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

Liquid-liquid phase separation regulates alpha-synuclein aggregate and mitophagy in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease in the world, and alpha-synuclein (α-syn) abnormal aggregate and mitochondrial dysfunction play a crucial role in its pathological development. Recent studies have revealed that proteins can form condensates through liquid-liquid phase separation (LLPS), and LLPS has been found to be widely present in α-syn aberrant aggregate and mitophagy-related protein physiological processes. This review summarizes the occurrence of α-syn LLPS and its influencing factors, introduces the production and transformation of the related protein LLPS during PINK1-Parkin-mediated mitophagy, hoping to provide new ideas and methods for the study of PD pathology.

Influence of ion specificity and concentration on the conformational transition of intrinsically disordered sheep prion peptide

The structural sensitivity of the IDPs with the ions has been observed experimentally; however, it is still unclear how the presence of different metal ions affects structural stability. We performed atomistic molecular dynamics simulation of sheep prion peptide (142-167) in presence of different monovalent, divalent ions at various concentrations to find out the effect of the size, charge, and ionic concentration on the structure of the peptide. It is found that Li + ions have a higher survival probability compared to Na + , K + and Mg2 + affecting the solvation structure of the protein leading to the alpha-helix structure. At high concentration, due to the increase in the ion-solvent and ion-counter interactions, the effect of the ions is screened on the surface of the protein and hence no ion specificity is observed. This study demonstrates how ions can be used to regulate the protein structure and function that can help in designing drugs.

Calcium promotes α-synuclein liquid-liquid phase separation to accelerate amyloid aggregation

α-Synuclein (α-Syn) is an aggregation-prone protein whose accumulation in Lewy bodies leads to neurodegenerative diseases like Parkinson's disease (PD). Calcium plays a critical role in neurons, and calcium dysregulation is one of the risk factors of PD. It is known that Ca²⁺ interacts with α-Syn and affects its assembly. However, how Ca²⁺ regulates α-Syn aggregation remains unclear. Here, we reported that Ca²⁺ accelerates α-Syn amyloid aggregation through the modulation of protein phase separation. We observed that Ca²⁺ promotes the formation of α-Syn liquid droplets but does not change the protein fluidity inside the droplets. Further studies showed Ca²⁺-involved α-Syn droplets are still able to fuse. A metal chelator eliminated Ca²⁺-induced enlargement of α-Syn droplets, suggesting the influence of Ca²⁺ on α-Syn assembly could be reversed at the stage of liquid-liquid phase separation (LLPS). Interestingly, our data showed Ca²⁺ still promoted α-Syn phase separation in the presence of the lipid membranes. In addition, Ca²⁺/α-syn droplets could efficiently recruit lipid vesicles to the surface of these condensates. Our findings demonstrate that Ca²⁺ facilitates α-Syn phase separation to accelerate amyloid aggregation and pave the path for understanding the implications of Ca²⁺ in α-Syn accumulation and PD.

Binding Mechanisms of Amyloid-like Peptides to Lipid Bilayers and Effects of Divalent Cations

In several neurodegenerative diseases, cell toxicity can emerge from damage produced by amyloid aggregates to lipid membranes. The details accounting for this damage are poorly understood including how individual amyloid peptides interact with phospholipid membranes before aggregation. Here, we use all-atom molecular dynamics simulations to investigate the molecular mechanisms accounting for amyloid-membrane interactions and the role played by calcium ions in this interaction. Model peptides known to self-assemble into amyloid fibrils and bilayer made from zwitterionic and anionic lipids are used in this study. We find that both electrostatic and hydrophobic interactions contribute to peptide-bilayer binding. In particular, the attraction of peptides to lipid bilayers is dominated by electrostatic interactions between positive residues and negative phosphate moieties of lipid head groups. This attraction is stronger for anionic bilayers than for zwitterionic ones. Hydrophobicity drives the burial of nonpolar residues into the interior of the bilayer producing strong binding in our simulations. Moreover, we observe that the attraction of peptides to the bilayer is significantly reduced in the presence of calcium ions. This is due to the binding of calcium ions to negative phosphate moieties of lipid head groups, which leaves phospholipid bilayers with a net positive charge. Strong binding of the peptide to the membrane occurs less frequently in the presence of calcium ions and involves the formation of a "Ca²⁺ bridge".

Amyloid-Mediated Mechanisms of Membrane Disruption

Protein aggregation and amyloid formation are pathogenic events underlying the development of an increasingly large number of human diseases named “proteinopathies”. Abnormal accumulation in affected tissues of amyloid β (Aβ) peptide, islet amyloid polypeptide (IAPP), and the prion protein, to mention a few, are involved in the occurrence of Alzheimer’s (AD), type 2 diabetes mellitus (T2DM) and prion diseases, respectively. Many reports suggest that the toxic properties of amyloid aggregates are correlated with their ability to damage cell membranes. However, the molecular mechanisms causing toxic amyloid/membrane interactions are still far to be completely elucidated. This review aims at describing the mutual relationships linking abnormal protein conformational transition and self-assembly into amyloid aggregates with membrane damage. A cross-correlated analysis of all these closely intertwined factors is thought to provide valuable insights for a comprehensive molecular description of amyloid diseases and, in turn, the design of effective therapies.

DNA-Mediated Stack Formation of Nanodiscs

Membrane-scaffolding proteins (MSPs) derived from apolipoprotein A-1 have become a versatile tool in generating nano-sized discoidal membrane mimetics (nanodiscs) for membrane protein research. Recent efforts have aimed at exploiting their controlled lipid protein ratio and size distribution to arrange membrane proteins in regular supramolecular structures for diffraction studies. Thereby, direct membrane protein crystallization, which has remained the limiting factor in structure determination of membrane proteins, would be circumvented. We describe here the formation of multimers of membrane-scaffolding protein MSP1D1-bounded nanodiscs using the thiol reactivity of engineered cysteines. The mutated positions N42 and K163 in MSP1D1 were chosen to support chemical modification as evidenced by fluorescent labeling with pyrene. Minimal interference with the nanodisc formation and structure was demonstrated by circular dichroism spectroscopy, differential light scattering and size exclusion chromatography. The direct disulphide bond formation of nanodiscs formed by the MSP1D1_N42C variant led to dimers and trimers with low yield. In contrast, transmission electron microscopy revealed that the attachment of oligonucleotides to the engineered cysteines of MSP1D1 allowed the growth of submicron-sized tracts of stacked nanodiscs through the hybridization of nanodisc populations carrying complementary strands and a flexible spacer.

Amyloid Oligomers: A Joint Experimental/Computational Perspective on Alzheimer's Disease, Parkinson's Disease, Type II Diabetes, and Amyotrophic Lateral Sclerosis

Protein misfolding and aggregation is observed in many amyloidogenic diseases affecting either the central nervous system or a variety of peripheral tissues. Structural and dynamic characterization of all species along the pathways from monomers to fibrils is challenging by experimental and computational means because they involve intrinsically disordered proteins in most diseases. Yet understanding how amyloid species become toxic is the challenge in developing a treatment for these diseases. Here we review what computer, in vitro, in vivo, and pharmacological experiments tell us about the accumulation and deposition of the oligomers of the (Aβ, tau), α-synuclein, IAPP, and superoxide dismutase 1 proteins, which have been the mainstream concept underlying Alzheimer's disease (AD), Parkinson's disease (PD), type II diabetes (T2D), and amyotrophic lateral sclerosis (ALS) research, respectively, for many years.

Calcium, Bioenergetics, and Parkinson's Disease

Degeneration of substantia nigra (SN) dopaminergic (DAergic) neurons is responsible for the core motor deficits of Parkinson’s disease (PD). These neurons are autonomous pacemakers that have large cytosolic Ca²⁺ oscillations that have been linked to basal mitochondrial oxidant stress and turnover. This review explores the origin of Ca²⁺ oscillations and their role in the control of mitochondrial respiration, bioenergetics, and mitochondrial oxidant stress.

Mechanisms of aggregation and fibril formation of the amyloidogenic N-terminal fragment of apolipoprotein A-I

The N-terminal (1-83) fragment of the major constituent of plasma high-density lipoprotein, apolipoprotein A-I (apoA-I), strongly tends to form amyloid fibrils, leading to systemic amyloidosis. Here, using a series of deletion variants, we examined the roles of two major amyloidogenic segments (residues 14-22 and 50-58) in the aggregation and fibril formation of an amyloidogenic G26R variant of the apoA-I 1-83 fragment (apoA-I 1-83/G26R). Thioflavin T fluorescence assays and atomic force microscopy revealed that elimination of residues 14-22 completely inhibits fibril formation of apoA-I 1-83/G26R, whereas Δ32-40 and Δ50-58 variants formed fibrils with markedly reduced nucleation and fibril growth rates. CD measurements revealed structural transitions from random coil to β-sheet structures in all deletion variants except for the Δ14-22 variant, indicating that residues 14-22 are critical for the β-transition and fibril formation. Thermodynamic analysis of the kinetics of fibril formation by apoA-I 1-83/G26R indicated that both nucleation and fibril growth are enthalpically unfavorable, whereas entropically, nucleation is favorable, but fibril growth is unfavorable. Interestingly, the nucleation of the Δ50-58 variant was entropically unfavorable, indicating that residues 50-58 entropically promote the nucleation step in fibril formation of apoA-I 1-83/G26R. Moreover, a residue-level structural investigation of apoA-I 1-83/G26R fibrils with site-specific pyrene labeling indicated that the two amyloidogenic segments are in close proximity to form an amyloid core structure, whereas the N- and C-terminal tail regions are excluded from the amyloid core. These results provide critical insights into the aggregation mechanism and fibril structure of the amyloidogenic N-terminal fragment of apoA-I.

Effects of alpha-synuclein post-translational modifications on metal binding

Parkinson's disease is the second most common neurodegenerative disorder worldwide. Neurodegeneration in this pathology is characterized by the loss of dopaminergic neurons in the substantia nigra, coupled with cytoplasmic inclusions known as Lewy bodies containing α-synuclein. The brain is an organ that concentrates metal ions, and there is emerging evidence that a break-down in metal homeostasis may be a critical factor in a variety of neurodegenerative diseases. α-synuclein has emerged as an important metal-binding protein in the brain, whereas these interactions play an important role in its aggregation and might represent a link between protein aggregation, oxidative damage, and neuronal cell loss. Additionally, α-synuclein undergoes several post-translational modifications that regulate its structure and physiological function, and may be linked to the aggregation and/or oligomer formation. This review is focused on the interaction of this protein with physiologically relevant metal ions, highlighting the cases where metal-AS interactions profile as key modulators for its structural, aggregation, and membrane-binding properties. The impact of α-synuclein phosphorylation and N-terminal acetylation in the metal-binding properties of the protein are also discussed, underscoring a potential interplay between PTMs and metal ion binding in regulating α-synuclein physiological functions and its role in pathology. This article is part of the Special Issue "Synuclein".

Intrinsically disordered proteins in synaptic vesicle trafficking and release

The past few years have resulted in an increased awareness and recognition of the prevalence and roles of intrinsically disordered proteins and protein regions (IDPs and IDRs, respectively) in synaptic vesicle trafficking and exocytosis and in overall synaptic organization. IDPs and IDRs constitute a class of proteins and protein regions that lack stable tertiary structure, but nevertheless retain biological function. Their significance in processes such as cell signaling is now well accepted, but their pervasiveness and importance in other areas of biology are not as widely appreciated. Here, we review the prevalence and functional roles of IDPs and IDRs associated with the release and recycling of synaptic vesicles at nerve terminals, as well as with the architecture of these terminals. We hope to promote awareness, especially among neuroscientists, of the importance of this class of proteins in these critical pathways and structures. The examples discussed illustrate some of the ways in which the structural flexibility conferred by intrinsic protein disorder can be functionally advantageous in the context of cellular trafficking and synaptic function.

Amyloid growth and membrane damage: Current themes and emerging perspectives from theory and experiments on Aβ and hIAPP

Alzheimer's Disease (AD) and Type 2 diabetes mellitus (T2DM) are two incurable diseases both hallmarked by an abnormal deposition of the amyloidogenic peptides Aβ and Islet Amyloid Polypeptide (IAPP) in affected tissues. Epidemiological data demonstrate that patients suffering from diabetes are at high risk of developing AD, thus making the search for factors common to the two pathologies of special interest for the design of new therapies. Accumulating evidence suggests that the toxic properties of both Aβ or IAPP are ascribable to their ability to damage the cell membrane. However, the molecular details describing Aβ or IAPP interaction with membranes are poorly understood. This review focuses on biophysical and in silico studies addressing these topics. Effects of calcium, cholesterol and membrane lipid composition in driving aberrant Aβ or IAPP interaction with the membrane will be specifically considered. The cross correlation of all these factors appears to be a key issue not only to shed light in the countless and often controversial reports relative to this area but also to gain valuable insights into the central events leading to membrane damage caused by amyloidogenic peptides. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

C-terminal calcium binding of α-synuclein modulates synaptic vesicle interaction

Alpha-synuclein is known to bind to small unilamellar vesicles (SUVs) via its N terminus, which forms an amphipathic alpha-helix upon membrane interaction. Here we show that calcium binds to the C terminus of alpha-synuclein, therewith increasing its lipid-binding capacity. Using CEST-NMR, we reveal that alpha-synuclein interacts with isolated synaptic vesicles with two regions, the N terminus, already known from studies on SUVs, and additionally via its C terminus, which is regulated by the binding of calcium. Indeed, dSTORM on synaptosomes shows that calcium mediates the localization of alpha-synuclein at the pre-synaptic terminal, and an imbalance in calcium or alpha-synuclein can cause synaptic vesicle clustering, as seen ex vivo and in vitro. This study provides a new view on the binding of alpha-synuclein to synaptic vesicles, which might also affect our understanding of synucleinopathies.

The active role of Ca2+ ions in Aβ-mediated membrane damage

Calcium ions inhibits Aβ induced membrane poration by small-sized oligomers but significantly foster fiber-dependent membrane disruption.

Membrane Binding and Oligomerization of the Lipopeptide A54145 Studied by Pyrene Fluorescence

A54145 is a lipopeptide antibiotic related to daptomycin that permeabilizes bacterial cell membranes. Its action requires both calcium and phosphatidylglycerol in the target membrane, and it is accompanied by the formation of membrane-associated oligomers. We here probed the interaction of A54145 with model membranes composed of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol, using the steady-state and time-resolved fluorescence of a pyrene-labeled derivative (Py-A54145). In solution, the labeled peptide was found to exist as a monomer. Its membrane interaction occurred in two stages that could be clearly distinguished by varying the calcium concentration. In the first stage, which was observed between 0.15 and 1 mM calcium, Py-A54145 bound to the membrane, as indicated by a strong increase in pyrene monomer emission. At the same calcium concentration, excimer emission increased also, suggesting that Py-A54145 had oligomerized. A global analysis of the time-resolved pyrene monomer and excimer fluorescence confirmed that Py-A54145 forms oligomers quantitatively and concomitantly with membrane binding. When calcium was raised beyond 1 mM, a distinct second transition was observed that may correspond to a doubling of the number of oligomer subunits. The collective findings confirm and extend our understanding of the action mode of A54145 and daptomycin.

Pyrene-Apelin Conjugation Modulates Fluorophore- and Peptide-Micelle Interactions

Bioactive apelin peptide forms ranging in length from 12 to 55 amino acids bind to and activate the apelin receptor (AR or APJ), a class A G-protein coupled receptor. Apelin-12, -17, and -36 isoforms, named according to length, with an additional N-terminal cysteine residue allowed for regiospecific and efficient conjugation of pyrene maleimide. Through steady-state fluorescence spectroscopy, the emission properties of pyrene in aqueous buffer were compared to those of the pyrene-apelin conjugates both without and with zwitterionic or anionic micelles. Pyrene photophysics are consistent with an expected partitioning into the hydrophobic micellar cores, while pyrene-apelin conjugation prevented this partitioning. Apelin, conversely, is expected to preferentially interact with anionic micelles; pyrene-apelin conjugates appear to lose preferential interaction. Finally, Förster resonance energy transfer between pyrene and tryptophan residues in the N-terminal tail and first transmembrane segment (the AR55 construct, comprising residues 1-55 of the AR) was consistent with efficient nonspecific pyrene-apelin conjugate binding to micelles rather than direct, specific apelin-AR55 binding. This approach provides a versatile fluorophore conjugation strategy for apelin, particularly valuable given that even a highly hydrophobic fluorophore is not deleterious to peptide behavior in membrane-mimetic micellar systems.

Molecular Understanding of Aβ-hIAPP Cross-Seeding Assemblies on Lipid Membranes

Amyloid-β (Aβ) and human islet polypeptide (hIAPP) are the causative agents responsible for Alzheimer's disease (AD) and type II diabetes (T2D), respectively. While numerous studies have reported the cross-seeding behavior of Aβ and hIAPP in solution, little effort has been made to examine the cross-seeding of Aβ and hIAPP in the presence of cell membranes, which is more biologically relevant to the pathological link between AD and T2D. In this work, we computationally study the cross-seeding and adsorption behaviors of Aβ and hIAPP on zwitterionic POPC and anionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylglycerol (POPG) mixed bilayers using all-atom molecular dynamics (MD) simulations, particularly aiming to the effects of the initial orientation of the Aβ-hIAPP assembly and the lipid composition of cell membranes on mutual structural and interaction changes in both Aβ-hIAPP assembly and lipid bilayers at the atomic level. Aβ-hIAPP cross-seeding assembly always preferred to adopt a specific orientation and interface to associate with both lipid bilayers strongly via the N-terminal strands of Aβ. Such membrane-bound orientation explains experimental observation that hybrid Aβ-hIAPP fibrils on cell membranes showed similar morphologies to pure hIAPP fibrils. Moreover, Aβ-hIAPP assembly, regardless of its initial orientations, interacted more strongly with POPC/POPG bilayer than POPC bilayer, indicating that electrostatic interactions are the major forces governing peptide-lipid interactions. Strong electrostatic interactions were also attributed to the formation of Ca²⁺ bridges connecting both negatively charged Glu of Aβ and PO₄ head groups of lipids, which facilitate the association of Aβ-hIAPP with the POPC/POPG bilayer. It was also found that the strong peptide-lipid binding reduced lipid fluidity. Both facts imply that Aβ-hIAPP assembly may induce cell damage by altering calcium homeostasis and cell membrane phase. This work provides a better fundamental understanding of cross-seeding of Aβ and hIAPP on cell membranes and a potential pathological link between AD and T2D.

Mechanisms of alpha-synuclein action on neurotransmission: cell-autonomous and non-cell autonomous role

Mutations and duplication/triplication of the alpha-synuclein (αSyn)-coding gene have been found to cause familial Parkinson’s disease (PD), while genetic polymorphisms in the region controlling the expression level and stability of αSyn have been identified as risk factors for idiopathic PD, pointing to the importance of wild-type (wt) αSyn dosage in the disease. Evidence that αSyn is present in the cerebrospinal fluid and interstitial brain tissue and that healthy neuronal grafts transplanted into PD patients often degenerate suggests that extracellularly-released αSyn plays a role in triggering the neurodegenerative process. αSyn’s role in neurotransmission has been shown in various cell culture models in which the protein was upregulated or deleted and in knock out and transgenic animal, with different results on αSyn’s effect on synaptic vesicle pool size and mobilization, αSyn being proposed as a negative or positive regulator of neurotransmitter release. In this review, we discuss the effect of αSyn on pre- and post-synaptic compartments in terms of synaptic vesicle trafficking, calcium entry and channel activity, and we focus on the process of exocytosis and internalization of αSyn and on the spreading of αSyn-driven effects due to the presence of the protein in the extracellular milieu.

Protein Misfolding in Lipid-Mimetic Environments

Among various cellular factors contributing to protein misfolding and subsequent aggregation, membranes occupy a special position due to the two-way relations between the aggregating proteins and cell membranes. On one hand, the unstable, toxic pre-fibrillar aggregates may interact with cell membranes, impairing their functions, altering ion distribution across the membranes, and possibly forming non-specific membrane pores. On the other hand, membranes, too, can modify structures of many proteins and affect the misfolding and aggregation of amyloidogenic proteins. The effects of membranes on protein structure and aggregation can be described in terms of the "membrane field" that takes into account both the negative electrostatic potential of the membrane surface and the local decrease in the dielectric constant. Water-alcohol (or other organic solvent) mixtures at moderately low pH are used as model systems to study the joint action of the local decrease of pH and dielectric constant near the membrane surface on the structure and aggregation of proteins. This chapter describes general mechanisms of structural changes of proteins in such model environments and provides examples of various proteins aggregating in the "membrane field" or in lipid-mimetic environments.

α-Synuclein forms non-selective cation channels and stimulates ATP-sensitive potassium channels in hippocampal neurons

KEY POINTS

In Parkinson's disease, the protein α-synuclein (αS) is produced within neurons and also appears in the extracellular fluid. In this study in hippocampal neurons, αS formed non-selective cation channels with multiple levels of conductance and rectification depending on their insertion site. αS channels induced local spontaneous increases in intracellular Na(+) and Ca(2+), depolarized neurons, augmented bursting activity and stimulated the opening of ATP-sensitive K(+) channels. Non-selective channels were also observed in neurons transfected with either wild-type or mutant A53T αS, and after extracellular application of these proteins. The properties of αS channels in neuronal membranes suggest that extracellular αS is more toxic than αS produced within neurons. In Parkinson's disease and several other neurodegenerative diseases, the protein α-synuclein (αS) is produced within neurons and accumulates in the extracellular fluid. Several mechanisms of αS action are proposed, one of which is the formation of cation-permeable pores that may mediate toxicity. αS induces non-selective cation channels in lipid bilayers, but whether this occurs in living neurons and which properties the channels possess have not yet been examined. In this study the properties of αS channels in dissociated hippocampal neurons are documented. In cell-attached recordings the incorporation of αS into membranes was driven by applied negative potentials. These channels exhibited multiple levels of conductance (30, 70 and 120 pS at -100 mV) and inward rectification. The persistent activity of αS channels induced local changes in intracellular Na(+) and Ca(2+), depolarized neurons and augmented bursting activity. αS channels formed by adding αS to the intracellular membrane in inside-out patches exhibited outward rectification. αS channels were equally permeable to Na(+), K(+) and Ca(2+). These channels were also observed in neurons transfected with wild-type or mutant A53T αS, and after extracellular application of wild-type or mutant A53T αS proteins. Opening of αS channels stimulated opening of ATP-sensitive K(+) (KATP ) channels and did not interfere with the activity of delayed rectifier K(+) channels. The properties of αS channels in neuronal membranes suggest stronger toxicity of extracellularly applied αS than intracellular αS. Enhancement of neuronal excitability and distortions in ion homeostasis may underlie the toxic effects of αS that can be dampened by KATP channels.

Ca2+-dependent intracellular drug delivery system developed with "raspberry-type" particles-on-a-particle comprising mesoporous silica core and α-synuclein-coated gold nanoparticles

For the development of an intracellular cargo release system with mesoporous silica nanoparticles (MSN), gold nanoparticles coated with an amyloidogenic protein of α-synuclein were employed to prepare a protein-mediated nanocomposite into the "raspberry-type" particles-on-a-particle (PoP). The PoPs were successfully fabricated only at pH 4.4 by yielding the MSN coverage to 75.3% with 5 nm gold nanoparticles covalently coated with a mutant form of α-synuclein containing a cysteine residue at the C-terminus. The entrapped cargo of rhodamine 6G was shown to be selectively released from PoPs upon exposure to divalent cations including the α-synuclein-specific pathophysiological ligand of Ca(2+). Intracellular uptake of the PoPs preloaded with doxorubicin as an anticancer drug and its subsequent Ca(2+)-dependent release were demonstrated with HeLa cells in the presence of intracellular Ca(2+)-regulating agents. Therefore, the fabrication of PoPs with the self-interactive protein of α-synuclein is expected to serve as a platform technology for preparation of diversified nanocomposites with various nanoparticles and/or bioactive molecules for eventual applications in the areas of theranostics.

Environmental neurotoxic challenge of conditional alpha-synuclein transgenic mice predicts a dopaminergic olfactory-striatal interplay in early PD

The olfactory bulb (OB) is one of the first brain regions in Parkinson's disease (PD) to contain alpha-synuclein (α-syn) inclusions, possibly associated with nonmotor symptoms. Mechanisms underlying olfactory synucleinopathy, its contribution to progressive aggregation pathology and nigrostriatal dopaminergic loss observed at later stages, remain unclear. A second hit, such as environmental toxins, is suggestive for α-syn aggregation in olfactory neurons, potentially triggering disease progression. To address the possible pathogenic role of olfactory α-syn accumulation in early PD, we exposed mice with site-specific and inducible overexpression of familial PD-linked mutant α-syn in OB neurons to a low dose of the herbicide paraquat. Here, we found that olfactory α-syn per se elicited structural and behavioral abnormalities, characteristic of an early time point in models with widespread α-syn expression, including hyperactivity and increased striatal dopaminergic marker. Suppression of α-syn reversed the dopaminergic phenotype. In contrast, paraquat treatment synergistically induced degeneration of olfactory dopaminergic cells and opposed the higher reactive phenotype. Neither neurodegeneration nor behavioral abnormalities were detected in paraquat-treated mice with suppressed α-syn expression. By increasing calpain activity, paraquat induced a pathological cascade leading to inhibition of autophagy clearance and accumulation of calpain-cleaved truncated and insoluble α-syn, recapitulating biochemical and structural changes in human PD. Thus our results underscore the primary role of proteolytic failure in aggregation pathology. In addition, we provide novel evidence that olfactory dopaminergic neurons display an increased vulnerability toward neurotoxins in dependence to presence of human α-syn, possibly mediating an olfactory-striatal dopaminergic network dysfunction in mouse models and early PD.

Structural and dynamical insights into the membrane-bound α-synuclein

Membrane-induced disorder-to-helix transition of α-synuclein, a presynaptic protein, has been implicated in a number of important neuronal functions as well as in the etiology of Parkinson’s disease. In order to obtain structural insights of membrane-bound α-synuclein at the residue-specific resolution, we took advantage of the fact that the protein is devoid of tryptophan and incorporated single tryptophan at various residue positions along the sequence. These tryptophans were used as site-specific markers to characterize the structural and dynamical aspects of α-synuclein on the negatively charged small unilamellar lipid vesicles. An array of site-specific fluorescence readouts, such as the spectral-shift, quenching efficiency and anisotropy, allowed us to discern various features of the conformational rearrangements occurring at different locations of α-synuclein on the lipid membrane. In order to define the spatial localization of various regions of the protein near the membrane surface, we utilized a unique and sensitive indicator, namely, red-edge excitation shift (REES), which originates when a fluorophore is located in a highly ordered micro-environment. The extent of REES observed at different residue positions allowed us to directly identify the residues that are localized at the membrane-water interface comprising a thin (∼ 15 Å) layer of motionally restrained water molecules and enabled us to construct a dynamic hydration map of the protein. The combination of site-specific fluorescence readouts allowed us to unravel the intriguing molecular details of α-synuclein on the lipid membrane in a direct model-free fashion. Additionally, the combination of methodologies described here are capable of distinguishing subtle but important structural alterations of α-synuclein bound to different negatively charged lipids with varied head-group chemistry. We believe that the structural modulations of α-synuclein on the membrane could potentially be related to its physiological functions as well as to the onset of Parkinson’s diseases.

Ca(2+) modulating α-synuclein membrane transient interactions revealed by solution NMR spectroscopy

α-Synuclein is involved in Parkinson's disease and its interaction with cell membrane is crucial to its pathological and physiological functions. Membrane properties, such as curvature and lipid composition, have been shown to affect the interactions by various techniques, but ion effects on α-synuclein membrane interactions remain elusive. Ca(2+) dynamic fluctuation in neurons plays important roles in the onset of Parkinson's disease and its influx is considered as one of the reasons to cause cell death. Using solution Nuclear Magnetic Resonance (NMR) spectroscopy, here we show that Ca(2+) can modulate α-synuclein membrane interactions through competitive binding to anionic lipids, resulting in dissociation of α-synuclein from membranes. These results suggest a negative modulatory effect of Ca(2+) on membrane mediated normal function of α-synuclein, which may provide a clue, to their dysfunction in neurodegenerative disease.

Modulation of human α-synuclein aggregation by a combined effect of calcium and dopamine

Parkinson's disease is characterized by the deposition of aggregated α-syn and its familial mutants into Lewy bodies leading to death of dopaminergic neurons. α-syn is involved in Ca(II) and dopamine (DA) signaling and their adequate balance inside neuronal cytoplasm is essential for maintaining healthy dopaminergic neurons. We have probed the binding energetics of Ca(II) and DA to human α-syn and its familial mutants A30P, A53T and E46K using isothermal titration calorimetry and have investigated the conformational and aggregation aspects using circular dichroism and fluorescence spectroscopy. While binding of Ca(II) to α-syn and its familial mutants was observed to be endothermic in nature, interaction of DA with α-syn was not detectable. Ca(II) enhanced fibrillation of α-syn and its familial mutants while DA promoted the formation of oligomers. However, Ca(II) and DA together critically favored the formation of protofibrils that are more cytotoxic than the mature fibrils. Using fluorescently labeled cysteine mutant A90C, we have shown that different aggregating species of α-syn formed in the presence of Ca(II) and DA are internalized into the human neuroblastoma cells with different rates and are responsible for the differential cytotoxicity depending on their nature. The findings put together suggest that an interplay between the concentrations of Ca(II), DA and α-syn can critically regulate the formation of various aggregating species responsible for the survival of dopaminergic neurons. Modulating this balance leading to either complete suppression of α-syn aggregation or promoting the formation of mature fibrils could be used as a strategy for the development of drugs to cure Parkinson's disease.

Physiological phenotype and vulnerability in Parkinson's disease

This review will focus on the principles underlying the hypothesis that neuronal physiological phenotype-how a neuron generates and regulates action potentials-makes a significant contribution to its vulnerability in Parkinson's disease (PD) and aging. A cornerstone of this hypothesis is that the maintenance of ionic gradients underlying excitability can pose a significant energetic burden for neurons, particularly those that have sustained residence times at depolarized membrane potentials, broad action potentials, prominent Ca(2+) entry, and modest intrinsic Ca(2+) buffering capacity. This energetic burden is shouldered in neurons primarily by mitochondria, the sites of cellular respiration. Mitochondrial respiration increases the production of damaging superoxide and other reactive oxygen species (ROS) that have widely been postulated to contribute to cellular aging and PD. Many of the genetic mutations and toxins associated with PD compromise mitochondrial function, providing a mechanistic linkage between known risk factors and cellular physiology that could explain the pattern of pathology in PD. Because much of the mitochondrial burden created by this at-risk phenotype is created by Ca(2+) entry through L-type voltage-dependent channels for which there are antagonists approved for human use, a neuroprotective strategy to reduce this burden is feasible.

Binding of polarity-sensitive hydrophobic ligands to erythroid and nonerythroid spectrin: fluorescence and molecular modeling studies

We have used three polarity-sensitive fluorescence probes, 6-propionyl 2-(N,N-dimethyl-amino) naphthalene (Prodan), pyrene and 8-anilino 1-naphthalene sulphonic acid, to study their binding with erythroid and nonerythroid spectrin, using fluorescence spectroscopy. We have found that both bind to prodan and pyrene with high affinities with apparent dissociation constants (Kd) of .50 and .17 μM, for prodan, and .04 and .02 μM, for pyrene, respectively. The most striking aspect of these bindings have been that the binding stoichiometry have been equal to 1 in erythroid spectrin, both in dimeric and tetrameric form, and in tetrameric nonerythroid spectrin. From an estimate of apparent dielectric constants, the polarity of the binding site in both erythroid and nonerythroid forms have been found to be extremely hydrophobic. Thermodynamic parameters associated with such binding revealed that the binding is favored by positive change in entropy. Molecular docking studies alone indicate that both prodan and pyrene bind to the four major structural domains, following the order in the strength of binding to the Ankyrin binding domain > SH3 domain > Self-association domain > N-terminal domain of α-spectrin of both forms of spectrin. The binding experiments, particularly with the tetrameric nonerythroid spectrin, however, indicate more toward the self association domain in offering the unique binding site, since the binding stoichiometry have been 1 in all forms of dimeric and tetrameric spectrin, so far studied by us. Further studies are needed to characterize the hydrophobic binding sites in both forms of spectrin.

Fluorescence analysis of the lipid binding-induced conformational change of apolipoprotein E4

Apolipoprotein (apo) E is thought to undergo conformational changes in the N-terminal helix bundle domain upon lipid binding, modulating its receptor binding activity. In this study, site-specific fluorescence labeling of the N-terminal (S94) and C-terminal (W264 or S290) helices in apoE4 by pyrene maleimide or acrylodan was employed to probe the conformational organization and lipid binding behavior of the N- and C-terminal domains. Guanidine denaturation experiments monitored by acrylodan fluorescence demonstrated the less organized, more solvent-exposed structure of the C-terminal helices compared to the N-terminal helix bundle. Pyrene excimer fluorescence together with gel filtration chromatography indicated that there are extensive intermolecular helix-helix contacts through the C-terminal helices of apoE4. Comparison of increases in pyrene fluorescence upon binding of pyrene-labeled apoE4 to egg phosphatidylcholine small unilamellar vesicles suggests a two-step lipid-binding process; apoE4 initially binds to a lipid surface through the C-terminal helices followed by the slower conformational reorganization of the N-terminal helix bundle domain. Consistent with this, fluorescence resonance energy transfer measurements from Trp residues to acrylodan attached at position 94 demonstrated that upon binding to the lipid surface, opening of the N-terminal helix bundle occurs at the same rate as the increase in pyrene fluorescence of the N-terminal domain. Such a two-step mechanism of lipid binding of apoE4 is likely to apply to mostly phospholipid-covered lipoproteins such as VLDL. However, monitoring pyrene fluorescence upon binding to HDL(3) suggests that not only apoE-lipid interactions but also protein-protein interactions are important for apoE4 binding to HDL(3).

The association between glaucoma prevalence and supplementation with the oxidants calcium and iron

PURPOSE

To investigate the relationship between supplementary consumption of the oxidants calcium and iron and the prevalence of glaucoma.

METHODS

This cross-sectional study included 3833 participants in the National Health and Nutrition Examination Survey (NHANES) for 2007 and 2008, ≥ 40 years of age, who reported a presence or absence of glaucoma. Participants were interviewed regarding the use of dietary supplements and antacids during the preceding 30-day period. Data pertaining to the supplementary intake of calcium and iron was aggregated and divided into quintiles. Information regarding the presence or absence of glaucoma and demographics, comorbidities, and health-related behavior was obtained via interview.

RESULTS

Participants who consumed ≥ 800 mg/d of supplementary calcium or ≥ 18 mg/d of supplementary iron had significantly higher odds of having been diagnosed with glaucoma than did those who had not consumed supplementary calcium or iron, after adjustment for potential confounders (odds ratio [OR] 2.44, 95% confidence interval [CI] 1.25-4.76 for calcium; OR 3.80, 95% CI 1.79-8.06 for iron). Concurrent consumption of both calcium and iron above these levels was associated with still greater odds of having been diagnosed with glaucoma (OR 7.24, 95% CI 2.42-21.62). A clear dose-response relationship between quintiles of supplementary calcium or iron intake and glaucoma prevalence was not found.

CONCLUSIONS

These results suggest that there may be a threshold intake of iron and calcium above which there is an increased risk of development of glaucoma. Prospective longitudinal studies are needed, to assess whether oxidant intake is a risk factor for development and progression of glaucoma.

Depth of α-synuclein in a bilayer determined by fluorescence, neutron reflectometry, and computation

α-Synuclein (α-syn) membrane interactions are implicated in the pathogenesis of Parkinson's disease. Fluorescence and neutron reflectometry (NR) measurements reveal that α-syn penetrates ∼9-14 Å into the outer leaflet of the bilayer, with a substantial portion of the membrane-bound polypeptide extending into the aqueous solvent. For the first time, to our knowledge, we used NR to obtain direct quantitative evidence of α-syn-induced membrane thinning. To examine the effect of specific residues on membrane penetration depths, we used a series of W4-containing N-terminal peptides. We identified that the first 15 residues (P15) nearly recapitulate the features of the full-length protein (i.e., partition constants, molecular mobility, and insertion of the W4 side chain into the bilayer), and found that as few as the first four N-terminal residues are sufficient for vesicle binding. Although at least one imperfect amphipathic repeat sequence (KAKEGV) is required for α-helical formation, secondary structural formation has little effect on membrane affinity. To develop an N-terminal α-syn model for bilayer interactions, we performed molecular-dynamics simulations of the P15 peptide submerged in a bilayer. The simulation results are highly consistent with experimental data indicating a broad low-energy region (8.5-14.5 Å) for W4 insertion.

Molecular insights into Parkinson's disease

Mutations in SNCA, PINK1, parkin, and DJ-1 are associated with autosomal-dominant or autosomal-recessive forms of Parkinson's disease (PD), the second most common neurodegenerative disorder. Studies on the structural and functional properties of the corresponding gene products have provided significant insights into the molecular underpinnings of familial PD and the much more common sporadic forms of the disease. Here, we review recent advances in our understanding of four PD-related gene products: α-synuclein, parkin, PINK1, and DJ-1. In Part 1, we review new insights into the role of α-synuclein in PD. In Part 2, we summarize the latest developments in understanding the role of mitochondrial dysfunction in PD, emphasizing the role of the PINK1/parkin pathway in regulating mitochondrial dynamics and mitophagy. The role of DJ-1 is also discussed. In Part 3, we point out converging pathways and future directions.

Α-synuclein misfolding and Parkinson's disease

Substantial evidence links α-synuclein, a small highly conserved presynaptic protein with unknown function, to both familial and sporadic Parkinson's disease (PD). α-Synuclein has been identified as the major component of Lewy bodies and Lewy neurites, the characteristic proteinaceous deposits that are the hallmarks of PD. α-Synuclein is a typical intrinsically disordered protein, but can adopt a number of different conformational states depending on conditions and cofactors. These include the helical membrane-bound form, a partially-folded state that is a key intermediate in aggregation and fibrillation, various oligomeric species, and fibrillar and amorphous aggregates. The molecular basis of PD appears to be tightly coupled to the aggregation of α-synuclein and the factors that affect its conformation. This review examines the different aggregation states of α-synuclein, the molecular mechanism of its aggregation, and the influence of environmental and genetic factors on this process.

Pyrene: a probe to study protein conformation and conformational changes

The review focuses on the unique spectral features of pyrene that can be utilized to investigate protein structure and conformation. Pyrene is a fluorescent probe that can be attached covalently to protein side chains, such as sulfhydryl groups. The spectral features of pyrene are exquisitely sensitive to the microenvironment of the probe: it exhibits an ensemble of monomer fluorescence emission peaks that report on the polarity of the probe microenvironment, and an additional band at longer wavelengths, the appearance of which reflects the presence of another pyrene molecule in spatial proximity (~10 Å). Its high extinction coefficient allows us to study labeled proteins in solution at physiologically relevant concentrations. The environmentally- and spatially-sensitive features of pyrene allow monitoring protein conformation, conformational changes, protein folding and unfolding, protein-protein, protein-lipid and protein-membrane interactions.

Role of metal ions in aggregation of intrinsically disordered proteins in neurodegenerative diseases

Neurodegenerative diseases constitute a set of pathological conditions originating from the slow, irreversible, and systematic cell loss within the various regions of the brain and/or the spinal cord. Depending on the affected region, the outcomes of the neurodegeneration are very broad and diverse, ranging from the problems with movements to dementia. Some neurodegenerative diseases are associated with protein misfolding and aggregation. Many proteins that misfold in human neurodegenerative diseases are intrinsically disordered; i.e., they lack a stable tertiary and/or secondary structure under physiological conditions in vitro. These intrinsically disordered proteins (IDPs) functionally complement ordered proteins, being typically involved in regulation and signaling. There is accumulating evidence that altered metal homeostasis may be related to the progression of neurodegenerative diseases. This review examines the effects of metal ion binding on the aggregation pathways of IDPs found in neurodegenerative diseases.

The N-terminus of the intrinsically disordered protein α-synuclein triggers membrane binding and helix folding

Alpha-synuclein (αS) is a 140-amino-acid protein that is involved in a number of neurodegenerative diseases. In Parkinson's disease, the protein is typically encountered in intracellular, high-molecular-weight aggregates. Although αS is abundant in the presynaptic terminals of the central nervous system, its physiological function is still unknown. There is strong evidence for the membrane affinity of the protein. One hypothesis is that lipid-induced binding and helix folding may modulate the fusion of synaptic vesicles with the presynaptic membrane and the ensuing transmitter release. Here we show that membrane recognition of the N-terminus is essential for the cooperative formation of helical domains in the protein. We used circular dichroism spectroscopy and isothermal titration calorimetry to investigate synthetic peptide fragments from different domains of the full-length αS protein. Site-specific truncation and partial cleavage of the full-length protein were employed to further characterize the structural motifs responsible for helix formation and lipid-protein interaction. Unilamellar vesicles of varying net charge and lipid compositions undergoing lateral phase separation or chain melting phase transitions in the vicinity of physiological temperatures served as model membranes. The results suggest that the membrane-induced helical folding of the first 25 residues may be driven simultaneously by electrostatic attraction and by a change in lipid ordering. Our findings highlight the significance of the αS N-terminus for folding nucleation, and provide a framework for elucidating the role of lipid-induced conformational transitions of the protein within its intracellular milieu.

Membrane Permeabilization by Oligomeric α-Synuclein: In Search of the Mechanism

Background

The question of how the aggregation of the neuronal protein α-synuclein contributes to neuronal toxicity in Parkinson's disease has been the subject of intensive research over the past decade. Recently, attention has shifted from the amyloid fibrils to soluble oligomeric intermediates in the α-synuclein aggregation process. These oligomers are hypothesized to be cytotoxic and to permeabilize cellular membranes, possibly by forming pore-like complexes in the bilayer. Although the subject of α-synuclein oligomer-membrane interactions has attracted much attention, there is only limited evidence that supports the pore formation by α-synuclein oligomers. In addition the existing data are contradictory.

Methodology/Principal Findings

Here we have studied the mechanism of lipid bilayer disruption by a well-characterized α-synuclein oligomer species in detail using a number of in vitro bilayer systems and assays. Dye efflux from vesicles induced by oligomeric α-synuclein was found to be a fast all-or-none process. Individual vesicles swiftly lose their contents but overall vesicle morphology remains unaltered. A newly developed assay based on a dextran-coupled dye showed that non-equilibrium processes dominate the disruption of the vesicles. The membrane is highly permeable to solute influx directly after oligomer addition, after which membrane integrity is partly restored. The permeabilization of the membrane is possibly related to the intrinsic instability of the bilayer. Vesicles composed of negatively charged lipids, which are generally used for measuring α-synuclein-lipid interactions, were unstable to protein adsorption in general.

Conclusions/Significance

The dye efflux from negatively charged vesicles upon addition of α-synuclein has been hypothesized to occur through the formation of oligomeric membrane pores. However, our results show that the dye efflux characteristics are consistent with bilayer defects caused by membrane instability. These data shed new insights into potential mechanisms of toxicity of oligomeric α-synuclein species.

Apolipoprotein E LDL receptor-binding domain-containing high-density lipoprotein: a nanovehicle to transport curcumin, an antioxidant and anti-amyloid bioflavonoid

Curcumin is an antioxidant and anti-inflammatory bioflavonoid that has been recently identified as an anti-amyloid agent as well. To make it more available in its potent form as a potential amyloid disaggregation agent, we employed high-density lipoproteins (HDL), which are lipid-protein complexes that transport plasma cholesterol, to transport curcumin. The objective of this study was to employ reconstituted HDL containing human apoE3 N-terminal (NT) domain, as a vehicle to transport curcumin. The NT domain serves as a ligand to mediate binding and uptake of lipoprotein complexes via the low-density lipoprotein receptor (LDLr) family of proteins located at the cell surface. Reconstituted HDL was prepared with phospholipids and recombinant apoE3-NT domain in the absence or presence of curcumin. Non-denaturing polyacrylamide gel electrophoresis indicated that the molecular mass and Stokes' diameter of HDL bearing curcumin were ~670kDa and ~17nm, respectively, while electron microscopy revealed the presence of discoidal particles. Fluorescence emission spectra of HDL bearing (the intrinsically fluorescent) curcumin indicated that the wavelength of maximal fluorescence emission (λ(max)) of curcumin was ~495nm, which is highly blue-shifted compared to λ(max) of curcumin in solvents of varying polarity (λ(max) ranging from 515-575nm) or in aqueous buffers. In addition, an enormous enhancement in fluorescence emission intensity was noted in curcumin-containing HDL compared to curcumin in aqueous buffers. Curcumin fluorescence emission was quenched to a significant extent by lipid-based quenchers but not by aqueous quenchers. These observations indicate that curcumin has partitioned efficiently into the hydrophobic milieu of the phospholipid bilayer of HDL. Functional assays indicated that the LDLr-binding ability of curcumin-containing HDL with apoE3-NT is similar to that of HDL without curcumin. Taken together, we report that apoE-containing HDL has a tremendous potential as a 'nanovehicle' with a homing device to transport curcumin to target sites.

Tryptophan probes at the alpha-synuclein and membrane interface

Understanding how environmental factors affect the conformational dynamics of alpha-synuclein (alpha-syn) is of great importance because the accumulation and deposit of aggregated alpha-syn in the brain are intimately connected to Parkinson's disease etiology. Measurements of steady-state and time-resolved fluorescence of single tryptophan-containing alpha-syn variants have revealed distinct phospholipid vesicle and micelle interactions at residues 4, 39, 94, and 125. Our circular dichroism data confirm that Trp mutations do not affect alpha-syn membrane binding properties (apparent association constant K(a)app approximately 1 x 10(7) M(-1) for all synucleins) saturating at an estimated lipid-to-protein molar ratio of 380 or approximately 120 proteins covering approximately 7% of the surface area of an 80 nm diameter vesicle. Fluorophores at positions 4 and 94 are the most sensitive to the lipid bilayer with pronounced spectral blue-shifts (W4: Delta(lambda)max approximately 23 nm; W94: Delta(lambda)max approximately 10 nm) and quantum yield increases (W4, W94: approximately 3 fold), while W39 and W125 remain primarily water-exposed. Time-resolved fluorescence data show that all sites (except W125) have subpopulations that interact with the membrane.

Adsorption of alpha-synuclein on lipid bilayers: modulating the structure and stability of protein assemblies

The interaction of alpha-synuclein with phospholipid membranes has been examined using supported lipid bilayers and epi-fluorescence microscopy. The membranes contained phosphatidylcholine (PC) and phosphatidic acid (PA), which mix at physiological pH. Upon protein adsorption, the lipids undergo fluid-fluid phase separation into PC-rich and PA-rich regions. The protein preferentially adsorbs to the PA-rich regions. The adsorption and subsequent aggregation of alpha-synuclein was probed by tuning several parameters: the charge on the lipids, the charge on the protein, and the screening environment. Conditions which promoted the greatest extent of adsorption resulted in structurally heterogeneous aggregates, while comparatively homogeneous aggregates were observed under conditions whereby adsorption did not occur as readily. Our observation that different alterations to the system lead to different degrees of aggregation and different aggregate structures poses a challenge for drug discovery. Namely, therapies aimed at neutralizing alpha-synuclein must target a broad range of potentially toxic, membrane-bound assemblies.

Pyrene fluorescence analysis offers new insights into the conformation of the lipoprotein-binding domain of human apolipoprotein E

The C-terminal domain (CT) of apolipoprotein E (apoE), a critical protein involved in cholesterol transport in the plasma and brain, plays an important role in high-affinity lipoprotein binding. Although high-resolution structural information is available for the N-terminal domain of apoE, the structural organization of the CT (residues 201-299) is largely unknown. In this study, we employ site-specific fluorescence labeling with pyrene maleimide to gain insight into the structure and conformation of apoE CT in its naturally self-associated state in buffer at physiologically relevant concentrations (5-50 microg/mL). Pyrene is a highly sensitive fluorophore that reports on spatial proximity between desired sites by displaying unique spectral features. Pyrene was covalently attached to single cysteine-containing recombinant human apoE CT at position 223 or 255 to probe the first predicted helical segment and at position 277 to monitor the terminal predicted helical segment. Regardless of the location of the probe, all three pyrene-labeled apoE CT variants display an intense and dramatic fluorescence excimer band at 460 nm, a signature feature of pyrene, which indicates that two pyrene moieties are within 10 A of each other. In addition, an intense peak at 387 nm (indicative of a highly hydrophobic environment) was noted in all cases. Fluorescence emission quenching by potassium iodide indicates that the accessibility to the probes was restricted at these locations. The possibility that the hydrophobicity of the pyrene moiety was the driving force for helix-helix interaction was excluded because pyrene located at position 209, which is predicted to be located in a nonhelical segment, did not display the above intense unique features. Lastly, denaturation studies suggest that the terminal helix unfolds prior to the first predicted helix in apoE CT. Our studies indicate that there are extensive intermolecular helix-helix contacts throughout the entire CT in the lipid-free state with two apoE CT molecules oriented parallel to each other to form a dimer, which dimerizes further to yield a tetramer. Such an organization allows helix-helix interactions to be replaced by helix-lipid interactions upon encountering a lipoprotein surface, with the terminal helix likely initiating the binding interaction. This study presents the possibility of employing pyrene fluorophores as powerful new alternatives to obtain conformational information of proteins at physiologically relevant concentrations.

Deamidation of alpha-synuclein

The rates of deamidation of alpha-synuclein and single Asn residues in 13 Asn-sequence mutants have been measured for 5 x 10(-5)M protein in both the absence and presence of 10(-2)M sodium dodecyl sulfate (SDS). In the course of these experiments, 370 quantitative protein deamidation measurements were performed and 37 deamidation rates were determined by ion cyclotron resonance Fourier transform mass spectrometry, using an improved whole protein isotopic envelope method and a mass defect method with both enzymatic and collision-induced fragmentation. The measured deamidation index of alpha-synuclein was found to be 0.23 for an overall deamidation half-time of 23 days, without or with SDS micelles, owing primarily to the deamidation of Asn(103) and Asn(122). Deamidation rates of 15 Asn residues in the wild-type and mutant proteins were found to be primary sequence controlled without SDS. However, the presence of SDS micelles slowed the deamidation rates of nine N-terminal region Asn residues, caused by the known three-dimensional structures induced through protein binding to SDS micelles.

Calbindin-1 association and Parkinson's disease

BACKGROUND AND PURPOSE

Calcium levels have been proposed to play an important role in the selective vulnerability of nigrostriatal dopaminergic neurons in Parkinson's disease (PD). Recently, an association was reported between the calcium buffer, calbindin (rs1805874) and risk of PD in a Japanese patient-control series.

METHODS

We genotyped rs1805874 in four independent Caucasian patient-control series (1543 PD patients, 1771 controls).

RESULTS

There was no evidence of an association between rs1805874 and disease risk in individual populations or in the combined series (odds ratio: 1.04, 95% CI: 0.82-1.31, P = 0.74).

DISCUSSION

Our study shows there is no association between rs1805874 and risk for PD in four Caucasian populations. This suggests the effect of calbindin on PD risk displays population specificity.