Pathogenic effects of D23N Iowa mutant amyloid β-protein

Cerebral amyloid beta -protein angiopathy (CAA) is a key pathological feature of patients with Alzheimer's disease and certain related disorders. In these conditions the CAA is characterized by the deposition of A beta within the cerebral vessel wall and, in severe cases, hemorrhagic stroke. Several mutations have been identified within the A beta region of the A beta protein precursor (A beta PP) gene that appear to enhance the severity of CAA. We recently described a new mutation within the A beta regio (D23N) of A beta PP that is associated with severe CAA in a Iowa kindred (Grabowski, T. J., Cho, H. S., Vonsattel, J. P. G., Rebeck, G. W., and Greenberg, S. M. (2001) Ann. Neurol 49, 697-705). In the present study, we investigated the effect of this new D23N mutation on the processing of A beta PP and the pathogenic properties of A beta. Neither the D23N Iowa mutation nor the E22Q Dutch mutation affected the amyloidogenic processing of A beta PP expressed in H4 cells. The A21G Flemish mutation, in contrast, resulted in a 2.3-fold increase in secreted A beta peptide. We also tested synthetic wild-type and mutant A beta 40 peptides for fibrillogenesis and toxicity toward cultured human cerebrovascular smooth muscle (HC-SM) cells. The E22Q Dutch, D23N Iowa, and E22Q,D23N Dutch/Iowa double mutant A beta 40 peptides rapidly assembled in solution to form fibrils, whereas wild-type and A21G Flemish A beta 40 peptides exhibited little fibril formation. Similarly, the E22Q Dutch and D23N Iowa A beta 40 peptides were found to induce robust pathologic responses in cultured HCSM cells, including elevated levels of cell-associated A beta PP, proteolytic breakdown of smooth muscle cell a-actin, and cell death. Double mutant E22Q,D23N Dutch/Iowa A beta 40 was more potent than either single mutant form of AP in causing pathologic responses in HCSM cells. These data suggest that the different CAA mutations in A beta PP may exert their pathogenic effects through different mechanisms. Whereas the A21G Flemish mutation appears to enhance A beta production, the E22Q Dutch and D23N Iowa mutations enhance fibrillogenesis and the pathogenicity of Ap toward HCSM cells.