m3 Muscarinic Acetylcholine Receptor Regulation in the Airway (2002)

The m3 muscarinic acetylcholine receptor (m3 mAChR) plays an important role in airway function by mediating the effects of acetylcholine on multiple airway cell types. m3 mAChRs expressed in airway smooth muscle (ASM) cells promote increased ASM tension, and, therefore, airway narrowing, in response to acetylcholine release from postganglionic parasympathetic nerves innervating the airway (1). In addition, m3 mAChRs have been implicated in the regulation of mucous secretion in submucosal glands (2) and in chemotactic mediator release in alveolar macrophages (3). Thus, multiple cellular functions that impact resistance to airflow are under the control of m3 mAChRs. Accordingly, insight into the mechanisms by which m3 mAChRs are activated and regulated in the airway are of potential value in understanding obstructive airway diseases and their effective management. In 1987 two groups, working independently, cloned the cDNA encoding the m3 mAChR (4, 5). As was the case with the cloning of other G protein‐coupled receptors (GPCRs), this facilitated extensive investigation into the pharmacology, signaling, and regulation of the receptor by enabling its heterologous expression in various cell culture systems. Although the m3 mAChR has the capacity to activate multiple signaling pathways in various cell types (6‐9), activation of phospholipase C (PLC) via the intermediary heterotrimeric G protein Gq is the predominant pathway through which the m3 mAChR regulates important airway cell functions such as ASM contraction (10, 11) (Figure 1). PLC activation induces protein kinase C (PKC) activation and inositol 1,4,5- trisphosphate (IP 3 ) generation, which serve to increase intracellular Ca 2 � and sensitize and activate the cell’s contractile machinery (12, 13). This basic signaling paradigm is also common to other GPCRs (e.g., the H1 histamine and cysteinyl leukotriene-1 receptors) whose stimulation also induces ASM contraction. m3 mAChR‐mediated transmembrane signaling has been shown to be a highly regulated progress. Regulation of the activities of Gq and PLC, via either their phosphorylation, subcellular localization, or changes in expression levels, has been shown to affect signaling via the m3 mAChR and other Gq-coupled receptors (9, 14). Similar mechanisms are employed at the receptor locus to modulate m3 mAChR signaling (Figure 2). Rapid desensitization of the m3 mAChR, defined as a loss of agonist-stimulated G protein activation or phosphoinositide generation, has been observed in various cell types (15‐17). As is the case with many GPCRs, the m3 mAChR is subject to phosphorylation by GPCR kinases (GRKs) upon binding agonist (17‐19). Phosphorylation by GRKs promotes receptor desensitization by partially uncoupling the receptor from G protein (reviewed in Refs. 20, 21). Interestingly, the m3 mAChR does not appear to be phosphorylated or regulated by protein kinase A or PKC (as are many other GPCRs), although a role for casein kinase 1 � in regulation of agonist-dependent m3 mAChR phosphorylation and desensitization has been demonstrated (22). Phosphorylation by GRKs promotes binding of arrestin molecules to the receptor, which more effectively uncouples the receptor from G protein by sterically inhibiting the receptor‐G protein interaction. For numerous GPCRs, GRK-mediated arrestin binding also initiates receptor endocytosis/internalization or “sequestration” (from G protein), which occurs via the association of the receptor/arrestin complex with components of clathrin-coated pits (23). m3 mAChRs have been shown to undergo agonist-dependent sequestration in multiple cell types, although conflicting data exist regarding the role of arrestins in this process, with some studies suggesting that an arrestin-independent mechanism of m3 mAChR sequestration may exist (24, 25).