High-Pressure Response of Amyloid Folds

Abstract

The abnormal protein aggregates in progressive neurodegenerative disorders, such as Alzheimer's, Parkinson's and prion diseases, adopt a generic structural form called amyloid fibrils. The precise amyloid fold can differ between patients and these differences are related to distinct neuropathological phenotypes of the diseases. A key focus in current research is the molecular mechanism governing such structural diversity, known as amyloid polymorphism. In this review, we focus on our recent work on recombinant prion protein (recPrP) and the use of pressure as a variable for perturbing protein structure. We suggest that the amyloid polymorphism is based on volumetric features. Accordingly, pressure is the thermodynamic parameter that fits best to exploit volume differences within the states of a chemical reaction, since it shifts the equilibrium constant to the state that has the smaller volume. In this context, there are analogies with the process of correct protein folding, the high pressure-induced effects of which have been studied for more than a century and which provides a valuable source of inspiration. We present a short overview of this background and review our recent results regarding the folding, misfolding, and aggregation-disaggregation of recPrP under pressure. We present preliminary experiments aimed at identifying how prion protein fibril diversity is related to the quaternary structure by using pressure and varying protein sequences. Finally, we consider outstanding questions and testable mechanistic hypotheses regarding the multiplicity of states in the amyloid fold.

Keywords: amyloid; high pressure; polymorphism; prion.

Figure 1

Pressure dissociates PrP fibrils and alters their macrostructure. Negative stained transmission electron microscopy micrographs of (a) untreated (8 h at 25 °C) PrP fibrils; and (c) after a cycle of compression (360 MPa, 8 h)/decompression at 25 °C. Scale bar, 500 nm. (b) The kinetics of the pressure-induced PrP fibril dissociation were recorded as a decrease in light scattering. Fibril fraction was calculated from the amount of resolubilized PrP rescued after the pressure treatment, as judged from the absorbance at 280 nm of the supernatants obtained after removing fibrils by centrifugation. Dashed lines, linear fits to the data.

Figure 2

Pressure alters the size-distribution of PrP fibrils. Sedimentation velocity centrifugation in density gradients of mouse (black) and bank vole (red) PrP fibrils labelled with Alexa Fluor 488. The sedimentogram of non-treated fibrils is denoted in solid circles (right Y-axis) and those obtained after a cycle of compression (360 MPa)/decompression at 25 °C are shown in open circles (right Y-axis).

Figure 3

Schematic representation of a hypothetical effect of pressure on the energy landscape of an amyloidogenic protein. The conformational selection phenomenon assumes that the new thermodynamic condition reached under pressure will preferentially stabilize the amyloid state (i.e., conformation) with the highest packing density, resulting in a population shift of the ensemble toward the state showing the deepest energy minimum.