Differential Binding of Vascular Cell-Derived Proteoglycans (Perlecan, Biglycan, Decorin, and Versican) to the Beta-Amyloid Protein of Alzheimer′s Disease

Previous studies have demonstrated the immunolocalization of perlecan, a specific heparan sulfate proteoglycan, to the beta-amyloid protein (Aβ)-containing amyloid deposits within the walls of blood vessels (i,e,, congophilic angiopathy) in Alzheimer′s disease (AD) brain. In the present investigation, the differential binding of previously characterized endothelial cell (EC)- and smooth muscle cell (SMC)-derived PGs to Aβ was examined to determine whether the accumulation of Aβ in cerebrovascular amyloid deposits may be due to its interactions with perlecan. Pretreatment of AA amyloidotic splenic and liver tissue sections with synthetic Aβ (1-28) produced strong immunoreactivity with Aβ antibodies at tissue sites enriched in perlecan which was partially removed by pretreatment with heparitinase, but not by chondroitin ABC lyase. [35S]-Sulfate labeled proteoglycans (PGs) derived from cultured ECs and SMCs bound to affinity columns containing Aβ (1-28) or (1-40), with virtually no binding to Aβ (40-1) (reverse peptide), beta-amyloid precursor protein (410-429), or bovine serum albumin. Characterization of EC and SMC PGs bound to Aβ (1-28) revealed strong binding by perlecan, weak binding by decorin and biglycan, two dermatan sulfate proteoglycans, and lack of binding by versican/PG-M, a large chondroitin sulfate proteoglycan. finding of 125I-labeled perlecan to Aβ (1-28) was strongly inhibited by isolated perlecan and to a lesser extent by heparin, but not by chondroitin-6-sulfate or unsulfated dextran sulfate, Heparitinase treatment decreased, but did not eliminate the binding of 125I-labeled perlecan to Aβ (1-28). Scatchard analysis of the interaction of Aβ (1-28)- and EC-derived perlecan in solid-phase assays indicated high-affinity (Kd = 8.3 × 10−11 M) and lower-affinity (Kd = 4.2 × 10−8 M) binding sites, with approximately 1 mol of perlecan binding 1.8 mol of Aβ. A significant decrease in binding of EC-derived perlecan to Aβ (1-28) was observed when a sequence within the putative heparin-binding motif of Aβ (His13His14Gln15Lys16) was replaced by the uncharged peptide sequence, Gly13Gly14Gln15Gly16, indicating a perlecan binding site on Aβ near the postulated alpha-secretase site (at Lys-16). Overall, the results indicate that specific vascular cell-derived PGs differentially interact with Aβ, and that the interactions of highest affinity occur between Aβ and binding sites on both the core protein and glycosaminoglycan chains of perlecan, The selective affinity of vascular cell-derived perlecan for Aβ may account for the accumulation of Aβ in conjunction with perlecan in cerebrovascular amyloid deposits in AD brain.