Endothelial cell dysfunction in response to intracellular overexpression of amyloid precursor protein

Vascular damage as a result of amyloid protein (Aβ) accumulation is observed in brain vasculature of patients with Alzheimer\u27s disease and cerebral amyloid angiopathy (CAA). Aβ is a proteolytic product of amyloid precursor protein (APP) which is expressed in many cell types but it is not clear which cell type is the source of toxic Aβ production. Exposure of vascular endothelial and smooth muscle cells to exogenous Aβ results in toxicity in both cell types. To determine the effect of endogenous overexpression of APP in endothelial and smooth muscle cells plasmids containing wild type APP cDNA or a Dutch mutant variant (associated with CAA) were stably introduced into bovine aortic endothelial (BAE) cells and bovine smooth muscle (BSM) cells. A significant morphological change was observed in endothelial cells expressing either wild type or mutant APP cDNA These cells exhibited a large number of vacuoles followed by gradual cell death. Determination of eel! viability (MTT assay) confirmed these observations and demonstrated the toxic effect of the amyloid expression in endothelial cells. Intracellular coexpression of plasmids containing free radical scavengers cDNA (either metallothionein or manganese Superoxide dismutase), abolished the toxic effect of APP in endothelial cells. These results suggest that APP overexpression exerts its toxic effect on endothelial cells via generation of free radicals. In striking contrast the expression of the wild type or mutant APP in smooth muscle cells had no observed toxic effects, even when cells were maintained in culture as long as 3 months post transfection. Similar expression levels of the APP proteins in both transfected endothelial and smooth muscle cells was confirmed by immunohistochemistry. Thus the endothelial and smooth muscle cell components of the vasculature differentially respond to an endogenous overexpression of APP, and endothelial cells are specifically susceptible to increased endogenous APP levels