Abstract 287: Formation of Temporary Niches Is Required for Bone Marrow Cells To Adopt the Cardiomyogenic Fate In Vivo
The possibility that adult bone marrow cells (BMCs) retain a remarkable degree of developmental plasticity and acquire the cardiomyocyte lineage after infarction has been challenged. For this reason, c-kit-positive-BMCs for myocardial regeneration were obtained from male transgenic mice carrying 3 different constructs:
EGFP driven by the ubiquitous β-actin promoter;
EGFP driven by the cardiac specific α-myosin-heavy-chain (α-MHC) promoter; and
c-myc-tagged nuclear-targeted-Akt transgene driven by the α-MHC-promoter.
With each cell type, a total of 60,000 BMCs were injected in four sites of the border zone of female infarcted wild type mice. Within 12– 48 hours, donor BMCs homed to the border zone. Junctional and adhesion complexes, made by connexins and cadherins, were detected between male BMCs and male BMCs and female myocytes and fibroblasts forming temporary niches within the recipient myocardium. A large proportion of engrafted BMCs was cycling as documented by BrdU and Ki67 labeling. Phospho-H3 was also detected in dividing BMCs. Conversely, in the absence of engraftment, BMCs died by apoptosis. Because of these two variables, in each case, ~25% or 14,000 of the 60,000 injected BMCs were present in the myocardium at 2 days. At 12 hours, BMCs were mostly CD45 positive but at 24–36 hours, a large subset of BMCs was CD45 negative and the absence of the CD45 epitope was even more evident at 48 hours. The synthesis of the gap junction channel protein connexin 43 preceded the downregulation of CD45. The progressive loss of the hematopoietic phenotype was associated with the appearance of transcription factors specific of myocytes, such as Nkx2.5, smooth muscle cells (SMCs), such as GATA6, and endothelial cells (ECs), such as Ets1. The acquisition of the cardiogenic lineage occurred only in engrafted cells expressing connexin 43. Additionally, cytoplasmic proteins specific of myocytes, SMCs and ECs began to appear during the conversion of BMCs to the cardiogenic fate. Thus, c-kit-positive-BMCs engraft, proliferate and acquire the cardiomyocyte and coronary vascular lineages in the infarcted heart. The process of BMC transdifferentiation requires the formation of temporary niches prior to cell growth and specification.