Soluble amyloid precursor protein-alpha mediates post-ischemic neovascularization after myocardial infarction

Abstract Background Amyloid precursor protein (APP) gives rise to amyloid-β peptides and thus has a key role in the pathogenesis of Alzheimer´s disease. APP is also highly expressed outside the brain including cells of the cardiovascular system, in particular endothelial cells (1). However, the role of APP and its proteolytic fragments in cardiovascular function and disease remains unknown. Purpose The main purpose of this study was to understand the function APP and the close homologue, amyloid precursor-like protein 2 (APLP2) in endothelial cells. Methods To test the potential role of endothelial APP and APLP2 in vivo, we generated inducible endothelium-specific APP/APLP2-double deficient mice (EC-APP/APLP2-KO) by crossing App fl/fl; Aplp2 fl/fl mice with the Cdh5-CreERT2 line. Animals were analyzed after exposure to myocardial infarction (MI). Human umbilical vein endothelial cells were used to study the mechanisms underlying APP/APLP2-mediated endothelial function in vitro. Results APP and APLP2 were found to be upregulated in cardiac endothelial cells after MI both in mice and humans. Loss of APP and APLP2 in endothelial cells had no effect on the basal heart function in mice. But, when subjected to MI, approximately 40 % of the EC-APP/APLP2-KO mice died within 3 weeks after MI in contrast to control animals, where hardly any death occurred. Survival of endothelium-specific APP- or APLP2-single knockouts was indistinguishable from that of control animals after MI. The surviving EC-APP/APLP2-KO mice showed larger infarct scars than the control animals. Magnetic resonance imaging and echocardiography at 3 weeks after MI revealed that EC-APP/APLP2-KO had more severe contractile dysfunction than the control mice (left ventricular ejection fraction 32.8 % vs. 44.9 %, respectively). As the underlying cause of this we identified a strong impairment of post-ischemia neovascularization in the absence of endothelial APP and APLP2. We found that soluble APP-α (APPsα) generated by non-amyloidogenic processing was the predominant form of APP in cardiac endothelial cells after MI. We therefore tested whether the expression of APPsα was able to rescue the observed phenotype of EC-APP/APLP2-KO after MI. Indeed, APPsα knock-in mice that solely express APPsα and no other APP fragments or cell surface APP showed the same phenotype as wild-type animals, indicating that APPsα is sufficient to mimic the function of APP. Under in vitro conditions, we found that APPsα was able to promote tube formation of cultured endothelial cells. APPsα also induced downstream signalling events such as AKT, ERK, and p38 activation which are reminiscent of growth factor induced signaling processes. Conclusions APPsα mediates post-ischemia neovascularization in the heart by exerting growth factor-like activity in endothelial cells, thereby protecting against cardiac injury.