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

Cell augmentation strategies for cardiac stem cell therapies

Myocardial infarction (MI) has been the primary cause of death in developed countries, resulting in a major psychological and financial burden for society. Current treatments for acute MI are directed toward rapid restoration of perfusion to limit damage to the myocardium, rather than promoting tissue regeneration and subsequent contractile function recovery. Regenerative cell therapies (CTs), in particular those using multipotent stem cells (SCs), are in the spotlight for treatment post‐MI. Unfortunately, the efficacy of CTs is somewhat limited by their poor long‐term viability, homing, and engraftment to the myocardium. In response, a range of novel SC‐based technologies are in development to provide additional cellular modalities, bringing CTs a step closer to the clinic. In this review, the current landscape of emerging CTs and their augmentation strategies for the treatment post‐MI are discussed. In doing so, we highlight recent advances in cell membrane reengineering via genetic modifications, recombinant protein immobilization, and the utilization of soft biomimetic scaffold interfaces.

Paracrine Mechanisms Involved in Mesenchymal Stem Cell Differentiation into Cardiomyocytes

Cardiovascular Disease (CVD) is one of the world-wide healthcare problem that involves the heart or blood vessels. CVD includes myocardial infarction and coronary artery diseases (CAD). Dysfunctional myocardial cells are leading causes of low cardiac output or ventricular dysfunction after cardiac arrest and may contribute to the progression of CVD which could not generate new cardiomyocytes in human adult heart. The mesenchymal stem cells (MSCs) which are present in adult marrow can self-renew and have the capacity of differentiation into multiple types of cells including cardiomyocytes. Recent biochemical analyses greatly revealed that several regulators of MSCs, such as HGF, PDGF, Wnt, and Notch-1 signaling pathways have been shown to be involved in the proliferation and differentiation into cardiomyocytes. Preclinical studies are paving the way for further applications of MSCs in the repair of myocardial infarction. In this study, we discuss and summarize the paracrine mechanisms involved in MSCs differentiation into cardiomyocytes.

Concise review: skeletal muscle stem cells and cardiac lineage: potential for heart repair

Valuable and ample resources have been spent over the last two decades in pursuit of interventional strategies to treat the unmet demand of heart failure patients to restore myocardial structure and function. At present, it is clear that full restoration of myocardial structure and function is outside our reach from both clinical and basic research studies, but it may be achievable with a combination of ongoing research, creativity, and perseverance. Since the 1990s, skeletal myoblasts have been extensively investigated for cardiac cell therapy of congestive heart failure. Whereas the Myoblast Autologous Grafting in Ischemic Cardiomyopathy (MAGIC) trial revealed that transplanted skeletal myoblasts did not integrate into the host myocardium and also did not transdifferentiate into cardiomyocytes despite some beneficial effects on recipient myocardial function, recent studies suggest that skeletal muscle-derived stem cells have the ability to adopt a cardiomyocyte phenotype in vitro and in vivo. This brief review endeavors to summarize the importance of skeletal muscle stem cells and how they can play a key role to surpass current results in the future and enhance the efficacious implementation of regenerative cell therapy for heart failure.

Muscle-derived stem cell sheets support pump function and prevent cardiac arrhythmias in a model of chronic myocardial infarction

Direct intracardiac cell injection for heart repair is hindered by numerous limitations including: cell death, poor spreading of the injected cells, arrhythmia, needle injury, etc. Tissue-engineered cell sheet implantation has the potential to overcome some of these limitations. We evaluated whether the transplantation of a muscle-derived stem cell (MDSC) sheet could improve the regenerative capacity of MDSCs in a chronic model of myocardial infarction. MDSC sheet-implanted mice displayed a reduction in left ventricle (LV) dilation and sustained LV contraction compared with the other groups. The MDSC sheet formed aligned myotubes and produced a significant increase in capillary density and a reduction of myocardial fibrosis compared with the other groups. Hearts transplanted with the MDSC sheets did not display any significant arrhythmias and the donor MDSC survival rate was higher than the direct myocardial MDSC injection group. MDSC sheet implantation yielded better functional recovery of chronic infarcted myocardium without any significant arrhythmic events compared with direct MDSC injection, suggesting this cell sheet delivery system could significantly improve the myocardial regenerative potential of the MDSCs.

From pluripotency to distinct cardiomyocyte subtypes

Differentiated adult cardiomyocytes (CMs) lack significant regenerative potential, which is one reason why degenerative heart diseases are the leading cause of death in the western world. For future cardiac repair, stem cell-based therapeutic strategies may become alternatives to donor heart transplantation. The principle of reprogramming adult terminally differentiated cells (iPSC) had a major impact on stem cell biology. One can now generate autologous pluripotent cells that highly resemble embryonic stem cells (ESC) and that are ethically inoffensive as opposed to human ESC. Yet, due to genetic and epigenetic aberrations arising during the full reprogramming process, it is questionable whether iPSC will enter the clinic in the near future. Therefore, the recent achievement of directly reprogramming fibroblasts into cardiomyocytes via a milder approach, thereby avoiding an initial pluripotent state, may become of great importance. In addition, various clinical scenarios will depend on the availability of specific cardiac cellular subtypes, for which a first step was achieved via our own programming approach to achieve cardiovascular cell subtypes. In this review, we discuss recent progress in the cardiovascular stem cell field addressing the above mentioned aspects.

Engineering biomaterials to integrate and heal: the biocompatibility paradigm shifts

This article focuses on one of the major failure routes of implanted medical devices, the foreign body reaction (FBR)--that is, the phagocytic attack and encapsulation by the body of the so-called "biocompatible" biomaterials comprising the devices. We then review strategies currently under development that might lead to biomaterial constructs that will harmoniously heal and integrate into the body. We discuss in detail emerging strategies to inhibit the FBR by engineering biomaterials that elicit more biologically pertinent responses.

Drug discovery models and toxicity testing using embryonic and induced pluripotent stem-cell-derived cardiac and neuronal cells

Development of induced pluripotent stem cells (iPSCs) using forced expression of specific sets of transcription factors has changed the field of stem cell research extensively. Two important limitations for research application of embryonic stem cells (ESCs), namely, ethical and immunological issues, can be circumvented using iPSCs. Since the development of first iPSCs, tremendous effort has been directed to the development of methods to increase the efficiency of the process and to reduce the extent of genomic modifications associated with the reprogramming procedure. The established lineage-specific differentiation protocols developed for ESCs are being applied to iPSCs, as they have great potential in regenerative medicine for cell therapy, disease modeling either for drug development or for fundamental science, and, last but not least, toxicity testing. This paper reviews efforts aimed at practical development of iPSC differentiation to neural/cardiac lineages and further the use of these iPSCs-derived cells for drug development and toxicity testing.

Terminal differentiation is not a major determinant for the success of stem cell therapy - cross-talk between muscle-derived stem cells and host cells

We have found that when muscle-derived stem cells (MDSCs) are implanted into a variety of tissues only a small fraction of the donor cells can be found within the regenerated tissues and the vast majority of cells are host derived. This observation has also been documented by other investigators using a variety of different stem cell types. It is speculated that the transplanted stem cells release factors that modulate repair indirectly by mobilizing the host's cells and attracting them to the injury site in a paracrine manner. This process is loosely called a 'paracrine mechanism', but its effects are not necessarily restricted to the injury site. In support of this speculation, it has been reported that increasing angiogenesis leads to an improvement of cardiac function, while inhibiting angiogenesis reduces the regeneration capacity of the stem cells in the injured vascularized tissues. This observation supports the finding that most of the cells that contribute to the repair process are indeed chemo-attracted to the injury site, potentially through host neo-angiogenesis. Since it has recently been observed that cells residing within the walls of blood vessels (endothelial cells and pericytes) appear to represent an origin for post-natal stem cells, it is tempting to hypothesize that the promotion of tissue repair, via neo-angiogenesis, involves these blood vessel-derived stem cells. For non-vascularized tissues, such as articular cartilage, the regenerative property of the injected stem cells still promotes a paracrine, or bystander, effect, which involves the resident cells found within the injured microenvironment, albeit not through the promotion of angiogenesis. In this paper, we review the current knowledge of post-natal stem cell therapy and demonstrate the influence that implanted stem cells have on the tissue regeneration and repair process. We argue that the terminal differentiation capacity of implanted stem cells is not the major determinant of the cells regenerative potential and that the paracrine effect imparted by the transplanted cells plays a greater role in the regeneration process.

Stem cells in the treatment of patients with coronary heart disease. Part I

Heart failure (HF) is one of the leading death causes in patients with myocardial infarction (MI). The modern methods of reperfusion MI therapy, such as thrombolysis, surgery and balloon revascularization, even when performed early, could fail to prevent the development of large myocardial damage zones, followed by HF. Therefore, the researches have been searching for the methods which improve functional status of damaged myocardium. This review is focused on stem cell therapy, a method aimed at cardiac function restoration. The results of experimental and clinical studies on stem cell therapy in coronary heart disease are presented. Various types of stem cells, used for cellular cardiomyoplasty, are characterised. The methods of cell transplantation into myocardium and potential adverse effects of stem cell therapy are discussed.

Next generation of electrosprayed fibers for tissue regeneration

Electrospinning is a widely established polymer-processing technology that allows generation of fibers (in nanometer to micrometer size) that can be collected to form nonwoven structures. By choosing suitable process parameters and appropriate solvent systems, fiber size can be controlled. Since the technology allows the possibility of tailoring the mechanical properties and biological properties, there has been a significant effort to adapt the technology in tissue regeneration and drug delivery. This review focuses on recent developments in adapting this technology for tissue regeneration applications. In particular, different configurations of nozzles and collector plates are summarized from the view of cell seeding and distribution. Further developments in obtaining thick layers of tissues and thin layered membranes are discussed. Recent advances in porous structure spatial architecture parameters such as pore size, fiber size, fiber stiffness, and matrix turnover are summarized. In addition, possibility of developing simple three-dimensional models using electrosprayed fibers that can be utilized in routine cell culture studies is described.

Cell-based therapy for chronic ischemic heart disease--a clinical perspective

In patients with ischemic heart disease, the goal of cell therapy is to improve perfusion and function of the damaged heart muscle. For this review, we selected articles that reported the findings from the major clinical studies of cardiovascular stem cell therapy in patients with chronic ischemic heart disease. Because of the current status of development of clinical investigation in this field, all relevant studies were included. Initial clinical trials have shown that adult cell-based therapy is safe and may improve the quality of life and the functional status of patients with chronic myocardial ischemia. Adult bone marrow mononuclear cells have been most frequently used in cardiac cell therapy trials to date, but new cell types are now being assessed in both preclinical and clinical studies. Although not well defined, mechanisms underlying the benefits associated with cell therapy are most likely multiple and include a paracrine effect. Cell therapy in patients with chronic ischemic heart disease has been shown to be safe and feasible. Initial data have shown that cell therapy with autologous bone marrow cells is associated with modest functional improvements. This finding needs to be confirmed in subsequent phase 2 and 3 trials.

Fabrication and characterization of bio-engineered cardiac pseudo tissues

We report on fabricating functional three-dimensional (3D) tissue constructs using an inkjet based bio-prototyping method. With the use of modified inkjet printers, contractile cardiac hybrids that exhibit the forms of the 3D rectangular sheet and even the 'half heart' (with two connected ventricles) have been fabricated by arranging alternate layers of biocompatible alginate hydrogels and mammalian cardiac cells according to pre-designed 3D patterns. In this study, primary feline adult and H1 cardiomyocytes were used as model cardiac cells. Alginate hydrogels with controlled micro-shell structures were built by spraying cross-linkers in micro-drops onto un-gelled alginic acid. The cells remained viable in constructs as thick as 1 cm due to the programmed porosity. Microscopic and macroscopic contractile functions of these cardiomyocyte constructs were observed in vitro. These results suggest that the inkjet bio-prototyping method could be used for hierarchical design of functional cardiac pseudo tissues, balanced with porosity for mass transport and structural support.

Isolation of a slowly adhering cell fraction containing stem cells from murine skeletal muscle by the preplate technique

This protocol details a procedure, known as the modified preplate technique, which is currently used in our laboratory to isolate muscle cells on the basis of selective adhesion to collagen-coated tissue culture plates. By employing this technique to murine skeletal muscle, we have been able to isolate a rapidly adhering cell (RAC) fraction within the earlier stages of the process, whereas a slowly adhering cell (SAC) fraction containing muscle-derived stem cells is obtained from the later stages of the process. This protocol outlines the methods and materials needed to isolate RAC and SAC populations from murine skeletal muscle. The procedure involves mechanical and enzymatic digestion of skeletal muscle tissue with collagenase XI, dispase and trypsin followed by plating the resultant muscle slurry on collagen type I-coated flasks where the cells adhere at different rates. The entire preplate technique requires 5 d to obtain the final preplate SAC population. Two to three additional days are usually required before this population is properly established. We also detail additional methodologies designed to further enrich the resultant cell population by continuing the modified preplating process on the SAC population. This process is known as replating and requires further time.

Successful myoblast transplantation in rat tongue reconstruction

BACKGROUND

Controversy exists regarding the success of myoblast transplantation. The purpose of this study was to determine the survival of transplanted myoblasts in a rat tongue reconstruction model by using fluorescently labeled myoblasts and surgical stains to mark the location of the pocket into which transplanted cells were delivered. We evaluated tongue histology after myoblast transplantation under the hypothesis that myoblast transplantation will promote muscle regeneration and result in minimal scar tissue formation.

METHODS

Sterile solutions of 1:10 India ink, 1% methylene blue, and 1% crystal violet were applied to the inner lining of a left-sided mucosa-sparing hemiglossectomy pocket. After air-drying, the hemiglossectomy defect was filled with collagen gel and closed. The tongues were evaluated histologically at 6 weeks. Next, myoblasts were cultured and labeled with three commercially available fluorescent dyes, 5-chloromethyl-fluorescein diacetate (CMFDA), chloromethylbenzamido (CM-DiI), and fluorescently labeled microspheres (FLMs), to determine which would optimally label myoblasts in a tongue reconstruction model. Next, Lewis rats underwent left hemiglossectomy, and the created pockets were coated with 1:10 India ink. Control animals received collagen gel alone, whereas experimental animals received labeled myoblast/collagen constructs into the tongue defect. Tongues were harvested at intervals to determine the presence of labeled fluorescent cells, the relative numbers of viable myoblasts, and the degree of scarring.

RESULTS

India ink coating of the hemiglossectomy pocket caused minimal inflammation and lasted longer than the other tested dyes. CMFDA and FLMs both successfully label myoblasts for transplantation. In vivo, donor cells were observed in all specimens at week 0 with increasing numbers of cells and muscle formation, determined by desmin immunofluorescence, after 6 weeks. There was less scar tissue contracture in the experimental group and a significant increase in the amount of desmin-stained muscle in the surgical defect.

CONCLUSIONS

India ink is an appropriate vehicle for intra-operative marking of a hemiglossectomy cavity. The introduction of myoblast/collagen constructs into the rat hemiglossectomy defect increases the amount of regenerated muscle, results in less scar contracture, and may increase meaningful tongue function.

Cell colonization in degradable 3D porous matrices

Cell colonization is an important in a wide variety of biological processes and applications including vascularization, wound healing, tissue engineering, stem cell differentiation and biosensors. During colonization porous 3D structures are used to support and guide the ingrowth of cells into the matrix. In this review, we summarize our understanding of various factors affecting cell colonization in three-dimensional environment. The structural, biological and degradation properties of the matrix all play key roles during colonization. Further, specific scaffold properties such as porosity, pore size, fiber thickness, topography and scaffold stiffness as well as important cell material interactions such as cell adhesion and mechanotransduction also influence colonization.

Stem Cell Therapy: A Hope for Dying Hearts

The restoration of functional myocardium following heart failure still remains a formidable challenge among researchers. Irreversible damage caused by myocardial infarction is followed by left ventricular remodeling. The current pharmacologic and interventional strategies fail to regenerate dead myocardium and are usually insufficient to meet the challenge caused by necrotic cardiac myocytes. There is growing evidence, suggesting that the heart has the ability to regenerate through the activation of resident cardiac stem cells or through the recruitment of a stem cell population from other tissues such as bone marrow. These new findings belie the earlier conception about the poor regenerating ability of myocardial tissue. Stem cell therapy is a promising new approach for myocardial repair. However, it has been limited by the paucity of cell sources for functional human cardiomyocytes. Moreover, cells isolated from different sources exhibit idiosyncratic characteristics including modes of isolation, ease of expansion in culture, proliferative ability, characteristic markers, etc., which are the basis for several technical manipulations to achieve successful engraftment. Clinical trials show some evidence for the successful integration of stem cells of extracardiac origin in adult human heart with an improved functional outcome. This may be attributed to the discrepancies in the methods of detection, study subject selection (early or late post transplantation), presence of inflammation, and false identification of infiltrating leukocytes. This review discusses these issues in a comprehensive manner so that their physiological significance in animal as well as in human studies can be better understood.

Morphology and ultrastructure of differentiating three-dimensional mammalian skeletal muscle in a collagen gel

Because previous studies of three-dimensional skeletal muscle cultures have shown limited differentiation, the goal of this study was to establish conditions that would produce mature sarcomeres in a mammalian-derived skeletal muscle construct. We evaluated the differentiation of bioartificial muscles generated from C(2)C(12) myoblasts in a collagen gel cultured under steady, passive tension for up to 36 days. Staining for alpha-actinin, myosin, and F-actin indicated the presence of striated fibers as early as 6 days post-differentiation. Electron microscopy at 16 days post-differentiation revealed multinucleated myotubes with ordered, striated myofibers. At 33 days, the cultures contained collagen fibers and showed localization of paxillin at the fiber termini, suggesting that myotendinous junctions were forming. The present study demonstrates mature muscle synthesis in a three-dimensional system using a pure mammalian myoblast cell line. Our results suggest that this culture model can be used to evaluate the effects of various mechanical and biochemical cues on muscle development under normal and pathological conditions.

Myogenic precursor cells in craniofacial muscles

Craniofacial skeletal muscles (CskM), including the masticatory (MM), extraocular (EOM) and laryngeal muscles (LM), have a number of properties that set them apart from the majority of skeletal muscles (SkM). They have embryological origins that are distinct from musculature elsewhere in the body, they express a number of immature myosin heavy chain isoforms and maintain increased and distinct expression of a number of myogenic growth factors and their receptors from other adult SkMs. Furthermore, it has recently been demonstrated that unlike limb SkM, normal adult EOM and LM retain a population of activated satellite cells, the regenerative cell in adult SkM. In order to maintain this proliferative pool throughout life, CSkM may contain more satellite cells and/or more multipotent precursor cells that may be more resistant to apoptosis than those found in limb muscle. A further exciting question is whether this potentially more active muscle precursor cell population could be utilized not only for SkM repair, but be harnessed for repair or reconstruction of other tissues, such as nervous tissue or bone. This is a highly attractive speculation as the innate regenerative capacity of craniofacial muscles would ensure the donor tissue would not have compromised future function.

Human embryonic stem cell-derived cardiomyocytes can be maintained in defined medium without serum

Current procedures for the maintenance of cardiomyocytes from human embryonic stem (hES) cells rely on either co-culture with mouse cells or medium containing fetal bovine serum (FBS). Due to exposure to animal products, these methods carry the risk of potential pathogen contamination and increased immunogenicity. Additionally, FBS introduces inherent variability in the cultures due to the inevitable differences in serum lots. Here we investigated whether a defined serum-free medium containing creatine, carnitine, taurine, and insulin (CCTI) could maintain hES cell-derived cardiomyocytes. We show that hES cell-derived cardiomyocytes maintained in the CCTI medium in the absence of any feeders exhibit similar phenotypes to those maintained in serum, as indicated by the following observations: (1) comparable levels of cardiac gene transcription were found in cells grown in serum-containing medium versus those in the CCTI medium; (2) cardiomyocyte-associated proteins were expressed in cells cultured in the CCTI medium; (3) beating cells in the CCTI medium responded to pharmacological agents in a dose-dependent manner; and (4) the vast majority of the beating embryoid bodies displayed ventricular-like action potentials (APs), and the ventricular cells in serum-containing medium and the CCTI medium had indistinguishable AP properties. Therefore, culturing hES cell-derived cardiomyocytes in serum-free medium as described here should facilitate the use of the cells for in vitro and in vivo applications.

Cardiac bodies: a novel culture method for enrichment of cardiomyocytes derived from human embryonic stem cells

Human embryonic stem (hES) cell-derived cardiomyocytes hold great promise for cardiovascular regenerative medicine. However, this application faces a number of challenges, including generating cardiomyocytes of adequate purity. With current protocols being used by several laboratories, cardiomyocyte differentiation from hES cells occurs at low frequency and results in a mixture of differentiated cells. Here we describe a novel method for enrichment of cardiomyocytes. Cardiomyocytes were isolated from embryoid body (EB) outgrowths by Percoll separation and then enriched by culturing the aggregates of cells (termed cardiac bodies, CBs) in suspension. The majority of CBs showed contractility after 1 week in culture and were positive for multiple cardiomyocyte- associated proteins. Enrichment of cardiomyocytes was evident by the increase in the expression of cardiac alpha and beta myosin heavy chains (alpha and betaMHC) in CBs in suspension culture compared to unpurified EB outgrowths. Flow cytometry analysis showed that 35-66% of the cells in CBs were positive for sarcomeric myosin heavy chain (sMHC) or cardiac troponin T (cTnT) expression. In addition, dissociated CBs were capable of reassociating into contracting aggregates in suspension and recovering contractility after the individual cells were replated onto matrix-coated surfaces. These data suggest that the CB method is a useful approach for the generation of cardiomyocytes at an adequate purity for cardiovascular therapies.

Cardiac conduction through engineered tissue

In children, interruption of cardiac atrioventricular (AV) electrical conduction can result from congenital defects, surgical interventions, and maternal autoimmune diseases during pregnancy. Complete AV conduction block is typically treated by implanting an electronic pacemaker device, although long-term pacing therapy in pediatric patients has significant complications. As a first step toward developing a substitute treatment, we implanted engineered tissue constructs in rat hearts to create an alternative AV conduction pathway. We found that skeletal muscle-derived cells in the constructs exhibited sustained electrical coupling through persistent expression and function of gap junction proteins. Using fluorescence in situ hybridization and polymerase chain reaction analyses, myogenic cells in the constructs were shown to survive in the AV groove of implanted hearts for the duration of the animal's natural life. Perfusion of hearts with fluorescently labeled lec-tin demonstrated that implanted tissues became vascularized and immunostaining verified the presence of proteins important in electromechanical integration of myogenic cells with surrounding re-cipient rat cardiomyocytes. Finally, using optical mapping and electrophysiological analyses, we provide evidence of permanent AV conduction through the implant in one-third of recipient animals. Our experiments provide a proof-of-principle that engineered tissue constructs can function as an electrical conduit and, ultimately, may offer a substitute treatment to conventional pacing therapy.

Paracrine action enhances the effects of autologous mesenchymal stem cell transplantation on vascular regeneration in rat model of myocardial infarction

BACKGROUND

There are several reports that engrafted mesenchymal stem cells (MSCs) stimulate angiogenesis in the ischemic heart, but the mechanism remains controversial. We hypothesize that transplantation of MSCs enhances vascular regeneration through a paracrine action.

METHODS

A transmural myocardial infarction was created by ligation of the left anterior descending coronary artery in rats. Those with an ejection fraction less than 0.70 1 week after myocardial infarction were included. Autologous MSCs (1 x 10(7); 0.2 mL) or culture medium (0.2 mL) was injected intramyocardially into the periinfarct zone (50 microL/injection at four sites; n = 20/group). At 2 weeks after transplantation, Western blot analysis was used to assay the paracrine factors and proapoptotic proteins. Echocardiography to assess heart function was performed on additional groups at 8 weeks after implantation.

RESULTS

The angiogenic factors basic fibroblast growth factor, vascular endothelial growth factor, and stem cell homing factor (stromal cell-derived factor -1alpha) increased in the MSC-treated hearts compared with medium-treated hearts. This was accompanied by a downregulation of proapoptotic protein Bax in ischemic myocardium. Similarly, capillary density increased about 40% in MSC-treated hearts compared with medium-treated hearts (p = 0.001). Left ventricular contractility, indicated by fractional shortening, improved in MSC-treated hearts at 2 months after implantation (MSCs: 48.6% +/- 19.9%; medium: 18.7% +/- 6.4%; p = 0.004).

CONCLUSIONS

Autologous MSC transplantation attenuates left ventricular remodeling and improves cardiac performance. The major mechanism appears to be paracrine action of the engrafted cells, increasing angiogenesis and cytoprotection.

Muscling in on stem cells

Skeletal muscle is one of the few adult tissues that possesses the capacity for regeneration (restoration of lost functional tissue) as opposed to repair. This capacity is due to the presence of 'muscle stem cells' known as satellite cells. Detailed investigation of these cells over the past 50 years has revealed that both these and other cells within the skeletal muscle complex are capable of regenerating both muscle and other cell types as well. Here, we review this information, and suggest that skeletal muscle is an exciting reservoir of cells for regenerating skeletal muscle itself, as well as other cell types.

Postnatal myocardial augmentation with skeletal myoblast–based fetal tissue engineering

BACKGROUND

Cardiac anomalies constitute the most common birth defects, many of which involve variable myocardial deficiencies. Therapeutic options for structural myocardial repair remain limited in the neonatal population. This study was aimed at determining whether engineered fetal muscle constructs undergo milieu-dependent transdifferentiation after cardiac implantation, thus becoming a potential means to increase/support myocardial mass after birth.

METHODS

Myoblasts were isolated from skeletal muscle specimens harvested from fetal lambs, labeled by transduction with a retrovirus-expressing green fluorescent protein, expanded in vitro, and then seeded onto collagen hydrogels. After birth, animals underwent autologous implantation of the engineered constructs (n = 8) onto the myocardium as an onlay patch. Between 4 and 30 weeks postoperatively, implants were harvested for multiple analyses.

RESULTS

Fetal and postnatal survival rates were 89% and 100%, respectively. Labeled cells were identified within the implants at all time points by immunohistochemical staining for green fluorescent protein. At 24 and 30 weeks postimplantation, donor cells double-stained for green fluorescent protein and Troponin I, while losing skeletal (type II) myosin expression.

CONCLUSIONS

Fetal skeletal myoblasts engraft in native myocardium up to 30 weeks after postnatal, autologous implantation as components of engineered onlay patches. These cells also display evidence of time-dependent transdifferentiation toward a cardiomyocyte-like lineage. Further analysis of fetal skeletal myoblast-based constructs for the repair of congenital myocardial defects is warranted.

Human Adult Bone Marrow Mesenchymal Stem Cells Repair Experimental Conduction Block in Rat Cardiomyocyte Cultures

OBJECTIVES

We evaluated whether human adult bone marrow-derived mesenchymal stem cells (hMSCs) could repair an experimentally induced conduction block in cardiomyocyte cultures.

BACKGROUND

Autologous stem cell therapy is a novel treatment option for patients with heart disease. However, detailed electrophysiological characterization of hMSCs is still lacking.

METHODS

Neonatal rat cardiomyocytes were seeded on multi-electrode arrays. After 48 h, abrasion of a 200- to 450-microm-wide channel caused conduction block. Next, we applied adult hMSCs (hMSC group, n = 8), human skeletal myoblasts (myoblast group, n = 7), rat cardiac fibroblasts (fibroblast group, n = 7), or no cells (control group, n = 7) in a channel-crossing pattern. Cross-channel electrical conduction was analyzed after 24 and 48 h. Intracellular action potentials of hMSCs and cardiomyocytes were recorded. Immunostaining for connexins and intercellular dye transfer (calcein) assessed the presence of functional gap junctions.

RESULTS

After creation of conduction block, two asynchronously beating fields of cardiomyocytes were present. Application of hMSCs restored synchronization between the two fields in five of eight cultures after 24 h. Conduction velocity across hMSCs (0.9 +/- 0.4 cm/s) was approximately 11-fold slower than across cardiomyocytes (10.4 +/- 5.8 cm/s). No resynchronization occurred in the myoblast, fibroblast, or control group. Intracellular action potential recordings indicated that conduction across the channel presumably occurred by electrotonic impulse propagation. Connexin-43 was present along regions of hMSC-to-cardiomyocyte contact, but not along regions of cardiomyocyte-to-myoblast or cardiomyocyte-to-fibroblast contact. Calcein transfer from cardiomyocytes to hMSCs was observed within 24 h after co-culture initiation.

CONCLUSIONS

Human mesenchymal stem cells are able to repair conduction block in cardiomyocyte cultures, probably through connexin-mediated coupling.

Generation, culture, and differentiation of human embryonic stem cells for therapeutic applications

Embryonic stem (ES) cells, derived from the inner cell mass of the mammalian blastocyst, can continuously proliferate in an undifferentiated state and can also be induced to differentiate into a desired cell lineage. These abilities make ES cells an appealing source for cell replacement therapies, the study of developmental biology, and drug/toxin screening studies. As compared to mouse ES cells, human ES cells have only recently been derived and studied. Although there are many differences in properties between mouse and human ES cells, the study of mouse ES cells has provided important insights into human ES cell research. In this review, we describe the advantages and disadvantages of methods used for human ES cell derivation, the expansion of human ES cells, and the current status of human ES cell differentiation research. In addition, we discuss the endeavor that scientists have undertaken toward the therapeutic application of these cells, which includes therapeutic cloning and the improvement of human ES cell culture conditions.

Non-invasive analysis of myoblast transplants in rodent cardiac muscle

BACKGROUND

Magnetic resonance imaging (MRI) of magnetically labeled stem cells is a non-invasive approach that can provide images with high spatial resolution. We evaluated the ability of a commercially available, Food and Drug Administration (FDA) approved contrast agent to allow the monitoring of myoblast transplants in the rodent heart.

METHODS AND RESULTS

Primary rat myoblasts were efficiently labeled by incubation with ferumoxide-polycation complexes and labeled cells retained their normal capacity to generate mature myotubes. Intra-cellular iron-oxide accumulation resulted in MRI contrast changes, allowing for three-dimensional, non-invasive detection of labeled cells in the rodent myocardium. Histological analysis of hearts injected with labeled myoblasts or control, non-viable myoblasts revealed that areas of MRI contrast changes corresponded to iron contained within engrafted myotubes and scavenger cells up to two months post-injection.

CONCLUSIONS

The high sensitivity of MR imaging will allow for non-invasive studies of cardiac stem cell migration and homing. Additional techniques are in development to non-invasively determine stem cell engraftment rates, viability and differentiation.

Modulation of gene expression in neonatal rat cardiomyocytes by surface modification of polylactide-co-glycolide substrates

Myocardial tissue engineering presents a potential treatment option for heart disease. Cardiomyocytes isolated at various stages of development retain the ability to form contractile networks in vitro, which suggests that it should be possible to reconstitute viable myocardium given the appropriate architecture, stimuli, and cardiomyogenic cell source. This study investigates the effects of modifying substrate surface energy (by plasma etching) and protein coating (by fibronectin adsorption) on neonatal rat ventricular myocyte (NRVM) function. Primary NRVMs were cultured for 96 h on modified and control films of a common degradable polymer, polylactide-co-glycolide. Cultures were analyzed for cell spreading, protein content, and mRNA expression of atrial natriuretic factor and beta-myosin heavy chain. The results demonstrate that NRVMs cultured on etched films significantly increased in spreading, myofibril development, protein content, and gene expression of atrial natriuretic factor and beta-myosin heavy chain compared with unetched films, and that this surface energy effect is overwhelmed by the addition of fibronectin. Conclusions from this study are that surface energy and protein adsorption influence the gene expression of adherent NRVMs, and may be important for modulating the function of engineered myocardium.

Transcatheter Transplantation of Autologous Skeletal Myoblasts in Postinfarction Patients With Severe Left Ventricular Dysfunction

PURPOSE

To report a case-controlled safety and feasibility study of transcatheter transplantation of autologous skeletal myoblasts as a stand-alone procedure in patients with ischemic heart failure.

METHODS

Six men (mean age 66.2+/-7.2 years) were eligible for transcatheter transplantation of autologous skeletal myoblasts cultured from quadriceps muscle biopsies. Six other men (mean age 65.7+/-6.3 years) were selected as matched controls (no muscle biopsies). A specially designed injection catheter was advanced through a femoral sheath into the left ventricle cavity, where myoblasts in solution (0.2 mL/injection) were injected into the myocardium via a 25-G needle. At baseline and in follow-up, both groups underwent Holter monitoring, a 6-minute walk test, New York Heart Association (NYHA) class determination, and echocardiography with dobutamine challenge.

RESULTS

Skeletal myoblast transplantation was technically successful in all 6 patients with no complications; 19+/-10 injections were performed per patient (210 x 10(6)+/-150 x 10(6) cells implanted per patient). Left ventricular ejection fraction (LVEF) rose from 24.3%+/-6.7% at baseline to 32.2%+/-10.2% at 12 months after myoblast implantation (p=0.02 versus baseline and p<0.05 versus controls); in matched controls, LVEF decreased from 24.7%+/-4.6% to 21.0%+/-4.0% (p=NS). Walking distance and NYHA functional class were significantly improved at 1 year (p=0.02 and p=0.001 versus baseline, respectively), whereas matched controls were unchanged.

CONCLUSIONS

Transcatheter transplantation of autologous skeletal myoblasts for severe left ventricular dysfunction in postinfarction patients is feasible, safe, and promising. Scrutiny with randomized, double-blinded, multicenter trials appears warranted.

Autologous Skeletal Myoblasts for Myocardial Regeneration

We overview the current knowledge about the use of skeletal myoblasts in regeneration of infarcted myocardium. Myoblasts are attractive candidates for cell source for cardiomyoplasty in chronic postmyocardial injury as indicated by experimental and initial clinical experience. We also review the recent developments in skeletal myoblasts transplantation techniques with special attention to percutaneous transvenous approach to deliver therapeutic agents into myocardium from the lumen of coronary veins under intravascular guidance.

Mesenchymal stem cells and their potential as cardiac therapeutics

Mesenchymal stem cells (MSCs) represent a stem cell population present in adult tissues that can be isolated, expanded in culture, and characterized in vitro and in vivo. MSCs differentiate readily into chondrocytes, adipocytes, osteocytes, and they can support hematopoietic stem cells or embryonic stem cells in culture. Evidence suggests MSCs can also express phenotypic characteristics of endothelial, neural, smooth muscle, skeletal myoblasts, and cardiac myocyte cells. When introduced into the infarcted heart, MSCs prevent deleterious remodeling and improve recovery, although further understanding of MSC differentiation in the cardiac scar tissue is still needed. MSCs have been injected directly into the infarct, or they have been administered intravenously and seen to home to the site of injury. Examination of the interaction of allogeneic MSCs with cells of the immune system indicates little rejection by T cells. Persistence of allogeneic MSCs in vivo suggests their potential "off the shelf" therapeutic use for multiple recipients. Clinical use of cultured human MSCs (hMSCs) has begun for cancer patients, and recipients have received autologous or allogeneic MSCs. Research continues to support the desirable traits of MSCs for development of cellular therapeutics for many tissues, including the cardiovascular system. In summary, hMSCs isolated from adult bone marrow provide an excellent model for development of stem cell therapeutics, and their potential use in the cardiovascular system is currently under investigation in the laboratory and clinical settings.

Cell-based myocardial repair: How should we proceed?

Cell-based myocardial repair and regeneration heralds a new frontier in the treatment of cardiovascular disease. It provides an unprecedented opportunity to treat the underlying loss of cardiomyocytes that occurs after myocardial injury and that results in the cascade of events leading to heart failure. Yet, even as it progresses to the clinic, much remains to be understood about this technology. For example, controversies exist over the specific cells to be used, the cell dosages needed, how cells will impact the electrical activity of the myocardium, and even whether transplanted cells can actually improve myocardial function. We can perhaps answer these questions more quickly and more effectively - and thus benefit patients more rapidly - if we learn from the successes and failures of our gene therapy colleagues and take a prudent, step-wise approach from bench to bedside. To do so, we need only to promise what we can deliver, to do careful science, and then to deliver well on our promises. Although cellular cardiomyoplasty (cell transplantation for cardiac repair) shows great early clinical promise, its future as a new frontier in the treatment for cardiovascular disease will rest heavily on how we move forward in the next few years. Its success will heavily depend upon conducting carefully controlled, randomized double-blind clinical trials with appropriate endpoints, in the right patients. Choice of cell type, and mode of cell delivery, will also have to be considered, and may have to be matched to the patient. Irrespective of cell type, we can also be assured that cells offer both an opportunity for tissue repair and the potential for not yet understood outcomes. As with any frontier, there will be pitfalls and consequences to be considered that may surpass those of previous endeavors. But so too is the potential for previously unimagined success at treating the leading cause of death in the western world. In short, the promise for cardiovascular cell therapy is too great to be spoiled by ill-designed attempts that forget to account for both the natural propensities of cells and of the myocardium.

Bone Marrow Cells and Myocardial Regeneration

Hematopoietic stem cell (HSC) plasticity and its clinical application have been studied profoundly in the past few years. Recent investigations indicate that HSC and other bone marrow stem cells can develop into other tissues. Because of the high morbidity and mortality of myocardial infarction and other heart disorders, myocardial regeneration is a good example of the clinical application of HSC plasticity in regenerative medicine. Preclinical studies in animals suggest that the use of this kind of treatment can reconstruct heart blood vessels, muscle, and function. Some clinical study results have been reported in the past 2 years. In 2003, reports of myocardial regeneration treatment increased significantly. Other studies include observations on the cell surface markers of transplanted cells and treatment efficacy. Some investigations, such as HSC testing, have focused on clinical applications using HSC plasticity and bone marrow transplantation to treat different types of disorders. In this review, we focus on the clinical application of bone marrow cells for myocardial regeneration.

Bone Marrow Cells in the Repair and Modulation of Heart and Blood Vessels: Emerging Opportunities in Native and Engineered Tissue and Biomechanical Materials

Adult bone marrow-derived stem/progenitor cells have traditionally been considered to be tissue-specific cells with limited capacity for differentiation. However, recent discoveries have generated tremendous excitement regarding possible applications of stem cells, particularly bone marrow-derived stem cells, in the treatment of human diseases. The potential ability to regenerate cells of various different lineages has raised the therapeutic possibility of using these bone marrow-derived stem cells as a source of cells for tissue repair and regeneration. Tissue engineering is a rapidly expanding interdisciplinary field aimed at restoring function to tissues through the delivery of constructs which become integrated into the patient. The use of bone marrow-derived stem cells provides a less invasive source for cells applicable to tissue engineering, including cardiovascular tissues such as heart valves, blood vessels, and myocardium. Although these strategies are in the early stages of development, they are conceptually promising and offer important insights into the future treatment of various cardiovascular ailments.

[Stem cells to regenerate cardiac tissue in heart failure]

Myocardial regeneration is one of the most promising therapeutic strategies for heart failure patients. Many experimental studies have demonstrated that different types of stem cell can differentiate into myocardial cells and tissues necessary for regeneration of the damaged myocardium, while studies in experimental animals suggest that muscle (myoblast), bone marrow (mesenchymal, endothelial or hematopoietic progenitors) and even heart cells can help to improve heart contractility in vivo. These findings have led several groups to undertake studies in patients with myocardial infarction. However, the use of cellular therapy in clinical trials is not without controversy, mainly related with the need for better knowledge before these therapeutic strategies are used in clinical practice. Although significant enhancement of our knowledge of the processes involved is fundamental, we do not consider it unreasonable to initiate clinical trials in which specific questions are posed, whose answers will allow us to make further progress.

Improved regional left ventricular function after successful satellite cell grafting in rabbits with myocardial infarction

OBJECTIVE

To evaluate whether satellite cells injected into infarct areas in rabbits remain viable during 6 weeks follow-up and can improve cardiac function as assessed by echocardiography.

METHODS

Myocardial infarction was induced in 16 New Zealand white rabbits, by ligation of the marginalis sinistra artery. Tissue from gluteus muscle biopsies was dissected into small pieces and cultured. Within 2-3 weeks the cells were expanded by 2-3 orders of magnitude and were fluorescent labeled. Single cell pellets for resuspension at >10(6)/1 ml were directly injected into the infarct areas in 8 rabbits. In 8 additional rabbits, 1 ml saline was injected (control). Regional left ventricular function was assessed weekly by 2-D echocardiography until animals were sacrificed. Analysis was performed blind and independently by two experienced echocardiographers, based on the American Society of Echocardiography scheme.

RESULTS AND DISCUSSION

Six treated and five control rabbits completed the study. One week after the artery occlusion, left ventricular function scoring did not differ between groups, mean 8.7+/-1.6 vs 8.3+/-1.9 (P=0.74). At 6 weeks post-injection, echocardiographic score was significantly better in the treated group, mean 2.6+/-0.9 vs 6.9+/-2.1 (P=0.002). The treated group showed significant gradual segmental improvement between the first week up to week 6. After sacrifice, macro and microscopic transmural areas showed typical changes of myocardial infarction. Histochemical staining identified viable grafted cells in high density 6 weeks post-transplantation in all grafted hearts.

CONCLUSION

Autologous satellite cells (skeletal myofiber), can be successfully grafted into rabbit hearts following myocardial infarction and may induce improved regional left ventricular function.

Heterogeneous proliferative potential in regenerative adult newt cardiomyocytes

Adult newt cardiomyocytes, in contrast to their mammalian counterparts, can proliferate after injury and contribute to the functional regeneration of the heart. In order to understand the mechanisms underlying this plasticity we performed longitudinal studies on single cardiomyocytes in culture. We find that the majority of cardiomyocytes can enter S phase, a process that occurs in response to serum-activated pathways and is dependent on the phosphorylation of the retinoblastoma protein. However, more than half of these cells stably arrest at either entry to mitosis or during cytokinesis, thus resembling the behaviour observed in mammalian cardiomyocytes. Approximately a third of the cells progress through mitosis and may enter successive cell divisions. When cardiomyocytes divided more than once, the proliferative behaviour of sister cells was significantly correlated, in terms of whether they underwent a subsequent cell cycle, and if so, the duration of that cycle. These observations suggest a mechanism whereby newt heart regeneration depends on the retention of proliferative potential in a subset of cardiomyocytes. The regulation of the remaining newt cardiomyocytes is similar to that described for their mammalian counterparts, as they arrest during mitosis or cytokinesis. Understanding the nature of this block and why it arises in some but not other newt cardiomyocytes may lead to an augmentation of the regenerative potential in the mammalian heart.

Mesenchymal progenitor cells differentiate into an endothelial phenotype, enhance vascular density, and improve heart function in a rat cellular cardiomyoplasty model

BACKGROUND

Cellular cardiomyoplasty is a promising approach to improve postinfarcted cardiac function. The differentiation pathways of engrafted mesenchymal progenitor cells (MPCs) and their effects on the left ventricular function in a rat myocardial infarct heart model were analyzed.

METHODS AND RESULTS

A ligation model of left coronary artery of Lewis rats was used. MPCs were isolated by bone marrow cell adherence. Seven days after ligation, MPCs labeled with 4',6-diamidino-2'-phenylindole were injected into the infarcted myocardium (n=8). Culture medium was injected in the infarcted myocardium of control animals (n=8). Thirty days after implantation, immunofluorescence studies revealed some engrafted cells expressing a smooth muscle phenotype (alpha SM actin+), as similarly observed in culture. Other engrafted cells lost their smooth muscle phenotype and acquired an endothelial phenotype (CD31+). Furthermore, vessel density was augmented in the MPC group in comparison with the control group. After 30 days, echocardiography showed an improvement on left ventricular performance in the MPCs compared with the control group.

CONCLUSIONS

In vivo administration of syngenic MPCs into a rat model of myocardial infarcted heart was safety demonstrated. Some engrafted cells appeared to differentiate into endothelial cells and loss their smooth muscle phenotype. MPC engraftment might to contribute to the improvement on the cardiac function in such a setting.

An overview of stem cell research and regulatory issues

Stem cells are noted for their ability to self-renew and differentiate into a variety of cell types. Some stem cells, described as totipotent cells, have tremendous capacity to self-renew and differentiate. Embryonic stem cells have pluripotent capacity, able to form tissues of all 3 germ layers but unable to form an entire live being. Research with embryonic stem cells has enabled investigators to make substantial gains in developmental biology, therapeutic tissue engineering, and reproductive cloning. However, with these remarkable opportunities many ethical challenges arise, which are largely based on concerns for safety, efficacy, resource allocation, and methods of harvesting stem cells. Discussing the moral and legal status of the human embryo is critical to the debate on stem cell ethics. Religious perspectives and political events leading to regulation of stem cell research are presented and discussed, with special attention directed toward the use of embryonic stem cells for therapeutic and reproductive cloning. Adult stem cells were previously thought to have a restricted capacity to differentiate; however, several reports have described their plasticity potential. Furthermore, there have been close ties between the behavior of stem cells and cancer cells. True eradication of cancer will require a deeper understanding of stem cell biology. This article was written to inform medical scientists and practicing clinicians across the spectrum of medical education about the research and regulatory issues affecting the future of stem cell therapy.

Osteogenic stem cells and orthopedic engineering: summary and update

The use of osteogenic stem cells or osteoprogenitors to reconstruct skeletal tissues is a popular area of research investigation with high potential for successful use of tissue-engineering principles in orthopedics. Recent studies demonstrate the migration of marrow-derived stem cells to skeletal sites and the proliferation and differentiation at local tissue sites and support possibilities for assessing the successful uses of human osteoprogenitors in the treatment of bone deficiency diseases. In addition, the development of gene therapy procedures in these and other conditions is now considered an achievable goal with the use of these primitive marrow cells.