Mediatophore regulates acetylcholine release from T cells

Contributions of Non-Neuronal Cholinergic Systems to the Regulation of Immune Cell Function, Highlighting the Role of α7 Nicotinic Acetylcholine Receptors

Loewi's discovery of acetylcholine (ACh) release from the frog vagus nerve and the discovery by Dale and Dudley of ACh in ox spleen led to the demonstration of chemical transmission of nerve impulses. ACh is now well-known to function as a neurotransmitter. However, advances in the techniques for ACh detection have led to its discovery in many lifeforms lacking a nervous system, including eubacteria, archaea, fungi, and plants. Notably, mRNAs encoding choline acetyltransferase and muscarinic and nicotinic ACh receptors (nAChRs) have been found in uninnervated mammalian cells, including immune cells, keratinocytes, vascular endothelial cells, cardiac myocytes, respiratory, and digestive epithelial cells. It thus appears that non-neuronal cholinergic systems are expressed in a variety of mammalian cells, and that ACh should now be recognized not only as a neurotransmitter, but also as a local regulator of non-neuronal cholinergic systems. Here, we discuss the role of non-neuronal cholinergic systems, with a focus on immune cells. A current focus of much research on non-neuronal cholinergic systems in immune cells is α7 nAChRs, as these receptors expressed on macrophages and T cells are involved in regulating inflammatory and immune responses. This makes α7 nAChRs an attractive potential therapeutic target.

The cholinergic pathway transmits signals of neuropeptide F to regulate feeding of Ostrinia furnacalis larvae

BACKGROUND

Feeding is the basis of animal survival and reproduction. In insects, the neuropeptide F (NPF), a homologous polypeptide of NPY in vertebrates, plays an important role in regulation of feeding behavior. However, relatively little has been known about the molecular mechanism of feeding.

RESULTS

In this study, we show that the cholinergic pathway is very important in signaling transmission of NPF feeding regulation in Ostrinia furnacalis larvae, in which the choline acetyltransferase (ChAT), the vesicular acetylcholine transporter (vAChT) in presynaptic membrane and the nicotinic acetylcholine receptor (nAChR) in postsynaptic membrane are positively regulated by NPF, while the ace1 and ace2 encoding the acetylcholinesterase (AChE) are negatively regulated by NPF, leading to a balance of acetylcholine (ACh)-the excitatory transmitter. More, the cholinergic pathway further transmits signaling to the downstream pathways of the phosphoInositide-3 kinase (PI3K) and the cAMP responsive element binding protein (CREB), respectively.

CONCLUSION

The cholinergic transmission, positively regulated by NPF, is involved in feeding of O. furnacalis larvae via downstream PI3K and the CREB pathways, respectively. The deexcitation of cell cholinergic pathway or inhibition of PI3K and CREB lead to decreases of larval feeding amount. © 2023 Society of Chemical Industry.

Reversible binding of divalent cations to Ductin protein assemblies-A putative new regulatory mechanism of membrane traffic processes

Ductins are a family of homologous and structurally similar membrane proteins with 2 or 4 trans-membrane alpha-helices. The active forms of the Ductins are membranous ring- or star-shaped oligomeric assemblies and they provide various pore, channel, gap-junction functions, assist in membrane fusion processes and also serve as the rotor c-ring domain of V-and F-ATPases. All functions of the Ductins have been reported to be sensitive to the presence of certain divalent metal cations (Me2+), most frequently Cu2+ or Ca2+ ions, for most of the better known members of the family, and the mechanism of this effect is not yet known. Given that we have earlier found a prominent Me2+ binding site in a well-characterised Ductin protein, we hypothesise that certain divalent cations can structurally modulate the various functions of Ductin assemblies via affecting their stability by reversible non-covalent binding to them. A fine control of the stability of the assembly ranging from separated monomers through a loosely/weakly to tightly/strongly assembled ring might render precise regulation of Ductin functions possible. The putative role of direct binding of Me2+ to the c-ring subunit of active ATP hydrolase in autophagy and the mechanism of Ca2+-dependent formation of the mitochondrial permeability transition pore are also discussed.

Regulation of Immune Functions by Non-Neuronal Acetylcholine (ACh) via Muscarinic and Nicotinic ACh Receptors

Acetylcholine (ACh) is the classical neurotransmitter in the cholinergic nervous system. However, ACh is now known to regulate various immune cell functions. In fact, T cells, B cells, and macrophages all express components of the cholinergic system, including ACh, muscarinic, and nicotinic ACh receptors (mAChRs and nAChRs), choline acetyltransferase, acetylcholinesterase, and choline transporters. In this review, we will discuss the actions of ACh in the immune system. We will first briefly describe the mechanisms by which ACh is stored in and released from immune cells. We will then address Ca²⁺ signaling pathways activated via mAChRs and nAChRs on T cells and B cells, highlighting the importance of ACh for the function of T cells, B cells, and macrophages, as well as its impact on innate and acquired (cellular and humoral) immunity. Lastly, we will discuss the effects of two peptide ligands, secreted lymphocyte antigen-6/urokinase-type plasminogen activator receptor-related peptide-1 (SLURP-1) and hippocampal cholinergic neurostimulating peptide (HCNP), on cholinergic activity in T cells. Overall, we stress the fact that ACh does not function only as a neurotransmitter; it impacts immunity by exerting diverse effects on immune cells via mAChRs and nAChRs.

Acetylcholinesterase and human cancers

The enzyme acetylcholinesterase (AChE) is a serine hydrolase whose primary function is to degrade acetylcholine (ACh) and terminate neurotransmission. Apart from its role in synaptic transmission, AChE has several "non-classical" functions in non-neuronal cells. AChE is involved in cellular growth, apoptosis, drug resistance pathways, response to stress signals and inflammation. The observation that the functional activity of AChE is altered in human tumors (relative to adjacent matched normal tissue) has raised several intriguing questions about its role in the pathophysiology of human cancers. Published reports show that AChE is a vital regulator of oncogenic signaling pathways involving proliferation, differentiation, cell-cell adhesion, migration, invasion and metastasis of primary tumors. The objective of this book chapter is to provide a comprehensive overview of the contributions of the AChE-signaling pathway in the growth of progression of human cancers. The AChE isoforms, AChE-T, AChE-R and AChE-S are robustly expressed in human cancer cell lines as well in human tumors (isolated from patients). Traditionally, AChE-modulators have been used in the clinic for treatment of neurodegenerative disorders. Emerging studies reveal that these drugs could be repurposed for the treatment of human cancers. The discovery of potent, selective AChE ligands will provide new knowledge about AChE-regulatory pathways in human cancers and foster the hope of novel therapies for this disease.

Acetylcholine From Tuft Cells: The Updated Insights Beyond Its Immune and Chemosensory Functions

Tuft cells, rare solitary chemosensory cells, are distributed in mucosal epithelium throughout mammalian organs. Their nomenclatures are various in different organs and may be confused with other similar cells. Current studies mainly focus on their chemosensory ability and immune functions in type 2 inflammation. Several state-of-the-art reviews have already systematically discussed their role in immune responses. However, given that tuft cells are one of the crucial components of non-neuronal cholinergic system, the functions of tuft cell derived acetylcholine (ACh) and the underlying mechanisms remain intricate. Existing evidence demonstrated that tuft cell derived ACh participates in maintaining epithelial homeostasis, modulating airway remodeling, regulating reflexes, promoting muscle constriction, inducing neurogenic inflammation, initiating carcinogenesis and producing ATP. In this review, the ACh biosynthesis pathways and potential clinical applications of tuft cells have been proposed. More importantly, the main pathophysiological roles and the underlying mechanisms of tuft cell derived ACh are summarized and discussed.

Cholinergic leukocytes in sepsis and at the neuroimmune junction in the spleen

The spleen is a key participant in the pathophysiology of sepsis and inflammatory disease. Many splenocytes exhibit a cholinergic phenotype, but our knowledge regarding their cholinergic biology and how they are affected by sepsis is incomplete. We evaluated effects of acute sepsis on the spleen using the cecal ligation and puncture (CLP) model in C57BL/6 and ChAT^BAC-eGFP mice. Quantification of cholinergic gene expression showed that choline acetyltransferase and vesicular acetylcholine transporter (VAChT) are present and that VAChT is upregulated in sepsis, suggesting increased capacity for release of acetylcholine (ACh). High affinity choline transporter is not expressed but organic acid transporters are, providing additional mechanisms for release. Flow cytometry studies identified subpopulations of cholinergic T and B cells as well as monocytes/macrophages. Neither abundance nor GFP intensity of cholinergic T cells changed in sepsis, suggesting that ACh synthetic capacity was not altered. Spleens have low acetylcholinesterase activity, and the enzyme is localized primarily in red pulp, characteristics expected to favor cholinergic signaling. For cellular studies, ACh was quantified by mass spectroscopy using d₄-ACh internal standard. Isolated splenocytes from male mice contain more ACh than females, suggesting the potential for gender-dependent differences in cholinergic immune function. Isolated splenocytes exhibit basal ACh release, which can be increased by isoproterenol (4 and 24 h) or by T cell activation with antibodies to CD3 and CD28 (24 h). Collectively, these data support the concept that sepsis enhances cholinergic function in the spleen and that release of ACh can be triggered by stimuli via different mechanisms.

Functions of acetylcholine-producing lymphocytes in immunobiology

Recent advances in neuroscience and immunology have shown that cholinergic signals are vital in the regulation of inflammation and immunity. Choline acetyltransferase⁺ (ChAT⁺) lymphocytes have the capacity to biosynthesize and release acetylcholine, the cognate ligand for cholinergic receptors. Acetylcholine-producing T cells relay neural signals in the 'inflammatory reflex' that regulate cytokine release in spleen. Mice deficient in acetylcholine-producing T cells have increased blood pressure, show reduced local vasodilatation and viral control in lymphocytic choriomeningitis virus infection, and display changes in gut microbiota compared with littermates. These observations indicate that ChAT⁺ lymphocytes play physiologically important roles in regulation of inflammation and anti-microbial defense. However, the full scope and importance of ChAT⁺ lymphocytes in immunity and vascular biology remains to be elucidated. Here, we review key findings in this emerging area.

Acetylcholine signaling system in progression of lung cancers

The neurotransmitter acetylcholine (ACh) acts as an autocrine growth factor for human lung cancer. Several lines of evidence show that lung cancer cells express all of the proteins required for the uptake of choline (choline transporter 1, choline transporter-like proteins) synthesis of ACh (choline acetyltransferase, carnitine acetyltransferase), transport of ACh (vesicular acetylcholine transport, OCTs, OCTNs) and degradation of ACh (acetylcholinesterase, butyrylcholinesterase). The released ACh binds back to nicotinic (nAChRs) and muscarinic receptors on lung cancer cells to accelerate their proliferation, migration and invasion. Out of all components of the cholinergic pathway, the nAChR-signaling has been studied the most intensely. The reason for this trend is due to genome-wide data studies showing that nicotinic receptor subtypes are involved in lung cancer risk, the relationship between cigarette smoke and lung cancer risk as well as the rising popularity of electronic cigarettes considered by many as a "safe" alternative to smoking. There are a small number of articles which review the contribution of the other cholinergic proteins in the pathophysiology of lung cancer. The primary objective of this review article is to discuss the function of the acetylcholine-signaling proteins in the progression of lung cancer. The investigation of the role of cholinergic network in lung cancer will pave the way to novel molecular targets and drugs in this lethal malignancy.

Ultrafast and Slow Cholinergic Transmission. Different Involvement of Acetylcholinesterase Molecular Forms

Acetylcholine (ACh), an ubiquitous mediator substance broadly expressed in nature, acts as neurotransmitter in cholinergic synapses, generating specific communications with different time-courses. (1) Ultrafast transmission. Vertebrate neuromuscular junctions (NMJs) and nerve-electroplaque junctions (NEJs) are the fastest cholinergic synapses; able to transmit brief impulses (1-4 ms) at high frequencies. The collagen-tailed A12 acetylcholinesterase is concentrated in the synaptic cleft of NMJs and NEJs, were it curtails the postsynaptic response by ultrafast ACh hydrolysis. Here, additional processes contribute to make transmission so rapid. (2) Rapid transmission. At peripheral and central cholinergic neuro-neuronal synapses, transmission involves an initial, relatively rapid (10-50 ms) nicotinic response, followed by various muscarinic or nicotinic effects. Acetylcholinesterase (AChE) being not concentrated within these synapses, it does not curtail the initial rapid response. In contrast, the late responses are controlled by a globular form of AChE (mainly G4-AChE), which is membrane-bound and/or secreted. (3) SlowAChsignalling. In non-neuronal systems, in muscarinic domains, and in most regions of the central nervous system (CNS), many ACh-releasing structures (cells, axon terminals, varicosities, boutons) do not form true synaptic contacts, most muscarinic and also part of nicotinic receptors are extra-synaptic, often situated relatively far from ACh releasing spots. A12-AChE being virtually absent in CNS, G4-AChE is the most abundant form, whose function appears to modulate the "volume" transmission, keeping ACh concentration within limits in time and space.

Cholinergic modulation of the immune system presents new approaches for treating inflammation

The nervous system and immune system have broad and overlapping distributions in the body, and interactions of these ubiquitous systems are central to the field of neuroimmunology. Over the past two decades, there has been explosive growth in our understanding of neuroanatomical, cellular, and molecular mechanisms that mediate central modulation of immune functions through the autonomic nervous system. A major catalyst for growth in this field was the discovery that vagal nerve stimulation (VNS) caused a prominent attenuation of the systemic inflammatory response evoked by endotoxin in experimental animals. This effect was mediated by acetylcholine (ACh) stimulation of nicotinic receptors on splenic macrophages. Hence, the circuit was dubbed the "cholinergic anti-inflammatory pathway". Subsequent work identified the α7 nicotinic ACh receptor (α7nAChR) as the crucial target for attenuation of pro-inflammatory cytokine release from macrophages and dendritic cells. Further investigation made the important discovery that cholinergic T cells within the spleen and not cholinergic nerve cells were the source of ACh that stimulated α7 receptors on splenic macrophages. Given the important role that inflammation plays in numerous disease processes, cholinergic anti-inflammatory mechanisms are under intensive investigation from a basic science perspective and in translational studies of animal models of diseases such as inflammatory bowel disease and rheumatoid arthritis. This basic work has already fostered several clinical trials examining the efficacy of VNS and cholinergic therapeutics in human inflammatory diseases. This review provides an overview of basic and translational aspects of the cholinergic anti-inflammatory response and relevant pharmacology of drugs acting at the α7nAChR.

The cholinergic anti-inflammatory pathway: An innovative treatment strategy for neurological diseases

Acetylcholine (ACh), as a classical neurotransmitter, regulates the neuronal network in response to internal and external stimuli. In recent decades, the biology of ACh has been endowed with unparalleled new insights, especially with respect to cholinergic anti-inflammatory properties in non-neuronal cells. In fact, a mechanism frequently referred to as the "cholinergic anti-inflammatory pathway" has been termed to describe interactions between the central nervous system (CNS) and the immune system via vagus nerve. As well documented, immune cells express choline acetyltransferase, a direct synthetase for ACh, and other corresponding cholinergic components. Alternatively, the ACh released from immune cells or cholinergic neurons modulates immune function in an autocrine/paracrine manner by acting on its receptors. Moreover, muscarinic or nicotinic ACh receptors on various immune cells and CNS glial cells administer the work of their respective agonists, causing functional and biochemical changes. In this review, we focus on the anti-inflammatory benefits of non-neuronal and neuronal ACh as a means of providing new insights into treating inflammation-related neurological diseases, as exemplified by those described herein.

Acetylcholine released from T cells regulates intracellular Ca2+, IL-2 secretion and T cell proliferation through nicotinic acetylcholine receptor

AIMS

T lymphocytes synthesize acetylcholine (ACh) and express muscarinic and nicotinic ACh receptors (mAChR and nAChR, respectively) responsible for increases in the intracellular Ca²⁺ concentration ([Ca²⁺]_i). Our aim in the present study was to assess whether autocrine ACh released from T lymphocytes regulates their physiological functions.

MAIN METHODS

MOLT-3 human leukemic cell line and murine splenocytes were loaded with fura-2 to monitor [Ca²⁺]_i changes in the absence or presence of several AChR antagonists, including mecamylamine, methyllycaconitine and scopolamine. Real-time PCR and ELISA were performed to measure interleukin-2 (IL-2) mRNA and protein levels.

KEY FINDINGS

T lymphocytes constitutively produce sufficient amounts of ACh to elicit autocrine changes in [Ca²⁺]_i. These autocrine ACh-evoked [Ca²⁺]_i transients were mediated by nAChRs and then influx of extracellular Ca²⁺. Mecamylamine, a nAChR inhibitor, suppressed not only these [Ca²⁺]_i transients, but also IL-2 release and T cell proliferation.

SIGNIFICANCE

Here, we confirmed that T lymphocytes utilize ACh as a tool to interact with each other and that autocrine ACh-activated nAChRs are involved in cytokine release and cell proliferation. These findings suggest the possibility that nAChR agonists and antagonists and smoking are able to modulate immune function, which in turn suggests the therapeutic potential of immune activation or suppression using nAChR agonists or antagonists.

Dysregulated Homeostasis of Acetylcholine Levels in Immune Cells of RR-Multiple Sclerosis Patients

Multiple sclerosis (MS) is characterized by pro-inflammatory cytokine production. Acetylcholine (ACh) contributes to the modulation of central and peripheral inflammation. We studied the homeostasis of the cholinergic system in relation to cytokine levels in immune cells and sera of relapsing remitting-MS (RR-MS) patients. We demonstrated that lower ACh levels in serum of RR-MS patients were inversely correlated with the increased activity of the hydrolyzing enzymes acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). Interestingly, the expression of the ACh biosynthetic enzyme and the protein carriers involved in non-vesicular ACh release were found overexpressed in peripheral blood mononuclear cells of MS patients. The inflammatory state of the MS patients was confirmed by increased levels of TNFα, IL-12/IL-23p40, IL-18. The lower circulating ACh levels in sera of MS patients are dependent on the higher activity of cholinergic hydrolyzing enzymes. The smaller ratio of ACh to TNFα, IL-12/IL-23p40 and IL-18 in MS patients, with respect to healthy donors (HD), is indicative of an inflammatory environment probably related to the alteration of cholinergic system homeostasis.

Vesicular acetylcholine transporter knock down-mice are more susceptible to inflammation, c-Fos expression and sickness behavior induced by lipopolysaccharide

In addition to the well-known functions as a neurotransmitter, acetylcholine (ACh) can modulate of the immune system. Nonetheless, how endogenous ACh release inflammatory responses is still not clear. To address this question, we took advantage of an animal model with a decreased ACh release due a reduction (knockdown) in vesicular acetylcholine transporter (VAChT) expression (VAChT-KD(HOM)). These animals were challenged with lipopolysaccharide (LPS). Afterwards, we evaluated sickness behavior and quantified systemic and cerebral inflammation as well as neuronal activation in the dorsal vagal complex (DVC). VAChT-KD(HOM) mice that were injected with LPS (10mg/kg) showed increased mortality rate as compared to control mice. In line with this result, a low dose of LPS (0.1mg/kg) increased the levels of pro-inflammatory (TNF-α, IL-1β, and IL-6) and anti-inflammatory (IL-10) cytokines in the spleen and brain of VAChT-KD(HOM) mice in comparison with controls. Similarly, serum levels of TNF-α and IL-6 were increased in VAChT-KD(HOM) mice. This excessive cytokine production was completely prevented by administration of a nicotinic receptor agonist (0.4mg/kg) prior to the LPS injection. Three hours after the LPS injection, c-Fos expression increased in the DVC region of VAChT-KD(HOM) mice compared to controls. In addition, VAChT-KD(HOM) mice showed behavioral changes such as lowered locomotor and exploratory activity and reduced social interaction after the LPS challenge, when compared to control mice. Taken together, our results show that the decreased ability to release ACh exacerbates systemic and cerebral inflammation and promotes neural activation and behavioral changes induced by LPS. In conclusion, our findings support the notion that activity of cholinergic pathways, which can be modulated by VAChT expression, controls inflammatory and neural responses to LPS challenge.

Cholinergic Machinery as Relevant Target in Acute Lymphoblastic T Leukemia

Various types of non-neuronal cells, including tumors, are able to produce acetylcholine (ACh), which acts as an autocrine/paracrine growth factor. T lymphocytes represent a key component of the non-neuronal cholinergic system. T cells-derived ACh is involved in a stimulation of their activation and proliferation, and acts as a regulator of immune response. The aim of the present work was to summarize the data about components of cholinergic machinery in T lymphocytes, with an emphasis on the comparison of healthy and leukemic T cells. Cell lines derived from acute lymphoblastic leukemias of T lineage (T-ALL) were found to produce a considerably higher amount of ACh than healthy T lymphocytes. Additionally, ACh produced by T-ALL is not efficiently hydrolyzed, because acetylcholinesterase (AChE) activity is drastically decreased in these cells. Up-regulation of muscarinic ACh receptors was also demonstrated at expression and functional level, whereas nicotinic ACh receptors seem to play a less important role and not form functional channels in cells derived from T-ALL. We hypothesized that ACh over-produced in T-ALL may act as an autocrine growth factor and play an important role in leukemic clonal expansion through shaping of intracellular Ca²⁺ signals. We suggest that cholinergic machinery may be attractive targets for new drugs against T-ALL. Specifically, testing of high affinity antagonists of muscarinic ACh receptors as well as antagomiRs, which interfere with miRNAs involved in the suppression of AChE expression, may be the first choice options.

Acetylcholine and acetylcarnitine transport in peritoneum: Role of the SLC22A4 (OCTN1) transporter

A suitable experimental tool based on proteoliposomes for assaying Organic Cation Transporter Novel member 1 (OCTN1) of peritoneum was pointed out. OCTN1, recently acknowledged as acetylcholine transporter, was immunodetected in rat peritoneum. Transport was assayed following flux of radiolabelled TEA, acetylcholine or acetylcarnitine in proteoliposomes reconstituted with peritoneum extract. OCTN1 mediated, besides TEA, also acetylcholine and a slower acetylcarnitine transport. External sodium inhibited acetylcholine uptake but not its release from proteoliposomes. Differently, sodium did not affect acetylcarnitine uptake. These results suggested that physiologically, acetylcholine should be released while acetylcarnitine was taken up by peritoneum cells. Transport was impaired by OCTN1 inhibitors, butyrobetaine, spermine, and choline. Biotin was also found as acetylcholine transport inhibitor. Anti-OCTN1 antibody specifically inhibited acetylcholine transport confirming the involvement of OCTN1. The transporter was also immunodetected in human mesothelial primary cells. Extract from these cells was reconstituted in proteoliposomes. Transport features very similar to those found with rat peritoneum were observed. Validation of the proteoliposome model for peritoneal transport study was then achieved assaying transport in intact mesothelial cells. TEA, butyrobetaine and Na(+) inhibited acetylcholine transport in intact cells while efflux was Na(+) insensitive. Therefore transport features in intact cells overlapped those found in proteoliposomes.

Non-neuronal cholinergic system in regulation of immune function with a focus on α7 nAChRs

In 1929, Dale and Dudley described the first reported natural occurrence of acetylcholine (ACh) in an animal's body. They identified this ACh in the spleens of horses and oxen, which we now know suggests possible involvement of ACh in the regulation of lymphocyte activity and immune function. However, the source and function of splenic ACh were left unexplored for several decades. Recent studies on the source of ACh in the blood revealed ACh synthesis catalyzed by choline acetyltransferase (ChAT) in CD4(+) T cells. T and B cells, macrophages and dendritic cells (DCs) all express all five muscarinic ACh receptor subtypes (mAChRs) and several subtypes of nicotinic AChRs (nAChRs), including α7 nAChRs. Stimulation of these mAChRs and nAChRs by their respective agonists causes functional and biochemical changes in the cells. Using AChR knockout mice, we found that M(1)/M(5) mAChR signaling up-regulates IgG(1) and pro-inflammatory cytokine production, while α7 nAChR signaling has the opposite effect. These findings suggest that ACh synthesized by T cells acts in an autocrine/paracrine fashion at AChRs on various immune cells to modulate immune function. In addition, an endogenous allosteric and/or orthosteric α7 nAChR ligand, SLURP-1, facilitates functional development of T cells and increases ACh synthesis via up-regulation of ChAT mRNA expression. SLURP-1 is expressed in CD205(+) DCs residing in the tonsil in close proximity to T cells, macrophages and B cells. Collectively, these findings suggest that ACh released from T cells along with SLURP-1 regulates cytokine production by activating α7 nAChRs on various immune cells, thereby facilitating T cell development and/or differentiation, leading to immune modulation.

Expression of choline and acetylcholine transporters in synovial tissue and cartilage of patients with rheumatoid arthritis and osteoarthritis

Increasing evidence is showing that the non-neuronal cholinergic system plays an important role in the pathology of rheumatoid arthritis (RA). Choline transport into the cell is the rate-limiting step for the synthesis of acetylcholine (ACh), which can be released directly or in vesicles from the cell. However, in the human joint little is known about choline import or the release of ACh from the cell. Thus, we analyze the expression of members of the organic cation transporter (OCT), of the newly discovered choline transporter-like (CTL) family and of classical neuronal components such as the high-affinity choline transporter (CHT1) and the vesicular ACh transporter (VAChT) in the synovium and cartilage of the human hip joint from patients with osteoarthritis (OA) and RA. OCT1, OCT3 and OCTN1 and all members of the CTL family were expressed in synovial and cartilage samples. The expression of CTL1 and CTL2 was localized in synovial macrophages and fibroblasts. CHT1 mRNA expression was detectable only in the synovium, whereas VAChT was completely absent in all samples. Therefore, in the human joint, choline transport into the cell and the release of ACh seems to be mediated mainly by members of the OCT and CTL family. Expression of transporters appears not to be influenced by the pathological state, as no differences have been detected between joints from OA or RA patients. Importantly, however, all necessary components for choline import and the release of non-neuronal ACh are present in the human joint.

Reconciling neuronally and nonneuronally derived acetylcholine in the regulation of immune function

Immune cells, including lymphocytes, express muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs, respectively), and agonist stimulation of these AChRs causes functional and biochemical changes in the cells. The origin of the ACh that acts on immune cell AChRs has remained unclear until recently, however. In 1995, we identified choline acetyltransferase mRNA and protein in human T cells, and found that immunological T cell activation potentiated lymphocytic cholinergic transmission by increasing ACh synthesis and AChR expression. We also found that M(1) /M(5) mAChR signaling upregulates IgG(1) and proinflammatory cytokine production, whereas α7 nAChR signaling has the opposite effect. These findings suggest that ACh synthesized by T cells acts as an autocrine and/or paracrine factor via AChRs on immune cells to modulate immune function. In addition, a recently discovered endogenous allosteric α7 nAChR ligand, SLURP-1, also appears to be involved in modulating normal T cell function.

Rethinking inflammation: neural circuits in the regulation of immunity

Neural reflex circuits regulate cytokine release to prevent potentially damaging inflammation and maintain homeostasis. In the inflammatory reflex, sensory input elicited by infection or injury travels through the afferent vagus nerve to integrative regions in the brainstem, and efferent nerves carry outbound signals that terminate in the spleen and other tissues. Neurotransmitters from peripheral autonomic nerves subsequently promote acetylcholine-release from a subset of CD4(+) T cells that relay the neural signal to other immune cells, e.g. through activation of α7 nicotinic acetylcholine receptors on macrophages. Here, we review recent progress in the understanding of the inflammatory reflex and discuss potential therapeutic implications of current findings in this evolving field.

The mechanisms and possible sites of acetylcholine release during chick primary sensory neuron differentiation

AIMS

In this study, we evaluated the ability of differentiating embryonic chick DRG neurons to release and respond to acetylcholine (ACh). In particular, we investigated the neuronal soma and neurites as sites of ACh release, as well as the mechanism(s) underlying this release.

MAIN METHODS

ACh release from DRG explants in the Campenot chambers was measured by a chemiluminescent assay. Real-time PCR analysis was used to evaluate the expression of ChAT, VAChT, mediatophore and muscarinic receptor subtypes in DRGs at different developmental stages.

KEY FINDINGS

We found that ACh is released both within the central and lateral compartments of the Campenot chambers, indicating that ACh might be released from both the neuronal soma and fibers. Moreover, we observed that the expression of the ChAT and mediatophore increases during sensory neuron differentiation and during the post-hatching period, whereas VAChT expression decreases throughout development. Lastly, the kinetics of the m2 and m3 transcripts appeared to change differentially compared to the m4 transcript during the same developmental period.

SIGNIFICANCE

The data obtained demonstrate that the DRG sensory neurons are able to release ACh and to respond to ACh stimulation. ACh is released both by the soma and neurite compartments. The contribution of the mediatophore to ACh release appears to be more significant than that of VAChT, suggesting that the non-vesicular release of ACh might represent the preferential mechanism of ACh release in DRG neurons and possibly in non-cholinergic systems.

Regulatory mechanisms of acetylcholine synthesis and release by T cells

AIMS

Muscarinic and nicotinic acetylcholine (ACh) receptors are expressed in immune cells. ACh synthesized by choline acetyltransferase (ChAT) and released in T cells binds to these receptors. Furthermore, we have recently demonstrated the involvement of mediatophore, a homooligomer of a 16-kDa proteolipid subunit of vacuolar H(+)-ATPase, in ACh release from T cells. In this study, we investigated the effects of phorbol 12-myristate 13-acetate (PMA), dibutyryl cAMP (dbcAMP) and FK506, an immunosuppressant calcineurin inhibitor, on lymphocytic cholinergic activity in T cells.

MAIN METHODS

We determined the content and release of ACh in human leukemic T cell line MOLT-3 cells using a sensitive and specific radioimmunoassay for ACh. In addition, expression of ChAT mRNA and ChAT activity were investigated using reverse-transcription-polymerase chain reaction and Fonnum method, respectively.

KEY FINDINGS

Phytohemagglutinin (PHA), a T-cell activator, up-regulated ChAT mRNA expression, synthesis and release of ACh. PMA, a protein kinase C (PKC) activator, and dbcAMP, a protein kinase A (PKA) activator, also increased ChAT activity and ACh synthesis by up-regulating ChAT gene expression. FK506 inhibited PHA-induced up-regulation of ChAT mRNA expression, suggesting the involvement of calcineurin-mediated pathways in ChAT gene transcription.

SIGNIFICANCE

Activation of PKC and PKA up-regulates ACh synthesis in T cells, and immunological activation triggers ChAT gene transcription through calcineurin-mediated pathways.