Dynamics and Cleavability at the alpha-cleavage site of APP(684-726) in different lipid environments

Obesity during preclinical Alzheimer's disease development exacerbates brain metabolic decline

Alzheimer's disease (AD) is the most common form of dementia. Obesity in middle age increases AD risk and severity, which is alarming given that obesity prevalence peaks at middle age and obesity rates are accelerating worldwide. Midlife, but not late-life obesity increases AD risk, suggesting that this interaction is specific to preclinical AD. AD pathology begins in middle age, with accumulation of amyloid beta (Aβ), hyperphosphorylated tau, metabolic decline, and neuroinflammation occurring decades before cognitive symptoms appear. We used a transcriptomic discovery approach in young adult (6.5 months old) male and female TgF344-AD rats that overexpress mutant human amyloid precursor protein and presenilin-1 and wild-type (WT) controls to determine whether inducing obesity with a high-fat/high-sugar "Western" diet during preclinical AD increases brain metabolic dysfunction in dorsal hippocampus (dHC), a brain region vulnerable to the effects of obesity and early AD. Analyses of dHC gene expression data showed dysregulated mitochondrial and neurotransmission pathways, and up-regulated genes involved in cholesterol synthesis. Western diet amplified the number of genes that were different between AD and WT rats and added pathways involved in noradrenergic signaling, dysregulated inhibition of cholesterol synthesis, and decreased intracellular lipid transporters. Importantly, the Western diet impaired dHC-dependent spatial working memory in AD but not WT rats, confirming that the dietary intervention accelerated cognitive decline. To examine later consequences of early transcriptional dysregulation, we measured dHC monoamine levels in older (13 months old) AD and WT rats of both sexes after long-term chow or Western diet consumption. Norepinephrine (NE) abundance was significantly decreased in AD rats, NE turnover was increased, and the Western diet attenuated the AD-induced increases in turnover. Collectively, these findings indicate obesity during prodromal AD impairs memory, potentiates AD-induced metabolic decline likely leading to an overproduction of cholesterol, and interferes with compensatory increases in NE transmission.

A tailored tetravalent peptide displays dual functions to inhibit amyloid β production and aggregation

Inhibition of amyloid-β peptide (Aβ) accumulation in the brain is a promising approach for treatment of Alzheimer's disease (AD). Aβ is produced by β-secretase and γ-secretase in endosomes via sequential proteolysis of amyloid precursor protein (APP). Aβ and APP have a common feature to readily cluster to form multimers. Here, using multivalent peptide library screens, we identified a tetravalent peptide, LME-tet, which binds APP and Aβ via multivalent interactions. In cells, LME-tet-bound APP in the plasma membrane is transported to endosomes, blocking Aβ production through specific inhibition of β-cleavage, but not γ-cleavage. LME-tet further suppresses Aβ aggregation by blocking formation of the β-sheet conformation. Inhibitory effects are not observed with a monomeric peptide, emphasizing the significance of multivalent interactions for mediating these activities. Critically, LME-tet efficiently reduces Aβ levels in the brain of AD model mice, suggesting it may hold promise for treatment of AD.

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.

β-Amyloid aggregation and heterogeneous nucleation

In this article, we consider the role of heterogeneous nucleation in β-amyloid aggregation. Heterogeneous nucleation is more common and occurs at lower levels of supersaturation than homogeneous nucleation. The nucleation period is also the stage at which most of the polymorphism of amyloids arises, this being one of the defining features of amyloids. We focus on several well-known heterogeneous nucleators of β-amyloid, including lipid surfaces, especially those enriched in gangliosides and cholesterol, and divalent metal ions. These two broad classes of nucleators affect β-amyloid particularly in light of the amphiphilicity of these peptides: the N-terminal region, which is largely polar and charged, contains the metal binding site, whereas the C-terminal region is aliphatic and is important in lipid binding. Notably, these two classes of nucleators can interact cooperatively, aggregation begetting greater aggregation.

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.

Impact of bilayer lipid composition on the structure and topology of the transmembrane amyloid precursor C99 protein

C99 (also known as β-CTF) is the 99 residue transmembrane C-terminal domain (residues 672–770) of the amyloid precursor protein and is the immediate precursor of the amyloid-β (Aβ) polypeptides. To test the dependence of the C99 structure on the composition of the host model membranes, NMR studies of C99 were conducted both in anionic lyso-myristoylphosphatidylglycerol (LMPG) micelles and in a series of five zwitterionic bicelle compositions involving phosphatidylcholine and sphingomyelin in which the acyl chain lengths of these lipid components varied from 14 to 24 carbons. Some of these mixtures are reported for the first time in this work and should be of broad utility in membrane protein research. The site-specific backbone ¹⁵N and ¹H chemical shifts for C99 in LMPG and in all five bicelle mixtures were seen to be remarkably similar, indicating little dependence of the backbone structure of C99 on the composition of the host model membrane. However, the length of the transmembrane span was seen to vary in a manner that alters the positioning of the γ-secretase cleavage sites with respect to the center of the bilayer. This observation may contribute to the known dependency of the Aβ42-to-Aβ40 production ratio on both membrane thickness and the length of the C99 transmembrane domain.

Cholesterol as a co-solvent and a ligand for membrane proteins

As of mid 2013 a Medline search on "cholesterol" yielded over 200,000 hits, reflecting the prominence of this lipid in numerous aspects of animal cell biology and physiology under conditions of health and disease. Aberrations in cholesterol homeostasis underlie both a number of rare genetic disorders and contribute to common sporadic and complex disorders including heart disease, stroke, type II diabetes, and Alzheimer's disease. The corresponding author of this review and his lab stumbled only recently into the sprawling area of cholesterol research when they discovered that the amyloid precursor protein (APP) binds cholesterol, a topic covered by the Hans Neurath Award lecture at the 2013 Protein Society Meeting. Here, we first provide a brief overview of cholesterol-protein interactions and then offer our perspective on how and why binding of cholesterol to APP and its C99 domain (β-CTF) promotes the amyloidogenic pathway, which is closely related to the etiology of Alzheimer's disease.

Effect of Different Phospholipids on α-Secretase Activity in the Non-Amyloidogenic Pathway of Alzheimer's Disease

Alzheimer’s disease (AD) is characterized by extracellular accumulation of amyloid-β peptide (Aβ), generated by proteolytic processing of the amyloid precursor protein (APP) by β- and γ-secretase. Aβ generation is inhibited when the initial ectodomain shedding is caused by α-secretase, cleaving APP within the Aβ domain. Therefore, an increase in α-secretase activity is an attractive therapeutic target for AD treatment. APP and the APP-cleaving secretases are all transmembrane proteins, thus local membrane lipid composition is proposed to influence APP processing. Although several studies have focused on γ-secretase, the effect of the membrane lipid microenvironment on α-secretase is poorly understood. In the present study, we systematically investigated the effect of fatty acid (FA) acyl chain length (10:0, 12:0, 14:0, 16:0, 18:0, 20:0, 22:0, 24:0), membrane polar lipid headgroup (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine), saturation grade and the FA double-bond position on α-secretase activity. We found that α-secretase activity is significantly elevated in the presence of FAs with short chain length and in the presence of polyunsaturated FAs, whereas variations in the phospholipid headgroups, as well as the double-bond position, have little or no effect on α-secretase activity. Overall, our study shows that local lipid membrane composition can influence α-secretase activity and might have beneficial effects for AD.

Evidence from solid-state NMR for nonhelical conformations in the transmembrane domain of the amyloid precursor protein

The amyloid precursor protein (APP) is subject to proteolytic processing by γ-secretase within neuronal membranes, leading to Alzheimer's disease-associated β-amyloid peptide production by cleavage near the midpoint of the single transmembrane (TM) segment of APP. Conformational properties of the TM segment may affect its susceptibility to γ-secretase cleavage, but these properties have not been established definitively, especially in bilayer membranes with physiologically relevant lipid compositions. In this article, we report an investigation of the APP-TM conformation, using (13)C chemical shifts obtained with two-dimensional solid-state NMR spectroscopy as site-specific conformational probes. We find that the APP-TM conformation is not a simple α-helix, particularly at 37°C in multilamellar vesicles with compositions that mimic the composition of neuronal cell membranes. Instead, we observe a mixture of helical and nonhelical conformations at the N- and C-termini and in the vicinity of the γ-cleavage site. Conformational plasticity of the TM segment of APP may be an important factor in the γ-secretase cleavage mechanism.

Structural and dynamic study of the transmembrane domain of the amyloid precursor protein

Alzheimer's disease affects people all over the world, regardless of nationality, gender or social status. An adequate study of the disease requires essential understanding of the molecular fundamentals of the pathogenesis. The amyloid β-peptide, which forms amyloid plaques in the brain of people with Alzheimer's disease, is the product of sequential cleavage of a single-span membrane amyloid precursor protein (APP). More than half of the APP mutations found to be associated with familial forms of Alzheimer's disease are located in its transmembrane domain. The pathogenic mutations presumably affect the structural-dynamic properties of the APP transmembrane domain by changing its conformational stability and/or lateral dimerization. In the present study, the structure and dynamics of the recombinant peptide corresponding to the APP fragment, Gln686-Lys726, which comprises the APP transmembrane domain with an adjacent N-terminal juxtamembrane sequence, were determined in the membrane mimetic environment composed of detergent micelles using NMR spectroscopy. The structure obtained in dodecylphosphocholine micelles consists of two α-helices: a short surface-associated juxtamembrane helix (Lys687-Asp694) and a long transmembrane helix (Gly700-Leu723), both connected via a mobile loop region. A minor bend of the transmembrane α-helix is observed near the paired residues Gly708-Gly709. A cholesterol-binding hydrophobic cavity is apparently formed under the loop region, where the juxtamembrane α-helix comes into contact with the membrane surface near the N-terminus of the transmembrane α-helix.

Direct binding of cholesterol to the amyloid precursor protein: An important interaction in lipid-Alzheimer's disease relationships?

It is generally believed that cholesterol homoeostasis in the brain is both linked to and impacted by Alzheimer's disease (AD). For example, elevated levels of cholesterol in neuronal plasma and endosome membranes appear to be a pro-amyloidogenic factor. The recent observation that the C-terminal transmembrane domain (C99, also known as the beta-C-terminal fragment, or beta-CTF) of the amyloid precursor protein (APP) specifically binds cholesterol helps to tie together previously loose ends in the web of our understanding of Alzheimer's-cholesterol relationships. In particular, binding of cholesterol to C99 appears to favor the amyloidogenic pathway in cells by promoting localization of C99 in lipid rafts. In turn, the products of this pathway-amyloid-beta and the intracellular domain of the APP (AICD)-may down-regulate ApoE-mediated cholesterol uptake and cholesterol biosynthesis. If confirmed, this negative-feedback loop for membrane cholesterol levels has implications for understanding the function of the APP and for devising anti-amyloidogenic preventive strategies for AD.

Structures of beta-amyloid peptide 1-40, 1-42, and 1-55-the 672-726 fragment of APP-in a membrane environment with implications for interactions with gamma-secretase

Aggregation of Amyloid beta (Abeta) peptide has been linked to the neurodegenerative Alzheimer's Disease and implicated in other amyloid diseases including cerebral amyloid angiopathy. Abeta peptide is generated by cleavage of the amyloid precursor protein (APP) by transmembrane proteases. It is crucial to determine the structures of beta-amyloid peptides in a membrane to provide a molecular basis for the cleavage mechanism. We report the structures of amyloid beta peptide (Abeta(1-40) and Abeta(1-42)) as well as the 672-726 fragment of APP (referred to as Abeta(1-55)) in a membrane environment determined by replica-exchange molecular dynamics simulation. Abeta(1-40) is found to have two helical domains A (13-22) and B(30-35) and a type I beta-turn at 23-27. The peptide is localized at the interface between membrane and solvent. Substantial fluctuations in domain A are observed. The dominant simulated tertiary structure of Abeta(1-40) is observed to be similar to the simulated Abeta(1-42) structure. However, there are differences observed in the overall conformational ensemble, as characterized by the two-dimensional free energy surfaces. The fragment of APP (Abeta(1-55)) is observed to have a long transmembrane helix. The position of the transmembrane region and ensemble of membrane structures are elucidated. The conformational transition between the transmembrane Abeta(1-55) structure, prior to cleavage, and the Abeta(1-40) structure, following cleavage, is proposed.

Structural studies of the transmembrane C-terminal domain of the amyloid precursor protein (APP): does APP function as a cholesterol sensor?

The amyloid precursor protein (APP) is subject to alternative pathways of proteolytic processing, leading either to production of the amyloid-beta (Abeta) peptides or to non-amyloidogenic fragments. Here, we report the first structural study of C99, the 99-residue transmembrane C-terminal domain of APP liberated by beta-secretase cleavage. We also show that cholesterol, an agent that promotes the amyloidogenic pathway, specifically binds to this protein. C99 was purified into model membranes where it was observed to homodimerize. NMR data show that the transmembrane domain of C99 is an alpha-helix that is flanked on both sides by mostly disordered extramembrane domains, with two exceptions. First, there is a short extracellular surface-associated helix located just after the site of alpha-secretase cleavage that helps to organize the connecting loop to the transmembrane domain, which is known to be essential for Abeta production. Second, there is a surface-associated helix located at the cytosolic C-terminus, adjacent to the YENPTY motif that plays critical roles in APP trafficking and protein-protein interactions. Cholesterol was seen to participate in saturable interactions with C99 that are centered at the critical loop connecting the extracellular helix to the transmembrane domain. Binding of cholesterol to C99 and, most likely, to APP may be critical for the trafficking of these proteins to cholesterol-rich membrane domains, which leads to cleavage by beta- and gamma-secretase and resulting amyloid-beta production. It is proposed that APP may serve as a cellular cholesterol sensor that is linked to mechanisms for suppressing cellular cholesterol uptake.