Beta 2-microglobulin: a new form of amyloid protein associated with chronic hemodialysis

Elevated concentration of beta2-microglobulin among patients with carpal tunnel syndrome in the course of primary Sjögren syndrome - a prospective observational study on 50 patients

INTRODUCTION

Sjögren's syndrome (SS) is a chronic autoimmune disease characterized by lymphocytic infiltrates in the exocrine glands. Carpal tunnel syndrome (CTS) is suggested to be more frequent among SS patients than in the general population. The aim of this study was to seek associations between the CTS and the laboratory and clinical findings of SS patients.

METHODS

Fifty patients diagnosed with primary SS (pSS) were examined. Clinical evaluation by a rheumatologist and electrophysiological studies were conducted. Data on laboratory tests results was collected. Control group consisted of 50 sex and age-matched individuals with osteoarthritis (OA).

RESULTS

Out of 50 patients in the study group 27 (54%) were diagnosed with CTS. The prevalence of CTS among 50 individuals in the control group was 8%. Among pSS patients with CTS the joint involvement was not more common than in those from the non-CTS group [15 vs. 13 (p = 0.945)]. There was an expected difference in sleep disorders [18 vs. 9 (p = 0.012)] and paresthesia [23 vs. 13 (p = 0.024)]. The major finding was a significant difference in elevated beta2-microglobulin (B2MG) [23 vs. 13 (p = 0.024)]. Other studied factors, suggested in the literature as significant in the pSS-related neuropathy, were not statistically different between the groups.

CONCLUSION

Our study confirms that CTS is more prevalent among pSS patients than in the general population and suggests that a new approach is required towards the pathogenesis of this phenomenon. We hypothesize that CTS is more associated with an overall disease activity than joint involvement as such.

Disease-relevant β2-microglobulin variants share a common amyloid fold

β2-microglobulin (β2m) and its truncated variant ΔΝ6 are co-deposited in amyloid fibrils in the joints, causing the disorder dialysis-related amyloidosis (DRA). Point mutations of β2m result in diseases with distinct pathologies. β2m-D76N causes a rare systemic amyloidosis with protein deposited in the viscera in the absence of renal failure, whilst β2m-V27M is associated with renal failure, with amyloid deposits forming predominantly in the tongue. Here we use cryoEM to determine the structures of fibrils formed from these variants under identical conditions in vitro. We show that each fibril sample is polymorphic, with diversity arising from a 'lego-like' assembly of a common amyloid building block. These results suggest a 'many sequences, one amyloid fold' paradigm in contrast with the recently reported 'one sequence, many amyloid folds' behaviour of intrinsically disordered proteins such as tau and Aβ.

Review of Isoflavones and Their Potential Clinical Impacts on Cardiovascular and Bone Metabolism Markers in Peritoneal Dialysis Patients

Cardiovascular disease is the most important cause of mortality in patients with chronic kidney disease, including patients undergoing peritoneal dialysis. Oxidative stress, systemic and vascular inflammation, and lipid abnormalities are important causes of cardiovascular disease in these patients. Bone disorders are also a common complication in dialysis patients and can lead to bone fractures, decreased quality of life, vascular calcification, cardiovascular disease, and increased mortality. Studies in non-uremic populations have shown that soy isoflavones have beneficial effects on oxidative stress, inflammation, lipid abnormalities, and markers of bone metabolism; however, very few studies in this field have been conducted with peritoneal dialysis patients. This paper reviews the key data regarding the effects of soy isoflavones on cardiovascular disease and bone markers and discusses the role of this nutraceutical in preventing and managing the complications of peritoneal dialysis.

Dimers of D76N-β2-microglobulin display potent antiamyloid aggregation activity

Self-association of WT β_2-microglobulin (WT-β₂m) into amyloid fibrils is associated with the disorder dialysis related amyloidosis. In the familial variant D76N-β₂m, the single amino acid substitution enhances the aggregation propensity of the protein dramatically and gives rise to a disorder that is independent of renal dysfunction. Numerous biophysical and structural studies on WT- and D76N-β₂m have been performed in order to better understand the structure and dynamics of the native proteins and their different potentials to aggregate into amyloid. However, the structural properties of transient D76N-β₂m oligomers and their role(s) in assembly remained uncharted. Here, we have utilized NMR methods, combined with photo-induced crosslinking, to detect, trap, and structurally characterize transient dimers of D76N-β₂m. We show that the crosslinked D76N-β₂m dimers have different structures from those previously characterized for the on-pathway dimers of ΔN6-β₂m and are unable to assemble into amyloid. Instead, the crosslinked D76N-β₂m dimers are potent inhibitors of amyloid formation, preventing primary nucleation and elongation/secondary nucleation when added in substoichiometric amounts with D76N-β₂m monomers. The results highlight the specificity of early protein-protein interactions in amyloid formation and show how mapping these interfaces can inform new strategies to inhibit amyloid assembly.

The residues 4 to 6 at the N-terminus in particular modulate fibril propagation of β-microglobulin

The ΔN6 truncation is the main posttranslational modification of β-microglobulin (βM) found in dialysis-related amyloid. Investigation of the interaction of wild-type (WT) βM with N-terminally truncated variants is therefore of medical relevance. However, it is unclear which residues among the six residues at the N-terminus are crucial to the interactions and the modulation of amyloid fibril propagation of βM. We herein analyzed homo- and heterotypic seeding of amyloid fibrils of WT human βM and its N-terminally-truncated variants ΔN1 to ΔN6, lacking up to six residues at the N-terminus. At acidic pH 2.5, we produced amyloid fibrils from recombinant, WT βM and its six truncated variants, and found that ΔN6 βM fibrils exhibit a significantly lower conformational stability than WT βM fibrils. Importantly, under more physiological conditions (pH 6.2), we assembled amyloid fibrils only from recombinant, ΔN4, ΔN5, and ΔN6 βM but not from WT βM and its three truncated variants ΔN1 to ΔN3. Notably, the removal of the six, five or four residues at the N-terminus leads to enhanced fibril formation, and homo- and heterotypic seeding of ΔN6 fibrils strongly promotes amyloid fibril formation of WT βM and its six truncated variants, including at more physiological pH 6.2. Collectively, these results demonstrated that the residues 4 to 6 at the N-terminus particularly modulate amyloid fibril propagation of βM and the interactions of WT βM with N-terminally truncated variants, potentially indicating the direct relevance to the involvement of the protein's aggregation in dialysis-related amyloidosis.

Chronic Kidney Disease-Associated Itch (CKD-aI) in Children-A Narrative Review

Chronic kidney disease (CKD) is a condition of widespread epidemiology and serious consequences affecting all organs of the organism and associated with significant mortality. The knowledge on CKD is rapidly evolving, especially concerning adults. Recently, more data is also appearing regarding CKD in children. Chronic itch (CI) is a common symptom appearing due to various underlying dermatological and systemic conditions. CI may also appear in association with CKD and is termed chronic kidney disease-associated itch (CKD-aI). CKD-aI is relatively well-described in the literature concerning adults, yet it also affects children. Unfortunately, the data on paediatric CKD-aI is particularly scarce. This narrative review aims to describe various aspects of CKD-aI with an emphasis on children, based on the available data in this population and the data extrapolated from adults. Its pathogenesis is described in details, focusing on the growing role of uraemic toxins (UTs), as well as immune dysfunction, altered opioid transmission, infectious agents, xerosis, neuropathy and dialysis-associated aspects. Moreover, epidemiological and clinical aspects are reviewed based on the few data on CKD-aI in children, whereas treatment recommendations are proposed as well, based on the literature on CKD-aI in adults and own experience in managing CI in children.

Beta-2 Microglobulin Amyloidosis: Past, Present, and Future

Almost half a century has elapsed since the first description of dialysis-related amyloidosis (DRA), a disorder caused by excessive accumulation of β-2 microglobulin (B2M). Within that period, substantial advances in RRT occurred. These improvements have led to a decrease in the incidence of DRA. In many countries, DRA is considered a "disappearing act" or complication. Although the prevalence of patients living with RRT increases, not all will have access to kidney transplantation. Consequently, the number of patients requiring interventions for treatment of DRA is postulated to increase. This postulate has been borne out in Japan, where the number of patients with ESKD requiring surgery for carpal tunnel continues to increase. Clinicians treating patients with ESKD have treatment options to improve B2M clearance; however, there is a need to identify ways to translate improved B2M clearance into improved quality of life for patients undergoing long-term dialysis.

Looking Beyond the Core: The Role of Flanking Regions in the Aggregation of Amyloidogenic Peptides and Proteins

Amyloid proteins are involved in many neurodegenerative disorders such as Alzheimer’s disease [Tau, Amyloid β (Aβ)], Parkinson’s disease [alpha-synuclein (αSyn)], and amyotrophic lateral sclerosis (TDP-43). Driven by the early observation of the presence of ordered structure within amyloid fibrils and the potential to develop inhibitors of their formation, a major goal of the amyloid field has been to elucidate the structure of the amyloid fold at atomic resolution. This has now been achieved for a wide variety of sequences using solid-state NMR, microcrystallography, X-ray fiber diffraction and cryo-electron microscopy. These studies, together with in silico methods able to predict aggregation-prone regions (APRs) in protein sequences, have provided a wealth of information about the ordered fibril cores that comprise the amyloid fold. Structural and kinetic analyses have also shown that amyloidogenic proteins often contain less well-ordered sequences outside of the amyloid core (termed here as flanking regions) that modulate function, toxicity and/or aggregation rates. These flanking regions, which often form a dynamically disordered “fuzzy coat” around the fibril core, have been shown to play key parts in the physiological roles of functional amyloids, including the binding of RNA and in phase separation. They are also the mediators of chaperone binding and membrane binding/disruption in toxic amyloid assemblies. Here, we review the role of flanking regions in different proteins spanning both functional amyloid and amyloid in disease, in the context of their role in aggregation, toxicity and cellular (dys)function. Understanding the properties of these regions could provide new opportunities to target disease-related aggregation without disturbing critical biological functions.

Mass spectrometry of urinary β2 microglobulin oligomer in patients with proteinuria

Urinary β2 microglobulin (β2-m) is a marker of renal tubule dysfunction; however, β2-m might become degraded under acidic conditions. To confirm the degradation and consequent deactivation of β2-m under acidic conditions, we used matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) to detect the levels and forms of β2-m in the urine samples of patients with high proteinuria (n = 21) and healthy subjects (n = 6). β2-m was purified in crude form using immunoprecipitation. A signal of 11.74 kDa, corresponding to the molecular weight of β2-m, was detected in all samples. In addition, several high-molecular-weight proteins were detected in a patient as integrals of the intensity at 11.74 kDa. These results indicate that posttranslational modifications of β2-m might be involved in the pathological process of proteinuria. Therefore, MS can be used for monitoring proteinuria and predicting the risk of progression.

Hypoalbuminemia: a price worth paying for improved dialytic removal of middle-molecular-weight uremic toxins?

Hemodiafiltration (HDF) increases the removal of middle-molecular-weight uremic toxins and may improve outcomes in patients with end-stage kidney disease (ESKD), but it requires complex equipment and comes with risks associated with infusion of large volumes of substitution solution. New high-flux hemodialysis membranes with improved diffusive permeability profiles do not have these limitations and offer an attractive alternative to HDF. However, both strategies are associated with increased albumin loss into the dialysate, raising concerns about the potential for decreased serum albumin concentrations that have been associated with poor outcomes in ESKD. Many factors can contribute to hypoalbuminemia in ESKD, including protein energy wasting, inflammation, volume expansion, renal loss and loss into the dialysate; of these factors, loss into the dialysate is not necessarily the most important. Furthermore, recent studies suggest that mild hypoalbuminemia per se is not an independent predictor of increased mortality in dialysis patients, but in combination with inflammation it is a poor prognostic sign. Thus, whether hypoalbuminemia predisposes to increased morbidity and mortality may depend on the presence or absence of inflammation. In this review we summarize recent findings on the role of dialysate losses in hypoalbuminemia and the importance of concomitant inflammation on outcomes in patients with ESKD. Based on these findings, we discuss whether hypoalbuminemia may be a price worth paying for increased dialytic removal of middle-molecular-weight uremic toxins.

Measuring the Energy Barrier of the Structural Change That Initiates Amyloid Formation

Obtaining kinetic and thermodynamic information for protein amyloid formation can yield new insight into the mechanistic details of this biomedically important process. The kinetics of the structural change that initiates the amyloid pathway, however, has been challenging to access for any amyloid protein system. Here, using the protein β-2-microglobulin (β2m) as a model, we measure the kinetics and energy barrier associated with an initial amyloidogenic structural change. Using covalent labeling and mass spectrometry, we measure the decrease in solvent accessibility of one of β2m's Trp residues, which is buried during the initial structural change, as a way to probe the kinetics of this structural change at different temperatures and under different amyloid forming conditions. Our results provide the first-ever measure of the activation barrier for a structural change that initiates the amyloid formation pathway. The results also yield new mechanistic insight into β2m's amyloidogenic structural change, especially the role of Pro32 isomerization in this reaction.

Epigallocatechin-3-gallate Inhibits Cu(II)-Induced β-2-Microglobulin Amyloid Formation by Binding to the Edge of Its β-Sheets

Epigallocatechin-3-gallate (EGCG) is a catechin found in green tea that can inhibit the amyloid formation of a wide variety of proteins. EGCG's ability to prevent or redirect the amyloid formation of so many proteins may reflect a common mechanism of action, and thus, greater molecular-level insight into how it exerts its effect could have broad implications. Here, we investigate the molecular details of EGCG's inhibition of the protein β-2-microglobulin (β2m), which forms amyloids in patients undergoing long-term dialysis treatment. Using size-exclusion chromatography and a collection of mass spectrometry-based techniques, we find that EGCG prevents Cu(II)-induced β2m amyloid formation by diverting the normal progression of preamyloid oligomers toward the formation of spherical, redissolvable aggregates. EGCG exerts its effect by binding with a micromolar affinity (K_d ≈ 5 μM) to the β2m monomer on the edge of two β-sheets near the N-terminus. This interaction destabilizes the preamyloid dimer and prevents the formation of a tetramer species previously shown to be essential for Cu(II)-induced β2m amyloid formation. EGCG's binding at the edge of the β-sheets in β2m is consistent with a previous hypothesis that EGCG generally prevents amyloid formation by binding cross-β-sheet aggregation intermediates.

The Effect of Dialysate Glucose on Phagocyte Superoxide Generation in Capd Patients

Objective

In the present study, we investigated the influence of dialysate glucose on superoxide (02) generation by peripheral and peritoneal phagocytes in continuous ambulatory peritoneal dialysis (CAPD) patients.

Design

Peripheral polymorphonuclear leukocytes (PMNL) and mononuclear leukocytes (MNL), and peritoneal cells were isolated from peripheral blood and peritoneal effluents, respectively, and their oxidative metabolism was assessed by measuring 02 generation after stimulation with a soluble stimulant [phorbol myristate acetate (PMA), 1 mg/mL, Sigma Chemical, St. Louis, MO, U.S.A.] using the chemiluminescence method. Dialysate glucose effect on 02 generation was also studied in vitro by exposing peripheral PMNL and MNL from healthy controls to peritoneal dialysis fluid (PDF) containing glucose or amino acids at a neutral pH for different time periods.

Results

The amount of 02 generation by both peripheral and peritoneal phagocytes in CAPD patients was significantly higher than that in the control, and the response was greater in patients who were dialyzed with high glucose dialysate than those using low glucose dialysate. In an in vitro study, all incubated cells, except the control, showed suppression of 02 generation in the early dwell time (2 hr), and subsequently showed increased responses (peaking at 6 hr), although lower in degree than those observed in vivo. In contrast, amino acid-based PDF exhibited no such effect on 02 generation at identical pH with similar or lower osmolality. Furthermore, the respective increased or decreased oxidative responses with the increased or decreased PDF glucose concentrations in the same patient confirmed the positive effect of PDF glucose on phagocyte 02 generation.

Conclusion

It is suggested that increased 02 generation by peritoneal and circulating phagocytes in CAPD patients is at least partly due to the enhancement of hexose monophosphate shunt activity by increasing glucose metabolism in phagocytes, and the increased 02 generation might be involved in long-term complications of CAPD. Therefore, a suitable alternative osmotic agent is needed to provide a more physiological environment to minimize the adverse effects of glucose on cell functions.

Beta-2 Microglobulin in Peritoneal Dialysis Patients: Serum Levels and Peritoneal Clearances

Beta-2 microglobulin (B2M) is amyloidogenic in long-term hemodialysis patients, with amyloid deposition manifesting as lytic bone lesions, carpal tunnel syndrome, destructive arthropathies, tenosynovitis, and pathologic fractures. To study the behavior of this protein in the peritoneal dialysis population, serum levels of B2M from 14 chronic peritoneal dialysis (CPD) patients (4IPD, 10 CAPD) were compared to those of 15 chronic hemodialysis patients, and peritoneal clearances were measured in 9 CAPD patients. Standard cuprophan dialyzers were used for hemodialysis.

Serum B2M levels were significantly lower in the peritoneal dialysis group (mean ± SD 73.2 ± 20.9 mg/L) than in the hemodialysis group (100.3 ± 24.7 mg/L, p < .004). When CAPD patients alone were compared to the hemodialysis patients, lower serum B2M levels were again apparent, with mean 68.7 ± 16.4 mg/L (p ≤ .002). Mean serum B2M in IPD patients (84.6 ± 28.9 mg/L) did not differ statistically from either the CAPD or the hemodialysis group.

Peritoneal clearance of B2M, urea nitrogen, and creatinine over a 6 h exchange were obtained in 9 CAPD patients without peritonitis. Mean clearance (±SD) of B2M was 0.9 ± 0.4 ml/min/1.73 m2, urea nitrogen 5.3 ± 0.3 ml/min/1.73 m2, and creatinine 4.2 ± 0.8 ml/min/1.73 m2. Mean loss of B2M via the peritoneal cavity was 19.9 ± 6.6 mg/2 L-exchange/1.73 m2 (range 7.7 to 26.2 mg/2 L-exchange/1.73 m2).

Decreased serum B2M in peritoneal dialysis patients is consistent with increased clearance by the peritoneal membrane versus standard cellulosic hemodialysis membranes. Whether use of CPD rather than hemodialysis can prevent or even treat dialysis-associated amyloidosis (AB2M) remains speculative.

Beta-2 Microglobulin in Patients on Peritoneal Dialysis and Hemodialysis

Serum β2 microglobulin ( β2μ) levels were determined in 62 patients on chronic dialysis, divided according to the type of dialysis-cuprophane hemodialysis, chronic ambulatory peritoneal dialysis (CAPD), or CAPD staned after 76 ± 47 months on cuprophane hemodialysis-and to residual urine output greater than 400 mLlday or less than 10 mL/day. In addition, for patients on CAPD, peritoneal excretion, peritoneal clearance, and urinary excretion of the protein were determined. In anuric patients serum β2μ levels were significantly higher in HD than in CAPD. In patients with residual urine output, serum concentrations of the microprotein were similar in HD and in CAPD. Significant differences were observed in β2μ serum levels and peritoneal clearances in patients switched to CAPD from hemodialysis as compared to those staning with CAPD. Peritoneal clearances of the microprotein was slightly and non-significantly greater in patients with urine output than in anuric patients.

Beta-2 Microglobulin Amyloidosis in a Patient on LongTerm Continuous Ambulatory Peritoneal Dialysis (CAPD)

A 61-year-old female patient with ESRD due to analge sic nephropathy and treated solely with CAPD for 81 months is described. During this period she developed peritonitis 4 times. She eventually died of acute pulmonary edema due to coronary heart disease. The autopsy showed an analgesic nephropathy with contracted kidneys, diffuse peritoneal fibrosis, and articular and periarticular amyloidosis of AB2m-type. This form of amyloidosis is well known after hemodialysis; it was, however, not observed heretofore after sole long-term CAPD.

Beta-2 Microglobulin Removal during Continuous Ambulatory Peritoneal Dialysis (CAPD)

Beta-2 microglobulin (B2M) handling in continuous ambulatory peritoneal dialysis (CAPD) was characterized in acute and chronic clinical studies. Average clearance rate was 0.7 mLlmin and mean mass transfer coefficient, KoA, was calculated to be 0.95 cm2lmin; these values are in the range expected from extrapolation of published data for other large solutes. In chronic studies with both anuric and oliguric populations, CAPD was shown to be much more effective than conventional hemodialysis in removing B2M and, in fact, CAPD removal rates were equivalent to those reported for high flux dialysis therapies. However, this greater extraction was not associated with any clinically significant reduction in circulating plasma concentrations. These trends remained valid in both the anuric and oliguric subsets of the study population.

Collagen I Weakly Interacts with the β-Sheets of β2-Microglobulin and Enhances Conformational Exchange To Induce Amyloid Formation

Amyloidogenesis is significant in both protein function and pathology. Amyloid formation of folded, globular proteins is commonly initiated by partial or complete unfolding. However, how this unfolding event is triggered for proteins that are otherwise stable in their native environments is not well understood. The accumulation of the immunoglobulin protein β₂-microglobulin (β₂m) into amyloid plaques in the joints of long-term hemodialysis patients is the hallmark of dialysis-related amyloidosis (DRA). While β₂m does not form amyloid unassisted near neutral pH in vitro, the localization of β₂m deposits to joint spaces suggests a role for the local extracellular matrix (ECM) proteins, specifically collagens, in promoting amyloid formation. Indeed, collagen and other ECM components have been observed to facilitate β₂m amyloid formation, but the large size and anisotropy of the complex, combined with the low affinity of these interactions, have limited atomic-level elucidation of the amyloid-promoting mechanism(s) by these molecules. Using solution NMR approaches that uniquely probe weak interactions in large molecular weight complexes, we are able to map the binding interfaces on β₂m for collagen I and detect collagen I-induced μs–ms time-scale dynamics in the β₂m backbone. By combining solution NMR relaxation methods and ¹⁵N-dark-state exchange saturation transfer experiments, we propose a model in which weak, multimodal collagen I−β₂m interactions promote exchange with a minor population of amyloid-competent species to induce fibrillogenesis. The results portray the intimate role of the environment in switching an innocuous protein into an amyloid-competent state, rationalizing the localization of amyloid deposits in DRA.

Structural Heterogeneity in the Preamyloid Oligomers of β-2-Microglobulin

In dialysis patients, the protein β2-microglobulin (β2m) forms amyloid fibrils in a condition known as dialysis-related amyloidosis. To understand the early stages of the amyloid assembly process, we have used native electrospray ionization (ESI) together with ion mobility mass spectrometry (IM-MS) to study soluble preamyloid oligomers. ESI-IM-MS reveals the presence of multiple conformers for the dimer, tetramer, and hexamer that precede the Cu(II)-induced amyloid assembly process, results which are distinct from β2m oligomers formed at low pH. Experimental and computational results indicate that the predominant dimer is a Cu(II)-bound structure with an antiparallel side-by-side configuration. In contrast, tetramers exist in solution in both Cu(II)-bound and Cu(II)-free forms. Selective depletion of Cu(II)-bound species results in two primary conformers-one that is compact and another that is more expanded. Molecular modeling and molecular dynamics simulations identify models for these two tetrameric conformers with unique interactions and interfaces that enthalpically compensate for the loss of Cu(II). Unlike with other amyloid systems in which conformational heterogeneity is often associated with different amyloid morphologies or off-pathway events, conformational heterogeneity in the tetramer seems to be a necessary aspect of Cu(II)-induced amyloid formation by β2m. Moreover, the Cu(II)-free models represent a new advance in our understanding of Cu(II) release in Cu(II)-induced amyloid formation, laying a foundation for further mechanistic studies as well as development of new inhibition strategies.

Membrane innovation: closer to native kidneys

Modern methods in analytical biochemistry have established that uraemia is associated with the retention of proteins, both in their native state and post-translationally modified, over a wide range of molecular weights up to 60 kDa. Evidence is accumulating that these higher molecular weight retention solutes are important uraemic toxins, and therapies such as online haemodiafiltration (HDF), which enhance their removal, are associated with improved outcomes. However, HDF has limitations regarding cost, clinical implementation and the need for an external source of sterile substitution solution to maintain fluid balance. New membranes that have a solute removal profile more closely approaching that of the glomerular filtration barrier when used for conventional haemodialysis, while at the same time not allowing the passage of clinically significant amounts of beneficial proteins, are needed to address these limitations. Tighter control of the molecular characteristics of the polymers used for membrane fabrication, along with the introduction of additives and improvements in the manufacturing process, has led to membranes with a tighter pore size distribution that allows the use of an increased absolute pore size without leaking substantial amounts of albumin. At the same time, the wall thickness and internal diameter of membrane fibres have been decreased, enhancing convective transport within the dialyser without the need for an external source of substitution solution. These new expanded range membranes provide a solute removal profile more like that of the native kidney than currently available membranes when used in conventional haemodialysis.

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.

Biochemical evidence of cell starvation in diabetic hemodialysis patients

Recently, the ratio of patients with diabetes mellitus (DM) among hemodialysis (HD) patients has increased to become the largest sub-population. Their prognoses are significantly worse than those of patients without diabetes (non-DM). In the present study, 10 DM patients who did not take meals and 10 non-DM patients who took meals during HD sessions were investigated. The time courses of the change in plasma levels of metabolites during HD were determined. DM patients exhibited decreased plasma levels of lactate, pyruvate and alanine and dramatically increased levels of ketone bodies. At the end of HD, the plasma levels of lactate, pyruvate, alanine and ketone body were 0.46 ± 0.07, 0.026 ± 0.01, 0.12 ± 0.04 and 0.26 ± 0.04 mM (mean ± standard error), respectively. The profile was ‘hypolactatemia and hyperketonemia’, indicating non-homeostasis. Glycolysis and tricarboxylic acid cycle were suppressed, and the oxidation of fatty acid was accelerated, indicating starvation, even though high amounts of glucose (150 mg/dl) in dialysate were supplied continuously to the bloodstream. In contrast, the plasma levels of lactate, pyruvate, and alanine in the non-DM patients were increased, with the levels of ketone body remaining low during HD to maintain homeostasis, indicating accelerated glycolysis. Furthermore, their plasma levels of insulin increased from 8.1 ± 1.4 to 19.8 ± 3.4 μU/ml, which indicated endogenous secretion stimulated by glucose in dialysate and meal intake. In contrast, in the DM patients, the levels decreased from 19.2 ± 3.4 to 5.5 ± 1.1 μU/ml. This value was the lower limit of the normal range. The depletion of the insulin through extracorporeal circulation may inhibit the transportation of glucose from the blood into the muscles, with the consequence of cell starvation. Such cell starvation along with lipolysis every two days may accelerate proteolysis and affect the prognosis of DM patients.

Beta-2-Microglobulin Adsorption and Release in-Vitro: Influence of Membrane Material, Osmolality and Heparin

We studied the effects of cuprammonium rayon (CR), polyacrylonitrile (PAN), polysulfone (PS), changes in osmolality, and heparin dosage on beta-2-microglobulin (b2M) handling in an in-vitro model that excluded convective transport and minimized diffusive transport. Both PAN and PS exhibited high adsorption capacity for b2M. Osmolality changes had no effect on b2M adsorption or release. CR membrane adsorption was minimal but increased slightly when higher heparin doses were used. In experiments with CR and high heparin doses (4 U/ml) b2M release occurred during the first 15 minutes of in-vitro dialysis, but this increase was inhibited by removing the leukocytes from the blood, indicating that b2M is released from leukocytes.

Optimal Reuse of Cuprammonium Rayon Hollow-Fibre Dialyzers

The optimal number of times a hollow-fibre cuprammonium rayon (Terumo TAF) dialyzer could be used was determined in twelve patients. Each dialyzer was reused six times. There was a highly significant decrease in neutrophil count (46.9± 6.9%, mean ± s.e.m.), platelet count (9.6± 3.8%, mean ± s.e.m.) and serum C3 concentration (10.25± 2.4%, mean ± s.e.m.) 30 minutes after the commencement of dialysis. The intensity of these changes decreased with the reuse of the dialyzers up to the fourth time. There was a small, but significant increase in serum β2-microglobulin concentrations after a 6-hour dialysis. The difference between pre- and post-dialysis serum β2-microglobulin concentrations appeared to be greater during the 5th and the 6th use (18.8% and 20.5% vs 5.0% during the first use). The clearance rates of urea and creatinine showed a trend to decrease from the fourth use onwards and the ultrafiltration coefficient was significantly reduced from the third use (8.5± 4.0 mean ± s.e.m.) onwards so that at the sixth use it was only 75.8± 4.8%, (mean ± s.e.m.) of the value at the first use. It is concluded that the optimal number of uses a cuprammonium rayon hollow-fibre dialyzer can be put to is four.

Factors Affecting β2-Microglobulin Plasma Concentration during Hemodialysis

We studied ten patients on hemodialysis (HD) treated in sequence with cuprophan (CU), ethylenevinylalcohol (EVAL), polyacrylonitrile (A-69) and polysulphone (PSP) membrane dialyzers. ß2-microglobulin (ß2m) was measured by radioimmunoassay in plasma and dialysate samples. Plasma concentrations were corrected for changes in extracellular volume (ECV). We also studied adsorption in vitro by incubating the above membranes with I-125-labelled ß2m. There were no changes in ß2m plasma concentration after HD with CU dialyzers, but a significant decrease was observed with the other membranes tested. Filtration of ß2m across the dialyzer was absent with CU and minimal with EVAL. However, large amounts were recovered from dialysate with the high-permeability dialyzers, AN-69 and PSP. In vitro studies showed that maximal adsorption capacity was obtained with AN-69 (73%) compared to 9% with CU, 4% with EVAL and 7% with PSP. In summary, ß2m clearance with PSP is achieved through greater removal of this protein by mass transport across the membrane. The mechanism by which ß2m is removed from blood during AN-69 dialysis seems to include both adsorption to and filtration by the membrane itself.

Red Blood Cell Function and β2 Microglobulin Kinetics during Cuprophan Hemodialysis: A Hypothesis

The exact role of β2microglobulin (β2m) in dialysis amyloidosis is yet not known. Local release of β2m from the endothelial cells of the lung and other tissues as a consequence of acute-phase activation due to the contact of blood with membranes has still to be considered a possible pathogenetic factor in this syndrome. β2m kinetics and decrease of glutathione content in RBC were studied in 41 chronic hemodialysis patients during cuprophan dialysis. The latter test reflects the RBC scavenger function for free oxygen radicals. Only 30% of patients showed a clinically significant increase in β2m. The change in β2m in this group between the start and 180 minutes, corrected for plasma volume, was 23.1±3.6% and the change in gluthathione content between the start and 15 minutes was 4.5±3.4%. In these patients there was a significant correlation between β2m production and decrease of gluthathione (R= -0.61, p= 0.0299). It is possible that the production of free oxygen radicals during bioincompatible dialysis leads to cellular toxicity with release ofβ2m which may be prevented to some extent by the scavenger role of RBC.

Using Ultrapure Water in Hemodialysis Delays Carpal Tunnel Syndrome

Since 1977, our patients have undergone chronic HD with ultra-pure dialysate (UPD), defined as having endotoxin levels below 0.008 ng/ml and less than 1 bacteria/ml of dialysate. We evaluated the incidence of carpal tunnel syndrome (CTS) in three groups of patients. Group I (GI), 84 patients, dialysed for 6.1 ± 3.2 years (mean ± SD) with UPD only; Group II (GII), 39 patients, first dialysed for 3.7 ± 2.3 years with non-UPD and afterwards for 8.4 ± 2.1 years with UPD; Group III (G III), 103 patients treated for 6 ± 5.9 years exclusively with non-UPD. All patients were dialysed with cuprophan or cellulose acetate membranes.

Results, expressed by Kaplan-Meier actuarial survival curves as the percent of patients without CTS, show that CTS occurred significantly less in GI than in GIII. This may be due to less stimulation of monocytes resulting from the absence of bacteria, endotoxins and pyrogens in the dialysate which would reduce the stimulation of cytokines release, interleukin 1 and 6, and tumor necrosis factor, known to stimulate β2 microglobulin synthesis.

Serum Alpha-1-Antitrypsin in Hemodialysis Patients with Dialysis Arthropathy

Dialysis arthropathy is the most prominent dialysis-related amyloidosis feature. Alpha-1-antitrypsin (alpha-1-proteinase inhibitor) is the major circulating antiprotease. Twenty-three otherwise uncomplicated hemodialysis patients with well-documented dialysis arthropathy had a significantly (p < 0.05) lower serum mean concentration, 1,960 ± 410.4 mg/I of alpha-1-antitrypsin than 47 patients with no joint symptoms who had a mean concentration of 2,256.6 ± 424.5 mg/I. Decreased levels of the substance were detected in 13 (56.5%) of the 23 patients with dialysis arthropathy and in 13 (27.6%) of those 47 with no joint symptoms, the incidence in the former group being significantly (p < 0.05) higher than in the latter. In the dialysis arthropathy group, serum alpha-1-antitrypsin levels correlated inversely (r = −0.54, p < 0.01) with the dialysis duration and directly (r = 0.413, p < 0.05) with the corresponding beta-2-microglobulin determinations. We speculate that reduced antiprotease activity may play a role in amyloidogenesis in the setting of long-term hemodialysis.

Amyloid Bone Disease and Highly Permeable Synthetic Membranes

The effect of different highly permeable membranes on amyloid bone disease (ABD) was retrospectively evaluated in patients on renal replacement therapy (RRT) in our Unit with a dialytic age of more than 4 years. A group of 36 patients (age 60 ± 12 years) after a variable period (28 ± 29 months) on hemodialysis with cuprophane membrane (CU-HD), were moved to HDF with a reinfusate volume of 22 ± 1 1/session, for a period of 65 ± 26 months using the following membranes: AN69 1.6 m2, PAN 1.8, PMMA 2.1, PS 1.3, polyamide (PA) 1.3 and 1.6. Bone x-rays of wrists, hips and shoulders were taken annually and the presence of ABD was evaluated according to generally accepted criteria. ABD occurred in 4 patients after a period of 73 ± 30 months on CU-HD only; it developed in 4/7 patients an AN69, in 4/6 on PAN, in 3/5 on PMMA, in 3/5 on PS; no patient of the 13 on PA developed ABD. Comparing patients on PA with those on other synthetic membranes, no significant difference was found in dialysis time (73 ± 19 vs 83 ± 28 months) as well as in age (59 ± 13 vs 61 ± 11 years) at ABD on set, when present. These data strongly encourage prospective studies enrolling more patients for a longer period of observation in order to evaluate possible differences on ABD development among various synthetic membranes.

Predilution Hemofiltration: A New Technology Applied to an Old Therapy

Postdilution hemofiltration (HF) as practised during the 80's is today associated with limitations of a medical, practical and economical nature. High blood flow rates are required to generate sufficient ultrafiltrate in order to achieve a clearance of small solutes comparable to hemodialysis within a reasonable time. High hematocrit and large body weight lead to extended treatment times. IV-quality solution is required in large volumes. This makes the use of bicarbonate difficult and has placed HF among the most expensive renal replacement therapies. These limitations can be resolved by performing HF in a predilution mode using an on-line prepared infusion solution. Diluting the blood before filtration increases the filtration fraction and the clearance of all solutes which are sieved by the membrane. Comparing pre- to postdilution at similar blood flow rates, the clearance may increase by 50% but three times as much infusion solution is required. To make predilution economically viable, the on-line preparation of the infusion solution is necessary, and this facilitates the use of bicarbonate. Compared to other dialysis therapies this new form of HF offers unequalled solute removal over a large molecular range.

Using Covalent Labeling and Mass Spectrometry To Study Protein Binding Sites of Amyloid Inhibiting Molecules

Amyloid aggregates are associated with several debilitating diseases, and there are numerous efforts to develop small molecule treatments against these diseases. One challenge associated with these efforts is determining protein binding site information for potential therapeutics because amyloid-forming proteins rapidly form oligomers and aggregates, making traditional protein structural analysis techniques challenging. Using β-2-microglobulin (β2m) as a model amyloid-forming protein along with two recently identified small molecule amyloid inhibitors (i.e., rifamycin SV and doxycycline), we demonstrate that covalent labeling and mass spectrometry (MS) can be used to map small-molecule binding sites for a rapidly aggregating protein. Specifically, three different covalent labeling reagents, namely diethylpyrocarbonate, 2,3-butanedione, and the reagent pair EDC/GEE, are used together to pinpoint the binding sites of rifamycin SV, doxycycline, and another molecule, suramin, which binds but does not inhibit Cu(II)-induced β2m amyloid formation. The labeling results reveal binding sites that are consistent with the known effects of these molecules on β2m amyloid formation and are in general agreement with molecular docking results. We expect that this combined covalent labeling approach will be applicable to other protein/small molecule systems that are difficult to study by traditional means.

Severe Destructive Tendinopathy in the Wrist Due to Dialysis-Related Amyloidosis

Dialysis-related amyloidosis (DRA) is a specific subtype of amyloidosis with several clinical presentations. Herein we report a case of severe destructive tendinopathy around the wrist associated with long-standing hemodialysis (HD). A 63-year-old female patient who had been on regular HD for 23 years suffered from symptoms of pain and a palpable mass around the wrist. Magnetic resonance imaging showed an ill-defined soft tissue mass around the extensor tendons that partially invaded the wrist joint. We performed surgical excision and tenolysis for the mass. The operative finding revealed an ill-defined yellowish soft tissue mass extensively invading the extensor tendons and wrist joint. The tendon fibers were severely fibrillated and showed impending rupture due to the infiltrated mass. Histological examination showed DRA. Amyloidosis-induced tendinopathy should be considered when a patient on long-term HD complains of an unusual mass-like lesion in the extremity and/or nonspecific joint stiffness. Early surgical intervention improves patient's disability and decreases the risk of spontaneous tendon rupture.

Small molecule-mediated inhibition of β-2-microglobulin-based amyloid fibril formation

In dialysis patients, β-2 microglobulin (β2m) can aggregate and eventually form amyloid fibrils in a condition known as dialysis-related amyloidosis, which deleteriously affects joint and bone function. Recently, several small molecules have been identified as potential inhibitors of β2m amyloid formation in vitro Here we investigated whether these molecules are more broadly applicable inhibitors of β2m amyloid formation by studying their effect on Cu(II)-induced β2m amyloid formation. Using a variety of biophysical techniques, we also examined their inhibitory mechanisms. We found that two molecules, doxycycline and rifamycin SV, can inhibit β2m amyloid formation in vitro by causing the formation of amorphous, redissolvable aggregates. Rather than interfering with β2m amyloid formation at the monomer stage, we found that doxycycline and rifamycin SV exert their effect by binding to oligomeric species both in solution and in gas phase. Their binding results in a diversion of the expected Cu(II)-induced progression of oligomers toward a heterogeneous collection of oligomers, including trimers and pentamers, that ultimately matures into amorphous aggregates. Using ion mobility mass spectrometry, we show that both inhibitors promote the compaction of the initially formed β2m dimer, which causes the formation of other off-pathway and amyloid-incompetent oligomers that are isomeric with amyloid-competent oligomers in some cases. Overall, our results suggest that doxycycline and rifamycin are general inhibitors of Cu(II)-induced β2m amyloid formation. Interestingly, the putative mechanism of their activity is different depending on how amyloid formation is initiated with β2m, which underscores the complexity of how these structures assemble in vitro.

Increased β-Sheet Dynamics and D-E Loop Repositioning Are Necessary for Cu(II)-Induced Amyloid Formation by β-2-Microglobulin

β-2-Microglobulin (β2m) forms amyloid fibrils in the joints of patients undergoing dialysis treatment as a result of kidney failure. One of the ways in which β2m can be induced to form amyloid fibrils in vitro is via incubation with stoichiometric amounts of Cu(II). To better understand the structural changes caused by Cu(II) binding that allow β2m to form amyloid fibrils, we compared the effect of Ni(II) and Zn(II) binding, which are two similarly sized divalent metal ions that do not induce β2m amyloid formation. Using hydrogen/deuterium exchange mass spectrometry (HDX/MS) and covalent labeling MS, we find that Ni(II) has little effect on β2m structure, despite binding in the same region of the protein as Cu(II). This observation indicates that subtle differences in the organization of residues around Cu(II) cause distant changes that are necessary for oligomerization and eventual amyloid formation. One key difference that we find is that only Cu(II), not Ni(II) or Zn(II), is able to cause the cis-trans isomerization of Pro32 that is an important conformational switch that initiates β2m amyloid formation. By comparing HDX/MS data from the three metal-β2m complexes, we also discover that increased dynamics in the β-sheet formed by the A, B, D, and E β strands of the protein and repositioning of residues in the D-E loop are necessary aspects of β2m forming an amyloid-competent dimer. Altogether, our results reveal new structural insights into the unique effect of Cu(II) in the metal-induced amyloid formation of β2m.

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.

Comparison of Long-term Complications in Patients on Haemodialysis and Peritoneal Dialysis Longer than 10 Years

INTRODUCTION

Depending on developments in dialysis techniques and new treatment strategies for comorbid diseases, life expectancy has increased. As a result, dialysis related long term complications could be seen more frequently. We investigated and compared long term complications of the Haemodialysis (HD) and Peritoneal Dialysis (PD) in patients with history if either mode at least 10years.

MATERIALS AND METHODS

A 13HD & 16PD patients were included to the study. Basic demographic parameters and prevalence of cardiovascular diseases (CVD), uraemic peripheral neuropathy (PNP), parathyroid adenoma, parathyroidectomy and acquired cystic disease (ACD) were assessed.

RESULTS

HD patients were older than PD patients (p=0.035) and duration of dialysis was longer in HD patients (p=0.001). CVD was present in 18 patients (9 HD, 9 PD). There was no difference in presence of CVD between HD and PD patients (p=0.455). Valvular diseases (n=15), diastolic dysfunction (n=8), left ventricular hypertrophy (n=5), ischemic heart disease (n=3) and congestive heart failure (n=1) were investigated. Uraemic peripheral neuropathy was observed in 14 of the patients (8 HD, 6 PD patients). Eight patients had mixed type sensory motor neuropathy and 3 patients had mixed type sensorial neuropathy, 2 patients had demyelinating PNP, 1 patient had axonal PNP and 3 of them had CTS related to peripheral neuropathy. Parathyroid adenoma was detected in 4 patients (2 HD, 2 PD) and 3 patients (1 HD, 2 PD) had history of parathyroidectomy. Serum phosphate and iPTH levels were higher in HD patients (p=0.003, p=0.04, respectively). ACD was detected in 14 patients (7 HD, 7 PD). There was no difference between PD and HD patients (p=0.75).

CONCLUSION

HD patients were older than PD patients and had longer duration of dialysis. The prevalence of long term complications was similar in HD and PD modalities. CVD especially valvular diseases were common complication in both modalities.