Abstract
Background-
Although there is significant enthusiasm for cell-based therapies, whether administered cells differentiate into cardiomyocytes (CM) or exert their effects solely by paracrine signaling remains controversial. We hypothesized that allogeneic bone marrow-derived mesenchymal stem cells (aMSCs) regenerate the infarcted heart by cell autonomous mechanisms involving differentiation into myocytes, endothelial cells, vascular smooth muscle cells and the stimulation of endogenous precursor cell proliferation.
Methods and Results
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Seventeen female swine underwent myocardial infarction. Three days later, they received intramyocardial injections of male GFP
pos
aMSCs (n=7), placebo (n=7) or no injection (n=3). Animals were euthanized at 24h (n=1 aMSCs, n=1 placebo), 72h (n=3 aMSCs, n=3 placebo) and 2 weeks (n=3 aMSCs, n=3 placebo, n=3 controls) after injections. Within 24h, engrafted aMSCs exhibited lineage commitment into CM (GATA4, α-sarcomeric actinin) but not vascular precursors. By 72h, CM differentiation rates increased substantially (GFP
pos
/GATA
pos
: 23.4±3.2% of GFP
pos
cells at 72h
vs
13.4±5.4% at 24h) as well as vascular lineage committment (GFP
pos
/KDR/Factor VIII
pos
: 1.3±0.4%). Importantly, aMSCs migrated throughout ischemic but not viable myocardium. Two weeks later, MSCs had differentiated into mature vessels and CM. Cardiac regeneration was accompanied by recruitment of endogenous c-kit
pos
cells at infarct (IZ) and border (BZ) zones of the treated animals, compared both to placebo and control (p≤0.001). In contrast to the untreated animals, c-kit cells were localized in clusters in the IZ and BZ of the treated hearts and formed connexin-43 and N-cadherin connections with each other and the GFP
pos
aMSCs, resembling stem cell niches. The number of the c-kit
pos
/GATA-4
pos
cells within the niches was significantly higher at the BZ of the treated hearts (3.9±1.6%) compared to placebo and control (p≤0.05), indicative of an endogenous repair mechanism.
Conclusions-
Together these findings offer important mechanistic insight into the basis of aMSCs ability to achieve post-MI cardiac regeneration, and illustrate that the cells exert both cell dependent and host dependent effects.