title: Bone marrow-derived mesenchymal stromal cells express cardiac-specific markers, retain the stromal phenotype, and do not become functional cardiomyocytes in vitro.
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 green fluorescent protein (GFP) under the control of the cardiac-specific alpha-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 alpha-actinin. The mRNA expression for selected cardiac-specific genes (atrial natriuretic factor, Nkx2.5, and alpha-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(+)-alpha-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.
authors.affiliation: Department of Physiology, Heart and Stroke/Richard Lewar Centre, University Health Network, Princess Margaret Hospital, Ontario Cancer Institute, Toronto, Ontario, Canada.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Rose
authors.forename: Huijie
authors.initials: H
authors.lastname: Jiang
authors.forename: Xinghua
authors.initials: X
authors.lastname: Wang
authors.forename: Simone
authors.initials: S
authors.lastname: Helke
authors.forename: James N
authors.initials: JN
authors.lastname: Tsoporis
authors.forename: Nanling
authors.initials: N
authors.lastname: Gong
authors.forename: Stephanie C J
authors.initials: SC
authors.lastname: Keating
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Parker
authors.forename: Peter H
authors.initials: PH
authors.lastname: Backx
authors.forename: Armand
authors.initials: A
authors.lastname: Keating
mesh_terms: D000818:Animals; D000943:Antigens, Differentiation / Q000378:metabolism*; D001854:Bone Marrow Cells / Q000166:cytology* / Q000502:physiology; D002454:Cell Differentiation / Q000502:physiology; D002459:Cell Fusion; D018920:Coculture Techniques; D005260:Female; D049452:Green Fluorescent Proteins / Q000096:biosynthesis / Q000235:genetics; D016130:Immunophenotyping; D008297:Male; D059630:Mesenchymal Stem Cells / Q000166:cytology* / Q000502:physiology; D051379:Mice; D008822:Mice, Transgenic; D032383:Myocytes, Cardiac / Q000166:cytology* / Q000502:physiology; D018995:Myosin Heavy Chains / Q000235:genetics; D018408:Patch-Clamp Techniques; D011401:Promoter Regions, Genetic; D051381:Rats; D017154:Stromal Cells / Q000166:cytology / Q000502:physiology; D024723:Ventricular Myosins / Q000235:genetics
chemical_list: D000943:Antigens, Differentiation; D049452:Green Fluorescent Proteins; D024723:Ventricular Myosins; D018995:Myosin Heavy Chains