Hematopoietic Transdifferentiation of Muscle-Derived Cells after In Vivo Transient Expression of MSX1 Transcription Factor.
Authors: Nobuyoshi Masaharu , Kume Akihiro , Mizukami Hiroaki , Matsushita Takashi , Okada Takashi , Urabe Masashi , Ohgoshi Yuichi , Endo Takashi , Ozawa Keiya 
Published: October 2019

Abstract
After a considerable dispute, it was shown that cells with hematopoietic activity in muscles are derived from hematopoietic organ. Transdifferentiation is not a frequent event, if any, and such a phenomenon cannot be applied to tissue regeneration easily. To achieve a breakthrough, we explored the possibility that genetic manipulation may enhance the efficiency of transdifferentiation. In this regard, there is a notable report that transient overexpression of a homeobox-containing transcriptional repressor Msx1 in muscles generated abundant mononuclear cells (MNCs) capable of differentiating into myotubes, chondrocytes, adipocytes and osteocytes. That is, enforced Msx1 expression caused dedifferentiation of myotubes, and subsequent Msx1 suppression induced redifferentiation (Odelberg SJ et al, Cell103:1099). Recently, we found that similar dedifferentiation-redifferentiation events also occur in vivo after transient Msx1 expression in muscles using adeno-associated virus (AAV) vectors (Endo T et al, manuscript in preparation). Since virtually all of AAV vector-mediated transgenes exist as episomes, they gradually disappear as the host cells divide. In the present study, we took advantage of this feature of AAV vectors; muscle-derived MNCs would lose Msx1 transgene through cell divisions after dedifferentiation, and a proper differentiation cue might redirect these undifferentiated cells into the hematopoietic lineage as well. AAV vector expressing Msx1 (AAV/msx1) was injected into tibialis anterior muscles of C57BL/6 mice. Flow cytometric analysis revealed that MNCs from AAV/msx1-treated muscles contained a considerable number of cells expressing hematopoietic stem cell markers. CD34−/c-Kit+ cells (1.3±0.9% of MNCs in control muscles) were increased in AAV/msx1-treated muscles (13.6±6.0% at 4 weeks). CD45+/Sca-1+ cells (2.4±0.4% in control muscles) were also increased in the AAV/msx1-treated muscle, peaking at 2 weeks (36.0±8.1%; P =.01). To evaluate hematopoietic activity, MNCs were cultured in methylcellulose medium containing stem cell factor, IL-3, IL-6 and erythropoietin. After AAV/msx1 injection, colony-forming cells in the muscles were gradually increased, reaching a peak at 3 weeks (48.9±24.1/muscle), in contrast to very few progenitors detected in control muscles (1.4±3.0/muscle; P <.01). Finally, in vivo reconstitution activity was evaluated by transplanting MNCs from AAV/msx1-treated muscles of Ly5.2 mice to irradiated congenic mice. About 3x105 muscle-derived cells (Ly5.2) and 2x105 fresh Ly5.1 bone marrow cells were cotransplanted into lethally irradiated Ly5.1 recipients. Among 5 mice transplanted, a robust engraftment of Ly5.2 cells was observed in 2 animals. These transplants showed a very high chimerism of Ly 5.2 (55.2% at 8 months and 41.7% at 6 months, respectively). Furthermore, in the secondary bone marrow transplantation from the former mouse to a Ly5.1/5.2 heterozygous recipient, the donor cell chimerism was even higher (82.9% at 7 months). These results suggest that enforced Msx1 expression can reprogram muscle cells into multipotential cells capable of differentiation into the hematopoietic lineage. This novel technology would be applied to the treatment of acquired bone marrow failure using genetically normal hematopoietic stem cells derived from the patient muscle.