Structural Characterization of the Amyloid Precursor Protein Transmembrane Domain and Its γ-Cleavage Site

Cholesterol-dependent amyloid β production: space for multifarious interactions between amyloid precursor protein, secretases, and cholesterol

Amyloid β is considered a key player in the development and progression of Alzheimer's disease (AD). Many studies investigating the effect of statins on lowering cholesterol suggest that there may be a link between cholesterol levels and AD pathology. Since cholesterol is one of the most abundant lipid molecules, especially in brain tissue, it affects most membrane-related processes, including the formation of the most dangerous form of amyloid β, Aβ42. The entire Aβ production system, which includes the amyloid precursor protein (APP), β-secretase, and the complex of γ-secretase, is highly dependent on membrane cholesterol content. Moreover, cholesterol can affect amyloidogenesis in many ways. Cholesterol influences the stability and activity of secretases, but also dictates their partitioning into specific cellular compartments and cholesterol-enriched lipid rafts, where the amyloidogenic machinery is predominantly localized. The most complicated relationships have been found in the interaction between cholesterol and APP, where cholesterol affects not only APP localization but also the precise character of APP dimerization and APP processing by γ-secretase, which is important for the production of Aβ of different lengths. In this review, we describe the intricate web of interdependence between cellular cholesterol levels, cholesterol membrane distribution, and cholesterol-dependent production of Aβ, the major player in AD.

The Progress of Label-Free Optical Imaging in Alzheimer's Disease Screening and Diagnosis

As the major neurodegenerative disease of dementia, Alzheimer's disease (AD) has caused an enormous social and economic burden on society. Currently, AD has neither clear pathogenesis nor effective treatments. Positron emission tomography (PET) and magnetic resonance imaging (MRI) have been verified as potential tools for diagnosing and monitoring Alzheimer's disease. However, the high costs, low spatial resolution, and long acquisition time limit their broad clinical utilization. The gold standard of AD diagnosis routinely used in research is imaging AD biomarkers with dyes or other reagents, which are unsuitable for in vivo studies owing to their potential toxicity and prolonged and costly process of the U.S. Food and Drug Administration (FDA) approval for human use. Furthermore, these exogenous reagents might bring unwarranted interference to mechanistic studies, causing unreliable results. Several label-free optical imaging techniques, such as infrared spectroscopic imaging (IRSI), Raman spectroscopic imaging (RSI), optical coherence tomography (OCT), autofluorescence imaging (AFI), optical harmonic generation imaging (OHGI), etc., have been developed to circumvent this issue and made it possible to offer an accurate and detailed analysis of AD biomarkers. In this review, we present the emerging label-free optical imaging techniques and their applications in AD, along with their potential and challenges in AD diagnosis.

Structural Studies Providing Insights into Production and Conformational Behavior of Amyloid-β Peptide Associated with Alzheimer's Disease Development

Alzheimer's disease is the most common type of neurodegenerative disease in the world. Genetic evidence strongly suggests that aberrant generation, aggregation, and/or clearance of neurotoxic amyloid-β peptides (Aβ) triggers the disease. Aβ accumulates at the points of contact of neurons in ordered cords and fibrils, forming the so-called senile plaques. Aβ isoforms of different lengths are found in healthy human brains regardless of age and appear to play a role in signaling pathways in the brain and to have neuroprotective properties at low concentrations. In recent years, different substances have been developed targeting Aβ production, aggregation, interaction with other molecules, and clearance, including peptide-based drugs. Aβ is a product of sequential cleavage of the membrane glycoprotein APP (amyloid precursor protein) by β- and γ-secretases. A number of familial mutations causing an early onset of the disease have been identified in the APP, especially in its transmembrane domain. The mutations are reported to influence the production, oligomerization, and conformational behavior of Aβ peptides. This review highlights the results of structural studies of the main proteins involved in Alzheimer's disease pathogenesis and the molecular mechanisms by which perspective therapeutic substances can affect Aβ production and nucleation.

The C99 domain of the amyloid precursor protein resides in the disordered membrane phase

Processing of the amyloid precursor protein (APP) via the amyloidogenic pathway is associated with the etiology of Alzheimer's disease. The cleavage of APP by β-secretase to generate the transmembrane 99-residue C-terminal fragment (C99) and subsequent processing of C99 by γ-secretase to yield amyloid-β (Aβ) peptides are essential steps in this pathway. Biochemical evidence suggests that amyloidogenic processing of C99 occurs in cholesterol- and sphingolipid-enriched liquid-ordered phase membrane rafts. However, direct evidence that C99 preferentially associates with these rafts has remained elusive. Here, we tested this by quantifying the affinity of C99-GFP for raft domains in cell-derived giant plasma membrane vesicles (GPMVs). We found that C99 was essentially excluded from ordered domains in vesicles from HeLa cells, undifferentiated SH-SY5Y cells, or SH-SY5Y-derived neurons; instead, ∼90% of C99 partitioned into disordered domains. The strong association of C99 with disordered domains occurred independently of its cholesterol-binding activity or homodimerization, or of the presence of the familial Alzheimer disease Arctic mutation (APP E693G). Finally, through biochemical studies we confirmed previous results, which showed that C99 is processed in the plasma membrane by α-secretase, in addition to the well-known γ-secretase. These findings suggest that C99 itself lacks an intrinsic affinity for raft domains, implying that either i) amyloidogenic processing of the protein occurs in disordered regions of the membrane, ii) processing involves a marginal subpopulation of C99 found in rafts, or iii) as-yet-unidentified protein-protein interactions with C99 in living cells drive this protein into membrane rafts to promote its cleavage therein.

Revealing the Mechanisms of Synergistic Action of Two Magainin Antimicrobial Peptides

The study of peptide-lipid and peptide-peptide interactions as well as their topology and dynamics using biophysical and structural approaches have changed our view how antimicrobial peptides work and function. It has become obvious that both the peptides and the lipids arrange in soft supramolecular arrangements which are highly dynamic and able to change and mutually adapt their conformation, membrane penetration, and detailed morphology. This can occur on a local and a global level. This review focuses on cationic amphipathic peptides of the magainin family which were studied extensively by biophysical approaches. They are found intercalated at the membrane interface where they cause membrane thinning and ultimately lysis. Interestingly, mixtures of two of those peptides namely magainin 2 and PGLa which occur naturally as a cocktail in the frog skin exhibit synergistic enhancement of antimicrobial activities when investigated together in antimicrobial assays but also in biophysical experiments with model membranes. Detailed dose-response curves, presented here for the first time, show a cooperative behavior for the individual peptides which is much increased when PGLa and magainin are added as equimolar mixture. This has important consequences for their bacterial killing activities and resistance development. In membranes that carry unsaturations both peptides align parallel to the membrane surface where they have been shown to arrange into mesophases involving the peptides and the lipids. This supramolecular structuration comes along with much-increased membrane affinities for the peptide mixture. Because this synergism is most pronounced in membranes representing the bacterial lipid composition it can potentially be used to increase the therapeutic window of pharmaceutical formulations.

A theoretical assessment of structure determination of multi-span membrane proteins by oriented sample solid-state NMR spectroscopy

Oriented sample solid state NMR (OS-ssNMR) spectroscopy allows direct determination of the structure and topology of membrane proteins reconstituted into aligned lipid bilayers. While OS-ssNMR theoretically has no upper size limit, its application to multi-span membrane proteins has not been established since most studies have been restricted to single or dual span proteins and peptides. Here, we present a critical assessment of the application of this method to multi-span membrane proteins. We used molecular dynamics simulations to back-calculate [¹⁵N-¹H] separated local field (SLF) spectra from a G protein-coupled receptor (GPCR) and show that fully resolved spectra can be obtained theoretically for a multi-span membrane protein with currently achievable resonance linewidths.

Structure and function of the perivascular fluid compartment and vertebral venous plexus: Illumining a novel theory on mechanisms underlying the pathogenesis of Alzheimer's, cerebral small vessel, and neurodegenerative diseases

Blood dynamically and richly supplies the cerebral tissue via microvessels invested in pia matter perforating the cerebral substance. Arteries penetrating the cerebral substance derive an investment from one or two successive layers of pia mater, luminally apposed to the pial-glial basal lamina of the microvasculature and abluminally apposed to a series of aquaporin IV-studded astrocytic end feet constituting the soi-disant glia limitans. The full investment of successive layers forms the variably continuous walls of the periarteriolar, pericapillary, and perivenular divisions of the perivascular fluid compartment. The pia matter disappears at the distal periarteriolar division of the perivascular fluid compartment. Plasma from arteriolar blood sequentially transudates into the periarteriolar division of the perivascular fluid compartment and subarachnoid cisterns in precession to trickling into the neural interstitium. Fluid from the neural interstitium successively propagates into the venules through the subarachnoid cisterns and perivenular division of the perivascular fluid compartment. Fluid fluent within the perivascular fluid compartment flows gegen the net direction of arteriovenular flow. Microvessel oscillations at the central tendency of the cerebral vasomotion generate corresponding oscillations of within the surrounding perivascular fluid compartment, interposed betwixt the abluminal surface of the vessels and internal surface of the pia mater. The precise microanatomy of this most fascinating among designable spaces has eluded the efforts of various investigators to interrogate its structure, though most authors non-consensusly concur the investing layers effectively and functionally segregate the perivascular and subarachnoid fluid compartments. Enlargement of the perivascular fluid compartment in a variety of neurological disorders, including senile dementia of the Alzheimer's type and cerebral small vessel disease, may alternately or coordinately constitute a correlative marker of disease severity and a possible cause implicated in the mechanistic pathogenesis of these conditions. Venular pressures modulating oscillatory dynamic flow within the perivascular fluid compartment may similarly contribute to the development of a variety among neurological disorders. An intimate understanding of subtle features typifying microanatomy and microphysiology of the investing structures and spaces of the cerebral microvasculature may powerfully inform mechanistic pathophysiology mediating a variety of neurovascular ischemic, neuroinfectious, neuroautoimmune, and neurodegenerative diseases.

Bicelles Rich in both Sphingolipids and Cholesterol and Their Use in Studies of Membrane Proteins

How the distinctive lipid composition of mammalian plasma membranes impacts membrane protein structure is largely unexplored, partly because of the dearth of isotropic model membrane systems that contain abundant sphingolipids and cholesterol. This gap is addressed by showing that sphingomyelin and cholesterol-rich (SCOR) lipid mixtures with phosphatidylcholine can be cosolubilized by n-dodecyl-β-melibioside to form bicelles. Small-angle X-ray and neutron scattering, as well as cryo-electron microscopy, demonstrate that these assemblies are stable over a wide range of conditions and exhibit the bilayered-disc morphology of ideal bicelles even at low lipid-to-detergent mole ratios. SCOR bicelles are shown to be compatible with a wide array of experimental techniques, as applied to the transmembrane human amyloid precursor C99 protein in this medium. These studies reveal an equilibrium between low-order oligomer structures that differ significantly from previous experimental structures of C99, providing an example of how ordered membranes alter membrane protein structure.

Far from Inert: Membrane Lipids Possess Intrinsic Reactivity That Has Consequences for Cell Biology

In this article, it is hypothesized that a fundamental chemical reactivity exists between some non-lipid constituents of cellular membranes and ester-based lipids, the significance of which is not generally recognized. Many peptides and smaller organic molecules have now been shown to undergo lipidation reactions in model membranes in circumstances where direct reaction with the lipid is the only viable route for acyl transfer. Crucially, drugs like propranolol are lipidated in vivo with product profiles that are comparable to those produced in vitro. Some compounds have also been found to promote lipid hydrolysis. Drugs with high lytic activity in vivo tend to have higher toxicity in vitro. Deacylases and lipases are proposed as key enzymes that protect cells against the effects of intrinsic lipidation. The toxic effects of intrinsic lipidation are hypothesized to include a route by which nucleation can occur during the formation of amyloid fibrils.

O-GlcNAcylation as a Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia and the number of elderly patients suffering from AD has been steadily increasing. Despite worldwide efforts to cope with this disease, little progress has been achieved with regard to identification of effective therapeutics. Thus, active research focusing on identification of new therapeutic targets of AD is ongoing. Among the new targets, post-translational modifications which modify the properties of mature proteins have gained attention. O-GlcNAcylation, a type of PTM that attaches O-linked β-N-acetylglucosamine (O-GlcNAc) to a protein, is being sought as a new target to treat AD pathologies. O-GlcNAcylation has been known to modify the two important components of AD pathological hallmarks, amyloid precursor protein, and tau protein. In addition, elevating O-GlcNAcylation levels in AD animal models has been shown to be effective in alleviating AD-associated pathology. Although studies investigating the precise mechanism of reversal of AD pathologies by targeting O-GlcNAcylation are not yet complete, it is clearly important to examine O-GlcNAcylation regulation as a target of AD therapeutics. This review highlights the mechanisms of O-GlcNAcylation and its role as a potential therapeutic target under physiological and pathological AD conditions.

Conformational Dynamics of Transmembrane Domain 3 of Presenilin 1 Is Associated with the Trimming Activity of γ-Secretase

γ-Secretase is an intramembrane-cleaving protease that generates the toxic species of the amyloid-β peptide (Aβ) that is responsible for the pathology of Alzheimer disease. The catalytic subunit of γ-secretase is presenilin 1 (PS1), which is a polytopic membrane protein with a hydrophilic catalytic pore. The length of the C terminus of Aβ is proteolytically determined by its processive trimming by γ-secretase, although the precise mechanism still remains largely unknown. Here, we identified that transmembrane domain (TMD) 3 of human PS1 is involved in the formation of the intramembranous hydrophilic pore. Notably, the water accessibility of TMD3 was greatly altered by point mutations and compounds, which modify γ-secretase activity. The changes in the water accessibility of TMD3 was also correlated with Aβ42 production. Moreover, crosslinking between TMD3 and TMD7 resulted in a loss of sensitivity to a γ-secretase modulator that reduces Aβ42 production. Therefore, our findings indicate that the conformational dynamics of TMD3 is a prerequisite for regulation of the Aβ trimming activity of γ-secretase.SIGNIFICANCE STATEMENT Modulation of γ-secretase activity to reduce the level of toxic amyloid-β species is thought to be a therapeutic strategy for Alzheimer disease. However, the detailed mechanism of the regulation of amyloid-β production, as well as the structure-and-activity relationship of γ-secretase remains unclear. Here we identified that the water accessibility around transmembrane domain 3 in presenilin 1 was increased along with a reduction in toxic amyloid-β production. Our findings demonstrate how the structure of presenilin 1 dynamically changes during amyloid-β production, and provides insights toward the development of treatments against Alzheimer disease.

Structure, Topology, and Dynamics of Membrane-Inserted Polypeptides and Lipids by Solid-State NMR Spectroscopy: Investigations of the Transmembrane Domains of the DQ Beta-1 Subunit of the MHC II Receptor and of the COP I Protein p24

MHC class II receptors carry important function in adaptive immunity and their malfunctioning is associated with diabetes type I, chronic inflammatory diseases and other autoimmune diseases. The protein assembles from the DQ alpha-1 and DQ beta-1 subunits where the transmembrane domains of these type I membrane proteins have been shown to be involved in homo- and heterodimer formation. Furthermore, the DQ alpha 1 chain carries a sequence motif that has been first identified in the context of p24, a protein involved in the formation of COPI vesicles of the intracellular transport machinery, to specifically interact with sphingomyelin-C18 (SM-C18). Here we investigated the membrane interactions and dynamics of DQ beta-1 in liquid crystalline POPC phospholipid bilayers by oriented ¹⁵N solid-state NMR spectroscopy. The ¹⁵N resonances are indicative of a helical tilt angle of the membrane anchor sequence around 20°. Two populations can be distinguished by their differential dynamics probably corresponding the DQ beta-1 mono- and homodimer. Whereas, this equilibrium is hardly affected by the addition of 5 mole% SM-C18 a single population is visible in DMPC lipid bilayers suggesting that the lipid saturation is an important parameter. Furthermore, the DQ alpha-1, DQ beta-1 and p24 transmembrane helical domains were reconstituted into POPC or POPC/SM-C18 lipid bilayers where the fatty acyl chain of either the phosphatidylcholine or of the sphingolipid have been deuterated. Interestingly in the presence of both sphingolipid and polypeptide a strong decrease in the innermost membrane order of the POPC palmitoyl chain is observed, an effect that is strongest for DQ beta-1. In contrast, for the first time the polypeptide interactions were monitored by deuteration of the stearoyl chain of SM-C18. The resulting ²H solid-state NMR spectra show an increase in order for p24 and DQ alpha-1 which both carry the SM recognition motif. Thereby the data are suggestive that SM-C18 together with the transmembrane domains form structures imposing positive curvature strain on the surrounding POPC lipids. This effect is attenuated when SM-C18 is recognized by the protein.

Familial L723P Mutation Can Shift the Distribution between the Alternative APP Transmembrane Domain Cleavage Cascades by Local Unfolding of the Ε-Cleavage Site Suggesting a Straightforward Mechanism of Alzheimer's Disease Pathogenesis

Alzheimer's disease is an age-related pathology associated with accumulation of amyloid-β peptides, products of enzymatic cleavage of amyloid-β precursor protein (APP) by secretases. Several familial mutations causing early onset of the disease have been identified in the APP transmembrane (TM) domain. The mutations influence production of amyloid-β, but the molecular mechanisms of this effect are unclear. The "Australian" (L723P) mutation located in the C-termini of APP TM domain is associated with autosomal-dominant, early onset Alzheimer's disease. Herein, we describe the impact of familial L723P mutation on the structural-dynamic behavior of APP TM domain studied by high-resolution NMR in membrane-mimicking micelles and augmented by molecular dynamics simulations in explicit lipid bilayer. We found L723P mutation to cause local unfolding of the C-terminal turn of the APP TM domain helix and increase its accessibility to water required for cleavage of the protein backbone by γ-secretase in the ε-site, thus switching between alternative ("pathogenic" and "non-pathogenic") cleavage cascades. These findings suggest a straightforward mechanism of the pathogenesis associated with this mutation, and are of generic import for understanding the molecular-level events associated with APP sequential proteolysis resulting in accumulation of the pathogenic forms of amyloid-β. Moreover, age-related onset of Alzheimer's disease can be explained by a similar mechanism, where the effect of mutation is emulated by the impact of local environmental factors, such as oxidative stress and/or membrane lipid composition. Knowledge of the mechanisms regulating generation of amyloidogenic peptides of different lengths is essential for development of novel treatment strategies of the Alzheimer's disease.

Solid-State NMR Investigations of the MHC II Transmembrane Domains: Topological Equilibria and Lipid Interactions

The major histocompatibility complex class II (MHC II) membrane proteins are key players in the adaptive immune response. An aberrant function of these molecules is associated with a large number of autoimmune diseases such as diabetes type I and chronic inflammatory diseases. The MHC class II is assembled from DQ alpha 1 and DQ beta 1 which come together as a heterodimer through GXXXG-mediated protein-protein interactions and a highly specific protein-sphingomyelin-C18 interaction motif located on DQA1. This association can have important consequences in regulating the function of these membrane proteins. Here, we investigated the structure and topology of the DQA1 and DQB1 transmembrane helical domains by CD-, oriented ²H and ¹⁵N solid-state NMR spectroscopies. The spectra at peptide-to-lipid ratios of 0.5 to 2 mol% are indicative of a topological equilibrium involving a helix crossing the membrane with a tilt angle of about 20° and another transmembrane topology with around 30° tilt. The latter is probably representing a dimer. Furthermore, at the lowest peptide-to-lipid ratio, a third polypeptide population becomes obvious. Interestingly, the DQB1 and to a lesser extent the DQA1 transmembrane helical domains exhibit a strong fatty acyl chain disordering effect on the inner segments of the ²H-labelled palmitoyl chain of POPC bilayers. This phosphatidylcholine disordering requires the presence of sphingomyelin-C18 suggesting that the ensemble of transmembrane polypeptide and sphingolipid exerts positive curvature strain.

Modulating Hinge Flexibility in the APP Transmembrane Domain Alters γ-Secretase Cleavage

Intramembrane cleavage of the β-amyloid precursor protein C99 substrate by γ-secretase is implicated in Alzheimer's disease pathogenesis. Biophysical data have suggested that the N-terminal part of the C99 transmembrane domain (TMD) is separated from the C-terminal cleavage domain by a di-glycine hinge. Because the flexibility of this hinge might be critical for γ-secretase cleavage, we mutated one of the glycine residues, G38, to a helix-stabilizing leucine and to a helix-distorting proline. Both mutants impaired γ-secretase cleavage and also altered its cleavage specificity. Circular dichroism, NMR, and backbone amide hydrogen/deuterium exchange measurements as well as molecular dynamics simulations showed that the mutations distinctly altered the intrinsic structural and dynamical properties of the substrate TMD. Although helix destabilization and/or unfolding was not observed at the initial ε-cleavage sites of C99, subtle changes in hinge flexibility were identified that substantially affected helix bending and twisting motions in the entire TMD. These resulted in altered orientation of the distal cleavage domain relative to the N-terminal TMD part. Our data suggest that both enhancing and reducing local helix flexibility of the di-glycine hinge may decrease the occurrence of enzyme-substrate complex conformations required for normal catalysis and that hinge mobility can thus be conducive for productive substrate-enzyme interactions.

Investigations of the Structure, Topology, and Interactions of the Transmembrane Domain of the Lipid-Sorting Protein p24 Being Highly Selective for Sphingomyelin-C18

The p24 proteins play an important role in the secretory pathway where they selectively connect various cargo to other proteins, thereby being involved in the controlled assembly and disassembly of the coat protein complexes and lipid sorting. Recently, a highly selective lipid interaction motif has been identified within the p24 transmembrane domain (TMD) that recognizes the combination of the sphingomyelin headgroup and the exact length of the C18 fatty acyl chain (SM-C18). Here, we present investigations of the structure, dynamics, and sphingomyelin interactions of the p24 transmembrane region using circular dichroism, tryptophan fluorescence, and solid-state nuclear magnetic resonance (NMR) spectroscopies of the polypeptides and the surrounding lipids. Membrane insertion and/or conformation of the TMD is strongly dependent on the membrane lipid composition where the transmembrane helical insertion is strongest in the presence of 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC) and SM-C18. By analyzing solid-state NMR angular restraints from a large number of labeled sites, we have found a tilt angle of 19° for the transmembrane helical domain at a peptide-to-lipid ratio of 1 mol %. Only minor changes in the solid-state NMR spectra are observed due to the presence of SM-C18; the only visible alterations are associated with the SM-C18 recognition motif close to the carboxy-terminal part of the hydrophobic transmembrane region in the proximity of the SM headgroup. Finally, the deuterium order parameters of POPC- d₃₁ were nearly unaffected by the presence of SM-C18 or the polypeptide alone but decreased noticeably when the sphingomyelin and the polypeptide were added in combination.

Influence of the familial Alzheimer's disease-associated T43I mutation on the transmembrane structure and γ-secretase processing of the C99 peptide

Extracellular deposition of β-amyloid (Aβ) peptides in the brain is a hallmark of Alzheimer's disease (AD). Upon β-secretase-mediated cleavage of the β C-terminal fragment (β-CTF) from the Aβ precursor protein, the γ-secretase complex produces the Aβ peptides associated with AD. The familial T43I mutation within the transmembrane domain of the β-CTF (also referred to as C99) increases the ratio between the Aβ42 and Aβ40 peptides largely due to a decrease in Aβ40 formation. Aβ42 is the principal component of amyloid deposits within the brain parenchyma, and an increase in the Aβ42/Aβ40 ratio is correlated with early-onset AD. Using NMR and FTIR spectroscopy, here we addressed how the T43I substitution influences the structure of C55, the minimal sequence containing the entire extracellular and transmembrane (TM) domains of C99 needed for γ-secretase processing. 13C NMR chemical shifts indicated that the T43I substitution increases helical structure within the TM domain of C55. These structural changes were associated with a shift of the C55 dimer to the monomer and an increase in the tilt of the TM helix relative to the membrane normal in the T43I mutant compared with that of WT C55. The A21G (Flemish) mutation was previously found to increase secreted Aβ40 levels; here, we combined this mutation in the extracellular domain of C99 with T43I and observed that the T43I/A21G double mutant decreases Aβ40 formation. We discuss how the observed structural changes in the T43I mutant may decrease Aβ40 formation and increase the Aβ42/Aβ40 ratio.

The Mechanisms of Action of Cationic Antimicrobial Peptides Refined by Novel Concepts from Biophysical Investigations

Even 30 years after the discovery of magainins, biophysical and structural investigations on how these peptides interact with membranes can still bear surprises and add new interesting detail to how these peptides exert their antimicrobial action. Early on, using oriented solid-state NMR spectroscopy, it was found that the amphipathic helices formed by magainins are active when being oriented parallel to the membrane surface. More recent investigations indicate that this in-planar alignment is also found when PGLa and magainin in combination exert synergistic pore-forming activities, where studies on the mechanism of synergistic interaction are ongoing. In a related manner, the investigation of dimeric antimicrobial peptide sequences has become an interesting topic of research which bears promise to refine our views how antimicrobial action occurs. The molecular shape concept has been introduced to explain the effects of lipids and peptides on membrane morphology, locally and globally, and in particular of cationic amphipathic helices that partition into the membrane interface. This concept has been extended in this review to include more recent ideas on soft membranes that can adapt to external stimuli including membrane-disruptive molecules. In this manner, the lipids can change their shape in the presence of low peptide concentrations, thereby maintaining the bilayer properties. At higher peptide concentrations, phase transitions occur which lead to the formation of pores and membrane lytic processes. In the context of the molecular shape concept, the properties of lipopeptides, including surfactins, are shortly presented, and comparisons with the hydrophobic alamethicin sequence are made.

Lipid bilayer position and orientation of novel carprofens, modulators of γ-secretase in Alzheimer's disease

γ-Secretase is an integral membrane protein complex and is involved in the cleavage of the amyloid precursor protein APP to produce amyloid-β peptides. Amyloid-β peptides are considered causative agents for Alzheimer's disease and drugs targeted at γ-secretase are investigated as therapeutic treatments. We synthesized new carprofen derivatives, which showed γ-secretase modulating activity and determined their precise position, orientation, and dynamics in lipid membranes by combining neutron diffraction, solid-state NMR spectroscopy, and molecular dynamics simulations. Our data indicate that the carprofen derivatives are inserted into the membrane interface, where the exact position and orientation depends on the lipid phase. This knowledge will help to understand the docking of carprofen derivatives to γ-secretase and in the design of new potent drugs. The approach presented here promises to serve as a general guideline how drug/target interactions in membranes can be analyzed in a comprehensive manner.

Dissecting conformational changes in APP's transmembrane domain linked to ε-efficiency in familial Alzheimer's disease

The mechanism by which familial Alzheimer's disease (FAD) mutations within the transmembrane domain (TMD) of the Amyloid Precursor Protein (APP) affect ε-endoproteolysis is only poorly understood. Thereby, mutations in the cleavage domain reduce ε-efficiency of γ-secretase cleavage and some even shift entry into production lines. Since cleavage occurs within the TMD, a relationship between processing and TMD structure and dynamics seems obvious. Using molecular dynamic simulations, we dissect the dynamic features of wild-type and seven FAD-mutants into local and global components. Mutations consistently enhance hydrogen-bond fluctuations upstream of the ε-cleavage sites but maintain strong helicity there. Dynamic perturbation-response scanning reveals that FAD-mutants target backbone motions utilized in the bound state. Those motions, obscured by large-scale motions in the pre-bound state, provide (i) a dynamic mechanism underlying the proposed coupling between binding and ε-cleavage, (ii) key sites consistent with experimentally determined docking sites, and (iii) the distinction between mutants and wild-type.

Impact of membrane curvature on amyloid aggregation

The misfolding, amyloid aggregation, and fibril formation of intrinsically disordered proteins/peptides (or amyloid proteins) have been shown to cause a number of disorders. The underlying mechanisms of amyloid fibrillation and structural properties of amyloidogenic precursors, intermediates, and amyloid fibrils have been elucidated in detail; however, in-depth examinations on physiologically relevant contributing factors that induce amyloidogenesis and lead to cell death remain challenging. A large number of studies have attempted to characterize the roles of biomembranes on protein aggregation and membrane-mediated cell death by designing various membrane components, such as gangliosides, cholesterol, and other lipid compositions, and by using various membrane mimetics, including liposomes, bicelles, and different types of lipid-nanodiscs. We herein review the dynamic effects of membrane curvature on amyloid generation and the inhibition of amyloidogenic proteins and peptides, and also discuss how amyloid formation affects membrane curvature and integrity, which are key for understanding relationships with cell death. Small unilamellar vesicles with high curvature and large unilamellar vesicles with low curvature have been demonstrated to exhibit different capabilities to induce the nucleation, amyloid formation, and inhibition of amyloid-β peptides and α-synuclein. Polymorphic amyloidogenesis in small unilamellar vesicles was revealed and may be viewed as one of the generic properties of interprotein interaction-dominated amyloid formation. Several mechanical models and phase diagrams are comprehensively shown to better explain experimental findings. The negative membrane curvature-mediated mechanisms responsible for the toxicity of pancreatic β cells by the amyloid aggregation of human islet amyloid polypeptide (IAPP) and binding of the precursors of the semen-derived enhancer of viral infection (SEVI) are also described. The curvature-dependent binding modes of several types of islet amyloid polypeptides with high-resolution NMR structures are also discussed.