Bilayer nanotubes and helical ribbons formed by hydrated galactosylceramides: acyl chain and headgroup effects

The HEAT repeat protein HPO-27 is a lysosome fission factor

Lysosomes are degradation and signalling centres crucial for homeostasis, development and ageing1. To meet diverse cellular demands, lysosomes remodel their morphology and function through constant fusion and fission2,3. Little is known about the molecular basis of fission. Here we identify HPO-27, a conserved HEAT repeat protein, as a lysosome scission factor in Caenorhabditis elegans. Loss of HPO-27 impairs lysosome fission and leads to an excessive tubular network that ultimately collapses. HPO-27 and its human homologue MROH1 are recruited to lysosomes by RAB-7 and enriched at scission sites. Super-resolution imaging, negative-staining electron microscopy and in vitro reconstitution assays reveal that HPO-27 and MROH1 self-assemble to mediate the constriction and scission of lysosomal tubules in worms and mammalian cells, respectively, and assemble to sever supported membrane tubes in vitro. Loss of HPO-27 affects lysosomal morphology, integrity and degradation activity, which impairs animal development and longevity. Thus, HPO-27 and MROH1 act as self-assembling scission factors to maintain lysosomal homeostasis and function.

Glucosylceramide Associated with Gaucher Disease Forms Amyloid-like Twisted Ribbon Fibrils That Induce α-Synuclein Aggregation

A major risk factor for Gaucher's disease is loss of function mutations in the GBA1 gene that encodes lysosomal β-glucocerebrosidase, resulting in accumulation of glucosylceramide (GlcCer), a key lysosomal sphingolipid. GBA1 mutations also enhance the risk for Parkinson's disease, whose hallmark is the aggregation of α-synuclein (αSyn). However, the role of accumulated GlcCer in αSyn aggregation is not completely understood. Using various biophysical assays, we demonstrate that GlcCer self-assembles to form amyloid-like fibrillar aggregates in vitro. The GlcCer assemblies are stable in aqueous media of different pH and exhibit a twisted ribbon-like structure. Near lysosomal pH GlcCer aggregates induced αSyn aggregation and stabilized its nascent oligomers. We found that several bona fide inhibitors of proteinaceous amyloids effectively inhibited aggregation of GlcCer. This study contributes to the growing evidence of cross-talk between proteinaceous amyloids and amyloid-like aggregates of metabolites accumulated in diseases and suggests these aggregates as therapeutic targets.

Wrapping axons in mammals and Drosophila: Different lipids, same principle

Plasma membranes of axon-wrapping glial cells develop specific cylindrical bilayer membranes that surround thin individual axons or axon bundles. Axons are wrapped with single layered glial cells in lower organisms whereas in the mammalian nervous system, axons are surrounded with a characteristic complex multilamellar myelin structure. The high content of lipids in myelin suggests that lipids play crucial roles in the structure and function of myelin. The most striking feature of myelin lipids is the high content of galactosylceramide (GalCer). Serological and genetic studies indicate that GalCer plays a key role in the formation and function of the myelin sheath in mammals. In contrast to mammals, Drosophila lacks GalCer. Instead of GalCer, ceramide phosphoethanolamine (CPE) has an important role to ensheath axons with glial cells in Drosophila. GalCer and CPE share similar physical properties: both lipids have a high phase transition temperature and high packing, are immiscible with cholesterol and form helical liposomes. These properties are caused by both the strong headgroup interactions and the tight packing resulting from the small size of the headgroup and the hydrogen bonds between lipid molecules. These results suggest that mammals and Drosophila wrap axons using different lipids but the same conserved principle.

Modulation of tubular microstructural self-assembly in galactosylceramides: Influence of N-linked fatty acyl chains

Our recent studies have indicated that aqueous dispersions of galactosylceramides (GalCers) containing nervonoyl [24:1Δ15(CiS)] acyl chains, which constitute the major unsaturated acyl moieties in bovine brain GalCer, form tubular bilayers. In an effort to define the structural parameters that modulate tubule formation, we synthesized a series.of GalCers having successively shortercismonounsaturated acyl chains than the tubule-forming 24:1 GalCer and investigated the resulting microstructural self-assemblies by freeze fracture and deep etch electron microscopy.

GalCers having W-linked erucic [22:1Δ13(CiS)], eicosenoic [20:Δ11(CiS)], or oleic [18:Δ9(CiS)] acyl chains were synthesized and purified as previously reported for 24:1 GalCer. Dispersions were prepared by hydrating dry lipid at >90°C with phosphate buffer (pH 6.6) followed by vigorous vortexing and three freeze-thaw cycles. Samples were cryo-fixed from room temperature (22°C) by plunging into liquid propane cooled by liquid nitrogen, were fractured in a Balzers 300 freeze fracture apparatus at -120°C, and were etched for 6 min. at -100°C.

Cryo-EM structure of the MinCD copolymeric filament from Pseudomonas aeruginosa at 3.1 Å resolution

Positioning of the division site in many bacterial species relies on the MinCDE system, which prevents the cytokinetic Z-ring from assembling anywhere but the mid-cell, through an oscillatory diffusion-reaction mechanism. MinD dimers bind to membranes and, via their partner MinC, inhibit the polymerization of cell division protein FtsZ into the Z-ring. MinC and MinD form polymeric assemblies in solution and on cell membranes. Here, we report the high-resolution cryo-EM structure of the copolymeric filaments of Pseudomonas aeruginosa MinCD. The filaments consist of three protofilaments made of alternating MinC and MinD dimers. The MinCD protofilaments are almost completely straight and assemble as single protofilaments on lipid membranes, which we also visualized by cryo-EM.

Structural basis for the clamping and Ca2+ activation of SNARE-mediated fusion by synaptotagmin

Synapotagmin-1 (Syt1) interacts with both SNARE proteins and lipid membranes to synchronize neurotransmitter release to calcium (Ca2+) influx. Here we report the cryo-electron microscopy structure of the Syt1-SNARE complex on anionic-lipid containing membranes. Under resting conditions, the Syt1 C2 domains bind the membrane with a magnesium (Mg2+)-mediated partial insertion of the aliphatic loops, alongside weak interactions with the anionic lipid headgroups. The C2B domain concurrently interacts the SNARE bundle via the 'primary' interface and is positioned between the SNAREpins and the membrane. In this configuration, Syt1 is projected to sterically delay the complete assembly of the associated SNAREpins and thus, contribute to clamping fusion. This Syt1-SNARE organization is disrupted upon Ca2+-influx as Syt1 reorients into the membrane, likely displacing the attached SNAREpins and reversing the fusion clamp. We thus conclude that the cation (Mg2+/Ca2+) dependent membrane interaction is a key determinant of the dual clamp/activator function of Synaptotagmin-1.

Formation of tubules and helical ribbons by ceramide phosphoethanolamine-containing membranes

Ceramide phosphoethanolamine (CPE), a major sphingolipid in invertebrates, is crucial for axonal ensheathment in Drosophila. Darkfield microscopy revealed that an equimolar mixture of bovine buttermilk CPE (milk CPE) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (diC18:1 PC) tends to form tubules and helical ribbons, while pure milk CPE mainly exhibits amorphous aggregates and, at low frequency, straight needles. Negative staining electron microscopy indicated that helices and tubules were composed of multilayered 5–10 nm thick slab-like structures. Using different molecular species of PC and CPE, we demonstrated that the acyl chain length of CPE but not of PC is crucial for the formation of tubules and helices in equimolar mixtures. Incubation of the lipid suspensions at the respective phase transition temperature of CPE facilitated the formation of both tubules and helices, suggesting a dynamic lipid rearrangement during formation. Substituting diC18:1 PC with diC18:1 PE or diC18:1 PS failed to form tubules and helices. As hydrated galactosylceramide (GalCer), a major lipid in mammalian myelin, has been reported to spontaneously form tubules and helices, it is believed that the ensheathment of axons in mammals and Drosophila is based on similar physical processes with different lipids.

CNS myelin sheath is stochastically built by homotypic fusion of myelin membranes within the bounds of an oligodendrocyte process

Myelin - the multilayer membrane that envelops axons - is a facilitator of rapid nerve conduction. Oligodendrocytes form CNS myelin; the prevailing hypothesis being that they do it by extending a process that circumnavigates the axon. It is pertinent to ask how myelin is built because oligodendrocyte plasma membrane and myelin are compositionally different. To this end, we examined oligodendrocyte cultures and embryonic avian optic nerves by electron microscopy, immuno-electron microscopy and three-dimensional electron tomography. The results support three novel concepts. Myelin membranes are synthesized as tubules and packaged into "myelinophore organelles" in the oligodendrocyte perikaryon. Myelin membranes are matured in and transported by myelinophore organelles within an oligodendrocyte process. The myelin sheath is generated by myelin membrane fusion inside an oligodendrocyte process. These findings abrogate the dogma of myelin resulting from a wrapping motion of an oligodendrocyte process and open up new avenues in the quest for understanding myelination in health and disease.

Self-assembly of polyisoprenoyl gemcitabine conjugates: influence of supramolecular organization on their biological activity

An amphiphilic prodrug of gemcitabine, a cytidine analogue used clinically against various tumors, had been previously synthesized by covalent coupling to squalene, a natural isoprenoid chain. The resulting bioconjugate self-assembled spontaneously in water as nanoparticles, displaying an impressive activity both in vitro and in vivo. The aim of the present study was to determine the influence of the length of the isoprene moiety on the structure of the nanoparticles, in an attempt to establish a relationship between the chemical structure of the prodrug, its supramolecular organization, and its pharmacological activity. Remarkably, gemcitabine-squalene and gemcitabine-5-isoprenes, which differ only in the position of two methyl groups on the hydrophobic chain, displayed different supramolecular organizations and different anticancer activities on some cell lines. This difference in activity was related to the ability of nanoparticles to be internalized by cells.

Influence of intracellular membrane pH on sphingolipid organization and membrane biophysical properties

Glucosylceramide (GlcCer) is a signaling lipid involved in the regulation of several cellular processes. It is present in different organelles, including the plasma membrane, Golgi apparatus, endoplasmic reticulum, and lysosomes. Accordingly, GlcCer is exposed to different pH environments in each organelle, which may lead to alterations in its properties and lateral organization and subsequent biological outcome. In this study, we addressed the effect of pH on the biophysical behavior of this lipid and other structurally related sphingolipids (SLs). Membranes composed of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and C16-GlcCer, sphingomyelin, and different acyl chain ceramides were characterized by fluorescence spectroscopy, confocal microscopy, and surface pressure-area measurements under neutral and acidic conditions. The results show that changing the pH from 7.4 to 5.5 has a larger impact on C16-GlcCer-containing membranes compared to other SLs. In addition, acidification mainly affects the organization and packing properties of the GlcCer-enriched gel phase, suggesting that the interactions established by the glucose moiety, in the GlcCer molecule, are those most affected by the increase in the acidity. These results further highlight the role of GlcCer as a modulator of membrane biophysical properties and will possibly contribute to the understanding of its biological function in different organelles.

High yield formation of lipid bilayer shells around silicon nanowires in aqueous solution

The combination of nanoscaled materials and biological self-assembly is a key step for the development of novel approaches for biotechnology and bionanoelectronic devices. Here we propose a route to merge these two subsystems and report on the formation of highly concentrated aqueous solutions of silanized silicon nanowires wrapped in a lipid bilayer shell. We developed protocols and investigated the dynamics of lipid films on both planar surfaces and silicon nanowires using fluorescence recovery after photobleaching, demonstrating fully intact and fluid bilayers without the presence of a lipid molecule reservoir. Finally, the experimental setup allowed for in situ observation of spontaneous bilayer formation around the nanowire by lipid diffusion from a vesicle to the nanowire. Such aqueous solutions of lipid coated nanowires are a versatile tool for characterization purposes and are relevant for newly emerging bioinspired electronics and nanosensorics.

Fabrication of microspheres via solvent volatization induced aggregation of self-assembled nanomicellar structures and their use as a pH-dependent drug release system

A series of oleamide derivatives, (C(18)H(34)NO)(2)(CH(2))(n) [n = 2 (1a), 3 (1b), 4 (1c), or 6 (1d); C(18)H(34)NO = oleic amide fragment] and (C(18)H(34)NO)(CH(2))(6)NH(2) (2), have been synthesized and their self-assembly is investigated in ethanol/water media. Self-assembly of 1a and 1b in ethanol/water (1/0.1 v/v) solution (5 mg mL(-1)) yields microspheres (MSs) with the average diameter ∼10 μm via a gradual temperature reduction and solvent volatilization process. Under the same self-assembly conditions, microrods (average diameter ∼6 μm and several tens of micrometers in length), micronecklace-like, and shape-irregular microparticles are formed from the self-assembly of 1c, 1d, and 2, respectively. The kinetics of evolution for their self-assemblies by dynamic light scattering technique and in situ observation by optical microscopy reveals that the microstructures formation is from a well-behaved aggregation of nanoscale micelles induced by solvent volatilization. The FT-IR and temperature-dependent (1)H-NMR spectra demonstrate the hydrogen bonding force and π-π stacking, which drove the self-assembly of all oleamide derivatives in ethanol/water. Among the fabricated microstructures, the MSs from 1a exhibit the best dispersity, which thus have been used as a scaffold for the in vitro release of doxorubicin. The results demonstrate a pH-sensitive release process, enhanced release specifically at low pH 5.2.

Bionanoelectronics

Every cell in a living organisms performs a complex array of functions using a vast arsenal of proteins, ion channels, pumps, motors, signaling molecules, and cargo carriers. With all the progress that humankind has made to date in the development of sophisticated machinery and computing capabilities, understanding and communicating with living systems on that level of complexity lags behind. A breakthrough in these capabilities could only come if a way is found to integrate biological components into artificial devices. The central obstacle for this vision of bionanoelectronics is the absence of a versatile interface that facilitates two-way communication between biological and electronic structures. 1D nanomaterials, such as nanotubes and nanowires, open up the possibility of constructing tight interfaces that could enable such bidirectional flow of information. This report discusses the overall progress in building such interfaces on the level of individual proteins and whole cells and focuses on the latest efforts to create device platforms that integrate membrane proteins, channels, and pumps with nanowire bioelectronics.

Direct visualization of the lateral structure of porcine brain cerebrosides/POPC mixtures in presence and absence of cholesterol

We studied the thermal behavior of membranes composed of mixtures of natural cerebrosides (from porcine brain) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with and without cholesterol, using differential scanning calorimetry, Fourier transform infrared spectroscopy, and confocal/multiphoton fluorescence microscopy. The POPC/cerebroside mixture display solid ordered/liquid disordered phase coexistence in a broad range of compositions and temperatures in agreement with previous results reported for POPC/(bovine brain)cerebrosides. The observed phase coexistence scenario consists of elongated, micrometer-sized cerebroside-rich solid ordered domains that span the bilayer, embedded in a POPC-rich liquid disordered phase. The data obtained from differential scanning calorimetry and Fourier transform infrared spectroscopy was in line with that obtained in the microscopy experiments for the binary mixture, except at very high cerebroside molar fractions (0.8-0.9) were some differences are observed. Cholesterol incorporation exerts strong changes on the lateral organization of POPC/porcine brain cerebroside membranes. At intermediate cholesterol concentrations (10-25 mol %) the solid ordered/liquid disordered phase coexistence scenario gradually transform to a solid ordered/liquid ordered one. Above 25 mol % of cholesterol two distinct regions with liquid ordered phase character are visualized in the membrane until a single liquid ordered phase forms at 40 mol % cholesterol. The observed cholesterol effect largely differs from that reported for POPC/porcine brain ceramide, reflecting the impact of the sphingolipids polar headgroup on the membrane lateral organization.

Undulating tubular liposomes through incorporation of a synthetic skin ceramide into phospholipid bilayers

Nonspherical liposomes were prepared by doping L-alpha-phosphatidylcholine (PC) with ceramide VI (a skin lipid). Cryo-transmission electron microscopy shows the liposome shape changing from spherical to an undulating tubular morphology, when the amount of ceramide VI is increased. The formation of tubular liposomes is energetically favorable and is attributed to the association of ceramide VI with PC creating regions of lower curvature. Since ceramides are the major component of skin lipids in the stratum corneum, tubular liposomes containing ceramide may potentially serve as self-enhanced nanocarriers for transdermal delivery.

Thermotropic behavior and lateral distribution of very long chain sphingolipids

Sphingolipids containing very long acyl chains are abundant in certain specialized tissues and minor components of plasma membranes in most mammalian cells. There are cellular processes in which these sphingolipids are required, and the function seems to be mediated through sphingolipid-rich membrane domains. This study was conducted to explore how very long acyl chains of sphingolipids influence their lateral distribution in membranes. Differential scanning calorimetry showed that 24:0- and 24:1-sphingomyelins, galactosylceramides and glucosylceramides exhibited complex thermotropic behavior and partial miscibility with palmitoyl sphingomyelin. The T(m) was decreased by about 20 degrees C for all 24:1-sphingolipids compared to the corresponding 24:0-sphingolipids. The ability to pack tightly with ordered and extended acyl chains is a necessity for membrane lipids to partition into ordered domains in membranes and thus the 24:1-sphingolipids appeared less likely to do so. Fluorescence quenching measurements showed that the 24:0-sphingolipids formed ordered domains in multicomponent membranes, both as the only sphingolipid and mixed with palmitoyl sphingomyelin. These domains had a high packing density which appeared to hinder the partitioning of sterols into them, as reported by the fluorescent cholesterol analog cholestatrienol. 24:0-SM was, however, better able to accommodate sterol than the glycosphingolipids. The 24:1-sphingolipids could, depending on head group structure, either stabilize or disrupt ordered sphingolipid/cholesterol domains. We conclude that very long chain sphingolipids, when present in biological membranes, may affect the physical properties of or the distribution of sterols between lateral domains. It was also evident that not only the very long acyl chain but also the specific molecular structure of the sphingolipids was of importance for their membrane properties.

Membrane domain formation, interdigitation, and morphological alterations induced by the very long chain asymmetric C24:1 ceramide

Ceramide (Cer) is involved in the regulation of several biological processes, such as apoptosis and cell signaling. The alterations induced by Cer in the biophysical properties of membranes are thought to be one of the major routes of Cer action. To gain further knowledge about the alterations induced by Cer, membrane reorganization by the very long chain asymmetric nervonoylceramide (NCer) was studied. The application of an established fluorescence multiprobe approach, together with x-ray diffraction, differential scanning calorimetry, and confocal fluorescence microscopy, allowed the characterization of NCer and the determination of the phase diagram of palmitoyloleoylphosphatidylcholine (POPC)/NCer binary mixtures. Nervonoylceramide undergoes a transition from a mixed interdigitated gel phase to a partially interdigitated gel phase at approximately 20 degrees C, and a broad main transition to the fluid phase at approximately 52 degrees C. The solubility of NCer in the fluid POPC is low, driving gel-fluid phase separation, and the binary-phase diagram is characterized by multiple and large coexistence regions between the interdigitated gel phases and the fluid phase. At 37 degrees C, the relevant phases are the fluid and the partially interdigitated gel. Moreover, the formation of NCer interdigitated gel phases leads to strong morphological alterations in the lipid vesicles, driving the formation of cochleate-type tubular structures.

Exotic aqueous behavior of synthetic lipids: formation of vesicular nanotubes

The work reported herein deals with the synthesis and the aggregation behavior studies of synthetic lipids bearing a non-ionic polar head made up of a tris(hydroxymethyl) aminomethane (tris) moiety linked with an aminoglycerol interface. The hydrophobic chains with variable lengths were grafted onto the hydroxyl functions of the aminoglycerol residue through ester or carbamate bonds. Tiny chemical modifications within this family of non-ionic surfactants brought about major variations in their aggregation behavior. They formed vesicles, tubules, and also small stable end-capped tubules - called vesicular nanotubes -, when the polar head bore two heptadecyl chains linked through a carbamate bond. Various techniques (nanosizer measurements, freeze fracture electron microscopy (FFEM), transmission electron microscopy (TEM), carboxyfluorescein (CF)) encapsulation were used to specify the structure of these assemblies. Notably, the vesicular nanotubes exhibited a small size, a fair polydispersity, great stability in an aqueous solution (up to 1 year) and a good efficiency to entrap and slowly release a probe such as carboxyfluoresceine: all these properties are perfectly suitable for their use as potential drug carriers.

Functional One-Dimensional Lipid Bilayers on Carbon Nanotube Templates

Use of biological machines and environments in novel bioinorganic nanostructures is critical for development of new types of biosensors, bio-NEMS devices, and functional materials. Lipid bilayers that mimic a cell membrane have already played an important role in such applications. We present supported lipid bilayers that spontaneously assemble in a continuous nanoshell around a template of a carbon nanotube wrapped with hydrophilic polymer cushion layers. We demonstrate that such 1-D lipid membranes are fluid and can heal defects, even over repeated damage-recovery cycles. A simple diffusion model can describe mobility of lipid molecules in these 1-D nanoshells. These structures could lead to the development of new classes of biosensors and bioelectronic devices.

Molecular structure of glucopyranosylamide lipid and nanotube morphology

A series of glucopyranosylamide lipids, N-(X-octadecenoyl)-beta-D-glucopyranosylamine [X = 13-cis (1), 11-cis (2), 9-cis (3), 6-cis (4), and 9-cis,12-cis (5)] and their saturated homologue N-octadecanoyl-beta-d-glucopyranosylamine (6), which differ in the position of a cis double bond in the C18 hydrocarbon chains, have been synthesized. The effect of the cis double bond position on the chiral self-assembly of each glycolipid has been examined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV, and circular dichroism (CD). The 11-cis derivative 2 was observed to self-assemble in water to form a uniform hollow cylinder structure with about 200-nm outer diameters in >98% yields. The obtained nanotubes from 2 showed the narrowest distribution of outer diameters and also gave a negative CD band around 234-236 nm, showing the largest CD intensity among the glycolipids investigated. Thus, we found that the position of a cis double bond significantly influences the homogeneity of the outer diameters as well as growth behavior of the self-assembled nanotube structures. Chiral molecular packing driven by a possible bending structure of the unsaturated glycolipids is playing a critical role in determining tubular morphology through molecular self-assembly.

Partitioning of pyrene-labeled phospho- and sphingolipids between ordered and disordered bilayer domains

Here we have studied how the length of the pyrene-labeled acyl chain (n) of a phosphatidylcholine, sphingomyelin, or galactosylceramide affects the partitioning of these lipids between 1), gel and fluid domains coexisting in bovine brain sphingomyelin (BB-SM) or BB-SM/spin-labeled phosphatidylcholine (PC) bilayers or 2), between liquid-disordered and liquid-ordered domains in BB-SM/spin-labeled PC/cholesterol bilayers. The partitioning behavior was deduced either from modeling of pyrene excimer/monomer ratio versus temperature plots, or from quenching of the pyrene monomer fluorescence by spin-labeled PC. New methods were developed to model excimer formation and pyrene lipid quenching in segregated bilayers. The main result is that partition to either gel or liquid-ordered domains increased significantly with increasing length of the labeled acyl chain, probably because the pyrene moiety attached to a long chain perturbs these ordered domains less. Differences in partitioning were also observed between phosphatidylcholine, sphingomyelin, and galactosylceramide, thus indicating that the lipid backbone and headgroup-specific properties are not severely masked by the pyrene moiety. We conclude that pyrene-labeled lipids could be valuable tools when monitoring domain formation in model and biological membranes as well as when assessing the role of membrane domains in lipid trafficking and sorting.

Interactions between glucosylceramide and galactosylceramide I(3) sulfate and microstructures formed

The monohexoside glycosphingolipids (GSLs), galactosylceramide (GalC), glucosylceramide (GluC), and their sulfated forms are abundant in cell membranes from a number of tissues. Carbohydrate-carbohydrate interactions between the head groups of some GSLs can occur across apposed membranes and may be involved in cell-cell interactions. In the present study, the ability of GluC to participate in trans interactions with galactosylceramide I(3) sulfate (CBS) was investigated by transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. Gaucher's spleen GluC had polymorphic phase behavior; in its metastable state, it formed large wrinkled vesicles. It transformed to a stable state via an intermediate state in which the surface of the vesicles consisted of narrow ribbons. In the stable state, the narrow ribbons split off from the surface to form membrane fragments and flat and helical ribbons. The strength of the intermolecular hydrogen bonding interactions between the carbonyls increased in the order metastable<intermediate<stable state. Aqueous dispersions of GluC and CBS were combined to allow trans carbohydrate-carbohydrate interaction across apposed bilayers, or the lipids were premixed in a solvent before hydration to allow lateral cis interactions. Premixed dispersions of NFA-GluC and CBS remained in the metastable state even when incubated under stable state conditions. When NFA-GluC dispersions were combined with CBS dispersions, they had a small effect on each other's amide groups in the metastable state. Furthermore, conversion of NFA-GluC to the stable state was inhibited, although it reached the intermediate state, suggesting that some degree of trans interaction between these two lipids occurred.

Nanotubules formed by highly hydrophobic amphiphilic alpha-helical peptides and natural phospholipids

We previously reported that the 18-mer amphiphilic alpha-helical peptide, Hel 13-5, consisting of 13 hydrophobic residues and five hydrophilic amino acid residues, can induce neutral liposomes (egg yolk phosphatidylcholine) to adopt long nanotubular structures and that the interaction of specific peptides with specific phospholipid mixtures induces the formation of membrane structures resembling cellular organelles such as the Golgi apparatus. In the present study we focused our attention on the effects of peptide sequence and chain length on the nanotubule formation occurring in mixture systems of Hel 13-5 and various neutral and acidic lipid species by means of turbidity measurements, dynamic light scattering measurements, and electron microscopy. We designed and synthesized two sets of Hel 13-5 related peptides: 1) Five peptides to examine the role of hydrophobic or hydrophilic residues in amphiphilic alpha-helical structures, and 2) Six peptides to examine the role of peptide length, having even number residues from 12 to 24. Conformational, solution, and morphological studies showed that the amphiphilic alpha-helical structure and the peptide chain length (especially 18 amino acid residues) are critical determinants of very long tubular structures. A mixture of alpha-helix and beta-structures determines the tubular shapes and assemblies. However, we found that the charged Lys residues comprising the hydrophilic regions of amphiphilic structures can be replaced by Arg or Glu residues without a loss of tubular structures. This suggests that the mechanism of microtubule formation does not involve the charge interaction. The immersion of the hydrophobic part of the amphiphilic peptides into liposomes initially forms elliptic-like structures due to the fusion of small liposomes, which is followed by a transformation into tubular structures of various sizes and shapes.

Induction of neuron-like tubes and liposome networks by cooperative effect of gangliosides and phospholipids

Although there is a rather large abundance of gangliosides in neurons, their functional role is still unclear. We focused on a physicochemical role of gangliosides in the formation of tubular structures, such as axons or dendrites in neurons. When a ganglioside, GM3, was added to cell-size liposomes that consisted of dioleoylphosphatidyl-choline, tubular structures were induced and liposome networks connected by the tubes were observed by differential interference microscopy and fluorescence microscopy. The potential for various gangliosides to induce tubes was dependent on the structures of their hydrophilic head group. With a large excess of gangliosides, the tubes are destabilized and small fragments, or micelles, are generated. The phenomenon was suggested by physical model calculation. Gangliosides may play a role as building material in neural unique tubular structures.

Lactosylceramide: effect of acyl chain structure on phase behavior and molecular packing

Lactosylceramide (LacCer) is a pivotal intermediate in the metabolism of higher gangliosides, localizes to sphingolipid-sterol "rafts," and has been implicated in cellular signaling. To provide a fundamental characterization of LacCer phase behavior and intermolecular packing, LacCer containing different saturated (16:0, 18:0, 24:0) or monounsaturated (18:1(Delta9), 24:1(Delta15)) acyl chains were synthesized and studied by differential scanning calorimetry and Langmuir film balance approaches. Compared to related sphingoid- and glycerol-based lipids, LacCers containing saturated acyl chains display relatively high thermotropic and pressure-induced transitions. LacCer monolayer films are less elastic in an in-plane sense than sphingomyelin films, but are somewhat more elastic than galactosylceramide films. Together, these findings indicate that the disaccharide headgroup only marginally disrupts gel phase packing and orients more perpendicular than parallel to the interface. This contrasts the reported behavior of digalactosyldiglycerides with saturated acyl chains. Introducing single cis double bonds into the LacCer acyl chains dramatically lowers the high thermotropic and pressure-induced transitions. Greater reductions occur when cis double bonds are located near the middle of the acyl chains. The results are discussed in terms of how an extended disaccharide headgroup can enhance interactions among naturally abundant LacCers with saturated acyl chains.

Elongated supramolecular assemblies in drug delivery

This review presents different lipid-based elongated microstructures: tubules, cochleate cylinders and ribbons. Their composition, process of preparation and the mechanism behind their formation is discussed as well as their use as a drug delivery system.

Snap-shots of clathrin-mediated endocytosis

Clathrin-mediated endocytosis is one of the major entry routes into a eukaryotic cell. It is driven by protein components that aid the selection of cargo and provide the mechanical force needed to both deform the plasma membrane and detach a vesicle. Clathrin-coated vesicles were first observed by electron microscopy in the early 1960s. In subsequent years, many of the characteristic intermediates generated during vesicle formation have been trapped and observed. A variety of electron microscopy techniques, from the analysis of sections through cells to the study of endocytic intermediates formed in vitro, have led to the proposition of a sequence of events and of roles for different proteins during vesicle formation. In this article, these techniques and the insights gained are reviewed, and their role in providing snap-shots of the stages of endocytosis in atomic detail is discussed.

Freeze-fracture studies on lipids and membranes

Freeze-fracture electron microscopy is especially useful for investigation of lipid structures by the advantageous fracture course within hydrophobic zones. Freezing is, on the other hand, a restriction because the structures of lamellar and non-lamellar phase states with disordered acyl chains (L(alpha), H(II,) cubic) are difficult to preserve. An important aspect of this method is therefore the lipid structure of phase states with ordered acyl chains (crystal, gel), and with a different degree of hydration. Freeze-fracture of pure lipid systems creates a valid representation of the structure of non-lamellar phases and of the general structure of the "lamellar" lipid bilayer, and lamellar phases with characteristic deformations (ripples, curvatures, plane sectors) can be identified. Fracture through the hydrophobic bilayer centre of biological membranes reveals characteristic protein components, the intramembraneous particles (IMPs). The lateral distribution of the IMPs is a helpful marker for fluid and rigid phase states, also without deformation of the lamella. The overall history and the present state of knowledge concerning the different structures revealed by the freeze-fracture and freeze-etch techniques in lipid systems, and to a limited extent in biological membranes, is reviewed, taking into account studies from our own laboratory.

Two-dimensional crystallogenesis of transmembrane proteins

Two-dimensional crystallogenesis is a crucial step in the long road that leads to the determination of macromolecules structure via electron crystallography. The necessity of having large and highly ordered samples can hold back the resolution of structural works for a long time, and this, despite improvements made in electron microscopes or image processing. Today, finding good conditions for growing two-dimensional crystals still rely on either "biocrystallo-cooks" or on lucky ones. The present review presents the field by first describing the different crystals that one can encounter and the different crystallisation methods used. Then, the effects of different components (such as protein, lipids, detergent, buffer, and temperature) and the different methods (dialysis, hydrophobic adsorption) are discussed. This discussion is punctuated by correspondences made to the world of three-dimensional crystallogenesis. Finally, a guide for setting up 2D crystallogenesis experiments, built on the discussion mentioned before, is proposed to the reader. More than giving recipes, this review is meant to open up the discussions in this field.

Polymorphic phases of galactocerebrosides: spectroscopic evidence of lamellar crystalline structures

Fourier transform infrared spectroscopy was applied to study the structural and thermal properties of bovine brain galactocerebroside (GalCer) containing amide linked non-hydroxylated or alpha-hydroxy fatty acids (NFA- and HFA-GalCer, respectively). Over the temperature range 0-90 degrees C, both GalCer displayed complex thermal transitions, characteristic of polymorphic phase behavior. Upon heating, aqueous dispersions of NFA- and HFA-GalCer exhibited high order-disorder transition temperatures near 80 and 72 degrees C, respectively. En route to the chain melting transition, the patterns of the amide I band of NFA-GalCer were indicative of two different lamellar crystalline phases, whereas those of HFA-GalCer were suggestive of lamellar gel and crystalline bilayers. Cooling from the liquid-crystalline phase resulted in the formation of another crystalline phase of NFA-GalCer and a gel phase of HFA-GalCer, with a phase transition near 62 and 66 degrees C, respectively. Prolonged incubation of GalCer bilayers at 38 degrees C revealed conversions among lamellar crystalline phases (NFA-GalCer) or between lamellar gel and crystalline bilayer structures (HFA-GalCer). Spectral changes indicated that the temperature and/or time induced formation of the lamellar crystalline structures of NFA- and HFA-GalCer was accompanied by partial dehydration and by rearrangements of the hydrogen bonding network and bilayer packing mode of GalCer.

Continuous and highly variable rate controlled release of model drugs from sphingolipid-based complex high axial ratio microstructures

Sphingolipids have been synthesized that contain as polar headgroups, model drugs ester-linked to the primary hydroxyl group of the ceramide core. These lipids, when allowed to self assemble below their chain-melting temperatures, either as single molecular species or in combination with other sphingolipid-derived amphiphiles, are shown to form supramolecular assemblies of varying morphologies including complex high axial ratio microstructures (CHARMs). Within these microstructures, the lipid esters are highly resistant to hydrolysis as compared to the esters dispersed as solitary monomers in aqueous solution or in a matrix of fluid phosphatidylcholine vesicles. The rate of headgroup hydrolysis within CHARMs may be manipulated over a broad range (days to years) by varying the length of the amide-linked fatty acyl chain in the ceramide core or the distance between the ester and the C-1 ceramide of the core. These microstructures, which have exceptionally high surface area display of attached headgroups, may be useful for controlled release of pharmacological agents.

The relationship between the structure of the headgroup of sphingolipids and their ability to form complex high axial ratio microstructures

Ceramides with chemically modified polar headgroups were prepared and examined for their ability to form complex high axial ratio microstructures (CHARMS), potential drug delivery vehicles. In general, if the modified ceramide had either a hydrogen bond donor or acceptor at C-1 and C-3, including hydrophobic or hydrophilic groups attached to C-1 microstructures formed. Tolerated groups include amides, esters, sulfonates, and ethers. If modification at C-3 added significant bulk (greater than four carbons regardless of hydrophilicity), then amorphous aggregates formed. Ceramides with C-1 and C-3 bridged through a cyclic structure also made microstructures. By using a sphingolipid with an amine headgroup, CHARMs may be modified covalently after formation.

Trans interactions between galactosylceramide and cerebroside sulfate across apposed bilayers

The two glycosphingolipids galactosylceramide (GalC) and its sulfated form, cerebroside sulfate (CBS), are present at high concentrations in the multilayered myelin sheath and are involved in carbohydrate-carbohydrate interactions between the lipid headgroups. In order to study the structure of the complex of these two glycolipids by Fourier transform infrared (FTIR) spectroscopy, GalC dispersions were combined with CBS dispersions in the presence and absence of Ca(2+). The FTIR spectra indicated that a strong interaction occurred between these glycolipids even in the absence of Ca(2+). The interaction resulted in dehydration of the sulfate, changes in the intermolecular hydrogen bonding interactions of the sugar and other oxygens, decreased intermolecular hydrogen bonding of the amide C==O of GalC and dehydration of the amide region of one or both of the lipids in the mixture, and disordering of the hydrocarbon chains of both lipids. The spectra also show that Ca(2+) interacts with the sulfate of CBS. Although they do not reveal which other groups of CBS and GalC interact with Ca(2+) or which groups participate in the interaction between the two lipids, they do show that the sulfate is not directly involved in interaction with GalC, since it can still bind to Ca(2+) in the mixture. The interaction between these two lipids could be either a lateral cis interaction in the same bilayer or a trans interaction between apposed bilayers. The type of interaction between the lipids, cis or trans, was investigated using fluorescent and spin-label probes and anti-glycolipid antibodies. The results confirmed a strong interaction between the GalC and the CBS microstructures. They suggested further that this interaction caused the CBS microstructures to be disrupted so that CBS formed a single bilayer around the GalC multilayered microstructures, thus sequestering GalC from the external aqueous phase. Thus the CBS and GalC interacted via a trans interaction across apposed bilayers, which resulted in dehydration of the headgroup and interface region of both lipid bilayers. The strong interaction between these lipids may be involved in stabilization of the myelin sheath.

Modulation of nanotube formation by structural modifications of sphingolipids

Galactosylceramides (GalCers) containing nervonoyl (24:1(Delta15(cis))) acyl chains have the capacity to assemble into nanotubular microstructures in excess water (. Biophys. J. 69:1976-1986). To define the structural parameters that modulate nanotube formation, GalCer derivatives were synthesized that contained cis monounsaturated acyl chains with the formula X:1((X-9)). X indicates the total acyl carbon number (24, 22, 20, or 18), and 1 indicates a single cis double bond, the location of which is designated by the superscript (X-9). Deep etching of freeze-fractured 24:1(Delta15(cis)) GalCer dispersions followed by replica production and transmission electron microscopic analysis confirmed nanotube morphology (25-30-nm diameter). Control experiments revealed that tubule formation was promoted by cooling through the main enthalpic phase transition coupled with repetitive freeze-thaw cycling. Imparting a negative charge to the sugar headgroup of 24:1(Delta15)GalCer via sulfate dramatically altered mesomorpholgy and resulted in myelinic-like, multilamellar structures. Removal of the sugar headgroup (24:1(Delta15)Cer) resulted in flattened cylindrical structures with a cochleate appearance. Compared to these large-scale changes in morphology, more subtle changes were induced by structural changes in the acyl chain of 24:1(Delta15)GalCer. 22:1(Delta13)GalCer dispersions consisted of long, smooth tubules (35-40-nm diameters) with a strong tendency to self-align into bundle-like aggregates. In contrast, the microstructures formed by 20:1(Delta11)GalCer resembled helical ribbons with a right-handed twist. Ribbon widths averaged 30-35 nm, with helical pitches of 80-90 nm. 18:1(Delta9)GalCer displayed a variety of morphologies, including large-diameter multilamellar cylinders and liposome-like structures, as well as stacked, plate-like arrays. The results are discussed within the context of current theories of lipid tubule formation.

Self-assembly of chlorophenols in water

In saturated solutions of some di- and trichlorophenols, structures with complex morphologies, consisting of thin, transparent sheets often coiling into helices and ultimately twisting into filaments, were observed under the optical microscope. Freeze-fracture electron microscopy, x-ray diffraction, phase diagrams, and molecular modeling were performed to elucidate the observed phenomena. Here, we present evidence that the chlorophenols studied, when interacting with water, self-assemble into bilayers. The fact that some chlorophenols form the same supramolecular structures as those described previously for structurally nonrelated surfactants sheds light on the mechanisms of self-assembly.

Thermotropic behavior of galactosylceramides with cis-monoenoic fatty acyl chains

To define the thermotropic behavior of galactosylceramides (GalCer) containing cis monounsaturated acyl chains, N-X:1Delta(X-9) cis galactosylsphingosines (GalSph) were synthesized (where X=24, 22, 20, or 18) and investigated by differential scanning calorimetry (DSC). After hydration of dried glycolipid, aqueous dispersions were prepared by repetitive heating and freeze-thaw cycles. The DSC data clearly showed that introducing a single cis double bond into the acyl chain of GalCer lowers the transition temperature of the main endothermic peak and affects the kinetics of formation of various metastable and stable gel phases. More importantly, the data emphasize the role that double bond location in concert with acyl chain length play in modulating the thermotropic behavior of GalCers. In contrast to the 18:1 GalCer and 20:1 GalCer endotherms which remain unchanged after identical repetitive heating scans and low temperature incubations, the thermotropic responses of 22:1 GalCer and 24:1 GalCer depended directly upon incubation time at lower temperatures following a heating scan. Only after extended incubation (4-5 days) did the endotherms revert to behavior observed during the initial heating scan that followed sample preparation by cyclic heating and freeze-thaw methods. The extended incubation times required for 22:1 GalCer and 24:1 GalCer to assume their more stable packing motifs appear to be consistent with nucleation events that promote transbilayer interdigitation. Yet, due to the slow kinetics of the process, the presence of cis monounsaturation in very long acyl chains that are common to GalCer may effectively inhibit transbilayer lipid interdigitation under physiological conditions.

Lipid nanotubes as substrates for helical crystallization of macromolecules

A general approach for crystallization of proteins in a fast and simple manner would be of immense interest to biologists studying protein structure-function relationships. Here, we describe a method that we have developed for promoting the formation of helical arrays of proteins and macromolecular assemblies. Electron micrographs of the arrays are suitable for helical image analysis and three-dimensional reconstruction. We show that hydrated mixtures of the glycolipid galactosylceramide (GalCer) and derivatized lipids or charged lipids form unilamellar nanotubules. The tubules bind proteins in a specific manner via high affinity ligands on the polar head groups of the lipid or via electrostatic interactions. By doping the GalCer with a novel nickel-containing lipid, we have been able to form helical arrays of two histidine-tagged proteins. Similarly, doping with a biotinylated lipid allows crystallization of streptavidin. Finally, three proteins with affinity for positively or negatively charged lipid layers formed helical arrays on appropriately charged tubules. The generality of this method may allow a wide variety of proteins to be crystallized on lipid nanotubes under physiological conditions.

Transmission electron microscopy and light scattering studies on the interaction of a nonionic/anionic surfactant mixture with phosphatidylcholine liposomes

The interaction of an equimolecular mixture of nonylphenol polyethoxylated [NP(EO)10] and sodium dodecyl sulfate (SDS) surfactants with phosphatidylcholine (PC) liposomes was studied by means of transmission electron microscopy (TEM) and changes in the mean particle size (quasielastic light scattering; QELS) and in the static light scattering (SLS) of the system during liposome solubilization. A good correlation was found between the TEM diameter of particles and the mean hydrodynamic diameter (HD) determined by QELS. The aggregates resulting in this interaction were dependent on the surfactant concentration in the system. Thus, an initial vesicle growth occurred when the surfactant concentration was 15.98 mol%, together with the formation of a very small percentage of smaller particles. Additional surfactant amounts (28.32 mol%) led first to the formation of largest vesicles (HD 418 nm) and second to a fall in the vesicle diameter and in the SLS of the system. Thus, for 38.27 mol%, the TEM picture still showed the presence of vesicles, albeit with signs of disintegration. When additional amounts of surfactant were added to the system, the size curve started to show a bimodal distribution. Thus, for 51.81 mol% surfactant concentration, a sharp curve appeared at 51 nm, corresponding to the formation of small particles and TEM pictures clearly showed vesicle disintegration with formation of tubular structures. It is noteworthy that additional surfactant amounts (from 52 to 60 mol%) led to the formation of unclosed multilayered structures together with smaller aggregates. The gradual increase in the proportion of these smaller aggregates (mixed micelles) led to the complete solubilization of liposomes.

Assembly Properties of a Glycoprotein Produced by Pseudoalteromonas antarctica, NF3

The self-assembly properties of an extracellular material of glycoprotein character produced by a new Gram-negative species, NF3, Pseudoalteromonas antarctica, isolated from muddy soil samples of Antarctica have been investigated. The aggregation behavior of this exopolymer was studied directly by transmission electron microscopy (TEM) and analysis of digitalized TEM images of its aqueous dispersions before and after sonication. Increasing amounts of glycoprotein (GP) in water led to an abrupt decrease in the dispersion surface tensions up to a GP concentration of about 0.20 mg/ml (from 72 to 47 mN m-1), followed by an almost constant surface tension value. The size distribution curves of the aggregates formed always showed a bimodal distribution. The mean size of these two aggregates increased as GP concentration increased (first peak from 120 to 140 nm and second peak from 500 to 700 nm), reaching in both cases almost a constant value also for 0.20 mg of GP/ml of water. TEM images of unsonicated aqueous GP dispersions at concentrations lower and higher than 0.20 mg/ml always showed the coexistence of concentric multilamellar and small unilamellar aggregates, the small particles being the dominant class in the first case. Sonication of these dispersions revealed that each lamella of the initial multilamellar structures was made up of various subunits of coiled coil, whereas the smaller particles were not composed of these subunits. Profiles from digitalized TEM images of unsonicated and sonicated dispersions confirm that each lamella of large aggregates was composed of three subunits. Copyright 1997Academic Press

Formation of high-axial-ratio-microstructures from natural and synthetic sphingolipids

Amphiphiles that form high-axial-ratio-microstructures (HARMs) are being considered as novel materials for controlled release of drugs and other biologically functional molecules. HARMs consisting of tubules, ribbons, solid rods and helices are formed from sphingolipids by addition of water to a solution of amphiphile in DMF. Single molecular species of galactocerebroside (GalCer) containing long unsaturated fatty acid chains or natural GalCer containing mixed-length, non-hydroxy fatty acids (NFA-GalCer) or alpha-hydroxy fatty acids (HFA-GalCer) form cylindrical structures. In contrast, single molecular species of GalCer containing long saturated fatty acids form ribbons and helices. GalCer HARMs are typically under 100 nm in diameter and have lengths of several microns. The importance of the amide of GalCer for HARM formation was evaluated using psychosine, which forms solid fibers, whereas sphingosine and an analog of GalCer in which the amide is reduced to a secondary amine form amorphous aggregates. Single molecular species of ceramide containing long unsaturated fatty acid chains form cylindrical structures, whereas those with long saturated fatty acids form ribbons and helices. Short chain saturated ceramide also forms cylindrical structures. GalCer analogs with N-acetyl-glycine in place of the galactose form fibers whereas those with N-acetyl-proline yield amorphous material. The N-acetyl-proline-containing amphiphile can de doped into pure GalCer or NFA-GalCer without perturbing tubule formation.