Production of recombinant human beta2-microglobulin for scintigraphic diagnosis of amyloidosis in uremia and hemodialysis

Beta-2 Microglobulin Amyloidosis: Past, Present, and Future

Almost half a century has elapsed since the first description of dialysis-related amyloidosis (DRA), a disorder caused by excessive accumulation of β-2 microglobulin (B2M). Within that period, substantial advances in RRT occurred. These improvements have led to a decrease in the incidence of DRA. In many countries, DRA is considered a "disappearing act" or complication. Although the prevalence of patients living with RRT increases, not all will have access to kidney transplantation. Consequently, the number of patients requiring interventions for treatment of DRA is postulated to increase. This postulate has been borne out in Japan, where the number of patients with ESKD requiring surgery for carpal tunnel continues to increase. Clinicians treating patients with ESKD have treatment options to improve B2M clearance; however, there is a need to identify ways to translate improved B2M clearance into improved quality of life for patients undergoing long-term dialysis.

Mechanisms of amyloid formation revealed by solution NMR

Amyloid fibrils are proteinaceous elongated aggregates involved in more than fifty human diseases. Recent advances in electron microscopy and solid state NMR have allowed the characterization of fibril structures to different extents of refinement. However, structural details about the mechanism of fibril formation remain relatively poorly defined. This is mainly due to the complex, heterogeneous and transient nature of the species responsible for assembly; properties that make them difficult to detect and characterize in structural detail using biophysical techniques. The ability of solution NMR spectroscopy to investigate exchange between multiple protein states, to characterize transient and low-population species, and to study high molecular weight assemblies, render NMR an invaluable technique for studies of amyloid assembly. In this article we review state-of-the-art solution NMR methods for investigations of: (a) protein dynamics that lead to the formation of aggregation-prone species; (b) amyloidogenic intrinsically disordered proteins; and (c) protein-protein interactions on pathway to fibril formation. Together, these topics highlight the power and potential of NMR to provide atomic level information about the molecular mechanisms of one of the most fascinating problems in structural biology.

Expression in E. coli and purification of the fibrillogenic fusion proteins TTR-sfGFP and β2M-sfGFP

The possibility of obtaining recombinant fibrillogenic fusion proteins such as transthyretin (TTR) and β2-microglobulin (β2M) with a superfolder green fluorescent protein (sfGFP) was studied. According to the literature data, sfGFP is resistant to denaturating influences, does not aggregate during renaturation, possesses improved kinetic characteristics of folding, and folds well when fused to different polypeptides. The corresponding DNA constructs for expression in Escherichia coli were created. It could be shown that during expression of these constructs in E. coli, soluble forms of the fusion proteins are synthesized. Efficient isolation of the fusion proteins was performed with the help of nickel-affinity chromatography. For this purpose a polyhistidine sequence (6-His-tag) was incorporated into the C-terminus of the sfGFP. We could show that the purified fusion proteins contained full-size sequences of the most amyloidogenic TTR variant, TTR(L55P) and β2M, and also sfGFP possessing fluorescent properties. In the course of fibrillogenesis both fusion proteins demonstrated their ability to form fibrils that were clearly detectable by atomic force microscopy. Furthermore, with the help of confocal microscopy we were able to reveal structures (exhibiting fluorescence) that are formed during fibrillogenesis. Thus, the use of sfGFP has made it possible to avoid formation of inclusion bodies (IB) during the synthesis of recombinant fusion proteins and to obtain soluble forms of TTR(L55P) and β2M that are suitable for further studies.

Towards an understanding of the structural molecular mechanism of β2-microglobulin amyloid formation in vitro

Deriving a complete understanding of protein self-association into amyloid fibrils across multiple distance and time scales is an enormous challenge. At small length scales, a detailed description of the partially folded protein ensemble that participates in self-assembly remains obscure. At larger length scales, amyloid fibrils are often heterogeneous, can form along multiple pathways, and are further complicated by phenomena such as phase-separation. Over the last 5 years, we have used an array of biophysical approaches in order to elucidate the structural and molecular mechanism of amyloid fibril formation, focusing on the all beta-sheet protein, beta(2)-microglubulin (beta(2)m). This protein forms amyloid deposits in the human disease 'dialysis-related amyloidosis' (DRA). We have shown that under acidic conditions beta(2)m rapidly associates in vitro to form amyloid-like fibrils that have different morphological properties, but which contain an underpinning cross-beta structure. In this review, we discuss our current knowledge of the structure of these fibrils, as well as the structural, kinetic and thermodynamic relationship between fibrils with different morphologies. The results provide some of the first insights into the shape of the self-assembly free-energy landscape for this protein and highlight the parallel nature of the assembly process. We include a detailed description of the structure and dynamics of partially folded and acid unfolded species of beta(2)m using NMR, and highlight regions thought to be important in early self-association events. Finally, we discuss briefly how knowledge of assembly mechanisms in vitro can be used to inform the design of therapeutic strategies for this, and other amyloid disorders, and we speculate on how the increasing power of biophysical approaches may lead to a fuller description of protein self-assembly into amyloid in the future.

Imaging in dialysis spondyloarthropathy

Destructive spondyloarthropathy has recently been described in patients who undergo maintenance hemodialysis for chronic renal disease. The condition most frequently involves the lower segment of the cervical spine, although the craniocervical junction also may be affected. Although the pathogenesis of destructive spondyloarthropathy remains unclear, the disorder is thought to relate to a hemodialysis-associated amyloidosis. It appears that the disease correlates with the duration of hemodialysis, although it has been reported in patients with chronic renal insufficiency not associated with hemodialysis. Radiographic features simulate those of an infectious process, encompassing a range of abnormalities from superficial erosions to large bony defects. Computed tomography (CT) images reveal osteolytic areas, with bone sclerosis of adjacent vertebral endplates, and minimal osteophytosis. The intervertebral spaces appear narrow or obliterated. On magnetic resonance imaging (MRI), the disorder may show the imaging characteristics of spondylodiskitis. The absence of high signal intensity on T2-weighted images generally helps to eliminate the diagnosis of an infection. With progression of the disease, collapse of a vertebral body and spinal instability may occur. Severe complications of destructive spondyloarthropathy in long-term dialysis patients may include spinal cord compromise, necessitating surgical decompression, with or without spinal stabilization.

Evaluation of 99mTc-MAMA-chrysamine G as an in vivo probe for amyloidosis

To date, systemic amyloidosis is diagnosed histologically using Congo red staining or in vivo using iodine-123 labelled serum amyloid P component (123I-SAP) scintigraphy. We developed 99mTc-MAMA-CG, a 99mTc-labelled derivative of the lipophilic Congo red analogue chrysamine G (CG), as a possible alternative to 123I-SAP. In vivo 99mTc-MAMA-CG scintigraphy, performed in chickens with spontaneous joint amyloidosis, resulted as soon as 10 min after injection in scintigraphic images showing uptake of activity in amyloid-loaded organs (liver, joints). One of these chickens was studied also with 123I-SAP resulting in scintigraphic images revealing 123I-SAP binding to amyloid deposits in the liver. However, up to 11 h after injection no radioactivity was visible in the amyloid positive joints. In vitro autoradiography, performed on sections of chicken joints with Enterococcus faecalis induced amyloid arthropathy (chjAA), demonstrated the failure of 99mTc-MAMA-CG to bind significantly to amyloid deposits in the presence of 10 microM Congo red The specificity of 99mTc-MAMA-CG localisation was also established by the absence of 99mTc-MAMA-CG binding in non-amyloidotic organs in vitro and in vivo. 99mTc-MAMA-CG did not show any sign of acute toxicity. These findings establish the usefulness of 99mTc-MAMA-CG as a non-invasive in vivo diagnostic probe in chickens with amyloid arthropathy and suggest that it may also be applicable to human amyloidosis.

Imaging techniques in the diagnosis of dialysis-related amyloidosis

beta(2)-microglobulin amyloidosis (A beta(2)M) is a major determinant of morbidity in patients on dialysis treatment. Symptoms of A beta(2)M amyloid are mainly related to (peri-)articular amyloid deposition. Imaging techniques [i.e., joint ultrasonography, X-ray, computed tomography (CT), or magnetic resonance imaging (MRI) findings], as well as conventional bone scans, are helpful in the screening of local lesions but are relatively nonspecific and/or not sensitive enough. Scintigraphic techniques using radiolabeled serum amyloid P component (SAP) or the radiolabeled A beta(2)M precursor protein, beta(2)M, generate more specific results. A beta(2)M deposits have been visualized in several long-term hemodialysis patients by using (123)I-labeled SAP. However, this scan did not show tracer accumulation in some frequently involved sites such as hips or shoulders, and frequently labeled the spleen, which is usually spared from A beta(2)M deposits. Improvements in technical sensitivity and specificity could be achieved by scanning with (131)I-labeled beta(2)M: this technique detected tracer accumulations corresponding to the typical distribution pattern of A beta(2)M. Further, both the radiation exposure and the optical resolution of this latter scan have been refined by substituting (111)In for (131)I. In a final step we generated recombinant human beta(2)M (rh beta(2)M). While (111)In rh beta(2)M again failed to show significant tracer accumulation over joint regions in patients on short-term hemodialysis without evidence of A beta(2)M, local tracer accumulations similar to those observed with natural, (111)In-labeled beta(2)M could be demonstrated in long-term hemodialysis patients with evidence of A beta(2)M. In conclusion, scintigraphy for A beta(2)M with (111)In-labeled rh beta(2)M provides a homogeneous and safe recombinant protein source and represents a suitable detection method of beta(2)M amyloid deposits in dialysis patients.

Recombinant versus natural human 111In-beta2-microglobulin for scintigraphic detection of Abeta2m amyloid in dialysis patients

BACKGROUND

We previously introduced scintigraphy with 131I-labeled beta2-microglobulin (beta2m), purified from uremic hemofiltrate, that is, "natural" beta2m, to specifically detect beta2m-associated amyloidosis (Abeta2m) in hemodialysis (HD) patients.

METHODS

To improve the safety and resolution of the scan, we covalently bound the chelator diethylenetriaminepentaacetic acid to natural beta2m to allow radiolabeling with 111In. In a second step, we generated and evaluated the usage of recombinant human beta2m (rhbeta2m) for scintigraphy.

RESULTS

Using natural 111In-labeled beta2m, eight patients on HD for 0 to 17 years, without evidence of Abeta2m, were scanned. Whole-body scintigraphy at 48 to 72 hours postinjection revealed no significant tracer accumulation over joint regions. In contrast, nine patients on HD for 10 to 21 years with clinical, radiological, or histologic (N = 4) evidence of Abeta2m showed selective tracer uptake over various joint regions. Tracer accumulation in visceral organs, which could not be related to tracer elimination or metabolism, was not detected. Compared with the previous 131I beta2m scan, scintigraphy with 111In-labeled beta2m offered highly improved image contrast, increased sensitivity, and a 50 to 70% reduction of the radiation exposure. Scanning with 111In-labeled recombinant human beta2m was performed in six patients: No significant tracer accumulation was observed over joint regions in two patients on short-term HD without evidence of Abeta2m; in contrast, local tracer accumulations similar to those observed with natural beta2m could be demonstrated in four long-term (10 to 27 years) HD patients with clinical, radiological, and histologic (N = 1) evidence of Abeta2m.

CONCLUSION

Scintigraphy for Abeta2m with 111In-labeled rhbeta2m provides a homogenous and safe recombinant protein source and leads to enhanced sensitivity and lower radiation exposure.