Figure 1

Gq-coupled receptor signaling in airway smooth muscle. Airway smooth muscle (ASM) is innervated by postganglionic parasympathetic nerves that release acetylcholine (acting on m3 mAChRs) to control resting ASM tone. In addition to the m3 muscarinic acetylcholine receptor (mAChR), other Gq-coupled coupled receptors are expressed in ASM (see Table 1), and can similarly mediate contraction and other depicted ASM functions. Transmembrane signaling of G protein-coupled receptors (GPCRs) involves sequential activation of receptor, G protein, and effector. Upon agonist binding, the receptor undergoes a conformational change exposing a high-affinity binding site for a G-protein in its GDP-bound inactive state. The receptor specifically interacts with the C-terminus of the α subunit of the G-protein heterotrimer. G-protein binding to receptor releases the nucleotide leaving an empty nucleotide binding pocket readily occupied by GTP, which exists at a higher cytosolic concentration than GDP. This exchange of the G-protein-bound GDP for GTP induces a conformational change in the switch region of Gα and causes the dissociation of Gα from the Gβγ dimer. The Gβ and Gγ subunits are tightly associated and remain anchored into the lipid bilayer due to the prenylation of the Gγ subunit – a permanent lipid modification. In the case of Gαq, the GTP-bound Gα q-protein's effector interaction domain is exposed and activates phospholipase C (PLC). PLC promotes the hydrolysis of phosphoinositol 4,5-bisphosphate (PIP2) into the intracellular messengers 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃). DAG remains membrane bound and promotes the translocation of protein kinase C (PKC) from the cytoplasm to the membrane and its subsequent activation. Activated PKC is capable of phosphorylating a number of substrates including calponin; PKC-mediated phosphorylation of calponin results in a loss of calponin's ability to inhibit actomyosin ATPase [30, 269]. PKC also phosphorylates intermediates of MAPK signaling pathways, which activate various gene transcription factors involved in promoting ASM growth. Gq-coupled receptors are also able to impact receptor tyrosine kinase-induced ASM growth via a synergistic activation of p70S6K. Both PKC and p42/p44 MAPK phosphorylate and stimulate the catalytic activity of phospholipase A2 (PLA2). Calcium binding to PLA2 triggers its association with the plasma or nuclear membrane and the subsequent cleaving and release of arachadonic acid (AA). The conversion of AA to prostaglandins and thromboxanes is facilitated by cyclo-oxygenase-2, a highly regulated enzyme upregulated by pro-inflammatory agents including lipopolysaccharide, cytokines and growth factors. The other product of PIP₂ hydrolysis, IP₃, translocates and binds to IP₃ receptors located on sarcoplasmic calcium stores. Activation of IP₃ receptors results in the opening of Ca²⁺ channels and calcium efflux into the cytosol. Intracellular calcium stores are the major source of elevated calcium mediating ASM contraction, although influx from receptor-operated calcium channels can contribute. The rise in intracellular calcium promotes calcium binding to calmodulin forming calcium-calmodulin complexes that activate myosin light chain kinase (MLCK). MLCK phosphorylates myosin light chains and enables actin to activate the myosin ATPase activity required for cross-bridge cycling and contraction. Via its interaction with various guanine-nucleotide exchange factors for Rho (RhoGEFs), Gq has also recently been shown to activate the small G protein Rho [270]. In ASM, Gq-mediated activation of Rho has been implicated in regulating actin cytoskeletal rearrangement [40]. Rho is also a key mediator of calcium sensitization – a phenomenon observed following stimulation with numerous GPCRs whereby heightened contractile effects can be induced for a given level of calcium mobilization. Rho activates Rho kinase, which in turn phosphorylates the myosin binding subunit of myosin light chain phosphatase (MLCP) to inhibit phosphatase activity, resulting in net increased phosphorylation of myosin light chain (MLC) and an associated increase in cross-bridge cycling [271]. Although activation of G_12/13 is most commonly associated with Rho activity, studies of ASM suggest that Gq and Gi can also participate in Rho-mediated functions [40, 272, 273].