Acceleration of human myoblast fusion by depolarization: graded Ca2+ signals involved

Abstract

We have previously shown that human myoblasts do not fuse when their voltage fails to reach the domain of a window T-type Ca(2+) current. We demonstrate, by changing the voltage in the window domain, that the Ca(2+) signal initiating fusion is not of the all-or-none type, but can be graded and is interpreted as such by the differentiation program. This was carried out by exploiting the properties of human ether-a-go-go related gene K(+) channels that we found to be expressed in human myoblasts. Methanesulfonanilide class III antiarrhythmic agents or antisense-RNA vectors were used to suppress completely ether-a-go-go related gene current. Both procedures induced a reproducible depolarization from -74 to -64 mV, precisely in the window domain where the T-type Ca(2+) current increases with voltage. This 10 mV depolarization raised the cytoplasmic free Ca(2+) concentration, and triggered a tenfold acceleration of myoblast fusion. Our results suggest that any mechanism able to modulate intracellular Ca(2+) concentration could affect the rate of myoblast fusion

The calcineurin pathway links hyperpolarization (Kir2.1)-induced Ca2+ signals to human myoblast differentiation and fusion

“…We have shown previously that one of the earliest events of human myoblast differentiation is a membrane hyperpolarization resulting from to Kir2.1 K + channel activity (Fischer-Lougheed et al, 2001;Liu et al, 2003). The goal of the present work was to uncover the molecular link between the Kir2.1-induced hyperpolarization and the expression of myogenin and MEF2, two major transcription factors of the differentiation process.…”

“…Early activation of calcineurin during human myoblast differentiation requires a membrane hyperpolarization Our previous work on human myoblasts suggested that membrane hyperpolarization generated by Kir2.1 channels increases intracellular Ca 2+ and that this step is essential to allow myoblast differentiation to proceed (Arnaudeau et al, 2006;Bijlenga et al, 2000;Liu et al, 2003). As calcineurin is a Ca 2+ -dependent phosphatase involved in human myoblast differentiation, we tested whether its activity is controlled by a Ca 2+ signal that could be induced by the hyperpolarization.…”

“…Differentiation of human myoblasts requires a hyperpolarization of their membrane resting potential to approximately -70 mV (Fischer-Lougheed et al, 2001;Liu et al, 2003). Preventing this hyperpolarization impedes both expression and activity of myogenin and MEF2, indicating that it is a prerequisite for differentiation (Konig et al, 2004).…”

“…Preventing this hyperpolarization impedes both expression and activity of myogenin and MEF2, indicating that it is a prerequisite for differentiation (Konig et al, 2004). We proposed that Kir2.1-linked hyperpolarization initiates the differentiation process by increasing cytoplasmic free Ca 2+ (Arnaudeau et al, 2006;Bijlenga et al, 2000;Liu et al, 2003). The question is what are the signal transduction pathways downstream of this cytoplasmic Ca 2+ signal that initiate human myoblast differentiation?…”

Potassium currents in human myogenic cells from healthy and congenital myotonic dystrophy foetuses

“…Therefore, it has been proposed that cell hyperpolarization, associated with the expression of two K + currents, I K(NI) , a non-inactivating delayed rectifier, and I K(IR) , an inward rectifier, is involved in the mechanism of Ca 2+ influx, an event observed prior to the cell fusion of human myoblasts [23]. Cells expressing the K NI and K IR channels are then able to achieve a more negative RMP than proliferating cells, allowing T-type Ca 2+ channels to conduct a Ca 2+ window current [25]. Since these voltage-gated K + currents are indeed crucial for cell fusion, we decided to determine their activity in DM1 muscle cells.…”