The Amyloid Precursor Protein Family and its Functional Role in Endothelium

Abstract

Amyloid Precursor Protein (APP), Amyloid Precursor-like Protein 1 (APLP1) and APLP2, are part of the APP protein family. They are characterized as type 1 single-pass transmembrane proteins. APP and APLP2 are ubiquitously expressed, whereas APLP1 is restricted to neuronal tissue. Consequently, I decided to focus solely on APP and APLP2 in my studies. The strong conservation of APP and APLP2 from invertebrates to humans implies they have fundamental roles in cellular function. They show high structural similarities. However, the amyloid β (Aβ) sequence, which is known to play a critical role in the development of Alzheimer’s disease, is not expressed in APLP2. Several studies have suggested that APP and APLP2 might act like cell surface receptor-like proteins or ligands. The suggested functions range from playing a role in transcriptional regulation to synaptic functions or involvement in cell adhesion. In contrast to the single App or Aplp2 knockout mice, gene deficiency for both genes is perinatal lethal. Therefore, it is assumed that both genes have, at least in part, overlapping functions during development. Apart from its involvement in Alzheimer’s disease, the function of APP and APLP2 remain poorly understood. According to our RNA sequencing data, APP and APLP2 are among the most highly expressed genes in endothelial cells. Therefore, I hypothesized that they have a critical, yet unknown role in either the development or functionality of blood vessels. The aim of this study was to unravel the function of APP and APLP2 within the endothelium. APP and APLP2 can potentially affect a wide array of endothelial cell-related processes, such as inflammation, vessel remodeling and regulation of blood homeostasis and vascular permeability. To identify the most suitable mouse model for App and/or Aplp2 endothelial-specific knockout (KO) mice, I initiated a series of in vitro experiments. The particular focus on the role of APP and/or APLP2 in angiogenesis stemmed from my intriguing observation of compromised tube formation in HUVECs subsequent to the double knockdown (KD) of both APP and APLP2 in an in vitro matrigel assay. As a next step, I used different in vivo models to investigate a potential role of the protein family in angiogenesis in vivo. Hereby, I observed fewer muscle fibers, less proliferation and a dilated vessel perimeter in double KO mice 28 days after femoral artery ligation. However, no significant differences directly related to the KO of App and Aplp2 in the endothelium were detected. Overall, these results describe an important role of APP in angiogenesis in vitro that could not be compensated by APLP2. However, the underlying mechanisms in vivo appear to be much more complex and could potentially be compensated by as-yet-unkown backup mechanisms. Future studies in other tissues and other species would certainly help to unravel the physiological role of App and Aplp2 within the endothelium.