The human heart is a pump that works only when its internal electrical system coordinates both its filling and its capacity to eject blood. This critical electrical timing is coordinated by many different ion channels, and this study looks at the one known as IKs. Mutations in its α subunit, KCNQ1, constitute the majority of cases of the disorder long QT syndrome (LQT–1). Here we have examined properties of human cardiac cells during very early stages of development and found evidence for the manner in which the subunits of IKs assemble; our data suggest that this assembly may be flexible and may change during development and/or disease.
Human embryonic stem cells (hESCs) are an important cellular model for studying ion channel function in the context of a human cardiac cell and will provide a wealth of information about both heritable arrhythmias and acquired electrophysiological disorders. However, detailed electrophysiological characterization of the important cardiac ion channels has been so far overlooked. Because mutations in the gene for the IKsα subunit, KCNQ1, constitute the majority of long QT syndrome (LQT–1) cases, we have carried out a detailed biophysical analysis of this channel expressed in hESCs to establish baseline IKs channel biophysical properties in cardiac myocytes derived from hESCs (hESC–CMs). IKs channels are heteromultimeric proteins consisting of four identical α–subunits (KCNQ1) assembled with auxiliary β–subunits (KCNE1). We found that the half–maximal IKs activation voltage in hESC–CMs and in myocytes derived from human induced pluripotent stems cells (hiPSC–CMs) falls between that of KCNQ1 channels expressed alone and with full complement of KCNE1, the major KCNE subunit expressed in hESC–CMs as shown by qPCR analysis. Overexpression of KCNE1 by transfection of hESC–CMs markedly shifted and slowed native IKs activation implying assembly of additional KCNE1 subunits with endogenous channels. Our results in hESC–CMs, which indicate an IKs subunit stoichiometry that can be altered by variable KCNE1 expression, suggest the possibility for variable IKs function in the developing heart, in different tissues in the heart, and in disease. This establishes a new baseline for IKs channel properties in myocytes derived from pluripotent stem cells and will guide future studies in patient–specific hiPSCs.