Abstract
The beta-amyloid precursor protein (APP) is a ubiquitous, highly conserved secretory glycoprotein that is expressed at high levels in mammalian brain by neurons, astrocytes, and activated microglia. Secreted APP (APPs) is generated by the cleavage of APP within the beta- amyloid (A beta) portion of its ectodomain. The formation and secretion of APPs can be increased by activation of particular neurotransmitter receptors and subsequent protein phosphorylation. We found that tissue slices from rat cortex, hippocampus, striatum, and cerebellum secrete APPs in vitro. APPs secretion was enhanced by electrical stimulation, but was not associated with a general increase in the release of total protein, lactate dehydrogenase (LDH) activity, or neuronal cell adhesion molecules. The pharmacological profile of stimulation-induced APPs secretion suggests complex interactions between muscarinic receptor subtypes in the tissue slices: in the unstimulated state, activation of Muscarinic M1 receptors increased APPs release while nonspecific activation of multiple muscarinic receptors had little effect on APPs release; in electrically stimulated slices, nonspecific inhibition of muscarinic receptors blunted the increase in APPs secretion. The nonspecific muscarinic agonist carbachol increased APPs secretion only in the presence of an M2 receptor antagonist, suggesting that activation of M2 receptors suppresses APPs formation. These data indicate that secretory APP processing in brain includes depolarization- enhanced cleavage of the cell-associated holoprotein within its ectodomain, and that the net effect of depolorization involves several subtypes of acetylcholine receptors.
