A Mixture of U.S. Food and Drug Administration-Approved Monoaminergic Drugs Protects the Retina From Light Damage in Diverse Models of Night Blindness.

Purpose: The purpose of this study was to test the extent of light damage in different models of night blindness and apply these paradigms in testing the therapeutic efficacy of combination therapy by drugs acting on the Gi, Gs, and Gq protein-coupled receptors. Methods: Acute bright light exposure was used to test susceptibility to light damage in mice lacking the following crucial phototransduction proteins: rod transducin (GNAT1), cone transducin (GNAT2), visual arrestin 1 (ARR1), and rhodopsin kinase 1 (GRK1). Mice were intraperitoneally injected with either vehicle or drug combination consisting of metoprolol (ß1-receptor antagonist), bromocriptine (dopamine family-2 receptor agonist) and tamsulosin (α1-receptor antagonist) before bright light exposure. Light damage was primarily assessed with optical coherence tomography and inspection of cone population in retinal whole mounts. Retinal inflammation was assessed in a subset of experiments using autofluorescence imaging by scanning laser ophthalmoscopy and by postmortem inspection of microglia and astrocyte activity. Results: The Gnat1-/- mice showed slightly increased susceptibility to rod light damage, whereas the Gnat2-/- mice were very resistant. The Arr1-/- and Grk1-/- mice were sensitive for both rod and cone light damage and showed robust retinal inflammation 7 days after bright light exposure. Pretreatment with metoprolol + bromocriptine + tamsulosin rescued the retina in all genetic backgrounds, starting at doses of 0.2 mg/kg metoprolol, 0.02 mg/kg bromocriptine, and 0.01 mg/kg tamsulosin in the Gnat1-/- mice. The therapeutic drug doses increased in parallel with light-damage severity. Conclusions: Our results suggest that congenital stationary night blindness and Oguchi disease patients can be at an elevated risk of the toxic effects of bright light. Furthermore, systems pharmacology drug regimens that stimulate Gi signaling and attenuate Gs and Gq signaling present a promising disease-modifying therapy for photoreceptor degenerative diseases.

Identifying Key Networks Linked to Light-Independent Photoreceptor Degeneration in Visual Arrestin 1 Knockout Mice.

When visual arrestin 1 (ARR1, S-antigen, 48 KDa protein) was genetically knocked out in mice (original Arr1 -/- , designated Arr1 -/-A ), rod photoreceptors degenerated in a light-dependent manner. Subsequently, a light-independent cone dystrophy was identified with minimal rod death in ARR1 knockout mice (Arr1 -/-A Arr4 +/+, designated Arr1 -/-B ), which were F2 littermates from breeding the original Arr1 -/-A and cone arrestin knockout 4 (Arr4 -/- ) mice. To resolve the genetic and phenotypic differences between the two ARR1 knockouts, we performed Affymetrix™ exon array analysis to focus on the potential differential gene expression profile and to explore the molecular and cellular pathways leading to this observed susceptibility to cone dystrophy in Arr1 -/-B compared to Arr1 -/-A or control Arr1 +/+ Arr4 +/+ (wild type [WT]). Only in the Arr1 -/-B retina did we observe an up-regulation of retinal transcripts involved in the immune response, inflammatory response and JAK-STAT signaling molecules, OSMRß and phosphorylation of STAT3. Of these responses, the complement system was significantly higher, and a variety of inflammatory responses by complement regulation and anti-inflammatory cytokine or factors were identified in Arr1 -/-B retinal transcripts. This discovery supports that Arr1 -/-B has a distinct genetic background from Arr1 -/-A that results in alterations in its retinal phenotype leading to susceptibility to cone degeneration induced by inappropriate inflammatory and immune responses.

ß-Arrestin 2 dependence of δ opioid receptor agonists is correlated with alcohol intake.

BACKGROUND AND PURPOSE: δ Opioid receptor agonists are being developed as potential treatments for depression and alcohol use disorders. This is particularly interesting as depression is frequently co-morbid with alcohol use disorders. Yet we have previously shown that δ receptor agonists range widely in their ability to modulate alcohol intake; certain δ receptor agonists actually increase alcohol consumption in mice. We propose that variations in ß-arrestin 2 recruitment contribute to the differential behavioural profile of δ receptor agonists. EXPERIMENTAL APPROACH: We used three diarylmethylpiperazine-based non-peptidic δ receptor selective agonists (SNC80, SNC162 and ARM390) and three structurally diverse δ receptor agonists (TAN-67, KNT127 and NIH11082). We tested these agonists in cAMP and ß-arrestin 2 recruitment assays and a behavioural assay of alcohol intake in male C57BL/6 mice. We used ß-arrestin 2 knockout mice and a model of depression-like behaviour to further study the role of ß-arrestin 2 in δ receptor pharmacology. KEY RESULTS: All six tested δ receptor agonists were full agonists in the cAMP assay but displayed distinct ß-arrestin 2 recruitment efficacy. The efficacy of δ receptor agonists to recruit ß-arrestin 2 positively correlated with their ability to increase alcohol intake (P < 0.01). The effects of the very efficacious recruiter SNC80 on alcohol intake, alcohol place preference and depression-like behaviour were ß-arrestin 2-dependent. CONCLUSIONS AND IMPLICATIONS: Our finding that δ receptor agonists that strongly recruit ß-arrestin 2 can increase alcohol intake carries important ramifications for drug development of δ receptor agonists for treatment of alcohol use disorders and depressive disorders.

ß-Arrestin 2 Promotes Hepatocyte Apoptosis by Inhibiting Akt Protein.

Recent studies reveal that multifunctional protein ß-arrestin 2 (Arrb2) modulates cell apoptosis. Survival and various aspects of liver injury were investigated in WT and Arrb2 KO mice after bile duct ligation (BDL). We found that deficiency of Arrb2 enhances survival and attenuates hepatic injury and fibrosis. Following BDL, Arrb2-deficient mice as compared with WT controls displayed a significant reduction of hepatocyte apoptosis as demonstrated by the TUNEL assay. Following BDL, the levels of phospho-Akt and phospho-glycogen synthase kinase 3ß (GSK3ß) in the livers were significantly increased in Arrb2 KO compared with WT mice, although p-p38 increased in WT but not in Arrb2-deficient mice. Inhibition of GSK3ß following BDL decreases hepatic apoptosis and decreased p-p38 in WT mice but not in Arrb2 KO mice. Activation of Fas receptor with Jo2 reduces phospho-Akt and increases apoptosis in WT cells and WT mice but not in Arrb2-deficient cells and Arrb2-deficient mice. Consistent with direct interaction of Arrb2 with and regulating Akt phosphorylation, the expression of a full-length or N terminus but not the C terminus of Arrb2 reduces Akt phosphorylation and coimmunoprecipates with Akt. These results reveal that the protective effect of deficiency of Arrb2 is due to loss of negative regulation of Akt due to BDL and decreased downstream GSK3ß and p38 MAPK signaling pathways.

Long-Acting Beta Agonists Enhance Allergic Airway Disease.

Asthma is one of the most common of medical illnesses and is treated in part by drugs that activate the beta-2-adrenoceptor (ß2-AR) to dilate obstructed airways. Such drugs include long acting beta agonists (LABAs) that are paradoxically linked to excess asthma-related mortality. Here we show that LABAs such as salmeterol and structurally related ß2-AR drugs such as formoterol and carvedilol, but not short-acting agonists (SABAs) such as albuterol, promote exaggerated asthma-like allergic airway disease and enhanced airway constriction in mice. We demonstrate that salmeterol aberrantly promotes activation of the allergic disease-related transcription factor signal transducer and activator of transcription 6 (STAT6) in multiple mouse and human cells. A novel inhibitor of STAT6, PM-242H, inhibited initiation of allergic disease induced by airway fungal challenge, reversed established allergic airway disease in mice, and blocked salmeterol-dependent enhanced allergic airway disease. Thus, structurally related ß2-AR ligands aberrantly activate STAT6 and promote allergic airway disease. This untoward pharmacological property likely explains adverse outcomes observed with LABAs, which may be overcome by agents that antagonize STAT6.

Spinophilin Is Indispensable for the α2B Adrenergic Receptor-Elicited Hypertensive Response.

The α2 adrenergic receptor (AR) subtypes are important for blood pressure control. When activated, the α2A subtype elicits a hypotensive response whereas the α2B subtype mediates a hypertensive effect that counteracts the hypotensive response by the α2A subtype. We have previously shown that spinophilin attenuates the α2AAR-dependent hypotensive response; in spinophilin null mice, this response is highly potentiated. In this study, we demonstrate that spinophilin impedes arrestin-dependent phosphorylation and desensitization of the α2BAR subtype by competing against arrestin binding to this receptor subtype. The Del301-303 α2BAR, a human variation that shows impaired phosphorylation and desensitization and is linked to hypertension in certain populations, exhibits preferential interaction with spinophilin over arrestin. Furthermore, Del301-303 α2BAR-induced ERK signaling is quickly desensitized in cells without spinophilin expression, showing a profile similar to that induced by the wild type receptor in these cells. Together, these data suggest a critical role of spinophilin in sustaining α2BAR signaling. Consistent with this notion, our in vivo study reveals that the α2BAR-elicited hypertensive response is diminished in spinophilin deficient mice. In arrestin 3 deficient mice, where the receptor has a stronger binding to spinophilin, the same hypertensive response is enhanced. These data suggest that interaction with spinophilin is indispensable for the α2BAR to elicit the hypertensive response. This is opposite of the negative role of spinophilin in regulating α2AAR-mediated hypotensive response, suggesting that spinophilin regulation of these closely related receptor subtypes can result in distinct functional outcomes in vivo. Thus, spinophilin may represent a useful therapeutic target for treatment of hypertension.

Protective Role of ß-arrestin2 in Colitis Through Modulation of T-cell Activation.

ß-arrestin2 (ß-arr2), identified as a scaffolding protein in G-protein-coupled receptor desensitization, is a negative regulator of inflammation in polymicrobial sepsis. In this study, we wanted to investigate the role of ß-arr2 in intestinal inflammation, a site of persistent microbial stimulation. In the absence of ß-arr2, mice exhibited greater extent of mucosal inflammation determined by cellular infiltration and expression of inflammatory mediators even under homeostatic conditions. Furthermore, ß-arr2-deficient mice were more susceptible to dextran sulfate sodium-induced colitis as demonstrated by greater body weight loss, higher disease activity index, and shortened colon as compared with wild-type mice. We also show that T cells from ß-arr2 knockout mice exhibit altered activation status under both basal and colitic conditions, implicating their involvement in disease induction. Further assessment of the role of ß-arr2 in intrinsic T-cell differentiation confirmed its importance in T-cell polarization. Using the T-cell transfer model of colitis, we demonstrate that T-cell-specific ß-arr2 is important in limiting colitic inflammation; however, it plays a paradoxical role in concurrent systemic wasting disease. Together, our study highlights a critical negative regulatory role of ß-arr2 in intestinal inflammation and demonstrates a distinct role of T-cell-specific ß-arr2 in systemic wasting disease.

Circulating Exosomes Induced by Cardiac Pressure Overload Contain Functional Angiotensin II Type 1 Receptors.

BACKGROUND: Whether biomechanical force on the heart can induce exosome secretion to modulate cardiovascular function is not known. We investigated the secretion and activity of exosomes containing a key receptor in cardiovascular function, the angiotensin II type I receptor (AT1R). METHODS AND RESULTS: Exosomes containing AT1Rs were isolated from the media overlying AT1R-overexpressing cells exposed to osmotic stretch and from sera of mice undergoing cardiac pressure overload. The presence of AT1Rs in exosomes was confirmed by both electron microscopy and radioligand receptor binding assays and shown to require ß-arrestin2, a multifunctional adaptor protein essential for receptor trafficking. We show that functional AT1Rs are transferred via exosomes in an in vitro model of cellular stretch. Using mice with global and cardiomyocyte conditional deletion of ß-arrestin2, we show that under conditions of in vivo pressure overload the cellular source of the exocytosis of exosomes containing AT1R is the cardiomyocyte. Exogenously administered AT1R-enriched exosomes target cardiomyocytes, skeletal myocytes, and mesenteric resistance vessels and are sufficient to confer blood pressure responsiveness to angiotensin II infusion in AT1R knockout mice. CONCLUSIONS: AT1R-enriched exosomes are released from the heart under conditions of in vivo cellular stress to likely modulate vascular responses to neurohormonal stimulation. In the context of the whole organism, the concept of G protein-coupled receptor trafficking should consider circulating exosomes as part of the reservoir of functional AT1Rs.

ß-Arrestin2 encourages inflammation-induced epithelial apoptosis through ER stress/PUMA in colitis.

ß-Arrestins (ß-arrs) are regulators and mediators of G protein-coupled receptor signaling, and accumulating evidence suggests that they are functionally involved in inflammation and autoimmune diseases. However, the effect of ß-arrs is unclear in inflammatory bowel disease (IBD), and the role of ß-arr2 is unknown in ulcerative colitis (UC) and Crohn's disease (CD). The aim of this study is to investigate whether ß-arr2 encourages inflammation-induced epithelial apoptosis through endoplasmic reticulum (ER) stress/p53-upregulated modulator of apoptosis (PUMA) in colitis. In the present study, the results showed that ß-arr2 was increased in specimens from patients with UC or CD. Furthermore, a ß-arr2 deficiency significantly repressed intestinal inflammation, ameliorated colitis, and alleviated mucosal apoptosis in mice. In addition, the targeted deletion of ß-arr2 depressed ER stress, inhibited PUMA, and downregulated PUMA-mediated mitochondrial apoptotic signaling in colitis. ß-Arr2, an important modulator of G protein-coupled receptor function, binds eIF2α to activate ER stress signaling. Furthermore, the knockdown of PUMA dramatically prevented ß-arr2-induced apoptosis via alleviating ER stress in vitro. The results suggest that ß-arr2 encourages inflammation-induced epithelial apoptosis through ER stress/PUMA in colitis and that ß-arr2 is a potential therapeutic target for colitis.

Functional selectivity of GPCR signaling in animals.

At one time, G protein-coupled receptors were envisioned to simply relay either inhibitory or stimulatory binary signals through engaging particular G proteins. These receptors are now viewed as complex, multidimensional triggers of a variety of potential signaling cascades. This review will showcase current attempts to elucidate biased signaling and functional selectivity in tissues and organs as well as in the whole animal. In addition, it will emphasize the challenges that are inherent in attributing bias in a living system as well as offer opinions as to the manner in which these problems may be approached.

The fission yeast ß-arrestin-like protein Any1 is involved in TSC-Rheb signaling and the regulation of amino acid transporters.

Rheb GTPase and the Tsc1-Tsc2 protein complex, which serves as a GTPase-activating protein for Rheb, have crucial roles in the regulation of cell growth in response to extracellular conditions. In Schizosaccharomyces pombe, Rheb and Tsc1-Tsc2 regulate cell cycle progression, the onset of meiosis and the uptake of amino acids. In cells lacking Tsc2 (Δtsc2), the amino acid transporter Aat1, which is normally expressed on the plasma membrane under starvation conditions, is confined to the Golgi. Here, we show that the loss of either pub1(+), encoding an E3 ubiquitin ligase, or any1(+), encoding a ß-arrestin-like protein, allows constitutive expression of Aat1 on the plasma membrane in Δtsc2 cells, suggesting that Pub1 and Any1 are required for localization of Aat1 to the Golgi. Subsequent analysis revealed that, in the Golgi, Pub1 and Any1 form a complex that ubiquitylates Aat1. Physical interaction of Pub1 and Any1 is more stable in Δtsc2 cells than in wild-type cells and is independent of Tor2 activity. These results indicate that the TSC-Rheb signaling pathway regulates the localization of amino acid transporters via Pub1 and Any1 in a Tor2-independent manner. Our study demonstrates that, unlike in budding yeast (in which Rsp5 and ARTs, a pair of proteins analogous to Pub1 and Any1, respectively, primarily act to reduce expression of the transporters on plasma membrane when nutrients are abundant), the primary role of fission yeast Pub1 and Any1 is to store the transporter in the Golgi under nutrient-rich conditions.

Loss of ß-arrestin 2 exacerbates experimental autoimmune encephalomyelitis with reduced number of Foxp3+ CD4+ regulatory T cells.

ß-Arrestins are well-known regulators and mediators of G protein-coupled receptor signalling, and accumulating evidence reveals that they are functionally involved in inflammation and autoimmune diseases. Of the two ß-arrestins, ß-arrestin 1 is documented to play regulatory roles in an animal model of multiple sclerosis (MS), whereas the role of ß-arrestin 2 is less clear. Here, we show that ß-arrestin 2-deficient mice displayed the exacerbated and sustained symptoms of experimental autoimmune encephalomyelitis (EAE), an animal model of MS. At the cellular level, deficiency of ß-arrestin 2 led to a decreased number of Foxp3(+) CD4(+) regulatory T (Treg) cells in peripheral lymphoid organs of EAE mice. Consistently, our in vitro observations also revealed that loss of ß-arrestin 2 impaired the conversion of Foxp3(-) CD4(+) T cells into Foxp3(+) CD4(+) inducible Treg cells. Taken together, our data suggest that ß-arrestin 2 plays a regulatory role in MS, that is opposite to that of ß-arrestin 1, in autoimmune diseases such as MS, which is at least partially through regulation of iTreg cell differentiation.

Loss of ß-arrestin2 mediates pancreatic-islet dysfunction in mice.

Insulin resistance and defective insulin secretion are two major factors contributing to the pathogenesis of type 2 diabetes. ß-Arrestin2 is known to interact with numerous signaling molecules. Our previous study demonstrated that ß-arrestin2 regulates insulin sensitivity in both skeletal muscle and liver, yet its role in insulin secretion remains elusive. In this study, we found that ß-arrestin2 was abundantly expressed in mouse pancreatic beta cells, while its expression was significantly decreased in obese and diabetic mouse models. Hyperglycemic clamp study showed that the acute and late phase of insulin secretion were impaired in ß-arrestin2 knockout mice. Ex vivo study showed that ß-arrestin2 deficient pancreatic islets exhibited blunted glucose-stimulated insulin secretion. Further analysis demonstrated the number of docked insulin granules in ß-arrestin2 deficient islets was markedly decreased compared to wild-type islets, while insulin content and beta cell mass remained unchanged. Our study establishes a new role for ß-arrestin2 in beta-cell functions, and suggests that the down regulation of ß-arrestin2 may contribute to impaired insulin secretion in type 2 diabetes.

G protein-coupled receptor kinase 6 deficiency promotes angiogenesis, tumor progression, and metastasis.

G protein-coupled receptor kinases (GRKs) phosphorylate the activated form of G protein-coupled receptors leading to receptor desensitization and downregulation. We have recently shown that the chemokine receptor, CXCR2, couples to GRK6 to regulate cellular responses including chemotaxis, angiogenesis, and wound healing. In this study, we investigate the role of GRK6 in tumorigenesis using murine models of human lung cancer. Mice deficient in GRK6 (GRK6(-/-)) exhibited a significant increase in Lewis lung cancer growth and metastasis relative to control littermates (GRK6(+/+)). GRK6 deletion had no effect on the expression of proangiogenic chemokine or vascular endothelial growth factor, but upregulated matrix metalloproteinase (MMP)-2 and MMP-9 release, tumor-infiltrating PMNs, and microvessel density. Because ß-arrestin-2-deficient (ßarr2(-/-)) mice exhibited increased Lewis lung cancer growth and metastasis similar to that of GRK6(-/-), we developed a double GRK6(-/-)/ßarr2(-/-) mouse model. Surprisingly, GRK6(-/-)/ßarr2(-/-) mice exhibited faster tumor growth relative to GRK6(-/-) or ßarr2(-/-) mice. Treatment of the mice with anti-CXCR2 Ab inhibited tumor growth in both GRK6(-/-) and GRK6(-/-)/ßarr2(-/-) animals. Altogether, the results indicate that CXCR2 couples to GRK6 to regulate angiogenesis, tumor progression, and metastasis. Deletion of GRK6 increases the activity of the host CXCR2, resulting in greater PMN infiltration and MMP release in the tumor microenvironment, thereby promoting angiogenesis and metastasis. Because GRK6(-/-)/ßarr2(-/-) showed greater tumor growth relative to GRK6(-/-) or ßarr2(-/-) mice, the data further suggest that CXCR2 couples to different mechanisms to mediate tumor progression and metastasis.

Deficiency of ß-arrestin1 ameliorates collagen-induced arthritis with impaired TH17 cell differentiation.

Rheumatoid arthritis (RA) is an inflammatory disease in which interleukin 17 (IL-17)-producing T helper 17 (T(H)17) cells have been critically involved. We show that in patients with RA, the expression of a multifunctional regulator ß-arrestin1 was significantly up-regulated in peripheral and synovial CD4(+) T cells, which correlated well with active phases of RA. In collagen-induced arthritis, deficiency of ß-arrestin1 ameliorated disease with decreased T(H)17 cell differentiation, proinflammatory cytokine production, synovitis, and cartilage and bone destruction. Further mechanistic study reveals that ß-arrestin1 promoted signal transducer and activator of transcription 3 (STAT3) activation required for T(H)17 cell differentiation through scaffolding the interaction of Janus kinase 1 and STAT3. These findings indicate a critical role for ß-arrestin1 in the pathogenesis of collagen-induced arthritis and T(H)17 cell differentiation and suggest ß-arrestin1 as a potential diagnostic biomarker and therapeutic target for RA.

Evidence against a role for ß-arrestin1 in STAT1 dephosphorylation and the inhibition of interferon-γ signaling.

Signal transducer and activator of transcription 1 (STAT1) is activated by tyrosine phosphorylation upon interferon-γ (IFNγ) stimulation, which results in the expression of genes with antiproliferative and immunomodulatory functions. The inactivation of STAT1 occurs through tyrosine dephosphorylation by the tyrosine phosphatase TC45. It was proposed that recruitment of TC45 required the direct interaction of STAT1 with the scaffold protein ß-arrestin1, making ß-arrestin1 an essential negative regulator of STAT1 and IFNγ signaling (Mo et al., 2008). We tested the relevance of ß-arrestin1 for STAT1 activity. Our results do not confirm ß-arrestin1 as a STAT1-interacting protein. The STAT1 phosphorylation/dephosphorylation cycle was found to be unaffected by both the overexpression and the genetic deletion of ß-arrestin1. Accordingly, ß-arrestin1 did not inhibit STAT1 transcriptional activity or the induction of IFNγ target genes in response to IFNγ. Our data indicate that ß-arrestin1 is dispensable for STAT1 dephosphorylation and the termination of IFNγ signaling.

ß-Arrestin-2 deficiency attenuates abdominal aortic aneurysm formation in mice.

RATIONALE: Abdominal aortic aneurysms (AAAs) are a chronic inflammatory vascular disease for which pharmacological treatments are not available. A mouse model of AAA formation involves chronic infusion of angiotensin II (AngII), and previous studies indicated a primary role for the AngII type 1a receptor in AAA formation. ß-arrestin (ßarr)-2 is a multifunctional scaffolding protein that binds G-protein-coupled receptors such as AngII type 1a and regulates numerous signaling pathways and pathophysiological processes. However, a role for ßarr2 in AngII-induced AAA formation is currently unknown. OBJECTIVE: To determine whether ßarr2 played a role in AngII-induced AAA formation in mice. METHODS AND RESULTS: Treatment of ßarr2(+/+) and ßarr2(-/-) mice on the hyperlipidemic apolipoprotein E-deficient (apoE(-/-)) background or on normolipidemic C57BL/6 background with AngII for 28 days indicated that ßarr2 deficiency significantly attenuated AAA formation. ßarr2 deficiency attenuated AngII-induced expression of cyclooxygenase-2, monocyte chemoattractant protein-1, macrophage inflammatory protein 1α, and macrophage infiltration. AngII also increased the levels of phosphorylated extracellular signal-regulated kinase 1/2 in apoE(-/-)/ßarr2(+/+) aortas, whereas ßarr2 deficiency diminished this increase. Furthermore, inhibition of extracellular signal-regulated kinase 1/2 activation with CI1040 (100 mg/kg per day) reduced the level of AngII-induced cyclooxygenase-2 expression in apoE(-/-)/ßarr2(+/+) mice to the level observed in apoE(-/-)/ßarr2(-/-) mice. AngII treatment also increased matrix metalloproteinase expression and disruption of the elastic layer in apoE(-/-)/ßarr2(+/+) aortas, and ßarr2 deficiency reduced these effects. CONCLUSIONS: ßarr2 contributes to AngII-induced AAA formation in mice by phosphorylated extracellular signal-regulated kinase 1/2-mediated cyclooxygenase-2 induction and increased inflammation. These studies suggest that for the AngII type 1a receptor, G-protein-independent, ßarr2-dependent signaling plays a major role in AngII-induced AAA formation.

ß-Arrestin-1 deficiency protects mice from experimental colitis.

ß-Arrestins are intracellular scaffolding proteins that modulate specific cell signaling pathways. Recent studies, in both cell culture and in vivo models, have demonstrated an important role for ß-arrestin-1 in inflammation. However, the role of ß-arrestin-1 in the pathogenesis of inflammatory bowel disease (IBD) is not known. Our goal was to investigate the role of ß-arrestin-1 in IBD using mouse models of colitis. To this end, we subjected wild-type (WT) and ß-arrestin-1 knockout (ß-arr-1(-/-)) mice to colitis induced by trinitrobenzenesulfonic acid or dextran sulfate sodium and examined the clinical signs, gross pathology, and histopathology of the colon, as well as inflammatory components. The ß-arr-1(-/-) mice displayed significantly attenuated colitis, compared with WT mice, in both models. Consistent with the phenotypic observations, histological examination of the colon revealed attenuated disease pathology in the ß-arr-1(-/-) mice. Our results further demonstrate that ß-arr-1(-/-) mice are deficient in IL-6 expression in the colon, but have higher expression of the anti-inflammatory IL-10 family of cytokines. Our results also demonstrate diminished ERK and NFκB pathways in the colons of ß-arr-1(-/-) mice, compared with WT mice. Taken together, our results demonstrate that decreased IL-6 production and enhanced IL-10 and IL-22 production in ß-arrestin-1-deficient mice likely lead to attenuated gut inflammation.

ß-arrestin-selective G protein-coupled receptor agonists engender unique biological efficacy in vivo.

Biased G protein-coupled receptor agonists are orthosteric ligands that possess pathway-selective efficacy, activating or inhibiting only a subset of the signaling repertoire of their cognate receptors. In vitro, D-Trp(12),Tyr(34)-bPTH(7-34) [bPTH(7-34)], a biased agonist for the type 1 PTH receptor, antagonizes receptor-G protein coupling but activates arrestin-dependent signaling. In vivo, both bPTH(7-34) and the conventional agonist hPTH(1-34) stimulate anabolic bone formation. To understand how two PTH receptor ligands with markedly different in vitro efficacy could elicit similar in vivo responses, we analyzed transcriptional profiles from calvarial bone of mice treated for 8 wk with vehicle, bPTH(7-34) or hPTH(1-34). Treatment of wild-type mice with bPTH(7-34) primarily affected pathways that promote expansion of the osteoblast pool, notably cell cycle regulation, cell survival, and migration. These responses were absent in ß-arrestin2-null mice, identifying them as downstream targets of ß-arrestin2-mediated signaling. In contrast, hPTH(1-34) primarily affected pathways classically associated with enhanced bone formation, including collagen synthesis and matrix mineralization. hPTH(1-34) actions were less dependent on ß-arrestin2, as might be expected of a ligand capable of G protein activation. In vitro, bPTH(7-34) slowed the rate of preosteoblast proliferation, enhanced osteoblast survival when exposed to an apoptotic stimulus, and stimulated cell migration in wild-type, but not ß-arrestin2-null, calvarial osteoblasts. These results suggest that bPTH(7-34) and hPTH(1-34) affect bone mass in vivo through predominantly separate genomic mechanisms created by largely distinct receptor-signaling networks and demonstrate that functional selectivity can be exploited to change the quality of G protein-coupled receptor efficacy.

APJ acts as a dual receptor in cardiac hypertrophy.

Cardiac hypertrophy is initiated as an adaptive response to sustained overload but progresses pathologically as heart failure ensues. Here we report that genetic loss of APJ, a G-protein-coupled receptor, confers resistance to chronic pressure overload by markedly reducing myocardial hypertrophy and heart failure. In contrast, mice lacking apelin (the endogenous APJ ligand) remain sensitive, suggesting an apelin-independent function of APJ. Freshly isolated APJ-null cardiomyocytes exhibit an attenuated response to stretch, indicating that APJ is a mechanosensor. Activation of APJ by stretch increases cardiomyocyte cell size and induces molecular markers of hypertrophy. Whereas apelin stimulates APJ to activate Gαi and elicits a protective response, stretch signals in an APJ-dependent, G-protein-independent fashion to induce hypertrophy. Stretch-mediated hypertrophy is prevented by knockdown of ß-arrestins or by pharmacological doses of apelin acting through Gαi. Taken together, our data indicate that APJ is a bifunctional receptor for both mechanical stretch and the endogenous peptide apelin. By sensing the balance between these stimuli, APJ occupies a pivotal point linking sustained overload to cardiomyocyte hypertrophy.