Abstract P1-09-08: Development and characterization of PTEN IHC assay for testing breast cancer patients specimens

Background: Phosphatase and tensin homolog (PTEN) is a tumor suppressor gene that is a major negative regulator of the Phosphatidylinositol 3-kinase (PI3K) pathway. Loss of PTEN protein expression has been mechanistically linked to tumor progression. Blocking the PI3K pathway might inhibit the growth and proliferation of cells that have deletions in PTEN. Thus characterization of PTEN expression in patient tumor samples may assist prediction of potential response to PI3K inhibitor therapies.

Methods: We developed and validated an immunohistochemical assay on Ventana BenchMark XT to detect PTEN in formalin fixed and paraffin-embedded (FFPE) tissue by utilizing a rabbit monoclonal antibody (clone 138G6) from Cell Signaling Technologies that recognizes the carboxy-terminal domain of PTEN. PTEN immunohistochemical staining was performed on 1577 breast tumor specimens to determine PTEN protein loss. A subset of these cases (n=663) was also assessed for PTEN mutation.

Results: Cellular localization of PTEN expression was observed in both the cytoplasm and the nucleus. Both cellular compartments were scored and used in the staining intensity determination. We observed that PTEN staining is sensitive to variation in tissue handling, fixation and antigen retrieval. PTEN staining was affected significantly by antigen retrieval method. Optimal staining conditions were determined to be 1:60 antibody dilution using Citrate pH 6.0 as antigen retrieval buffer. In a cohort of 1577 hormone receptor positive HER2-negative locally advanced or metastatic breast cancer patients, loss of PTEN protein was observed in 8.6% (135/1577) of patients. There was a positive correlation between PTEN mutation rate (17 out of 66 PTEN IHC positive cases vs 14 out of 587 negative cases) and loss of PTEN protein expression. PTEN loss was observed to correlate with better clinical response to PI3K inhibitor.

Conclusions: Pre-analytical handling of samples is important for PTEN IHC staining. PTEN mutations and insertions/deletions contribute to PTEN protein loss. This study validates a simple method to interrogate PTEN status in clinical specimens and supports the utility of this test in selecting patients who are likely to respond to PI3K inhibitor treatment.

Citation Format: Hua Gong, Emmanuel Pacia, Sharmila Manjeshwar, Beiru Chen, Xun Li, Bashar Dabbas, Jelveh Lameh, Naveen Babbar, Shabnam Tangri. Development and characterization of PTEN IHC assay for testing breast cancer patients specimens [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-09-08.

Abstract 2299: Development of a novel next-generation sequencing (NGS)-based assay for measurable residual disease (MRD) in FLT3-ITD acute myeloid leukemia (AML) and its potential clinical application in patients

Introduction The presence of MRD in patients with AML who are in morphologic remission has been shown to be a powerful predictor of eventual relapse. FMS-like tyrosine kinase 3 (FLT3) internal tandem duplications (ITD) confer a negative prognostic impact by increasing risk of relapse. The ability to detect FLT3-ITD mutations in remission bone marrow specimens is hampered by the limited sensitivity at 1% of PCR-based assays. To address such limitations, we developed a novel NGS-based MRD assay for the detection of FLT3-ITD mutations.

Method Genomic DNA was isolated from bone marrow (BM) aspirates or peripheral blood (PB) samples. PCR was performed to amplify exons 13 to 15 of the FLT3 gene. Highly diverse NGS libraries were then generated and sequenced using Illumina’s sequencer. Using a custom bioinformatics approach, unique FLT3-ITD mutations of varying lengths were identified and mutant allelic frequency calculated. The assay was validated using clinical samples and contrived samples. For accuracy assessment, 30 samples (including PB or BM remnant patient DNA and cell line DNA, above DNA diluted in normal DNA) were included. Data were compared with a Fragment Size Analysis by Capillary Electrophoresis assay. To assess precision, the assay was validated at multiple levels evaluating intra-assay, inter-assay, inter-operator, inter-instrument and inter-reagent lot precision. DNA samples from selected mutant cell lines representing different FLT3-ITD lengths were spiked into normal DNA to evaluate assay sensitivity and linearity.

Summary The ideal DNA input range was established as 300 ng to 500 ng. In all FLT3-ITD-positive cell line samples covering diverse FLT3-ITD lengths (6 bp to 156 bp), the FLT3-ITD MRD NGS assay showed 100% concordance with the reference assay. The assay is also capable of detecting ITDs as expected after the samples were diluted in normal DNA to mimic samples from patients in remission. All acceptance criteria for the different precision parameters were met. The lower limit of detection of 0.013% regardless of ITD length was established but the assay was capable of detecting FLT3-ITD mutations at a level as low as 0.003% without false-positive results. The assay was linear (R2 = 0.958) down to FLT3-ITD allele frequency levels of 0.035% or the lower limit of quantitation.

Conclusion Analytical validation results established the role of this NGS-based MRD assay for the clinical management of FLT3-ITD AML. The FLT3-ITD MRD NGS assay demonstrated high sensitivity and high specificity by detecting the unique length of each patient’s mutation with no false-positive results in expected negative samples. This assay might be helpful in defining the depth of remission, identifying persistent disease, and helping to guide decision making in the use of FLT3 inhibitors as continuation therapy.

Citation Format: Wenge Shi, Christian Laing, Wei Ding, Marc Mycoco, Jelveh Lameh, Reinhold Pollner. Development of a novel next-generation sequencing (NGS)-based assay for measurable residual disease (MRD) in FLT3-ITD acute myeloid leukemia (AML) and its potential clinical application in patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2299.

Quantitative Spatial Profiling of PD-1/PD-L1 Interaction and HLA-DR/IDO-1 Predicts Improved Outcomes of Anti-PD-1 Therapies in Metastatic Melanoma

Purpose: PD-1/L1 axis-directed therapies produce clinical responses in a subset of patients; therefore, biomarkers of response are needed. We hypothesized that quantifying key immunosuppression mechanisms within the tumor microenvironment by multiparameter algorithms would identify strong predictors of anti-PD-1 response.Experimental Design: Pretreatment tumor biopsies from 166 patients treated with anti-PD-1 across 10 academic cancer centers were fluorescently stained with multiple markers in discovery (n = 24) and validation (n = 142) cohorts. Biomarker-positive cells and their colocalization were spatially profiled in pathologist-selected tumor regions using novel Automated Quantitative Analysis algorithms. Selected biomarker signatures, PD-1/PD-L1 interaction score, and IDO-1/HLA-DR coexpression were evaluated for anti-PD-1 treatment outcomes.Results: In the discovery cohort, PD-1/PD-L1 interaction score and/or IDO-1/HLA-DR coexpression was strongly associated with anti-PD-1 response (P = 0.0005). In contrast, individual biomarkers (PD-1, PD-L1, IDO-1, HLA-DR) were not associated with response or survival. This finding was replicated in an independent validation cohort: patients with high PD-1/PD-L1 and/or IDO-1/HLA-DR were more likely to respond (P = 0.0096). These patients also experienced significantly improved progression-free survival (HR = 0.36; P = 0.0004) and overall survival (HR = 0.39; P = 0.0011). In the combined cohort, 80% of patients exhibiting higher levels of PD-1/PD-L1 interaction scores and IDO-1/HLA-DR responded to PD-1 blockers (P = 0.000004). In contrast, PD-L1 expression was not predictive of survival.Conclusions: Quantitative spatial profiling of key tumor-immune suppression pathways by novel digital pathology algorithms could help more reliably select melanoma patients for PD-1 monotherapy. Clin Cancer Res; 24(21); 5250-60. ©2018 AACR.

Abstract 853: Novel quantitative multiplexed PD-1/PD-L1 immunohistochemistry test provides superior prediction of treatment response in melanoma patients

While PD-1/L1 axis-targeted therapies have shown promising clinical responses, their use in combination therapies is associated with both benefits and safety concerns. Response rates for single-agent anti-PD-1 therapies are significantly higher in biomarker positive patients, therefore there is a need to utilize predictive diagnostics to enhance benefit-risk profiles and guide treatment decisions. To address this, we developed a novel quantitative multiplexed immunohistochemistry assay that provides objective quantitation of PD-L1 positive cells, but more importantly assesses interactions with immune cells (PD-1+ or CD8+) in formalin-fixed paraffin embedded (FFPE) clinical specimens. The clinical validity of this assay was verified in a small series of melanoma patients treated with anti-PD1 targeted therapies/agents.

FFPE melanoma tissues from patients who received anti-PD-1 therapy were fluorescently stained with a combination of anti-PD-1, PD-L1, and S100 antibodies plus DAPI or a combination of anti-CD8, PD-L1, and S100 antibodies plus DAPI. Each slide was then imaged on Vectra platform and the frequencies of biomarker positive cells (PD-L1, PD-1, and CD8) and their interaction scores were objectively evaluated using proprietary Automated Quantitative Analysis (AQUA®) algorithms. Analytical sensitivity, precision, and accuracy were established using standardized PD-L1 and PD-1 tissue control arrays composed of cell lines and lymphoid organs, while range of biomarker expression was verified on archived melanoma clinical specimens (n = 30), including samples taken from melanoma patients prior to anti-PD-1 therapies (n = 21).

Frequencies of PD-L1 positive cells could be accurately quantified within 1% to 100% range in predefined control cell line mixtures. PD-L1 and PD-1 measurements were highly reproducible (R2 = 0.98 and 0.97, respectively). A broad range of PD-L1 and PD-1 expression and interaction scores were observed in archival clinical specimens (n = 53). In a cohort of 21 advanced melanoma patients treated with nivolumab (n = 5) or pembrolizumab (n = 16), the PD-1/L1 interaction score was found to reliably distinguish responders from non-responders (p = 0.03) while PD-L1 alone (p = 0.15) or CD-8 alone (p = 0.23) did not. Additionally, patients exhibiting higher PD-1/L1 interaction scores had superior response rates (78% vs. 17%, p = 0.03). Responders experienced significantly longer median progression-free survival (177 vs. 85 days, p = 0.014), and fewer deaths (22% vs 58%) compared with patients having lower PD-1/L1 interaction scores.

In terms of diagnostic utility, the PD-1/L1 multiplex test showed superior predictive power (78% Positive Predictive Value, 83% Negative Predictive Value) compared with PD-L1 expression alone. Additional studies are underway to fully establish diagnostic utility and aid in treatment guidance.

Citation Format: Jennifer Bordeaux, Douglas Johnson, Jeffrey Sosman, Ju Young Kim, Christine Vaupel, Bashar Dabbas, Justin Cates, Jeff Hall, Jelveh Lameh, Shabnam Tangri, Naveen Dakappagari. Novel quantitative multiplexed PD-1/PD-L1 immunohistochemistry test provides superior prediction of treatment response in melanoma patients. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 853.

Abstract 1302: High complexity flow cytometry panels to monitor target expression, T-cell activation and suppresssion by novel immunotherapies in hematomalignancy clinical trials

The renaissance of cancer immunotherapies and the positive clinical responses observed with chimeric antigen receptor modified T cells in the hematomalignancy setting has stimulated substantial interest in monitoring immune cell activation and suppression to determine efficacy, prognosis and safety in new agent investigational trials. However, immunophenotyping capacity has been limited and required multi-test tube panels. To address the growing needs of clinical trials, we have developed three fit-for-purpose high complexity (10 or more markers) T-cell phenotyping flow cytometry panels on a qualified LSR FortessaTM platform that enables detection of up to 15 markers in a single test tube. The first panel allows identification of multiple phenotypes along the T-cell differentiation pathway, namely, T-naive (TN), T-stem cell memory (TSCM), T-effector memory (TEM) and T-central memory (TCM) and T-effector memory RA+ (TEMRA). The second panel identifies the most common helper T-cell phenotypes such as Th1, Th2, Treg, and Th17. A third panel determines the functional status of T-cells (e.g., activation vs. suppression) but also enables quantitation of important checkpoint inhibitor expression (e.g., PD-1) on T-cells of interest. These high complexity flow cytometry panels can serve as powerful tools for comprehensive examination of T-cells in a small volume of patient specimen. We believe these new flow cytometry panels will have a substantial impact on the determination of efficacy and safety correlates of immunomodulating agents administered alone or in combination to patients with leukemia.

Citation Format: Ghanashyam Sarikonda, Devika Ashok, Anil Pahuja, Jelveh Lameh, Shabnam Tangri, Naveen Dakappagari. High complexity flow cytometry panels to monitor target expression, T-cell activation and suppresssion by novel immunotherapies in hematomalignancy clinical trials. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1302. doi:10.1158/1538-7445.AM2015-1302

Abstract 2840: Evaluation of HER-2 RNA and protein levels in a large cohort of breast cancer specimens to support development of a diagnostic immunofluorescence assay quantified by AQUA® Technology

With the advent of multiple HER-2 targeting agents, the need for a diagnostic test that accurately predicts the levels of HER2 in newly diagnosed breast cancer patients has never been greater. The goal of this study was to develop an immunofluorescence based assay to objectively and reproducibly quantify HER-2 protein level in Formalin-Fixed Paraffin-Embedded (FFPE) specimens using AQUA Technology and to provide a binary readout, thus eliminating reflex testing and enabling quick treatment decisions.

Methods: Three well known HER-2 antibody clones, A0485, CB11, and SP3 were evaluated across a three-log dilution series under four different antigen retrieval conditions on a tissue microarray (TMA) containing 80 breast cancer cases with known central HER2 immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) amplification status. The top six immunofluorescence assay combinations out of 108 conditions were quantified by AQUA scoring based on the their ability to accurately separate negative and positive cases using receiver operating characteristic (ROC) analysis. These top six assay conditions were then assessed on an independent TMA containing 94 breast cancer cases and their AQUA scores were compared to HER2 protein levels determined by reverse phase protein microarray (RPMA®), HER2 mRNA levels assessed by RNAscope® as well as to the current “gold standard” IHC and FISH amplification.

Results: Top three conditions clearly segregating HER2 positive and HER2 negative breast cancer cases were selected and analyzed on an independent TMA. Conditions associated with both SP3 and A0485 antibodies demonstrated high correlations with both HER2 protein levels determined by RPMA analysis and HER2 mRNA levels assessed by RNAscope (spearman's rho >0.85).

Conclusion: Assessment of HER2 status using AQUA Technology was confirmed using two expression platforms, including both protein (RPMA) and RNA (RNAscope) supporting the utilization of the AQUA methodology for diagnostic test development.

Citation Format: Jennifer Bordeaux, Krupa Chandrasekaran, Sue Beruti, Mike Nerenberg, Corinne Ramos, David Rimm, Jelveh Lameh, Naveen Dakappagari. Evaluation of HER-2 RNA and protein levels in a large cohort of breast cancer specimens to support development of a diagnostic immunofluorescence assay quantified by AQUA® Technology. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2840. doi:10.1158/1538-7445.AM2014-2840

Abstract 3734: Development of a nanostring copy number assay for a customized 55 gene panel using challenging formalin-fixed paraffin-embedded (ffpe) tumor samples

Introduction: Gene amplification/deletion is a common tactic employed by tumor cells to proliferate uncontrollably. Multiple methods have been developed to determine gene copy numbers, such as FISH, qPCR, and, more recently, Next Generation Sequencing (NGS). However, these methods have limitations: FISH and NGS are very labor intensive; and FISH and qPCR can detect only a limited number of targets. Therefore, it is imperative to develop methods that are more user-friendly and have increased multiplexing capabilities to detect copy number variations (CNV). Using a novel digital barcode technology, NanoString Technologies (Seattle, WA) has developed the nCounter® technology to meet these needs. This new technology will be especially valuable after the recent FDA approval of the ProsignaTM assay, a breast cancer gene signature assay for subtype classification to help guide treatment decisions. In the BioPharma group at Genoptix, Inc., Carlsbad, CA, we have started to assess the nCounter technology for copy number determinations in a CLIA laboratory setting.

Methods: A cancer gene CNV panel composed of 55 genes was custom designed and evaluated. The CNV results of a subset of genes generated from our gene panel using the nCounter technology were compared with the results generated from qPCR, digital droplet PCR (ddPCR), FISH, publications, and NGS. Furthermore, assay robustness was tested by using different amounts and quality of input FFPE DNA. An initial comparison on the CNVs obtained from genomic DNA digested by Alu1 enzyme or sonicated by Bioruptor® Pico (Diagenode Inc, Denville, NJ). Finally, the effects of different DNA isolation methods on copy number determinations were also evaluated.

Results: Overall, the copy numbers generated using nCounter technology were concordant with results from the abovementioned methods as analyzed by comparing selected genes with known CNVs. A wide range of FFPE DNA input (200ng-4ug) could be tolerated by the assay. Based on a limited number of samples, DNA isolation methods, such as Qiagen, Promega Maxwell® CSC, or Roche Cobas®, have minimal effect on copy numbers.

Conclusions: With the short hands-on time, relative high level of automation, and no amplification requirements, nCounter technology is able to generate CNVs in good agreement with other established CNV methods. This new 55 cancer gene CNV assay may be useful in clinical trials to assist in patient stratification and care.

Citation Format: Tingdong Tang, Wenge Shi, Loretta Hipolito, Julie Mayer, Jelveh Lameh, Shabnam Tangri, Reinhold Pollner. Development of a nanostring copy number assay for a customized 55 gene panel using challenging formalin-fixed paraffin-embedded (ffpe) tumor samples. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3734. doi:10.1158/1538-7445.AM2014-3734

Abstract LB-96: A novel blood preservation system to study oncogenic signaling pathway biomarkers by flow cytometry in leukemia/lymphoma clinical trials

Propelled by the advent of new technologies and an evolving regulatory landscape, the desire to personalize cancer treatments has never been greater. There is a critical need to reliably evaluate target inhibition and pharmacodynamic activity of investigational drugs in biologically relevant compartments. However, the ability to fulfill such a task in global clinical trials is complicated by varying technical expertise available at clinical sites and considerations about specimen stability for centralized laboratory analysis. We developed a novel formalin-based preservation method that enables specimen stabilization in a single step, requiring less than 20 minutes, without the need for specialized training or instrumentation. Stabilized specimens can be frozen for shipping and batched analysis at a later time point, in a specialized laboratory. Using this preservation method, we developed a flow assay enabling identification of multiple cell-types, and quantification of intracellular biomarkers in target cellular compartments, in both the peripheral blood (PB) and bone marrow (BM). We present pharmacodynamic (PD) data for a novel anticancer investigational agent intended for acute myeloid leukemia and multiple myeloma and its effect on multiple target biomarkers of the PI3K signaling pathway, including phosphorylation of the S6 ribosomal protein (S6). Our novel fixation method allowed detection of pS6 modulation in a dose dependent manner in both tumor cells and PB or BM in response to the novel investigational agent. Assay sensitivity and concordance were evaluated by comparing flow assay with western blot analysis, with each assay performed at a different site using the same batched frozen samples; exceptional preservation of phosphorylated proteins for more than 72 hours could be observed, when frozen immediately following fixation. In conclusion, our novel preservation method enables reliable quantification of signaling biomarkers in centralized laboratories at different time points post sample (PB or BM) acquisition requiring minimal processing at collection sites.

Citation Format: Anil Pahuja, Abdel Saci, Shyam Sarikonda, Armin Graber, Benjamin Lee, Jelveh Lameh, Shabnam Tangri, Naveen Dakappagari. A novel blood preservation system to study oncogenic signaling pathway biomarkers by flow cytometry in leukemia/lymphoma clinical trials. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-96. doi:10.1158/1538-7445.AM2014-LB-96

Abstract 2843: Development of a binary diagnostic immunofluorescence assay by AQUA® technology for accurate detection of HER-2 levels in breast cancer specimens

HER2 is a prognostic and predictive marker for breast cancer patients and its expression is routinely evaluated by immunochemistry (IHC). Scoring of IHC slides is prone to operator subjectivity and equivocal results make selection of appropriate therapy difficult, ultimately affecting patient outcomes. We describe development and validation of a reproducible quantitative immunofluorescence assay to accurately assess HER-2 levels in Formalin-Fixed Paraffin-Embedded (FFPE) breast cancer specimens by AQUA Technology that avoids the ambiguity of equivocal results and enables critical treatment decisions.

Methods: A tissue microarray (n=80) composed of breast cancer cases with known IHC and fluorescence in situ hybridization (FISH) status was used to characterize and optimize assay performance for three HER-2 antibody clones; A0485, CB11, and SP3. Eight dilutions of each antibody were tested with four different antigen retrieval conditions. A total of 108 assay conditions were evaluated by receiver operator characteristic (ROC) analysis for sensitivity and specificity. Equal weight was given to sensitivity and specificity to select the most robust assay. The top six assay conditions were then assessed using 45 whole tissue breast cancer specimens to identify one condition with highest sensitivity and specificity. The selected assay condition was evaluated on an additional 90 breast cancer specimens (training set) annotated for IHC and FISH scores to determine the binary cut point for the HER-2 AQUA assay®, the cut point was then confirmed on a validation set composed of 90 independent breast cancer specimens. The final assay was analytically validated in accordance with College of American Pathologists (CAP) guidelines utilizing over 120 independent breast cancer specimens.

Results: Based on an evaluation of over 400 breast cancer specimens with nearly equal distribution of 0, 1+, 2+ and 3+ cases, HER-2 antibody clone, SP3, which recognizes the extracellular domain of the receptor, clearly segregated HER2 positive specimens from HER2 negative breast cancer cases.

Conclusion: The availability of a highly reproducible quantitative binary test facilitates rapid treatment decisions with appropriate HER-2 targeting biologics on the market.

Citation Format: Lakshmi Krupa Chandrasekaran, Jennifer Bordeaux, Sue Beruti, Naveen Dakappagari, Mike Nerenberg, Jelveh Lameh, Armin Graber, David Rimm, Bruce Robbins, Nagesh Rao. Development of a binary diagnostic immunofluorescence assay by AQUA® technology for accurate detection of HER-2 levels in breast cancer specimens. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2843. doi:10.1158/1538-7445.AM2014-2843

Neuropeptide FF receptors have opposing modulatory effects on nociception

The role of neuropeptide FF (NPFF) and its analogs in pain modulation is ambiguous. Although NPFF was first characterized as an antiopioid peptide, both antinociceptive and pronociceptive effects have been reported, depending on the route of administration. Currently, two NPFF receptors, termed FF1 and FF2, have been identified and cloned, but their roles in pain modulation remain elusive because of the lack of availability of selective compounds suitable for systemic administration in in vivo models. Ligand-binding studies confirm ubiquitous expression of both subtypes in brain, whereas only FF2 receptors are expressed spinally. This disparity in localization has served as the foundation of the hypothesis that FF1 receptors mediate the pronociceptive actions of NPFF. We have identified novel small molecule NPFF receptor agonists and antagonists with varying degrees of FF2/FF1 functional selectivity. Using these pharmacological tools in vivo has allowed us to define the roles of NPFF receptor subtypes as pertains to the modulation of nociception. We demonstrate that selective FF2 agonism does not modulate acute pain but instead ameliorates inflammatory and neuropathic pains. Treatment with a nonselective FF1/FF2 agonist potentiates allodynia in neuropathic rats and increases sensitivity to noxious thermal and to non-noxious mechanical stimuli in normal rats in an FF1 antagonist-reversible manner. Treatment with FF1 antagonists reversed established mechanical allodynia, indicating the possibility of increased NPFF tone through FF1 receptors. In conclusion, we provide evidence for the opposing roles of NPFF receptors and highlight selective FF2 agonism and/or selective FF1 antagonism as potential targets warranting further investigation.

Differential regulation of muscarinic M1 receptors by orthosteric and allosteric ligands

Background

Activation of muscarinic M₁ receptors is mediated via interaction of orthosteric agonists with the acetylcholine binding site or via interaction of allosteric agonists with different site(s) on the receptor. The focus of the present study was to determine if M₁ receptors activated by allosteric agonists undergo the same regulatory fate as M₁ receptors activated by orthosteric agonists.

Results

The orthosteric agonists carbachol, oxotremorine-M and pilocarpine were compared to the allosteric agonists AC-42, AC-260584, N-desmethylclozapine and xanomeline. All ligands activated M₁ receptors and stimulated interaction of the receptors with β-arrestin-1. All ligands reduced cell surface binding and induced the loss of total receptor binding. Receptor internalization was blocked by treatment with hypertonic sucrose indicating that all ligands induced formation of clathrin coated vesicles. However, internalized receptors recycled to the cell surface following removal of orthosteric, but not allosteric agonists. Whereas all ligands induced loss of cell surface receptor binding, no intracellular vesicles could be observed after treatment with AC-260584 or xanomeline. Brief stimulation of M₁ receptors with AC-260584 or xanomeline resulted in persistent activation of M₁ receptors, suggesting that continual receptor signaling might impede or delay receptor endocytosis into intracellular vesicles.

Conclusion

These results indicate that allosteric agonists differ from orthosteric ligands and among each other in their ability to induce different regulatory pathways. Thus, signaling and regulatory pathways induced by different allosteric ligands are ligand specific.

Identification and characterization of novel small-molecule protease-activated receptor 2 agonists

We report the first small-molecule protease-activated receptor (PAR) 2 agonists, AC-55541 [N-[[1-(3-bromo-phenyl)-eth-(E)-ylidene-hydrazinocarbonyl]-(4-oxo-3,4-dihydro-phthalazin-1-yl)-methyl]-benzamide] and AC-264613 [2-oxo-4-phenylpyrrolidine-3-carboxylic acid [1-(3-bromo-phenyl)-(E/Z)-ethylidene]-hydrazide], each representing a distinct chemical series. AC-55541 and AC-264613 each activated PAR2 signaling in cellular proliferation assays, phosphatidylinositol hydrolysis assays, and Ca(2+) mobilization assays, with potencies ranging from 200 to 1000 nM for AC-55541 and 30 to 100 nM for AC-264613. In comparison, the PAR2-activating peptide 2-furoyl-LIGRLO-NH(2) had similar potency, whereas SLIGRL-NH(2) was 30 to 300 times less potent. Neither AC-55541 nor AC-264613 had activity at any of the other PAR receptor subtypes, nor did they have any significant affinity for over 30 other molecular targets involved in nociception. Visualization of EYFP-tagged PAR2 receptors showed that each compound stimulated internalization of PAR2 receptors. AC-55541 and AC-264613 were well absorbed when administered intraperitoneally to rats, each reaching micromolar peak plasma concentrations. AC-55541 and AC-264613 were each stable to metabolism by liver microsomes and maintained sustained exposure in rats, with elimination half-lives of 6.1 and 2.5 h, respectively. Intrapaw administration of AC-55541 or AC-264613 elicited robust and persistent thermal hyperalgesia and edema. Coadministration of either a tachykinin 1 (neurokinin 1) receptor antagonist or a transient receptor potential vanilloid (TRPV) 1 antagonist completely blocked these effects. Systemic administration of either AC-55541 or AC-264613 produced a similar degree of hyperalgesia as was observed when the compounds were administered locally. These compounds represent novel small-molecule PAR2 agonists that will be useful in probing the physiological functions of PAR2 receptors.

Pharmacology of N-desmethylclozapine

Currently available treatments for schizophrenia have limited efficacy and are generally poorly tolerated. However, among these antipsychotic agents, clozapine stands apart in having generally superior motoric tolerability and efficacy. One intriguing possibility, based on clinical correlations, receptor activity profiles and studies with animal models predictive of antipsychotic or cognitive action is that the activity of N-desmethylclozapine (NDMC), a major metabolite of clozapine, may, at least in part, underlie the unique efficacy of clozapine. In this review we compare the pharmacological properties of NDMC to those of clozapine and consider how they may contribute to the overall clinical properties of clozapine. We also consider whether NDMC, in its own right, might be a superior antipsychotic drug.

Transcutaneous Blood Gas CO2 Monitoring of Induced Ventilatory Depression in Mice

We assessed a simple, noninvasive method of monitoring transcutaneous partial pressure of CO2 (Ptcco2) in mice to determine whether it would provide an accurate and reproducible method to assess ventilatory depression in mice. To this end, Ptcco2 and Paco2 (partial pressure of arterial CO2) measurements were performed on isoflurane-anesthetized male C57Bl/6 mice breathing differing percentages of CO2 or fentanyl, a known ventilatory depressive drug. All doses of fentanyl produced a sharp increase in Ptcco2 values within 20 min with difference in Ptcco2 values between saline and all fentanyl groups being statistically significant (P < 0.0001). A good correlation between Paco2 and Ptcco2 values was established (r2 = 0.91). A Bland-Altman analysis likewise found that Ptcco2 measurements in the mice reliably and accurately reflected their Paco2 values. Therefore, under controlled conditions, Ptcco2 measurements were found to reliably reflect Paco2 values in mice. Consequently, the Ptcco2 method can be used as a means to rapidly and quantitatively assess the ventilatory depressive properties of a wide spectrum of drugs, under varying conditions in numerous mouse models.

Pharmacological and Behavioral Profile of N-(4-Fluorophenylmethyl)-N-(1-methylpiperidin-4-yl)-N′-(4-(2-methylpropyloxy)phenylmethyl) Carbamide (2R,3R)-Dihydroxybutanedioate (2:1) (ACP-103), a Novel 5-Hydroxytryptamine2A Receptor Inverse Agonist

The in vitro and in vivo pharmacological properties of N-(4-fluorophenylmethyl)-N-(1-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)phenylmethyl)carbamide (2R,3R)-dihydroxybutanedioate (2:1) (ACP-103) are presented. A potent 5-hydroxytryptamine (5-HT)(2A) receptor inverse agonist ACP-103 competitively antagonized the binding of [(3)H]ketanserin to heterologously expressed human 5-HT(2A) receptors with a mean pK(i) of 9.3 in membranes and 9.70 in whole cells. ACP-103 displayed potent inverse agonist activity in the cell-based functional assay receptor selection and amplification technology (R-SAT), with a mean pIC(50) of 8.7. ACP-103 demonstrated lesser affinity (mean pK(i) of 8.80 in membranes and 8.00 in whole cells, as determined by radioligand binding) and potency as an inverse agonist (mean pIC(50) 7.1 in R-SAT) at human 5-HT(2C) receptors, and lacked affinity and functional activity at 5-HT(2B) receptors, dopamine D(2) receptors, and other human monoaminergic receptors. Behaviorally, ACP-103 attenuated head-twitch behavior (3 mg/kg p.o.), and prepulse inhibition deficits (1-10 mg/kg s.c.) induced by the 5-HT(2A) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride in rats and reduced the hyperactivity induced in mice by the N-methyl-d-aspartate receptor noncompetitive antagonist 5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate; MK-801) (0.1 and 0.3 mg/kg s.c.; 3 mg/kg p.o.), consistent with a 5-HT(2A) receptor mechanism of action in vivo and antipsychotic-like efficacy. ACP-103 demonstrated >42.6% oral bioavailability in rats. Thus, ACP-103 is a potent, efficacious, orally active 5-HT(2A) receptor inverse agonist with a behavioral pharmacological profile consistent with utility as an antipsychotic agent.

Pharmacological profiles of opioid ligands at kappa opioid receptors

Background

The aim of the present study was to describe the activity of a set of opioid drugs, including partial agonists, in a human embryonic kidney cell system stably expressing only the mouse κ-opioid receptors. Receptor activation was assessed by measuring the inhibition of cyclic adenosine mono phosphate (cAMP) production stimulated by 5 μM forskolin. Intrinsic activities and potencies of these ligands were determined relative to the endogenous ligand dynorphin and the κ agonist with the highest intrinsic activity that was identified in this study, fentanyl.

Results

Among the ligands studied naltrexone, WIN 44,441 and dezocine, were classified as antagonists, while the remaining ligands were agonists. Intrinsic activity of agonists was assessed by determining the extent of inhibition of forskolin-stimulated cAMP production. The absolute levels of inhibition of cAMP production by each ligand was used to describe the rank order of intrinsic activity of the agonists; fentanyl = lofentanil ≥ hydromorphone = morphine = nalorphine ≥ etorphine ≥ xorphanol ≥ metazocine ≥ SKF 10047 = cyclazocine ≥ butorphanol > nalbuphine. The rank order of affinity of these ligands was; cyclazocine > naltrexone ≥ SKF 10047 ≥ xorphanol ≥ WIN 44,441 > nalorphine > butorphanol > nalbuphine ≥ lofentanil > dezocine ≥ metazocine ≥ morphine > hydromorphone > fentanyl.

Conclusion

These results elucidate the relative activities of a set of opioid ligands at κ-opioid receptor and can serve as the initial step in a systematic study leading to understanding of the mode of action of these opioid ligands at this receptor.

The role of M1 muscarinic receptor agonism of N-desmethylclozapine in the unique clinical effects of clozapine

RATIONALE

Clozapine is a unique antipsychotic, with efficacy against positive symptoms in treatment-resistant schizophrenic patients, and the ability to improve cognition and treat the negative symptoms characteristic of this disease. Despite its unique clinical actions, no specific molecular mechanism responsible for these actions has yet been described.

OBJECTIVES AND METHODS

To comprehensively profile a large library of neuropsychiatric drugs, including most antipsychotics, at human monoamine receptors using R-SAT, an in vitro functional assay.

RESULTS

Profiling revealed that N-desmethylclozapine (NDMC), the principal metabolite of clozapine, but not clozapine itself, is a potent and efficacious muscarinic receptor agonist, a molecular property not shared by any other antipsychotic. To further explore the role of NDMC muscarinic receptor agonist properties in mediating the physiological actions of clozapine, systemically administered NDMC was found to stimulate the phosphorylation of mitogen-activated protein kinase (MAP kinase) in mouse CA1 hippocampal neurons, an effect that was blocked by scopolamine, confirming central M1 muscarinic receptor agonist activity in vivo. Lastly, an analysis of clozapine and NDMC serum levels in schizophrenic patients indicated that high NDMC/clozapine ratios better predicted improvement in cognitive functioning and quality of life than the levels of either compound alone.

CONCLUSIONS

The muscarinic receptor agonist activities of NDMC are unique among antipsychotics, and provide a possible molecular basis for the superior clinical effects of clozapine pharmacotherapy.

Pharmacology of polymorphic variants of the human 5-HT1A receptor

The 5-HT1A receptor is a critical mediator of serotonergic (5-HT) function. We have identified 13 potential single nucleotide polymorphisms resulting in amino acid changes throughout the human 5-HT1A receptor. The pharmacological profiles of these 13 polymorphic variants were then characterized using a high-throughput assay based on ligand-dependent transformation of NIH/3T3 cells. The majority of the polymorphic variants displayed wild-type pharmacological profiles in response to a panel of well-established agonists at the 5-HT1A receptor. However, the A50V polymorphic variant, which had an alanine to valine substitution in transmembrane 1, exhibited a loss of detectable response to 5-HT. Interestingly, all other agonists tested, including buspirone, lisuride, and (+)8-OH-DPAT, exhibited efficacies similar to that of the wild-type receptor. The competitive antagonist, methiothepin, also displayed a 19-fold decrease in potency at the A50V variant receptor. However, both 5-HT and methiothepin were able to compete for [3H]WAY-100635 binding to the A50V variant with affinities similar to the wild-type receptor. Moreover, the Bmax of [3H]WAY-100635 binding was 14-fold lower for the A50V variant than for the wild-type receptor. Thus, the A50V receptor variant exhibited ligand-specific functional alterations in addition to lower expression levels. These data suggest a previously unappreciated role for transmembrane 1 in mediating 5-HT response at the 5-HT1A receptor. Furthermore, individuals that potentially harbor the A50V polymorphism might display aberrant affective behaviors and altered responses to drugs targeting the 5-HT1A receptor.

Activity of opioid ligands in cells expressing cloned mu opioid receptors

BACKGROUND

The aim of the present study was to describe the activity of a set of opioid drugs, including partial agonists, in a cell system expressing only mu opioid receptors. Receptor activation was assessed by measuring the inhibition of forskolin-stimulated cyclic adenosine mono phosphate (cAMP) production. Efficacies and potencies of these ligands were determined relative to the endogenous ligand beta-endorphin and the common mu agonist, morphine.

RESULTS

Among the ligands studied naltrexone, WIN 44,441 and SKF 10047, were classified as antagonists, while the remaining ligands were agonists. Agonist efficacy was assessed by determining the extent of inhibition of forskolin-stimulated cAMP production. The rank order of efficacy of the agonists was fentanyl = hydromorphone = beta-endorphin > etorphine = lofentanil = butorphanol = morphine = nalbuphine = nalorphine > cyclazocine = dezocine = metazocine >or= xorphanol. The rank order of potency of these ligands was different from that of their efficacies; etorphine > hydromorphone > dezocine > xorphanol = nalorphine = butorphanol = lofentanil > metazocine > nalbuphine > cyclazocine > fentanyl > morphine >>>> beta-endorphin.

CONCLUSION

These results elucidate the relative activities of a set of opioid ligands at mu opioid receptor and can serve as the initial step in a systematic study leading to understanding of the mode of action of opioid ligands at this receptor. Furthermore, these results can assist in understanding the physiological effect of many opioid ligands acting through mu opioid receptors.

Activation profiles of opioid ligands in HEK cells expressing delta opioid receptors

BACKGROUND

The aim of the present study was to characterize the activation profiles of 15 opioid ligands in transfected human embryonic kidney cells expressing only delta opioid receptors. Activation profiles of most of these ligands at delta opioid receptors had not been previously characterized in vitro. Receptor activation was assessed by measuring the inhibition of forskolin-stimulated cAMP production.

RESULTS

Naltrexone and nalorphine were classified as antagonists at delta opioid receptor. The other ligands studied were agonists at delta opioid receptors and demonstrated IC50 values of 0.1 nM to 2 microM, maximal inhibition of 39-77% and receptor binding affinities of 0.5 to 243 nM. The rank order of efficacy of the ligands tested was metazocine = xorphanol > or = fentanyl = SKF 10047 = etorphine = hydromorphone = butorphanol = lofentanil > WIN 44,441 = Nalbuphine = cyclazocine > or = met-enkephalin >> morphine = dezocine. For the first time these data describe and compare the function and relative efficacy of several ligands at delta opioid receptors.

CONCLUSIONS

The data produced from this study can lead to elucidation of the complete activation profiles of several opioid ligands, leading to clarification of the mechanisms involved in physiological effects of these ligands at delta opioid receptors. Furthermore, these data can be used as a basis for novel use of existing opioid ligands based on their pharmacology at delta opioid receptors.

Characterization of the RDC1 gene which encodes the canine omolog of a proposed human VIP receptor Expression does not correlate with an increase in VIP binding sites

We have isolated a portion of the canine gene encoding the orphan receptor RDC1 [1]. The complete coding sequence is contained in a single exon, and an intron divides the 5' untranslated region of RDC1 mRNA. The RDC1 protein is 94% homologous to the gene product of GPRN1, which has been proposed to serve as a VIP receptor when expressed in CHO-K1 and COS-7 cells (Sreedharan, S.P. et al. (1991) Proc. Natl. Acad. Sci. USA 88, 4986-4990). Northern analysis indicates that CHO-K1 cells endogenously express a 2.1 kb RDC1 mRNA. However, while CHO-K1 cells possess detectable low affinity [125I]VIP binding sites, VIP binding is not altered in membranes of CHO-K1 cells expressing varying amounts of the RDC1 gene construct. Further, endogenous VIP binding is not increased by transient expression of RDC1 in COS-7 cells. Taken together, the data suggest that RDC1 is not a canine homolog of the proposed VIP receptor.

Influence of benzodiazepine binding site ligands on fear-conditioned contextual memory

Eight compounds that bind to the benzodiazepine binding site on the gamma-amino butyric acid(A) (GABA(A)) receptor were assessed for their influence on contextual memory, an aspect of memory affected in various cognitive disorders including Alzheimer's disease. Using a Pavlovian fear-conditioning paradigm, each ligand was evaluated in C57Bl/6 mice in regards to its direct affect on contextual memory and whether the ligand could attenuate scopolamine-induced contextual memory impairment. Of the eight ligands tested, one impaired contextual memory (agonist), six attenuated scopolamine-induced contextual memory impairment (inverse agonists), and one antagonized the ability of an inverse agonist to attenuate scopolamine-induced contextual memory impairment. Hence, further demonstrating the bi-directional influence benzodiazepine binding site ligands are able to exert on memory modulation. This study serves as an initial starting point in the development of pharmacological tools to be used in deciphering how GABA(A) receptors influence contextual memory.

Characterization of novel benzodiazepine ligands in Spodotera frugiperda (Sf-9) insect cells

The goals of the present work were to characterize the binding profile of nine benzodiazepine ligands in Spodotera frugiperda (Sf-9) insect cells expressing specific gamma aminobutyric acid (A) (GABA(A)) receptor subunit combinations and compare the affinities to those for the receptors in the rat cerebellum. Three recombinant baculovirus constructs, each harboring a different GABA(A) receptor subunit, were introduced into insect cells by simultaneous infection. Saturation and competition binding assays were carried out in membranes from Sf-9 cells infected with either alpha1beta2gamma2 or alpha6beta2gamma2 subunit combinations. The affinities of the ligands to the alpha1beta2gamma2 or alpha6beta2gamma2 receptors expressed in Sf-9 cells were similar to the affinities previously determined for the alpha1 or alpha6 subunit-containing GABA(A) receptors in the rat cerebellum, respectively, thus confirming the previously assigned receptor types in the cerebellum.

Characterization of benzodiazepine receptors in the cerebellum

1. The goals of the work reported here were to further characterize benzodiazepine/GABA(A) (BDZR) receptor heterogeneity in the cerebellum and to measure the affinities and selectivities of structurally diverse benzodiazepines at each site identified. 2. Five chemical families were included in these studies. These were 1,4-benzodiazepines (flunitrazepam), imidazobenzodiazepines (RO15-1788 and RO15-4513 and RO16-6028), beta-carbolines (Abecarnil) and pyrazoloquinolines (CGS 8216, CGS 9895 and CGS 9896). 3. Saturation and competition binding assays were combined with powerful data analysis software developed in our laboratory. Among the capabilities of this software is the identification of multiple binding sites for a cold ligand using a non-selective labeled ligand that binds with equal, but high, affinity to all the binding sites 4. Saturation binding assays using either [3H]-RO15-1788 or [3H]-RO15-4513 revealed only one apparent binding site, with a higher affinity for RO15-4513 than for RO15-1788. However, using [3H]-RO15-4513 for the competition binding studies in the cerebellum, together with our data analysis software, led to the identification of two distinct binding sites with equal densities for the diverse benzodiazepines studied. 5. In rat cerebellum one of the sites identified corresponds to GABA(A) receptors exhibiting alpha1 subunit pharmacology and the other to GABA(A) receptors exhibiting alpha6 subunit pharmacology. In general, the diverse families of BDZR ligands studied had much lower affinities for the alpha6 containing receptors.

Unraveling the identity of benzodiazepine binding sites in rat hipppocampus and olfactory bulb

The goals of the work reported here were (i) to identify distinct GABA(A)/benzodiazepine receptors in the rat hippocampus and olfactory bulb using receptor binding assays, and (ii) to determine the affinities and selectivities of benzodiazepine receptor ligands from structurally diverse chemical families at each site identified. These studies were aided by the use of software AFFINITY ANALYSIS SYSTEM, developed in our laboratory for analysis of receptor binding data that allows the determination of receptor heterogeneity using non-selective radioligands. Saturation binding assays using [3H]RO15-4513 (ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1, 5-a]-[1,4]benzodiazepine-3-carboxylate) revealed two binding sites in each of these two tissues. The higher affinity site corresponds to alpha(5) subunit-containing GABA(A) receptor and the lower affinity site to a combination of alpha(1), alpha(2), and alpha(3) subunit-containing receptors. These results should be useful in the challenging task of identifying the various functional GABA(A) receptors in the central nervous system, and in providing a link between receptor affinities and in vivo activities of the GABA(A)/benzodiazepine receptor ligands studied.

Differential regulation of muscarinic M1 and M3 receptors by a putative phosphorylation domain

A motif consisting of several serine residues flanked N-terminally by acidic residues occurs in the third intracellular loop of both muscarinic M1 and M3 receptors (287SerLeuThrSerSer291 and 349SerAlaSerSer352, respectively). We examined the role of these domains in modulating agonist-induced desensitization and receptor trafficking, and for the muscarinic M3 receptor, we assessed the contribution of phosphorylation to receptor regulation. Mutation of the above residues did not affect desensitization of phosphoinositide hydrolysis signaling for either the muscarinic M1 or M3 receptor and did not alter the agonist-induced phosphorylation state of the muscarinic M3 receptor. Mutation of this domain (349SerAlaSerSer352/349AlaAlaAlaAla352) in the muscarinic M3 receptor completely abrogated receptor internalization and subsequently, down-regulation. Mutation of the analogous domain (287SerLeuThrSerSer291/287AlaLeuAlaAlaAla291) in the muscarinic M1 receptor had no obvious effect on internalization, but led to a more rapid down-regulation. Thus, these serine-rich regions are not required for receptor desensitization, but are differentially involved in receptor trafficking for the muscarinic M1 and M3 receptors.

Role of signal transduction in internalization of the G protein-coupled receptor for parathyroid hormone (PTH) and PTH-related protein

For G protein-coupled receptors, limited information is available on the role of agonist binding or of the second-messenger products of receptor signaling on receptor endocytosis. We explored this problem using the opossum PTH/PTH-related protein (PTHrP) receptor, a prototypical Class II G protein-coupled receptor, as a model. In one approach, we evaluated the endocytic properties of mutated forms of the opossum PTH/PTHrP receptor that we had previously shown to be impaired in their ability to initiate agonist-induced signaling when expressed in COS-7 cells. A point mutation in the third cytoplasmic loop (K382A) that severely impairs PTH/PTHrP receptor signaling significantly reduced internalization, whereas two mutant receptors that displayed only partial defects in signaling were internalized normally. To explore more directly the role of second-messenger pathways, we used a cleavable biotinylation method to assess endocytosis of the wild-type receptor stably expressed in human embryonic kidney (HEK) 293 cells. A low rate of constitutive internalization was detected (<5% over a 30-min incubation at 37 C); the rate of receptor internalization was enhanced about 10-fold by the receptor agonists PTH(1-34) or PTHrP(1-34), whereas the receptor antagonist PTH(7-34) had no effect. Forskolin treatment produced a minimal increase in constitutive receptor endocytosis, and the protein kinase (PK)-A inhibitor H-89 failed to block agonist-stimulated endocytosis. Similarly, activation of PK-C, by treatment with phorbol 12-myristate 13-acetate, elicited only a minimal increase in constitutive receptor endocytosis; and blockade of the PK-C pathway, by treatment with a bisindolylmaleimide, failed to inhibit agonist-induced receptor endocytosis. Immunofluorescence confocal microscopic studies of PTH/PTHrP receptor internalization confirmed the results using receptor biotinylation. These findings suggest that: 1) agonist binding is required for the efficient endocytosis of the PTH/PTHrP receptor; 2) receptor activation (agonist-induced receptor conformational change) and/or coupling to G proteins plays a critical role in receptor internalization; and 3) activation of PK-A and PK-C is neither necessary nor sufficient for agonist-stimulated receptor internalization.

Sequestration of dopamine D2 receptors depends on coexpression of G-protein-coupled receptor kinases 2 or 5

We examined the agonist-dependent sequestration/internalization of dopamine D2 receptor (the long form D2L and short form D2S), which were transiently expressed in COS-7 and HEK 293 cells with or without G-protein-coupled receptor kinases (GRK2 or GRK5). Sequestration was assessed quantitatively by loss of [3H] sulpiride-binding activity from the cell surface and by transfer of [3H] spiperone-binding activity from the membrane fraction to the light vesicle fraction in sucrose-density gradients. In COS-7 cells expressing D2 receptors alone, virtually no sequestration was observed with or without dopamine (< 4%). When GRK2 was coexpressed, 50% of D2S receptors and 36% of D2L receptors were sequestered by treatment with 10(-4) M dopamine for 2 h, whereas no sequestration was observed in cells expressing the dominant negative form of GRK2 (DN-GRK2). When GRK5 was coexpressed, 36% of D2S receptors were sequestered following the same treatment. The agonist-dependent and GRK2-dependent sequestration of D2S receptors was reduced markedly in the presence of hypertonic medium containing 0.45 M sucrose, suggesting that the sequestration follows the clathrin pathway. Internalization of D2S receptors was also assessed by immunofluorescence confocal microscopy. Translocation of D2 receptors from the cell membrane to intracellular vesicles was observed following the treatment with dopamine from HEK 293 cells only when GRK2 was coexpressed. D2S receptors expressed in HEK 293 cells were shown to be phosphorylated by GRK2 in an agonist-dependent manner. These results indicate that the sequestration of D2 receptors occurs only through a GRK-mediated pathway.

Internalization and down-regulation of human muscarinic acetylcholine receptor m2 subtypes. Role of third intracellular m2 loop and G protein-coupled receptor kinase 2

Internalization and down-regulation of human muscarinic acetylcholine m2 receptors (hm2 receptors) and a hm2 receptor mutant lacking a central part of the third intracellular loop (I3-del m2 receptor) were examined in Chinese hamster ovary (CHO-K1) cells stably expressing these receptors and G protein-coupled receptor kinase 2 (GRK2). Agonist-induced internalization of up to 80-90% of hm2 receptors was demonstrated by measuring loss of [3H]N-methylscopolamine binding sites from the cell surface, and transfer of [3H]quinuclidinyl benzilate binding sites from the plasma membrane into the light-vesicle fractions separated by sucrose density gradient centrifugation. Additionally, translocation of hm2 receptors with endocytic vesicles were visualized by immunofluorescence confocal microscopy. Agonist-induced down-regulation of up to 60-70% of hm2 receptors was demonstrated by determining the loss of [3H]quinuclidinyl benzilate binding sites in the cells. The half-time (t1/2) of internalization and down-regulation in the presence of 10(-4) M carbamylcholine was estimated to be 9.5 min and 2.3 h, respectively. The rates of both internalization and down-regulation of hm2 receptors in the presence of 10(-6) M or lower concentrations of carbamylcholine were markedly increased by coexpression of GRK2. Agonist-induced internalization of I3-del m2 receptors was barely detectable upon incubation of cells for 1 h, but agonist-induced down-regulation of up to 40-50% of I3-del m2 receptors occurred upon incubation with 10(-4) M carbamylcholine for 16 h. However, the rate of down-regulation was lower compared with wild type receptors (t1/2 = 9.9 versus 2.3 h). These results indicate that rapid internalization of hm2 receptors is facilitated by their phosphorylation with GRK2 and does not occur in the absence of the third intracellular loop, but down-regulation of hm2 receptors may occur through both GRK2-facilitating pathway and third intracellular loop-independent pathways.

Antibody to epitope tag induces internalization of human muscarinic subtype 1 receptor

We have studied the effect of an antibody against the epitope EYMPME on the internalization of the human muscarinic cholinergic receptor hm1 tagged with the epitope at the amino terminus. The antibody to the tag induces internalization of the hm1 receptor within minutes after exposure of human embryonic kidney 293 cells transfected with the tagged receptor. This antibody-induced internalization is reversible following removal of the antibody. In contrast to hm1 internalization induced by the agonist carbachol, internalization induced by antibody is not blocked by the muscarinic antagonist atropine. The mechanism of antibody-mediated internalization does not appear to involve receptor dimerization by the antibody, as Fab fragments derived from the antibody also induce internalization. The pathway of antibody-induced internalization, similar to the agonist-induced process, is mediated by clathrin-coated vesicles. Furthermore, antibody treatment does not result in any second messenger production, as measured by phosphoinositide accumulation. Our data show that internalization of a G protein-coupled receptor can be triggered by interaction of the amino terminus of the receptor with an exogenous ligand and can occur independently of second messenger production. This result suggests that the receptor can exist in multiple conformations, each mediating distinct downstream events.

A constitutively internalizing and recycling mutant of the mu-opioid receptor

Internalization and recycling of G protein-coupled receptors (GPCRs), such as the mu-opioid receptor, largely depend on agonist stimulation, whereas certain other receptor types recycle constitutively, e.g., the transferrin receptor. To investigate structural domains involved in mu-opioid receptor internalization, we constructed two truncation mutants bracketing a Ser/Thr-rich domain (354ThrSerSerThrIleGluGlnGlnAsn362) unique to the C-terminus of the mu-opioid receptor (mutants Trunc354 and Trunc363). Ligand binding did not differ substantially, and G protein coupling was slightly lower for these mu-receptor constructs, in particular for Trunc363. To permit localization of the receptor by immunocytochemistry, an epitope tag was added to the N-terminus of the wild-type and mutant receptors. Both the wild-type mu-opioid receptor and Trunc363 resided largely at the plasma membrane and internalized into vesicles upon stimulation with the agonist [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin. Internalization occurred into vesicles that contain transferrin receptors, as shown previously, as well as clathrin, but not caveolin. In contrast, even without any agonist present, Trunc354 colocalized in intracellular vesicles with clathrin and transferrin receptors, but not caveolin. On blocking internalization by hyperosmolar sucrose or acid treatment, Trunc354 translocated to the plasma membrane, indicating that the mutant internalized into clathrin-coated vesicles and recycled constitutively. Despite agonist-independent internalization of Trunc354, basal G protein coupling was not elevated, suggesting distinct mechanisms for coupling and internalization. Furthermore, a portion of the C-terminus, particularly the Ser/Thr domain, appears to suppress mu-receptor internalization, which can be overcome by agonist stimulation. These results demonstrate that a mutant GPCR can be constructed such that internalization, normally an agonist-dependent process, can occur spontaneously without concomitant G protein activation.

Residues specifically involved in down-regulation but not internalization of the m1 muscarinic acetylcholine receptor

Human m1 muscarinic acetylcholine receptor mutants were screened to determine receptor domains and cellular pathways relevant to down-regulation. Mutations in the second intracellular loop and the junctions of the third intracellular loop of the receptor, where a role for receptor activation or internalization had been previously demonstrated in HEK293 cells, were selected for this study. To assess receptor down-regulation, the m1 receptor mutants were transfected into Chinese hamster ovary cells. Because receptor internalization is expected to precede down-regulation, mutants displaying intact internalization were selected to permit interpretation of mutational effects on down-regulation alone. Four mutations were identified that specifically impaired down-regulation without altering receptor internalization: V127A, I211A, E360A, and K362A. The results define new receptor domains in the second intracellular loop and the junctions of the third intracellular loop that are involved in down-regulation. These same four mutants were also defective in signaling via the phospholipase C and the adenylyl cyclase pathways and in G protein activation, as measured by [35S]GTP gamma S binding. However, the level of second messenger stimulation correlated poorly with the extent of down-regulation. In summary, several mutations of the m1 receptor selectively affect down-regulation, demonstrating that internalization and down-regulation represent distinct events driven by different cellular mechanisms.

Human muscarinic cholinergic receptor Hm1 internalizes via clathrin-coated vesicles

The mechanism by which muscarinic receptors internalize upon agonist exposure is poorly understood. To determine the endocytic pathways responsible for muscarinic receptor internalization, we have stably transfected human embryonic kidney (HEK 293) cells with the Hm1 (human muscarinic subtype 1) receptor tagged at the amino terminus with the epitope EYMPME. The subcellular location of the receptor was visualized by immunofluorescence confocal microscopy and quantified with the use of binding studies. The receptor redistributed into intracellular compartments following agonist treatment. This process was reversible upon removal of agonist and inhibited by antagonist. Acid treatment of the cells, which disrupts internalization via clathrin-coated vesicles, inhibited carbachol-stimulated internalization. Phorbol 12-myristate 13-acetate, on the other hand, which inhibits caveolae-mediated endocytosis, had no effect on carbachol-induced endocytosis. Double-labeling confocal microscopy was used to characterize the intracellular vesicles containing Hm1 receptor following agonist treatment. The Hm1 receptor was shown to be colocalized with clathrin and alpha-adaptin, a subunit of the AP2 adaptor protein which links endocytosed proteins with clathrin in the intracellular vesicles. In addition, endosomes containing Hm1 also contained the transferrin receptor, which internalizes via clathrin-coated vesicles. In contrast, caveolin, the protein that comprises caveolae, did not colocalize with Hm1 in intracellular vesicles following agonist treatment, indicating that caveolae are not involved in the agonist-induced internalization of Hm1. These results indicate that agonist-induced internalization of the Hm1 receptor occurs via clathrin-coated vesicles in HEK cells.

Phosphorylation and agonist-specific intracellular trafficking of an epitope-tagged mu-opioid receptor expressed in HEK 293 cells

We expressed the cloned mu-opioid receptor (muR) in high abundance (5.5 x 10(6) sites/cell) with an amino-terminal epitope tag (EYMPME) in human embryonic kidney 293 cells. The epitope-tagged receptor (EE-muR) was similar to the untagged mu R ligand binding and agonist-dependent inhibition of cyclic AMP accumulation. By confocal microscopy, the labeled receptor was shown to be largely confined to the plasma membrane. Pretreatment with morphine failed to affect the cellular distribution of the receptor as judged by immunofluorescence and tracer binding studies. In contrast, exposure to the mu-specific peptide agonist [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAMGO) caused strong labeling of endocytic vesicles, indicating extensive agonist-induced cellular redistribution of EE-muR. Tracer binding studies suggested partial net internalization and a small degree of down-regulation caused by DAMGO. EE-muR-containing membranes were solubilized in detergent [3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate] and immunoprecipitated by an anti-epitope monoclonal antibody. Immunoblotting revealed a prominent band at approximately 70 kDa with weaker bands at approximately 65 kDa. EE-muR was labeled with [gamma-32P]ATP in permeabilized cells, immunoprecipitated, and analyzed by polyacrylamide gel electrophoresis autoradiography. A prominent band at 65-70 kDa indicated the presence of basal receptor phosphorylation occurring in the absence of agonist, which was enhanced approximately 1.8-fold with the addition of morphine. In conclusion, intracellular trafficking of the muR appears to depend on the agonist, with morphine and DAMGO having markedly different effects. Unlike other G protein-coupled receptors, basal phosphorylation is substantial, even in the absence of agonist.

Activating and inactivating mutations in N- and C-terminal i3 loop junctions of muscarinic acetylcholine Hm1 receptors

The N- and C-terminal junctions of the third intracellular loop (i3) of G protein-coupled receptors play a role in the coupling process. We had previously constructed two triple point alanine mutants of the i3 junction of the muscarinic Hm1 receptor, W209A/I211A/Y212A and E360A/K362A/T366A, which are defective in mediating carbachol stimulation of phosphatidylinositol (PI) turnover (Moro, O., Lameh, J., Högger, P., and Sadée, W. (1993) J. Biol. Chem. 268, 22273-22276). Each of the corresponding six single point mutations were constructed to determine residues crucial to receptor coupling. Mutants W209A and T366A were similar to or only slightly less effective than wild type Hm1 in stimulating PI turnover. In the N-terminal junction, I211A and Y212A were defective in coupling, and I211A was even more defective than the corresponding triple mutant. Therefore, the triple mutation compensated at least partially for the effect of these two single point mutations. In the C-terminal i3 loop junction, mutant K362A was again more strongly defective than the corresponding triple mutant. In contrast, mutation E360A was found to be activating, leading to elevated PI turnover in the absence of agonist and sensitization toward carbachol activation. Activating mutations in the C-terminal i3 loop junction have been reported previously for the adrenergic receptors, but E360A represents the first muscarinic receptor with substantial basal activity. The effects of the single point mutations observed in this study were not readily predictable from similar mutations from closely related G protein-coupled receptors despite sequence conservation in the i3 loop junctions. Our results caution against defining precise coupling domains in these regions by mutagenesis results.

Overlapping multi-site domains of the muscarinic cholinergic Hm1 receptor involved in signal transduction and sequestration

Alanine mutagenesis scanning of the intracellular portion of the human muscarinic cholinergic Hm1 receptor was performed to identify domains mediating agonist induced receptor sequestration. Using these multiple alanine point mutants of Hm1, we had previously identified several receptor domains in the intracellular loops i1-3 that play a role in coupling to phosphatidyl inositol turnover, most notably, a lipophilic residue, Leu-131, in the conserved i2 loop domain DRYXXVXXPL (Moro, O., Lameh, J., Hogger, P., and Sadée, W. (1993) J. Biol. Chem. 268, 6862-6865). We now demonstrate that alanine substitutions in three of these domains, i.e. middle of the i2 loop and both junctions of the i3 loop, also result in defective sequestration (loss of surface receptor sites accessible to a polar tracer) in transfected human kidney U293 cells. The i2 loop was studied further by single point mutations. The strongest impairment of sequestration occurred with mutant L131A which was also highly defective in phosphatidyl inositol (PI) coupling. Substitution of Leu-131 with several distinct amino acids indicated that a bulky lipophilic residue is required for sequestration in this position, as shown for coupling to PI turnover. Further, the double point mutation, V127A/L131A, almost completely suppressed both sequestration and coupling of Hm1. In the beta 2 adrenoceptor, alanine substitution of the i2 residue Phe-139, equivalent to Leu-131 in Hm1, also resulted in impaired coupling to adenylyl cyclase and sequestration, indicating a general role for this conserved i2 loop residue in both processes. The combined results show that the multi-site domain involved in signal transduction of Hm1 is similar to and overlaps with that involved in sequestration. However, three Hm1 mutants that were moderately deficient in stimulating PI turnover displayed normal sequestration, suggesting distinct mechanisms. We propose that cellular mediators of receptor sequestration are structurally similar or identical to the heterotrimeric G proteins.

Hydrophobic amino acid in the i2 loop plays a key role in receptor-G protein coupling

Signal transduction of the heptahelical G protein-coupled receptors (GPCRs) involves multiple receptor domains, but a universal consensus domain for coupling has not yet been defined. Alanine mutagenesis scanning was performed on the intracellular loops and the COOH tail of the human muscarinic cholinergic receptor (Hm1) to identify coupling domains. Stimulation of phosphatidylinositol (PI) turnover was determined after transfection of the alanine mutants into U293 human embryonic kidney cells. Alanine substitutions in four regions (loops i1, i2, and NH2 and COOH junctions of i3) impaired coupling efficiency by approximately 50% or more, but the strongest reduction (> 80%) resulted from alanine replacement of a single amino acid, leucine 131. This residue is located in the middle of the second intracellular loop (i2), within the highly conserved GPCR motif (DRYXXV(I)XXPL). The position equivalent to Leu-131 in Hm1 contains a bulky hydrophobic amino acid (L, I, V, M, or F) in nearly all cloned GPCRs. Substitution of Leu-131 with polar amino acids (aspartate and asparagine) also resulted in strongly defective coupling, whereas phenylalanine (found in the equivalent position in the beta 2 adrenoceptor) can replace leucine without losing PI coupling ability of Hm1. Alanine substitution of the corresponding amino acid in the Hm3 receptor (L174A) also inhibited agonist-stimulated PI turnover, while replacing Phe-139 with alanine in the beta 2 adrenoceptor suppressed stimulation of adenylyl cyclase. We propose that a bulky hydrophobic amino acid in the middle of the i2 loop serves as a general site relevant to G protein coupling, whereas coupling selectivity is governed by other receptor domains.

Serine- and threonine-rich domain regulates internalization of muscarinic cholinergic receptors

Upon agonist exposure, most membrane receptors internalize into the cell as part of an adaptation process. Receptor domains that mediate internalization have been defined for several receptor classes, e.g. growth factor and transport receptors, but not yet for any of the numerous members of the family of G protein-coupled receptors (GPCRs), having seven putative transmembrane helices. With the use of deletion mutations, we previously showed that a small region in the middle of the third intracellular loop (i3) appears to be required for the agonist-induced internalization of the Hm1 muscarinic cholinergic receptor (Lameh, J., Philip, M., Sharma, Y. K., Moro, O., Ramachandran, J., and Sadée, W. (1992) J. Biol. Chem. 267, 13406-13412). Using point mutations, we now demonstrate that domain 286-292 (ESLTSSE) is required, with TSS playing a crucial role. Although the i3 loops of GPCRs share minimal overall sequence identity, even among closely related subtypes, small domains containing multiple S/T residues are also present in several other GPCRs, e.g. peptide, catechol, and all the muscarinic cholinergic receptor subtypes. S/T-->A mutations in analogous i3 domains also prevented receptor internalization in the case of Hm3 and attenuated internalization of Hm2. We propose that an S/T-rich domain in the i3 loop, possibly via phosphorylation reactions, regulates one pathway of GPCR internalization.