The consequences of reducing expression of the α7 nicotinic receptor by RNA interference and of stimulating its activity with an α7 agonist in SH-SY5Y cells indicate that this receptor plays a neuroprotective role in connection with the pathogenesis of Alzheimer's disease

Alterations in Neuronal Nicotinic Acetylcholine Receptors in the Pathogenesis of Various Cognitive Impairments

Cognitive impairment is a typical symptom of both neurodegenerative and certain other diseases. In connection with these different pathologies, the etiology and neurological and metabolic changes associated with cognitive impairment must differ. Until these characteristics and differences are understood in greater detail, pharmacological treatment of the different forms of cognitive impairment remains suboptimal. Neurotransmitter receptors, including neuronal nicotinic acetylcholine receptors (nAChRs), dopamine receptors, and glutamine receptors, play key roles in the functions and metabolisms of the brain. Among these, the role of nAChRs in the development of cognitive impairment has attracted more and more attention. The present review summarizes what is presently known concerning the structure, distribution, metabolism, and function of nAChRs, as well as their involvement in major cognitive disorders such as Alzheimer's disease, Parkinson's disease, vascular dementia, schizophrenia, and diabetes mellitus. As will be discussed, the relevant scientific literature reveals clearly that the α4β2 and α7 nAChR subtypes and/or subunits of the receptors play major roles in maintaining cognitive function and in neuroprotection of the brain. Accordingly, focusing on these as targets of drug therapy can be expected to lead to breakthroughs in the treatment of cognitive disorders such as AD and schizophrenia.

Unraveling the molecular interactions between α7 nicotinic receptor and a RIC3 variant associated with backward speech

Recent work putatively linked a rare genetic variant of the chaperone Resistant to Inhibitors of acetylcholinesterase (RIC3) (NM_024557.4:c.262G > A, NP_078833.3:p.G88R) to a unique ability to speak backwards, a language skill that is associated with exceptional working memory capacity. RIC3 is important for the folding, maturation, and functional expression of α7 nicotinic acetylcholine receptors (nAChR). We compared and contrasted the effects of RIC3G88R on assembly, cell surface expression, and function of human α7 receptors using fluorescent protein tagged α7 nAChR and Förster resonance energy transfer (FRET) microscopy imaging in combination with functional assays and 125I-α-bungarotoxin binding. As expected, the wild-type RIC3 protein was found to increase both cell surface and functional expression of α7 receptors. In contrast, the variant form of RIC3 decreased both. FRET analysis showed that RICG88R increased the interactions between RIC3 and α7 protein in the endoplasmic reticulum. These results provide interesting and novel data to show that a RIC3 variant alters the interaction of RIC3 and α7, which translates to decreased cell surface and functional expression of α7 nAChR.

Real-time tilting and twisting motions of ligand-bound states of α7 nicotinic acetylcholine receptor

The α7 nicotinic acetylcholine receptor is a member of the nicotinic acetylcholine receptor family and is composed of five α7 subunits arranged symmetrically around a central pore. It is localized in the central nervous system and immune cells and could be a target for treating Alzheimer's disease and schizophrenia. Acetylcholine is a ligand that opens the channel, although prolonged application rapidly decreases the response. Ivermectin was reported as one of the positive allosteric modulators, since the binding of Ivermectin to the channel enhances acetylcholine-evoked α7 currents. One research has suggested that tilting motions of the nicotinic acetylcholine receptor are responsible for channel opening and activation. To verify this hypothesis applies to α7 nicotinic acetylcholine receptor, we utilized a diffracted X-ray tracking method to monitor the stable twisting and tilting motion of nAChR α7 without a ligand, with acetylcholine, with Ivermectin, and with both of them. The results show that the α7 nicotinic acetylcholine receptor twists counterclockwise with the channel transiently opening, transitioning to a desensitized state in the presence of acetylcholine and clockwise without the channel opening in the presence of Ivermectin. We propose that the conformational transition of ACh-bound nAChR α7 may be due to the collective twisting of the five α7 subunits, resulting in the compression and movement, either downward or upward, of one or more subunits, thus manifesting tilting motions. These tilting motions possibly represent the transition from the resting state to channel opening and potentially to the desensitized state.

Discoveries and future significance of research into amyloid-beta/α7-containing nicotinic acetylcholine receptor (nAChR) interactions

Initiated by findings that Alzheimer's disease is associated with a profound loss of cholinergic markers in human brain, decades of studies have examined the interactions between specific subtypes of nicotinic acetylcholine receptors and amyloid-β [derived from the amyloid precursor protein (APP), which is cleaved to yield variable isoforms of amyloid-β]. We review the evolving understanding of amyloid-β's roles in Alzheimer's disease and pioneering studies that highlighted a role of nicotinic acetylcholine receptors in mediating important aspects of amyloid-β's effects. This review also surveys the current state of research into amyloid-β / nicotinic acetylcholine receptor interactions. The field has reached an exciting point in which common themes are emerging from the wide range of prior research and a range of accessible, relevant model systems are available to drive further progress. We highlight exciting new areas of inquiry and persistent challenges that need to be considered while conducting this research. Studies of amyloid-β and the nicotinic acetylcholine receptor populations that it interacts with provide opportunities for innovative basic and translational scientific breakthroughs related to nicotinic receptor biology, Alzheimer's disease, and cholinergic contributions to cognition more broadly.

Physiologic Functions and Therapeutic Applications of α7 Nicotinic Acetylcholine Receptor in Brain Disorders

Accumulating data suggest that α7 nicotinic acetylcholine receptors (α7nAChRs) are an important therapeutic target for the treatment of Alzheimer's disease (AD) and schizophrenia. The homopentameric ligand-gated ion channel α7nAChR consists of five identical α7 subunits that are encoded by the CHRNA7 (cholinergic receptor nicotinic alpha7 subunit) gene. Moreover, α7nAChRs are densely distributed throughout the hippocampus, cortex, and thalamus brain regions, but sparsely in the striatum, forebrain, and medulla. Compared with other nAChRs, α7nAChR binds with low affinity to the naturally occurring neurotransmitter acetylcholine and the non-specific exogenous agonist nicotine, and with high affinity to the specific antagonists α-bungarotoxin and methyllycaconitine. Reports indicate that α7nAChR plays important roles in neurotransmitter release, cognitive functioning, and the cholinergic anti-inflammatory response. Genetic variations that alter CHRNA7 mRNA and protein expression or cause α7nAChR dysfunction are associated with many brain disorders. Our previous studies revealed that α7nAChR exerts neuroprotection in AD by acting as a cargo receptor for binding the autophagosomal marker protein LC3 and engulfing extracellular neurotoxic Aβ_1-42 during autophagic degradation of the α7nAChR-Aβ_1-42 complex. However, the role of α7nAChRs in other diseases remains unknown. Here, we review and summarize the essential characteristics and current findings concerning α7nAChRs in four common brain diseases (AD, Parkinson's disease, schizophrenia, and depression), which may elucidate the role of α7nAChRs and inform innovative research and novel treatments that target α7nAChRs in brain disease.

Antioxidant Systems, lncRNAs, and Tunneling Nanotubes in Cell Death Rescue from Cigarette Smoke Exposure

Cigarette smoke is a rich source of carcinogens and reactive oxygen species (ROS) that can damage macromolecules including DNA. Repair systems can restore DNA integrity. Depending on the duration or intensity of stress signals, cells may utilize various survival and adaptive mechanisms. ROS levels are kept in check through redundant detoxification processes controlled largely by antioxidant systems. This review covers and expands on the mechanisms available to cigarette smoke-exposed cancer cells for restoring the redox balance. These include multiple layers of transcriptional control, each of which is posited to be activated upon reaching a particular stress threshold, among them the NRF2 pathway, the AP-1 and NF-kB pathways, and, finally, TP53, which triggers apoptosis if extreme toxicity is reached. The review also discusses long noncoding RNAs, which have been implicated recently in regulating oxidative stress-with roles in ROS detoxification, the inflammatory response, oxidative stress-induced apoptosis, and mitochondrial oxidative phosphorylation. Lastly, the emerging roles of tunneling nanotubes in providing additional mechanisms for metabolic rescue and the regulation of redox imbalance are considered, further highlighting the expanded redox reset arsenal available to cells.

Genetic deletion of α7 nicotinic acetylcholine receptors induces an age-dependent Alzheimer's disease-like pathology

The accumulation of amyloid-beta peptide (Aβ) and the failure of cholinergic transmission are key players in Alzheimer's disease (AD). However, in the healthy brain, Aβ contributes to synaptic plasticity and memory acting through α7 subtype nicotinic acetylcholine receptors (α7nAChRs). Here, we hypothesized that the α7nAChR deletion blocks Aβ physiological function and promotes a compensatory increase in Aβ levels that, in turn, triggers an AD-like pathology. To validate this hypothesis, we studied the age-dependent phenotype of α7 knock out mice. We found that α7nAChR deletion caused an impairment of hippocampal synaptic plasticity and memory at 12 months of age, paralleled by an increase of Amyloid Precursor Protein expression and Aβ levels. This was accompanied by other classical AD features such as a hyperphosphorylation of tau at residues Ser 199, Ser 396, Thr 205, a decrease of GSK-3β at Ser 9, the presence of paired helical filaments and neurofibrillary tangles, neuronal loss and an increase of GFAP-positive astrocytes. Our findings suggest that α7nAChR malfunction might precede Aβ and tau pathology, offering a different perspective to interpret the failure of anti-Aβ therapies against AD and to find novel therapeutical approaches aimed at restoring α7nAChRs-mediated Aβ function at the synapse.

Current opinion in neurological manifestations of SARS-CoV-2 infection

Neurological symptoms occur in approximately one-third of hospitalized patients with coronavirus disease 2019 (COVID-19). Among these symptoms, hypoxic encephalopathy develops in one-fifth of severe cases, while ischemic strokes due to thrombotic complications are common in one-third of COVID-19 intensive care patients. Brain involvement of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) is eventuated by several routes, including hematogenous spread, transsynaptic entry through infected neurons, olfactory nerve, ocular epithelium, vascular endothelium, and impaired blood–brain barrier. Besides the high angiotensin-converting enzyme-2 (ACE2) binding affinity, and FURIN preactivation, SARS-CoV-2 maintains efficient neuronal entry while evading immune surveillance by using basigin and neuropilin-1 receptors. However, the neurological manifestations and their pathogenic mechanisms are still debated in COVID-19 patients.

Expression levels of the α7 nicotinic acetylcholine receptor in the brains of patients with Alzheimer's disease and their effect on synaptic proteins in SH-SY5Y cells

Alzheimer's disease (AD) is a chronic neurodegenerative, and abnormal aggregation of the neurotoxic β amyloid (Aβ) peptide is an early event in AD. The present study aimed to determine the correlation between the nicotinic acetylcholine receptor α7 subunit (α7 nAChR) and Aβ in the brains of patients with AD, and to investigate whether the increased expression levels of the α7 nAChR could alter the neurotoxicity of Aβ. The expression levels of α7 nAChR and Aβ in the brains of patients with AD and healthy brains were analyzed using immunofluorescence. Moreover, SH‑SY5Y cells were used to stably overexpress or silence α7 nAChR expression levels, prior to the treatment with or without 1 µmol/l Aβ1‑42 oligomer (AβO). The mRNA and protein expression levels of α7 nAChR, synaptophysin (SYP), postsynaptic density of 95 kDa (PSD‑95) and synaptosomal‑associated protein of 25 kDa (SNAP‑25) were subsequently analyzed using reverse transcription‑quantitative PCR and western blotting. In addition, the concentration of acetylcholine (ACh) and the activity of acetylcholinesterase (AChE) were analyzed using spectrophotometry, while the cell apoptotic rate was determined using flow cytometry. The expression of Aβ in the brains of patients with AD was found to be significantly increased, whereas the expression of α7 nAChR was significantly decreased compared with the healthy control group. In vitro, the expression levels of α7 nAChR were significantly increased or decreased following the overexpression or silencing of the gene, respectively. Consistent with these observations, the mRNA and protein expression levels of SYP, PSD‑95 and SNAP‑25 were also significantly increased following the overexpression of α7 nAChR and decreased following the genetic silencing of the receptor. In untransfected or negative control cells, the expression levels of these factors and the apoptotic rate were significantly reduced following the exposure to AβO, which was found to be attenuated by α7 nAChR overexpression, but potentiated by α7 nAChR RNA silencing. However, no significant differences were observed in either the ACh concentration or AChE activity following transfection. Collectively, these findings suggested that α7 nAChR may protect the brains of patients with AD against Aβ, as α7 nAChR overexpression increased the expression levels of SYP, SNAP‑25 and PSD‑95, and attenuated the inhibitory effect of Aβ on the expression of these synaptic proteins and cell apoptosis. Overall, this indicated that α7 nAChR may serve an important neuroprotective role in AD.

Effect of α7nAChR on learning and memory dysfunction in a rat model of diffuse axonal injury

Diffuse axonal injury (DAI) is the predominant effect of severe traumatic brain injury and significantly contributes to cognitive deficits. The mechanisms that underlie these cognitive deficits are often associated with complex molecular alterations. α7nAChR, one of the abundant and widespread nicotinic acetylcholine receptors (nAChRs) in the brain, plays important physiological functions in the central nervous system. However, the relationship between temporospatial alterations in the α7nAChR and DAI-related learning and memory dysfunction are not completely understood. Our study detected temporospatial alterations of α7nAChR in vulnerable areas (hippocampus, internal capsule, corpus callosum and brain stem) of DAI rats and evaluated the development and progression of learning and memory dysfunction via the Morris water maze (MWM). We determined that α7nAChR expression in vulnerable areas was mainly reduced at the recovery of DAI in rats. Moreover, the escape latency of the injured group increased significantly and the percentages of the distance travelled and time spent in the target quadrant were significantly decreased after DAI. Furthermore, α7nAChR expression in the vulnerable area was significantly positively correlated with MWM performance after DAI according to regression analysis. In addition, we determined that a selective α7nAChR agonist significantly improved learning and memory dysfunction. Rats in the α7nAChR agonist group showed better learning and memory performance than those in the antagonist group. These results demonstrate that microstructural injury-induced alterations of α7nAChR in the vulnerable area are significantly correlated with learning and memory dysfunctions after DAI and that augmentation of the α7nAChR level by its agonist contributes to the improvement of learning and memory function.

Perioperative activation of spinal α7 nAChR promotes recovery from preoperative stress-induced prolongation of postsurgical pain

Preoperative stress could delay the recovery of postoperative pain and has been reported to be a risk factor for chronic postsurgical pain. As stress could facilitate the proinflammatory activation of microglia, we hypothesized that these cells may play a vital role in the development of preoperative stress-induced pain chronification after surgery. Our experiments were conducted in a rat model that consists of a single prolonged stress (SPS) procedure and plantar incision. A previous SPS exposure induced anxiety-like behaviors, prolonged incision-induced mechanical allodynia, and potentiated the activation of spinal microglia. Based on the results from ex vivo experiments, spinal microglia isolated from SPS-exposed rats secreted more proinflammatory cytokines upon challenge with LPS. Our results also demonstrated that microglia played a more important role than astrocytes in the initiation of SPS-induced prolongation of postsurgical pain. We further explored the therapeutic potential of agonism of α7 nAChR, an emerging anti-inflammatory target, for SPS-induced prolongation of postsurgical pain. Multiple intrathecal (i.t.) injections of PHA-543613 (an α7 nAChR agonist) or PNU-120596 (a type II positive allosteric modulator) during the perioperative period shortened the duration of postsurgical pain after SPS and suppressed SPS-potentiated microglia activation, but their effects were abolished by pretreatment with methyllycaconitine (an α7 nAChR antagonist; i.t.). Based on the results from ex vivo experiments, the anti-inflammatory effects of PHA-543613 and PNU-120596 may have been achieved by the direct modulation of microglia. In conclusion, stress-induced priming of spinal microglia played a key role in the initiation of preoperative stress-induced prolongation of postsurgical pain, and PHA-543613 and PNU-120596 may be potential candidates for preventing pain chronification after surgery.

Arctic Aβ40 blocks the nicotine-induced neuroprotective effect of CHRNA7 by inhibiting the ERK1/2 pathway in human neuroblastoma cells

Amyloid β protein (Aβ) plays a central role in Alzheimer's disease (AD) pathogenesis. Point mutations in the Aβ sequence, which cluster around the central hydrophobic core of the peptide, are associated with familial AD (FAD). Several mutations have been identified, with the Arctic mutation exhibiting a purely cognitive phenotype that is typical of AD. Our previous findings suggest that Arctic Aβ40 binds to and aggregates with CHRNA7, thereby inhibiting the calcium response and signaling pathways downstream of the receptor. Activation of CHRNA7 is neuroprotective both in vitro and in vivo. Therefore, in the present study, we investigated whether Arctic Aβ40 affects neuronal survival and/or death via CHRNA7. Using human neuroblastoma SH-SY5Y cells, we found that the neuroprotective function of CHRNA7 is blocked by CHRNA7 knockdown using RNA interference. Furthermore, Arctic Aβ40 blocked the neuroprotective effect of nicotine by inhibiting the ERK1/2 pathway downstream of CHRNA7. Moreover, we show that ERK1/2 activation mediates the neuroprotective effect of nicotine against oxidative stress. Collectively, our findings further our understanding of the molecular pathogenesis of Arctic FAD.

Molecular Mechanisms Regulating LPS-Induced Inflammation in the Brain

Neuro-inflammation, one of the pathogenic causes of neurodegenerative diseases, is regulated through the cholinergic anti-inflammatory pathway via the α7 nicotinic acetylcholine receptor (α7 nAChR). We previously showed that either bacterial lipopolysaccharide (LPS) or immunization with the α7(1-208) nAChR fragment decrease α7 nAChRs density in the mouse brain, exacerbating chronic inflammation, beta-amyloid accumulation and episodic memory decline, which mimic the early stages of Alzheimer's disease (AD). To study the molecular mechanisms underlying the LPS and antibody effects in the brain, we employed an in vivo model of acute LPS-induced inflammation and an in vitro model of cultured glioblastoma U373 cells. Here, we report that LPS challenge decreased the levels of α7 nAChR RNA and protein and of acetylcholinesterase (AChE) RNA and activity in distinct mouse brain regions, sensitized brain mitochondria to the apoptogenic effect of Ca(2+) and modified brain microRNA profiles, including the cholinergic-regulatory CholinomiRs-132/212, in favor of anti-inflammatory and pro-apoptotic ones. Adding α7(1-208)-specific antibodies to the LPS challenge prevented elevation of both the anti-inflammatory and pro-apoptotic miRNAs while supporting the resistance of brain mitochondria to Ca(2+) and maintaining α7 nAChR/AChE decreases. In U373 cells, α7-specific antibodies and LPS both stimulated interleukin-6 production through the p38/Src-dependent pathway. Our findings demonstrate that acute LPS-induced inflammation induces the cholinergic anti-inflammatory pathway in the brain, that α7 nAChR down-regulation limits this pathway, and that α7-specific antibodies aggravate neuroinflammation by inducing the pro-inflammatory interleukin-6 and dampening anti-inflammatory miRNAs; however, these antibodies may protect brain mitochondria and decrease the levels of pro-apoptotic miRNAs, preventing LPS-induced neurodegeneration.

Identification and in vitro pharmacological characterization of a novel and selective α7 nicotinic acetylcholine receptor agonist, Br-IQ17B

Aim:

Alpha7-nicotinic acetylcholine receptor (α7 nAChR) is a ligand-gated Ca²⁺-permeable ion channel implicated in cognition and neuropsychiatric disorders. Activation of α7 nAChR improves learning, memory, and sensory gating in animal models. To identify novel α7 nAChR agonists, we synthesized a series of small molecules and characterized a representative compound, Br-IQ17B, N-[(3R)-1-azabicyclo[2,2,2]oct-3-yl]-5-bromoindolizine-2-carboxamide, which specifically activates α7 nAChR.

Methods:

Two-electrode voltage clamp (TEVC) recordings were primarily used for screening in Xenopus oocytes expressing human α7 nAChR. Assays, including radioisotope ligand binding, Western blots, whole-cell recordings of hippocampal culture neurons, and spontaneous IPSC recordings of brain slices, were also utilized to evaluate and confirm the specific activation of α7 nAChR by Br-IQ17B.

Results:

Br-IQ17B potently activates α7 nAChR with an EC₅₀ of 1.8±0.2 μmol/L. Br-IQ17B is selective over other subtypes such as α4β2 and α3β4, but it blocks 5-HT_3A receptors. Br-IQ17B displaced binding of the α7 blocker [³H]-MLA to hippocampal crude membranes with a K_i of 14.9±3.2 nmol/L. In hippocampal neurons, Br-IQ17B evoked α7-like currents that were inhibited by MLA and enhanced in the presence of the α7 PAM PNU-120596. In brain slice recordings, Br-IQ17B enhanced GABAergic synaptic transmission in CA1 neurons. Mechanistically, Br-IQ17B increased ERK1/2 phosphorylation that was MLA-sensitive.

Conclusion:

We identified the novel, potent, and selective α7 agonist Br-IQ17B, which enhances synaptic transmission. Br-IQ17B may be a helpful tool to understand new aspects of α7 nAChR function, and it also has potential for being developed as therapy for schizophrenia and cognitive deficits.

Identification and Characterization of a G Protein-binding Cluster in α7 Nicotinic Acetylcholine Receptors

α7 nicotinic acetylcholine receptors (nAChRs) play an important role in synaptic transmission and inflammation. In response to ligands, this receptor channel opens to conduct cations into the cell but desensitizes rapidly. In recent studies we show that α7 nAChRs bind signaling proteins such as heterotrimeric GTP-binding proteins (G proteins). Here, we demonstrate that direct coupling of α7 nAChRs to G proteins enables a downstream calcium signaling response that can persist beyond the expected time course of channel activation. This process depends on a G protein-binding cluster (GPBC) in the M3-M4 loop of the receptor. A mutation of the GPBC in the α7 nAChR (α7345-348A) abolishes interaction with Gαq as well as Gβγ while having no effect on receptor synthesis, cell-surface trafficking, or α-bungarotoxin binding. Expression of α7345-348A, however, did significantly attenuate the α7 nAChR-induced Gαq calcium signaling response as evidenced by a decrease in PLC-β activation and IP3R-mediated calcium store release in the presence of the α7 selective agonist choline. Taken together, the data provides new evidence for the existence of a GPBC in nAChRs serving to promote intracellular signaling.

Structure-activity relationship studies of SEN12333 analogues: determination of the optimal requirements for binding affinities at α7 nAChRs through incorporation of known structural motifs

Alpha7 nicotinic acetylcholine receptors (nAChRs) have implications in the regulation of cognitive processes such as memory and attention and have been identified as a promising therapeutic target for the treatment of the cognitive deficits associated with schizophrenia and Alzheimer's disease (AD). Structure affinity relationship studies of the previously described α7 agonist SEN12333 (8), have resulted in the identification of compound 45, a potent and selective agonist of the α7 nAChR with enhanced affinity and improved physicochemical properties over the parent compound (SEN12333, 8).

Stoichiometry for drug potentiation of a pentameric ion channel

Drug modulation of ion channels is a powerful means to alter physiological responses for therapeutic benefit, yet the structural bases of modulation remain poorly understood. Here we study potentiation of nicotinic α7 acetylcholine receptors, which are emerging drug targets in several neurological disorders. α7 receptors are ligand-gated ion channels composed of five identical subunits, each bearing a site for the potentiating drug PNU-120596 (PNU). How the individual subunits contribute to PNU potentiation is not known. Taking advantage of a PNU-resistant mutant, we generated receptors composed of normal and PNU-resistant subunits and tagged one of the subunits with conductance mutations to report subunit stoichiometry. We then used patch clamp recording to monitor PNU potentiation of single α7 receptors with defined stoichiometry in real time. We find that potentiation depends steeply on the number of PNU-resistant subunits and that four, and possibly five, subunits must be sensitive to PNU for potentiation to occur. Thus, by monitoring the activity of every possible subunit combination, our findings predict that at the macroscopic level, PNU potentiation is highly cooperative.

Synthesis and evaluation of new radioligands [(11)C]A-833834 and [(11)C]A-752274 for positron-emission tomography of α7-nicotinic acetylcholine receptors

INTRODUCTION

α7-nicotinic acetylcholine receptor (α7-nAChR) is one of the major neuronal nAChR subtypes. α7-nAChR is involved in variety of neuronal processes and disorders including schizophrenia and Alzheimer's disease. A number of α7-nAChR PET radioligands have been developed, but a quality radiotracer remains to be discovered.

METHODS

High binding affinity α7-nAChR ligands A-833834 and A-752274 were radiolabeled with (11)C. Baseline and blockade biodistribution studies in the mouse brain of [(11)C]A-833834 (5-(6-(5-[(11)C]methylhexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl)pyridazin-3-yl)-1H-indole) and [(11)C]A-752274 (2-(6-[(11)C]methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-7-(6-methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-9H-fluoren-9-one) were performed. [(11)C]A-752274 was evaluated in a baseline baboon PET study.

RESULTS

[(11)C]A-833834 and [(11)C]A-752274 were synthesized by radiomethylation of corresponding des-methyl precursors. The radioligands were prepared with radiochemical yield of 12%-32%, high specific radioactivity (330-403GBq/μmol) and radiochemical purity>95%. Dissection studies with [(11)C]A-833834 demonstrated low specific α7-nAChR binding in the mouse brain. [(11)C]A-752274 specifically (~50%) labeled α7-nAChR in the mouse thalamus. However, [(11)CA-752274 exhibited low brain uptake in baboon (%SUV<100).

CONCLUSION

Two novel α7-nAChR ligands radioligands were synthesized and studied in animals. Specific binding of [(11)C]A-833834 in the mouse brain is low due to the insufficient binding affinity of the radioligand. The very high binding affinity [(11)C]A-752274 exhibited good specific binding in the α7-nAChR-rich mouse brain regions. The low uptake of [(11)C]A-752274 in the baboon brain is due to its high hydrophilicity, rapid metabolism or other properties. Future development of α7-nAChR PET radioligands will be based on compounds with high binding affinities and good blood-brain barrier permeability.

Preventing expression of the nicotinic receptor subunit α7 in SH-SY5Y cells with interference RNA indicates that this receptor may protect against the neurotoxicity of Aβ

The present aim was to characterize the influence of the α7 nicotinic acetylcholine receptor (nAChR) on BACE, the enzyme that cleaves the amyloid precursor protein (APP) at the β-site, as well as on the oxidative stress induced by amyloid-β peptide (Aβ). To this end, human neuroblastoma SH-SY5Y cells were transfected with siRNAs targeting the α7 nAChR subunit and/or exposed to Aβ1-42. For α7 nAChR, BACE1 (cleaving at the β-site of APP) and BACE2 (cleaving within the Aβ domain), α-secretase (ADAM10), and the two components of γ-secretase, PS and NCT, the mRNA and protein levels were determined by real-time PCR and Western blotting, respectively. The level of Aβ1-42 in the cell culture medium was determined by an ELISA procedure. The extent of lipid peroxidation and activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) were assayed spectrophotometrically. In the transfected SH-SY5Y cells, expression of α7 nAChR was reduced; the level of BACE1 increased and that of BACE2 decreased; the amount of ADAM10 lowered; and the level of PS raised. Moreover, the level of Aβ1-42 in the culture medium was elevated. Treatment of non-transfected cells with Aβ elevated the level of malondialdehyde (MDA) and lowered the activities of SOD and GSH-Px and these changes were potentiated by inhibiting expression of α7 nAChR. These results indicate that α7 nAChR plays a significant role in amyloidogenic metabolism of APP and the oxidative stress evoked by Aβ, suggesting that this receptor might help protect against the neurotoxicity of Aβ.

Consequences of linker length alteration of the α7 nicotinic acetylcholine receptor (nAChR) agonist, SEN12333

A series of ligands based on SEN12333, containing either contracted or elongated alkyl chains, were synthesized and evaluated in molecular docking studies against a homology model of the α7 nicotinic acetylcholine receptor (nAChR) subtype. The predicted binding of all ligands was highly similar, with the exception of the analog containing a 5 methylene unit spacer. However, in vitro competition binding assays revealed that the ligands possessed dissimilar binding affinities, with a K(i) range of more than an order of magnitude (K(i)=0.50 to >10 μM), and only SEN12333 itself exhibited functional activity at the α7 nAChR.

3-(2,4-dimethoxybenzylidene)-anabaseine: a promising candidate drug for Alzheimer's disease?

3-(2,4-dimethoxybenzylidene)-anabaseine is an analog of the paralytic alkaloid, anabaseine, from the ribbon worms Amphiporus sp., that shows numerous properties, in particular an agonist activity on alpha 7 nicotinic acetylcholine receptors. This article reviews these properties and explains to what extent they could be valuable to control symptomatology and/or neurodegeneration in Alzheimer's disease.

α7 nicotinic ACh receptors as a ligand-gated source of Ca(2+) ions: the search for a Ca(2+) optimum

The spatiotemporal distribution of cytosolic Ca(2+) ions is a key determinant of neuronal behavior and survival. Distinct sources of Ca(2+) ions including ligand- and voltage-gated Ca(2+) channels contribute to intracellular Ca(2+) homeostasis. Many normal physiological and therapeutic neuronal functions are Ca(2+)-dependent, however an excess of cytosolic Ca(2+) or a lack of the appropriate balance between Ca(2+) entry and clearance may destroy cellular integrity and cause cellular death. Therefore, the existence of optimal spatiotemporal patterns of cytosolic Ca(2+) elevations and thus, optimal activation of ligand- and voltage-gated Ca(2+) ion channels are postulated to benefit neuronal function and survival. Alpha7 nicotinic -acetylcholine receptors (nAChRs) are highly permeable to Ca(2+) ions and play an important role in modulation of neurotransmitter release, gene expression and neuroprotection in a variety of neuronal and non-neuronal cells. In this review, the focus is placed on α7 nAChR-mediated currents and Ca(2+) influx and how this source of Ca(2+) entry compares to NMDA receptors in supporting cytosolic Ca(2+) homeostasis, neuronal function and survival.

Single-channel and structural foundations of neuronal α7 acetylcholine receptor potentiation

Potentiation of neuronal nicotinic acetylcholine receptors by exogenous ligands is a promising strategy for treatment of neurological disorders including Alzheimer's disease and schizophrenia. To gain insight into molecular mechanisms underlying potentiation, we examined ACh-induced single-channel currents through the human neuronal α7 acetylcholine receptor in the presence of the α7-specific potentiator PNU-120596 (PNU). Compared to the unusually brief single-channel opening episodes elicited by agonist alone, channel opening episodes in the presence of agonist and PNU are dramatically prolonged. Dwell time analysis reveals that PNU introduces two novel components into open time histograms, indicating at least two degrees of PNU-induced potentiation. Openings of the longest potentiated class coalesce into clusters whose frequency and duration change over a narrow range of PNU concentration. At PNU concentrations approaching saturation, these clusters last up to several minutes, prolonging the submillisecond α7 opening episodes by several orders of magnitude. Mutations known to reduce PNU potentiation at the whole-cell level still give rise to multisecond-long single-channel clusters. However mutation of five residues lining a cavity within each subunit's transmembrane domain abolishes PNU potentiation, defining minimal structural determinants of PNU potentiation.

The coumarin scopoletin potentiates acetylcholine release from synaptosomes, amplifies hippocampal long-term potentiation and ameliorates anticholinergic- and age-impaired memory

In a previous study the simple, naturally derived coumarin scopoletin (SCT) was identified as an inhibitor of acetylcholinesterase (AChE), using a pharmacophore-based virtual screening approach. In this study the potential of SCT as procholinergic and cognition-enhancing therapeutic was investigated in a more detailed way, using different experimental approaches like measuring newly synthesized acetylcholine (ACh) in synaptosomes, long-term potentiation (LTP) experiments in hippocampal slices, and behavior studies. SCT enhanced the K⁺-stimulated release of ACh from rat frontal cortex synaptosomes, showing a bell-shaped dose effect curve (E_max: 4 μM). This effect was blocked by the nicotinic ACh receptor (nAChR) antagonists mecamylamine (MEC) and dihydro-β-erythroidine (DHE). The nAChR agonist (and AChE inhibitor) galantamine induced a similar increase in ACh release (E_max: 1 μM). SCT potentiated LTP in hippocampal slices of rat brain. The high-frequency stimulation (HFS)-induced, N-methyl-D-aspartate (NMDA) receptor dependent LTP of field excitatory postsynaptic potentials at CA3-CA1 synapses was greatly enhanced by pre-HFS application of SCT (4 μM for 4 min). This effect was mimicked by nicotine (2 μM) and abolished by MEC, suggesting an effect on nAChRs. SCT did not restore the total inhibition of LTP by NMDA receptor antagonist d, l-2-amino-5-phosphonopentanoic acid (AP-5). SCT (2 μg, i.c.v.) increased T-maze alternation and ameliorated novel object recognition of mice with scopolamine-induced cholinergic deficit. It also reduced age-associated deficits in object memory of 15–18-month-old mice (2 mg/kg sc). Our findings suggest that SCT possesses memory-improving properties, which are based on its direct nAChR agonistic activity. Therefore, SCT might be able to rescue impaired cholinergic functions by enhancing nAChR-mediated release of neurotransmitters and promoting neural plasticity in hippocampus.

Activation of α7 nicotinic receptor affects APP processing by regulating secretase activity in SH-EP1-α7 nAChR-hAPP695 cells

Multiple lines of evidence have implicated that nicotinic acetylcholine receptor (nAChR) may be an important therapeutic target for the treatment of Alzheimer's disease (AD). Although there are reports suggesting a link between alpha7 nAChR subtype and AD, there has been little report on the mechanism. The present study investigates whether and how α7 nAChR activation affects APP695 processing in SH-EP1 cell model. Cell line co-expressing α7 nAChR gene and human amyloid precursor protein 695 (hAPP695) gene were constructed by stable transfection. Expression of β-amyloid, α-form of secreted APP (αAPPs) and APP1695 was measured by ELISA, western blotting and real-time PCR respectively. Additionally, α, β, and γ-secretase activities were also analyzed in constructed SH-EP1-α7 nAChR-hAPP695 cell line. The results showed that SH-EP1-α7 nAChR-hAPP695 cell line, expressing both hAPP695 gene and α7 nAChR subtype gene, was constructed successfully. The secreted Aβ was decreased and αAPPs was significantly increased by non-selective nAChR agonist nicotine (10 μM) and specific α7 nAChR agonist GTS-21 (1 μM), and APP expression was not affected. Furthermore, specific α7 nAChR antagonist methyllycaconitine (MLA) reversed the alterations induced by activation of α7 nAChR. CTF-α was increased and CTF-γ was decreased when treated with nicotine (10 μM). In addition, the results of enymatic activity analysis showed that nicotine (1μM) and GTS-21 (0.1, 1 μM) decreased γ-secretase activity, but has no effects on α-secretase activity and β-secretase activity. Our findings demonstrate that, through regulating γ-secretase activity, α7 nAChR activation reduces APP processing in amyloidogenic pathway, and at the same time enhances APP processing in non-amyloidogenic pathway. The constructed SH-EP1-α7 nAChR-hAPP695 cell line might be useful for screening specific nAChR agonists against AD.

Amyloid precursor protein gene mutated at Swedish 670/671 sites in vitro induces changed expression of nicotinic acetylcholine receptors and neurotoxicity

In order to investigate the influence of amyloid precursor protein (APP) over-expression on the levels of nicotinic acetylcholine receptors (nAChRs), the pCDNA 3.0 carrying the Swedish 670/671 APP double mutation (APP(SWE)) gene was transfected into human neuroblastoma (SH-SY5Y) cells and primary culture of rat hippocampal neurons. The mRNA level of APP, and nAChR α3, α4 and α7 subunits were detected by real-time PCR, and their corresponding proteins as well as α-secreted APP (αAPPs) by Western blotting. [3H]Epibatidine binding sites were measured by the receptor binding assay. The results showed that significantly concomitant increases in mRNA and protein levels of SH-SY5Y cells and primary cultured neurons transfected with APP(SWE) were observed. Interestingly, a decreased αAPPs level was detected in both cells treated with APP(SWE) transfection. In addition, decreases in mRNA and protein levels of α3 nAChR subunit in SH-SY5Y cells or α4 subunit in primary cultured neurons with APP(SWE) transfection were observed. For α7 nAChR, the increased protein and mRNA levels were found in SH-SY5Y cells and primary cultured neurons with APP(SWE) transfection. The number of cholinergic receptor binding site of [3H]epibatidine was decreased in the SH-SY5Y cells transfected with APP(SWE). Elevations in the activities of AChE and BuChE and in the level of lipid peroxidation were detected in both types of cultured cells transfected with APP(SWE). These results indicated that the over-expression of APP(SWE) gene can influence the expression of nAChRs and resulted in neurotoxicity, in which this process might play an important role in the pathogenesis of Alzheimer's disease.

DMXB (GTS-21) ameliorates the cognitive deficits in beta amyloid(25-35(-) ) injected mice through preventing the dysfunction of alpha7 nicotinic receptor

Intracerebroventricular injection of beta-amyloid(25-35) (Abeta(25-35)) in mice leads to cognitive deficits with the dysfunction of alpha7 nicotinic acetylcholine receptor (alpha7nAChR) within 1-2 weeks in a dose-dependent manner. The present study focused on the effect of DMXB, a selective alpha7nAChR agonist, on Abeta(25-35) (3 nmol)-impaired spatial memory and alpha7nAChR function. We found that the treatment with DMXB on days 1-10 after Abeta(25-35) injection dose-dependently prevented Abeta(25-35)-induced impairment of acquisition performance and probe trail test in Morris water maze. Importantly, the treatment with DMXB (1 mg/kg) perfectly prevented Abeta(25-35)-induced depression of alpha7nAChR response, which was associated with improving the probability of presynaptic glutamate release and the induction of high-frequency stimulation (HFS)-dependent long-term potentiation (LTP) in hippocampal Schaffer collaterale-CA1 synapse. Furthermore, although either the basal level of extracellular signal-regulated kinase 2 (ERK2) or its phosphorylation in the hippocampus had no difference between control and Abeta(25-35) mice, the Abeta(25-35) injection significantly attenuated HFS-triggered increase in ERK2 phosphorylation. The treatment with DMXB also rescued the ERK2 phosphorylation triggered by HFS in Abeta(25-35) mice that is required for LTP induction. This study firstly provides in vivo evidence that the anti-amnesic effect of DMXB is likely due to preventing the Abeta(25-35)-induced dysfunction of alpha7nAChR.

Targeting Abeta and tau in Alzheimer's disease, an early interim report

The amyloid beta (Abeta) and tau proteins, which misfold, aggregate, and accumulate in the Alzheimer's disease (AD) brain, are implicated as central factors in a complex neurodegenerative cascade. Studies of mutations that cause early onset AD and promote Abeta accumulation in the brain strongly support the notion that inhibiting Abeta aggregation will prevent AD. Similarly, genetic studies of frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17 MAPT) showing that mutations in the MAPT gene encoding tau lead to abnormal tau accumulation and neurodegeneration. Such genetic studies clearly show that tau dysfunction and aggregation can be central to neurodegeneration, however, most likely in a secondary fashion in relation to AD. Additional pathologic, biochemical, and modeling studies further support the concept that Abeta and tau are prime targets for disease modifying therapies in AD. Treatment strategies aimed at preventing the aggregation and accumulation of Abeta, tau, or both proteins should therefore be theoretically possible, assuming that treatment can be initiated before either irreversible damage is present or downstream, self-sustaining, pathological cascades have been initiated. Herein, we will review recent advances and also potential setbacks with respect to the myriad of therapeutic strategies that are designed to slow down, prevent, or clear the accumulation of either "pathological" Abeta or tau. We will also discuss the need for thoughtful prioritization with respect to clinical development of the preclinically validated modifiers of Abeta and tau pathology. The current number of candidate therapies targeting Abeta is becoming so large that a triage process is clearly needed to insure that resources are invested in a way such that the best candidates for disease modifying therapy are rapidly moved toward clinical trials. Finally, we will discuss the challenges for an appropriate "triage" after potential disease modifying therapies targeting tau and Abeta have entered clinical trials.

Is modulation of nicotinic acetylcholine receptors by melatonin relevant for therapy with cholinergic drugs?

Melatonin, the darkness hormone, synchronizes several physiological functions to light/dark cycle. Besides the awake/sleep cycle that is intuitively linked to day/night, daily variations in memory acquisition and innate or acquired immune responses are some of the major activities linked to melatonin rhythm. The daily variation of these complex processes is due to changes in specific mechanisms. In the last years we focused on the influence of melatonin on the expression and function of nicotinic acetylcholine receptors (nAChRs). Melatonin, either "in vivo" or "in vitro", increases, in a selective manner, the efficiency of alpha-bungarotoxin (alpha-BTX)-sensitive nAChRs. Melatonin's effect on receptors located in rat sympathetic nerve terminals, cerebellum, skeletal muscle and chick retina, was tested. We observed that melatonin is essential for the development of alpha-BTX-sensitive nAChRs, and important for receptor maintenance in aging models. Taking into account that both melatonin and alpha-7 nAChRs (one of the subtypes sensitive to alpha-BTX) are involved in the development of Alzheimer's disease, here we discuss the possibility of a therapeutic strategy focused on both melatonin replacement and its potential association with cholinergic drugs.

Alzheimer's disease as homeostatic responses to age-related myelin breakdown

The amyloid hypothesis (AH) of Alzheimer's disease (AD) posits that the fundamental cause of AD is the accumulation of the peptide amyloid beta (Aβ) in the brain. This hypothesis has been supported by observations that genetic defects in amyloid precursor protein (APP) and presenilin increase Aβ production and cause familial AD (FAD). The AH is widely accepted but does not account for important phenomena including recent failures of clinical trials to impact dementia in humans even after successfully reducing Aβ deposits. Herein, the AH is viewed from the broader overarching perspective of the myelin model of the human brain that focuses on functioning brain circuits and encompasses white matter and myelin in addition to neurons and synapses. The model proposes that the recently evolved and extensive myelination of the human brain underlies both our unique abilities and susceptibility to highly prevalent age-related neuropsychiatric disorders such as late onset AD (LOAD). It regards oligodendrocytes and the myelin they produce as being both critical for circuit function and uniquely vulnerable to damage. This perspective reframes key observations such as axonal transport disruptions, formation of axonal swellings/sphenoids and neuritic plaques, and proteinaceous deposits such as Aβ and tau as by-products of homeostatic myelin repair processes. It delineates empirically testable mechanisms of action for genes underlying FAD and LOAD and provides "upstream" treatment targets. Such interventions could potentially treat multiple degenerative brain disorders by mitigating the effects of aging and associated changes in iron, cholesterol, and free radicals on oligodendrocytes and their myelin.

Nicotinic acetylcholine receptor signalling: roles in Alzheimer's disease and amyloid neuroprotection

Alzheimer's disease (AD), the major contributor to dementia in the elderly, involves accumulation in the brain of extracellular plaques containing the beta-amyloid protein (Abeta) and intracellular neurofibrillary tangles of hyperphosphorylated tau protein. AD is also characterized by a loss of neurons, particularly those expressing nicotinic acetylcholine receptors (nAChRs), thereby leading to a reduction in nAChR numbers. The Abeta(1-42) protein, which is toxic to neurons, is critical to the onset and progression of AD. The discovery of new drug therapies for AD is likely to be accelerated by an improved understanding of the mechanisms whereby Abeta causes neuronal death. We examine the evidence for a role in Abeta(1-42) toxicity of nAChRs; paradoxically, nAChRs can also protect neurons when activated by nicotinic ligands. Abeta peptides and nicotine differentially activate several intracellular signaling pathways, including the phosphatidylinositol 3-kinase/v-akt murine thymoma viral oncogene homolog pathway, the extracellular signal-regulated kinase/mitogen-activated protein kinase, and JAK-2/STAT-3 pathways. These pathways control cell death or survival and the secretion of Abeta peptides. We propose that understanding the differential activation of these pathways by nicotine and/or Abeta(1-42) may offer the prospect of new routes to therapy for AD.

Vasoactive intestinal peptide-induced neuritogenesis in neuroblastoma SH-SY5Y cells involves SNAP-25

Vasoactive intestinal peptide (VIP) is a neuropeptide known to regulate proliferation and differentiation in normal and tumoral cells. We previously reported that VIP induced neuritogenesis in human neuroblastoma SH-SY5Y cells cultured in serum-free medium. This neuritogenesis was associated with a regulated expression of neuronal cytoskeleton markers. To further characterize the neuroblastic cell differentiation induced by VIP in human SH-SY5Y cells, we investigated expression of synaptosomal-associated protein of 25 kDa (SNAP-25), a protein implicated in exocytosis associated with different processes, including neurite outgrowth. Western immunoblotting and real-time RT-PCR analyses revealed that VIP increased expression of the SNAP-25 protein and the level of both SNAP-25a and SNAP-25b mRNA isoforms. Immunofluorescence experiments indicated that SNAP-25 was mainly located in neurites and at the plasma membrane in SH-SY5Y cells treated with VIP. RNA interference experiments demonstrated that SNAP-25 was involved in VIP-induced neuritogenesis. In conclusion, SNAP-25 is up-regulated and implicated in neuritogenesis in human neuroblastoma SH-SY5Y cells treated with the neuropeptide VIP.

Cross-talk between oxidative stress and modifications of cholinergic and glutaminergic receptors in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder, and its pathogenesis is likely to be associated with multiple etiologies and mechanisms in which oxidative stress and deficits of neurotransmitter receptors may play important roles. It has been indicated that a high level of free radicals can influence the expressions of nicotinic receptors (nAChRs), muscarinic receptors (mAChRs), and N-methyl-D-aspartate (NMDA) receptors, exhibiting disturbances of cellular membrane by lipid peroxidation, damages of the protein receptors by protein oxidation, and possible modified gene expressions of these receptors by DNA oxidation. nAChRs have shown an antioxidative effect by a direct or an indirect pathway; mAChR stimulation may generate reactive oxygen species, which might be a physiological compensative reaction, or improve oxidative stress; and high stimulation to NMDA receptors can increase the sensitivity of oxidative stress of neurons. This review may provide complemental information for understanding the correlation between oxidative stress and changed cholinergic and glutaminergic receptors in AD processing, and for revealing the underlying molecular mechanisms of these factors in the multiple etiologies and pathophysiology of the disorder.

Construction of SH-EP1-alpha4beta2-hAPP695 cell line and effects of nicotinic agonists on beta-amyloid in the cells

(1) Nicotinic acetylcholine receptors in central nervous system are thought to be new targets for Alzheimer's disease. However, the most involved nicotinic receptor subtype in Alzheimer's disease is unclear. alpha4beta2 receptor is the most widely spread subtype in brain, involving in several important aspects of cognitive and other functions. We constructed cell line by transfecting human amyloid precursor protein (695) gene into SH-EP1 cells which have been transfected with human nicotinic receptor alpha4 subunit and beta2 subunit gene, to observe effects of alpha4beta2 receptors activation on beta-amyloid, expecting to provide a new cell line for drug screening and research purpose. (2) Liposome transfection was used to express human amyloid precursor protein (695) gene in SH-EP1-alpha4beta2 cells. Function of the transfected alpha4beta2 receptors was tested by patch clamp. Effects of nicotine and epibatidine (selective alpha4beta2 nicotinic receptor agonist) on beta-amyloid were detected by Western blot and ELISA. Effects of nicotine and epibatidine on amyloid precursor protein (695) mRNA level were measured using real-time PCR. (3) Human amyloid precursor protein (695) gene was stably expressed in SH-EP1-alpha4beta2 cells; Nicotine (1 muM) and epibatidine (0.1 muM) decreased intracellular and secreted beta-amyloid in the cells; and activation of alpha4beta2 receptors did not affect amyloid precursor protein (695) mRNA level. (4) These results suggest that the constructed cell line, expressing both amyloid precursor protein (695) gene and human nicotinic receptor alpha4 subunit and beta2 subunit gene, might be useful for screening specific nicotinic receptor agonists against Alzheimer's disease. Alteration of Abeta level induced by activation of alpha4beta2 nAChR in our study might occur at a post-translational level.