Preservation of Gi-protein inhibited adenylyl cyclase activity in the brains of patients with Alzheimer's disease

Compartmentalized Signaling in Aging and Neurodegeneration

The cyclic AMP (cAMP) signalling cascade is necessary for cell homeostasis and plays important roles in many processes. This is particularly relevant during ageing and age-related diseases, where drastic changes, generally decreases, in cAMP levels have been associated with the progressive decline in overall cell function and, eventually, the loss of cellular integrity. The functional relevance of reduced cAMP is clearly supported by the finding that increases in cAMP levels can reverse some of the effects of ageing. Nevertheless, despite these observations, the molecular mechanisms underlying the dysregulation of cAMP signalling in ageing are not well understood. Compartmentalization is widely accepted as the modality through which cAMP achieves its functional specificity; therefore, it is important to understand whether and how this mechanism is affected during ageing and to define which is its contribution to this process. Several animal models demonstrate the importance of specific cAMP signalling components in ageing, however, how age-related changes in each of these elements affect the compartmentalization of the cAMP pathway is largely unknown. In this review, we explore the connection of single components of the cAMP signalling cascade to ageing and age-related diseases whilst elaborating the literature in the context of cAMP signalling compartmentalization.

Alterations in cyclic nucleotide signaling are implicated in healthy aging and age-related pathologies of the brain

It is not only important to consider how hormones may change with age, but also how downstream signaling pathways that couple to hormone receptors may change. Among these hormone-coupled signaling pathways are the 3',5'-cyclic guanosine monophosphate (cGMP) and 3',5'-cyclic adenosine monophosphate (cAMP) intracellular second messenger cascades. Here, we test the hypothesis that dysfunction of cAMP and/or cGMP synthesis, execution, and/or degradation occurs in the brain during healthy and pathological diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. Although most studies report lower cyclic nucleotide signaling in the aged brain, with further reductions noted in the context of age-related diseases, there are select examples where cAMP signaling may be elevated in select tissues. Thus, therapeutics would need to target cAMP/cGMP in a tissue-specific manner if efficacy for select symptoms is to be achieved without worsening others.

Phosphodiesterase Inhibitors for Alzheimer's Disease: A Systematic Review of Clinical Trials and Epidemiology with a Mechanistic Rationale

BACKGROUND

Preclinical studies, clinical trials, and reviews suggest increasing 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) with phosphodiesterase inhibitors is disease-modifying in Alzheimer's disease (AD). cAMP/protein kinase A (PKA) and cGMP/protein kinase G (PKG) signaling are disrupted in AD. cAMP/PKA and cGMP/PKG activate cAMP response element binding protein (CREB). CREB binds mitochondrial and nuclear DNA, inducing synaptogenesis, memory, and neuronal survival gene (e.g., brain-derived neurotrophic factor) and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α). cAMP/PKA and cGMP/PKG activate Sirtuin-1, which activates PGC1α. PGC1α induces mitochondrial biogenesis and antioxidant genes (e.g.,Nrf2) and represses BACE1. cAMP and cGMP inhibit BACE1-inducing NFκB and tau-phosphorylating GSK3β.

OBJECTIVE AND METHODS

We review efficacy-testing clinical trials, epidemiology, and meta-analyses to critically investigate whether phosphodiesteraseinhibitors prevent or treat AD.

RESULTS

Caffeine and cilostazol may lower AD risk. Denbufylline and sildenafil clinical trials are promising but preliminary and inconclusive. PF-04447943 and BI 409,306 are ineffective. Vinpocetine, cilostazol, and nicergoline trials are mixed. Deprenyl/selegiline trials show only short-term benefits. Broad-spectrum phosphodiesterase inhibitor propentofylline has been shown in five phase III trials to improve cognition, dementia severity, activities of daily living, and global assessment in mild-to-moderate AD patients on multiple scales, including the ADAS-Cogand the CIBIC-Plus in an 18-month phase III clinical trial. However, two books claimed based on a MedScape article an 18-month phase III trial failed, so propentofylline was discontinued. Now, propentofylline is used to treat canine cognitive dysfunction, which, like AD, involves age-associated wild-type Aβ deposition.

CONCLUSION

Phosphodiesterase inhibitors may prevent and treat AD.

Cyclic nucleotide signaling changes associated with normal aging and age-related diseases of the brain

Deficits in brain function that are associated with aging and age-related diseases benefit very little from currently available therapies, suggesting a better understanding of the underlying molecular mechanisms is needed to develop improved drugs. Here, we review the literature to test the hypothesis that a break down in cyclic nucleotide signaling at the level of synthesis, execution, and/or degradation may contribute to these deficits. A number of findings have been reported in both the human and animal model literature that point to brain region-specific changes in Galphas (a.k.a. Gαs or Gsα), adenylyl cyclase, 3',5'-adenosine monophosphate (cAMP) levels, protein kinase A (PKA), cAMP response element binding protein (CREB), exchange protein activated by cAMP (Epac), hyperpolarization-activated cyclic nucleotide-gated ion channels (HCNs), atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), soluble and particulate guanylyl cyclase, 3',5'-guanosine monophosphate (cGMP), protein kinase G (PKG) and phosphodiesterases (PDEs). Among the most reproducible findings are 1) elevated circulating ANP and BNP levels being associated with cognitive dysfunction or dementia independent of cardiovascular effects, 2) reduced basal and/or NMDA-stimulated cGMP levels in brain with aging or Alzheimer's disease (AD), 3) reduced adenylyl cyclase activity in hippocampus and specific cortical regions with aging or AD, 4) reduced expression/activity of PKA in temporal cortex and hippocampus with AD, 5) reduced phosphorylation of CREB in hippocampus with aging or AD, 6) reduced expression/activity of the PDE4 family in brain with aging, 7) reduced expression of PDE10A in the striatum with Huntington's disease (HD) or Parkinson's disease, and 8) beneficial effects of select PDE inhibitors, particularly PDE10 inhibitors in HD models and PDE4 and PDE5 inhibitors in aging and AD models. Although these findings generally point to a reduction in cyclic nucleotide signaling being associated with aging and age-related diseases, there are exceptions. In particular, there is evidence for increased cAMP signaling specifically in aged prefrontal cortex, AD cerebral vessels, and PD hippocampus. Thus, if cyclic nucleotide signaling is going to be targeted effectively for therapeutic gain, it will have to be manipulated in a brain region-specific manner.

Modulation of the endocannabinoid system: neuroprotection or neurotoxicity?

There is now a large volume of data indicating that compounds activating cannabinoid CB(1) receptors, either directly or indirectly by preventing the breakdown of endogenous cannabinoids, can protect against neuronal damage produced by a variety of neuronal "insults". Given that such neurodegenerative stimuli result in increased endocannabinoid levels and that animals with genetic deletions of CB(1) receptors are more susceptible to the deleterious effects of such stimuli, a case can be made for an endogenous neuroprotective role of endocannabinoids. However, this is an oversimplification of the current literature, since (a) compounds released together with the endocannabinoids can contribute to the neuroprotective effect; (b) other proteins, such as TASK-1 and PPARalpha, are involved; (c) the CB(1) receptor antagonist/inverse agonist rimonabant has also been reported to have neuroprotective properties in a number of animal models of neurodegenerative disorders. Furthermore, the CB(2) receptor located on peripheral immune cells and activated microglia are potential targets for novel therapies. In terms of the clinical usefulness of targeting the endocannabinoid system for the treatment of neurodegenerative disorders, data are emerging, but important factors to be considered are windows of opportunity (for acute situations such as trauma and ischemia) and the functionality of the target receptors (for chronic neurodegenerative disorders such as Alzheimer's disease).

Cortical M1 receptor concentration increases without a concomitant change in function in Alzheimer's disease

Although the M(1) muscarinic receptor is a potential therapeutic target for Alzheimer's disease (AD) based on its wide spread distribution in brain and its association with learning and memory processes, whether its receptor response is altered during the onset of AD remains unclear. A novel [(35)S]GTPgammaS binding/immunocapture assay was employed to evaluated changes in M(1) receptor function in cortical tissue samples harvested from people who had no cognitive impairment (NCI), mild cognitive impairment (MCI), or AD. M(1) function was stable across clinical groups. However, [(3)H]-oxotremorine-M radioligand binding studies revealed that the concentration of M(1) cortical receptors increased significantly between the NCI and AD groups. Although M(1) receptor function did not correlate with cognitive function based upon mini-mental status examination (MMSE) or global cognitive score (GCS), functional activity was negatively correlated with the severity of neuropathology determined by Braak staging and NIA-Reagan criteria for AD. Since M(1) agonists have the potential to modify the pathologic hallmarks of AD, as well as deficits in cognitive function in animal models of this disease, the present findings provide additional support for targeting the M(1) receptor as a potential therapeutic for AD.

G protein-coupled receptor systems and their lipid environment in health disorders during aging

Cells, tissues and organs undergo phenotypic changes and deteriorate as they age. Cell growth arrest and hyporesponsiveness to extrinsic stimuli are all hallmarks of senescent cells. Most such external stimuli received by a cell are processed by two different cell membrane systems: receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). GPCRs form the largest gene family in the human genome and they are involved in most relevant physiological functions. Given the changes observed in the expression and activity of GPCRs during aging, it is possible that these receptors are directly involved in aging and certain age-related pathologies. On the other hand, both GPCRs and G proteins are associated with the plasma membrane and since lipid-protein interactions regulate their activity, they can both be considered to be sensitive to the lipid environment. Changes in membrane lipid composition and structure have been described in aged cells and furthermore, these membrane changes have been associated with alterations in GPCR mediated signaling in some of the main health disorders in elderly subjects. Although senescence could be considered a physiologic process, not all aging humans develop the same health disorders. Here, we review the involvement of GPCRs and their lipid environment in the development of the major human pathologies associated with aging such as cancer, neurodegenerative disorders and cardiovascular pathologies.

Effects of single and continuous administration of amyloid beta-peptide (25-35) on adenylyl cyclase activity and the somatostatinergic system in the rat frontal and parietal cortex

It is unknown whether the amyloid beta-peptide (Abeta), a principal component found in extracellular neuritic plaques in the brain of patients with Alzheimer's disease (AD), is capable of altering adenylyl cyclase (AC) activity and the somatostatin (SRIF) receptor-effector system in the cerebral cortex of the patients. Therefore, the objective of this study was to investigate the effect of the beta fragment, beta (25-35), on AC activity and the somatostatinergic system in the rat frontoparietal cortex. A single dose of beta (25-35) (10microg) injected intracerebroventricularly significantly decreased the density of SRIF receptors (27.4%) and increased their affinity (32.2%) in the frontoparietal cortex. The inhibitory effect of SRIF on basal and forskolin (FK)-stimulated AC activity was significantly lower in the beta (25-35)-treated rats when compared with controls. beta (25-35) did not modify Gialpha1, Gialpha2 nor Gialpha3 levels in membranes from the frontoparietal cortex. Continuous infusion of the peptide induced a decrease in the SRIF receptor density in this brain area to a similar extent as that observed 14 days after the single administration of the peptide. Likewise, this treatment decreased the SRIF receptor density in the frontal cortex (15.3%) and parietal cortex (27.2%). This effect was accompanied by a decrease in the SRIF-mediated inhibition of FK-stimulated AC activity (from 41.6% to 25.6%) in the frontal cortex as well by a decrease in basal AC activity (from 36.9% to 31.6%) and FK-stimulated AC activity (from 35.6% to 27.1%) in the parietal cortex. Continuous infusion of Abeta (25-35) had no effect on Gialpha1, Gialpha2 or Gialpha3 levels in membranes from frontal and parietal cortex. However, this treatment caused a decrease in SRIF-like immunoreactivity content in the parietal (38.9%) and frontal (20.4%) cortex. These results suggest that Abeta might be involved in the alterations of somatostatinergic system reported in AD.

Design, synthesis, and preliminary pharmacological evaluation of a set of small molecules that directly activate gi proteins

Heterotrimeric G proteins play a pivotal role in the communication of cells with the environment. G proteins are stimulated by cell surface receptors (GPCR) that catalyze the exchange of GDP, bound to Galpha subunit, with GTP and can per se be the target of drugs. Based on the structure of two nonpeptidic modulators of Gi proteins, a series of new molecules characterized by a long hydrophobic chain and at least two nitrogen atoms protonated at physiological pH was designed. The compounds were tested for their ability to stimulate binding of GTPgammaS to recombinant Gi proteins. Gi activation properties were also evaluated by inhibition of adenylyl cyclase activity in intact lymphocytes. Most compounds were able to stimulate GTPgammaS binding and to inhibit cAMP production at micromolar doses. Among the active compounds, 34 showed good efficacy and was the most potent compound studied, particularly on alpha(o) subtype; its regioisomer, 36, was the most efficacious one. Compound 7 showed also an interesting profile as it showed selectivity toward the alpha(o) subtype, in both efficacy and potency. Some of the compounds synthesized and found to be active may be useful leads to develop more potent and selective Gi protein modulators.

Effects of age and spatial learning on adenylyl cyclase mRNA expression in the mouse hippocampus

Adenylyl cyclase (AC) subtypes have been implicated in memory processes and synaptic plasticity. In the present study, the effects of aging and learning on Ca2+/calmodulin-stimulable AC1, Ca2+-insensitive AC2 and Ca2+/calcineurin-inhibited AC9 mRNA level were compared in the dorsal hippocampus of young-adult and aged C57BL/6 mice using in situ hybridization. Both AC1 and AC9 mRNA expression were downregulated in aged hippocampus, whereas AC2 mRNA remained unchanged, suggesting differential sensitivities to the aging process. We next examined AC mRNA expression in the hippocampus after spatial learning in the Morris water maze. Acquisition of the spatial task was associated with an increase of AC1 and AC9 mRNA levels in both young-adult and aged groups, suggesting that Ca2+-sensitive ACs are oppositely regulated by aging and learning. However, aged-trained mice had reduced AC1 and AC9, but greater AC2, mRNA levels relative to young-trained mice and age-related learning impairments were correlated with reduced AC1 expression in area CA1. We suggest that reduced levels of hippocampal AC1 mRNA may greatly contribute to age-related defects in spatial memory.

Melatonin signaling dysfunction in adolescent idiopathic scoliosis

STUDY DESIGN

In vitro assays were performed with bone-forming cells isolated from 41 patients with adolescent idiopathic scoliosis and 17 control patients exhibiting another type of scoliosis or none.

OBJECTIVE

To determine whether a dysfunction of the melatonin-signaling pathway in tissues targeted by this hormone is involved in adolescent idiopathic scoliosis.

SUMMARY OF BACKGROUND DATA

Pinealectomy in chicken has led to the formation of a scoliotic deformity, thereby suggesting that a melatonin deficiency may be at the source of adolescent idiopathic scoliosis. However, the relevance of melatonin in the etiopathogenesis of that condition is controversial because most studies have reported no significant change in circulating levels of melatonin in patients with adolescent idiopathic scoliosis.

METHODS

Primary osteoblast cultures prepared from bone specimens obtained intraoperatively during spine surgeries were used to test the ability of melatonin and Gpp(NH)p, a GTP analogue, to block cAMP accumulation induced by forskolin. In parallel, melatonin receptor and Gi protein functions were evaluated by immunohistochemistry and by coimmunoprecipitation experiments.

RESULTS

The cAMP assays demonstrated that melatonin signaling was impaired in osteoblasts isolated from adolescent idiopathic scoliosis patients to different degrees allowing their classification in 3 distinct groups based on their responsiveness to melatonin or Gpp(NH)p.

CONCLUSION

Melatonin signaling is clearly impaired in osteoblasts of all patients with adolescent idiopathic scoliosis tested. Classification of patients with adolescent idiopathic scoliosis in 3 groups based on functional in vitro assays suggests the presence of distinct mutations interfering with the melatonin signal transduction. Posttranslational modifications affecting Gi protein function, such as serine residues phosphorylation, should be considered as one possible mechanism in the etiopathogenesis of AIS.

G-protein alpha-subunit levels in hippocampus and entorhinal cortex of brains staged for Alzheimer's disease neurofibrillary and amyloid pathologies

G-protein alpha-subunits (Galphao, Galphai, Galphas, Galphaq) and adenylyl cyclase (AC) I and II isoforms were quantified in hippocampus and entorhinal cortex from 22 cases staged for Alzheimer's disease (AD) pathologies according to Braak and Braak. Hippocampal Galphai levels declined significantly with neurofibrillary staging, whereas AC I levels in this region increased. Significant amyloid stage-related reductions of Galphai were seen in both the hippocampus and entorhinal cortex. The hippocampus also showed a significant reduction of Galphao with amyloid staging. It is concluded that levels of inhibitory G-protein subunits Galphao, and in particular Galphai, decrease in parallel to the extent of AD pathology.

Quantitative autoradiography of [3H]forskolin binding sites in post-mortem brain staged for Alzheimer's disease neurofibrillary changes and amyloid deposits

Adenylyl cyclase (AC) signal transduction has been shown to be affected in Alzheimer's disease (AD). Deficits have been described in different components of the system, from the receptor to the effector level. [3H]forskolin is a diterpene that binds with high affinity to AC. In the present report, we used autoradiography to study [3H]forskolin binding to sections of entorhinal cortex and hippocampus from 23 cases staged for AD pathology according to Braak and Braak [Acta Neuropathol. 82 (1991) 239-259]. This protocol defines six stages according to neurofibrillary changes, which start in the entorhinal region (stages I-II), spread to the hippocampus (stages III-IV) and finally to the isocortical areas (stages V-VI). The amyloid classification includes three stages in which the basal isocortex is first affected (stage A), followed by other isocortical association areas (stage B) and finally the primary isocortical areas (stage C). We also studied the effects of the GTP-analogue Gpp[NH]p on binding, in order to detect changes in G-protein-AC coupling. We used two different concentrations of Gpp[NH]p, that were previously reported to inhibit and stimulate [3H]forskolin binding via Gi and Gs, respectively. Results showed that [3H]forskolin binding declined significantly with staging for neurofibrillary changes only in the entorhinal region (P < 0.05, ANOVA). In addition, the decrease in [3H]forskolin binding observed in the presence of 1 microM Gpp[NH]p diminished significantly with staging in the entorhinal region (P < 0.05, ANOVA). No significant changes were seen with amyloid staging, with the exception of the CA1 subfield of the hippocampus, where [3H]forskolin binding in the absence of Gpp[NH]p was significantly decreased at stage B compared with all other stages (P < 0.05, ANOVA). In conclusion, our results showed a very limited decrease in [3H]forskolin binding with the progression of AD pathology, suggesting that the AC levels may be largely preserved in the disease. The specific change in the effect of a low concentration of Gpp[NH]p on the binding could indicate the loss of Ca2+/calmodulin-sensitive AC isoforms in AD.

Dendritic changes in Alzheimer's disease and factors that may underlie these changes

It seems likely that the Alzheimer disease (AD)-related dendritic changes addressed in this article are induced by two principally different processes. One process is linked to the plastic response associated with deafferentation, that is, long-lasting transneuronally induced regressive changes in dendritic geometry and structure. The other process is associated with severe alterations of the dendritic- and perikaryal cytoskeleton as seen in neurons with the neurofibrillary pathology of AD, that is, the formation of paired helical filaments formed by hyperphosphorylated microtubule-associated protein tau. As the development of dendritic and cytoskeletal abnormalities are at least mediated by alterations in signal transduction, this article also reviews changes in signal pathways in AD. We also discuss transgenic approaches developed to model and understand cytoskeletal abnormalities.

Ca2+/CaM-sensitive adenylyl cyclase activity is decreased in the Alzheimer's brain: possible relation to type I adenylyl cyclase

Immunoreactivities of four subtypes of adenylyl cyclase (AC) (types I, II, IV and V/VI), and basal, forskolin- and Mn(2+)-stimulated AC activities with or without calcium and calmodulin (Ca2+/CaM) were estimated in parietal cortex membranes from cases with dementia of the Alzheimer type (DAT) and age-matched controls. Immunoreactivities of AC-I and AC-II were significantly decreased, but those of AC-IV and AC-V/VI did not change in DAT brains. There was a significant correlation of AC-I immunoreactivity with Ca2+/CaM-sensitive AC activity, but not with the Ca2+/CaM-insensitive activity. Ca2+/CaM-sensitive AC activity was significantly lower in DAT than in the control, indicating that impairment of Ca2+/CaM-sensitive AC-I is clearly involved in the pathophysiology of DAT.

Decreased beta-adrenoceptor-stimulated adenylyl cyclase activity in lymphocytes from Alzheimer's disease patients

Previous studies have shown that in Alzheimer's disease post-mortem brain there are disruptions of both beta1-adrenoceptor-G-protein coupling and G-protein stimulation of adenylyl cyclase activity. Decreased beta-adrenoceptor stimulated adenylyl cyclase activity has also been shown in Alzheimer's disease primary skin fibroblasts. In the present study, we determined the regulation of adenylyl cyclase in Alzheimer's disease patients using an easily accessible tissue source, namely peripheral blood lymphocytes. beta-Adrenoceptor- and forskolin-stimulated adenylyl cyclase activities were investigated in lymphocytes from 12 Alzheimer's disease and 12 carefully matched and selected control subjects. No significant differences were found in basal or forskolin-stimulated enzyme activities between Alzheimer's disease and control lymphocytes. In contrast, isoprenaline-stimulated adenylyl cyclase activities were significantly lower in the Alzheimer's disease groups, as compared to controls. These results indicate that there is a widespread disruption of beta-adrenoceptor-G-protein-enzyme coupling in different tissues from Alzheimer's disease patients, and that adenylyl cyclase disturbances previously reported in Alzheimer's disease brain do not occur as a consequence of disease pathology or of terminal illness.

Impaired G-protein-stimulated adenylyl cyclase activity in Alzheimer's disease brain is not accompanied by reduced cyclic-AMP-dependent protein kinase A activity

Previous studies have shown that the regulation of adenylyl cyclase activity is disrupted in Alzheimer's disease postmortem brain. In the present study, we determined whether disrupted adenylyl cyclase is accompanied by altered cAMP-dependent protein kinase activity in Alzheimer's disease superior temporal cortex and cerebellum. GTP gamma S-stimulated adenylyl cyclase activity was significantly lower in Alzheimer's disease superior temporal cortex, but not cerebellum, compared to values from a series of matched control cases. Neither basal or forskolin-stimulated adenylyl cyclase activities were significantly different between the Alzheimer's disease and control brain regions. No significant differences were seen in either particulate or soluble fraction cAMP-dependent protein kinase activities between the Alzheimer's disease and control brain regions. It is concluded that disrupted adenylyl cyclase signalling in Alzheimer's disease brain occurs specifically at the level of Gs-protein-enzyme interactions and is not accompanied by an altered cAMP-dependent protein kinase activity.

Neurotransmitters, signal transduction and second-messengers in Alzheimer's disease

It has long been assumed that widespread changes in postsynaptic neurotransmitter receptor function are not a feature of the disrupted neurotransmission seen in the brains with Alzheimer's disease (AD). However, recent evidence from postmortem brain and fibroblast studies suggests that both the neurotransmitter receptor/G-protein-modulated adenylyl cyclase and the phosphatidylinositol hydrolysis signal transduction cascades are disrupted in AD. Such disruptions may severely limit the functional integrity of key receptor types and undermine pharmacological attempts to ameliorate disease symptomatology through neurotransmitter replacement strategies. The involvement of some signalling mechanisms in the regulation of beta-amyloid precursor protein metabolism suggests also that disrupted signal transduction may exacerbate AD pathology.

Disturbances in signal transduction mechanisms in Alzheimer's disease

Many of the treatments directed towards alleviation of symptoms in Alzheimer's disease assume that target receptor systems are functionally intact. However, there is now considerable evidence that this is not the case. In human post-mortem brain tissue samples, the function of the GTP-binding protein Gs in regulating adenylyl cyclase is severely disabled, whereas that of Gi is intact. This difference in the function of the two G-protein types is also found in G-protein regulation of high- and low-affinity receptor recognition site populations. Measurement of G-protein densities using selective antibodies has indicated that the dysfunction in Gs-stimulation of cAMP production correlates with the ratio of the large to small molecular weight isoforms of the Gs alpha subunit. With respect to intracellular second messenger effects, there is a dramatic decrease in the density of brain receptor recognition sites for Ins(1,4,5)P3 that is not accompanied by a corresponding change in the Ins(1,3,4,5)P4 recognition site density. Protein kinase C function is also altered in Alzheimer's disease, a finding that may be of importance for the control of beta-amyloid production. These studies indicate that signal transduction processes are severely compromised in Alzheimer's disease. Some of these disturbances are also seen in cultured fibroblasts from Alzheimer's disease patients, indicating that they are neither restricted to areas of histopathological change, nor non-specific changes found late in the course of the disease. Cellular models to investigate the relation between amyloid production and deficits in signal transduction are also discussed.

Preservation of acetylcholine muscarinic M2 receptor G-protein interactions in the neocortex of patients with Alzheimer's disease

The efficacy of acetylcholine muscarinic M2 receptor-G protein coupling was investigated in Alzheimer's disease and control neocortical membranes by measuring the effects of MgCl2 and 5'-guanylylimidodiphosphate (Gpp[NH]p) on high-affinity [3H]oxotremorine-M ([3H]OXO-M) binding. MgCl2 gave similar enhancements of [3H]OXO-M binding in Alzheimer's disease and control occipital cortex. In contrast, MgCl2 enhanced [3H]OXO-M binding was significantly higher in Alzheimer's disease superior temporal cortex, compared to controls. MgCl2 enhanced [3H]OXO-M binding in both the occipital and temporal cortices of the Alzheimer's disease cases was reversed to control levels by Gpp[NH]p. It is concluded that the number of high-affinity muscarinic M2 sites is increased in Alzheimer's disease superior temporal, but not occipital, cortex and that M2 sites in both regions maintain an efficient G-protein coupling.

Adenylyl cyclase activity in Alzheimer's disease brain: stimulatory and inhibitory signal transduction pathways are differently affected

Adenylyl cyclase (AC) activity was studied in post mortem hippocampus and cerebellum from eight patients with Alzheimer's disease/senile dementia of the Alzheimer type (AD/SDAT) and seven non-demented control patients. AC was stimulated via stimulatory guanine nucleotide binding proteins (Gs) using guanosine triphosphate (GTP) and GppNHp (both 10(-4) M) or directly with either forskolin (10(-4) M) or Mn2+ (10(-2) M). Inhibition of AC via A1-receptors was performed with N6-cyclohexyladenosine (CHA) under basal conditions and in the presence of forskolin (10(-5) M). In both brain regions AC activity was significantly reduced in AD/SDAT when compared to controls. Under basal conditions and after stimulation via Gs mean reduction in hippocampus and cerebellum was 47.7% and 58.2%, respectively. The reduction was less pronounced after direct activation of the AC, amounting to 21.8% in hippocampus and 28.1% in cerebellum. CHA inhibited basal and forskolin-stimulated AC concentration-dependently by about 20% (basal) and 30% (forskolin). Inhibition by CHA was similar in hippocampus and cerebellum and tended to be more pronounced in AD/SDAT than in controls. Since the reduction of AC activity in AD/SDAT is greater after stimulation via Gs than after direct activation of the catalytic subunit, we suggest that both Gs and the catalytic subunit seem to be impaired. The fact that CHA-mediated inhibition of AC is not significantly different in AD/SDAT and controls, indicates that in contrast to Gs-, inhibitory G-proteins (Gi) coupling to AC remains intact in Alzheimer's disease.

Impaired phosphoinositide hydrolysis in Alzheimer's disease brain

The effect of Alzheimer's disease (AD) on the activity of the phosphoinositide second messenger system was studied by measuring the hydrolysis of [3H]phosphatidylinositol (PI) by membranes from postmortem human prefrontal cortex. The activity of phospholipase C was similar in AD and control tissue. Activation with GTP gamma S and with carbachol demonstrated less [3H]PI hydrolysis in AD than control membranes. The concentration of Gq/11, the G-proteins most likely functional in phosphoinositide metabolism, was unchanged in AD compared with controls, indicating that function of the receptor-G-protein complex rather than the G-protein concentration was the site of the impairment in AD. These results indicate that postsynaptic muscarinic receptor responses are impaired in AD, a finding that may explain, in part, the limited therapeutic responses achieved by administration of cholinomimetics to patients with AD. Also, this assay provides a means to identify cholinomimetics that are most effective in activating muscarinic receptor-coupled phosphoinositide hydrolysis in human brain, agents which should have the greatest potential for providing therapeutic responses in AD.

Regionally selective alterations in G protein subunit levels in the Alzheimer's disease brain

In the present study the relative densities of a number of G protein subunits were quantified in membranes prepared from the hippocampus, temporal cortex and angular gyrus of Alzheimer's disease and control post-mortem brain by immunoblotting with specific polyclonal antisera against Gs alpha, Gi alpha, Gi alpha-1, G(o) alpha and G beta protein subunits. In addition, basal, Gs-stimulated and Gi-inhibited adenylyl cyclase activities were measured in the same hippocampal membrane samples. Densitometric analysis of the immunoblot data revealed a 58% reduction in the levels of Gi alpha, and a 75% reduction in the levels of Gi alpha-1, in the Alzheimer's disease temporal cortex. Gi alpha levels were reduced, by 37% in the angular gyrus of the Alzheimer's disease cases. The ratio of large to small molecular weight isoforms of the Gs alpha subunit was significantly increased in both the hippocampus and the angular gyrus of the Alzheimer's disease samples when compared to control values, although the difference in individual Gs alpha isoform levels did not attain statistical significance when comparing groups. No statistically significant differences were observed in G(o) alpha or G beta levels when comparing control and Alzheimer's disease cases. Gs-stimulated adenylyl cyclase activity was significantly reduced in the Alzheimer's disease samples compared to controls, whereas Gi-inhibited adenylyl cyclase activity was unchanged. No significant differences were observed between the control and Alzheimer's disease samples for either basal or forskolin stimulated adenylyl cyclase activity. The ratio of hippocampal Gs-stimulated to basal adenylyl cyclase activity correlated significantly with the large to small Gs alpha subunit ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist-induced, GTP-dependent phosphoinositide hydrolysis in postmortem human brain membranes

Membranes prepared from postmortem human brain were used to measure the activities of three components of the phosphoinositide second messenger system. [3H]Phosphatidylinositol ([3H]PI) hydrolysis was stimulated by directly activating phospholipase C with calcium, by activating guanine nucleotide-binding proteins (G proteins) with guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) or with AIF4, and by receptors activated with several agonists (in the presence of GTP gamma S), including (in order of increasing magnitudes of responses) carbachol, pilocarpine, histamine, trans-1-aminocyclopentyl-1,3-dicarboxylic acid (a selective excitatory amino acid metabotropic receptor agonist), serotonin, and ATP. Gq/11 was identified as the G protein most likely to mediate [3H]PI hydrolysis in human brain membranes based on the findings that this process was not impaired by pretreatment with pertussis toxin and it was inhibited by antibodies specific for the alpha-subunit of Gq/11 but not by antibodies for G0 or Gi1. The effects of postmortem delay on [3H]PI hydrolysis were examined by studying tissues obtained 6-21 h postmortem. A slight increase in basal [3H]PI hydrolysis was associated with increased postmortem time, suggesting a slow loss of the normal inhibitory control of phospholipase C. GTP gamma S-stimulated [3H]PI hydrolysis was unaffected by postmortem times within this range, but carbachol-induced [3H]PI hydrolysis tended to decrease with increasing postmortem times. These results demonstrate that the entire phosphoinositide complex remains functional and experimentally detectable in postmortem human brain membranes. This method provides a means to study the function, regulation, effects of diseases, and responses to drugs of the phosphoinositide system in human brain.

Differential alterations of ion channel binding sites in temporal and occipital regions of the cerebral cortex in Alzheimer's disease

Three ion channel binding sites were examined by means of quantitative ligand binding autoradiography in temporal and occipital cortex from 9 patients with neuropathologically confirmed Alzheimer's disease (AD) and 7 matched control subjects. The following ligands were used: 125I-apamin to label a population of Ca(2+)-sensitive K+ channels; [3H]PN200-110 to label L-type voltage-sensitive Ca2+ channels and [3H]glibenclamide to label ATP-sensitive K+ channels. Ion channel binding sites were compared to: choline acetyltransferase (ChAT) activity and plaque densities measured in the same tissue. In the temporal cortex in AD 125I-apamin binding was increased compared to controls (e.g. superficial layers: control = 0.71 +/- 0.07; AD = 1.02 +/- 0.07, mean +/- S.E.M. pmol/g tissue). In contrast, in adjacent sections [3H]glibenclamide binding was reduced in AD compared to controls (e.g. superficial layers: control = 25.3 +/- 1.7; AD = 17.9 +/- 1.4 pmol/g tissue). [3H]PN200-110 binding in temporal cortex was not altered in AD compared to controls. In the occipital cortex 125I-apamin binding was increased in AD while both [3H]glibenclamide and [3H]PN-200-110 binding sites in this cortical area were not different from controls. Plaque density (per mm2) was higher in temporal (e.g. layers I-III, 43 +/- 6) than in occipital cortex (layers I-III, 27 +/- 4) in the AD patients while ChAT activity was reduced by 40% in temporal cortex and by 50% in occipital cortex compared to controls. The results suggest that the three ion channel binding sites are located on structural elements in the brain which are differentially affected by the pathophysiology of AD.

Alterations in the activity of adenylate cyclase and high affinity GTPase in Alzheimer's disease

The aim of this study was to assess the effect of Alzheimer's disease has on the functional integrity of several signal transduction proteins. The relative levels of the G-protein alpha subunits Gs alpha-L, Gs alpha-S, Gi alpha-2 and G(o) alpha were measured by western blotting and found to be unchanged in membranes prepared from Alzheimer-diseased frontal cortex or hippocampus compared to control brains. However the activity of the G-protein associated enzyme, high affinity GTPase, was found to be reduced in the frontal cortex (reduced by 25%) and by a similar magnitude in the hippocampus (reduced by 27%) of Alzheimer subjects. The same membrane preparations were also assayed for the activity of adenylate cyclase. Basal enzyme activity was not significantly altered in Alzheimer diseased hippocampus, but was markedly reduced (by 45%) in the frontal cortex. The ability of fluoride and aluminium ions to stimulate adenylate cyclase was not significantly changed in either brain region. This suggests that G-proteins, especially Gs, are still able to interact with this enzyme. These results indicate that although the presence of Alzheimer's disease does not significantly alter G-protein levels, changes have taken place in the overall activity of these proteins. However this alteration does not affect their ability to stimulate adenylate cyclase activity.

Disrupted beta 1-adrenoceptor-G protein coupling in the temporal cortex of patients with Alzheimer's disease

The efficacy of beta 1-adrenoceptor-G protein coupling was studied in postmortem temporal cortex synaptic membranes from a series of control and Alzheimer's disease subjects. For the control cases, the non-hydrolysable GTP analogue 5'-guanylylimidodiphosphate (Gpp[NH]p) gave a significant reduction in the affinity of the agonist isoprenaline to displace binding of the radiolabelled antagonist (+/)-4-(3-t-butylamino-2-hydroxypropoxy)[5,7-3H]benzimidazol-2-one ([3H]CGP-12177). This effect was attributed to the conversion of high agonist-affinity sites to a lower-affinity state and was not found for the Alzheimer's disease cases. These data indicate that a disruption of beta 1-adrenoceptor-G protein coupling occurs in the temporal cortex of Alzheimer's disease patients.