Site-specific Effects of Peptide Lipidation on β-Amyloid Aggregation and Cytotoxicity

Chemically modified and conjugated antimicrobial peptides against superbugs

Antimicrobial resistance (AMR) is one of the greatest threats to human health that, by 2050, will lead to more deaths from bacterial infections than cancer. New antimicrobial agents, both broad-spectrum and selective, that do not induce AMR are urgently required. Antimicrobial peptides (AMPs) are a novel class of alternatives that possess potent activity against a wide range of Gram-negative and positive bacteria with little or no capacity to induce AMR. This has stimulated substantial chemical development of novel peptide-based antibiotics possessing improved therapeutic index. This review summarises recent synthetic efforts and their impact on analogue design as well as their various applications in AMP development. It includes modifications that have been reported to enhance antimicrobial activity including lipidation, glycosylation and multimerization through to the broad application of novel bio-orthogonal chemistry, as well as perspectives on the direction of future research. The subject area is primarily the development of next-generation antimicrobial agents through selective, rational chemical modification of AMPs. The review further serves as a guide toward the most promising directions in this field to stimulate broad scientific attention, and will lead to new, effective and selective solutions for the several biomedical challenges to which antimicrobial peptidomimetics are being applied.

Site-Specific Alkylation of the Islet Amyloid Polypeptide Accelerates Self-Assembly and Potentiates Perturbation of Lipid Membranes

The accumulation of insoluble amyloids in the pancreatic islets is a pathological hallmark of type II diabetes and correlates closely with the loss of β-cell mass. The predominant component of these amyloid deposits is the islet amyloid polypeptide (IAPP). The factors contributing to the conversion of IAPP from a monomeric bioactive peptide hormone into insoluble amyloid fibrils remain partially elusive. In this study, we investigated the effect of the oxidative non-enzymatic post-translational modification induced by the reactive metabolite 4-hydroxynonenal (HNE) on IAPP aggregation and cytotoxicity. Incubation of IAPP with exogenous HNE accelerated its self-assembly into β-sheet fibrils and led to the formation of a Michael adduct on the His-18 side chain. To model this covalent modification, the imidazole N(π) position of histidine was alkylated using a close analogue of HNE, the octyl chain. IAPP lipidated at His-18 showed a hastened random coil-to-β-sheet conformational conversion into fibrillar assemblies with a distinct morphology, a low level of binding to thioflavin T, and a high surface hydrophobicity. Introducing an octyl chain on His-18 enhanced the ability of the peptide to perturb synthetic lipid vesicles, to permeabilize the plasma membrane, and to induce the death of pancreatic β-cells. Alkylated IAPP triggered the self-assembly of unmodified IAPP by prompting primary nucleation and increased its capacity to perturb the plasma membrane, indicating that only a small proportion of the modified peptide is necessary to shift the balance toward the formation of proteotoxic species. This study underlines the importance of studying IAPP post-translational modifications induced by oxidative metabolites in the context of pancreatic amyloids.

Nanodroplet Oligomers (NanDOs) of Aβ40

Using atomic force microscopy (AFM) and nuclear magnetic resonance (NMR), we describe small Aβ40 oligomers, termed nanodroplet oligomers (NanDOs), which form rapidly and at Aβ40 concentrations too low for fibril formation. NanDOs were observed in putatively monomeric solutions of Aβ40 (e.g., by size exclusion chromatography). Video-rate scanning AFM shows rapid fusion and dissolution of small oligomer-sized particles, of which the median size increases with peptide concentration. In NMR (¹³C HSQC), a small number of chemical shifts changed with a change in peptide concentration. Paramagnetic relaxation enhancement NMR experiments also support the formation of NanDOs and suggest prominent interactions in hydrophobic domains of Aβ40. Addition of Zn²⁺ to Aβ40 solutions caused flocculation of NanDO-containing solutions, and selective loss of signal intensity in NMR spectra from residues in the N-terminal domain of Aβ40. NanDOs may represent the earliest aggregated form of Aβ40 in the aggregation pathway and are akin to premicelles in solutions of amphiphilies.

Effects of combination antiretroviral drugs (cART) on hippocampal neuroplasticity in female mice

The incidence of HIV-associated neurocognitive disorder (HAND) continues despite the introduction of combination antiretroviral drugs (cART). Several studies have reported the neurotoxicity of individual antiretroviral drugs (monotherapy), while the common approach for HIV treatment is through cART. Hence, the current study investigated the effects of long-term exposure to cART on cognitive function, oxidative damage, autophagy, and neuroplasticity in the hippocampus of mice. Female Balb/c mice received a once-a-day oral dose of cART composed of emtricitabine + tenofovir disoproxil fumarate or vehicle for 8 weeks. On week 7 of drug administration, all mice were assessed for spatial learning in the Morris water maze (MWM), and then on week 8, mice were sacrificed, and hippocampal tissue dissected from the brain. For biochemical analyses, we measured the concentration of 4-hydroxynonenal, and the expression of autophagic marker LC3B, synaptophysin, and brain-derived neurotrophic factor (BDNF) in the hippocampus. Our results showed that cART exposure increased escape latency in the MWM test. The cART-treated mice also showed increased 4-hydroxynonenal concentration and expression of LC3B. Furthermore, cART treatment decreased the expression of synaptophysin and BDNF. These findings further support the evidence that cART may be neurotoxic and therefore may play a role in the neuropathogenesis of HAND.

Modulation of Antimicrobial Peptide Conformation and Aggregation by Terminal Lipidation and Surfactants

The function and properties of peptide-based materials depend not only on the amino acid sequence but also on the molecular conformations. In this paper, we chose a series of peptides G_m(XXKK)_nX-NH₂ (m = 0, 3; n = 2, 3; X = I, L, and V) as the model molecules and studied the conformation regulation through N-terminus lipidation and their formulation with surfactants. The structural and morphological transition of peptide self-assemblies have also been investigated via transmission electron microscopy, atomic force microscopy, circular dichroism spectroscopy, and small-angle neutron scattering. With the terminal alkylation, the molecular conformation changed from random coil to β-sheet or α-helix. The antimicrobial activities of alkylated peptide were different. C₁₆-G₃(IIKK)₃I-NH₂ showed antimicrobial activity against Streptococcus mutans, while C₁₆-(IIKK)₂I-NH₂ and C₁₆-G₃(IIKK)₂I-NH₂ did not kill the bacteria. The surfactant sodium dodecyl sulfonate could rapidly induce the self-assemblies of alkylated peptides (C₁₆-(IIKK)₂I-NH₂, C₁₆-G₃(IIKK)₂I-NH₂, C₁₆-G₃(VVKK)₂V-NH₂) from nanofibers to micelles, along with the conformation changing from β-sheet to α-helix. The cationic surfactant hexadecyl trimethyl ammonium bromide made the lipopeptide nanofibers thinner, and nonionic surfactant polyoxyethylene (23) lauryl ether (C₁₂EO₂₃) induced the nanofibers much more intensively. Both the activity and the conformation of the α-helical peptide could be modulated by lipidation. Then, the self-assembled morphologies of alkylated peptides could also be further regulated with surfactants through hydrophobic, electrostatic, and hydrogen-bonding interactions. These results provided useful strategies to regulate the molecular conformations in peptide-based material functionalization.

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.

Early involvement of lysosome dysfunction in the degeneration of cerebral cortical neurons caused by the lipid peroxidation product 4-hydroxynonenal

Free radical-mediated oxidative damage to proteins, lipids, and DNA occurs in neurons during acute brain injuries and in neurodegenerative disorders. Membrane lipid peroxidation contributes to neuronal dysfunction and death, in part by disrupting neuronal ion homeostasis and cellular bioenergetics. Emerging findings suggest that 4-hydroxynonenal (HNE), an aldehyde produced during lipid peroxidation, impairs the function of various proteins involved in neuronal homeostasis. Here we tested the hypothesis that HNE impairs the cellular system that removes damaged proteins and organelles, the autophagy-lysosome pathway in rat primary cortical neurons. We found that HNE, at a concentration that causes apoptosis over a 48-72 h period, increases protein levels of LC3 II and p62 and within 1 and 4 h of exposure, respectively; LC3 II and p62 immunoreactive puncta were observed in the cytoplasm of HNE-treated neurons at 6 h. The extent of up-regulation of p62 and LC3 II in response to HNE was not affected by co-treatment with the lysosome inhibitor bafilomycin A1, suggesting that the effects of HNE on autophagy were secondary to lysosome inhibition. Indeed, we found that neurons exposed to HNE exhibit elevated pH levels, and decreased protein substrate hydrolysis and cathepsin B activity. Neurons exposed to HNE also exhibited the accumulation of K63-linked polyubiquitinated proteins, which are substrates targeted for lysosomal degradation. Moreover, we found that the levels of LAMP2a and constitutively active heat-shock protein 70, and numbers of LAMP2a-positive lysosomes, are decreased in neurons exposed to HNE. Our findings demonstrate that the lipid peroxidation product HNE causes early impairment of lysosomes which may contribute to the accumulation of damaged and dysfunctional proteins and organelles and consequent neuronal death. Because impaired lysosome function is increasingly recognized as an early event in the neuronal death that occurs in neurodegenerative disorders, our findings suggest a role for HNE in such lysosomal dysfunction.

Lipidation Effect on Surface Adsorption and Associated Fibrillation of the Model Protein Insulin

Lipidation of proteins is used in the pharmaceutical field to increase the therapeutic efficacy of proteins. In this study, we investigate the effect of a 14-carbon fatty acid modification on the adsorption behavior of human insulin to a hydrophobic solid surface and the subsequent fibrillation development under highly acidic conditions and elevated temperature by comparing to the fibrillation of human insulin. At these stressed conditions, the lipid modification accelerates the rate of fibrillation in bulk solution. With the use of several complementary surface-sensitive techniques, including quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM), and neutron reflectivity (NR), we show that there are two levels of structurally different protein organization at a hydrophobic surface for both human insulin and the lipidated analogue: a dense protein layer formed within minutes on the surface and a diffuse outer layer of fibrillar structures which took hours to form. The two layers may only be weakly connected, and proteins from both layers are able to desorb from the surface. The lipid modification increases the protein surface coverage and the thickness of both layer organizations. Upon lipidation not only the fibrillation extent but also the morphology of the fibrillar structures changes from fibril clusters on the surface to a more homogeneous network of fibrils covering the entire hydrophobic surface.

Engineering amyloid-like assemblies from unstructured peptides via site-specific lipid conjugation

Aggregation of amyloid beta (Aβ) into oligomers and fibrils is believed to play an important role in the development of Alzheimer’s disease (AD). To gain further insight into the principles of aggregation, we have investigated the induction of β-sheet secondary conformation from disordered native peptide sequences through lipidation, in 1–2% hexafluoroisopropanol (HFIP) in phosphate buffered saline (PBS). Several parameters, such as type and number of lipid chains, peptide sequence, peptide length and net charge, were explored keeping the ratio peptide/HFIP constant. The resulting lipoconjugates were characterized by several physico-chemical techniques: Circular Dichroism (CD), Attenuated Total Reflection InfraRed (ATR-IR), Thioflavin T (ThT) fluorescence, Dynamic Light Scattering (DLS), solid-state Nuclear Magnetic Resonance (ssNMR) spectroscopy and Electron Microscopy (EM). Our data demonstrate the generation of β-sheet aggregates from numerous unstructured peptides under physiological pH, independent of the amino acid sequence. The amphiphilicity pattern and hydrophobicity of the scaffold were found to be key factors for their assembly into amyloid-like structures.

Covalently attached fatty acyl chains alter the aggregation behavior of an amyloidogenic peptide derived from human β(2)-microglobulin

Aggregation of a polypeptide chain into highly ordered amyloid aggregates is a complex process. Various factors, both extrinsic and intrinsic to the polypeptide chain, have been shown to perturb this process, leading to a drastic change in the amyloidogenic behavior, which is reflected in the polymorphism of amyloid aggregates at various levels of self-assembly. In this paper, we have investigated the ability of covalently linked long-chain fatty acids in modulating the self-assembly of an aromatic amino acid-rich highly amyloidogenic sequence derived from the amino acid region 59-71 of human β2-microglobulin by thioflavin T (ThT) fluorescence microscopy, circular dichroism, and fluorescence spectroscopy. Our results indicate that under identical conditions of dissolution and concentration, each peptide enhances the fluorescence of ThT. However, the aggregates are morphologically distinct. For the same peptide, the aggregate morphologies are dependent on peptide concentration. Further, an optimum concentration, which varies with solution ionic strength, is required for the formation of fibrillar aggregates. We show that covalent modification of this amyloidogenic sequence, with long-chain fatty acids, affects the way the higher order amyloid structures assemble from the cross-β units, in fatty acyl chain-dependent and position-dependent manner. Our data indicate that noncovalent interactions leading to amyloid fibril formation can be modulated by the hydrophobicity of covalently attached long-chain fatty acids resulting in self-assembly of the peptide chain to form nonfibrillar aggregates.

Peptide lipidation stabilizes structure to enhance biological function

Medicines that decrease body weight and restore nutrient tolerance could improve human diabetes and obesity treatment outcomes. We developed lipid-acylated glucagon analogs that are co-agonists for the glucagon and glucagon-like peptide 1 receptors, and stimulate weight loss and plasma glucose lowering in pre-diabetic obese mice. Our studies identified lipid acylation (lipidation) can increase and balance in vitro potencies of select glucagon analogs for the two aforementioned receptors in a lipidation site-dependent manner. A general capacity for lipidation to enhance the secondary structure of glucagon analogs was recognized, and the energetics of this effect quantified. The molecular structure of a lipid-acylated glucagon analog in water was also characterized. These results support that lipidation can modify biological activity through thermodynamically-favorable intramolecular interactions which stabilize structure. This establishes use of lipidation to achieve specific pharmacology and implicates similar endogenous post-translational modifications as physiological tools capable of refining biological action in means previously underappreciated.

Oxidative stress and cell membranes in the pathogenesis of Alzheimer's disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimer's disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimer's disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Modulation of lipid peroxidation and mitochondrial function improves neuropathology in Huntington's disease mice

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder. Oxidative damage has been associated with pathological neuronal loss in HD. The therapeutic modulation of oxidative stress and mitochondrial function using low molecular weight compounds may be an important strategy for delaying the onset and slowing the progression of HD. In the present study, we found a marked increase of 4-hydroxy-2-nonenal (4-HNE) adducts, a lipid peroxidation marker, in the caudate and putamen of HD brains and in the striatum of HD mice. Notably, 4-HNE immunoreactivity was colocalized with mutant huntingtin inclusions in the striatal neurons of R6/2 HD mice. Administration of nordihydroguaiaretic acid (NDGA), an antioxidant that functions by inhibiting lipid peroxidation, markedly reduced 4-HNE adduct formation in the nuclear inclusions of R6/2 striatal neurons. NDGA also protected cultured neurons against oxidative stress-induced cell death by improving ATP generation and mitochondrial morphology and function. In addition, NDGA restored mitochondrial membrane potential, mitochondrial structure, and synapse structure in the striatum of R6/2 mice and increased their lifespan. The present findings suggest that further therapeutic studies using NDGA are warranted in HD and other neurodegenerative diseases characterized by increased oxidative stress and altered mitochondrial function.

Immunological origin and functional properties of catalytic autoantibodies to amyloid beta peptide

OBJECTIVES

Objectives The objectives of this study are to (1) evaluate the ability of the immune system to synthesize specific antibodies that catalyze the degradation of amyloid beta peptide (Abeta) and to (2) evaluate the prospect of developing a catalytic IVIG (CIVIG) formulation for therapy of Alzheimer's disease (AD).

CONCLUSIONS

Polyclonal autoantibodies from humans without dementia hydrolyzed Abeta specifically. The catalytic activity improved as a function of age. Patients with AD produced catalytic antibodies at increased levels. IgM-class antibodies expressed the activity at levels superior to IgGs. Production of catalytic autoantibodies appears to be an innate immunity function with adaptive improvements occurring upon Abeta overexpression, which suggests a beneficial function of the catalytic activity. The catalytic autoantibodies impeded Abeta aggregation, dissolved preformed Abeta aggregates, and inhibited Abeta cytotoxicity in tissue culture. Recombinant catalytic antibodies from a human library have been identified, validating the phenomenon of antibody-catalyzed Abeta cleavage. As a single catalyst molecule inactivates multiple Abeta molecules, catalytic antibodies may clear Abeta efficiently. IVIG did not cleave Abeta, indicating the importance of purification procedures that maintain catalytic site integrity. Traditional Abeta-binding antibodies form immune complexes that can induce inflammatory reaction and vascular dysfunction. Catalysts do not form stable immune complexes, minimizing these risks. Criteria appropriate for developing a CIVIG formulation with potential therapeutic utility are discussed, including isolation of the Abeta-specific catalytic subsets present in IgM and IgG from human blood.

Seeded growth of beta-amyloid fibrils from Alzheimer's brain-derived fibrils produces a distinct fibril structure

Studies by solid-state nuclear magnetic resonance (NMR) of amyloid fibrils prepared in vitro from synthetic 40-residue beta-amyloid (Abeta(1-40)) peptides have shown that the molecular structure of Abeta(1-40) fibrils is not uniquely determined by amino acid sequence. Instead, the fibril structure depends on the precise details of growth conditions. The molecular structures of beta-amyloid fibrils that develop in Alzheimer's disease (AD) are therefore uncertain. We demonstrate through thioflavin T fluorescence and electron microscopy that fibrils extracted from brain tissue of deceased AD patients can be used to seed the growth of synthetic Abeta(1-40) fibrils, allowing preparation of fibrils with isotopic labeling and in sufficient quantities for solid-state NMR and other measurements. Because amyloid structures propagate themselves in seeded growth, as shown in previous studies, the molecular structures of brain-seeded synthetic Abeta(1-40) fibrils most likely reflect structures that are present in AD brain. Solid-state (13)C NMR spectra of fibril samples seeded with brain material from two AD patients were found to be nearly identical, indicating the same molecular structures. Spectra of an unseeded control sample indicate greater structural heterogeneity. (13)C chemical shifts and other NMR data indicate that the predominant molecular structure in brain-seeded fibrils differs from the structures of purely synthetic Abeta(1-40) fibrils that have been characterized in detail previously. These results demonstrate a new approach to detailed structural characterization of amyloid fibrils that develop in human tissue, and to investigations of possible correlations between fibril structure and the degree of cognitive impairment and neurodegeneration in AD.