Chronic heart failure(CHF) has emerged as a major worldwide epidemic. Recently, a fundamental shift in the underlying etiology of CHF is becoming evident, in which the most common cause is no longer hypertension or valvular disease, but rather long-term survival after acute myocardial infarction (AMI)[1,2].
The costs of this syndrome, both in economic and personal terms, are considerable [3]. American Heart Association statistics indicate that CHF affects 4.7 million patients in the United States and is responsible for approximately one million hospitalizations and 300,000 deaths annually.
The total annual costs associated with this disorder have been estimated to exceed $22 billion. The societal impact of CHF is also remarkable. Patients with CHF often suffer a greatly compromised quality of life. About 30% of diagnosed individuals (i.e.,1.5 million in U.S.) experience difficulty breathing with little or no physical exertion, and are very restricted in their daily functions. This forced sedentary lifestyle inevitably leads to further physical and mental distress.
The CHF problem is growing worse. While CHF already represents one of our greatest health care problems, it is expected to become even more severe in the future. By 2010, the number of patients suffering from HF will have grown to nearly 7 million, a more than 40% increase.
Coronary artery disease (CAD) is the cause of CHF in the majority of patients, and CHF is the only mode of CAD presentation associated with increasing incidence and mortality.
However, it is evident, running through the different therapeutical strategies of CHF, that the appropriate treatment of patients with ischemic heart failure is still unknown [4,5].
After myocardial infarction, injured cardiomyocytes are replaced by fibrotic tissue promoting the development of heart failure. Cell transplantation has emerged as a potential therapy and stem cells may be an important and powerful cellular source.
Cell transplantation represents the last frontier within the treatment of cardiac diseases. Cell transplantation is currently generating a great deal of interest since the replacement of akinetic scar tissue by viable myocardium should improve cardiac function, impede progressive LV remodelling, and revascularize ischemic area. The goals of cell therapy are multiple and non exclusive, leading to the formation of a new tissue.
One should expect to replace a scar tissue by living cells and/or to block or reverse the remodelling process or change its nature and/or to restore the contractility of the cardiac tissue and/or to induce neoangiogenesis that would favour the recruitment of hibernating cardiomyocytes or to enhance transplanted cell engraftment, survival, function, and, ultimately, synergistic interaction with resident cells.
From the first paper published in 1992 that has documented the potentials of the transplantation of autologous skeletal muscle to treat the damage induced by acute myocardial infarction [5], innumerable techniques, types of cells, myocardial pathologies, and techniques of implantation have been reported, greatly expanding this innovative and appealing field of search in cardiovascular medicine.
Different stem cell populations have been intensively studied in the last decade as a potential source of new cardiomyocytes to ameliorate the injured myocardium, compensate for the loss of ventricular mass and contractility and eventually restore cardiac function. An array of cell types has been explored in this respect, including skeletal muscle, bone marrow derived stem cells, embryonic stem cells (ESC) and more recently cardiac progenitor cells. The best-studied cell types are mouse and human ESC cells, which have undisputedly been demonstrated to differentiate into cardiomyocyte and vascular lineages and have been of great help to understand the differentiation process of pluripotent cells. However, due to their immunogenicity, risk of tumor development and the ethical challenge arising from their embryonic origin, they do not provide a suitable cell source for a regenerative therapy approach.
Embryonic stem cells can differentiate into true cardiomyocytes, making them in principle an unlimited source of transplantable cells for cardiac repair, although immunological and ethical constraints exist. Somatic stem cells are an attractive option to explore for transplantation as they are autologous, but their differentiation potential is more restricted than embryonic stem cells. Currently, the major sources of somatic cells used for basic research and in clinical trials originate from the bone marrow. The differentiation capacity of different populations of bone marrow-derived stem cells into cardiomyocytes has been studied intensively. Only mesenchymal stem cells seem to form cardiomyocytes, and only a small percentage of this population will do so in vitro or in vivo. A newly identified cell population isolated from cardiac tissue, called cardiac progenitor cells, holds great potential for cardiac regeneration.
New approaches for cardiac repair have been enabled by the discovery that the heart contains its own reservoir of stem cells. These cells are positive for various stem/progenitor cell markers, are self-renewing, and exhibit multilineage differentiation potential. Recently has been developed a method for ex vivo expansion of cardiac-derived stem cells from human myocardial biopsies with a view to subsequent autologous transplantation for myocardial regeneration.
Despite original promises and expectations, current evidences of stem cell transplantation are still weak and controversial. The use of trypsin to detach the cells from the culture dish disrupts their microintercellular communication and extracellular matrix, restricts cell survival and growth, and thus appears deleterious to cell transplantation theraphy. Intercellular communication factors play a key role in cell adhesion, migration, proliferation, differentiation, and death and must be maintained for optimal cellular benefits. Therefore, alternative line of research are being explored, particularly in the field of techniques of cell implantation and engraftment.
Besides direct implantation or myocardial colonization by bone marrow stimulation, epicardial application of cell-delivering systems (scaffold and patches) have gained popularity due to the possibility to apply selectively a cell-containing device which may gradually release the chosen cell type, alone or in combination with trophic substances.
The scaffolds have proven to be successful in this respect and may represent a valid alternative to coronary, intra-myocardial, or venous injection of stem cells, or to stem cell stimulating factors.
Several materials have been assessed for generate scaffold.Li and associates produced 3D contractile cardiac grafts using gelatin sponges and synthetic biodegradable polymers [6]. Leor and colleagues reported the formation of bioengineered cardiac grafts with 3-D alginate scaffolds [7] Eschenhagen and coworkers engineered 3-D heart tissue by gelling a mixture of cardiomyocytes and collagen [8]. Robinson et al experimented urinary bladder matrix (UBM) and demonstrated UBM superiority to synthetic material for cardiac patching and trends toward myocardial replacement at 3 months [9].
Biological patches may, moreover, show enormous advantages, particularly in congenital diseases, where the existence of a growing tissue might reduce or limit the postoperative complications linked to not-growing material, ultimately leading to stenosis or patient/material mismatch with the need of replacement with all the risks related to redo surgery.
The engineered heart tissue survived and matured after implantation on uninjured hearts. Shimizu and colleagues have developed a novel approach of culturing cell sheets without scaffolds using a temperature-responsive polymer [10]. Several cell sheets were layered on top of each other to create thicker grafts. Ishii et al as an alternative approach, developed an in vitro system for creating sheets of cardiomyocytes on a mesh consisting of ultrafine fibers. This device consists of a thin, highly porous, nonwoven fibrous mesh stretched across a wire ring. This novel scaffold can be fabricated in specific shapes and is easy to handle. However, thicker grafts are required to obtain sufficient function. It is hypothesized that a clinically relevant cardiac graft will require a vasculature to provide sufficient perfusion of oxygenated blood. As an intermediate step toward a thick, vascularized cardiac graft, it is important to assess the ability to increasing the thickness without a vasculature and determine the maximum thickness before core ischemia is observed in the graft. So isessential the development of a multilayer system as an intermediate step toward functional cardiac grafts.
Kochupura et al matured a novel finding that a tissue-engineered myocardialpatch (TEMP) derived from extra cellular matrix (ECM ) contributes to regional function 8 weeks after implantation in the canine heart [11]. In addition, they confirmed cardiomyocyte population of ECM. The etiology of these cells has been under investigation, with possible explanations including the deposition of circulating bone marrow-derived progenitor cells and the fusion of cardiac progenitor cells with host cells.
The regional mechanical benefit with ECM patch report an active contraction of the ECM and not passive elastic recoil. This contraction is also in synchrony with native myocardium. Microscopic evaluation of Dacron patches did not demonstrate the presence of cardiomyocytes nor do the mechanical data indicate that Dacron implantation contributes to regional function. Increasing the number of ECM layers could be an alternative, but it is unclear if the physiological benefit, ie, cardiomyocyte population, would still be evident. Grossly, Dacron elicited far greater fibrosis than ECM, correlating with more mediastinal adhesions and epicardial connective tissue deposition. On placement, the Dacron patch was clearly under tension. In sharp contrast, ECM triggered far less fibrosis. The patch was neither wrinkled nor aneurismal and appeared to share the same surface tension as adjacent native myocardium. Finally, after removal of adhesions, it was difficult to grossly distinguish ECM from native myocardium The quantitative and qualitative differences between ECM and Dacron could be explained by an inherent ability of ECM to house cellular elements that facilitate remodeling.
The modulus of elasticity of Dacron is at least 4 orders of magnitude greater than healthy myocardium, ie, Dacron is stiffer than myocardium. Thus, the use of Dacron as a myocardial patch may have a "tethering effect" that would reduce the mechanical function of surrounding myocardium. Furthermore, the cellular response to Dacron was primarily diffuse fibroblast proliferation, an observation also seen with remodeling after myocardial infarction. In contrast, ECM stimulated less fibrosis and was populated by different cell types, including cardiomyocytes.
Atkins et al have shown that the reduction of infarct stiffness via cell transplantation leads to increased diastolic function [12]. Similarly, Quarterman et al created a detailed finite element model to show that cell transplantation alone will result in changes in compliance that result in mechanical benefit [13]. The potential clinical applications of ECM as a scaffold are many and would have a powerful impact on the management of cardiac disease. These would include instances in which Dacron is presently used as a myocardial patch: repair of ventricular aneurysms, repair of congenital heart defects, and most recently, surgical restoration of a dyskinetic or akinetic ventricle. By its contribution to regional systolic function, ECM provides true restoration of the ventricle rather than nonfunctional substitution of defective tissue, as is the case with Dacron.
Limits and Perspectives
The use of scaffold for tissue engineering is supportive for myocardial regeneration but subject to biocompatibility, biodegradability, and cytotoxicity, including inflammatory response and surface adhesion molecule loss issues, and this limits its efficacy. Eliminating such disadvantages, is necessary to establish cell sheet engineering technology without using scaffolds. The engineered cell sheets from this technique showed preserved cellular communication junctions, endogenous extracellular matrix, and integrative adhesive agents. Nonligature implantation of these engineered neonatal cardiomyocyte sheets to infarcted myocardium showed their integration with impaired myocardium and improved cardiac performance. For clinical application, use of skeletal myoblasts averts ethical and cell source issues. Recent findings suggested that locally or transgenically delivered stromal-derived factor 1 (SDF-1) expression plays a role in mobilizing and recruiting stem cells with neovascularization [14]. Because SDF-1 is secreted in skeletal muscle tissue, grafted myoblasts might beneficially attract hematopoietic stem cells (HSCs) to home in the infarct heart area for heart regeneration and angiogenesis [15].
The use of engineered patches, therefore, represents an appealing frontier which, in several formats, may provide material and solutions for some complex and inoperable disease. These devices, appropriately designed, may also allow the release of any kind of compounds and material, from cells to drugs, from factor to solution, ad programmed speed, ranging from transient and quick release (high biodegradability) to slow release (low degradability, several months). Last, but not least, the material chosen for realising such a device may also represent a containing structure, variably ranging from pure passive to slightly active action, which may play a role in the mechanical effect on cardiac dilatation in the case of heart containment procedure.
Finally, some treatments, particularly drug-related, showed promising results, but the potential disadvantages of systemic administration hampered a clinical or wider application. The possibility to deliver a specifc agent only locally, with obvious reduction in systemic effects, might be appealing and allow higher and focused concentration only to the target organ, that is the heart, or area of the heart.
