Initiation of Embryonic Cardiac Pacemaker Activity by Inositol 1,4,5-Trisphosphate–dependent Calcium Signaling

Abstract

In the adult, the heart rate is driven by spontaneous and repetitive depolarizations of pacemaker cells to generate a firing of action potentials propagating along the conduction system and spreading into the ventricles. In the early embryo before E9.5, the pacemaker ionic channel responsible for the spontaneous depolarization of cells is not yet functional. Thus the mechanisms that initiate early heart rhythm during cardiogenesis are puzzling. In the absence of a functional pacemaker ionic channel, the oscillatory nature of inositol 1,4,5-trisphosphate (InsP 3 )-induced intracellular Ca 2؉ signaling could provide an alternative pacemaking mechanism. To test this hypothesis, we have engineered pacemaker cells from embryonic stem (ES) cells, a model that faithfully recapitulates early stages of heart development. We show that InsP 3 -dependent shuttle of free Ca 2؉ in and out of the endoplasmic reticulum is essential for a proper generation of pacemaker activity during early cardiogenesis and fetal life.

Developmental Changes in Cardiomyocytes Differentiated from Human Embryonic Stem Cells: A Molecular and Electrophysiological Approach

“…This isoform is indeed reported to be more sensitive to intracellular cAMP compared to HCN1. Our results, however, do not exclude the possibility that other mechanisms controlling pacemaker activity, such as inositol triphosphate-dependent calcium signaling [39], may contribute to isoprenaline-mediated effect in clusters of beating CMs.…”

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

“…Likely candidates could be the hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels that are responsible for pacemaking currents (I f ) in adult tissue [96]. However, the spontaneous Ca 2+ oscillations appear to precede the appearance of substantial HCN-mediated currents, and the activity persists in the presence of I f inhibitors [97]. Furthermore, repetitive Ca 2+ transients could still be observed in cells that were fully depolarized (using K + application), thereby precluding the involvement of membrane potential oscillations [98].…”

“…Furthermore, repetitive Ca 2+ transients could still be observed in cells that were fully depolarized (using K + application), thereby precluding the involvement of membrane potential oscillations [98]. In contrast, transfection of early stem cell-derived myocytes with antisense cDNA to reduce expression of type 1 IP 3 Rs, or application of an IP 3 R antagonist, significantly reduced the spontaneous beating [92,97]. The effect of antagonizing IP 3 Rs diminishes over time [99], suggesting that IP 3 -driven signals progressively give way to other mechanisms that generate Ca 2+ oscillations [100].…”

“…However, there are reports suggesting that IP 3 Rs are also expressed in other cardiac regions/cell types, such as papillary muscle [48,159].It has also been proposed that IP 3 Rs are functional in nodal cells within the developing atria. Isolation of atrial cells from E14.5 mouse embryos revealed a minor population of spontaneously active myocytes (presumed to be pacemaking cells).Transfection of an enzyme that metabolizes IP 3 , and thereby prevents its Ca 2+ -mobilising action, or an anti-IP 3 R antibody, inhibited the spontaneous activity [97]. In the sinus node of the adult mammalian heart, spontaneously active pacemaker cells make use of cyclic subsarcolemmal RyR-mediated SR Ca 2+ release to activate depolarizing NCX current, which contributes to diastolic depolarization and pacemaker activity (for review see [160]).…”

Maintenance Of Calcium Homeostasis in Embryonic Stem Cell-Derived Cardiomyocytes

“…These were demonstrated to be functional at two time-points; application of 5μM of IP 3 was able to induce Ca 2+ release from the SR in CMs derived from mouse embryos at E5.5 and E8.5. Application of 5μM xestospongin C, an IP 3 R antagonist, to mouse embryos at E10 diminished its Ca 2+ spiking; washing the drug out allowed slow recovery of this spiking activity (Mery et al, 2005). Likewise, application of 5μM xestospongin C also abrogated the Ca 2+ spiking activity observed in CGR8 mESC-derived CMs at day 8-10 post-differentiation.…”