Anfinsen showed that a protein's fold is specified by its sequence. Although it is clear why mutant proteins form amyloid, it is harder to rationalize why a wild-type protein adopts a native conformation in most individuals, but it misfolds in a minority of others, in what should be a common extracellular environment. This discrepancy suggests that another event likely triggers misfolding in sporadic amyloid disease. One possibility is that an abnormal metabolite, generated only in some individuals, covalently modifies the protein or peptide and causes it to misfold, but evidence for this is sparse. Candidate metabolites are suggested by the recently appreciated links between Alzheimer's disease (AD) and atherosclerosis, known chronic inflammatory metabolites, and the newly discovered generation of ozone during inflammation. Here we report detection of cholesterol ozonolysis products in human brains. These products and a related, lipid-derived aldehyde covalently modify A␤, dramatically accelerating its amyloidogenesis in vitro, providing a possible chemical link between hypercholesterolemia, inflammation, atherosclerosis, and sporadic AD. Anfinsen's classic experiments demonstrated that a protein's amino acid sequence specifies its conformation (1). These ideas were extended to explain the misfolding susceptibility of mutant proteins associated with a growing number of familial amyloid diseases (2-5). Although it is thus clear why mutant proteins might be more susceptible to misfolding, it is harder to understand why a wild-type protein or peptide adopts a native conformation in some individuals but it misfolds in others in what should be a common extracellular environment, leading to sporadic amyloid diseases. This discrepancy suggests that other events likely trigger misfolding in sporadic amyloid disease, but their nature remains elusive.The misfolding of secreted amyloid ␤ peptides (A␤) 39-43 residues in length is linked by a plethora of evidence to the pathology of Alzheimer's disease (AD) (6, 7). A␤ misfolding occurs when the soluble, monomeric, extracellular ensemble of extended conformations and low M r oligomers is transformed first into spherical assemblies, then into a number of intermediates, and lastly into fibrillar cross ␤-sheet quaternary structures known as amyloid (8)(9)(10)(11)(12). Amyloid fibrils and related structures recruit soluble A␤ to the aggregate by a seeded polymerization mechanism (10). The direct neurotoxicity of A␤ aggregates (8, 13) combined with their role in mediating chronic inflammation by microglia (14) and complement cascade activation (15) suggests that aggregation then mediates inflammation (16), which in turn promotes aggregation, in a vicious cycle of AD pathology.It is known that atherosclerosis and AD share many risk factors, including hypercholesterolemia and inflammation. The apoE-4 allele, which exacerbates hypercholesterolemia, has been linked to AD by data from both epidemiological and transgenic mouse studies (17)(18)(19)(20). It has also recently been shown t...

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contributed equally to this work Hereditary familial amyloidosis of Finnish type (FAF) leading to amyloid in the peripheral and central nervous systems stems from deposition of a 71 residue fragment generated from the D187N/Y variants of plasma gelsolin by two sequential endoproteolytic events. We identify the protease accomplishing the ®rst cleavage as furin, a proprotein convertase. Endoproteolysis of plasma gelsolin occurs in the transGolgi network due to the inability of the FAF variants to bind and be stabilized by Ca 2+ . Secretion and processing of the FAF variants by furin can be uncoupled by blocking the convergence of the exocytic pathway transporting plasma gelsolin and the endocytic recycling of furin. We propose that coincidence of membrane traf®cking pathways contributes to the development of proteolysis-initiated amyloid disease.

Amyloid diseases like Alzheimer's disease and familial amyloidosis of Finnish type (FAF) stem from endoproteolytic cleavage of a precursor protein to generate amyloidogenic peptides that accumulate as amyloid deposits in a tissue-specific manner. FAF patients deposit both 8 and 5 kDa peptides derived from mutant (D187Y/N) plasma gelsolin in the extracellular matrix (ECM). The first of two aberrant sequential proteolytic events is executed by furin to yield a 68 kDa (C68) secreted fragment. We now identify the metalloprotease MT1-matrix metalloprotease (MMP), an integral membrane protein active in the ECM, as a protease that processes C68 to the amyloidogenic peptides. We further demonstrate that ECM components are capable of accelerating gelsolin amyloidogenesis. Proteolysis by MT1-MMP-like proteases proximal to the unique chemical environment of the ECM offers an explanation for the tissue-specific deposition observed in FAF and provides critical insight into new therapeutic strategies.

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