Rab GTPases bind at a common site within the angiotensin II type I receptor carboxyl-terminal tail: evidence that Rab4 regulates receptor phosphorylation, desensitization, and resensitization

TRAP-induced PAR1 expression with its mechanism during AMI in a rat model

BACKGROUND AND OBJECTIVE

Protease-activated receptor 1 (PAR1) is crucial in individuals with acute myocardial infarction (AMI). The continuous and prompt PAR1 activation mainly dependent on PAR1 trafficking is essential for the role of PAR1 during AMI in which cardiomyocytes are in hypoxia. However, the PAR1 trafficking in cardiomyocytes specially during the hypoxia is still unclear.

METHODS AND RESULT

A rat AMI model was created. PAR1 activation with thrombin-receptor activated peptide (TRAP) had a transient effect on cardiac function in normal rats but persistent improvement in rats with AMI. Cardiomyocytes from neonatal rats were cultured in a normal CO2 incubator and a hypoxic modular incubator chamber. The cells were then subjected to western blot for the total protein expression and staining with fluorescent reagent and antibody for PAR1 localization. No change in total PAR1 expression following TRAP stimulation was observed; however, it led to increased PAR1 expression in the early endosomes in normoxic cells and decreased expression in the early endosomes in hypoxic cells. Under hypoxic conditions, TRAP restored the PAR1 expression on both cell and endosomal surfaces within an hour by decreasing Rab11A (8.5-fold; 179.93 ± 9.82% of the normoxic control group, n = 5) and increasing Rab11B (15.5-fold) expression after 4 h of hypoxia. Similarly, Rab11A knockdown upregulated PAR1 expression under normoxia, and Rab11B knockdown downregulated PAR1 expression under both normoxic and hypoxic conditions. Cardiomyocytes knocked out of both Rab11A, and Rad11B lost the TRAP-induced PAR1 expression but still exhibited the early endosomal TRAP-induced PAR1 expression under hypoxia.

CONCLUSIONS

TRAP-mediated activation of PAR1 in cardiomyocytes did not alter the total PAR1 expression under normoxic conditions. Instead, it triggers a redistribution of PAR1 levels under normoxic and hypoxic conditions. TRAP reverses the hypoxia-inhibited PAR1 expression in cardiomyocytes by downregulating Rab11A expression and upregulating Rab11B expression.

Endocytosis and signaling of angiotensin II type 1 receptor

A vasoactive octapeptide angiotensin II (Ang II) hormone is the key regulator of the renin-angiotensin system (RAS). It binds with the two different plasma membrane receptors like angiotensin II type 1 (AT1) and type 2 (AT2) and consequence various biological responses occur. Further, AT1 has two subtypes such as AT1A and AT1B. These angiotensin receptors are classified to be G protein-coupled receptors (GPCRs). The main constituent of RAS is the AT1 receptor (AT1R), and its activation, signal transduction, and regulation have been extensively studied. After Ang II stimulation, the ligand-receptor complexes internalized and trafficked through the early endosome, recycling endosome, and some receptors skipped the recycling endosome and trafficked to the lysosome for metabolic degradation. Moreover, some short sequence motifs located in the carboxyl-terminus (CT) of the receptor play a vital role in the internalization, phosphorylation, subcellular trafficking, signaling, and desensitization. Furthermore, in endocytosis, the various proteins interact with the CT region of the receptor. This chapter highlights the basic mechanism of AT1 receptor internalization, trafficking and signaling in both physiological and pathophysiological conditions.

Cell Trafficking and Function of G Protein-coupled Receptors

The G protein-coupled receptors (GPCRs) are plasma membrane proteins that function as sensors of changes in the internal and external milieux and play essential roles in health and disease. They are targets of hormones, neurotransmitters, local hormones (autacoids), and a large proportion of the drugs currently used as therapeutics and for "recreational" purposes. Understanding how these receptors signal and are regulated is fundamental for progress in areas such as physiology and pharmacology. This review will focus on what is currently known about their structure, the molecular events that trigger their signaling, and their trafficking to endosomal compartments. GPCR phosphorylation and its role in desensitization (signaling switching) are also discussed. It should be mentioned that the volume of information available is enormous given the large number and variety of GPCRs. However, knowledge is fragmentary even for the most studied receptors, such as the adrenergic receptors. Therefore, we attempt to present a panoramic view of the field, conscious of the risks and limitations (such as oversimplifications and incorrect generalizations). We hope this will provoke further research in the area. It is currently accepted that GPCR internalization plays a role signaling events. Therefore, the processes that allow them to internalize and recycle back to the plasma membrane are briefly reviewed. The functions of cytoskeletal elements (mainly actin filaments and microtubules), the molecular motors implicated in receptor trafficking (myosin, kinesin, and dynein), and the GTPases involved in GPCR internalization (dynamin) and endosomal sorting (Rab proteins), are discussed. The critical role phosphoinositide metabolism plays in regulating these events is also depicted.

Roles of receptor phosphorylation and Rab proteins in G protein-coupled receptor function and trafficking

The G Protein-Coupled Receptors form the most abundant family of membrane proteins and are crucial physiological players in the homeostatic equilibrium, which we define as health. They also participate in the pathogenesis of many diseases and are frequent targets of therapeutic intervention. Considering their importance, it is not surprising that different mechanisms regulate their function, including desensitization, resensitization, internalization, recycling to the plasma membrane, and degradation. These processes are modulated in a highly coordinated and specific way by protein kinases and phosphatases, ubiquitin ligases, protein adaptors, interaction with multifunctional complexes, molecular motors, phospholipid metabolism, and membrane distribution. This review describes significant advances in the study of the regulation of these receptors by phosphorylation and endosomal traffic (where signaling can take place); we revisited the bar code hypothesis and include two additional observations: a) that different phosphorylation patterns seem to be associated with internalization and endosome sorting for recycling or degradation, and b) that, surprisingly, phosphorylation of some G protein-coupled receptors appears to be required for proper receptor insertion into the plasma membrane. Significance Statement G protein-coupled receptor phosphorylation is an early event in desensitization/ signaling switching, endosomal traffic, and internalization. These events seem crucial for receptor responsiveness, cellular localization, and fate (recycling/ degradation) with important pharmacological/ therapeutic implications. Phosphorylation sites vary depending on the cells in which they are expressed and on the stimulus that leads to such covalent modification. Surprisingly, evidence suggests that phosphorylation also seems to be required for proper insertion into the plasma membrane for some receptors.

Differential barcoding of opioid receptors trafficking

Over the past several years, studies have highlighted the δ-opioid receptor (DOPr) as a promising therapeutic target for chronic pain management. While exhibiting milder undesired effects than most currently prescribed opioids, its specific agonists elicit effective analgesic responses in numerous animal models of chronic pain, including inflammatory, neuropathic, diabetic, and cancer-related pain. However, as compared with the extensively studied μ-opioid receptor, the molecular mechanisms governing its trafficking remain elusive. Recent advances have denoted several significant particularities in the regulation of DOPr intracellular routing, setting it apart from the other members of the opioid receptor family. Although they share high homology, each opioid receptor subtype displays specific amino acid patterns potentially involved in the regulation of its trafficking. These precise motifs or "barcodes" are selectively recognized by regulatory proteins and therefore dictate several aspects of the itinerary of a receptor, including its anterograde transport, internalization, recycling, and degradation. With a specific focus on the regulation of DOPr trafficking, this review will discuss previously reported, as well as potential novel trafficking barcodes within the opioid and nociceptin/orphanin FQ opioid peptide receptors, and their impact in determining distinct interactomes and physiological responses.

Agonist-activated glucagon receptors are deubiquitinated at early endosomes by two distinct deubiquitinases to facilitate Rab4a-dependent recycling

The glucagon receptor (GCGR) activated by the peptide hormone glucagon is a seven-transmembrane G protein-coupled receptor (GPCR) that regulates blood glucose levels. Ubiquitination influences trafficking and signaling of many GPCRs, but its characterization for the GCGR is lacking. Using endocytic colocalization and ubiquitination assays, we have identified a correlation between the ubiquitination profile and recycling of the GCGR. Our experiments revealed that GCGRs are constitutively ubiquitinated at the cell surface. Glucagon stimulation not only promoted GCGR endocytic trafficking through Rab5a early endosomes and Rab4a recycling endosomes, but also induced rapid deubiquitination of GCGRs. Inhibiting GCGR internalization or disrupting endocytic trafficking prevented agonist-induced deubiquitination of the GCGR. Furthermore, a Rab4a dominant negative (DN) that blocks trafficking at recycling endosomes enabled GCGR deubiquitination, whereas a Rab5a DN that blocks trafficking at early endosomes eliminated agonist-induced GCGR deubiquitination. By down-regulating candidate deubiquitinases that are either linked with GPCR trafficking or localized on endosomes, we identified signal-transducing adaptor molecule-binding protein (STAMBP) and ubiquitin-specific protease 33 (USP33) as cognate deubiquitinases for the GCGR. Our data suggest that USP33 constitutively deubiquitinates the GCGR, whereas both STAMBP and USP33 deubiquitinate agonist-activated GCGRs at early endosomes. A mutant GCGR with all five intracellular lysines altered to arginines remains deubiquitinated and shows augmented trafficking to Rab4a recycling endosomes compared with the WT, thus affirming the role of deubiquitination in GCGR recycling. We conclude that the GCGRs are rapidly deubiquitinated after agonist-activation to facilitate Rab4a-dependent recycling and that USP33 and STAMBP activities are critical for the endocytic recycling of the GCGR.

Dysregulated Plasma Membrane Turnover Underlying Dendritic Pathology in Neurodegenerative Diseases

Due to their enormous surface area compared to other cell types, neurons face unique challenges in properly handling supply and retrieval of the plasma membrane (PM)-a process termed PM turnover-in their distal areas. Because of the length and extensiveness of dendritic branches in neurons, the transport of materials needed for PM turnover from soma to distal dendrites will be inefficient and quite burdensome for somatic organelles. To meet local demands, PM turnover in dendrites most likely requires local cellular machinery, such as dendritic endocytic and secretory systems, dysregulation of which may result in dendritic pathology observed in various neurodegenerative diseases (NDs). Supporting this notion, a growing body of literature provides evidence to suggest the pathogenic contribution of dysregulated PM turnover to dendritic pathology in certain NDs. In this article, we present our perspective view that impaired dendritic endocytic and secretory systems may contribute to dendritic pathology by encumbering PM turnover in NDs.

In vivo mapping of a GPCR interactome using knockin mice

With over 30% of current medications targeting this family of proteins, G-protein-coupled receptors (GPCRs) remain invaluable therapeutic targets. However, due to their unique physicochemical properties, their low abundance, and the lack of highly specific antibodies, GPCRs are still challenging to study in vivo. To overcome these limitations, we combined here transgenic mouse models and proteomic analyses in order to resolve the interactome of the δ-opioid receptor (DOPr) in its native in vivo environment. Given its analgesic properties and milder undesired effects than most clinically prescribed opioids, DOPr is a promising alternative therapeutic target for chronic pain management. However, the molecular and cellular mechanisms regulating its signaling and trafficking remain poorly characterized. We thus performed liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses on brain homogenates of our newly generated knockin mouse expressing a FLAG-tagged version of DOPr and revealed several endogenous DOPr interactors involved in protein folding, trafficking, and signal transduction. The interactions with a few identified partners such as VPS41, ARF6, Rabaptin-5, and Rab10 were validated. We report an approach to characterize in vivo interacting proteins of GPCRs, the largest family of membrane receptors with crucial implications in virtually all physiological systems.

L-type prostaglandin D synthase regulates the trafficking of the PGD2 DP1 receptor by interacting with the GTPase Rab4

Accumulating evidence indicates that G protein-coupled receptors (GPCRs) interact with Rab GTPases during their intracellular trafficking. How GPCRs recruit and activate the Rabs is unclear. Here, we report that depletion of endogenous L-type prostaglandin D synthase (L-PGDS) in HeLa cells inhibited recycling of the prostaglandin D₂ (PGD₂) DP1 receptor (DP1) to the cell surface after agonist-induced internalization and that L-PGDS overexpression had the opposite effect. Depletion of endogenous Rab4 prevented l-PGDS-mediated recycling of DP1, and l-PGDS depletion inhibited Rab4-dependent recycling of DP1, indicating that both proteins are mutually involved in this pathway. DP1 stimulation promoted its interaction through its intracellular C terminus with Rab4, which was increased by l-PGDS. Confocal microscopy revealed that DP1 activation induces l-PGDS/Rab4 co-localization. l-PGDS/Rab4 and DP1/Rab4 co-immunoprecipitation levels were increased by DP1 agonist treatment. Pulldown assays with purified GST-l-PGDS and His₆-Rab4 indicated that both proteins interact directly. l-PGDS interacted preferentially with the inactive, GDP-locked Rab4S22N variant rather than with WT Rab4 or with constitutively active Rab4Q67L proteins. Overexpression and depletion experiments disclosed that l-PGDS partakes in Rab4 activation following DP1 stimulation. Experiments with deletion mutants and synthetic peptides revealed that amino acids 85-92 in l-PGDS are involved in its interaction with Rab4 and in its effect on DP1 recycling. Of note, GTPγS loading and time-resolved FRET assays with purified proteins suggested that l-PGDS enhances GDP-GTP exchange on Rab4. Our results reveal how l-PGDS, which produces the agonist for DP1, regulates DP1 recycling by participating in Rab4 recruitment and activation.

Mechanisms of the anterograde trafficking of GPCRs: Regulation of AT1R transport by interacting proteins and motifs

Anterograde cell surface transport of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum (ER) through the Golgi apparatus represents a crucial checkpoint to control the amount of the receptors at the functional destination and the strength of receptor activation-elicited cellular responses. However, as compared with extensively studied internalization and recycling processes, the molecular mechanisms of cell surface trafficking of GPCRs are relatively less defined. Here, we will review the current advances in understanding the ER-Golgi-cell surface transport of GPCRs and use angiotensin II type 1 receptor as a representative GPCR to discuss emerging roles of receptor-interacting proteins and specific motifs embedded within the receptors in controlling the forward traffic of GPCRs along the biosynthetic pathway.

Time-Resolved Tracking of Separately Internalized Neuropeptide Y2 Receptors by Two-Color Pulse-Chase

Internalization and intracellular trafficking of G protein-coupled receptors (GPCR) plays an important role in the signal transduction. These processes are often highly dynamic and take place rapidly. In the past 10 years, it became obvious that internalized GPCRs are also capable of signaling via arrestin or heterotrimeric G proteins within the endosomal compartment. Real-time imaging of receptors in living cells can help to evaluate the temporal and spatial localization. We achieved a two-color pulse-chase labeling approach, which allowed the tracking of the human neuropeptide Y₂ receptor (hY₂R) in the same cell at different times. The ability to visualize the internalization pathway of two separately labeled and separately stimulated subsets of hY₂R in a time-resolved manner revealed a rapid trafficking. Fusion of the two hY₂R subsets was already observed 10 min after stimulation in the early endosomal compartment without subsequent separation of the fused receptor populations. The results demonstrate that the cells do not discriminate between receptors that were stimulated and internalized at different time points.

Identification of α-helix 4 (α4) of Rab11a as a novel Rab11-binding domain (RBD): Interaction of Rab11a with the Prostacyclin Receptor

The cellular trafficking of numerous G protein-coupled receptors (GPCRs) is known to be regulated by Rab proteins that involves a direct protein:protein interaction between the receptor and the GTPase. In the case of the human prostacyclin receptor (hIP), it undergoes agonist-induced internalization and subsequent Rab11a-dependent recyclization involving an interaction between a Rab11-binding domain (RBD) localized within its carboxyl-tail domain with Rab11a. However, the GPCR-interacting domain on Rab11a itself is unknown. Hence, we sought to identify the region within Rab11a that mediates its interaction with the RBD of the hIP. The α4 helix region of Rab11 was identified as a novel binding domain for the hIP, a site entirely distinct from the Switch I/Switch II -regions that act as specific binding domain for most other Rab and Ras-like GTPase interactants. Specifically, Glu¹³⁸ within α4 helix of Rab11a appears to contact with key residues (e.g. Lys³⁰⁴) within the RBD of the hIP, where such contacts differ depending on the agonist-activated versus -inactive status of the hIP. Through mutational studies, supported by in silico homology modelling of the inactive and active hIP:Rab11a complexes, a mechanism is proposed to explain both the constitutive and agonist-induced binding of Rab11a to regulate intracellular trafficking of the hIP. Collectively, these studies are not only the first to identify α4 helix of Rab11a as a protein binding domain on the GTPase but also reveal novel mechanistic insights into the intracellular trafficking of the hIP, and potentially of other members of the GPCR superfamily, involving Rab11-dependent mechanisms.

The role of small GTPases and EPAC-Rap signaling in the regulation of the blood-brain and blood-retinal barriers

Maintenance and regulation of the vascular endothelial cell junctional complex is critical for proper barrier function of the blood-brain barrier (BBB) and the highly related blood-retinal barrier (BRB) that help maintain proper neuronal environment. Recent research has demonstrated that the junctional complex is actively maintained and can be dynamically regulated. Studies focusing on the mechanisms of barrier formation, maintenance, and barrier disruption have been of interest to understanding development of the BBB and BRB and identifying a means for therapeutic intervention for diseases ranging from brain tumors and dementia to blinding eye diseases. Research has increasingly revealed that small GTPases play a critical role in both barrier formation and disruption mechanisms. This review will summarize the current data on small GTPases in barrier regulation with an emphasis on the EPAC-Rap1 signaling pathway to Rho in endothelial barriers, as well as explore its potential involvement in paracellular flux and transcytosis regulation.

Using nanoBRET and CRISPR/Cas9 to monitor proximity to a genome-edited protein in real-time

Bioluminescence resonance energy transfer (BRET) has been a vital tool for understanding G protein-coupled receptor (GPCR) function. It has been used to investigate GPCR-protein and/or -ligand interactions as well as GPCR oligomerisation. However the utility of BRET is limited by the requirement that the fusion proteins, and in particular the donor, need to be exogenously expressed. To address this, we have used CRISPR/Cas9-mediated homology-directed repair to generate protein-Nanoluciferase (Nluc) fusions under endogenous promotion, thus allowing investigation of proximity between the genome-edited protein and an exogenously expressed protein by BRET. Here we report BRET monitoring of GPCR-mediated β-arrestin2 recruitment and internalisation where the donor luciferase was under endogenous promotion, in live cells and in real time. We have investigated the utility of CRISPR/Cas9 genome editing to create genome-edited fusion proteins that can be used as BRET donors and propose that this strategy can be used to overcome the need for exogenous donor expression.

Protein Kinase C Activation Promotes α1B-Adrenoceptor Internalization and Late Endosome Trafficking through Rab9 Interaction. Role in Heterologous Desensitization

Upon agonist stimulation, α_1B-adrenergic receptors couple to G_q proteins, calcium signaling and protein kinase C activation; subsequently, the receptors are phosphorylated, desensitized, and internalized. Internalization seems to involve scaffolding proteins, such as β-arrestin and clathrin. However, the fine mechanisms that participate remain unsolved. The roles of protein kinase C and the small GTPase, Rab9, in α_1B-AR vesicular traffic were investigated by studying α_1B-adrenergic receptor-Rab protein interactions, using Förster resonance energy transfer (FRET), confocal microscopy, and intracellular calcium quantitation. In human embryonic kidney 293 cells overexpressing Discosoma spp. red fluorescent protein (DsRed)-tagged α_1B-ARs and enhanced green fluorescent protein--tagged Rab proteins, pharmacological protein kinase C activation mimicked α_1B-AR traffic elicited by nonrelated agents, such as sphingosine 1-phosphate (i.e., transient α_1B-AR-Rab5 FRET signal followed by a sustained α_1B-AR-Rab9 interaction), suggesting brief receptor localization in early endosomes and transfer to late endosomes. This latter interaction was abrogated by blocking protein kinase C activity, resulting in receptor retention at the plasma membrane. Similar effects were observed when a dominant-negative Rab9 mutant (Rab9-GDP) was employed. When α_1B-adrenergic receptors that had been mutated at protein kinase C phosphorylation sites (S396A, S402A) were used, phorbol ester-induced desensitization of the calcium response was markedly decreased; however, interaction with Rab9 was only partially decreased and internalization was observed in response to phorbol esters and sphingosine 1-phosphate. Finally, Rab9-GDP expression did not affect adrenergic-mediated calcium response but abolished receptor traffic and altered desensitization. Data suggest that protein kinase C modulates α_1B-adrenergic receptor transfer to late endosomes and that Rab9 regulates this process and participates in G protein-mediated signaling turn-off.

Role of FQQI motif in the internalization, trafficking, and signaling of guanylyl-cyclase/natriuretic peptide receptor-A in cultured murine mesangial cells

Binding of the cardiac hormone atrial natriuretic peptide (ANP) to transmembrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), produces the intracellular second messenger cGMP in target cells. To delineate the critical role of an endocytic signal in intracellular sorting of the receptor, we have identified a FQQI (Phe(790), Gln(791), Gln(792), and Ile(793)) motif in the carboxyl-terminal region of NPRA. Mouse mesangial cells (MMCs) were transiently transfected with the enhanced green fluorescence protein (eGFP)-tagged wild-type (WT) and mutant constructs of eGFP-NPRA. The mutation FQQI/AAAA, in the eGFP-NPRA cDNA sequence, markedly attenuated the internalization of mutant receptors by almost 49% compared with the WT receptor. Interestingly, we show that the μ1B subunit of adaptor protein-1 binds directly to a phenylalanine-based FQQI motif in the cytoplasmic tail of the receptor. However, subcellular trafficking indicated that immunofluorescence colocalization of the mutated receptor with early endosome antigen-1 (EEA-1), lysosome-associated membrane protein-1 (LAMP-1), and Rab 11 marker was decreased by 57% in early endosomes, 48% in lysosomes, and 42% in recycling endosomes, respectively, compared with the WT receptor in MMCs. The receptor containing the mutated motif (FQQI/AAAA) also produced a significantly decreased level of intracellular cGMP during subcellular trafficking than the WT receptor. The coimmunoprecipitation assay confirmed a decreased level of colocalization of the mutant receptor with subcellular compartments during endocytic processes. The results suggest that the FQQI motif is essential for the internalization and subcellular trafficking of NPRA during the hormone signaling process in intact MMCs.

Recycling and Endosomal Sorting of Protease-activated Receptor-1 Is Distinctly Regulated by Rab11A and Rab11B Proteins

Protease-activated receptor-1 (PAR1) is a G protein-coupled receptor that undergoes proteolytic irreversible activation by coagulant and anti-coagulant proteases. Given the irreversible activation of PAR1, signaling by the receptor is tightly regulated through desensitization and intracellular trafficking. PAR1 displays both constitutive and agonist-induced internalization. Constitutive internalization of PAR1 is important for generating an internal pool of naïve receptors that replenish the cell surface and facilitate resensitization, whereas agonist-induced internalization of PAR1 is critical for terminating G protein signaling. We showed that PAR1 constitutive internalization is mediated by the adaptor protein complex-2 (AP-2), whereas AP-2 and epsin control agonist-induced PAR1 internalization. However, the mechanisms that regulate PAR1 recycling are not known. In the present study we screened a siRNA library of 140 different membrane trafficking proteins to identify key regulators of PAR1 intracellular trafficking. In addition to known mediators of PAR1 endocytosis, we identified Rab11B as a critical regulator of PAR1 trafficking. We found that siRNA-mediated depletion of Rab11B and not Rab11A blocks PAR1 recycling, which enhanced receptor lysosomal degradation. Although Rab11A is not required for PAR1 recycling, depletion of Rab11A resulted in intracellular accumulation of PAR1 through disruption of basal lysosomal degradation of the receptor. Moreover, enhanced degradation of PAR1 observed in Rab11B-deficient cells is blocked by depletion of Rab11A and the autophagy related-5 protein, suggesting that PAR1 is shuttled to an autophagic degradation pathway in the absence of Rab11B recycling. Together these findings suggest that Rab11A and Rab11B differentially regulate intracellular trafficking of PAR1 through distinct endosomal sorting mechanisms.

Subcellular trafficking of guanylyl cyclase/natriuretic peptide receptor-A with concurrent generation of intracellular cGMP

Atrial natriuretic peptide (ANP) modulates blood pressure and fluid volume by activation of natriuretic peptide receptor-A (NPRA). Immunofluorescence (IF) studies reveal that NPRA is internalized and redistributed into subcellular compartments with concurrent production of cGMP.

Atrial natriuretic peptide (ANP) activates guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), which lowers blood pressure and blood volume. The objective of the present study was to visualize internalization and trafficking of enhanced GFP (eGFP)-tagged NPRA (eGFP–NPRA) in human embryonic kidney-293 (HEK-293) cells, using immunofluorescence (IF) and co-immunoprecipitation (co-IP) of eGFP–NPRA. Treatment of cells with ANP initiated rapid internalization and co-localization of the receptor with early endosome antigen-1 (EEA-1), which was highest at 5 min and gradually decreased within 30 min. Similarly, co-localization of the receptor was observed with lysosome-associated membrane protein-1 (LAMP-1); however, after treatment with lysosomotropic agents, intracellular accumulation of the receptor gradually increased within 30 min. Co-IP assays confirmed that the localization of internalized receptors occurred with subcellular organelles during the endocytosis of NPRA. Rab 11, which was used as a recycling endosome (Re) marker, indicated that ∼20% of receptors recycled back to the plasma membrane. ANP-treated cells showed a marked increase in the IF of cGMP, whereas receptor was still trafficking into the intracellular compartments. Thus, after ligand binding, NPRA is rapidly internalized and trafficked from the cell surface into endosomes, Res and lysosomes, with concurrent generation of intracellular cGMP.

New insights in the regulation of Rab GTPases by G protein-coupled receptors

Cargo-mediated regulation of vesicular transport has received great attention lately. Rab GTPases, forming the largest branch of the Ras GTPase superfamily, regulate almost every step of vesicle-mediated trafficking. Growing evidence suggests that mutations, aberrant expression, and altered post-translational modifications of Rab GTPases are associated with human diseases. However, their regulatory mechanisms and how they are connected to cargo proteins are still poorly understood. Accumulating data indicate that G protein-coupled receptors (GPCRs) directly associate with Rab GTPases and that these interactions dictate receptor trafficking. Yet, it remained unclear whether the receptors could regulate the targeting and activity of Rab GTPases in various cell compartments. It is only in recent years that experimental studies showed that GPCR signaling and interaction with Rab-associated regulatory proteins modulate the localization and activity of Rab GTPases. This research is revealing novel regulatory mechanisms of these small GTPases and should contribute to the progress in effective drug development. Recently published in the Journal of Cell Science, Lachance et al. present a novel role for ubiquitylation of Rab11a by a β2AR/HACE1 complex in regulating Rab11a activity and β2AR trafficking.

Mechanism of erythropoietin regulation by angiotensin II

Erythropoietin (EPO) is the primary regulator of red blood cell development. Although hypoxic regulation of EPO has been extensively studied, the mechanism(s) for basal regulation of EPO are not well understood. In vivo studies in healthy human volunteers and animal models indicated that angiotensin II (Ang II) and angiotensin converting enzyme inhibitors regulated blood EPO levels. In the current study, we found that Ang II induced EPO expression in situ in murine kidney slices and in 786-O kidney cells in culture as determined by reverse transcription polymerase chain reaction. We further investigated the signaling mechanism of Ang II regulation of EPO in 786-O cells. Pharmacological inhibitors of Ang II type 1 receptor (AT1R) and extracellular signal-regulated kinase 1/2 (ERK1/2) suppressed Ang II transcriptional activation of EPO. Inhibitors of AT2R or Src homology 2 domain-containing tyrosine phosphatase had no effect. Coimmunoprecipiation experiments demonstrated that p21Ras was constitutively bound to the AT1R; this association was increased by Ang II but was reduced by the AT1R inhibitor telmisartan. Transmembrane domain (TM) 2 of AT1R is important for G protein-dependent ERK1/2 activation, and mutant D74E in TM2 blocked Ang II activation of ERK1/2. Ang II signaling induced the nuclear translocation of the Egr-1 transcription factor, and overexpression of dominant-negative Egr-1 blocked EPO promoter activation by Ang II. These data identify a novel pathway for basal regulation of EPO via AT1R-mediated Egr-1 activation by p21Ras-mitogen-activated protein kinase/ERK kinase-ERK1/2. Our current data suggest that Ang II, in addition to regulating blood volume and pressure, may be a master regulator of erythropoiesis.

Rab family proteins regulate the endosomal trafficking and function of RGS4

RGS4, a heterotrimeric G-protein inhibitor, localizes to plasma membrane (PM) and endosomal compartments. Here, we examined Rab-mediated control of RGS4 internalization and recycling. Wild type and constitutively active Rab5 decreased RGS4 PM levels while increasing its endosomal targeting. Rab5, however, did not appreciably affect the PM localization or function of the M1 muscarinic receptor (M1R)/Gq signaling cascade. RGS4-containing endosomes co-localized with subsets of Rab5-, transferrin receptor-, and Lamp1/Lysotracker-marked compartments suggesting RGS4 traffics through PM recycling or acidified endosome pathways. Rab7 activity promoted TGN association, whereas Rab7(dominant negative) trapped RGS4 in late endosomes. Furthermore, RGS4 was found to co-localize with an endosomal pool marked by Rab11, the protein that mediates recycling/sorting of proteins to the PM. The Cys-12 residue in RGS4 appeared important for its Rab11-mediated trafficking to the PM. Rab11(dominant negative) decreased RGS4 PM levels and increased the number of RGS4-containing endosomes. Inhibition of Rab11 activity decreased RGS4 function as an inhibitor of M1R activity without affecting localization and function of the M1R/Gq signaling complex. Thus, both Rab5 activation and Rab11 inhibition decreased RGS4 function in a manner that is independent from their effects on the localization and function of the M1R/Gq signaling complex. This is the first study to implicate Rab GTPases in the intracellular trafficking of an RGS protein. Thus, Rab GTPases may be novel molecular targets for the selective regulation of M1R-mediated signaling via their specific effects on RGS4 trafficking and function.

Regulation of G protein-coupled receptor trafficking and signaling by Rab GTPases

Rab GTPases play an essential role in the regulation of intracellular transport including the budding, tethering, and fusion of vesicles as well as organelle motility. The regulation of G protein-coupled receptor (GPCR) trafficking by Rab GTPases has traditionally been regarded as a non-specific process that facilitates the movement of the receptors between intracellular membrane compartments. Thus, alterations in GPCR signal transduction and trafficking following the overexpression of constitutively active and dominant negative Rabs were originally considered to be solely the passive by-product of perturbations in intracellular compartmental dynamics. Recently, an explosion of experimental studies has provided increasingly convincing evidence that receptor trafficking actively affects the signal transduction of cargo proteins and that the signaling of GPCR vesicular cargo can in turn modulate Rab GTPase regulated intracellular transport processes. This research is revealing how different Rabs coordinate with themselves and other regulatory molecules to mediate protein trafficking, as well as uncovers novel functions for traditional Rabs, while illustrating the active role these trafficking molecules play in pathology of disease. Recently published in the Journal of Neuroscience, Esseltine et al., present a novel role for the typified exocytic small G protein Rab8 in the intracellular trafficking and signal transduction of metabotropic glutamate receptor 1.

Rab GTPases regulate endothelial cell protein C receptor-mediated endocytosis and trafficking of factor VIIa

Recent studies have established that factor VIIa (FVIIa) binds to the endothelial cell protein C receptor (EPCR). FVIIa binding to EPCR may promote the endocytosis of this receptor/ligand complex. Rab GTPases are known to play a crucial role in the endocytic and exocytic pathways of receptors or receptor/ligand complexes. The present study was undertaken to investigate the role of Rab GTPases in the intracellular trafficking of EPCR and FVIIa. CHO-EPCR cells and human umbilical vein endothelial cells (HUVEC) were transduced with recombinant adenoviral vectors to express wild-type, constitutively active, or dominant negative mutant of various Rab GTPases. Cells were exposed to FVIIa conjugated with AF488 fluorescent probe (AF488-FVIIa), and intracellular trafficking of FVIIa, EPCR, and Rab proteins was evaluated by immunofluorescence confocal microscopy. In cells expressing wild-type or constitutively active Rab4A, internalized AF488-FVIIa accumulated in early/sorting endosomes and its entry into the recycling endosomal compartment (REC) was inhibited. Expression of constitutively active Rab5A induced large endosomal structures beneath the plasma membrane where EPCR and FVIIa accumulated. Dominant negative Rab5A inhibited the endocytosis of EPCR-FVIIa. Expression of constitutively active Rab11 resulted in retention of accumulated AF488-FVIIa in the REC, whereas expression of a dominant negative form of Rab11 led to accumulation of internalized FVIIa in the cytoplasm and prevented entry of internalized FVIIa into the REC. Expression of dominant negative Rab11 also inhibited the transport of FVIIa across the endothelium. Overall our data show that Rab GTPases regulate the internalization and intracellular trafficking of EPCR-FVIIa.

Differential endosomal sorting of a novel P2Y12 purinoreceptor mutant

P2Y12 receptor internalization and recycling play an essential role in ADP-induced platelet activation. Recently, we identified a patient with a mild bleeding disorder carrying a heterozygous mutation of P2Y12 (P341A) whose P2Y12 receptor recycling was significantly compromised. Using human cell line models, we identified key proteins regulating wild-type (WT) P2Y12 recycling and investigated P2Y12 -P341A receptor traffic. Treatment with ADP resulted in delayed Rab5-dependent internalization of P341A when compared with WT P2Y12 . While WT P2Y12 rapidly recycled back to the membrane via Rab4 and Rab11 recycling pathways, limited P341A recycling was observed, which relied upon Rab11 activity. Although minimal receptor degradation was evident, P341A was localized in Rab7-positive endosomes with considerable agonist-dependent accumulation in the trans-Golgi network (TGN). Rab7 activity is known to facilitate recruitment of retromer complex proteins to endosomes to transport cargo to the TGN. Here, we identified that P341A colocalized with Vps26; depletion of which blocked limited recycling and promoted receptor degradation. This study has identified key points of divergence in the endocytic traffic of P341A versus WT-P2Y12 . Given that these pathways are retained in human platelets, this research helps define the molecular mechanisms regulating P2Y12 receptor traffic and explain the compromised receptor function in the platelets of the P2Y12 -P341A-expressing patient.

Rab8 modulates metabotropic glutamate receptor subtype 1 intracellular trafficking and signaling in a protein kinase C-dependent manner

Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors (GPCRs) that are activated by glutamate, the primary excitatory neurotransmitter in the CNS. Alterations in glutamate receptor signaling are implicated in neuropathologies such as Alzheimer's disease, ischemia, and Huntington's disease among others. Group 1 mGluRs (mGluR1 and mGluR5) are primarily coupled to Gα(q/11) leading to the activation of phospholipase C and the formation of diacylglycerol and inositol 1,4,5-trisphosphate, which results in the release of intracellular calcium stores and protein kinase C (PKC) activation. Desensitization, endocytosis, and recycling are major mechanisms of GPCR regulation, and the intracellular trafficking of GPCRs is linked to the Rab family of small G proteins. Rab8 is a small GTPase that is specifically involved in the regulation of secretory/recycling vesicles, modulation of the actin cytoskeleton, and cell polarity. Rab8 has been shown to regulate the synaptic delivery of AMPA receptors during long-term potentiation and during constitutive receptor recycling. We show here that Rab8 interacts with the C-terminal tail of mGluR1a in an agonist-dependent manner and plays a role in regulating of mGluR1a signaling and intracellular trafficking in human embryonic kidney 293 cells. Specifically, Rab8 expression attenuates mGluR1a-mediated inositol phosphate formation and calcium release from mouse neurons in a PKC-dependent manner, while increasing cell surface mGluR1a expression via decreased receptor endocytosis. These experiments provide us with an understanding of the role Rabs play in coordinated regulation of mGluR1a and how this impacts mGluR1a signaling.

G-protein-coupled receptor interaction with small GTPases

In addition to heterotrimeric G-proteins, Ras-like small GTPases are also involved in regulating physiological functions of the G-protein-coupled receptor (GPCR) superfamily. In particular, Rab and ARF GTPases function either as "traffic cops" to coordinate receptor targeting to specific locations or as "signal transducers" to directly mediate receptor signal propagation. As revealed in protein-protein interaction assays, GPCRs may use specific motifs to physically interact with small GTPases, providing important insights into the underlying molecular mechanisms. In this chapter, we describe coimmunoprecipitation and GST fusion protein pull-down approaches to study the GPCR-small GTPase interaction, by focusing on the interaction of α(2B)- and β(2)-adrenegic receptors with the small GTPases Rab8 and ARF1.

Ubiquitination and activation of a Rab GTPase promoted by a β2-Adrenergic Receptor/HACE1 complex

We and others have shown that trafficking of G protein-coupled receptors is regulated by Rab GTPases. Cargo-mediated regulation of vesicular transport has received great attention lately. Rab GTPases, forming the largest branch of the Ras GTPase superfamily, regulate almost every step of vesicle-mediated trafficking. Rab GTPases are well-recognized targets of human diseases but their regulation and the mechanisms connecting them to cargo proteins are still poorly understood. Herein, we show by overexpression/depletion studies that HACE1, a HECT domain-containing ubiquitin ligase, promotes the recycling of the β2-adrenergic receptor (β2AR), a prototypical G protein-coupled receptor, through a Rab11a-dependent mechanism. Interestingly, the β2AR in conjunction with HACE1 triggered ubiquitination of Rab11a, as observed by Western blot analysis. LC-MS/MS experiments determined that Rab11a is ubiquitnatied on Lys145. A Rab11a-K145R mutant failed to undergo β2AR/HACE1-induced ubiquitination and inhibited the HACE1-mediated recycling of the β2AR. Rab11a, but not Rab11a-K145R, was activated by β2AR/HACE1 indicating that ubiquitination of Lys145 is involved in Rab11a activation. β2AR/HACE1 co-expression also potentiated ubiquitination of Rab6a and Rab8a, but not of other Rab GTPases that were tested. We report a novel regulatory mechanism of Rab GTPases by their ubiquitination with associated functional effects demonstrated on Rab11a. This partakes into a new pathway whereby a cargo protein, like a G protein-coupled receptor, can regulate its own trafficking by inducing the ubiquitination and activation of a Rab GTPase.

β-Arrestins: modulators of small GTPase activation and function

Most cellular events responsible for accurate G protein-coupled receptor trafficking involve small GTP-binding proteins. For example, trafficking of receptors via the endocytic and exocytic pathways requires activation of ADP-ribosylation factors and Rab proteins, while receptor-mediated complex responses such as migration are well characterized to be dependent upon Rho family members. Because β-arrestin proteins are recruited to activated receptors and now considered as key signaling molecules, whether they act to control small GTPase activity remains a subject of great interest. Over the years, considerable evidence has suggested that β-arrestins and GTPases might be effectors of the same signaling pathways. One example is the roles of both β-arrestin and Ras, the prototypical GTPase, in coordinating activation of mitogen-activated protein kinase. Recently developed tools effective in suppressing the expression of β-arrestins will help define whether they are essential for small G protein activation. Furthermore, novel approaches to identify protein complexes will greatly advance our understanding of the possible cross talk between β-arrestin and small GTPases.

Modulating neuromodulation by receptor membrane traffic in the endocytic pathway

Cellular responsiveness to many neuromodulators is controlled by endocytosis of the transmembrane receptors that transduce their effects. Endocytic membrane trafficking of particular neuromodulator receptors exhibits remarkable diversity and specificity, determined largely by molecular sorting operations that guide receptors at trafficking branchpoints after endocytosis. In this Review, we discuss recent progress in elucidating mechanisms mediating the molecular sorting of neuromodulator receptors in the endocytic pathway. There is emerging evidence that endocytic trafficking of neuromodulator receptors, in addition to influencing longer-term cellular responsiveness under conditions of prolonged or repeated activation, may also affect the acute response. Physiological and pathological consequences of defined receptor trafficking events are only now being elucidated, but it is already apparent that endocytosis of neuromodulator receptors has a significant impact on the actions of therapeutic drugs. The present data also suggest, conversely, that mechanisms of receptor endocytosis and molecular sorting may themselves represent promising targets for therapeutic manipulation.

A hypothetical model of cargo-selective rab recruitment during organelle maturation

Rabs constitute a group of small GTPases that confer directionality to intracellular vesicle transport by promoting on the membrane of transport vesicles in the formation of specific protein complexes allowing for efficient fusion with a selected set of target organelles. The molecular mechanism controlling recruitment of the correct Rab at the right time is not fully understood. We propose a model according to which the residence time of a given Rab on the membrane of an organelle is determined by its transient trapping into a Rab effector complex (REC) composed of cargo receptor, SNAREs and further effectors. The stability of REC is controlled by the conformational state of the receptor which may change due to binding and release of cargo or changes in the luminal ion milieu. We use a conceptual mathematical model to calculate temporal changes in the Rab decoration of an organelle brought about by exchange with a cytosolic pool of Rabs or alternatively by budding and uptake of Rab-carrying vesicles. Considering the time-dependent drop in pH as one crucial factor for the conformational change of endocytic cargo receptors, our model provides a good quantitative description of the switch from Rab5 to Rab7 during the early-to-late endosome transition and correctly explains the arrest of this transition at insufficient luminal acidification. Model simulations suggest that a switch from one Rab to another may be continuous or abrupt. We discuss mechanisms, e.g. specific signalling pathways, which may restore an arrested organelle maturation.

Regulation of GPCR activity, trafficking and localization by GPCR-interacting proteins

GPCRs represent the largest family of integral membrane proteins and were first identified as receptor proteins that couple via heterotrimeric G-proteins to regulate a vast variety of effector proteins to modulate cellular function. It is now recognized that GPCRs interact with a myriad of proteins that not only function to attenuate their signalling but also function to couple these receptors to heterotrimeric G-protein-independent signalling pathways. In addition, intracellular and transmembrane proteins associate with GPCRs and regulate their processing in the endoplasmic reticulum, trafficking to the cell surface, compartmentalization to plasma membrane microdomains, endocytosis and trafficking between intracellular membrane compartments. The present review will overview the functional consequence of β-arrestin, receptor activity-modifying proteins (RAMPS), regulators of G-protein signalling (RGS), GPCR-associated sorting proteins (GASPs), Homer, small GTPases, PSD95/Disc Large/Zona Occludens (PDZ), spinophilin, protein phosphatases, calmodulin, optineurin and Src homology 3 (SH3) containing protein interactions with GPCRs.

Endothelin-converting enzyme-1 actions determine differential trafficking and signaling of corticotropin-releasing factor receptor 1 at high agonist concentrations

CRF receptor 1 (CRF(1)), a key neuroendocrine mediator of the stress response, has two known agonists corticotropin-releasing factor (CRF) and urocortin 1 (Ucn1). Here we report that endothelin-converting enzyme-1 (ECE-1) differentially degrades CRF and Ucn1; ECE-1 cleaves Ucn1, but not CRF, at critical residue Arginine-34/35', which is essential for ligand-receptor binding. At near K(D) agonist concentration (30 nm), both Ucn1- and CRF-mediated Ca(2+) mobilization are ECE-1 dependent. Interestingly, at high agonist concentration (100 nm), Ucn1-mediated Ca(2+) mobilization remains ECE-1 dependent, whereas CRF-mediated mobilization becomes independent of ECE-1 activity. At high agonist concentration, ECE-1 inhibition disrupted Ucn1-, but not CRF-induced CRF(1) recycling and resensitization, but did not prolong the association of CRF(1) with β-arrestins. RNA interference-mediated knockdown of Rab suggests that both Ucn1- and CRF-induced CRF(1) resensitization is dependent on activity of Rab11, but not of Rab4. CRF(1) behaves like a class A G protein-coupled receptor with respect to transient β-arrestins interaction. We propose that differential degradation by ECE-1 is a novel mechanism by which CRF(1) receptor is protected from overactivation by physiologically relevant high concentrations of higher affinity ligand to mediate distinct resensitization and downstream signaling.

Regulation of β2-adrenergic receptor maturation and anterograde trafficking by an interaction with Rab geranylgeranyltransferase: modulation of Rab geranylgeranylation by the receptor

Previous reports by us and others demonstrated that G protein-coupled receptors interact functionally with Rab GTPases. Here, we show that the β(2)-adrenergic receptor (β(2)AR) interacts with the Rab geranylgeranyltransferase α-subunit (RGGTA). Confocal microscopy showed that β(2)AR co-localizes with RGGTA in intracellular compartments and at the plasma membrane. Site-directed mutagenesis revealed that RGGTA binds to the L(339)L(340) motif in the β(2)AR C terminus known to be involved in the transport of the receptor from the endoplasmic reticulum to the cell surface. Modulation of the cellular levels of RGGTA protein by overexpression or siRNA-mediated knockdown of the endogenous protein demonstrated that RGGTA has a positive role in the maturation and anterograde trafficking of the β(2)AR, which requires the interaction of RGGTA with the β(2)AR L(339)L(340) motif. Furthermore, the β(2)AR modulates the geranylgeranylation of Rab6a, Rab8a, and Rab11a, but not of other Rab proteins tested in this study. Regulation of Rab geranylgeranylation by the β(2)AR was dependent on the RGGTA-interacting L(339)L(340) motif. Interestingly, a RGGTA-Y107F mutant was unable to regulate Rab geranylgeranylation but still promoted β(2)AR maturation, suggesting that RGGTA may have functions independent of Rab geranylgeranylation. We demonstrate for the first time an interaction between a transmembrane receptor and RGGTA which regulates the maturation and anterograde transport of the receptor, as well as geranylgeranylation of Rab GTPases.

Functional relevance of biased signaling at the angiotensin II type 1 receptor

Angiotensin II type 1 receptor antagonists (AT1R blockers, or ARBs) are used commonly in the treatment of cardiovascular disorders such as heart failure and hypertension. Their clinical success arises from their ability to prevent deleterious Gα(q) protein activation downstream of AT1R, which leads to a decrease in morbidity and mortality. Recent studies have identified AT1R ligands that concurrently inhibit Gα(q) protein-dependent signaling and activate Gα(q) protein-independent/β-arrestin-dependent signaling downstream of AT1R, events that may actually improve cardiovascular performance more than conventional ARBs. The ability of such ligands to induce intracellular signaling events in an AT1R-β-arrestin-dependent manner while preventing AT1R-Gα(q) protein activity defines them as biased AT1R ligands. This mini-review will highlight recent studies that have defined biased signaling at the AT1R and discuss the possible clinical relevance of β-arrestin-biased AT1R ligands in the cardiovascular system.

Yellow submarine of the Wnt/Frizzled signaling: submerging from the G protein harbor to the targets

The Wnt/Frizzled signaling pathway plays multiple functions in animal development and, when deregulated, in human disease. The G-protein coupled receptor (GPCR) Frizzled and its cognate heterotrimeric Gi/o proteins initiate the intracellular signaling cascades resulting in cell fate determination and polarization. In this review, we summarize the knowledge on the ligand recognition, biochemistry, modifications and interacting partners of the Frizzled proteins viewed as GPCRs. We also discuss the effectors of the heterotrimeric Go protein in Frizzled signaling. One group of these effectors is represented by small GTPases of the Rab family, which amplify the initial Wnt/Frizzled signal. Another effector is the negative regulator of Wnt signaling Axin, which becomes deactivated in response to Go action. The discovery of the GPCR properties of Frizzled receptors not only provides mechanistic understanding to their signaling pathways, but also paves new avenues for the drug discovery efforts.