The structural core of prion disease

Transmissible spongiform encephalopathies (TSEs) or prion diseases are serious neurological ailments, in which the brain tissue deteriorates by progressive loss of brain cells which results in the loss of a wide variety of brain functions, including memory, speech and locomotion. Similar conditions can be observed in patients with Alzheimer’s disease (AD). In TSEs, Alzheimer’s disease, and some more protein folding diseases a key factor in the pathogenesis is the deposition of aggregated aberrant proteins, often accompanied by amyloid fibril formation. In prion diseases the pathogenic particle is a malformed version of the prion protein (PrP) in which the endogenous prion protein (PrPC) is transformed into a malformed pathogenic version (PrPSc) with a higher beta-sheet content compared with PrPC. The conformational change of PrPC into PrPSc is caused by an other PrPSc molecule which functions as a template forcing the PrPC into the PrPSc form. The malformed PrPSc accumulates in amyloidogenic fibrils which form insoluble aggregates. These aggregates are associated with the loss of brain cells. Structural knowledge of the malformed protein is essential for the development of therapeutics. Therefore, two new in vitro assays are developed to study PrP and A-beta peptide fibril formation. Both assays depend on reproducible fibril formation. This is achieved either by drying the peptides prior to fibril specific staining, or the addition of polyanionic compounds which act as fibril promoting scaffolds. The assays allowed us to study the fibril interfering properties of tetracyclic compounds and beta-breaker peptides and the fibril promoting activity of C1q and SAP. Most likely these assays are more widely applicable for other fibrillogenic proteins and peptides. Furthermore, the assays helped to validate the theoretical two-rung left-handed beta-helix model of the PrPSc core. According to this model each PrPSc monomer contributes two beta-helical rungs to the fibril. Two cyclized human PrP peptides, corresponding to rung one and rung two of the left-handed beta-helical core of the human PrPSc fibril, show spontaneous cooperative fibril growth in vitro. The unique approach of stacking two different peptides was also used to develop peptide-based therapeutics. The strategy, coined as “stack-and-stop”, is based on a combination of a fibril stimulating peptide and a fibril stopper peptide. The study of fibril inhibiting peptides requires a controllable fibril growth. Therefore the stimulator peptide was designed as an optimal left-handed-beta-helical fold that can serve as a template for fibril growth initiation. The inhibiting peptide was designed to bind to the exposed rung, but frustrate the propagation of the fibril growth. The single inhibitory peptide hardly shows inhibition, but the combination of the inhibitory with the stimulatory peptide showed complete inhibition of the fibril growth. The unique strategy based on stimulatory and inhibitory peptides seems a powerful new approach to study amyloidogenic fibril structures in general and could prove useful for the development of therapeutics