An interaction between basement membrane and Alzheimer amyloid precursor proteins suggests a role in the pathogenesis of Alzheimer's disease.
BACKGROUND
Extracellular matrix proteins (ECMPs) of the basement membrane type, such as the heparan sulfate proteoglycan perlecan, laminin, entactin, collagen IV, and fibronectin are present in and have been implicated in the genesis of amyloids. As in many forms of amyloid, perlecan, laminin, collagen IV, and fibronectin are present in Alzheimer deposits. We have previously demonstrated high-affinity interactions between Alzheimer amyloid precursor proteins (beta PP-695, -751, and -770), and perlecan or laminin. With a view to examining our hypothesis that beta PP:ECMP interactions are involved in Alzheimer's amyloidogenesis, additional studies have now been performed examining the interactions of the beta PPs with entactin, fibronectin, and collagen IV, the influence each of the ECMPs has on the binding of the others to beta PPs, and the effect of beta PPs on interactions among the various ECMPs.


METHODS
A modified solid-phase enzyme-linked immunosorbent assay was used to assess the binding of the various ECMPs to the beta PPs. One element was immobilized on plastic, and another element, operationally defined as a ligand, was incubated in solution at various concentrations over the immobilized protein. To evaluate the effect of one ECMP on the binding of other ECMPs to beta PP, the beta PP was immobilized and the binding of the "ligand" ECMP was assessed in the presence of a single concentration of a second "competitor" ECMP. Similarly, in evaluating the effect of beta PPs on the binding of ECMPs to each other, one ECMP was immobilized and the binding of a second ECMP "ligand" was assessed in the presence of a fixed concentration of beta PP "competitor."


RESULTS
As in the case of perlecan and laminin, each of the ECMPs bound to the beta PPs with high affinity (Kd values in the nanomolar range). The binding of entactin to beta PPs was stimulated by collagen IV but was markedly inhibited by laminin, perlecan, and fibronectin. Conversely, the presence of entactin inhibited the binding of perlecan, laminin, and fibronectin to beta PPs. Moreover, the presence of beta PPs usually interfered with the binding of ECMPs to each other. Generally, in all binding assays, beta PP-751 and -770, behaved in similar ways, but beta PP-695, the brain-specific form, exhibited unique characteristics.


CONCLUSIONS
These binding data may reflect the normal interactions of beta PPs with ECMPs. However, the fact that beta PPs interfere with the normal interactions between ECMPs themselves, a process that spontaneously generates a basement membrane, suggests that aspects of ECMP:beta PP binding may be a pathologic part of the amyloidogenic process in Alzheimer's disease.
Narindrasorasak
S
S
Department of Pathology, Queen's University, Ontario, Canada.
Altman
R A
RA
Gonzalez-DeWhitt
P
P
Greenberg
B D
BD
Kisilevsky
R
R
D000544:Alzheimer Disease / Q000378:metabolism*; D016564:Amyloid beta-Protein Precursor / Q000378:metabolism*; D001485:Basement Membrane / Q000378:metabolism*; D003094:Collagen / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D016326:Extracellular Matrix Proteins / Q000378:metabolism*; D005353:Fibronectins / Q000378:metabolism; D019812:Heparan Sulfate Proteoglycans; D006497:Heparan Sulfate / Q000378:metabolism; D006801:Humans; D007797:Laminin / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000378:metabolism; D011485:Protein Binding / Q000502:physiology; D011509:Proteoglycans / Q000378:metabolism
D016564:Amyloid beta-Protein Precursor; D003094:Collagen; D016326:Extracellular Matrix Proteins; D005353:Fibronectins; D019812:Heparan Sulfate Proteoglycans; D006497:Heparan Sulfate; D007797:Laminin; D008562:Membrane Glycoproteins; D011509:Proteoglycans; C029967:nidogen; C071927:perlecan