Patent Publication Number: US-2009226949-A1

Title: Single-cell biosensor for the measurement of GPCR ligands in a test sample

Description:
This application is a Continuation of U.S. patent application Ser. No. 10/161,916, filed Jun. 4, 2002, which claims priority under 35 U.S.C. § 119(e) to U.S. Ser. No. 60/295,945 filed Jun. 5, 2001, and is a continuation-in-part claiming priority under 35 U.S.C. § 120 to U.S. Ser. No. 09/631,468 filed Aug. 3, 2000, which is a continuation of U.S. Ser. No. 09/233,530 filed on Jan. 20, 1999, now U.S. Pat. No. 6,110,693, which is a continuation of U.S. Ser. No. 08/869,568 filed on Jun. 5, 1997, now U.S. Pat. No. 5,891,646, the contents of which are hereby incorporated by reference in their entireties. 
    
    
     This work was supported by National Institutes of Health Grants DK 02544, HL 61365, and NS 19576, and therefore the government may have certain rights to the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods of analyzing the presence and concentration of a GPCR ligand in test samples, including biological and environmental samples. Preferably, the present invention relates to the detection of multiple GPCR ligands in a test sample, wherein the test sample may be heterogeneous. The present invention provides improved methods of disease diagnosis, as well as detection of harmful chemicals, such as insecticides, neurotoxins, and chemicals used in bioterrorism. 
     BACKGROUND 
     G protein-coupled receptors (GPCRs) are cell surface proteins that translate hormone or ligand binding into intracellular signals. GPCRs are found in all animals, insects, and plants. GPCR signaling plays a pivotal role in regulating various physiological functions including phototransduction, olfaction, neurotransmission, vascular tone, cardiac output, digestion, pain, and fluid and electrolyte balance. Although they are involved in various physiological functions, GPCRs share a number of common structural features. They contain seven membrane domains bridged by alternating intracellular and extracellular loops and an intracellular carboxyl-terminal tail of variable length. 
     GPCRs have been implicated in a number of disease states, including, but not limited to cardiac indications such as angina pectoris, essential hypertension, myocardial infarction, supraventricular and ventricular arrhythmias, congestive heart failure, atherosclerosis, renal failure, diabetes, respiratory indications such as asthma, chronic bronchitis, bronchospasm, emphysema, airway obstruction, upper respiratory indications such as rhinitis, seasonal allergies, inflammatory disease, inflammation in response to injury, rheumatoid arthritis, chronic inflammatory bowel disease, glaucoma, hypergastrinemia, gastrointestinal indications such as acid/peptic disorder, erosive esophagitis, gastrointestinal hypersecretion, mastocytosis, gastrointestinal reflux, peptic ulcer, Zollinger-Ellison syndrome, pain, obesity, bulimia nervosa, depression, obsessive-compulsive disorder, organ malformations (for example, cardiac malformations), neurodegenerative diseases such as Parkinson&#39;s Disease and Alzheimer&#39;s Disease, multiple sclerosis, Epstein-Barr infection and cancer. 
     The magnitude of the physiological responses controlled by GPCRs is linked to the balance between GPCR signaling and signal termination. The signaling of GPCRs is controlled by a family of intracellular proteins called arresting. Arrestins bind activated GPCRs, including those that have been agonist-activated and especially those that have been phosphorylated by G protein-coupled receptor kinases (GRKs). 
     The abnormal regulation of hormones that bind to G protein-coupled receptors underlies the pathogenesis of many diseases. The ability to measure serum and tissue levels of these regulators, while clinically and scientifically desirable, is presently limited to very specialized biochemical and immunochemical assays. 
     Altered concentrations of a GPCR ligand in a biological sample may be indicative of a disease state. Altered concentrations of a GPCR ligand in an environmental sample may indicate the presence of harmful chemicals. There is a need for highly sensitive and specific methods for the quantitative detection of GPCR ligands in a heterogeneous sample, as well as methods for the detection of the multiple bioactive isoforms of a GPCR ligand in a heterogeneous sample. Sensitive, rapid methods of analyzing the presence of GPCR ligands in heterogeneous samples, both biological and environmental, would improve disease diagnosis and the detection of harmful compounds in the environment. 
     SUMMARY 
     A first aspect of the present invention is a method of detecting a GPCR ligand in a test sample. The test sample is a biological sample, an environmental sample, or a sample derived from a biological sample or an environmental sample. Preferably, a cell is provided that includes at least one GPCR and an arrestin. The test sample is provided and the cell is exposed to the test sample. The cellular distribution of the GPCR or arrestin in the presence of the test sample is determined. The cellular distribution of the GPCR or the arrestin in the presence of the test sample may be compared to the cellular distribution of the GPCR or arrestin in the absence of the test sample. 
     In one aspect of the present invention, the GPCR or the arrestin is detectably labeled, another endogenous molecule is detectably labeled, or another exogenous molecule is detectably labeled. The distribution of the detectably labeled molecules represents the cellular distribution of the GPCR or the arrestin proteins. 
     In a further aspect, the cellular distribution of the GPCR or arrestin is determined at different time points after exposure to the test sample. The cellular distribution of the GPCR or arrestin is determined after exposure to different concentrations of the test sample. The cellular distribution of the detectably labeled molecules may be quantified. 
     In an additional aspect, the concentration of the ligand in the test sample is quantified by comparing the cellular distribution of the GPCR of arrestin in the presence of the test sample to the cellular distribution of the GPCR or arrestin in the presence of a known concentration of the ligand. 
     The biological sample provided as the test sample may be serum, tissue, blood, urine, or derived therefrom. 
     In a further aspect, the GPCR is CCK-B or CCK-A. The ligand may be gastrin, preprogastrin, cleaved preprogastrin, gastrin-34, gastrin-17, pentagastrin, progastrin, glycine-extended gastrin-17, glycine-extended gastrin-34, gastrin-71, gastrin-6, hG17, a compound with an amidated tetrapeptide of the sequence Trp-Met-Asp-Phe-NH 2 , or another bioactive isoform of gastrin. 
     In a further aspect, the GPCR is a muscarinic receptor. The ligand may be acetylcholine. 
     In an additional aspect, the labeled molecule may be localized in the cytosol, plasma membrane, clathrin-coated pits, endocytic vesicles, or endosomes. An increase in the local concentration of the labeled molecule results in an increase in the local signal intensity. The signal intensity of the labeled molecule in the plasma membrane, clathrin-coated pits, endocytic vesicles, or endosomes may be increased with respect to the level of signal intensity in the cytosol. The local signal intensity may be increased in the presence of increased concentration of ligand in the test sample. 
     In a further aspect, the concentration of the ligand in the test sample indicates a disease state. The concentration of the ligand in the test sample may indicate the presence of a compound in the test sample that alters the ligand concentration. The concentration of the ligand in the test sample indicates the presence of a compound in the test sample that modifies acetylcholine. The concentration of the ligand in the test sample indicates the presence of a compound that inhibits an acetylcholinesterase. 
     The detectable molecule may be a radioisotope, an epitope tag, an affinity label, an enzyme, a fluorescent group, or a chemiluminescent group. The molecule may be detectably labeled due to its interaction with another molecule, which may be detectable labeled. 
     The present invention relates to a method of monitoring a GPCR ligand in a mammal. The test sample is a biological sample derived from the mammal. Preferably, a cell is provided that includes at least one GPCR and an arrestin. The test sample is provided and the cell is exposed to the test sample. The cellular distribution of the GPCR or arrestin in the presence of the test sample is determined. The concentration of the ligand in the test sample may be quantified by comparing the cellular distribution of the GPCR or arrestin in the presence of the test sample to the cellular distribution of the GPCR or arrestin in the presence of a known concentration of the ligand. This method may be used to monitor a clinical condition, which may indicate the presence of a disease state, or may indicate that the subject has a disorder or is at risk for developing a disorder. The clinical condition monitored may be gastrointestinal cancer, hypergastrinemia, atrophic gastritis, gastric ulcers, malignant tumors, or other GPCR-related disease. The mammal may be on prolonged acid suppressive medications. 
     In a further aspect of the present invention, the provided cell may express a protein that increases the internalization of the GPCR. The GPCR may itself be modified, resulting in an increased concentration of the labeled molecule at the plasma membrane, clathrin-coated pits, endocytic vesicles, or endosomes. The provided cell may express a G protein-coupled receptor kinase (GRK). 
     One aspect of the present invention is a single cell biosensor. This biosensor includes a cell which overexpresses arrestin and at least one GPCR, wherein the GPCR, the arrestin, or the cell is detectably labeled for monitoring internalization of the GPCR. A further aspect of the present invention is a method of detecting a GPCR ligand in a test sample, wherein the test sample is a biological sample, an environmental sample, or a sample derived therefrom. In this method, the single cell biosensor is provided, the biosensor is exposed to the test sample, and the cellular distribution of the GPCR or arrestin in the presence of the test sample is determined. In the single cell biosensor, the GPCR may be a CCK-A, a CCK-B, or a muscarinic receptor. The arrestin may be conjugated to a Green Fluorescent Protein. The biosensor may have increased sensitivity due to longer incubation time, increased concentration of test sample, GPCR mutation, or GPCR antibodies. 
     The present invention is related to a method of altering GPCR internalization, comprising providing to the cells an effective amount of an antagonist of CCK-B. 
     In the methods of the present invention, the cellular distribution may be visualized by flow cytometry or fluorescence confocal microscopy. A computer may analyze an image of the cellular distribution and the distribution may be quantified. The test sample to be analyzed may comprise a ligand of the GPCR, or an antagonist of the GPCR. 
     The present invention is related to a method of detecting a compound which modulates a GPCR ligand in a test sample. The test sample is a biological sample, an environmental sample, or a sample derived from a biological sample or an environmental sample. Preferably, a cell is provided that includes at least one GPCR and an arrestin. The test sample is provided and the cell is exposed to the test sample. The cellular distribution of the GPCR or arrestin in the presence of the test sample is determined. The cellular distribution of the GPCR or the arrestin in the presence of the test sample may indicate the presence of a compound which modulates a GPCR ligand. 
     The present invention is related to a method of detecting a compound which modulates a GPCR ligand in a test sample. The test sample is a biological sample, an environmental sample, or a sample derived from a biological sample or an environmental sample. Preferably, a cell is provided that includes at least one GPCR and an arrestin. The test sample is provided and the cell is exposed to the test sample. The cellular distribution of the GPCR or arrestin in the presence of the test sample is determined. 
     The present invention is related to a method of continuous screening of GPCR ligands in a test sample. The test sample is a biological sample, an environmental sample, or a sample derived from a biological sample or an environmental sample. Preferably, a cell is provided that includes at least one GPCR and an arrestin. The test sample is provided and the cell is exposed to the test sample. The cellular distribution of the GPCR or arrestin in the presence of the test sample is determined. Then, the cell is replaced with another cell comprising a GPCR and an arrestin. 
     A further aspect of the present invention is a method of detecting an inhibitor of acetylcholinesterase in a test sample. The test sample is a biological sample, an environmental sample, or a sample derived from a biological sample or an environmental sample. Preferably, a cell is provided that expresses a muscarinic receptor and an arrestin. A mixture, containing a test sample, acetylcholinesterase, and an agonist of the muscarinic receptor, is provided. The agonist is sensitive to acetylcholinesterase. The cell is exposed to the mixture. The cellular distribution of the muscarinic receptor or arrestin in the presence of the test sample is determined. The agonist may be acetylcholine. 
     In an aspect of the present invention, the test sample may contain acetylcholine and acetylcholinesterase. The test sample may contain an agonist. The ligand may have been identified, and multiple bioactive isoforms of the GPCR ligand in the test sample may be detected. 
     In a further aspect of the invention, the test sample may be derived from a mammal with hypergastrinemia. The gastrin concentration in the test sample may be less than 10 nM. The test sample may be heterogeneous. 
     In an aspect of the present invention, the cellular distribution may determined after 15-30 minutes of exposure to the test sample. The cellular distribution may be determined after 1 hour of exposure to the test sample. The cell may be exposed to the test sample at a temperature of approximately 37° C. 
     The present invention is related to a method of detecting a compound that modulates GPCR internalization in a test sample. The test sample is a biological sample, an environmental sample, or a sample derived from a biological sample or an environmental sample. Preferably, a cell is provided that includes at least one GPCR and an arrestin. The test sample is provided and the cell is exposed to the test sample. The cellular distribution of the GPCR or arrestin in the presence of the test sample is determined. 
     In a further aspect, the present invention is related to a method of detecting a compound that modulates GPCR internalization in a test sample. The test sample is a biological sample, an environmental sample, or a sample derived from a biological sample or an environmental sample. Preferably, a cell is provided that includes at least one GPCR and an arrestin. The cell is exposed to an agonist. The test sample is provided and the cell is exposed to the test sample. The cellular distribution of the GPCR or arrestin in the presence of the test sample is determined. 
     A further aspect of the present invention is a bioarray containing at least one single cell biosensor. The bioarray may detect multiple GPCR ligands. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       The objects and advantages of the invention will be understood by reading the following detailed description in conjunction with the drawings in which: 
         FIG. 1  is an illustrative, non-limiting list of known GPCRs with which the present invention may be used is contained in  FIG. 1 . The receptors are grouped according to classical divisions based on structural similarities and ligands. 
         FIG. 2  lists some of the over 40 different GPCRs that may associate with arrestin and subsequently internalize. This may be visualized using expressed GPCRs and fusion proteins between arrestin and a green fluorescent protein. 
         FIG. 3A  illustrates the amino acid and nucleic acid sequences of the homo sapiens muscarinic receptor 1, Accession NM — 000738.  FIG. 3B  illustrates the amino acid and nucleic acid sequences of the homo sapiens muscarinic receptor 2, Accession NM — 000739.  FIG. 3C  illustrates the amino acid and nucleic acid sequences of the homo sapiens muscarinic receptor 3, Accession NM — 000740.  FIG. 3D  illustrates the amino acid and nucleic acid sequences of the homo sapiens muscarinic receptor 4, Accession NM — 000741.  FIG. 3E  illustrates the amino acid and nucleic acid sequences of the homo sapiens acetylcholinesterase (YT blood group), Accession XM — 36148.  FIG. 3F  illustrates the amino acid and nucleic acid sequences of the human cholecystokinin A receptor, Accession L13605.  FIG. 3G  illustrates the amino acid and nucleic acid sequences of the homo sapiens cholecystokinin B receptor, Accession NM — 000731. Amino acid sequences are listed in the amino-terminal to carboxy-terminal orientation. Nucleic acid sequences are listed in the 5′ 3′ orientation. 
         FIG. 4  illustrates the uniformity of arrestin-GFP and CCK-B receptor expression in cells by flow cytometry and arrestin-GFP translocation.  FIG. 4A  shows the relative expression of arrestin-GFP in cells belonging to Clone A.  FIG. 4B  shows fluorescence images of a field of cells from Clone A before (left panel) and after treatment (right panel) with 10 nM hG17 for 5 minutes at room temperature. 
         FIG. 5  shows the characterization of ligand binding and second messenger response in a cell line expressing arrestin-GFP and the CCK-B receptor. As shown in  FIG. 5A , cells from Clone A were incubated with increasing concentrations of [ 3 H]CCK-8 in order to determine the average CCK-B receptor expression per cell and the receptor affinity for [ 3 H]CCK-8.  FIG. 5B  shows that Clone A cells were exposed to increasing concentrations of hG17 peptide in order to evaluate the IP3 second messenger response. The inset shows the competitive displacement of [ 3 H]CCK8 by hG17 from this cell line.  FIG. 5C  shows the fluorescence images of cells from Clone A that were treated with vehicle (upper left panel), or treated for one hour with 10 nM of the agonist hG17 (upper right panel), or with 10 nM hG17 plus 1 μM of the CCK-A antagonist devazepide (L-364,718, lower left panel); or with 10 nM hG17 plus 1 μM, of the CCK-B antagonist (lower right panel). 
         FIG. 6  illustrates the dose response to hG17 at five minutes in an HEK-293 cell line containing arrestin-GFP and the CCK-B receptor.  FIG. 6A  is a representative experiment depicting the arrestin-GFP translocation of Clone A cells that were exposed to various concentrations of hG17 for 5 minutes. In  FIG. 6B , the fractional amount of arrestin-GFP lost after 5 minutes from the cell cytosol was used to generate a sigmoid dose response curve for the increasing concentrations of hG17 shown in the graph in A.  FIG. 6C  illustrates images from an experiment demonstrating the response of Clone A cells exposed to hG17 for one hour at 37° C. 
         FIG. 7  shows the calculation of the Fluorescence Signal from the Distribution of βarrestin2-GFP.  FIG. 7A  shows the distribution of βarrestin-GFP fluorescence in cells stably expressing the βarrestin2-GFP fusion protein and receptor was visualized before and after a 30-minute agonist treatment. In  FIG. 7B , a histogram of the pixel count versus pixel intensity (green curve in the graph at the lower left) was generated using a representative control cell. 
         FIG. 8  shows the dose response to pentagastrin in an HEK-293 cell line containing arrestin-GFP and the CCK-B receptor.  FIG. 8A  shows images from a representative experiment depicting the response of Clone A cells that were exposed to pentagastrin at 37° C. for two hours. The graph in  FIG. 8B  depicts the increase in the normalized sum of pixel intensity (TI/TF) above a threshold value (Methods) for images obtained at each concentration of ligand. 
         FIG. 9  shows the dose response at one hour in a clonal cell line containing arrestin-GFP and the CCK-B receptor.  FIG. 9A  illustrates the representative image of Clone A cells incubated for one hour with a 1:1 dilution of serum that was obtained from a patient with hypergastrinemia.  FIG. 9B  shows the dose response curve to hG17 of Clone A cells computed from the imaged translocation data obtained at one hour and analyzed as in  FIG. 7 . Between 9 and 16 separate images were analyzed for each hG17 concentration and the patient&#39;s serum (arrow). 
         FIG. 10  illustrates the Internalization of muscarinic receptor in present of acetylcholine. HEK-293 cells expressing arrestin-GFP and the human muscarinic receptor type 1 conjugated with the Vasopressin carboxyl-terminal tail were exposed to micromolar concentrations of acetylcholine. Arrestin-GFP was observed at the membrane edge or in vesicles, in response to acetylcholine. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention there may be employed conventional molecular biology, microbiology, immunology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Sambrook et al, “Molecular Cloning: A Laboratory Manual” (3 rd  edition, 2001); “Current Protocols in Molecular Biology” Volumes I-IV [Ausubel, R. M., ed. (2002 and updated bimonthly)]; “Cell Biology: A Laboratory Handbook” Volumes I-III [J. E. Celis, ed. (1994)]; “Current Protocols in Immunology” Volumes I-IV [Coligan, J. E., ed. (2002 and updated bimonthly)]; “Oligonucleotide Synthesis” (M. J. Gait ed. 1984); “Nucleic Acid Hybridization” [B. D. Hames &amp; S. J. Higgins eds. (1985)]; “Transcription And Translation” [B. D. Hames &amp; S. J. Higgins, eds. (1984)]; “Culture of Animal Cells, 4 th  edition” [R. I. Freshney, ed. (2000)]; “Immobilized Cells And Enzymes” [IRL Press, (1986)]; B. Perbal, “A Practical Guide To Molecular Cloning” (1988);  Using Antibodies: A Laboratory Manual: Portable Protocol No. I , Harlow, Ed and Lane, David (Cold Spring Harbor Press, 1998);  Using Antibodies: A Laboratory Manual , Harlow, Ed and Lane, David (Cold Spring Harbor Press, 1999); “G Protein-Coupled Receptors” [T. Haga, et al., eds. (1999)]. 
     Unless otherwise stated, the following terms used in the specification and claims have the meanings given below: 
     “hG17” is the human gastrin-17 amino acid peptide. It may be produced in a human, another organism, such as  E. coli , yeast, mouse, or it may be synthesized chemically. 
     “RIA”, or radioimmunoassay, is an antibody-based method of detecting a particular compound in a sample. Presently, serum gastrin measurements are performed by RIA using antibodies directed against one or more distinct gastrin isoforms. 
     A “neurotoxin” is any compound that has the ability to damage or destroy nerve tissues. Of particular relevance to the present invention are compounds which inhibit acetylcholinesterase. Normally, acetylcholinesterase breaks down acetylcholine, a natural ligand of the muscarinic receptor. Nerve toxins which inhibit acetylcholinesterase prevent the normal degradation of acetylcholine. The present invention can be used to detect the presence of nerve toxins which inhibit acetylcholinesterase by detecting the acetylcholine concentration in a sample. Compounds that inhibit acetylcholinesterase include organophosphate insecticides such as diazinon and the neurotoxin sarin. 
     “Acetylcholine” is a neurotransmitter and functions at least at neuromuscular synapses, which are synapses between neurons and cardiac, smooth, and skeletal muscle, as well as at a variety of neuron-neuron synapses in the central and peripheral nervous systems. It is synthesized in nerve terminals from acetylCoA and choline, in a reaction catalyzed by the enzyme choline acetyltransferase. 
     A “muscarinic receptor” is a GPCR which is located at least in many brain neurons, sympathetic neurons, smooth muscle, gland cells and heart cells. Muscarinic receptor is meant to include muscarinic acetylcholine receptor, muscarinic cholinergic receptor, other references for muscarinic receptors, including sub-types 1, 2, 3, 4, and other sub-types known to those of skill in the art. The term muscarinic receptor includes, but is not limited to, muscarinic receptor sequences of homo sapiens, eukaryota, metazoa, chordata, craniata, vertebrata, euteleostomi, mammalia, eutheria, primates, catarrhini, homimidae, homo, and others. Acetylcholine is an agonist of the muscarinic receptor. 
     “Acetylcholinesterase” is the enzyme which degrades acetylcholine into acetate and choline. This enzyme is clustered at high concentrations in the synaptic cleft. 
     An “acetylcholinesterase inhibitor” is a compound that inhibits the activity of acetylcholinesterase. Compounds that inhibit acetylcholinesterase include organophosphate insecticides such as diazinon and the neurotoxin sarin. 
     “Insecticides” include compounds which are nerve toxins. Insecticides, including organophosphate insecticides such as diazinon, may be acetylcholinesterase inhibitors. 
     A “bioassay” is the use of a physiological response to assay for a biologically active compound. 
     A “biosensor” utilizes a biological process or component to detect the presence of compound. The single-cell biosensors of the present invention are cells which include a GPCR and an arrestin. By exposing the cells to a heterogeneous sample and monitoring the GPCR or arrestin response to the sample, they are useful for the detection of a GPCR ligand in a heterogeneous sample. 
     “ZE syndrome” is Zollinger-Ellison syndrome, which is caused by a gastrin producing tumor. 
     “Biological sample” is intended to include tissues, cells and/or biological fluids isolated from a subject, as well as tissues, cells and fluids present within a subject; wherein said sample can be blood, serum, a urine sample, a fecal sample, a tumor sample, a cellular wash, an oral sample, sputum, biological fluid, a tissue extract, freshly harvested cells, or cells which have been incubated in tissue culture. The biological sample may be selected from the group consisting of whole blood, serum, plasma, saliva, urine, sweat, ascitic fluid, peritoneal fluid, synovial fluid, amniotic fluid, cerebrospinal fluid, skin biopsy, and the like. The biological sample may includes serum, whole blood, plasma, lymph and ovarian follicular fluid as well as other circulatory fluid and saliva, mucus secretion, and respiratory fluid or fractionated portions thereof. The sample may be extracted, untreated, treated, diluted or concentrated from a patient. 
     “Biologically active” and “bioactive” are used interchangeably herein to refer to a compound, compound fragment, or compound isoform which has biological activity. Preferably, biologically active or bioactive is used to describe a GPCR ligand, or ligand isoforms, which have the ability to bind a GPCR. 
     A “replicon” is any genetic element (e.g., plasmid, chromosome, virus) that functions as an autonomous unit of DNA replication in vivo; i.e., capable of replication under its own control. 
     A “vector” is a replicon, such as plasmid, phage or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment. 
     A “DNA molecule” refers to the polymeric form of deoxyribonucleotides (adenine, guanine, thymine, or cytosine) in its either single stranded form, or a double-stranded helix. This term refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alia, in linear DNA molecules (e.g., restriction fragments), viruses, plasmids, and chromosomes. In discussing the structure of particular double-stranded DNA molecules, sequences may be described herein according to the normal convention of giving only the sequence in the 5′ to 3′ direction along the nontranscribed strand of DNA (i.e., the strand having a sequence homologous to the mRNA). 
     An “origin of replication” refers to those DNA sequences that participate in DNA synthesis. 
     A DNA “coding sequence” is a double-stranded DNA sequence which is transcribed and translated into a polypeptide in vivo when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a start codon at the 5′ (amino) terminus and a translation stop codon at the 3′ (carboxyl) terminus. A coding sequence can include, but is not limited to, prokaryotic sequences, cDNA from eukaryotic mRNA, genomic DNA sequences from eukaryotic (e.g., mammalian) DNA, and even synthetic DNA sequences. A polyadenylation signal and transcription termination sequence will usually be located 3′ to the coding sequence. 
     Transcriptional and translational control sequences are DNA regulatory sequences, such as promoters, enhancers, polyadenylation signals, terminators, and the like, that provide for the expression of a coding sequence in a host cell. 
     A “promoter sequence” is a DNA regulatory region capable of binding RNA polymerase in a cell and initiating transcription of a downstream (3′ direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bounded at its 3′ terminus by the transcription initiation site and extends upstream (5′ direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter sequence will be found a transcription initiation site (conveniently defined by mapping with nuclease S1), as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. Eukaryotic promoters will often, but not always, contain “TATA” boxes and “CAT” boxes. Prokaryotic promoters contain Shine-Dalgarno sequences in addition to the −10 and −35 consensus sequences. 
     An “expression control sequence” is a DNA sequence that controls and regulates the transcription and translation of another DNA sequence. A coding sequence is “under the control” of transcriptional and translational control sequences in a cell when RNA polymerase transcribes the coding sequence into mRNA, which is then translated into the protein encoded by the coding sequence. 
     A “signal sequence” can be included before the coding sequence. This sequence encodes a signal peptide, N-terminal to the polypeptide, that communicates to the host cell to direct the polypeptide to the cell surface or secrete the polypeptide into the media, and this signal peptide is clipped off by the host cell before the protein leaves the cell. Signal sequences can be found associated with a variety of proteins native to prokaryotes and eukaryotes. 
     The term “oligonucleotide,” as used herein in referring to the probe of the present invention, is defined as a molecule comprised of two or more ribonucleotides, preferably more than three. Its exact size will depend upon many factors which, in turn, depend upon the ultimate function and use of the oligonucleotide. 
     The term “primer” as used herein refers to an oligonucleotide, whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product, which is complementary to a nucleic acid strand, is induced, i.e., in the presence of nucleotides and an inducing agent such as a DNA polymerase and at a suitable temperature and pH. 
     The primer may be either single-stranded or double-stranded and must be sufficiently long to prime the synthesis of the desired extension product in the presence of the inducing agent. The exact length of the primer will depend upon many factors, including temperature, source of primer and use of the method. For example, for diagnostic applications, depending on the complexity of the target sequence, the oligonucleotide primer typically contains 15-25 or more nucleotides, although it may contain fewer nucleotides. 
     The primers herein are selected to be “substantially” complementary to different strands of a particular target DNA sequence. This means that the primers must be sufficiently complementary to hybridize with their respective strands. Therefore, the primer sequence need not reflect the exact sequence of the template. For example, a non-complementary nucleotide fragment may be attached to the 5′ end of the primer, with the remainder of the primer sequence being complementary to the strand. Alternatively, non-complementary bases or longer sequences can be interspersed into the primer, provided that the primer sequence has sufficient complementarity with the sequence of the strand to hybridize therewith and thereby form the template for the synthesis of the extension product. 
     As used herein, the terms “restriction endonucleases” and “restriction enzymes” refer to bacterial enzymes, each of which cut double-stranded DNA at or near a specific nucleotide sequence. 
     A cell has been “transformed” by exogenous or heterologous DNA when such DNA has been introduced inside the cell. The transforming DNA may or may not be integrated (covalently linked) into chromosomal DNA making up the genome of the cell. In prokaryotes, yeast, and mammalian cells for example, the transforming DNA may be maintained on an episomal element such as a plasmid. With respect to eukaryotic cells, a stably transformed cell is one in which the transforming DNA has become integrated into a chromosome so that it is inherited by daughter cells through chromosome replication. This stability is demonstrated by the ability of the eukaryotic cell to establish cell lines or clones comprised of a population of daughter cells containing the transforming DNA. A “clone” is a population of cells derived from a single cell or common ancestor by mitosis. A “cell line” is a clone of a primary cell that is capable of stable growth in vitro for many generations. 
     Two DNA sequences are “substantially homologous” when at least about 75% (preferably at least about 80%, and most preferably at least about 90 or 95%) of the nucleotides match over the defined length of the DNA sequences. Sequences that are substantially homologous can be identified by comparing the sequences using standard software available in sequence data banks, or in a Southern hybridization experiment under, for example, stringent conditions as defined for that particular system. For example, stringent conditions may include hybridization 6×SSC or 6×SSPE at 68° C. for 1 hour to 3 days. Defining appropriate hybridization conditions is within the skill of the art. See, e.g., Sambrook et al, “Molecular Cloning: A Laboratory Manual” (3 rd  edition, 2001), supra; DNA Cloning, Vols. I &amp; II, supra; Nucleic Acid Hybridization, supra. 
     It should be appreciated that also within the scope of the present invention are DNA sequences having the same amino acid sequence as SEQ ID NO: 1, 3, 5, 7, 9, 11, and 13, but which are degenerate to SEQ ID NO: 1, 3, 5, 7, 9, 11, and 13. By “degenerate to” is meant that a different three-letter codon is used to specify a particular amino acid. 
     “Arrestin” means all types of naturally occurring and engineered variants of arrestin, including, but not limited to, visual arrestin (sometimes referred to as Arrestin 1), β arrestin 1 (sometimes referred to as Arrestin 2), and βarrestin 2 (sometimes referred to as Arrestin 3). 
     “βARK1” is a GRK termed β-adrenergic receptor kinase 1, also called GRK2. 
     “βAR” is a GPCR termed a β-adrenergic receptor. 
     “Gastrin receptors” are GPCRs, preferably CCK-A and CCK-B, that bind gastrin. CCK-A and CCK-B, the cholecystokinin A and B receptors, are GPCRs that bind gastrin, cholecystokinin, and similar ligands. 
     “Internalization” of a GPCR is the intracellular translocation of a GPCR. Internalization includes the translocation of a GPCR to clathrin-coated pits, endocytic vesicles, and endosomes. 
     “Carboxyl-terminal tail” means the carboxyl-terminal tail of a GPCR. The carboxyl-terminal tail of many GPCRs begins shortly after the conserved NPXXY motif that marks the end of the seventh transmembrane domain (i.e. what follows the NPXXY motif is the carboxyl-terminal tail of the GPCR). The carboxyl-terminal tail may be relatively long (approximately tens to hundreds of amino acids), relatively short (approximately tens of amino acids), or virtually non-existent (less than approximately ten amino acids). As used herein, “carboxyl-terminal tail” shall mean all three variants (whether relatively long, relatively short, or virtually non-existent). 
     “Class A receptors” preferably do not translocate arrestin to endocytic vesicles or endosomes in HEK-293 cells. 
     “Class B receptors” preferably do translocate arrestin to endocytic vesicles or endosomes in HEK-293 cells. 
     “DACs” mean any desensitization active compounds. Desensitization active compounds are any compounds that influence the GPCR desensitization mechanism by either stimulating or inhibiting the process. DACs influence the GPCR desensitization pathway by acting on any cellular component of the process, as well as any cellular structure implicated in the process, including but not limited to, arresting, GRKs, GPCRs, PI3K, AP-2 protein, clathrin, protein phosphatases, and the like. DACs may include, but are not limited to, compounds that inhibit arrestin translocating to a GPCR, compounds that inhibit arrestin binding to a GPCR, compounds that stimulate arrestin translocating to a GPCR, compounds that stimulate arrestin binding to a GPCR, compounds that inhibit GRK phosphorylation of a GPCR, compounds that stimulate GRK phosphorylation of a GPCR, compounds that inhibit protein phosphatase dephosphorylation of a GPCR, compounds that stimulate protein phosphatase dephosphorylation of a GPCR, compounds that regulate the release of arrestin from a GPCR, antagonists of a GPCR, inverse agonists and the like. DACs preferably inhibit or stimulate the GPCR desensitization process without binding to the same ligand binding site of the GPCR as traditional agonists and antagonists of the GPCR. DACs act independently of the GPCR, i.e., they do not have high specificity for one particular GPCR or one particular type of GPCRs. 
     “Detectable molecule” means any molecule capable of detection by spectroscopic, photochemical, biochemical, immunochemical, electrical, radioactive, and optical means, including but not limited to, fluorescence, phosphorescence, and bioluminescence and radioactive decay. Detectable molecules include, but are not limited to, GFP, luciferase, β-galactosidase, rhodamine-conjugated antibody, and the like. Detectable molecules include radioisotopes, epitope tags, affinity labels, enzymes, fluorescent groups, chemiluminescent groups, and the like. Detectable molecules include molecules which are directly or indirectly detected as a function of their interaction with other molecule(s). 
     “GFP” means Green Fluorescent Protein which refers to various naturally occurring forms of GFP which may be isolated from natural sources or genetically engineered, as well as artificially modified GFPs. GFPs are well known in the art. See, for example, U.S. Pat. Nos. 5,625,048; 5,777,079; and 6,066,476. It is well understood in the art that GFP is readily interchangeable with other fluorescent proteins, isolated from natural sources or genetically engineered, including but not limited to, yellow fluorescent proteins (YFP), red fluorescent proteins (RFP), cyan fluorescent proteins (CFP), blue fluorescent proteins, luciferin, UV excitable fluorescent proteins, or any wave-length in between. As used herein, “GFP” shall mean all fluorescent proteins known in the art. 
     “Unknown or Orphan Receptor” means a GPCR whose function and/or ligands are unknown. 
     “NPXXY motif” means a conserved amino acid motif that marks the end of the seventh transmembrane domain. The conserved amino acid motif begins with asparagine and proline followed by two unspecified amino acids and then a tyrosine. The two unspecified amino acids may vary among GPCRs but the overall NPXXY motif is conserved. 
     In referring to a polypeptide, “downstream” means toward a carboxyl-terminus of an amino acid sequence, with respect to the amino-terminus. In referring to a polynucleotide, “downstream” means in the 3′ direction. 
     In referring to a polypeptide, “upstream” means toward an amino-terminus of an amino acid sequence, with respect to the carboxyl-terminus. In referring to a polynucleotide, “upstream” means in the 5′ direction. 
     Amino acid substitutions may also be introduced to substitute an amino acid with a particularly preferable property. For example, a Cys may be introduced a potential site for disulfide bridges with another Cys. A His may be introduced as a particularly “catalytic” site (i.e., His can act as an acid or base and is the most common amino acid in biochemical catalysis). Pro may be introduced because of its particularly planar structure, which induces β-turns in the protein&#39;s structure. 
     Two amino acid sequences are “substantially homologous” when at least about 70% of the amino acid residues (preferably at least about 80%, and most preferably at least about 90 or 95%) are identical, or represent conservative substitutions. 
     A “heterologous” region of the DNA construct is an identifiable segment of DNA within a larger DNA molecule that is not found in association with the larger molecule in nature. Thus, when the heterologous region encodes a mammalian gene, the gene will usually be flanked by DNA that does not flank the mammalian genomic DNA in the genome of the source organism. Another example of a heterologous coding sequence is a construct where the coding sequence itself is not found in nature (e.g., a cDNA where the genomic coding sequence contains introns, or synthetic sequences having codons different than the native gene). Allelic variations or naturally-occurring mutational events do not give rise to a heterologous region of DNA as defined herein. Heterologous DNA may include, but is not limited to, DNA from a heterologous species (“foreign DNA”), as described in U.S. Pat. No. 6,331,415, which is incorporated by reference herein. 
     A DNA sequence is “operatively linked” to an expression control sequence when the expression control sequence controls and regulates the transcription and translation of that DNA sequence. The term “operatively linked” includes having an appropriate start signal (e.g., ATG) in front of the DNA sequence to be expressed and maintaining the correct reading frame to permit expression of the DNA sequence under the control of the expression control sequence and production of the desired product encoded by the DNA sequence. If a gene that one desires to insert into a recombinant DNA molecule does not contain an appropriate start signal, such a start signal can be inserted in front of the gene. 
     “Hybridization” means hydrogen bonding, which may be Watson-Crick, Hoogsteen or reversed Hoogsteen hydrogen bonding, between complementary nucleoside or nucleotide bases. For example, adenine (A) and thymine (T) are complementary nucleobases which pair through the formation of hydrogen bonds. 
     The term “standard hybridization conditions” refers to salt and temperature conditions substantially equivalent to 6×SSC and 68° C. for both hybridization and wash. However, one skilled in the art will appreciate that such “standard hybridization conditions” are dependent on particular conditions including the concentration of sodium and magnesium in the buffer, nucleotide sequence length and concentration, percent mismatch, percent formamide, and the like. Also important in the determination of “standard hybridization conditions” is whether the two sequences hybridizing are RNA-RNA, DNA-DNA or RNA-DNA. Such standard hybridization conditions are easily determined by one skilled in the art according to well known formulae, wherein hybridization is typically 10-20° C. below the predicted or determined Tm with washes of higher stringency, if desired. These conditions are described in Protocol 10 of Sambrook et al, “Molecular Cloning: A Laboratory Manual” (3 rd  edition, 2001). 
     By “animal” is meant any member of the animal kingdom including vertebrates (e.g., frogs, salamanders, chickens, or horses) and invertebrates (e.g., worms, etc.). “Animal” is also meant to include “mammals.” Preferred mammals include livestock animals (e.g., ungulates, such as cattle, buffalo, horses, sheep, pigs and goats), as well as rodents (e.g., mice, hamsters, rats and guinea pigs), canines, felines, primates, lupine, camelid, cervidae, rodent, avian and ichthyes. 
     “Antagonist(s)” include all agents that interfere with wild-type and/or modified GPCR binding to an agonist, wild-type and/or modified GPCR desensitization, wild-type and/or modified GPCR binding arrestin, wild-type and/or modified GPCR endosomal localization, internalization, and the like, including agents that affect the wild-type and/or modified GPCRs as well as agents that affect other proteins involved in wild-type and/or modified GPCR signaling, desensitization, endosomal localization, resensitization, and the like. 
     “GPCR” means G protein-coupled receptor and includes GPCRs naturally occurring in nature, as well as GPCRs which have been modified. Such modified GPCRs are described in U.S. Ser. No. 09/993,844 filed on Nov. 5, 2001 and U.S. Ser. No. 10/054,616 filed on Jan. 22, 2002 which is incorporated herein by reference in its entirety. 
     “Abnormal GPCR desensitization” and “abnormal desensitization” mean that the GPCR desensitization pathway is disrupted such that the balance between active receptor and desensitized receptor is altered with respect to wild-type conditions. There may be more active receptor than normal or there may be more desensitized receptor than wild-type conditions. Abnormal GPCR desensitization may be the result of a GPCR that is constitutively active or constitutively desensitized, leading to an increase above normal in the signaling of that receptor or a decrease below normal in the signaling of that receptor. 
     “Concurrent administration,” “administration in combination,” “simultaneous administration,” or “administered simultaneously” mean that the compounds are administered at the same point in time or sufficiently close in time that the results observed are essentially the same as if the two or more compounds were administered at the same point in time. 
     “Conserved abnormality” means an abnormality in the GPCR pathway, including but not limited to, abnormalities in GPCRs, GRKs, arresting, AP-2 protein, clathrin, protein phosphatase and the like, that may cause abnormal GPCR signaling. This abnormal GPCR signaling may contribute to a GPCR-related disease. 
     “Desensitized GPCR” means a GPCR that presently does not have ability to respond to agonist and activate conventional G protein signaling. Desensitized GPCRs of the present invention do not properly respond to agonist, are phosphorylated, bind arrestin, constitutively localize in clathrin-coated pits, and/or constitutively localize to endocytic vesicles or endosomes. 
     “Desensitization pathway” means any cellular component of the desensitization process, as well as any cellular structure implicated in the desensitization process and subsequent processes, including but not limited to, arresting, GRKs, GPCRs, AP-2 protein, clathrin, protein phosphatases, and the like. In the methods of assaying of the present invention, the polypeptides may be detected, for example, in the cytoplasm, at a cell membrane, in clathrin-coated pits, in endocytic vesicles, endosomes, any stages in between, and the like. 
     “GPCR signaling” means GPCR induced activation of G proteins. This may result in, for example, cAMP production. 
     “G protein-coupled receptor kinase” (GRK) includes any kinase that has the ability to phosphorylate a GPCR. 
     “G protein-coupled receptor phosphatase” includes any phosphatase that has the ability to dephosphorylate a GPCR. 
     “ Homo sapien  GPCR” means a naturally occurring GPCR in a  Homo sapien.    
     “Inverse agonist” means a compound which, upon binding to the GPCR, inhibits the basal intrinsic activity of the GPCR. An inverse agonist is a type of antagonist. 
     An “isolated” or “purified” nucleic acid molecule or protein, biologically active portion thereof, or antibody is substantially free of other cellular material, or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized. Preferably, an “isolated” nucleic acid is free of sequences (preferably protein encoding sequences) that naturally flank the nucleic acid (i.e., sequences located at the 5 and 3 ends of the nucleic acid) in the genomic DNA of the organism from which the nucleic acid is derived. For purposes of the invention, “isolated” when used to refer to nucleic acid molecules, excludes isolated chromosomes. For example, in various embodiments, the isolated nucleic acid molecule can contain less than about 5 kb, 4 kb, 3 kb, 2 kb, 1 kb, 0.5 kb, or 0.1 kb of nucleotide sequences that naturally flank the nucleic acid molecule in genomic DNA of the cell from which the nucleic acid is derived. A protein that is substantially free of cellular material includes preparations of protein having less than about 30%, 20%, 10%, or 5% (by dry weight) of another protein. When the protein or biologically active portion thereof is recombinantly produced, preferably, culture medium represents less than about 30%, 20%, 10%, or 5% of the volume of the protein preparation. When protein is produced by chemical synthesis, preferably the protein preparations have less than about 30%, 20%, 10%, or 5% (by dry weight) of chemical precursors or non-protein chemicals. 
     “Modified GRK” means a GRK modified such that it alters desensitization. 
     “Naturally occurring GPCR” means a GPCR that is present in nature. 
     “Odorant ligand” means a ligand compound that, upon binding to a receptor, leads to the perception of an odor including a synthetic compound and/or recombinantly produced compound including agonist and antagonist molecules. 
     “Odorant receptor” means a receptor protein normally found on the surface of olfactory neurons which, when activated (normally by binding an odorant ligand) leads to the perception of an odor. 
     “Sensitized GPCR” means a GPCR that presently has ability to respond to agonist and activate conventional G protein signaling. 
     GPCRs and Desensitization 
     The present invention is generally directed to the detection of a GPCR ligand present in a heterogeneous solution that contains one or more GPCR ligands. For example, such solutions could include serum, blood, another biological sample, or an environmental sample. 
     G protein-coupled receptors (GPCRs) regulate a wide variety of physiological processes and are important targets for clinical drug discovery. GPCRs function in vivo as sensitive plasma-membrane sensors that sample the extracellular environment for biologically active molecules. They transduce the binding event across the plasma membrane by interacting with one or more of the numerous classes of intracellular G proteins. 
     The exposure of a GPCR to agonist produces rapid attenuation of its signaling ability that involves uncoupling of the receptor from its cognate heterotrimeric G-protein. The cellular mechanism mediating agonist-specific or homologous desensitization is a two-step process in which agonist-occupied receptors are phosphorylated by a G protein-coupled receptor kinases (GRKs) and then bind an arrestin protein. 
     It has been discovered that after agonists bind GPCRs, G-protein coupled receptor kinases (GRKs) phosphorylate intracellular domains of GPCRs. After phosphorylation, an arrestin protein associates with the GRK-phosphorylated receptor and uncouples the receptor from its cognate G protein. The interaction of the arrestin with the phosphorylated GPCR terminates GPCR signaling and produces a non-signaling, desensitized receptor. 
     The arrestin bound to the desensitized GPCR targets the GPCR to clathrin-coated pits for endocytosis (i.e., internalization) by functioning as an adaptor protein, which links the GPCR to components of the endocytic machinery, such as adaptor protein-2 (AP-2) and clathrin. The internalized GPCRs are dephosphorylated and are recycled back to the cell surface desensitized. The stability of the interaction of arrestin with the GPCR is one factor which dictates the rate of GPCR dephosphorylation, recycling, and resensitization. The involvement of GPCR phosphorylation and dephosphorylation in the desensitization process has been exemplified in U.S. Ser. No. 09/993,844, filed Nov. 5, 2001, the disclosure of which is hereby incorporated by reference in its entirety. 
     The abnormal regulation of hormones that bind to G protein-coupled receptors underlies the pathogenesis of many diseases. The ability to measure serum and tissue levels of these regulators, while clinically and scientifically desirable, is presently limited to very specialized biochemical and immunochemical assays. The present invention provides generalized methods that evaluate a process common to GPCR activity, providing useful methods for the screening and diagnosis of GPCR-based disease. Additionally, the present invention provides a method of screening a sample, biological, environmental, or the like, for compounds which alter GPCR ligands. 
     The present inventors have harnessed this desensitization process common among GPCRs to develop a method of detecting the presence of a GPCR ligand in a biological or environmental solution. The present invention is related to methods of detecting the concentration, presence, absence, or altered concentration of GPCR agonists, ligands, antagonists, or related compounds in a biological sample, environmental sample such as water or soil, or other solution. 
     In one embodiment of the present invention, a biosensor is employed. The biosensor is a host cell or cells that include a GPCR and arrestin. In the presence of agonist, the arrestin binds the GPCR, and the GPCR is internalized. Preferably, this process is visualized by the detection of the arrestin or the GPCR. The biosensor may be used to detect the presence of an agonist in a test sample, such as a biological or environmental sample. The detection of agonists by biosensors of the present invention is useful in disease diagnosis, as well as in the detection of dangerous compounds in the environment. 
     The present inventors determined that the host cells including a GPCR and arrestin could be used to detect various ligands of a GPCR in a test sample. The biosensors of the present invention are useful for the detection of multiple bioactive isoforms of a ligand in a test sample. In U.S. Ser. No. 09/993,844 filed on Nov. 5, 2001, U.S. Ser. No. 10/054,616 filed on Jan. 22, 2002, and U.S. Ser. No. 10/101,235 filed on Mar. 19, 2002, which are hereby incorporated by reference in their entirety, cells expressing GPCRs and arrestin-GFP were used for the identification of a ligand, or antagonist, in a solution. In the present invention, the present inventors determined that cells expressing GPCRs and arrestin-GFP were useful for the detection of all bioactive isoforms of a ligand in the test sample, not just one ligand isoform. The present invention is useful for determining the concentration of all bioactive isoforms, not just one, of a ligand in a test sample. 
     In a preferred embodiment of the present invention, the test samples are heterogeneous. They may include various proteins and compounds. They may include multiple isoforms of a GPCR ligand. 
     The methods of the present invention present a number of advantages over current methods of detecting GPCR ligands in a test sample. The present methods are highly sensitive and specific. In the methods of the present invention, the GPCRs detect the various bioreactive ligand species in the sample, as opposed to other antibody-based methods, such as RIA, which detect only the ligand species with the reactive epitope. Additionally, the present method broadly applies to all GPCRs and is easily adapted for the various GPCRs. 
     A particular strength of the bioassay of the present invention is the virtual elimination of all false positive results. In a standard RIA any epitope capable of interacting with antisera could produce a positive reading. In contrast, the bioassay of the present invention, employing a GPCR, measures bioactivity rather than immunoreactivity. A ligand-receptor interaction that results in arrestin translocation is biologically relevant regardless of the immunological properties of the ligand. The ability to determine the degree of biological activity in the absence of radioactivity in a serum or tissue sample is a much-needed laboratory addition for identifying disease pathology or predicting potential complications arising from abnormal hormone concentrations. 
     As opposed to other methods, the methods of the present invention are not subject to cross-reactivity with other compounds in the sample. The methods of the present invention are specific for the detection of GPCR ligands which are biologically active and do not cross-react with compounds in the sample which are not biologically active. Additionally, as opposed to the methods of the present inventions, other methods of detection do not have the ability to detect all of the bioactive isoforms of a ligand in a test sample. 
     Detecting Gastrin 
     A number of disease conditions are associated with abnormal regulation of GPCR ligand concentration. The present invention provides a method of detecting the presence, absence, concentration, or change in concentration of a GPCR ligand. Using such methods, the present invention provides methods of diagnosing a disease or a disease-causative state. 
     Clinical assays are often hampered by an inability to diagnose disease when it exists, a false negative result, or inappropriately indicating pathology, a false positive result. The methods of the present invention are resistant to false negative results because the methods involve the detection of all bioactive isoforms of the GPCR ligands. For example, the present inventors have determined that the methods of the present invention detected all bioactive isoforms of gastrin rather than just immunoactive forms of gastrin, and the methods are resistant to false negative results. This is particularly evident from the robust response observed in response to pentagastrin, a potent receptor agonist that is not detectable by the immunological assays. The experimental sensitivity for detecting endogenous ligand using hG17 as a standard was approximately 100-200 pM (200-400 pg/ml of hG17), a range approximating the upper limit of normal as defined by RIA (&lt;200 pg/ml, approximately 50 to 90 pM). Various strategies to increase the sensitivity include blocking receptor recycling to allow for more internalization, sample concentration, or receptor modification by mutagenesis to increase affinity. 
     One embodiment of the present invention is the diagnosis of hypergastrinemia by analyzing the location of the CCK-B GPCR after exposure to gastrin in a test sample. Cells are provided that express the CCK-B GPCR and arrestin. The GPCR or arrestin may be detectably labeled. These cells are exposed to a test sample, and subsequent changes in the location of the GPCR or arrestin are analyzed. Such analyses may be quantitative and may indicate the concentration of gastrin, or biologically-active isoforms, in a test sample. As discussed below, the gastrin concentration may be indicative of a disease condition, such as hypergastrinemia. 
     Gastrin is a ligand which binds a GPCR, and is the major hormonal regulator of gastric acid secretion. Two major forms of gastrin are secreted (Gastrin-34 and Gastrin-17), however, all gastrins have an amidated tetrapeptide (Trp-Met-Asp-Phe-NH 2 ) at the carboxyl terminus, which imparts full biological activity. The vast majority of gastrin is produced in endocrine cells of the gastric antrum. Progastrin is known to be expressed in a number of mammalian tissues: the gastrin antrum, jejunum, ileum, colon, and pancreas of the gastrointestinal tract; the ovaries, testicles, and spermatozoa of the genital tract; the cerebellum, vagus nerve, hypothalamus, pituitary, and adrenal medulla of the neuroendocrine tissue; and the bronchial mucosa of the respiratory tract, although it may be expressed in other tissues as well. 
     Gastrin is a member of the cholecystokinin (CCK) family of gastrointestinal (GI) peptides, hormones that bind to CCK-A and CCK-B receptors, GPCRs found in the GI tract and brain. The cloning and characterization of the CCK-B receptor as a typical heptahelical G protein-coupled receptor (GPCR) has provided a valuable tool in the study of gastrin. The human CCK-B receptor has a nanomolar affinity for gastrin and cholecystokinin. The circulating levels of CCK are beyond detection by conventional radioimmunoassay (RIA), but the major biologically active forms of gastrin, gastrin-17 and gastrin-34 that are secreted into the blood are immunologically detectable by RIA. Presently, serum gastrin measurements are performed by radioimmunoassay using antibodies directed against one or more distinct gastrin isoforms. Occasionally, antisera may show cross-reactivity to gastrin precursors or other serum proteins that vary in their biological potency or have no biological consequence related to CCK-B receptor signaling. Alternatively, patients have presented with symptoms of hypergastrinemia, and/or known gastrin-secreting tumors where the RIA determinations of serum gastrin were normal. This has lead to the hypothesis that certain tumors may produce non-RIA detectable gastrin variants. 
     The two major biologically active forms of gastrin, 17 and 34 amino acids in length, are produced by enzymatic digestion of preprogastrin and secreted into the blood by gastric antral G cells. Gastrin primarily regulates the release of stomach acid and the growth of GI mucosa, and its oversecretion is associated with enterochromaffin cell hyperplasia and tumors. 
     In the endoplasmic reticulum, the signal peptide of preprogastrin is cleaved resulting in progastrin. Further enzymatic modification of progastrin in the Golgi generates products which are packaged into secretory granules. A number of secretory granule products are derived from preprogastrin: progastrin, glycine-extended gastrin-17, glycine-extended gastrin-34, gastrin-71, gastrin-34, gastrin-17, and gastrin-6. 
     Hypergastrinemia is associated with GI malignancies and consequently serum gastrin levels are routinely measured in clinical practice. Hypergastrinemia may occur in pathophysiologic states and serum gastrin levels can also become elevated in patients on prolonged acid suppressive medications. Presently, serum gastrin measurements are performed by radioimmunoassay (RIA), using antibodies directed against one or more distinct gastrin isoforms. Occasionally, antisera may show cross-reactivity to gastrin precursors or other serum proteins that vary in their biological potency or have no biological consequence related to CCK-B receptor signaling. Alternatively, patients have presented with symptoms of hypergastrinemia, and/or known gastrin-secreting tumors where the RIA determinations of serum gastrin were normal. This has lead to the hypothesis that certain tumors may elaborate non-RIA detectable gastrin variants. 
     Detecting Acetylcholine 
     In another embodiment of the present invention, a number of chemical/biological agents of interest to the military and civilian communities may be sensed readily by the described sensors. The present invention may be used to detect biological agents, toxins, neurotoxins, nerve gases, and the like. The ability to rapidly and accurately detect and quantify biologically relevant molecules with high sensitivity is a central issue for medical technology, national security, public safety, environmental safety and civilian and military medical diagnostics. 
     Such a biosensor for the detection of agents, such as bioterrorism agents, in the environment provides a number of advantages over present detection methods. First, the assay is sensitive and, since based on the biological activity of the ligand, detects the presence of any bioactive variants of the ligand of interest. Secondly, the assay is quantitative and can detect altered or minimal concentrations of the ligand in the sample. The assay also can be monitored in a continuous fashion. Additionally, sample preparation is quite straightforward: a sample need only be suspended in an aqueous solution for detection. 
     A number of neurotoxins including sarin and organophosphate insecticides, for example diazinon, inhibit acetylcholinesterase, an enzyme which inactivates the neurotransmitter acetylcholine. In vivo, compounds which decrease acetylcholine esterase activity result in an increase in the concentration of acetylcholine in the synaptic cleft, producing excessive nerve excitation. Levels of acetylcholine in a test sample can be used to monitor acetylcholinesterase activity, and detect the presence of acetylcholinesterase inhibitors. One aspect of the present invention is the use of a single cell biosensor expressing the muscarinic receptor as a method of detecting the presence of acetylcholine esterase inhibitors in the environmental or a biological sample. 
     Another embodiment of the invention pertains to field-testing of environmental conditions. Automated sensing of environmental conditions, including the presence of natural chemicals, industrial wastes, and biological/chemical warfare agents is possible using an embodiment of the invention. Uploading of test results via radio transmission may provide remote sensing capabilities, and may provide response capabilities through human or central computer directed action. Response instructions may then be downloaded either to the sensing site or to another strategic response position. Such a system may be useful, for example, in determining the presence of toxins in a public water supply, and the subsequent centralized-directed cessation of water flow from the supply pool. 
     Described above are embodiments of the present invention employing the CCK-B or the muscarinic GPCRs. Additionally, the present invention encompasses biosensors employing any GPCR. By fluorescence microscopy, GPCR association with arrestin and subsequent internalization at least 40 different GPCRs using fusion proteins between arrestin and a green fluorescent protein is possible ( FIG. 2 ). Each of these cells, as well as other like cells, is a useful biosensor of the present invention. 
     Methods of the Present Invention 
     The present invention provides highly specific, sensitive, generalized, and quantitative methods of analyzing the presence of GPCR-binding compounds in samples. 
     One embodiment of the present invention is a method of detecting a GPCR ligand in a test sample. Most preferably, this method comprises the steps of (a) providing a cell including a GPCR and an arrestin; (b) exposing the cell to the test sample; and (c) determining the cellular distribution of the GPCR or arrestin in the presence of the test sample, wherein the test sample is, or is derived from, a biological sample or an environmental sample. 
     In a preferred embodiment, the cellular distribution of the GPCR or arrestin in the presence of the test sample is compared to the cellular distribution of the GPCR or arrestin in the absence of the test sample. Different concentrations of the test sample may be analyzed. The cellular distribution may be determined at different time points after exposure to the test sample. 
     In one embodiment of the present invention, the GPCR or arrestin is detectably labeled, other endogenous molecules are detectably labeled, or exogenous molecules are detectably labeled. The distribution of the detectably labeled molecules represents the cellular distribution of the GPCR or arrestin. The distribution of the GPCR or arrestin may indicate the extent to which the GPCR is internalized. The cellular distribution of the detectably labeled molecule may be quantified. 
     In one embodiment of the present invention, a sample may include a known concentration of the ligand. By comparing the cellular distribution of the detectably labeled molecule in the presence of the test sample to the distribution in the presence of known concentrations of ligand, the concentration of ligand in the test sample may be determined. 
     The test sample may be a biological sample or an environmental sample. The biological sample may be or may be derived from serum, tissue, blood, or urine. 
     In one embodiment, the GPCR is CCK-B or CCK-A. The ligand may be gastrin, preprogastrin, cleaved preprogastrin, gastrin-34, gastrin-17, pentagastrin, progastrin, glycine-extended gastrin-17, glycine-extended gastrin-34, gastrin-71, or gastrin-6. In one embodiment, the GPCR is a muscarinic receptor. The ligand may be acetylcholine. 
     In a preferred embodiment, the labeled molecule is localized in the cytosol, clathrin-coated pits, the plasma membrane, endocytic vesicles, or endosomes. An increase in the local concentration of the labeled molecule results in a local increase in signal intensity. The signal intensity in the plasma membrane, clathrin-coated pits, endocytic vesicles, or endosomes may be greater than the signal intensity in the cytosol. The local signal intensity may be increased or decreased in the presence of increased or decreased amounts of a compound, such as a ligand, agonist, or antagonist. 
     In a preferred embodiment, the concentration of a ligand in the test sample indicates a disease state. The concentration of the ligand in the test sample may indicate the presence of a compound in the test sample that alters the ligand concentration. The ligand concentration may indicate the presence of a compound in the test sample that modifies acetylcholine or inhibits acetylcholinesterase. 
     In a preferred embodiment, the detectable molecule is a radioisotope, an epitope tag, an affinity label, an enzyme, a fluorescent group, or a chemiluminescent group. In one embodiment, a molecule may be detectably labeled due to its interaction with another molecule which is detectably labeled. 
     One embodiment of the present invention is a method of monitoring a GPCR ligand in mammals, wherein the analysis of the ligand concentration in a test sample is based on the binding of the ligand to the GPCR. This method may be used to monitor a clinical condition and/or may indicate the presence of a disease state. The clinical condition may indicate that the subject has a disorder or is at risk for developing a disorder. The clinical condition may be gastrointestinal cancer, hypergastrinemia, atrophic gastritis, gastric ulcers, or malignant tumors. 
     Arrestin coupled to a detectable molecule may be detected and monitored as it functions in the GPCR pathway. The location of the arrestin may be detected, for example, evenly distributed in the cell cytoplasm, concentrated at a cell membrane, concentrated in clathrin-coated pits, localized in endocytic vesicles or endosomes, and the like. The proximity of arrestin to a GPCR may be monitored, as well as the proximity to any other cell structure. 
     Preferably, the arrestin, the GPCR, and/or the arrestin/GPCR complex may be detected in endocytic vesicles or endosomes. The arrestin, the GPCR, and/or the arrestin/GPCR complex thus may be detected in endocytic vesicles or endosomes absence of agonist. The association of arrestin with a GPCR in endocytic vesicles or endosomes may give a strong, readily recognizable signal that persists for extended periods of time. Under magnification of 40× objective lens, the signal may be doughnut-like in appearance. The signal resulting from the compartmentalization of arrestin and GPCR colocalized in endocytic vesicles or endosomes is typically easy to detect. Similarly, blocking this association is easy to detect. Examples of detection methods are described herein. Such methods include, for example. polarization microscopy, BRET, FRET, evanescent wave excitation microscopy, and standard or confocal microscopy. 
     One embodiment of the present invention is a method of measuring the gastrin concentration in a test sample. By employing cell lines permanently expressing the gastrin receptor (CCK-B) and a fusion protein consisting of β-arrestin 2 and green fluorescent protein (GFP), the present inventors have constructed a single cell biosensor for the measurement of serum gastrin. The quantitative redistribution of arrestin-GFP in response to agonist-activated gastrin receptors was measured by analysis of cell images obtained by fluorescence confocal microscopy, and provided a sensitive and specific determination of receptor activation. Such a single cell biosensor is a practical means to measure the bioactive serum concentration of gastrin, allowing the diagnosis of hypergastrinemia. 
     In one embodiment, the presence of GPCR ligand in the test sample indicates the presence of a disease, that a subject has a disorder, or is at risk for getting a disorder. Alternatively, the absence, altered concentration, or other alteration of the GPCR ligand may indicate the presence of a disease, that a subject has a disorder, or is at risk for getting a disorder. 
     In one embodiment, the analysis of the GPCR ligand indicates the presence, absence, enhancement, inhibition, or other alteration of a compound that alters the GPCR ligand. The analysis may indicate the presence, absence, altered concentration, or other alteration of the ligand. The compound that alters the GPCR ligand may be an enzyme, an inhibitor, an activator, a small molecule, or other compound that directly affects the GPCR ligand. The compound that alters the GPCR ligand may be an enzyme, an inhibitor, an activator, a small molecule, or other compound that indirectly affects the GPCR ligand. 
     In a specific embodiment, the GPCR is the muscarinic receptor and the method of determining the concentration of acetylcholine in a test sample. By employing cell lines transiently transfected with the muscarinic receptor and a fusion protein consisting of β-arrestin 2 and green fluorescent protein, the present inventors have constructed a single cell biosensor for the measurement of acetylcholine in a sample. 
     Acetylcholine, the ligand of the muscarinic receptor, is altered by acetylcholinesterase. In the presence of acetylcholinesterase, the concentration of acetylcholine in a test sample is decreased. A decrease in the amount of acetylcholine in a test sample decreased the amount of internalization of the muscarinic receptor, as visualized by the decreased internalization of the arrestin-GFP conjugate. 
     An additional embodiment of the present invention is related to methods of increasing the sensitivity of the above methods. In one aspect, as described in U.S. Ser. No. 09/993,844 filed on Nov. 5, 2001, which is hereby incorporated by reference in its entirety, the GPCR itself may be modified in its C-terminal tail such that it has enhanced phosphorylation sites. The sensitivity of the assay may also be increased with GRK over-expression. The biosensor may be exposed to the test sample for longer periods of time, or at increased concentrations, in order to increase the signal. 
     Expression of the Proteins 
     Another feature of this invention is the expression of the DNA sequences encoding a GPCR and/or arrestin in a cell to form a biosensor, as disclosed herein. As is well known in the art, DNA sequences may be expressed by operatively linking them to an expression control sequence in an appropriate expression vector and employing that expression vector to transform an appropriate unicellular host. 
     Such operative linking of a DNA sequence to an expression control sequence, of course, includes, if not already part of the DNA sequence, the provision of an initiation codon, ATG, in the correct reading frame upstream of the DNA sequence. 
     A wide variety of host/expression vector combinations may be employed in expressing the DNA sequences. Useful expression vectors, for example, may consist of segments of chromosomal, non-chromosomal and synthetic DNA sequences. Suitable vectors include derivatives of SV40 and known bacterial plasmids, e.g.,  E. coli  plasmids col El, pCR1, pBR322, pMB9 and their derivatives, plasmids such as RP4; phage DNAs, e.g., the numerous derivatives of phage λ, e.g., NM989, and other phage DNA, e.g., M13 and filamentous single stranded phage DNA; yeast plasmids such as the 2μ plasmid or derivatives thereof, vectors useful in eukaryotic cells, such as vectors useful in insect or mammalian cells; vectors derived from combinations of plasmids and phage DNAs, such as plasmids that have been modified to employ phage DNA or other expression control sequences; and the like. 
     Any of a wide variety of expression control sequences—sequences that control the expression of a DNA sequence operatively linked to it—may be used in these vectors to express the DNA sequences. Such useful expression control sequences include, for example, the early or late promoters of SV40, CMV, vaccinia, polyoma or adenovirus, the lac system, the trp system, the TAC system, the TRC system, the LTR system, the major operator and promoter regions of phage λ, the control regions of fd coat protein, the promoter for 3-phosphoglycerate kinase or other glycolytic enzymes, the promoters of acid phosphatase (e.g., Pho5), the promoters of the yeast α-mating factors, and other sequences known to control the expression of genes of prokaryotic or eukaryotic cells or their viruses, and various combinations thereof. 
     A wide variety of unicellular host cells are also useful in expressing the DNA sequences. These hosts may include well known eukaryotic and prokaryotic hosts, such as strains of  E. coli, Pseudomonas, Bacillus, Streptomyces , fungi such as yeasts, plant cells, nematode cells, and animal cells, such as HEK-293, CHO, RI.I, B-W and L-M cells, African Green Monkey kidney cells (e.g., COS 1, COS 7, BSC1, BSC40, and BMT10), insect cells (e.g., Sf9), and human cells and plant cells in tissue culture. However, mammalian cells are preferred for creating the biosensors of the invention. 
     It will be understood that not all vectors, expression control sequences and hosts will function equally well to express the DNA sequences. Neither will all hosts function equally well with the same expression system. However, one skilled in the art will be able to select the proper vectors, expression control sequences, and hosts without undue experimentation to accomplish the desired expression without departing from the scope of this invention. For example, in selecting a vector, the host must be considered because the vector must function in it. The vector&#39;s copy number, the ability to control that copy number, and the expression of any other proteins encoded by the vector, such as antibiotic markers, will also be considered. 
     In selecting an expression control sequence, a variety of factors will normally be considered. These include, for example, the relative strength of the system, its controllability, and its compatibility with the particular DNA sequence or gene to be expressed, particularly as regards potential secondary structures. Suitable unicellular hosts will be selected by consideration of, e.g., their compatibility with the chosen vector, their secretion characteristics, their ability to fold proteins correctly, and their fermentation requirements, as well as the toxicity to the host of the product encoded by the DNA sequences to be expressed, and the ease of purification of the expression products. 
     Considering these and other factors a person skilled in the art will be able to construct a variety of vector/expression control sequence/host combinations that will express the DNA sequences on fermentation or in large scale animal culture. 
     As mentioned above, a DNA sequence encoding a modified GPCR can be prepared synthetically rather than cloned. The DNA sequence can be designed with the appropriate codons for the GPCR amino acid sequence. In general, one will select preferred codons for the intended host if the sequence will be used for expression. The complete sequence is assembled from overlapping oligonucleotides prepared by standard methods and assembled into a complete coding sequence. See, e.g., Edge, Nature, 292:756 (1981); Nambair et al., Science, 223:1299 (1984); Jay et al., J. Biol. Chem., 259:6311 (1984). 
     Synthetic DNA sequences allow convenient construction of genes which will express GPCR analogs or “muteins”. Alternatively, DNA encoding muteins can be made by site-directed mutagenesis of native or modified GPCR genes or cDNAs, and muteins can be made directly using conventional polypeptide synthesis. 
     A general method for site-specific incorporation of unnatural amino acids into proteins is described in Christopher J. Noren, Spencer J. Anthony-Cahill, Michael C. Griffith, Peter G. Schultz, Science, 244:182-188 (April 1989). This method may be used to create analogs with unnatural amino acids. 
     Conjugates 
     The cells used in the methods of assaying of the present invention may comprise a conjugate of an arrestin protein and a detectable molecule, a conjugate of a GPCR and a detectable molecule, a conjugate of any member of a GPCR/arrestin complex and a detectable molecule, a conjugate of a detectable molecule and a molecule that interacts with any member of a GPCR/arrestin complex, and the like. The detectable molecule allows detection of molecules interacting with the detectable molecule, as well as the molecule itself. 
     All forms of arrestin, naturally occurring and engineered variants, including but not limited to, visual arrestin, β-arrestin 1 and β-arrestin 2, may be used in the present invention. GPCRs may interact to a detectable level with all forms of arrestin. 
     Detectable molecules that may be used include, but are not limited to, molecules that are detectable by spectroscopic, photochemical, biochemical, immunochemical, electrical, radioactive, and optical means, including but not limited to bioluminescence, phosphorescence, and fluorescence. These detectable molecules should be a biologically compatible molecule and should not compromise the biological function of the molecule and must not compromise the ability of the detectable molecule to be detected. Preferred detectable molecules are optically detectable molecules, including optically detectable proteins, such that they may be excited chemically, mechanically, electrically, or radioactively to emit fluorescence, phosphorescence, or bioluminescence. More preferred detectable molecules are inherently fluorescent molecules, such as fluorescent proteins, including, for example, Green Fluorescent Protein (GFP). The detectable molecule may be conjugated to the arrestin protein by methods as described in Barak et al. (U.S. Pat. Nos. 5,891,646 and 6,110,693). The detectable molecule may be conjugated at the front-end, at the back-end, or in the middle. 
     The GPCRs may also be conjugated with a detectable molecule. Preferably, the carboxyl-terminus of the GPCR is conjugated with a detectable molecule. If the GPCR is conjugated with a detectable molecule, proximity of the GPCR with the arrestin may be readily detected. In addition, if the GPCR is conjugated with a detectable molecule, compartmentalization of the GPCR with the arrestin may be readily confirmed. The detectable molecule used to conjugate with the GPCRs may include those as described above, including, for example, optically detectable molecules, such that they may be excited chemically, mechanically, electrically, or radioactively to emit fluorescence, phosphorescence, or bioluminescence. Preferred optically detectable molecules may be detected by immunofluorescence, luminescence, fluorescence, and phosphorescence. 
     For example, the GPCRs may be antibody labeled with an antibody conjugated to an immunofluorescence molecule or the GPCRs may be conjugated with a luminescent donor. In particular, the GPCRs may be conjugated with, for example, luciferase, for example,  Renilla  luciferase, or a rhodamine-conjugated antibody, for example, rhodamine-conjugated anti-HA mouse monoclonal antibody. Preferably, the carboxyl-terminal tail of the GPCR may be conjugated with a luminescent donor, for example, luciferase. The GPCR, preferably the carboxyl-terminal tail, also may a be conjugated with GFP as described in L. S. Barak et al. Internal Trafficking and Surface Mobility of a Functionally Intact β2-Adrenergic Receptor-Green Fluorescent Protein Conjugate, Mol. Pharm. (1997) 51, 177-184. 
     Cell Types and Substrates 
     The cells of the present invention express at least one GPCR, and arrestin, wherein at least one of the molecules is detectably labeled. Cells useful in the present invention include eukaryotic and prokaryotic cells, including, but not limited to, bacterial cells, yeast cells, fungal cells, insect cells, nematode cells, plant cells, and animal cells. Suitable animal cells include, but are not limited to, HEK cells, HeLa cells, COS cells, U2OS cells and various primary mammalian cells. An animal model expressing a conjugate of an arrestin and a detectable molecule throughout its tissues or within a particular organ or tissue type, may also be used in the present invention. 
     A substrate may have deposited thereon a plurality of cells of the present invention. The substrate may be any suitable biologically substrate, including but not limited to, glass, plastic, ceramic, semiconductor, silica, fiber optic, diamond, biocompatible monomer, or biocompatible polymer materials. 
     Methods of Detection 
     Methods of detecting the intracellular location of the detectably labeled arrestin, the intracellular location of a detectably labeled GPCR, or interaction of the detectably labeled arrestin, or other member of GPCR/arrestin complex with a GPCR or any other cell structure, including for example, the concentration of arrestin or GPCR at a cell membrane, colocalization of arrestin with GPCR in endosomes, and concentration of arrestin or GPCR in clathrin-coated pits, and the like, will vary dependent upon the detectable molecule(s) used. 
     One skilled in the art readily will be able to devise detection methods suitable for the detectable molecule(s) used. For optically detectable molecules, any optical method may be used where a change in the fluorescence, bioluminescence, or phosphorescence may be measured due to a redistribution or reorientation of emitted light. Such methods include, for example, polarization microscopy, BRET, FRET, evanescent wave excitation microscopy, and standard or confocal microscopy. 
     In a preferred embodiment arrestin may be conjugated to GFP and the arrestin-GFP conjugate may be detected by confocal microscopy. In another preferred embodiment, arrestin may conjugated to a GFP and the GPCR may be conjugated to an immunofluorescent molecule, and the conjugates may be detected by confocal microscopy. In an additional preferred embodiment, arrestin may conjugated to a GFP and the carboxy-terminus of the GPCR may be conjugated to a luciferase and the conjugates may be detected by bioluminescence resonance emission technology. In a further preferred embodiment arrestin may be conjugated to a luciferase and GPCR may be conjugated to a GFP, and the conjugates may be detected by bioluminescence resonance emission technology. The methods of the present invention are directed to detecting GPCR activity. The methods of the present invention allow enhanced monitoring of the GPCR pathway in real time. 
     In a preferred embodiment, the localization pattern of the detectable molecule is determined. In a further preferred embodiment, alterations of the localization pattern of the detectable molecule may be determined. The localization pattern may indicated cellular localization of the detectable molecule. Certain methods of detection are described in U.S. Ser. No. 10/095,620, filed Mar. 12, 2002, which claims priority to U.S. Provisional Patent Application No. 60/275,339, filed Mar. 13, 2001, the contents of which are incorporated by reference in their entirety. 
     Molecules may also be detected by their interaction with another detectably labeled molecule, such as an antibody. 
     Test Kits 
     The present invention includes test kits for analysis of test samples. Most preferably, the test kits would be useful for determining the GPCR ligand concentration in a biological or environmental sample. Even more preferably, the test kit would include a host cell expressing a GPCR and arrestin, and method of determining ligand concentration in sample. 
     EXAMPLES 
     The invention will be further explained by the following illustrative examples which are intended to be non-limiting. 
     Example 1 
     Materials &amp; Methods 
     Human Embryonic Kidney Cells (HEK-293) were obtained from the American Type Culture Collection (ATCC, Manassas, Va.). Media, fetal bovine serum (FBS), and antibiotics were purchased from MediaTech (Herndon, Va.) and Gibco Invitrogen Corp (Carlsbad, Calif.). 96 well glass plates were obtained from Whatman (Clifton, N.J.) and binding resins from BioRad Laboratories (Hercules, Calif.). The CCK-A receptor antagonist devazepide and a specific CCK-B receptor antagonist were used. Pentagastrin was obtained from Sigma. 
     Membrane Preparation/Binding—All steps were performed at 4° C. Membrane binding was performed in triplicate as described in Shetzline et al.,  J. Biol. Chem.  273:6756-6752 (1998). Membrane fractions were assayed immediately or stored at −80° C. Saturation or competition binding was performed with [ 3 H]CCK-8 polypeptide (Peninsula Labs, San Carlos, Calif.), with non-specific binding determined in the presence of 1 μM hG17. Competitive binding was performed using hG17. Data were analyzed using Graph Pad-Prism. 
     Cloning of the Human Gastrin Receptor: The human gastrin/CCK-B receptor cDNA was amplified from a human brain cDNA library (Clontech) using two oligonucleotide primers matching the 5′ and 3′ ends of the coding region, a sense oligonucleotide (5′-GCGCCC GCTAGC ACCGCCATGGAGCTGCTAAAGCTGAACCGG) (SEQ ID NO. 17) with a Nhel restriction site, and an antisense oligonucleotide (5′ GCGCCC GGTACC TCAGCCAGGGCCCAGT-GTGCTGAT) (SEQ ID NO. 18) with a Kpnl restriction site. The 1.4 kb amplified CCK-B receptor DNA band was subcloned into the pcDNA3.1-ZEO(−) expression vector (Invitrogen) at Nhel and Kpnl and verified on an ABI 377 fluorescent sequencer. 
     Cell Culture and Transfection: Transient transfection in HEK-293 cells was done as described in Walker et al.,  J. Biol. Chem.  274:31515-23. 
     Inositol Phosphate Determination—HEK 293 cells expressing the CCK-B receptors were plated into 12-well plates coated with 25 μg/ml Poly-D-Lysine (Sigma Cat# P-6407, St. Louis, Mo.) and incubated overnight at 37° C. in MEM containing 10% FBS (fetal bovine serum). They were next placed for 24 hours in labeling media (1 μCi/0.5 mL/well of [ 3 H]inositol in 5% FBS/MEM/Gentamicin), washed with MEM, 20 mM HEPES, pH 7.40, 20 mM LiCl for 5 minutes at 37° C., and treated with agonist. The reactions were stopped by addition of 500 μL of ice-cold 0.8 M HClO 4 , the cells gently agitated at 4° C. for 30-60 minutes and the cell lysate then added to polypropylene tubes. 200 μL of neutralizing solution (0.72 M KOH/0.6 M KCO 3 ) was added to each tube and the tubes remained at 4° C. until analysis. BioRad AG-1×8 Resin (200-400 mesh) columns (BioRad Econo-Pac) were prepared with 1 ml of 50% slurry each in order to assay IP3 activity. Each column was washed twice with 10 ml of 18 megaohm deionized water, 800 μL of lysate was added, and after 5-10 minutes the columns were again washed twice with 10 mL of water. Samples were eluted into scintillation vials containing 15 mL of Lefko-Fluor (Research Products International Mt. Prospect, Ill.) using 3.5 mL of a 1 M ammonium formate/0.1 M formic acid solution. Fifty μL lysate samples corresponding to each fraction were also counted to determine total radioactivity uptake per sample. 
     Fluorescence Confocal Microscopy and Data Analysis: The measurements of arrestin-GFP translocation for quantitative determinations of dose responses or of patient serum levels were done in the following manner. Cells permanently expressing the human CCK-B receptor and arrestin-GFP were seeded at 20,000 cells per well in 96 well, glass bottomed plates in 200 μL of MEM supplemented with 10% fetal bovine serum. 100 μl was removed and replaced by media containing a known concentration of hG17 peptide, pentagastrin, or a known volume of patient serum. Fluorescence cell images were obtained with a Zeiss LSM-510 confocal microscope. The dynamic translocation of arrestin-GFP over 5 minutes was analyzed as described. Static cell images obtained after one-two hour incubations were analyzed using the computer program IP LABS (Scanalytics, Fairfax, Va. 22031). 
     The analysis of translocation proceeded as follows. An average and standard deviation of pixel intensity was determined for images of untreated cells containing arrestin-GFP. An intensity corresponding to 3 standard deviations above the mean for these cells was set as the threshold to define translocation. To determine the subset of pixels representing translocated arrestin-GFP, only pixels that had at least one neighbor above this threshold were counted. This second restriction was set to eliminate noise. The measured amount of translocation, TI, in an image was then calculated to be sum of the intensities from pixels representative of translocation. The number of cells in an image could vary, but the total amount of fluorescence obtained from an image was independent of the distribution of chromophore and remained constant over time. To correct for the variation in the number of cells contained in different images, the calculated translocation for an image was normalized by the total image fluorescence, TF (i.e. sum of intensities for all pixels in the image). The computed translocation was defined as TI/TF. The mean intensity of the untreated cells was set to fall within the bottom 10% of the dynamic range of the microscope imaging system in order to avoid clipping the signal from areas with large amounts of receptor/arrestin-GFP complexes. Data are presented as mean±SEM. 
     Evaluation of the Signal to Noise (See  FIG. 7 ) In order to evaluate the signal/noise ratio, the following two assumptions were made about the experimental system; (1) the sum of the intensities over all pixels, is independent of time and redistribution of βarrestin2-GFP and (2) the intensity distribution of cell fluorescence is gaussian and is given by the normalized probability distribution: 
     
       
         
           
             
               
                 
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     For a homogeneous line of cells described by a narrow gaussian distribution of intensity the ratio of the mean intensity to its standard deviation 
     
       
         
           
             
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     can be chosen greater than 4 by appropriately adjusting the range of the imaging system. The term √{square root over (2)}·[1+erf(I o /σ] 1/2  is then approximately equal to 2. The fraction of translocated receptors represented by the term h varies between 0 (no translocation) and 1 (100% translocation) whereas the ratio of the volumes (areas) due to redistribution of the arrestin-GFP 
     
       
         
           
             
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     may vary between 5-100 depending upon the imaging system and the identity of the cellular compartment containing the translocated arrestins (for example, membrane, coated pits, or endosomes).
 
The term in the denominator
 
     
       
         
           
             
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     equals 0.014 and 0.00070 for the threshold intensity set to b=2 and b=3 standard deviations above the mean intensity respectively. Therefore the signal to noise, S/N, can easily exceed 10 3 -10 4  for very homogeneous populations of cells with even minimal amounts of translocation 
     
       
         
           
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     Even though an inhomogeneous fluorescence distribution or background fluorescence will cause the intensity profile to depart from this ideal case, the analysis indicates that translocation by simply considering intensity changes can be a very sensitive method for evaluating receptor behavior. Moreover, the use of simple pattern recognition algorithms could provide even greater discrimination of translocated arrestin and be useful for cell populations that are not homogenous such as in transient transfections. 
     Patient serum collection and serum determination by RIA: The Institutional Review Board approved this study at Duke University Medical Center. Patient serum was obtained from patients scheduled for serum gastrin analysis as requested by their primary care provider. Patients signed informed consent and an additional sample of serum was drawn for use in this study. Conventional RIA determinations were performed at Mayo Medical Laboratories, Rochester, Minn. 
     Example 2 
     Cells Expressing the CCK-B Receptor and Arrestin-GFP Respond to the Presence of Synthetic Human Gastrin-17 Peptide (hG17): Determination of the Uniformity of Arrestin-GFP and CCK-B Receptor Expression in Cells by Flow Cytometry and Arrestin-GFP Translocation 
     To simplify quantitative analysis of arrestin-GFP redistribution, cells which respond to ligand in an identical manner were established. In order to achieve a large degree of uniformity in receptor and arrestin expression among the entire cell population, an HEK-293 cell line permanently expressing the CCK-B receptor and arrestin-GFP was established. The degree of homogeneity of arrestin expression within the clone (Clone A) used in this study was determined by flow cytometry ( FIG. 4A ) and confirmed by fluorescence ( FIG. 4B , Left Panel). The general ability of cells to respond to the presence of synthetic human gastrin-17 peptide (hG17) by redistributing arrestin-GFP is shown in  FIG. 4B  (Right Panel). 
     In  FIG. 4A , the relative expression of arrestin-GFP in cells belonging to Clone A was determined using a Becton Dickenson FACScan flow cytometer. The x-axis is logarithmic in the relative cell intensity and the y-axis indicates the number of cells (Counts) at that intensity. Ninety-nine percent of the cell population was within the bounds of the bar seen above the intensity profile.  FIG. 4B  shows fluorescence images of a field of cells from Clone A before (left panel) and after treatment (right panel) with 10 nM hG17 for 5 minutes at room temperature. 
     Example 3 
     Characterization of Ligand Binding and Second Messenger Response 
     Clone A was further characterized my measuring the binding of the gastrin peptide agonists [ 3 H]-CCK8 and hG17, and determining the hG17-mediated second messenger response. Saturation binding with [ 3 H]-CCK8 showed that the cells expressed (14±1) pmol CCK-B receptor/mg cell protein and had an affinity for [ 3 H]-CCK8 of (9.0±1.6) nM ( FIG. 5A ). Human gastrin-17 stimulation of inositol phosphate (IP3) production yielded an EC 50 =(3.2±0.7) nM ( FIG. 5B ). The inset in  FIG. 5B  shows the competitive displacement of [ 3 H]-CCK8 by hG17. The EC 50  was (28±5) nM and the Kd of hG17 for the CCK-B receptor was (17±3.5) nM. 
       FIG. 4  shows that 10 nM hG17 produced a measurable arrestin-GFP redistribution to the plasma membrane even after five minutes. After 30-60 minutes of exposure to hG17, endosomes containing arrestin-GFP became visible ( FIG. 5C  Upper Right Panel). This translocation was blocked completely by addition of 10 μM of the specific CCK-B receptor antagonist ( FIG. 5C  Lower Left Panel), but was not blocked by 10 μM of the closely related CCK-A receptor antagonist devazepide ( FIG. 5C  Lower Right Panel). The data in  FIG. 5C  confirm that the CCK-B receptor is a class B GPCR, since it promotes arrestin internalization into endosomes. 
       FIG. 5A  shows that cells from Clone A were incubated with increasing concentrations of [ 3 H]CCK-8 in order to determine the average CCK-B receptor expression per cell and the receptor affinity for [ 3 H]CCK-8. Total binding of [ 3 H]CCK-8, □; specific binding of [ 3 H]CCK-8, ▪; non-specific binding in the presence of excess (1 μM) unlabeled hG17, ∘. In  FIG. 5B , Clone A cells were exposed to increasing concentrations of hG 17 peptide in order to evaluate the IP3 second messenger response. The inset shows the competitive displacement of [ 3 H]CCK8 by hG17 from this cell line. Data are presented as mean±SEM.  FIG. 5C  shows the fluorescence images of cells from Clone A that were treated with vehicle (upper left panel), or treated for one hour with 10 nM of the agonist hG17 (upper right panel), or with 10 nM hG17 plus 1 μM of the CCK-A antagonist devazepide (L-364,718, lower left panel); or with 10 nM hG17 plus 1 μM, of the CCK-B antagonist (lower right panel). 
     Example 4 
     Dose Response to hG17 and Analysis of Serum Samples from a Patient with Hypergastrinemia 
     Using increasing concentrations of hG17, the time-dependent loss of cytoplasmic arrestin-GFP was measured in order to determine if the pharmacology of arrestin redistribution correlated with that of IP3 production. Sequential fluorescence images of ligand-treated cells grown in 96 well plates were obtained in 30-second intervals over 5 minutes by confocal microscopy and analyzed by measuring the loss of cytosolic fluorescence. The time and dose dependence of arrestin redistribution for increasing concentrations of hG17 is plotted in  FIG. 6A . From this data a dose response curve was calculated which resulted in an EC 50  for translocation of 4.2±1.5 nM ( FIG. 6B ), in agreement with the IP3 results. 
     Serum samples from a patient with hypergastrinemia were evaluated in addition to hG17 using this 5 minutes assay paradigm ( FIG. 6A ). Arrestin translocated in response to the serum, but the measured response occurred in a range approximating the lower limits of assay sensitivity for hG17. In order to increase assay sensitivity at concentrations near or below 1 nM agonist, arrestin-GFP redistribution in response to hG17 after 1-2 hours of incubation at 37° C. was directly measured.  FIG. 6C  shows representative cell fields that were exposed to increasing concentrations of hG17. Vesicles are readily apparent at concentrations below 10 nM hG17. 
       FIG. 6A  illustrates a representative experiment depicting the response of Clone A cells that were exposed to various concentrations of hG17 for 5 minutes. Fluorescence images were obtained every 30 seconds and analyzed for arrestin-GFP translocation. Nine to eleven cells were analyzed for each time point for each curve. The graph shows the fractional amount of arrestin-GFP remaining in the cytosol as a function of time.  FIG. 6B  shows that the fractional amount of arrestin-GFP lost after 5 minutes from the cell cytosol was used to generate a sigmoid dose response curve for the increasing concentrations of hG17 shown in the graph in A. Data are presented as mean±SEM.  FIG. 6C  shows images from an experiment demonstrating the response of Clone A cells exposed to hG17 for one hour at 37° C. 
     Example 5 
     Calculation of the Fluorescence Signal from the Distribution of βarrestin2-GFP 
     The methodology for these measurements of Example 4 is described in  FIG. 7 . The signal over background obtained by simply measuring fluorescent intensities was quite large due to the concentration of translocated arrestins in small volumes, as shown in  FIG. 7B  where a 120-fold increase was observed. 
       FIG. 7A  shows the distribution of βarrestin-GFP fluorescence in cells stably expressing the arrestin2-GFP fusion protein. The receptor was visualized before and after a 30-minute agonist treatment. Image intensity was acquired using 8 bits per pixel, yielding a grayscale range of intensities from 0-255. Relative intensities greater than 255 were clipped and set equal to 255. The upper and lower panels show the same field of cells. The cells appear brighter in the upper panels because the average pixel intensity in the upper images was selected to fall midway between 0-255. In the lower panels the mean pixel intensity falls within the bottom 12% of the dynamic range. The selection of a higher mean intensity caused many of the pixels representing translocated βarrestin2-GFP in the right upper panel to be clipped at a relative brightness of 255. This resulted in an underestimate of the amount of agonist-induced translocation, which is avoided in the lower images due to the selection of a smaller mean intensity.  FIG. 7B  illustrates a histogram of the pixel count versus pixel intensity (green curve in the graph at the lower left) was generated using a representative control cell. The first minimal intensity peak represents background and the second peak is green fluorescent protein. The mean cell intensity plus three standard deviations (&gt;99 th  percentile) was selected as a threshold to separate the fluorescence signal of the untranslocated cytosolic βarrestin, from the Parrestin that translocated with the receptor into vesicles. Pixels with intensities above this baseline (magenta curve corresponds to vesicles in the treated cells) are indicated by the magenta color overlay in both the control and treated images (upper left an right panels). Note the correspondence between the magenta-colored pixels in the upper right image of  FIG. 7B  and the βarrestin-GFP-containing endocytic vesicles in the lower right image of  FIG. 7A . Comparison of the total pixel intensity from pixels above the baseline for the two images is depicted in the lower right graph and shows a 120-fold increase in the fluorescence signal in the treated cells. Image data were analyzed by the computer program IP labs. 
     Example 6 
     Detection of Immunologically Undetectable Gastrin Receptor Agonists in Serum: Dose Response to Pentagastrin 
     There have been numerous reports of clinical hypergastrinemia in the absence of elevated serum gastrin levels. Consequently, RIA detection of serum gastrin may miss a group of patients with immunologically undetectable gastrin receptor agonists. Therefore the effects on translocation of the CCK-B receptor agonist pentagastrin, which is not detectable by conventional RIA, were evaluated. As illustrated in  FIG. 8 , the response of the biosensor to increasing amounts of pentagastrin produced a dose-response with an EC 50  for translocation of 2.4±1.9 nM, similar to the reported Kd of 1 nM for pentagastrin binding to the CCK-B receptor and 3.9 nM for polyphosphoinositide turnover. 
       FIG. 8A  shows images from a representative experiment depicting the response of Clone A cells that were exposed to pentagastrin at 37° C. for two hours. Fluorescence images were analyzed for arrestin-GFP translocation as described in Methods. The graph in  FIG. 8B  depicts the increase in the normalized sum of pixel intensity (TI/TF) above a threshold value (Methods) for images obtained at each concentration of ligand. Data are representative of two experiments, each with eight to ten separate images and are presented as mean±SEM. 
     Example 7 
     Detection of Gastrin from Patient with Hypergastrinemia: Dose Response at One Hour 
     The ability of the biosensor to respond to various synthetic gastrin isoforms suggested its potential to measure the multitude of bioactive forms of gastrin contained in human serum. The upper panels in  FIG. 9A  show the cellular response to a patient serum sample (documented hypergastrinemia by RIA of 5000 pg/ml, range of 3,400 to 6,600 pg/ml) after one hour of incubation. Arrestin-GFP was seen at the plasma membrane and in vesicles. The amount of translocated arrestin-GFP was compared to an hG17 standard curve generated from the data represented by the images of  FIG. 6C . The EC 50  of the hG17 dose response curve was (0.80±0.25) nM ( FIGS. 6B and 6C ), and the patient&#39;s bioactive gastrin serum concentration was determined to be 0.63 nM±0.16 nM (see arrow  FIG. 9B ). 
     Depicted in  FIG. 9A  is a representative image of Clone A cells incubated for one hour with a 1:1 dilution of serum that was obtained from a patient with hypergastrinemia.  FIG. 9B  shows a dose response curve to hG 17 of Clone A cells computed from the imaged translocation data obtained at one hour and analyzed as in  FIG. 7 . Between 9 and 16 separate images were analyzed for each hG17 concentration and the patient&#39;s serum (arrow). Data are presented as mean±SEM. 
     The present inventors demonstrated that the agonist-mediated arrestin interaction with the CCK-B receptor mirrored the pharmacology of CCK-B receptor signaling using a biosensor consisting of a cell permanently expressing CCK-B receptors and βarrestin2-GFP (arrestin-GFP). Moreover, this biosensor was used to determine the serum gastrin concentration of a patient with hypergastrinemia. All bioactive gastrin isoforms, including those not identified with conventional RIA, were detected with methods of the present invention. 
     These data show that the CCK-B receptor underwent agonist-mediated arrestin regulation. The IP3 receptor signaling was activated at the same gastrin and pentagastrin concentrations that produced arrestin translocation. The residues in gastrin that produce CCK-B receptor conformations capable of desensitization likely reside in the gastrin terminal pentapeptide. 
     After the CCK-B receptor bound agonist and arrestin initiated receptor internalization, the CCK-B receptor moved into the cell via arrestin-mediated clathrin-coated vesicular pathway. The CCK-B receptor was shown to be a “Class B” GPCR and subsequent receptor endocytosis may initiate secondary (or intracellular) signaling events, for example MAP kinase activation. 
     This pharmacology was exploited to construct a single cell biosensor to measure serum concentrations of gastrin. Given the large number of GPCRs that desensitize by arrestin, biosensors have been similarly constructed for other GPCR ligands; additional biosensors will be constructed. The novelty of this biological approach is that combinations of G protein-coupled receptors and fluorescent proteins form some of the most sensitive biosensor arrays developed and provide a mechanism to detect thousands of natural and synthetic compounds. 
     An area where synthetic compounds are clinically useful is in cancer chemotherapy. In particular, heptagastrin conjugated to an ellipticine moiety was used to kill tumors expressing the CCK-B receptor. The toxicity of this agent was shown to require receptor endocytosis, which based on our findings, was most likely arrestin dependent. This agent was administered to mice in concentrations well within the measurable range of our biosensor. The present invention has broad applications for the evaluation of newly designed drugs where the pharmacology is unknown and where serum or tissue levels need to be determined. 
     GPCRs signal the presence of bioactive substances. The present inventors exploited the common biochemical paradigm for terminating GPCR signaling and determined that GPCRs can also be used to detect those substances for clinical diagnosis. 
     Example 7 
     Muscarinic Receptor is Internalized in the Presence of Acetylcholine 
     HEK-293 cells expressing the human muscarinic receptor type 1 conjugated with the vasopressin carboxyl tail were exposed to micromolar concentrations of acetylcholine. The translocation of arrestin-GFP was determined. Arrestin-GFP was observed at the membrane edge or in vesicles, as shown in  FIG. 10 . 
     The biosensor expressing the muscarinic receptor and arrestin-GFP was useful for the detection of acetylcholine in a sample, as indicated by the agonist-induced internalization of arrestin-GFP. 
     Example 8 
     Acetylcholinesterase Inhibits Acetylcholine Induced Internalization of Muscarinic Receptor 
     HEK-293 cells were incubated in Minimal Essential Media containing 10% Fetal Bovine Serum (FBS). The cells had been transiently transfected with cDNA to induce the expression of arrestin-GFP and the human muscarinic receptor type 1 conjugated with the vasopressin carboxyl tail. In the presence of 10% FBS, no arrestin-GFP translocation was observed after the cells were exposed for up to 30 minutes to concentrations of acetylcholine in the range of 10-100 micromolar. However, millimolar amounts of acetylcholine did produce arrestin-GFP translocation. When cells were exposed to 10-20 micromolar concentrations of acetylcholine in the absence of serum, arrestin-GFP translocated readily to the cell membrane. Acetylcholinesterase, an enzyme known to degrade acetylcholine, is a common component of serum, including FBS. The acetylcholinesterase broke down the acetylcholine, the ligand of the muscarinic receptor, thereby preventing acetylcholine-induced internalization of the muscarinic receptor and arrestin. Exceeding large amounts of acetylcholine (millimolar) in the presence of acetylcholinesterase were able to produce only a transient amount of arrestin internalization. In contrast, a much smaller concentration of acetylcholine (10-20 micromolar) was able to produce a robust response when serum was absent from the media. This suggests that arrestin translocation can be used to assay serum for inhibitors of acetylcholinesterase, as a potent inhibitor such as an organophosphate compound would produce effects similar to removing the serum and all its constituent ingredients entirely from the environment of the test cell containing arrestin-GFP and the acetylcholine-exposed muscarinic receptor. 
     Example 9 
     Use of Muscarinic Acetyl Choline Receptor to Screen for Acetylcholinesterase Inhibitors 
     A sample containing the putative inhibitor is extracted into an appropriate solvent, in one instance this may be an aqueous buffer. The extract either is diluted or combined with a buffer containing acetylcholinesterase protein and an agonist to the muscarinic receptor such as acetylcholine chloride sensitive to the acetylcholinesterase. This mixture containing the agonist, acetylcholinesterase, and the putative inhibitor is allowed to incubate for a given period of time between zero and a few hours, between 5 and 60 minutes is most practical, and then placed in contact with a cell containing the muscarinic receptor with its natural tail or the tail interchanged with a high affinity tail such as from the vasopressin receptor and arrestin-GFP. If a putative inhibitor of acetylcholinesterase is present, the acetylcholine chloride will not be broken down and translocation of the arrestin-GFP to the plasma membrane or endosomes will occur due to the activation of the receptor by the acetylcholine. If an inhibitor of the acetylcholinesterase is not present, the acetylcholine will be degraded and a lesser amount or no amount of arrestin-GFP translocation will occur. This assay can be used to assess commonly used inhibitors of the acetylcholinesterase enzyme in the environment such as the organophosphate insecticides, for example diazinon (EPA Completes Risk Assessment and Announces Risk Reduction Agreement for the Pesticide Diazinon. On Dec. 5, 2000, EPA released its revised risk assessment and announced an agreement with registrants to phase out/eliminate certain uses of the organophosphate pesticide diazinon.), and more potent inhibitors not commonly found such as derivatives the neurotoxin sarin. A particular use of such an assay system could be the continuous monitoring of a municipal water system for insecticides and like compounds by continuously adding aliquots of water, premixed with an acetylcholine chloride like agonist and acetylcholinesterase, to chambers with cells containing arrestin-GFP and the muscarinic receptor, and observing the cells for loss of inhibition of arrestin-GFP translocation. The assays can be performed on a high throughput basis by instruments that are commercially available for this purpose. Another use of this assay would be to assess a person&#39;s physiological exposure to compounds that inhibit acetylcholinesterase by measuring the presence of these compounds in serum or tissue. For example a drop of blood could directly be placed in a well containing a cell exposed to acetylcholine chloride and possessing, arrestin-GFP and the muscarinic acetylcholine receptor. Human blood or serum normally contains sufficient acetylcholinesterase to rapidly degrade acetylcholine. Thus, a loss of inhibition of the ability of the cell to translocate arrestin-GFP would be indicative of the presence of an acetylcholinesterase inhibitor. 
     While the invention has been described and illustrated herein by references to various specific material, procedures and examples, it is understood that the invention is not restricted to the particular material combinations of material, and procedures selected for that purpose. Numerous variations of such details can be implied as will be appreciated by those skilled in the art. 
     The following is a list of documents related to the above disclosure and particularly to the experimental procedures and discussions. The following documents, as well as any documents referenced in the foregoing text, should be considered as incorporated by reference in their entirety.
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