Scientists at the Institute of Molecular Biotechnology (IMBA) of the Austrian Academy of Sciences have developed a multi-chamber organoid that mirrors the heart’s intricate structure. The human cardioid platform recapitulates the development of all major embryonic heart compartments, including right and left ventricles, atria, outflow tract, and atrioventricular canal. The developers suggest the technology will help scientists generate a screening platform for use in drug development, toxicology studies, and to help understand heart development.

Headed by Sasha Mendjan, PhD, the IMBA team reported on their development in Cell, in a paper titled, “Multi-chamber cardioids unravel human heart development and cardiac defects.” In their paper, the team concluded, “… we established a multi-chamber cardioid platform that unravels how interacting chambers coordinate contractions and how mutations, drugs, and environmental factors impact specific regions of the developing heart.”

Cardiovascular disease is the leading cause of death worldwide, but there are only a few new therapies on the horizon, and development of new treatments faces a bottleneck, in that there is no physiological model of the entire human heart. Similarly, congenital heart disease (CHD) is the most common human development birth defect, and the most prevalent cause of embryonic and fetal mortality, the authors continued. Again, therapies are few and far between, as we know little why such defects arise. For about 56% of diagnosed CHD cases, the underlying cause is unknown but is assumed to originate from undiscovered genetic mutations, environmental factors, or a combination of both, the authors wrote. “ … because the human embryonic heart is inaccessible and the impacts of mutations, drugs, and environmental factors on the specialized functions of different heart compartments are not captured by in vitro models, determining the underlying causes is difficult.”

What is missing in understanding both heart disease and cardiac malformations is a model comprising the major regions of the human heart, the investigators continued. “To identify possible causes and preventive measures, we need models encompassing all compartments of the developing human heart.” Mendjan and the IMBA team have now developed what they suggest is the first physiological organoid model that includes all the principal developing heart structures, allowing researchers to study cardiac disease and development.

In 2021, the Mendjan lab reported on the first chamber-like organoid heart model formed from human induced pluripotent stem cells. These self-organizing cardioids recapitulated the development of the heart’s left ventricular chamber in the very early days of embryogenesis. “These cardioids were a proof-of-principle and an important step forward,” said Mendjan. “While most adult diseases affect the left ventricle, which pumps oxygenated blood through the body, congenital defects affect mostly other heart regions essential to establish and maintain circulation.”

For their new study, the IMBA team expanded on the previous work. The researchers first derived organoid models of each developing heart structure individually. “Then we asked: If we let all these organoids co-develop together, do we get a heart model that co-ordinately beats like the early human heart?” Mendjan explained.

After growing left and right ventricular and atrial organoids together, the researchers were in for a surprise. “Indeed, an electrical signal spread from the atrium to the left and then the right ventricular chambers—just like in early fetal heart development in animals,” Mendjan commented. “We now observed this fundamental process in a human heart model for the first time, with all its chambers.”  The authors further wrote, “When we placed different cardioid subtypes together on day 3.5, they co-developed to form a structural connection after 24 h. Still, they maintained their distinct identities and compartments … Cardioids only co-developed when combined on day 3.5, electrochemically connected, and contracted in a coordinated manner by day 6.5, demonstrating functional interaction.”

While the previous cardioid model allowed the researchers to study the chamber’s shape and tissue organization, the newly developed multi-chamber cardioids enabled them to go beyond, studying how regional gene expression differences lead to specific chamber contraction patterns and intricate communication between them.

Cross-section of a multi-chamber cardioid, with the atrial organoid in cyan, the left ventricular organoid in white and the right ventricular organoid in magenta. Cross-section emphasises the cavities inside the multi-chamber cardioid.
Cross-section of a multi-chamber cardioid, with the atrial organoid in cyan, the left ventricular organoid in white, and the right ventricular organoid in magenta. Cross-section emphasizes the cavities inside the multi-chamber cardioid. [Tobias Illmer/IMBA]

The researchers have already gained insight into early heart development, particularly how the human heart starts beating—which has not been understood so far. “We saw that as the organoid chambers developed, they performed an intricate dance of lead and follow,” noted Alison Deyett, a PhD student in the Mendjan group and one of the study’s first authors. “At first, the left ventricular chamber leads the budding right ventricular and atrium chambers at its rhythm. Then, as the atrium develops—two days later—the ventricles follow the atrial lead. This mirrors what is seen in animals before the final leaders, the pacemakers, control the heart rhythm.”

In addition to studying human development, multi-chamber cardioids enable researchers to investigate chamber-specific defects. In a proof-of-principle, the Mendjan team set up a screening platform for defects, in which they study how known teratogens and mutations affect hundreds of heart organoids simultaneously. “Currently, we still miss human systems to investigate, in a high-throughput and easily quantifiable manner, whether teratogens cause compartment-specific cardiac defects,” they pointed out.

Thalidomide, a well-known teratogen in humans, and retinoid derivatives—used in treatments against leukemia, psoriasis, and acne—are known to cause severe heart defects in the fetus. Both teratogens induced similar, severe compartment-specific defects in the heart organoids. In a similar way, mutations in three cardiac transcription factor (TF) genes led to chamber-specific defects seen in human development. “… the cardioid platform can be employed to dissect human stage- and compartment-specific genetic cardiac defects of specification, morphogenesis, and function without compensatory mechanisms present in the embryo,” they stated in summary. “Our tests show that multi-chamber cardioids recapitulate embryonic heart development and can uncover disruptive effects on the whole heart with high specificity. We do this using a holistic approach, looking at multiple readouts simultaneously,” Mendjan further noted.

Multi-chambered cardioids can be cultured in high-throughput in different combinations. Close-up shows the cross-section of a multi-chamber cardioid, with the atrial organoid in cyan, the left ventricular organoid in white and the right ventricular organoid in magenta.
Multi-chambered cardioids can be cultured in high-throughput in different combinations. Close-up shows the cross-section of a multi-chamber cardioid, with the atrial organoid in cyan, the left ventricular organoid in white, and the right ventricular organoid in magenta. [Tobias Ilmer, Alison Ann Deyett/IMBA]

In the future, multi-chamber heart organoids could be used for toxicology studies and to develop new drugs with heart chamber-specific effects. “… the platform allows us to screen for specific drug effects in single cardioids, within interacting subcompartments, and in a whole multi-chamber cardioid,” they claimed. “For example, atrial arrhythmias are widespread, but we currently don’t have good drugs to treat it,” Mendjan added. “One reason is that no models existed comprising all regions of the developing heart working in a coordinated manner—so far.”  And although heart defects are common, including as the leading cause of miscarriages, the individual origin often remains unknown.

Heart organoids developed from patient-derived stem cells could, in the future, give insight into the developmental defect and how it may be treated and prevented. The Mendjan group is particularly interested in using multi-chamber heart organoids to understand heart development further. “We now have a basis to investigate the heart’s further growth and regenerative potential.”

“Together, these results validate that we can discern early developmental effects of known teratogenic and arrhythmogenic drugs and therapeutic agents in a human multi-compartment cardiac platform and relate these to cardiac defects observed in patients,” the investigators concluded. “Thus, our work has broad implications for studying the effects on human cardiac biology in contexts ranging from therapeutic development to environmental studies.”

IMBA has granted an exclusive license of the multi-chamber cardiac organoid technology to, an IMBA spin-out co-founded by Mendjan.

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