Columbia University researchers recently generated beating cardiac tissue from induced pluripotent stem cells, human cells that are able to differentiate into nearly any cell type. Using physical conditioning, the researchers produced samples with the hallmarks of mature heart tissue with just four weeks of cell culture. The work paves a concrete pathway to functional heart-on-a-chip platforms.
Organ-on-a-chip technologies, which replicate small samples of functional organ tissues on microfluidic chips for experiments, hold great promise for studying disease progression and drug toxicity. But one of the most important organs has been one of the most difficult to emulate: the heart.
Today, hearts-on-a-chip are moving closer to reality. Researchers recently generated beating cardiac tissue from induced pluripotent stem cells, human cells that are able to differentiate into nearly any cell type. Using physical conditioning, the scientists produced samples with the hallmarks of mature heart tissue with just four weeks of cell culture.
Their work offers a concrete pathway to functional heart-on-a-chip platforms. The researchers have started a company, TARA Biosystems, New York, NY, to create these cardiac models for drug toxicity testing and disease research.
TARA Biosystems is aiming to be “the place to go for cardiotoxic studies,” says Gordana Vunjak-Novakovic, a professor of biomedical engineering at Columbia University and co-founder of TARA Biosystems.
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While delivering tissue samples in a month is a boon to production, the platform also purports to generate more mature cardiac cells for experiments. Scientists have previously differentiated stem cells into cardiac cells, or cardiomyocytes. Those cardiomyocytes, however, function the way fetal heart tissue does, beating weakly and using energy inefficiently.
Mature cardiac cells, on the other hand, take on a more rod-like shape, express different genes, and handle calcium differently than their fetal counterparts. These factors all improve the heart’s function.
The difficulties of transitioning heart cells and tissue to maturity — a process that takes roughly nine months, or a bit more for a human fetus — prevent scientists from using heart tissue models to study the progression of adult heart disease or test how drugs interact with heart tissue.
To encourage stem cell-derived cardiomyocytes to mature, Vunjak-Novakovic and her team use a mix of electrical and mechanical signaling, beginning on day seven of cell culture. Rigorous conditioning pushes the cells to differentiate into a better emulation of cardiac tissue; timing is key.
“Only these young cells that are still plastic are able to respond,” says Kacey Ronaldson-Bouchard, a co-founder of TARA Biosystems. The TARA platform starts with a fibrin hydrogel seeded with cardiomyocytes, says Ronaldson-Bouchard. Each sample is roughly 10 millimeters long, 5 millimeters wide, and 1.5 millimeters thick.
Carbon fibers deliver electrical signals to each sample, which pushes the cells to beat the way a pacemaker regulates the heartbeat with rhythmic voltage.
And because the cells are situated in wells between two elastomeric pillars, each contraction requires the samples to push against the polymer columns. The pace of the electrical signaling and the stiffness of the pillars are based on native heart conditions.
Perhaps most importantly, this cardiomyocyte conditioning ramps up over time. The cells’ electrical signal starts around 2 Hz and gradually increases to 6 Hz. “By forcing [the cells] to work hard, we actually force them to build the ultrastructure they need,” Vunjak-Novakovic says.
The researchers used transmission electron microscopy to verify that cells conditioned this way take on appropriate ultrastructure, developing elongated muscle fibers. The correctly-timed electrical and mechanical stimulation drove cells to express similar genes to mature cells, building up their ultrastructure while also improving their efficiency. And these cardiomyocytes exhibit one of the hallmarks of mature cardiac tissue: When they contract more frequently, their beating is also more forceful.
Because these are mature cardiomyocytes that began as stem cells, this method produces samples that model the patient’s unique cardiac tissue. That allows for a personalized look at cardiac response to drugs, either to treat disease or to check for toxicity.
These stem cell-generated cardiomyocytes don’t produce quite as much force as Ronaldson-Bouchard would have expected, but she said that may not matter. Developing cells that capture essential heart functions quickly will probably be more of a priority for TARA’s clients.
“The longer you keep cells in culture, the more time and money you spend,” Ronaldson-Bouchard says. “So we want to reduce these times to make it more useful for industry and discovery.”
TARA’s platform has already shown promise for drug screening. TARA’s other cofounder, Milica Radisic, published a study last year using these types of cardiac tissue samples to examine kinase inhibitors, a family of drugs to treat cancer. The study was well-received; TARA raised $9 million of funding at the end of 2017.
The team is developing other tissues — skin, bone, liver — and linking them together, “so these tissues are connected into the platforms like Legos,” Vunjak-Novakovic says. The team hopes to eventually broaden their platform to other organs, moving beyond organ-on-a-chip into more comprehensive territory that approaches person-on-a-chip.
Vunjak-Novakovic’s lab has also developed a new type of stem cell treatment to regenerate heart muscle. Typically, most stem cells delivered to a beating heart wash away within two hours, and those that do form heart muscle often contract with different rhythm than the heart, potentially causing arrhythmia. Even so, stem cells deliver benefits to the heart. This has led researchers to develop the “hit and run” hypothesis that says the cells deliver key regulatory molecules before leaving the injury process.
“Consistent with this hypothesis, we postulated that the benefits of cell therapy of the heart could be coming from the secreted bioactive molecules, such as micro RNAs, rather than the cells themselves,” Vunjak-Novakovic says. “So we explored whether the benefits of cell therapy of the injured heart could be achieved without using the cells.”
She focused on cell secretions of extracellular vesicles, which are filled with micro RNAs and other biochemicals. She then applied the vesicles to a collagen patch to mice with damaged hearts for four weeks. The patch improved function and healing, and at the end of the experiment the damaged hearts were functioning as well as undamaged control hearts.
The vesicles are easy to isolate from heart cells and easy to freeze and store. They could be used immediately, unlike heart cells that take weeks or even months to grow. Since they do not use whole cells, regulatory approval might be easier as well.
Menaka Wilhelm is an independent technical writer.
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