An ether ‐à‐go‐go K⁺ current, I_h‐eag, contributes to the hyperpolarization of human fusion‐competent myoblasts

Abstract

The skeletal muscle fibre is a postmitotic multinucleated cell formed by the fusion of mononucleated myoblasts. Myoblast fusion is essential to skeletal muscle development and repair, and understanding this process is important for myoblastbased grafts or gene therapies. Our recent work on human fusion-competent myoblasts indicates that an early step in the fusion process is a hyperpolarization of the resting potential. This hyperpolarization is a two-step mechanism involving the sequential expression of two voltage-gated potassium currents. The first is a non-inactivating delayed rectifier current (IK(NI)) that hyperpolarizes the cell to an intermediate resting potential of approximately −32 mV (Bernheim et al. 1996). Then, slightly before fusion, an inward rectifier current is expressed (IK(IR)), which drives the potential further down to approximately −65 mV, i.e. similar to that measured for multinucleated myotubes ).We recently isolated the full-length coding region of a human ether-à-go-go K¤ channel (h_eag) from myoblasts undergoing differentiation . We describe here the properties of the current elicited by h_eag and present data indicating that it constitutes the K(NI) channel responsible for the first hyperpolarization step linked to myoblast differentiation. METHODS Dissociation and cell culturesSamples of human skeletal muscle were obtained during corrective orthopaedic surgery of young patients (9 months to 17 years old) without any known neuromuscular disease, in accordance with the guidelines of the ethical committee of the University Hospital of Geneva (written informed consent was obtained from patients or their legal guardians). Myoblasts (Baroffio et al. 1993) and fusioncompetent myoblasts (Krause et al. 1995) were prepared as Forced expression of h_eag hyperpolarizes undifferentiated myoblasts from −9 to −50 mV, the threshold for the activation of both Ih_eag and IK(NI). Similarly, the higher the density of IK(NI), the more hyperpolarized the resting potential of fusion-competent myoblasts. 6. It is concluded that h_eag constitutes the channel underlying IK(NI) and that it contributes to the hyperpolarization of fusion-competent myoblasts. To our knowledge, this is the first demonstration of a physiological role for a mammalian eag K¤ channel.8405

Ether à go-go Potassium Channels as Human Cervical Cancer Markers

“…On the other hand, EAG mRNA is expressed in several cancer cell lines including HeLa, MCF-7, SHSY-5Y, and IGR1 from carcinoma of the cervix, breast tumor, neuroblastoma, and melanoma, respectively (9,10). Despite the major expression of EAG in normal brain (9), endogenous EAG-mediated currents have been reported only in myoblasts (6) and in the tumoral cell lines SHSY-5Y (4), MCF-7 (11), and IGR1 (10). Eag is expressed in the tumor cell line HeLa (9); however, no EAG-mediated currents have been described in these cells.…”

“…Retinoic acid down-regulates hEAG current in neuroblastoma cells (4). hEAG is transiently expressed before myoblasts fusion, which is a cell cyclerelated event (5,6). hEAG current decreases during M phase and is modulated by cytoskeletal elements (7).…”

“…In particular, some members of the ether à go-go (EAG) potassium channels family are modulated through the cell cycle (2)(3)(4)(5)(6)(7)(8) and have been suggested to be involved in tumorigenesis (9 -17). Rat EAG channels expressed in frog oocytes display rectification induced by mitosis-promoting factor activation (2), and their conducting properties change during cell cycle (3).…”

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

“…Once activated, these cells start to proliferate as myoblasts and then differentiate and fuse into newly formed myotubes or damaged fibers. Electrophysiological studies performed on healthy human myoblasts indicated that the appearance of voltage-gated K + channels and the setting of the resting membrane potential (RMP) in the vicinity of -60 mV are processes associated with cell fusion [21][22][23][24]. 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].…”

Ion Channels as Promising Therapeutic Targets for Melanoma

“…No information about its expression in melanocyte has been reported. While its function in the brain remains unknown, Eag1 has been involved in skeletal muscle development, the channel being transiently expressed in the onset of myoblasts fusion (Bijlenga et al, 1998). Remarkably, Eag1 is associated with tumours development in patients and animals.…”