Restoring the resting potential brings maturity to induced stem cell–derived human cardiac myocytes

Abstract

Reprogramming of mature human cells to produce the equivalent of embryonic stem cells offers exciting possibilities for therapeutic tissue repair and artificial organs. A more immediate benefit is the possibility of testing potential new drugs and other therapies in human cells of any type that can easily be created in vitro without the use of human blastocysts and with a minimum of invasive procedures in patients. Fundamental research in embryology lead to the identification of growth factors that could be used to force cultured embryonic stem cells to differentiate into mature adult cells including atrial and ventricular cardiac myocytes. This work grew out of the initial approach of allowing the harvesting and subculturing regions of embryos growing in vitro that showed spontaneous beating. A spectacular advance came in 2006 when Takahashi and Yamanaka 1 showed that mouse or human adult fibroblasts could revert to pluripotent stem cells when exposed to 4 factors: Oct3/4, Sox2, c-Myc, and Klf4 under embryonic stem cell culture conditions. In 2012, Yamanaka received the Nobel Prize in Medicine or Physiology for this work, which he shared with Gurdon, who showed in l962 that the specialization of mature cells can be reversed by transplantation of a frog intestinal cell nucleus into a frog egg. By the time of this award, substantial work by others had shown what additional factors or conditions are needed to cause the so-called induced pluripotent stem (IPS) cells to differentiate into adult cardiac myocytes or other cellular components including vascular and endothelial cells. 2 In the heart, embryonic development and tissue differentiation is accompanied by important changes in the electrical activity that alter the cardiac action potential and underlying ionic currents. While the genome must contain sequence information for all ion channels in the heart and other tissues, the expression of the ion channels varies with the stage of embryonic development and with anatomic location. In the embryonic hearts of vertebrates, the action potential of the immature ventricle is similar to that found in the adult sinoatrial node. Specifically, there is spontaneous diastolic depolarization (the pacemaker potential) with a relatively low maximum diastolic potential in immature atrial and ventricular cells, which disappears during development. The upstroke of the action potential is mediated by calcium channels in the early embryo and by sodium channels in the late embryo and adult. The action potential upstroke becomes increasingly rapid during embryonic development, as demonstrated by Nathan and DeHaan 3 in the chick heart. Mature atrial and ventricular cells have a resting potential that is due to predominant permeability to potassium and is approximately � 90 mV, which is the potassium equilibrium