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

¹ Departments of Physiology and Medicine, Heart and Stroke/Richard Lewar Centre, University Health Network, University of Toronto, Toronto, Ontario, Canada
² Cell Therapy Program, Princess Margaret Hospital/Ontario Cancer Institute, University of Toronto, Toronto, Ontario, Canada
³ Division of Cardiology, St. Michael's Hospital, University of Toronto, Toronto, Ontario, Canada

^* To whom correspondence should be addressed. E-mail: rob.rose{at}utoronto.ca.

Correspondence may also be addressed to Armand Keating at armand.keating@uhn.on.ca and Peter Backx at p.backx@utoronto.ca.

	Abstract

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 GFP under the control of the cardiacspecific -myosin heavy chain promoter were cocultured with male rat embryonic cardiomyocytes (rCMs) for 5–15 days. After 5 days in co-culture, 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 co-culture 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 a MSC population displaying plasticity towards the cardiomyocyte lineage while retaining mesenchymal stromal cell properties, including a non-excitable electrophysiological phenotype. The demonstration of a MSC population phenotyping of MSCs co-cultured with cardiomyocytes 2 co-expressing 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.