Ion occupancy of the channel pore is critical for triggering excitation-transcription (ET) coupling
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During membrane depolarization the voltage-gated calcium channel (VGCC) activates gene expression in excitable cells by means of a signal-transduction pathway termed excitation transcription (ET) coupling. The L-type calcium channel Cav1.2 can drive nuclear activity by either the ERK-CREB pathway, the Ca2+/calmodulin-dependent protein kinase II (CaMKII) cascade, or via the Ca2+-dependent protein phosphatase calcineurin. The ERK-CREB pathway mediates nuclear activity via a direct interaction of the intracellular β subunit of VGCC with the Ras/GRF1 complex. Here we show that ET coupling in HEK293 cells transfected with wt Cav1.2 or the Timothy mutant Cav1.2G406R is mediated by substituting Ca2+ with the impermeable lanthanum (La3+). In the absence of extracellular Ca2+ or La3+, ET coupling was not triggered. This implies that cation occupancy of the selectivity filter, as opposed to calcium influx, plays an essential role in depolarization triggered signaling to the nucleus. ET coupling triggered by membrane depolarization in Cav1.2 transfected HEK293 cells and neuroendocrine PC12 cells was also supported by substituting Ba2+ for Ca2+ as the charge carrier. Since Ba2+ ions do not bind to calmodulin this implies activation of ET coupling via a Ca2+/calmodulin-independent pathway. Together, these results suggest a model whereby nuclear signaling through the ERK-CREB pathway is driven by voltage-dependent conformational change that requires channel pore occupancy and is Ca2+ influx-independent. This model is also consistent with the previous observation that ET coupling can be driven by the Ca2+-impermeable Cav1.2L745P mutant. Thus, the conversion of synaptic stimuli to transcriptional activation is mediated by the metabotropic function (Ca2+-inflow independent) of Cav1.2, similar to the ion-influx independent depolarization-triggered transmitter release and transcription activation mediated by the NMDA receptors.
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