Protein kinase A phosphorylation enhances sodium channel currents in Xenopus oocytes

Abstract

The voltage-sensitive rat brain sodium channel is known to be phosphorylated by adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase A (PKA), but the functional significance of that phosphorylation is unknown. We have shown that rat brain sodium channel currents expressed in Xenopus oocytes were enhanced by induction of PKA activity. Stimulation of the beta 2-adrenergic receptor or treatment with dibutyryl cAMP resulted in increased sodium current amplitudes without affecting the voltage dependence of channel activation or inactivation. These increases were completely blocked by preinjection of protein kinase inhibitor, a specific inhibitor of PKA. Injection of phosphatase into the oocytes resulted in a significant decrease in sodium current amplitude, indicating that phosphorylation is important for basal levels of sodium channel activity in oocytes. The enhancement was specific for the rat brain IIA sodium channel, because currents expressed from the rat muscle microI sodium channel were not enhanced by the same procedures. These data demonstrate a modulatory role of PKA phosphorylation on brain sodium channel function and suggest a means by which the electrical excitability of cells may be regulated.

Analysis of Mutant Platelet-Derived Growth Factor Receptors Expressed in PC12 Cells Identifies Signals Governing Sodium Channel Induction during Neuronal Differentiation

“…Consistent with this possibility, expression of an activated form of Raf-1 (30) in PC12 cells results in sustained activation of MAPK and a minor, though significant, increase in Na ϩ current density, without concomitant increases in ␣-subunit mRNA levels (15). Translational and posttranslational events appear to be developmentally regulated, rate-limiting steps in the expression of ␣ subunits in the brain and retina (66,85), and efficient expression of functional Na ϩ channels appears to require subunit assembly, glycosylation, and phosphorylation (23,67,83; for a review, see reference 5). If activation of the Src family of kinases normally influences one of these events, the loss of this signal may decrease the induction of functional Na ϩ channel expression without affecting the induction of type II ␣-subunit mRNA.…”

Structure and function of voltage‐gated sodium channels

“…Interestingly, when the cardiac channel is phosphorylated by PKA, the whole-cell conductance increases, suggesting the specific pattern of phosphorylation is responsible for the functional effect (Schreibmayer et al 1994;Frohnwieser, Weigl & Schreibmayer, 1995;Murphy et al 1996;Frohnwieser, Chen, Schreibmayer & Kallen, 1997). The function of the skeletal muscle isoform of the Na¤ channel is not affected by PKA (Smith & Goldin, 1992, despite the fact that this channel is an excellent substrate for PKA-mediated phosphorylation. ) Indeed, the importance of Na¤ channel phosphorylation is not always clear, and caution should be exercised when attempting to relate phenomena in heterologous expression systems to more physiological settings (and vice versa).…”

Activity‐dependent regulation of the potassium channel subunits Kv1.1 and Kv3.1

“…Intracellular levels of cyclic AMP (cAMP), whose synthesis by adenylyl cyclase can be driven by activity or neurotransmitter activation (Offord and Catterall, 1989;Zirpel et al, 1998), can regulate voltage-gated K ϩ and Na ϩ currents in a variety of cell types (Offord and Catterall, 1989;Smith and Goldin, 1992;Bosma et al, 1993;McAnelly and Zakon, 1996;Yuhi et al, 1996;Allen et al, 1998;Golowasch et al, 1999), and cAMP can regulate the expression of K ϩ channels at the transcriptional level, via cAMP-responsive elements (CREs) within ion channel genes (Mori et al, 1993;Gan et al, 1996). Heterologous expression of Kv1.1 subunits in frog oocytes and mammalian cell lines has shown that changes in intracellular [cAMP] trigger changes in Kv1.1 protein and the ionic currents mediated by these channels (Bosma et al, 1993;Levin et al, 1995).…”