RGK protein-mediated impairment of slow depolarization- dependent Ca 2%2b entry into developing myotubes

Three physiological functions have been described for the skeletal muscle 1,4-dihydropyridine receptor (CaV1.1): (1) voltage-sensor for excitation-contraction (EC) coupling, (2) L-type Ca2%2b channel, and (3) voltage-sensor for slow depolarization-dependent Ca2%2b entry. Members of the RGK (Rad, Rem, Rem2, Gem/Kir) family of monomeric GTP-binding proteins are potent inhibitors of the former two functions of CaV1.1. However, it is not known whether the latter function that has been attributed to Ca V1.1 is subject to modulation by RGK proteins. Thus, the purpose of this study was to determine whether Rad, Gem and/or Rem inhibit the slowly developing, persistent Ca2%2b entry that is dependent on the voltage-sensing capability of CaV1.1. As a means to investigate this question, Venus fluorescent protein-fused RGK proteins (V-Rad, V-Rem and V-Gem) were overexpressed in normal mouse myotubes. We observed that such overexpression of V-Rad, V-Rem or V-Gem in myotubes caused marked changes in morphology of the cells. As shown previously for YFPRem, both L-type current and EC coupling were also impaired greatly in myotubes expressing either V-Rad or V-Gem. The reductions in L-type current and EC coupling were paralleled by reductions in depolarization-induced Ca2%2b entry. Our observations provide the first evidence of modulation of this enigmatic Ca2%2b entry pathway peculiar to skeletal muscle. © 2014 Landes Bioscience.

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