Patent Publication Number: US-7910705-B2

Title: Stable cell lines expressing HERG1a and HERG1b

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 10/976,122, filed Oct. 28, 2004, now U.S. Pat. No. 7,510,874 which claims the benefit of Provisional Application 60/515,158 filed Oct. 28, 2003, the disclosures of each of which are hereby incorporated herein by reference as if set forth in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with United States government support awarded by the following agency: NIH Grant Number HL55973. The United States government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Ion channels expressed in the plasma membrane of excitable tissues (including heart) regulate the function of the tissues. Ion channels can comprise alpha, beta and auxiliary subunits. The alpha subunits are largely responsible for determining overall biophysical properties of the channels, such as ion selectivity, gating and drug sensitivity, whereas beta or auxiliary subunits modify these properties in important ways. Voltage-gated potassium channels comprise four alpha subunits that assemble into a pseudosyrmmetric array (MacKinnon, 1991), thereby providing the opportunity for heterogeneity by mixing of related subunits to form heterotetrameric channels (Christie et al., 1990; Isacoff et al., 1990). The potential for complexity and heterogeneity increases substantially when beta or auxiliary subunits are also present (England et al., 1995). 
     Cardiac I Kr  is a rapidly-inactivating potassium current first identified by its sensitivity to the methanesulfonanilide drug E-403 1 (Sanguinetti, M. C. and N. K. Jurkiewicz, 1990). Compared to all other known potassium currents, I Kr  has a unique functional profile characterized by the suppression of current during depolarization and large, rebounding tail currents produced upon repolarization. Currents are suppressed during depolarization because channels open only briefly and then rapidly inactivate. Upon repolarization, channels recover rapidly from inactivation and revisit the open state. Because deactivation is slow, the channels linger in this highly stable open state and produce the resurgent current that is a hallmark of I Kr . Moreover, the sensitivity to E-4031 and other antiarrhythmic drugs is unique to I Kr . 
     Currents with comparable biophysical and pharmacological properties are produced when HERG1, a gene encoding an inwardly rectifying potassium channel that was cloned from human hippocampus (Warmke and Ganetzky, 1994), is transiently expressed in  Xenopus  oocytes, suggesting that HERG1 is a central component of the channels that give rise to the I Kr  currents (Sanguinetti et al., 1995; Trudeau et al., 1995). Trudeau, M. C., et al., “HERG, a Human Inward Rectifier in the Voltage-Gated Potassium Channel Family,”  Science  269:92 (1995), incorporated by reference as if set forth herein in its entirety, described the HERG gene and also depicted the inwardly rectifying HERG currents and a gating model in the same paper. 
     Families with a form of inherited (familial) Long QT Syndrome (LQTS-2) have mutations the HERG1 gene (Curran et al., 1995). LQTS-2 is a life-threatening illness characterized by polymorphic ventricular arrhythmias known as torsades de pointes (Roden, 1993). Undiagnosed or untreated, LQTS often leads to sudden death by young adulthood. The expression studies of Trudeau et al. (1995), defining HERG as the primary component underlying I Kr , thus explained the underlying cause of LQTS-2 as a loss of I Kr . 
     More clinically prevalent than familial LQTS is an acquired form of the disease caused by block of I Kr  currents by a surprising variety of drugs, including antiarrhythmic drugs such as dofetilide (Tikosyn®) (Snyders and Chaudhary, 1996), the antihistamines terfenadine (Seldane®) (Roy et al., 1996; Suessbrich et al., 1996) and astemizole (Hismanal®) (Zhou et al., 1999b), the gastric motility drug cisapride (Propulsid®) (Mohammad et al., 1997; Rampe et al., 1997), and cocaine (Zhang S, 2001). An estimated 1-8% of the general public is susceptible to acquired LQTS. Despite their therapeutic value, several of these drugs have been withdrawn from the market because of an unacceptable risk of torsades. As a result, to avoid the risk of torsades and the lost investment associated with withdrawal of a drug from the market, standard pharmaceutical industry practice today dictates that all pharmaceutics in development are screened against cultured cells that express HERG1 in the cell membranes with monitoring for changes in potassium channel behavior. Commercially available HERG-expressing cell lines express only HERG1a channel subunits that assemble into HERG1 channels. 
     While it is accepted that I Kr  channels primarily contain HERG 1 subunits, the precise composition of these channels is unknown. The discovery of alternative HERG1a and HERG1b transcripts encoded by the HERG1 gene in human heart (Lees-Miller et al., 1997; London et al., 1997; Kupershmidt et al., 1998; London et al., 1998, each incorporated by reference as if set forth herein in its entirety), raised the possibility that alpha subunits other than HERG1a contribute to the I Kr  channels. 
     The proteins encoded by the HERG1a and HERG1b transcripts differ only at their amino termini, as shown in the attached Sequence Listing. The longer amino terminus of HERG1a confers slow deactivation; the shorter amino terminus of HERG1b confers rapid deactivation, relative to HERG1a. When transiently expressed together in a heterologous  Xenopus  oocyte system, the two subunits assemble to form heteromeric channels that produce currents with unique, intermediate deactivation properties that cannot be explained by the algebraic summation of two homomeric populations of channels. HERG1a DNA and amino acid sequences (SEQ ID NO:1 and 2, respectively) can be found at GenBank Accession No. NM — 000238, and HERG1b DNA and amino acid sequences (SEQ ID NO:3 and 4, respectively) can be found at GenBank Accession No. NM — 172057). The understanding of the art in this regard is presented in London, B. et al., “Two Isoforms of the Mouse Ether-a-go-go-Related Gene Co-assemble to Form Channels With Properties Similar to the Rapidly Activating Component of the Cardiac Delayed Rectifier K +  Current,”  Circ. Res.,  81:870 (1997), which is incorporated by reference as if set forth herein in its entirety. 
     Although HERG1b transcripts have been observed in human heart tissue, until now there was no convincing evidence for the existence in the heart of HERG1b protein, nor was there a consensus as to whether HERG1a and HERG1b channel subunits co-assemble in the heart in vivo. It has heretofore been presumed that HERG channels in cardiac myocytes are uniformly formed of HERG1a subunits and a host of such HERG1a-containing cell lines are available for testing, as described. Even so, the potassium ion channel behavior of HERG1a-containing cell lines does not fully match the behavior of I Kr  currents observed in cardiac myocytes. Additionally, from the prior work in  Xenopus  oocytes one cannot predict co-assembly of HERG1a and HERG1b subunits, let alone production of an I Kr  current, in the membranes of mammalian cells, particularly upon heritable maintenance and expression of HERG1a and HERG1b in such cells. Understanding cardiac I Kr  physiology and the disease mechanisms of HERG-linked congenital and acquired LQTS necessitates approximating the native state in heterologous systems as closely as possible. It would be desirable to provide improved cell lines for pharmacologic testing, where the improved cell lines mirror the potassium ion channel behavior and subunit composition found in cardiac myocytes. 
     BRIEF SUMMARY OF THE INVENTION 
     ERG is understood by the skilled person to refer to the ether-a-go-go related gene, and ERG to the corresponding protein, identified in various mammalian, non-mammalian, and non-vertebrate species. HERG and HERG refer to the human ERG homolog, and corresponding protein, respectively. Reference herein to HERG refers to the human ether-a-go-go related gene while ERG indicates the homolog in lower mammals. When discussing an anti-ERG antibody, the applicants intend that the antibody reacts across species and interacts with the ERG protein (or ERG1a or ERG1b subunits) from both human and non-human animals. There is very close sequence similarity between the genes and the encoded proteins in higher and lower mammals, as well as interspecies cross-reactivity of isoform-specific antibodies. 
     The present invention relates to the direct demonstration by the inventors that both the HERG1a and HERG1b proteins are present in the cell membranes of heart tissue of human and non-human animals, and further that when both proteins are stably expressed in a mammalian cell line, a single antibody raised specifically against either the HERG1b or HERG1a subunits co-precipitates both the HERG1a and the HERG1b subunits. The inventors have demonstrated that ERG1a and ERG1b subunits of non-human animals are also expressed in non-human animal cardiac tissue. 
     Further, HERG1a and HERG1b expression in a heterologous mammalian cell system produced current with the characteristic hallmark pharmacological and biophysical properties of native I Kr  channels, namely sensitivity to a methanesulfonanilide drug, suppression of current during depolarization and large, rebounding tail currents produced upon repolarization. These hallmarks, and the ability to distinguish an I Kr  current from a non-I Kr  current, are understood by the skilled artisan familiar with the papers by Sanguinetti, M C and Jurkiewicz (1990), Sanguinetti, M C et al. (1995) and by Trudeau, M C, et al. (1995), each incorporated by reference herein as if set forth in its entirety. 
     A first aspect of the invention follows from these demonstrations. In accord with the first aspect, the invention is summarized in that cardiac I Kr  can be recapitulated in a line of cultured mammalian cells, notably human cells, having a cell membrane that comprises HERG potassium ion channels having HERG1a and HERG1b subunit components, wherein the HERG1a and HERG1b subunit components are heterologous to the line of cultured cells where the cultured cells do not contain the subunit components until polynucleotides encoding the components are provided in the cells. A cell line thus produced is useful for screening of a pharmacologic agent for an effect on potassium ion channel behavior, in the manner that cell lines comprising HERG1a channels alone are now used. A heterologous subunit is expressed in the line of cultured cells as a result of stable and heritable transfer of a subunit-encoding polynucleotide into cells used to produce the cell line. Relatedly, a line of cultured cells having a HERG1b subunit component without a HERG1a subunit component, wherein the HERG1b subunit component is heterologous to the cells, is also useful for evaluating the potassium ion channels formed in, and the membrane potential of, such cells, as a tool to evaluate ion channels in patients having a truncated HERG1a subunit or no HERG1a subunit and to screen and develop compounds that may be effective in enhancing HERG1b surface expression and thus rescuing the mutant phenotype. While it will be appreciated that mammalian cell lines comprising other ERG orthologs can be prepared and used as described herein, such cells are of less commercial interest than cells expressing HERG, as it is the behavior of the human ion channels in human cells that is of interest when screening prospective new drugs for use in humans. However, such cells could find utility in screening of veterinary pharmaceuticals for possible effects upon ERG ion channels in non-human animals. While the application is generally directed to HERG-expressing cells, it is understood that ERG-expressing cells are also within the scope of the disclosure. 
     A second aspect of the invention is summarized in that a method for screening a pharmacologic agent for an effect on potassium ion channel behavior includes the steps of (1) establishing a baseline potassium ion channel behavior of a line of cultured cells having a cell membrane that comprises HERG potassium ion channels having HERG1a and HERG1b subunit components, (2) exposing the line of cultured cells to a pharmacologic agent, (3) determining the potassium ion channel behavior of the exposed cells, (4) comparing the potassium ion channel behavior of the exposed cells to the baseline potassium ion channel behavior, and (5) determining whether the pharmacologic agent affects the potassium ion channel behavior of the cells. Ion channel behavior can be assessed using any conventional electrophysiological approach such as a square voltage clamp protocol or an action potential clamp protocol. The latter is advantageous as it approximates the physiological behavior of the ion channels in cardiac tissue, particularly with regard to the opening and closing of the channels, and thereby yields more meaningful information about the binding of a pharmacologic agent to an open channel and the associated risk to an individual. 
     In a related aspect, the invention is further summarized in that a method for establishing a line of cultured cells includes the steps of (1) introducing into an expression-competent cell line one or more expression vectors containing polynucleotides that encode HERG1b (and, optionally, HERG1a) under the control of an upstream transcriptional promoter and an optional downstream polyA addition sequence under conditions favoring transcription of one or more transcripts from the vector and translation from the transcripts to yield HERG1a and HERG1b subunits for co-assembly into heterotetrameric HERG channels in the membranes of the cells, (2) selecting cells that express either the HERG1b subunit alone or the HERG1a and HERG1b subunits, and (3) expanding a single cell clone to establish the line of cultured cells. To confirm expression of the HERG1a and HERG1b in the cultured cell line, levels of HERG subunit expression can be evaluated in the line using biochemical or electrophysiological methods or both. 
     In still another related aspect, the invention is further summarized in that in the method for establishing a line of cultured cells, the polynucleotides that encode HERG1a and HERG1b are the published polynucleotides that encode conventional HERG1a and HERG1b, wherein such sequences are presented herein in SEQ ID NO:1 and SEQ ID NO:3. Optionally, additional related cell lines in keeping with the invention can be established by substituting for the conventional HERG1a- or HERG1b -encoding polynucleotides, or both, related polynucleotides carrying mutations known in the art to be associated with LQTS. The skilled artisan will appreciate that the evaluation of a pharmacologic agent can be customized for a particular individual if the HERG profile (protein sequences of the HERG1a and HERG1b subunits) in the tested line of cultured cells matches or substantially matches the HERG profile of the individual. 
     These findings have significant implications for N-terminal mutations that are causally associated with LQTS. Approximately 20% of LQTS-2 mutations reside in the N terminus of HERG1a, where they can truncate the protein, alter gating properties, and/or cause trafficking deficiencies. Since HERG1a and HERG1b are alternate transcripts produced by the HERG1 gene, mutations in exons encoding the HERG1a N-terminus are not likely to affect the production of wild-type HERG1b from this gene. These findings suggest the importance of screening LQTS patients for mutations in the HERG1 b-specific exon and for assessing the disease mechanism of all mutations in heterologous expression systems in which HERG1a and HERG1b are co-expressed. 
     In another aspect, the invention relates to a polyclonal or monoclonal antibody specific for the ERG1b isoform. In this regard, the inventors have produced a polyclonal antibody specific for the ERG1b isoform. It is the only such antibody known to be in existence, and it is useful in many applications, especially for establishing and characterizing the cell lines of the invention and for localizing the ERG1b isoform in vivo. The anti-ERG1b antibody recognizes HERG1b (from humans) and ERG1b (from non-human species). 
     In a related aspect, the invention further relates to a polyclonal or monoclonal antibody specific for the ERG1a isoform, the antibody being raised against an epitope in the ERG1a subunit, where the epitope shares amino acid similarity with the ERG1b epitope used to produce the ERG1b antibody and has a Jameson-Wolf antigenic index greater than 1. A portion of ERG1a between amino acids 264 and 286, inclusive, has these attributes. Interestingly, the characteristics of this epitope are also found in the 1a/1b C-terminal epitope and the HERG1b N-terminal epitope, used to raise the ERG-KA antibody and the ERG1b antibody, respectively, but in no other contiguous ˜20 amino acid long portion of the HERG1a or HERG1b protein. In particular, these regions are characterized by a first pair of basic amino acid residues (such as conservatively related arginine or lysine residues) spaced apart by a single residue and a second pair of adjacent basic residues separated from the aforementioned pair by three to six amino acid residues. 
     In another related aspect, the cultured cells of the invention can also be employed in a screen for anti-cancer HERG blocker drugs, since it has been reported that HERG channels are upregulated in tumor cells and that proliferation of tumor cells is blocked by HERG blockers (Crociani et al., 2003). 
     It is an object of the invention to provide a line of cultured cells for screening of pharmacologic agents for an effect on potassium ion channel behavior where the line of cells recapitulates native cardiac I Kr  or the I Kr  observed in cardiac cells having mutant HERG channels. 
     It is a feature of the invention that the cultured cells have a cell membrane that comprises HERG potassium ion channels having native or mutant HERG1a and HERG1b subunit components. 
     Other objects, advantages and features of the invention will become apparent upon consideration of the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  illustrates the schematic topology of N-terminal regions of HERG1a and HERG1b proteins showing divergent N termini (hatched) and identical regions (white and black). Predicted molecular mass for ERG1a is 127 kD, and 90 kD for ERG1b. 
         FIG. 2  illustrates an overlay of normalized currents recorded from HEK-293 cells expressing HERG1a, HERG1b and both HERG1a/1b. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Conventional or native HERG1a and HERG1b channel subunits are encoded by the HERG1 gene and arise as the result of differential splicing. The polynucleotide that encodes native HERG1a is published at the National Center for Biotechnology Information website and is presented herein at SEQ ID NO:1. The polynucleotide that encodes native HERGlb is published at the National Center for Biotechnology Information website and is presented herein at SEQ ID NO: 3. Additionally, several hundred separate mutations in the HERG1a polynucleotides (often in the portion shared by HERG1a and HERG1b) known to cause various changes in cardiac I Kr , are also known, and additional mutations in HERG1 will undoubtedly be reported. The skilled artisan is in possession of various databases of such mutations (as well as polymorphisms). For example, the Working Group on Arrhythmias of the European Society of Cardiology maintains a database that is periodically updated on its website Also, the Statens Serum Institute maintains a very similar database on its website. The current data are provided herein at Table 1, but it will be understood that the skilled artisan can be readily apprised of further cataloged mutations in HERG by consulting the literature or database of such mutations. Additionally, one or a plurality of mutations in either HERG1a, HERG1b, or both, can be incorporated using standard tools of the skilled molecular biologist into one or more of the HERG1 subunits. 
     It will be understood by the skilled artisan that a line of cultured cells produced according to the invention can comprise in the cell membrane a conventional HERG1b subunit or a mutant HERG1b subunit, optionally in combination with a conventional or mutant HERG1a subunit. It will be appreciated that if the cell line is used in a screening method as described herein, then it is desirable for the cell line to express both the HERG1b and the HERG1a subunits. It is of particular interest to evaluate the potassium ion channel behavior of cell lines carrying conventional HERG1a and HERG1b. Also of significant interest is the potassium ion channel behavior of cell lines carrying conventional HERG1b in combination with HERG1a mutants, even more particularly those HERG1a mutants that differ from conventional HERG1a in the N-terminal portion of the protein not shared with HERG1 b. Further, when screening ion channels characteristic of HERG1a-defective mutants, a cell line that comprises HERG1b channels alone is indicated. A HERG1a-defective mutant subunit can be expressed in such cells to simulate channel formation and behavior in native tissue. The skilled artisan will appreciate that mutations in HERG1a- or HERG1b (which can be deletions, insertions, or substitutions) can alter, or can have no effect upon, the activity of the subunits or the channels formed therefrom. The attached list of mutations give a flavor for the types of mutations contemplated, but is not to be considered an exhaustive list. 
     Any of the aforementioned polynucleotides can be cloned into an expression vector (or plurality of expression vectors) engineered to support expression from the polynucleotides. Suitable expression vectors comprise a transcriptional promoter active in a recipient cell upstream of the HERG1 polynucleotide and can optionally comprise a polyA-addition sequence downstream of the polynuceotide. 
     Suitable commercially available expression vectors are pcDNA3.1 and pcDNA3.1zeo (Invitrogen), which differ from one another in that pcDNA3.1 includes sequences conferring resistance to neomycin while pcDNA3.1zeo includes sequences conferring resistance to zeocin. The polyA-addition sequences, not required for expression, can be excised from these vectors by digesting both with ApaI (nuc. 1002) and BbsI (nuc. 1217), respectively, filling in, and self-ligating. The vectors can be prepared to receive the HERG1a or HERG1b polynucleotides, by cleavage with EcoRI and BamHI. For convenience during the subsequent selection step, HERG1a can then be cloned into the cleaved pcDNA3.1vector; HERG1b can be cloned into the cleaved pcDNA3.1zeo vector. In addition, HERG1a and 1b polynucleotides can be ligated into the two multiple cloning sites of a vector with an internal ribosomal entry site, such as pIRES (Novagen), which allows for production of two separate proteins from a single transcript. Use of this vector to produce clonal cell lines allows for selection with a single antibiotic. 
     The vector(s) can be introduced (or co-introduced) by, for example, transfection or lipofection, into cells competent to receive and express the HERG1 subunits in their cell membrane. A commercially lipofection kit, such as a kit available for this purpose from Mirus Corporation, Madison, Wis., can be employed. Preferably, the recipient cells do not natively contain HERG1 subunits in their cell membranes, so that the presence of HERG1 channels in the membrane is completely attributable to expression from the introduced expression vector. Suitable recipient cells are human embryonic kidney cells such as cells of the HEK-293 cell line, commercially available from the American Type Culture Collection (Accession Number CRL-1573). 
     Later, preferably about 24 hours later, cells can be harvested, distributed into wells and grown in selective media. In the exemplified embodiment, a selective medium suitable for selecting cells carrying the HERG1a-vector contains neomycin at 500 micrograms/ml, a medium suited for carrying the HERG1b-vector contains zeocin at 100 micrograms/ml, and a medium suited for growing cells carrying both vectors contains both antibiotics. Cells can be grown under selection for 2-3 weeks until the wells are confluent. Resulting clonal lines (24-48 for each type) can be examined biochemically or electrophysiologically to confirm the presence of the HERG1 channel subunit(s) and the level(s) of HERG produced. 
     EXAMPLES 
     Materials and Methods 
     Cell lines and Antibodies. Human embryonic kidney 293 (HEK-293) cell lines stably expressing conventional HERG1a have been previously described by Zhou, Z., et al.,  Biophys J.  74, 230-241 (1998), and by Furutani, M., et al.,  Circulation  99:2290-2294 (1999), each incorporated by reference as if set forth in its entirety. Cell lines stably expressing HERG1a and HERG1b were prepared by transfecting HEK-293/HERG1a stable cells with HERG1b containing a Kozak consensus sequence cloned into the Bam HI/Eco RI sites of pcDNA3.1zeo (Invitrogen, Carlsbad, Calif.). Separate cell colonies were selected after plating cells at low density and grown in media containing 100 μg/ml Zeocin, 500 μg/ml neomycin for selection. All HEK-293 cells were cultured in Dulbecco&#39;s modified Eagle&#39;s medium at 37° C. 
     A polyclonal antibody (termed “ERG-KA”) raised against a peptide having an amino acid sequence of CRQRKRKLSFRRRTDKDTEQ (corresponding to amino acids 883 through 901 of SEQ ID NO:1 plus a non-essential N-terminal cysteine residue provided to permit coupling of the peptide to an immunogenic carrier) can co-precipitate both HERG1a and HERG1b subunits from cardiac myocytes of the HERG channel protein and is diagnostic for the presence of both HERG1a and HERG1b in HERG channels in vivo or in cultured cell lines in vitro. See Roti Roti, E. et al., “Interaction with GM130 during HERG Ion Channel Trafficking,”  J. B. C.,  277:47779 (2002), incorporated herein by reference as if set forth in its entirety. ERG1 isoform-specific antibodies were produced by Bethyl Laboratories (Montgomery, Tex., USA) in rabbits. Antisera were affinity purified using the same peptides employed in immunization. An immunogenic ERG1b peptide was amino acids 12-25 (GALRPRAQKGRVRR) of SEQ ID NO:4 (HERG1 b). The ERG1b antibody was raised against CGALRPRAQKGRVRR, corresponding to the aforementioned amino acids 12-25 plus a non-essential N-terminal cysteine residue provided to permit coupling of the peptide to an immunogenic carrier. An immunogenic ERG1a peptide was amino acids 140-153 (SPAHDTNHRGPPTS) (Neoclone, Madison, Wis.) of SEQ ID NO:2 (HERG1a). A HERG1a-specific antibody raised in goat (HERG N-20) was purchased from Santa Cruz Biotechnologies (Santa Cruz, Calif.). Horseradish Peroxidase-(HRP-) coupled secondary antibodies were purchased from Pierce (Rockford, Ill.) and Santa Cruz Biotechnology (Santa Cruz, Calif.). Fluorophore-coupled secondary antibodies were purchased from Molecular Probes (Lake Oswego, Oreg.). 
     Cardiac Tissue Preparation. Human male ventricular lysate was purchased from ProSci Inc. (Poway, Calif.). Canine ventricular myocytes were isolated from mongrel males and enzymatically treated as described by He, J. Q., et al. (2001), incorporated herein by reference as if set forth in its entirety. Sprague-Dawley rat ventricles were excised from anesthetized adult males after injection of sodium Pentobarbital (100 mg/kg body weight intraperitoneal) as described by He, J. Q, et al. Rat ventricular myocytes were prepared using the same procedure as described for the canine tissue. All procedures were approved by the Research Animal Resources Center (RARC) at UW-Madison. 
     Cell Membrane Protein Preparations. Membranes were prepared from myocytes or ventricular tissue after suspension in homogenization buffer (in mM: 25 Tris-HCl, pH 7.4; 10 NaEGTA; 20 NaEDTA). All buffers used in this procedure contained the following protease inhibitor cocktail: 5 μg/ml aprotinin, 50 μg/ml 1,10 phenanthroline, 0.7 μg/ml pepstatin A, 1.56 μg/ml benzamidine, and 1× Complete minitab (Roche, Indianapolis, Ind.). Suspensions were homogenized using a Polytron homogenizer at setting 6 for two bursts of 15 seconds each, followed by sonication on ice twice at an amplitude of 20 for 20 seconds each. Suspensions were spun at 2,000×g at 4° C. for 10 minutes to remove cellular debris. The supernatants were subjected to further centrifugation at 40,000×g for 30 minutes at 4° C. The resultant pellet was solubilized on a rotary shaker at 4° C. for 2 hours, in either Triton buffer (in mM: 150 NaCl; 25 Tris-HCl, pH 7.4; 20 NaEDTA; 10 NaEGTA; 5 glucose, and 1% v/v Triton X-100), or RIPA buffer (in mM: 150 NaCl; 50 Tris-HCl, pH 7.4, 1 NaEDTA, and 1% v/v Triton X-100, 1% v/v sodium deoxycholate, 0.1% v/v sodium dodecylsulfate). Samples were then spun at 10,000×g to remove insoluble material. Cell line membrane pellets were prepared by washing plates gently with PBS, aspirating, and adding either Triton buffer or RIPA buffer. Cells were then scraped, collected in a microfuge tube, and sonicated on ice twice at an amplitude of 20 for 20 seconds each. The suspension was rotated at 4° C. for 2 hours and then centrifuged at 10,000×g for 10 minutes to remove insoluble material. Protein concentrations of all samples were determined using a modified Bradford assay (DC Protein Assay, Bio-Rad, Hercules, Calif.). 
     Biochemical Analysis. Membrane proteins were deglycosylated using PNGase F and Endoglycosidase H (Roche Applied Science, Indianapolis, Ind.) as described by Zhou, Z., et al. (1998), supra, and by Zhou, Z., et al., J Biol Chem 273, 21061-21066 (1998), incorporated herein by reference as if set forth in its entirety. Proteins were denatured at 60° C. to avoid thermal aggregation at higher temperatures. To determine which proteins were expressed on the surface membrane, proteins were surface biotinylated using sulfo-NHS-LC-Biotin reagent. Briefly, 100 mm tissue culture dishes with growth at 70-80% confluency were rinsed three times with cold PBS, and incubated with freshly prepared Biotin reagent (5 mg/ml) in PBS for 45 minutes at 4° C. Cells were then rinsed once with 25 mM Tris-HCl (pH 7.5) to quench the reaction, followed by three washes with cold PBS. Membrane proteins were prepared as indicated above. 
     Western Blot Analysis. Membrane proteins (cell lines 2-10 μg/lane; heart lysates 30-50 μg/lane) were separated on 7.5% SDS-polyacrylamide electrophoresis gels along with prestained molecular weight markers (Bio-Rad, Hercules, Calif.), and were then transferred to PVDF membranes (Immobilon-P, Bedford, Mass.) for 1 hour at 100 mV. Western blots were blocked, probed, and analyzed as described. For peptide block experiments, 5 μl antibody was incubated with 10 μg peptide in 100 μl TBS for 6 hours at 4° C., then centrifuged at 10,000×g for 20 min. The supernatant was carefully removed and used to probe Western blots. Western blot controls include probing blots with secondary antibody alone, and peptide block of primary antibody. In the case of heart lysates, a lane containing HERG1a/1b cell membrane preparation was included as a positive control. 
     Co-Immunoprecipitation. Membrane lysates (cell lines: 100-200 μg/reaction; heart lysates: 500-1000 μg/reaction) in 1 ml TBS (150 mM NaCl, 25 mM Tris-HCl, pH 7.4) were cleared with 50 μl Protein A or G sepharose beads (Amersham, Palatine, Ill.) on immunoprecipitating (IP) antibody; Protein A was used for rabbit and Protein G for goat IP antibodies. Cleared lysates were incubated with antibody (anti-ERG1b at 1/100 or N-20 at 1/20) on a rotating platform for 3-16 hours at 4° C. 50 μl Protein A or G coupled beads were added and samples were incubated at 4° C. for an additional 1-3 h. Beads were collected by centrifugation at 10,000×g, and washed three times with 150 mM NaCl, 25 mM Tris-HCl, pH 7.4, 5 mM NaEDTA, 1% (v/v) Trition X-100, followed by one wash with 150 mM NaCl, 25 mM Tris-HCl, pH 7.4. Proteins were eluted with 200 ng/ml antibody-specific peptide for 1 hour at 4° C. Samples were centrifuged at 10,000×g and the supernatant was collected. 100 μl LSB (25 mM Tris-HCl, pH 6.8, 2% v/v sodium dodecylsulfate, 10% glycerol) was added to the beads to elute any proteins that remained bound. Additional controls included lysates processed without antibody. Eluted proteins were Western blotted as described above. 
     Immunohistochemistry. Isolated canine myocytes were fixed in 2% paraformaldehyde-PBS, pH 7.4 for 10 minutes at room temperature, and were washed 3× in PBS (pH 7.4). Myocytes were then either stored at 4° C. (for up to 8 weeks) or processed immediately. Myocytes were washed once in PBS (pH 7.4)+1% Triton X-100, and permeabilized in PBS (pH 7.4)+0.5% Triton X-100 for 10 minutes at room temperature followed by incubation in 0.75% glycine-PBS (pH 7.4) for 10 minutes at room temperature to quench any free aldehydes, and incubation in blocking buffer (PBS, pH 7.4, +0.1% Tween-20+10% donkey serum+2% BSA) for 2 hours at 4° C., with rotation. Cells were washed 3× with PBS (pH 7.4)+0.1% Tween-20, and divided into 0.5 ml aliquots. Each myocyte aliquot was incubated overnight at 4° C. in diluted primary antibody. ERG1b antibodies were diluted 1:1000, ERG1a antibodies (N-20), 1:10, and myosin binding protein C antibodies, 1:500. Myocytes were washed 3×1 hour in PBS (pH 7.4)+0.1% Tween-20. Secondary antibodies were diluted in PBS (pH 7.4)+0.1% Tween-20+5% BSA, and spun to remove any aggregates. Myocytes were suspended in 0.5 ml of diluted secondary antibody and incubated in the dark 2 hours at room temperature with rotation. Donkey anti-rabbit Alexa 488 and donkey anti-goat Alexa 568 antibodies were diluted 1:1000. Myocytes were washed briefly 3× with PBS (pH 7.4)+0.1% Tween-20 followed by two 1 hour washes with PBS, pH 7.4 and were stored at 4° C. until viewed on a Zeiss Axiovert 200 with a 63× objective. Optical sectioning was accomplished using the Apotome, and 3D rendering done within Axiovision software. Fluorescent excitation-emission filter set for Alexa 488 (excitation 450-490 nm: emission 515-565 nm) and Alexa 568 (excitation 500-639 nm, emission 560-700 nm) do not overlap. Species specificity of secondary antibodies was confirmed by incubating cells probed with one primary with secondary antibody raised against the other species. No signal was detected demonstrating each secondary is species specific. Secondary alone controls were also used to ensure signal was specific. 
     Results 
     On a Western blot of rat heart tissue, the ERG-KA antibody identified three bands at 160, 120, and 95 kD. The two higher molecular mass bands are consistent in size with maturely glycosylated and unglycosylated rat ERG1a, respectively. The 95 kD band is consistent in size with ERG1b protein produced in heterologous expression systems (see below) but had not been previously observed in native tissue. The 95 kD band cannot represent ERG-USO, another HERG1 transcript that produces a protein of approximately the same size, since ERG-USO does not contain the C terminal sequence against which the ERG-KA antibody was raised. 
     To test the hypothesis that the 95 kD band represents ERG1b, Western blots of membrane proteins prepared from HEK-293 cells stably expressing HERG1a and HERG1b were evaluated with the ERG1a- and ERG1b N termini-specific antibodies. The ERG-KA antibody recognized bands at 155 and 135 kD, consistent with previously published results identifying these bands as mature and immature HERG1 glycoforms, respectively. As expected, blots probed with the ERG1a-specific antibody recognized the 155 and 135 kD bands but not the three lower-mass bands. The 155 and 135 kD bands were eliminated upon incubation of the ERG1a antisera with the antigenic HERG1a peptide prior to probing the blots. Notably, cells expressing HERG1a alone produced only the 155 and 135 kD bands, representing the mature and immature HERG1a species, respectively. 
     ERG-KA also recognized three lower molecular mass bands at 95, 85 and 80 kD. The ERG1b-specific antibody recognized the 95, 85, and 80 kD bands but not the two higher-mass HERG1a bands. These bands were similarly eliminated by preincubation of the antisera with the antigenic HERG1b peptide. These data show that ERG-KA antisera recognize both HERG1a and HERG1b isoforms, and that ERG1a and HERG1b antisera are specific for their corresponding isoforms. 
     Membrane proteins from stable HEK-293 HERG1a/1b cell lines were incubated with glycosidases to determine if the multiple HERG1b bands on Western blots correspond to different glycoforms, as shown previously for the HERG1a 155 and 135 kD bands. Removing all glycans from the HERG1b proteins by incubating membrane preparations with PNGase F reduced the higher molecular mass HERG1b species to a single 80 kD band. Digestion with Endoglycosidase H, which removes only glycans that are attached in the ER but not yet processed in the Golgi, reduced the 85 kD band to 80 kD but left the 95 kD band unaltered. Thus, the 95 kD band represents the maturely glycosylated (Golgi-processed) HERG1b isoform, the 85 kD band the core glycosylated, ER-retained form, and the 80 kD band the unglycosylated form. To determine if the mature HERG1b glycoform is expressed on the cell surface, where it could contribute to HERG1 currents, surface proteins were biotinylated prior to cell lysis. Biotinylated proteins were affinity purified with streptavidin beads, Western blotted, and probed with ERG-KA antisera. Like HERG1a, only the maturely glycosylated HERG1b (95 kD) protein band was biotinylated, showing that it is expressed on the cell surface in HEK-293 cells. 
     In Western blots from two separate human ventricular membrane preparations, the ERG-KA antibody revealed bands at 140, 120, 94 and 83 kD. The 140 and 120 kD bands are consistent with previous reports from human tissue and represent the maturely glycosylated and unglycosylated HERG1a, respectively. The HERG1 b-specific antibody recognized the 94 and 83 kD bands, demonstrating that ERG1b protein is expressed in human ventricle. 
     ERG1b was also observed in Western blots of canine ventricular tissue. There the ERG-KA antibody consistently recognized proteins at 160-165 and 90-95 kD and less consistently at 140-145, 115-125 and 80-85 kD. High molecular weight bands at 165 and 140 kD were recognized by both ERG-KA and ERG1a antibodies and thus represent ERG1a isoforms. The ERG1b antibody recognized the bands at 95 and 83 kD, which were also recognized by ERG-KA, demonstrating that these bands represent ERG1b isoforms. These data show conclusively both ERG1a and ERG1b proteins are expressed within the ventricle across a range of mammalian species. 
     The antibodies were also characterized by immunocytochemistry using confocal microscopy. The HERG-KA antibody detected HERG1a and HERG1b with a very high sensitivity and little background. The HERG1 b-specific antibody stained cells expressing HERG1b but not those expressing HERG1a. The ERG1a antibody gave a high background in immunocytochemistry. 
     Co-Immunoprecipitation Evidence For of HERG1a and HERG1b Heteromerization in Animal Tissue and in HEK-293 Cells 
     Immunoprecipitation studies confirmed that HERG1a and HERG1b co-assemble in heterologous expression systems and in animal tissue. Bidirectional co-immunoprecipitations were carried out in four separate canine cardiac membrane preparations to confirm in vivo assembly. Extracts of fresh, solubilized canine heart tissue were incubated with the HERG1b-specific antibody conjugated to sepharose beads. The beads were concentrated by centrifugation and washed, and the bound proteins were eluted, size-fractionated by SDS-PAGE and transferred to PVDF membrane. At least two membranes were prepared for each eluate. One was probed with the HERG1b-specific antibody to confirm HERG1b immunoprecipitation. The second blot probed with the ERG1a-specific antibody showed HERG1a mature bands, demonstrating HERG1a associates with HERG1b in vivo. The immunoprecipitated proteins were visualized with the HERG-KA antibody, identifying both the precipitating and the associating subunits. The ERG1a-specific antibody immunoprecipitates both mature and immature HERG1a, and co-immunoprecipitates HERG1b. Interestingly, the ERG1a antibody enriched for the mature HERG1b species, relative to its abundance in lysate. Signals were visualized on blots using ECL (Amersham) chemiluminescent detection. The converse IP experiment using bead-bound ERG1a-specific antibody was run to confirm the first IP results. A no-antibody, bead-only control was included in each immunoprecipitation experiment to control for nonspecific precipitation. 
     Similar results were obtained from one human myocyte preparation immunoprecipitated with the ERG1a specific antibody. These data show that HERG1a and HERG1b proteins associate in mammalian ventricular myocytes in vivo. 
     To confirm that ERG1a and HERG1b isoforms co-assemble to form heteromeric channels in native tissues, we demonstrated that both ERG1a and ERG1b isoforms are present in rat and canine heart lysates, and that they can be visualized in confocal immunofluorescence microscopy images as puncta along the myocyte sarcolemma, characteristic of a T-tubular distribution. Localization of ERG1a and ERG1b to T-tubular structures in canine ventricular myocytes is consistent with electron microscopy studies in rat myocytes showing ERG1 protein predominantly localized to the T-tubules, where it could regulate action potential duration at the site of excitation-contraction coupling. The signal is similar for both fixed and live cells. No signal was observed when probed only with the secondary antibody. These data suggests that both subunits express at the surface membrane in a similar pattern. 
     To characterize ERG1 localization more precisely, we stained myocytes concurrently with ERG1a and Myosin binding protein C (MyBP-C) antisera. Three-dimensional images were rendered from a stack of deconvolved two-dimensional immunofluorescent images. MyBP-C signal, in green, appeared as a repeating pattern of doublets separated by regions devoid of fluorescence that span the cell&#39;s width. MyBP-C signal localized to the myosin-containing sarcomere A-band; the unstained areas, between doublets, represent M-lines. ERG1a fluorescent signal, shown in red, was seen in I-bands, adjacent to A-bands. Both Z-lines and T-tubules were located in the I-band. The punctate red ERG1a signal extended in columns from the cell surface to the interior, as expected of a T-tubular-restricted protein, where it borders the green MyBP-C signal. These data indicate ERG1 signal in canine myocytes is consistent with a T-tubular distribution. 
     Co-Assembly of HERG1a and HERG1b Subunits in Stably Transfected Cell Lines. 
     HERG1a and HERG1b subunits can be co-immunoprecipitated by antibodies specific to either ERG1a or ERG1b subunits from HEK-293 cells. Membrane currents resulting from the co-assembly of these two subunits display characteristic I Kr  current profile and sensitivity to E-4031 and antiarrhythmic drug quinidine. 
       FIG. 2  illustrates an overlay of normalized currents recorded from HEK-293 cells expressing HERG1a, HERG1b and both HERG1a/1b. The current of the cells that express HERG1a and HERG1b has the characteristic electrophysiological properties of an I Kr  current. 
     The present invention is not intended to be limited to the foregoing embodiments, but rather to encompass all such modifications and variations as come within the scope of the appended claims.