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

Myoblast cell grafting into heart muscle: cellular biology and potential applications

This review surveys a wide range of cellular and molecular approaches to strengthening the injured or weakened heart, focusing on strategies to replace dysfunctional, necrotic, or apoptotic cardiomyocytes with new cells of mesodermal origin. A variety of cell types, including myogenic cell lines, adult skeletal myoblasts, immoratalized atrial cells, embryonic and adult cardiomyocytes, embryonic stem cells, tetratoma cells, genetically altered fibroblasts, smooth muscle cells, and bone marrow-derived cells have all been proposed as useful cells in cardiac repair and may have the capacity to perform cardiac work. We focus on the implantation of mesodermally derived cells, the best developed of the options. We review the developmental and cell biology that have stimulated these studies, examine the limitations of current knowledge, and identify challenges for the future, which we believe are considerable.

Cell cycle control in the terminally differentiated myocyte. A platform for myocardial repair?

Currently available pharmaceuticals exert beneficial effects on morbidity and mortality in heart failure. Only cardiac transplantation, however, provides a definitive solution to the irreversible loss of cardiomyocytes in the failing heart. The limited availability of donor hearts leaves the vast majority of afflicted patients in need. The need for innovative approaches to improve care for these patients is apparent.

Myocardial tissue engineering with autologous myoblast implantation

OBJECTIVE

Implanting myoblasts derived from autologous skeletal muscle, that is, satellite cells, for myocardial replacement has many advantages when compared with implanting either fetal cardiac myocytes (ethical and donor availability issues) or established cell lines (oncogenicity). Furthermore, autologous myoblasts do not require immunosuppression. The feasibility of satellite cell differentiation into muscle fibers, after implantation into the myocardium, was confirmed by means of a unique cell-labeling technique.

METHODS

Myoblasts (satellite cells) isolated from the skeletal muscle of adult rats are labeled with 4',6-diamidino-2-phenylindone, which binds to DNA and to the protein tubulin to form a fluorescent complex, and implanted into the left ventricular wall of isogenic rats. The specimens are harvested 1 to 4 weeks after myoblast implantation. Histologic sections are examined under a fluorescent microscope.

RESULTS

The labeling efficiency of satellite cells with 4',6-diamidino-2-phenylindole is nearly 100%. In 4 specimens, the progressive differentiation of implanted myoblasts into fully developed striated muscle fibers can be observed.

CONCLUSION

Our earlier studies of autologous myoblast implantation into the cryoinjured myocardium of dogs suggested that these cells could differentiate into cardiac myocytes. However, it had been difficult to firmly establish these findings with the use of cell markers, thereby proving that the neomyocardium had indeed been derived from the implanted myoblasts. In this study, using 4',6-diamidino-2-phenylindole as a satellite cell marker, we were able to demonstrate that the implanted satellite cells did in fact differentiate into fully developed, labeled muscle fibers. Because of the obvious advantages of using autologous donor myoblasts, the clinical application of this approach may provide a novel strategy for the future management of heart failure.

Bioreactor perfusion system for the long-term maintenance of tissue-engineered skeletal muscle organoids

Three-dimensional skeletal muscle organ-like structures (organoids) formed in tissue culture by fusion of proliferating myoblasts into parallel networks of long, unbranched myofibers provide an in vivo-like model for examining the effects of growth factors, tension, and space flight on muscle cell growth and metabolism. To determine the feasibility of maintaining either avian or mammalian muscle organoids in a commercial perfusion bioreactor system, we measured metabolism, protein turnover. and autocrine/paracrine growth factor release rates. Medium glucose was metabolized at a constant rate in both low-serum- and serum-free media for up to 30 d. Total organoid noncollagenous protein and DNA content decreased approximately 22-28% (P < 0.05) over a 13-d period. Total protein synthesis rates could be determined accurately in the bioreactors for up to 30 h and total protein degradation rates could be measured for up to 3 wk. Special fixation and storage conditions necessary for space flight studies were validated as part of the studies. For example, the anabolic autocrine/paracrine skeletal muscle growth factors prostaglandin F2alpha (PGF2alpha) and insulin-like growth factor-1 (IGF-1) could be measured accurately in collected media fractions, even after storage at 37 degrees C for up to 10 d. In contrast, creatine kinase activity (a marker of cell damage) in collected media fractions was unreliable. These results provide initial benchmarks for long-term ex vivo studies of tissue-engineered skeletal muscle.

In vivo analysis of microvascular injury after myocardial cryothermia

We studied microvascular injury after myocardial cryothermia in rats using intravital fluorescence microscopic techniques. Cryolesions were induced to the right ventricle by freezing with -160 degrees C (probe diameter: 5 mm) for a total of 5 min. Fluorescence microscopy was performed at 15, 30, 60, 90, and 120 min as well as at 3 and 7 days after cryothermia. Analysis of the epicardial microvasculature 15 min after cryothermia revealed an area of 24.6 +/- 3.8 mm2 of nonperfused tissue, which was reduced to 5.3 +/- 1.5 mm2 (P < 0.05) after the initial 2-h observation period. Vital microscopic images of reperfused tissue characteristically demonstrated extravasation of the macromolecular fluorescent tracer FITC-dextran (21.7 +/- 3.4 mm2), suggesting substantial loss of endothelial integrity. In vivo propidium iodide staining confirmed membrane damage of microvascular endothelial cells. Three days after cryoinjury the area of nonperfused tissue was reduced further to 1.1 +/- 0.4 mm2 in the center of the lesion, while the area of perfused tissue with disruption of endothelial integrity was found significantly increased to 47.4 +/- 5.9 mm2 (P < 0.05) toward the periphery. Analysis at 7 days revealed endothelial repair at the periphery of the cryolesion, but now a central necrotic area was found demarcated (nonperfused), presenting with a size (26.0 +/- 3.5 mm2) similar to that shown during the very early (15 min) reperfusion period. Our study demonstrates recovery of microvascular perfusion during the first hours and days after myocardial cryothermia. This is, however, associated with endothelial injury, i.e., damage of plasma membrane and loss of barrier function. Infarction with capillary perfusion failure is evident at 7 days with a size which strikingly corresponds to the sizeof nonperfused tissue observed immediately after cryointervention.

Efficiency of a high-titer retroviral vector for gene transfer into skeletal myoblasts

BACKGROUND

Genetic transformation of skeletal myoblasts for myocardial repair is dependent on an efficient gene transfer system that integrates the genes of interest into the genome of the target cell and its progeny. The aim of this investigation was to evaluate the use of a new retrovirally based gene transfer system for this purpose.

METHODS

MFGnlslacZ retroviral vector, packaged in high-titer, split-genome packaging cell line (FLYA4) was used to transduce the skeletal myoblast cell line L6. L6 cells, cultured in 10% fetal calf serum, were transduced with the MFGnlslacZ vector by means of filtered supernatant from FLYA4 cells. Transduced L6 cells were divided into four groups. Group I cells were fixed as myoblasts 3 days after transduction. Group II cells were allowed to differentiate into myotubes. Group III cells were split every 3 days for 4 months. Group IV cells were split as in group III but then allowed to differentiate into myotubes. All samples were fixed and stained for beta-galactosidase activity. The effects on gene transfer of transforming growth factor-beta, insulin-like growth factor-I, and platelet-derived growth factor were determined by spectrophotometric assay of beta-galactosidase activity in cells transduced in the presence or absence of serum with 0 to 200 ng/ml of each growth factor.

RESULTS

Morphometric analysis showed that 66.3% +/- 3% to 69.6% +/- 6% of cells in group I to IV expressed the lacZ reporter gene. In the presence of serum, transforming growth factor-beta significantly inhibited gene transfer, whereas insulin-like growth factor-I and platelet-derived growth factor significantly enhanced gene transfer. In absence of serum, however, only platelet-derived growth factor enhanced retrovirally mediated gene transfer into skeletal myoblasts.

CONCLUSION

MFG retroviral vectors packaged in FLYA4 cells are efficient in gene transfer into skeletal myoblasts and result in transgenic expression that is maintained after repeated cell division, differentiation, or both. Platelet-derived growth factor enhances retrovirally mediated gene transfer into skeletal myoblasts.

Molecular remodeling of cardiac contractile function

A number of techniques are now available that allow the contractile apparatus of the heart to be altered in a defined manner. This review focuses on those approaches that result in germ-line transmission of the remodeling event(s). Thus the desired modifications can be propagated stably throughout multiple generations and result in the creation of stable, new animal models. Necessarily, such stable changes need to be performed at the level of the genome, and two distinct but complementary approaches have been developed: transgenesis and gene targeting. Each results in the stable modification of the mammalian genome. Via gene targeting or gene ablation of sequences encoding various components of the sarcomere, the contractile apparatus of the heart can be altered dramatically. Ablating a gene may lead to a loss in function, which can help establish a function of the candidate sequence. Gene targeting can also be used to effect changes in the sequences encoding a functional domain of the contractile protein or at a single-amino acid residue, resulting in the establishment of precise structure-function relationships. With the use of transgenesis, the contractile apparatus of the heart can also be significantly remodeled. These approaches are rapidly creating a group of animals in which altered contractile protein complements will lead to a fundamental understanding of the structure-function relationships that underlie the function of the heart at the molecular, biochemical, whole organ, and whole animal levels.

Molecular and cellular prospects for repair, augmentation, and replacement of the failing heart

Molecular and cellular biology offer the promise of new approaches to the treatment of heart failure. This article discusses the basic science background, the current state of investigation, and the potential for therapeutic application of these new sciences. It also emphasizes the limitations and unknowns in this frontier. Three approaches are presented: First, increasing the number of myocytes in the heart, previously held to be untenable because postnatal cardiomyocytes do not divide, may be possible by regulating the cell cycle to reinduce cardiac growth. Also, nonmyocytes extant in the heart may be coaxed into differentiating into cardiomyocytes, or exogenous muscle cells may be grafted into the myocardium. Second, cardiac function may be augmented by molecular therapies that increase contractile protein function or regulate beta-adrenergic receptors or Ca++ channels. Third, improved prospects for transplantation of the failed heart may occur by genetic modification of a xenograft donor heart that reduces the chance of immune rejection by the human recipient. The formulation for the successful application of any of these therapies depends on not only the creativity of scientists but also the wisdom of physicians.

Skeletal myoblast transplantation for repair of myocardial necrosis

Myocardial infarcts heal by scarring because myocardium cannot regenerate. To determine if skeletal myoblasts could establish new contractile tissue, hearts of adult inbred rats were injured by freeze-thaw, and 3-4.5 x 10(6) neonatal skeletal muscle cells were transplanted immediately thereafter. At 1 d the graft cells were proliferating and did not express myosin heavy chain (MHC). By 3 d, multinucleated myotubes were present which expressed both embryonic and fast fiber MHCs. At 2 wk, electron microscopy demonstrated possible satellite stem cells. By 7 wk the grafts began expressing beta-MHC, a hallmark of the slow fiber phenotype; coexpression of embryonic, fast, and beta-MHC continued through 3 mo. Transplanting myoblasts 1 wk after injury yielded comparable results, except that grafts expressed beta-MHC sooner (by 2 wk). Grafts never expressed cardiac-specific MHC-alpha. Wounds containing 2-wk-old myoblast grafts contracted when stimulated ex vivo, and high frequency stimulation induced tetanus. Furthermore, the grafts could perform a cardiac-like duty cycle, alternating tetanus and relaxation, for at least 6 min. Thus, skeletal myoblasts can establish new muscle tissue when grafted into injured hearts, and this muscle can contract when stimulated electrically. Because the grafts convert to fatigue-resistant, slow twitch fibers, this new muscle may be suited to a cardiac work load.

Myocardial infarction as a problem of growth control: cell cycle therapy for cardiac myocytes?

Pump failure after myocardial infarction ultimately can be ascribed, in large part, to the inability of ventricular muscle to regenerate functional mass through cell proliferation. Recent studies using adenoviral gene transfer have provided direct evidence for the operation of two growth-suppressing pathways in cardiac muscle, via "pocket proteins," including the retinoblastoma gene product, and via a less well understood protein, p300. An understanding of molecular mechanisms that confer a virtually irreversible lock to the proliferative cell cycle in "postmitotic" cardiac muscle, together with improved means for delivery of exogenous genes to the heart, suggests the long-term potential for manipulating cardiac growth to achieve a therapeutic benefit.

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.