Characterization of the Alzheimer's Disease-associated CLAC Protein and Identification of an Amyloid β-Peptide-binding Site

Abstract

Amyloid ␤-peptide (A␤) deposition into amyloid plaques is one of the invariant neuropathological features of Alzheimer's disease. Other proteins co-deposit with A␤ in plaques, and one recently identified amyloidassociated protein is the collagen-like Alzheimer amyloid plaque component CLAC. It is not known how CLAC deposition affects A␤ plaque genesis and the progress of the disease. Here, we studied the in vitro properties of CLAC purified from a mammalian expression system. CLAC displays features characteristic of a collagen protein, e.g. it forms a partly protease-resistant triple-helical structure, exhibits an intermediate affinity for heparin, and is glycosylated. Purified CLAC was also used to investigate the interaction between CLAC and A␤. Using a solid-phase binding assay, we show that CLAC bound with a similar affinity to aggregates formed by A␤-(1-40) and A␤-(1-42) and that the interaction was impaired by increasing salt concentrations. An 8-residue-long sequence located in non-collagenous domain 2 of CLAC was found to be crucial for the interaction with A␤. These findings may be useful for future therapeutic interventions aimed at finding compounds that modulate the binding of CLAC to A␤ deposits.Alzheimer's disease (AD) 1 is a progressive neurodegenerative disorder characterized by selective neuronal loss associated with intracellular neurofibrillary tangle formation and extracellular amyloid plaques (1). The major constituents of the amyloid plaques are fibrils formed from the 40 -42-residue amyloid ␤-peptide (A␤) (2). A␤ is generated from the type I transmembrane Alzheimer amyloid precursor protein by two consecutive proteolytic cleavages. The aspartyl protease ␤-secretase BACE generates the N terminus of A␤, whereas the C terminus results from the proteolytic action of the ␥-secretase complex (3). Data from genetic and biochemical studies suggest that accumulation of the longer and more amyloidogenic species of A␤, A␤42, is a primary event in the development of AD (4). For instance, autosomal dominant forms of early-onset familial AD appear to result from an increased production of A␤42 (5), and total levels of brain A␤42 correlate with cognitive decline in AD (6).Several proteins other than A␤ have been shown by immunohistochemical methods to be associated with AD amyloid plaques (7). Some of these plaque-associated proteins, e.g. apolipoprotein J (8, 9), apolipoprotein E (10), and ␣ 1 -antichymotrypsin (11), modulate peptide aggregation. In addition, constituents of the extracellular brain matrix, e.g. the heparan sulfate proteoglycans (12-14), laminin (15, 16), and collagen type IV (17), accumulate in amyloid plaques and bind to A␤. The appearance of some of these plaque-associated proteins may be indicative of a local inflammatory response to the amyloid, and others may promote A␤ aggregation or stabilize the amyloid plaques. Recently, a novel plaque-associated protein, CLAC (collagen-like Alzheimer amyloid plaque component), was identified using antibodies raised against insoluble amyloid fr...

Collagens in the developing and diseased nervous system

“…Both the furin cleaved secreted (sCLAC/sCollagenXXV) and the membrane tethered (pCLAC/pCollagenXXV) forms can bind to Ab deposits. In vitro, two binding sites for Ab were identified on CLAC, one in the collagenous domain 1 and one in the non collagenous 2 domain [116,117]. It was shown that CLAC could favour the resistance of Ab aggregates to protease [118] and two studies converge to show that CLAC reduces Ab fibril growth [117,119].…”

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

“…This is consistent with our preliminary immunohistochemical data that ectodomain fragments of type XIII or type XXIII collagens do not appear to be associated with senile plaque amyloid. 2 Recently Soderberg et al (32) reported that another positively charged amino acid-rich domain of CLAC, LIKRRLIK, within the non-collagenous domain 2, is the major binding site for A␤ (32); they deleted or replaced these amino acid sequences with the homologous region of type XIII collagen that contains smaller number of basic amino acids compared with CLAC and found a reduced A␤ binding. This domain seemed to bind to A␤ irrespective of the triple-helix formation of CLAC because synthetic peptides mimicking these sequences were also bound to A␤.…”

Brain-Specific Deletion of Extracellular Signal-Regulated Kinase 2 Mitogen-Activated Protein Kinase Leads to Aberrant Cortical Collagen Deposition

“…64 Intriguingly, a nonamyloid protein found in a subset of Alzheimer's plaques was found to be a modified form of a transmembrane collagen. [65][66][67] The presence of parenchymal fibrillar collagen has been reported in human temporal lobe epilepsy. 68 Most importantly, inhibition of collagen synthesis after brain lesion prevented glial scar formation and promoted axonal regeneration and recovery of function.…”