Protein Misfolding and Aggregation-Related Disease

Aggregation-related diseases include Alzheimer's, Amyotrophic Lateral Sclerosis (ALS), Huntington's disease, and the Prion diseases such as Creutzfeld-Jacob disease (CJD). They all have no cure at present. These diseases in humans range from rare in the case of CJD (with an incidence rate of about 1 in a million humans) to common in the case of Alzheimer's (about 10% of those over 65 and about 50% over 85). However they all share a common general mechanism involving neurodegeneration: healthy functional protein misfolds and aggregates, during which process oligomeric species recruit and induce healthy protein to misfold via a template-directed process. Intriguingly, essentially all the neurodegerative disease-related proteins are involved in metal binding and metal-mediated catalysis. For example Superoxide dismutase (SOD1), the homodimeric protein whose misfolding is responsible for the symptoms of ALS, normally functions as an anti-oxidant by converting negatively-charged superoxide radicals to hydrogen peroxide and molecular oxygen through a two-step catalytic process involving the successive reduction and oxidation of a copper ion embedded in the core of the protein. This rate for this reaction is among the fastest known for all enzymes making the reaction essentially diffusion-limited.

Using antibodies designed to mimic the action of misfolded prion (PrPSc), our experimental collaborators have been able to induce healthy prion (PrPc) to misfold, and have dissected which regions are involved in misfolding [1]. These regions turn our to have thermodynamic signatures that are computationally tractable using statistical mechanics analysis along with a proper energy function [2].

One mystery that emerged from the experiments was a kind of "action at a distance" problem wherein an antibody that bound to a region on the N-terminal unstructured tail of PrPc induced the misfolding (as probed by exposure of previously buried epitopes to subsequence diagnostic antibody binding) of a region on the first C-terminal alpha-helix.

We hypothesized in [1] that the disordered N-terminal tail is not in a random-coil state, but instead must exist in a more condensed phase around the structured domain of the protein, similar to a "molten shell".

This condensed halo of polymer is not sufficiently structured to yield any significant NMR constraints, but must contribute to the stability of the C-terminal structured domain through the collective effect of transient interactions again imperceptible to conventional NMR analysis. The C-terminal domain can be thought of as an "avidity-enhanced structure".

References:

[1] Li L, Guest W, Huang A, Plotkin SS, Cashman N, "Immunological mimicry of PrPc-PrPSc interactions: Antibody-induced PrP misfolding" Protein Engineering, Design and Selection 22(8):523-529 (2009).
  
[2] Guest W, Cashman N, Plotkin SS, "Electrostatics in the Stability and Misfolding of the Prion Protein: Salt Bridges, Self-Energy, and Solvation" Biochem. Cell Biol 88, 371–381 (2010)

[3] Guest W, Plotkin SS, Cashman NR, "Toward A Mechanism of Prion Misfolding and Structural Models of PrPSc: Current Knowledge and Future Directions", J Toxicol Env Health Part A, 74:154–160 (2011)

[4] Grad LI, Guest W, Yanai A, Pokrishevsky E, O’Neill MA, Gibbs E, Semenchenko V, Yousefi M, Wishart DS, Plotkin SS, Cashman NR, "Intermolecular transmission of superoxide dismutase 1 misfolding in living cells" Proc. Natl. Acad. Sci, 108, 16398–16403 (2011)