Cathepsin B prevents cell death by fragmentation and destruction of pathological amyloid fibrils.

Amyloid fibrils cause organ and tissue dysfunction in numerous severe diseases. Despite the prevalence and severity of amyloidoses, there is still no effective and safe anti-amyloid therapy. This study investigates the impact of cysteine protease cathepsin B (CTSB) on amyloids associated with Alzheimer's and Parkinson's diseases, hemodialysis, and lysozyme amyloidosis. We analyzed the effect of CTSB on the size, structure, and proteotoxicity of amyloid fibrils formed from alpha-synuclein, abeta peptide (1-42), insulin, and lysozyme using a combination of spectroscopic, microscopic, electrophoretic, and colorimetric methods. Our comprehensive research revealed a dual effect of CTSB on amyloid fibrils. Firstly, CTSB induced amyloid fragmentation while preserving their ordered morphology, and, secondly, it "loosened" the tertiary structure of amyloids and reduced the regularity of the secondary structure. This dual mechanism of action was universal across fibrils associated with different pathologies, although the disruption efficacy and predominant type of degradation products depended on the amyloids' structure, size, and clustering. Notably, CTSB-induced irreversible degradation significantly reduced the toxicity for immortalized and primary cell lines of low-clustered fibrils, such as alpha-synuclein amyloids associated with Parkinson's disease. These findings enhance our understanding of how endogenous CTSB may regulate amyloid content at the molecular level in different neuropathologies. In addition, our results suggest the potential of CTSB as a component of anti-amyloid drugs in combination with agents that enhance the accessibility of proteolytic sites within amyloid clots and reduce these clusters stability.