Alpha bungarotoxin-1.4 nm gold: a novel conjugate for visualising the precise subcellular distribution of alpha 7* nicotinic acetylcholine receptors

Chemical Synthesis of a Functional Fluorescent-Tagged α-Bungarotoxin

α-bungarotoxin is a large, 74 amino acid toxin containing five disulphide bridges, initially identified in the venom of Bungarus multicinctus snake. Like most large toxins, chemical synthesis of α-bungarotoxin is challenging, explaining why all previous reports use purified or recombinant α-bungarotoxin. However, only chemical synthesis allows easy insertion of non-natural amino acids or new chemical functionalities. Herein, we describe a procedure for the chemical synthesis of a fluorescent-tagged α-bungarotoxin. The full-length peptide was designed to include an alkyne function at the amino-terminus through the addition of a pentynoic acid linker. Chemical synthesis of α-bungarotoxin requires hydrazide-based coupling of three peptide fragments in successive steps. After completion of the oxidative folding, an azide-modified Cy5 fluorophore was coupled by click chemistry onto the toxin. Next, we determined the efficacy of the fluorescent-tagged α-bungarotoxin to block acetylcholine (ACh)-mediated currents in response to muscle nicotinic receptor activation in TE671 cells. Using automated patch-clamp recordings, we demonstrate that fluorescent synthetic α-bungarotoxin has the expected nanomolar affinity for the nicotinic receptor. The blocking effect of fluorescent α-bungarotoxin could be displaced by incubation with a 20-mer peptide mimicking the α-bungarotoxin binding site. In addition, TE671 cells could be labelled with fluorescent toxin, as witnessed by confocal microscopy, and this labelling was partially displaced by the 20-mer competitive peptide. We thus demonstrate that synthetic fluorescent-tagged α-bungarotoxin preserves excellent properties for binding onto muscle nicotinic receptors.

Cholinergic Synaptic Homeostasis Is Tuned by an NFAT-Mediated α7 nAChR-Kv4/Shal Coupled Regulatory System

Homeostatic synaptic plasticity (HSP) involves compensatory mechanisms employed by neurons and circuits to preserve signaling when confronted with global changes in activity that may occur during physiological and pathological conditions. Cholinergic neurons, which are especially affected in some pathologies, have recently been shown to exhibit HSP mediated by nicotinic acetylcholine receptors (nAChRs). In Drosophila central neurons, pharmacological blockade of activity induces a homeostatic response mediated by the Drosophila α7 (Dα7) nAChR, which is tuned by a subsequent increase in expression of the voltage-dependent K_v4/Shal channel. Here, we show that an in vivo reduction of cholinergic signaling induces HSP mediated by Dα7 nAChRs, and this upregulation of Dα7 itself is sufficient to trigger transcriptional activation, mediated by nuclear factor of activated T cells (NFAT), of the K_v4/Shal gene, revealing a receptor-ion channel system coupled for homeostatic tuning in cholinergic neurons.

Eadaim et al. show that in vivo reduction of cholinergic signaling in Drosophila neurons induces synaptic homeostasis mediated by Dα7 nAChRs. This upregulation of Dα7 induces K_v4/Shal gene expression mediated by nuclear factor of activated T cells (NFAT), revealing a receptor-ion channel system coupled for homeostatic tuning in cholinergic neurons.

Cholinergic Modulation of Neuroinflammation: Focus on α7 Nicotinic Receptor

All nervous system pathologies (e.g., neurodegenerative/demyelinating diseases and brain tumours) develop neuroinflammation, a beneficial process during pathological events, aimed at removing damaged cells, toxic agents, and/or pathogens. Unfortunately, excessive inflammation frequently occurs during nervous system disorders, becoming a detrimental event capable of enhancing neurons and myelinating glial cell impairment, rather than improving their survival and activity. Consequently, targeting the neuroinflammation could be relevant for reducing brain injury and rescuing neuronal and glial cell functions. Several studies have highlighted the role of acetylcholine and its receptors in the regulation of central and peripheral inflammation. In particular, α7 nicotinic receptor has been described as one of the main regulators of the “brain cholinergic anti-inflammatory pathway”. Its expression in astrocytes and microglial cells and the ability to modulate anti-inflammatory cytokines make this receptor a new interesting therapeutic target for neuroinflammation regulation. In this review, we summarize the distribution and physiological functions of the α7 nicotinic receptor in glial cells (astrocytes and microglia) and its role in the modulation of neuroinflammation. Moreover, we explore how its altered expression and function contribute to the development of different neurological pathologies and exacerbate neuroinflammatory processes.

Cholinergic Homeostatic Synaptic Plasticity Drives the Progression of Aβ-Induced Changes in Neural Activity

Homeostatic synaptic plasticity (HSP) is the ability of neurons to exert compensatory changes in response to altered neural activity. How pathologically induced activity changes are intertwined with HSP mechanisms is unclear. We show that, in cholinergic neurons from Drosophila, beta-amyloid (Aβ) peptides Aβ40 and Aβ42 both induce an increase in spontaneous activity. In a transgenic line expressing Aβ42, we observe that this early increase in spontaneous activity is followed by a dramatic reduction in spontaneous events, a progression that has been suggested to occur in cholinergic brain regions of mammalian models of Alzheimer's disease. We present evidence that the early enhancement in synaptic activity is mediated by the Drosophila α7 nicotinic acetylcholine receptor (nAChR) and that, later, Aβ42-induced inhibition of synaptic events is a consequence of Dα7-dependent HSP mechanisms induced by earlier hyperactivity. Thus, while HSP may initially be an adaptive response, it may also drive maladaptive changes and downstream pathologies.

FluoroNanogold: an important probe for correlative microscopy

Correlative microscopy is a powerful imaging approach that refers to observing the same exact structures within a specimen by two or more imaging modalities. In biological samples, this typically means examining the same sub-cellular feature with different imaging methods. Correlative microscopy is not restricted to the domains of fluorescence microscopy and electron microscopy; however, currently, most correlative microscopy studies combine these two methods, and in this review, we will focus on the use of fluorescence and electron microscopy. Successful correlative fluorescence and electron microscopy requires probes, or reporter systems, from which useful information can be obtained with each of the imaging modalities employed. The bi-functional immunolabeling reagent, FluoroNanogold, is one such probe that provides robust signals in both fluorescence and electron microscopy. It consists of a gold cluster compound that is visualized by electron microscopy and a covalently attached fluorophore that is visualized by fluorescence microscopy. FluoroNanogold has been an extremely useful labeling reagent in correlative microscopy studies. In this report, we present an overview of research using this unique probe.

Live Imaging of Nicotine Induced Calcium Signaling and Neurotransmitter Release Along Ventral Hippocampal Axons

Sustained enhancement of axonal signaling and increased neurotransmitter release by the activation of pre-synaptic nicotinic acetylcholine receptors (nAChRs) is an important mechanism for neuromodulation by acetylcholine (ACh). The difficulty with access to probing the signaling mechanisms within intact axons and at nerve terminals both in vitro and in vivo has limited progress in the study of the pre-synaptic components of synaptic plasticity. Here we introduce a gene-chimeric preparation of ventral hippocampal (vHipp)-accumbens (nAcc) circuit in vitro that allows direct live imaging to analyze both the pre- and post-synaptic components of transmission while selectively varying the genetic profile of the pre- vs post-synaptic neurons. We demonstrate that projections from vHipp microslices, as pre-synaptic axonal input, form multiple, reliable glutamatergic synapses with post-synaptic targets, the dispersed neurons from nAcc. The pre-synaptic localization of various subtypes of nAChRs are detected and the pre-synaptic nicotinic signaling mediated synaptic transmission are monitored by concurrent electrophysiological recording and live cell imaging. This preparation also provides an informative approach to study the pre- and post-synaptic mechanisms of glutamatergic synaptic plasticity in vitro.

α7 Nicotinic receptor agonist enhances cognition in aged 3xTg-AD mice with robust plaques and tangles

Alzheimer disease (AD) is a progressive neurodegenerative disorder with associated memory loss, spatial disorientation, and other psychiatric problems. Cholinergic system dysfunction is an early and salient feature of AD, and enhancing cholinergic signaling with acetylcholinesterase inhibitors is currently the primary strategy for improving cognition. The beneficial effects of acetylcholinesterase inhibitors, however, are typically short-lived and accompanied by adverse effects. Recent evidence suggests that activating α7 nicotinic acetylcholine receptors (α7 nAChR) may facilitate the specific modulation of brain cholinergic signaling, leading to cognitive enhancement and possibly to amelioration of AD pathologic findings. In the present study, we determined the effect of long-term treatment with the selective α7 nAChR agonist A-582941 in aged 3xTg-AD mice with robust AD-like pathology, which is particularly significant not only because this is the only mouse model that co-develops amyloid plaques and neurofibrillary tangles but also because it enabled us to explore whether A-582941 is able to restore brain function after the severe damage associated with AD. Analysis of β-amyloid deposits, tau phosphorylation, and inflammatory cells revealed that, overall, pathologic findings were unchanged. Rather, α7 nAChR activation induced expression of c-Fos and brain-derived neurotrophic factor and phosphorylation of cyclic adenosine monophosphate response element binding and neurotrophic tyrosine receptor kinase type 2. More important, A-582941 completely restored cognition in aged 3xTg-AD mice to the level of that in age-matched nontransgenic mice. These novel findings indicate that activating α7 nAChR is a promising treatment for cognitive impairment in AD.

Butyrylcholinesterase genotype and gender influence Alzheimer's disease phenotype

Retrospective data are presented to support a spectrum of early Alzheimer's disease (AD) along a continuum defined by gender and genotype. The putative neurodegenerative mechanisms driving distinct phenotypes at each end of the spectrum are glial hypoactivity associated with early failure of synaptic cholinergic neurotransmission and glial overactivation associated with loss of neural network connectivity due to accelerated age-related breakdown of myelin. In early AD, male butyrylcholinesterase K-variant carriers with one or two apolipoprotein ɛ4 alleles have prominent medial temporal atrophy, synaptic failure, cognitive decline, and accumulation of aggregated beta-amyloid peptide. Increasing synaptic acetylcholine in damaged but still functional cholinergic synapses improves cognitive symptoms, whereas increasing the ability of glia to support synapses and to clear beta-amyloid peptide might be disease-modifying. Conversely, chronic glial overactivation can also drive degenerative processes and in butyrylcholinesterase K-variant negative females generalized glial overactivation may be the main driver from mild cognitive impairment to AD. Females are more likely than males to have accelerated age-related myelin breakdown, more widespread white matter loss, loss of neural network connectivity, whole brain atrophy, and functional decline. Increasing extracellular acetylcholine levels blocks glial activation, reduces myelin loss and damage to neural network connectivity, and is disease-modifying. Between extremes characterized by gender, genotype, and age, pathophysiology may be mixed and this spectrum may explain much of the heterogeneity of amnestic mild cognitive impairment. Preservation of the functional integrity of the neural network may be an important component of strengthening cognitive reserve and significantly delaying the onset and progression of dementia, particularly in females. Prospective confirmation of these hypotheses is required. Implications for future research and therapeutic opportunities are discussed.

Preparation and high-resolution microscopy of gold cluster labeled nucleic acid conjugates and nanodevices

Nanogold and undecagold are covalently linked gold cluster labels which enable the identification and localization of biological components with molecular precision and resolution. They can be prepared with different reactivities, which means they can be conjugated to a wide variety of molecules, including nucleic acids, at specific, unique sites. The location of these sites can be synthetically programmed in order to preserve the binding affinity of the conjugate and impart novel characteristics and useful functionality. Methods for the conjugation of undecagold and Nanogold to DNA and RNA are discussed, and applications of labeled conjugates to the high-resolution microscopic identification of binding sites and characterization of biological macromolecular assemblies are described. In addition to providing insights into their molecular structure and function, high-resolution microscopic methods also show how Nanogold and undecagold conjugates can be synthetically assembled, or self-assemble, into supramolecular materials to which the gold cluster labels impart useful functionality.

Amyloid beta from axons and dendrites reduces local spine number and plasticity

Excessive synaptic loss is thought to be one of the earliest events in Alzheimer's disease. Amyloid beta (Abeta), a peptide secreted in an activity-modulated manner by neurons, has been implicated in the pathogenesis of Alzheimer's disease by removing dendritic spines, sites of excitatory synaptic transmission. However, issues regarding the subcellular source of Abeta, as well as the mechanisms of its production and actions that lead to synaptic loss, remain poorly understood. In rat organotypic slices, we found that acute overproduction of either axonal or dendritic Abeta reduced spine density and plasticity at nearby ( approximately 5-10 mum) dendrites. The production of Abeta and its effects on spines were sensitive to blockade of action potentials or nicotinic receptors; the effects of Abeta (but not its production) were sensitive to NMDA receptor blockade. Notably, only 30-60 min blockade of Abeta overproduction permitted induction of plasticity. Our results indicate that continuous overproduction of Abeta at dendrites or axons acts locally to reduce the number and plasticity of synapses.

Presence of alpha7 nicotinic acetylcholine receptors on dorsal root ganglion neurons proved using knockout mice and selective alpha-neurotoxins in histochemistry

In complex tissues where multiple subtypes of nicotinic acetylcholine receptors (nAChRs) are expressed, immunohistochemistry has been the most popular tool for investigation of nAChR subunit distribution. However, recent studies with nAChR subunit knockout mice demonstrated that a large panel of antibodies is unsuitable. Thus, we aimed to develop a histochemical method for selective labeling of alpha7 nAChR with neurotoxins, utilizing alpha7 nAChR-transfected cells, dorsal root ganglia (DRG) and spinal cord from wild-type and knockout mouse. The specificity of Alexa Fluor 488-conjugated alpha-bungarotoxin (Alexa-alphaBgt) was demonstrated in binding to alpha7-transfected cells inhibited by long-chain alpha-cobratoxin (CTX), but not short-chain alpha-neurotoxin II (NTII). In contrast, binding to Torpedo muscle-type nAChRs and to motor end plates in mouse tongue sections was prevented by both CTX and NTII. In tissue sections of DRG, expressing all neuronal nAChR subunits, only CTX precluded Alexa-alphaBgt labeling of neurons, with no staining for alpha7 nAChR knockout tissue. It proved that alpha7 nAChRs are the major alphaBgt-binding sites in mouse DRG. Corresponding results were obtained for terminals in the spinal cord. Thus, we present a protocol utilizing Alexa-alphaBgt and non-labeled CTX/NTII that allows specific histochemical detection of alpha7 nAChR with a spatial resolution at the level of single axon terminals.

Influence of Neuronal Nicotinic Receptors over Nicotine Addiction and Withdrawal

Cigarette smoking represents an enormous, global public health threat. Nearly five million premature deaths during a single year are attributable to smoking. Despite the resounding message of risks associated with smoking and numerous public health initiatives, cigarette smoking remains the most common preventable cause of disease in the United States. Fortunately, even in an adult smoker, smoking cessation can reverse many of the potential harmful effects. The symptoms associated with nicotine withdrawal represent the major obstacle to smoking cessation. This minireview examines the roles of various nicotinic receptors in the mechanisms of nicotine dependence, discusses the potential role of the habenula-interpeduncular nucleus axis in nicotine withdrawal, and highlights nicotinic receptors containing the β4 subunit as a potential pharmacological target for smoking cessation strategies.

Alpha7 nicotinic acetylcholine receptor expression in Alzheimer's disease: receptor densities in brain regions of the APP(SWE) mouse model and in human peripheral blood lymphocytes

The brains of people with Alzheimer's disease (AD) display several characteristic pathological features, including deposits (plaques) of beta-amyloid 1-42 (Abeta1-42), intraneuronal accumulations (tangles) of hyperphosphorylated tau, degeneration of the basal forebrain cholinergic pathway, and gliosis. Abeta1-42 plaques develop in specific brain regions, including hippocampus and cortex, as well as in the vasculature. Abeta1-42 might promote neurodegeneration through the induction of free radicals and disruption of Ca2+ homeostasis, giving rise to the symptoms of AD. Abeta1-42 interacts with the alpha7 subtype of the nicotinic acetylcholine receptor (alpha7 nAChR), which is widely expressed throughout the central and peripheral nervous systems, as well as in several nonneuronal loci, such as epithelial cells, lymphoid tissues, and peripheral blood lymphocytes. Western blot and autoradiographic analyses indicate that the alpha7 nAChR subunit protein is up-regulated in human brain samples from Alzheimer patients, as well as in animal models of AD (Dineley et al., 2001; Bednar et al., 2002), and might be involved in nicotine-mediated reduction of Abeta1-42 deposition (Hellstrom et al., 2004), although the nature of this relationship remains ill-defined. We have undertaken a semiquantitative histological evaluation of alpha7 nAChR expression in a mouse model of AD pathology, as well as a comparison of alpha7 nAChR levels in lymphocytes from AD patients and control subjects.

Evaluating the suitability of nicotinic acetylcholine receptor antibodies for standard immunodetection procedures

Nicotinic acetylcholine receptors play important roles in numerous cognitive processes as well as in several debilitating central nervous system (CNS) disorders. In order to fully elucidate the diverse roles of nicotinic acetylcholine receptors in CNS function and dysfunction, a detailed knowledge of their cellular and subcellular localizations is essential. To date, methods to precisely localize nicotinic acetylcholine receptors in the CNS have predominantly relied on the use of anti-receptor subunit antibodies. Although data obtained by immunohistology and immunoblotting are generally in accordance with ligand binding studies, some discrepancies remain, in particular with electrophysiological findings. In this context, nicotinic acetylcholine receptor subunit-deficient mice should be ideal tools for testing the specificity of subunit-directed antibodies. Here, we used standard protocols for immunohistochemistry and western blotting to examine the antibodies raised against the alpha3-, alpha4-, alpha7-, beta2-, and beta4-nicotinic acetylcholine receptor subunits on brain tissues of the respective knock-out mice. Unexpectedly, for each of the antibodies tested, immunoreactivity was the same in wild-type and knock-out mice. These data imply that, under commonly used conditions, these antibodies are not suited for immunolocalization. Thus, particular caution should be exerted with regards to the experimental approach used to visualize nicotinic acetylcholine receptors in the brain.

Precise localization of alpha7 nicotinic acetylcholine receptors on glutamatergic axon terminals in the rat ventral tegmental area

Alpha7 neuronal nicotinic acetylcholine receptors (nAChRs) constitute one of the predominant nAChR subtypes in the mammalian brain. Within the ventral tegmental area (VTA), nicotine application, paired with postsynaptic stimulation, contributes to a form of long-term potentiation, an effect attributed to presynaptic alpha7 nAChRs on glutamatergic afferents (Mansvelder and McGehee, 2000). The aim of this study was to examine the precise subcellular distribution of alpha7 nAChRs in the adult rat VTA to establish whether these receptors are indeed present on glutamatergic axon terminals and to determine their relationship with cholinergic afferents. The spatial relationship between alpha7 nAChRs, labeled using the alpha7 nAChR-specific antagonist alpha-bungarotoxin, and the local neurochemical environment was investigated by the application of multiple labeling strategies with antibodies against tyrosine hydroxylase, vesicular glutamate transporters (VGluTs), vesicular acetylcholine transporter, and glial fibrillary acidic protein. alpha7 nAChRs were localized at both somatodendritic and presynaptic loci within the VTA: on subpopulations of dopaminergic and nondopaminergic neurons and glutamatergic and nonglutamatergic terminals. There was no detectable alpha7 nAChR expression within astrocytes in the VTA. Most alpha7 nAChRs were cytoplasmic (82%), and the remainder were associated with the plasma membrane. Most presynaptic receptors (75%) were on glutamatergic axon terminals, with similar levels of alpha-bungarotoxin binding present on both VGluT1- and VGluT2-immunoreactive boutons. Both preembedding and postembedding electron microscopy revealed that presynaptic alpha7 nAChRs are often located at extrasynaptic (27%) and perisynaptic (61%) loci. alpha7 nAChRs were not associated with cholinergic synapses, consistent with their activation by a paracrine mode of acetylcholine or choline delivery.