Expression of mRNAs in human thymus coding for the alpha 3 subunit of a neuronal acetylcholine receptor

An independent non-neuronal cholinergic system in lymphocytes

Acetylcholine (ACh) is a well characterized neurotransmitter occurring throughout the animal kingdom. In addition, both muscarinic and nicotinic ACh receptors have been identified on lymphocytes of various origin, and their stimulation by muscarinic or nicotinic agonists elicits a variety of functional and biochemical effects. It was thus initially postulated that the parasympathetic nervous system may play a role in modulating immune system function. However, ACh in the blood has now been localized to lymphocytes; indeed expression of choline acetyltransferase (ChAT), an ACh synthesizing enzyme, has been shown in human blood mononuclear leukocytes, human leukemic T-cell lines and rat lymphocytes. Stimulation of T-lymphocytes with phytohemagglutinin activates the lymphoid cholinergic system, as evidenced by increased synthesis and release of ACh and increased expression of mRNAs encoding ChAT and ACh receptors. The observation that M3 muscarinic receptor stimulation by ACh and other agonists increases the intracellular free Ca2+ concentration and upregulates c-fos gene expression strongly argues that ACh, synthesized and released from T-lymphocytes, acts as an autocrine and/or paracrine factor regulating immune function. These findings present a compelling picture in which immune function is, at least in part, under the control of an independent lymphoid cholinergic system.

Neuronal nicotinic receptors in non-neuronal cells: new mediators of tobacco toxicity?

The nicotinic acetylcholine receptors are prototypic ionotropic receptors that mediate fast synaptic transmission. However, also non-excitable cells, and particularly the tegumental cells that line external and internal body surfaces, express acetylcholine receptors of neuronal type sensitive to nicotine. Bronchial epithelial cells, endothelial cells of blood vessels and skin keratinocytes express neuronal nicotinic receptors composed of alpha(3), alpha(5), beta(2) and beta(4) subunits, similar to those expressed in sympathetic ganglia, and neuronal nicotinic receptors composed of alpha(7) subunits. Neuronal nicotinic receptors in tegumental cells are involved in modulating cell shape and motility, and therefore in maintaining the integrity of the surfaces lined by those cells. Neuronal nicotinic receptors in non-neuronal tissues may modulate other functions, including cell proliferation and differentiation. Acetylcholine is synthesized, secreted and degraded by a variety of cells, including the tegumental cells that express neuronal nicotinic receptors. Thus, acetylcholine may function as a local "hormone" that is able to modulate cell functions that require fast adaptation to new conditions. The presence of neuronal nicotinic receptors sensitive to nicotine in tissues known to be involved in tobacco toxicity, like bronchi and blood vessels, raises the possibility that they mediate some of the toxic effects of smoking.

Thymocytes and cultured thymic epithelial cells express transcripts encoding alpha-3, alpha-5 and beta-4 subunits of neuronal nicotinic acetylcholine receptors: preferential transcription of the alpha-3 and beta-4 genes by immature CD4 + 8 + thymocytes

Thymic tissues express transcripts encoding the alpha-3, alpha-5 and beta-4 subunits of nicotinic neuronal acetylcholine receptors (AcChRs) suggesting that neuronal AcChRs similar to those expressed in ganglia are expressed in the thymus. Transcription occurs in both isolated thymocytes and thymic epithelial cells. RT-PCR analyses of thymocyte subsets indicate that immature CD4 + 8 + thymocytes express higher levels of the alpha-3 and beta-4 transcripts than more mature thymocytes. Compared to freshly isolated thymocytes, peripheral blood lymphocytes do not express alpha-3 and beta-4 AcChR subunit transcripts. Cultured thymocytes rapidly down-regulate transcription of the alpha-3 and beta-4 AcChR subunit genes by a process that is not reversed by stimulation with phytohemagglutinin and IL-2. Thus our results indicate that there is transcriptional regulation of neuronal AcChR subunit genes during the process of thymocyte maturation and that factors within the thymic microenvironment influence expression of the alpha-3 and beta-4 AcChR subunit genes by developing T cells.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

Myasthenia gravis associated with small-cell carcinoma of the lung

A 49-year-old man complained of a 3-month history of progressive generalized muscle weakness. He was diagnosed as having small-cell lung carcinoma at the same time. He received an intravenous injection of edrophonium chloride with remarkable improvement of muscle strength. Electromyographic studies revealed a compound muscle action potential that decreased after repetitive stimulation. These findings were considered representative of myasthenia gravis (MG), and inconsistent with Eaton-Lambert syndrome. The appearance of MG with small-cell lung carcinoma seems to be very rare, but possible.

Myf-4 does not mediate AChR receptor subunit mRNA expression in thymic tissues

We investigated whether a MyoD gene family member plays a role in the expression of AChR or AChR-like proteins in human thymus. We amplified from thymic tissue, mRNA of subunits of the fetal- and adult-type AChR, and Myf-4, a gene product that appears to regulate AChR expression. RNA extracted from paraffin-embedded thymic tissue of five myasthenics and five nonmyasthenics was subjected to reverse transcription followed by polymerase chain reaction using primers specific for these mRNAs. Thymic RNA from eight of ten patients contained transcripts of the alpha- and epsilon-subunits (specific for the adult-type AChR). Presence of these transcripts did not correlate with thymic pathology or clinical presentation. No gamma-subunit (specific for the fetal-type AChR) or Myf-4 transcripts were found. Our results indicate that mRNA for the adult-type AChR is expressed in thymic tissue and is expressed by mechanisms not involving Myf-4. Thymic AChR subunits may be the primary epitope that initiates the immune response in MG against the adult-type AChR.

Nicotinic neuronal acetylcholine receptor alpha-3 subunit transcription in normal and myasthenic thymus

Thymic transcription of the alpha-3 subunit of the AChR was studied through sequencing and PCR analysis of thymic cDNA clones, Northern blotting, and ribonuclease protection assays. This analysis revealed at least three, 3' end sequence variants for the alpha-3 subunit as well as a variant that results from the alternative splicing of an antisense 122 bp Alu sequence between exons 5 and 6 of the normal transcript. The spliced Alu sequence not only shifts the exon 6 reading frame but also carries an in-frame stop codon. If translated, this variant transcript would produce a truncated peptide lacking the fourth transmembrane domain of the subunit and carrying a carboxy terminus dodecapeptide not found in any other known AChR subunit sequence. The putative variant subunit may lack biological activity and should differ antigenically from its normal counterpart. In comparing the normal, the MG hypertrophic, and the MG thymoma for transcription of the alpha-3 subunit and its 122 bp variant, it was found that there were no qualitative or quantitative changes in alpha-3 transcript expression in the MG hypertrophic thymi. Thymomas, however, showed an overall decrease in alpha-3 transcription and a comparative increase in beta-amyloid precursor transcription. The decrease in the levels of alpha-3 transcription in thymomas may be related to the proliferation of thymic epithelial cells.

Desensitization of central cholinergic mechanisms and neuroadaptation to nicotine

This review focuses on neuroadaptation to nicotine. The first part of the paper delineates some possible general mechanisms subserving neuroadaptation to commonly abused drugs. The postulated role of the mesocorticolimbic neuroanatomical pathway and drug-receptor desensitization mechanisms in the establishment of tolerance to, dependence on, and withdrawal from psychoactive drugs are discussed. The second part of the review deals with the pharmacological effects of nicotine at both pre- and postsynaptic locations within the central nervous system, and the still-perplexing upregulation of brain nicotine-binding sites seen after chronic nicotine administration. A special emphasis has been put on desensitization of presynaptic cholinergic mechanisms, and postsynaptic neuronal nicotinic-receptor function and its modulation by endogenous substances. A comparison with the inactivation process occurring at peripheral nicotinic receptors is also included. Finally, a hypothesis on the possible connections between desensitization of central cholinergic mechanisms and neuroadaptation to nicotine is advanced. A brief comment on the necessity of fully understanding the effects of nicotine on the developing nervous system closes this work.