Abstract 1414: Pim-1 Stimulates Cardiac Progenitor Cell Proliferation And Asymmetric Division
Cardiac progenitor cells (CPCs) blunt cardiomyopathic damage and increase survival following adoptive transfer into hearts subjected to myocardial infarction (MI), but the initial survival, persistence, and long term engraftment of the donated cell population remains problematic. Previous studies from our group have demonstrated that transgenes driven by the α-myosin heavy chain gene promoter are expressed in the CPC population allowing for enhanced proliferation and survival. This study details a genetic engineering strategy to augment the salutary effects of CPCs through the use of a serine/threonine kinase named Pim-1 that promotes cellular proliferation and survival. Transgenic mice created with cardiac-specific Pim-1 overexpression (Pim-wt) exhibit enhanced Pim-1 activity in both cardiomyocytes and CPCs, both of which show increased proliferative activity assessed using BrdU or Ki-67 markers relative to non-transgenic (NTG) controls. However, CPC population number was not increased in the Pim-wt hearts during normal postnatal growth or after infarction challenge, suggesting that Pim-1 expression promotes asymmetric division resulting in maintenance of the CPC pool as well as expansion of the cardiomyocyte population. Localization and quantitation of cell fate determinants Numb and α-adaptin by confocal microscopy were employed to assess levels of asymmetric division in the CPC population. Polarization of Numb in mitotic phospho-histone positive cells demonstrates asymmetric division in 65% of the CPC population in hearts of Pim-wt mice versus 26% in NTG hearts after infarction challenge. Similarly, Pim-wt hearts had fewer cells with uniform α-adaptin staining indicative of symmetrically dividing CPCs, with in 36% of the CPCs versus vs. 73% in NTG sections. These findings define a mechanistic basis for enhanced myocardial regeneration in transgenic mice overexpressing Pim-1 kinase in the myocardial lineage cells.