Increased susceptibility to ischemic brain damage in transgenic mice overexpressing the amyloid precursor protein

Abstract

We studied the role of the amyloid precursor protein (APP) in ischemic brain damage using transgenic mice overexpressing APP. The middle cerebral artery (MCA) was occluded in FVB/N mice expressing APP695.SWE (Swedish mutation) and in nontransgenic littermates. Infarct volume (cubic millimeters) was assessed 24 hr later in thionin-stained brain sections. The infarct produced by MCA occlusion was enlarged in the transgenics (+32 +/- 6%; n = 12; p < 0. 05; t test). Measurement of APP by ELISA revealed that, although relatively high levels of Abeta were present in the brain of the transgenics (Abeta1-40 = 80 +/- 19 pmol/g; n = 6), there were no differences between ischemic and nonischemic hemispheres (p > 0.05). The reduction in cerebral blood flow produced by MCA occlusion at the periphery of the ischemic territory was more pronounced in APP transgenics (-42 +/- 8%; n = 9) than in controls (-20 +/- 8%; n = 9). Furthermore, the vasodilatation produced by neocortical application of the endothelium-dependent vasodilator acetylcholine (10 microM) was reduced by 82 +/- 5% (n = 8; p < 0.05) in APP transgenics. The data demonstrate that APP overexpression increases the susceptibility of the brain to ischemic injury. The effect is likely to involve the Abeta-induced disturbance in endothelium-dependent vascular reactivity that leads to more severe ischemia in regions at risk for infarction. The cerebral vascular actions of peptides deriving from APP metabolism may play a role in the pathogenic effects of APP.

Fig. 1.

Effect of MCA occlusion on cerebral ischemic damage in FVB/N transgenic mice overexpressing APP₆₉₅.SWE.A, MCA occlusion resulted in larger infarcts in APP transgenics than in nontransgenic littermates (p < 0.05, t test), an effect that persists after correction for ischemic swelling (corrected). B, Rostrocaudal distribution of the ischemic lesion produced by MCA occlusion in APP transgenics and nontransgenic littermates. The enlargement of the lesion occurs both in the anterior and posterior regions of the infarct (p < 0.05).

Fig. 2.

Brain sections from representative animals indicating the distribution of the ischemic lesion in nontransgenic mice (Non-Tg) and in FVB/N transgenic mice overexpressing APP₆₉₅.SWE [APP Tg (FVB/N HuAPP₆₉₅.SWE)]. The area recruited into infarction in the transgenics involves the periphery of the lesion uniformly at all rostrocaudal levels.

Fig. 3.

Effect of MCA occlusion on Aβ concentration in the ischemic territory in FVB/N transgenic mice overexpressing APP₆₉₅.SWE. Aβ was measured 24 hr after MCA occlusion using an ELISA-based method (for details, see Materials and Methods). Transgene-derived Aβ [(1–40), (1–42)] was present in the brain. However, the Aβ concentration did not differ between ischemic and nonischemic hemispheres (p > 0.05, paired t test). The ratio between the 1–40 and 1–42 peptides [(1–42)/(1–40)×100] also did not change.

Fig. 4.

Effect of MCA occlusion on CBF in FVB/N transgenic mice overexpressing APP₆₉₅.SWE and nontransgenic littermates. A, The reduction in CBF in the center of the ischemic core is not different between transgenic and nontransgenic mice. B, In the ischemic penumbra, however, CBF is relatively more reduced in APP transgenics (p < 0.05, t test).C, Mean arterial pressure is not different between APP transgenics and nontransgenic littermates.

Fig. 5.

Effect of topical cortical application of ACh or SNAP on the CBF in APP transgenics and nontransgenic littermates. Mice were anesthetized and artificially ventilated. End-tidal CO₂ was monitored and maintained between 2.2 and 2.6%, a value that corresponded at a P_CO2 of 35–38 mmHg (see Materials and Methods). Arterial pressure was 94 ± 8 mmHg in APP transgenics and 84 ± 6 in nontransgenics. The vasodilatation produced by ACh was markedly reduced in APP transgenics (p < 0.001, t test), whereas the response to SNAP was not statistically different between transgenic and nontransgenic mice.