Stem cell researchers and cardiologists from the University of Wisconsin-Madison have designed a new and improved protocol to turn embryonic and induced pluripotent stem cells into heart muscle cells.
The study leader, Sean Palecek, who is also professor of chemical and biological engineering at the University of Wisconsin-Madison, and his colleagues Timothy Kamp, professor of cardiology at UW School of Medicine and Public Health, and Xiaojun Lian, a UW graduate student, have developed a technique for efficient and abundant production of heart muscle cells. This technique will provide scientists a better and more abundant source of material for drug studies and a better model system to study diseases and heart pathologies.
Heart muscle cells (also known as cardiomyocytes) are essential cells that compose the beating heart. However, it is rather difficult to make large quantities of them. Typically, cultured heart muscle cells only survive or a short period time, which greatly complicates using them for any experiments or drug tests. Now, however, these UW researchers have devised an inexpensive method for developing an abundance of heart muscle cells in the laboratory.
Cardiologist Timothy Kamp explained: “Many forms of heart disease are due to the loss or death of functioning cardiomyocytes, so strategies to replace heart cells in the diseased heart continue to be of interest. For example, in a large heart attack up to 1 billion cardiomyocytes die. The heart has a limited ability to repair itself, so being able to supply large numbers of potentially patient-matched cardiomyocytes could help.”
Why is their method so much more efficient? The UW research team discovered that by changing a signaling pathway called Wnt pathway, they could guide the stem cells to differentiate into heart muscle cells. All they had to do was turn the Wnt pathway on and off at different times by using two small molecules.
The Wnt signaling pathway is an extremely common signaling pathway that exists in virtually all multicellular organisms and is used multiple times during development. Wnt signaling begins with the secretion of a small protein can a Wnt protein. Wnt proteins are produced by cells to send signals to nearby cells. When the cells receiving the signal are bound by the Wnt protein, a series of events are set into motion inside the cell. The receptor that binds the Wnt protein consists of a protein that is a member of the Frizzled gene family. Frizzled receptors bind the Wnt protein in combination with another protein called LRP. The binding of Wnt, LRP and Frizzled brings an internal protein called Disheveled to the membrane. Once Disheveled come to the membrane, it becomes activated. How this activation occurs in still unclear, but Disheveled inhibits GSK-3 (glycogen synthase kinase-3). GSK-3 normally attaches phosphate groups to a protein called beta-catenin. This phosphate group attachment marks beta-catenin for destruction, but once GSK-3 is inhibited by activated Disheveled, beta-catenin is no longer destroyed and when the levels of beta-catenin build up in the cytoplasm, it goes to the nucleus where it combines with another protein called TCF and regulates gene expression. Once again we see that a signal transduction pathway begins at the cell surface and results in changes in gene expression.
“Our protocol is more efficient and robust,” said Palecek. “We have been able to reliably generate greater than 80 percent cardiomyocytes in the final population while other methods produce about 30 percent cardiomyocytes with high batch-to-batch variability.”
Palacek continued: “The biggest advantage of our method is that it uses small molecule chemicals to regulate biological signals. It is completely defined, and therefore more reproducible. And the small molecules are much less expensive than protein growth factors.”
Kamp noted that the “fact that turning on and off one master signaling pathway in the cells can orchestrate the complex developmental dance completely is a remarkable finding as there are many other signaling pathways and molecules involved.”
This protocol has the capacity to revolutionized the use of heart muscle cells for drug testing. Also, because the Wnt signaling pathway is required during heart development, this protocol also has the ability to clarify the exact role of this pathway during heart differentiation. Finally, if stem cells are eventually used for therapeutic purposes, this protocol or one like it will certainly be employed to convert stem cells into heart muscle cells.