Asymmetry of lipid organization in cholinergic synaptic vesicle membranes

Interplay between Copper, Phosphatidylserine, and α-Synuclein Suggests a Link between Copper Homeostasis and Synaptic Vesicle Cycling

Copper homeostasis is critical to the functioning of the brain, and its breakdown is linked with many brain diseases. Copper is also known to interact with the negatively charged lipid, phosphatidylserine (PS), as well as α-synuclein, an aggregation-prone protein enriched in the synapse, which plays a role in synaptic vesicle docking and fusion. However, the interplay between copper, PS lipid, and α-synuclein is not known. Herein, we report a detailed and predominantly kinetic study of the interactions among these three components pertinent to copper homeostasis and neurotransmission. We found that synaptic vesicle-mimicking small unilamellar vesicles (SUVs) can sequester any excess free Cu2+ within milliseconds, and bound Cu2+ on SUVs can be reduced to Cu+ by GSH at a nearly constant rate under physiological conditions. Moreover, we revealed that SUV-bound Cu2+ does not affect the binding between wild-type α-synuclein and SUVs but affect that between N-terminal acetylated α-synuclein and SUVs. In contrast, Cu2+ can effectively displace both types of α-synuclein from the vesicles. Our results suggest that synaptic vesicles may mediate copper transfer in the brain, while copper could participate in synaptic vesicle docking to the plasma membrane via its regulation of the interaction between α-synuclein and synaptic vesicle.

Macrophages and Natural Killers Degrade α-Synuclein Aggregates

Amyloid oligomers and fibrils are protein aggregates that exert a high cell toxicity. Efficient degradation of these protein aggregates can minimize the spread and progression of neurodegeneration. In this study, we investigate the properties of natural killer (NK) cells and macrophages in the degradation of α-synuclein (α-Syn) aggregates grown in a lipid-free environment and in the presence of phosphatidylserine and cholesterol (PS/Cho), which are lipids that are directly associated with the onset and progression of Parkinson's disease. We found that both types of α-Syn aggregates were endocytosed by neurons, which caused strong damage to cell endosomes. Our results also indicated that PS/Cho vesicles drastically increased the toxicity of α-Syn fibrils formed in their presence compared to the toxicity of α-Syn aggregates grown in a lipid-free environment. Both NK cells and macrophages were able to degrade α-Syn and α-Syn/Cho monomers, oligomers, and fibrils. Quantitative analysis of protein degradation showed that macrophages demonstrated substantially more efficient internalization and degradation of amyloid aggregates in comparison to NK cells. We also found that amyloid aggregates induced the proliferation of macrophages and NK cells and significantly changed the expression of their cytokines and chemokines.

Cholesterol and Sphingomyelin Uniquely Alter the Rate of Transthyretin Aggregation and Decrease the Toxicity of Amyloid Fibrils

Transthyretin (TTR) is a small tetrameric protein that aggregates, forming highly toxic oligomers and fibrils. In the blood and cerebrospinal fluid, TTR can interact with various biomolecules, phospho- and sphingolipids, and cholesterol on the red blood cell plasma membrane. However, the role of these molecules in TTR aggregation remains unclear. In this study, we investigated the extent to which phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (Cho), important components of plasma membranes, could alter the rate of TTR aggregation. We found that PC and SM inhibited TTR aggregation whereas Cho strongly accelerated it. The presence of these lipids during the stage of protein aggregation uniquely altered the morphology and secondary structure of the TTR fibrils, which changed the toxicity of these protein aggregates. These results suggest that interactions of TTR with red blood cells, whose membranes are rich with these lipids, can trigger irreversible aggregation of TTR and cause transthyretin amyloidosis.

Unsaturated fatty acids uniquely alter aggregation rate of α-synuclein and insulin and change the secondary structure and toxicity of amyloid aggregates formed in their presence

Docosahexaenoic (DHA) and arachidonic acids (ARA) are omega-3 and omega-6 long-chain polyunsaturated fatty acids (LCPUFAs). These molecules constitute a substantial portion of phospholipids in plasma membranes. Therefore, both DHA and ARA are essential diet components. Once consumed, DHA and ARA can interact with a large variety of biomolecules, including proteins such as insulin and α-synuclein (α-Syn). Under pathological conditions known as injection amyloidosis and Parkinson's disease, these proteins aggregate forming amyloid oligomers and fibrils, toxic species that exert high cell toxicity. In this study, we investigate the role of DHA and ARA in the aggregation properties of α-Syn and insulin. We found that the presence of both DHA and ARA at the equimolar concentrations strongly accelerated aggregation rates of α-Syn and insulin. Furthermore, LCPUFAs substantially altered the secondary structure of protein aggregates, whereas no noticeable changes in the fibril morphology were observed. Nanoscale Infrared analysis of α-Syn and insulin fibrils grown in the presence of both DHA and ARA revealed the presence of LCPUFAs in these aggregates. We also found that such LCPUFAs-rich α-Syn and insulin fibrils exerted significantly greater toxicities compared to the aggregates grown in the LCPUFAs-free environment. These findings show that interactions between amyloid-associated proteins and LCPUFAs can be the underlying molecular cause of neurodegenerative diseases.

Lipids uniquely alter the secondary structure and toxicity of amyloid beta 1-42 aggregates

Abrupt aggregation of amyloid β1-42 (Aβ) peptide is a hallmark of Alzheimer's disease (AD), a severe pathology that affects more than 44 million people worldwide. A growing body of evidence suggests that lipids can uniquely alter rates of Aβ1-42 aggregation. However, it remains unclear whether lipids only alter rates of protein aggregation or also uniquely modify the secondary structure and toxicity of Aβ1-42 oligomers and fibrils. In this study, we investigated the effect of phosphatidylcholine (PC), cardiolipin (CL), and cholesterol (Chol) on Aβ1-42 aggregation. We found that PC, CL and Chol strongly accelerated the rate of fibril formation compared to the rate of Aβ1-42 aggregation in the lipid-free environment. Furthermore, anionic CL enabled the strongest acceleration of Aβ1-42 aggregation compared to zwitterionic PC and uncharged Chol. We also found that PC, CL and Chol uniquely altered the secondary structure of early-, middle- and late-stage Aβ1-42 aggregates. Specifically, CL and Chol drastically increased the amount of parallel β-sheet in Aβ1-42 oligomers and fibrils grown in the presence of these lipids. This caused a significant increase in the toxicity of Aβ : CL and Aβ : Chol compared to the toxicity of Aβ : PC and Aβ1-42 aggregates formed in the lipid-free environment. These results demonstrate that toxicity of Aβ aggregates correlates with the amount of their β-sheet content, which, in turn, is determined by the chemical structure of lipids present at the stage of Aβ1-42 aggregation.

Protein-to-lipid ratio uniquely changes the rate of lysozyme aggregation but does not significantly alter toxicity of mature protein aggregates

Irreversible aggregation of misfolded proteins is the underlying molecular cause of numerous pathologies, including diabetes type 2, Alzheimer's, and Parkinson's diseases. Such an abrupt protein aggregation results in the formation of small oligomers that can propagate into amyloid fibrils. A growing body of evidence suggests that protein aggregation can be uniquely altered by lipids. However, the role of the protein-to-lipid (P:L) ratio on the rate of protein aggregation, as well as the structure and toxicity of corresponding protein aggregates remains poorly understood. In this study, we investigate the role of the P:L ratio of five different phospho- and sphingolipids on the rate of lysozyme aggregation. We observed significantly different rates of lysozyme aggregation at 1:1, 1:5, and 1:10 P:L ratios of all analyzed lipids except phosphatidylcholine (PC). However, we found that at those P:L ratios, structurally and morphologically similar fibrils were formed. As a result, for all studies of lipids except PC, mature lysozyme aggregates exerted insignificantly different cell toxicity. These results demonstrate that the P:L ratio directly determines the rate of protein aggregation, however, has very little if any effect on the secondary structure of mature lysozyme aggregates. Furthermore, our results point to the lack of a direct relationship between the rate of protein aggregation, secondary structure, and toxicity of mature fibrils.

Elucidation of the Effect of Phospholipid Charge on the Rate of Insulin Aggregation and Structure and Toxicity of Amyloid Fibrils

The plasma membrane is a dynamic structure that separates the cell interior from the extracellular space. The fluidity and plasticity of the membrane determines a large number of physiologically important processes ranging from cell division to signal transduction. In turn, membrane fluidity is determined by phospholipids that possess different charges, lengths, and saturation states of fatty acids. A growing body of evidence suggests that phospholipids may play an important role in the aggregation of misfolded proteins, which causes pathological conditions that lead to severe neurodegenerative diseases. In this study, we investigate the role of the charge of the most abundant phospholipids in the plasma membrane: phosphatidylcholine and phosphatidylethanolamine, zwitterions: phosphatidylserine and phosphatidylglycerol, lipids that possess a negative charge, and cardiolipin that has double negative charge on its polar head. Our results show that both zwitterions strongly inhibit insulin aggregation, whereas negatively charged lipids accelerate fibril formation. We also found that in the equimolar presence of zwitterions insulin yields oligomers that exert significantly lower cell toxicity compared to fibrils that were grown in the lipid-free environment. Such aggregates were not formed in the presence of negatively charged lipids. Instead, long insulin fibrils that had strong cell toxicity were grown in the presence of such negatively charged lipids. However, our results showed no correlation between the charge of the lipid and secondary structure and toxicity of the aggregates formed in its presence. These findings show that the secondary structure and toxicity are determined by the chemical structure of the lipid rather than by the charge of the phospholipid polar head.

Lipids uniquely alter rates of insulin aggregation and lower toxicity of amyloid aggregates

Amyloid formation is a hallmark of many medical diseases including diabetes type 2, Alzheimer's and Parkinson diseases. Under these pathological conditions, misfolded proteins self-assemble forming oligomers and fibrils, structurally heterogeneous aggregates that exhibit a large variety of shapes and forms. A growing body of evidence points to drastic changes in the lipid profile in organs affected by amyloidogenic diseases. In this study, we investigated the extent to which individual phospho- and sphingolipids, as well as their mixtures can impact insulin aggregation. Our results show that lipids and their mixtures uniquely alter rates of insulin aggregation simultaneously changing the secondary structure of protein aggregates that are grown in their presence. These structurally different protein-lipid aggregates impact cell viability to different extent while using distinct mechanisms of toxicity. These findings suggest that irreversible changes in lipid profiles of organs may trigger formation of toxic protein species that in turn are responsible for the onset and progression of amyloidogenic diseases.

Amyloid aggregates exert cell toxicity causing irreversible damages in the endoplasmic reticulum

Amyloid oligomers and fibrils are protein aggregates that cause an onset and progression of many neurodegenerative diseases, diabetes type 2 and systemic amyloidosis. Although a growing body of evidence shows that oligomers and fibrils trigger mitochondrial dysfunction simultaneously enhancing production of reactive oxygen species, exact mechanisms by which these protein aggregates exert their toxicities remain unclear. In this study, we used advanced microscopic and spectroscopic methods to examine topography and structure of insulin aggregates grown in the lipid-free environment, as well as in the presence of major classes of phospho- and sphingolipids. We also employed a set of molecular markers to determine the extent to which insulin aggregates induce a damage of cell endoplasmic reticulum (ER), an important cell organelle used for calcium storage, protein synthesis and folding. Our results show that insulin aggregates activate the expression of Activating Transcription Factor 6 (ATF6), a transmembrane protein that is involved in unfolded protein response (UPR) of the stressed ER. At the same time, two other ER transmembrane proteins, Inositol Requiring 1 (IRE1α) and eLF2a, the product of PKR-like ER kinase (PERK), exhibited very low expression levels. Furthermore, amyloid aggregates trigger an expression of the 78-kDa glucose-regulated protein GRP78, which is also involved in the UPR. We also observed UPR-induced expression of a proapoptotic transcription factor CHOP, which, in turn, regulates expression of caspase 3 kinase and BCL2 protein family members, including the ER localized Bax. These findings show that insulin oligomers and fibrils induce UPR-associated ER stress and ultimately fatal changes in cell homeostasis.

Lipids uniquely alter secondary structure and toxicity of lysozyme aggregates

Abrupt aggregation of misfolded proteins is a hallmark of the large group of amyloid pathologies that include diabetes type 2, Alzheimer and Parkinson's diseases. Protein aggregation yields oligomers and fibrils, β-sheet-rich structures that exert cell toxicity. Microscopic examination of amyloid deposits reveals the presence of lipids membranes, which suggests that lipids can be involved in the process of pathogenic protein assembly. In this study, we show that lipids can uniquely alter the aggregation rates of lysozyme, a protein that is associated with systemic amyloidosis. Specifically, cardiolipin (CL), ceramide (CER), and sphingomyelin (SM) accelerate, phosphatidylcholine (PC) strongly inhibits, whereas phosphatidylserine (PS) has no effect on the rate of protein aggregation. Furthermore, lipids uniquely alter the secondary structure of lysozyme aggregates. Furthermore, we found that lysozyme aggregates grown in the presence of CL, CER, SM, PS, and CL:PC mixtures exert significantly lower production of reactive oxygen species and mitochondrial dysfunction compared to lysozyme:PC aggregates and lysozyme fibrils grown in the lipid-free environment. These findings suggest that a change in the lipid composition of cell membranes, which is taken place upon neurodegeneration, may trigger the formation of toxic protein species that otherwise would not be formed.

Charge of Phospholipids Determines the Rate of Lysozyme Aggregation but Not the Structure and Toxicity of Amyloid Aggregates

Biophysical properties of plasma membranes are determined by a chemical structure of phospholipids, including saturation of fatty acids and charge of polar heads of these molecules. Phospholipids not only determine fluidity and plasticity of membranes but also play an important role in abrupt aggregation of misfolded proteins. In this study, we investigate the role of the charge of the most abundant phospholipids in the plasma membrane on the aggregation properties of the lysozyme. We found that the charge of phospholipids determines the aggregation rate of lysozyme and the morphology of the protein aggregates. However, the secondary structure and toxicity of these protein specimens are determined by the chemical nature rather than the charge of phospholipids. These findings show that the charge of phospholipids can be a key factor that determines the stability and aggregation mechanism of amyloidogenic proteins.

Unsaturation in the Fatty Acids of Phospholipids Drastically Alters the Structure and Toxicity of Insulin Aggregates Grown in Their Presence

Lipid bilayers play an important role in the pathological assembly of amyloidogenic proteins and peptides. This assembly yields oligomers and fibrils, which are highly toxic protein aggregates. In this study, we investigated the role of saturation in fatty acids of two phospholipids that are present in cell membranes. We found that unsaturated cardiolipin (CL) drastically shortened the lag phase of insulin aggregation. Furthermore, structurally and morphologically different aggregates were formed in the presence of unsaturated CL vs saturated CL. These aggregates exerted drastically different cell toxicity. Both saturated and unsaturated phosphatidylcholine (PC) were able to inhibit insulin aggregation equally efficiently. Similar to CL, structurally different aggregates were formed in the presence of saturated and unsaturated PC. These aggregates exerted different cell toxicities. These results show that unsaturated phospholipids catalyze the formation of more toxic amyloid aggregates comparing to those formed in the presence of saturated lipids.

The Degree of Unsaturation of Fatty Acids in Phosphatidylserine Alters the Rate of Insulin Aggregation and the Structure and Toxicity of Amyloid Aggregates

Phosphatidylserine (PS) in the plasma membrane plays an important role in cell signaling and apoptosis. Cell degeneration is also linked to numerous amyloid diseases, pathologies that are associated with aggregation of misfolded proteins. In this work, we examine the effect of both saturated PS (DMPS) and unsaturated PS (DOPS and POPS) on the aggregation properties of insulin, as well as the structure and toxicity of insulin aggregates formed in the presence of these phospholipids. We found that the degree of unsaturation of fatty acids in PS alters the rate of insulin aggregation. We also found that toxicity of insulin-DMPS aggregates is significantly lower than the toxicity of DOPS- and POPS-insulin fibrils, whereas all these lipid-containing aggregates exert lower cell toxicity than insulin fibrils grown in a lipid-free environment.

A Perspective: Active Role of Lipids in Neurotransmitter Dynamics

Synaptic neurotransmission is generally considered as a function of membrane-embedded receptors and ion channels in response to the neurotransmitter (NT) release and binding. This perspective aims to widen the protein-centric view by including another vital component—the synaptic membrane—in the discussion. A vast set of atomistic molecular dynamics simulations and biophysical experiments indicate that NTs are divided into membrane-binding and membrane-nonbinding categories. The binary choice takes place at the water-membrane interface and follows closely the positioning of the receptors’ binding sites in relation to the membrane. Accordingly, when a lipophilic NT is on route to a membrane-buried binding site, it adheres on the membrane and, then, travels along its plane towards the receptor. In contrast, lipophobic NTs, which are destined to bind into receptors with extracellular binding sites, prefer the water phase. This membrane-based sorting splits the neurotransmission into membrane-independent and membrane-dependent mechanisms and should make the NT binding into the receptors more efficient than random diffusion would allow. The potential implications and notable exceptions to the mechanisms are discussed here. Importantly, maintaining specific membrane lipid compositions (MLCs) at the synapses, especially regarding anionic lipids, affect the level of NT-membrane association. These effects provide a plausible link between the MLC imbalances and neurological diseases such as depression or Parkinson’s disease. Moreover, the membrane plays a vital role in other phases of the NT life cycle, including storage and release from the synaptic vesicles, transport from the synaptic cleft, as well as their synthesis and degradation.

Extracellular vesicles released by J774A.1 macrophages reduce the bacterial load in macrophages and in an experimental mouse model of tuberculosis

Background

Tuberculosis is the leading cause of death by an infectious microorganism worldwide. Conventional treatment lasts at least six months and has adverse effects; therefore, it is important to find therapeutic alternatives that reduce the bacterial load and may reduce the treatment duration. The immune response against tuberculosis can be modulated by several mechanisms, including extracellular vesicles (EVs), which are nano-sized membrane-bound structures that constitute an efficient communication mechanism among immune cells.

Methods

The EVs released by the J774A.1 mouse macrophage cell line, both spontaneously (S-EV) and after infection with Mycobacterium tuberculosis H37Rv (Mtb-EV), were purified by ultra-centrifugation and size-exclusion chromatography. The size distribution and chemical composition of these EVs were evaluated, and their effect on the bacterial load and the production of cytokines was determined in both in vitro and in vivo models of M. tuberculosis infection.

Results

Mtb-EV are larger than S-EV, they contain M. tuberculosis-specific antigens (not detected in EVs released from M. fortuitum-infected J774A.1 cells) and are rich in phosphatidylserine, present in their outer membrane layer. S-EV, but not Mtb-EV, reduced the bacterial load and the production of MCP-1 and TNF-α in M. tuberculosis-infected macrophages, and these effects were reversed when phosphatidylserine was blocked with annexin V. Both S-EV and Mtb-EV significantly reduced the lung bacterial load in mice infected with M. tuberculosis after 60 days of treatment, but they had no effect on survival or on the lung pneumonic area of these mice.

Conclusion

J774A.1 macrophages infected with M. tuberculosis H37Rv released EVs that differed in size and phosphatidylserine content from spontaneously released EVs, and these EVs also had different biological effects: S-EV reduced the mycobacterial load and the cytokine production in vitro (through a phosphatidylserine-dependent mechanism), while both EVs reduced the lung bacterial load in vivo. These results are the basis for further experiments to evaluate whether EVs improve the efficiency of the conventional treatment for tuberculosis.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

Calcium Assists Dopamine Release by Preventing Aggregation on the Inner Leaflet of Presynaptic Vesicles

In this study, the dopamine-lipid bilayer interactions were probed with three physiologically relevant ion compositions using atomistic molecular dynamics simulations and free energy calculations. The in silico results indicate that calcium is able to decrease significantly the binding of dopamine to a neutral (zwitterionic) phosphatidylcholine lipid bilayer model mimicking the inner leaflet of a presynaptic vesicle. We argue that the observed calcium-induced effect is likely in crucial role in the neurotransmitter release from the presynaptic vesicles docked in the active zone of nerve terminals. The inner leaflets of presynaptic vesicles, which are responsible for releasing neurotransmitters into the synaptic cleft, are mainly composed of neutral lipids such as phosphatidylcholine and phosphatidylethanolamine. The neutrality of the lipid head group region, enhanced by a low pH level, should limit membrane aggregation of transmitters. In addition, the simulations suggest that the high calcium levels inside presynaptic vesicles prevent even the most lipophilic transmitters such as dopamine from adhering to the inner leaflet surface, thus rendering unhindered neurotransmitter release feasible.

RIBEYE(B)-domain binds to lipid components of synaptic vesicles in an NAD(H)-dependent, redox-sensitive manner

Synaptic ribbons are needed for fast and continuous exocytosis in ribbon synapses. RIBEYE is a main protein component of synaptic ribbons and is necessary to build the synaptic ribbon. RIBEYE consists of a unique A-domain and a carboxyterminal B-domain, which binds NAD(H). Within the presynaptic terminal, the synaptic ribbons are in physical contact with large numbers of synaptic vesicle (SV)s. How this physical contact between ribbons and synaptic vesicles is established at a molecular level is not well understood. In the present study, we demonstrate that the RIBEYE(B)-domain can directly interact with lipid components of SVs using two different sedimentation assays with liposomes of defined chemical composition. Similar binding results were obtained with a SV-containing membrane fraction. The binding of liposomes to RIBEYE(B) depends upon the presence of a small amount of lysophospholipids present in the liposomes. Interestingly, binding of liposomes to RIBEYE(B) depends on NAD(H) in a redox-sensitive manner. The binding is enhanced by NADH, the reduced form, and is inhibited by NAD⁺, the oxidized form. Lipid-mediated attachment of vesicles is probably part of a multi-step process that also involves additional, protein-dependent processes.

Getting to the Outer Leaflet: Physiology of Phosphatidylserine Exposure at the Plasma Membrane

Phosphatidylserine (PS) is a major component of membrane bilayers whose change in distribution between inner and outer leaflets is an important physiological signal. Normally, members of the type IV P-type ATPases spend metabolic energy to create an asymmetric distribution of phospholipids between the two leaflets, with PS confined to the cytoplasmic membrane leaflet. On occasion, membrane enzymes, known as scramblases, are activated to facilitate transbilayer migration of lipids, including PS. Recently, two proteins required for such randomization have been identified: TMEM16F, a scramblase regulated by elevated intracellular Ca(2+), and XKR8, a caspase-sensitive protein required for PS exposure in apoptotic cells. Once exposed at the cell surface, PS regulates biochemical reactions involved in blood coagulation, and bone mineralization, and also regulates a variety of cell-cell interactions. Exposed on the surface of apoptotic cells, PS controls their recognition and engulfment by other cells. This process is exploited by parasites to invade their host, and in specialized form is used to maintain photoreceptors in the eye and modify synaptic connections in the brain. This review discusses what is known about the mechanism of PS exposure at the surface of the plasma membrane of cells, how actors in the extracellular milieu sense surface exposed PS, and how this recognition is translated to downstream consequences of PS exposure.

Restoring synaptic vesicles during compensatory endocytosis

In the CNS (central nervous system), nerve cells communicate by transmitting signals from one to the next across chemical synapses. Electrical signals trigger controlled secretion of neurotransmitter by exocytosis of SV (synaptic vesicles) at the presynaptic site. Neurotransmitters diffuse across the synaptic cleft, activate receptor channels in the receiving neuron at the postsynaptic site, and thereby elicit a new electrical signal. Repetitive stimulation should result in fast depletion of fusion-competent SVs, given their limited number in the presynaptic bouton. Therefore, to support repeated rounds of release, a fast trafficking cycle is required that couples exocytosis and compensatory endocytosis. During this exo-endocytic cycle, a defined stoichiometry of SV proteins has to be preserved, that is, membrane proteins have to be sorted precisely. However, how this sorting is accomplished on a molecular level is poorly understood. In the present chapter we review recent findings regarding the molecular composition of SVs and the mechanisms that sort SV proteins during compensatory endocytosis. We identify self-assembly of SV components and individual cargo recognition by sorting adaptors as major mechanisms for maintenance of the SV protein complement.

Biophysics of α-synuclein membrane interactions

Membrane proteins participate in nearly all cellular processes; however, because of experimental limitations, their characterization lags far behind that of soluble proteins. Peripheral membrane proteins are particularly challenging to study because of their inherent propensity to adopt multiple and/or transient conformations in solution and upon membrane association. In this review, we summarize useful biophysical techniques for the study of peripheral membrane proteins and their application in the characterization of the membrane interactions of the natively unfolded and Parkinson's disease (PD) related protein, α-synuclein (α-syn). We give particular focus to studies that have led to the current understanding of membrane-bound α-syn structure and the elucidation of specific membrane properties that affect α-syn-membrane binding. Finally, we discuss biophysical evidence supporting a key role for membranes and α-syn in PD pathogenesis. This article is part of a Special Issue entitled: Membrane protein structure and function.

Measurement of the membrane curvature preference of phospholipids reveals only weak coupling between lipid shape and leaflet curvature

In biological processes, such as fission, fusion and trafficking, it has been shown that lipids of different shapes are sorted into regions with different membrane curvatures. This lipid sorting has been hypothesized to be due to the coupling between the membrane curvature and the lipid's spontaneous curvature, which is related to the lipid's molecular shape. On the other hand, theoretical predictions and simulations suggest that the curvature preference of lipids, due to shape alone, is weaker than that observed in biological processes. To distinguish between these different views, we have directly measured the curvature preferences of several lipids by using a fluorescence-based method. We prepared small unilamellar vesicles of different sizes with a mixture of egg-PC and a small mole fraction of N-nitrobenzoxadiazole (NBD)-labeled phospholipids or lysophospholipids of different chain lengths and saturation, and measured the NBD equilibrium distribution across the bilayer. We observed that the transverse lipid distributions depended linearly on membrane curvature, allowing us to measure the curvature coupling coefficient. Our measurements are in quantitative agreement with predictions based on earlier measurements of the spontaneous curvatures of the corresponding nonfluorescent lipids using X-ray diffraction. We show that, though some lipids have high spontaneous curvatures, they nevertheless showed weak curvature preferences because of the low values of the lipid molecular areas. The weak curvature preference implies that the asymmetric lipid distributions found in biological membranes are not likely to be driven by the spontaneous curvature of the lipids, nor are lipids discriminating sensors of membrane curvature.

Presynaptic defects underlying impaired learning and memory function in lipoprotein lipase-deficient mice

Lipoprotein lipase (LPL) is predominantly expressed in adipose and muscle where it plays a crucial role in the metabolism of triglyceride-rich plasma lipoproteins. LPL is also expressed in the brain with highest levels found in the pyramidal cells of the hippocampus, suggesting a possible role for LPL in the regulation of cognitive function. However, very little is currently known about the specific role of LPL in the brain. We have generated a mouse model of LPL deficiency which was rescued from neonatal lethality by somatic gene transfer. These mice show no exogenous and endogenous LPL expression in the brain. To study the role of LPL in learning and memory, the performance of LPL-deficient mice was tested in two cognitive tests. In a water maze test, LPL-deficient mice exhibited increased latency to escape platform and increased mistake frequency. Decreased latency to platform in the step-down inhibitory avoidance test was observed, consistent with impaired learning and memory in these mice. Transmission electron microscopy revealed a significant decrease in the number of presynaptic vesicles in the hippocampus of LPL-deficient mice. The levels of the presynaptic marker synaptophysin were also reduced in the hippocampus, whereas postsynaptic marker postsynaptic density protein 95 levels remained unchanged in LPL-deficient mice. Theses findings indicate that LPL plays an important role in learning and memory function possibly by influencing presynaptic function.

The Gaussian curvature elastic energy of intermediates in membrane fusion

The Gaussian curvature elastic energy contribution to the energy of membrane fusion intermediates has usually been neglected because the Gaussian curvature elastic modulus, kappa, was unknown. It is now possible to measure kappa for phospholipids that form bicontinuous inverted cubic (Q(II)) phases. Here, it is shown that one can estimate kappa for lipids that do not form Q(II) phases by studying the phase behavior of lipid mixtures. The method is used to estimate kappa for several lipid compositions in excess water. The values of kappa are used to compute the curvature elastic energies of stalks and catenoidal fusion pores according to recent models. The Gaussian curvature elastic contribution is positive and similar in magnitude to the bending energy contribution: it increases the total curvature energy of all the fusion intermediates by 100 units of k(B)T or more. It is important to note that this contribution makes the predicted intermediate energies compatible with observed lipid phase behavior in excess water. An order-of-magnitude fusion rate equation is used to estimate whether the predicted stalk energies are consistent with the observed rates of stalk-mediated processes in pure lipid systems. The current theory predicts a stalk energy that is slightly too large, by approximately 30 k(B)T, to rationalize the observed rates of stalk-mediated processes in phosphatidylethanolamine or N-monomethylated dioleoylphosphatidylethanolamine systems. Despite this discrepancy, the results show that models of fusion intermediate energy are accurate enough to make semiquantitative predictions about how proteins mediate biomembrane fusion. The same rate model shows that for proteins to drive biomembrane fusion at observed rates, they have to perform mediating functions corresponding to a reduction in the energy of a purely lipidic stalk by several tens of k(B)T. By binding particular peptide sequences to the monolayer surface, proteins could lower fusion intermediate energies by altering the elastic constants of the patches of lipid monolayer that form the stalk. Here, it is shown that if peptide binding changes kappa or some other combinations of local elastic constants by only tens of percents, the stalk energy and the energy of catenoidal fusion pores would decrease by tens of k(B)T relative to the pure lipid value. This is comparable to the required mediating effect. The curvature energies of stalks and catenoidal fusion pores have almost the same dependence on monolayer elastic constants as the curvature energies of the rhombohedral and Q(II) phases; respectively. The effects of isolated fusion-relevant peptides on the energies of these intermediates can be determined by studying the effects of the peptides on the stability of rhombohedral and Q(II) phases.

Alpha-synuclein selectively binds to anionic phospholipids embedded in liquid-disordered domains

Previous studies indicate that binding of alpha-synuclein to membranes is critical for its physiological function and the development of Parkinson's disease (PD). Here, we have investigated the association of fluorescence-labeled alpha-synuclein variants with different types of giant unilamellar vesicles using confocal microscopy. We found that alpha-synuclein binds with high affinity to anionic phospholipids, when they are embedded in a liquid-disordered as opposed to a liquid-ordered environment. This indicates that not only electrostatic forces but also lipid packing and hydrophobic interactions are critical for the association of alpha-synuclein with membranes in vitro. When compared to wild-type alpha-synuclein, the disease-causing alpha-synuclein variant A30P bound less efficiently to anionic phospholipids, while the variant E46K showed enhanced binding. This suggests that the natural association of alpha-synuclein with membranes is altered in the inherited forms of Parkinson's disease.

Productive hemifusion intermediates in fast vesicle fusion driven by neuronal SNAREs

An in vitro fusion assay uses fluorescence microscopy of labeled lipids to monitor single v-SNARE vesicle docking and fusion events on a planar lipid bilayer containing t-SNAREs. For vesicles and bilayer comprising phosphatidylcholine (POPC, 84-85% by mol) and phosphatidylserine (DOPS, 15% by mol), previous work demonstrated prompt, full fusion (tau(fus) = 25 ms). Substitution of 20-60% phosphatidylethanolamine (DOPE) for phosphatidylcholine in the v-SNARE vesicle with either 0 or 20% DOPE included in the t-SNARE bilayer gives rise to hemifusion events. Labeled lipids diffuse into the planar bilayer as two temporally distinct waves, presumably hemifusion of the outer leaflet followed by inner leaflet (core) fusion. The fusion kinetics with DOPE is markedly heterogeneous. Some vesicle/docking site pairs exhibit prompt, full fusion while others exhibit hemifusion. Hemifusion events are roughly half productive (leading to subsequent core fusion within 20 s) and half dead-end. In qualitative accord with expectations from studies of protein-free vesicle-vesicle fusion, the hemifusion rate k(hemi) is 15-20 times faster than the core fusion rate k(core), and the fraction of hemifusion events increases with increasing percentage of DOPE. This suggests similar underlying molecular pathways for protein-free and neuronal SNARE-driven fusion. Removal of phosphatidylserine from the v-SNARE vesicle has no effect on docking or fusion.

Quantitative Proteomics Analysis of Detergent-resistant Membranes from Chemical Synapses

Synaptic vesicles (SVs) in the central nervous system upon stimulation undergo rapid calcium-triggered exoendocytic cycling within the nerve terminal that at least in part depends on components of the clathrin- and dynamin-dependent endocytosis machinery. How exocytic SV fusion and endocytic retrieval are temporally and spatially coordinated is still an open question. One possibility is that specialized membrane microdomains characterized by their high content in membrane cholesterol may assist in the spatial coordination of synaptic membrane protein recycling. Quantitative proteomics analysis of detergent-resistant membranes (DRMs) isolated from rat brain synapses or cholesterol-depleted control samples by liquid chromatography-tandem mass spectrometry identified a total of 159 proteins. Among these 122 proteins were classified as cholesterol-dependent DRM or DRM-associated proteins, many of which with proven or hypothesized functions in exoendocytic vesicle cycling including clathrin, the clathrin adaptor complex AP-2, and a variety of SV proteins. In agreement with this, SV membrane and endocytic proteins displayed a partial resistance to extraction with cold Triton X-100 in cultured rat hippocampal neurons where they co-localized with labeled cholera toxin B, a marker for cholesterol-enriched DRMs. Moreover SV proteins formed cholesterol-dependent complexes in CHAPS-extracted synaptic membrane lysates. Our combined data suggest that lipid microdomains may act as spatial coordinators for exoendocytic vesicle cycling at synapses.

Calcium-Triggered Membrane Interaction of the α-Synuclein Acidic Tail†

Alpha-synuclein (alpha-syn) is a 140-residue protein that aggregates in intraneuronal inclusions called Lewy bodies in Parkinson's disease (PD). It is composed of an N-terminal domain with a propensity to bind lipids and a C-terminal domain rich in acidic residues (the acidic tail). The objective of this study was to examine the effect of Ca(2+) on the acidic tail conformation in lipid-bound alpha-syn. We exploit the extreme sensitivity of the band III fluorescence emission peak of the pyrene fluorophore to the polarity of its microenvironment to monitor subtle conformational response of the alpha-syn acidic tail to Ca(2+). Using recombinant human alpha-syn bearing a pyrene to probe either the N-terminal domain or the acidic tail, we noted that lipid binding resulted in an increase in band III emission intensity in the pyrene probe tagging the N-terminal domain but not that in the acidic tail. This suggests that the protein is anchored to the lipid surface via the N-terminal domain. However, addition of Ca(2+) caused an increase in band III emission intensity in the pyrene tagging the acidic tail, with a corresponding increased susceptibility to quenching by quenchers located in the lipid milieu, indicative of lipid interaction of this domain. Taken together with the increased beta-sheet content of membrane-associated alpha-syn in the presence of Ca(2+), we propose a model wherein initial lipid interaction occurs via the N-terminal domain, followed by a Ca(2+)-triggered membrane association of the acidic tail as a potential mechanism leading to alpha-syn aggregation. These observations have direct implications in the role of age-related oxidative stress and the attendant cellular Ca(2+) dysregulation as critical factors in alpha-syn aggregation in PD.

A study of the regional effects of alpha-synuclein on the organization and stability of phospholipid bilayers

Associations between the protein alpha-synuclein (alpha-syn) and presynaptic vesicles have been implicated in synaptic plasticity and neurotransmitter release and may also affect how the protein aggregates into fibrils found in Lewy bodies, the cellular inclusions associated with neurodegenerative diseases. This work investigated how alpha-syn interacts with model phospholipid membranes and examined what effect protein binding has upon the physical properties of lipid bilayers. Wide line 2H and 31P NMR spectra of phospholipid vesicles revealed that alpha-syn associates with membranes containing lipids with anionic headgroups and can disrupt the integrity of the lipid bilayer, but the protein has little effect on membranes of zwitterionic phosphatidylcholine. A peptide, alpha-syn(10-48), which corresponds to the lysine-rich N-terminal region of alpha-syn, was found to associate with lipid headgroups with a preference for a negative membrane surface charge. Another peptide, alpha-syn(120-140), which corresponds to the glutamate-rich C-terminal region, also associates weakly with lipid headgroups but with a slightly higher affinity for membranes with no net surface charge than for negatively charged membrane surfaces. Binding of alpha-syn(10-48) and alpha-syn(120-140) to the lipid vesicles did not disrupt the lamellar structure of the membranes, but both peptides appeared to induce the lateral segregation of the lipids into clusters of acidic lipid-enriched and acidic lipid-deficient domains. From these findings, it is speculated that the N-terminal and C-terminal domains of full-length alpha-syn might act in concert to organize the membrane components during normal protein function and perhaps play a role in presynaptic vesicle synthesis, maintenance, and fusion.

Binding of α-Synuclein Affects the Lipid Packing in Bilayers of Small Vesicles

The intracellular deposition of fibrillar aggregates of alpha-synuclein is a characteristic feature of Parkinson disease. Alternatively, as a result of its unusual conformational plasticity, alpha-synuclein may fold into an amphipathic helix upon contact with a lipid-water interface. Using spin label ESR and fluorescence spectroscopy, we show here that alpha-synuclein affects the lipid packing in small unilamellar vesicles. The ESR hyperfine splittings of spin-labeled phospholipid probes revealed that alpha-synuclein induces chain ordering at carbon 14 of the acyl chains below the chain melting phase transition temperature but not in the liquid crystalline state of electroneutral vesicle membranes. Binding of alpha-synuclein leads to an increase in the temperature and cooperativity of the phase transition according to the fluorescence anisotropy of the hydrophobic polyene 1,6-diphenylhexatriene and of the fluorescence emission maxima of the amphiphilic probe 6-dodecanoyl-2-dimethylaminonaphthalene. Binding parameters were obtained from the fluorescence anisotropy measurements in combination with our previous determinations by titration calorimetry (Nuscher, B., Kamp, F., Mehnert, T., Odoy, S., Haass, C., Kahle, P. J., and Beyer, K. (2004) J. Biol. Chem. 279, 21966-21975). We also show that alpha-synuclein interacts with vesicle membranes containing sphingomyelin and cholesterol. We propose that the protein is capable of annealing defects in curved vesicle membranes, which may prevent synaptic vesicles from premature fusion.

Cholesterol facilitates the native mechanism of Ca2+-triggered membrane fusion

The process of regulated exocytosis is defined by the Ca2+-triggered fusion of two apposed membranes, enabling the release of vesicular contents. This fusion step involves a number of energetically complex steps and requires both protein and lipid membrane components. The role of cholesterol has been investigated using isolated release-ready native cortical secretory vesicles to analyze the Ca2+-triggered fusion step of exocytosis. Cholesterol is a major component of vesicle membranes and we show here that selective removal from membranes, selective sequestering within membranes, or enzymatic modification causes a significant inhibition of the extent, Ca2+ sensitivity and kinetics of fusion. Depending upon the amount incorporated, addition of exogenous cholesterol to cholesterol-depleted membranes consistently recovers the extent, but not the Ca2+ sensitivity or kinetics of fusion. Membrane components of comparable negative curvature selectively recover the ability to fuse, but are unable to recover the kinetics and Ca2+ sensitivity of vesicle fusion. This indicates at least two specific positive roles for cholesterol in the process of membrane fusion: as a local membrane organizer contributing to the efficiency of fusion, and, by virtue of its intrinsic negative curvature, as a specific molecule working in concert with protein factors to facilitate the minimal molecular machinery for fast Ca2+-triggered fusion.

Lipid regulation of the synaptic vesicle cycle

Membrane vesicle cycling is orchestrated through the combined actions of proteins and lipids. At neuronal synapses, this orchestration must meet the stringent demands of speed, fidelity and sustainability of the synaptic vesicle cycle that mediates neurotransmission. Historically, the lion's share of the attention has been focused on the proteins that are involved in this cycle; but, in recent years, it has become clear that the previously unheralded plasma membrane and vesicle lipids are also key regulators of this cycle. This article reviews recent insights into the roles of lipid-modifying enzymes and lipids in the acute modulation of neurotransmission.

Mutation E46K increases phospholipid binding and assembly into filaments of human α-synuclein

Missense mutations (A30P and A53T) in alpha-synuclein and the overproduction of the wild-type protein cause familial forms of Parkinson's disease and dementia with Lewy bodies. Alpha-synuclein is the major component of the filamentous Lewy bodies and Lewy neurites that define these diseases at a neuropathological level. Recently, a third missense mutation (E46K) in alpha-synuclein was described in an inherited form of dementia with Lewy bodies. Here, we have investigated the functional effects of this novel mutation on phospholipid binding and filament assembly of alpha-synuclein. When compared to the wild-type protein, the E46K mutation caused a significantly increased ability of alpha-synuclein to bind to negatively charged liposomes, unlike the previously described mutations. The E46K mutation increased the rate of filament assembly to the same extent as the A53T mutation. Filaments formed from E46K alpha-synuclein often had a twisted morphology with a cross-over spacing of 43 nm. The observed effects on lipid binding and filament assembly may explain the pathogenic nature of the E46K mutation in alpha-synuclein.

Alpha-synuclein has a high affinity for packing defects in a bilayer membrane: a thermodynamics study

A number of neurodegenerative disorders, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, are characterized by the intracellular deposition of fibrillar aggregates that contain a high proportion of alpha-synuclein (alphaS). The interaction with the membrane-water interface strongly modulates folding and aggregation of the protein. The present study investigates the lipid binding and the coil-helix transition of alphaS, using titration calorimetry, differential scanning calorimetry, and circular dichroism spectroscopy. Titration of the protein with small unilamellar vesicles composed of zwitterionic phospholipids below the chain melting temperature of the lipids yielded exceptionally large exothermic heat values. The sigmoidal titration curves were evaluated in terms of a simple model that assumes saturable binding sites at the vesicle surface. The cumulative heat release and the ellipticity were linearly correlated as a result of simultaneous binding and helix folding. There was no heat release and folding of alphaS in the presence of large unilamellar vesicles, indicating that a small radius of curvature is necessary for the alphaS-membrane interaction. The heat release and the negative heat capacity of the protein-vesicle interaction could not be attributed to the coil-helix transition of the protein alone. We speculate that binding and helix folding of alphaS depends on the presence of defect structures in the membrane-water interface, which in turn results in lipid ordering in the highly curved vesicular membranes. This will be discussed with regard to a possible role of the protein for the stabilization of synaptic vesicle membranes.

Selective prion protein binding to synaptic components is modulated by oxidative and nitrosative changes induced by copper(II) and peroxynitrite in cholinergic synaptosomes, unveiling a role for calcineurin B and thioredoxin

Choline acetyltransferase (ChAT) and choline transport are decreased after nitrosative stress. ChAT activity is altered in scrapie-infected neurons, where oxidative stress develops. Cellular prion protein (PrPc) may play a neuroprotective function in participating in the redox control of neuronal environment and regulation of copper metabolism, a role impaired when PrPc is transformed into PrPSc in prion pathologies. The complex cross-talk between PrPc and cholinergic neurons was analyzed in vitro using peroxynitrite and Cu2+ treatments on nerve endings isolated from Torpedo marmorata, a model of the motoneuron pre-synaptic element. Specific interactions between solubilized synaptic components and recombinant ovine prion protein (PrPrec) could be demonstrated by Biacore technology. Peroxynitrite abolished this interaction in a concentration-dependent way and induced significant alterations of neuronal targets. Interaction was restored by prior addition of peroxynitrite trapping agents. Cu2+ (in the form of CuSO4) treatment of synaptosomes triggered a milder oxidative effect leading to a bell-shaped increase of PrPrec binding to synaptosomal components, counteracted by the natural thiol agents, glutathione and thioredoxin. Copper(II)-induced modifications of thiols in several neuronal proteins. A positive correlation was observed between PrPrec binding and immunoreactive changes for calcineurin B and its partners, suggesting a synergy between calcineurin complex and PrP for copper regulation.

Transbilayer asymmetry of phospholipids in the plasma membrane regulates exocytotic release in mast cells

To investigate the role of the asymmetric distribution of phospholipids of the plasma membrane in exocytosis, we examined the effects of disruption of this asymmetrical distribution of lipids on exocytotic release from mast cells (RBL-2H3). Lipid scramblase, which is activated by divalent cations and catalyzes the transbilayer movement of phospholipids, was overexpressed in mast cells. Exogenous lipid scramblase was expressed in the plasma membrane and the cytoplasm. Activation of scramblase by divalent cations disrupted the asymmetrical distribution of phospholipids in the plasma membrane. Exocytotic release induced by calcium ionophore and phorbol ester was significantly inhibited in the cells transfected with wild-type scramblase. This inhibition was observed with time lag of about 5 min. Furthermore, when the asymmetric distribution of lipids was disrupted before induction of exocytosis, the inhibition of exocytotic release was obvious from the beginning without time lag. These results suggest that the asymmetric distribution of phospholipids in the plasma membrane plays an essential role in fusion between secretory granules and the plasma membrane. This finding also demonstrates that the transbilayer asymmetry of phospholipids regulates exocytosis and gives a new insight into the significance of lipid asymmetry in the plasma membrane.

Chemical control of phospholipid distribution across bilayer membranes

Most biological membranes possess an asymmetric transbilayer distribution of phospholipids. Endogenous enzymes expend energy to maintain the arrangement by promoting the rate of phospholipid translocation, or flip-flop. Researchers have discovered ways to modify this distribution through the use of chemicals. This review presents a critical analysis of the phospholipid asymmetry data in the literature followed by a brief overview of the maintenance and physiological consequences of phospholipid asymmetry, and finishes with a list of chemical ways to alter phospholipid distribution by enhancement of flip-flop.

alpha-Synuclein membrane interactions and lipid specificity

With the discovery of missense mutations (A53T and A30P) in alpha-synuclein (alpha-Syn) in several families with early onset familial Parkinson's disease, alpha-Syn aggregation and fibril formation have been thought to play a role in the pathogenesis of alpha-synucleinopathies, such as Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. As previous reports have suggested that alpha-Syn plays a role in lipid transport and synaptic membrane biogenesis, we investigated whether alpha-Syn binds to a specific lipid ligand using thin layer chromatography overlay and examined the changes in its secondary structure using circular dichroism spectroscopy. alpha-Syn was found to bind to acidic phospholipid vesicles and this binding was significantly augmented by the presence of phosphatidylethanolamine, a neutral phospholipid. We further examined the interaction of alpha-Syn with lipids by in situ atomic force microscopy. The association of soluble wild-type alpha-Syn with planar lipid bilayers resulted in extensive bilayer disruption and the formation of amorphous aggregates and small fibrils. The A53T mutant alpha-Syn disrupted the lipid bilayers in a similar fashion but at a slower rate. These results suggest that alpha-Syn membrane interactions are physiologically important and the lipid composition of the cellular membranes may affect these interactions in vivo.

How do presynaptic PLA2 neurotoxins block nerve terminals?

Snake presynaptic neurotoxins with phospholipase A2 activity block nerve terminals in an unknown way. Here, we propose that they enter the lumen of synaptic vesicles following endocytosis and hydrolyse phospholipids of the inner leaflet of the membrane. The transmembrane pH gradient drives the translocation of fatty acids to the cytosolic monolayer, leaving lysophospholipids on the lumenal layer. Such vesicles are highly fusogenic and release neurotransmitter upon fusion with the presynaptic membrane, but cannot be retrieved because of the high local concentration of fatty acids and lysophospholipids, which prevents vesicle neck closure.

Stabilization of alpha-synuclein secondary structure upon binding to synthetic membranes

alpha-Synuclein is a highly conserved presynaptic protein of unknown function. A mutation in the protein has been causally linked to Parkinson's disease in humans, and the normal protein is an abundant component of the intraneuronal inclusions (Lewy bodies) characteristic of the disease. alpha-Synuclein is also the precursor to an intrinsic component of extracellular plaques in Alzheimer's disease. The alpha-synuclein sequence is largely composed of degenerate 11-residue repeats reminiscent of the amphipathic alpha-helical domains of the exchangeable apolipoproteins. We hypothesized that alpha-synuclein should associate with phospholipid bilayers and that this lipid association should stabilize an alpha-helical secondary structure in the protein. We report that alpha-synuclein binds to small unilamellar phospholipid vesicles containing acidic phospholipids, but not to vesicles with a net neutral charge. We further show that the protein associates preferentially with vesicles of smaller diameter (20-25 nm) as opposed to larger (approximately 125 nm) vesicles. Lipid binding is accompanied by an increase in alpha-helicity from 3% to approximately 80%. These observations are consistent with a role in vesicle function at the presynaptic terminal.

Binding of substance P agonists to lipid membranes and to the neurokinin-1 receptor

Three new analogues of the neuropeptide substance P (SP) were synthesized. The C-terminal message segment was made more hydrophilic in (Arg9)SP or more hydrophobic in (Nle9)SP. In (AcPro2, Arg9)SP the charge at the N-terminal address segment was reduced, while that of the message segment was increased. The rationale underlying these substitutions was to correlate the physical-chemical properties of the SP-analogues, in particular their lipid-induced conformation and membrane-binding affinity, with receptor binding and functional activity. In solution, all three analogues exhibited random coil conformations as evidenced by circular dichroism spectroscopy. Addition of SDS micelles induced partially alpha-helical structures. The same structure was also produced by negatively charged lipid vesicles for (AcPro2, Arg9)SP and (Arg9)SP whereas both alpha-helix-like structures and beta-sheet structures were observed for SP and (Nle9)SP. The measurement of the Gibbs adsorption isotherms and monolayer expansion studies provided quantitative data on the surface area requirement and on the membrane penetration area of the SP analogues. The thermodynamic parameters for lipid binding were determined with monolayer expansion for measurements and high-sensitivity titration calorimetry. The apparent binding constants, Kapp, for membranes containing 100% POPG were of the order of 10(3)- 10(5) M(-1). The binding was due to electrostatic attraction of the cationic peptides to the negatively charged membrane surface. The intrinsic (hydrophobic) binding constants, obtained after correcting for electrostatic effects, were much smaller with Kp=10+/- 1 M(-1) for (Arg9)SP, 9 +/- 1 M(-1) for (AcPro2, Arg9)SP, and 39 +/- 3 M(-1) for (Nle9)SP. The measurement of the binding affinities to the NK-1 receptor and of the in vitro activities showed that all three peptides behaved as agonists. Their binding affinity to the neurokinin-1 receptor decreased with the size of the side chains at position 9 of the amino acid sequence but was independent of the cationic charge of the peptides. The fact that even the highly charged (Arg9)SP has agonistic activity provides evidence that the binding epitope at the receptor is in a rather hydrophilic environment. This finding is in agreement with the low hydrophobic binding constants and the weak penetration of the three peptides into negatively charged membranes. It argues against a membrane mediated receptor mechanism and suggests that the agonist approaches the receptor binding, site from the aqueous phase.

Esophageal cancer phospholipid characterization by 31P NMR

Phospholipid extracts of surgical tissue specimens from 18 patients, consisting of normal esophagus, distal esophageal tumor and normal stomach, were analyzed using 31P NMR. The prominent phospholipids detected in these tissues included cardiolipin (CL), phosphatidylethanolamine plasmalogen, phosphatidylethanolamine (PE), phosphatidylserine (PS), sphingomyelin (SPH), phosphatidylinositol (PI), phosphatidylcholine plasmalogen and phosphatidylcholine (PC). Very small quantities of the phospholipids lysophosphatidylcholine, phosphatidic acid, phosphatidylglycerol, and an uncharacterized phospholipid at -0.13 delta also were detected in some of the 54 tissue specimens analyzed. The mean relative concentrations of these phospholipids, in mole percentages of total detected phosphorus, were determined from the acquired spectra and used to differentiate among the three tissue groups. The relative concentrations of the following phospholipids differed significantly (p < 0.001) among the respective tissue groups: normal esophagus vs esophageal tumor, PS, SPH, PI, PC; normal esophagus vs normal stomach, CL, PE, PS, SPH; esophageal tumor vs normal stomach, CL, PE. Membrane phospholipids implicated in modulating the growth and metastases of tumors of epithelial origin can be profiled to discriminate among normal esophagus, distal esophageal tumor and normal stomach using 31P NMR.

Effects of exercise training and anabolic steroids on plantaris and soleus phospholipids: a 31P nuclear magnetic resonance study

1. The purpose of this study was to examine the effect of exercise, anabolic steroid treatment, and a combination of both treatments on the phospholipid composition of predominantly fast twitch (plantaris) and slow twitch (soleus) skeletal muscles. The 4 experimental groups analyzed were sedentary control (C), steroid-treated (S), exercise-trained (E), and exercise plus steroid-treated (ES). 2. Among the 11 phospholipids quantitated, for the plantaris muscle, phosphatidylcholine was reduced in ES relative to C, while phosphatidylethanolamine and phosphatidylethanolamine plasmalogen were elevated in E and ES relative to C. For the soleus muscle, phosphatidylserine was reduced in S and E relative to C, and cardiolipin was elevated in E relative to C. 3. Of the 27 metabolic indices calculated for the plantaris, 15 changed significantly among E and ES relative to S and C, while for the soleus, only three indices changed among the four groups, two among E and ES relative to S and C and one between S and C. 4. For the plantaris muscle, the results are consistent with an exercise-induced alteration of membrane phospholipid composition that increases ion translocation activity. For the soleus muscle, this membrane alteration essentially does not take place. 5. Steroid treatment had little to no statistically significant effect on plantaris and soleus muscle phospholipid systems, regardless of the imposed regimen.

Phosphatidylcholine asymmetry in electroplax from the electric eel: use of a phosphatidylcholine exchange protein

Phosphatidylcholine asymmetry in the inner and outer leaflets of the plasma membrane bilayer of the innervated and noninnervated surfaces of the electroplax cell was determined, using a phosphatidylcholine exchange protein. The exchange protein from bovine liver catalyzed the exchange of phosphatidylcholine from small unilamellar vesicles to the outer monolayer of the plasma membrane bilayer. The exchange protein did not penetrate to the inner monolayer of the plasma membrane, did not modify the permeability of the electroplax, and did not alter the phospholipid or cholesterol content of the electroplax. In the innervated plasma membrane, 42% of the phosphatidylcholine is in the outer leaflet, 33% is in the inner leaflet, and 25% is inaccessible to the exchange protein. Corresponding values for the noninnervated plasma membrane are 56, 26, and 18%, respectively. These results are similar to phosphatidylcholine asymmetry in other biological membranes. This unique cell can be used as a model to test the effects on phospholipid asymmetry of compounds that act on the membrane.

Phospholipid transverse diffusion in synaptosomes: evidence for the involvement of the aminophospholipid translocase

We have studied in Torpedo marmorata electric organ synaptosomes the equilibration kinetics of spin-labeled phospholipid analogues initially incorporated into the outer plasma membrane monolayer. As assayed by evoked releases of both ATP and acetylcholine, the nerve endings were closed vesicles containing an energy source. The aminophospholipids (phosphatidylethanolamine and phosphatidylserine) were translocated toward the inner membrane leaflet faster and to a higher extent than their choline-containing counterparts (phosphatidylcholine and sphingomyelin). This difference was abolished by incubation of synaptosomal membranes with N-ethylmaleimide, suggesting that the accumulation of aminophospholipids in the inner layer was driven by a protein. This phenomenon is comparable with what was described in plasma membranes of other eucaryotic cells (erythrocyte, lymphocyte, platelet, fibroblast), and thus we would suggest that an aminophospholipid translocase, capable of moving the aminophospholipids from the outer to the inner layer at the expense of ATP, is also present in the synaptosomal plasma membrane.

Transmembrane movements of lipids

Membranes allow the rapid passage of unchanged lipids. Phospholipids on the other hand diffuse very slowly from one monolayer to another with a half-time of several hours. This slow spontaneous movement in a pure lipid bilayer can be selectively modulated in biological membranes by intrinsic proteins. In microsomes, and probably in bacterial membranes, non-specific phospholipid flippases allow the rapid redistribution of newly synthesized phospholipids. In eukaryotic plasma membranes, aminophospholipid translocase selectively pumps phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the outer to the inner leaflet and establishes a permanent lipid asymmetry. The discovery of an aminophospholipid translocase in chromaffin granules proves that eukaryotic organelles may also contain lipid translocators.