Amyloid-beta interactions with chondroitin sulfate-derived monosaccharides and disaccharides. implications for drug development

Future proofing of chondroitin sulphate production: Importance of sustainability and quality for the end-applications

Chondroitin sulphates (CSs) are the most well-known glycosaminoglycans (GAGs) found in any living organism, from microorganisms to invertebrates and vertebrates (including humans), and provide several health benefits. The applications of CSs are numerous including tissue engineering, osteoarthritis treatment, antiviral, cosmetics, and skincare applications. The current commercial production of CSs mostly uses animal, bovine, porcine, and avian tissues as well as marine organisms, marine mammals, sharks, and other fish. The production process consists of tissue hydrolysis, protein removal, and purification using various methods. Mostly, these are chemical-dependent and are complex, multi-step processes. There is a developing trend for abandonment of harsh extraction chemicals and their substitution with different green-extraction technologies, however, these are still in their infancy. The quality of CSs is the first and foremost requirement for end-applications and is dependent on the extraction and purification methodologies used. The final products will show different bio-functional properties, depending on their origin and production methodology. This is a comprehensive review of the characteristics, properties, uses, sources, and extraction methods of CSs. This review emphasises the need for extraction and purification processes to be environmentally friendly and gentle, followed by product analysis and quality control to ensure the expected bioactivity of CSs.

Increased Cerebral Serum Amyloid A2 and Parameters of Oxidation in Arylsulfatase B (N-Acetylgalactosamine-4-Sulfatase)-Null Mice

Background

Chondroitin sulfate and chondroitin sulfate proteoglycans have been associated with Alzheimer's disease (AD), and the impact of modified chondroitin sulfates is being investigated in several animal and cell-based models of AD. Published reports have shown the role of accumulation of chondroitin 4-sulfate and decline in Arylsulfatase B (ARSB; B-acetylgalactosamine-4-sulfatase) in other pathology, including nerve injury, traumatic brain injury, and spinal cord injury. However, the impact of ARSB deficiency on AD pathobiology has not been reported, although changes in ARSB were associated with AD in two prior reports. The enzyme ARSB removes 4-sulfate groups from the non-reducing end of chondroitin 4-sulfate and dermatan sulfate and is required for their degradation. When ARSB activity declines, these sulfated glycosaminoglycans accumulate, as in the inherited disorder Mucopolysaccharidosis VI.

Objective

Reports about chondroitin sulfate, chondroitin sulfate proteoglycans, and chondroitin sulfatases in AD were reviewed.

Methods

Measurements of SAA2, iNOS, lipid peroxidation, chondroitin sulfate proteoglycan 4 (CSPG4), and other parameters were performed in cortex and hippocampus from ARSB-null mice and controls by QRT-PCR, ELISA, and other standard assays.

Results

SAA2 mRNA expression and protein, CSPG4 mRNA, chondroitin 4-sulfate, and iNOS were increased significantly in ARSB-null mice. Measures of lipid peroxidation and redox state were significantly modified.

Conclusion

Findings indicate that decline in ARSB leads to changes in expression of parameters associated with AD in the hippocampus and cortex of the ARSB-deficient mouse. Further investigation of the impact of decline in ARSB on the development of AD may provide a new approach to prevent and treat AD.

Increased Cerebral Serum Amyloid A2 and Parameters of Oxidation in Arylsulfatase B (N-Acetylgalactosamine-4-Sulfatase)-Null Mice

Introduction

Chondroitin sulfate and chondroitin sulfate proteoglycans have been associated with Alzheimer's Disease (AD), and the impact of modified chondroitin sulfates is being investigated in several animal and cell-based models of AD. Published reports have shown the role of accumulation of chondroitin 4-sulfate and decline in Arylsulfatase B (ARSB; B-acetylgalactosamine-4-sulfatase) in other pathology, including nerve injury, traumatic brain injury, and spinal cord injury. However, the impact of ARSB deficiency on AD pathobiology has not been reported, although changes in ARSB were associated with AD in two prior reports. The enzyme ARSB removes 4-sulfate groups from the non-reducing end of chondroitin 4-sulfate and dermatan sulfate and is required for their degradation. When ARSB activity declines, these sulfated glycosaminoglycans accumulate, as in the inherited disorder Mucopolysaccharidosis VI.

Methods

Reports about chondroitin sulfate, chondroitin sulfate proteoglycans and chondroitin sulfatases in Alzheimer's Disease were reviewed. Measurements of SAA2, iNOS, lipid peroxidation, chondroitin sulfate proteoglycan 4, and other parameters were performed in cortex and hippocampus from ARSB-null mice and controls by QRT-PCR, ELISA, and other standard assays.

Results

SAA2 mRNA expression and protein, CSPG4 mRNA, chondroitin 4-sulfate and i-NOS were increased significantly in ARSB-null mice. Measures of lipid peroxidation and redox state were significantly modified.

Discussion

Findings indicate that decline in ARSB leads to changes in expression of parameters associated with AD in the hippocampus and cortex of the ARSB-deficient mouse.

Conclusions

Further investigation of the impact of decline in ARSB on the development of AD may provide a new approach to prevent and treat AD.

Heparin-Assisted Amyloidogenesis Uncovered through Molecular Dynamics Simulations

Glycosaminoglycans (GAGs), in particular, heparan sulfate and heparin, are found colocalized with Aβ amyloid. They have been shown to enhance fibril formation, suggesting a possible pathological connection. We have investigated heparin's assembly of the KLVFFA peptide fragment using molecular dynamics simulation, to gain a molecular-level mechanistic understanding of how GAGs enhance fibril formation. The simulations reveal an exquisite process wherein heparin accelerates peptide assembly by first "gathering" the peptide molecules and then assembling them. Heparin does not act as a mere template but is tightly coupled to the peptides, yielding a composite protofilament structure. The strong intermolecular interactions suggest composite formation to be a general feature of heparin's interaction with peptides. Heparin's chain flexibility is found to be essential to its fibril promotion activity, and the need for optimal heparin chain length and concentration has been rationalized. These insights yield design rules (flexibility; chain-length) and protocol guidance (heparin:peptide molar ratio) for developing effective heparin mimetics and other functional GAGs.

Characterizing Heparin Tetrasaccharides Binding to Amyloid-Beta Peptide

The aggregation of β-amyloid peptide (Aβ) is one potential cause for Alzheimer’s disease (AD). Heparin can either promote or inhibit Aβ aggregation. The sulfation pattern and chain size determine its binding affinity and its role. Using 2D-NMR analysis and molecular modelling, the binding motif of heparin oligoaccharides to Aβ was determined to be HexA-GlcNS-IdoA2S-GlcNS6S. Iduronic acid epimerization and 6-O-sulfation are key factors for the binding affinity, while 3-O-sulfation of Arixtra (heparin pentasaccharide) is not involved in the binding to Aβ. Hydrogen-deuterium exchange mass spectrometry (HDX-MS) was used to study the glycosaminoglycan (GAG)-peptide complex and identified V12HHQKL17 as the binding site of GAG at Aβ. Furthermore, an MTT assay was applied to evaluate the anti-Aβ fibril formation function of heparin tetrasaccharide, and indicated that the heparin tetrasaccharide with the defined sequence represents a promising inhibitor of Aβ aggregation.

Circadian control of heparan sulfate levels times phagocytosis of amyloid beta aggregates

Alzheimer's Disease (AD) is a neuroinflammatory disease characterized partly by the inability to clear, and subsequent build-up, of amyloid-beta (Aβ). AD has a bi-directional relationship with circadian disruption (CD) with sleep disturbances starting years before disease onset. However, the molecular mechanism underlying the relationship of CD and AD has not been elucidated. Myeloid-based phagocytosis, a key component in the metabolism of Aβ, is circadianly-regulated, presenting a potential link between CD and AD. In this work, we revealed that the phagocytosis of Aβ42 undergoes a daily circadian oscillation. We found the circadian timing of global heparan sulfate proteoglycan (HSPG) biosynthesis was the molecular timer for the clock-controlled phagocytosis of Aβ and that both HSPG binding and aggregation may play a role in this oscillation. These data highlight that circadian regulation in immune cells may play a role in the intricate relationship between the circadian clock and AD.

Glucosamine and Its Analogues as Modulators of Amyloid-β Toxicity

In Alzheimer's disease (AD), amyloid-β (Aβ) oligomers are considered key mediators of synaptic dysfunction and cognitive impairment. These unstable intermediate Aβ species can interfere with different cellular organelles, leading to neuronal cell death, through the formation of Ca2+-permeable membrane pores, impairment in the levels of acetylcholine neurotransmitters, increased insulin resistance, promotion of pro-inflammatory cascades, among others. Based on a series of evidences that indicate the key role of glycosaminoglycans (GAGs) in amyloid plaque formation, we evaluated the capacity of four monosaccharides, i.e., glucosamine (GlcN), N-acetyl glucosamine (GlcNAc), glucosamine-6-sulfate (GlcN6S), and glucosamine-6-phosphate (GlcN6P), to reduce the Aβ-mediated pathological hallmarks. The tested monosaccharides, in particular, GlcN6S and GlcN6P, were able to interact with Aβ aggregates, reducing neuronal cell death, Aβ-mediated damage to the cellular membrane, acetylcholinesterase activity, insulin resistance, and pro-inflammation levels.

Glycosaminoglycans in Neurodegenerative Diseases

Glycosaminoglycans (GAGs) are linear polysaccharides that consist of alternating disaccharides sequences of uronic acids and/or galactose hexamino sugars most of which are sulfated. GAGs are ubiquitously expressed on the cell surface, in the intracellular milieu and in the extracellular matrix of all animal cells. Thus, GAGs exhibit many essential roles in a variety of physiological and pathological processes. The targets of GAGs are GAG-binding proteins and related proteins that are of significant interest to both the academic community and in the pharmaceutical industry. In this review, the structures of GAGs, their binding proteins, and analogs are presented that further the development of GAGs and their analogs for the treatment of neurodegenerative diseases agents.

Study on the relationships between molecular weights of chondroitin sulfate oligosaccharides and Aβ-induced oxidative stress and the related mechanisms

In the present study, we studied anti-Alzheimer's disease (AD) activities of chondroitin sulfate (CS) oligosaccharides with different molecular weights. CS from shark cartilage was degraded by a recombinant CS endolyase, chondroitinase ABC I (CHSase ABC I), and CS disaccharide (DP2), tetrasaccharide (DP4), hexasaccharide (DP6), octasaccharide (DP8), decasaccharide (DP10) and dodecasaccharide (DP12) were obtained by separation with gel filtration. Anti-AD activities of CS oligosaccharides were assessed using Aβ-injured SH-SY5Y cells and BV2 cells. It was shown that CS oligosaccharides could block Aβ-induced oxidative stress, mitochondrial dysfunction and activation of intrinsic apoptotic pathway for SH-SY5Y cells. Furthermore, these activities increased with the increase of molecular weights. For Aβ-injured BV2 cells, CS oligosaccharides inhibited oxidative stress, the production of proinflammatory cytokines and the activation of toll-like receptor pathway, and CS DP2 had the best activity among them. In conclusion, CS oligosaccharides suppressed Aβ-induced oxidative stress and relevant injury in vitro, and these effects had different relationships with the molecular weights of CS oligosaccharides for different cell lines, which might be caused by different mechanisms.

A game-theoretic approach to deciphering the dynamics of amyloid-β aggregation along competing pathways

Aggregation of amyloid-β (Aβ) peptides is a significant event that underpins Alzheimer's disease (AD). Aβ aggregates, especially the low-molecular weight oligomers, are the primary toxic agents in AD pathogenesis. Therefore, there is increasing interest in understanding their formation and behaviour. In this paper, we use our previously established results on heterotypic interactions between Aβ and fatty acids (FAs) to investigate off-pathway aggregation under the control of FA concentrations to develop a mathematical framework that captures the mechanism. Our framework to define and simulate the competing on- and off-pathways of Aβ aggregation is based on the principles of game theory. Together with detailed simulations and biophysical experiments, our models describe the dynamics involved in the mechanisms of Aβ aggregation in the presence of FAs to adopt multiple pathways. Specifically, our reduced-order computations indicate that the emergence of off- or on-pathway aggregates are tightly controlled by a narrow set of rate constants, and one could alter such parameters to populate a particular oligomeric species. These models agree with the detailed simulations and experimental data on using FA as a heterotypic partner to modulate the temporal parameters. Predicting spatio-temporal landscape along competing pathways for a given heterotypic partner such as lipids is a first step towards simulating scenarios in which the generation of specific 'conformer strains' of Aβ could be predicted. This approach could be significant in deciphering the mechanisms of amyloid aggregation and strain generation, which are ubiquitously observed in many neurodegenerative diseases.

Expression, Purification and Characterization of Chondroitinase AC II from Marine Bacterium Arthrobacter sp. CS01

Chondroitinase (ChSase), a type of glycosaminoglycan (GAG) lyase, can degrade chondroitin sulfate (CS) to unsaturate oligosaccharides, with various functional activities. In this study, ChSase AC II from a newly isolated marine bacterium Arthrobacter sp. CS01 was cloned, expressed in Pichia pastoris X33, purified, and characterized. ChSase AC II, with a molecular weight of approximately 100 kDa and a specific activity of 18.7 U/mg, showed the highest activity at 37 °C and pH 6.5 and maintained stability at a broad range of pH (5–7.5) and temperature (below 35 °C). The enzyme activity was increased in the presence of Mn²⁺ and was strongly inhibited by Hg²⁺. Moreover, the kinetic parameters of ChSase AC II against CS-A, CS-C, and HA were determined. TLC and ESI-MS analysis of the degradation products indicated that ChSase AC II displayed an exolytic action mode and completely hydrolyzed three substrates into oligosaccharides with low degrees of polymerization (DPs). All these features make ChSase AC II a promising candidate for the full use of GAG to produce oligosaccharides.

Glycosaminoglycans have variable effects on α-synuclein aggregation and differentially affect the activities of the resulting amyloid fibrils

Parkinson's disease is mainly a sporadic disorder in which both environmental and cellular factors play a major role in the initiation of this disease. Glycosaminoglycans (GAG) are integral components of the extracellular matrix and are known to influence amyloid aggregation of several proteins, including α-synuclein (α-Syn). However, the mechanism by which different GAGs and related biological polymers influence protein aggregation and the structure and intercellular spread of these aggregates remains elusive. In this study, we used three different GAGs and related charged polymers to establish their role in α-Syn aggregation and associated biological activities of these aggregates. Heparin, a representative GAG, affected α-Syn aggregation in a concentration-dependent manner, whereas biphasic α-Syn aggregation kinetics was observed in the presence of chondroitin sulfate B. Of note, as indicated by 2D NMR analysis, different GAGs uniquely modulated α-Syn aggregation because of the diversity of their interactions with soluble α-Syn. Moreover, subtle differences in the GAG backbone structure and charge density significantly altered the properties of the resulting amyloid fibrils. Each GAG/polymer facilitated the formation of morphologically and structurally distinct α-Syn amyloids, which not only displayed variable levels of cytotoxicity but also exhibited an altered ability to internalize into cells. Our study supports the role of GAGs as key modulators in α-Syn amyloid formation, and their distinct activities may regulate amyloidogenesis depending on the type of GAG being up- or down-regulated in vivo.

Epigallocatechin-3-gallate remodels apolipoprotein A-I amyloid fibrils into soluble oligomers in the presence of heparin

Amyloid deposits of WT apolipoprotein A-I (apoA-I), the main protein component of high-density lipoprotein, accumulate in atherosclerotic plaques where they may contribute to coronary artery disease by increasing plaque burden and instability. Using CD analysis, solid-state NMR spectroscopy, and transmission EM, we report here a surprising cooperative effect of heparin and the green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG), a known inhibitor and modulator of amyloid formation, on apoA-I fibrils. We found that heparin, a proxy for glycosaminoglycan (GAG) polysaccharides that co-localize ubiquitously with amyloid in vivo, accelerates the rate of apoA-I formation from monomeric protein and associates with insoluble fibrils. Mature, insoluble apoA-I fibrils bound EGCG (K_D = 30 ± 3 μm; B_max = 40 ± 3 μm), but EGCG did not alter the kinetics of apoA-I amyloid assembly from monomer in the presence or absence of heparin. EGCG selectively increased the mobility of specific backbone and side-chain sites of apoA-I fibrils formed in the absence of heparin, but the fibrils largely retained their original morphology and remained insoluble. By contrast, fibrils formed in the presence of heparin were mobilized extensively by the addition of equimolar EGCG, and the fibrils were remodeled into soluble 20-nm-diameter oligomers with a largely α-helical structure that were nontoxic to human umbilical artery endothelial cells. These results argue for a protective effect of EGCG on apoA-I amyloid associated with atherosclerosis and suggest that EGCG-induced remodeling of amyloid may be tightly regulated by GAGs and other amyloid co-factors in vivo, depending on EGCG bioavailability.

Fatty Acid Concentration and Phase Transitions Modulate Aβ Aggregation Pathways

Aggregation of amyloid β (Aβ) peptides is a significant event that underpins Alzheimer disease (AD) pathology. Aβ aggregates, especially the low-molecular weight oligomers, are the primary toxic agents in AD and hence, there is increasing interest in understanding their formation and behavior. Aggregation is a nucleation-dependent process in which the pre-nucleation events are dominated by Aβ homotypic interactions. Dynamic flux and stochasticity during pre-nucleation renders the reactions susceptible to perturbations by other molecules. In this context, we investigate the heterotypic interactions between Aβ and fatty acids (FAs) by two independent tool-sets such as reduced order modelling (ROM) and ensemble kinetic simulation (EKS). We observe that FAs influence Aβ dynamics distinctively in three broadly-defined FA concentration regimes containing non-micellar, pseudo-micellar or micellar phases. While the non-micellar phase promotes on-pathway fibrils, pseudo-micellar and micellar phases promote predominantly off-pathway oligomers, albeit via subtly different mechanisms. Importantly off-pathway oligomers saturate within a limited molecular size, and likely with a different overall conformation than those formed along the on-pathway, suggesting the generation of distinct conformeric strains of Aβ, which may have profound phenotypic outcomes. Our results validate previous experimental observations and provide insights into potential influence of biological interfaces in modulating Aβ aggregation pathways.

Molecular interactions of amyloid nanofibrils with biological aggregation modifiers: implications for cytotoxicity mechanisms and biomaterial design

Amyloid nanofibrils are ubiquitous biological protein fibrous aggregates, with a wide range of either toxic or beneficial activities that are relevant to human disease and normal biology. Protein amyloid fibrillization occurs via nucleated polymerization, through non-covalent interactions. As such, protein nanofibril formation is based on a complex interplay between kinetic and thermodynamic factors. The process entails metastable oligomeric species and a highly thermodynamically favoured end state. The kinetics, and the reaction pathway itself, can be influenced by third party moieties, either molecules or surfaces. Specifically, in the biological context, different classes of biomolecules are known to act as catalysts, inhibitors or modifiers of the generic protein fibrillization process. The biological aggregation modifiers reviewed here include lipid membranes of varying composition, glycosaminoglycans and metal ions, with a final word on xenobiotic compounds. The corresponding molecular interactions are critically analysed and placed in the context of the mechanisms of cytotoxicity of the amyloids involved in diverse pathologies and the non-toxicity of functional amyloids (at least towards their biological host). Finally, the utilization of this knowledge towards the design of bio-inspired and biocompatible nanomaterials is explored.

Learning from Synthetic Models of Extracellular Matrix; Differential Binding of Wild Type and Amyloidogenic Human Apolipoprotein A-I to Hydrogels Formed from Molecules Having Charges Similar to Those Found in Natural GAGs

Among other components of the extracellular matrix (ECM), glycoproteins and glycosaminoglycans (GAGs) have been strongly associated to the retention or misfolding of different proteins inducing the formation of deposits in amyloid diseases. The composition of these molecules is highly diverse and a key issue seems to be the equilibrium between physiological and pathological events. In order to have a model in which the composition of the matrix could be finely controlled, we designed and synthesized crosslinked hydrophilic polymers, the so-called hydrogels varying the amounts of negative charges and hydroxyl groups that are prevalent in GAGs. We checked and compared by fluorescence techniques the binding of human apolipoprotein A-I and a natural mutant involved in amyloidosis to the hydrogel scaffolds. Our results indicate that both proteins are highly retained as long as the negative charge increases, and in addition it was shown that the mutant is more retained than the Wt, indicating that the retention of specific proteins in the ECM could be part of the pathogenicity. These results show the importance of the use of these polymers as a model to get deep insight into the studies of proteins within macromolecules.

Molecular Origins of the Compatibility between Glycosaminoglycans and Aβ40 Amyloid Fibrils

The Aβ peptide forms extracellular plaques associated with Alzheimer's disease. In addition to protein fibrils, amyloid plaques also contain non-proteinaceous components, including glycosaminoglycans (GAGs). We have shown previously that the GAG low-molecular-weight heparin (LMWH) binds to Aβ40 fibrils with a three-fold-symmetric (3Q) morphology with higher affinity than Aβ40 fibrils in alternative structures, Aβ42 fibrils, or amyloid fibrils formed from other sequences. Solid-state NMR analysis of the GAG-3Q fibril complex revealed an interaction site at the corners of the 3Q fibril structure, but the origin of the binding specificity remained obscure. Here, using a library of short heparin polysaccharides modified at specific sites, we show that the N-sulfate or 6-O-sulfate of glucosamine, but not the 2-O-sulfate of iduronate within heparin is required for 3Q binding, indicating selectivity in the interactions of the GAG with the fibril that extends beyond general electrostatic complementarity. By creating 3Q fibrils containing point substitutions in the amino acid sequence, we also show that charged residues at the fibril three-fold apices provide the majority of the binding free energy, while charged residues elsewhere are less critical for binding. The results indicate, therefore, that LMWH binding to 3Q fibrils requires a precise molecular complementarity of the sulfate moieties on the GAG and charged residues displayed on the fibril surface. Differences in GAG binding to fibrils with distinct sequence and/or structure may thus contribute to the diverse etiology and progression of amyloid diseases.

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.

Modulation of amyloid assembly by glycosaminoglycans: from mechanism to biological significance

Glycosaminoglycans (GAGs) are long and unbranched polysaccharides that are abundant in the extracellular matrix and basement membrane of multicellular organisms. These linear polyanionic macromolecules are involved in many physiological functions from cell adhesion to cellular signaling. Interestingly, amyloid fibrils extracted from patients afflicted with protein misfolding diseases are virtually always associated with GAGs. Amyloid fibrils are highly organized nanostructures that have been historically associated with pathological states, such as Alzheimer’s disease and systemic amyloidoses. However, recent studies have identified functional amyloids that accomplish crucial physiological roles in almost all living organisms, from bacteria to insects and mammals. Over the last 2 decades, numerous reports have revealed that sulfated GAGs accelerate and (or) promote the self-assembly of a large diversity of proteins, both inherently amyloidogenic and non-aggregation prone. Despite the fact that many studies have investigated the molecular mechanism(s) by which GAGs induce amyloid assembly, the mechanistic elucidation of GAG-mediated amyloidogenesis still remains the subject of active research. In this review, we expose the contribution of GAGs in amyloid assembly, and we discuss the pathophysiological and functional significance of GAG-mediated fibrillization. Finally, we propose mechanistic models of the unique and potent ability of sulfated GAGs to hasten amyloid fibril formation.

How Glycosaminoglycans Promote Fibrillation of Salmon Calcitonin

Glycosaminoglycans (GAGs) bind all known amyloid plaques and help store protein hormones in (acidic) granular vesicles, but the molecular mechanisms underlying these important effects are unclear. Here we investigate GAG interactions with the peptide hormone salmon calcitonin (sCT). GAGs induce fast sCT fibrillation at acidic pH and only bind monomeric sCT at acidic pH, inducing sCT helicity. Increasing GAG sulfation expands the pH range for binding. Heparin, the most highly sulfated GAG, binds sCT in the pH interval 3-7. Small angle x-ray scattering indicates that sCT monomers densely decorate and pack single heparin chains, possibly via hydrophobic patches on helical sCT. sCT fibrillates without GAGs, but heparin binding accelerates the process by decreasing the otherwise long fibrillation lag times at low pH and accelerates fibril growth rates at neutral pH. sCT·heparin complexes form β-sheet-rich heparin-covered fibrils. Solid-state NMR reveals that heparin does not alter the sCT fibrillary core around Lys(11) but makes changes to Val(8) on the exterior side of the β-strand, possibly through contacts to Lys(18) Thus GAGs significantly modulate sCT fibrillation in a pH-dependent manner by interacting with both monomeric and aggregated sCT.

Protective effects of low molecular weight chondroitin sulfate on amyloid beta (Aβ)-induced damage in vitro and in vivo

In the present study, we investigated the effects of low molecular weight chondroitin sulfate (LMWCS) on amyloid beta (Aβ)-induced neurotoxicity in vitro and in vivo. The in vitro results showed that LMWCS blocked Aβ25-35-induced cell viability loss and apoptosis, decreased intracellular calcium concentration, reactive oxygen species (ROS) levels, the mitochondrial membrane potential (MMP) depolarization, and the protein expression of Caspase-3. During in vivo experiments, LMWCS improved the cognitive impairment induced by Aβ1-40, increased the level of choline acetyltransferase (ChAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), and decreased the level of malondialdehyde (MDA) and acetylcholinesterase (AChE) in the mouse brain. Moreover, LMWCS decreased the density of pyramidal cells of CA1 regions, and suppressed the protein expression of Bax/Bcl-2 and Caspase-3, -9 in the hippocampus of mice. In conclusion, LMWCS possessed neuroprotective properties against toxic effects induced by Aβ peptides both in vitro and in vivo, which might be related to anti-apoptotic activity. LMWCS might be a useful preventive and therapeutic compound for Alzheimer's disease.

The effect of glycosaminoglycans (GAGs) on amyloid aggregation and toxicity

Amyloidosis is a protein folding disorder in which normally soluble proteins are deposited extracellularly as insoluble fibrils, impairing tissue structure and function. Charged polyelectrolytes such as glycosaminoglycans (GAGs) are frequently found associated with the proteinaceous deposits in tissues of patients affected by amyloid diseases. Experimental evidence indicate that they can play an active role in favoring amyloid fibril formation and stabilization. Binding of GAGs to amyloid fibrils occurs mainly through electrostatic interactions involving the negative polyelectrolyte charges and positively charged side chains residues of aggregating protein. Similarly to catalyst for reactions, GAGs favor aggregation, nucleation and amyloid fibril formation functioning as a structural templates for the self-assembly of highly cytotoxic oligomeric precursors, rich in β-sheets, into harmless amyloid fibrils. Moreover, the GAGs amyloid promoting activity can be facilitated through specific interactions via consensus binding sites between amyloid polypeptide and GAGs molecules. We review the effect of GAGs on amyloid deposition as well as proteins not strictly related to diseases. In addition, we consider the potential of the GAGs therapy in amyloidosis.

Heparin nanoparticles for β amyloid binding and mitigation of β amyloid associated cytotoxicity

Accumulation of β amyloid (Aβ) in the brain is believed to play a key role in the pathology of Alzheimer's disease. Glycosaminoglycans on surface of neuronal cells can serve as nucleation sites to promote plaque formation on cell surface. To mimic this process, magnetic nanoparticles coated with heparin have been synthesized. The heparin nanoparticles were demonstrated to bind with Aβ through a variety of techniques including enzyme-linked immunosorbent assay, gel electrophoresis, and thioflavin T assay. The nanoparticle exhibited little toxicity to neuronal cells and at the same time can effectively protect them from Aβ induced cytotoxicity. These results suggest that heparin nanoparticles can be a very useful tool for Aβ studies.

Glyconanoparticle aided detection of β-amyloid by magnetic resonance imaging and attenuation of β-amyloid induced cytotoxicity

The development of a noninvasive method for the detection of Alzheimer's disease is of high current interest, which can be critical in early diagnosis and in guiding treatment of the disease. The aggregates of β-amyloid are a pathological hallmark of Alzheimer's disease. Carbohydrates such as gangliosides have been shown to play significant roles in initiation of amyloid aggregation. Herein, we report a biomimetic approach using superparamagnetic iron oxide glyconanoparticles to detect β-amyloid. The bindings of β-amyloid by the glyconanoparticles were demonstrated through several techniques including enzyme linked immunosorbent assay, gel electrophoresis, tyrosine fluorescence assay, and transmission electron microscopy. The superparamagnetic nature of the nanoparticles allowed easy detection of β-amyloid both in vitro and ex vivo by magnetic resonance imaging. Furthermore, the glyconanoparticles not only were nontoxic to SH-SY5Y neuroblastoma cells but also greatly reduced β-amyloid induced cytotoxicity to cells, highlighting the potential of these nanoparticles for detection and imaging of β-amyloid.

Neuronal Transcytosis of WGA Conjugated Protein

Neuronal transcytosis was observed at the stage when no neurotransmitter was released after the injection of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP; WGA = 22 kDa, HRP = 40 kDa) into the vagus nerve. The co-injection of Rab3A-siRNA with WGA-HRP into the vagus nerve was performed to further examine this phenomenon. This co-injection resulted in the transcytosis of WGA-HRP, both of the passing type, by which it crossed the synapses, and of the secretion type followed by endocytosis of postsynaptic membranes. These findings raised the possibility in vivo that WGA plays an important role in the transcytosis of protein. Therefore, WGA may be a valuable tool for therapeutic drug targeting via transcytosis. The ability of WGA-conjugated Amyloid ß (WGA-Aß) to decrease amyloid deposits in Alzheimer’s disease was investigated. The conjugation of WGA to amyloid-ß (1-40) (Aß; 5 kDa) was confirmed. WGA-Aß was then shown to move to terminals by axonal flow in vivo as well as WGA-HRP. WGA-Aß was also observed in the nodose ganglion cells and terminals after injections of fluorescent Aß (FAß) into the vagus nerve and fluorescent WGA (FWGA) into the common carotid artery. These studies suggested that WGA-Aß could be localized to solitary neurons via transcytosis.

Proteoglycans in the central nervous system: role in development, neural repair, and Alzheimer's disease

Proteoglycans (PGs) are major components of the cell surface and extracellular matrix and play critical roles in development and maintenance of the central nervous system (CNS). PGs are a family of proteins, all of which contain a core protein to which glycosaminoglycan side chains are covalently attached. PGs possess diverse physiological roles, particularly in neural development, and are also implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). The main functions of PGs in the CNS are reviewed as are the roles of PGs in brain injury and in the development or treatment of AD.

Sulfated glycosaminoglycans accelerate transthyretin amyloidogenesis by quaternary structural conversion

Glycosaminoglycans (GAGs), which are found in association with all extracellular amyloid deposits in humans, are known to accelerate the aggregation of various amyloidogenic proteins in vitro. However, the precise molecular mechanism(s) by which GAGs accelerate amyloidogenesis remains elusive. Herein, we show that sulfated GAGs, especially heparin, accelerate transthyretin (TTR) amyloidogenesis by quaternary structural conversion. The clustering of sulfate groups on heparin and its polymeric nature are essential features for accelerating TTR amyloidogenesis. Heparin does not influence TTR tetramer stability or TTR dissociation kinetics, nor does it alter the folded monomer-misfolded monomer equilibrium directly. Instead, heparin accelerates the conversion of preformed TTR oligomers into larger aggregates. The more rapid disappearance of monomeric TTR in the presence of heparin likely reflects the fact that the monomer-misfolded amyloidogenic monomer-oligomer-TTR fibril equilibria are all linked, a hypothesis that is strongly supported by the light scattering data. TTR aggregates prepared in the presence of heparin exhibit a higher resistance to trypsin and proteinase K proteolysis and a lower exposure of hydrophobic side chains comprising hydrophobic clusters, suggesting an active role for heparin in amyloidogenesis. Our data suggest that heparin accelerates TTR aggregation by a scaffold-based mechanism, in which the sulfate groups comprising GAGs interact primarily with TTR oligomers through electrostatic interactions, concentrating and orienting the oligomers, facilitating the formation of higher molecular weight aggregates. This model raises the possibility that GAGs may play a protective role in human amyloid diseases by interacting with proteotoxic oligomers and promoting their association into less toxic amyloid fibrils.

A computational approach for identifying the chemical factors involved in the glycosaminoglycans-mediated acceleration of amyloid fibril formation

Background

Amyloid fibril formation is the hallmark of many human diseases, including Alzheimer's disease, type II diabetes and amyloidosis. Amyloid fibrils deposit in the extracellular space and generally co-localize with the glycosaminoglycans (GAGs) of the basement membrane. GAGs have been shown to accelerate the formation of amyloid fibrils in vitro for a number of protein systems. The high number of data accumulated so far has created the grounds for the construction of a database on the effects of a number of GAGs on different proteins.

Methodology/Principal Findings

In this study, we have constructed such a database and have used a computational approach that uses a combination of single parameter and multivariate analyses to identify the main chemical factors that determine the GAG-induced acceleration of amyloid formation. We show that the GAG accelerating effect is mainly governed by three parameters that account for three-fourths of the observed experimental variability: the GAG sulfation state, the solute molarity, and the ratio of protein and GAG molar concentrations. We then combined these three parameters into a single equation that predicts, with reasonable accuracy, the acceleration provided by a given GAG in a given condition.

Conclusions/Significance

In addition to shedding light on the chemical determinants of the protein∶GAG interaction and to providing a novel mathematical predictive tool, our findings highlight the possibility that GAGs may not have such an accelerating effect on protein aggregation under the conditions existing in the basement membrane, given the values of salt molarity and protein∶GAG molar ratio existing under such conditions.

Amyloid formation by the model protein muscle acylphosphatase is accelerated by heparin and heparan sulphate through a scaffolding-based mechanism

Amyloid formation is the hallmark of many diseases. The propensity of a protein to aggregate depends on a number of biological factors like the presence of sulphated polysaccharides termed as glycosaminoglycans (GAGs). Here we assessed whether the polymeric nature of GAGs is responsible for their protein aggregation-promoting effect. We studied the effect of different monosaccharide derivatives, featuring the main characteristics of heparin and heparan sulphate (HS) building blocks, on the aggregation kinetics of human muscle acylphosphatase (mAcP), a useful model protein for these studies. We observed that while heparin and HS changed the mAcP aggregation kinetic profile, the monosaccharide derivatives had no effect, whatever their concentration could be and both when they are studied separately or in combination. In contrast, heparin fragments with six or more monosaccharides reproduced the effects of HS and in part those of heparin. We conclude that the effect of heparin and HS on protein aggregation arises from the clustering and regular distribution of their composing units on a polymeric structure. We propose a model in which heparin and HS promote mAcP aggregation through a scaffolding-based mechanism, in which the regularly spaced sulphate moieties of the polymer interact with mAcP molecules increasing their local concentration and facilitating their orientation.

Abeta aggregation and possible implications in Alzheimer's disease pathogenesis

Amyloid β protein (Aβ) has been associated with Alzheimer's disease (AD) because it is a major component of the extracellular plaque found in AD brains. Increased Aβ levels correlate with the cognitive decline observed in AD. Sporadic AD cases are thought to be chiefly associated with lack of Aβ clearance from the brain, unlike familial AD which shows increased Aβ production. Aβ aggregation leading to deposition is an essential event in AD. However, the factors involved in Aβ aggregation and accumulation in sporadic AD have not been completely characterized. This review summarizes studies that have examined the factors that affect Aβ aggregation and toxicity. By necessity these are studies that are performed with recombinant-derived or chemically synthesized Aβ. The studies therefore are not done in animals but in cell culture, which includes neuronal cells, other mammalian cells and, in some cases, non-mammalian cells that also appear susceptible to Aβ toxicity. An understanding of Aβ oligomerization may lead to better strategies to prevent AD.

Binding of anti-prion agents to glycosaminoglycans: Evidence from electronic absorption and circular dichroism spectroscopy

The polyanionic glycosaminoglycans (GAGs) are intimately involved in the pathogenesis of protein conformational disorders such as amyloidosis and prion diseases. Several cationic agents are known to exhibit anti-prion activity but their mechanism of action is poorly understood. In this study, UV absorption and circular dichroism (CD) spectroscopic techniques were used to investigate the interaction between heparin and chondroitin-6-sulfate and anti-prion drugs including acridine, quinoline, and phenothiazine derivatives. UV band hypochromism of (+/-)-quinacrine, (+/-)-primaquine, tacrine, quinidine, chlorpromazine, and induced CD spectra of (+/-)-quinacrine upon addition of GAGs provided evidence for the GAG binding of these compounds. The association constants (approximately 10(6)-10(7)M(-1)) estimated from the UV titration curves show high-affinity drug-heparin interactions. Ionic strength-dependence of the absorption spectra suggested that the interaction between GAGs and the cationic drugs is principally electrostatic in nature. Drug binding differences of heparin and chondroitin-6-sulfate were attributed to their different negative charge density. These results call the attention to the alteration of GAG-prion/GAG-amyloid interactions by which these compounds might exert their anti-prion/anti-amyloidogenic activities.

Glycosaminoglycans are part of amyloid fibrils: ultrastructural evidence in avian AA amyloid stained with cuprolinic blue and labeled with immunogold

In domestic brown layer fowl, reactive amyloidosis of internal organs, such as liver and spleen, and of the joints is a common disorder. In a variety of amyloid types including the AA-amyloid of the chicken, in addition to amyloid fibrils, proteoglycans and glycosaminoglycans (GAGs) are found on immunohistochemistry or after extraction. The aim of the present report is to study amyloid fibrils for the ultrastructural location of GAGs by cuprolinic blue staining and immunogold labeling. Rabbit antichicken AA antiserum was used for the immunogold labeling on conventionally embedded and cryoembedded liver tissue and revealed similar results. Therefore conventional blocks could be used for further analysis. Cuprolinic blue staining was performed on blocks of joint tissue in which clearly discernable rod-shaped glycoproteins were encountered in between collagen fibrils. Moreover, it appeared to stain larger deposits which might represent amyloid. Postlabeling with the immunogold method of the cuprolinic blue-stained tissue proved that cuprolinic blue positive fibrils represented AA-amyloid fibrils. Therefore, it was concluded that the GAGs which appeared to colocalize with the fibrillar microanatomy of amyloid, represent a structural part of the amyloid fibrils and that the avian amyloid fibrils may be considered as a pathological proteoglycan.

New pathways in drug discovery for alzheimer’s disease

Specific treatments for Alzheimer's disease (AD) were first introduced in the 1990s using the acetyl-cholinesterase inhibitors. More recently, the N-methyl-D-aspartate (NMDA) antagonist memantine has become available. Although these treatments do provide a modest improvement in the cognitive abnormalities present in AD, their pharmacology is based on manipulation of neurotransmitter systems, and there is no compelling evidence that they interfere with the underlying pathogenic process. Pathologic and genetic data have led to the hypothesis that a peptide called amyloid ss(Abeta) plays a primary role in the pathophysiology of AD. Several investigational therapies targeting Abeta are now undergoing clinical trials. This paper reviews the available data regarding Abeta-directed therapies that are in the clinic and summarizes the approach to biomarkers and clinical trial designs that can provide evidence of modification of the underlying disease process.

Biophysical characterization of longer forms of amyloid beta peptides: possible contribution to flocculent plaque formation

Alzheimer's disease is characterized by amyloid deposits in the parenchyma and vasculature of the brain. The plaques are mainly composed of amyloid beta (Abeta) peptides ending in residues 40 and 42. Novel longer Abeta peptides were found in brain homogenates of mouse models of Alzheimer's disease and human brain tissue of patients carrying the familial amyloid precursor protein V717F mutation. The biophysical characteristics of these longer Abeta peptides and their role in plaque formation are not understood. We chose to focus our studies on Abeta peptides ending in residues Ile45, Val46 and Ile47 as these peptides were identified in human brain tissue. A combination of circular dichroism and electron microscopy was used to characterize the secondary and tertiary structures of these peptides. All three longer Abeta peptides consisted mainly of a beta-sheet secondary structure. Electron microscopy demonstrated that these beta-structured peptides formed predominantly amorphous aggregates, which convert to amyloid fibres over extended time periods. As these longer peptides may act as seeds for the nucleation of fibrils composed predominantly of shorter amyloid peptides, these interactions were studied. All peptides accelerated the random to beta-structural transitions and fibril formation of Abeta40 and 42.

Kinetic analysis of the polymerization and depolymerization of β2-microglobulin-related amyloid fibrils in vitro

beta(2)-Microglobulin-related (Abeta(2)M) amyloidosis is a serious complication in patients on long-term dialysis, and partial unfolding of beta(2)-microglobulin (beta(2)-m) is believed to be prerequisite to its assembly into Abeta(2)M amyloid fibrils. Many kinds of amyloid-associated molecules (e.g., apolipoprotein E (apoE), glycosaminoglycans (GAGs), proteoglycans (PGs)) may contribute to the development of Abeta(2)M amyloidosis. The formation of Abeta(2)M amyloid fibrils in vitro was first observed at low pH (2.0-3.0). Very recently, low concentrations of 2,2,2-trifluoroethanol (TFE) and the sub-micellar concentration of sodium dodecyl sulfate, a model for anionic phospholipids, have been reported to cause the extension of Abeta(2)M amyloid fibrils at a neutral pH, inducing partial unfolding of beta(2)-m and stabilization of the fibrils. Moreover, apoE, GAGs and PGs were found to stabilize Abeta(2)M amyloid fibrils at a neutral pH, forming a stable complex with the fibrils. Some GAGs, especially heparin enhanced the fibril extension in the presence of TFE at a neutral pH. Some PGs, especially biglycan also induced the polymerization of acid-denatured beta(2)-m. These findings are consistent with the hypothesis that in vivo, specific molecules that affect the conformation and stability of beta(2)-m and amyloid fibrils will have significant effects on the deposition of Abeta(2)M amyloid fibrils.

Contribution of simple saccharides to the stabilization of amyloid structure

The use of osmolytes or chaperones to stabilize proteins/peptides that misfold in neurodegenerative diseases is an attractive concept for drug development. We have investigated the role of a series of small carbohydrates for protection of the natively structured Alzheimer's amyloid-beta peptides (Abeta). Using circular dichroism spectroscopy to follow the beta-structural transitions and electron microscopy to examine tertiary structural characteristics, we demonstrate that the hydrogen bonding capacity of the carbohydrate determines the inhibition or promotion of fibrillogenesis. Three sugar molecules that vary only in their distribution of potential H-bonding partners promote various structural changes in Abeta. Two of these sugar molecules are excluded from Abeta during aggregation and promote mature fibre growth, while the other binds Abeta promoting nucleation and the accumulation of protofibrils. Our studies suggest that utilization of a combinatorial strategy to alter H-bonding capacity across a simple carbohydrate molecule may represent a novel drug design strategy.

Interaction of human and mouse Aβ peptides

Transgenic mice over-expressing mutant human amyloid precursor protein have become an important tool for research on Alzheimer's disease (AD) and, in particular, for therapeutic screening. Many models have reported formation of amyloid plaques with age as is detected in AD. However, the plaques generated in transgenic mice are more soluble than human plaques. Differences in solubility may occur for a number of reasons; one proposal is the presence of murine Abeta peptides within the CNS milieu. Here, we report the interaction of human and murine Abeta peptides, Abeta40 and Abeta42, utilizing a fluorescence assay to monitor formation of mixed pre-fibrillar aggregates, electron microscopy to examine morphological characteristics and detergent solubility to monitor stability. Our results demonstrate that interspecies Abeta aggregates and fibres are readily formed and are more stable than homogenous human fibres. Furthermore, these results suggest that the presence of endogenous murine Abeta in human APP transgenic mice does not account for the increased solubility of plaques.

Wnt-1 expression in PC12 cells induces exon 15 deletion and expression of L-APP

The metabolism of amyloid precursor protein (APP) is central to Alzheimer's disease pathogenesis. Recent data have linked APP and presenilin to the Wnt/wingless signaling pathway. To assess affects of Wnt stimulation on APP isoform expression, we infected PC12 cells and C57MG cells with a retrovirus containing murine Wnt-1. In PC12 cells, Wnt-1 expression is associated with induction of exon 15 deletion from APP mRNA and expression of L-APP. Our data suggest that APP isoform expression is regulated, in part, by the Wnt/wingless signaling pathway.

Glycosaminoglycan and proteoglycan inhibit the depolymerization of beta2-microglobulin amyloid fibrils in vitro

BACKGROUND

Although several kinds of evidence suggest that glycosaminoglycans (GAGs) and proteoglycans (PGs) may contribute to the development of beta2-microglobulin-related (Abeta2m) amyloidosis, the precise roles of these molecules for the development of Abeta2m amyloidosis are poorly understood.

METHODS

We investigated the effects of GAGs and PGs on the depolymerization of Abeta2m amyloid fibrils at a neutral pH, as well as on the formation of the fibrils at an acidic pH in vitro, using fluorescence spectroscopy with thioflavin T and electron microscopy.

RESULTS

Depolymerization of Abeta2m amyloid fibrils at pH 7.5 at 37 degrees C was inhibited dose-dependently by the presence of some GAGs (heparin, dermatan sulfate, or heparan sulfate) or PGs (biglycan, decorin, or keratan sulfate proteoglycan). Electron microscopy revealed that a significant amount of Abeta2m amyloid fibrils remained in the reaction mixture with some lateral aggregation. Second, when monomeric beta2m was incubated with aggrecan, biglycan, decorin, or heparin at pH 2.5 at 37 degrees C for up to 21 days, the thioflavin T fluorescence increased depending on dose and time. Electron microscopy revealed the formation of rigid and straight fibrils similar to Abeta2m amyloid fibrils in beta2m incubated with biglycan for 21 days.

CONCLUSION

These results suggest that some GAGs and PGs could enhance the deposition of Abeta2m amyloid fibrils in vivo, possibly by binding directly to the surface of the fibrils and stabilizing the conformation of beta2m in the fibrils, as well as by acting as a scaffold for the polymerization of beta2m into the fibrils.