Angiotensin II receptor endocytosis involves two distinct regions of the cytoplasmic tail. A role for residues on the hydrophobic face of a putative amphipathic helix

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.

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.

Angiotensin Converting Enzyme Regulates Cell Proliferation and Migration

Background

The angiotensin-I converting enzyme (ACE) plays a central role in the renin-angiotensin system, acting by converting the hormone angiotensin-I to the active peptide angiotensin-II (Ang-II). More recently, ACE was shown to act as a receptor for Ang-II, and its expression level was demonstrated to be higher in melanoma cells compared to their normal counterparts. However, the function that ACE plays as an Ang-II receptor in melanoma cells has not been defined yet.

Aim

Therefore, our aim was to examine the role of ACE in tumor cell proliferation and migration.

Results

We found that upon binding to ACE, Ang-II internalizes with a faster onset compared to the binding of Ang-II to its classical AT1 receptor. We also found that the complex Ang-II/ACE translocates to the nucleus, through a clathrin-mediated process, triggering a transient nuclear Ca²⁺ signal. In silico studies revealed a possible interaction site between ACE and phospholipase C (PLC), and experimental results in CHO cells, demonstrated that the β3 isoform of PLC is the one involved in the Ca²⁺ signals induced by Ang-II/ACE interaction. Further studies in melanoma cells (TM-5) showed that Ang-II induced cell proliferation through ACE activation, an event that could be inhibited either by ACE inhibitor (Lisinopril) or by the silencing of ACE. In addition, we found that stimulation of ACE by Ang-II caused the melanoma cells to migrate, at least in part due to decreased vinculin expression, a focal adhesion structural protein.

Conclusion

ACE activation regulates melanoma cell proliferation and migration.

Angiotensin II receptors and peritoneal dialysis-induced peritoneal fibrosis

The vasoactive hormone angiotensin II initiates its major hemodynamic effects through interaction with AT1 receptors, a member of the class of G protein-coupled receptors. Acting through its AT1R, angiotensin II regulates blood pressure and renal salt and water balance. Recent evidence points to additional pathological influences of activation of AT1R, in particular inflammation, fibrosis and atherosclerosis. The transcription factor nuclear factor κB, a key mediator in inflammation and atherosclerosis, can be activated by angiotensin II through a mechanism that may involve arrestin-dependent AT1 receptor internalization. Peritoneal dialysis is a therapeutic modality for treating patients with end-stage kidney disease. The effectiveness of peritoneal dialysis at removing waste from the circulation is compromised over time as a consequence of peritoneal dialysis-induced peritoneal fibrosis. The non-physiological dialysis solution used in peritoneal dialysis, i.e. highly concentrated, hyperosmotic glucose, acidic pH as well as large volumes infused into the peritoneal cavity, contributes to the development of fibrosis. Numerous trials have been conducted altering certain components of the peritoneal dialysis fluid in hopes of preventing or delaying the fibrotic response with limited success. We hypothesize that structural activation of AT1R by hyperosmotic peritoneal dialysis fluid activates the internalization process and subsequent signaling through the transcription factor nuclear factor κB, resulting in the generation of pro-fibrotic/pro-inflammatory mediators producing peritoneal fibrosis.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Distinct regions in the C-Terminus required for GLP-1R cell surface expression, activity and internalisation

The glucagon-like peptide-1 (GLP-1) receptor (GLP-1R), an important drug target in the treatment of type 2 diabetes, is a G-protein coupled receptor (GPCR) that mediates insulin secretion by GLP-1. The N-terminus controls GLP-1R biosynthetic trafficking to the cell surface but the C-terminus involvement in that trafficking is unknown. The aim of this study was to identify distinct regions within the C-terminal domain required for human GLP-1R (hGLP-1R) cell surface expression, activity and internalisation using a number of C-terminal deletions and site-directed mutations. The results of this study revealed that the residues 411-418 within the C-terminal domain of the hGLP-1R are critical in targeting the newly synthesised receptor to the plasma membrane. The residues 419-430 are important for cAMP producing activity of the receptor, most likely by coupling to Gαs. However, the residues 431-450 within the C-terminus are essential for agonist-induced hGLP-1R internalisation. In conclusion, these findings demonstrate the hGLP-1R has distinct regions within the C-terminal domain required for its cell surface expression, activity and agonist-induced internalisation.

A single mutation in helix 8 enhances the angiotensin II type 1a receptor transport and signaling

The amphipathic helix 8 in the membrane-proximal C-terminus is a structurally conserved feature of class A seven transmembrane-spanning G protein-coupled receptors (GPCRs). Mutations of this helical motif often cause receptor misfolding, defective cell surface transport and dysfunction. Surprisingly, we demonstrated here that a single point mutation at Lys308 in helix 8 markedly enhanced the steady-state surface density of the angiotensin II type 1a receptor (AT1aR). Consistent with the enhanced cell surface expression, Lys308 mutation significantly augmented AT1aR-mediated mitogen-activated protein kinase ERK1/2 activation, inositol phosphate production, and vascular smooth muscle cell migration. This mutation also increased the overall expression of AT1aR without altering receptor degradation. More interestingly, Lys308 mutation abolished AT1aR interaction with β-COP, a component of COPI transport vesicles, and impaired AT1aR responsiveness to the inhibition of Rab6 GTPase involved in the Golgi-to-ER retrograde pathway. Furthermore, these functions of Lys308 were largely dependent on its positively charged property. These data reveal previously unappreciated functions of helix 8 and novel mechanisms governing the cell surface transport and function of AT1aR.

Structure of the Angiotensin receptor revealed by serial femtosecond crystallography

Angiotensin II type 1 receptor (AT(1)R) is a G protein-coupled receptor that serves as a primary regulator for blood pressure maintenance. Although several anti-hypertensive drugs have been developed as AT(1)R blockers (ARBs), the structural basis for AT(1)R ligand-binding and regulation has remained elusive, mostly due to the difficulties of growing high-quality crystals for structure determination using synchrotron radiation. By applying the recently developed method of serial femtosecond crystallography at an X-ray free-electron laser, we successfully determined the room-temperature crystal structure of the human AT(1)R in complex with its selective antagonist ZD7155 at 2.9-Å resolution. The AT(1)R-ZD7155 complex structure revealed key structural features of AT(1)R and critical interactions for ZD7155 binding. Docking simulations of the clinically used ARBs into the AT(1)R structure further elucidated both the common and distinct binding modes for these anti-hypertensive drugs. Our results thereby provide fundamental insights into AT(1)R structure-function relationship and structure-based drug design.

Scanning mutagenesis in a yeast system delineates the role of the NPxxY(x)(5,6)F motif and helix 8 of the adenosine A(2B) receptor in G protein coupling

The adenosine receptor subfamily includes four subtypes: the A1, A2A, A2B and A3 receptors, which all belong to the superfamily of G protein-coupled receptors (GPCRs). The adenosine A2B receptor is the least investigated of the adenosine receptors, and the molecular mechanisms of its activation have hardly been explored. We used a single-GPCR-one-G protein yeast screening method in combination with mutagenesis studies, molecular modeling and bio-informatics to investigate the importance of the different amino acid residues of the NPxxY(x)6F motif and helix 8 in the human adenosine A2B receptor (hA2BR) activation. A scanning mutagenesis protocol was employed, yielding 11 single mutations and one double mutation of the NPxxY(x)6F motif and 16 single mutations of helix 8. The amino acid residues P287(7.50), Y290(7.53), R293(7.56) and I304(8.57) were found to be essential, since mutation of these amino acid residues to alanine led to a complete loss of function. Western blot analysis showed that mutant receptor R293(7.56)A was not expressed, whereas the other proteins were. Amino acid residues that are also important in receptor activation are: N286(7.49), V289(7.52), Y292(7.55), N294(8.47), F297(8.50), R298(8.51), H302(8.55) and R307(8.60). The mutation Y290(7.53)F lost 50% of efficacy, while F297(8.50)A behaved similar to wild type receptor. The double mutation, Y290(7.53)F/F297(8.50)Y, lost around 70% of efficacy and displayed a lower potency for the reference agonist 5'-(N-ethylcarboxamido)adenosine (NECA). This study provides new insight into the molecular interplay and impact of TM7 and helix 8 for hA2B receptor activation, which may be extrapolated to other adenosine receptors and possibly to other GPCRs.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

New insights and perspectives on intrarenal renin-angiotensin system: focus on intracrine/intracellular angiotensin II

Although renin, the rate-limiting enzyme of the renin-angiotensin system (RAS), was first discovered by Robert Tigerstedt and Bergman more than a century ago, the research on the RAS still remains stronger than ever. The RAS, once considered to be an endocrine system, is now widely recognized as dual (circulating and local/tissue) or multiple hormonal systems (endocrine, paracrine and intracrine). In addition to the classical renin/angiotensin I-converting enzyme (ACE)/angiotensin II (Ang II)/Ang II receptor (AT₁/AT₂) axis, the prorenin/(Pro)renin receptor (PRR)/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, and the Ang IV/AT₄/insulin-regulated aminopeptidase (IRAP) axis have recently been discovered. Furthermore, the roles of the evolving RAS have been extended far beyond blood pressure control, aldosterone synthesis, and body fluid and electrolyte homeostasis. Indeed, novel actions and underlying signaling mechanisms for each member of the RAS in physiology and diseases are continuously uncovered. However, many challenges still remain in the RAS research field despite of more than one century's research effort. It is expected that the research on the expanded RAS will continue to play a prominent role in cardiovascular, renal and hypertension research. The purpose of this article is to review the progress recently being made in the RAS research, with special emphasis on the local RAS in the kidney and the newly discovered prorenin/PRR/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, the Ang IV/AT₄/IRAP axis, and intracrine/intracellular Ang II. The improved knowledge of the expanded RAS will help us better understand how the classical renin/ACE/Ang II/AT₁ receptor axis, extracellular and/or intracellular origin, interacts with other novel RAS axes to regulate blood pressure and cardiovascular and kidney function in both physiological and diseased states.

Heteromerization of angiotensin receptors changes trafficking and arrestin recruitment profiles

The cardiovascular hormone angiotensin II (AngII) exerts its actions via two G protein-coupled receptor (GPCR) subtypes, AT(1) and AT(2), which often display antagonistic functions. Methodological constraints have so far precluded detailed analyses of the ligand-dependency, cellular localization, and functional relevance of AngII receptor interactions in live cells. In this study, we utilize a protein-fragment complementation assay (PCA) and GPCR-Heteromer Identification Technology (GPCR-HIT) to provide the first detailed investigation of the ligand-dependency and cellular localization of AngII receptor interactions in human embryonic kidney 293 cells. Fluorescent-tagged receptor constructs for PCA and GPCR-HIT displayed normal affinity and selectivity for AngII (AT(1): IC(50)=1.0-1.6nM; AT(2): IC(50)=2.0-3.0nM). Well-characterized angiotensin receptor interactions were used as positive and negative controls to demonstrate the sensitivity and specificity of these fluorescence-based assays. We report that AT(1)-AT(2) receptor heteromers form constitutively, are localized to the plasma membrane and perinuclear compartments, and do not internalize following AngII stimulation despite arrestin being recruited specifically to the heteromer. Our findings using novel fluorescence-based technologies reveal a previously unrecognized mechanism of angiotensin receptor cross-talk involving cross-inhibition of AT(1) receptor internalization through heteromerization with the AT(2) receptor subtype.

Somatostatin receptor-3 mediated intracellular signaling and apoptosis is regulated by its cytoplasmic terminal

In the present study, we describe the role of cytoplasmic terminal (C-tail) domain in regulating coupling to adenylyl cyclase, signaling, and apoptosis in human embryonic kidney (HEK-293) cells transfected with wild type (wt)-hSSTR3 and C-tail deleted mutants. Cells transfected with wt-hSSTR3 and C-tail mutants show comparable membrane expression; however, display decreased expression in presence of agonist. wt-hSSTR3 exists as preformed homodimer at cell surface in basal conditions and decreases in response to agonist. Cells expressing C-tail mutants also show evidence of homodimerization with the same intensity as wt-hSSTR3. The agonist-dependent inhibition of cyclic adenosine monophosphate (cAMP) was lost in cells expressing C-tail mutants. Agonist treatment in cells expressing wt-hSSTR3 resulted in inhibition of cell proliferation, increased expression of PARP-1, and TUNEL positivity in proliferating cell nuclear antigen (PCNA)-positive cells. The agonist mediated increase in membrane expression of protein tyrosine phosphatase (PTP) seen with wt-hSSTR3 was diminished in C-tail mutants, which was accompanied with the loss of receptor's ability to induce apoptosis. Taken together, our data provide new insights into C-tail-dependent regulation of cell signaling and apoptosis by hSSTR3.

C-tail mediated modulation of somatostatin receptor type-4 homo- and heterodimerizations and signaling

Somatostatin receptors show great diversity in response to agonist mediated receptor-specific homo- and heterodimerizations. Here, using photobleaching-fluorescence resonance energy transfer, immunocytochemistry, western blot and co-immunoprecipitation, we investigated dimerization, trafficking, coupling to adenylyl cyclase and signaling of human somatostatin receptor-4 (hSSTR4) in HEK-293 cells. We also determined the role of the C-tail of hSSTR4 on physiological responses of the cells. wt-hSSTR4 exogenously expressed in HEK-293 cells exhibits constitutive dimerization, inhibits forskolin-stimulated cAMP, and displays agonist dependent changes in pERK1/2 and pERK5 expressions. Upon C-tail deletion, the receptor loses membrane expression and ability to dimerize and inhibition of cAMP and pERK5 however, displays several-fold increases in the expression of pERK1/2. Chimeric hSSTR4 with the C-tail of hSSTR5 functions like wt-hSSTR4, in contrast, with the C-tail of hSSTR1 functions like C-tail deleted hSSTR4. hSSTR4 dimerization and signaling are associated with increased cyclin-dependent-kinase p27(kip1) expression and inhibition of the cell proliferation. We also report heterodimerization between hSSTR4/hSSTR5, but not between hSSTR4/hSSTR1, with significant changes in receptor functions. Taken together, these data define a novel mechanism for the role of hSSTR4 in cell proliferation and modulation of signaling pathways.

Role of helix 8 in G protein-coupled receptors based on structure-function studies on the type 1 angiotensin receptor

G protein-coupled receptors (GPCRs) are transmembrane receptors that convert extracellular stimuli to intracellular signals. The type 1 angiotensin II receptor is a widely studied GPCR with roles in blood pressure regulation,water and salt balance and cell growth. The complex molecular and structural changes that underpin receptor activation and signaling are the focus of intense research. Increasingly, there is an appreciation that the plasma membrane participates in receptor function via direct, physical interactions that reciprocally modulate both lipid and receptor and provide microdomains for specialized activities. Reversible protein:lipid interactions are commonly mediated by amphipathic -helices in proteins and one such motif - a short helix, referred to as helix VIII/8 (H8), located at the start of the carboxyl (C)-terminus of GPCRs - is gaining recognition for its importance to GPCR function. Here, we review the identification of H8 in GPCRs and examine its capacity to sense and interact with diverse proteins and lipid environment, most notably with acidic lipids that include phosphatidylinositol phosphates.

Prolonged RXFP1 and RXFP2 signaling can be explained by poor internalization and a lack of beta-arrestin recruitment

Relaxin induces sustained physiological responses, which brings into question the deactivation processes typical of most G protein-coupled receptors (GPCR) for its receptor, relaxin family peptide receptor 1 (RXFP1). Here, we examined relaxin-dependent phosphorylation of RXFP1 and the related insulin-like peptide 3 (INSL3) receptor, RXFP2, as well as the capacity of these receptors to recruit beta-arrestins and internalize in response to ligand stimulation. We confirmed in human embryonic kidney (HEK)-293T cells, expressing RXFP1 or RXFP2, that both receptors elicit prolonged cAMP responses up to 6 h after stimulation. Receptors immunoprecipitated from (32)P metabolically labeled cells were used to investigate the agonist-specific phosphorylation. Rapid and robust receptor phosphorylation was not observed for either RXFP1 or RXFP2, although some (32)P-incorporation was observed at 30 min; however, this was not statistically significant. In accord with this result, RXFP1 and RXFP2 demonstrated poor internalization in response to relaxin or INSL3, as compared with the angiotensin II type 1 receptor (AT(1)R), which undergoes rapid and robust phosphorylation and internalization in response to angiotensin II. Additionally, coexpression of GPCR kinases has no effect on the rate of internalization for either RXFP1 or RXFP2. Confocal microscopy was used to follow the trafficking of green fluorescent protein-labeled beta-arrestins after receptor activation. Neither RXFP1 nor RXFP2 activation results in recruitment of beta-arrestins to the cell surface, whereas AT(1)R rapidly recruits both beta-arrestins-1 and -2. The apparent lack of classical regulation for RXFP1 and RXFP2 provides the molecular basis for the prolonged signaling and physiological actions of relaxin and related peptides.

The cytoplasmic domain of the hyaluronan receptor for endocytosis (HARE) contains multiple endocytic motifs targeting coated pit-mediated internalization

The hyaluronic acid (HA) receptor for endocytosis (HARE) is the primary scavenger receptor for HA and chondroitin sulfates in mammals. The two human isoforms of HARE (full-length 315-kDa and a 190-kDa proteolytic cleavage product), which are type I single-pass membrane proteins, are highly expressed in sinusoidal endothelial cells of lymph nodes, liver, and spleen. Their identical HARE cytoplasmic domains contain four candidate AP-2/clathrin-mediated endocytic signaling motifs as follows: YSYFRI(2485), FQHF(2495), NPLY(2519), and DPF(2534) (315-HARE numbering). Stably transfected cells expressing 190-HARE(DeltaYSYFRI), 190-HARE(DeltaFQHF), or 190-HARE(DeltaNPLY) (lacking Motifs 1, 2, or 3) had decreased (125)I-HA endocytosis rates of approximately 49, approximately 39, and approximately 56%, respectively (relative to wild type). In contrast, 190-HARE(DeltaDPF) cells (lacking Motif 4) showed no change in HA endocytic rate. Deletions of motifs 1 and 2 or of 1, 2, and 4 decreased the rate of HA endocytosis by only approximately 41%. Endocytosis was approximately 95% decreased in mutants lacking all four motifs. Cells expressing a 190-HARE(Y2519A) mutant of the NPLY motif retained 85-90% of wild type endocytosis, whereas this mutation in the triple motif deletant decreased endocytosis to approximately 7% of wild type. Tyr in NPLY(2519) is thus important for endocytosis. All HARE mutants showed similar HA binding and degradation of the internalized HA, indicating that altering endocytic motifs did not affect ectodomain binding of HA or targeting of internalized HA to lysosomes. We conclude that, although NPLY may be the most important motif, it functions together with two other endocytic motifs; thus three signal sequences (YSYFRI, FQHF, and NPLY) provide redundancy to mediate coated pit targeting and endocytosis of HARE.

Internalisation of the mu-opioid receptor by endomorphin-1 and leu-enkephalin is dependant on aromatic amino acid residues

The opioid receptor system in the central nervous system controls a number of physiological processes, most notably pain. However, most opioids currently available have a variety of side-effects as well as exhibiting tolerance. Tolerance is most likely to be a complex phenomenon, however, the role of receptor internalisation is thought to play a crucial role. In this study, we examined the role of aromaticity in ligand-mediated receptor internalisation of the mu-opioid receptor (MOPR). These studies show that the amount of receptor internalisation may be dependant on the amphiphilicity of the ligand. Specifically, deletion of the C-terminus aromatic residues of endomorphin 1, particularly tryptophan reduces receptor-mediated internalisation whilst the addition of tryptophan within the enkephalin sequence increases receptor internalisation and decreases tolerance.

Role of the carboxyl terminal di-leucine in phosphorylation and internalization of C5a receptor

The carboxyl tail of G protein-coupled receptors contains motifs that regulate receptor interactions with intracellular partners. Activation of the human neutrophil complement fragment C5a receptor (C5aR) is terminated by phosphorylation of the carboxyl tail followed by receptor internalization. In this study, we demonstrated that bulky hydrophobic residues in the membrane-proximal region of the C5aR carboxyl tail play an important role in proper structure and function of the receptor: Substitution of leucine 319 with alanine (L319A) resulted in receptor retention in the endoplasmic reticulum, whereas a L318A substitution allowed receptor transport to the cell surface, but showed slow internalization upon activation, presumably due to a defect in phosphorylation by both PKC and GRK. Normal agonist-induced activation of ERK1/2 and intracellular calcium release suggested that the L318A mutation did not affect receptor signaling. Binding of GRK2 and PKCbetaII to intracellular loop 3 of C5aR in vitro indicated that mutagenesis of L318 did not affect kinase binding. Limited proteolysis with trypsin revealed a conformational difference between wild type and mutant receptor. Our studies support a model in which the L318/L319 stabilizes an amphipathic helix (Q305-R320) in the membrane-proximal region of C5aR.

The Angiotensin II AT1 Receptor Structure-Activity Correlations in the Light of Rhodopsin Structure

The most prevalent physiological effects of ANG II, the main product of the renin-angiotensin system, are mediated by the AT1 receptor, a rhodopsin-like AGPCR. Numerous studies of the cardiovascular effects of synthetic peptide analogs allowed a detailed mapping of ANG II's structural requirements for receptor binding and activation, which were complemented by site-directed mutagenesis studies on the AT1 receptor to investigate the role of its structure in ligand binding, signal transduction, phosphorylation, binding to arrestins, internalization, desensitization, tachyphylaxis, and other properties. The knowledge of the high-resolution structure of rhodopsin allowed homology modeling of the AT1 receptor. The models thus built and mutagenesis data indicate that physiological (agonist binding) or constitutive (mutated receptor) activation may involve different degrees of expansion of the receptor's central cavity. Residues in ANG II structure seem to control these conformational changes and to dictate the type of cytosolic event elicited during the activation. 1) Agonist aromatic residues (Phe8 and Tyr4) favor the coupling to G protein, and 2) absence of these residues can favor a mechanism leading directly to receptor internalization via phosphorylation by specific kinases of the receptor's COOH-terminal Ser and Thr residues, arrestin binding, and clathrin-dependent coated-pit vesicles. On the other hand, the NH2-terminal residues of the agonists ANG II and [Sar1]-ANG II were found to bind by two distinct modes to the AT1 receptor extracellular site flanked by the COOH-terminal segments of the EC-3 loop and the NH2-terminal domain. Since the [Sar1]-ligand is the most potent molecule to trigger tachyphylaxis in AT1 receptors, it was suggested that its corresponding binding mode might be associated with this special condition of receptors.

Identification of a putative nuclear localization sequence within ANG II AT(1A) receptor associated with nuclear activation

Angiotensin II (ANG II) type 1 (AT(1)) receptors, similar to other G protein-coupled receptors, undergo desensitization and internalization, and potentially nuclear localization, subsequent to agonist interaction. Evidence suggests that the carboxy-terminal tail may be involved in receptor nuclear localization. In the present study, we examined the carboxy-terminal tail of the receptor for specific regions responsible for the nuclear translocation phenomenon and resultant nuclear activation. Human embryonic kidney cells stably expressing either a wild-type AT(1A) receptor-green fluorescent protein (AT(1A)R/GFP) construct or a site-directed mutation of a putative nuclear localization sequence (NLS) [K307Q]AT(1A)R/GFP (KQ/AT(1A)R/GFP), were examined for differences in receptor nuclear trafficking and nuclear activation. Receptor expression, intracellular signaling, and ANG II-induced internalization of the wild-type/GFP construct and of the KQ/AT(1A)R/GFP mutant was similar. Laser scanning confocal microscopy showed that in cells expressing the AT(1A)R/GFP, trafficking of the receptor to the nuclear area and colocalization with lamin B occurred within 30 min of ANG II (100 nM) stimulation, whereas the KQ/AT(1A)R/GFP mutant failed to demonstrate nuclear localization. Immunoblotting of nuclear lysates with an anti-GFP antibody confirmed these observations. Nuclear localization of the wild-type receptor correlated with increase transcription for both EGR-1 and PTGS-2 genes while the nuclear-deficient KQ/AT(1A)R/GFP mutant demonstrated increases for only the EGR-1 gene. These results suggest that a NLS (KKFKKY; aa307-312) is located within the cytoplasmic tail of the AT(1A) receptor and that nuclear localization of the receptor corresponds with specific activation of transcription for the COX-2 gene PTGS-2.

Development of hypertension and kidney hypertrophy in transgenic mice overexpressing ARAP1 gene in the kidney

Angiotensin II regulates blood pressure via activation of the type 1 receptor. We previously identified a novel angiotensin II type 1 receptor-associated protein and demonstrated that it promotes receptor recycling to the plasma membrane. To delineate the pathophysiological function of the ARAP1 in the kidneys, we generated transgenic mice that overexpress rat ARAP1 cDNA specifically in proximal tubules and tested the hypothesis that proximal tubule-specific overexpression of ARAP1 causes hypertension. Two lines of male transgenic mice, 650 and 670, displayed kidney-specific transgene expression. Systolic blood pressure was significantly elevated by &20 to 25 mm Hg in these lines of mice at 20 weeks of age compared with their nontransgenic litter mates. Urine volume, but not water intake, was significantly decreased in both lines compared with nontransgenic controls. The kidney/body weight ratio was significantly increased in both lines compared with their nontransgenic litter mates at 12 and 20 weeks of age. In contrast, no difference was observed in the ratio of brain, spleen, heart, and testis to body weight between male transgenic and nontransgenic animals. Inhibitions of the renin-angiotensin system completely normalized the systolic blood pressure of transgenic mice. Moreover, low salt intake prevented the development of hypertension, whereas high salt intake exacerbated the increase in blood pressure in transgenic mice. Therefore, our data show that proximal tubule-specific overexpression of ARAP1 leads to hypertension, suggesting that renal ARAP1 plays an important role in the regulation of blood pressure and renal function via activation of the intrarenal renin-angiotensin system.

The complement inhibitor, CRIT, undergoes clathrin-dependent endocytosis

Complement C2 receptor inhibitor trispanning (CRIT) is a receptor for the second component of complement and is found in various tissues and hemopoietic cells. On binding to CRIT, C2 cannot be activated to potentially form a variant-C3 convertase as it is rendered non-cleavable by C1s. CRIT thus limits the amount of C3 convertase formed on the cell surface. In this study we have shown, using flow cytometry and immunofluorescence microscopy, that human CRIT undergoes endocytosis from the plasma membrane. The endocytosis, possibly ligand mediated, occurs via clathrin-coated pits as it can be inhibited by prior incubation of cells in hypertonic medium or with chlorpromazine, at 37 degrees C. However, inhibition of endocytosis was not possible after treatment with nystatin, or filipin, inhibitors of caveolae/raft-dependent endocytosis. In the presence of C2 alone, CRIT associates with the adapter protein, beta-arrestin-2, and whether in association with C2 or not, then appears in the perinuclear region, but does not appear to be translocated into the nucleus. Apart from the C3aR and C5aR that internalize the anaphylatoxic peptides, this is the first report of the internalization via the clathrin pathway of a receptor for a complement serum protein.

Regulatory mechanisms underlying pheromone biosynthesis activating neuropeptide (PBAN)-induced internalization of the Bombyx mori PBAN receptor

Internalization of the Bombyx mori pheromone biosynthesis activating neuropeptide receptor (PBANR) has been attributed to the presence of a 67 amino acid C-terminal extension absent in PBANRs from Helicoverpa. To identify the structural motif(s) responsible for internalization, a series of truncation mutants fused with enhanced green fluorescent protein were constructed and transiently expressed in insect Sf9 cells. Confocal microscopy analyses revealed that truncation at Gly357 severely inhibited internalization while truncation at Gln367 did not, indicating that the PBANR internalization motif resides between Gly357-Gln367. Alanine substitution studies suggest that Tyr360 and Leu363 may constitute a YXXL endosomal targeting motif that facilitates endocytosis, however, this motif does not appear to be the primary determinant; an indication that multiple sites are involved. Furthermore, we determined that internalization of the PBANR proceeds via a clathrin-dependent pathway, is dependent on the influx of extracellular calcium, and likely does not involve a G protein-coupled receptor kinase.

Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C

The 5-hydroxytryptamine2A (5-HT2A) receptor is a G(q/11)-coupled serotonin receptor that activates phospholipase C and increases diacylglycerol formation. In this report, we demonstrated that calmodulin (CaM) co-immunoprecipitates with the 5-HT2A receptor in NIH-3T3 fibroblasts in an agonist-dependent manner and that the receptor contains two putative CaM binding regions. The putative CaM binding regions of the 5-HT2A receptor are localized to the second intracellular loop and carboxyl terminus. In an in vitro binding assay peptides encompassing the putative second intracellular loop (i2) and carboxyl-terminal (ct) CaM binding regions bound CaM in a Ca2+-dependent manner. The i2 peptide bound with apparent higher affinity and shifted the mobility of CaM in a nondenaturing gel shift assay. Fluorescence emission spectral analyses of dansyl-CaM showed apparent K(D) values of 65 +/- 30 nM for the i2 peptide and 168 +/- 38 nM for the ct peptide. The ct CaM-binding domain overlaps with a putative protein kinase C (PKC) site, which was readily phosphorylated by PKC in vitro. CaM binding and phosphorylation of the ct peptide were found to be antagonistic, suggesting a putative role for CaM in the regulation of 5-HT2A receptor phosphorylation and desensitization. Finally, we showed that CaM decreases 5-HT2A receptor-mediated [35S]GTPgammaS binding to NIH-3T3 cell membranes, supporting a possible role for CaM in regulating receptor-G protein coupling. These data indicate that the serotonin 5-HT2A receptor contains two high affinity CaM-binding domains that may play important roles in signaling and function.

Evaluation of the Membrane-binding Properties of the Proximal Region of the Angiotensin II Receptor (AT1A) Carboxyl Terminus by Surface Plasmon Resonance

The proximal region of the angiotensin II receptor (AT1A) carboxyl-terminus (known as helix VIII) is important for receptor function. In this study, we used surface plasmon resonance (SPR) to examine the interaction of helix VIII-derived peptides with three model lipid membranes. The membrane-binding properties of these synthetic peptides, as well as a series of peptide analogues with modified amino acid sequences, could be explained by both amino acid sequence and kinetic binding data by SPR. The helix VIII peptides showed a higher affinity for lipid membranes that contained negatively charged phospholipid, rather than zwitterionic phospholipid. The findings of an SPR study may be useful for estimating the cooperative binding of intracellular receptor domains with G proteins and the components of the lipid bilayer.

Helix I of beta-arrestin is involved in postendocytic trafficking but is not required for membrane translocation, receptor binding, and internalization

beta-Arrestins bind to phosphorylated, seven-transmembrane-spanning, G protein-coupled receptors (GPCRs), including the type 1 angiotensin II receptor (AT(1)R), to promote receptor desensitization and internalization. The AT(1) R is a class B GPCR that recruits both beta-arrestin1 and beta-arrestin2, forming stable complexes that cotraffic to deep-core endocytic vesicles. beta-Arrestins contain one amphipathic and potentially amphitropic (membrane-targeting) alpha-helix (helix I) that may promote translocation to the membrane or influence receptor internalization or trafficking. Here, we investigated the trafficking and function of beta-arrestin1 and beta-arrestin2 mutants bearing substitutions in both the hydrophobic and positively charged faces of helix I. The level of expression of these mutants and their cytoplasmic localization (in the absence of receptor activation) was similar to wild-type beta-arrestins. After angiotensin II stimulation, both wild-type and beta-arrestin mutants translocated to the cell membrane, although recruitment was weaker for mutants of the hydrophobic face of helix I. For all beta-arrestin mutants, the formation of deep-core vesicles was less observed compared with wild-type beta-arrestins. Furthermore, helix I conjugated to green fluorescent protein is not membrane-localized, suggesting that helix I, in isolation, is not amphitropic. Bioluminescence resonance energy transfer analysis revealed that both wild-type and beta-arrestin mutants retained a capacity to interact with the AT(1)R, although the interaction with the mutants was less stable. Finally, wild-type and mutant beta-arrestins fully supported receptor internalization in human embryonic kidney cells and mouse embryonic fibroblasts deficient in beta-arrestin1 and -2. Thus, helix I is implicated in postmembrane trafficking but is not strongly amphitropic.

Megalin binds and internalizes angiotensin II

Megalin is an abundant membrane protein heavily involved in receptor-mediated endocytosis. The major functions of megalin in vivo remain incompletely defined as megalin typically faces specialized milieus such as glomerular filtrate, airways, epididymal fluid, thyroid colloid, and yolk sac fluid, which lack many of its known ligands. In the course of studies on ANG II internalization, we were surprised when only part of the uptake of labeled ANG II into immortalized yolk sac cells (BN-16 cells) was blocked by specific peptide inhibitors and direct competitors of the angiotensin type 1 receptor. This led us to test if megalin was a receptor for ANG II. Four lines of direct evidence demonstrate that megalin and, to a lesser extent, its chaperone protein cubilin are receptors for ANG II. First, in BN-16 cells anti-megalin and anti-cubilin antisera interfere with ANG II uptake. Second, also in BN-16 cells, pure ANG II competes for uptake of a known megalin ligand. Third, in proximal tubule cell brush-border membrane vesicles extracted from mice, anti-megalin antisera interfere with ANG II binding. Fourth, purified megalin binds ANG II directly in surface plasmon resonance experiments. The finding that megalin is a receptor for ANG II suggests a major new function for the megalin pathway in vivo. These results also indicate that ANG II internalization in some tissues is megalin dependent and that megalin may play a role in regulating proximal tubule ANG II levels.

Cloning and characterization of the pheromone biosynthesis activating neuropeptide receptor from the silkmoth, Bombyx mori. Significance of the carboxyl terminus in receptor internalization

In most Lepidoptera, pheromone biosynthesis is regulated by a neuropeptide termed pheromone biosynthesis activating neuropeptide (PBAN). Although much is known about the cellular targets of PBAN, identification and functional characterization of the PBAN receptor (PBANR) has proven to be elusive. Given the sequence similarity between the active C-terminal regions of PBAN and neuromedin U, it was hypothesized that their respective receptors might also be similar in structure (Park, Y., Kim, Y. J., and Adams, M. E. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 11423-11428). Consequently, utilizing primers constructed from the conserved regions of insect neuromedin U receptor homologues, a full-length 2780-nucleotide clone encoding a 46-kDa G protein-coupled receptor was amplified from a Bombyx mori pheromone gland cDNA library. Tissue distribution analyses revealed that the receptor transcript is specific to the pheromone gland where it undergoes significant up-regulation in the day preceding eclosion. When transiently expressed in Sf9 cells, the B. mori PBANR responds to PBAN by mobilizing extracellular calcium in a dose-dependent manner. Confocal microscopic studies demonstrated the specificity of enhanced green fluorescent protein-tagged B. mori PBANR for PBAN and showed that PBAN induces internalization of the PBANR.PBAN complex. The rapid onset of internalization is mediated by a 67-amino acid C-terminal extension absent in the cloned Helicoverpa zea PBANR, which suggests that receptor internalization in that species likely utilizes a different mechanism. From these results, we have concluded that the cloned receptor gene encodes the B. mori PBANR and that it is both structurally and functionally distinct from the H. zea PBANR.

Agonist-dependent internalization of the angiotensin II type one receptor (AT1): role of C-terminus phosphorylation in recruitment of β-arrestins

Beta-arrestins play a role in AT1 endocytosis by binding the cytoplasmic, C-terminus region T332-S338, the major site of angiotensin II (Ang II)-induced phosphorylation. However, the processes responsible for recruiting beta-arrestin to the activated receptor are poorly defined. In this study, we used CHO-K1 and HEK 293 cells expressing wild-type or mutant AT1 to investigate two possibilities: activated AT1 induces global relocation of beta-arrestins to the plasma membrane or the phosphorylated C-terminus acts as bait to attract beta-arrestins. Results obtained using high osmolarity and dominant-negative beta-arrestin confirmed that internalization of AT1 in both CHO-K1 and HEK 293 cells is predominately via clathrin-mediated endocytosis involving beta-arrestin, and substitution of T332, S335, T336 and S338 with alanine to preclude phosphorylation markedly attenuated AT1 internalization. Confocal microscopy revealed that wild-type AT1 induced a time-dependent translocation of GFP-tagged beta-arrestins 1 and 2 to the cell surface. In contrast, the TSTS/A mutant did not traffic beta-arrestin 1 at all, and only trafficked beta-arrestin 2 weakly. Results of rescue-type experiments were consistent with the idea that both beta-arrestins are able to interact with the non-phosphorylated receptor, albeit with much lower affinity and beta-arrestin 1 less so than beta-arrestin 2. In conclusion, this study shows that the high affinity binding of beta-arrestins to the phosphorylated C-terminus is the predominant mechanism of agonist-induced beta-arrestin recruitment to the cell surface and AT1 receptor.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

“Network Leaning” as a Mechanism of Insurmountable Antagonism of the Angiotensin II Type 1 Receptor by Non-peptide Antagonists

A mechanistic understanding of the insurmountable antagonism of the angiotensin II type 1 (AT(1)) receptor could be fundamental in the quest for discovery and improvement of drugs. Candesartan and EXP3174 are competitive, reversible insurmountable antagonists of the AT(1) receptor. They contain di-acidic substitutions, whereas the surmountable antagonist, losartan, contains only one acidic group. We tested the hypothesis that these two classes of ligands interact with the AT(1) receptor through similar but not identical bonds and that the differences in the acid-base group contacts are critical for insurmountable antagonism. By pharmacological characterization of site-directed AT(1) receptor mutants expressed in COS1 cells we show that specific interactions with Gln(257) in transmembrane 6 distinguishes insurmountable antagonists and that abolishing these interactions transforms insurmountable to surmountable antagonism. In the Q257A mutant, the dissociation rate of [(3)H]candesartan is 2.8-fold more than the rate observed with wild type, and the association rate was reduced 4-fold lower than the wild type. The pattern of antagonism of angiotensin II concentration-response in the Q257A mutant pretreated with EXP3174 and candesartan is surmountable. We propose that leaning ability of insurmountable antagonists on Gln(257) in the wild-type receptor is the basis of an antagonist-mediated conformational transition, which is responsible for both slow dissociation and inhibition of maximal IP response.

Structural determinants of agonist-induced signaling and regulation of the angiotensin AT1 receptor

Angiotensin II (Ang II) regulates aldosterone secretion by stimulating inositol phosphate production and Ca(2+) signaling in adrenal glomerulosa cells via the G(q)-coupled AT(1) receptor, which is rapidly internalized upon agonist binding. Ang II also binds to the heptahelical AT(2) receptor, which neither activates inositol phosphate signaling nor undergoes receptor internalization. The differential behaviors of the AT(1) and AT(2) receptors were analyzed in chimeric angiotensin receptors created by swapping the second (IL2), the third (IL3) intracellular loops and/or the cytoplasmic tail (CT) between these receptors. When transiently expressed in COS-7 cells, the chimeric receptors showed only minor alterations in their ligand binding properties. Measurements of the internalization kinetics and inositol phosphate responses of chimeric AT(1A) receptors indicated that the CT is required for normal receptor internalization, and IL2 is a determinant of G protein activation. In addition, the amino-terminal portion of IL3 is required for both receptor functions. However, only substitution of IL2 impaired Ang II-induced ERK activation, suggesting that alternative mechanisms are responsible for ERK activation in signaling-deficient mutant AT(1) receptors. Substitution of IL2, IL3, or CT of the AT(1A) receptor into the AT(2) receptor sequence did not endow the latter with the ability to internalize or to mediate inositol phosphate signaling responses. These data suggest that the lack of receptor internalization and inositol phosphate signal generation by the AT(2) receptor is a consequence of its different activation mechanism, rather than the inability of its cytoplasmic domains to couple to intracellular effectors.

The Type 1 Angiotensin II Receptor Tail Affects Receptor Targeting, Internalization, and Membrane Fusion Properties

Endocytosis modulates cell responses by removing and recycling receptors from the cell surface. Type I angiotensin II receptors (AT1R) are somewhat unique in that they are expressed at apical (AP) and basolateral (BL) membranes in proximal tubule cells and both receptor sites undergo endocytosis. We analyzed AT1R cytoplasmic (-COOH) tail deletion mutants to determine whether classic AT1R endocytosis motifs functioned similarly in polarized cells and simultaneously altered receptor properties. Serially truncating the AT1R tail had little effect on AP/BL AT1R distribution as determined by 125I-angiotensin II binding in LLCPK(Cl4) cells transfected with an AT1R transcript. AP AT1R expression required the proximal 12 amino acids in the AT1R-COOH tail. Deleting all but the proximal 12 aa of the AT1R-COOH tail (T316L mutant) decreased AP AT1R internalization at 20 min (17 +/- 6%; p < 0.05 versus full-length; n = 5) and inhibited AP AT1R-stimulated arachidonic acid release (counts released per milligram of protein at 20 min: full-length, 18,762 +/- 4018; T316L, 2430 +/- 1711; n = 4; p < 0.02). Endosomal fusion assays were performed using peptide sequences of regions in the AT1R tail involved in endocytosis (YFLQLLKYIPP [LL] and LSTKMSTLSY [STL]). Peptide STL significantly inhibited endosomal fusion (22 +/- 10% of control; n = 5; p < 0.05 versus positive control). Peptide LL had no significant inhibitory effect. AT(1)R in polarized cells contain dominant endocytosis signals but these motifs do not correlate with AP or BL AT1R expression. Moreover, peptide sequences within the AT1R-COOH tail necessary for endocytosis also modulate endosomal fusion properties.

Type 1 angiotensin II receptor-associated protein ARAP1 binds and recycles the receptor to the plasma membrane

The carboxyl terminus of the type 1 angiotensin II receptor (AT(1)) plays an important role in receptor phosphorylation, desensitization, and internalization. The yeast two-hybrid system was employed to isolate proteins associated with the carboxyl terminal region of the AT(1A) receptor. In the present study, we report the isolation of a novel protein, ARAP1, which promotes recycling of AT(1A) to the plasma membrane in HEK-293 cells. ARAP1 cDNA encodes a 493-amino-acid protein and its mRNA is ubiquitously expressed in rat tissues. A complex of ARAP1 and AT(1A) was observed by immunoprecipitation and Western blotting in HEK-293 cells. In the presence of ARAP1, recycled AT(1A) showed a significant Ca(2+) release response to a second stimulation by Ang II 30 min after the first treatment. Immunocytochemical analysis revealed co-localization of recycled AT(1A) and ARAP1 in the plasma membrane 45 min after the initial exposure to Ang II. Taken together, these results indicate a role for ARAP1 in the recycling of the AT(1) receptor to the plasma membrane with presumable concomitant recovery of receptor signal functions.

Interaction of angiotensin II with the C-terminal 300-320 fragment of the rat angiotensin II receptor AT1a monitored by NMR

Interaction between angiotensin II (Ang II) and the fragment peptide 300-320 (fCT300-320) of the rat angiotensin II receptor AT1a was demonstrated by relaxation measurements, NOE effects, chemical shift variations, and CD measurements. The correlation times modulating dipolar interactions for the bound and free forms of Ang II were estimated by the ratio of the nonselective and single-selective longitudinal relaxation rates. The intermolecular NOEs observed in NOESY spectra between HN protons of 9Lys(fCT) and 6His(ang), 10Phe(fCT) and 8Phe(ang), HN proton of 3Tyr(fCT) and Halpha of 4Tyr(ang), 5Phe(fCT)Hdelta and Halpha of 4Tyr(ang) indicated that Ang II aromatic residues are directly involved in the interaction, as also verified by relaxation data. Some fCT300-320 backbone features were inferred by the CSI method and CD experiments revealing that the presence of Ang II enhances the existential probability of helical conformations in the fCT fragment. Restrained molecular dynamics using the simulated annealing protocol was performed with intermolecular NOEs as constraints, imposing an alpha-helix backbone structure to fCT300-320 fragment. In the built model, one strongly preferred interaction was found that allows intermolecular stacking between aromatic rings and forces the peptide to wrap around the 6Leu side chain of the receptor fragment.

Caveolin interacts with the angiotensin II type 1 receptor during exocytic transport but not at the plasma membrane

The mechanisms involved in angiotensin II type 1 receptor (AT1-R) trafficking and membrane localization are largely unknown. In this study, we examined the role of caveolin in these processes. Electron microscopy of plasma membrane sheets shows that the AT1-R is not concentrated in caveolae but is clustered in cholesterol-independent microdomains; upon activation, it partially redistributes to lipid rafts. Despite the lack of AT1-R in caveolae, AT1-R.caveolin complexes are readily detectable in cells co-expressing both proteins. This interaction requires an intact caveolin scaffolding domain because mutant caveolins that lack a functional caveolin scaffolding domain do not interact with AT1-R. Expression of an N-terminally truncated caveolin-3, CavDGV, that localizes to lipid bodies, or a point mutant, Cav3-P104L, that accumulates in the Golgi mislocalizes AT1-R to lipid bodies and Golgi, respectively. Mislocalization results in aberrant maturation and surface expression of AT1-R, effects that are not reversed by supplementing cells with cholesterol. Similarly mutation of aromatic residues in the caveolin-binding site abrogates AT1-R cell surface expression. In cells lacking caveolin-1 or caveolin-3, AT1-R does not traffic to the cell surface unless caveolin is ectopically expressed. This observation is recapitulated in caveolin-1 null mice that have a 55% reduction in renal AT1-R levels compared with controls. Taken together our results indicate that a direct interaction with caveolin is required to traffic the AT1-R through the exocytic pathway, but this does not result in AT1-R sequestration in caveolae. Caveolin therefore acts as a molecular chaperone rather than a plasma membrane scaffold for AT1-R.

Endocytosis of the AT1A angiotensin receptor is independent of ubiquitylation of its cytoplasmic serine/threonine-rich region

Agonist-induced internalisation of the rat type 1A (AT(1A)) angiotensin II receptor is associated with phosphorylation of a serine/threonine-rich region in its cytoplasmic tail. In yeast, hyperphosphorylation of the alpha-factor pheromone receptor regulates endocytosis of the receptor by facilitating the monoubiquitylation of its cytoplasmic tail on lysine residues. The role of receptor ubiquitylation in AT(1A) receptor internalisation was evaluated by deletion or replacement of lysine residues in its agonist-sensitive serine/threonine-rich region. Expression of such receptor mutants in CHO cells showed that these modifications had no detectable effect on the angiotensin II-induced endocytosis of the AT(1A) receptor. Furthermore, fusion of ubiquitin in-frame to an internalisation-deficient AT(1A) receptor mutant with a truncated carboxyl-terminal tail did not restore the endocytosis of the resulting chimeric receptor. No impairment of receptor internalisation was observed after substitution of all lysine residues in the serine/threonine-rich region at saturating angiotensin II concentrations, where endocytosis occurs by a beta-arrestin and dynamin independent mechanism. Taken together, these data demonstrate that ubiquitylation of the cytoplasmic serine/threonine-rich region of the AT(1A) receptor on lysine residues is not required for its agonist-induced internalisation, and suggest that endocytosis of mammalian G protein-coupled receptors (GPCRs) occurs by a different mechanism than that of yeast GPCRs.

Homologous regulation of the heptahelical D1A receptor responsiveness: specific cytoplasmic tail regions mediate dopamine-induced phosphorylation, desensitization and endocytosis

In the present study, we investigate the role of specific cytoplasmic tail (CT) regions of the D1A receptor in mediating dopamine (DA)-induced phosphorylation, desensitization and endocytosis. Results obtained in human embryonic kidney (HEK) cells expressing the wild-type (WT) or truncation forms (Delta425, Delta379 and Delta351) of the D1A receptor show that sequences located downstream of Gly379 regulate DA-mediated phosphorylation-dependent desensitization of D1A receptors. However, the longer truncation mutant Delta351 failed to undergo detectable DA-induced phosphorylation while exhibiting DA-induced desensitization features similar to the shorter truncation mutant Delta379. These data potentially suggest a novel role for a receptor phosphorylation-independent process in the DA-promoted D1A subtype desensitization. Our immunofluorescence data also suggest that sequences located between Cys351 and Gly379 play an important role in DA-mediated receptor endocytosis. Additionally, time-course studies were done in intact cells expressing WT or truncation receptors to measure the observed rate constant for adenylyl cyclase (AC) activation or k(obs), a parameter linked to the receptor-G protein coupling status. In agreement with the desensitization data, Delta425- and Delta379-expressing cells exhibit an increase of kobs in comparison with WT-expressing cells. Nevertheless, Delta351-expressing cells, which harbor similar desensitization features of Delta379-expressing cells, display no change in k(obs) when compared with WT-expressing cells. Our results suggest that a defective DA-induced endocytosis may hamper Delta351 resensitization and concomitant increase in k(obs). Thus, our study showing that specific D1A receptor CT sequences regulate DA-induced phosphorylation, desensitization, and endocytosis highlights the underlying molecular complexity of signaling at dopaminergic synapses.

Role of internalization in AT(1A) receptor function in proximal tubule epithelium

Angiotensin II (ANG II), acting through angiotensin type I (AT(1)) receptors on apical and basolateral surfaces of proximal tubule epithelial cells, increases sodium reabsorption in proximal tubules. Apical and basolateral receptors internalize after exposure to ANG II, but the role of internalization in receptor signaling and transport is not well defined. To determine the role of receptor internalization in ANG II-mediated receptor signaling and sodium transport, we stably expressed full-length and truncated AT(1A) receptors in opossum kidney cells. After stimulation with ANG II, wild-type receptors on apical and basolateral surfaces rapidly internalized, inhibited adenylate cyclase, and increased transcellular sodium transport. Truncation of the cytoplasmic tail of the AT(1A) receptor (TL314) resulted in receptors that were expressed on apical and basolateral surfaces but did not internalize, inhibit adenylate cyclase, or increase sodium transport. Because the cytoplasmic tail contains putative G protein coupling sites, mutant receptors that leave G protein interaction sites intact were designed. Cells expressing the truncation (TK333) or deletion (Del 315-329) also failed to internalize. When ANG II was added to basolateral surfaces of TK333 or Del 315-329, adenylate cyclase activity was inhibited and sodium transport was increased. In contrast, apical addition of ANG II was not associated with decreases in adenylate cyclase or increases in sodium transport. In conclusion, internalization pathways are important for AT(1A) receptor function in polarized proximal tubule epithelial cells. Apical AT(1A) receptors internalize before they interact with G proteins and signal cAMP. In contrast, basolateral AT(1A) receptors interact with G proteins and signal cAMP without internalizing.

Agonist-induced internalization of the platelet-activating factor receptor is dependent on arrestins but independent of G-protein activation. Role of the C terminus and the (D/N)PXXY motif

As with most G-protein-coupled receptors, repeated agonist stimulation of the platelet-activating factor receptor (PAFR) results in its desensitization, sequestration, and internalization. In this report, we show that agonist-induced PAFR internalization is independent of G-protein activation but is dependent on arrestins and involves the interaction of arrestins with a limited region of the PAFR C terminus. In cotransfected COS-7 cells, both arrestin-2 and arrestin-3 could be coimmunoprecipitated with PAFR, and agonist stimulation of PAFR induced the translocation of both arrestin-2 and arrestin-3. Furthermore, coexpression of arrestin-2 with PAFR potentiated receptor internalization, whereas agonist-induced PAFR internalization was inhibited by a dominant negative mutant of arrestin-2. The coexpression of a minigene encoding the C-terminal segment of the receptor abolished PAF-induced arrestin translocation and inhibited PAFR internalization. Using C terminus deletion mutants, we determined that the association of arrestin-2 with the receptor was dependent on the region between threonine 305 and valine 330 because arrestin-2 could be immunoprecipitated with the mutant PAFRstop330 but not PAFRstop305. Consistently, stop330 could mediate agonist-induced arrestin-2 translocation, whereas stop305 could not. Two other deletion mutants with slightly longer regions of the C terminus, PAFRstop311 and PAFRstop317, also failed to induce arrestin-2 translocation. Finally, the PAFR mutant Y293A, containing a single substitution in the putative internalization motif DPXXY in the seventh transmembrane domain (which we had shown to be able to internalize but not to couple to G-proteins) could efficiently induce arrestin translocation. Taken together, our results indicate that ligand-induced PAFR internalization is dependent on arrestins, that PAFR can associate with both arrestin-2 and -3, and that their translocation involves interaction with the region of residues 318-330 in the PAFR C terminus but is independent of G-protein activation.

Association of beta-Arrestin 1 with the type 1A angiotensin II receptor involves phosphorylation of the receptor carboxyl terminus and correlates with receptor internalization

Arrestins bind to phosphorylated G protein-coupled receptors and participate in receptor desensitization and endocytosis. Although arrestins traffic with activated type 1 (AT(1A)) angiotensin II (AngII) receptors, the contribution of arrestins to AT(1A) receptor internalization is controversial, and the physical association of arrestins with the AT(1A) receptor has not been established. In this study, by coimmunoprecipitating AT(1A) receptors and beta-arrestin 1, we provide direct evidence for an association between arrestins and the AT(1A) receptor that was agonist- and time-dependent and contingent upon the level of beta-arrestin 1 expression. Serial truncation of the receptor carboxyl terminus resulted in a graded loss of beta-arrestin 1 association, which correlated with decreases in receptor phosphorylation. Truncation of the AT(1A) receptor to lysine(325) prevented AngII-induced phosphorylation and beta-arrestin 1 association as well as markedly inhibiting receptor internalization, indicating a close correlation between these receptor parameters. AngII-induced association was also dramatically reduced in a phosphorylation- and internalization-impaired receptor mutant in which four serine and threonine residues in the central portion of the AT(1A) receptor carboxyl terminus (Thr(332), Ser(335), Thr(336), Ser(338)) were substituted with alanine. In contrast, substitutions in another serine/threonine-rich region (Ser(346), Ser(347), Ser(348)) and at three PKC phosphorylation sites (Ser(331), Ser(338), Ser(348)) had no effect on AngII-induced beta-arrestin 1 association or receptor internalization. While AT(1A) receptor internalization could be inhibited by a dominant-negative beta-arrestin 1 mutant (beta arr1(319-418)), treatment with hyperosmotic sucrose to inhibit internalization did not abrogate the differences in arrestin association observed between the wild-type and mutant receptors, indicating that arrestin binding precedes, and is not dependent upon, receptor internalization. Interestingly, a substituted analog of AngII, [Sar(1)Ile(4)Ile(8)]-AngII, which promotes robust phosphorylation of the receptor but does not activate receptor signaling, stimulated strong beta-arrestin 1 association with the full-length AT(1A) receptor. These results identify the central portion of the AT(1A) receptor carboxyl terminus as the important determinant for beta-arrestin 1 binding and internalization and indicate that AT(1A) receptor phosphorylation is crucial for beta-arrestin docking.

ANG II type 1 receptor downregulation does not require receptor endocytosis or G protein coupling

ANG II type 1 (AT(1)) receptors respond to sustained exposure to ANG II by undergoing downregulation of absolute receptor numbers. It has been assumed previously that downregulation involves endocytosis. The present study hypothesized that AT(1) receptor downregulation occurs independently of receptor endocytosis or G protein coupling. Mutant AT(1) receptors with carboxy-terminal deletions internalized <5% of radioligand compared with 65% for wild-type AT(1) receptors. The truncated AT(1) receptors retained the ability to undergo downregulation. These data suggest the existence of an alternative pathway to AT(1) receptor degradation that does not require endocytosis, per se. Point mutations in either the second transmembrane region or second intracellular loop impaired G protein (G(q)) coupling. These receptors exhibited a biphasic pattern of downregulation. The earliest phase of downregulation (0-2 h) was independent of coupling to G(q), but no additional downregulation was observed after 2 h of ANG II exposure in the receptors with impaired coupling to G(q). These data suggest that coupling to G(q) is required for the later phase (2-24 h) of AT(1) receptor downregulation.

Ligand-induced endocytosis and nuclear localization of angiotensin II receptors expressed in CHO cells

A construct (AT1R-NF) containing a "Flag" sequence added to the N-terminus of the rat AT1 receptor was stably expressed in Chinese hamster ovary cells and quantified in the cell membrane by confocal microscopy after reaction with a fluorescein-labeled anti-Flag monoclonal antibody. Angiotensin II bound to AT1R-NF and induced endocytosis with a half-time of 2 min. After 60-90 min, fluorescence accumulated around the cell nucleus, suggesting migration of the ligand-receptor complex to the nuclear membrane. Angiotensin antagonists also induced endocytosis, suggesting that a common step in the transduction signal mechanism occurring after ligand binding may be responsible for the ligand-receptor complex internalization.