- Open Access
Human iPS cell-engineered three-dimensional cardiac tissues perfused by capillary networks between host and graft
© The Author(s) 2018
- Received: 17 June 2018
- Accepted: 7 August 2018
- Published: 10 October 2018
Stem cell-based cardiac regenerative therapy is expected to be a promising strategy for the treatment of severe heart diseases. Pluripotent stem cells enabled us to reconstruct regenerated myocardium in injured hearts as an engineered tissue aiming for cardiac regeneration. To establish a long-term survival of transplanted three-dimensional (3D) engineered heart tissues in vivo, it is indispensable to induce microcapillaries into the engineered tissues after transplantation. Using temperature-responsive culture surface, we have developed pluripotent stem cell-derived cardiac tissue sheets including multiple cardiac cell lineages. The application of gelatin hydrogel microsphere between the cell sheet stacks enabled us to generate thick stacked cell sheets with functional vascular network in vivo. Another technology to generate 3D engineered cardiac tissues using cardiac cells and biomaterials also validated successful induction of vascular network originated from both host and graft-derived vascular cells.
- Capillary network
- 3D cardiac tissues
- Human iPS cells
- Cardiac regeneration
Stem cell-based cardiac regeneration is a rapidly expanding paradigm to deliver therapeutic approaches for severe cardiac disorders resistant to current therapies [1, 2]. The discovery of human induced pluripotent stem cells (iPSCs)  opened the door toward in vitro formulation of human myocardium aiming for cardiac regeneration. Engineered three-dimensional (3D) myocardial tissue constructs generated from human iPSCs including multiple cardiovascular lineage constructs are more likely to replicate the dynamic organization and function of native myocardium and have emerged as a robust methodology to accomplish myocardial regeneration in animal heart disease models [4–7]. In the context of cell transplantation to the heart, the 3D construct is reported to be advantageous over single cell injection into the myocardium because of the avoidance of mechanical loss related to the cell injection  and/or the higher survival efficiency in vivo .
In addition with the biophysical advantages of 3D structure in cell retainment after transplantation as described above, the introduction of microcapillaries is an important factor for long-term survival of the transplanted tissue. It is assumed that the transplanted tissue survives only through the direct diffusion of oxygen and nutrition at the initial stage of the transplantation, and vascular formation perfusing the whole engineered tissue would be indispensable for the long-term survival. It means that the successful cardiac regenerative strategy requires re-vascularization mechanisms to validate long-term myocardial regeneration.
Cell sheet-based thick cardiac tissues
Cell sheet formulation is one of the principal methods to generate 3D tissues from single cells . Okano et al. reported a novel method to generate cell sheets using poly (N-isopropylacrylamide) (PIPAAm), a temperature-responsive polymer which changes the property of culture surface from hydrophobic to hydrophilic along with the lowering of the temperature which enables us to collect the confluent cell culture as a cell-sheet shape preserving attachment molecule and extracellular proteins without enzymatical digestion or physical damage . Using this method, we have reported a formulation of human iPS cell-derived “cardiac tissue sheet (CTS)” including multiple cardiac cell lineages including cardiomyocytes and vascular cells (vascular endothelial cells, and mural cells), and a successful human myocardial regeneration and functional recovery mainly mediated by paracrine mechanisms such as angiogenesis in a rat myocardial infarction model . However, the extent of the engraftment was not fully satisfactory requiring additional strategies to enhance the regenerative capacity.
Another method to induce vascular networks into the cell sheet-based 3D structures is to generate “vascularized cardiac cell sheets” using bioreactors [14, 15]. In the study, the authors developed a bioreactor system using a femoral muscle-based and a synthetic collagen gel-based vascular beds which can provide fair perfusion throughout the 12-layered cell sheets. This novel system may also serve as a technology to induce vascular networks inside the engineered tissues.
Biomaterial-supported engineered cardiac tissue
Another format of 3D cardia tissue is biomaterials-supported engineered tissues which strongly support the structural stiffness of the artificial tissue structure [4, 16–18]. Taking advantages of a 3D cardiac tissue formation technology using rat  or chick  embryonic cardiac cells and a combination of biomaterials (collagen I, Matrigel), we have developed self-pulsating human iPS-derived engineered cardiac tissues (hiPSC-ECTs) with cylindrical  and mesh-like  shapes. The incorporation of multiple cardiac cell lineages into the hiPSC-ECTs enhanced the tissue function including tissue stiffness evaluated by the measurement of Young’s modulus, cardiomyocyte alignment, and sarcomeric ultrastructural maturation shown by transmission electron microscopy. The formulation of multiple lineages also validated a better force-frequency relationship which is known to be a parameter for tissue maturation .
Mechanisms and significance of perfusion among engineered 3D constructs in vivo
In the present review, we introduced various strategies to induce microcapillaries for 3D engineered cardiac tissues aiming to promote the effectiveness of stem cell-based cardiac regenerative therapy. Further investigations for the post-transplantation vascularization are anticipated.
HM and JKY wrote the manuscript. Both authors read and approved the final manuscript.
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