G protein-coupled receptor endocytosis in ADP-ribosylation factor 6-depleted cells

Regulation of receptor trafficking by GRKs and arrestins

To ensure that extracellular stimuli are translated into intracellular signals of appropriate magnitude and specificity, most signaling cascades are tightly regulated. One of the major mechanisms involved in the regulation of G protein-coupled receptors (GPCRs) involves their endocytic trafficking. GPCR endocytic trafficking entails the targeting of receptors to discrete endocytic sites at the plasma membrane, followed by receptor internalization and intracellular sorting. This regulates the level of cell surface receptors, the sorting of receptors to degradative or recycling pathways, and in some cases the specific signaling pathways. In this chapter we discuss the mechanisms that regulate receptor endocytic trafficking, emphasizing the role of GPCR kinases (GRKs) and arrestins in this process.

Somatodendritic localization of EFA6A, a guanine nucleotide exchange factor for ADP-ribosylation factor 6, and its possible interaction with α-actinin in dendritic spines

EFA6A is a member of the guanine nucleotide exchange factors that can specifically activate ADP ribosylation factor 6 (ARF6). In this study, we identified alpha-actinin-1 as a possible interacting protein with EFA6A by the yeast two-hybrid screening with its C-terminal region as bait. The central region of alpha-actinin-1 containing a part of spectrin repeat 1 and spectrin repeats 2-3 is responsible for this interaction. In the hippocampal formation, EFA6A immunoreactivity occurred at a high level as numerous fine puncta in the strata oriens, radiatum, lacunosum-moleculare of the hippocampal CA1-3 subfields and the dentate molecular layer, whereas the immunoreactivity was faint in the neuronal cell layers and the stratum lucidum, the mossy fiber-recipient layer of the CA3 subfield. Double-immunofluorescent analyses revealed a partial overlapping of EFA6A and alpha-actinin at the dendritic spines of in vivo and cultured hippocampal neurons. Our present findings suggest that EFA6A may form a protein complex with alpha-actinin and activate ARF6 in close proximity of the actin cytoskeleton and membrane proteins in the dendritic spines.

Agonist mediated internalization of M2 mAChR is beta-arrestin-dependent

Background

Muscarinic acetylcholine receptors (mAChRs) undergo agonist-promoted internalization, but evidence suggesting that the mechanism of internalization is β-arrestin dependent has been contradictory and unclear. Previous studies using heterologous over-expression of wild type or dominant-negative forms of β-arrestins have reported that agonist-promoted internalization of M₂ mAChRs is a β-arrestin- and clathrin-independent phenomenon. In order to circumvent the complications associated with the presence of endogenous β-arrestin that may have existed in these earlier studies, we examined agonist-promoted internalization of the M₂ mAChR in mouse embryonic fibroblasts (MEFs) derived from β-arrestin knockout mice that lack expression of either one or both isoforms of β-arrestin (β-arrestin 1 and 2).

Results

In wild type MEF cells transiently expressing M₂ mAChRs, 40% of surface M₂ mAChRs underwent internalization and sorted into intracellular compartments following agonist stimulation. In contrast, M₂ mAChRs failed to undergo internalization and sorting into intracellular compartments in MEF β-arrestin double knockout cells following agonist stimulation. In double knockout cells, expression of either β-arrestin 1 or 2 isoforms resulted in rescue of agonist-promoted internalization. Stimulation of M₂ mAChRs led to a stable co-localization with GFP-tagged β-arrestin within endocytic structures in multiple cell lines; the compartment to which β-arrestin localized was determined to be the early endosome. Agonist-promoted internalization of M₂ mAChRs was moderately rescued in MEF β-arrestin 1 and 2 double knockout cells expressing exogenous arrestin mutants that were selectively defective in interactions with clathrin (β-arrestin 2 ΔLIELD), AP-2 (β-arrestin 2-F391A), or both clathrin/AP-2. Expression of a truncated carboxy-terminal region of β-arrestin 1 (319–418) completely abrogated agonist-promoted internalization of M₂ mAChRs in wild type MEF cells.

Conclusion

In summary, this study demonstrates that agonist-promoted internalization of M₂ mAChRs is β-arrestin- and clathrin-dependent, and that the receptor stably co-localizes with β-arrestin in early endosomal vesicles.

EFA6 regulates endosomal trafficking and affects early endosomes in polarized MDCK cells

The small-GTPase family of ADP ribosylation factors (ARFs) recruit coat proteins to promote vesicle budding. ARFs are activated by an association with sec7-containing exchange factors which load them with GTP. In epithelial cells, the small GTPase ARF6 operates within the endocytic system and has been shown to associate with ARNO to promote apical endocytosis and early to late endosomal trafficking. EFA6 has been shown to stimulate tight-junction formation and maintenance. Here, we show that in polarized epithelial MDCK cells, EFA6 is localized to early endosomes, causes their dramatic enlargement, and promotes basolateral targeting of IgA, which is normally targeted to the apical PM. These results suggest that the physiological function of ARF6 within the endocytic system is regulated by the exchange factor it associates with.

Endogenous ARF6 interacts with Rac1 upon angiotensin II stimulation to regulate membrane ruffling and cell migration

ARF6 and Rac1 are small GTPases known to regulate remodelling of the actin cytoskeleton. Here, we demonstrate that these monomeric G proteins are sequentially activated when HEK 293 cells expressing the angiotensin type 1 receptor (AT(1)R) are stimulated with angiotensin II (Ang II). After receptor activation, ARF6 and Rac1 transiently form a complex. Their association is, at least in part, direct and dependent on the nature of the nucleotide bound to both small G proteins. ARF6-GTP preferentially interacts with Rac1-GDP. AT(1)R expressing HEK293 cells ruffle, form membrane protrusions, and migrate in response to agonist treatment. ARF6, but not ARF1, depletion using small interfering RNAs recapitulates the ruffling and migratory phenotype observed after Ang II treatment. These results suggest that ARF6 depletion or Ang II treatment are functionally equivalent and point to a role for endogenous ARF6 as an inhibitor of Rac1 activity. Taken together, our findings reveal a novel function of endogenously expressed ARF6 and demonstrate that by interacting with Rac1, this small GTPase is a central regulator of the signaling pathways leading to actin remodeling.

ARF proteins: roles in membrane traffic and beyond

The ADP-ribosylation factor (ARF) small GTPases regulate vesicular traffic and organelle structure by recruiting coat proteins, regulating phospholipid metabolism and modulating the structure of actin at membrane surfaces. Recent advances in our understanding of the signalling pathways that are regulated by ARF1 and ARF6, two of the best characterized ARF proteins, provide a molecular context for ARF protein function in fundamental biological processes, such as secretion, endocytosis, phagocytosis, cytokinesis, cell adhesion and tumour-cell invasion.

Role of the conserved NPxxY motif of the 5-HT2A receptor in determining selective interaction with isoforms of ADP-ribosylation factor (ARF)

In this study we have shown that N376 to D mutation in the conserved NPxxY motif within the carboxy terminal tail domain (CT) of the 5-HT2A receptor alters the binding preference of GST-fusion protein constructs of the CT domain from ARF1 to an alternative isoform, ARF6. These findings were corroborated by experiments investigating co-immunoprecipitation of the wild type (WT) and N376D mutant of the 5-HT2A receptor with ARF1 or 6 or dominant negative ARF1/6 constructs co-expressed in COS7 cells. In functional assays of 5-HT-induced phospholipase D (PLD) activation responses of the WT receptor were inhibited by a dominant negative mutant of ARF1 but not ARF6, whereas responses of the N376D mutant were strongly inhibited by negative mutant ARF6. No equivalent effect of the ARF mutants was seen on phospholipase C activation. In experiments assaying 5-HT-induced increases in [35S]GTPgammaS binding to ARF 1/6 immunoprecipitates as a measure of ARF activation, increased ARF6 activation was seen only with the mutant receptor. When cellular PLD responses of other NPxxY- or a DPxxY-containing GPCRs were measured in the presence of dominant negative ARF1/6 constructs, the majority, but not all, fitted the pattern exemplified by the 5-HT2A receptor and its N376D mutant. These data suggest that the presence of the N or a D in this highly conserved motif is an important, but not exclusive, determinant of which ARF isoform interacts with the GPCR.

Role of phospholipase D2 in the agonist-induced and constitutive endocytosis of G-protein coupled receptors

We have recently shown that the mu-opioid receptor [MOR1, also termed mu-opioid peptide (MOP) receptor] is associated with the phospholipase D2 (PLD2), a phospholipid-specific phosphodiesterase located in the plasma membrane. We further demonstrated that, in human embryonic kidney (HEK) 293 cells co-expressing MOR1 and PLD2, treatment with (D-Ala2, Me Phe4, Glyol5)enkephalin (DAMGO) led to an increase in PLD2 activity and an induction of receptor endocytosis, whereas morphine, which does not induce opioid receptor endocytosis, failed to activate PLD2. In contrast, a C-terminal splice variant of the mu-opioid receptor (MOR1D, also termed MOP(1D)) exhibited robust endocytosis in response to both DAMGO and morphine treatment. We report here that MOR1D also mediates an agonist-independent (constitutive) PLD2-activation facilitating agonist-induced and constitutive receptor endocytosis. Inhibition of PLD2 activity by over-expression of a dominant negative PLD2 (nPLD2) blocked the constitutive PLD2 activation and impaired the endocytosis of MOR1D receptors. Moreover, we provide evidence that the endocytotic trafficking of the delta-opioid receptor [DOR, also termed delta-opioid peptide (DOP) receptor] and cannabinoid receptor isoform 1 (CB1) is also mediated by a PLD2-dependent pathway. These data indicate the generally important role for PLD2 in the regulation of agonist-dependent and agonist-independent G protein-coupled receptor (GPCR) endocytosis.

ARF6 activation by Galpha q signaling: Galpha q forms molecular complexes with ARNO and ARF6

G protein-coupled receptors (GPCRs) are widely expressed hepta-helical receptors with tightly regulated pleiotropic effects. ADP-Ribosylation Factor 6 (ARF6) plays an important role in GPCR trafficking and is the subject of intense research. However, the mechanisms underlying activation and regulation of ARF6 by GPCRs are poorly characterized. Here we report that Galpha(q) signaling leads to the activation of ARF6. Stimulation of the TPbeta receptor triggered ARF6 activation which was completely inhibited by the RGS domain of GRK2 known to specifically bind and sequester Galpha(q). Co-immunoprecipitation studies revealed that ARNO (a guanine nucleotide exchange factor for ARF6) and ARF6 formed complexes preferentially with activated Galpha(q) compared to non-activated Galpha(q). Formation of the Galpha(q) complexes with ARNO and ARF6 was detected early and was optimal after 30 min of receptor stimulation corresponding with the profile of ARF6 activation. Interestingly, binding experiments using purified proteins showed that Galpha(q) interacted directly with ARNO. Galpha(q)-dependent TPbeta receptor-mediated activation of ARF6 resulted in phosphoinositol-4,5-bisphosphate production which was potently inhibited by dominant negative mutants of ARNO and ARF6. Furthermore, our data show that the expression of ARNO and ARF6 promoted, whereas dominant negative mutants of these proteins inhibited the internalization of the TPbeta receptor. This further elucidates our previous data on the PLCbeta- and PKC-independent mechanism involved in Galpha(q)-mediated internalization of the TPbeta receptor. Taken altogether, our results support a novel model where activated Galpha(q) forms molecular complexes with ARNO and ARF6, possibly through a direct interaction with ARNO, leading to ARF6 activation.

ADP-ribosylation factor 6 regulation of phosphatidylinositol-4,5-bisphosphate synthesis, endocytosis, and exocytosis

Unlike other members of the ADP-ribosylation factor (ARF) family, Arf6 is localized to the plasma membrane and endosomes, and regulates membrane traffic from and into the plasma membrane. Arf6 regulates a clathrin-independent endocytic membrane recycling pathway in nonpolarized cells and clathrin-dependent endocytosis in polarized cells. It also regulates recycling endosome traffic back to the plasma membrane as well as dense-core vesicle exocytosis in neuroendocrine cells. A key effector for Arf6 is phosphatidylinositol 4-monophosphate 5-kinase, which catalyzes plasma membrane synthesis of phosphatidylinositol-4,5-bisphosphate (PIP2), a common required cofactor for several endocytic and exocytic membrane trafficking pathways. Long-term expression of a constitutively active Arf6 mutant in cells can lead to the depletion of PIP2 from the plasma membrane, its accumulation in intracellular vacuoles, and the inhibition of PIP2-dependent membrane trafficking at the plasma membrane.

Arf6 plays an early role in platelet activation by collagen and convulxin

Small GTPases play critical roles in hemostasis, though the roster of such molecules in platelets is not complete. In this study, we report the presence of Ras-related GTPases of the ADP-ribosylation factor (Arf) family. Platelets contain Arf1 or 3 and Arf6, with the latter being predominantly membrane associated. Using effector domain pull-down assays, we show, counter to other GTPases, that Arf6-GTP is present in resting platelets and decreases rapidly upon activation with collagen or convulxin. This decrease does not completely rely on secondary agonists (ADP and thromboxane A2) or require integrin signaling. The decrease in free Arf6-GTP temporally precedes activation of Rho family GTPases (RhoA, Cdc42, and Rac1). Using a membrane-permeant, myristoylated peptide, which mimics the N-terminus of Arf6, we show that the Arf6-GTP decrease is essential for collagen- and convulxin-induced aggregation, platelet adherence, and spreading on collagen-coated glass. Treatment with this peptide also affects the activation of Rho family GTPases, but has little effect on RalA and Rap1 or on agonist-induced calcium mobilization. These data show that Arf6 is a key element in activation through GPVI, and is required for activation of the Rho family GTPases and the subsequent cytoskeletal rearrangements needed for full platelet function.

Thyrotropin receptor trafficking relies on the hScrib-betaPIX-GIT1-ARF6 pathway

G protein-coupled receptors are regulated by ligand stimulation, endocytosis, degradation of recycling to the cell surface. Little information is available on the molecular mechanisms underlying G protein-coupled receptors recycling. We have investigated recycling of the G protein-coupled thyroid stimulating hormone receptor (TSHR) and found that it relies on hScrib, a membrane-associated PDZ protein. hScrib directly binds to TSHR, inhibits basal receptor endocytosis and promotes recycling, and thus TSHR signalling, at the cell membrane. We previously demonstrated that hScrib is associated with a betaPIX-GIT1 complex comprised of a guanine nucleotide exchange factor and a GTPase-activating protein for ADP ribosylation factors that is involved in vesicle trafficking. We used dominant-negative constructs and small interfering RNA to show that TSHR recycling is regulated by the interaction between hScrib and betaPIX, and by the activity of GIT1. In addition, ARF6, a major target for GIT1, is activated during TSH stimulation of HEK293 and FRTL-5 thyroid cells, and plays a key role in TSHR recycling. Thus, we have uncovered an hScrib-betaPIX-GIT1-ARF6 pathway devoted to TSHR trafficking and function.

Gonadotropin receptors: role of post-translational modifications and post-transcriptional regulation

This review focuses on the post-translational modifications of LH and FSH receptors and recent studies on the regulation of LH receptor expression mediated by an RNA binding protein. Both LH and FSH receptors undergo extensive post-translational modifications. N-linked glycosylation occurs co-translationally and plays a role in the maturation and processing of the receptor, while palmitoylation is involved in receptor endocytosis and post-endocytic trafficking. A third type of post-translational modification is phosphorylation and its function has been reviewed. Finally, the regulation of LH receptor at the mRNA level by an RNA binding protein is discussed in the context of ovarian function.

The small G-protein Arf6GTP recruits the AP-2 adaptor complex to membranes

The small GTP-binding protein ADP-ribosylation factor 6 (Arf6) is involved in plasma membrane/endosomes trafficking. However, precisely how the activation of Arf6 regulates vesicular transport is still unclear. Here, we show that, in vitro, recombinant Arf6GTP recruits purified clathrin-adaptor complex AP-2 (but not AP-1) onto phospholipid liposomes in the absence of phosphoinositides. We also show that phosphoinositides and Arf6 tightly cooperate to translocate AP-2 to the membrane. In vivo, Arf6GTP (but not Arf6GDP) was found associated to AP-2. The expression of the GTP-locked mutant of Arf6 leads to the plasma membrane redistribution of AP-2 in Arf6GTP-enriched areas. Finally, we demonstrated that the expression of the GTP-locked mutant of Arf6 inhibits transferrin receptor internalization without affecting its recycling. Altogether, our results demonstrated that Arf6GTP interacts specifically with AP-2 and promotes its membrane recruitment. These findings strongly suggest that Arf6 plays a major role in clathrin-mediated endocytosis by directly controlling the assembly of the AP-2/clathrin coat.

Centaurin-α1, an ADP-Ribosylation Factor 6 GTPase Activating Protein, Inhibits β2-Adrenoceptor Internalization

The small GTP-binding protein ADP ribosylation factor 6 (ARF6) has recently been implicated in the internalization of G protein-coupled receptors (GPCRs), although its precise molecular mechanism in this process remains unclear. We have recently identified centaurin alpha(1) as a GTPase activating protein (GAP) for ARF6. In the current study, we characterized the effects of centaurin alpha(1) on the agonist-induced internalization of the beta(2)-adrenoceptor transiently expressed in human embryonic kidney (HEK) 293 cells. Using an enzyme-linked immunosorbent assay as well as confocal imaging of cells, we found that expression of centaurin alpha(1) strongly inhibited the isoproterenol-induced internalization of beta(2)-adrenoceptor. On the other hand, expression of functionally inactive versions of centaurin alpha(1), including an R49C mutant, which has no catalytic activity, and a double pleckstrin homology (PH) mutant (DM; R148C/R273C), which has mutations in both the PH domains of centaurin alpha(1), rendering it unable to translocate to the cell membrane, were unable to inhibit beta(2)-adrenoceptor internalization. In addition, a constitutively active version of ARF6, ARF6Q67L, reversed the ability of centaurin alpha(1) to inhibit beta(2)-adrenoceptor internalization. Finally, expression of centaurin alpha(1) also inhibited the agonist-induced internalization of beta(2)-adrenoceptor endogenously expressed in HEK 293 cells, whereas the R49C and DM mutant versions of centaurin alpha(1) had no effect. Together, these data indicate that by acting as an ARF6 GAP, centaurin alpha(1) is able to switch off ARF6 and so inhibit its ability to mediate beta(2)-adrenoceptor internalization. Thus, ARF6 GAPs, such as centaurin alpha(1), are likely to play a crucial role in GPCR trafficking by modulating the activity of ARF6.