Seminar

Rationally designed, structure-based vaccine candidates for prion diseases: A valid approach for Alzheimer’s and Parkinson’s disease?

Holger Wille, PhD

All prion diseases occur due to the misfolding of the cellular prion protein (PrPC) into the infectious conformer (PrPSc), which is prone to aggregate into amyloid fibrils. No cures or prophylactic vaccines exist for any prion disease, resulting in an invariably fatal outcome for an infected host. Although the high-resolution structure of PrPSc has eluded experimental determination due to its insolubility and propensity to aggregate, the repeating nature of PrPSc aggregates (e.g. amyloid fibrils) has allowed the collection of lower-resolution structural data via X-ray fiber diffraction, cryo-electron microscopy, and other techniques, all of which indicated a four-rung β-solenoid architecture for PrPSc. The structure of the prion domain of the fungal HET-s prion protein was determined by solid-state NMR spectroscopy to contain a two-rung β-solenoid structure. This structural similarity made the HET-s prion domain an ideal target to create a conformational mimic for the structure of PrPSc. By doubling the coding region of the HET-s prion domain, connected by a flexible linker, a four-rung β-solenoid variant was engineered, termed HET-2s. The similarity of the β-solenoid fold allowed us to use HET-2s as an innocuous scaffold for the rational design of structure-based vaccine candidates targeting PrPSc. The alternating position of amino acid side chains within the β-strands (inwards vs outwards facing) was used to strategically place predicted surface residues of PrPSc onto the surface of HET-2s. Constructs were expressed in E. coli, purified, and assembled into amyloid fibrils morphologically indistinguishable from native HET-s fibrils. Vaccine candidates showing good fibrillization, and therefore adopting the proper β-solenoid fold, were used to immunize both Prnp-/-and FVB wild type mice. The resulting anti-sera were titre-tested against the original antigen, before being tested for specificity against PrPSc. One of our constructs, containing just seven PrP-based residues from before the hydrophobic region and the first α-helix, resulted an immune response that recognized brain homogenate from prion infected individual, but did not bind uninfected brain homogenate. The efficacy of the vaccine candidates will be tested in oral prion-infection models. Moreover, we have established hybridoma cell clones expressing PrPSc-specific monoclonal antibodies from the spleens of the immunized mice. Lastly, it has not escaped our attention that this approach of engineering the surface of an innocuous amyloid scaffold could be used to generate immunological mimics (i.e. vaccine candidates) targeting more common neurodegenerative diseases such as Alzheimer’s or Parkinson’s disease.