Alpha 2-macroglobulin protects some of the protein constituents of dialysis-associated amyloidosis from protease degradation

Light chain amyloidosis: Where are the light chains from and how they play their pathogenic role?

Amyloid light-chain (AL) amyloidosis is a plasma-cell dyscrasia, as well as the most common type of systematic amyloidosis. Pathogenic plasma cells that have distinct cytogenetic and molecular properties secrete an excess amount of amyloidogenic light chains. Assisted by post-translational modifications, matrix components, and other environmental factors, these light chains undergo a conformational change that triggers the formation of amyloid fibrils that overrides the extracellular protein quality control system. Moreover, the amyloidogenic light-chain itself is cytotoxic. As a consequence, organ dysfunction is caused by both organ architecture disruption and the direct cytotoxic effect of amyloidogenic light chains. Here, we reviewed the molecular mechanisms underlying this sequence of events that ultimately leads to AL amyloidosis and also discuss current in vitro and in vivo models, as well as relevant novel therapeutic approaches.

Design and testing of biological scaffolds for delivering reparative cells to target sites in the lung

This study summarizes the development and testing of a scaffold to promote engraftment of cells in the distal lung. A fibrinogen-fibronectin-vitronectin hydrogel (FFVH) was developed and optimized with respect to its mechanical and biological properties for this application. In vitro, FFVH scaffolds promoted attachment, histiotypic growth and expression of basement membrane proteins by primary ovine lung mesenchymal cells derived from lung biopsies. In vivo testing was then performed to assess the ability of FFVHs to promote cell engraftment in the sheep lung. Treatment with autologous cells delivered using FFVH was clinically well tolerated. Cells labelled with a fluorescent dye (PKH-26) were detected at treatment sites after 1 month. Tissue mass (assessed by CT imaging) and lung perfusion (assessed by nuclear scintigraphy) were increased at emphysema test sites. Post-treatment histology demonstrated cell proliferation and increased elastin expression without scarring or collapse. No treatment-related pathology was observed at healthy control sites. FFVH scaffolds promote cell attachment, spreading and extracellular matrix expression in vitro and apparent engraftment in vivo, with evidence of trophic effects on the surrounding tissue. Scaffolds of this type may contribute to the development of cell-based therapies for patients with end-stage pulmonary diseases.

Disorder-to-order conformational transitions in protein structure and its relationship to disease

Function in proteins largely depends on the acquisition of specific structures through folding at physiological time scales. Under both equilibrium and non-equilibrium states, proteins develop partially structured molecules that being intermediates in the process, usually resemble the structure of the fully folded protein. These intermediates, known as molten globules, present the faculty of adopting a large variety of conformations mainly supported by changes in their side chains. Taking into account that the mechanism to obtain a fully packed structure is considered more difficult energetically than forming partially "disordered" folding intermediates, evolution might have conferred upon an important number of proteins the capability to first partially fold and-depending on the presence of specific partner ligands-switch on disorder-to-order transitions to adopt a highly ordered well-folded state and reach the lowest energy conformation possible. Disorder in this context can represent segments of proteins or complete proteins that might exist in the native state. Moreover, because this type of disorder-to-order transition in proteins has been found to be reversible, it has been frequently associated with important signaling events in the cell. Due to the central role of this phenomenon in cell biology, protein misfolding and aberrant disorder-to-order transitions have been at present associated with an important number of diseases.

[BIG-H3 protein: mutation of codon 124 and corneal amyloidosis]

In 1997, a group of hereditary corneal dystrophies was related to mutations in the TGFBI (BIGH3) gene. Within this group, some corneal dystrophies present particular biochemical features in that they are characterized by corneal amyloid deposition. Contrary to clinical and genetic knowledge, the biochemical characteristics of the encoded protein (Big-h3) and the mechanisms of its amyloid conversion remain unclear. We review the current knowledge on the Big-h3 protein and focus on the behavior of the codon 124 region. We discuss this protein's mechanisms of amyloid conversion from our results and previous reports as well as from other types of amyloidosis. These data provide a better understanding of the putative processes leading to the phenotypic variations linked with their respective codon 124 mutation.

Circulating level of α2-macroglobulin–β2-microglobulin complex in hemodialysis patients

BACKGROUND

The presence of alpha2-macroglobulin (alpha2M) in amyloid tissue from patients with dialysis-related amyloidosis (DRA) was demonstrated by Argilés et al in 1989. Thereafter, the formation of the complex of beta2-microglobulin (beta2m) with alpha2m was confirmed directly by in vitro study. In Alzheimer's disease, complex formation of amyloid beta-peptide and alpha2M is considered to play an important role in the pathogenesis by modifying the degradation processes of amyloid protein. Thus, we hypothesized that the alpha2M-beta2m complex is an important factor in the pathogenesis of DRA as well. Here, we measured the circulating levels of alpha2M-beta2m complex in the maintenance hemodialysis patients and discussed about its clinical significance in DRA.

METHODS

One hundred and thirty-seven hemodialysis patients and 11 prehemodialysis chronic renal failure (CRF) patients were included in this study. The affinity of purified alpha2M for beta2m was confirmed by a highly sensitive 27 MHz quartz crystal microbalance (QCM). The presence of circulating alpha2M-beta2m complex was analyzed by immunoblotting analysis. Furthermore, the serum levels of alpha2M-beta2m complex were measured by sandwich enzyme immunoassay.

RESULTS

QCM analysis revealed the high affinity of alpha2M for beta2m. The presence of circulating alpha2M-beta2m complex was detected in two out of a total 11 prehemodialysis CRF patients and in 95 out of the total of 137 hemodialysis patients. None of the healthy subjects, however, were observed to present with any alpha2M-beta2m complex. Serum levels of the alpha2M-beta2m complex were correlated to the duration of hemodialysis (P= 0.043). Serum levels of the alpha2M-beta2m complex were significantly higher in patients with high DRA score than in patients with negative DRA score (P= 0.018). Moreover, serum levels of the alpha2M-beta2m complex showed significantly lower in the hemodiafiltration patients compared to the hemodialysis patients (P= 0.002) and showed a strong correlation with DRA score in hemodialysis patients excluding 11 hemodiafiltration patients (P= 0.0004).

CONCLUSION

This study is the first to demonstrate the presence of circulating alpha2M-beta2m complex in hemodialysis patients. Furthermore, we observed the correlation between serum levels of alpha2M-beta2m complex and clinical characteristics of DRA. Thus we concluded that a formation of an alpha2M-beta2m complex may be implicated in DRA.

Dynamic of beta(2)-microglobulin fibril formation and reabsorption: the role of proteolysis

Dialysis-related amyloidosis (DRA) is caused by the deposition, in target tissues, of beta(2)-microglobulin (beta(2)M) in fibrillar conformation. Several reports indicate that fibrillar beta(2)M is chemically heterogeneous and such heterogeneity is partially related to the presence of truncated species of the protein. In association with the full-length species, a beta(2)M isoform lacking six N-terminal residues is present in all the samples of our collection of ex vivo fibrils. The pattern of proteolytic cleavage in amyloidosis and in other diseases is completely different, as demonstrated by the absence in fibrillar beta(2)M of the cleavage at lysine 58, which is contrary to that described in rheumatoid arthritis and other diseases. The role of limited proteolysis of beta(2)M in the pathogenesis of the disease is uncertain. However, we have shown that the apparently minor modification of the intact protein, such as the removal of N-terminal hexapeptide, is capable of dramatically affecting its stability, protection from proteolytic digestion, and enhance its capacity to make in vitro amyloid fibrils. The structure, folding dynamic, and function of the truncated species of beta(2)M, peculiar of DRA, could shed new light on the mechanism of beta(2)M fibril formation and reabsorption.

beta2-microglobulin-derived amyloidosis: an update

The present review attempts to summarize recent developments in the field of beta2-microglobulin-derived amyloidosis (A(beta2)m amyloidosis) in patients on chronic dialysis therapy. A key factor in the pathogenesis is the uremic retention of the precursor molecule, beta2-microglobulin (beta2m). However, secondary modifications of the molecule such as limited proteolysis, conformational changes, and the formation of advanced glycation end products have also been described. Finally, in order to explain the striking predilection of the disease for synovial and periarticular structures, a role of local predisposing factors within the synovial membrane (for example, of the particular constituents of the extracellular matrix) must also be postulated. With respect to clinical symptomatology, recent data have confirmed that clinically manifest signs of the amyloidosis represent only the tip of the iceberg, since histologically amyloid deposition is much more widespread. Noninvasive diagnosing of the disease has been advanced by technical changes of the beta2m scintigraphy. Finally, there is accumulating evidence that prevention of the disease not only includes the usage of high-flux synthetic membranes for hemodialysis or hemodiafiltration, but that other factors contribute to the clinical manifestations of amyloidosis such as the dialysate composition and its microbacteriological quality. Such factors, which have changed over the last years as part of general improvements in dialysis care, may explain why the prevalence of the amyloidosis appears to decrease.