Prions

Recent advances in cellular models for discovering prion disease therapeutics

INTRODUCTION

Prion diseases are a group of rare and lethal rapidly progressive neurodegenerative diseases arising due to conversion of the physiological cellular prion protein into its pathological counterparts, denoted as "prions". These agents are resistant to inactivation by standard decontamination procedures and can be transmitted between individuals, consequently driving the irreversible brain damage typical of the diseases.

AREAS COVERED

Since its infancy, prion research has mainly depended on animal models for untangling the pathogenesis of the disease as well as for the drug development studies. With the advent of prion-infected cell lines, relevant animal models have been complemented by a variety of cell-based models presenting a much faster, ethically acceptable alternative.

EXPERT OPINION

To date, there are still either no effective prophylactic regimens or therapies for human prion diseases. Therefore, there is an urgent need for more relevant cellular models that best approximate in vivo models. Each cellular model presented and discussed in detail in this review has its own benefits and limitations. Once embarking in a drug screening campaign for the identification of molecules that could interfere with prion conversion and replication, one should carefully consider the ideal cellular model.

Yeast Models for Amyloids and Prions: Environmental Modulation and Drug Discovery

Amyloids are self-perpetuating protein aggregates causing neurodegenerative diseases in mammals. Prions are transmissible protein isoforms (usually of amyloid nature). Prion features were recently reported for various proteins involved in amyloid and neural inclusion disorders. Heritable yeast prions share molecular properties (and in the case of polyglutamines, amino acid composition) with human disease-related amyloids. Fundamental protein quality control pathways, including chaperones, the ubiquitin proteasome system and autophagy are highly conserved between yeast and human cells. Crucial cellular proteins and conditions influencing amyloids and prions were uncovered in the yeast model. The treatments available for neurodegenerative amyloid-associated diseases are few and their efficiency is limited. Yeast models of amyloid-related neurodegenerative diseases have become powerful tools for high-throughput screening for chemical compounds and FDA-approved drugs that reduce aggregation and toxicity of amyloids. Although some environmental agents have been linked to certain amyloid diseases, the molecular basis of their action remains unclear. Environmental stresses trigger amyloid formation and loss, acting either via influencing intracellular concentrations of the amyloidogenic proteins or via heterologous inducers of prions. Studies of environmental and physiological regulation of yeast prions open new possibilities for pharmacological intervention and/or prophylactic procedures aiming on common cellular systems rather than the properties of specific amyloids.

Unified theoretical description of the kinetics of protein aggregation

Solution conditions chosen for the production of amyloid can also promote formation of significant extents of amorphous protein aggregate. In one interpretation, the amyloid and amorphous aggregation pathways are considered to be in competition with each other. An alternative conceptualization involves considering amorphous aggregation as an obligatory intermediate process of the amyloid formation pathway. Here, we review recently developed macroscopic-level theories of protein aggregation that unify these two competing models into a single paradigm. Key features of the unified model included (1) a description of the amorphous aggregate as a second liquid phase with the degree of liquid-like character determined by the mobility of the monomer within it, and (2) heterogeneous growth pathways based on nucleation, growth, and fragmentation of amyloid occurring within different phases and at their interfacial boundary. Limiting-case behaviors of the protein aggregation reaction, either singly involving amyloid or amorphous aggregate production, and mixed-case behaviors, involving competitive and/or facilitated growth of amorphous and amyloid species, are presented and reviewed in context. This review principally describes an approach developed by Hirota and Hall 2019 (Hirota, N. and Hall, D. 2019. Protein Aggregation Kinetics: A Unified Theoretical Description. Chapter 7 of 'Protein Solubility and Amorphous Aggregation: From Academic Research to Applications in Drug Discovery and Bioindustry' edited by Y. Kuroda and F. Arisaka. CMC Publishers). Sections of that work are translated from the original Japanese and republished here with the full permission of CMC Publishing Corporation.

Genetic Methods for Studying Yeast Prions

The recognition that certain long-known nonchromosomal genetic elements were actually prions was based not on the specific phenotypic manifestations of those elements, but rather on their unusual genetic properties. Here, we outline methods of prion assay, methods for showing the nonchromosomal inheritance, and methods for determining whether a nonchromosomal trait has the unusual characteristics diagnostic of a prion. Finally, we discuss genetic methods often useful in the study of yeast prions.