Reprogramming is the process of taking and adult cell and changing its cell fate so that it becomes another type of adult cell. For the past decade, stem cells researchers have tried to reprogram the identity of all kinds of cell types. One of the most sought-after reprogramming events is the production of heart cells, since they could be used to treat patients who have had a heart attack. Researchers at the Perelman School of Medicine at the University of Pennsylvania, Philadelphia, have demonstrated the direct conversion of a non-heart cell type into a heart cell by means of RNA transfer. Since molecules called messenger RNAs contain the information for the synthesis of specific proteins, investigators were able to change a brain cell called an astrocyte (a star-shaped brain cell) and a fibroblast (a skin cell), into a heart cell, by using mRNAs.
The scientists put an excess of heart cell mRNAs into either astrocytes or fibroblasts using lipid-mediated transfection. The host cell responds to these RNAs by transdifferentiating into another cell type. These RNA molecules direct DNA in the host nucleus to change the cell’s RNA populations to that of the destination cell type (heart cell, or cardiomyocyte),and this, in turn changes the phenotype of the host cell into the destination cell. James Eberwine, PhD, the Elmer Holmes Bobst Professor of Pharmacology, who was involved with this study, said, “What’s new about this approach for heart-cell generation is that we directly converted one cell type to another using RNA, without an intermediate step.”
The method the group used in this study is called Transcriptome Induced Phenotype Remodeling, or TIPeR. This technique is distinct from the induced pluripotent stem cell (iPS) approach used by many labs in that host cells do not have to be dedifferentiated to an embryonic, pluripotent state and then redifferentiated with growth factors to the destination cell type. TIPeR is more similar to prior nuclear transfer work in which the nucleus of one cell is transferred into another cell where upon the transferred nucleus then directs the cell to change its phenotype based upon the RNAs that are made. The cardiomyocyte work follows directly from earlier work from the Eberwine lab, where neurons were converted into Astrocytes using the TIPeR process.
The team first extracted mRNA from a heart cell, and then used TIPeR to put it into host cells. Because there are now so many more heart-cell mRNAs versus astrocyte or fibroblast mRNAs, the transfected RNAs take over the indigenous RNA population. The heart-cell mRNAs are translated into heart-cell proteins in the cell cytoplasm. These heart-cell proteins then influence gene expression in the host nucleus so that heart-cell genes are turned on and heart-cell-enriched proteins are made.
To track the change from an astrocyte to heart cell, the team looked at the new cells’ RNA profile by means of single cell microarray analysis. They also assayed the cell shape; and immunological and electrical properties of the cells. While TIPeR-generated cardiomyocytes are of significant use in fundamental science it is easy to envision their potential use to screen for heart cell therapeutics. What’s more, creation of cardiomyoctes from patients would certainly permit personalized screening for efficacy of drug treatments; screening of new drugs; and potentially as a cellular therapeutic.