Muscarinic Acetylcholine Receptors Modulate Interleukin-6 Production and Immunoglobulin Class Switching in Daudi Cells

Muscarinic acetylcholine receptors regulate inflammatory responses through arginases 1/2 in zebrafish

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in various effector cells and have been proved to play vital roles in smooth muscle contraction and digestive secretion. However, there are relatively few literatures revealing the roles of mAChRs in inflammatory processes, and its underlying regulatory mechanisms have not been elucidated. Taking the advantages of live imaging of zebrafish, we found that inhibition of mAChRs resulted in increased neutrophils recruitment and proinflammatory cytokines expression, whereas activation of mAChRs led to opposite outcome. Subsequently, we found that mAChRs regulated the expression of arginases (args), and pharmacological intervention of args level could reverse the effects of mAChRs on neutrophils migration and cytokines expression, suggesting that args are important downstream proteins of mAChRs that mediate the regulation of inflammatory response. In this study, we identified args as novel downstream proteins of mAChRs in inflammatory responses, providing additional evidence for system immune regulation of cholinergic receptors.

Non-neuronal Cholinergic Muscarinic Acetylcholine Receptors in the Regulation of Immune Function

Immune cells such as T and B cells, monocytes and macrophages all express most of the cholinergic components of the nervous system, including acetylcholine (ACh), choline acetyltransferase (ChAT), high affinity choline transporter, muscarinic and nicotinic ACh receptors (mAChRs and nAChRs, respectively), and acetylcholinesterase (AChE). Because of its efficient cleavage by AChE, ACh synthesized and released from immune cells acts only locally in an autocrine and/or paracrine fashion at mAChRs and nAChRs on themselves and other immune cells located in close proximity, leading to modification of immune function. Immune cells generally express all five mAChR subtypes (M₁-M₅) and neuron type nAChR subunits α2-α7, α9, α10, β2-β4. The expression pattern and levels of mAChR subtypes and nAChR subunits vary depending on the tissue involved and its immunological status. Immunological activation of T cells via T-cell receptor-mediated pathways and cell adhesion molecules upregulates ChAT expression, which facilitates the synthesis and release of ACh. At present, α7 nAChRs expressed in macrophages are receiving much attention because they play a central role in anti-inflammatory cholinergic pathways. However, it now appears that through modification of cytokine synthesis, G_q/11-coupled mAChRs play a prominent role in regulation of T cell proliferation and differentiation and B cell immunoglobulin class switching. It is anticipated that greater understanding of G_q/11-coupled mAChRs on immune cells will provide an opportunity to develop new and effective treatments for immunological disorders.

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.

Cholinergic System and Its Therapeutic Importance in Inflammation and Autoimmunity

Neurological and immunological signals constitute an extensive regulatory network in our body that maintains physiology and homeostasis. The cholinergic system plays a significant role in neuroimmune communication, transmitting information regarding the peripheral immune status to the central nervous system (CNS) and vice versa. The cholinergic system includes the neurotransmitter\ molecule, acetylcholine (ACh), cholinergic receptors (AChRs), choline acetyltransferase (ChAT) enzyme, and acetylcholinesterase (AChE) enzyme. These molecules are involved in regulating immune response and playing a crucial role in maintaining homeostasis. Most innate and adaptive immune cells respond to neuronal inputs by releasing or expressing these molecules on their surfaces. Dysregulation of this neuroimmune communication may lead to several inflammatory and autoimmune diseases. Several agonists, antagonists, and inhibitors have been developed to target the cholinergic system to control inflammation in different tissues. This review discusses how various molecules of the neuronal and non-neuronal cholinergic system (NNCS) interact with the immune cells. What are the agonists and antagonists that alter the cholinergic system, and how are these molecules modulate inflammation and immunity. Understanding the various functions of pharmacological molecules could help in designing better strategies to control inflammation and autoimmunity.