The single amino acid mutation G26R in human apolipoprotein A-I (apoA-IIowa) is the first mutation that was associated with familial AApoA1 amyloidosis. The N-terminal fragments (amino acid residues 1–83) of apoA-I containing this mutation deposit as amyloid fibrils in patients’ tissues and organs, but the mechanisms of cellular degradation and cytotoxicity have not yet been clarified. In this study, we demonstrated degradation of apoA-IIowa fibrils via the autophagy-lysosomal pathway in human embryonic kidney 293 cells. ApoA-IIowa fibrils induced an increase in lysosomal pH and the cytosolic release of the toxic lysosomal protease cathepsin B. The mitochondrial dysfunction caused by apoA-IIowa fibrils depended on cathepsin B and was ameliorated by increasing the degradation of apoA-IIowa fibrils. Thus, although apoA-IIowa fibril transport to lysosomes and fibril degradation in lysosomes may have occurred, the presence of an excess number of apoA-IIowa fibrils, more than the lysosomes could degrade, may be detrimental to cells. Our results thus provide evidence that the target of apoA-IIowa fibrils is lysosomes, and we thereby gained a novel insight into the mechanism of AApoA1 amyloidosis.

Highly sulfated domains of heparan sulfate (HS), alias HS S-domains, consist of consecutive clusters of a trisulfated disaccharide unit [iduronic acid (2S)-glucosamine (NS, 6S)]. 1 These HS S-domains are a major determinant of specific interactions between HS and its protein ligands, including growth factors, chemokines, and morphogens, and thus they regulate HS functions in diverse biological processes. 2,3 In addition to being involved in biological processes, HS is associated with various amyloid deposits, including those in transthyretin amyloidosis (ATTR amyloidosis), 4 Alzheimer disease (AD), 5e7 light-chain amyloidosis, 8 and amyloid A (AA) amyloidosis, 9 which suggests that HS may affect amyloid deposition and amyloidosis pathology. Indeed, HS and heparin, whose major components are HS S-domains, reportedly promoted fibrillogenesis of various amyloidogenic proteins, such as transthyretin (TTR) in ATTR amyloidosis, 4,10 amyloid b (Ab) in AD, 11e13 cellular prion, Ig light chain in light-chain amyloidosis, 14 and serum AA in AA amyloidosis. 15e17 Also, HS at the cell surface mediated interactions of cells with and cytotoxicity of amyloid fibrils or protein aggregates, including those of t protein and a-synuclein, 18 Ab, 19 Supported in part by the Japan Society for the Promotion of Science grants-in-aid for young scientists B-15K19488 (K.N.