Perfusion-decellularized matrix: using nature's platform to engineer a bioartificial heart. Nat Med. 2008;14:213-221. [PMID: 18193059 DOI: 10.1038/nm1684]

[Heart tissue from embryonic stem cells]

Embryonic stem cells can give rise to all somatic cells, making them an attractive cell source for tissue engineering applications. The propensity of cells to form tissue-like structures in a culture dish has been well documented. We and others made use of this intrinsic property to generate bioartificial heart muscle. First proof-of-concept studies involved immature heart cells mainly from fetal chicken, neonatal rats and mice. They eventually provided evidence that force-generating heart muscle can be engineered in vitro. Recently, the focus shifted to the application of stem cells to eventually enable the generation of human heart muscle and reach following long-term goals: (1) development of a simplified in vitro model of heart muscle development; (2) generation of a human test-bed for drug screening and development; (3) allocation of surrogate heart tissue to myocardial repair applications. This overview will provide the background for cell-based myocardial repair, introduce the main myocardial tissue engineering concepts, discuss the use of embryonic and non-embryonic stem cells, and lays out the potential direct and indirect therapeutic use of human tissue engineered myocardium.

Cardiac regenerative medicine

Severe heart failure is associated with damage to the myocardium that is irreversible with current medical therapies. Recent experimental and clinical studies, however, have opened the possibility of solving many of the associated problems, making this an exciting and tangible goal. There are many potential cell sources for regenerative cardiac medicine, including bone marrow stem cells, endothelial progenitor cells, skeletal myocytes, adult cardiac stem cells, and embryonic stem (ES) cells. Although ES cells are highly proliferative and suitable for mass production, they are not autologous, and an efficient protocol is yet to be established to ensure selective cardiomyocyte induction. Recent studies have successfully established inducible pluripotent stem (iPS) cells from mouse and human fibroblasts by the gene transfer of 4 transcription factors that are strongly expressed in ES cells: Oct3/4, Sox2, Klf4 and c-Myc. iPS cells can differentiate into all 3 germ layer-derived cells and are syngeneic, indicating that they can become an ideal cell source for regenerative medicine. Despite these successes, the accumulating evidence from fields as diverse as developmental biology, stem cell biology and tissue engineering must be integrated to achieve the full potential of cardiac regenerative medicine.

Extracellular matrix profiles in the progression to heart failure. European Young Physiologists Symposium Keynote Lecture-Bratislava 2007

The myocardial extracellular matrix (ECM), which preserves the geometry and integrity of the myocardium, is a dynamic structure whose component proteins are maintained by a finely controlled homeostatic balance between deposition and degradation. One of the key targets in cardiology is the elucidation of the molecular mechanisms which mediate pathological remodelling of this matrix causing the transition from compensatory hypertrophy to congestive decompensated heart failure. In response to injury or increased workload, cardiac remodelling including myocyte hypertrophy, develops as the heart attempts to compensate for increased wall stresses. Persistence of these stresses over extended time periods leads to disruption of ECM homeostasis resulting in irreversible maladaptive cardiac remodelling, ventricular dilatation and finally heart failure. ECM remodelling is regulated by the matrix metalloproteinases (MMPs) and their endogenous inhibitors (TIMPs). Clinical studies and experimental models of cardiac disease states have reported alterations in the balance between the MMPs and TIMPs in the failing heart and crucially at intermediate time points in the progression to failure. This article reviews the recent clinical, genetic and experimental approaches employed to compare ECM, MMP and TIMP profiles in healthy, compensated and failing hearts and identifies common themes in the perturbation of ECM homeostasis in the transition to heart failure.

Stemming heart failure with cardiac- or reprogrammed-stem cells

Despite extensive efforts to control myocyte growth by genetic targeting of the cell cycle machinery and small molecules for cardiac repair, adult myocytes themselves appeared to divide a limited number of times in response to a variety of cardiac muscle stresses. Rare tissue-resident stem cells are thought to exist in many adult organs that are capable of self-renewal and differentiation and possess a range of actions that are potentially therapeutic. Recent studies suggest that a population of cardiac stem cells (CSCs) is maintained after cardiac development in the adult heart in mammals including human beings; however, homeostatic cardiomyocyte replacement might be stem cell-dependent, and functional myocardial regeneration after cardiac muscle damage is not yet considered as sufficient to fully maintain or reconstitute the cardiovascular system and function. Although it is clear that adult CSCs have limitations in their capabilities to proliferate extensively and differentiate in response to injury in vivo for replenishing mature car-diomyocytes and potentially function as resident stem cells. Transplantation of CSCs expanded ex vivo seems to require an integrated strategy of cell growth-enhancing factor(s) and tissue engineering technologies to support the donor cell survival and subsequent proliferation and differentiation in the host microenvironment. There has been substantial interest regarding the evidence that mammalian fibroblasts can be genetically reprogrammed to induced pluripotent stem (iPS) cells, which closely resemble embryonic stem (ES) cell properties capable of differentiating into functional cardiomyocytes, and these cells may provide an alternative cell source for generating patient-specific CSCs for therapeutic applications.

Drawbacks to stem cell therapy in cardiovascular diseases

Stem cells seem to have unlimited potential for repairing injured tissues derived from cardiovascular diseases. Much as the initial euphoria over preclinical models has ushered in some skepticism, several reports have advised caution against over exuberance, as cellular therapy has both theoretical and reported safety concerns. Embryonic stem cells, skeletal myoblasts, bone marrow-derived stem cells and mesenchymal stem cells are current candidates for cell therapy in end-stage cardiovascular diseases. However, before large-scale clinical trials can take place, a few safety concerns have to be clarified, such as atherogenesis, postangioplasty or stenting restenosis, tumorigenesis, stem cell metastasis, stem cell-mobilized cytokine-related complications and arrhythmogenesis. In this review, potential ways to overcome these issues are discussed, including medical and gene manipulations, dedicated cell-purification techniques, antiarrhythmic cell therapy design, and new concepts such as using natural constructs. With these safety issues under control, stem cell therapy still has a promising future in the next decade.

Synthetic morphology: prospects for engineered, self-constructing anatomies

This paper outlines prospects for applying the emerging techniques of synthetic biology to the field of anatomy, with the aim of programming cells to organize themselves into specific, novel arrangements, structures and tissues. There are two main reasons why developing this hybrid discipline--synthetic morphology--would be useful. The first is that having a way to engineer self-constructing assemblies of cells would provide a powerful means of tissue engineering for clinical use in surgery and regenerative medicine. The second is that construction of simple novel systems according to theories of morphogenesis gained from study of real embryos will provide a means of testing those theories rigorously, something that is very difficult to do by manipulation of complex embryos. This paper sets out the engineering requirements for synthetic morphology, which include the development of a library of sensor modules, regulatory modules and effector modules that can be connected functionally within cells. A substantial number of sensor and regulatory modules already exist and this paper argues that some potential effector modules have already been identified. The necessary library may therefore be within reach. The paper ends by suggesting a set of challenges, ranging from simple to complex, the achievement of which would provide valuable proofs of concept.

Myocardial tissue engineering

INTRODUCTION

Regeneration of the infarcted myocardium after a heart attack is one of the most challenging aspects in tissue engineering. Suitable cell sources and optimized biocompatible materials must be identified.

SOURCES OF DATA

In this review, we briefly discuss the current therapeutic options available to patients with heart failure post-myocardial infarction. We describe the various strategies currently proposed to encourage myocardial regeneration, with focus on the achievements in myocardial tissue engineering (MTE). We report on the current cell types, materials and methods being investigated for developing a tissue-engineered myocardial construct.

AREAS OF AGREEMENT

Generally, there is agreement that a 'vehicle' is required to transport cells to the infarcted heart to help myocardial repair and regeneration.

AREAS OF CONTROVERSY

Suitable cell source, biomaterials, cell environment and implantation time post-infarction remain obstacles in the field of MTE.

GROWING POINTS

Research is being focused on optimizing natural and synthetic biomaterials for tissue engineering. The type of cell and its origin (autologous or derived from embryonic stem cells), cell density and method of cell delivery are also being explored.

AREAS TIMELY FOR DEVELOPING RESEARCH

The possibility is being explored that materials may not only act as a support for the delivered cell implants, but may also add value by changing cell survival, maturation or integration, or by prevention of mechanical and electrical remodelling of the failing heart.