Broken but not beaten: Challenge of reducing the amyloids pathogenicity by degradation

[Display omitted] •Presumably highly stable, amyloids are actually susceptible to degradation by many physical and chemical effects.•Amyloid degradation is rarely complete, resulting in aggregates with different properties.•Degradation products often outperform intact amyloids in cytotoxic, seeding, and infectious activity.•Fibril-destabilizing medications can induce severe side effects.•Anti-fibrillogenic drugs also must be tested for their effect on mature amyloids. The accumulation of ordered protein aggregates, amyloid fibrils, accompanies various neurodegenerative diseases (such as Parkinson's, Huntington's, Alzheimer's, etc.) and causes a wide range of systemic and local amyloidoses (such as insulin, hemodialysis amyloidosis, etc.). Such pathologies are usually diagnosed when the disease is already irreversible and a large amount of amyloid plaques have accumulated. In recent years, new drugs aimed at reducing amyloid levels have been actively developed. However, although clinical trials have demonstrated a reduction in amyloid plaque size with these drugs, their effect on disease progression has been controversial and associated with significant side effects, the reasons of which are not fully understood. The purpose of this review is to summarize extensive array of data on the effect of exogenous and endogenous factors (physico-mechanical effects, chemical effects of low molecular weight compounds, macromolecules and their complexes) on the structure and pathogenicity of mature amyloids for proposing future directions of the development of effective and safe anti-amyloid therapeutics. Our analysis show that destruction of amyloids is in most cases incomplete and degradation products often retain the properties of amyloids (including high and sometimes higher than fibrils, cytotoxicity), accelerate amyloidogenesis and promote the propagation of amyloids between cells. Probably, the appearance of protein aggregates, polymorphic in structure and properties (such as amorphous aggregates, fibril fragments, amyloid oligomers, etc.), formed because of uncontrolled degradation of amyloids, may be one of the reasons for the ambiguous effectiveness and serious side effects of the anti-amyloid drugs. This means that all medications that are supposed to be used both for degradation and slow down the fibrillogenesis must first be tested on mature fibrils: the mechanism of drug action and cytotoxic, seeding, and infectious activity of the degradation products mu