APP and Alzheimer disease: Thus, two major soluble factors involved in AD pathogenesis, the amyloid ß-peptide (Aß) and the secreted form of ß-amyloid precursor protein (APP) (Cline et al., 2018), stimulate SR expression and promote d-serine release in microglial cell cultures whereas these subtypes of glial cells do not normally produce the amino acid (Wu et al., 2004, 2007).