Amyloid disorders cause debilitating illnesses through the formation of toxic protein aggregates. The mechanisms of amyloid toxicity and the nature of species responsible for mediating cellular dysfunction remain unclear. Here, using β 2 -microglobulin (β 2 m) as a model system, we show that the disruption of membranes by amyloid fibrils is caused by the molecular shedding of membrane-active oligomers in a process that is dependent on pH. Using thioflavin T (ThT) fluorescence, NMR, EM and fluorescence correlation spectroscopy (FCS), we show that fibril disassembly at pH 6.4 results in the formation of nonnative spherical oligomers that disrupt synthetic membranes. By contrast, fibril dissociation at pH 7.4 results in the formation of nontoxic, native monomers. Chemical cross-linking or interaction with hsp70 increases the kinetic stability of fibrils and decreases their capacity to cause membrane disruption and cellular dysfunction. The results demonstrate how pH can modulate the deleterious effects of preformed amyloid aggregates and suggest why endocytic trafficking through acidic compartments may be a key factor in amyloid disease.amyloid | fibrils | disassembly | oligomer | membrane disruption

Background: The causative agents and pathological mechanisms of amyloid disease are poorly understood.Results: β2-Microglobulin amyloid fibrils display length-dependent internalization, alter trafficking of lysosomal membrane proteins, and inhibit the degradation of proteins by lysosomes.Conclusion: Amyloid fibrils act as nanoparticles that disrupt membrane trafficking and protein degradation.Significance: Fibril length, by determining access to intracellular compartments, may contribute to amyloid disease.

Severe α1-antitrypsin deficiency results from the Z allele (Glu342Lys) that causes the accumulation of homopolymers of mutant α1-antitrypsin within the endoplasmic reticulum of hepatocytes in association with liver disease. We have used a DNA-encoded chemical library to undertake a high throughput screen to identify small molecules that bind to, and stabilise Z α1-antitrypsin. The lead compound blocks Z α1-antitrypsin polymerisation in vitro, reduces intracellular polymerisation and increases the secretion of Z α1-antitrypsin three-fold in mammalian cells including an iPSC model of disease. Crystallographic and biophysical analyses demonstrate that GSK716 and related molecules bind to a cryptic binding pocket, negate the local effects of the Z mutation and stabilise the bound state against progression along the polymerization pathway. Oral dosing of transgenic mice at 100 mg/kg three times a day for 20 days increased the secretion of Z α1-antitrypsin into the plasma by 7-fold. There was no observable clearance of hepatic inclusions with respect to controls. This study provides proof-of-principle that ‘mutation ameliorating’ small molecules are a viable approach to treat protein conformational diseases.