Overexpression of BACE1 stimulates spontaneous basal secretion in PC12 cells

Although alterations in the function of the neurotransmitter system have been implicated in the pathology of Alzheimer's disease (AD), the mechanisms that underlie this pathological change are not well understood. Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) is a key protease in the generation of beta-amyloid, an important trigger protein in the pathogenesis of AD. The expression and activity of BACE1 are increased in the brains of sporadic AD patients, and a role for BACE1 in neurotransmission has been suggested recently. This study examines whether BACE1 plays a role in regulated exocytosis in PC12 cells. Treatment of PC12 cells with a beta-secretase inhibitor reduced stimulus-dependent secretion of neurotransmitters, suggesting a potential role of BACE1 in regulated exocytosis. Using transfected human growth hormone as a reporter for a regulated secretory pathway in PC12 cells, we found that the transient overexpression of BACE1 increased basal secretion in the absence of a stimulus and reduced stimulus-dependent secretion in intact PC12 cells. In digitonin-permeabilized PC12 cells, an overexpression of BACE1 enhanced the Ca2+-independent and ATP-independent component of the secretory pathway. Furthermore, expression of the glycosylation-deficient mutant of BACE1, BACE1N354Q, led to an elevation of basal secretions over that by BACE1 wild-type, suggesting a role of BACE1 glycosylation in basal secretion. These results demonstrate an unknown role for BACE1 in secretion, and suggest that elevated levels of BACE1 in AD brains may contribute to the altered neurotransmitter pathology of AD through stimulation of spontaneous basal secretion under resting conditions.