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TISSUE-SPECIFIC STEM CELLS

Bone Marrow-Derived Mesenchymal Stromal Cells Express Cardiac-Specific Markers, Retain the Stromal Phenotype, and Do Not Become Functional Cardiomyocytes In Vitro

Departments of ^aPhysiology and
^bMedicine, Heart and Stroke/Richard Lewar Centre, University Health Network,
^cCell Therapy Program, Princess Margaret Hospital/Ontario Cancer Institute,
^dDivision of Cardiology, St. Michael's Hospital, University of Toronto, Toronto, Ontario, Canada

Key Words. Mesenchymal stromal cells • Mesenchymal stem cells • Action potential • Electrophysiology • Ion channels • Cell transdifferentiation

Correspondence: Correspondence: Armand Keating, M.D., Princess Margaret Hospital, Suite 5-303, 610 University Avenue Toronto, Ontario M5G 2M9, Canada. Telephone: 416-946-4595; Fax: 416-946-4530; e-mail: armand.keating{at}uhn.on.ca; or Peter Backx, Ph.D., DVM, University of Toronto, Room 68, Fitzgerald Building, 150 College St., Toronto, Ontario M5S 3E2, Canada. Telephone: 416-946-8112; Fax: 416-946-8380; e-mail: p.backx{at}utoronto.ca

Received on April 1, 2008; accepted for publication on July 28, 2008.

First published online in STEM CELLS EXPRESS  August 7, 2008.

Although bone marrow-derived mesenchymal stromal cells (MSCs) may be beneficial in treating heart disease, their ability to transdifferentiate into functional cardiomyocytes remains unclear. Here, bone marrow-derived MSCs from adult female transgenic mice expressing green fluorescent protein (GFP) under the control of the cardiac-specific -myosin heavy chain promoter were cocultured with male rat embryonic cardiomyocytes (rCMs) for 5–15 days. After 5 days in coculture, 6.3% of MSCs became GFP⁺ and stained positively for the sarcomeric proteins troponin I and -actinin. The mRNA expression for selected cardiac-specific genes (atrial natriuretic factor, Nkx2.5, and -cardiac actin) in MSCs peaked after 5 days in coculture and declined thereafter. Despite clear evidence for the expression of cardiac genes, GFP⁺ MSCs did not generate action potentials or display ionic currents typical of cardiomyocytes, suggesting retention of a stromal cell phenotype. Detailed immunophenotyping of GFP⁺ MSCs demonstrated expression of all antigens used to characterize MSCs, as well as the acquisition of additional markers of cardiomyocytes with the phenotype CD45^–-CD34⁺-CD73⁺-CD105⁺-CD90⁺-CD44⁺-SDF1⁺-CD134L⁺-collagen type IV⁺-vimentin⁺-troponin T⁺-troponin I⁺--actinin⁺-connexin 43⁺. Although cell fusion between rCMs and MSCs was detectable, the very low frequency (0.7%) could not account for the phenotype of the GFP⁺ MSCs. In conclusion, we have identified an MSC population displaying plasticity toward the cardiomyocyte lineage while retaining mesenchymal stromal cell properties, including a nonexcitable electrophysiological phenotype. The demonstration of an MSC population coexpressing cardiac and stromal cell markers may explain conflicting results in the literature and indicates the need to better understand the effects of MSCs on myocardial injury.

Disclosure of potential conflicts of interest is found at the end of this article.

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