Retroviral vectors containing putative internal ribosome entry sites: development of a polycistronic gene transfer system and applications to human gene therapy

Recombinant retroviral vectors producing multicistronic mRNAs were constructed. Picornavirus putative internal ribosome entry sites (IRES) were used to confer cap-independent translation of an internal cistron. Internal cistrons were engineered by ligation of various lengths of the IRES of encephalomyocarditis (EMC) virus or polio virus to the E. coli chloramphenicol acetyltransferase (CAT) gene. The IRES/CAT fusions were introduced into retroviral vectors 3' to the translation stop codon of the neomycin phosphotransferase (NEO) gene, and the molecular constructs transfected into retroviral vector packaging lines. Retroviral vector producer cells efficiently express the internal CAT gene product only when the full length IRES is used. Both the EMC/CAT and polio/CAT retroviral vectors produced high titer vector supernatant capable of productive transduction of target cells. To test the generality of this gene transfer system, a retroviral vector containing an IRES fusion to the human adenosine deaminase (ADA) gene was constructed. Producer cell supernatant was used to transduce NIH/3T3 cells, and transduced cells were shown to express NEO, and ADA. Novel three-gene-containing retroviral vectors were constructed by introducing the EMC/ADA fusion into either an existing internal-promoter-containing vector, or a polio/CAT bicistronic vector. Producer cell clones of the three-gene vectors synthesize all three gene products, were of high titer, and could productively transduce NIH/3T3 cells. By utilizing cap-independent translation units, IRES vectors can produce polycistronic mRNAs which enhance the ability of retroviral-mediated gene transfer to engineer cells to produce multiple foreign proteins.

HIV-1 protease cleaves actin during acute infection of human T-lymphocytes

Actin, one of the most abundant proteins of the cell, is hydrolyzed by the human immunodeficiency virus type 1 (HIV-1) protease during acute infection of cultured human T lymphocytes. The actin fragments produced during the course of infection are identical to those obtained by recombinant HIV-1 protease digests of (1) a lysate from uninfected T lymphocytes and (2) globular actin itself. Hydrolysis by the HIV-1 protease of physiologically important host cellular proteins during infection may have important consequences relative to viral pathogenesis.

Frame-shift mutations within the vaccinia virus A-type inclusion protein gene

The genetic basis for the failure of vaccinia virus (strain WR) to form a full-length 150 kiloDalton (kDa) A-type inclusion protein was determined by sequencing a 4.1-kb pair segment of DNA and analyzing its transcription products. Open reading frames predicted to encode slightly overlapping 84.5- and 27.1-kDa proteins homologous to contiguous N-terminal segments of the A-type inclusion protein of cowpox virus were found. A putative deletion of two adjacent nucleotides occurring within several consecutive AG repeats and an insertion of 8 nucleotides accounted for the first and second reading frame shifts, respectively. Additional small mutations affecting reading frames were present in the C-terminal region of the gene. The vaccinia and cowpox virus mRNAs encoding the disparate size A-type inclusion proteins were similar in length, had equivalent 5' and 3' ends, and were expressed late in infection indicating the absence of mutations affecting transcriptional signals.

Identification and expression of rpo19, a vaccinia virus gene encoding a 19-kilodalton DNA-dependent RNA polymerase subunit

The vaccinia virus DNA-dependent RNA polymerase subunit gene rpo19 was identified, and its expression was examined at RNA and protein levels. Antibody to the multisubunit RNA polymerase purified from virions reacted with a polypeptide with an apparent Mr of 21,000 that was synthesized in reticulocyte lysates programmed with (i) mRNA from infected cells that was isolated by hybridization to DNA subclones of the viral genomic HindIII A fragment and (ii) mRNA made in vitro by transcription of the viral open reading frame A6R. Polyclonal antiserum, raised to a recombinant protein product of the A6R open reading frame which could encode an 18,996-Da protein with an acidic N terminus, reacted with Mr-21,000 and -22,000 polypeptides that cosedimented with purified RNA polymerase. Internal sequencing of the two polypeptides confirmed that both were encoded by A6R, and the gene was named rpo19 to indicate the predicted molecular mass of the polypeptide in kilodaltons. Immunoblotting and metabolic labeling of infected cell proteins indicated that synthesis of the Mr-21,000 polypeptide started early and continued throughout virus infection, whereas the Mr-22,000 form appeared late following DNA replication. RNA analyses suggested that the rpo19 mRNA was expressed from a dual early/late promoter and that the protein-coding region of the mRNA was directly preceded by a short 5' poly(A) leader, apparently initiated within the TAAATG motif at the beginning of the open reading frame.

Hepatitis B virus (HBV)-specific cytotoxic T-cell (CTL) response in humans: characterization of HLA class II-restricted CTLs that recognize endogenously synthesized HBV envelope antigens

In this study, we show that CD4+, hepatitis B virus (HBV) envelope-specific T-cell clones produced by stimulation with a particulate antigen preparation are able to recognize and kill not only autologous antigen-presenting cells incubated with exogenous HBV envelope antigens but also autologous HLA class II-positive cells expressing endogenously synthesized HBV envelope antigens following infection with recombinant vaccinia viruses or transfection with recombinant Epstein-Barr virus expression vectors. Experiments with lysosomotropic agents and brefeldin A suggest that the endosomal compartment is likely involved in the processing of endogenously synthesized viral proteins for recognition by CD4+ T cells. Our study indicates that HBV envelope-specific, HLA class II-restricted CD4+ cytotoxic T lymphocytes can potentially participate in the immune clearance of HBV-infected cells and the pathogenesis of hepatocellular injury in hepatitis B.

Vaccinia virus complement-control protein prevents antibody-dependent complement-enhanced neutralization of infectivity and contributes to virulence

The role of a viral gene product in evasion of the host immune response was investigated. The antibody-dependent complement-enhanced neutralization of vaccinia virus infectivity was prevented by the culture medium from vaccinia virus-infected cells. The vaccinia virus complement-control protein (VCP) was identified as the secreted product of vaccinia virus gene C21L and has homology to a group of eukaryotic genes encoding regulators of complement activation. Thus, the culture medium from cells infected with a C21L deletion mutant was VCP deficient and had little or no effect on antibody-dependent complement-enhanced neutralization. In addition, the anticomplement effect was associated with the C21L-encoded protein partially purified from the medium of cells infected with wild-type virus. Antibody-dependent, complement-enhanced neutralization of vaccinia virus occurred with a complement source that was deficient in the classical pathway complement component C4 and required the alternative pathway complement factor B. Furthermore, the presence of VCP abrogated the complement-enhanced neutralization in C4-deficient serum. Together with previous hemolysis data, the present result suggests that VCP can inhibit both the classical and alternative pathways of complement activation. Skin lesions caused by the C21L deletion mutant were smaller than those caused by wild-type virus, demonstrating an important role for VCP in virulence. The C21L deletion mutant also was attenuated in C4-deficient guinea pigs, consistent with in vitro studies. Vaccinia virus appears to have acquired the ability to regulate the complement cascade for the purpose of evading the host immune response.

Cell-specific posttranslational events affect functional expression at the plasma membrane but not tetrodotoxin sensitivity of the rat brain IIA sodium channel alpha-subunit expressed in mammalian cells

The rat brain IIA Na+ channel alpha-subunit was expressed and studied in mammalian cells. Cells were infected with a recombinant vaccinia virus (VV) carrying the bacteriophage T7 RNA polymerase gene and were transfected with cDNA encoding the IIA Na+ channel alpha-subunit under control of a T7 promoter. Whole-cell patch-clamp recording showed that functional IIA channels were expressed efficiently (approximately 10 channels/microns2 in approximately 60% of cells) in Chinese hamster ovary (CHO) cells and in neonatal rat ventricular myocytes but were expressed poorly in undifferentiated BC3H1 cells and failed to express in Ltk- cells. However, voltage-dependent Drosophila Shaker H4 K+ channels and Escherichia coli beta-galactosidase were expressed efficiently in all four cell types with VV vectors. Because RNA synthesis probably occurs without major differences in the cytoplasm of all infected cell types under the control of the T7 promoter and T7 polymerase, we conclude that cell type-specific expression of the Na+ channel probably reflects differences at posttranslational steps. The gating properties of the IIA Na+ currents expressed in cardiac myocytes differed from those expressed in CHO cells; most noticeably, the IIA Na+ currents displayed more rapid macroscopic inactivation when expressed in cardiac myocytes. These differences also suggest cell-specific posttranslational modifications. IIA channels were blocked by approximately 90% by 90 nM TTX when expressed either in CHO cells or in cardiac myocytes; the latter also continued to display endogenous TTX-resistant Na+ currents. Therefore, the TTX binding site of the channel is not affected by cell-specific modifications and is encoded by the primary amino acid sequence.

Activity of CD4-Pseudomonas exotoxin against cells expressing diverse forms of the HIV and SIV envelope glycoproteins

CD4(178)-PE40 is a genetically engineered hybrid toxin containing a portion of human CD4 linked to the translocation and ADP-ribosylation domains of Pseudomonas exotoxin A. In vitro, the molecule has been shown to selectively kill cells expressing the envelope glycoproteins of human immunodeficiency virus (HIV) or simian immunodeficiency virus (SIV), and to inhibit HIV spread. In this report we examine the activity of the hybrid toxin against cells expressing diverse forms of the HIV and SIV envelope glycoproteins, encoded by recombinant vaccinia virus vectors. The activity of CD4(178)-PE40 was found to be unaffected by mutations in the HIV-1 or HIV-2 envelope glycoprotein genes, which prevent normal proteolytic processing of the corresponding gp160 precursor molecules. Cells expressing a mutant HIV-1 envelope glycoprotein lacking most of the cytoplasmic tail of the gp41 transmembrane subunit were also sensitive to the hybrid toxin. Most interestingly, HIV-1, HIV-2, and SIVmac envelope glycoprotein molecules known to have widely differing affinities for CD4 were found to be comparably effective at mediating sensitivity to CD4(178)-PE40. By virtue of its ability to kill infected cells, the hybrid toxin inhibited the spread of SIVmac in vitro. These results indicate that CD4(178)-PE40 is active against cells expressing HIV and SIV envelope glycoproteins with a diverse array of structural differences.

Specific lysis of human immunodeficiency virus type 1-infected cells by a HLA-A3.1-restricted CD8+ cytotoxic T-lymphocyte clone that recognizes a conserved peptide sequence within the gp41 subunit of the envelope protein

A HLA-A3.1-restricted CD8+ cytotoxic T-cell clone, E7.20, that lyses cells infected with human immunodeficiency virus type 1 was isolated from an infected individual. The epitope was localized to amino acids 768-778 (RLRDLLLIVTR, NL43 env sequence) of the cytoplasmic domain of gp41 by successive use of a panel of recombinant vaccinia viruses that express truncated env genes and synthetic peptides. The epitope is conserved on 7 (NL43, BRU, HXB2, BRVA, SC, JH3, and JFL) of 13 human immunodeficiency virus type 1 isolates from North America. Synthetic peptides of this region of strains RF and CDC4 are also recognized by E7.20 despite a nonconservative Thr----Val or Thr----Ala change at amino acid 777; however, an MN peptide, which has four amino acid substitutions, was not reactive. The epitope recognized by E7.20 has a predicted hydrophobic alpha-helical structure, with three contiguous Leu residues followed by Ile and Val at amino acids 772-776. Cytotoxicity was restricted by HLA-A3.1 using allogeneic target cells that shared HLA class I antigens with the donor and an HLA-A and -B negative human plasma cell line transfected with the HLA-A3.1 gene. The transfected cells were infectable by human immunodeficiency virus type 1 strains IIIB and MN but only the former virus sensitized them to killing by E7.20. The ability of E7.20 to specifically lyse a human lymphocyte line infected with a human immunodeficiency virus type 1 strain carrying the conserved epitope is consistent with an important role for cytotoxic T cells in controlling infection.

Vaccinia virus morphogenesis is interrupted when expression of the gene encoding an 11-kilodalton phosphorylated protein is prevented by the Escherichia coli lac repressor

A conditional lethal vaccinia virus mutant, which constitutively expresses the Escherichia coli lac repressor and has the lac operator controlling the F18R gene (the 18th open reading frame of the HindIII F fragment of the vaccinia virus strain WR genome) encoding an 11-kDa protein, was previously shown to be dependent on the inducer isopropyl-beta-D-thiogalactoside (IPTG) for replication (Y. Zhang and B. Moss, Proc. Natl. Acad. Sci. USA 88:1511-1515, 1991). Further studies indicated that the yield of infectious virus could be regulated by titration with IPTG and that virus production was arrested by IPTG removal at appropriate times. Under nonpermissive conditions, an 11-kDa protein reactive with antiserum raised to a previously described DNA-binding phosphoprotein (S. Y. Kao and W. R. Bauer, Virology 159:399-407, 1987) was not synthesized, indicating that the latter is the product of the F18R gene. In the absence of IPTG, replication of viral DNA and the subsequent resolution of concatemeric DNA molecules appeared normal. Omission of IPTG did not alter the kinetics of early and late viral protein synthesis, although the absence of the 11-kDa polypeptide was noted by labeling infected cells with [35S]methionine or [32P]phosphate. Pulse-chase experiments revealed that proteolytic processing of the major viral structural proteins, P4a and P4b, was inhibited under nonpermissive conditions, suggesting a block in virus maturation. Without addition of IPTG, the failure of virus particle formation was indicated by sucrose gradient centrifugation of infected cell lysates and by the absence of vaccinia virus-mediated pH-dependent cell fusion. Electron microscopic examination of infected cells revealed that immature virus particles, with aberrant internal structures, accumulated when synthesis of the 11-kDa DNA-binding protein was prevented.

Live vaccinia virus recombinants expressing herpes simplex virus genes

Vaccinia virus recombinants expressing antigens from herpes simplex virus (HSV) have been tested as potential live virus vaccines for prevention of HSV infection. We describe three vaccinia virus/HSV recombinants. The first expresses the HSV-1 glycoprotein D (vaccinia/gD), the second expresses the HSV-1 glycoprotein B (vaccinia/gB), and the third expresses both the HSV-1 glycoprotein D and the influenza A hemagglutinin (vaccinia/HSVgD/influenza). Mice immunized with vaccinia/gD or vaccinia/gB developed antibodies capable of neutralizing HSV in vitro and were protected against both lethal and latent infection with HSV. Protection against HSV challenge persisted for greater than 1 year in mice immunized with vaccinia/gD. The immune response to HSV in mice immunized with vaccinia/gD could be increased by a booster vaccination with vaccinia/gD. However, the immune response to HSV was decreased in animals immunized with a vaccinia recombinant that expressed non-HSV genes before vaccination with vaccinia/gD. In separate experiments, a bivalent vaccinia recombinant, vaccinia/HSVgD/influenza, was constructed and was found to be comparable to the vaccinia/gD single recombinant in immunogenicity and protective efficacy against lethal HSV challenge. We conclude that vaccinia/HSV recombinants can provide protection against HSV infection in mice and that these recombinants may provide an alternative approach in the development of a live virus vaccine against HSV.

Extracellular vaccinia virus formation and cell-to-cell virus transmission are prevented by deletion of the gene encoding the 37,000-Dalton outer envelope protein

There are two types of infectious vaccinia virus particles: intracellular naked virions and extracellular enveloped virions (EEV). To determine the biological role of the enveloped form of vaccinia virus, we produced and characterized a mutant that is defective in EEV formation. The strategy involved replacement by homologous recombination of the gene F13L, encoding a 37,000-Da protein (VP37) that is specific for the outer envelope of EEV, with a selectable antibiotic resistance marker, the Escherichia coli gpt gene. Initial experiments, however, suggested that such a mutation was lethal or prevented plaque formation. By employing a protocol consisting of high-multiplicity passages of intracellular virus from the transfected cells and then limiting dilution cloning, we succeeded in isolating the desired mutant, which was defective in production of plaques and extracellular virus but made normal amounts of intracellular naked virions. Electron microscopic examination indicated that the mutant virus particles, unlike wild type, were neither wrapped with Golgi-derived membranes nor associated with the cell surface. The absence of VP37 did not prevent the transport of the viral hemagglutinin to the plasma membrane but nevertheless abrogated both low-pH- and antibody-mediated cell fusion. These results indicate that VP37 is required for EEV formation and also plays a critical role in the local cell-to-cell transmission of vaccinia virus, perhaps via enveloped virions attached to or released from the cell membrane. By contrast, a mutated virus with a deletion of the K4L open reading frame, which is a homolog of the VP37 gene, was not defective in formation of plaques or EEV.

Poly(A) polymerase and a dissociable polyadenylation stimulatory factor encoded by vaccinia virus

mRNA made in eukaryotic cells typically has a 3' poly(A) tail that is added posttranscriptionally. To investigate mechanisms by which 3' poly(A) is formed, we identified the genes for the two vaccina virus-encoded polypeptides, VP55 and VP39. Primer-dependent polyadenylation activity was associated exclusively with purified VP55-VP39 heterodimer, which, although stable to column chromatography and glycerol gradient sedimentation, was readily dissociated by antibody to an N-terminal peptide of VP55. Poly(A) polymerase activity was associated with immunopurified VP55, but not with immunopurified or chromatographically purified VP39. VP39 was, however, required for the formation of long poly(A) molecules, in conjunction with either purified VP55 or low concentrations of the heterodimer, and was shown to bind free poly(A). Thus, a catalytic polypeptide and a dissociable poly(A)-binding stimulatory factor each contribute to poly(A) tail formation. No prokaryotic or eukaryotic homologs of either polypeptide were detected in sequence data bases, consistent with the absence of previously reported poly(A) polymerase genes from any source.

Studies using a recombinant vaccinia virus expressing the circumsporozoite protein of Plasmodium berghei

A recombinant vaccinia virus was constructed which expressed the circumsporozoite protein of Plasmodium berghei. Four different strains of mice belonging to different haplotypes were immunized with the recombinant virus. The antibody response to the circumsporozoite protein as well as to vaccinia virus varied among the strains, independently of each other. The anti-circumsporozoite protein titers were comparable to that obtained on immunization with irradiated sporozoites. Spleen cells from H2d mice immunized with P. berghei sporozoites showed a significant proliferative response when cultured in vitro with a low multiplicity of the recombinant vaccinia virus. A weak cytotoxic T lymphocyte response specifically targeting the circumsporozoite protein could be identified in spleens of BALB/c (H2d) mice immunized with vaccinia virus when BALB 3T3 cells transformed with a plasmid expressing the circumsporozoite protein under control of the simian virus 40 promoter were used as target cells in the cytotoxic T lymphocyte assay. However, none of the recombinant virus-immunized animals could be protected from a challenge of sporozoites even at the lowest dose of parasite used.

Sequence analysis, expression, and deletion of a vaccinia virus gene encoding a homolog of profilin, a eukaryotic actin-binding protein

A 4,500-bp BamHI fragment, located within the HindIII A segment of the vaccinia virus genome, was found to contain eight potential coding regions for polypeptides of 78 to 346 amino acids. The open reading frames with 133, 346, and 125 codons were homologous to profilin (an actin-binding protein), 3-beta-hydroxysteroid dehydrogenase, and Cu-Zn superoxide dismutase, respectively. Sequence alignments indicated that the vaccinia virus and mammalian profilins were more closely related to each other than to known profilins of other eukaryotes. The expression and possible role of the profilin homolog in the virus replicative cycle were therefore investigated. Antibody raised to Escherichia coli expressed vaccinia virus profilin was used to demonstrate the synthesis of the 15-kDa polypeptide at late times after vaccinia virus infection of mammalian cells. The protein accumulated in the cytoplasm, but only trace amounts remained associated with highly purified virions. The isolation of vaccinia virus mutants (in strains WR and IHD-J), with nearly the entire profilin gene replaced by the E. coli gpt gene, indicated that the protein is not essential for infectivity. The characteristic vaccinia virus-induced changes in actin fibers, seen by fluorescence microscopy, occurred in cells infected with the mutant. Moreover, the virus-encoded profilin homolog was not required for actin-associated events, including intracellular virus movement to the periphery of the cell, formation of specialized microvilli, or release of mature virions, as shown by electron microscopy and yields of infectious intra- and extracellular virus.

Promoter DNA contacts made by the vaccinia virus early transcription factor

Vaccinia virus RNA polymerase requires the heterodimeric protein, vaccinia early transcription factor (VETF), for transcription of early gene templates in vitro. We have analyzed the vaccinia growth factor promoter sequences interacting with VETF at the nucleotide level and provide evidence that the factor contacts the DNA at two separate sites. DNase I protection analysis showed that VETF was found to nucleotides -12 to -29 relative to the transcription initiation site, and also to nucleotides +8 to +10 downstream of the initiation site. The importance of both binding sites for stable complex formation was supported by methylation interference analysis. Using synthetic oligonucleotides encoding different parts of the vaccinia growth factor promoter, it was shown that nucleotides down-stream of the transcription initiation site are required for stable complex formation. Competition binding experiments demonstrated that only the upstream binding site contributes significantly to binding specificity. Binding to two separated DNA sequences results in a bend in the promoter DNA as demonstrated by electrophoretic mobility shift analysis of permuted DNA fragments. These findings suggest that VETF activates transcription by sequence specific binding and structural alteration of the promoter DNA helix.

An HLA-C-restricted CD8+ cytotoxic T-lymphocyte clone recognizes a highly conserved epitope on human immunodeficiency virus type 1 gag

A unique epitope on the gag protein of human immunodeficiency virus type 1 (HIV-1), located at amino acid 145 to 150, has been mapped by using a CD8+ cytotoxic T-lymphocyte (CTL) clone. This epitope is highly conserved among 18 HIV-1 strains. The HIV-1 gag-specific human leukocyte antigen (HLA) class I-restricted CD8+ CTL clone was generated from fresh peripheral blood mononuclear cells of an HIV-seropositive donor by stimulation with gamma-irradiated allogeneic peripheral blood mononuclear cells in the presence of an anti-CD3 monoclonal antibody and recombinant interleukin-2. This gag-specific CTL clone killed autologous target cells infected with a recombinant vaccinia virus containing the gag gene of HIV-1 and target cells pulsed with an authentic p24gag construct expressed in Escherichia coli. Fine specificity was determined by using a panel of overlapping 30-amino-acid-long synthetic peptides and subsequently using smaller peptides to precisely map the CTL domain on p24. The epitope is on a highly conserved region, and it overlaps with a major B-cell epitope of gag. This CD8+ T-cell epitope is restricted by HLA-Cw3, which has not been previously identified as a restricting element for human CTL responses.

Identification, sequence, and expression of the gene encoding a Mr 35,000 subunit of the vaccinia virus DNA-dependent RNA polymerase

The gene rpo35, encoding a subunit of the vaccinia virus DNA-dependent RNA polymerase, was identified, and its RNA and protein products were characterized. An Mr 35,000 polypeptide, which bound antibody to the purified RNA polymerase, was synthesized in reticulocyte lysates programmed with viral mRNA that hybridized to a 2,300-base pair segment of the viral genome. Determination of the sequence of the DNA segment revealed four potential protein coding regions, none of which had evident similarity to any described RNA polymerase subunit of prokaryotes or eukaryotes. One open reading frame that could encode a 35,400-Da protein was identified as rpo35 on the basis of mRNA hybridization, cell-free translation, and immunoprecipitation. The identification was confirmed by sequencing tryptic peptides of the authentic Mr 35,000 RNA polymerase subunit. Antiserum to the purified recombinant protein, expressed in bacteria, reacted specifically with a Mr 35,000 polypeptide that was detected starting 2 h after virus infection and that co-sedimented with RNA polymerase purified from virions. RNA analyses indicated that the 5'-end of an early transcript started 25 nucleotides upstream of rpo35, which is consistent with the location of an early promoter consensus sequence.

Heterologously expressed serotonin 1A receptors couple to muscarinic K+ channels in heart

In cardiac atrial cells, muscarinic acetylcholine receptors activate a K+ current directly via a guanine nucleotide-binding protein (G protein). Serotonin type 1A receptors may activate a similar pathway in hippocampal neurons. To develop a system in which receptor/G protein/K+ channel coupling can be experimentally manipulated, we have used a highly efficient recombinant vaccinia virus vector system to express human serotonin 1A receptors in primary cultures of rat atrial myocytes. The expressed 1A receptors activated the inwardly rectifying K+ conductance that is normally activated by the endogenous muscarinic acetylcholine receptors. Maximal responses to either agonist occluded further activation by the other agonist. The average activation time constants for serotonin were about 5 times slower than for acetylcholine. The data support suggestions that the intracellular signaling pathway from seven-helix receptors to G proteins and directly to ion channels is widespread in excitable cells. After a fraction of the G proteins are activated irreversibly by guanosine 5'-[gamma-thio]triphosphate, subsequent transduction proceeds more efficiently. One possible interpretation is that multiple G-protein molecules are required to activate each channel. Vaccinia virus expression vectors are thus useful for expressing seven-helix receptors in primary cultures of postmitotic cells and have provided a heterologous expression system for the signaling pathway from seven-helix receptors to G proteins and directly to ion channels.