Amyloid beta protein is the major protein component of neuritic plaque
s found in the brain of Alzheimer's disease. The activation of phospho
lipase D by amyloid beta protein (25-35), quisqualate and phorbol 12,1
3-dibutyrate was investigated in LA-N-2 cells by measuring phosphatidy
lethanol formation. The activation of phospholipase D by quisqualate a
nd A beta P (25-35) was calcium-independent. The A beta P (25-35) and
quisqualate activation of phospholipase D appeared to be mediated thro
ugh a pertussis toxin-sensitive GTP-binding protein. Phospholipase D a
ctivation by A beta P (25-35), quisqualate and phorbol dibutyrate was
not blunted by the protein kinase C inhibitors, staurosporine, H-7 and
RO-31-8220. However, it was abolished by overnight exposure to phorbo
l dibutyrate. This activation of phospholipase D was prevented by the
tyrosine kinase inhibitor, genistein but not by tyrophostin A. Several
excitatory amino acid antagonists were tested for their ability to pr
event the phospholipase D activation by quisqualate and A beta P (25-3
5). Only NBQX was effective with an IC50 of 75 mu M for A beta P (25-3
5) and quisqualate. Activation of phospholipase D by A beta P or quisq
ualate was absent in LA-N-2 cells previously desensitized by quisquala
te or A beta P (25-35), but the activation by phorbol dibutyrate was u
naltered. The responsiveness to A beta P and quisqualate in previously
desensitized cells reappeared subsequent to a period of resensitizati
on. The observations with the antagonist NBQX, and the desensitization
and resensitization experiments, are consistent with a receptor occup
ancy mediated activation of phospholipase D by quisqualate and by A be
ta P (25-35).