Serum beta 2-microglobulin in patients with monoclonal gammopathies

Beta-2-microglobulin concentrations were determined in serum samples from 45 patients with benign and malignant monoclonal gammopathies. In the group of patients suffering from multiple myeloma or Waldenstr?m macroglobulinemia the mean beta 2-microglobulin level was significantly higher than in the group with monoclonal gammopathy of undetermined significance. Values above 3 mg/L were highly indicative of a neoplastic process and were observed in all the Waldenstr?m patients and in greater than 90% of myeloma patients. No significant correlation was noticed between beta 2-microglobulin and monoclonal protein levels in any of the groups examined.     

Isolation and properties of goat beta 2-microglobulin

Goat beta 2-microglobulin was isolated and purified from colostrum. Comparisons of the amino acid composition and amino-terminal sequence of the goat protein with the bovine and human homologues, indicates a high degree of similarity. Both goat and bovine beta 2-microglobulins differ slightly in composition from the human molecule, most notably in threonine and proline values. For the first 32 residues, bovine and goat differ only at two positions, one of which is a valyl/isoleucyl substitution consistent with the amino acid compositions. The equivalent goat/human sequence comparison shows seven differences. Immunological studies, using the ELISA method, also confirm the close relatedness of goat and bovine beta 2-microglobulin and their more distant relatedness to the human homologue.     

Structure and significance of beta2-microglobulin.

beta2-Microglobulin shares many structural features with the homology regions of the immunoglobulins. Particularly significant is the fact that its amino acid sequence is homologous to the sequences of the constant regions of both classes of light chains (kappa and lambda) and to the constant homology regions of at least three classes (gamma, mu and epsilon) of heavy chains, especially the carboxyl-terminal regions Cgamma3 Cmu4 and Cepsilon4. Molecules similar to human beta2-microglobulin have been found in other vertebrate species. The properties of beta2-microglobulin suggest that the gene for this protein may have evolved from a precursor gene that by duplication gave rise to immunoglobulin light and heavy chains. Furthermore, the observation that beta2-microglobulin is synthesized by and appears on the surfaces of a variety of cell types, including nonlymphoid cells, suggests that the concepts derived from analysis of the immune system may be applicable to other areas of cell biology. In particular, the close association of this immunoglobulin-like molecule with the histocompatibility antigens has a number of implications for the origin, structure, and function of these as well as other cell surface glycoproteins.     

Proteomics of beta2-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.     

Characterization of the beta2-microglobulin gene of the horse

A clone containing beta(2)-microglobulin (beta(2)-m), the light chain of the major histocompatibility complex class I cell surface molecule, was isolated from an equine bacterial artificial chromosome library. This clone was used as a template for polymerase chain reaction (PCR) and unidirectional sequencing to elucidate the genomic sequence and intron/exon boundaries. We obtained 7,000 bases of sequence, extending from 1,100 nucleotides (nt) upstream of the coding region start through 1,698 nt downstream of the stop codon. The sequence contained regulatory elements in the region upstream of the coding sequence similar to those of the beta(2)-m gene of other species. The beta(2)-m gene was localized to horse chromosome ECA1q23-q25 by fluorescent in situ hybridization. This was confirmed by synteny mapping on a (horse x mouse) somatic cell hybrid panel. The sequence and intron/exon boundaries determined were used to design PCR primers to amplify and sequence the coding region of the beta(2)-m gene in other equids, including five breeds of domestic horse, one Przewalski's horse, five domestic donkeys and five zebras. A high degree of conservation was found among equids, illustrated by >98% (349/354) identity at the nucleotide level and 95% (113/118) at the amino acid level, because of non-synonymous nucleotide substitutions. The promoter detected in the region upstream of the coding sequence was subcloned and used in chloramphenicol acetyl transferase (CAT) assays to demonstrate the presence of a functional promoter. This study provides tools for the analysis of regulation of not only the horse beta(2)-m gene, but also for any genes dependent upon beta(2)-m for expression.     

Isolation and immunoenzyme determination of human beta 2-microglobulin

Two procedures for isolation of homogeneous beta 2-microglobulin from urine of patients with systemic lupus erythematosus were developed: a procedure for isolation of beta 2-microglobulin with two-stage gel chromatography on Sephacryl S-200 and a procedure for isolation of homogeneous beta 2-microglobulin with affinity chromatography on rabbit polyclonal antibodies. The microglobulins isolated with the two procedures showed identical physicochemical properties and were used in development of a competitive immunoenzymatic assay method for determination of beta 2-microglobulin in the blood. The method was approved for determination of beta 2-microglobulin in blood serum of patients.     

Ultrasonication-induced amyloid fibril formation of beta2-microglobulin

To obtain insight into the mechanism of fibril formation, we examined the effects of ultrasonication, a strong agitator, on beta2-microglobulin (beta2-m), a protein responsible for dialysis-related amyloidosis. Upon sonication of an acid-unfolded beta2-m solution at pH 2.5, thioflavin T fluorescence increased markedly after a lag time of 1-2 h with a simultaneous increase of light scattering. Atomic force microscopy images showed the formation of a large number of short fibrils 3 nm in diameter. When the sonication-induced fibrils were used as seeds in the next seeding experiment at pH 2.5, a rapid and intense formation of long fibrils 3 nm in diameter was observed demonstrating seed-dependent fibril growth. We then examined the effects of sonication on the native beta2-m at neutral pH, conditions under which amyloid deposits occur in patients. In the presence of 0.5 mm sodium dodecyl sulfate, a model compound of potential trigger and stabilizer of amyloid fibrils in patients, a marked increase of thioflavin T fluorescence was observed after 1 day of sonication at pH 7.0. The products of sonication caused the accelerated fibril formation at pH 7.0. Atomic force microscopy images showed that the fibrils formed at pH 7.0 have a diameter of more than 7 nm, thicker than those prepared at pH 2.5. These results indicate that ultrasonication is one form of agitation triggering the formation of amyloid fibrils of beta2-m, producing fibrils adapted to the respective pH.     

Diurnal variations of serum beta 2-microglobulin in multiple myeloma

The chronobiological circadian behaviour in serum levels of beta 2-microglobulin has been investigated in three groups of subjects: (A) 6 healthy controls; (B) 6 patients with untreated multiple myeloma; (C) 6 patients with multiple myeloma in complete remission after polychemotherapy. From all subjects, under the same standard life conditions, venous blood samples were drawn at 4-hour intervals starting from midnight during the span of a whole day. Circulating serum beta 2-microglobulin levels were determined by RIA method. The time-related data were analyzed by chronograms and the "mean-group cosinor" method. A significant circadian rhythm for serum beta 2-microglobulin was detected in the control group, with a peak in the morning hours, and in untreated patients, with a peak in the afternoon hours. No significant rhythm was found in treated patients with multiple myeloma. A significant mesor reduction was noted in patients with complete remission, correlated with the absence of circadian rhythm, in respect to untreated patients. These data suggest that serum levels of beta 2-microglobulin could be related to the neoplastic plasma cell proliferation and to the effect of therapy, and that the circadian evaluation could be used as a guide in monitoring myeloma patients.     

Limited proteolysis in the investigation of beta2-microglobulin amyloidogenic and fibrillar states

Amyloid fibrils of patients treated with regular haemodialysis essentially consists of beta2-microglobulin (beta2-m) and its truncated species DeltaN6beta2-m lacking six residues at the amino terminus. The truncated fragment shows a higher propensity to self-aggregate and constitutes an excellent candidate for the analysis of a protein in the amyloidogenic conformation. The surface topology and the conformational analysis of native beta2-m and the truncated DeltaN6beta2-m species both in the soluble and in the fibrillar forms were investigated by the limited proteolysis/mass spectrometry strategy. The conformation in solution of a further truncated mutant DeltaN3beta2-m lacking three residues at the N-terminus was also examined. This approach appeared particularly suited to investigate the regions that are solvent-exposed, or flexible enough to be accessible to protein-protein interactions and to describe the conformation of transient intermediates. Moreover, proteolysis experiments can also be tailored to investigate amyloid fibrils by discriminating the protein regions constituting the unaccessible core of the fibrils and those still flexible and exposed to the solvent. Although native beta2-m and DeltaN3beta2-m shared essentially the same conformation, significative structural differences exist between the native and the DeltaN6beta2-m proteins in solution with major differences located at the end moiety of strand V and subsequent loop with strand VI and at both the N- and C-termini of the proteins. On the contrary, an identical distribution of preferential proteolytic sites was observed in both proteins in the fibrillar state, which was nearly superimposible to that observed for the soluble form of DeltaN6beta2-m. These data revealed that synthetic fibrils essentially consists of an unaccessible core comprising residues 20-87 of the beta2-m protein with exposed and flexible N- and C-terminal ends. Moreover, proteolytic cleavages observed in vitro at Lys 6 and Lys 19 reproduce specific cleavages that have to take place in vivo to generate the truncated forms of beta2-m occurring in natural fibrils. On the basis of these results, a molecular mechanism for fibril formation has been proposed.     

Role of beta2-microglobulin in the intracellular processing of HLA antigens

The biosynthesis of HLA-A, -B, and -C antigens was examined in the two lymphoblastoid cell lines DAUDI and RAJI. In RAJI cells the HLA-A, -B, and -C antigen heavy chains become core-glycosylated in the endoplasmic reticulum as evidenced by their sensitivity to endo-H digestion and tunicamycin treatment. Beta2-Microglobulin is present in excess in the endoplasmic reticulum of the RAJI cells and associates with the heavy chain at the time of synthesis of the heavy chain. Pulse-chase experiments demonstrated that the RAJI HLA-A, -B, and -C antigen heavy chains become terminally glycosylated since their changed characteristics included resistance to endo-H digestion, sensitivity to neuraminidase treatment, and incorporation fucose. DAUDI HLA-A, -B, and -C antigen heavy chains are synthesized normally and become core-glycosylated but not terminally glycosylated. Other glycosylated cell surface proteins, like the HLA-DR antigens, display normal glycosylation in DAUDI cells. Therefore it is unlikely that the absence of terminally glycosylated HLA-A, -B, and -C antigen heavy chains is the result of a general defect in the biosynthetic machinery of DAUDI cells. However, DAUDI cells lack the ability to synthesize beta2-microglobulin, the common subunit of all HLA-A, -B, and -C antigens. Therefore, it seems reasonable to conclude that beta2-microglobulin is of importance for intracellular transport of newly synthesized HLA-A, -B, and -C antigens.     

Structure of wild-type and mutant mouse beta 2-microglobulin genes

beta 2-Microglobulin is encoded by a single gene per haploid genome in the mouse. This gene was cloned and sequenced and was shown to consist of four coding regions separated by three intervening sequences. Most of the protein (amino acids 3-95) is encoded in a single uninterrupted unit flanked by intervening sequences. In this respect, the genetic organization is similar to that of the evolutionarily related H-2 and immunoglobulin domains. A mutant cell line selected for loss of cell-surface expression of the thymus leukemic antigen and failing to express surface beta 2-microglobulin and H-2 was found to contain no detectable cytoplasmic beta 2-microglobulin. The genetic defect in this mutant was analyzed by Southern blots, which indicated that there are defects in both copies of the beta 2-microglobulin gene present in this cell. The concomitant loss of three cell-surface antigens was thereby linked to genetic defects in the beta 2-microglobulin locus.     

Identity of the HL-A common portion fragment and human beta2-microglobulin

Glycogen synthetase D from the 17,000 × g supernatant of a homogenate of human polymorphonuclear leukocytes has been purified to a specific activity of 7,4 units/mg protein in a single step, chromatography on Concanavalin A bound to agarose (Con A-Sepharose). The overall recovery of the enzyme was 66% and the entire procedure requires only 3–4 hours. After an in vitro D to I conversion, glycogen synthetase I was purified to a specific activity of 11,5 units/mg protein in a similar procedure.     

Dynamics in the unfolded state of beta2-microglobulin studied by NMR

Many proteins form amyloid-like fibrils in vitro under conditions that favour the population of partially folded conformations or denatured state ensembles. Characterising the structural and dynamic properties of these states is crucial towards understanding the mechanisms of self-assembly in amyloidosis. The aggregation of beta2-microglobulin (beta2m) into amyloid fibrils in vivo occurs in the condition known as dialysis-related amyloidosis (DRA) and the protein has been shown to form amyloid-like fibrils under acidic conditions in vitro. We have used a number of 1H-15N nuclear magnetic resonance (NMR) experiments in conjunction with site-directed mutagenesis to study the acid-unfolded state of beta2m. 15N NMR transverse relaxation experiments reveal that the acid-denatured ensemble, although predominantly unfolded at the N and C termini, contains substantial non-native structure in the central region of the polypeptide chain, stabilised by long-range interactions between aromatic residues and by the single disulphide bond. Relaxation dispersion studies indicate that the acid-unfolded ensemble involves two or more distinct species in conformational equilibrium on the micro- to millisecond time-scale. One of these species appears to be hydrophobically collapsed, as mutations in an aromatic-rich region of the protein, including residues that are solvent-exposed in the native protein, disrupt this structure and cause a consequent decrease in the population of this conformer. Thus, acid-unfolded beta2m consists of a heterogeneous ensemble of rapidly fluctuating species, some of which contain stable, non-native hydrophobic clusters. Given that amyloid assembly of beta2m proceeds with lag kinetics under the conditions of this study, a rarely populated species such as a conformer with non-native aromatic clustering could be key to the initiation of amyloidosis.     

Dissolution of beta2-microglobulin amyloid fibrils by dimethylsulfoxide

Increasing numbers of proteins have been found to aggregate into insoluble fibers, collectively referred to as amyloid fibrils. To address the conformational stability of amyloid fibrils, we studied the effects of dimethylsulfoxide (DMSO), 2,2,2-trifluoroethanol (TFE), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) on beta(2)-microglobulin amyloid fibrils by circular dichroism, thioflavin T fluorescence, light scattering, and electron microscopy. When measured by circular dichroism and thioflavin T fluorescence, HFIP, and TFE dissolved the fibrils, producing predominantly helical conformations. However, these alcohols did not dissolve the amyloid fibrils completely as monitored by light scattering and electron microscopy. On the other hand, DMSO completely dissolved the amyloid fibrils although a high concentration [i.e., 80% (v/v)] was required. These results are consistent with the important role of hydrogen bonds in stabilizing amyloid fibrils.