3.3 Overexpression of Lf in astrocytes inhibits APP phosphorylation in APP/PS1 mice
Given that the astrocytes could secrete Lf into neurons (Fig. 2), we, therefore, evaluated the key proteins involved in APP-processing. As shown in Fig. 3A, the APP phosphorylation at Thr668 was dramatically inhibited in the APP/PS1/Lf mice compared to the APP/PS1 mice, whereas the critical enzymes for APP-processing such as ADAM10, BACE1 and PS1 remained similar. Immunostaining also showed that the p-APP expression was highly associated with Aβ deposition, and was reduced in the neurons in APP/PS1/Lf mouse cortex (Fig. 3B, Supplementary Fig. 1). Inhibition of p-APP expression was demonstrated to repress the BACE1-mediated Aβ production (Lee et al., 2003). As expected, the production of BACE1-mediated APP cleavage, sAPPβ, was considerably reduced in APP/PS1/Lf mice (Fig. 3C), indicating that the APP amyloidogenic pathway was inhibited by the overexpression of Lf in astrocytes in APP/PS1 mice.
Aβ homeostasis is also modulated by the capacity of Aβ clearance in the brain. We, therefore, analyzed the Aβ efflux protein LRP1 and its ligand APOE, the Aβ-degrading enzyme IDE, and the Aβ influx protein RAGE. Although APOE was significantly increased in the Astro-Lf mice compared to the WT mice, the expressions of APOE in APP/PS1/Lf mice and APP/PS1 mice were similar (Fig. 3D). Additionally, the RAGE expressions were unchanged in the different gene type mice, while overexpression of Lf in astrocytes effectively rescued the decreased expression of LRP1 and IDE in APP/PS1 mice (Fig. 3D). These data suggested that overexpression of Lf in astrocytes inhibited Aβ burden via repressing APP phosphorylation and promoting Aβ clearance in APP/PS1 mouse brains.