MAPT and Alzheimer disease: Key studies by Faria et al., Laske et al. and Ng et al. reported increased BDNF plasma and serum levels in the early stages of AD and MCI; however, levels of BDNF have been reported to decrease with elevated levels of Aβ, GSK3β, tau and cortisol in the later stages of AD progression [33,34,35,36,64,70,128,129,130,131,132,133,134,135,136,137,138].