A mammalian-specific Alex3/Gαq protein complex regulates mitochondrial trafficking, dendritic complexity, and neuronal survival.

Mitochondrial dynamics and trafficking are essential to provide the energy required for neurotransmission and neural activity. We investigated how G protein-coupled receptors (GPCRs) and G proteins control mitochondrial dynamics and trafficking. The activation of Gαq inhibited mitochondrial trafficking in neurons through a mechanism that was independent of the canonical downstream PLCß pathway. Mitoproteome analysis revealed that Gαq interacted with the Eutherian-specific mitochondrial protein armadillo repeat-containing X-linked protein 3 (Alex3) and the Miro1/Trak2 complex, which acts as an adaptor for motor proteins involved in mitochondrial trafficking along dendrites and axons. By generating a CNS-specific Alex3 knockout mouse line, we demonstrated that Alex3 was required for the effects of Gαq on mitochondrial trafficking and dendritic growth in neurons. Alex3-deficient mice had altered amounts of ER stress response proteins, increased neuronal death, motor neuron loss, and severe motor deficits. These data revealed a mammalian-specific Alex3/Gαq mitochondrial complex, which enables control of mitochondrial trafficking and neuronal death by GPCRs.

Kazrin promotes dynein/dynactin-dependent traffic from early to recycling endosomes.

Kazrin is a protein widely expressed in vertebrates whose depletion causes a myriad of developmental defects, in part derived from altered cell adhesion and migration, as well as failure to undergo epidermal to mesenchymal transition. However, the primary molecular role of kazrin, which might contribute to all these functions, has not been elucidated yet. We previously identified one of its isoforms, kazrin C, as a protein that potently inhibits clathrin-mediated endocytosis when overexpressed. We now generated kazrin knock-out mouse embryonic fibroblasts to investigate its endocytic function. We found that kazrin depletion delays juxtanuclear enrichment of internalized material, indicating a role in endocytic traffic from early to recycling endosomes. Consistently, we found that the C-terminal domain of kazrin C, predicted to be an intrinsically disordered region, directly interacts with several early endosome (EE) components, and that kazrin depletion impairs retrograde motility of these organelles. Further, we noticed that the N-terminus of kazrin C shares homology with dynein/dynactin adaptors and that it directly interacts with the dynactin complex and the dynein light intermediate chain 1. Altogether, the data indicate that one of the primary kazrin functions is to facilitate endocytic recycling by promoting dynein/dynactin-dependent transport of EEs or EE-derived transport intermediates to the recycling endosomes.

Gq Signaling in Autophagy Control: Between Chemical and Mechanical Cues.

All processes in human physiology relies on homeostatic mechanisms which require the activation of specific control circuits to adapt the changes imposed by external stimuli. One of the critical modulators of homeostatic balance is autophagy, a catabolic process that is responsible of the destruction of long-lived proteins and organelles through a lysosome degradative pathway. Identification of the mechanism underlying autophagic flux is considered of great importance as both protective and detrimental functions are linked with deregulated autophagy. At the mechanistic and regulatory levels, autophagy is activated in response to diverse stress conditions (food deprivation, hyperthermia and hypoxia), even a novel perspective highlight the potential role of physical forces in autophagy modulation. To understand the crosstalk between all these controlling mechanisms could give us new clues about the specific contribution of autophagy in a wide range of diseases including vascular disorders, inflammation and cancer. Of note, any homeostatic control critically depends in at least two additional and poorly studied interdependent components: a receptor and its downstream effectors. Addressing the selective receptors involved in autophagy regulation is an open question and represents a new area of research in this field. G-protein coupled receptors (GPCRs) represent one of the largest and druggable targets membrane receptor protein superfamily. By exerting their action through G proteins, GPCRs play fundamental roles in the control of cellular homeostasis. Novel studies have shown Gαq, a subunit of heterotrimeric G proteins, as a core modulator of mTORC1 and autophagy, suggesting a fundamental contribution of Gαq-coupled GPCRs mechanisms in the control of this homeostatic feedback loop. To address how GPCR-G proteins machinery integrates the response to different stresses including oxidative conditions and mechanical stimuli, could provide deeper insight into new signaling pathways and open potential and novel therapeutic strategies in the modulation of different pathological conditions.

Gαq activation modulates autophagy by promoting mTORC1 signaling.

The mTORC1 node plays a major role in autophagy modulation. We report a role of the ubiquitous Gαq subunit, a known transducer of plasma membrane G protein-coupled receptors signaling, as a core modulator of mTORC1 and autophagy. Cells lacking Gαq/11 display higher basal autophagy, enhanced autophagy induction upon different types of nutrient stress along with a decreased mTORC1 activation status. They are also unable to reactivate mTORC1 and thus inactivate ongoing autophagy upon nutrient recovery. Conversely, stimulation of Gαq/11 promotes sustained mTORC1 pathway activation and reversion of autophagy promoted by serum or amino acids removal. Gαq is present in autophagic compartments and lysosomes and is part of the mTORC1 multi-molecular complex, contributing to its assembly and activation via its nutrient status-sensitive interaction with p62, which displays features of a Gαq effector. Gαq emerges as a central regulator of the autophagy machinery required to maintain cellular homeostasis upon nutrient fluctuations.

An Overview on G Protein-coupled Receptor-induced Signal Transduction in Acute Myeloid Leukemia.

BACKGROUND: Acute Myeloid Leukemia (AML) is a genetically heterogeneous disease characterized by uncontrolled proliferation of precursor myeloid-lineage cells in the bone marrow. AML is also characterized by patients with poor long-term survival outcomes due to relapse. Many efforts have been made to understand the biological heterogeneity of AML and the challenges to develop new therapies are therefore enormous. G Protein-coupled Receptors (GPCRs) are a large attractive drug-targeted family of transmembrane proteins, and aberrant GPCR expression and GPCR-mediated signaling have been implicated in leukemogenesis of AML. This review aims to identify the molecular players of GPCR signaling, focusing on the hematopoietic system, which are involved in AML to help developing novel drug targets and therapeutic strategies. METHODS: We undertook an exhaustive and structured search of bibliographic databases for research focusing on GPCR, GPCR signaling and expression in AML. RESULTS AND CONCLUSION: Many scientific reports were found with compelling evidence for the involvement of aberrant GPCR expression and perturbed GPCR-mediated signaling in the development of AML. The comprehensive analysis of GPCR in AML provides potential clinical biomarkers for prognostication, disease monitoring and therapeutic guidance. It will also help to provide marker panels for monitoring in AML. We conclude that GPCR-mediated signaling is contributing to leukemogenesis of AML, and postulate that mass spectrometrybased protein profiling of primary AML cells will accelerate the discovery of potential GPCR related biomarkers for AML.

Phosphorylation of filamin A regulates chemokine receptor CCR2 recycling.

Proper endosomal trafficking of ligand-activated G-protein-coupled receptors (GPCRs) is essential to spatiotemporally tune their physiological responses. For the monocyte chemoattractant receptor 2 (CCR2B; one of two isoforms encoded by CCR2), endocytic recycling is important to sustain monocyte migration, whereas filamin A (FLNa) is essential for CCL2-induced monocyte migration. Here, we analyze the role of FLNa in the trafficking of CCR2B along the endocytic pathway. In FLNa-knockdown cells, activated CCR2B accumulated in enlarged EEA-1-positive endosomes, which exhibited slow movement and fast fluorescence recovery, suggesting an imbalance between receptor entry and exit rates. Utilizing super-resolution microscopy, we observed that FLNa-GFP, CCR2B and ß2-adrenergic receptor (ß2AR) were present in actin-enriched endosomal microdomains. Depletion of FLNa decreased CCR2B association with these microdomains and concomitantly delayed CCR2B endosomal traffic, without apparently affecting the number of microdomains. Interestingly, CCR2B and ß2AR signaling induced phosphorylation of FLNa at residue S2152, and this phosphorylation event was contributes to sustain receptor recycling. Thus, our data strongly suggest that CCR2B and ß2AR signals to FLNa to stimulate its endocytosis and recycling to the plasma membrane.

Gastrin-stimulated Gα13 Activation of Rgnef Protein (ArhGEF28) in DLD-1 Colon Carcinoma Cells.

The guanine nucleotide exchange factor Rgnef (also known as ArhGEF28 or p190RhoGEF) promotes colon carcinoma cell motility and tumor progression via interaction with focal adhesion kinase (FAK). Mechanisms of Rgnef activation downstream of integrin or G protein-coupled receptors remain undefined. In the absence of a recognized G protein signaling homology domain in Rgnef, no proximal linkage to G proteins was known. Utilizing multiple methods, we have identified Rgnef as a new effector for Gα13 downstream of gastrin and the type 2 cholecystokinin receptor. In DLD-1 colon carcinoma cells depleted of Gα13, gastrin-induced FAK Tyr(P)-397 and paxillin Tyr(P)-31 phosphorylation were reduced. RhoA GTP binding and promoter activity were increased by Rgnef in combination with active Gα13. Rgnef co-immunoprecipitated with activated Gα13Q226L but not Gα12Q229L. The Rgnef C-terminal (CT, 1279-1582) region was sufficient for co-immunoprecipitation, and Rgnef-CT exogenous expression prevented Gα13-stimulated SRE activity. A domain at the C terminus of the protein close to the FAK binding domain is necessary to bind to Gα13. Point mutations of Rgnef-CT residues disrupt association with active Gα13 but not Gαq. These results show that Rgnef functions as an effector of Gα13 signaling and that this linkage may mediate FAK activation in DLD-1 colon carcinoma cells.

Time-dependent inhibitory effects of cGMP-analogues on thrombin-induced platelet-derived microparticles formation, platelet aggregation, and P-selectin expression.

In platelets, nitric oxide (NO) activates cGMP/PKG signalling, whereas prostaglandins and adenosine signal through cAMP/PKA. Cyclic nucleotide signalling has been considered to play an inhibitory role in platelets. However, an early stimulatory effect of NO and cGMP-PKG signalling in low dose agonist-induced platelet activation have recently been suggested. Here, we investigated whether different experimental conditions could explain some of the discrepancy reported for platelet cGMP-PKG-signalling. We treated gel-filtered human platelets with cGMP and cAMP analogues, and used flow cytometric assays to detect low dose thrombin-induced formation of small platelet aggregates, single platelet disappearance (SPD), platelet-derived microparticles (PMP) and thrombin receptor agonist peptide (TRAP)-induced P-selectin expression. All four agonist-induced platelet activation phases were blocked when platelets were costimulated with the PKG activators 8-Br-PET-cGMP or 8-pCPT-cGMP and low-doses of thrombin or TRAP. However, extended incubation with 8-Br-PET-cGMP decreased its inhibition of TRAP-induced P-selectin expression in a time-dependent manner. This effect did not involve desensitisation of PKG or PKA activity, measured as site-specific VASP phosphorylation. Moreover, PKG activators in combination with the PKA activator Sp-5,6-DCL-cBIMPS revealed additive inhibitory effect on TRAP-induced P-selectin expression. Taken together, we found no evidence for a stimulatory role of cGMP/PKG in platelets activation and conclude rather that cGMP/PKG signalling has an important inhibitory function in human platelet activation.

A new non-canonical pathway of Gα(q) protein regulating mitochondrial dynamics and bioenergetics.

Contrary to previous assumptions, G proteins do not permanently reside on the plasma membrane, but are constantly monitoring the cytoplasmic surfaces of the plasma membrane and endomembranes. Here, we report that the Gαq and Gα11 proteins locate at the mitochondria and play a role in a complex signaling pathway that regulates mitochondrial dynamics. Our results provide evidence for the presence of the heteromeric G protein (Gαq/11ßγ) at the outer mitochondrial membrane and for Gαq at the inner membrane. Both localizations are necessary to maintain the proper equilibrium between fusion and fission; which is achieved by altering the activity of mitofusin proteins, Drp1, OPA1 and the membrane potential at both the outer and inner mitochondrial membranes. As a result of the absence of Gαq/11, there is a decrease in mitochondrial fusion rates and a decrease in overall respiratory capacity, ATP production and OXPHOS-dependent growth. These findings demonstrate that the presence of Gαq proteins at the mitochondria serves as a physiological function: stabilizing elongated mitochondria and regulating energy production in Drp1 and Opa1 dependent mechanisms. This thereby links organelle dynamics and physiology.

Gαq signalling: the new and the old.

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cß (PLCß), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCß-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

Filamin A-hinge region 1-EGFP: a novel tool for tracking the cellular functions of filamin A in real-time.

BACKGROUND: Filamin A (FLNa) is an actin-crosslinking protein necessary for stabilizing the cell surface, organizing protrusive activity and for promoting efficient cellular translocation. Recently, our group demonstrated the requirement of FLNa for the internalization of the chemokine receptor CCR2B. METHODOLOGY AND PRINCIPAL FINDINGS: In order to study the role of FLNa in vitro and in real-time, we have developed a fluorescent FLNa-EGFP construct. In this novel imaging tool, we introduced the EGFP-tag inside the flexible hinge 1 region of FLNa between two calpain cleavage sites. Our findings indicate that the FLNa-EGFP construct was correctly expressed, cleaved by calpain and colocalized with actin filaments as shown by immunostaining experiments in the human melanoma cell lines A7 (FLNa-repleted) and M2 (FLNa-deficient). In addition, scanning-electron microscopy (SEM) and micropatterning studies also provided clear evidence that the cell rigidity was restored. FLNa-EGFP allowed us to demonstrate the interaction of FLNa with the chemokine receptor CCR2B in endocytic vesicles after CCL2 ligand stimulation. Through live-cell imaging studies we show that the CCR2B receptor in Rab5-positive vesicles moves along filamin A-positive fibers. SIGNIFICANCE: Taken together, these results outline the functionality of the FLNa-EGFP and the importance of filamin A for receptor internalization and movement into endocytic vesicles.

Gα(12) binds to the N-terminal regulatory domain of p120(ctn), and downregulates p120(ctn) tyrosine phosphorylation induced by Src family kinases via a RhoA independent mechanism.

p120 Catenin (p120(ctn)) regulates cadherin stability, and thus facilitates strong cell-cell adhesion. Previously, we demonstrated that Gα(12) interacts with p120(ctn). In the present study, we have delineated a region of p120(ctn) that binds to Gα(12). We report that the N-terminal region of p120(ctn) (amino acids 1-346) is necessary and sufficient for the interaction. While the coiled-coiled domain and a charged region, comprising a.a 102-120, were found to be dispensable, amino acids 121-323 were required for p120(ctn) binding to Gα(12). This region harbors the phosphorylation domain of p120(ctn) and has been postulated as important for RhoA regulation. Downregulation of Src family kinase-induced tyrosine phosphorylation of p120(ctn) was observed in the presence of activated Gα(12). This down-regulation was triggered by three different Gα(12) mutants uncoupled from RhoA signalling. Furthermore, a dominant active form of RhoA did not reduce Src-induced phosphoryaltion of p120(ctn). In summary, our results suggest that Gα(12) binds to p120(ctn) and modulates its phosphorylation status through a Rho-independent mechanism. Gα(12) emerges as an important regulator of p120(ctn) function, and possibly of cadherin-mediated adhesion and/or cell motility.

Filamin a binds to CCR2B and regulates its internalization.

The chemokine (C-C motif) receptor 2B (CCR2B) is one of the two isoforms of the receptor for monocyte chemoattractant protein-1 (CCL2), the major chemoattractant for monocytes, involved in an array of chronic inflammatory diseases. Employing the yeast two-hybrid system, we identified the actin-binding protein filamin A (FLNa) as a protein that associates with the carboxyl-terminal tail of CCR2B. Co-immunoprecipitation experiments and in vitro pull down assays demonstrated that FLNa binds constitutively to CCR2B. The colocalization of endogenous CCR2B and filamin A was detected at the surface and in internalized vesicles of THP-1 cells. In addition, CCR2B and FLNa were colocalized in lamellipodia structures of CCR2B-expressing A7 cells. Expression of the receptor in filamin-deficient M2 cells together with siRNA experiments knocking down FLNa in HEK293 cells, demonstrated that lack of FLNa delays the internalization of the receptor. Furthermore, depletion of FLNa in THP-1 monocytes by RNA interference reduced the migration of cells in response to MCP-1. Therefore, FLNa emerges as an important protein for controlling the internalization and spatial localization of the CCR2B receptor in different dynamic membrane structures.

G protein-coupled receptor kinase 2 negatively regulates chemokine signaling at a level downstream from G protein subunits.

The G protein-coupled receptor kinase 2 (GRK2) phosphorylates and desensitizes ligand-activated G protein-coupled-receptors. Here, evidence is shown for a novel role of GRK2 in regulating chemokine-mediated signals. The presence of increased levels of GRK2 in human embryonic kidney (HEK) 293 cells produced a significant reduction of the extracellular signal-regulated kinase (ERK) response to CCL2. This effect is independent of its role in receptor phosphorylation because the kinase-deficient mutant GRK2K220R was able to reduce this response, and ERK activation by CCR2BIX, a phosphorylation-defective receptor mutant, was also inhibited by GRK2. Constructs containing the Galpha(q)-binding RGS-like RH domain of GRK2 or its Gbetagamma-binding domain could not reproduce the inhibition, thus revealing that GRK2 acts downstream of G proteins. Interestingly, chemokine-driven mitogen-activated protein kinase kinase (MEK) stimulation is not affected in cells overexpressing GRK2 or GRK2K220R or in splenocytes from heterozygous GRK2 mice, where reduced kinase levels correlate with enhanced ERK activation by chemokines. We find GRK2 and MEK in the same multimolecular complex, thus suggesting a mechanism for GRK2 regulation of ERK activity that involves a direct or coordinate interaction with MEK. These results suggest an important role for GRK2 in the control of chemokine induction of ERK activation at the level of the MEK-ERK interface.

Proteasome involvement in the degradation of the G(q) family of Galpha subunits.

Metabolically unstable proteins are involved in a multitude of regulatory networks, including those that control cell signaling, the cell cycle and in many responses to physiological stress. In the present study, we have determined the stability and characterized the degradation process of some members of the G(q) class of heterotrimeric G proteins. Pulse-chase experiments in HEK293 cells indicated a rapid turnover of endogenously expressed Galpha(q) and overexpressed Galpha(q) and Galpha(16) subunits. Pretreatment with proteasome inhibitors attenuated the degradation of both G alpha subunits. In contrast, pretreatment of cells with inhibitors of lysosomal proteases and nonproteasomal cysteine proteases had very little effect on the stability of the proteins. Significantly, the turnover of these proteins is not affected by transient activation of their associated receptors. Fractionation studies showed that the rates of Galpha(q) and Galpha16 degradation are accelerated in the cytosol. In fact, we show that a mutant Galpha(q) which lacks its palmitoyl modification site, and which is localized almost entirely in the cytoplasm, has a marked increase in the rate of degradation. Taken together, these results suggest that the G(q) class proteins are degraded through the proteasome pathway and that cellular localization and/or other protein interactions determine their stability.

A role for Galpha12/Galpha13 in p120ctn regulation.

The catenin p120 (p120ctn) is an armadillo repeat domain protein that binds to cadherins and has been shown to facilitate strong cell-cell adhesion. We have investigated a possible link between heterotrimeric G proteins and p120ctn, and found that both Galpha12 and Galpha13 can completely and selectively abrogate the p120ctn-induced branching phenotype in different cell types. Consistent with these observations, the expression of Galpha12 or Galpha13 compensates for the reduction of Rho activity induced by p120ctn. On the other hand, p120ctn can be selectively coimmunoprecipitated with Galpha12, and the coimmunoprecipitation was favored by activation of the G protein. A specific interaction between p120ctn and Galpha12Q231L was also observed in in vitro binding experiments. In addition, p120ctn can be immunoprecipitated along with Galpha12Q231L in L cells in absence of E-cadherin. Interestingly, the expression of Galpha12Q231L increases the amount of p120ctn associated with E-cadherin. These findings demonstrate that Galpha12 and p120ctn are binding partners, and they also suggest a role for Galpha12 in regulating p120ctn activity and its interaction with cadherins. We propose that the Galpha12-p120ctn interaction acts as a molecular switch, which regulates cadherin-mediated cell-cell adhesion.

Signaling pathways for monocyte chemoattractant protein 1-mediated extracellular signal-regulated kinase activation.

G protein-coupled receptors (GPCRs) initiate diverse down-stream signaling events in response to ligand stimulation, as rapid activation of the extracellular signal-regulated kinase ERK1 and ERK2. The chemokine monocyte chemoattractant protein-1 (MCP-1) is the agonist for several chemokine receptors that belong to the GPCR superfamily, CCR2 being the most important. Stimulation of mitogen-activated protein kinases (MAPKs) by MCP-1 has been implicated in integrin activation and chemotaxis, but the molecular pathways down-stream of the receptors remain unclear. To dissect the cascade of events leading to MAPK activation upon CCR2 receptor stimulation, several specific inhibitors and mutants of signal transduction proteins were used in monocytic cells endogenously expressing CCR2 and/or in human embryonic kidney-293 cells transfected with CCR2B receptors and epitope-tagged ERK1. We show that ERK activation by MCP-1 involves heterotrimeric Gi protein subunits, protein kinase C, phosphoinositide-3-kinase, and Ras. On the other hand, the activity of cytosolic tyrosine kinases, epidermal growth factor receptor transactivation, or variations in intracellular calcium levels are not required for the mitogenic activation elicited by MCP-1. In addition, we find that internalization of CCR2B itself is not necessary for efficient MCP-1-induced activation of ERK, although a dynamin mutant partially inhibits ERK stimulation. These results suggest that different parallel pathways are being activated that lead to the full activation of the mitogen-activated protein kinase cascade and that internalization of other signaling proteins but not of the receptor is required for complete ERK activation.