MAPT and Alzheimer disease: Several in vitro studies where tau is overexpressed suggest that this microtubule-associated protein is secreted via exosomes (Chai et al., 2012; Simon et al., 2012), and transmitted trans-synaptically through the extracellular space (Lee et al., 2012; Liu et al., 2012), which proposes a mechanism for the progressive spreading of tauopathy in the brain of AD patients.