Amyloid precursor protein metabolism and biomarker of amyloid pathology. Schematic representation of amyloid precursor protein (APP) metabolism and biomarker of amyloid pathology in various of the intracellular, membrane, extracellular, cerebrospinal fluid (CSF) and plasma compartments. Color pallet indicates processes that have increased, declined or remained unchanged in Alzheimer's disease (AD). Note that in AD higher amyloid-beta (Aβ1-42) retention in the extracellular compartment (brain tissue) due to peptide aggregation leads to declines of Aβ1-42 in the CSF. In the plasma, it is debatable whether declines are present in individuals with AD.