Engineered Cardiac Tissues Generated from Immature Cardiac and Stem Cell-Derived Cells: Multiple Approaches and Outcomes
The translation of in vitro engineered cardiac tissues (ECTs) from immature cardiac and stem cell-derived cells toward clinical therapies is benefiting from the following major advances: (1) rapid progress in the generation of immature cardiac cells from the cardiac and noncardiac cells of multiple species including normal and disease human cells, (2) incorporation of multiple cell lineages into 3D tissues, (3) multiple scalable 3D formulations including injectable gels and implantable tissues, and (4) insights into the regulation of cardiomyocyte proliferation and functional maturation. These advances are based on insights gained from investigating the regulation of cardiac morphogenesis and adaptation. Our lab continues to explore this approach, including changes in gene expression that occur in response to mechanical loading and tyrosine kinase inhibition, the incorporation of vascular fragments into ECTs, and the fabrication of porous implantable electrical sensors for in vitro conditioning and postimplantation testing. Significant challenges remain including optimizing ECT survival postimplantation and limited evidence of ECT functional coupling to the recipient myocardium. One clear focus of current research is the optimization and expansion of the cellular constituents, including CM, required for clinical-grade ECTs. Another major area of investigation will be large animal preclinical models that more accurately represent human CV failure and that can generate data in support of regulatory approval for phase I human clinical trials. The generation of reproducible human ECTs creates the opportunity to develop in vitro myocardial surrogate tissues for novel drug therapeutics and toxicity assays.
KeywordsCardiomyocytes Cardiac repair and regeneration Engineered cardiac tissues Stem cells
Following significant cardiac injury, the postnatal human heart lacks the ability to restore lost myocardium, resulting in an adaptive response that often ultimately leads to progressive cardiac dysfunction, morbidity, and mortality. There are currently many strategies for cardiac “cellular therapy” undergoing both preclinical and clinical trials [1, 2, 3, 4]. While there has been modest success with improvement in cardiac function in some of the early human clinical trials, it is clear that injected or implanted cells do not survive, and functional improvement occurs via paracrine mechanisms. In contrast, rapid advances in tissue engineering over the past two decades have resulted in the generation of functional, multicellular, 3D cardiac tissues with the potential for translation to human cardiac repair and regeneration [5, 6, 7, 8]. This chapter provides a concise overview of some of the key issues in the generation, maturation, and translation of these engineered cardiac tissues (ECTs).
46.2 A Broad View of Bioengineering Cardiac Tissues
The bioengineering process for complex tissues begins with an understanding of the cellular and noncellular constituents of the target tissue . For replacement myocardium, the major cellular constituents include cardiomyocytes (CM), fibroblasts, and vessel-associated cells. There are also numerous extracellular matrix (ECM) constituents including collagen, fibronectin, laminin, and multiple growth factors bound within the ECM. Of course, the neonatal myocardium and the adult myocardium have vastly different profiles for cellular and noncellular constituents, architecture, and biomechanical properties. While all currently successful ECTs are constructed using immature cells and simplified ECM components, the target tissue is usually mature myocardium. The success of ECT survival, integration, and functional maturation depends on the ability of these ECT constituents to acquire “mature” fates.
46.3 Immature Cells for Engineered Cardiac Tissues
46.4 Various Formulations for Engineered Cardiac Tissues
The constructs used for cardiac tissue repair include the implantation of multicellular cardiospheres , various formulations of 2D cellular sheets [19, 20, 21, 22], and various formulations of 3D tissues [5, 6, 7, 8, 13]. The composition of the noncellular constituents varies from minimal constituents for cardiosphere clusters to a range of ECM components [23, 24] and growth factors [25, 26, 27] selected for their ability to facilitate CM survival and functional maturation. Some of the ECT formulations allow for in vitro preconditioning strategies that can stimulate cell proliferation and/or maturation [5, 6]. While there can be wide variation in the formulation of ECTs used for preclinical studies, all constituents used to generate ECTs for human use are requir ed to conform to strict FDA regulatory guidelines that include the elimination of all sources for potential infectious agents and/or toxins and the generation of clinical-use materials using good manufacturing practices .
46.5 In Vitro ECT Findings
46.6 In Vivo ECT Findings
Second, the in vivo results are often reported after relatively short periods that may not reflect long-term, sustained functional recovery . Finally, the acute surgical models of cardiac injury in preclinical models often do not fully represent the human disease state with medical comorbidities and recurrent episodes of ischemia/injury. To date there are too few studies in large animal models to validate the feasibility of scaling up CM and ECT production for cardiac repair and to compare direct cell delivery strategies to ECT implantation strategies for short- and longer-term efficacy.
46.7 Future Directions
One clear focus of current and future research is the optimization and expansion of the cellular constituents, including CM, required for clinical-grade ECTs. Ultimately, these cells will need to be from human sources and have minimal immunogenic profiles. Another major area of investigation will be large animal preclinical models that more accurately represent human CV failure and that can generate data in support of regulatory approval for phase I human clinical trials. Related to these large animal models will be innovations in the ability to manufacture large-scale ECTs and to implant them with minimally invasive techniques. Although it is beyond the scope of this chapter, the generation of reproducible human ECTs creates the opportunity to develop in vitro myocardial surrogate tissue s for novel drug therapeutics and toxicity assays.
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