MAPT and Alzheimer disease: Since glycogen synthase kinase 3-beta (GSK3- beta) is known to play a crucial role in the pathology of AD by hyperphosphorylation of tau protein [40] and by influencing the BACE1-mediated cleavage of APP [41], it is of great importance that the activity of GSK3-beta in SH-SY5Y is not altered during co-cultivation with PBECs as shown in Figure 2 B (109% compared to SH-SY5Y mono-cultures).