Cyclic AMP-dependent protein kinase (PKA) and protein kinase C phosphorylate sites in the amino acid sequence corresponding to the M3/M4 cytoplasmic domain of alpha4 neuronal nicotinic receptor subunits

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.

Evaluation of the Phosphoproteome of Mouse Alpha 4/Beta 2-Containing Nicotinic Acetylcholine Receptors In Vitro and In Vivo

Activation of nicotinic acetylcholine receptors containing α4 and β2 subunits (α4/β2* nAChRs) in the mammalian brain is necessary for nicotine reinforcement and addiction. We previously identified interactions between α4/β2* nAChRs and calcium/calmodulin-dependent protein kinase II (CaMKII) in mouse and human brain tissue. Following co-expression of α4/β2 nAChR subunits with CaMKII in HEK cells, mass spectrometry identified 8 phosphorylation sites in the α4 subunit. One of these sites and an additional site were identified when isolated α4/β2* nAChRs were dephosphorylated and subsequently incubated with CaMKII in vitro, while 3 phosphorylation sites were identified following incubation with protein kinase A (PKA) in vitro. We then isolated native α4/β2* nAChRs from mouse brain following acute or chronic exposure to nicotine. Two CaMKII sites identified in HEK cells were phosphorylated, and 1 PKA site was dephosphorylated following acute nicotine administration in vivo, whereas phosphorylation of the PKA site was increased back to baseline levels following repeated nicotine exposure. Significant changes in β2 nAChR subunit phosphorylation were not observed under these conditions, but 2 novel sites were identified on this subunit, 1 in HEK cells and 1 in vitro. These experiments identified putative CaMKII and PKA sites on α4/β2* nAChRs and novel nicotine-induced phosphorylation sites in mouse brain that can be explored for their consequences on receptor function.

Nicotinic modulation of hippocampal cell signaling and associated effects on learning and memory

The hippocampus is a key brain structure involved in synaptic plasticity associated with long-term declarative memory formation. Importantly, nicotine and activation of nicotinic acetylcholine receptors (nAChRs) can alter hippocampal plasticity and these changes may occur through modulation of hippocampal kinases and transcription factors. Hippocampal kinases such as cAMP-dependent protein kinase (PKA), calcium/calmodulin-dependent protein kinases (CAMKs), extracellular signal-regulated kinases 1 and 2 (ERK1/2), and c-jun N-terminal kinase 1 (JNK1), and the transcription factor cAMP-response element-binding protein (CREB) that are activated either directly or indirectly by nicotine may modulate hippocampal plasticity and in parallel hippocampus-dependent learning and memory. Evidence suggests that nicotine may alter hippocampus-dependent learning by changing the time and magnitude of activation of kinases and transcription factors normally involved in learning and by recruiting additional cell signaling molecules. Understanding how nicotine alters learning and memory will advance basic understanding of the neural substrates of learning and aid in understanding mental disorders that involve cognitive and learning deficits.

Engineering α4β2 nAChRs with reduced or increased nicotine sensitivity via selective disruption of consensus sites in the M3-M4 cytoplasmic loop of the α4 subunit

The α4β2 neuronal nicotinic acetylcholine receptor (nAChR) plays a crucial role in nicotine addiction. These receptors are known to desensitize and up-regulate after chronic nicotine exposure, but the mechanism remains unknown. Currently, the structure and functional role of the intracellular domains of the nAChR are obscure. To study the effect of subunit phosphorylation on α4β2 nAChR function and expression, eleven residues located in the M3-M4 cytoplasmic loop were mutated to alanine and aspartic acid. Two-electrode voltage clamp and 125I-labeled epibatidine binding assays were performed on Xenopus oocytes to assess agonist activation and receptor expression. When ACh was used as an agonist, a decrease in receptor activation was observed for the majority of the mutations. When nicotine was used as an agonist, four mutations exhibited a statistically significant hypersensitivity to nicotine (S438D, S469A, Y576A, and S589A). Additionally, two mutations (S516D and T536A) that displayed normal activation with ACh displayed remarkable reductions in sensitivity to nicotine. Binding assays revealed a constitutive up-regulation in these two nicotine mutations with reduced nicotine sensitivity. These results suggest that consensus phosphorylation residues in the M3-M4 cytoplasmic loop of the α4 subunit play a crucial role in regulating α4β2 nAChR agonist selectivity and functional expression. Furthermore, these results suggest that disruption of specific interactions at PKC putative consensus sites can render α4β2 nAChRs almost insensitive to nicotine without substantial effects on normal AChR function. Therefore, these PKC consensus sites in the M3-M4 cytoplasmic loop of the α4 nAChR subunit could be a target for smoking cessation drugs.

Rare human nicotinic acetylcholine receptor α4 subunit (CHRNA4) variants affect expression and function of high-affinity nicotinic acetylcholine receptors

Nicotine, the primary psychoactive component in tobacco smoke, produces its behavioral effects through interactions with neuronal nicotinic acetylcholine receptors (nAChRs). α4β2 nAChRs are the most abundant in mammalian brain, and converging evidence shows that this subtype mediates the rewarding and reinforcing effects of nicotine. A number of rare variants in the CHRNA4 gene that encode the α4 nAChR subunit have been identified in human subjects and appear to be underrepresented in a cohort of smokers. We compared three of these variants (α4R336C, α4P451L, and α4R487Q) to the common variant to determine their effects on α4β2 nAChR pharmacology. We examined [(3)H]epibatidine binding, interacting proteins, and phosphorylation of the α4 nAChR subunit with liquid chromatography and tandem mass spectrometry (LC-MS/MS) in HEK 293 cells and voltage-clamp electrophysiology in Xenopus laevis oocytes. We observed significant effects of the α4 variants on nAChR expression, subcellular distribution, and sensitivity to nicotine-induced receptor upregulation. Proteomic analysis of immunopurified α4β2 nAChRs incorporating the rare variants identified considerable differences in the intracellular interactomes due to these single amino acid substitutions. Electrophysiological characterization in X. laevis oocytes revealed alterations in the functional parameters of activation by nAChR agonists conferred by these α4 rare variants, as well as shifts in receptor function after incubation with nicotine. Taken together, these experiments suggest that genetic variation at CHRNA4 alters the assembly and expression of human α4β2 nAChRs, resulting in receptors that are more sensitive to nicotine exposure than those assembled with the common α4 variant. The changes in nAChR pharmacology could contribute to differences in responses to smoked nicotine in individuals harboring these rare variants.

Neuronal nicotinic receptors in sleep-related epilepsy: studies in integrative biology

Although Mendelian diseases are rare, when considered one by one, overall they constitute a significant social burden. Besides the medical aspects, they propose us one of the most general biological problems. Given the simplest physiological perturbation of an organism, that is, a single gene mutation, how do its effects percolate through the hierarchical biological levels to determine the pathogenesis? And how robust is the physiological system to this perturbation? To solve these problems, the study of genetic epilepsies caused by mutant ion channels presents special advantages, as it can exploit the full range of modern experimental methods. These allow to extend the functional analysis from single channels to whole brains. An instructive example is autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), which can be caused by mutations in neuronal nicotinic acetylcholine receptors. In vitro, such mutations often produce hyperfunctional receptors, at least in heterozygous condition. However, understanding how this leads to sleep-related frontal epilepsy is all but straightforward. Several available animal models are helping us to determine the effects of ADNFLE mutations on the mammalian brain. Because of the complexity of the cholinergic regulation in both developing and mature brains, several pathogenic mechanisms are possible, which also present different therapeutic implications.

The multifunctional Ca(2+)/calmodulin-dependent protein kinase II delta (CaMKIIδ) phosphorylates cardiac titin's spring elements

Titin-based passive stiffness is post-translationally regulated by several kinases that phosphorylate specific spring elements located within titin's elastic I-band region. Whether titin is phosphorylated by calcium/calmodulin dependent protein kinase II (CaMKII), an important regulator of cardiac function and disease, has not been addressed. The aim of this work was to determine whether CaMKIIδ, the predominant CaMKII isoform in the heart, phosphorylates titin, and to use phosphorylation assays and mass spectrometry to study which of titin's spring elements might be targeted by CaMKIIδ. It was found that CaMKIIδ phosphorylates titin in mouse LV skinned fibers, that the CaMKIIδ sites can be dephosphorylated by protein phosphatase 1 (PP1), and that under baseline conditions, in both intact isolated hearts and skinned myocardium, about half of the CaMKIIδ sites are phosphorylated. Mass spectrometry revealed that both the N2B and PEVK segments are targeted by CaMKIIδ at several conserved serine residues. Whether phosphorylation of titin by CaMKIIδ occurs in vivo, was tested in several conditions using back phosphorylation assays and phospho-specific antibodies to CaMKIIδ sites. Reperfusion following global ischemia increased the phosphorylation level of CaMKIIδ sites on titin and this effect was abolished by the CaMKII inhibitor KN-93. No changes in the phosphorylation level of the PEVK element were found suggesting that the increased phosphorylation level of titin in IR (ischemia reperfusion) might be due to phosphorylation of the N2B element. The findings of these studies show for the first time that titin can be phosphoryalated by CaMKIIδ, both in vitro and in vivo, and that titin's molecular spring region that determines diastolic stiffness is a target of CaMKIIδ.

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

Nicotine-induced up regulation of α4β2 neuronal nicotinic receptors is mediated by the protein kinase C-dependent phosphorylation of α4 subunits

Sustained exposure to nicotine is well known to increase the cell surface density of α4β2* neuronal nicotinic receptors both in vivo and in vitro, but the cellular mechanisms mediating this effect are equivocal. Using a pharmacological approach to investigate the effects of nicotine on receptor subunit expression and phosphorylation in SH-EP1 cells expressing human α4 and β2 nicotinic receptor subunits, we have demonstrated that incubation with nicotine for 24 h increased the expression of immature and mature forms of both α4 and β2 subunits in a concentration-dependent manner, and that inhibition of protein kinase C (PKC), but not cAMP-dependent protein kinase (PKA) inhibited the nicotine-induced increased expression of subunits. Incubation of cells with nicotine for 24 h also increased the phosphorylation of immature forms of α4 subunits similar to that induced by activation of either PKC or PKA. When cells were preincubated with nicotine, the PKC-mediated increased phosphorylation was inhibited; the PKA-mediated phosphorylation was unaltered. The phosphopeptide maps for immature α4 subunits following nicotine exposure or PKC activation were identical, and phosphoamino acid analyses indicated phosphorylation on serine residues only. Results indicate that nicotine-induced up regulation of α4β2 neuronal nicotinic receptors involves a PKC-dependent mechanism and likely reflects the ability of nicotine to activate PKC, leading to the phosphorylation of immature α4 subunits, promoting subunit assembly and receptor maturation. Because up regulation of these receptors has been implicated to mediate tolerance, locomotor sensitization and addiction to nicotine, results identify a potential new target for modulating the effects of nicotine on the brain.

Cyclic AMP-dependent protein kinase A and protein kinase C phosphorylate alpha4beta2 nicotinic receptor subunits at distinct stages of receptor formation and maturation

Neuronal nicotinic receptor alpha4 subunits associated with nicotinic alpha4beta2 receptors are phosphorylated by cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC), but the stages of receptor formation during which phosphorylation occurs and the functional consequences of kinase activation are unknown. SH-EP1 cells transfected with DNAs coding for human alpha4 and/or beta2 subunits were incubated with (32)Pi, and PKA or PKC was activated by forskolin or phorbol 12,13-dibutyrate, respectively. Immunoprecipitation and immunoblotting of proteins from cells expressing alpha4beta2 receptors or only alpha4 subunits were used to identify free alpha4 subunits, and alpha4 subunits present in immature alpha4beta2 complexes and mature alpha4beta2 pentamers containing complex carbohydrates. In the absence of kinase activation, phosphorylation of alpha4 subunits associated with mature pentamers was three times higher than subunits associated with immature complexes. PKA and PKC activation increased phosphorylation of free alpha4 subunits on different serine residues; only PKC activation phosphorylated subunits associated with mature alpha4beta2 receptors. Activation of both PKA and PKC increased the density of membrane-associated receptors, but only PKC activation increased peak membrane currents. PKA and PKC activation also phosphorylated beta2 subunits associated with mature alpha4beta2 receptors. Results indicate that activation of PKA and PKC leads to the phosphorylation alpha4beta2 receptors at different stages of receptor formation and maturation and has differential effects on the expression and function of human alpha4beta2 receptors.

Contribution of position alpha4S336 on functional expression and up-regulation of alpha4beta2 neuronal nicotinic receptors

Phosphorylation of the nicotinic acetylcholine receptor (nAChR) is believed to play a critical role in its nicotine-induced desensitization and up-regulation. We examined the contribution of a consensus PKC site in the alpha4 M3/M4 intracellular loop (alpha4S336) on the desensitization and up-regulation of alpha4beta2 nAChRs expressed in oocytes. Position alpha4S336 was replaced with either alanine to abolish potential phosphorylation at this site or with aspartic acid to mimic phosphorylation at this same site. Mutations alpha4S336A and alpha4S336D displayed a threefold increase in the ACh-induced response and an increase in ACh EC(50). Epibatidine binding revealed a three and sevenfold increase in surface expression for the alpha4S336A and alpha4S336D mutations, respectively, relative to wild-type, therefore, both mutations enhanced expression of the alpha4beta2 nAChR. Interestingly, the EC(50)'s and peak currents for nicotine activation remained unaffected in both mutants. Both mutations abolished the nicotine-induced up-regulation that is normally observed in the wild-type. The present data suggest that adding or removing a negative charge at this phosphorylation site cannot be explained by a simple straightforward on-and-off mechanism; rather a more complex mechanism(s) may govern the functional expression of the alpha4beta2 nAChR. Along the same line, our data support the idea that phosphorylation at multiple consensus sites in the alpha4 subunit could play a remarkable role on the regulation of the functional expression of the alpha4beta2 nAChR.

Cognitive deficits in schizophrenia: focus on neuronal nicotinic acetylcholine receptors and smoking

Patients with schizophrenia present with deficits in specific areas of cognition. These are quantifiable by neuropsychological testing and can be clinically observable as negative signs. Concomitantly, they self-administer nicotine in the form of cigarette smoking. Nicotine dependence is more prevalent in this patient population when compared to other psychiatric conditions or to non-mentally ill people. The target for nicotine is the neuronal nicotinic acetylcholine receptor (nAChR). There is ample evidence that these receptors are involved in normal cognitive operations within the brain. This review describes neuronal nAChR structure and function, focusing on both cholinergic agonist-induced nAChR desensitization and nAChR up-regulation. The several mechanisms proposed for the nAChR up-regulation are examined in detail. Desensitization and up-regulation of nAChRs may be relevant to the physiopathology of schizophrenia. The participation of several subtypes of neuronal nAChRs in the cognitive processing of non-mentally ill persons and schizophrenic patients is reviewed. The role of smoking is then examined as a possible cognitive remediator in this psychiatric condition. Finally, pharmacological strategies focused on neuronal nAChRs are discussed as possible therapeutic avenues that may ameliorate the cognitive deficits of schizophrenia.

The nicotinic acetylcholine receptor subunits Mdalpha5 and Mdbeta3 on autosome 1 of Musca domestica are not involved in spinosad resistance

Spinosad is a relatively new insecticide that exerts its toxic action via nicotinic acetylcholine receptors (nAChRs). Spinosad resistance in house flies appears to be due to an altered target site. To evaluate the molecular basis of spinosad resistance, two nAChR subunit genes, Mdalpha5 and Mdbeta3 were cloned and characterized from an insecticide-susceptible (aabys) and spinosad resistant (rspin) strain of the house fly, Musca domestica. The Mdalpha5 and Mdbeta3 cDNAs encode proteins of 781 and 432 amino acid residues, respectively. Phylogenetic analysis with insect nAChR subunits suggested that Mdalpha5 and Mdbeta3 are most closely related to Dalpha5 and Dbeta3 of Drosophila melanogaster, respectively. Mdbeta3 is intronless, which is unique among all previously described nAChR genes. A-to-I RNA editing was found at 13 sites in Mdalpha5, eleven of which resulted in amino acid substitutions. No evidence for A-to-I RNA editing was found in Mdbeta3. Mdalpha5 expression, quantified by real time PCR, was 340- and 23-fold higher in the head and thorax than in the abdomen. Mdbeta3 expression was more uniform, being only 2.4-fold higher in the head and 1.4-fold lower in the thorax, compared to the abdomen. There was no difference in the expression of Mdalpha5 and Mdbeta3 between the aabys and rspin strains. Although Mdalpha5 and Mdbeta3 both map to the same chromosome as spinosad resistance, there were no unique features of either gene in rspin, relative to the aabys strain. This suggests neither Mdalpha5 nor Mdbeta3 is responsible for spinosad resistance in house flies.

Cyclic AMP-dependent protein kinase (PKA) phosphorylates Ser362 and 467 and protein kinase C phosphorylates Ser550 within the M3/M4 cytoplasmic domain of human nicotinic receptor alpha4 subunits

Studies have suggested that the expression, translocation, and function of alpha4beta2 nicotinic receptors may be modulated by alpha4 subunit phosphorylation, but little direct evidence exists to support this idea. The objective of these experiments was to identify specific serine/threonine residues on alpha4 subunits that are phosphorylated in vivo by cAMP-dependent protein kinase and protein kinase C (PKC). To accomplish this, DNAs coding for human alpha4 subunits containing alanines in place of serines/threonines predicted to represent phosphorylation sites were constructed, and transiently transfected with the DNA coding for wild-type beta2 subunits into SH-EP1 cells. Cells were pre-incubated with (32)Pi and incubated in the absence or presence of forskolin or phorbol 12,13-dibutyrate. Immunoprecipitated alpha4 subunits were subjected to immunoblot, autoradiographic and phosphoamino acid analyses, and two-dimensional phosphopeptide mapping. Results confirmed the presence of two alpha4 protein bands, a major band of 71/75 kDa and a minor band of 80/85 kDa. Phosphoamino acid analysis of the major band indicated that only serine residues were phosphorylated. Phosphopeptide maps demonstrated that Ser362 and 467 on the M3/M4 cytoplasmic domain of the alpha4 subunit represent major cAMP-dependent protein kinase phosphorylation sites, while Ser550 also contained within this major intracellular loop is a major site for protein kinase C phosphorylation.

The nicotinic acetylcholine receptor subunit Mdalpha6 from Musca domestica is diversified via post-transcriptional modification

Recent studies showed that deletion of a nicotinic acetylcholine receptor (nAChR) subunit gene, Dalpha6 in Drosophila melanogaster results in a strain that is resistant to spinosad, indicating that Dalpha6 is important for the toxic action of this insecticide. To determine if spinosad resistance in house flies was due to a mutation(s) of Mdalpha6 (the orthologue of Dalpha6 from house flies), cDNAs were cloned and characterized from an insecticide-susceptible and a spinosad-resistant strain of the house fly, Musca domestica. The cDNAs contain a 1470-bp open reading frame encoding 490 amino acid residues, 415-bp 5' untranslated region (UTR) and a polymorphic 3'-UTR of approximately 371 bp. The predicted mature protein possesses 468 amino acid residues, has the typical features of a nAChR alpha subunit and is 97% identical to Dalpha6. Quantitative real-time PCR analysis revealed that Mdalpha6 was expressed in the head and the thorax at 1300- and 26-fold higher levels, respectively, than in the abdomen. There was no difference in the expression level of Mdalpha6 between spinosad-resistant and susceptible strains. Ten isoforms arising from alternative splicing were characterized, with isoform II being most common. A-to-I RNA editing was examined and found at 12 sites: editing at 11 of these sites resulted in an amino acid substitution. Mdalpha6 is linked to autosome 1 (spinosad resistance was previously shown to be linked to autosome 1). Single nucleotide polymorphisms, alternative splicing, mRNA levels and A-to-I RNA editing were compared between head and thorax and between insecticide-susceptible and spinosad-resistant strains. These comparisons indicate that Mdalpha6 is not responsible for spinosad resistance in house flies.

Characterization of the nicotinic acetylcholine receptor subunit gene Mdalpha2 from the house fly, Musca domestica

A nicotinic acetylcholine receptor (nAChR) subunit gene, Mdalpha2, was isolated and characterized from the house fly, Musca domestica. This is the first nAChR family member cloned from house flies. Mdalpha2 had a cDNA of 2,607 bp, which included a 696 bp 5'-untranslated region (UTR), an open reading frame of 1,692 bp, and a 219 bp 3'-UTR. Its deduced amino acid sequence possesses the typical characteristics of nAChRs. Mdalpha2 genomic sequence was 11.2 kb in length in the aabys strain and 10.9 kb in the OCR strain, including eight exons and seven introns. Based on the deduced amino acid sequence, Mdalpha2 had the closest phylogenetic relationship to the Drosophila melanogaster Dalpha2 and Anopheles gambiae Agamalpha2, and a similar genomic structure to Dalpha2. Quantitative real-time PCR analysis showed that Mdalpha2 is expressed in the head and the thorax at 150- and 8.5-fold higher levels than in the abdomen. Linkage analysis of a Mdalpha2 polymorphism indicates this gene is on autosome 2. The importance of these results in understanding the diversity and phylogenetic relationships of insect nAChRs, the physiology of nAChRs in the house fly, and the utility of nAChR sequences in resistance detection/monitoring is discussed.

Novel G423S Mutation of Human α7 Nicotinic Receptor Promotes Agonist-Induced Desensitization by a Protein Kinase C-Dependent Mechanism

The alpha7 nicotinic acetylcholine receptor subunit (CHRNA7) gene harbors a high degree of polymorphism. In this study, we found a novel variant (1267 G to A) in exon 10 of the CHRNA7 gene in a Japanese population. This variant results in glycine-to-serine substitution at position 423 (G423S) located in the large cytoplasmic loop of the protein. To clarify the possibility that the G423S mutation alters the pharmacological properties of alpha7 receptors, acetylcholine (ACh)-elicited current through alpha7-G423S mutant receptors expressed in Xenopus laevis oocytes was measured using the two-electrode voltage-clamp technique. We found that the current elicited by ACh (1 mM, 5 s) through alpha7-G423S receptors, but not through alpha7 receptors, was significantly decreased by treatment with a protein kinase C activator, phorbol-12-myristate-13-acetate (PMA, 10-30 nM). In addition, PMA (10 nM) selectively promoted a progressive decrease in alpha7-G423S current induced by repetitive application of ACh pulses (1 mM, 0.1 s, 0.17-0.33 Hz) compared with alpha7 current. PMA also enhanced the inactivation of alpha7-G423S mutant receptors induced by a prolonged application of choline (30 microM) without affecting alpha7 receptor responses. Western blot analysis showed that the treatment with PMA (30 nM) increased the serine phosphorylation level of the alpha7-G423S mutant receptors but not that of the wild-type receptors. These findings demonstrate that the G423S mutation promotes receptor desensitization by a protein kinase C-dependent mechanism. Thus, we provide the first evidence that a variant in the human CHRNA7 gene alters the function of alpha7 nicotinic receptors.

Sgbeta1, a novel locust (Schistocerca gregaria) non-alpha nicotinic acetylcholine receptor-like subunit with homology to the Drosophila melanogaster Dbeta1 subunit

The cloning, sequencing and functional expression of Sgbeta1, a novel locust (Schistocerca gregaria) non-alpha nicotinic acetylcholine receptor (nAChR) subunit is described. This subunit shows 80% identity with the Drosophila melanogaster Dbeta1 and 92% identity with the Locusta migratoria beta1, non-alpha subunits but only 38% identity to Sgalpha1 (also referred to as alphaL1), a previously cloned S. gregaria nAChR alpha-subunit. When expressed in Xenopus laevis oocytes, Sgbeta1 does not respond to nicotine. Responses to nicotine are observed, however, in oocytes co-expressing Sgalpha1 and Sgbeta1, but the pharmacology is indistinguishable from that of currents produced by expressing Sgalpha1 alone. We conclude that either Sgbeta1 does not co-assemble with Sgalpha1, or that it is unable to contribute to the functional properties of the receptor, in the Xenopus oocyte expression system.

Desensitized nicotinic receptors in brain

Desensitization is an intriguing characteristic of ligand-gated channels, whereby a decrease or loss of biological response occurs following prolonged or repetitive stimulation. Nicotinic acetylcholine receptors (nAChRs), as a member of transmitter gated ion channels family, also can be desensitized by continuous or repeated exposure to agonist. Desensitization of nicotinic receptors can occur as a result of extended nicotine exposure during smoking or prolonged acetylcholine when treatment of Alzheimer's disease (AD) with cholinesterase inhibitors, or anticholinesterase agent poisoning. Studies from our lab have shown that nAChRs desensitization is not a nonfunctional state and we proposed that desensitized nAChRs could increase sensitivity of brain muscarinic receptor to its agonists. Here, we will review the regulation of nicotinic receptor desensitization and discuss the important biological function of desensitized nicotinic receptors in light of our previous studies. These studies provide the critical information for understanding the importance of nicotinic receptors desensitization in both normal physiological processing and in various disease states.

Activities of cAMP-dependent protein kinase and protein kinase C are modulated by desensitized nicotinic receptors in the rat brain

When rats were treated with different dosages of nicotine, nicotinic acetylcholine receptors (nAChRs) were observed in activated, sub-acute desensitized, acute desensitized, and chronic desensitized states. The activities of cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) were assayed in the respective rat brains. The results showed that the activities of PKA and PKC could not be modified when brain nAChRs were in an activated state. However, the activities of PKA and PKC decreased when brain nAChRs were in a sub-acute state, an acute state or a chronic desensitized state induced by repeated administration of nicotine. These results suggest that desensitized nAChRs in the rat brain can inhibit the activities of PKA and PKC, which may be responsible for nicotine dependence.

Nicotinic acetylcholine receptors: from structure to brain function

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system where they are involved in fast synaptic transmission. nAChRs are pentameric structures that are made up of combinations of individual subunits. Twelve neuronal nAChR subunits have been described, alpha2-alpha10 and beta2-beta4; these are differentially expressed throughout the nervous system and combine to form nAChRs with a wide range of physiological and pharmacological profiles. The nAChR has been proposed as a model of an allosteric protein in which effects arising from the binding of a ligand to a site on the protein can lead to changes in another part of the molecule. A great deal is known about the structure of the pentameric receptor. The extracellular domain contains binding sites for numerous ligands, which alter receptor behavior through allosteric mechanisms. Functional studies have revealed that nAChRs contribute to the control of resting membrane potential, modulation of synaptic transmission and mediation of fast excitatory transmission. To date, ten genes have been identified in the human genome coding for the nAChRs. nAChRs have been demonstrated to be involved in cognitive processes such as learning and memory and control of movement in normal subjects. Recent data from knockout animals has extended the understanding of nAChR function. Dysfunction of nAChR has been linked to a number of human diseases such as schizophrenia, Alzheimer's and Parkinson's diseases. nAChRs also play a significant role in nicotine addiction, which is a major public health concern. A genetically transmissible epilepsy, ADNFLE, has been associated with specific mutations in the gene coding for the alpha4 or beta2 subunits, which leads to altered receptor properties.

Phosphorylation of the alpha4 subunit of human alpha4beta2 nicotinic receptors: role of cAMP-dependent protein kinase (PKA) and protein kinase C (PKC)

This study determined whether the alpha4 subunit of human alpha4beta2 neuronal nicotinic receptors is phosphorylated in situ by cyclic AMP-dependent protein kinase (PKA) or protein kinase C (PKC). To accomplish this, human cloned epithelial cells stably transfected with the human alpha4beta2 nicotinic receptor (SH-EP1-halpha4beta2) were incubated with 32P-orthophosphate to label endogenous ATP stores, and the phosphorylation of alpha4 subunits was determined in the absence or presence of PKA or PKC activation. Autoradiographs and immunoblots indicated that alpha4 subunits immunoprecipitated from a membrane preparation of SH-EP1-halpha4beta2 cells exhibited a single 32P-labeled band corresponding to the alpha4 subunit protein; no signals were associated with untransfected SH-EP1 cells. The alpha4 subunits from SH-EP1-halpha4beta2 cells incubated in the absence of the activators exhibited a basal level of phosphorylation that was decreased in the presence of the PKA inhibitor H-89 (5 microM), but unaltered in the presence of the PKC inhibitor Ro-31-8220 (0.1 microM). Activation of PKA by forskolin (10 microM), dibutyryl-cAMP (1 mM), or Sp-8-Br-cAMP (1 mM) enhanced phosphorylation nearly threefold; the inactive isomer, Rp-8-Br-cAMP (1 mM) had no effect. In addition, the forskolin effect was totally blocked by the PKA inhibitor H-89 (5 microM). Activation of PKC by the phorbol esters PDBu (200 nM) or PMA (200 nM) increased alpha4 subunit phosphorylation approximately twofold, and the PDBu effect was blocked by the selective PKC inhibitor Ro-31-8220 (0.1 microM). These findings indicate that the alpha4 subunit of human alpha4beta2 nicotinic receptors is phosphorylated in situ by PKA and PKC.

Phosphorylation sites within alpha4 subunits of alpha4beta2 neuronal nicotinic receptors: a comparison of substrate specificities for cAMP-dependent protein kinase (PKA) and protein kinase C (PKC)

The present study determined whether putative phosphorylation sites within the M3/M4 cytoplasmic domain of the human alpha4 subunit of alpha4beta2 neuronal nicotinic receptors are substrates for cAMP-dependent protein kinase (PKA) or protein kinase C (PKC). Five peptides corresponding to predicted phosphorylation sequences were synthesized, and phosphorylation was compared with standard peptide substrates for each kinase, that is, Kemptide for PKA and glycogen synthase (GS) 1-8 for PKC. VRCRSRSI had the highest affinity for PKA, with a Km of 44.5 microM; Kemptide had a Km of 7.7 microM. LMKRPSVVK and KARSLSVQH were also phosphorylated by PKA, but had lower affinities of 593 microM and 2896 microM, respectively. LMKRPSVVK had the highest affinity for PKC with a Km of 182 microM; GS 1-8 had a Km of 2.1 microM. VRCRSRSI had a comparative affinity for PKC with a Km of 327 microM. PCKCTCKK was not phosphorylated by PKA, but was a substrate for PKC with a Km of 1392 microM, whereas PGPSCKSP was not phosphorylated by either kinase. Based on these findings, results suggest that Ser-362 and Ser-486 on the human alpha4 subunit may be phosphorylated by either PKA or PKC, Ser-467 is a putative PKA site, and Thr-532 represents a likely PKC substrate; Ser-421 does not appear to be phosphorylated by either kinase.

Desensitization of neuronal nicotinic receptors

The loss of functional response upon continuous or repeated exposure to agonist, desensitization, is an intriguing phenomenon if not as yet a well-defined physiological mechanism. However, detailed evaluation of the properties of desensitization, especially for the superfamily of ligand-gated ion channels, reveals how the nervous system could make important use of this process that goes far beyond simply curtailing excessive receptor stimulation and the prevention of excitotoxicity. Here we will review the mechanistic basis of desensitization and discuss how the subunit-dependent properties and regulation of nicotinic acetylcholine receptor (nAChR) desensitization contribute to the functional diversity of these channels. These studies provide the essential framework for understanding how the physiological regulation of desensitization could be a major determinant of synaptic efficacy by controlling, in both the short and long term, the number of functional receptors. This type of mechanism can be extended to explain how the continuous occupation of desensitized receptors during chronic nicotine exposure contributes to drug addiction, and highlights the potential significance of prolonged nAChR desensitization that would also occur as a result of extended acetylcholine lifetime during treatment of Alzheimer's disease with cholinesterase inhibitors. Thus, a clearer picture of the importance of nAChR desensitization in both normal information processing and in various diseased states is beginning to emerge.

Conditional rescue of protein kinase C epsilon regulates ethanol preference and hypnotic sensitivity in adult mice

Conventional gene targeting is a powerful tool to study the influence of specific genes on behavior. However, conclusions relevant for adult animals are limited by consequences of gene loss during development. Mice lacking protein kinase C epsilon (PKCepsilon) consume less alcohol and show greater acute sensitivity to alcohol than do wild-type mice. There are no selective inhibitors of PKCepsilon that can be administered systemically and cross the blood-brain barrier to test whether these phenotypes result from loss of PKCepsilon during development or in adulthood. Here we used conditional expression of PKCepsilon in the basal forebrain, amygdala, and cerebellum to rescue wild-type responses to alcohol in adult PKCepsilon(-/-) mice. Subsequent suppression of transgenic PKCepsilon restored PKCepsilon(-/-) behaviors. These findings establish that PKCepsilon signaling in the adult brain regulates alcohol consumption and sensitivity. If this extends to humans, then PKCepsilon inhibitors might prove useful as novel therapeutics for alcoholism.

Identification of three cAMP-dependent protein kinase (PKA) phosphorylation sites within the major intracellular domain of neuronal nicotinic receptor alpha4 subunits

This study determined whether all protein kinase A (PKA) and protein kinase C (PKC) phosphorylation sites on the alpha4 subunit of rat alpha4beta2 neuronal nicotinic receptors could be localized to the M3/M4 cytoplasmic domain of the protein, and investigated specific amino acid substrates for the kinases through two-dimensional phosphopeptide mapping and site-directed mutagenesis. Experiments were conducted using alpha4beta2 receptors expressed in Xenopus oocytes and a fusion protein corresponding to the M3/M4 cytoplasmic domain of alpha4 (alpha4(333-594) ). When oocytes expressing alpha4beta2 receptors were incubated with [(32) P]orthophosphate in order to label endogenous ATP stores, phosphorylation of alpha4 subunits was evident. Incubation of either immunoprecipitated receptors or the fusion protein with [(32) P]ATP and either PKA or PKC followed by trypsinization of the samples demonstrated that the kinases phosphorylated alpha4 subunits on multiple phosphopeptides, and that the phosphorylated full-length alpha4 protein and fusion protein produced identical phosphopeptide maps. Site-directed mutagenesis of Ser365, Ser472 and Ser491 to alanines in the fusion protein eliminated phosphopeptides phosphorylated by PKA, but not by PKC. Other mutations investigated, Ser470, Ser493, Ser517 and Ser590, did not alter the phosphopeptide maps. Results indicate that Ser365, Ser472 and Ser491 on neuronal nicotinic receptor alpha4 subunits are phosphorylated by PKA and are likely to represent post-translational regulatory sites on the receptor.

Nicotinic acetylcholine receptors in autonomic ganglia

Although alpha3beta4 subunit combination is clearly prevalent in the nAChRs of autonomic ganglia neurons, the ganglia are strikingly different in the ratio of neurons containing each particular nAChR subunit, as found with immunohistochemical methods and from the analysis of the effects of nAChR subunit-specific antibodies on the ACh-induced membrane currents. In particular, the number of neurons containing alpha3, alpha4, alpha5 or alpha7 subunits is by about three times higher in sympathetic ganglia than in parasympathetic ganglia. This difference may explain why the parasympathetic and sympathetic ganglia markedly differ in their pharmacology. Still, alpha7 subunit makes the highest contribution to ACh-induced membrane current. No correlation between the physiological functions of the ganglia and subunit composition of their nAChRs has been found as yet. High permeability for Ca2+ should permit the nAChRs with alpha7 subunits to influence a variety of Ca2+-dependent events in autonomic neurons. As found with biochemical methods and site-directed mutagenesis, the ACh binding site is formed in the alpha/beta subunits interface by multiple loops containing cysteine, tyrosine and tryptophan amino residues as important for ACh binding. Likewise, both alpha and beta subunits are important for the effects of blocking agents on nAChRs. As found by electrophysiological methods, each neuron of sympathetic and parasympathetic ganglia, as a rule, possesses nAChRs of two groups, "fast" and "slow", with the mean duration of the burst of single channel openings ranging approximately from 5 to 10 and from 25 to 45 ms, respectively. These groups of channels differ from each other with their pharmacology. The burst-like activity of autonomic nAChRs channels is possible only if the disulfide bonds are left intact, otherwise only single openings of the channel are observed. The ionic channel of a nAChRs pentamer is formed by M2 transmembrane segments arranging glutamate, serine, threonine, leucine, and valine rings critical for channel conductance and ionic selectivity. In particular, the mutations V251T and E237A, and insertion of proline or alanine, convert a cation-selective channel into an anion-selective one. The open-channel blockers bind to the nAChR channel at the level where the channel diameter is nearly 12 A, both for "fast" and "slow" channel groups.

Characterization of human alpha4beta2 neuronal nicotinic receptors stably expressed in SH-EP1 cells

These studies characterized human alpha4beta2 neuronal nicotinic receptors stably expressed in a human epithelial cell line (SH-EP1). Receptors in transfected SH-EPI-halpha4beta2 cells were functional, as determined by increases in intracellular Ca2+ in response to a nicotine stimulus. Nicotine increased Fura-2 fluorescence in a concentration-dependent manner with an apparent EC50 of 2.4 microM, a response that was blocked by the specific antagonist mecamylamine. When cells were incubated in 50 nM nicotine for 24 hours, the Ca2+ response inactivated by 44%, an effect that recovered within 24 hours. SH-EP1-halpha4beta2 cells expressed a single class of high affinity binding sites for [3H]cytisine with a Kd of 0.63 +/- 0.08 nM and a Bmax of 6,797 +/- 732 femtomoles/mg protein. Incubation of cells with 50 nM nicotine for 24 hours increased the Bmax by 45% without changing affinity, a concentration-dependent effect with an EC50, of 58.6 nM. The nicotine-induced up regulation was reversible, and control values were achieved within 24 hours. Results indicate that SH-EPI-halpha4beta2 cells may be a good model system to study regulation of human alpha4beta2 receptors, the most abundant nicotinic receptor subtype in brain.