Bradycardic mice undergo effective heart rate improvement after specific homing to the sino-atrial node and differentiation of adult muscle derived stem cells.

Current treatment for heart automaticity disorders still lack a safe and efficient source of stem cells to bring about biological pacemaking. Since adult Muscle-Derived Stem Cells (MDSC) show multi- lineage differentiation in vitro including into spontaneously beating cardiomyocytes, we questioned whether MDSCs could effectively differentiate into cardiac pacemakers, a specific population of myocytes producing electrical impulses in the sino-atrial node of adult heart. We show here that beating cardiomyocytes, differentiated from MDSC in vitro, exhibit typical characteristics of cardiac pacemakers: the expression of Hcn4, Tbx3 and Islet1, as well as spontaneous calcium transients and hyperpolarization-activated funny current, a unique signature of sino-atrial pacemakers. Pacemaker-like myocytes differentiated in vitro from Cav1.3 -deficient mouse MDSC produced slower Ca2+ transients, consistent with the reduction of native pacemaker activity in these mice. In vivo, systemic injection of undifferentiated wild type MDSCs into bradycardic mutant Cav1.3-/- mice was ensued by their migration and homing to the sino-atrial node area within 48h and differentiation into Cav1.3 -expressing pacemaker-like myocytes within 10 days, a process accompanied by a significant improvement of the heart rate after 10 days that was maintained for up to 5 weeks. These findings identify MDSCs as directly transplantable stem cells that efficiently engraft, differentiate and improve heart rhythm in a mouse model of congenital bradycardia.