Functional activation of Gαq coupled to 5-HT2A receptor and M1 muscarinic acetylcholine receptor in postmortem human cortical membranes

Serotonin Receptors as a Potential Target in the Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia worldwide that has an increasing impact on aging societies. Besides its critical role in the control of various physiological functions and behavior, brain serotonin (5-HT) system is involved in the regulation of migration, proliferation, differentiation, maturation, and programmed death of neurons. At the same time, a growing body of evidence indicates the involvement of 5-HT neurotransmission in the formation of insoluble aggregates of β-amyloid and tau protein, the main histopathological signs of AD. The review describes the role of various 5-HT receptors and intracellular signaling cascades induced by them in the pathological processes leading to the development of AD, first of all, in protein aggregation. Changes in the functioning of certain types of 5-HT receptors or associated intracellular signaling mediators prevent accumulation of β-amyloid plaques and tau protein neurofibrillary tangles. Based on the experimental data, it can be suggested that the use of 5-HT receptors as new drug targets will not only improve cognitive performance in AD, but will be also important in treating the causes of AD-related dementia.

Understanding How Physical Exercise Improves Alzheimer’s Disease: Cholinergic and Monoaminergic Systems

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder, characterized by the accumulation of proteinaceous aggregates and neurofibrillary lesions composed of β-amyloid (Aβ) peptide and hyperphosphorylated microtubule-associated protein tau, respectively. It has long been known that dysregulation of cholinergic and monoaminergic (i.e., dopaminergic, serotoninergic, and noradrenergic) systems is involved in the pathogenesis of AD. Abnormalities in neuronal activity, neurotransmitter signaling input, and receptor function exaggerate Aβ deposition and tau hyperphosphorylation. Maintenance of normal neurotransmission is essential to halt AD progression. Most neurotransmitters and neurotransmitter-related drugs modulate the pathology of AD and improve cognitive function through G protein-coupled receptors (GPCRs). Exercise therapies provide an important alternative or adjunctive intervention for AD. Cumulative evidence indicates that exercise can prevent multiple pathological features found in AD and improve cognitive function through delaying the degeneration of cholinergic and monoaminergic neurons; increasing levels of acetylcholine, norepinephrine, serotonin, and dopamine; and modulating the activity of certain neurotransmitter-related GPCRs. Emerging insights into the mechanistic links among exercise, the neurotransmitter system, and AD highlight the potential of this intervention as a therapeutic approach for AD.

5-HT2A receptor-mediated Gαq/11 activation in psychiatric disorders: A postmortem study

OBJECTIVES

Serotonin-2A (5-HT_2A) receptors play an important role in the regulation of many brain functions that are disturbed in patients with such psychiatric diseases as mood disorders and schizophrenia. The objective of this study was to evaluate 5-HT_2A receptor-mediated signalling pathway through Gα_q/11 activation in psychiatric patients by using post-mortem brain samples.

METHODS

Functional activation of Gα_q/11 proteins coupled to 5-HT_2A receptors was determined by means of [³⁵S]GTPγS binding/immunoprecipitation assay in post-mortem prefrontal cortex of psychiatric patients diagnosed as bipolar disorder (BP), major depressive disorder (MDD), and schizophrenia, and individually matched controls. The effects of antipsychotic treatment as well as suicide were also analysed.

RESULTS

There was no significant difference in maximum percent increase (%E_max) or slope factor among the four groups. The negative logarithm of concentration eliciting the half-maximal effect (pEC₅₀) was significantly reduced in BP and schizophrenia patients as compared to controls. These alterations were attributable to antipsychotic medication. The pEC₅₀ values in 'non-suicide' group of schizophrenia, but not in 'suicide' group, were significantly reduced as compared with controls.

CONCLUSIONS

Altered 5-HT_2A receptor-mediated signalling pathway through Gα_q/11 proteins in prefrontal cortex might be apparently involved in pathophysiology and pharmacotherapy of BP and schizophrenia. In schizophrenic patients, these alterations as a result of successful treatment with antipsychotic agents may help in prevention of suicidal behaviour.

Functional approaches to the study of G-protein-coupled receptors in postmortem brain tissue: [35S]GTPγS binding assays combined with immunoprecipitation

G-protein-coupled receptors (GPCRs) have an enormous biochemical importance as they bind to diverse extracellular ligands and regulate a variety of physiological and pathological responses. G-protein activation measures the functional consequence of receptor occupancy at one of the earliest receptor-mediated events. Receptor coupling to G-proteins promotes the GDP/GTP exchange on Gα subunits. Thus, modulation of the binding of the poorly hydrolysable GTP analog [³⁵S]GTPγS to the Gα-protein subunit can be used as a functional approach to quantify GPCR interaction with agonist, antagonist or inverse agonist drugs. In order to determine receptor-mediated selective activation of the different Gα-proteins, [³⁵S]GTPγS binding assays combined with immunodetection by specific antibodies have been developed and applied to physiological and pathological brain conditions. Currently, immunoprecipitation with magnetic beads and scintillation proximity assays are the most habitual techniques for this purpose. The present review summarizes the different procedures, advantages and limitations of the [³⁵S]GTPγS binding assays combined with selective Gα-protein sequestration methods. Experience of functional coupling of several GPCRs to different Gα-proteins and recommendations for optimal performance in brain membranes are described. One of the biggest opportunities opened by these techniques is that they enable evaluation of biased agonism in the native tissue, which results in high interest in drug discovery. The available results derived from application of these functional methodologies to study GPCR dysfunctions in neuro-psychiatric disorders are also described. In conclusion, [³⁵S]GTPγS binding combined with antibody-mediated immunodetection represents an useful method to separately evaluate the functional activity of drugs acting on GPCRs over each Gα-protein subtype.

5-HT2A receptor- and M1 muscarinic acetylcholine receptor-mediated activation of Gαq/11 in postmortem dorsolateral prefrontal cortex of opiate addicts

BACKGROUND

Chronic exposure to opiates causes the development of tolerance and physical dependence as well as persistent brain neuroplasticity. Despite a wealth of postmortem human studies for opiate addicts, little direct information regarding the functional status of serotonergic and cholinergic receptor-mediated signaling pathways in the human brain of opiate addicts is yet available.

METHODS

Functional activation of Gα_q/11 proteins coupled to 5-HT_2A and M₁ type muscarinic acetylcholine receptor (mAChR) was assessed by using the method named [³⁵S]GTPγS binding/immunoprecipitation in frontal cortical membrane preparations from postmortem human brains obtained from opiate addicts and matched controls.

RESULTS

Concentration-response curves for 5-HT and carbachol in individual subjects were analyzed according to a nonlinear regression model, which generated the values of maximum percent increase (%E_max), negative logarithm of the half-maximal effect (pEC₅₀) and slope factor. As for 5-HT_2A receptor-mediated Gα_q/11 activation, the %E_max values were reduced significantly and the pEC₅₀ values were decreased significantly in opiate addicts as compared to the control group. Regarding carbachol-induced Gα_q/11 activation, no significant difference in %E_max or pEC₅₀ values was detected between the both groups, whereas the slope factor was increased significantly in opiate addicts as compared to the control group.

CONCLUSION

Our data demonstrate that the signaling pathways mediated by Gα_q/11 proteins coupled with 5-HT_2A receptors and M₁ mAChRs in prefrontal cortex are functionally altered in opiate addicts in comparison with control subjects. These alterations may underpin some aspects of addictive behavior to opiate as well as neuropsychological consequences or comorbid mental disorders associated with opioid use.

Use of bladder antimuscarinics is associated with an increased risk of dementia: a retrospective population-based case-control study

The association between bladder antimuscarinic use and dementia development is unclear. We used data from the Taiwan National Health Insurance Research Database to determine the association between the exposure dose and duration of bladder antimuscarinics and the subsequent dementia risk. We enrolled participants aged 55 years or more and defined a dementia cohort (International Classification of Diseases, Ninth Revision, Clinical Modification codes 290, 294.1, and 331.0). We used a propensity score matching method, and randomly enrolled two controls without dementia. We evaluated dementia risk with respect to the exposure dose and duration of treatment with seven bladder antimuscarinics (oxybutynin, propiverine, tolterodine, solifenacin, trospium, darifenacin, and fesoterodine) used for at least 1 year before the index date, after adjusting for age, sex, comorbidities, and medications. The dementia risk was 2.46-fold (95% confidence interval: 2.22–2.73) higher in Taiwanese patients who used bladder antimuscarinics for ≥ 1 year than in those who were not exposed to this treatment. The risk proportionally increased with increasing doses of antimuscarinics for less than 4 years. Taiwanese patients aged 55 years or more on bladder antimuscarinics exhibited a higher risk of dementia. Additional studies in other countries are required to determine whether this result is valid worldwide.

Fundamental features of receptor-mediated Gαi/o activation in human prefrontal cortical membranes: A postmortem study

To elucidate possible abnormalities in transmembrane signal transduction in psychiatric diseases, use of autopsy brain is a feasible approach. However, postmortem studies should be interpreted with caution concerning such factors as age, gender, psychotropic drug history, agonal state, postmortem delay (PMD), and storage period. In this study, agonist-induced [³⁵S]GTPγS binding was performed in postmortem dorsolateral prefrontal cortical membranes of 40 control subjects. In addition to the previously reported G protein-coupled receptor (GPCR)-mediated G_i/o activation, κ-opioid receptor-mediated [³⁵S]GTPγS binding was detected by using U-50,448. The responses elicited by 16 different agonists were determined, and the effects of several factors were investigated. Gender difference was negligible. Concentration-response curve of histamine H₃ receptor-mediated [³⁵S]GTPγS binding was shifted rightward in the subjects with some drugs detected at toxicological screening. Age-related alterations were minimal, except for the age-dependent supersensitivity of μ-opioid receptor-mediated Gα_i/o activation, revealed by endomorphin-1- and DAMGO-stimulated [³⁵S]GTPγS binding. Age-related increase in %E_max values was also detected as to DPDPE-induced [³⁵S]GTPγS binding through δ-opioid receptors. With an exception of NOP receptor/G-protein coupling, GPCR-mediated [³⁵S]GTPγS binding is relatively stable irrespective of PMD or storage period. There were many positive correlations among the %E_max values for different receptor subtypes, which might reflect formation of heterodimer complex of such GPCRs coupled to the same G_i/o proteins. These results provide us with important fundamental data in the future project using human postmortem brains from patients with psychiatric disorders.

Functional coupling of M1 muscarinic acetylcholine receptor to Gαq/11 in dorsolateral prefrontal cortex from patients with psychiatric disorders: a postmortem study

Accumulating studies have implicated intracellular signaling through muscarinic acetylcholine receptors (mAChRs) in psychiatric illness. In the present study, carbamylcholine chloride (carbachol)-induced Gα_i/o and Gα_q/11 activation was identified in postmortem human prefrontal cortical membranes. The following two sample cohorts were used: subjects [1], consisting of 40 controls without neuropsychiatric disorders, and subjects [2], consisting of 20 with bipolar disorder (BP), 20 major depressive disorder (MDD), 20 schizophrenia, and 20 controls, strictly sex- and age-matched. Carbachol-stimulated [³⁵S]GTPγS binding to human brain membranes was assessed by the two methods, i.e., conventional method using filtration techniques (Gα_i/o activation coupled to M₂/M₄ mAChRs) applied to subjects [1], and [³⁵S]GTPγS binding/immuno precipitation assay (Gα_q/11 activation coupled to M₁ mAChR) applied to subjects [1] and [2]. The concentration eliciting the half-maximal effect (EC₅₀), maximum percent increase (%E_max), and slope factor were obtained from concentration-response curve of carbachol-induced Gα_i/o and Gα_q/11 activation. The pEC₅₀ values of both carbachol-induced Gα_i/o and Gα_q/11 activations in subjects [1] were significantly correlated, though its implications or underlying molecular processes are unclear. The results of M₁ mAChR-mediated Gα_q/11 activation in subjects [2] indicated no significant disorder-specific alterations. However, the distribution patterns of the pEC₅₀ values showed unequal variances among the groups. There was a significant inverse correlation between the %E_max values and the pEC₅₀ values in subjects with schizophrenia, but not in those with BP or MDD, or controls. These data support the notion that schizophrenia patients consist of biologically heterogeneous subgroups with respect to M₁ mAChR-mediated signaling pathways.

Optimization and pharmacological characterization of receptor-mediated Gi/o activation in postmortem human prefrontal cortex

The biochemical abnormalities in transmembrane signal transduction mediated through G protein-coupled receptors (GPCRs) have been postulated as underlying pathophysiology of psychiatric diseases such as schizophrenia and mood disorders. In the present study, the experimental conditions of agonist-induced [35 S]GTPγS binding in postmortem human brain membranes were optimized, and the responses induced by a series of agonists were pharmacologically characterized. The [35 S]GTPγS binding assay was performed in postmortem human prefrontal cortical membranes by means of filtration techniques, and standardized as to GDP concentration, membrane protein content, MgCl2 and NaCl concentrations in assay buffer, incubation period and effect of white matter contamination. Under the standard assay conditions, the specific [35 S]GTPγS binding was stimulated by the addition of 15 compounds in a concentration-dependent manner. Of these agonists, R(+)-8-OH-DPAT, UK-14,304, DAMGO and DPDPE showed apparently biphasic concentration-response curves. As for these four responses, only higher-potency site was pharmacologically characterized. The receptors involved in the responses investigated were 5-HT1A receptor (probed with R(+)-8-OH-DPAT or 5-HT), α2A -adrenoceptor (UK-14,304 or (-)-epinephrine), M2 /M4 mAChRs (carbachol), adenosine A1 receptor (adenosine), histamine H3 receptor (histamine), group II mGlu (l-glutamate), GABAB receptor (baclofen), μ-opioid receptor (DAMGO or endomophin-1), δ-opioid receptor (DPDPE or SNC-80) and NOP (nociceptin). Although dopamine also activated specific [35 S]GTPγS binding, this response was likely mediated via α2A -adrenoceptor, but not dopamine receptor subtypes. The present study provides us with fundamental aspects of the strategy for elucidation of probable abnormalities of neural signalling mediated by G proteins activated through multiple GPCRs in the brain of psychiatric patients.

Functional coupling between adenosine A1 receptors and G-proteins in rat and postmortem human brain membranes determined with conventional guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding or [35S]GTPγS/immunoprecipitation assay

Adenosine signaling plays a complex role in multiple physiological processes in the brain, and its dysfunction has been implicated in pathophysiology of neuropsychiatric diseases such as schizophrenia and affective disorders. In the present study, the coupling between adenosine A₁ receptor and G-protein was assessed by means of two [³⁵S]GTPγS binding assays, i.e., conventional filtration method and [³⁵S]GTPγS binding/immunoprecipitation in rat and human brain membranes. The latter method provides information about adenosine A₁ receptor-mediated Gα_i-3 activation in rat as well as human brain membranes. On the other hand, adenosine-stimulated [³⁵S]GTPγS binding determined with conventional assay derives from functional activation of Gα_i/o proteins (not restricted only to Gα_i-3) coupled to adenosine A₁ receptors. The determination of adenosine concentrations in the samples used in the present study indicates the possibility that the assay mixture under our experimental conditions contains residual endogenous adenosine at nanomolar concentrations, which was also suggested by the results on the effects of adenosine receptor antagonists on basal [³⁵S]GTPγS binding level. The effects of adenosine deaminase (ADA) on basal binding also support the presence of adenosine. Nevertheless, the varied patterns of ADA discouraged us from adding ADA into assay medium routinely. The concentration-dependent increases elicited by adenosine were determined in 40 subjects without any neuropsychiatric disorders. The increases in %E_max values determined by conventional assay according to aging and postmortem delay should be taken into account in future studies focusing on the effects of psychiatric disorders on adenosine A₁ receptor/G-protein interaction in postmortem human brain tissue.