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Skeletal muscle stem cells do not transdifferentiate into cardiomyocytes after cardiac grafting
Hans Reinecke1, Veronica Poppa, Charles E Murry
1Department of Pathology, University of Washington, Seattle, Washington 98195-7335, USA. hreineck@u.washington.edu
Insights
Skeletal muscle stem cells grafted into the heart form mature skeletal muscle, not new heart cells. These grafts do not express cardiac markers, indicating transdifferentiation into cardiomyocytes does not occur.
Area of Science:
- Regenerative Medicine
- Cardiovascular Biology
- Muscle Physiology
Background:
- Skeletal muscle stem cells (satellite cells) are explored for cardiac repair.
- Previous studies suggested potential transdifferentiation into cardiomyocytes, but lacked rigorous confirmation.
Purpose of the Study:
- To determine the in vivo fate of adult rat satellite cell-derived grafts in the heart.
- To rigorously assess for transdifferentiation into cardiomyocytes and expression of cardiac markers.
Main Methods:
- Adult rat satellite cells were tagged with bromodeoxyuridine (BrdU) in vitro.
- Cells were grafted into syngeneic rat hearts and analyzed at 4 and 12 weeks.
- Graft cells were examined for skeletal and cardiac marker expression, including myosin heavy chain (MHC) isoforms and cardiac troponin I.
Main Results:
- Grafts formed multinucleated, cross-striated myofibers expressing fast skeletal myosin heavy chain (MHC).
- No significant expression of cardiac markers (alpha-MHC, cardiac troponin I, atrial natriuretic peptide) was detected.
- Grafts expressed beta-MHC at 4 weeks, but atrophied and downregulated it by 12 weeks.
- Intercalated disk proteins (N-cadherin, connexin43) were absent, preventing electromechanical coupling.
Conclusions:
- Satellite cells differentiate into mature skeletal muscle tissue within the heart.
- Transdifferentiation of satellite cells into cardiomyocytes does not occur under these conditions.
- Skeletal muscle grafts do not integrate functionally or electrically with host cardiac tissue.
Abstract:
Skeletal muscle cell-derived grafts in the heart may benefit myocardial performance after infarction. Several studies have suggested that skeletal muscle stem cells (satellite cells) from adult muscle undergo transdifferentiation into cardiomyocytes after grafting into the heart, but expression of cardiac markers in graft cells has not been rigorously confirmed. To determine the fate of satellite cell-derived grafts in the heart, adult rat satellite cells were tagged in vitro with bromodeoxyuridine (BrdU) and grafted into normal hearts of syngeneic rats. At 4 and 12 weeks the graft cells formed multinucleated, cross-striated myofibers that expressed fast skeletal myosin heavy chain (MHC), thus indicating a mature skeletal muscle phenotype. Double staining for the BrdU tag and cardiac-specific markers was employed to identify transdifferentiation. Aside from four questionable cells, none of the 11 grafts examined expressed alpha-MHC, cardiac troponin I, or atrial natriuretic peptide. At 4 weeks, grafts expressed beta -MHC, a hallmark of slow twitch myofibers. By 12 weeks, however, the myofibers had atrophied and downregulated beta-MHC. Grafts never expressed the intercalated disk proteins N-cadherin or connexin43, hence electromechanical coupling did not occur. In conclusion, satellite cells differentiate into mature skeletal muscle and do not express cardiac-specific genes after grafting into the heart. Thus, transdifferentiation into cardiomyocytes did not occur.