beta(2)-adrenergic receptor signaling and desensitization elucidated by quantitative modeling of real time cAMP dynamics
ABSTRACT G protein-coupled receptor signaling is dynamically regulated by multiple feedback mechanisms, which rapidly attenuate signals elicited by ligand stimulation, causing desensitization. The individual contributions of these mechanisms, however, are poorly understood. Here, we use an improved fluorescent biosensor for cAMP to measure second messenger dynamics stimulated by endogenous beta(2)-adrenergic receptor (beta(2)AR) in living cells. beta(2)AR stimulation with isoproterenol results in a transient pulse of cAMP, reaching a maximal concentration of approximately 10 microm and persisting for less than 5 min. We investigated the contributions of cAMP-dependent kinase, G protein-coupled receptor kinases, and beta-arrestin to the regulation of beta(2)AR signal kinetics by using small molecule inhibitors, small interfering RNAs, and mouse embryonic fibroblasts. We found that the cAMP response is restricted in duration by two distinct mechanisms in HEK-293 cells: G protein-coupled receptor kinase (GRK6)-mediated receptor phosphorylation leading to beta-arrestin mediated receptor inactivation and cAMP-dependent kinase-mediated induction of cAMP metabolism by phosphodiesterases. A mathematical model of beta(2)AR signal kinetics, fit to these data, revealed that direct receptor inactivation by cAMP-dependent kinase is insignificant but that GRK6/beta-arrestin-mediated inactivation is rapid and profound, occurring with a half-time of 70 s. This quantitative system analysis represents an important advance toward quantifying mechanisms contributing to the physiological regulation of receptor signaling.
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ABSTRACT: The second messengers cAMP and cGMP transduce many neuromodulatory signals from hormones and neurotransmitters into specific functional outputs. Their production, degradation and signaling are spatiotemporally regulated to achieve high specificity in signal transduction. The development of genetically encodable fluorescent biosensors has provided researchers with useful tools to study these versatile second messengers and their downstream effectors with unparalleled spatial and temporal resolution in cultured cells and living animals. In this review, we introduce the general design of these fluorescent biosensors and describe several of them in more detail. Then we discuss a few examples of using cyclic nucleotide fluorescent biosensors to study regulation of neuronal function and finish with a discussion of advances in the field. Although there has been significant progress made in understanding how the specific signaling of cyclic nucleotide second messengers is achieved, the mechanistic details in complex cell types like neurons are only just beginning to surface. Current and future fluorescent protein reporters will be essential to elucidate the role of cyclic nucleotide signaling dynamics in the functions of individual neurons and their networks.Frontiers in Cellular Neuroscience 11/2014; 8(395). DOI:10.3389/fncel.2014.00395 · 4.18 Impact Factor
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ABSTRACT: G-protein Coupled Receptors (GPCRs) are involved in a variety of disease processes and comprise major drug targets. However, the complexity of integral membrane proteins such as GPCRs makes the identification of their interacting partners and subsequent drug development challenging. A comprehensive understanding of GPCR protein interaction networks is needed to design effective therapeutic strategies to inhibit these drug targets. Here, we developed a novel split-ubiquitin membrane yeast two-hybrid (MYTH) technology called CHIP-MYTH, which allows the unbiased characterization of interaction partners of full-length GPCRs in a drug-dependent manner. This was achieved by coupling DNA microarray technology to the MYTH approach, which allows a quantitative evaluation of interacting partners of a given integral membrane protein in the presence or absence of drug. As a proof of principle, we applied the CHIP-MYTH approach to the human ß2-adrenergic receptor (ß2AR), a target of interest in the treatment of asthma, chronic obstructive pulmonary disease (COPD), neurological disease, cardiovascular disease, and obesity. A CHIP-MYTH screen was performed in the presence or absence of salmeterol, a long-acting ß2AR-agonist. Our results suggest that ß2AR activation with salmeterol can induce the dissociation of heterotrimeric G-proteins, G, into G and G subunits, which in turn activates downstream signaling cascades. Using CHIP-MYTH, we confirmed previously known and identified novel ß2AR interactors involved in GPCR-mediated signaling cascades. Several of these interactions were confirmed in mammalian cells using LUMIER and co-immunoprecipitation assays. In summary, the CHIP-MYTH approach is ideal for conducting comprehensive PPI screenings of full-length GPCRs in the presence or absence of drugs, thus providing a valuable tool to further our understanding of GPCR-mediated signaling.Biochemical and Biophysical Research Communications 03/2014; 445(4). DOI:10.1016/j.bbrc.2014.02.033 · 2.28 Impact Factor
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ABSTRACT: The cattle tick Rhipicephalus microplus, is one of the most damaging livestock ectoparasites. Tropical tick infestation limits the introduction of high-yield bovine varieties because they do not have immunity to the diseases transmitted by these ectoparasites. This tick is usually controlled with chemical acaricides but their indiscriminate use has created resistant populations. The discovery of new molecules that can be used for tick control is urgent. Based on the knowledge that octopamine, a biogenic amine analog to epinephrine, is central to the regulation of oviposition in several studied arthropods and that an imbalance in octopamine release causes sterility in a Drosophila model. Tyramine, octopamine and epinastine and 83 adrenergic compounds classified by their effect in the vertebrate systems were screened for their ability to block oviposition in Rhipicephalus microplus. Of these molecules, we found that 10 alpha-agonists, 3 alpha-antagonists, 5 beta-adrenergic agonists, 7 beta-antagonists and Norepinephrine were able to inhibit oviposition in this tick at pharmacological concentrations. Surprisingly, tyramine appears to be more potent than octopamine. The probable physiological causes of this inhibition are discussed. Our results suggest that although there are alpha adrenergic-like receptors in the tick, they do not behave in a manner completely analogous to their vertebrate counterparts.