Effect of Pax3 Overexpression on the Engraftment of Mesenchymal Stem Cells in Dystrophic Mice.

Mesenchymal stem cells (MSCs) residing within the bone marrow (BM) not only support hematopoiesis, but also differentiate into multiple lineages, including fat, bone, and cartilage. Because MSCs are multipotent and have a great expansion capacity in vitro, these cells are attractive candidates for clinical applications to repair or regenerate damaged tissues of mesenchymal origin. However, treatment of muscle degenerative diseases with MSCs has been hampered by the poor differentiation of MSCs into the muscle lineage. To date most in vitro methods require the presence of strong non-physiological agents, such as azacytidine. Accordingly, limited engraftment has been observed following the transplantation of BM into dystrophic mice (less than 4%). In the present study we explored the potential of Pax3, the master regulator of the embryonic myogenic program, to promote myogenic differentiation from human BM-derived MSCs. Upon genetic modification with lentiviral vectors encoding Pax3-GFP or GFP only (vector control), MSCs were evaluated for their potential to differentiate towards muscle in vitro. Real time PCR analyses revealed up-regulation of several myogenic regulatory factors in Pax3-transduced MSC cultures in various differentiation conditions, whereas control GFP-MSCs showed undetectable levels. When transplanted into cardiotoxin-injured tibialis anterior (TA) muscles of Rag2^−/− γc^−/− immunodeficient as well as dystrophic mice, superior engraftment was observed with Pax3-transduced MSCs (31% vs. 12% and 12% vs. 6.5%, respectively). To assess whether such levels of engraftment are therapeutically relevant, functional analyses of transplanted muscles are required. We are currently undertaking these experiments.