Substitutions at codon 22 of Alzheimer's Aβ peptide induce diverse conformational changes and apoptotic effects human cerebral endothelial cells

Cerebral amyloid angiopathy is commonly associated with normal aging and Alzheimer's disease and it is also the principal feature of hereditary cerebral hemorrhage with amyloidosis Dutch type, a familial condition associated to a point mutation G to C at codon 693 of the amyloid β (Aβ) precursor protein gene resulting in a Glu to Gln substitution at position 22 of the Aβ (E22Q). The patients carrying the AβE22Q variant usually present with lobar cerebral hemorrhages before 50 years of age. A different mutation described in several members of three Italian kindred who presented with recurrent hemorrhagic strokes late in life, between 60 and 70 years of age, also associated with extensive cerebrovascular amyloid deposition has been found at the same position 22, this time resulting in a Glu to Lys substitution (E22K). We have compared the secondary structure, aggregation, and fibrillization properties of the two Aβ40 variants and the wild type peptide. Using flow cytometry analysis after staining with propidium iodide and anuexin V, we also evaluated the cytotoxic effects of the peptides on human cerebral endothelial cells in culture. Under the conditions tested, the E22Q peptide exihibited the highest content of β-sheet conformation and the fastest aggregation/fibrillization properties. The Dutch variant also induced apoptosis of cerebral endothelial cells at a concentration of 25 μM, whereas the wild type Aβ and the E22K mutant had no effect. The data suggest that different amino acids at position 22 confer distinct structural properties to the peptides that appear to influence the onset and aggressiveness of the disease rather than the phenotype.