Quantitative analysis of agonist-dependent parathyroid hormone receptor trafficking in whole cells using a functional green fluorescent protein conjugate

Large G protein α-subunit XLαs limits clathrin-mediated endocytosis and regulates tissue iron levels in vivo

Alterations in the activity/levels of the extralarge G protein α-subunit (XLαs) are implicated in various human disorders, such as perinatal growth retardation. Encoded by GNAS, XLαs is partly identical to the α-subunit of the stimulatory G protein (Gsα), but the cellular actions of XLαs remain poorly defined. Following an initial proteomic screen, we identified sorting nexin-9 (SNX9) and dynamins, key components of clathrin-mediated endocytosis, as binding partners of XLαs. Overexpression of XLαs in HEK293 cells inhibited internalization of transferrin, a process that depends on clathrin-mediated endocytosis, while its ablation by CRISPR/Cas9 in an osteocyte-like cell line (Ocy454) enhanced it. Similarly, primary cardiomyocytes derived from XLαs knockout (XLKO) pups showed enhanced transferrin internalization. Early postnatal XLKO mice showed a significantly higher degree of cardiac iron uptake than wild-type littermates following iron dextran injection. In XLKO neonates, iron and ferritin levels were elevated in heart and skeletal muscle, where XLαs is normally expressed abundantly. XLKO heart and skeletal muscle, as well as XLKO Ocy454 cells, showed elevated SNX9 protein levels, and siRNA-mediated knockdown of SNX9 in XLKO Ocy454 cells prevented enhanced transferrin internalization. In transfected cells, XLαs also inhibited internalization of the parathyroid hormone and type 2 vasopressin receptors. Internalization of transferrin and these G protein-coupled receptors was also inhibited in cells expressing an XLαs mutant missing the Gα portion, but not Gsα or an N-terminally truncated XLαs mutant unable to interact with SNX9 or dynamin. Thus, XLαs restricts clathrin-mediated endocytosis and plays a critical role in iron/transferrin uptake in vivo.

GPCR Signaling and Trafficking: The Long and Short of It

Emerging findings disclose unexpected components of G protein-coupled receptor (GPCR) signaling and cell biology. Select GPCRs exhibit classical signaling, that is restricted to cell membranes, as well as newly described persistent signaling that depends on internalization of the GPCR bound to β-arrestins. Termination of non-canonical endosomal signaling requires intraluminal acidification and sophisticated protein trafficking machineries. Recent studies reveal the structural determinants of the trafficking chaperones. This review summarizes advances in GPCR signaling and trafficking with a focus on the parathyroid hormone receptor (PTHR) as a prototype, and on the actin-sorting nexin 27 (SNX27)-retromer tubule (ASRT) complex, an endosomal sorting hub responsible for recycling and preservation of cell surface receptors. The findings are integrated into a model of PTHR trafficking with implications for signal transduction, bone growth, and mineral ion metabolism.

Quantitative Cell-Based High-Content Screening for Vasopressin Receptor Agonists Using Transfluor®Technology

The authors demonstrate the use of a simple, universal G-protein-coupled receptor (GPCR) assay to screen for agonists for a specific GPCR. Cells stably expressing a green fluorescent protein (GFP)-labeled β-arrestin fusion protein and the vasopressin V2 receptor (V2R) were used in a high-content screening (HCS) assay to screen a small peptide library for V2R agonists. Cells were treated with the peptides at a final concentration of 500 nM for 30min. Agonist stimulation causes V2R internalization into endosomes. GFP-β-arrestin remains associated with the V2R in endosomes, resulting in a fluorescent pattern of intracellular spots. Assay plates were automatically imaged and quantitatively analyzed using an HCS imaging platformand a fast turnkey image analysis application optimized for detection of receptor activation and intracellular spots. Hits were further evaluated to determine their potency. The combination of unique biology, automated high-content analysis, and a powerful means of validating hits results in better leads.

Parathyroid Hormone-related Peptide (hPTHrP) and Parathyroid Hormone-related Peptide Receptor Type 1 (PTHR1) Expression in Human Thymus

Parathyroid hormone-related peptide (hPTHrP) is expressed in human tissues and regulates cellular proliferation, differentiation, and apoptosis by an autocrine/paracrine loop. In rodent thymus, both parathormone and parathyroid hormone-related peptide (PTHrP) are expressed by thymic epithelial cells (TECs). The present study demonstrated by RT-PCR and immunohistochemistry that hPTHrP and parathyroid hormone-related peptide receptor type 1 (PTHR1) were expressed in human thymus at both RNA and protein levels. hPTHrP was expressed mainly in the thymic medulla by epithelial (cytokeratin-positive), mature dendritic (CD40+/86+) and plasmacytoid interleukin (IL)-3Rα+ cells. This protein was also present in some cells forming Hassall's bodies and a few subcapsular and cortical TECs. PTHR1 was expressed by scattered subcapsular and cortical TECs and by rare TECs in the medulla. Thymocytes did not express either hPTHrP or PTHR1. Primary cultures of human TECs revealed the presence of both hPTHrP and PTHR1 mRNAs, confirming the capacity of TECs to synthesize both peptides. Moreover, synthetic (1–39) hPTHrP peptide administered on cultured TECs induced the expression of IL-6 mRNA, suggesting that hPTHrP can regulate thymic functions by inducing in TECs the expression of IL-6, which is involved in the development and maturation of thymocytes.

Actin-Sorting Nexin 27 (SNX27)-Retromer Complex Mediates Rapid Parathyroid Hormone Receptor Recycling

The G protein-coupled parathyroid hormone receptor (PTHR) regulates mineral-ion homeostasis and bone remodeling. Upon parathyroid hormone (PTH) stimulation, the PTHR internalizes into early endosomes and subsequently traffics to the retromer complex, a sorting platform on early endosomes that promotes recycling of surface receptors. The C terminus of the PTHR contains a type I PDZ ligand that binds PDZ domain-containing proteins. Mass spectrometry identified sorting nexin 27 (SNX27) in isolated endosomes as a PTHR binding partner. PTH treatment enriched endosomal PTHR. SNX27 contains a PDZ domain and serves as a cargo selector for the retromer complex. VPS26, VPS29, and VPS35 retromer subunits were isolated with PTHR in endosomes from cells stimulated with PTH. Molecular dynamics and protein binding studies establish that PTHR and SNX27 interactions depend on the PDZ recognition motif in PTHR and the PDZ domain of SNX27. Depletion of either SNX27 or VPS35 or actin depolymerization decreased the rate of PTHR recycling following agonist stimulation. Mutating the PDZ ligand of PTHR abolished the interaction with SNX27 but did not affect the overall rate of recycling, suggesting that PTHR may directly engage the retromer complex. Coimmunoprecipitation and overlay experiments show that both intact and mutated PTHR bind retromer through the VPS26 protomer and sequentially assemble a ternary complex with PTHR and SNX27. SNX27-independent recycling may involve N-ethylmaleimide-sensitive factor, which binds both PDZ intact and mutant PTHRs. We conclude that PTHR recycles rapidly through at least two pathways, one involving the ASRT complex of actin, SNX27, and retromer and another possibly involving N-ethylmaleimide-sensitive factor.

Ubiquitination-deubiquitination balance dictates ligand-stimulated PTHR sorting

Parathyroid hormone receptors (PTHR) are promptly internalized upon stimulation by activating (PTH[1-84], PTH[1-34]) and non-activating (PTH[7-84], PTH[7-34]) ligands. Here, we characterized the mechanism regulating the sorting of internalized receptors between recycling and degradative pathways. PTHR recycles faster after challenge with PTH(1-34) than with PTH(7-34). PTHR recycling is complete by 2 h after PTH(1-34) stimulation, but incomplete at this time in cells treated with PTH(7-34). The slower and incomplete recycling induced by PTH(7-34) is due to proteasomal degradation. Both PTH(1-34) and PTH(7-34) induced PTHR polyubiquitination. Ubiquitination by PTH(1-34) was transient, whereas receptor ubiquitination after PTH(7-34) was sustained. PTH(1-34), but not PTH(7-34), induced expression of the PTHR-specific deubiquitinating enzyme USP2. Overexpression of USP2 prevented PTH(7-34)-induced PTHR degradation. We conclude that PTH(1-34) promotes coupled PTHR ubiquitination and deubiquitination, whereas PTH(7-34) activates only ubiquitination, thereby leading to PTHR downregulation. These findings may explain PTH resistance in diseases associated with elevated PTH(7-84) levels.

Dynamic Na+-H+ exchanger regulatory factor-1 association and dissociation regulate parathyroid hormone receptor trafficking at membrane microdomains

Na/H exchanger regulatory factor-1 (NHERF1) is a cytoplasmic PDZ (postsynaptic density 95/disc large/zona occludens) protein that assembles macromolecular complexes and determines the localization, trafficking, and signaling of select G protein-coupled receptors and other membrane-delimited proteins. The parathyroid hormone receptor (PTHR), which regulates mineral ion homeostasis and bone turnover, is a G protein-coupled receptor harboring a PDZ-binding motif that enables association with NHERF1 and tethering to the actin cytoskeleton. NHERF1 interactions with the PTHR modify its trafficking and signaling. Here, we characterized by live cell imaging the mechanism whereby NHERF1 coordinates the interactions of multiple proteins, as well as the fate of NHERF1 itself upon receptor activation. Upon PTHR stimulation, NHERF1 rapidly dissociates from the receptor and induces receptor aggregation in long lasting clusters that are enriched with the actin-binding protein ezrin and with clathrin. After NHERF1 dissociates from the PTHR, ezrin then directly interacts with the PTHR to stabilize the PTHR at the cell membrane. Recruitment of β-arrestins to the PTHR is delayed until NHERF1 dissociates from the receptor, which is then trafficked to clathrin for internalization. The ability of NHERF to interact dynamically with the PTHR and cognate adapter proteins regulates receptor trafficking and signaling in a spatially and temporally coordinated manner.

Actin cytoskeleton-dependent Rab GTPase-regulated angiotensin type I receptor lysosomal degradation studied by fluorescence lifetime imaging microscopy

The dynamic regulation of the cellular trafficking of human angiotensin (Ang) type 1 receptor (AT1R) is not well understood. Therefore, we investigated the cellular trafficking of AT1R-enhanced green fluorescent protein (EGFP) (AT1R-EGFP) heterologously expressed in HEK293 cells by determining the change in donor lifetime (AT1R-EGFP) in the presence or absence of acceptor(s) using fluorescence lifetime imaging-fluorescence resonance energy transfer (FRET) microscopy. The average lifetime of AT1R-EGFP in our donor-alone samples was ∼2.33 ns. The basal state lifetime was shortened slightly in the presence of Rab5 (2.01±0.10 ns) or Rab7 (2.11±0.11 ns) labeled with Alexa 555, as the acceptor fluorophore. A 5-min Ang II treatment markedly shortened the lifetime of AT1R-EGFP in the presence of Rab5-Alexa 555 (1.78±0.31 ns) but was affected minimally in the presence of Rab7-Alexa 555 (2.09±0.37 ns). A 30-min Ang II treatment further decreased the AT1R-EGFP lifetime in the presence of both Rab5- and Rab7-Alexa 555. Latrunculin A but not nocodazole pretreatment blocked the ability of Ang II to shorten the AT1R-EGFP lifetime. The occurrence of FRET between AT1R-EGFP (donor) and LAMP1-Alexa 555 (acceptor) with Ang II stimulation was impaired by photobleaching the acceptor. These studies demonstrate that Ang II-induced AT1R lysosomal degradation through its association with LAMP1 is regulated by Rab5/7 via mechanisms that are dependent on intact actin cytoskeletons.

Functional analysis of a type 1 parathyroid hormone receptor intracellular tail mutant [KRK(484-6)AAA]: effects on second messenger generation and cellular targeting

The parathyroid hormone receptor type 1 (PTHR1) is activated by parathyroid hormone (PTH) and PTH-related protein (PTHrP) and primarily signals via intracellular pathways involving adenylyl cyclase and phospholipase C. The intracellular tail domain of the PTHR1 contributes to G protein subunit coupling that is important for second messenger signalling. In addition, the intracellular domain has a potential nuclear localization sequence (NLS) that, if functional, could point to an intracrine role for the receptor. In the present study, we have utilized 2 sets of constructs that employ either a [KRK(484-486)AAA](3Ala) mutation in the putative NLS or the non-mutant counterpart and included (a) the full-length rat PTHR1 with FLAG and c-myc epitope tags at the N-terminus and C-terminus, respectively (designated as PTHR1(3Ala)-TAG and PTHR1-TAG); and (b) only the putative NLS-containing intracellular domain (471-488), with green fluorescent protein (GFP) fused to the C-terminus (designated as GFP-(3Ala)471-488 or GFP-471-488). Porcine kidney LLC-PK1 cells stably expressing the PTHR1(3Ala)-TAG exhibited reduced signalling via both cAMP and cytosolic calcium transients in spite of greater cell surface expression relative to cells expressing PTHR1-TAG. We also examined the ability of the intracellular tail to influence the cellular localization of a heterologous protein. LLC-PK1 cells transiently transfected with GFP-471-488, exhibited increased fluorescence within the nucleus, relative to cells transfected with GFP alone that was not observed when cells were transiently transfected with the mutated construct, GFP-(3Ala)471-488. However, LLC-PK1 cells transiently transfected with either the full-length PTHR1-TAG or the PTHR1(3Ala)-TAG constructs did not exhibit nuclear localization of these receptors. Moreover, mouse osteoblast-like cells (MC3T3-E1) transiently expressing PTHR1-TAG also failed to demonstrate nuclear localization, although both full-length PTHR1 constructs exhibited plasma membrane immunofluorescence in both cell lines. Thus, the 484-486 sequence is critical for the full signalling responsiveness of the intact PTHR1, but the putative nuclear localization signal may not function as such within the intact receptor.

Effect of CCR5 receptor antagonists on endocytosis of the human CCR5 receptor in CHO-K1 cells

BACKGROUND AND PURPOSE

The CCR5 chemokine receptor is a member of the G protein-coupled receptor (GPCR) family that is expressed by macrophages, memory T-lymphocytes and dendritic cells and is activated by chemotactic proteins (e.g. MIP-1alpha [CCL3], MIP-1beta [CCL4] and RANTES [CCL5]). CCR5 is also the principal co-receptor for macrophage-tropic strains of human immunodeficiency virus-1 (HIV-1) and some chemokines can inhibit HIV-1 infection by stimulating CCR5 receptor endocytosis. The aim of this study was to evaluate the effect of CCR5 antagonists on CCR5 endocytosis.

EXPERIMENTAL APPROACH

The effects of CCR5 agonists and antagonists on receptor internalization in CHO cells, expressing a C-terminal green fluorescent protein-tagged human CCR5 receptor (CCR5-GFP), were quantified using a confocal imaging plate reader.

KEY RESULTS

MIP-1alpha [CCL3], MIP-1beta [CCL4] and RANTES [CCL5] were all able to stimulate potently the internalization of CCR5-GFP. This effect was inhibited by the non-peptide antagonist TAK 779. The CCR5 peptide antagonist met-RANTES antagonized MIP-1alpha-mediated increases in intracellular free calcium but was also able to stimulate a substantial internalization of the human CCR5-GFP receptor. However, CHO cells exhibited an aminopeptidase activity that was able to metabolize sufficient met-RANTES into an agonist metabolite capable of stimulating calcium mobilization via CCR5 receptors in naïve cells.

CONCLUSIONS AND IMPLICATIONS

These data suggest that there is an endogenous aminopeptidase activity on the surface of CHO cells, that produces a slow internalization of the receptor following a time-dependent conversion of receptor-bound met-RANTES from a CCR5 receptor antagonist into a CCR5 agonist molecule.

NHERF1 Regulates Parathyroid Hormone Receptor Membrane Retention without Affecting Recycling

Na/H exchange regulatory factor-1 (NHERF1) is a PDZ protein that regulates trafficking of several G protein-coupled receptors. The phenotype of NHERF1-null mice suggests that the parathyroid hormone (PTH) receptor (PTH1R) is the principal GPCR interacting with NHERF1. The effect of NHERF1 on receptor recycling is unknown. Here, we characterized NHERF1 effects on PTH1R membrane tethering and recycling by radio-ligand binding and recovery after maximal receptor endocytosis. Using Chinese hamster ovary cells expressing the PTH1R, where NHERF1 expression could be induced by tetracycline, NHERF1 inhibited PTH1R endocytosis and delayed PTH1R recycling. NHERF1 also inhibited PTH-induced receptor internalization in MC4 osteoblast cells. Reducing constitutive NHERF1 levels in HEK-293 cells with short hairpin RNA directed against NHERF1 augmented PTH1R endocytosis in response to PTH. Mutagenesis of the PDZ-binding domains or deletion of the MERM domain of NHERF1 demonstrated that both are required for inhibition of endocytosis and recycling. Likewise, an intact COOH-terminal PDZ recognition motif in PTH1R is needed. The effect of NHERF1 on receptor internalization and recycling was not associated with altered receptor expression or binding, activation, or phosphorylation but involved beta-arrestin and dynamin. We conclude that NHERF1 inhibits endocytosis without affecting PTH1R recycling in MC4 and PTH1R-expressing HEK-293 cells. Such an effect may protect against PTH resistance or PTH1R down-regulation in certain cells harboring NHERF1.

Quantitative characterization of mitosis-blocked tetraploid cells using high content analysis

A range of cellular evidence supporting a G1 tetraploidy checkpoint was obtained from different assay methods including flow cytometry, immunoblotting, and microscopy. Cancer research would benefit if these cellular properties could instead be measured by a single, quantitative, automated assay method, such as high content analysis (HCA). Thus, nocodazole-treated cells were fluorescently labeled for different cell cycle-associated properties, including DNA content, retinoblastoma (Rb) and histone H3 phosphorylation, p53 and p21(WAF1) expression, nuclear and cell sizes, and cell morphology, and automatically imaged, analyzed, and correlated using HCA. HCA verified that nocodazole-induced mitosis block resulted in tetraploid cells. Rb and histone H3 were maximally hyperphosphorylated by 24 h of nocodazole treatment, accompanied by cell and nuclear size decreases and cellular rounding. Cells remained tetraploid and mononucleated with longer treatments, but other targets reverted to G1 levels, including Rb and histone H3 dephosphorylation accompanied by cellular respreading. This was accompanied by increased p53 and p21(WAF1) expression levels. The range of effects accompanying nocodazole-induced block of mitosis and the resulting tetraploid cells' reversal to a pseudo-G1 state can be quantitatively measured by HCA in an automated manner, recommending this assay method for the large-scale biology challenges of modern cancer drug discovery.

In vivo imaging of green fluorescent protein-expressing cells in transgenic animals using fibred confocal fluorescence microscopy

Animal imaging requires the use of reliable long-term fluorescence methods and technology. The application of confocal imaging to in vivo monitoring of transgene expression within internal organs and tissues has been limited by the accessibility to these sites. We aimed to test the feasibility of fibred confocal fluorescence microscopy (FCFM) to image in situ green fluorescent protein (GFP) in cells of living animals. We used transgenic rabbits expressing the enhanced GFP (eGFP) gene. Detailed tissue architecture and cell morphology were visualised and identified in situ by FCFM. Imaging of vasculature by using FCFM revealed a single blood vessel or vasculature network. We also used non-transgenic female rabbits mated with transgenic males to visualise eGFP expression in extra-foetal membranes and the placenta. Expression of the eGFP gene was confirmed by FCFM. This new imaging technology offers specific characteristics: a way to gain access to organs and tissues in vivo, sensitive detection of fluorescent signals, and cellular observations with rapid acquisition at near real time. It allows an accurate visualisation of tissue anatomical structure and cell morphology. FCFM is a promising technology to study biological processes in the natural physiological environment of living animals.

A live cell, image-based approach to understanding the enzymology and pharmacology of 2-bromopalmitate and palmitoylation

The addition of a lipid moiety to a protein increases its hydrophobicity and subsequently its attraction to lipophilic environments like membranes. Indeed most lipid-modified proteins are localized to membranes where they associate with multiprotein signaling complexes. Acylation and prenylation are the two common categories of lipidation. The enzymology and pharmacology of prenylation are well understood but relatively very little is known about palmitoylation, the most common form of acylation. One distinguishing characteristic of palmitoylation is that it is a dynamic modification. To understand more about how palmitoylation is regulated, we fused palmitoylation substrates to fluorescent proteins and reported their subcellular distribution and trafficking. We used automated high-throughput fluorescence microscopy and a specialized computer algorithm to image and measure the fraction of palmitoylation reporter on the plasma membrane versus the cytoplasm. Using this system we determined the residence half-life of palmitate on the dipalmitoyl substrate peptide from GAP43 as well as the EC(50) for 2-bromopalmitate, a common inhibitor of palmitoylation.

Characterization of agonist-induced motilin receptor trafficking and its implications for tachyphylaxis

The motilin receptor (MR) is a member of the seven-transmembrane receptor family and is expressed throughout the gastrointestinal tract of humans and other species. Motilin, the natural MR peptide ligand, has profound stimulatory effects on gastrointestinal contractility, indicating a therapeutic potential for MR modulators. However, long-term clinical use of certain MR agonists is limited by tachyphylaxis, a reduced responsiveness to repeated compound exposure. This study was meant to characterize the ligand-induced endocytosis of MR and to test whether receptor trafficking contributes to tachyphylaxis. A cell-based assay was developed by fusing a green fluorescent protein (GFP) moiety to the motilin receptor, and high-content biology instrumentation was used to quantify time and dose dependence of MR-GFP endocytosis. Maximal internalization of MR-GFP was induced after 45 min of constant exposure to 80 nM motilin. This process was disrupted by nocodazole, suggesting an essential role for microtubules. Internalized MR-GFP vesicles disappeared within 15 to 45 min of motilin withdrawal but did not overlap with the lysosomal compartment, indicating that MR-GFP escaped degradation and was recycled back to the plasma membrane. It is noteworthy that the kinetics of MR-GFP redistribution varied substantially when stimulated with motilin, erythromycin, 6,9-hemiacetal 8,9-anhydro-4''-deoxy-3'-N-desmethyl-3'-N-ethylerythromycin B (ABT-229), or N-[(1S)-1-[[[(1S)-1-(aminocarbonyl)-3-phenylpropyl]amino]carbonyl]-3-phenylpropyl]-2'-(1,3-benzodioxol-5-ylmethyl)tetrahydro-1',3'-dioxo-spiro[piperidine-4,5'(6'H)-[1H][1,2,4]triazolo[1,2-a]pyridazine]-8'-carboxamide (BMS-591348) at equipotent doses for Ca(2+)-mobilization. Retardation of the intracellular MR-GFP sorting cycle seemed to correlate with the tachyphylaxis-inducing properties of each compound, but not its EC(50). These results indicate that MR internalization, desensitization, and resensitization are ligand-dependent and that appropriate screening strategies may enable the development of small molecule agonists with ideal combinations of these distinct properties.

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.

Response to continuous and pulsatile PTH dosing: a mathematical model for parathyroid hormone receptor kinetics

In this paper, we propose a mathematical model for parathyroid hormone receptor (PTH1R) kinetics, focusing on the receptor's response to PTH dosing to discern bone formation responses from bone resorption. The PTH1R is a major target for new osteoporosis treatments, as pulsatile PTH dosing has been shown to induce net bone formation in both animals and humans, and PTH(1-34) was recently FDA approved for the treatment of post-menopausal osteoporosis. PTH has also been shown to cause net bone loss when given continuously, so that the net action of PTH on bone is dependent on the dosing pattern. We have developed a simplified two-state receptor kinetics model for the PTH1R, based on the concepts of Segel et al., to distinguish the activity of active and inactive receptor and receptor-ligand complexes. The goal is to develop a plausible model of the minimal essential biological relationships necessary for understanding the responses to PTH dosing. A two-state model is able to effectively discriminate between continuous and pulsatile PTH dosing using the active species as surrogates for the downstream anabolic response. For continuous PTH dosing, the model predicts a desensitized system dominated by the inactive receptor and complex, consistent with downstream net bone loss that has been demonstrated experimentally. Using pulsatile PTH dosing, the model system predicts a highly sensitized state dominated by the active receptor and complex, corresponding to net bone formation. These results are consistent with the hypothesis that the kinetics of the receptor plays a critical role in the downstream effects of PTH dosing. Moreover, these results indicate that within a range of biologically relevant PTH doses, the two-state model is able to capture the differential behavior of the system for both continuous and pulsatile PTH dosing. The development of such a model provides a rational basis for developing more biologically extensive models that may support the design of optimal dosing strategies for PTH-based anti-osteoporosis treatments. Moreover, this model provides a unique starting point from which to design experiments investigating PTH receptor biology.

Green fluorescent proteins in receptor research: An emerging tool for drug discovery

In the last five years, green fluorescent protein (GFP) has emerged from being a mere curiosity to become a reliable tool for molecular pharmacological research. GFP produces an intense and stable green fluorescence noncatalytically by absorbing blue light maximally at 395 nm and emitting green light with a peak at 509 nm. It consists of 238 amino acids and its molecular mass is 27-30 kDa. GFP fluorescence occurs without cofactors and this property allows GFP fluorescence to be utilised in nonnative organisms, wherein it can be used as a reporter. This use of GFP permits real-time analysis of receptor dynamics. The emitted fluorescence can be used as a nontoxic marker and detected using fluorescence-activated cell sorting (FACS), thus avoiding any staining procedure, expensive mRNA analysis or hazardous radiolabeled binding assays. The potential value of GFP has also been recognized in orphan receptor research, where various GFP-tagged therapeutic proteins have been constructed in an attempt to identify the endogenous ligand(s). These chimeric proteins have been used to determine the site and time course of receptor expression and to relate receptor dynamics with therapeutic outcome. The preparation of new GFP constructs for identifying germ layer cells (endodermal, ectodermal, and mesodermal), as well as neuronal, haematopoietic, endothelial, and cartilage cells, has provided a useful battery of tissue/receptor-specific screening assays for new chemical entities. Genetically engineered cells with GFP expression have provided a valuable tool for automated analysis, and can be adapted for high-throughput systems. GFP is being increasingly utilised for the study of receptor dynamics, where, having already proved beneficial, it will likely continue to contribute towards the search for new classes of drugs, as well as to "de-orphaning" orphan receptors.

A Quantitative Cell-Based High-Content Screening Assay for the Epidermal Growth Factor Receptor-Specific Activation of Mitogen-Activated Protein Kinase

The complexity of mitogen-activated protein kinase (MAPK) signaling pathways and their activation by different stimuli makes assaying the activation of particular MAPKs by specific receptors a challenging problem. The multiplexing capability of quantitative high-content screening (HCS) assays enables the simultaneous monitoring and correlation, in the same cell, of an MAPK's specific activation with a particular receptor's post-signaling behavior, such as its internalization. We demonstrate a cell-based HCS assay to quantify the epidermal growth factor (EGF) receptor-specific activation of the MAPK ERK. Activation was quantified by measuring immunofluorescently labeled phosphorylated extracellular signal-regulated protein kinases (ERK) in the nucleus. Specificity of ERK activation by the EGF receptor was simultaneously confirmed in the same cell by quantitatively monitoring fluorescent EGF's internalization and subsequent intracellular degradation. Quantitative analysis of the temporal behavior of these two activities showed that phosphorylated ERK's accumulation in the nucleus peaked at 5 min before falling to basal levels by 30 min. Cellular accumulation of fluorescent EGF was slower, peaking around 30 min, before being degraded. This assay strategy can serve as a paradigm to study other signaling pathways and their activation by specific receptors. The flexibility and multiplexing capability of HCS assays allow the use of additional targets to further qualify the specificity of response by including other MAPKs or receptors, to rule out cross-talk from competing signaling pathways, or to simultaneously monitor toxicity effects of compounds. This automated, non-subjective, easy-to-use assay procedure provides information rich, quantitative results, and demonstrates the potential of the HCS assay approach in deconvolving intracellular signaling pathways.

Advances in high content screening for drug discovery

Cell-based target validation, secondary screening, lead optimization, and structure-activity relationships have been recast with the advent of HCS. Prior to HCS, a computational approach to the characterization of the functions of specific target proteins and other cellular constituents, along with whole-cell functions employing fluorescence cell-based assays and microscopy, required extensive interaction among the researcher, instrumentation, and software tools. Early HCS platforms were instrument-centric and addressed the need to interface fully automated fluorescence microscopy, plate-handling automation, and seamless image analysis. HCS has since evolved into an integrated solution for accelerated drug discovery by encompassing the workflow components of assay and reagent design, robust instrumentation for automated fixed-end-point and live cell kinetic analysis, generalized and specific BioApplication software (Cellomics, Pittsburgh, PA) modules that produce information on drug responses from cell image data, and informatics/bioinformatics solutions that build knowledge from this information while providing a means to globalize HCS throughout an entire organization. This review communicates how these recent advances are incorporated into the drug discovery workflow by presenting a real-world use case.

Ligand Dependency of 5-Hydroxytryptamine 2C Receptor Internalization

Agonist-induced internalization of G protein-coupled receptors (GPCRs) is a well characterized phenomenon believed to contribute to receptor desensitization. The 5-hydroxytryptamine (5-HT)2C subtype of serotonin receptor is a GPCR that we have shown to internalize upon agonist incubation. In this study, we have examined the effects of 5-HT2C receptor agonists serotonin, Ro 60-0175 [(S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine], and WAY-161503 [(4aR)-8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one]; partial agonists mCPP [1-(m-chlorophenyl)piperazine] and DOI [(+)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane]; inverse agonists SB-206553 [N-3-pyridinyl-3,5-dihydro-5-methylbenzo(1,2-b:4,5-b')dipyrrole-1(2H)carboxamide] and mianserin; and neutral antagonists SB-242084 [6-chloro-5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-indoline] and 5-methoxygramine on the internalization of a C-terminal green fluorescent protein (GFP)-tagged 5-HT2C receptor (VSV isoform) expressed in transiently transfected human embryonic kidney cells. We detected internalization with an automated, cell-based fluorescence-imaging system (Arrayscan) and monitored function with intracellular Ca2+ measurements (flourometric imaging plate reader). The 5-HT2C-GFP construct exhibited appropriate pharmacology, and we observed that although all three agonists resulted in similar magnitudes of dose-dependent internalization, the partial agonists resulted in approximately 50% less internalization, and the inverse agonists and neutral antagonists failed to induce internalization. These results were confirmed by confocal microscopy. They demonstrate that the 5-HT2C receptor is internalized by incubation with agonists and partial agonists but not with inverse agonists or neutral antagonists.

Protein tyrosine phosphatase-1B dephosphorylation of the insulin receptor occurs in a perinuclear endosome compartment in human embryonic kidney 293 cells

Protein tyrosine phosphatase-1B (PTP-1B) is a negative regulator of insulin signaling. It is thought to carry out this role by interacting with and dephosphorylating the activated insulin receptor (IR). However, little is known regarding the nature of the cellular interaction between these proteins, especially because the IR is localized to the plasma membrane and PTP-1B to the endoplasmic reticulum. Using confocal microscopy and fluorescence resonance energy transfer (FRET), the interaction between PTP-1B and the IR was examined in co-transfected human embryonic kidney 293 cells. Biological activities were not significantly affected for either PTP-1B or the IR with the fusion of W1B-green fluorescent protein (GFP) to the N terminus of PTP-1B (W1B-PTP-1B) or the fusion of Topaz-GFP to the C terminus of the IR (Topaz-IR). FRET between W1B and Topaz was monitored in cells transfected with either wild type PTP-1B (W1B-PTP-1B) or the substrate-trapping form PTP-1B(D181A) (W1B-PTP-1B(D181A)) and Topaz-IR. Co-expression of W1B-PTP-1B with Topaz-IR resulted in distribution of Topaz-IR to the plasma membrane, but no FRET was obtained upon insulin treatment. In contrast, co-expression of W1B-PTP-1B(D181A) with Topaz-IR caused an increase in cytosolic Topaz-IR fluorescence and, in some cells, a significant basal FRET signal, suggesting that PTP-1B is interacting with the IR during its synthesis. Stimulation of these cells with insulin resulted in a rapid induction of FRET that increased over time and was localized to a perinuclear spot. Co-expression of Topaz-IR with a GFP-labeled RhoB endosomal marker and treatment of the cells with insulin identified a perinuclear endosome compartment as the site of localization. Furthermore, the insulin-induced FRET could be prevented by the treatment of the cells with a specific PTP-1B inhibitor. These results suggest that PTP-1B appears not only to interact with and dephosphorylate the insulin-stimulated IR in a perinuclear endosome compartment but is also involved in maintaining the IR in a dephosphorylated state during its biosynthesis.

PTH/PTH-related protein receptor interacts directly with Tctex-1 through its COOH terminus

COOH-terminal cytoplasmic domains of G protein-coupled receptors (GPCRs) have been shown to carry determinants that control their cell surface localization, internalization, and recycling. In attempts to seek cellular proteins that mediate these processes of PTH/PTH-related protein receptor (PTHR), one of the class B GPCRs, we have found that Tctex-1, a 14kDa light chain of cytoplasmic dynein motor complex, interacts with the COOH-terminal tail of the receptor. A 34-amino-acid stretch of the receptor responsible for binding to Tctex-1 has a bipartite structure consisting of a motif previously implicated in binding of some proteins to Tctex-1 and a putative new consensus sequence. Site-directed mutations or a 20-amino-acid deletion in the bipartite consensus binding sequence abolished the association of the PTHR COOH terminus with Tctex-1 in vitro. A GFP-fused mutant PTHR impaired in binding to Tctex-1 expressed in MDCK cells showed a decreased rate of internalization in response to PTH compared to that of the wild type.

High-content assays for ligand regulation of G-protein-coupled receptors

High-content assays rely on the imaging of cellular events. They can be used to monitor the activation of G-protein-coupled receptors (or other receptors), their internalization into the cell, or alterations in their amount. In addition, multiplexed assays can provide further information about the characteristics of the receptor. Recent improvements in throughput using high-content screening platforms means that such assays are now an integral element of functional analysis in the drug discovery process.

High-content profiling of drug-drug interactions: cellular targets involved in the modulation of microtubule drug action by the antifungal ketoconazole

Drug-drug interactions play an important role in the discovery and development of therapeutic agents. High-content profiling was developed to unravel the complexity of these interactions by providing multiparameter measurements of target activity at the cellular and subcellular levels. Two microtubule drugs, vinblastine and curacin A, were shown to modulate multiple cellular processes, including nuclear condensation, the activation of the extracellular signal-regulated kinase pathway as measured by RSK90 phosphorylation, and the regulation of the microtubule cytoskeleton as measured in detergent-extracted cells. The heterogeneity of the response, addressed through population analysis and multiparameter comparisons within single cells, was consistent with vinblastine and curacin A having similar effects on nuclear morphology and 90 kDa ribosomal s6 kinase (RSK90) phosphorylation despite having distinct effects on the microtubule cytoskeleton. Ketoconazole, originally developed as an antifungal agent, exhibited concentration-dependent inhibitory and potentiating effects on both drugs in HeLa and PC-3 cells at concentration ranges near the plasma levels of ketoconazole attained in human subjects. Thus, high-content profiling was used to dissect the cellular and molecular responses to interacting drugs and is therefore a potentially important tool in the selection, characterization, and optimization of lead therapeutic compounds.

Interaction of the parathyroid hormone receptor with the 14-3-3 protein

The receptor for parathyroid hormone (PTH) and PTH-related protein (PTHrP) regulates calcium homeostasis, bone remodeling and skeletal development. 14-3-3 proteins bind to signaling proteins and act as molecular scaffolds and regulators of subcellular localization. We show that the parathyroid hormone receptor (PTHR) interacts with 14-3-3 and the proteins colocalize within the cell. 14-3-3 interacts with the C-terminal tail of the receptor containing a consensus 14-3-3 binding motif, but additional binding sites are also used. Protein kinase-A treatment of the receptor and especially the C-terminal tail reduces 14-3-3 binding. The expressed C-terminal tail is primarily localized in the nucleus, supporting the function of a putative nuclear localization signal that could be involved in the previously described nuclear localization of PTHR. The observed interaction between PTHR and the 14-3-3 protein implies that 14-3-3 could contribute to regulation of PTHR signaling.