Currently, bone marrow derived endothelial progenitor cells (human CD34+ cells, EPC) are being used clinically to improve vascularization in patients with ischemic heart disease. While it is generally accepted that CD34+ cells predominantly work through a paracrine mechanism, there exists no convincing evidence that these cells trans-differentiate into functional cardiomyocytes (CMC). Since ischemic heart disease leads to substantial loss of CMC, improving cardiomyogenic plasticity of an existing autologous cell therapy is of obvious import. EPC and CMC both differentiate from a common mesodermal progenitor however; during EC-specific lineage differentiation, CMC specific genes are epigenetically silenced. We hypothesized that reprogramming of CD34+ cells using small molecules targeting key epigenetic repressive marks may recapitulate their cardiomyogenic potential.
Human CD34+ EPCs were treated with inhibitors of histone deacetylases (valproic acid) for 24 hours followed by an additional 24 hours with the DNA methyltransferase inhibitor (5-Azacytidine). This forty-eight hour treatment led to the reactivation of pluripotency associated and CMC specific mRNA expression while EC specific gene expression was maintained. Intra-myocardial transplantation of a sub-therapeutic dose of reprogrammed CD34+ cells in an acute myocardial infarction mouse model showed significant improvement in LV function compared to the same number of control CD34+ cells that are therapeutically equivalent to no treatment at all. This was histologically supported by de novo CMC differentiation. In addition to increased cardiomyogenic plasticity, drug treatment also enhanced the inherent therapeutic capacity of the CD34+ cells as shown by reduced fibrosis, increased capillary density, increased proliferation, increased cell survival and increased secretion of angiogenic factors.
Taken together, our results suggest that epigenetically reprogrammed CD34+ cells are “super-CD34+ cells” that have an enhanced paracrine effect, display a more plastic phenotype and improve post-infarct cardiac repair by both neo-cardiomyogenesis and neovascularization.
Accelerated cellular senescence as underlying mechanism for functionally impaired bone marrow-derived progenitor cells in ischemic heart disease