Collagenous Alzheimer amyloid plaque component assembles amyloid fibrils into protease resistant aggregates

Recently, a novel plaque-associated protein, collagenous Alzheimer amyloid plaque component (CLAC), was identified in brains from patients with Alzheimer's disease. CLAC is derived from a type II transmembrane collagen precursor protein, termed CLAC-P (collagen XXV). The biological function and the contribution of CLAC to the pathogenesis of Alzheimer's disease and plaque formation are unknown. In vitro studies indicate that CLAC binds to fibrillar, but not to monomeric, amyloid beta-peptide (A beta). Here, we examined the effects of CLAC on A beta fibrils using assays based on turbidity, thioflavin T binding, sedimentation analysis, and electron microscopy. The incubation of CLAC with preformed A beta fibrils led to increased turbidity, indicating that larger aggregates were formed. In support of this contention, more A beta was sedimented in the presence of CLAC, as determined by gel electrophoresis. Moreover, electron microscopy revealed an increased amount of A beta fibril bundles in samples incubated with CLAC. Importantly, the frequently used thioflavin T-binding assay failed to reveal these effects of CLAC. Digestion with proteinase K or trypsin showed that A beta fibrils, incubated together with CLAC, were more resistant to proteolytic degradation. Therefore, CLAC assembles A beta fibrils into fibril bundles that have an increased resistance to proteases. We suggest that CLAC may act in a similar way in vivo.