Transient hypoxia induces sequestration of M1 and M2 muscarinic acetylcholine receptors

Live-cell microscopy or fluorescence anisotropy with budded baculoviruses-which way to go with measuring ligand binding to M4 muscarinic receptors?

M4 muscarinic acetylcholine receptor is a G protein-coupled receptor (GPCR) that has been associated with alcohol and cocaine abuse, Alzheimer's disease, and schizophrenia which makes it an interesting drug target. For many GPCRs, the high-affinity fluorescence ligands have expanded the options for high-throughput screening of drug candidates and serve as useful tools in fundamental receptor research. Here, we explored two TAMRA-labelled fluorescence ligands, UR-MK342 and UR-CG072, for development of assays for studying ligand-binding properties to M4 receptor. Using budded baculovirus particles as M4 receptor preparation and fluorescence anisotropy method, we measured the affinities and binding kinetics of both fluorescence ligands. Using the fluorescence ligands as reporter probes, the binding affinities of unlabelled ligands could be determined. Based on these results, we took a step towards a more natural system and developed a method using live CHO-K1-hM4R cells and automated fluorescence microscopy suitable for the routine determination of unlabelled ligand affinities. For quantitative image analysis, we developed random forest and deep learning-based pipelines for cell segmentation. The pipelines were integrated into the user-friendly open-source Aparecium software. Both image analysis methods were suitable for measuring fluorescence ligand saturation binding and kinetics as well as for screening binding affinities of unlabelled ligands.

GPCR kinase knockout cells reveal the impact of individual GRKs on arrestin binding and GPCR regulation

G protein-coupled receptors (GPCRs) activate G proteins and undergo a complex regulation by interaction with GPCR kinases (GRKs) and the formation of receptor-arrestin complexes. However, the impact of individual GRKs on arrestin binding is not clear. We report the creation of eleven combinatorial HEK293 knockout cell clones lacking GRK2/3/5/6, including single, double, triple and the quadruple GRK knockout. Analysis of β-arrestin1/2 interactions for twelve GPCRs in our GRK knockout cells enables the differentiation of two main receptor subsets: GRK2/3-regulated and GRK2/3/5/6-regulated receptors. Furthermore, we identify GPCRs that interact with β-arrestins via the overexpression of specific GRKs even in the absence of agonists. Finally, using GRK knockout cells, PKC inhibitors and β-arrestin mutants, we present evidence for differential receptor-β-arrestin1/2 complex configurations mediated by selective engagement of kinases. We anticipate our GRK knockout platform to facilitate the elucidation of previously unappreciated details of GRK-specific GPCR regulation and β-arrestin complex formation.

miRNA-132/212 Gene-Deletion Aggravates the Effect of Oxygen-Glucose Deprivation on Synaptic Functions in the Female Mouse Hippocampus

Cerebral ischemia and its sequelae, which include memory impairment, constitute a leading cause of disability worldwide. Micro-RNAs (miRNA) are evolutionarily conserved short-length/noncoding RNA molecules recently implicated in adaptive/maladaptive neuronal responses to ischemia. Previous research independently implicated the miRNA-132/212 cluster in cholinergic signaling and synaptic transmission, and in adaptive/protective mechanisms of neuronal responses to hypoxia. However, the putative role of miRNA-132/212 in the response of synaptic transmission to ischemia remained unexplored. Using hippocampal slices from female miRNA-132/212 double-knockout mice in an established electrophysiological model of ischemia, we here describe that miRNA-132/212 gene-deletion aggravated the deleterious effect of repeated oxygen-glucose deprivation insults on synaptic transmission in the dentate gyrus, a brain region crucial for learning and memory functions. We also examined the effect of miRNA-132/212 gene-deletion on the expression of key mediators in cholinergic signaling that are implicated in both adaptive responses to ischemia and hippocampal neural signaling. miRNA-132/212 gene-deletion significantly altered hippocampal AChE and mAChR-M1, but not α7-nAChR or MeCP2 expression. The effects of miRNA-132/212 gene-deletion on hippocampal synaptic transmission and levels of cholinergic-signaling elements suggest the existence of a miRNA-132/212-dependent adaptive mechanism safeguarding the functional integrity of synaptic functions in the acute phase of cerebral ischemia.

Dysfunctional nucleus tractus solitarius: its crucial role in promoting neuropathogenetic cascade of Alzheimer's dementia--a novel hypothesis

The pathophysiological mechanism(s) underlying Alzheimer's disease (AD) still remain unclear, and no disease-modifying or prophylactic therapies are currently available. Unraveling the fundamental neuropathogenesis of AD is an important challenge. Several studies on AD have suggested lesions in a number of CNS areas including the basal forebrain, hippocampus, entorhinal cortex, amygdale/insula, and the locus coeruleus. However, plausible unifying studies on the upstream factors that involve these heterogeneous regions and herald the onset of AD pathogenesis are not available. The current article presents a novel nucleus tractus solitarius (NTS) vector hypothesis that underpins several disparate biological mechanisms and neural circuits, and identifies relevant hallmarks of major presumptive causative factor(s) linked to the NTS, in older/aging individuals. Aging, obesity, infection, sleep apnea, smoking, neuropsychological states, and hypothermia-all activate inflammatory cytokines and oxidative stress. The synergistic impact of systemic proinflammatory mediators activates microglia and promotes neuroinflammation. Acutely, the innate immune response is protective defending against pathogens/toxins; however, when chronic, it causes neuroinflammation and neuronal dysfunction, particularly in brainstem and neocortex. The NTS in the brainstem is an essential multiple signaling hub, and an extremely important central integration site of baroreceptor, chemoreceptor, and a multitude of sensory afferents from gustatory, gastrointestinal, cardiac, pulmonary, and upper airway systems. Owing to persistent neuroinflammation, the dysfunctional NTS exerts deleterious impact on nucleus ambiguus, dorsal motor nucleus of vagus, hypoglossal, parabrachial, locus coeruleus and many key nuclei in the brainstem, and the hippocampus, entorhinal cortex, prefrontal cortex, amygdala, insula, and basal forebrain in the neocortex. The neuronal and synaptic dysfunction emanating from the inflamed NTS may affect its interconnected pathways impacting almost the entire CNS--which is already primed by neuroinflammation, thus promoting cognitive and neuropsychiatric symptoms. The upstream factors discussed here may underpin the neuropathopgenesis of AD. AD pathology is multifactorial; the current perspective underscores the value of attenuating disparate upstream factors--in conjunction with anticholinesterase, anti-inflammatory, immunosuppressive, and anti-oxidant pharmacotherapy. Amelioration of the NTS pathology may be of central importance in countering the neuropathological cascade of AD. The NTS, therefore, may be a potential target of novel therapeutic strategies.

Antiamnesic and neuroprotective effects of the aminotetrahydrofuran derivative ANAVEX1-41 against amyloid beta(25-35)-induced toxicity in mice

The antiamnesic and neuroprotective activities of the new aminotetrahydrofuran derivative tetrahydro-N,N-dimethyl-5,5-diphenyl-3-furanmethanamine hydrochloride (ANAVEX1-41), a nonselective muscarinic receptor ligand and sigma1 protein activator, were examined in mice injected intracerebroventricularly with amyloid beta(25-35) (Abeta(25-35)) peptide (9 nmol). Abeta(25-35) impaired significantly spontaneous alternation performance, a spatial working memory, and passive avoidance response. When ANAVEX1-41 (1-1000 microg/kg i.p.) was administered 7 days after Abeta(25-35), ie, 20 min before the behavioral tests, it significantly reversed the Abeta(25-35)-induced deficits, the most active doses being in the 3-100 microg/kg range. When the compound was preadministered 20 min before Abeta(25-35), ie, 7 days before the tests, it prevented the learning impairments at 30-100 microg/kg. Morphological analysis of corticolimbic structures showed that Abeta(25-35) induced a significant cell loss in the CA1 pyramidal cell layer of the hippocampus that was prevented by ANAVEX1-41 (100 microg/kg). Increased number of glial fibrillary acidic protein immunopositive cells in the retrosplenial cortex or throughout the hippocampus revealed an Abeta(25-35)-induced inflammation that was prevented by ANAVEX1-41. The drug also prevented the parameters of Abeta(25-35)-induced oxidative stress measured in hippocampus extracts, ie, the increases in lipid peroxidation and protein nitration. ANAVEX1-41, however, failed to prevent Abeta(25-35)-induced caspase-9 expression. The compound also blocked the Abeta(25-35)-induced caspase-3 expression, a marker of apoptosis. Both the muscarinic antagonist scopolamine and the sigma1 protein inactivator BD1047 prevented the beneficial effects of ANAVEX1-41 (30 or 100 microg/kg) against Abeta(25-35)-induced learning impairments, suggesting that muscarinic and sigma1 targets are involved in the drug effect. A synergic effect could indeed account for the very low active doses measured in vivo. These data outline the therapeutic potential of ANAVEX1-41 as a neuroprotective agent in Alzheimer's disease.

G-protein-coupled receptor phosphorylation: where, when and by whom

Almost all G-protein coupled receptors (GPCRs) are regulated by phosphorylation and this process is a key event in determining the signalling properties of this receptor super-family. Receptors are multiply phosphorylated at sites that can occur throughout the intracellular regions of the receptor. This diversity of phospho-acceptor sites together with a lack of consensus phosphorylation sequences has led to the suggestion that the precise site of phosphorylation is not important in the phosphorylation-dependent regulation of GPCR function but rather it is the increase in bulk negative charge of the intracellular face of the receptor which is the significant factor. This review investigates the possibility that the multi-site nature of GPCR phosphorylation reflects the importance of specific phosphorylation events which mediate distinct signalling outcomes. In this way receptor phosphorylation may provide for a flexible regulatory mechanism that can be tailored in a tissue specific manner to regulate physiological processes. By understanding the flexible nature of GPCR phosphorylation if may be possible to develop agonists or allosteric modulators that promote a subset of phosphorylation events on the target GPCR and thereby restrict the action of the drug to a particular receptor mediated signalling response.

Prenatal exposure to nicotine with associated in utero hypoxia decreased fetal brain muscarinic mRNA in the rat

Prenatal exposure to nicotine can be associated with fetal abnormal development and brain damage. This study determined the effect of administration of nicotine with associated in utero hypoxia in maternal rats from early, middle, and late gestation on fetal blood hemoglobin, and expression of cholinergic receptor subtypes in the fetal brain. Our results demonstrated that maternal subcutaneous nicotine from the early gestation increased fetal hemoglobin and hematocrit, associated with reduction of PO(2). Although exposure to nicotine during late gestation had no effects on fetal brain weight, nicotine administration from the early gestation significantly decreased fetal brain muscarinic receptor (M1, M2, M3, and M4) mRNA expression, associated with restricted brain growth. Nicotine-altered muscarinic receptor subtype expression in the fetal forebrain and hindbrain showed regional differences. In addition, there were gestational differences for fetal brain muscarinic suppression by prenatal nicotine. Together, the results demonstrate that nicotine-induced in utero hypoxia is associated with poor development of muscarinic receptors in the fetal brain and restricted brain growth, and that either prolonged prenatal exposure to nicotine or critical "window" period for the brain development during pregnancy may play a role in prenatal nicotine-induced fetal muscarinic-receptor deficiency in the fetal brain.