APP and Alzheimer disease: Several studies suggest that amyloid precursor protein (APP) contributes to synapse homeostasis (for reviews see Hoe et al., 2012; Müller et al., 2017), and although the evidence is not entirely consistent, this possibility has intuitive appeal because of APP’s involvement in diseases of cognition (e.g., Alzheimer’s disease, Lewy body dementia, and cerebral amyloid angiopathy)(Müller et al., 2017).