One way to overcome the skin barrier is to apply chemical permeation enhancers, which can modify the SC structure. In this paper, we investigated the molecular effect of three different types of glycols in SC: dipropylene glycol (diPG), propylene glycol (PG), and butylene glycol (BG). The aim is to understand how these molecules influence the molecular mobility and structure of the SC components, and to relate the molecular effects to the efficiency of these molecules as permeation enhancers. We used complementary experimental techniques, including natural abundance ¹³C NMR spectroscopy and wide-angle X-ray diffraction to characterize the molecular consequences of these compounds at different doses in SC at 97% RH humidity and 32 °C. In addition, we study the permeation enhancing effects of the same glycols in comparable conditions using Raman spectroscopy. Based on the results from NMR, we conclude that all three glycols cause increased mobility in SC lipids, and that the addition of glycols has an effect on the keratin filaments in similar manner as Natural Moisturizing Factor (NMF). The highest mobility of both lipids and amino acids can be reached with BG, which is followed by PG. It is also shown that one reaches an apparent saturation level for all three chemicals in SC, after which increased addition of the compound does not lead to further increase in the mobility of SC lipids or protein components. The examination with Raman mapping show that BG and PG give a significant permeation enhancement as compared to SC without any added glycol at corresponding conditions. Finally, we observe a non-monotonic response in permeation enhancement with respect to the concentration of glycols, where the highest concentration does not give the highest permeation. This is explained by the dehydration effects at highest glycol concentrations. In summary, we find a good correlation between the molecular effects of glycols on the SC lipid and protein mobility, and macroscopic permeation enhances of the same molecules.

Vesicles Balance Osmotic Stress with Bending Energy That Can Be Released to Form Daughter Vesicles

The bending energy of the lipid membrane is central to biological processes involving vesicles, such as endocytosis and exocytosis. To illustrate the role of bending energy in these processes, we study the response of single-component giant unilamellar vesicles (GUVs) subjected to external osmotic stress by glucose addition. For osmotic pressures exceeding 0.15 atm, an abrupt shape change from spherical to prolate occurs, showing that the osmotic pressure is balanced by the free energy of membrane bending. After equilibration, the external glucose solution was exchanged for pure water, yielding rapid formation of monodisperse daughter vesicles inside the GUVs through an endocytosis-like process. Our theoretical analysis shows that this process requires significant free energies stored in the deformed membrane to be kinetically allowed. The results indicate that bending energies stored in GUVs are much higher than previously implicated, with potential consequences for vesicle fusion/fission and the osmotic regulation in living cells.

On the Cluster Formation of α-Synuclein Fibrils

The dense accumulation of α-Synuclein fibrils in neurons is considered to be strongly associated with Parkinson’s disease. These intracellular inclusions, called Lewy bodies, also contain significant amounts of lipids. To better understand such accumulations, it should be important to study α-Synuclein fibril formation under conditions where the fibrils lump together, mimicking what is observed in Lewy bodies. In the present study, we have therefore investigated the overall structural arrangements of α-synuclein fibrils, formed under mildly acidic conditions, pH = 5.5, in pure buffer or in the presence of various model membrane systems, by means of small-angle neutron scattering (SANS). At this pH, α-synuclein fibrils are colloidally unstable and aggregate further into dense clusters. SANS intensities show a power law dependence on the scattering vector, q, indicating that the clusters can be described as mass fractal aggregates. The experimentally observed fractal dimension was d = 2.6 ± 0.3. We further show that this fractal dimension can be reproduced using a simple model of rigid-rod clusters. The effect of dominatingly attractive fibril-fibril interactions is discussed within the context of fibril clustering in Lewy body formation.

Purification and HDL-like particle formation of apolipoprotein A-I after co-expression with the EDDIE mutant of Npro autoprotease

Apolipoprotein A-I (ApoA-I) is the major protein constituent of high-density lipoprotein particles, and as such is involved in cholesterol transport and activation of LCAT (the lecithin:cholesterol acyltransferase). It may also form amyloidal deposits in the body, showing the multifaceted interactions of ApoA-I. In order to facilitate the study of ApoA-I in various systems, we have developed a protocol based on recombinant expression in E. coli. ApoA-I is protected from degradation by driving its expression to inclusion bodies using a tag: the EDDIE mutant of Npro autoprotease from classical swine fever virus. Upon refolding, EDDIE will cleave itself off from the target protein. The result is a tag-free ApoA-I, with its N-terminus intact. ApoA-I was then purified using a five-step procedure composed of anion exchange chromatography, immobilized metal ion affinity chromatography, hydrophobic interaction chromatography, boiling and size exclusion chromatography. This led to protein of high purity as confirmed with SDS-PAGE and mass spectrometry. The purified ApoA-I formed discoidal objects in the presence of zwitterionic phospholipid DMPC, showing its retained function of interacting with lipids. The protocol was also tested by expression and purification of two ApoA-I mutants, both of which could be purified in the same manner as the wildtype, showing the robustness of the protocol.

Skin hydration as a tool to control the distribution and molecular effects of intermediate polarity compounds in intact stratum corneum

The barrier function of the skin is mainly assured by its outermost layer, stratum corneum (SC), which consists of dead keratin-filled cells embedded in a lipid matrix. The skin is daily exposed to an environment with changing conditions in terms of hydration and different chemicals. Here we investigate how a molecule that has reasonable solubility in both hydrophobic and hydrophilic environments can be directed to certain regions in SC by changing the skin hydration. We use 1,2,3-trimethoxy propane (TMP) as a model substance and solid-state NMR on natural abundance ¹³C to obtain atomically resolved information on the molecular dynamics of TMP as well as SC lipid and protein components at varying hydration conditions. Upon dehydration, TMP redistributes from the hydrophilic corneocytes to the hydrophobic SC lipid regions. In this way, TMP can act to prevent the fluid-solid lipid transition in drying conditions and be present in the corneocytes in more humid conditions. Hydration can thereby be used as a switch to control the location and action of TMP or similar compounds in complex materials like SC. The general principles described here can also have impact on other applications including lipid-based formulations in food, drug delivery and cosmetics.

Cooperativity of α-Synuclein Binding to Lipid Membranes

Cooperative binding is a key feature of metabolic pathways, signaling, and transport processes. It provides tight regulation over a narrow concentration interval of a ligand, thus enabling switching to be triggered by small concentration variations. The data presented in this work reveal strong positive cooperativity of α-synuclein binding to phospholipid membranes. Fluorescence cross-correlation spectroscopy, confocal microscopy, and cryo-TEM results show that in excess of vesicles α-synuclein does not distribute randomly but binds only to a fraction of all available vesicles. Furthermore, α-synuclein binding to a supported lipid bilayer observed with total internal reflection fluorescence microscopy displays a much steeper dependence of bound protein on total protein concentration than expected for independent binding. The same phenomenon was observed in the case of α-synuclein binding to unilamellar vesicles of sizes in the nm and μm range as well as to flat supported lipid bilayers, ruling out that nonuniform binding of the protein is governed by differences in membrane curvature. Positive cooperativity of α-synuclein binding to lipid membranes means that the affinity of the protein to a membrane is higher where there is already protein bound compared to a bare membrane. The phenomenon described in this work may have implications for α-synuclein function in synaptic transmission and other membrane remodeling events.

Transient Lipid-Protein Structures and Selective Ganglioside Uptake During α-Synuclein-Lipid Co-aggregation

α-Synuclein is a membrane-interacting protein involved in Parkinson's disease. Here we have investigated the co-association of α-synuclein and lipids from ganglioside-containing model membranes. Our study relies on the reported importance of ganglioside lipids, which are found in high amounts in neurons and exosomes, on cell-to-cell prion-like transmission of misfolded α-synuclein. Samples taken along various stages of the aggregation process were imaged using cryogenic transmission electron microscopy, and the composition of samples corresponding to the final state analyzed using NMR spectroscopy. The combined data shows that α-synuclein co-assembles with lipids from the ganglioside (GM1)-containing model membranes. The lipid-protein samples observed during the aggregation process contain non-vesicular objects not present at the final stage, thus capturing the co-existence of species under non-equilibrium conditions. A range of different lipid-protein co-assemblies are observed during the time course of the reaction and some of these appear to be transient assemblies that evolve into other co-aggregates over time. At the end of the aggregation reaction, the samples become more homogeneous, showing thin fibrillar structures heavily decorated with small vesicles. From the NMR analysis, we conclude that the ratio of GM1 to phosphatidyl choline (PC) in the supernatant of the co-aggregated samples is significantly reduced compared to the GM1/PC ratio of the lipid dispersion from which these samples were derived. Taken together, this indicates a selective uptake of GM1 into the fibrillar aggregates and removal of GM1-rich objects from the solution.

NACore Amyloid Formation in the Presence of Phospholipids

Amyloids are implicated in many diseases, and disruption of lipid membrane structures is considered as one possible mechanism of pathology. In this paper we investigate interactions between an aggregating peptide and phospholipid membranes, focusing on the nanometer-scale structures of the aggregates formed, as well as on the effect on the aggregation process. As a model system, we use the small amyloid-forming peptide named NACore, which is a fragment of the central region of the protein α-synuclein that is associated with Parkinson's disease. We find that phospholipid vesicles readily associate with the amyloid fibril network in the form of highly distorted and trapped vesicles that also may wet the surface of the fibrils. This effect is most pronounced for model lipid systems containing only zwitterionic lipids. Fibrillation is found to be retarded by the presence of the vesicles. At the resolution of our measurements, which are based mainly on cryogenic transmission electron microscopy (cryo-TEM), X-ray scattering, and circular dichroism (CD) spectroscopy, we find that the resulting aggregates can be well fitted as linear combinations of peptide fibrils and phospholipid bilayers. There are no detectable effects on the cross-β packing of the peptide molecules in the fibrils, or on the thickness of the phospholipid bilayers. This suggests that while the peptide fibrils and lipid bilayers readily co-assemble on large length-scales, most of them still retain their separate structural identities on molecular length-scales. Comparison between this relatively simple model system and other amyloid systems might help distinguish aspects of amyloid-lipid interactions that are generic from aspects that are more protein specific. Finally, we briefly consider possible implications of the obtained results for in-vivo amyloid toxicity.

The plant dehydrin Lti30 stabilizes lipid lamellar structures in varying hydration conditions

A major challenge to plant growth and survival are changes in temperature and diminishing water supply. During acute temperature and water stress, plants often express stress proteins, such as dehydrins, which are intrinsically disordered hydrophilic proteins. In this article, we investigated how the dehydrin Lti30 from Arabidopsis thaliana stabilizes membrane systems that are exposed to large changes in hydration. We also compared the effects of Lti30 on membranes with those of the simple osmolytes urea and trimethylamine N-oxide. Using X-ray diffraction and solid-state NMR, we studied lipid-protein self-assembly at varying hydration levels. We made the following observations: 1) the association of Lti30 with anionic membranes relies on electrostatic attraction, and the protein is located in the bilayer interfacial membrane region; 2) Lti30 can stabilize the lamellar multilayer structure, making it insensitive to variations in water content; 3) in lipid systems with a composition similar to those present in some seeds and plants, dehydrin can prevent the formation of nonlamellar phases upon drying, which may be crucial for maintaining membrane integrity; and 4) Lti30 stabilizes bilayer structures both at high and low water contents, whereas the small osmolyte molecules mainly prevent dehydration-induced transitions. These results corroborate the idea that dehydrins are part of a sensitive and multifaceted regulatory mechanism that protects plant cells against stress.

Quantification of the amount of mobile components in intact stratum corneum with natural-abundance 13C solid-state NMR

The outermost layer of the skin is the stratum corneum (SC), which is mainly comprised of solid proteins and lipids. Minor amounts of mobile proteins and lipids are crucial for the macroscopic properties of the SC, including softness, elasticity and barrier function. Still this minor number of mobile components are not well characterized in terms of structure or amount. Conventional quantitative direct polarization (Q-DP) 13C solid-state NMR gives signal amplitudes proportional to concentrations, but fails to quantify the SC mobile components because of spectral overlap with the overwhelming signals from the solids. Spectral editing with the INEPT scheme suppresses the signals from solids, but also modulates the amplitudes of the mobile components depending on their values of the transverse relaxation times T2, scalar couplings JCH, and number of covalently bound hydrogens nH. This study describes a quantitative INEPT (Q-INEPT) method relying on systematic variation of the INEPT timing variables to estimate T2, JCH, nH, and amplitude for each of the resolved resonances from the mobile components. Q-INEPT is validated with a series of model systems containing molecules with different hydrophobicity and dynamics. For selected systems where Q-DP is applicable, the results of Q-INEPT and Q-DP are similar with respect to the linearity and uncertainty of the obtained molar ratios. Utilizing a reference compound with known concentration, we quantify the concentrations of mobile lipids and proteins within the mainly solid SC. By melting all lipids at high temperature, we obtain the total lipid concentration. These Q-INEPT results are the first steps towards a quantitative understanding of the relations between mobile component concentrations and SC macroscopic properties.

Self-Assembly in Ganglioside‒Phospholipid Systems: The Co-Existence of Vesicles, Micelles, and Discs

Ganglioside lipids have been associated with several physiological processes, including cell signaling. They have also been associated with amyloid aggregation in Parkinson’s and Alzheimer’s disease. In biological systems, gangliosides are present in a mix with other lipid species, and the structure and properties of these mixtures strongly depend on the proportions of the different components. Here, we study self-assembly in model mixtures composed of ganglioside GM1 and a zwitterionic phospholipid, 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC). We characterize the structure and molecular dynamics using a range of complementary techniques, including cryo-TEM, polarization transfer solid state NMR, diffusion NMR, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and calorimetry. The main findings are: (1) The lipid acyl chains are more rigid in mixtures containing both lipid species compared to systems that only contain one of the lipids. (2) The system containing DOPC with 10 mol % GM1 contains both vesicles and micelles. (3) At higher GM1 concentrations, the sample is more heterogenous and also contains small disc-like or rod-like structures. Such a co-existence of structures can have a strong impact on the overall properties of the lipid system, including transport, solubilization, and partitioning, which can be crucial to the understanding of the role of gangliosides in biological systems.

Lipid Dynamics and Phase Transition within α-Synuclein Amyloid Fibrils

The deposition of coassemblies made of the small presynaptic protein, α-synuclein, and lipids in the brains of patients is the hallmark of Parkinson's disease. In this study, we used natural abundance ¹³C and ³¹P magic-angle spinning nuclear magnetic resonance spectroscopy together with cryo-electron microscopy and differential scanning calorimetry to characterize the fibrils formed by α-synuclein in the presence of vesicles made of 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine or 1,2-dilauroyl-sn-glycero-3-phospho-L-serine. Our results show that these lipids coassemble with α-synuclein molecules to give thin and curly amyloid fibrils. The coassembly leads to slower and more isotropic reorientation of lipid molecular segments and a decrease in both the temperature and enthalpy of the lipid chain-melting compared with those in the protein-free lipid lamellar phase. These findings provide new insights into the properties of lipids within protein-lipid assemblies that can be associated with Parkinson's disease.

Assembling responsive microgels at responsive lipid membranes

Directed colloidal self-assembly at fluid interfaces can have a large impact in the fields of nanotechnology, materials, and biomedical sciences. The ability to control interfacial self-assembly relies on the fine interplay between bulk and surface interactions. Here, we investigate the interfacial assembly of thermoresponsive microgels and lipogels at the surface of giant unilamellar vesicles (GUVs) consisting of phospholipids bilayers with different compositions. By altering the properties of the lipid membrane and the microgel particles, it is possible to control the adsorption/desorption processes as well as the organization and dynamics of the colloids at the vesicle surface. No translocation of the microgels and lipogels through the membrane was observed for any of the membrane compositions and temperatures investigated. The lipid membranes with fluid chains provide highly dynamic interfaces that can host and mediate long-range ordering into 2D hexagonal crystals. This is in clear contrast to the conditions when the membranes are composed of lipids with solid chains, where there is no crystalline arrangement, and most of the particles desorb from the membrane. Likewise, we show that in segregated membranes, the soft microgel colloids form closely packed 2D crystals on the fluid bilayer domains, while hardly any particles adhere to the more solid bilayer domains. These findings thus present an approach for selective and controlled colloidal assembly at lipid membranes, opening routes toward the development of tunable soft materials.

Influence of polar co-solutes and salt on the hydration of lipid membranes

The influence of the co-solutes TMAO, urea, and NaCl on the hydration repulsion between lipid membranes is investigated in a combined experimental/simulation approach.

The Impact of Nonequilibrium Conditions in Lung Surfactant: Structure and Composition Gradients in Multilamellar Films

The lipid-protein mixture that covers the lung alveoli, lung surfactant, ensures mechanical robustness and controls gas transport during breathing. Lung surfactant is located at an interface between water-rich tissue and humid, but not fully saturated, air. The resulting humidity difference places the lung surfactant film out of thermodynamic equilibrium, which triggers the buildup of a water gradient. Here, we present a millifluidic method to assemble multilamellar interfacial films from vesicular dispersions of a clinical lung surfactant extract used in replacement therapy. Using small-angle X-ray scattering, infrared, Raman, and optical microscopies, we show that the interfacial film consists of several coexisting lamellar phases displaying a substantial variation in water swelling. This complex phase behavior contrasts to observations made under equilibrium conditions. We demonstrate that this disparity stems from additional lipid and protein gradients originating from differences in their transport properties. Supplementing the extract with cholesterol, to levels similar to the endogenous lung surfactant, dispels this complexity. We observed a homogeneous multilayer structure consisting of a single lamellar phase exhibiting negligible variations in swelling in the water gradient. Our results demonstrate the necessity of considering nonequilibrium thermodynamic conditions to study the structure of lung surfactant multilayer films, which is not accessible in bulk or monolayer studies. Our reconstitution methodology also opens avenues for lung surfactant pharmaceuticals and the understanding of composition, structure, and property relationships at biological air-liquid interfaces.

Ganglioside lipids accelerate α-synuclein amyloid formation

The deposition of α-synuclein fibrils is one hallmark of Parkinson's disease. Here, we investigate how ganglioside lipids, present in high amounts in neurons and exosomes, influence the aggregation kinetics of α-synuclein. Gangliosides, as well as, other anionic lipid species with small or large headgroups were found to induce conformational changes of α-synuclein monomers and catalyse their aggregation at mildly acidic conditions. Although the extent of this catalytic effect was slightly higher for gangliosides, the results imply that charge interactions are more important than headgroup chemistry in triggering aggregation. In support of this idea, uncharged lipids with large headgroups were not found to induce any conformational change and only weakly catalyse aggregation. Intriguingly, aggregation was also triggered by free ganglioside headgroups, while these caused no conformational change of α-synuclein monomers. Our data reveal that partially folded α-synuclein helical intermediates are not required species in triggering of α-synuclein aggregation.

Evaporation, diffusion and self-assembly at drying interfaces

Water evaporation from complex aqueous solutions leads to the build-up of structure and composition gradients at their interface with air. We recently introduced an experimental setup for quantitatively studying such gradients and discussed how structure formation can lead to a self-regulation mechanism for controlling water evaporation through self-assembly. Here, we provide a detailed theoretical analysis using an advection/diffusion transport equation that takes into account thermodynamically non-ideal conditions and we directly relate the theoretical description to quantitative experimental data. We derive that the concentration profile develops according to a general square root of time scaling law, which fully agrees with experimental observations. The evaporation rate notably decreases with time as t-1/2, which shows that diffusion in the liquid phase is the rate limiting step for this system, in contrast to pure water evaporation. For the particular binary system that was investigated experimentally, which is composed of water and a sugar-based surfactant (α-dodecylmaltoside), the interfacial layer consists in a sequence of liquid crystalline phases of different mesostructures. We extract values for mutual diffusion coefficients of lamellar, hexagonal and micellar cubic phases, which are consistent with previously reported values and simple models. We thus provide a method to estimate the transport properties of oriented mesophases. The macroscopic humidity-independence of the evaporation rate up to 85% relative humidities is shown to result from both an extremely low mutual diffusion coefficient and the large range of water activities corresponding to relative humidities below 85%, at which the lamellar phase exists. Such a humidity self-regulation mechanism is expected for a large variety of complex system.

Aggregate Size Dependence of Amyloid Adsorption onto Charged Interfaces

Amyloid aggregates are associated with a range of human neurodegenerative disorders, and it has been shown that neurotoxicity is dependent on aggregate size. Combining molecular simulation with analytical theory, a predictive model is proposed for the adsorption of amyloid aggregates onto oppositely charged surfaces, where the interaction is governed by an interplay between electrostatic attraction and entropic repulsion. Predictions are experimentally validated against quartz crystal microbalance-dissipation experiments of amyloid beta peptides and fragmented fibrils in the presence of a supported lipid bilayer. Assuming amyloids as rigid, elongated particles, we observe nonmonotonic trends for the extent of adsorption with respect to aggregate size and preferential adsorption of smaller aggregates over larger ones. Our findings describe a general phenomenon with implications for stiff polyions and rodlike particles that are electrostatically attracted to a surface.

Skin hydration: interplay between molecular dynamics, structure and water uptake in the stratum corneum

Hydration is a key aspect of the skin that influences its physical and mechanical properties. Here, we investigate the interplay between molecular and macroscopic properties of the outer skin layer – the stratum corneum (SC) and how this varies with hydration. It is shown that hydration leads to changes in the molecular arrangement of the peptides in the keratin filaments as well as dynamics of C-H bond reorientation of amino acids in the protruding terminals of keratin protein within the SC. The changes in molecular structure and dynamics occur at a threshold hydration corresponding to ca. 85% relative humidity (RH). The abrupt changes in SC molecular properties coincide with changes in SC macroscopic swelling properties as well as mechanical properties in the SC. The flexible terminals at the solid keratin filaments can be compared to flexible polymer brushes in colloidal systems, creating long-range repulsion and extensive swelling in water. We further show that the addition of urea to the SC at reduced RH leads to similar molecular and macroscopic responses as the increase in RH for SC without urea. The findings provide new molecular insights to deepen the understanding of how intermediate filament organization responds to changes in the surrounding environment.

Diffusion through Pig Gastric Mucin: Effect of Relative Humidity

Mucus covers the epithelium found in all intestinal tracts, where it serves as an important protecting barrier, and pharmaceutical drugs administrated by the oral, rectal, vaginal, ocular, or nasal route need to penetrate the mucus in order to reach their targets. Furthermore, the diffusion in mucus as well as the viscosity of mucus in the eyes, nose and throat can change depending on the relative humidity of the surrounding air. In this study we have investigated how diffusion through gels of mucin, the main protein in mucus, is affected by changes in ambient relative humidity (i.e. water activity). Already a small decrease in water activity was found to give rise to a significant decrease in penetration rate through the mucin gel of the antibacterial drug metronidazole. We also show that a decrease in water activity leads to decreased diffusion rate in the mucin gel for the fluorophore fluorescein. This study shows that it is possible to alter transport rates of molecules through mucus by changing the water activity in the gel. It furthermore illustrates the importance of considering effects of the water activity in the mucosa during development of potential pharmaceuticals.

Cyclic and Linear Monoterpenes in Phospholipid Membranes: Phase Behavior, Bilayer Structure, and Molecular Dynamics

Monoterpenes are abundant in essential oils extracted from plants. These relatively small and hydrophobic molecules have shown important biological functions, including antimicrobial activity and membrane penetration enhancement. The interaction between the monoterpenes and lipid bilayers is considered important to the understanding of the biological functions of monoterpenes. In this study, we investigated the effect of cyclic and linear monoterpenes on the structure and dynamics of lipids in model membranes. We have studied the ternary system 1,2-dimyristoyl-sn-glycero-3-phosphocholine-monoterpene-water as a model with a focus on dehydrated conditions. By combining complementary techniques, including differential scanning calorimetry, solid-state nuclear magnetic resonance, and small- and wide-angle X-ray scattering, bilayer structure, phase transitions, and lipid molecular dynamics were investigated at different water contents. Monoterpenes cause pronounced melting point depression and phase segregation in lipid bilayers, and the extent of these effects depends on the hydration conditions. The addition of a small amount of thymol to the fluid bilayer (volume fraction of 0.03 in the bilayer) leads to an increased order in the acyl chain close to the bilayer interface. The findings are discussed in relation to biological systems and lipid formulations.

Assembly of RNA nanostructures on supported lipid bilayers

The assembly of nucleic acid nanostructures with controlled size and shape has large impact in the fields of nanotechnology, nanomedicine and synthetic biology. The directed arrangement of nano-structures at interfaces is important for many applications. In spite of this, the use of laterally mobile lipid bilayers to control RNA three-dimensional nanostructure formation on surfaces remains largely unexplored. Here, we direct the self-assembly of RNA building blocks into three-dimensional structures of RNA on fluid lipid bilayers composed of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or mixtures of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) and cationic sphingosine. We demonstrate the stepwise supramolecular assembly of discrete building blocks through specific and selective RNA-RNA interactions, based on results from quartz crystal microbalance with dissipation (QCM-D), ellipsometry, fluorescence recovery after photobleaching (FRAP) and total internal reflection fluorescence microscopy (TIRF) experiments. The assembly can be controlled to give a densely packed single layer of RNA polyhedrons at the fluid lipid bilayer surface. We show that assembly of the 3D structure can be modulated by sequence specific interactions, surface charge and changes in the salt composition and concentration. In addition, the tertiary structure of the RNA polyhedron can be controllably switched from an extended structure to one that is dense and compact. The versatile approach to building up three-dimensional structures of RNA does not require modification of the surface or the RNA molecules, and can be used as a bottom-up means of nanofabrication of functionalized bio-mimicking surfaces.

Acceleration of α-synuclein aggregation by exosomes

Exosomes are small vesicles released from cells into extracellular space. We have isolated exosomes from neuroblastoma cells and investigated their influence on the aggregation of α-synuclein, a protein associated with Parkinson disease pathology. Using cryo-transmission electron microscopy of exosomes, we found spherical unilamellar vesicles with a significant protein content, and Western blot analysis revealed that they contain, as expected, the proteins Flotillin-1 and Alix. Using thioflavin T fluorescence to monitor aggregation kinetics, we found that exosomes catalyze the process in a similar manner as a low concentration of preformed α-synuclein fibrils. The exosomes reduce the lag time indicating that they provide catalytic environments for nucleation. The catalytic effects of exosomes derived from naive cells and cells that overexpress α-synuclein do not differ. Vesicles prepared from extracted exosome lipids accelerate aggregation, suggesting that the lipids in exosomes are sufficient for the catalytic effect to arise. Using mass spectrometry, we found several phospholipid classes in the exosomes, including phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, and the gangliosides GM2 and GM3. Within each class, several species with different acyl chains were identified. We then prepared vesicles from corresponding pure lipids or defined mixtures, most of which were found to retard α-synuclein aggregation. As a striking exception, vesicles containing ganglioside lipids GM1 or GM3 accelerate the process. Understanding how α-synuclein interacts with biological membranes to promote neurological disease might lead to the identification of novel therapeutic targets.

Controlling interfacial film formation in mixed polymer-surfactant systems by changing the vapor phase

Here we show that transport-generated phase separation at the air-liquid interface in systems containing self-assembling amphiphilic molecules and polymers can be controlled by the relative humidity (RH) of the air. We also show that our observations can be described quantitatively with a theoretical model describing interfacial phase separation in a water gradient that we published previously. These phenomena arises from the fact that the water chemical potential corresponding to the ambient RH will, in general, not match the water chemical potential in the open aqueous solution. This implies nonequilibrium conditions at the air-water interface, which in turn can have consequences on the molecular organization in this layer. The experimental setup is such that we can control the boundary conditions in RH and thereby verify the predictions from the theoretical model. The polymer-surfactant systems studied here are composed of polyethylenimine (PEI) and hexadecyltrimethylammonium bromide (CTAB) or didecyldimethylammonium bromide (DDAB). Grazing-incidence small-angle X-ray scattering results show that interfacial phases with hexagonal or lamellar structure form at the interface of dilute polymer-surfactant micellar solutions. From spectroscopic ellipsometry data we conclude that variations in RH can be used to control the growth of micrometer-thick interfacial films and that reducing RH leads to thicker films. For the CTAB-PEI system, we compare the phase behavior of the interfacial phase to the equilibrium bulk phase behavior. The interfacial film resembles the bulk phases formed at high surfactant to polymer ratio and reduced water contents, and this can be used to predict the composition of interfacial phase. We also show that convection in the vapor phase strongly reduces film formation, likely due to reduction of the unstirred layer, where diffusive transport is dominating.

Stratum corneum molecular mobility in the presence of natural moisturizers

The outermost layer of the skin, the stratum corneum (SC), is a lipid-protein membrane that experiences considerable osmotic stress from a dry and cold climate. The natural moisturizing factor (NMF) comprises small and polar substances, which like osmolytes can protect living systems from osmotic stress. NMF is commonly claimed to increase the water content in the SC and thereby protect the skin from dryness. In this work we challenge this proposed mechanism, and explore the influence of NMF on the lipid and protein components in the SC. We employ natural-abundance (13)C solid-state NMR methods to investigate how the SC molecular components are influenced by urea, glycerol, pyrrolidone carboxylic acid (PCA), and urocanic acid (UCA), all of which are naturally present in the SC as NMF compounds. Experiments are performed with intact SC, isolated corneocytes and model lipids. The combination of NMR experiments provides molecularly resolved qualitative information on the dynamics of different SC lipid and protein components. We obtain completely novel molecular information on the interaction of these NMF compounds with the SC lipids and proteins. We show that urea and glycerol, which are also common ingredients in skin care products, increase the molecular mobility of both SC lipids and proteins at moderate relative humidity where the SC components are considerably more rigid in the absence of these compounds. This effect cannot be attributed to increased SC water content. PCA has no detectable effect on SC molecular mobility under the conditions investigated. It is finally shown that the more apolar compound, UCA, specifically influences the mobility of the SC lipid regions. The present results show that the NMF components act to retain the fluidity of the SC molecular components under dehydrating conditions in such a way that the SC properties remain largely unchanged as compared to more hydrated SC. These findings provide a new molecular insight into how small polar molecules in NMF and skin care products act to protect the human skin from drying.

Membrane interaction of α-synuclein in different aggregation states

Aggregated α-synuclein in Lewy bodies is one of the hallmarks of Parkinson's disease (PD). Earlier observations of α-synuclein aggregates in neurons grafted into brains of PD patients suggested cell-to-cell transfer of α-synuclein and a prion-like mechanism. This prompted the current investigation of whether α-synuclein passes over model phospholipid bilayers. We generated giant unilamellar vesicles (GUVs) containing a small amount of a lipid-conjugated red emitting dye (rhodamine B) and varied the membrane charge by using different molar ratios of DOPC and DOPS or cardiolipin. We then used confocal fluorescence microscopy to examine how monomer, fibril as well as on-pathway α-synuclein species labeled with a green emitting fluorophore (Alexa488) interacted with the phospholipid bilayers of the GUV. We defined conditions that yielded reproducible aggregation kinetics under basal conditions and with none or moderate shaking. We found that on-pathway α-synuclein species and equilibrium amyloid aggregates, but not α-synuclein monomers, bound to lipid membranes. α-Synuclein was particularly strongly associated with GUVs containing the anionic lipids cardiolipin or DOPS, whereas it did not associate with GUVs containing only zwitterionic DOPC. We found that α-synuclein progressively aggregated at the surface of the GUVs, typically in distinct domains rather than uniformly covering the membrane, and that both lipid and protein were incorporated in the aggregates. Importantly, we never observed transport of α-synuclein over the GUV bilayer. This suggests that α-synuclein transport over membranes requires additional molecular players and that it might rely on active transport.