Three-dimensional structure of beta 2-microglobulin

β₂-microglobulin forms three-dimensional domain-swapped amyloid fibrils with disulfide linkages

β₂-microglobulin (β₂m) is the light chain of the type I major histocompatibility complex. It deposits as amyloid fibrils within joints during long-term hemodialysis treatment. Despite the devastating effects of dialysis-related amyloidosis, full understanding of how fibrils form from soluble β₂m remains elusive. Here we show that β₂m can oligomerize and fibrillize via three-dimensional domain swapping. Isolating a covalently bound, domain-swapped dimer from β₂m oligomers on the pathway to fibrils, we were able to determine its crystal structure. The hinge loop that connects the swapped domain to the core domain includes the fibrillizing segment LSFSKD, whose atomic structure we also determined. The LSFSKD structure reveals a class 5 steric zipper, akin to other amyloid spines. The structures of the dimer and the zipper spine fit well into an atomic model for this fibrillar form of β₂m, which assembles slowly under physiological conditions.

Glycosaminoglycans enhance the fibrillation propensity of the β2-microglobulin cleavage variant--ΔK58-β2m

Dialysis related amyloidosis (DRA) is a serious complication to long-term hemodialysis treatment which causes clinical symptoms such as carpal tunnel syndrome and destructive arthropathies. The disease is characterized by the assembly and deposition of β2-microglobulin (β2m) predominantly in the musculoskeletal system, but the initiating events leading to β2m amyloidogenesis and the molecular mechanisms underlying amyloid fibril formation are still unclear. Glycosaminoglycans (GAGs) and metal ions have been shown to be related to the onset of protein aggregation and to promote de novo fiber formation. In this study, we show that fibrillogenesis of a cleavage variant of β2m, ΔK58-β2m, which can be found in the circulation of hemodialysis patients and is able to fibrillate at near-physiological pH in vitro, is affected by the presence of copper ions and heparan sulfate. It is found that the fibrils generated when heparan sulfate is present have increased length and diameter, and possess enhanced stability and seeding properties. However, when copper ions are present the fibrils are short, thin and less stable, and form at a slower rate. We suggest that heparan sulfate stabilizes the cleaved monomers in the early aggregates, hereby promoting the assembly of these into fibrils, whereas the copper ions appear to have a destabilizing effect on the monomers. This keeps them in a structure forming amorphous aggregates for a longer period of time, leading to the formation of spherical bodies followed by the assembly of fibrils. Hence, the in vivo formation of amyloid fibrils in DRA could be initiated by the generation of ΔK58-β2m which spontaneously aggregate and form fibrils. The fibrillogenesis is enhanced by the involvement of GAGs and/or metal ions, and results in amyloid-like fibrils able to promote the de novo formation of β2m amyloid by a scaffold mechanism.

Characterization of the functional domain of β2-microglobulin from the Asian seabass, Lates calcarifer

Background

β2-Microglobulin (β₂M) is the light chain of major histocompatibility class I (MHC I) that binds non-covalently with the α heavy chain. Both proteins attach to the antigen peptide, presenting a complex to the T cell to be destroyed via the immune mechanism.

Methodology/Principal Findings

In this study, a cDNA sequence encoding β₂M in the Asian seabass (Lates calcarifer) was identified and analyzed using in silico approaches to predict and characterize its functional domain. The β₂M cDNA contains an open reading frame (ORF) of 351 bases with a coding capacity of 116 amino acids. A large portion of the protein consists of the IG constant domain (IGc1), similar to β₂M sequences from other species studied thus far. Alignment of the IGc1 domains of β₂M from L. calcarifer and other species shows a high degree of overall conservation. Seven amino acids were found to be conserved across taxa whereas conservation between L. calcarifer and other fish species was restricted to 14 amino acids at identical conserved positions.

Conclusion/Significance

As the L. calcarifer β₂M protein analyzed in this study contains a functional domain similar to that of β₂M proteins in other species, it can be postulated that the β₂M proteins from L. calcarifer and other organisms are derived from a common ancestor and thus have a similar immune function. Interestingly, fish β₂M genes could also be classified according to the ecological habitat of the species, i.e. whether it is from a freshwater, marine or euryhaline environment.

Crystal structure of a bony fish beta2-microglobulin: insights into the evolutionary origin of immunoglobulin superfamily constant molecules

Three-dimensional structures of beta(2)-microglobulin (beta2m) from chicken and various mammals have been described previously, but aside from genomic sequences, very little is known about the three-dimensional structures of beta2m in species other than warm-blooded vertebrates. Here, we present the first three-dimensional structure of beta2m from bony fish grass carp (Ctid-beta2m), resolved at 2.1 A. The key structural differences between this new structure and previously published structures are two new hydrogen bonds at positions Ile(37) and Glu(38) in strand C and Lys(66) in strand E, and a hydrophobic pocket around the center of the protein found in Ctid-beta2m. Importantly, Ctid-beta2m has a short D strand and a long loop between stands C and D, rather than the flexible region found in other beta2m structures that serves as a putative binding region for the major histocompatibility complex heavy chain. Comparing the Ctid-beta2m structure with those of bovine and human beta2ms, the Calpha root mean square deviation of the latter are 1.3 A and 1.8 A, respectively. Compared with the constant domains of Lamprey T cell receptor-like receptor (Lamp-TCRLC) and Amphioxus V and C domain-bearing protein (Amphi-VCPC), Ctid-beta2m exhibits very different topology. The three-dimensional structures of domains predicted from Amphi-VCPC/Lamp-TCRLC are distinctly lacking in strand A of beta2ms. There are 18 amino acids at the N terminus of Amphi-VCPC that may have evolved into strand A of beta2ms. A mutation in the BC loops of Amphi-VCPC may have led to the novel topology found in beta2m. Based on these results, Ctid-beta2m may well reflect evolutionary characteristics of ancestral C set molecules.

A generic mechanism of beta2-microglobulin amyloid assembly at neutral pH involving a specific proline switch

Although numerous measurements of amyloid assembly of different proteins under distinct conditions in vitro have been performed, the molecular mechanisms underlying the specific self-association of proteins into amyloid fibrils remain obscure. Elucidating the nature of the events that initiate amyloid formation remains a particularly difficult challenge because of the heterogeneity and transient nature of the species involved. Here, we have used site-directed mutagenesis to create five proline to glycine variants in the naturally amyloidogenic protein beta2-microglobulin (beta2m). One of these variants, P5G, allowed us to isolate and characterise an intermediate containing a non-native trans Pro32 backbone conformation, a feature that is known to be required for amyloid elongation at neutral pH. By analysing oligomerisation and amyloid formation using analytical size-exclusion chromatography, multi-angle static light-scattering, analytical ultracentrifugation, circular dichroism and thioflavin T fluorescence we reveal a pathway for beta2m amyloid assembly at pH 7.5 that does not require the addition of metal ions, detergents, co-solvents or other co-factors that have been used to facilitate amyloid formation at physiological pH and temperature. Assembly is shown to involve the transient formation of a non-native monomer containing a trans P32 backbone conformation. This is followed by the formation of dimeric species and higher molecular mass oligomers that accumulate before the development of amyloid fibrils. On the basis of these results, we propose a generic mechanism for beta2m fibrillogenesis at neutral pH that is consistent with the wide range of published studies of this protein. In this mechanism, amyloid formation is initiated by a specific cis to trans proline switch, the rate of which we show to be controlled by the amino acid sequence proximal to P32 and to the applied solution conditions.

HDX-ESI-MS reveals enhanced conformational dynamics of the amyloidogenic protein beta(2)-microglobulin upon release from the MHC-1

The light chain of the major histocompatibility complex class 1 (MHC-1), the protein beta(2)-microglobulin (beta(2)m), has amyloidogenic properties that arise only upon its dissociation from the MHC-1. Here hydrogen/deuterium exchange electrospray ionization mass spectrometry (HDX-ESI-MS) has been used to compare the solution dynamics of beta(2)m in its MHC-1 bound state compared with those of beta(2)m as a free monomer. The capability of tandem mass spectrometry to dissociate the MHC-1 into its individual constituents in the gas phase following deuterium incorporation in solution has permitted the direct observation of the exchange properties of MHC-1 bound beta(2)m for the first time. The HDX-ESI-MS data show clearly that the H-->D exchange of MHC-1 bound beta(2)m follows EX2 kinetics and that about 20 protons remain protected from exchange after 17 days. Free from the MHC-1, monomeric beta(2)m exhibits significantly different HDX behavior, which encompasses both EX1 and EX2 kinetics. The EX2 kinetics indicate a tenfold increase in the rate of exchange compared with MHC-1 bound beta(2)m, with just 10 protons remaining protected from EX2 exchange and therefore exchanging only via the EX1 mechanism. The EX1 kinetics observed for unbound beta(2)m are consistent with unfolding of its exchange-protected core with a t(1/2) of 68 min (pH 7, 37 degrees C). Thus, upon dissociation from the stabilizing influence of the MHC-1, free beta(2)m becomes highly dynamic and undergoes unfolding transitions that result in an aggregation-competent protein.

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.

Molecular characterization and expression analysis of beta2-microglobulin in large yellow croaker Pseudosciaena crocea

Beta(2)-microglobulin (beta(2)m), a protein necessary for proper folding, peptide binding, and surface display of class I antigens plays an important role in immune response. The full-length cDNA containing beta(2)m was cloned from the spleen cDNA library of large yellow croaker Pseudosciaena crocea (Pscr-beta ( 2 ) m) by expressed sequence tag (EST) analysis. The Pscr-beta ( 2 ) m is 926 nucleotides (nt) long, including an open reading frame (ORF) of 348 nt encoding a polypeptide of 116 amino acids (aa). The deduced Pscr-beta ( 2 ) m possessed all characteristic domains of beta(2)m in other species, including a 16-aa leader peptide and a typical immunoglobulin (Ig) and major histocompatibility complex protein (MHC) signature YSCRVTH at residues 81-87. Homology modeling showed that the 3D structure of Pscr-beta ( 2 ) m protein is similar to that of human beta(2)m, except for a beta-strand (G) being lost in Pscr-beta ( 2 ) m due to amino acid deletion (positions 94-95). Tissue expression profile analysis revealed that the Pscr-beta ( 2 ) m was constitutively expressed in all tissues examined, such as kidney, spleen, liver, gills, heart, intestine, brain, and muscle, although at different levels. Upon stimulation with poly(I:C) or inactivated trivalent bacterial vaccine, the expression of Pscr-beta ( 2 ) m was significantly up-regulated in intestine, kidney and spleen at 24 h post-induction, and increase of Pscr-beta ( 2 ) m transcripts was also observed in liver post-induction with poly(I:C). Real-time PCR further revealed that the expression of Pscr-beta ( 2 ) m in intestine, kidney and spleen tissues was differentially regulated by poly(I:C) and bacterial vaccine during 72 h of induction. These results suggested that Pscr-beta ( 2 ) m might be involved in both antiviral and antibacterial mechanisms in large yellow croaker.

Cloning and characterization of ovine beta2-microglobulin cDNAs

Beta-2-microglobulin (beta(2)m) is the light chain of the major histocompatibility complex (MHC) class I cell surface heterodimer. beta(2)m is well conserved across most species with few polymorphisms seen within species. The aims of this study were to clone and express ovine beta(2)m and investigate if allelic variation of ovine beta(2)m exists. Ovine beta(2)m clones were isolated from five sheep of three breeds by reverse-transcription polymerase chain reaction (RT-PCR). Sequence analysis showed that four ovine beta(2)m sequences were obtained. Within breeds and individual animals there was evidence of allelic variation of ovine beta(2)m. An expression system was established to express one of the alleles with an ovine MHC class I cDNA clone in human embryo kidney cells (HEK293) and quail cells (QT35). Transfection experiments showed that ovine beta(2)m was expressed and directed the expression of ovine MHC class I heavy chain to the cell surface of the transfected cells. Both bovine and human beta(2)m supported ovine MHC class I heavy chain cell surface expression.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of grass carp beta2-microglobulin

Beta(2)-microglobulin (beta(2)m) is an essential subunit of MHC I molecules; it stabilizes the structure of MHC I and plays a pivotal role in coreceptor recognition. To date, structures of beta(2)m have been solved for three different mammals: human, mouse and cattle. In order to illuminate the molecular evolutionary origin of beta2m, an understanding of its structure in lower vertebrates becomes important. Here, grass carp (Ctenopharyngodon idellus) beta(2)m (Ctid-beta(2)m) was expressed, purified and crystallized. Diffraction data were collected to a resolution of 2.5 A. The crystal belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 38.72, b = 40.65, c = 71.12 A. The Matthews coefficient and the solvent content were calculated to be 2.56 A Da(-1) and 52.07%, respectively, for one molecule per asymmetric unit. The structure has been solved by molecular replacement using monomeric human beta(2)m as a model.

Structure of Interacting Segments in the Growing Amyloid Fibril of β2-Microglobulin Probed with IR Spectroscopy

Inter-segmental interaction at the growing tip of the amyloid fibril of beta2-microglobulin (beta2m) was investigated using IR microscopy. Cross-seeded fibril formation was implemented, in which the amyloid fibril of the #21-31 fragment of beta2m (fA[#21-31]) was generated on the beta2m amyloid fibril (fA[beta2m]) as a seed. Differences between the IR spectra of the cross-seeded fibril and those of the seed were attributed to the contribution from the tip, whose structure is discussed. The results indicated that 6.5 +/- 1.0 out of 11 residues of the fA[#21-31] tip on fA[beta2m] are contained in a beta-sheet at pH 2.5, which was smaller than the corresponding value (7.5 +/- 1.1 residues) of the spontaneous fA[#21-31] at pH 2.5. The tip was suggested to have a planar structure, indicating the planarity of the interacting segment. The N-terminal region of fA[#21-31] in the fibril is more exposed to the solvent than that in the tip, and vice versa for the C-terminal region. This is consistent with the different protonation levels of these regions, and the direction of peptide in the fibrils is determined from these results.

Simulation of pH-dependent edge strand rearrangement in human beta-2 microglobulin

Amyloid fibrils formed from unrelated proteins often share morphological similarities, suggesting common biophysical mechanisms for amyloidogenesis. Biochemical studies of human beta-2 microglobulin (beta2M) have shown that its transition from a water-soluble protein to insoluble aggregates can be triggered by low pH. Additionally, biophysical measurements of beta2M using NMR have identified residues of the protein that participate in the formation of amyloid fibrils. The crystal structure of monomeric human beta2M determined at pH 5.7 shows that one of its edge beta-strands (strand D) adopts a conformation that differs from other structures of the same protein obtained at higher pH. This alternate beta-strand arrangement lacks a beta-bulge, which may facilitate protein aggregation through intermolecular beta-sheet association. To explore whether the pH change may yield the observed conformational difference, molecular dynamics simulations of beta2M were performed. The effects of pH were modeled by specifying the protonation states of Asp, Glu, and His, as well as the C terminus of the main chain. The bulged conformation of strand D is preferred at medium pH (pH 5-7), whereas at low pH (pH < 4) the straight conformation is observed. Therefore, low pH may stabilize the straight conformation of edge strand D and thus increase the amyloidogenicity of beta2M.

Solution structure of β2-microglobulin and insights into fibrillogenesis

The solution structure of human beta(2)-microglobulin (beta(2)-m) was determined by (1)H NMR spectroscopy and restrained modeling calculations. Compared to the crystal structure of type I major histocompatibility complex (MHC-I), where the protein is associated to the heavy-chain component, several differences are observed, i.e., increased separation between strands A and B, displacements of strand C' and loop DE, shortening of strands D and E. These modifications can be considered as the prodromes of the amyloid transition. Even minor charge changes in response to pH, as is the case with H31 imidazole protonation, trigger the transition that starts with unpairing of strand A. The same mechanism accounts for the partial unfolding and fiber formation subsequent to Cu(2+) binding which is shown to occur primarily at H31. Solvation of the protected regions in MHC-I decreases the tertiary packing by breaking the contiguity of the surface hydrophobic patches via surface charge cluster. Mutants or truncated forms of beta(2)-m can be designed to remove the instability from H31 titration or to enhance the instability through surface charge suppression. By monitoring the conformational evolution of wild-type protein and variants thereof, either in response or absence of external perturbation, valuable insights into intermediate structure and fibrillogenesis mechanisms are gained.

The three-dimensional structure of beta2 microglobulin: results from X-ray crystallography

beta2-microglobulin, the light chain component of the major histocompatibility complex I, is involved in the development of DRA, an amyloid deposition disease occurring in man. Specifically, the beta2-microglobulin component, dissociated form the complex heavy chain, gives rise to amyloidogenic deposits in the joints of patients exposed to long dialysis periods. beta2-microglobulin three-dimensional structure is based on an antiparallel beta-barrel fold, with immunoglobulin domain topology, displaying structural flexibility in the crystal and NMR structures so fare determined. The structural bases of amyloidogenic potential in beta2-microglobulin can be related to local unfolding, to the tendency to aggregate laterally through non-compensated beta-strands, and partly also to its trend towards N-terminal proteolytic degradation. Such trends emerge quite clearly from inspection of a limited number of crystal structures of beta2-microglobulin as an isolated chain, separated form the major histocompatibility complex I heavy chain.

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.

Conformational constraints for amyloid fibrillation: the importance of being unfolded

Recent reports give strong support to the idea that amyloid fibril formation and the subsequent development of protein deposition diseases originate from conformational changes in corresponding amyloidogenic proteins. In this review, recent findings are surveyed to illustrate that protein fibrillogenesis requires a partially folded conformation. This amyloidogenic conformation is relatively unfolded, and shares many structural properties with the pre-molten globule state, a partially folded intermediate frequently observed in the early stages of protein folding and under some equilibrium conditions. The inherent flexibility of such an intermediate is essential in allowing the conformational rearrangements necessary to form the core cross-beta structure of the amyloid fibril.

beta2-microglobulin H31Y variant 3D structure highlights the protein natural propensity towards intermolecular aggregation

beta2-Microglobulin (beta2m) is the non-covalently bound light chain of the human class I major histocompatibility complex (MHC-I). The natural turnover of MHC-I gives rise to the release of beta2m into plasmatic fluids and to its catabolism in the kidney. beta2m dissociation from the heavy chain of the complex is a severe complication in patients receiving prolonged hemodialysis. As a consequence of renal failure, the increasing beta2m concentrations can lead to deposition of the protein as amyloid fibrils. Here we characterize the His31-->Tyr human beta2m mutant, a non-natural form of beta2m that is more stable than the wild-type protein, displaying a ten-fold acceleration of the slow phase of folding. We report the 2.9A resolution crystal structure and the NMR characterization of the mutant beta2m, focussing on selected structural features and on the molecular packing observed in the crystals. Juxtaposition of the four mutant beta2m molecules contained in the crystal asymmetric unit, and specific hydrogen bonds, stabilize a compact protein assembly. Conformational heterogeneity of the four independent molecules, some of their mutual interactions and partial unpairing of the N-terminal beta-strand in one protomer are in keeping with the amyloidogenic properties displayed by the mutant beta2m.

Beta2-microglobulin aberrations in diffuse large B-cell lymphoma of the testis and the central nervous system

Human leukocyte antigen (HLA) class I molecules are expressed on the surface of all nucleated cells and present antigenic peptides to cytotoxic T cells, thereby playing an important role in initiating the cellular anti-tumor immune response. We previously reported that loss of HLA class I expression in diffuse large B-cell lymphoma (DLBCL) of the central nervous system (CNS) and the testis is a common event. Loss of expression and mutations of the light chain of the HLA class I molecule, beta(2)-microglobulin (beta(2)m) have been described in a variety of human tumors and cell lines. In our study, we screened 15 DLBCL cases with a combined loss of HLA class I and beta(2)m expression for mutations in the latter gene by direct sequencing. Frame shift mutations in repetitive sequences within the beta(2)m gene leading to loss of functional beta(2)m were detected in 2 cases. Loss of heterozygosity (LOH) and fluorescent in situ hybridization (FISH) analysis for chromosome 15 exhibited loss of the remaining copy of the beta(2)m gene in both cases but also hemizygous deletions and monosomies in 6 additional cases. Since similar mutations in the beta(2)m gene have been associated with microsatellite instability (MSI), we used 8 markers to study MSI involvement in DLBCL. Low MSI was more frequent (33%) as compared to nodal DLBCL (n=15) but did not correlate with the beta(2)m mutations. Our data indicate that multiple mechanisms lead to downregulation of beta(2)m and concomitant loss of HLA class I expression in DLBCL.

Crystal structure of monomeric human beta-2-microglobulin reveals clues to its amyloidogenic properties

Dissociation of human beta-2-microglobulin (beta(2)m) from the heavy chain of the class I HLA complex is a critical first step in the formation of amyloid fibrils from this protein. As a consequence of renal failure, the concentration of circulating monomeric beta(2)m increases, ultimately leading to deposition of the protein into amyloid fibrils and development of the disorder, dialysis-related amyloidosis. Here we present the crystal structure of a monomeric form of human beta(2)m determined at 1.8-A resolution that reveals remarkable structural changes relative to the HLA-bound protein. These involve the restructuring of a beta bulge that separates two short beta strands to form a new six-residue beta strand at one edge of this beta sandwich protein. These structural changes remove key features proposed to have evolved to protect beta sheet proteins from aggregation [Richardson, J. & Richardson, D. (2002) Proc. Natl. Acad. Sci. USA 99, 2754-2759] and replaces them with an aggregation-competent surface. In combination with solution studies using (1)H NMR, we show that the crystal structure presented here represents a rare species in solution that could provide important clues about the mechanism of amyloid formation from the normally highly soluble native protein.

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.

The solution structure of human beta2-microglobulin reveals the prodromes of its amyloid transition

The solution structure of human beta2-microglobulin (beta2-m), the nonpolymorphic component of class I major histocompatibility complex (MHC-I), was determined by (1)H NMR spectroscopy and restrained modeling calculations. Compared to previous structural data obtained from the NMR secondary structure of the isolated protein and the crystal structure of MHC-I, in which the protein is associated to the heavy-chain component, several differences are observed. The most important rearrangements were observed for (1) strands V and VI (loss of the C-terminal and N-terminal end, respectively), (2) interstrand loop V-VI, and (3) strand I, including the N-terminal segment (displacement outward of the molecular core). These modifications can be considered as the prodromes of the amyloid transition. Solvation of the protected regions in MHC-I decreases the tertiary packing by breaking the contiguity of the surface hydrophobic patches at the interface with heavy chain and the nearby region at the surface charge cluster of the C-terminal segment. As a result, the molecule is placed in a state in which even minor charge and solvation changes in response to pH or ionic-strength variations can easily compromise the hydrophobic/hydrophilic balance and trigger the transition into a partially unfolded intermediate that starts with unpairing of strand I and leads to polymerization and precipitation into fibrils or amorphous aggregates. The same mechanism accounts for the partial unfolding and fiber formation subsequent to Cu(2+) binding, which is shown to occur primarily at His 31 and involve partially also His 13, the next available His residue along the partial unfolding pathway.

Conformational intermediate of the amyloidogenic protein beta 2-microglobulin at neutral pH

Aggregation and fibrillation of beta(2)-microglobulin are hallmarks of dialysis-related amyloidosis. We characterize perturbations of the native conformation of beta(2)-microglobulin that may precede fibril formation. For a beta(2)-microglobulin variant cleaved at lysine 58, we show using capillary electrophoresis that two conformers spontaneously exist in aqueous buffers at neutral pH. Upon treatment of wild-type beta(2)-microglobulin with acetonitrile or trifluoroethanol, two conformations were also observed. These conformations were in equilibrium dependent on the sample temperature and the percentage of organic solvent present. Circular dichroism showed a loss of beta-structures and gain of alpha-helices. Reversal to the native conformation occurred when removing the organics. Affinity capillary electrophoresis experiments showed increased specific interactions of the nonnative beta(2)-microglobulin conformation with the dyes 8-anilino-1-naphthalene sulfonic acid and Congo red. The observations may relate to early folding events prior to amyloid fibrillation and facilitate the development of methods to detect and inhibit pro-amyloid protein and peptide conformations.

Pore formation by beta-2-microglobulin: a mechanism for the pathogenesis of dialysis associated amyloidosis

Beta-2 microglobulin (beta 2M, molecular weight 10,000) is a 99 residue immune system protein which is part of the MHC Class I complex whose role is to present antigens to T cells. beta 2M serum levels rise dramatically in renal failure, and a syndrome called "dialysis associated amyloidosis" occurs with time in a majority of hemodialysis patients who exhibit beta 2M amyloid deposits in joints, bone and other organs. beta 2M can also induce Ca++ efflux from calvariae, collagenase production, and bone resorption. We report here that beta 2M formed relatively nonselective, long-lived, voltage independent ion channels in planar phospholipid bilayer membranes at physiologically relevant concentrations. The channels were inhibited by Congo red and blocked by zinc suggesting that they exist in an aggregated beta sheet state as is common with other amyloid fibril forming peptides. Multiple single channel conductances were seen suggesting that various oligomers of beta 2M may be capable of forming channel structures. We suggest that beta 2M channel formation may account for some of the pathophysiologic effects seen in dialysis associated amyloidosis. These findings lend further weight to the "channel hypothesis" of amyloid pathogenesis.

Causal relationship between conformational change and inhibition of domain functions of glycoxidative fibronectin

Glycoxidative modification of various body proteins, including fibronectin (FN), has been shown to change their structural and functional properties, and be implicated in pathogenesis of diabetic complications. Little is known about the role of secondary structure of glycoxidative FN (gFN) in its domain functions. gFN was prepared by incubation with 25 and 200 mM glucose in 0.2 M sodium phosphate buffer at 37 degrees C on a shaking plate under aerobic and sterile conditions for various time intervals up to 49 days, being defined as gFN25 and gFN200, respectively. Unmodified FN (uFN) was prepared by incubation in 0.2 M sodium phosphate buffer without any glucose at 4 degrees C for 49 days. The extent of glycoxidative modification was examined using a noncompetitive enzyme-linked immunosorbent assay with an antibody against N(epsilon) -(carboxymethyl)lysine (CML), one of the major glycoxidation products. The binding activities of uFN and gFN to collagen, gelatin and heparin were determined by a solid phase enzyme immunoassay or heparin-affinity HPLC. Cell attachment was estimated by the extent of adhesion of FITC-labeled smooth muscle cells to uFN or gFN. Conformational change in gFN was detected by SDS-polyacrylamide gel electrophoresis and spectroscopy (circular dichroism). CML was detected in gFN25 and gFN200 after 49 and 21 days of incubation, respectively. Levels of CML were about six-fold higher in gFN200 than in gFN25 after 49 days. Both gFN25 and gFN200 showed a significant decrease in the ability of binding to collagen and gelatin after 7 days of incubation. The binding activity for heparin was significantly decreased in both gFN25 and gFN200 after one day. Cell attachment activity was reduced to 89% and 76% of the unmodified form in both gFN25 and gFN200 after 49 days, respectively. High molecular weight materials were found in gFN25 and gFN200 after 21 and 7 days, respectively. CD spectrum showed that gFN25 had lost its native conformation after 3 days of incubation, depending upon the concentration and incubation interval of the applied glucose. These in vitro results suggest that the loss of native conformation may reduce the domain functions of gFN, including binding activity to macromolecular ligands and cell attachment, and may play a major role in the pathogenesis of diabetic complications.

Role of beta(2)-microglobulin in the immune response in renal osteodystrophy

Renal osteodystrophy is the major cause of skeletal morbidity in dialysis patients. It is characterized by beta(2)-microglobulin (beta(2)M) amyloid deposition at the osteoarticular sites and a destructive arthropathy. beta(2)M is present on the surface of all nucleated cells as the small extracellular subunit of the major histocompatibility complex (MHC) class I molecule and actively participates in the immune response. Accumulating evidence suggests that beta(2)M plays a key role in the development of renal osteodystrophy through a T cell-mediated inflammatory immune mechanism.

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.

Expression of the adenovirus receptor and its interaction with the fiber knob

The coxsackievirus group B (CVB) and adenovirus (Ad) receptor (HCVADR, formerly HCAR) is a cell surface protein with two immunoglobulin-like regions (IG1 and IG2) that serves as a receptor for two structurally unrelated viruses. We have established the tissue distribution of the receptor in the rodent by immunohistochemistry and show that the receptor is broadly expressed during embryonic development in the central and peripheral nervous systems and in several types of epithelial cells. The tissue distribution is more restricted in the adult but remains high mainly in epithelial cells. Using site-directed mutagenesis, based on computer modeling of the IG1 region, Ad5 binding could be inhibited but CVB attachment was unaffected. A double amino acid substitution in a three-stranded anti-parallel beta sheet that may form a face of the receptor completely inhibited Ad5 binding. Therefore, we conclude that the molecular interactions critical for Ad5 binding to HCVADR do not overlap with those of CVB3. In fact a specific antibody interfering with only CVB binding recognizes the IG2 domain in the receptor, suggesting that the CVB interacts with this region or an overlap between the IG1 and the IG2 regions.

Clinical investigation of the role of membrane structure on blood contact and solute transport characteristics of a cellulose membrane

Regenerated cellulose membranes contain cellulose chains with crystalline and amorphous regions in the direction of extrusion. A study was undertaken to investigate if reduced contact surface arising from alteration of pore size alters biocompatibility (complement activation (C3a and C5a) and neutropenia) and solute transport. The average pore size for the membrane studied (RC HP400A) was 7.23 compared to 2.76 nm for the standard membrane (Cuprophan). C3a levels rose to 6861+/-1595 compared to 2723+/-1228 ng/ml for Cuprophan at 15 min after initial blood contact (P < 0.0001). C5a levels also rose to 30.1+/-11.9 compared to 21.3+/-6.6 ng/ml for Cuprophan (P = 0.18). Both fractions gradually returned to baseline levels thereafter. Circulating white cell count fell rapidly over the same time period to 39+/-17% of the baseline value by 15 min and was similar to Cuprophan (27.5+/-11.2%) (P = 0.25). A small (< 10%) change in platelet numbers was noted for both membranes. Removal of urea (60 Da) was independent of pore size; however, the RC HP400A removed r2 microglobulin (11818 Da). These findings indicate that pore distribution fails to influence material-induced complement activation but influences large solute transport.

Functional comparison of bovine, murine, and human beta2-microglobulin: interactions with murine MHC I molecules

Fetal calf serum is a well known source of bovine beta2-microglobulin (beta2m) which can exchange with endogenous beta2m from, as well as promote peptide binding to, class I major histocompatibility (MHC I) molecules on cells cultured in vitro. Recombinant bovine beta2m was expressed and purified for direct functional comparison to human and murine beta2m for interactions with murine MHC I molecules H-2Kb, Db, Kd, Ld, and Dd. Bovine and human beta2m were equivalent in stabilizing MHC I heavy chains and facilitating peptide loading, suggesting similar affinities for murine MHC I heavy chains. The activity of murine beta2m was significantly weaker, consistent with previous work that demonstrated the lower affinity of murine human beta2m for murine heavy chains compared to human beta2m. Analysis of bovine beta2m in fetal calf serum revealed ten-fold higher concentrations than in adult bovine serum, levels shown to significantly affect MHC I stability and peptide loading. The ramifications for the study of MHC I molecules from cells in culture and the evolutionary implications of the higher affinity interactions of human and bovine beta2m are discussed.

Common core structure of amyloid fibrils by synchrotron X-ray diffraction

Tissue deposition of normally soluble proteins as insoluble amyloid fibrils is associated with serious diseases including the systemic amyloidoses, maturity onset diabetes, Alzheimer's disease and transmissible spongiform encephalopathy. Although the precursor proteins in different diseases do not share sequence homology or related native structure, the morphology and properties of all amyloid fibrils are remarkably similar. Using intense synchrotron sources we observed that six different ex vivo amyloid fibrils and two synthetic fibril preparations all gave similar high-resolution X-ray fibre diffraction patterns, consistent with a helical array of beta-sheets parallel to the fibre long axis, with the strands perpendicular to this axis. This confirms that amyloid fibrils comprise a structural superfamily and share a common protofilament substructure, irrespective of the nature of their precursor proteins.

Plasma protein adsorption to highly permeable hemodialysis membranes

Although membrane adsorption of plasma proteins is one of several factors determining the biocompatibility and mass transfer characteristics of a hemodialyzer, this process has not been evaluated rigorously. We performed an equilibrium and kinetic analysis of the binding of proteins of differing molecular weight to highly permeable membranes of differing hydrophobicity and surface change. Hydrophobic, anionic polyacrylonitrile (PAN) and hydrophilic, uncharged cellulose triacetate (CT) membrane fragments were incubated in buffer containing radioiodinated beta 2-microglobulin (beta 2m) or bovine serum albumin (BSA). From an initial solution concentration of 50 mg/liter, both membranes adsorbed significantly more beta 2m than BSA at equilibrium (PAN, 352 +/- 30 vs. 32.1 +/- 2.4 ng; CT, 87.0 +/- 0.6 vs. 30.8 +/- 1.7 ng). These results were consistent with membrane pore exclusion of BSA. Comparison of the slopes of the equilibrium isotherm lines (concentration range, 0 to 220 mg/liter) showed the PAN binding affinity for beta 2m and BSA was 28 and 1.4 times that of CT, respectively. In kinetic studies, the approach to equilibrium versus (time)1/2 was assessed. For all protein-membrane combinations, this relationship was linear, consistent with a diffusion-controlled process. This latter characteristic permitted the determination of beta 2m membrane diffusivity values for both PAN and CT, which were found to be 0.30 and 3.25 x 10(-7) cm2/sec, respectively. These data suggest membrane hydrophobicity more significantly influences the binding of low-molecular weight proteins than that of pore-excluded proteins. In addition, these results demonstrate electrostatic membrane-protein interactions may influence the kinetics of both the adsorption and transmembrane mass transfer of plasma proteins.

Beta 2-microglobulin co-distributes with the heavy chain of the intestinal IgG-Fc receptor throughout the transepithelial transport pathway of the neonatal rat

Maternal IgG crosses the proximal small intestine of the suckling rat by receptor-mediated endocytosis and transepithelial transport. The Fc receptor resembles the major histocompatibility complex class I antigens in that it consists of two subunits: a transmembrane glycoprotein (gp50) in association with beta 2-microglobulin. We used immunofluorescence microscopy and quantitative immunogold cytochemistry to study the subcellular distribution of the two subunits. In mature absorptive cells both subunits were colocalized in each of the membrane compartments that mediate transcytosis of IgG. IgG administered in situ apparently caused both subunits to concentrate within endocytic pits of the apical plasma membrane, suggesting that ligand causes redistribution of receptors at this site. These results support a model for transport in which IgG is transferred across the cell as a complex with both subunits. During absorptive cell differentiation, gp50 and beta 2-microglobulin showed nearly identical patterns of increased expression that accompanied the development of the apical endocytic apparatus and terminal web. However, absorptive cells in weanling rats expressed no detectable gp50 and only low levels of beta 2-microglobulin in the Golgi region and on the basolateral plasma membrane where class I antigens would likely reside. Thus, beta 2-microglobulin has a novel distribution unrelated to its function as a subunit of the class I antigens. The co-expression of the two receptor subunits is restricted to neonatal epithelial cells engaged in IgG transport and is coordinately regulated during absorptive cell differentiation and during postnatal intestinal development.

Dialysis-associated amyloidosis

Dialysis-related arthropathy represents a major complication of uremic patients treated by hemodialysis or other renal replacement therapies. Nearly 10 years ago, this syndrome was shown to be associated with a new type of amyloid, mainly composed of beta-2 microglobulin (beta 2-M). Retention of the beta 2-M protein due to chronic renal failure, although unquestionably a prerequisite for the occurrence of beta 2-M amyloidosis, appears not to be the unique pathogenetic factor involved in this complication. A role has also been attributed to an enhanced local or systemic generation of inflammatory mediators, an increased production of beta 2-M, and an altered metabolism of the molecule including partial proteolysis and glycation. It is possible that factors related to renal replacement therapy such as dialysis membrane biocompatibility also play a role. However, the clarification of the precise underlying mechanism(s) awaits further study. Because dialysis technology has progressed considerably during the last decade, a significant beta 2-M removal can be achieved at present using high-flux dialyzers. Moreover, a marked reduction in bioincompatibility during the dialysis procedure as manifested by activation of complement and stimulation of mononuclear blood cells can now be attained. Future studies will tell whether technical progress in dialysis technique results in a decrease in the incidence of symptomatic dialysis-associated amyloidosis.

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.

Synthesis of beta 2-microglobulin in lymphocyte culture: role of hemodialysis, dialysis membranes, dialysis-amyloidosis, and lymphokines

Dialysis-amyloidosis (A beta 2M) is a recently recognized chronic complication in long-term dialysis patients, apparently effecting 5% to 10% of all dialysis patients. In 1985, Gejyo et al (Biochem Biophys Res Commun 129:701-706, 1085) and Shirahama et al (Lab Invest 53:705-709, 1985) identified beta 2-microglobulin (beta 2-M) as the major constituent protein of this unique type of systemic amyloidosis. The specific pathogenesis of A beta 2M remains unknown, although beta 2-M has been clearly identified as playing a central role as the amyloidogenic protein. To investigate the factors responsible for in vitro beta 2-M synthesis, we studied beta 2-M production by lymphocyte cultures obtained from dialysis patients and grown under a variety of different conditions, and compared the results to a control group of subjects with normal renal function. We could not demonstrate any stimulatory influence on beta 2-M synthesis by the hemodialysis treatment, the type of dialysis membrane used, or the clinical presence of A beta 2M. Rather, dialysis membranes, sterilized with ethylene oxide or gamma rays, added to the lymphocyte cultures exerted a strong dose-dependent inhibitory effect on beta 2-M synthesis. From the results of this study, we conclude that peripheral blood lymphocytes in uremic patients synthesize beta 2-M normally and that the direct interaction between circulating lymphocytes and the dialysis membrane that occurs during hemodialysis does not seem to contribute directly to beta 2-M synthesis.

Lichenoid skin lesions as a sign of beta 2-microglobulin-induced amyloidosis in a long-term haemodialysis patient

We report a case of beta 2-microglobulin-induced amyloidosis. The patient was a 40-year-old man suffering from non-amyloid nephropathy, who had been treated by haemodialysis for 20 years. Lichenoid skin lesions, consisting of groups of pin-head-sized shiny papules, were present on the arms and trunk. On histological examination, amyloid deposits were present, principally in the dermal papillae, but also around the sweat ducts and hair follicles. The amyloid displayed potassium-permanganate-resistant Congo red affinity, and green birefringence under polarized light. Immunohistochemically, beta 2-microglobulin was demonstrated in the lesions, confirming that they were a manifestation of beta 2-microglobulin-associated amyloidosis. Skin lesions of this type have not been reported previously in beta 2-microglobulin-associated amyloidosis.

Cloning of the beta 2-microglobulin gene in the zebrafish

The beta 2-microglobulin (beta 2m) is a protein found in the serum in a free form and on the cell surface in a form noncovalently associated with the alpha chain of the class I major histocompatibility complex (Mhc) molecules. In mammals, the beta 2m-encoding gene (B2m) is found on a chromosome different from the Mhc proper. We have isolated and characterized the B2m gene of the zebrafish, Brachydanio rerio, family Cyprinidae. We obtained both cDNA and genomic clones of the Brre-B2m gene. The cDNA clones contained the entire coding sequence, the entire 3' untranslated (UT) region, and at least part of the 5'UT region. The genomic clone contained the entire Brre-B2m gene. The coding sequence specifies 97 amino acid residues of the mature protein so that the zebrafish beta 2m is two residues shorter than human and one residue shorter than cattle, fowl, or turkey beta 2m (codons at positions 85 and 86 have been deleted in the Brre-B2m gene). The amino acid and nucleotide sequence similarities between zebrafish and human beta 2m (B2m) are 45% and 59%, respectively. Approximately 24% of the positions are invariant and an additional 9% show only conservative substitutions in comparisons which include all known beta 2m sequences (fish, avian, and mammalian). Most of the conserved positions are in the beta strands (some 47% of the beta-strand positions are conserved in the three vertebrate classes). The Brre-B2m gene consists of four exons separated by three introns. All of the introns are considerably shorter than the corresponding introns in the mammalian B2m genes. The coding sequences of the cDNA and the genomic clones are almost identical but the sequences of the 3'UT regions differ at 1.7% of the sites, suggesting that the genes borne by these clones might have diverged at least 0.7 million years (my) ago. In contrast to the human B2m gene, the Brre-B2m gene shows no bias in the distribution of the CpG dinucleotides: the dinucleotides are distributed evenly along the entire available sequence. The haploid genome of the zebrafish contains only one copy of the B2m gene.

Demonstration of plasma proteinase inhibitors in beta 2-microglobulin amyloid deposits

beta 2-microglobulin-related amyloidosis (A beta 2M) represents a frequent complication in long-term dialysis patients. Although the pathogenetic mechanism has yet to be fully understood, it is known that amyloid fibrils usually consist of intact molecules of beta 2-microglobulin (beta 2m). Plasma proteinase inhibitors (PPI) are a broad family of glycoproteins with the function of eliminating unwanted proteolysis of serine proteases. Their role in amyloidogenesis has become a subject of intense discussion, especially since the recent identification of alpha 1-antichymotrypsin in the beta-protein amyloid deposits of Alzheimer's disease. We evaluated immunohistochemically and biochemically the presence and distribution of several PPIs (alpha 1-proteinase inhibitor, alpha 1-antichymotrypsin, antithrombin III, alpha 2-macroglobulin and tissue inhibitor metalloproteinase) and amyloid P component in A beta 2M deposits in osteo-articular and visceral tissues from dialysis patients with amyloidosis, as well as two carpal tunnel synovia from non-dialysis patients and one Alzheimer's brain as controls. The immunohistochemical study demonstrated that all but one (anti-alpha 1-antichymotrypsin) of the PPI antibodies tested showed varying degrees of positive reaction against A beta 2M deposits. All the antibodies (including anti-alpha 1-antichymotrypsin) also reacted to some extent with other non-amyloid visceral and connective tissue elements diffusely and/or selectively. Among them, only the reaction of anti-amyloid P component had significantly distinctive localization to A beta 2M deposits, which were identified in adjacent serial sections by Congo red staining and immunohistochemical reaction against anti-beta 2m.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro spontaneous synthesis of beta 2-microglobulin amyloid fibrils in peripheral blood mononuclear cell culture

beta 2-microglobulin-related amyloidosis (A beta 2M), in long-term dialysis patients, is a new and frequent complication for which the pathogenesis remains unknown. The authors documented, by light and high resolution electron microscopy, the spontaneous polymerization of beta 2-microglobulin to amyloid fibrils in mononuclear cell culture supernatants from dialysis patients. These data provide significant information about the pathogenesis of dialysis-amyloidosis, revealing an unusual and different fibrillogenic mechanism for beta 2-microglobulin and dialysis-amyloidosis than for other forms of amyloidosis. beta 2-microglobulin does not appear to require a proteolytic process before polymerization into amyloid fibrils and deposits. This study represents the first cell culture system in which beta 2-microglobulin amyloid fibrils have been spontaneously created.