Guest blog written by Karen Heyman, a freelance science writer
Scientists in Sanford-Burnham’s NCI-designated Cancer Center have discovered key steps in the process by which mature cells revert to stem cells. The work of Tariq Rana, Ph.D., and his colleagues, published in the April 3 issue of Cell Stem Cell, should lead to faster production of stem cells and a greater understanding of the cellular chain of events involved in stem-cell biology and cancer.
In 2006, Japan’s Shinya Yamanaka demonstrated that only four genes were required to reset a mature cell that had already differentiated into a specific cell type back to the immature state of pluripotency, from which it could become nearly any cell in the body. This new class of cells was dubbed “induced pluripotent stem cells” or iPSCs.
These cells, for which Yamanaka would later share the Nobel Prize, provide enormous clinical and research opportunities. For biomedical investigators, such as the faculty at Sanford-Burnham, iPSCs are a tool to study disease progression. In the laboratory, afflicted cells are reverted back to their original healthy state, so scientists can then observe their transformation by conditions such as cancer and neurodegenerative disorders.
For patients, iPSCs may one day be used to grow new tissues for transplantation, without risk of immune rejection. The practical challenge to that great promise of regenerative medicine is that it is currently prohibitively difficult to generate the millions of iPSCs that would be required to create a biological structure.
Despite the apparent simplicity of a four-gene “recipe,” an iPSC, like all cellular processes, is really the end-point of a complicated relay involving thousands of molecular events. Key to passing signals down that relay are proteins called kinases, which are as vital to the body as nails are to a house, underlying basic functions ranging from metabolic balance to neurotransmission.
“Given how important kinases are to cell signaling, we theorized they must play a pivotal role in pluripotency and determining cell fates during differentiation,” said Rana.
Using the four genes, the team derived almost 5,000 iPSCs from mouse skin cells in the course of their experiments. To understand the role of kinases, they employed the painstaking technique of RNA interference (RNAi) or gene silencing, which required preparing 3,686 viral vectors to target each one of the 734 kinases involved in the iPSC process. They discovered 59 that they dubbed “barrier kinases,” because they prevent cells from reverting all the way back to the pluripotent state.
Confirming their results to produce human iPSCs, the team concluded that certain barrier kinases interrupt the development of the cell’s cytoskeleton, the mobile tapestry of protein fibers that controls the shape and movement of a cell and the structures within it. The mechanism is essentially a toggle system: Increasing the amount of particular barrier kinases enables the formation of a healthy cytoskeleton, while decreasing them disrupts the structure.
Knowing the role of particular kinases in blocking the construction of the cytoskeleton is a first step in investigating potential therapies for cancer. “From these 59 kinases, one can understand both the signaling networks needed to change cell type, as well as pathways involved in cancer progression,” said Rana. “Thus, we can potentially gain insights into how to possibly slow a tumor’s growth or metastasis.”
For regenerative medicine, knowing what levels of barrier kinase affect whether cells will be formed, means a potential opportunity to vastly increase iPSC yields. In fact, Rana’s laboratory has also employed a different method to achieve the same aim. “Whether using small molecules as we did in our previous paper, or RNAi as we did here, the notion of barrier kinases is quite clear because inhibition of several kinases led to enhanced production of iPSCs,” said Rana.
Both studies underscore the importance of basic biomedical research as the foundation of clinical advances. “This helps us to understand which mechanisms and pathways that are involved in the establishment of the cytoskeleton play a role in diseases,” said Rana. “As a long-term goal, we could envision developing RNAis or small molecules to therapeutically target these pathways.”
A link to the paper can be found at http://www.sciencedirect.com/science/article/pii/S1934590914000952.