Purification and complete amino acid sequence of novel beta 2-microglobulin

Implications of Metal Binding and Asparagine Deamidation for Amyloid Formation

Increasing evidence suggests that amyloid formation, i.e., self-assembly of proteins and the resulting conformational changes, is linked with the pathogenesis of various neurodegenerative disorders such as Alzheimer’s disease, prion diseases, and Lewy body diseases. Among the factors that accelerate or inhibit oligomerization, we focus here on two non-genetic and common characteristics of many amyloidogenic proteins: metal binding and asparagine deamidation. Both reflect the aging process and occur in most amyloidogenic proteins. All of the amyloidogenic proteins, such as Alzheimer’s β-amyloid protein, prion protein, and α-synuclein, are metal-binding proteins and are involved in the regulation of metal homeostasis. It is widely accepted that these proteins are susceptible to non-enzymatic posttranslational modifications, and many asparagine residues of these proteins are deamidated. Moreover, these two factors can combine because asparagine residues can bind metals. We review the current understanding of these two common properties and their implications in the pathogenesis of these neurodegenerative diseases.

Uremic Toxicity: Present State of the Art

The uremic syndrome is a complex mixture of organ dysfunctions, which is attributed to the retention of a myriad of compounds that under normal condition are excreted by the healthy kidneys (uremic toxins). In the area of identification and characterization of uremic toxins and in the knowledge of their pathophysiologic importance, major steps forward have been made during recent years. The present article is a review of several of these steps, especially in the area of information about the compounds that could play a role in the development of cardiovascular complications. It is written by those members of the Uremic Toxins Group, which has been created by the European Society for Artificial Organs (ESAO). Each of the 16 authors has written a state of the art in his/her major area of interest.

Role of Site-Specific Asparagine Deamidation in Islet Amyloid Polypeptide Amyloidogenesis: Key Contributions of Residues 14 and 21

Deamidation of an asparagine residue is a spontaneous non-enzymatic post-translational modification that results in the conversion of asparagine into a mixture of aspartic acid and isoaspartic acid. This chemical conversion modulates protein conformation and physicochemical properties, which could lead to protein misfolding and aggregation. In this study, we investigated the effects of site-specific Asn deamidation on the amyloidogenicity of the aggregation-prone peptide islet amyloid polypeptide (IAPP). IAPP is a 37-residue peptidic hormone whose deposition as insoluble amyloid fibrils is closely associated with type 2 diabetes. Asn residues were successively substituted with an Asp or isoAsp, and amyloid formation was evaluated by a thioflavin T fluorescence assay, circular dichroism spectroscopy, atomic force microscopy, and transmission electron microscopy. Whereas deamidation at position 21 inhibited IAPP conformational conversion and amyloid formation, the N14D mutation accelerated self-assembly and led to the formation of long and thick amyloid fibrils. In contrast, IAPP was somewhat tolerant to the successive deamidation of Asn residues 22, 31, and 35. Interestingly, a small molar ratio of IAPP deamidated at position 14 promoted the formation of nucleating species and the elongation from unmodified IAPP. Besides, using the rat pancreatic β cell line INS-1E, we observed that site-specific deamidation did not significantly alter IAPP-induced toxicity. These data indicate that Asn deamidation can modulate IAPP amyloid formation and fibril morphology and that the site of modification plays a critical role. Above all, this study reinforces the notion that IAPP amyloidogenesis is governed by precise intermolecular interactions involving specific Asn side chains.

Evaluation of capillary electrophoresis-mass spectrometry for the analysis of the conformational heterogeneity of intact proteins using beta2-microglobulin as model compound

In this work we explored the feasibility of different CE-ESI-MS set-ups for the analysis of conformational states of an intact protein. By using the same background electrolyte at quasi physiological conditions (50 mM ammonium bicarbonate, pH 7.4) a sequential optimization was carried out, initially by evaluating a sheath-liquid interface with both a single quadrupole (SQ) and a time-of-flight (TOF) mass spectrometer; then a sheathless interface coupled with high-resolution QTOF MS was considered. Beta₂-microglobulin has been taken as a model, as it is an amyloidogenic protein and its conformational changes are strictly connected to the onset of a disease. The separation of two conformers at dynamic equilibrium is achieved all the way down to the MS detection. Notably, the equilibrium ratio of the protein conformers is maintained in the electrospray source after CE separation. Strengths and weaknesses of each optimized set-up are emphasized and their feasibility in unfolding studies is evaluated. In particular, ESI-TOF MS can assign protein forms that differ by 1 Da only and sheathless interfacing is best suited to preserve protein structure integrity. This demonstrates the CE-ESI-MS performance in terms of separation, detection and characterization of conformational species that co-populate a protein solution.

Decoding the Structural Bases of D76N ß2-Microglobulin High Amyloidogenicity through Crystallography and Asn-Scan Mutagenesis

D76N is the first natural variant of human β-2 microglobulin (β2m) so far identified. Contrary to the wt protein, this mutant readily forms amyloid fibres in physiological conditions, leading to a systemic and severe amyloidosis. Although the Asp76Asn mutant has been extensively characterized, the molecular bases of its instability and aggregation propensity remain elusive. In this work all Asp residues of human β2m were individually substituted to Asn; D-to-N mutants (D34N, D38N, D53N, D59N, D96N and D98N) were characterised in terms of thermodynamic stability and aggregation propensity. Moreover, crystal structures of the D38N, D53N, D59N and D98N variants were solved at high-resolution (1.24–1.70 Å). Despite showing some significant variations in their thermal stabilities, none showed the dramatic drop in melting temperature (relative to the wt protein) as observed for the pathogenic mutant. Consistently, none of the variants here described displayed any increase in aggregation propensity under the experimental conditions tested. The crystal structures confirmed that D-to-N mutations are generally well tolerated, and lead only to minor reorganization of the side chains in close proximity of the mutated residue. D38N is the only exception, where backbone readjustments and a redistribution of the surface electrostatic charges are observed. Overall, our results suggest that neither removing negative charges at sites 34, 38, 53, 59, 96 and 98, nor the difference in β2m pI, are the cause of the aggressive phenotype observed in D76N. We propose that the dramatic effects of the D76N natural mutation must be linked to effects related to the crucial location of this residue within the β2m fold.

Quantitative analysis of deamidation and isomerization in β2-microglobulin by 18O labeling

Deamidation of asparagine residues in proteins via the formation of a 5-membered succinimide ring intermediate is a nonenzymatic intramolecular reaction and, in general, occurs most rapidly at an Asn-Gly sequence. A protein containing this sequence would, therefore, be susceptible to modification, and the result would produce a structural alteration in the molecule. An Asn would be replaced with an Asp, resulting in an increase in the overall negative charge on the molecule but also an isomerization to isoAsp. Despite the fact that such a structural replacement could affect the functional properties of a protein, estimating the susceptibility of the Asn-Gly sequence to deamidation/isomerization remains a difficult task. This is especially true for proteins that are subjected to enzymatic digestion during their characterization, since the above transformation could occur spontaneously during this treatment. To address this issue, we applied a stable-isotope (18)O-labeling method combined with nano-LC-MS/MS to examine the susceptibility of two Asn-Gly sites in β2-microglobulin (β2m) to the reaction. The method permits the reaction occurring in a protein to be distinguished from that during enzymatic treatment. When β2m was incubated for 60 days at 37 °C, deamidation at Asn17-Gly and Asn42-Gly with half-lives of 33 and 347 days occurred, respectively. Moreover, a comparison of the deamidated products to synthetic peptides revealed that 44% of the Asp17 and 96% of the Asp42 had been converted into isoAsp forms. Interestingly, such structurally altered β2m showed a specific affinity for divalent Cu(2+) ions, which is thought to be a candidate for initiating fibril formation.

β(2)-microglobulin amyloidosis

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

Understanding the complex mechanisms of β2-microglobulin amyloid assembly

Several protein misfolding diseases are associated with the conversion of native proteins into ordered protein aggregates known as amyloid. Studies of amyloid assemblies have indicated that non-native proteins are responsible for initiating aggregation in vitro and in vivo. Despite the importance of these species for understanding amyloid disease, the structural and dynamic features of amyloidogenic intermediates and the molecular details of how they aggregate remain elusive. This review focuses on recent advances in developing a molecular description of the folding and aggregation mechanisms of the human amyloidogenic protein β(2)-microglobulin under physiologically relevant conditions. In particular, the structural and dynamic properties of the non-native folding intermediate I(T) and its role in the initiation of fibrillation and the development of dialysis-related amyloidosis are discussed.

beta(2)-microglobulin: from physiology to amyloidosis

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

Role of conformational change in the C-terminus of beta2-microglobulin in dialysis-related amyloidosis

BACKGROUND

Beta(2)-microglobulin (beta(2)m) has been identified as the precursor protein of dialysis-related amyloidosis (DRA), which is a serious complication for haemodialysis (HD) patients. However, mechanisms underlying beta(2)m amyloid fibril formation remains to be elucidated. We previously demonstrated, in amyloid deposits from HD patients, a conformational isoform of beta(2)m with an unfolded C-terminus. However, no direct experiments have previously been performed to address whether unfolded beta(2)m in the C-terminus may be prone to form amyloid fibrils.

METHODS

To evaluate roles of C-terminal amino acids in beta(2)m-induced amyloid formation, we generated six types of recombinant beta(2)m with amino acid substitutions in the C-terminal region. To investigate their conformational change and amyloidogenicity, we measured circular dichroism spectra, the fluorescence intensity of tryptophan and thioflavin-T (ThT) of the recombinant beta(2)m. To analyse morphological change of beta(2)m, we performed electron microscopy (EM) on the samples with elevated ThT fluorescence intensity. We used ultrasonication to enhance beta(2)m destabilization of the protein.

RESULTS

Beta(2)M Trp95Leu and Arg97Ala showed conformational changes and increased their amyloidgenicity compared with beta(2)m wild-type (WT). With ultrasonication, beta(2)m Trp95Leu and Arg97Ala generated more amyloid fibrils than did beta(2)m WT even in physiological solution. EM showed that beta(2)m formed amorphous debris containing typical amyloid fibrils at 24 hours, when ThT fluorescence intensity was three-fold lower than that at six hours.

CONCLUSIONS

Conformational changes in the C-terminus of beta(2)m may play an important role in DRA and that ultrasonication is useful for analysis of beta(2)m amyloidogenesis.

Gastrointestinal beta2microglobulin amyloidosis in hemodialysis patients: biochemical analysis of amyloid proteins in small formalin-fixed paraffin-embedded tissue specimens

We present here a first report on the biochemical analysis of intestinal amyloid deposits found in two cases of hemodialysis-related amyloidosis. A new microtechnique was applied for extraction and immunochemical/chemical characterization of amyloid proteins in small amounts of fixed tissue, thus allowing precise identification of beta2microglobulin amyloid (Abeta2M) in both cases studied. The molecular mass of the identified amyloid beta2M was close to that of intact beta2M (12 kDa), with no evidence of the products of proteolytic fragmentation of these molecules. The isoelectrofocusing of the purified Abeta2M demonstrated a shift to more acidic pI as compared to the normal beta2M analyzed under the same experimental conditions. The obtained data suggest that the intestinal amyloid deposits in dialysis-related amyloidosis contain disease-specific beta2M isoforms, which could play a role in the pathogenesis of amyloid disease. The new methodology used might be useful in obtaining precise diagnosis of amyloidosis that is necessary for appropriate therapy, and also provide new important information on the chemical structure of amyloid proteins.

Proteomics of β2-microglobulin amyloid fibrils

Knowledge on the chemical structure of beta2-microglobulin in natural amyloid fibrils is quite limited because of the difficulty in obtaining tissue samples suitable for biochemical studies. We have reviewed the available information on the chemical modifications and we present new data of beta2-microglobulin extracted from non-osteotendinous tissues. beta2-microglobulin can accumulate in these compartments after long-term haemodialysis but rarely forms amyloid deposits. We confirm that truncation at the N-terminus is an event specific to beta2-microglobulin derived from fibrils but is not observed in the beta2-microglobulin from plasma or from the insoluble non-fibrillar material deposited in the heart and spleen. We also confirm the partial deamidation of Asn 17 and Asn 42, as well as the oxidation of Met 99 in fibrillar beta2-microglobulin. Other previously reported chemical modifications cannot be excluded, but should involve less than 1-2% of the intact molecule.

Quantification of cleaved beta2-microglobulin in serum from patients undergoing chronic hemodialysis

BACKGROUND

Patients on chronic hemodialysis are prone to develop amyloid deposits of misfolded beta(2)-microglobulin (beta(2)M) in osteoarticular tissues. beta(2)M with various deletions/truncations and chemical modifications has been found together with structurally intact beta(2)M in extracts of beta(2)M amyloid fibrils. The state of the circulating population of beta(2)M molecules has not been characterized previously with high-resolution methods.

METHODS

We used immunoaffinity-liquid chromatography-mass spectrometry analysis of serum samples to examine whether structurally modified beta(2)M is generated in the circulation. In addition, we developed an immunoassay for the quantification of a cleaved beta(2)M variant in biological fluids based on novel monoclonal antibodies and applied this assay to patient and control sera.

RESULTS

A specific alteration compatible with the generation of lysine-58-cleaved and truncated beta(2)M (DeltaK58-beta(2)M) was found in the sera of many (20%-40%) dialysis patients but not in control sera or sera from patients with cerebral amyloidosis (Alzheimer disease). Applied to patient sera, specific immunoassays revealed that dialysis, as expected, significantly lowered the total beta(2)M concentration, but the concentrations of DeltaK58-beta(2)M remained unchanged after dialysis. The results also show that patients dialyzed with less biocompatible membranes have higher serum concentrations of cleaved beta(2)M (mean, 8.5, 1.8, and 0.7 mg/L in cuprophane membrane-dialyzed, polysulfone membrane-dialyzed, and control sera, respectively).

CONCLUSIONS

This study for the first time demonstrates and assigns the structure of a specific beta(2)M variant in sera from dialysis patients. Because this variant is conformationally unstable in vitro, it may be involved in in vivo amyloidogenesis.

Studies on unfolded β2-microglobulin at C-terminal in dialysis-related amyloidosis

BACKGROUND

In 1997, Stoppini et al reported that monoclonal antibody specific to the C-terminal 92-99 of beta(2)-microglobulin (beta(2)m) had been capable of inhibiting fibrillogenesis of beta(2)m in vitro. Meanwhile, recent studies have indicated that an acidifying procedure can unfold conformation of the precursor protein, leading to fibril formation of beta(2)m as well as a transthyretin.

METHODS

We thus prepared monoclonal antibody specific to the C-terminal 92-99 (mAb 92-99), and investigated its reactivity in plasma ultrafiltrate and amyloid tissues from 18 hemodialysis patients with dialysis-related amyloidosis (DRA).

RESULTS

beta(2)m extracted from ultrafiltrate showed no reaction for mAb 92-99, whereas acidified beta(2)m from ultrafiltrate showed a reaction for mAb 92-99. Similarly, a homogenate of carpal amyloid tissues showed a strong reaction for mAb 92-99 on immunoblotting. Immunohistochemical study showed also a distinct staining for mAb 92-99 in 7 Congophilic specimens from DRA patients. More interestingly, staining for mAb 92-99 could be found in most, though not all, non-Congophilic tissues.

CONCLUSION

This study demonstrates that the monoclonal antibody specific to the C-terminal 92-99 of beta(2)m can detect the conformational intermediate in amyloidogenesis of beta(2)m ex vivo, and demonstrates that an unfolded beta(2)m at C-terminal could be found not only in Congophilic area but even in non-Congophilic area as well.

A systematic investigation into the effect of protein destabilisation on beta 2-microglobulin amyloid formation

Beta-2-microglobulin (beta(2)m) has been shown to form amyloid fibrils with distinct morphologies under acidic conditions in vitro. Short, curved fibrils (<600 nm in length), form rapidly without a lag phase, with a maximum rate at pH 3.5. By contrast, fibrils with a long (approximately 1 microm), straight morphology are produced by incubation of the protein at pH< or =3.0. Both fibril types display Congo red birefringence, bind Thioflavin-T and have X-ray fibre diffraction patterns consistent with a cross-beta structure. In order to investigate the role of different partially folded states in generating fibrils of each type, and to probe the effect of protein stability on amyloid formation, we have undertaken a detailed mutagenesis study of beta(2)m. Thirteen variants containing point mutations in different regions of the native protein were created and their structure, stability and fibril forming propensities were investigated as a function of pH. By altering the stability of the native protein in this manner, we show that whilst destabilisation of the native state is important in the generation of amyloid fibrils, population of specific denatured states is a pre-requisite for amyloid formation from this protein. Moreover, we demonstrate that the formation of fibrils with different morphologies in vitro correlates with the relative population of different precursor states.

Total and corrected antioxidant capacity in hemodialyzed patients

BACKGROUND

Oxidative stress may play a critical role in the vascular disease of end stage renal failure and hemodialysis patients. Studies, analyzing either discrete analytes and antioxidant substances, or the integrated total antioxidant activity of human plasma during hemodialysis, give contradictory results.

METHODS

Recently, we have introduced a new automated method for the determination of Total Antioxidant Capacity (TAC) of human plasma. We have serially measured TAC and corrected TAC (cTAC: after subtraction of the interactions due to endogenous uric acid, bilirubin and albumin) in 10 patients before the onset of the dialysis session, 10 min, 30 min, 1 h, 2 h and 3 h into the procedure and after completion of the session.

RESULTS

Our results indicate that TAC decreases, reaching minimum levels at 2 h. However, corrected TAC increases with t1/2 of about 30 min. We then repeated the measurements in 65 patients undergoing dialysis with different filters (36 patients with ethylene vinyl alcohol copolymer resin filter -Eval-, 23 patients with two polysulfone filters -10 with F6 and 13 with PSN140-, and 6 patients with hemophan filters). Three specimens were collected (0, 30, 240 min). The results of this second group confirm our initial results, while no significant difference was observed using either filter.

CONCLUSIONS

Our results are discussed under the point of view of possible mechanisms of modification of endogenous antioxidants, and the interaction of lipid- and water-soluble antioxidants.

The elephant in uremia: oxidant stress as a unifying concept of cardiovascular disease in uremia

Cardiovascular disease is the leading cause of mortality in uremic patients. In large cross-sectional studies of dialysis patients, traditional cardiovascular risk factors such as hypertension and hypercholesterolemia have been found to have low predictive power, while markers of inflammation and malnutrition are highly correlated with cardiovascular mortality. However, the pathophysiology of the disease process that links uremia, inflammation, and malnutrition with increased cardiovascular complications is not well understood. We hereby propose the hypothesis that increased oxidative stress and its sequalae is a major contributor to increased atherosclerosis and cardiovascular morbidity and mortality found in uremia. This hypothesis is based on studies that conclusively demonstrate an increased oxidative burden in uremic patients, before and particularly after renal replacement therapies, as evidenced by higher concentrations of multiple biomarkers of oxidative stress. This hypothesis also provides a framework to explain the link that activated phagocytes provide between oxidative stress and inflammation (from infectious and non-infections causes) and the synergistic role that malnutrition (as reflected by low concentrations of albumin and/or antioxidants) contributes to the increased burden of cardiovascular disease in uremia. We further propose that retained uremic solutes such as beta-2 microglobulin, advanced glycosylated end products (AGE), cysteine, and homocysteine, which are substrates for oxidative injury, further contribute to the pro-atherogenic milieu of uremia. Dialytic therapy, which acts to reduce the concentration of oxidized substrates, improves the redox balance. However, processes related to dialytic therapy, such as the prolonged use of catheters for vascular access and the use of bioincompatible dialysis membranes, can contribute to a pro-inflammatory and pro-oxidative state and thus to a pro-atherogenic state. Anti-oxidative therapeutic strategies for patients with uremia are in their very early stages; nonetheless, early studies demonstrate the potential for significant efficacy in reducing cardiovascular complications.

Cleaved beta 2-microglobulin partially attains a conformation that has amyloidogenic features

beta(2)-Microglobulin, a small protein localized in serum and on cell surfaces, can adopt specific aggregating conformations that generate amyloid in tissues and joints as a complication to long-term hemodialysis. We characterize a proteolytic variant of beta(2)-microglobulin (cleaved after Lys(58)) that as a trimmed form (Lys(58) is removed) can be demonstrated in the circulation in patients with chronic disease. An unexpected electrophoretic heterogeneity of these two cleaved variants was demonstrated by capillary electrophoresis under physiological conditions. Each separated into a fast and a slow component while appearing homogeneous, except for a fraction of oxidized species detected by other techniques. The two components had different binding affinities for heparin and for the amyloid-specific dye Congo red, and the equilibrium between the two forms was dependent on solvent conditions. Together with analysis of the differences in circular dichroism, the results suggest that beta(2)-microglobulin cleaved after Lys(58) readily adopts two equilibrium conformations under native conditions. In the cleaved and trimmed beta(2)-microglobulin that appears in vivo, the less populated conformation is characterized by an increased affinity for Congo red. These observations may help elucidate why beta(2)-microglobulin polymerizes as amyloid in chronic hemodialysis and facilitate the search for means to inhibit this process.

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.

Filtration of native and glycated beta2-microglobulin by charged and neutral dialysis membranes

BACKGROUND

It has been postulated that protein glycation and formation of advanced glycation end products (AGE) are among toxic factors in chronic uremia, whether the renal disease is of diabetic or nondiabetic origin. In this setting, AGE-modified beta2-microglobulin (beta2m) may favor dialysis beta2m-related dialysis amyloidosis. Consequently, efficient removal of modified beta2m by highly permeable dialysis membranes is as important as removal of native beta2m to postpone the development of dialysis amyloidosis.

METHODS

To define the role of dialysis membrane surface electronegativity on plasma protein transfer, an in vitro model was used to test the interactions of native and glycated beta2m with various highly permeable dialysis membranes. An experimental circuit with minidialyzers was used. The neutral high-flux polysulfone membrane (PS), the electronegative polymethylmetacrylate membrane (PMMA), the electronegative AN69 membrane and a modified AN69 membrane, the surface of which was neutralized with polyethyleneimine (AN69-PEI), were tested using both native beta2m and the more acidic glycated beta2m. Protein mass transfer and binding to the membrane were measured.

RESULTS

Mass transfer of glycated beta2m was significantly decreased through all membranes tested when compared with native beta2m. This result was due to the increased molecular weight of beta2m, which became less permeable to porous membranes, whereas adsorption of both native and glycated beta2m to membranes, due to ionic interactions, decreased similarly with AN69 and AN69-PEI, but remained unchanged with PS and PMMA. Moreover, surface neutralization of AN69 membrane did not alter its core binding capacity, since beta2m absorption accounted for 98 and 97% and glycated beta2m for 83.7 and 81.4% of the protein removed with AN69 and AN69-PEI, respectively.

CONCLUSION

Clearance of glycated beta2m through highly permeable neutral and negatively charged membranes was lower than that of native beta2m, reflecting a decreased sieving coefficient for the neoformed higher molecular weight and conformationally altered molecule. The binding capacity of the neutral PS was roughly half that of the charged membranes. Neutralizing surface electronegativity of the AN69 membrane with PEI did not alter its binding capacity. These results suggest that it would be useful for dialysis protocols to include comparative studies of both serum native and modified beta2m in order to prevent beta2m-amyloidosis.

Pathogenesis of beta(2)-microglobulin amyloidosis: role of monocytes/macrophages

beta(2)-microglobulin (beta(2)M) amyloidosis (A beta(2)M) is a serious, often incapacitating complication for patients undergoing long-term hemodialysis. Amyloid deposits composed of beta(2)M fibrils as the major constituent protein are mainly localized in joints and periarticular bone and lead to chronic arthralgias, carpal tunnel syndrome, and eventually destructive arthropathy. Although recent histologic studies have shown the accumulation of monocytes/macrophages around amyloid deposits, the factor(s) causing their infiltration and pathologic involvement have yet to be fully elucidated. Immunohistochemical staining reveals that macrophages in tenosynovial tissues express CD13, CD14, CD33, HLA-DR, and CD68 antigens on their surfaces and express interleukin (IL)-1 beta, tumor necrosis factor (TNF)-alpha, and IL-6. Many of these cells also express LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18), and VLA-4 (CD49d/CD29) on their surfaces. AGE-modified beta(2)M enhances chemotaxis of monocytes and stimulates macrophages to release bone-resorbing cytokines, such as IL-1 beta, TNF-alpha and IL-6. Via a RAGE-mediated pathway, AGE-modified, but not unmodified beta(2)M, significantly delays constitutive apoptosis of human peripheral blood monocytes. Monocytes survival in an advanced glycation end product (AGE) beta(2)M-containing microenvironment is associated with their phenotypic alteration into macrophage-like cells that generate more reactive oxygen species and elaborate greater quantities of IL-1 beta and TNF-alpha. Thus through regulation of their survival and differentiation, AGE beta(2)M in amyloid deposits may be able to influence the presence and quantity of infiltrated monocytes, and hence their biologic effects.

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.

Purification of bovine urinary beta2-microglobulin and its biochemical characteristics

In this study, bovine beta2-m was purified from urine by ion-exchange chromatography and gel chromatography, and the characteristics were compared with those of colostral beta2-m by the immunological reactivity, isoelectric points, peptide map, and amino acid sequence. The characteristics of purified urinary beta2-m were consistent with those of the colostral beta2-m. The urinary and colostral beta2-m possessed the same polypeptide chain consisting of 98 amino acids, and its molecular weight is 11.8 kDa. Furthermore, four isoforms of beta2-m were found. The isoelectric points were different from each other.

Alterations in nonenzymatic biochemistry in uremia: origin and significance of "carbonyl stress" in long-term uremic complications

Advanced glycation end products (AGEs), formed during Maillard or browning reactions by nonenzymatic glycation and oxidation (glycoxidation) of proteins, have been implicated in the pathogenesis of several diseases, including diabetes and uremia. AGEs, such as pentosidine and carboxymethyllysine, are markedly elevated in both plasma proteins and skin collagen of uremic patients, irrespective of the presence of diabetes. The increased chemical modification of proteins is not limited to AGEs, because increased levels of advanced lipoxidation end products (ALEs), such as malondialdehydelysine, are also detected in plasma proteins in uremia. The accumulation of AGEs and ALEs in uremic plasma proteins is not correlated with increased blood glucose or triglycerides, nor is it determined by a decreased removal of chemically modified proteins by glomerular filtration. It more likely results from increased plasma concentrations of small, reactive carbonyl precursors of AGEs and ALEs, such as glyoxal, methylglyoxal, 3-deoxyglucosone, dehydroascorbate, and malondialdehyde. Thus, uremia may be described as a state of carbonyl overload or "carbonyl stress" resulting from either increased oxidation of carbohydrates and lipids (oxidative stress) or inadequate detoxification or inactivation of reactive carbonyl compounds derived from both carbohydrates and lipids by oxidative and nonoxidative chemistry. Carbonyl stress in uremia may contribute to the long-term complications associated with chronic renal failure and dialysis, such as dialysis-related amyloidosis and accelerated atherosclerosis. The increased levels of AGEs and ALEs in uremic blood and tissue proteins suggest a broad derangement in the nonenzymatic biochemistry of both carbohydrates and lipids.

Polymerization of normal and intact beta 2-microglobulin as the amyloidogenic protein in dialysis-amyloidosis

The primary structure of beta 2-microglobulin (beta 2m), the major constituent protein of beta 2-microglobulin amyloidosis (A beta 2m) or dialysis-amyloidosis, was initially shown to be identical to serum beta 2m, thereby strongly suggesting the polymerization of intact beta 2m in tissues. Recent biochemical data have been controversial, showing beta 2m acidic isoforms, fragmentation and amino acid sequence alteration of deposited beta 2m. The aim of this study was to reinvestigate beta 2m amyloid deposits for the presence of beta 2m fragments and/or amino acid sequence alteration. Four amyloid-laden tissues (3 femoral bone amyloid cysts and 1 heart tissue) from dialysis patients were used to isolate amyloidogenic beta 2m. Amyloid fibrils were isolated using the classic water extraction method, and purified in 6 M guanidine on a gel-filtration column. The protein was further purified on 17% SDS-PAGE gel, and transferred to a nitrocellulose membrane for immunostaining with antihuman beta 2m. beta 2m samples were microsequenced using the standard 03RPTH program on a 470A gas-phase sequencer, and HPLC was performed after digestion with trypsin. Two peaks were obtained with the gel filtration column, the second corresponding by molecular weight to beta 2m. SDS-PAGE analysis of this peak under reducing conditions, demonstrated one major band at 12,000 Da and a minor band at 25,000 Da (monomer and dimer), and no lower molecular weight bands were observed. The 12 kDa band was micro-sequenced and the amino acid sequence corresponded to that of normal beta 2m through the 40th residue. Amino acid sequence analysis showed no difference from normal beta 2m in any of the beta 2m proteins contained in the amyloid deposits isolated from the four studied tissues. Also, the HPLC profile of the four protein samples were strictly normal and identical to a commercial preparation of beta 2m. The present study demonstrates that beta 2m molecules polymerized in amyloid fibrils and deposits are intact and have a normal amino acid sequence, and produced by a specific and unique fibrillogenetic mechanism, which does not require proteolytic processing from the precursor protein to the amyloid fibrils.

Purification of bovine beta 2-microglobulin from colostrum and its complete amino acid sequence

Bovine beta 2-microglobulin (beta 2-m) was purified from colostrum milk in two chromatographic steps: anion-exchange chromatography, and gel filtration. The amino acid sequence was determined to confirm that the purified protein was beta 2-m. A molecular weight of 11.8 kDa beta 2-m was estimated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Bovine beta 2-m consists of 98 amino acid residues and contains one disulfide linkage connecting residues 25 and 79. The amino acid sequence determined by this study is different from a previously published sequence at three sites, but agrees with the amino acid sequence from cDNA. Thus, we can conclude that the amino acid sequence determined in this study is correct.

Monocyte/macrophage response to beta 2-microglobulin modified with advanced glycation end products

We recently found that acidic beta 2-microglobulin (beta 2m), a major isoform of beta 2m in amyloid fibrils of patients with dialysis-related amyloidosis (DRA), contained early Amadori products and advanced glycation end products (AGEs) formed nonenzymatically between sugar and protein. Further analysis revealed that acidic beta 2m induces monocyte chemotaxis and macrophage secretion of bone-resorbing cytokines, suggesting the involvement of acidic beta 2m in the pathogenesis of DRA. Acidic beta 2m, however, is a mixture of heterogeneous molecular adducts due to various types of modification. In the present study, we investigated the modification responsible for the biological activity of acidic beta 2m toward monocytes/macrophages. The presence of a fair amount of beta 2m species with deamidation was detected in acidic beta 2m isolated from urine of non-diabetic long-term hemodialysis patients, but deamidated beta 2m had no biological activity. In contrast, normal beta 2m acquired the activity upon incubation with glucose in vitro. Among the glycated beta 2m, the pigmented and fluorescent beta 2m that formed after a long incubation period, that is, AGE-modified beta 2m, exhibited biological activity, whereas beta 2m modified with Amadori products, major Maillard products in acidic beta 2m, had no such activity. These findings suggest that AGEs, although only a minor constituent of acidic beta 2m, are responsible for monocyte chemotaxis and macrophage secretion of cytokines, implicating the contribution of AGEs to bone and joint destruction in DRA.

Beta 2 microglobulin isoforms in healthy individuals and in amyloid deposits

beta 2 microglobulin (beta 2m) is classically known to have isoforms with isoelectric points (pI) 5.7 and 5.3. New isoforms of beta 2m with lower pI, probably due to modifications with advanced glycation end products, were found in the amyloid deposits of dialysis related amyloidosis (DRA), and they were proposed as the amyloidogenic forms of beta 2m. The other modifications in beta 2m from amyloid deposits are partial proteolysis and single amino acid replacement (Asn by ASp at position 17). However, there are no data on the sequence of the different isoforms of beta 2 m from amyloid deposits. Amyloid deposits surgically obtained from the carpal tunnel from 13 dialysis treated patients and urine from 10 healthy volunteers and 5 living-related kidney donors were analyzed for beta 2m content. Two-dimensional gel electrophoresis (2D-PAGE) of beta 2m from amyloid deposits showed the presence of four or more isoforms with pIs < 5.7. All the spots migrating at 12 kDa Mr region and between 4 and 6 pH reacted with rabbit anti-human beta 2m antibody by Western blotting, confirming that they were beta 2m isoforms. beta 2m isoforms from the amyloid deposits were then separately purified with an IEF column (PB94, Pharmacia) for analysis. Enough quantities of three pure beta 2m isoforms could be obtained in two cases. The sequence analysis showed an intact N-terminus in all the isoforms. There was Asn in the 17th residue in all the isoforms sequenced. 2D-PAGE of urine from 8 out of the 10 healthy volunteers showed the presence of beta 2m. In two of them beta 2m also displayed four different isoforms. At least four isoforms were observed in urine of all the kidney donors. The present study shows that the elution peaks of three different beta 2m isoforms in gel isoelectrofocusing contain beta 2m with intact N-terminus. None of them have deamidated their 17th residue. More importantly, the beta 2m isoforms with lower pI are not specific for amyloidosis as they were found in urine from kidney donors and in normal volunteers. These results bring into question the hypothesis that dialysis related amyloidosis is due to the known modifications on beta 2m. They suggest that the precipitation of beta 2m into amyloid fibrils should result from the interaction of beta 2m with other factors with amyloid enhancing activity.

Amino acid sequence of a modified beta 2-microglobulin in renal failure patient urine and long-term dialysis patient blood

We isolated and analyzed beta 2-microglobulin (BM) in urine of two renal failure patients and in ultrafiltrate from long-term dialysis patients. In both groups of patients, we found the native BM with pI 5.7 and some modified BM with pI 5.3. The amino acid sequence of the modified BM was determined and compared with that of the native BM; the N-terminal isoleucine residue was missing and the second glutamine residue was cyclized into pyroglutamic acid in the modified BM.

Glycation of human beta 2-microglobulin in patients with hemodialysis-associated amyloidosis: identification of the glycated sites

beta 2-Microglobulin (beta 2M) is a major component forming amyloid deposits in patients with hemodialysis-associated amyloidosis (HAA), a serious complication of long-term hemodialysis. Recently, we demonstrated that beta 2M modified with the Maillard reaction is a definite constituent of amyloid deposits in patients with HAA. Our further study demonstrated that this modified beta 2M induces not only chemotaxis of monocytes but also secretion of tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6 from macrophages, suggesting the potential link of glycation of beta 2M by the Maillard reaction to the pathogenesis of HAA. The present study was undertaken to identify the glycated site(s) of beta 2M purified from long-term hemodialysis patients as well as beta 2M incubated with glucose in vitro. Borotritide-treated beta 2M was cleaved by endoproteinase Lys-C, and peptides were isolated by reverse-phase high-performance liquid chromatography, followed by amino acid sequence analysis and fast atom bombardment mass spectrometry to identify the glycated site. The glycated sites of beta 2M formed in vivo were found to be almost the same as those of glycated beta 2M in vitro. The primary glycated site was the alpha-amino group of the amino terminal isoleucine. Other minor sites were the epsilon-amino groups of Lys-19, -41, -48, -58, -91, and -94. Computer graphics of the three-dimensional structure of beta 2M suggested that the high specificity for the glycated site at Ile-1 may be explained by its high solvent accessibility and the nearby imidazole group of His-31 as an acid-base catalyst of the Amadori rearrangement.

Excess-substrate inhibition in enzymology and high-dose inhibition in pharmacology: a reinterpretation [corrected]

A kinetic model, called the Recovery Model, which incorporates an obligatory recovery phase of fixed duration (tr) in the operation cycle of a macromolecule (enzyme, receptor) is proposed. Binding of a ligand (substrate, agonist) during tr disturbs the recovery process and causes inhibition (substrate inhibition, agonist autoinhibition). A quantitative stochastic analysis of a minimal version of the Recovery Model reveals that (1) plotting the response versus the logarithm of the ligand concentration never yields a strictly symmetrical bell-shaped dose-response curve, (2) the position and shape of the descent of the dose-response curve can vary greatly in dependence of the kinetic parameters of the system, and (3) a minimal steepness of the descent with a Hill coefficient of 1 exists provided that the response can be totally inhibited by high ligand concentrations. The Recovery Model is equally applicable to macromolecules that can bind single or multiple ligands, and suggests new ways to explain such diverse phenomena as partial agonism, pulse generation, desensitization, memory effects and ultrasensitivity. In addition, substrate inhibition and agonist autoinhibition are regarded as phenomena closely related to other kinds of non-Michaelian behaviour because of a common temporal mechanism, namely the temporal interference of arriving ligand molecules with timing-sensitive phases of the operation cycle.

Molecular variants of beta 2-microglobulin in renal insufficiency

Many patients with renal insufficiency treated by dialysis for more than 10 years have tissue deposits of amyloid material containing polymerized beta 2-microglobulin (beta 2m). The mechanisms of beta 2m polymerization and degradation remain unknown. In biological fluids (serum and urine) from haemodialysis patients and in dialysis fluids from patients treated by chronic ambulatory peritoneal dialysis (CAPD), we have characterized different molecular forms of beta 2m, including proteolytic split products. beta 2m isoforms of pI 5.7, 5.3 and 4.5-5.0 were isolated from urine and CAPD fluid. The pI 5.3 beta 2m, but not the other forms, was recovered both as monomers and as dimers. Such dimers were also detected in serum from patients but not from healthy controls. pI 5.3 and 5.7 beta 2m isoforms were found to be nearly identical by mass spectrometry and by their amino acid sequences. The amino acid sequence of the 43 N-terminal amino acids of beta 2m of pI 5.0 showed identity with the corresponding region of pI 5.7 beta 2m. Fragments recovered from CAPD fluid were similar to proteolytic fragments generated from pure pI 5.7 beta 2m by incubation in mouse ascitic fluid at acidic pH. Furthermore, pure pI 5.7 beta 2m was converted into more acidic forms of 12 kDa upon incubation in mouse ascitic fluid at acid pH. beta 2m dimers found in serum may represent a precursor of amyloid fibrils.

beta 2-Microglobulin modified with advanced glycation end products is a major component of hemodialysis-associated amyloidosis

beta 2-Microglobulin (beta 2M) is a major constituent of amyloid fibrils in hemodialysis-associated amyloidosis, a complication of long-term hemodialysis patients. Amyloid fibril proteins were isolated from connective tissues forming carpal tunnels in hemodialysis patients with carpal tunnel syndrome. Two-dimensional polyacrylamide gel electrophoresis and Western blotting demonstrated that most of the beta 2M forming amyloid fibrils exhibited a more acidic pI value than normal beta 2M. This acidic beta 2M was also found in a small fraction of beta 2M in sera and urine from these patients, whereas heterogeneity was not observed in healthy individuals. We purified acidic and normal beta 2M from the urine of long-term hemodialysis patients and compared their physicochemical and immunochemical properties. Acidic beta 2M, but not normal beta 2M, was brown in color and fluoresced, both of which are characteristics of advanced glycation end products (AGEs) of the Maillard reaction. Immunochemical studies showed that acidic beta 2M reacted with anti-AGE antibody and also with an antibody against an Amadori product, an early product of the Maillard reaction, but normal beta 2M did not react with either antibody. Incubating normal beta 2M with glucose in vitro resulted in a shift to a more acidic pI, generation of fluorescence, and immunoreactivity to the anti-AGE antibody. The beta 2M forming amyloid fibrils also reacted with anti-AGE antibody. These data provided evidence that AGE-modified beta 2M is a dominant constituent of the amyloid deposits in hemodialysis-associated amyloidosis.