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Beta-2-microglobulin (B2M) is synthesized by all nucleated cells and forms part of the major histocompatibility complex (MHC) class-1 present on cell surfaces, which presents peptide fragments to cytotoxic CD8+ T-lymphocytes, or by association with CD1, antigenic lipids to natural killer T-cells. Knockout of B2M results in loss of these functions and severe combined immunodeficiency. Plasma levels of this protein are low in healthy serum, but are elevated up to 50-fold in some pathologies including chronic kidney disease and multiple myeloma, where it has both diagnostic and prognostic value. High levels of the protein are associated with amyloid formation, with such deposits containing significant levels of modified or truncated protein. In the current study we examine the chemical and structural changes induced of B2M generated by both inflammatory oxidants (HOCl and ONOOH), and photo-oxidation (1O2) which is linked with immunosuppression. Oxidation results in oligomer formation, with this occurring most readily with HOCl and 1O2, and a loss of native protein conformation. LC-MS analysis provided evidence for nitrated (from ONOOH), chlorinated (from HOCl) and oxidized residues (all oxidants) with damage detected at Tyr, Trp, and Met residues, together with cleavage of the disulfide (cystine) bond. An intermolecular di-tyrosine crosslink is also formed between Tyr10 and Tyr63. The pattern of these modifications is oxidant specific, with ONOOH inducing a greater range of modifications than HOCl. Comparison of the sites of modification with regions identified as amyloidogenic indicate significant co-localization, consistent with the hypothesis that oxidation may contribute, and predispose B2M, to amyloid formation.
Prostaglandins (PGs) are important mediators of inflammation in arthritis. We evaluated the role of the cycloxygenase-2 (COX-2) enzyme, which regulates PG biosynthesis, in osteoarthropathy associated with hemodialysis-associated amyloidosis (HAA) by characterizing COX-2 expression in beta 2-microglobulin-treated human synovial cells.
In this report we provide evidence for the expression of antigenic epitopes on mouse beta 2-microglobulin(b) (beta 2mb) that result from assembly with cognate H-2 class I heavy chains. For the cell line 69.9.15 (beta 2ma x beta 2mb), which expresses a mutant cytosolic form of H-2Kb and wild-type H-2Db, flow cytometry with rabbit antiserum against mouse beta 2m displayed beta 2m expression by cells grown in the presence or absence of fetal calf serum. By contrast, the epitopes identified by the beta 2mb-specific monoclonal antibody (mAb) S19.8 and clone 23 were not expressed by 69.9.15 cells grown in serum-containing conditions, and although S19.8 reactivity was weakly recovered by culture in the absence of serum, no such reactivity was observed with clone 23. Strong expression of these epitopes was achieved following transfection of 69.9.15 cells with the wild-type H-2Kb gene, indicating that the beta 2mb epitopes defined by mAb S19.8 and clone 23 were expressed when beta 2mb was assembled with an appropriate heavy chain. In support of this conclusion, we observed the recovery of the S19.8 and clone 23 epitopes by in vitro assembly of H-2Kb heavy chains with beta 2mb in the presence of the VSV N protein p52-59; however, such epitopes were expressed neither by beta 2mb prior to heterodimer assembly nor by non-conformed beta 2mb present in tissue culture supernatants recovered from H-2 class I surface positive cells. Taken together, these data indicate that in addition to the property of beta 2m to modify the antigenicity of the MHC class I heavy chains, beta 2m epitopes are induced in a reciprocal manner by assembly with MHC class I heavy chain molecules.
Cellular immunity is dependent on major histocompatibility complex (MHC) class I molecules enabling cytotoxic T cell recognition of malignant and infected cells. Loading of antigenic peptides onto MHC class I is assisted by a peptide-loading protein complex including tapasin. We found that tapasin expression is enhanced by beta 2-microglobulin via both transcriptional and post-transcriptional mechanisms. In addition, using conditions which preserve the tapasin-ERp57 disulfide-bonded conjugate, we demonstrated that beta 2-microglobulin increases tapasin-containing protein complexes, and reduces the level of MHC class I/ERp57 complexes lacking tapasin. Overall, our results provide a new perspective on the regulation of tapasin expression and association.
Amyloid fibrils are ordered polymers in which constituent polypeptides adopt a non-native fold. Despite their importance in degenerative human diseases, the overall structure of amyloid fibrils remains unknown. High-resolution studies of model peptide assemblies have identified residues forming cross-beta-strands and have revealed some details of local beta-strand packing. However, little is known about the assembly contacts that define the fibril architecture. Here we present a set of three-dimensional structures of amyloid fibrils formed from full-length beta(2)-microglobulin, a 99-residue protein involved in clinical amyloidosis. Our cryo-electron microscopy maps reveal a hierarchical fibril structure built from tetrameric units of globular density, with at least three different subunit interfaces in this homopolymeric assembly. These findings suggest a more complex superstructure for amyloid than hitherto suspected and prompt a re-evaluation of the defining features of the amyloid fold.
The objectives of this study were to evaluate urinary beta-2-microglobulin (β2M) levels in long-term childhood cancer survivors and to establish its association with anticancer drug-induced nephrotoxicity. The study consisted of 165 childhood cancer survivors (CCS) who were in continuous complete remission. We reported that CCS had a significantly higher level of β2M (p < 0.001) and β2M/Cr. ratio (p < 0.05) than healthy peers. Among all participants, 24 (14.5%) had decreased eGFR (<90 mL/min/1.73 m2). A significant positive correlation between β2M/Cr. ratio and body mass index (coef. 14.48, p = 0.046) was found. Furthermore, higher levels of urinary β2M were detected among CCS with a longer follow-up time (over 5 years) after treatment. Subjects with decreased eGFR showed statistically higher urinary β2M levels (20.06 ± 21.56 ng/mL vs. 8.55 ± 3.65 ng/mL, p = 0.007) compared with the healthy peers. Twelve survivors (7.2%) presented hyperfiltration and they had higher urinary β2M levels than CCS with normal glomerular filtration (46.33 ± 93.11 vs. 8.55 ± 3.65 ng/mL, p = 0.029). This study did not reveal an association between potential treatment-related risk factors such as chemotherapy, surgery, radiotherapy, and the urinary β2M level. The relationship between treatment with abdominal radiotherapy and reduced eGFR was confirmed (p < 0.05). We demonstrated that urinary beta-2-microglobulin may play a role in the subtle kidney injury in childhood cancer survivors; however, the treatment-related factors affecting the β2M level remain unknown. Further prospective studies with a longer follow-up time are needed to confirm the utility of urinary β2M and its role as a non-invasive biomarker of renal dysfunction.
The rat monoclonal antibody LMR-12 was shown earlier to react with a plasma membrane protein, upregulated in multidrug-resistant cell lines. In this study, we observed distinct LMR-12 staining in 36 out of 55 non-drug-selected tumour cell lines, including melanomas, renal cell-, colon- and lung carcinomas, whereas in other tumour types, such as leukaemia and ovarian cancer, LMR-12 staining was generally low or absent. The cDNA encoding the LMR-12 antigen was isolated from a library of the multidrug-resistant human fibrosarcoma cell line HT1080/DR4 by expression cloning in MOP8 cells. Sequence analysis showed that the LMR-12 antigen is identical to the major histocompatibility complex class I molecule beta 2-microglobulin (beta2-m). The LMR-12/ beta2-m staining results were confirmed by mRNA microarray data from an independent National Cancer Institute study, as well as by newly obtained reverse transcriptase polymerase chain reaction data. Further analysis of the microarray data showed that beta2-m levels closely reflected levels of major histocompatibility complex class I heavy chains and the transporter associated with antigen processing. Since the ABC transporter associated with antigen processing was previously shown to contribute to multidrug-resistance, it may very well be that the observed LMR-12/ beta2-m levels are secondary to (elevated) levels of the transporter associated with antigen processing. A perspective arising from the present study is that drug resistant tumour cells may, by having elevated levels of major histocompatibility complex related molecules, be particular good candidates for alternative therapeutic therapies, such as cytotoxic T cell mediated immune-therapies.
The bottom-up design of smart nanodevices largely depends on the accuracy by which each of the inherent nanometric components can be functionally designed with predictive methods. Here, we present a rationally designed, self-assembled nanochip capable of capturing a target protein by means of pre-selected binding sites. The sensing elements comprise computationally evolved peptides, designed to target an arbitrarily selected binding site on the surface of beta-2-Microglobulin (β2m), a globular protein that lacks well-defined pockets. The nanopatterned surface was generated by an atomic force microscopy (AFM)-based, tip force-driven nanolithography technique termed nanografting to construct laterally confined self-assembled nanopatches of single stranded (ss)DNA. These were subsequently associated with an ssDNA-peptide conjugate by means of DNA-directed immobilization, therefore allowing control of the peptide's spatial orientation. We characterized the sensitivity of such peptide-containing systems against β2m in solution by means of AFM-based differential topographic imaging and surface plasmon resonance (SPR) spectroscopy. Our results show that the confined peptides are capable of specifically capturing β2m from the surface-liquid interface with micromolar affinity, hence providing a viable proof-of-concept for our approach to peptide design.
Plasma concentrations of beta 2 microglobulin (B2M), the light chain of the class I major histocompatibility complex, were measured serially in 26 patients undergoing allogeneic bone marrow transplantation (BMT). The concentrations fell after conditioning treatment, and recovered when the marrow was transplanted. Bacterial infection did not influence B2M concentration, but nine of 22 episodes of acute graft versus host disease were associated with raised concentrations. Increased plasma B2M concentrations were also a feature of eight episodes of chronic graft versus host disease, and these fell after treatment. Reactivation of herpes simplex, varicella zoster, or cytomegalovirus infections were also accompanied by raised B2M concentrations. Three patients with cytomegalovirus pneumonitis had high concentrations of plasma B2M, the rise starting between five and 22 days before onset of symptoms. Although it is non-specific, serial measurement of plasma B2M in patients undergoing BMT may be clinically useful in monitoring chronic graft versus host disease.
Beta-2 microglobulin (?2m) is the light chain of class I major histocompatibility complex (MHC-I). Beta2m is an intrinsically amyloidogenic protein that can assemble into amyloid fibrils in a concentration dependent manner. Beta2m is accumulated in serum of haemodialysed patients, and deposited in the skeletal joints, causing dialysis related amyloidosis. Recent reports suggested that the loop comprised between beta2m strands D and E is crucial for protein stability and for beta2m propensity to aggregate as cross-beta structured fibrils. In particular, the role of Trp60 for beta2m stability has been highlighted by showing that the Trp60-->Gly beta2m mutant is more thermo-stable and less prone to aggregation than the wild type protein. On the contrary the Asp59-->Pro beta2m mutant shows lower Tm and stronger tendency to fibril aggregation. To further analyse such properties, the Trp60-->Val beta2m mutant has been expressed and purified; the propensity to fibrillar aggregation and the folding stability have been assessed, and the X-ray crystal structure determined to 1.8A resolution. The W60V mutant structural features are discussed, focusing on the roles of the DE loop and of residue 60 in relation to ?2m structure and its amyloid aggregation trends.
Beta 2-microglobulin amyloidosis (A beta 2m) is a serious complication for patients undergoing long-term dialysis. beta 2-microglobulin modified with advanced glycation end products (beta 2m-AGE) is a major component of the amyloid in A beta 2m. It is not completely understood whether beta 2m-AGE plays an active role in the pathogenesis of A beta 2m, or if its presence is a secondary event of the disease. beta 2-microglobulin amyloid is mainly located in tendon and osteo-articular structures that are rich in collagen, and local fibroblasts constitute the principal cell population in the synthesis and metabolism of collagen. Recent identification of AGE binding proteins on human fibroblasts lead to the hypothesis that the fibroblast may be a target for the biological action of beta 2m-AGE. The present study demonstrated that two human fibroblast cell lines exhibited a decrease in procollagen type I mRNA and type I collagen synthesis after exposure to beta 2m-AGE for 72 hours. Similar results were observed using AGE-modified albumin. Antibody against the RAGE, the receptor for AGE, attenuated this decrease in synthesis, indicating that the response was partially mediated by RAGE. In addition, antibody against epidermal growth factor (EGF) attenuated the decrease in type I procollagen mRNA and type I collagen induced by beta 2m-AGE, suggesting that EGF acts as an intermediate factor. These findings support the hypothesis that beta 2m-AGE actively participates in connective tissue and bone remodeling via a pathway involving fibroblast RAGE, and at least one interposed mediator, the growth factor EGF.
β2-microglobulin has been increasingly investigated as a diagnostic marker of kidney function and a prognostic marker of adverse outcomes. To date, non-renal determinants of β2-microglobulin levels have not been well described. Non-renal determinants are important for the interpretation and appraisal of the diagnostic and prognostic value of any endogenous kidney function marker.
Spontaneous aggregation of folded and soluble native proteins in vivo is still a poorly understood process. A prototypic example is the D76N mutant of beta-2 microglobulin (β2m) that displays an aggressive aggregation propensity. Here we investigate the dynamics of β2m by X-ray crystallography, solid-state NMR, and molecular dynamics simulations to unveil the effects of the D76N mutation. Taken together, our data highlight the presence of minor disordered substates in crystalline β2m. The destabilization of the outer strands of D76N β2m accounts for the increased aggregation propensity. Furthermore, the computational modeling reveals a network of interactions with residue D76 as a keystone: this model allows predicting the stability of several point mutants. Overall, our study shows how the study of intrinsic dynamics in crystallo can provide crucial answers on protein stability and aggregation propensity. The comprehensive approach here presented may well be suited for the study of other folded amyloidogenic proteins.
Colorectal cancer (CRC) is the fourth most common non-cutaneous malignancy in the United States and the second most frequent cause of cancer-related death. One of the most important determinants of CRC survival is lymph node metastasis. To determine whether molecular markers might be prognostic for lymph node metastases, we measured by quantitative real-time RT-PCR the expression levels of 15 cancer-associated genes in formalin-fixed paraffin-embedded primary tissues derived from stage I-IV CRC patients with (n=20) and without (n=18) nodal metastases. Using the mean of the 15 genes as an internal reference control, we observed that low expression of beta(2)microglobulin (B2M) was a strong prognostic indicator of lymph node metastases (area under the curve (AUC)=0.85; 95% confidence interval (CI)=0.69-0.94). We also observed that the expression ratio of B2M/Spint2 had the highest prognostic accuracy (AUC=0.87; 95% CI=0.71-0.96) of all potential two-gene combinations. Expression values of Spint2 correlated with the mean of the entire gene set at an R(2) value of 0.97, providing evidence that Spint2 serves not as an independent prognostic gene, but rather as a reliable reference control gene. These studies are the first to demonstrate a prognostic role of B2M at the mRNA level and suggest that low B2M expression levels might be useful for identifying patients with lymph node metastasis and/or poor survival.
Beta-2 microglobulin (β2m) is an essential component of the major histocompatibility complex (MHC) class I proteins and in the nervous system β2m is predominantly expressed in motor neurons. As β2m can promote nerve regeneration, we investigated its potential role in amyotrophic lateral sclerosis (ALS) by investigating its expression level as well as the effect of genetically removing β2m on the disease process in mutant superoxide dismutase 1 (SOD1 (G93A) ) mice, a model of ALS. We observed a strong upregulation of β2m in motor neurons during the disease process and ubiquitous removal of β2m dramatically shortens the disease duration indicating that β2m plays an essential and positive role during the disease process. We hypothesize that β2m contributes to plasticity that is essential for muscle reinnervation. Absence of this plasticity will lead to faster muscle denervation and counteracting this process could be a relevant therapeutic target.
Beta-2-microglobulin (B2M), a light chain subunit of the major histocompatibility complex (MHC) class I complex, has been implicated in tumorigenesis. However, whether it is expressed in different epithelial-type ovarian tumours remains unknown. This study was performed to examine the expression of B2M in different histopathological types of ovarian tumours, to explore the function of B2M in ovarian cancer (OC) cells and to investigate the mechanisms underlying the regulation of B2M by the TGF-β signaling pathway.
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