This is an experimental medicine study to evaluate the kinetics of cerebrospinal fluid (CSF) biomarkers in subjects with Alzheimer's disease (AD) or progressive supranuclear palsy (PSP) compared to healthy controls using a heavy water (2H2O) labeling method. This study is exploring the time profile of appearance and disappearance of pulse deuterium-labeled cargo proteins in CSF of subjects with AD and/or PSP, which is different from healthy controls, due to deficits in fast axonal transport.