Imaging stem cell differentiation for cell-based tissue repair

Mesenchymal stem cells (MSCs) can differentiate into a number of tissue lineages and possess great potential in tissue regeneration and cell-based therapy. For bone fracture or cartilage wear and tear, stem cells need to be delivered to the injury site for repair. Assessing engraftment of the delivered cells and their differentiation status is crucial for the optimization of novel cell-based therapy. A longitudinal and quantitative method is needed to track stem cells transplanted/implanted to advance our understanding of their therapeutic effects and facilitate improvements in cell-based therapy. Currently, there are very few effective noninvasive ways to track the differentiation of infused stem cells. A brief review of a few existing approaches, mostly using transgenic animals, is given first, followed by newly developed in vivo imaging strategies that are intended to track implanted MSCs using a reporter gene system. Specifically, marker genes are selected to track whether MSCs differentiate along the osteogenic lineage for bone regeneration or the chondrogenic lineage for cartilage repair. The general strategy is to use the promoter of a differentiation-specific marker gene to drive the expression of an established reporter gene for noninvasive and repeated imaging of stem cell differentiation. The reporter gene system is introduced into MSCs by way of a lenti-viral vector, which allows the use of human cells and thus offers more flexibility than the transgenic animal approach. Imaging osteogenic differentiation of implanted MSCs is used as a demonstration of the proof-of-principle of this differentiation-specific reporter gene approach. This framework can be easily extended to other cell types and for differentiation into any other cell lineage for which a specific marker gene (promoter) can be identified.