G protein-coupled receptors: heterologous regulation of homologous desensitization and its implications

Milk fat-globule epidermal growth factor 8: A potential Regulator of Cutaneous Wound Healing

Destroying the integrity of the skin may causes disability and even death from injury or illness. Wound healing is a core mechanism to maintain skin barrier function. Milk fat-globule epidermal growth factor 8 (MFG-E8) is a key factor in wound healing and is involved in regulating blood coagulation, mediating macrophage uptake of apoptotic cells, shifting macrophages from an inflammatory to an anti-inflammatory phenotype, promoting angiogenesis, enhancing vascular endothelial growth factor (VEGF) signaling, and assisting wound tissue perfusion. However, these abilities are dysregulated in pathological conditions, such as glucose disorders and ischemic injury. Restricted application of exogenous MFG-E8 can restore function and play a beneficial role in cutaneous wound healing.

Hyperinsulinemia promotes heterologous desensitization of β2 adrenergic receptor in airway smooth muscle in obesity

β-Adrenergic receptor (β-AR) agonists are the most common clinical bronchodilators for asthma. Obesity influences asthma severity and may impair response to β-AR agonists. Previous studies show that in obese mice, hyperinsulinemia plays a crucial role in β-AR desensitization in the heart. We therefore investigated whether insulin promotes β-AR desensitization in airway smooth muscle (ASM) and compromises airway relaxation responsiveness to β-AR agonists. We found that human ASM cells and mouse airway tissues exposed to insulin exhibit impaired β₂ AR-induced cAMP accumulation and airway relaxation. This impaired relaxation is associated with insulin-induced phosphorylation and expression of phosphodiesterase 4D (PDE4D) through transactivation of a G protein-coupled receptor kinase 2 (GRK2)-dependent β₂ AR-G_i -ERK1/2 cascade. Both acute and chronic pharmacological inhibition of PDE4 effectively reversed impaired β₂ AR-mediated ASM relaxation in an obesity mouse model induced by a high fat diet. Collectively, these findings reveal that cross talk between insulin and β₂ AR signaling promotes ASM β₂ AR desensitization in obesity through upregulation of PDE4D phosphorylation and expression. Our results identify a novel pathway of asthma pathogenesis in patients with obesity/metabolic syndrome, in which the GRK2-mediated signaling can be a potential therapeutic modality to prevent or treat β₂ AR desensitization in ASM. Moreover, PDE4 inhibitors may be used as efficacious therapeutic agents for asthma in obese and diabetic subjects.

Designer Approaches for G Protein-Coupled Receptor Modulation for Cardiovascular Disease

The new horizon for cardiac therapy may lie beneath the surface, with the downstream mediators of G protein–coupled receptor (GPCR) activity. Targeted approaches have shown that receptor activation may be biased toward signaling through G proteins or through GPCR kinases (GRKs) and β-arrestins, with divergent functional outcomes. In addition to these canonical roles, numerous noncanonical activities of GRKs and β-arrestins have been demonstrated to modulate GPCR signaling at all levels of receptor activation and regulation. Further, research continues to identify novel GRK/effector and β-arrestin/effector complexes with distinct impacts on cardiac function in the normal heart and the diseased heart. Coupled with the identification of once orphan receptors and endogenous ligands with beneficial cardiovascular effects, this expands the repertoire of GPCR targets. Together, this research highlights the potential for focused therapeutic activation of beneficial pathways, with simultaneous exclusion or inhibition of detrimental signaling, and represents a new wave of therapeutic development.

The evidence on tiotropium bromide in asthma: from the rationale to the bedside

Severe and poorly controlled asthma still accounts for a great portion of the patients affected. Disease control and future risk management have been identified by international guidelines as the main goals in patients with asthma. The need for new treatment approaches has led to reconsider anticholinergic drugs as an option for asthma treatment. Tiotropium is the first anticholinergic drug that has been approved for children and adults with poorly controlled asthma and is currently considered as an option for steps 4 and 5 of the Global Initiative for Asthma. In large randomized clinical trials enrolling patients with moderate to severe asthma, add-on therapy with tiotropium has demonstrated to be efficacious in improving lung function, decreasing risk of exacerbation and slowing the worsening of disease; accordingly, tiotropium demonstrated to be non inferior compared to long acting beta-agonists in the maintenance treatment along with medium to high inhaled corticosteroids. In view of the numerous ancillary effects acting on inflammation, airway remodeling, mucus production and cough reflex, along with the good safety profile and the broad spectrum of efficacy demonstrated in different disease phenotypes, tiotropium can represent a beneficial alternative in the therapeutic management of poorly controlled asthma. The present extensive narrative review presents the pharmacological and pathophysiological basis that guided the rationale for the introduction of tiotropium in asthma treatment algorithm, with a particular focus on its conventional and unconventional effects; finally, data on tiotropium efficacy and safety. from recent randomized clinical trials performed in all age categories will be extensively discussed.

Heterologous desensitization of cardiac β-adrenergic signal via hormone-induced βAR/arrestin/PDE4 complexes

AIMS

Cardiac β-adrenergic receptor (βAR) signalling is susceptible to heterologous desensitization by different neurohormonal stimuli in clinical conditions associated with heart failure. We aim to examine the underlying mechanism of cross talk between βARs and a set of G-protein coupled receptors (GPCRs) activated by hormones/agonists.

METHODS AND RESULTS

Rat ventricular cardiomyocytes were used to determine heterologous phosphorylation of βARs under a series of GPCR agonists. Activation of Gs-coupled dopamine receptor, adenosine receptor, relaxin receptor and prostaglandin E2 receptor, and Gq-coupled α1 adrenergic receptor and angiotensin II type 1 receptor promotes phosphorylation of β1AR and β2AR at putative protein kinase A (PKA) phosphorylation sites; but activation of Gi-coupled α2 adrenergic receptor and activation of protease-activated receptor does not. The GPCR agonists that promote β2AR phosphorylation effectively inhibit βAR agonist isoproterenol-induced PKA phosphorylation of phospholamban and contractile function in ventricular cardiomyocytes. Heterologous GPCR stimuli have minimal to small effect on isoproterenol-induced β2AR activation and G-protein coupling for cyclic adenosine monophosphate (cAMP) production. However, these GPCR stimuli significantly promote phosphorylation of phosphodiesterase 4D (PDE4D), and recruit PDE4D to the phosphorylated β2AR in a β-arrestin 2 dependent manner without promoting β2AR endocytosis. The increased binding between β2AR and PDE4D effectively hydrolyzes cAMP signal generated by subsequent stimulation with isoproterenol. Mutation of PKA phosphorylation sites in β2AR, inhibition of PDE4, or genetic ablation of PDE4D or β-arrestin 2 abolishes this heterologous inhibitory effect. Ablation of β-arrestin 2 or PDE4D gene also rescues β-adrenergic stimuli-induced myocyte contractile function.

CONCLUSIONS

These data reveal essential roles of β-arrestin 2 and PDE4D in a common mechanism for heterologous desensitization of cardiac βARs under hormonal stimulation, which is associated with impaired cardiac function during the development of pathophysiological conditions.

Purinergic P2Y6 receptors: A new therapeutic target of age-dependent hypertension

Aging has a remarkable effect on cardiovascular homeostasis and it is known as the major non-modifiable risk factor in the development of hypertension. Medications targeting sympathetic nerve system and/or renin-angiotensin-aldosterone system are widely accepted as a powerful therapeutic strategy to improve hypertension, although the control rates remain unsatisfactory especially in the elder patients with hypertension. Purinergic receptors, activated by adenine, uridine nucleotides and nucleotide sugars, play pivotal roles in many biological processes, including platelet aggregation, neurotransmission and hormone release, and regulation of cardiovascular contractility. Since clopidogrel, a selective inhibitor of G protein-coupled purinergic P2Y₁₂ receptor (P2Y₁₂R), achieved clinical success as an anti-platelet drug, P2YRs has been attracted more attention as new therapeutic targets of cardiovascular diseases. We have revealed that UDP-responsive P2Y₆R promoted angiotensin type 1 receptor (AT1R)-stimulated vascular remodeling in mice, in an age-dependent manner. Moreover, the age-related formation of heterodimer between AT1R and P2Y₆R was disrupted by MRS2578, a P2Y₆R-selective inhibitor. These findings suggest that P2Y₆R is a therapeutic target to prevent age-related hypertension.

High-fat diet induces protein kinase A and G-protein receptor kinase phosphorylation of β2 -adrenergic receptor and impairs cardiac adrenergic reserve in animal hearts

Key points

Patients with diabetes show a blunted cardiac inotropic response to β‐adrenergic stimulation despite normal cardiac contractile reserve.

Acute insulin stimulation impairs β‐adrenergically induced contractile function in isolated cardiomyocytes and Langendorff‐perfused hearts.

In this study, we aimed to examine the potential effects of hyperinsulinaemia associated with high‐fat diet (HFD) feeding on the cardiac β₂‐adrenergic receptor signalling and the impacts on cardiac contractile function.

We showed that 8 weeks of HFD feeding leads to reductions in cardiac functional reserve in response to β‐adrenergic stimulation without significant alteration of cardiac structure and function, which is associated with significant changes in β₂‐adrenergic receptor phosphorylation at protein kinase A and G‐protein receptor kinase sites in the myocardium.

The results suggest that clinical intervention might be applied to subjects in early diabetes without cardiac symptoms to prevent further cardiac complications.

Abstract

Patients with diabetes display reduced exercise capability and impaired cardiac contractile reserve in response to adrenergic stimulation. We have recently uncovered an insulin receptor and adrenergic receptor signal network in the heart. The aim of this study was to understand the impacts of high‐fat diet (HFD) on the insulin–adrenergic receptor signal network in hearts. After 8 weeks of HFD feeding, mice exhibited diabetes, with elevated insulin and glucose concentrations associated with body weight gain. Mice fed an HFD had normal cardiac structure and function. However, the HFD‐fed mice displayed a significant elevation of phosphorylation of the β₂‐adrenergic receptor (β₂AR) at both the protein kinase A site serine 261/262 and the G‐protein‐coupled receptor kinase site serine 355/356 and impaired adrenergic reserve when compared with mice fed on normal chow. Isolated myocytes from HFD‐fed mice also displayed a reduced contractile response to adrenergic stimulation when compared with those of control mice fed normal chow. Genetic deletion of the β₂AR led to a normalized adrenergic response and preserved cardiac contractile reserve in HFD‐fed mice. Together, these data indicate that HFD promotes phosphorylation of the β₂AR, contributing to impairment of cardiac contractile reserve before cardiac structural and functional remodelling, suggesting that early intervention in the insulin–adrenergic signalling network might be effective in prevention of cardiac complications in diabetes.

Patients with diabetes show a blunted cardiac inotropic response to β‐adrenergic stimulation despite normal cardiac contractile reserve.

Acute insulin stimulation impairs β‐adrenergically induced contractile function in isolated cardiomyocytes and Langendorff‐perfused hearts.

In this study, we aimed to examine the potential effects of hyperinsulinaemia associated with high‐fat diet (HFD) feeding on the cardiac β₂‐adrenergic receptor signalling and the impacts on cardiac contractile function.

We showed that 8 weeks of HFD feeding leads to reductions in cardiac functional reserve in response to β‐adrenergic stimulation without significant alteration of cardiac structure and function, which is associated with significant changes in β₂‐adrenergic receptor phosphorylation at protein kinase A and G‐protein receptor kinase sites in the myocardium.

The results suggest that clinical intervention might be applied to subjects in early diabetes without cardiac symptoms to prevent further cardiac complications.

Activation of the central melanocortin system chronically reduces body mass without the necessity of long-term caloric restriction

Melanotan II (MTII) is a potent appetite suppressor that rapidly reduces body mass. Given the rapid loss of anorexic response upon chronic MTII treatment, most investigations have focused on the initial physiological adaptations. However, other evidence supports MTII as a long-term modulator of energy balance that remains to be established. Therefore, we examined the chronic effects of MTII on energy homeostasis. MTII (high or low dose) or artificial cerebrospinal fluid (aCSF) was infused into the lateral ventricle of the brain of 6-month-old F344BN rats (6-7/group) over 40 days. MTII suppressed appetite in a dose-dependent manner (P < 0.05). Although food intake promptly rose back to control level, body mass was persistently reduced in both MTII groups (P < 0.01). At day 40, both MTII groups displayed lower adiposity than the aCSF animals (P < 0.01). These results show that MTII chronically reduces body mass without the requirement of long-term caloric restriction. Our study proposes that food restriction helps initiate mass loss; however, combined with a secondary pharmacological approach preserving a negative energy balance state over time may help combat obesity.

Hypocretin/Orexin Peptides Alter Spike Encoding by Serotonergic Dorsal Raphe Neurons through Two Distinct Mechanisms That Increase the Late Afterhyperpolarization

Orexins (hypocretins) are neuropeptides that regulate multiple homeostatic processes, including reward and arousal, in part by exciting serotonergic dorsal raphe neurons, the major source of forebrain serotonin. Here, using mouse brain slices, we found that, instead of simply depolarizing these neurons, orexin-A altered the spike encoding process by increasing the postspike afterhyperpolarization (AHP) via two distinct mechanisms. This orexin-enhanced AHP (oeAHP) was mediated by both OX1 and OX2 receptors, required Ca(2+) influx, reversed near EK, and decayed with two components, the faster of which resulted from enhanced SK channel activation, whereas the slower component decayed like a slow AHP (sAHP), but was not blocked by UCL2077, an antagonist of sAHPs in some neurons. Intracellular phospholipase C inhibition (U73122) blocked the entire oeAHP, but neither component was sensitive to PKC inhibition or altered PKA signaling, unlike classical sAHPs. The enhanced SK current did not depend on IP3-mediated Ca(2+) release but resulted from A-current inhibition and the resultant spike broadening, which increased Ca(2+) influx and Ca(2+)-induced-Ca(2+) release, whereas the slower component was insensitive to these factors. Functionally, the oeAHP slowed and stabilized orexin-induced firing compared with firing produced by a virtual orexin conductance lacking the oeAHP. The oeAHP also reduced steady-state firing rate and firing fidelity in response to stimulation, without affecting the initial rate or fidelity. Collectively, these findings reveal a new orexin action in serotonergic raphe neurons and suggest that, when orexin is released during arousal and reward, it enhances the spike encoding of phasic over tonic inputs, such as those related to sensory, motor, and reward events.

SIGNIFICANCE STATEMENT

Orexin peptides are known to excite neurons via slow postsynaptic depolarizations. Here we elucidate a significant new orexin action that increases and prolongs the postspike afterhyperpolarization (AHP) in 5-HT dorsal raphe neurons and other arousal-system neurons. Our mechanistic studies establish involvement of two distinct Ca(2+)-dependent AHP currents dependent on phospholipase C signaling but independent of IP3 or PKC. Our functional studies establish that this action preserves responsiveness to phasic inputs while attenuating responsiveness to tonic inputs. Thus, our findings bring new insight into the actions of an important neuropeptide and indicate that, in addition to producing excitation, orexins can tune postsynaptic excitability to better encode the phasic sensory, motor, and reward signals expected during aroused states.

Signals and Receptors

Communication between cells in a multicellular organism occurs by the production of ligands (proteins, peptides, fatty acids, steroids, gases, and other low-molecular-weight compounds) that are either secreted by cells or presented on their surface, and act on receptors on, or in, other target cells. Such signals control cell growth, migration, survival, and differentiation. Signaling receptors can be single-span plasma membrane receptors associated with tyrosine or serine/threonine kinase activities, proteins with seven transmembrane domains, or intracellular receptors. Ligand-activated receptors convey signals into the cell by activating signaling pathways that ultimately affect cytosolic machineries or nuclear transcriptional programs or by directly translocating to the nucleus to regulate transcription.

Prostaglandin E2-stimulated prostanoid EP4 receptors induce prolonged de novo prostaglandin E2 synthesis through biphasic phosphorylation of extracellular signal-regulated kinases mediated by activation of protein kinase A in HCA-7 human colon cancer cells

Approximately two decades have passed since E-type prostanoid 4 (EP4) receptors were cloned, and the signaling pathways mediated by these receptors have since been implicated in cancer development through the alliance of Gαi-protein/phosphatidylinositol 3-kinase (PI3K)/extracellular signal-regulated kinases (ERKs) activation. Although prostanoid EP4 receptors were initially identified as Gαs-coupled receptors, the specific/distinctive role(s) of prostanoid EP4 receptor-induced cAMP/protein kinase A (PKA) pathways in cancer development have not yet been elucidated in detail. We previously reported using HCA-7 human colon cancer cells that prostaglandin E2 (PGE2)-stimulated prostanoid EP4 receptors induced cyclooxygenase-2 (COX-2) as an initiating event in development of colon cancer. Moreover, this induction of COX-2 was mediated by transactivation of epidermal growth factor (EGF) receptors. However, direct activation of EGF receptors by EGF also induced similar amounts of COX-2 in this cell line. Thus, the emergence of unique role(s) for prostanoid EP4 receptors is expected by clarifying the different signaling mechanisms between PGE2-stimulated prostanoid EP4 receptors and EGF-stimulated EGF receptors to induce COX-2 and produce PGE2. We here demonstrated that prostanoid EP4 receptor activation by PGE2 in HCA-7 cells led to PKA-dependent re-activation of ERKs, which resulted in prolonged de novo synthesis of PGE2. Although EGF-stimulated EGF receptors in cells also induced COX-2 and the de novo synthesis of PGE2, the activation of this pathway was transient and not mediated by PKA. Therefore, the novel mechanism underlying prolonged de novo synthesis of PGE2 has provided an insight into the importance of prostanoid EP4 receptor-mediated Gαs-protein/cAMP/PKA pathway in development of colon cancer.

Pharmacologic rationale underlying the therapeutic effects of tiotropium/olodaterol in COPD

Bronchodilators are the most important drugs used for the treatment of chronic obstructive pulmonary disease (COPD). In particular, these therapeutic agents are mostly long-acting compounds utilized via inhalation, and include LAMA (long-acting muscarinic receptor antagonists) and LABA (long-acting β₂-adrenoceptor agonists). Because LAMA and LABA induce bronchodilation by distinct mechanisms of action, LABA/LAMA combinations provide a reciprocal potentiation of the pharmacological effects caused by each component. Hence, many COPD patients who do not achieve a satisfactory control of their symptoms using a single, either LAMA or LABA bronchodilator, can experience relevant benefits with the use of LAMA/LABA fixed combinations. Many different LAMA/LABA combinations have been recently developed and evaluated in randomized clinical trials. In this context, our review focuses on the pharmacological mechanisms underpinning the bronchodilation elicited by the LAMA tiotropium bromide and the LABA olodaterol. We also discuss the results of the most important clinical studies carried out in COPD patients to assess the efficacy and safety of tiotropium/olodaterol combinations.

MFG-E8 inhibits neutrophil migration through αvβ₃-integrin-dependent MAP kinase activation

We have previously demonstrated the involvement of milk fat globule-epidermal growth factor-factor 8 (MFG-E8) in reducing neutrophil infiltration in a murine model of acute lung injury (ALI). In the present study, we aimed to delineate the mechanisms through which MFG-E8 attenuates neutrophil migration. Recombinant human MFG-E8 (rhMFG-E8) was expressed and purified in our facility. The human differentiated neutrophil cell line, dHL-60, was treated with rhMFG-E8 and cell migration assay was performed in a Boyden chamber using recombinant interleukin-8 (IL-8) as the chemoattractant. Surface CXCR2 and intracellular G protein-coupled receptor kinase 2 (GRK2) levels were evaluated by flow cytometry or western blot analysis. The levels of mitogen-activated protein (MAP) kinases were determined by western blot analysis. Treatment with rhMFG-E8 resulted in a significant inhibition of dHL-60 cell migration in a dose-dependent manner. There was a 46% decrease in CXCR2 expression in the rhMFG-E8-treated dHL-60 cells, which was associated with a 32% increase in GRK2 expression. In the dHL-60 cells, treatment with rhMFG-E8 promoted the phosphorylation of p38 and extracellular signal-regulated kinase (ERK) within 10–30 min. The use of SB203580, a p38 inhibitor, and PD98059, an ERK inhibitor, resulted in the restoration of dHL-60 cell migration which was significantly inhibited treatment with rhMFG-E8. Furthermore, blocking the MFG-E8 receptors, α_vβ₃/α_vβ₅-integrins, by anti-α_v-integrin neutralizing antibody (Ab) inhibited the activation of p38 and ERK, and reversed the rhMFG-E8-induced inhibition of dHL-60 cell migration. Finally, treatment of the dHL-60 cells with SB203580 and PD98059 neutralized the rhMFG-E8-induced downregulation of CXCR2 expression and upregulation of GRK2 expression, as well as the inhibitory effects on cell migration. Our findings reveal a novel mechanism of action of MFG-E8 through which it inhibits neutrophil migration through α_vβ₃-integrin-dependent MAP kinase activation.

Pharmacologic rationale, efficacy and safety of the fixed-dose co-formulation of indacaterol and glycopyrronium

Chronic obstructive pulmonary disease (COPD) is a widespread respiratory disorder, usually characterized by progressive and poorly reversible airflow limitation. Inhaled long-acting bronchodilators, namely LABA (long-acting β2-adrenergic agonists) and LAMA (long-acting muscarinic receptor antagonists) are the mainstay of COPD treatment. Because the symptoms of many patients with COPD do not satisfactorily improve by using a single, either LABA or LAMA bronchodilator, the synergism of action resulting from the combination of the different bronchodilating mechanisms activated by LABA and LAMA, respectively, can significantly contribute to a better disease control. Based on these clinical and pharmacological considerations, several LABA/LAMA fixed-dose combinations have been developed and experimentally evaluated. Within such a context, the drug co-formulation containing indacaterol and glycopyrronium is probably the LABA/LAMA association which has been most extensively studied during the last few years.

Mechanisms of fast desensitization of GABA(B) receptor-gated currents

GABA(B) receptors (GABA(B)Rs) regulate the excitability of most neurons in the central nervous system by modulating the activity of enzymes and ion channels. In the sustained presence of the neurotransmitter γ-aminobutyric acid, GABA(B)Rs exhibit a time-dependent decrease in the receptor response-a phenomenon referred to as homologous desensitization. Desensitization prevents excessive receptor influences on neuronal activity. Much work focused on the mechanisms of GABA(B)R desensitization that operate at the receptor and control receptor expression at the plasma membrane. Over the past few years, it became apparent that GABA(B)Rs additionally evolved mechanisms for faster desensitization. These mechanisms operate at the G protein rather than at the receptor and inhibit G protein signaling within seconds of agonist exposure. The mechanisms for fast desensitization are ideally suited to regulate receptor-activated ion channel responses, which influence neuronal activity on a faster timescale than effector enzymes. Here, we provide an update on the mechanisms for fast desensitization of GABA(B)R responses and discuss physiological and pathophysiological implications.

Identification and functional characterization of the phosphorylation sites of the neuropeptide FF2 receptor

The neuropeptide FF2 (NPFF2) receptor belongs to the rhodopsin family of G protein-coupled receptors and mediates the effects of several related RFamide neuropeptides. One of the main pharmacological interests of this system resides in its ability to regulate endogenous opioid systems, making it a potential target to reduce the negative effects of chronic opioid use. Phosphorylation of intracellular residues is the most extensively studied post-translational modification regulating G protein-coupled receptor activity. However, until now, no information concerning NPFF2 receptor phosphorylation is available. In this study, we combined mass spectrometric analysis and site-directed mutagenesis to analyze for the first time the phosphorylation pattern of the NPFF2 receptor and the role of the various phosphorylation sites in receptor signaling, desensitization, and trafficking in a SH-SY5Y model cell line. We identified the major, likely GRK-dependent, phosphorylation cluster responsible for acute desensitization, (412)TNST(415) at the end of the C terminus of the receptor, and additional sites involved in desensitization ((372)TS(373)) and internalization (Ser(395)). We thus demonstrate the key role played by phosphorylation in the regulation of NPFF2 receptor activity and trafficking. Our data also provide additional evidence supporting the concept that desensitization and internalization are partially independent processes relying on distinct phosphorylation patterns.

Downregulation of kinin B1 receptor function by B2 receptor heterodimerization and signaling

Signaling through the G protein-coupled kinin receptors B1 (kB1R) and B2 (kB2R) plays a critical role in inflammatory responses mediated by activation of the kallikrein-kinin system. The kB2R is constitutively expressed and rapidly desensitized in response to agonist whereas kB1R expression is upregulated by inflammatory stimuli and it is resistant to internalization and desensitization. Here we show that the kB1R heterodimerizes with kB2Rs in co-transfected HEK293 cells and natively expressing endothelial cells, resulting in significant internalization and desensitization of the kB1R response in cells pre-treated with kB2R agonist. However, pre-treatment of cells with kB1R agonist did not affect subsequent kB2R responses. Agonists of other G protein-coupled receptors (thrombin, lysophosphatidic acid) had no effect on a subsequent kB1R response. The loss of kB1R response after pretreatment with kB2R agonist was partially reversed with kB2R mutant Y129S, which blocks kB2R signaling without affecting endocytosis, or T342A, which signals like wild type but is not endocytosed. Co-endocytosis of the kB1R with kB2R was dependent on β-arrestin and clathrin-coated pits but not caveolae. The sorting pathway of kB1R and kB2R after endocytosis differed as recycling of kB1R to the cell surface was much slower than that of kB2R. In cytokine-treated human lung microvascular endothelial cells, pre-treatment with kB2R agonist inhibited kB1R-mediated increase in transendothelial electrical resistance (TER) caused by kB1R stimulation (to generate nitric oxide) and blocked the profound drop in TER caused by kB1R activation in the presence of pyrogallol (a superoxide generator). Thus, kB1R function can be downregulated by kB2R co-endocytosis and signaling, suggesting new approaches to control kB1R signaling in pathological conditions.

Bronchoprotection by olodaterol is synergistically enhanced by tiotropium in a guinea pig model of allergic asthma

The novel once-daily β₂-agonist bronchodilator drug olodaterol has recently been shown to be effective in patients with allergic asthma for >24 hours. An increased cholinergic tone common to these patients may decrease the effectiveness of β₂-agonists. This could provide a rationale for combination therapy with olodaterol and the long-acting anticholinergic tiotropium to aim for a once-daily treatment regimen. In guinea pigs, we evaluated the protective effects of olodaterol, alone and in combination with tiotropium, on airway responsiveness to histamine, which is partially mediated by a cholinergic reflex mechanism. In addition, using a guinea pig model of acute allergic asthma, we examined the cooperative effects of these bronchodilators on allergen-induced early (EAR) and late (LAR) asthmatic reactions, airway hyper-responsiveness (AHR) to histamine, and airway inflammation. It was demonstrated that the protective effect of olodaterol against histamine-induced bronchoconstriction was synergistically enhanced and prolonged in the presence of tiotropium. In addition, tiotropium synergistically augmented both the reversal of and the protection against the allergen-induced AHR after the EAR by olodaterol. Olodaterol and tiotropium were highly effective in inhibiting the magnitude of the allergen-induced EAR and LAR, and both reactions were fully inhibited by the combination of these drugs. It is remarkable that these effects were not associated with an effect on inflammatory cell infiltration in the airways. In conclusion, the results indicate that combination therapy with olodaterol and tiotropium may be highly effective in the treatment of allergen-induced asthmatic reactions and AHR.

A new perspective on muscarinic receptor antagonism in obstructive airways diseases

Acetylcholine has traditionally only been regarded as a neurotransmitter of the parasympathetic nervous system, causing bronchoconstriction and mucus secretion in asthma and COPD by muscarinic receptor activation on airway smooth muscle and mucus-producing cells. Recent studies in experimental models indicate that muscarinic receptor stimulation in the airways also induces pro-inflammatory, pro-proliferative and pro-fibrotic effects, which may involve activation of airway structural and inflammatory cells by neuronal as well as non-neuronal acetylcholine. In addition, mechanical changes caused by muscarinic agonist-induced bronchoconstriction may be involved in airway remodeling. Crosstalk between muscarinic receptors and β2-adrenoceptors on airway smooth muscle causes a reduced bronchodilator response to β2-agonists, and a similar mechanism could possibly apply to the poor inhibition of inflammatory and remodeling processes by these drugs. Collectively, these findings provide novel perspectives for muscarinic receptor antagonists in asthma and COPD, since these drugs may not only acutely affect cholinergic airways obstruction, but also have important beneficial effects on β2-agonist responsiveness, airway inflammation and remodeling. The clinical relevance of these findings is presently under investigation and starting to emerge.

Marked resistance of femoral adipose tissue blood flow and lipolysis to adrenaline in vivo

AIMS/HYPOTHESIS

Fatty acid entrapment in femoral adipose tissue has been proposed to prevent ectopic fat deposition and visceral fat accumulation, resulting in protection from insulin resistance. Our objective was to test the hypothesis of femoral, compared with abdominal, adipose tissue resistance to adrenergic stimulation in vivo as a possible mechanism.

METHODS

Regional fatty acid trafficking, along with the measurement of adipose tissue blood flow (ATBF) with (133)Xe washout, was studied with the arteriovenous difference technique and stable isotope tracers in healthy volunteers. Adrenergic agonists (isoprenaline, adrenaline [epinephrine]) were infused either locally by microinfusion or systemically. Local microinfusion of adrenoceptor antagonists (propranolol, phentolamine) was used to characterise specific adrenoceptor subtype effects in vivo.

RESULTS

Femoral adipose tissue NEFA release and ATBF were lower during adrenaline stimulation than in abdominal tissue (p < 0.001). Mechanistically, femoral adipose tissue displayed a dominant α-adrenergic response during adrenaline stimulation. The α-adrenoceptor blocker, phentolamine, resulted in the 'disinhibition' of the femoral ATBF response to adrenaline (p < 0.001).

CONCLUSIONS/INTERPRETATION

Fatty acids, once stored in femoral adipose tissue, are not readily released upon adrenergic stimulation. Femoral adipose tissue resistance to adrenaline may contribute to the prevention of ectopic fatty acid deposition.

Muscarinic receptors on airway mesenchymal cells: novel findings for an ancient target

Since ancient times, anticholinergics have been used as a bronchodilator therapy for obstructive lung diseases. Targets of these drugs are G-protein-coupled muscarinic M(1), M(2) and M(3) receptors in the airways, which have long been recognized to regulate vagally-induced airway smooth muscle contraction and mucus secretion. However, recent studies have revealed that acetylcholine also exerts pro-inflammatory, pro-proliferative and pro-fibrotic actions in the airways, which may involve muscarinic receptor stimulation on mesenchymal, epithelial and inflammatory cells. Moreover, acetylcholine in the airways may not only be derived from vagal nerves, but also from non-neuronal cells, including epithelial and inflammatory cells. Airway smooth muscle cells seem to play a major role in the effects of acetylcholine on airway function. It has become apparent that these cells are multipotent cells that may reversibly adopt (hyper)contractile, proliferative and synthetic phenotypes, which are all under control of muscarinic receptors and differentially involved in bronchoconstriction, airway remodeling and inflammation. Cholinergic contractile tone is increased by airway inflammation associated with asthma and COPD, resulting from exaggerated acetylcholine release as well as increased expression of contraction related proteins in airway smooth muscle. Moreover, muscarinic receptor stimulation promotes proliferation of airway smooth muscle cells as well as fibroblasts, and regulates cytokine, chemokine and extracellular matrix production by these cells, which may contribute to airway smooth muscle growth, airway fibrosis and inflammation. In line, animal models of chronic allergic asthma and COPD have recently demonstrated that tiotropium may potently inhibit airway inflammation and remodeling. These observations indicate that muscarinic receptors have a much larger role in the pathophysiology of obstructive airway diseases than previously thought, which may have important therapeutic implications.

Milk fat globule-epidermal growth factor-factor 8 attenuates neutrophil infiltration in acute lung injury via modulation of CXCR2

Excessive neutrophil infiltration to the lungs is a hallmark of acute lung injury (ALI). Milk fat globule epidermal growth factor-factor 8 (MFG-E8) was originally identified for phagocytosis of apoptotic cells. Subsequent studies revealed its diverse cellular functions. However, whether MFG-E8 can regulate neutrophil function to alleviate inflammation is unknown. We therefore aimed to reveal MFG-E8 roles in regulating lung neutrophil infiltration during ALI. To induce ALI, C57BL/6J wild-type (WT) and Mfge8(-/-) mice were intratracheally injected with LPS (5 mg/kg). Lung tissue damage was assessed by histology, and the neutrophils were counted by a hemacytometer. Apoptotic cells in lungs were determined by TUNEL, whereas caspase-3 and myeloperoxidase activities were assessed spectrophotometrically. CXCR2 and G protein-coupled receptor kinase 2 expressions in neutrophils were measured by flow cytometry. Following LPS challenge, Mfge8(-/-) mice exhibited extensive lung damage due to exaggerated infiltration of neutrophils and production of TNF-α, MIP-2, and myeloperoxidase. An increased number of apoptotic cells was trapped into the lungs of Mfge8(-/-) mice compared with WT mice, which may be due to insufficient phagocytosis of apoptotic cells or increased occurrence of apoptosis through the activation of caspase-3. In vitro studies using MIP-2-mediated chemotaxis revealed higher migration of neutrophils of Mfge8(-/-) mice than those of WT mice via increased surface exposures to CXCR2. Administration of recombinant murine MFG-E8 reduces neutrophil migration through upregulation of GRK2 and downregulation of surface CXCR2 expression. Conversely, these effects could be blocked by anti-α(v) integrin Abs. These studies clearly indicate the importance of MFG-E8 in ameliorating neutrophil infiltration and suggest MFG-E8 as a novel therapeutic potential for ALI.

GRK2 protein-mediated transphosphorylation contributes to loss of function of μ-opioid receptors induced by neuropeptide FF (NPFF2) receptors

Neuropeptide FF (NPFF) interacts with specific receptors to modulate opioid functions in the central nervous system. On dissociated neurons and neuroblastoma cells (SH-SY5Y) transfected with NPFF receptors, NPFF acts as a functional antagonist of μ-opioid (MOP) receptors by attenuating the opioid-induced inhibition of calcium conductance. In the SH-SY5Y model, MOP and NPFF(2) receptors have been shown to heteromerize. To understand the molecular mechanism involved in the anti-opioid activity of NPFF, we have investigated the phosphorylation status of the MOP receptor using phospho-specific antibody and mass spectrometry. Similarly to direct opioid receptor stimulation, activation of the NPFF(2) receptor by [D-Tyr-1-(NMe)Phe-3]NPFF (1DMe), an analog of NPFF, induced the phosphorylation of Ser-377 of the human MOP receptor. This heterologous phosphorylation was unaffected by inhibition of second messenger-dependent kinases and, contrarily to homologous phosphorylation, was prevented by inactivation of G(i/o) proteins by pertussis toxin. Using siRNA knockdown we could demonstrate that 1DMe-induced Ser-377 cross-phosphorylation and MOP receptor loss of function were mediated by the G protein receptor kinase GRK2. In addition, mass spectrometric analysis revealed that the phosphorylation pattern of MOP receptors was qualitatively similar after treatment with the MOP agonist Tyr-D-Ala-Gly (NMe)-Phe-Gly-ol (DAMGO) or after treatment with the NPFF agonist 1DMe, but the level of multiple phosphorylation was more intense after DAMGO. Finally, NPFF(2) receptor activation was sufficient to recruit β-arrestin2 to the MOP receptor but not to induce its internalization. These data show that NPFF-induced heterologous desensitization of MOP receptor signaling is mediated by GRK2 and could involve transphosphorylation within the heteromeric receptor complex.

Heterologous down-regulation of angiotensin type 1 receptors by purinergic P2Y2 receptor stimulation through S-nitrosylation of NF-kappaB

Cross-talk between G protein-coupled receptor (GPCR) signaling pathways serves to fine tune cellular responsiveness by neurohumoral factors. Accumulating evidence has implicated nitric oxide (NO)-based signaling downstream of GPCRs, but the molecular details are unknown. Here, we show that adenosine triphosphate (ATP) decreases angiotensin type 1 receptor (AT(1)R) density through NO-mediated S-nitrosylation of nuclear factor κB (NF-κB) in rat cardiac fibroblasts. Stimulation of purinergic P2Y(2) receptor by ATP increased expression of inducible NO synthase (iNOS) through activation of nuclear factor of activated T cells, NFATc1 and NFATc3. The ATP-induced iNOS interacted with p65 subunit of NF-κB in the cytosol through flavin-binding domain, which was indispensable for the locally generated NO-mediated S-nitrosylation of p65 at Cys38. β-Arrestins anchored the formation of p65/IκBα/β-arrestins/iNOS quaternary complex. The S-nitrosylated p65 resulted in decreases in NF-κB transcriptional activity and AT(1)R density. In pressure-overloaded mouse hearts, ATP released from cardiomyocytes led to decrease in AT(1)R density through iNOS-mediated S-nitrosylation of p65. These results show a unique regulatory mechanism of heterologous regulation of GPCRs in which cysteine modification of transcriptional factor rather than protein phosphorylation plays essential roles.

Label-Free Receptor Assays

Label-free biosensors offer integrated, kinetic and multi-parametric measures of receptor biology and ligand pharmacology in whole cells. Being highly sensitive and pathway-unbiased, label-free receptor assays can be used to probe the systems cell biology including pleiotropic signaling of receptors, and to characterize the functional selectivity and phenotypic pharmacology of ligand molecules. These assays provide a new dimension for elucidating receptor biology and for facilitating drug discovery.

Aminergic control and modulation of honeybee behaviour

Biogenic amines are important messenger substances in the central nervous system and in peripheral organs of vertebrates and of invertebrates. The honeybee, Apis mellifera, is excellently suited to uncover the functions of biogenic amines in behaviour, because it has an extensive behavioural repertoire, with a number of biogenic amine receptors characterised in this insect.In the honeybee, the biogenic amines dopamine, octopamine, serotonin and tyramine modulate neuronal functions in various ways. Dopamine and serotonin are present in high concentrations in the bee brain, whereas octopamine and tyramine are less abundant. Octopamine is a key molecule for the control of honeybee behaviour. It generally has an arousing effect and leads to higher sensitivity for sensory inputs, better learning performance and increased foraging behaviour. Tyramine has been suggested to act antagonistically to octopamine, but only few experimental data are available for this amine. Dopamine and serotonin often have antagonistic or inhibitory effects as compared to octopamine.Biogenic amines bind to membrane receptors that primarily belong to the large gene-family of GTP-binding (G) protein coupled receptors. Receptor activation leads to transient changes in concentrations of intracellular second messengers such as cAMP, IP(3) and/or Ca(2+). Although several biogenic amine receptors from the honeybee have been cloned and characterised more recently, many genes still remain to be identified. The availability of the completely sequenced genome of Apis mellifera will contribute substantially to closing this gap.In this review, we will discuss the present knowledge on how biogenic amines and their receptor-mediated cellular responses modulate different behaviours of honeybees including learning processes and division of labour.

Complexity in signal transduction

This review is focused on complexity in cell signaling. Signaling experiments have demonstrated that many different stimuli activate the same signaling pathways yet result in different outcomes. Differences in the cellular machinery between cells explain response variation between cell types, but for a single cell type an appealing explanation is still lacking. The kinetic disconnect between cell signal transduction and a cellular action is highlighted; possible explanations for this disconnect, such as a series of cascading autocrine signaling molecules and new research suggesting that cells experience multiple rounds of reactivation in numerous cell signaling pathways, are reviewed. Additionally, evidence that kinase pathways exhibit frequency and amplitude modulation is examined. From this review, a new model of signal transduction is proposed whereby multiple signal transduction pathways are reactivated over time to orchestrate unique outcomes in physiological processes.

Sphingosine-1-phosphate receptors: zooming in on ligand-induced intracellular trafficking and its functional implications

Regulatory processes including receptor phosphorylation and intracellular trafficking, also referred to as receptor internalization, are important processes to terminate G protein-coupled receptor (GPCR) signaling. Compelling evidence now indicates that internalization of a receptor is not necessarily the endpoint of signaling, but can also be the beginning of the activation of intracellular signaling pathways. Sphingosine-1-phosphate (S1P) receptors, which are activated by the endogenous phospholipid S1P, belong to the family of GPCRs. Interestingly, there is evidence indicating differential intracellular trafficking of one of the S1P receptor subtypes, the S1P1 receptor, upon agonist activation by either S1P or the synthetic agonist FTY720-P. Moreover, the differential effect of FTY720-P on S1P1 receptor regulation has been suggested to be the mechanism of action of this drug, which is now in Phase III clinical trials for the treatment of multiple sclerosis. It is thus of importance to get a good insight into the regulation of S1P receptors. This review therefore gives a detailed overview about the current state of knowledge on S1P receptor internalization and its functional implications, including some data on nuclear signaling of S1P receptors.

CCR5 ligands modulate CXCL12-induced chemotaxis, adhesion, and Akt phosphorylation of human cord blood CD34+ cells

CXCL12 and its receptor CXCR4 play an important role in hematopoietic stem/progenitor cell (HSPC) migration from and retention within the bone marrow. HSPCs are very selective in their chemotactic response and undergo chemotaxis only in response to CXCL12. In addition to CXCR4, HSPCs express receptors for various other chemokines; however, the role of these receptors is not well understood. Freshly isolated CD34(+) cells (highly enriched for HSPCs) from cord blood (CB) express low levels of CCR5; however, if the cells were washed with acidic buffer before Ab staining to remove any ligand bound to CCR5, then nearly 80% of CD34(+) CB cells were found to express CCR5 on the cell surface. Although none of the CCR5 ligands investigated in this study (CCL3, CCL4, and CCL5) induced chemotaxis, at relatively high concentrations they transiently enhanced CXCL12-mediated chemotaxis of CD34(+) CB cells. In contrast, CXCL12-mediated adhesion of cells to VCAM-1-coated surfaces was reduced if CD34(+) CB cells were pretreated with these CCR5 ligands for 15 min. The effect of these chemokines on CXCL12-mediated responses was not at the level of CXCR4 expression, but on downstream signaling pathways elicited by CXCL12. Pretreatment with CCR5 chemokines enhanced CXCL12-mediated Akt phosphorylation, but down-modulated calcium flux in CD34(+) CB cells. Modulation of CXCL12-mediated responses of CD34(+) cells by CCR5 chemokines provides a possible mechanism that underlies movement of HSPCs during inflammation.

A role for inflammatory mediators in heterologous desensitization of CysLT1 receptor in human monocytes

Cysteinyl-leukotrienes (cysteinyl-LT) are rapidly generated at sites of inflammation and, in addition to their role in asthma, rhinitis, and other immune disorders, are increasingly regarded as significant inflammatory factors in cancer, gastrointestinal, cardiovascular diseases. We recently demonstrated that in monocyte/macrophage-like U937 cells, extracellular nucleotides heterologously desensitize CysLT(1) receptor (CysLT(1)R)-induced Ca(2+) transients. Given that monocytes express a number of other inflammatory and chemoattractant receptors, this study was aimed at characterizing transregulation between these different stimuli. We demonstrate that in U937 cells and in primary human monocytes, a series of inflammatory mediators activating G(i)-coupled receptor (FPR1, BLT(1)) desensitize CysLT(1)R-induced Ca(2+) response unidirectionally through activation of PKC. Conversely, PAF-R, exclusively coupled to G(q), cross-desensitizes CysLT(1)R without the apparent involvement of any kinase. Interestingly, G(s)-coupled receptors (beta(2)AR, H(1/2)R, EP(2/4)R) are also able to desensitize CysLT(1)R response through activation of PKA. Heterologous desensitization seems to affect mostly the G(i)-mediated signaling of the CysLT(1)R. The hierarchy of desensitization among agonists may be important for leukocyte signal processing at the site of inflammation. Considering that monocytes/macrophages are likely to be the major source of cysteinyl-LT in many immunological and inflammatory processes, shedding light on how their receptors are regulated will certainly help to better understand the role of these cells in orchestrating this complex network of integrated signals.

The KDEL receptor: new functions for an old protein

The KDEL receptor is a seven-transmembrane-domain protein that was first described about 20 years ago. Its well-known function is to retrotransport chaperones from the Golgi complex to the endoplasmic reticulum. Recent studies, however, have suggested that the KDEL receptor has additional functions. Indeed, we have demonstrated that chaperone-bound KDEL receptor triggers the activation of Src family kinases on the Golgi complex. This activity is essential in the regulation of Golgi-to-plasma membrane transport. However, the identification of different KDEL receptor interactors that are inconsistent with these established functions opens the possibility of further receptor activities.

Label-free optical biosensor for probing integrative role of adenylyl cyclase in G protein-coupled receptor signaling

Adenylyl cyclase is considered as an integrator for receptor signaling. However, its integrative role in receptor signaling is largely studied at the level of point of contacts in complex pathways. Here we used forskolin as a pharmacological probe and the resonant waveguide grating (RWG) biosensor to examine the signal integration of G protein-coupled receptors (GPCRs) at the cyclase-cyclic AMP-PKA module. The biosensor is a refractive index sensitive optical biosensor that is capable of detecting ligand-induced dynamic mass redistribution in cells without labels and cellular manipulations. Stimulation of seven cell lines with forskolin led to distinct optical responses, indicative of distinct expressions and/or organization of cyclase isoforms. The forskolin response in A431 was sensitive to the activities of protein kinase A, Rho kinase, and MAP kinases. Desensitization assays showed that the forskolin pretreatment heterologously desensitized G(s) signaling, partially attenuated G(q) signaling, but had complicate impacts on G(i) signaling. This study documents the integrative role of adenylyl cyclase in GPCR signaling and the power of forskolin as a pharmacological probe to differentiate receptor signaling using the label-free biosensor cellular assays.

RGS13 controls g protein-coupled receptor-evoked responses of human mast cells

IgE-mediated mast cell degranulation and release of vasoactive mediators induced by allergens elicits allergic responses. Although G protein-coupled receptor (GPCR)-induced signals may amplify IgE-dependent degranulation, how GPCR signaling in mast cells is regulated remains incompletely defined. We investigated the role of regulator of G protein signaling (RGS) proteins in the modulation of these pathways in human mast cells. Several RGS proteins were expressed in mast cells including RGS13, which we previously showed inhibited IgE-mediated mast cell degranulation and anaphylaxis in mice. To characterize how RGS13 affects GPCR-mediated functions of human mast cells, we analyzed human mast cell lines (HMC-1 and LAD2) depleted of RGS13 by specific small interfering RNA or short hairpin RNA and HMC-1 cells overexpressing RGS13. Transient RGS13 knockdown in LAD2 cells lead to increased degranulation to sphingosine-1-phosphate but not to IgE-Ag or C3a. Relative to control cells, HMC-1 cells stably expressing RGS13-targeted short hairpin RNA had greater Ca(2+) mobilization in response to several natural GPCR ligands such as adenosine, C5a, sphingosine-1-phosphate, and CXCL12 than wild-type cells. Akt phosphorylation, chemotaxis, and cytokine (IL-8) secretion induced by CXCL12 were also greater in short hairpin RGS13-HMC-1 cells compared with control. RGS13 overexpression inhibited CXCL12-evoked Ca(2+) mobilization, Akt phosphorylation and chemotaxis. These results suggest that RGS13 restricts certain GPCR-mediated biological responses of human mast cells.

Duplexed label-free G protein--coupled receptor assays for high-throughput screening

This article describes duplexed label-free optical biosensor cellular assays for simultaneously assaying 2 endogenous receptors, the G(q)-coupled histamine receptor (H( 1)) and the G(s)-coupled beta(2)-adrenergic receptor (beta(2)AR), in A431 cells. The biosensor cellular assays consist of 2 sequential steps-an initial agonist screening using Sigma LOPAC (Library of Pharmaceutically Active Compounds) and a subsequent antagonist screening using a solution mixture containing the H(1) agonist histamine and the beta(2)AR agonist epinephrine. Results showed that costimulating A431 cells with histamine and epinephrine led to an optical response additive to individual responses. The agonist screening not only identified all full agonists for both the H(1) and beta(2) receptors, but also detected pathway-biased ligands for the beta(2)AR. Furthermore, the succeeding antagonist screening documented all known antagonists in the library for either the H(1) or beta(2) receptors. This is the 1st demonstration of a single cellular assay that is capable of screening ligands against 2 GPCRs coupled to distinct G proteins, and highlights the power of pathway-unbiased and label-free biosensor cellular assays for GPCR screens.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

Norepinephrine-mediated regulation of 5HT1 receptor functioning in human platelets

Adaptive changes in serotonergic 5HT1 receptor signalling are believed to underlie the therapeutic effectiveness of antidepressant drugs. Since cells are continuously exposed to neurotransmitters/neuromodulators, spatially and temporally integrated, the responsiveness of a receptor system is dependent upon the physio-pathological state of the cell and the interaction between different neurotransmitters. In the present work, we investigated heterologous regulation of 5HT1 receptors induced by norepinephrine (NE) in human platelets. NE platelet treatment induced a time and concentration dependent 5HT1 receptor desensitisation mediated by both alpha and beta receptors through activation of intracellular protein kinases. In particular NE, through PKC activation, regulated 5HT1 receptor phosphorylation on threonine residues, causing in turn serotonin receptor-G protein uncoupling and functional responsiveness drop. These results suggest that high NE levels (released i.e. during stress disorders) may play an important role in regulating the 5HT1 responsiveness and in controlling effectiveness of drugs acting on these neurotransmitter systems.

Desensitization of herpesvirus-encoded G protein-coupled receptors

Members of the herpesvirus family, including human cytomegalovirus (HCMV) and Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8), encode G protein-coupled receptor (GPCR) homologs, which strongly activate classical G protein signal transduction networks within the cell. In animal models of herpesvirus infection, the viral GPCRs appear to play physiologically important roles by enabling viral replication within tropic tissues and by promoting reactivation from latency. While a number of studies have defined intracellular signaling pathways activated by herpesviral GPCRs, it remains unclear if their physiological function is subjected to the process of desensitization as observed for cellular GPCRs. G protein-coupled receptor kinases (GRK) and arrestin proteins have been recently implicated in regulating viral GPCR signaling; however, the role that these desensitization proteins play in viral GPCR function in vivo remains unknown. Here, we review what is currently known regarding viral GPCR desensitization and discuss potential biological ramifications of viral GPCR regulation by the host cell desensitization machinery.

Exercise-mediated regulation of Hsp70 expression following aerobic exercise training

An issue central to understanding the biological benefits associated with regular exercise training is to elucidate the intracellular mechanisms governing exercise-conferred cardioprotection. Heat shock proteins (HSPs), most notably the inducible 70-kDa HSP family member Hsp70, are believed to participate in the protection of the myocardium during cardiovascular stress. Following acute exercise, activation of PKA mediates the suppression of an intermediary protein kinase, ERK1/2, which phosphorylates and suppresses the activation of the heat shock transcription factor 1 (HSF1). However, following exercise training, ERK1/2 has been reported to regulate the transcriptional activation of several genes involved in cell growth and proliferation and has been shown to be associated with training-mediated myocardial hypertrophy. The present project examined the transcriptional activation of hsp70 gene expression in acutely exercised (60 min at 30 m/min) naïve sedentary and aerobically trained (8 wk, low intensity) male Sprague-Dawley rats. Following acute exercise stress, no significant differences were demonstrated in the expression of myocardial Hsp70 mRNA and activation of PKA between sedentary and trained animals. However, trained animals elicited expression of the hsp70 gene (P < 0.05) in the presence of elevated ERK1/2 activation. Given the association of ERK1/2 and the suppression of hsp70 gene expression following acute exercise in naïve sedentary rats, these results suggest that training results in adaptations that allow for the simultaneous initiation of both proliferative and protective responses. While it is unclear what factors are associated with this training-related shift, increases in HSF1 DNA binding affinity (P < 0.05) and posttranscriptional modifications of the Hsp70 transcript are suggested.

Roles of Protein Kinase C and Actin-Binding Protein 280 in the Regulation of Intracellular Trafficking of Dopamine D3 Receptor

D3 dopamine receptor (D3R) is expressed mainly in parts of the brain that control the emotional behaviors. It is believed that the improper regulation of D3R is involved in the etiology of schizophrenia. Desensitization of D3R is weakly associated with G protein-coupled receptor kinase (GRK)/β-arrestin-directed internalization. This suggests that there might be an alternative pathway that regulates D3R signaling. This report shows that D3R undergoes robust protein kinase C (PKC)-dependent sequestration that is accompanied by receptor phosphorylation and the desensitization of signaling. PKC-dependent D3R sequestration, which was enhanced by PKC-β or -δ, was dynamin dependent but independent of GRK, β-arrestin, or caveolin 1. Site-directed mutagenesis of all possible phosphorylation sites within the intracellular loops of D3R identified serine residues at positions 229 and 257 as the critical amino acids responsible for phorbol-12-myristate-13-acetate (PMA)-induced D3R phosphorylation, sequestration, and desensitization. In addition, the LxxY endocytosis motif, which is located between residues 252 and 255, was found to play accommodating roles for PMA-induced D3R sequestration. A continuous interaction with the actin-binding protein 280 (filamin A), which was previously known to interact with D3R, is required for PMA-induced D3R sequestration. In conclusion, the PKC-dependent but GRK-/β-arrestin-independent phosphorylation of D3R is the main pathway responsible for the sequestration and desensitization of D3R. Filamin A is essential for both the efficient signaling and sequestration of D3R.