BDNF and Alzheimer disease: Moreover, we were particularly interested to study whether changes in OMI/HTRA2 expression levels correlated with the levels of different cholinergic markers, AD-related genes such as amyloid precursor protein (APP) and microtubule-associated protein tau (MAPT), and neurotrophic factors such as NGF and brain-derived neurotrophic factor (BDNF).