Pathogenic Effects of D23N Iowa Mutant Amyloid β-Protein

Abstract

Cerebral amyloid β-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 Aβ within the cerebral vessel wall and, in severe cases, hemorrhagic stroke. Several mutations have been identified within the Aβ region of the Aβ protein precursor (AβPP) gene that appear to enhance the severity of CAA. We recently described a new mutation within the Aβ region (D23N) of AβPP that is associated with severe CAA in an 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 AβPP and the pathogenic properties of Aβ. Neither the D23N Iowa mutation nor the E22Q Dutch mutation affected the amyloidogenic processing of AβPP expressed in H4 cells. The A21G Flemish mutation, in contrast, resulted in a 2.3-fold increase in secreted Aβ peptide. We also tested synthetic wild-type and mutant Aβ40 peptides for fibrillogenesis and toxicity toward cultured human cerebrovascular smooth muscle (HCSM) cells. The E22Q Dutch, D23N Iowa, and E22Q,D23N Dutch/Iowa double mutant Aβ40 peptides rapidly assembled in solution to form fibrils, whereas wild-type and A21G Flemish Aβ40 peptides exhibited little fibril formation. Similarly, the E22Q Dutch and D23N Iowa Aβ40 peptides were found to induce robust pathologic responses in cultured HCSM cells, including elevated levels of cell-associated AβPP, proteolytic breakdown of smooth muscle cell α-actin, and cell death. Double mutant E22Q,D23N Dutch/Iowa Aβ40 was more potent than either single mutant form of Aβ in causing pathologic responses in HCSM cells. These data suggest that the different CAA mutations in AβPP may exert their pathogenic effects through different mechanisms. Whereas the A21G Flemish mutation appears to enhance Aβ production, the E22Q Dutch and D23N Iowa mutations enhance fibrillogenesis and the pathogenicity of Aβ toward HCSM cells.