The effects of an ideal beta-turn on beta-2 microglobulin fold stability

Antibodies and protein misfolding: From structural research tools to therapeutic strategies

Protein misfolding disorders, including the neurodegenerative conditions Alzheimer's disease (AD) and Parkinson's disease (PD) represent one of the major medical challenges or our time. The underlying molecular mechanisms that govern protein misfolding and its links with disease are very complex processes, involving the formation of transiently populated but highly toxic molecular species within the crowded environment of the cell and tissue. Nevertheless, much progress has been made in understanding these events in recent years through innovative experiments and therapeutic strategies, and in this review we present an overview of the key roles of antibodies and antibody fragments in these endeavors. We discuss in particular how these species are being used in combination with a variety of powerful biochemical and biophysical methodologies, including a range of spectroscopic and microscopic techniques applied not just in vitro but also in situ and in vivo, both to gain a better understanding of the mechanistic nature of protein misfolding and aggregation and also to design novel therapeutic strategies to combat the family of diseases with which they are associated. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.

Secondary structure in the core of amyloid fibrils formed from human β₂m and its truncated variant ΔN6

Amyloid fibrils formed from initially soluble proteins with diverse sequences are associated with an array of human diseases. In the human disorder, dialysis-related amyloidosis (DRA), fibrils contain two major constituents, full-length human β₂-microglobulin (hβ₂m) and a truncation variant, ΔN6 which lacks the N-terminal six amino acids. These fibrils are assembled from initially natively folded proteins with an all antiparallel β-stranded structure. Here, backbone conformations of wild-type hβ₂m and ΔN6 in their amyloid forms have been determined using a combination of dilute isotopic labeling strategies and multidimensional magic angle spinning (MAS) NMR techniques at high magnetic fields, providing valuable structural information at the atomic-level about the fibril architecture. The secondary structures of both fibril types, determined by the assignment of ∼80% of the backbone resonances of these 100- and 94-residue proteins, respectively, reveal substantial backbone rearrangement compared with the location of β-strands in their native immunoglobulin folds. The identification of seven β-strands in hβ₂m fibrils indicates that approximately 70 residues are in a β-strand conformation in the fibril core. By contrast, nine β-strands comprise the fibrils formed from ΔN6, indicating a more extensive core. The precise location and length of β-strands in the two fibril forms also differ. The results indicate fibrils of ΔN6 and hβ₂m have an extensive core architecture involving the majority of residues in the polypeptide sequence. The common elements of the backbone structure of the two proteins likely facilitates their ability to copolymerize during amyloid fibril assembly.

Nanobodies as structural probes of protein misfolding and fibril formation

The deposition of peptides and proteins as amyloid fibrils is a common feature of nearly 50 medical -disorders affecting the brain or a variety of other organs and tissues. These disorders, which include Alzheimer's disease, Parkinson's disease, the prion diseases, and type II diabetes, have an enormous impact on the public health and economy of the modern world. Extensive research is therefore taking place to determine the underlying molecular mechanisms and determinants of the pathological conversion of amyloidogenic proteins from their soluble forms into fibrillar structures. The use of molecular probes and biophysical techniques, such as X-ray crystallography and particularly NMR spectroscopy, are allowing detailed analysis of the mechanism of fibril formation and of the underlying structural and chemical features of the associated pathogenicity. Nanobodies, the antigen-binding domains derived from camelid heavy-chain antibodies, are excellent tools to probe protein aggregation as a result of their exquisite specificity and high affinity and stability, along with their ease of expression and small size; the latter in particular allows them to be used very efficiently in combination with NMR spectroscopy and X-ray crystallography. In this chapter we present an overview of how nanobodies are being used to obtain detailed information on the mechanisms of amyloid formation and on the nature and origin of their links with human diseases.

Structure, stability, and aggregation of β-2 microglobulin mutants: insights from a Fourier transform infrared study in solution and in the crystalline state

β-2 microglobulin (β2m) is an amyloidogenic protein involved in dialysis-related amyloidosis. We report here the study of the structural properties of the protein in solution and in the form of single crystals by Fourier transform infrared (FTIR) spectroscopy and microspectroscopy. The investigation has been extended to four β2m mutants previously characterized by x-ray crystallography: Asp(53)Pro, Asp(59)Pro, Trp(60)Gly, and Trp(60)Val. These variants displayed very similar three-dimensional structures but different thermal stability and aggregation propensity, investigated here by FTIR spectroscopy. For each variant, appreciable spectral differences were found between the protein in solution and in single crystals, consisting in a downshift of the main β-sheet band and in better resolved turn and loop bands, indicative of reduced protein secondary structure dynamics in the crystalline state. Notably, the well-resolved spectra of the β2m crystalline variants enabled us to identify structural differences induced by the single amino acid mutations. Such differences encompass turn and loop structures that might affect the stability and aggregation propensity of the investigated β2m variants. This study highlights the potential of FTIR microspectroscopy to acquire useful structural information on protein crystals, complementary to the crystallographic analyses.

β(2)-microglobulin amyloidosis

Dialysis-related amyloidosis (DRA) is a clinical syndrome of pain, loss of function and other symptoms due to the deposition of amyloid consisting of β(2)-microglobulin (β(2)m) in the musculoskeletal system. The condition is seen in patients who suffer from chronic kidney disease and are treated with hemodialysis for a long time. Even though β(2)m easily can be manipulated to form amyloid in laboratory experiments under non-physiological conditions the precise mechanisms involved in the formation of β(2)m-amyloid in patients with DRA have been difficult to unravel. The current knowledge which is reviewed here indicates that conformational fluctuations centered around the D-strand, the DE-loop, and around the cis-configured Pro32 peptide bond are involved in β(2)m amyloidosis. Also required are highly increased concentrations of circulating β(2)m and possibly various post-translational modifications mediated by the pro-inflammatory environment in uremic blood, together with the influence of divalent metal ions (specifically Cu(2 +)), uremic toxins, and dialysis-enhanced redox-processes. It seems plausible that domain-swapped β(2)m dimers act as building blocks of β-spine cross-β -sheet fibrils consisting of otherwise globular, roughly natively folded protein. An activated complement system and cellular activation perpetuate these reactions which due to the affinity of β(2)m-amyloid for the collagen of synovial surfaces result in the DRA syndrome.

Understanding the complex mechanisms of β2-microglobulin amyloid assembly

Several protein misfolding diseases are associated with the conversion of native proteins into ordered protein aggregates known as amyloid. Studies of amyloid assemblies have indicated that non-native proteins are responsible for initiating aggregation in vitro and in vivo. Despite the importance of these species for understanding amyloid disease, the structural and dynamic features of amyloidogenic intermediates and the molecular details of how they aggregate remain elusive. This review focuses on recent advances in developing a molecular description of the folding and aggregation mechanisms of the human amyloidogenic protein β(2)-microglobulin under physiologically relevant conditions. In particular, the structural and dynamic properties of the non-native folding intermediate I(T) and its role in the initiation of fibrillation and the development of dialysis-related amyloidosis are discussed.