Wolfgang Platzer

GPR55 ligands promote receptor coupling to multiple signalling pathways

Background and purpose:  Although GPR55 is potently activated by the endogenous lysophospholipid, L‐α‐lysophosphatidylinositol (LPI), it is also thought to be sensitive to a number of cannabinoid ligands, including the prototypic CB1 receptor antagonists AM251 and SR141716A (Rimonabant®). In this study we have used a range of functional assays to compare the pharmacological activity of selected cannabinoid ligands, AM251, AM281 and SR141716A with LPI in a HEK293 cell line engineered to stably express recombinant, human GPR55.

GPR55 regulates cannabinoid 2 receptor-mediated responses in human neutrophils

The directional migration of neutrophils towards inflammatory mediators, such as chemokines and cannabinoids, occurs via the activation of seven transmembrane G protein coupled receptors (7TM/GPCRs) and is a highly organized process. A crucial role for controlling neutrophil migration has been ascribed to the cannabinoid CB2 receptor (CB2R), but additional modulatory sites distinct from CB2R have recently been suggested to impact CB2R-mediated effector functions in neutrophils. Here, we provide evidence that the recently de-orphanized 7TM/GPCR GPR55 potently modulates CB2R-mediated responses. We show that GPR55 is expressed in human blood neutrophils and its activation augments the migratory response towards the CB2R agonist 2-arachidonoylglycerol (2-AG), while inhibiting neutrophil degranulation and reactive oxygen species (ROS) production. Using HEK293 and HL60 cell lines, along with primary neutrophils, we show that GPR55 and CB2R interfere with each others signaling pathways at the level of small GTPases, such as Rac2 and Cdc42. This ultimately leads to cellular polarization and efficient migration as well as abrogation of degranulation and ROS formation in neutrophils. Therefore, GPR55 limits the tissue-injuring inflammatory responses mediated by CB2R, while it synergizes with CB2R in recruiting neutrophils to sites of inflammation.

Heteromerization of GPR55 and cannabinoid CB2 receptors modulates signalling

Heteromerization of GPCRs is key to the integration of extracellular signals and the subsequent cell response via several mechanisms including heteromer‐selective ligand binding, trafficking and/or downstream signalling. As the lysophosphatidylinositol GPCR 55 (GPR55) has been shown to affect the function of the cannabinoid receptor subtype 2 (CB2 receptor) in human neutrophils, we investigated the possible heteromerization of CB2 receptors with GPR55.

The G-protein Coupled Receptor Associated Sorting Protein GASP-1 Regulates the Signalling and Trafficking of the Viral Chemokine Receptor US28

Human cytomegalovirus (HCMV) encodes the seven transmembrane (7TM)/G‐protein coupled receptor (GPCR) US28, which signals and endocytoses in a constitutive, ligand‐independent manner. Here we show that, following endocytosis, US28 is targeted to the lysosomes for degradation as a consequence of its interaction with the GPCR‐associated sorting protein‐1 (GASP‐1). We find that GASP‐1 binds to US28 in vitro and that disruption of the GASP‐1/US28 interaction by either (i) overexpression of dominant negative cGASP‐1 or by (ii) shRNA knock‐down of endogenous GASP‐1 is sufficient to inhibit the lysosomal targeting of US28 and slow its post‐endocytic degradation. Furthermore, we found that GASP‐1 affects US28‐mediated signalling. The knock‐down of endogenous GASP‐1 impairs the US28‐mediated Gαq/PLC/inositol phosphate (IP) accumulation as well as the activation of the transcription factors Nuclear Factor–κB (NF‐κB) and cyclic AMP responsive element binding protein (CREB). Overexpression of GASP‐1 enhances both IP accumulation and transcription factor activity. Thus, GASP‐1 is an important cellular determinant that not only regulates the post‐endocytic trafficking of US28, but also regulates the signalling capacities of US28.

D-type prostanoid receptor enhances the signaling of chemoattractant receptor–homologous molecule expressed on TH2 cells

BACKGROUND Prostaglandin (PG) D(2) is substantially involved in allergic responses and signals through the 7 transmembrane-spanning/G protein-coupled receptors, chemoattractant receptor-homologous molecule expressed on T(H)2 cells (CRTH2), and D-type prostanoid (DP) receptor. OBJECTIVE Although the proinflammatory function of CRTH2 is well recognized and CRTH2 is hence considered an important emerging pharmacotherapeutic target, the role of the DP receptor in mediating the biological effects of PGD(2) in patients with allergic inflammation has remained unclear. METHODS The cross-talk of CRTH2 and DP receptors was investigated by using both a recombinant HEK293 cell model and human eosinophils in Ca(2+) mobilization assays, coimmunoprecipitation, Western blotting, radioligand binding, and immunofluorescence. RESULTS We show that CRTH2 and DP receptors modulate one anothers signaling properties and form CRTH2/DP heteromers without altering their ligand-binding capacities. We find that the DP receptor amplifies the CRTH2-induced Ca(2+) release from intracellular stores and coincidentally forfeits its own signaling potency. Moreover, desensitization or pharmacologic blockade of the DP receptor hinders CRTH2-mediated signal transduction. However, CRTH2 internalization occurs independently of the DP receptor. In cells that express both receptors, pharmacologic blockade of Gα(q/11) proteins abolishes the Ca(2+) response to both CRTH2 and DP agonists, whereas inhibition of Gα(i) proteins selectively attenuates the CRTH2-mediated response but not the DP signal. CONCLUSION Our data demonstrate the capacity of DP receptors to amplify the biological response to CRTH2 activation. Therefore the CRTH2/DP heteromer might not only represent a functional signaling unit for PGD(2) but also a potential target for the development of heteromer-directed therapies to treat allergic diseases.

Activation of EP4 receptors prevents endotoxin‐induced neutrophil infiltration into the airways and enhances microvascular barrier function

Pulmonary vascular dysfunction is a key event in acute lung injury. We recently demonstrated that PGE2, via activation of E‐prostanoid (EP)4 receptors, strongly enhances microvascular barrier function in vitro. The aim of this study was to investigate the beneficial effects of concomitant EP4 receptor activation in murine models of acute pulmonary inflammation.

The GPCR-associated sorting protein 1 regulates ligand-induced down-regulation of GPR55

BACKGROUND AND PURPOSE Many GPCRs, including the CB1 cannabinoid receptor, are down‐regulated following prolonged agonist exposure by interacting with the GPCR‐associated sorting protein‐1 (GASP‐1). The CB1 receptor antagonist rimonabant has also recently been described to be an agonist at GPR55, a cannabinoid‐related receptor. Here we investigated the post‐endocytic properties of GPR55 after agonist exposure and tested whether GASP‐1 is involved in this process.

EP4 receptor prevents endotoxin‐induced neutrophil infiltration into the airways and enhances microvascular barrier function

Pulmonary vascular dysfunction is a key event in acute lung injury. We recently demonstrated that PGE2, via activation of E‐prostanoid (EP)4 receptors, strongly enhances microvascular barrier function in vitro. The aim of this study was to investigate the beneficial effects of concomitant EP4 receptor activation in murine models of acute pulmonary inflammation.

A Biased Non-Gαi OXE-R Antagonist Demonstrates That Gαi Protein Subunit Is Not Directly Involved in Neutrophil, Eosinophil, and Monocyte Activation by 5-Oxo-ETE

Gαi-coupled chemoattractant receptors, such as the 5-oxo-6E,8Z,11Z,14Z-eicosatetraenoic acid (5-oxo-ETE) receptor (OXE-R), are able to switch on Gαiβγ protein-dependent and β-arrestin–related signaling traits. However, which of these signaling pathways are truly important for the chemoattractant functions in leukocytes is not clarified yet. As we recently reported, Gue1654 is a unique Gβγ-biased OXE-R antagonist having no inhibitory activity on Gαi-related signaling, which makes Gue1654 an unprecedented tool for assessing the involvement of G protein subunits in chemoattractant receptor function. β-arrestin2 recruitment was studied in OXE-R–overexpressing HEK293 cells using bioluminescence resonance energy transfer assays. Activation of leukocytes was assessed by flow cytometric assays and by immunofluorescence microscopy. Leukocyte capture to endothelial cells was addressed under physiological flow conditions. We found that Gue1654 blocks β-arrestin2 recruitment in HEK293 cells overexpressing OXE-R and ERK1/2 phosphorylation in human eosinophils and neutrophils. Furthermore, Gue1654 was able to prevent several 5-oxo-ETE–triggered functional events in eosinophils and neutrophils, such as activation of CD11b/CD18 integrins, oxidative burst, actin polymerization, and interaction with endothelial cells. In addition, Gue1654 completely prevented 5-oxo-ETE–induced Ca2+ flux and chemotaxis of human primary monocytes. All of these leukocyte responses to 5-oxo-ETE, except ERK1/2 phosphorylation and oxidative burst, were likewise prevented by pertussis toxin. Therefore, we conclude that chemoattractant receptors require Gαi subunits only as adaptors to transactivate the Gβγ heteromers, which then act responsible for cell activation. Finally, our data characterize Gue1654 as a non-Gαi–biased antagonist of OXE-R that provides a new basis for therapeutic intervention in inflammatory diseases that involve activation of eosinophils, neutrophils, and monocytes.

The Role of PGE 2 in Alveolar Epithelial and Lung Microvascular Endothelial Crosstalk

Disruption of the blood-air barrier, which is formed by lung microvascular endothelial and alveolar epithelial cells, is a hallmark of acute lung injury. It was shown that alveolar epithelial cells release an unidentified soluble factor that enhances the barrier function of lung microvascular endothelial cells. In this study we reveal that primarily prostaglandin (PG) E2 accounts for this endothelial barrier-promoting activity. Conditioned media from alveolar epithelial cells (primary ATI-like cells) collected from BALB/c mice and A549 cells increased the electrical resistance of pulmonary human microvascular endothelial cells, respectively. This effect was reversed by pretreating alveolar epithelial cells with a cyclooxygenase-2 inhibitor or by blockade of EP4 receptors on endothelial cells, and in A549 cells also by blocking the sphingosine-1-phosphate1 receptor. Cyclooxygenase-2 was constitutively expressed in A549 cells and in primary ATI-like cells, and was upregulated by lipopolysaccharide treatment. This was accompanied by enhanced PGE2 secretion into conditioned media. Therefore, we conclude that epithelium-derived PGE2 is a key regulator of endothelial barrier integrity via EP4 receptors under physiologic and inflammatory conditions. Given that pharmacologic treatment options are still unavailable for diseases with compromised air-blood barrier, like acute lung injury, our data thus support the therapeutic potential of selective EP4 receptor agonists.