CLAC Binds to Amyloid β Peptides through the Positively Charged Amino Acid Cluster within the Collagenous Domain 1 and Inhibits Formation of Amyloid Fibrils

Abstract

CLAC (collagenous Alzheimer amyloid plaque component) is a proteolytic fragment derived from a novel membrane-bound collagen, CLAC-P/collagen type XXV, that deposits in senile plaques associated with amyloid ␤ peptides (A␤) in the brains of patients with Alzheimer's disease. We previously showed that CLAC binds to the fibrillized form of A␤ in vitro, although the mechanism and the subdomains that mediate interaction of CLAC with A␤ as well as the effect of binding of CLAC on amyloid fibril formation remain unknown. Here we show that the collagenous domain 1 of CLAC, which is rich in positively charged amino acid residues, mediates its interaction with A␤ and that this binding is mediated by an electrostatic interaction and requires formation of the triple helix structure of CLAC. The soluble form of CLAC purified from the media of cells transfected with CLAC-P inhibited fibrillization of A␤ in vitro, especially in its elongation phase. These results suggest the antiamyloidogenic roles of CLAC in the pathophysiology of Alzheimer's disease.Alzheimer's disease (AD) 1 is an elderly onset neurodegenerative disease causing dementia that is pathologically characterized by a massive accumulation of amyloid deposits in senile plaques or cerebrovasculature and of tau-rich neurofibrillary lesions (1). The principal component of the amyloid deposits is amyloid ␤ peptide (A␤), which is proteolytically produced from a transmembrane precursor, ␤APP, as 38 -43-amino acid fragments (2, 3). A␤ secreted from cells exhibits a heterogeneity in its C-terminal extent; A␤42 ending at the 42nd residue, which comprises ϳ10% of A␤40 in total secreted A␤, aggregates much faster than A␤40 (4) and deposits initially as diffuse-type plaques in AD or Down's syndrome brains (5). Moreover, missense mutations in ␤APP, presenilin 1 and presenilin 2 genes linked to early onset familial AD increase the production of A␤42 (6 -8). These findings collectively suggest that aggregation and deposition of A␤ are closely linked to the pathogenesis of AD.A number of non-A␤ proteinaceous components, e.g. apolipoprotein E (apoE), ␣1-antichymotrypsin, ␣ 2 -macroglobulin, complement component C1q, amyloid P component, have been identified associated with senile plaque amyloids (9), some of which have been shown to affect fibrillization of A␤ (10). Genetic polymorphism of apoE, especially the ⑀4 genotype, is known as the major genetic risk factor of AD (11), and accumulating evidence from experimental studies suggests that apoE may affect the fibrillization and deposition of A␤, thereby leading to AD; ablation of apoE gene reduced A␤ deposition in transgenic mice overexpressing human ␤APP, whereas further transgenic supplementation of human apoE ⑀4 gene accelerated deposition of ␤-sheet-rich A␤ (12). In vitro studies, however, showed that apoE also has an inhibitory effect on fibrillization of A␤ (13,14), implicating apoE in various aspects of ␤-amyloid formation in AD. These previous results emphasize the compelling need for the analysis of effects of other no...

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