Disease modeling and functional screening using engineered heart tissue

Highlights

• A concise historical perspective on the evolution of cardiac tissue engineering.

• Reveal the potential of bioengineering for studying cellular interactions.

• Discuss advantages of human engineered heart tissue for disease modeling.

• Summarize strategies for drug screening using engineered heart tissue.

Recent advances in cardiac tissue engineering and pluripotent stem cell technologies are providing unprecedented access to human heart tissue with in vivo-like structural and functional properties. Engineered heart tissue (EHT) is emerging as a powerful technology platform for regenerative medicine, as well as for in vitro modeling of human physiological and pathophysiological processes. Here, we discuss the potential of EHT for studying cellular interactions within the heart and for modeling genetic and environmental causes of cardiovascular disease. In addition, we highlight the utility of EHT for drug screening and the identification of novel therapeutic modalities.

Introduction

The translation of promising drug candidates into clinical therapeutics is a time-consuming and costly process, with 9 out of 10 candidates failing to progress from Phase I trials to clinical approval [1, 2]. Furthermore, many targets identified using cell culture or small-animal models fail to translate in pre-clinical testing due to lack of efficacy. There is a need to improve our current methods of modeling human biology and disease, which may lead to the identification of better candidates for clinical translation. One approach is to use tissue engineering to develop 3D tissues and screening platforms that more closely mimic in vivo physiological and pathophysiological processes. Currently, tissue engineered products can be stratified into two primary applications: firstly, making new tissue for implantation and replacement/repair of lost or damaged organs, or secondly, creation of engineered constructs to mimic the native tissue as an in vitro model system. It is important to highlight that these two different applications may require very different tissue engineering strategies. Implantation requires integration with the host, sufficient tissue to replace the lost or damaged tissue, a safe and defined product and, additionally, may require factors to tailor the potentially hostile new environment and ensure long-term functionality [3]. However, when creating a tissue as an in vitro model system, the tissue needs to re-capitulate the functional and physiological properties of interest, and the tissue needs to be amenable to manipulation (e.g. genetic or pharmacological manipulation) to study the effects of biological factors. In addition, it may also be beneficial for these model systems to be scaled down to sizes amenable to screening to reduce the costs associated with producing the tissue. Whilst a model tissue will never be exactly the same as its in vivo counterpart, a good model only needs to be well suited to addressing the particular physiological or disease processes in question. As the famous statistician George E.P. Box once wrote, ‘essentially all models are wrong, but some are useful’ [4].

Recent advances in the re-capitulation of in vivo-like structure and function with in vitro organoids is providing more accurate models of human biology and disease. The term organoid was originally used to describe tumors, which ‘resemble an organ in structural appearance or qualities’ (Marriam-Webster Medical Dictionary). However, it is now also widely used to describe 3D engineered tissues or cultures that re-capitulate their in vivo counterparts, ‘ex vivo multicellular fragments that contain the major cell types of a particular organ and approximate its in vivo organization’ [5]. The development of multiple organ systems is becoming widespread with: liver [6], intestinal [7, 8], stomach [9], optic cup [10], cerebral [11, 12], lung [13], kidney [14], and heart [15^••] organoids having been developed and used as model systems. The development of engineered heart tissue (EHT) has progressed steadily over 20 years, making it one of the most studied organoid models to date.

Tissue engineering was originally dominated by the ideology of seeding polymer constructs with cells. In the cardiac field, this approach was one of the first to be employed [16, 17] and has been widely used over time [18, 19, 20, 21, 22] (reviewed in [23]). Additionally, other methodologies such as micro-patterns/devices [24, 25, 26], cell sheets [27], and de-cellularised hearts [28] have also been used to create cardiomyocyte cultures for modeling or regenerative approaches. However, most studies have achieved in vivo-like cardiac organoids by embedding the cells in extracellular matrix hydrogels, first pioneered by the Eschenhagen group [29]. Multiple natural matrices have now been used to embed the cells, the most widespread being collagen I, Matrigel and/or fibrin [15••, 30••, 31••, 32, 33]. Embedding the cells in a bioactive hydrogel enables the cells to not only re-arrange to form organoid structures [15^••], but also enables the cells themselves, rather than the substrate [18], to dictate the mechanical properties of the EHT [34], which is critical for optimal cardiomyocyte function [34, 35]. Additionally, mechanical loading directly imparts load onto the cells and cardiomyocytes in the EHT [34], and greatly improves the structure and function of EHT [36, 37]. The in vivo-like properties, and the recent ability to make human EHT using pluripotent stem cell-derived cardiac cells [31••, 38, 39, 40, 41••, 42] (Figure 1), have made EHTs an attractive platform for multiple modeling applications over the past few years, as summarized and discussed in this review. Please note that while the term EHT specifically refers to the fabrication method depicted in Figure 1, we have used the term EHT to describe all engineered culture formats in this review for simplicity.