The tell-tale heart: Firefly genes for stem cell tracking

by Chris Kamel

As mentioned in a previous post, in order to fully understand what's happening when stem cells are used for in clinical or in vivo research, we need some way to track them, preferably in a non-invasive manner. This can be done with either reporter genes or some sort of traceable label, and would allow researchers to watch the cells as they migrate and differentiate in the body or research animal. Using luciferase, the protein that makes fireflies glow, researchers have been able to non-invasively image cardiac stem cells in a living mouse, according to research published in Stem Cells and Development.

In the study, researchers generated mouse ES cells expressing luciferase under the control of a cardiac-specific promoter (Ncx-1). Under these conditions, the stem cells appear normal until they differentiate into heart cells. When heart genes are expressed (specifically Ncx-1), these cells will turn on the firefly luciferase, which can be made to glow, allowing tracking of the cells.

The researchers first demonstrated this in vitro, showing that only ES cells that differentiated into cardiac cells had bioluminescence. When injected into the heart, the cells were seen in the chest cavity of mice as early as four days, persisting for as many as 24 days. The cells were able to align with existing tissue and maintain a cardiac muscle phenotype.

This work was done with otherwise healthy, newborn mice, but the authors hope to use this method to see how stem cells behave in adult mouse models of heart disease. This system provides a non-invasive way to monitor stem cells in individual live animals, allowing scientists to track their migration and differentiation when designing new protocols and testing them in pre-clinical studies. Ultimately, it could be used as a way to follow patient progress without the need for surgery. This method could even be adapted for use in other organs by using different tissue-specific promoters.

Another interesting finding was that the procedure worked better when the number of cells injected was lower, and when differentiation was initiated before injection. With a high number of undifferentiated cells, many remained undifferentiated and spread outside the cardiac region into lung and other tissue. At a moderate dose, the cells were more localized to the heart. Triggering differentiation before implantation improved the formation of heart tissue while reducing the chances of tumour formation. This kind of information about dosing and safety is only available through pre-clinical and clinical trials, such as the current Geron stem cell trial that recently treated its first patient, and mark the difference between legitimate therapies and those offered by less scrupulous "clinics" that offer untested "cures" to the hopeful and desperate.