Cardiovascular Organoids/3D Models Review Series: An Introduction

Even with the vast amount of research focused on uncovering the etiology and molecular mechanisms that drive the progression of cardiovascular disease, along with concerted public health efforts to mitigate environmental contributors, and the groundbreaking drug discoveries made in recent years, heart disease is still the leading cause of death in the United States.¹ This year marks the Centennial anniversary of the founding of the American Heart Association, as well as the platinum anniversary of the publication of Circulation Research, and looking back to our first issue we see that the topics of 70 years ago are just as relevant today. The manuscripts in that first issue covered studies that investigated the differences in vasoreactivity of the vascular beds,² the impact of estrogen in atherosclerosis,³ the importance of ATP in cardiac contraction,⁴ and contractility of what we now know to be vascular smooth muscle cells.⁵ Scrolling through these seminal studies, it is striking to see the variety of models utilized—humans, dogs, cats, and chickens—while not a single study used rodents, and the methods to culture vascular cells in vitro had not been established yet. While larger mammals more fully recapitulate human biology there are many reasons why research has moved away from them. Studies utilizing mice began in earnest in the early 1900’s but they did not become the primary in vivo system of study until the early 1990s; their advantages owing in part to the advances in genetic engineering, the relative speed of their reproduction, and lower cost and smaller footprint to house these smaller mammals.⁶ Mice are still less costly and easier to house and care for than larger mammals, but they are by no means inexpensive. And in many cases, such as with monogenetic disease and age-related vascular remodeling, mice do not naturally recapitulate the pathogenesis and time course of the human maladies,^7,8 and these differences contribute to the high failure rate of novel therapies that move from the lab to the clinic.⁹

While animal models are indeed useful and essential tools, how can we fill in the gaps of their limitations? One answer lies in our ability to coax cells to self-assemble into three dimensional structures that can recapitulate human biology and physiology, which can serve as a cheaper and more efficient means for modelling vascular disease and for things like drug testing. In this review series we cover the current state of knowledge in the field of cardiovascular organoids and 3D cell culture models.

In Modelling heart diseases on chip: advantages and future opportunities Mourad et al¹⁰ discuss the current state of heart-on-a-chip (HOC) technology. HOC tech utilizes scaffolds built by techniques such as 3D printing, electrospinning, or thermoplastic micromilling, which provide an external structure for cells to adhere to and build upon. Sensors can be implanted in scaffolds to measure functions such as contractility, and the closed system enables fluids to be circulated. The shapes embedded in the scaffolds may also direct cell behavior and phenotype. That these scaffolds are manufactured enable stringent reproducibility of the final shape of the tissues made, and thus may be ideal for drug testing or determining therapies in personalized medicine approaches. Mourad et al cover the latest information on the use of HOC in modeling arrhythmia, cardiac fibrosis, myocardial infarction and ischemia and reperfusion injury, inherited cardiomyopathies, and COVID-associated myocarditis.

During development, cardiac progenitor cells divide and differentiate to produce cardiomyocytes (CMs) that proliferate and grow in size. As they mature CMs elongate and organize their contractile apparatus and mitochondria, and couple to their neighboring CMs. With this maturation a switch is thrown, and adult cardiomyocytes are rendered unable to proliferate. This proliferation-maturation dichotomy (PMD) is the topic addressed in the review by Singh et al, Proliferation and Maturation: Janus and the art of cardiac tissue engineering.¹¹ In it they present our current understanding of the cellular, molecular, and energetic changes that occur in PMD, as well as how other cells, substrates, and environments impact this transition. They present the case that a more comprehensive understanding of PMD and the switch from an immature to mature CM will enable more physiologically relevant and reproducible 3D models and organoid tissue.

Within this reading audience, perhaps we can all agree that the heart is the most important organ, however, without the blood vessels the heart cannot do its job. In Blood vessel organoids for development and disease¹² Salewskij and Penninger discuss the history that led to advances that enabled the generation of vascular organoids. In their review they cover the complexity of modeling organ-specific vasculature and emphasize there is still much to learn about the interplay between various cell types involved and the difference between say, the endothelial cells residing in the arteries vs the veins vs the lymphatics.

Myocardial and endocardial tissues are derived from the mesoderm however developmental and lineage-tracing studies show that the endoderm helps to inform cardiac tissue differentiation. In Alliance of Heart and Endoderm: Multilineage Organoids to Model Co-development¹³ Ng et al describe what is known regarding the coordination between the cardiac mesoderm and endoderm during development, and discuss how leveraging this knowledge is critical for enhancing the development of multilineage organoids that more fully recapitulate human cardiac cell biology and heart physiology.

Together these four articles provide a comprehensive overview of the most recent advances in the field of cardiovascular organoids and 3D culturing methods, the first review series on this burgeoning technology in Circulation Research. Importantly, the compiling of this series serves to identify gaps in knowledge regarding the state of 3D tissue modeling and organoid development and we hope it also inspires others to consider using these systems in their own research.