This elegant study by Eichner et al. considerably advances atomic-level understanding of the molecular features that underlie human β₂-microglobulin (β₂m) amyloidogenecity, and should facilitate the development of novel therapeutics for dialysis-related amyloidosis (DRA). Using NMR, they solved a solution structure of an amyloidogenic intermediate of N-terminal truncated β₂m (ΔN6), a protein variant that is present in renal amyloid deposits of DRA patients. Furthermore, using NMR, Eichner et al. established specific conformational changes in full-length β₂m that arose from bimolecular interactions with ΔN6 that presumably reflected the truncated protein’s prion-like seeding of β₂m fibrils. Their findings reinforce that invaluable insight on the molecular basis of amyloidogenic polypeptides can be obtained by probing, at the atomic level, the native and non-native structures and aggregation propensities of pathogenically relevant variants (unmodified and post-translationally modified forms).

In contrast with the central...  Read more

This elegant study by Eichner et al. considerably advances atomic-level understanding of the molecular features that underlie human β₂-microglobulin (β₂m) amyloidogenecity, and should facilitate the development of novel therapeutics for dialysis-related amyloidosis (DRA). Using NMR, they solved a solution structure of an amyloidogenic intermediate of N-terminal truncated β₂m (ΔN6), a protein variant that is present in renal amyloid deposits of DRA patients. Furthermore, using NMR, Eichner et al. established specific conformational changes in full-length β₂m that arose from bimolecular interactions with ΔN6 that presumably reflected the truncated protein’s prion-like seeding of β₂m fibrils. Their findings reinforce that invaluable insight on the molecular basis of amyloidogenic polypeptides can be obtained by probing, at the atomic level, the native and non-native structures and aggregation propensities of pathogenically relevant variants (unmodified and post-translationally modified forms).

In contrast with the central pathogenic role that soluble Aβ oligomers are believed to have in Alzheimer’s disease, the pathogenic process in DRA is likely to be the abundant renal deposition of amyloid that contains β₂m fibrils and various accessory molecules. Similar with other amyloidogenic polypeptides, β₂m aggregates to form amyloid fibrils via distinct pathways that generate heterogeneous assemblies in dynamic equilibria (1). Nevertheless, this study, and others, suggests that discrete assemblies of ΔN6 are relevant to the disease process. First, greater than 20 percent of β₂m in renal amyloid deposits of DRA patients is proteolyzed and contains the truncated ΔN6 form (2). Second, solid-state NMR studies indicate that the non-native trans-prolyl peptide bond at position 32, which is present in the ΔN6 intermediate, is also present in in-vitro generated β₂m fibrils (3). Third, ΔN6 forms amyloid fibrils more readily than the full-length protein (4). Fourth, ΔN6 interacts more strongly with collagen than the full-length protein, and β₂m amyloid deposits more readily in collagen-rich joints (5). Lastly, Eichner et al. showed that the trans-prolyl ΔN6 intermediate acted in a prion-like manner by specifically seeding fibril formation by the less amyloidogenic full-length β₂m. If, indeed, ΔN6 assemblies are a primary culprit for DRA, a novel therapeutic strategy may be reagents that specifically inhibit proteolysis of the N-terminal of β₂m.

As mentioned above, more than one assembly of β₂m is likely to contribute to DRA. Thus, it would be interesting to establish if the ΔN6 intermediate studied by Eichner et al., or an equivalent full-length β₂m assembly, is a pre-nucleus for fibrils or a direct precursor for oligomers that are formed by domain-swapped β₂m variants (6,7). Alternatively, the latter conformers may form amyloid fibrils by distinct pathways that reflect the heterogeneous nature of β₂m fibrils (8). Lastly, the finding by Eichner et al. that non-native structure in the ΔN6 intermediate is anchored by a propyl-peptide rearrangement is similar to the central role that proline has in stabilizing non-native conformations of several other amyloidogenic proteins, for example, the variable light chain from the κ4 Bence Jones protein, Len (9).

References:
1. Platt GW, Radford SE. Glimpses of the molecular mechanisms of beta2-microglobulin fibril formation in vitro: aggregation on a complex energy landscape. FEBS Lett. 2009 Aug 20;583(16):2623-9. Abstract

2. Bellotti V, Stoppini M, Mangione P, Sunde M, Robinson C, Asti L, Brancaccio D, Ferri G. Beta2-microglobulin can be refolded into a native state from ex vivo amyloid fibrils. Eur J Biochem. 1998 Nov 15;258(1):61-7. Abstract

3. Barbet-Massin E, Ricagno S, Lewandowski JR, Giorgetti S, Bellotti V, Bolognesi M, Emsley L, Pintacuda G. Fibrillar vs crystalline full-length beta-2-microglobulin studied by high-resolution solid-state NMR spectroscopy. J Am Chem Soc. 2010 Apr 28;132(16):5556-7. Abstract

4. Esposito G, Michelutti R, Verdone G, Viglino P, Hernández H, Robinson CV, Amoresano A, Dal Piaz F, Monti M, Pucci P, Mangione P, Stoppini M, Merlini G, Ferri G, Bellotti V. Removal of the N-terminal hexapeptide from human beta2-microglobulin facilitates protein aggregation and fibril formation. Protein Sci. 2000 May;9(5):831-45. Abstract

5. Giorgetti S, Rossi A, Mangione P, Raimondi S, Marini S, Stoppini M, Corazza A, Viglino P, Esposito G, Cetta G, Merlini G, Bellotti V. Beta2-microglobulin isoforms display an heterogeneous affinity for type I collagen. Protein Sci. 2005 Mar;14(3):696-702. Abstract

6. Domanska K, Vanderhaegen S, Srinivasan V, Pardon E, Dupeux F, Marquez JA, Giorgetti S, Stoppini M, Wyns L, Bellotti V, Steyaert J. Atomic structure of a nanobody-trapped domain-swapped dimer of an amyloidogenic {beta}2-microglobulin variant. Proc Natl Acad Sci U S A. 2011 Jan 25;108(4):1314-9. Abstract

7. Liu C, Sawaya MR, Eisenberg D. ß2-microglobulin forms three-dimensional domain-swapped amyloid fibrils with disulfide linkages. Nat Struct Mol Biol. 2011 Jan;18(1):49-55. Abstract

8. Radford SE, Gosal WS, Platt GW. Towards an understanding of the structural molecular mechanism of beta(2)-microglobulin amyloid formation in vitro. Biochim Biophys Acta. 2005 Nov 10;1753(1):51-63. Abstract

9. O'Nuallain B, Allen A, Ataman D, Weiss DT, Solomon A, Wall JS. Phage display and peptide mapping of an immunoglobulin light chain fibril-related conformational epitope. Biochemistry. 2007 Nov 13;46(45):13049-58. Abstract

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