Atrial natriuretic factor-SV40 T antigen transgenes produce tumors and cardiac arrhythmias in mice.

Transgenic mice that carry fusions between the transcriptional regulatory sequences of atrial natriuretic factor (a hormone intimately involved in the regulation of blood pressure) and those encoding SV40 T antigen (an oncoprotein) were generated. Although both atria express the fusion gene, the pathological response to T antigen is asymmetrical. The right atrium undergoes a several hundredfold increase in mass while the left atrium remains relatively normal in size. Hyperplasia is accompanied by a progressive increase in both the frequency and severity of abnormalities in the atrial conduction system, which ultimately result in death.

Formation of nascent intercalated disks between grafted fetal cardiomyocytes and host myocardium.

Fetal cardiomyocytes isolated from transgenic mice carrying a fusion gene of the alpha-cardiac myosin heavy chain promoter with a beta-galactosidase reporter were examined for their ability to form stable intracardiac grafts. Embryonic day 15 transgenic cardiomyocytes delivered directly into the myocardium of syngeneic hosts formed stable grafts, as identified by nuclear beta-galactosidase activity. Grafted cardiomyocytes were observed as long as 2 months after implantation, the latest date assayed. Intracardiac graft formation did not induce overtly negative effects on the host myocardium and was not associated with chronic immune rejection. Electron microscopy revealed the presence of nascent intercalated disks connecting the engrafted fetal cardiomyocytes and the host myocardium. These results suggest that intracardiac grafting might provide a useful approach for myocardial repair, provided that the grafted cells can contribute to myocardial function.

Differentiation and long-term survival of C2C12 myoblast grafts in heart.

We have assessed the ability of skeletal myoblasts to form long-term, differentiated grafts in ventricular myocardium. C2C12 myoblasts were grafted directly into the heart of syngeneic mice. Viable grafts were observed as long as 3 mo after implantation. Immunohistological analyses revealed the presence of differentiated myotubes that stably expressed the skeletal myosin heavy chain isoform. Thymidine uptake studies indicated that virtually all of the grafted skeletal myocytes were withdrawn from the cell cycle by 14 d after grafting. Graft myocytes exhibited ultrastructural characteristics typical of differentiated myotubes. Graft formation and the associated myocardial remodeling did not induce overt cardiac arrhythmia. This study indicates that the myocardium can serve as a stable platform for skeletal myoblast grafts. The long-term survival, differentiated phenotype, and absence of sustained proliferative activity observed in myoblast grafts raise the possibility that similar grafting approaches may be used to replace diseased myocardium. Furthermore, the genetic tractability of myoblasts could provide a useful means for the local delivery of recombinant molecules to the heart.

Genetically selected cardiomyocytes from differentiating embronic stem cells form stable intracardiac grafts.

This study describes a simple approach to generate relatively pure cultures of cardiomyocytes from differentiating murine embryonic stem (ES) cells. A fusion gene consisting of the alpha-cardiac myosin heavy chain promoter and a cDNA encoding aminoglycoside phosphotransferase was stably transfected into pluripotent ES cells. The resulting cell lines were differentiated in vitro and subjected to G418 selection. Immunocytological and ultrastructural analyses demonstrated that the selected cardiomyocyte cultures (> 99% pure) were highly differentiated. G418 selected cardiomyocytes were tested for their ability to form grafts in the hearts of adult dystrophic mice. The fate of the engrafted cells was monitored by antidystrophin immunohistology, as well as by PCR analysis with primers specific for the myosin heavy chain-aminoglycoside phosphotransferase transgene. Both analyses revealed the presence of ES-derived cardiomyocyte grafts for as long as 7 wk after implantation, the latest time point analyzed. These studies indicate that a simple genetic manipulation can be used to select essentially pure cultures of cardiomyocytes from differentiating ES cells. Moreover, the resulting cardiomyocytes are suitable for the formation of intracardiac grafts. This selection approach should be applicable to all ES-derived cell lineages.

Targeted expression of transforming growth factor-beta 1 in intracardiac grafts promotes vascular endothelial cell DNA synthesis.

Intracardiac grafts comprised of genetically modified skeletal myoblasts were assessed for their ability to effect long-term delivery of recombinant transforming growth factor-beta (TGF-beta) to the heart. C2C12 myoblasts were stably transfected with a construct comprised of an inducible metallothionein promoter fused to a modified TGF-beta 1 cDNA. When cultured in medium supplemented with zinc sulfate, cells carrying this transgene constitutively secrete active TGF-beta 1. These genetically modified myoblasts were used to produce intracardiac grafts in syngeneic C3Heb/FeJ hosts. Viable grafts were observed as long as three months after implantation, and immunohistological analyses of mice maintained on water supplemented with zinc sulfate revealed the presence of grafted cells which stably expressed TGF-beta 1. Regions of apparent neovascularization, as evidenced by tritiated thymidine incorporation into vascular endothelial cells, were observed in the myocardium which bordered grafts expressing TGF-beta 1. The extent of vascular endothelial cell DNA synthesis could be modulated by altering dietary zinc. Similar effects on the vascular endothelial cells were not seen in mice with grafts comprised of nontransfected cells. This study indicates that genetically modified skeletal myoblast grafts can be used to effect the local, long-term delivery of recombinant molecules to the heart.

Stable fetal cardiomyocyte grafts in the hearts of dystrophic mice and dogs.

This report documents the formation of stable fetal cardiomyocyte grafts in the myocardium of dystrophic dogs. Preliminary experiments established that the dystrophin gene product could be used to follow the fate of engrafted cardiomyocytes in dystrophic mdx mice. Importantly, ultrastructural analyses revealed the presence of intercalated discs consisting of fascia adherens, desmosomes, and gap junctions at the donor-host cardiomyocyte border. To determine if isolated cardiomyocytes could form stable intracardiac grafts in a larger species, preparations of dissociated fetal canine cardiomyocytes were delivered into the hearts of adult CXMD (canine X-linked muscular dystrophy) dogs. CXMD dogs, like Duchenne muscular dystrophy patients and mdx mice, fail to express dystrophin in both cardiac and skeletal muscle. Engrafted fetal cardiomyocytes, identified by virtue of dystrophin immunoreactivity, were observed to be tightly juxtaposed with host cardiomyocytes as long as 10 wk after engraftment, the latest date analyzed. Confocal laser scanning microscopy revealed the presence of connexin43, a major constituent of the gap junction, at the donor-host cardiomyocyte border. The presence of intracardiac grafts was not associated with arrhythmogenesis in the CXMD model. These results indicate that fetal cardiomyocyte grafting can successfully augment cardiomyocyte number in larger animals.

Cyclin D1 overexpression promotes cardiomyocyte DNA synthesis and multinucleation in transgenic mice.

D-type cyclin/cyclin-dependent kinase (CDK) complexes regulate transit through the restriction point of the cell cycle, and thus are required for the initiation of DNA synthesis. Transgenic mice which overexpress cyclin D1 in the heart were produced to determine if D-type cyclin deregulation would alter myocardial development. Cyclin D1 overexpression resulted in a concomitant increase in CDK4 levels in the adult myocardium, as well as modest increases in proliferating cell nuclear antigen and CDK2 levels. Flow cytometric and morphologic analyses of dispersed cell preparations indicated that the adult transgenic cardiomyocytes had abnormal patterns of multinucleation. Histochemical analyses confirmed a marked increase in number of cardiomyocyte nuclei in sections prepared from the transgenic mice as compared with those from control animals. Tritiated thymidine incorporation analyses revealed sustained cardiomyocyte DNA synthesis in adult transgenic hearts.

Expression of tissue-specific Ren-1 and Ren-2 genes of mice: comparative analysis of 5'-proximal flanking regions.

All inbred strains of mice carry the Ren-1 structural gene, which encodes the renin-1 isozyme, the classical renin activity found in kidneys. In addition, some strains carry a second renin structural gene, Ren-2, which encodes the predominantly expressed submaxillary gland renin isozyme, renin-2. Ren-1 and Ren-2 exhibit markedly different patterns of tissue-specific expression. In an effort to understand the molecular basis for this differential expression, detailed analysis of the genomic sequences corresponding to the Ren-1 and Ren-2 genes, and the transcripts originating from these loci, was undertaken. Sequence analysis of regions proximal to the structural genes indicated the presence of eucaryotic consensus sequences for transcription. These sequence motifs were strongly conserved between Ren-1 and Ren-2. Approximately 150 bases upstream from the major transcription initiation site, significant differences between these genes were apparent, including the presence of a repetitive DNA element in the Ren-2 copy as well as other breaks in homology and sequence curiosities. Strong homology between Ren-1 and Ren-2 resumed at a point ca. 200 bases further upstream on Ren-1. S1 analysis of submaxillary gland and kidney RNA populations indicated that the majority of transcripts initiate at homologous positions on Ren-1 and Ren-2. On a per cell basis, the accumulation of Ren-1 transcripts in the kidney and Ren-2 transcripts in the submaxillary gland are probably equivalent. These results suggest that it is tissue-specific utilization of the homologous start sites that is critical to their differential patterns of expression. Models which can account for this observation are presented. Interestingly, we found a minor fraction of transcripts initiating 5' to the major transcription start site. These transcripts encoded an open reading frame which may add an additional 23 amino acids to the N-terminus of the renin precursor.

Coexpression of mutant p53 and p193 renders embryonic stem cell-derived cardiomyocytes responsive to the growth-promoting activities of adenoviral E1A.

Expression of adenoviral E1A in cardiomyocytes results in the activation of DNA synthesis followed by apoptosis. In contrast, expression of simian virus 40 large T antigen induces sustained cardiomyocyte proliferation. Previous studies have shown that T antigen binds to 2 proapoptotic proteins in cardiomyocytes, namely the p53 tumor suppressor and p193 (a new member of the BH3-only proapoptosis subfamily). Structure-function analyses identified a p193 C-terminal truncation mutant that encodes prosurvival activity. This mutant was used to test the role of p193 in E1A-induced cardiomyocyte apoptosis. E1A induced apoptosis in cardiomyocytes derived from differentiating embryonic stem cells. Expression of the prosurvival p193 mutant alone or a mutant p53 alone did not block E1A-induced apoptosis. In contrast, combinatorial expression of mutant p193 and mutant p53 blocked E1A-induced apoptosis, resulting in a proliferative response indistinguishable from that seen with T antigen. These results confirm the hypothesis that there are 2 proapoptotic pathways, encoded by p53 and p193, respectively, which restrict cardiomyocyte cell cycle activity in differentiating embryonic stem cell cultures. Furthermore, these results explain in molecular terms the phenotypic differences of E1A versus T-antigen gene transfer in cardiomyocytes.

Enhanced cardiomyocyte DNA synthesis during myocardial hypertrophy in mice expressing a modified TSC2 transgene.

Tuberous sclerosis complex (TSC) is a rare genetic disorder characterized by the appearance of benign tumors in multiple organs, including the heart. Disease progression is accompanied by homozygous mutation at 1 of 2 loci (designated TSC1 or TSC2), leading to the suggestion that these genes function as tumor suppressors. In this study, we generated a series of TSC2 cDNAs in which one or more structural motifs were deleted, with the hope that expression of the modified gene product would override the growth-inhibitory activity of the endogenous TSC2 gene product. Several of the modified cDNAs enhanced growth rate, increased endocytosis, and promoted aberrant protein trafficking when expressed in NIH-3T3 cells, thereby mimicking phenotypes typical of TSC2-deficient cells. Surprisingly, targeted expression of the most potent TSC2 cDNA to the heart did not perturb cardiac development. However, the level of cardiomyocyte DNA synthesis in adult transgenic mice was elevated >35-fold during isoproterenol-induced hypertrophy compared with their nontransgenic siblings. These results suggest that alteration of TSC2 gene activity in combination with beta-adrenergic stimulation can reactivate the cell cycle in a limited number of terminally differentiated adult cardiomyocytes.

Cardiac repair by embryonic stem-derived cells.

Cell transplantation approaches offer the potential to promote regenerative growth of diseased hearts. It is well established that donor cardiomyocytes stably engraft into recipient hearts when injected directly into the myocardial wall. Moreover, the transplanted donor cardiomyocytes participate in a functional syncytium with the host myocardium. Thus, transplantation of donor cardiomyocytes resulted in at least partial restoration of lost muscle mass. It is also well established that embryonic stem (ES) cells differentiate into cells of ecto-, endo-, and mesodermal lineages when cultured under appropriate conditions in vitro. Robust cardiomyogenic differentiation was frequently observed in spontaneously differentiating ES cultures. Cellular, molecular and physiologic analyses indicated that ES-derived cells were bona fide cardiomyocytes, with in vitro characteristics typical for cells obtained from early stages of cardiac development. Thus, ES-derived cardiomyocytes constitute a viable source of donor cells for cell transplantation therapies.

Heritable lympho-epithelial thymoma resulting from a transgene insertional mutation.

Thymoma is the most common tumor of the anterior-superior mediastinum. We have identified a line of transgenic mice which spontaneously and heritably develop thymomas at a very high penetrance. The available data suggest that thymoma formation in these mice results as a consequence of transgene insertional mutagenesis. Immune histologic analyses indicate that the thymomas are of epithelial cell origin. Survival studies indicate that tumor progression is more aggressive in females as compared to males (73.9 vs 41.7% mortality at 20 weeks of age, respectively). Fluorescent in situ hybridizations have localized the transgene integration site to the F2-G region of mouse chromosome 2. Translocation encompassing the syntenic region in humans has been implicated in lympho-epithelial thymoma. These animals may constitute a useful resource for the identification of gene(s) which participate in thymoma progression, as well as a model system for screening anti-thymoma therapeutic agents.

Cardiovascular research in transgenic animals.

The ability to alter specifically the expression or activity of known gene products in intact animals provides a powerful experimental system to study pathophysiology. These animal models can now be readily generated using transgenic technology. Traditional transgenic approaches rely on gain-of-function alterations, and several experimental paradigms have recently been established based on this attribute. These include assessment of promoter function, overexpression of physiologically important gene products, targeted oncogenesis, and insertional mutagenesis. Technical and practical issues relevant to the generation of such models are discussed here. In addition, several recent transgenic models germane to the cardiovascular system are presented. These experiments provide exciting new model systems to study clinical issues that heretofore have been difficult, if not impossible, to address.

Atrial natriuretic factor and transgenic mice.

Atrial natriuretic factor (ANF) is a peptide hormone that induces potent but transient hypotensive and natriuretic responses on short-term administration. The role of the hormone in long-term cardiovascular regulation has remained elusive in part because of the temporal limitations of long-term infusion models and the extremely short half-life of the molecule in vivo. To circumvent these temporal limitations, a transgenic mouse model was developed that exhibits lifelong elevated plasma ANF levels. These mice are chronically hypotensive, with arterial pressures averaging 20 to 30 mm Hg less than those observed in nontransgenic siblings. In contrast, no obvious natriuretic or diuretic phenotype was observed in transgenic animals housed in metabolic cages. Thus, the mice adequately compensate for the renal effects but not the hemodynamic effects of the hormone. The ANF transgenic mice provide a tractable model system with which to study the consequences of long-term alterations of ANF expression in vivo.

Hypotension in transgenic mice expressing atrial natriuretic factor fusion genes.

Chronic regulation of the cardiovascular system by atrial natriuretic factor was investigated by generating transgenic mice with elevated hormone levels in the systemic circulation. A fusion gene comprising the mouse transthyretin promoter and mouse atrial natriuretic factor structural sequences was designed so as to target hormone expression to the liver. Hepatic expression of atrial natriuretic factor was detectable as early as embryonic day 15 in transgenic animals. In adult transgenic mice, plasma immunoreactive atrial natriuretic factor concentration was elevated at least eightfold as compared with nontransgenic littermates. The mean arterial pressure of conscious transgenic mice was 75.5 +/- 0.9 mm Hg, significantly less than that of nontransgenic siblings (103.9 +/- 2.0 mm Hg). This difference in mean arterial pressure was not accompanied by significant changes in several other physiological parameters, including heart rate, plasma and urinary electrolytes, water intake, and urine volume. This study demonstrates that a chronic elevation of plasma atrial natriuretic factor decreases arterial blood pressure without inducing diuresis and natriuresis in transgenic mice and also illustrates the value of the transgenic approach for the study of the cardiovascular system.

Tissue and gene specificity of mouse renin expression.

The Ren-1 locus of mice encodes the protease renin, which with converting enzyme processes angiotensinogen to the potent vasopressor angiotensin II. Some strains of mice appear to carry a duplication of the renin structural gene (Ren-2) near the Ren-1 locus. Strains with the gene duplication can exhibit as much as 100-fold higher levels of submaxillary gland renin compared to strains with a single gene copy. In contrast, kidney renin levels appear to be unaffected by the gene duplication. Sequence analysis of a 319 bp renin cDNA recombinant isolated from a kidney library from the two-gene strain DBA/2Ha corresponds to a transcript of the Ren-1 gene. Moreover, a single base substitution of A for G at residue #996 in the kidney renin mRNA creates a potential glycosylation recognition site that may, in part, account for the differential glycosylation of kidney and submaxillary gland renins. In addition, our tissue surveys indicate that mature mRNAs from the Ren loci are detectable in adrenal gland and testes, as well as sublingual and parotid salivary glands, and reveal length variation for the renin transcripts in at least the submaxillary gland.

Cloning of cDNA to a yeast viral double-stranded RNA and comparison of three viral RNAs.

We have constructed recombinant DNA clones containing small complementary DNA (cDNA) sequences homologous to portions of a 4.8-kb yeast viral double-stranded RNA (dsRNA) (L1) that codes for the viral capsid polypeptide. Neither the viral dsRNA nor its in vitro transcript is polyadenylated; hence the cDNAs were synthesized by reverse transcriptase on the in vitro mRNA transcript made by the viral transcriptase, using sheared salmon sperm DNA as a random primer. This is the first reported cloning of cDNA homologous to a viral double-stranded RNA. This method should be of general utility for dsRNA viruses, since all have a capsid-associated transcriptase activity. The lengths of the overlapping cDNA inserts varied from 100 to 800 bp. About 40% of them mapped to the 5' end of the in vitro transcript, and these have been ordered. At least 1485 bp of this end of L1 is represented in the cloned cDNAs characterized. Using the cloned cDNAs as probes, we have shown that the L dsRNAs of two viral subtypes are similar at the transcription initiation site and dissimilar elsewhere.

Assessment of donor bone marrow cell-derived chimerism in transplantation tolerance using transgenic mice.

Allospecific skin graft prolongation can be induced in mice using antithymocyte globulin and allospecific donor bone marrow cells (DBMC). This enhancement may be due to the persistence of chimeric cells of donor origin in the host. In this study, we systematically investigated DBMC-derived chimerism in various lymphoid and nonlymphoid tissues over time. To do this, transgenic mice were used as a source of DBMC to clearly distinguish chimerism due only to the injected DBMC. Chimerism in various tissues was assessed at several times points after DBMC infusion by polymerase chain reaction amplification of tissue DNA using transgene specific primers. A cDNA probe specific for the transgene was used to demonstrate DBMC-derived chimerism in polymerase chain reaction products by the method of Southern. Although chimerism was initially detectable in most tissues tested 1 day after DBMC infusion, the presence of chimeric cells generally diminished over time. At 4 weeks or longer, chimerism was consistently confined to recipient skin. Furthermore, the chimeric cells in recipient skin persisted even after the allograft was rejected. In contrast to chimerism in recipient skin, chimerism became undetectable in donor skin as early as 2 weeks after DBMC infusion. The loss of chimerism in donor skin showed a temporal correlation with a reduction of chimerism in host bone marrow and lymphoid tissues that preceded rejection in all experiments by a range of 7-14 days. The use of DBMC from transgenic mice allowed a unique opportunity to monitor the kinetics of DBMC-derived chimerism. The presence of chimerism in the skin of mice in temporal association with chronic allograft rejection suggests that chimerism per se is not a reliable index of allogeneic unresponsiveness.

Scalable production of embryonic stem cell-derived cardiomyocytes.

Cardiomyocyte transplantation could offer a new approach to replace scarred, nonfunctional myocardium in a diseased heart. Clinical application of this approach would require the ability to generate large numbers of donor cells. The purpose of this study was to develop a scalable, robust, and reproducible process to derive purified cardiomyocytes from genetically engineered embryonic stem (ES) cells. ES cells transfected with a fusion gene consisting of the alpha-cardiac myosin heavy chain (MHC) promoter driving the aminoglycoside phosphotransferase (neomycin resistance) gene were used for cardiomyocyte enrichment. The transfected cells were aggregated into embyroid bodies (EBs), inoculated into stirred suspension cultures, and differentiated for 9 days before selection of cardiomyocytes by the addition of G418 with or without retinoic acid (RA). Throughout the culture period, EB and viable cell numbers were measured. In addition, flow cytometric analysis was performed to monitor sarcomeric myosin (a marker for cardiomyocytes) and Oct-4 (a marker for undifferentiated ES cells) expression. Enrichment of cardiomyocytes was achieved in cultures treated with either G418 and retinoic acid (RA) or with G418 alone. Eighteen days after differentiation, G418-selected flasks treated with RA contained approximately twice as many cells as the nontreated flasks, as well as undetectable levels of Oct-4 expression, suggesting that RA may promote cardiac differentiation and/or survival. Immunohistological and electron microscopic analysis showed that the harvested cardiomyocytes displayed many features characteristic of native cardiomyocytes. Our results demonstrate the feasibility of large-scale production of viable, ES cell-derived cardiomyocytes for tissue engineering and/or implantation, an approach that should be transferable to other ES cell derived lineages, as well as to adult stem cells with in vitro cardiomyogenic activity.

Engineering the cardiovascular system. Blood pressure regulation.

We have generated several lineages of transgenic mice that exhibit chronic elevations in the steady-state concentration of atrial natriuretic factor (ANF) in the peripheral circulation. ANF, a peptide hormone synthesized primarily by atrial cardiomyocytes, is a potent natriuretic and diuretic. ANF also reduces blood pressure transiently when acutely administered. To address the potential role of ANF in chronic cardiovascular regulation, we generated transgenic mice that express the ANF gene in the liver. The fusion genes comprised either the mouse transthyretin (TTR) or rat phosphoenolpyruvate carboxykinase (PEPCK) promoters fused to the mouse ANF structural gene and were designed to target to the liver constitutive and inducible expression of pre-pro-ANF, respectively. Transgenic animals harboring the TTR-ANF fusion gene expressed chimeric ANF transcripts exclusively in the liver. In contrast, mice harboring the PEPCK-ANF fusion gene did not express detectable amounts of ANF mRNA in liver even after induction (24-hour fasting). In the TTR-ANF mice, hepatic and plasma immunoreactive ANF concentrations were proportional to the concentration of hepatic ANF transcripts. Moreover, mean arterial blood pressure recorded in conscious transgenic mice was inversely proportional to hepatic ANF expression. These transgenic models demonstrate that chronically elevated ANF concentration can induce sustained hypotension.