Global small-angle scattering data analysis of inverted hexagonal phases

Metal ion cofactors modulate integral enzyme activity by varying differential membrane curvature stress

Metal ions are well-known cofactors of protein function and stability. In the case of the integral membrane enzyme OmpLA (outer membrane phospholipase A) the active dimer is stabilized by calcium ions. We studied the lipid hydrolysis kinetics of OmpLA in charge-neutral and charged membranes with symmetric or asymmetric transbilayer lipid distributions. In charge-neutral membranes, OmpLA was more active in symmetric bilayers due to the lower differential curvature stress between membrane leaflets. Strikingly, this behavior was completely reversed in charged bilayers. Measurements revealed intrinsic molecular shape changes in the charged lipids upon addition of calcium. This effectively reduces the differential curvature stress in charged asymmetric membranes leading to increased protein activity. This conclusion is further supported by similar effects observed upon the addition of sodium ions, which also alter the shape of the lipids, but do not specifically interact with the protein. Additional lipid-protein interactions likely contribute to this phenomenon. Our findings demonstrate that ion cofactors not only interact directly with membrane proteins but also modulate protein activity indirectly by altering the effective molecular shape of charged lipid species.

Setting the curve: the biophysical properties of lipids in mitochondrial form and function

Mitochondrial membranes are defined by their diverse functions, complex geometries, and unique lipidomes. In the inner mitochondrial membrane, highly curved membrane folds known as cristae house the electron transport chain and are the primary sites of cellular energy production. The outer mitochondrial membrane is flat by contrast, but is critical for the initiation and mediation of processes key to mitochondrial physiology: mitophagy, interorganelle contacts, fission and fusion dynamics, and metabolite transport. While the lipid composition of both the inner mitochondrial membrane and outer mitochondrial membrane have been characterized across a variety of cell types, a mechanistic understanding for how individual lipid classes contribute to mitochondrial structure and function remains nebulous. In this review, we address the biophysical properties of mitochondrial lipids and their related functional roles. We highlight the intrinsic curvature of the bulk mitochondrial phospholipid pool, with an emphasis on the nuances surrounding the mitochondrially-synthesized cardiolipin. We also outline emerging questions about other lipid classes - ether lipids, and sterols - with potential roles in mitochondrial physiology. We propose that further investigation is warranted to elucidate the specific properties of these lipids and their influence on mitochondrial architecture and function.

Optical and molecular features of negatively curved surfaces created by POPE lipids: A crucial role of the initial conditions

Membrane fusion is closely related to plasma membrane domains rich in cone-shaped phosphatidylethanolamine (PE) lipids that can reverse membrane curvature under certain conditions. The phase transition of PE-based lipid membranes from the lamellar fluid phase (Lα) to the inverse hexagonal phase (HII) is commonly taken as a general model in reconstructing the membrane fusion pathway, and whose structural features have been mostly described so far using structural and microscopic techniques. The aim of this paper is to decipher the optical and molecular features of Lβ → Lα and especially of Lα → HII transition of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) lipids at pH = 7.0 when they are initially prepared in the form of both multi- and unilamellar liposomes (MLVs and LUVs). The distinction between optical properties of MLS- and LUVs-derived HII phase, provided from turbidity-sensitive temperature-dependent UV-Vis spectra, was attributed to different formation mechanisms of HII phase. Most importantly, from FTIR spectroscopic data of POPE lipids in Lβ (15 °C), Lα (50 °C) and HII (85 °C) phases we identified the changes in molecular features of POPE lipids during phase transitions. Among the latter, by far the most significant is different hydration pattern of POPE lipids in MLVs- and LUVs-derived HII phase which extends from the polar-apolar interface all the way to the terminal amino group of the POPE lipid, along with the changes in the conformation of glycerol backbone as evidenced by the signature of α-methylene groups. Molecular dynamics simulations confirmed higher water penetration in HII phase and provided insight into hydrogen bonding patterns.

Homeocurvature adaptation of phospholipids to pressure in deep-sea invertebrates

Hydrostatic pressure increases with depth in the ocean, but little is known about the molecular bases of biological pressure tolerance. We describe a mode of pressure adaptation in comb jellies (ctenophores) that also constrains these animals' depth range. Structural analysis of deep-sea ctenophore lipids shows that they form a nonbilayer phase at pressures under which the phase is not typically stable. Lipidomics and all-atom simulations identified phospholipids with strong negative spontaneous curvature, including plasmalogens, as a hallmark of deep-adapted membranes that causes this phase behavior. Synthesis of plasmalogens enhanced pressure tolerance in Escherichia coli, whereas low-curvature lipids had the opposite effect. Imaging of ctenophore tissues indicated that the disintegration of deep-sea animals when decompressed could be driven by a phase transition in their phospholipid membranes.

The rise of FTIR spectroscopy in the characterization of asymmetric lipid membranes

In contrast to symmetric unilamellar liposomes (sLUVs) prepared from a mixture of different lipids, asymmetric ones (aLUVs) with different lipid composition in the inner and outer membrane leaflets are more suitable model systems of eukaryotic plasma membranes. However, apart from the challenging preparation of asymmetric liposomes and small amounts of obtained asymmetric unilamellar liposomes (aLUVs), a major drawback is the qualitative characterization of asymmetry, as each of the techniques used so far has certain limitations. In this regard, we prepared aLUVs composed dominantly of DPPC(out)/DPPS(in) lipids and, along with 1H NMR and DSC characterization, we showed for the first time how FTIR spectroscopy can be used in the presence of (a)symmetry between DPPC/DPPS lipid bilayers. Using second derivative FTIR spectra we demonstrated not only that the hydration of lipids glycerol backbone and choline moiety of DPPC differs in s/aLUVs, but in addition that the lateral interactions between hydrocarbon chains during the phase change display different trend in s/aLUVs. Molecular dynamics simulations confirmed different chain ordering and packing between s/a bilayers, with a significant influence of temperature, i.e. membrane phase.

From angular to round: in depth interfacial analysis of binary phosphatidylethanolamine mixtures in the inverse hexagonal phase

Packing stress in the lipidic inverse hexagonal HII phase arises from the necessity of the ideally cylinder-shaped micelles to fill out the hexagonally-shaped Wigner-Seitz unit cell. Thus, hydrocarbon chains stretch towards the corners and compress in the direction of the flat side of the hexagonal unit cell. Additionally, the lipid/water interface deviates from being perfectly circular. To study this packing frustration in greater detail, we have doped 1-palmitoyl-2-oleoyl-sn-phosphatidylethanolamine (POPE) with increasing molar concentrations of 1,2-palmitoyl-sn-phosphatidylethanolamine (DPPE: 0 to 15 mol%). Due to its effectively longer hydrophobic tails, DPPE tends to aggregate in the corner regions of the unit cell, and thus, increases the circularity of the lipid/water interface. From small angle X-ray diffraction (SAXD) we determined electron density maps. Using those, we analysed the size, shape and homogeneity of the lipid/water interface as well as that of the methyl trough region. At 6 and 9 mol% DPPE the nanotubular water core most closely resembles a circle; further to this, in comparison to its neighbouring concentrations, the 9 mol% DPPE sample has the smallest water core area and smallest number of lipids per circumference, best alleviating the packing stress. Finally, a three-water layer model was applied, discerning headgroup, perturbed and free water, demonstrating that the hexagonal phase is most stable in the direction of the flat faces (compression zones) and least stable towards the vertices of the unit cell (decompression zones).

Small-Angle Neutron Scattering Study of a Phosphatidylcholine-Phosphatidylethanolamine Mixture

The properties of single-component phospholipid lipid bilayers have been extensively characterized. Natural cell membranes are not so simple, consisting of a diverse mixture of lipids and proteins. While having detailed structural information on complex membranes would be useful for understanding their structure and function, experimentally characterizing such membranes at a level of detail applied to model phospholipid bilayers is challenging. Here, small-angle neutron scattering with selective deuteration was used to characterize a binary lipid mixture composed of 1,2-dimyristoyl-3-sn-glycero-phosphatidylcholine and 1,2-dimyristoyl-3-sn-glycero-phosphatidylethanolamine. The data analysis provided the area per lipid in each leaflet as well as the asymmetry of the composition of the inner and outer leaflets of the bilayer. The results provide new insight into the structure of the lipid bilayer when this lipid mixture is used to prepare vesicles.

Cristae formation is a mechanical buckling event controlled by the inner membrane lipidome

Cristae are high curvature structures in the inner mitochondrial membrane (IMM) that are crucial for ATP production. While cristae-shaping proteins have been defined, analogous mechanisms for lipids have yet to be elucidated. Here we combine experimental lipidome dissection with multi-scale modeling to investigate how lipid interactions dictate IMM morphology and ATP generation. When modulating phospholipid (PL) saturation in engineered yeast strains, we observed a surprisingly abrupt breakpoint in IMM topology driven by a continuous loss of ATP synthase organization at cristae ridges. We found that cardiolipin (CL) specifically buffers the IMM against curvature loss, an effect that is independent of ATP synthase dimerization. To explain this interaction, we developed a continuum model for cristae tubule formation that integrates both lipid and protein-mediated curvatures. The model highlighted a snapthrough instability, which drives IMM collapse upon small changes in membrane properties. We also showed that CL is essential in low oxygen conditions that promote PL saturation. These results demonstrate that the mechanical function of CL is dependent on the surrounding lipid and protein components of the IMM.

Distributing aminophospholipids asymmetrically across leaflets causes anomalous membrane stiffening

We studied the mechanical leaflet coupling of prototypic mammalian plasma membranes using neutron spin-echo spectroscopy. In particular, we examined a series of asymmetric phospholipid vesicles with phosphatidylcholine and sphingomyelin enriched in the outer leaflet and inner leaflets composed of phosphatidylethanolamine/phosphatidylserine mixtures. The bending rigidities of most asymmetric membranes were anomalously high, exceeding even those of symmetric membranes formed from their cognate leaflets. Only asymmetric vesicles with outer leaflets enriched in sphingolipid displayed bending rigidities in conformity with these symmetric controls. We performed complementary small-angle neutron and x-ray experiments on the same vesicles to examine possible links to structural coupling mechanisms, which would show up in corresponding changes in membrane thickness. In addition, we estimated differential stress between leaflets originating either from a mismatch of their lateral areas or spontaneous curvatures. However, no correlation with asymmetry-induced membrane stiffening was observed. To reconcile our findings, we speculate that an asymmetric distribution of charged or H-bond forming lipids may induce an intraleaflet coupling, which increases the weight of hard undulatory modes of membrane fluctuations and hence the overall membrane stiffness.

Cr(vi) permanently binds to the lipid bilayer in an inverted hexagonal phase throughout the reduction process

Cr(vi) is a harmful, carcinogenic agent with a high permeability rate throughout the lipid membranes. In an intracellular environment and during interactions with cellular membranes, it undergoes an instant reduction to lower oxidation states throughout radical states, recognized as the most dangerous factor for cells. The cellular membrane is the most visible cellular organelle in the interior and exterior of a cell. In this study, liposomes and non-lamellar inverted hexagonal phase lipid structures based on phosphoethanolamine (PE) were used as model cellular bilayers because of their simple composition, preparation procedure, and the many other properties of natural systems. The lipid membranes were subjected to 0.075 mM Cr(vi) for 15 min, after which the Cr content was removed via dialysis. This way, the remaining Cr content could be studied qualitatively and quantitatively. Using the combined XRF/XAS/EPR approach, we revealed that some Cr content (Cr(iii) and Cr(vi)) was still present in the samples even after long-term dialysis at a temperature significantly above the phase transition for the chosen liposome. The amount of bound Cr increased with increasing PE and -C[double bond, length as m-dash]C- bond content in lipid mixtures. Internal membrane order decreased in less fluid membranes, while in more liquified ones, internal order was only slightly changed after subjecting them to the Cr(vi) agent. The results suggest that the inverted hexagonal phase of lipid structures is much more sensitive to oxidation than the lamellar lipid phase, which can play an important role in the strong cytotoxicity of Cr(vi).

Interaction of detergent with complex mimics of bacterial membranes

Detergents are valuable tools to extract membrane proteins for biophysical, biochemical, and structural scrutiny. The detergent-driven solubilization of bilayers made from a single lipid species is commonly described in terms of pseudo-phase diagrams and a three-stage model accounting for three ranges comprising (i) intact vesicles, (ii) vesicle/micelle co-existence, or (iii) mixed micelles. Moreover, the pseudo-phase boundaries thus determined can often be quantitatively rationalized in terms of the molecular shapes of the lipid and the detergent used. Yet, it has remained unclear to what extent this approach can be applied to multi-component lipid membranes that more closely mimic the compositional complexity of cellular membranes. Here, we studied how lipid mixtures composed of palmitoyl oleoyl phosphatidylethanolamine (POPE), palmitoyl oleoyl phosphatidylglycerol (POPG), and tetraoleoyl cardiolipin (TOCL) are solubilized by the commonly used zwitterionic detergent lauryldimethylamine N-oxide using isothermal titration calorimetry. While phase diagrams of the diverse lipid mixtures showed the typical ranges of the three-stage model, we found that POPG-rich POPE/POPG bilayers are more difficult to solubilize than POPG-poor POPE/POPG bilayers. In turn, POPE/POPG/TOCL bilayers became increasingly resistant to detergent with increasing TOCL content. Since POPG is nearly cylindrically shaped and TOCL adopts inverted cone-like shapes under current buffer conditions, our solubilization data do not align with shape-based arguments. Instead, additional electrostatic interactions between lipids and detergents lead to non-additive mixing behavior affecting the resilience of complex lipid bilayers against solubilization.

Allosteric modulation of integral protein activity by differential stress in asymmetric membranes

The activity of integral membrane proteins is tightly coupled to the properties of the surrounding lipid matrix. In particular, transbilayer asymmetry, a hallmark of all plasma membranes, might be exploited to control membrane-protein activity. Here, we hypothesized that the membrane-embedded enzyme outer membrane phospholipase A (OmpLA) is susceptible to the lateral pressure differences that build up between such asymmetric membrane leaflets. Upon reconstituting OmpLA into synthetic, chemically well-defined phospholipid bilayers exhibiting different lateral pressure profiles, we indeed observed a substantial decrease in the enzyme's hydrolytic activity with increasing membrane asymmetry. No such effects were observed in symmetric mixtures of the same lipids. To quantitatively rationalize how the differential stress in asymmetric lipid bilayers inhibits OmpLA, we developed a simple allosteric model within the lateral pressure framework. Thus, we find that membrane asymmetry can serve as the dominant factor in controlling membrane-protein activity, even in the absence of specific, chemical cues or other physical membrane determinants such as hydrophobic mismatch.

Antimicrobial peptide activity in asymmetric bacterial membrane mimics

We report on the response of asymmetric lipid membranes composed of palmitoyl oleoyl phosphatidylethanolamine and palmitoyl oleoyl phosphatidylglycerol, to interactions with the frog peptides L18W-PGLa and magainin 2 (MG2a), as well as the lactoferricin derivative LF11-215. In particular we determined the peptide-induced lipid flip-flop, as well as membrane partitioning of L18W-PGLa and LF11-215, and vesicle dye-leakage induced by L18W-PGLa. The ability of L18W-PGLa and MG2a to translocate through the membrane appears to correlate with the observed lipid flip-flop, which occurred at the fastest rate for L18W-PGLa. The higher structural flexibility of LF11-215 in turn allows this peptide to insert into the bilayers without detectable changes of membrane asymmetry. The increased vulnerability of asymmetric membranes to L18W-PGLa in terms of permeability, appears to be a consequence of tension differences between the compositionally distinct leaflets, but not due to increased peptide partitioning.

Intrinsic lipid curvatures of mammalian plasma membrane outer leaflet lipids and ceramides

We developed a global X-ray data analysis method to determine the intrinsic curvatures of lipids hosted in inverted hexagonal phases. In particular, we combined compositional modelling with molecular shape-based arguments to account for non-linear mixing effects of guest-in-host lipids on intrinsic curvature. The technique was verified by all-atom molecular dynamics simulations and applied to sphingomyelin and a series of phosphatidylcholines and ceramides with differing composition of the hydrocarbon chains. We report positive lipid curvatures for sphingomyelin and all phosphatidylcholines with disaturated and monounsaturated hydrocarbons. Phosphatidylcholines with diunsaturated hydrocarbons in turn yielded intrinsic lipid curvatures with negative values. All ceramides, with chain lengths varying between C2:0 and C24:0, displayed significant negative lipid curvature values. Moreover, we report non-additive mixing for C2:0 ceramide and sphingomyelin. This suggests for sphingolipids that in addition to lipid headgroup and hydrocarbon chain volumes also lipid-specific interactions are important contributors to membrane curvature stress.

Structure and Interdigitation of Chain-Asymmetric Phosphatidylcholines and Milk Sphingomyelin in the Fluid Phase

We addressed the frequent occurrence of mixed-chain lipids in biological membranes and their impact on membrane structure by studying several chain-asymmetric phosphatidylcholines and the highly asymmetric milk sphingomyelin. Specifically, we report trans-membrane structures of the corresponding fluid lamellar phases using small-angle X-ray and neutron scattering, which were jointly analyzed in terms of a membrane composition-specific model, including a headgroup hydration shell. Focusing on terminal methyl groups at the bilayer center, we found a linear relation between hydrocarbon chain length mismatch and the methyl-overlap for phosphatidylcholines, and a non-negligible impact of the glycerol backbone-tilting, letting the sn1-chain penetrate deeper into the opposing leaflet by half a CH2 group. That is, penetration-depth differences due to the ester-linked hydrocarbons at the glycerol backbone, previously reported for gel phase structures, also extend to the more relevant physiological fluid phase, but are significantly reduced. Moreover, milk sphingomyelin was found to follow the same linear relationship suggesting a similar tilt of the sphingosine backbone. Complementarily performed molecular dynamics simulations revealed that there is always a part of the lipid tails bending back, even if there is a high interdigitation with the opposing chains. The extent of this back-bending was similar to that in chain symmetric bilayers. For both cases of adaptation to chain length mismatch, chain-asymmetry has a large impact on hydrocarbon chain ordering, inducing disorder in the longer of the two hydrocarbons.

Lipid monolayer spontaneous curvatures: A collection of published values

Lipid monolayer spontaneous curvatures (or lipid intrinsic curvatures) are one of several material properties of lipids that enable the stored curvature elastic energy in a lipid aggregate to be determined. Stored curvature elastic energy is important since it can modulate the function of membrane proteins and plays a role in the regulatory pathways of phospholipid homeostasis. Due to the large number of different lipid molecules that might theoretically exist in nature, very few lipid spontaneous curvatures have been determined. Herein the values of lipid spontaneous curvatures that exist in the literature are collected, alongside key experimental details. Where possible, trends in the data are discussed and finally, obvious gaps in the knowledge are signposted.

Increasing complexity in small-angle X-ray and neutron scattering experiments: from biological membrane mimics to live cells

Small-angle X-ray and neutron scattering are well-established, non-invasive experimental techniques to interrogate global structural properties of biological membrane mimicking systems under physiologically relevant conditions. Recent developments, both in bottom-up sample preparation techniques for increasingly complex model systems, and in data analysis techniques have opened the path toward addressing long standing issues of biological membrane remodelling processes. These efforts also include emerging quantitative scattering studies on live cells, thus enabling a bridging of molecular to cellular length scales. Here, we review recent progress in devising compositional models for joint small-angle X-ray and neutron scattering studies on diverse membrane mimics - with a specific focus on membrane structural coupling to amphiphatic peptides and integral proteins - and live Escherichia coli. In particular, we outline the present state-of-the-art in small-angle scattering methods applied to complex membrane systems, highlighting how increasing system complexity must be followed by an advance in compositional modelling and data-analysis tools.

Structural Characterization of Biomaterials by Means of Small Angle X-rays and Neutron Scattering (SAXS and SANS), and Light Scattering Experiments

Scattering techniques represent non-invasive experimental approaches and powerful tools for the investigation of structure and conformation of biomaterial systems in a wide range of distances, ranging from the nanometric to micrometric scale. More specifically, small-angle X-rays and neutron scattering and light scattering techniques represent well-established experimental techniques for the investigation of the structural properties of biomaterials and, through the use of suitable models, they allow to study and mimic various biological systems under physiologically relevant conditions. They provide the ensemble averaged (and then statistically relevant) information under in situ and operando conditions, and represent useful tools complementary to the various traditional imaging techniques that, on the contrary, reveal more local structural information. Together with the classical structure characterization approaches, we introduce the basic concepts that make it possible to examine inter-particles interactions, and to study the growth processes and conformational changes in nanostructures, which have become increasingly relevant for an accurate understanding and prediction of various mechanisms in the fields of biotechnology and nanotechnology. The upgrade of the various scattering techniques, such as the contrast variation or time resolved experiments, offers unique opportunities to study the nano- and mesoscopic structure and their evolution with time in a way not accessible by other techniques. For this reason, highly performant instruments are installed at most of the facility research centers worldwide. These new insights allow to largely ameliorate the control of (chemico-physical and biologic) processes of complex (bio-)materials at the molecular length scales, and open a full potential for the development and engineering of a variety of nano-scale biomaterials for advanced applications.

Vitamin E Promotes the Inverse Hexagonal Phase via a Novel Mechanism: Implications for Antioxidant Role

Vitamin E (α-tocopherol) and a range of other biological compounds have long been known to promote the H_II (inverted hexagonal) phase in lipids. Now, it has been well established that purely hydrophobic lipids such as dodecane promote the H_II phase by relieving extensive packing stress. They do so by residing deep within the hydrocarbon core. However, we argue from X-ray diffraction data obtained with 1-palmitoyl-2-oleoylphosphatidylcholine (POPE) and 1,2-dioleoylphosphatidylcholine (DOPE) that α-tocopherol promotes the H_II phase by a different mechanism. The OH group on the chromanol moiety of α-tocopherol anchors it near the aqueous interface. This restriction combined with the relatively short length of α-tocopherol (as compared to DOPE and POPE) means that α-tocopherol promotes the H_II phase by relieving compressive packing stress. This observation offers new insight into the nature of packing stress and lipid biophysics. With the deeper understanding of packing stress offered by our results, we also explore the role that molecular structure plays in the primary function of vitamin E, which is to prevent the oxidation of polyunsaturated membrane lipids.