Current Drug Targets - Volume 13, Issue 5, 2012

The α7-nicotinic acetylcholine receptor (α7-nAChR) is widely known as a neurotransmitter receptor in nervous systems. α7-nAChR is also present in a variety of non-neuronal tissues, where it has been implicated in the regulation of essential cellular functions including proliferation, survival, differentiation and communication. We have recently found in breast cancer that α7-nAChR is involved in the proliferation of cancer stem cells, which constitute a minor subpopulation responsible for tumor development and metastasis. Since growing evidence suggests that α7-nAChR is present not only in mature tissues and organs but also in undifferentiated stem cells and progenitor cells, α7-nAChR emerges as a key mediator in the regulation of self-renewal and differentiation. We provide here an overview of the recent works on the expression and function of α7-nAChR in normal and cancer stem cells, and their relevance to disease-related cellular dysfunction. Understanding the role of α7-nAChR in stem cells would be of great interest for its application potential in drug discovery and in regenerative medicine.

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In the years from 1856 to 1936, when the Nobel Prize for Physiology/Medicine was awarded to Dale and Loewi " for their discoveries relating to chemical transmission of nerve impulses" , the nicotinic acetylcholine receptor (nAChR) emerged from an assumption to a reality. Its biochemical isolation in 1970 represents a major breakthrough in pharmacology. The α7-nAChR subunit forms homo-oligodimeric nAChR with unique distinctive properties, such as high permeability to calcium and modulation by the extracellular calcium concentrations, the possibility of binding two-five molecules of agonist, function modulation via phosphorylation and/or via calcium-dependent serine/threonine kinases and modulating transmitter release and activation of GABAergic interneurons. In the brain, the α7-nAChR plays several important functions running from synaptic plasticity, regulation of neuronal growth, differentiation and survival to enhance learning and cognition. The detection of its occurrence on non-neuronal cells raises question related to their specific activity, since in these cells it appears involved in modulation of cell death, migration and signaling. Its unbalance might involve it in different diseases such as Alzheimer, Parkinson and cancer. However, the peculiarity of α7-nAChR offers rational bases to develop new drugs and new therapeutic strategies. In conclusion, α7-nAChR roughly in 150 years of life, instead to be an old actor became an important player in regulating cell signaling.

The alpha7 nicotinic acetylcholine receptor (α7 nAChR) is widely distributed in the human brain and has been implicated in a number of human central nervous system (CNS) diseases, including Alzheimer’s and Parkinson’s disease, schizophrenia and autism. Recently, new roles for α7 nAChRs in lung cancer and heart disease have been elucidated. Despite the importance of this receptor in human pathology, many technical difficulties are still encountered when investigating the role of α7 nAChRs. Electrophysiological analysis of the receptor upon heterologous expression or in human tissues was limited by the fast desensitization of α7-mediated nicotinic currents and by tissue availability. In addition, animal models for the human diseases related to α7 nAChRs have long been unavailable. The recent development of new imaging and analysis approaches such as PET and receptor microtransplantation have rendered the study of α7 nAChRs increasingly feasible, paving new roads to the design of therapeutic drugs. This review summarizes the current knowledge and recent findings obtained by these novel approaches.

Nicotinic acetylcholine receptors play a major role in the regulation of electrochemical synapses at neuromuscular junctions. During the early stages of Paracentrotus lividus development, the nicotinic receptor-like molecules are found and localized by use of the specific blocker, -bungarotoxin, and by α-7 subunit immunoreactivity. Both the methods identify and localize the nicotinic receptor-like molecules at the sites where active changes in ionic intracellular concentration take place. These are well known to lead either fertilization, sperm propulsion or co-ordinated ciliary movement. After neural differentiation, immunoreactivity for the α-7 subunit is localized mainly in ganglia, ectoderm ciliary bands and in the motile cells forming the gut wall. Both α-bungarotoxin binding sites and α-7 subunits are also localized at the cells linked to the skeletal rods, performing the small movements which drive the swimming direction in the water column. The localization of these molecules paves the way to a speculation on their function and possible role in neurogenesis as well as neurodegeneration.

Molecular imaging of brain structures by highly sensitive non-invasive techniques offers unique possibilities in the understanding of physiological and pathological processes in the central nervous system. In particular, the quantitative analysis by positron emission tomography (PET) of α7 nicotinic acetylcholine receptors (α7 nAChR), which are involved in different signalling pathways in the brain, is assumed to provide important information on the relation between receptor dysfunction and the pathogeneses of neuropsychiatric brain diseases, but the applicability of this imaging approach is still hampered due to insufficient imaging agents. This paper presents the recent efforts made to develop PET radiotracers targeting α7 nAChR as well as the current state of the evaluation of the most promising radiolabelled compounds in animal models and humans.

Nicotinic α7 receptors have been shown in a variety of studies with animal models to play important roles in diverse components of cognitive function, including learning, memory and attention. Mice with α7 receptor knockouts show impairments in memory. Selective α7 agonists significantly improve learning, memory and attention. α7 receptors in limbic structures such as the hippocampus and amygdala have been demonstrated to play critical roles in memory. Blockade of α7 receptors in these areas cause memory impairments. In the brains of people with schizophrenia α7 receptors are impaired. This may be related to pronounced cognitive impairments seen with schizophrenia. There has been a major effort to develop α7 nicotinic agonists for helping to reverse cognitive impairment. These receptors are a promising target for development of therapeutic treatments for a variety of diseases of cognitive impairment including Alzheimer’s disease, attention deficit hyperactivity disorder (ADHD) and schizophrenia.

Nicotine is well known for its deleterious effects on human health, and it has long been known that nicotine interacts with the stress axis in both man and in laboratory animals. Nicotine also has beneficial effects upon cognition, and an emerging literature has demonstrated that it may play a protective or palliative role in diseases such as Alzheimer’s disease and schizophrenia. Recent advances have permitted scientists to identify the specific subtypes of nicotinic receptors responsible for the drugs varied physiological effects. The α7 subunit of the nicotinic acetylcholine receptor (NAchRα7), has been identified as a significant mediator of nicotine’s interactions with the stress axis and human disease. The NAchRα7 has also been shown to have neuroprotective effects via multiple pathways, making it a logical target for the treatment of a number of brain disorders.

One of the early signs of Alzheimer’s disease is the impairment in hippocampus-based episodic memory function, which is improved through the enhancement of cholinergic transmission. Several studies suggest that α7 nicotinic receptor (nAChR) activation represents a useful therapeutic strategy for the cognitive impairments associated with early Alzheimer’s disease as the α7 subtype of nicotinic acetylcholine receptors are expressed by basal forebrain cholinergic projection neurons as well as by their targets in the hippocampus. The current model for the cholinergic deficit in Alzheimer’s disease posits that inappropriate accumulation of misfolded oligomeric aggregates of β-amyloid peptide leads to the dysfunction of the signaling mechanisms that support the cholinergic phenotype; this is manifested as an altered function of nicotinic acetylcholine receptors and the nerve-growth factor trophic support system that results in the loss of cholinergic markers and eventually cholinergic neurons from the basal forebrain cholinergic system. A view was confounded by the fact that α7 nAChRs and β-amyloid peptides have been shown to interact in vitro and in vivo, including human post-mortem AD brain. This review will begin with a brief overview of the basal forebrain cholinergic system, followed by a discussion of the current knowledge of the cholinergic deficit in Alzheimer’s disease, then a summary of the cholinergic phenotype observed in transgenic Alzheimer’s disease mouse models. We will also present our recent findings that support our hypothesis that the α7 nicotinic acetylcholine receptor performs both the neurotrophic and neuroprotective roles in the maintenance of the cholinergic phenotype and discusses potential mechanisms and implications for Alzheimer’s disease therapy.

Parkinson’s disease (PD) is characterized by relatively selective degeneration of dopaminergic neurons in the substantia nigra and loss of dopamine in the striatum. More than 50 epidemiological studies confirmed the low incidence of PD in smokers. Examining the distribution of subtypes of nicotinic acetylcholine receptors (nAChRs) in dopaminergic neurons of nigrostriatal system and its change in PD patients is quite important to elucidate possible neuroprotective cascade triggered by nicotine. Evidences of nAChR-mediated protection against neurotoxicity induced by rotenone, 6- hydroxydopamine (6-OHDA), and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are briefly reviewed. In rotenone- and 6-OHDA-induced PD models, nAChR-mediated neuroprotection was blocked not only by α4β2 but also by α7 nAChR antagonists. The survival signal transduction, α7 nAChR-Src family-PI3K-Akt/PKB cascade and subsequent upregulation of Bcl-2, would lead to neuroprotection. These findings suggest that nAChR-mediated neuroprotection is achieved through subtypes of nAChRs and common signal cascades. An early diagnosis and protective therapy with specific nAChR modulations could be effective in delaying the progression of PD.

Traumatic brain injury (TBI) is a significant public health concern worldwide for which there is no cure. Once trauma has occurred, multiple biochemical pathways are set into motion that leads to a chronic, neurodegenerative condition. Two of the most widely studied pathological pathways are excitotoxicity and inflammation, processes that are influenced by α7 nicotinic acetylcholine receptors (nAChR). Previous studies have found a bilateral decrease in α7 nAChR expression in regions of the cortex and hippocampus that occurs in relation to injury severity. Subsequent studies showed that this decrease was evident in some parts of the hippocampus as early as 1 hour post-injury and remained decreased through 21 days. Other ligand-gated ion channels, such as non-α7 nAChRs and n-methyl-D-aspartate (NMDA) receptors did not show a similar widespread and consistent pattern of change following TBI, nor did the G-protein coupled muscarinic acetylcholine receptors, suggesting that the α7 nAChR could be a key mediator in the pathophysiology of traumatic brain injury. In addition to its expression in the brain, the α7 nAChR has been found outside of the central nervous system (CNS) on many different cell types, including peripheral blood leukocytes, where they have a role in the cholinergic antiinflammatory pathway, and have recently been identified on platelets where they may have a role in activation. How these receptors are regulated in response to injury has not been investigated, but could potentially serve as a marker of neurodegeneration as has been done in Alzheimer's disease and schizophrenia. In this review, we will detail the role of α7 nAChR following TBI as well as explore the evidence of this receptor subtype in regards to blood component (leukocytes and platelets) involvement and the potential influence TBI has on peripheral expression and function.

The presence of memory impairment and cognitive deficits in the Alzheimer’s disease (AD), dementia with Lewy bodies (DLB), and Pick’s disease (PiD) has been associated to dysfunction of cholinergic transmission, possibly due to the loss of cholinergic neurons and to the elimination of nAChR in dementia patients. Alternative hypotheses take into account molecular interactions of the β-amyloid peptide Aβ with nAChR, which may lead to deregulation of the receptor function. Genetic polymorphisms of CHRNA7 and CHRFAM7A, a fusion gene containing a partial CHRNA7 duplication, have been investigated as possible susceptibility traits to dementia, potentially useful either to identify high risk individuals or as therapeutic targets. To summarize the existing evidence, a systematic re-evaluation of published papers has been performed (PubMed database, no language restriction, updated to 1st August 2011). Eleven articles reporting data on genetic variations in CHRNA7 or CHRFAM7 and risk of dementia fulfilled selection criteria and were evaluated. Published evidence on the association between variations in CHRNA7 or CHRFAM7A and the risk of dementia is still sparse and inconclusive. Further studies are needed to establish whether some polymorphisms may really affect the probability of developing AD or other forms of dementia. Additional and more conclusive results may come from the ongoing GWAS studies investigating high numbers of genetic variants in large samples, that have the potential to assess the role of genetic susceptibility in dementia.

The alkaloid nicotine, a major addictive component of tobacco, exerts anti-inflammatory and immunemodulating activities on multiple cell types, such as T cells, B cells, dendritic cells, mononuclear phagocytes and polymorphonuclear leukocytes, in lung, spleen, liver, kidney and gastrointestinal tract. In addition, nicotine may blunt pro-inflammatory cytokine release, with prominent effects on T helper type 1 (Th1) and Th17 cytokines. The nonneuronal α7-nicotinic cholinergic receptors are a primary target for nicotine through the JAK2 and STAT3/NF-κB pathways, ultimately mediating the inhibition of pro-inflammatory gene transcription. The present paper reviews the growing evidence in favor of detrimental as well as beneficial effects of nicotine and other α7-nicotininc receptor agonists in pre-clinical models of organ-specific and systemic inflammatory and autoimmune diseases. These data may portend favorable implications for the targeted treatment of chronic and debilitating human disorders, such as diabetes, arthritis, asthma and inflammatory bowel disease, with α7-selective ligands.

This review examines the role of α7 nAChR in different types of airway epithelial cells of the normal human bronchial tree. In each of these cells α7 nAChR activation elicits a specific effect. The effect is essentially mitogenic, whereas in the airway basal cells are antiproliferative. It is postulated that α7 nAChR may have mitogenic or antiproliferative signals differentially activated in different types of airway cells and under exposure to exogenous stimuli such as nicotine.

Cigarette smoking is strongly correlated with many diseases like cancer, cardiovascular disease and macular degeneration. Nicotine, the main active and addictive component of tobacco smoke has recently been shown to enhance angiogenesis in many experimental systems and animal models. The pro-angiogenic activity of nicotine is mediated by nicotinic acetylcholine receptors, particularly the alpha 7 subunit, that are expressed on a variety of non-neuronal cells including those in the vasculature such as endothelial cells and smooth muscle cells. The present review focuses on the role of α7nAChR in mediating the pro-angiogenic effects of nicotine and describes the molecular mechanisms involved in nicotine-induced angiogenesis as well as epithelial to mesenchymal transition. These observations on nicotine function highlight the therapeutic potential of α7nAChR agonists and antagonists for combating angiogenesis related diseases.

Exposure to tobacco products is responsible for the majority of all human cancers. Nicotinic acetylcholine receptors (nAChRs) were identified as early as 1989 as important regulators of cancer cells. In analogy to its function in the brain, the homomeric α7nAChR has “accelerator function” on the most common human cancers by stimulating the synthesis and release of excitatory neurotransmitters (serotonin in small cell lung cancer, noradrenaline/adrenaline in most other cancers) that drive cell proliferation, migration, angiogenesis, neurogenesis and metastasis while inhibiting apoptosis. These effects are not only caused by α7nAChRs expressed in cancer cells but also by α7nAChRs in ganglia and nerves of the sympathetic part of the autonomic nervous system that release noradrenaline/adrenaline into the tumor environment. In the nervous system, α7nAChR protein undergoes paradoxical upregulation without concomitant desensitization upon chronic exposure to nicotine. The same phenomenon has been reported for α7nAChR expressed in cancer cells of the lungs and pancreas where chronic nicotine or nicotine-derived nitrosamines upregulated the receptor protein, resulting in hyperactivity of its effectors. Strategies that target the α7nAChR for cancer intervention are highly promising but should aim to reduce signaling downstream of the receptor rather than blocking the receptor because of its numerous vital functions in the mammalian organism.

This paper discusses the potential therapeutic effect of α7-nAChR antagonists for NSCLC (non small cell lung cancer) and MPM (malignant pleural mesothelioma). This therapeutic approach is based on the experimental observations that: (a) functional α7-nAChR are expressed in NSCLC and MPM cells, (b) the activation of these receptors by agonists, namely nicotine, induces cell proliferation and inhibits apoptosis, whereas antagonists have a pro-apoptotic effect. Among competitive α7-nAChR antagonists, d-tubocurarine and -cobratoxin (α-CbT), from the snake venom of Naja, emerged as possible drug candidates. However, some aspects of the samples must be particularly taken into account, such as the particular nature of the sample. Thus, when using natural compounds purified from snake venom, it is important to take into account the factors such as whether the venom sample was derived from different animals, purified by different methods, or contained contaminants of the same molecular weight. Finally, biological activity may be different for different batches, which could also have been stored under different conditions (e.g. temperature, dilution, suspension medium etc.). These factors, affecting the experimental results, are also discussed.

The nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels involved in cognitive processes and are associated with brain disorders which makes them interesting drug targets. This article presents a general overview of the receptor to introduce the α7 nAChR as a drug target. The advances in understanding of the structure/function properties of the nAChR produced during the last decade are detailed as they are crucial for rational drug design. The allosteric properties of the nAChR will also be described because they also have important consequences for drug design.

The α7 nicotinic acetylcholine receptor (nAChR) is a promising drug target for a number of diseases ranging from schizophrenia and Alzheimer’s disease to chronic pain and inflammatory diseases. Focusing on the central nervous system, we describe how endogenous and experimental compounds and proteins regulate expression and function of the α7 nAChR. Drug development efforts have hitherto focused on direct manipulation of the α7 nAChR, but it is still not clear, whether agonism/antagonism or allosteric modulation is preferable as a potential drug therapy. In addition, the action of such compounds in vivo is highly dependent on α7 nAChR-interacting proteins, such as RIC-3 and lynx1, which modulate expression and function of the receptor. These regulatory proteins are often not expressed in in vitro models used to study α7 nAChR function, and it is not known to what extent they are involved in diseases such as schizophrenia and Alzheimer’s disease. Furthermore, α7 nAChR agonists and allosteric modulators differentially alter expression and functionality of the α7 nAChR with repeated administration, which suggests that there may be fundamentally different outcomes of long-term administration with the Finally, we describe the special case of Aβ1-42 binding to the α7 nAChR, which may pose a unique challenge to drug development of α7 nAChR-specific ligands for Alzheimer’s disease. Hopefully, a greater knowledge of the many factors influencing α7 nAChR function as well as an increasing pipeline of specific drug candidates, enabling a more subtle manipulation of α7 nAChR function, may facilitate α7 nAChR drug development efforts.

The α7-nAChR plays critical roles in numerous organs and cells by regulating highly organ and cell typespecific functions. In this special issue different Authors have contributed to clarify the different roles played by the α7- nAChR. Post-translational processes such as receptor “underactivation” or “overactivation” are associated in the central nervous system with brain disorders including neurodegeneration, while also contributing to the regulation of nonneuronal cells and cancers derived from them. Current advances in the knowledge of α7-nAChR biology encourage the exploitation of this receptor as a therapeutic target for a variety of diseases, including Alzheimer’s, disease, Parkinson’s disease, cognitive decline, pain and cancer.