Patent Publication Number: US-2016237136-A1

Title: NOVEL CELL LINES EXPRESSING ENaC and METHODS USING THEM

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application 61/062,371, filed Jan. 25, 2008, and U.S. Provisional Application 61/063,219, filed Feb. 1, 2008, which applications are both incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to ENaC and cells and cell lines stably expressing ENaC or proteolyzed forms of ENaC. The invention further provides methods of making such cells and cell lines. The ENaC cells and cell lines provided herein are useful in identifying modulators of ENaC or proteolyzed forms of ENaC. 
     BACKGROUND 
     Epithelial sodium channels (ENaC) are multimeric, transmembrane channels located in the apical membranes of a wide variety of epithelial tissues, including the kidneys, lungs and the colon, where they play ah important role in homeostasis by regulating salt/water reabsorption. The channels comprise three subunits: alpha or delta, beta and gamma. Modulators of ENaC have therapeutic potential in diseases that are related to ion conductance through ENaC channels, such as hypertension, pulmonary edema and cystic fibrosis. 
     The discovery of new and improved therapeutics that specifically target ENaC family members has been hampered by the lack of robust, physiologically relevant cell-based systems and more especially such systems that are amenable to high through-put formats for identifying and testing ENaC modulators. Cell-based systems are preferred for drug discovery and validation because they provide a functional assay for a compound as opposed to cell-free systems, which only provide a binding assay. Moreover, cell-based systems have the advantage of simultaneously testing cytotoxicity. Ideally, cell-based systems should also stably and constitutively express the target protein. It is also desirable for a cell-based system to be reproducible. With ENaC, this was not possible because ENaC is a heteromultimeric protein and ENaC expression was cytotoxic. The present invention addresses these problems. 
     SUMMARY OF THE INVENTION 
     We have discovered new and useful cells and cell lines that express functional ENaC comprising an alpha or delta subunit, a beta subunit and a gamma subunit. The cells in the cell line may (a) be eukaryotic cells, (b) be mammalian cells, (c) not express ENaC endogenously, or (d) any combination of (a), (b) and (c). In some embodiments, the cells are CHO cells. In other embodiments, the cells are capable of forming polarized monolayers. In the cells or cell lines of the invention, the ENaC may be mammalian, such as mouse, rat or human, preferably human. One or more ENaC subunits may be chimeric, i.e., comprising sequences from two or more sources which can be different species. In some embodiments, a membrane potential dye assay using the cells or cell lines of the invention and NaCl as an agonist has a Z′ value of at least 0.8. The cells or cell lines may be stable in culture media without antibiotics. In some embodiments, one or more ENaC subunits are expressed from an introduced nucleic acid encoding it. In some embodiments, one or more ENaC subunits are expressed from an endogenous nucleic acid by gene activation. 
     In another aspect of the invention, the cells or cell lines express a human ENaC alpha subunit. An ENaC alpha subunit may be a polypeptide having the amino acid sequence set forth in SEQ ID NO: 7, SEQ ID NO: 30, SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21; a polypeptide with 95% sequence identity to any one of SEQ ID NOS: 7, 10, 19, 20 and 21 and that is blocked by amiloride; a polypeptide encoded by a nucleotide that hybridizes to any one of SEQ ID NOS: 1, 4 and 22 under stringent conditions; or a polypeptide that is an allelic variant of any one of SEQ ID NOS: 7, 10, 19, 20 and 21. The ENaC alpha subunit may also be encoded by a nucleic acid having the sequence set forth in SEQ ID NO: 1, SEQ ID NO: 4, and SEQ ID NO: 22; a nucleic acid that hybridizes to any one of SEQ ID NOS: 1,4, and 22 under stringent conditions; a nucleic acid that encodes the polypeptide of any one of SEQ ID NOS: 7, 10, 19, 20 and 21; a nucleic acid with at least 95% sequence identity to any one of SEQ ID NOS: 1,4 and 22; or a nucleic acid that is an allelic variant of any one of SEQ ID NOS: 1,4 and 22. 
     In another aspect of the invention, the cells or cell lines express a human ENaC beta subunit. An ENaC beta subunit may be a polypeptide having the amino acid sequence set forth in SEQ ID NO: 8, SEQ ID NO: 11; a polypeptide with 95% sequence identity to anyone of SEQ ID NOS: 8 and 11 and that is blocked by amiloride; a polypeptide encoded by a nucleic acid that hybridizes to any one of SEQ ID NOS: 2, 5 and 23; or a polypeptide that is an allelic variant of any one of SEQ ID NOS: 8 and 11. The ENaC beta subunit may be encoded by a nucleic acid having the sequence set forth in SEQ ID NO: 2, SEQ ID NO: 5, and SEQ ID NO: 23; a nucleic acid that hybridizes to any one of SEQ ID NOS: 2, 5 and 23 under stringent conditions; a nucleic acid that encodes the polypeptide of any one of SEQ ID NOS: 8 and 11; a nucleic acid with at least 95% sequence identity to any one of SEQ ID NOS: 2, 5 and 23; or a nucleic acid that is an allelic variant of any one of SEQ ID NOS: 2, 5 and 23. 
     In another aspect, the cells or cell lines of the invention express a human ENaC gamma subunit. A gamma ENaC may be a polypeptide having the amino acid sequence set forth in SEQ ID NO: 9, SEQ ID NO: 12; a polypeptide with 95% sequence identity to any one of SEQ ID NOS: 9 and 12 and that is blocked by amiloride; a polypeptide encoded by a nucleic acid that hybridizes to any one of SEQ ID NOS: 3, 6 and 24; and a polypeptide that is an allelic variant of any one of SEQ ID NOS: 9 and 12. An ENaC gamma subunit may be encoded by a nucleic acid having the sequence set forth in SEQ ID NO: 3, SEQ ID NO: 6, and SEQ ID NO: 24; a nucleic acid that hybridizes to any one of SEQ ID NOS: 3,6 and 24 under stringent conditions; a nucleic acid that encodes the polypeptide of any one of SEQ ID NOS: 9 and 12; a nucleic acid with at least 95% sequence identity to any one of SEQ ID NOS: 3, 6 and 24; and a nucleic acid that is an allelic variant of any one of SEQ ID NOS: 3, 6 and 24. 
     In another aspect, the cells or cell lines of the invention express a human ENaC delta subunit. A delta ENaC may be a encoded by a nucleic acid having the sequence set forth in SEQ ID NO: 24; a nucleic acid that hybridizes to SEQ ID NO: 24 under stringent conditions; a nucleic acid with at least 95% sequence identity to SEQ ID NO: 24; or a nucleic acid that is an allelic variant of any one of SEQ ID NO: 24. 
     In some embodiments, the cells or cell lines of the invention are treated with a protease so that the ENaC has a proteolyzed form. In some embodiments, the protease is trypsin. 
     In another aspect, the invention provides a method for producing the cells or cell lines of the invention comprising the steps of: (a) introducing a first vector comprising a nucleic acid encoding an ENaC alpha or delta subunit, a second vector comprising a nucleic acid encoding an ENaC beta subunit and a third vector comprising a nucleic acid encoding an ENaC gamma subunit into a host cell; (b) introducing into the host cell produced in step (a) a first signaling probe that detects the expression of the ENaC alpha or delta subunit, a second signaling probe that detects the expression of the ENaC beta subunit and a third signaling probe that detects the expression of the ENaC gamma subunit, (c) isolating a cell that expresses the ENaC alpha or delta subunit, the ENaC beta subunit and the ENaC gamma subunit. In some embodiments, the method comprises the additional step of generating a cell line from a cell isolated in step (c). In some embodiments, the host cells (a) are eukaryotic cells; (b) are mammalian cells; (c) do not express ENaC endogenously; or (d) any combination of (a), (b) and (c). 
     In some embodiments, the method produces an ENaC-expressing cell line wherein the ENaC alpha subunit comprises the amino acid sequence set forth in SEQ ID NO: 7, the ENaC beta subunit comprises the amino acid sequence set forth in SEQ ID NO: 8 and the ENaC gamma subunit comprises the amino acid sequence set forth in SEQ ID NO: 9. In other embodiments, the method produces an ENaC-expressing cell line wherein the ENaC alpha subunit is encoded by a nucleic acid comprising SEQ ID NO: 1, the ENaC beta subunit is encoded by a nucleic acid comprising SEQ ID NO: 2 and the ENaC gamma subunit is encoded by a nucleic acid comprising SEQ ID NO: 3. 
     In some embodiments, the method of producing cells and cell lines of the invention utilizes flow cytometry to isolate a cell that expresses the ENaC alpha or delta subunit, the ENaC beta subunit and the ENaC gamma subunit. In some embodiments, the method utilizes a signaling probe that fluoresces upon binding to a target sequence and uses a fluorescence activated cell sorter to isolate a cell. 
     In another aspect, the invention provides a method for identifying a modulator of an ENaC function comprising the step of exposing a cell or cell line of the invention to a test compound and detecting a change in an ENaC function. In some embodiments, the detecting step is selected from a membrane potential assay, electrophysiology assay, a binding assay or an Ussing chamber assay. The modulator may be an ENaC inhibitor, antagonist, partial antagonist, an ENaC agonist, partial agonist or potentiator. Test compounds used in the method may include a small molecule, a chemical moiety, a polypeptide, or an antibody. In other embodiments, the test compound is in a library of compounds. The library may be a small molecule library, a combinatorial library, a peptide library or an antibody library. In some embodiments, the modulator identified with the method of the invention is selective for an enzymatically modified form of ENaC. 
     In another aspect, the invention provides a method for producing a cell or cell line comprising an enzymatically modified form of ENaC, by exposing a cell or cell line of the invention to a protease, wherein at least one enzymatically modified form of ENaC is generated. In some embodiments, the protease is trypsin. 
     In another aspect, the invention provides a method of culturing a cell or cell line of the invention that expresses an ENaC comprising the step of culturing cells in media with reduced sodium. In some embodiments, the concentration of sodium in the media is between 9 mM and 140 mM sodium. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  ENaC induces cytotoxicity.  FIG. 1  depicts cells that were transfected with either vectors containing ENaC subunits (right panel) or empty vector controls (left panel). Light microscopy showed increased cell death in ENaC-expressing cells. 
         FIG. 2  ENaC induces cell swelling.  FIG. 2  depicts CHO cells that were transfected with either vectors containing ENaC subunits (right panel) or empty vector controls (left panel). Cells expressing ENaC exhibited increased surface area and volume. 
         FIG. 3  Characterization of a stable ENaC cell line.  FIG. 3A  is a series of histograms showing cells expressing, ENaC alpha, beta and gamma subunits as detected by FACS.  FIG. 3B  depicts the relative expression level of ENaC alpha, beta and gamma subunits.  FIG. 3C  shows data for the ability of untransfected cells, and cells having various combinations of ENaC subunits to take up sodium and lithium in the presence or absence of amiloride (antagonist). 
         FIG. 4  Response of an ENaC cell line in a functional assay.  FIG. 4  shows representative traces from the ENaC membrane potential assay in the presence of added NaCl alone or NaCl plus amiloride. EC50 values for NaCl and IC50 values for amiloride are shown. 
         FIG. 5  Assay Consistency.  FIG. 5  shows that ENaC-expressing cells subjected to a membrane potential dye assay have a Z′ of 0.85. 
         FIG. 6  Identification of Novel ENaC Modulators  FIG. 6  shows traces produced by a test compound with ENaC inhibiting activity (left panel) and by a compound with ENaC potentiating activity (right panel), identified using an ENaC cell line of the invention in a membrane potential assay. 
         FIG. 7  Proteolysis of ENaC generates novel isoforms.  FIG. 7A  shows the membrane potential response for 24 forms of proteolyzed ENaC.  FIG. 7B  shows the activation of a membrane potential dye in response to the addition of sodium to cell lines expressing each of the 24 proteolyzed variants and the non-proteolyzed form. 
         FIG. 8  Amiloride selectively inhibits the non-proteolyzed form of ENaC compared to one of the proteolyzed forms of ENaC (ENaC-P).  FIG. 8  is a dose response curve of amiloride for both non-proteolyzed (filled squares) and one proteolyzed form (open circles) of ENaC. A concentration sufficient to almost completely inhibit non-proteolyzed ENaC was not effective to inhibit Form 24. 
         FIG. 9  Proteolyzed forms of ENaC are differentially blocked by different compounds including amiloride (Form 24).  FIG. 9  shows graphs depicting the response of 24 proteolyzed forms of ENaC in response to different compounds. 
         FIG. 10  High-Throughput data confirmed by Ussing Chamber assays.  FIG. 10  shows the results of Ussing Chamber assays for potentiating compounds (upper panels) and inhibitory compounds (lower panel). 
         FIG. 11  Taste testing of compounds that selectively potentiate either the non-proteolyzed (NP) or one proteolyzed (P) form of ENaC.  FIG. 11  shows the results of a taste test for controls and potentiators of a proteolyzed form of ENaC. The effects of compounds on salt taste was assessed by having taste panelists compare saline solutions with or without compound. Potentiators of the proteolyzed form of ENaC were selected as enhancing salt taste compared to control solutions containing NaCl without the potentiators more often than potentiators of the non-proteolyzed form. 
     
    
    
     DETAILED DISCLOSURE 
     Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Exemplary methods and materials are described below, although methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention. All publications and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. Although a number of documents are cited herein, this citation docs not constitute an admission that any of these documents forms part of the common general knowledge in the art. Throughout this specification and claims, the word “comprise,” or variations such as “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular. The materials, methods, and examples are illustrative only and not intended to be limiting. 
     In order that the present invention may be more readily understood, certain terms are first defined. Additional definitions are set forth throughout the detailed description. 
     The term “stable” or “stably expressing” is meant to distinguish the cells and cell lines of the invention from cells with transient expression as the terms “stable expression” and “transient expression” would be understood by a person of skill in the art. 
     The term “cell line” or “clonal cell line” refers to a population of cells that are all progeny of a single original cell. As used herein, cell lines are maintained in vitro in cell culture and may be frozen in aliquots to establish banks of clonal cells. 
     The term “stringent conditions” or “stringent hybridization conditions” describe temperature and salt conditions for hybridizing one or more nucleic acid probes to a nucleic acid sample and washing off probes that have not bound specifically to target nucleic acids in the sample. Stringent conditions are known to those skilled in the art and can be found in  Current Protocols in Molecular Biology , John Wiley &amp; Sons, N.Y. (1989), 6.3.1-6.3.6. Aqueous and nonaqueous methods are described in that reference and either can be used. An example of stringent hybridization conditions is hybridization in 6×SSC at about 45° C., followed by at least one wash in 0.2×SSC, 0.1% SDS at 60° C. A further example of stringent hybridization conditions is hybridization in 6×SSC at about 45° C., followed by at least one wash in 0.2×SSC, 0.1% SDS at 65° C. Stringent conditions include hybridization in 0.5M sodium phosphate, 7% SDS at 65° C., followed by at least one wash at 0.2×SSC, 1% SDS at 65° C. 
     The phrase “percent identical” or “percent identity” in connection with amino acid and/or nucleic acid sequences refers to the similarity between at least two different sequences. This percent identity can be determined by standard alignment algorithms, for example, the Basic Local Alignment Tool (BLAST) described by Altshul et al. ((1990) J. Mol. Biol., 215:403-410); the algorithm of Needleman et al. ((1970) J. Mol. Biol., 48:444-453); or the algorithm of Meyers et al. ((1988) Comput. Appl. Biosci., 4: 11-17). A set of parameters may be the Blosum 62 scoring matrix with a gap penalty of 12, a gap extend penalty of 4, and a frameshift gap penalty of 5. The percent identity between two amino acid or nucleotide sequences can also be determined using the algorithm of E. Meyers and W. Miller ((1989) CABIOS, 4:11-17) that has been incorporated into the ALIGN program (version 2.0), using a PAM120 weight residue table, a gap length penalty of 12 and a gap penalty of 4. The percent identity is usually calculated by comparing sequences of similar length. Protein analysis software matches, similar sequences using measures of similarity assigned to various substitutions, deletions and other modifications, including conservative amino acid substitutions. For instance, the GCG Wisconsin Package (Accelrys, Inc.) contains programs such as “Gap” and “Bestfit” that can be used with default parameters to determine sequence identity between closely related polypeptides, such as homologous polypeptides from different species of organisms or between a wild type protein and a mutein thereof. See, e.g., GCG Version 6.1. Polypeptide sequences also can be compared using FASTA using default or recommended parameters. A program in GCG Version 6.1. FASTA (e.g., FASTA2 and FASTA3) provides alignments and percent sequence identity of the regions of the best overlap between the query and search sequences (Pearson, Methods Enzymol. 183:63-98 (1990); Pearson, Methods Mol. Biol. 132:185-219 (2000)). The length of polypeptide sequences compared for identity will generally be at least about 16 amino acid residues, usually at least about 20 residues, more usually at least about 24 residues, typically at least about 28 residues, and preferably more than about 35 residues. The length of a DNA sequence compared for identity will generally be at least about 48 nucleic acid residues, usually at least about 60 nucleic acid residues, more usually at least about 72 nucleic acid residues, typically at least about 84 nucleic acid residues, and preferably more than about 105 nucleic acid residues. 
     The phrase “substantially as set out,” “substantially identical” or “substantially homologous” in connection with an amino acid nucleotide sequence means that the relevant amino acid or nucleotide sequence will be identical to or have insubstantial differences (through conserved amino acid substitutions) in comparison to the sequences that are set out. Insubstantial differences include minor amino acid changes, such as 1 or 2 substitutions in a 50 amino acid sequence of a specified region. 
     The terms “potentiator”, “agonist” or “activator” refer to a compound or substance that increases ion conductance via an ENaC. As used herein, a potentiator or activator may act upon all or upon a specific subset of ENaCs, ENaC subunits or proteolytic isoforms of ENaC. 
     The terms “inhibitor”, “antagonist” or “blocker” refers to a compound or substance that decreases ion conductance via an ENaC. As used herein, an inhibitor or blocker may act upon all or upon a specific subest of ENaCs, ENaC subunits or proteolytic isoforms of ENaC. 
     The term “modulator” refers to a compound or substance that alters the structure, conformation, biochemical or biophysical properties or functionality of an ENaC either positively or negatively. The modulator can be an ENaC agonist (potentiator or activator) or antagonist (inhibitor or blocker), including partial agonists or antagonists, selective agonists or antagonists and inverse agonists, and can be an allosteric modulator. A substance or compound is a modulator even if its modulating activity changes under different conditions or concentrations or with respect to different forms (i.e., proteolyzed forms) of ENaC. As used herein, a modulator may affect the ion conductance of an ENaC, the response of an ENaC to another regulatory compound or the selectivity of an ENaC. A modulator may also change the ability of another modulator to affect the function of an ENaC A modulator may act upon all or upon a specific subset of ENaCs, ENaC subunits, or proteolytic isoforms of ENaC. Modulators include, but are not limited to, potentiators, activators, inhibitors, agonists, antagonists and blockers. 
     The phrase “functional ENaC” refers to an ENaC that comprises at least an alpha or delta, beta and a gamma substances and that responds to a known activator such as sodium or a known inhibitor, such as amiloride in substantially the same way as ENaC in a cell that normally expresses ENaC without engineering. ENaC behavior can be determined by, for example, physiological activities and pharmacological responses. Physiological activities include, but are not limited to sodium conductance and lithium conductance. Pharmacological responses include, but are not limited to, inhibition by amiloride. 
     A “heterologous” or “introduced” ENaC subunit means that the ENaC subunit is encoded by a polynucleotide introduced into a host cell. 
     This application relates to novel cells and cell lines that have been engineered to express three or more epithelial sodium channel (ENaC) subunits. In some embodiments, the cells or cell lines express three ENaC subunits: an alpha or delta subunit, a beta subunit, and a gamma subunit. In some embodiments, the novel cells or cell lines of the invention express a functional ENaC. In some embodiments the ENaC expressing cells or cell lines are enzymatically treated to generate enzymatically modified forms of ENaC. In other aspects, the invention provides methods of making and using the novel cells and cell lines. 
     ENaC is a protein that is present in many mammalian tissues, including epithelial cells of the kidney, lung, colon, CNS/brain, heart, vasculature, neurons of arterial baroreceptors, endothelial and vascular smooth muscle cells, eye, mouth, tongue and fallopian tubes. Without being bound by theory, we believe that ENaC dysregulation, dysfunction or improper proteolytic processing may be linked to many disease states including pulmonary disorders such as cystic fibrosis and chronic obstructive pulmonary disease, cardiovascular disorders, renal disorders and infertility. 
     ENaC is a membrane spanning multimeric protein channel, comprising alpha, beta and gamma subunits. Various stochiometries may be possible, including but not limited to two alpha, one beta, and one gamma; three of each subunit; or one each of each subunit. Delta subunits substitute for the alpha subunits in some or all channels in some tissue types. 
     According to one embodiment of the invention, the novel cells and cell lines are triply transfected with a nucleic acids individually encoding a ENaC alpha subunit, an ENaC beta subunit, and an ENaC gamma subunit on the same or separate vectors. The novel cell lines of the invention stably express the introduced ENaC subunits. In another embodiment, the novels cells and cells lines have three subunits activated for expression by gene activation. The invention also provides cell lines stably expressing ENaC subunits that have been enzymatically (such as proteolytically) treated to generate enzymatically modified forms of ENaC. The cells and cell lines of the invention comprising intact or enzymatically modified forms of ENaC can be used to identify modulators of ENaC function, including modulators that are specific for a particular ENaC enzymatically modified form. The cells and cell lines can thus be used to obtain information about the properties, activities and roles of individual enzymatically modified forms of ENaC and to identify ENaC modulators with activity for all enzymatically modified forms or on a particular form or subset of forms. These modulators are useful as therapeutics that target differentially modified ENaC forms in disease states or tissues. Because the disregulation of proteolysis of ENaC in vivo, for example, may contribute to an undesired activity or disease state, cells and cell lines of this invention also can be used to screen for modulators that reduce or promote proteolytic processing for therapeutic use where altered of proteolysis may be desired. The cells and cell lines also are useful to identify modulators that have activity with only a subset of proteolytic isoforms of ENaC. 
     ENaC also has been proposed as a mammalian salt taste receptor. Within the oral cavity, there are many active proteases including salivary enzymes and bacterially derived proteins, consistent with the proteolysis of ENaC in the oral cavity. As shown herein ( FIGS. 7-9 ), differentially proteolyzed forms of ENaC show varied sensitivity to amiloride, a known blocker of ENaC. Some proteolyzed forms of ENaC also show varied sensitivity to amiloride compared to non-proteolyzed forms of ENaC. Some forms have little or no sensitivity to amiloride. For example, one or a combination of proteolyzed ENaC forms may account for the human salt taste sensory data on the partial but not full block of salt taste by amiloride. Also, ENaC is expressed in various tissues where it has been implicated in diseases where ENaC inhibitors would be expected to be of therapeutic value. Therefore, the use of other (non-amiloride) modulators may be of therapeutic value. 
     This invention also identifies and solves a difficulty in generating stable ENaC expressing cells and cell lines. As disclosed herein, we have discovered that expression of ENaC is cytotoxic in several host cell lines tested. ENaC expression led to swelling and bursting, resulting in ENaC cytotoxicity. It was further discovered that growing cells and cell lines in low-sodium media alleviated ENaC toxicity and allowed ENaC expressing cells and cell lines to behave in a physiological manner. 
     In a first aspect, the invention provides cells and cell lines that stably express ENaCs. In some embodiments, the expressed ENaCs conduct sodium and lithium ions and are modulated by amiloride. In further embodiments, the ENaC cells and cell lines of the invention have enhanced properties compared to cells and cell lines made by conventional methods. For example, the ENaC cells and cell lines have enhanced stability of expression (even when maintained in culture without selective antibiotics) and result in high Z′ values. Cells and cell lines of the invention can also be treated with enzymes such as trypsin to generate, for example, proteolyzed forms of ENaC. In other aspects, the invention provides methods of making and using the ENaC cells and cell lines. 
     In various embodiments, the cell or cell line of the invention expresses ENaC alpha, beta and gamma subunits at a consistent level of expression for at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, 40, 45, 50; 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200 days or over 200 days, where consistent expression refers to a level of expression that does not vary by more than:
 
1%, 2%, 3%, 4%, 5%, 6%, 7%, 8% 9% or 10% over 2 to 4 days of continuous cell culture; 2%, 4%, 6%, 8%, 10% or 12% over 5 to 15 days of continuous cell culture; 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18% or 20% over 16 to 20 days of continuous cell culture; 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24% over 21 to 30 days of continuous cell culture; 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30% over 30 to 40 days of continuous cell culture; 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30% over 41 to 45 days of continuous cell culture; 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30% over 45 to 50 days of continuous cell culture; 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30% or 35% over 45 to 50 days of continuous cell culture, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30% over 50 to 55 days of continuous cell culture, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30% or 35% over 50 to 55 days of continuous cell culture; 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%, 30%, 35% or 40% over 55 to 75 days of continuous cell culture; 1%, 2%, 3%, 4%, 5%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45% over 75 to 100 days of continuous cell culture; 1%, 2%, 3%, 4%, 5%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45% over 101 to 125 days of continuous cell culture; 1%, 2%, 3%, 4%, 5%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45% over 126 to 150 days of continuous cell culture; 1%, 2%, 3%, 4%, 5%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45% over 151 to 175 days of continuous cell culture; 1%, 2%, 3%, 4%, 5%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45% over 176 to 200 days of continuous cell culture; 1%, 2%, 3%, 4%, 5%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45% over more than 200 days of continuous cell culture.
 
     According to the invention, the ENaC expressed by a cell or cell line can be from any mammal, including rat, mouse, rabbit, goat, dog, cow, pig or primate. The alpha (and delta), beta and gamma subunits can be from the same or different species. In a preferred embodiment, the ENaC is human ENaC, including human alpha and/or human delta, human beta, and human gamma subunits. Also within the invention are engineered cells and cell lines stably expressing a moleculed selected from the group consisting of: Unc-105, MEC-4, MEC-10, MEC-2, RPK, FaNaC(Ha), FaNaC(Ht), ASIC1a, ASIC1b, ASIC2a, ASIC3, ASIC1a/2a, ASIC2a/2b, ASIC2a/3, ASIC2b/3, BLINaC, hINaC. 
     In some embodiments, a cell or cell line of the invention may comprise a nucleic acid sequence that encodes any one of a human ENaC alpha subunit (SEQ ID NO: 1); a human ENaC beta subunit (SEQ ID NO: 2); and a human ENaC gamma subunit (SEQ ID NO: 3); or a rat ENaC alpha subunit (SEQ ID NO: 4); a rat ENaC beta subunit (SEQ ID NO: 5); and a rat ENaC gamma subunit (SEQ ID NO: 6); or either a mouse ENaC alpha subunit (SEQ ID NO: 22) or a mouse ENaC delta subunit (SEQ ID NO: 25); a mouse ENaC beta subunit (SEQ ID NO: 23); and a mouse ENaC gamma subunit (SEQ ID NO: 24). In other embodiments, a cell or cell line of the invention may comprise an amino acid that is a human ENaC alpha subunit isoform 1 (SEQ ID NO: 7), a human ENaC alpha subunit isoform 2 (SEQ ID NO: 19), a human ENaC alpha subunit isoform 3 (SEQ ID NO: 20), or a human ENaC alpha subunit isoform 4 (SEQ ID NO: 21); a human ENaC beta subunit (SEQ ID NO: 8); and a human ENaC gamma subunit (SEQ ID NO: 9); or a rat ENaC alpha subunit (SEQ ID NO: 10); a rat ENaC beta subunit (SEQ ID NO: 11); or a rat ENaC gamma subunit (SEQ ID NO: 12). 
     The nucleic acid encoding the ENaC alpha (and delta), beta, or gamma subunit can be genomic DNA or cDNA. In some embodiments, the nucleic acid encoding the ENaC subunit comprises one or more substitutions, mutations or deletions, as compared to a wild-type ENaC, that may or may not result in an amino acid substitution. In some embodiments, the nucleic acid is a fragment of the nucleic acid sequence provided. Such ENaCs that are fragments or have such modifications retain at least one biological property of an ENaC, e.g., its ability to conduct sodium, conduct lithium, or be modulated by amiloride. The invention encompasses cells and cell lines stably expressing a subunit-encoding nucleotide sequence that is at least about 85% identical to a sequence disclosed herein. In some embodiments, the subunit-encoding sequence identity is at least 85%, 90%, 95%, 96%, 97%, 98%, 99% or higher compared to a subunit sequence provided herein. The invention also encompasses cells and cell lines wherein a nucleic acid encoding an ENaC subunit hybridizes under stringent conditions to a nucleic acid provided herein encoding the subunit. 
     In some embodiments, the cell or cell line comprises an ENaC subunit-encoding nucleic acid sequence comprising a substitution compared to a sequence provided herein by at least one but less than 10,20, 30, or 40 nucleotides, up to or equal to 1%, 5%, 10% or 20% of the nucleotide sequence or from a sequence substantially identical thereto (e.g., a sequence at least 85%, 90%, 95%, 96%, 97%, 98%, 99% or higher identical thereto, or that is capable of hybridizing under stringent conditions to the sequences disclosed). In some embodiments, the cell or cell line comprises an ENaC subunit-encoding nucleic acid sequence comprising an insertion into or deletion from the sequences provided herein by less than 10,20, 30, or 40 nucleotides up to or equal to 1%, 5%, 10% or 20% of the nucleotide sequence or from a sequence substantially identical thereto (e.g., a sequence at least 85%, 90%, 95%, 96%, 97%, 98%, 99% or higher identical thereto, or that is capable of hybridizing under stringent conditions to the sequences disclosed). The substitutions, insertions and deletions described herein may occur in any of the polynucleotides encoding ENaC subunits in the cells or cell lines of the invention. 
     In some embodiments, where the nucleic acid substitution or modification results in an amino acid change, such as an amino acid substitution, the native amino acid may be replaced by a conservative or non-conservative substitution. In some embodiments, the sequence identity between the original and modified polypeptide sequence can differ by about 1%, 5%, 10% or 20% of the polypeptide sequence or from a sequence substantially identical thereto (e.g., a sequence at least 85%, 90%, 95%, 96%, 97%, 98%, 99% or higher identical thereto). Those of skill in the art will understand that a conservative amino acid substitution is one in which the amino acid side chains are similar in structure and/or chemical properties and the substitution should not substantially change the structural characteristics of the parent sequence. In embodiments comprising a nucleic acid comprising a mutation, the mutation may be a random mutation or a site-specific mutation. 
     Conservative modifications will produce ENaCs having functional and chemical characteristics similar to those of the unmodified ENaC. A “conservative amino acid substitution” is one in which an amino acid residue is substituted by another amino acid residue having a side chain R group with similar chemical properties to the parent amino acid residue (e.g., charge or hydrophobicity). In general, a conservative amino acid substitution will not substantially change the functional properties of a protein. In cases where two or more amino acid sequences differ from each other by conservative substitutions, the percent sequence identity or degree of similarity may be adjusted upwards to correct for the conservative nature of the substitution. Means for making this adjustment are well-known to those of skill in the art. See, e.g., Pearson, Methods Mol. Biol. 243:307-31 (1994). 
     Examples of groups of amino acids that have side chains with similar chemical properties include 1) aliphatic side chains: glycine, alanine, valine, leucine, and isoleucine; 2) aliphatic-hydroxyl side chains: serine and threonine; 3) amide-containing side chains: asparagine and glutamine; 4) aromatic side chains: phenylalanine, tyrosine, and tryptophan; 5) basic side chains: lysine, arginine, and histidine; 6) acidic side chains: aspartic acid and glutamic acid; and 7) sulfur-containing side chains: cysteine and methionine. Preferred conservative amino acids substitution groups are: valine-leucine-isoleucine, phenylalanine-tyrosine, lysine-arginine, alanine-valine, glutamate-aspartate, and asparagine-glutamine. Alternatively, a conservative amino acid substitution is any change having a positive value in the PAM250 log-likelihood matrix disclosed in Gonnet et al., Science 256:1443-45 (1992). A “moderately conservative” replacement is any change having a nonnegative value in the PAM250 log-likelihood matrix. 
     Any ENaC alpha subunit from any species may be co-expressed with any ENaC beta subunit from any species, and any ENaC gamma subunit from any species in a cell or cell line of the invention. Similarly, any ENaC delta subunit from any species may be co-expressed with any ENaC beta subunit from any species, and any ENaC gamma subunit from any species in a cell line of the invention. In some embodiments, an ENaC subunit may be a chimeric subunit comprising sequences form two or more species. In some embodiments, the novel cell and cell line stably expresses human ENaC subunits, for example a cell or cell line that expresses at least one human ENaC alpha subunit (SEQ ID NO: 7); at least one human ENaC beta subunit (SEQ ID NO: 8); and at least one human ENaC gamma subunit (SEQ ID NO: 9). In some embodiments, the novel cell line is triply transfected to expresses a human ENaC alpha subunit, a human ENaC beta subunit and a human ENaC gamma subunit. 
     The invention further encompasses cells or cell lines that comprise an ENaC in which one or more subunits have a proteolyzed form and further encompasses a collection or panel of cell lines each comprising a different proteolyzed isoform. Such cells, cell lines and collections are useful to determine the activity of a proteolyzed isoform and the differential activity of a modulator on different proteolyzed isoforms of ENaC. In particular embodiments, ENaC expressing cells are exposed to one or more proteases at varying concentrations for varying times. In some embodiments the protease concentration is above zero but less than 0.1 ng/ml, about 0.1 ng/ml; 1 ng/ml; 10 ng/ml; 50 ng/ml; 100 ng/ml; 250 ng/ml; 500 ng/ml, 750 ng/ml, 1 ng/ml, 1.5 ng/ml, 2 ng/ml, 2.5 ng/ml, 3 ng/ml, 4 μg/ml, 5 ng/ml, 6 ng/ml, 7 ng/ml, 8 μg/ml, 9 ng/ml, 10 ng/ml, 100 ng/ml, 0.5 mg/ml, 1 mg/ml, 1.5 mg/ml, 2 mg/ml, 2.5 mg/ml, 3 mg/ml, 3.5 mg/ml, 4 mg/ml, 4.5 mg/ml, 5 mg/ml, 5.5 mg/ml, 6 mg/ml, 6.5 mg/ml, 7 mg/ml, 7.5 mg/ml, 8 mg/ml, 8.5 g/ml, 9 mg/ml, 9.5 mg/ml, 10 mg/ml or even greater concentrations or any concentration in between those listed. That is, the enzyme concentration may be in picomolar up to millimolar range. In some embodiments, ENaC expressing cells are exposed to protease for about 1 sec, 5 sec, 10 sec, 15 sec, 30 sec, 45 sec, 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, 15 min, 20 min, 30 min, 60 min, 90 min, longer than 90 min, or any length of time in between. Non-limiting examples of proteases include furin, trypsin, chymotrypsin, elastase, thrombin, plasmin, kallikrein, and acrosin, PACE4, PC5/6B, PC7, channel-activating protease-1 (CAP-1), prostatin, mCAP-2, human transmembrane protease serine 4 (TMPRSS4), mCAP-3, or a protease within saliva, stomach secretions or bacterial lysates, or any protease which hydrolyzes ENaC, or a combination of these. In other embodiments, enzymatically modified or proteolyzed forms of ENaC are generated by transfecting a cell or cell line expressing ENaC with a gene encoding the enzyme (e.g., protease) and stimulating or inducing its expression. Proteolyzed forms of ENaC or of an ENaC subunit may be characterized, for example, by analysis of peptides released into the media following proteolysis, such as by mass spectrometry, by analysis of the proteolyzed ENaC such as by Western blot or other immunodetection or by other suitable means that are well known in the art. 
     In some embodiments, the ENaC subunit-encoding nucleic acid sequence further comprises a tag. Such tags may encode, for example, a HIS tag, a myc tag, a hemagglutinin (HA) tag, protein C, VSV-G, FLU, yellow fluorescent protein (YFP), green fluorescent protein, FLAG, BCCP, maltose binding protein tag, Nus-tag, Softag-1, Softag-2, Strep-tag, S-tag, thioredoxin, GST, V5, TAP or CBP. A tag may be used as a marker to determine ENaC expression levels, intracellular localization, protein-protein interactions, ENaC regulation, or ENaC function. Tags may also be used to purify or fractionate ENaC. 
     Host cells used to produce a cell or cell line of the invention may express in their native state one or more endogenous ENaC subunits or lack expression of any ENaC subunit. The host cell may be a primary, germ, or stem cell, including an embryonic stem cell. The host cell may also be an immortalized cell. Primary or immortalized host cells may be derived from mesoderm, ectoderm or endoderm layers of eukaryotic organisms. The host cell may be endothelial, epidermal, mesenchymal, neural, renal, hepatic, hematopoietic, or immune cells. For example, the host cells may be intestinal crypt or villi cells, clara cells, colon cells, intestinal cells, goblet cells, enterochromafin cells, enteroendocrine cells. The host cells may be eukaryotic, prokaryotic, mammalian, human, primate, bovine, porcine, feline, rodent, marsupial, murine or other cells. The host cells may also be nonmammalian, such as yeast, insect, fungus, plant, lower eukaryotes and prokaryotes. Such host cells may provide backgrounds that are more divergent for testing ENaC modulators with a greater likelihood for the absence of expression products provided by the cell that may interact with the target. In preferred embodiments, the host cell is a mammalian cell. Examples of host cells that may be used to produce a cell or cell line of the invention include but are not limited to: Chinese hamster ovary (CHO) cells, established neuronal cell lines, pheochromocytomas, neuroblastomas fibroblasts, rhabdomyosarcomas, dorsal root ganglion cells, NS0 cells, CV-1 (ATCC CCL 70), COS-1 (ATCC CRL 1650), COS-7 (ATCC CRL 1651), CHO-K1 (ATCC CCL 61), 3T3 (ATCC CCL 92), NIH/3T3 (ATCC CRL 1658), HeLa (ATCC CCL 2), C127I (ATCC CRL 1616), BS-C-1 (ATCC CCL 26), MRC-5 (ATCC CCL 171), L-cells, HEK-293 (ATCC CRL1573) and PC12 (ATCC CRL-1721), HEK293T (ATCC CRL-11268), RBL (ATCC CRL-1378), SH-SY5Y (ATCC CRL-2266), MDCK (ATCC CCL-34), SJ-RH30 (ATCC CRL-2061), HepG2 (ATCC HB-8065), ND7/23 (ECACC 92090903), CHO (ECACC 85050302), Vero (ATCC CCL 81), Caco-2 (ATCC HTB 37), K562 (ATCC CCL 243), Jurkat (ATCC TIB-152), Per.C6 (Crucell, Leiden, The Netherlands), Huvec (ATCC Human Primary PCS 100-010, Mouse CRL 2514, CRL 2515, CRL 2516), HuH-7D12 (ECACC 01042712), 293 (ATCC CRL 10852), A549 (ATCC CCL 185), IMR-90 (ATCC CCL 186), MCF-7 (ATC HTB-22), U-2 OS (ATCC HTB-96), T84 (ATCC CCL 248), or any established cell line (polarized or nonpolarized) or any cell line available from repositories such as American Type Culture Collection (ATCC, 10801 University Blvd. Manassas, Va. 20110-2209 USA) or European Collection of Cell Cultures (ECACC, Salisbury Wiltshire SP4 0JG England). In some embodiments, the host cell is not a CHO cell. 
     In one embodiment, the host cell is an embryonic stem cell that is then used as the basis for the generation of transgenic animals. In some embodiments one or more subunits may be expressed with desired temporal and/or tissue specific expression. Embryonic stem cells stably expressing at least one ENaC subunit, or all three ENaC subunits, and preferably a functional introduced ENaC, may be implanted into organisms directly, or their nuclei may be transferred into other recipient cells and these may then be implanted, or they may be used to create transgenic animals. 
     As will be appreciated by those of skill in the art, any vector that is suitable for use with the host cell may be used to introduce a nucleic acid encoding an ENaC subunit into the host cell. The vectors comprising the various ENaC subunits may be the same type or may be of different types. Examples of vectors that may be used to introduce the ENaC subunit encoding nucleic acids into host cells include but are not limited to plasmids, viruses, including retroviruses and lentiviruses, cosmids, artificial chromosomes and may include for example, pCMVScript, pcDNA3.1 Hygro, pcDNA3.1neo, pcDNA3.1puro, pSV2neo, pIRES puro, pSV2 zeo. In some embodiments, the vectors comprise expression control sequences such as constitutive or conditional promoters. One of ordinary skill in the art will be able to select such sequences. For example, suitable promoters include but are not limited to GMV, TK, SV40 and EF-1α. In some embodiments, the promoters are inducible, temperature regulated, tissue specific, repressible, heat-shock, developmental, cell lineage specific, eukaryotic, prokaryotic or temporal promoters or a combination or recombination of unmodified or mutagenized, randomized, shuffled sequences of any one or more of the above. In other embodiments, ENaC is expressed by gene activation or when a gene encoding an ENaC subunit is episomal. Nucleic acids encoding ENaC subunits are preferably constitutively expressed. 
     In some embodiments, the vector lacks a selectable marker or drug resistance gene. In other embodiments, the vector optionally comprises a nucleic acid encoding a selectable marker such as a protein that confers drug or antibiotic resistance. Each vector for a sequence encoding a different ENaC subunit may have the same or a different drug resistance or other selectable marker. If more than one of the drug resistance markers are the same, simultaneous selection may be achieved by increasing the level of the drug. Suitable markers will be well-known to those of skill in the art and include but are not limited to genes conferring resistance to any one of the following: Neomycin/G418, Puromycin, hygromycin, Zeocin, methotrexate and blasticidin. Although drug selection (or selection using any other suitable selection marker) is not a required step, it may be used to enrich the transfected cell population for stably transfected cells, provided that the transfected constructs are designed to confer drug resistance. If subsequent selection of cells expressing at least three ENaC subunits is accomplished using signaling probes, selection too soon following transfection can result in some positive cells that may only be transiently and not stably transfected. However, this can be minimized by allowing sufficient cell passage allowing for dilution of transient expression in transfected cells. 
     In some embodiments, the vector comprises a nucleic acid sequence encoding an RNA tag sequence. “Tag sequence” refers to a nucleic acid sequence that is an expressed RNA or portion of an RNA that is to be detected by a signaling probe. Signaling probes may detect a variety of RNA sequences. Any of these RNAs may be used as tags. Signaling probes may be directed against the RNA tag by designing the probes to include a portion that is complementary to the sequence of the tag. The tag sequence may be a 3′ untranslated region of the plasmid that is cotranscribed and comprises a target sequence for signaling probe binding. The RNA encoding the gene of interest may include the tag sequence or the tag sequence may be located within a 5′-untranslated region or 3-untranslated region. In some embodiments, the tag is not with the RNA encoding the gene of interest. The tag sequence can be in frame with the protein-coding portion of the message of the gene or out of frame with it, depending on whether one wishes to tag the protein produced. Thus, the tag sequence does not have to be translated for detection by the signaling probe. The tag sequences may comprise multiple target sequences that are the same or different, wherein one signaling probe hybridizes to each target sequence. The tag sequences may encode an RNA having secondary structure. The structure may be a three-arm junction structure. Examples of tag sequences that may be used in the invention, and to which signaling probes may be prepared, include but are not limited to the RNA transcript of epitope tags such as, for example, a HIS tag, a myc tag, a hemagglutinin (HA) tag, protein C, VSV-G, FLU, yellow fluorescent protein (YFP), green fluorescent protein, FLAG, BCCP, maltose binding protein tag, Nus-tag, Softag-1, Softag-2, Strep-tag, S-tag, thioredoxin, GST, V5, TAP or CBP. As described herein, one of ordinary skill in the art could create his or her own RNA tag sequences. 
     In another aspect of the invention, cells and cell lines of the invention have enhanced stability as compared to cells and cell lines produced by conventional methods. To identify stable expression, a cell or cell line&#39;s expression of each ENaC subunit is measured over a timecourse and the expression levels are compared. Stable cell lines will continue expressing ENaC alpha or delta, beta and gamma subunits throughout the timecourse. In some aspects of the invention, the timecourse may be for at least one week, two weeks, three weeks, etc., or at least one month, or at least two, three, four, five, six, seven, eight or nine months, or any length of time in between. Isolated cells and cell lines can be further characterized, such as by qRT-PCR and single end-point RT-PCR to determine the absolute amounts and relative amounts of each ENaC subunit being expressed. In some embodiments, stable expression is measured by comparing the results of functional assays over a timecourse. The measurement of stability based on functional assay provides the benefit of identifying clones that not only stably express the mRNA of the gene of interest, but also stably produce and properly process (e.g., post-translational modification, subunit assembly, and localization within the cell) the protein encoded by the gene of interest that functions appropriately. 
     Cells and cell lines of the invention have the further advantageous property of providing assays with high reproducibility as evidenced by their Z′ factor. See Zhang J H, Chung T D, Oldenburg K R, “A Simple Statistical Parameter for Use in Evaluation and Validation of High Throughput Screening Assays.”  J. Biomol. Screen.  1999; 4(2):67-73. Z′ values pertain to the quality of a cell or cell line because it reflects the degree to which a cell or cell line will respond consistently to modulators. Z′ is a statistical calculation that takes into account the signal-to-noise range and signal variability (i.e., from well to well) of the functional response to a reference compound across a multiwell plate. Z′ is calculated using data obtained from multiple wells with a positive control and multiple wells with a negative control. The ratio of their combined standard deviations multiplied by three to the difference in their mean values is subtracted from one to give the Z′ factor, according the equation below: 
         Z ′ factor=1−((3σ positive control +3σ negative control )/(μ positive control −μ negative control ))
 
     The theoretical maximum Z′ factor is 1.0, which would indicate an ideal assay with no variability and limitless dynamic range. As used herein, a “high Z′” refers to a Z′ factor of Z′ of at least 0.6, at least 0.7, at least 0.75 or at least 0.8, or any decimal in between 0.6 and 1.0. In the case of a complex target such as ENaC, a high Z′ means a Z′ of at least 0.4 or greater. A score less than 0 is undesirable because it indicates that there is overlap between positive and negative controls. In the industry, for simple cell-based assays, Z′ scores up to 0.3 are considered marginal scores, Z′ scores between 0.3 and 0.5 are considered acceptable, and Z′ scores above 0.5 are considered excellent Cell-free or biochemical assays may approach higher Z′ scores, but Z′ scores for cell-based systems tend to be lower because cell-based systems are complex. 
     As those of ordinary skill in the art will recognize, historically, cell-based assays using cells expressing even a single chain protein do not typically achieve a Z′ higher than 0.5 to 0.6. Cells with engineered expression (either with introduced coding sequences or gene activation methods) of multi-subunit proteins, if even reported, would be lower due to their added complexity. Such cells would not be reliable to use in an assay because the results are not reproducible. Cells and cell lines of the invention, on the other hand, have high Z′ values and advantageously produce consistent results in assays. ENaC expression cells and cell lines of the invention provided the basis for high throughput screening (HTS) compatible assays because they generally have Z′ factors at least 0.7. In some aspects of the invention, the cells and cell lines result in Z′ of at least 0.3, at least 0.4, at least 0.5, at least 0.6, at least 0.7, or at least 0.8. Even Z′ values of at least 0.3-0.4 for ENaC-expressing cell are advantageous because ENaC is a multigene target, such as ENaC, because historically, such targets have been difficult to express. In other aspects of the invention, the cells and cell lines of the invention result in a Z′ of at least 0.7, at least 0.75 or at least 0.8 maintained for multiple passages, e.g., between 5-20 passages, including any integer in between 5 and 20. In some aspects of the invention, the cells and cell lines result in a Z′ of at least 0.7, at least 0.75 or at least 0.8 maintained for 1, 2,3,4 or 5 weeks or 2,3,4, 5, 6, 7, 8 or 9 months, including any period of time in between. 
     Also according to the invention, cells and cell lines that express a form of a naturally occurring ENaC, as well as cells and cell lines that express a proteolyzed form of ENaC, can be characterized for sodium ion conductance and lithium ion conductance. In some embodiments, the cells and cell lines of the invention express ENaC with “physiologically relevant” activity. As used herein, physiological relevance refers to a property of a cell or cell line expressing an ENaC whereby the ENaC conducts sodium and lithium ions as a naturally occurring ENaC of the same type (e.g., an αβγ-ENaC behaves as an αβγ-ENaC and a δβγ-ENaC behaves as a δβγ-ENaC) and responds to modulators in the same ways that naturally occurring ENaC of the same type is modulated by the same compounds. ENaC cells and cell lines of this, invention, including some proteolyzed forms of ENaC, preferably demonstrate comparable function to cells that normally express native ENaC in a suitable assay, such as a membrane potential assay using sodium as the ion conducted by ENaC, a membrane potential assay using lithium as the ion conducted by ENaC, electrophysiology or by inhibition with amiloride. Such comparisons are used to determine a cell or cell line&#39;s physiological relevance. “Sip and spit” taste tests using a panel of trained taste testers also may be used to further validate ENaC physiological relevance in cells and cell lines of the invention. The results of sip and spit taste tests using modulators identified via screening of non-proteolyzed or proteolyzed forms of ENaC can be used to validate the physiological relevance of these proteolyzed forms. 
     In some embodiments, the cells and cell lines of the invention have increased sensitivity to modulators of ENaC. Cells and cell lines of the invention respond to modulators and conduct sodium and lithium ions with physiological range EC 50  or IC 50  values for ENaC. As used herein, EC 50  refers to the concentration of a compound or substance required to induce a half-maximal activating response in the cell or cell line. As used herein, IC 50  refers to the concentration of a compound or substance required to induce a half-maximal inhibitory response in the cell or cell line. EC 50  and IC 50  values may be determined using techniques that are well-known in the art, for example, a dose-response curve that correlates the concentration of a compound or substance to the response of the ENaC-expressing cell line. Sodium chloride and lithium chloride EC 50  and IC 50  values for the ENaC-expressing cells and cell lines of the invention were within the ranges established for naturally occurring ENaCs. For example, the EC 50  for sodium chloride in a cell line of the invention is about 50 mM, the EC 50  for lithium chloride is about 35 mM, and the IC 50  for amiloride is sub-micromolar to low micromolar, e.g. single digit micromolar, for example, 0.7 μM or less. 
     A further advantageous property of the ENaC cells and cell lines of the inventions, flowing from the physiologically relevant function of the ENaC is that modulators identified in initial screening are functional in secondary functional assays, e.g., Ussing chamber assays or electrophysiology. As those of ordinary skill in the art will recognize, compounds identified in initial screening assays typically must be modified, such as by combinatorial chemistry, medicinal chemistry or synthetic, chemistry, for their derivatives or analogs to be functional in secondary functional assays. However, due to the high physiological relevance of the present ENaC cells and cell lines, many compounds identified therewith are functional without modification. 
     One aspect of the invention provides a collection of clonal cells and cell lines, each expressing the same ENaC, or different ENaCs, including different proteolyzed forms of ENaC subunits. The collection may include, for example, cells or cell lines expressing combinations of different subunits, or full length or fragments of subunits. The collection may also include, for example, proteolyzed forms of ENaC. 
     To make cells and cell lines of the invention, one can use, for example, the technology described in U.S. Pat. No. 6,692,965 and International Patent Publication WO/2005/079462. Both of these documents are incorporated herein by reference in their entirety for all purposes. This technology provides real-time assessment of millions of cells such that any desired number of clones (from hundreds to thousands of clones) may be selected. Using cell sorting techniques, such as flow cytometric cell sorting (e.g., with a FACS machine) or magnetic cell sorting (e.g., with a MACS machine), one cell per well may be automatically deposited with high statistical confidence in a culture vessel (such as a 96 well culture plate). The speed and automation of the technology allows multigene cell lines to be readily isolated. 
     Using the technology, the RNA sequence for each ENaC subunit may be detected using a signaling probe, also referred to as a molecular beacon or fluorogenic probe. In some embodiments, the molecular beacon recognizes a target tag sequence as described above. In another embodiment, the molecular beacon recognizes a sequence within the ENaC subunit itself. Signaling probes may be directed against the RNA tag or ENaC subunit sequence by designing the probes to include a portion that is complementary to the RNA sequence of the tag or the ENaC subunit, respectively. 
     Nucleic acids comprising a sequence encoding an ENaC subunit, or the sequence of an ENaC subunit and a tag sequence, and optionally a nucleic acid encoding a selectable marker may be introduced into selected host cells by well known methods. The methods include but not limited to transfection, viral delivery, protein or peptide mediated insertion, coprecipitation methods, lipid based delivery reagents (lipofection), cytofection, lipopolyamine delivery, dendrimer delivery reagents, electroporation or mechanical delivery. Examples of transfection reagents are GENEPORTER, GENEPORTER2, LIPOFECTAMINE, LIPOFECTAMINE 2000, FUGENE 6, FUGENE HD, TFX-10, TFX-20, TFX-50, OLIGOFECTAMINE, TRANSFAST, TRANSFECTAM, GENESHUTTLE, TROJENE, GENESILENCER, X-TREMEGENE, PERFECTIN, CYTOFECTIN, SIPORT, UNIFECTOR, SIFECTOR, TRANSIT-LT1, TRANSIT-LT2, TRANSIT-EXPRESS, IFECT, RNAI SHUTTLE, METAFECTENE, LYOVEC, LIPOTAXI, GENEERASER, GENEJUICE, CYTOPURE, JETSI, JETPEI, MEGAFECTIN, POLYFECT, TRANSMESSANGER, RNAiFECT, SUPERFECT, EFFECTENE, TF-PEI-KIT, CLONFECTIN, AND METAFECTINE. 
     Following introduction of the ENaC coding sequences or the ENaC activation sequences into host cells and optional subsequent drug selection, molecular beacons (e.g., fluorogenic probes) are introduced into the cells and cell sorting is used to isolate cells positive for their signals. Multiple rounds of sorting may be carried out, if desired. In one embodiment, the flow cytometric cell sorter is a FACS machine. MACS (magnetic cell sorting) or laser ablation of negative cells using laser-enabled analysis and processing can also be used. According to this method, cells expressing at least one alpha or delta, one beta and one gamma subunit are detected and recovered The ENaC subunit sequences may be integrated at different locations of the genome in the cell. The expression level of the introduced genes encoding the ENaC subunits may vary based upon integration site. The skilled worker will recognize that sorting can be gated for any desired expression level. Further, stable cell lines may be obtained wherein one or more of the introduced genes encoding an ENaC subunit is episomal or results from gene activation. 
     Signaling probes useful in this invention are known in the art and generally are oligonucleotides comprising a sequence complementary to a target sequence and a signal emitting system so arranged that no signal is emitted when the probe is not bound to the target sequence and a signal is emitted when the probe binds to the target sequence. By way of non-limiting illustration, the signaling probe may comprise a fluorophore and a quencher positioned in the probe so that the quencher and fluorophore are brought together in the unbound probe. Upon binding between the probe and the target sequence, the quencher and fluorophore separate, resulting in emission of signal. International publication WO/2005/079462, for example, describes a number of signaling probes that may be used in the production of the cells and cell lines of this invention. Where tag sequences are used, the vector for each of the ENaC subunit can comprise the same or a different tag sequence. Whether the tag sequences are the same or different, the signaling probes may comprise different signal emitters, such as different colored fluorophores and the like so that (RNA) expression of each subunit may be separately detected. By way of illustration, the signaling probe that specifically detects ENaC alpha subunit mRNA can comprise a red fluorophore, the probe that detects the first ENaC beta subunit (RNA) can comprise a green fluorophore and the probe that detects the ENaC gamma subunit (RNA) can comprise a yellow fluorophore. Those of skill in the art will be aware of other means for differentially detecting the expression of the three subunits with a signaling probe in a triply transfected cell. 
     Nucleic acids encoding signaling probes may be introduced into the selected host cell by any of numerous means that will be well-known to those of skill in the art, including but not limited to transfection, coprecipitation methods, lipid based delivery reagents (lipofection), cytofection, lipopolyamine delivery, dendrimer delivery reagents, electroporation or mechanical delivery. Examples of transfection reagents are GENEPORTER, GENEPORTER2, LIPOFECTAMINE, LIPOFECTAMINE 2000, FUGENE 6, FUGENE HD, TFX-10, TFX-20, TFX-50, OLIGOFECTAMINE, TRANSFAST, TRANSFECTAM, GENESHUTTLE, TROJENE, GENESILENCER, X-TREMEGENE, PERFECTIN, CYTOFECTIN, SIPORT, UNIFECTOR, SIFECTOR, TRANSIT-LT1, TRANSIT-LT2, TRANSIT-EXPRESS, IFECT, RNAI SHUTTLE, METAFEGTENE, LYOVEC, LIPOTAXI, GENEERASER, GENEJUICE, CYTOPURE, JETSI, JETPEI, MEGAFECTIN, POLYFECT, TRANSMESSANGER, RNAiFECT, SUPERFECT, EFFECTENE, TF-PEI-KIT, CLONFECTIN, AND METAFECTINE. 
     In one embodiment, the signaling probes are designed to be complementary to either a portion of the RNA encoding a ENaC subunit or to portions of their 5′ or 3′ untranslated regions. Even if the signaling probe designed to recognize a messenger RNA of interest is able to detect endogenously expressed target sequences, the proportion of these sequences in comparison to the proportion of the sequence of interest produced by transfected cells is such that the sorter is able to discriminate the two cell types. 
     The expression level of an ENaC subunit may vary from cell or cell line to cell or cell line. The expression level in a cell or cell line also may decrease over time due to epigenetic events such as DNA methylation and gene silencing and loss of transgene copies. These variations can be attributed to a variety of factors, for example, the copy number of the transgene taken up by the cell, the site of genomic integration of the transgene, and the integrity of the transgene following genomic integration. One may use FACS or other cell sorting methods (i.e., MACS) to evaluate expression levels. Additional rounds of introducing signaling probes may be used, for example, to determine if and to what extent the cells remain positive over time for any one or more of the RNAs for which they were originally isolated. 
     In a further aspect of the invention, we discovered a novel method for producing stable, viable ENaC expressing cells. Cells engineered to express ENaC demonstrated increased cell death compared to control cells ( FIG. 1 ), indicating that ENaC expression has a cytotoxic effect. The cells also appeared swollen compared to control cells ( FIG. 2 ). Without being bound by theory, ENaC expressing cells may take up an amount of sodium ions that disrupts the osmotic gradient, causing increased water uptake and potential swelling and cell bursting. This cytotoxicity could explain why ENaC expressing cell lines have heretofore not been producible. To overcome this newly recognized problem, we identified modified culture media (with reduced ion conductance) that produced viable ENaC expressing cells with normal morphology. According to the invention, ENaC expressing cells are preferably grown in cell culture media that has been modified to reduce ion conductance by ENaC. 
     In a first embodiment, ion conductance is reduced by reducing ion concentrations (e.g., sodium) in the media. According to the invention, sodium concentration in media is optimized for a particular cell because cells vary in their genetic background and thus respond to sodium concentrations in different ways. Accordingly, to produce an ENaC cell or cell line of the invention, cells are grown in various media with several different concentrations of sodium (ranging from 9 mM to 300 mM). Different types of host cells, as well as different cells within one type of host cell, may be grown in media with different concentrations of sodium. Growing a variety of cells of the invention in media with different concentrations of sodium allows for production and selection of cells that are viable and stably express ENaC. The cells and cell lines of the invention may be cultured in various media with modified sodium concentrations at different times during the process of generating the cells or cell lines, including before transfection, between transfection and sorting, during sorting, during maintenance of the cells or cell lines after sorting, or any combination thereof. In some embodiments, the sodium ion concentration in the culture media is reduced. Culture media with reduced sodium can have a sodium concentration of about 140 mM, about 130 mM, about 120 mM, about 110 mM, about 100 mM, about 90 mM, about 80 mM, about 70 mM, about 60 mM, about 50 mM, about 40 mM, about 30 mM, about 20 mM, 10 mM, about 9 mM or less, or any concentration in between. In some embodiments, cells that tolerate sodium concentration in the media of 150 mM or greater are identified. In order to maintain osmolality, the culture media of the ENaC expressing cells and cell lines of the invention may have their sodium ions replaced with other cations. Non-limiting examples of such cations include ammonium, lithium, choline, and cesium. One skilled in the art will recognize appropriate sodium ion replacements for culture media. Ion conductance also may be reduced by adding an ENaC antagonist or inhibitor (e.g., amiloride) to normal or reduced sodium cell culture media. One skilled in the art will understand that different concentrations of antagonists/inhibitors may be used depending on the type of antagonist/inhibitor used and depending on the ion concentration in the media. In some embodiments, an ENaC antagonist or inhibitor and/or one or more protease inhibitors may be added to the media. 
     Once cells expressing ENaC are isolated, they may be cultured in the modified media with their respective different sodium concentrations for a length of time sufficient to produce and identify those stably expressing all three subunits. In one embodiment, isolated cells may be grown individually or pooled to give rise to populations of cells. Individual or multiple cells or cell lines may also be grown separately or pooled. If a pool of cells of cell lines is producing a desired activity, it can be further fractionated until the cell or cell line or set of cells or cell lines having this effect is identified. This may make it easier to maintain large numbers of cells and cell lines without the requirements for maintaining each separately. 
     A further advantageous property of the ENaC cells and cell lines of the invention is that they stably express at least one alpha or one delta, at least one beta and at least one gamma subunit in the absence of drug selection pressure. Thus, in preferred embodiments, cells and cell lines of the invention are maintained in culture without any selective drug. In further embodiments, cells and cell lines are maintained without any antibiotics. As used herein, cell maintenance refers to culturing cells after they have been selected as described above for their ENaC expression. Maintenance does not refer to the optional step of growing cells in a selective drug (e.g., an antibiotic) prior to cell sorting where drug resistance marker(s) introduced into the cells allow enrichment of stable transfectants in a mixed population. 
     Drug-free cell maintenance provides a number of advantages. For examples, drug-resistant cells do not always express the co-transfected transgene of interest at adequate levels, because the selection relics on survival of the cells that have taken up the drug resistant gene, with or without the transgene. Further, selective drugs are often mutagenic or otherwise interfere with the physiology of the cells, leading to skewed results in cell-based assays. For example, selective drugs may decrease susceptibility to apoptosis (Robinson et al., 1997), increase DNA repair and drug metabolism (Deffie et al., 1988), increase cellular pH (Thiebaut et al., 1990; Roepe et al., 1993; Simon et al., 1994), decrease lysosomal and endosomal pH (Schindler et al., 1996; Altan et al., 1998), decrease plasma membrane potential (Roepe et al., 1993), increase plasma membrane conductance to chloride (Gill et al., 1992) and ATP (Abraham et al., 1993), and increase rates of vesicle transport (Altan et al., 1999). Thus, the cells and cell lines of this invention allow screening assays that are free from any artifact caused by selective drugs. In some preferred embodiments, the cells and cell lines of this invention are not cultured with selective drugs such as antibiotics before or after cell sorting, so that cells and cell lines with desired properties are isolated by sorting, even when not beginning with an enriched cell population. 
     In another aspect, the invention provides methods of using the cells and cell lines of the invention. The cells and cell lines of the invention may be used in any application for which functional ENaC subunits, ENaC ion channels or proteolyzed forms of ENaC are needed. The cells and cell lines may be used, for example, but not limited to, in an in vitro cell-based assay or an in vivo assay where the cells are implanted in an animal (e.g., a non-human mammal) to, e.g., screen for ENaC modulators; produce protein for crystallography and binding studies; and investigate compound selectivity and dosing, receptor/compound binding kinetic and stability, and effects of receptor expression on cellular physiology (e.g., electrophysiology, protein trafficking, protein folding, and protein regulation); The cells and cell lines of the invention also can be used in knock down studies to study the roles of specific ENaC subunits. 
     Cells and cell lines expressing various combinations of subunits can be used separately or together to identify ENaC modulators, including those specific for a particular ENaC or a particular subunit or proteolytic isoform of ENaC and to obtain information about the activities of individual subunits or proteolytic isoforms. The present cells and cell lines may be used to identify the roles of different forms of ENaC in different ENaC pathologies by correlating the identity of in vivo forms of ENaC with the identify of known forms of ENaCs or proteolyzed ENaCs based on their response to various modulators. This allows selection of disease- or tissue-specific ENaC modulators for highly targeted treatment of such ENaC-related pathologies. 
     Modulators include any substance or compound that alters an activity of ENaC, an ENaC subunit or a proteolytic isoform of ENaC. The modulator can be an ENaC agonist (potentiator or activator) or antagonist (inhibitor or blocker), including partial agonists or antagonists, selective agonists or antagonists and inverse agonists, and can be an allosteric modulator. A substance or compound is a modulator even if its modulating activity changes under different conditions or concentrations or with respect to different forms (i.e., proteolyzed forms) of ENaC. In other aspects, a modulator may change the ability of another modulator to affect the function of an ENaC. For example, a modulator of a form of ENaC that is not inhibited by amiloride may render that form of ENaC susceptible to inhibition by amiloride. 
     To identify an ENaC modulator, one can expose a novel cell or cell line of the invention to a test compound under conditions in which the ENaC would be expected to be functional and then detect a statistically significant change (e.g., p&lt;0.05) in ENaC activity compared to a suitable control, e.g., cells that are not exposed to the test compound. Positive and/or negative controls using known agonists or antagonists and/or cells expressing different combinations of ENaC subunits or proteolytic isoforms may also be used. In some embodiments, the ENaC activity to be detected and/or measured is membrane depolarization, change in membrane potential, or fluorescence resulting from such membrane changes. One of ordinary skill in the art would understand that various assay parameters may be optimized, e.g., signal to noise ratio. 
     The invention also provides methods of identifying modulators specific for particular enzymatically modified (e.g., proteolyzed, phosphorylated, dephosphorylated, acetylated, ubiquitinated, SUMOylated, glycosylated, methylated, myrstiolated, or fanesylated) forms of ENaC to determine the tissue specificity of different ENaC enzymatically modified forms. Such information may be useful in identifying the relevant enzymatically modified forms of ENaC implicated in ENaC pathologies, (i.e., pathologies related to ion conductance through various ENaC channels) and selecting tissue specific compounds for the selective treatment of such pathologies or for the development of related compounds useful in those treatments. 
     In some embodiments, one or more cells or cell lines of the invention are exposed to a plurality of test compounds, for example, a library of test compounds. A library of test compounds can be screened using the cell lines of the invention to identify one or more modulators. The test compounds can be chemical moieties including small molecules, polypeptides, peptides, peptide mimetics, antibodies or antigen-binding portions thereof. In the case of antibodies, they may be non-human antibodies, chimeric antibodies, humanized antibodies, or fully human antibodies. The antibodies may be intact antibodies comprising a full complement of heavy and light chains or antigen-binding portions of any antibody, including antibody fragments (such as Fab and Fab, Fab′, F(ab′) 2 . Fd, Fv, dAb and the like), single chain antibodies (scFv), single domain antibodies, all or an antigen-binding portion of a heavy chain or light chain variable region. 
     In some embodiments, prior to exposure to a test compound, the cells of cell lines of the invention may be modified by pretreatment with, for example, enzymes, including mammalian or other animal enzymes, plant enzymes, bacterial enzymes, protein modifying enzymes and lipid modifying enzymes. Such enzymes can include, for example, kinases, proteases, phosphatases, glycosidases, oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases and the like. For example, in some embodiments, cells and cell lines are pretreated with at least one proteolytic enzyme such as trypsin or furin. Alternatively, the cells and cell lines may be exposed to the test compound first followed by treatment to identify compounds that alter the modification of the ENaC by the treatment. 
     In some embodiments, large compound collections are tested for ENaC modulating activity in a cell-based, functional, high-throughput screen (HTS), e.g., using 384 well format. In some embodiments, a test compound or multiple test compounds including a library of test compounds may be screened using more than one cell or cell line of the invention. If multiple cells or cell lines, each expressing a different naturally occurring or enzymatically modified ENaC molecule, are used, one can identify modulators that are effective on multiple ENaC isoforms or alternatively, modulators that are specific for a particular ENaC and that do not modulate other ENaC isoforms. In the case of a cell or cell line of the invention that expresses a human ENaC, one can expose the cells to a test compound to identify a compound that modulates ENaC activity (either increasing or decreasing) for use in the treatment of disease or condition characterized by undesired ENaC activity, or the decrease or absence of desired ENaC activity. Further, according to the methods of the invention, cells or cell lines of the invention can be used to identify compounds or substances that potentiate or inhibit salt taste for use in ingestible substances. 
     These and other embodiments of the invention may be further illustrated in the following non-limiting Examples. 
     EXAMPLES 
     Example 1 
     Generating Cells Expressing ENaC Alpha, Beta and Gamma Subunits 
     Transfection 
     ¶ Transfected cells were grown for at least one week, preferably at least 2 weeks prior to the addition of signaling probes. During the growth period, cells were transferred into four groups, each group with a different sodium concentration (approximately 50, 75, 90, 150 mM Na) to ensure that at least one group had optimal conditions for the experiment. Also during the growth period, antibiotics were added to the media (150 μg/ml Hygromycin, 500 μg/ml G418 and 5 ng/ml Puromycin). Following antibiotic selection, cells were passaged 1 to 10 times in the absence of antibiotics to allow time for expression that is not stable over the selected period of time to subside. 
     Selection of Cells 
     Cells were harvested and transfected with signaling probes (SEQ ID NO: 16-18). The cells were then dissociated and collected for analysis and sorted using a fluorescence activated cell sorter. Standard analytical methods were used to gate cells fluorescing above background and to isolate cells falling within that defined gate directly into 96-well plates. Cell sorting was operated to deposit one cell only per well. 
     Target Sequences Detected by Signaling Probes 
     
       
         
           
               
               
            
               
                   
                 Target 1 
               
               
                   
                 (SEQ ID NO: 13) 
               
               
                   
                 5′-GTTCTTAAGGCACAGGAACTGGGAC-3′ (beta) 
               
               
                   
                   
               
               
                   
                 Target 2 
               
               
                   
                 (SEQ ID NO: 14) 
               
               
                   
                 5′-GAAGTTAACCCTGTCGTTCTGCGAC-3′ (gamma) 
               
               
                   
                   
               
               
                   
                 Target 3 
               
               
                   
                 (SEQ ID NO: 3) 
               
               
                   
                 5′-GTTCTATAGGGTCTGCTTGTCGCTC-3′ (alpha) 
               
            
           
         
       
     
     Signaling Probes 
     Supplied as 100 μM stocks 
                    Signaling probe 1-binds (Target 1)       (SEQ ID NO: 6)       5′-Cy5 GCCAGTCCCAGTTCCTGTGCCTTAAGAACCTCGC        BHQ3 quench-3′               Signaling probe 2-binds (Target 2)       (SEQ ID NO: 7)       5′-Cy5.5 GCGAGTCGCAGAACGACAGGGTTAACTTCCTCGC        BHQ3 quench-3′            
Note that BHQ3 could be substituted with BHQ1 or a gold particle in Probe 1 or Probe 2.
 
     
       
         
           
               
               
            
               
                   
                 Signaling probe 3-binds (Target 3) 
               
               
                   
                 (SEQ ID NO: 18) 
               
               
                   
                 GCGAGAGCGACAAGCAGACCCTATAGAACCTCGC  
               
               
                   
                 BHQ1 quench-3  
               
            
           
         
       
     
     Note that BHQ1 could be substituted with BHQ2 or Dabcyl in Probe 3.
         Sorted cells were maintained in media groups as described above. Cells were maintained and were passaged as needed to prevent overconfluence.   Various cell properties were observed while the cells were growing and being passaged in their respective media. For example, cell adherence, viability and fragility (e.g., whether debris remained in media after passaging) were observed using optical microscopy. One cell line was selected with cells that were viable, adhered to the wells and were not fragile was a cell line that was derived from CHO cells that were transfected with all three ENaC subunits and maintained in media containing 90 mM sodium.       

     Example 2 
     Preparing Reduced Sodium Media 
     In initial experiments, we discovered that cells were dying and those that remained alive were enlarged. To generate viable cells with normal morphology that stably express ENaC, we grew ENaC expressing cells in media with various concentrations of sodium (ranging from 50 mM to 150 mM). We prepared these various media with differing sodium concentrations by combining two different stock media (complete media and choline chloride media) in various ratios. 
     We prepared “Complete media” as follows: we supplemented Nutrient Mixture F-12 HAM (Sigma N4888) with 10% Fetal Bovine Serum (Sigma F2442), 1% L-Glutamine (Sigma G7513), 1% HEPES (Sigma H0887) and 1% MEM Amino Acids (Sigma M7145). For a drug containing version, we added Hygromycin (150 jag/ml), G418 (500 μg/ml) and Puromycin (5 μg/ml). 
     We prepared a “Choline Chloride Media” as follows: To 500 ml of sterile deionized (DI) water in a 1 L beaker we added the following components, followed by an additional 500 ml of DI water and filtered: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Component 
                 g/L 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 CaCl2•2H2O 
                 0.265 
               
               
                   
                 MgSO4 (anhyd) 
                 0.09767 
               
               
                   
                 KCl 
                 0.4 
               
               
                   
                 Choline Chloride 
                 14.3 
               
            
           
           
               
               
               
               
            
               
                   
                 Choline Bicarbonate 
                 9.1 
                 ml 
               
            
           
           
               
               
               
            
               
                   
                 Phenol Red 
                 0.0159 
               
            
           
           
               
               
               
               
            
               
                   
                 L-Glutamine 200 mM 
                 20 
                 ml 
               
               
                   
                 D-glucose solution 45% 
                 10 
                 ml 
               
               
                   
                 HEPES 1M 
                 10 
                 ml 
               
               
                   
                 12.5X BME* 
                 80 
                 ml 
               
               
                   
                 500X vitamin solution** 
                 2 
                 ml 
               
               
                   
                 0.01 g/ml Fe(NO3)3•9H2O*** 
                 10 
                 μl 
               
               
                   
                   
               
               
                   
                 *To prepare 12.5X BME, we added the following to 100 ml of BME 50X solution: L-cysteine HCl (0.018 g), Glycine (0.0375 g), L-Histidine HCl monohydrat (0.0125 g), L-Serine (0.0525 g), L-Proline (0.043 g). 
               
               
                   
                 **To prepare 500X Vitamin Solution we added the following to 50 ml sterile DI water and filtered: Choline Chloride (0.1 g), Folic Acid (0.1 g), myo-Inositol (0.18 g), Nicotinamide (0.1 g), DL-Pantothenic Acid hemicalcium salt (0.1 g), Pyridoxine HCl (0.1 g), Riboflavin (0.01 g), Thiamine HCl (0.1 g). 
               
               
                   
                 ***To prepare 0.01 g/ml Fe(NO3) 3.9 H2O we added 0.1 g of Fe(NO3)3•9H2O to 10 ml sterile DI water and filtered. 
               
            
           
         
       
     
     For various “low-sodium media”, we combined Complete media and Choline Chloride media in varying ratios such that the concentration of sodium and the concentration of choline totaled 150 mM. Although those of skill in the art will understand from this disclosure how to prepare media with a range of sodium concentrations, by way of example, to make 90 mM sodium media, we combined 300 ml of Complete media, 180 ml Choline Chloride media, 20 ml Fetal bovine Serum, 2 ml 100× MEM Nonessential amino acid and 2 ml 100× Proline (3.4 g/L of proline diluted with Choline Chloride media). For drug containing low-sodium media, antibiotics were added to final concentrations of 150 ng/ml Hygromycin, 500 μg/ml G418 and 5 μg/ml Puromycin. Those of skill in the art will recognize that ions other than choline may be used to produce low-sodium media. For example, we also successfully used N-methyl-D-glucamine (NMDG) to substitute NaCl in media. 
     Example 3 
     Characterization of the Stable ENaC Expressing Cell Line 
     1. Confirmation and Quantification of Gene Expression. 
     Expression of the mRNAs encoding the three ENaC subunits was confirmed for the top responding ENaC clones by TaqMan and endpoint RT-PCR methods. Total RNA was extracted from the ENaC cell line for TaqMan analysis of gene expression using PRE-DEVELOPED TAQMAN GENE EXPRESSION ASSAY (Applied Biosystems). Relative expression levels over control cells are presented in  FIG. 3B . 
     Heterologous expression of the mRNAs encoding the three ENaC subunits was also confirmed by cell sorting using a FACS machine. Relative expression levels over control cells are presented in  FIG. 3A . 
     2. Membrane Potential Assay 
     The influx of positively-charged sodium ions following the activation of ENaC results in a change in the membrane potential of the cell. To test the activity of ENaC cell lines, we used a membrane potential assay. Such assays are well known in the art. See, e.g., Zheng, W. et al.,  Assays and Drug Development Technologies,  2:543-553 (2004); A. S. Waggoner,  Annu. Rev. Biophys. Bioeng.  8:47-68 (1979); Zochowski, M. et al.,  Biol. Bull.  198:1-21 (2000). 
     ENaC is an ion channel composed of alpha, beta and gamma subunits. We compared the activities of ENaC cell lines using transient transfection assays. As shown in  FIG. 3C , the activity of untransfected cells (“None”), cells transfected with all three ENaC subunits, and cells transfected with any combination of only two subunits was compared. Only cells transfected with all three subunits reported sodium uptake when exposed to 150 mM sodium. 
     We generated the ENaC expressing cell line from ENaC expressing cells that were transfected and grown in media with reduced sodium ion concentration, such as 90 mM sodium. We maintained the ENaC cell line both in media with 150 mM sodium media as well as in 90 mM sodium media. We characterized cells of the ENaC cell line maintained under both these conditions for their ability to conduct sodium ions, their ability to conduct lithium ions, and their inhibition with amiloride. As shown in Table 1, cells grown in complete media have EC50 and IC50 values that are significantly different from physiological ENaC values, while cells grown in reduce sodium media are physiologically and pharmacologically relevant. We also observed that the addition of antibiotics were required to maintain the consistency of the functional response of the ENaC cell line over time when the cells were maintained in 150 mM sodium media but not when they were maintained in 90 mM sodium media. These results demonstrated that a number of characteristics of the cells of the ENaC cell line are different when the cells are maintained in 90 mM versus 150 mM media. Only cells maintained in the 90 mM sodium media demonstrated ENaC activity or function that was stable, physiological and pharmacologically relevant, due to e.g. differences in the characteristics of the cells including host genome organization and gene expression in the different media conditions. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 150 mM Media 
                 90 mM Media 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 NaCl EC50 
                 80 
                 mM 
                 48 
                 mM 
               
               
                   
                 (approx) 
               
               
                   
                 LiCl EC50 
                 82 
                 mM 
                 33 
                 mM 
               
               
                   
                 (approx) 
               
               
                   
                 Amiloride IC50 
                 11.0 
                 uM 
                 0.64 
                 uM 
               
               
                   
                 (approx) 
               
               
                   
                   
               
            
           
         
       
     
     3. Determination of Z′ Value for ENaC Cell-Based Assay 
     The Z′ value for the optimized ENaC assay was calculated to achieve a value of 0.85, a surprisingly high value for a membrane potential assay expressing a previously intractable heteromultimeric ion channel target with associated cytotoxicity. 
     As shown in  FIG. 4 , testing of the cell line expressing ENaC in the membrane potential assay showed activation by sodium, and the sodium uptake was reduced, as expected, with amiloride, a known ENaC blocker. The EC50 value for sodium (48 mM) and the IC50 value for amiloride (0.56 μM) were consistent with reported values for ENaC. For the ENaC cell line, essentially all of the sodium response was blocked by amiloride. This was an important result to confirm that the assay responses are ENaC-specific. These data indicate that the ENaC expressing cells of the present invention are physiologically and pharmacologically relevant. 
     In order to test the reproducibility of the cell lines of the invention, the line was plated onto a 384-well plate and tested for its ability to conduct sodium. The cells were exposed to NaCl or buffer alone, and sodium uptake was measured and Z′ value was determined.  FIG. 5  demonstrates that the cells of each well of the 384-well plate responded similarly to sodium stimulation. A Z′ value of 0.85 was calculated, indicating that the cells of the invention demonstrate high reproducibility. 
     Example 4 
     High Throughput Screening and Identification of ENaC Modulators 
     Assay Buffer—Phosphate 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Chemical 
                 Final Concentration 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 CaCl2•2H20 
                 1.939 
                 mM 
               
               
                   
                 KCl 
                 5.365 
                 mM 
               
               
                   
                 KH2P04 
                 441 
                 μM 
               
               
                   
                 MgCl2•6H20 
                 492 
                 μM 
               
               
                   
                 MgS04•7H20 
                 406 
                 μM 
               
               
                   
                 Na2HP04• 
                 338.5 
                 μM 
               
               
                   
                 Glucose 
                 5.549 
                 mM 
               
               
                   
                 HEPES 
                 20 
                 mM 
               
               
                   
                 NaCl 
                 25 
                 mM 
               
               
                   
                 Choline Chloride 
                 125 
                 mM 
               
               
                   
                   
               
               
                   
                 Add components to 500 ml water and bring up to 1 L pH to 7.4 
               
            
           
         
       
     
     EnaC High-Throughput Screening Assay Protocol 
     On the day before assay, we harvested the cells from stock plates into 90 mM sodium media with no antibiotics and plated 1.5×104 cells in 50 into black clear-bottom 384 well assay plates (Coring 3712). The assay plates were maintained in a 37° C. cell culture incubator under 5% CO2 for 19-21 hours. The media was then removed from the assay plates and 20 μl blue membrane potential dye (Molecular Devices Inc) was added and the cells were incubated for 1 hr at room temperature. Test compounds were solubilized in dimethylsulfoxide, diluted in a phosphate buffer and loaded into 384 well polypropylene micro-titer plates. The cell and compound plates were loaded into an FDSS instrument (Hamamatsu) which measures fluorescence (relative fluorescence units) from the plates over time. Fluorescence values were continuously collected from the beginning of the experiment and following addition of test compounds to report baseline values and any changes due to the addition of test compounds. Test compounds in assay buffer or assay buffer alone were then added to the cells and fluorescence values were collected for two minutes. A solution of sodium chloride in assay buffer with reduced choline chloride was then added to the cells to raise the final sodium concentration by 75 mM. The activity of the compound was determined by measuring the change in fluorescence produced following the compound&#39;s addition to the cells and/or following subsequent sodium addition. 
     The high-throughput screening assay described above was used with a library of compounds to identify several novel ENaC blockers. Unexpectedly, several ENaC potentiators, no examples of which had been previously reported, were also identified in the high-throughput screening assay.  FIG. 6  demonstrates representative traces for both an ENaC inhibitor (left panel) and an ENaC potentiator (right panel). These data demonstrate that the ENaC expressing cells of the present invention are well suited for high-throughput screening. The cells of the present invention may also be used in high-throughput screening at the beginning of a drug candidate study to identify promising lead compounds. 
     Example 5 
     Enzymatically Modified Isoforms of ENaC 
     ENaC Isoform Specific High-Throughput Screening Assay Protocol 
     To investigate the effect of protease treatment on ENaC activity, we conducted the membrane potential assay as described in Example 3 above using a blue membrane potential (MBP) dye solution containing protease (trypsin) at a concentration that does not produce cell detachment. After a 1 hour incubation with a dye-protease solution at room temperature, the assay was run using the FDSS (Hamamatsu Corporation) as described above. 
     The experiment was performed using several treatment groups, each receiving a different concentration of trypsin (0 to 3 mg/ml). The cells were incubated with the trypsin for 1 hour at room temperature concurrently with membrane potential dye (standard protocol). Following incubation, the ion flux in response to challenge with sodium or sodium amiloride was reported with the standard two step membrane potential FDSS protocol. Signal (RFU) values were indicated at each dose of protease tested, averaged over nine independent experiments. As shown in  FIG. 7A , we were able to distinguish twenty-four proteolytically modified forms (twenty-five including the untreated form) with different activities, indicated by the peaks, valleys, and plateaus. We further characterized these proteolytically modified isoforms for their ability to conduct sodium.  FIG. 7B  demonstrates that each ENaC cleavage isoform has a unique capacity for sodium conductance. These data indicate that proteolytic cleavage of ENaC generates isoforms with pharmacologically distinct profiles (Table 2). Table 2 shows concentrations of trypsin at which different proteolyzed forms of ENaC have been defined. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Amiloride IC50 
               
               
                   
                 Isoform 
                 [Trypsin] mg/ml 
                 (approx.) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Form 1 
                 0.000025 
                 5.6 
               
               
                   
                 Form 2 
                 0.001250 
                 NA 
               
               
                   
                 Form 3 
                 0.001438 
                 8.4 
               
               
                   
                 Form 4 
                 0.002500 
                 NA 
               
               
                   
                 Form 5 
                 0.004140 
                 11.9 
               
               
                   
                 Form 6 
                 0.005000 
                 54.3 
               
               
                   
                 Form 7 
                 0.008280 
                 4.7 
               
               
                   
                 Form 8 
                 0.011500 
                 NA 
               
               
                   
                 Form 9 
                 0.016500 
                 8.7 
               
               
                   
                 Form 10 
                 0.023000 
                 NA 
               
               
                   
                 Form 11 
                 0.035100 
                 1.2 
               
               
                   
                 Form 12 
                 0.046875 
                 NA 
               
               
                   
                 Form 13 
                 0.066250 
                 16.3 
               
               
                   
                 Form 14 
                 0.093750 
                 0.44 
               
               
                   
                 Form 15 
                 0.187500 
                 1.3 
               
               
                   
                 Form 16 
                 0.225000 
                 0.82 
               
               
                   
                 Form 17 
                 0.420000 
                 6.2 
               
               
                   
                 Form 18 
                 0.562500 
                 0.41 
               
               
                   
                 Form 19 
                 0.600000 
                 2.4 
               
               
                   
                 Form 20 
                 0.750000 
                 NA 
               
               
                   
                 Form 21 
                 0.940000 
                 6.1 
               
               
                   
                 Form 22 
                 1.200000 
                 NA 
               
               
                   
                 Form 23 
                 1.680000 
                 2.1 
               
               
                   
                 Form 24 
                 2.250000 
                 NA 
               
               
                   
                   
               
            
           
         
       
     
     We further characterized the effect of amiloride on the proteolytically cleaved ENaC cells. The isoform specific assay described above was performed with MBP dye solution with trypsin concentrations as listed (Table 2). The cells were then exposed to sodium and subsequently to increasing doses of amiloride. IC50 values (Table 2) demonstrate that amiloride was variably effective at inhibiting some of the cleaved ENaC forms. IC50 values were not calculated for a subset of the forms where amiloride was not effective as a blocker at the concentrations tested (“NA”).  FIG. 8  demonstrates that amiloride effectively inhibits uncleaved ENaC, but is much less effective or not effective at inhibiting the cleaved ENaC form 24. These data confirm that ENaC cleavage isoforms are differentially inhibited. 
     We also tested the ability of novel ENaC inhibitors (identified in Example 4) to inhibit proteolytically cleaved ENaC. As described above, this experiment was performed using several treatment groups, each receiving a different concentration of trypsin (0 to 2.25 mg/ml). The cells were loaded with membrane potential dye concurrently with the trypsin. The cells were incubated with the trypsin for 1 hour at room temperature and assayed for activity with sodium alone or with sodium and an inhibitor. Dose response studies of Amiloride, as well as three compounds identified by the high-throughput screen, were performed at the end of the dye loading incubation in the standard two step membrane potential FDSS protocol. IC50 values were calculated and plotted as a function of protease concentration.  FIG. 9  demonstrates that proteolytically cleaved ENaC has variable sensitivity to inhibitory compounds. For instance, compound 1 (top left panel) is capable of inhibiting nearly all ENaC iso forms, while compound 2 (top right panel), compound 3 (bottom left panel), and amiloride (bottom right panel) each inhibit a unique subset of ENaC isoforms. These data further confirm that ENaC cleavage isoforms are differentially inhibited and indicate that the ENaC cells of the present invention can be used to identify isoform specific modulators of ENaC. The different isoforms of ENaC can be pursed in drug discovery to more effectively address the role of the different isoforms of ENaC or the effect or role of proteolysis in disease. 
     Example 6 
     Confirmation of ENaC Modulators by Ussing Chamber Analysis 
     We verified compounds identified using ENaC cell lines of the invention in a high-throughput screening membrane potential assay (with and without protease treatment), in an independent functional assay using human primary epithelial cells in an Ussing Chamber assay as follows. Human primary epithelial cells express endogenous ENaC and, thus, permit a comparison of the activity of ENaC in a cell or cell line of the invention with the activity of endogenously expressed ENaC. 
     Cells 
     Primary human bronchial epithelial (HBE) cells (Lonza) were cultured in bronchial epithelial cell growth medium supplemented with bovine pituitary extract, hydrocortisone, human recombinant epidermal growth factor, epinephrine, transferrin (10 μg/ml), insulin (5 μg/ml), retinoic acid, triiodothyronine, gentamicin, and amphotericin B according to manufacturer&#39;s instructions. The medium was changed every 3 days until the cells were 90% confluent. The cells were then passaged and seeded onto Snapwell culture inserts in differentiation medium containing 50% DMEM in bronchial epithelial cell growth medium with the supplements described above, but without triiodothyronine and at a final retinoic acid concentration of 50 nM (all-trans-retinoic acid; Sigma). Cells were seeded at a density of 1×10 4  cells/cm 2  onto 1.12-cm 2  Snapwell (Costar) inserts. HBE cells were maintained submerged for the first 7 days in culture; then they were exposed to an air-liquid interface for the remainder of the culture period. At all stages of culture, cells were maintained at 37° C. in 5% CO 2  in an air incubator. Four HBE cell donors were used for these studies. 
     Transepithelial Measurements 
     HBE cells on culture inserts were rinsed, mounted in an EasyMount Chamber System (Physiologic Instruments) and bathed with continuously gassed Ringer solution (5% CO 2  in O 2 , pH 7.4) maintained at 37° C. containing (in mM) 120 NaCl, 25 NaHCO 3 , 3.3 KH 2 PO 4 , 0.8 K 2 HPO 4 , 1.2 CaCl 2 , 1.2 MgCl 2 , and 10 glucose (all from Sigma). Electrodes [agar bridged (4% in 1 M KCl) Ag—AgCl] were used and the inserts were voltage clamped to 0 mV. Transepithelial current, voltage and resistance were measured every 10 seconds for the duration of the experiments Membranes with a resistance of &lt;200mOhms were discarded. The test compounds identified above were added to the apical surface of the membrane. All dosing was cumulative (final concentrations of compounds: 0.5 uM, 1 uM, 4.33 uM, 10 uM). Membranes receiving vehicle only were used as a control. 10 μM amiloride was added towards the end of the experiment, in order to determine, the magnitude of the ENaC current. 
     As shown in  FIG. 10 , the Ussing chamber assay confirmed the activity of both the positive (upper panels) and negative (lower panel) modulators that Were identified using an ENaC cell line of the invention in the ENaC HTS assay, including compounds that were identified by their selective activation of protease-treatment enzymatically modified ENaC isoforms. 
     Example 7 
     Taste Test Confirmation of Potentiators 
     To confirm the activity of ENaC potentiators identified using an ENaC cell line of the invention in the ENaC high-throughput screening assay, we conducted a standard “sip and spit” taste test. Tasters are screened for acuity and descriptive capabilities. All have been trained to detect and quantify taste perception. Briefly, subjects performed blind taste testing of a salt solution alone or with two different concentrations (5 and 10 ppm) of a test compound. Panelists are presented with coded paired samples labeled A and B. Each pair consists of a reference solution (salt solution alone) and a test solution (salt solution plus potentiator). Test solutions contain potentiator compounds at either of the two different concentrations. Subjects are asked to taste both samples and select the sample that tastes more salty. At least three out of four times, solutions containing the potentiator compound were detected as saltier than the solution alone. 
     Taste testing was used to assess the effect of potentiator compounds on salt taste. This included a set of compounds (P) that were active at the most highly proteolyzed form but not the non-proteolyzed form, as well as a set of compounds (NP) active at the non-proteolyzed form but not the most highly proteolyzed form. As shown in  FIG. 11 , on average the P compounds were more effective at potentiating salt taste than the NP compounds. These data are consistent with the presence of proteases in the oral cavity, and demonstrate that the cleaved isoforms of ENaC play a major role in detecting salty taste as well as or in addition to the non-cleaved isoform. By confirming the activity of the novel potentiators identified in Example 3, these data also confirm that the ENaC expressing cells of the present invention can be used in a high-throughput screening assay to identify novel modulators of ENaC activity.