Cardiomyocytes from Adult Stem Cells
The stem cell division of Cellectis is working with human pluripotent stem cells to create a cardiomyocyte platform for drug-discovery screening, said Peter Sartipy, Ph.D., senior principal scientist.
The process of creating induced pluripotent stem (iPS) cells involves reprogramming an adult cell to behave like an embryonic stem cell. Embryonic stem cells have the ability to differentiate into any type of cell, but they lose that ability as they mature.
It has been discovered only recently that it is possible to return an adult cell to a state of pluripotency resembling an embryonic cell. The trick, as described by scientists from Kyoto University in 2006, is to get the adult cell to express four genes that are not normally expressed in adult cells.
Cardiomyocytes are an ideal focus for iPS cell development, because it is very difficult to isolate them from primary human tissue. According to Dr. Sartipy, controlling the culture conditions in order to activate or block specific pathways at specific time points is the key to going from undifferentiated cells to early mesoderm cells, then to cardiac mesoderm, to cardiac precursors, and finally to fully differentiated cardiomyocytes. Small molecule growth factors are used at different stages of the process.
Cellectis compared the iPS cardiomyocytes to the current gold standard for safety pharmacology testing—the ex vivo rabbit model—as well as the dog model.
“The stem-cell-derived cardiomyocytes are giving representative results in nice correlation to the in vivo models,” said Dr. Sartipy, noting that the iPS cardiomyocyte system has the potential to replace or reduce the use of those animal models for testing of new compounds in development.
Cellectis has further preliminary data showing how the cells can be used to test drugs in a model of cardiac hypertrophy. Dr. Sartipy explained that scale up was a challenge that Cellectis successfully overcame.
Going forward, Cellectis will strive to create cells that fully resemble the adult phenotype in every way.
“The state of the art in this area is that you can produce cardiomyocytes with good functionality, but it is still not completely identical to what you see in vivo in humans,” said Dr. Sartipy.
Cardiomyocytes from Embryonic Cells
Cardiomyocytes have also been the focus of development at GE Healthcare, whose scientists have been exploring the use of the cells in drug discovery, drug development, and toxicology applications. Robert Graves, Ph.D., senior applications scientist, described toxicology testing of the company’s embryonic stem cell-derived cardiomyocytes.
Dr. Graves said that the currently used tests for cardiotoxicity rely on animal cell models, ex vivo tests, or animal tissues. “It’s well-known that there are problems with the predictivity of animal cells. At times it’s only 50 percent. You might as well toss a coin.”
Cardiomyocytes derived from human embryonic cells would be accurately predictive, as well as being a renewable resource producing a consistent supply of cells that can be used across a range of platforms, he contended.
Cardiotoxicity occurs through a number of mechanisms that interfere with heart function, such as blocking the electrophysiology, or the contractility, of the heart. Drugs can also be toxic to the mitochondria, which the heart requires in large quantities to supply its energy needs.
GE’s system makes use of high-content analysis through imaging. For example, changes in the size and shape of the mitochondria can indicate toxicity affecting these organelles.
Four-color imaging assays extract as many as 20 parameters from the assays. The process yields the same kind of toxicity data as a conventional assay, but it also has the potential to indicate the mechanism of toxicity.
In a collaborative study with Genentech, GE used its cardiomyocytes in conjunction with high-content analysis to look at specific sets of compounds. Data from the first set of 26 showed excellent correlation between assay and historical data, said Dr. Graves.
In a blinded analysis, GE scientists graded the compounds according to the assay parameters collected. Compounds recorded in the literature as having cardiotoxicity were in a group scored with the highest toxicity classification.
“We believe this was an indication that this cardiomyocyte-based assay was going to be more predictive than standard cell-based assays,” said Dr. Graves. “There’s a need for a sea change in toxicology, to take a more holistic approach, and not focus on single issues like hERG assays for cardiotoxicology.”