Plastic adenylyl cyclase.

Signal transduction via receptors that couple to guanine nucleotide binding proteins (G proteins) is pervasive throughout eukaryotic and prokaryotic biology. In humans, there are thought to be thousands of different G protein–coupled receptors (GPCRs), which carry out an amazingly diverse array of signaling, such as odorant detection, phototransduction, central nervous system neurotransmission, and sympathetic and parasympathetic nervous system functions, to name only a few broad categories. All GPCRs have a similar structure: an extracellular amino-terminus, seven transmembrane-spanning domains, three extracellular and three intracellular loops, and an intracellular carboxy terminus. In simplistic terms, GPCR signaling appears to be like that of a switch: an agonist binds to a receptor, whose conformation then favors binding and activation of a G protein, which then alters the activity of an effector such as an enzyme or channel. In keeping with the analogy to electronic circuits, several aspects of this signaling have become clear (Figure 1). First, GPCR signaling is not like that of a simple switch but rather more like a group of amplifiers , attenuators, and timers . Second, signal splitting occurs immediately after agonist binding, activating a number of different pathways relevant to asthma. In addition, there are multiple levels of feedback inhibition and potentiation. Also, these circuits have both series and parallel components with little in the way of pure signal isolation. Each signal is multistaged so that modulation can occur at various levels. Many of the GPCRs couple to three G-protein families: G s , G i , and G q . G s -coupled receptors are typified by the b 2 -adrenergic receptor ( b 2 AR), which serves to stimulate adenylyl cyclase. Such stimulation acts to convert adenotriphosphate (ATP) to cyclic adenosine monophosphate (cAMP), which then activates protein kinase A (PKA). PKA acts to inhibit the production of inositol phosphates, and phosphorylates myosin light-chain kinase, cell surface K 1 channels, a Na 1 /K 1 adenosine triphosphase (ATPase), phospholamban, and one or more pumps that lead to sarcoplasmic reticulum uptake of Ca 2 1 . The net effect is a decrease in intracellular Ca 2 1 and phosphorylation of contractile proteins, which results in smooth-muscle relaxation (1, 2). Receptors that couple to the inhibitory G protein G i (such as the M2 muscarinic receptor) inhibit adenylyl cyclase activity. This is particularly obvious when adenylyl cyclase is being activated; but even in the absence of stimulation, basal cAMP levels can be lowered by activation of G i -coupled receptors. Receptors coupling to the G q class of G proteins lead to activation of phospholipase C, with generation of 1,2-diacylglycerol and inositol 1,4,5 triphosphate (IP 3 ). The former activates some protein kinase C (PKC) isoforms, and the latter increases intracellular calcium via IP 3 receptors. The M3-muscarinic receptor and the cysteinyl leukotriene-1 receptor couple through G q . All G proteins consist of a , b , and g subunits, which are maintained in the heterotrimeric form in the absence of coupling to a receptor. Contact with the receptor in the agonist conformation results in the binding of the a subunit to the receptor and the dissociation of the bg subunits, which remain bound to each other. Traditionally, the G a subunits have been considered the primary signaling element because they interact with effectors such as adenylyl cyclase. It is now appreciated that G bg also initiates other signaling pathways independent from the a subunit (3). The intrinsic guanosine triphosphatase (GTPase) activity of activated G a , along with the action of a newly discovered group of proteins termed regulators of G protein signaling (RGS proteins), act to terminate the cycle, leading to dissociation of G a from the receptor and reformation of the inactive heterotrimer. The b 2 AR of airway smooth muscle is the target of b -agonists, used therapeutically for the treatment of bronchospasm in asthma and chronic obstructive pulmonary disease. As such, the mechanisms of b 2 AR regulation have been a subject of intense interest. Three broad categories of b 2 AR regulation have been identified: genetic (polymorphisms), agonist-promoted, and that due to crosstalk with other receptors (4). Rapid and long-term regulation of signaling by b 2 AR and other GPCRs serves an important adaptive function in that it permits the cell to integrate the myriad of signals being received via these receptors. As such, the plasticity of GPCR signaling makes it difficult to know at any given instant which regulatory factors are at play. Nevertheless, substantial progress has been made regarding regulation of b 2 AR function by agonist (5). Brief (seconds to minutes) agonist exposure results in phosphorylation of b 2 AR by PKA and the b AR kinase ( b ARK). Phosphorylation by the former kinase probably serves to directly depress coupling of the receptor to G s , whereas b ARK-mediated desensitization requires the binding of b -arrestins to the phosphorylated re( Received in original form September 9, 1999 )