Muscarinic receptor subtypes in subpopulations of human blood mononuclear cells as analyzed by RT-PCR technique

Immunological Tolerance in Liver Transplant Recipients: Putative Involvement of Neuroendocrine-Immune Interactions

The transplantation world changed significantly following the introduction of immunosuppressants, with millions of people saved. Several physicians have noted that liver recipients that do not take their medication for different reasons became tolerant regarding kidney, heart, and lung transplantations at higher frequencies. Most studies have attempted to explain this phenomenon through unique immunological mechanisms and the fact that the hepatic environment is continuously exposed to high levels of pathogen-associated molecular patterns (PAMPs) or non-pathogenic microorganism-associated molecular patterns (MAMPs) from commensal flora. These components are highly inflammatory in the periphery but tolerated in the liver as part of the normal components that arrive via the hepatic portal vein. These immunological mechanisms are discussed herein based on current evidence, although we hypothesize the participation of neuroendocrine-immune pathways, which have played a relevant role in autoimmune diseases. Cells found in the liver present receptors for several cytokines, hormones, peptides, and neurotransmitters that would allow for system crosstalk. Furthermore, the liver is innervated by the autonomic system and may, thus, be influenced by the parasympathetic and sympathetic systems. This review therefore seeks to discuss classical immunological hepatic tolerance mechanisms and hypothesizes the possible participation of the neuroendocrine-immune system based on the current literature.

Increased expression of blood muscarinic receptors in patients with reflex syncope

AIMS

Pathophysiology of reflex syncope is not fully understood but a vagal overactivity might be involved in this syncope. Previously, overexpression of muscarinic M2 receptors and acetylcholinesterase was found in particular in the heart and in lymphocytes of rabbits with vagal overactivity as well as in hearts of Sudden Infant Death Syndromes. The aim of this present study was to look at M2 receptor expression in blood of patients with reflex syncope. The second objective was to measure acetylcholinesterase expression in these patients.

METHODS AND RESULTS

136 subjects were enrolled. This monocenter study pooled 45 adults exhibiting recurrent reflex syncope compared with 32 healthy adult volunteers (18-50 years) and 38 children exhibiting reflex syncope requiring hospitalization compared with 21 controls (1-17 years). One blood sample was taken from each subject and blood mRNA expression of M2 receptors was assessed by qRT-PCR. Taking into account the non-symmetric distributions of values in both groups, statistical interferences were assessed using bayesian techniques. A M2 receptor overexpression was observed in adult and pediatric patients compared to controls. The medians [q1;q3] were 0.9 [0.3;1.9] in patients versus 0.2 [0.1;1.0] in controls; the probability that M2 receptor expression was higher in patients than in controls (Pr[patients>controls]) was estimated at 0.99. Acetylcholinesterase expression was also increased 0.7 [0.4;1.6] in patients versus 0.4 [0.2;1.1] in controls; the probability that acetylcholinesterase expression was higher in patients than in controls (Pr[patients>controls]) was estimated at 0.97. Both in adults and children, the expression ratio of M2 receptors over acetylcholinesterase was greater in the patient group compared with the control group.

CONCLUSION

M2 receptor overexpression has been detected in the blood of both, adults and children, exhibiting reflex syncope. As in our experimental model, i.e. rabbits with vagal overactivity, acetylcholinesterase overexpression was associated with M2 receptor overexpression. For the first time, biological abnormalities are identified in vagal syncope in which only clinical signs are, so far, taken into account for differential diagnosis and therapeutic management. Further work will be needed to validate potential biomarkers of risk or severity associated with the cholinergic system.

Modulation of the extraneuronal cholinergic system on main innate response leukocytes

The expression of elements of the cholinergic system has been demonstrated in non-neuronal cells, such as immune cells, where acetylcholine modulates innate and adaptive responses. However, the study of the non-neuronal cholinergic system has focused on lymphocyte cholinergic mechanisms, with less attention to its role of innate cells. Considering this background, the aims of this review are 1) to review information regarding the cholinergic components of innate immune system cells; 2) to discuss the effect of cholinergic stimuli on cell functions; 3) and to describe the importance of cholinergic stimuli on host immunocompetence, in order to set the base for the design of intervention strategies in the biomedical field.

Cigarette smoking promotes inflammation in patients with COPD by affecting the polarization and survival of Th/Tregs through up-regulation of muscarinic receptor 3 and 5 expression

Background

CD4⁺ T cells in the lung are involved in the pathogenesis of chronic obstructive pulmonary disease (COPD), although CD4⁺ T cell subsets and the direct effect of smoking on these cells, especially the expression of MRs, have not been comprehensively examined.

Methods

First, circulating CD4⁺ T cell subsets in healthy nonsmokers, patients with SCOPD and patients with AECOPD were evaluated by flow cytometry. Then, differentiation experiments were carried out using RT-PCR, and Ki-67/Annexin V antibodies were used to measure proliferation and apoptosis. We also explored the impact of CSE on the differentiation and survival of CD4⁺Th/Tregs and examined the expression of MRs in healthy nonsmokers and patients with SCOPD.

Results

We found the percentages of circulating Th1 and Th17 cells were increased in patients with AECOPD, while the percentage of Th2 cells was decreased in patients with SCOPD. The percentages of Th10 cells were decreased in both patients with SCOPD and patients with AECOPD, while the percentages of Tregs were increased. In addition, the percentages of CD4⁺α-7⁺ T cells were decreased in patients with SCOPD and patients with AECOPD. However, only the decrease observed in patients with AECOPD was significant. In vitro studies also revealed MR expression affected the polarization of T cells, with different CD4⁺ T cell subtypes acquiring different MR expression profiles. The addition of CSE facilitated CD4⁺ T cell polarization towards pro-inflammatory subsets (Th1 and Th17) and affected the survival of CD4⁺ T cells and Treg cells by up-regulating the expression of MR3 and 5, resulting in an imbalance of CD4⁺ T cell subsets.

Conclusions

Our findings suggest an imbalance of circulating CD4⁺ T cell subsets is involved in COPD pathogenesis in smokers. Cigarette smoking may contribute to this imbalance by affecting the polarization and survival of Th/Tregs through the up-regulation of MR3 and MR5.

Muscarinic receptor agonists and antagonists: effects on inflammation and immunity

In this chapter, we will review what is known about muscarinic regulation of immune cells and the contribution of immune cell muscarinic receptors to inflammatory disease and immunity. In particular, immune cell expression of cholinergic machinery, muscarinic receptor subtypes and functional consequences of agonist stimulation will be reviewed. Lastly, this chapter will discuss the potential therapeutic effects of selective antagonists on immune cell function and inflammatory disease in recent animal studies and human clinical trials.

Differential expression of muscarinic acetylcholine receptor subtypes in Jurkat cells and their signaling

Muscarinic acetylcholine receptors expression and signaling in the human Jurkat T cell line were investigated. Semiquantitative real-time PCR and radioligand binding studies, using a wide set of antagonist compounds, showed the co-existence of M(3), M(4), and M(5) subtypes. Stimulation of these subpopulations caused a concentration and time- dependent activation of second messengers and ERK signaling pathways, with a major contribution of the M(3) subtype in a G(q/11)-mediated response. In addition, we found that T-cell stimulation leads to increased expression of M(3) and M(5) both at transcriptional and protein levels in a PLC/PKCθ dependent manner. Our data clarifies the functional role of AChR subtypes in Jurkat cells and pave the way to future studies on the potential cross-talk among these subpopulations and their regulation of T lymphocytes immune function.

Muscarinic acetylcholine receptor localization and activation effects on ganglion response properties

PURPOSE

The activation and blockade of muscarinic acetylcholine receptors (mAChRs) affects retinal ganglion cell light responses and firing rates. This study was undertaken to identify the full complement of mAChRs expressed in the rabbit retina and to assess mAChR distribution and the functional effects of mAChR activation and blockade on retinal response properties.

METHODS

RT-PCR, Western blot analysis, and immunohistochemistry were used to identify the complement and distribution of mAChRs in the rabbit retina. Extracellular electrophysiology was used to determine the effects of the activation or blockade of mAChRs on ganglion cell response properties.

RESULTS

RT-PCR of whole neural retina resulted in the amplification of mRNA transcripts for the m1 to m5 mAChR subtypes. Western blot and immunohistochemical analyses confirmed that all five mAChR subtypes were expressed by subpopulations of bipolar, amacrine, and ganglion cells in the rabbit retina, including subsets of cells in cholinergic and glycinergic circuits. Nonspecific muscarinic activation and blockade resulted in the class-specific modulation of maintained ganglion cell firing rates and light responses.

CONCLUSIONS

The expression of mAChR subtypes on subsets of bipolar, amacrine, and ganglion cells provides a substrate for both enhancement and suppression of retinal responses via activation by cholinergic agents. Thus, the muscarinic cholinergic system in the retina may contribute to the modulation of complex stimuli. Understanding the distribution and function of mAChRs in the retina has the potential to provide important insights into the visual changes that are caused by decreased ACh in the retinas of Alzheimer's patients and the potential visual effects of anticholinergic treatments for ocular diseases.

Role of acetylcholine in nitric oxide production in the salamander retina

Although acetylcholine is one of the most widely studied neurotransmitters in the retina, many questions remain about its downstream signaling mechanisms. In this study we initially characterized the cholinergic neurotransmitter system in the salamander retina by localizing a variety of cholinergic markers. We then examined the link between both muscarinic and nicotinic receptor activation and nitric oxide production by using immunocytochemistry for cyclic guanosine monophosphate (cGMP) as an indicator. We found a large increase in cGMP-like immunoreactivity (cGMP-LI) in the inner retina in response to muscarinic (but not nicotinic) receptor activation. Based on the amplification of mRNA transcripts, receptor immunocytochemistry, and the use of selective antagonists, we identified these receptors as M2 muscarinic receptors. Using double-labeling techniques, we established that these increases in cGMP-LI were seen in GABAergic but not cholinergic amacrine cells, and that the increases were blocked by inhibitors of nitric oxide production. The creation of nitric oxide in response to cholinergic receptor activation may provide a mechanism for modulating the well-known mutual interactions of acetylcholine-glycine-GABA in the inner retina. As GABA and glycine are the primary inhibitory neurotransmitters in the retina, signaling pathways that modulate their levels or release will have major implications for the processing of complex stimuli by the retina.

Methylmercury interaction with lymphocyte cholinergic muscarinic receptors in developing rats

Cerebral cholinergic muscarinic receptors (MR) have been suggested as one of the sensitive biochemical endpoints of the central nervous system altered by developmental exposure to the widespread seafood contaminant methylmercury (MeHg). In adult rats, MeHg has been shown to alter MR binding both in the brain and lymphocytes, supporting the use of MR in blood cells as a surrogate marker of CNS changes. The effects of MeHg have been evaluated on rat lymphocyte MR binding (using [3H]QNB as specific muscarinic ligand) in vivo (after perinatal exposure) and in vitro. For comparison, in vitro studies were also performed on human lymphocytes. Exposure to 1 mg MeHg/kg/day during pregnancy and lactation (from GD7 to PND7) significantly enhanced lymphocyte MR density in both adult and young rats 21 days after delivery, with a more pronounced effect in the mothers (B(max) increase of 139%) than in the male offspring (+49%) and female offspring (+73%) as compared with their respective controls (33+/-4, 41+/-8, and 37+/-4 fmol/million cells), in accordance with the higher Hg levels detected in the adult blood (11.3+/-2.2 microg/mL) than in pups (1.3+/-0.4 microg/L in both genders). A lower MeHg dose (0.5 mg/kg/day) was without any effect on lymphocyte MRs. In in vitro studies, MeHg was an almost equipotent inhibitor of (3)H-QNB binding to rat and human lymphocyte MRs (IC50 values were 4.1+/-0.29, 5.2+/-0.51, and 5.0+/-0.9 microM for total rat lymphocytes, rat T lymphocytes, and total human lymphocytes, respectively). Notably, the IC50 values for MeHg to lymphocyte MRs were comparable to the Hg levels reached in blood (5-50 microM) of the PND21 rats exposed to MeHg. The finding that the MR binding is a target for the effects of MeHg in peripheral blood cells is in accordance with our previous data in brain [Coccini et al., 2006. Effects of developmental co-exposure to methylmercury and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) on cholinergic muscarinic receptors in rat brain. Neurotoxicology, in press], and supports the use of this peripheral endpoint as a biomarker of MeHg-induced cerebral muscarinic alterations. The similarity of MeHg IC50 binding data between human and rat in peripheral tissues suggests the possible application of such biomarker to humans exposed to environmental chemicals.

Selective increases in the cytokine, TNFalpha, in the prefrontal cortex of PCP-treated rats and human schizophrenic subjects: influence of antipsychotic drugs

The psychotomimetic drug phencyclidine (PCP) induces symptoms closely related to those of schizophrenia in humans. In order to test the hypothesis that cytokines may be involved in the aetiology and treatment of schizophrenia, this study investigated the levels of cytokine mRNAs in rat brain after acute and chronic administration of PCP, in the presence and absence of antipsychotic drugs. The levels of the mRNAs encoding TNF, IL-2, IL-6, TGF 1, 2, 3, IL-3 and GM-CSF were measured in the prefrontal cortex, cortex, hippocampus, ventral and dorsal striatum regions of male hooded Long Evans rats after acute drug administration. Antipsychotic drugs and PCP significantly reduced the levels of TNF in the prefrontal cortex compared to vehicle-treated animals, whilst other cytokines remained unchanged. In addition, significant reductions in the levels of TNF mRNA in the prefrontal cortex still occurred 24h after acute PCP administration. However, levels of TNF mRNA were restored to control values after chronic PCP treatment, whereas increased expression was detected in animals co-administered with haloperidol. Levels of TNF mRNA were also found to be significantly increased in the prefrontal cortex of schizophrenic subjects. The relationship between TNF levels and schizophrenia are discussed.

Hydrolytic and nonenzymatic functions of acetylcholinesterase comodulate hemopoietic stress responses

Glucocorticoid-initiated granulocytosis, excessive proliferation of granulocytes, persists after cortisol levels are lowered, suggesting the involvement of additional stress mediator(s). In this study, we report that the stress-induced acetylcholinesterase variant, AChE-R, and its cleavable, cell-penetrating C-terminal peptide, ARP, facilitate granulocytosis. In postdelivery patients, AChE-R-expressing granulocyte counts increased concomitantly with serum cortisol and AChE activity levels, yet persisted after cortisol had declined. Ex vivo, mononuclear cells of adult peripheral blood responded to synthetic ARP26 by overproduction of hemopoietically active proinflammatory cytokines (e.g., IL-6, IL-10, and TNF-alpha). Physiologically relevant ARP26)levels promoted AChE gene expression and induced the expansion of cultured CD34+ progenitors and granulocyte maturation more effectively than cortisol, suggesting autoregulatory prolongation of ARP effects. In vivo, transgenic mice overexpressing human AChE-R, unlike matched controls, showed enhanced expression of the myelopoietic transcription factor PU.1 and maintained a stable granulocytic state following bacterial LPS exposure. AChE-R accumulation and the consequent inflammatory consequences can thus modulate immune responses to stress stimuli.

Lymphocyte muscarinic receptors and platelet monoamine oxidase-B as biomarkers of CNS function: effects of age and gender in healthy humans

Lymphocyte cholinergic muscarinic receptors (MRs) and platelet monoamine oxidase-B (MAO-B) activity are considered surrogate markers of the same parameters in the central nervous system. Lymphocyte MR binding and platelet MAO-B activity were measured in a consistent number of healthy human adults and analysed according to gender and age. The mean value±S.D. of MR binding neither differed between males (12.2±10.0fmol/10(6)cells, range: 0.5-37.9, n=86) and females (10.7±9.7fmol/10(6)cells, range: 0.5-39.7, n=69) nor among age groups. MAO-B activity was significantly higher in women (geometric mean: 11.3nmol/mgprotein/h, with 65% of values from 7.3 to 17.6; n=43), than in men (7.7nmol/mgprotein/h, with 65% of values from 4.5 to 13; n=95). Males aged 56-66 years displayed a higher, though not statistically significant, basal enzyme activity than younger subjects. Altogether these data indicate gender-related differences in MAO activity, but not in MR binding, and inter-individual differences in the basal values of both peripheral blood markers in healthy subjects.

Detection of anti-type 3 muscarinic acetylcholine receptor autoantibodies in the sera of Sjögren's syndrome patients by use of a transfected cell line assay

OBJECTIVE

Sjögren's syndrome (SS) is an autoimmune disease affecting primarily the salivary and lacrimal glands, leading to dry mouth and dry eyes. Recent studies have suggested that autoantibodies reactive with the type 3 muscarinic acetylcholine receptors (M3Rs) expressed on salivary and lacrimal gland cells may be highly specific for SS. To test this hypothesis, we constructed a cell line expressing the human M3R gene in order to screen for anti-M3R autoantibodies in sera from SS patients.

METHODS

Complementary DNA encoding the open-reading frame (ORF) of the human M3R gene was amplified, ligated into the pcDNA5/FRT/V5-His-TOPO TA vector, and then used to transform Escherichia coli bacteria. Plasmid DNA containing the M3R ORF with the correct orientation was transfected into Flp-In Chinese hamster ovary (CHO) cells using Flp recombinase-mediated site-specific recombination. An M3R-transfected CHO cell line, selected and propagated in hygromycin, was used to detect anti-M3R autoantibodies in SS patient and healthy control sera by flow cytometry.

RESULTS

Testing of sera for the presence of anti-M3R autoantibodies bound to CHO-transfected cells revealed the presence of anti-M3R autoantibodies in SS patients (9 of 11) but not in healthy controls (0 of 11). Although the anti-M3R autoantibodies detected in patient sera were of multiple isotypes, the most consistently detected were IgG1, IgG3, and IgA.

CONCLUSION

Using a newly constructed cell line expressing human M3R, anti-M3R autoantibodies were easily detected in sera from SS patients. These autoantibodies were skewed toward the IgG1, IgG3, and IgA isotypes, probably recognizing a tertiary epitope created by extracellular domains of the receptor protein. Anti-M3R autoantibodies represent a highly promising clinical marker for the identification of SS.

Potential clozapine target sites on peripheral hematopoietic cells and stromal cells of the bone marrow

The antipsychotic drug clozapine, acts via interaction with selective neurotransmitter receptor systems. Its use however, is associated with life-threatening agranulocytosis. The mechanism by which this occurs and its possible relationship with the drug's atypicality remain unclear. As a first step in identifying mechanistic pathways involved, profiling of neurotransmitter receptors on human neutrophils, mononuclear and bone marrow stromal cells as putative targets for clozapine-mediated toxicity was undertaken. Expression of mRNA encoding dopaminergic d2, d3, d4; serotonergic 5ht2a, 5ht2c, 5ht3, 5ht6, 5ht7; adrenergic alpha1a, alpha2; histaminergic h1 and muscarinic m1, m2, m3, m4, m5 receptors was analyzed by reverse transcription-polymerase chain reaction methods. While 5ht2c, 5ht6, m1 and m2 mRNA were undetected, the presence of the other receptors indicates sites at which clozapine could bind and induce toxicity of neutrophils and stromal components which regulate granulopoiesis. The functional significance of differential receptor expression while unknown, may argue for neural regulation of hematopoiesis.

[An independent, non-neuronal cholinergic system in lymphocytes and its roles in regulation of immune function]

Acetylcholine (ACh) is classically thought of as a neurotransmitter in mammalian species. However, lymphocytes express most of the cholinergic components found in the nervous system, including ACh, choline acetyltransferase (ChAT), high-affinity choline transporter, and acetylcholinesterase as well as both muscarinic and nicotinic ACh receptors (mAChRs and nAChRs, respectively). Activation of T cells via the T cell receptor/CD3 complex, contact of T cells with antigen presenting cells, or activation of the adenylyl cyclase pathway in T cells modulates cholinergic activity, as evidenced by up-regulation of ChAT and M(5) mAChR mRNA expression. Stimulation of mAChRs on T and B cells with ACh or another mAChR agonists elicits intracellular Ca(2+) signaling, up-regulation of c-fos expression, increased nitric oxide synthesis and interleukin-2-induced signal transduction via M(3) and M(5) mAChR-mediated pathways. Acute stimulation of nAChRs with ACh or nicotine causes rapid and transient Ca(2+) signaling in T and B cells, probably via alpha7 nAChRs subunit-mediated pathways. Chronic nicotine stimulation, by contrast, down-regulates nAChR expression and suppresses T cell activity. Abnormalities in lymphocytic cholinergic system have been seen in animal models of immune deficiency and immune acceleration. Collectively, these data provided a compelling picture in which immune function is, at least partly, under the control of an independent, non-neuronal cholinergic system in lymphocytes.

The lymphocytic cholinergic system and its contribution to the regulation of immune activity

Lymphocytes express most of the cholinergic components found in the nervous system, including acetylcholine (ACh), choline acetyltransferase (ChAT), high affinity choline transporter, muscarinic and nicotinic ACh receptors (mAChRs and nAChRs, respectively), and acetylcholinesterase. Stimulation of T and B cells with ACh or another mAChR agonist elicits intracellular Ca2+ signaling, up-regulation of c-fos expression, increased nitric oxide synthesis and IL-2-induced signal transduction, probably via M3 and M5 mAChR-mediated pathways. Acute stimulation of nAChRs with ACh or nicotine causes rapid and transient Ca2+ signaling in T and B cells, probably via alpha7 nAChR subunit-mediated pathways. Chronic nicotine stimulation, by contrast, down-regulates nAChR expression and suppresses T cell activity. Activation of T cells with phytohemagglutinin or antibodies against cell surface molecules enhances lymphocytic cholinergic transmission by activating expression of ChAT and M5 mAChR, which is suggestive of local cholinergic regulation of immune system activity. This idea is supported by the facts that lymphocytic cholinergic activity reflects well the changes in immune system function seen in animal models of immune deficiency and immune acceleration. Collectively, these data provide a compelling picture in which lymphocytes constitute a cholinergic system that is independent of cholinergic nerves, and which is involved in the regulation of immune function.

Upregulation of mRNA encoding the M5 muscarinic acetylcholine receptor in human T- and B-lymphocytes during immunological responses

Lymphocytes possess an independent, non-neuronal cholinergic system. Moreover, both T- and B-lymphocytes express multiple muscarinic acetylcholine receptors (mAChR). To obtain a better understanding of the regulatory mechanisms governing mAChR gene expression in the lymphocytic cholinergic system, we examined the effects of lymphocyte activation on expression of mAChR mRNA. Stimulation of T- and B-lymphocytes, respectively, with T-cell activator phytohemagglutinin and B-cell activator Staphylococcus aureus Cowan I upregulated M5 mAChR mRNA expression in the CEM human leukemic T-cell line and in the Daudi B-cell line, which served as models of lymphocytes. In striking contrast, M3 and M4 mAChR mRNA expression was not affected in either cell line. Nonetheless, stimulating lymphocytes with phorbol 12-myristate 13-acetate, a protein kinase C activator, plus ionomycin, a calcium ionophore, upregulated expression of both M3 and M5 mAChR mRNA. This represents the first demonstration that immunological stimulation leads to M5 mAChR gene expression in lymphocytes.

Immunochemical and immunocytochemical characterization of cholinergic markers in human peripheral blood lymphocytes

Cholinergic markers and the expression of M(2)-M(5) muscarinic cholinergic receptor subtypes were investigated in human peripheral blood lymphocytes by Western blot analysis and immunocytochemistry. The totality of peripheral blood lymphocytes express acetylcholine (ACh) immunoreactivity, choline acetyltransferase (ChAT), acetylcholinesterase (AChE), vesicular ACh transporter (VAChT) and M(2)-M(5) muscarinic cholinergic receptor protein immunoreactivity. Western blot analysis performed independently on T and B lymphocytes using anti-ChAT and anti-AChE antibodies revealed labelling of single bands of approximately 68-70 and 70 kDa, respectively, whereas VAChT was bound to two bands of approximately 80 and 45 kDa. The pattern of immunoblotting was similar in membranes of lymphocytes and striatum, used as a reference brain tissue. Western blot analysis using anti M(2)-M(5) receptor antibodies revealed labelling of single bands of approximately 55, 85-90, 50 and 81 kDa, respectively. Confocal laser immunofluorescence showed the localization of ACh and VAChT immunoreactivity in punctiform areas likely corresponding to cytoplasmic vesicles. ChAT and AChE were diffused to the cytoplasm and plasma membrane. Muscarinic receptor immunoreactivity was located in lymphocyte plasma membrane. Although the role of lymphocyte cholinergic system is still unclear, the demonstration of cholinergic markers in T and B human blood lymphocytes supports the view that a cholinergic systems may contribute to the regulation of immune function. The characterization of these cholinergic markers may also contribute to define if their evaluation can be used for assessing the status of brain cholinergic system.

Expression of peripheral blood lymphocyte muscarinic cholinergic receptor subtypes in airway hyperresponsiveness

The expression of muscarinic cholinergic receptor subtypes was investigated in peripheral blood lymphocytes (PBL) of bronchial asthma patients by a combined kinetic and equilibrium labeling technique for radioligand binding assay of muscarinic cholinergic receptor subtypes and by receptor immunochemistry and immunocytochemistry. An increased expression of M2 and to a lesser extent of M5 receptors and no changes of M4 receptor were observed in PBL of asthmatics compared to control individuals. The increase was related to bronchial hyperresponsiveness detected by methacholine challenge test. Analysis of M3 receptor expression revealed biphasic changes, with a decreased receptor density in patients with normal, mild and moderate responses to methacholine test and a recovery to levels similar to those found in healthy individuals in severe responders to methacholine test. The demonstration of a different expression of lymphocyte muscarinic receptors in asthma suggests that cholinergic system may participate to a molecular framework influencing immune functions in asthma.

Structure and transcription of the human m3 muscarinic receptor gene

We have isolated and characterized the human m3 muscarinic receptor gene and its promoter. Using 5' rapid amplification of cDNA ends (RACE), internal polymerase chain reaction (PCR), and homology searching to identify EST clones, we determined that the cDNA encoding the m3 receptor comprises 4,559 bp in 8 exons, which are alternatively spliced to exclude exons 2, 4, 6, and/or 7; the receptor coding sequence occurs within exon 8. Analysis of P1 artificial chromosome (PAC) and bacterial artificial chromosome (BAC) clones and of PCR- amplified genomic DNA, and homology searching of human chromosome 1 sequence provided from the Sanger Centre (Hinxton, Cambridge, UK) revealed that the m3 muscarinic receptor gene spans at least 285 kb. A promoter fragment containing bp -1240 to +101 (relative to the most 5' transcription start site) exhibited considerable transcriptional activity during transient transfection in cultured subconfluent, serum-fed canine tracheal myocytes, and 5' deletion analysis of promoter function revealed the presence of positive transcriptional regulatory elements between bp -526 and -269. Sequence analysis disclosed three potential AP-2 binding sites in this region; five more AP-2 consensus binding motifs occur between bp -269 and +101. Cotransfection with a plasmid expressing human AP-2alpha substantially increased transcription from m3 receptor promoter constructs containing 526 or 269 bp of 5' flanking DNA. Furthermore, m3 receptor promoter activity was enhanced by long-term serum deprivation of canine tracheal myocytes, a treatment that is known to increase AP-2 transcription-promoting activity in these cells. Together, these data suggest that expression of the human m3 muscarinic receptor gene is regulated in part by AP-2 in airway smooth muscle.

Roles of muscarinic acetylcholine receptors in interleukin-2 synthesis in lymphocytes

Receptors for many neurotransmitters including catecholamines and acetylcholine (ACh) have been detected on the cell surface of lymphocytes. It has been demonstrated that a human T cell line synthesizes ACh and suggested that ACh may be an autacoid modulating T cell-dependent immune responses. However, the biochemical interactions of the ACh system with the immune system have not been elucidated in detail. We have shown that m1 and m2 muscarinic receptor mRNAs are expressed in human peripheral blood lymphocytes and in human T cell line Jurkat cells and that pretreatment of these cells with a muscarinic receptor agonist enhances interleukin-2 (IL-2) production. We also postulated possible intracellular signaling pathways via which muscarinic receptors regulate IL-2 production in Jurkat cells. The findings suggest that M1 muscarinic receptors are involved in muscarinic receptor-mediated enhancement of IL-2 production in Jurkat cells and that the transcription factor AP-1 and pathways via mitogen-activated protein kinase (MAPK)/extracellular signal regulated protein kinase and c-Jun N-terminal kinase, but not via p38 MAPK, may be involved in the muscarinic receptor-mediated enhancement of IL-2 production. Our findings demonstrate a neuro-immune interaction through muscarinic receptor signaling in immune cells.

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.

Calcium signaling and c-Fos gene expression via M3 muscarinic acetylcholine receptors in human T- and B-cells

We previously showed that blood acetylcholine (ACh) originates mainly from T-lymphocytes, and that stimulation of muscarinic ACh receptors (mAChRs) induces Ca2+ oscillations and up-regulates c-fos gene expression in both T- and B-lymphocytes. In the present study, we investigated which mAChR subtypes are involved in Ca2+ signaling and c-fos gene expression in human T- (CEM) and B- (Daudi) cells. Stimulation of mAChRs with 100 microM oxotremorine-M, an M1/M3 agonist, increased levels of intracellular free Ca2+ ([Ca2+]i) and c-fos mRNA expression in both cell lines. 4-DAMP, an M3 antagonist, more effectively blocked the oxotremorine-M-induced increase in [Ca2+]i than pirenzepine and telenzepine, M1-receptor antagonists; AF-DX 116, an M2 antagonist; hexahydrosiladifenidol, a weak M3 antagonist; or hexamethonium and d-tubocurarine, nicotinic receptor antagonists. McN-A-343 (100 microM), a partial M1-receptor agonist, had no apparent effect on [Ca2+]i in either cell line. The oxotremorine-M-induced up-regulation of c-fos transcription was inhibited by 4-DAMP, but not by pirenzepine or AF-DX 116. Our findings thus suggest that ACh released from T-lymphocytes acts as an autocrine/paracrine factor, transmitting a Ca2+-dependent signal to the nuclei of T- and B-lymphocytes via M3 receptors.

Low-level exposure to methylmercury modifies muscarinic cholinergic receptor binding characteristics in rat brain and lymphocytes: physiologic implications and new opportunities in biologic monitoring

Methylmercury (MeHg) affects several parameters of cholinergic function. These alterations are thought to play a role in MeHg neurotoxicity. In vitro experiments have indicated that MeHg acts as a strong competitive inhibitor of radioligand binding to muscarinic cholinergic receptors (mAChRs) in rat brain. Furthermore, rat brain mAChRs share several pharmacologic characteristics of similar receptors present on lymphocytes. Using the muscarinic antagonist [(3)H]quinuclidinyl benzilate (QNB) to label receptors, we investigated the in vivo interactions of MeHg with rat brain mAChRs. We also investigated whether MeHg-induced central mAChR changes are reflected by similar alterations in splenic lymphocytes. Exposure to low doses of MeHg--0.5 or 2 mg/kg/day in drinking water--for 16 days significantly increased (20-44% of control) mAChRs density (B(max)) in the hippocampus and cerebellum without affecting receptor affinity (K(d)). The effect of MeHg did not occur immediately; it was not apparent until 2 weeks after the termination of treatment. No significant changes in [(3)H]QNB binding were observed in the cerebral cortex. In splenic lymphocytes, mAChR density was remarkably increased (95-198% of control) by day 14 of MeHg exposure and remained enhanced 14 days after the cessation of treatment. These results suggest up-regulation of mAChRs in selected brain regions (hippocampus and cerebellum) after prolonged low-level ingestion of MeHg in rats. These cerebral effects are delayed in onset and are preceded by a marked increase in density of mAChRs on lymphocytes. In chronic MeHg exposure, peripheral lymphocytes may represent a sensitive target for the interaction of MeHg with mAChRs and, therefore, may be predictive indicators of later adaptive response involving cerebral mAChRs. Additionally, the effect of MeHg on lymphocyte mAChRs in vivo indicates that this receptor system should be investigated further as a possible target for MeHg immunotoxicity.

Radioligand binding assay of M1-M5 muscarinic cholinergic receptor subtypes in human peripheral blood lymphocytes

Analysis of lymphocyte muscarinic cholinergic receptors using quantitative techniques such as radioligand binding assay is made difficult due to the low density of these sites and the lack of subtype-specific selectivity of most available muscarinic ligands. In this study, a combined kinetic and equilibrium labeling technique recently developed for brain tissue was used for labeling the five muscarinic cholinergic receptor subtypes in intact human peripheral blood lymphocytes. No specific muscarinic M1 receptor binding was detectable in human peripheral blood lymphocytes using [3H]-pirenzepine as a ligand. Labeling of M2-M5 muscarinic receptors using [3H]N-methyl-scopolamine (NMS) by occluding various receptor subtypes with muscarinic antagonist and mamba venom resulted in the labeling of M2-M5 receptors in brain as well as in human peripheral blood lymphocytes. The relative density of different receptor subtypes was M3 > M5 > M4 > M2. The development of a reproducible technique for assaying muscarinic cholinergic receptor subtypes expressed by human peripheral blood lymphocytes may contribute to clarify their role in lymphocyte function.

Relative amounts of mRNA encoding four subtypes of muscarinic receptors (m2-m5) in human peripheral blood mononuclear cells

It is known that lymphocytes express functional muscarinic cholinergic receptors. In this study, RT-PCR method was applied to study the presence and relative levels of mRNA encoding muscarinic receptor subtypes in human peripheral blood mononuclear cells (PBMCs). Our results, confirmed by DNA sequencing, demonstrate the presence of m2, m3, m4, and m5 receptor subtypes in human PBMCs. The relative levels of muscarinic receptor subtypes fit the following pattern: m3 > m5 > m4 > m2. Our data provide strong evidence confirming previous pharmacological studies that suggested the existence of several subtypes of muscarinic receptors on human PBMCs. We cannot exclude the possibility that expression of receptor subtype depends on the lineage and/or activation status of the cell.

Diversity of mRNA expression for muscarinic acetylcholine receptor subtypes and neuronal nicotinic acetylcholine receptor subunits in human mononuclear leukocytes and leukemic cell lines

Previously, we reported that various levels of acetylcholine (ACh), currently known as a neurotransmitter, are detectable in the blood of several mammals including humans and that most blood ACh originates from T-lymphocytes. To investigate whether ACh in the blood acts on lymphocytes and participates in the modulation of immune responses, we have analyzed the expression of mRNA for muscarinic (Ms) ACh receptor subtypes and nicotinic (Nc) ACh receptor subunits using reverse transcription-polymerase chain reaction (RT-PCR) methods. The cells tested were human peripheral mononuclear leukocytes (MNLs) from seven healthy donors and eight leukemic cell lines, as models of lymphocytes. We detected mRNA expression for various neuronal Nc receptor subunits and Ms receptor subtypes in all of the MNL samples and in all of the cell lines tested. However, the expression pattern of mRNA for neuronal Nc receptor subunits (alpha2-alpha7 and beta2-beta4) and Ms receptor subtypes (m1-m5) varied among the individuals and cell lines. No expression of mRNA for three muscle-type Nc receptor subunits (alpha1, beta1 and epsilon) was observed in the MNLs and cell lines. These results indicate that both neuronal-type Nc and Ms ACh receptors are present on the surface of lymphocytes.

Constitutive muscarinic receptors are involved in the growth and differentiation of friend erythroleukemia cells

Binding experiments with the specific muscarinic ligand [3H]N-methylscopolamine (3H-NMS) have shown the presence of constitutive muscarinic acetylcholine receptors (mAChR) on Friend murine erythroleukemia cells (MELC). Competition experiments with a panel of specific antagonists indicated that the mAChR were predominantly of the M3 subtype. This was confirmed by the rt-PCR analysis of mRNA levels for M1-M5 AChR. Uninduced MELC expressed approximately 2,100 and 1,200 binding sites per cell of growing and resting populations, respectively. The dissociation constant (K(D)) for 3H-NMS was in the picomolar range. The modulation of mAChR upon induction suggested that MELC growth and maturation might be under control of a cholinergic system since mAChR were markedly decreased or virtually absent in MELC induced to terminal division by dimethyl sulfoxide (DMSO) or hexamethylene bisacetamide (HMBA), respectively. In turn, the number of mAChR on MELC committed to polyploidization by colcemid was either increased over or maintained at the control levels when receptor densities were expressed per cell or surface unit (square micrometers), respectively. Moreover, the muscarinic agonist carbachol was found to inhibit MELC differentiation by decreasing by approximately 35% the amount of benzidine-positive (B+) cells in HMBA-induced cultures and, to a lesser degree, also AChE levels. The carbachol effect on erythroid differentiation was reverted by atropine that was found to restore the original amount of B+ cells, while it reduced acetylcholinesterase (AChE) to levels of approximately 66% of control. Such a selective atropine-mediated inhibition of AChE expression was observed also in HMBA-induced MELC supplemented with the antagonist. These results have suggested that mAChR on MELC are functional and might play a role in modulating the expression of either the erythroid or megakaryocytic traits of these cells.

Constitutive expression of mRNA for the same choline acetyltransferase as that in the nervous system, an acetylcholine-synthesizing enzyme, in human leukemic T-cell lines

Both muscarinic and nicotinic acetylcholine (ACh) receptors are known to be present on the surface of lymphocytes. We have shown that variable amounts of ACh are detectable in the blood of various mammals including humans, and a major portion of blood ACh is localized in circulating mononuclear leukocytes in humans. In order to investigate which types of blood cell are the source of ACh in human blood, expression of mRNA for choline acetyltransferase (ChAT, EC 2.3.1.6), which catalyzes ACh synthesis, was analyzed using human leukemic cell lines as models of lymphocytes and the reverse transcription-polymerase chain reaction (RT-PCR) method. We observed that mRNA for the same ChAT as that in the nervous system is expressed constitutively in all the T-cell lines tested, but not in B-, pre-lymphoma or monocytic cell lines. Furthermore, only T-cell lines showed high ACh-synthesizing activities and intracellular ACh contents. These results suggest that the major portion of ACh in the circulating blood originates from T-lymphocytes.

Identification and mapping of keratinocyte muscarinic acetylcholine receptor subtypes in human epidermis

Acetylcholine mediates cell-to-cell communications in the skin. Human epidermal keratinocytes respond to acetylcholine via two classes of cell-surface receptors, the nicotinic and the muscarinic cholinergic receptors. High affinity muscarinic acetylcholine receptors (mAChR) have been found on keratinocyte cell surfaces at high density. These receptors mediate effects of muscarinic drugs on keratinocyte viability, proliferation, adhesion, lateral migration, and differentiation. In this study, we investigated the molecular structure of keratinocyte mAChR and their location in human epidermis. Polymerase chain reaction amplification of cDNA sequences uniquely present within the third cytoplasmic loop of each subtype demonstrated the expression of the m1, m3, m4, and m5 mAChR subtypes. To visualize these mAChR, we raised rabbit anti-sera to synthetic peptide analogs of the carboxyl terminal regions of each subtype. The antibodies selectively bound to keratinocyte mAChR subtypes in immunoblotting membranes and epidermis, both of which could be abolished by preincubating the anti-serum with the peptide used for immunization. The immunofluorescent staining patterns produced by each antibody in the epidermis suggested that the profile of keratinocyte mAChR changes during epidermal turnover. The semiquantitative analysis of fluorescence revealed that basal cells predominantly expressed m3, prickle cells had equally high levels of m4 and m5, and granular cells mostly possessed m1. Thus, the results of this study demonstrate for the first time the presence of m1, m3, m4, and m5 mAChR in epidermal keratinocytes. Because keratinocytes express a unique combination of mAChR subtypes at each stage of their development in the epidermis, each receptor may regulate a specific cell function. Hence, a single cytotransmitter, acetylcholine, and muscarinic drugs may exert different biologic effects on keratinocytes at different stages of their maturation.

Acetylcholine synthesis and muscarinic receptor subtype mRNA expression in T-lymphocytes

We used a sensitive and specific radioimmunoassay for acetylcholine (ACh), and detected significant amounts of ACh in the blood of various mammals, including humans. About 60% of human blood ACh was localized in mononuclear leukocytes. Human leukemic T-cell lines, used as T-lymphocyte models, contained both ACh and choline acetyltransferase (ChAT) activity. Furthermore, ChAT mRNA and protein were detected in the T-cell line MOLT-3. Phytohemagglutinin, a T-cell activator, increased both synthesis and release of ACh by MOLT-3 cells. Muscarinic receptor subtype mRNA expression was confirmed in various T-cell lines. These findings indicate that ACh synthesized by ChAT in T-lymphocytes acts on the muscarinic receptors on lymphocytes in autocrine and/or paracrine pathways and suggest that ACh in blood functions as a modulator of T-cell-dependent immune responses.

Induction of choline acetyltransferase mRNA in human mononuclear leukocytes stimulated by phytohemagglutinin, a T-cell activator

The induction of mRNA for choline acetyltransferase (ChAT), which catalyzes acetylcholine (ACh) synthesis was investigated in human mononuclear leukocytes (MNL) stimulated by phytohemagglutinin (PHA), a T-cell activator, using the reverse transcription-polymerase chain reaction. Stimulation of MNL by PHA induced the expression of ChAT mRNA, and potentiated ACh synthesis. ChAT mRNA induction required more time than the induction of interleukin-2 mRNA. Expression of the gene encoding the vesicular ACh transporter, which mediates ACh transport in cholinergic neurons, was not observed in PHA-stimulated MNL, suggesting that the mechanisms controlling ACh release from T-lymphocytes differ from those in cholinergic neurons. These findings demonstrate that activation of T-lymphocytes up-regulates ACh synthesis in the blood, and suggest that ACh plays an important role as a neuroimmunomodulator besides its role as a neurotransmitter.

Muscarinic receptor subtype selective toxins

The muscarinic acetylcholine receptors are monomeric proteins with seven hydrophobic, membrane spanning helices, and share a common evolutionary origin with the other members of the superfamily of membrane proteins known as seven-helix receptors. The amino acid sequences of five different muscarinic acetylcholine receptors, called m1, m2, m3, m4 and m5 have been determined. The five subtypes are expressed to different extent in different tissues. A large number of low molecular ligands for muscarinic receptors are known, but they bind to all five subtypes of receptors and only a few of them have a slightly higher (five-six fold) affinity for one of the subtypes, e.g. pirenzepine for M1 (1) and tripitramine for M2 receptors (2). Several neurotoxins have been isolated from snake venoms and used as pharmacological tools. Mambas, African snakes of genus Dendroaspis, have toxins that recognize muscrinic receptors and some of these muscarinic toxins are the most selective ligands for M1 and M4 receptors known to date.