Highly Sensitized Patients Listed for Heart After Liver Transplantation With or Without Domino.

For patients with end stage heart disease and borderline hemodynamics, high HLA allosensitization presents a barrier for heart transplantation in a timely manner. Conventional desensitization protocols are inadequate in this context due to time constraints and for the most highly reactive immunologically. We previously reported performing heart after liver transplant with domino liver transplant (HALT-D) on a single patient without liver disease. We describe this patient's course to date as well as four subsequent patients listed for this novel therapy. This experience demonstrates that the liver effectively confers immunoprotection to the heart for patients with high-titer, preformed antibodies. This strategy may provide some measure of equity for demographic groups previously disadvantaged for heart transplantation due to allosensitization.

When cardiomyopathy, cancer, and COVID-19 collide: A case report.

Malignancy has historically prohibited solid organ transplant; however, patients with effectively treated, favorable-risk cancers should not necessarily be eliminated as transplant candidates. These cases require careful review by a multidisciplinary team. Here, we report the case of a woman with end-stage heart failure undergoing heart transplant evaluation during the COVID pandemic who was found to have early-stage, hormone receptor-positive breast cancer. Given her favorable cancer-related prognosis, a multidisciplinary committee recommended lumpectomy, accelerated partial breast irradiation, and adjuvant aromatase inhibitor therapy for definitive treatment to allow for consideration of orthotopic heart transplant.

Accuracy of Doppler blood pressure measurement in continuous-flow left ventricular assist device patients.

AIMS: Accurate blood pressure (BP) measurement in continuous-flow ventricular assist device (CF-VAD) patients is imperative to reduce stroke risk. This study assesses the accuracy of the Doppler opening pressure method compared with the gold standard arterial line method in CF-VAD patients. METHODS AND RESULTS: In a longitudinal cohort of HeartMate II and HVAD patients, arterial line BP and simultaneously measured Doppler opening pressure were obtained. Overall correlation, agreement between Doppler opening pressure and arterial line mean vs. systolic pressure, and the effect of arterial pulsatility on the accuracy of Doppler opening pressure were analysed. A total of 1933 pairs of Doppler opening pressure and arterial line pressure readings within 1 min of each other were identified in 154 patients (20% women, mean age 55 ± 15, 50% HeartMate II and 50% HVAD). Doppler opening pressure had good correlation with invasive mean arterial pressure (r = 0.742, P < 0.0001) and more closely approximated mean than systolic BP (mean error 2.4 vs. -8.4 mmHg). Arterial pulsatility did not have a clinically significant effect on the accuracy of the Doppler opening pressure method. CONCLUSIONS: Doppler opening pressure should be the standard non-invasive method of BP measurement in CF-VAD patients.

Cardiac macrophages adopt profibrotic/M2 phenotype in infarcted hearts: Role of urokinase plasminogen activator.

BACKGROUND: Macrophages (mac) that over-express urokinase plasminogen activator (uPA) adopt a profibrotic M2 phenotype in the heart in association with cardiac fibrosis. We tested the hypothesis that cardiac macs are M2 polarized in infarcted mouse and human hearts and that polarization is dependent on mac-derived uPA. METHODS: Studies were performed using uninjured (UI) or infarcted (MI) hearts of uPA overexpressing (SR-uPA), uPA null, or nontransgenic littermate (Ntg) mice. At 7days post-infarction, cardiac mac were isolated, RNA extracted and M2 markers Arg1, YM1, and Fizz1 measured with qrtPCR. Histologic analysis for cardiac fibrosis, mac and myofibroblasts was performed at the same time-point. Cardiac macs were also isolated from Ntg hearts and RNA collected after primary isolation or culture with vehicle, IL-4 or plasmin and M2 marker expression measured. Cardiac tissue and blood was collected from humans with ischemic heart disease. Expression of M2 marker CD206 and M1 marker TNFalpha was measured. RESULTS: Macs from WT mice had increased expression of Arg1 and Ym1 following MI (41.3±6.5 and 70.3±36, fold change vs UI, n=8, P<0.007). There was significant up-regulation of cardiac mac Arg1 and YM1 with MI in both WT and uPA null mice (n=4-9 per genotype and condition). Treatment with plasmin increased expression of Arg1 and YM1 in cultured cardiac macs. Histologic analysis revealed increased density of activated fibroblasts and M2 macs in SR-uPA hearts post-infarction with associated increases in fibrosis. Cardiac macs isolated from human hearts with ischemic heart disease expressed increased levels of the M2 marker CD206 in comparison to blood-derived macs (4.9±1.3). CONCLUSIONS: Cardiac macs in mouse and human hearts adopt a M2 phenotype in association with fibrosis. Plasmin can induce an M2 phenotype in cardiac macs. However, M2 activation can occur in the heart in vivo in the absence of uPA indicating that alternative pathways to activate plasmin are present in the heart. Excess uPA promotes increased fibroblast density potentially via potentiating fibroblast migration or proliferation. Altering macrophage phenotype in the heart is a potential target to modify cardiac fibrosis.

Right ventricular sarcoidosis: is it time for updated diagnostic criteria?

A 55-year-old woman with a history of complete heart block, atrial flutter, and progressive right ventricular failure was referred to our tertiary care center to be evaluated for cardiac transplantation. The patient's clinical course included worsening right ventricular dysfunction for 3 years before the current evaluation. Our clinical findings raised concerns about arrhythmogenic right ventricular cardiomyopathy. Noninvasive imaging, including a positron emission tomographic scan, did not reveal obvious myocardial pathologic conditions. Given the end-stage nature of the patient's right ventricular failure and her dependence on inotropic agents, she underwent urgent listing and subsequent heart transplantation. Pathologic examination of the explanted heart revealed isolated right ventricular sarcoidosis with replacement fibrosis. Biopsy samples of the cardiac allograft 6 months after transplantation showed no recurrence of sarcoidosis. This atypical presentation of isolated cardiac sarcoidosis posed a considerable diagnostic challenge. In addition to discussing the patient's case, we review the relevant medical literature and discuss the need for updated differential diagnostic criteria for end-stage right ventricular failure that mimics arrhythmogenic right ventricular cardiomyopathy.

Human embryonic-stem-cell-derived cardiomyocytes regenerate non-human primate hearts.

Pluripotent stem cells provide a potential solution to current epidemic rates of heart failure by providing human cardiomyocytes to support heart regeneration. Studies of human embryonic-stem-cell-derived cardiomyocytes (hESC-CMs) in small-animal models have shown favourable effects of this treatment. However, it remains unknown whether clinical-scale hESC-CM transplantation is feasible, safe or can provide sufficient myocardial regeneration. Here we show that hESC-CMs can be produced at a clinical scale (more than one billion cells per batch) and cryopreserved with good viability. Using a non-human primate model of myocardial ischaemia followed by reperfusion, we show that cryopreservation and intra-myocardial delivery of one billion hESC-CMs generates extensive remuscularization of the infarcted heart. The hESC-CMs showed progressive but incomplete maturation over a 3-month period. Grafts were perfused by host vasculature, and electromechanical junctions between graft and host myocytes were present within 2 weeks of engraftment. Importantly, grafts showed regular calcium transients that were synchronized to the host electrocardiogram, indicating electromechanical coupling. In contrast to small-animal models, non-fatal ventricular arrhythmias were observed in hESC-CM-engrafted primates. Thus, hESC-CMs can remuscularize substantial amounts of the infarcted monkey heart. Comparable remuscularization of a human heart should be possible, but potential arrhythmic complications need to be overcome.

Electrical Integration of Human Embryonic Stem Cell-Derived Cardiomyocytes in a Guinea Pig Chronic Infarct Model.

BACKGROUND: Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) were recently shown to be capable of electromechanical integration following direct injection into intact or recently injured guinea pig hearts, and hESC-CM transplantation in recently injured hearts correlated with improvements in contractile function and a reduction in the incidence of arrhythmias. The present study was aimed at determining the ability of hESC-CMs to integrate and modulate electrical stability following transplantation in a chronic model of cardiac injury. METHODS AND RESULTS: At 28 days following cardiac cryoinjury, guinea pigs underwent intracardiac injection of hESC-CMs, noncardiac hESC derivatives (non-CMs), or vehicle. Histology confirmed partial remuscularization of the infarct zone in hESC-CM recipients while non-CM recipients showed heterogeneous xenografts. The 3 experimental groups showed no significant difference in the left ventricular dimensions or fractional shortening by echocardiography or in the incidence of spontaneous arrhythmias by telemetric monitoring. Although recipients of hESC-CMs and vehicle showed a similar incidence of arrhythmias induced by programmed electrical stimulation at 4 weeks posttransplantation, non-CM recipients proved to be highly inducible, with a ∼3-fold greater incidence of induced arrhythmias. In parallel studies, we investigated the ability of hESC-CMs to couple with host myocardium in chronically injured hearts by the intravital imaging of hESC-CM grafts that stably expressed a fluorescent reporter of graft activation, the genetically encoded calcium sensor GCaMP3. In this work, we found that only ∼38% (5 of 13) of recipients of GCaMP3+ hESC-CMs showed fluorescent transients that were coupled to the host electrocardiogram. CONCLUSIONS: Human embryonic stem cell-derived cardiomyocytes engraft in chronically injured hearts without increasing the incidence of arrhythmias, but their electromechanical integration is more limited than previously reported following their transplantation in a subacute injury model. Moreover, non-CM grafts may promote arrhythmias under certain conditions, a finding that underscores the need for input preparations of high cardiac purity.

Effects of cell grafting on coronary remodeling after myocardial infarction.

BACKGROUND: With recent advances in therapeutic applications of stem cells, cell engraftment has become a promising therapy for replacing injured myocardium after infarction. The survival and function of injected cells, however, will depend on the efficient vascularization of the new tissue. Here we describe the arteriogenic remodeling of the coronary vessels that supports vascularization of engrafted tissue postmyocardial infarction (post-MI). METHODS AND RESULTS: Following MI, murine hearts were injected with a skeletal myoblast cell line previously shown to develop into large grafts. Microcomputed tomography at 28 days postengraftment revealed the 3-dimensional structure of the newly formed conducting vessels. The grafts elicited both an angiogenic response and arteriogenic remodeling of the coronary arteries to perfuse the graft. The coronaries upstream of the graft also remodeled, showing an increase in branching, and a decrease in vascular density. Histological analysis revealed the presence of capillaries as well as larger vascular lumens within the graft. Some graft vessels were encoated by smooth muscle α-actin positive cells, implying that vascular remodeling occurs at both the conducting arterial and microvascular levels. CONCLUSIONS: Following MI and skeletal myoblast engraftment, the murine coronary vessels exhibit plasticity that enables both arteriogenic remodeling of the preexisting small branches of the coronary arteries and development of large and small smooth muscle encoated vessels within the graft. Understanding the molecular mechanisms underlying these 2 processes suggests mechanisms to enhance the therapeutic vascularization in patients with myocardial ischemia.

Delivery of basic fibroblast growth factor with a pH-responsive, injectable hydrogel to improve angiogenesis in infarcted myocardium.

A pH- and temperature-responsive, injectable hydrogel has been designed to take advantage of the acidic microenvironment of ischemic myocardium. This system can improve therapeutic angiogenesis methods by providing spatio-temporal control of angiogenic growth factor delivery. The pH- and temperature-responsive random copolymer, poly(N-isopropylacrylamide-co-propylacrylic acid-co-butyl acrylate) (p[NIPAAm-co-PAA-co-BA]), was synthesized by reversible addition fragmentation chain transfer polymerization. This polymer was a liquid at pH 7.4 and 37 °C but formed a physical gel at pH 6.8 and 37 °C. Retention of biotinylated basic fibroblast growth factor (bFGF) between 0 and 7 days after injection into infarcted rat myocardium was 10-fold higher with hydrogel delivery versus saline. Following 28 days of treatment in vivo, capillary and arteriolar densities were increased 30-40% by polymer + bFGF treatment versus saline + bFGF or polymer-only controls. Treatment with polymer + bFGF for 28 days resulted in a 2-fold improvement in relative blood flow to the infarct region versus day 0, whereas saline + bFGF or polymer-only had no effect. Fractional shortening determined by echocardiography was significantly higher following treatment with polymer + bFGF (30 ± 1.4%) versus saline (25 ± 1.2%) and polymer alone (25 ± 1.8%). By responding to local changes in pH- and temperature in an animal model of ischemia, this hydrogel system provided sustained, local delivery of bFGF, improved angiogenesis, and achieved therapeutic effects in regional blood flow and cardiac function.

Sildenafil reverses cardiac dysfunction in the mdx mouse model of Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is a progressive and fatal genetic disorder of muscle degeneration. Patients with DMD lack expression of the protein dystrophin as a result of mutations in the X-linked dystrophin gene. The loss of dystrophin leads to severe skeletal muscle pathologies as well as cardiomyopathy, which manifests as congestive heart failure and arrhythmias. Like humans, dystrophin-deficient mice (mdx mice) show cardiac dysfunction as evidenced by a decrease in diastolic function followed by systolic dysfunction later in life. We have investigated whether sildenafil citrate (Viagra), a phosphodiesterase 5 (PDE5) inhibitor, can be used to ameliorate the age-related cardiac dysfunction present in the mdx mice. By using echocardiography, we show that chronic sildenafil treatment reduces functional deficits in the cardiac performance of aged mdx mice, with no effect on normal cardiac function in WT controls. More importantly, when sildenafil treatment was started after cardiomyopathy had developed, the established symptoms were rapidly reversed within a few days. It is recognized that PDE5 inhibitors can have cardioprotective effects in other models of cardiac damage, but the present study reports a prevention and reversal of pathological cardiac dysfunction as measured by functional analysis in a mouse model of DMD. Overall, the data suggest that PDE5 inhibitors may be a useful treatment for the cardiomyopathy affecting patients with DMD at early and late stages of the disease.

The role of macrophage-derived urokinase plasminogen activator in myocardial infarct repair: urokinase attenuates ventricular remodeling.

Cardiac plasmin activity is increased following myocardial ischemia. To test the hypothesis that macrophage-derived uPA is a key mediator of repair following myocardial infarction, we performed myocardial infarction on mice with macrophage-specific over-expression of uPA (SR-uPA mice). SR-uPA(+/0) mice and wild-type littermates were sacrificed at 5 days or 4 weeks after infarction and cardiac content of macrophages, collagen, and myofibroblasts was quantified. Cardiac function and dimensions were assessed by echocardiography at baseline and at 4 weeks post-infarction. At 4 weeks after myocardial infarction, macrophage counts were increased in SR-uPA(+/0) mice in the infarct (13.1 vs. 4.9%, P<0.001) and distant uninfarcted regions (5.9 vs. 2.4%, P<0.001). Infarct scar was thicker in SR-uPA(+/0) mice (0.54+/-0.03 mm vs. 0.45+/-0.03 mm, P<0.05) and infarct cardiac collagen content was increased (72.4+/-3.3% vs. 63.0+/-3.6%, P<0.06). Functionally, these changes resulted in mildly improved fractional shortening in SR-uPA(+/0) mice compared to controls (24.6+/-1.68 vs. 19.8+/-1.3%, P=0.03). At 5 days after infarction there was increased collagen content in the scar without increases in macrophages or myofibroblasts. To understand the mechanisms by which macrophage-derived uPA increases collagen, cardiac fibroblasts were treated with macrophage-conditioned medium or plasmin and expression of ColIalpha1 measured by qPCR. Conditioned media from SR-uPA(+/0) or plasmin-treated non-transgenic macrophages but not plasmin alone increased collagen expression in isolated cardiac fibroblasts. We hypothesize that plasmin generation in the heart in response to injury may induce activation of macrophages to a profibrotic phenotype to allow rapid formation of collagenous scar.

Cardiogenic differentiation and transdifferentiation of progenitor cells.

In recent years, cell transplantation has drawn tremendous interest as a novel approach to preserving or even restoring contractile function to infarcted hearts. A typical human infarct involves the loss of approximately 1 billion cardiomyocytes, and, therefore, many investigators have sought to identify endogenous or exogenous stem cells with the capacity to differentiate into committed cardiomyocytes and repopulate lost myocardium. As a result of these efforts, dozens of stem cell types have been reported to have cardiac potential. These include pluripotent embryonic stem cells, as well various adult stem cells resident in compartments including bone marrow, peripheral tissues, and the heart itself. Some of these cardiogenic progenitors have been reported to contribute replacement muscle through endogenous reparative processes or via cell transplantation in preclinical cardiac injury models. However, considerable disagreement exists regarding the efficiency and even the reality of cardiac differentiation by many of these stem cell types, making these issues a continuing source of controversy in the field. In this review, we consider approaches to cell fate mapping and establishing the cardiac phenotype, as well as the present state of the evidence for the cardiogenic and regenerative potential of the major candidate stem cell types.

Cardiomyocytes derived from human embryonic stem cells in pro-survival factors enhance function of infarcted rat hearts.

Cardiomyocytes derived from human embryonic stem (hES) cells potentially offer large numbers of cells to facilitate repair of the infarcted heart. However, this approach has been limited by inefficient differentiation of hES cells into cardiomyocytes, insufficient purity of cardiomyocyte preparations and poor survival of hES cell-derived myocytes after transplantation. Seeking to overcome these challenges, we generated highly purified human cardiomyocytes using a readily scalable system for directed differentiation that relies on activin A and BMP4. We then identified a cocktail of pro-survival factors that limits cardiomyocyte death after transplantation. These techniques enabled consistent formation of myocardial grafts in the infarcted rat heart. The engrafted human myocardium attenuated ventricular dilation and preserved regional and global contractile function after myocardial infarction compared with controls receiving noncardiac hES cell derivatives or vehicle. The ability of hES cell-derived cardiomyocytes to partially remuscularize myocardial infarcts and attenuate heart failure encourages their study under conditions that closely match human disease.

Chemical dimerization of fibroblast growth factor receptor-1 induces myoblast proliferation, increases intracardiac graft size, and reduces ventricular dilation in infarcted hearts.

The ability to control proliferation of grafted cells in the heart and consequent graft size could dramatically improve the efficacy of cell therapies for cardiac repair. To achieve targeted graft cell proliferation, we created a chimeric receptor (F36Vfgfr-1) composed of a modified FK506-binding protein (F36V) fused with the cytoplasmic domain of the fibroblast growth factor receptor-1 (FGFR-1). We retrovirally transduced mouse C2C12 and MM14 skeletal myoblasts with this construct and treated them with AP20187, a dimeric F36V ligand ("dimerizer"), in vitro and in vivo to induce receptor dimerization. Dimerizer treatment in vitro activated the mitogen-activated protein kinase pathway and induced proliferation in myoblasts expressing F36Vfgfr-1 comparable with the effects of basic FGF. Wild-type myoblasts did not respond to dimerizer. Subcutaneous grafts composed of myoblasts expressing F36Vfgfr-1 showed a dose-dependent increase in DNA synthesis with dimerizer treatment. When myoblasts expressing F36Vfgfr-1 were injected into infarcted hearts of nude mice, dimerizer treatment resulted in a dose-dependent increase in graft size, from 20 +/- 3 to 42.9 +/- 4.3% of the left ventricle. Blinded echocardiographic analysis demonstrated that larger graft size was associated with a dose-dependent reduction in ventricular dilation after myocardial infarction, although animals with the largest grafts showed an increased incidence of ventricular tachycardia. Thus, selective proliferation of genetically modified graft cells can be induced with a systemically administered synthetic molecule in vitro or in vivo. Control of intramyocardial graft size by this approach may allow optimization of cell-based therapy to obtain desired cardiac function postinfarction.

Transplantation of undifferentiated murine embryonic stem cells in the heart: teratoma formation and immune response.

Embryonic stem (ES) cells are promising for cardiac repair, but directing their differentiation toward cardiomyocytes remains challenging. We investigated whether the heart guides ES cells toward cardiomyocytes in vivo and whether allogeneic ES cells were immunologically tolerated. Undifferentiated mouse ES cells consistently formed cardiac teratomas in nude or immunocompetent syngeneic mice. Cardiac teratomas contained no more cardiomyocytes than hind-limb teratomas, suggesting lack of guided differentiation. ES cells also formed teratomas in infarcted hearts, indicating injury-related signals did not direct cardiac differentiation. Allogeneic ES cells also caused cardiac teratomas, but these were immunologically rejected after several weeks, in association with increased inflammation and up-regulation of class I and II histocompatibility antigens. Fusion between ES cells and cardiomyocytes occurred in vivo, but was rare. Infarct autofluorescence was identified as an artifact that might be mistaken for enhanced GFP expression and true regeneration. Hence, undifferentiated ES cells were not guided toward a cardiomyocyte fate in either normal or infarcted hearts, and there was no evidence for allogeneic immune tolerance of ES cell derivatives. Successful cardiac repair strategies involving ES cells will need to control cardiac differentiation, avoid introducing undifferentiated cells, and will likely require immune modulation to avoid rejection.

rAAV6-microdystrophin preserves muscle function and extends lifespan in severely dystrophic mice.

Mice carrying mutations in both the dystrophin and utrophin genes die prematurely as a consequence of severe muscular dystrophy. Here, we show that intravascular administration of recombinant adeno-associated viral (rAAV) vectors carrying a microdystrophin gene restores expression of dystrophin in the respiratory, cardiac and limb musculature of these mice, considerably reducing skeletal muscle pathology and extending lifespan. These findings suggest rAAV vector-mediated systemic gene transfer may be useful for treatment of serious neuromuscular disorders such as Duchenne muscular dystrophy.

The effects of atorvastatin (10 mg) on systemic inflammation in heart failure.

In observational studies, statins are associated with lower mortality in patients with heart failure (HF), including those with nonischemic HF. Such benefits could be related to anti-inflammatory effects; however, the effects of statins on systemic inflammation in HF are not well-established. We conducted a 16-week, single-center, randomized, double-blind, placebo-controlled, crossover clinical trial of the effects of atorvastatin 10 mg/day on concentrations of systemic inflammatory markers in 22 patients with HF (including 20 with nonischemic HF) with New York Heart Association class II or III symptoms and left ventricular ejection fraction of <40%. The absolute and percentage of changes in inflammatory marker levels were evaluated using analysis of variance. Statin treatment reduced the concentrations of soluble tumor necrosis factor receptor-1 by 132 pg/ml (p = 0.04) and 8% (p = 0.056), C-reactive protein by 1.6 mg/L (p = 0.006) and 37% (p = 0.0002), and, after adjustment for treatment order, endothelin-1 by 0.21 pg/ml (p = 0.007) and 17% (p = 0.01). In post hoc analyses, the reduction in tumor necrosis factor receptor-1 levels was highest among patients with elevated levels at baseline (at or higher than the median of 1,055 pg/ml, p interaction = 0.001), among whom statin therapy reduced the levels by 306 pg/ml (p <0.001) and 22% (p <0.001). Statin treatment did not significantly affect the levels of other inflammatory markers, including interleukin-6 and brain natriuretic peptide. In conclusion, short-term atorvastatin therapy reduced the levels of several important inflammatory markers in patients with HF.

Extracardiac progenitor cells repopulate most major cell types in the transplanted human heart.

BACKGROUND: Extracardiac progenitor cells are capable of repopulating cardiomyocytes at very low levels in the human heart after injury. Here, we explored the extent of endothelial, smooth muscle, and Schwann cell chimerism in patients with sex-mismatched (female-to-male) heart transplants. METHODS AND RESULTS: Autopsy specimens from 5 patients and endomyocardial biopsies from 7 patients were used for this study. Endothelial, vascular smooth muscle, and Schwann cells were stained with antibodies against CD31 or Ulex europaeus lectin, smooth muscle alpha-actin, and S-100, respectively, and the Y chromosome was identified with in situ hybridization. Biopsy specimens from 1, 4, 6, and 12 months and 5 and 10 years after heart transplantation were evaluated. Y-positive cells were counted by conventional bright-field microscopy and confirmed by confocal microscopy. Endothelial cells showed the highest degree of chimerism, averaging 24.3+/-8.2% from extracardiac sources. Schwann cells showed the next highest chimerism, averaging 11.2+/-2.1%; vascular smooth muscle cells averaged 3.4+/-1.8%. All 3 cell types showed substantially higher chimerism than we previously observed for cardiomyocytes (0.04+/-0.05%). Endothelial chimerism was much higher in the microcirculation than in larger vessels. Analysis of serial endomyocardial biopsies revealed that high levels of endothelial chimerism occurred as early as 1 month after transplantation (22+/-6.6%) with no significant increases even up to 10 years after cardiac transplantation. CONCLUSIONS: Extracardiac progenitor cells are capable of repopulating most major cell types in the heart, but they do so with varying frequency. The signals for endothelial progenitor recruitment occur early and could relate to injury during allograft harvest or transplantation. The high degree of endothelial chimerism may have immune implications such as for myocardial rejection or graft vasculopathy.

Gene transfer of connexin43 into skeletal muscle.

Cellular cardiomyoplasty using skeletal myoblasts may be beneficial for infarct repair. One drawback to skeletal muscle cells is their lack of gap junction expression after differentiation, thus preventing electrical coupling to host cardiomyocytes. We sought to overexpress the gap junction protein connexin43 (Cx43) in differentiated skeletal myotubes, using retroviral, adenoviral, and plasmid-mediated gene transfer. All strategies resulted in overexpression of Cx43 in cultured myotubes, but expression of Cx43 from constitutive viral promoters caused significant death upon differentiation. Dye transfer studies showed that surviving myotubes contained functional gap junctions, however. Retrovirally transfected myoblasts did not express Cx43 after grafting into the heart, possibly due to promoter silencing. Adenovirally transfected myoblasts expressed abundant Cx43 after forming myotubes in cardiac grafts, but grafts showed signs of injury at 1 week and had died by 2 weeks. Interestingly, transfection of already differentiated myotubes with adenoviral Cx43 was nontoxic, implying a window of vulnerability during differentiation. To test this hypothesis, Cx43 was expressed from the muscle creatine kinase (MCK) promoter, which is active only after myocyte differentiation. The MCK promoter resulted in high levels of Cx43 expression in differentiated myotubes but did not cause cell death during differentiation. MCK-Cx43-transfected myoblasts formed viable cardiac grafts and, in some cases, Cx43-expressing myotubes were in close apposition to host cardiomyocytes, possibly allowing electrical coupling. Thus, high levels of Cx43 during skeletal muscle differentiation cause cell death. When, however, expression of Cx43 is delayed until after differentiation, using the MCK promoter, myotubes are viable and express gap junction proteins after grafting in the heart. This strategy may permit electrical coupling of skeletal and cardiac muscle for cardiac repair.