Ischemia of rat brain decreases pertussis toxin-catalyzed [32P]ADP ribosylation of GTP-binding proteins (Gi1 and G0) in membranes

Effects of postmortem interval, age, and Alzheimer's disease on G-proteins in human brain

Heterotrimeric G-proteins are critical components in many receptor-coupled signal transduction systems, and altered levels and functions of G-proteins have been implicated in several neurological disorders, including Alzheimer's disease. Investigations in postmortem human brain provide a direct approach to study G-protein involvement in neurological disorders. Therefore, the effects of postmortem interval, aging, and Alzheimer's disease on G-protein levels were determined in postmortem human brain and an assay to measure activation of G-proteins was developed. Within the postmortem interval range of 5 to 21 h, the levels of G alpha i1, G alpha i2, G alpha s, and G beta were stable, whereas G alpha q and G alpha o decreased slightly, in human prefrontal cortex. In subjects aged 19 to 100 y, decreased levels of G alpha q and G alpha o were significantly correlated with increased age, but levels of the other G-protein subunits did not vary. In Alzheimer's disease prefrontal cortex, superior temporal gyrus, and occipital cortex, all G-protein subunit levels were equivalent to those in matched controls except for a slight deficit in G alpha i1. An ELISA assay using selective antibodies was used to measure [35S]GTP gamma S binding to G alpha o and G alpha i1. Binding was proportional to the concentration of GTP-gamma S and was concentration-dependently stimulated by mastoparan equivalently in control and Alzheimer's disease prefrontal cortical membranes.

Alterations in cyclic AMP generation and G protein subunits following transient ischemia in gerbil hippocampus

We examined alterations in the cyclic AMP generating system and G protein subunits in gerbil hippocampus following 10 min of transient ischemia. In hippocampal slices, basal and isoproterenol- and forskolin-stimulated cyclic AMP accumulations were markedly increased at 6 and 24 h after ischemia. Interestingly, both the inhibition of forskolin-stimulated cyclic AMP and the potentiation of beta-adrenoceptor-stimulated cyclic AMP by a gamma-aminobutyric acidB receptor agonist were attenuated at these time points. Ischemia did not affect the immunolabeling of any of the G protein alpha subunits; only that of beta subunits was significantly decreased, by 28.2%, 4 days after ischemia. In contrast, pertussis toxin-catalyzed [32P]ADP ribosylation declined progressively during the late recirculation period, reaching a significant reduction (25.4%) at 6 h after ischemia. These results suggest that ischemia affects the heterotrimeric conformation (alpha beta gamma) of Gi/Go during the recirculation period, thereby leading to increased cyclic AMP production. Because cyclic AMP-dependent protein kinase A modulates the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-kainate receptor channels, postischemic sensitization of the cyclic AMP generating system may contribute to neuronal degeneration in the hippocampus.

The role of GTP binding proteins in ischemic brain damage: autoradiographic and histopathological study

Cerebral ischemia produces perturbation of signal transduction systems in neurons. In order to estimate the contribution of guanine nucleotide-binding protein (G-protein) to hippocampal neuronal death, the effect of pertussis toxin (PTX) on the CA1 pyramidal cell damage after transient forebrain ischemia in rats was examined. PTX was administered 3 days before 20 min of transient forebrain ischemia. PTX injection into the CA1 subfield failed to alter the number of ischemic-damaged CA1 pyramidal cells. In contrast, ventricular PTX injection exacerbated CA1 pyramidal cell damage. We also studied postischemic alteration of GTP binding sites in the hippocampal formation using quantitative in vitro autoradiography. Autoradiographic imaging demonstrated predominant distribution of GTP binding sites in synaptic areas in the hippocampus. No significant change of GTP binding activity was observed in the hippocampus until 2 days after recirculation. Seven days after ischemia, when the CA1 pyramidal cells were depleted, the GTP binding sites of the strata oriens and radiatum in the CA1 subfield had reduced by 32% and 31%, respectively. In contrast, GTP binding in the CA3 subfield and the dentate gyrus remained unaltered throughout the reperfusion period. These results suggest that the amount of G-proteins as estimated by GTP binding remained unaltered in the hippocampus during the early recirculation period, when the CA1 pyramidal cells were morphologically intact, and that signal transduction pathways mediated by Gi and Go do not play a major role in delayed death of the CA1 pyramidal cells.