Intracellular and cell-surface distribution of amyloid precursor protein in cortical astrocytes

Amyloid peptides that aggregate to form plaques in Alzheimer's disease are derived from secretory processing of the amyloid precursor protein (APP). T ransport of APP to the cell surface may be prerequisite for non-amyloidogen ic ARP processing and the secretion of soluble APP (APPs), while missorting or reinternalization of APP to intracellular compartments can promote amyl oid formation. In cultured astrocytes, APP mRNA and holoprotein are increas ed by elevations in cAMP levels, and 8-Bromo-cAMP promotes process formatio n on these cells. We now report that treatment of cultured astrocytes with 8-Bromo-cAMP increased intracellular and cell surface APP in the soma and p erinuclear region as detected by immunolabeling with monoclonal antibody 22 C11 and polyclonal antibody Kunitz-type protease inhibitor (KPI) (against t he N-terminus and KPI domain of APP, respectively) and led to intense but d iscontinuous labelling of APP on the surface of astrocytic processes. North ern and Western blot analyses confirmed that 8-Bromo-cAMP treatment of cult ured astrocytes also increased APP mRNA and KPI-containing APP holoprotein, implying that the intense APP immunolabeling observed in 8-Bromo-cAMP trea ted astrocytes was not derived from truncated forms of APP (e.g., APPs), bu t reflected high levels of APP holoprotein containing intact amyloid peptid es. Discontinuous cell surface staining in process-bearing astrocytes may b e caused by miscompartmentalization of APP related to rearrangement of the cytoskeleton. Inasmuch as intracellular APP is not accessible for non-amylo idogenic processing, we suggest that the increased immunoreactivity of intr acellular APP in process-bearing astrocytes may predispose the cells to inc reased amyloid production. (C) 1999 Elsevier Science Inc.