Phosphoinositide hydrolysis, G alpha q, phospholipase C, and protein kinase C in post mortem human brain: effects of post mortem interval, subject age, and Alzheimer's disease

Abstract

Influences of post mortem time interval, subject age and Alzheimer's disease were investigated on several components of the phosphoinositide second messenger system, including stimulation of [3H]phosphatidylinositol hydrolysis by GTP[S] and several receptor agonists and the levels of Galphaq, beta, delta and gamma subtypes of phospholipase C, and five protein kinase C isoforms, in membranes prepared from post mortem human prefrontal cortex. Most of these components were stable with post mortem delays in the range of 5-21 h, but decreases of Galphaq and the alpha and xi protein kinase C subtypes were detected. Within the subject age range of 19-100 years, G-protein- and agonist-induced [3H]phosphatidylinositol hydrolysis decreased, as did levels of Galphaq, but the levels of phospholipase C and protein kinase C subtypes were generally unchanged. In Alzheimer's disease, compared with age- and post mortem interval-matched controls, there was a decrease in [3H]phosphatidylinositol hydrolysis stimulated by G-proteins and by several receptor agonists, but the levels of Galphaq and most of the phospholipase C and protein kinase C isoforms were unaffected. The greatest deficits, which were >50%, occurred with GTP[S]- and carbachol-induced [3H]phosphatidylinositol hydrolysis, indicating that this G-protein function and the response to cholinergic stimulation are significantly impaired in Alzheimer's disease. In summary a comprehensive assessment of several components of the phosphoinositide second messenger system was made in post mortem human brain. Most elements were stable within the post mortem interval range of 5-21 h, lending validity to measurements using these tissues. Significant age-related reductions in several components were identified, indicating loss of responses with increasing age. Most importantly, severe reductions in responses to several stimuli were found in Alzheimer's disease brain, deficits in signal transduction which may contribute to impaired cognition and to the limited therapeutic responses to drugs, such as those used to activate cholinergic receptors coupled with the phosphoinositide system.