Biochemical identification of a linear cholesterol-binding domain within Alzheimer's β amyloid peptide

New insights in lipid metabolism: potential therapeutic targets for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is increasingly recognized as being intertwined with the dysregulation of lipid metabolism. Lipids are a significant class of nutrients vital to all organisms, playing crucial roles in cellular structure, energy storage, and signaling. Alterations in the levels of various lipids in AD brains and dysregulation of lipid pathways and transportation have been implicated in AD pathogenesis. Clinically, evidence for a high-fat diet firmly links disrupted lipid metabolism to the pathogenesis and progression of AD, although contradictory findings warrant further exploration. In view of the significance of various lipids in brain physiology, the discovery of complex and diverse mechanisms that connect lipid metabolism with AD-related pathophysiology will bring new hope for patients with AD, underscoring the importance of lipid metabolism in AD pathophysiology, and promising targets for therapeutic intervention. Specifically, cholesterol, sphingolipids, and fatty acids have been shown to influence amyloid-beta (Aβ) accumulation and tau hyperphosphorylation, which are hallmarks of AD pathology. Recent studies have highlighted the potential therapeutic targets within lipid metabolism, such as enhancing apolipoprotein E lipidation, activating liver X receptors and retinoid X receptors, and modulating peroxisome proliferator-activated receptors. Ongoing clinical trials are investigating the efficacy of these strategies, including the use of ketogenic diets, statin therapy, and novel compounds like NE3107. The implications of these findings suggest that targeting lipid metabolism could offer new avenues for the treatment and management of AD. By concentrating on alterations in lipid metabolism within the central nervous system and their contribution to AD development, this review aims to shed light on novel research directions and treatment approaches for combating AD, offering hope for the development of more effective management strategies.

Towards a Unitary Hypothesis of Alzheimer's Disease Pathogenesis

The "amyloid cascade" hypothesis of Alzheimer's disease (AD) pathogenesis invokes the accumulation in the brain of plaques (containing the amyloid-β protein precursor [AβPP] cleavage product amyloid-β [Aβ]) and tangles (containing hyperphosphorylated tau) as drivers of pathogenesis. However, the poor track record of clinical trials based on this hypothesis suggests that the accumulation of these peptides is not the only cause of AD. Here, an alternative hypothesis is proposed in which the AβPP cleavage product C99, not Aβ, is the main culprit, via its role as a regulator of cholesterol metabolism. C99, which is a cholesterol sensor, promotes the formation of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a cholesterol-rich lipid raft-like subdomain of the ER that communicates, both physically and biochemically, with mitochondria. We propose that in early-onset AD (EOAD), MAM-localized C99 is elevated above normal levels, resulting in increased transport of cholesterol from the plasma membrane to membranes of intracellular organelles, such as ER/endosomes, thereby upregulating MAM function and driving pathology. By the same token, late-onset AD (LOAD) is triggered by any genetic variant that increases the accumulation of intracellular cholesterol that, in turn, boosts the levels of C99 and again upregulates MAM function. Thus, the functional cause of AD is upregulated MAM function that, in turn, causes the hallmark disease phenotypes, including the plaques and tangles. Accordingly, the MAM hypothesis invokes two key interrelated elements, C99 and cholesterol, that converge at the MAM to drive AD pathogenesis. From this perspective, AD is, at bottom, a lipid disorder.

Real-time monitoring of the amyloid β1-42 monomer-to-oligomer channel transition using a lipid bilayer system

This study describes the observation of the transformation of monomeric amyloid β1-42 (Aβ42) into oligomers in a lipid membrane utilizing a lipid bilayer system for electrophysiological measurement. The relevance of oligomers and protofibrils in Alzheimer's disease (AD) is underscored given their significant neurotoxicity. By closely monitoring the shift of Aβ42 from its monomeric state to forming oligomeric channels in phospholipid membranes, we noted that this transformation transpired within a 2-h frame. We manipulated the lipid membrane's constitution with components such as glycerophospholipid, porcine brain total lipid extract, sphingomyelin (SM), and cholesterol (Chol.) to effectively imitate nerve cell membranes. Interesting findings showcased Chol.'s ability to foster stable oligomeric channel formation in the lipid membrane, with SM and GM1 lipids potentially enhancing channel formation as well. Additionally, the study identified the potential of a catechin derivative, epigallocatechin gallate (EGCG), in obstructing oligomerization. With EGCG present in the outer solution of the Aβ42-infused membrane, a noteworthy reduction in channel current was observed, suggesting the successful inhibition of oligomerization. This conclusion held true in both, prior and subsequent, stages of oligomerization. Our findings shed light on the toxicity of oligomers, promising invaluable information for future advancements in AD treatment strategies.

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.

Amyloid Precursor Protein Changes Arrangement in a Membrane and Its Structure Depending on the Cholesterol Content

One of the hallmarks of Alzheimer's disease (AD) is the accumulation of amyloid beta (Aβ) peptides in the brain. The processing of amyloid precursor protein (APP) into Aβ is dependent on the location of APP in the membrane, membrane lipid composition and, possibly, presence of lipid rafts. In this study, we used atomic force microscopy (AFM) to investigate the interaction between transmembrane fragment APP672-726 (corresponding to Aβ1-55) and its amyloidogenic mutant L723P with membranes combining liquid-ordered and liquid-disordered lipid phases. Our results demonstrated that most of the APP672-726 is located either in the liquid-disordered phase or at the boundary between ordered and disordered phases, and hardly ever in rafts. We did not notice any major changes in the domain structure induced by APP672-726. In membranes without cholesterol APP672-726, and especially its amyloidogenic mutant L723P formed annular structures and clusters rising above the membrane. Presence of cholesterol led to the appearance of concave membrane regions up to 2 nm in depth that were deeper for wild type APP672-726. Thus, membrane cholesterol regulates changes in membrane structure and permeability induced by APP that might be connected with further formation of membrane pores.

White matter injury, cholesterol dysmetabolism, and APP/Abeta dysmetabolism interact to produce Alzheimer's disease (AD) neuropathology: A hypothesis and review

We postulate that myelin injury contributes to cholesterol release from myelin and cholesterol dysmetabolism which contributes to Abeta dysmetabolism, and combined with genetic and AD risk factors, leads to increased Abeta and amyloid plaques. Increased Abeta damages myelin to form a vicious injury cycle. Thus, white matter injury, cholesterol dysmetabolism and Abeta dysmetabolism interact to produce or worsen AD neuropathology. The amyloid cascade is the leading hypothesis for the cause of Alzheimer's disease (AD). The failure of clinical trials based on this hypothesis has raised other possibilities. Even with a possible new success (Lecanemab), it is not clear whether this is a cause or a result of the disease. With the discovery in 1993 that the apolipoprotein E type 4 allele (APOE4) was the major risk factor for sporadic, late-onset AD (LOAD), there has been increasing interest in cholesterol in AD since APOE is a major cholesterol transporter. Recent studies show that cholesterol metabolism is intricately involved with Abeta (Aβ)/amyloid transport and metabolism, with cholesterol down-regulating the Aβ LRP1 transporter and upregulating the Aβ RAGE receptor, both of which would increase brain Aβ. Moreover, manipulating cholesterol transport and metabolism in rodent AD models can ameliorate pathology and cognitive deficits, or worsen them depending upon the manipulation. Though white matter (WM) injury has been noted in AD brain since Alzheimer's initial observations, recent studies have shown abnormal white matter in every AD brain. Moreover, there is age-related WM injury in normal individuals that occurs earlier and is worse with the APOE4 genotype. Moreover, WM injury precedes formation of plaques and tangles in human Familial Alzheimer's disease (FAD) and precedes plaque formation in rodent AD models. Restoring WM in rodent AD models improves cognition without affecting AD pathology. Thus, we postulate that the amyloid cascade, cholesterol dysmetabolism and white matter injury interact to produce and/or worsen AD pathology. We further postulate that the primary initiating event could be related to any of the three, with age a major factor for WM injury, diet and APOE4 and other genes a factor for cholesterol dysmetabolism, and FAD and other genes for Abeta dysmetabolism.

Amyloids on Membrane Interfaces: Implications for Neurodegeneration

Membrane interfaces are vital for various cellular processes, and their involvement in neurodegenerative disorders such as Alzheimer's and Parkinson's disease has taken precedence in recent years. The amyloidogenic proteins associated with neurodegenerative diseases interact with the neuronal membrane through various means, which has implications for both the onset and progression of the disease. The parameters that regulate the interaction between the membrane and the amyloids remain poorly understood. The review focuses on the various aspects of membrane interactions of amyloids, particularly amyloid-β (Aβ) peptides and Tau involved in Alzheimer's and α-synuclein involved in Parkinson's disease. The genetic, cell biological, biochemical, and biophysical studies that form the basis for our current understanding of the membrane interactions of Aβ peptides, Tau, and α-synuclein are discussed.

Amyloid β, Lipid Metabolism, Basal Cholinergic System, and Therapeutics in Alzheimer’s Disease

The presence of insoluble aggregates of amyloid β (Aβ) in the form of neuritic plaques (NPs) is one of the main features that define Alzheimer’s disease. Studies have suggested that the accumulation of these peptides in the brain significantly contributes to extensive neuronal loss. Furthermore, the content and distribution of cholesterol in the membrane have been shown to have an important effect on the production and subsequent accumulation of Aβ peptides in the plasma membrane, contributing to dysfunction and neuronal death. The monomeric forms of these membrane-bound peptides undergo several conformational changes, ranging from oligomeric forms to beta-sheet structures, each presenting different levels of toxicity. Aβ peptides can be internalized by particular receptors and trigger changes from Tau phosphorylation to alterations in cognitive function, through dysfunction of the cholinergic system. The goal of this review is to summarize the current knowledge on the role of lipids in Alzheimer’s disease and their relationship with the basal cholinergic system, as well as potential disease-modifying therapies.

Cholesterol as a key player in amyloid β-mediated toxicity in Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disorder that is one of the most devastating and widespread diseases worldwide, mainly affecting the aging population. One of the key factors contributing to AD-related neurotoxicity is the production and aggregation of amyloid β (Aβ). Many studies have shown the ability of Aβ to bind to the cell membrane and disrupt its structure, leading to cell death. Because amyloid damage affects different parts of the brain differently, it seems likely that not only Aβ but also the nature of the membrane interface with which the amyloid interacts, helps determine the final neurotoxic effect. Because cholesterol is the dominant component of the plasma membrane, it plays an important role in Aβ-induced toxicity. Elevated cholesterol levels and their regulation by statins have been shown to be important factors influencing the progression of neurodegeneration. However, data from many studies have shown that cholesterol has both neuroprotective and aggravating effects in relation to the development of AD. In this review, we attempt to summarize recent findings on the role of cholesterol in Aβ toxicity mediated by membrane binding in the pathogenesis of AD and to consider it in the broader context of the lipid composition of cell membranes.

Free Cholesterol Accelerates Aβ Self-Assembly on Membranes at Physiological Concentration

The effects of membranes on the early-stage aggregation of amyloid β (Aβ) have come to light as potential mechanisms by which neurotoxic species are formed in Alzheimer’s disease. We have shown that direct Aβ-membrane interactions dramatically enhance the Aβ aggregation, allowing for oligomer assembly at physiologically low concentrations of the monomer. Membrane composition is also a crucial factor in this process. Our results showed that apart from phospholipids composition, cholesterol in membranes significantly enhances the aggregation kinetics. It has been reported that free cholesterol is present in plaques. Here we report that free cholesterol, along with its presence inside the membrane, further accelerate the aggregation process by producing aggregates more rapidly and of significantly larger sizes. These aggregates, which are formed on the lipid bilayer, are able to dissociate from the surface and accumulate in the bulk solution; the presence of free cholesterol accelerates this dissociation as well. All-atom molecular dynamics simulations show that cholesterol binds Aβ monomers and significantly changes the conformational sampling of Aβ monomer; more than doubling the fraction of low-energy conformations compared to those in the absence of cholesterol, which can contribute to the aggregation process. The results indicate that Aβ-lipid interaction is an important factor in the disease prone amyloid assembly process.

Effect of Cholesterol Molecules on Aβ1-42 Wild-Type and Mutants Trimers

Alzheimer’s disease displays aggregates of the amyloid-beta (Aβ) peptide in the brain, and there is increasing evidence that cholesterol may contribute to the pathogenesis of the disease. Though many experimental and theoretical studies have focused on the interactions of Aβ oligomers with membrane models containing cholesterol, an understanding of the effect of free cholesterol on small Aβ42 oligomers is not fully established. To address this question, we report on replica exchange with a solute tempering simulation of an Aβ42 trimer with cholesterol and compare it with a previous replica exchange molecular dynamics simulation. We show that the binding hot spots of cholesterol are rather complex, involving hydrophobic residues L17–F20 and L30–M35 with a non-negligible contribution of loop residues D22–K28 and N-terminus residues. We also examine the effects of cholesterol on the trimers of the disease-causing A21G and disease-protective A2T mutations by molecular dynamics simulations. We show that these two mutations moderately impact cholesterol-binding modes. In our REST2 simulations, we find that cholesterol is rarely inserted into aggregates but rather attached as dimers and trimers at the surface of Aβ42 oligomers. We propose that cholesterol acts as a glue to speed up the formation of larger aggregates; this provides a mechanistic link between cholesterol and Alzheimer’s disease.

An Overview of Several Inhibitors for Alzheimer's Disease: Characterization and Failure

Amyloid beta (Aβ) oligomers are the most neurotoxic aggregates causing neuronal death and cognitive damage. A detailed elucidation of the aggregation pathways from oligomers to fibril formation is crucial to develop therapeutic strategies for Alzheimer's disease (AD). Although experimental techniques rely on the measure of time- and space-average properties, they face severe difficulties in the investigation of Aβ peptide aggregation due to their intrinsically disorder character. Computer simulation is a tool that allows tracing the molecular motion of molecules; hence it complements Aβ experiments, as it allows to explore the binding mechanism between metal ions and Aβ oligomers close to the cellular membrane at the atomic resolution. In this context, integrated studies of experiments and computer simulations can assist in mapping the complete pathways of aggregation and toxicity of Aβ peptides. Aβ oligomers are disordered proteins, and due to a rapid exploration of their intrinsic conformational space in real-time, they are challenging therapeutic targets. Therefore, no good drug candidate could have been identified for clinical use. Our previous investigations identified two small molecules, M30 (2-Octahydroisoquinolin-2(1H)-ylethanamine) and Gabapentin, capable of Aβ binding and inhibiting molecular aggregation, synaptotoxicity, intracellular calcium signaling, cellular toxicity and memory losses induced by Aβ. Thus, we recommend these molecules as novel candidates to assist anti-AD drug discovery in the near future. This review discusses the most recent research investigations about the Aβ dynamics in water, close contact with cell membranes, and several therapeutic strategies to remove plaque formation.

Influential role of 7-Ketocholesterol in the progression of Alzheimer's disease

Millions of people are affected by neurodegenerative diseases worldwide. They occur due to the loss of brain functions or peripheral nervous system dysfunction. If untreated, prolonged condition ultimately leads to death. Mostly they are associated with stress, altered cholesterol metabolism, inflammation and organelle dysfunction. Endogenous cholesterol and phospholipids in brain undergo auto-oxidation by enzymatic as well as non-enzymatic modes leading to the formation of by-products such as 4-hydroxynonenal and oxysterols. Among various oxysterols, 7-ketocholesterol (7KCh) is one of the major toxic components involved in altering neuronal lipid metabolism, contributing to inflammation and nerve cell damage. More evidently 7KCh is proven to induce oxidative stress and affects membrane permeability. Loss in mitochondrial membrane potential affects metabolism of cell organelles such as lysosomes and peroxisomes which are involved in lipid and protein homeostasis. This in turn could affect amyloidogenesis, tau protein phosphorylation and accumulation in pathological conditions of neurodegenerative diseases. Lipid alterations and the consequent pathogenic protein accumulation, results in the damage of cell organelles and microglial cells. This could be a reason behind disease progression and predominantly reported characteristics of neurodegenerative disorders such as Alzheimer's disease. This review focuses on the role of 7KCh mediated neurodegenerative Alzheimer's disease with emphasis on alterations in the lipid raft microdomain. In addition, current trends in the significant therapies related to 7KCh inhibition are highlighted.

Beta-amyloid pore linked to controlled calcium influx into the cell: A new paradigm for Alzheimer's Disease

Despite tremendous worldwide efforts, clinical trials assessing Alzheimer's disease (AD)‐related therapeutics have been relentlessly unsuccessful. Hence, there is an urgent need to challenge old hypotheses with novel paradigms. An emerging concept is that the amyloid‐beta (Aβ) peptide, which was until recently deemed a major player in the cause of AD, may instead modulate synaptic plasticity and protect against excitotoxicity. The link between Aβ‐mediated synaptic plasticity and Aβ trafficking is central for understanding AD pathogenesis and remains a perplexing relationship. The crossover between Aβ pathological and physiological roles is subtle and remains controversial. Based on existing literature, as a signaling molecule, Aβ is proposed to modulate its own turnover and synaptic plasticity through what is currently believed to be the cause of AD: the transient formation of pore‐like oligomers. A change of perspective regarding how Aβ pores exert a protective function will unavoidably revolutionize the entire field of anti‐amyloid drug development.

Therapeutic Uses of Bacterial Subunit Toxins

The B subunit pentamer verotoxin (VT aka Shiga toxin-Stx) binding to its cellular glycosphingolipid (GSL) receptor, globotriaosyl ceramide (Gb₃) mediates internalization and the subsequent receptor mediated retrograde intracellular traffic of the AB5 subunit holotoxin to the endoplasmic reticulum. Subunit separation and cytosolic A subunit transit via the ER retrotranslocon as a misfolded protein mimic, then inhibits protein synthesis to kill cells, which can cause hemolytic uremic syndrome clinically. This represents one of the most studied systems of prokaryotic hijacking of eukaryotic biology. Similarly, the interaction of cholera AB5 toxin with its GSL receptor, GM1 ganglioside, is the key component of the gastrointestinal pathogenesis of cholera and follows the same retrograde transport pathway for A subunit cytosol access. Although both VT and CT are the cause of major pathology worldwide, the toxin–receptor interaction is itself being manipulated to generate new approaches to control, rather than cause, disease. This arena comprises two areas: anti neoplasia, and protein misfolding diseases. CT/CTB subunit immunomodulatory function and anti-cancer toxin immunoconjugates will not be considered here. In the verotoxin case, it is clear that Gb₃ (and VT targeting) is upregulated in many human cancers and that there is a relationship between GSL expression and cancer drug resistance. While both verotoxin and cholera toxin similarly hijack the intracellular ERAD quality control system of nascent protein folding, the more widespread cell expression of GM1 makes cholera the toxin of choice as the means to more widely utilise ERAD targeting to ameliorate genetic diseases of protein misfolding. Gb₃ is primarily expressed in human renal tissue. Glomerular endothelial cells are the primary VT target but Gb₃ is expressed in other endothelial beds, notably brain endothelial cells which can mediate the encephalopathy primarily associated with VT2-producing E. coli infection. The Gb₃ levels can be regulated by cytokines released during EHEC infection, which complicate pathogenesis. Significantly Gb₃ is upregulated in the neovasculature of many tumours, irrespective of tumour Gb₃ status. Gb₃ is markedly increased in pancreatic, ovarian, breast, testicular, renal, astrocytic, gastric, colorectal, cervical, sarcoma and meningeal cancer relative to the normal tissue. VT has been shown to be effective in mouse xenograft models of renal, astrocytoma, ovarian, colorectal, meningioma, and breast cancer. These studies are herein reviewed. Both CT and VT (and several other bacterial toxins) access the cell cytosol via cell surface ->ER transport. Once in the ER they interface with the protein folding homeostatic quality control pathway of the cell -ERAD, (ER associated degradation), which ensures that only correctly folded nascent proteins are allowed to progress to their cellular destinations. Misfolded proteins are translocated through the ER membrane and degraded by cytosolic proteosome. VT and CT A subunits have a C terminal misfolded protein mimic sequence to hijack this transporter to enter the cytosol. This interface between exogenous toxin and genetically encoded endogenous mutant misfolded proteins, provides a new therapeutic basis for the treatment of such genetic diseases, e.g., Cystic fibrosis, Gaucher disease, Krabbe disease, Fabry disease, Tay-Sachs disease and many more. Studies showing the efficacy of this approach in animal models of such diseases are presented.

Array-Based Discriminative Optical Biosensors for Identifying Multiple Proteins in Aqueous Solution and Biofluids

Identification of proteins is an important issue both in medical research and in clinical practice as a large number of proteins are closely related to various diseases. Optical sensor arrays with recognition ability have been flourished to apply for distinguishing multiple chemically or structurally similar analytes and analyzing unknown or mixed samples. This review gives an overview of the recent development of array-based discriminative optical biosensors for recognizing proteins and their applications in real samples. Based on the number of sensor elements and the complexity of constructing array-based discriminative systems, these biosensors can be divided into three categories, which include multi-element-based sensor arrays, environment-sensitive sensor arrays and multi-wavelength-based single sensing systems. For each strategy, the construction of sensing platform and detection mechanism are particularly introduced. Meanwhile, the differences and connections between different strategies were discussed. An understanding of these aspects may help to facilitate the development of novel discriminative biosensors and expand their application prospects.

Molecular evolution of a collage of cholesterol interaction motifs in transmembrane helix V of the serotonin1A receptor

The human serotonin_1A receptor is a representative member of the superfamily of G protein-coupled receptors (GPCRs) and an important drug target for neurological disorders. Using a combination of biochemical, biophysical and molecular dynamics simulation approaches, we and others have shown that membrane cholesterol modulates the organization, dynamics and function of vertebrate serotonin_1A receptors. Previous studies have shown that the cytoplasmic portion of transmembrane helix V (TM V) and the extramembraneous intracellular loop 3 are critical for G-protein coupling, phosphorylation and desensitization of the receptor. We have recently resolved a collage of putative cholesterol interaction motifs from the amino acid sequence overlapping this region. In this paper, we explore the sequence plasticity of this fragment that may have adapted to altered membrane lipidome, after vertebrates evolved from primordial invertebrates. Since invertebrates have lower levels of membrane cholesterol relative to vertebrates, we compared TM V sequence fragments from invertebrate serotonin₁ receptors with vertebrate orthologs to infer the sequence plasticity in TM V. We report that the average number of cholesterol interaction motifs in TM V for diverse phyla represents an increasing trend that could mirror vertebrate evolution from primordial invertebrates. By statistical modeling, we propose that the collage of cholesterol interaction motifs in TM V of the human serotonin_1A receptor may have evolved from rudimentary collages, reminiscent of primordial invertebrate orthologs. Taken together, we propose that a repertoire of cholesterol-philic nonsynonymous substitutions may have enhanced collage complexity in TM V during vertebrate evolution.

Lipid rafts and neurodegeneration: structural and functional roles in physiologic aging and neurodegenerative diseases

Lipid rafts are small, dynamic membrane areas characterized by the clustering of selected membrane lipids as the result of the spontaneous separation of glycolipids, sphingolipids, and cholesterol in a liquid-ordered phase. The exact dynamics underlying phase separation of membrane lipids in the complex biological membranes are still not fully understood. Nevertheless, alterations in the membrane lipid composition affect the lateral organization of molecules belonging to lipid rafts. Neural lipid rafts are found in brain cells, including neurons, astrocytes, and microglia, and are characterized by a high enrichment of specific lipids depending on the cell type. These lipid rafts seem to organize and determine the function of multiprotein complexes involved in several aspects of signal transduction, thus regulating the homeostasis of the brain. The progressive decline of brain performance along with physiological aging is at least in part associated with alterations in the composition and structure of neural lipid rafts. In addition, neurodegenerative conditions, such as lysosomal storage disorders, multiple sclerosis, and Parkinson's, Huntington's, and Alzheimer's diseases, are frequently characterized by dysregulated lipid metabolism, which in turn affects the structure of lipid rafts. Several events underlying the pathogenesis of these diseases appear to depend on the altered composition of lipid rafts. Thus, the structure and function of lipid rafts play a central role in the pathogenesis of many common neurodegenerative diseases.jlr;61/5/636/F1F1f1.

Cholesterol Modulates the Formation of the Aβ Ion Channel in Lipid Bilayers

The misfolding of amyloid beta (Aβ) is one of the predominant hallmarks in the pathology of Alzheimer's disease (AD). In this study, we showed that the formation of the Aβ ion channel on the membrane depended on the cholesterol concentration. From a mechanical aspect, we found that cholesterol levels affected the stability and assembly of lipid bilayers. Measurements on planar lipid bilayers indicated that a small amount of cholesterol interacted with Aβ proteins and promoted the insertion process. Conversely, high cholesterol integrated the lipid bilayer and exerted an opposite effect on Aβ insertion. The Aβ ion channel was then detected by graphene-based field-effect transistors. Results demonstrated that the Aβ ion channel promoted a Ca²⁺ flux in the presence of 15% cholesterol but prevented a Ca²⁺ flux in high cholesterol. Thus, cholesterol had a complex impact on the Aβ ion channel that can be described as two different effects. First, a small amount of cholesterol interacted with Aβ and facilitated the Aβ ion channel formation in the membrane. Second, a large amount of cholesterol did not induce the ion flux in the membrane, which can be explained by the cholesterol damage to the regular distribution of the lipid bilayer. Overall, this study suggested a possible approach to consider cholesterol levels for the treatment of AD patients.

β-Amyloid Peptide: the Cell Compartment Multi-faceted Interaction in Alzheimer's Disease

Alzheimer's disease (AD) is the most widespread form of dementia, characterized by memory loss and reduction of cognitive functions that strongly interfere with normal daily life. Numerous evidences show that aggregates of the amyloid beta peptide, formed by 39 to 42 amino acid residues (Aβ39-43), from soluble small oligomers to large fibrils are characteristic markers of this pathology. However, AD is a complex disease and its neurodegenerative molecular mechanism is not yet fully understood. Growing evidence suggests a link between Aβ polymorphic nature, oligomers and fibrils, and specific mechanisms of neurodegeneration. The Aβ variable nature and its multiplicity of interactions with different proteins and organelles reflect the complexity of this pathology. In this review, we analyze the effects of the interaction between Aβ peptide and different cellular compartments in relation to the different kinds and sizes of amyloid aggregates. In particular, Aβ interaction with different cell structures such as the plasma membrane, mitochondria, lysosomes, nucleus, and endoplasmic reticulum is discussed. Further, we analyze the Aβ peptide ability to modify the structure and function of the target organelle, inducing alteration of its physiological role thus contributing to the pathological event. Dysfunction of cellular components terminating with the activation of the cellular death mechanism and subsequent neurodegeneration is also taken into consideration.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

Emerging Diversity in Lipid-Protein Interactions

Membrane lipids interact with proteins in a variety of ways, ranging from providing a stable membrane environment for proteins to being embedded in to detailed roles in complicated and well-regulated protein functions. Experimental and computational advances are converging in a rapidly expanding research area of lipid-protein interactions. Experimentally, the database of high-resolution membrane protein structures is growing, as are capabilities to identify the complex lipid composition of different membranes, to probe the challenging time and length scales of lipid-protein interactions, and to link lipid-protein interactions to protein function in a variety of proteins. Computationally, more accurate membrane models and more powerful computers now enable a detailed look at lipid-protein interactions and increasing overlap with experimental observations for validation and joint interpretation of simulation and experiment. Here we review papers that use computational approaches to study detailed lipid-protein interactions, together with brief experimental and physiological contexts, aiming at comprehensive coverage of simulation papers in the last five years. Overall, a complex picture of lipid-protein interactions emerges, through a range of mechanisms including modulation of the physical properties of the lipid environment, detailed chemical interactions between lipids and proteins, and key functional roles of very specific lipids binding to well-defined binding sites on proteins. Computationally, despite important limitations, molecular dynamics simulations with current computer power and theoretical models are now in an excellent position to answer detailed questions about lipid-protein interactions.

Characterization of Lipid-Protein Interactions and Lipid-Mediated Modulation of Membrane Protein Function through Molecular Simulation

The cellular membrane constitutes one of the most fundamental compartments of a living cell, where key processes such as selective transport of material and exchange of information between the cell and its environment are mediated by proteins that are closely associated with the membrane. The heterogeneity of lipid composition of biological membranes and the effect of lipid molecules on the structure, dynamics, and function of membrane proteins are now widely recognized. Characterization of these functionally important lipid-protein interactions with experimental techniques is however still prohibitively challenging. Molecular dynamics (MD) simulations offer a powerful complementary approach with sufficient temporal and spatial resolutions to gain atomic-level structural information and energetics on lipid-protein interactions. In this review, we aim to provide a broad survey of MD simulations focusing on exploring lipid-protein interactions and characterizing lipid-modulated protein structure and dynamics that have been successful in providing novel insight into the mechanism of membrane protein function.

Structure of amyloid β25-35 in lipid environment and cholesterol-dependent membrane pore formation

The amyloid β (Aβ) peptide and its shorter variants, including a highly cytotoxic Aβ_25–35 peptide, exert their neurotoxic effect during Alzheimer’s disease by various mechanisms, including cellular membrane permeabilization. The intrinsic polymorphism of Aβ has prevented the identification of the molecular basis of Aβ pore formation by direct structural methods, and computational studies have led to highly divergent pore models. Here, we have employed a set of biophysical techniques to directly monitor Ca²⁺-transporting Aβ_25–35 pores in lipid membranes, to quantitatively characterize pore formation, and to identify the key structural features of the pore. Moreover, the effect of membrane cholesterol on pore formation and the structure of Aβ_25–35 has been elucidated. The data suggest that the membrane-embedded peptide forms 6- or 8-stranded β-barrel like structures. The 8-stranded barrels may conduct Ca²⁺ ions through an inner cavity, whereas the tightly packed 6-stranded barrels need to assemble into supramolecular structures to form a central pore. Cholesterol affects Aβ_25–35 pore formation by a dual mechanism, i.e., by direct interaction with the peptide and by affecting membrane structure. Collectively, our data illuminate the molecular basis of Aβ membrane pore formation, which should advance both basic and clinical research on Alzheimer’s disease and membrane-associated pathologies in general.

Challenges and approaches to understand cholesterol-binding impact on membrane protein function: an NMR view

Experimental evidence for a direct role of lipids in determining the structure, dynamics, and function of membrane proteins leads to the term 'functional lipids'. In particular, the sterol molecule cholesterol modulates the activity of many membrane proteins. The precise nature of cholesterol-binding sites and the consequences of modulation of local membrane micro-viscosity by cholesterol, however, is often unknown. Here, we review the current knowledge of the interaction of cholesterol with transmembrane proteins, with a special focus on structural aspects of the interaction derived from nuclear magnetic resonance approaches. We highlight examples of the importance of cholesterol modulation of membrane protein function, discuss the specificity of cholesterol binding, and review the proposed binding motifs from a molecular perspective. We conclude with a short perspective on what could be future trends in research efforts targeted towards a better understanding of cholesterol/membrane protein interactions.

Gangliosides in Membrane Organization

Since the structure of GM1 was elucidated 55years ago, researchers have been attracted by the sialylated glycans of gangliosides. Gangliosides head groups, protruding toward the extracellular space, significantly contribute to the cell glycocalyx; and in certain cells, such as neurons, are major determinants of the features of the cell surface. Expression of glycosyltransferases involved in the de novo biosynthesis of gangliosides is tightly regulated along cell differentiation and activation, and is regarded as the main metabolic mechanism responsible for the acquisition of cell-specific ganglioside patterns. The resulting sialooligosaccharides are characterized by a high degree of geometrical complexity and by highly dynamic properties, which seem to be functional for complex interactions with other molecules sitting on the same cellular membrane (cis-interactions) or soluble molecules present in the extracellular environment, or molecules associated with the surface of other cells (trans-interactions). There is no doubt that the multifaceted biological functions of gangliosides are largely dependent on oligosaccharide-mediated molecular interactions. However, gangliosides are amphipathic membrane lipids, and their chemicophysical, aggregational, and, consequently, biological properties are dictated by the properties of the monomers as a whole, which are not merely dependent on the structures of their polar head groups. In this chapter, we would like to focus on the peculiar chemicophysical features of gangliosides (in particular, those of the nervous system), that represent an important driving force determining the organization and properties of cellular membranes, and to emphasize the causal connections between altered ganglioside-dependent membrane organization and relevant pathological conditions.

Sterol Structure Strongly Modulates Membrane-Islet Amyloid Polypeptide Interactions

Amyloid formation has been implicated in a wide range of human diseases, and the interaction of amyloidogenic proteins with membranes are believed to be important for many of them. In type-2 diabetes, human islet amyloid polypeptide (IAPP) forms amyloids, which contribute to β-cell death and dysfunction in the disease. IAPP-membrane interactions are potential mechanisms of cytotoxicity. In vitro studies have shown that cholesterol significantly modulates the ability of model membranes to induce IAPP amyloid formation and IAPP-mediated membrane damage. It is not known if this is due to the general effects of cholesterol on membranes or because of specific sterol-IAPP interactions. The effects of replacing cholesterol with eight other sterols/steroids on IAPP binding to model membranes, membrane disruption, and membrane-mediated amyloid formation were examined. The primary effect of the sterols/steroids on the IAPP-membrane interactions was found to reflect their effect upon membrane order as judged by fluorescence anisotropy measurements. Specific IAPP-sterol/steroid interactions have smaller effects. The fraction of vesicles that bind IAPP was inversely correlated with the sterols/steroids' effect on membrane order, as was the extent of IAPP-induced membrane leakage and the time to form amyloids. The correlation between the fraction of vesicles binding IAPP and membrane leakage was particularly tight, suggesting the restriction of IAPP to a subset of vesicles is responsible for incomplete leakage.

Membrane Binding and Pore Formation by a Cytotoxic Fragment of Amyloid β Peptide

Amyloid β (Aβ) peptide contributes to Alzheimer's disease by a yet unidentified mechanism. In the brain tissue, Aβ occurs in various forms, including an undecapeptide Aβ_25-35, which exerts a neurotoxic effect through the mitochondrial dysfunction and/or Ca²⁺-permeable pore formation in cell membranes. This work was aimed at the biophysical characterization of membrane binding and pore formation by Aβ_25-35. Interaction of Aβ_25-35 with anionic and zwitterionic membranes was analyzed by microelectrophoresis. In pore formation experiments, Aβ_25-35 was incubated in aqueous buffer to form oligomers and added to Quin-2-loaded vesicles. Gradual increase in Quin-2 fluorescence was interpreted in terms of membrane pore formation by the peptide, Ca²⁺ influx, and binding to intravesicular Quin-2. The kinetics and magnitude of this process were used to evaluate the rate constant of pore formation, peptide-peptide association constants, and the oligomeric state of the pores. Decrease in membrane anionic charge and high ionic strength conditions significantly suppressed membrane binding and pore formation, indicating the importance of electrostatic interactions in these events. Circular dichroism spectroscopy showed that Aβ_25-35 forms the most efficient pores in β-sheet conformation. The data are consistent with an oligo-oligomeric pore model composed of up to eight peptide units, each containing 6-8 monomers.

Cholesterol Changes the Mechanisms of Aβ Peptide Binding to the DMPC Bilayer

Using isobaric-isothermal all-atom replica-exchange molecular dynamics (REMD) simulations, we investigated the equilibrium binding of Aβ10-40 monomers to the zwitterionic dimyristoylphosphatidylcholine (DMPC) bilayer containing cholesterol. Our previous REMD simulations, which studied binding of the same peptide to the cholesterol-free DMPC bilayer, served as a control, against which we measured the impact of cholesterol. Our findings are as follows. First, addition of cholesterol to the DMPC bilayer partially expels the Aβ peptide from the hydrophobic core and promotes its binding to bilayer polar headgroups. Using thermodynamic and energetics analyses, we argued that Aβ partial expulsion is not related to cholesterol-induced changes in lateral pressure within the bilayer but is caused by binding energetics, which favors Aβ binding to the surface of the densely packed cholesterol-rich bilayer. Second, cholesterol has a protective effect on the DMPC bilayer structure against perturbations caused by Aβ binding. More specifically, cholesterol reduces bilayer thinning and overall depletion of bilayer density beneath the Aβ binding footprint. Third, we found that the Aβ peptide contains a single cholesterol binding site, which involves hydrophobic C-terminal amino acids (Ile31-Val36), Phe19, and Phe20 from the central hydrophobic cluster, and cationic Lys28 from the turn region. This binding site accounts for about 76% of all Aβ-cholesterol interactions. Because cholesterol binding site in the Aβ10-40 peptide does not contain the GXXXG motif featured in cholesterol interactions with the transmembrane domain C99 of the β-amyloid precursor protein, we argued that the binding mechanisms for Aβ and C99 are distinct reflecting their different conformations and positions in the lipid bilayer. Fourth, cholesterol sharply reduces the helical propensity in the bound Aβ peptide. As a result, cholesterol largely eliminates the emergence of helical structure observed upon Aβ transition from a water environment to the cholesterol-free DMPC bilayer. We explain this effect by the formation of hydrogen bonds between cholesterol and the Aβ backbone, which prevent helix formation. Taken together, we expect that our simulations will advance understanding of a molecular-level mechanism behind the role of cholesterol in Alzheimer's disease.

Anandamide-ceramide interactions in a membrane environment: Molecular dynamic simulations data

Anandamide is a lipid neurotransmitter that interacts with various plasma membrane lipids. The data here consists of molecular dynamics simulations of anandamide, C18-ceramide and cholesterol performed in vacuo and within a hydrated palmitoyl-oleoyl-phosphatidylcholine (POPC)/cholesterol membrane. Several models of anandamide/cholesterol and anandamide/ceramide complexes are presented. The energy of interaction and the nature of the intermolecular forces involved in each of these complexes are detailed. The impact of water molecules hydrating the POPC/cholesterol membrane for the stability of the anandamide/cholesterol and anandamide/ceramide complexes is also analyzed. From a total number of 1920 water molecules stochatiscally merged with the lipid matrix, 48 were eventually redistributed around the polar head groups of the anandamide/ceramide complex, whereas only 15 reached with the anandamide/cholesterol complex. The interpretation of this dataset is presented in the accompanying article "Ceramide binding to anandamide increases its half-life and potentiates its cytotoxicity in human neuroblastoma cells" [1].

Amyloid Beta Peptide Folding in Reverse Micelles

Previously published experimental studies have suggested that when the 40-residue amyloid beta peptide is encapsulated in a reverse micelle, it folds into a structure that may nucleate amyloid fibril formation (Yeung, P. S.-W.; Axelsen, P. H. J. Am. Chem. Soc. 2012, 134, 6061 ). The factors that induce the formation of this structure have now been identified in a multi-microsecond simulation of the same reverse micelle system that was studied experimentally. Key features of the polypeptide-micelle interaction include the anchoring of a hydrophobic residue cluster into gaps in the reverse micelle surface, the formation of a beta turn at the anchor point that brings N- and C-terminal segments of the polypeptide into proximity, high ionic strength that promotes intramolecular hydrogen bond formation, and deformation of the reverse micelle surface to facilitate interactions with the surface along the entire length of the polypeptide. Together, these features cause the simulation-derived vibrational spectrum to red shift in a manner that reproduces the red-shift previously reported experimentally. On the basis of these findings, a new mechanism is proposed whereby membranes nucleate fibril formation and facilitate the in-register alignment of polypeptide strands that is characteristic of amyloid fibrils.

Amyloid plaques beyond Aβ: a survey of the diverse modulators of amyloid aggregation

Aggregation of the amyloid-β (Aβ) peptide is strongly correlated with Alzheimer's disease (AD). Recent research has improved our understanding of the kinetics of amyloid fibril assembly and revealed new details regarding different stages in plaque formation. Presently, interest is turning toward studying this process in a holistic context, focusing on cellular components which interact with the Aβ peptide at various junctures during aggregation, from monomer to cross-β amyloid fibrils. However, even in isolation, a multitude of factors including protein purity, pH, salt content, and agitation affect Aβ fibril formation and deposition, often producing complicated and conflicting results. The failure of numerous inhibitors in clinical trials for AD suggests that a detailed examination of the complex interactions that occur during plaque formation, including binding of carbohydrates, lipids, nucleic acids, and metal ions, is important for understanding the diversity of manifestations of the disease. Unraveling how a variety of key macromolecular modulators interact with the Aβ peptide and change its aggregation properties may provide opportunities for developing therapies. Since no protein acts in isolation, the interplay of these diverse molecules may differentiate disease onset, progression, and severity, and thus are worth careful consideration.

Cellular cholesterol homeostasis and Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in older adults. Currently, there is no cure for AD. The hallmark of AD is the accumulation of extracellular amyloid plaques composed of amyloid-β (Aβ) peptides (especially Aβ1-42) and neurofibrillary tangles, composed of hyperphosphorylated tau and accompanied by chronic neuroinflammation. Aβ peptides are derived from the amyloid precursor protein (APP). The oligomeric form of Aβ peptides is probably the most neurotoxic species; its accumulation eventually forms the insoluble and aggregated amyloid plaques. ApoE is the major apolipoprotein of the lipoprotein(s) present in the CNS. ApoE has three alleles, of which the Apoe4 allele constitutes the major risk factor for late-onset AD. Here we describe the complex relationship between ApoE4, oligomeric Aβ peptides, and cholesterol homeostasis. The review consists of four parts: 1) key elements involved in cellular cholesterol metabolism and regulation; 2) key elements involved in intracellular cholesterol trafficking; 3) links between ApoE4, Aβ peptides, and disturbance of cholesterol homeostasis in the CNS; 4) potential lipid-based therapeutic targets to treat AD. At the end, we recommend several research topics that we believe would help in better understanding the connection between cholesterol and AD for further investigations.

Hybrid In Silico/In Vitro Approaches for the Identification of Functional Cholesterol-Binding Domains in Membrane Proteins

In eukaryotic cells, cholesterol is an important regulator of a broad range of membrane proteins, including receptors, transporters, and ion channels. Understanding how cholesterol interacts with membrane proteins is a difficult task because structural data of these proteins complexed with cholesterol are scarce. Here, we describe a dual approach based on in silico studies of protein-cholesterol interactions, combined with physico-chemical measurements of protein insertion into cholesterol-containing monolayers. Our algorithm is validated through careful analysis of the effect of key mutations within and outside the predicted cholesterol-binding site. Our method is illustrated by a complete analysis of cholesterol-binding to Alzheimer's β-amyloid peptide, a protein that penetrates the plasma membrane of brain cells through a cholesterol-dependent process.

The dynamic binding of cholesterol to the multiple sites of C99: as revealed by coarse-grained and all-atom simulations

The multi-site cholesterol binding model of C99.

Common molecular mechanism of amyloid pore formation by Alzheimer's β-amyloid peptide and α-synuclein

Calcium-permeable pores formed by small oligomers of amyloid proteins are the primary pathologic species in Alzheimer’s and Parkinson’s diseases. However, the molecular mechanisms underlying the assembly of these toxic oligomers in the plasma membrane of brain cells remain unclear. Here we have analyzed and compared the pore-forming capability of a large panel of amyloid proteins including wild-type, variant and truncated forms, as well as synthetic peptides derived from specific domains of Aβ1-42 and α-synuclein. We show that amyloid pore formation involves two membrane lipids, ganglioside and cholesterol, that physically interact with amyloid proteins through specific structural motifs. Mutation or deletion of these motifs abolished pore formation. Moreover, α-synuclein (Parkinson) and Aβ peptide (Alzheimer) did no longer form Ca²⁺-permeable pores in presence of drugs that target either cholesterol or ganglioside or both membrane lipids. These results indicate that gangliosides and cholesterol cooperate to favor the formation of amyloid pores through a common molecular mechanism that can be jammed at two different steps, suggesting the possibility of a universal therapeutic approach for neurodegenerative diseases. Finally we present the first successful evaluation of such a new therapeutic approach (coined “membrane therapy”) targeting amyloid pores formed by Aβ1-42 and α-synuclein.

The pleiotropic effects of omega-3 docosahexaenoic acid on the hallmarks of Alzheimer's disease

Among omega-3 polyunsaturated fatty acids (PUFAs), docosahexaenoic acid (DHA, 22:6n-3) is important for adequate brain development and cognition. DHA is highly concentrated in the brain and plays an essential role in brain functioning. DHA, one of the major constituents in fish fats, readily crosses the blood-brain barrier from blood to the brain. Its critical role was further supported by its reduced levels in the brain of Alzheimer's disease (AD) patients. This agrees with a potential role of DHA in memory, learning and cognitive processes. Since there is yet no cure for dementia such as AD, there is growing interest in the role of DHA-supplemented diet in the prevention of AD pathogenesis. Accordingly, animal, epidemiological, preclinical and clinical studies indicated that DHA has neuroprotective effects in a number of neurodegenerative conditions including AD. The beneficial effects of this key omega-3 fatty acid supplementation may depend on the stage of disease progression, other dietary mediators and the apolipoprotein ApoE genotype. Herein, our review investigates, from animal and cell culture studies, the molecular mechanisms involved in the neuroprotective potential of DHA with emphasis on AD.

A mirror code for protein-cholesterol interactions in the two leaflets of biological membranes

Cholesterol controls the activity of a wide range of membrane receptors through specific interactions and identifying cholesterol recognition motifs is therefore critical for understanding signaling receptor function. The membrane-spanning domains of the paradigm neurotransmitter receptor for acetylcholine (AChR) display a series of cholesterol consensus domains (referred to as “CARC”). Here we use a combination of molecular modeling, lipid monolayer/mutational approaches and NMR spectroscopy to study the binding of cholesterol to a synthetic CARC peptide. The CARC-cholesterol interaction is of high affinity, lipid-specific, concentration-dependent, and sensitive to single-point mutations. The CARC motif is generally located in the outer membrane leaflet and its reverse sequence CRAC in the inner one. Their simultaneous presence within the same transmembrane domain obeys a “mirror code” controlling protein-cholesterol interactions in the outer and inner membrane leaflets. Deciphering this code enabled us to elaborate guidelines for the detection of cholesterol-binding motifs in any membrane protein. Several representative examples of neurotransmitter receptors and ABC transporters with the dual CARC/CRAC motifs are presented. The biological significance and potential clinical applications of the mirror code are discussed.

Interactions of the N-terminal domain of human islet amyloid polypeptide with lipid membranes: the effect of cholesterol

Cholesterol facilitates the insertion and aggregation of hIAPP_1–19 in membrane and the CARC motif mediates the peptide–cholesterol interaction.

Effects of Charged Cholesterol Derivatives on Aβ40 Amyloid Formation

Understanding of the mechanistic progess of amyloid-β peptide (Aβ) aggregation is critical for elucidating the underlying pathogenesis of Alzheimer's disease (AD). Herein, we report for the first time the effects of two cholesterol derivatives, negatively charged cholesterol sulfate (cholesterol-SO4) and positively charged 3β-[N-(dimethylaminoethane)carbamoyl]-cholesterol (DC-cholesterol), on the fibrillization of Aβ40. Our results demonstrate that both of the nonvesicular forms of cholesterol-SO4 and DC-cholesterol moderately accelerate the aggregation rate of Aβ40. This effect is similar to that observed for unmodified cholesterol, indicating the importance of hydrophobic interactions in binding of Aβ40 to these steroid molecules. Furthermore, we show that the vesicles formed at higher concentrations of anionic cholesterol-SO4 facilitate Aβ40 aggregation rate markedly. In contrast, the cationic DC-cholesterol vesicles show the ability to inhibit Aβ40 fibril formation under appropriate experimental conditions. The results suggest that the electrostatic interactions between Aβ40 and the charged vesicles can be of great importance in regulating Aβ40-vesicle interaction. Our results also indicate that the structural properties of the aggregates of the cholesterol derivatives, including the surface charge and the size of the vesicles, are critical in regulating the effects of these vesicles on Aβ40 aggregation kinetics.

Lipid insertion domain unfolding regulates protein orientational transition behavior in a lipid bilayer

We have used coarse-grained (CG) and united atom (UA) molecular dynamics simulations to explore the mechanisms of protein orientational transition of a model peptide (Aβ42) in a phosphatidylcholine/cholesterol (PC/CHO) lipid bilayer. We started with an inserted state of Aβ42 containing a folded (I) or unfolded (II) K28-A42 lipid insertion domain (LID), which was stabilized by the K28-snorkeling and A42-anchoring to the PC polar groups in the lipid bilayer. After a UA-to-CG transformation and a 1000ns-CG simulation for enhancing the sampling of protein orientations, we discovered two transitions: I-to-"deep inserted" state with disrupted K28-snorkeling and II-to-"deep surface" state with disrupted A42-anchoring. The new states remained stable after a CG-to-UA transformation and a 200ns-UA simulation relaxation. Significant changes in the cholesterol-binding domain of Aβ42 and protein-induced membrane disruptions were evident after the transitions. We propose that the conformation of the LID regulates protein orientational transitions in the lipid membrane.

The influence of cholesterol on membrane protein structure, function, and dynamics studied by molecular dynamics simulations

The plasma membrane, which encapsulates human cells, is composed of a complex mixture of lipids and embedded proteins. Emerging knowledge points towards the lipids as having a regulating role in protein function. Furthermore, insight from protein crystallography has revealed several different types of lipids intimately bound to membrane proteins and peptides, hereby possibly pointing to a site of action for the observed regulation. Cholesterol is among the lipid membrane constituents most often observed to be co-crystallized with membrane proteins, and the cholesterol levels in cell membranes have been found to play an essential role in health and disease. Remarkably little is known about the mechanism of lipid regulation of membrane protein function in health as well as in disease. Herein, we review molecular dynamics simulation studies aimed at investigating the effect of cholesterol on membrane protein and peptide properties. This article is part of a Special Issue entitled: Lipid-protein interactions.

Functional competition within a membrane: Lipid recognition vs. transmembrane helix oligomerization

Binding of specific lipids to large, polytopic membrane proteins is well described, and it is clear that such lipids are crucial for protein stability and activity. In contrast, binding of defined lipid species to individual transmembrane helices and regulation of transmembrane helix monomer-oligomer equilibria by binding of distinct lipids is a concept, which has emerged only lately. Lipids bind to single-span membrane proteins, both in the juxta-membrane region as well as in the hydrophobic membrane core. While some interactions counteract transmembrane helix oligomerization, in other cases lipid binding appears to enhance oligomerization. As reversible oligomerization is involved in activation of many membrane proteins, binding of defined lipids to single-span transmembrane proteins might be a mechanism to regulate and/or fine-tune the protein activity. But how could lipid binding trigger the activity of a protein? How can binding of a single lipid molecule to a transmembrane helix affect the structure of a transmembrane helix oligomer, and consequently its signaling state? These questions are discussed in the present article based on recent results obtained with simple, single-span transmembrane proteins. This article is part of a Special Issue entitled: Lipid-protein interactions.