Ultrastructural organization of hemodialysis-associated beta 2-microglobulin amyloid fibrils

Extracellular matrix components modulate different stages in β2-microglobulin amyloid formation

Amyloid deposition of WT human β2-microglobulin (WT-hβ2m) in the joints of long-term hemodialysis patients is the hallmark of dialysis-related amyloidosis. In vitro, WT-hβ2m does not form amyloid fibrils at physiological pH and temperature unless co-solvents or other reagents are added. Therefore, understanding how fibril formation is initiated and maintained in the joint space is important for elucidating WT-hβ2m aggregation and dialysis-related amyloidosis onset. Here, we investigated the roles of collagen I and the commonly administered anticoagulant, low-molecular-weight (LMW) heparin, in the initiation and subsequent aggregation phases of WT-hβ2m in physiologically relevant conditions. Using thioflavin T fluorescence to study the kinetics of amyloid formation, we analyzed how these two agents affect specific stages of WT-hβ2m assembly. Our results revealed that LMW-heparin strongly promotes WT-hβ2m fibrillogenesis during all stages of aggregation. However, collagen I affected WT-hβ2m amyloid formation in contrasting ways: decreasing the lag time of fibril formation in the presence of LMW-heparin and slowing the rate at higher concentrations. We found that in self-seeded reactions, interaction of collagen I with WT-hβ2m amyloid fibrils attenuates surface-mediated growth of WT-hβ2m fibrils, demonstrating a key role of secondary nucleation in WT-hβ2m amyloid formation. Interestingly, collagen I fibrils did not suppress surface-mediated assembly of WT-hβ2m monomers when cross-seeded with fibrils formed from the N-terminally truncated variant ΔN6-hβ2m. Together, these results provide detailed insights into how collagen I and LMW-heparin impact different stages in the aggregation of WT-hβ2m into amyloid, which lead to dramatic effects on the time course of assembly.

The structure of a β2-microglobulin fibril suggests a molecular basis for its amyloid polymorphism

All amyloid fibrils contain a cross-β fold. How this structure differs in fibrils formed from proteins associated with different diseases remains unclear. Here, we combine cryo-EM and MAS-NMR to determine the structure of an amyloid fibril formed in vitro from β₂-microglobulin (β₂m), the culprit protein of dialysis-related amyloidosis. The fibril is composed of two identical protofilaments assembled from subunits that do not share β₂m's native tertiary fold, but are formed from similar β-strands. The fibrils share motifs with other amyloid fibrils, but also contain unique features including π-stacking interactions perpendicular to the fibril axis and an intramolecular disulfide that stabilises the subunit fold. We also describe a structural model for a second fibril morphology and show that it is built from the same subunit fold. The results provide insights into the mechanisms of fibril formation and the commonalities and differences within the amyloid fold in different protein sequences.

Light chain amyloidosis: Where are the light chains from and how they play their pathogenic role?

Amyloid light-chain (AL) amyloidosis is a plasma-cell dyscrasia, as well as the most common type of systematic amyloidosis. Pathogenic plasma cells that have distinct cytogenetic and molecular properties secrete an excess amount of amyloidogenic light chains. Assisted by post-translational modifications, matrix components, and other environmental factors, these light chains undergo a conformational change that triggers the formation of amyloid fibrils that overrides the extracellular protein quality control system. Moreover, the amyloidogenic light-chain itself is cytotoxic. As a consequence, organ dysfunction is caused by both organ architecture disruption and the direct cytotoxic effect of amyloidogenic light chains. Here, we reviewed the molecular mechanisms underlying this sequence of events that ultimately leads to AL amyloidosis and also discuss current in vitro and in vivo models, as well as relevant novel therapeutic approaches.

Preclinical Validation of the Heparin-Reactive Peptide p5+14 as a Molecular Imaging Agent for Visceral Amyloidosis

Amyloid is a complex pathologic matrix comprised principally of paracrystalline protein fibrils and heparan sulfate proteoglycans. Systemic amyloid diseases are rare, thus, routine diagnosis is often challenging. The glycosaminoglycans ubiquitously present in amyloid deposits are biochemically and electrochemically distinct from those found in the healthy tissues due to the high degree of sulfation. We have exploited this unique property and evaluated heparin-reactive peptides, such as p5+14, as novel agents for specifically targeting and imaging amyloid. Herein, we demonstrate that radiolabeled p5+14 effectively bound murine AA amyloid in vivo by using molecular imaging. Biotinylated peptide also reacted with the major forms of human amyloid in tissue sections as evidenced immunohistochemically. Furthermore, we have demonstrated that the peptide also binds synthetic amyloid fibrils that lack glycosaminoglycans implying that the dense anionic motif present on heparin is mimicked by the amyloid protein fibril itself. These biochemical and functional data support the translation of radiolabeled peptide p5+14 for the clinical imaging of amyloid in patients.

Characterization of the response of primary cells relevant to dialysis-related amyloidosis to β2-microglobulin monomer and fibrils

The formation of insoluble amyloid fibrils is associated with an array of devastating human diseases. Dialysis-related amyloidosis (DRA) is a severe complication of hemodialysis that results in the progressive destruction of the bones and joints. Elevated concentrations of β₂-microglobulin (β₂m) in the serum of subjects on hemodialysis promote the formation of amyloid fibrils in the osteoarticular tissues, but the cellular basis for the destruction of these tissues in DRA is poorly understood. In this study we performed a systematic analysis of the interaction of monomeric and fibrillar β₂m with primary human cells of the types present in the synovial joints of subjects with DRA. Building upon observations that macrophages infiltrate β₂m amyloid deposits in vivo we demonstrate that monocytes, the precursors of macrophages, cannot degrade β₂m fibrils, and that both monomeric β₂m and fibrillar β₂m are cytotoxic to these cells. β₂m fibrils also impair the formation of bone resorbing osteoclasts from monocytes and reduce the viability of osteoblasts, the cell type that produces bone. As a consequence, we predict that β₂m amyloid will disrupt the remodelling of the bone, which is critical for the maintenance of this tissue. Moreover, we show that β₂m fibrils reduce the viability of chondrocytes, rationalizing the loss of cartilage in DRA. Together, our observations demonstrate that β₂m cytotoxicity has multiple cellular targets in the osteoarticular tissues and is likely to be a key factor in the bone and joint destruction characteristic of DRA.

Nanoporous protein matrix made of amyloid fibrils of β2-microglobulin

Amyloid fibrils are considered as novel nanomaterials because of their nanoscale width, a regular constituting structure of cross β-sheet conformation, and considerable mechanical strength. By using an amyloidogenic protein of β(2)-microglobulin (β(2)M) related to dialysis-related amyloidosis, nanoporous protein matrix has been prepared. The β(2) M granules made of around 15 monomers showed an average size of 23.1 nm. They formed worm-like fibrils at pH 7.4 in 20 mM sodium phosphate containing 0.15 M NaCl following vigorous nondirectional shaking incubation, in which they became laterally associated and interwound to generate the porous amyloid fibrillar matrix with an average pore size of 30-50 nm. This nanoporous protein matrix was demonstrated to be selectively disintegrated by reducing agents, such as tris-(2-carboxyethyl) phosphine. High surface area with nanopores on the surface has been suggested to make the matrix of β(2) M amyloid fibrils particularly suitable for applications in the area of nanobiotechnology including drug delivery and tissue engineering.

Heparan sulfate proteoglycans in amyloidosis

Amyloidosis is a generic term for a group of diseases characterized by deposits in different organ systems of insoluble materials composed mainly of distinct fibrillar proteins named amyloid. Besides amyloid, heparan sulfate proteoglycan (HSPG), is commonly found in most amyloid deposits, suggesting that HS/HSPG may be functionally involved in the pathogenesis of amyloidosis. HS or HSPG is found to interact with a number of amyloid proteins, displaying a promoting effect on amyloid fibrilization in vitro. In addition, HS is reported to be involved in processing amyloid precursor proteins and mediate amyloid toxicity. Although little is known about the in vivo mechanisms regarding the codeposition of HS with amyloid proteins in different amyloid diseases, experiments carried out in animal models, especially in transgenic mouse model where HS molecular structure is modified, support an active role for HS in amyloidogenesis. Further experimental evidence is required to strengthen these in vivo findings at a molecular level. Animal models that express mutant forms of HS due to knockout of the enzymes involved in glycosaminoglycan (GAG) biosynthesis are expected to provide valuable tools for studying the implications of HS, as well as other GAGs, in amyloid disorders.

Cartilage biomarkers in hemodialysis patients and the effect of beta2-microglobulin on articular chondrocytes

OBJECTIVE

Dialysis-related amyloidosis (DRA) is a severe complication of maintenance hemodialysis (HD). Given the predominant deposition of beta(2)-microglobulin (beta2m) fibrils on articular cartilage in early DRA, we investigated the significance of beta2m and its relationship to distinct cartilage biomarkers in early DRA diagnosis in HD patients. Furthermore, we assessed the effects of beta2m on articular chondrocytes in vitro.

METHODS

Serum samples from 133 patients were collected before and after HD. Type II collagen cleavage product (C2C), procollagen II c-propeptide (CPII), aggrecan chondroitin sulfate 846 epitope (CS-486) and cartilage oligomeric matrix protein (COMP) levels were determined by enzyme-linked immunosorbent assay. Primary bovine articular chondrocytes were cultured as monolayers and incubated with beta2m at 1.5mg/l and 20mg/l. Cartilage glucosaminoglycan synthesis was measured by [(35)S]sulfate incorporation. mRNA expression of interleukin (IL)-1beta, matrix metalloproteinases (MMPs)-3 and -9 was measured by reverse-transcriptase polymerase chain reaction (RT-PCR).

RESULTS

Incubation with beta2m at 20mg/l significantly decreased matrix biosynthesis. PCR analysis revealed an increase of IL-1beta, as well as MMPs-3 and -9 on the mRNA level. C2C/CPII, CS-486 and COMP levels were increased only in a subset of patients without a significant correlation with beta2m concentrations. A subgroup analysis elucidated an increase in type II collagen degradation during the first years of HD, as shown by the elevation of C2C/CPII ratio.

CONCLUSION

beta2m exerted anti-anabolic effects on articular chondrocytes in vitro and might be involved in cartilage degradation in HD patients. beta2m serum levels, however, did not reflect cartilage degradation in DRA. The assessment of C2C/CPII, CS-486 or COMP concentrations apparently has minor relevance in DRA diagnosis in HD patients. However, the increased type II collagen breakdown within 5 years after HD onset possibly mirrors the early stages of DRA. Thus, the C2C/CPII ratio could be employed in longitudinal studies, since it may reflect a risk for DRA related arthropathy development in a subset of patients.

Specific glycosaminoglycans promote unseeded amyloid formation from β2-microglobulin under physiological conditions

Dialysis-related amyloidosis (DRA) is a complication of hemodialysis where beta2-microglobulin (beta2m) forms plaques mainly in cartilaginous tissues. The tissue-specific deposition, along with a known intransigence of pure beta2m to form fibrils in vitro at neutral pH in the absence of preformed fibrillar seeds, suggests a role for factors within cartilage in enhancing amyloid formation from this protein. To identify these factors, we determined the ability of a derivative lacking the N-terminal six amino acids found in ex vivo beta2m amyloid deposits to form amyloid fibrils at pH 7.4 in the absence of fibrillar seeds. We show that the addition of the glycosaminoglycans (GAGs) chrondroitin-4 or 6-sulfate to fibril growth assays results in the spontaneous generation of amyloid-like fibrils. By contrast, no fibrils are observed over the same time course in the presence of hyaluronic acid, a nonsulfated GAG that is abundant in cartilaginous joints. Based on the observation that hyaluronic acid has no effect on fibril stability, while chrondroitin-6-sulfate decreases the rate of fibril disassembly, we propose that the latter GAG enhances amyloid formation by stabilizing the rare fibrils that form spontaneously. This leads to the accumulation of beta2m in fibrillar deposits. Our data rationalize the joint-specific deposition of beta2m amyloid in DRA, suggesting mechanisms by which amyloid formation may be promoted.

Heparan sulfate as a therapeutic target in amyloidogenesis: prospects and possible complications

Amyloid formation in vivo is a much more complicated process than studies of in vitro protein/peptide fibrillogenesis would lead one to believe. Amyloidogenesis in vivo involves multiple components, some no less important than the amyloidogenic protein/peptides themselves, and each of these components, and its role in the pathogenetic steps toward amyloid deposition could, theoretically, be a therapeutic target. Herein we use the definition of amyloid as it was originally described, discuss the similarities and differences between amyloid in vivo and in vitro, address the potential role of the extracellular matrix in in vivo amyloidogenesis by focusing on a specific component, namely heparan sulfate proteoglycan, and describe studies illustrating that heparan sulfate is a valid target for anti-amyloid therapy. In light of experimental and recent clinical results obtained from studies addressing heparan sulfate's role in amyloid deposition additional novel anti-amyloid therapeutic targets will be proposed. Lastly, given the multiple roles that heparan sulfate plays in organ development, and organ and cell function, potential side effects of targeting heparan sulfate biosynthesis for therapeutic purposes are considered.

Dialysis-related amyloidosis: late finding or hidden epidemic?

Dialysis-related amyloidosis is a complication of end-stage renal disease (ESRD) that results from retention of beta2-microglobulin (beta2M) and its deposition as amyloid fibrils into osteoarticular tissue. The clinical manifestations usually develop after several years of dialysis dependence and include carpal tunnel syndrome, destructive arthropathy, and bone cysts and fractures. High-flux membranes, daily dialysis, and hemofiltration all would be expected to delay the onset of dialysis-related amyloidosis because, to varying degrees, each increases the clearance of beta2M from the plasma. Thus what is currently a late complication of ESRD might become an even later complication as dialysis practices change. The significance of histologically evident but clinically silent beta2M amyloid, detectable not only in osteoarticular tissue but also in blood vessels, is unclear. Accumulating evidence that amyloidogenic proteins have direct and specific effects on cell processes irrespective of the extent of amyloid deposition raises the possibility that early, clinically silent beta2M amyloid deposits have unrecognized importance.

Alpha-sheet: The toxic conformer in amyloid diseases?

A novel secondary structure, the alpha-sheet, was identified through molecular dynamics (MD) simulations of various proteins associated with amyloid diseases under amyloidogenic conditions. The structure was first predicted by Pauling and Corey, and it has been directly observed in crystal structures of "nonnatural peptides". There are occurrences of alpha-strands and alpha-sheets in the Protein Data Bank, but they are rare. We propose that alpha-sheet is formed during the conformational changes associated with amyloidosis and that it may represent the toxic conformer. Here, structural properties of the alpha-sheet, background information, and experimental support for this novel structure are presented. Finally we speculate about the possible role of this conformation in disease.

Topo-optical visualization reactions of carbohydrate-containing amyloid deposits in the respiratory tract

Staining with Congo red according to is the most commonly used method for the demonstration of amyloid, but structures other than amyloid can give false-positive results. To overcome this problem, introduced an aqueous Congo red staining with gum arabic as the mounting medium, which we have used in this and previous publications. Most histochemical studies on amyloid deposits to date have concentrated on conventional methods including staining with thioflavine, sirius red, alcian blue, methyl and crystal violet. In this study, we used topo-optical reactions with thiazine dyes on both the light and polarization microscopic level to establish the structure, distribution and location of carbohydrate components that occur within amyloid deposits, especially in the respiratory tract. Topo-optical staining reactions for the qualitative analysis of carbohydrate components in amyloid deposits included (1) reactions that identify the carbohydrate residues, (2) reactions that detect sialic acids and, (3) methods that visualize glycosaminoglycans. In conclusion, a comparison of consecutive serial sections stained with Congo red, aldehyde bisulfite toluidine blue reaction, sialic acid-specific topo-optical reaction, toluidine blue topo-optical reaction and chemically intensified basophilic reaction showed correlative staining patterns and anisotropic effects, corresponding to a close pathomorphological relationship between amyloid fibrils, periodate reactive carbohydrates, including sialic acids, and glycosaminoglycans.

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.

Kinetically Controlled Thermal Response of β2-Microglobulin Amyloid Fibrils

Calorimetric measurements were carried out using a differential scanning calorimeter in the temperature range from 10 to 120 degrees C for characterizing the thermal response of beta2-microglobulin amyloid fibrils. The thermograms of amyloid fibril solution showed a remarkably large decrease in heat capacity that was essentially released upon the thermal unfolding of the fibrils, in which the magnitude of negative heat capacity change was not explicable in terms of the current accessible surface area model of protein structural thermodynamics. The heat capacity-temperature curve of amyloid fibrils prior to the fibril unfolding exhibited an unusual dependence on the fibril concentration and the heating rate. Particularly, the heat needed to induce the thermal response was found to be linearly dependent on the heating rate, indicating that its thermal response is under a kinetic control and precluding the interpretation in terms of equilibrium thermodynamics. Furthermore, amyloid fibrils of amyloid beta peptides also exhibited a heating rate-dependent exothermic process before the fibril unfolding, indicating that the kinetically controlled thermal response may be a common phenomenon to amyloid fibrils. We suggest that the heating rate-dependent negative change in heat capacity is coupled to the association of amyloid fibrils with characteristic hydration pattern.

Formation of experimental murine AA amyloid fibrils in SAP-deficient mice: high resolution ultrastructural study

Previously, the role of the serum amyloid P component (SAP) in the deposition of murine AA amyloid has been examined in SAP-deficient mice in which the deposition was significantly retarded. In this study, AA amyloid fibrillogenesis in SAP-deficient mice was examined ultrastructurally. The fibrils of wild type mice were made up of a microfibril-like main body composed of SAP, chondroitin sulfate proteoglycan (CSPG), and outermost heparan sulfate proteoglycan (HSPG), and associated on its surface were 3 nm wide AA protein 'helical rods', a possible suitable form for Congo red staining. In SAP-deficient mice, fibrils of a similar appearance were also noted among an overwhelming amount of amorphous material, but the AP-containing main body of the fibril was replaced by elongated irregular aggregates of CSPG. The mechanism of retardation of AA amyloid induction in SAP-deficient mice has not yet been clear. It may be caused by possible slower formation of a 'substitute' core. Also, slower formation of AA helical rods may be possible due to the difference in the core material to which AA protein is attached. If it is so, it may limit the extent of Congo red staining, resulting in underestimation of the actual amount of AA protein.

Molecular interactions in the formation and deposition of beta2-microglobulin-related amyloid fibrils

In beta2-microglobulin-related (A beta2M) amyloidosis, a serious complication in patients on long-term dialysis, partial unfolding of beta2-microglobulin (beta2-m) is believed to be prerequisite to its assembly into A beta2M amyloid fibrils. Many kinds of amyloid-associated molecules, (e.g., apolipoprotein E (apoE), glycosaminoglycans (GAGs), proteoglycans (PGs)) may contribute to the development of A beta2M amyloidosis. In 1990s, the formation of A beta2M 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 A beta2M amyloid fibrils at a neutral pH, inducing partial unfolding of beta2-m and stabilization of the fibrils. Moreover, apoE, GAGs, and PGs were found to stabilize A beta2M 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 beta2-m. These findings are consistent with the hypothesis that in vivo, specific molecules that affect the conformation and stability of beta2-m and amyloid fibrils will have significant effects on the deposition of A beta2M amyloid fibrils.

Amyloid accomplices and enforcers

Amyloid-related diseases are often ascribed to protein "misfolding." Yet in the absence of high-resolution structures for mature fibrils or intermediates, the connection between the mechanism of amyloid formation and protein folding remains tenuous. The simplistic view of amyloid fibrillogenesis as a homogeneous self-assembly process is being increasingly challenged by observations that amyloids interact with a variety of cofactors including metals, glycosaminoglycans, glycoproteins such as serum amyloid P and apolipo-protein E, and constituents of basement membranes such as perlecan, laminin, and agrin. These "pathological chaperones" have effects that range from mediating the rate of amyloid fibril formation to increasing the stability of amyloid deposits, and may contribute to amyloid toxicity. An increasing appreciation of the role of accessory molecules in amyloid etiology has paved the way to novel diagnostics and therapeutic strategies.

Up-regulation of the extracellular matrix remodeling genes, biglycan, neutrophil gelatinase-associated lipocalin, and matrix metalloproteinase-9 in familial amyloid polyneuropathy

Familial amyloid polyneuropathy (FAP) is characterized by extracellular deposition of transthyretin (TTR) aggregates and amyloid fibrils, particularly in the peripheral nervous system (PNS) and is accompanied with changes in connective tissue. Given the invasiveness of nerve biopsy, FAP salivary glands (SGs) were used in microarray analysis; biglycan and neutrophil gelatinase-associated lipocalin (NGAL), two genes related to extracellular matrix (ECM) remodeling were overexpressed in FAP. Results were validated by RT-PCR and immunohistochemistry both in SG and in nerve biopsies of different stages of disease progression. Matrix metalloproteinase-9 (MMP-9), which exists as a complex with NGAL, was also increased in FAP and in vitro degraded TTR aggregates and fibrils; however in the presence of serum amyloid P, a universal amyloid component, TTR fibrils became resistant to MMP-9 proteolysis. Biglycan, NGAL, and MMP-9 are transcriptionally up-regulated by NF-kappaB, a transcription factor that is activated in FAP nerves and SG. Given the relationship between inflammation and ECM remodeling, and the increase of proinflammatory cytokines in FAP, IL-10 expression in FAP nerves was investigated; IL-10 increased after fibril deposition, suggesting a balance between proinflammatory and anti-inflammatory mechanisms. Changes in ECM-related proteins and inflammatory events may be relevant for therapy in FAP and other neurodegenerative disorders.

Pauling and Corey's alpha-pleated sheet structure may define the prefibrillar amyloidogenic intermediate in amyloid disease

Transthyretin, beta(2)-microglobulin, lysozyme, and the prion protein are four of the best-characterized proteins implicated in amyloid disease. Upon partial acid denaturation, these proteins undergo conformational change into an amyloidogenic intermediate that can self-assemble into amyloid fibrils. Many experiments have shown that pH-mediated changes in structure are required for the formation of the amyloidogeneic intermediate, but it has proved impossible to characterize these conformational changes at high resolution using experimental means. To probe these conformational changes at atomic resolution, we have performed molecular dynamics simulations of these proteins at neutral and low pH. In low-pH simulations of all four proteins, we observe the formation of alpha-pleated sheet secondary structure, which was first proposed by L. Pauling and R. B. Corey [(1951) Proc. Natl. Acad. Sci. USA 37, 251-256]. In all beta-sheet proteins, transthyretin and beta(2)-microglobulin, alpha-pleated sheet structure formed over the strands that are highly protected in hydrogen-exchange experiments probing amyloidogenic conditions. In lysozyme and the prion protein, alpha-sheets formed in the specific regions of the protein implicated in the amyloidogenic conversion. We propose that the formation of alpha-pleated sheet structure may be a common conformational transition in amyloidosis.

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.

Hierarchical assembly of beta2-microglobulin amyloid in vitro revealed by atomic force microscopy

The kinetics of spontaneous assembly of amyloid fibrils of wild-type beta(2)-microglobulin (beta(2)M) in vitro, under acid conditions (pH 2.5) and low ionic strength, has been followed using thioflavin-T (ThT) binding. In parallel experiments, the morphology of the different fibrillar species present at different time-points during the growth process were characterised using tapping-mode atomic force microscopy (TM-AFM) in air and negative stain electron microscopy (EM). The thioflavin-T assay shows a characteristic lag phase during which the nucleation of fibrils occurs before a rapid growth in fibril density. The volume of fibrils deposited on mica measured from TM-AFM images at each time-point correlates well with the fluorescence data. TM-AFM and negative-stain EM revealed the presence of various kinds of protein aggregates in the lag phase that disappear concomitantly with a rise in the density of amyloid fibrils, suggesting that these aggregates precede fibril growth and may act as nucleation sites. Three distinct morphologies of mature amyloid fibrils were observed within a single growth experiment, as observed previously for the wild-type protein and the variant N17D. Additional supercoiled morphologies of the lower-order fibrils were observed. Comparative height analysis from the TM-AFM data allows each of the mature fibril types and single protofilaments to be identified unambiguously, and reveals that the assembly occurs via a hierarchy of morphological states.

Amyloidogenesis: historical and modern observations point to heparan sulfate proteoglycans as a major culprit

Amyloids are complex tissue deposits and each type is identified by one of 22 different proteins or peptides which become re-folded into non-native conformational intermediates and then assemble into fibrils of a highly regular structure. All amyloid deposits also contain apolipoprotein E (apoE) as well as the basement membrane (BM) components, serum amyloid P and heparan sulfate proteoglycans (HSPG), perlecan or agrin. These BM components likely contribute to the overall organization of amyloid fibrils and HSPG has been further implicated in the genesis of amyloid. A growing body of evidence, summarized in this review, suggests that heparan sulfate (HS) promotes fibrillogenesis by associating with the amyloid precursors and inducing the conformational change required for their assembly into fibrils. HS also remains associated with the nascent fibrils contributing to its stability. These activities of HS are likely mediated through specific binding sites on the precursor proteins which appear to have sequence characteristics that are unique to amyloid.

Early pathogenesis of cardiac amyloid deposition in senile systemic amyloidosis: close relationship between amyloid deposits and the basement membranes of myocardial cells

Despite a number of in vitro studies of transthyretin (TTR) amyloidogenesis the early stage of in vivo amyloidogenesis in the human heart is largely unknown. A heart with a mild degree of cardiac amyloidosis removed from a 90-year old woman at autopsy was selected for analysis. The genotype of the TTR was the wild type. An immunohistochemical study with anti-TTR antibody was performed on serial paraffin sections, and 17 TTR-positive lesions less than 50 micro m in diameter consisting of 13 interstitial and 4 vascular lesions were identified. The early interstitial lesions start as thick membranous deposits between interfacing myocardial cells. They are Congophilic with green birefringence and positive for apolipoprotein E but negative for amyloid P component. The TTR-positive amyloid extends along intercellular spaces and becomes larger, involving several myocardial fibers. The media is the initial site of arteriolar involvement. According to the in vitro studies of amyloid fibrillogenesis, the most critical step is formation of the nucleus under supersaturated conditions. The supersaturated conditions are speculated to be achieved by binding to proteoglycans or lipid membranes. Our results indicate that the basement membrane of myocardial cells is the initial site of amyloid deposition, providing a suitable place for concentration of TTR.

Basement membrane and beta amyloid fibrillogenesis in Alzheimer's disease

High-resolution ultrastructural and immunohistochemical studies revealed that in situ beta amyloid fibrils of Alzheimer's disease were made up of a core consisting of a solid column of amyloid P component (AP) and associated chondroitin sulfate proteoglycan, and a heparan sulfate proteoglycan surface layer with externally associated fine filaments of beta protein. The main body of beta amyloid fibrils closely resembled that of microfibrils. Abundant microfibrils were reported to be present at the basement membrane of capillaries with "leaky" blood-urine or blood-air barriers. Similarly, abundant microfibril-like beta amyloid fibrils are formed at the microvascular basement membrane in cerebrovascular amyloid angiopathy with altered blood-brain barrier. Since AP is an indispensable major component of microfibrils and microfibril-like structures, the formation of microfibrils may depend on, among other factors, the availability of AP. Thus, in beta amyloid fibrillogenesis fibrils may be built around AP which continuously leaks out from circulation into vascular basement membrane, and beta amyloid fibrils may be regarded as pathologically altered basement membrane-associated microfibrils. With no source of AP around them, senile plaque fibrils may also be derived from perivascular amyloid.

Basement membrane of mouse bone marrow sinusoids shows distinctive structure and proteoglycan composition: a high resolution ultrastructural study

Venous sinusoids in bone marrow are the site of a large-scale traffic of cells between the extravascular hemopoietic compartment and the blood stream. The wall of the sinusoids consists solely of a basement membrane interposed between a layer of endothelial cells and an incomplete covering of adventitial cells. To examine its possible structural specialization, the basement membrane of bone marrow sinusoids has now been examined by high resolution electron microscopy of perfusion-fixed mouse bone marrow. The basement membrane layer was discontinuous, consisting of irregular masses of amorphous material within a uniform 60-nm-wide space between apposing endothelial cells and adventitial cell processes. At maximal magnifications, the material was resolved as a random arrangement of components lacking the "cord network" formation seen in basement membranes elsewhere. Individual components exhibited distinctive ultrastructural features whose molecular identity has previously been established. By these morphological criteria, the basement membrane contained unusually abundant chondroitin sulfate proteoglycan (CSPG) revealed by 3-nm-wide "double tracks," and moderate amounts of both laminin as dense irregular coils and type IV collagen as 1-1.5-nm-wide filaments, together with less conspicuous amounts of amyloid P forming pentagonal frames. In contrast, 4.5-5-nm-wide "double tracks" characteristic of heparan sulfate proteoglycan (HSPG) were absent. The findings demonstrate that, in comparison with "typical" basement membranes in other tissues, the bone marrow sinusoidal basement membrane is uniquely specialized in several respects. Its discontinuous nature, lack of network organization, and absence of HSPG, a molecule that normally helps to maintain membrane integrity, may facilitate disassembly and reassembly of basement membrane material in concert with movements of adventitial cell processes as maturing hemopoietic cells pass through the sinusoidal wall: the exceptionally large quantity of CSPG may represent a reservoir of CD44 receptor for use in hemopoiesis.

Beta(2)-microglobulin and its deamidated variant, N17D form amyloid fibrils with a range of morphologies in vitro

Amyloid fibrils formed by incubation of recombinant wild-type human beta(2)-microglobulin (beta(2)M) ab initio in vitro at low pH and high ionic strength are short and highly curved. By contrast, fibrils extracted from patients suffering from haemodialysis-related amyloidosis and those formed by seeding growth of the wild-type protein in vitro with fibrils ex vivo are longer and straighter than those previously produced ab initio in vitro. Here we explore the effect of growth conditions on morphology of beta(2)M fibrils formed ab initio in vitro from the wild-type protein, as well as a variant form of beta(2)M in which Asn17 is deamidated to Asp (N17D). We show that deamidation results in significant destabilisation of beta(2)M at neutral pH. Despite this, acidification is still necessary to form amyloid from the mutant protein in vitro. Interestingly, at low pH and low ionic strength long, straight fibrils of recombinant beta(2)M are formed in vitro. The fibrils comprise three distinct morphological types when examined using electron microscopy (EM) and atomic force microscopy (AFM) that vary in periodicity and the number of constituent protofibrils. Using kinetic experiments we suggest that the immature fibrils observed previously do not represent intermediates in the assembly of fully mature amyloid, at least under the conditions studied here.

Role of the single disulphide bond of beta(2)-microglobulin in amyloidosis in vitro

The aggregation of beta(2)-microglobulin (beta(2)m) into amyloid fibrils occurs in the condition known as dialysis-related amyloidosis (DRA). The protein has a beta-sandwich fold typical of the immunoglobulin family, which is stabilized by a highly conserved disulphide bond linking Cys25 and Cys80. Oxidized beta(2)m forms amyloid fibrils rapidly in vitro at acidic pH and high ionic strength. Here we investigate the role of the single disulphide bond of beta(2)m in amyloidosis in vitro. We show that reduction of the disulphide bond destabilizes the native protein such that non-native molecules are populated at neutral pH. These species are prone to oligomerization but do not form amyloid fibrils when incubated for up to 8 mo at pH 7.0 in 0.4 M NaCl. Over the pH range 4.0-1.5 in the presence of 0.4 M NaCl, however, amyloid fibrils of reduced beta(2)m are formed. These fibrils are approximately 10 nm wide, but are shorter and assemble more rapidly than those produced from the oxidized protein. These data show that population of non-native conformers of beta(2)m at neutral pH by reduction of its single disulphide bond is not sufficient for amyloid formation. Instead, association of one or more specific partially unfolded molecules formed at acid pH are necessary for the formation of beta(2)m amyloid in vitro. Further experiments will now be needed to determine the role of different oligomeric species of beta(2)m in the toxicity of the protein in vivo.

Kidney dialysis-associated amyloidosis: a molecular role for copper in fiber formation

In the US alone, more than 250,000 people have impaired renal function that necessitates treatment by dialysis. A debilitating complication of long-term treatment is the deposition of beta2-microglobulin (beta2m) as amyloid fibers within the joint space. However, the intrinsic propensity of isolated beta2m protein to initiate in vitro fiber formation is negligible under conditions matched to the neutral pH and ionic conditions of serum. Here, we present evidence for a novel interaction between beta2m and Cu(2+) at a concentration within institutionally recommended limits for this metal ion in dialysate solution. Mass spectrometry, using electrospray ionization from native conditions, demonstrates that the binding of Cu(2+) is specific over Ca(2+) or Zn(2+). Despite maintaining a native-like conformation upon Cu(2+) binding, the folded protein is unusually destabilized against thermal and urea denaturation. We further demonstrate that destabilization by Cu(2+) uniquely promotes de novo fiber formation at 37 degrees C and neutral pH. Since the incidence of amyloidosis is dramatically reduced upon elimination of copper from dialysis membranes, our results provide a molecular understanding for dialysis-associated amyloid formation by beta2m.

Pathological aspects of beta(2)-microglobulin amyloidosis

Histology remains the gold standard to diagnose beta(2)-microglobulin amyloidosis (A beta(2)M). Two diagnostic criteria are required: positive Congo red staining with typical birefringence under polarized light and immunostaining of amyloid deposits with a labeled anti-beta(2)M antibody. A beta(2)M is preferentially located in the joints. Small deposits are also found in various organs, mainly the heart and gastrointestinal tract. Pathologic studies have demonstrated a high prevalence of articular A beta(2)M early in the course of hemodialysis and peritoneal dialysis, antedating clinical manifestations by several years. The stages of beta(2)M amyloid formation have been delineated: beta(2)M amyloid deposits first on the surface of the cartilage, in the absence of macrophages (stage 1), and subsequently involves capsules and synovia (stage 2), with eventual recruitment of macrophages around large beta(2)M amyloid deposits (stage 3). Clinical manifestations are likely associated with the inflammation observed in stage 3. The factors triggering the fibrillar precipitation of beta(2)M remain unknown. Macrophages do not play a role: their presence is the consequence rather than the cause of beta(2)M amyloid deposits. Several substances coprecipitated with beta(2)M amyloid have been incriminated: highly sulfated glycosaminoglycans such as chondroitin or keratan sulfate, antiproteases such as alpha(2)-macroglobulin, and apolipoprotein E. As yet, no definitive conclusion has been reached.

beta2-microglobulin-derived amyloidosis: an update

The present review attempts to summarize recent developments in the field of beta2-microglobulin-derived amyloidosis (A(beta2)m amyloidosis) in patients on chronic dialysis therapy. A key factor in the pathogenesis is the uremic retention of the precursor molecule, beta2-microglobulin (beta2m). However, secondary modifications of the molecule such as limited proteolysis, conformational changes, and the formation of advanced glycation end products have also been described. Finally, in order to explain the striking predilection of the disease for synovial and periarticular structures, a role of local predisposing factors within the synovial membrane (for example, of the particular constituents of the extracellular matrix) must also be postulated. With respect to clinical symptomatology, recent data have confirmed that clinically manifest signs of the amyloidosis represent only the tip of the iceberg, since histologically amyloid deposition is much more widespread. Noninvasive diagnosing of the disease has been advanced by technical changes of the beta2m scintigraphy. Finally, there is accumulating evidence that prevention of the disease not only includes the usage of high-flux synthetic membranes for hemodialysis or hemodiafiltration, but that other factors contribute to the clinical manifestations of amyloidosis such as the dialysate composition and its microbacteriological quality. Such factors, which have changed over the last years as part of general improvements in dialysis care, may explain why the prevalence of the amyloidosis appears to decrease.

Ultrastructure and composition of basement membranes in the tooth

The tooth, the hardest organ in the body, is known to be formed through highly elaborate, unique processes of differentiation and development. Basement membranes play critical roles in fundamentally important biological processes such as growth and differentiation, and for better understanding of the mechanism of development and maintenance of the tooth, specializations of tooth basement membranes are reviewed in detail in relation to their roles. The basement membrane at such diverse locations in the tooth as the inner enamel epithelium, maturation-stage ameloblasts, and junctional epithelium at the dentogingival border are specialized in their own highly unique ways for anchoring, firm binding, or mediation in the transport of substances. Thus, the role of basement membranes in the developing and mature tooth is manifold and for these roles individual basement membranes are specialized in their own specific ways which are rare or not seen in nondental tissues, and these specializations are essential for successful development and maintenance of the tooth.

Pathogenesis of beta2-microglobulin amyloidosis

Hemodialysis-related amyloidosis is a relatively new form of systemic amyloidosis, with beta2-microglobulin (B2M) being identified as the major constituent protein. Most of the clinical findings are related to amyloid deposition in osseo-articular tissues. B2M amyloid deposits first appear in the cervical intervertebral discs, which are well known to be susceptible to mechanical stress. A close relationship between changes of microenvironment caused by such stress and amyloid deposition is highly suggested. In advanced cases, an inflammatory reaction composed of macrophages, multinucleated giant cells, and granulation tissue, is observed around the amyloid deposits. Purified amyloid protein is native B2M, and mutations and proteolysis are not believed to be important for its deposition. Plasma levels of B2M are elevated as much as 5-10 times because of the inability of hemodialysis equipment removal of B2M from blood plasma, the duration being very important for B2M amyloid fibrillogenesis. Heparan sulfate proteoglycans, perlecan, is increased at the same sites of amyloid deposits from the early stages. In B2M amyloidosis, an increase of heparan sulfate proteoglycans is observed in the vascular wall and synovium, but in the discs, ligaments and cartilage, there is an increase of chondroitin sulfate proteoglycans predominantly. B2M has an affinity for heparan sulfate proteoglycans, although it is weaker than that for laminin and type IV collagen. This is related to the interactions between negative charges of sulfate groups of proteoglycans and positive charges of basic amino acids in N-terminal side of B2M. Increased cytokines production in the synovium, induced by advanced glycation end products as well as elevated plasma levels, is also linked to inflammatory reactions. Increased expression of matrix metalloproteinases (MMP), especially MMP-1 and -9, is related to the destructive changes of the bone and cartilage. The decrease of plasma levels by high flux membrane and control of inflammatory reactions are very important for prevention of B2M amyloidosis.

Ultrastructural organization of amyloid fibrils by atomic force microscopy

Atomic force microscopy has been employed to investigate the structural organization of amyloid fibrils produced in vitro from three very different polypeptide sequences. The systems investigated are a 10-residue peptide derived from the sequence of transthyretin, the 90-residue SH3 domain of bovine phosphatidylinositol-3'-kinase, and human wild-type lysozyme, a 130-residue protein containing four disulfide bridges. The results demonstrate distinct similarities between the structures formed by the different classes of fibrils despite the contrasting nature of the polypeptide species involved. SH3 and lysozyme fibrils consist typically of four protofilaments, exhibiting a left-handed twist along the fibril axis. The substructure of TTR(10-19) fibrils is not resolved by atomic force microscopy and their uniform appearance is suggestive of a regular self-association of very thin filaments. We propose that the exact number and orientation of protofilaments within amyloid fibrils is dictated by packing of the regions of the polypeptide chains that are not directly involved in formation of the cross-beta core of the fibrils. The results obtained for these proteins, none of which is directly associated with any human disease, are closely similar to those of disease-related amyloid fibrils, supporting the concept that amyloid is a generic structure of polypeptide chains. The detailed architecture of an individual fibril, however, depends on the manner in which the protofilaments assemble into the fibrillar structure, which in turn is dependent on the sequence of the polypeptide and the conditions under which the fibril is formed.

Review: amyloidogenesis-unquestioned answers and unanswered questions

Current assumptions and conclusions in several active areas of amyloid research are examined to see how consistent the data from chosen in vitro and in vivo model systems are with clinical and anatomic observations. These areas include the assembly of amyloid-like fibrils in vitro, the nucleation phenomenon, amyloid fibril structure in vivo and in vitro, common structural components of the amyloids, and the regression of tissue amyloid and proteolysis of amyloid proteins. Divergences and congruencies are highlighted, which in turn suggests caution in the interpretation of present data, greater collaboration and communication among investigators, and, additional areas and techniques for investigation.

Review: history of the amyloid fibril

Rudolph Virchow, in 1854, introduced and popularized the term amyloid to denote a macroscopic tissue abnormality that exhibited a positive iodine staining reaction. Subsequent light microscopic studies with polarizing optics demonstrated the inherent birefringence of amyloid deposits, a property that increased intensely after staining with Congo red dye. In 1959, electron microscopic examination of ultrathin sections of amyloidotic tissues revealed the presence of fibrils, indeterminate in length and, invariably, 80 to 100 A in width. Using the criteria of Congophilia and fibrillar morphology, 20 or more biochemically distinct forms of amyloid have been identified throughout the animal kingdom; each is specifically associated with a unique clinical syndrome. Fibrils, also 80 to 100 A in width, have been isolated from tissue homogenates using differential sedimentation or solubility. X-ray diffraction analysis revealed the fibrils to be ordered in the beta pleated sheet conformation, with the direction of the polypeptide backbone perpendicular to the fibril axis (cross beta structure). Because of the similar dimensions and tinctorial properties of the fibrils extracted from amyloid-laden tissues and amyloid fibrils in tissue sections, they have been assumed to be identical. However, the spatial relationship of proteoglycans and amyloid P component (AP), common to all forms of amyloid, to the putative protein only fibrils in tissues, has been unclear. Recently, it has been suggested that, in situ, amyloid fibrils are composed of proteoglycans and AP as well as amyloid proteins and thus resemble connective tissue microfibrils. Chemical and physical definition of the fibrils in tissues will be needed to relate the in vitro properties of amyloid protein fibrils to the pathogenesis of amyloid fibril formation in vivo.

Basement membranes, microfibrils and beta amyloid fibrillogenesis in Alzheimer's disease: high resolution ultrastructural findings

It is known that beta amyloid fibrils are deposited at the basement membrane of the cerebromicrovasculature in the brains of patients with Alzheimer's disease, and the assembly of the fibrils may be in continuation with the core of senile plaques. The fibrils accumulate in a manner similar to that in which microfibrils accumulate in the glomerular basement membrane of the rat kidney during long-term experimental diabetes, and in the alveolar-capillary basement membrane of the normal lung. beta amyloid fibrils in-situ are known to be about 10 nm wide tubular structures and they closely resemble connective tissue microfibrils. Our recent high resolution ultrastructural studies combined with immunogold labeling demonstrated that beta amyloid fibrils in-situ are indeed microfibril-like structures, and the beta protein is associated with their surface in the form of loose assemblies of 1 nm wide flexible filaments. Thus, the result of this study indicates that in-situ a major component of the beta amyloid deposit is the microfibril-like structure. The elucidation of the mechanism of cerebral beta amyloid fibrillogenesis in Alzheimer's disease may therefore require understanding the mechanism of 'normal' microfibrils biogenesis.

Impaired lysosomal processing of beta2-microglobulin by infiltrating macrophages in dialysis amyloidosis

BACKGROUND

Macrophages may participate in amyloid fibril formation by processing the protein precursor. Although this theory seems to apply for amyloidosis, in which proteolytic cleavage is a prerequisite for amyloid fibril formation, it has not been demonstrated for beta2-microglobulin (beta2m) amyloidosis. We aimed to establish the role played by macrophages in beta2m amyloidosis.

METHODS

We used a double immunogold electron microscopy technique, including mouse antihuman CD68, rabbit antihuman beta2m, amyloid P component, and lysosome-associated membrane protein (LAMP-1) antibodies. Differential density labeling studies of beta2m and amyloid P component were performed extra- and intracellularly to assess protein processing by macrophages.

RESULTS

The cells surrounding amyloid fibrils were found to be mostly CD68 positive, suggesting that they were of monocyte-macrophage lineage. Intracellular accumulation of amyloid fibrils was also observed; these fibrils were constantly surrounded by LAMP-1-linked gold particles, demonstrating that intracellular beta2m was almost exclusively lysosomal. The rough-surface endoplasmic reticulum was not labeled by beta2m antibody, suggesting that there was no active synthesis of beta2m by the cells. As a marker of endocytosis, protruded cytoplasmic processes in close relation with the intracellular accumulations of beta2m amyloid fibrils were observed. No difference in density labeling (extracellular vs. intracellular) was observed for beta2m, whereas intracellular P component labeling was significantly decreased.

CONCLUSIONS

All of these data are strongly suggestive of phagocytosis and not synthesis of amyloid fibrils by macrophages. Further, they demonstrate an impaired lysosomal processing specific for beta2m, as other compounds of the amyloid fibrils (P component) are significantly cleared.

Ultrastructure of familial amyloid polyneuropathy amyloid fibrils: examination with high-resolution electron microscopy

The ultrastructure of familial amyloid polyneuropathy (FAP) amyloid fibrils was examined with high-resolution electron microscopy and immunolabeling. Sural nerve biopsies from FAP (Met 30) patients as well as control tissues were prepared for thin-section observations. Extracellular spaces in the vicinity of myelinated and unmyelinated peripheral nerves were found to be filled with amyloid fibrils as well as with deposits of an "amorphous" material. The fibril was composed of a surface layer and a core. The surface layer was made up of heparan sulfate proteoglycan and was externally associated with a loose assembly of 0.5- to 1-nm-wide filaments. The core was a microfibril-like structure in which amyloid P component was enclosed in a tight helical structure by chondroitin sulfate proteoglycan. Immunogold labeling showed that the peripheral fine filaments were composed of transthyretin. The dimensions of the transthyretin filament suggest that its basic unit is a modified monomer. The deposited amorphous material was a mixture of individual components of the fibril. These results suggest that the main body of FAP amyloid fibrils is similar to that of recently observed fibrils of experimental murine AA and hemodialysis-associated amyloid as well as of connective tissue microfibrils. The differences in the fibrils of these various types of amyloid are in the peripheral filaments which are composed of a protein specific to each type of amyloid.