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

Extracellular Matrix Remodeling Alleviates Memory Deficits in Alzheimer's Disease by Enhancing the Astrocytic Autophagy-Lysosome Pathway

Extracellular matrix (ECM) remodeling is strongly linked to Alzheimer's disease (AD) risk; however, the underlying mechanisms are not fully understood. Here, it is found that the injection of chondroitinase ABC (ChABC), mimicking ECM remodeling, into the medial prefrontal cortex (mPFC) reversed short-term memory loss and reduced amyloid-beta (Aβ) deposition in 5xFAD mice. ECM remodeling also reactivated astrocytes, reduced the levels of aggrecan in Aβ plaques, and enhanced astrocyte recruitment to surrounding plaques. Importantly, ECM remodeling enhanced the autophagy-lysosome pathway in astrocytes, thereby mediating Aβ clearance and alleviating AD pathology. ECM remodeling also promoted Aβ plaque phagocytosis by astrocytes by activating the astrocytic phagocytosis receptor MERTK and promoting astrocytic vesicle circulation. The study identified a cellular mechanism in which ECM remodeling activates the astrocytic autophagy-lysosomal pathway and alleviates AD pathology. Targeting ECM remodeling may represent a potential therapeutic strategy for AD and serve as a reference for the treatment of this disease.

Systemic Amyloidosis and Kidney Transplantation: An Update

Amyloidosis is a heterogeneous disorder characterized by abnormal protein aggregate deposition that often leads to kidney involvement and end-stage kidney disease. With advancements in diagnostic techniques and treatment options, the prevalence of patients with amyloidosis requiring chronic dialysis has increased. Kidney transplantation is a promising avenue for extending survival and enhancing quality of life in these patients. However, the complex and heterogeneous nature of amyloidosis presents challenges in determining optimal referral timing for transplantation and managing post-transplantation course. This review focuses on recent developments and outcomes of kidney transplantation for amyloidosis-related end-stage kidney disease. This review also aims to guide clinical decision-making and improve management of patients with amyloidosis-associated kidney disease, offering insights into optimizing patient selection and post-transplant care for favorable outcomes.

Hyaluronan-carnosine conjugates inhibit Aβ aggregation and toxicity

Alzheimer’s disease is the most common neurodegenerative disorder. Finding a pharmacological approach that cures and/or prevents the onset of this devastating disease represents an important challenge for researchers. According to the amyloid cascade hypothesis, increases in extracellular amyloid-β (Aβ) levels give rise to different aggregated species, such as protofibrils, fibrils and oligomers, with oligomers being the more toxic species for cells. Many efforts have recently been focused on multi-target ligands to address the multiple events that occur concurrently with toxic aggregation at the onset of the disease. Moreover, investigating the effect of endogenous compounds or a combination thereof is a promising approach to prevent the side effects of entirely synthetic drugs. In this work, we report the synthesis, structural characterization and Aβ antiaggregant ability of new derivatives of hyaluronic acid (Hy, 200 and 700 kDa) functionalized with carnosine (Car), a multi-functional natural dipeptide. The bioactive substances (HyCar) inhibit the formation of amyloid-type aggregates of Aβ₄₂ more than the parent compounds; this effect is proportional to Car loading. Furthermore, the HyCar derivatives are able to dissolve the amyloid fibrils and to reduce Aβ-induced toxicity in vitro. The enzymatic degradation of Aβ is also affected by the interaction with HyCar.

Galactosaminoglycans: Medical Applications and Drawbacks

Galactosaminoglycans (GalAGs) are sulfated glycans composed of alternating N-acetylgalactosamine and uronic acid units. Uronic acid epimerization, sulfation patterns and fucosylation are modifications observed on these molecules. GalAGs have been extensively studied and exploited because of their multiple biomedical functions. Chondroitin sulfates (CSs), the main representative family of GalAGs, have been used in alternative therapy of joint pain/inflammation and osteoarthritis. The relatively novel fucosylated chondroitin sulfate (FCS), commonly found in sea cucumbers, has been screened in multiple systems in addition to its widely studied anticoagulant action. Biomedical properties of GalAGs are directly dependent on the sugar composition, presence or lack of fucose branches, as well as sulfation patterns. Although research interest in GalAGs has increased considerably over the three last decades, perhaps motivated by the parallel progress of glycomics, serious questions concerning the effectiveness and potential side effects of GalAGs have recently been raised. Doubts have centered particularly on the beneficial functions of CS-based therapeutic supplements and the potential harmful effects of FCS as similarly observed for oversulfated chondroitin sulfate, as a contaminant of heparin. Unexpected components were also detected in CS-based pharmaceutical preparations. This review therefore aims to offer a discussion on (1) the current and potential therapeutic applications of GalAGs, including those of unique features extracted from marine sources, and (2) the potential drawbacks of this class of molecules when applied to medicine.

Protective effect of extractive and biotechnological chondroitin in insulin amyloid and advanced glycation end product-induced toxicity

Glycosaminoglycans are extracellular matrix components related to several biological functions and diseases. Chondroitin sulfate is a sulphated glycosaminoglycan synthesized as part of proteoglycan molecules. They are frequently associated with amyloid deposits and possess an active role in amyloid fibril formation. Recently, a neuroprotective effect of extracellular matrix components against amyloid toxicity and oxidative stress has been reported. Advanced glycation end products (AGEs), the end products of the glycation reaction, have been linked to amyloid-based neurodegenerative disease as associated with oxidative stress and inflammation. In this study we have analyzed the effect of chondroitin sulfate isolated from different species, in comparison with a new biotechnological unsulfated chondroitin, in the amyloid aggregation process of insulin, as well as the ability to prevent the formation of AGEs and related toxicity. The results have showed a determining role of chondroitin sulfate groups in modulating insulin amyloid aggregation. In addition, both sulfated and unsulfated chondroitins have shown protective properties against amyloid and AGEs-induced toxicity. These data are very relevant as a protective effect of these glycosaminoglycans in the AGE-induced toxicity was never observed before. Moreover, considering the issues related to the purity and safety of chondroitin from natural sources, this study suggests a new potential application for the biotechnological chondroitin.

Heparan sulfate S-domains and extracellular sulfatases (Sulfs): their possible roles in protein aggregation diseases

Highly sulfated domains of heparan sulfate (HS), also known as HS S-domains, consist of repeated trisulfated disaccharide units [iduronic acid (2S)-glucosamine (NS, 6S)-]. The expression of HS S-domains at the cell surface is determined by two mechanisms: tightly regulated biosynthetic machinery and enzymatic remodeling by extracellular endoglucosamine 6-sulfatases, Sulf-1 and Sulf-2. Intracellular or extracellular deposits of misfolded and aggregated proteins are characteristic of protein aggregation diseases. Although proteins can aggregate alone, deposits of protein aggregates in vivo contain a number of proteinaceous and non-protein components. HS S-domains are one non-protein component of these aggregated deposits. HS S-domains are considered to be critical for signal transduction of several growth factors and several disease conditions, such as tumor progression, but their roles in protein aggregation diseases are not yet fully understood. This review summarizes the current understanding of the possible roles of HS S-domains and Sulfs in the formation and cytotoxicity of protein aggregates.

Molecular mechanisms underlying uremic toxin-related systemic disorders in chronic kidney disease: focused on β2-microglobulin-related amyloidosis and indoxyl sulfate-induced atherosclerosis-Oshima Award Address 2016

Uremic toxins are linked to chronic kidney disease (CKD)-related systemic diseases. β₂-Microglobulin (β₂-m), a water-soluble, middle-sized molecule, is associated with mortality and dialysis-related amyloidosis (DRA). DRA occurs in long-term dialysis patients, with β₂-m amyloid deposited mainly in osteoarticular tissues. We investigated a model of β₂-m amyloid fibril extension at neutral pH in the presence of trifluoroethanol or sodium dodecyl sulfate. Using this model, some biological molecules, including glycosaminoglycans and lysophospholipids, were found to be chaperones for β₂-m amyloid fibril extension. Several protein-bound solutes, such as indoxyl sulfate (IS) and p-cresyl sulfate, are independent risk factors for cardiovascular disease in CKD patients, especially those undergoing dialysis. We investigated kidney injury-induced acceleration of atherosclerosis in association with macrophage phenotypic change to a proinflammatory state as well as increased IS deposition in lesions in an animal model. IS directly induced macrophage inflammation and impaired cholesterol efflux to high-density lipoprotein (HDL) in vitro. In addition, a clinical study showed that HDL isolated from CKD patients induced proinflammatory reactions and impaired cholesterol efflux to macrophages. These findings suggest that protein-bound solutes, including IS, will induce dysfunction of both macrophages and HDL in atherosclerotic lesions. To remove uremic toxins efficiently, we demonstrated the potential efficacy of oral charcoal adsorbent and hexadecyl-immobilized cellulose beads in hemodialysis patients. These findings suggest that uremic toxins induce various CKD-related systemic disorders, and further therapeutic strategies will be needed to reduce uremic toxins enough and improve life expectancy in CKD patients.

Probing amylin fibrillation at an early stage via a tetracysteine-recognising fluorophore

Amyloid fibrillation is a nucleation-dependent process known be involved in the development of more than 20 progressive and chronic diseases. The detection of amyloid formation at the nucleation stage can greatly advance early diagnoses and treatment of diseases. In this work, we developed a new assay for the early detection of amylin fibrillation using the biarsenical dye 4,5-bis(1,3,2-dithiarsolan-2-yl)fluorescein (FlAsH), which could recognise tetracysteine motifs and transform from non-fluorescent form into strongly fluorescent complexes. Due to the close proximity of two cysteine residues within the hydrophilic domain of amylin, a non-contiguous tetracysteine motif can form upon amylin dimerisation or oligomerisation, which can be recognised by FlAsH and emit strong fluorescence. This enables us to report the nucleation-growth process of amylin without modification of the protein sequence. We showed that the use of this assay not only allowed the tracking of initial nucleation events, but also enabled imaging of amyloid fibrils and investigation of the effects of amyloid inhibitor/modulator toward amylin fibrillation.

Heparin-induced amyloid fibrillation of β2 -microglobulin explained by solubility and a supersaturation-dependent conformational phase diagram

Amyloid fibrils are fibrillar deposits of denatured proteins associated with amyloidosis and are formed by a nucleation and growth mechanism. We revisited an alternative and classical view of amyloid fibrillation: amyloid fibrils are crystal-like precipitates of denatured proteins formed above solubility upon breaking supersaturation. Various additives accelerate and then inhibit amyloid fibrillation in a concentration-dependent manner, suggesting that the combined effects of stabilizing and destabilizing forces affect fibrillation. Heparin, a glycosaminoglycan and anticoagulant, is an accelerator of fibrillation for various amyloidogenic proteins. By using β₂ -microglobulin, a protein responsible for dialysis-related amyloidosis, we herein examined the effects of various concentrations of heparin on fibrillation at pH 2. In contrast to previous studies that focused on accelerating effects, higher concentrations of heparin inhibited fibrillation, and this was accompanied by amorphous aggregation. The two-step effects of acceleration and inhibition were similar to those observed for various salts. The results indicate that the anion effects caused by sulfate groups are one of the dominant factors influencing heparin-dependent fibrillation, although the exact structures of fibrils and amorphous aggregates might differ between those formed by simple salts and matrix-forming heparin. We propose that a conformational phase diagram, accommodating crystal-like amyloid fibrils and glass-like amorphous aggregates, is important for understanding the effects of various additives.

α-synuclein aggregation and its modulation

Parkinson's disease (PD) is a neurological disorder marked by the presence of cytoplasmic inclusions, Lewy bodies (LBs) and Lewy neurites (LNs) as well as the degeneration of dopamine producing neurons in the substantia nigra region of the brain. The LBs and LNs in PD are mainly composed of aggregated form of a presynaptic protein, α-synuclein (α-Syn). However, the mechanisms of α-Syn aggregation and actual aggregated species responsible for the degeneration of dopaminergic neurons have not yet been resolved. Despite the fact that α-Syn aggregation in LBs and LNs is crucial and mutations of α-Syn are associated with early onset PD, it is really a challenging task to establish a correlation between α-Syn aggregation rate and PD pathogenesis. Regardless of strong genetic contribution, PD is mostly sporadic and familial forms of the disease represent only a minor part (<10%) of all cases. The complexity in PD further increases due to the involvement of several cellular factors in the pathogenesis of the disease as well as the environmental factors associated with the risk of developing PD. Therefore, effect of these factors on α-Syn aggregation pathway and how these factors modulate the properties of wild type (WT) as well as mutated α-Syn should be collectively taken into account. The present review specifically provides an overview of recent research on α-Syn aggregation pathways and its modulation by several cellular factors potentially relevant to PD pathogenesis. We also briefly discuss about effect of environmental risk factors on α-Syn aggregation.

Lessons from treatment resistant hyperlipidaemia

A 68-year-old woman was referred to Lipid Clinic with sudden deterioration of previously well-controlled primary hyperlipidaemia. Investigations revealed nephrotic range proteinuria, leading to urgent renal biopsy and a diagnosis of amyloidosis. Chemotherapy was successful in stabilising renal function, reducing proteinuria and eliminating serum paraprotein. The resistant hyperlipidaemia subsequently resolved. Whilst hyperlipidaemia is pathognomonic of nephrotic syndrome, it is rarely the first characteristic identified by clinicians, often preceded by the identification of oedema or proteinuria. This case is an unusual example of a nephrotic syndrome presenting to Lipid Clinic as a resistant primary hyperlipidaemia, and highlights the importance of considering superimposed secondary causes of hyperlipidaemia in treatment resistant cases.

Molecular pathogenesis of human amyloidosis: Lessons from β2 -microglobulin-related amyloidosis

Amyloidosis refers to a group of diseases with amyloid fibrils deposited in various organs and is classified into more than 30 diseases in humans based on the kind of amyloid protein. In order to elucidate the molecular pathogenesis of human amyloidosis, we studied the molecular mechanism of amyloid fibril formation in vitro. We first developed a novel fluorometric method to determine amyloid fibrils in vitro based on the unique characteristics of thioflavin T. We next proposed a nucleation-dependent polymerization model to explain the general mechanism of amyloid fibril formation in vitro. Based on this model, we characterized the biological molecular interactions that promote or inhibit amyloid fibril formation in vitro and developed models of pathological molecular environment for inducing human β2-microglobulin-related amyloidosis in long-term hemodialysis patients. We also proposed a novel and attractive cytotoxic mechanism of β2-microglobulin amyloid fibrils, that is, the disruption of endosomal/lysosomal membranes by endocytosed amyloid fibrils. These findings may be useful to elucidate the molecular pathogenesis of other kinds of human amyloidosis.

Substitution of proline32 by α-methylproline preorganizes β2-microglobulin for oligomerization but not for aggregation into amyloids

Conversion of soluble folded proteins into insoluble amyloids generally proceeds in three distinct mechanistic stages: (1) initial protein misfolding into aggregation-competent conformers, (2) subsequent formation of oligomeric species and, finally, (3) self-assembly into extended amyloid fibrils. In the work reported herein, we interrogated the amyloidogenesis mechanism of human β2-microglobulin (β2m), which is thought to be triggered by a pivotal cis-trans isomerization of a proline residue at position 32 in the polypeptide, with nonstandard amino acids. Using chemical protein synthesis we prepared a β2m analogue in which Pro32 was replaced by the conformationally constrained amino acid α-methylproline (MePro). The strong propensity of MePro to adopt a trans prolyl bond led to enhanced population of a non-native [trans-MePro32]β2m protein conformer, which readily formed oligomers at neutral pH. In the presence of the antibiotic rifamycin SV, which inhibits amyloid growth of wild-type β2m, [MePro32]β2m was nearly quantitatively converted into different spherical oligomeric species. Self-assembly into amyloid fibrils was not observed in the absence of seeding, however, even at low pH (<3), where wild-type β2m spontaneously forms amyloids. Nevertheless, we found that aggregation-preorganized [MePro32]β2m can act in a prion-like fashion, templating misfolded conformations in a natively folded protein. Overall, these results provide detailed insight into the role of cis-trans isomerization of Pro32 and ensuing structural rearrangements that lead to initial β2m misfolding and aggregation. They corroborate the view that conformational protein dynamics enabled by reversible Pro32 cis-trans interconversion rather than simple population of the trans conformer is critical for both nucleation and subsequent growth of β2m amyloid structures.

How our bodies fight amyloidosis: effects of physiological factors on pathogenic aggregation of amyloidogenic proteins

The process of protein aggregation from soluble amyloidogenic proteins to insoluble amyloid fibrils plays significant roles in the onset of over 30 human amyloidogenic diseases, such as Prion disease, Alzheimer's disease and type 2 diabetes mellitus. Amyloid deposits are commonly found in patients suffered from amyloidosis; however, such deposits are rarely seen in healthy individuals, which may be largely attributed to the self-regulation in vivo. A vast number of physiological factors have been demonstrated to directly affect the process of amyloid formation in vivo. In this review, physiological factors that influence amyloidosis, including biological factors (chaperones, natural antibodies, enzymes, lipids and saccharides) and physicochemical factors (metal ions, pH environment, crowding and pressure, etc.), together with the mechanisms underlying these proteostasis effects, are summarized.

Calcium binding to beta-2-microglobulin at physiological pH drives the occurrence of conformational changes which cause the protein to precipitate into amorphous forms that subsequently transform into amyloid aggregates

Using spectroscopic, calorimetric and microscopic methods, we demonstrate that calcium binds to beta-2-microglobulin (β2m) under physiological conditions of pH and ionic strength, in biological buffers, causing a conformational change associated with the binding of up to four calcium atoms per β2m molecule, with a marked transformation of some random coil structure into beta sheet structure, and culminating in the aggregation of the protein at physiological (serum) concentrations of calcium and β2m. We draw attention to the fact that the sequence of β2m contains several potential calcium-binding motifs of the DXD and DXDXD (or DXEXD) varieties. We establish (a) that the microscopic aggregation seen at physiological concentrations of β2m and calcium turns into actual turbidity and visible precipitation at higher concentrations of protein and β2m, (b) that this initial aggregation/precipitation leads to the formation of amorphous aggregates, (c) that the formation of the amorphous aggregates can be partially reversed through the addition of the divalent ion chelating agent, EDTA, and (d) that upon incubation for a few weeks, the amorphous aggregates appear to support the formation of amyloid aggregates that bind to the dye, thioflavin T (ThT), resulting in increase in the dye's fluorescence. We speculate that β2m exists in the form of microscopic aggregates in vivo and that these don't progress to form larger amyloid aggregates because protein concentrations remain low under normal conditions of kidney function and β2m degradation. However, when kidney function is compromised and especially when dialysis is performed, β2m concentrations probably transiently rise to yield large aggregates that deposit in bone joints and transform into amyloids during dialysis related amyloidosis.

Accumulation of advanced glycation end products and beta 2-microglobulin in fibrotic thickening of the peritoneum in long-term peritoneal dialysis patients

Characteristics of pathological alterations in long-term peritoneal dialysis (PD) are thickening of submesothelial compact (SMC) zone, small-vessel vasculopathy, and loss of mesothelial cells. Bioincompatible PD fluid plays crucial roles in peritoneal injury. Encapsulating peritoneal sclerosis (EPS), a rare and serious complication, occurred in patients on long-term PD or frequent peritonitis episodes, and ~50 % of EPS developed after PD cessation. We hypothesized that PD-related peritoneal injury factors induced by bioincompatible PD fluid accumulated in the peritoneum and might induce EPS. We therefore examined the accumulation of advanced glycation end products (AGE) and beta 2-microglobulin (β2M) in peritoneum and evaluated the relationship between their accumulation, clinical parameters, and outcome after PD cessation. Forty-five parietal peritoneal specimens were obtained from 28 PD patients, 14 uremic patients, and three patients with normal kidney function. The peritoneal equilibration test was used for peritoneal function. AGE- and β2M-expressing areas were found in vascular walls, perivascular areas, and the deep layer of the SMC in short-term PD patients and extended over the entire SMC in long-term patients. Peritonitis and prolonged PD treatment aggravated peritoneal thickening and the proportion of AGE-expressing areas. The proportion of β2M-expressing areas was increased in long-term PD patients. Thickening of the SMC and the proportions of AGE- and β2M-expressing areas were not related to ascites or EPS after PD withdrawal. It appears that the increased proportion of AGE and β2M deposition induced by long-term exposure of PD fluid may be a marker of peritoneal injury.

The extracellular chaperone haptoglobin prevents serum fatty acid-promoted amyloid fibril formation of β2-microglobulin, resistance to lysosomal degradation, and cytotoxicity

Fibril formation of β2-microglobulin and associated inflammation occur in patients on long term dialysis. We show that the plasma protein haptoglobin prevents the fatty acid-promoted de novo fibril formation of β2-microglobulin even at substoichiometric concentration. The fibrils are cytotoxic, and haptoglobin abolishes the cytotoxicity by preventing fibril formation. Haptoglobin does not alleviate the cytotoxicity of preformed fibrils. Fibrillar β2-microglobulin is resistant to lysosomal degradation. However, the species of β2-microglobulin populated in the presence of haptoglobin is susceptible to degradation. We observed that haptoglobin interacts with oligomeric prefibrillar species of β2-microglobulin but not with monomeric or fibrillar β2-microglobulin that may underlie the molecular mechanism. 1,1'-Bis(4-anilino)naphthalene-5,5'-disulfonic acid cross-linking to haptoglobin significantly compromises its chaperone activity, suggesting the involvement of hydrophobic surfaces. Haptoglobin is an acute phase protein whose level increases severalfold during inflammation, where local acidosis can occur. Our data show that haptoglobin prevents fibril formation of β2-microglobulin under conditions of physiological acidosis (between pH 5.5 and 6.5) but with relatively decreased efficiency. However, compromise in its chaperone activity under these conditions is more than compensated by its increased level of expression under inflammation. Erythrolysis is known to release hemoglobin into the plasma. Haptoglobin forms a 1:1 (mol/mol) complex with hemoglobin. This complex, like haptoglobin, interacts with the prefibrillar species of β2-microglobulin, preventing its fibril formation and the associated cytotoxicity and resistance to intracellular degradation. Thus, our study demonstrates that haptoglobin is a potential extracellular chaperone for β2-microglobulin even in moderately acidic conditions relevant during inflammation, with promising therapeutic implications in β2-microglobulin amyloid-related diseases.

Capillary electrophoretic profile of β2-microglobulin intermediate associated with hemodialysis

We previously identified an intermediate β(2)-microglobulin (I-β(2) m), which is an amyloidogenic β(2) m variant, via capillary electrophoresis (CE) and reported hemodialysis (HD)-associated variations in the serum concentrations of each β(2) m component, including that found in the rebound phase. Recent research has indicated that I-β(2) m can bind, via the SO(3)(-) moiety, with glycosaminoglycan or proteoglycan, which are major components of interstitial tissue. Because alterations in I-β(2) m are likely to be important in view of the possible accumulation of amyloidogenic precursor proteins in the interstitial space, we studied the I-β(2) m profile as related to HD. We used CE to determine the I-β(2) m profile both at the start and at the end of HD and during the rebound phase in 12 HD patients. We found both an unfolded β(2) m and a destructured I-β(2) m. More important, two peaks appeared in the rebound phase, one suggesting a refolding and one suggesting an irreversible destruction. Given that the intercompartmental transfer coefficient for β(2) m is 1.0, our results indicated concomitant processes occurring after HD: refolding of the β(2) m conformation and trapping of destructured I-β(2) m in the extravascular space. Because the trapping of destructured I-β(2) m supposedly leads to accumulation of β(2) m in the interstitial space, we have proposed a new concept-a "shuttle" concept-for amyloid formation from β(2) m in the HD setting.

Rapid oligomer formation of human muscle acylphosphatase induced by heparan sulfate

Many human diseases are caused by the conversion of proteins from their native state into amyloid fibrils that deposit in the extracellular space. Heparan sulfate, a component of the extracellular matrix, is universally associated with amyloid deposits and promotes fibril formation. The formation of cytotoxic prefibrillar oligomers is challenging to study because of its rapidity, transient appearance and the heterogeneity of species generated. The process is even more complex with agents such as heparan sulfate. Here we have used a stopped-flow device coupled to turbidometry detection to monitor the rapid conversion of human muscle acylphosphatase into oligomers with varying heparan sulfate and protein concentrations. We also analyzed mutants of the 15 basic amino acids of acylphosphatase, identifying the residues primarily involved in heparan sulfate-induced oligomerization of this protein and tracing the process with unprecedented molecular detail.

Glycosaminoglycans (GAGs) suppress the toxicity of HypF-N prefibrillar aggregates

A group of diverse human pathologies is associated with proteins unable to retain their native state and convert into prefibrillar and fibrillar amyloid aggregates that are then deposited in the extracellular space. Glycosaminoglycans (GAGs) have been found to physically associate with these deposits and also to promote their formation in vitro. However, the effect of GAGs on the toxicity of these aggregates has been investigated in only one protein system, the amyloid β peptide associated with Alzheimer's disease. In this study, we investigate whether GAGs affect the toxicity of the N-terminal domain of Escherichia coli HypF (HypF-N) oligomers on Chinese hamster ovarian cells and the mechanism by which such suppression is mediated. The results show that heparin and other GAGs inhibit the toxicity observed by HypF-N oligomers in a dose-dependent manner. GAGs were not found to bind preformed HypF-N oligomers, change their morphological and structural characteristics or disaggregate them. Nevertheless, they were found to bind to the cells' surface and prevent the interaction of the oligomers with the cells. Overall, the results indicate that GAGs have a generic ability to inhibit the toxicity of aberrant protein oligomers and that such toxicity suppression can occur through different mechanisms, such as through binding to the oligomers with consequent loss of interaction of the oligomers to the GAGs present on the cell surface, as proposed previously for amyloid β aggregates, or through mechanisms independent of direct GAG-oligomer binding, as shown here for HypF-N aggregates.

β(2)-microglobulin amyloidosis

Dialysis-related amyloidosis (DRA) is a clinical syndrome of pain, loss of function and other symptoms due to the deposition of amyloid consisting of β(2)-microglobulin (β(2)m) in the musculoskeletal system. The condition is seen in patients who suffer from chronic kidney disease and are treated with hemodialysis for a long time. Even though β(2)m easily can be manipulated to form amyloid in laboratory experiments under non-physiological conditions the precise mechanisms involved in the formation of β(2)m-amyloid in patients with DRA have been difficult to unravel. The current knowledge which is reviewed here indicates that conformational fluctuations centered around the D-strand, the DE-loop, and around the cis-configured Pro32 peptide bond are involved in β(2)m amyloidosis. Also required are highly increased concentrations of circulating β(2)m and possibly various post-translational modifications mediated by the pro-inflammatory environment in uremic blood, together with the influence of divalent metal ions (specifically Cu(2 +)), uremic toxins, and dialysis-enhanced redox-processes. It seems plausible that domain-swapped β(2)m dimers act as building blocks of β-spine cross-β -sheet fibrils consisting of otherwise globular, roughly natively folded protein. An activated complement system and cellular activation perpetuate these reactions which due to the affinity of β(2)m-amyloid for the collagen of synovial surfaces result in the DRA syndrome.

Heparin binds 8 kDa gelsolin cross-β-sheet oligomers and accelerates amyloidogenesis by hastening fibril extension

Glycosaminoglycans (GAGs) are highly sulfated linear polysaccharides prevalent in the extracellular matrix, and they associate with virtually all amyloid deposits in vivo. GAGs accelerate the aggregation of many amyloidogenic peptides in vitro, but little mechanistic evidence is available to explain why. Herein, spectroscopic methods demonstrate that GAGs do not affect the secondary structure of the monomeric 8 kDa amyloidogenic fragment of human plasma gelsolin. Moreover, monomerized 8 kDa gelsolin does not bind to heparin under physiological conditions. In contrast, 8 kDa gelsolin cross-β-sheet oligomers and amyloid fibrils bind strongly to heparin, apparently because of electrostatic interactions between the negatively charged polysaccharide and a positively charged region of the 8 kDa gelsolin assemblies. Our observations are consistent with a scaffolding mechanism whereby cross-β-sheet oligomers, upon formation, bind to GAGs, accelerating the fibril extension phase of amyloidogenesis, possibly by concentrating and orienting the oligomers to more efficiently form amyloid fibrils. Notably, heparin decreases the 8 kDa gelsolin concentration necessary for amyloid fibril formation, likely a consequence of fibril stabilization through heparin binding. Because GAG overexpression, which is common in amyloidosis, may represent a strategy for minimizing cross-β-sheet oligomer toxicity by transforming them into amyloid fibrils, the mechanism described herein for GAG-mediated acceleration of 8 kDa gelsolin amyloidogenesis provides a starting point for therapeutic strategy development. The addition of GAG mimetics, small molecule sulfonates shown to reduce the amyloid load in animal models of amyloidosis, to a heparin-accelerated 8 kDa gelsolin aggregation reaction mixture neither significantly alters the rate of amyloidogenesis nor prevents oligomers from binding to GAGs, calling into question their commonly accepted mechanism.

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.

Charge distribution and amyloid fibril formation: insights from genetically engineered model systems

The influence of electrostatic interactions on protein amyloidogenesis has been investigated using de novo designed repetitive polypeptides YEHK21 [GH6[(GA)3GY(GA)3GE(GA)3GY(GA)3GE]21GAH6] and YE8 [GH6[(GA)3GY(GA)3GE]8GAH6]. The beta-sheet forming polypeptides were designed with identical beta-strands but with variable substitution at the turns that enable precise location of charged residues (Topilina et al. Biopolymers 2007, 86 (4), 261-264; Topilina et al. Biopolymers 2010, submitted for publication; Topilina et al. Biomacromolecules 2006, 7 (4), 1104-11). Solubility, folding, and aggregation of YEHK21 and YE8 were shown to be controlled by charge distribution. Under those conditions favoring the development of charge, YEHK21 and YE8 have significant propensities to form intermolecular beta-sheet assemblies illustrating the potential of charged polypeptide chains to form ordered amyloid aggregates even in the absence of additional environmental factors such as the presence of polyelectrolytes, salts, and so on.

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.

Kinetic analysis of amyloid formation in the presence of heparan sulfate: faster unfolding and change of pathway

A number of human diseases are associated with the conversion of proteins from their native state into well defined fibrillar aggregates, depositing in the extracellular space and generally termed amyloid fibrils. Heparan sulfate (HS), a glycosaminoglycan normally present in the extracellular matrix, has been found to be universally associated with amyloid deposits and to promote amyloid fibril formation by all studied protein systems. We have studied the impact of HS on the amyloidogenesis of human muscle acylphosphatase, monitoring the process with an array of techniques, such as normal and stopped-flow far-UV circular dichroism, thioflavin T fluorescence, static and dynamic light scattering, and atomic force microscopy. The results show that HS accelerates the conversion of the studied protein from the native state into the amyloidogenic, yet monomeric, partially folded state. They also indicate that HS does not simply accelerate the conversion of the resulting partially folded state into amyloid species but splits the process into two distinct pathways occurring in parallel: a very fast phase in which HS interacts with a fraction of protein molecules, causing their rapid aggregation into ThT-positive and beta-sheet containing oligomers, and a slow phase resulting from the normal aggregation of partially folded molecules that cannot interact with HS. The HS-mediated aggregation pathway is severalfold faster than that observed in the absence of HS. Two aggregation phases are generally observed when proteins aggregate in the presence of HS, underlying the importance of a detailed kinetic analysis to fully understand the effect of this glycosaminoglycan on amyloidogenesis.

Recent progress in understanding dialysis-related amyloidosis

Dialysis-Related Amyloidosis (DRA) is a general amyloidosis, which is specifically found in CKD stage 5 patients. DRA causes various osteoarticular lesions in dialysis patients, and therefore it is not practical to regard this condition separately from chronic kidney disease-mineral and bone disorder (CKD-MBD), at least from the viewpoint of daily clinical practice. However, it is still controversial whether this disease condition should be included in CKD-MBD. Recently, a better understanding of the pathogenesis of DRA has been obtained by examination of beta(2)-microglobulin-related amyloid fibril formation, extension, and depolymerization in vitro. Apoliprotein E, proteoglycans, and glycosaminoglycans stabilize the amyloid fibrils. In addition, some lysophospholipids and non-esterified fatty acids accelerate amyloid fibril formation and extension under physiological conditions in vitro. Those molecules may enhance the amyloid deposition in vivo. The frequency and severity of osteoarticular disorders that may be associated with DRA accelerate with the duration of dialysis therapy. We have shown that patients undergoing dialysis therapy for 30 years or more survive with serious complications from osteoarticular disorders. DRA is one of the most harmful osteoarticular complications with regard to the maintenance of daily activities and quality of life in patients undergoing long-term dialysis therapy, in addition to the classical complications of CKD-MBD.

Mutational analysis of the aggregation-prone and disaggregation-prone regions of acylphosphatase

We have performed an extensive mutational analysis of aggregation and disaggregation of amyloid-like protofibrils of human muscle acylphosphatase. Our findings indicate that the regions that promote aggregation in 25% (v/v) 2,2,2 trifluoroethanol (TFE) are different from those that promote disaggregation under milder conditions (5% TFE). Significant changes in the rate of disaggregation of protofibrils in 5% TFE result not only from mutations situated in the regions of the sequence that play a key role in the mechanism of aggregation in 25% TFE, but also from mutations located in other regions. In order to rationalise these results, we have used a modified version of the Zyggregator aggregation propensity prediction algorithm to take into account structural rearrangements of the protofibrils that may be induced by changes in solution conditions. Our results suggest that a wider range of residues contributes to the stability of the aggregates in addition to those that play an important kinetic role in the aggregation process. The mutational approach described here is capable of providing residue-specific information on the structure and dynamics of amyloid protofibrils under conditions close to physiological and should be widely applicable to other systems.

beta(2)-microglobulin: from physiology to amyloidosis

beta(2)-microglobulin (beta(2)m) is capable of forming amyloid in osteoarticular structures in kidney failure patients that undergo chronic hemodialysis treatment. Although sophisticated analytical methods have yielded comprehensive data about the conformation of the native protein both as a monomer and as the light chain of the type I major histocompatibility complex, the cause and mechanisms leading to the transformation of beta(2)m into amyloid deposits in patients with dialysis-related amyloidosis are unsettled. The impact on conformational stability of various truncations, cleavages, amino acid substitutions, and divalent cations, especially Cu(2+), however, are highly relevant for understanding beta(2)m unfolding pathways leading to amyloid formation. This review describes the current knowledge about such conformationally destabilizing and amyloidogenic factors and links these to the structure and function of beta(2)m in normal physiology and pathology. Tables listing modifications of beta(2)m found in amyloid from patients and a systematic overview of laboratory conditions conducive to beta(2)m-fibrillogenesis are also included.

Growth of β2-microglobulin-related amyloid fibrils by non-esterified fatty acids at a neutral pH

Aβ2M (β2-microglobulin-related) amyloidosis is a frequent and serious complication in patients on long-term dialysis. Partial unfolding of β2-m (β2-microglobulin) may be essential to its assembly into Aβ2M amyloid fibrils in vivo. Although SDS around the critical micelle concentration induces partial unfolding of β2-m to an α-helix-containing aggregation-prone amyloidogenic conformer and subsequent amyloid fibril formation in vitro, the biological molecules with similar activity under near-physiological conditions are still unknown. The effect of various NEFAs (non-esterified fatty acids), which are representative anionic amphipathic compounds in the circulation, on the growth of Aβ2M amyloid fibrils at a neutral pH was examined using fluorescence spectroscopy with thioflavin T, CD spectroscopy, and electron microscopy. Physiologically relevant concentrations of laurate, myristate, oleate, linoleate, and mixtures of palmitate, stearate, oleate and linoleate, induced the growth of fibrils at a neutral pH by partially unfolding the compact structure of β2-m to an aggregation-prone amyloidogenic conformer. In the presence of human serum albumin, these NEFAs also induced the growth of fibrils when their concentrations exceeded the binding capacity of albumin, indicating that the unbound NEFAs rather than albumin-bound NEFAs induce the fibril growth reaction in vitro. These results suggest the involvement of NEFAs in the development of Aβ2M amyloidosis, and in the pathogenesis of Aβ2M amyloidosis.

Amyloid peptides and proteins in review

Amyloids are filamentous protein deposits ranging in size from nanometres to microns and composed of aggregated peptide beta-sheets formed from parallel or anti-parallel alignments of peptide beta-strands. Amyloid-forming proteins have attracted a great deal of recent attention because of their association with over 30 diseases, notably neurodegenerative conditions like Alzheimer's, Huntington's, Parkinson's, Creutzfeldt-Jacob and prion disorders, but also systemic diseases such as amyotrophic lateral sclerosis (Lou Gehrig's disease) and type II diabetes. These diseases are all thought to involve important conformational changes in proteins, sometimes termed misfolding, that usually produce beta-sheet structures with a strong tendency to aggregate into water-insoluble fibrous polymers. Reasons for such conformational changes in vivo are still unclear. Intermediate aggregated state(s), rather than precipitated insoluble polymeric aggregates, have recently been implicated in cellular toxicity and may be the source of aberrant pathology in amyloid diseases. Numerous in vitro studies of short and medium length peptides that form amyloids have provided some clues to amyloid formation, with an alpha-helix to beta-sheet folding transition sometimes implicated as an intermediary step leading to amyloid formation. More recently, quite a few non-pathological amyloidogenic proteins have also been identified and physiological properties have been ascribed, challenging previous implications that amyloids were always disease causing. This article summarises a great deal of current knowledge on the occurrence, structure, folding pathways, chemistry and biology associated with amyloidogenic peptides and proteins and highlights some key factors that have been found to influence amyloidogenesis.

Heparin strongly enhances the formation of beta2-microglobulin amyloid fibrils in the presence of type I collagen

The tissue specificity of fibrillar deposition in dialysis-related amyloidosis is most likely associated with the peculiar interaction of beta2-microglobulin (beta2-m) with collagen fibers. However, other co-factors such as glycosaminoglycans might facilitate amyloid formation. In this study we have investigated the role of heparin in the process of collagen-driven amyloidogenesis. In fact, heparin is a well known positive effector of fibrillogenesis, and the elucidation of its potential effect in this type of amyloidosis is particularly relevant because heparin is regularly given to patients subject to hemodialysis to prevent blood clotting. We have monitored by atomic force microscopy the formation of beta2-m amyloid fibrils in the presence of collagen fibers, and we have discovered that heparin strongly accelerates amyloid deposition. The mechanism of this effect is still largely unexplained. Using dynamic light scattering, we have found that heparin promotes beta2-m aggregation in solution at pH 6.4. Morphology and structure of fibrils obtained in the presence of collagen and heparin are highly similar to those of natural fibrils. The fibril surface topology, investigated by limited proteolysis, suggests that the general assembly of amyloid fibrils grown under these conditions and in vitro at low pH is similar. The exposure of these fibrils to trypsin generates a cleavage at the C-terminal of lysine 6 and creates the 7-99 truncated form of beta2-m (DeltaN6beta2-m) that is a ubiquitous constituent of the natural beta2-m fibrils. The formation of this beta2-m species, which has a strong propensity to aggregate, might play an important role in the acceleration of local amyloid deposition.

Procollagen C-proteinase enhancer-1 (PCPE-1) interacts with beta2-microglobulin (beta2-m) and may help initiate beta2-m amyloid fibril formation in connective tissues

Dialysis related amyloidosis (DRA) is a progressive and serious complication in patients under long-term hemodialysis and mainly leads to osteo-articular diseases. Although beta(2)-microglobulin (beta2-m) is the major structural component of beta2-m amyloid fibrils, the initiation of amyloid formation is not clearly understood. Here, we have identified procollagen C-proteinase enhancer-1 (PCPE-1) as a new interacting protein with beta2-m by screening a human synovium cDNA library. The interaction of beta2-m with full-length PCPE-1 was confirmed by immunoprecipitation, solid-phase binding and pull-down assays. By yeast two-hybrid analysis and pull-down assay, beta2-m appeared to interact with PCPE-1 via the NTR (netrin-like) domain and not via the CUB (C1r/C1s, Uegf and BMP-1) domain region. In synovial tissues derived from hemodialysis patients with DRA, beta2-m co-localized and formed a complex with PCPE-1. beta2-m did not alter the basal activity of bone morphogenetic protein-1/procollagen C-proteinase (BMP-1/PCP) nor BMP-1/PCP activity enhanced by PCPE-1. PCPE-1 did not stimulate beta2-m amyloid fibril formation from monomeric beta2-m in vitro under acidic and neutral conditions as revealed by thioflavin T fluorescence spectroscopy and electron microscopy. Since PCPE-1 is abundantly expressed in connective tissues rich in type I collagen, it may be involved in the initial accumulation of beta2-m in selected tissues such as tendon, synovium and bone. Furthermore, since such preferential deposition of beta2-m may be linked to subsequent beta2-m amyloid fibril formation, the disruption of the interaction between beta2-m and PCPE-1 may prevent beta2-m amyloid fibril formation and therefore PCPE-1 could be a new target for the treatment of DRA.

Changes in major proteins in the embryonic capsule during immobilization (fixation) of the conceptus in the third week of pregnancy in the mare

During the third week of pregnancy, the equine conceptus is enclosed within a capsule, the glycan composition of which changes at around day 16 (ovulation = day 0) when the conceptus becomes immobilized (fixed) in the uterine lumen. Our objective was to characterize the process of fixation by identifying changes in major capsule-associated proteins. Individual equine conceptuses (n = 55) were collected transcervically by uterine lavage between days 13.5 and 26.5. Major proteins extracted from capsules were compared with those in fluids from the uterus and yolk sac by SDS-PAGE. Until day 14, a major capsule-associated protein that migrated at approximately 10 kDa was identified by N-terminal sequencing as equine beta2 microglobulin (beta2M). During fixation, beta2M in the capsule underwent limited proteolysis to an approximately 8 kDa form lacking nine amino acids from the N terminus, and was subsequently degraded. Expression of beta2M mRNA was detected in the yolk-sac wall tissues and endometrium between days 13.5 and 17.5. During this period, beta2M in the capsule was evidently not part of a complex with major histocompatibility complex class 1 heavy alpha chain bands because these were undetectable in the capsule and uterine lavage. Uterocalin (p19) was detected in uterine lavage and capsule throughout fixation, but in yolk-sac fluid only before fixation. These studies indicate that intact beta2M is a major protein associated with the embryonic capsule before fixation, after which it undergoes limited proteolysis to a truncated approximately 8 kDa form that remains in the capsule after the conceptus is immobilized.

Investigation into the role of macrophages in the formation and degradation of beta2-microglobulin amyloid fibrils

Dialysis related amyloidosis is a serious complication of long-term hemodialysis in which beta(2)-microglobulin (beta(2)m) forms amyloid fibrils that deposit predominantly in cartilaginous tissues. How these fibrils form in vivo, however, is poorly understood. Here we perform a systematic investigation into the role of macrophages in the formation and degradation of beta(2)m amyloid fibrils, building on observations that macrophages are found in association with beta(2)m amyloid deposits in vivo and that these cells contain intra-lysosomal beta(2)m amyloid. In live cell imaging experiments we demonstrate that macrophages internalize monomeric beta(2)m, whereupon it is sorted to lysosomes. At lysosomal pH beta(2)m self-associates in vitro to form amyloid-like fibrils with an array of morphologies as visualized by atomic force microscopy. Cleavage of the monomeric protein by both macrophages and lysosomal proteases isolated from these cells results in the rapid degradation of the monomeric protein, preventing amyloid formation. Incubation of macrophages with preformed fibrils revealed that macrophages internalize amyloid-like fibrils formed extracellularly, but in marked contrast with the monomeric protein, the fibrils were not degraded within macrophage lysosomes. Correspondingly beta(2)m fibrils were highly resistant to degradation by high concentrations of lysosomal proteases isolated from macrophages. Despite their enormous degradative capacity, therefore, macrophage lysosomes cannot ameliorate dialysis-related amyloidosis by degrading pre-existing amyloid fibrils, but lysosomal proteases may play a protective role by eliminating amyloid precursors before beta(2)m fibrils can accumulate in what may represent an otherwise fibrillogenic environment.