14 February 2009. Now hear this. There’s new support for the idea that antibodies that stabilize normal prion proteins offer hope of fighting Creutzfeldt-Jakob and other prion-based diseases. In the 9 February PNAS online, researchers led by John Collinge at the Medical Research Council Prion Unit, London, and S. Samar Hasnain at the Daresbury Laboratory, Warrington, England, detail x-ray crystallographic analysis of a prion-antibody complex. They found that a monoclonal antibody with high therapeutic potential binds with high affinity to normal prion (PrPc) rather than a toxic form (PrPSc). The finding may help researchers develop better antibodies for passive immunotherapy and helps explain why certain mutations in the prion proteins increase susceptibility to prion-based disease.
Prion diseases, which include mad cow disease (or bovine spongiform encephalopathy, BSE) and scrapie in sheep, are caused by the Dr. Jekyll/Mr. Hyde-like prions. The normally innocuous proteins assume a toxic form following a rare, though potentially lethal shape shift. Toxic PrPSc proteins form amyloids and also have a penchant for transmogrifying normal prions, setting off a self-perpetuating, neurodegenerative spiral. The bulk of evidence indicates that this self-catalytic conversion is what makes prions infectious and that stabilizing the normal PrPc might be a key to controlling prion diseases. The new research supports this idea.
First author Svetlana Antonyuk and colleagues tested monoclonal antibodies previously made against α- and β-PrP, which have the same basic conformations as PrPc and PrPSc, respectively. They found that antibodies that had the highest affinity for PrP were by far the most potent at inhibiting prion propagation in ScN2a neuroblastoma cells. The most potent antibody, ICSM 18, was previously shown to prevent prion disease in mice (see White et al., 2003).
To determine what kind of structure these antibodies recognize, the researchers took the Fab fragments (containing the antigen binding site) of the most potent antibody (ICSM 18) and crystallized it in the presence of a truncated form of recombinant human PrP (PrP119-231) containing the antibody binding site. That epitope is in residues 143-156, which make up helix H1 of the protein. That α helix is thought to contribute to prion toxicity by undergoing a conformational change to β-sheet structure that helps drive conversion to PrPSc. The crystal structure shows a significant stabilization of the H1 helix. It also shows that amino acids 197-205, which form part of a disordered segment of human PrPc that may promote β-sheet formation, are in close proximity to the heavy chain of the antibody. “The physical proximity of the Fab H chain to a region of PrP that is a possible site for β-sheet formation suggests that the complex may provide structural inhibition by burying the ‘active’ residues at this interface,” write the authors (see image below).
A Packed House
The crystal structure shows light (magenta) and heavy (cyan) chains of the monoclonal antibody packed together with normal PrP (green). Image credit: Samar Hasnain, University of Liverpool, U.K.
The 1-to-2 antibody-to-prion complex further shows an interaction between two β-sheet strands (amino acids 129-131 and 161-163) that have been found sidling up to each other in structures obtained in the absence of antibody (see Haire et al., 2004). The authors suggest that this is hardly coincidence and may be related to prion propagation. That idea is supported by the fact that polymorphisms in amino acid 129 have a dramatic effect on prion transmission and pathogenesis. Heterozygosity at this position is protective against prion disease in humans, while transmission of BSE from cows to humans appears to occur exclusively in people who are homozygous for methionine at that position. “Heterozygosity is protective owing to the requirement for sequence homogeneity in forming ordered, self-replicating particles,” suggest the authors.
In other prion news this week, researchers led by Stanley Prusiner at the University of California, San Francisco, report that different polyoxometalates (POMs), such as phosphotungstate anions, influence whether prions form into amyloid-like rods or two-dimensional crystals. It is not clear by what mechanism POMs work their magic, or if there may be POMs that can be used to create 3D crystals for ultrastructural analysis. The authors do not address whether POMs might offer any therapeutic value.—Tom Fagan.
Antonyuk SV, Trevitt CR, Strange RW, Jackson GS, Sangar D, Batchelor M, Cooper S, Fraser C, Jones S, Georgiou T, Khalili-Shirazi A, Clarke AR, Hasnain SS, Collinge J. Crystal structure of human prion protein bound to a therapeutic antibody. PNAS early edition. 2009, February 9. Abstract
Wille H, Shanmugam M, Murugesu M, Ollesch J, Stubbs G, Long JR, Safar JG, Prusiner SB. Surface charges of polyoxometalates modulate polymerization of the scrapie prion protein. PNAS early edition, 2009, February 9. Abstract