Abstract 14700: Notch1 Overexpression Reprograms Adult Myocytes to an Immature Functional Phenotype
In the adult heart Notch1 favors the early commitment of cardiac stem cells to the myocyte lineage and maintains the newly formed cells in a highly proliferative state, commonly seen in transit amplifying myocytes. The aim of this study was to determine whether Notch1 overexpression was capable of inducing adult myocytes to re-enter a more primitive physiological phenotype with properties of newly formed fetal-neonatal cells. To test this hypothesis, α-MHC-N1ICD mice were utilized because, in these animals, the expression of the active fragment of Notch1 (N1ICD) and GFP are driven by a tamoxifen-inducible α-MHC promoter. Upon Cre recombination, a pool of myocytes expressed the N1ICD-GFP transgene, allowing us to define their electrophysiological and molecular characteristics. The protocols employed included immunolabeling, patch-clamp and quantitative RT-PCR. N1ICD overexpression was associated with a 40% upregulation of the cell cycle protein Ki67, documenting that this myocyte population conserved a higher replicative state. Since the process of myocyte maturation typically shows changes in electrical behavior, the action potential and ionic currents of N1ICD/GFP-positives (NGPos) and N1ICD/GFP-negative (NGNeg) cells were evaluated. NGPos myocytes manifested a prolonged action potential, resulting in an 11-, 8- and 3-fold increase in the time to 30%, 50% and 70% repolarization, respectively. The transient outward potassium current (Ito) is developmentally regulated and is responsible for the early repolarization phase of the action potential; Ito density was reduced by >70% in NGPos myocytes, pointing to Ito as the ionic mechanism responsible for the prolongation of the action potential in myocytes with Notch1 overexpression. These electrophysiological parameters were consistent with changes in the relative proportion of transcripts for Kv1.4, Kv4.2 and Kv4.3, the molecular substrate of Ito. In conclusion, Notch1 activation reprograms the electrical and mechanical properties of adult cardiomyocytes which acquire a more immature cell phenotype.
- © 2010 by American Heart Association, Inc.