Regulation of the neuronal nicotinic acetylcholine receptor by SRC family tyrosine kinases

Regulation of nicotinic acetylcholine receptors by post-translational modifications

Nicotinic acetylcholine receptors (nAChRs) comprise a family of pentameric ligand-gated ion channels widely distributed in the central and peripheric nervous system and in non-neuronal cells. nAChRs are involved in chemical synapses and are key actors in vital physiological processes throughout the animal kingdom. They mediate skeletal muscle contraction, autonomic responses, contribute to cognitive processes, and regulate behaviors. Dysregulation of nAChRs is associated with neurological, neurodegenerative, inflammatory and motor disorders. In spite of the great advances in the elucidation of nAChR structure and function, our knowledge about the impact of post-translational modifications (PTMs) on nAChR functional activity and cholinergic signaling has lagged behind. PTMs occur at different steps of protein life cycle, modulating in time and space protein folding, localization, function, and protein-protein interactions, and allow fine-tuned responses to changes in the environment. A large body of evidence demonstrates that PTMs regulate all levels of nAChR life cycle, with key roles in receptor expression, membrane stability and function. However, our knowledge is still limited, restricted to a few PTMs, and many important aspects remain largely unknown. There is thus a long way to go to decipher the association of aberrant PTMs with disorders of cholinergic signaling and to target PTM regulation for novel therapeutic interventions. In this review we provide a comprehensive overview of what is known about how different PTMs regulate nAChR.

A Multi-Target and Multi-Channel Mechanism of Action for Jiawei Yinhuo Tang in the Treatment of Social Communication Disorders in Autism: Network Pharmacology and Molecular Docking Studies

Background

Autism spectrum disorder (ASD) is a highly heterogeneous neurodevelopmental disorder with complex pathogenesis. Currently, the pathogenesis of ASD is not fully understood. Moreover, current treatments do not effectively alleviate the primary symptoms of ASD social disorder (SCDA). Jiawei Yinhuo Tang (JWYHT) is an improved version of the classic prescription Yinhuo Tang. Although this medication has been shown to improve social behavior in ASD patients, the mechanism by which it works remains unknown.

Methods

In this study, network pharmacology bioinformatics analysis was used to identify the key targets, biological functions, and signal pathways of JWYHT in SCDA. Then, molecular docking and molecular dynamic simulation were used to validate the activity and stability of the active ingredient and the target protein during the binding process.

Results

The analysis identified 157 key targets and 9 core targets of JWYHT (including proto-oncogene (FOS), caspase 3 (CASP3), mitogen-activated protein kinase-3 (MAPK3), interleukin-6 (IL6), mitogen-activated protein kinase-1 (MAPK1), tumor necrosis factor (TNF), mitogen-activated protein kinase-8 (MAPK8), AKT serine/threonine kinase 1 (AKT1), and 5-hydroxytryptamine receptor 1B (5HT1B)) in SCDA. In addition, the Kyoto Encyclopedia of Gene and Genome results, as well as the staggering network analyses, revealed 20 biological processes and 20 signal pathways targeted by JWYHT in SCDA. Finally, molecular docking analysis was used to determine the binding activity of the main active components of JWYHT to the key targets. The binding activity and stability of methyl arachidonate and MAPK8 were demonstrated using molecular dynamics simulation.

Conclusion

This study demonstrates that JWYHT regulates neuronal development, synaptic transmission, intestinal and cerebral inflammatory response, and other processes in SCDA.

Addiction-related neuroadaptations following chronic nicotine exposure

The addiction-relevant molecular, cellular, and behavioral actions of nicotine are derived from its stimulatory effects on neuronal nicotinic acetylcholine receptors (nAChRs) in the central nervous system. nAChRs expressed by dopamine-containing neurons in the ventral midbrain, most notably in the ventral tegmental area (VTA), contribute to the reward-enhancing properties of nicotine that motivate the use of tobacco products. nAChRs are also expressed by neurons in brain circuits that regulate aversion. In particular, nAChRs expressed by neurons in the medial habenula (mHb) and the interpeduncular nucleus (IPn) to which the mHb almost exclusively projects regulate the "set-point" for nicotine aversion and control nicotine intake. Different nAChR subtypes are expressed in brain reward and aversion circuits and nicotine intake is titrated to maximally engage reward-enhancing nAChRs while minimizing the recruitment of aversion-promoting nAChRs. With repeated exposure to nicotine, reward- and aversion-related nAChRs and the brain circuits in which they are expressed undergo adaptations that influence whether tobacco use will transition from occasional to habitual. Genetic variation that influences the sensitivity of addiction-relevant brain circuits to the actions of nicotine also influence the propensity to develop habitual tobacco use. Here, we review some of the key advances in our understanding of the mechanisms by which nicotine acts on brain reward and aversion circuits and the adaptations that occur in these circuits that may drive addiction to nicotine-containing tobacco products.

Tyrosine phosphorylation differentially fine-tunes ionotropic and metabotropic responses of human α7 nicotinic acetylcholine receptor

The α7 nicotinic acetylcholine receptor is involved in neurological, neurodegenerative, and inflammatory disorders. It operates both as a ligand-gated cationic channel and as a metabotropic receptor in neuronal and non-neuronal cells. As protein phosphorylation is an important cell function regulatory mechanism, deciphering how tyrosine phosphorylation modulates α7 dual ionotropic/metabotropic molecular function is required for understanding its integral role in physiological and pathological processes. α7 single-channel activity elicited by ACh appears as brief isolated openings and less often as episodes of few openings in quick succession. The reduction of phosphorylation by tyrosine kinase inhibition increases the duration and frequency of activation episodes, whereas the inhibition of phosphatases has the opposite effect. Removal of two tyrosine residues at the α7 intracellular domain recapitulates the effects mediated by tyrosine kinase inhibition. The tyrosine-free mutant receptor shows longer duration-activation episodes, reduced desensitization rate and significantly faster recovery from desensitization, indicating that phosphorylation decreases α7 channel activity by favoring the desensitized state. However, the mutant receptor is incapable of triggering ERK1/2 phosphorylation in response to the α7-agonist. Thus, while tyrosine phosphorylation is absolutely required for α7-triggered ERK pathway, it negatively modulates α7 ionotropic activity. Overall, phosphorylation/dephosphorylation events fine-tune the integrated cell response mediated by α7 activation, thus having a broad impact on α7 cholinergic signaling.

Membrane protein-regulated networks across human cancers

Alterations in membrane proteins (MPs) and their regulated pathways have been established as cancer hallmarks and extensively targeted in clinical applications. However, the analysis of MP-interacting proteins and downstream pathways across human malignancies remains challenging. Here, we present a systematically integrated method to generate a resource of cancer membrane protein-regulated networks (CaMPNets), containing 63,746 high-confidence protein-protein interactions (PPIs) for 1962 MPs, using expression profiles from 5922 tumors with overall survival outcomes across 15 human cancers. Comprehensive analysis of CaMPNets links MP partner communities and regulated pathways to provide MP-based gene sets for identifying prognostic biomarkers and druggable targets. For example, we identify CHRNA9 with 12 PPIs (e.g., ERBB2) can be a therapeutic target and find its anti-metastasis agent, bupropion, for treatment in nicotine-induced breast cancer. This resource is a study to systematically integrate MP interactions, genomics, and clinical outcomes for helping illuminate cancer-wide atlas and prognostic landscapes in tumor homo/heterogeneity.

Tyrosine Phosphorylation Determines Afterdischarge Initiation by Regulating an Ionotropic Cholinergic Receptor

Changes to neuronal activity often involve a rapid and precise transition from low to high excitability. In the marine snail, Aplysia, the bag cell neurons control reproduction by undergoing an afterdischarge, which begins with synaptic input releasing acetylcholine to open an ionotropic cholinergic receptor. Gating of this receptor causes depolarization and a shift from silence to continuous action potential firing, leading to the neuroendocrine secretion of egg-laying hormone and ovulation. At the onset of the afterdischarge, there is a rise in intracellular Ca²⁺, followed by both protein kinase C (PKC) activation and tyrosine dephosphorylation. To determine whether these signals influence the acetylcholine ionotropic receptor, we examined the bag cell neuron cholinergic response both in culture and isolated clusters using whole-cell and/or sharp-electrode electrophysiology. The acetylcholine-induced current was not altered by increasing intracellular Ca²⁺ via voltage-gated Ca²⁺ channels, clamping intracellular Ca²⁺ with exogenous Ca²⁺ buffers, or activating PKC with phorbol esters. However, lowering phosphotyrosine levels by inhibiting tyrosine kinases both reduced the cholinergic current and prevented acetylcholine from triggering action potentials or afterdischarge-like bursts. In other systems, acetylcholine receptors are often modulated by multiple signals, but bag cell neurons appear to be more restrictive in this regard. Prior work finds that, as the afterdischarge proceeds, tyrosine dephosphorylation leads to biophysical alterations that promote persistent firing. Because this firing is subsequent to the cholinergic input, inhibiting the acetylcholine receptor may represent a means of properly orchestrating synaptically induced changes in excitability.

Uncontrolled SFK-mediated protein trafficking in prion and Alzheimer's disease

Prions and Amyloid beta (Aβ) peptides induce synaptic damage via complex mechanisms that include the pathological alteration of intracellular signaling cascades. The host-encoded cellular prion protein (PrP^C) acts as a high-affinity cell surface receptor for both toxic species and it can modulate the endocytic trafficking of the N-methyl D-aspartate (NMDA) receptor and E-cadherin adhesive complexes via Src family kinases (SFKs). Interestingly, SFK-mediated control of endocytosis is a widespread mechanism used to regulate the activity of important transmembrane proteins, including neuroreceptors for major excitatory and inhibitory neurotransmitters. Here we discuss our recent work in zebrafish and accumulating evidence suggesting that subversion of this pleiotropic regulatory mechanism by Aβ oligomers and prions explains diverse neurotransmission deficits observed in human patients and mouse models of prion and Alzheimer's neurodegeneration. While Aβ, PrP^C and SFKs constitute potential therapeutic targets on their own, drug discovery efforts might benefit significantly from aiming at protein-protein interactions that modulate the endocytosis of specific SFK targets.

Regulation of pannexin channels in the central nervous system by Src family kinases

Pannexins form single membrane channels that regulate the passage of ions, small molecules and metabolites between the intra- and extracellular compartments. In the central nervous system, these channels are integrated into numerous signaling cascades that shape brain physiology and pathology. Post-translational modification of pannexins is complex, with phosphorylation emerging as a prominent form of functional regulation. While much is still not known regarding the specific kinases and modified amino acids, recent reports support a role for Src family tyrosine kinases (SFK) in regulating pannexin channel activity. This review outlines the current evidence supporting SFK-dependent pannexin phosphorylation in the CNS and examines the importance of these modifications in the healthy and diseased brain.

Mechanisms of growth-promoting and tumor-protecting effects of epithelial nicotinic acetylcholine receptors

Although the role of nicotine as a carcinogen is debatable, it is widely accepted that it contributes to cancer by promoting growth and survival of mutated cell clones and protecting them from the chemo- and radiotherapy-induced apoptosis. On the cell membrane (cm), the nicotinic acetylcholine (ACh) receptors (nAChRs) implement upregulation of proliferative and survival genes. Nicotine also can permeate cells and activate mitochondrial (mt)-nAChRs coupled to inhibition of the mitochondrial permeability transition pore (mPTP) opening, thus preventing apoptosis. In this study, we sought to pin down principal mechanisms mediating the tumor-promoting activities of nicotine resulting from activation of cm- and mt-nAChRs in oral and lung cancer cells, SCC25 and SW900, respectively. Activated cm-nAChRs were found to form complexes with receptors for EGF and VEGEF via the α7 and β2 nAChR subunits, respectively, whereas activated mt-nAChRs physically associated with the intramitochondrial protein kinases PI3K and Src via the α7 and β4 subunits. This was associated with upregulated expression of cyclin D1/activation of ERK1/2 and inhibition of mPTP opening, respectively, as well as upregulated proliferation and resistance to H(2)O(2)-induced apoptosis. The molecular synergy between cm-nAChRs and growth factor receptors helps explain how one biological mediator, such as ACh, can modulate activity of the other, such as a growth factor, and vice versa. Establishment of functional coupling of mt-nAChRs to regulation of mPTP opening provides a novel mechanism of nicotine-dependent protection from cell death. Further elucidation of this novel mechanism of tumor-promoting activities of nicotine should have a strong translational impact, because extraneuronal nAChRs may provide a novel molecular target to prevent, reverse, or retard progression of both nicotine-related and unrelated cancers.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Nicotinic Acetylcholine Receptor (nAChR) Dependent Chorda Tympani Taste Nerve Responses to Nicotine, Ethanol and Acetylcholine

Nicotine elicits bitter taste by activating TRPM5-dependent and TRPM5-independent but neuronal nAChR-dependent pathways. The nAChRs represent common targets at which acetylcholine, nicotine and ethanol functionally interact in the central nervous system. Here, we investigated if the nAChRs also represent a common pathway through which the bitter taste of nicotine, ethanol and acetylcholine is transduced. To this end, chorda tympani (CT) taste nerve responses were monitored in rats, wild-type mice and TRPM5 knockout (KO) mice following lingual stimulation with nicotine free base, ethanol, and acetylcholine, in the absence and presence of nAChR agonists and antagonists. The nAChR modulators: mecamylamine, dihydro-β-erythroidine, and CP-601932 (a partial agonist of the α3β4* nAChR), inhibited CT responses to nicotine, ethanol, and acetylcholine. CT responses to nicotine and ethanol were also inhibited by topical lingual application of 8-chlorophenylthio (CPT)-cAMP and loading taste cells with [Ca²⁺]_i by topical lingual application of ionomycin + CaCl₂. In contrast, CT responses to nicotine were enhanced when TRC [Ca²⁺]_i was reduced by topical lingual application of BAPTA-AM. In patch-clamp experiments, only a subset of isolated rat fungiform taste cells exposed to nicotine responded with an increase in mecamylamine-sensitive inward currents. We conclude that nAChRs expressed in a subset of taste cells serve as common receptors for the detection of the TRPM5-independent bitter taste of nicotine, acetylcholine and ethanol.

Neuron membrane trafficking and protein kinases involved in autism and ADHD

A brain-enriched multi-domain scaffolding protein, neurobeachin has been identified as a candidate gene for autism patients. Mutations in the synaptic adhesion protein cell adhesion molecule 1 (CADM1) are also associated with autism spectrum disorder, a neurodevelopmental disorder of uncertain molecular origin. Potential roles of neurobeachin and CADM1 have been suggested to a function of vesicle transport in endosomal trafficking. It seems that protein kinase B (AKT) and cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) have key roles in the neuron membrane trafficking involved in the pathogenesis of autism. Attention deficit hyperactivity disorder (ADHD) is documented to dopaminergic insufficiencies, which is attributed to synaptic dysfunction of dopamine transporter (DAT). AKT is also essential for the DAT cell-surface redistribution. In the present paper, we summarize and discuss the importance of several protein kinases that regulate the membrane trafficking involved in autism and ADHD, suggesting new targets for therapeutic intervention.

Src kinases regulate de novo actin polymerization during exocytosis in neuroendocrine chromaffin cells

The cortical actin network is dynamically rearranged during secretory processes. Nevertheless, it is unclear how de novo actin polymerization and the disruption of the preexisting actin network control transmitter release. Here we show that in bovine adrenal chromaffin cells, both formation of new actin filaments and disruption of the preexisting cortical actin network are induced by Ca²⁺ concentrations that trigger exocytosis. These two processes appear to regulate different stages of exocytosis; whereas the inhibition of actin polymerization with the N-WASP inhibitor wiskostatin restricts fusion pore expansion, thus limiting the release of transmitters, the disruption of the cortical actin network with cytochalasin D increases the amount of transmitter released per event. Further, the Src kinase inhibitor PP2, and cSrc SH2 and SH3 domains also suppress Ca²⁺-dependent actin polymerization, and slow down fusion pore expansion without disturbing the cortical F-actin organization. Finally, the isolated SH3 domain of c-Src prevents both the disruption of the actin network and the increase in the quantal release induced by cytochalasin D. These findings support a model where a rise in the cytosolic Ca²⁺ triggers actin polymerization through a mechanism that involves Src kinases. The newly formed actin filaments would speed up the expansion of the initial fusion pore, whereas the preexisting actin network might control a different step of the exocytosis process.

Connections of nicotine to cancer

This Opinion article discusses emerging evidence of direct contributions of nicotine to cancer onset and growth. The list of cancers reportedly connected to nicotine is expanding and presently includes small-cell and non-small-cell lung carcinomas, as well as head and neck, gastric, pancreatic, gallbladder, liver, colon, breast, cervical, urinary bladder and kidney cancers. The mutagenic and tumour-promoting activities of nicotine may result from its ability to damage the genome, disrupt cellular metabolic processes, and facilitate growth and spreading of transformed cells. The nicotinic acetylcholine receptors (nAChRs), which are activated by nicotine, can activate several signalling pathways that can have tumorigenic effects, and these receptors might be able to be targeted for cancer therapy or prevention. There is also growing evidence that the unique genetic makeup of an individual, such as polymorphisms in genes encoding nAChR subunits, might influence the susceptibility of that individual to the pathobiological effects of nicotine. The emerging knowledge about the carcinogenic mechanisms of nicotine action should be considered during the evaluation of regulations on nicotine product manufacturing, distribution and marketing.

Are nicotinic acetylcholine receptors coupled to G proteins?

It was, until recently, accepted that the two classes of acetylcholine (ACh) receptors are distinct in an important sense: muscarinic ACh receptors signal via heterotrimeric GTP binding proteins (G proteins), whereas nicotinic ACh receptors (nAChRs) open to allow flux of Na+, Ca2+, and K+ ions into the cell after activation. Here we present evidence of direct coupling between G proteins and nAChRs in neurons. Based on proteomic, biophysical, and functional evidence, we hypothesize that binding to G proteins modulates the activity and signaling of nAChRs in cells. It is important to note that while this hypothesis is new for the nAChR, it is consistent with known interactions between G proteins and structurally related ligand-gated ion channels. Therefore, it underscores an evolutionarily conserved metabotropic mechanism of G protein signaling via nAChR channels.

Modulation of nicotinic receptor channels by adrenergic stimulation in rat pinealocytes

Melatonin secretion from the pineal gland is triggered by norepinephrine released from sympathetic terminals at night. In contrast, cholinergic and parasympathetic inputs, by activating nicotinic cholinergic receptors (nAChR), have been suggested to counterbalance the noradrenergic input. Here we investigated whether adrenergic signaling regulates nAChR channels in rat pinealocytes. Acetylcholine or the selective nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) activated large nAChR currents in whole cell patch-clamp experiments. Norepinephrine (NE) reduced the nAChR currents, an effect partially mimicked by a β-adrenergic receptor agonist, isoproterenol, and blocked by a β-adrenergic receptor antagonist, propranolol. Increasing intracellular cAMP levels using membrane-permeable 8-bromoadenosine (8-Br)-cAMP or 5,6-dichlorobenzimidazole riboside-3',5'-cyclic monophosphorothioate (cBIMPS) also reduced nAChR activity, mimicking the effects of NE and isoproterenol. Further, removal of ATP from the intracellular pipette solution blocked the reduction of nAChR currents, suggesting involvement of protein kinases. Indeed protein kinase A inhibitors, H-89 and Rp-cAMPS, blocked the modulation of nAChR by adrenergic stimulation. After the downmodulation by NE, nAChR channels mediated a smaller Ca(2+) influx and less membrane depolarization from the resting potential. Together these results suggest that NE released from sympathetic terminals at night attenuates nicotinic cholinergic signaling.

α7 nicotinic acetylcholine receptors control cytochrome c release from isolated mitochondria through kinase-mediated pathways

Nicotinic acetylcholine receptors are ligand-gated ion channels found in the plasma membrane of both excitable and non-excitable cells. Previously we reported that nicotinic receptors containing α7 subunits were present in the outer membranes of mitochondria to regulate the early apoptotic events like cytochrome c release. Here we show that signaling of mitochondrial α7 nicotinic receptors affects intramitochondrial protein kinases. Agonist of α7 nicotinic receptors PNU 282987 (30 nM) prevented the effect of phosphatidyl inositol-3-kinase inhibitor wortmannin, which stimulated cytochrome c release in isolated mouse liver mitochondria, and restored the Akt (Ser 473) phosphorylation state decreased by either 90 μM Ca(2+) or wortmannin. The effect of PNU 282987 was similar to inhibition of calcium-calmodulin-dependent kinase II (upon 90 μM Ca(2+)) or of Src kinase(s) (upon 0.5mM H2O2) and of protein kinase C. Cytochrome c release from mitochondria could be also attenuated by α7 nicotinic receptor antagonist methyllicaconitine or α7-specific antibodies. Allosteric modulator PNU 120526 (1 μM) did not improve the effect of agonist PNU 282987. Acetylcholine (1 μM) and methyllicaconitine (10nM) inhibited superoxide release from mitochondria measured according to alkalization of Ca(2+)-containing medium. It is concluded that α7 nicotinic receptors regulate mitochondrial permeability transition pore formation through ion-independent mechanism involving activation of intramitochondrial PI3K/Akt pathway and inhibition of calcium-calmodulin-dependent or Src-kinase-dependent signaling pathways.

Gene-centric analysis of serum cotinine levels in African and European American populations

To date, most genetic association studies of tobacco use have been conducted in European American subjects using the phenotype of smoking quantity (cigarettes per day). However, smoking quantity is a very imprecise measure of exposure to tobacco smoke constituents. Analyses of alternate phenotypes and populations may improve our understanding of tobacco addiction genetics. Cotinine is the major metabolite of nicotine, and measuring serum cotinine levels in smokers provides a more objective measure of nicotine dose than smoking quantity. Previous genetic association studies of serum cotinine have focused on individual genes. We conducted a genetic association study of the biomarker in African American (N=365) and European American (N=315) subjects from the Coronary Artery Risk Development in Young Adults study using a chip containing densely-spaced tag SNPs in ∼2100 genes. We found that rs11187065, located in the non-coding region (intron 1) of insulin-degrading enzyme (IDE), was the most strongly associated SNP (p=8.91 × 10(-6)) in the African American cohort, whereas rs11763963, located on chromosome 7 outside of a gene transcript, was the most strongly associated SNP in European Americans (p=1.53 × 10(-6)). We then evaluated how the top variant association in each population performed in the other group. We found that the association of rs11187065 in IDE was also associated with the phenotype in European Americans (p=0.044). Our top SNP association in European Americans, rs11763963 was non-polymorphic in our African American sample. It has been previously shown that psychostimulant self-administration is reduced in animals with lower insulin because of interference with dopamine transmission in the brain reward centers. Our finding provides a platform for further investigation of this, or additional mechanisms, involving the relationship between insulin and self-administered nicotine dose.

Chaperoning α7 neuronal nicotinic acetylcholine receptors

The α7 subtype of nicotinic acetylcholine receptors (AChRs) is one of the most abundant members of the Cys-loop family of receptors present in the central nervous system. It participates in various physiological processes and has received much attention as a potential therapeutic target for a variety of pathologies. The importance of understanding the mechanisms controlling AChR assembly and cell-surface delivery lies in the fact that these two processes are key to determining the functional pool of receptors actively engaged in synaptic transmission. Here we review recent studies showing that RIC-3, a protein originally identified in the worm Caenorhabditis elegans, modulates the expression of α7 AChRs in a subtype-specific manner. Potentiation of AChR expression by post-transcriptional events is also critically assessed.

Abelson family tyrosine kinases regulate the function of nicotinic acetylcholine receptors and nicotinic synapses on autonomic neurons

Abelson family kinases (AFKs; Abl1, Abl2) are non-receptor tyrosine kinases (NRTKs) implicated in cancer, but they also have important physiological roles that include regulating synaptic structure and function. Recent studies using Abl-deficient mice and the antileukemia drug STI571 [imatinib mesylate (Gleevec); Novartis], which potently and selectively blocks Abl kinase activity, implicate AFKs in regulating presynaptic neurotransmitter release in hippocampus and postsynaptic clustering of nicotinic acetylcholine receptors (nAChRs) in muscle. Here, we tested whether AFKs are relevant for regulating nAChRs and nAChR-mediated synapses on autonomic neurons. AFK immunoreactivity was detected in ciliary ganglion (CG) lysates and neurons, and STI571 application blocked endogenous Abl tyrosine kinase activity. With similar potency, STI571 specifically reduced whole-cell current responses generated by both nicotinic receptor subtypes present on CG neurons (α3*- and α7-nAChRs) and lowered the frequency and amplitude of α3*-nAChR-mediated excitatory postsynaptic currents. Quantal analysis indicated that the synaptic perturbations were postsynaptic in origin, and confocal imaging experiments revealed they were unaccompanied by changes in nAChR clustering or alignment with presynaptic terminals. The results indicate that in autonomic neurons, Abl kinase activity normally supports postsynaptic nAChR function to sustain nAChR-mediated neurotransmission. Such consequences contrast with the influence of Abl kinase activity on presynaptic function and synaptic structure in hippocampus and muscle, respectively, demonstrating a cell-specific mechanism of action. Finally, because STI571 potently inhibits Abl kinase activity, the autonomic dysfunction side effects associated with its use as a chemotherapeutic agent may result from perturbed α3*- and/or α7-nAChR function.

Src family kinase potentiates the activity of nicotinic acetylcholine receptor in rat autonomic ganglion innervating urinary bladder

Src family kinases (SFKs), one of the tyrosine kinase groups, are primary regulators of signal transductions that control cellular functions such as cell proliferation, differentiation, survival, metabolism, and other important roles of the cell. One of the crucial functions of SFKs is to regulate the activities of various neuronal channels. In this study, we investigated the modulatory action of SFK on nicotinic acetylcholine receptors (nAChRs) expressed in rat major pelvic ganglion (MPG) neurons innervating the urinary bladder. PP1 and PP2 (5 μM), selective Src-kinase inhibitors, attenuated ACh-induced ionic currents and [Ca²+](i) transients in MPG neurons, whereas PP3, an inactive analogue, had no effect. Blocking the tyrosine kinase activity of Src kinase by pp60 c-src inhibitory peptide also reduced the ACh-induced currents. Conversely, sodium orthovanadate (200 μM), a tyrosine phosphatase inhibitor, significantly augmented the ACh-induced currents. In the kinase assay, the activities of SFKs in MPG neurons were also inhibited by PP2, but not by PP3. These data suggests that SFKs may have a facilitative role on the synaptic transmission in rat pelvic autonomic ganglion.

PACAP/PAC1R signaling modulates acetylcholine release at neuronal nicotinic synapses

Neuropeptides collaborate with conventional neurotransmitters to regulate synaptic output. Pituitary adenylate cyclase-activating polypeptide (PACAP) co-localizes with acetylcholine in presynaptic nerve terminals, is released by stimulation, and enhances nicotinic acetylcholine receptor- (nAChR-) mediated responses. Such findings implicate PACAP in modulating nicotinic neurotransmission, but relevant synaptic mechanisms have not been explored. We show here that PACAP acts via selective high-affinity G-protein coupled receptors (PAC(1)Rs) to enhance transmission at nicotinic synapses on parasympathetic ciliary ganglion (CG) neurons by rapidly and persistently increasing the frequency and amplitude of spontaneous, impulse-dependent nicotinic excitatory postsynaptic currents (sEPSCs). Of the canonical adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP/PAC(1)R signaling, only AC-generated signals are critical for synaptic modulation since the increases in sEPSC frequency and amplitude were mimicked by 8-Bromo-cAMP, blocked by inhibiting AC or cAMP-dependent protein kinase (PKA), and unaffected by inhibiting PLC. Despite its ability to increase agonist-induced nAChR currents, PACAP failed to influence nAChR-mediated impulse-independent miniature EPSC amplitudes (quantal size). Instead, evoked transmission assays reveal that PACAP/PAC(1)R signaling increased quantal content, indicating that it modulates synaptic function by increasing vesicular ACh release from presynaptic terminals. Lastly, signals generated by the retrograde messenger, nitric oxide- (NO-) are critical for the synaptic modulation since the PACAP-induced increases in spontaneous EPSC frequency, amplitude and quantal content were mimicked by NO donor and absent after inhibiting NO synthase (NOS). These results indicate that PACAP/PAC(1)R activation recruits AC-dependent signaling that stimulates NOS to increase NO production and control presynaptic transmitter output at neuronal nicotinic synapses.

Structural basis of activation of cys-loop receptors: the extracellular-transmembrane interface as a coupling region

Cys-loop receptors mediate rapid transmission throughout the nervous system by converting a chemical signal into an electric one. They are pentameric proteins with an extracellular domain that carries the transmitter binding sites and a transmembrane region that forms the ion pore. Their essential function is to couple the binding of the agonist at the extracellular domain to the opening of the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50 A to the gate is therefore central for the understanding of the receptor function. A step forward toward the identification of the structures involved in gating has been given by the recently elucidated high-resolution structures of Cys-loop receptors and related proteins. The extracellular-transmembrane interface has attracted attention because it is a structural transition zone where beta-sheets from the extracellular domain merge with alpha-helices from the transmembrane domain. Within this zone, several regions form a network that relays structural changes from the binding site toward the pore, and therefore, this interface controls the beginning and duration of a synaptic response. In this review, the most recent findings on residues and pairwise interactions underlying channel gating are discussed, the main focus being on the extracellular-transmembrane interface.

Cellular distribution of the nicotinic acetylcholine receptor alpha7 subunit in rat hippocampus

The hippocampus is a region of the mammalian brain that has been extensively studied due to its role in many forms of memory. To better understand hippocampal function, significant attention has focused upon the cellular distribution of ligand-gated ion channels. Despite strong cholinergic innervation from the basal forebrain and a dense expression of nicotinic acetylchoine receptors (nAChRs), the cellular distribution of subunits forming these receptors has received little attention. We used organotypic hippocampal slice cultures (OHSCs) to study native alpha7 subunits, which, unlike other nAChR subunits, form a homomeric receptor. Cell-surface biotinylation, cross-linking of surface proteins, and sub-cellular fractionation all revealed a very limited presence of the subunit at the plasma membrane. In contrast, subunits of other receptors displayed significant surface expression. Notably, subunits in adult hippocampal tissue were distributed in a fashion similar to that observed in OHSCs. To monitor alpha7 subunits contained in functional nAChRs, a colourimetric assay using alpha-bungarotoxin (a specific alpha7 nAChR antagonist) was developed, and revealed a majority of binding at the cell surface. To change alpha7 subunit distribution, OHSCs were treated with compounds known to affect other ionotropic receptors-insulin, genistein, and elevated external K(+); however, neither subunit surface expression nor antagonist binding was affected. Our data reveal that hippocampal neurons possess a large internal population of alpha7 subunits under basal conditions, which persists during stimuli affecting tyrosine phosphorylation or neuronal activity. The nature of the internal pool of alpha7 subunits remains to be determined, but should have important implications for hippocampal activity.

Recent advances in understanding the structure of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs), members of the Cys-loop ligand-gated ion channels (LGICs) superfamily, are involved in signal transduction upon binding of the neurotransmitter acetylcholine or exogenous ligands, such as nicotine. nAChRs are pentameric assemblies of homologous subunits surrounding a central pore that gates cation flux, and are expressed at the neuromuscular junction and in the nervous system and several nonneuronal cell types. The 17 known nAChR subunits assemble into a variety of pharmacologically distinct receptor subtypes. nAChRs are implicated in a range of physiological functions and pathophysiological conditions related to muscle contraction, learning and memory, reward, motor control, arousal, and analgesia, and therefore present an important target for drug research. Such studies would be greatly facilitated by knowledge of the high-resolution structure of the nAChR. Although this information is far from complete, important progress has been made mainly based on electron microscopy studies of Torpedo nAChR and the high-resolution X-ray crystal structures of the homologous molluscan acetylcholine-binding proteins, the extracellular domain of the mouse nAChR alpha1 subunit, and two prokaryotic pentameric LGICs. Here, we review some of the latest advances in our understanding of nAChR structure and gating.

Coupling of ionic events to protein kinase signaling cascades upon activation of alpha7 nicotinic receptor: cooperative regulation of alpha2-integrin expression and Rho kinase activity

Defining the signaling mechanisms and effector proteins mediating phenotypic and mechanical plasticity of keratinocytes (KCs) during wound epithelialization is one of the major goals in epithelial cell biology. The acetylcholine (ACh)-gated ion channels, or nicotinic ACh receptors (nAChRs), mediate the nicotinergic signaling that controls crawling locomotion of KCs. To elucidate relative contributions of the ionic and protein kinase-mediated events elicited due to activation of alpha7 nAChRs, we quantitated expression of alpha2-integrin gene at the mRNA and protein levels and also measured Rho kinase activity in KCs stimulated with the alpha7 agonist AR-R17779 while blocking the Na+ or Ca2+ entry and/or inhibiting signaling kinases. The results demonstrated the existence of the two-component signaling systems coupling the ionic events and protein kinase signaling cascades downstream of alpha7 nAChR to simultaneous up-regulation of alpha2-integrin expression and activation of Rho kinase. The Raf/MEK1/ERK1/2 cascade up-regulating alpha2-integrin was activated due to both Ca2+-dependent recruitment of Ca2+/calmodulin-dependent protein kinase II and protein kinase C and Ca2+-independent activation of Ras. Likewise the phosphatidylinositol 3-kinase-mediated activation of Rho kinase was elicited due to both Ca2+ entry-dependent involvement of Ca2+/calmodulin-dependent protein kinase II and Ca2+-independent activation of Jak2. Thus, although the initial signals emanating from activated alpha7 nAChR are different in nature the pathways intersect at common effector molecules providing for a common end point effect. This novel paradigm of nAChR-mediated coordination of the ionic and metabolic signaling events can allow an auto/paracrine ACh to simultaneously alter gene expression and induce reciprocal changes in the cytoskeleton and contractile system of KCs required to compete a particular step of wound epithelialization.

Neuronal nicotinic receptors as novel targets for inflammation and neuroprotection: mechanistic considerations and clinical relevance

A number of studies have confirmed the potential for neuronal nicotinic acetylcholine receptor (NNR)-mediated neuroprotection and, more recently, its anti-inflammatory effects. The mechanistic overlap between these pathways and the ubiquitous effects observed following diverse insults suggest that NNRs modulate fundamental pathways involved in cell survival. These results have wide-reaching implications for the design of experimental therapeutics that regulate inflammatory and anti-apoptotic responses through NNRs and represent an initial step toward understanding the benefits of novel therapeutic strategies for the management of central nervous system disorders that target neuronal survival and associated inflammatory processes.

Receptor-mediated tobacco toxicity: acceleration of sequential expression of alpha5 and alpha7 nicotinic receptor subunits in oral keratinocytes exposed to cigarette smoke

Tobacco products and nicotine alter the cell cycle and lead to squamatization of oral keratinocytes (KCs) and squamous cell carcinoma. Activation of nicotinic acetylcholine receptors (nAChRs) elicits Ca(2+) influx that varies in magnitude between different nAChR subtypes. Normal differentiation of KCs is associated with sequential expression of the nAChR subtypes with increasing Ca(2+) permeability, such as alpha5-containing alpha3 nAChR and alpha7 nAChR. Exposure to environmental tobacco smoke (ETS) or an equivalent concentration of nicotine accelerated by severalfold the alpha5 and alpha7 expression in KCs, which could be abolished by mecamylamine and alpha-bungarotoxin with different efficacies, suggesting the following sequence of autoregulation of the expression of nAChR subtypes: alpha3(beta2/beta4) > alpha3(beta2/beta4)alpha5 > alpha7 > alpha7. This conjecture was corroborated by results of quantitative assays of subunit mRNA and protein levels, using nAChR-specific pharmacologic antagonists and small interfering RNAs. The genomic effects of ETS and nicotine involved the transcription factor GATA-2 that showed a multifold increase in quantity and activity in exposed KCs. Using protein kinase inhibitors and dominant negative and constitutively active constructs, we characterized the principal signaling cascades mediating a switch in the nAChR subtype. Cumulative results indicated that the alpha3(beta2/beta4) to alpha3(beta2/beta4)alpha5 nAChR transition predominantly involved protein kinase C, alpha3(beta2/beta4)alpha5 to alpha7 nAChR transition-Ca(2+)/calmodulin-dependent protein kinase II and p38 MAPK, and alpha7 self-up-regulation-the p38 MAPK/Akt pathway, and JAK-2. These results provide a mechanistic insight into the genomic effects of ETS and nicotine on KCs and characterize signaling pathways mediating autoregulation of stepwise overexpression of nAChR subtypes with increasing Ca(2+) permeability in exposed cells. These observations have salient clinical implications, because a switch in the nAChR subunit composition can bring about a corresponding switch in receptor function, leading to profound pathobiologic effects observed in KCs exposed to tobacco products.

Src-family protein tyrosine kinase negatively regulates cerebellar long-term depression

Protein phosphorylation is a major mechanism for the regulation of synaptic transmission. Previous studies have shown that several serine/threonine kinases are involved in the induction of long-term depression (LTD) at excitatory synapses on a Purkinje neuron (PN) in the cerebellum. Here, we show that Src-family protein tyrosine kinases (SFKs) are involved in the regulation of the LTD induction. Intracellular application of c-Src suppressed LTD. We also show that application of a SFK-selective inhibitor PP2 recovered LTD from the suppression caused by the inhibition of mGluR1 activity. These results indicate that SFKs negatively regulate the LTD induction at excitatory synapses on a cerebellar PN.

Effects of genistein and herbimycin, tyrosine kinase inhibitors, on catecholamine release in bovine adrenal chromaffin cells

1 The effects of genistein and herbimycin, tyrosine kinase inhibitors, on catecholamine (CA) release were examined in bovine adrenal chromaffin cells. 2 In intact cells, genistein (10-100 microm) and herbimycin (3-30 microm) inhibited CA release induced by acetylcholine (ACh; 100 microm) or the nicotinic receptor stimulant 1,1-dimethyl-4-phenyl-piperazinium (DMPP; 10 microm), but did not affect CA release induced by high K+ (40 mm). 3 Genistein and herbimycin inhibited (45)Ca2+ uptake induced by ACh (100 microm). 4 Neither genistein nor herbimycin affected [(3)H]nicotine binding with nicotinic receptors. 5 In beta-escin-permeabilized cells, neither genistein nor herbimycin affected CA release induced by Ca2+ (1 microm). 6 These results suggest that protein tyrosine kinase plays the facilitatory role in the regulation of CA release induced by nicotinic receptor stimulation in stimulus-secretion coupling of bovine adrenal chromaffin cells.

Differential coupling of M1 muscarinic and alpha7 nicotinic receptors to inhibition of pemphigus acantholysis

The mechanisms mediating and regulating assembly and disassembly of intercellular junctions is a subject of intensive research. The IgG autoantibodies produced in patients with the immunoblistering skin disease pemphigus vulgaris (PV) can induce keratinocyte (KC) dyshesion (acantholysis) via mechanisms that involve signaling kinases targeting intercellular adhesion molecules, thus providing a useful model to study the physiologic regulation of KC cohesion. Previous studies showed that activation of Src and protein kinase C are the earliest events in the PV IgG-induced intracellular phosphorylation cascades and that cholinergic agonists are effective for treating patients with pemphigus. In this study, we sought to elucidate the molecular mechanisms allowing cholinergic agonists to inhibit PV IgG-induced acantholysis and phosphorylation of KC adhesion molecules. The extent of acantholysis in KC monolayers correlated closely with the degree of PV IgG-induced phosphorylation of p120- and beta-catenins, with classic isoforms of protein kinase C mediating serine phosphorylation of beta-catenin and Src-tyrosine phosphorylation of p120-catenin. The M(1) muscarinic agonist pilocarpine blocked phosphorylation of both catenins, which could be abolised by the M(1) antagonist MT7. The alpha7 nicotinic agonist AR-R17779 inhibited phosphorylation of P120-cateinin. The alpha7 antagonist methyllycaconitine abolished the effect of AR-R17779. Okadaic acid abrogated protective effects of agonists on phosphorylation of beta-catenin, and pervanadate, on that of p120-catenin. Stimulation of KCs with pilocarpine significantly (p < 0.05) elevated both serine/threonine and tyrosine phosphatase activities in KCs. AR-R17779 both stimulated tyrosine phosphatase and decreased PV IgG-induced Src activity. Methyllycaconitine released Src activity in intact KCs and caused acantholysis. Thus, downstream signaling from M(1) abolished PV IgG-dependent catenin phosphorylation due to activation of both serine/threonine and tyrosine phosphatases, whereas alpha7 action involved both activation of tyrosine phosphatase and inhibition of Src. These findings identified novel paradigm of regulation of signaling kinases associated with cholinergic receptors and provided mechanistic explanation of therapeutic activity of cholinomimetics in PV patients.

Brevetoxin-induced phosphorylation of Pyk2 and Src in murine neocortical neurons involves distinct signaling pathways

Brevetoxins (PbTx-1 to PbTx-10) are potent lipid soluble polyether neurotoxins produced by the marine dinoflagellate Karenia brevis. Brevetoxins bind to site 5 of the alpha-subunit of voltage-gated sodium channels (VGSCs) and augment Na(+) influx. In neocortical neurons brevetoxins elevate intracellular Ca(2+) and augment NMDA receptor signaling. In this study, we explored the effects of PbTx-2 on Pyk2 and Src activation in neocortical neurons. We found that both Pyk2 and Src were activated following PbTx-2 exposure. PbTx-2-induced Pyk2 Tyr402 phosphorylation was dependent on elevation of Ca(2+) influx through NMDA receptors. Moreover, Pyk2 Tyr402 phosphorylation was also found to require PKC activation inasmuch as RO-31-8425 and GF 109203x both attenuated the response. In contrast, PbTx-2-induced Src Tyr416 phosphorylation involved a Gq-coupled receptor inasmuch as U73122, a specific PLC inhibitor, abolished the response. This Gq-coupled receptor appears to be mGluR 5. The PKCdelta inhibitor rottlerin abolished PbTx-2-induced Src activation demonstrating that this isoform of PKC is involved in the activation of Src by PbTx-2. Considered together these data suggest that although activation of neuronal Pyk2 and Src result from PbTx-2 stimulation of VGSC, engagement of these two non-receptor tyrosine kinases involves distinct signaling pathways.

alpha5 integrin signaling regulates the formation of spines and synapses in hippocampal neurons

The actin-based dynamics of dendritic spines play a key role in synaptic plasticity, which underlies learning and memory. Although it is becoming increasingly clear that modulation of actin is critical for spine dynamics, the upstream molecular signals that regulate the formation and plasticity of spines are poorly understood. In non-neuronal cells, integrins are critical modulators of the actin cytoskeleton, but their function in the nervous system is not well characterized. Here we show that alpha5 integrin regulates spine morphogenesis and synapse formation in hippocampal neurons. Knockdown of alpha5 integrin expression using small interfering RNA decreased the number of dendritic protrusions, spines, and synapses. Expression of constitutively active or dominant negative alpha5 integrin also resulted in alterations in the number of dendritic protrusions, spines, and synapses. alpha5 integrin signaling regulates spine morphogenesis and synapse formation by a mechanism that is dependent on Src kinase, Rac, and the signaling adaptor GIT1. Alterations in the activity or localization of these molecules result in a significant decrease in the number of spines and synapses. Thus, our results point to a critical role for integrin signaling in regulating the formation of dendritic spines and synapses in hippocampal neurons.

Chronic nicotine doses down-regulate PDE4 isoforms that are targets of antidepressants in adolescent female rats

BACKGROUND

Previous data in humans and animal models has suggested connections between anxiety, depression, smoking behavior, and nicotine dependence. The importance of these connections has been confirmed by clinical studies that led to the recent FDA approval of an anti-depressant (Zyban) for use in human smoking cessation programs. Other anti-depressants (such as rolipram) specifically inhibit PDE4 phosphodiesterases.

METHODS

We used DNA microarrays to discover gene expression changes in adolescent female rats following chronic nicotine treatments, and real-time PCR assays to confirm and extend those results.

RESULTS

We found a consistent decrease in the mRNA levels encoded by the Pde4b gene in nucleus accumbens, prefrontal cortex, and hippocampus of adolescent female rats treated with .24 mg/day nicotine, and in prefrontal cortex of adolescent female rats treated with .12 mg/day nicotine. We further show that each of these brain areas produced a different profile of Pde4b isoforms.

CONCLUSIONS

Chronic nicotine treatments produce a dose-dependent down-regulation of Pde4b, which may have an antidepressant effect. This is the first report of a link between nicotine dependence and phosphodiesterase gene expression. Our results also add to the complex interrelationships between smoking and schizophrenia, because mutations in the PDE4B gene are associated with schizophrenia.

Interleukin-1β Released by gp120 Drives Neural Death through Tyrosine Phosphorylation and Trafficking of NMDA Receptors

Interleukin-1beta is a proinflammatory cytokine implicated under pathological conditions involving NMDA receptor activation, including the AIDS dementia complex (HAD). No information is available on the molecular mechanisms recruited by native interleukin-1beta produced under this type of condition. Using a sandwich co-culture of primary hippocampal neurons and glia, we investigated whether native interleukin-1beta released by HIV-gp120-activated glia (i) affects NMDAR functions and (ii) the relevance on neuronal spine density and survival, two specific traits of HAD. Increased phosphorylation of NR2B Tyr-1472 was observed after 24 h of exposure of neurons to 600 pm gp120. This effect occurred only when neurons were treated in the presence of glial cells and was abolished by the interleukin-1 receptor antagonist (IL-1ra). Gp120-induced phosphorylation of NR2B resulted in a sustained elevation of intracellular Ca(2+) in neurons and in a significant increase of NR2B binding to PSD95. Increased intracellular Ca(2+) was prevented by 10 mum ifenprodil, that selectively inhibits receptors containing the NR2B, by interleukin-1ra and by Ca-pYEEIE, a Src family SH2 inhibitor peptide. These last two inhibitors, prevented also NR2B binding to PSD95. Finally, gp120 reduced by 35% of the total PSD95 positive spine density after 48 h of treatment and induced by 30% of the neuronal death. Again, both of these effects were blocked by Ca-pYEEIE. Altogether, our data show that gp120 releasing interleukin-1beta from glia increases tyrosine phosphorylation of NMDAR. Thus, tyrosine phosphorylation may contribute to the sensitization of the receptor increasing its function and synaptic localization. Both of these effects are relevant for neurodegeneration.

Enhancement of insulin-induced PI3K/Akt/GSK-3beta and ERK signaling by neuronal nicotinic receptor/PKC-alpha/ERK pathway: up-regulation of IRS-1/-2 mRNA and protein in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells treated with nicotine (10 microm for 24 h), phosphorylation of Akt, glycogen synthase kinase-3beta (GSK-3beta) and extracellular signal-regulated kinase (ERK)1/2 induced by insulin (100 nm for 10 min) was enhanced by approximately 62%, without altering levels of these protein kinases. Nicotine produced time (> 12 h)- and concentration (EC(50) 3.6 and 13 microm)-dependent increases in insulin receptor substrate (IRS)-1 and IRS-2 levels by approximately 125 and 105%, without altering cell surface density of insulin receptors. In these cells, insulin-induced tyrosine phosphorylation of IRS-1/IRS-2 and recruitment of phosphoinositide 3-kinase (PI3K) to IRS-1/IRS-2 were augmented by approximately 63%. The increase in IRS-1/IRS-2 levels induced by nicotine was prevented by nicotinic acetylcholine receptor (nAChR) antagonists, the Ca(2+) chelator 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetra-acetic acid tetrakis-acetoxymethyl ester, cycloheximide or actinomycin D. Nicotine increased IRS-1 and IRS-2 mRNA levels by approximately 57 and approximately 50%, and this was prevented by conventional protein kinase C (cPKC) inhibitor Gö6976, or ERK kinase inhibitors PD98059 and U0126. Nicotine phosphorylated cPKC-alpha, thereby increasing phosphorylation of ERK1/ERK2, as demonstrated by using Gö6976, PD98059 or U0126. Selective activation of cPKC-alpha by thymeleatoxin mimicked these effects of nicotine. Thus, stimulation of nAChRs up-regulated expression of IRS-1/IRS-2 via Ca(2+)-dependent sequential activation of cPKC-alpha and ERK, and enhanced insulin-induced PI3K/Akt/GSK-3beta and ERK signaling pathways.

Selective modulation of neuronal nicotinic acetylcholine receptor channel subunits by Go-protein subunits

G-protein modulation of neuronal nicotinic acetylcholine receptor (nAChR) channels in rat intrinsic cardiac ganglia was examined using dialyzed whole-cell and excised membrane patch-recording configurations. Cell dialysis with GTPgammaS increased the agonist affinity of nAChRs, resulting in a potentiation of nicotine-evoked whole-cell currents at low concentrations. ACh- and nicotine-evoked current amplitudes were increased approximately twofold in the presence of GTPgammaS. In inside-out membrane patches, the open probability (NP(o)) of nAChR-mediated unitary currents was reversibly increased fourfold after bath application of 0.2 mm GTPgammaS relative to control but was unchanged in the presence of GDPbetaS. The modulation of nAChR-mediated whole-cell currents was agonist specific; currents evoked by the cholinergic agonists ACh, nicotine, and 1,1-dimethyl-4-phenylpiperazinium iodide, but not cytisine or choline, were potentiated in the presence of GTPgammaS. The direct interaction between G-protein subunits and nAChRs was examined by bath application of either G(o)alpha or Gbetagamma subunits to inside-out membrane patches and in glutathione S-transferase pull-down and coimmunoprecipitation experiments. Bath application of 50 nm Gbetagamma increased the open probability of ACh-activated single-channel currents fivefold, whereas G(o)alpha (50 nm) produced no significant increase in NP(o). Neuronal nAChR subunits alpha3-alpha5 and beta2 exhibited a positive interaction with G(o)alpha and Gbetagamma, whereas beta4 and alpha7 failed to interact with either of the G-protein subunits. These results provide evidence for a direct interaction between nAChR and G-protein subunits, underlying the increased open probability of ACh-activated single-channel currents and potentiation of nAChR-mediated whole-cell currents in parasympathetic neurons of rat intrinsic cardiac ganglia.

Rapid upregulation of alpha7 nicotinic acetylcholine receptors by tyrosine dephosphorylation

Alpha7 nicotinic acetylcholine receptors (nAChRs) modulate network activity in the CNS. Thus, functional regulation of alpha7 nAChRs could influence the flow of information through various brain nuclei. It is hypothesized here that these receptors are amenable to modulation by tyrosine phosphorylation. In both Xenopus oocytes and rat hippocampal interneurons, brief exposure to a broad-spectrum protein tyrosine kinase inhibitor, genistein, specifically and reversibly potentiated alpha7 nAChR-mediated responses, whereas a protein tyrosine phosphatase inhibitor, pervanadate, caused depression. Potentiation was associated with an increased expression of surface alpha7 subunits and was not accompanied by detectable changes in receptor open probability, implying that the increased function results from an increased number of alpha7 nAChRs. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated exocytosis was shown to be a plausible mechanism for the rapid delivery of additional alpha7 nAChRs to the plasma membrane. Direct phosphorylation/dephosphorylation of alpha7 subunits was unlikely because mutation of all three cytoplasmic tyrosine residues did not prevent the genistein-mediated facilitation. Overall, these data are consistent with the hypothesis that the number of functional cell surface alpha7 nAChRs is controlled indirectly via processes involving tyrosine phosphorylation.

PACAP support of neuronal survival requires MAPK- and activity-generated signals

Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in the parasympathetic ciliary ganglion (CG) and modulates nicotinic acetylcholine receptor function. PACAP also provides trophic support, promoting partial survival of CG neurons in culture and full survival when accompanied by membrane depolarization. We probed the adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP to determine their respective roles in supporting neuronal survival and examined their interaction with signals generated by membrane activity. While PLC-dependent signaling was dispensable, AC-generated signals proved critical for PACAP to support neuronal survival. Specifically, PACAP-supported survival was mimicked by 8Br-cAMP and blocked by inhibiting either PKA or the phosphorylation of mitogen-activated protein kinase (MAPK). The ability of PACAP to promote survival was additionally dependent on spontaneous activity as blocking Na+ or Ca2+ channel currents completely abrogated trophic effects. Our results underscore the importance of coordinated MAPK- and activity-generated signals in transducing neuropeptide-mediated parasympathetic neuronal survival.

Alpha7 neuronal nicotinic acetylcholine receptors are negatively regulated by tyrosine phosphorylation and Src-family kinases

Nicotine, a component of tobacco, is highly addictive but possesses beneficial properties such as cognitive improvements and memory maintenance. Involved in these processes is the neuronal nicotinic acetylcholine receptor (nAChR) alpha7, whose activation triggers depolarization, intracellular signaling cascades, and synaptic plasticity underlying addiction and cognition. It is therefore important to investigate intracellular mechanisms by which a cell regulates alpha7 nAChR activity. We have examined the role of phosphorylation by combining molecular biology, biochemistry, and electrophysiology in SH-SY5Y neuroblastoma cells, Xenopus oocytes, rat hippocampal interneurons, and neurons from the supraoptic nucleus, and we found tyrosine phosphorylation of alpha7 nAChRs. Tyrosine kinase inhibition by genistein decreased alpha7 nAChR phosphorylation but strongly increased acetylcholine-evoked currents, whereas tyrosine phosphatase inhibition by pervanadate produced opposite effects. Src-family kinases (SFKs) directly interacted with the cytoplasmic loop of alpha7 nAChRs and phosphorylated the receptors at the plasma membrane. SFK inhibition by PP2 [4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine] or SU6656 (2,3-dihydro-N,N-dimethyl-2-oxo-3-[(4,5,6,7-tetrahydro-1H-indol-2-yl)methylene]-1H-indole-5-sulfonamide) increased alpha7 nAChR-mediated responses, whereas expression of active Src reduced alpha7 nAChR activity. Mutant alpha7 nAChRs lacking cytoplasmic loop tyrosine residues because of alanine replacement of Tyr-386 and Tyr-442 were more active than wild-type receptors and insensitive to kinase or phosphatase inhibition. Because the amount of surface alpha7 receptors was not affected by kinase or phosphatase inhibitors, these data show that functional properties of alpha7 nAChRs depend on the tyrosine phosphorylation status of the receptor and are the result of a balance between SFKs and tyrosine phosphatases. These findings reveal novel regulatory mechanisms that may help to understand nicotinic receptor-dependent plasticity, addiction, and pathology.

Inhibition by SEA0400, a Selective Inhibitor of Na+/Ca2+ Exchanger, of Na+-Dependent Ca2+ Uptake and Catecholamine Release in Bovine Adrenal Chromaffin Cells

The effects of SEA0400, a selective inhibitor of the Na(+)/Ca(2+) exchanger (NCX), on Na(+)-dependent Ca(2+) uptake and catecholamine (CA) release were examined in bovine adrenal chromaffin cells that were loaded with Na(+) by treatment with ouabain and veratridine. SEA0400 inhibited Na(+)-dependent (45)Ca(2+) uptake and CA release, with the IC(50) values of 40 and 100 nM, respectively. The IC(50) values of another NCX inhibitor KB-R7943 were 1.8 and 3.7 microM, respectively. These results indicate that SEA0400 is about 40 times more potent than KB-R7943 in inhibiting NCX working in the reverse mode. In intact cells, SEA0400 and KB-R7943 inhibited CA release induced by acetylcholine and DMPP. The IC(50) values of SEA0400 were 5.1 and 4.5 microM and the values of KB-R7943 were 2.6 and 2.1 microM against the release induced by acetylcholine and DMPP, respectively, indicating that the potency of SEA0400 is about a half of that of KB-R7943 in inhibiting the nicotinic receptor-mediated CA release. The binding of [(3)H]nicotine with nicotinic receptors was inhibited by SEA0400 (IC(50) = 90 microM) and KB-R7943 (IC(50) = 12 microM). From these results, it is concluded that unlike KB-R7943, SEA0400 has a potent and selective action on NCX in bovine adrenal chromaffin cells.

Smoking, the missing drug interaction in clinical trials: ignoring the obvious

Tobacco use is universally recognized as the foremost preventable cause of cancer in the United States and globally and is responsible for 30% of all cancer-related deaths in the United States. Tobacco use, including exposure to secondhand smoke has been implicated as a causal or contributory agent in an ever-expanding list of cancers, including lung, oral cavity and pharynx, pancreas, liver, kidney, ureter, urinary bladder, uterine cervix, and myeloid leukemia. In addition to and independent of the etiologic effects of tobacco carcinogens in numerous cancers, there is a growing literature on the direct and indirect effects of smoking on treatment efficacy (short-term and long-term outcomes), toxicity and morbidity, quality of life (QOL), recurrence, second primary tumors (SPT), and survival time as summarized below. Oncology health professionals have called for increased advocacy for tobacco control. Despite the critical relevance of smoking to cancer outcomes, most oncology clinical trials do not collect data on smoking history and status unless the malignancy is widely acknowledged as smoking related (e.g., lung or head and neck cancer). Usually, these data are collected only at trial registration. Changes in smoking status during treatment or follow-up are monitored in very few trials and are infrequently reported in sample descriptions or included in analysis plans as a potential moderator of outcomes. Based on mounting evidence that tobacco use affects cancer treatment outcomes and survival, we recommend that smoking history and status be systematically collected as core data in all oncology clinical trials: at diagnosis, at trial registration, and throughout treatment and follow-up to long-term survival or death. We feel that the inclusion and analysis of such data in clinical trials will add important information to the interpretation of outcomes and the development of scientific knowledge in this area. Smoking status has been called another "vital sign" because of its relevance to a patient's immediate medical condition. We explain the critical value of knowing the smoking status of every patient with cancer at every visit by providing a brief overview of the following research findings: (a) the effects of tobacco use on cancer treatment and outcome; (b) recent findings on the role of nicotine in malignant processes; (c) some unexpected results concerning tobacco status, treatment, and disease outcome; and (d) identifying key questions that remain to be addressed. We provide a suggested set of items for inclusion in clinical trial data sets that also are useful in clinical practice.

Agrin mediates a rapid switch from electrical coupling to chemical neurotransmission during synaptogenesis

In contrast to its well-established actions as an organizer of synaptic differentiation at the neuromuscular junction, the proteoglycan agrin is still in search of a function in the nervous system. Here, we report an entirely unanticipated role for agrin in the dual modulation of electrical and chemical intercellular communication that occurs during the critical period of synapse formation. When applied at the developing splanchnic nerve–chromaffin cell cholinergic synapse in rat adrenal acute slices, agrin rapidly modified cell-to-cell communication mechanisms. Specifically, it led to decreased gap junction–mediated electrical coupling that preceded an increase in nicotinic synaptic transmission. This developmental switch from predominantly electrical to chemical communication was fully operational within one hour and depended on the activation of Src family–related tyrosine kinases. Hence, agrin may play a pivotal role in synaptogenesis in promoting a rapid switch between electrical coupling and synaptic neurotransmission.

Nicotinic acetylcholine receptor subunits and associated proteins in human sperm

We demonstrated previously the involvement of a nicotinic acetylcholine receptor containing an alpha7 subunit in the human sperm acrosome reaction (a modified exocytotic event essential to fertilization). Here we report the presence in human sperm of alpha7, alpha9, alpha3, alpha5, and beta4 nicotinic acetylcholine receptor subunits and the following proteins known to be associated with the receptor in the somatic cell: rapsyn and the tyrosine kinases c-SRC and FYN. The alpha7 subunit appears to exist as a homomer in the posterior post-acrosomal and neck regions of sperm and is probably linked to the cytoskeleton via rapsyn. The alpha3, alpha5, and beta4 subunits are present in the sperm flagellar mid-piece of sperm and possibly exist as alpha3alpha5beta4 and/or alpha3beta4 channels. The alpha9 subunit is present in the sperm mid-piece. We detected the FYN and c-SRC tyrosine kinases in the flagellar mid-piece region. Both co-precipitated only with the nicotinic acetylcholine receptor beta4 subunit. Immunolocalization with a C-terminal SRC kinase antibody, which recognizes several members of SRC kinase family, detected a SRC kinase co-localized with the alpha7 subunit in the neck region of sperm. Immunoprecipitation studies with that antibody demonstrated that the alpha7 subunit is associated with a SRC kinase. Antagonists of tyrosine phosphorylation inhibited the acetylcholine-initiated acrosome reaction, suggesting the involvement of a SRC kinase in the acrosome reaction.

Src in synaptic transmission and plasticity

In the central nervous system (CNS), Src and other Src family kinases are widely expressed and are abundant in neurons. Src has been implicated in proliferation and differentiation during the development of the CNS. But Src is highly expressed in fully differentiated neurons in the developed CNS, implying additional functions of this kinase. Over the past decade, a large body of evidence has accumulated showing that a main function of Src is to upregulate the activity of N-methyl-D-aspartate (NMDA) receptors and other ion channels. NMDA receptors (NMDARs) are a principal subtype of glutamate receptors, which mediate fast excitatory transmission at most central synapses. In this review, we focus on Src as a regulator of NMDARs and on the role of Src in NMDAR-dependent synaptic plasticity. We also describe recent studies that give insights into the regulation of Src itself at glutamatergic synapses. By upregulating the function of NMDARs, Src gates the production of NMDAR-dependent synaptic potentiation and plasticity. Thus, Src may be critical for processes underlying physiological plasticity, including learning and memory, and pathological plasticity, such as pain and epilepsy.